Synthesis and structures of bis(dithiolene)-tungsten(IV) complexes related to the active sites of tungstoenzymes.

Article abstract of DOI:10.1021/ic991153u

Recent protein crystallographic results on tungsten enzymes and primary sequence relationships between certain molybdenum and tungsten enzymes provoke interest in the generalized bis(dithiolene) complexes [WIV(QR)(S2C2R'2)2]1- and [WVIO(QR)(S2C2R'2)2]1- (Q = O, S, Se) as minimal representations of enzyme sites. The existence and stability of W(IV) complexes have been explored by synthesis. Reaction of [W(CO)2(S2C2Me2)2] (1) with PhO- results in complete CO substitution to give [W(OPh)(S2C2Me2)2]1- (2). Reaction of 1 with PhQ- affords the monocarbonyls [W(CO)(QPh)(S2C2Me2)2]1- (Q = S (3), Se (5)). The use of sterically demanding 2,4,6-Pri3C6H2Q- also yields monocarbonyls, [W(CO)(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1- (Q = S (4), Se (6)). The X-ray structures of square pyramidal 2 and trigonal prismatic 3-6 (with unidentate ligands cis) are described. The tendency to substitute one or both carbonyl ligands in 1 in the formation of [MIV(QAr)(S2C2Me2)2]1- and [MIV(CO)(QAr)(SeC2Me2)2]1- with M = Mo and W is related to the M-Q bond length and ligand steric demands. The results demonstrate a stronger binding of CO by W(IV) than Mo(IV), a behavior previously demonstrated by thermodynamic and kinetic features of zerovalent carbonyl complexes. Complexes 3-6 can be reversibly reduced to W(III) at approximately -1.5 V versus SCE. On the basis of the potential for 2(-2.07 V), monocarbonyl ligation stabilizes W(III) by approximately 500 mV. This work is part of a parallel investigation of the chemistry of bis(dithiolene)-molybdenum (Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263) and -tungsten complexes related to enzyme active sites.

Full text of DOI:10.1021/ic991153u

Inorg. Chem. 2000, 39, 1275-1281
1275
Synthesis and Structures of Bis(dithiolene)-Tungsten(IV) Complexes Related to the Active
Sites of Tungstoenzymes
Kie-Moon Sung and R. H. Holm*
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138
ReceiVed September 28, 1999
Recent protein crystallographic results on tungsten enzymes and primary sequence relationships between certain
molybdenum and tungsten enzymes provoke interest in the generalized bis(dithiolene) complexes [WIV(QR)-
(S2C2R′2)2]1- and [WVIO(QR)(S2C2R′2)2]1- (Q ) O, S, Se) as minimal representations of enzyme sites. The
existence and stability of W(IV) complexes have been explored by synthesis. Reaction of [W(CO)2(S2C2Me2)2]
(1) with PhO- results in complete CO substitution to give [W(OPh)(S2C2Me2)2]1- (2). Reaction of 1 with PhQ-
affords the monocarbonyls [W(CO)(QPh)(S2C2Me2)2]1- (Q ) S (3), Se (5)). The use of sterically demanding
2,4,6-Pri3C6H2Q- also yields monocarbonyls, [W(CO)(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1- (Q ) S (4), Se (6)). The
X-ray structures of square pyramidal 2 and trigonal prismatic 3-6 (with unidentate ligands cis) are described.
The tendency to substitute one or both carbonyl ligands in 1 in the formation of [MIV(QAr)(S2C2Me2)2]1- and
[MIV(CO)(QAr)(S2C2Me2)2]1- with M ) Mo and W is related to the M-Q bond length and ligand steric demands.
The results demonstrate a stronger binding of CO by W(IV) than Mo(IV), a behavior previously demonstrated by
thermodynamic and kinetic features of zerovalent carbonyl complexes. Complexes 3-6 can be reversibly reduced
to W(III) at approximately -1.5 V versus SCE. On the basis of the potential for 2 (-2.07 V), monocarbonyl
ligation stabilizes W(III) by ∼500 mV. This work is part of a parallel investigation of the chemistry of bis-
(dithiolene)-molybdenum (Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263) and -tungsten
complexes related to enzyme active sites.
Introduction
All tungstoenzymes recognized thus far have been isolated
from thermophilic bacteria and hyperthermophilic archaea and
contain the pterin dithiolene cofactor depicted in Figure 1.1-3
The enzymes have been organized into two principal families,
AOR (aldehyde oxidoreductases) and F(M)DH (formate dehy-
drogenases, FDH, and N-formylmethanofuran dehydrogenases,
FMDH). FDH functions in the first step of the conversion of
carbon dioxide to acetate in acetogens, and FMDH is involved
in the conversion of carbon dioxide to methane in methanogens.
The crystal structures of two enzymes isolated from Pyrococcus
furiosus, aldehyde oxidoreductase4-6 and formaldehyde oxi-
doreductase,7 have been determined by Rees and co-workers.
The active sites of both enzymes contain a tungsten atom
chelated by two cofactor ligands, with additional coordination
thought to involve one or two oxygen atoms. Owing to the
Figure 1. Structure of the pterin dithiolene cofactor ligand and putative
minimal active site structures for members of the AOR (a,d) and F(M)-
DH (b,c,e,f) enzyme families.
difficulty of locating light atom scatterers in the presence of
tungsten and the possibility of site heterogeneity, the binding
of oxygenous ligands is not well defined. The most recent
EXAFS results for AOR, quoted by Johnson et al.,2 are in
substantial agreement with the X-ray structure and include one
W-O interaction at 1.75 Å and possible W-O/N coordination
at 1.97 Å.
In terms of primary sequences, there appears to be little
relationship between the AOR and F(M)DH families.2 However,
there is substantial homology between the FM(D)H family and
the molybdenum-containing DMSO reductases (DMSORs),2,8
which are a structurally better defined family of enzymes. The
possibility emerges that active FM(D)H enzymes resemble
(1) Kletzin, A.; Adams, M. W. W. FEMS Microbiol. ReV. 1996, 18, 5.
(2) Johnson, M. K.; Rees, D. C.; Adams, M. W. W. Chem. ReV. 1996,
96, 2817.
(3) Hagen, W. R.; Arendsen, A. F. Struct. Bonding 1998, 90, 161.
(4) Chan, M. K.; Mukund, S.; Kletzin, A.; Adams, M. W. W.; Rees, D.
C. Science 1995, 267, 1463.
(5) Schindelin, H.; Kisker, C.; Rees, D. C. J. Biol. Inorg. Chem. 1997, 2,
773.
(6) Rees, D. C.; Hu, Y.; Kisker, C.; Schindelin, H. J. Chem. Soc., Dalton
Trans. 1997, 3909.
(7) Hu, Y.; Faham, S.; Roy, R.; Adams, M. W. W.; Rees, D. C. J. Mol.
Biol. 1999, 286, 899.
(8) Hille, R. Chem. ReV. 1996, 96, 2757.
10.1021/ic991153u CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/23/2000

1276 Inorganic Chemistry, Vol. 39, No. 6, 2000
Sung and Holm
Rhodobacter sphaeroides DMSOR9,10 by having two pterin
dithiolenes and one endogenous protein ligand in the W(IV,
VI) states and no oxo ligand in the W(IV) state. The protein
ligands are serinate for DMSOR,9,10 cysteinate for dissimilatory
nitrate reductase,11 and cysteinate or selenocysteinate for
FDH12-14 and FMDH. Under the Hille classification,8 all of
these enzymes are included in the Mo-DMSOR family. Putative
minimal active site structures are shown as a-f in Figure 1.
Consequently, the desoxo and monooxo complexes [WIV(QR)-
(S2C2R2)2]1- and [WVIO(QR)(S2C2R2)2]1- (Q ) O, S, Se),
respectively, are potentially relevant in determining intrinsic
properties of active sites.
Table 1. Functional Groups in Bis(dithiolene)-Tungsten (IV, V,
VI) Complexes
Interest in the foregoing species has led us to explore, rather
generally, the types of mononuclear tungsten complexes acces-
sible under the restriction of bis(dithiolene) ligation.15-17
Currently known complexes15-21 are summarized in Table 1 in
terms of functional groups, that is, metal and nondithiolene
ligands potentially subject to further reaction. The entries are
limited to complexes that have been isolated in substance. Only
the groups WIV,VO, WVIO2, and WVIO(S2) were characterized
before the beginning of our investigation of the synthesis and
structures of bis(dithiolene) complexes. None of the complete
ligation modes in active site structures a-f (Figure 1) were
represented prior to the inception of the present investigation.
We herein describe the initial results from a synthetic search
for the reduced sites a-c. Inasmuch as the corresponding
molybdenum sites are of direct pertinence to the structure and
function of members of the Mo-DMSOR family,8 a similar
synthetic exploration involving bis(dithiolene)Mo complexes has
been undertaken and is described elsewhere.22,23
a This work; R ) aryl. b Not proven by a structure determination.
c Ambiguous oxidation state.
(MeCN)3] and purified by column chromatography.17 The compounds
NaSPh and NaSePh were prepared by the reactions of NaOMe with
benzenethiol and benzeneselenol, respectively, in methanol. Metathesis
of these salts with Et4NCl in acetonitrile afforded (Et4N)(SPh) and
(Et4N)(SePh), which were recrystallized from acetonitrile/ether. The
compound NaSC6H2-2,4,6-Pri3 was prepared from 2,4,6-triisopropyl-
benzenethiol24 by the method employed for NaSPh. Bis(2,4,6-triiso-
propylphenyl)diselenide was obtained by a published method.25 The
solvent removal and drying steps were performed in vacuo. The
filtrations were through Celite; the compounds were isolated by
filtration.
Experimental Section
Preparation of Compounds. All operations were carried out under
a pure dinitrogen atmosphere using an inert atmosphere box or Schlenk
techniques. Solvents were distilled and dried by the appropriate methods
and stored over 4 Å molecular sieves in a drybox. [W(CO)2(S2C2Me2)2]
was synthesized by ligand transfer from [Ni(S2C2Me2)2] to [W(CO)3-
(Et4N)[W(OPh)(S2C2Me2)2]. To a dark violet solution of 120 mg
(0.25 mmol) of [W(CO)2(S2C2Me2)2] in 3 mL of THF was added 29.1
mg (0.25 mmol) of NaOPh. The reaction mixture, which became green-
brown within 1 min, was stirred for 90 min, treated with 42.0 mg (0.25
mmol) of Et4NCl, and stirred for an additional 10 min. The solvent
was removed to leave a dark brown solid, which was dissolved in 1.5
mL of acetonitrile. The mixture was filtered and ether (15 mL) was
layered on the filtrate, causing the separation of brown needlelike
crystals over 1 d. This material was collected, washed with ether (3 ×
1 mL), and dried to afford the product as 107 mg (66%) of brown
crystals. 1H NMR (CD3CN, anion): δ 2.61 (s, 12), 6.42 (d, 2), 6.80 (t,
1), 7.01 (t, 2). Absorption spectrum (acetonitrile): λmax (ꢀM) 250
(15 000), 299 (12 000), 319 (sh, 9500), 347 (sh, 4100), 394 (2100),
478 (sh, 610), 572 (290) nm. Anal. Calcd for C22H37NOS4W: C, 41.05;
H, 5.79; N, 2.18; S, 19.92. Found: C, 41.04; H, 5.96; N, 2.22; S, 19.77.
(9) Schindelin, H.; Kisker, C.; Hilton, J.; Rajagopalan, K. V.; Rees, D.
C. Science 1996, 272, 1615.
(10) George, G. N.; Hilton, J.; Temple, C.; Prince, R. C.; Rajagopalan, K.
V. J. Am. Chem. Soc. 1999, 121, 1256. This article contains a
discussion of reported differences in active site structures of two
DMSORs from different organisms and differences in structural results
for enzymes from the same organism.
(11) Dias, J. M.; Than, M. E.; Humm, A.; Huber, R.; Bourenkov, G. P.;
Bartunik, H. D.; Bursakov, S.; Calvete, J.; Caldeira, J.; Carneiro, C.;
Moura, J. J. G.; Moura, I.; Roma˜o, M. J. Structure 1999, 7, 65.
(12) Boyington, J. C.; Gladyshev, V. N.; Khangulov, S. V.; Stadtman, T.
C.; Sun, P. D. Science 1997, 275, 1305.
(13) George, G. N.; Colangelo, C. M.; Dong, J.; Scott, R. A.; Khangulov,
S. V.; Gladyshev, V. N.; Stadtman, T. C. J. Am. Chem. Soc. 1998,
120, 1267.
(14) George, G. N.; Costa, C.; Moura, J. J. G.; Moura, I. J. Am. Chem.
Soc. 1999, 121, 2625.
(15) Donahue, J. P.; Lorber, C.; Nordlander, E.; Holm, R. H. J. Am. Chem.
Soc. 1998, 120, 3259.
(16) Lorber, C.; Donahue, J. P.; Goddard, C. A.; Nordlander, E.; Holm, R.
H. J. Am. Chem. Soc. 1998, 120, 8102.
(17) Goddard, C. A.; Holm, R. H. Inorg. Chem. 1999, 38, 5389.
(18) Ueyama, N.; Oku, H.; Nakamura, A. J. Am. Chem. Soc. 1992, 114,
7310.
(Et4N)[W(CO)(SPh)(S2C2Me2)2]. Method a. The compound Ph-
SSPh (22.0 mg, 0.10 mmol) was treated with 0.20 mL of a 1.0 M
solution of LiBEt3H in THF. The solution was transferred into a solution
of 96.2 mg (0.20 mmol) of [W(CO)2(S2C2Me2)2] in 1 mL of THF,
producing an immediate green color. The reaction mixture was stirred
for 2 h, a solution of 34.2 mg (0.21 mmol) of Et4NCl was added, and
the mixture was stirred for an additional 10 min. The solvent was
removed, the solid residue was treated with 1.5 mL of acetonitrile,
and the mixture was filtered. Ether (60 mL) was layered onto the filtrate,
causing the separation of dark green block-shaped crystals over 2 d.
The solid was collected, washed with ether (3 × 4 mL), and dried to
(19) Das, S. K.; Biswas, D.; Maiti, R.; Sarkar, S. J. Am. Chem. Soc. 1996,
118, 1387.
(20) Davies, E. S.; Aston, G. M.; Beddoes, R. L.; Collison, D.; Dinsmore,
A.; Docrat, A.; Joule, J. A.; Wilson, C. R.; Garner, C. D. J. Chem.
Soc., Dalton Trans. 1998, 3647.
(21) Schrauzer, G. N.; Mayweg, V. P.; Heinrich, W. J. Am. Chem. Soc.
1966, 88, 5174.
(22) Donahue, J. P.; Goldsmith, C. R.; Nadiminti, U.; Holm, R. H. J. Am.
Chem. Soc. 1998, 120, 12 869.
(24) Blower, P. J.; Dilworth, J. R.; Hutchinson, J.; Zubieta, J. A. J. Chem.
Soc., Dalton Trans. 1985, 1533.
(25) Bochmann, M.; Webb, K. J.; Hursthouse, M. B.; Mazid, M. J. Chem.
Soc., Dalton Trans. 1991, 2317.
(23) Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263.

Synthesis of Bis(dithiolene)-Tungsten(IV)
Inorganic Chemistry, Vol. 39, No. 6, 2000 1277
give the product as 82.1 mg (59%) of dark green crystals. IR (KBr):
ν
Chart 1. Designation of Bis(dithiolene)-Tungsten(IV)
Complexes
CO 1933 cm-1. 1H NMR (CD3CN, anion): δ 2.49 (s, 12), 7.02 (t, 1),
7.18 (t, 2), 7.48 (d, 2). Absorption spectrum (acetonitrile): λmax (ꢀM)
273 (12 000), 334 (6700), 373 (sh, 6300), 436 (8200), 596 (3400) nm.
Anal. Calcd for C23H37NOS5W: C, 40.17; H, 5.42; N, 2.04; S, 23.31.
Found: C, 39.98; H, 5.33; N, 1.94; S, 23.08.
Method b. To a solution of 78.7 mg (0.17 mmol) of [W(CO)2(S2C2-
Me2)2] in 1 mL of THF was added a solution of 97.5 mg (0.41 mmol)
of (Et4N)(SPh) in 1 mL of acetonitrile. The dark green solution was
stirred for 1 h, the solvents were removed, and the residue was dissolved
in 2 mL of acetonitrile. The solution was filtered; 15 mL of ether was
added to the filtrate, causing precipitation of a dark green crystalline
solid over 3 d. This material was isolated, washed with ether (3 × 1
mL), and dried in vacuo, leading to 46.7 mg (41%) of product as a
dark green solid. This material is spectroscopically identical to the
product of the preceding method.
crystals of (Et4N)[2] (brown needles), (Et4N)[3] and (Et4N)[5] (thin
dark green plates), and (Et4N)[4] and (Et4N)[6] (dark green blocks)
were obtained by layering ether on the corresponding acetonitrile
solutions and allowing the solutions to stand over 2-3 days. The
crystals were coated in grease and mounted on a Siemens (Bruker)
SMART CCD area detector instrument with Mo KR radiation. The
data were collected at 213 K with ω scans of 0.3° per frame, with 30
s frames, such that 1271 frames were collected for a hemisphere of
data. The first 50 frames were recollected at the end of the data
collection to monitor for decay; no significant decay was detected for
any compound. Data out to 2θ of 56° were used for the compounds
(Et4N)[2,4,6], but for (Et4N)[3,5], the data were used only out to 2θ of
45° because of the low-quality high-angle data. Cell parameters were
retrieved using SMART software and refined using SAINT software
on all observed reflections between 2θ of 3° and the upper thresholds.
Data reduction was performed with the SAINT software, which corrects
for Lorentz polarization and decay. Absorption corrections were applied
using SADABS, as described by Blessing.26 The space groups for all
of the compounds were assigned unambiguously by analysis of
symmetry and systematic absences determined by the program XPREP.
The crystal parameters are listed in Table 2.
All of the structures were solved by the direct method with SHELXS-
97 and subsequently refined against all data in the 2θ ranges by full-
matrix least squares on F2 using SHELXL-97. Asymmetric units contain
one ((Et4N)[2,4,6]), three ((Et4N)[3]), and four ((Et4N)[5]) formula
weights. All of the methylene groups of the cation in (Et4N)[2] and
three methylene groups of one of the cations in (Et4N)[5] in the
asymmetric unit were disordered over two sites and were refined with
site occupancy factors of 0.83 and 0.55, respectively. All non-hydrogen
atoms, including those of the disordered cations, were refined aniso-
tropically. Hydrogen atoms were attached at idealized positions on
carbon atoms and were checked for missing symmetry by the program
PLATON. The final agreement factors are given in Table 2. (See the
Supporting Information.)
(Et4N)[W(CO)(SC6H2-2,4,6-Pri3)(S2C2Me2)2]. To a solution of 57.6
mg (0.22 mmol) of NaSC6H2-2,4,6-Pri3 in 3 mL of acetonitrile was
added 106 mg (0.22 mmol) of [W(CO)2(S2C2Me2)2]. A green color
developed immediately. The reaction mixture was stirred for 1 h, a
solution of 37.4 mg (0.23 mmol) of Et4NCl was added, and the mixture
was stirred for an additional 10 min. The solvent was removed, and
the dark green residue was dissolved in 2 mL of acetonitrile. The
mixture was filtered, and 90 mL of ether was added. Dark green
blocklike crystals separated over 3 d. The solid was collected by
filtration, washed with ether (3 × 6 mL), and dried in vacuo to afford
the product as 76.1 mg (42%) of green crystals. IR (KBr): νCO 1953
1
cm-1. H NMR (CD3CN, anion): δ 1.00 (d, 12),1.23 (d, 6), 2.48 (s,
12), 2.89 (septet, 1), 3.16 (septet, 2), 6.95 (s, 2). Absorption spectrum
(acetonitrile): λmax (ꢀM) 291 (11 000), 344 (7200), 438 (6400), 590
(2800) nm. Anal. Calcd for C32H55NOS5W: C, 47.22; H, 6.81; N, 1.72;
S, 19.70. Found: C, 47.09; H, 7.02; N, 1.74; S, 19.48.
(Et4N)[W(CO)(SePh)(S2C2Me2)2]. Method a. The procedure is
analogous to method (a) for (Et4N)[W(CO)(SPh)(S2C2Me2)2], but with
the use of PhSeSePh instead. When conducted on a 0.18 mmol scale
of [W(CO)2(S2C2Me2)2], the product was isolated as a dark green
1
crystalline solid in 41% yield. IR (KBr): νCO 1925 cm-1. H NMR
(CD3CN, anion): δ 2.53 (s, 12), 7.03 (t, 1), 7.09 (t, 2), 7.44 (d, 2).
Absorption spectrum (acetonitrile): λmax (ꢀM) 282 (14 000), 334 (sh,
7600), 373 (sh, 6600), 442 (8900), 591 (3600) nm. Anal. Calcd for
C23H37NOS4SeW: C, 37.60; H, 5.08; N, 1.91; S, 17.46; Se, 10.75.
Found: C, 37.55; H, 5.12; N, 1.89; S, 17.38; Se, 10.84.
Method b. The procedure is analogous to method (b) for (Et4N)-
[W(CO)(SPh)(S2C2Me2)2] but with the use of (Et4N)(SePh) instead.
When conducted on a 0.22 mmol scale of [W(CO)2(S2C2Me2)2], the
product was isolated as a dark green crystalline solid in 68% yield.
This material is spectroscopically identical to the product of the
preceding method.
Other Physical Measurements. All measurements were performed
under anaerobic conditions. Absorption spectra were recorded with a
Varian Cary 50 Bio spectrophotometer. The 1H NMR spectra were
obtained with Bruker AM 400N/500N spectrometers. IR spectra were
measured with KBr pellets in a Nicolet Impact 400 or a Nicolet Nexus
470 FT-IR instrument. Cyclic voltammograms and differential potential
voltammograms were recorded with a PAR Model 263 potentiostat/
galvanostat, using Pt disk working electrode and 0.1 M Bu4NPF6
supporting electrolyte in MeCN solution. Potentials were directly
measured with and referenced to the saturated calomel electrode (SCE).
(Et4N)[W(CO)(SeC6H2-2,4,6-Pri3)(S2C2Me2)2]. Di-(2,4,6-triisopro-
pylphenyl)diselenide (22.5 mg, 0.040 mmol) was added to 0.08 mL of
a 1.0 M solution of LiBEt3H in THF. The light yellow solution was
transferred to a solution of 37.8 mg (0.080 mmol) of [W(CO)2(S2C2-
Me2)2] in 1 mL of THF. A green color developed immediately. The
reaction mixture was stirred for 1 h, a solution of 12.5 mg (0.075 mmol)
of Et4NCl in 1 mL of acetonitrile was added, and the mixture was stirred
for an additional 10 min. The solvents were removed leaving a dark
green residue, which was dissolved in 2 mL of acetonitrile. The solution
was filtered, and 90 mL of ether was layered onto the filtrate. Over 3
d, dark green blocklike crystals deposited. This material was collected,
washed with ether (3 × 6 mL), and dried in vacuo. The product was
Results and Discussion
In seeking a convenient starting material for bis(dithiolene)-
tungsten complexes, we have utilized the trigonal prismatic
dicarbonyl complex 1. This compound was first prepared by
Schrauzer et al.21 in low yield by means of a photochemical
reaction between [W(CO)6] and [Ni(S2C2R2)2] (R ) Me, Ph).
We have improved the synthesis by the use of [W(CO)3-
(MeCN)3];27 the reaction system does not require irradiation
and affords 1 in yields of ∼60-70%.17 The lability of the
carbonyl ligands was demonstrated by displacement with
obtained as 38.9 mg (57%) of green crystals. IR (KBr): νCO 1953 cm-1
.
1H NMR (CD3CN, anion): δ 1.03 (d, 12), 1.25 (d, 6), 2.48 (s, 12),
2.89 (septet, 1), 3.18 (septet, 2), 6.97 (s, 2). Absorption spectrum
(acetonitrile): λmax (ꢀM) 298 (13 000), 339 (8700), 441 (8600), 610
(3800) nm. Anal. Calcd for C32H55NOS4SeW: C, 44.65; H, 6.44; N,
1.63; S, 14.90; Se, 9.17. Found: C, 44.53; H, 6.51; N, 1.59; S, 14.11;
Se, 9.12.
In the sections that follow, tungsten complexes are referred to by
the numerical designations in Chart 1.
X-ray Structure Determinations. The five compounds listed in
Table 2 were structurally identified by X-ray crystallography. Suitable
(26) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(27) Tate, D. P.; Knipple, W. R.; Augl, J. M. Inorg. Chem. 1962, 1, 433.

1278 Inorganic Chemistry, Vol. 39, No. 6, 2000
Table 2. Crystallographic Dataa
Sung and Holm
(Et4N)[2]
(Et4N)[3]
(Et4N)[4]
(Et4N)[5]
(Et4N)[6]
formula
fw
cryst syst
space grp
Z
C22H37NOS4W
643.62
monoclinic
P21/n
C23H37NOS5W
687.69
triclinic
P1h
6
C32H55NOS5W
813.92
orthorhombic
Pbcn
C23H37NOS4SeW
734.59
triclinic
P1h
8
C32H55NOS4SeW
860.82
orthorhombic
Pbcn
4
8
8
a (Å)
b (Å)
c (Å)
8.7440(3)
24.2608(3)
12.7236(4)
10.1257(2)
16.1756(1)
25.8216(5)
88.915(1)
82.50
84.242(1)
4171.87(12)
1.642
25.4814(5)
16.9145(2)
17.1093(4)
19.2516(4)
19.2618(3)
19.3633(4)
103.497(1)
104.794(1)
116.170(1)
5713.85(19)
1.708
25.5714(7)
16.8328(4)
17.2163(5)
R (deg)
â (deg)
γ (deg)
V (Å3)
102.844(2)
2631.60(13)
1.624
7374.2(2)
1.466
7410.6(3)
1.543
d
calc (g/cm3)
2θ range (deg)
3-56
0.0360 (0.0687)
3-45
0.0386 (0.0742)
3-56
0.0612 (0.1132)
3-45
0.0479 (0.0716)
3-56
0.0286 (0.0728)
c
R1b (wR2 )
a Obtained with graphite-monochromatized Mo KR (λ ) 0.710 73 Å) radiation at 213 K. b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) {∑[w(Fo
-
2
2
Fc2)2]/∑[w(Fo )2]}1/2
.
fication. For this purpose, UV-vis spectra are included in
Figures 5 and 6.
Oxygen Ligation. Reaction of 1 in THF with NaOPh affords
the phenolate complex 2 (66%), which manifests a square-
pyramidal structure with the desired W-OPh ligation (Figure
3). As is typical of five-coordinate [ML(S2C2R2)2]1-,2- com-
plexes (M ) Mo,22,23,29,30 W16-20), the two chelate rings are
not coplanar and, in this case, are disposed at a dihedral angle
θd ) 127.3°. The metal atom is displaced from the S4 least-
squares plane toward the axial ligand by δ ) 0.78 Å. The W-O
bond length is 1.861(3) Å. The mean chelate ring bond distances
of 1.321 Å (C-C) and 1.779(5) (C-S) Å indicate two
enedithiolate ligands and thus the formulation [WIV(OPh)-
(S2C2Me2)2]1-, rather than a W(II) dithione/enedithiolate de-
scription. The molecule is structurally similar to [Mo(OC6H3-
2,6-Pri2)(S2C2Me2)2]1-, for which θd ) 125.7° and δ ) 0.82
Å.23 The spectra of 2 and its molybdenum analogue (Figure 5)
are quite distinct. The spectrum of 2 appears to be a blue-shifted
version of the spectrum of [Mo(OPh)(S2C2Me2)2]1-, but the
exact band correlations require spectral assignments that are not
yet available. Complex 2 is intended to be an approach to
serinate-bound site a, the existence of which would be appear
to be feasible. However, we note that 2 and its molybdenum
analogue are extremely sensitive to small amounts of water.
The hydrolysis product has not been characterized.
Sulfur and Selenium Ligation. Complex 1 in THF reacts
instantly with benzenethiolate or benzeneselenolate in acetoni-
trile, when the latter is added to the reaction mixture as a salt
or generated in situ by reduction of PhSSPh or PhSeSePh in
THF. Green complexes 3 (59%) and 5 (68%) are obtained as
monocarbonyls. These species exhibit characteristic absorption
spectra, that of 5 being red-shifted compared to 3 (Figure 6).
The compounds [Mo(CO)(QPh)(S2C2Me2)2]1- (Q ) S, Se) are
formed under comparable conditions.23 In an attempt to prepare
carbonyl-free complexes, the reactions of 1 and the sterically
bulky ligands 2,4,6-triisopropylbenzenethiolate and 2,4,6-tri-
isopropylbenzeneselenolate were examined. With ligand/1 mol
ratios in the range from 1:1 to 2.4:1 and reaction times of about
1 h, the green monocarbonyl complex 4 (42%) and 6 (57%)
were obtained. Compared to 3 and 5, the absorption spectra of
these complexes are very similar; the νCO values are increased
Figure 2. Synthesis of bis(dithiolene)W(IV) complexes 2-6 by ligand
substitution of the dicarbonyl complex 1.
hydroxide,sulfide,andselenidetoformtheseries[WQ(S2C2Ph2)2]2-
(Q ) O, S, Se).17 The corresponding complex [Mo(CO)2(S2C2-
Me2)2] is a versatile precursor for the formation of a large family
of bis(dithiolene)Mo complexes with ligation modes resembling
those of W(IV) in Figure 1.23 Species derived from [M(CO)2-
(S2C2Me2)2] have the attractive feature of binding by a dialkyl-
dithiolene, in this sense, closely related to the cofactor ligand
(Figure 1). Much of our previous work has utilized the ligand
benzene-1,2-dithiolate.16 Although it is of the dithiolene type
and entirely suitable for structural studies, as in our recent X-ray
absorption investigation,28 this ligand should induce electronic
properties including redox potentials that depart to an extent
from those intrinsic to the cofactor ligand.
To examine the viability of molecules based on W(IV) sites
a-c (Figure 1), reactions of 1 with arene oxide, sulfide, and
selenide ligands have been investigated in THF or acetonitrile
and are summarized in Figure 2. Reaction products 2-6 are
diamagnetic. The structures of the five complexes are shown
in Figures 3 and 4; metric data are collected in Table 3. Because
dithiolene complexes are strongly colored, the usually highly
featured absorption spectra provide a simple means of identi-
(29) Boyde, S.; Ellis, S. R.; Garner, C. D.; Clegg, W. J. Chem. Soc., Chem.
Commun. 1986, 1541.
(30) Davies, E. S.; Beddoes, R. L.; Collison, D.; Dinsmore, A.; Docrat,
A.; Joule, J. A.; Wilson, C. R.; Garner, C. D. J. Chem. Soc., Dalton
Trans. 1997, 3985.
(28) Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. H.; Hedman,
B.; Hodgson, K. O. J. Am. Chem. Soc. 1991, 121, 10297.

Synthesis of Bis(dithiolene)-Tungsten(IV)
Inorganic Chemistry, Vol. 39, No. 6, 2000 1279
Figure 4. Structures of selenolate-ligated complexes 5 and 6, showing
50% probability ellipsoids and atom labeling schemes.
Table 3. Selected Bond Distances (Å) and Angles (deg)
2
3a
4
5a
6
WsQb
WsS1
WsS2
WsS3
WsS4
mean of 4
WsC1
C1sO1
SsCc
1.861(3) 2.438(2) 2.450(2) 2.547(1) 2.568(1)
2.329(1) 2.387(2) 2.382(2) 2.396(3) 2.331(1)
2.331(1) 2.351(2) 2.374(2) 2.365(3) 2.403(1)
2.311(1) 2.378(2) 2.336(2) 2.393(3) 2.378(1)
2.319(1) 2.360(2) 2.410(2) 2.349(3) 2.368(1)
2.323(8) 2.37(1)
2.03(1)
1.12(1)
1.779(5) 1.74(1)
2.38(3)
2.38(2) 2.37(3)
2.022(7) 2.02(1) 2.017(3)
1.154(8) 1.12(1) 1.139(4)
1.756(5) 1.74(1) 1.751(6)
CsCc,d
WsQsCb
1.321
1.341
1.341
1.354
1.337
146.7(3) 114.0(3) 110.3(2) 110.6(3) 108.2(1)
Figure 3. Structures of phenolate-ligated complex 2 and thiolate-ligated
complexes 3 and 4, showing 50% probability ellipsoids and atom
labeling schemes.
S1sWsS2e 81.98(5) 80.43(8) 79.67(6) 79.8(1) 82.21(3)
S3sWsS4e 82.34(5) 80.41(8) 82.10(6) 80.19(9) 79.81(3)
S1sWsS4f 141.82(4) 137.50(8) 147.3(2) 131.4(1) 142.08(3)
S2sWsS3f 139.99(5) 138.51(8) 141.28(6) 146.1(1) 137.57(3)
by 20-28 cm-1, presumably because of the higher basicity of
the substituted ligands. Subsequently, reactions of the substituted
ligands were conducted with the mol ratio ligand/1 of 5:1 for 4
h in acetonitrile/THF. The solvent was removed in vacuo, and
the absorption and IR spectra of the residue were examined.
These are identical to those of 4 and 6, prepared and isolated
as described. Clearly, carbonyl-free complexes are not favored
under these more forcing conditions.
δg
θd
0.779
127.3
0.849
124.0
0.830
125.3
0.665
124.4
0.650
126.3
h
a One of three (3) or four (5) anions in the asymmetric; dimensions
of other independent anions are not significantly different. b Q ) O,
S, or Se; unidentate ligand. c Mean value. d Chelate ring. e Chelate bite
angle. f Transoid angle. g Perpendicular displacement of W atom from
S4 least-squares plane. h Dihedral angle between WS2 planes.
The structures of complexes 3/4 (Figure 3) and 5/6 (Figure
4) are quite similar. The set of four possesses the following
properties, evident from the data in Table 3: (i) The stereo-
chemistry is distorted trigonal prismatic, as indicated by angles
between coordination planes (θd ) 124-126°) and transoid
S-W-S angles, involving atoms of different ligands (131-
147°). For a trigonal prism, θd ) 120°, and the transoid angle
is 136°. (ii) Ligand C-C and C-S bond distances point to an
enedithiolate/W(IV) formulation. (iii) Mean W-S distances are
essentially constant (2.37-2.38 Å) and are larger than the value
for 2 (2.323(8) Å) because of the higher coordination number.
They correspond closely to other five-coordinate WIV-S(dithi-

1280 Inorganic Chemistry, Vol. 39, No. 6, 2000
Sung and Holm
Table 4. Metal-Oxygen/Sulfur/Selenium Bond Lengths (Å) (x )
unknown compound)
[M(CO)(QAr)
(S2C2Me2)2]1-
[M(QAr)
(S2C2Me2)2]1-
-QAr
Moa
W
Moa
W
OPh
x
x
x
x
b
1.861(3)
c
x
x
x
x
OC6H3-2,6-Pri2
SPh
1.867(8)
x
2.338(1)
x
x
2.469(1)
d
2.585(1)
2.580(3)
2.438(2)
2.450(2)
2.547(1)
2.568(1)
SC6H2-2,4,6-Pri3
SePh
SeC6H2-2,4,6-Pri3
a Ref 23. b Not determined. c Not prepared. d Detected but not iso-
lated.
Table 5. Tungsten (IV/III) Redox Potentials in Acetonitrile
complex
E1/2, Va (∆Ep, mV)
Figure 5. Absorption spectra of [M(OPh)(S2C2Me2)2]1- (M ) Mo,
W (2)) in acetonitrile solutions.
[W(OPh)(S2C2Me2)2]1-
-2.07b
[Mo(OPh)(S2C2Me2)2]1-
-1.78b
[W(CO)(SPh)(S2C2Me2)2]1-
-1.45 (100)
-1.54 (166)
-1.45 (90)
-1.57 (90)
[W(CO)(SC6H2-2,4,6-Pri3)(S2C2Me2)2]1-
[W(CO)(SePh)(S2C2Me2)2]1-
[W(CO)(SeC6H2-2,4,6-Pri3)(S2C2Me2)2]1-
a E1/2 ) (Epc + Epa)/2, 100 mV/s, vs SCE. b Irreversible; values from
differential pulse voltammetry.
Pri3)(S2C2Me2)2]1- (green-brown) has been observed only as an
intermediate and reverts to five-coordinate [Mo(SC6H2-2,4,6-
Pri3)(S2C2Me2)2]1- (red-brown) upon workup, which includes
solvent removal in vacuo. In contrast, selenium-ligated [Mo-
(CO)(SeC6H2-2,4,6-Pri3)(S2C2Me2)2]1- (green-brown) is readily
isolated under the same workup conditions. A carbonyl-free
molybdenum selenolate complex has not been observed in the
reaction system, nor has it been obtained by thermolysis or
photolysis of the carbonyl complex. Similarly, tungsten complex
6, with the triisopropylbenzeneselenoate ligand, is entirely stable.
One likely factor affecting the stability of molybdenum
carbonyl adducts is the length of the Mo-QAr bond. The short
Mo-O bond and the multiple orientations of the diisopropyl-
phenolate group by rotations about the Mo-O and C-O bonds
should discourage carbonyl binding at a cis position. The longer
Mo-S bond and similar orientational mobility do allow
formation of a labile monocarbonyl but also enable the isolation
of the carbonyl-free complex. The Mo-Se bond is 0.11-0.12
Å longer, and, even with the very bulky triisopropyl benzene-
selenoate ligand, the carbonyl complex is stable. The same steric
factors must apply to the isostructural tungten complexes. The
stability of monocarbonyls 4 and 6, containing the sterically
demanding triisopropylphenyl group, reflects a larger intrinsic
tendency of tungsten to bind carbon monoxide. This effect has
been observed in other systems as both a thermodynamic and
a kinetic property. For example, the bond dissociation energy
of [W(CO)6] is ∼5 kcal/mol larger than that found for [Mo-
(CO)6], as studied by laser pyrolysis.31 Also, carbonyl subsitution
reactions in M(0) complexes are usually markedly slower for
tungsten than molybdenum.32 We are unaware of additional
comparative reactivity observations for Mo(IV) and W(IV) other
than redox potentials.33
Figure 6. Absorption spectra of [W(CO)(QPh)(S2C2Me2)2]1- (Q ) S
(3), Se (5)) in acetonitrile solutions.
olene) values (2.33-2.39 Å16-20). (iv) The difference between
W-SAr bond distances in 3 and 4 (0.01 Å) is not significant,
and between W-Se bond distances in 5 and 6 (0.02 Å) is slight,
indicating that the more voluminous triisopropylphenyl group
does not significantly elongate these bonds. With the exception
of certain conformational differences, analogous molybdenum23
and tungsten complexes are isostructural and nearly isometric.
The stereochemistry of six-coordinate complexes, such as 1 and
3-6, places unidentate ligands in cis positions without excep-
tion. The trans isomer of either octahedral or trigonal prismatic
geometry has never been observed.
In the context of active-site structural analogues, non-carbonyl
complexes are obviously more desirable than monocarbonyls.
Several interesting differences have emerged, thus far, in the
comparative chemistry of molybdenum23 and tungsten bis-
(dithiolenes) with arene oxide, thiolate, and selenolate (ArQ-)
ligands. Metal-ligand bond lengths for monocarbonyl and non-
carbonyl complexes of molybdenum and tungsten are collected
in Table 4, which makes evident the complexes that have not
yet been prepared. We note certain dimensional factors that
apparently contribute to the existence or nonexistence of the
compounds included in Table 4. In the synthesis of oxygen-
ligated complexes 2, [Mo(OPh)(S2C2Me2)2]1-, and [Mo(OC6H3-
2,6-Pri2)(S2C2Me2)2]1-, no carbonyl intermediate or product has
been detected. Passage of CO through an acetonitrile solution
of 2 generates the extremely labile green chromophore
[W(CO)(OPh)(S2C2Me2)2]1-, whose formation is reversible. In
comparison, sulfur-ligated 3 and [Mo(CO)(SPh)(S2C2Me2)2]1-
are stable isolable compounds. However, [Mo(CO)(SC6H2-2,4,6-
Tungsten(IV/III) redox potentials for complexes 2-6 are
given in Table 5. The potential for 2 and its molybdenum
counterpart are so negative as to preclude the formation of a
(31) Lewis, K. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. Soc. 1984,
106, 3905.
(32) Howell, J. A. S.; Burkinshaw, P. M. Chem. ReV. 1983, 83, 557.
(33) Tucci, G. C.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1998, 37,
1602, and references therein.

Synthesis of Bis(dithiolene)-Tungsten(IV)
Inorganic Chemistry, Vol. 39, No. 6, 2000 1281
bis(dithiolene)M(III) species under physiological conditions. The
potential order W < Mo is observed without exception in
constant coordination environments.33 The retention of a car-
bonyl in the coordination sphere affords chemically reversible
cyclic voltammograms (ipc/ip ≈ 1, not shown) and stabilizes
the W(III) state by ∼500 mV. The existence of bis(dithiolene)
monocarbonyl complexes of both metals raises the interesting
question as to whether reduced enzymes will bind carbon
monoxide.
This work represents our initial exploration of the formation
and stability of bis(dithiolene)W(IV) complexes related to sites
a-c (Figure 1). Further development of structural and reactivity
analogues of enzyme sites necessitates the examination of the
oxidative chemistry, both the oxygen atom and the electron
transfer, potentially leading to W(VI) sites d-f. Such
studies, also involving bis(dithiolene)Mo(IV) complexes, are in
progress.
Acknowledgment. This research was supported by NSF
Grant No. CHE 95-23830. We thank Dr. C. A. Goddard and B.
S. Lim for experimental assistance and useful discussions.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of the five compounds
in Table 2. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC991153U