Synthesis and characterization of low valent vanadium thiolate complexes

Article abstract of DOI:10.1016/j.poly.2008.04.039

The reaction of the bulky lithium terphenyl thiolate LiSAr′ (Ar^′ = C₆ H₃ - 2, 6 - (C₆ H₃ - 2, 6 -Pr₂ⁱ)₂) with VI₂(THF)₂ in THF resulted in the formation of two new V(II) and V(III) thiolate complexes. The addition of one equivalent of LiSAr′ to VI₂(THF)₄ led to the isolation of the dimeric V(II) species {V(μ-SAr′)I}₂ (2); a rare example of a structurally characterized V(II) thiolate. The thiolate ligands bridge the two vanadium centers, which are also bonded to a terminal iodide. In addition, there is an η⁶-interaction between a flanking aryl ring from a terphenyl substituent with each metal. The attempted preparation of the vanadium dithiolate V(SAr′)₂ by reaction of VI₂(THF)₂ with two equivalents of LiSAr′ led to disproportionation which resulted in the isolation of the V(III) product V(SAr′)₂I (3) in which the vanadium center is coordinated by two sulfurs, an iodine, and a weaker interaction to a flanking aryl ring of the terphenyl group.

Full text of DOI:10.1016/j.poly.2008.04.039

Polyhedron 27 (2008) 2337–2340
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of low valent vanadium thiolate complexes
*
Andrew D. Sutton, James C. Fettinger, Brian D. Rekken, Philip P. Power
Department of Chemistry, One Shields Avenue, University of California, Davis, CA 95616, USA
a r t i c l e i n f o
a b s t r a c t
The reaction of the bulky lithium terphenyl thiolate LiSAr0 (Ar0 ¼ C6H3-2; 6-ðC6H3-2; 6-Pr Þ2) with
i
Article history:
2
Received 14 January 2008
Accepted 30 April 2008
Available online 10 June 2008
VI2(THF)2 in THF resulted in the formation of two new V(II) and V(III) thiolate complexes. The addition
l
of one equivalent of LiSAr0 to VI2(THF)4 led to the isolation of the dimeric V(II) species {V( -SAr0)I}2
(2); a rare example of a structurally characterized V(II) thiolate. The thiolate ligands bridge the two vana-
dium centers, which are also bonded to a terminal iodide. In addition, there is an
g
6-interaction between
Keywords:
Vanadium
Thiolate
a flanking aryl ring from a terphenyl substituent with each metal. The attempted preparation of the vana-
dium dithiolate V(SAr0)2 by reaction of VI2(THF)2 with two equivalents of LiSAr0 led to disproportionation
which resulted in the isolation of the V(III) product V(SAr0)2I (3) in which the vanadium center is coordi-
nated by two sulfurs, an iodine, and a weaker interaction to a flanking aryl ring of the terphenyl group.
Ó 2008 Elsevier Ltd. All rights reserved.
Terphenyl
1. Introduction
nitrogenases, although less well understood than their molybde-
num analogues [6–10,21], are believed to have a similar coordina-
Transition metal thiolate and related derivatives [1–4] have
been extensively studied because of their relevance for biological
systems [5,6]. In many instances, they can serve as structural mod-
els for active sites in numerous metal–sulfur proteins such as fer-
redoxins [7,8] or nitrogenases [9–11], which feature cysteine
residues as ligands [6]. However, the preparation of the simplest
neutral homoleptic transition metal thiolates has often been diffi-
tion environment to Mo in Mo-based nitrogenases, with low
oxidation state (II–IV) oxidation states [11,18–20] wherein the me-
tal is coordinated to several sulfur atoms. We now report on our
initial investigations which have resulted in the synthesis of two
low coordinate V(II) and V(III) thiolate complexes.
2. Experimental
cult because of extensive M-(l-SR)-M thiolate bridging which
results in the formation of intractable oligomers or polymers. The
2.1. General procedures
use of sufficiently bulky ligands [12,13] such as —SC6H2-2;
4; 6-Bu3 (–SMes*) [14,15] and –SC6H2-2,4,6-Ph3 [16] mitigates
t
All manipulations were carried out with use of modified
Schlenk techniques under an argon atmosphere or in a Vacuum
Atmospheres HE-43 drybox. All solvents were dried by the method
of Grubbs and degassed three times (freeze–thaw) prior to use,
VI2(THF)4 was synthesized according to the literature procedure
[22]. 1H NMR spectra were recorded on a Varian 300 MHz instru-
ment and referenced to the residual protio benzene in the C6D6 sol-
vent. Melting points were recorded in glass capillaries sealed under
N2 or Ar and are uncorrected. UV–Vis data were recorded on a Hit-
achi-1200 spectrometer. Solution magnetic moment measure-
ments were carried out by the method of Evans as a solution in
C6D6 [23].
extensive bridging interactions so that low coordinate species,
exemplified by the dimeric transition metal complexes
[{M(SMes*)2}2] (M = Mn, Fe, Co) [15] and [{Fe(SC6H2-2,4,6-
Ph3)2}2] [16], can be obtained. In a more recent development, the
use of the very hindered thiolate derivative, –SAr* (Arꢀ ¼
i
C6H3-2; 6-ðC6H2-2; 4; 6-Pr3 Þ2) has led to the isolation of a series
of soluble, monomeric first row transition metal(II) dithiolates
M(SAr*)2 (M = Cr, Mn, Fe, Co, Ni and Zn) in which the metals are
nominally two-coordinate [17]. These complexes exhibit linear
(or nearly linear) S–M–S units with varying degrees of weak inter-
action between the metal center and the flanking aryl rings of the
terphenyl ligands. We wished to extend these results to vanadium
via the synthesis and characterization of low valent vanadium thi-
olates of formula V(SAr)2 (Ar = bulky terphenyl). Vanadium thiol-
ates have been studied because of their potential relevance to
the function of vanadium nitrogenase enzymes [18–20]. Vanadium
2.2. Ar0SH (1)
The terphenyl thiol Ar0SH was synthesized in an analogous
manner to that described for the related thiol HSC6H3-2;
6ðC6H2-2; 4; 6-Pr3 Þ2 [24]. Yield: 72%, m.p. 165–168 °C. 1H NMR
i
(C6D6, 400 MHz, 25 °C):
o-CH(CH3)2), 1.32 (d, JHH = 6.8 Hz, 12H, o-CH(CH3)2), 2.83 (sept,
d
1.11 (d, 3JHH = 6.8 Hz 12 H,
* Corresponding author.
E-mail address: pppower@ucdavis.edu (P.P. Power).
3
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.04.039

2338
A.D. Sutton et al. / Polyhedron 27 (2008) 2337–2340
3JHH = 6.9 Hz, 4H, o-CH(CH3)2), 3.08 (S, 1H, –SH), 6.96 (1H p-C6H3),
suitable crystal was selected, attached to a glass fiber, and placed
in the cold temperature stream as previously described [25]. The
data were collected near 90 K using a Bruker APEX (2) or a SMART
3
3
i
7.21 (d, JHH = 7.6 Hz, 2H, m-C6H3-2; 6-Pr2 ), 7.31 (d, JHH = 7.6 Hz,
2H, p-C6H3-2; 6-Pr2 . 13C{1H} NMR (C6D6, 100.59 MHz): d 24.00,
i
d
24.92 (o-CH(CH3)2), 31.14 (o-CH(CH3)2), 123.67 (m-C6H3-2;
1000 (3) diffractometer and Mo Ka (k = 0.71073 Å) radiation.
i
i
6-Pr2 ), 124.36 (i-C6H3), 129.20 (p-C6H3-2; 6-Pr2 , 129.9 (m-C6H3),
135.61 (i-C6H3-2; 6-Pr2 ), 138.00 (p-C6H3) 138.68 (o-C6H3-2;
6-Pr2 ), 147.26 (o-C6H3).
Absorption corrections were applied using SADABS [26]. The struc-
tures were solved by use of direct methods in SHELXS [27] and re-
fined by the full-matrix least-squares procedure in SHELXL. All non
hydrogen atoms were refined anisotropically, while hydrogens
were placed at calculated positions and included in the refinement
by using a riding model. Some details of the data collection and
refinement are given in Table 1. Further details are in the supple-
mentary material.
i
i
2.3. [V(l
-SAr0)I]2 (2)
(LiSAr0)2 (generated in solution from 0.872 g, 2.0 mmol and
LiBun (1.6 M in hexanes, 1.25 mL)), in THF (25 mL), was added
dropwise to a stirred suspension of VI2(THF)4 (1.186 g, 2.0 mmol)
in THF (25 mL) cooled to ca. 0 °C. The resulting dark, brown-red
solution was stirred overnight while it was allowed to warm to
room temperature. The solvent was removed under reduced pres-
sure and the resulting dark red solid was extracted with hexane
(30 mL) and filtered. The volume of the filtrate was reduced to
ca. 5 mL under reduced pressure to afford dark red, X-ray quality
crystals of 2 after storage for 3 days at ca. ꢁ20 °C. Yield: 0.768 g
(63%) m.p. = 230 °C (dec). 1H NMR (400 MHz, C6D6, 25 °C): d = 7.1
(br, s), 6.9 (br, s), 3.7 (br, s), 2.7 (br, m), 1.3 (br, d), 1.0 (br, d).
3. Results and discussion
The use of sterically encumbering ligands has played a key role
in the stabilization of low coordinate (coordination number 63)
compounds throughout the periodic table [28]. Early work focused
on bulky ligands attached via a second row element C, N, or O in al-
kyl, aryl, amido, alkoxo on aryloxo ligands. The application of li-
gands based on elements from the third or later rows, involving
for example, silyl, phosphido, or thiolate ligands, was a later devel-
opment for a number of reasons that included synthetic and steric
considerations as well as weaker metal–ligand bonding [28].
Although there is now a vast chemistry of transition metal thiolates,
the fact that the sulfur can carry only one organic substituent has
limited their application in the stabilization of low coordination
numbers. We have recently used the very large terphenyl thiolate
UV/Vis (hexanes; kmax, nm (
e
, molꢁ1 L cmꢁ1)): 282 (7300), 384
(2500), 466 (2000). leff = 3.61lB/vanadium.
2.4. [V(SAr0)2I] (3)
A THF (40 mL) solution of (LiSAr0)2 (1.744 g, 4.0 mmol) in THF
(50 mL) was added dropwise to a stirred suspension of VI2(THF)4
(1.186 g, 2.0 mmol) in THF (25 mL) with cooling in an ice bath.
The resulting dark green solution was allowed to warm to room
temperature and stirred overnight. The solvent was removed under
reduced pressure and resulting dark green solid was extracted with
hexane (40 mL) and filtered. The volume of the solution was re-
duced to ca. 10 mL which afforded green, X-ray quality crystals
of 3 after 7 days at ca. 7 °C. Yield 0.552 g (23%) m.p. = 174–178 °C
(dec). 1H NMR (300 MHz, C6D6, 25 °C): d = 7.10 (br, d), 2.95 (br,
ligand —SC6H3-2; 6ðC6H2-2; 4; 6-Pr3 Þ2 (–SAr*) [24] to synthesize a
i
series of divalent transition metal complexes (M(SAr*)2 (M = Cr,
Mn, Fe, Co)) [17] and their zinc analogue Zn(SAr*)2. These feature
linear or almost linear metal coordination for Cr, Mn, Co, Ni and
Zn whereas Fe(SAr*)2 displays a bent (S–Fe–S = 151.48(2)°) geome-
try and an apparent tendency to rearrange to a sandwich structure
involving metal–aryl ring coordination [17]. A feature of the com-
plexes is that, with the exception of the unique iron derivative, all
the secondary metal–ligand interactions (mainly to ipso-carbons
from a flanking aryl ring of the terphenyl ligand) are weak and ex-
ceed 2.5 Å, with the longest being observed for Mn(SAr*)2 (Mn–
S=2.951(2) Å). Such distances exceed those in the corresponding
s), 1.21 (br, s), 1.05 (br, s). UV/Vis (hexanes; kmax
molꢁ1 L cmꢁ1)): 328 (3050), 390 (2000), 440 (1400), 638 (1600).
eff = 2.64 B/vanadium.
, nm (e,
l
l
g5 or
For example, the Cr–C interaction is 2.502(2) Å in Cr(SAr*)2 whereas
in Cr(
5-C5H5)2 the Cr–C distance is 2.142(3) Å and the Cr–centroid
g
6-bonded metal arene complexes by a significant margin.
2.5. X-ray data collection
g
X-ray quality crystals of 2 or 3 were removed from the Schlenk
distance is 1.61 Å [29]. The above findings encouraged us to extend
the approach to vanadium and it was hoped that the employment of
a bulky terphenyl thiolate ligand would allow us to obtain the first
example of a two-coordinate V(II) species.
tube and immediately covered with a layer of hydrocarbon oil. A
Table 1
Selected crystallographic data and collection parameters for 2 and 3
The reaction of VI2(THF)4 with one or two equivalents of LiSAr*
did not afford readily characterizable products. We turned instead
to the related –SAr0 ligand which has similar steric properties to –
SAr* and can be synthesized by a similar route. It differs from Ar*
only in the absence of para-Pri substituents from the flanking aryl
rings. The addition of one equivalent of LiSAr0 to a suspension of
VI2(THF)4 in THF at 0 °C (Eq. (1)) afforded a dark red-brown solu-
tion from which deep red crystals of 2 could be isolated in
2
3
Formula
fw
Color, habit
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
b (°)
C
60H74S2V2I2
C71H98IOS2V
1209.45
green, rod
monoclinic
P21/n
1214.99
red, plate
monoclinic
P21/c
13.086(1)
13.278(1)
16.507(2)
90
13.4040(5)
34.915(1)
13.8562(5)
90
0
ꢂC;THF
2VI2ðTHFÞ þ 2LiSAr0
½Vð
l
-SAr0ÞIꢃ2 þ 2Lil
ð1Þ
(°)
ꢀꢀꢀ!
4
105.170(2)
90
92.949(1)
90
c
(°)
63% yield. These were characterized by UV–Vis spectroscopy, mag-
netic studies and X-ray crystallography. The results of the diffrac-
tion study of 2 are illustrated in Fig. 1. It represents a very rare
example of a structurally characterized vanadium (II) thiolate
[4,30–33]. It is dimeric and the metals are linked by thiolate bridges
to form a V2S2 core structure. Each vanadium is also coordinated to
a terminal iodide. The V2S2 core is characterized by an inversion
center with internal S–V–S and V–S–V angles of 100.86(4)° and
V (Å3)
2768(4)
2
6476.1(4)
4
Z
dcalc (Mg/m3)
h Range (°)
1.458
1.240
2.00–27.50
1.565
1.17–25.25
0.735
l
(mmꢁ1
)
Number of observed (I > 2
R1 (observed data)
wR2 (all data)
r
(I)) data
4435
7976
0.0422
0.0897
0.0487
0.1384

A.D. Sutton et al. / Polyhedron 27 (2008) 2337–2340
2339
Table 3
Selected bond lengths (Å) and angles (°) for 3
V(1)–I(1)
V(1)–Ct
V(1)–S(1)
V(1)–S(2)
S(1)–C(1)
S(2)–C(31)
2.6659(8)
1.972(3)
2.292(1)
2.284(1)
1.771(4)
1.786(4)
S(1)–V(1)–S(2)
S(1)–V(1)–I(1)
S(2)–V(1)–I(1)
C(1)–S(1)–V(1)
C(31)–S(2)–V(1)
108.85(5)
90.10(4)
96.77(4)
106.8(2)
119.3(2)
0
ꢂC;THF
VI2ðTHFÞ þ 2LiSAr0
½VðSAr0Þ Iꢃ þ 2LiI þ VðIÞ
ð2Þ
ꢀꢀꢀ!
4
2
P
VS2I array is pyramidal ( °V = 295.72(5)) rather than planar and
there is an
g
6-interaction involving the flanking aryl ring of the ter-
phenyl substituent and the vanadium center. The V–(Ct) distance
(1.972(3) Å) is significantly longer than it is in 2, however. This is
especially noteworthy in view of the fact that effective ionic radius
of six-coordinate V3+ (0.64 Å) is smaller that of V2+ (0.79 Å) [37]
which may lead to an expectation for a shorter V3+-centroid contact.
On the other hand, the higher electron deficiency and reduced
crowding of the metal center in 2 may account for the shorter
V(II)–centroid distance. Nonetheless, inspection of the bond dis-
tances in the interacting and non interacting aryl rings of the S(I)
ligand shows the average C–C distance in the former 1.405 Å are
longer than those in the latter consistent with a weakening of the
CC-bonding. The V–S–C angle for the interacting terphenyl
V(I)–S(I)–C(1) = 106.8(2)° is ca. 12.5° narrower than the 119.3(2)
observed for the thiolate ligand. The difference in the Vn+ effective
ionic radii is indeed reflected in the V–S bond lengths, 2.292(1)
and 2.284(1) Å, which are over 0.1 Å shorter than those in 2. The
V–S distances are also shorter than those in [V(tipt)3(THF)2]
(tipt = 2,4,6-tri-isoproplbenzenethiolate) [38] (2.308(1), 2.334(1),
2.320(1) Å) or the 2.364 Å in [V(SNC9H6)3] [32]. The V–I distance
2.6659(8) Å is also shorter in 3, but the shortening is not as great
as it is for the V–S bonds. It may be compared with the V–I distance
in [VI(Ind)2] (2.794(1) Å) [39].
Fig. 1. Thermal ellipsoid (30%) plot of [V(
ðC6H3-2; 6-Pr2 Þ2). Hydrogen atoms are not shown.
l
-SAr0)I]2 (2) (Ar0 ¼ C6H3-2; 6-
i
79.14(4)°, respectively. The V–I bonds subtend an angle of
100.37(3)° with respect to the V2S2 plane. In addition there is an
g
6-interaction between a flanking aryl ring of the terphenyl ligand
and each vanadium center. This interaction is relatively short (V(1)–
Ct = 1.909(2) Å, Ct = centroid) and is comparable to that observed in
vanadocene (1.917 (2) Å) [30] (1.962 Å), but marginally longer than
that in V(g
5-C5Me5)2 (1.873(2) Å) [31]. The data thus indicate the
presence of a relatively strong vanadium–aryl ring interaction in
2. This is also supported by the difference in the average C–C bond
lengths in the interacting (1.415 Å avg.) and non interacting
(1.401 Å avg.) aryl ring. The V–S bond lengths are 2.395(1) and
2.412(1) Å. These may be compared with the V(II)–S distances in
[V(Spy)2(TMEDA)] (Spy = pyridine-2-thiolate, TMEDA = tetrameth-
ylethylenediamine; V–S = 2.252(2) and 2.547(2) Å) [33], [V(Spy)3]ꢁ
(V–S = 2.545 Å (avg.)) [31] and in [V(SC6H3-2,6-Cl2)2(TMEDA)] (V–
S = 2.464(2) and 2.468(2) Å) [34]. The metal centers in these com-
plexes are six-coordinate or quasi six-coordinate, and, if the V–aryl
interaction in 2 is viewed as occupying three coordination sites, the
metal center in 2 may also be regarded as having six-coordination.
In view of this, the V–S bond lengths in 2 may be regarded as being
somewhat shorter than expected especially since the thiolate
groups are bridging. It may be that the presence of a halogen ligand
in 2, which increases the positive charge on the metal, may cause
the shorter V–S bond. The V–I bond length in 2 (2.7228(8) Å) is a lit-
tle shorter than that in vanadium (II) complexes [VI2(depe)2]
(depe = bis(diethylphosphino)ethane) [22,30] (2.779(2) Å), [VI2(TH-
F)([9]aneS3)][35] ([9]aneS3 = 1,4,7-trithiacyclononane) (2.787(2)
and 2.798(3) Å) and cis-VI2(DME)2 [36] (2.811(2) and 2.805(2) Å)
(see Tables 2 and 3)
The magnetic moments of the two compounds were measured
in C6D6 by the Evans’ method. These measurements afforded
magnetic moments consistent with d3 (1,
l
eff = 3.67lB per vana-
dium) and d2 (2,
leff = 2.64lB) electron configurations. Both values
indicate that there is little orbital contribution to the magnetic mo-
ment and that there is little coupling between the vanadiums in 2
(V–V = 3.063(1) Å). The presence of unpaired electrons gave rise to
The addition of freshly prepared Ar0SLi to VI2(THF)4 suspended
in THF in a 2:1 ratio results in the formation of the monomeric
vanadium (III) thiolate [V(SAr0)2I] 3, which presumably arises via
a disproportionation process. Each vanadium center is coordinated
to a terminal iodide and two thiolate ligands. However, the coordi-
nation geometry of the (see Fig. 2)
Table 2
Selected bond lengths (Å) and angles (°) for 2
V(1)–I(1)
2.7228(8)
1.909(2)
2.396(1)
2.413(1)
1.793(4)
3.063(1)
S(1)–V(1)–S(1)#1
S(1)–V(1)–I(1)
S(1)#1–V(1)–I(1)
V(1)–S(1)–V(1)#1
V(1)–C(1)–S(1)
100.86(4)
96.84(3)
96.33(3)
79.14(4)
104.44(1)
V(1)–Ct01
V(1)–S(1)
V(1)–S(1)#1
S(1)–C(1)
V(1)–V(1)#
Fig. 2. Thermal ellipsoid (30%) plot of V(SAr0)2I ꢄ hexane ꢄ THF (2 ꢄ hexane ꢄ THF)
(Ar0 ¼ C6H3-2; 6-ðC6H3-2; 6-Pr Þ2). Hydrogen atoms and solvent molecules are not
i
2
shown.

2340
A.D. Sutton et al. / Polyhedron 27 (2008) 2337–2340
broad and largely featureless paramagnetically shifted 1H NMR
spectra for both 2 and 3.
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4. Conclusions
The synthesis of 2 and 3 demonstrates the ability of bulky ter-
phenyl thiolates to stabilize early transition metal complexes in
low oxidation states. Nonetheless, a vanadium dithiolate V(SAr0),
analogous to those reported for M(SAr*)2 (M = Cr, Mn, Fe, Co, Ni,
Zn), could not be isolated by a similar syntheticroute due to a dispro-
portionation reaction. However, the isolation of the iodide 2 in good
yield may prove to be a useful reagent for further synthesis and the
study of additional reactivity. For example, it may be useful as a pre-
cursor for the construction of V–V bonds via reductive coupling. In
addition, two-coordinate V(II) may yet be obtainable from 2 with
use of another less reducing coligand. Attempts to isolate such com-
plexes are in hand.
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183.
We would like to thank Dr. Eric Rivard for assistance in the
magnetic measurements and useful discussions, and the Petroleum
Research Fund administered by the American Chemical Society and
the National Science Foundation.
Supplementary data
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CCDC 671397 and 671398 contain the supplementary crystallo-
graphic data for 2 and 3. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
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