Steric titration of arylthiolate coordination modes at pseudotetrahedral nickel(II) centers

Article abstract of DOI:10.1021/ic901347p

Several derivatives of the pseudotetrahedral phenylthiolate complex Tp ^Me,MsNi-SPh (1), Tp^Me,Me- = hydrotris(3,5dimethyl-1- pyrazolyl)borate, were prepared incorporating substituted arylthlolates, including a series of orthosubstituted ligands Tp^Me,MeNi-SR (R = 2,6-Me₂C₆H₃,2; 2,4,6-Me₃C ₆H₂,3; 2,4,6- Pr₃C₆H₂,4; and 2,6-Ph₂C₆H₃,5) and a series of para-substituted complexes (R = C₆H₄-4-OMe, 6; C ₆H₄-4-Me, 7; and C₆H₄-4-CI, 8). The products were characterized by ¹H NMR and UV-vis spectroscopy. Spectra of 6-8 were consistent with retention of a common structure across the para-substituted series with modest perturbation of the spectral features of 1 assisting their assignment. In contrast, spectra of 2-5 were indicative of a significant change in configuration across the orthodisubstituted series. The structure of complex 5 was determined by X-ray crystallography and a distinctive arylthiolate ligation mode was found, in which the N₃S ligand field was significantly distorted toward a sawhorse, compared to a more common trigonal pyramidal shape (e.g., 1). Moreover, the arylthiolate substituent rotated from a vertical orientation co-directional with the pyrazole rings and disposed between two of them in 1, to a horizontal orientation perpendicular to and over a single pyrazole ring in 5. This reorientation is necessary to accommodate the large ortho substituants of the latter complex. The divergent Ni-S coordination modes result in distinct ¹H NMR and electronic spectra that were rationalized by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. These results demonstrate rich coordination chemistry for arylthlolates that can be elicited by steric manipulation at the periphery of pseudotetrahedral ligand fields.

Full text of DOI:10.1021/ic901347p

Inorg. Chem. 2010, 49, 457–467 457
DOI: 10.1021/ic901347p
Steric Titration of Arylthiolate Coordination Modes at Pseudotetrahedral Nickel(II)
Centers
Swarup Chattopadhyay,† Tapash Deb,† Jeffrey L. Petersen,‡ Victor G. Young Jr.,§ and Michael P. Jensen*,†
†Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, ‡C. Eugene Bennett
§
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, and X-ray
Crystallographic Laboratory, Department of Chemistry, University of Minnesota, Minneapolis,
Minnesota 55455
Received July 10, 2009
Several derivatives of the pseudotetrahedral phenylthiolate complex TpMe,MeNi-SPh (1), TpMe,Me- = hydrotris(3,5-
dimethyl-1-pyrazolyl)borate, were prepared incorporating substituted arylthiolates, including a series of ortho-
substituted ligands TpMe,MeNi-SR (R = 2,6-Me2C6H3, 2; 2,4,6-Me3C6H2, 3; 2,4,6- iPr3C6H2, 4; and 2,6-Ph2C6H3, 5)
and a series of para-substituted complexes (R = C6H4-4-OMe, 6; C6H4-4-Me, 7; and C6H4-4-Cl, 8). The products were
characterized by 1H NMR and UV-vis spectroscopy. Spectra of 6-8 were consistent with retention of a common
structure across the para-substituted series with modest perturbation of the spectral features of 1 assisting their
assignment. In contrast, spectra of 2-5 were indicative of a significant change in configuration across the ortho-
disubstituted series. The structure of complex 5 was determined by X-ray crystallography and a distinctive arylthiolate
ligation mode was found, in which the N3S ligand field was significantly distorted toward a sawhorse, compared to a
more common trigonal pyramidal shape (e.g., 1). Moreover, the arylthiolate substituent rotated from a vertical
orientation co-directional with the pyrazole rings and disposed between two of them in 1, to a horizontal orientation
perpendicular to and over a single pyrazole ring in 5. This reorientation is necessary to accommodate the large ortho
substituents of the latter complex. The divergent Ni-S coordination modes result in distinct 1H NMR and electronic
spectra that were rationalized by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.
These results demonstrate rich coordination chemistry for arylthiolates that can be elicited by steric manipulation at the
periphery of pseudotetrahedral ligand fields.
1. Introduction
with sulfur ligands and cofactors.8 These structures pose
questions regarding dynamics and reactivity of novel active
sites. As small molecule modeling is an established paradigm
to examine these questions, we are accordingly interested
in developing coordination chemistry of paramagnetic nickel
complexes with sulfur ligands.9-11
The initial focus of our efforts has been pseudotetrahedral
arylthiolate complexes supported by tris(pyrazolyl)borates.9
A key advantage afforded by these tripodal supporting
ligands is the ability to add substituents to the facially
tridentate pyrazole rings, enabling manipulation of electronic
and steric properties of the ligand.12 As the substituents are
disposed “vertically” (i.e., perpendicular to the N3 donor
plane), a pocket is formed about a fourth binding site for a
Active site nickel ions are found in a number of bacterial
and archaebacterial enzymes that catalyze organometallic
reactions, including H2 oxidation,1 methanesynthesis,2,3 CO2
reduction, and acetate synthesis.2,4,5 This enzymology is
environmentally significant in the global carbon cycle,2 and
may also represent precedent for sustainable energy technol-
ogies.6,7 Recent structural work has elucidated a variety of
active sites for nickel, typically with low-coordinate ligand
fields in non-classical geometries, valence and spin states, and
*To whom correspondence should be addressed. E-mail: jensenm@
ohio.edu.
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Rev. 2007, 107, 4273–4303.
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(5) Drennan, C. L.; Doukov, T. I.; Ragsdale, S. W. J. Biol. Inorg. Chem.
2004, 9, 511–515.
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Dalton Trans. 2003, 4147–4151.
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(9) Chattopadhyay, S.; Deb, T.; Ma, H.; Petersen, J. L.; Young, V. G.,
Jr.; Jensen, M. P. Inorg. Chem. 2008, 47, 3384–3392.
(10) Ma, H.; Chattopadhyay, S.; Petersen, J. L.; Jensen, M. P. Inorg.
Chem. 2008, 47, 7966–7968.
(11) Ma, H.; Wang, G.; Yee, G. T.; Petersen, J. L.; Jensen, M. P. Inorg.
Chim. Acta 2009, 362, 4563–4569.
(12) Trofimenko, S. Chem. Rev. 1993, 93, 943–980.
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Rev. 2009, 38, 89–99.
r
2009 American Chemical Society
Published on Web 12/15/2009
pubs.acs.org/IC

458 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chattopadhyay et al.
co-ligand. This effect enables rudimentary control of
co-ligand bonding through steric contact, in modest analogy
to typically elegant allosteric active site control achieved in
metalloenzymes through all levels of protein structure.
We previously observed that the relatively large substitu-
ents of hydrotris(3-phenyl,5-methylpyrazolyl)borate (i.e.,
TpPh,Me) forced significant bending of a Ni(II)-SAr bond;
the observed bond angle increased from 103.84(8)ꢀ for
TpMe,MeNi-SPh (1) to 116.51(7)ꢀ for TpPh,MeNi-S-2,6-
Me2C6H3 (9) in respective X-ray crystal structures.9 The
arylthiolate coordination was otherwise quite similar in the
two complexes, with the thiophenolate sulfur displaced off
the 3-fold axis toward one pyrazole, and the aryl substituent
oriented vertically (i.e., aligned between 3-substituents on the
other two pyrazole rings). Several related complexes of
various tripodal borate ligands have been reported. Similar
vertical geometries were observed for TpiPr,iPrNi-SC6F5,13 an
analogous tripodal borate complex with thioether donors,14
and its C6H5S- congener.14 In contrast, a tripodal borate
with diphenylphosphinomethyl donor arms yielded a differ-
ent motif, with the sulfur atom disposed between donors and
the substituent significantly rotated toward horizontal,15 as
typically found for complexes with planar tetradentate sup-
porting ligands.16 Another class of [Ni(SAr)4]2- complexes
exhibits tetragonal (D2d) distortion,17-20 and a linear Ni(SAr)2
complex has also been reported.21 The distinctive features of
these structures demonstrate disparate coordination modes
for arylthiolates in pseudotetrahedral Ni(II) complexes.
In the present work, we probed arylthiolate ligation at
TpMe,MeNiII centers by selectively modifying the arylthiolate
on the parent TpMe,MeNi-SPh complex (1), either with
increasingly bulky ortho groups (2-5), or with electronically
significant para substituents (6-8). These complexes demon-
strate a primary role for steric interaction with the tris-
(pyrazolyl) pocket in eliciting two disparate arylthiolate
bonding modes, which have been structurally characterized
and distinguished by spectroscopy and density functional
theory (DFT) calculations.
NMR method in CDCl3 at 295 K.22 UV-visible-NIR spectra
were recorded on an Agilent HP-8453 diode-array spectrophot-
ometer. Elemental analyses were performed by Atlantic
Microlabs, Inc. (Norcross, GA). TpMe,MeNiCl was prepared
by metathesis of anhydrous NiCl2 and TlTpMe,Me in MeOH/
CH2Cl2 (Caution: thallium salts are extremely toxic and must be
properly handled and disposed of).23 2,6-Diphenyl- and 2,4,6-
tris(isopropyl)phenylthiol were prepared by literature
procedures,24-26 and other thiols were obtained from a com-
mercial vendor (Aldrich). The thiols were converted to respec-
tive sodium salts by titration of NaNH2 in toluene, and reacted
with TpMe,MeNiCl in tetrahydrofuran (THF) to obtain target
complexes in 65-80% yields following toluene extraction as
previously described.9 Characterizations of product complexes
are summarized below. All other materials were obtained from
commercial vendors as ACS reagent-grade or better and used as
received, except for drying of solvents by routine techniques.
TpMe,MeNi-SC6H5 (1).9 Anal. Calcd (found) for C21H27-
BN6NiS: C, 54.23 (54.64); H, 5.85 (5.98); N 18.07 (18.24).
UV-vis (CH2Cl2, λmax, nm; ε, mM-1 cm-1): 354 (1.6); 464
(2.4); 506 (2.6); 614 (sh, 0.2); 836 (0.2); 970 (sh, 0.1). 1H NMR
(CDCl3, 295 K; δ, ppm): 77.2 (3H, 4-pz); 23.3 (2H, meta); 6.6
(9H, 5-Me); -7.1 (9H, 3-Me); -10.7 (1H, B-H); -18.9 (2H,
ortho); -26.6 (1H, para); μeff = 2.92 μB.
TpMe,MeNi-S-2,6-Me2C6H3 (2). Anal. Calcd (found) for
C23H31BN6NiS H2O: C, 54.05 (54.24); H, 6.51 (6.71); N 16.44
3
(16.57). UV-vis (toluene, λmax, nm; ε, mM-1 cm-1): 342 (0.9);
368 (0.9); 411 (1.1); 471 (sh, 1.4); 511 (2.2); 640 (0.2); 856 (0.1);
958 (0.1). 1H NMR (CDCl3; δ, ppm): 75.3 (3H, 4-pz); 31.7 (6H,
ortho); 25.2 (2H, meta); 5.5 (9H, 5-Me); -7.5 (9H, 3-Me); -10.4
(1H, para); -11.3 (1H, B-H).
TpMe,MeNi-S-2,4,6-Me3C6H2 (3).9 Anal. Calcd (found) for
C24H33BN6NiS: C, 56.84 (56.75); H, 6.56 (6.59); N 16.57 (17.32).
UV-vis (CH2Cl2, λmax, nm; ε, mM-1 cm-1): 343 (1.1); 371 (1.1);
1
406 (1.2); 467 (sh, 1.5); 514 (2.7); 636 (sh, 0.2); 861 (0.2). H
NMR (CDCl3; δ, ppm): 75.0 (3H, 4-pz); 33.3 (6H, ortho); 25.8
(3H, para); 25.3 (2H, meta); 5.2 (9H, 5-Me); -7.5 (9H, 3-Me);
-11.3 (1H, B-H); μeff = 3.01 μB.
TpMe,MeNi-S-2,4,6-iPr3C6H2 (4). Anal. Calcd (found) for
C30H45BN6NiS: C, 60.94 (61.03); H, 7.67 (7.83); N 14.21
(14.25). UV-vis (CH2Cl2, λmax, nm; ε, mM-1 cm-1): 338 (sh,
1.1); 414 (2.2); 516 (2.8); 665 (0.3); 870 (0.2); 954 (0.1). 1H NMR
(CDCl3; δ, ppm): 75.3 (3H, 4-pz); 24.5 (2H, meta); 19.7 (2H, 2,6-
CHMe2); 8.8 (1H, 4-CHMe2); 8.0 (12H, 2,6-CHMe2); 5.3 (9H, 5-
Me); 2.3 (6H, 2,6-CHMe2); -8.1 (9H, 3-Me); -11.8 (1H, B-H).
TpMe,MeNi-S-2,6-Ph2C6H3 (5). Anal. Calcd (found) for
C33H35BN6NiS: C, 64.21 (64.27); H, 5.72 (5.79); N 13.62
(13.57). UV-vis (CH2Cl2, λmax, nm; ε, mM-1 cm-1): 357 (sh,
0.8); 422 (1.8); 516 (1.8); 657 (0.3); 867 (0.2); 942 (0.1). 1H NMR
(CDCl3; δ, ppm): 76.5 (3H, 4-pz); 24.6 (2H, meta); 15.1 (4H, 2,6-
ortho); 7.4 (2H, 2,6-para); 6.8 (4H, 2,6-meta); 5.9 (9H, 5-Me); 1.4
(1H, para); -9.4 (9H, 3-Me); -11.7 (1H, B-H); μeff = 2.89 μB.
TpMe,MeNi-S-C6H4-4-OCH3 (6). Anal. Calcd (found) for
C22H29BN6NiOS: C, 53.37 (53.46); H, 5.90 (5.89); N 16.98
(17.21). UV-vis (CH2Cl2, λmax, nm; ε, mM-1 cm-1): 348
(1.8); 484 (sh, 2.4); 517 (3.1); 843 (0.2); 1000 (sh, 0.1). 1H
NMR (CDCl3; δ, ppm): 75.5 (3H, 4-pz); 22.5 (2H, meta); 6.3
(9H, 5-Me); -6.9 (9H, 3-Me); -8.3 (3H, OMe); -10.3 (1H, B-
H); -21.0 (2H, ortho).
2. Experimental Methods
2.1. General and Synthetic Procedures. All manipulations
were carried out under an inert atmosphere of prepurified argon,
either in a glovebox (MBraun Unilab) or using Schlenk techni-
ques. 1H NMR data were recorded on a Varian Unity 500
spectrometer and processed using the MestReNova 5.1.0 soft-
ware suite (Mestrelab Research, Santiago de Compostela,
Spain); spectra were referenced internally to residual solvent.
Solution magnetic moments were determined by the Evans
(13) Matsunaga, Y.; Fujisawa, K.; Ibi, N.; Miyashita, Y.; Okamoto, K.-i.
Inorg. Chem. 2005, 44, 325–335.
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(16) Fox, D. C.; Fiedler, A. T.; Halfen, H. L.; Brunold, T. C.; Halfen, J. A.
J. Am. Chem. Soc. 2004, 126, 7627–7638.
TpMe,MeNi-S-C6H4-4-CH3 (7). Anal. Calcd (found) for
C22H29BN6NiS: C, 55.16 (55.18); H, 6.10 (6.16); N 17.54
(17) Swenson, D.; Baenziger, N. C.; Coucouvanis, D. J. Am. Chem. Soc.
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Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 459
(17.63). UV-vis (CH2Cl2, λmax, nm; ε, mM-1 cm-1): 353 (1.7);
475 (2.4); 513 (3.2); 624 (sh, 0.2); 841 (0.1); 970 (sh, 0.1). H
equatorial or axial sulfur ligation and horizontal or vertical
substituent orientations was obtained by manipulating the
Naxial-Ni-S-Cipso and Ni-S-Cipso-Cortho torsion angles of 50
with subsequent geometry optimization under appropriate
symmetry (i.e., C1 or Cs) without additional constraints. Finally,
a 100 model optimized under experimental C1 symmetry was also
included in TD-DFT calculations. Spin-unrestricted geometry
optimizations, single point and TD-DFT33,34 calculations were
performed using the Vosko-Wilk-Nussair LDA exchange-
correlation functional,35 the Becke-Perdew GGA correction,36,37
and the Slater-type orbital TZP (triple-ζ with single
polarization) basis set available in the ADF library, with frozen
atomic cores and default convergence criteria. A solvation
model and relativistic correction were not applied. Spin-re-
stricted calculations on the free phenylthiolate anion were
performed similarly, starting from a suitable crystal structure
fragment.
1
NMR (CDCl3; δ, ppm): 75.7 (3H, 4-pz); 38.1 (3H, para-CH3);
23.1 (2H, meta); 6.4 (9H, 5-Me); -6.9 (9H, 3-Me); -10.4 (1H,
B-H); -19.9 (2H, ortho).
TpMe,MeNi-S-C6H4-4-Cl (8). Anal. Calcd (found) for
C21H26BClN6NiS: C, 50.50 (50.65); H, 5.25 (5.35); N 16.83
(16.70). UV-vis (CH2Cl2, λmax, nm; ε, mM-1 cm-1): 361 (1.9);
464 (2.7); 510(2.8); 615 (sh, 0.2); 835 (0.2); 970 (sh, 0.1). 1H NMR
(CDCl3; δ, ppm): 77.0 (3H, 4-pz); 23.7 (2H, meta); 7.1 (9H, 5-
Me); -6.9 (9H, 3-Me); -10.5 (1H, B-H); -20.3 (2H, ortho).
2.2. X-ray Crystallography. The structure of TpMe,MeNi-S-
2,6-Ph2C6H3 2.0THF (5) was solved at the University of Min-
3
nesota (VGY). Suitable crystals were grown from a concen-
trated THF solution cooled to -36 ꢀC. An orange block (0.35 ꢀ
0.30 ꢀ 0.30 mm) was placed onto the tip of a 0.1 mm diameter
glass capillary and mounted on a Bruker AXS diffractometer
equipped with a CCD area detector.27 The data collection was
˚
3. Results and Discussion
carried out at 123(2) K using Mo KR radiation (λ = 0.71073 A,
graphite monochromator). The intensity data were corrected for
absorption and decay (SADABS).28 Final cell constants were
calculated from 2761 strong reflections from the actual data
collection after integration (SAINT).29 The structure was solved
and refined using Bruker SHELXTL.30 The space group Pnma
was determined based on systematic absences and intensity
statistics. A direct-methods solution was calculated which pro-
vided most non-hydrogen atoms from the E-map; full-matrix
least-squares difference Fourier cycles were performed which
located the remaining non-hydrogen atoms. All non-hydrogen
atoms were refined with anisotropic displacement parameters.
All hydrogen atoms were placed in ideal positions and refined as
riding atoms with relative isotropic displacement parameters.
The molecule is bisected by the crystallographic mirror plane so
that half appears in one asymmetric unit. There is also one
molecule of THF solvent per asymmetric unit disordered over
two positions in the same region of space in a 0.68:0.32 ratio.
There is some indication that at least one THF fragment also has
“envelope flap” disorder. It appears that the THF oxygen atoms
were correctly placed for both disordered fragments. A total of
46783 reflections were collected, with 4718 being independent
(Rint = 0.0323) and 4062 observed. The refinement converged
to give R1 = 0.0411, wR2 = 0.1042 (all data), GoF = 1.122.
Crystal data: C41H51BN6NiO2S2, formula weight 761.46, ortho-
3.1. General Comments. We prepared several deriva-
tives of TpMe,MeNi-SPh (1), our previously reported
parent complex,9 modified along steric and electronic
vectors: a series of complexes with enhanced bulk at the
ortho arylthiolate positions, TpMe,MeNi-SAr [Ar = C6H5
(1), 2,6-Me2-C6H3 (2), 2,4,6-Me3-C6H2 (3), 2,4,6-iPr3-
C6H2 (4), 2,6-Ph2-C6H3 (5)]; and a second series with
para substituents, TpMe,MeNi-S-C6H4-4-X [X = OCH3
(6), CH3 (7), H (1), and Cl (8)], of varying electronic
donor strength. The complexes were prepared from
TpMe,MeNiCl by metatheses with respective sodium thio-
late salts in THF, resulting in appearance of intense
coloration due to characteristic Ni-SAr LMCT bands
(vide infra). The complexes were characterized by ele-
mental analyses, 1H NMR and UV-vis-NIR spectrosco-
py. In all cases, the results were consistent with
pseudotetrahedral complexes exhibiting a paramagnetic
(S = 1) d8 electron configuration. The structure of 5 was
also determined.
3.2. X-ray Crystallography. The previously reported
structures of TpMe,MeNiSPh (1) and the bulkier analogue
TpPh,MeNi-S-2,6-Me2C6H3 (9) consisted of distorted
pseudotetrahedral N3S ligand fields with the Ni-S bond
˚
˚
rhombic, Pnma, a = 15.984(2) A, b = 23.499(3) A, c =
3
10.685(1) A, V = 4013.4(8) A , Z = 4, Dcalc = 1.260 g/cm3,
˚
˚
μ = 5.78 cm-1
.
tilted off the 3-fold H-B Ni axis particularly toward
3 3 3
2.3. DFT Calculations. All calculations were carried out using
the Amsterdam Density Functional software package (ver-
sion 2007.01, Scientific Computing and Modeling NV).31,32
Atomic coordinates were imported from the experimental
structures of 1 and 5 and reduced to simple TpNi-SPh models
10 and 50 by replacement of the pyrazole and ortho-arylthiolate
substituents with hydrogen atoms in ideal positions. The model
geometries were optimized under imposed Cs symmetry, with
the Ni-S-Cipso angle of 50 constrained to the experimental
value to prevent inward bending in the absence of the 3-pyrazole
and ortho-arylthiolate substituents. Furthermore, a full series of
configurational models spanning arylthiolate coordination with
one of the pyrazolyl arms, giving a trigonal pyramidal
shape with an N2S equatorial plane.9 The arylthiolate
substituents were disposed over unoccupied axial sites
and nearly vertical between 3-pyrazole substituents on
opposing equatorial donor arms. In the present work, we
introduced steric bulk at the arylthiolate ortho positions
while retaining the smaller 3-pyrazole methyl substitu-
ents. This elicited a distinct arylthiolate coordination
mode, revealed by the crystal structure of TpMe,Me
-
Ni-S-2,6-Ph2C6H3 (5), which is distorted by off-axis
thiolate bending toward a sawhorse geometry, with axial
sulfur and pyrazole donors (i.e., S1 and N2) and an open
equatorial site (Figure 1). The Ni-S bond in 5 is bent
between pyrazole donors, and the arylthiolate substituent
is displaced backward over the third pyrazole and the
(27) SMART, V5.054; Bruker Analytical X-ray Systems: Madison, WI, 2001.
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(29) SAINTþ, V6.45; Bruker Analytical X-Ray Systems: Madison, WI,
2003.
(30) SHELXTL, V6.14; Bruker Analytical X-Ray Systems: Madison, WI,
2000.
(31) ADF 2007.01, Scientific Computing and Modelling NV; Vrije
Universiteit: Amsterdam, The Netherlands, 2007.
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460 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chattopadhyay et al.
bendings are limited to opposing motions of the sulfur
toward or away from a particular pyrazole ring. These
will respectively lead to a unique N-Ni-S angle that is
smaller or larger than the average umbrella angle.
Relevant geometric parameters for the seven structu-
rally characterized arylthiolate complexes of Ni(II) sup-
ported by tripodal borate ligands can be summarized
within this model (Table 1).9,13-15 A significant umbrella
distortion is indicated in each case by the pseudotetra-
hedral angle formed between the 3-fold H-B Ni axis
3 3 3
and the coordinate bond vectors to the borate tripod
donor atoms (L) ranging from 120.4ꢀ to 124.8ꢀ compared
to the ideal tetrahedral value of 109.47ꢀ. Further distor-
tion by off-axis or sawhorse bending of the arylthiolate is
indicated by distribution of L-Ni-S angles about this
value. As predicted in limiting cases, one of the three
L-Ni-S angles is unique, being smaller (e.g., 9) or larger
(e.g., 5) than the pseudotetrahedral angle.
Figure 1. ORTEP diagram (50% ellipsoids) of TpMe,MeNi-S-2,6-
Ph2C6H3 (5). Selected bond lengths (A): Ni1-N4, 1.995(1); Ni1-N2,
2.001(2); Ni1-S1, 2.2589(6); S1-C11, 1.785(2). Bond angles (deg):
N4-Ni1-N40, 92.06(8); N4-Ni1-N2, 91.20(5); N4-Ni1-S1,
115.47(4); N2-Ni1-S1, 140.01(6); Ni1-S1-C11, 100.11(7).
˚
The sawhorse distortion has been structurally analyzed
in terms of a cis-divacant octahedron (Figure 2, left),39,40
in which Ni-S bending toward an edge of the L3 donor
triangle leads to a staggered geometry, illustrated for the
core structure of 5 (Figure 2, top). Off-axis bending can be
interpreted in the opposite direction, toward a vertex that
yields an eclipsed geometry typified by 9 (Figure 2,
bottom). Since a bent arylthiolate ligand can donate only
four electrons, these distortions can be alternatively
considered on the surface of a monovacant trigonal
bipyramid (Figure 2, right),13,14,38,42 overlooking the
constrained bite of the equatorial nitrogens. Neverthe-
less, bending of sulfur toward distinct axial or equatorial
sites gives the same two limiting geometries. The Ni-S
bond is bent directly over a donor in the trigonal/eclipsed/
equatorial geometry, and directly away in the sawhorse/
staggered/axial geometry. Equivalence of the octahedral
and trigonal bipyramidal models can be observed by
rotation of the corresponding core structures of 5 and 9
about the axial pyrazole nitrogen. The trigonal bipyr-
amidal nomenclature (i.e., axial and equatorial) seems the
most descriptive of the ligand field and will be used
exclusively henceforth, with “axial” coordination denot-
ing distortion toward a trigonal bipyramidal vertex as
vacant equatorial site, rotating into a rigorously horizon-
tal orientation (molecular Cs symmetry is enforced by a
crystallographic mirror plane). The outer 2,6-phenyl ring
planes are rotated 55.89ꢀ from coplanarity, giving short
contacts between each of the pyrazole 3-methyl substitu-
ents and an opposing aromatic ring on the arylthiolate,
˚
3.330 and 3.503 A to the inner and outer centroids,
respectively. The unique N-Ni-S angle from the axial
pyrazole increases from 113.26(6)ꢀ in 1 to 140.01(6)ꢀ in 5,
with further opening possibly constrained by the equatorial
3-methyl substituents, given short S HC contacts of
˚
3 3 3
2.883 A. The Ni-S bond length is slightly longer in 5 than
˚
in 1, 2.2589(6) versus 2.216(1) A, while the Ni-S-Ar angle
is less obtuse, 100.11(7) versus 103.84(8)ꢀ.
3.3. Ni-SAr Structural Configurations. Tris(pyrazo-
lyl)borates and related tripodal ligands have been referred
to as “tetrahedral enforcers” for the four-coordinate,
half-sandwich geometry they support.38-40 However,
the tetrahedral ligand fields are typically distorted
(hence “pseudotetrahedral”), and several unique distor-
tional modes have been identified by symmetry mapping
of four-coordinate metal centers.41 A universal Td f C3v
umbrella distortion is enforced by the constrained ligand
bite, in which N-Ni-N angles are reduced to about 90ꢀ,
opposed to a site along the H-B Ni umbrella axis.
3 3 3
The limiting axial and equatorial configurations just
defined are further differentiated by orthogonal orienta-
tions of the aryl substituent (Figure 2). The substituent is
usually folded backward over the vacated site and rotated
to minimize steric contact with the opposing tripodal
ligand substituents. The axial thiolate configurations of
5 and the PhBP3Ni-S-C6H4-4-tBu complex reported by
MacBeth, et al.15 are associated with a “horizontal”
disposition of the aryl substituent, denoted by large
Ni-S-Cipso-Cortho torsion angles (Table 1), while an
equatorial, vertical orientation is observed in the other
structures.
and the angle to the trigonal H-B Ni axis is concor-
3 3 3
dantly increased to about 124ꢀ for the tris(pyrazo-
lyl)borate ligand. Such facially tridentate ligands are
perhaps more appropriately assigned as filling one face
of an octahedron, particularly when an apical co-ligand
can donate six electrons utilizing orthogonal lone pairs of
π symmetry, such as CO or halide anions.38-40 The borate
chelate also constrains Td f D2d spread and plier distor-
tions, the former having been observed for [Ni(SAr4)]2-
complexes,17-20 but the apical arylthiolate ligand is still
susceptible to Td f Cs off-axis and Td f C2v sawhorse
bendings.41 With three positions defined by constrained
facial pyrazolyl nitrogens, the off-axis and sawhorse
The magnitudes of distortions of tetracoordinate com-
plexes from tetrahedral toward a trigonal monopyramid
have been quantified either by a τ parameter ranging from
zero to unity, based on the difference of apical and basal
bond angles,42 or inversely by metal separation from an
(38) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 7148–7165.
(39) Kersten, J. L.; Kucharczyk, R. R.; Yap, G. P. A.; Rheingold, A. L.;
Theopold, K. H. Eur. J. Chem. 1997, 3, 1668–1674.
ꢀ ꢁ
(40) Detrich, J. L.; Konecny, R.; Vetter, W. M.; Doren, D.; Rheingold,
€
(42) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Munck, E.; Holland, P. L.
J. Am. Chem. Soc. 2004, 126, 4522–4523.
A. L.; Theopold, K. H. J. Am. Chem. Soc. 1996, 118, 1703–1712.
(41) Cirera, J.; Alemany, P.; Alvarez, S. Eur. J. Chem. 2004, 10, 190–207.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 461
Table 1. Structural Parameters for Ni(II) Arylthiolate Complexes of Tripodal Borato Ligands
umbrella angle
(deg)a
Ni-S-Cipso-Cortho
torsion angles (deg)
b
d
τ4
τc
ref.
˚
d (A)
complex
L-Ni-S angles (deg)
TpPh,MeNi-S-2,6-Me2C6H3 (9)
124.8
135.73(5), 129.95(5), 103.05(5)
130.29(2), 128.52(2), 105.13(2)
132.7(1), 128.9(1), 106.8(1)
132.8(1), 128.4(1), 106.4(1)
129.94(4), 129.00(4), 104.49(4)
134.96(6), 122.69(6), 113.26(6)
140.01(6), 115.47(4), 115.47(4)
151.84(3), 109.79(3), 101.54(3)
15.14, 15.19
5.35, 7.15
10.41, 11.62
10.84, 11.90
1.03, 1.66
16.11, 16.64
89.46, 89.46
67.32, 68.66
0.20
0.28
0.28
0.81
0.74
0.72
0.67
0.70
0.70
9
14
13
PhTttBuNi-SC6F5f
122.9
124.0
123.8
122.3
124.0
124.2
120.4
e
TpiPr,iPrNi-SC6F5
PhTttBuNi-SPhe
0.31
0.37
0.42
0.32
0.69
0.60
g
0.72
0.73
0.74
0.70
14
9
h
15
TpMe,MeNi-SPh (1)
TpMe,MeNi-S-2,6-Ph2C6H3 (5)
PhBP3Ni-S-C6H4-4-tBui
g
a The average umbrella angle between the 3-fold H-B Ni axis and the L-Ni bond vectors(i.e., 109.47ꢀ for Td symmetry). b Displacement of Ni from
3 3 3
an equatorial L2S plane, see ref 13. c [Σ—(basal-basal) - Σ—(apical-basal)]/90, see ref 42. d (360ꢀ - R - β)/141ꢀ, where R and β are the two largest
interligand angles, see ref 43. e PhTttBu = phenyltris(tert-butylthiomethyl)borate, [PhB(CH2StBu)3]-. f Two independent molecules. g Undefined. h This
work. i PhBP3 = phenyltris(diphenylphosphinomethyl)borate, [PhB(CH2PPh2)3]-.
thiolate of 5 adopts an axial/horizontal configuration. We
hypothesize that the equatorial/vertical configuration lies
somewhat lower in energy for 1, but is destabilized by
enhanced steric contact with ortho-phenyl substituents
in 5, thus effecting a conformational switch (Figure 3).
This suggests a dynamic equilibrium between the config-
urations can be manipulated by changing the size of
added ortho arylthiolate substituents. Spectroscopic evi-
dence supporting this interpretation was obtained across
the series 1-5 encompassing added 2,6-Me2, -iPr2, and -
Ph2 arylthiolate disubstitution of 1.
3.4. 1H NMR Spectroscopy. NMR spectra obtained
for paramagnetic complexes 1-8 were consistent with
their formulations, notwithstanding significant para-
Figure 2. Illustration of sawhorse (top) and off-axis bending (bottom)
on a cis-divacant octahedron (left column) and monovacant trigonal
bipyramid (right column), as seen in two orientations of the core
structures for 5 (top row) and 9 (bottom row), with ortho substituents
on the arylthiolate rings and all but the ipso carbon of the 3-phenyl
substituents of 9 omitted for clarity.
magnetic shifts induced by unpaired electrons (S = 1)
on high-spin Ni(II). A prominent feature common to all
the spectra was a single resonance for three equivalent
4-pyrazole protons with a pronounced downfield shift
(Figure 4). Equivalent 3- and 5-methyl resonances were
also observed much closer to the diamagnetic region, with
the former significantly broadened by the proximal para-
magnet. A borohydride resonance and additional signals
consistent with the various aryl substituents were also
observed. Compared to the static crystal structures,
equivalence of all three pyrazoles and both arylthiolate
faces is indicative of rapid migration of the arylthiolate on
the NMR time scale, minimally between equivalent bind-
equatorial N2S basal plane.13 A uniquely large N-Ni-S
angle consistent with a sawhorse geometry gives an un-
defined τ value. However, a different τ4 parameter defined
by the two largest bond angles encompasses both distor-
tions, such that unity corresponds to a tetrahedron
(reduced to 0.78-0.85 by the umbrella distortion), while
a sawhorse and a trigonal pyramid are independently
defined as 0.64 (with 90ꢀ N-Ni-N angles).43 These
various parameters are also summarized for each tripodal
arylthiolate complex (Table 1).
As discussed previously, the off-axis and sawhorse
bendings are energetically favored for a d8 electron con-
figuration by reduction of destabilizing overlap in the
filled Ni-S dσ-pσ* interaction.40 While electronic in
origin, the direction and magnitude of the bending can
be manipulated by steric contacts. We previously demon-
strated enhanced trigonal bending from 1 to 9 (τ = 0.60
and 0.81, respectively, Table 1) by substituting bulky
3-pyrazole substituents (i.e., TpPh,Me) proximal to added
ortho-methyl substituents on the arylthiolate ring.9 The
rigid 3-phenyl rings enforced retention of a vertical 2,6-
xylyl substituent orientation in 9, leaving the sulfur in an
equatorial configuration with enhanced off-axis bending.
With retention of the smaller TpMe,Me ligand substituents,
rotation of the arylthiolate ring is enabled, and the bulky
ing sites about the 3-fold H-B Ni axis.
3 3 3
The alternating upfield and downfield shifts for the
arylthiolate protons of 1, as well as reversal of ortho and
para polarization with methyl substitution in 3, are con-
sistent with hyperfine contact shifting because of electron
spin delocalization onto the arylthiolate by a π-polariza-
tion pathway.9 The para protons of 2 and 5 exhibit a
striking attenuation of upfield shift, consistent with
quenching of the π-polarization upon orthogonal rota-
tion of the aryl substituent evident between the crystal
structures of 1 and 5. Thus, the solid state structures of 1
and 5 are plausibly maintained as predominant confor-
mations in solution, with 2 partitioning in a dynamic
equilibrium between these two limits. The 3-pyrazole
methyl protons also exhibited a much smaller monotonic
increase in upfield shift with increasing steric bulk of 1-5,
which may represent differential diamagnetic shifts arising
from aromatic ring currents on the proximal thiolate,
consistent with the proposed conformation change. In
contrast, the meta protons of 1-5 maintain a nearly
(43) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955–964.

462 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chattopadhyay et al.
Figure 3. Space-filling models of 1 (left) and 5 (right), demonstrating a
dominant steric role for ortho substitution in effecting an observed
conformational switch in arylthiolate coordination.
Figure 5. UV-visible-NIR spectra (CH2Cl2 solutions, 297 K) of 1
(black), 3 (green), 4 (red), and 5 (blue).
Figure 4. 1H NMR spectra of 1, 2, and 5 (from bottom to top,
respectively) recorded in CDCl3 at 295 K. The peak marked (*) is residual
solvent; peak assignments not labeled are listed in the experimental
section. Arrows indicate shift of the resonance assigned to the para
position of the arylthiolate rings.
constant downfield shift, indicative of a second mechan-
ism, such as direct spin delocalization through σ bonds.
J-coupling between the assigned meta and para protons
of 2 and 5 were established by COSY techniques (see
Supporting Information).
The series of para-substituted arylthiolate complexes
6-8 was also examined. Not surprisingly, 1H NMR
spectra were nearly identical to that of 1, absent the
upfield para proton resonance and addition of either a
broad upfield methoxy resonance in 6 or a downfield
methyl resonance in 7. The remaining resonances exhibi-
ted comparable chemical shifts to those of 1, except for a
slight upfield shift of the 4-pyrazole protons in 6 and 7.
These observations demonstrate the absence of signifi-
cant electronic effects on Ni-S bonding, with the struc-
ture of 1 retained over the series 6-8.
3.5. Electronic Spectroscopy. The previously reported
UV-vis-NIR spectrum of 1 in CH2Cl2 revealed multiple
transitions, with three strong bands (354, 464, and
506 nm) apparently exhibiting S-Ni LMCT character.9
The three spin-allowed ligand field transitions of a tetra-
hedral d8 metal ion were split by the effective Cs symme-
try, yielding several relatively weak and broad bands
at lower energy (614, 836, 970, and 1580 nm) and
Figure 6. UV-visible-NIR spectra of 3 in toluene as a function of
temperature from 287 to 343 K. Spectra were normalized for changes in
solvent density.46 Raw data are shown in Supporting Information.
obfuscating more precise assignment. Analysis of absorp-
tion and MCD spectra for TpiPr,iPrNi-SC6F5 in cyclohex-
ane yielded qualitatively similar assignments,44 and
comparable ligand field bands under C3v symmetry were
also reported for TpMe,MeNiCl in CH2Cl2.45
Compared to 1, the ortho-disubstituted arylthiolate
complexes 3-5 yielded dramatically different UV-vis
spectra (Figure 5). The 506 nm CT band was retained,
but the 464 and 354 nm bands seemed to progressively
collapse in 3-5, while a new feature appeared at inter-
mediate wavelengths. The ligand field bands also shifted
dramatically, with emergence of a broad band near 660 nm.
Taken together, the overlaid spectra give the appearance
of a titration plot, presumably representing distributions
of the two distinct conformations already elucidated.
Complexes 1 and 5 would retain the respective equator-
ial/vertical and axial/horizontal configurations defined in
the solid state, while 3 and 4 with intermediate steric bulk
would partition between these configurations. Of course,
the features of 3-5 would display minor differences in
energy and extinction because of chemical modification
(44) 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.
(45) Desrochers, P. J.; Telser, J.; Zvyagin, S. A.; Ozarowski, A.; Krzystek,
J.; Vicic, D. A. Inorg. Chem. 2006, 45, 8930–8941.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 463
Figure 7. UV-visible-NIR spectra (CH2Cl2 solutions, 297 K) of 1
(black), 6 (blue), 7 (red), and 8 (green).
of the arylthiolate, and legitimate isosbestic points are not
observed.
Figure 8. Relevant frontier orbitals of the free PhS- ligand, separated
by symmetry class. The dashed horizontal line separates occupied and
empty orbitals.
To test this hypothesis, spectra of the mesityl complex 3
were recorded in toluene over a temperature range of
287-343 K (Figure 6). Fully reversible temperature-
dependent absorption changes were observed with at least
four isosbestic points, consistent with perturbation of a
two-state equilibrium. In particular, a CT feature at 412 nm
emerged with increasing temperature at the expense of
flanking bands, analogous to the preceding comparison
of 1 and 5 (Figure 5). The spectral features of 3 show
surprisingly little difference between CH2Cl2 and toluene
solutions despite a significant change in solvent polarity.
To further probe the charge transfer features of 1,
spectra were obtained for the series of para-substituted
arylthiolate complexes TpMe,MeNi-S-C6H4-p-X (X =
OMe, 6; Me, 7; H, 1; and Cl, 8, Figure 7). All four spectra
yielded similar ligand field bands, exhibiting only a slight
red-shift in the order 8 < 1 < 7 < 6, paralleling the
overall donor strength of the arylthiolates. This result is
again indicative of a common solution structure. In
contrast, the strong charge transfer features at 464 and
506 nm were more noticeably red-shifted, with the former
band showing a larger effect, in the order 1 < 8 < 7 < 6,
following relative π-donor strengths consistent with
S-Ni pπ-dπ* LMCT character. Finally, the weaker CT
band at 354 nm displayed a reversed red-shift pattern, 6 <
7 < 1 < 8, with retention of the poorly resolved fine
structure. The contrasting behaviors of the three strong
CT bands indicate complexity in the nickel-arylthiolate
bonding interaction.
donor orbitals. The metal and thiolate fragments are first
discussed separately, and then together as Ni-SR com-
plexes. An expanded range of limiting arylthiolate con-
figurations incorporating 10 and 50 was defined on the
basis of Ni-SAr bending and scaled by calculated relative
energies, and TD-DFT calculations were performed to
assign CT features in electronic spectra.
3.6.1. Arylthiolate Donor Orbitals. Spin-restricted cal-
culations were performed for the free C6H4S- anion after
geometry optimization under C2v symmetry (Figure 8).
Of the three sulfur 3p orbitals available for bonding with
nickel, one is coincident with the S-Cipso bond vector and
significantly stabilized (15a1). The other two comprise
nearly degenerate lone pairs at high energy (i.e., HOMO
and HOMO-1), one coplanar with the phenyl ring (9b1,
93% S), and the other perpendicular to the ring and
delocalized onto it. The symmetry and intermediate en-
ergy of the latter atomic orbital with respect to the one-
€
and two-node Huckel π orbitals on the substituent ring
result in bonding (3b2, 16% S), non-bonding (4b2, 69% S),
and antibonding (5b2, 13% S) combinations. The 4b2
HOMO is destabilized by only 0.07 eV relative to 9b1.
These two donor orbitals have been referred to respectively
as πop and πip in previous work,15,21,47,48 although one of
these must adopt a pseudo-σ orientation for a bent thio-
late,44 depending on the configurational geometry. The 3b2
orbital is stabilized by 2.06 eV compared to the 4b2 HOMO,
3.6. DFT Calculations. To further rationalize the
divergent arylthiolate configurations and spectroscopic
results observed for 1 and 5, spin-unrestricted DFT
calculations were performed on simplified TpNiSPh com-
putational models 10 and 50 constructed from the respec-
tive crystal structures by substitution of hydrogen atoms
for pyrazole and arylthiolate substituents and geometry
optimization under imposed Cs symmetry. Orbital ener-
gies and allowed electronic transitions were calculated for
both models. Of principal interest in Ni-SAr bonding are
eight β-spin frontier orbitals corresponding to the five d
orbitals split by the ligand field and three arylthiolate
€
and by 0.37 eV relative to its symmetry-isolated Huckel
congener (1a2). Despite this stabilization and consequent
attenuation of S 3p contribution, the 3b2 orbital exhibits
the same symmetry as πop, and subsequent calculations on
the arylthiolate complexes suggest a role for this third
donor orbital in differentiating the bonding and spectros-
copy of the two arylthiolate conformations of 1 and 5 (vide
infra).
3.6.2. Nickel Acceptor d Orbitals. Ligand field splitting
of d orbitals for a pseudotetrahedral Ni(II) ion can be
(47) Davis, M. I.; Orville, A. M.; Neese, F.; Zaleski, J. M.; Lipscomb,
J. D.; Solomon, E. I. J. Am. Chem. Soc. 2002, 124, 602–614.
(48) Ghosh, S.; Cirera, J.; Vance, M. A.; Ono, T.; Fujisawa, K.; Solomon,
E. I. J. Am. Chem. Soc. 2008, 130, 16262–16273.
(46) Kashiwagi, H.; Hashimoto, T.; Tanaka, Y.; Kubota, H.; Makita, T.
Int. J. Thermophysics 1982, 3, 201–215.

464 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chattopadhyay et al.
of the spectroscopic differences between 1 and 5 in their
distinctive configurations. The two highest molecular
orbitals (i.e., 41a0 and 22a00 in 10 and 38a0 and 25a00 in 50)
are primarily nickel-centered d orbitals destabilized by
strong Ni-N σ* interactions, and are unoccupied, con-
sistent with a d8 electron count for Ni(II) and a para-
magnetic, S = 1 ground state in both complexes.
Three β-spin frontier orbitals exhibit Ni-S π-bonding
symmetry in the axial/horizontal thiolate coordination of
50 (23a00-25a00; Figure 10, right). The highest orbital is
unoccupied and metal-centered (25a00: 59% Ni dyz; 19%
S pz), representing Ni-S dπ-pπ* character. The inter-
mediate orbital represents the πip orbital with a minor
non-bonding metal contribution (24a00: 22% Ni dxz, 15%
dyz; 47% S pz). The lowest orbital is a metal centered
π-bond (23a00: 41% Ni dxz; 22% S pz). Overlap in this
orbital is favorably directed toward the axial sulfur, but
minimized in the empty 25a00 antibonding orbital that is
directed toward the vacant equatorial site. Significant
Ni-S π covalency is therefore indicated.
Figure 9. Qualitative symmetry-derived ligand field splittings for tetra-
hedral (left), axial pseudotetrahedral (center), and bent arylthiolate
geometries (right).
qualitatively anticipated by symmetry reduction from Td,
with upper t2 (dxy, dxz, dyz) and lower e (dx2-y2, and dz2
)
sets (Figure 9, left). Umbrella distortion yields C3v sym-
metry with reduced N-Ni-N angles, causing axial de-
stabilization that is offset in the perpendicular equatorial
plane. This gives rise to an axial dz2 orbital sandwiched
between e sets (dxz,dyz over dxy,dx2-y2; Figure 9, center).45
The former exhibit π symmetry toward the axial ligand,
while the filled dz2 orbital assumes σ symmetry, and the
equatorial dxy,dx2-y2 pair is non-bonding.15,45 Off-axis or
sawhorse bending further reduces symmetry to Cs, break-
ing the equatorial degeneracy. The e orbital pairs
are slightly split; within each pair, one orbital adopts
σ symmetry while the other exhibits π overlap (Figure 6,
right). Consequent elaboration of σ interactions stabilizes
the distortion. The standard setting of the Cs point group
invokes an x,y mirror plane containing the nickel-sulfur
bond. The dxy, dx2-y2, and dz2 orbitals are symmetric
under reflection and encompass Ni-SPh σ bonding under
a0 symmetry, while antisymmetric dxz, dyz orbitals are
π bonding under a00 symmetry.
3.6.3. Ni-SAr Bonding. Spin unrestricted calculations
were performed on TpNiSPh models 10 and 50 under
enforced Cs symmetry, focusing on eight β-spin molecular
orbitals corresponding to the five nickel d orbitals and
three arylthiolate donor orbitals. These are illustrated for
both models (Figure 10). Bonding in 10 and 50 were
considered in detail to elucidate the differences between
the diametrically opposed equatorial/vertical and axial/
horizontal configurations. Previous calculations on
TpNi-SC6F5 in the equatorial/vertical coordination
mode analogous to 10 assigned thiolate πip and πop donor
orbitals intermediate energies with respect to the d orbital
manifold; the πop orbital was stabilized by a pπ-dπ*
interaction with a half-filled Ni d orbital, while the
perpendicular πip orbital was destabilized by a fil-
led-filled interaction of pseudo-σ symmetry.44 A quali-
tatively similar electronic structure was found for 10
(Figure 10, left), with the arylthiolate πop and πip donor
orbitals (21a00 and 40a0, respectively) retaining analogous
bonding roles. In the axial/horizontal configuration of 50,
these donor orbitals (24a00 and 37a0) necessarily exchange
their roles because of orthogonal rotation of the arylthio-
late substituent, and bonding overlap is altered by dis-
placement of the sulfur atom within the ligand field.
These combined symmetry effects must be the root cause
The five other relevant β-spin orbitals of 50 (32a0 and
35a0-38a0) exhibit a0 symmetry appropriate for Ni-S σ
bonding. Two of the orbitals are primarily metal-centered
(36a0: 39% dz2, 31% dx2-y2; 38a0: 44% dx2-y2, 21% dz2) and
present nodal planes toward the sulfur atom, rendering
them essentially non-bonding (<4% S px). Similar to
25a00, the latter is destabilized by a Ni-N σ* interaction
and is unoccupied. The third d orbital (35a0: 39% dxy,
12% dz2) is properly aligned to overlap with the πop
orbital (37a0: 33% dxy; 28% S px). However, this orbital
is lacking any sulfur contribution (ca. 1% S px), despite
obvious delocalization onto the aryl substituent. The
third arylthiolate donor orbital, 0specifically the one-node
€
Huckel donor orbital 3b2 (32a : 13% dxy; 33% S px),
brackets the d manifold from below, and with the πop
donor orbital above, forms a stack of three σ orbitals with
bonding, non-bonding, and antibonding character. The
σ bonding can be described as a three-center interaction
between nickel, sulfur, and the aryl substituent. The
intermediate non-bonding orbital (35a0) exhibits signifi-
cant Ni d character and a node on the central sulfur atom,
while the bonding orbital (32a0) is fully delocalized and
stabilized by an additional 0.8 eV below0 the symmetry-
€
isolated one-node Huckel congener (21a , not shown in
Figure 10; cf., 1a2 in Figure 8). As all three orbitals are
filled, net Ni-S dσ-pσ* bonding is not obtained.
The difference in arylthiolate coordination in 10 com-
pared to 50 is displacement of sulfur from an axial to an
equatorial position, with a concordant back-flip and
orthogonal rotation of the aryl substituent from horizon-
tal to vertical. This perturbs several aspects of Ni-SAr
bonding elucidated for 50 (Figure 10, left). Aryl substitu-
ent rotation swaps the pseudo-σ and π bonding roles of
the thiolate πop and πip donor orbitals (21a00 and 40a0 in 10,
respectively); the third thiolate 3b2 donor orbital is also
transformed from σ to π symmetry in 10 (14a00), stabilizing
the overall σ manifold. The Ni-S π bonding is also
perturbed by hopping of sulfur from an axial to an
equatorial position, which enhances overlap in the empty
antibonding orbital (22a00) at the expense of the filled
bonding orbital (20a00) and destabilizes both orbitals. The
opposing effects on σ- and π-bonding yield a relatively
small difference in total bonding energy between 10 and 50,

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Inorganic Chemistry, Vol. 49, No. 2, 2010 465
Figure 10. Relevant β-spin frontier orbitals for 10 (left) and 50 (right). Orbitals are distinguished by parity under imposed Cs symmetry, with a0 (red) and a00
(blue) representations corresponding to respective σ and π overlap along the Ni-S bond vector.
this additional factor yields the aforementioned sixteen
orientations (i.e., equatorial/axial ꢀ horizontal/vertical ꢀ
0/90/180/270ꢀ). However, the 90ꢀ and 270ꢀ rotations are
enantiomorphous, so only 12 configurations are energe-
tically unique. Such rotation is evident in the structure of
PhBP3Ni-S-C6H4-4-tBu reported by MacBeth, et al.15
Even for the miminal TpNiSPh models with no substit-
uents, only six of these configurations are sterically
accessible as local minima (Figure 11), excluding from
consideration the thirteenth umbrella configuration, with
a linear thiolate and both πip and πop in a π-bonding
orientation. Added substituents presumably would
further reduce the number of assessible configurations.
Optimization of the six model conformations suggests a
relatively flat energy surface within 13 KJ/mol of the
global minimum; the absolute accuracy is unknown,
although calculations on isomeric configurations of the
same complex would presumably cancel some systematic
error. Three of these six configurations have been ob-
served experimentally. Five of the seven known tripodal
thiolate structures adopt the equatorial/vertical/0ꢀ con-
Figure 11. Sterically accessible limiting configurations for a simplified
TpNiSPh model. Horizontal translation between columns corresponds to
figuration (Table 1),9,13,14 which is calculated to lie only
90ꢀ rotation about the S-Cipso bond, transposing the πip and πop thiolate
5-8 KJ/mol below the axial/horizontal configurations
donor orbitals, and vertical translation down a columncorresponds to90ꢀ
adopted by 5 and PhBP3Ni-S-C6H4-4-tBu.15 Only small
rotation about the Ni-S bond, transposing Ni dπ* acceptor orbitals.
Number in brackets indicates the relative calculated energy in KJ/mol.
enhancement of steric contact at the ortho position should
suffice to tip the minimum from the equatorial/vertical
configuration observed for 1 into the axial/horizontal
configuration of 5. The modest substitution of 2-4 would
accordingly enable equilibration between both conforma-
tions. Other configurations have yet to be observed,
including an axial/vertical/180ꢀ structure identified as
the global minimum for TpNiSPh. However, this config-
uration was recently observed in a crystal structure of a
tripodal copper(II) phenolate.48
3.6.5. TD-DFT Calculations and Electronic Spectra.
TD-DFT calculations were performed on models 100
and 50 under respective experimental C1 and Cs symmetry,
to provide a basis for assignment of the observed CT
bands already described in electronic spectra (Figure 12).
The vacant β-spin orbitals of σ symmetry (41a0 and 38a0 in
supporting a relatively flat energy surface consistent with
the dynamic structural rearrangement in solution, as
evidenced in NMR spectra.
3.6.4. Conformational Energetics. Including the re-
spective equatorial/vertical and axial/horizontal config-
urations of 10 and 50, a total of 16 limiting bonding
configurations are possible for a bent thiolate. The sulfur
can adopt axial or equatorial positions, and orthogonal
rotation between the vertical and horizontal orientations
exchanges the respective σ and π overlap symmetries of
the πip and πop donor orbitals. Furthermore, the sub-
stituent can rotate away from the Ni-S bond in 90ꢀ
increments, exchanging π overlap with orthogonal nickel
d acceptor orbitals (e.g., 25a00 and 38a0 of 50, Figure 10);

466 Inorganic Chemistry, Vol. 49, No. 2, 2010
Chattopadhyay et al.
interaction with the nearby CT transition (63a r 59a;
22a00 r 20a00) at 612 nm. The remaining features are
pyrazole LMCT and ligand field bands comparable to
those described for 50.
The calculations qualitatively reproduce the differences
observed between the CT spectra of 1 and 5, which arise
from the difference in Ni-S π overlap in their respective
equatorial and axial conformations. This demonstrates
that UV-visible-NIR spectroscopy can distinguish the
various Ni-SAr configurations (Figure 5). The assign-
ments just given also rationalize the comparative shifts in
band energies across the Hammett series 6-8 (Figure 7).
However, while calculated ligand field energies appear to
be reasonably accurate, Ni-S LMCT energies calculated
herein depart from experimental results by an average of
0.6 ( 0.2 eV, in line with the present state of the art.34 The
low symmetry can also enable configuration interactions,
and the energies of the relevant orbitals will be perturbed
by arylthilolate substitution, a continuum of possible
substituent rotations, steric interactions and σ* interac-
tions with the supporting tripodal ligand. Hence, while
unique TD-DFT results were computed for the various
limiting conformations of TpNiSPh (see Supporting
Information), these may not be fully predictive of spectra
for structures yet to be obtained experimentally.
4. Conclusions
We have elicited structurally distinct axial/horizontal and
equatorial/vertical configurations for bent arylthiolate li-
gands at a pseudotetrahedral Ni(II) center using steric inter-
actions. The resulting conformers can be distinguished by
spectroscopy in solution, and we rationalized this result on
the basis of divergent electronic structures arising from
differential Ni-S π overlap. Sterically unhindered arylthio-
late ligands coordinate to a [TpMe,MeNi(II)]þ center in an
equatorial position within an axially vacant trigonal bipyr-
amidal ligand field. Steric contact from increasingly bulky
ortho substitution displaces the arylthiolate ligand into the
axial position. Limiting equatorial and axial configurations
were structurally and spectroscopically defined for 1 and 5,
while complexes 2-4 appear to be distributed between the
limiting structures in solution. Thus, the series of complexes
1-5 can be taken to be a steric titration, in which increasing
size of the ortho arylthiolate disubstitution (i.e., H, Me, iPr,
Ph) destabilizes preferred equatorial/vertical coordination.
In contrast, sterically innocent para modification with sub-
stituents of varying electronic donor strength (i.e., OMe, Me,
Cl) effect informative spectroscopic perturbation of CT
features in 6-8 without altering the coordination geometry
from that of 1.
Figure 12. TD-DFT calculated spectra of 100 (bottom) and 50 (top).
Ligand field and S-Ni LMCT bands are labeled with predominant
orbital contributions. For convenience, calculated lines are presented as
Gaussian curves with line widths of 3 nm (corresponding to the vertical
intensity scale) and 50 nm (vertical scale exaggerated with offset).
10 and 50, respectively) exhibit little contribution from the
thiolate sulfur (3.1 and 3.7% S p, respectively). The TD-
DFT calculations accordingly predict that the strongest
transitions arise within the Ni-S π bond manifolds. The
next set of empty acceptor orbitals are π* orbitals loca-
lized on the pyrazole and arylthiolate substituent rings; as
these lie 3.5-3.8 eV above the highest occupied orbitals,
they would support MLCT and π-π* transitions deeper
into the UV.
The three-orbital π stack in 50 yields two CT excitations
(Figure 12, top), one at 806 nm (25a00 r 24a00) and the
other at 482 nm (25a00 r 23a00). Several weaker pyrazole-
centered LMCT bands are calculated to fall in the range
of about 400-600 nm;49 two such transitions are evident
on the high-energy side of the 482 nm LMCT band. The
three most prominent ligand field bands fall at 1261, 712,
and 571 nm. Calculations on 100 (Figure 12, bottom) also
predict two strong CT bands arising from the Ni-S π
interaction at 674 (63a r 60a, corresponding to 22a00 r
21a00 under Cs symmetry) and 560 nm (63a r 58a), plus a
third band at 359 nm arising from the 3b2 donor orbital
(63a r 49a; 22a00 r 14a00). Owing to the reduced
symmetry, the intermediate band is actually a ligand
field transition that gains intensity by configuration
Only a handful of other pseudotetrahedral Ni(II) arylthio-
late complexes have been structurally characterized, and the
majority of these are tetragonally distorted (i.e., D2d
symmetry) [Ni(SR)4]2- salts.17-20 Also noteworthy is a linear
Ni(SAr)2 complex.21 The others are monothiolate complexes
supportedbyanionic tripodal borateligands. These examples
include
PhB(CH2PPh2)3Ni-S-4-C6H4Bu,15
TpiPr,iPrNi-
SC6F5,13 PhB(CH2StBu)3Ni-SC6F5,14 PhB(CH2StBu)3Ni-
SC6H5,14 and TpPh,MeNi-S-2,6-Me2C6H3 (9).9 The first com-
plex was the initial example of an axial/horizontal config-
uration like 5, but also uniquely twisted toward a 90ꢀ
sidesaddle configuration. The arylthiolate is bent off-axis
(49) Randall, D. W.; DeBeer George, S.; Hedman, B.; Hodgson, K. O.;
Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 11620–11631.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 467
into an equatorial/vertical orientation analogous to 1 in all
the other complexes. In the last complex, the relatively large
3-Ph pyrazole moieties enforce retention of a vertical orienta-
tion even with the same ortho substituents as 2.
and approaching η2 coordination. The geometry and
bonding of this complex are analogous to equatorial/
vertical arylthiolate coordination, and an axial conforma-
tion with a reversed alkylperoxo ligand can be predicted on
this basis.
In concert with these results, our study suggests an
expanded range of bonding configurations is accessible to
arylthiolates and isolobal ligands at pseudotetrahedral metal
centers. Structuralcharacterization of disparate coordination
geometries underscores the potential to elicit rich coordina-
tion chemistry by steric manipulation at the periphery of
these ligand fields. As the conformers can be distinguished by
routine spectroscopy, a full range of ligand configurations
can be eventually observed and their effects on complex
reactivity elucidated.
The recent structural characterization of two Cu(II) phe-
nolate complexes, TpiPr,iPrCu-O-C6H4-4-F and TptBu,iPr
-
Cu-O-C6H4-4-F, which are sterically distinguished by the
proximal 3-pyrazole positions rather than substitution on the
phenolate co-ligand, provides another relevant example.48
The phenolate ligand binds axially in both complexes; how-
ever, in the TpiPr,iPr complex, the substituent is again twisted
toward a sidesaddle orientation, while the bulkier 3-tBu
substituents force the latter complex into an axial/vertical/
180ꢀ conformation. This switches phenolate donor orbitals
between the complexes, and as for 1 and 5, significant
differences were observed in the electronic spectra of these
complexes.
Acknowledgment. This work was supported by startup
funding provided by Ohio University (M.P.J.).
More recently, structural characterization of a pseudo-
tetrahedral alkylperoxo complex TpiPr,iPrNi-OOtBu was
reported.50 The Ni-O bond was displaced from the
Supporting Information Available: Miscellaneous spectro-
scopic data, TD-DFT spectra and Cartesian coordinates for
six geometry optimized conformations of TpNiSPh (.pdf).
Crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
H-B Ni axis toward one pyrazole, with the OR
3 3 3
substituent folded back between the other two pyrazoles
(50) Hikichi, S.; Okuda, H.; Ohzu, Y.; Akita, M. Angew. Chem., Int. Ed.
2009, 48, 188–191.