Redox-Active Ligands
Q. Michaudel
"Although we loved these ligands, by the end of the 1960s, we knew that they were
“guilty” as charged."
Introduction: the Wacker example and non-innocent ligands
H. B. Gray, Inorg. Chem., 2011, 50, 974.
OH
O2
Pd(II)
Ph
Baran Lab GM 2013-10-19
Modern experimental (EPR, Raman, X-ray crystallography...) and computational
methods can often identify the appropriate oxidation state of the metal and the ligand.
Biological relevance: Several enzymes contain non-innocent ligands like molybdenum
oxotransferases (see Michaudel Group Meeting 2012)
HO
O
O
Ph
O
Me
+
Pd(0)
Ph
HN
H2O
CHO
molybdopterin
O
Pd
N
N
OR
R = Me, H
N
N
Inorg. Chim. Acta, 2011, 370, 374.
Appending quinone moieties into a palladium complex could improve the rate of the
reaction by proximity effect. However, both NHC complexes (R = Me or H) failed to
catalyze oxidation of styrene, as well as the oxidized (quinone) version.
Introducing a redox-active substructure in a ligand is not that simple!
O
+ e–
O
O
– e–
O
– e–
O
Dithiolene:
S
II
S
semiquinone
O
O
L
S
S'
M+n
L–2
S'
M+(n-2) L
low energy M
high energy M
oxidation state
oxidation state
(analogous to releasing electrons by the ligand)
S
X
M
L
S
S
M+(n+1) L
induce radical-type reactivity
on the substrate-ligand
2
– e–
S
II
S
– e–
S
Ni
S
P
increased Lewis acidity on the metal– e–
ligand involved in substrate
S X
(analogous for basicity)
bond making/breaking
spectator ligand M+n L
actor ligand
catecholate
Ni
S
O
O
N
O
H H
Four Different Categories of Redox-Active Ligands :
Non-Innocent ("Suspect") Ligands (Jørgensen, 1966):
Ligands that have an ambiguous oxidation state:
NO+/NO /NO– or O2/O2 –/O22–
+ e–
S
For two recent and exhaustive reviews, see Crabtree, Chem. Soc. Rev., 2013, 42,
1440; de Bruin, ACS Catal., 2012, 2, 270.
M+n
O
S
Redox-active ligands recently regained an increasing attention from the inorganic
community and have been the topic of a recent Forum issue in Inorganic Chemistry
(Chirik, Inorg. Chem., 2011, 50 (20), 9737–9914.) and a special issue in the European
Journal of Inorganic Chemistry (de Bruin, Eur. J. Inorg. Chem., 2012, 3, 340–580.)
Br
Br
orthoquinone
N
RO
OR
RO
H2N
H H
N
+ e–
S
II
S
Ni
S
+ e–
S
S
The complex is better described by radical ligands than oxidation of Ni(II) to Ni(IV)
This group meeting will not discuss:
* electrochemical recognition of cations or anions by a ligand.
* redox-switchable hemilabile ligands or reactive fragment redox-switchable ligands.
(For a review, see: Mirkin, Angew. Chem., Int. Ed., 1998, 37, 894).
1
Redox-Active Ligands
Q. Michaudel
Baran Lab GM 2013-10-19
Biomimetic Alcohol Oxidation
Actor Ligands:
R
RO
O II S
II
Cu
Cu
S
O
OR
R
Cooperative Ligand-Centered Reactivity:
Mechanism of Galactose-Oxidase
O2
Tyr495
2 R'2CHOH
Wieghardt, Angew.
Chem., Int. Ed., 1998, 37,
2217.
R'
H2O2
His591
O
N
His694
NH
II
H2O
H
GAO
RCH2OH + O2
RCHO + H2O2
Tyr272
N Cu O
HN
Net reaction:
S
Cys228
Inactive
HO
Tyr
Tyr
O
Active
RCH2OH
II
H2O
N
R'
R'
H2O
O
H
R
R'
R'
RH
II
H
RCH2OH
S
N Cu O
air, r.t., 12 h
S
N
O
air, r.t., 12 h
O2
HO
R
OH
R
R
R
R = Me, 61%
R = Ph, 68%
First biomimetic complexes of Galactose Oxidase
(various R and R' groups).
Only oxidizes activated alcohols (benzylic or allylic).
See Stack, J. Am. Chem. Soc. 1996, 118, 13097 and
Science 1998, 279, 537.
N
I
N Cu O
H
H
S
N
S
N
Cu
R
PCET: Proton Coupled Electron Transfer
tBu
Tyr
II
O
tBu
HO
N Cu O
O
tBu
O II O II S
Cu
Cu
S
O
O
tBu
R = Me, 63%
R = Ph, 60%
RCHO
R2CHOH
Tyr
HO
PCET
tBu
tBu
H
R
RCHO
Tyr
H
cat. (0.1 mol%)
H
O
S
H2O
N
2
tBu
N Cu O
cat. (0.1 mol%)
HO
tBu
RO II O II SH
Cu
Cu
S
O
OR
OH
H2O2
II
R
RO II O II S
Cu
Cu
S
O
OR
R
Primary alcohols are oxidized to aldehydes
with the same system.
N
PCET
N Cu O
S
R'
HO
II
N Cu O
R'
R'
Tyr
HO
N
H
OR H O
S
RO
O II
II
Cu
Cu
S
O
OR
R H
H O RH O
– e–
HO
R'
R'
H
R
O
RO
S
II
II
Cu
Cu
S
O
OR
R
H
R' R'
Que, Nature 2008, 455, 333.
O
R'
R
O
R'
2
Redox-Active Ligands
Q. Michaudel
Baran Lab GM 2013-10-19
HQ2– + RCHO
KOtBu
M = Cu or Zn
SQ
N
tBu
N
tBuOH
N
I
Ir
tBu
II
H2O2
HN
M
O
O
O2
+ RCH2OH
tBu
tBu
HN
I
N
N
Ir
– RCH2OH
I
N
Ir
O
H
BQ
N
tBu
N
N
tBu
tBu
II
N
SQ
M
M
O
H
R
tBu
II
O
O
H
tBu
tBu
tBu
e– transfer
(fast)
O
I
H
H
O
N
II
N
R
RCH2OH
tBu
M
O O
HOH
tBu
R
tBu
Wieghardt, J. Am. Chem.
Soc. 1999, 121, 9599
Substrate
H
EPR spectroscopy, magnetic and electrochemical data provides evidence of ligand
participation. It is also of note that Zn2+ is d10 and cannot accommodate any redox
catalysis. Catalyst is more efficient with Cu than with Zn and even MeOH can be
oxidized (>95%). The active catalyst was prepared by oxidation with ferrocenium:
[M(L)] + [Fc]PF6
[M(L')]PF6 + Fc
Cat. (0.01 mol%)
KOtBu (0.03 mol%)
BQ (1.2 equiv.)
O
OH
O > 98%
(3 h)
N
Substrate
94%
(3 min)
OH
OH
Me
N
CH3OH
Angew. Chem., Int.
Ed., 2007, 46, 3567.
CH2O
Yield
(Time)
Product
OH
O
64%
(4 h)
OH Me
O
OH
O
OH
Me O
HN
I
Ir
N
NH2
BQ = Benzoquinone, SQ = Semiquinone,
HQ2– = Hydroquinone
CH3CH2OH
CH3CHO
94%
(4 h)
> 98%
(2 h)
OH
O
Me
N
(gram scale, but
GC/NMR yield)
RCHO
PhCl, 80 °C, Ar
Yield
(Time)
Product
I
RCH2OK
H
r.d.s.
N
Ir
RCHO
tBu
N
N
Ir
tBu
H
R
N
O
H
OH
OH
O
> 98%
(1 h)
> 98%
(10 min)
BQ (2
equiv.)
> 98%
(5 min)
BQ (2
equiv.)
3
Redox-Active Ligands
Q. Michaudel
Mechanism of Water Oxidation with Ru "Blue Dimer": Redox-Active Ligand or Not?
4
O
V
L2Ru
H
O
O
4
V
O
IV
RuL2
L2Ru
O
O
O
H
O2
Purification of Ethene Gas Streams:
IV
O
III
L2Ru
OH
H2O
III
F3C
S
4
O
V
V
L2Ru
O N
O
H
O
O
IV
IV
Ru
L2Ru
OH N
H
O
H
H
O
H
HO
HO
S
Ph2P
III
L2Ru
H2O N
H2O
O2
O
S
F3C
S
II
S
CF3
S
CF3
Ni
Science 2001, 291, 106.
MCS = Multi-Component Stream
H2C
S
III
Re
S
Ph2P
Ph
[C]
+ e–
III
F3C
H
4
Ru
CF3
CH2
P
L N
S
Improved system: Fixation of ethene faster with oxidized complex [C]2+ and release
requires less energy with to reduce complex [D]2+ into [D].
4
H2O
CH2
+
MCS
IV
Ph
O
H2C
Ru
Hurst, Inorg. Chem., 2008, 47, 1753.
III
CF3
MCS
electrochemical –
e
reduction
L N
OH N
HO
H2C
H2O
L2Ru
S
H2C CH2
4
L N
O
CF3
CF3
N
4
Ru
S
F3C
N
V
S
S
Ni
L=
L2Ru
II
II
S
F3C
OH2
H2O
S
Ni
electrochemical
oxidation
RuL2
Meyer, J. Am. Chem. Soc., 2000, 122, 8464.
L N
F3C
e–
4
RuL2
H
Baran Lab GM 2013-10-19
CH2
S
Ph2P
P
K1
Ph
– e–
S
Re
S
Ph2P
Ph
[D]
+ e–
– e–
L N
III
Ru
H2C
H2O N
[D]+
K2
H
O
O
H
+ e–
– e–
+ e–
H2C
The original Meyer mechanism is still regarded as more likely, but the Hurst
mechanism is still under consideration.
CH2
[C]+
– e–
CH2
[D]2+
[C]2+
K3
J. Am. Chem. Soc., 2009, 131, 64.
K3 > K2 > K1
4
Redox-Active Ligands
Q. Michaudel
Baran Lab GM 2013-10-19
Cyclopropanation with Redox Non-Innocent Carbene:
Cooperative Substrate-Centered Radical-Type Reactivity:
R'
Carbenoid Redox-Active Actor Ligand:
MII, d7
RO2C
M
M
H
R
R
H
N
py
py
dπ
dπ
MII
N
N
N
III
Co
N
CO2R
2
H
R'
N
N2
N
III
Co
N
4
N
CO2R
H
Electronic configuration of metalloradicals of group 9
N
II
Co
N
N
π
MIII
M = Rh, Ir, Co
H
π* (SOMO)
+
Chem. Soc. Rev., 2013, 42, 1440
N2
N
R'
RO2C
dz2
dz2
CO2R
N II N
Co
N
N
MIII, d6
N
J. Am. Chem. Soc., 2010, 132, 10891.
C–H Amination of Benzylic C–H Bonds:
N
Me
II
Ir N
N
CO2Et
Me
Me
N2
MeCN
– H2C
2
N
N
Me
Me
MeO2C
III
Ir
N
N
N
N
N
E
N
N
III
Co
N
N
MeO2C
H
Ar
Me
Chem.–Eur. J., 2008, 14, 7594.
Me
H
Reactivity of radical carbenoid is different from both Fischer (electrophilic, carboncentered LUMO) and Schrock (nucleophilic, carbon-centered HOMO) carbenes.
Ph
Me
(10 equiv.)
N3
N
NH
Me
III
Ir N
N
II
Co
N
O
Ar
Me
Me
N
Me
MeO
N
Me
N
N
N
H
MeO
III
Ir N
Me
EtO2C
Me
CH2
Ar
O
Me
N
N
N
E = CO2Me
N
N
III
Co
N
N2
N
J. Am. Chem. Soc., 2011, 133, 12264.
TrocN3 (1 equiv.)
Co(TPP) (5 mol %)
40 °C, 48 h, N2, 69%
II
Co
N
N
N
N
N
N
NHTroc
Ph
Me
Organometallics 2010, 29, 389.
5
Redox-Active Ligands
Q. Michaudel
Another C–H Amination System:
Intermolecular Version:
Baran Lab GM 2013-10-19
R3
Mes
R1
N
II
H
Betley, J. Am. Chem.
Soc., 2011, 133, 4917.
N
Fe
Ph
H
N3
N3
R2
N3
Ph
Product
Boc
N
+ PhMe
(solvent)
Ph
60 °C, 12 h, N2
N
H
H
Me
N3
H
N3
R
23 °C, benzene
N
II
Fe
N
Me
N3
Boc2O
65 °C, 12 h
Cl
Fe
H
H
R
N3
Me
17%
N3
E (1 equiv.)
Boc2O (1 equiv.)
Boc
N
H
N
II
Fe
N
Me
H
R
radical
rebound
Boc
N
N3
Me
CoII(ttp) (5 mol%)
KOH (10 equiv.)
t
I BuOH (10 equiv.)
PhH (100 equiv.)
Me
Boc Me
N
Me
same conditions
C–H Arylation of Benzene:
Cl NH
R
67%
(45%)
Me
Me
Fe
BocHN
benzene,
23 °C, 12 h
same conditions
N
Me
Me
H
N3
III
OTMS
Boc
Me S N
75%
Ph
93%ee
H
Me
Cl
47%
68%
OTMS
N3
Boc
N
Yield
N3
Ph R
Boc
Me
N
Me
N2
E
H
Product
Boc
N
O
Boc
N
49%
Me
III
N
Cl OEt2
R
Me
H
H
H
H
H
H
Me
Betley, Science 2013, 340, 591.
Cl
H
Ph
N3
72%
Boc
N
Me
O
Ph
H
H
Intramolecular Version:
Cl
Boc
N
N3
R4
Substrate
H
[Fe]
R2
R3
Yield
57%
Boc
R5
N
R6
R1
benzene,
65 °C, 12 h
R4
Substrate
Et2O Cl
E (10–20 mol%)
Boc2O (1 equiv.)
R5 R6
N2, dark, 200 °C, Me
3.5 h, 88%
(82%)
tBu
Me
Me
Eur. J. Inorg. Chem.
2012, 485.
HBoc
N
Me
(47%)
Me
1.0:1.5
CoII(ttp)
Me
Ph
CoIII(ttp)
Me
ttp = tetratolylporphyrin
6
Redox-Active Ligands
Q. Michaudel
Spectator Ligands:
Baran Lab GM 2013-10-19
Redox Switch Polymerization:
N
O
Modification of the Lewis Acid-Base Properties of the Metal
H2 oxidation:
Cp*
III
III
AgBF4
N
F3C
tBu
Ir
Cp*2Fe
N
O
F3C
tBu
Ag
A
tBu
tBu
Cp*
[Ox + e–] + 2H+
III
Ir
[Ox] + 2B
Ox = AgBF4 or B
O
tBu
AgOTf
N
O
B
tBu
Fe
N
O
Ti
OiPr
tBu
Fe
N
TBP
F3C
tBu
tBu
tBu
N
Me
tBu
A (0.33 equiv.)
AgBF4 (2 equiv.)
TBP (2 equiv.)
J. Am. Chem. Soc.,
2008, 130, 788.
Fe
OiPr
increased Ir
Lewis acidity
H2
H2
N
O
Ti
OiPr
tBu
Fe
Cp*
Ir
O
OiPr
O
O
H2
2
O
O
P
Co
I
RhSn
P
Ph
Ph
P
Co
–
I
RhSn
e–
H2
P
Ph
Ph
increased phosphine
Lewis basicity
S = Me2CO
J. Am. Chem. Soc.,
2006, 128, 7410.
Me
Me
Ph
Ph
+ e–
n
Me
or
/
Ph
Ph
O
Redox-switches could be
useful for block
copolymerization.
Hydrogenation
/
Me
O
cat.
H+
1.5 h, r.t.
2
O
OH
or
Me
Ph2P
Me
N
J. Am. Chem. Soc., 1995,117, 3617.
Cobaltocene reduction enhances electron density on Rh, which facilitates the oxidative
addition of H2 (r.d.s.) and leads to 16-fold rate increase compared to the oxidized form
of the complex. Interestingly, the oxidized complex catalyzes the hydrosilylation of
alkene faster than the reduced form.
Fe
Ph2P
tBu
O
M
N
OR
Ph2P
O
FcBArF
CoCp2
tBu
tBu
N
O
M
Fe
N
OR
Ph2P
J. Am. Chem. Soc.,
2011, 133, 9278
O
BArF
tBu
For M = Y, R = tBu,
ferrocenium stops
lactide polymerization.
While, for M = In, R =
Ph, it is the opposite
behavior! R.d.s. is
different depending
on which complex is
used.
7
Redox-Active Ligands
Q. Michaudel
Redox Non-Innocent Ligands as Electron-Reservoirs:
Redox-Active Ligands Confer Nobility on Base-Metal Catalysts:
Baran Lab GM 2013-10-19
Formal [2 + 2] cycloadditions:
Precious metals most efficiently catalyze two electron reactions, since they typically
undergo ±2 e–oxidation state changes (IrI, IrIII, IrV or Pd0, PdII, PdIV) whereas basemetal undergo ±1 e–oxidation state changes (CoI, CoII, CoIII or FeII, FeIII). However,
the price of precious metals is increasing due to their low abundance.
Me
iPr
Me
N
II
N
Fe
N2
iPr
N
N2
iPr
F
reduced
ligand, FeII
iPr
Abundance in Earth's crust (ppb by weight)
X
Me
Me
N
N
II
Fe
Me
N
Me
N
N
Ar
Ar
Source: http://www.webelements.com/periodicity/abundance_crust/
X
II
Fe
N
Ar
Ar
oxidized
ligand, FeII
Chirik and Wieghardt have proposed that redox active ligands may be able to confer
nobility on base metals by playing the role of electron sinks.
reduced
ligand, FeII
X
X
F (10 mol%)
Science, 2010, 327, 794.
X
X
23 °C
Bis(aryliminopyridine) Pincer Ligand:
Conversion > 90% for X = CH2, NHBn, NHtBu, C(CO2Et)2
after 5h (< 30 min for amine compounds)
Chirik, J. Am. Chem. Soc., 2006, 128, 13340.
F' (2.5 mol%)
Me
iPr
II
N
Fe
Br
iPr
0.5% Na(Hg)
or 2 NaC10H8
Me
N
N
iPr
Br
iPr
Me
iPr
N2
Me
N
II
N
Fe
N2
iPr
+
N
iPr
23 °C, 16 h
95%
F' (2.5 mol%)
Me
N2
+
23 °C, 16 h
95%
iPr
Via:
This ligand framework can store two reducing equivalents in an iron(II) complex. It
has been used for several reactions:
* Formal [2 + 2] cycloadditions
R=H
reductive
elimination
(For F' structure, see next slide)
N
N
Fe
Me
N
* Hydrogenation and hydrosilylation
* Oxidative additions into C–C bonds
Me
Me
Me
* 1,6-enyne reactions
Me
Me
Ar
R = Me
β-elimination
R
then
reductive
elimination
Me
Me
Chirik, J. Am. Chem. Soc. 2011, 133, 8858.
8
Redox-Active Ligands
Q. Michaudel
1,6 enyne reactions:
F (5 mol%)
E
E
23 °C, H2 (4 atm)
PhH, 23 °C
Me
E = NTs, R = H, 79% (1h)
R = Me, 79% (3h)
E = O, R = H, 95% (6h)
R = Me, 62% (3h)
E = C(EtCO2)2, R = H, 74% (3h)
(TMSO)2MeSiH
F' (0.004 mol%)
5
Me
Me
R
II
N
Fe
E
5
Me
Ph
Me
23 °C, 30 min,
> 98%
Si
N F
Ar
Fe
N
N
N
Me
Fe
R
N
Ph
N
F': R = Me
F": R = Et
R
iPr
R
Me
N
N
Ar
Me
N2
N2
iPr
N
N
OTMS
OTMS
(TMSO)2MeSiH
F' (0.004 mol%)
Me
N
Me
23 °C, neat
15 min, > 98%
N
OTMS
OTMS
Si
N
Chirik, J. Am. Chem. Soc.,
2009, 131, 8772.
iPr
N
Anti Markovnikov Hydrosilylation:
R
R
Baran Lab GM 2013-10-19
F
iPr
O
E
(TMSO)2MeSiH
F' (0.26 mol%)
Me
O
8.9
60 °C, 60 min,
> 98%
Me
OTMS
OTMS
Si
O
O
Me
8.9
Chirik, Science, 2012, 335, 567.
Me
Ar
N
II
N
Fe
H
H
Me
Me
N
R Ar
Me
N
N
II
Fe
Enantioselective Hydrogenation:
Me
N
R Ar
Ar
N
N
H2
E
iPr
N
Cy
R1
Me
F
iPr
– 2 N2
22 °C, H2 (4 atm)
PhH, 24 h
R2
R1
R2
R1 = Ar, R2 = Alk
Up to > 98% yield, 96% ee
G
Hydrogenation and hydrosilylation:
Me
G (5 mol%)
Co
Me
E
Me
Chirik, J. Am. Chem. Soc., 2012, 134, 4561.
Me
[Fe]
This catalyst enables the
hydrogenation of olefins in the
presence of unprotected
amines, various carbonyls,
ethers and fluorinated
hydrocarbons.
Reduction of Azides:
N3
[Fe]
F (10 mol%)
[Fe]
Me
H
[Fe] H
Chirik, J. Am. Chem. Soc., 2004, 126, 13794;
Organometallics, 2008, 27, 1470.
NH2
23–65 °C, H2 (4 atm)
Me
H2
R
R:
R
2,6-iPr
2
>
2,5-tBu
2
> 2,6-Et2 >>> 2,4,6-Me3
Chirik, J. Am. Chem. Soc., 2006, 128, 5302.
Me
9
Redox-Active Ligands
Q. Michaudel
Oxidative additions into C–C bonds:
Me
iPr
II
N
Fe
Me
iPr
N
23 °C, 2 h
N2
N2
Negishi-type cross coupling:
Me
N
Ar
N
III
Fe
Cl–
EtCl
Me
N
N
Ar
tBu
O
iPr
iPr
Baran Lab GM 2013-10-19
tBu
III
tBu
O
Co
tBu
O
N
Ph
Ph
N
III
tBu
Co
O
N
Ph
tBu
In this case, both ligand and metal give one electron. This is another way for this
bis(imino)pyridine frame to adjust to the electronic requirements of a metal
complex and a specific redox process.
Me
tBu
Ph
N
tBu
Chirik, J. Am. Chem. Soc., 2012, 134, 17125.
J. Am. Chem. Soc., 2010,
132, 14358.
C–C Bond Formation:
This paper set a key precedent for the field of redox-active ligands, because it
showcases a reductive elimination from a ZrIV d0 complex!!
2
Ph Ph
tBuN
Zr
Ph Ph
IV
O
O tBuN
tBu
[Cp2Fe]PF6
(2 equiv.)
tBu
tBu
tBu
tBuN
Heyduk, J. Am. Chem. Soc., 2006, 128, 8410.
Me Me
tBuN
tBu
tBu
Zr
Reaction also works with hexylzinc bromide in similar yields.
O
tBu
tBu
THF
O
reduced
ligand, ReV
Ph3P
O
O
O
O
V Re
O
O
O
O
O
O
O
tBuN
tBu
Zr IV
O tBuN
2
Ph3P
O
oxidized
ligand, ReV
H
tBu
O
tBu
O
VII
O
O
tBu
O
O
O O
tBu
tBu
O
V Re
H
IV
O tBuN
J. Am. Chem. Soc., 2010, 132, 3879.
Dioxygen Fixation:
Ph Ph
(yield: 74%)
"Authentic carbon-carbon bond-forming
reductive elimination reactions are
uncommon outside of the platinum group
triad."
PhZnBr
Miscellaneous Reactions:
O tBuN
tBu
tBu
Zr IV
PhEt + [ZnBr]+
(yield: 10-15%)
The dimethyl equivalent yields 23% of a mixture
of methane (79%) and ethane (21%), so about
5% yield for the Me–Me coupling.
reduced
ligand, ReVII
O
Re
O
O
O
O
Re
V
O
O
O
O
V Re
O
O
O
O
O
O
oxidized
ligand, ReV
The ligand facilitates spin-crossover in the formally “spin-forbidden” reaction between
triplet oxygen and the closed-shell (singlet) d2 rhenium(V).
10
Redox-Active Ligands
Q. Michaudel
Nitrene Transfer:
MeO
The elegant work of Milstein and Tanaka on base assisted redox-active ligand catalysis
for alcohol oxidation is also notable:
iPr
N
tBu
N
C
N
IV
2
Cl
NH2
Zr
CNtBu
N
R
Baran Lab GM 2013-10-19
N
RN3
III
Ru
N
iPr
N
NH
– H+
N
O
N
III
Ru
+ H+
O
N
N
N
N
O
II
Ru
N
O
N
O
O
MeO
MeO
N2
CNtBu
iPr
tBu
MeO
iPr
N
N
IV
tBu
tBu
tBu
tBu
tBu
1/2 MeOH
N
Cl
N
C
N
Zr
N
N R
iPr
MeO
Cl
IV
tBu
Zr
Tanaka, J. Am. Chem. Soc., 2003, 125, 6729.
NR
CNtBu
R1NH2 + R2CH2OH
(1 equiv.) (1 equiv.)
N
iPr
MeO
MeO
J (0.1 mol%)
R1NHCOR2 + 2H2
PhMe, reflux
up to 96%
R1, R2 = Alk, Ar
N
iPr
R = Ad, tBu
N
N
Cl
NR
J
NtBu
Co
tBu
tBu
Me
"The redox activity of iminopyridine ligands may play a role in
effecting efficient catalysis".
N
Me
I
Ar
Four-Electron Oxidative Formation of Aryl Diazenes Using a Tantalum
Redox-Active Ligand Complex: Heyduk, Angew. Chem., Int. Ed., 2008, 47, 4715.
Ar
Iron Diazoalkane Chemistry: N–N Bond Hydrogenation and Intramolecular C–H
Activation: Chirik, J. Am. Chem. Soc., 2007, 129, 7212.
N
Fe
Cl2
Further Reading:
Catalytic Reactivity of a Zirconium(IV) Redox-Active Ligand Complex with
1,2-Diphenylhydrazine: Heyduk, J. Am. Chem. Soc., 2008, 130, 2728.
Ritter, J. Am. Chem. Soc., 2009, 131, 12915.
Me
O
2
“Oxidative Addition” to a Zirconium(IV) Redox-Active Ligand Complex: Heyduk,
Inorg. Chem., 2005, 44, 5559.
Coupling of tBuNC with AdN3 reacted to completion in 2 h at 55 °C with 10 mol% of
catalyst.
I (4 mol%)
Mg (10 mol%)
Me
HBPin (1.2
equiv.)
Me Et2O, 23 °C, BPin
3 h, 80 °C
H
N
O
Milstein, Science, 2009, 324, 74.
Heyduk, Chem. Sci., 2011, 2, 166.
MeO
PtBu
Ru
Et2N
iPr
Hydroboration of 1,3 Diene:
II
N
N
Zr
NH2
Ru
N
IV
1/2 HCOH
Ar = 3,5-dimethylphenyl
Reaction of a Redox-Active Ligand Complex of Nickel with Dioxygen Probes
Ligand-Radical Character: J. Am. Chem. Soc., 2009, 131, 15582.
11