Bivalent, trivalent, and tetravalent nickel complexes with a common tridentate deprotonated pyridine bis-amide ligand. molecular structures of nickel(II) and nickel(IV) and redox activity

Article abstract of DOI:10.1021/ic980672e

Using a tridentate bis-amide ligand 2,6-bis[N-(phenyl)carbamoyl]pyridine (H₂L), in its deprotonated form, nickel complexes in three consecutive oxidation states [Et₄N]₂[Ni^IIL₂]·H ₂I (1), [Et₄N][Ni^IIIL₂]·H₂O (2), and [Ni^IVL₂]· 0.75H₂O (3) have been prepared, and 1 and 3 structurally characterized. These X-ray structures represent first crystallographically characterized NiN₆ coordination sphere, with a common pyridine bis-amide ligand. Complex 1 crystallizes in the orthorhombic space group Pccn, a = 10.175(2) A?, b = 20.834(3) A?, c = 23.765(4) A?, Z = 4, and 3 crystallizes in the monoclinic space group P2₁/a, a - 14.874(7) A?, b = 13.300(4) A?, c = 16.604(5) A?, β -99.678(3)°, Z = 4. Considerable distortion is observed with the average distances being Ni-N_amide 2.131(8) A? and Ni-N_py 1.994(7) A? for 1 and Ni-N_amide 1.946(8) A? and Ni-N_py 1.846(8) A? for 3. The observation of short axial M-N_py and long equatorial M-N_amide bonds (tetragonally compressed octahedral geometry) is caused by the steric requirement of the ligand. Magnetic susceptibility measurements (63-300 K) reveal that the spin states of nickel centers in 1 and 2 are S = 1 and S = 1/2, respectively. Complex 3 is diamagnetic. In their absorption spectra (MeCN), 1 exhibits a d-d transition at 854 nm; 2 and 3 display LMCT transitions at 449 nm with a shoulder at 636 nm and at 480 nm with a shoulder at 730 nm, respectively. The nickel(III) complex 2 exhibits a rhombic EPR signal (g values: 2.149, 2.115, and 2.034), showing that the metal center is the primary residence site of the unpaired electron. Cyclic voltammetric measurements of 1 in MeCN solution at a glassy carbon electrode exhibit two chemically reversible (i_p,a/i_P,c ≈ 1) and electrochemically quasireversible (ΔE_P = 100 mV) oxidative responses: a Ni^III-Ni^II couple (E_1/2 = 0.05 V vs SCE) and a Ni^IV-Ni^III couple (E_1/2 = 0.51 V vs SCE). A one-electron chemical oxidation of yellowish brown 1 was achieved in a two-phase solvent mixture H₂O-CH₂-Cl₂ with [Fe(η⁵-C₅H₅)₂][PF₆], which led to the isolation of reddish brown 2. A two-electron chemical oxidation of 1 was readily achieved in MeCN with ceric ammonium nitrate to afford dark violet crystals of 3. For 1 a linear correlation between the Ni^III-Ni^II reduction potentials and the reciprocal of solvent dielectric constants is obtained.

Full text of DOI:10.1021/ic980672e

1388
Inorg. Chem. 1999, 38, 1388-1393
Bivalent, Trivalent, and Tetravalent Nickel Complexes with a Common Tridentate
Deprotonated Pyridine Bis-Amide Ligand. Molecular Structures of Nickel(II) and
Nickel(IV) and Redox Activity
Apurba Kumar Patra and Rabindranath Mukherjee*
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
ReceiVed June 16, 1998
Using a tridentate bis-amide ligand 2,6-bis[N-(phenyl)carbamoyl]pyridine (H2L), in its deprotonated form, nickel
complexes in three consecutive oxidation states [Et4N]2[NiIIL2]‚H2O (1), [Et4N][NiIIIL2]‚H2O (2), and [NiIVL2]‚
0.75H2O (3) have been prepared, and 1 and 3 structurally characterized. These X-ray structures represent first
crystallographically characterized NiN6 coordination sphere, with a common pyridine bis-amide ligand. Complex
1 crystallizes in the orthorhombic space group Pccn, a ) 10.175(2) Å, b ) 20.834(3) Å, c ) 23.765(4) Å, Z )
4, and 3 crystallizes in the monoclinic space group P21/a, a ) 14.874(7) Å, b ) 13.300(4) Å, c ) 16.604(5) Å,
â ) 99.678(3)°, Z ) 4. Considerable distortion is observed with the average distances being Ni-Namide 2.131(8)
Å and Ni-Npy 1.994(7) Å for 1 and Ni-Namide 1.946(8) Å and Ni-Npy 1.846(8) Å for 3. The observation of
short axial M-Npy and long equatorial M-Namide bonds (tetragonally compressed octahedral geometry) is caused
by the steric requirement of the ligand. Magnetic susceptibility measurements (63-300 K) reveal that the spin
states of nickel centers in 1 and 2 are S ) 1 and S ) 1/2, respectively. Complex 3 is diamagnetic. In their absorption
spectra (MeCN), 1 exhibits a d-d transition at 854 nm; 2 and 3 display LMCT transitions at 449 nm with a
shoulder at 636 nm and at 480 nm with a shoulder at 730 nm, respectively. The nickel(III) complex 2 exhibits
a rhombic EPR signal (g values: 2.149, 2.115, and 2.034), showing that the metal center is the primary residence
site of the unpaired electron. Cyclic voltammetric measurements of 1 in MeCN solution at a glassy carbon electrode
exhibit two chemically reversible (ip,a/ip,c ≈ 1) and electrochemically quasireversible (∆Ep ) 100 mV) oxidative
responses: a NiIII-NiII couple (E1/2 ) 0.05 V vs SCE) and a NiIV-NiIII couple (E1/2 ) 0.51 V vs SCE). A
one-electron chemical oxidation of yellowish brown 1 was achieved in a two-phase solvent mixture H2O-CH2-
Cl2 with [Fe(η5-C5H5)2][PF6], which led to the isolation of reddish brown 2. A two-electron chemical oxidation
of 1 was readily achieved in MeCN with ceric ammonium nitrate to afford dark violet crystals of 3. For 1 a linear
correlation between the NiIII-NiII reduction potentials and the reciprocal of solvent dielectric constants is obtained.
Introduction
It has been well-documented that amidato-N ligands7,14,15
have the ability to stabilize high formal oxidation states of metal
ions, due to their good σ-donor properties of the deprotonated
nitrogen. Since strong donor ligands reduce the oxidizing
properties of high-valent complexes,15 salicylamide15,16 and
pyridine amide17-22 ligands which resist oxidative degradation,
afford very stable (characterized by X-ray crystallography)
complexes with a wide range of metal ions. Very recently, we
found23 that simple, easily synthesizable, tridentate dianionic
bis-amide ligand L(2-) [H2L ) 2,6-bis(N-phenylcarbamoyl)-
Stabilization of nickel in its higher (>2) oxidation state1-3
was a challenge to inorganic chemists during seventies and
eighties. The ligands used to fulfill the electronic requirements
of Ni(III) or Ni(IV) states were primarily oximes1-6 or depro-
tonated amides.7-9 In recent years, synthesis of trivalent nickel
complexes10-12 has attracted attention to bioinorganic chemists
to provide models,11,12 for the Ni sites in [NiFe]-hydrogenases.13
(1) (a) Nag, K.; Chakravorty, A. Coord. Chem. ReV. 1980, 33, 87. (b)
Chakravorty, A. Isr. J. Chem. 1985, 25, 99.
(11) (a) Kru¨ger, H.-J.; Holm, R. H. Inorg. Chem. 1987, 26, 3645. (b) Kru¨ger,
H.-J.; Holm, R. H. J. Am. Chem. Soc. 1990, 112, 2955. (c) Kru¨ger,
H.-J.; Peng, G.; Holm, R. H. Inorg. Chem. 1991, 30, 734.
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Soc. 1992, 114, 9666. (b) Marganian, C. A.; Vazir, H.; Baidya, N.;
Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 1995, 117,
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(13) (a) Cammack, R. AdV. Inorg. Chem. 1988, 32, 297. (b) Moura, J. J.
G.; Moura, I.; Teixeira, M.; Xavier, A. V.; Fauque, G. D.; Legall, J.
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(3) Lappin, A. G.; McAuley, A. AdV. Inorg. Chem. 1988, 32, 241.
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(b) Sproul, G.; Stucky, G. D. Inorg. Chem. 1973, 12, 2898.
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Soc., Dalton Trans. 1983, 1225. (b) Bemtgen, J.-M.; Gimpert, H.-R.;
von Zelewsky, A. Inorg. Chem. 1983, 22, 3576.
(7) Margerum, D. W. Pure Appl. Chem. 1983, 55, 23.
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(b) Bossu, F. P.; Margerum, D. W. Inorg. Chem. 1977, 16, 1210. (c)
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(14) Sigel, H.; Martin, R. B. Chem. ReV. 1982, 82, 385.
(15) Collins, T. J. Acc. Chem. Res. 1994, 27, 279 and references therein.
(16) (a) Koikawa, M.; Gotoh, M.; O˜ kawa, H.; Kida, S.; Kohzuma, T. J.
Chem. Soc., Dalton Trans. 1989, 1613. (b) Koikawa, M.; O˜ kawa, H.;
Maeda, Y.; Kida, S. Inorg. Chim. Acta 1992, 194, 75.
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10.1021/ic980672e CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/13/1999

Multivalent Nickel Complexes
Inorganic Chemistry, Vol. 38, No. 7, 1999 1389
solid [Et4N]Cl‚xH2O (111 mg, 0.67 mmol), and stirring was continued
for 12 h. Removal of the solvent was followed by addition of MeCN
(10 mL) and filtration. Reducing the volume of the solution down to
∼3 mL and addition of diethyl ether (∼8 mL) to it resulted in the
crystallization of a dark brownish yellow compound (90 mg; ∼56%).
Recrystallization from MeCN-EtOAc (vapor diffusion) afforded
crystalline solid suitable for structural studies.
pyridine] is capable of stabilizing trivalent state of iron and
Characterization. Anal. Calcd for C54H68N8O5Ni: C, 67.03; H, 7.03;
N, 11.59. Found: C, 66.98; H, 6.99; N, 11.56. Conductivity (MeCN;
10-3 M solution at 298 K): ΛM ) 222; (in DMF) 121; (in DMSO) 65;
(in H2O) 272 Ω-1 cm2 mol-1. Absorption spectrum [λmax, nm (ꢀ, M-1
cm-1): (in MeCN) 225 sh (52 640), 249 (38 000), 286 sh (30 100),
352 sh (8070), 854 (87); (in DMF) 375 sh (8170), 480 (790), 863 (85);
(in DMSO) 857 (93); (in H2O) 815 (145)].
cobalt to a considerable extent, in their bis-chelate complexes.
Most interestingly, these low-spin complexes exhibit tetragonal
distortion with two unusually short axial bonds. The present
work stems from our search for preparing nickel analogues using
L(2-) in unusual stereochemistry and high oxidation state.
Here we report the synthesis and characterization of highly
stable nickel complexes in all the three oxidation states, [Et4N]2-
[NiIIL2]‚H2O (1), [Et4N][NiIIIL2]‚H2O (2), and [NiIVL2]‚0.75H2O
(3). This work presents, to the best of our knowledge, the first
structural study on tetragonally compressed nickel(II) complex
1 as well as nickel(IV) complex 3, with a common deprotonated
amide ligand L(2-), providing NiN6 coordination sphere. The
present comparative studies offer the first direct evidence for
the influence of coordination geometry on the electrochemical
behavior of Ni(II), Ni(III), and Ni(IV), with a given bis-amide
ligand.
(b) [Et4N][NiL2]‚H2O (2). To a magnetically stirred solution of
[Et4N]2[NiL2]‚H2O (50 mg, 0.052 mmol) in distilled water (1 mL) was
added solid [Fe(η5-C5H5)2][PF6] (20.5 mg, 0.062 mmol) followed by
immediate addition of CH2Cl2 (25 mL). The reddish-brown reaction
mixture thus formed was stirred for 30 min. The CH2Cl2 layer was
then separated. The water layer was washed with CH2Cl2 (2 × 5 mL)
and all of the CH2Cl2 extracts were then collected. After addition of
anhydrous Na2SO4 it was left for 3 h. After removal of inorganic solid
the filtrate was concentrated to ∼1-2 mL. Addition of EtOAc (∼3
mL) precipitated a brown microcrystalline compound. The solid was
collected by filtration, washed with EtOAc, and finally with diethyl
ether. This crude product was contaminated with [Et4N][PF6]. Recrys-
tallization was achieved by vapor diffusion of diethyl ether into an
MeCN solution to afford highly crystalline brown compound (yield:
16 mg, ∼38%).
Characterization. Anal. Calcd for C46H48N7O5Ni: C, 65.97; H, 5.74;
N, 11.71. Found: C, 65.87; H, 5.80; N, 11.68. Conductivity (MeCN,
10-3 M solution at 298 K): ΛM ) 165 Ω-1 cm2 mol-1. Absorption
spectrum [λmax, nm (ꢀ, M-1 cm-1): (in MeCN) 275 sh (20 320), 304
(17 000), 449 (7140), 636 sh (1960)].
Experimental Section
Materials and Reagents. All chemicals were obtained from
commercial sources and used as received. Solvents were dried/purified
as reported previously.23 The ligand 2,6-bis(N-phenylcarbamoyl)pyridine
and tetra-n-butylammonium perchlorate (TBAP) were prepared as
before.23 [Fe(η5-C5H5)2][PF6] was prepared following a reported
procedure.24
Synthesis of Metal Complexes. (a) [Et4N]2[NiL2]‚H2O (1). The
ligand H2L (100 mg, 0.315 mmol) was dissolved in dinitrogen flushed
N,N′-dimethylformamide (DMF) (5 mL) and to it was added solid NaH
(35 mg, 1.46 mmol), resulting in a light yellow solution. NiCl2‚6H2O
(40 mg, 0.168 mmol) was dissolved in DMF (5 mL) separately. The
latter solution was then added to the ligand solution using a cannula
under a dinitrogen atmosphere. The resulting brownish yellow solution
was magnetically stirred for 2 h at 298 K in the air. To this was added
(c) [NiL2]‚0.75 H2O (3). A solution of [Et4N]2[NiL2]‚H2O (45 mg,
0.047 mmol) in MeCN (1 mL) was added dropwise to a stirred solution
of [NH4]2[Ce(NO3)6] (75 mg, 0.137 mmol) within a time span of 15
min. The violet colored microcrystalline compound started precipitating
out. The mixture was stirred for a further 45 min and then filtered.
The collected solid was washed with cold MeCN (1 mL). Recrystal-
lization was achieved by diethyl ether diffusion into a CHCl3 solution
of the complex. Block-like single crystals were formed which were
filtered and vacuum-dried (yield: 22 mg, ∼61%).
Characterization. Anal. Calcd for C38H27.5N6O4.75Ni: C, 64.94; H,
3.92; N, 11.96. Found: C, 64.91; H, 3.93; N, 12.00. Conductivity
(MeCN, 10-3 M solution at 298 K): ΛM ) 15 Ω-1 cm2 mol-1
.
Absorption spectrum [λmax, nm (ꢀ, M-1 cm-1): (in MeCN) 299 sh
(18) (a) Che, C.-M.; Ma, Ji-X.; Wong, W.-T.; Lai, T.-F.; Poon, C.-K. Inorg.
Chem. 1988, 27, 2547. (b) Mak, S.-T.; Yam, V. W.-W.; Che, C.-M.
J. Chem. Soc., Dalton Trans. 1990, 2555. (c) Mak, S.-T.; Wong, W.-
T.; Yam, V. W.-W.; Lai, T.-F.; Che, C.-M. J. Chem. Soc., Dalton
Trans. 1991, 1915. (d) Che, C.-M.; Leung, W.-H.; Li, C.-K.; Cheng,
H.-Y.; Peng, S.-M. Inorg. Chim. Acta 1992, 196, 43. (e) Ko, P.-H.;
Chen, T.-Y.; Zhu, J.; Cheng, K.-F.; Peng, S.-M.; Che, C.-M. J. Chem.
Soc., Dalton Trans. 1995, 2215 and references therein.
(19) (a) Yang, Y.; Diederich, F.; Valentine, J. S. J. Am. Chem. Soc. 1991,
113, 7195. (b) Wocadlo, S.; Massa, W.; Folgado, J.-V. Inorg. Chim.
Acta 1993, 207, 199 and references therein.
(20) (a) Ray, M.; Mukherjee, R.; Richardson, J. F.; Buchanan, R. M. J.
Chem. Soc., Dalton Trans. 1993, 2451. (b) Ray, M.; Mukherjee, R.;
Richardson, J. F.; Mashuta, M. S.; Buchanan, R. M. J. Chem. Soc.,
Dalton Trans. 1994, 965 and references therein.
(21) (a) Brown, S. J.; Tao, X.; Stephan, D. W.; Mascharak, P. K. Inorg.
Chem. 1986, 25, 3377. (b) Tao, X.; Stephan, D. W.; Mascharak, P.
K. Inorg. Chem. 1987, 26, 754. (c) Delany, K.; Arora, S. K.;
Mascharak, P. K. Inorg. Chem. 1988, 27, 705. (d) Chavez, F. A.;
Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 1996, 35, 1410. (e)
Chavez, F. A.; Nguyen, C. V.; Olmstead, M. M.; Mascharak, P. K.
Inorg. Chem. 1996, 35, 6282.
(22) Redmore, S. M.; Rickard, C. E. F.; Webb, S. J.; Wright, L. J. Inorg.
Chem. 1997, 36, 4743.
(23) Ray, M.; Ghosh, D.; Shirin, Z.; Mukherjee, R. Inorg. Chem. 1997,
36, 3568 and references therein.
(11 770), 480 (13 700), 730 sh (4820)].
Physical Measurements. Solution electrical conductivity measure-
ments were carried out with an Elico (Hyderabad, India) Type CM-82
T conductivity bridge. Spectroscopic data were obtained by using the
following instruments: infrared spectra, Perkin-Elmer M-1320; elec-
tronic spectra, Perkin-Elmer Lambda 2; X-band EPR spectra, Varian
E-109 C; 1H NMR spectra, Bru¨ker WP-80 (80 MHz) NMR spectrom-
eter.
Variable-temperature (63-300 K) solid-state magnetic susceptibility
measurements were done by the Faraday technique using a locally built
magnetometer.23 The set up consists of an electromagnet with constant
gradient pole caps (Polytronic Corporation, Mumbai, India), Sartorius
M25-D/S balance (Germany), a closed cycle refrigerator, and a Lake
Shore temperature controller (Cryo Industries, USA). All measurements
were made at a fixed main field strength of ∼10 kG. Solution-state
magnetic susceptibility was obtained by the NMR technique of Evans25
in MeCN with a PMX- 60 JEOL (60 MHz) NMR spectrometer.
Susceptibilities were corrected for diamagnetic contribution which was
calculated to be -388 × 10-6 cm3 mol-1 for 1 and -276 × 10-6 cm3
mol-1 for 2 by using literature values.26
(24) Ercolani, C.; Gardini, M.; Pennesi, G.; Rossi, G.; Russo, U. Inorg.
Chem. 1988, 27, 422.
(25) Evans, D. F. J. Chem. Soc. 1959, 2003.
(26) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203.

1390 Inorganic Chemistry, Vol. 38, No. 7, 1999
Patra and Mukherjee
Table 2. Selected Bond Lengths (Å) and Angles (deg) of
Table 1. Crystal Data for [Et4N]2[NiL2]‚H2O (1) and
[NiL2]‚0.75H2O (3)
[Et4N]2[NiL2]‚H2O (1) and [NiL2]‚0.75H2O (3)
1
3
1
3
empirical formula
fw
temp. (°C)
radiation (λ, Å)
space group
a, Å
C54H68N8O5Ni
966.69
20
Mo KR (0.710 73)
Pccn (No. 56)
10.175(2)
20.834(3)
23.765(4)
C38H27.5N6O4.75Ni
702.19
20
Ni-N(1)
Ni-N(2)
Ni-N(3)
2.127(8)
1.994(7)
2.135(8)
Ni-N(1)
Ni-N(2)
Ni-N(3)
Ni-N(4)
Ni-N(5)
Ni-N(6)
N(1)-Ni-N(2)
N(1)-Ni-N(3)
N(1)-Ni-N(4)
N(1)-Ni-N(5)
N(1)-Ni-N(6)
N(2)-Ni-N(3)
N(2)-Ni-N(4)
N(2)-Ni-N(5)
N(2)-Ni-N(6)
N(3)-Ni-N(4)
N(3)-Ni-N(5)
N(3)-Ni-N(6)
N(4)-Ni-N(5)
N(4)-Ni-N(6)
N(5)-Ni-N(6)
1.940(7)
1.852(8)
1.953(7)
1.949(8)
1.840(8)
1.942(8)
81.9(3)
164.3(3)
90.4(3)
98.2(3)
91.7(3)
82.4(3)
97.2(3)
179.8(4)
97.5(3)
90.6(3)
97.4(3)
91.2(3)
82.7(3)
165.3(3)
82.6(3)
Mo KR (0.710 73)
P21/a (No. 14)
14.874(7)
13.300(4)
16.604(5)
99.678(3)
3237.93(2)
4
1.443
0.65
0.061
0.064
b, Å
c, Å
N(1)-Ni-N(2)
N(1)-Ni-N(2′)
N(1)-Ni-N(3)
N(1)-Ni-N(3′)
N(2)-Ni-N(3)
N(2)-Ni-N(3′)
N(1)-Ni-N(1′)
N(2)-Ni-N(2′)
N(3)-Ni-N(3′)
76.0(3)
100.8(3)
153.5(3)
97.2(3)
77.6(3)
105.7(3)
89.2(3)
175.6(4)
88.4(3)
â, deg
V, Å3
5037.85(1)
4
1.276
0.44
0.069
0.044
Z
F (calcd), g cm-3
µ (Mo KR), mm-1
R(Fo)a
Rw(Fo)b
2
a R ) ∑(Fo - Fc)/∑Fo. b Rw ) [∑w(Fo - Fc)2/∑wFo ]1/2, w ) 1/σ(F).
Cyclic voltammetric measurements were performed by using a PAR
model 370 electrochemistry system, consisting of a model 174A
polarographic analyzer with a model 175 universal programmer. A PAR
model G0021 glassy carbon electrode was used as the working
electrode. All potentials were measured referenced to the saturated
calomel electrode (SCE) at 298 K; no corrections were made for
junction potentials.
of 1 was performed in MeCN using 3.0 equiv of ceric
ammonium nitrate a net two-electron oxidation occurred,
resulting in the generation of a violet solution. Usual workup
of this solution produced dark violet microcrystals of the nickel-
(IV) complex [NiL2]‚0.75 H2O 3. The absence of ν(N-H) in
the IR spectra of 1-3 confirmed that the ligand is coordinated
in the deprotonated form. For all the three complexes ν(OH)
absorption of water of crystallization was observed in the region
3400-3500 cm-1. Elemental analyses, IR, and solution electrical
conductivity data (1, 1:2 electrolyte; 2, 1:1 electrolyte; 3,
nonelectrolyte in MeCN)29 are in agreement with the above
formulations.
Among polarizable anionic ligands known to stabilize high
oxidation states of nickel, deprotonated oximes deserve special
mention. Chakravorty and co-workers prepared Ni(II), Ni(III),
and Ni(IV) complexes with NiN6 coordination sphere using
tridentate and hexadentate amino-imino-oxime ligands.1-3 From
the standpoint of present investigation, the complexes with
pyridine-2,6-bis(acetyloxime) (H2dapo) reported by Drago et
al. are worth mentioning.4 This ligand forms six-coordinate Ni-
(II) and Ni(IV) complexes, which are isolated in the solid state.
The structure of the Ni(IV) complex [Ni(dapo)2] has been
demonstrated crystallographically. Holm et al.11b demonstrated
the existence of the intermediate Ni(III) species, [Ni(dapo)2]-
from coulometric generation and subsequent characterization
by EPR spectroscopy.
Crystal Structure Determinations. A yellow block-shaped crystal
of [Et4N]2[NiL2]‚H2O 1 (dimensions: 0.3 × 0.2 × 0.1 mm) and blue
well-formed crystal of [NiL2]‚0.75H2O 3 (dimensions: 0.2 × 0.2 ×
0.1 mm) were used for data collection. Diffracted intensities were
collected on an Enraf Nonius CAD4-Mach diffractometer using
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). Lattice
parameters for the complex was obtained from least-squares analyses
of 25 machine-centered reflections. Three standard reflections were
measured at every hour to monitor instrument and crystal stability.
Intensity data were corrected for Lorentz and polarization effects;
analytical absorption corrections were applied. The XTAL3.2 program
package27 was used in absorption and all subsequent calculations. The
linear absorption coefficients, neutral atom scattering factors for the
atoms, and anomalous dispersion corrections for non-hydrogen atoms
were taken from ref 28. The structures were solved by the direct method
and successive difference Fourier syntheses. The function minimized
during full-matrix least-squares refinement was ∑w(Fo - Fc)2 where
w ) 1/σ(F). The hydrogen atoms of the water molecules were located
by a Fourier difference map. The positions of the hydrogen atoms were
calculated assuming ideal geometries of the atom concerned and their
positions and thermal parameters were not refined. All other atoms
were refined with anisotropic thermal parameters. Crystal data and a
summary of experimental results are presented in Table 1. Selected
bond distances and angles are listed in Table 2. The rest of the
crystallographic data have been submitted as Supporting Information.
Description of the Structures. (a) [Et4N]2[NiL2]‚H2O (1).
A view of the anion is presented in Figure 1. The nickel center
is coordinated to two L(2-) ligands. The Ni atom sits on an
imposed C2 axis and is coordinated by four deprotonated amide
nitrogens in the equatorial plane [N(1) and N(3) and their
symmetry related] and two pyridyl nitrogens [N(2) and its
symmetry related] in the axial positions. The Ni atom is
constrained by symmetry to lie on the equatorial plane. The
arrangement of the ligands in 1 is meridional, as that observed
with its Fe(III) and cobalt(III) analogues, [Et4N][FeL2]‚1.5H2O
and [Et4N][CoL2]‚2H2O.23
Results and Discussion
Synthesis. The synthesis of the nickel(II) compound [Et4N]2-
[NiL2]‚H2O 1 involved initial anaerobic reaction of Na2L with
NiCl2‚6H2O in DMF at 298 K to form a brownish yellow
complex. Addition of [Et4N]Cl‚xH2O and usual workup afforded
yellow air-stable crystals. Clean one-electron chemical oxidation
of 1 in 1:25 H2O-CH2Cl2 solvent mixture (v/v) by 1.2 equiv
of [Fe(η5-C5H5)2][PF6] generated a reddish brown solution,
which on usual workup led to the isolation of the nickel(III)
complex, [Et4N][NiL2]‚H2O (2). When the chemical oxidation
The geometry of the NiIIN6 coordination polyhedron is
appreciably compressed octahedral (Table 2). Significant devia-
tion from 90° of the bond angles involving the chelation is
(27) Hall, S. R.; Flack, H. D.; Stewart, J. M. (Eds.) The XTAL 3.2 Reference
Manual; Universities of Western Australia, Geneva, and Maryland:
1992.
(28) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974; Vol. IV.
(29) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.

Multivalent Nickel Complexes
Inorganic Chemistry, Vol. 38, No. 7, 1999 1391
Figure 2. View of the metal coordination environment in the crystal
of [NiL2]‚0.75H2O (3), showing atom-labeling scheme and 50%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
Figure 1. View of the molecular structure of the anion of [Et4N]2-
[NiL2]‚H2O (1), showing atom-labeling scheme and 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity. Unlabeled atoms
are related to labeled atoms by the crystallographic 2-fold axis.
the pyridine ring are planar, the two phenyl rings, however,
make an angle of ∼66° to each other and angles of ∼104° and
∼97° with the central pyridine ring for one ligand; for the other
ligand the two phenyl rings make an angle of ∼109° to each
other and angles of ∼104° and ∼85° with the central pyridine
ring.
Interestingly, in 3 all the C-O bond lengths are not similar.
For one ligand the average distance is 1.22 Å, but for the other
the distances are dissimilar. In fact, one distance is 1.21 Å and
the other is slightly longer 1.23 Å, implying a partial single
bond character (the normal range of bond lengths are CdO
(≈1.20 Å) and C-O (≈1.43 Å)).30 For an N-coordinated and
therefore deprotonated amide group it is expected that the
negative charge would be delocalized along the amide C-N
and C-O bonds, leading to two resonance forms which have
observed (Table 2) presumably due to formation of five-
membered chelate rings with extended conjugation, as observed
for the bis-chelates of iron(III) and cobalt(III).23 Further evidence
of strain in the chelate rings arises from the following observa-
tion. Though in each ligand the two N-phenyl rings and the
pyridine ring are planar, the two phenyl rings, however, make
an angle of ∼56° to each other and angles of ∼122° and ∼129°
with the central pyridine ring.
The average NiII-Namide distance in 1 (2.131 Å) is appreciably
longer, compared to those in square-planar Ni(II) complexes
with pyridine amide ligands (1.839-1.918 Å).17 A very interest-
ing feature of this structure is the NiII-Npy bond length (1.994
Å), which is appreciably shorter (∼0.05 Å) than that observed
for [Et4N]2[NiII(pdtc)2] (pdtc ) pyridine-2,6-(monothiocarboxy-
late)(2-)), with NiN4S2 coordination sphere.11b It is worth noting
here that the NiII-Npy bond lengths for the square planar NiIIN4
complexes with pyridine amide ligands span in the range 1.944-
1.960 Å.17a-c Due to the strong donor capacity of deprotonated
amide N donors providing four negative charges on the
equatorial plane, it is expected to have shorter Ni-Namide bond
lengths. The opposite effect observed here is the result of steric
predominance over electronic effect, as observed for Fe(III) and
Co(III) complexes of L(2-).23 A similar situation with excep-
tionally short NiII-Npy bond distance of 1.805 Å has recently
been reported for a square planar pyridyl diamide complex with
NiIIN3O coordination sphere.17d
(b) [NiL2]‚0.75H2O (3). A view of the metal environment
in 3 is shown in Figure 2. To the best of our knowledge, it is
the first crystallographically characterized neutral six-coordinate
nickel(IV) complex with a deprotonated amide ligand. Compar-
ing the molecular structures of 1 and 3 it is revealed that the
coordination environment around nickel centers is closely
similar. The average Ni-Npy distances 1.994 Å for 1 and 1.846
Å for 3 and Ni-Namide distances 2.131 Å for 1 and 1.946 Å for
3 are clearly distinguishable. The Ni-Npy bond length of 3 can
be compared with Ni(dapo)2 having NiIVN6 coordination sphere.
The bond length for the present complex is shorter by ∼0.13
Å.4b This significant difference in bonding parameters adds
credence to our +4 oxidation state assignment of nickel in 3.
As that in 1, though in each ligand the two N-phenyl rings and
different C-N and C-O bond lengths. In resonance form I,
C-N has single-bond character, C-O has double-bond char-
acter, and the Ni-Namide bond is expected to be relatively short.
Conversely, in resonance form II, C-N has double bond
character, C-O single bond character and the Ni-Namide bond
is expected to be longer. These bonds can also be intermediate
between double and single bonds. Out of four Ni-Namide bonds,
one ligand gives distances of 1.942 and 1.949 Å and the other
ligand gives distances of 1.940 and 1.953 Å. The evidence for
the presence of a partial negative charge on the amide oxygen
O(3) (the one which gives rise to Ni-Namide bond length of
1.953 Å) 3 comes from the presence of a hydrogen-bonded water
molecule, in close proximity. The O(1w)‚‚‚O(3) distance of 2.96
(2) Å is typical of systems with strong hydrogen-bonding, O-H‚
‚‚O.30b
(30) (a) Machida, R.; Kimura, E.; Kushi, Y. Inorg. Chem. 1986, 25, 3461.
(b) McLachlan, G. A.; Brudenell, S. J.; Fallon, G. D.; Martin, R. L.;
Spiccia, L.; Tiekink, E. R. T. J. Chem. Soc., Dalton Trans. 1995, 439.

1392 Inorganic Chemistry, Vol. 38, No. 7, 1999
Patra and Mukherjee
Figure 4. EPR spectrum of a polycrystalline sample of 2 at 77 K.
have investigated its temperature-dependent magnetic suscep-
tibility behavior. The effective magnetic moment of 2 in the
solid-state at 300 K was 2.14 µB, attesting the presence of only
one unpaired electron with appreciable orbital contribution.31
The result followed the Curie-Weiss law. In MeCN solution
(300 K), µeff ) 2.16 µB.
To clarify the ambiguous situation between nickel(II)-
stabilized ligand radicals and authentic nickel(III) complexes,
EPR spectral behavior of 2 was investigated. Solid samples of
2 at 300 K exhibits predominantly an axial spectrum with g )
2.129 and g| ) 2.049. However, at 77 K the spectrum is cleanly
rhombic with three principal g values of 2.149, 2.115, and 2.034
(Figure 4). The average g value (gav ) 2.100) indicates that the
unpaired electron is associated primarily with the nickel ion. If
ligand oxidation had occurred, as is a possibility given the
electrochemical behavior of [MIIIL2]- (M ) Fe, Co),23 the
paramagnetic product would be a nitrogen- or carbon-centered
radical complex of nickel(II). Such species would be expected
to exhibit g values close to the spin-only value of 2.002. The
large orbital contribution to paramagnetism actually observed
suggests that the unpaired electron is located primarily on the
metal ion consistent with the trivalent oxidation state. The nature
of the observed nonaxial spectrum strongly suggests that the
rhombic field splits the perpendicular absorption into overlap-
ping gxx and gyy components, the corresponding parallel absorp-
tion being represented by gzz. Hence we can write “g|” ) 2.034
and “g ” ) 1/2(gxx + gyy) ) 2.132. The spectral characteristics
Figure 3. Electronic absorption spectra of [Et4N]2[NiL2]‚H2O (1) (top)
and [Et4N][NiL2]‚H2O (2) (solid line) and [NiL2]‚0.75H2O (3) (dashed
line) in MeCN solutions.
Absorption Spectra. The absorption spectra of 1-3 in
MeCN are displayed in Figure 3. It is the easiest means of their
characterization. The yellowish orange solution of the Ni(II)
complex 1 exhibits a crystal field transition at 854 nm, which
could be assigned as ν1 band (3A2g
f
3T2g in octahedral
parentage). This feature remains invariant to change in solvents
such as DMF, DMSO, and water. The reddish brown Ni(III)
complex 2 is characterized by its LMCT transition at 449 nm
with a shoulder at 636 nm. The spectrum of 3 is dominated by
the intense LMCT band at 480 nm with a shoulder at 730 nm,
which accounts for the violet color of the complex.
3+
Ground States of [NiL2]z. (a) [NiL2]-2. The effective
magnetic moment of 1 in MeCN solution is 3.09 µB (300 K),
corresponding to an S ) 1 state, with an appreciable orbital
contribution.31 The magnetism of its polycrystalline Et4N+ salt
was investigated over the temperature range 63-300 K to define
the spin-state and allow structural inferences. The compound
follows the Curie-Weiss law. The magnetic moment value of
3.12 µB at 300 K of polycrystalline 1 is in conformity with the
solution-state data.
is closely similar to that observed for Ni(bpyO2)3 (bpyO2 )
2,2′-bipyridine 1,1′-dioxide).32 The present behavior with g >
2
g| is consistent with a dz ground state in a tetragonally distorted
octahedral geometry with elongation of the axial bonds.1-3 This
is in sharp contrast to what we have observed (tetragonal
compression) in the X-ray structures of Ni(II) and Ni(IV)
complexes (vide supra). Thus on removal of an electron from
a Ni(II) complex 1 there is a structural change. In frozen MeCN
solution the spectrum is isotropic (g ) 2.113).
(c) [NiL2]. In CDCl3 solution complex 3 displays a clean 1H
NMR spectrum (Figure S1, Supporting Information) closely
(b) [NiL2]-. Because we were unable to grow single crystals
for this compound, structural information is lacking. Hence we
(31) Drago, R. S. In Physical Methods in Chemistry; Saunders: Philadel-
phia, 1977; Chapter 11, pp 411-435.
(32) Bhattacharya, S.; Mukherjee, R.; Chakravorty, A. Inorg. Chem. 1986,
25, 3448.

Multivalent Nickel Complexes
Inorganic Chemistry, Vol. 38, No. 7, 1999 1393
shift (270 mV) in the E1/2 value for the NiIII-NiII couple is
observed.33 It is worth noting here that, though a similar trend
was observed, the E1/2 value for the FeIII-FeII couple of [FeL2]-,
however, shifts more anodically. This investigation reveals that
the Ni(II) state of the complex [NiL2]2- is systematically better
stabilized due to enhanced solvation, as the dielectric constant
of the medium is increased. Interestingly, the NiIII-NiII reduc-
tion potentials correlate linearly with inverse of the dielectric
constant of the medium (Figure S2, Supporting Information),
in line with the free energy of solvation as predicted by the
Born equation.34 A similar plot was obtained for [FeIIIL2]-.23
Summary
The following are the principal findings and conclusions of
this investigation.
(1) To the best of our knowledge, this report documents the
first examples of synthesis and characterization of nickel
complexes in three oxidation states +2, +3, and +4 with a
common deprotonated bis-amide ligand.
(2) The X-ray structures of nickel(II) and nickel(IV) com-
plexes reveal that in both the cases the metal ion is coordinated
in tetragonally compressed geometry with four comparatively
longer Ni-Namide distances and two very short Ni-Npy bonds.
The two-electron oxidation of a Ni(II) complex 1 to a Ni(IV)
complex 3 is accompanied by an expected decrease in the Ni-N
bond lengths, as revealed by X-ray structural analysis.
(3) The Ni(II) complex 1 is paramagnetic with respect to two
Figure 5. Cyclic voltammogram (50 mV/s) of a 1.03 mM solution of
[Et4N]2[NiL2]‚H2O (1) at a glassy carbon electrode in acetonitrile (0.15
M in TBAP).
similar to that observed for [CoIIIL2]-,23 attesting its diamagnetic
ground state.
Redox Properties. (a) Electron-Transfer Reactions. To
investigate the extent of stabilization of the nickel(II) state
toward oxidation and whether a possibility corresponding to the
accessibility of higher oxidation states could be achieved, cyclic
voltammetric (CV) studies on 1 were performed.
1
unpaired electrons (S ) 1). The Ni(III) complex 2 has S ) /2
state, as determined by temperature-dependent magnetic sus-
ceptibility. EPR spectroscopy reveals that the unpaired electron
2
is in a dz orbital, suggesting tetragonally elongated geometry
CV scan of 1 in MeCN at a glassy carbon electrode shows
two consecutive quasireversible couples at E1/2 ) 0.05 and 0.51
V vs SCE, with ∆Ep values of 100 mV (Figure 5). The one-
electron nature of each redox response has been confirmed by
comparison of current height23 with the redox response of a
sample of [Et4N][FeL2]‚1.5H2O, under the same experimental
conditions. The pertinent redox couples are represented in the
following electron-transfer series (eq 1).
of the Ni(III) state. The Ni(IV) complex 3 is diamagnetic.
(4) Redox properties of these complexes unravel the attain-
ment of a three-member electron-transfer series with oxidation
states of nickel shuttling between +2, +3, and +4. This unique
property of clean redox transformations has allowed us to
synthesize genuine Ni(III) and Ni(IV) complexes. The E1/2
values for the NiIII-NiII redox process (0.05 V vs SCE) as well
as NiIV-NiIII process (0.51 V vs SCE) are apparently the lowest
for a NiN6 complex having deprotonated amide coordination.
(5) Due to their fascinating redox properties, easily synthe-
sizable high-valent nickel complexes 2 and 3, are ideally suited
to be used as one-electon and two-electron potent outer-sphere
oxidants, respectively.
-
-
[NiIVL2] y+e z [NiIIIL2]- y+e z [NiIIL2]2-
(1)
-e-
-e-
The observation of two clean well-separated oxidative redox
responses was the key to our choice of chemical oxidizing agents
in the successful isolation of nickel(III) as well as nickel(IV)
complexes 2 and 3.
CV studies on isolated nickel(III) and nickel(IV) complexes
2 and 3 exhibit identical behavior to that observed for complex
1. This documents the purity and authenticity of the isolated
nickel(III) and nickel(IV) complexes. The observed stabilizing
potential of L(2-) toward Ni(III) and Ni(IV) states must be
due to the presence of four deprotonated amide nitrogen
coordination.
Within NiN6 family of complexes with amide ligands, to the
best of our knowledge, the present NiIII-NiII potential is the
lowest.1-3 It is worth mentioning here that the nickel(IV)
complex of Chakravorty et al. undergoes two successive one-
electron reductions in MeCN, with the NiIII/II potential at -0.20
V and NiIV/NiIII potential at 0.30 V vs SCE.1b
Acknowledgment. This work is supported by grants from
the Council of Scientific & Industrial Research (CSIR) and
Department of Science and Technology (DST), Government of
India, New Delhi. The financial assistance of the National X-ray
diffractometer facility by DST at this Department is gratefully
acknowledged.
Supporting Information Available: Experimental details and
crystallographic structure determination information for 1 and 3, in CIF
format (Table S1-S16), magnetic data for 1 and 2 (Tables S17 and
S18), 1H NMR spectrum of 3 in CDCl3 (Figure S1), and least-squares
plot of E1/2 (NiIII-NiII) of 1 vs reciprocal of dielectric constant of the
solvents (Figure S2). This material is available free of charge via the
Internet at http://pubs.acs.org.
IC980672E
(b) Solvent Dependency of the Redox Processes. Due to
its ready solubility in many solvents we investigated the redox
behavior of 1 as a function of solvent: MeCN, DMF, DMSO,
and water. In going from DMF to water, an appreciable anodic
(33) The shift in the NiIV-NiIII couple does not follow any trend: 0.66 V
(water), 0.58 V (DMSO), 0.51 V (MeCN), 0.58 V (DMF).
(34) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry for
Chemists; Wiley: New York, 1974; pp 186, 204-207.