Structural, spectroscopic, and electrochemical properties of a series of high-spin thiolatonickel(II) complexes

Article abstract of DOI:10.1021/ic701743z

A series of high-spin thiolatonickel(II) complexes, [PhTt ^tBu]Ni(SR) (PhTt^tBu = phenyltris((tert-butylthio)methyl) borate; 2, R = triphenylmethyl; 3, R = pentafluorophenyl; 4, R = phenyl), were synthesized via the reaction of [PhTt^tBu]-Ni(NO₃) (1) with thiols (RSH) in the presence of triethylamine. The [PhTt^tBu]Ni(SR) products were isolated and characterized by various physicochemical measurements including X-ray diffraction analyses. These thiolatonickel-(II) complexes have a distorted trigonal pyramidal geometry with somewhat different τ values: 0.80 and 0.90 for two crystalline phases of 2, 0.74 for 3, and 0.69 for 4, where τ is a normalized measure of pyramidalization (τ = 0 for tetrahedron, τ = 1 for trigonal pyramid). The electronic absorption spectra display characteristic sulfur-to-nickel(II) charge transfer (CT) bands at 532 nm (7500 M^-1 cm^-1) for 2, 510 nm (4800 M^-1 cm ^-1) for 3, and 569 nm (4100 M^-1 cm^-1) for 4. The cyclic voltammograms show a quasi-reversible redox couple at E_1/2 = -1.11 V for 2, and reversible redox couples at E_1/2 = -1.03 V for 3 and E_1/2 = -1.17 V for 4 (vs Fc⁺/Fc). Correlation between the τ value and the CT intensity was observed: the strong CT intensity results from the high τ value, which provides for strong orbital overlap (2 > 3 > 4). Additionally, the CT transition energy correlates with the reduction potential: both the CT transition energy and potential decrease in the order 3 > 2 > 4, consistent with the influence of decreasing electron withdrawing abilities, R = pentafluorophenyl > triphenylmethyl > phenyl. The three thiolatonickel complexes exhibit dramatically different thermal stabilities. Complex 4 is the least stable, undergoing decomposition to [κ²-PhBt^tBuSPh] Ni(η²-CH₂SBu^t) (5) via net exchange of Ni-SPh and B-CH₂SBu^t groups.

Full text of DOI:10.1021/ic701743z

Inorg. Chem. 2007, 46, 11308−11315
Structural, Spectroscopic, and Electrochemical Properties of a Series of
High-Spin Thiolatonickel(II) Complexes
Jaeheung Cho, Glenn P. A. Yap, and Charles G. Riordan*
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
Received September 5, 2007
A series of high-spin thiolatonickel(II) complexes, [PhTttBu]Ni(SR) (PhTttBu
)
phenyltris((tert-butylthio)methyl)borate;
2, R
) triphenylmethyl; 3, R ) pentafluorophenyl; 4, R )
phenyl), were synthesized via the reaction of [PhTttBu]-
Ni(NO3) (1) with thiols (RSH) in the presence of triethylamine. The [PhTttBu]Ni(SR) products were isolated and
characterized by various physicochemical measurements including X-ray diffraction analyses. These thiolatonickel-
(II) complexes have a distorted trigonal pyramidal geometry with somewhat different
τ values: 0.80 and 0.90 for
two crystalline phases of 2, 0.74 for 3, and 0.69 for 4, where is a normalized measure of pyramidalization (τ )
τ
0 for tetrahedron, τ ) 1 for trigonal pyramid). The electronic absorption spectra display characteristic sulfur-to-
1
1
nickel(II) charge transfer (CT) bands at 532 nm (7500 M- cm-1) for 2, 510 nm (4800 M- cm-1) for 3, and 569
nm (4100 M- cm-1) for 4. The cyclic voltammograms show a quasi-reversible redox couple at E1 2 ) −1.11 V for
1
/
2, and reversible redox couples at E1 2 ) −1.03 V for 3 and E1 2 ) −1.17 V for 4 (vs Fc+/Fc). Correlation between
/
/
the
τ value and the CT intensity was observed: the strong CT intensity results from the high τ value, which
provides for strong orbital overlap (2 > 3 > 4). Additionally, the CT transition energy correlates with the reduction
potential: both the CT transition energy and potential decrease in the order 3 > 2 > 4, consistent with the influence
of decreasing electron withdrawing abilities, R
) pentafluorophenyl > triphenylmethyl > phenyl. The three thiolatonickel
complexes exhibit dramatically different thermal stabilities. Complex 4 is the least stable, undergoing decomposition
2-PhBttBuSPh]Ni( 2-CH2SBut) (5) via net exchange of Ni CH2SBut groups.
to [κ η −SPh and B−
Introduction
intermediate capable of hydroxylation reactions.4 More recent
discoveries include nickel metalloproteins with diverse
functions including superoxide processing (superoxide dis-
mutase),5 hydrogen oxidation (hydrogenases),6 reductive
methane generation (methyl coenzyme M reductase),7 and
acetate synthesis (acetyl coenzyme A synthase).8 Our un-
derstanding of the fundamental role(s) of nickel-thiolate
linkages, particularly in high-spin states, in these contexts
is only beginning to emerge.8b
The chemistry of transition metal-thiolate complexes is
of fundamental importance in biology.1,2 A number of
metalloprotein active sites feature metal-thiolate linkages
as key elements seminal to their structure and function. While
thiolates exhibit a propensity to bridge between and among
metal sites, protein site isolation2 allows for a number of
mononuclear metal thiolates, with the blue copper proteins
archetypal. Indeed, the unusual bonding characteristic of the
Cu2+-thiolate, i.e., high covalency, is responsible for the
unique spectroscopic features of the sites and for providing
an efficient pathway for electron transfer.3 In a different
context, the axial thiolate ligand in cytochrome P450
engenders an electronic structure that favors an oxoiron(IV)
Preparative chemistry has played a central role in advanc-
ing understanding. Kitajima and co-workers synthesized the
first Cu2+-thiolate complexes outside of a protein matrix,
(4) Meunier, B.; Visser, S. P. d.; Shaik, S. Chem. ReV. 2004, 104, 3947-
3980.
(5) Barondeau, D. P.; Kassmann, C. J.; Bruns, C. K.; Tainer, J. A.; Getzoff,
E. D. Biochemistry 2004, 43, 8038-8047.
(6) Volbeda, A.; Frontecilla-Camps, J. C. Coord. Chem. ReV. 2005, 249,
1609-1619.
(7) Ragsdale, S. W. Chem. ReV. 2006, 106, 3317-3337.
(8) (a) Riordan, C. G. J. Biol. Inorg. Chem. 2004, 9, 509-510, and
associated minireviews. (b) Funk, T.; Gu, W.; Friedrich, S.; Wang,
H.; Gencic, S.; Grahame, D. A.; Cramer, S. P. J. Am. Chem. Soc.
2004, 126, 88-95.
* E-mail: riordan@udel.edu.
(1) Stiefel, E. I., Matsumoto, K., Eds. Transition Metal Sulfur Chemistry;
American Chemical Society: Washington, D.C., 1996.
(2) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96,
2239-2314.
(3) Solomon, E. I.; Szilagyi, R. K.; DeBeer George, S.; Basumallick, L.
Chem. ReV. 2004, 104, 419-458.
11308 Inorganic Chemistry, Vol. 46, No. 26, 2007
10.1021/ic701743z CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/15/2007

High-Spin Thiolatonickel(II) Complexes
by employing thiolates less prone to oxidation. [TpiPr2]Cu-
(SCPh3) and [TpiPr2]Cu(SC6F5) successfully replicate certain
structural and spectroscopic features of the blue copper active
sites.9 More recently, Tolman and co-workers prepared a
â-diketiminate supported Cu2+ model featuring both thiolate
and thioether donors.10 The complex displays perturbed
spectral features in accord with its distorted geometry.
Given the longstanding interest in thiolato-nickel chem-
istry,11 it is perhaps surprising that only recently have high-
spin, four-coordinate complexes possessing a single thiolate
donor been reported. Fujisawa and co-workers described the
complex [TpiPr2]Ni(SC6F5) as a member of a series of
isostructural transition metal complexes.12 Note that the C6F5
substituent is the same as that utilized by Kitajima and
Tolman due to its reluctance to undergo oxidation. Peters’
laboratory characterized a related complex supported by his
PhBP3 tripod, [PhBP3]Ni(SC6H4p-But).13 It is of interest that
these room-temperature stable complexes provide the op-
portunity to evaluate a single nickel-thiolate interaction in
high-spin complexes.
Our current efforts toward the investigation of thiolato-
nickel complexes have yielded a series of high-spin com-
plexes, [PhTttBu]Ni(SR) (R ) CPh3 for 2, C6F5 for 3, and
Ph for 4), where [PhTttBu] is phenyltris((tert-butylthio)-
methyl)borate, by the reaction of [PhTttBu]Ni(NO3) (1) with
thiols (RSH) and triethylamine. Specifically, we have evalu-
ated the effect of the thiolate substituents on the structural,
spectroscopic, and electrochemical properties and thermal
stability of the complexes. Further motivation for these
studies stem from our prior efforts to install strong σ-donor
ligands, e.g., alkyl, aryl, and hydride, into the [PhTttBu]Ni
fragment, strategies that generally led to reductive B-C
degradation14 without evidence for intermediates. Herein, we
have characterized one such intermediate, 4, and its rear-
ranged product, [κ2-PhBttBuSPh]Ni(η2-CH2SBut) (5).
purification. The syntheses of the thiolato-nickel(II) complexes
were all carried out either in an Ar glovebox or under N2 using
standard Schlenk techniques. Elemental analyses were performed
at Desert Analytics, Inc., Tucson, AZ.
Physical Measurements. UV-vis spectra were measured with
a HP 8453 diode array spectrometer. 1H NMR spectra were recorded
on a 400 MHz Bruker DRX spectrometer. Chemical shifts (δ) were
referenced to the residual proton peak in the deuterated solvent.
Solid-state magnetic moments were determined using a Johnson
Matthey magnetic susceptibility balance calibrated with mercury-
(II) tetrathiocyanatocobaltate(II).
Cyclic voltammetry (CV) experiments were performed on a BAS
50W electrochemical analyzer in tetrahydrofuran (THF) containing
0.1 M electrolyte (Bu4NPF6) and 3 mM sample. The cell was housed
in an Ar-filled Vacuum Atmospheres glovebox. A carbon working
electrode was polished with an alumina (0.06 µm) paste and then
rinsed with THF before use. The counter electrode was a Pt wire.
The Ag/Ag+ couple was used as a reference electrode, and the
potentials were determined using the ferrocenium/ferrocene (Fc+/
Fc) couple as an internal reference
[PhTttBu]Ni(NO3) (1). [PhTttBu]Tl (1.20 g, 2 mmol) was added
to a THF solution (100 mL) of Ni(NO3)2‚6H2O (0.58 g, 2 mmol).
The resulting solution was stirred for 3 h, affording a green solution
and a white precipitate. The reaction mixture was dried over Na2-
SO4 and filtered through Celite. The solvent was removed under
vacuum yielding 1 as a green solid, which was recrystallized from
pentane to afford crystals suitable for X-ray analysis. Crystalline
yield: 0.74 g (71%). 1H NMR (C6D6, 400 MHz): δ 17.4 (br,
(CH3)3C), 8.4 (br, (o-C6H5)B), 7.6 (t, (p-C6H5)B), 7.4 (br, (m-
C6H5)B). UV-vis (λmax (nm) (ꢀ (M-1 cm-1)) in toluene): 355
(2900), 403 (2000), 695 (100), 866 (54). µeff ) 3.28 µB. Anal. Calcd
for C21H38BNO3S3Ni: C, 48.67; H, 7.39; N, 2.70. Found: C, 48.53;
H, 7.24; N, 2.86.
[PhTttBu]Ni(SCPh3) (2). A mixture of 1 (0.052 g, 0.1 mmol)
and Ph3CSH (0.028 g, 0.1 mmol) was dissolved in 5 mL of acetone.
To this yellow-green solution was added triethylamine (14 µL, 0.1
mmol), producing a dark violet solution. The resulting solution was
left for a few days at -25 °C, giving 2 as dark red crystals suitable
1
for X-ray analysis. Crystalline yield: 0.027 g (37%). H NMR
Experimental Section
(C6D6, 400 MHz): δ 30.2 (br, (o-C6H5)C), 16.9 (br, (CH3)3C), 8.6
(br, (o-C6H5)B + (p-C6H5)C), 8.1 (br, (p-C6H5)B + (m-C6H5)C),
7.6 (br, (m-C6H5)B). UV-vis (λmax (nm) (ꢀ (M-1 cm-1)) in
toluene): 391 (2900), 532 (7500), 855 (400). µeff ) 3.06 µB. Anal.
Calcd for C40H53BS4Ni: C, 65.67; H, 7.30. Found: C, 65.43; H,
7.54.
Synthesis. [PhTttBu]Tl was synthesized according to the literature
method.15 [PhTttBu]Ni(NO3) was first prepared by Schebler.16 Other
reagents were purchased commercially and used without further
(9) (a) Kitajima, N.; Fujisawa, K.; Moro-oka, Y. J. Am. Chem. Soc. 1990,
112, 3210-3212. (b) Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-
oka, Y. J. Am. Chem. Soc. 1992, 114, 9232-9233. (c) Qiu, D.;
Kilpatrick, L.; Kitajima, N.; Spiro, T. G. J. Am. Chem. Soc. 1994,
116, 2585-2590. (d) Randall, D. W.; DeBeer George, S.; Hedman,
B.; Hodgson, K. O.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc.
2000, 122, 11620-11631.
[PhTttBu]Ni(SC6F5) (3). C6F5SH (20 µL, 0.1 mmol) was added
to a pentane solution of 1 (0.052 g, 0.1 mmol). To this light red
solution was added triethylamine (14 µL, 0.1 mmol), producing a
dark violet solution. The resulting solution was left for a few days
at -25 °C, giving 3 as dark red crystals suitable for X-ray analysis.
(10) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 2000, 122, 6331-
1
Crystalline yield: 0.035 g (53%). H NMR (C6D6, 400 MHz): δ
6332.
18.7 (br, (CH3)3C), 9.2 (br, (o-C6H5)B), 7.7 (br, (p-C6H5)B), 7.6
(br, (m-C6H5)B). UV-vis (λmax (nm) (ꢀ (M-1 cm-1)) in toluene):
390 (3200), 510 (4800), 820 (350). µeff ) 3.30 µB. Anal. Calcd for
C27H38BF5S4Ni: C, 49.48; H, 5.84. Found: C, 49.75; H, 6.10.
[PhTttBu]Ni(SPh) (4). PhSH (11 µL, 0.1 mmol) was added to a
pentane solution of 1 (0.052 g, 0.1 mmol) at -40 °C. To this
yellow-green solution was added triethylamine (14 µL, 0.1 mmol),
producing a dark purple solution. The resulting solution was left
for a few days at -25 °C, giving 4 as dark red crystals suitable for
X-ray analysis. Crystalline yield: 0.031 g (55%). 1H NMR (C6D6,
400 MHz): δ 32.2 (br, (m-C6H5)S), 17.0 (br, (CH3)3C), 8.9 (br,
(o-C6H5)B), 7.6 (br, (p-C6H5)B), 7.6 (br, (m-C6H5)B), -39.0 (br,
(11) Kruger, H. J.; Peng, G.; Holm, R. H. Inorg. Chem. 1991, 30, 734-
742.
(12) Matsunaga, Y.; Fujisawa, K.; Ibi, N.; Miyashita, Y.; Okamoto, K.
Inorg. Chem. 2005, 44, 325-335, and references therein.
(13) MacBeth, C. E.; Thomas, J. C.; Betley, T. A.; Peters, J. C. Inorg.
Chem. 2004, 43, 4645-4662.
(14) Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands,
L. M.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001,
123, 331-332.
(15) Schebler, P. J.; Riordan, C. G.; Guzei, I. A.; Rheingold, A. L. Inorg.
Chem. 1998, 37, 4754-4755.
(16) Schebler, P. J. The Synthesis and Coordination Chemistry of Ferro-
cenyltris((methylthio)methyl)borate and Phenyltris((tert-butylthio)-
methyl)borate. Dcotoral (Ph.D.) dissertation. University of Delaware,
Newark, DE, 2000.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11309

Cho et al.
Table 1. Crystallographic Data for Complexes 1-5
1
2â
3
4
5
formula
fw
cryst syst
space group
a, Å
C21H38BNNiO3S3
518.22
triclinic
C40H53BNiS4
731.58
monoclinic
P21
9.593(6)
14.124(7)
14.545(8)
90
99.73(1)
90
C27H38BF5NiS4
655.33
monoclinic
P21/n
13.093(4)
14.447(5)
16.340(5)
90
100.908(5)
90
C27H43B NiS4
565.37
triclinic
C27H43BNiS4
565.37
monoclinic
P21/n
10.858(6)
19.215(9)
14.368(8)
90
90.151(10)
90
2998(3)
4
P1h
P1h
10.134(4)
11.592(4)
12.326(5)
91.425(4)
111.515(4)
104.426(4)
1293.8(8)
2
9.724(4)
11.597(5)
13.026(6)
88.138(15)
87.704(15)
86.385(15)
1464.1(11)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
1942.4(18)
2
3034.9(17)
4
T, K
D
120
1.330
120
1.251
120
1.434
120
1.282
120
1.253
calcd, g/cm-1
2θ range, deg
µ (Mo KR), mm-1
R1, wR2
1.83 - 28.22
1.013
0.0255, 0.0676
2.02 - 25.00
0.741
0.0459, 0.1149
1.83 - 28.21
0.962
0.0338, 0.0839
1.57 - 28.33
0.962
0.0489, 0.0952
1.77- 28.12
0.940
0.0554, 0.1387
a Quantity minimized ) R(wF2) ) ∑[w(Fo - Fc )2]/∑[(wFo )2]1/2; R - ∑∆/∑(Fo), ∆ ) |(Fc - Fo)|.
2
2
2
Scheme 1
group P1h for 1 and 4 and P21 for 2r yielded chemically reasonable
and computationally stable results of refinement. Samples of 2r
deposited from solution as weakly diffracting crystals with diffrac-
tion maxima of 45° in 2θ. Crystallographic results for 2r are
contained in the Supporting Information. Samples of 2â, grown
under similar conditions, afforded superior quality diffraction data.
One of the thioether arms (C5-S1-C11-C12-C13-C14) in the
structure of 2â is disordered in two positions with refined site
occupancy of 60/40. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were
treated as idealized contributions. Structure factors are contained
in the SHELXTL 6.12 program library (Sheldrick, G., Bruker-AXS,
2001). The CIFs are available from the Cambridge Crystallographic
Data Center under the depositary numbers CCDC 659304-659309.
Scheme 2
Results and Discussion
Synthesis of High-Spin Thiolatonickel Complexes.
Synthetic procedures for a series of thiolato-nickel(II)
complexes are summarized in Scheme 1. The starting
material, [PhTttBu]Ni(NO3) (1), was prepared via the meta-
thetical reaction of [PhTttBu]Tl and Ni(NO3)2‚6H2O in THF.
The resulting yellow-green solid is stable to air and moisture
and freely soluble in a range of solvents, including pentane
and chlorinated hydrocarbons. The reaction of 1 with 1 equiv
each of Ph3CSH and triethylamine in acetone at room
temperature caused a color change to dark violet. From the
reaction mixture, [PhTttBu]Ni(SCPh3) (2) was isolated at a
temperature below -25 °C.
(o-C6H5)S), -45.7 (br, (p-C6H5)S). UV-vis (λmax (nm) (ꢀ (M-1
cm-1)) in toluene): 381 (2500), 429 (2500), 569 (4100), 825 (390).
µeff ) 3.34 µB. Anal. Calcd for C27H43BS4Ni: C, 57.36; H, 7.67.
Found: C, 57.13; H, 7.49.
[PhBttBuSPh]Ni(η2-CH2SBut) (5). Complex 4 (0.057 g, 0.1
mmol) was dissolved in 15 mL of pentane at room temperature
giving a dark purple solution, which was stirred for several hours
affording a brown solution. The resulting solution was left for a
few days at -25 °C, giving 5 as brown crystals suitable for X-ray
1
analysis. Crystalline yield: 0.026 g (45%). H NMR (C6D6, 400
[PhTttBu]Ni(SC6F5) (3) was obtained by a method similar
to that for 2 with the corresponding thiol, C6F5SH, in pentane.
MHz): δ 7.91 (d, (o-C6H5)B), 7.74 (d, (o-C6H5)S), 7.38 (t, (m-
C6H5)B), 7.24 (t, (p-C6H5)B), 6.98 (m, (m-C6H5)S + (p-C6H5)S),
2.57 (d, BCH2), 2.45 (s, BCH2), 2.33 (d, BCH2), 1.69 (d, NiCH2),
1.52 (d, NiCH2), 1.34 (s, (CH3)3), 1.28 (s, (CH3)3), 0.98 (s, (CH3)3).
Anal. Calcd for C27H43BS4Ni: C, 57.36; H, 7.67. Found: C, 56.92;
H, 7.63.
X-ray Crystallography. Crystals were selected, mounted with
viscous oil on glass fibers, and flash-cooled to the data collection
temperature. Unit cell parameters (Table 1) were obtained from 60
data frames, 0.3° ω, from three different sections of the Ewald
sphere. Two different crystalline phases of 2 (r and â) were
obtained and characterized. The systematic absences in the dif-
fraction data were consistent for the space groups P21 and P21/m
for 2r and uniquely consistent for P21/n for 3 and 5. No symmetry
higher than triclinic was observed for 1 and 4. Solution in the space
Figure 1. Thermal ellipsoid plot of [PhTttBu]Ni(NO3) (1) at the 50%
probability level. Hydrogen atoms are omitted for clarity.
11310 Inorganic Chemistry, Vol. 46, No. 26, 2007

High-Spin Thiolatonickel(II) Complexes
Figure 2. Thermal ellipsoid plots of (a) [PhTttBu]Ni(SCPh3) (2â), (b) [PhTttBu]Ni(SC6F5) (3), and (c) [PhTttBu]Ni(SPh) (4) at the 50% probability level. One
of the two positionally disordered thioether fragments for 2â is shown with the other contributing form contained in the Supporting Information. Hydrogen
atoms are omitted for clarity.
Figure 3. Thermal ellipsoid plot of [[κ2-PhBttBuSPh]Ni(η2-CH2SBut) (5)
at the 50% probability level. Hydrogen atoms are omitted for clarity.
However, [PhTttBu]Ni(SPh) (4) could not be obtained under
similar experimental conditions, due to its thermal instability,
vide infra. Similar instability was noted for certain thiola-
Figure 4. Electronic absorption spectra of 2-4 in toluene at room
tocopper(II) complexes of [TpiPr2] due to an internal redox
temperature.
reaction.9b Complex 4 was obtained successfully by the low-
temperature reaction of 1 with 1 equiv each of PhSH and
triethylamine, which led to a dark purple species at temper-
atures below -40 °C. The thermal decomposition of 4 at
sesses a four-coordinate nickel(II) ion consisting of three
thioether sulfurs (S1 and S2 are in the equatorial positions,
whereas S3 is axial) from [PhTttBu] and the thiolate sulfur
from triphenylmethylthiolate. A normalized measure of
room temperature cleanly gave a nickel alkylation product,
pyramidalization τ is used to describe the distortion of
[κ2-PhBttBuSPh]Ni(η2-CH2SBut) (5), via net exchange of Ni-
tetrahedral coordination along a pseudo-C3 axis (τ ) 0 for
SPh and B-CH2SBut groups, as shown in Scheme 2.
tetrahedron, τ ) 1 for trigonal pyramid with flat base).19
Structural Description. Complexes 1-5 were character-
The pyramidalization of 2 (τ ) 0.90) indicates the nickel
ized successfully by X-ray diffraction analyses. The molec-
stereochemistry is quite distorted away from tetrahedral and,
ular structures of 1-5 are depicted in Figures 1-3, with
thus, is better described as trigonal-pyramidal. The Ni-Sthiolate
selected bond distances and angles contained in Table 2.
bond distance of 2.1668(15) Å is in the range of those for
Complex 1 has a five-coordinate nickel(II) with a monoan-
thiolato-transition metal complexes (2.07-2.39 Å).12,20 The
ionic face-capping [PhTttBu] and a symmetrically coordinated
Ni-S4-C22 angle of 116.54(13)° is comparable to that in
bidentate nitrate, comparable to that observed for [TptBu]Ni-
[TpiPr2]Cu(SCPh3) (124°).9c
(NO3).17 The nickel geometry is best described as square-
The coordination geometry of 3 is best described as
pyramidal with two oxygens from the nitrate occupying the
trigonal-pyramidal with a τ value of 0.74, which is smaller
basal plane.
than that of 2. The nickel(II) ligation is composed of three
thioether sulfurs from [PhTttBu] and the thiolate sulfur from
the pentafluorophenylthiolate. The Ni-Sthiolate bond distance
Two different crystalline phases of 2 (r and â) were
analyzed by single-crystal X-ray diffraction. Results from
the higher quality crystal, 2â, are contained in the paper with
the data from the weaker diffracting crystal phase, 2r,18
contained in the Supporting Information. Complex 2 pos-
(19) The parameter τ is defined as τ ) [Σ( Lbasal-M-Lbasal) - Σ( Lbasal
-
M-Laxial)]/90 in: Vela, J.; Stoian, S.; Flaschenriem, C. J.; Mu¨nck,
E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522-4523.
(20) (a) Zang, Y.; Que, L., Jr. Inorg. Chem. 1995, 34, 1030-1035. (b)
Fox, D. C.; Fiedler, A. T.; Halfen, H. L.; Brunold, T. C.; Halfen, J.
A. J. Am. Chem. Soc. 2004, 126, 7627-7638. (c) Fiedler, A. T.;
Halfen, H. L.; Halfen, J. A.; Brunold, T. C. J. Am. Chem. Soc. 2005,
127, 1675-1689.
(17) Han, R.; Parkin, G. J. Am. Chem. Soc. 1991, 113, 9707-9708.
(18) Complex 2r crystallized in the space group P21/n with the following
unit cell parameters: a ) 9.397(4) Å, b ) 16.170(7) Å, c ) 25.002-
(9) Å, b ) 90.272(14)°, V ) 3799(3) Å3, and Z ) 4.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11311

Cho et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1, 2â, 3, 4, and 5
2â
1
3
4
5
Ni-S1
Ni-S2
Ni-S3
Ni-O1
Ni-O2
2.3003(6)
2.3213(7)
2.3185(7)
2.0873(11)
2.0907(12)
Ni-S1
Ni-S2
Ni-S3
Ni-S4
Ni-C23
2.3596(19)
2.3232(13)
2.3409(14)
2.1668(15)
2.2847(7)
2.2910(7)
2.3289(8)
2.2355(9)
2.2848(12)
2.2948(12)
2.2991(13)
2.1868(12)
2.2288(11)
2.1571(12)
2.1408(14)
1.914(3)
S1-Ni-O1
S1-Ni-O2
S1-Ni-S2
S1-Ni-S3
S2-Ni-O1
S2-Ni-O2
S2-Ni-S3
S3-Ni-O1
S3-Ni-O2
O1-Ni-O2
109.17(4)
102.11(4)
94.98(3)
94.86(3)
96.49(4)
155.82(3)
93.93(3)
152.76(3)
101.45(4)
61.85(5)
S1-Ni-S2
S1-Ni-S3
S1-Ni-S4
S2-Ni-S3
S2-Ni-S4
S3-Ni-S4
S4-C23-Ni
C23-Ni-S3
C23-Ni-S4
C23-S4-Ni
82.69(5)
105.72(6)
138.18(6)
91.22(5)
128.98(4)
100.57(5)
96.49(3)
93.25(2)
130.29(2)
89.96(2)
128.52(2)
105.13(2)
94.89(4)
92.91(3)
129.94(4)
94.47(3)
129.00(4)
104.49(4)
92.45(5)
113.44(5)
71.55(13)
103.90(11)
50.43(11)
58.01(10)
[TpiPr2] ligand as summarized in Table 3. The spectrum of 2
shows an intense band at 532 nm (ꢀ ) 7500 M-1 cm-1),
which is at higher energy compared to [TpiPr2]Cu(SCPh3)
(λmax ) 625 nm, ꢀ ) 6600 M-1 cm-1).9b The absorption band
of [TpiPr2]Cu(SCPh3) has previously been assigned to the
Table 3. Electronic Absorption Spectral Data for Thiolatonickel(II) and
-copper(II) Complexes
λ
max (nm) (ꢀ (M-1 cm-1))
ref
nickel complexes
2
3
4
532 (7500)
510 (4800)
569 (4100)
434 (3100)
this work
this work
this work
12, 20
S
thiolate f Cu(II) charge-transfer (CT) transition arising from
the relatively high covalency of the Cu-Sthiolate bond.9d
The spectrum of 3 exhibits an intense band at 510 nm (ꢀ
) 4800 M-1 cm-1), at lower energy than that of [TpiPr2]Ni-
(SC6F5) (λmax ) 434 nm, ꢀ ) 3100 M-1 cm-1).12 Such an
[TpiPr2]Ni(SC6F5)
copper complexes
[TpiPr2]Cu(SCPh3)
[TpiPr2]Cu(SC6F5)
625 (6600)
665 (5960)
9b, 9d
9b, 12, 20
of 2.2355(9) Å is comparable to that found in [TpiPr2]Ni-
(SC6F5) (2.259 Å) and longer than that of 2 (2.1668(15) Å).9b
The Ni-S4-C27 angle of 110.48(7)° is smaller than that
in 2 (116.54(13)°).
intense band has been assigned to the Sthiolate pπ f Ni(II)
21
2
2
dx -y CT transition on the basis of theoretical calculations.
The lower CT transition energy of 3 is due to the thioether
donors of [PhTttBu]. The thioether sulfur p orbitals mix with
the Ni-Sthiolate bonding orbitals such that the energy of the
CT is reduced. A similar situation has been described for
the O f Ni CT bands in the bis-µ-oxo complexes, [(PhTttBu)-
Ni]2(µ-O)2.22 Nonetheless, the CT transition energies for the
thiolato-nickel complexes of 2 and 3 having [PhTttBu] are
higher than those of the corresponding thiolato-copper
complexes of [TpiPr2]Cu(SCPh3) and [TpiPr2]Cu(SC6F5) hav-
ing [TpiPr],9b because the d orbital energy of nickel(II) is
much higher than that of copper(II).
Complex 4 has a coordination geometry similar to those
of 2 and 3, albeit with the smallest τ value of the series at
0.69. The Ni-Sthiolate bond distance of 2.1868(12) Å is
slightly longer than that of 2 (2.1668(15) Å) and shorter than
that of 3 (2.2355(9) Å). The Ni-S4-C27 angle of 111.62-
(11)° is only slightly larger than that of 3 (110.48(7)°).
Complex 5 possesses a square-planar stereochemistry in
which one of the side arms of the [PhTttBu] ligand has been
transferred entirely to the nickel yielding the three-membered
thianickelacycle ring. The phenylthiolate group bridges the
nickel and boron resulting in a five-membered chelate ring.
The Ni-C23 (1.914(3) Å) and Ni-S4 (2.1408(14) Å) bond
distances are comparable to those found in [κ2-PhTttBu]Ni(η2-
CH2SBut) (1.939 and 2.256 Å, respectively).14
The spectrum of 4 shows an intense band at 569 nm (ꢀ )
4100 M-1 cm-1), which is at lower energy than those of 2
and 3. Thus, the trend in CT transition energies in thiola-
tonickel complexes is 3 > 2 > 4. CT transition energies of
thiolatonickel complexes having relatively strong electron
withdrawing groups are higher than those having relatively
weak electron withdrawing groups. This may be ascribed to
a decrease in the sulfur pπ orbital energy due to the orbital
stabilization provided by electron withdrawing groups.
The geometry of the thiolatonickel complexes, as reflected
in the τ values, plays an important role in determining the
Spectroscopic Characterization of 2-4. The electronic
absorption spectra of 2-4 were measured in toluene (300-
1100 nm) at room temperature, Figure 4. Spectroscopic
properties of 2-4 are qualitatively similar to those of
analogous thiolatocopper(II) complexes supported by the
2
2
intensity of the Sthiolate pπ f Ni(II) dx -y CT transition. The
trend in the CT intensities, 2 > 3 > 4, Figure 4, parallels
that found in the τ values, (2 (0.90) > 3 (0.74) > 4 (0.69)).
The complex with the largest τ value displays the most
(21) Gorelsky, S. I.; Basumallick, L.; Vura-Weis, J.; Sarangi, R.; Hodgson,
K. O.; Hedman, B.; Fujisawa, K.; Solomon, E. I. Inorg. Chem. 2005,
44, 4947-4960.
(22) Schenker, R.; Mandimutsira, B. S.; Riordan, C. G.; Brunold, T. C. J.
Am. Chem. Soc. 2002, 124, 13842-13855.
Figure 5. Orientation of π orbitals involved in the CT transition at limiting
values of τ.19
11312 Inorganic Chemistry, Vol. 46, No. 26, 2007

High-Spin Thiolatonickel(II) Complexes
Figure 6. 1H NMR spectra of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5 in benzene-d6 at room temperature.
intense CT transition. The explanation is depicted pictorially
in Figure 5. The Sthiolate pπ orientation parallel to the dx -y
be ∼4.2 µB at room temperature, but distortions reduce the
value into the range from 4.0 to 3.0 µB.24 These results show
good agreement with the coordination geometries of 2-4
discussed in the structural description, vide supra.
2
2
plane provides for significant orbital overlap. Thus, the high
τ value, along with the high CT intensity, is indicative of a
high degree of Ni-S π covalency. In addition, the relatively
large difference in CT intensity of 2 when compared with 3
and 4 can be attributed to the aryl groups of the latter
complexes which provide a means for delocalizing the
thiolate electron density.
Magnetic Properties. The room-temperature effective
magnetic moments are 3.06 µB for 2, 3.30 µB for 3, and 3.34
µB for 4, values consistent with an S ) 1 ground state. These
values are similar to those reported for Ni(II)-substituted
azurin (3.2 µB) and a synthetic model complex (3.30 µB,
[TpiPr]Ni(SC6F5)).20,23 In general, the effective magnetic
moment of an idealized tetrahedral Ni(II) complex should
The 1H NMR spectra of complexes 1-5 are displayed in
Figure 6. The spectrum of 1 shows a relatively broad peak
at δ ) 17.40 and three resonances in the range δ ) 8.42-
7.37 with an intensity ratio of 27:5, which are assignable to
the chemical shifts of tert-butyl and phenyl protons, respec-
tively. The chemical shift for the methylene protons was not
detected presumably due to broadening beyond detection by
(23) Blaszak, J. A.; Ulrich, E. L.; Markley, J. L.; McMillin, D. R.
Biochemistry 1982, 21, 6253-6258.
(24) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York 1999;
p 841.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11313

Cho et al.
the paramagnetic center. These features are similar to those
of 3-fold symmetric nickel(II) and cobalt(II) complexes
having [PhTttBu] ligation, e.g., [PhTttBu]NiCl.15 A single set
of the tert-butyl protons observed also indicates that 1 has
3-fold rotational symmetry in solution.
The chemical shifts of the protons of [PhTttBu] in 2-4 are
very similar to those of 1. The chemical shifts of phenyl
protons of the coordinated thiolates in 2 and 4 exhibit
significant upfield and downfield shifts. For 2, the signal at
δ ) 30.17 is assigned to the o-protons, which are closest to
the paramagnetic center, on the basis of integration of two
protons and line broadening, the latter attributed to a spin
delocalization pathway. The p- and m-protons are assigned
to the peaks at δ ) 8.55 and 8.14, respectively.
For 4, there is one downfield feature at δ ) 32.16 and
two upfield-shifted peaks at δ ) -38.97 and -45.70. A
similar chemical shift pattern has been observed in [Fe(TPA)-
(SPh)]+ (δ ) 22.6 for m-H, -19.5 for o-H, and -26.0 for
p-H).20a These patterns are characteristic of spin polarization
pathway, which results in an alternating pattern of parallel
and antiparallel spins on adjacent nuclei along chemical
bonds, affording alternating downfield- and upfield-shifted
peaks, respectively.25 Thus, the chemical shifts of phenylthi-
olate protons in 4 are assigned as δ ) 32.16 for m-H, -38.97
for o-H, and -45.70 for p-H.
Figure 7. Cyclic voltammograms of 2-4 in THF (1 mM) containing 0.1
M n-Bu4NPF6 at room temperature (working electrode, glassy carbon;
counter electrode, Pt; reference electrode, Ag/Ag+; scan rate, 100 mV).
The spectrum of 5 exhibits sharp resonances in the δ )
0-10 region, in support of its diamagnetic ground state. The
Table 4. Electrochemical Data for the Reduction of Thiolatonickel(II)
Complexes 2-4 in THF at Room Temperature at Scan Rate ) 0.1 V
1
analysis of the 2-D H NMR COSY spectrum permits for
assignment of the chemical shifts of two phenyl groups in
the range δ ) 7.91-6.98 (Figure S6 of the Supporting
Information). The singlet at δ ) 2.45 is assigned to
methylene protons of the uncoordinated thioether group of
[PhBttBuSPh], whereas the four doublets at δ ) 2.57, 2.33,
1.69, and 1.52 are assigned to diastereotopic methylene
complex
E1/2(II/I) (∆E) vs Fc+/Fc (V)
2
3
4
-1.11 (∼0.15)
-1.03 (0.10)
-1.17 (0.09)
stabilizes the nickel(II) oxidation state. A similar trend is
observed for the CT transition energies, vide supra.
protons of the coordinated thioether side arm of [PhBttBu
-
Thermal Decomposition of 4. The thiolatonickel(II)
complexes exhibit dramatically different thermal stabilities
with the order decreasing 3 > 2 > 4. Complex 4 readily
decomposes to 5 within several hours at room temperature
via net exchange of Ni-SPh and B-CH2SBut groups. X-ray
diffraction analysis and 1H NMR spectral studies show that
5 consists of a η2-CH2SBut ligand and a SPh functionalized
borate ligand, which is coordinated to the nickel in a κ2
fashion. One plausible mechanism for the formation of 5
from 4 under consideration is intramolecular nucleophilic
substitution (SN2) of a methylene group at boron by SPh.
The resulting three-membered thianickelacycle ring favors
square-planar geometry to give 5. Previously, we have noted
that a related thianickelacycle is a thermodynamic sink when
handling [PhTttBu]NiCl under reducing conditions in the
absence of suitable nickel(I) traps.14 To the best of our
knowledge, incorporation of a thiolato group into a borate
ligand is not precedented.14 Somewhat similar rearrangements
involving hydridotris(pyrazolyl)borate ligands and Zr-CH2-
Ph 26 and Sm-CCPh 27 moieties have been reported. Com-
SPh] and η2-CH2SBut, respectively. The three distinct tert-
butyl group protons resonate at δ ) 1.34, 1.28, and 0.98.
Cyclic Voltammetry of 2-4. The cyclic voltammograms
(CV) of the thiolatonickel(II) complexes in THF exhibit a
quasi-reversible redox couple at E1/2 ) -1.11 V for 2, and
reversible redox couples at E1/2 ) -1.03 V for 3 and E1/2
)
-1.17 V for 4 (vs Fc+/Fc), as shown in Figure 7 with data
collected in Table 4. The redox couples correspond to the
one-electron reduction of the Ni(II) state to the Ni(I) state.
Peters’ [PhBP3]Ni(SC6H4p-But) displays a reversible wave
at the same potential as 2 indicating that the tris(phosphino)-
borate and tris(thioether)borate stabilize the lower oxidation
states to similar extents.13 The data show that the presence
of the electron withdrawing group on the thiolato group
causes a positive shift of E1/2 (Ni(II)/Ni(I)) values (3 > 2 >
4), consistent with the expected effect of increasing electron
withdrawing abilities pentafluorophenyl > triphenylmethyl
> phenyl. Such a positive shift is in line with the general
trend that the electron withdrawing group differentially
(25) Ming, L.-J. In Physical Methods in Bioinorganic Chemistry: Spec-
troscopy and Magnetism; Que, L., Jr., Ed.; University Science
Books: Sausalito, CA, 2000; pp 375-464.
(26) Lee, H.; Jordan, R. F. J. Am. Chem. Soc. 2005, 127, 9384-9385.
(27) Lin, G.; McDonald, R.; Takats, J. Organometallics 2000, 19, 1814-
1816.
11314 Inorganic Chemistry, Vol. 46, No. 26, 2007

High-Spin Thiolatonickel(II) Complexes
pared to the pentafluorophenylthiolato group in 3, the
phenylthiolato group in 4 is more nucleophilic, consistent
with different thermal stabilities of the thiolatonickel com-
plexes (3 > 4). We consider Ni-SPh bond homolysis, i.e.,
a radical mechanism, unlikely because addition of CO, an
established trap of the nickel(I) fragment, [PhTttBu]Ni, to
solutions of 4 does not impact the course of decomposition.
The unexpected formation of 5 suggests that other interesting
insertion chemistry leading to modified ligands may be
possible in this system, a topic for future investigation.
Complex 2 decomposes over a period of days in solution,
leading to the disulfido dimer, [(PhTttBu)Ni]2(µ-η2:η2-S2), the
subject of a separate report. Complex 3 decomposes only at
elevated temperatures, to an intractable product.
spectroscopic properties (CT intensity) shows the strong CT
transition intensity is derived from the high τ value, which
reflects enhanced orbital overlap (2 > 3 > 4). The presence
of the electron withdrawing group on the thiolato group
makes the thiolatonickel(II) complexes easy to reduce in the
order 3 > 2 > 4, which is in line with the Sthiolate pπ f
Ni(II) dx -y CT transition energy. Thermal decomposition
of 4 exhibits unusual nickel alkylation. Incorporation of SPh
to the borato ligand of [PhTttBu] and activation of one of
methylene groups of [PhTttBu] yield 5.
2
2
Acknowledgment. The National Science Foundation
provided financial support for these studies through Grant
CHE-0518508.
Supporting Information Available: Thermal ellipsoid plots
including complete atom labeling and X-ray crystallographic details
(CIF files) for 1-5 and a COSY NMR spectrum of 5. This material
is available free of charge via the Internet at http://pubs.acs.org.
Conclusions
A series of the thiolatonickel(II) complexes, 2-4, were
successfully isolated and characterized by various physico-
chemical methods including X-ray crystallographic analysis.
The relationship between structural factors (τ value) and
IC701743Z
Inorganic Chemistry, Vol. 46, No. 26, 2007 11315