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Lecture 10a
Phosphine Complexes
Introduction I
• In order to understand what a ligand does, one has to
look at its electronic and its steric properties
• The reaction conditions (kinetic and thermodynamic
control) during the reaction determine the configuration
is observed in the product (cis-trans, fac-mer)
• In many cases, there is an equilibrium in solution, which
can be detected by NMR or infrared spectroscopy
• The polarity of the solvent determines which product
precipitates i.e., SnCl4(THT)2: dichloromethane (trans),
pentane (cis)
Introduction II
• L as p-complex only (C2H4, alkenes)
• Ligands like ethylene form strong p-complexes with low-valent
metals
• The HOMO is the C=C p-bond, which is used to form the
M-L s-bond
• Often times, there is also a back-bonding into the p*-orbital
of the C=C bond (free C2H4: 134 pm, complex: 137 pm)
• Example: Zeise’s salt (K[PtCl3(h2-C2H4)]),
Introduction III
• L as s-complex only (H2)
• Molecular hydrogen does not exhibit a lone pair or
a p-bond, yet it binds to some metal centers as intact
molecule (meaning it does not perform an oxidative
addition!)
• The s-bond of the H2 molecule is the electron donor
in this bond (red bond), while the s*-orbital acts as
an acceptor for the back-bonding (blue bond)
• In order to maximize the overlap, the H2-molecule
binds side-on
• Example: [W(h2-H2)(CO)3(PR3)2] (free H2: 74 pm,
complex: 75.5 pm)
• Sigma complexes are also found for C-H, Si-H, B-H
and M-H groups
Introduction IV
•
L as s-donor only (NH3, NR3)
• The metal has to exhibit a medium or high oxidation state
in order for these complexes to be stable
• Metal acts as a hard acid and the ligand as a hard base
• Examples: [M(NH3)4]2+ (M=Cu, Zn),
[M(NH3)6]2+ (M=Co, Ni)
• Many of the complexes are very colorful
Introduction V
•
L as s- and p-donor (H2O, OH-, OR-, NR2-, F-)
• The metal has to exhibit a medium or high oxidation state
in order for these complexes to be stable
• The ligand acts as very hard base and the metal as hard
acid
• Examples: [Ni(H2O)6]2+, [CoF6]3-, [Sn(OH)6]2-
Introduction VI
• L as s-donor and p*-acceptor (CO, CN-, NO)
• The metal has to exhibit a low oxidation state in order for these
complexes to be stable
• The s-bond is formed from the sp-orbital of the carbon atom with
a suitable empty d-orbital of the metal while the p-back bond is
formed by the interaction of a filled d-orbital of the metal with
the p*-orbital of the carbonyl group
• The ligand and the metal act as base
• Examples: Mo(CO)6, [Fe(CN)6]4-, [Co(NO)4]
Phosphines I
•
L as s-donor and s*-acceptor (PR3)
• In the older literature, phosphine ligands
are often referred to as p-acceptors
• In the more recent literature (after 1980),
they are usually referred to as s*-acceptor
• As electron-withdrawing groups (i.e.,
halogen atoms) are placed on the
phosphorus atom, the s-donating capacity
of the phosphine ligand tends to decrease
• At the same time, the energy of the
s*-orbital on phosphorous is lowered
in energy, providing an increase in
backbonding ability (p-acid)
Phosphines II
• The degree of p-acidity largely depends on the substituents
on the phosphorus atom
• While alkyl phosphines are weak p-acids, the acidity increases
for aryl, dialkylamino and alkoxy phosphines
• The extreme cases are PCl3 and PF3, which is equivalent to CO
in its p-acidity because more electronegative elements on the
phosphorous atom stabilize the s-bond and lower the energy
of the s*-orbital (see diagram)
• The contribution of the phosphorus atom to the s*-orbital
increases and the size of the orbital pointing towards the metal
as well allowing for a better overlap
• Based on this argument, the order of p-acidity of phosphines
is
• PMe3 < PAr3 < P(OMe)3 < P(OAr)3 < PCl3 < PF3 ≈ CO
Phosphines III
• Aside of the p-acidity, the steric impact of the phosphine
ligand has to be considered as well
• C.A. Tolman (Chem. Rev. 1977, 77, 313) summarizes the
electronic parameters and cone angles of phosphine ligands:
• The electronic parameter can be adjusted by changing the
R-group (see above). Stronger donor groups increase the electron
density on the metal atom, which is capable of more
backbonding to ligands like CO, CN-, etc.
• Tolman observed for Ni(CO)3L that the carbonyl stretching
frequency decreases as the donor ability of the R-group
increases (i.e., PCy3 (2056 cm-1) vs. P(OMe)3 (2070 cm-1) vs.
PF3 (2111 cm-1)).
Phosphines IV
• The second important parameter is the steric demand, which can also be
controlled by changing the R-group.
• Very bulky phosphines often favor low-coordinate compounds, which
can coordinate additional small ligand as observed in catalytic cycles
• Metals like Mo and W can coordinate up to six PMe3 ligands (i.e., M(PMe3)6)),
while a maximum of four PPh3 ligands (i.e., M(PPh3)4, M=Pd, Cu+, Ag+, Au+)
or two PCy3 ligands (i.e., Cu+, Ag+, Au+, Ni2+, Pd2+, Pt2+) can be coordinated
to a metal center
• Thus, the bulkiness of the phosphine ligand can be quantified by its cone
angle (Q)
• The observed cone angles for phosphines range from Q=87o (PH3) to
Q=212o (P(mes)3) (neither one is shown in the diagram below).
• The cone angles for PMe3, PPh3 and PCy3 are Q=118o, Q=145o and
Q=170o, respectively, consistent with the observations above.
• Generally, phosphines with aryl groups or highly branched alkyl
chains exhibit large cone angles while phosphite have much smaller
cone angles
Phosphines V
Phosphines VI
• The ability of a metal to perform backbonding can
easily be tuned by manipulating the electronic
effect of the phosphine ligand.
• For instance, a change of the ligand from PBu3 to
P(OiPr)3, which possess virtually identical cone angles,
decreases the ability of the metal for backbonding as
can be seen from the higher carbonyl stretching
frequency in Ni(CO)3L.
• If the same electronic effect is desired but a larger cone
angle to lower the number of coordinated ligands, one
could move from PBu3 to P(iPr)3, which exhibits a 30o
larger cone angle, but is electronically speaking
identical.
Mo(CO)5L Complexes I
• These complexes can easily be prepared from
Mo(CO)6 by the reaction with one equivalent
of L
• The resulting compounds exhibit colors
ranging from white to red depending in the
ligand L
• 95Mo-NMR and infrared spectroscopy can be
used to assess the effect of the ligand L on the
metal and the remaining CO ligands
Mo(CO)5L Complexes II
•
95Mo-NMR
studies have shown that the
chemical shift varies significantly with
the ligand
• Ligands that are good s-donors, but poor
or no p-acceptor causing a significant
decrease in the HOMO-LUMO gap, which
results in a deshielding of the Mo-nucleus
• Ligands that are s-donors and good
p-acceptor i.e., PF3 and P(OR)3 are
comparable to the CO ligand itself
Ligand
d(ppm)
Piperidine -1433
CH3CN
-1440
PBr3
-1396
PCl3
-1523
PCl2Ph
-1615
PClPh2
-1702
PPh3
-1743
P(OPh)3
-1819
PF3
-1860
Mo(CO)6
-1857
Mo(CO)5L Complexes III
Mo-C (trans)
C-O (trans)
PPh3
256.0
199.6
114.1
P(2-MeOPh)3
258.8
198.1
114.3
P(2,4,6-MeOPh)3
263.6
197.4
114.5
P(NC5H10)3
260.5
198.5
114.0
PCy3
259.4
197.2
115.4
PMe3
250.8
198.4
115.1
PCl3
237.9
203.5
113.0
Mo-C(trans) vs. Mo-P
204
Mo-C(trans)
Mo-P
R² = 0.8273
202
200
198
196
235
240
245
250
255
260
Mo-P (pm)
• If the phosphine ligand is a good p-acid, the Mo-P bond is
very short (i.e. PCl3) and the Mo-C bond is fairly long
• If donor groups are attached to the phenyl group, the
Mo-P bond length increases while the Mo-C bond length
increases because the phosphorus atom becomes a weaker
p-acid
265
Cone Angle
• For the sequence, the chemical shift
in the 95Mo-NMR spectrum follows
a straight trend
• The chemical shift depends linearly
with the cone angle of the phosphine
• The weaker of a p-acid the phosphine
is, the more negative the chemical
shift is because the Mo-atom is more
shielded
• The comparison of the first and the
second diagram shows that a larger
number of phosphine groups increases
the effect (~200-300 ppm/group)
Mo(CO)5(PPhxCl(3-x))
-1200
-1300
120
125
130
135
140
145
150
-1400
-1500
PCl3
-1600
PPhCl2
-1700
-1800
PPh2Cl
R² = 0.972
PPh3
Mo(CO)4(PPhxCl(3-x))2
-1200
120
PCl3
125
130
135
140
145
150
-1300
PPhCl2
-1400
-1500
PPh2Cl
-1600
-1700
R² = 0.9373
PPh3
Mo(CO)4L2 and Mo(CO)3L3 Complexes
• The di- and trisubstituted compounds
exhibits the same trends like the
monosubstituted compounds, just to
a much larger degree i.e., L=PPh3
(d=-1743ppm, -1556 ppm, -1265
ppm)
• Note that all disubstituted compounds
are in cis-configuration while the
trisubstituted compounds are in
fac-configuration.
Ligand
Mo(CO)4L2
Mo(CO)3L3
Piperidine
-1093
CH3CN
-1307
PBr3
-977
PCl3
-1206
-910
PCl2Ph
-1369
-1124
PClPh2
-1522
-1320
PPh3
-1556
-1265
PF3
-1860
-1860
P(OPh)3
-1819
Mo(CO)6
-1857
-1112
M(CO4)(PR3)2 Complexes
• The s*-backbonding is also controlled by the metal
involved
P(OMe)3
Ph2PCH2PPh2 Ph2PCH2CH2PPh2
Ligand
140.0
-23.6
-12.5
Cis-Cr(CO)4(PR3) 2
172.5
25.4
79.4
Cis-Mo(CO)4(PR3) 2
165. 0
0.0
54.7
Cis-W(CO)4(PR3) 2
141.5
-23.7
40.1
• The backbonding increases in the sequence from Cr
to W.
Catalysis I
• Wilkinson’s catalyst (RhCl(PPh3)3)
• It is obtained by the reaction of RhCl3
with four equivalents of
triphenylphosphine as a red-violet solid
(note that the phosphine acts as ligand and
as reducing reagent here)
• It exhibits a square-planar coordination of
around the Rh(I)-ion (d8)
• It catalyzes the hydrogenation of alkenes
• The complex itself is the 16 VE system
Catalysis II
• Step 1:The dissociation of one
triphenylphosphine ligands to
give 14 VE complexes
• Step 2: Oxidation addition of
H2 to the metal (cis)
• Step 3: The π-complexation of
alkene to the metal
• Step 4: Intramolecular hydride
transfer (olefin insertion)
• Step 5: Reductive elimination
results in extrusion of the
alkane product
Catalysis III
• When the triphenylphosphine ligands are replaced by
chiral phosphines (i.e., DIPAMP), the catalyst
becomes chiral and converts prochiral alkenes into
enantiomerically enriched alkanes via the process
called asymmetric hydrogenation (i.e., L-DOPA
process, Monsanto)