Nickel complexes of bidentate [S2] and [SN] borato ligands

Article abstract of DOI:10.1021/ic010966r

Nickel(II) complexes of the monoanionic borato ligands [Ph₂B(CH₂SCH₃)₂] (abbreviated Ph₂Bt), [Ph₂B(CH₂S^tBu)₂] (Ph₂Bt^tBu), [Ph₂B(1-pyrazolyl)(CH₂SCH₃)], and [Ph₂B(1-pyrazolyl)(CH₂S^tBu)] have been prepared and characterized. While [Ph₂Bt] formed the square planar homoleptic complex, [Ph₂Bt]₂Ni, the larger [S₂] ligand with tert-butyl substituents, [Ph₂Bt^tBu], yielded an unexpected organometallic derivative, [Ph₂Bt^tBu]Ni(η²-CH ₂SBu^t) resulting from B-C bond rupture. The analogous thiametallacycle derived from the [S₃] ligand, [PhB(CH₂S^tBu)₃] (PhTt^tBu), has been structurally authenticated (Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands, L.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 331). The [SN] borato ligands formed exclusively the cis stereoisomers upon reaction with Ni(II) sources, [Ph₂B(1-pyrazolyl)(CH₂SR)]₂Ni. Analysis of the Ni(II/I) reduction potentials by cyclic voltammetry revealed a ～600 mV anodic shift upon replacement of two thioether donors ([Ph₂-Bt]₂Ni) with two pyrazolyl donors ([Ph₂B(1-pyrazolyl)(CH₂SCH₃)]₂Ni) consistent with the all thioether environment stabilizing the lower oxidation state of nickel.

Full text of DOI:10.1021/ic010966r

Inorg. Chem. 2002, 41, 1383−1390
Nickel Complexes of Bidentate [S ] and [SN] Borato Ligands
2
Pinghua Ge, Arnold L. Rheingold, and Charles G. Riordan*
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
Received September 12, 2001
t
Nickel(II) complexes of the monoanionic borato ligands [Ph2B(CH SCH ) ] (abbreviated Ph2Bt), [Ph2B(CH SBu)2]
2
3 2
2
(Ph2BttBu), [Ph2B(1-pyrazolyl)(CH SCH )], and [Ph2B(1-pyrazolyl)(CH SBu)] have been prepared and characterized.
t
2
3
2
While [Ph2Bt] formed the square planar homoleptic complex, [Ph2Bt]2Ni, the larger [S ] ligand with tert-butyl
2
t
substituents, [Ph2BttBu], yielded an unexpected organometallic derivative, [Ph2BttBu]Ni(η2-CH SBu), resulting from
2
t
B−C bond rupture. The analogous thiametallacycle derived from the [S ] ligand, [PhB(CH SBu)3] (PhTttBu), has
3
2
been structurally authenticated (Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands, L.; Incarvito, C.
D.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 331). The [SN] borato ligands formed exclusively the cis
stereoisomers upon reaction with Ni(II) sources, [Ph2B(1-pyrazolyl)(CH SR)]2Ni. Analysis of the Ni(II/I) reduction
2
potentials by cyclic voltammetry revealed a ∼600 mV anodic shift upon replacement of two thioether donors ([Ph2-
Bt]2Ni) with two pyrazolyl donors ([Ph2B(1-pyrazolyl)(CH SCH )]2Ni) consistent with the all thioether environment
2
3
stabilizing the lower oxidation state of nickel.
Introduction
(pz)Bt] forms the derivative, cis-[Ph(pz)Bt]2Ni, whereas
[Ph(pztBu)BttBu] yields monomeric [Ph(pztBu)BttBu]NiCl. In the
case of a ligand of intermediate bulk, [Ph(pz)BttBu], the
resulting nickel complex is the five coordinate, chloride-
bridged dimer, {[Ph(pz)BttBu]NiCl}2, in the solid state while
monomeric in solution.8
Our laboratory has been interested in the coordination
chemistry of tridentate borato ligands, [PhTtR] and [Ph(pzR′)-
BtR], which present [S3] or [S2N] donors, respectively, to
metal ions, Chart 1.1-6 The ligand design permits stereo-
electronic control of resulting metal complexes by appropri-
ate choice of the substituents on sulfur and/or the pyrazolyl
ring. To date, we have prepared [PhTtR] derivatives where
the sulfur substituent R is methyl, phenyl, para-tolyl, iso-
propyl, or tert-butyl. The first three ligands yield complexes
of the form [PhTtR]2M where two ligands are accommodated
at a single metal ion.2 Alternatively, the [PhTtiPr] and
[PhTttBu] ligands, because of the larger size of the sulfur
substituents, promote formation of [PhTtR]MX complexes.7
The latter species are ripe for further synthetic elaboration
at the M-X unit. Similar steric control is achieved in the
mixed donor [S2N] ligands, [Ph(pz)BtR]. For example, [Ph-
Beyond the aforementioned coordination chemistry ob-
servations, current applications of these ligands include the
preparation of functional models for mononuclear zinc
enzyme active sites,9 the development the nickel, cobalt, and
iron organometallic chemistry,10 and nickel-promoted di-
oxygen activation11 supported by these thioether-rich ligands.
With respect to organometallic transformations, we have
demonstrated that the B-CH2 bonds of [PhTttBu] are
susceptible to cleavage under certain conditions. For ex-
ample, reaction of [PhTttBu]NiCl with CH3Li produced the
novel thiametallacycle, [κ2-PhTttBu]Ni(η2-CH2SBut), via such
a ligand degradation pathway.10 Mechanistic studies con-
firmed that the thiametallacycle arises from alkylation of
[PhTttBu]NiCl by excess borato ligand generated during the
reaction. To date, this represents the only example of
instability of the [PhTtR] ligands.
* To whom correspondence should be addressed. E-mail: riordan@
udel.edu.
(1) Ge, P.; Riordan, C. G.; Haggerty, B. S.; Rheingold, A. L. J. Am. Chem.
Soc. 1994, 116, 8406-8407.
(2) Ohrenberg, C.; Ge, P.; Schebler, P.; Riordan, C. G.; Yap, G. P. A.;
Rheingold, A. L. Inorg. Chem. 1996, 35, 749-754.
(3) Ohrenberg, C.; Riordan, C. G.; Liable-Sands, L.; Rheingold, A. L.
Coord. Chem. ReV. 1998, 174, 301-311.
(4) Schebler, P.; Riordan, C. G.; Liable-Sands, L.; Rheingold, A. L. Inorg.
Chim. Acta 1998, 270, 543-549.
(8) Chiou, S.-J.; Riordan, C. G. Unpublished results.
(9) Chiou, S.-J.; Innocent, J.; Lam, K.-C.; Riordan, C. G.; Liable-Sands,
L.; Rheingold, A. L. Inorg. Chem. 2000, 39, 4347-4353.
(10) Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands,
L.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123,
331-332.
(5) Chiou, S.-J.; Ge, P.; Riordan, C. G.; Liable-Sands, L.; Rheingold, A.
L. Chem. Commun. 1999, 159-160.
(6) Ohrenberg, C.; Riordan, C. G.; Liable-Sands, L. M.; Rheingold, A.
L. Inorg. Chem. 2001, 40, 4276-4283.
(7) Schebler, P. J.; Riordan, C. G.; Guzei, I.; Rheingold, A. L. Inorg.
Chem. 1998, 37, 4754-4755.
(11) Mandimutsira, B. S.; Yamarik, J. L.; Brunold, T. C.; Gu, W.; Cramer,
S. P.; Riordan, C. G. J. Am. Chem. Soc. 2001, 123, 9194-9195.
10.1021/ic010966r CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/16/2002
Inorganic Chemistry, Vol. 41, No. 6, 2002 1383

Ge et al.
Chart 1
Concurrent with our studies of the tridentate ligands, we
groups is convenient and highly reproducible. Provided the
introduction of the LiCH2SR is conducted at low temperature,
there is no evidence for the formation of either of the
undesired [S2] or [N2] borato ligands. Similar preparative
approaches have led to the isolation of the analogous [S2N]
borato ligands, [Ph(pzR)BtR′].
have been developing synthetic strategies for the correspond-
ing bidentate borato ligands, [Ph2BtR] and [Ph2B(pzR)(CH2-
SR)], Chart 1.5,12 As anionic ligands, comparison can be made
to related monoanionic, bidentate ligands such as bis-
(pyrazolyl)borates, â-diketonates, and the recently popular-
ized â-diketiminates.13 Herein, we report the preparation,
characterization, and comparative electrochemical properties
of nickel complexes of the borato ligands [Ph2Bt], [Ph2BttBu],
[Ph2B(pz)(CH2SCH3)], and [Ph2B(pz)(CH2S(CH2SBut)]. The
preparations of [Ph2Bt]2Ni and [Ph2B(pz)(CH2SCH3)]2Ni
have been communicated.5,12
Preparation of Metal Complexes. The synthesis of
the square planar nickel complexes is outlined in Scheme 1.
Each complex was accessible from either NiCl2‚6 H2O or
Ni(BF4)2‚6 H2O and 2 molar equiv of the corresponding
ligand salt in methanol. The resulting red solids were
1
diamagnetic as judged by their sharp H NMR resonances,
and therefore, each species possesses a square planar
structure. [Ph2Bt]2Ni, [Ph2B(pz)(CH2SCH3)]2Ni, and [Ph2B(pz)-
(CH2SBut)]2Ni are stable and highly soluble in a range of
noncoordinating organic solvents including chlorinated hy-
drocarbons, acetone, benzene, and nitromethane. For the two
complexes derived from the [SN] ligands, [Ph2B(pz)(CH2-
SCH3)]2Ni and [Ph2B(pz)(CH2SBut)]2Ni, cis and trans iso-
mers are possible. Proton NMR spectra of these complexes
displayed a single set of resonances in accord with either
the existence of a single isomer or facile equilibration (on
the NMR time scale) between isomers. Low-temperature
spectra are consistent with the former interpretation. That
is, [Ph2B(pz)(CH2SCH3)]2Ni and [Ph2B(pz)(CH2SBut)]2Ni are
present as single isomers. Each is assigned as the cis structure
on the basis of the following considerations. The bis[S2N]
analogues of nickel, cobalt, and iron, [Ph(pz)Bt]2M, for which
the molecular structures have been elucidated, exist solely
as the cis isomers.5 This observation suggests an electronic
preference for pyrazolyl substituents to reside trans to the
thioether, rather than trans to another pyrazolyl group. A
similar electronic preference should persist in [Ph2B(pz)(CH2-
SCH3)]2Ni favoring the cis form. While in the absence of a
crystal structure this analysis is reasonable for [Ph2B(pz)(CH2-
SCH3)]2Ni, it might not appear as satisfying for [Ph2B(pz)(CH2-
SBut)]2Ni in which the cis isomer would necessarily place
two tert-butyl thioether donors in adjacent coordination sites.
However, careful inspection of X-ray structures of the related
complexes [Ph2Bt]2Ni and [Ph2Bt]2Pd14 show that the sulfur
substituents are projected in roughly orthogonal directions.
That is, one S-C vector lies in the metal’s coordination
plane, while the other is perpendicular to the metal’s plane.
Consequently, van der Waals’ contacts between adjacent tert-
butyl thioether groups in [Ph2B(pz)(CH2SBut)]2Ni should be
minimal. On the basis of these considerations and the present
lack of crystals suitable for X-ray analysis, we are left to
Results and Discussion
Ligand Syntheses. The [S2] ligands, [Ph2Bt] and [Ph2-
BttBu], were prepared conveniently following protocols
developed in this laboratory for the tridentate ligands,
[PhTtR].6 The requisite sulfide, CH3SR (R ) CH3, But), was
readily and quantitatively deprotonated by n-BuLi in the
presence of TMEDA. The resulting carbanion, LiCH2SR, was
added slowly to Ph2BBr yielding the desired product.
Aqueous workup with either [Bu4N]Br or CsCl as the cation
source yielded the ligands, [Bu4N]Ph2Bt and [Cs]Ph2BttBu
,
as air-stable, microcrystalline white solids in moderate yields
(45-80%). The reactivity and properties of the ligand
prepared with either countercation were indistinguishable.
The salts are soluble in chlorinated hydrocarbons, THF, and
acetone. Complete spectroscopic data are contained in the
Experimental Section.
The [SN] borato ligands, containing one thioether and one
pyrazolyl donor, were prepared using a stepwise, in situ
procedure. Addition of equimolar LiCH2SR (R ) CH3, But)
to Ph2BBr at -78 °C in THF effected the incorporation of
one thioether substituent to the borane. The presumed
product, Li[Ph2B(Br)(CH2SR)], was not isolated; however,
its formation was inferred from the identity of the final [SN]
product. To the THF solution was introduced 1 equiv of Lipz.
The resulting mixture was stirred for 12 h, after which time
the desired ligand was precipitated in yields of 40-65% by
addition of aqueous [Bu4N]Br. The white solids, [Bu4N]-
[Ph2B(pz)(CH2SCH3)] and [Bu4N][Ph2B(pz)(CH2SBut)], are
soluble in chlorinated hydrocarbons, THF, and acetone.
Complete spectroscopic data are contained in the Experi-
mental Section. The direct preparation of the [SN] ligands
by sequential introduction of the thioether and pyrazolyl
(12) Ge, P.; Riordan, C. G.; Yap, G. P. A.; Rheingold, A. L. Inorg. Chem.
1996, 35, 5408-5409.
(13) For one recent example: Kim, W. K.; Fevola, M. J.; Liable-Sands, L.
M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17,
4541-4543.
(14) Schebler, P. J.; Rheingold, A. L.; Riordan, C. G. Unpublished results.
1384 Inorganic Chemistry, Vol. 41, No. 6, 2002

Nickel Complexes of Bidentate Borato Ligands
Scheme 1
lated by [Ph2BttBu]. Alternatively, the two-coordinate nickel
complex generated by chloride abstraction by Cs+ (as shown
in Scheme 2) may be the active electrophile. In either
scenario, alkylation yields the thiametallacycle and Ph2B(CH2-
SBut).
Scheme 2
Molecular Structures. The solid-state structure of [Ph2-
Bt]2Ni is contained in Figure 1 with crystallographic and
metric parameters in Tables 1 and 2, respectively. The Ni
ion resides on a crystallographic inversion center which
renders trans thioethers metrically equivalent and ensures a
planar ligation sphere. The Ni-S bond lengths of 2.200(1)
and 2.240(1) Å are consistent with previously reported
Ni(II)-thioether distances in square planar geometries and
are ∼0.1 Å longer than Ni(II)-S(thiolate) bond lengths.16
The bite angle of the borato ligand is slightly less than 90 °;
S(1)-Ni-S(2) ) 86.31(4)°. The six-membered chelate
ring resides in a twisted boat conformation which orients
one of the methyl groups (C(15)) essentially perpendicular
to the NiS4 plane (displacement from S4 plane, 1.66 Å) while
the other (C(16)) lies nearly in the NiS4 plane (displacement
from S4 plane, 0.11 Å). This disposition of the chelate ring
places the phenyl substituents in distinct positions. One (Pheq)
is directed away from the Ni while the other (Phax) is located
directly above the NiS4 plane. This latter orientation results
in phenyl canopies protecting the open axial coordination
sites with the Phax centroid-Ni distance of 3.79 Å. A similar
placement of pseudoaxial phenyl groups has been noted in
the molecular structure of [Ph2B(pz)2]2Ni, in which the Pheq
centroid-Ni distance is 3.45 Å.17 Variable temperature
proton NMR studies in CD2Cl2 established a facile equilibra-
tion of the Pheq and Phax groups in [Ph2Bt]2Ni with an
activation energy (Ea) of 9 kcal/mol. The proposed mecha-
nism of equilibration entails concerted isomerization of the
chelate rings from one twist boat to another as depicted in
Scheme 3. The small value of Ea precludes alternative
pathways involving Ni-S bond breaking.
propose [Ph2B(pz)(CH2SCH3)]2Ni and [Ph2B(pz)(CH2S-
(CH3)3)]2Ni form the cis isomers exclusively.
Reaction of 2 equiv of Cs[Ph2BttBu] with NiCl2 generated
a single, diamagnetic product in high yield (76%), Scheme
1. In stark contrast with the preparation of [Ph2Bt]2Ni, the
material generated was not the simple homoleptic complex,
[Ph2BttBu]2Ni. Rather, the nickel thiametallacycle, [Ph2-
BttBu]Ni(η2-CH2SBut), was produced. The species contains
one [Ph2BttBu] borato ligand and an alkyl thioether derived
from degradation of the second equivalent of [Ph2BttBu]. The
tridentate ligand, [PhTttBu], reacts analogously with NiCl2
yielding [κ2-PhTttBu]Ni(η2-CH2SBut), in which the borato
ligand is bound through two sulfurs.10 Alternatively, reduc-
tion of [PhTttBu]NiCl generates [κ2-PhTttBu]Ni(η2-CH2SBut)
in approximately 40% yield. In the latter reaction, the
proposed reduction results in the release of free borato ligand
which subsequently alkylates [PhTttBu]NiCl. The proposed
mechanism for formation of [Ph2BttBu]Ni(η2-CH2SBut) is
contained in Scheme 2. Coordination of one [Ph2BttBu] results
in a three-coordinate [Ph2BttBu]NiCl.15 This 14-electron
intermediate should be sufficiently electrophilic to be alky-
(15) An elegant example of three-coordinate Ni(II): Mindiola, D. J.;
(16) Halcrow, M. A.; Christou, G. Chem. ReV. 1994, 94, 2421-2481.
(17) Cotton, F. A.; Murillo, C. A. Inorg. Chim. Acta 1976, 17, 121-124.
Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623-4624.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1385

Ge et al.
Table 1. Crystallographic Data for [Ph2Bt]2Ni, [Ph2BttBu]Ni(η2-CH2SBut), [Bu4N]2[trans-(Ph2Bt)2NiBr2] and [Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2]
[Ph2Bt]2Ni
[Ph2BttBu]Ni(η2-CH2SBut)
[Bu4N]2[trans-(Ph2Bt)2NiBr2]
[Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2]
formula
fw
C32H40B2NiS4
633.21
C27H43BNiS3
533.31
C64H112B2Br2N2NiS4
781.87
C68H115B2N5NiS6
1275.34
space group
color, habit
a, Å
b, Å
c, Å
P21/n
P21/n
Pna21
green/yellow plate
22.2330(4)
10.3879(4)
29.6866(6)
P1h
orange block
10.3144(9)
9.365(2)
orange blade
9.7472(2)
15.0844(2)
19.4406(2)
yellow blade
13.42540(10)
13.6055(2)
21.8006(2)
76.653(1)
86.038(1)
76.233(1)
3762.79(7)
2
16.793(1)
R, deg
â, deg
107.624(9)
94.9262(4)
γ, deg
V, Å3
1546.0(3)
2
2847.82(6)
4
6856.2(3)
4
Z,
F (calcd), g cm-3
µ(Mo KR), cm-1
temp, K
diffractometer
radiation
R(F), %a
R(wF2), %a
1.360
9.19
293(2)
1.244
9.14
173(2)
1.480
16.08
213(2)
1.126
4.64
173(2)
Siemens P4/CCD
Mo KR (λ ) 0.71073 Å)
4.10
4.04
8.53
7.06
19.08
5.49
14.08
19.26
a Quantity minimized ) R(wF2) ) ∑[w(Fo - Fc )2]/∑[(wFo )2]1/2; R ) ∑|Fo - Fc|/(Fo) × 100%.
2
2
2
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for
[Ph2Bt]2Ni, [Ph2BttBu]Ni(η2-CH2SBut), [Bu4N]2[trans-(Ph2Bt)2NiBr2],
and [Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2]
[Ph2Bt]2Ni
Ni-S(1)
2.244(1)
2.200(1)
1.799(4)
1.818(4)
1.806(4)
1.808(4)
86.31(4)
S(1A)-Ni-S(2)
C(15)-S(1)-C(13)
C(15)-S(1)-Ni
C(13)-S(1)-Ni
C(16)-S(2)-C(14)
C(16)-S(2)-Ni
C(14)-S(2)-Ni
93.69(4)
101.8(2)
107.3(2)
111.2(2)
101.0(2)
116.0(2)
106.3(1)
Ni-S(2)
S(1)-C(15)
S(1)-C(13)
S(2)-C(16)
S(2)-C(14)
S(1)-Ni-S(2)
[Ph2BttBu]Ni(η2-CH2SBut)
Ni-C(23)
1.926(5)
2.232(2)
2.178(2)
2.148(2)
49.6(1)
S(3)-Ni-S(1)
S(2)-Ni-S(1)
C(1)-S(1)-C(15)
C(1)-S(1)-Ni
C(15)-S(1)-Ni
C(2)-S(2)-Ni
C(23)-S(3)-Ni
C(24)-S(3)-Ni
108.70(7)
95.59(7)
105.0(3)
110.0(2)
109.3(3)
109.5(2)
58.5(2)
Ni-S(1)
Ni-S(2)
Ni-S(3)
C(23)-Ni-S(3)
C(23)-Ni-S(2)
S(3)-Ni-S(2)
C(23)-Ni-S(1)
107.2(1)
152.79(8)
157.2(1)
113.9(2)
[Bu4N]2[trans-(Ph2Bt)2NiBr2]
Ni-S(2)
2.470(2)
2.470(2)
2.476(2)
2.476(2)
2.591(1)
2.625(1)
176.98(7)
88.70(7)
92.95(7)
90.87(6)
87.55(7)
S(1)-Ni-S(4)
S(2)-Ni-Br(1)
S(3)-Ni-Br(1)
S(1)-Ni-Br(1)
S(4)-Ni-Br(1)
S(2)-Ni-Br(2)
S(3)-Ni-Br(2)
S(1)-Ni-Br(2)
S(4)-Ni-Br(2)
S(1)-Ni-S(4)
178.40(7)
88.68(5)
88.81(5)
89.41(5)
92.13(5)
91.21(5)
91.32(5)
89.65(5)
88.81(5)
178.40(7)
Ni-S(3)
Ni-S(1)
Ni-S(4)
Ni-Br(1)
Ni-Br(2)
S(2)-Ni-S(3)
S(2)-Ni-S(1)
S(3)-Ni-S(1)
S(2)-Ni-S(4)
S(3)-Ni-S(4)
[Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2]
Ni-N(2)
2.041(3)
2.049(3)
2.451(1)
2.462(1)
2.470(1)
2.486(1)
93.6(1)
88.9(1)
91.9(1)
88.8(1)
88.9(1)
N(2)-Ni-S(2)
N(1)-Ni-S(2)
S(1)-Ni-S(2)
S(3)-Ni-S(2)
N(2)-Ni-S(4)
N(1)-Ni-S(4)
S(1)-Ni-S(4)
S(3)-Ni-S(4)
S(2)-Ni-S(4)
C(33)-N(1)-Ni
C(34)-N(2)-Ni
177.7(1)
88.7(1)
91.38(3)
90.91(3)
87.8(1)
177.8(1)
89.90(3)
89.29(3)
89.92(3)
153.2(3)
167.7(3)
Figure 1. Thermal ellipsoid plots of [Ph2Bt]2Ni (A) and [Ph2BttBu]Ni(η2-
Ni-N(1)
CH2SBut) (B) at 30% probability level; hydrogen atoms not shown.
Ni-S(1)
The structure of [Ph2BttBu]Ni(η2-CH2SBut), shown in
Figure 1, highlights the unexpected consequence of position-
ing tert-butyl substituents on the sulfur donors of the
bidentate borato ligand, [Ph2BtR]. The borato ligand is
coordinated in the bidentate mode with the chelate ring in
the twist boat conformation. The Ni-S(1) bond distance for
the thioether trans to the alkyl group is longer (2.232(2) Å)
than the other, mutually trans thioethers NisS (2.178(2) and
2.149(2) Å) because of the trans influence of the alkyl
substituent. The Ni-C(23) bond distance is 1.926(5) Å.
Expectedly, the NisSsC angle of the metallacycle is acute
Ni-S(3)
Ni-S(2)
Ni-S(4)
N(2)-Ni-N(1)
N(2)-Ni-S(1)
N(1)-Ni-S(1)
N(2)-Ni-S(3)
N(1)-Ni-S(3)
S(1)-Ni-S(3)
177.57(4)
at 58.5(2)°. The solid-state structure shows a slight twist from
square planar, 14°, a consequence of close contact between
the phenyl substituent of the borato ligand and the tert-butyl
of the alkyl thioether fragment. Metric parameters agree well
1386 Inorganic Chemistry, Vol. 41, No. 6, 2002

Nickel Complexes of Bidentate Borato Ligands
Scheme 3
Table 3. Cyclic Voltammetry Data for [Ph2Bt]2Ni,
couple. No anodic peaks were noted up to 1.2 V. Comparison
of the reductive peak currents with internal ferrocene
corroborates the assignment of one-electron processes. Only
[Ph2Bt]2Ni shows a quasireversible signal at modest scan
velocities (<1 V/s). The other three complexes are reduced
irreversibly on the electrochemical time scale in scan rate
dependent processes. The trend in reduction potentials
supports the propensity with which thioethers stabilize the
lower valent states of nickel.22 That is, the Ni[S4] complex,
[Ph2Bt]2Ni, has the most accessible Ni(I) state at -1.13 V.
The two Ni[S2N2] complexes are reduced at similar potentials
which are ∼600 mV more negative than those for [Ph2Bt]2Ni.
The complex with the least polarizable ligands, [Bp*]2Ni, is
the most difficult of the four to reduce with a potential of
less than -2.4 V. The assignment of the cathodic waves to
Ni(I) formation is supported by chemical reduction of [Ph2-
Bt]2Ni with Na/Hg. The resulting pale yellow material, Na-
[(Ph2Bt)2Ni], has been characterized as an authentic Ni(I)
complex on the basis of its rhombic ESR signal (g ) 2.27,
2.17, 2.09) and its Ni L-edge X-ray absorption spectrum.23
The integrated intensity of the latter is diagnostic for
assigning the nickel oxidation state. Recently, we have
demonstrated that reduction of [PhTttBu]NiCl (E1/2 ) -1.30
mV) in the presence of suitable donor ligands (CO, CNR,
PMe3) generates isolable Td Ni(I) derivatives, [PhTttBu]Ni-
(L).10
[Ph2B(pz)(CH2SCH3)]2Ni, [Ph2B(pz)(CH2SBut)]2Ni, and [Bp*]2Ni
cmpd
[Ph2Bt]2Ni
Ef,a V ∆Ep, mV Ip,a/Ip,c
-1.13 140 0.92
quasireversible
irreversible
irreversible
irreversible
[Ph2B(pz)(CH2SCH3)]2Ni -1.69
[Ph2B(pz)(CH2SBut)]2Ni -1.73
[Bp*]2Ni28
-2.42
a
Vs Fc/Fc+ in CH2Cl2.
with those determined for the related complex, [κ2-PhTttBu]-
Ni(η2-CH2SBut), prepared similarly.10 The latter complex
contains the potentially tridentate borato ligand coordinated
via only two thioethers. Related η2-CH2SCH3 ligated com-
plexes of nickel have been reported, for example, (PPh3)-
Ni(Cl)(η2-CH2SCH3),18 although this report represents the
first nickel derivative to be structurally authenticated. Cat-
ionic [(PPh3)2Pd(η2-CH2SCH3)]+ and neutral [Cp2Ti(PMe3)-
(η2-CH2SCH3)] are selected examples from either end of the
transition series for which molecular structures have been
determined.19,20 The C(23)sS(3) bond distance of 1.720(5)
Å in [Ph2BttBu]Ni(η2-CH2SBut) is midway between that of a
CsS single bond (1.82 Å) and a CdS double bond (1.61
Å), suggesting some contribution from resonance structure
B, the methylene tert-butylsulfonium ion, should be consid-
ered.
Reactivity of [Ph2Bt]2Ni. The ligand substitution chem-
istry of square planar [Ph2Bt]2Ni is summarized in Scheme
4. Brick red [Ph2Bt]2Ni dissolved in CH3CN to form a
yellowish-green solution with optical characteristics in accord
with six-coordinate nickel formulated as [Ph2Bt]2Ni(CH3-
CN)2; d-d transitions occur at 593 and 940 nm. Formation
is reversible as solvent removal under vacuum generates the
red starting material. Similar observations made in pyridine
are suggestive of reversible [Ph2Bt]2Ni(py)2 production. [Ph2-
Bt]2Ni reacts with 2 molar equiv of [Bu4N]Br to form yellow-
green [Bu4N]2[trans-(Ph2Bt)2NiBr2]. The molecular structure
has been established by X-ray diffraction, Figure 2, with
selected metric parameters in Table 2. To accommodate the
axial bromide donors, the two borato chelate rings adopt twist
chair conformations which place the phenyl groups away
Bond valence analysis of [Ph2BttBu]Ni(η2-CH2SBut) yields
a C-S bond order of 1.24 in agreement with substantial
double bond character and in accord with contribution from
resonance form B.21
Electrochemical Properties. Cyclic voltammetry data for
[Ph2Bt]2Ni, [Ph2B(pz)(CH2SCH3)]2Ni, [Ph2B(pz)(CH2SBut)]2-
Ni, and [Bp*]2Ni (Bp*, dihydrobis(3,5-dimethylpyrazolyl)-
borate) in CH2Cl2 are contained in Table 3. Formal potentials
are referenced to Fc/Fc+. Each complex displays a cathodic
wave at negative potentials assigned to the Ni(II)/Ni(I)
(18) Davidson, J. G.; Barefield, E. K.; VanDerveer, D. G. Organometallics
1985, 4, 1178-1184.
(19) Miki, K.; Kai, Y.; Yasuoka, N.; Kasai, N. Bull. Chem. Soc. Jpn. 1981,
54, 3639-3647.
(22) Musie, G.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1998,
37, 302-310.
(23) Wang, H.; Ge, P.; Riordan, C. G.; Brooker, S.; Woomer, C. G.; Collins,
T.; Balasubramanian, M.; Melendres, C.; Graudejus, O.; Bartlett, N.;
Cramer, S. P. J. Phys. Chem. 1998, 102, 8343-8346.
(20) Park, J. W.; Henling, L. M.; Schaefer, W. P.; Grubbs, R. H.
Organometallics 1990, 9, 1650-1656.
(21) Thorp, H. H. Inorg. Chem. 1992, 31, 1585-1588.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1387

Ge et al.
Scheme 4
IR spectrum at 2090 and 2084 cm-1 supporting the cis
stereochemistry. The similar intensities of the symmetric and
asymmetric vibrational modes indicate orthogonal thiocy-
anate vectors.25 In contrast, the trans isomer would have a
very weak or unobservable symmetric νCN mode (IR forbid-
den). The structure of [Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2] is
depicted in Figure 2. The thiocyanates are coordinated via
nitrogen with Ni-N-C angles of 167.7° and 153.2 °. The
Ni-S bonds trans to the thiocyanates are slightly longer,
2.47 and 2.49 Å, than the mutually trans Ni-S thioethers,
2.45 and 2.46 Å. The borato ligands are bound in different
conformations. As in [Bu4N]2[trans-(Ph2Bt)2NiBr2], the che-
late rings have opened from the twist boat configuration of
[Ph2Bt]2Ni to accommodate two additional ligands. One is
in a chair, and the other, a twist chair orientation. Perhaps
the most unusual aspect of the structure is the cis stereo-
chemistry. As described above, [Bu4N]2[trans-(Ph2Bt)2NiBr2]
exists as the trans isomer. Furthermore, the neutral analogue,
(MeSCH2CH2SMe)2Ni(NCS)2, forms exclusively the trans
isomer.24 The isomeric preference may be energetically small,
although the trans form of [Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2]
has not been detected. In the cis isomer, the N-bound
thiocyanates occupy positions trans to the soft thioether
ligands rather than mutually trans. This may be the source
of the preference for the cis isomer.
Figure 2. Thermal ellipsoid plots of anions of [Bu4N]2[trans-(Ph2-
Bt)2NiBr2] (A) and [Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2] (B) at 30% probability
level; hydrogen atoms not shown.
from the nickel. Consequently, the Ni-B distance increases
from 3.41 to 4.13 Å. The structure is metrically similar to
neutral trans-(MeSCH2CH2SMe)2NiBr2.24 In the anion, the
Ni-S bond lengths are similar with an average distance of
2.47 Å, 0.25 Å longer than in the four coordinate precursor.
Addition of excess [Bu4N]SCN to [Ph2Bt]2Ni in THF
yields [Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2] as a blue crystalline
solid. Two equal intensity νCN vibrations are present in the
Summary
Bidentate borato ligands possessing two thioether donors
[S2] and one thioether, one pyrazole donor [SN] are syntheti-
cally accessible in multigram quantities. Nickel complexes
of four such ligands have been prepared and fully character-
(24) Kockerling, M.; Henkel, G. Z. Naturforsch., B: Chem. Sci. 1996, 51,
178-186.
(25) Cotton, F. A.; Wilkinson, G. In AdVanced Inorganic Chemistry, 4th
ed.; John Wiley & Sons: New York, 1988; p 1036-1037.
1388 Inorganic Chemistry, Vol. 41, No. 6, 2002

Nickel Complexes of Bidentate Borato Ligands
through Celite, and the product precipitated by addition of aqueous
CsCl (2.1 g, 12.5 mmol). The white precipitate was isolated by
filtration, washed with H2O (2 × 30 mL), and dried under vacuum.
ized. For the [S2] ligands, the sulfur substituent impacts
greatly product formation. [Ph2Bt] yields the homoleptic
complex, [Ph2Bt]2Ni, which reacts further with bromide or
thiocyanate to form the corresponding six-coordinate species.
Alternatively, [Ph2BttBu] does not generate the homoleptic
complex. Rather, [Ph2BttBu]Ni(η2-CH2SBut) is formed via
borato ligand B-C bond cleavage: a reaction that is
promoted to reduce steric congestion at nickel. Formation
of the thiametallacycle is the outcome of other nickel-based
reactions including reduction of [PhTttBu]NiCl. Electrochemi-
cal analysis of a series of neutral, square planar Ni(II)
complexes highlights the propensity with which the thioether
donors stabilize the Ni(I) state. [Ph2Bt]2Ni is reduced at a
potential 1.2 V more accessible than that of [Bp*]2Ni with
the mixed [SN] donor complexes, [Ph2B(pz)(CH2SCH3)]2-
Ni and [Ph2B(pz)(CH2SBut)]2Ni, exhibiting reductions mid-
way between these two extremes.
1
Yield: 2.9 g (46%). H NMR (CD3NO2): δ 7.38 (br, m-C6H5, 4
H), 7.01 (t, o-C6H5, 4 H), 6.85 (t, p-C6H5, 2 H), 2.13 (q, BCH2,
2JB-H ) 4.5 Hz, 4 H), 1.26 (s, CH3, 18 H). 13C NMR (CD3NO2):
δ 135.2 (m-C6H5), 127.3 (o-C6H5), 123.7 (p-C6H5), 40.5 (C(CH3)3),
1
31.4 (CH3), 26.2 (q, BCH2, JB-C ) 162 Hz). Anal. Calcd for
C22H32BCsS2: C, 52.4%; H, 6.40%. Found: C, 52.0%; H, 6.70%.
[Bu4N][Ph2B(pz)(CH2SCH3)]. (CH3)2S (3.5 mL, 47.6 mmol) and
TMEDA (0.1 mL, 0.65 mmol) were charged into a 300-mL flask
under a N2 atmosphere. n-BuLi (10 mL, 2.5 M in hexanes) was
added dropwise via syringe over 5 min and stirred for 1 h. The
mixture was heated to 40 °C for 1 h to remove excess (CH3)2S.
The solution was cooled to -78 °C and 30 mL of THF added.
This solution was added slowly to Ph2BBr (6.12 g, 25 mmol) in
30 mL THF. The mixture was allowed to warm to 25 °C and stirred
for 1 h. In a separate flask, pyrazole (1.7 g, 25 mmol) was dissolved
in 20 mL of THF. Upon cooling to -78 °C, n-BuLi (10 mL, 2.5
M in hexanes) was added dropwise via syringe over 5 min. At 25
°C, the Li pyrazolate solution was transferred to the borane solution
and stirred for 12 h. The reaction was terminated by addition of 30
mL of H2O. Volatile organics were removed by rotary evaporation,
the aqueous solution was filtered through Celite, and the product
precipitated by addition of aqueous [Bu4N]Br (8.06 g, 25 mmol).
The white precipitate was isolated by filtration, washed with H2O
Experimental Section
Materials and Methods. Unless otherwise stated, all reagents
were obtained from commercial sources and used without further
purification. When dry solvents were required, they were distilled
under N2 and dried as indicated. Hexanes, Et2O, THF, toluene,
benzene, and pentanes were freshly distilled over Na/benzophenone.
Acetonitrile, CH2Cl2, pyridine, and TMEDA were distilled over
CaH2, the latter two under reduced pressure. HPLC grade acetone
(e0.05% H2O) and benzonitrile (e0.005% H2O) were purchased
from Acros Organics, Fisher Scientific. HPLC grade acetonitrile
(e0.001% H2O) was purchased from Burdick and Jackson, Inc.
Ph2BBr,26 ButSCH3,27 and [Bu4N]Ph2Bt12 were prepared according
to procedures previously outlined in the literature. All ligands and
metal complexes were dried under vacuum at refluxing acetone
temperatures unless otherwise stated. Elemental analyses were
performed by Desert Analytics, Inc., Tuscon, AZ. Electronic spectra
were recorded with a Hewlett-Packard 8453 diode array spectro-
photometer. NMR spectra were recorded on a 400 MHz Bru¨ker
spectrometer equipped with a Sun workstation, a Bru¨ker AM-250,
or a Bru¨ker AC-250 spectrometer. Samples were referenced to the
residual protio solvent signal. Cyclic voltammetry was performed
on a BAS 50W system. All experiments were performed in an Ar-
filled glovebox in a cell consisting of a glassy carbon or platinum
disk working electrode (r ) 1 mm), Pt wire counter electrode, and
Ag/AgCl reference electrode. Solutions contained 0.1 M electrolyte
([Bu4N][PF6], dried prior to use) and 10 mM sample. Potentials
were referenced to internal Fc/Fc+ (+410 mV vs Ag/AgCl).
Caution. (CH3)2S is a flammable liquid and presents a pungent
odor. The deprotonation reaction should be Vented through an
aqueous solution of NaOCl.
1
(2 × 30 mL), and dried under vacuum. Yield: 8.6 g (65%). H
NMR (CD3NO2): δ 7.50 (s, pz, 1 H), 7.28 (s, pz, 1 H), 7.24 (d,
m-C6H5, 4 H), 7.03 (t, o-C6H5, 4 H), 6.93 (t, p-C6H5, 2 H), 6.00 (s,
pz, 1H), 3.25 (t, NCH2, 8 H), 2.45 (s, BCH2, 2 H), 1.97 (s, SCH3,
3 H), 1.72 (p, CH2, 8 H), 1.41 (h, CH2, 8 H), 0.98 (t, CH3, 12 H).
13C NMR (CD3NO2): δ 137.0 (pz), 134.1 (pz), 133.2 (m-C6H5),
126.0 (o-C6H5), 123.3 (p-C6H5), 101.5 (pz), 58.0 (NCH2), 24.0
(CH2), 20.4 (SCH3), 19.6 (CH2), 13.7 (CH3). Anal. Calcd for C33H54-
BN3S: C, 74.0%; H, 10.16%; N, 7.84%. Found: C, 74.0%; H,
10.16%; N, 7.66%.
[Bu4N][Ph2B(pz)(CH2SBut)]. CH3SBut (2.7, 25.9 mmol) and
TMEDA (0.1 mL, 0.65 mmol) were charged into a 300-mL flask
under a N2 atmosphere. n-BuLi (10 mL, 2.5 M in hexanes) was
added dropwise via syringe over 5 min and allowed to stir for 8 h.
The solution was cooled to -78 °C and 30 mL of THF added.
This solution was added slowly to Ph2BBr (6.12 g, 25 mmol) in
30 mL of THF. The mixture was brought to 25 °C and stirred for
1 h. In a separate flask, pyrazole (1.7 g, 25 mmol) was dissolved
in 20 mL of THF. Upon cooling to -78 °C, n-BuLi (10 mL, 2.5
M in hexanes) was added dropwise via syringe over 5 min. At 25
°C, the Li pyrazolate solution was transferred to the borane solution
and stirred for 12 h. The reaction was terminated by addition of 30
mL of H2O. Volatile organics were removed by rotary evaporation,
the aqueous solution was filtered through Celite, and the product
precipitated by addition of aqueous [Bu4N]Br (8.06 g, 25 mmol).
The white precipitate was isolated by filtration, washed with H2O
Cs[Ph2BttBu]. CH3SBut (2.65 g, 25.4 mmol) and TMEDA (0.1
mL, 0.65 mmol) were charged into a 300-mL flask under a N2
atmosphere. n-BuLi (10 mL, 2.5 M in hexanes) was added dropwise
via syringe over 5 min and allowed to stir for 8 h. The solution
was cooled to -78 °C and 20 mL of THF added followed by Ph2-
BBr (3.06 g, 12.5 mmol) in 20 mL benzene. The mixture was
allowed to warm to 25 °C and stirred for 48 h. The reaction was
terminated by addition of 30 mL of H2O. Volatile organics were
removed by rotary evaporation, the aqueous solution was filtered
1
(2 × 30 mL), and dried under vacuum. Yield: 6.2 g (43%). H
NMR (CDCl3): δ 7.92 (s, pz, 1 H), 7.45 (s, pz, 1 H), 7.35 (d,
m-C6H5, 4 H), 7.06 (t, o-C6H5, 4 H), 6.95 (t, p-C6H5, 2 H), 6.14 (s,
pz, 1H), 2.51 (t, NCH2, 8 H), 2.40 (s, BCH2, 2 H), 1.28 (s, SCH3,
3 H), 1.22 (m, CH2, 16 H), 0.93 (t, CH3, 12 H). 13C NMR (CD3-
NO2): δ 138.2 (pz), 134.4 (pz), 135.3 (m-C6H5), 127.1 (o-C6H5),
124.6 (p-C6H5), 102.5 (pz), 60.0 (NCH2), 40.8 (C(CH3)3), 31.1
(CH2), 24.8 (C(CH3)3), 20.7 (CH2), 13.9 (CH3). Anal. Calcd for
C36H60BN3S: C, 74.8%; H, 10.5%; N, 7.27%. Found: C, 74.8%;
H 10.3%; N, 7.27%.
(26) Eisch, J.; King, R. B. In Organometallic Syntheses; Academic Press:
New York, 1981; pp 126-127.
(27) Vogel, A. I.; Cowan, D. M. J. Chem. Soc. 1943, 18.
(28) Kokusen, H.; Sohrin, Y.; Matsui, M.; Hata, Y.; Hasegawa, H. J. Chem.
Soc., Dalton Trans. 1996, 195-201.
[Ph2Bt]2Ni. [Ni(H2O)6](BF4)2 (500 mg, 1.47 mmol) was dis-
solved in 50 mL of methanol. To the solution was added solid
Inorganic Chemistry, Vol. 41, No. 6, 2002 1389

Ge et al.
[Bu4N]Ph2Bt (1.56 g, 2.95 mmol) resulting in the slow precipitation
of a brick red solid. The red solid was collected via filtration,
washed with 10 mL of methanol, and dried in vacuo. Yield: 744
mg (80%). UV-vis (CH2Cl2) λmax (ꢀ, M-1cm-1): 354 nm (7300),
412 nm (7800). 1H NMR (CDCl3): δ 7.26 (br, m-C6H5, 8 H), 7.20
(t, o-C6H5, 8 H), 7.06 (t, p-C6H5, 4 H), 1.82 (s, CH2 and CH3, 20
H). 13C NMR (CD2Cl2): δ 133.0 (m-C6H5), 128.1 (o-C6H5), 125.0
(p-C6H5), 22.0 (CH3). Anal. Calcd for C32H40B2S4Ni: C, 60.7%;
H, 6.37%. Found: C, 60.5%; H, 6.17%.
[Ph2BttBu]Ni(η2-CH2SBut). Anhydrous NiCl2 (100 mg, 0.77
mmol) was charged into a flask and suspended in 20 mL of THF.
Cs[Ph2BttBu] (776 mg, 1.54 mmol) was added as a solid, resulting
in a color change to red, yielding a homogeneous solution after 24
h. The solvent was removed under vacuum and the resulting residue
dissolved in 10 mL of methylene chloride, filtered, and concentrated
to a volume of 2 mL. Slow solvent evaporation yielded red crystals.
Yield: 302 mg (76%). Similar yields were obtained using the
[Bu4N] salt of the [Ph2BttBu] ligand. UV-vis (THF) λmax (ꢀ, M-1
cm-1): 300 nm (4000), 470 nm (400). 1H NMR (CD3CN): δ 7.25
(br, m-C6H5, 4 H), 7.04 (t, o-C6H5, 4 H), 6.89 (t, p-C6H5, 2 H),
1.42 (s, C(CH3)3, 18 H), 1.33 (br m, CH2, 6 H), 1.13 (s, C(CH3)3,
9 H). Anal. Calcd for C27H43BS3Ni: C, 60.8%; H, 8.13%. Found:
C, 59.7%; H, 7.99%.
[Ph2B(pz)(CH2SCH3)]2Ni. [Ni(H2O)6](BF4)2 (200 mg, 0.59
mmol) was dissolved in 20 mL of methanol. To the solution was
added solid [Bu4N][Ph2B(pz)(CH2SCH3)] (630 mg, 1.18 mmol)
resulting in the slow precipitation of a brick red solid. The red solid
was collected via filtration, washed with 10 mL methanol, and dried
in vacuo. Yield: 243 mg (61%). UV-vis (THF) λmax (ꢀ, M-1 cm-1):
321 nm (4200), 486 nm (250). 1H NMR (CDCl3): δ 7.50 (br, pz
+ C6H5, 6 H), 7.19 (m, pz + C6H5, 7 H), 6.87 (t, p-C6H5, 2 H),
6.01 (t, pz, 1 H), 2.04 (d, BCH2, 2 H), 1.33 (s, SCH3, 6 H), 1.10
(d, BCH2, 2 H). Anal. Calcd for C34H36B2N4S2Ni: C, 63.3%; H,
5.62%; N, 8.68%. Found: C, 63.1%; H, 5.34%; N, 8.68%.
[Ph2B(pz)(CH2SBut)]2Ni. [Ni(H2O)6](BF4)2 (200 mg, 0.59 mmol)
was dissolved in 20 mL of methanol. To the solution was added
solid [Bu4N][Ph2B(pz)(CH2SBut)] (680 mg, 1.18 mmol) resulting
in the slow precipitation of a brick red solid. The red solid was
collected via filtration, washed with 10 mL of methanol, and dried
in vacuo. Yield: 232 mg (54%). UV-vis (THF) λmax (ꢀ, M-1 cm-1):
321 nm (4200), 486 nm (250). 1H NMR (CDCl3): δ 7.45 (m, pz
+ C6H5, 3 H), 7.33 (d, pz, 2 H), 7.14 (m, p-C6H5, 4 H), 7.06 (t, pz,
1 H), 6.88 (t, C6H5, 2 H), 5.93 (t, pz, 2 H) 1.82 (d, BCH2, 2 H),
1.43 (d, BCH2, 1 H), 1.33 (s, SC(CH3)3, 18 H). Anal. Calcd for
C40H48B2N4S2Ni: C, 65.9%; H, 6.63%; N, 7.68%. Found: C,
62.0%; H, 6.94%; N, 7.24%. Multiple attempts using microcrys-
talline samples to obtain satisfactory %C analyses yielded repro-
ducibly low values consistent with incomplete combustion.
[Bu4N]2[trans-(Ph2Bt)2NiBr2]. [Ph2Bt]2Ni (200 mg, 0.32 mmol)
was added to 20 mL of ethylene glycol dimethyl ether (DME). To
the suspended solution was added [Bu4N]Br (205 mg, 0.63 mmol)
slowly. The reaction mixture was stirred for 12 h resulting in a
color change of the suspension from red to yellow-green. The
yellow-green solid was collected by filtration. The product, [Bu4N]2-
[trans-(Ph2Bt)2NiBr2], was isolated as yellow-green crystals by
diffusing ether into its concentrated acetone solution. Crystalline
yield: 176 mg (43%). UV-vis (acetone) λmax (ꢀ, M-1cm-1): 360
nm (3800), 558 nm (130), 662 nm (60), 962 nm (100). Anal. Calcd
for C64H112B2Br2N2S4Ni: C, 60.2; H, 8.83; N, 2.19. Found: C,
60.2; H, 9.14; N, 2.17. µeff ) 2.6 µB in acetone.
homogeneous green solution. The green solution was stirred for 4
h. The solvent was removed in vacuo. The remaining green oil
was dissolved into 10 mL of ethanol and filtered. Slow evaporation
afforded blue crystals. Crystalline yield: 234 mg (68%). UV-vis
(CH3NO2) λmax (ꢀ, M-1 cm-1): 340 nm (4000), 625 nm (120), 990
nm (100). Anal. Calcd for C66H112B2N4S6Ni: C, 64.2; H, 9.15; N,
4.54. Found: C, 61.1; H, 9.35; N, 4.57. Multiple attempts using
crystalline samples to obtain satisfactory %C analyses yielded
reproducibly low values consistent with incomplete combustion.
µeff ) 2.7 µB in CD3NO2. FT-IR (KBr): νCN ) 2090, 2084 cm-1
.
Variable Temperature 1H NMR Studies of [Ph2Bt]2Ni. Proton
NMR spectra of [Ph2Bt]2Ni were recorded in CD2Cl2 from 203 to
303 K in 10 K intervals. To determine the exchange rate constant,
k, the following expressions were used in (i) the stopped exchange
limit, ∆ν ) k/π; (ii) the intermediate exchange regime, k ) 2.2δν;
(iii) the fast exchange regime, ∆ν ) π(δν)2/2k with ∆ν ) line
width and δν ) the chemical shift difference of exchange sites.
The rate constants were fit to the Arrhenius expression (ln k against
1/T) yielding the activation energy, Ea.
Crystallographic Structural Determinations. Suitable air-stable
crystals were selected and mounted on thin glass fibers using epoxy.
Air-sensitive crystals were handled under an inert atmosphere and
sealed in thin walled capillary tubes. Preliminary unit cell deter-
minations were obtained by harvesting reflections from three
orthogonal sets of 15 frames, using 0.3° ω scans. These results
were confirmed by refinement of unit-cell parameters during
integration. The unit cell for [Ph2Bt]2Ni was determined using least
squares refinement of 25 reflections with 2° e 2θ e 25°.
Crystallographic information is summarized in Table 1. All
structures were solved using direct methods. Non-hydrogen atoms
were located by difference Fourier synthesis and were refined
anisotropically. Hydrogen atoms were added at calculated positions
and treated as isotropic contributions with thermal parameters
defined as 1.2 or 1.5 times that of the parent atom. All software
and sources of scattering factors are contained in SHELXTL
program library (version 5.10, G. Sheldrick, Bruker-AXS, Madison,
WI.)
The systematic absences in diffraction data for [Ph2Bt]2Ni, [Ph2-
BttBu]Ni(η2-CH2SBut), [Bu4N]2[trans-(Ph2Bt)2NiBr2], and [Bu4N]2-
[cis-(Ph2Bt)2Ni(NCS)2] were consistent with the reported space
groups. The E-statistics for [Bu4N]2[trans-(Ph2Bt)2NiBr2] suggested
a noncentrosymmetric space group. The correct absolute structure
was unambiguously determined; Flack parameter ) 0.00(6).
[Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2] cocrystallizes with one molecule of
acetonitrile in the lattice. The structure of [Ph2Bt]2Ni was com-
municated previously. The crystallographic data are included in
Table 1 for comparison. The ligand Bu4N[Ph2Bt] has been
structurally characterized. A thermal ellipsoid plot and crystal-
lographic tables are included as Supporting Information.
Acknowledgment. We thank Julie Dupont for the prepa-
ration and electrochemical characterization of [Bp*]2Ni. We
gratefully acknowledge the financial support provided by the
National Science Foundation (NYI and CHE-9974628 to
C.G.R.).
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
[Bu4N]2[cis-(Ph2Bt)2Ni(NCS)2]. [Ph2Bt]2Ni (200 mg, 0.32 mmol)
was added to 20 mL of CH3CN. To the suspended solution was
added [Bu4N]SCN (950 mg, 3.2 mmol), slowly resulting in a
IC010966R
1390 Inorganic Chemistry, Vol. 41, No. 6, 2002