Access to formally Ni(i) states in a heterobimetallic NiZn system

Article abstract of DOI:10.1039/c2sc21231e

Heterobimetallic NiZn complexes featuring metal centers in distinct coordination environments have been synthesized using diimine-dioxime ligands as binucleating scaffolds. A tetramethylfuran-containing ligand derivative enables a stable one-electron-reduced S = 1/2 species to be accessed using Cp ₂Co as a chemical reductant. The resulting pseudo-square planar complex exhibits spectroscopic and crystallographic characteristics of a ligand-centered radical bound to a Ni(ii) center. Upon coordination of a π-acidic ligand such as PPh₃, however, a five-coordinate Ni(i) metalloradical is formed. The electronic structures of these reduced species provide insight into the subtle effects of ligand structure on the potential and reversibility of the Ni^II/I couple for complexes of redox-active tetraazamacrocycles.

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EDGE ARTICLE
Access to formally Ni(I) states in a heterobimetallic NiZn system†
Christopher Uyeda and Jonas C. Peters*
Received 10th August 2012, Accepted 10th September 2012
DOI: 10.1039/c2sc21231e
Heterobimetallic NiZn complexes featuring metal centers in distinct coordination environments have
been synthesized using diimine–dioxime ligands as binucleating scaffolds. A tetramethylfuran-
containing ligand derivative enables a stable one-electron-reduced S ¼ 1/2 species to be accessed using
2
Cp Co as a chemical reductant. The resulting pseudo-square planar complex exhibits spectroscopic and
crystallographic characteristics of a ligand-centered radical bound to a Ni(II) center. Upon
coordination of a p-acidic ligand such as PPh , however, a five-coordinate Ni(I) metalloradical is
3
formed. The electronic structures of these reduced species provide insight into the subtle effects of
II/I
ligand structure on the potential and reversibility of the Ni couple for complexes of redox-active
tetraazamacrocycles.
Introduction
stability under catalytic conditions. Complexes featuring
conjugated p-systems in the ligand framework, including dii-
10
mine–dioxime ligands (Fig. 1), have attracted interest due to
Metalloenzymes that mediate multiproton and multielectron
+
1
2
3
4
redox processes, such as the reduction of H , O , N , and CO
2
2
2
their relatively anodic reduction potentials. Nevertheless, while
6
a number of solution-phase electrochemical and spectroscopic
11
studies have been reported, isolable and structurally charac-
commonly utilize polymetallic reaction sites, a characteristic that
engenders greater redox flexibility than is possible at a single
metal center, both in the range of accessible potentials and the
number of stored reducing or oxidizing equivalents. Addition-
ally, the binding and activation of small molecule substrates may
be facilitated by simultaneous interaction with multiple transi-
tion metals. For example, the C-cluster of NiFe CO dehydro-
genase, which catalyzes the reversible oxidation of CO to form
CO , has been crystallographically characterized with CO
terized reduced nickel
12
4
N -macrocycles are relatively
uncommon, particularly for those cases in which the ligand is
redox active.
An understanding of the specific factors that influence the
II/I
potential of the Ni couple and the stability of various inter-
mediates in catalytic systems is predicated on a detailed
description of the reduced state. We, therefore, have pursued the
synthesis of S ¼ 1/2 formally Ni(I) species in a series of hetero-
bimetallic NiZn complexes supported by diimine–dioxime
ligands. Significant spin density localized in p-systems of these
ligands causes hydrogen-atom abstraction pathways to be facile
for the parent, methyl-substituted complexes; however, a modi-
fied tetramethylfuran-containing ligand framework is described
that sufficiently stabilizes these species to allow for isolation and
characterization by a variety of spectroscopic and crystallo-
graphic techniques. Insights from these studies are used to
provide a rationale for the impact of subtle changes in macro-
cycle structure on the observed potential and reversibility of the
2
2
bound simultaneously through carbon to a reduced nickel center
4
and through oxygen to a Lewis-acidic iron site. The develop-
ment of well-defined synthetic systems that accommodate two
metal centers in different coordination environments can facili-
5
tate the study of these cooperativity effects.
Tetraazamacrocyclic complexes of nickel have been studied
extensively as electroreduction catalysts for a variety of
+
6
7
8
2
substrates including H , CO , and alkyl electrophiles. A
prototypical example is the nickel tetrapyrrole cofactor F430,
which is the site at which S-methyl coenzyme M is catalytically
9
reduced to generate methane. Key intermediates in these
processes are paramagnetic reduced states, formulated either
as Ni(I) metalloradicals or Ni(II)-stabilized ligand-centered
radicals. The nature of these species dictates the potentials at
which they are accessible as well as their reactivity and
II/I
Ni redox couples.
Division of Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena, CA, 91125, USA. E-mail: jpeters@caltech.edu
†
Electronic supplementary information (ESI) available: Synthetic
procedures, spectroscopic and crystallographic details, and calculated
geometries. CCDC 895612–895620. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2sc21231e
Fig. 1 Diimine–dioxime complexes of nickel containing either a
2
bridging proton (1 and 2) or BF (3 and 4) group.
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bond, has one long and one short N–O distance at 1.361(1) and
ꢀ
.308(1) A respectively. A summary of some of the key metrical
Results and discussion
1
Synthesis and structural characterization of NiZn complexes
parameters is shown in Table 1.
As compared to complexes derived from 1,3-diaminopropane,
the two-carbon tether in the 1,2-diaminoethane-derived
Based on precedence by Chaudhuri et al. for the synthesis of
13
related copper complexes, substitution of the bridging proton
Me
Me
2+
complexes enforces shorter Ni–N distances—by approximately
ꢀ
.05 A on average. Additional effects of the shorter tether length
in [Ni( doenH)]ClO
ion was accomplished by deprotonation with excess Et
presence of [Zn(H O) ](ClO
Zn coordination sphere and prevent the formation of dimeric
species, 1,4,7-trimethyl-1,4,7-triazacyclononane (Me TACN)
4 4
(1) and [Ni( dopnH)]ClO (2) for a Zn
0
3
N in the
are a compressed N2–Ni–N4 angle and expanded N1–Ni–N3
angle, which result in an increase in the distance between the
oximato oxygen atoms. These structural differences between 5
and 6 are manifested in their relative susceptibility toward pro-
tonolysis. When treated with 10 equivalents of AcOH, complex 6
is completely converted to the proton-bridged species 2, while
complex 5 is stable under these conditions. This observation can
be rationalized by the relative strength of the intramolecular
hydrogen-bonding interactions in 1 and 2, with greater O–O
separation resulting in weaker hydrogen-bonds. Consistent with
2
6
4 2
) (Scheme 1). In order to fill out the
3
was employed as a capping ligand. The resulting NiZn
1
complexes, 5 and 6, are diamagnetic, and the H-NMR reso-
nances corresponding to the methyl and methylene groups of the
Me TACN ligand appear as two singlets at room temperature in
3
CD CN, suggestive of rapid fluxional processes. Positive ions
3
Me Me
corresponding to [Ni( doen/ dopn)Zn(Me TACN)(ClO )]
+
3
4
were observed by ESI-MS, confirming the presence of one nickel
and one zinc atom.
this hypothesis, the bridging proton in complex 1 is in rapid
1
CN on the H-NMR
exchange with residual water in CD
3
Orange-red crystals of 5 and 6 grown from MeCN solutions
were analysed by X-Ray diffraction (XRD). Solid state struc-
tures reveal a square planar coordination environment for nickel,
consistent with these complexes being diamagnetic in solution
chemical shift timescale at room temperature, whereas a sharp
downfield resonance at 18.3 ppm is observed for complex 2 under
the same conditions.
In order to explore whether these bimetallic platforms are
capable of supporting ligands that span the two metal centers,
(
Fig. 2). In both cases, zinc is six-coordinate, associating a
molecule of MeCN. In order to accommodate the zinc ion, the
separation between the oxygen atoms of the oximato groups
+
complex 5 was treated with n-Bu N salts of OAc or NO2 to
ꢀ
ꢀ
4
ꢀ
increases by 0.35 to 0.4 A relative to the corresponding proton-
ꢀ
bridged complexes. Zinc is displaced approximately 1 A above
yield the corresponding dark-brown monocationic complexes.
1
ꢀ
Complex 7 exhibits strong IR bands at 1569 and 1400 cm
,
assigned to the antisymmetric and symmetric stretching vibra-
4
the mean NiN plane, and the distances between the Ni and Zn
ꢀ
are 3.6 A. The short N–O bond lengths of the oximato groups,
ꢀ
ranging from 1.309(3) to 1.316(3) A, suggest a significant degree
ꢀ1
tions of the acetate group. The Dy of 169 cm falls within the
15
typical range for syn–syn bridging carboxylates. This coordi-
nation mode was confirmed by XRD (Fig. 3). While multiple
bridging geometries have been observed in bimetallic m-NO
ꢀ
of p-bonding. By comparison, N–O distances of 1.340–1.367 A
1
4
2
were observed for the BF
2
-bridged complex 4. The proton-
16
1
1
complexes, the m-(h -N:h -O) motif is favored in the solid-state
for 8. Coordination of one of the nitrite oxygen atoms to
bridged complex 1, which features a non-symmetrical hydrogen-
zinc induces a moderate asymmetry in the N–O bond distances
ꢀ
(
D z 0.05 A).
Electrochemical and chemical reduction
In order to compare the electrochemistry of complexes 5 and 6 to
2
the corresponding proton and BF -bridged complexes, cyclic
voltammetry (CV) studies were conducted in MeCN. Complex 5
ꢀ1
Me
Scheme 1 Preparation of [Ni( doen)Zn] (5) and [Ni( dopn)Zn] (6).
2+
Me
2+
exhibits a reversible reduction (100 mV s scan rate) at ꢀ0.98 V
vs. SCE, followed by a quasi-reversible reduction at ꢀ1.42 V
2
+
Fig. 5). Despite the fact that the Zn ion causes complex 5 to be
(
ꢁ
overall dicationic, the E for the first reduction is cathodically
ꢀ
Table 1 Selected bond distances (A) from solid-state structures
Zn–mean
Mean Ni–N O–O
N–O
Ni–Zn
4
N plane
1
2
5
6
7
8
1.833
1.884
1.860
1.914
1.857
1.867
2.613(1) 1.361(1), 1.308(1)
2.420 1.339, 1.333
3.035(2) 1.312(3), 1.318(3) 3.5919(4) 0.916
2.758(3) 1.309(3), 1.316(3) 3.6150(4) 1.266
3.089(3) 1.320(3), 1.303(4) 3.4755(8) 1.023
3.032(4) 1.327(5), 1.319(4) 3.4467(6) 1.276
—
—
—
—
a
4ꢀ
Fig. 2 Solid-state structures of (a) 5 and (b) 6 excluding ClO coun-
terions and non-coordinated solvent molecules (thermal ellipsoids at 50%
ꢁ
ꢀ
probability). Selected bond distances (A) and angles ( ) for 5: Ni1–N1,
b
1.884(2); Ni1–N2, 1.835(3); Ni1–N3, 1.885(2); Ni1–N4, 1.836(2); N1–Ni–
ꢀ
N3, 106.40(7); N2–Ni–N4, 86.3(1). Selected bond distances (A) and
a
b
From ref. 6b. Bond metrics for one of two crystallographically distinct
molecules in the asymmetric unit are shown.
ꢁ
angles ( ) for 6: Ni1–N1, 1.923(2); Ni1–N2, 1.905(2); Ni1–N3, 1.911(2);
Ni1–N4, 1.917(2); N1–Ni–N3, 99.49(9); N2–Ni–N4, 96.5(1).
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Scheme 2 Hydrogen-atom and proton abstraction reactions.
Dihydrogen was not detected by gas chromatography as a
byproduct of this process, suggesting that the fraction of metal-
containing product not observed by NMR is the hydrogen-atom
2
acceptor in this reaction. When reduction with Cp Co was con-
ducted in the presence of TEMPO, the formation of 9 was nearly
quantitative, and an equimolar amount of TEMPO–H was
generated. This result suggests that the C–H bond dissociation
energies for the methyl substituents are lowered significantly in
the reduced complex, a likely indication of significant radical
character in the ligand. Compound 9 can alternatively be
accessed directly by treatment of the Ni(II) complex 5 with one
equivalent of KOt-Bu.
Fig. 3 Association of bridging anions to complex 5 and solid-state
4ꢀ
structures of (a) 7 and (b) 8 excluding ClO counterions (thermal
ellipsoids at 50% probability). Only one of two crystallographically
distinct molecules in the asymmetric unit is shown for 8. Selected bond
ꢀ
distances (A) for 7: Ni1–O3, 2.173(3); Zn1–O4, 2.060(2); O3–C20,
ꢀ
.239(4); O4–C20, 1.260(4). Selected bond distances (A) for 8: Ni1–N8,
1
2
.209(3); Zn1–O3, 2.148(3); N8–O3, 1.266(5); N8–O4, 1.236(4).
The observation of this ligand-based reactivity is significant in
+
the context of H reduction catalysis and constitutes a Faradaic
shifted by 270 mV relative to the proton-bridged, monocationic
complex 1. The shift observed for 6 relative to 2 is of a similar
magnitude. This effect may reflect the relatively modest Lewis
process that does not generate H
diimine–dioxime ligands evolve H
2
. While cobalt complexes of
in high yield under a variety
2
2
+
acidity of pseudo-octahedral Zn
ions, particularly when
of conditions, the corresponding nickel complexes generally
6b
coordinated to multiple amine donors. This hypothesis is sup-
ported by the relatively short N–O distances in the NiZn
complexes. The expanded macrocycle core size upon substitution
of a proton for a larger metal ion may also contribute to this
shift. It is noteworthy in this context that the complexes derived
from the longer diaminopropane tether are reduced at more
negative potentials than those derived from diaminoethane
exhibit comparatively low Faradaic efficiency.
TMF
2+
doen)Zn] (10)
Synthesis and electrochemistry of [Ni(
In order to circumvent hydrogen-atom transfer pathways from
17
the ligand, the tetramethylfuran-derived NiZn complex 10,
lacking hydrogen-atoms adjacent to the imines, was prepared.
While many of the major features of the solid-state structure are
similar to complex 5, a noteworthy difference is that zinc is nearly
(
Table 2).
When a chemical reduction of complex 5 was attempted using
coplanar with the NiN
4
unit, and the separation between the
ꢀ
one equivalent of Cp Co, a green, diamagnetic product was
2
oximato oxygen atoms is correspondingly greater by 0.1 A
(Fig. 4).
instead observed to form within 15 min at room temperature.
1
The H-NMR spectrum of this product is consistent with the
In contrast to complexes 5 and 6, both the first and second
reduction waves for 10 are electrochemically reversible (100 mV
enamide structure, 9, shown in Scheme 2: the methylene groups
of the two-carbon tether appear as non-equivalent triplets, and
two singlets are observed, at 4.01 and 3.75 ppm, in a region that is
typical for olefinic protons.
ꢀ
1
ꢁ
s
scan rate) with E values of ꢀ0.76 and ꢀ1.21 V vs. SCE
(
Fig. 5). These potentials are approximately 200 mV more
1
positive than for the methyl-substituted complex 5. One possible
The enamide product 9, formed in 71% yield by H-NMR
explanation for this shift is an inductive electron-withdrawing
integration against an internal standard, is presumed to arise by
hydrogen-atom abstraction from the putative reduced species.
18
effect of the furan oxygens, which may be particularly
a
Table 2 First and second reduction potentials (V) vs. SCE
ꢁ
n/nꢀ1
ꢁ
nꢀ1/nꢀ2
)
E ([Ni]
)
E ([Ni]
1
2
5
6
1
ꢀ0.71
ꢀ0.86
ꢀ0.98
ꢀ1.08
ꢀ0.76
ꢀ1.42
ꢀ1.39
b
ꢀ1.42
b
ꢀ1.43
0
ꢀ1.21
a
0
.5 mM concentration of the complex in MeCN; 0.1 M [n-Bu
atmosphere;
00 mV s scan rate; internally referenced to the Fc/Fc redox couple at
4 4
N][ClO ]
supporting electrolyte; glassy carbon working electrode; N
1
2
4ꢀ
4ꢀ
ꢀ1
+
Fig. 4 Solid-state structure of 10 excluding ClO and BPh counter-
ions and non-coordinated solvent molecules (thermal ellipsoids at 50%
probability).
b
+
0.38 V vs. SCE. Reduction waves are quasi-reversible.
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Fig. 5 Cyclic voltammograms for 0.5 mM (a) 5 and (b) 10 in MeCN.
Scans that extend past the second reduction wave are in black, and a scan
that extends only past the first reduction wave is in red. (0.1 M [n-Bu
4
N]
[
ClO ] supporting electrolyte; glassy carbon working electrode; N
4
2
+
ꢀ1
atmosphere; 100 mV s scan rate; internally referenced to the Fc/Fc
redox couple at +0.38 V vs. SCE).
4ꢀ
Fig. 6 Solid-state structures of (a) 11 and (b) 12 excluding BPh
counterions and non-coordinated solvent molecules (thermal ellipsoids at
ꢁ
significant in cases where reduction is ligand based. Alterna-
tively, the shorter C–C bond distance between the imine and
oxime groups enforced by the five-membered ring may lower the
energy of the ligand-based p* orbital, allowing it to be populated
at less reducing potentials.
50% probability). Selected bond lengths ( Aꢀ ) and angles ( ) for 11: Ni1–
ꢀ
-plane, 0.021; S N–Ni1–N, 360.0. Selected bond lengths (A) and
mean N
4
ꢁ
angles ( ) for 12: Ni1–P1, 2.2263(5); Ni1–mean N
4
-plane, 0.666; S N–
Ni1–N, 334.4. Experimental (black, top) and simulated (red, bottom)
X-band EPR spectra for (c) 11 (2-MeTHF/THF frozen solution, 77 K)
and (d) 12 (2-MeTHF/THF frozen solution, 50 K). Simulated parameters
for 11: giso ¼ 2.020. Simulated parameters for 12: g
k
t
¼ 2.208, g ¼ 2.044.
TMF
2+
doen)Zn] and PPh
Chemical reduction of [Ni(
coordination
3
The metrical parameters associated with the solid-state struc-
ture of 11 support the electronic configuration suggested by the
EPR data. The Ni–N distances are contracted relative to the
more oxidized species, 10, consistent with greater electrostatic
attraction between the metal cation and the increased negative
charge on the ligand framework. This propensity for shorter
Ni–N bonds in the reduced state provides an explanation for the
observed effect of macrocycle core size on the reduction potential
of Ni(II) diimine–dioxime complexes: the diaminoethane-derived
complex 5 exhibits both shorter Ni–N distances and an anodi-
cally-shifted reduction potential than for 6. This trend addi-
Reduction of complex 10 using Cp
2
Co yielded a dark green,
paramagnetic species that was isolated in 81% yield (Scheme 3).
The reduced complex, 11, exhibits a sharp, nearly isotropic EPR
signal at g ¼ 2.02. This spectrum is detectable in 2-MeTHF/THF
solution at room temperature and in a frozen glass at 77 K. The
narrow line width, lack of anisotropy, and g-factor near the free
electron value are suggestive of a ligand-centered radical with
minimal spin population at the nickel center.
Room temperature diffusion of pentane into concentrated
solutions of complex 11 in THF produced plate-like, green-
brown crystals. Characterization by XRD revealed a structure in
which zinc is five-coordinate. Combustion analysis of bulk
samples isolated at room temperature also indicates a formula
lacking associated solvent molecules. Although crystals obtained
in this manner were sufficient to establish connectivity between
atoms, reliable metrical parameters could not be obtained. In
order to further characterize the reduced complex, crystalliza-
tions were conducted at ꢀ30 ꢁC under otherwise identical
conditions, yielding the THF adduct shown in Fig. 6.
tionally extends to the proton and BF -bridged complexes.
2
Distortions in the ligand geometry are also consistent with a
delocalized organic radical: as compared to complex 10, C–N
distances for the imine and oxime groups in 11 are elongated, and
the intervening C–C bonds are contracted. No significant
differences were observed in the bond metrics for the two halves
of the macrocycle suggesting that the unpaired spin is distributed
over both ligand p-systems. The reduced C–N bond order of
the imine group is also apparent in the IR spectrum: the imine
ꢀ
1
C–N stretching band at 1657 cm for complex 10 is shifted to
ꢀ
1
560 cm in complex 11.
1
Gagne has reported EPR studies of a series of reduced nickel
11a
tetraazamacrocycle complexes. For ligands containing conju-
gated p-systems, reduction was observed to be primarily ligand-
centered. By contrast, complexes containing amine and isolated
imine donors exhibited axial EPR spectra that were attributed to
Ni(I) species. Consistent with this assignment, structural
Scheme 3 Chemical reduction and coordination of PPh
3
.
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ꢀ
five-coordinate and positioned 0.67 A above the N basal plane.
4
The Ni–N distances are significantly elongated relative to the
four-coordinate complexes in either oxidation state, consistent
with population of a d
x
2
ꢀy
2
orbital. The relevant C–N and C–C
distances are similar to complex 10 and indicate that the oxida-
tion state of the ligand is largely unchanged.
Computational electronic structure model
More detailed insight into the electronic structure of the S ¼ 1/2
reduced complex 11 and the PPh adduct 12 was obtained by
3
calculating the singly occupied molecular orbitals (SOMO) and
spin densities using DFT methods (B3LYP/6-31G(d)). Tetra-
methylfuran groups were truncated to methyl substituents and
outer-sphere anions were excluded in these calculations. The
computationally optimized geometries accurately reproduce the
decrease in Ni–N distances upon reduction as well as the changes
in the C–N, C–C, and N–O bond distances of the ligand
p-systems. A more detailed comparison between experimental
and computational values for key metrical parameters is shown
in Fig. 8.
Fig. 7 Nickel K-edge spectra of 10 (top, red) and 11 (bottom, black).
characterization of four-coordinate diimine–diamine and tetra-
amine Ni(I) complexes by Fujita revealed significantly elongated
Ni–N distances relative to the Ni(II) complexes of the same
12b,12c
ligand.
This structural change can be readily interpreted as a
consequence of an electron residing in a s-anti-bonding orbital
of d
2
2
parentage. Accordingly, tetraamine complexes of nickel
For complex 11, the Mulliken spin population on nickel was
calculated to be 0.02, indicating negligible metalloradical char-
acter (Fig. 9). The SOMO has p-anti-bonding character between
the carbon and nitrogen atoms of the imine and oxime groups,
and p-bonding character between the intervening C–C bond,
accounting for the observed geometric changes upon reduction.
x ꢀy
are reduced at more positive potentials as the macrocycle core
19
size is increased —an opposing trend to what is observed for
diimine–dioxime complexes.
Nickel K-edge data (Fig. 7) are also consistent with the
reduction of 10 being primarily ligand-based. For related square
20
21
planar
tetraazamacrocycle
and
dithiolene/dithiolato
The square pyramidal PPh adduct, by contrast, was calculated
3
complexes of nickel, the pre-edge feature, assigned to a 1s / 4p
z
to have a 1.12 spin population on nickel. The SOMO is derived
transition, has been used as a measure of the effective nuclear
charge at the metal center. The energies for this transition are
nearly coincident at 8333 eV for complexes 10 and 11. By
comparison, a Ni(I) tetramethylcyclam complex investigated by
from a d
2
2
orbital and is s-anti-bonding with respect to the
x ꢀy
Ni–N bonds. For both reduced complexes, elongation of the
oximato N–O bonds can be rationalized by the p-anti-bonding
character in the calculated SOMOs.
20
Fujita et al. exhibited a significantly lower energy for both the
pre-edge feature (approximately 8328 eV) and the subsequent
rising edge.
TMF
+
Hydrogen-atom abstraction reactivity of [Ni(
doen)Zn]
The addition of PPh
3
to solutions of complex 11 in THF
In order to assess the stability of complex 11 toward hydrogen-
caused a subtle color change to green-blue. A frozen THF/2-
MeTHF solution of the resulting adduct exhibited an axial EPR
atom abstraction, reaction with TEMPO was conducted in
CD CN. The formation of TEMPO–H was observed by
3
1
signal (g
k
pattern, attributed to the four roughly equivalent equatorial
¼ 2.208, g
t
¼ 2.044) with a 12-line hyperfine coupling
H-NMR, and two diamagnetic products were detected in 55%
and 13% yield (Scheme 4). For the major product, the presence of
seven well-resolved peaks corresponding to the furan methyl
nitrogen atoms (Fig. 6). In the solid-state structure of 12, nickel is
ꢀ
Fig. 8 Comparison of selected bond lengths (A) from solid-state structures (black) for (a) 10, (b) 11, and (c) 12. For 11, metrical parameters are shown
for only one of the two crystallographically distinct molecules in the asymmetric unit. Calculated values from computationally optimized structures (red)
at the B3LYP/6-31G(d) level of DFT are shown for comparison.
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Conclusions
Bimetallic complexes containing a low-valent, reduced nickel
center in combination with a Lewis-acidic zinc site can be
synthetically accessed using the dianionic form of diimine–
dioxime ligands. These scaffolds afford sufficient geometric
flexibility to accommodate a variety of bridging and non-
bridging ligands, with metal–metal distances ranging from 3.45
ꢀ
to 3.78 A. The incorporation of zinc in the place of a bridging
II/I
proton results in a significant cathodic shift in the Ni couple,
illustrating the utility of varying this substituent in order to tune
the overall redox potential of the complex.
Reduced states of nickel diimine–dioxime complexes are
accessible at modest potentials owing to the presence of low-lying
ligand-based p-orbitals. The resulting four-coordinate, S ¼ 1/2
species exhibits characteristic features of a ligand-centered
radical bound to Ni(II), yet is capable of associating p-acidic
ligands such as PPh3 to form five-coordinate Ni(I) metal-
loradicals. The ability of a single platform to reversibly access
multiple oxidation states and conformations at minimal energetic
cost is a key characteristic of redox catalysts that are capable of
operating without the need for large thermodynamic or kinetic
driving forces. The application of the complexes described here
as bifunctional electrocatalysts is an ongoing area of research.
Fig. 9 Calculated SOMO for (a) 11 and (b) 12, and spin density plots for
c) 11 and (d) 12. Geometries were optimized at the B3LYP/6-31G(d)
(
level of DFT and verified by frequency analysis.
Acknowledgements
Scheme 4 Hydrogen-atom abstraction from the reduced complex.
This work was supported by the NSF Center for Chemical
Innovation: Powering the Planet grant CHE-0802907, and by the
Gordon and Betty Moore Foundation. We thank Larry Henling
and Charlene Tsay for assistance with crystallography, Dr
Angelo Di Bilio for EPR measurements, and Dr Limei Zhang
and Dr Jens Kaiser for XAS measurements. We acknowledge the
Gordon and Betty Moore Foundation, the Beckman Institute,
and the Sanofi-Aventis BRP at Caltech for their generous
support of the Molecular Observatory at Caltech. SSRL is
operated for the DOE and supported by its Office of Biological
and Environmental Research, and by the NIH, NIGMS
substituents indicates a non-symmetrical structure, and the two
singlets at 4.85 and 5.35 ppm suggest unsaturation in the ligand.
The identity of this species was established by XRD analysis of
crystals obtained from concentrated THF solutions (Fig. 10).
The unusual cyclopropane motif presumably arises by H-atom
abstraction from a methyl substituent and cyclization onto the
ꢀ
adjacent imine carbon. The short C17–C18 distance of 1.36 A is
consistent with a double bond. By comparison, the correspond-
ꢀ
ing C–C distance in complex 10 is 1.54 A. While the identity of
(
including P41GM103393) and the NCRR (P41RR001209).
the minor product has not been definitively established, a plau-
1
sible structure based on H-NMR data and analogy to complex
1
3 is the cyclopropane-containing product with a saturated two-
Notes and references
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