Steric and electronic effects in the formation and carbon disulfide reactivity of dinuclear nickel complexes supported by bis(iminopyridine) ligands

Article abstract of DOI:10.1021/om400187v

We are developing bimetallic platforms for the cooperative activation of heteroallenes. Toward this goal, we designed a new family of bis(iminopyridine) ((N,N′-1,1′-(1,4-phenylene)bis(N-(pyridin-2-ylmethylene)methanamine) and N,N′-1,1′-(1,4-phenylene)bis(N-(1-(pyridin-2-yl)ethylidene) methanamine)) dinickel complexes, synthesized their CS₂ compounds, and studied their reactivity. Bis(iminopyridine) ligands L react with Ni(COD)₂ to form Ni₂(L)₂ complexes or Ni ₂(L)(COD)₂ complexes as a function of the steric and electronic properties of the ligand precursor. Product structures disclosed an anti geometry in the Ni₂(L)(COD)₂ species and helical (anti) structures for Ni₂(L)₂ complexes. Carbon disulfide adducts Ni₂(L)(CS₂)₂ were obtained in good yields upon addition of CS₂ to Ni₂(L)(COD)₂ or in a one-pot reaction of L with 2 equiv of both Ni(COD)₂ and CS ₂. Ni₂(L)(CS₂)₂ complexes are highly flexible, displaying both syn and anti conformations (shortest S- - -S separations of 5.0 and 9.5 A, respectively) in the solid state. DFT calculations demonstrate virtually no energy difference between the two conformations. Electrochemical studies of the Ni₂(L)(CS ₂)₂ complexes displayed two ligand-based reductions and a broad CS₂-based oxidation. Chemical oxidation with [FeCp ₂]⁺ liberated free CS₂. The addition of NHC (NHC = 1,3-di-tert-butylimidazolin-2-ylidene) to Ni₂(L)(CS ₂)₂ yielded Ni₂(NHC)₂(CS ₂)₂, in which both carbon disulfide ligands are bridging two Ni centers.

Full text of DOI:10.1021/om400187v

Article
pubs.acs.org/Organometallics
Steric and Electronic Effects in the Formation and Carbon Disulfide
Reactivity of Dinuclear Nickel Complexes Supported by
Bis(iminopyridine) Ligands
Amarnath Bheemaraju,† Jeffrey W. Beattie,† Erwyn G. Tabasan,† Philip D. Martin,† Richard L. Lord,*,‡
and Stanislav Groysman*,†
†Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States
‡Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States
S
* Supporting Information
ABSTRACT: We are developing bimetallic platforms for the
cooperative activation of heteroallenes. Toward this goal, we
designed a new family of bis(iminopyridine) ((N,N′-1,1′-(1,4-
phenylene)bis(N-(pyridin-2-ylmethylene)methanamine) and
N,N′-1,1′-(1,4-phenylene)bis(N-(1-(pyridin-2-yl)ethylidene)-
methanamine)) dinickel complexes, synthesized their CS2
compounds, and studied their reactivity. Bis(iminopyridine)
ligands L react with Ni(COD)2 to form Ni2(L)2 complexes or Ni2(L)(COD)2 complexes as a function of the steric and electronic
properties of the ligand precursor. Product structures disclosed an anti geometry in the Ni2(L)(COD)2 species and helical (anti)
structures for Ni2(L)2 complexes. Carbon disulfide adducts Ni2(L)(CS2)2 were obtained in good yields upon addition of CS2 to
Ni2(L)(COD)2 or in a one-pot reaction of L with 2 equiv of both Ni(COD)2 and CS2. Ni2(L)(CS2)2 complexes are highly
flexible, displaying both syn and anti conformations (shortest S- - -S separations of 5.0 and 9.5 Å, respectively) in the solid state.
DFT calculations demonstrate virtually no energy difference between the two conformations. Electrochemical studies of the
Ni2(L)(CS2)2 complexes displayed two ligand-based reductions and a broad CS2-based oxidation. Chemical oxidation with
[FeCp2]+ liberated free CS2. The addition of NHC (NHC = 1,3-di-tert-butylimidazolin-2-ylidene) to Ni2(L)(CS2)2 yielded
Ni2(NHC)2(CS2)2, in which both carbon disulfide ligands are bridging two Ni centers.
of dinuclear systems in which the two heteroallene substrates
are close enough to react with each other. Furthermore, the
electronic structure of the heteroallene adduct is of primary
importance as it determines its reactivity. The electronic
structure of a bound heteroallene is determined in part by the
ancillary ligand. The current investigation focuses on
iminopyridine chelates. Our choice of the iminopyridine
ancillary ligand results from its redox-active nature that helps
to stabilize low-oxidation-state metal precursors and to mediate
electron transfer upon binding and reduction of substrates.11
Toward this goal, we have recently reported two dinucleating
bis(iminopyridine) ligands, L1 and L2 (see Figure 1).10 We
discovered that the treatment of the ligand with Ni(COD)2
affords two different products: the expected complex Ni2(L1)-
(COD)2 (1a) for L1 and the bis(homoleptic) complex Ni2(L2)2
(2b/2c) for L2.10a We also reported that Ni2(L1)(COD)2 reacts
with 2 equiv of carbon disulfide to form Ni2(L1)(CS2)2.10b To
decipher the steric and electronic effects guiding the formation
and the reactivity of the dinuclear nickel complexes, we
designed several bis(iminopyridine) ligands featuring different
substituents in the ligand’s framework. In this work we
interrogate the formation, properties, and reactivity of the
INTRODUCTION
■
Dinuclear complexes offer an attractive strategy for the
cooperative binding and activation of small molecules.1,2 In
particular, heteroallene activation is a multielectron process and
the cooperative action of several metals broadens the number
of oxidation states available for the heteroallene reduction.3−6
Recently, several groups reported CO2 activation and reduction
using dinuclear and polynuclear complexes.7−9 Several different
approaches were tested. Thomas and co-workers described a
heterodinuclear system containing a metal−metal bond that
oxidatively adds CO2.7 Berben and co-workers described
electrocatalytic reduction of CO2 to formate by the
tetrametallic cluster [HFe4N(CO)12]¯.8 Hazari and co-workers
have investigated insertion of CO2 into Pd(I) bridging allyl
dimers.9
We are designing homodinuclear metal complexes for the
cooperative activation of heteroallenes (CO2 and CS2).10 Our
systems feature metal centers that are positioned close to each
other but are connected by a flexible linker and feature no
direct metal−metal bond. Our goal is to achieve the reductive
transformation of two heteroallene molecules by the cooper-
ative action of two metal centers, brought together by a
dinucleating ligand. Possible products of this bimetallic
reductive transformation include oxalate (tetrathiooxalate)
and carbonate (trithiocarbonate). This goal requires the design
Received: March 6, 2013
© XXXX American Chemical Society
A
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156.96, 155.03, 138.59, 137.70, 131.96, 128.85, 128.62, 121.03, 119.08,
114.55, 65.16, 55.86. HRMS (ESI): calcd for [C34H30N4O2 + H]+
527.2447, found 527.2438. Mp: 204 °C.
6-(2,4,6-Triisopropylphenyl)-2-pyridinecarboxaldehyde. This
compound was prepared in a way similar to that for the previously
reported 4-fluoro-2′,4′,6′-triisopropylbiphenyl-3-carbaldehyde.12
Under a nitrogen atmosphere, 6-bromo-2-pyridinecarboxaldehyde
(1.60 g, 8.60 mmol), 2,4,6-triisopropylphenylboronic acid (3.20 g,
12.89 mmol), 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl
(SPhos; 0.60 g, 1.48 mmol), and K3PO4 (20 g, excess base) were
added to a 250 mL Schlenk flask. The flask was evacuated and refilled
with nitrogen three times. Toluene (35 mL) and Pd(OAc)2 (0.38 g,
1.71 mmol) were added to the flask. The reaction mixture was heated
to 110 °C for 48 h. After that, the reaction mixture was cooled to room
temperature and diethyl ether (50 mL) was added. The resulting
reaction mixture was filtered through a thin pad of silica gel, and the
resulting filtrate was concentrated in vacuo. The crude product was
purified by silica gel chromatography (5/95 ether/hexanes) to afford
the product 6-(2,4,6-triisopropylphenyl)-2-pyridinecarboxaldehyde
Figure 1. Synthesis of ligands L1−L7.
1
(0.80 g, 30%) as a white solid. H NMR (C6D6, 400 MHz): δ 10.16
(s, 1H), 7.63 (d, J = 7.2, 1H), 7.20 (s, 2H), 6.99 (d, J = 7.2, 1H), 6.96
(q, J = 7.6, 1H), 2.86 (m,, 1H), 2.55 (m, 2H), 1.27 (d, J = 6.8, 6H),
1.12 (t, J = 6.8, 12H). 13C NMR (C6D6, 75 MHz): δ 193.57, 161.57,
153.50, 149.98, 147.14, 136.89, 129.36, 128.73, 128.62, 128.14, 121.37,
119.47, 35.29. 31.31, 24.83, 24.71, 24.38; HRMS (ESI): calcd for
[C21H27NO + H]+ 310.2171, found 310.2181. Mp: 199 °C.
open-chain Ni2(L)(COD)2-type complexes vs bis(homoleptic)
Ni2L2-type complexes. In addition, we report the straightfor-
ward one-pot synthesis of Ni2(L)(CS2)2 complexes that does
not require prior isolation of the Ni2(L)(COD)2 complexes.
Electrochemical properties and the reactivity of Ni2(L)(CS2)2
complexes are presented. The structure of the syn-Ni2(L)-
(CS2)2-type complex is reported and compared with the
structure of the anti-Ni2(L)(CS2)2-type complex. The con-
formational stability of the arms in this bimetallic complex is
interrogated using DFT.
L4. A 20 mL methanol solution of 6-(2,4,6-triisopropylphenyl)-2-
pyridinecarboxaldehyde (39 mg, 0.12 mmol) was added to a 20 mL
methanol solution of p-xylylenediamine (85 mg, 0.06 mmol). The
resulting solution was stirred and refluxed for 4 h. The reaction
mixture was cooled to room temperature. White powder was separated
from the solution by filtration, washed with cold methanol, and dried
1
to give L4 (88 mg, 98%) as a white solid. H NMR (CD2Cl2, 400
EXPERIMENTAL SECTION
MHz): δ 8.48 (s, 2H), 8.03 (d, J = 8.0, 2H), 7.78 (t, J = 7.6, 2H), 7.37
(s, 4H), 7.28 (d, J = 7.6, 2H), 7.08 (s, 4H), 4.85 (s, 4H), 2.92 (m, 2H),
2.48 (m, 4H), 1.26 (d, J = 6.8, 12H), 1.07 (dd, J = 3.6, 2.8, 24H). 13C
NMR (CD2Cl2, 75 MHz): δ 163.99, 160.18, 154.94, 149.69, 146.97,
138.72, 136.79, 136.73, 128.82, 126.69, 121.22, 119.08, 65.22, 35.07,
30.91, 30.27, 24.46, 24.28. HRMS (ESI): calcd for [C50H62N4 + H]+
719.5053, found 719.5063. Mp: 255 °C.
■
General Considerations. All reactions involving metal complexes
were executed in a nitrogen-filled glovebox. p-Xylylenediamine, 6-(4-
methoxyphenyl)-2-pyridinecarboxaldehyde, 6-methoxy-2-pyridinecar-
boxaldehyde, 4-(6-formylpyridin-2-yl)benzonitrile, 2-bromopyridine-
6-carboxaldehyde, 2,4,6-triisopropylphenylboronic acid, 6-
(trifluoromethyl)pyridine-2-carboxaldehyde, bis(cyclooctadiene)-
nickel(0) (Ni(COD)2), 1,3-di-tert-butylimidazolin-2-ylidene (NHC),
carbon disulfide, and 13C-labeled carbon disulfide were purchased from
Aldrich, Strem, or TCI America and used as received. L1 and L2 were
synthesized as previously described.10 All solvents were purchased
from Fisher Scientific and were of HPLC grade. The solvents were
purified using an MBRAUN solvent purification system and stored
over 3 Å molecular sieves. Compounds were routinely characterized by
1H NMR, 13C{1H} NMR (13C NMR thereafter), and 19F NMR
spectroscopy, X-ray crystallography, and elemental analyses. Selected
compounds were characterized by mass spectrometry (ESI). NMR
spectra of all compounds were recorded at the Lumigen Instrument
Center (Wayne State University) on a Varian Mercury 400 NMR
spectrometer in C6D6, (CD3)2SO or CD2Cl2 at room temperature.
Chemical shifts and coupling constants (J) are reported in parts per
million (δ) and hertz, respectively. Low-resolution mass spectra were
obtained at the Lumigen Instrument Center utilizing a Waters
Micromass ZQ mass spectrometer (direct injection, with capillary at
3.573 kV and cone voltage of 20.000 V). Only selected peaks in the
mass spectra are reported below. Elemental analyses were performed
by Midwest Microlab LLC.
L5. A 20 mL solution of 4-(6-formyl-2-pyridinyl)benzonitrile (200
mg, 0.961 mmol) was added to a 20 mL methanol solution of p-
xylylenediamine (65.7 mg, 0.481 mmol). The resulting solution was
stirred and refluxed for 4 h. The reaction mixture was cooled to room
temperature. White precipitate was separated from the solution by
filtration, washed with cold methanol, and dried to give L5 (240 mg,
97%). 1H NMR (CD2Cl2, 400 MHz): δ 8.56 (s, 2H), 8.19 (d, J = 8.4,
4H), 8.06 (dd, J = 6.4, 1.2, 2H), 7.87 (t, J = 7.6, 2H), 7.82 (dd, J = 8.0,
1.6, 2H), 7.77 (d, J = 8.4, 4H), 7.37 (s, 4H), 4.89 (d, J = 1.2, 4H). 13C
NMR (CD2Cl2, 75 MHz): δ 163.30, 155.58, 155.18, 143.51, 138.52,
138.19, 133.14, 128.93, 127.94, 122.38, 121.02, 119.29, 113.15, 65.16.
HRMS (ESI) calcd for [C34H24N6 + H]+ 517.2141, found 517.2141.
Mp: 206 °C.
L6. A 10 mL methanol solution of 6-methoxy-2-pyridinecarbox-
aldehyde (250 mg, 1.82 mmol) was added to a 10 mL methanol
solution of p-xylylenediamine (124 mg, 0.910 mmol). The resulting
solution was stirred and refluxed for 4 h. The reaction mixture was
cooled to room temperature. White powder was separated from the
solution by filtration, washed with cold methanol, and dried to yield L6
(308 mg, 91%) as a white solid. 1H NMR (C6D6, 400 MHz): δ 8.36 (s,
2H), 7.79 (d, J = 7.6, 2H), 7.25 (s, 4H), 7.01 (t, J = 8.0, 2H), 6.54 (d, J
= 8.0, 2H), 4.60 (s, 4H), 3.81 (s, 6H). 13C NMR (C6D6, 75 MHz): δ
164.66, 163.0, 153.43, 139.28, 138.76, 128.96, 114.52, 112.69, 64.21,
53.44. HRMS (ESI): calcd for [C22H22N4O2 + H]+ 375.1821, found
375.1821. Mp: 121 °C.
Synthesis and Characterization of Compounds. L3. A 50 mL
methanol solution of 6-(4-methoxyphenyl)-2-pyridinecarboxaldehyde
(3.61 g, 33.8 mmol) was added to a 50 mL methanol solution of p-
xylylenediamine (2.3 g, 16.9 mmol). The resulting solution was stirred
and refluxed for 4 h. The white cloudy reaction mixture was cooled to
room temperature. A white solid was isolated by filtration and dried to
L7. A 50 mL solution of 6-(trifluoromethyl)-2-pyridinecarboxalde-
hyde (1.00 g, 5.71 mmol) was added to a 50 mL methanol solution of
p-xylylenediamine (0.389 g, 2.85 mmol). The resulting solution was
stirred and refluxed for 4 h. The reaction mixture was cooled to room
temperature. White precipitate was separated from the solution by
1
give L3 (4.28 g, 81%). H NMR (CD2Cl2, 400 MHz): δ 8.55 (s, 2H),
8.01 (d, J = 8.4, 4H), 7.92 (d, J = 6.4, 2H), 7.77 (t, J = 7.2, 2H), 7.71
(d, J = 7.6, 2H), 7.37 (s, 4H), 6.99 (d, J = 7.2, 4H), 4.87 (d, J = 1.6,
4H), 3.86 (s, 6H). 13C NMR (CD2Cl2, 75 MHz): δ 163.89, 161.21,
B
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filtration, washed with cold methanol, and dried to give L7 (1.01 g,
79%). H NMR (C6D6, 400 MHz): δ 8.26 (s, 2H), 7.94 (d, J = 8.0,
2H), 7.19 (s, 4H), 6.99 (d, J = 7.6, 2H), 6.82 (t, J = 8.0, 2H), 4.49 (d, J
= 1.2, 4H). 13C NMR (C6D6, 75 MHz): δ 161.95, 155.96, 148.40,
148.06, 138.42, 137.93, 129.02, 123.67, 121.43, 121.41, 65.02. 19F
NMR (C6D6, 400 MHz): δ −67.75. HRMS (ESI): calcd for
[C22H16N4F6+H]+ 451.1357, found 451.1362. Mp: 146 °C.
mmol) in 5 mL of toluene. The reaction mixture was stirred for 2 h to
give a purple solution. The solvent was removed under high vacuum.
The resulting solid was dissolved in an additional 10 mL of THF,
filtered, and dried in vacuo. The purple solid was washed with hexane
(5 mL) and diethyl ether (5 mL) and dried to give the mixture of
Ni2(L1)2, Ni2(L2)2, and Ni2(L1)(L2). 1H NMR (C6D6, 400 MHz):
Ni2(L1)2 (isomer a), δ 10.17 (d, J = 5.6, 4H), 9.16 (s, 4H), 7.48−7.58
(m, 12H), 7.12 (s, 8H), 5.71 (d, J = 13.6, 4H), 4.85−5.0 (dd, J = 13.6,
10.0, 4H); Ni2(L1)2 (isomer b), δ 10.14 (d, J = 4.8, 4H), 9.04 (s, 4H),
7.11 (s, 8H), 6.82−6.92 (m, 12H), 5.40 (d, J = 13.6, 4H), 4.85−5.0
(dd, J = 13.6, 10.0, 4H); Ni2(L2)2 (isomer a), δ 10.36 (d, J = 6.0, 4H),
7.59 (m, 8H), 7.25 (s, 8H), 6.76 (d, J = 8.0, 4H), 6.58 (d, J = 14.4,
4H), 5.16 (d, J = 13.6, 4H), −0.41 (s, 12H); Ni2(L2)2 (isomer b), δ
10.32 (d, J = 6.4, 4H), 7.57 (m, 8H), 6.97 (s, 8H), 6.48 (d, J = 8.0,
4H), 6.45 (d, J = 7.2, 4H), 5.09 (d, J = 13.6, 4H), −0.52 (s, 12H);
Ni2(L1)(L2) (isomer a), δ 10.39 (d, J = 5.6, 2H), 10.28 (m, 2H), 9.20
(s, 2H), 7.48−7.31 (m, 12H), 7.27 (s, 4H), 7.04 (s, 4H), 5.66 (d, J =
12.8, 4H), 5.28 (d, J = 14.8, 4H), −0.34 (s, 6H); Ni2(L1)(L2) (isomer
b), δ 10.26 (m, 2H), 10.23 (m, 2H), 9.19 (s, 2H), 7.48−7.31 (m,
12H), 7.09 (s, 4H), 4.84 (d, J = 13.2, 4H), 4.78 (d, J = 13.6, 4H),
−0.51 (s, 6H). MS (ESI): calcd for [Ni2(L2)2]+ 800.3, found 800.3;
calcd for [Ni2(L1)(L2)]+ 772.2, found 772.2; calcd for [Ni(L2)2]+
742.3, found 742.3; calcd for [Ni(L1)(L2)]+ 714.3, found 714.3; calcd
for [Ni(L1)2]+ 686.2, found 686.3; calcd for [Ni(L2) + H]+ 401.1,
found 401.2; calcd for [Ni(L1)]+ 372.1, found 372.3.
1
Ni2(L3)2 (3b,c). A suspension of L3 (62 mg, 0.12 mmol) in 3 mL of
THF was added at room temperature to a 3 mL solution of
bis(cyclooctadiene)nickel(0) (Ni(COD)2; 65 mg, 0.24 mmol) in THF
over the course of 2 h. The reaction mixture was stirred for 24 h to
give a purple solution. The solvent was removed under vacuum. The
resultant solid was washed with hexane (20 mL) and dissolved in THF
(3 mL). The solvent was removed under vacuum to give a dark purple
solid. The solid was dissolved in THF (1 mL), ether was added (5
mL), and the resulting mixture was kept at −40 °C for 2 days. The
purple crystals that separated were washed with cold ether and dried to
afford pure Ni2(L3)2 (42 mg, 0.072 mmol, 60%). 1H NMR (C6D6, 400
MHz): isomer a, δ 9.12 (s, 4H), 8.63 (d, J = 8.0, 4H), 8.10 (d, J = 6.0,
4H), 7.99 (t, J = 7.2, 4H), 6.94 (s, 8H), 6.68 (d, J = 8.4, 8H), 6.55 (d, J
= 10.4, 8H), 5.57 (s, 4H, 1,5-cyclooctadiene), 4.90 (d, J = 13.2, 4H),
3.96 (d, J = 13.4, 4H), 3.22 (s, 12H), 2.20 (s, 8H, 1,5-cyclooctadiene);
isomer b, δ 8.86 (s, 4H), 8.51 (d, J = 8.8, 4H), 8.08 (d, J = 6.0, 4H),
7.94 (t, J = 6.8, 4H), 6.92 (s, 8H), 6.64 (d, J = 8.0, 8H), 6.53 (d, J =
8.8, 8H), 4.06 (d, J = 13.6, 4H), 3.72 (d, J = 13.6, 4H), 3.27 (s, 12H).
13C NMR (C6D6, 75 MHz): δ 163.87, 163.54, 162.76, 159.58, 159.40,
144.73, 143.86, 138.78, 138.59, 135.44, 134.60, 133.58, 132.81, 129.03,
126.57, 126.12, 122.35, 121.68, 117.65, 116.98, 114.77, 113.0, 68.93,
68.30, 68.15, 66.25, 55.12, 55.07, 15.93. MS (ESI): calcd for Ni2(L3)2
([C68H60N8O4Ni2]+) 1168.34, found 1168.51. Anal. Calcd for
C68H60N8O4Ni2: C, 69.7; H, 5.1; N, 9.6. Found: C, 69.2; H, 5.5; N,
9.2.
Ni2(L2)(CS2)2 (2d). A solution of L2 (50 mg, 0.15 mmol) in 3 mL of
THF was added at room temperature to a 3 mL solution of Ni(COD)2
(80 mg, 0.29 mmol) in THF. The resulting mixture was stirred for 2 h.
After that, CS2 was added (0.29 mmol, 0.34 mL, 0.83 M in THF),
causing precipitation of a purple solid, and the resulting mixture was
stirred overnight. Purple solid was separated from the solution, washed
with ether (10 mL), and dried to yield pure Ni2(L2)(CS2)2 (86 mg,
1
Ni2(L4)2 (4b). A suspension of L4 (70 mg, 0.10 mmol) in 3 mL of
THF was added at room temperature to a 3 mL solution of Ni(COD)2
(27 mg, 0.10 mmol) in THF. The reaction mixture was stirred for 24 h
to give a purple solution. The solvent was removed under vacuum.
The residue was dissolved in hexane (3 mL) and stored at −30 °C to
give black crystals in two crops (combined yield 21 mg, 0.027 mmol,
0.14 mmol, 94%). H NMR (DMSO-d6, 400 MHz): δ 9.50 (m, 2H),
8.24 (t, J = 8.0, 2H), 8.09 (d, J = 8.0, 2H), 7.89 (t, J = 6.4, 2H), 7.48 (s,
4H), 5.32 (s, 4H), 2.46 (s, 6H). 13C NMR (DMSO-d6, 75 MHz): δ
270.56 (corresponding to 13CS2 in Ni2(L2)(CS2)2). MS (ESI): calcd
for Ni2(L2)(CS2) ([C23H22N4Ni2S2 + H]+), 535.01 found 534.81.
Anal. Calcd for C24H22N4Ni2S4: C, 47.1; H, 3.6; N, 9.2. Found: C,
47.1; H, 3.7; N, 9.1.
1
27% yield). H NMR (C6D6, 400 MHz): δ 9.68 (s, 4H), 8.09 (t, J =
Ni2(L3)(CS2)2 (3d). A suspension of L3 (64 mg, 0.12 mmol) in 3 mL
of THF was added at room temperature to a 3 mL solution of
Ni(COD)2 (65 mg, 0.24 mmol) in THF over the course of 1 h, and
the reaction mixture was stirred for an additional 1 h. To the resulting
purple solution was added CS2 (0.095 mmol, 0.67 mL, 0.15 M in
THF), leading to the formation of a dark precipitate. The reaction
mixture was stirred for 12 h. Ether (10 mL) was added, and a dark
brown solid was separated. The solid was washed with THF (15 mL)
and ether (10 mL) and dried to afford Ni2(L3)(CS2)2 (49 mg, 0.064
8.0, 4H), 7.92 (d, J = 6.8, 4H), 7.27 (d, J = 1.6, 4H), 7.26 (s, 8H), 6.59
(dd, J = 9.2, 1.2, 4H), 5.12 (d, J = 13.6, 4H), 4.51 (m, 4H), 3.93 (d, J =
13.6, 4H), 2.53 (m, 8H), 1.34 (m, 48H), 1.15 (m, 24H) (note: δ 8.60
(s, 2H), 8.16 (d, J = 7.6, 2H), 7.23 (s, 8H), 7.10 (t, J = 8.0, 2H), 7.00
(d, J = 7.6, 2H), 4.57 (s, 4H), 2.86 (p, J = 7.2, 6.8, 2H), 2.73 (p, J =
6.8, 6.8, 4H), 1.28 (d, J = 6.8, 12H), and 1.17 (dd, J = 11.6, 7.2, 24H)
are peaks corresponding to free L4). MS (ESI): calcd for Ni2(L4)2
([C100H124N8Ni2]+) 1552.87, found 1553.07. Anal. Calcd for
C100H124N4Ni2: C, 77.2; H, 8.0; N, 7.2. Found: C, 76.9; H, 8.2; N, 7.0.
Ni2(L4)(COD)2 (4a). A suspension of L4 (33 mg, 0.046 mmol) in 3
mL of THF was added at room temperature to a 3 mL solution of
Ni(COD)2 (28 mg, 0.10 mmol) in THF. The reaction mixture was
stirred for 24 h to give a dark purple solution. The solvent was
removed under vacuum. The resulting solid was dissolved in hexane
(10 mL), and the solvent was removed. Recrystallization of the
product from ether at −35 °C over 2 days yielded Ni2(L4)(COD)2 as
purple crystals (19 mg, 0.018 mmol, 39% yield). 1H NMR (C6D6, 400
MHz): δ 8.56 (s, 2H), 7.44 (t, J = 7.6, 2H), 7.40 (s, 4H), 7.33 (dd, J =
5.6, 1.2, 2H), 7.30 (s, 4H), 6.91 (dd, J = 6.8, 0.8, 2H), 5.57 (s, 4H),
3.90 (s, 8H), 3.06 (m, 4H), 2.97 (m, 2H), 2.67 (m, 4H), 1.62−1.50
(m, 8H), 1.27 (dd, J = 20.0, 6.4, 24H), 1.13 (d, J = 6.4, 12H). 13C
NMR (C6D6, 75 MHz): δ 161.68, 150.15, 149.04, 147.56, 146.03,
140.66, 138.12, 129.14, 128.85, 128.62, 128.14, 125.80, 125.35, 124.52,
121.50, 90.02, 83.10, 81.51, 66.28, 66.25, 35.53, 31.37, 31.20, 30.67,
30.52, 28.71, 27.50, 24.90, 22.01, 15.93. MS (ESI): calcd for
Ni2(L4)(COD) ([C58H74N4Ni2]+) 942.4620, found 942.5277. Anal.
Calcd for C66H86N4Ni2: C, 75.3; H, 8.2; N, 5.3. Found: C, 75.1; H, 8.0;
N, 5.4.
1
mmol, 53% yield). H NMR (DMF-d7, 400 MHz): δ 8.62 (s, 2H),
8.17 (d, J = 8.0, 4H), 8.03 (m, 2H), 7.96 (m, 4H), 7.45 (s, 4H), 7.11
(d, J = 8.0, 4H), 4.94 (s, 4H), 3.89 (s, 6H). 13C NMR (DMSO-d6, 75
MHz): δ 264.41. Anal. Calcd for C36H30N4Ni2O2S4: C, 54.3; H, 3.8; N,
7.0. Found: C, 53.9; H, 3.5; N, 6.6.
Ni2(L4)(CS2)2 (4d). A suspension of L4 (34 mg, 0.047 mmol) in 3 mL
of THF was added at room temperature to a 3 mL solution of
Ni(COD)2 (26 mg, 0.095 mmol) in THF. To the resulting mixture
was added CS2 (0.095 mmol, 0.67 mL, 0.15 M in THF). The reaction
mixture was stirred for 24 h to give a dark blue solution. The solvent
was removed under vacuum. The resulting solid was washed
successively with hexane (10 mL), ether (3 mL), and toluene (3
mL) and dried to afford Ni2(L4)(CS2)2 (22 mg, 0.022 mmol, 47%). 1H
NMR (DMSO-d6, 400 MHz): δ 8.99 (s, 2H), 8.19 (t, J = 7.2, 2H),
7.95 (d, J = 8.0, 2H), 7.84 (d, J = 8.4, 2H), 7.45 (s, 4H), 7.07 (s, 4H),
5.40 (s, 4H), 2.91 (m, 2H), 2.24 (m, 4H), 1.15−1.25 (m, 24H), 1.00
(d, J = 8.0, 12H); 13C NMR (DMSO-d6, 75 MHz): δ 265.44 (also the
peak for unbound 13CS2 at δ 192.62 is due to decomposition of the
product in DMSO-d6). Anal. Calcd for C52H62N4Ni2S4: C, 63.2; H,
6.3; N, 5.7. Found: C, 63.1; H, 6.2; N, 5.5.
[Ni2(L1)2]/[Ni2(L2)2]/[Ni2(L1)(L2)] (1b,c/2b,c/5b,c). A solution of L2
(15 mg, 0.043 mmol) in toluene (5 mL) was added dropwise at room
temperature to a stirred solution of Ni2(L1)(COD)2 (30 mg, 0.046
Reaction of Ni2(L2)(13CS2)2 (2d) with (FeCp2)(PF6). A purple
suspension of Ni2(L2)(13CS2)2 (16 mg, 0.026 mmol) in 2 mL of
C
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position while L6 and L7 feature more compact, electronically
diverse substituents at the 2′-position of the pyridine: the
electron-rich OMe group and the electron-withdrawing CF3
group, respectively. L1 and L2 have been previously reported, by
us and others,10,13 while L3−L7 have not been previously
synthesized. The synthesis of L1−L7 was accomplished by the
reflux of 2 equiv of the respective aldehyde/ketone with 1,4-
phenylenedimethanamine. All the aldehydes were obtained
commercially except for 6-(2,4,6-triisopropylphenyl)-
picolinaldehyde. This aldehyde was prepared as described in
Figure 2. We have also attempted to synthesize L8, that features
CD3CN was treated at room temperature with a blue solution of
(FeCp2)(PF6) (1 equiv, 17 mg, 0.026 mmol) in CD3CN (1 mL). The
color changed to brown. The resulting mixture was stirred for 1 h. The
1
1
reaction was followed by H and 13C NMR. The H NMR (CD3CN,
400 MHz) spectrum showed a single resonance attributable to FeCp2
at δ 4.16. The 13C NMR spectrum (CD3CN, 75 MHz) showed two
resonances, attributable to 13CS2 and Fe(C5H5)2, observed at δ 193.65
and 68.81 ppm, respectively.
1
Reaction of Ni2(L2)(13CS2)2 with CoCp*2. The H NMR (CD3CN,
400 MHz) spectrum showed no peaks corresponding to Ni2(L2)-
(13CS2)2 or CoCp*2. The 13C NMR (CD3CN, 75 MHz) spectrum
demonstrated two resonances (δ 263.6 and 282.9 ppm) attributable to
the 13C-labeled carbons that originate in 13CS2.
Ni2(NHC)2(CS2)2 (6). A purple suspension of Ni2(L2)(CS2)2 (66 mg,
0.11 mmol) in 2 mL of CD3CN was treated at room temperature with
1,3-di-tert-butylimidazolin-2-ylidene (NHC, 40 mg, 0.22 mmol) in
THF (1 mL). The mixture changed to red-brown. The resulting
mixture was stirred for 2 h and filtered, and the volatiles were removed.
The residue was extracted with THF (5 mL), and the solution was
concentrated to ca. 1 mL and layered with ether (10 mL).
Recrystallization overnight at −33 °C formed Ni2(NHC)2(CS2)2 as
purple crystals (24 mg, 0.038 mmol, 35% yield). 1H NMR (C6D6, 400
MHz): δ 6.51 (s, 2H), 1.69 (s, 18H). 13C NMR (C6D6, 400 MHz,
Figure 2. Synthesis of 6-(2,4,6-triisopropylphenyl)picolinaldehyde.
1 3 CS2 -labeled sample):
δ 283.51. Anal. Calcd for
C24H40N4Ni2S4·C4H8O: C, 47.9; H, 6.9; N, 8.0. Found: C, 47.6; H,
6.6; N, 8.8.
an electron-withdrawing CF3 group at the imino carbon
position. However, although ligand formation occurs after
prolonged reflux, we were not able to isolate it in a pure form.
Therefore, this ligand will not be discussed.
X-ray Crystallographic Details. Structures of compounds 2d, 3b,
4a,b, 6, and L2 were confirmed by X-ray analysis; the structures of 1d
and 2b,c were previously reported. Table S1 (Supporting Information)
presents selected structural and refinement data for compounds L2, 2d,
3b, 4a,b, and 6. The crystals were mounted on a Bruker APEXII/
Kappa three-circle goniometer platform diffractometer equipped with
an APEX-2 detector. A graphic monochromator was employed for
wavelength selection of the Mo Kα radiation (λ = 0.71073 Å). The
data were processed and refined using the program SAINT supplied by
Siemens Industrial Automation. Structures were solved by direct
methods in SHELXS and refined by standard difference Fourier
techniques in the SHELXTL program suite (version 6.10, G. M.
Sheldrick and Siemens Industrial Automation, 2000). Hydrogen atoms
were placed in calculated positions using the standard riding model
and refined isotropically; all other atoms were refined anisotropically.
The asymmetric units of L2, 4a, and 6 contain only half of the
centrosymmetric molecule. The structure of 3b was of somewhat low
quality due to the poorly diffracting crystals. The structure contained
two methoxy groups that were found to be disordered over two
positions. The disorder was satisfactorily modeled. In addition, the
structure contained one hexane molecule per asymmetric unit. The
structure of 4a contained one ether molecule. The structure of 4b
contained three partially disordered solvent molecules that were
modeled as hexane (crystallization solvent) with partial occupancies.
The structure of 2d was of very low quality due to the small crystal
size, poor diffraction, and twinned nature of the crystals. Therefore,
only the overall connectivity and the approximate metal−metal
separation of 2d are discussed in the paper.
The ligands were characterized by 1H, 13C{1H} (13C
thereafter), and 19F (where applicable) NMR spectroscopy
and by high-resolution mass spectrometry (HRMS); the
spectra can be found in the Supporting Information. L2 was
also characterized by X-ray crystallography (Figure S1,
Supporting Information). The structure of L2 (alongside the
previously reported structure of L1)13a provides information
about the carbon−nitrogen (C4−N2, 1.275(4) Å) and the
carbon−carbon (C3−C4, 1.496(5) Å) bond distances un-
perturbed by bound metals and therefore serves to calibrate
redox effects on the iminopyridine chelate. As in the structure
of L1, L2 displays an anti conformation for the nitrogens of the
imine and the pyridine. The two sides of the ligands are also
anti.
The ligand precursors were characterized by cyclic
voltammetry (see section 5 of the Supporting Information for
details). Figure 3 shows the cyclic voltammograms (CVs) of the
ligand precursors in DMF (except for L4, due to insufficient
solubility). The ligands’ CVs exhibit two irreversible reduction
waves at potentials higher than −2.3 V. Following the cathodic
sweep, an irreversible oxidation is observed between −1.5 and
−1.1 V depending on the ligand. Comparison of the reduction
potentials of the ligands with sterically similar, but electronically
diverse, substituents indicates that the ligands possessing
electron-withdrawing groups are somewhat easier to reduce.
Thus, the first reduction of L1 peaks at −2.4 V (onset at −2.1
V), whereas for L2 the corresponding potential is −2.7 V (onset
at −2.4 V). Such a difference may explain the disparity in the
reactivity of these sterically comparable ligands with Ni-
(COD)2: whereas L1 undergoes fast reaction with Ni(COD)2
that forms Ni2(L1)(COD)2, the reaction of L2 is significantly
slower and results in the formation of Ni2(L2)2 species. Other
ligands (L5 vs L3, L7 vs L6) display an overall similar trend,
although ortho substitution results in less pronounced changes
in the reduction potential.
RESULTS AND DISCUSSION
■
Ligand Synthesis and Characterization. To evaluate
steric and electronic effects in the bis(iminopyridine) frame-
work, we have synthesized ligands L1−L7 (Figure 1). Electronic
effects in the iminopyridine chelate were manipulated via
substitution at two positions: the imino carbon position and the
pyridine 2′ (ortho)-position. Steric effects were evaluated by
employing various bulky groups at the pyridine 2′-position. L1
is a prototypical ligand featuring hydrogens only. L2 has methyl
groups at the imino carbons. L3 and L5 bear bulky electron-rich
4-methoxyphenyl and bulky electron-withdrawing 4-cyano-
phenyl groups at the pyridine 2′-position, respectively. L4 has
very bulky 2,4,6-triispropylphenyl groups at the pyridine 2′-
Synthesis of the Ni2(L)(COD)2 and Ni2(L)2 Complexes.
Figure 4 summarizes the syntheses and the reactivity of the
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Figure 3. Cyclic voltammograms of L1−L3 and L5−L7 ligand precursors in DMF (0.1 M [NBu4](PF6) supporting electrolyte, 25 °C, platinum
working electrode, 100 mV/s scan rate).
metal compounds described in this paper; the abbreviations are
shown in Table 1. Treatment of L1 with 2 equiv of Ni(COD)2
leads cleanly to the formation of Ni2(L1)(COD)2 (1a) in 63%
yield, whereas treatment of L1 with 1 equiv of Ni(COD)2 forms
Ni2(L1)2 (1b,c).10a In contrast, treatment of L2 with either 1 or
2 equiv of Ni(COD)2 invariably forms the bis(homoleptic)
complex Ni2(L2)2 (2b,c). Since both ligands feature similar
steric parameters, we postulated that the origin of the difference
in their reactivity is electronic: the Me group in the imino
carbon position creates a relatively electron-rich iminopyridine
chelate. To further evaluate the impact of the electronic effects
on L reactivity, we compared the reactivity of L3 and L5. L3 and
L5 both contain comparatively bulky, but electronically diverse,
p-methoxyphenyl and p-cyanophenyl groups at the 2′-position
of the pyridine. L5 failed to lead to an isolable product,
presumably undergoing activation of the cyano group at Ni(0).
The electron-rich and more robust L3, on the other hand, led
cleanly to the formation of the bis(homoleptic) complexes
Ni2(L3)2 (3b,c), independent of the ligand-to-metal ratio. NMR
of the crude product contained only species attributable to
Ni2(L3)2 and Ni(COD)2, implying that no other products were
formed. The product can be recrystallized from THF/ether at
−40 °C to give brown crystals of Ni2(L3)2 in 60% yield. Next,
we compared the reactivity of L6 and L7, featuring an electron-
rich and electron-withdrawing OMe and CF3 group at the
pyridine 2′-position. The crude reaction mixture of L6 with 2
equiv of Ni(COD)2 indicated the presence of the bis-
(homoleptic) complexes. However, the products were not
stable and could not be isolated. L7 underwent fast reaction
with Ni(COD)2, as indicated by an immediate color change to
violet. However, it failed to form either Ni2(L)(COD)2 or
Ni2(L)2 species. Instead, it forms different diamagnetic
products whose natures are still elusive to us. Finally, L4 tested
mostly the steric effect, featuring 2,4,6-triisopropylphenyl
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Figure 4. Schematic representation of the synthetic routes toward the dinickel compounds described in this paper.
with Ni(COD)2. Slow addition of L4 to 2 equiv of Ni(COD)2
in toluene forms mostly Ni2(L)(COD)2, on the basis of the
NMR spectrum of the resulting product. Pure Ni2(L4)(COD)2
was obtained by recrystallization from ether and isolated as
purple crystals in 39% yield. Its structure was confirmed by X-
ray crystallography. Treatment of L4 with 1 equiv of Ni(COD)2
forms mostly the bis(homoleptic) complex Ni2(L4)2. Ni2(L4)2
was obtained by recrystallization from hexane at −40 °C as
blue-violet crystals in 27% yield, and its structure was also
verified by X-ray crystallography. In addition, its purity was
confirmed by elemental analysis. Ni2(L4)2 is unstable in
solution: analytically pure samples of Ni2(L4)2 in C6D6
demonstrate progressive disappearance of the Ni2(L4)2-
attributed signals coupled with the increase in the intensity of
free ligand signals. After eight hours at room temperature, ca.
30% of the complex remains.
Table 1. Designation of Compounds and Their Abbreviation
Ni2(L1)(COD)2
syn-Ni2(L1)2
anti-Ni2(L1)2
Ni2(L1)(CS2)2
syn-Ni2(L2)2
anti-Ni2(L2)2
Ni2(L2)(CS2)2
syn-Ni2(L3)2
anti-Ni2(L3)2
Ni2(L3)(CS2)2
Ni2(L4)(COD)2
Ni2(L4)2
1a
1b
1c
1d
2b
2c
2d
3b
3c
3d
4a
4b
4d
5b
5c
6
Ni2(L4)(CS2)2
syn-Ni2(L1)(L2)
anti-Ni2(L1)(L2)
Ni2(NHC)2(CS2)2
Spectroscopic and Electrochemical Characterization
of the Ni2(L)(COD)2 and Ni2(L)2 complexes. The complexes
were characterized by NMR spectroscopy and mass spectrom-
etry. Two isolated Ni2(L)(COD)2 complexes (1a and 4a) both
groups at the 2′-position of the pyridine. The presence of the
bulky groups leads to a well-behaved reactivity of the ligand
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display C2v/C2h symmetry on the NMR time scale at room
temperature: one set of signals is observed for both arms of the
dinuclear system, the protons of the central benzene ring
appear as a singlet, and the methylene bridge protons give rise
to a singlet as well. COD protons, on the other hand, appear as
three multiplets (broad signals are observed for 4a). Such a
spectrum is consistent with the complex constantly residing in
either the syn (C2v), or the anti (C2h) conformation or
undergoing fast equilibration between syn and anti conforma-
tions (see below for DFT calculations of the stability of
different conformers of Ni2L(COD)2 species). The lack of
symmetry at a given Ni center causes COD signals to appear as
three multiplets. Ni2(L)2 complexes, on the other hand,
demonstrate AB signals for the methylene benzyl signals,
being consistent with restricted rotation around the benzyl
methylene bond. Two stereoisomers are observed in NMR
spectra of the Ni2(L2)2 and Ni2(L3)2 complexes 2b,c and 3b,c.
On the basis of the crystal structure of Ni2(L2)2,10a we
previously correlated these stereoisomers with the syn and anti
isomers. A single stereoisomer is observed for Ni2(L4)2 (4b),
possibly as a result of steric pressure that makes the syn isomer
unstable. Molecular ions [Ni2(L)2]+ are observed in the mass
spectra of all the Ni2(L)2 compounds. For Ni2(L)(COD)2
compounds, the molecular ions appear to be unstable under
ionizing conditions and are detected only at a very low
intensity. Interestingly, [Ni2(L)(COD)]+ compounds are
observed at significantly higher intensities.
However, Ni2(L)(COD)2 complexes demonstrate broader
peaks, presumably due to the conformational lability of these
complexes.
Ligand Lability As Demonstrated by the Reaction of
Ni2(L1)(COD)2 with L2. We have previously interrogated the
nature of the L binding to the Ni centers by theoretical
methods. DFT calculations demonstrated that each iminopyr-
1
idine unit has a /2− charge in the bis(homoleptic) complexes
of the Ni2(L)2 type.10a This finding implies that the ligand
should be relatively labile. We decided to probe the lability of
the bis(iminopyridine) ligand by a ligand competition experi-
ment. We treated the Ni2(L1)(COD)2 complex with 1 equiv of
L2. We postulated that if L1 is strongly bound to Ni as a 1−
ligand, then the reaction should lead cleanly to the formation of
Ni2(L1)(L2). Labile binding, on the other hand, should result in
the formation of a mixture of products. The reaction outcome
was analyzed by NMR spectroscopy and mass spectrometry
(MS). NMR and MS both indicate that the formation of all
1
possible products takes place. The H NMR spectrum displays
features consistent with the previously characterized Ni2(L1)2
and Ni2(L2)2 compounds (see the Supporting Information for
details). It addition, it contains signals attributable to the syn
and anti isomers of the mixed-ligand product, Ni2(L1)(L2). The
mass spectrum provides further indication for the proposed
mixture of products (Figure 6). The spectrum displays three
Cyclic voltammograms of the homoleptic complexes Ni2(L)2
(L = L1, L2, L3) demonstrate similar features. Figure 5 shows
Figure 5. Cyclic voltammogram of Ni2(L2)2 and Ni2(L3)2 in THF (0.1
M [NBu4](PF6), 25 °C, platinum working electrode, 100 mV/s scan
rate).
CVs of Ni2(L2)2 and Ni2(L3)2 complexes. Cyclic voltammetry
of all species demonstrates reduction (between −2.3 and −2.6
V) and reversible oxidation (−1.1 to −1.2 V). Comparison of
these results with the electrochemical studies on the
mononuclear bis(iminopyridine)Ni complex11a suggests that
these events are ligand-based. Wieghardt and co-workers have
observed an additional reversible peak at −0.57 V for the
mononuclear bis(iminopyridine)Ni complex,11a which was
attributed to the metal-centered oxidation (NiII/NiI). In our
complexes, an additional oxidation is observed for the Ni2(L3)2
complex only. The Ni2(L)(COD)2 complexes show similar
redox properties (see the Supporting Information for details).
One reduction and two oxidation events are observed.
Figure 6. Mass spectrum of the reaction products [Ni(L2)2],
[Ni2(L1)(L2)], and ([Ni2(L1)2] + [Ni(L1)2]).
intense peaks. The peak at m/z 800.2 agrees with the predicted
+
spectrum for Ni2(L2)2 , and the peak at m/z 772 corresponds
to Ni2(L1)(L2)+. The peak at m/z 742.3 may correspond to the
[Ni2(L1)(L2) − 2H]+ species or to the overlap of the mono-Ni
Ni(L2)2 species (m/z 742.3) and Ni2(L1)2 (m/z 744.3).
Structures of the Ni2(L)2 and Ni2L(COD)2 Complexes.
Selected compounds were characterized by X-ray crystallog-
+
+
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raphy. The structures are presented in Figures 7−9, and the
relevant bond distances and angles are tabulated in Table 2.
Figure 9. Structure of Ni2(L4)(COD)2 (4a), with 50% probability
ellipsoids.
Figure 7. Structure of Ni2(L3)2 (3b), with 30% probability ellipsoids.
Table 2. Selected Bond Distances (Å) and Angles (deg)
a
a
b
CN
C−C
IP−Ni−IP
Ni−Nim
Ni−Npy
2b,c
3b
4b
4a
1.331(3)
1.31(1)
1.426(4)
1.39(1)
52(2)
63(3)
1.908(3)
1.89(1)
1.921(5)
1.95(1)
1.314(3)
1.309(3)
1.275(4)
1.256(2)
1.415(3)
1.422(3)
1.496(5)
1.472(2)
69 (1)
1.906(4)
1.925(2)
1.999(9)
2.008(2)
L2
L1
a
b
Average and standard deviation of all the relevant distances. Average
and standard deviation of (NCCN)−Ni−(NCCN) dihedral angle.
the iminopyridine planes increases for the bulkier groups. A
comparison of the CN and C−C bond distances in the metal
complexes with the corresponding distances in L1 and L2 clearly
indicates substantial reduction of the iminopyridine unit.11
Synthesis and Structures of the Ni2(L)(CS2)2 Com-
plexes. We have previously reported that the reaction of
Ni2(L1)(COD)2 with 2 equiv of carbon disulfide forms
Ni2(L1)(CS2)2 (1d) in high yield.10b We were not able to
isolate Ni2(L)(COD)2 for other ligands (except for L4).
However, Ni2(L)2 complexes were shown to be labile in
solution and the reaction of Ni2(L2)2 with diphenylacetylene
(DPA) was demonstrated to form Ni2(L)(DPA)2 as one of the
products.10a On the basis of these observations, we surmised
that combining Ni(COD)2, L, and CS2 may still form the
desired product. Gratifyingly, the addition of 2 equiv of carbon
disulfide to a mixture of Ni(COD)2 (2 equiv) and L2 led to the
formation of bright purple Ni2(L2)(CS2)2 (2d; 94% yield). A
similar protocol formed purple-brown Ni2(L3)(CS2)2 (3d; 53%
yield) and blue Ni2(L4)(CS2)2 (4d; 47% yield).
Figure 8. Structure of Ni2(L4)2 (4b), with 50% probability ellipsoids.
The structures of the bis(homoleptic) Ni2(L3)2 and Ni2(L4)2
complexes are precedented by the structure of Ni2(L2)2 (that is
also included in Table 2). Unlike the structure of Ni2(L2)2,
which contained two stereoisomers, Ni2(L3)2 exists as a single
stereoisomer in the solid state (Figure 7). Dissolution of the
crystals in C6D6 solution re-forms two stereoisomers. A single
solid-state isomer of Ni2(L4)2 (Figure 8) is consistent with a
single isomer in solution. The structure of Ni2(L4)(COD)2
(4a) represents the first example of a crystallographically
characterized bis(COD) complex in our system (Figure 9). The
two parts of the dinuclear complex are in the anti conformation,
and the Ni centers are in an approximately tetrahedral
geometry. Several trends can be discerned from Table 2.
Whereas the Ni−N(imine) bonds are unaffected by the steric
bulk of the iminopyridine chelate, Ni−N(pyridine) bonds
gradually increase upon an increase in the steric bulk of the
pyridine 2′-substituent. Similarly, the dihedral angle between
1
These CS2 complexes were characterized by H and 13C
NMR spectroscopy, mass spectrometry, and elemental analysis.
13C NMR spectra of the 13CS2-labeled samples display a
resonance around 267 ppm that is characteristic of the metal-
bound CS2 group.6,10 In addition, the structure of Ni2(L2)-
(CS2)2 (2d) has been confirmed by an X-ray structure
determination. The structure of 2d is of low quality, but it
clearly demonstrates the connectivity pattern and the positions
of the two metal centers. Figure 10 displays the structure of 2d,
along with the previously reported structure of 1d.10b Both
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Figure 10. Structures of the bimetallic CS2 complexes Ni2(L1)(CS2)2
(1d, top) and Ni2(L2)(CS2)2 (2d, bottom).
structures contain two distorted-square-planar nickel(II)
centers each coordinating side-on-bound carbon disulfide.
Whereas the structure of 1d had an anti conformation of the
two CS2-bound nickel centers, the heteroallene adducts in 2d
are syn. Accordingly, C(CS2)- - -C(CS2) separations are 11.2 Å
for the anti (1d) species and 7.6 Å for the syn (2d) structure.
This finding provides crystallographic evidence that the two
heteroallenes bound by the bis(iminopyridine) dinickel system
can be in the vicinity of each other.
Electrochemistry of CS2 Complexes. Ni2(L)(CS2)2
complexes were characterized by cyclic voltammetry. CVs of
1d, 2d, and 4d are presented in Figure 11. We were not able to
obtain a reliable CV of 3d, due to its extremely low solubility.
In the cathodic sweep, two ligand-based reduction events are
observed for all of the complexes: a quasi-reversible reduction,
followed by an irreversible reduction. The overall pattern of the
electrochemical events described for 1d, 2d, and 4d is similar to
that for the Ni2(L)2 and Ni2(L)(COD)2 complexes. The
characteristic difference is that the first reversible event is a
reduction in the case of Ni2(L)(CS2)2 complexes but an
oxidation for the Ni2(L)2 and Ni2(L)(COD)2 complexes. This
difference results from the fact that the iminopyridine unit is
fully oxidized in Ni2(L)(CS2)2,10b but partially reduced in
Ni2(L)2 and Ni2(L)(COD)2.10a In addition, an irreversible
oxidation event is observed in all the Ni2(L)(CS2)2 complexes.
As the only possible location for the oxidation event is at the η2-
bound CS2, we postulated that an oxidation of the reduced
carbon disulfide leads to a chemical transformation.
Figure 11. Cyclic voltammograms of Ni2(L1)(CS2)2, Ni2(L2)(CS2)2,
and Ni2(L4)(CS2)2 in DMF (0.1 M [NBu4](PF6) supporting
electrolyte, 25 °C, platinum working electrode, 100 mV/s scan rate).
CD3CN with 2 equiv of (FeCp2)(OTf) forms FeCp2 and
liberates 13CS2. No free ligand was observed by NMR
spectroscopy, suggesting that the [Ni2(L)] fragment remains
intact. These findings are consistent with our previous report
on the oxidation of Ni2(L1)(13CS2)2 in DMSO.10b Reduction of
Ni2(L2)(13CS2)2 in CD3CN with 2 equiv of Co(Cp*)2 leads to
the formation of [Co(Cp*)2]+ (identified by NMR spectros-
copy) and the formation of a new 13C NMR signal at 283 ppm.
We were not able to identify the nature of the resulting Ni
product.
We have also investigated the reaction of Ni2(L2)(13CS2)2
with N-heterocyclic carbene. Addition of 2 equiv of NHC
(NHC = 1,3-di-tert-butylimidazolin-2-ylidene) to a stirred
suspension of 2d forms a purple-pink solution. Recrystallization
of the residue from THF/ether at −40 °C leads to the isolation
1
of 6 in ca. 40% yield. The H NMR spectrum of the product
contains two resonances (both singlets) attributable to the
NHC protons. The 13C NMR spectrum contains a new 13CS2
resonance at 283 ppm. X-ray structure determination reveals a
[Ni2(μ2-CS2)2] core supported by NHC ligands (Figure 12).
The topology of the core has been precedented for the
phosphine ligands14 but has not been isolated for the N-
heterocyclic carbenes. Compound 6 demonstrates the shortest
distance between the CS2 carbons (3.15 Å) obtained so far in
our research, and we are investigating its electronic structure
and reactivity. It is worth noting that the reaction of Ni(COD)2
with NHC and CS2 in the absence of L2 forms a mixture of two
Reactivity of CS2 Complexes. Following the electro-
chemical experiments, we carried out chemical oxidation and
reduction. For that purpose, we decided to use Ni2(L2)(13CS2)2
complexes: this complex has higher solubility than previously
studied Ni2(L1)(13CS2)2 or Ni2(L3)(13CS2)2, and 13C-labeled
carbon disulfide enables convenient monitoring of the reaction
by 13C NMR spectroscopy. Oxidation of Ni2(L2)(13CS2)2 in
I
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Organometallics
Article
We also explored the thermodynamics of various conformers
of the dinuclear species Ni2(L)(COD)2 and Ni2(L)(CS2)2.
Details of these calculations are included in the Supporting
Information, but we find no significant thermodynamic
difference between the syn and anti conformers. This was
expected for the bis-CS2 complex, since both the syn and anti
conformers were observed crystallographically. We speculated
that the Ni2(L)(COD)2 complexes could experience steric
conflicts in the syn conformer, since only the anti conformer
was observed crystallographically. As Figure 14 demonstrates,
Figure 12. X-ray structure of 7, with 50% probability ellipsoids.
compounds, one of them being 6 (see Figures S41 and S42 in
the Supporting Information). Therefore, the dinucleating ligand
L2 provides a template for the clean formation of the dinuclear
complex 6.
DFT Calculations. To evaluate the possible thermodynamic
influence of ligand substituents on the reactivity of these
bis(iminopyridine) complexes, we evaluated the thermody-
namics of COD substituting one iminopyridine using density
functional theory (see section 7 of the Supporting Information
for details):
[Ni(Lm)2] + COD ⇆ [Ni(Lm)(COD)] + Lm
Five ligands were studied: L1m with R1 = R2 = H, L2m with R1 =
H, R2 = Me, L3m with R1 = H, R2 = CF3, L4m with R1 = Me, R2
= H, and L5m with R1 = CF3, R2 = H (see Figure 13). These
Figure 14. Image of the lowest energy syn conformer of Ni2(L2)-
(COD)2.
however, there is sufficient flexibility in the bis(iminopyridine)
ligands for the Ni-COD moieties to avoid one another. The Ni
centers are well separated at 8.49 Å. The closest COD H···H
separation occurs at 4.24 Å. This syn complex is computed to
be 0.55 kcal/mol more stable than the lowest energy anti
conformer. Within the limits of our DFT methodology this
energy difference is insignificant and suggests that the
observation of only the anti conformer experimentally may
have more to do with packing effects than an intrinsic stability
of one conformer vs the other.
Figure 13. Model ligands with substituents.
data represent both electron-donating and -withdrawing groups
at the 2′-position of pyridine (L4m/L5m) and the imine carbon
(L2m/L3m). A summary of the thermodynamics is included in
Table 3. The displacement of an iminopyridine by COD is
Table 3. Thermodynamics of Ligand Exchange Predicted by
DFT
SUMMARY AND CONCLUSIONS
■
In the present study we investigated (i) steric and electronic
effects in the formation of Ni2(L)(COD)2 complexes vs
Ni2(L)2, (ii) properties and the L ligand lability in the resulting
species, (iii) formation, structures, and electrochemical proper-
ties of the Ni2(L)(CS2)2 complexes, (iv) reactivity of the
Ni2(L)(CS2)2 complexes. Reactivity studies disclose that
ligands featuring electron-donating groups react more slowly
with Ni(COD)2 and lead preferentially to the formation of
Ni2(L)2 complexes. DFT calculations agree with the conclusion
that the electron-withdrawing groups in the imine position
provide some stabilization to Ni(L)(COD) complexes vs
Ni2(L)2 complexes. Structural and theoretical data for Ni2(L)-
(COD)2 are consistent with the free rotation of the ligand
chelating units vs each other. We also discovered that prior
isolation of Ni2(L)(COD)2 complexes is unnecessary to form
Ni2(L)(CS2)2, as these species are formed in a one-pot reaction
between 2 equiv of Ni(COD)2, L, and 2 equiv of CS2.
Spectroscopic, crystallographic, and theoretical data all agree
with the lack of thermodynamic difference between syn and anti
conformers in Ni2(L)(CS2)2 complexes. Electrochemical
studies of Ni2(L)(CS2)2 reveal two ligand-based reductions
and a CS2-based oxidation. Chemical reduction results in the
oxidation of [CS2]2− to [CS2]0 followed by its liberation from
reaction
energy (kcal/mol)
[Ni(L1m)2] + COD ⇆ [Ni(L1m)(COD)] + L1m
[Ni(L2m)2] + COD ⇆ [Ni(L2m)(COD)] + L2m
[Ni(L3m)2] + COD ⇆ [Ni(L3m)(COD)] + L3m
[Ni(L4m)2] + COD ⇆ [Ni(L4m)(COD)] + L4m
[Ni(L5m)2] + COD ⇆ [Ni(L5m)(COD)] + L5m
17.01
19.03
17.09
18.68
18.87
predicted to be unfavorable for each bis(iminopyridine)
complex. However, the least unfavorable reaction is for the
ligand with hydrogens at R1 and R2, which is most similar to the
experimental ligand L1. Both of the ligands with imine
substituents show less favorable displacement by COD and
the bis(iminopyridine) compounds show a noticeably longer
Cpyr−Cim bond length of ∼1.45 (L2m/L3m) vs 1.43 Å (L1m).
Thus, any substituent that provides polarizability for the radical
iminopyridine anion10a seems to disfavor formation of the
COD complex. The pyridyl substituents do not show a simple
trend in the ligand displacement thermodynamics (L4m/L5m).
These structures demonstrate much larger interligand dihedral
angles due to the steric conflicts of the substituents. These
sterics influence the ligand exchange thermodynamics in a
nontrivial way.
J
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Organometallics
Article
Soc. 1983, 105, 5914. (h) Angamuthu, R.; Byers, P.; Lutz, M.; Spek, A.
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the metal. Reaction of Ni2(L)(CS2)2 with the N-heterocyclic
carbene NHC forms Ni2(NHC)2(CS2)2. Overall, this research
indicates that the iminopyridine ligand is labile at Ni(I) and
Ni(II) centers and does not provide necessary stabilization for
Ni in the oxidation states required for the overall catalytic cycle
(especially for Ni(II)). In addition, the bidentate nature of the
iminopyridine enables formation of stable and unreactive η2
adducts of CS2, which may preclude its activation. Formation of
Ni2(NHC)2(CS2)2 containing nearby positioned carbon
disulfides represents a new promising avenue in this project.
Its electronic structure and reactivity will be investigated. In
addition, we are currently studying the formation and reactivity
of the M2(L)(CO2)2 and M2(L)(oxalate)2 complexes (M = Ni,
Cu), hoping to shed light on the nature of CO2 and oxalate
binding and transformation in this system.
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ASSOCIATED CONTENT
* Supporting Information
■
S
(7) Krogman, J. P.; Foxman, B. M.; Thomas, C. M. J. Am. Chem. Soc.
2011, 133, 14582.
(8) Rail, M. D.; Berben, L. A. J. Am. Chem. Soc. 2011, 133, 18577.
(9) (a) Hruszkewycz, D. P.; Wu, J.; Green, J. C.; Hazari, N.;
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Wu, J.; Hazari, N.; Incarvito, C. D. J. Am. Chem. Soc. 2011, 133, 328.
Figures, tables, and CIF files giving NMR spectra, X-ray data,
and Cartesian coordinates for all computed structures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
(10) (a) Bheemaraju, A.; Lord, R. L.; Muller, P.; Groysman, S.
̈
Organometallics 2012, 31, 2120. (b) Bheemaraju, A.; Beattie, J. W.;
Lord, R. L.; Martin, P. D.; Groysman, S. Chem. Commun. 2012, 48,
9595.
(11) (a) Lu, C. C.; Bill, E.; Weyhermuller, T.; Bothe, E.; Wieghardt,
̈
ACKNOWLEDGMENTS
■
K. J. Am. Chem. Soc. 2008, 130, 3181. (b) Lu, C. C.; Weyhermueller,
T.; Bill, E.; Wieghardt, K. Inorg. Chem. 2008, 48, 6005. (c) van Gastel,
M.; Lu, C. C.; Wieghardt, K.; Lubitz, W. Inorg. Chem. 2009, 48, 2626.
(d) McDaniel, A. M.; Tseng, H.-W.; Hill, E. A.; Damrauer, N. H.;
Rappe, A. K.; Shores, M. P. Inorg. Chem. 2013, 52, 1368. (e) Myers, T.
W.; Berben, L. A. Inorg. Chem. 2012, 51, 1480. (f) Summerscales, O.
T.; Myers, T. W.; Berben, L. A. Organometallics 2012, 31, 3463.
(g) Myers, T. W.; Kazem, N.; Stoll, S.; Britt, R. D.; Shanmugam, M.;
Berben, L. A. J. Am. Chem. Soc. 2011, 133, 8662. (h) Nayek, H. P.;
Arleth, N.; Trapp, I.; Loeble, M.; Ona-Burgos, P.; Kuzdrowska, M.;
Lan, Y.; Powell, A. K.; Breher, F.; Roesky, P. W. Chem. Eur. J. 2011, 17,
10814. (i) Trifonov, A. A.; Gudilenkov, I. D.; Larionova, J.; Luna, C.;
Fukin, G. K.; Cherkasov, A. V.; Poddel’sky, A. I.; Druzhkov, N. O.
Organometallics 2009, 28, 6707.
(12) Aharoni, A.; Vidavsky, Y.; Diesendruck, C. E.; Ben-Asuly, A.;
Goldberg, I.; Lemcoff, N. G. Organometallics 2011, 30, 1607.
(13) (a) Li, C.; Sun, F.-A.; He, M.-Y.; Xu, H.; Chen, Q. Acta
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We thank Wayne State University (S.G.) and Grand Valley
State University (R.L.L.) for funding, Dr. Lew Hryhorczuk for
the experimental assistance, and Prof. H. B. Schlegel for helpful
discussions. Computational resources were housed and
maintained by the Wayne State Grid. Erwyn Tabasan thanks
the Office of Undergraduate Research at Wayne State
University for the Undegraduate Research and Creative
Projects Award.
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dx.doi.org/10.1021/om400187v | Organometallics XXXX, XXX, XXX−XXX