Download Organometallic Chemistry

Organometallic Chemistry
JHU Course 030.442
Prof. Kenneth D. Karlin
Spring, 2010
Kenneth D. Karlin
Department of Chemistry, Johns Hopkins University
[email protected]
http://www.jhu.edu/~chem/karlin/
Organometallic Chemistry
030.442 Spring 2010
Prof. Kenneth D. Karlin [email protected]
Class Meetings: TTh, 12:00 – 1:15 pm ++
Remsen Hall 347
p. 1
Textbook – Organometallic Chemistry (2nd Edition)
Gary O. Spessard, Gary L. Miessler
Oxford University Press
Course Construction: Homeworks, Midterm Exam(s), Oral Presentations (Grads)
TA: Craig Bettenhausen ([email protected])
Course Information:
http://www.jhu.edu/~chem/karlin/
Rough Syllabus Most or all of these topics
• Introduction, History/Key advances • Reaction Types
Oxidative Addition
• Transition Metals, d-electrons
Reductive elimination
–
• Bonding, 18 e Rule (EAN Rule)
Insertion – Elimination
Nucleophilic/electrophilic Rxs.
• Ligand Types / Complexes
• Types of Compounds
• Catalysis – Processes
M-carbonyls, M-alkyls/hydrides Wacker oxidation
Monsanto acetic acid synthesis
M-olefins/arenes Hydroformylation
M-carbenes (alkylidenes alkylidynes) Polymerization- Olefin metathesis
Water gas-shift reaction
Other Fischer-Tropsch reaction
p. 2
p. 3
Reaction Examples
•  Oxidative Addition
Reductive Elimination
Vaskaʼs complex
• Carbonyl Migratory Insertion
CH3Mn(CO)5
CO
O
CH3CMn(CO)5
• Reaction of Coordinated Ligands
O
(Iron pentacarbonyl)
(CO)4Fe–C O + :OH– ––––> (CO)4Fe
––––––>
(CO)4Fe–H
+
CO2
O
H
Reaction Examples - continued
p. 4
• Wacker Oxidation
C2H4 (ethylene) + ½ O2 –––> CH3CH(O) (acetaldehyde) Pd catalyst, Cu (co-catalyst)
• Monsanto Acetic Acid Synthesis
CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)
• Ziegler-Natta catalysts – Stereoregular polymerization of 1-alkenes (α-olefins)
1963 Nobel Prize
n CH2=CHR –––> –[CH2-CHR]n–
Catalyst: Ti compounds and organometalllic Al compound (e.g.,
(C2H5)3Al ) • Olefin metathesis – variety of metal complexes
2005 Nobel Prize – Yves Chauvin, Robert H. Grubbs, Richard R. Schrock
p. 5
Organo-transition Metal Chemistry History-Timeline
•  Main-group Organometallics
1760 - Cacodyl – tetramethyldiarsine, from Co-mineral with arsenic
1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)
n-Butyl-lithium
•  1827 – “Zeiseʼs salt” - K+ [(C2H4)PtCl3]–
Synthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl
Bonding- Dewar-Chatt-Duncanson model
p. 6
Organo-transition Metal Chemistry History-Timeline (cont.)
1863 - 1st metal-carbonyl, [PtCl2(CO)2]
1890 – L. Mond, (impure) Ni + xs CO –––> Ni(CO)4 (highly toxic)
1900 – M catalysts; organic hydrogenation (---> food industry, margerine)
1930 – Lithium cuprates, Gilman regent, formally R2Cu–Li+
1951 – Ferrocene discovered. 1952 -- Sandwich structure proposed
(Cp)2Fe
Cp = cyclopentadienyl anion)
(h5-C5H5)2Fe
(pentahapto)
Solid-state
structure
Ferrocene was first prepared unintentionally. Pauson and Kealy, cyclopentadieny-MgBr and
FeCl3 (goal was to prepare fulvalene) But, they obtained a light orange powder of "remarkable
stability.”, later accorded to the aromatic character of Cp– groups. The sandwich compound structure
was described later; this led to new metallocenes chemistry (1973 Nobel prize, Wilkinson & Fischer).
The Fe atom is assigned to the +2 oxidation state (Mössbauer spectroscopy).
The bonding nature in (Cp)2Fe allows the Cp rings to freely rotate, as observed by NMR
spectroscopy and Scanning Tunneling Microscopy. ----> Fluxional behavior. (Note: Fe-C bond
distances are 2.04 Å).
p. 7
Organo-transition Metal Chemistry History-Timeline (cont.)
1955 - Cotton and Wilkinson (of the Text) discover organometallic-complex
fluxional behavior (stereochemical non-rigidity)
The capability of a molecule to undergo fast and reversible intramolecular isomerization, the energy
barrier to which is lower than that allowing for the preparative isolation of the individual isomers at
room temperature. It is conventional to assign to the stereochemically non-rigid systems those
compounds whose molecules rearrange rapidly enough to influence NMR line shapes at
temperatures within the practical range (from –100 °C to +200 °C ) of experimentation. The energy
barriers to thus defined rearrangements fall into the range of 5-20 kcal/mol (21-85 kJ/mol).
Aside:
Oxidation State
18-electron Rule
p. 8
Fluxional behavior; stereochemical non-rigidity (cont.)
Butadiene iron-tricarbonyl
Xray- 2 COʼs equiv, one diff., If retained in solution, expect,
2:1 for 13-C NMR. But, see only 1 peak at RT. Cooling
causes a change to the 2:1 ratio expected. Two possible explanations: (1) Dissociation and re-association or (2) rotation of the Fe(CO)3 moiety so that COʼs become equiv.
Former seems not right, because for example addition of PPh3 does NOT result in substitution to give
(diene)M(CO)2PPh3.
Note: You can substitute PPh3 for CO, but that requires
either high T or hv. So, the equivalency of the CO groups
is due to rotation without bond rupture, pseudorotation.
13C-NMR
spectra
CO region, only
p. 9
Berry Pseudorotation
Pseudorotation: Ligands 2 and 3 move from axial to equatorial
positions in the trigonal bipyramid whilst ligands 4 and 5 move from
equatorial to axial positions. Ligand 1 does not move and acts as a
pivot.
At the midway point (transition state) ligands 2,3,4,5 are
equivalent, forming the base of a square pyramid. The motion is
equivalent to a 90° rotation about the M-L1 axis.
Molecular
examples could be PF5 or Fe(CO)5.
p. 10
The Berry mechanism, or Berry pseudorotation mechanism, is a type of
vibration causing molecules of certain geometries to isomerize by exchanging the
two axial ligands for two of the equatorial ones. It is the most widely accepted
mechanism for pseudorotation. It most commonly occurs in trigonal bipyramidal
molecules, such as PF5, though it can also occur in molecules with a square
pyramidal geometry.
The process of pseudorotation occurs when the two axial ligands close like a
pair of scissors pushing their way in between two of the equatorial groups which
scissor out to accommodate them. This forms a square based pyramid where the
base is the four interchanging ligands and the tip is the pivot ligand, which has not
moved. The two originally equatorial ligands then open out until they are 180
degrees apart, becoming axial groups perpendicular to where the axial groups
were before the pseudorotation.
Organo-transition Metal Chemistry History-Timeline (cont.)
p. 11
1961 – D. Hodgkin, X-ray structure – Coenzyme Vitamin B12 (see other page)
Oldest organometallic complex (because biological) (see other
page)
R H
H R
Catalysis
C C
C C
of 1,2-shifts
H H
H H
(mutases)
or
(homocysteine) RSH
Homocysteine
methylation
[B12CoIII-CH3]+
Methylmalonyl-CoA
––> Succinyl-CoA
(CoA = coenzyme A)
RSCH3 (methionine)
[B12CoI]–
1963 - Ziegler/Natta Nobel Prize, polymerization catalysts
1964 - Fischer, 1st Metal-carbene complex
1965 – Cyclobutadieneiron tricarbonyl, (C4H4)Fe(CO)3
– theory before experiment
(C4H4) is anti-aromatic (4 π-electrons)
With
-Fe(CO)3,
C4H4 behaves as aromatic 1965 – Wilkinson hydrogenation catalyst, Rh(PPh3)3Cl
1971 – Monsanto Co. – Rh catalyzed acetic acid synthesis
p.12
Vitamin B-12 Co-enzyme
Vitamin B-12 is a water soluble vitamin, one of the eight B vitamins. It is normally involved in
the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but
also fatty acid synthesis and energy production.
Vitamin B-12 is the name for a class of chemically-related compounds, all of which have
vitamin activity. It is structurally the most complicated vitamin. A common synthetic form of the
vitamin, cyanocobalamin (R = CN), does not occur in nature, but is used in many
pharmaceuticals, supplements and as food additive, due to its stability and lower cost. In the
body it is converted to the physiological forms, methylcobalamin (R = CH3) and
adenosylcobalamin, leaving behind the cyanide.
5-deoxyadenosyl group
p. 13
Organo-transition Metal Chemistry History-Timeline (cont.)
1973 – Commercial synthesis of L-Dopa (Parkinsonʼs drug)
asymmetric catalytic hydrogenation
2001 Nobel Prize – catalytic asymmetric synthesis, W. S. Knowles (Monsanto
Co.)
R. Noyori,, (Nagoya, Japan), K. B. Sharpless
(Scripps, USA)
1982, 1983 – Saturated hydrocarbon oxidative addition, including methane
1983 – Agostic interactions (structures)
p. 14
AGOSTIC INTERACTIONS:
Agostic – derived from Greek word for "to hold on to oneself”
C-H bond on a ligand that undergoes an interaction with the metal
complex resembles the transition state of an oxidative addition or
reductive elimination reaction.
Detected by NMR spectroscopy, X-ray diffraction
Compound above: Mo–H = 2.1 angstroms, IR bands were observed at
2704 and 2664 cm–1 and the agostic proton was observed at –3.8 ppm.
The two hydrogens on the agostic methylene are rapidly switching
between terminal and agostic on the NMR time scale.
p. 15
Organometallic Chemistry
Definition: Definition of an organometallic compound Anything with M–R bond R = C, H (hydride)
Metal (of course) Periodic Table – down & left
electropositive element (easily loses electrons)
NOT: • Complex which binds ligands via, N, O, S, other
M-carboxylates, ethylenediamine, water
• M–X where complex has organometallic behavior, reactivity patterns
e.g., low-valent
Oxidation State
M
–N
R'
R''
Charge left on central metal as the ligands are removed in their ʻusualʼ
closed shell configuration (examples to follow).
dn
for compounds of transition elements
N d < (N+1) s or (N+1) p in compounds
e.g., 3 d < 4 s or 4 p
d
d
n computation – very important in transition metal chemistry
n
zero oxidation state of M in M-complex has a
configuration d n where n is the group #.
Examples: Mo(CO)6
Mo(0) d
n
= d 6 (CO, neutral)
HCo(CO)4 H is hydride, H–, --> --> Co(I), d n = d 8 Group 5
Group 6
Group 7
V(CO)6– Cr(CO)6
Mn(CO)6+
V(–1)
Cr(0) Mn(+1)
d6
d 6 d 6
Isoelectronic and isostructural compounds (importance of d n)
Effective Atomic # Rule; 18-Electron Rule (Noble gas formalism)
# of electrons in next inert gas =
# Metal valence electrons + σ (sigma) electrons from ligands
Rule: For diamagnetic (spin-paired) mononuclear complexes in
organotransition metal compounds, one never exceeds the E.A.N.
p. 16
p. 17
d6
Cr(CO)6
(CO)6
Cr --->
6 electrons
e– - pairs from 6 ligands 12 electrons
––> to [Ar] configuration
18 electrons
(will see more in M.O. diagram)
Consequence of EAN Rule:
leads to prediction of maximum in coordination # Max coordination # = (18 – n) / 2 n is from d n
.
d n
10
8
6
4
2
0
Max Coord # 4 5
6
7
8
9
– Change in 2-electrons results in change of only one in Coord. #
– Any Coord. # less than Max # ---> “coordinatively unsaturated” –2e– +CO
Fe(CO)42– 2e–
–CO
Fe(CO)5
18 e–
18 e–
Fe(–2)
Fe(0)
d 10
d 8
4-coord 5-coord
both Coord. Saturated p. 18
[ReH9]2–
e.g., as Ba2+ salt
Re(VII), (Mn,Tc, Re triad)
d 0, 9 hydride ligands; CN = 9
Geometry: Face capped trigonal prism
A compound not obeying an rules
Fe5(CO)15C
Iron-carbonyl carbide
p. 19
Eighteen-Electron Rule - Examples
Co(NH3)63+
Cr(CO)6
Obey 18-electron rule for different reasons
Carbonyl Compounds in Metal-Metal Bonded Complexes
less straightforward
Fe2(CO)9
[π-Cp)Cr(CO)3]2
Co2(CO)8
(2 isomers)
p. 20
d6 Octahedral
maximum of 6 coordinate
eg
M+
M+
Free
ion
spherical
Δo
six
point
charges
spherically
distributed
t2g
octahedral
ligand
9ield
M+
M+
Free
ion
spherical
t2
Δt
four
point
charges
spherically
distributed
e
tetrahedral
ligand
9ield
p. 21
Picture of Octahedral Complex
Various representations
(ignore “s orbital”
lower
case
letters
for
orbital
dz2,
dx2-y2
(e2g)
(destabilized)
spherical
9ield
of
6
charges
10Dq
or
Δo
Oh
dxy,
dxz,
dyz
(t2g)
(stabilized)
p. 22
The five d-orbitals form a set of two bonding molecular orbitals (eg set
with the dz2 and the dx2-y2), and a set of three non-bonding orbitals
(t2g set with the dxy, dxz, and the dyz orbitals). eg orbitals point at ligands (antibonding)
appropriate symmetry for σ-bonds to ligands
σ-bonds will be six d2sp3 hybrids
ndz2, ndx2-y2, (n+1)s, (n+1)px,py,pz
t2g orbital set left as non-bonding
p. 23
p. 24
Standard MO diagram for
Octahedral ML6 complexes
with σ-donor ligands
e.g., [Co(NH3)6]3+ (18 e–)
e.g., W(Me)6 (12 e–)
Case I
Electron-configuration unrelated to 18–-Rule
1st Row-Complexes with “weak ligands”
Δo small or relatively small, eg* only weakly antibonding
No restriction on # of d-electrons –– 12 to 22 electrons
p. 25
Case II Compounds which follow rule insofar as they
p. 26
never exceed the 18-e– rule
• Metal in high oxidation state Δo is large(r) (for a given ligand)
radius is small –-> ligands approach closely ––> stronger bonding • 2nd or 3rd Row Metal - 4d, 5d
Δo is large(er) (for a given ligand); d-orbitals larger, more diffuse.
Complex
d n
Total e–Complex
ZrF62– ZrF73– Zr(C2O4)44–
WCl6 WCl6– WCl62– TcF62– 0
0
0
0
1
2
3
12
14
16
12
13
14
15
d n
OsCl62– W(CN)83–
W(CN)64–
PtF6
PtF6– PtF62– PtCl42– Less than 18 e–, but rarely exceed 18 e–
Total e–
4
1
2
4
5
6
8
16
17
18
16
17
18
16
p. 27
Similar Result if ligands are high in Spectrochemical Series
e.g., CN– Δo is larger
V(CN)63–
Cr(CN)63–
Mn(CN)63–
Fe(CN)63–
Fe(CN)63–
Co(CN)63–
d2 d3 d4
d5
d6 d6 Less than or equal to 6 d-electrons
eg* not occupied
however Co(II) d7 ––> Co(CN)53–
Ni(II) d8 ––> Ni(CN)42– and
Ni(CN)53–
Can have less than maximum # of non-bonding (t2g) electrons, because they
are nonbonding. Addition or removal of e– has little effect on complex stability
p. 28
Δo can get (or is) very small with π-donor ligands
F– example (could be Cl–, H2O, OH–, etc.)
a)  Filled p-orbitals are the only orbitals
capable of π-interactions
i)  1 lone pair used in σ-bonding
ii)  Other lone pairs π-bond
b)  The filled p-orbitals are lower in
energy than the metal t2g set
c)  Bonding Interaction
i.  3 new bonding MOʼs filled by
Fluorine electrons
ii.  3 new antibonding MOʼs form t2g*
set contain d-electrons
iii.  Δo is decreased (weak field)
d)  Ligand to metal (L M) π-bonding
i.  Weak field, π-donors: F, Cl, H2O
ii.  Favors high spin complexes
p. 29
Metal
Orbital
s
T1u
A1g
Eg
T2g
4p
4s
Molecular
Orbitals
Ligand
Orbitals
focus
on
this
part
only
Δo
eg
(σ*)
t2g
(π*)
both
sets
of
d
orbitals
are
driven
↑
in
energy
due
to
lower
lying
ligand
orbitals
T1g,T2g
3d
t2g
(π)
eg
(σ)
A1g
T1u,T2u
π‐orbitals
px,
py
T1u σ‐orbital
E pz
g
p. 30
Have discussed σ-donor and π-donor – now π-acceptor
antibonding
eg
(σ*)
eg
(σ*)
eg
(σ*)
Δo
t2g
(π)
M‐L
bonding
Δo
t2g
(n.b.)
non‐bonding
σ-donor
π‐acceptor
largest
separation
between
sets
of
d‐orbitals
intermediate
separation
Δo
t2g
(π*)
both
are
antibonding
π-donor
smallest
separation
Metal Orbitals
Molecular Orbitals
(only consider the d
orbitals – 4s and 4p
orbitals not included
in the analysis)
Ligand Orbitals
t2g
(π*)
T1g, T2g
T1u, T2u
eg (σ* M-L)
Mo(CO)6
p. 31
CASE III
π*
orbitals on CO
L high in spectrochemical series:
(6 x 2 each orthogonal)
CO, NO, CN–, PR3, CNR
π-acid ligands – π-acceptors
Eg
T2g
Can form strong π-bonds
18 e– rule followed rigorously
Δo
4d
t2g (π)
σ
orbitals on CO
(6 x 1 each)
A1g
Orbitals on M used in such π-bonding
T1u
are just those which are non-bonding
Eg
eg (σ
M-L)
Result: Increase in Δo
Imperative to not
Have electrons in eg* orbitals
Want to maximize occupation of t2g
because they are stabilizing
p. 32
p. 33
Implications of 18e– Rule for Complexes with π-accepting ligands
In octahedral geometry almost always have 6 d-electrons
12 electrons from ligands
Other cases: # d-electrons and coordination # complementary
• Coordination # exactly determined by electron-configuration and vice-versa
BrMn(CO)5 (d ?)
(see previous notes)
I2Fe(CO)4 (d ?)
Fe(CO)5 (d ?)
All 18-electron
Ni(PF3)4 (d ?)
When M has odd electron ––––> metal-metal bond (often bridging COʼs)
Mn2(CO)10
Co2(CO)8
Some 17 electron species known: V(CO)6 d 5
Mo(CO)2(diphos)2]+ d 5
See MO diagram: Want to fill stable MOʼsʼ there is a large gap to LUMO
p. 34
Major Exception: d 8 square-planar complexes
As one goes across periodic table, d and p orbital energy
Level splitting gets larger – hard to use p orbitals for σ-bonding
Common to have 4-coordinate SP complexes – dsp2 hybridization dx2-y2
Which d-orbitals?
e g
Common for:
dxy
Δo
Rh(I), Ir(I)
Pd(II), Pt(II)
dz2
t2g
dxz
dyz (degenerate )
ML6
ML4
Rationalize d-orbital splittings
look at d-orbital pictures/axes
p. 35
p. 36
Again, examples of complexes:
dn
C.N.
Coord. Geom. Example(s)
d10
4
Td
Ni(CO)4, Cu(py)41+
d10
3
Trig.planar
d10
2
Linear d8
5
TBP
d8
4
(square) planar d4
7
capped octahedral
d2
8
sq. antiprism
d0
9
D3h symmetry Pt(PPh3)3
(PPh3)AuX, Cu(py)2+
Fe(PF3)5
Rh(PPh3)2(CO)Cl (trans)
Mo(CO)5X2
ReH5(PMePh2)3, Mo(CN)84–
tricapped trig. prism
[ReH9]2–
p. 37
LIGANDS in Organometallic Chemistry:
Ligands,
charge,
coordination # (i.e., denticity)
X
SnCl3 H (hydride)
Ar
RC(O) (acyl)
R3E (E = P, As, Sb, N)
CO
RNC (isonitrile, isocyanide)
R 2N
N2
R2C (cabenoid, carbene)
C3H5–
(π-allyl)
π-C5H5 (π-Cp) π-C3H3 (cyclopropenium, +)
ArN2+ (diazonium)
R2P
C2H4 (olefin, alkene) C4H4 (cyclobutadiene) benzene (arenes)
CH3 (alkyl, perfluoroalkyl)
R2C2 (acetylene)
CH=CH-CH2– (σ-allyl)
π-C7H7 (tropylium)
O (O-atom; oxide)
NO (nitrosyl)
p. 38
Carbon Monoxide – exceedingly important ligand CO-derivatives known for all transition metals
Structurally interesting, important industrially, catalytic Rxs
Source of pure metal: Ni (Mond); Fe contaminated with Cu, purify via Fe(CO)5
Fe & Ni only metals that directly react with CO
Source of oxygen in organics: RC(O)H, RC(O)OH, esters
Processes: hydroformylation, MeOH ––> acetic acid
double insertion into olefins, hydroquinone synthesis (acetylene + CO;
Ru catalyst), acrylic acid synthesis (acetylene, CO, Ni catalyst)
Fischer Tropsch Rx: CO + H2 ––> ––> CnH2n+2 + H2O
Most of these involve CO “insertion”
p. 39
Metal-Carbonyl Synthesis:
Reduction of available (in our O2-environment) metal salts,
e.g., MX2, MʼX3, other (e.g., carbonates)
M-carbonyls generally in low-valent oxidation states
––––> “Reductive Carbonylation”
Reductants: CO itself ( ––> CO2), H2, Na-dithionite
Some Reactions: WMe6 + xs CO –––> W(CO)6 +
NiO + H2 (400 °C) + CO ––> Ni(CO)4
3 Me2CO
Re2O7 + xs CO ––> (OC)5Re–Re(CO)5 + 7 CO2
Cl– acceptor/reductant
RhCl3 + CO + pressure + (Cu, Ag, Cd, Zn) –––> Rh4(CO)12 or Rh6(CO)16
Structures Possible: X-ray diffraction, Infrared spectroscopy
Ni(CO)4
Td 2058 cm–1
Fe(CO)5
M(CO)6
D3h Oh
2013, 2034 cm–1
2000 cm–1 p. 40
H3B–CO = 2164 cm–1
no backbonding possible
13C
NMR spectroscopy of M-CO fragments: 180 – 250 ppm
Useful to use 13C enriched carbon monoxide
Can be useful to observed “coupling” to other spin active nuclei,
e.g., 103Rh or 13P
Metal-Carbonyl Structures (cont.):
Polynuclear Metal-Carbonyls
p. 41
p. 42
p. 43
p. 44
p. 45
The backbonding between the metal and the CO ligand,
where the metal donates electron density to the CO ligand
forms a dynamic synergism between the metal and ligand,
which gives unusual stability to these compounds.
O:
M=C=O
:
Valence Bond formalism: M–C
+
:
–
p. 46
C–O stretching frequencies, ν(C-O) Put more electron density on metal
– by charge
– by ligands which cannot π-accept
Remaining COʼs have to take up the charge (e–-density) on the metal
See effects on ν(C-O).
Ni(CO)4
[Co(CO)4]–
Fe(CO)42–
2057 cm–1
1886 cm–1
1786 cm–1
–––––––> –––––––> more –ve charge
Mn(dien)(CO)3+
Cr(dien)(CO)3 2020, 1900 cm–1
~1900, 1760 cm–1 (dien not π-acceptor)
~
p. 47
Reactions of Metal-Carbonyl Complexes
Substitution of CO:
PX3, PR3, P(OR)3, SR2, NR3, pyridine, OR2, RNC, RCN, olefins, NO
Examples:
––heat or hv––> [Fe(CO)3L2] + 2 CO
Fe(CO)5 + 2 L (or L2)
Mo(CO)6 + cycloheptatriene –heat or hv––> [Mo(cht)(CO)3] + 3 CO
Cr(CO)6 + arene ––heat or hv––> [Cr(arene)(CO)3] + 3 CO
Fe(CO)5 + 2 H–C=C–H
L = PPh3 (trans)
––heat or hv––> [Fe(CO)3(C5H4O)] + CO
Oxidation – Carbonyl Halides
Mn2(CO)10 + Br2
––heat ––> Mn(CO)5Br
[FeCp*(CO)2]2 + Br2
2 PtCl2 + 2 CO
O
––heat ––>
Fe CO
CO
[FeCp*(CO)2Br] –––> [Pt(CO)(Cl2]2 (µ2-Cl)2
CO
p. 48
Reactions of Metal-Carbonyl Complexes (cont.)
Nucleophilic Attack – Reactions with bases
Previous – Fe(CO)5 + hydroxide
+ PF6–
OC
Fe
+
CO
Na+BH4–
THF
–80 °C
OC
H
Fe
C
CO
CO
O
OC
Fe
H
O–
Fischer type
M-carbene
CO
O
M(CO)n + R3N+O–
––––> (CO)n–1M–
C
Use: liberate M(CO)3 groups; or oxidize M
–––> M(CO)n–1 + R3N + CO2
ON+R3
Fe2(CO)9 + 4 OH– ––––––> Fe2(CO)82–
Cr(CO)6
+ 3 KOH
Cr(CO)6
+ BH4–
––––––> KHCr(CO)5 + K2CO3 + H2O
––––––> [(CO)5Cr–H–Cr(CO)5]–
p. 49
Reactions of Metal-Carbonyl Complexes (cont.)
Alkyl Metal Carbonyls
NaMn(CO)5 + CH3X
––––> Mn(CO)5CH3
NaMn(CO)5 + RC(O)Cl
––––> Mn(CO)5C(O)R
NaCo(CO)4 + (C2F5C(O))2O (anhydride) –––> Co(CO)4C(O)C2F5 +
Metal-Olefin Complexes
Zeiseʼs Salt, Pt-olefin complex
Ag-(triflate) + C2H4 ––––>
[PtCl4]2– +
C2H4 ––––>
[(C2H4)Ag-OSO2CF3]
[PtCl3(C2H4)]– + Cl–
FeCp(CO)2I + C2H4 + AgBF4 –––> [FeCp(CO)2(C2H4)]BF4 + AgI
Fp-CH2-CH=CH2 + H+
Fp-CHMe2 + Ph3C+BF4–
–––> [Fp-CH2-CH=CH2]+ –––> [Fp-CH2-CH=CH2]+ Fp = (Cp)Fe(CO)2–
[ (Cp)FeI(CO)2• or (Cp)FeII(CO)2+ ]
p. 50
Dewar-Chatt-Duncanson
model for
M-Olefin Bonding
Not unlike M-CO bonding
π and π* alkene orbitals Proper symmetry; good overlap
σ -Bond – πolefin + empty d-orbital
π – bond – π*olefin + filled d-orbital
Overall double-bond character
M-olefin bonding reduces C–C bond-strength
p. 51
(η2-C70)Ir(X)(CO)(PPh3)2
C60 - Buckminsterfullerene – Buckyballs
(Soccer-ball 6- and 5-membered rings) Fullerenes
Buckyferrocene Olefin σ-donation to M, AND π-donation to π* by metal leads to
reduction in C–C bond strength.
p. 52
Also, for longer C–C distances, olefin is no longer planar. can regard
metal-olefin as a metallacyclopropane (sp3 carbons)
Keq –––> M-olefin + L
M–L + olefin
<–––
Studied with Pd(II), Ni(0), Rh(I)
Keq (M-olefin bond-strength) smaller for sterically hindered olefins
Keq increased by e– -withdrawing substituents (-CN, -carboxyl)
Keq decreased by e– -donating substituents
Back donation from M into olefin π*-orbital; predominant in M-olefin bonding.
Ni(0) – d10 – olefin cannot donate to the metal
Electronic effect less pronounced with metal with less d e– -density
Ni(0) > Fe(0) > Rh(I) > Pt(II)
p. 53
Also follow by IR spectroscopy:
Ethylene, 1623 cm–1; Zeiseʼs salt, 1516 cm–1
Olefin coordination tendencies:
p. 54
Tend to be perpendicular to plane of Square-Planar Complexes
In the plane, for trigonal or TBP compounds
(Not relevant for octahedral complexes)
In solution, olefins are not in fixed orientations – olefins rotate
(Cp)Rh(C2H4)2
–20 °C
Cp : inner : outer
“inner” and “outer” Hʼs
= 5 :
4
: 4
i o RT two C2H4 peaks strongly broadened
non-equiv Hʼs exchange at rate intermediate on NMR time scale
+ 57 °C
two C2H4 peaks coalesce to one
(Cp)Rh
i
o
Exchange fast. NMR cannot distinguish between non-equiv Hʼs Cp remains singlet throughout whole T range
p. 55
Two modes for rotation consistent with NMR spectroscopic data
C
M
M
C
ON
OC
C
Propeller like movement
C
PPh3
H''
Os
H''
Hʼ ʻs are equiv to each other; same with Hʼʼ ʻs
but
Hʼ ʻs are different from Hʼʼ ʻs
H'
PPh3
H'
• Rotation about C–C axis would not change situation
• Propeller movement would exchange non-equiv hydrogens
•NMR spectroscopy shows two separate peaks at –90 °C
• they coalesce at –65 °C
–––> Propeller like movement is operative
Measured barrier to rotation ~ 50-60 kJ/mole for C2H4
No rotation for CF2=CF2 and (NC)2C=C(CN)2
Stronger π-bonding restricts rotation
p. 56
Olefin metal complexes have a considerable use in organic synthesis
Metal alters chemical behavior of olefins
Metal can activate, deactivate or protect double bond for electrophilic
or nucleophilic attack
Resolve optical or geometric isomers
direct stereospecific attack
aromatic or de-aromatize appropriate systems
Can effect olefin metathesis reactions - polymerizations
Example
O
Cp
OC
C
Fe
Fe
CO
Cp
C
O
2 HBF4
Et2O
(CO)2CpFeCl
+
or
(CO)2CpFe–
[Cp(CO)2Fe–
90 %
]+ BF4–
O
THF
0°C to 25 °C
30 min
NaI
Cp(CO)2Fe
O–
olefin liberated
acetone
Use method to reduce epoxides stereospecifically to olefins with retention of configuration
p. 57
Another Important Reaction of Olefins
Wacker Process – Hoechst-Wacker Process
Ethylene Oxidation - German Invention
1st Homogeneous Catalytic Process with organometallic (R-Pd)
compound used on an industrial scale (related to hydroformylation)
Net Reactions:
[PdCl4]2– + C2H4 + H2O –––> CH3CHO + Pd + 2HCl + 2Cl–
Pd + 2CuCl2 + 2Cl–
–––>
[PdCl4]2– + 2CuCl
2CuCl + ½ O2 + 2HCl –––> 2CuCl2 + H2O
==============================================
C2H4 + ½ O2
–––> CH3CHO
p. 58
p. 59
p. 60
Mechanism summary
Several interesting key points:
(1) there is no H/D exchange seen in this reaction. Reaction runs with C2D4 in
water generate CD3CDO, and runs with C2H4 in D2O generate CH3CHO. Thus,
keto-enol tautomerization is not a possible mechanistic step.
(2) There is a negligible kinetic isotope effect with fully deuterated reactants (k H/k
D=1.07). Hence, it is inferred that hydride transfer is not a rate-determining step.
(3) a significant competitive isotope effect with C2H2D2, (k H/k D= ~1.9), suggests
that the rate determining step should be prior to oxidized product formation.
The bulk of mechanistic studies on the Wacker Process debated whether
nucleophilic attack occurred via an external (anti-addition) pathway or via an
internal (syn-addition) pathway. In summary, it was determined that syn-addition occurs under lowchloride reaction concentrations (< 1 mol/L, industrial process conditions), while
anti-addition occurs under high-chloride (> 3 mol/L) reaction concentrations.
However, the exact pathway and the reason for this switching of pathways is still
unknown.
p. 61
Another key step in the Wacker process is the migration of the hydrogen from
oxygen to chlorine and formation of the C-O double bond. This step is generally
regarded to proceed through a so-called β-hydride elimination with a fourmembered cyclic transition state:
One in silico study[JACS,2006] argues that the transition state for this reaction
step is unfavorable (activation energy 36.6 kcal/mol) and proposes an alternative
reductive elimination reaction mechanism in which the proton directly attaches
itself to chlorine with an activation energy of 18.8 kcal/mol. The proposed reaction
step gets assistance from a water molecule acting as a catalyst.
Pd(0)
Reoxidation
Must be complicated
p. 62
This and the
next 5 slides
are due to
Darren Achey
and
Byron Farnum
2
Febʼ09
Shows only Syn mechanism - Nucleophilic attack by OH- ligand
p. 63
Indicates Syn and Anti mechanisms – dependence on Cl- concentration
p. 64
Beyramabadi, S. A.; Eshtiagh-Hosseini, H.; Housaindokht, M. R.; Morsali, A.;
Organometallics, 2008, 27, 72-79.
Syn
additi
on
- Water-Chain mechanism
- Compared Syn vs. Anti mechanisms
for rate determining step
- All DFT calculations
- Accounted for kinetic isotope effect data
for O-D vs O-H bond breaking
Anti
additi
on
Syn
p. 65
Transition State
Syn-Product
p. 66
Anti
Lower Activation
Barrier
Concluded to be the
mechanism of ratedetermining step
Transition State
Anti-Product
p. 67
Kieth, J. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A.; Henry, P. M.; Organometallics,
Feb. 2009
- Rebuked the article by Beyramabadi et. al.
- Emphasized the well established nature of the syn mechanism at low [Cl-]
(Standard Conditions)
Mech depends highly on [Cl-] and [CuCl2] LL – syn mechanism
HL – Isomerization
HH – anti mechanism w/ chlorohydrin products
Allyl Ligand Organometallic Complexes
unidentate 2-e– anionic ligand
rarely observed form [CH2=CHCH2Co(CN)5]3–
[CH2=CHCH2Mn(CO)5] C–C stretch ~ 1620 cm–1
p. 68
alkyl + neutral alkene (2-e– )––> bidentate
most common structure
behaves as delocalized π-system
3 (4) electrons now valence electrons
[(η3-C3H5)PdCl]2
[(π-C3H5)PdCl]2
∠C-C-C ~ 120 ° (sp2). C…C(observed) = 1.40-1.43 Å; C–C = 1.54 Å; C=C = 1.34 Å
Allyl Ligand-M Complexes (continued)
p. 69
[(h3-C3H5)PdCl]2 approximately square-planar (allyl as bidentate)
16-electron system
Pd(II), d8, 2 Clʼs (4 e–), allyl is 2 e–) (can think about allyl as 3e–)
Metal interaction with allyl ψ3:
Always M-to-allyl ligand Can be M-to-L (L-anion) or L-to-M (allyl-cation)
Maximize bonding for allyl ψ2: want terminal Cʼs in the PdCl2Pd plane. So Allyl plane tilts wrt PdCl2Pd plane from 90° to ~ 110 ° (central C bent away); moves terminal Cʼs closer to M Always Ligand to M. To maximize bonding:
PdCl2Pd plane cuts π-allyl skeleton~ 2/3 of the distance (center of gravity) from the central C-atom towards the terminal Cʼs
Allyl Ligand-M Complexes (continued)
p. 70
Tilted allyl group: Organometallics 1985, 4, 285: Neutron diffraction structure
(able to locate Hʼs) of a Ni-allyl complex. Hmeso and Hsyn are bent towards M (7°
and 13° from planar); Hanti are bent 31° away. (D.Astruc text – says opposite)
Typical static 1H-NMR of trihapto allyl: Hanti at 1 - 3 ppm, Hsyn at 2 - 5 ppm
and Hmeso at4 - 6.5 ppm. There is no syn-anti proton-proton coupling. In the 13C-NMR, terminal Cʼs at 80 - 90 ppm; Central C, 110-130 ppm.
Allyl ligands can be fluxional on the NMR time scale; see “exchange” of Hʼs.
Allyl Ligand-M Complexes (continued)
p. 71
Syntheses of Allyl-Metal complexes
From alkene-complex, H-attack on the metal
Overall 1,3-hydrogen shift
Nucleophilic (electrophilic) substitution using allylic substrate
Oxidative addition of allylic substrate to low-valent metal
Protonation or insertion of a 1,3-diene complex
Allyl Ligand-M Complexes (continued)
p. 72
Allyl-M (π-to-σ; η3-to-η1) interconversions important to catalysis/synthesis,
as a way to create a vacent coordination site
and a way to exert fluxional
behavior
With excess of PPh3, can observe η1-intermediate by IR spectroscopy
1600-1650 cm–1
[Mn(CO)5]– + C3H5Cl –––> (η1-C3H5)Mn(CO)
Δ or hν 5
18-electron species ––––> (η3-C3H5)Mn(CO)4 + CO
Allyl Ligand-M Complexes (continued)
p. 73
Some reactions:
Nucleophilic attack:
can be stereoselective
maybe useful
Insertion reaction
Reductive elimination
p. 74
Digression – A Metallocene of a Different Kind
Uranocene – Bis(cyclooctatetraene-dianion)-uranium
Considerations of Aromaticity
METAL HYDRIDE COMPLEXES
Important class of compounds
p. 75
(relate to M-olefin, M-R compounds)
Hydrogenation (stereospecific), hydrogen storage (H2 economy), catalysis
Unstable
hydrides discovered in 1930ʼs
H2Fe(CO)4,
HCo(CO)4
not understood
trans H-Pt(Cl)(PEt3)2 1957 breakthrough, J. Chatt (UK)
discovered by accident, found good prep later
cis-PtCl2(PEt3)2 + N2H4
–––>
trans H-Pt(Cl)(PEt3)2 + N2 could be sublimed strong, sharp Pt-H stretch in infrared spectrum
confirmed by replacement by D
Later, an X-ray structure was obtained, ––> ʻnormalʼ Pt(II) complex Using M-H stretch, trans H-Pt(Cl)(PEt3)2 used to measure “trans effect” confirmed later via studies on actual relative rates of substitution (will discuss)
Metal-Hydride Complexes (continued)
IR/Raman Spectroscopy
more intense than ν(C-H), ~ 3000 cm–1
p. 76
Terminal M–H ν(M-H) 1900 ± 300 cm–1
weaker than ν(C-O), ν(N-N), ν(N-C), RNC
Bridging M–H
1000 ± 300 cm–1
(broad; v1/2 ~ 100 cm–1)
Metal-Hydride Complexes (continued)
Aspects of Bonding
p. 77
Metal-Hydride Complexes (continued)
p. 78
Problem: Given a complex formulated as Ru(H)(CO)(Cl)(PPh3)3 IR bands are observed at 2020 cm–1 and 1933 cm–1. How can you assign
the bands either to the Ru–H or C-O stretching frequencies?
M–Hterminal and M–CO terminal IR stretches are in similar regions of
spectrum. Thus, cannot assign directly. A solution would be to prepare
Ru(D)(CO)(Cl)(PPh3)3 or Ru(H)(13CO)(Cl)(PPh3)3, because IR bands would
undergo an isotope shift. Heavy isotope substitution reduces frequency of corresponding vibration;
reduced mass, µ, in Hookeʼs Law, increases
{Force constant k doesnʼt change; bond strengths change little with isotope
substitution.}
Hookeʼs Law:
Metal-Hydride Complexes (continued)
ν1 / ν2
=
µ 2 / µ1
νRu-D / νRu-H = µRu-H / µRu-H
For vibration at 2000 cm–1:
p. 79
1/2
1/2
~ 0.71
2000 cm–1 x (0.71) = 1420 cm–1
For Ru(D)(CO)(Cl)(PPh3)3: Should observe Ru–D at ~1420 cm–1
ν(13C–16O) / ν(12C–16O) = 0.978
For a Metal-Carbonyl at: ν(12O–16O) = 2000 cm–1
Δ(13C–O) ~ 44 cm–1
ν(18O–18O) / ν(16O–16O) = 0.9
For a Metal-Peroxide: ν(16O–16O) ~ 800 cm–1
Δ(18O2) ~ 48 cm–1
p. 80
Metal-Hydride Complexes (continued)
NMR spectroscopy. high fields, delta (δ) 15-30 ppm relative to TMS
along with coupling constants (e.g., JP,H) e.g., distinguish cis vs. trans.
useful for sterochemical analysis.
Other book: The chemical shift range for hydrides is approximately +25 to
–60 ppm. The downfield shifts are most common in d0, d10 and early
transition metal cases whereas those with other dn counts and late transition
metals tend to be upfield of zero. Coupling to other spin active nuclei such
as 31P often makes structural assignments unambiguous.
Xray diffraction: near other heavy atoms, position often inferred
Other ligands bend towards position of ʻhydrogenʼ, because it is small
Neutron diffraction; finding of an atom proportional to Z (X-ray, it is Z2)
HMn(CO)5 Neutron diffraction study)
Mn–H = 1.6 Å, which
equals sum of the covalent radii.
TRANS – EFFECT (INFLUENCE) in INORGANIC CHEMISTRY
p. 81
Trans effect (influence) - trans effect is the labilization of ligands
trans to certain other ligands, which can thus be regarded as trans
directing ligand. It is attributed to electronic effects and it is most notable
in square planar complexes, In addition to this kinetic trans effect, trans
ligands also have an influence on the ground state of the molecule,
notably on bond lengths and stability
Some authors prefer the term trans influence to distinguish it
from the kinetic effect, while others use more specific terms such as
structural trans effect (i.e., elongated trans M-L distances) or
thermodynamic trans effect.
The intensity of the trans effect (as measured by the increase in
rate of substitution of the trans ligand) follows this sequence:
F−, H2O, OH− < NH3 < py < Cl− < Br− < I−, SCN−, NO2−, SC(NH2)2, Ph− <
SO32− < PR3, AsR3, SR2, CH3− < H−, NO, CO, CN−, C2H4
Established by substitution kinetic measurements, M–H stretch (as mentioned
above), or other observations
p. 82
Classic example of the trans effect: the synthesis of cis-platin.
Starting from PtCl42−, the first NH3 ligand is added to any of the four
equivalent positions at random, but the second NH3 is added cis to the first
one, because Cl− has a larger trans effect than NH3. If, on the other hand,
one starts from Pt(NH3)42+, the trans product is obtained instead.
Cl
Cl
2–
+ NH3
PtII
Cl
Cl
H3N
NH3
PtII
H3N
– Cl–
NH3
Cl
NH3
1–
+ NH3
PtII
Cl
– Cl–
Cl
NH3
Cl
PtII
Cl
Cis
2+
+
Cl–
–NH3
H3N
Cl
PtII
H3N
NH3
1+
+ Cl–
–NH3
NH3
Cl
H3N
PtII
Cl
NH3
Trans
Metal-Hydride Complexes (continued)
In hydrido carbonyls, get mixing of ν(M-CO) and ν(M-H) modes,
especially when CO and H are trans. So deuterate to shift M-D to lower
energy and separate out (and less mode mixing).
p. 83
Metal-Hydride Complexes – More Syntheses by Protonation
[Cp2Re]– + H+ –––>
HCp2Re
H
H–Ir(CO)L3 +
HCl
–––> L
OC
Mn
+
H
L Cl
L
+
Fe
Mn(CO)5–
Ni{P(OEt)3}4
H+
+
+ H+
+
–––>
Fe H
H–Mn(CO)5
H+
Ni(H){P(OEt)3}4
p. 84
p. 85
Metal-Dihydrogen (H2)
Complexes
Figure:
ORTEP Drawing
Neutron diffraction study, 30 K
W(CO)3(PnPr)3)2(η2-H2)
Intact H–H bond
Elongated (by ~ 20 %)
to 0.82(1) Å
(lower PR3 disordered)
p. 86
Metal-Dihydrogen (H2) Complexes – (continued)
H
LnM
H
Elongated H–H bond:
H2 is not physisorbed
but chemisorbed
H2 bond “activated”
toward breaking
H
H
LnM
H
dihydrogen
complex
LnM
H
dihydride
complex
“This initially
enigmatic
Interaction lies at the heart of all
Interactions of
sigma bonds
X–Y with metals” (G.Kubas)
p. 87
Metal-Dihydrogen (H2) Complexes – (continued)
H2-binding is reversible (to this analogue with P(Cy)3)
“relevant to new materials for hydrogen storage” ?
p. 88
Metal-Dihydrogen (H2) Complexes – (continued)
p. 89
Metal-Dihydrogen (H2) Complexes – (continued)
p. 90
Metal-Dihydrogen (H2) Complexes – (Syntheses)
p. 91
Metal-Dihydrogen (H2) Complexes –> Hydrides
M-H2 complexes also called “non-classical” hydrides
Metal-Dihydrogen (H2) Complexes – (H–H bonding / NMR)
p. 92
Solution 1H NMR spectra of η2-H2 ligands normally give broad
uncoupled signals throughout a large range of chemical shifts (2.5 to
–31 ppm) that can overlap with those for classical hydrides. NMR
can be used to determine dHH in solution by two different techniques
involving measurement of either JHD or relaxation time, T1. JHD for
the HD isotopomer of an H2 complex is the premier diagnostic for H2
versus hydride coordination. The 1H-NMR signal for an HD complex
becomes a 1:1:1 triplet (D has I = 1 : (2I + 1) with a much narrower
line width and is direct proof of the existence of an H2 ligand, since
classical hydrides do not show significant JHD because no residual
H–D bond is present. JHD for HD gas is 43 Hz, the maximum value
(dHD ) 0.74 Å), and lower values (20 – 34 Hz) represent
proportionately longer (shorter) dHD. JHD determined in solution
correlates well with dHH in the solid state, and both Morris and
Heinekey developed empirical relationships:
dHH = 1.42 – 0.0167(JHD) Å
dHH = 1.44 – 0.0168(JHD) Å
(Morris)
(Heinekey)
p. 93
Metal-Dihydrogen (H2) Complexes – (continued)
Dynamics in M–H2 and M–H Complexes
Metal-Dihydrogen (H2) Complexes –dynamics (cont)
p. 94
Hydrogenase Metalloenzymes
p. 95
Redox enzymes: billions of years old – found in microorganisms
Catalyze complete reversible interconversion of H2 & H+ / e–
H2 as energy source or
dispose of excess electrons via H2 release
High turnover rates: 104 turnovers/s
H2
2 H+ + 2e–
True equilibrium; position (e)affected by H2 pressure
H2 + D2O
HD + HDO (rx observed)
pH dependent ––> infer that H2 is split heterolytically at metal(s)
Hydrogenase – (continued)
Understanding the mechanism of hydrogenase might
help scientists design clean biological energy sources,
such as algae, that produce hydrogen
p. 96
p. 97
Fe – Fe Hydrogenase
Molecular “wire”
p. 98
Hydrogenase – (continued)
Active-site attached
only at one point
Iron-Iron bond:
Site of H2
heterolysis
CO and CN ligands
On
low-spin
Fe(II)
Possible Mechanism for Hydrogenase: H2
2H+ +
p. 99
2e–
Proposed H2ase Mechanism
With dithiolate bridge
Based on DFT Calculations
Overall charges not shown
p. 100
Proposed H2ase Mechanism
aza-dithiolate bridge
DFT calculations
Overall charges not shown
p. 101
p. 102
Hydride transfers to M, resulting olefin may or may not stay coordinated
Requirements – (i) Vacant site. (ii) complex usually has less than 18e–,
Otherwise a 20 electron complex results immediately
Beta-hydride Elimination
Mechanism –––> Four-center transition state inferred
H
M
CH2
CH2
H
M
C
C
H
H
H
M
C
H2
CH2
H H
Olefin-insertion – microscopic reverse reaction
critical step in olefin polymerization
β-hydride elimination is a termination step in olefin polymerization
Known structures (X-ray and/or Neutron diffraction),
Supporting the proposed four-center transition state
Stable M-alkyls – No beta-hydride elimination
CMe3
M
M
M
neophyl
neopentyl
CMe3
M
benzyl
Rh(III)
d 6
low-spin
SiMe3
CMe2Ph
"silyl-neopentyl"
M
R
M
norbornyl
alkynyl
Ex: Stabilize M-alkyl-to β-hydride
elimination: have a stable
complex where ligands do not
come off to create vacant site,
that which is needed
α-hydride elimination Alpha-hydride elimination is the transfer of a hydride (hydrogen atom) from the
alpha-position on a ligand to the metal center. The process can be thought of as a
type of oxidative addition reaction as the metal center is oxidized by two electrons
(Eq 1). As the reaction involves a formal oxidation of the metal, alpha-elimination
can not occur in a d0 or d1 metal complex. In these cases, a variant called alphaabstraction can occur. Alpha-abstraction does not result in a change of oxidation
state and the alpha-hydrogen is transferred directly to an adjacent ligand instead of
the metal center (Eq 2):
Delta and gamma eliminations also exist
INSERTION REACTIONS
U = an unsaturated ligand
Insertion Reaction Net Result:
Decrease in coordination, formation of new U–X bond
Reverse reaction referred to as deinsertion
When deinsertion group is an olefin –––> β-hydride elimination
Migratory Insertions Anionic and neutral couple too form a new anionic ligand
{makes the neutral ligand (e.g., CO) more electrophilic and Susceptible to nucleophilic
attack by the anionic ligand.}
Mechanistic Considerations: CO Insertion or Me Migration
Mechanistic Considerations: CO Insertion or Me Migration
For a generic insertion of CO into a metal alkyl bond, one can envision
two mechanistic extremes, one in which the methyl migrates to the
carbonyl and a second in which the carbonyl moves and inserts into the
metal-methyl bond. Either way, this generates an open coordination
site denoted here as the small box:
Label on one CO (13CO) cis to an acyl group, can differentiate the two possible
mechanisms. If the CO moves during the deinsertion, then it can only move to a cis
position, displacing another CO in the process. As there are four cis CO's and only
one of them is labeled with 13CO, then we would expect to remove the labeled
carbonyl 25% of the time. If the methyl group moves, it can also displace the 13CO
25% of the time. However, if it moves into one of the other three cis positions, it can
do so in a cis and trans fashion with respect to the 13CO, something that can be
detected spectroscopically:
This subtle but important difference was studied by Calderazzo (see Ang.
Chem. Int. Ed. Eng. 1977, 16, 299 for his classic IR spectroscopy study)
who examined the reverse reaction, deinsertion of CO from a metal acyl
complex. By the Principle of Microscopic Reversibility, the insertion and
deinsertion must follow the same mechanistic route, only in different directions.
CO
25%
OC
OC
CO
OC
OC
Mn
CO
CO
CH3
CO
CO
OC
OC
Mn
CO
Mn
– CO 50%
CH3
OC
OC
CO
Mn
OC
OC
CO
CH3
CO
CO
25%
CH3
CO
CO
O
CO
Mn
CO
CO
CH3
Most systems undergo migration; both mechanisms can occur
82 % yield
95 % e.e.
Inversion on Fe
Et migrated
Retention on Fe
CO migrated
Optically active compound.
CH3NO2, MeCN, Me-migration
DMSO, DMF, proplyene carbonate, HMPA CO migration
Possible intervention of η2-acyl-intermediate could make the interpretation of which is the migrating group
“less than definitive”– RBJordanp. 170
Fe
P
O
C
CH3
Electron-transfer Induced Insertion Reaction Chemistry
1 atm CO
CpFe(CO)(PPh3)Me
N.R. after 5 days (rate < 10-7s-1)
0° C, CH2Cl2
CO
few % Cp2Fe+
Ferrocinium cation
O
(Ph3P)(CO)CpFe
C
complete in <2 min under similar conditions
rate enhancement ~ 107!!
Me
Mechanism:
– e–
CpFe(CO)(PPh3)Me
[CpFe(CO)(PPh3)Me]+
CO
(Ph3P)(CO)CpFe
O
O
C
C
Me
18 e–
(Ph3P)(CO)CpFe
17 e–
Me
(oxidizes starting material)
Alkene (Olefin) Migratory Insertions
This is the basis for almost all transition metal-based polymerization
catalysts. A polymerization rxn is just many, many migratory insertions
of an alkene and alkyl (the growing polymer chain) interspaced with
alkene ligand addition reactions. Alkynes can also do migratory insertions
to produce vinyl groups:
Oxidative Addition Reactions
Oxidative addition is formally the microscopic reverse of reductive
elimination, and it is not surprising that a series of reactions
involving an oxidative addition, a rearrangement and then a
reductive elimination form the basis for a variety of
industrially important catalytic cycles
Oxidative addition reactions are most facile when there is a good twoelectron redox couple. In other words, both the starting and final
oxidation states are relatively stable. For example, oxidative addition
from Ir(I) to Ir(III) is common but an oxidative addition from Fe(III) to
Fe(V), while possible, is generally unlikely
The more reduced a metal center is (also electron-rich because of
ligands), the greater the reactivity towards oxidative addition.
The likelihood of oxidative addition of A–B to a metal, M, depends on
the relative strengths of the A–B, M–A and M–B bonds. For example,
oxidative addition of an alkane is much less common than oxidative
addition of an alkyl halide. For the alkane case, the C-H bond is fairly
strong compared to the M-H and M-R (R = alkyl) bonds
Vaska Complex Oxidative Addition Reactions
Reductive Elimination Reactions
Eliminating
groups
must be
cis
But important; C–H “activation”
If we consider that the DH-H = 104 kcal/mol and that the DM-H is 50-60 kcal/mol we
see that these are essentially balanced and there should be no thermodynamic
preference for a dihydride versus a reduced metal center.
But DR-H is typically 100 kcal/mol versus a metal alkyl bond strength of 30 to 40
kcal/mol. We see that the thermodynamic situation is again approximately balanced
with a slight preference for the forward reaction.
DR-R is typically around 90 kcal/mol, so for two alkyl substituents, there is a strong
thermodynamic driving force for the reaction to go to the right. C-C bond activation is
unusually rare, but more examples continue to be found.
Transition Metal Carbene Complexes
Carbenes: neutral divalent six-electron carbon atom species
Ground-state, single or triplet, depends on R and Rʼ
Transition Metal Carbene Complexes – 2
M-carbon double bonds ––> Metal-carbene complexes – 2 types
Fischer carbene complexes (right)
low oxidation state M; heteroatoms at carbene carbon atom
E.O. Fischer (1st Carbene complex (1964, then Nobel Prize with
Wilkinson, for metallocenes)
Schrock carbene complexes:
higher oxidation state; C or H substituents at carbene C-atom
“alkylidene complex” Richard Schrock MIT, 2005 Nobel Prize for olefin metathesis
(shared with Robert Grubbs (Cal Tech) and Y. Chauvin (France).
Transition Metal Carbene Complexes – 3
MO/AO perspective: one lone pair is donated from the singlet carbene to an
empty d-orbital on the metal (red), and a lone pair is back-donated from a filled
metal orbital into a vacant pz orbital on carbon (blue). There is competition for this
vacant orbital by the lone pair(s) on the heteroatom, consistent with our second
resonance structure. Overall, bonding resembles that of carbon monoxide. Therefore, carbene
ligands are usually thought of as neutral species, unlike dianionic Schrock
alkylidenes (which usually lack electrons for back-donation). However, electron counting is just a formalism!
Transition Metal Carbene Complexes – 4
• Nucleophilic attack on coordinated carbonyl, then alkylation
• Protonation (akylation) of neutral acyl-complex
Transition Metal Carbene Complexes – 5
• Addition of ROH to a coordinated isocyanide ligand
• Synthesis of a N-heterocyclic Carbene Complex. NHCʼs are useful coligands (ancillary ligands) in reactive transition-metal carbene complexes.
Transition Metal Carbene Complexes – 6
• The first “non-stabilized” (i.e., with C-heteroatom) metal carbene
complex).
• Schrock Carbene Complexes:
α-deprotonation of a metal alkyl
Transition Metal Carbene/Carbyne Complexes – 7 Metal-alkyl hydride abstraction:
Metal Carbyne Complexes – Metal-carbon triple bond Made serendipitously
Transition Metal Carbyne Complexes – 8 Reactions of Fischer Carbenes
• Heteratom substitution – reflects electrophilic nature of the ʻCʼ-atom
Methyl group C–H acidity is enhanced because the carbanion formed is
stabilized by the carbene group:
M-carbenes generally not good carbene transfer agents, however:
A ʻfreeʼ carbene is not likely formed
Reactions of Fischer Carbenes: continued
• Olefin Metathesis or Cyclopropanation – Olefin [2 + 2] addition to Mcarbene gives a metallocyclobutane. Then, a retro [2 + 2] could give olefin
metathesis. However, cyclopropane formation is more (energetically) likely:
Reaction
stereospecific
First isolated
metallocyclobutane
complex
Grubbs, 1980
Reactions of Fischer or Schrock Carbenes
Another example: if you replace CO by PPh3 ––> proceeds with enantiomeric excess
Schrock carbenes are more reactive than Fischer carbenes
The alkylidene carbon is nucleophilic Reactions of Schrock Carbenes
Reactions of Schrock Carbenes - continued
Tebbe chemistry – Tebbe reagent
Reactions of Schrock Carbenes - continued
Carbene Migratory Insertions:
If we electron-count the carbene as a dianionic ligand
we are reacting a monoanionic ligand (X) with a
dianionic ligand (carbene) to make a new monoanionic ligand.
Now formally a reductive coupling reaction (since the metal is being
reduced and we are coupling together two ligands). One Can/Should consider the carbene (or alkylidene) as a neutral ligand.
For X = H–, the reverse reaction is called an α-hydride abstraction or
elimination. Carbene Migratory Insertion:
Note: This is Schrock type M-carbene complex, where carbene carbon is
not electrophilic (like CO is traditional migration reactions). The reaction is
probably aided by the overall metal complex positive charge.
Alkene/Olefin Metathesis Reaction Types
Very useful in natural product syntheses
Very useful in polymer syntheses
Olefin Metathesis Catalysts (there are many others)
From, “Organometallic chemistry and catalysis”, Didier Astruc (Springer, 2007)
From, “Organometallic chemistry and catalysis”, Didier Astruc (Springer, 2007)
Chauvin Mechanism of Alkene Metathesis
D2C=CD2 is the product from the deuterated substrate, only.
From 1:1 substrate mixture, of you get 1:2:1 mixture of ehtylenes
ROMP Examples
Schrock asymmetric catalyst (based on BINOL)
First ROMP commercial catalyst
ROMP & RCM
ROMP incredibly useful/practical in materials/polymer syntheses
Ring closing metathesis (RCM) incredibly useful in natural product and pharmaceutical syntheses.
Hepatitis B
protease inhibitor
Cp*2TaV
H
Cp*2TaIII
CH2
H2O
H2O
H2O
SiH4
Oxidative
Addition
CH4 +
H2 + Cp*2TaV
CH3
OH
CH2
Cp*2TaIII
H
Cp*2TaV
H
O
CH3
SiH3
Alpha Elimination
Cp*2
TaV
H
O
Migratory Insertion may be disfavored (equilibrium lies to
to the left) due to higher energy of the insertion product
(right side). BUT - this higher energy may favor
reactivity with the insertion product
Pairwise Mechanism:
R
R
'
R'
R
R'
R
M
R
M
R'
R
M
R'
R
R'
Carbene "stepwise" mechanism:
M
M
CH2
+
R
R
H2C
R'
R
+
CH
R'
R'
R'
M
R'
+
M
R
R
R'
R
Distinguished by an experiment with 1:1 mixture of:
Metathesis products:
from 100% protio
+
substrate
These olefins do not
back-react with catylst
(i.e. are not metathezised) due to
A: low concentration of ethylene in reaction mixture
B: stabilization of phenanthene
+
D2
D2
D
D
By Pairwise Mechanism
(after initiation)
M
or
M
D
D
+ H2C
M
Predicts 1:1 product distribution
CH2
CH2
H2C
M
CH2
+
M
CH2
Stepwise
+
+ D2C
M
CH2
CD2
H2C
CD2
M
CD2
+
D2C
M
M
+
CD2
CD2
+
This leads to the observed product distribution:
1:2:1
H2C=CH2 : H2C=CD2 : D2C=CD2
Homogeneous Catalysis
EXAMPLES:
Wacker Process oxidation
C2H4 (ethylene) + ½ O2 –––> CH3C(O)H (acetaldehyde) (Pd, Cu)
Monsanto acetic acid synthesis
CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)
Alkene Hydrogenation
CH3CH2=CH2 + H2 ––> CH3CH2CH3 (all gases) ΔGf° = – 20.6 kcal/mol
Hydroformylation
Polymerization
Ziegler-Natta (1963 Nobel Prize)
Olefin metathesis
n CH2=CHR
––>
–[CH2-CHR]n–
(Chauvin, Schrock, Grubbs, 2005 Nobel Prize)
Water gas-shift reaction
H2O + CO –––> H2 + CO2 (all gases) ΔGf° = – 6.9 kcal/mol
Fischer-Tropsch reaction
Coupling reactions
Homogeneous Catalysis
Catalyst:
Increases overall rate of a reaction
Does not change equilibrium position
Significantly lowers activation energy
Is not used up in the reaction
Normally interacts with substrate –––> alternative reaction pathway
A Catalyst Significantly Lowers the Reaction Activation Energy
Products of un-catalyzed and catalyzed reactions may be different
• Rate of all of the steps of the reaction are the “same” for catalytic cycle
But, of course, the slowest individual step (“weakest link”) dictates the Rx rate
i.e., “turnover limiting step”
• TON, turnover #; # of reactant molecules the catalyst converts to product
• TOF, turnover frequency; TON per unit time
To achieve high TON, reactants and products cannot bind too tightly to M
• A catalyst may influence initial product distribution, giving preferential formation
of a less thermodynamically stable product, i.e., “selectivity”
Kinetic Competence
Catalysis is a kinetic phenomenon:
Activity may rely on minor (even minuscule) component (of catalyst or intermediate)
Danger in relying too much on spectroscopic studies of catalytic systems
where you “see” only major components
Must demonstrate a given step is ʻkinetically competent” to carry out the reaction.
•• proposed intermediates reacts sufficiently fast to account for product formation
One issue is (has been) catalyst decomposition to metal, M(0) --- > heterogeneous catalysis
ʻhomogeneous catalyst is heterogeneous catalyst in disguiseʼ (Crabtree)
Examples; hydrogenation catalysts are Pt group metal (Ru, Os, Rh, Ir, Pd, Pt) halides.
MX2 in polar solvent with H2 (g) --- > colloidal metal particles (heterog.)
Test: Add Hg(l), which selectively poisons any heterog. Pt group M –Absorption to
ʻactive sitesʼ
The Monsanto acetic acid process
is the major commercial production
method for acetic acid. Methanol,
which can be generated from
synthesis gas ("syn gas", a CO/H2
mixture), is reacted with carbon
monoxide/catalyst ––> acetic acid. In
essence, you have the insertion of
carbon monoxide into the C-O bond
of methanol, i.e. the carbonylation of
methanol.
This process operates at a pressure
of 30–60 atm and a temperature of
150–200 °C and gives a selectivity
greater than 99%.
Limitations/Drawbacks
1. Rhodium is an expensive
starting material. 1 mole of
RhCl3•3H2O costs ~ $30,000!
2. I2 is cheap (about $20 per mole),
but is extremely corrosive. Other
halogens or halogen substitutes do
not work nearly as well.
The Monsanto process has largely
been supplanted by the Cativa
process, a similar iridium-based
process developed by BP Chemicals
Ltd which is more economical and
environmentally friendly
“The Cativa process: A method for
the production of acetic acid by
carbonylation
of
MeOH.
The
technology, similar to the Monsanto
process, was developed by BP
Chemicals and is under license by
BP Plc.[1] [2]The process is based
on an iridium-containing catalyst,
such as the complex [Ir(CO)2I2]−.
The Cativa and Monsanto
processes are similar; they can use
the same chemical plant. Initial
studies by Monsanto had shown that
iridium to be less active than the
rhodium for the carbonylation of
methanol. Subsequent research,
however, showed that the iridium
catalyst could be promoted by
ruthenium, and this combination
leads to a catalyst that is superior to
the rhodium-based systems. The
switch from rhodium to iridium also
allows the use of less water in the
reaction mixture. This change
reduces the number of drying
columns necessary, decreases byproducts formation, and suppresses
the water gas shift reaction.
Furthermore, the process allows a
higher catalyst loading. Compared
with the Monsanto process, the
Cativa process generates less
propionic acid by-product.”
Homogeneous Hydrogenation Catalysis
Wilkinson’s Catalyst (1966)
Nobel Laureate (1973; on another subject)
Catalyst Synthesis:
RhCl3(H2O)3 + 4 PPh3 –––> Formally: double bond
attacks H+
RhCl(PPh3)3 + O=PPh3 + 2 HCl + 2 H2O
Chiral Molecules – Enantiomers with different Biological Effects
(R)-Limonene smells of oranges (S)-limonene smells of lemons.
Insects use chiral chemical messengers (pheromones) as sex attractants; one of
the enantiomers of the insect pheromone, olean, attracts male fruit flies, while its
mirror image operates on the female of the species.
Most drugs consist of chiral molecules. Since a drug must match the receptor
in the cell, it is often only one of the enantiomers that is of interest (active).
In the 1960ʼs, the drug thalidomide was prescribed to alleviate morning
sickness in pregnant woman. Tragically, the drug also caused deformities in the
limbs of children born by these woman. May have been the “wrong” enantiomer (?). In drug development, pharmaceutical companies are required to
carefully purify and test both enantiomers.
Catalytic Asymmetric Syntheses
Early success, 1968:
Dr William S. Knowles, Monsanto Company, St Louis, USA
Following (i)  Wilkinson catalyst discovery, and (ii) synthetic methods for chiral phosphines,
Knowlesʼ strategy: replace triphenylphosphine in Wilkinsonʼs catalyst with the
enantiomer of a known chiral phosphine and hydrogenate a prochiral olefin.
• Knowlesʼs catalytic asymmetric hydrogenation of α-phenylacrylic acid
using a rhodium catalyst containing (-)-methylpropylphenylphosphine
(69% ee) gave (+)-hydratropic acid in 15% ee.
Industrial synthesis of the rare amino acid L-DOPA
Proved useful in the treatment of Parkinsonʼs disease
Early BIG Success in Catalytic Asymmetric Syntheses – Knowles (Monsanto)
enamide
protected AA
Monsanto Process - first commercialized catalytic asymmetric synthesis
employing a chiral transition metal complex. In operation since 1974.
Mechanism (J. Halpern): Asymmetric Hydrogenation of Enamides
COD or
other
weak
ligand(s)
dissociated
rate limiting
Stepwise addition of H
Rh(III)-alkyl complex
Noyoriʼs (R. Noyori, Nagoya U.) General Hydrogenation Catalysts
1. Developed Widely Useful BINAP Chelating Diphosphine
Noyori’s (R. Noyori, Nagoya U.) General Hydrogenation Catalysts
2. Application
Noyori’s catalyst
Noyori’s (R. Noyori, Nagoya U.) General Hydrogenation Catalysts
3.  Application – Expand synthetic organic utility:
With Ru, hydrogenation of carbonyl group (rather than olefin)
The (R)-BINAP-Ru-(II)-catalyzed hydrogenation of acetol to (R)-1,2-
Propanediol: Used for the industrial synthesis of antibacterial levofloxacin.
2001 Nobel Prize in Chemistry
Catalytic asymmetric synthesis
Dr William S. Knowles, Monsanto Company, St Louis, USA;
Professor Ryoji Noyori, Nagoya University, Nagoya, Japan:
Professor K. Barry Sharpless, The Scripps Research Institute, La Jolla, CA USA.
Nobel Prize ”their development of catalytic asymmetric synthesis”. Knowles and Noyori receive half the Prize for: “ their work on chirally catalysed hydrogenation reactions” and Sharpless, the other half of the Prize for: ”his work on chirally catalyzed oxidation reactions”.
Key: enantiopure dialkyltartrate ligands for Ti
Also, developed Osmium catalyzed asymmetric dihyroxylation of olefins.
Chemistry Nobel Prizes: Four (4) in Organometallic Chemistry:
Ziegler-Natta, Wilkinson, 2001/Enantioselective organic rxs., 2005/ Olefin Metathesis
Water gas-shift reaction
H2O + CO <––––> H2 + CO2 kcal/mol
(all gases) ΔGf° = – 6.9
Thermal or Photochemical Water-Gas Shift Reaction
The water-gas shift reaction (WGS) is widely employed in industry to enrich
the hydrogen content in water gas (synthesis gas; Syngas; H2(g)/CO(g) ) after
the steam reforming of methane. The WGS reaction is typically performed at
high temperatures over heterogeneous iron oxide or copper oxide catalysts.
Interest in the WGS shift reaction under mild, homogeneous conditions has
been long-standing. Many soluble transition metal carbonyl complexes show
activity for thermal WGS catalysis, usually in basic media. WGS activity is
promoted photocatalytically, where the photons are typically used to open
coordination sites by the expulsion of CO or photoextrusion of H2 from the
transition metal center.
Proposed mechanism for the thermal (or hv) WGS reaction catalyzed by
homoleptic Group 6 carbonyls. Chem. Rev. 2007, 107, 4022. A. J. Esswein & D. G. Nocera
*
hv
1 photon
Photochemical
Water-Gas Shift
Reaction
bipy; 2,2’-bipyridine