Synthesis of a vanadium(III) tris(arylthiolato) complex and its reactions with azide and azo compounds: Formation of a sulfenamide complex via cleavage of an azo N=N bond

Article abstract of DOI:10.1021/ic048721c

The tris(arylthiolate) vanadium(III) complex (1) has been synthesized in good yield. This complex is found to undergo CH activation across a V-S bond in the presence of TMEDA to give a cyclometalated species along with free arylthiol. Complex 1 behaves as a two-electron reductant toward Ad-N ₃, yielding an imide complex. Treatment of 1 with azobenzene produces an imidesulfenamide compound, in which an azo N=N bond cleavage takes place concomitant with formation of a V=N and an S-N bond.

Full text of DOI:10.1021/ic048721c

Inorg. Chem. 2005, 44, 175−177
Synthesis of a Vanadium(III) Tris(arylthiolato) Complex and Its
Reactions with Azide and Azo Compounds: Formation of a Sulfenamide
Complex via Cleavage of an Azo N N Bond
d
Takashi Komuro,† Tsukasa Matsuo,† Hiroyuki Kawaguchi,*,† and Kazuyuki Tatsumi‡
Coordination Chemistry Laboratories, Institute for Molecular Science, Myodaiji,
Okazaki 444-8787, Japan, and Research Center for Materials Science and
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, Furo-cho,
Chikusa-ku, Nagoya 464-8602, Japan
Received September 14, 2004
The tris(arylthiolate) vanadium(III) complex (1) has been synthe-
sized in good yield. This complex is found to undergo CH activation
synthesize reactive low-coordination complexes in a sulfur-
rich ligation environment.4 In this Communication, we report
the synthesis of a mononuclear vanadium(III) complex
having the [SC6H3-2,6-(SiMe3)2]- ()[SAr]-) ligands5 and
its reactivity studies with azide and azo compounds. Pre-
liminary data suggest that the reaction proceeds through a
cooperative activation sequence involving both vanadium and
sulfur centers.
Treatment of VCl3(THF)3 with 3 equiv of LiSAr in toluene
at 60 °C gave a dark orange solution. Solvent removal
followed by hexamethyldisiloxane (HMDSO) extraction and
crystallization produced V(SAr)3(THF) (1) as dark orange
crystals in 80% yield (Scheme 1). The magnetic moment
for 1 (µeff ) 2.8 µB) by the Evans method is consistent with
a high-spin d2 electronic configuration.
Formulation of 1 as a 4-coordinate monomer was con-
firmed by an X-ray crystal structure (Figure 1). Despite the
expected steric preference for the 4-coordinate metal center
to adopt a tetrahedral geometry, 1 has a trigonal monopy-
ramidal structure.6 The THF is at the vertex of the pyramid,
and trigonal ligation to vanadium is provided by the three
thiolate sulfurs with the mean deviation from the VS3 plane
less than 0.02 Å. The vacant site trans to the THF ligand is
occupied by agostic C-H bonds from a SiMe group of the
thiolate ligand [V-C(7)agostic, 2.611(3) Å].7 The reality of
the agostic interaction in 1 is supported by the V-S-C(ipso)
across a V
cyclometalated species along with free arylthiol. Complex 1 behaves
as a two-electron reductant toward Ad N3, yielding an imide
complex. Treatment of 1 with azobenzene produces an imide
sulfenamide compound, in which an azo N N bond cleavage takes
place concomitant with formation of a V N and an S N bond.
−S bond in the presence of TMEDA to give a
−
−
d
d
s
Thiolate complexes continue to attract considerable at-
tention due to their unique chemical properties and structural
diversity.1 These complexes are also useful tools for under-
standing the active sites of metalloenzymes and the surfaces
of metal sulfides.2 Since thiolate groups meet the electronic
and steric requirements necessary to stabilize a wide variety
of metal complexes, they have been used as auxiliary ligands.
On the other hand, thiolate complexes are known to undergo
chemistry at the sulfur center, including oxidation/reduction
and protonation/deprotonation, a complement to traditional
chemistry for these complexes centered on the metal.2,3
Recently, we have begun to investigate coordination chem-
istry of sterically hindered thiolate ligands, aiming to
* To whom correspondence should be addressed. E-mail: hkawa@
ims.ac.jp.
† Institute for Molecular Science.
‡ Nagoya University.
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10.1021/ic048721c CCC: $30.25
Published on Web 12/21/2004
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 2, 2005 175

COMMUNICATION
Scheme 1
Figure 2. Structure of 2. One set of the disordered trimethylsilyl group
Si(4) is shown. Selected interatomic distances (Å) and angles (deg): V-S(1)
2.356(1), V-S(2) 2.380(1), V-N(1) 2.291(4), V-N(2) 2.249(5), V-C(7)
2.066(4); S(1)-V-S(2) 90.55(4), S(1)-V-N(1) 168.9(1), S(1)-V-C(7)
97.3(1), N(1)-V-N(2) 79.6(1).
complicated mixture. On the other hand, inspection of the
1H NMR spectrum of paramagnetic 1 revealed the presence
of a significant amount of free ArSH besides a set of poorly
resolved broad resonances. Elimination of ArSH from 1
presumably occurs via CH activation across a V-S bond,
in which a thiolate ligand undergoes cyclometalation with
an agostic SiMe group to provide a bidentate bit-ArS ligand.
This type of cyclometalation is quite commonplace for the
relatively acidic SiMe groups.10,11 Relief of the crowded
coordination sphere about the metal center may be important
as a driving force in the reaction.
angle being 15-20° larger than the corresponding angles of
the nonagostic thiolate ligands. In addition, the V-S distance
involving the agostic group is ca. 0.05 Å shorter than those
to the ordinary thiolates. It was reported that two THF
molecules could be accommodated by the V(III) center
having three less sterically hindered arylthiolate ligands.8
Evidence for the presence of a cyclometalated species is
provided by the observation that it may be trapped by
TMEDA to afford paramagnetic V(bit-SAr)(SAr)(TMEDA)
(2) (µeff ) 2.7 µB) as a reddish brown powder in 76% isolated
yield. The solid-state structure (Figure 2) contains a 5-co-
ordinate vanadium complex in an approximately trigonal
bipyramidal environment with the bit-SAr sulfur [S(1)] and
one of the TMEDA nitrogen donors [N(1)] occupying the
axial sites. The cyclometalated V-C(7) distance of 2.066-
(4) Å is placed at the short end of the reported V(III)-
alkyls.11,12
In complex 1, THF is to be considered as a labile ligand
for purposes of substituting with stronger and smaller donor
ligands.13 Furthermore, on the basis of the above results, we
Figure 1. Structure of 1. Selected interatomic distances (Å) and angles
(deg): V-S(1) 2.277(1), V-S(2) 2.329(1), V-S(3) 2.328(1), V-O
2.111(2); S(1)-V-S(2) 119.98(3), S(1)-V-S(3) 123.79(3), S(2)-V-S(3)
116.10(3), S(1)-V-O 85.99(5), S(2)-V-O 99.81(6), S(3)-V-O 88.40(5).
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N.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1997, 119, 10104-
10115. (b) Song, J.-I.; Berno, P.; Gambarotta, S. J. Am. Chem. Soc.
1994, 116, 6927-6928. (c) Buijink, J. K. F.; Meetsma, A.; Teuben,
J. H. Organometallics 1993, 12, 2004-2005. (d) Cummins, C. C. Prog.
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Organometallics 1999, 18, 4608-4613. (b) Berno, P.; Minhas, R.;
Hao, S.; Gambarotta, S. Organometallics 1994, 13, 1052-1054.
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2004, 23, 3914-3920.
Although some 3-coordinated V(III) fragments are known
to be very reactive,9 attempts to isolate the desolvated
complex [V(SAr)3] have met with failure. The coordinated
THF was not lost when 1 was recrystalized from HMDSO.
Prolonged reflux of a toluene solution of 1 resulted in a
(8) Randall, C. R.; Armstrong, W. H. J. Chem. Soc., Chem. Commun.
1988, 986-987.
176 Inorganic Chemistry, Vol. 44, No. 2, 2005

COMMUNICATION
postulated that one of sterically demanding thiolates may
potentially dissociate from the metal center during reac-
tions to open up reversibly a larger reactive site. In order to
prove the possibility of labile coordination of THF and/or
[ArS]-, we studied the reactions of 1 with 1-adamantyl azide
(Ad-N3) and azobenzene. Treatment of 1 with 1 equiv of
Ad-N3 in toluene at room temperature was found to produce
the diamagnetic imide complex V(SAr)3(NAd) (3) in 73%
yield with evolution of dinitrogen. The molecular structure
of 3 shows the vanadium center to possess a pseudotetra-
hedral geometry, and the V(SAr)3 fragment remains intact
during the course of the reaction. The formation of 3 results
in oxidation of V(III) to V(V). Accordingly, the average V-S
distance decreases from 2.311 Å in 1 to 2.277 Å in 3. The
short V-N distance of 1.639(2) Å is clearly indicative of a
V-Nimide multiple bond.14 In contrast to 1, the imide complex
3 contains no agostic interaction.
Figure 3. Structure of 4. Selected interatomic distances (Å) and angles
(deg): V-S(1) 2.570(1), V-S(2) 2.270(1), V-N(1) 1.861(3), V-N(2)
1.649(3), V-C(7) 2.029(3), S(1)-N(1) 1.698(3), S(1)-V-N(1) 41.32(8),
N(1)-V-C(7) 105.4(1), S(1)-V-C(7) 93.2(1), V-N(2)-C(31) 173.2(2).
to give an azo adduct intermediate that subsequently
undergoes cleavage of an N-N bond by formation of an
N-S bond, generating an imide and a sulfenamide group.
Obviously, other mechanisms can be considered, including
initial formation of an imide complex analogous to 3 via
bimolecular activation of azobenzene.18 However, no reaction
of 3 with azobenzene was observed.
In conclusion, we have synthesized and characterized a
tris(thiolato) complex of vanadium(III), 1, which behaves
as a two-electron reductant toward Ad-N3 to give an imide
complex 3. In the reaction of 1 with azobenzenene, both V
and thiolate S atoms are found to engage in cleavage of an
azo NdN bond to produce an imide-sulfenamide complex
4 concomitant with cyclometalation of coordinated ArS- and
elimination of free ArSH. Further studies of this and related
systems are in progress.
Compound 1 reacted with 1 equiv of PhNdNPh in
refluxing toluene, producing the sulfenamide V(V) complex
V(NPh)(bit-PhNSAr)(SAr) (4) in 37% isolated yield.15 The
1
aliphatic region of the H NMR spectrum of 4, which
contains three peaks assigned to SiMe3 groups and two peaks
assigned to a SiMe2 group, is consistent with C1 symmetry
and indicates hindered rotation about the V-SAr bond. The
methylene protons of the V-CH2-Si group appear as a pair
of doublets at δ 3.81 and 2.22. A crystal structure determi-
nation of 4 revealed that the pseudotetrahedral coordination
sphere about vanadium is composed of an imide group, a
thiolate ligand, and a newly formed methylene-sulfenamide
bidentate ligand (Figure 3). Within the sulfenamide moiety,
both N(1) and S(1) are bound to the V atom [1.861(3) and
2.570(1) Å, respectively]. The S(1)-N(1) distance of 1.698(3)
Å is typical of sulfenamide complexes.16 The V-N(2)
distance of 1.649(3) Å is similar to that of 3.
The formation of the imide-sulfenamide complex 4 via
scission of an NdN bond is noteworthy. Cleavage of azo
substrates has been observed in low-valent transition-metal
chemistry,17 but this is the first documented example of an
azo NdN cleavage coupled with formation of a sulfenamide
ligand. Although the detailed mechanism of the reaction
remains a matter of speculation, the cyclometalated V(III)
species could be trapped by azobenzene instead of TMEDA
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research [14703008 and 14078101
(Priority Areas “Reaction Control of Dynamic Complexes”)]
from Ministry of Education, Culture, Sports, Science, and
Technology, Japan. T.K. is grateful to JSPS for fellowship
support (16-2303).
Supporting Information Available: Crystallographic data,
synthetic procedures, and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC048721C
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Inorganic Chemistry, Vol. 44, No. 2, 2005 177