Selective Organometallic Catalysis:
systems and advanced techniques
Redox-active and
chemically reactive ligands
Peter Budzelaar, for a large part based on
material by David Herbert
University of Naples
"Federico II"
Redox-active and chemically reactive ligands
Core article:
Lyaskovskyy, V.; de Bruin, B., Acs Catalysis 2012, 2, 270-279
Redox Non-Innocent Ligands:
Versatile New Tools to Control Catalytic Reactions
Important topics:
• Formal oxidation states
• Bonding and "real" oxidation states
• Metal-centered and ligand-centered reactivity
Redox-active and chemically reactive ligands
2
Formal oxidation states
Organometallic chemists find the concepts of electron count
and formal oxidation state very useful.
Once you know in what oxidation state a metal is, you
roughly know what chemistry to expect.
The electron count helps predict how reactive a compound is.
Rules for assigning electron count and formal oxidation state
are simple and usually unambiguous.
Ph3P
Cl
16-e Pd2+
Pd
Ph3P
Cl
Redox-active and chemically reactive ligands
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Formal oxidation states
Sometimes things are not so clear.
We can think of the NO ligand in Co(CO)3(NO) as "NO+" or
"NO-":
CO
CO
N O
NO+ bound as a 2-e donor
(or NO as 3-e donor):
expect linear M-N-O
CO
OC Co
OC
NO
N
O
NO- bound as a 2-e donor
(or NO as 1-e donor):
expect bent M-N-O
OC
Co
+
OC
N
O
16-e Co-
18-e Co-
CO
OC
Co
+
OC
OC
Co
OC
N
N
O
14-e Co+
O
CO
OC Co
OC
N O
16-e Co+
The geometry is often informative but not always decisive.
Redox-active and chemically reactive ligands
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Redox-active ligands
The formal ambiguity has a real chemical cause:
oxidizing or reducing the ligand is easy.
The final "equilibrium" depends on the environment of the
metal and its ease of oxidation/reduction. Just looking at the
structure is not enough to assign oxidation state.
Ligands behaving like this (i.e. accepting or providing extra
electrons) are called "non-innocent" or "redox-active".
These additional electrons could potentially be used to do
useful chemistry.
Redox-active and chemically reactive ligands
5
Example: Fe(CO)3(NO)-
Teller, R. G.; Finke, R. G.; Collman, J. P.; Chin, H. B.; Bau, R.,
J. Am. Chem. Soc. 1977, 99, 1104-1111
1.172
1.213
1.151
Clarkson, L. M.; Clegg, W.; Hockless, D. C. R.; Norman, N. C.,
Acta Crystallographica Section C-Crystal Structure Communications 1992, 48, 236-239
Redox-active and chemically reactive ligands
6
Example: Fe(CO)3(NO)Fe-N-O: nearly linear. Would suggest NO+, 18-e Fe-2
But N-O too long (1.213 Å) and too weak (nCO 1647 cm-1),
more compatible with NO-.
Mössbauer indicates Fe(0).
DFT suggests: two Fe-N p-bonds, no s-bond.
Klein, J.; Miehlich, B.; Holzwarth, M. S.; Bauer, M.; Milek, M.;
Khusniyarov, M. M.; Knizia, G.; Werner, H. J.; Plietker, B.,
Angew. Chem., Int. Ed. 2014, 53, 1790-1794
Redox-active and chemically reactive ligands
7
Redox-active ligands
There are many redox-active ligands.
Just a few examples:
M
H
H2
M
H
N
N
M
M
N
Al
Al
M
N
tBu
tBu
O
tBu
O
M(n+2)
tBu
M
N
tBu
O
M(n+1)
tBu
N
tBu
Redox-active and chemically reactive ligands
M(n)
tBu
N
tBu
8
Why the interest?
Oxidative addition and reductive elimination are key steps in
nearly every complex catalytic cycle.
Classical oxidative addition needs a redox-active metal:
LxMn+ +
X
X
LxMn+2
Y
Y
A redox-active ligand can take over the redox role from the
X
metal: L z+Mn+ + X
L z+2Mn
x
Y
x
Y
This means we could use e.g. Ti4+ to do oxidative addition!
Redox-active and chemically reactive ligands
9
Potential roles of non-innocent ligands
Innocent ligands:
"just sitting there and being pretty"
(steric and electronic tuning of metal environment)
Electronic non-innocence:
the ligand accepts/releases electrons
(acts as electron reservoir)
Chemical non-innocence:
the ligand undergoes chemical changes
(makes or breaks bonds)
Redox-active and chemically reactive ligands
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Specific roles of non-innocent ligands
1. Oxidation/reduction of the ligand tunes the electronic
properties (i.e., Lewis acidity/basicity) of the metal
2. The ligand acts as an electron reservoir
3. The ligand acquires radical character and actively
participates in the making and breaking of chemical bonds
during catalysis
4. The ligand induces (radical-type) activation of the
substrate which itself acts as a redox non-innocent ligand
Redox-active and chemically reactive ligands
11
Tuning the metal centre
+ BF4F 3C
N
Ag
Ir
H2
O
[1+]
[1] not reactive to H2
In [1+] the metal is more
Lewis acidic, reacts with H2
Overall reaction:
H2 + 2 AgBF4 → 2 H+ + 2 BF4- + 2 Ag
AgBF4
+ BF4-
F 3C
F3C
N
H2
N
Ir
Ir
O
O
[1+.H2]
[1]
(ox+e) + 2 HB+
ox + 2 B
Ringenberg, M. R.; Kokatam, S. L.;
Heiden, Z. M.; Rauchfuss, T. B.,
J. Am. Chem. Soc. 2008, 130, 788-789
Ringenberg, M. R.; Rauchfuss, T. B.,
Eur. J. Inorg. Chem. 2012, 490-495
Redox-active and chemically reactive ligands
12
Act as electron reservoir
THF
THF
Zr O
O
N
Cl
Cl2
Zr O
O
N
Cl
N
N
Oxidation happens at the ligands.
That is why it works even for a d0 metal.
Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F., Inorg. Chem. 2005, 44, 5559-5561
Redox-active and chemically reactive ligands
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Act as electron reservoir
Iron stays Fe2+ throughout.
The "reductive coupling"
forming the product is possible
because the ligand accepts the electrons.
X
N
N
N
Ar
N
L
Ar
N
Fe
L
Fe
N
Ar
Chirik, P. J.; Wieghardt, K.,
Science 2010, 327, 794-795
Ar
X
N
X
N
Fe
Ar
N
Ar
X
Redox-active and chemically reactive ligands
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A redox-active substrate...
R
FeII
N
N
N2
Fe
Cl
N
FeIV
RN 3
N
Fe
N
FeII
N
FeIII
OEt2
N
Fe
Fe
N
Cl
R
Cl
R
Cl
N
RNH
Ph
N
N
Ph
CH3
R
CH2
FeIII
N
N
Fe
N
H
Cl
Ph
CH2
King, E. R.; Hennessy, E. T.; Betley, T. A., J. Am. Chem. Soc. 2011, 133, 4917-4923
Redox-active and chemically reactive ligands
15
How do we know about electron transfer?
• Ligand geometry
N
N
N
N
M
Ar
N
N
Ar
M
Ar
N
N
Ar
p*a
M
Ar
N
Ar
p*b
Populating p*a and/or p*b lengthens the C=N bonds
and shortens the Cim-Cpy bonds.
• Magnetic properties
EPR, magnetic moments,...
It is not just about numbers of electrons, also about spin state !
• Oxidation state indicators
XPS, Mössbauer
Redox-active and chemically reactive ligands
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Analysis of ligand geometry
N
N
Ar
N
N
N
Ar
C=N 1.274 Å
Cim-Cpy 1.489 Å
Ar
Fe
N
N
N
Ar
N 2 N2
C=N 1.33 and 1.38 Å
Cim-Cpy 1.43 Å
Fe2+
Ar
Fe
N 2 N2
N
Ar
Ar
C=N 1.32 Å
Cim-Cpy 1.44 Å
Fe3+
N
N
Fe
N
N
N
Ar
Ar
Darmon, J. M.; Stieber, S. C. E.; Sylvester, K. T.; Fernandez, I.; Lobkovsky, E.; Semproni, S. P.; Bill, E.;
Wieghardt, K.; DeBeer, S.; Chirik, P. J., J. Am. Chem. Soc. 2012, 134, 17125-17137
Redox-active and chemically reactive ligands
Fe
N
Ar
17
Demonstrating ligand non-innocence
Use an "innocent" metal (Zn, Al)
N
N
N
Zn
N
N
Zn
N
N
N
b-diiminate ligands are typically innocent
N
N
N
N
N
N
N
N
Zn
Zn
e-
N
N
N
N
Zn
e-
2N
N
N
N
Zn
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Formazanate ligands accept an additional electron
Chang, M. C.; Dann, T.; Day, D. P.; Lutz, M.; Wildgoose, G. G.; Otten, E.,
Angew. Chem., Int. Ed. 2014, 53, 4118-4122
Redox-active and chemically reactive ligands
18
Quantifying non-innocence
Mössbauer spectroscopy sees a signal for every type of
nucleus and typically produces two parameters: Isomer Shift
and Quadrupole Splitting. In favourable cases one also
sees Magnetic Splitting. Comparison with literature values
often allows assignment of oxidation state and spin state.
Not all elements allow easy Mössbauer detection. In fact, of
the transition metals only 57Fe is commonly studied.
Redox-active and chemically reactive ligands
19
Using Mössbauer
High-spin (S = 2)
Fe(IV)=O:
 0.02 mm/s
D 0.43 mm/s
Redox-active and chemically reactive ligands
20
Using Mössbauer
Fe(II), SFe = 1; SPDI = 1
Fe(III), spin-crossover
Fe(III), SFe = 5/2 (high spin)
Darmon, J. M.; Stieber, S. C. E.; Sylvester, K. T.; Fernandez, I.; Lobkovsky, E.; Semproni, S. P.; Bill, E.;
Wieghardt, K.; DeBeer, S.; Chirik, P. J., J. Am. Chem. Soc. 2012, 134, 17125-17137
Redox-active and chemically reactive ligands
21
Quantifying non-innocence
X-ray photoelectron spectroscopy (XPS) looks at the
energies of core electrons. These are influenced by the
oxidation state of the element, with a shift of about 1 eV per
step of oxidation state.
XPS produces only a single "shift" value per type of core
electron,* which allows assessment of oxidation state but
usually not of spin state. More information, but less easily
interpreted, can be obtained from Xray Adsorption Near Edge
Spectroscopy (XANES).
* This is an oversimplification...
Redox-active and chemically reactive ligands
22
Using XPS
• Basically, the photo-electric effect
• Look at levels of core electrons
one peak for every type
• Surface-sensitive technique
Redox-active and chemically reactive ligands
23
Using XPS
Assigning "real" or "spectroscopic" oxidation states
Complex
Binding Energy (eV)
Ox State
TiO2
TiCl3
TiO
LTiCl3
LTiCl2
LTi(Cl)(R)
LTi(Cl)(Ph)
LTiMe2
LTiR2
458.8
457.8
454.7
457.7
458.7
458.4
458.6
458.4
458.4
Ti(IV)
Ti(III)
Ti(II)
Ti(III)
Ti(IV)
Ti(IV)
Ti(IV)
Ti(IV)
Ti(IV)
N
N
N
L
R = CH2SiMe3
Rahimi, N.; Budzelaar, P.H.M. (unpublished)
Redox-active and chemically reactive ligands
24
XANES
The "edge" says something
about oxidation state.
Here, differences are large enough
to conclude (PDI)FeCl2 and
(PDI)Fe(biphenyl) contain Fe in
different oxidation states.
The "wiggles" contain
information about the local
environment of the metal atom.
http://www.chem.ucalgary.ca/research/groups/faridehj/xas.pdf
Redox-active and chemically reactive ligands
25
Analysis case study: (PNN)Fe complexes
N
R 2P
Fe
OC
N
or
N
CO Ar
R 2P
Fe
OC
?
N
CO Ar
Compare different analysis methods:
ligand-centered (geometry)
metal-centered (Mössbauer, XPS)
Butschke, B.; Fillman, K. L.; Bendikov, T.; Shimon, L. J. W.; Diskin-Posner, Y.; Leitus, G.;
Gorelsky, S. I.; Neidig, M. L.; Milstein, D., Inorg. Chem. 2015, 54, 4909-4926
Redox-active and chemically reactive ligands
26
Ligand geometry
1.377
Geometry:
N
R 2P
Fe
OC
1.410
N
1.339
compatible with transfer
of one full electron (or more?)
CO Ar
Redox-active and chemically reactive ligands
27
Mössbauer
• Data for 2 very similar to those for (PDI)Fe(CO)2.
• Interpreted as compatible with Fe(0) or LS-Fe2+.
Ambiguous!
Redox-active and chemically reactive ligands
28
XPS
• Complexes 2 and 7 have lower oxidation state (+1 ?)
than (PNN)FeX2 complexes.
Redox-active and chemically reactive ligands
29
DFT study
• All calculations indicate a standard "closed-shell"
electronic structure.
• This indicates that the metal-ligand interaction should be
interpreted in terms of bonding-backbonding rather than
single or multiple electron transfer.
• Both models can result in similar geometry changes.
• The PNN ligand appears to be innocent here;
the carbonyl ligands may be to blame for that!
• Nowadays, any bonding analysis based on geometry
and/or spectroscopy should be combined with some kind
of computational study!
Redox-active and chemically reactive ligands
30
Backbonding vs electron transfer
Electron transfer: when there is not much overlap between the relevant
metal and ligand orbitals.
Normally, a full electron (or two of them!) transfer.
L p*
M 3d
L p*
M 3d
Backbonding: when there is larger overlap.
The resulting MO can contain metal and ligand contributions in any ratio.
M 3d
L p*
M 3d
L p*
For geometry effects, it does not matter whether L p* becomes populated
through orbital mixing or through electron transfer.
Redox-active and chemically reactive ligands
31
Using Chemical Non-innocence
H
H
t
P Bu2
N
Ru
N
Et2 H
Pt Bu2
- H2
N
CO
H2
Ru
CO
N
Et2
Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D., Angew. Chem., Int. Ed. 2006, 45, 1113-1115
Redox-active and chemically reactive ligands
32
Using Chemical Non-innocence
H
Pt Bu2
H
t
P Bu2
N
Ru
RCOOR'
RCH2OH
N
CO
N
Et2 H
Ru
CO
O
H2
Et2N
R
H
H
H
H
Pt Bu2
t
P Bu2
Ru
CO
N
O
Ru
H
R
Ru
Pt Bu2
O
Et2N
N
H
N
CO
H
Et2N
N
Et 2
OR'
CO
R
H
H
Pt Bu2
N
Ru
H2
H
Pt Bu2
CO
N
O
RCH(OH)(OR')
Et 2N
- R'OH
R
Ru
CO
N
Et2 H
OR'
H
RCHO
Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D., Angew. Chem., Int. Ed. 2006, 45, 1113-1115
Redox-active and chemically reactive ligands
33
Bifunctional Catalysis
A complex that has both a Lewis-acidic and a Lewis-basic
site available for (catalytic) reactions.
Term coined by Noyori et al for (transfer) hydrogenation of
ketones using Ru/diphosphine/diamine catalysts.
O
H
P
H
H2
N
O
P
Ru
P
H
N
H
Ru
Cl N
H2
P
Cl N
H2
H
N
P
Ru
H
OH
P
Cl N
H2
Ohkuma, T.; Ooka, H.; Ikariya, T.; Noyori, R., J. Am. Chem. Soc. 1995, 117, 10417-10418
Noyori, R.; Ohkuma, T., Angew. Chem., Int. Ed. 2001, 40, 40-73
Noyori, R.; Yamakawa, M.; Hashiguchi, S., J. Org. Chem. 2001, 66, 7931-7944
Redox-active and chemically reactive ligands
34
Bifunctional Catalysis
The cycle for H2 hydrogenation is slightly more complex.
Variation 1: (assisted by external base)
O
H
P
H
H2
N
O
P
Ru
P
H
N
H
Ru
Cl N
H2
P
H
N
P
Ru
Cl N
H2
- H
OH
B-
P
Cl N
H2
H+
- HB
P
H2
H2
N
Ru
P
Cl N
H2
H2
H2
N
P
Ru
P
Cl N
H2
Redox-active and chemically reactive ligands
35
Bifunctional Catalysis
The cycle for H2 hydrogenation is slightly more complex.
Variation 2: (heterolytic H2 cleavage)
O
H
P
P
H
H2
N
Cl N
H2
H
- H
Cl N
H2
OH
H
N
P
Ru
Ru
P
N
Ru
O
P
H
H
P
H
N
Ru
P
P
H
Cl N
H2
H2
Cl N
H2
Redox-active and chemically reactive ligands
36
Why Bifunctional Catalysis ?
Aldehydes and ketones normally form s-bound complexes
with transition metals.
The resulting geometry is far from ideal for transfer of hydride
via a "classical" mechanism.
H
M
H
H
M
O
H
H
H
M
O
O
O
H2
O
M
O
M
H
- HO
O
H
H
Redox-active and chemically reactive ligands
37
Why Bifunctional Catalysis ?
In bifunctional catalysis, the substrate attaches at two points
to the catalyst, resulting (in favourable cases) in improved
chiral recognition.
O
H
P
H
N
H
Ru
P
Cl N
H2
Redox-active and chemically reactive ligands
38
Related to Bifunctional Catalysis...
In Sharpless epoxidation, the substrate OH group provides
an additional attachment point for the catalyst
Ti(Oi Pr)4
diethyl tartrate
OH
O
HO
COOEt
HO
COOEt
OH
t BuOOH
diethyl tartrate
O
OH
O
O
OH
t Bu
Ti
Katsuki, T.; Sharpless, K. B., J. Am. Chem. Soc. 1980, 102, 5974-5976
Redox-active and chemically reactive ligands
39