New synthetic routes to a disulfidodinickel(II) complex: Characterization and reactivity of a Ni2(μ-η2:η2- S2) core

Article abstract of DOI:10.1021/ic800321x

Activation of elemental sulfur by the monovalent nickel complex [PhTt ^tBu]Ni(CO) [PhTt^tBu = phenyl{tris[(tert-butylmethyl)thio] methyl}borate] generates the disulfidodinickel(II) complex 2. This species is alternatively accessible via thermal decomposition of [PhTt^tBu] Ni(SCPh₃). Spectroscopic, magnetic, and X-ray diffraction studies establish that 2 contains a μ-η²:η²-S ₂ ligand that fosters antiferromagnetic exchange coupling between the Ni^II ions. This observation is in contrast to the lighter congener, oxygen, which strongly favors the bis(μ-oxo)dinickel(III) structure. 2 oxidizes PPh₃ to SPPh₃ and reacts with O₂, generating several products, one of which has been identified as [(PhTt ^tBu)Ni]₂(μ-S) (3).

Full text of DOI:10.1021/ic800321x

Inorg. Chem. 2008, 47, 3931-3933
New Synthetic Routes to a Disulfidodinickel(II) Complex:
Characterization and Reactivity of a Ni2(µ-η2:η2-S2) Core
Jaeheung Cho,†,‡ Katherine M. Van Heuvelen,§ Glenn P. A. Yap,† Thomas C. Brunold,§
and Charles G. Riordan*,†
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716,
and Department of Chemistry, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706
Received February 21, 2008
Activation of elemental sulfur by the monovalent nickel complex
[PhTttBu]Ni(CO) [PhTttBu
phenyl{tris[(tert-butylmethyl)thio]-
Alternative preparative routes include atom transfer from
cyclohexene sulfide and 1,2-elimination from (dcpe)Ni-
(SH)Ph, generating an inferred terminal sulfide, (dcpe)NiS,
that dimerizes to a di-µ-sulfido complex.3 Two of the Ni2(µ-
η2:η2-S2) complexes have been structurally characterized, a
dinickel(II) species with N donors7 and the mixed-valent,
nickel(II)-nickel(I) complex ligated by a triphosphine
ligand.8 However, neither the reactivity nor the full magnetic
properties10 of these complexes have been reported. Also,
we are unaware of reports detailing the synthesis of nickel
sulfide complexes via the reaction of nickel(I) and sulfur.
Analogous copper sulfide chemistry has seen a renaissance
with the identification of the Cu4(µ4-S) (Cuz) cluster in the
enzyme nitrous oxide reductase.11 For example, Karlin and
co-workers reported the synthesis of Cu2(µ-1,2-S2)12 and
Cu2(µ-η2:η2-S2)13 complexes as well as their reactivity with
exogenous substrates. Tolman’s laboratory has highlighted
ligand structural effects on the Cu2(µ-η2:η2-S2) core bond-
ing14,15 as well as prepared higher nuclearity species contain-
)
methyl}borate] generates the disulfidodinickel(II) complex 2. This
species is alternatively accessible via thermal decomposition of
[PhTttBu]Ni(SCPh3). Spectroscopic, magnetic, and X-ray diffraction
2
2
studies establish that 2 contains a µ-η :η -S2 ligand that fosters
antiferromagnetic exchange coupling between the NiII ions. This
observation is in contrast to the lighter congener, oxygen, which
strongly favors the bis(µ-oxo)dinickel(III) structure. 2 oxidizes PPh3
to SPPh3 and reacts with O2, generating several products, one of
which has been identified as [(PhTttBu)Ni]2(µ-S) (3).
The activation of O2 by monovalent nickel complexes has
proven to be a productive approach to the generation of new,
metastable NixOy species that include side-on and end-on
superoxo and trans-µ-1,2-peroxo motifs.1 Initial investiga-
tions focused primarily on exploring the geometric and
electronic structures of these compounds, characteristics that
are fundamental in understanding their stability and reactivity.
Recently, we have turned our attention to the heavier
congener sulfur, with the expectation that a similarly diverse
range of structures may be kinetically accessible via the
analogous synthetic route, i.e., reaction of a nickel(I)
precursor with elemental sulfur. These new structure types
are anticipated to display novel magnetic properties and
reactivity. For example, nickel sulfides are effective catalysts
for hydrogenation2 and hydrodesulfurization3 and have been
explored as cathode materials for rechargeable lithium
batteries.4 Previous reports of soluble nickel sulfide com-
plexes include sulfido-5,6 and disulfido-bridged7–9 species in
which the bridges are most commonly derived from hydrogen
sulfide (or its conjugate bases) via acid-base reactions.
(4) Han, S. C.; Kim, H. S.; Song, M. S.; Kim, J. H.; Ahn, H. J.; Lee,
J. Y. J. Alloys Compd. 2003, 351, 273.
(5) (a) Kruger, T.; Krebs, B.; Henkel, G. Angew. Chem., Int. Ed. Engl.
1989, 28, 61. (b) Oster, S. S.; Lachicotte, R. J.; Jones, W. D. Inorg.
Chim. Acta 2002, 330, 118.
(6) Mealli, C.; Midollini, S.; Sacconi, L. Inorg. Chem. 1978, 17, 632.
(7) Pleus, R. J.; Waden, H.; Saak, W.; Haase, D.; Pohl, S. J. Chem. Soc.,
Dalton Trans. 1999, 2601.
(8) Mealli, C.; Midollini, S. Inorg. Chem. 1983, 22, 2785.
(9) Tremel, W.; Henkel, G. Inorg. Chem. 1988, 27, 3896.
(10) The bulk magnetic moment and 77 K EPR spectrum of the mixed-
valence complex were reported.
(11) Chen, P.; Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. Angew. Chem.
2004, 116, 4224.
(12) (a) Maiti, D.; Woertink, J. S.; Vance, M. A.; Milligan, A. E.; Sarjeant,
A. N. N.; Solomon, E. I.; Karlin, K. D. J. Am. Chem. Soc. 2007, 129,
8882. (b) Helton, M. E.; Chen, P.; Paul, P. P.; Tyeklar, Z.; Sommer,
R. D.; Zakharov, L. N.; Rheingold, A. L.; Solomon, E. I.; Karlin,
K. D. J. Am. Chem. Soc. 2003, 125, 1160.
*
To whom correspondence should be addressed. E-mail:
riordan@udel.edu.
†
University of Delaware.
Current address: Department of Chemistry and Nano Science, Center
‡
for Biomimetic Systems, Ewha Womans University, Seoul 120-750, Korea.
(13) Helton, M. E.; Maiti, D.; Zakharov, L. N.; Rheingold, A. L.; Porco,
J. A.; Karlin, K. D. Angew. Chem., Int. Ed. 2006, 45, 1138.
(14) Brown, E. C.; Bar-Nahum, I.; York, J. T.; Aboelella, N. W.; Tolman,
W. B. Inorg. Chem. 2007, 46, 486.
§
University of WisconsinsMadison.
(1) Kieber-Emmons, M. T.; Riordan, C. G. Acc. Chem. Res. 2007, 40, 618.
(2) Olivas, A.; Cruz-Reyes, J.; Petranovskii, V.; Avalos, M.; Fuentes, S.
J. Vac. Sci. Technol., A 1998, 16, 3515.
(15) Brown, E. C.; Aboelella, N. W.; Reynolds, A. M.; Aullon, G.; Alvarez,
S.; Tolman, W. B. Inorg. Chem. 2004, 43, 3335.
(3) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 7606.
10.1021/ic800321x CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 10, 2008 3931
Published on Web 04/15/2008

COMMUNICATION
Scheme 1
Figure 1. Structure of 2. Thermal ellipsoids are at the 50% level, and H
atoms are omitted. Selected bond lengths (Å) and angles (deg): S4-S4*
2.177(2), Ni-S1 2.304(1), Ni-S2 2.315(1), Ni-S3 2.311(1), Ni-S4
2.242(1), Ni-S4* 2.248(1), Ni · · ·Ni* 3.926; Ni-S4-Ni* 121.99(4),
S4-Ni-S4* 58.01(4).
ing [Cu3(µ-S)2]3+,16 [Cu4(µ-S2)2]4+, and/or [Cu6(µ-S2)4]4+
cores.17 Detailed electronic structure descriptions for several
of these complexes have very recently been presented.18
Herein,we report the structural and spectroscopic char-
acterization of a µ-η2:η2-disulfidodinickel(II) complex,
[(PhTttBu)Ni]2(µ-η2:η2-S2) (2), prepared by following either
of two synthetic routes: (i) reaction of S8 with the nickel(I)
complex [PhTttBu]Ni(CO)19 or (ii) the thermal decomposition
of the nickel(II) complex [PhTttBu]Ni(SCPh3)20 (1) via C-S
bond cleavage (Scheme 1). The latter route is inspired by
the seminal study of Kitajima and Fujisawa21 in which trityl
thiolate decomposition was shown to be an effective prepara-
tive strategy for the synthesis of a Cu2S2 core.22 The
structural and spectroscopic characterizations of 2, and its
reactivity with PPh3 and O2, the latter leading to a µ-sulfi-
dodinickel complex, are detailed.
Figure 2. Electronic spectra of 2 (red line) and 3 (green line) in chloroform
at room temperature. The inset shows the rR spectra (λex ) 472 nm) of frozen
chloroform solutions (77 K) of 2 (red line for 32S-2; blue line for 34S-2).
Stirring a solution of 1 under N2 at room temperature for
several hours afforded a color change from dark violet to
dark brown, signifying the formation of 2 via C-S bond
rupture (Scheme 1).21 This complex is also accessible from
the reaction of [PhTttBu]Ni(CO) with S8, although this
reaction requires 1 week to reach completion. In contrast to
the latter reaction, the reactions of copper(I) precursors with
S8 and the oxygenation of [PhTttBu]Ni(CO) proceed within
minutes to hours. The optical spectra of samples of 2
prepared by either route are indistinguishable. An X-ray
diffraction study revealed that 2 consists of a centrosym-
metric Ni2(µ-η2:η2-S2) core in a slightly distorted square-
pyramidal geometry (τ ) 0.1),23 as shown in Figure 1. The
S-S bond distance, 2.177(2) Å, is shorter than that found
in {[(cyclam)Ni]2(µ-η2:η2-S2)}2+ [2.297(4) Å]7 and {[(triph-
os)Ni]2(µ-η2:η2-S2)}+ [2.208(4) Å].8 The elongated S-S
distances found in the latter two species point to even more
reduced (S2)2- linkages resulting from greater back-donation
from the electron-rich metal to the S2 σ* orbital. Population
of this S2 antibonding orbital serves to weaken and lengthen
the S-S bond. This increased electron density derives, in
large part, from the higher coordination number provided
by cyclam in {[(cyclam)Ni]2(µ-η2:η2-S2)}2+ and the NiIINiI
valency in {[(triphos)Ni]2(µ-η2:η2-S2)}+.
The optical spectrum of 2 displays an intense band
centered at λmax ) 466 nm (ε ) 22 000 M-1 cm-1) and a
shoulder at ∼560 nm (ε ) 4700 M-1 cm-1) (Figure 2), which
are tentatively assigned as the disulfide π*σ f NiII and π*ν
II
2
2
f Ni dx sy charge-transfer (CT) transitions, respectively,
from comparison with the absorption spectra of analogous
Cu2(µ-η2:η2-S2) complexes.22 The resonance Raman (rR)
spectrum (λex ) 472 nm; T ) 77 K) of 2 in chloroform
exhibits an isotopically sensitive band at 446 cm-1
[∆(32S-34S) ) 9 cm-1; Figure 2 (inset)]. On the basis of its
isotope shift and the fact that it is strongly resonance-
enhanced for excitation into the dominant disulfide π*σ f
NiII absorption band at 466 nm, this feature is assigned to
the S-S stretching mode of the Ni2(µ-η2:η2-S2) core. The
energy of this band is within the range of the νS-S values
reported for other transition-metal disulfido complexes
(424-613 cm-1).14 The observed isotopic shift is smaller
than that predicted for a diatomic harmonic oscillator (∼13
cm-1), which may be attributed to mixing between the S-S
and Ni-S stretching motions. The relatively low νS-S
frequency is consistent with the larger S-S bond distance
in 2 than in most previously characterized disulfidodimetal
complexes. Indeed, these data are correlated via the empirical
relationship known as Badger’s rule, which has recently been
applied by Tolman to disulfidodimetal complexes.14 The low
(16) Brown, E. C.; York, J. T.; Antholine, W. E.; Ruiz, E.; Alvarez, S.;
Tolman, W. B. J. Am. Chem. Soc. 2005, 127, 13752.
(17) York, J. T.; Bar-Nahum, I.; Tolman, W. B. Inorg. Chem. 2007, 46, 8105.
(18) Sarangi, R.; York, J. T.; Helton, M. E.; Fujisawa, K.; Karlin, K. D.;
Tolman, W. B.; Hodgson, K. O.; Hedman, B.; Solomon, E. I. J. Am.
Chem. Soc. 2008, 130, 676.
(19) 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.
(20) Cho, J.; Yap, G. P. A.; Riordan, C. G. Inorg. Chem. 2007, 46, 11308.
(21) Fujisawa, K.; Moro-oka, Y.; Kitajima, N. J. Chem. Soc., Chem.
Commun. 1994, 623. This complex was subsequently prepared by the
reaction of copper(I) and S8.22
(22) Chen, P.; Fujisawa, K.; Helton, M. E.; Karlin, K. D.; Solomon, E. I.
J. Am. Chem. Soc. 2003, 125, 6394.
(23) Addison, A. W.; Rao, T. N.; Reedijk, J.; Vanrijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349.
3932 Inorganic Chemistry, Vol. 47, No. 10, 2008

COMMUNICATION
elongation of the Ni · · · Ni distance (from 3.926 to 4.143 Å).
1
3 was characterized by electronic absorption and H NMR
spectroscopies. A very intense optical band centered at λmax
) 494 nm (ε ) 70 000 M-1 cm-1) is tentatively assigned as
a µ-sulfido f NiII CT transition (Figure 2). 3 was found to
possess a diamagnetic ground state at room temperature, as
1
evidenced by its H NMR spectrum and the lack of a
magnetic moment. Collectively, these results suggest that 3
is composed of two antiferromagnetically coupled high-spin
(S ) 1) NiII ions, similar to 2.
Figure 3. Structure of 3. Thermal ellipsoids are at the 50% level, and H
atoms are omitted. Selected bond lengths (Å) and angles (deg): Ni-S1
2.2869(6), Ni-S2 2.2948(7), Ni-S3 2.2839(6), Ni-S4 2.0714(4), Ni · · ·Ni*
4.143; Ni-S4-Ni* 180.
Interestingly, the reactivity of 2 with O2 is quite different
from that reported for its copper analogues. In particular,
the Cu2(µ-η2:η2-S2) complex prepared by Karlin’s group,
which possesses a neutral tridentate N-donor ligand, reacts
with O2 to yield a µ-η2:η2-peroxocopper(II) species at low
temperature,13 whereas 2 does not react with O2 at low
temperatures, conditions under which the bis(µ-oxo)dinick-
el(III) complex persists.27 Alternatively, Tolman’s Cu2(µ-
η2:η2-S2) complexes that are supported by anionic bidentate
N-donor ligands do not react with O2.14 Although the details
concerning the oxygenation of 2 including the establishment
of the major product(s) remain to be established, additional
reactivity studies were carried out to obtain insight into the
character of the Ni2(µ-η2:η2-S2) core in 2 (Scheme 1). The
reaction of 2 with 2 equiv of PPh3 generates SdPPh3
quantitatively (see the Supporting Information for details).
Thus, the electrophilic character of 2 is similar to that
reported recently for copper analogues.13,14
In summary, complex 2 is accessible via two independent
and complementary synthetic routes, namely, the activation
of elemental sulfur by nickel(I) and the thermal decomposi-
tion of 1 via C-S bond cleavage. This report establishes
the reaction of nickel(I) with elemental sulfur as a new
synthetic strategy for the preparation of sulfidonickel adducts.
In contrast to oxygenation, which leads to the thermally
unstable dinickel(III) complex [(PhTttBu)Ni](µ-O)2, 2 contains
a distinctly stable dichalcogenide core. As such, 2 serves as
a model for hitherto unobserved µ-η2:η2-peroxodinickel
complexes. Exposure of 2 to O2 yields the Ni2(µ-S) complex
3, albeit in low yield. Efforts to identify the major product(s)
of this reaction remain ongoing. The reactivity of 2 toward
O2 is distinct from that reported for analogous copper
complexes. Further elucidation of the magnetic properties,
electronic structures, and reactivities of 2 and 3 is underway.
νS-S value and long S-S distance place 2 near one end of
this correlation, lending support for the (S2)2- formalism.
Complex 2 is electron paramagnetic resonance (EPR) silent
1
at 77 K and exhibits a well-resolved H NMR spectrum in
the δ ) 0-10 region at room temperature (see the Supporting
Information). The resonances for the tert-butyl and phenyl
protons were observed at δ ) 1.68 and in the δ ) 7.51s7.12
range, respectively. These observations suggest that 2 pos-
sesses a diamagnetic ground state. However, a relatively
broad (∆ν1/2 ) 54 Hz) peak at δ ) 7.67 assigned to the
methylene protons displays an upfield shift with decreasing
temperature, characteristic of anti-Curie behavior. A similar
temperature dependence was also noted for the tert-butyl
resonances, indicating that 2 is composed of two high-spin
(S ) 1) NiII ions that are antiferromagnetically coupled to
yield an S ) 0 ground state. From a quantitative analysis of
the temperature dependence of the NMR spectral data, we
estimate that J ) -476(3) cm-1 (H ) -2JS1 ·S2).24 Similarly,
large exchange coupling constants have been estimated for
µ-η2:η2-disulfidodicopper(II) complexes, i.e., |2J| g 600
cm-1.15,21
Complex 2 reacts under aerobic conditions to generate the
diamagnetic species [(PhTttBu)Ni]2(µ-S) (3) in low crystalline
yield (6%).25 As in the case of 2, 3 can be accessed by two
distinct synthetic pathways; the independent reaction of
[PhTttBu]Ni(NO3) with Na2S yields a product that is spec-
troscopically indistinguishable from that obtained by expos-
ing 2 to air. The X-ray structure of 3 revealed that the two
nickel centers are bridged by a single sulfido ligand and adopt
a distorted tetrahedral geometry (τ ) 0.27)26 (Figure 3). The
Ni-S4 bond distance of 2.0714(4) Å and Ni-S-Ni angle
of 180° are comparable to the analogous structural parameters
reported for {[(p3)Ni]2(µ-S)}2+ [p3 ) CH3C(CH2PPh2)3] of
2.034 Å and 180°, respectively.6 The Ni-S bond distance
in 3 [2.0714(4) Å] is significantly shorter than those in 2
[2.242(1) and 2.248(1) Å], while the opening of the
Ni-S-Ni angle (from 121.99° to 180°) gives rise to an
Acknowledgment is made to the NSF (Grant CHE-
0518508) by C.G.R. and to the NSF Graduate Research
Fellowship Program by K.M.V.H. We thank Matt Kieber-
1
Emmons for fitting the variable-temperature H NMR data.
Supporting Information Available: Synthetic procedures and
characterization data (PDF) and X-ray diffraction refinement data
for 2 and 3 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(24) The 1H NMR spectral data were recorded between 213 and 313 K
and fitted to the effective Hamiltonian as detailed in the Supporting
Information.
(25) The reaction of 2 with O2 produces at least two products; the minor
species is 3. The major species (>80%), as deduced by 1H NMR
spectroscopy, displays paramagnetically shifted resonances typical for
a [PhTttBu]NiII species. Efforts to identify this product continue.
(26) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Mu¨nck, E.; Holland, P. L.
J. Am. Chem. Soc. 2004, 126, 4522.
IC800321X
(27) Mandimutsira, B. S.; Yamarik, J. L.; Brunold, T. C.; Gu, W.; Cramer,
S. P.; Riordan, C. G. J. Am. Chem. Soc. 2001, 123, 9194.
Inorganic Chemistry, Vol. 47, No. 10, 2008 3933