Synthesis of homo- and heterobimetallic NiII-MII (M = Fe, Co, Ni, Zn) complexes based on an unsymmetric ligand framework: Structures, spectroscopic features, and redox properties

Article abstract of DOI:10.1016/j.ica.2014.07.018

Several homo- and heterobimetallic Ni^II-M^II complexes (M^II = Fe, Co, Ni, Zn) supported by an unsymmetric polydentate ligand (L₁^3-) are reported (L₁^3- is the trianion of 2-[bis(2-

Full text of DOI:10.1016/j.ica.2014.07.018

Inorganica Chimica Acta 421 (2014) 559–567
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
II
II
Synthesis of homo- and heterobimetallic Ni –M (M = Fe, Co, Ni, Zn)
complexes based on an unsymmetric ligand framework: Structures,
spectroscopic features, and redox properties
⇑
Denan Wang, Sergey V. Lindeman, Adam T. Fiedler
Department of Chemistry, Marquette University, Milwaukee, WI 53201, United States
a r t i c l e i n f o
a b s t r a c t
II
II
II
Article history:
Several homo- and heterobimetallic Ni –M complexes (M = Fe, Co, Ni, Zn) supported by an unsymmet-
Received 9 July 2014
Accepted 12 July 2014
Available online 21 July 2014
3ꢀ
3ꢀ
1
ric polydentate ligand (L
1
) are reported (L
is the trianion of 2-[bis(2-hydroxy-3,5-tert-butyl-
3ꢀ
phenyl)aminomethyl]-4-methyl-6-[(2-pyridylmethyl)iminomethyl]phenol). The L
1
chelate provides
} site
with equimolar amounts of Ni and M salts provides bimetallic complexes in which
two distinct coordination environments: a planar tridentate {N
2
O} site (A) and a tetradentate {NO
3
3
ꢀ
II
II
(
B). Reaction of L
1
Keywords:
Ni complexes
Heterobimetallic complexes
Electronic absorption spectroscopy
Electrochemistry
II
II
the Ni ion exclusively occupies the tetragonal A-site and the M ion is found in the tripodal B-site. X-ray
crystal structures revealed that the two metal centers are bridged by the central phenolate donor of
3
ꢀ
L
1
and an anionic X-ligand, where X =
l
-1,1-acetate, hydroxide, or methoxide. The metal ions are sep-
X
arated by 3.0–3.1 Å in the M
A
M
B
structures, where M and M indicate the ion located in the A and B
A
B
sites, respectively, and X represents the second bridging ligand. Analysis of magnetic data and
UV–Vis–NIR spectra indicate that, in all cases, the two metal ions adopt high-spin states in solution.
II
II
II
The Ni
nolate-rich B-site are reduced at lower potentials. Significantly, the Ni
labile coordination sites in a meridional geometry, which are generally occupied by solvent-derived
A
centers undergo one-electron reduction at ꢀ1.17 V vs. SCE, while the Ni and Co ions in the phe-
II
A
center possesses three open or
X
ligands in the crystal structures. The NiM
B
complexes serve as structural mimics of heterometallic
Ni-containing sites in biology, such as the C-cluster of carbon monoxide dehydrogenase (CODH).
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
2
as a Lewis acid, stabilizing the critical NiCO Fe intermediate and
facilitating cleavage of the CO bond. Finally, the A-cluster of
acetyl-coenzyme A synthase (ACS) consists of a dinickel unit linked
Nickel centers in metalloenzymes are often embedded in poly-
nuclear, heterometallic frameworks that facilitate the activation of
small molecules via multiple electron and proton transfers [1,2].
Three examples are shown in Scheme 1. [NiFe]-hydrogenases fea-
4 4
to a [Fe S ] cubane via a bridging cysteine residue. In each case, the
presence of a metal center adjacent to the redox-active Ni site is
critical for efficient catalysis.
ture an unusual heterobimetallic active site with a [NiFe(
l
-SCys
)
2
]
These biological precedents suggest that multinuclear Ni-con-
taining complexes may have applications as synthetic catalysts
for H generation and CO activation. In an effort to model the
2 2
core; in the available X-ray structures, the Niꢁ ꢁ ꢁFe distance ranges
from 2.5 to 2.9 Å [3,4]. While only the Ni site is redox-active under
II
catalytic conditions, the Fe center is believed to play an important
NiFe1 unit of the C-cluster, Holm and coworkers generated a series
II
role in the binding and heterolytic bond cleavage of H
2
[5,6]. The C-
of bio-inspired complexes in which a Ni center is bound to a 2,6-
II
cluster of carbon monoxide dehydrogenase (CODH), shown in
pyridinedicarboxamidate pincer and a second M ion is coordi-
ꢀ
+
Scheme 1, catalyzes the reversible reaction: CO
CO + H O [7,8]. Based on crystallographic data, it appears that sub-
strate binding and activation occurs at the heterobimetallic NiFe1
unit highlighted in Scheme 1 [9–12], while the [Fe ] component
is only involved in shuttling electrons. The Fe1 center likely serves
2
+ 2e + 2H ?
nated to a pendant chelate (M = Mn, Fe, and Cu) [13,14]. The two
metal centers are bridged by a single hydroxo, cyano, or formato
ligand; for the hydroxo-bridged complexes, X-ray analysis revealed
Niꢁ ꢁ ꢁM distances near 3.7 Å and NiOM angles of ꢂ140°. More
recently, Uyeda and Peters used a dimine-dioxime ligand to gener-
2
3 4
S
II
ate a heterobimetallic NiZn complex [15]. The square-planar Ni
center is linked to a [Zn (Me
2
+
3
-TACN)] unit via two oxime
⇑
Corresponding author. Fax: +1 414 288 7066.
E-mail address: adam.fiedler@marquette.edu (A.T. Fiedler).
bridges (Me -TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane).
3
http://dx.doi.org/10.1016/j.ica.2014.07.018
020-1693/Ó 2014 Elsevier B.V. All rights reserved.
0

5
60
D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
Scheme 1.
The addition of acetate or nitrite anions yielded triply-bridged
structures exhibiting Niꢁ ꢁ ꢁZn distances of ꢂ3.45 Å. While numer-
ous heterobimetallic NiM complexes (where M is a biologically-
relevant transition metal) exist in the literature [16–23], their
revealed that a second bridging ligand is also present:
l-1,1-ace-
tate, hydroxide, or methoxide. As shown in Scheme 2, the series
X
A B A B
of complexes are labeled M M , where M and M identify the
divalent ions located in the A and B sites, respectively, and X rep-
potential relevance to CODH and CO
explored.
2
reduction has not been
resents the second bridging ligand. The presence of two mono-
atomic bridging groups leads to short M –M distances of 3.0–
A B
II
II
In this manuscript, we report our initial efforts to generate
homo- and heterobimetallic complexes containing a coordinatively
unsaturated Ni center in close proximity to a second MII site
3.1 Å. Both metal centers possess open or labile coordination sites
for potential use in substrate binding and activation.
II
In addition to X-ray structural characterization, we have
employed electrochemical and spectroscopic methods to analyze
the electronic properties of the dinuclear complexes. Specifically,
insights into the ligand-field environment were obtained using
electronic absorption spectroscopy, and the redox potentials of
the metal centers were probed with cyclic and square-wave vol-
tammetry. Collectively, these results provide a foundation for
future studies that will examine the reactivity of this new class
of heterobimetallic complexes.
(
M = Fe, Co, Ni, and Zn). Our studies have employed the unsymmet-
3
ꢀ
ric ligand (L
1
) shown in Scheme 2, which contains a central phe-
nolate group capable of bridging the metal ions. Related ligands
have been used to model the homo- and heterodinuclear active
3ꢀ
sites of various metalloenzymes [24–26]. The L
1
ligand provides
two distinct coordination environments: one bearing a Schiff-base
moiety linked to a 2-pyridinylmethyl group (site A), and the other
consisting of a tertiary amine with two additional phenolate
II
donors (site B). Site A is well-suited for Ni coordination, while
the trisphenolate site binds the second divalent metal ion. The phe-
nolate-rich chelate is intended to depress the redox potentials of
the metal centers, thereby enhancing the potential reactivity of
2
. Experimental
2.1. Materials and physical methods
2
the complexes towards electrophilic substrates, such as CO . X-
ray crystal structures of the bimetallic complexes, described below,
Reagents and solvents were purchased from commercial sources
and used as received, unless otherwise noted. The air-sensitive com-
OAc
plex NiFe
was synthesized and handled under inert atmosphere
using a Vacuum Atmospheres Omni-Lab glovebox. Elemental anal-
yses were performed at Midwest Microlab, LLC in Indianapolis, IN.
Two instruments were used to measure electronic absorption spec-
tra: an Agilent 8453 diode array spectrophotometer and an Agilent
Cary 5000 UV–Vis–NIR spectrophotometer. Fourier-transform
infrared (FTIR) spectra of solid samples were obtained with a
Thermo Scientific Nicolet iS5 FTIR spectrometer equipped with the
1
13
iD3 attenuated total reflectance accessory. H and C NMR spectra
were recorded at room temperature with a Varian 400 MHz spec-
trometer. Voltammetric measurements employed an epsilon EC
potentiostat (iBAS) at a scan rate of 100 mV/s with 100 mM (NBu
4
)-
PF as the supporting electrolyte. The three-electrode cell contained
6
a Ag/AgCl reference electrode, a platinum auxiliary electrode, and a
glassy carbon working electrode. Potentials were referenced to the
+
/0
ferrocene/ferrocenium (Fc ) couple, which has an E1/2 value of
0.45 V versus the standard calomel electrode (SCE) in DMF [27].
+
Solid-state magnetic susceptibility measurements were performed
at room temperature using an AUTO balance manufactured by
Sherwood Scientific. Solution-state magnetic moments were
measured in chloroform using the Evans NMR method.
2.2. 3-(Bromomethyl)-2-hydroxy-5-methylbenzaldehyde (I)
Modifying
30 mL, 47% in H
a
published procedure [28]. hydrobromic acid
2
O) was added to a mixture of 5-methylsalicylal-
(
dehyde (3.0 g, 22 mmol), paraformaldehyde (0.99 g, 33 mmol)
and 10 drops of fuming sulfuric acid. The resulting solution was
stirred at 70 °C for 6 h, resulting in formation of a white precipitate.
After cooling to room temperature, the solid was isolated by filtra-
tion and washed with cold water (20 mL). The white product was
Scheme 2.

D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
561
dried under vacuum and used without further purification.
0.434 mmol) and one equivalent of the appropriate M
B
(OAc)
2
salt.
1
Yield = 4.6 g, 91%. H NMR (CDCl
3
) d: 2.34 (s, 3H), 4.55 (s, 2H),
The mixture was stirred overnight and the solvent removed under
vacuum. The resulting solid was taken up in THF and filtered
through Celite to remove sodium salts. Evaporation of solvent pro-
vided the crude solid. The methods used to grow crystals of the
complexes are described below.
7
.33 (d, J = 1.4 Hz, 1H), 7.42 (d, J = 1.4 Hz, 1H), 9.85 (s, 1H), 11.30
1
3
(
3
s, 1H). C NMR (CDCl ) d: 20.4, 26.9, 120.7, 126.2, 129.5, 134.4,
1
39.1, 157.7, 196.7.
2.3. Bis(3,5-di-tert-butyl-2-hydroxybenzyl)amine (II) [29]
1
)(l )
-OAc)] (NiFeOAc
2.7. [NiFe(L
A mixture of 2,4-di-tert-butylphenol (2.0 g, 10 mmol) and hexa-
methylenetetramine (2.80 g, 20 mmol) was refluxed for 2 h in for-
mic acid (100 mL, 85%). The reaction yielded crystals of the
benzoxazine intermediate (1.85 g) that were subsequently isolated
by filtration. This material was dissolved in ethylene glycol
X-ray quality crystals were obtained from a 1:1 mixture of ace-
tone:MeCN. Yield: 40%. Material for elemental analysis was puri-
fied by recrystallization in a 1:1 mixture of MeCN and MeOH.
ꢀ1
Anal. Calc. for C49
3 7 W
H69FeN NiO (M = 926.63 g mol ): C, 63.51;
(
100 mL) and hydrochloric acid (37%; 40 mL) and heated overnight
H, 7.51; N, 4.53. Found: C, 63.48; H, 7.36; N, 4.80%. UV–Vis [kmax,
ꢀ1
ꢀ1
ꢀ1
at 130 °C. After cooling to room temperature, the hydrochloride
salt of the desired product (i.e., II–HCl) was collected by filtration,
washed with water, and dried under vacuum. The resulting solid
was slowly added to a stirred mixture of aqueous KOH (1.8 M;
nm (e, M cm ) in DMF]: 890 (18), 530 (1000). FTIR (cm , solid):
2951 (s), 2902 (m), 2864 (m), 1621 (m), 1603 (m), 1573 (s,
m[C@OOAc]).
leff = 5.02
B 3
l (Evans method, CHCl ).
3
0 mL) and diethyl ether (50 mL). The organic layer was separated,
dried over MgSO , and the solvent removed under vacuum to give
II as a pure white powder. Yield = 1.27 g, 56% overall. H NMR
CDCl ) d: 1.30 (s, 18H), 1.46 (s, 18H), 3.93 (s, 4H), 6.97 (s, 2H),
1
2.8. [NiCo(L )(l
-OAc)] (NiCoOAc
)
4
1
The crude solid was dissolved in a 1:1 mixture of MeCN and
(
3
MeOH, which yielded orange crystals after several days. Yield:
1
3
ꢀ1
7
1
.26 (s, 2H). C NMR (CDCl
3
) d: 29.8, 31.6, 34.2, 34.8, 51.4,
60%. Anal. Calc. for C49
H69CoN
3
NiO
7
(M
W
= 929.71 g mol ): C,
22.8, 123.5, 124.4, 135.9, 141.6, 152.6.
63.30; H, 7.48; N, 4.52. Found: C, 63.66; H, 7.43; N, 4.81. UV–
ꢀ1
ꢀ1
Vis–NIR [kmax, nm (
830 (7), 640 (sh), 580 (sh). FTIR (cm , solid): 2951 (s), 2902 (m),
864 (m), 1627 (w), 1605 (m), 1565 (s, [C@OOAc]). eff = 4.51
(solid state), 4.72 (Evans method, CHCl
e, M cm ) in DMF]: 1630 (6), 1050 (sh),
ꢀ
1
2
.4. Compound III
2
m
l
3
-(Bromomethyl)-2-hydroxy-5-methylbenzaldehyde (I, 2.14 g,
l
B
3
).
9
.4 mmol) was added dropwise to a solution of bis(3,5-di-tert-
2 1
2.9. [Ni (L )(l )
-OAc)] (NiNiOAc
butyl-2-hydroxybenzyl)amine (II; 4.24 g, 9.4 mmol) and triethyl-
amine (2.6 mL, 19 mmol) in THF (50 mL). The mixture was refluxed
overnight and then filtered to remove the salt byproduct. After
evaporation of solvent under vacuum, the crude product was iso-
lated as pale yellow solid. Purification by flash column chromatog-
Yellow crystals were grown from a concentrated solution in
1:1 MeCN:MeOH. Yield: 36%. Anal. Calc. for Ni
= 929.47 g mol ): C, 63.32; H, 7.48; N, 4.52. Found: C,
C
49
H
69
N
3
2 7
O
ꢀ1
(M
W
ꢀ1
ꢀ1
raphy (10:1 mixture of hexanes:EtOAc) provided III as a pure solid.
Yield = 3.0 g, 53%. H NMR (CDCl
62.52; H, 7.28; N, 4.84%. UV–Vis [kmax, nm (
DMF]: 866 (17), 740 (19), 500 (sh). FTIR (cm , solid): 2951 (s),
e
, M cm ) in
1
ꢀ1
3
) d: 1.28 (s, 18H), 1.35 (s, 18H),
2
.23 (s, 3H), 3.70 (s, 2H), 3.74 (s, 4H), 6.95 (d, J = 2.3 Hz, 2H), 7.08
2903 (m), 2864 (m), 1625 (m), 1605 (s), 1585 (s, m[C@OOAc]).
1
3
(
s, 1H), 7.19 (d, J = 2.3 Hz, 2H), 7.24 (s, 1H), 9.83 (s, 1H). C NMR
CDCl ) d: 20.1, 29.6, 31.7, 34.1, 34.9, 52.8, 58.2, 120.2, 121.8,
23.5, 125.0, 125.9, 129.2, 133.3, 135.9, 140.1, 141.3, 152.3,
58.1, 196.7.
leff = 3.66 (solid state), 3.82
l
B
3
(Evans method, CHCl ).
(
1
1
3
2.10. [NiZn(L
1
)(l
-OAc)] (NiZnOAc
)
The yellow solid was washed with cold MeOH, dissolved in CH2-
2
Cl , and layered with MeOH to provide X-ray quality crystals.
2.5. Ligand L
H
1 3
ꢀ1
Yield: 45%. Anal. Calc. for C49
69 3 7 W
H N NiO Zn (M = 936.16 g mol ):
Modifying a published procedure [30], 2-picolylamine (0.21 mL,
.0 mmol) was added to a solution of compound III (1.2 g,
.0 mmol) in methanol (20 mL). The mixture was refluxed over-
C, 62.87; H, 7.43; N, 4.49. Found: C, 62.67; H, 6.97; N, 4.62%. UV–
ꢀ1
ꢀ1
ꢀ1
2
2
vis [kmax, nm (e, M cm ) in DMF]: 869 (8), 575 (4). FTIR (cm ,
solid): 2951 (s), 2902 (m), 2863 (m), 1643 (m), 1604 (s), 1586 (s,
night under argon, causing the color to change to yellow. Removal
of solvent under vacuum provided L as yellow powder, which
was used without further purification. Yield = 1.33 g, 96%.
NMR (CDCl ) d: 1.27 (s, 18H), 1.34 (s, 18H), 2.19 (s, 3H), 3.73 (s,
m
[C@OOAc]).
l
eff = 2.69 (solid state), 2.67
B 3
l (Evans method, CHCl ).
1 3
H
1
-OAc)] (CoCoOAc
H
2.11. [Co (L
2
1
)(
l
)
3
2
7
7
8
3
1
1
2
H), 3.77 (s, 4H), 5.00 (s, 2H), 6.87 (s, 1H), 6.94 (d, J = 2.3 Hz, 2H),
.00 (s, 1H), 7.18 (d, J = 2.3 Hz, 2H), 7.29 (dd, J = 7.6, 5.0 Hz, 1H),
.48 (d, J = 7.6 Hz, 1H), 7.80 (t, J = 7.6 Hz, 1H), 8.24 (br s, 3H, OH),
Three equivalents of NaOMe (47 mg, 0.87 mmol) were added to
a stirred solution of L (200 mg, 0.29 mmol) in MeOH (10 mL).
After 10 min, two equivalents of Co(OAc) (144 mg,
ꢁ4H
1
H
3
2
2
O
.40(s, 1H), 8.61 (d, J = 5.0 Hz, 1H). 13C NMR (CDCl
) d: 20.2, 29.6,
0.58 mmol) were added and the resulting mixture was stirred
overnight. The solvent was evaporated under vacuum and the
crude material dissolved in THF (10 mL). After filtering through
Celite to remove sodium salts, the THF solvent was evaporated to
give a yellow solid that was taken up in a 1:1 mixture of
MeCN:MeOH. X-ray-quality crystals appeared after a few days.
3
1.7, 34.1, 34.9, 54.0, 58.3, 63.9, 118.2, 122.0, 122.1, 122.7, 123.4,
24.9, 125.0, 127.8, 131.6, 135.5, 136.0, 138.0, 141.2, 148.4,
ꢀ1
42.5, 157.6, 157.8, 167.3. FTIR (cm , solid): 2951 (s), 2904 (m),
866 (m), 1632 (m), 1590 (m).
2
.6. General procedure for synthesis of [NiM(L
1
)(
l
-OAc)] complexes
Yield = 111 mg, 41%. Anal. Calc. for C49
H
69Co
2
N
3
O
7
(M
W
= 929.95 -
OAc
ꢀ1
(NiM
B
)
g mol ): C, 63.29; H, 7.48; N, 4.52. Found: C, 63.33; H, 7.40; N,
ꢀ1
ꢀ1
4
.90%. UV–Vis–NIR [kmax, nm (
920 (3), 660 (sh), 580 (sh), 530 (sh). FTIR (cm , solid): 2952 (s),
2902 (m), 2865 (m), 1626 (w), 1602 (m), 1568 (s, [C@OOAc]).
eff = 5.31 (Evans method, CHCl ).
e, M cm ) in DMF]: 1585 (5),
ꢀ
1
The L
NaOMe (70 mg, 1.3 mmol) were stirred in MeOH (10 mL) for
0 min, followed by addition of Ni(OAc) (108 mg,
ꢁ4H
1 3
H ligand (300 mg, 0.434 mmol) and three equivalents of
m
1
2
2
O
l
l
B
3

5
62
D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
-OH)] (NiZnOH
)
Technologies) equipped with dual microfocus Cu/Mo X-ray
1
2.12. [NiZn(L )(l
sources, X-ray mirror optics, Atlas CCD detector, and low-temper-
ature Cryojet device. The data were processed with the CrysAlis
Pro package of programs (Agilent Technologies, 2011), followed
by an empirical multi-scan correction using the SCALE3 ABSPACK
routine. Structures were solved using the SHELXS program and
refined with SHELXL [31] in combination with the Olex2 crystallo-
graphic package [32]. In most cases, hydrogen atoms were local-
ized in difference syntheses of electron density but were refined
using appropriate geometric restrictions of the corresponding bond
lengths and bond angles within a riding/rotating model. X-ray
crystallographic parameters are provided in Table 1 and experi-
mental details are available in the corresponding CIFs.
The L
NEt (0.18 mL, 1.3 mmol) were dissolved in MeOH (10 mL), fol-
lowed by addition of equimolar amounts of Ni(ClO
ꢁ6H
159 mg, 0.434 mmol) and Zn(ClO O (162 mg, 0.434 mmol).
ꢁ6H
After stirring for one hour, one equivalent of (NEt )OH (0.434 mL,
.0 M solution in MeOH) was injected, resulting in formation of a
light brown precipitate. The solid was collected and washed with
cold methanol to provide the crude product as a yellow powder.
X-ray-quality crystals were grown by vapor diffusion of
pentane into a concentrated 1,2-dichloroethane (DCE) solution.
1 3
H ligand (300 mg, 0.434 mmol) and three equivalents of
3
4
)
2
2
O
(
4
)
2
2
4
1
Yield = 162 mg, 40%. Anal. Calc. for
C
45
H
59
N
3
NiO
4
ZnꢁDCE
ꢀ1
(
M = 929.00 g mol ): C, 60.76; H, 6.84; N, 4.52. Found: C,
W
ꢀ1
ꢀ1
6
0.50; H, 7.02; N, 4.92%. UV–Vis [kmax, nm (
e
, M cm ) in
3. Results and discussion
ꢀ1
DMF]: 848 (18), 510 (sh). FTIR (cm , solid): 2949 (s), 2903 (m),
2
2
864 (m), 1626 (m), 1601 (m), 1553 (w).
.53 (Evans method, CHCl ).
leff = 1.43 (solid state),
3.1. Ligand and complex syntheses
l
B
3
1 3
The novel L H ligand was prepared in four steps via the route
shown in Scheme 3. As demonstrated by Belostotskaya et al. the
Duff-like reaction of 2,4-di-tert-butylphenol with hexamethylene-
2 1 3
.13. [Ni (L )(l-OCH )
)] (NiNiOMe
2
2
tetramine in HCO H generates a benzoxazine intermediate that is
Ni(ClO
4
)
2
ꢁ6H
2
O
(211 mg, 0.578 mmol) and triethylamine
(200 mg,
converted to bis(3,5-di-tert-butyl-2-hydroxybenzyl)amine (II) in
ethylene glycol and HCl. Bromomethylation of 5-methylsalicylal-
dehyde provided compound I, which participated in a substitution
reaction with II under basic conditions to generate III in 53% yield.
In the final step, the Schiff base and pyridyl donors of L H were
1 3
incorporated by condensation of 2-picolylamine with the aldehyde
(
0
121 mL, 0.867 mmol) were added to a solution of L H
.29 mmol) in 10 mL of MeOH. After one hour, a single equivalent
of (NEt )OH (0.29 mmol) was added. The mixture was stirred over-
4
night, eventually giving rise to a dark yellow precipitate. The sol-
vent was removed under vacuum, and the resulting solid was
dissolved in THF. Insoluble salts were eliminated by filtration and
the THF solvent was removed to yield the crude brown product.
X-ray-quality crystals were grown from
solution of MeCN:MeOH. Yield = 147 mg, 57%. Anal. Calc. for
1 3
moiety of III.
OAc
The homo- and heterobimetallic complexes NiM
B
(Scheme 2)
1 3
H , Ni(OAc)2-
a concentrated 1:1
were prepared by mixing equimolar amounts of L
4H O, and the appropriate M(OAc) salt in MeOH (M = Fe, Co,
Ni, or Zn). Three equivalents of base (NaOMe) were required to
deprotonate the phenolic groups of L . The resulting complexes
are soluble in CH Cl and most polar aprotic solvents, but only
sparingly soluble in MeOH. High-quality crystals were generally
obtained from solvent mixtures involving MeOH and either CH Cl
B
ꢁ
2
2
ꢀ1
C
48
H
69
N
3
Ni
2 6 W
O (M = 901.46 g mol ): C, 63.95; H, 7.72; N, 4.66.
ꢀ1
ꢀ1
Found: C, 64.01; H, 7.68; N, 4.70%. UV–vis [kmax, nm (
e
, M cm
in DMF]: 890 (20), 760 (25). FTIR (cm , solid): 2950 (s), 2902 (m),
865 (m), 1634 (w), 1604 (m), 1573 (w). eff = 3.77 (Evans
method, CHCl ).
)
H
3
ꢀ1
1
2
2
2
l
l
B
3
2
2
OAc
or MeCN; the single exception was complex NiFe , which was
2.14. Crystallographic studies
crystallized from a 1:1 combination of acetone and MeCN.
Similar reaction conditions were used to generate the
complex, NiZn , with three crucial differences: (i) the M ions
l-OH
OH
II
X-ray diffraction (XRD) data were collected at 100 K with an
Oxford Diffraction SuperNova kappa-diffractometer (Agilent
were added as perchlorate salts, (ii) triethylamine (NEt ) was used
3
Table 1
Summary of X-ray crystallographic data collection and structure refinement.
NiFeOAcꢁ4 acetone NiCoOAc
NiNiOAc
NiZnOAc
CoCoOAc
69Co
929.93
monoclinic
P2 /n
NiZnOHꢁDCE
NiNiOMeꢁMeOH
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
59
H
89FeN
1130.89
monoclinic
P2
3
NiO11
C
49
H
69CoN
929.71
monoclinic
P2 /n
3
NiO
7
C
49
H
69
N
3
Ni
2
O
7
C
49
H
69
N
3
7
NiO Zn
C
49
H
N
2 3
O
7
C
47
H
63
N
Ni
3 2
O
4
2
Cl Zn
49 73 3 2 7
C H N Ni O
929.49
monoclinic
P2 /n
936.15
monoclinic
P2 /n
928.98
monoclinic
P2 /c
933.52
monoclinic
P2 /c
1
1
1
1
1
1
1
10.3503(2)
20.2208(3)
15.6385(3)
90
107.257(2)
90
3125.7(1)
2
1.202
10.21448(8)
20.4809(2)
22.7461(3)
90
97.7635(9)
90
4714.90(8)
4
1.310
10.1841(1)
20.5261(3)
22.8062(4)
90
98.208(1)
90
4718.6(1)
4
1.308
10.2153(1)
20.4672(3)
22.7709(3)
90
97.818(1)
90
4716.7(1)
4
1.318
10.2249(2)
20.6272(3)
22.7256(4)
90
97.741(2)
90
4749.0(1)
4
1.301
14.4458(3)
19.0800(2)
17.1363(2)
90
106.356(2)
90
4532.0(1)
4
1.362
20.8266(6)
13.0162(3)
18.9697(4)
90
110.255(3)
90
4824.4(2)
4
1.285
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
calc (g/cm3)
k (Å)
l
0.7107
0.589
1.5418
3.67
0.7107
0.851
1.5418
1.539
0.7107
0.751
1.5418
2.606
1.5418
1.378
(mm 1
ꢀ
)
h-Range (°)
5.6–59.0
36415
14441 (0.0338)
14441/7/714
1.042
7.8–147.2
45296
9423 (0.0284)
9423/0/574
1.041
5.7–58.9
40517
11527 (0.0312) 9435 (0.0312)
11527/0/574
1.067
0.0383/0.0880
0.0485/0.0936
5.8–147.8
45332
5.6–59.0
38008
11671 (0.0289) 9000 (0.0357)
11671/0/574
1.033
0.0365/0.0878
0.0475/0.0946
7.1–147.6
43809
8.2–147.2
45182
9558 (0.0416)
9558/0/579
1.043
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
)
9435/0/574
1.023
0.0339/0.0924
0.0392/0.0965
9000/5/559
1.031
0.0324/0.0847
0.0375/0.0888
2
Goodness-of-fit (GOF) on F
a
R
R
1
/wR
/wR
2
(I > 2
(all data)
r
(I))
0.0359/0.0768
0.0422/0.0813
0.0389/0.1097
0.0430/0.1132
0.0413/0.1137
0.0482/0.1212
a
1
2
P||F
|–|F
||/P
|F
o
|; wR
= [P
w(F
–F
) / w(F
P
a
2
2
2
2
)2]1/2.
R1 =
o
c
2
o
c
o

D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
563
Scheme 3.
to deprotonate L
1 3 4
H , and (ii) a single equivalent of (NEt )OH was
added to supply the bridging ligand and induce precipitation. X-
OH
ray quality crystals of NiZn
were obtained by vapor diffusion
of pentane into a concentrated DCE solution. The same approach
was used in our attempts to prepare the corresponding dinickel(II)
OH
complex (i.e., NiNi ), yet we were not able to grow suitable crys-
tals from mixtures of DCE (or DCM) and pentane. X-ray quality
crystals were obtained, however, from a mixture of MeCN and
MeOH. The resulting structure (vide infra) revealed that the NiII
centers are bridged by a solvent-derived methoxide ligand instead
OMe
of the intended hydroxide group, giving rise to complex NiNi
.
OH
This result indicates that the
l
-OH ligand of NiM
B
is readily
ꢀ
exchanged with OCH
3
in methanolic solution.
3.2. Crystallographic studies
Solid-state structures of the homo- and heterobimetallic com-
plexes were collected using X-ray diffraction. Details concerning
the crystallographic experiments and structure refinements are
OAc
OAc
OAc
provided in Table 1. Complexes NiCo , NiNi , and NiZn
yield quasi-isomorphous crystals in the monoclinic P2 /n space
crystallizes in the non-centro-
1
OAc
group, whereas complex NiFe
symmetric P2 group. The structure of NiZn , shown in Fig. 1,
is representative of the entire series. In each case, the trianionic
OAc
1
3
ꢀ
L
1
ligand supports a bimetallic core in which the metal centers
3
ꢀ
are bridged by two donors: the central phenolate of L
1
(O1)
and a -1,1-acetate group (O4). The complexes are neutral overall
l
II
due to the presence of the acetate ligand. As intended, the Ni cen-
ter occupies the ‘‘tetragonal’’ position (site A) defined by the Schiff-
base (N2) and pyridyl (N3) donors in addition to the two bridging
groups. Two solvent-derived ligands, either MeOH or H
2
O, bind to
ion is located in the ‘‘tripodal’’
five-coordinate position (site B) attached to the amino donor
II
Ni in a trans orientation. The M
B
Fig. 1. Thermal ellipsoid plots (50% probability) derived from the X-ray structure of
NiZn . Most hydrogen atoms and the tert-butyl substituents of the terminal
phenolate donors have been removed for clarity.
(
N1) and two terminal phenolates (O2 and O3).
Metal ion assignments were established by comparing R-factors
OAc
and thermal parameters for the four possible configurations (two
homobimetallic and two heterobimetallic). For complexes NiFe
and NiZn , the experimental data strongly support the metal ion
identities and positions indicated in Scheme 2; however, in the
case of NiCo , replacement of Co for Ni (or vice versa) causes little
change in R-factor. We therefore synthesized the analogous dico-
balt(II) complex, CoCo . Table 2 provides a quantitative compar-
ison of bond distances and angles in the NiNi , CoCo , and
OAc
OAc
II
OAc
is occupied by Co . For site A, the NiCo
matches NiNi
structure more closely
OAc
OAc
than CoCo
with respect to both bond lengths
OAc
OAc
and angles (Table 2). We are confident, therefore, that the NiCo
structure corresponds to a heterobimetallic complex with Ni in
site A and Co in site B.
II
OAc
II
OAc
OAc
As shown in Table 3, metric parameters for structures in the
OAc
OAc
NiCo
structures. The three complexes have identical ligand sets
NiM
B
series exhibit little variation across the series. The presence
ꢁ ꢁ ꢁM distances of
and quasi-isomorphous unit cells; thus, any structural differences
must be due to the presence of different metal ions in the A and
B sites. The B-site metric parameters in the CoCo
nearly identical to those observed for NiCo , with root-mean-
square (rms) deviations of merely 0.007 Å and 0.36° for metal–
ligand bond distances and angles, respectively. In contrast, the B-
of two monoatomic bridges results in short Ni
3.12 ± 0.02 Å and average bridge angles (Ni–O–M ) of 97°. The
Niꢁ ꢁ ꢁM distance is also reduced by the ‘‘puckering’’ of the [Ni–
O(1/4)–M ] diamond core, which deviates from planarity by nearly
A
B
B
OAc
structure are
B
OAc
B
19°. Each axial solvent ligand serves as an intramolecular hydrogen
bond (H-bond) donor: one to the terminal phenolate group (O3),
the other to the non-coordinating carbonyl moiety (O5) of acetate
(Fig. 1). These interactions are facilitated by the aforementioned
puckering of the central core, which tilts the phenolate acceptor
OAc
OAc
sites in the NiNi
and NiCo
structures are significantly differ-
ent (Table 2); in particular, the Ni2–N1 and Ni2–O4 distances are
shorter in NiNi
O3 angles shift by ꢂ5°. Thus, it is clear that the B-site in NiCo
OAc
B B
by ꢂ0.06 Å, and the O1–M –O2 and O1–M –
OAc
2
(O3) towards the H6–O6 donor of the proximal MeOH (or H O).

5
64
D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
Table 2
Root-mean-square Deviations in Metal–Ligand Bond Distances (Å) and Angles (deg)
OAc
OAc
OAc
for the A- and B-Sites within the NiCo , CoCo , and NiNi
Structures.
rms deviation from NiCoOAc structure
Site A
Site B
NiNiOAc
CoCoOAc
bond distances (Å)
bond angles (°)
bond distances (Å)
0.0140
1.14
0.0349
1.58
0.0400
2.54
0.0066
0.36
bond angles (°)
OAc
series, the NiII
center exists in a distorted octahe-
A
In the NiM
B
dral environment with trans angles of 174 ± 3° and cis angles rang-
ing between 82° and 98°. The average nickel-ligand bond distance
is 2.03 Å in the equatorial plane and 2.12 Å in the axial direction,
consistent with the presence of a high-spin (S = 1) center. This con-
eff
clusion is corroborated by the solid-state magnetic moment (l )
OAc
of 2.69
the M
cated by
l
B
measured for NiZn . The coordination geometries of
centers are best described as trigonal–bipyramidal, as indi-
-values greater than 0.50 ( = 0 for an ideal square-
B
Fig. 2. Thermal ellipsoid plot (50% probability) derived from the X-ray structure of
OH
s
s
NiZn ꢁDCE. The non-coordinating DCE molecule, most hydrogen atoms, and the
tert-butyl substituents of the terminal phenolate donors have been removed for
clarity. Selected bond lengths (Å): Ni1ꢁ ꢁ ꢁZn1 2.9967(4), Ni1–O1 1.844(1), Ni–1O4
pyramid and 1 for an ideal trigonal bipyramid; Table 3).[33] The
amino (N1) and acetate (O4) donors occupy the axial positions,
while the phenolate donors (O1–O3) constitute the equatorial
1.859(1), Ni1–N2 1.841(2), Ni1–N3 1.870(2), Zn1–O1 2.127(1), Zn1–O2 1.950(1),
Zn1–O3 1.964(1), Zn1–O4 2.054(1), Zn1–N1 2.138(1).
plane. For each complex, the M
variability, ranging from 1.96(3) Å for the terminal phenolates to
.17(4) Å for the axial M –O4 bond. The average M –L distance
of ꢂ2.05 Å, however, suggests that each M
ies is high-spin. Indeed, the magnetic moment of 4.51
B
–L distances display considerable
of NiZnOH were obtained from DCE/pentane, this structure lacks
2
B
B
II
OAc
OAc
B
ion in the NiM
measured
expected
B
ser-
the solvent-derived ligands found in the NiM
B
series. The NiA
OH
l
B
center of NiZn adopts a four-coordinate geometry with Ni –O/
A
OAc
for solid NiCo
for a molecule with uncoupled S = 1 and S = 3/2 centers.
The X-ray structure of NiZnOH, shown in Fig. 2, reveals a hetero-
is close to the spin-only value of 4.80
l
B
N bond lengths of 1.85 ± 0.02 Å – approximately 0.17 Å shorter
OAc
than the corresponding distances in the NiZn
structure. These
II
OH
parameters suggest that the NiA center in NiZn
is low-spin
3ꢀ
bimetallic [Ni(
l
-OH)Zn] core supported by the L
1
chelate. The
l
-
(S = 0), consistent with its square-planar coordination environ-
OH
OH ligand forms an intramolecular H-bond with the O3 atom of a
ment. However, the magnetic moment of NiZn powder is 1.43
OH
terminal phenolate donor. While the ZnNO
4
units in NiZn and
display similar metric parameters, the geometries of the
sites are quite different in the two structures. Since crystals
lB at room temperature, nearly half-way between the spin-only
limits of 0.0 (S = 0) and 2.83 lB (S = 1). Thus, it appears that a mix-
OAc
NiZn
II
II
Ni
A
ture of high- and low-spin NiA centers exist in the solid state.
Table 3
OAc
complexes (M
B
= Fe, Co, Ni, Zn) and NiNiOMe measured with X-ray diffraction.
Selected bond distances (Å) and bond angles (°) for NiM
B
NiFeOAc
NiCoOAc
NiNiOAc
NiZnOAc
NiNiOMe
Bond distances
Ni(1)ꢁ ꢁ ꢁM
B
(1)
3.1165(4)
1.976(2)
2.072(2)
2.030(2)
2.042(2)
2.117(2)
2.119(2)
3.0960(5)
1.981(2)
2.058(2)
2.032(1)
2.031(2)
2.116(2)
2.129(2)
3.1210(3)
1.979(2)
2.053(2)
2.007(1)
2.047(1)
2.100(1)
2.134(2)
3.1329(4)
1.976(2)
2.055(2)
2.025(1)
2.037(1)
2.107(1)
2.130(1)
3.0744(4)
2.014(2)
2.085(2)
2.030(1)
1.977(1)
2.124(2)
2.128(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–O(1)
Ni(1)–O(4)
Ni(1)–O(6)
Ni(1)–O(7)
M
B
M
B
M
B
M
B
M
B
(1)–N(1)
(1)–O(1)
(1)–O(2)
(1)–O(3)
(1)–O(4)
2.154(2)
2.116(2)
1.943(2)
1.983(2)
2.196(2)
2.094(2)
2.054(2)
1.940(1)
1.950(1)
2.199(1)
2.035(2)
2.070(1)
1.945(1)
1.959(1)
2.134(1)
2.095(1)
2.108(1)
1.933(1)
1.959(1)
2.174(1)
2.072(2)
2.059(1)
2.001(1)
1.999(1)
1.980(1)
Bond-angles
Ni(1)–O(1)–M
Ni(1)–O(4)–M
B
(1)
(1)
97.46(6)
94.62(6)
126.06(6)
109.43(6)
78.36(6)
88.03(6)
124.44(7)
97.42(6)
93.72(6)
90.50(6)
90.38(6)
165.84(6)
0.66
98.53(6)
94.02(6)
129.00(6)
110.52(6)
78.48(5)
90.34(6)
119.85(6)
95.17(5)
95.15(6)
87.96(6)
92.12(6)
168.05(6)
0.65
99.90(6)
96.55(6)
134.74(6)
105.88(6)
77.38(5)
90.05(6)
118.29(6)
95.03(5)
96.63(6)
86.73(5)
93.09(6)
166.84(6)
0.54
98.57(5)
96.07(5)
129.11(5)
108.73(5)
77.02(5)
89.06(5)
121.23(5)
95.33(5)
96.83(5)
87.31(5)
93.22(5)
165.43(5)
0.61
97.50(6)
101.97(6)
104.14(6)
136.31(6)
77.76(6)
89.21(6)
119.26(6)
92.65(6)
92.66(6)
94.56(6)
93.39(6)
166.78(6)
0.51
B
O(1)–M
O(1)–M
O(1)–M
O(1)–M
O(2)–M
O(2)–M
O(2)–M
O(3)–M
O(3)–M
O(4)–M
B
B
B
B
B
B
B
B
B
B
(1)–O(2)
(1)–O(3)
(1)–O(4)
(1)–N(1)
(1)–O(3)
(1)–O(4)
(1)–N(1)
(1)–O(4)
(1)–N(1)
(1)–N(1)
a
s-value
a
For a definition of the
s-value, see Ref. [26]. Five-coordinate complexes with ideal square-pyramidal geometries have s-values of 0.0, and those with ideal trigonal–
bipyramidal geometries have values of 1.0.

D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
565
A B
Fig. 3. Electronic absorption spectra of select M M
complexes measured at room temperature in DMF. The baselines of the NiNiOAc and NiZnOH spectra were shifted upward
X
slightly to enhance clarity. The modified spectra shown in the inset of (b) were obtained via electronic subtraction of the broad absorption feature that trails into the visible
region.
Regardless, in the NiZnOH structure, the short Ni
–O(1/4) distances
A
bring the two metal centers closer together, resulting in a Ni
distance of slightly less than 3.0 Å.
A
ꢁ ꢁ ꢁZn
B
As noted above, our attempts to isolate the dinickel(II) analog of
OH
NiZn
NiNi
were unsuccessful; however, X-ray quality crystals of
can be grown by dissolving the putative NiNi complex
OMe
OH
in a mixture of MeOH and MeCN. In the resulting structure, the
second bridging position is occupied by methoxide (Scheme 2),
OMe
but otherwise the geometry of NiNi
NiNi
is quite similar to that of
OAc
(Fig. S1). In both cases, the binding of solvent-derived
MeOH ligands gives rise to a six-coordinate, high-spin Ni center.
A
II
A comparison of metric parameters reveals only minor differences
between the two structures, although the Ni–OMe bond distances
are noticeably shorter than the Ni–OAc distances (Table 3). Inter-
OAc
estingly, whereas the NiNi
structure features two intramolecu-
lar H-bonds, the replacement of acetate with methoxide forces one
of the MeOH ligands to form an intermolecular H-bond with an
outersphere MeOH molecule. This solvate, in turn, serves as a H-
OMe
bond donor to the phenolate moiety of a second NiNi
Fig. S1).
complex
(
Fig. 4. Energy level diagram indicating the ligand-field states for a high-spin d7 ion
in three coordination environments: free ion, octahedral (O ), and trigonal–
bipyramidal (D3h). The experimentally-determined transition energies for the Co
3
.3. Magnetic, Spectroscopic, and Electrochemical Properties in
h
Solution
II
OAc
ꢀ1
ion in NiCo
are provided in wavenumbers (cm ). Partially adapted from
X
The magnetic moments of the NiM
tions were determined using the Evans NMR method. The NiZn
complexes exhibit eff values of 2.67 (X = OAc) and 2.53
X = OH), characteristic of S = 1 complexes. Thus, in contrast to its
B 3
complexes in CHCl solu-
reference 27. Not to scale.
X
II
are characteristic of six-coordinate Ni ions with partial pyridyl
l
l
B
OAc
coordination [34]. In the NiNi
spectrum, an ‘‘extra’’ ligand-field
(
II
II
OH
band arising from the second Ni center is apparent at 740 nm
solid-state structure, the Ni center in NiZn is unambiguously
high-spin in solution, even in a non-coordinating solvent like
CHCl
ing values were measured:
4
netic moments are slightly lower than the theoretical spin-only
values expected for uncoupled paramagnets, indicating the pres-
(Fig. 3(a)).
As shown in Fig. 3(b), both NiCoOAc and CoCoOAc exhibit two
3
. For complexes with two paramagnetic centers, the follow-
OAc
OMe
bands in the near infrared (NIR) region with k
max
= ꢂ1600 and
l
eff = 3.82 (NiNi ), 3.77 (NiNi
),
X
OAc
OAc
1050 nm. As the NiZn spectra are featureless in this region, these
B
.72 (NiCo ), and 5.02 l (NiFe ). These experimental mag-
II
band must arise from the Co center in the five-coordinate B-site.
II
In addition, multiple Co -derived features appear as shoulders in
OAc
OAc
the 500–650 nm region of the NiCo
and CoCo
spectra. These
ence of weak magnetic interactions between the metal centers.
X
peaks are clearly evident in the inset of Fig. 3(b), where the broad
absorption manifold that trails into the visible region has been
removed (electronically) to provide a level baseline.1 In order to
assign these features, we have built upon the ligand-field approach
UV–Vis absorption spectra of the reported NiM
B
complexes were
measured in DMF at room temperature; select data are shown in
Fig. 3. Typical for complexes with high-spin Ni centers, the NiZn
spectrum displays two bands in the visible region at 870 and 575 nm
II
OAc
II
= 8 M cm ) arising from the 3
ꢀ
1
ꢀ1
2g ? 3T2g and 3
3
developed by Ciampolini et al. in their seminal study of M ions in
(e
A
A
2g ? T1g(F)
trigonal bipyramidal complexes [35]. Assuming approximate D3h
transitions, respectively [34]. In addition, the spin-forbidden
00
3
1
OH
xz yz
geometry, the 3d orbitals split into three levels: E (d , d ),
A
2g ? E
g
transition is evident at 750 nm. Complex NiZn exhibits
0
2
E’(dxy, d
x
2
ꢀy
2
) and A (d ), listed in order of increasing energy. The
1 z
ligand-field transitions at similar energies, although the higher-
energy band is obscured by features attributed to
charge transfer (CT) transitions. Analysis of transition energies in
the NiZn
II
conversion from Oh to pseudo-D3h symmetry causes the T1g and
l-OH ? Ni
T
2g states to split into two components, as shown in Fig. 4. The
OAc
spectrum yields a ligand-field splitting (
D
o
) of
and
ꢀ1
ꢀ1
1
1500 cm
and Racah parameters of B = 730 cm
1
The background feature was modeled as a simple Gaussian curve in order to avoid
ꢀ
1
ꢀ1
C = 3870 cm (the free ion B-value is 1030 cm ). These parameters
introducing spurious features into the experimental spectrum.

5
66
D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
ground state term symbol for high-spin d7 ions is 4
A
0
. Using this
that the Ni2+/+ potential is ꢂ0.20 V lower in the B-site than the
2
ꢀ1
scheme, the band near 1600 nm (6250 cm ) can be assigned to
A-site, on account of the two terminal phenolate donors in the
4
0
4
00
OAc
the
A
2
? E (F) transition that, in the strong field limit, corre-
former. Interestingly, the CV and SWV data for CoCo
exhibit a
00
0
2
sponds to promotion of an E (dxz, dyz) electron to the E (dxy, dxꢀy
)
single cathodic feature at ꢀ1.44 V with approximately twice the
orbitals. The band at 1050 nm then arises from the 4
A
0
? E (F)
4
0
intensity of those observed for NiCo , suggesting that the two
OAc
2
II
OAc
transition, and the two peaks at 640 and 580 nm spectrum are
? E (P) transitions, respectively
these features appear in both spectra at roughly the same wave-
A B
Co ions are reduced at nearly identical potentials. The M M
4
0
4
0
(P) and 4
0
4
00
assigned to
A
2
? A
2
A
2
complexes are also susceptible to irreversible oxidations, as indi-
cated by the electrochemical data in Fig. S2; however, these anodic
features are less well-defined than their cathodic counterparts,
(
OAc
lengths). The weak peak at 610 nm in the NiCo
tentatively assigned to the spin-forbidden A
spectrum is
(G) transition,
4
0
2
0
OAc
2
? A
1
especially in the case of NiZn . Complexes containing a redox-
OAc
OAc
OAc
which gains intensity from the nearby spin-allowed transitions.
spectrum is more complex due to the
presence of a second Co center in the A-site. Based on spectra
reported for six-coordinate high-spin Co complexes [34], the
prominent band at 530 nm (18870 cm ) is assigned to the
F) ? T1g(P) transition, while the corresponding T1g (F) ? T2g(F)
active M
B
ion–NiNi , NiCo , and CoCo
– each display an
OAc
Analysis of the CoCo
irreversible peak near 0.30 V that is assigned to one-electron
II
II
B
oxidation of the M center. An additional irreversible feature is
II
evident near 0.75 V that, based on literature precedents [36–38],
ꢀ1
4
T
1g
likely arises from oxidation of one (or more) of the phenolate
4
4
4
3ꢀ
(
groups in L
1
to give a phenoxyl radical species.
transition (generally quite weak) is likely part of the broad absorp-
tion manifold centered at 1000 nm (Fig. 3(b)). Interestingly, the
4. Conclusions
OAc
NiCo
spectrum also exhibits a band at 530 nm, although its
OAc
intensity is much lower. This result suggests that our NiCo
sam-
– a possibility
previously considered in our analysis of the XRD data (vide supra).
In this manuscript, we have described the synthesis and coordi-
nation chemistry of a new unsymmetric ligand (L ) designed to
support heterobimetallic structures. The Schiff-base and pyridyl
OAc
ple is contaminated with a small amount of CoCo
1 3
H
OAc
II
The electrochemical properties of representative M
plexes were examined by voltammetric methods in DMF solutions
with 0.1 M (NBu )PF as the supporting electrolyte and scan rates
of 100 mV/s. As shown in Fig. 5, the cyclic voltammogram (CV) of
A
M
B
com-
donors of the tetragonal A-site favor Ni binding, while the pheno-
late donors of the tripodal B-site are capable of coordinating vari-
II
ous M ions (M = Fe, Co, Ni, Zn). In X-ray crystal structures of the
4
6
X
II
A
resulting M M
B
complexes, the two M centers are bridged by
OAc
3ꢀ
NiZn
displays a quasi-reversible redox couple at ꢀ1.17 V
the central phenolate donor of L
1,1-OAc, –OH, or –OCH
the solid-state structure of NiZn , the M ions in both coordina-
tion sites are high-spin, as determined by detailed analyses of
structural parameters, UV–Vis–NIR absorption spectra, and mag-
1
and an X anion, where X = l-
. With the exception of the Ni
II
(
D
E = 0.20 V) that corresponds to one-electron reduction of the
center (all potentials are relative to SCE). The CV’s of NiNi
3
A
center in
II
OAc
OH
II
Ni
and NiCo
Ni ion in the A-site, but a second peak from the redox-active M
ion is also apparent at more negative potentials (ꢀ1.36 V for
A
OAc
also feature a cathodic wave at ꢀ1.17 V due to the
II
B
II
netic susceptibility measurements. The Ni
A
centers undergo one-
OAc
OAc
II
II
NiNi
and 1.45 V for NiCo ). These redox event are very evi-
electron reduction at ꢀ1.17 V (versus SCE), while the Ni
B B
and Co
dent in the corresponding square-wave voltammograms (SWVs),
ions are reduced at lower potentials.
represented by the dotted lines in Fig. 5. The NiNiOAc data indicate
As stated in the Introduction, our efforts were inspired by the
active site of CODH, which utilizes a NiFe1 unit and a nearby
[
3 4 2 2
Fe S ] cluster to reversibly reduce CO to CO and H O. The bimetal-
lic complexes described here reproduce several key features of the
Ni–Fe1 fragment in CODH. Similar to enzymatic active site, the
X
II
NiM
B
complexes position a redox-active Ni center in close proxim-
ꢁ ꢁ ꢁM distances of
series are close to the average NiFe1
ity to a second transition metal; indeed, the Ni
A
B
X
3
.03.1 Å observed in the NiM
distance of ꢂ2.8 Å found in CODH [8]. It is our intention that the M
ion will facilitate CO activation by serving as a Lewis acid and (per-
haps) a source of reducing equivalents; thus, the B-site contains
three phenolate anions to depress the M redox potential. The Ni
center in CODH has three ‘‘permanent’’ ligands: the thiolate of
Cys526 and two sulfides from the [Fe ] cluster (Scheme 1). Like-
centers are bound to only three L -derived donors,
B
B
2
B
3 4
S
II
wise, the Ni
A
1
thereby providing three exchangeable coordination sites in a
meridional arrangement. The presence of cis-labile sites is expected
to be advantageous for CO binding and activation, since studies of
2
CODH suggest that the energetic barrier to C–O bond cleavage (typ-
ically the rate-determining step [39]) is lowered if the resulting
hydroxide is able to adopt a bridging position between two metal
centers [9,10]. Given these promising structural features, we are
X
currently exploring the ability of the NiM
rocatalysts for CO
active donors into the L
electron reservoirs in multi-electron processes, similar to the role
of the [Fe ] cluster in the catalytic cycle of CODH.
B
series to serve as elect-
2
reduction. We also plan to incorporate redox-
framework; such moieties could serve as
1
3 4
S
Fig. 5. Cyclic voltammograms (solid lines) of NiZnOAc, NiNiOAc, NiCoOAc, and
Acknowledgements
OAc
CoCo
4 6
collected in DMF with 0.1 M (NBu )PF as the supporting electrolyte and
scan rates of 100 mV/s. The corresponding square-wave voltammograms are
indicated by the dashed lines. In all cases the voltammogram was initiated by the
cathodic sweep. The solution concentrations were 2.0 mM.
This research is generously supported by Marquette University
and the U.S. National Science Foundation (NSF) (CHE-1056845).

D. Wang et al. / Inorganica Chimica Acta 421 (2014) 559–567
567
[
[
19] S. Yao, C. Herwig, Y. Xiong, A. Company, E. Bill, C. Limberg, M. Driess, Angew.
Chem., Int. Ed. 49 (2010) 7054.
20] A. Roth, A. Buchholz, M. Rudolph, E. Schuetze, E. Kothe, W. Plass, Chem.-Eur. J.
14 (2008) 1571.
Appendix A. Supplementary material
CCDC 1004508–1004514 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2014.07.018.
[21] K. Danjobara, Y. Mitsuka, Y. Miyasato, M. Ohba, H. Okawa, Bull. Chem. Soc. Jpn.
6 (2003) 1769.
[
7
22] S.K. Dutta, R. Werner, U. Floerke, S. Mohanta, K.K. Nanda, W. Haase, K. Nag,
Inorg. Chem. 35 (1996) 2292.
[23] T.R. Holman, C. Juarez-Garcia, M.P. Hendrich, L. Que Jr., E. Munck, J. Am. Chem.
Soc. 112 (1990) 7611.
[
[
24] M. Jarenmark, H. Carlsson, E. Nordlander, C.R. Chim. 10 (2007) 433.
25] C. Belle, J.-L. Pierre, Eur. J. Inorg. Chem. (2003) 4137.
References
[26] L.J. Daumann, G. Schenk, D.L. Ollis, L.R. Gahan, Dalton Trans. 43 (2014) 910.
[
[
27] N.G. Connelly, W.E. Geiger, Chem. Rev. 96 (1996) 877.
28] Q. Wang, C. Wilson, A.J. Blake, S.R. Collinson, P.A. Tasker, M. Schroeder,
Tetrahedron Lett. 47 (2006) 8983.
[
[
[
1] J.L. Boer, S.B. Mulrooney, R.P. Hausinger, Arch. Biochem. Biophys. 544 (2014)
42.
2] S.W. Ragsdale, E. Pierce, Biochim. Biophys. Acta, Proteins Proteomics 1784
2008) 1873.
3] A. Volbeda, M.-H. Charon, C. Piras, E.C. Hatchikian, M. Frey, J.C. Fontecilla-
Camps, Nature 373 (1995) 580.
1
[
[
29] I.S. Belostotskaya, N.L. Komissarova, T.I. Prokof’eva, L.N. Kurkovskaya, V.B.
Vol’eva, Russ. J. Org. Chem. 41 (2005) 703.
30] I.A. Koval, D. Pursche, A.F. Stassen, P. Gamez, B. Krebs, J. Reedijk, Eur. J. Inorg.
Chem. (2003) 1669.
(
[
[
31] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
32] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl.
Crystallogr. 42 (2009) 339.
[
[
4] A. Volbeda, J.C. Fontecilla-Camps, Coord. Chem. Rev. 249 (2005) 1609.
5] M. Bruschi, G. Zampella, P. Fantucci, G.L. De, Coord. Chem. Rev. 249 (2005)
1
620.
[
33] A.W. Addison, T.N. Rao, J. Reedijk, J. Vanrijn, G.C. Verschoor, J. Chem. Soc.,
Dalton Trans. (1984) 1349.
[
[
[
[
6] H. Ogata, W. Lubitz, Y. Higuchi, Dalton Trans. (2009) 7577.
7] C.L. Drennan, T.I. Doukov, S.W. Ragsdale, J. Biol. Inorg. Chem. 9 (2004) 511.
8] Y. Kung, C.L. Drennan, Curr. Opin. Chem. Biol. 15 (2011) 276.
9] A. Volbeda, J.C. Fontecilla-Camps, Dalton Trans. (2005) 3443.
[
[
[
34] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1968.
35] M. Ciampolini, N. Nardi, G.P. Speroni, Coord. Chem. Rev. 1 (1966) 222.
36] M.M. Allard, J.A. Sonk, M.J. Heeg, B.R. McGarvey, H.B. Schlegel, C.N. Verani,
Angew. Chem., Int. Ed. 51 (2012) 3178.
[
10] P. Amara, J.-M. Mouesca, A. Volbeda, J.C. Fontecilla-Camps, Inorg. Chem. 50
2011) 1868.
(
[
[
37] M. Lanznaster, H.P. Hratchian, M.J. Heeg, L.M. Hryhorczuk, B.R. McGarvey, H.B.
Schlegel, C.N. Verani, Inorg. Chem. 45 (2006) 955.
38] B. Adam, E. Bill, E. Bothe, B. Goerdt, G. Haselhorst, K. Hildenbrand, A.
Sokolowski, S. Steenken, T. Weyhermüller, K. Wieghardt, Chem. Eur. J. 3
[
[
[
[
[
[
11] J.-H. Jeoung, H. Dobbek, Science 318 (2007) 1461.
12] J.-H. Jeoung, H. Dobbek, J. Biol. Inorg. Chem. 17 (2012) 167.
13] D. Huang, R.H. Holm, J. Am. Chem. Soc. 132 (2010) 4693.
14] X. Zhang, D. Huang, Y.-S. Chen, R.H. Holm, Inorg. Chem. 51 (2012) 11017.
15] C. Uyeda, J.C. Peters, Chem. Sci. 4 (2013) 157.
16] S. Hazra, S. Bhattacharya, M.K. Singh, L. Carrella, E. Rentschler, T.
Weyhermueller, G. Rajaraman, S. Mohanta, Inorg. Chem. 52 (2013) 12881.
17] H. Kon, T. Nagata, Dalton Trans. 42 (2013) 5697.
(
1997) 308.
[
39] A.M. Appel, J.E. Bercaw, A.B. Bocarsly, H. Dobbek, D.L. DuBois, M. Dupuis, J.G.
Ferry, E. Fujita, R. Hille, P.J.A. Kenis, C.A. Kerfeld, R.H. Morris, C.H.F. Peden, A.R.
Portis, S.W. Ragsdale, T.B. Rauchfuss, J.N.H. Reek, L.C. Seefeldt, R.K. Thauer, G.L.
Waldrop, Chem. Rev. 113 (2013) 6621.
[
[
18] M. Jarenmark, M. Haukka, S. Demeshko, F. Tuczek, L. Zuppiroli, F. Meyer, E.
Nordlander, Inorg. Chem. 50 (2011) 3866.