Shuttling of nickel oxidation states in N4S2 coordination geometry versus donor strength of tridentate N2S donor ligands

Article abstract of DOI:10.1021/ic300606g

Seven bis-Ni^II complexes of a N₂S donor set ligand have been synthesized and examined for their ability to stabilize Ni ⁰, Ni^I, Ni^II and Ni^III oxidation states. Compounds 1-5 consist of mod

Full text of DOI:10.1021/ic300606g

Article
pubs.acs.org/IC
Shuttling of Nickel Oxidation States in N4S2 Coordination Geometry
versus Donor Strength of Tridentate N2S Donor Ligands
Sudip K. Chatterjee,† Suprakash Roy,† Suman Kumar Barman,‡ Ram Chandra Maji,†
Marilyn M. Olmstead,§ and Apurba K. Patra*,†
†Department of Chemistry, National Institute of Technology Durgapur, Mahatma Gandhi Avenue, Durgapur 713 209, India
‡Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
§Department of Chemistry, University of California, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: Seven bis-NiII complexes of a N2S donor set
ligand have been synthesized and examined for their ability to
stabilize Ni0, NiI, NiII and NiIII oxidation states. Compounds
1−5 consist of modifications of the pyridine ring of the
tridentate Schiff base ligand, 2-pyridyl-N-(2′-
methylthiophenyl)methyleneimine (XL1), where X = 6-H, 6-
Me, 6-p-ClPh, 6-Br, 5-Br; compound 6 is the reduced amine
form (L2); compound 7 is the amide analog (L3). The
compounds are perchlorate salts except for 7, which is neutral.
Complexes 1 and 3−7 have been structurally characterized.
Their coordination geometry is distorted octahedral. In the
case of 6, the tridentate ligand coordinates in a facial manner,
X
whereas the remaining complexes display meridional coordination. Due to substitution of the pyridine ring of L1, the Ni−Npy
distances for 1∼5 < 3 < 4 increase and UV−vis λmax values corresponding to the 3A2g(F)→3T2g(F) transition show an increasing
trend 1∼5 < 2 < 3 < 4. Cyclic voltammetry of 1−5 reveals two quasi-reversible reduction waves that correspond to NiII→NiI and
NiI→Ni0 reduction. The E1/2 for the NiII/NiI couple decreases as 1 > 2 > 3 > 4. Replacement of the central imine N donor in 1
by amine 6 or amide 7 N donors reveals that complex 6 in CH3CN exhibits an irreversible reductive response at Epc = −1.28 V,
Epa = +0.25 V vs saturated calomel electrode (SCE). In contrast, complex 7 shows a reversible oxidation wave at E1/2 = +0.84 V
(ΔEp = 60 mV) that corresponds to NiII→NiIII. The electrochemically generated NiIII species, [(L3)2NiIII]+ is stable, showing a
new UV−vis band at 470 nm. EPR measurements have also been carried out.
of NiII complexes with ligands containing such soft donor
INTRODUCTION
In nickel-containing enzymes, Ni can be either redox active or
■
atoms are known where the formation of a NiI species is
evident by chemical or electrochemical reduction.3,6 However,
inactive, and it is found in Sx, NxSy, or NxOy coordination
the main challenges are the tendency of the generated NiI
environments.1 In [Ni−Fe] hydrogenase, Ni oxidation states
species to disproportionate back to the NiII and Ni0 states5a,b
switch among +1, +2, and +3 during the reversible catalytic
and the formation of the NiII-ligand anion radical7 species
where the ligand system is more prone to be reduced than the
metal.
oxidation of dihydrogen.2 Therefore, model complexes for
which the stabilization of nickel in both lower and higher than
its normal +2 oxidation state required is of interest. Previous
work on model complexes of [Ni−Fe]hydrogenase, reported by
Mascharak and co-workers,3 revealed that the mentioned three
oxidation states of Ni are highly achievable in imine, pyridine
N, and thiolato S ligand donor sets. The strong σ-donor ability
of deprotonated amide4 and oxime5 ligands has successfully
been used to stabilize the higher oxidation states of nickel. For
example, the tridentate bis-amide ligand, 2,6-bis[N-(phenyl)-
carbamoyl]pyridine, forms NiII, NiIII, and NiIV oxidation
states,4b and electrosynthesis involving a thioether-azo-oxime
ligand5c yields a series of NiIIIN4S2 complexes. On the other
hand, the common softer donor atoms such as N of imine, S of
thioether/thiol, and P of phosphene have a σ-donor and a π-
acceptor property that stabilizes the NiI state. Several examples
Alkyl amine N are σ-donors, with no π-accepting properties,
and hence are not a good choice for stabilizating the NiI state
(high cathodic potential needed for NiII→NiI reduction).8
However, it has been noted that amine-N methylation
anodically shifts the reduction potential and also appreciably
increases the stability of electrochemically generated NiI
species.9 Thus, the ease of Ni’s oxidation state flipping and
its characteristic chemical/biological reactivity is strongly
influenced by the nature and number of various types of
donor atoms. For a clear understanding of the effect of a
Received: March 22, 2012
Published: June 29, 2012
© 2012 American Chemical Society
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CHCl3 and successively washed with distilled water, brine solution,
and finally distilled water. The organic layer separated out was then
dried over Na2SO4 for 1 h and filtered, and the solvent was completely
removed. The yellow oil kept at 4 °C overnight to get a yellow solid.
The solid was then recrystallized from diethyl ether that afforded
crystals of the respective Schiff base ligands.
2-Pyridyl-N-(2′-methylthiophenyl)methyleneimine, L1. Con-
densation of 1.0 g of pyridine-2-carboxaldehyde (9.33 mmol) with 1.30
g of 2-(methylthioaniline) (9.33 mmol) in 15 mL CH3OH yielded
1.80 g of L1 (yield: 84%).12
particular type of donor atom, it is desirable to vary the strength
of one donor atom while keeping the remaining donors fixed
and to examine the redox and spectral properties of these
complexes.
Series of NiN4S2 chromophores with common ligand
architectures are rare. To the best of our knowledge,
Chakravorty et al. have reported one such family of NiIIIN4S2
chromophores, stabilized in thioether-azo-oxime ligand envi-
ronment.5c In most of the NiN4S2 chromophores, the S ligation
comes from thiosemicarbazone and its derivatives,10 and fewer
examples of either thiol3a or thioether4a,5c,6g,h,11 S linkages to Ni
in combination with imine or amine or amide N donors are
known. Herein, we report the synthesis, structure, and
electrochemical and spectral properties of a series of NiIIN4S2
complexes. To vary the pyridine (py) N donor strength;
different ortho substituents on the pyridine ring, possessing +I/
−I inductive effects have been chosen. To probe the effect of
the middle N donor strength of the tridentate ligands the imine,
amine, and amide functional groups have been incorporated in
the ligand framework (Scheme 1). Redox and spectral
6 - M e t h y l - 2 - p y r i d y l - N - ( 2 ′ - m e t h y l t h i o p h e n y l ) -
methyleneimine, 6,MeL1. Condensation of 0.1 g of 6-methyl-
pyridine-2-carboxaldehyde (0.82 mmol) with 0.115 g of 2-(methyl-
thioaniline) (0.82 mmol) in 30 mL CH3OH yielded 0.16 g of 6,MeL1
(yield: 77%). Elemental analysis calcd for C14H14N2S (6,MeL1): C
69.39, H 5.82, N 11.56. Found: C 69.11, H 5.76, N 11.43; Selected IR
frequencies (KBr disk, cm−1): 3055(w), 2982(w), 2917(w), 1626(vs),
1588(vs), 1572(vs), 1466(vs), 1437(s), 1335(w), 1284(w), 1204(m),
1151(w), 1120(s), 1046(m), 988(m), 970(w), 861(w), 842(w),
803(s), 756(s), 732(s), 683(m), 644(w), 538(w), 521(w), 450(w).
1H NMR (300 MHz, CDCl3): δ 8.55 (1H, s, HCN), 8.15 (1H, d,
pyridine proton), 7.70 (1H, t, pyridine proton), 7.24−7.14 (4H, m,
phenyl ring proton), 7.08 (1H, d, pyridine ring proton), 2.62 (3H, s,
methyl group adjacent to pyridine N), 2.47 (3H, s, methyl group of S-
Me).
Scheme 1. Structural Representations of the Ligands
6-p-Chlorophenyl-2-pyridyl-N-(2′-methylthiophenyl)-
methyleneimine, 6,p‑ClPhL1. condensation of 0.50 g of pyridine-2-
carboxaldehyde (2.29 mmol) with 0.32 g of 2-(methylthioaniline)
(2.29 mmol) in 25 mL toluene yielded 0.71 g of 6,p‑ClPhL1 (yield: 90%).
Elemental analysis calcd for C19H16N2SCl: C 67.15, H 4.75, N 8.24.
Found: C 67.04, H 4.65, N 8.22. Selected IR frequencies (KBr disk,
cm−1): 3059(w), 2963(s), 2916(w), 1629(vs), 1588(vs), 1575(vs),
1466(s), 1450(vs), 1336(w), 1262(vs), 1205(m), 1181(m), 1037(w),
989(w), 972(w), 897(vw), 846(w), 803(m), 742(m), 696(m), 642(w),
1
571(vw), 554(vw), 450(vw). H NMR (300 MHz, CDCl3): δ 8.65
(1H, s, HCN), 8.29 (1H, d, pyridine proton), 8.01 (2H, d, pyridine/
p-ClPh proton), 7.88 (1H, t, pyridine ring proton), 7.78 (1H, d, p-
ClPh proton), 7.47 (2H, d, p-ClPh proton), 7.25−7.10 (4H, m, phenyl
ring proton), 2.49 (3H, s, methyl group of S-Me).
properties of these complexes clearly revealed that the pyridine,
imine N, and thioether S donor set may be able to stabilize the
NiI state. The electron withdrawing substituent (e.g., −Br) near
the pyridine N donor decreases its donor strength and shifts the
NiII→NiI reduction potential by 200 mV more anodic than the
unsubstituted one. It is also reported that the amide N, even
with the thioether S donor, can stabilize the NiIII state, whereas
the amine N and thioether S together only favor the NiII state.
6 - B r o m o - 2 - p y r i d y l - N - ( 2 ′ - m e t h y l t h i o p h e n y l ) -
methyleneimine, 6,BrL1. Condensation of 0.15 g of 6-bromo-
pyridine-2-carboxaldehyde (0.806 mmol) with 0.112 g of 2-
(methylthioaniline) (0.804 mmol) in 10 mL CH3OH yielded 0.21 g
of 6,BrL1 (yield: 85%). Elemental analysis calcd for C13H11N2SBr
(
6,BrL1): C 50.82, H 3.61, N 9.12. Found: C 50.38, H 3.57, N 9.01.
Selected IR frequencies (KBr disk, cm−1): 3067(w), 2981(w),
2917(w), 2858(vw), 1624(vs), 1576(s), 1555(s), 1465(m), 1436(s),
1337(w), 1265(w), 1202(w), 1156(m), 1122(m), 1077(m), 984(m),
975(w), 896(m), 841(w), 804(s), 761(s), 731(vs), 708(w), 673(w),
637(w), 548(w), 505(w), 452(w), 436(w). 1H NMR (300 MHz,
CDCl3): δ 8.53 (1H, s, HCN), 8.31 (1H, d, pyridine proton), 7.67−
7.96 (1H, m, pyridine proton), 7.58 (1H, d, pyridine proton), 7.28−
7.07 (4H, m, phenyl ring proton), 2.48 (3H, s, methyl group of S-Me).
5 - B r o m o - 2 - p y r i d y l - N - ( 2 ′ - m e t h y l t h i o p h e n y l ) -
methyleneimine, 5,BrL1. Condensation of 0.1 g of 5-bromo-pyridine-
2-carboxaldehyde (0.53 mmol) with 0.075 g of 2-(methylthioaniline)
(0.53 mmol) in 10 mL CH3OH yielded 0.122 g of 5,BrL1 (yield: 75%).
Elemental analysis calcd for C13H11N2SBr (5,BrL1): C 50.82, H 3.61, N
9.12. Found: C 50.68, H 3.55, N 9.08. Selected IR frequencies (KBr
disk, cm−1): 3056(w), 2984(w), 2917(w), 1623(vs), 1570(s),
1471(vs), 1440(s), 1366(m), 1350(m), 1260(m), 1192(w), 1083(s),
1006(s), 957(w), 880(m), 831(s), 757(s), 738(s), 715(m), 666(w),
EXPERIMENTAL SECTION
■
Materials and Reagents. Pyridine-2-carboxaldehyde, pyridine-2-
carboxylic acid, 2-(methylthio)aniline, triphenylphosphite, sodium
hydride, [Ni(H2O)6](ClO4)2, sodium borohydride, 6-bromo-pyri-
dine-2-aldehyde, 6-methyl-pyridine-2-aldehyde, 6-p-chlorophenyl-pyr-
idine-2-aldehyde, 5-bromo-pyridine-2-aldehyde, sodium perchlorate
(NaClO4) were purchased from Aldrich Chemical Co. and used
without further purification. CH3CN, CH3OH, C2H5OH, CHCl3,
pyridine (py), and diethyl ether (C2H5)2O used either for
spectroscopic studies or for syntheses were purified and dried
following standard procedures prior to use.
Synthesis Safety Note. Transition metal perchlorates should be
handled cautiously as these are hazardous and explosive upon heating.
No such happening is encountered in the present study.
1
631(m), 552(w), 492(s), 416(m). H NMR (300 MHz, CDCl3): δ
8.77 (1H, s, pyridine proton adjacent to N), 8.53 (1H, s, HCN),
8.23 (1H, d, pyridine proton), 7.96 (1H, d, pyridine proton), 7.37
(1H, t, pyridine proton), 7.28−7.09 (4H, m, phenyl ring proton), 2.48
(3H, s, methyl group of S-Me).
2-Pyridyl-N-(2′-methylthiophenyl)methylamine, L2. In a
round-bottom flask, 0.5 g (2.19 mmol) of L1 was dissolved in 30
mL of freshly distilled CH3OH with development of a light yellow
General Method of Schiff Base Ligand Syntheses. To a stirred
solution of pyridine-2-carboxaldehyde or of its substituted derivative in
freshly distilled methanol or toluene was added a methanol/toluene
solution of 2-(methylthioaniline). The reaction mixture was then
refluxed under nitrogen for 6 h. After cooling the reaction mixture to
room temperature, the solvent was removed completely using a rotary
evaporator to yield a thick yellow oil. The oil was then dissolved in
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color. The flask was then placed on an ice-water bath. Solid NaBH4
(1.25 g, 33 mmol) was added pinchwise to the vigorously stirred
yellow solution of L1. The solution became hazy and the yellow color
faded to almost colorless with effervescence of hydrogen. After
complete addition of NaBH4, the reaction mixture was heated to 40 °C
on a water bath for 30 min and was stirred further at room
temperature for 3 h. The solution was again placed in an ice-bath and
30 mL of brine solution was added to it, giving a pale yellowish white
precipitate. Methanol was then rotary evaporated, and the reaction
mixture was extracted with ether (3 × 30 mL). The yellow ether layer
was then washed with distilled water, and finally, it was dried over solid
Na2SO4 and filtered. Solvent removal was yielded 0.39 g of L2 as pale
yellow oil (Yield: 77%). Elemental analysis calcd for C13H14N2S (L2):
C 67.79, H 6.13, N 12.16. Found: C 67.71, H 5.96, N 12.11. Selected
IR frequencies (KBr disk, cm−1): 3358(s, amine-NH), 3066(m),
3012(w), 2920(m), 1588(vs), 1570(vs), 1478(vs), 1448(vs), 1353(w),
1320(s), 1285(m), 1161(m), 1089(w), 1040(s), 994(m), 969(m),
829(vs), 752(vs), 617(w), 464(m), 402(m). 1H NMR (300 MHz,
CDCl3): δ 8.59 (1H, d, pyridine proton adjacent to N), 7.61 (1H, t,
pyridine proton), 7.41 (1H, d, pyridine proton), 7.32 (1H, d, phenyl
ring proton), 7.09−7.28 (2H, m, phenyl ring proton), 6.65 (1H, t,
pyridine ring proton), 6.53 (1H, d, pyridine ring proton), 5.88 (1H,
broad s, amine), 4.53 (2H, s, −CH2−N), 2.37 (3H, s, methyl group of
S-Me).
[Ni(H2O)6](ClO4)2 (0.10 mmol) in 15 mL of CH3CN. Crystals
suitable for X-ray diffraction were grown by diffusion of ether into a
concentrated CH3CN solution of 2 at 4 °C. Elemental analysis calcd
for C28H28N4O8S2Cl2Ni, 2: C 45.34, H 3.78, N 7.55. Found: C 45.24,
H 3.71, N 7.49. Selected IR frequencies (KBr disk, cm−1): 3081(w),
3009(w), 2923(w), 1595(s), 1478(s), 1463(w), 1382(m), 1254(m),
1235(w), 1089(vs), 1008(m), 955(w), 874(w), 803(m), 777(m),
741(m), 636(m), 624(s), 580(w), 532(m), 474(w). EI mass spectrum
m/z (%): 398.87 ([Ni(6,MeL1)(ClO4)]+, 15) 243.02 ((6,MeL1+H)+,
100). Absorption spectrum [λmax, nm (ε, M−1 cm−1), in MeCN] 237
sh (25700), 248 sh (23760), 320 sh (21700), 339 (25140), 354 sh
(23420), 372 sh (14130), 870 (40).
[Ni(6,p‑ClPhL1)2](ClO4)2, 3. This compound was prepared similarly
(0.38 g, yield 55%), using 0.05 g of 6,p‑ClPhL1 (0.14 mmol) and 0.027 g
of [Ni(H2O)6](ClO4)2 (0.07 mmol) in 15 mL of a mixed solvent of
CH3CN/CH2Cl2 (4:1 v/v). Crystals suitable for X-ray diffraction were
grown from a concentrated CH3CN solution of 3, using ether diffusion
at 4 °C. Elemental analysis calcd for C38H30N4O8S2Cl4Ni, 3: C 48.48,
H 3.21, N 5.99. Found: C 48.37, H 3.11, N 5.88. Selected IR
frequencies (KBr disk, cm−1): 3073(m), 3011(w), 2917(w), 1595(s),
1561(m), 1480(s), 1450(s), 1369(m), 1257(w), 1232(m), 1096(vs),
1007(s), 949(w), 846(m), 807(s), 762(s), 750(s), 647(m), 620(s),
571(w), 533(m), 476(m), 425(w). EI mass spectrum m/z (%): 367.85
(M2+/2, 100) for [(6,p‑ClPhL1)2Ni]2+, 338.91 ((6,p‑ClPhL1+H)+, 40).
Absorption spectrum [λmax, nm (ε, M−1 cm−1), in MeCN] 205 sh
(66070), 243 (44565), 270 sh (33730), 347 (18400), 382 sh (10530),
880 (42).
N-2-Methylthiophenyl-2′-pyridinecarboxamide, L3. To a
stirred pyridine solution (10 mL) of picolinic acid (1.0 g, 8.13
mmol) was added dropwise a pyridine solution (2 mL) of 2-
(methylthioaniline) (1.13 g, 8.13 mmol). The reaction mixture was
stirred at 70 °C in an oil bath for 30 min. To this solution was added
triphenylphosphite (2.78 g, 8.13 mmol) dropwise, and the temperature
was raised to 110 °C and stirred for 6 h. The pyridine was removed
using rotary evaporator, and the light yellow oil thus obtained was kept
in the freezer overnight to obtain a white solid. The white solid was
washed with distilled water and then dissolved with warm C2H5OH/
H2O mixture, slow evaporation of which afforded 1.73 g of white
needlelike crystals of L3 (Yield: 87%). Elemental analysis calcd for
C13H12N2SO (L3): C 63.91, H 4.95, N 11.47. Found: C 63.85, H 4.96,
N 11.38. Selected IR frequencies (KBr disk, cm−1): 3278(s, amide-
NH), 3066(m), 2920(m), 1682(vs), 1592(s), 1578(vs), 1514(vs),
1467(m), 1440(s), 1426(s), 1302(s), 1280(m), 1135(w), 1114(m),
1089(m), 1059(m), 998(m), 977(m), 942(m), 897(m), 814(m),
[Ni(6,BrL1)2](ClO4)2, 4. This compound was prepared similarly (0.56
g, yield 79%) using 0.05 g of 6,BrL1 (0.16 mmol), 0.03 g of
[Ni(H2O)6](ClO4)2 (0.08 mmol) in 15 mL of CH3CN. Crystals
suitable for X-ray diffraction were grown from concentrated CH3CN
solution of 4 using ether diffusion at 4 °C. Elemental analysis calcd for
C26H22N4O8S2Cl2Br2Ni, 4: C 35.81, H 2.52, N 6.42. Found: C 35.76,
H 2.39, N 6.36. Selected IR frequencies (KBr disk, cm−1): 3077(m),
3011(w), 2926(m), 1587(s), 1547(s), 1475(s), 1437(s), 1370(m),
1361(w), 1235(m), 1088(br,vs), 1003(m), 956(w), 860(w), 802(m),
769(m), 734(m), 679(w), 624(s), 583(w), 568(w), 530(m), 472(w),
427(w). EI mass spectrum m/z (%): 308.91 ((6,BrL1+H)+, 100).
Absorption spectrum [λmax, nm (ε, M−1 cm−1), in MeCN] 248
(29125), 264 sh (25550), 293 (18470), 333 (15560), 351 (15910),
370 sh (12700), 890 (30).
749(vs), 691(vs), 621(w), 588(m), 468(m), 402(w). H NMR (300
[Ni(5,BrL1)2](ClO4)2·CH3CN, 5·CH3CN. This compound was prepared
similarly (0.54 g, yield 76%), using 0.05 g of 5,BrL1 (0.16 mmol) and
0.03 g of [Ni(H2O)6](ClO4)2 (0.08 mmol) in 15 mL of CH3CN.
Crystals suitable for X-ray diffraction were grown from a concentrated
CH3CN solution of 5, using ether diffusion at 4 °C. Elemental analysis
calcd for C28H25N5O8S2Br2Cl2Ni, 5·CH3CN: C 36.83, H 2.73, N 7.66.
Found: C 36.77, H 2.67, N 7.61. Selected IR frequencies (KBr disk,
cm−1): 3056(w), 3020(m), 2925(m), 2251(w, νCN of CH3CN),
1583(m), 1545(s), 1480(s), 1433(m), 1386(w), 1358(m), 1305(w),
1265(w),1232(m), 1092(br,vs), 1031(m), 972(w), 954(w), 916(m),
835(m), 772(s), 744(w), 643(w), 655(w), 624(s), 545(w), 507(m),
472(w), 424(w). EI mass spectrum m/z (%): 335.57 (M2+/2, 25) for
1
MHz, CDCl3): δ 11.04 (1H, s, amide-NH), 8.68 (1H, d, pyridine ring
proton), 8.56 (1H, d, pyridine proton), 8.30 (1H, d, phenyl ring
proton), 7.9 (1H, t, pyridine ring proton), 7.52 (2H, m, phenyl
protons), 7.34 (1H, t, pyridine ring proton), 7.26 (1H, s, phenyl
proton), 2.46 (3H, s, methyl group of S−Me).
Synthesis of Complexes. [Ni(L1)2](ClO4)2, 1. To the stirred
yellow solution of ligand L1 (50 mg, 0.210 mmol) in 10 mL dry
CH3CN was added dropwise a 5 mL CH3CN solution of
[Ni(H2O)6](ClO4)2 (40 mg, 0.10 mmol). The color of the solution
changed to yellowish red and was stirred for 2 h. To this solution was
then layered 20 mL of dry diethyl ether and kept at 4 °C temperature
overnight. A yellowish red microcrystalline solid precipitated, was
filtered, and was washed with ether. The solid was then redissolved in
5 mL CH3CN. Ether diffusion to this solution yielded 68 mg of
yellowish red needle-shaped crystals of 1 after two days at −10 °C
(Yield: 87%). Elemental analysis calcd for C26H24N4O8S2Cl2Ni, 1: C
43.72, H 3.39, N 7.84. Found: C 43.68, H 3.30, N 7.79. Selected IR
frequencies (KBr disk, cm−1): 3071(w), 2963(m), 2926(w), 1596(s),
1483(m), 1443(w), 1370(w), 1310(w), 1261(m),1241(w), 1097-
(br,vs), 1019(m), 960(w), 951(w), 803(m), 787(m), 772(m), 742(m),
645(w), 622(m), 504(m), 429(w), 418(w). EI (electron ionization)
mass spectrum m/z (%): 257.02 (M2+/2, 95) for [Ni (L1)2]2+, 229.06
((L1+H)+, 100). Absorption spectrum [λmax, nm (ε, M−1 cm−1), in
MeCN] 234 sh (30290), 246 sh (26160), 318 sh (22170), 333
(23790), 347 sh (21910), 366 sh (13650), 520 sh (170), 803 (46), 940
sh (22).
[(5,BrL1)2Ni]2+, 308.64 ((5,BrL1+H)+, 100). Absorption spectrum [λmax
,
nm (ε, M−1 cm−1), in MeCN] 270 (21770), 330 sh (21460), 342
(23520), 357 (22770), 380 sh (14200), 805 (45).
[Ni(L2)2](ClO4)2, 6. Ligand L2 (0.05 g, 0.217 mmol) was taken in a
Schlenk flask, and to it, 7 mL of dry CH3CN was added. To this pale
yellow clear solution was added dropwise a CH3CN solution (3 mL)
of [Ni(H2O)6](ClO4)2 (0.04 g, 0.106 mmol) under nitrogen
atmosphere. The color of the solution changed immediately to light
brownish yellow, which was allowed to stir for 2 h. Dry ether (25 mL)
was layered to this solution and kept at 4 °C. After 3 days, light violet
block shaped crystals obtained were filtered, washed with ether and
dried under vacuum (0.56 g, yield 72%). Elemental analysis calcd for
C26H28N4O8S2Cl2Ni, 6: C 43.48, H 3.93, N 7.80. Found: C 43.24, H
3.79, N 7.71. Selected IR frequencies (KBr disk, cm−1): 3190(s, νNH),
3084(w), 2975(w), 2935(w), 1609(s, def νNH), 1571(w), 1461(s),
1435(s), 1287(m), 1116(vs), 1049(s), 996(w), 973(m), 959(m),
929(m), 888(w), 811(m), 767(s), 737(m), 721(w), 750(s), 652(w),
[Ni(6,MeL1)2](ClO4)2, 2. This compound was prepared similarly (0.59
g, yield 77%) to 1 using 0.05 g of 6,MeL1 (0.20 mmol) and 0.038 g of
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Table 1. Data Collection and Structure Refinement Parameters for Complexes 1, 3−7
1
3
4
5·CH3CN
6
7
C26H24N4O8S2Cl2Ni C38H30N4O8S2Cl4Ni C26H22N4O8S2Cl2Br2Ni C28H25N5O8S2Br2Cl2Ni C26H28N4O8S2Cl2Ni C26H22N4O2S2Ni
mol wt
cryst. system
color
714.22
934.28
872.03
913.08
718.20
545.30
orthorhombic
reddish yellow
P212121
8.5779(6)
10.2497(14)
33.302(4)
90.0
orthorhombic
reddish yellow
Pca21
triclinic
monoclinic
red
monoclinic
violet
monoclinic
dark brown
P21/c
dark green
space group
a (Å)
P1
̅
P21/n
P21/c
21.635(5)
7.7408(17)
23.877(5)
90.0
8.618(5)
10.792(5)
17.819(9)
83.576(11)
75.352(18)
73.700(8)
1537.5(14)
2
12.2520(12)
19.3495(19)
15.0922(15)
90.0
8.8510(15)
11.746(2)
16.841(3)
90.0
11.7107(13)
15.9712(17)
13.3606(14)
90.0
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
90.0
90.0
107.613(2)
90.0
120.627(7)
90.0
110.296(3)
90.0
90.0
90.0
2928(5)
4
3998.7(15)
4
3410.2(6)
4
1506.6(5)
2
2343.7(4)
4
d
calc(g cm−3
)
1.620
1.552
1.884
1.778
1.583
1.547
θ range (deg)
2.08−26.14
1.044
1.71−18.85
0.914
2.25−32.08
3.596
1.76−27.88
3.247
2.23−25.24
1.015
1.85−19.39
7.051
μ (mm−1
)
F(000)
1464
1908
868
1824
740
2378
refl./params.
unique refl.
9089/427
3619
14015/517
2451
14721/445
13203
42868/453
7215
7831/196
2365
12279
1539
a
R1 [I > 2σ(I)]
0.0887
0.0659
0.0458
0.0537
0.0381
0.0340
b
wR2 [I > 2σ]
0.2172
0.1567
0.1266
0.1316
0.0941
0.0668
GOF
1.032
1.075
1.10
1.143
1.064
1.038
−3
res. dens. (eÅ
)
1.036/−1.079
0.795/−0.412
1.790/−1.018
2
2.701/−1.811
0.787/−0.306
0.222/−0.239
a
b
R1 = ∑∥Fo | − |Fc∥/∑|Fo|, wR2 = { ∑ [w(Fo −Fc2)2]/∑w[(Fo )2]}1/2
2
638(w), 623(s), 590(w), 556(w), 491(m), 460(w), 424(m). EI mass
spectrum m/z: 517.10 for [(L2)2Ni2+−H+]+, 231.09 [(L2+H)]+.
Absorption spectrum [λmax, nm (ε, M−1 cm−1), in MeCN] 257
(16332), 302 sh (5116), 370 sh (575), 555 (13), 890 (14).
mesh working electrode was used and solute concentration was kept of
∼1 × 10−3 or ∼3 × 10−4 molar.
X-ray Diffraction Studies. Crystals of 1, 3, 4, and 5 were grown by
diffusion of dry diethyl ether into the concentrated CH3CN solution of
the respective complexes at low temperatures (−10 °C for 1 and 4 °C
for 3−5). Violet block-shaped crystals of 6 and the deep brown
crystals of 7 were obtained by layering ether on top of the CH3CN or
DMF solution of 6 and 7, respectively, at 4 °C. Single-crystal intensity
measurements for 1 were collected at room temperature, while those
for 3, 6, and 7 were collected at 120 K and for 4 and 5 at 90 K with a
Bruker Smart APEX II CCD area detector using Mo Kα radiation (λ =
0.71073 Å) with a graphite monochromator. The cell refinement,
indexing, and scaling of the data set were carried out using Apex2
program.13 All structures were solved by direct methods with
SHELXS, and refined by full-matrix least-squares based on F2 with
SHELXL14 whereas the other calculations were performed using
WinGX, Version 1.80.05.15 The crystal of 4 was a pseudomerohedral
twin. One of the two perchlorate anions in 1, 4, and 5 was found to be
disordered over two orientations. No such disorder of perchlorate
anions has been encountered for complex 3 and 6. The positions of
the C-bound H atoms were calculated assuming ideal geometry and
refined using a riding model. Figures showing displacement parameters
were created with ORTEP3.16 Crystal data for the complexes 1 and 3−
7 are summarized in Table 1. Additional crystallographic data and
refinement details are available in the Supporting Information.
[Ni(L3)2], 7. The ligand L3 (0.15 g, 0.615 mmol) was dissolved in 10
mL of dry DMF and to it was added solid NaH (0.017 g, 0.738 mmol),
while a light yellow solution was generated. To the resulting solution
was then added dropwise a DMF solution (5 mL) of [Ni(H2O)6]-
(ClO4)2 (112 mg, 0.307 mmol). After complete addition, the solution
color changed to light green that, on warming at ∼60 °C in a water
bath for 30 min, changed to clear yellow solution. It was stirred for 2 h
at room temperature. The solution was concentrated to half of its
volume and was layered with ether and kept at 4 °C. After two days,
dark brown needle shaped crystals obtained were filtered off and
washed with ether and vacuum-dried (0.14 g, yield 85%). Elemental
analysis calcd for C26H22N4O2S2Ni, 7: C 57.27, H 4.07, N 10.27.
Found: C 57.20, H 3.99, N 10.22. Selected IR frequencies (KBr disk,
cm−1): 3057(w), 3001(w), 2931(w), 1611(vs, νCO), 1589(vs),
1574(vs), 1561(vs), 1465(vs), 1432(s), 1354(vs), 1289(s), 1265(m),
1089(w), 1045(m), 1023(w), 926(s), 822(m), 757(s), 695(m),
644(m), 513(m), 476(m), 458(w), 436(w), 406(w). EI mass spectrum
m/z (%): 545.12 (M+H)+, 100) for [{(L3)2Ni} + H]+, 245.10
(L3+H)+, 30). Absorption spectrum [λmax, nm (ε, M−1 cm−1), in
CH2Cl2] 319 (9715), 346 (10964), 380 sh (5220), 560 (27), 665 sh
(14), 851 (70).
Physical Measurements. The Fourier transform infrared spectra of
the ligand and the complexes were recorded on a Thermo Nicolet iS10
spectrometer using KBr pellets in the range 4000−400 cm−1. The
electronic spectra were recorded on an Agilent 8453 diode array
spectrophotometer. Elemental analyses were carried out on a Perkin-
Elmer 2400 series-II CHNS analyzer. Electron paramagnetic resonance
(EPR) spectra of the electrochemically generated NiIII and Ni(I)
species were obtained in CH3CN using a Bruker-EMX-1444 EPR
spectrometer at 77 K. Mass spectra were recorded on Micromass Q-
Tof micro. Solution conductivity was measured using a CHEMILINE
conductivity meter, CL220, 1H NMR spectra were recorded on Bruker
DPX-300. Redox potentials were measured using CHI 1120A
potentiometer. For constant potential electrolysis experiment platinum
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Condensation of
pyridine-2-carboxaldehyde or of its substituted forms with an
equimolar amount of 2-(methylthioaniline) in CH3OH or
toluene produced the ligands XL1 (Scheme 1) in high yield. All
the Schiff bases display an intense stretch in the range 1620−
1630 cm−1 and a medium intense stretch within the range 666−
696 cm−1 in their IR spectra17 that correspond to the νCN (of
imine) and νC−S (of thioether),respectively. Yellow thick oil of
L2 was obtained from the NaBH4 reduction of L1 in CH3OH
after usual workup. Presence of an intense peak at 3358 cm−1
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corresponding to νN−H is observed in the IR spectrum of L2.
The amide ligand L3 has been synthesized from the
condensation reaction of pyridine-2-carboxylic acid with 2-
(methylthioaniline) in pyridine at elevated temperature in
presence of triphenylphosphite. The presence of νN−H at 3278
cm−1 and νCO at 1682 cm−1 in the IR spectrum of L3 confirms
the amide functional group. To ensure the ligation of two
ligands to the NiII salt, [Ni(H2O)6](ClO4)2 was chosen as a
starting material. Dilute solutions of [Ni(H2O)6](ClO4)2 were
added dropwise to the ligand solutions (metal to ligand ratio
1:2) to yield the desired bis complexes.
2. Distorted octahedral coordination geometry for Ni is
observed in all cases. Two tridentate ligands L1 (Scheme 1)
X
Table 2. Selected Average Bond Distances (Å) of
[Ni(XL1)2](ClO4)2 Complexes 1, 3, 4, 5
1
3
4
5
X = 6-H X = 6-p-ClPh
X = 6-Br
X = 5-Br
Ni−N(py)
Ni−Ni(im)
Ni−S(thioether) 2.439(2)
2.083(7)
2.036(6)
2.110(12)
2.025(13)
2.475(4)
2.136(6)
2.041(3)
2.4229(13)
2.086(3)
2.030(3)
2.4383(11)
The red-shifted νCN and νC−S of the corresponding ligands
in complexes 1−5, found in the ranges 1583−1596 cm−1 and
636−647 cm−1, respectively, indicate imine N and thioether S
coordination to NiII . Complex 5, in addition exhibits a weak
stretch at 2251 cm−1, corresponds to νC≡N of a CH3CN
molecule present as a solvent of crystallization. The red-shifted
νN−H at 3190 cm−1 (for free L2 3358 cm−1) observed in the IR
spectrum of 6 indicates the amine N coordination. The IR
spectra of 1−6 feature an intense broad band at ∼1100 cm−1
that indicates the presence of perchlorate as counteranion.
Deprotonation of the amide proton of L3 using a strong base
like NaH in DMF and reaction with [Ni(H2O)6](ClO4)2
produced 7, the IR spectrum of which displays an intense
and red-shifted (νCO = 1682 cm−1 for L3) stretch at 1611 cm−1,
corresponding to νCO. Conductivity measurement of 7 in DMF
(Λ = ∼8 Ω Mol−1 cm−1) reveals its electroneutrality. The
absence of νN−H and the presence of red-shifted νCO strongly
support the amidato N− coordination to NiII. Solution
conductivity of 1−6 in CH3CN corroborates their 1:2
electrolytic behavior (Λ ranges from 220 to 245 Ω Mol−1
cm−1).18 Mass spectral results and microanalytical data, in
addition to the information of 1−7, further support their
formulations as stated.
each consisting of a pyridine N, an imine N and a thioether S
donor atom are arranged in a meridional fashion to NiII. The
Ni−Npy, Ni−Nimine, and Ni−S distances vary according to the
average Ni−Npy, Ni−Nimine, and Ni−S19,20 distances tabulated
in Table 2.
Scrutiny of the structural data reveal that the average Ni−Npy
distances follow the trend 1∼5 < 3 < 4 as a consequence a
X
weaker Npy donor strength of the respective L1 ligand.
[Ni(L2)2](ClO4)2, 6. An ORTEP diagram of the cationic part
of 6 with the atom labeling scheme is illustrated in Figure 2.
Structures of the Complexes: [Ni(XL1)2](ClO4)2 (1 and 3−
5). A perspective view of the cationic part of a representative
complex, [Ni(L1)2](ClO4)2, 1, with its atom labeling scheme is
shown in Figure 1, and figures for the other complexes, 3−5,
are given in the Supporting Information (Figures S1−S3).
Selected average bond distances and angles are listed in Table
Figure 2. Thermal ellipsoid (probability level 50%) plot of
[Ni(L2)2]2+ (cation 6) with the atom labeling scheme. H atoms are
omitted for the sake of clarity.
The structure has crystallographic inversion symmetry. The
secondary amine N apparently gives the ligand greater flexibility
than the imine N and allows for facial coordination of the two
L2 ligands. The Ni−N(am) distance is 2.082(2) Å, 0.046 Å
longer than the average Ni−N(im) distance of 1 and is
comparable to the Ni−N(am) distances of other reported NiII
complexes of amino-pyridine ligands.11a−e The Ni−N(py)
distance is 2.070(2) Å, and the Ni−S(thioether) distance is
2.4422(7) Å. The amine proton is hydrogen bonded to one of
the perchlorate oxygen atoms with the N2...O4 distance of
2.896(3) Å and N2−H2A...O4 angle of 177.0°.
[Ni(L3)2], 7. A perspective view of 7 with labeled atoms is
shown in Figure 3. Two L3 ligands are meridionally
coordinated via their pyridine N, amidato N, and thioether S
donors. The average Ni−Npy, Ni−Namide, and Ni−S distances
are 2.059(4) Å, 2.016(4) Å, and 2.4442(15) Å, respectively,
Figure 1. Thermal ellipsoid (probability level 50%) plot of
[Ni(L1)2]2+ (cation 1) with the atom labeling scheme. H atoms are
omitted for the sake of clarity.
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Table 3. Electronic Spectral Data of NiII Complexes, in
CH3CN at 298 K
compd
λmax, nm (ε M−1 cm−1
)
1
2
3
4
5
234 sh (30290), 246 sh (26160), 318 sh (22170), 333 (23790), 347
sh (21910), 366 sh (13650), 520 sh (166), 803 (46), 940 sh (22)
237 sh (25700), 248 sh (23760), 320 sh (21700), 339 (25140), 354
sh (23420), 372 sh (14130), 870 (40)
205 sh (66070), 243 (44565), 270 sh (33730), 347 (18400), 382 sh
(10530), 880 (42)
248 (29125), 264 sh (25550), 293 (18470), 333 (15560), 351
(15910), 370 sh (12700), 890 (30)
270 (21770), 330 sh (21460), 342 (23520), 357 (22770), 380 sh
(14200), 805 (45)
6
7
257 (16332), 302 sh (5116), 370 sh (575), 555 (13), 890 (14)
320 (10244), 341 (11060), 560 sh (27), 665 sh (14), 851 (65)
a
[319 (9715), 346 (10964), 560 sh (30), 665 sh (14), 851 (70)]
a
L1
253 (17650), 266 sh (17200), 365 (3870)
a
Measured in CH2Cl2
Figure 3. Thermal ellipsoid (probability level 50%) plot of [(L3)2Ni],
7, with the atom labeling scheme. H atoms are omitted for clarity.
for 1−4, respectively), and indicates weakening of the ligand
field strength, which is strongly supported from an increasing
trend of the average Ni−Npy distances (see Table 2). As the
average Ni−Npy distances in 1 and 5 are virtually identical a
very close ν1 value for these two compounds is expected and
observed (for 5, ν1= 12422 cm−1). Ligand 6,MeL1 with an
electron donating -Me substituent, ortho to the pyridine N is
expected to have better Npy donor strength than the
unsubstituted L1 and in consequence a higher ν1 value for 2
than 1. However, for 2, a 459 cm−1 lower value of ν1 than 1 is
observed suggests not only the electronic but also a steric factor
is operating for such lowering of ν1 value. This lowering of ν1
will be more prominent for a bulky −I substituent (−Br in 4)
than a +I substituent (−Me in 2) one, attached ortho to Npy.
Other higher energy transitions (see Table 4) are due to the
intra-ligand n→π* and π→π* charge transfer transitions.12,23
The spectra of 6 and 7 feature two distinct, well separated
(more than 250 nm), weak lower energy bands [λmax (ε in M−1
cm−1) for 6, 890 nm (57) and 555 nm (46); for 7, 851 nm (69)
and 560 nm (27)] correspond to ν1 and ν2 transitions,
respectively. As these bands are well separated, a little mixing of
these two transitions are expected that will provide a more
accurate value of the Racah parameter B24 and the
nephelauxetic ratio β,25 which measures the degree of covalency
for a particular complex. The β values for 6 and 7 are 0.84 and
0.64, respectively. The lower β value in 7 is due to the planarity
and conjugation of the pyridine and phenyl rings of the L3
ligand, unlike ligand L2 in 6.
comparable to those of reported N4S2NiII complexes of ligands
having similar types of donor atoms.4a,k The average Ni−
N(amide) distance is 0.02 Å and 0.066 Å shorter than the
average Ni−N(im) and Ni−N(am) distances found in 1 and 6
and indicates stronger donation of amidato N− than either the
imine or amine N donor. The N2−C6 and C6−O1 distances of
1.351(7) Å and 1.236(6) Å, respectively, support the
delocalization of the negative charge on deprotonated amidato
N atom over the three atoms (N2, C6, and O1) of the amide
functional group. Similar observations have previously been
reported.4b
Electronic Spectra. The electronic spectra taken in
CH3CN of complexes 1, 6, and 7 are shown in Figure 4, and
Figure 4. Electronic absorption spectra of [Ni(L1)2](ClO4)2, 1 (blue),
[Ni(L2)2](ClO4)2, 6 (pink), and [Ni(L3)2], 7 (orange) in CH3CN.
Redox Properties. To investigate the susceptibility toward
oxidation or reduction of the nickel(II) center in complexes 1−
7, the cyclic voltammograms (CV) were recorded for all the
complexes in CH3CN using same cell set up. A three electrode
cell set-up such as platinum, saturated calomel electrode (SCE),
and a platinum wire as a working, reference, and auxiliary
electrodes, respectively, has been used for measurement of the
potentials. Results are summarized in Table 4. Each of the
complexes 1−5 shows two quasi-reversible one electron
reduction waves correspond to NiII→NiI and subsequent
NiI→Ni0 transformation (Supporting Information, Figure S5).
Schiff base ligands, XL1, have pyridine, imine N, and thioether S
donor sites, all of which has a π-acceptance property; therefore,
they are expected to stabilize the lower oxidation states of
nickel, such as NiI and Ni0 states.
the spectral data of 1−7 are tabulated in Table 3. The spectra of
1−5 exhibit similar patterns in the 200−1100 nm region (see
Supporting Information, Figure S4). In the visible range,
compounds 1−5 display a broad and weak lower energy band
in the range 800−900 nm (for 1−5 this band are at 803, 870,
880, 890, and 805 nm respectively), attributable to
3A2 g(F)→3T2 g(F) transition in case of an octahedral NiII
parentage.21 Descending into a lower symmetry causes splitting
of both 3T2g(F) and 3T1g(F) terms due to spin−orbit coupling;
therefore, the lowest energy band, ν1, can be treated as an
envelope of several overlapped transitions, hence broad in
nature.22 The ν1 value linearly decreases for complexes 1→4
(ν1 = 12453 cm−1, 11494 cm−1, 11363 cm−1, and 11235 cm−1
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a
Table 4. Electrochemical Data for the Complexes 1−7 and Other Selected NiN4S2 Complexes
b
E1/2,V (ΔEp, mV)
c
NiI/Ni0
NiII/NiI or NiII/Ni0
NiII/NiIII
ref
1
−0.84 (131)
−0.88 (90)
−0.79 (72)
−0.78 (91)
−0.77 (102)
−0.65 (74)
−0.60 (91)
−0.54(85)
−0.45 (104)
−0.57 (97)
this work
this work
this work
this work
this work
this work
this work
5c
2
3
4
5
c
6
−1.28
7
+0.84 (60)
+0.45 (80)
+0.46(90)
+0.43 (80)
+0.45 (80)
[NiIII(Ph2L)]PF6
[NiIII(Ph3L)]PF6
[NiIII(Np2L)]PF6
[NiIII(Np3L)]PF6
[NiII(Atep)2](ClO4)2
[NiII(L4)2](ClO4)2
[NiII(dtdida)2](ClO4)2
[NiII(bpzctb)]
[NiII(bpctb)]
[NiII(SEtaaiNEt)2]2+
5c
5c
5c
d
−1.35
11b
e
−1.23 (140)
11d
+1.17 (70)
+1.04 (120)
+0.84 (200)
+0.80 (90)
11j
4a
4k, a
11k
a
b
Potentials are vs SCE (Fc/Fc+ couple in CH3CN, E1/2 = 0.43 V), scan rate 50 mV/s, supporting electrolyte, n-Bu4NClO4 (0.1 M). ΔEp = Epa − Epc.
c
d
e
Epc of NiII/Ni0. Epc of NiII/Ni0 vs Ag+. Potential is vs Fc/Fc+.
The negative potentials of 1 are more anodic than the
potential needed to reduce the ligand L1 (Epc = −1.72 V see
Supporting Information, Figure S6) in CH3CN. X-ray structural
(longer Ni−Npy distance) and electronic spectral (red-shifted
ν1 value) data of the complexes 1−5 revealed a decreasing
trend of Npy donor strength such as 1∼5 > 2 > 3 > 4 (vide
supra), in consequence, an easy NiII→NiI reduction; that is, a
more anodic E1/2 value for NiII/NiI couple is expected and
observed, shown in Figure 5. A 200 mV anodic shift of the E1/2
ligand(s) containing soft donors, so they can adopt the required
geometry needed to stabilize the NiI and Ni0 states.6a Quite in
contrast, in the present study, the donor sites of XL1 are
coplanar, restrained to occupy the fixed geometric sites and not
flexible enough to adopt Td geometry, required for stabilizing
Ni0. To the best of our knowledge, with planar ligand system
these complexes comprise the first example of a series of
NiN4S2 complexes where quasi-reversible redox responses
correspond to NiII→NiI and NiI→Ni0 transformations are
evident.
To clarify whether the first reduction is of metal or ligand
centered that will generate either a NiI species or a NiII-(L1•−)
species respectively, the controlled potential electrolysis at an
applied potential of −0.79 V and the EPR spectral measure-
ment of the electro-generated species in CH3CN at 77 K were
performed for a representative complex 1, associated with a low
ΔEp value (more toward reversible) of 74 mV. The EPR
spectrum of the electrochemically generated species displays a
rhombic anisotropic signal with three principal g values at
2.059, 2.010, and 1.915 (shown in Figure 6); those are far from
2.002 expected for a ligand radical anion indicating that the
unpaired electron primarily resides in the d orbital of the NiI
Figure 5. Cyclic voltammograms of 1 (blue), 2 (green), 3 (red), and 4
(black) in CH3CN containing 0.1 M N(n-Bu)4ClO4 as a supporting
electrolyte at 298 K at a platinum working electrode at a scan rate 50
mV s−1 using SCE as reference electrode.
value for NiII/NiI couple is evident, going from 1 (E1/2 = −0.65
V vs SCE) to 4 (E1/2 = −0.45 V vs SCE). To the best of our
knowledge, other reported NiIIN4S2 complexes display either an
irreversible reduction or their reduction is not reported or the
reduction is of ligand centered in origin.11k Low NiII→ NiI
reduction potential is reported for NiII complexes with flexible
Figure 6. X-band EPR spectrum of electrochemically generated NiI
species (from electrolysis of a CH3CN solution of 1) at 77 K
(conditions: frequency, 9.475 GHz; modulation, 5G; power, 0.189
mW).
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ion, not on the ligand frame. Furthermore, a five line splitting of
the g = 2.01 peak is due to the super hyperfine coupling
(coupling constant A∥ of 10 × 10−4 cm−1) of the unpaired
N
electron spin on NiI to the nuclear spin of two nitrogen atoms,
possibly of the two trans imine N of L1.3d The electronic
spectrum of the electrolyzed solution features a lower energy
band at ∼970 nm (shown in Figure 7), red-shifted than that of
Figure 8. Cyclic voltammograms (a→d, repetitive scans) of 6 in
CH3CN containing 0.1 M N(n-Bu)4ClO4 as supporting electrolyte at
298 K at a platinum working electrode at a scan rate 50 mV s−1 using
SCE as reference electrode, showing first scan (a: blue trace) and
transformation to [(L2)2Ni(CH3CN)2](ClO4)2 (b, c, d: red traces).
Figure 7. Electronic absorption spectra of 1 (black) and of the
coulometrically reduced species, [(L1)2NiI]+ (red) and ligand
decoordinated Ni0 species (blue) containing 0.1 M N(n-Bu)4ClO4 at
298 K in CH3CN.
Ni0 reduction. Flexibility of L2 coordination in 6 is capable of
stabilizing Ni0 state as shown in Scheme 2. During reoxidation
of Ni0→NiII the solvent molecules (CH3CN) and dangling
pyridine N donors (−NH−CH2-py) of L2 compete to occupy
two empty sites to fulfill hexa-coordination, preferred for NiII
state (B→A + C in Scheme 2). Most possibly due to this
reason, a broad anodic reoxidation response (with ipa ≪ ipc; or
ipc = ipa of +0.11 V peak + ipa of +0.25 V peak, see blue trace in
Figure 8) at Epa = +0.11 V with a shoulder at +0.25 V is
observed, owing to the formation of two NiII species, A and C,
shown in Scheme 2. Successive scans between +0.6 V → −1.6
V anodically shifted the Epc of NiII →Ni0 transformation from
−1.28 to −1.01 V (Figure 8). The Epc of −1.01 V is responsible
for C→D transformation. The solvated species C is expected to
reduce more easily than A where L2 is fully coordinated. Similar
CV scan profile for a NiII complex, [Ni(Atep)2](ClO4)2 (where
Atep = [(2′-aminophenylthio)ethyl)pyridine]) in CH3CN is
reported.11b The newly developed response at +0.45 V, possibly
is due to the reoxidation of a more solvated NiII species than D,
generated during the time of repetitive reduction−oxidation
processes.
The CV of 7 in CH3CN displays a reversible oxidative
response at E1/2 = +0.84 V vs SCE (Epa = +0.87 V, Epc = +0.81
V and ΔEp = 60 mV) that has been shown in Figure 9. No
other redox response within the potential window of −1.6 V to
+1.6 V has been observed. The peak current function, ip/ν1/2 is
independent of the scan rates shown in the inset of Figure 9.
The value of ipa/ipc ≈ 1 clearly indicates the reversibility of the
NiII/NiIII couple. At least two other NiIIN4S2 complexes,
namely [NiII(bpctb)]4k and [NiII(bpzctb)]4a (where H2bpctb
=1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane
and H2bpzctb =1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-
dithiobutane; H are dissociable amide protons) are reported
where a closely similar donor environment surrounding NiII
ion, as in 7, are there. [NiII(bpctb)] exhibits a quasireversible
oxidation virtually at the same potential but with a much larger
ΔEp value (200 mV) than 7 (see Table 4). Therefore,
the parent complex 1. Other higher energy transitions [λmax, nm
(ε, M−1 cm−1) for NiI species: 309 sh (17020), 330 (17560),
348 sh (15700), 366 sh (10530), 970 (100)] are similar to
those of 1. However, this electrochemically generated NiI
species is not stable enough, highly sensitive to dioxygen and
revert back with time to NiII along with partial complex
decomposition products, most possibly, due to the dispropor-
tionation of 2NiI →NiII + [Ni0 + L1] (vide infra). We have
attempted to investigate the behavior of the electrochemically
reduced species correspond to the second reduction wave of
E1/2 = −0.84 V. The original reddish brown solution of 1
changes to pale yellow upon electro-reduction at −1.2 V, the
electronic spectrum of which is identical to that of the ligand
(Figure 7). Therefore, the equation of electron transfer reaction
X
is as NiII + 2e− → Ni0 + XL1. Because L1 is planar, it can not
provide the preferred Td geometry for Ni0 state, hence
detached from metal.
The CV scans of 6 in CH3CN have been displayed in Figure
8 and the plausible transformations followed by electron
transfer reactions have been shown in Scheme 2. The first scan
(blue trace) was started from the resting potential of −0.4 V
toward anodic that does not show any response up to +0.6 V.
The reverse scan from +0.6 V → −1.6 V features a strong
reductive response at Epc = −1.28 V (close to the E1/2 value of
NiI→Ni0 reduction of complexes 1−5) correspond to the
NiII→Ni0 reduction (Scheme 2: A→B transformation). A
broad, anodic stripping potential at Epa = +0.11 V with a
shoulder at +0.25 V is observed as a result of the Ni0→NiII
reoxidation process (blue trace in Figure 8), which was absent
when the fresh solution was scanned from −0.4 V→+0.6 V.
This response has only been seen when the cathodic scan up
to a high negative potential of −1.6 V is performed that
generates Ni0 species which will get reoxidized back. Large ΔEp
value firmly indicates that this process is irreversible and a
remarkable structural reorganization takes place during NiII→
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Scheme 2. Structures of the Complexes Involved in the Most Plausible Pathway of Repetitive Electron Transfer Reaction of 6 in
MeCN
Figure 10. Electronic absorption spectra of 7 (black) and of the
coulometrically generated oxidized species [(L3)2NiIII]+ (red)
containing 0.1 M N(n-Bu)4ClO4 at 298 K. The arrow indicates the
Y-scale of the peaks from dashed line.
Figure 9. Cyclic voltammograms (scan rate 50 mV/s) of 7 in CH3CN
containing 0.1 M N(n-Bu)4ClO4 as supporting electrolyte at 298 K at a
platinum working electrode at a scan rate 50 mV s−1 using SCE as
reference electrode.
poorly resolved super-hyper-fine, nine-line splitting of the g =
2.04 component, shown in Figure 11, firmly supports the
coupling of the unpaired nickel eg electron spin to the nuclear
spin of four coordinated N (2nI +1 = 9 lines, where I =1)
Mukherjee and co-workers have revisited the stability of the
electrochemically generated NiIII species, [NiIII(bpctb)]+ and
compared its stability and other electronic properties (UV−vis,
EPR) to their electro-oxidized NiIII species, [NiIII(bpzctb)]+
derived from the NiII precursor [NiII(bpzctb)] where a lower
ΔEp value of 120 mV is evident for the NiII/NiIII redox
couple.4a A much lower ΔEp value of 60 mV for the NiII/NiIII
couple of 7, therefore, tempted us to investigate the stability
and electronic properties of the electrochemically oxidized
species [(L3)2Ni]+ in CH3CN. Constant potential electrolysis
at an applied potential of +1.2 V generates a dark brown color
that is stable at 298 K for 2−3 days. The electronic spectrum of
electrochemically generated species, [(L3)2NiIII]+ (Figure 10)
features a new band at 470 nm [λmax, nm (ε, M−1 cm−1) for NiIII
species: 335 sh (9990), 470 (1560), 860 (130); see Supporting
Information Figure S7 for UV−vis traces taken during the
course of coulometric oxidation], as reported.4a The EPR
spectrum of [(L3)2NiIII]+ displays an anisotropic rhombic
signal with three principal g values at 2.140, 2.097, and 2.044,
which are far from g = 2.002, supporting strongly the metal
centered oxidation and hence its formulation as [(L3)2NiIII]+
and not as [(L3•+)(L3)NiII]+. The trend of g values such as g⊥
donor atoms of two L3 ligands. The A∥ value of ∼12 × 10−4
N
cm−1 found for this super-hyper-fine splitting is similar to the
N
A∥ value reported before for Ni(I) complexes of general
formulas [Ni(terpy)(SR)2(CO)]− (where terpy = terpyridine,
R= 2,4,6-(i-pr)3C6H2 or C6F5),3d however, a bit less than the
value reported for NiIIIN4S2 complexes, which is ∼28 × 10−4
cm−1.5c
CONCLUSIONS
■
A series of seven bis-NiII pseudo-octahedral complexes based
on the use of N2S donor tridentate ligands have been
synthesized. These include five pyridine-substituted imines,
one amine, and one amido central N atom variants. To the best
of our knowledge, complexes 1−5 are the first examples of a
series of NiIIN4S2 complexes that display quasi-reversible
responses for NiII→ NiI reduction. Electrochemically generated
[NiI(L1)2]+ species display a rhombic EPR signal (g values at
2.059, 2.011, and 1.998). Imine and pyridine N and thioether S
donors are thus seen to stabilize the NiI state. In the present
< g∥ indicates a dx −y ground state of NiIII ion. Furthermore, a
2
2
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Figure 11. X-band EPR spectrum of electrochemically generated NiIII
species (from electrolysis of a CH3CN solution of 7) at 77 K
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study we have outlined a correlation between donor strength of
the N2S donor set and the accessibility of different oxidation
states of Ni.
ASSOCIATED CONTENT
* Supporting Information
■
S
ORTEP of 3, 4, and 5, cyclic voltammograms of 1−5 showing
both reduction waves, electronic spectra of 1−5 taken in
CH3CN in the range 200−1100 nm region, CV of L1, UV−vis
traces during coulometric oxidation of 7. X-ray crystallographic
data (CCDC number: 865967, 865969, 865966, 865965,
865964, and 865968 for 1, 3, 4, 5, 6, and 7, respectively).
This material is available free of charge via the Internet at
http://pubs.acs.org.
(7) (a) Muresan, N.; Chlopek, K.; Weyhermuller, T.; Neese, F.;
̈
Wieghardt, K. Inorg. Chem. 2007, 46, 5327. (b) Kadish, K. M.;
Franzen, M. M.; Han, B. C.; Araullo-McAdams, C.; Sazou, D. J. Am.
Chem. Soc. 1991, 113, 512.
AUTHOR INFORMATION
Corresponding Author
*Email: apurba.patra.nitdgp@gmail.com.
■
(8) (a) Hartman, J. R.; Vachet, R. W.; Callahan, J. H. Inorg. Chim.
Acta 2000, 297, 79. (b) Phillip, A. T.; Casey, A. T.; Thompson, C. R.
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86.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Department of Science and
Technology (DST), Government of India, (SR/S1/IC-35/
2007) is gratefully acknowledged. S. Chatterjee acknowledges
the support of CSIR for a junior research fellowship. We
sincerely acknowledge the AvH Foundation, Germany, for the
equipment donation grant to procure the CHI-1120A
spectroelectrochemical analyzer.
Gonzalez
́ ́
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