From a paramagnetic, mononuclear supersulfidonickel(II) complex to a diamagnetic dimer with a four-sulfur two-electron bond

Article abstract of DOI:10.1021/ja806101h

The first isolable side-on supersulfidonickel(II) complex 1₁ with the elusive S₂^-? ligand has been synthesized by facile oxidation of the corresponding nickel(I) precursor [LNi] with elemental sulfur. Remarkably, paramagnetic 1₁ associates voluntarily to give the diamagnetic dimer 1₂ with a four-sulfur two-electron bond as proven by X-ray diffraction analysis, spectroscopic measurements (¹H NMR, EPR, SQUID), and DFT calculations. Gentle sulfur transfer of 1₁ to Ph₃P or its reaction with [LNi] affords solely the genuine disulfide complex 3 having a Ni₂(μ-μ²:μ²-S₂) core. Copyright

Full text of DOI:10.1021/ja806101h

Published on Web 09/20/2008
From a Paramagnetic, Mononuclear Supersulfidonickel(II) Complex to a
Diamagnetic Dimer with a Four-Sulfur Two-Electron Bond
Shenglai Yao,† Carsten Milsmann,‡ Eckhard Bill,‡ Karl Wieghardt,‡ and Matthias Driess*,†
Metalorganics and Inorganic Materials, Institute of Chemistry, Technische UniVersita¨t Berlin, Strasse des 17,
Juni 135, Sekr. C2, D-10623 Berlin, Germany, and Max-Planck-Institut fu¨r Bioanorganische Chemie,
Stiftsstrasse 34-36, 45470 Mu¨lheim/Ruhr, Germany
Received August 4, 2008; E-mail: matthias.driess@tu-berlin.de
Transition-metal complexes of diatomic chalcogen ligands X2 are
of interest because of their role in oxidation processes in biological
and catalytic systems.1 Owing to their importance, respective peroxo
(O22-) as well as persulfido (disulfido) complexes (S22-) have attracted
much attention, resulting in a vast number of complexes for
structure-reactivity studies containing one or more X2 units with a
“side-on” or “end-on” coordination mode.2 In contrast, much less is
-
known about complexes containing the paramagnetic superoxo (O2
)
and supersulfido (S2-) ligand, respectively, despite the fact that they
can be key intermediates in metal-mediated X-X bond activation and
X-atom transfer reactions.3,4 Thus it is challenging in synthetic
chemistry to develop isolable superchalcogenidometal complexes
capable of soft X-atom transfer for chemoselective C-H bond
functionalization. Up to now, the elusive S2- radical anion could only
be detected by spectroscopy5a and only a few “end-on” supersulfido
complexes have been characterized structurally.4,5b,c A “side-on”
supersulfido complex is as yet unknown. Recently, we developed a
simple route to the first isolable superoxonickel complex [LNi(O2)]
bearing a tetracoordinate Ni(II) center by taking advantage of the
electronic nature of the bulky substituted, monoanionic ꢀ-diketiminate
ligand L-, CH{(CMe)(2,6-iPr2C6H3N)}2-.6 The latter complex results
from one-electron reduction of O2 by the corresponding Ni(I) precursor.
Its fascinating magnetic properties and reactivity prompted us to probe
whether the supersulfidonickel homologue 1 could be synthesized by
direct oxidation of the same Ni(I) precursor, [(LNi)2(toluene)] 2, with
elemental sulfur. Herein, we report the facile synthesis, the striking
structural features, and the reactivity of the first supersulfidonickel
complex 1 with the “side-on” S2- ligand.
Figure 1. Molecular structure of 12 in the crystal. Thermal ellipsoids are
drawn at 50% probability level. Hydrogen atoms are omitted for clarity.
(Atoms with a prime (′) have equivalent coordinates (-x + 2, -y + 1,
-z). For details see Supporting Information.
EI mass spectrometry. Its molecular structure has been established
by X-ray diffraction analysis (Figure 1). Unexpectedly, the com-
pound crystallizes in the form of centrosymmetric dimers (12)
featuring a four-membered, rectangular-shaped S4 ring with two
relatively short (S1-S2 194.4(2) pm) and two very long S-S
distances (S1-S2′ 277.7(2) pm). The S1-S2 distance is signifi-
cantly shorter than that in [(S2WS2)Ni(S2)]2- (203.8 pm),7a and those
values observed in related “side-on” disulfido complexes.1a Like-
wise, the S1-S2 distance is only slightly longer than the value for
the SdS double bond in S2 (189 pm) and in [S2I4]2+ (182 pm)7b
-
indicating significant double-bond character of the supersulfido S2
Reaction of 2 with S8 in a molar ratio of 2:1 in toluene at -60
°C slowly gives rise to a color change from red-brown to yellow-
brown. The complete conversion of 2 to 1 can be ensured by
warming up the mixture to room temperature (Scheme 1).
ligand. This is consistent with the ν(S-S) stretching vibration mode
of 606 cm-1 determined by IR spectroscopy.2c The much longer
S1-S2′ distance is substantially smaller than the sum of the van
der Waals radii (360 pm) and even shorter than the transannular
2+
S-S distance in S8 (288 pm)8a but similar to those in metal
Scheme 1. Synthesis of 1 and 3
complexes with a rectangular S4 unit.8b How could one explain
this peculiar association? Since 11 is isolobal with its paramagnetic
superoxo homologue,6 the SOMO of which is a singly occupied
π* orbital that is completely localized on the O2 ligand, the
dimerization of 11 leads to a stabilization by π*-π* interaction
between the larger sulfur atoms. This is supported by DFT
calculations (see below and Supporting Information). Accordingly,
the measurement of the effective magnetic moment of a solid
sample of 12 displays diamagnetic behavior. Apparently, the driving
force for the dimerization arises from the formation of a four-sulfur
two-electron bond. As expected, the geometric features of the LNiS2
subunit are identical to those of the oxygen homologue, consisting
of an almost planar six-membered C3N2Ni ring and a square-planar,
tetracoordinate low-spin Ni(II) atom. The Ni-S distances of
216.3(1)and216.6(1)pmareclosetothosevaluesin[(S2WS2)Ni(S2)]2-
(av 218.6 pm),7a but significantly shorter than those values in a
Recrystallization of the crude product from n-hexane solutions
at -20 °C affords dark brown, air-stable crystals of 1 in 87% yield.
Its composition is proven by elemental analysis (C, H, N, S) and
† Technische Universita¨t Berlin.
‡ Max-Planck-Institut fu¨r Bioanorganische Chemie
9
13536 J. AM. CHEM. SOC. 2008, 130, 13536–13537
10.1021/ja806101h CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Orbital Diagram for the Dimerization of 11 via
π*(SOMO)-π*(SOMO) Interaction with a Four-Sulfur Two-Electron
Bond in the Dimer 12
Figure 2. Molecular structure of compound 3. Thermal ellipsoids are drawn
at 50% probability level. Hydrogen atoms are omitted for clarity. For details
see Supporting Information.
bond distance of 205.1(1) pm is longer than that in 12, but shorter
than those observed in other complexes with a Ni2(µ-η2:η2-S2) core
[ranging from 217.7(2) to 229.7(4) pm].9a-d The shorter S-S
distance in 3 indicates a less activated S-S bond in the S22- bridge
owing to a smaller back-donation by the Ni centers compared to
those disulfido nickel complexes with higher coordination number
(>4) on nickel.9a
dinuclear, butterfly like Ni(II) disulfido complex with a Ni2(µ-η2:
η2-S2) core [224.2(1) and 224.8(1) pm],9a and in related diphos-
phinonickel(II) disulfides.9e Apparently, diamagnetic 12 dissociates
in d6-benzene solutions at ambient temperature to paramagnetic 11
as indicated by its 1H NMR spectrum displaying paramagnetically
shifted resonances similar to its oxygen homologue. X-band EPR
measurements of 1 in toluene solutions at room temperature reveal
a paramagnetic ground-state for 11 with spin S ) 1/2. The spectrum
in frozen solution at 50 K is rhombic and has principal g values of
Acknowledgment. We thank for financial support from the
Cluster of Excellence “Unifying Concepts in Catalysis” (EXC 314/
1; www.unicat.tu-berlin.de) by the Deutsche Forschungsgemein-
schaft. The work is dedicated to Professor Helmut Schwarz on the
occasion of his 65th birthday.
Supporting Information Available: Experimental details for the
synthesis and spectroscopic data of 1 and 3, DFT calculations, and
Cartesian coordinates of 11 and 12; crystallographic data for 12 and 3
(CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
2.148, 2.082, and 2.073. The corresponding average value, gav
)
2.101, resembles that of S2 in NaI.5a Since the intensity of the
low temperature spectrum reveals less than 5% spin concentration,
11 and 12 must be present in a thermal equilibirium, and predomi-
natly diamagnetic dimers 12 have been trapped upon freezing of
the toluene solutions.
-•
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