Download Redox Non-Innocent Ligands

Redox Non-Innocent Ligands
Meredith Eno
Topic Talk
January 9, 2015
1
Innocent vs. Non-innocent ligands
•
“ligands are innocent when they allow the oxidation states of the central
atoms to be defined” - C. K. Jørgensen, 1966
NH 3
H 3N
Cl
Co
H 3N
Cl
NH 3
Co(III), d6
•
H 3N
Cl
Pt
H 3N
Cl
Ph 3P
PPh 3
Rh
Cl
PPh 3
Rh(I), d8
Pt(II), d8
“non-innocent, has become synonymous with situation where with formal oxidation
state, determined by a prescribed set of rules, differs from an oxidation state
determined by experiment.” -Paul Chirik, 2011
O
O
N
M
N
M
N
M
NO +
NO
NO -
O
Chirik, P. J. Inorg. Chem., 2011, 50, 9737
Jorgensen, C. K. Coord. Chem. Rev., 1966, 1, 164
Kaim, W.; Schwederski, B. Coord. Chem. Rev.,2009, 254, 1580
2
Innocent vs. Non-innocent ligands
•
In the 1960’s metal dithiolene complexes were proposed to act noninnocently
!
!
!
!
!
!
•
Ph
NiS 2 +
Ph
Ph
toluene
120°C, 24h
S
S
Ph
S
Ph
Ni
Ph
S
NiC 28H 20S 4
black solid
Two possible electronic structures were considered for the isolated
complex
Ph
S
S
Ph
S
Ph
Ni
Ph
S
experimental
data
S
Ph
S
Ph
- square planar
Ni
- forms complexes with pyridine at rt
S
S
Ph
Ph
- paramagnetic at room temperature
Ni(0), d10
Ni(II), d8
Schrauzer, G. N.; Mayweg, V. J. Am. Chem. Soc. 1962, 84,3221–3221
Eisenberg, R., Gray, H. B. Inorg. Chem., 2011, 50, 9741
3
Techniques Used to Determine Electronic Structures
•
X-ray crystallography: researchers usually look for bond length
contractions or elongations on the ligand
!
•
Mössbauer Spectroscopy (1957): help to distinguish between
oxidation states of metals, and also coordination number
!
!
!
!
!
!
!
!
!
!
•
Infrared Spectroscopy: distinguish between oxidation states of metals
coordinated to ∏-accepting ligands
4
Techniques Used to Determine Electronic Structures
•
Electron Paramagnetic Resonance (EPR): used to study materials
with unpaired electrons, can help distinguish between metal based
radical or organic based radical
!
!
!
!
!
!
!
!
•
•
•
•
g-factor (proportionality factor):
measured at the middle of the
signal
g = 1.99-2.01 for organic radicals
g = 1.4-3.0 for transition metal
radicals
Computational Studies: Used along with experimental results to
determine; spin state, spin densities, broken symmetry calculations,
N
NH 2
NH 2
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M.; Chirik, P. J. Am. Chem. Soc. 2006, 128, 13901
5
Classes of Redox Active Ligands
A.! Spectator Ligand: Ligands accept/release electrons
I. Ligands act as electron reservoirs
S
L M +n
S' M +n L -2
S' M +(n-2) L
higher energy
(unstable)
oxidation state
lower energy
(stable) oxidation
state
II. Ligands modify the lewis acid/base properties of the metal
L
M +n
-e-
L• +
M +n
B. Actor Ligands: ligands form/break chemical bonds of the substrate
III. Ligand involved in making or breaking bonds of substrate
L M +n
S-X
X S
L M
IV. Ligand induces radical-type reactivity of substrate
L M +n
S
S M +(n+1) L
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
6
Classes of Redox Active Ligands
A.! Spectator Ligand: Ligands accept/release electrons
I. Ligands act as electron reservoirs
S
L M +n
S' M +n L -2
S' M +(n-2) L
higher energy
(unstable)
oxidation state
lower energy
(stable) oxidation
state
II. Ligands modify the lewis acid/base properties of the metal
L
M +n
-e-
L• +
M +n
B. Actor Ligands: ligands form/break chemical bonds of the substrate
III. Ligand involved in making or breaking bonds of substrate
L M +n
S-X
X S
L M
IV. Ligand induces radical-type reactivity of substrate
L M +n
S
S M +(n+1) L
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
7
[ONNO]-Type Ligands As Electron Reservoirs, !
Allowing New Reactivity
•
Zirconium imido complexes are common in a number of transformations: [2+2]
cycloadditions, hydroamination reactions, C-H bond activation etc. (usually redox
neutral processes)
HN Ar
R'
R
Zr N Ar
R
Zr(IV),
d0
Zr
Ar
N
NHAr
R
R'
R
Zr
Ar
N
R'
H 2NAr
reductive
elimination?
Zr II
Ar
N
+
R
R'
Zr (IV) reluctant to undergo R.E.
Zr(II) high in energy
Heyduk, A. F.; Zarkesh, R. A.; Nguyen, A. I. Inorganic Chemistry. 2011, 50, 9849
Meyer, K. E.; Walsh, P. J.; J. Am. Chem. Soc. 1995, 117, 974
8
[ONNO]-Type Pincer Ligands As Electron Reservoirs, !
Allowing New Reactivity
•
•
•
What if instead a Redox Ligand is employed to allow OA/RE at the ligand
!
!
Ligand Based Oxidation
!
O
O
O
!
-e
-e
II
II
MII
M
M
!
O
O
O
!
!
!
-2 eN N
!
N N
O
O
!
O
O
!
reduced ligand
oxidized ligand
!
[N 2O2red ]4[N 2O2ox]2!
!
Having a redox active ligand on Zr (also Ta) can allow new reactivity to occur, by
allowing Zr to stay in more stable Zr(IV) oxidation state
catchetol based ligands can dissociate readily after oxidation
Heyduk, A. F.; Zarkesh, R. A.; Nguyen, A. I. Inorganic Chemistry. 2011, 50, 9849
Meyer, K. E.; Walsh, P. J.; J. Am. Chem. Soc. 1995, 117, 974
9
[ONNO]-Type Ligands As Electron Reservoirs, !
Allowing New Reactivity
•
Oxidation State of Ligand Determined by Crystal Structure Analysis
H 4[N 2O2red ]
1. nBuLi (4 equiv)
2. ZrCl4(THF) 2
(55% yield)
[N 2O2red ]Zr(THF)3
1a
PhICl 2
(42% yield)
[N 2O2ox]ZrCl2(THF)
2
Bond
N
O
N
O
Bond Length (Å)
C(1)-C(6)
1.423
C(1)-N(1)
1.403
C(6)-N(2)
1.407
reduced ligand
[N 2O2red ]4-
N
O
N
O
Bond
Bond Length (Å)
C(1)-C(6)
1.445
C(1)-N(1)
1.347
C(6)-N(2)
1.333
oxidized ligand
[N 2O2ox]2Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am. Chem. Soc. 2008, 130, 2728
10
[ONNO]-Type Ligands As Electron Reservoirs, !
Allowing New Reactivity
Overall Reaction:
H
N
2
[N 2O2]Zr(THF)3 (10%)
N
H
NH 2
2
N
+
24 h
N
N
N
O
Zr
O
[N 2O2]Zr(THF)3
N
N
L Zr L
O
O
L
ZrIV
reduced ligand
PhNH 2
L
N
N
L Zr L
O
O
NH 2
ZrIV
Ph
reduced ligand
N
Ph
N
Ph
H
N
Ph
H
N
Ph
N
H
Ph
!
L
N
N
L Zr L
O
O
NH
Ph
NHPh
- second step shows “oxidative
addition” with d0 metal, electrons
come from the ligand
- the azobenzene is likely formed
through H-atom abstraction, with a
net “reductive elimination” occurring
at the ligand
!
ZrIV
reduced ligand
N
H
Ph
N
N
PhNH 2
L Zr L
O
O
N
Ph
ZrIV
oxidized ligand
11
Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A.
F. J. Am. Chem. Soc. 2008, 130, 2728
Cobalt Catalyzed Negishi Type Cross-Coupling
•
Soper and co-workers demonstrated with several stoichiometric experiment
that Co(III) complexes can perform net 2e- OA and RE reactions
tBu
tBu
Ph
O
N
Ph
CoIII
tBu
N
O
tBu
excess CH2Cl2
rt, 4 h
tBu
tBu
O
Co
N
Ph
C-N bond length: 1.379 Å
•
Cl
tBu
N
O
tBu
green solid
698 m/z
C-N bond length: 1.359 Å
Reaction of the alkyl cobalt with PhZnBr results in C-C bond formation
ZnBr
tBu
tBu
Ph
O
N
Ph
CoIII
tBu
N
O
tBu
tBu
EtBr
MeCN
tBu
O
Co
N
Ph
O
tBu
tBu
N
tBu
tBu
Ph
O
N
Ph
CoIII
tBu
N
O
tBu
(10-15% y)
Smith, A. L.; Hardcastle, K. I.; Soper, J. D. J. Am. Chem. Soc. 2010, 132, 14358
Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev., 2013, 42, 1440
12
Fe-based Bis(imino)pyridine (PDI) Ligands !
as Electron Reservoirs
•
Brookhart used PDI ligands in the 1990’s with Fe as polymerization catalysts, bulkier
ligands causing less chain transfer and allowing longer lived polymerization
!
!
!
!
600 psi
!
!
!
!
!
!
•
Fe cat:
Fe cat.
(0.5 µmol)
toluene, 60 °C, 12 min
iPr
n
up to 31,000 MW
N
N
N
Fe
X iPr
iPr
X
iPr
determined that
[PDI]FeX2 complex
was high spin
Since the 1990’s Fe-PDI complexes have been used for a number of reactions:
hydrogenation and hydroxylation of olefins, enyne cyclizations, and [2+2]
cycloaddtions
(1)
+
[Fe] (5 mol %)
rt, 16-24 h
95% yield
(2)
[Fe] (0.3 mol %)
4 atm H 2
rt, minutes
TOF = 1814 mol/hr
Small, B. L.; Brookhard, M.; Bennett, A. M. J. Am. Chem. Soc. 1998, 120, 4049
S. K. Russell, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2011, 133, 8858–8861.
S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004,126, 13794–13807.
13
Fe-based Bis(imino)pyridine (PDI) Ligands !
as Electron Reservoirs
•
The exact electronic structure of these catalysts weren’t determined until around
2006.
!
!
!
!
!
N
!
N
!Ar N Fe
Ar
Cl
!
Cl
!
1-Cl 2
! S=2, high spin
!
Fe(II), PDI 0
!
!
!
•
NaBEt 3H
N
-NaCl
Ar
N
N
Fe
Cl
1-Cl
S= 3/2
Ar
NaBEt 3H
1 atm N 2
-NaCl
N
Ar
N
N
Fe
N2
Ar
N2
1-(N 2)2
S=1
uncommon for Fe(0) complex
1-Cl2, has been previously characterized by Gibson and Brookhart, by X-ray
crystallography, Mössbauer, and DFT calculations
Small, B. L.; Brookhard, M.; Bennett, A. M. J. Am. Chem. Soc. 1998, 120, 4049
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M.; Chirik, P. J. Am. Chem. Soc. 2006, 128, 13901
14
Fe-based Bis(imino)pyridine (PDI) Ligands !
as Electron Reservoirs
N
Ar
N
Fe
N
Ar
Cl
1-Cl
S= 3/2
Determination of 1-Cl electronic structure
•
•
Mössbauer isomer shift and quadruple
splitting consistent with Fe(II) complex
DFT calculations using B3LYP functional,
was used to calculate two possible,
electronic structures for the 1-Cl complex.
• BS(4,1) was found to be 10
kcal/mol lower in energy than
BS(3,0).
• BS(4,1); likely, high spin Fe(II)
ion (SFe=2)
antiferromagnetically coupled
to a ligand radical (SL=1/2)
• bond lengths calculated with
BS(4,1) seem to correlate
closely to experimental values
δ= 0.77 mm/s
ΔΕQ= 0.73 mm/s
Key:
m= unpaired spin-up electrons
n= unpaired spin-down electrons
isomer shift (δ)
BS(m,n)
!
S= (m-n/2) quadropole splitting (ΔΕQ) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M.; Chirik, P. J. Am. Chem. Soc. 2006, 128, 13901
15
Fe-based Bis(imino)pyridine (PDI) Ligands !
as Electron Reservoirs
N
Ar
N
Fe
N
Ar
Cl
1-Cl
S= 3/2
Determination of 1-Cl electronic structure
•
DFT calculations using B3LYP functional, was used to calculate two possible, electronic
structures for the 1-Cl complex.
• BS(4,1) was found to be 10 kcal/mol lower in energy than BS(3,0).
• BS(4,1); likely, high spin Fe(II) ion (SFe=2) antiferromagnetically coupled to a ligand
radical (SL=1/2)
• bond lengths calculated with BS(4,1) seem to correlate closely to experimental
values
N
N
Ar
N
δ= 0.77 mm/s
ΔΕQ= 0.73 mm/s
Fe
N
Ar
Ar
N
FeII
N
Ar
Cl
Cl
1-Cl
1-Cl
S= 3/2
high spin Fe(II), PDI -
Key:
m= unpaired spin-up electrons
n= unpaired spin-down electrons
isomer shift (δ)
BS(m,n)
S= (m-n/2) quadropole splitting (ΔΕQ)
!
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M.; Chirik, P. J. Am. Chem. Soc. 2006, 128, 13901
16
Fe-based Bis(imino)pyridine (PDI) Ligands !
as Electron Reservoirs
N
Ar
N
N
Fe
N2
Ar
N2
Typical Fe(0) complexes have N2 stretches between 1950 and 2068 cm-1
•
1-(N 2)2
S=1
!
Pentane solution IR of 1-(N2)2 : 2073 cm-1 and 2132 cm-1 (more consistent with Fe(II))
•
!
1-(N2)2 has been reported previously to undergo loss of N2 to 4-coordinate complex,
making characterization difficult
•
!
1-DMAP complex was found to behave similarly to the 1-(N2)2 and many studies were
done on this complex to determine the electronic structure.
•
Experimental Bond Distances (Å):
N
Ar
N
Fe
N
Ar
N
1-DMAP
N
Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. Am. Chem. Soc. 2008, 128, 13340
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M.; Chirik, P. J. Am. Chem. Soc. 2006, 128, 13901
17
Bond
1-Cl 2
1-DMAP
N(1)-C(2)
1.301
1.350
C(8)-N(3)
1.313
1.358
C(2)-C(3)
1.453
1.414
C(7)-C(8)
1.443
1.406
N imine-Cimine
enlongated
Cipso-Cimine
contracted
Fe-based Bis(imino)pyridine (PDI) Ligands !
as Electron Reservoirs
N
Ar
N
N
Fe
N2
Ar
N2
Typical Fe(0) complexes have N2 stretches between 1950 and 2068 cm-1
•
1-(N 2)2
S=1
!
Pentane solution IR of 1-(N2)2 : 2073 cm-1 and 2132 cm-1 (more consistent with Fe(II))
•
!
1-(N2)2 has been reported previously to undergo loss of N2 to 4-coordinate complex,
making characterization difficult
•
!
N
N
N
N2 2
N2
1-DMAP complex was found to behave similarly to the 1-(N2)2 and many studies were
N Fe
N
N Fe
N
done on this complexArto determine theArelectronic Ar
structure. N
Ar
•
1-(N 2)2
N
Ar
N
Fe
N
2
1-(N2)2
Experimental Bond Distances (Å):
S=1
1-Cl2Bond
1-DMAP
intermediate spin,
Fe(II), PDI
2
Ar
N
1-DMAP
N
Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. Am. Chem. Soc. 2008, 128, 13340
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M.; Chirik, P. J. Am. Chem. Soc. 2006, 128, 13901
18
N(1)-C(2)
1.301
1.350
C(8)-N(3)
1.313
1.358
C(2)-C(3)
1.453
1.414
C(7)-C(8)
1.443
1.406
N imine-Cimine
enlongated
Cipso-Cimine
contracted
Fe-based Bis(imino)pyridine (PDI) Ligands !
as Electron Reservoirs
!
•
Fe-PDI catalyzed [2π + 2π] Cycloaddition of α,ω-Dienes
H
X
1-(N 2)2 (10 mol %)
23 °C, time
iPr
X
X
time
(min)
1
CH2
300
92
1.8
2
SiMe2
300
0
0
3
NH a
300
0
0
4
N-Bn
26
90
21b
5
N-tBu
<5
>95
>240c
6
NBoc
300
24
0.5
7
C(CO2Et) 2
141
>95
4
N
iPr
H
entry
N
conv.
-1
(%) TOF (h )
N
Fe
iPr
iPr
reduced ligand
[PDI] 2-, FeII
X
X
N
Ar
N
Fe
N
N
X
oxidized ligand
[PDI] 0, FeII
Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. Am. Chem. Soc. 2008, 128, 13340
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M.; Chirik, P. J. Am. Chem. Soc. 2006, 128, 13901
19
Ar
Ar
N
Fe
N
X
reduced ligand
[PDI] 2-,FeII
Ar
Aerobic Oxidation via Re(V)-Oxo Species !
with Catechol Ligands
•
Metal complexes can react with (triplet) dioxygen and generate oxo-ligands,
usually occurring via one-electron redox processes
!
O
O
[Fe ]III
O
[Fe]III
O2
•
II
[Fe]
O
[Fe]IV
!
[Fe]II
O
!
[Fe]III
Re(V) has a tendency to undergo 2e- processes, and therefore has a low
probability to form the [ReVI]-O-O• , addition of a redox active ligand will allow the
process to occur
H 2O +
Ph
OH
Ph
O
O
O Re
O O O
1
O
O
-
O
O
O
O Re O O
O O O
2
reduced ligand
Re V
O Re
O O O
reduced ligand
4
-
oxidized ligand
Re V
Re VII
2
O
O
O
Re
O
O
O Re O O
O
O O O
oxidized ligand
3
Re V
1
-
the catechol ligand allows
Re to maintain its +5
oxidation state and access
the superoxo species 2!
homolysis of the O-O bond
gives rise to desired oxo
species
other substrates require the
use of Br4cat for oxidation
Holm. R. H. Chem. Rev. 1987, 87, 1402
Lippert, C. A.; Riener, K.; Soper, J. D. Eur. J. Inorg. Chem. 2012, 554
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
20
!
Classes of Redox Active Ligands
A.! Spectator Ligand: Ligands accept/release electrons
I. Ligands act as electron reservoirs
S
L M +n
S' M +n L -2
S' M +(n-2) L
higher energy
(unstable)
oxidation state
lower energy
(stable) oxidation
state
II. Ligands modify the lewis acid/base properties of the metal
L
M +n
-e-
L• +
M +n
B. Actor Ligands: ligands form/break chemical bonds of the substrate
III. Ligand involved in making or breaking bonds of substrate
L M +n
S-X
X S
L M
IV. Ligand induces radical-type reactivity of substrate
L M +n
S
S M +(n+1) L
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
21
Oxidation of Dihydrogen by Cationic Ir-Complex
BF 4
•
•
•
•
Dithiolene and dioxalene complexes with IrCp* are not
R
R
E
E
very lewis acidic despite the fact that they are 16 e
Ir Cp*
Ir Cp*
E
E
complexes
R
R
DFT studies suggest the zwitterionic structure shown is
16 ethe main resonance contributor
E=O,S
These complexes show low affinity for donor ligands (MeCN, CO and also H2)
Thomas Rauchfuss et al. asked, can the reactivity of these ligands be turned on by a ligandlocalized oxidation
Ar
N
O
BF 4
Ar
Ir
AgBF4
CH2Cl2
1
N
O
Ir
[1] +
BF 4
Bond Lengths 1
N-C 1.33 Å
O-C 1.34 Å
AgBF4
CH2Cl2
Bond Lengths [1] +
N-C 1.34 Å
O-C 1.28 Å
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
Ringenberg, M. R.; Kokatam, S. L.; Heiden, Z. M.; Rauchfuss, T. B.J. Am. Chem. Soc. 2008, 130, 788
Don, M.-J.; Yang, K.; Bott, S. G.; Richmond, M. G.J. Orgnomet. Chem. 1997, 544, 15
22
Oxidation of Dihydrogen by Cationic Ir-Complex
•
•
Complex [1]+ is now a stronger lewis acid which will coordinate H2,
It is known that when H2 is coordinated to an electrophilic metal the acidity increases about
40 orders of magnitude.
F 3C
N
[Ox +
e- ]
+
O
2H+
Ir
AgBF4
1
Ag
[Ox] + 2B
B = 2,6-di-tbu-pyridine
Ox = Ag+ or 1
BF 4
BF 4
F 3C
N
F 3C
N
H2
O
[1•H 2]+
O
Ir
Ir
[1] +
H2
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
Ringenberg, M. R.; Kokatam, S. L.; Heiden, Z. M.; Rauchfuss, T. B.J. Am. Chem. Soc. 2008, 130, 788
Don, M.-J.; Yang, K.; Bott, S. G.; Richmond, M. G.J. Orgnomet. Chem. 1997, 544, 15
23
Redox Switching with Metallocene !
Catalysts-Olefin Hydrogenation
•
•
•
In olefin hydrogenation reactions, OA of H2 is frequently the rate determining
step.
In Rh(I) diphosphinoferrocene complexes, rates can be increased by
changing the R group of the phosphines
However, a similar effect can be seen when changing the charge of the
metalocene-based ligand
CoII
Ph 2
P
Rh(solv)
P
Ph 2
+
-e+e-
CoIII
•
Fe
Rh(NBD)
P
R R
R=Ph, tBu
2+
Ph 2
P
Rh(solv)
P
Ph 2
1 ox
1 red
•
RR
P
1red is a better hydrogenation catalyst (16
times faster rate) due to the more electron
rich Rh center
1ox is a faster more durable isomerization/
hydrosilation catalyst
1 ox (mol%)
Et 3SiH
THF, 20 °C
Et 3Si
Lorkovic, I. M., Duff, R. R., Wrighton, M. S. J. Am. Chem. Soc., 1995, 117, 3617
Luca, O. R., Crabtree, R. H. Chem. Soc. Rev., 2013, 42, 1440
Gregson, C. K. A., Gibson, V. C., Long, N. J. Marshall, E. L., Oxford, P. J., White, A. J. P. J. Am. Chem. Soc., 2006, 128, 7410
24
Redox Switching with Metallocene !
Catalysts-Polymeization
•
•
Redox switching could be a powerful strategy for formation of co-block polymers
In 2014 Diaconescu et al. demonstrated this concept in the formation of L-lactide and Ɛcaprolactone co-block polymers
tBu
tBu
S
O
Ti(OiPr) 2
FeII
O
S
tBu
O
1red
tBu
O
O
O
O
AcFcBAr F
CoCp2
O
tBu
tBu
S
O
Ti(OiPr) 2
FeIII
S
O
tBu
1 ox
tBu
!
Luca, O. R., Crabtree, R. H. Chem. Soc. Rev., 2013, 42, 1440
Gregson, C. K. A., Gibson, V. C., Long, N. J. Marshall, E. L., Oxford, P. J., White, A. J. P. J. Am. Chem. Soc., 2006, 128, 7410
25
Classes of Redox Active Ligands
A.! Spectator Ligand: Ligands accept/release electrons
I. Ligands act as electron reservoirs
S
L M +n
S' M +n L -2
S' M +(n-2) L
higher energy
(unstable)
oxidation state
lower energy
(stable) oxidation
state
II. Ligands modify the lewis acid/base properties of the metal
L
M +n
-e-
L• +
M +n
B. Actor Ligands: ligands form/break chemical bonds of the substrate
III. Ligand involved in making or breaking bonds of substrate
L M +n
S-X
X S
L M
IV. Ligand induces radical-type reactivity of substrate
L M +n
S
S M +(n+1) L
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
26
Redox active ligands in Enzyme Active !
Sites-Galactose Oxidase
(Y495)
O
Galactose Oxidase is a well studied
enzyme which catalyzes the oxidation of
D-galactose
!
! OH HO
!HO
O
OH
!
OH
!
•
HO
H 2O2
O
OH
OH
HO
Tyr
N CuII N
O
O
H
O
The oxygen centered radical promotes
oxidation of the alcohol through protoncoupled electron transfer
•
Bond making/breaking occurs between
the redox active ligand and the substrate
S
O
Tyr
HO
R
S
H 2O
S
HO
HO
N
(Y272)
(C228)
N CuII N
O
H 2O
active
H 2O
O2
•
CuII
e-
OH O
galactose oxidase
O2
(H591)
N
O
H 2O
inactive
(H694) N
•
Tyr
N CuII N
O
O
H
R H
S
Tyr
N
O
H
CuI
Hydrogen abstraction from alcohol
substrate is RDS (kH/kD 6 - 8)
O
R
Tyr
S
HO
Tyr
N CuII N
O
O
H
R H
S
Whittaker, J. W. Chem. Rev. 2003, 103, 2347
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
27
Redox Active Ligands in Bio-Inspired Dimerization of
Secondary Alcohols
•
!
!
!
!
!
•
•
•
•
•
•
tBu
Stack et al. had previously identified a
mononuclear copper complex which
catalyzes aerobic oxidation of activated
substrates
RO
S
tBu tBu
R
O
S
Cu
Cu
O
tBu
=
S
OR
O
R
S
tBu
CuII
O
O
tBu
tBu
N
N
O
CuII
tBu
Cu
O
O
Weighardt and coworkers discovered different
ligands scaffolds can lead to new reactivity
2-propanol and diphenylcarbinol form the 1,2diol as the only organic product
showed that monomeric form of the catalyst is
inactive
rate has a second order dependence on CuIIL
species
complex 2, is a diamagnetic species,
coordination of the phenoxyl group is evident
by the Raman Spectrum (ṽ (C-O·)=1451cm-1
Hydrogen abstraction from alcohol substrate
is RDS (kH/kD =8 for CH3CD2OH)
HO
R' R' R' R'
S
2H+
H
RO
S
O
R
O
Cu
Cu
4
S
Cu
Cu
O
R
1
O
R
Whittaker, J. W. Chem. Rev. 2003, 103, 2347
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
Chaudhuri, P.; Hess, M.; Florke, U.; Wieghardt, K. Angew. Chem. Int. Ed. 1998, 37, 2217
O2
OR
R'
R'
O
!
28
RO
OH
R'
R'
R
O
H 2O2
R
O
RO
S
S
OR
H
Cu
2
R' R' R' R'
H
R
O O O H
RO
S
Cu
Cu
OR
S
O
R
3
S
Cu
OR
O
R
HO
2
R'
H
R'
2H+
Redox Active Ligands in Bio-Inspired Oxidation of Primary
Alcohols with Ir-Aminyl Radical Complexes
•
•
Wanted to see if transition metals are able to be more active and chemoselective than copper
shown previously.
Rhodium-aminyl radical complexes had been shown to abstract hydrogen from a variety of
substrates previously by Grutzmacher et al.
HOtBu
KOtBu
+ OTf-
trop2dach
1
[Ir 2(µ-Cl)2(coe) 4]
AgOTf
NH
HN
N
HN
KOtBu
Ir
Ir
2
3
K+
N
N
Ir
BQ
4
RCHO + HQ 2SQ
mixture of BQ, SQ•-, and HQ2do not lead to oxidation
- no reaction occurs in the
absence of KOtBu
- could not proposed definite
electronic structure for 5
- no experimental evidence for 6
or 7!
- kH/kD for C6D5CD2OH ≈ 2
-
SQ
H R
K+
O
HN
N
N
N
Ir
Ir
5
7
4•K [18]crown6
H R
H
O
N
N
Ir
K+
KOCH 2R
6
Konigsmann, M.; Donati, N.; Stien, A.; Schonberg, H.; Harmer, A.; Sreekanth, A.; Grutzmacher, H. Angew. Chem. Int. Ed. 2007, 46, 3567
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
29
Redox Active Ligands in Bio-Inspired Oxidation of Primary
Alcohols with Ir-Aminyl Radical Complexes
R
OH
2 (0.01 mol%)
BQ (0.01 mol %)
KOtBu (0.03 mol %)
chlorobenzene, 80 °C
R
O
benzyl alcohol gives
TOF >150,000 h -1
Konigsmann, M.; Donati, N.; Stien, A.; Schonberg, H.; Harmer, A.; Sreekanth, A.; Grutzmacher, H. Angew. Chem. Int. Ed. 2007, 46, 3567
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
30
Classes of Redox Active Ligands
A.! Spectator Ligand: Ligands accept/release electrons
I. Ligands act as electron reservoirs
S
L M +n
S' M +n L -2
S' M +(n-2) L
higher energy
(unstable)
oxidation state
lower energy
(stable) oxidation
state
II. Ligands modify the lewis acid/base properties of the metal
L
M +n
-e-
L• +
M +n
B. Actor Ligands: ligands form/break chemical bonds of the substrate
III. Ligand involved in making or breaking bonds of substrate
L M +n
S-X
X S
L M
IV. Ligand induces radical-type reactivity of substrate
L M +n
S
S M +(n+1) L
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
31
Cyclopropanation by Cobalt(II)-Based Metalloradical
Typical Rhodium-Cat. Cyclopropanation
!
- Metal Carbenes in these cases are electrophilic (Fischer-type) metallocarbene
- Enhanced reactivity is seen with electron rich substrates, such as aromatic olefins
- Carbenes in these cases also suffer from dimerization side reactions
common diazo precursors and catalysts
widely accepted mechanism
O
L nRh
R
Y
N2
Y
R
L nRh
via
L nRh
O
Y
O
H
δ+
Intrieri, D,; Caselli, A.; Gallo, E. Eur. J. Inorg. Chem. 2011, 5071
H. Lu, W. I. Dzik, X. Xu, L. Wojtas, B. de Bruin, X. P. Zhang, J. Am. Chem. Soc. 2011, 133, 8518–8521
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
32
N2
H
R
Cyclopropanation by Cobalt(II)-Based Metalloradical
Cobalt (II) Porphyrins in Asymmetric Cyclopropanation
O
N2
R
R'
O
(1 equiv)
+
OEt (EDA)
-or-
[Co(1)] (1 mol %)
DMAP (0.5 equiv)
O
toluene, rt
R'
R''
O
O
R O
[Co(1)]
N2
OtBu (t-BDA)
(1.2 equiv)
!
- In these cases dimerization side
products are not an issue
- electron deficient olefins react with
high diasteroselectivity and high ee’s
- seem to go through a distinctly
different pathway than previously
studied Rh2 systems
Chen, Y.; Ruppel, J. V.; Zhang, X. P. J. Am. Chem. Soc. 2007,129,12074
H. Lu, W. I. Dzik, X. Xu, L. Wojtas, B. de Bruin, X. P. Zhang, J. Am. Chem. Soc. 2011, 133, 8518–8521
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
33
Cyclopropanation by Cobalt(II)-Based Metalloradical
Mechanistic Studies (Zhang and coworkers):
- previous studies have shown that the reaction shows first order rate dependence on [catalyst], [EDA],
and [styrene]
- groups have also previously proposed carbene-carbon centered radicals, in similar [Co(por)] systems,
based on IR streching frequencies of the CO stretching frequencies
!
!
!
!
!
- addition of TEMPO significantly slowed the reaction down, but no irreversible reaction occurred and no
intermediates could be identified
- despite other groups proposing the unique carbene-carbon radicals no groups had performed EPR
studies to try to identify the intermediate species
O
+
N2
toluene
OEt
?
(4 equiv)
[Co(1)]
Chen, Y.; Ruppel, J. V.; Zhang, X. P. J. Am. Chem. Soc. 2007,129,12074
H. Lu, W. I. Dzik, X. Xu, L. Wojtas, B. de Bruin, X. P. Zhang, J. Am. Chem. Soc. 2011, 133, 8518–8521
Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270
34
Cyclopropanation by Cobalt(II)-Based Metalloradical
EPR Studies (Zhang and coworkers):
!
!
!
!
!
!
!
!
• spectra w/o EDA is complicated and indicates 2-3 paramagnetic species.
• Co(TPP) species are known to form 1:1 and 1:2 adducts with toluene
• addition of EDA causes full conversion of the original complex, to a complicated mixture of 5 coordinate
species
!
Chen, Y.; Ruppel, J. V.; Zhang, X. P. J. Am. Chem. Soc. 2007,129,12074
H. Lu, W. I. Dzik, X. Xu, L. Wojtas, B. de Bruin, X. P. Zhang, J. Am. Chem. Soc. 2011, 133, 8518–8521
35
Cyclopropanation by Cobalt(II)-Based Metalloradical
EPR Studies (Zhang and coworkers):
!
!
!
!
!
!
!
!
EPR after the addition of EDA is a mixture of
three species (5:1:0.8)
• species I signals are similar to a weak field
ligand coordinating, such as H2O
• species II g-values are in the range of strong
field carbon adducts (such as CO)
• species III EPR parameters indicate organic
radical
•
!
N
!EtO O 2
! N N
! N CoN
!
I-coordinated
EDA
!
!
H
OEt
H
O
OEt
N
N
N Co
N
N
N
N Co
N
II-bridging carbene
III-carbene-radical
• In depth DFT calculations were used to further
confirm the structures presents
• species I could also be coordination through the
carbon or the dinitrogen
!
!
!
!
!
Chen,
Y.; Ruppel, J. V.; Zhang, X. P. J. Am. Chem. Soc. 2007,129,12074
H. Lu, W. I. Dzik, X. Xu, L. Wojtas,
! B. de Bruin, X. P. Zhang, J. Am. Chem. Soc. 2011, 133, 8518–8521
36
Cyclopropanation by Cobalt(II)-Based Metalloradical
Proposed Mechanism (Zhang and coworkers):
!
Chen, Y.; Ruppel, J. V.; Zhang, X. P. J. Am. Chem. Soc. 2007,129,12074
H. Lu, W. I. Dzik, X. Xu, L. Wojtas, B. de Bruin, X. P. Zhang, J. Am. Chem. Soc. 2011, 133, 8518–8521
37
Cobalt (II) Porphyrin-catalyzed Olefin Aziridination
tBu
tBu
O
O
HN
NH
N
Ar
(1 equiv)
+
O
Ar' S N 3
O
(5 equiv)
SO2Ar'
[Co(por)] (2 mol %)
chlorobenzene
40 °C, 18h
N
+ N2
N
Ar
Co
N
N
HN
NH
O
O
tBu
tBu
[Co(por)]
Ruppel, J. V.; Jones, J. E.; Huff, C. A.; Kamble, R. M.; Chen, Y.; Zhang, X.P. Org. Lett. 2008, 10, 1995
38
Cobalt (II) Porphyrin-catalyzed Olefin Aziridination
Mechanistic Studies (Zhang and coworkers):
- Similar to the cyclopropination mechanism involving the carbene radical, a nitrene radical has been
proposed as an intermediate in this reaction
- !Non-substituted Co(TPP) performs poorly in the reaction, does the hydrogen bonding capability of the
!substituent play an important role
!
!
Olivos Saurez, A. I.; Jiang, H.; Zhang, X. P.; de Bruin, B. Dalton Trans. 2011, 40, 5697
Ruppel, J. V.; Jones, J. E.; Huff, C. A.; Kamble, R. M.; Chen, Y.; Zhang, X.P. Org. Lett. 2008, 10, 1995
39
Cobalt (II) Porphyrin-catalyzed Olefin Aziridination
Mechanistic Studies (Zhang and coworkers):
!
!
SOMO plot of
nitrene radical
Olivos Saurez, A. I.; Jiang, H.; Zhang, X. P.; de Bruin, B. Dalton Trans. 2011, 40, 5697
Ruppel, J. V.; Jones, J. E.; Huff, C. A.; Kamble, R. M.; Chen, Y.; Zhang, X.P. Org. Lett. 2008, 10, 1995
40
Cobalt (II) Porphyrin-catalyzed Amination of C-H Bonds
•
originally reported in 2000 by Stefano Tollari and co-coworker, but the mechanism not
disclosed until 2011 by the Zhang and de Bruin groups
Me O
O
O
N3
Me
+
R
Co(TPP) (mol 10%)
110 °C, benzene
N
H
O
R
Lyaskovskyy, V.; Suarez, A. I. O.; Lu, H.; Jiang, H.;Zhang, X. P.; de Bruin, B. J. Am. Chem. Soc., 2011, 133, 12264
Cenini, S.; Gallo, E.; Penoni, A.; Ragaini, F.; Tollari, S. Chem. Commun., 2000, 2265.
Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev., 2013, 42, 1440
41
Conclusions
•
•
•
•
Redox active ligands are very diverse and perform in a number of
different ways
- “Spectator”-as electron reservoirs or changing lewis acidity or
basicity of metals
- “Actor”- ligand engage directly with the substrate
Allow first row transition metals participate in multi-electron processes
or maintain their most stable oxidation states throughout a catalytic
cycle
interest in redox-active ligands has aslo lead to the development of
better spectroscopic techniques and also new computational
methods
It is likely that new redox-active ligand scaffolds will be disclosed in
the future allowing new reactions to be discovered
42