Synthesis and structures of bis(dithiolene)tungsten(IV,VI) thiolate and selenolate complexes: Approaches to the active sites of molybdenum and tungsten formate dehydrogenases

Article abstract of DOI:10.1021/ic062441a

Formate dehydrogenases are molybdenum- or tungsten-containing enzymes that catalyze the oxidation of formate to carbon dioxide. Among the significant characteristics of the mononuclear active sites are coordination of two pyranopterindithiolene ligands and selenocysteinate to the metal in oxidation states IV-VI. The first detailed investigation of the synthesis and structures of bis(dithiolene)tungsten selenolate and analogous thiolate complexes of relevance to formate dehydrogenases has been undertaken. Some 17 complexes of the types [W^IV(QR)(S₂C₂Me₂) ₂]^-, [W^VIO(QR)(S₂C ₂Me₂)₂]^-, and [W^VIS(QR) (S₂C₂Me₂)₂]^- (Q = S, Se; R = tert-butyl, 1-adamantyl) and the desoxo species [W^VI(SR) (OSiR′₃)(S₂C₂Me₂) ₂] (R′ = Me, Ph) were prepared. Ten structures of representative members of these types were determined; W^IV complexes are square-pyramidal and W^VI complexes are six-coordinate, with geometries intermediate between octahedral and trigonal-prismatic. Selenolate complexes are less stable than similar thiolate species; decomposition products were identified as [W^V₂(μ₂-Q) ₂(S₂C₂Me₂)₂] ^2- and [W^IV,V₂(μ₂-Se)(S ₂C₂Me₂)₄]^_. The several [Mo^IV(QR)(S₂C₂Me₂)₂] ^- complexes prepared earlier and the tungsten compounds synthesized in this work form a family of molecules whose overall stereochemistry and metric features are those expected in the absence of protein structural constraints.

Full text of DOI:10.1021/ic062441a

Inorg. Chem. 2007, 46, 4090−4102
Synthesis and Structures of Bis(dithiolene)tungsten(IV,VI) Thiolate and
Selenolate Complexes: Approaches to the Active Sites of Molybdenum
and Tungsten Formate Dehydrogenases
Stanislav Groysman and R. H. Holm*
Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138
Received December 20, 2006
Formate dehydrogenases are molybdenum- or tungsten-containing enzymes that catalyze the oxidation of formate
to carbon dioxide. Among the significant characteristics of the mononuclear active sites are coordination of two
pyranopterindithiolene ligands and selenocysteinate to the metal in oxidation states IV−VI. The first detailed
investigation of the synthesis and structures of bis(dithiolene)tungsten selenolate and analogous thiolate complexes
of relevance to formate dehydrogenases has been undertaken. Some 17 complexes of the types [WIV(QR)(S2C2Me2)2]-,
[WVIO(QR)(S2C2Me2)2]-, and [WVIS(QR)(S2C2Me2)2]- (Q
species [WVI(SR)(OSiR
) S, Se; R ) tert-butyl, 1-adamantyl) and the desoxo
′
3)(S2C2Me2)2] (R′ ) Me, Ph) were prepared. Ten structures of representative members of
these types were determined; WIV complexes are square-pyramidal and WVI complexes are six-coordinate, with
geometries intermediate between octahedral and trigonal-prismatic. Selenolate complexes are less stable than
similar thiolate species; decomposition products were identified as [WV2 µ2-Q)2(S2C2Me2)2]2- and [WIV,V
( 2(µ2-Se)(S2C2-
Me2)4]-. The several [MoIV(QR)(S2C2Me2)2]- complexes prepared earlier and the tungsten compounds synthesized
in this work form a family of molecules whose overall stereochemistry and metric features are those expected in
the absence of protein structural constraints.
Introduction
the DMSOR family.3 FDHs are most frequently encountered
as molybdoenzymes. The active site structures of FDHs
obtained from protein crystallography7-10 and extended X-ray
absorption fine structure (EXAFS) analyses11,12 are schemati-
cally summarized in Figure 1, where it is seen that all but
one refer to Mo-FDHs. It is now apparent, long after their
recognition in 1973-1983,13,14 that a substantial number of
W-FDHs exist.15-19 In considering currently known FDH
As part of our elaboration of the synthetic, structural, and
reactivity analogues of the mononuclear active sites of
molybdenum and tungsten enzymes,1 we have turned our
attention to formate dehydrogenases (FDHs2). These enzymes
have been purified from eukaryotes, archaea, and bacteria
and catalyze the reaction HCO2 ) CO2 + H+ + 2e-.3-6
Although not an oxotransferase, FDH has been classified in
-
* To whom correspondence should be addressed. E-mail: holm@
chemistry.harvard.edu.
(7) Boyington, J. C.; Gladyshev, V. N.; Khangulov, S. V.; Stadtman, T.
C.; Sun, P. D. Science 1997, 275, 1305-1308.
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ReV. 2004, 104, 1175-1200.
(2) Abbreviations: Ad ) adamantyl; Ar ) 2,4,6-PriC6H2; bdt ) benzene-
1,2-dithiolate(2-); But3tach ) 1,3,5-tri-tert-butyl-1,3,5-triazacyclo-
hexane; DMSOR ) dimethyl sulfoxide reductase; FDH ) formate
dehydrogenase; Me3tacn ) 1,4,7-trimethyl-1,4,7-triazacyclononane;
mnt ) maleonitriledithiolate(2-); Q ) O, S, Se; Tp* ) tris(3,5-
dimethylpyrazolyl)hydroborate(1-)
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4090 Inorganic Chemistry, Vol. 46, No. 10, 2007
10.1021/ic062441a CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/14/2007

Molybdenum and Tungsten Formate Dehydrogenases
Figure 1. Schematic depictions of the active sites of FDHs obtained from X-ray diffraction or EXAFS analysis. Selected bond distances are indicated. In
b
c
d
e
f
the pyranopterindithiolene structure, R is a nucleotide. References: aref 7, ref 8, ref 11, ref 9, ref 12, ref 10.
site structures, there are three features of note. (i) The metal
atom is coordinated by two pyranopterindithiolene cofactor
ligands, a defining feature of the DMSOR family of
molybdenum enzymes. However, not all W-FDHs reported
have been shown to contain two such ligands. (ii) Sites
accommodate molybdenum and/or tungsten. Indeed, one
organism (DesulfoVibrio alaskensis) produces active Mo-
FDH and W-FDH isoenzymes.18 (iii) Selenocysteinate is
coordinated in the oxidized and reduced states, with one
possible exception8 (Figure 1).
A necessary step prior to the development of analogue
reaction systems is the synthesis of appropriate representa-
tions of the enzyme active sites. For nearly all native
tungstoenzymes, the full coordination unit in any oxidation
state has not been experimentally established. Molybdoen-
zyme members of the DMSOR family manifest terminal
binding of endogenous serinate, cysteinate, or selenocys-
teinate and exogenous hydroxide/water and probably sulfide/
hydrosulfide. Consequently, we have prepared and structur-
ally characterized extensive sets of five-coordinate (L′ absent)
and six-coordinate molybdenum and tungsten complexes
[MLL′(S2C2R2)2]z containing variously substituted alkox-
ide, phenolate, and thiolate ligands as well as other ligand
types. With M ) WIV-VI, ca. 25 complexes containing dif-
ferent ligand sets L and L,L′ have been prepared in this
laboratory1,20-22 and by others.23-25 During the course of that
work, we found selenolate complexes related to FDH sites
difficult to prepare. The only known examples of such com-
plexes are of the types [MoIV(SeR)(S2C2Me2)2]-,26 [MIV(CO)-
(SeR)(S2C2Me2)2]- (M ) Mo, W),27,28 and [WV(SeR)2(S2C2-
Me2)2]-.29 Consequently, the present investigation was
(20) Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 1302-1310.
(21) Wang, J.-J.; Kryatova, O.; Rybak-Akimova, E. V.; Holm, R. H. Inorg.
Chem. 2004, 43, 8092-8101.
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2988.
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E.; Fritsche, K.; Stams, A. J. M. Eur. J. Biochem. 2003, 270, 2476-
2485.
(23) Ueyama, N.; Oku, H.; Nakamura, A. J. Am. Chem. Soc. 1992, 114,
7310-7311.
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J. Biochem. 2003, 270, 325-333.
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118, 1387-1397.
(17) Graentzdoerffer, A.; Rauh, D.; Pich, A.; Andreesen, J. R. Arch.
Microbiol. 2003, 179, 116-130.
(25) Oku, H.; Ueyama, N.; Nakamura, A. Bull. Chem. Soc. Jpn. 1996, 69,
3139-3150.
(18) Brondino, C. D.; Passeggi, M. C. G.; Caldeira, J.; Almendra, M. J.;
Feio, M. J.; Moura, J. J. G.; Moura, I. J. Biol. Inorg. Chem. 2004, 9,
145-151.
(26) Lim, B. S.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1920-1930.
(27) Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275-1281.
(28) Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263-
273.
(19) Rauh, D.; Graentzdoerffer, A.; Granderath, K.; Andreesen, J. R.; Pich,
A. Eur. J. Biochem. 2004, 271, 212-219.
(29) Sung, K.-M.; Holm, R. H. Inorg. Chem. 2001, 40, 4518-4525.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4091

Groysman and Holm
h and filtered, and the solvent was removed from the filtrate. The
cloudy oil was dissolved in 10 mL of hexane, an insoluble material
was removed by filtration, and the solvent was removed to afford
the product as 180 mg (29%) of yellow oil. (Caution: malodorous!)
undertaken to develop synthetic routes to mononuclear bis-
(dithiolene)tungsten(IV,VI) selenolate complexes and to
determine their structures. We have concentrated on tungsten
rather than molybdenum in response to the existence of
W-FDHs (albeit less well-defined than Mo-FDHs) and
because of the instability of MoVI complexes containing
dimethyldithiolene ligands to autoreduction to MoV.26 As
noted in previous reports, this dialkyldithiolene ligand
provides an accurate simulation of the geometrical and
electronic structural features of the native pyranopterindithi-
olene ligand cofactor (Figure 1).
1
This compound was prepared earlier using ButLi at 0 °C. The H
NMR spectrum was the same as that of a sample prepared by that
method.33
Tungsten(IV) Complexes. (Et4N)[W(SBut)(S2C2Me2)2]. Method
1. To a solution of 95 mg (0.15 mmol) of (Et4N)[W(O2CBut)(S2C2-
Me2)2]20 in 2 mL of acetonitrile was added a solution of 27 mg
(0.17 mmol) of Me3SiSBut in 0.5 mL of acetonitrile. The reaction
mixture was stirred for 30 min, and the solvent was removed. The
brown solid was washed with 10 mL of ether and dissolved in 3
mL of 2:1 (v/v) THF/acetonitrile, and the solution was layered with
20 mL of ether. The mixture was allowed to stand for 12 h. The
pale-yellow solution was decanted, and the solid was washed with
5 mL of ether and dried, leading to 49 mg (53%) of product as a
yellow crystalline solid. Absorption spectrum (acetonitrile): λmax
(ꢀM) 311 (13 000), 343 (7000), 419 (4300), 501 (2400) nm. ES-
Experimental Section
Preparation of Compounds. All reactions and manipulations
were performed under anaerobic conditions using an inert atmo-
sphere box and standard Schlenk techniques. Acetonitrile and ether
were freshly purified using an Innovative Technology solvent
purification system. Methanol was distilled from magnesium and
THF, and hexanes were distilled from sodium benzophenone ketyl.
Adamantane-1-thiol was a commercial sample (Aldrich). All solvent
removal and drying steps were performed in vacuo. 1H NMR data
of salts refer to anions only. Compounds were identified by a
combination of their mass spectra (in which parent ions were
1
MS: m/z 510 (M-). H NMR (CD3CN): δ 0.97 (9), 2.81 (12).
Method 2. A solution of 25 mg (0.052 mmol) of [W(CO)2(S2C2-
Me2)2]34 in 1 mL of THF was added with stirring to a solution of
11 mg (0.050 mmol) of (Et4N)(SBut) in 1 mL of acetonitrile.
Vigorous gas evolution followed, and the mixture turned green.
After 90 min, 20 mL of ether was layered on the mixture, which
was allowed to stand for 12 h. The solid that separated was washed
with ether and recrystallized from THF/acetonitrile/ether to give
20 mg (60%) of yellow crystals, spectroscopically identical to the
product of method 1.
1
observed), H NMR spectra, and X-ray crystal structures. Several
representative compounds were subjected to elemental analysis.
2-Adamantylselenotrimethylsilane. A solution of 220 mg (0.51
mmol) of bis(2-adamantyl)diselenide26 in 10 mL of THF was treated
with 1.0 mL of a 1.0 M solution of LiEt3BH (2 equiv) in THF.
The reaction mixture changed from yellow to milky white with
vigorous gas evolution and was stirred for 2 h. Me3SiCl (0.13 mL,
1.0 mmol) was added, the mixture was stirred overnight and filtered,
and volatiles were removed from the filtrate. The remaining yellow
liquid was extracted with hexanes, the solution was filtered, and
hexane was evaporated to give the product as 118 mg (41%) of a
(Et4N)[W(S-1-Ad)(S2C2Me2)2]. A solution of 97 mg (0.20
mmol) of [W(CO)2(S2C2Me2)2] in 2 mL of THF was mixed with a
suspension of 39 mg (0.20 mmol) of NaS-1-Ad in 0.5 mL of
acetonitrile. Upon stirring, the mixture formed a homogeneous green
solution. After 30 min, 34 mg (0.20 mmol) of Et4NCl in 0.5 mL of
acetonitrile was added and stirring was continued for 5 h. The
solvent was removed, and the orange-brown solid was washed with
ether (6 mL) and dissolved in 3 mL of THF. Acetonitrile (1 mL)
was added; the solution was covered with 20 mL of ether and
allowed to stand for 20 h. The solid was collected, washed with
ether, and dried, giving the product as 89 mg (61%) of orange
crystalline solid. Absorption spectrum (acetonitrile): λmax (ꢀM) 310
(15 600), 343 (8200), 418 (4800), 502 (2650), 674 (650) nm. ES-
1
bright-yellow oil. H NMR (CDCl3): δ 0.35 (s, 9), 1.54 (d, 2),
1.79 (m, 10), 2.22 (d, 2), 3.45 (s, 1).
1-Adamantylselenotrimethylsilane. The preceding method was
used with bis(1-adamantyl)diselenide,30,31 which was prepared by
1
the procedure for the 2-adamantyl compound. H NMR (CDCl3):
δ 0.44 (s, 9), 1.71 (m, 6), 2.00 (br s, 3), 2.17 (m, 6).
1
MS: m/z 587 (M-). H NMR (CD3CN): δ 1.42-1.52 (m, 12),
tert-Butylthiotrimethylsilane. A mixture of 3.0 mL (0.027 mol)
of ButSH and 1.5 g (0.022 mol) of NaOEt in anhydrous methanol
was stirred for 2 h, and the solvent was removed. The residue was
suspended in acetonitrile, and 3.2 g (0.019 mol) of Et4NCl was
added. The mixture was stirred for 36 h and filtered, and the solvent
was removed from the filtrate. The oily residue was dried for 2 h,
leaving (Et4N)(SBut) as an off-white solid. This material (0.67 g,
3.1 mmol) was suspended in hexanes, and 0.33 g (3.0 mmol) of
Me3SiCl was added. The mixture was stirred for 5 h and filtered,
and the solvent was removed to give the product as a clear liquid.
1H NMR (CDCl3): δ 0.35 (s, 9), 1.45 (s, 9). This compound was
prepared earlier by a different method.32
tert-Butylselenotrimethylsilane. To an ethereal solution of But-
MgCl (3.0 mmol) was added 236 mg (3.0 mmol) of gray selenium.
As the mixture was stirred, the gray color gradually disappeared
and a milky-white precipitate developed in 1-2 h. After 7 h, 0.50
g (3.0 mmol) of Me3SiCl was added, the mixture was stirred for 4
1.76 (br s, 3), 2.80 (12). Anal. Calcd for C26H47NS5W: C, 43.50;
H, 6.60; N, 1.95; S, 22.34. Found: C, 43.37; H, 6.52; N, 1.92; S,
21.38.
(Et4N)[W(SeBut)(S2C2Me2)2]. To a solution of 102 mg (0.156
mmol) of (Et4N)[W(O2CBut)(S2C2Me2)2] in 2 mL of acetonitrile
was added 42 mg (0.20 mmol) of Me3SiSeBut in 0.5 mL of
acetonitrile. After 2 min, formation of an orange solid was observed.
The reaction mixture was stirred for 15 min, and the solvent was
removed. The brown solid was washed with 6 mL of ether and
dissolved in 3 mL of 2:1 (v/v) THF/acetonitrile, and the solution
was layered with 20 mL of ether. After 12 h, the solid that separated
was collected, washed with 10 mL of ether, and dried to afford the
product as 77 mg (72%) of a yellow-orange solid. Absorption
spectrum (acetonitrile): λmax (ꢀM) 318 (15 400), 382 (sh, 5500),
424 (5200), 507 (3200) nm. ES-MS: m/z 557 (M-). 1H NMR (CD3-
CN): δ 1.11 (9), 2.84 (12).
(Et4N)[W(Se-1-Ad)(S2C2Me2)2]. To a solution of 113 mg (0.181
mmol) of (Et4N)[W(O2CBut)(S2C2Me2)2] in 2 mL of acetonitrile
(30) Palacios, S. M.; Santiago, A. N.; Rossi, R. A. J. Org. Chem. 1984,
49, 4609-4613.
(31) Barton, D. H. R.; Fontana, G. Tetrahedron 1996, 52, 11163-11176.
(32) Kuwajima, I.; Abe, T. Bull. Chem. Soc. Jpn. 1978, 51, 2183-2184.
(33) Zhu, N.; Fenske, D. J. Chem. Soc., Dalton Trans. 1999, 1067-1075.
(34) Goddard, C. A.; Holm, R. H. Inorg. Chem. 1999, 38, 5389-5398.
4092 Inorganic Chemistry, Vol. 46, No. 10, 2007

Molybdenum and Tungsten Formate Dehydrogenases
was added 73 mg (0.25 mmol) of Me3SiSe-1-Ad. The color changed
to red-orange. and a bright-orange solid precipitated. The reaction
mixture was stirred for 10 min, and the solvent was removed. The
solid residue was washed with ether (10 mL) and dissolved in THF/
acetonitrile (2:1, v/v). The red-orange solution was filtered, and
ether was layered on the filtrate. After 2 h, the pale-brown solution
was decanted and the remaining solid was washed with ether (6
mL) and dried to afford the product as 100 mg (72%) of orange
needlelike crystals. Absorption spectrum (acetonitrile): λmax (ꢀM)
318 (18 900), 381 (sh, 5800), 424 (4900), 507 (2400). ES-MS: m/z
20 mL of cold ether was added, and the mixture was allowed to
stand at -30 °C for 1 day. The pale-purple solution was decanted
from a purple solid, which was washed with cold ether (2 × 3
mL) and dried. The product was obtained as 79 mg (93%) of purple
solid. Absorption spectrum (acetonitrile): λmax (ꢀM) 298 (sh, 8200),
406 (3750), 557 (3500), 686 (sh, 1650) nm. ES-MS: m/z 525 (M-).
1H NMR (CD3CN): δ 1.46 (9), 2.19 (12).
(Et4N)[WO(S-1-Ad)(S2C2Me2)2]. To a solution of 89 mg (0.12
mmol) of (Et4N)[W(S-1-Ad)(S2C2Me2)2] in 1 mL of acetonitrile
was added in one portion a solution of 47 mg (0.15 mmol) of Ph3-
AsO in 1 mL of THF. A color change to intense purple was
observed within 5 min. The reaction mixture was stirred for 30
min. Cold ether (30 mL) was added, and the mixture was allowed
to stand at -30 °C for 1 day. The pale-purple solution was decanted,
and the solid residue was washed with cold ether (2 × 3 mL) and
dried to afford the product as 76 mg (84%) of a purple solid.
Absorption spectrum (acetonitrile): λmax (ꢀM) 300 (sh, 8900), 421
(3750), 559 (4500), 689 (sh, 1900) nm. ES-MS: m/z 603 (M-). 1H
NMR (CD3CN): δ 1.64 (m, 6), 1.95 (br s, 3), 2.06 (m, 6), 2.18
(12).
1
635 (M-). H NMR (CD3CN): δ 1.50 (m, 6), 1.65 (m, 6), 1.76
(br s, 3), 2.83 (12).
(Et4N)[W(Se-2-Ad)(S2C2Me2)2]. To a solution of 47 mg (0.072
mmol) of (Et4N)[W(O2CBut)(S2C2Me2)2] in 1 mL of acetonitrile
was added a solution of 31 mg (0.11 equiv) of Me3SiSe-2-Ad in
0.5 mL of acetonitrile. Within 2 min, an orange solid appeared.
The reaction mixture was stirred for 3 min, and the solvent was
removed. The red-orange solid was washed with 10 mL of ether
and dissolved in 3 mL of 2:1 (v/v) THF/acetonitrile. The solution
was filtered, and ether was layered on the filtrate. After 3 h, the
brown solution was decanted; the solid was washed with 6 mL of
ether and dried to afford the product as 36 mg (65%) of red-orange
needles. Absorption spectrum (acetonitrile): λmax (ꢀM) 317 (15 200),
374 (sh, 5600), 425 (4300), 500 (1200) nm. ES-MS: m/z 635 (M-).
1H NMR (CD3CN): δ 1.35 (d, 2), 1.47 (d, 2), 1.61 (br s, 3), 1.70
(m, 2), 1.73 (br s, 3), 1.80 (d, 2), 2.85 (s, 12), 3.27 (s, 1). Anal.
Calcd for C26H47NS4SeW: C, 40.84; H, 6.19; N, 1.83. Found: C,
40.57; H, 6.37; N, 1.66.
(Et4N)[W(CO)(SeBut)(S2C2Me2)2]. To a solution of 50 mg (0.11
mmol) of [W(CO)2(S2C2Me2)2] in 2 mL of THF was added a
solution of 33 mg (0.13 mmol) of (Et4N)(SeBut) in 0.5 mL of
acetonitrile. Vigorous gas evolution occurred immediately, and the
reaction mixture became green. The mixture was stirred for 35 min,
and the solvent was removed. The solid was washed with 6 mL of
ether and dissolved in THF (ca. 3 mL), and the solution was covered
with 20 mL of ether. After 24 h, the solid that separated was
collected, washed with ether, and dried to give the product as 49
mg (65%) of a green microcrystalline solid. Absorption spectrum
(acetonitrile): λmax (ꢀM) 365 (5200), 424 (4500), 602 (3900) nm.
IR (KBr): νCO 1921 cm-1. ES-MS: m/z 585 (M-). 1H NMR (CD3-
CN): δ 1.65 (9), 2.48 (12).
(Et4N)[W(CO)(Se-1-Ad)(S2C2Me2)2]. To a bright-yellow solu-
tion of 35 mg (0.082 mmol) of bis(1-adamantyl)diselenide in 1 mL
of THF was added 0.13 mL of (0.13 mmol) of a 1.0 M solution of
LiEt3BH in THF. The yellow color disappeared as gas evolution
occurred. This solution was added in one portion to a solution of
62 mg (0.13 mmol) of [W(CO)2(S2C2Me2)2]. Vigorous gas evolution
occurred immediately, and the reaction mixture became green. After
30 min, a solution of 21 mg (0.13 mmol) of Et4NCl in 1 mL of
acetonitrile was added, the mixture was stirred for 10 min, and the
solvent was removed. The greenish-brown solid residue was washed
with ether and dissolved in ca. 3 mL of THF, and 20 mL of ether
was layered on the solution. Two crops of solid were isolated,
affording the product as 28 mg (27%) of green crystals. Absorption
spectrum (acetonitrile): λmax (ꢀM) 276 (sh, 9200), 317 (sh, 6560),
437 (5200), 616 (2600) nm. IR (KBr): νCO 1919 cm-1. ES-MS:
m/z 663 (M-), 635 ({M-CO}-). NMR (CD3CN): δ 1.75 (m, 6),
1.96 (br s, 3), 2.26 (br s, 6), 2.48 (12).
(Et4N)[WO(SeBut)(S2C2Me2)2]. To a solution of 38 mg (0.055
mmol) of (Et4N)[W(SeBut)(S2C2Me2)2] in 2 mL of acetonitrile was
added a solution of 22 mg (0.068 mmol) of Ph3AsO in 2 mL of
THF, resulting in a slow color change to brown. After 3 min, 20
mL of cold ether was added and the solution was allowed to stand
at -30 °C. After 3 h, the blue solution was separated from a brown
solid and maintained at -30 °C for 3 h. The blue microcrystalline
solid that separated was isolated, washed with 2 × 3 mL of cold
ether, and dried, affording 18 mg (47%) of product. Absorption
spectrum (acetonitrile): λmax (ꢀM) 304 (sh, 5800), 429 (2400), 581
1
(2950), 697 (sh, 1800) nm. ES-MS: 573 (M-). H NMR (CD3-
CN): δ 1.59 (9), 2.19 (12).
(Et4N)[WO(Se-1-Ad)(S2C2Me2)2]. A solution of 46 mg (0.060
mmol) of (Et4N)[W(Se-1-Ad)(S2C2Me2)2] in 1 mL of acetonitrile
was treated with a solution of 23 mg (0.072 mmol) of Ph3AsO in
1 mL of THF in one portion. A color change to blue was observed
within 1-2 min. The reaction mixture was stirred for 7 min, and
20 mL of cold ether was layered on the solution. The mixture was
left to stand at -30 °C for 1 day. The pale-blue solution was
decanted. The solid residue was washed with cold ether and dried
to yield the product as 25 mg (53%) of a gray-blue solid. Absorption
spectrum (acetonitrile): λmax (ꢀM) 299 (sh, 9200), 437 (sh, 3000),
1
587 (3200), 689 (sh, 2500) nm. ES-MS: m/z 651 (M-). H NMR
(CD3CN): δ 1.67 (m, 6), 1.95 (br s, 3), 2.18 (12), 2.20 (m, 6).
(Et4N)[WS(S-1-Ad)(S2C2Me2)2]. A solution of 30 mg (0.042
mmol) of (Et4N)[W(S-1-Ad)(S2C2Me2)2] in 1 mL of acetonitrile
was cooled to -30 °C. A solution of 16 mg (0.041 mmol) of Ph3-
SbS in 1 mL of THF at -30 °C was added in one portion with
stirring. A color change from orange-brown to purple was observed
within seconds. The reaction mixture was stirred for 2 min, 20 mL
of cold ether was added, and the mixture was allowed to stand at
-30 °C overnight. The solid that separated was washed with cold
ether and dried to give the product as 28 mg (89%) of a dark-
violet solid. Absorption spectrum (acetonitrile): λmax (ꢀM) 333
1
(9100), 374 (7500), 556 (6100) nm. ES-MS: m/z 619 (M-). H
NMR (CD3CN): δ 1.64 (br s, 6), 2.00 (br s, 9), 2.30 (s, 12). Anal.
Calcd. for C26H47NS6W: C, 41.64; H, 6.32; N, 1.87. Found: C,
41.54; H, 6.30; N, 1.90.
Tungsten(VI) Complexes. (Et4N)[WO(SBut)(S2C2Me2)2]. A
solution of 83 mg (0.13 mol) of (Et4N)[W(SBut)(S2C2Me2)2] in 1
mL of acetonitrile was treated with a solution of 50 mg (0.16 mmol)
of Ph3AsO in 2 mL of THF. A slow color change to intense purple
occurred over 5 min. The reaction mixture was stirred for 30 min,
(Et4N)[WS(Se-1-Ad)(S2C2Me2)2]. The preceding method was
used based on 43 mg (0.056 mmol) of (Et4N)[W(Se-1-Ad)(S2C2-
Me2)2]. A rapid color change to purple-blue occurred. The dark-
purple solid that separated in ca. 1 h after the addition of ether was
discarded, and the remaining blue-gray solution was cooled to -30
Inorganic Chemistry, Vol. 46, No. 10, 2007 4093

Groysman and Holm
°C. After 3 h, the pale-blue solution was decanted and the solid
that separated was washed with cold ether and dried. The product
was obtained as 19 mg (43%) of a gray-blue solid. Absorption
spectrum (acetonitrile): λmax (ꢀM) 333 (8000), 383 (sh, 5500), 573
more intense and more solid dissolved. The mixture was filtered,
and the solvent was removed from the filtrate, leaving a red-brown
solid. This material was washed with hexanes and ether and dried
to give the product as 22 mg (39%) of a dark-red solid. Absorption
spectrum (toluene): λmax (ꢀM) 386 (8100), 491 (6500), 579 (sh,
1
(5200) nm. ES-MS: m/z 667 (M-). H NMR (CD3CN): δ 1.69
1
(br s, 6), 1.95 (br s, 3), 2.13 (br s, 6), 2.30 (s, 12).
4500) nm. H NMR (C6D6): δ 1.38 (br, 6), 1.80 (br, 3), 1.92 (m,
[W(SBut)(OSiMe3)(S2C2Me2)2]. To a stirred suspension of 65
mg (0.100 mmol) of (Et4N)[WO(SBut)(S2C2Me2)2] in 2 mL of
toluene was added 0.24 mL (0.094 mmol) of a 0.39 M solution of
Me3SiCl in toluene. The reaction mixture was stirred for 1 h and
filtered, and the solvent was removed. The brown solid was
extracted with a minimal volume of hexane, the solution was
filtered, and the solvent was removed from the filtrate, leaving the
product as 20 mg (33%) of a brown solid. Absorption spectrum
(hexane): λmax (ꢀM) 302 (4650), 378 (4400), 487 (3500), 585 (2750)
6), 2.31 (12), 7.19 (m, H), 7.97 (m, 6).
Tungsten(V) Complexes. (Et4N)2[W2(µ2-S)2(S2C2Me2)4]. A
solution of 25 mg (0.052 mmol) of [W(CO)2(S2C2Me2)2] in 1 mL
of THF was added with stirring to a solution of 11 mg (0.050 mmol)
of (Et4N)(SBut) in 1 mL of acetonitrile. The mixture turned green
with vigorous gas evolution and was stirred overnight, during which
it assumed an orange-brown color. An 1H NMR spectrum of a solid
obtained from the reaction mixture indicated a ∼3:1 mixture of
[W(SBut)(S2C2Me2)2]- and the product. The reaction mixture was
stirred for 3 days, layered with 20 mL of ether, and allowed to
stand overnight. The precipitate was collected, washed with ether,
and dried to afford the product as 12 mg (41%) of a purple solid.
Absorption spectrum (acetonitrile): λmax (ꢀM) 267 (25 600), 336
(13 900), 404 (11 400), 492 (10 500), 560 (sh, 8800) nm. ES-MS:
m/z 1036.5 ({(Et4N)[W2S2(S2C2Me2)4]}-), 452 ([WS(S2C2Me2)2]-).
1H NMR (CD3CN): δ 2.26. This compound has also been prepared
by other methods.29
(Et4N)2[W2(µ2-Se)2(S2C2Me2)4]. A solution of 17 mg (0.026
mmol) of (Et4N)[W(SeBut)(S2C2Me2)2] in 2 mL of acetonitrile was
stirred for 20 h, during which the color slowly changed from red-
orange to brown-black. The solvent was removed, and the solid
residue was recrystallized from acetonitrile/ether to afford the
product as 13 mg (77%) of a purple-black solid. Absorption
spectrum (acetonitrile): λmax (ꢀM) 271 (26 300), 351 (14 200), 409
(13 100), 440 (14 500), 529 (12 000) nm. ES-MS: m/z 1128
({(Et4N)[W2Se2(S2C2Me2)4]}-), 500.3 ([WSe(S2C2Me2)2]-). 1H
NMR (CD3CN): δ 2.31.
(Et4N)2[MoS(S2C2Me2)2]. A dark-violet solution of 50 mg (0.13
mmol) of [Mo(CO)2(S2C2Me2)2]28 in 2 mL of THF was added in
one portion to a solution of 42 mg (0.26 mmol) of (Et4N)(SH) in
1 mL of acetonitrile. A color change to green and gas evolution
were observed. The reaction mixture was stirred for 1 h, the solvent
was removed, and the residue was dissolved in 2 mL of acetonitrile.
The solution was covered with THF. The solid that separated after
1 day was washed with THF and dried to give the product as 60
mg (74%) of a microcrystalline green-brown solid. Absorption
spectrum (acetonitrile): λmax (ꢀM) 268 (16 200), 310 (10 000), 398
(4600), 462 (2000) nm. ES-MS: m/z 626 ({(Et4N)2[MoS(S2C2-
Me2)2]}+). 1H NMR (CD3CN): δ 2.36 (br). Anal. Calcd for C24H52-
MoN2S5: C, 46.12; H, 8.39; N, 4.48; S, 25.65. Found: C, 45.91;
H, 8.12; N, 4.36; S, 24.66. E1/2 ) -0.65 V (2-/1-, acetonitrile).
X-ray Structure Determinations. The structures of the 14
compounds in Tables 1 and 2 were determined. For simplicity, the
compounds are referred to by the numerical designations of their
complexes in Table 3. Diffraction-quality crystals of 3-7 were
obtained by layering ether onto concentrated acetonitrile/THF
solutions and allowing the mixtures to stand overnight. Crystals of
10-12 were grown similarly but with mixtures maintained at
-30 °C for longer periods. Crystals of 19 were obtained by slow
concentration of a toluene solution, and those of 20 were obtained
from a saturated ether solution at -30 °C. Crystals of 21 formed
upon ether diffusion into an acetonitrile solution at room temper-
ature, and those of 22 and 23 were grown in a similar manner from
solutions at -30 °C.
1
nm. ES-MS: m/z 598.0 (M-). H NMR (C6D6): δ 0.36 (9), 1.45
(9), 2.35 (12). Over several days in solution (ether, hexane), the
complex partially decomposes, forming [W(S2C2Me2)3] (identified
1
by H NMR7).
[W(SC6H2-2,4,6-Pri3)(OSiMe3)(S2C2Me2)2]. To a stirred suspen-
sion of 79 mg (0.098 mmol) of purple (Et4N)[WO(SC6H2-2,4,6-
Pri3)(S2C2Me2)2]20 in 2 mL of toluene was added 0.22 mL of a 0.39
M solution of Me3SiCl (0.90 equiv) in toluene. The reaction mixture
was stirred for 40 min as the color gradually changed to brown
and was filtered, and the solvent was removed from the filtrate.
The red-brown solid contained the desired product and ca. 8%
[W(S2C2Me2)3] (1H NMR). The latter was removed by extraction
with a minimal volume of hexane, leaving the product as 30 mg
(45%) of a red-brown solid. Absorption spectrum (hexane): λmax
(ꢀM) 282 (sh, 7000), 391 (8150), 525 (sh, 4300), 600 (2900) nm.
1
ES-MS: m/z 745 (M-). H NMR (C6D6): δ 1.18 (d, 6), 1.32 (d,-
12), 2.38 (s, 12), 2.76 (sept, 1), 3.95 (sept, 2), 6.97 (s, 2).
[W(SC6H2-2,4,6-Pri3)2(S2C2Me2)2]. This compound was isolated
in small yield as a reaction byproduct and was identified by an
X-ray structure determination. To a stirred suspension of 44 mg
(0.054 mmol) of (Et4N)[WO(SC6H2-2,4,6-Pri3)(S2C2Me2)2] in 2 mL
of THF was added 0.07 mL of a 0.39 M solution of Me3SiCl (0.027
mmol). The reaction mixture immediately turned to red-brown, was
stirred for 45 min and filtered, and the solvent was removed from
the filtrate to give a brown solid. The solid was dissolved in ether
and maintained at -30 °C. After several days, brown crystals were
separated, washed with cold ether, and dissolved in hexanes. The
solution was filtered, and the solvent was removed from the filtrate
to given a brown solid, which was dried to give ∼4 mg of product.
ES-MS: m/z 745 (M-). 1H NMR (C6D6): δ 1.23 (d, 12), 1.35 (br
s, 24), 2.32 (s, 12), 2.80 (2), ∼3.5 (v br, 4), 6.95 (br, 4).
[W(S-1-Ad)(OSiMe3)(S2C2Me2)2]. To a stirred suspension of
25 mg (0.034 mmol) of purple (Et4N)[WO(S-1-Ad)(S2C2Me2)2] in
2 mL of toluene was added 0.09 mL (0.035 mmol) of a 0.39 M
solution of Me3SiCl in toluene. The reaction mixture, which slowly
assumed a red-brown color, was stirred for 1 h and filtered, and
the solvent was removed from the filtrate to give a brown
solid. The solid was extracted with hexane. Solvent was removed
from the solution to yield the product as 12 mg (52%) of a red-
brown solid. Absorption spectrum (hexanes): λmax (ꢀM) 282 (sh,
9700), 383 (7300), 441 (6000), 488 (sh, 5800), 579 (sh, 2900) nm.
1
ES-MS: m/z 676 (M-). H NMR (C6D6): δ 0.37 (s, 9), 1.45 (m,
6), 1.87 (br s, 3), 2.12 (s, 12), 2.33 (s, 12).
[W(S-1-Ad)(OSiPh3)(S2C2Me2)2]. To a stirred suspension of 54
mg (0.074 mmol) of (Et4N)[WO(S-1-Ad)(S2C2Me2)2] in 2 mL of
toluene was added a solution of 20 mg (0.066 mmol) of Ph3SiCl
in 1 mL of toluene. The reaction mixture immediately turned red
and was stirred for 15 min, during which the red color became
Crystals were coated in grease and mounted on a Siemens
(Bruker) SMART CCD instrument using Mo KR radiation. Data
were collected at 193 K with scans of 0.3°/frame for 10, 20, or 30
4094 Inorganic Chemistry, Vol. 46, No. 10, 2007

Molybdenum and Tungsten Formate Dehydrogenases
Table 1. Crystallographic Data and Structure Refinement Parameters for 3-7, 10, and 11
(Et4N)[11]
‚CH3CN
(Et4N)[3]
(Et4N)[4]
(Et4N)[5]
(Et4N)[6]
(Et4N)[7]
(Et4N)[10]
formula
fw
cryst syst
space group
Z
C20H41NS5W
639.69
monoclinic
P21/n
C26H47NS5W
717.80
monoclinic
P21/n
C20H41NS4SeW
686.59
monoclinic
P21/n
C26H47NS4SeW
764.70
tetragonal
I41/a
C26H47NS4SeW
764.70
tetragonal
I41/a
C20H41NOS5W
655.69
monoclinic
P21/n
C26H44NOS5W
793.86
triclinic
P1h
2
4
4
4
16
16
4
a, Å
b, Å
c, Å
10.888(1)
21.931(2)
11.311(1)
90
97.963(1)
90
2674.7(4)
1.589
4.716
10.894(1)
24.802(1)
11.476(1)
90
96.536(1)
90
3080.5(3)
1.548
4.104
10.895(3)
22.103(6)
11.330(3)
90
98.595(6)
90
2697.9(13)
1.690
5.947
36.01(2)
36.02(1)
9.885(8)
90
90
90
12816(15)
1.585
5.017
36.315(3)
36.315(3)
9.736(2)
90
90
90
12839(3)
1.582
5.008
9.504(1)
16.525(2)
17.415(3)
90
103.404(3)
90
2660.4(7)
1.637
4.746
11.011(1)
11.819(1)
14.844(1)
74.006(1)
88.728(1)
70.253(1)
1742.5(2)
1.513
R, deg
â, deg
γ, deg
V, Å3
d
calcd, g/cm3
µ, mm-1
3.639
2θ range, deg
R1a (wR2b), %
GOF (F2)
3.8-56.6
4.63 (10.41)
1.111
3.2-55.8
3.79 (7.52)
1.057
3.6-50.0
7.82 (14.11)
0.992
2.2-55.9
4.64 (11.41)
1.118
2.2-56.7
3.99 (10.25)
1.025
3.4-55.8
5.95 (9.22)
0.994
3.8-55.9
3.80 (8.64)
1.109
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )2/∑(Fo )2]}1/2
.
2
2
2
Table 2. Crystallographic Data and Structure Refinement Parameters for 12 and 19-24
(Et4N)2[21]
‚CH3CN
(Et4N)2[22]
‚CH3CN
(Et4N)2[23]
‚6CH3CN
(Et4N)[12]
19
20
(Et4N)[24]
formula
C20H41NOS4-
SeW
702.59
monoclinic
P21/n
C36H42OS5-
SiW
862.94
monoclinic
P21/c
C38H58S6W
C26H55Mo-
N3S5
665.97
triclinic
P1h
4
C36H70N4-
S10W2
1247.26
monoclinic
P21/n
C44H82N8-
S8Se2W2
1505.28
triclinic
P1
C24H44N-
S8SeW2
1049.74
monoclinic
P21/c
fw
891.05
cryst syst
space group
Z
triclinic
P1h
2
4
4
2
1
4
a, Å
b, Å
c, Å
9.521(1)
16.522(2)
17.570 (2)
90
103.435(2)
90
2688.2(5)
1.736
5.974
10.945(2)
12.904(2)
25.552(4)
90
91.022(3)
90
3608.1(9)
1.589
3.552
10.546(2)
14.131(2)
14.663(3)
96.490(4)
94.872(3)
102.656(3)
2104.8(6)
1.406
13.201(1)
13.747(1)
19.267(1)
95.866(1)
94.090(1)
90.356(1)
3468.9(4)
1.275
11.300(1)
15.347 (2)
14.930(2)
90
107.755(2)
90
2466.0(5)
1.680
5.114
10.971(2)
11.675(1)
13.732(2)
69.463(2)
68.356(2)
70.839(2)
1490.5(3)
1.677
10.035(2)
16.524(3)
20.663(4)
90
95.856(4)
90
3408(1)
2.046
8.320
R, deg
â, deg
γ, deg
V, Å3
d
calcd, g/cm3
µ, mm-1
3.066
0.698
5.394
2θ range, deg
R1a (wR2b), %
GOF (F2)
3.4-55.9
5.29 (9.97)
0.978
3.2-56.0
3.93 (9.37)
1.042
2.8-50.0
5.92 (14.3)
1.014
2.1-55.9
6.30 (11.63)
0.954
2.9-55.8
5.49 (10.23)
1.298
2.8-55.9
2.65 (7.00)
1.038
4.7-55.8
3.99 (7.14)
1.018
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )2/∑(Fo )2]}1/2
.
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 intensity decay; no significant decay was
observed from any compound. Data out to 2θ of 50° were used for
5 and 20 because of the somewhat lower quality of the high-angle
data; for other compounds, data out to 2θ of 56° were employed.
Cell parameters were retrieved using SMART software and refined
with SAINT software on all observed reflections in the 2θ range.
Data reduction was performed with SAINT, which corrects for
Lorentz polarization and decay. All space groups were assigned
by analysis of symmetry and systematic absences determined by
XPREP. Structures were solved by direct methods and SHELXL-
97 and refined against all data in the 2θ ranges by full-matrix least
squares on F2. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms at idealized positions were included in the final
refinements. The 1-Ad group of 4 was found in two overlapping
conformations that could be accurately modeled. The cation in 7
and one of the isopropyl groups in 20 were disordered; the disorders
were treated in terms of two conformations. The disorder of the
1-Ad group of 19 could not be resolved. Crystal parameters and
agreement factors are reported in Tables 1 and 2.35
Other Physical Measurements. All measurements were per-
formed under anaerobic conditions. 1H NMR spectra were recorded
with a Bruker DMX-500 or a Varian Mercury 300/400 spectrometer.
Absorption spectra were obtained with a Varian Cary 50 Bio
spectrophotometer, and infrared spectra were obtained with a
Nicolet 5PC FT-IR instrument. Electrospray mass spectra were
measured on acetonitrile solutions of salts or hexane solutions of
neutral compounds directly infused into an LCT mass spectrometer.
Cyclic voltammograms were recorded using a PAR model 263
potentiostat/galvanostat and a glassy-carbon working electrode, with
0.1 M (Bu4N)(PF6) supporting electrolyte; potentials are referenced
to the saturated calomel electrode.
Results and Discussion
The structural information in Figure 1 indicates that
suitable FDH active site analogues require the preparation
of reduced (MIV) and oxidized (MVI) bis(dithiolene) com-
plexes with variant additional ligation that includes selenolate
binding in all but one proposed case. Compiled in Table 3
are all previously known bis(dithiolene)molybdenum and
-tungsten monoselenolate complexes and their thiolate
counterparts and complexes 3-24 prepared in this investiga-
tion by the reactions in Schemes 1 and 2. No attempt has
(35) See paragraph at the end of this article for Supporting Information
available.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4095

Groysman and Holm
Table 3. Synthetic Bis(dithiolene)molybdenum and -tungsten
Complexes: Numerical Designations and Selected Examples Relevant to
Proposed or Established Coordination Units in Formate Dehydrogenases
Tungsten(IV) Complexes. (a) Synthesis and Structures.
Two approaches were employed utilizing as precursors the
dicarbonyl [W(CO)2(S2C2Me2)2] (1)34 and the carboxylate
[W(O2CBut)(S2C2Me2)2]- (2).20 One is based on previous
observations of the generalized reaction 1 (M ) Mo, W; Q
) O, S, Se). With M ) Mo, all reactants RO- and thiolates
and selenolates with bulky substituents (R ) 2,4,6-Pri3C6H2,
2-Ad) lead to complete carbonyl substitution (n ) 2)
MoIV/WIV
[Mo(SR)(S2C2Me2)2]-
[W(SR)(S2C2Me2)2]-
R ) 2-Ad,a
b
C6H2-2,4,6-Pri3
R ) But (3), 1-Ad (4),
C6H2-2,4,6-Pri3
c
[Mo(Se-2-Ad)(S2C2Me2)2]- a
[W(SeR)(S2C2Me2)2]-
R ) But (5), 1-Ad (6), 2-Ad (7)
[Mo(CO)(SeR)(S2C2Me2)2]-
[W(CO)(SeR)(S2C2Me2)2]-
R ) Ph, C6H2-2,4,6-Prt3
b
R ) But (8), 1-Ad (9),
[M(CO)2(S2C2Me2)2] + RQ- f
Ph, C6H2-2,4,6-Pri3
d
[MoS(S2C2Me2)2]2-
[WS(S2C2Me2)2]2- e
21
[M(CO)2-n(QR)(S2C2Me2)2]- + nCO (1)
WVI
whereas PhS- and PhSe- afford monocarbonyl products (n
) 1).26,28 With M ) W, the principal difference is that
monocarbonyls are formed with the same large thiolate and
selenolate reactants. These results have been rationalized on
the basis of the bond length order M-OR < M-SR <
M-SeR, leading to decreasing steric interaction between the
carbonyl and R substituent, and the bond strength order Mo-
CO < W-CO. In this work, we have employed the sterically
demanding groups R ) But and 1-Ad in an attempt to disrupt
carbonyl binding and otherwise stabilize the desired product
complexes in the WIV and WVI oxidation states. Schemes 1
and 2 separately display reactions involving these groups.
By means of reaction 1 with NaSR, thiolate complexes 3
and 4 were obtained in ca. 60% yield. However, reactions
with LiSeR afforded the green monocarbonyls 8 (65%) and
9 (27%) rather than the desired n ) 2 complexes. This
problem was addressed by means of reaction 2 (Q ) Se),
which afforded the red-orange selenolate complexes 5 (72%),
6 (72%), and 7 (65%).
[WO(SR)(S2C2Me2)2]-
R ) But (10), 1-Ad (11),
d
Ph, C6H2-2,4,6-Pri3
[WO(SeR)(S2C2Me2)2]-
[WS(SC6H2-2,4,6-Pri3)-
(S2C2Me2)2]- c
R ) But (12), 1-Ad (13)
[WS(S-1-Ad)(S2C2Me2)2]-
[WS(Se-1-Ad)(S2C2Me2)2]-
[W(SR)(OSiMe3)-
(S2C2Me2)2]-
14
15
R ) But (16),
C6H2-2,4,6-Pri3 (17)
R ) Me (18), Ph (19)
[W(S-1-Ad)(OSiR3)-
(S2C2Me2)2]
[W(SC6H2-2,4,6-Pri3)2-
(S2C2Me2)2]
20
WV,IV/V
22
[W2(µ2-S)2(S2C2Me2)4]2-
[W2(µ2-Se2)2(S2C2Me2)4]2
[W2(µ2-Se)(S2C2Me2)4]-
23
24
a Reference 26. b Reference 28. c Reference 20. d Reference 27. e Ref-
erence 29.
been made to prepare the highly unusual structures with
interligand Se-S bonds deduced from EXAFS analysis of
Escherichia coli FDHH and D. desulfuricans FDH. Note the
close structural and electronic relationship in the immediate
coordination environment between the 1,2-dimethyldithiolene
ligand and the chelating functionality of the pyranopterin-
dithiolene cofactor ligand. In the following sections, syn-
theses are considered in terms of the tungsten oxidation state;
all anionic complexes were isolated at Et4N+ salts in the
indicated yields. Synthesis descriptions extend in some cases
to identification of the decomposition products of isolated
complexes and reaction byproducts. Structural descriptions
are provided with the use of Figures 2-5 and Tables 4-6.
Metric parameters are limited to selected bond distances and
to angles that define the overall stereochemistry. Because
they are nearly constant over the set of complexes, certain
other parameters such as chelate bite angles (79-83°) and
atom deviations from nearly planar chelate rings are omit-
ted.35 Chelate ring C-C and S-C bond lengths35 fall in the
intervals of 1.32-1.36 and 1.71-1.77 Å, respectively, with
the large majority of values in the centers of the intervals.
As with all prior [ML(S2C2Me2)2]- and [MLL′(S2C2Me2)2]-
complexes, these values correspond to an enedithiolate(2-)
ligand description. In the latter complexes, we use the
transoid (interchelate) S-M-S angle as a shape parameter
for (distorted) octahedral vs trigonal-prismatic stereochem-
istry.36
[W(O2CBut)(S2C2Me2)2]- + RQSiMe3 f
[W(QR)(S2C2Me2)2]- + ButC(O)OSiMe3 (2)
Complex 3 (53%) was also prepared by an analogous
procedure (Q ) S). Structures of thiolate complex 3 and
selenolate complex 5 with the same R substituent are
compared in Figure 2 together with the structure of 6; metric
data for 3-7 are collected in Table 4. The complexes are
isostructural and apart from the 0.12-0.14 Å difference in
W-Q axial bond lengths are essentially isometric. They
adopt a square-pyramidal geometry with the tungsten atom
displaced 0.70-0.75 Å toward the axial ligand and the planar
chelate rings disposed at dihedral angles of 129-132°. With
these and earlier examples, it is evident that this stereochem-
istry, with idealized C4V symmetry of the WS4Q coordination
unit, is generic to [MIVL(S2C2R2)2]z complexes.
(b) Decomposition Products. In a qualitative examination
of stability under ambient anaerobic conditions, significant
differences were found between thiolate and selenolate
complexes with R ) But and 1-Ad. A solution of 4 in
acetonitrile was completely stable for at least 1 week,
whereas 3 in this solvent showed some decomposition at 3
h and was fully decomposed within 3 days. The isolated
(36) Beswick, C. L.; Schulman, J. M.; Stiefel, E. I. Prog. Inorg. Chem.
2004, 52, 55-110.
4096 Inorganic Chemistry, Vol. 46, No. 10, 2007

Molybdenum and Tungsten Formate Dehydrogenases
Scheme 1. Preparative Routes to Bis(dithiolene)tungsten(IV-VI) Thiolate and Selenoate Complexes with tert-Butyl Substituents, Based on
Dicarbonyl Complex 1 and Carboxylate Complex 2
product was the doubly sulfido-bridged WV dimer 22
(Scheme 1), demonstrated by an X-ray structure determina-
tion. (Et4N)2[22] (41%) was obtained from the reaction of 1
and (Et4N)(SBut) in acetonitrile after 3 days. For selenolate
complex 5 in acetonitrile, decomposition is even faster, being
complete in 1 day. The isolated product is the analogous
selenido-bridged dimer 23, identified by a structure deter-
mination (Figure 3). The complex was obtained in 77% yield
after 20 h in acetonitrile. Complexes 22 and 23 are
diamagnetic and isostructural (Table 5) and exhibit W‚‚‚W
separations too long for direct metal-metal interaction. The
present structure of 22 is a polymorph of the structure of
triclinic (Et4N)2[22]‚MeCN described elsewhere.29 In earlier
work, the related complex [W2(µ2-S)2(S2C2Ph)4]2- has been
obtained by a different procedure,34 and 22 was prepared by
the iodine oxidation of [WS(S2C2Me2)2]2-.29 Dimers 22 and
23 must arise from cleavage of C-Q bonds with concomitant
metal oxidation, as in the overall process 2RQ- + 2e- f
2Q2- + R-R. Apparent precedents are found in the reactions
of WV compounds with a thiolate or selenolate that lead to
products with WVI ) Q terminal groups.37-39
Attempts to prepare selenolate complexes 5 and 6 by
decarbonylation of the monocarbonyls 8 and 9, respectively,
under conditions of heating of acetonitrile solutions at 50
°C or allowing them to stand overnight, led to decomposition.
The product isolated from 8 is the dinuclear monoanion 24
of idealized C2 symmetry (Figure 3 and Table 4). This
complex includes two dithiolene chelate rings and is bridged
by one µ2-selenium atom and two sulfur atoms from
dithiolene ligands. The dithiolene bridging mode has been
previously observed in [Mo2O(S2C2Me2)4]-;22 we are un-
aware of a previous example of a WIVWV mixed-valent
dimer.
Molybdenum(IV) Sulfido Complex. Because of the
possible intervention of a five-coordinate MoIV-SH/OH site
in E. coli FDHH (Figure 1), the unprotonated version 21 of
the sulfide species was sought. This complex was obtained
(37) Kawaguchi, H.; Tatsumi, K. J. Am. Chem. Soc. 1995, 117, 3885-
3886.
(38) Kawaguchi, H.; Yamada, K.; Lang, J.-P.; Tatsumi, K. J. Am. Chem.
Soc. 1997, 119, 10346-10358.
(39) Kawaguchi, H.; Tatsumi, K. Chem. Commun. 2000, 1299-1300.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4097

Groysman and Holm
Scheme 2. Preparative Routes to Bis(dithiolene)tungsten(IV-VI) Thiolate and Selenoate Complexes with 1-Adamantyl Substituents, Based on
Dicarbonyl Complex 1 and Carboxylate Complex 2
by the reaction of [Mo(CO)2(S2C2Me2)2]28 with hydrosulfide
in acetonitrile and obtained in 74% yield. The compound
(Et4N)2[21]‚MeCN contains two inequivalent anions of
essentially identical dimensions in the asymmetric unit. The
structure of one anion is included in Figure 2. The square-
pyramidal complex is isostructural with [MoO(S2C2Me2)2]2- 28
and [WS(S2C2Me2)2]2-, which was prepared earlier by an
analogous reaction,29 and is characterized by an axial Mo-S
bond length of 2.172(2) Å (Table 3). [MoS(S4)2]2- 40 and
[MoS(CS4)2]2- 41,42 are the only examples of mononuclear
MoIV complexes with multiply bonded ModS units.
3. Dark-purple oxothiolate complexes 10 (93%) and 11
(84%) are produced by the reaction of 3 and 4, respectively,
with a slight excess of XO ) Ph3AsO, an oxo donor of broad
utility.21,43 Gray-blue oxoselenolate complexes 12 (47%) and
13 (53%) are prepared from 5 and 6, respectively, in a similar
manner but in substantially lower yields.
[W(QR)(S2C2Me2)2]- + XO/X′S f
[WO/S(QR)(S2C2Me2)2]- + X/X′ (3)
Analogous reactions of 4 and 6 with X′S ) Ph3SbS
afford the dark-violet sulfidothiolate complex 14 (89%)
and gray-blue sulfidoselenolate complex 15 (43%), respec-
tively. We have previously demonstrated the efficacy of
Ph3AsS as a sulfido transfer reagent in the preparation of
[MS(OR)(S2C2Me2)2]- complexes.21 Ph3SbS has been little
utilized in this regard but is a stronger sulfido donor than
Tungsten(VI) Complexes. (a) Oxo/Sulfido. As shown in
Schemes 1 and 2, all WVIO and WVIS complexes are prepared
from WIV precursors by the generalized atom transfer reaction
(40) Draganjac, M.; Simhon, E.; Chan, L. T.; Kanatzidis, M.; Baenziger,
N. C.; Coucouvanis, D. Inorg. Chem. 1982, 21, 3321-3332.
(41) Coucouvanis, D.; Draganjac, M. J. Am. Chem. Soc. 1982, 104, 6820-
6822.
(42) Coucouvanis, D.; Draganjac, M.; Koo, S. M.; Toupadakis, A.;
Hadjikyriacou, A. Inorg. Chem. 1992, 31, 1186-1196.
(43) Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571-589.
4098 Inorganic Chemistry, Vol. 46, No. 10, 2007

Molybdenum and Tungsten Formate Dehydrogenases
precedented but not entirely common mode of ligation in
high-valent synthetic complexes owing to the possible
reaction 2M-OH f M-O-M + H2O. In the present
context, M-OH groups (M ) Mo, W) are protonated
versions of the oxo group MdO. In this and earlier work,
we have not been able to isolate molecules containing the
group MIV-VI-OH derived from species with one MdO
group. Examples are restricted to the monoprotonation of
certain trioxo complexes to afford [LMO2(OH)]1+,0 (L )
But3tach, Me3tacn;48-50 Tp*51), which have been isolated and
structurally characterized. Electrophiles can attack dithiolene
complexes at a chelate ring sulfur atom or a multiply bonded
ligand.34,52 However, silyl reagents react exclusively at MIVO
or WVIO2 groups.53-55 As a first step, reaction 4 was
investigated with [WO(SAr)(S2C2Me2)2]1- and Me3SiCl.
Under optimized conditions, red-brown 17 (45%) was
obtained. The reaction was accompanied by the formation
of two byproducts. The tris(dithiolene) complex [W(S2C2-
Figure 2. Structures of square-pyramidal tungsten(IV) thiolate and
selenolate complexes and a molybdenum(IV) sulfido complex showing
partial atom labeling schemes and 50% probability ellipsoids: upper, 3 and
5; lower, 6 and 21.
1
Me2)3]56,57 was identified by an H NMR spectrum and a
crystal structure (not shown), and the bis(thiolate)bis-
(dithiolene) complex 20 was identified by a structure
determination (Figure 5 and Table 6). The latter, with cis
thiolate ligands, is formally related by a one-electron
Ph3AsS inasmuch as it will transfer sulfur to Ph3As and Ph2-
MeAs.44,45 An extensive compilation of sulfur transfer agents
has recently been presented by Donahue.46
[WO(SR)(S2C2Me2)2]- + R′3SiCl f
[W(OSiR′3)(SR)(S2C2Me2)2] + Cl- (4)
Complexes 10 and 11 are precedented only by
[WO(SAr)(S2C2Me2)2]-, and 14 is precedented only by
[WS(SAr)(S2C2Me2)2]-, both with a very large substituent
(Ar ) 2,4,6-Pri3C6H2)20 to promote stability. Selenolate
complexes 12, 13, and 15 are the first examples of the
general type [WVIQ(SeR)(S2C2R′2)2]- (Q ) O, S). The
structures of 10-12 are set out in Figure 4; metric parameters
are collected in Table 6. They are essentially isostructural
with one another and with [WO(OPh)(S2C2Me2)2]-, the
structure of which has been described.47 Complexes 10 and
12 provide another comparison of thiolate and selenolate
molecules at parity of the R substituent. The three molecules
have an irregular six-coordinate stereochemistry with cis oxo
and QBut ligands, an oxo trans effect of 0.09 Å, S-W-S
transoid angles of 156-158°, and WS2 dihedral angles of
93-96°. For octahedral/trigonal-prismatic stereochemistry,
these angles are 180°/136° and 90°/120°. In approximate
terms, the structures may be described as intermediate
between these two limits. The structures of sulfidothiolate
and sulfidoselenolate complexes were not determined. Dif-
fraction-quality crystals of (Et4N)[14] could not be
obtained. Attempts to obtain suitable crystals of (Et4N)[15]
from acetonitrile/ether instead afforded crystals of the
decomposition product (Et4N)2[22]. However, the proven
isostructural relationship of [WQ(SAr)(S2C2Me2)2]- (Q )
O, S)20 is entirely probable for the 1-Ad complexes 11, 14,
and 15.
oxidation to similarly configured [W(QPh)2(S2C2Me2)2]1-,
whose structures have been described.29 Oxothiolate com-
plexes 10 and 11 in reaction 4 afford the neutral complexes
16 (33%), 18 (52%), and 19 (39%) in moderate yield. The
structure of 19 (Figure 5 and Table 6) features mutually cis
thiolate and silyloxide ligands and, based on a trans angle
of 167°, approaches a (distorted) octahedral limit. Complexes
16-19, although stable enough to isolate, tend to decompose
even in a nonpolar solvent such as toluene, forming [W(S2C2-
Me2)3] among other products. They have no precedents
among bis(dithiolene) complexes. Attempts to prepare the
corresponding selenolate complexes by silylation of 12 or
13 were unsuccessful. The only product identified from these
reactions was the tris(dithiolene).
Comparative Features. The results described here con-
stitute the first concerted investigation of bis(dithiolene)
(48) Schreiber, P.; Wieghardt, K.; Nuber, B.; Weiss, J. Polyhedron 1989,
8, 1675-1682.
(49) Schreiber, P.; Wieghardt, K.; Nuber, B.; Weiss, J. Z. Anorg. Allg.
Chem. 1990, 587, 174-192.
(50) Partyka, D. V.; Staples, R. J.; Holm, R. H. Inorg. Chem. 2003, 42,
7877-7886.
(51) Eagle, A. A.; George, G. N.; Tiekink, E. R. T.; Young, C. G. Inorg.
Chem. 1997, 36, 472-479.
(52) Sellman, D.; Wemple, M. W.; Donaubauer, W.; Heinemann, F. W.
Inorg. Chem. 1997, 36, 1397-1402.
(53) Lorber, C.; Donahue, J. P.; Goddard, C. A.; Nordlander, E.; Holm, R.
H. J. Am. Chem. Soc. 1998, 120, 8102-8112.
(54) Donahue, J. P.; Goldsmith, C. R.; Nadiminti, U.; Holm, R. H. J. Am.
Chem. Soc. 1998, 120, 12869-12881.
(55) Musgrave, K. B.; Lim, B. S.; Sung, K.-M.; Holm, R. H.; Hedman,
B.; Hodgson, K. O. Inorg. Chem. 2000, 39, 5238-5247.
(56) Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1966, 88, 3235-
3242.
(b) Desoxo. As seen in Figure 1, a number of active site
structures are proposed to contain coordinated hydroxyl, a
(44) Baechler, R. D.; Stack, M.; Stevenson, K.; Van Valkenburgh, V.
Phosphorus, Sulfur Silicon Relat. Elem. 1990, 48, 49-52.
(45) Capps, K. B.; Wixmerten, B.; Bauer, A.; Hoff, C. D. Inorg. Chem.
1998, 37, 2861-2864.
(46) Donahue, J. P. Chem. ReV. 2006, 106, 4747-4783.
(47) Sung, K.-M.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1931-1943.
(57) Fomitchev, D. V.; Lim, B. S.; Holm, R. H. Inorg. Chem. 2001, 40,
645-654.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4099

Groysman and Holm
Table 4. Selected Structural Data (Å, Deg) for Square-Pyramidal WIV and MoIV Complexes 3-7 and 21
3
4
5
6
7
21
W/Mo-Sa
W/Mo-Qax
S-Cc
2.314(6)
2.303(2)
1.77(1)
1.33(1)
0.718
2.316(2)
2.296(1)
1.768(5)
1.327
0.707
132.2
2.311(4)
2.425(2)
1.77(2)
1.34(2)
0.701
2.310(4)
2.434(2)
1.76(1)
1.33(1)
0.746
2.311(3)
2.435(1)
1.77(1)
1.32(1)
0.728
2.362(7)
2.172(2)
1.774(3)
1.341(6)
0.815
b
C-Cc
δd
θe
131.2
132.4
129.2
130.5
125.7
a Mean values. b Q ) S (3, 4, 21), Se (5-7). c Chelate ring. d Perpendicular displacement of a tungsten atom from the S4 least-squares plane. e Dihedral
angle between WS2 planes.
Figure 3. Structures of selenide-bridged dinuclear tungsten complexes
showing partial atom labeling schemes and 50% probability ellipsoids:
upper, 23; lower, 24. The structure of 22 (not shown) is commensurate
with that of 23.
Figure 4. Structures of tungsten(VI)-oxothiolate and selenoate complexes
showing partial atom labeling schemes and 50% probability ellipsoids:
upper, 10; lower, 11 and 12.
Table 6. Selected Structural Data (Å, Deg) for WVI Complexes 10-12,
19, and 20
Table 5. Selected Structural Data for Dinuclear Complexes 22-24
22
23
24
10
11
12
19
20
W-Sa
2.41(2)
2.327(4)
2.41(2)
2.45(2)
2.36(1)
2.49(1)
2.47(2)
2.868(1)
1.76(1)
1.34(1)
70.38(2)
W-(µ2-Q)a,b
W-(µ2-SR)a
W-W
W-S1
W-S2
W-S3
W-S4
W-Qb
W-O
2.501(2)a 2.497(1)a 2.495(2)a 2.408(1) 2.378(2)
2.382(2) 2.383(1) 2.373(2) 2.383(1) 2.357(2)
2.406(2) 2.406(1) 2.406(2) 2.364(1) 2.378(2)
2.437(2) 2.424(1) 2.427(2) 2.403(1) 2.387(2)
2.435(2) 2.416(1) 2.564(1) 2.351(1) 2.368(9)c
1.717(6) 1.741(3) 1.729(6) 1.861(3)
2.989(1)
1.74(1)
1.340(1)
79.92(5)
156.58(7)c
155.5(3)
3.111(4)
1.75(3)
1.36(4)
78.5(1)
156.6(1)a
156(1)
S-Ca
C-Ca
W-Q-Wb
S-W-Sc
S-W-Qa,b,c
S-Cd
1.74(2)
1.35(1)
1.74(3)
1.34(1)
1.72(2)
1.33(1)
1.73(2)
1.337(7) 1.36(1)
1.71(2)
CsCd
O-W-S1
155.4(2) 154.9(1) 155.1(2) 156.9(1)
a Mean values. b Q ) S (22), Se (23, 24). c Transoid angle.
O-W-Qb
93.8(2)
95.2(1)
93.2(2)
97.8(1)
S2-W-S3e 155.93(8) 158.48(4) 156.55(8) 167.40(5) 153.37(8)f
S4-W-Qb,e 160.33(7) 157.78(4) 160.39(5) 154.48(5)
selenolate complexes of any type and, in particular, those
related in varying degrees to FDH active sites. Here we
compare certain related features of bis(dithiolene) complexes
in terms of metal and ligand and the structures of these
complexes and enzyme sites.
θg
95.2
93.5
96.4
91.6
59.0
a Trans to oxo. b Q ) S(R) (10, 11, 19), Se(R) (12). c Mean of W-S5/
S6. d Chelate ring. e Transoid angle. f Respective angles for S1-W-S4 and
S2-W-S5 are 139.06(9)° and 143.84(8)°. g Dihedral angle between WS2
planes.
(a) Molybdenum vs Tungsten. As noted earlier, tungsten
complexes were prepared because of the autoredox instability
of MoVI complexes containing electron-rich dithiolene
ligands.58 Molybdenum and tungsten complexes at strict
parity of oxidation state and ligation manifest certain
differences and similarities. Among the differences are the
potential order EW < EMo for a given redox couple,59
illustrated by the MIV/V potentials in acetonitrile for isos-
tructural oxo and sulfido complexes, including 21. The very
(58) A limited set of bis(dithiolene)molybdenum(VI) complexes have been
isolated with use of the more electronegative ligands bdt54,64,65 and
mnt.66 These ligands are less accurate representations of the chelate
portion of the pyranopterindithiolene cofactor ligand.
4100 Inorganic Chemistry, Vol. 46, No. 10, 2007

Molybdenum and Tungsten Formate Dehydrogenases
Figure 5. Structures of desoxo/sulfido tungsten(VI) complexes showing partial atom labeling schemes and 50% probability ellipsoids: right, 19;
left, 20.
small dependence on the axial ligand Q (20-30 mV) follows
from the symmetry-restricted interactions between the elec-
troactive dxy orbital and the filled s and p valence orbitals of
[MO(S2C2Me2)2]2- M ) Mo (-0.62 V),28 W (-0.91 V)34
[MS(S2C2Me2)2]2- M ) Mo (-0.65 V), W (-0.89 V)29
Q. A further difference is the rate constant order of kW
kMo for oxo transfer from substrate to metal and of kMo
>
>
kW for the reverse process.21,47,59,60 The most significant
similarity is a virtual identity: pairs of molybdenum and
tungsten complexes are isostructural and nearly isometric.
There are no exceptions. For example, in the six compara-
tive pairs of complexes [MQ(S2C2Me2)2]2- 28,34 and
[M(QR)(S2C2Me2)2]- 20,26-28 (Q)O,S)and[M(OSiR3)(bdt)2]-
and [MO(OSiR3)(bdt)2]-,53,54 differences in mean M-S
distances in chelate rings are ∼0.01 Å, in M-Q distances
∼0.04 Å, and in M-OSi distances ∼0.02 Å. Bond
angles also display very little variation. Under the foregoing
stricture, tungsten is a structural surrogate of molybdenum.
(b) Selenium vs Sulfur. In the present work, the most
important difference is that selenolate complexes are clearly
less stable than thiolate complexes in solution, as documented
above. The UV-vis spectra of dithiolene complexes are
generally dominated by ligand-to-metal charge transfer
absorptions involving dithiolene ligand orbitals with signifi-
cant sulfur character.61 All complexes prepared in this work
are intensely colored and exhibit such spectra. Among them,
pairs differing only in a selenium vs sulfur donor atom show
a systematic behavior in which all or nearly all of the
transitions in the selenolate complex are shifted to lower
energies. The effect is particularly clear in the 1-Ad
complexes comprising the WIV pair 4/6 and the WVI pair
14/15, whose spectra are shown in Figure 6. Spectral shifts
are those expected based on the optical electronegativity
order Se < S, but band assignments are required for an
Figure 6. Comparative UV-vis absorption spectra of pairs of [W(QR)-
(S2C2Me2)2]- and [WS(QR)(S2C2Me2)2]- complexes (Q ) S, Se) in
acetonitrile; λmax values are indicated: upper, 4 and 6; lower, 14 and 15.
evaluation of the origin of the shifts. A similar but less
clearly displayed effect is found in the series [Mo(Q-2-Ad)-
(S2C2Me2)2]- (Q ) O, S, Se).26 In the absence of crystal
structures, the spectra of 14 and 15 serve as characterization
information.
(c) Synthetic Complexes and Enzyme Sites. With
reference to Figure 1 and Table 3, [Mo(Se-2-Ad)(S2C2-
Me2)2]- and selenolate complexes 5-7 are analogues of the
originally described square-pyramidal formate-reduced site
of E. coli FDHH,7 and [Mo(SR)(S2C2Me2)2]-, 3, 4, and
[W(SAr)(S2C2Me2)2]- are their thiolate counterparts. Com-
plex 21 is an unprotonated form of the revised description
of the formate-reduced site without selenocysteinate
binding.8 Complexes 12 and 13 are unprotonated versions
of the oxidized sites of E. coli FDHH and FDHN and
D. gigas W-FDH with bound hydroxyl, and 10, 11, and
(59) Tucci, G. C.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1998, 37,
1602-1608.
(60) Jiang, J.; Holm, R. H. Inorg. Chem. 2005, 44, 1068-1072.
(61) Kirk, M. L.; McNaughton, R. L.; Helton, M. E. Prog. Inorg. Chem.
2004, 52, 111-212.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4101

Groysman and Holm
[WO(SAr)(S2C2Me2)2]- are their thiolate counterparts. The
latter are representations of the oxidized site of a mutant
FDH that is ca. 300 times less active than the wild-type
enzyme.62 Complex 15 simulates the unprotonated oxidized
sites of E. coli FDHH and D. gigas W-FDH with coordinated
hydrosulfide, and [WS(SAr)(S2C2Me2)2]- and 14 are thiolate
complements. Protonated MoVI and WVI enzyme sites are
desoxo/sulfido entities. Thiolate complexes 16-19 serve to
demonstrate that desoxo complexes, at least with WVI, are
sufficiently stable for isolation. Here a silyloxide group
mimics protonation. We have not prepared any complex
related to the dithionite-reduced six-coordinate form of D.
sulfuricans FDH deduced from EXAFS analysis. Given the
structural relationship between molybdenum and tungsten
complexes, the selenolate complexes are symmetrized rep-
resentations of enzyme sites whose metric features are or
approach those expected in the absence of the constraints
imposed by protein structure. In this context, thiolate
complexes serve as site representatives of the homologues
of selenocysteinate proteins that contain Cys instead of SeCys
at the active centers.63
that bis(dithiolene) selenolate complexes of molybdenum and
tungsten are synthetically accessible but are less stable than
analogous thiolate complexes. Indeed, the isolation of both
14 and 15, containing WVI bound to anionic reducing sulfur
or sulfur and selenium ligands, is notable. With this set of
new complexes in hand, the binding of formate and the
attainment of an FDH analogue reaction system are under
investigation.
Acknowledgment. This research was supported by NSF
Grants CHE 0237419 and 00547734. We thank J.-J. Wang
and Dr. C. Tessier for helpful discussions.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of the compounds
in Tables 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC062441A
(63) Gladyshev, V. N. Met. Ions Biol. Syst. 2002, 39, 655-672.
(64) Ueyama, N.; Oku, H.; Kondo, M.; Okamura, T.; Yoshinaga, N.;
Nakamura, A. Inorg. Chem. 1996, 35, 643-650.
This investigation and prior work (Table 3) demonstrate
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