Dioxotungsten(VI) complexes of hydrotris(3,5-dimethylpyrazol-1-yl)borate including the x-ray crystal structure of the tungsten selenophenolate complex cis-{HB(Me2C3N2H)3}WO 2(SePh)

Article abstract of DOI:10.1021/ic970544a

The syntheses, and spectroscopic, structural and electrochemical properties of cis-dioxotungsten(VI) complexes LWO₂X [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; X = Cl^-, NCS^-, OMe^-, O₂CH^-, OPh^-, SPh^-, S₂PPh₂-S^-, SePh^-] are described. Reaction of LWO₂Cl with group 1 salts M^IX and 18-crown-6 in refluxing toluene was employed in the syntheses of derivatives with X = NCS^-, OPh^-, SPh^-, and SePh^-, while reaction of LWO₂-(SePh) with methanol and LWO₂(SPh) with formic acid yielded LWO₂(OMe) and LWO₂(O₂CH), respectively. The complex LWO₂(S₂PPh₂-S) was prepared by reacting LW(S₂PPh₂-S)(CO)₂ with pyridine N-oxide. The complexes exhibit two ν(WO₂) infrared bands, at 935-960 cm^-1 and 900-915 cm^-1, and ¹H NMR spectra consistent with C_s symmetry. Orange crystals of cis-LWO₂(SePh) are monoclinic, space group P2_1/c, with a = 18.385(6) A?, b = 8.102(1) A?, c = 18.284(5) A?, β=117.15(2)°, V = 2423(2) A?³, and Z = 4. The structure was solved by direct methods and refined to R = 0.035 (R_w = 0.033) for 3668 reflections with I ≥ 3.0σ(I). The mononuclear complex exhibits a distorted octahedral coordination sphere composed of a selenophenolate ligand [W-Se = 2.535(1) A?], two terminal oxo groups [W=O = 1.716(4) and 1.721(4) A?], and a facially tridentate L ligand. In acetonitrile, the complexes undergo a one-electron reduction at very cathodic potentials (E_1/2 = -1.71 to E_pc = -1.05 V vs SCE), some 560-620 mV more negative than observed for analogous molybdenum complexes (Inorg. Chem. 1996, 35, 7508). The complexes are very stable and do not participate in clean oxygen atom transfer or coupled electron-proton transfer reactions.

Full text of DOI:10.1021/ic970544a

Inorg. Chem. 1997, 36, 6315-6322
6315
Dioxotungsten(VI) Complexes of Hydrotris(3,5-dimethylpyrazol-1-yl)borate Including the
X-ray Crystal Structure of the Tungsten Selenophenolate Complex
cis-{HB(Me2C3N2H)3}WO2(SePh)
Aston A. Eagle,1a Edward R. T. Tiekink,1b and Charles G. Young*,1a
School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia, and
Department of Chemistry, University of Adelaide, Adelaide, South Australia 5005, Australia
ReceiVed May 12, 1997X
The syntheses, and spectroscopic, structural and electrochemical properties of cis-dioxotungsten(VI) complexes
LWO2X [L ) hydrotris(3,5-dimethylpyrazol-1-yl)borate; X ) Cl-, NCS-, OMe-, O2CH-, OPh-, SPh-, S2PPh2-
S-, SePh-] are described. Reaction of LWO2Cl with group 1 salts MIX and 18-crown-6 in refluxing toluene was
employed in the syntheses of derivatives with X ) NCS-, OPh-, SPh-, and SePh-, while reaction of LWO2-
(SePh) with methanol and LWO2(SPh) with formic acid yielded LWO2(OMe) and LWO2(O2CH), respectively.
The complex LWO2(S2PPh2-S) was prepared by reacting LW(S2PPh2-S)(CO)2 with pyridine N-oxide. The
1
complexes exhibit two ν(WO2) infrared bands, at 935-960 cm-1 and 900-915 cm-1, and H NMR spectra
consistent with Cs symmetry. Orange crystals of cis-LWO2(SePh) are monoclinic, space group P21/c, with a )
18.385(6) Å, b ) 8.102(1) Å, c ) 18.284(5) Å, â ) 117.15(2)°, V ) 2423(2) Å3, and Z ) 4. The structure was
solved by direct methods and refined to R ) 0.035 (Rw ) 0.033) for 3668 reflections with I g 3.0σ(I). The
mononuclear complex exhibits a distorted octahedral coordination sphere composed of a selenophenolate ligand
[W-Se ) 2.535(1) Å], two terminal oxo groups [WdO ) 1.716(4) and 1.721(4) Å], and a facially tridentate L
ligand. In acetonitrile, the complexes undergo a one-electron reduction at very cathodic potentials (E1/2 ) -1.71
to Epc ) -1.05 V vs SCE), some 560-620 mV more negative than observed for analogous molybdenum complexes
(Inorg. Chem. 1996, 35, 7508). The complexes are very stable and do not participate in clean oxygen atom
transfer or coupled electron-proton transfer reactions.
Introduction
centers. Oxo complexes containing dithiolene and selenium
donor ligands would appear to be particularly relevant as enzyme
There is growing evidence for the presence of oxotungsten
centers in many pterin-containing tungsten enzymes.2,3 Indirect
evidence for oxo ligation was obtained from the crystal structure
of the aldehyde ferredoxin oxidoreductase from Pyrococcus
furiosus (P. furiosus, Pf-AOR)4 and earlier EXAFS studies of
an inactive form of this enzyme.5 Recent EPR, variable temper-
ature MCD, and further EXAFS studies of this and related
enzymes provide direct spectroscopic evidence for oxo ligation.
Thus, despite considerable heterogeneity at the tungsten center
at all oxidation levels, oxo and hydroxo (protonated oxo) ligands
are invoked for all the catalytically competent W(V) and W(VI)
forms of active Pf-AOR.6 However, some forms of this enzyme
are postulated to contain mercapto (protonated thio) ligands in
place of hydroxo ligands, and there has been speculation that
W-S redox and trithiolene ligation may be involved in the
generation of a high-potential form of the enzyme.6 Closely
related but less well-studied enzymes also appear to contain
oxotungsten centers; these include the formaldehyde ferredoxin
oxidoreductases from P. furiosus and Thermococcus litoralis
and various formate dehydrogenases (FDH).2,3 Advances in the
area of oxotungsten inorganic chemistry7,8 are essential to a full
understanding of the nature and biological role of these enzyme
models.
Several groups have developed significant structural and
functional models for various tungsten enzymes. Bis(dithiolene)
dioxo-W(VI) and oxo-W(V) and -W(IV) complexes of the
type [WVIO2(dt)2]2-, [WVO(dt)2]-, and [WIVO(dt)2]2- [dt )
2-
2-
2-
S2C2(CN)2 (mnt),9,10 S2C6H4 (bdt),11 S2C10H6 (ndt)12
]
have been reported by Sarkar and Nakamura and their respective
co-workers. The W(VI) complexes possess distorted octahedral
structures, while the W(V) and W(IV) complexes are square
pyramidal in geometry.9-12 The mnt complexes exhibit a variety
of reactions of biological relevance, including a ligand extrusion
reaction between [WO2(mnt)2]2- and MoO42-, the oxidation of
crotonaldehyde to crotonic acid (cf., AOR),10 and the reduction
-
of CO2/HCO3 to formate (eq 1, cf., FDH).9 The complex
-
[WO(mnt)2]2- + HCO3- f [WO2(mnt)2]2- + HCO2 (1)
[WO2(bdt)2]2- + PhCH(OH)COPh f
[WO(bdt)2]2- + PhCOCOPh + H2O (2)
[WVIO2(bdt)2]2- cleanly oxidizes benzoin to benzil in a reaction
X Abstract published in AdVance ACS Abstracts, December 1, 1997.
(1) (a) University of Melbourne. (b) University of Adelaide.
(2) Kletzin, A.; Adams, M. W. W. FEMS Microbiol. ReV. 1996, 18, 5.
(3) Johnson, M. K.; Rees, D. C.; Adams, M. W. W. Chem. ReV. 1996,
96, 2817.
(4) Chan, M. K.; Mukund, S.; Kletzin, A.; Adams, M. W. W.; Rees, D.
C. Science 1995, 267, 1463.
(5) George, G. N.; Prince, R. C.; Mukund, S.; Adams, M. W. W. J. Am.
Chem. Soc. 1992, 114, 3521.
(7) Reviews: (a) Dori, Z. Prog. Inorg. Chem. 1981, 28, 239. (b) Dori, Z.
In ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard,
R. D., McCleverty, J. A., Eds.; Pergamon: Oxford, U.K., 1987;
Chapter 37, p 973. (c) McCleverty, J. A. In Encyclopedia of Inorganic
Chemistry; King, R. B., Ed.; Wiley: Chichester, U.K., 1994, p 4240.
(8) (a) Mayer, J. M. Inorg. Chem. 1988, 27, 3899. (b) Nugent, W. A.;
Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley: New York, 1988.
(9) Sarkar, S.; Das, S. K. Proc. Indian Acad. Sci. 1992, 104, 533.
(10) Das, S. K.; Biswas, D.; Maiti, R.; Sarkar, S. J. Am. Chem. Soc. 1996,
118, 1387.
(6) Koehler, B. P.; Mukund, S.; Conover, R. C.; Dhawan, I. K.; Roy, R.;
Adams, M. W. W.; Johnson, M. K. J. Am. Chem. Soc. 1996, 118,
12391.
(11) Ueyama, N.; Oku, H.; Nakamura, A. J. Am. Chem. Soc. 1992, 114,
7310.
S0020-1669(97)00544-2 CCC: $14.00 © 1997 American Chemical Society

6316 Inorganic Chemistry, Vol. 36, No. 27, 1997
Eagle et al.
(eq 2) which closely resembles enzyme processes.11,13 On the
basis of an observed deuterium isotope effect, the rate determin-
ing step is proposed to involve abstraction of the R-hydrogen
dimethyl sulfoxide and lead to the generation of oxo-Mo(IV)
and oxo-hydroxo-Mo(V) species of biological relevance. An
extensive chemistry22-29 and a single model exhibiting the
important centers and reactions involved in dioxo-Mo(VI)-
containing enzymes28,29 have evolved from work in this area.
A number of mononuclear dioxo-hydrocarbyl complexes of
the type LMoO2R (R ) Me, CH2SiMe3) have also been
reported.30,31 The first mononuclear dioxo-W(VI) complex of
L to be reported was LWO2Cl.19 It has been employed in the
synthesis of dioxo-hydrocarbyl complexes, LWO2R (R ) Me,
Et, Ph),19,30,32 and converted to analogous oxo-thio and bis(thio)
derivatives, LWOSCl and LWS2Cl.19,20 As well, the selective
oxyfunctionalization of the LWO2R complexes by singlet
oxygen and dioxirane has been described by Sundermeyer and
co-workers.33 More recently, the dioxo-hydroxo- and trioxo-
W(VI) complexes LWO2(OH) and [LWO3]- have been prepared
and characterized.30,33,34
atom of the benzoin by an oxo ligand of [WO2(bdt)2]2- 13
The
.
[WO2(dt)2]2- complexes do not readily undergo classic oxygen
atom transfer reactions with, for example, PPh3. Finally, the
related non-dithiolenic complex [WO2{O2CC(S)Ph2}2]2- is
capable of oxidizing benzoin to benzil but does not react with
PPh3 or thiophenol at 25-60 °C (cf., Mo analogue, which
undergoes reactions with all three reagents).14 This complex
also catalyzes the oxidation of benzoin by nitrate.14 These very
important model studies have not been augmented by any
extensive, systematic studies of dioxo-W(VI) chemistry.
Here, we report the synthesis and spectroscopic, structural,
and electrochemical properties of a wide range of cis-dioxo-
tungsten(VI) complexes containing the tripodal N3-donor hy-
drotris(3,5-dimethylpyrazol-1-yl)borate ligand (L). The spec-
troscopic and electrochemical properties of the complexes as a
function of the ligands X are documented, and the crystal
structure of LWO2(SePh) is described. Aspects of the reactivity
of these complexes are also reported. The results are consistent
with the general chemical and electrochemical characteristics
of oxotungsten complexes, as defined by the pioneering work
of Spence,15 Wieghardt,16 Holm,17 and others.18 Further interest
in complexes of this type stems from their use as starting
materials for novel oxo-thio and bis(thio) complexes.19-21
A brief survey of the dioxomolybdenum and dioxotungsten
chemistry of L provides a context for this work. A wide variety
of molybdenum complexes of the type cis-LMoO2X have been
prepared as models for pterin-containing molybdenum enzymes
such as sulfite oxidase and nitrate reductase.22-29 These
complexes exhibit distorted octahedral structures and participate
in a number of important oxygen atom transfer (OAT)23-29 and
coupled electron-proton transfer (CEPT) reactions;26-29 these
reactions involve biological substrates such as nitrate and
Materials and Methods
Reagents (AR grade or better) were used as supplied or were purified
by standard procedures.35 Potassium thiocyanate was dried at 150 °C
under a dynamic vacuum for 15 h. Literature methods were used for
the preparations of KL,36 HS2PPh2,37 (NEt4)2[WO2(NCS)4],38 LWO2-
Cl,19,32 LWO2(OH),34 [WO2Cl2]n,39 and [WO2Br2]n.39 Unless stated,
reactions were performed under an atmosphere of pure dinitrogen,
employing standard Schlenk techniques; workups were performed in
air. Solvents were carefully dried, deoxygenated, and distilled before
use.35 Infrared spectra were obtained on Jasco A-302 or Perkin-Elmer
983G infrared spectrophotometers using pressed KBr disks with
polystyrene as reference. 1H NMR spectra were obtained using a Varian
Unity 300 MHz FT NMR spectrometer, and electronic spectra were
recorded on a Hitachi 150-20 UV spectrophotometer. Electron impact
(70 eV) mass spectra were obtained on a JEOL AX 505H mass
spectrometer. Column chromatography was performed using Merck
Artikel 7734 Kieselgel 60. Microanalyses were performed by Atlantic
Microlabs, Norcross, GA. Cyclic voltammetric samples were prepared
as 5-10 mM solutions in dried 0.1 M BunNBF4/acetonitrile and run
on a Cypress Electrochemical System II with 3 mm glassy carbon
working electrode and platinum auxiliary and reference electrodes.
Solutions were purged with dinitrogen before use and maintained under
a dinitrogen atmosphere during experiments. Reported peaks were
referenced to the saturated calomel electrode (SCE) by use of an internal
standard (ferrocene; 0.390 V vs SCE).40
(12) Oku, H.; Ueyama, N.; Nakamura, A. Chem. Lett. 1996, 31.
(13) Oku, H.; Ueyama, N.; Nakamura, A. Chem. Lett. 1996, 1131.
(14) Cervilla, A.; Llopis, E.; Ribera, A.; Dome´nech, A.; Sinn, E. J. Chem.
Soc., Dalton Trans. 1994, 3511.
(15) (a) Rice, C. A.; Kroneck, P. M. H.; Spence, J. T. Inorg. Chem. 1981,
20, 1996. (b) (Mo analogues) Taylor, R. D.; Street, J. P.; Minelli, M.;
Spence, J. T. Inorg. Chem. 1978, 17, 3207.
(16) Backes-Dahmann, G.; Wieghardt, K. Inorg. Chem. 1985, 24, 4049.
(17) Yu, S.; Holm, R. H. Inorg. Chem. 1989, 28, 4385.
(18) (a) Chen, G. J.-J.; McDonald, J. W.; Newton, W. E. Inorg. Chim.
Acta 1976, 19, L67. (b) Lee, S.; Staley, D. L.; Rheingold, A. L.;
Cooper, N. J. Inorg. Chem. 1990, 29, 4391. (c) Bradbury, J. R.;
Masters, A. F.; McDonell, A. C.; Brunette, A. A.; Bond, A. M.; Wedd,
A. G. J. Am. Chem. Soc. 1981, 103, 1959. (d) Heath, G. A.; Moock,
K. A.; Sharp, D. W. A.; Yellowlees, L. J. J. Chem. Soc., Chem.
Commun. 1985, 1503.
Syntheses
LWO2(NCS). Method 1. A suspension of LWO2Cl (0.250 g, 0.456
mmol), anhydrous KNCS (0.50 g, 5.1 mmol), and 18-crown-6 (5 mg)
was refluxed in toluene (15 mL) for 6 days. The solvent was removed
on a rotary evaporator, and the residue was dissolved in 1:1 dichlo-
romethane/water (100 mL). The dichloromethane phase was separated
(19) Eagle, A. A.; Tiekink, E. R. T.; Young, C. G. J. Chem. Soc., Chem.
Commun. 1991, 1746.
(30) Sundermeyer, J.; Putterlik, J.; Pritzkow, H. Chem. Ber. 1993, 126,
289.
(20) Eagle, A. A.; Harben, S. M.; Tiekink, E. R. T.; Young, C. G. J. Am.
Chem. Soc. 1994, 116, 9749.
(21) Eagle, A. A. Ph.D. Dissertation, University of Melbourne, 1996.
(22) (a) Enemark, J. H.; Young, C. G. AdV. Inorg. Chem. 1993, 40, 1. (b)
Young, C. G.; Wedd, A. G. J. Chem. Soc., Chem. Commun. 1997,
1251.
(31) Onishi, M.; Ikemoto, K.; Hiraki, K.; Koga, R. Chem. Lett. 1993, 66,
1849.
(32) Eagle, A. A.; Young, C. G.; Tiekink, E. R. T. Organometallics 1992,
11, 2934.
(33) Adam, W.; Putterlik, J.; Schuhmann, R. M.; Sundermeyer, J. Orga-
nometallics 1996, 15, 4586.
(23) Roberts, S. A.; Ortega, R. B.; Zolg, L. M.; Cleland, W. E., Jr.;
Enemark, J. H. Acta Crystallogr., Sect. C 1987, 43, 51.
(24) Roberts, S. A.; Young, C. G.; Cleland, W. E., Jr.; Ortega, R. B.;
Enemark, J. H. Inorg. Chem. 1988, 27, 3044.
(25) Roberts, S. A.; Young, C. G.; Kipke, C. A.; Cleland, W. E., Jr.;
Yamanouchi, K.; Carducci, M. D.; Enemark, J. H. Inorg. Chem. 1990,
29, 3650.
(34) Eagle, A. A.; George, G. N.; Tiekink, E. R. T.; Young, C. G. Inorg.
Chem. 1997, 36, 472.
(35) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: Oxford, U.K., 1988.
(36) Trofimenko, S. J. Am. Chem. Soc. 1967, 89, 6288.
(37) Higgins, W. A.; Vogel, P. W.; Craig, W. G. J. Am. Chem. Soc. 1955,
77, 1864.
(26) Xiao, Z.; Bruck, M. A.; Doyle, C; Enemark, J. H.; Grittini, C.; Gable,
R. W.; Wedd, A. G.; Young, C. G. Inorg. Chem. 1995, 34, 5950.
(27) Laughlin, L. J.; Young, C. G. Inorg. Chem. 1996, 35, 1050.
(28) Xiao, Z.; Young, C. G.; Enemark, J. H.; Wedd, A. G. J. Am. Chem.
Soc. 1992, 114, 9194.
(29) Xiao, Z.; Bruck, M. A.; Enemark, J. H.; Young, C. G.; Wedd, A. G.
Inorg. Chem. 1996, 35, 7508.
(38) Brisdon, B. J.; Edwards, D. A. Inorg. Nucl. Chem. Lett. 1974, 10,
301.
(39) (a) Tillack, J. Inorg. Synth. 1973, 14, 109. (b) Schrock, R. R.; DePue,
R. T.; Feldman, J.; Yap, K. B.; Yang, D. C.; Davis, W. M.; Park, L.;
DiMare, M.; Schofield, M.; Anhaus, J.; Walborsky, E.; Evitt, E.;
Kru¨ger, C.; Betz, P. Organometallics 1990, 9, 2262.
(40) Bashkin, J. K.; Kinlen, P. J. Inorg. Chem. 1990, 29, 4507.

LWO2X Complexes with a Tripodal N3-Donor Ligand
Inorganic Chemistry, Vol. 36, No. 27, 1997 6317
and dried over magnesium sulfate and then enriched in hexanes to
precipitate white microcrystals. Yield: 0.167 g (64%). The analytical
sample was recrystallized from 1,2-dichloroethane/hexane.
Table 1. Crystallographic Data and Refinement Details for
LWO2(SePh)
empirical formula C21H27BN6O2SeW
F
calcd, g cm-3
F(000)
1.834
1296
6.304
0.909/1
Method 2. A mixture of (NEt4)2[WO2(NCS)4] (1.000 g, 1.41 mmol)
and KL (0.498 g, 1.48 mmol) was refluxed in acetonitrile for 18 h,
and the resulting purple mixture was poured into water (100 mL). The
purple solid was filtered, washed with water and then methanol, and
recrystallized (3 times) from dichloromethane/methanol to give white
crystals. Yield: 0.119 g (15%). Anal. Calcd for LWO2(NCS)‚0.35C2H4-
Cl2, C16.7H23.4BCl0.7N7O2SW: C, 33.11; H, 3.89; N, 16.19. Found: C,
33.05; H, 3.84; N, 16.21 (the partial 1,2-dichloroethane of crystallization
was confirmed by NMR spectroscopy).
fw
669.1
space group
a, Å
b, Å
monoclinic P2
18.385(6)
8.102(1)
18.284(5)
117.15(2)
2423(2)
4
1/c
µ, cm-1
max/min transm
factors
no. of data measd 6161
no. of unique data 5983
no. of obsd data
c, Å
â, deg
V, Å3
Z
3668
[I g 3.0σ(I)]
h, k, l range
-23 < h < +21,
-10 < k < 0,
0 < l < +23
20
R
Rw
F
0.035
0.033
0.76
max, e Å-3
LWO2(OPh). A suspension of LWO2Cl (0.250 g, 0.456 mmol),
NaOPh (0.064 g, 0.55 mmol), and 18-crown-6 (5 mg) was refluxed in
toluene (15 mL) for 1.5 h and then filtered through a short bed of silica
gel. The silica was washed with dichloromethane (8 × 5 mL), and
the combined filtrate and washings were enriched in hexanes to
precipitate white microcrystals. Yield: 0.228 g (83%). Anal. Calcd
for C21H27BN6O3W: C, 41.61; H, 4.49; N, 13.87. Found: C, 41.67;
H, 4.51; N, 13.77.
T (°C)
NMR (MeOH-d4): δ 7.30 (m, 6H), 8.03 (m, 4H). Anal. Calcd for
C12H14NPS2: C, 53.91; H, 5.28; N, 5.24. Found: C, 53.71; H, 5.31;
N, 5.21.
LW(CO)2(S2PPh2). A mixture of NH4[S2PPh2] (1.234 g, 4.61
mmol) and LWI(CO)3 (3.194 g, 4.62 mmol) was heated in toluene (30
mL) at 80 °C with stirring. The flask was left open to the nitrogen
line. The reaction was stopped as soon as the dark color of LW(CO)3I
was exhausted, and a thick, bright orange mixture was present (ca. 70
min). Further heating was avoided to prevent formation of [LW(CO)2]2-
(µ-S).41 The mixture was stripped dry under a vacuum, and then the
residue was extracted with dichloromethane and filtered free of NH4I.
Enrichment of the filtrate with methanol gave an orange precipitate,
whose quality was improved by a second recrystallization. Yield: 3.147
g (88%). Smaller quantities were freed from traces of [LW(CO)2]2-
(µ-S) by eluting a dichloromethane solution on a 50 cm silica column
with 3:2 dichloromethane/hexanes. After elution of two green bands,
the eluent was changed to dichloromethane and the product was
collected as a saffron-orange band. An independent synthesis and
details of the characterisation of this complex appear elsewhere.42
LWO2(SPh). A suspension of LWO2Cl (0.500 g, 0.913 mmol), 18-
crown-6 (5 mg), and NaSPh (0.130 g, 0.984 mmol) was refluxed in
toluene (15 mL) for 13 h and then evaporated to dryness. The residue
was extracted with dichloromethane (20 mL) and filtered through a
short bed of silica gel, which was washed free of yellow color using
dichloromethane. The filtrate and washings were enriched in hexane,
and the resulting orange crystals were collected and washed with
hexane. Yield: 0.466 g (82%). Anal. Calcd for C21H27BN6O2SW:
C, 40.53; H, 4.37; N, 13.51; S, 5.15. Found: C, 40.28; H, 4.38; N,
13.44; S, 5.10. Electronic spectrum (CH2Cl2): λ, 415 (sh) nm (ꢀ, 670
M-1‚cm-1).
LWO2(SePh). A stirred solution of Ph2Se2 (0.285 g, 0.912 mmol)
in tetrahydrofuran (10 mL) was titrated with a tetrahydrofuran solu-
tion of LiEt3BH (1 equiv) until the yellow color disappeared. Under
vacuum, the solvent was removed and the flask was heated at
50 °C for 20 min to remove Et3B. The resulting LiSePh was dis-
solved in tetrahydrofuran (2 × 5 mL), and the solution was injected
onto solid LWO2Cl (0.500 g, 0.912 mmol). The resulting mixture was
stirred for 24 h, filtered free of salts, and then reduced to 2 mL under
vacuum. Orange crystals were precipitated with dry methanol.
Yield: 0.317 g (52%). Anal. Calcd for C21H27BN6O2SeW: C, 37.69;
H, 4.07; N, 12.56. Found: C, 37.75; H, 4.06; N, 12.58. Electronic
spectrum (CH2Cl2): λ, 435 nm (ꢀ, 615 M-1‚cm-1).
LWO2(OMe). A solution of LWO2(SePh) (0.110 g, 0.164 mmol)
in dichloromethane (20 mL) was treated with methanol (5 mL) and
stirred in air in a stoppered flask for 17 h. The resulting pale yellow
solution was reduced to dryness, and the residue was redissolved in
dichloromethane (3 mL) and eluted on a 20 cm silica gel column using
dichloromethane as eluent. The light yellow and following colorless
bands were discarded. The eluent was changed to tetrahydrofuran, and
the product was collected as a colorless band. Recrystallization from
dichloromethane/methanol produced white crystals. Yield: 0.076 g
(85%). Anal. Calcd for C16H25BN6O3W: C, 35.32; H, 4.63; N, 15.45.
Found: C, 35.44; H, 4.60; N, 15.34.
LWO2(S2PPh2). A solution of LW(CO)2(S2PPh2) (2.570 g, 3.27
mmol) and pyridine N-oxide (0.96 g, 10.1 mmol) was stirred in 1,2-
dichloroethane (20 mL) for 20 min and then at 55 °C for 40 min, and
then was stripped dry. Recrystallization of the residue from dichlo-
romethane/methanol gave white crystals. Yield: 1.85 g (74%). Anal.
Calcd for C27H32BN6O2PS2W: C, 42.54; H, 4.23; N, 11.02; S, 8.41.
Found: C, 42.66; H, 4.21; N, 11.11; S, 8.32.
Crystal Structure of LWO2(SePh). Orange crystals of LWO2-
(SePh) were grown in darkness under nitrogen, by slow diffusion of
dry methanol into a saturated dichloromethane solution of the complex.
Intensity data for a crystal 0.08 × 0.23 × 0.26 mm were measured at
room temperature (20 °C) on a Rigaku AFC6R diffractometer fitted
with graphite monochromatized Mo KR radiation, λ ) 0.710 73 Å.
The ω: 2θ scan technique was employed to measure 6161 data up to
a maximum Bragg angle of 27.5°. The data set was corrected for
Lorentz and polarization effects,43 and an empirical absorption correc-
tion was applied.44 Relevant crystal data are given in Table 1.
The structure was solved by direct methods employing SHELXS8645
and refined by a full-matrix least-squares procedure based on F.43
Non-H atoms were refined with anisotropic displacement parameters,
and H atoms were included in the model in their calculated positions
(C-H ) 0.97 Å). The refinement was continued until convergence,
employing σ weights (i.e. 1/σ2(F)) for 3668 data with I g 3.0σ(I). Final
refinement details are collected in Table 1, and the numbering scheme
employed is shown in Figure 1 (drawn with ORTEP46 at 35%
probability ellipsoids). The teXsan43 package, installed on an Iris Indigo
workstation, was employed for all calculations.
LWO2(O2CH). A solution of LWO2(SPh) (0.150 g, 0.241 mmol)
in toluene (10 mL) was treated with formic acid (10.0 µL, 0.265 mmol)
and stirred for 3 days. White crystals were steadily deposited. The
volume was reduced to 5 mL under vacuum, hexane (30 mL) was
added, and the white crystalline product was filtered and washed
with hexane. Yield: 0.131 g (97%). Anal. Calcd for LWO2(O2CH)‚
0.5CH2Cl2, C16.5H24BClN6O4W: C, 33.00; H, 4.03; N, 14.00. Found:
C, 33.28; H, 4.04; N, 13.92. The formation of a CH2Cl2 hemisolvate
from CH2Cl2/hexane was confirmed by NMR.
(41) Thomas, S.; Tiekink, E. R. T.; Young, C. G. Inorg. Chem. 1994, 33,
1416.
(42) Thomas, S. Ph.D. Dissertation, University of Melbourne, 1997.
(43) teXsan: Structure Analysis Software; Molecular Structure Corp.: The
Woodlands, TX, 1992.
(44) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, 39, 158.
(45) Sheldrick, G. M. SHELXS86, Program for the Automatic Solution of
Crystal Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(46) Johnson, C. K. ORTEP; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
NH4[S2PPh2]. Ammonia gas was passed through an aerobic solution
of HS2PPh2 (2.00 g, 7.99 mmol) in toluene (20 mL) for 5 min, and
then hexane (10 mL) was added. The white microcrystalline product
was filtered, washed with hexane, and recrystallized from dichlo-
romethane/hexanes. Yield: 2.14 g (100%). IR (KBr, cm-1) ν(NH),
3050 s, br, 1630 m, 1480 w, 1470 w; ν(P-Ph), 1430 s; δ(NH), 1400
s, br, 1305 w, 1165 w, 1095 s, 1070 w, 1025 w, 995 w, 745 m, 700 s,
690 s, 640 s, 610 s, 600 w; ν(PS), 560 s, 490 m, 480 m, 445 w. 1H

6318 Inorganic Chemistry, Vol. 36, No. 27, 1997
Eagle et al.
plexes such as (phen)WO2(SPh)2,47 WO2(S2CNR2)2,17,18a,b and
[WO2(bdt)2]2- 11
The displacement of a soft sulfur or selenium
.
donor ligand from [WOx]n+ centers may conceivably relate to
substrate binding or activation in tungsten enzymes.48,49 Finally,
dioxo complexes can be prepared by oxidative decarbonylation
of low-valent carbonyl complexes.42 The complex LWO2(S2-
PPh2) was not accessible from LWO2Cl but could be prepared
by oxidation of LW(CO)2(S2PPh2) with excess pyridine N-oxide
in 1,2-dichloroethane. Use of a single equivalent of pyridine
N-oxide led to the formation of red crystalline LWO(CO)(S2-
PPh2).50 The complex LWO2(OH) was also prepared by
oxidative decarbonylation of LWI(CO)3, in this case by dimethyl
sulfoxide.34
The diamagnetic, crystalline complexes are colorless (X )
NCS-, OMe-, OPh-, O2CH-, S2PPh2-S-) or orange (X ) SPh-,
SePh-). They are soluble in chlorinated solvents and tetrahy-
drofuran and are appreciably soluble in aromatic hydrocarbons.
The complexes LWO2(EPh) (E ) S, Se) are reasonably soluble
in alcohols and diethyl ether, but the other complexes are
insoluble in these solvents. All are insoluble in alkanes. These
complexes are indefinitely stable as solids and in solution, with
the exception of LWO2(SPh) and LWO2(SePh), which are
sensitive to water, alcohols, and acids. The complex LWO2-
(SPh) may be recrystallized in air from undried solvents but
LWO2(SePh) must be handled in and recrystallized from dry
solvents.
Microanalytical and mass spectrometric data were consistent
with the formulation of the complexes as monomers. Mass
spectra generally exhibited an [M]+ molecular ion peak cluster
and were devoid of peak clusters at higher m/z (Table 2). The
FAB mass spectrum of LWO2(O2CH) (in a thioglycerol matrix)
contained a molecular ion at m/z 558 (7%) and an [M - CO2]+
peak cluster at m/z 514 (22%).
The infrared spectra of the complexes (Table 2) exhibited
bands characteristic of L [ν(BH) ) 2540-2560 cm-1] and the
co-ligands X, as well as two strong bands at 935-960 cm-1
and 900-915 cm-1, assigned to the symmetric (A′) and
asymmetric (A′′) stretches of the cis-[WO2]2+ fragment,8
respectively. The thiocyanate ligand of LWO2(NCS) is presum-
ably N-bound, consistent with a strong ν(CN) infrared band at
2040 cm-1 51 and the previously reported structure of LMoO2-
(NCS).23
Figure 1. Molecular structure and atom labeling scheme for LWO2-
(SePh). The labeling of the atoms in the pyrazole rings containing N(21)
and N(31) parallels that shown for the ring containing N(11).
Results and Discussion
Synthesis and Characterization. Four different synthetic
strategies were employed in the synthesis of the LWO2X
complexes. The first, which proved to be very limited in scope,
involved the reaction of tungsten-containing starting materials
with KL. Thus, reaction of [WO2Cl2]n and KL in N,N-
dimethylformamide produced LWO2Cl in ca. 70% yield,19,32
while reaction of (NEt4)2[WO2(NCS)4] with KL in refluxing
acetonitrile produced LWO2(NCS) in 15% yield; the latter
reaction was complicated by the formation of large amounts of
unidentified, intensely-colored byproducts. The reaction of
[WO2Br2]n and KL in N,N-dimethylformamide or acetonitrile
did not provide access to the analogous bromo derivative. The
second strategy involved substitution of the chloride ligand of
LWO2Cl. The derivatives LWO2X were prepared in 52-83%
yields by reaction of LWO2Cl with alkali metal salts MIX (MIX
) KNCS, NaOPh, NaSPh, LiSePh). With one exception, the
formation of these derivatives required forcing conditions
(refluxing toluene) and was aided by small quantities of 18-
crown-6. The exception was LWO2(SePh), which formed
within 24 h when LWO2Cl and LiSePh were reacted (without
crown ether) at room temperature in tetrahydrofuran. A higher
yield of LWO2(NCS) (64%) was obtained by this method, but
the reaction was very sluggish (6 days). The scope of these
reactions was also limited as LWO2Cl failed to react with, e.g.,
NaOMe, Me3SiNEt2, AgF, NaF, KSeCN, KF, KOBut, KBr,
NaS2PPh2, NH4S2PPh2, or NaO2CH, despite prolonged reflux
in toluene and the presence of 18-crown-6. Analogous LMoO2X
(X ) OPh-, SPh-, S2PPh2-S-) complexes are formed by
reaction of LMoO2X (X ) Cl-, Br-) with NEt3 and HX,25,26
but similar reactions were unsucessful starting from LWO2Cl.
The third strategy involved the substitution by hard ligands of
the soft thiophenolate and selenophenolate ligands of LWO2-
(SPh) and LWO2(SePh). Complexes prepared in this manner
were generally not accessible by methathesis reactions involving
The 1H NMR spectra of the complexes were consistent with
molecular Cs symmetry (Table 2). As observed for LMoO2X
and LWO2R (R ) hydrocarbyl),24-33 the methine resonance of
the unique pyrazole ring is deshielded with respect to the other
methine protons for most of the new complexes. Exceptionally,
where X ) SPh- or SePh-, these resonances are coincident.
The 1H NMR signatures of proton-bearing X groups are given
in Table 2. Resonances typical of phenyl groups were observed
-
for the X ) EPh- and S2PPh2 complexes, the latter showing
features due to unresolved 31P coupling. The methoxide group
of LWO2(OMe) produces a singlet at δ 4.56 which is more
deshielded than those of {HB(Me2tz)3}MoO2(OMe) (δ 4.38,
HB(Me2tz)3 ) hydrotris(3,5-dimethyl-1,2,4-triazolyl)borate) and
LMoO2(OMe) (δ 4.24),26 indicating that the [LWO2]+ center
is more electron withdrawing than [LMoO2]+. The formate
1
LWO2Cl. Infrared and H NMR studies showed that LWO2-
(SePh) was converted in wet acetonitrile to LWO2(OH)34 and
that LWO2(SPh) reacted with phenol over 3 days in toluene to
give LWO2(OPh). The complexes LWO2(SPh) and LWO2-
(SePh) reacted in air with methanol to give LWO2(OMe) and
thiophenol or diphenyldiselenide, respectively; the reaction
involving LWO2(SePh) was the faster of the two. The formate
complex LWO2(O2CH) was obtained by reacting LWO2(SPh)
with 1.1 equiv of formic acid in toluene. While hard ligands
displaced soft ones at dioxo-W(VI) centers, they did not appear
to replace other hard ligands; thus, 1H NMR studies showed no
conversion of LWO2(OPh) to LWO2(OMe) in methanol over a
1 month period. Hard-for-soft ligand exchange may account
for the air and water sensitivity of dioxo-W(VI) S-donor com-
(47) Lang, R. F.; Ju, T. D.; Hoff, C. D.; Bryan, J. C.; Kubas, G. J. J. Am.
Chem. 1994, 116, 9747.
(48) Axley, M. J.; Bo¨ck, A.; Stadtman, T. C. Proc. Natl. Acad. Sci. U.S.A.
1991, 88, 8450.
(49) Bo¨ck, A. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.;
Wiley: Chichester, U.K., 1994; p 3700.
(50) Thomas, S.; Tiekink, E. R. T.; Young, C. G. Organometallics 1996,
15, 2428.
(51) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 3rd ed.; Wiley: New York, 1978; p 270.

LWO2X Complexes with a Tripodal N3-Donor Ligand
Inorganic Chemistry, Vol. 36, No. 27, 1997 6319
1H NMR spectrum, δ (multiplicity,a no. of H) (CDCl3)
Table 2. Characterization Data for LWO2X Complexes
infrared spectrum (KBr, cm-1
)
X
m/z [M]+ b
ν(BH)
ν(WO2)
X ligandc
L (methyl)
L (methine)
X ligandc
Cl-
548 (8%)
2560 m
960 s, 915 s
ν(WCl) 345 s
2.36 (3H), 2.38 (6H)
2.69 (3H), 2.70 (6H)
2.38 (3H), 2.38 (6H)
2.65 (9H)
2.35 (9H), 2.58 (6H)
2.78 (3H)
2.38 (6H), 2.39 (3H)
2.49 (6H), 2.71 (3H)
2.36 (6H), 2.39 (3H)
2.40 (6H), 2.85 (3H)
2.37 (3H), 2.39 (6H)
2.69 (3H), 2.76 (6H)
2.33 (3H), 2.36 (6H)
2.54 (6H), 2.60 (3H)
2.36 (3H), 2.39 (6H)
2.66 (3H), 2.82 (6H)
5.89 (2H)
5.92 (1H)
5.91 (2H)
5.94 (1H)
5.84 (2H)
5.89 (1H)
5.89 (2H)
5.97 (1H)
5.84 (2H)
5.94 (1H)
5.90 (3H)
NCS-
OMe-
O2CH-
OPh-
SPh-
571 (19%)
544 (27%)
2550 m
2555 m
2555 m
2540 m
2550 m
2550 m
2550 m
945 s, 900 vs
935 s, 900 vs
945 s, 910 vs
940 s, 900 vs
945 s, 905 vs
950 s, 905 vs
945 s, 905 vs
ν(NCS) 2040 vs
ν(W-OMe) 525 m
ν(CO) 1100 s
OCH3-, 4.56 (3H)
O2CH-, 8.77 (1H)
558 (7%)
ν(CdO) 1705 vs
ν(CO) 1185 br,s
1590 s, 1480 s
ν(CO) 1255 s
514 (22%)d
606 (31%)
phenyl, 6.97 (t, 1H)
7.13 (d, 2H), 7.32 (t, 2H)
phenyl, 7.10 (t, 1H)
7.33 (t, 2H), 7.74 (d, 2H)
phenyl, 7.43 (m, 6H)
8.11 (m, 4H)
622 (10%)
762 (14%)
668 (7%)
1580 m, 1475 w
-
S2PPh2
SePh-
ν(PPh) 1435 s
ν(PS) 525 s
1575 w, 1475 m
5.84 (2H)
5.86 (1H)
5.90 (3H)
phenyl, 7.12 (t, 1H)
7.30 (t, 2H), 7.46 (d, 2H)
a Singlet resonances unless otherwise indicated. b Electron impact mass spectral data, peak intensities indicated by percentage relative to strongest
peak at 100%. c Assignments given where possible. d Obtained using fast atom bombardment mass spectrometry, peak due to [M- CO2]+.
Table 3. Selected Interatomic Distances and Angles for
LWO2(SePh)
(1) Å is slightly shorter than the average from available
structures (of low-valent organometallic species rather than
oxoselenolate complexes).56-58 The W-N(11) and W-N(31)
distances are ca. 0.14 Å longer than the W-N(21) distance as
a result of the trans influence of the oxo ligands. The O(1)-
W-O(2) angle of 102.2(2)° enhances favorable π interactions
between the oxo and metal orbitals.8,59,60 The W-N(21) and
W-Se vectors are inclined away from the oxo ligands and an
N(21)-W-Se angle of 157.0 (1)° pertains. The oxo and
selenophenolate ligand are unsymmetrically disposed around the
pseudo-3-fold axis associated with the LW fragment with
O-W‚‚‚B(1) and Se-W‚‚‚B(1) angles of 122 and 109°,
respectively. The N(n1)-W‚‚‚B(1) angles fall in a narrow range
around 47°. The W atom lies 0.8759(3) Å out of the plane
defined by the O(1), O(2), and Se atoms and 1.5157(3) Å out
of the plane defined by the N(n1) atoms. The pseudomirror
plane of the molecule is defined by the pyrazole ring containing
N(11) and atoms W and Se; the N(21)-W-Se-C(41) torsion
angle is -174.8(4)°. The dihedral angle between this plane
and the plane through the phenyl group is 45.6°, and the
W-Se-C(41)-C(42) and W-Se-C(41)-C(46) torsion angles
are -27.0(7) and 154.2(6)°, respectively. It has been noted that
the aryl rings of thiophenolate ligands involved in significant
π donation prefer to lie coplanar with the M-S-C plane to
facilitate an extended π interaction between the aryl, sulfur, and
metal π orbitals.61,62 The observed skewing of the phenyl ring
from the M-Se-C plane suggests that it is not involved in π
bonding, consistent with the presence of strongly π-donating
oxo groups.
Bond Distances (Å)
W-Se
2.535(1)
1.716(4)
2.147(5)
1.940(7)
W-O(1)
W-N(11)
W-N(31)
1.721(4)
2.283(6)
2.288(5)
W-O(2)
W-N(21)
Se-C(41)
Bond Angles (deg)
Se-W-O(1)
99.7(2)
99.6(2)
83.1(1)
157.0(1)
84.5(1)
102.2(2)
166.6(2)
95.3(2)
O(1)-W-N(31)
O(2)-W-N(11)
O(2)-W-N(21)
O(2)-W-N(31)
N(11)-W-N(21)
N(11)-W-N(31)
N(21)-W-N(31)
W-Se-C(41)
87.3(2)
90.1(2)
94.1(2)
168.7(2)
78.5(2)
79.9(2)
78.9(2)
103.5(2)
Se-W-O(2)
Se-W-N(11)
Se-W-N(21)
Se-W-N(31)
O(1)-W-O(2)
O(1)-W-N(11)
O(1)-W-N(21)
proton of LWO2(O2CH) resonates at δ 8.77; similar chemical
shifts are observed for the formate resonance of Mo(CO)2-
(PEt3)2(O2CH)2 (δ 8.48) and Mo(CO)2(PPh3)2(O2CH)2 (δ 8.14),52
Mo(H)(O2CH)(PMe3)4 (δ 8.05),53 N,N-dimethylformamide (δ
7.90), and acetaldehyde (δ 9.80).54
The electronic spectra of the colorless complexes were de-
void of bands in the range 400-900 nm. The yellow-orange
colors of LWO2(SPh) and LWO2(SePh) arise from tailing of
intense ultraviolet bands into the visible region. The complex
LWO2(SPh) has a shoulder at 410 nm (ꢀ ) 690 M-1‚cm-1),
and LWO2(SePh) has a distinct band at 434 nm (ꢀ ) 610
M-1‚cm-1). These features are atributed to ligand-to-metal
charge-transfer transitions from filled Sp or Sep orbitals to empty
tungsten d orbitals; d f d transitions are impossible with these
formally d0 complexes.
Crystal Structure of LWO2(SePh). The molecular structure
of LWO2(SePh) is presented in Figure 1, and selected inter-
atomic distances and angles are listed in Table 3. The six-
coordinate complex exhibits a distorted octahedral geometry and
possesses approximate Cs symmetry. The tungsten center is
coordinated by a facially tridentate L ligand, two cis oxo ligands,
and a selenophenolate ligand. The W-O(1) and W-O(2)
distances are equal within experimental error and are in the range
typical of oxotungsten units.8,55 The W-Se distance of 2.535-
(55) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
(56) (η7-C7H7)W(CO)2(SePh): W-Se ) 2.588(2) Å, Se-C ) 1.954(18)
Å, W-Se-C ) 107.5(6)°. Rettenmeier, A.; Weidenhammer, K.;
Ziegler, M. L. Z. Anorg. Allg. Chem. 1981, 473, 91.
(57) CpW(CO)3(SeCH2Ph): W-Se ) 2.623(1) Å, Se-C ) 1.961(11) Å,
W-Se-C ) 109.8(4)°. Eikens, W.; Kienitz, C.; Jones, P. G.; Tho¨ne,
C. J. Chem. Soc., Dalton Trans. 1994, 3329.
(58) W(CO)2(PMe2Ph)2(SeC6H3Pri2)2 (two independent molecules): W-Se
) 2.506(1) and 2.583(1) Å, Se-C ) 1.945(8) and 1.950(7) Å,
W-Se-C ) 118.3(2) and 114.9(2)°. Burrow, T. E.; Hughes, D. L.;
Lough, A. J.; Maguire, M. J.; Morris, R. H.; Richards, R. L. J. Chem.
Soc., Dalton Trans. 1995, 1315.
(59) Tatsumi, K.; Hoffmann, R. Inorg. Chem. 1980, 19, 2656.
(60) Brower, D. C.; Templeton, J. L.; Mingos, D. M. P. J. Am. Chem. Soc.
1987, 109, 5203.
(61) Coucouvanis, D.; Swenson, D.; Baenziger, N. C.; Murphy, C.; Holah,
D. G.; Sfarnas, N.; Simopoulos, A.; Kostikas, A. J. Am. Chem. Soc.
1981, 103, 3350.
(52) Brower, D. C.; Winston, P. B.; Tonker, T. L.; Templeton, J. L. Inorg.
Chem. 1986, 25, 2883.
(53) Lyons, D.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, M. B. J.
Chem. Soc., Dalton Trans. 1984, 695.
(54) Williams, D. H.; Fleming, I. Spectroscopic Methods In Organic
Chemistry; McGraw-Hill: London, 1989.
(62) Ashby, M. T. Comments Inorg. Chem. 1990, 10, 297.

6320 Inorganic Chemistry, Vol. 36, No. 27, 1997
Table 4. Cyclic Voltammetric Data for LWO2X Complexesa
Eagle et al.
X
E1/2, V
Epc, V
Epa, V
Ipa/Ipc
∆Epp, mV
comments
other processes
-
S2PPh2
NCS-
Cl-
na
na
na
-1.25
na
-1.32
na
-1.05
-1.07
-1.21
-1.29
-1.34
-1.35
-1.50
-1.57
-1.76
no
no
no
-1.21
no
-1.29
no
0
0
0
na
na
na
89
na
64
na
84
92
irrev
irrev
irrev
quasi-rev
irrev
quasi-rev
irrev
rev
no
E1/2 ) -1.12 V
E1/2 ) -1.32 V
O2CH-
SePh-
SPh-
0.56
0
E1/2 ) -1.38 V
no
no
1.00b
0
OH- 34
OPh-
Epc ) -1.74 V
no
no
-1.53
-1.71
-1.48
-1.66
1.00
0.98
OMe-
rev
a At V ) 100 mV‚s-1. Compounds arranged in order of decreasing (more negative) Epc. Abbreviations: irrev ) irreversible, quasi-rev. )
quasi-reversible, rev ) reversible, na ) not applicable, no ) not observed. b This ratio diminishes at V < 100 mV‚s-1
.
The complex is isomorphous and isostructural with LMoO2-
(SPh),26 and both molecules, especially their LMO2 fragments,
are virtually superimposable (mean MO2 atom displacement )
0.017 Å). The EPh- fragments adopt the same orientation but
overlap less perfectly upon superposition of the molecules (mean
E and maximal C atom displacements, 0.13 and 0.32 Å,
respectively); as well, the W-Se and Mo-S vectors are virtually
collinear. The structure of LWO2(SePh) also closely resembles
those of other dioxo-pyrazolylborate complexes of tungsten.19,32
Complex LWO2(SePh) is the first oxo selenolate of tungsten
and only the second oxo selenolate of any metal to be
structurally characterized, the previous example being MoO-
{Ph2P(O)(CH2)2P(Ph)(CH2)2PPh2}(SeC6H2Me3)2.63 Indeed,
mononuclear early transition metal selenolates are scarce, and
the structures of only three tungsten complexes have been
reported.56-58
Electrochemistry. With few exceptions, one-electron reduc-
tion of cis-[MO2]2+ complexes (M ) Mo, W) leads to [MVO]3+
or [MV2O3]4+ species, rather than stable cis-[MVO2]+ com-
plexes.26,64 Elimination and/or condensation reactions account
for the formation of these thermodynamically favored species
upon reduction.26 However, ligands capable of retarding these
reactions have permitted the chemical or electrochemical
generation of long-lived, EPR-active cis-[MoVO2]+ complexes
and the observation of reversible, one-electron reduction
processes by cyclic voltammetry.25-29,65-68 Very recently,
compounds of the type CoCp2[LMoVO2X] (Cp ) η5-C5H5; X
) ER, E ) O, S; R ) alkyl, phenyl) have been isolated and
spectroscopically and structurally characterized.69,70 Typically,
cis-[WO2]2+ complexes are reduced at more cathodic potentials
than their molybdenum counterparts,9-11,15,16 and only one such
complex, [(Me3tcn)WO2Cl]Cl (Me3tcn ) 1,4,7-trimethyltriaza-
cyclononane), has been reported to undergo a reversible
reduction on the cyclic voltammetric time scale.16
assigned to a one-electron reduction to the corresponding anion,
[LWO2X]-. Typically, this process was irreversible or quasi-
reversible. In only two cases, LWO2(OPh) and LWO2(OMe),
was the process electrochemically reversible. For these com-
plexes, Ipa/Ipc values approached unity and both Ep and ∆Ep
were independent of scan rate V; ∆Ep values were close to that
observed for the standard [FeCp2]+/[FeCp2] one-electron couple
under the same conditions, and plots of Ipc vs V1/2 were linear
and passed through the origin. Electrochemically reversible one-
electron processes are consistent with these observations.71 The
analogous molybdenum complexes exhibit similar electrochemi-
cal behavior.29 At scan rates greater than 100 mV‚s-1, LWO2-
(SPh) undergoes an apparently reversible reduction with E1/2
) -1.32 V. At slower scan rates, peak current ratios steadily
diminish, indicating a quasi-reversible process.71 The irrevers-
ible reduction of LWO2(SePh) [cf., LWO2(OPh) and LWO2-
(SPh)] may be due to cleavage of the W-Se bond or of the
Se-C bond, as Se-C bond dissociation energies are substan-
tially less than those for S-C and O-C bonds.72 The
electrochemical behavior of LWO2(O2CH) is dependent on scan
rate; a single quasi-reversible process (E1/2 ) -1.25 V, ∆Epp
) 141 mV) is observed at 500 mV‚s-1, while, at slower scan
rates, a second process at E1/2 ) -1.38 V steadily replaces the
first. At V ) 30 mV‚s-1, the first process is irreversible and
the second approaches reversibility. The reduction of LWO2-
(S2PPh2) was irreversible at all scan rates. Four complexes (X
) NCS-, Cl-, O2CH-, OH-) underwent a second reductive
process at potentials more cathodic than the principal process.
By analogy with [MoO2]2+ electrochemistry,73 these processes
are attributed to the formation of µ-oxo-W(V) dimers and
possible further reduction to W(IV). All cyclic voltammograms
were devoid of features at anodic potentials.
Consistent with earlier results,9-11,15,16 the complexes LWO2X
undergo typically irreversible reduction at very negative po-
tentials (Epc ) -1.05 to E1/2 ) -1.71 V vs SCE). The Epc
values are ligand dependent (recognizing the caveat applying
to the Epc values for irreversible processes) and span a range of
710 mV (Table 4). This observation demonstrates that a high
degree of electronic fine tuning of the [WO2]2+ center is possible
by variation of co-ligands. The S2PPh2-, halide and pseudo-
halide complexes are the most readily reduced, the soft ligand
SePh- and SPh- complexes exhibit intermediate reduction
potentials, and the OPh- and OMe- complexes are the most
difficult to reduce. All the reduction potentials are more
We have used cyclic voltammetry to explore the electro-
chemistry of the LWO2X complexes. Electrochemical data are
summarized in Table 4. In acetonitrile, all complexes underwent
at least one reductive process at potentials more cathodic than
-1.05 V vs SCE. The principal (more positive) process is
(63) Boorman, P. M.; Kraatz, H.-B.; Parvez, M. Polyhedron 1993, 12, 601.
(64) Young, C. G.; Wedd, A. G. in Encyclopedia of Inorganic Chemistry;
King, R. B., Ed.; Wiley: Chichester, 1994; p 2330 and references
cited therein.
(65) Pickett, C.; Kumar, S.; Vella, P. A.; Zubieta, J. Inorg. Chem. 1982,
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Farchione, F.; Enemark, J. H.; Kristofzski, J.; Bruck, M. J. Am. Chem.
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(70) Xiao, Z.; Gable, R. W.; Wedd, A. G.; Young, C. G. J. Am. Chem.
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(71) Brown, E. R.; Large, R. F. In Techniques of Chemistry; Weissberger,
A., Rossiter, B. W., Eds.; Wiley: New York, 1971; Vol. I, Part IIA,
Chapter VI.
(68) Farchione, F.; Hanson, G. R.; Rodrigues, C. G.; Bailey, T. D.; Bagchi,
R. N.; Bond, A. M.; Pilbrow, J. R.; Wedd, A. G. J. Am. Chem. Soc.
1986, 108, 831.
(69) Xiao, Z.; Gable, R. W.; Wedd, A. G.; Young, C. G. J. Chem. Soc.,
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(72) (a) Batt, L. In The Chemistry of Organic Selenium and Tellurium
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1986;
Vol. 1, p 159. (b) Boorman, P. M.; Kraatz, H.-B.; Parvez, M. J. Chem.
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(73) Isbell, A. F., Jr.; Sawyer, D. T. Inorg. Chem. 1971, 10, 2449.

LWO2X Complexes with a Tripodal N3-Donor Ligand
Inorganic Chemistry, Vol. 36, No. 27, 1997 6321
version of dioxo-Mo(VI) and oxo-Mo(IV) complexes (eq 3,
PR3
2+
[MoVIO2]
y
z [MoIVO]2+
(3)
Me2SO
[MoVIO2]2+ + [MoIVO]2+ f {[MoVO]2(µ-O)}4+ (4)
co-ligands excluded) without the formation of dinuclear µ-oxo-
Mo(V) complexes via comproportionation of these species (eq
4). In contrast, dioxo-W(VI) complexes are reluctant to
participate in OAT reactions and are generally inert toward
reaction with phosphines.17,18a,b,77 We now describe attempts
to reduce the LWO2X complexes using phosphines and benzoin.
At sub-reflux temperatures in a variety of solvents, there was
Figure 2. Comparison of the reduction potentials of LMO2X complexes
in MeCN (except LMoO2Cl in CH2Cl2). The horizontal scale refers to
E1/2 or Epc values for the M(VI)/M(V) process, represented by vertical
bars for M ) Mo (bottom) and W (top) complexes, with specified X
ligands. The dashed lines connect the potentials of LMO2X complexes
having the same X ligand.
no reaction between LWO2Cl and PPh3 or PBun . This is in
3
direct contrast to the facile reactions of phosphines with the
dioxo-Mo(VI) and thio-W(VI) analogues, LMoO2X,25,26,28,29
LWOSX,21 and LWS2X.21 The breaking of ModO (bond
negative than those of potent biological reductants (e.g. NADPH,
E0′ ) -0.5 V; low-potential [Fe4S4]n+ centers, E0′ ) -0.705
V74 ), even allowing for significant overpotential effects and
the absence of water (E0′ ) -0.42 V). These considerations
suggest that the [WO2]2+/[WO2]+ redox couple is biologically
irrelevant; however, this does not preclude the biological
exploitation of [WO2]2+ centers, as other reductive mechanisms
are conceivable. Thus, [WO2(mnt)2]2- (Epc ) -1.50 V vs Ag/
AgCl) can be reduced to [WO(mnt)2]2- by compounds (H2S,
PhSH, dithiothreitol, dithionite) with E values significantly more
positive than -1.50 V.9-11 The facility of the latter reactions
was interpreted in terms of the formation of precursor complex-
substrate adducts with markedly more positive reduction
potentials.10
The availability29 of electrochemical data for the molybdenum
complexes LMoO2X (X ) NCS-, OMe-, SPh-, OPh-) obtained
under conditions identical to the data presented here, permits a
direct comparison of the the two series of compounds (an earlier
comparison contrasted the redox properties of LWO2X in
acetonitrile and LMoO2X in dichloromethane20). Comparing
E1/2 or Epc values as appropriate (Figure 2), the tungsten
complexes undergo reduction at more cathodic potentials (560-
620 mV more negative) than do their molybdenum analogues.
This is in keeping with previous results obtained on pairs of
Mo/W compounds or limited series of analogues.15,16 These
differences are a manifestation of the decrease in oxidizing
power down a group of the periodic table.18d In the case of
LMO2X complexes, they reflect the relatively high energy of
the LUMO orbital of the tungsten complexes compared to that
of the molybdenum analogues (the LUMO is a principally metal
based π* combination of dπ and oxo p ligand obitals). The
different redox potentials of Mo(VI) and W(VI) complexes
appear to dramatically influence enzyme turnover, the attenu-
ation of the normal activity of molybdoenzymes upon substitu-
tion of Mo by W being attributed to this influence.
strength ca. 98 kcal‚mol-1) and WdS (ca. 82-92 kcal‚mol-1 21
)
but not WdO (>138 kcal‚mol-1) bonds in reactions with PPh3
is in accord with thermodynamic expectations.82 Indeed, the
very similar bond energies of PdO (ca. 126-139 kcal‚mol-1 83
)
and WdO (>138 kcal‚mol-1) bonds may account for the
difficulty in reducing [WO2]2+ centers using phosphines. In
refluxing acetonitrile, LWO2Cl reacted with PPh3 over a 1 week
period to give a deep red solution, but the red species could
not be isolated or identified. A similar reaction in refluxing
tetrahydrofuran (PPh3, 3 days) resulted in the isolation of
unreacted LWO2Cl. In refluxing pyridine (115 °C), LWO2Cl
reacted with PPh3 in 3 h to produce an intensely colored maroon
solution. Chromatographic (aerobic or anaerobic) workup led
to the isolation of [LWO2]2(µ-O) (ca. 35%), blue LWOCl2,
white LWO2Cl, and a mixture of two unidentified orange oxo
species [ν(WdO) ) 965 (s) cm-1] (ca. 40 mg from 0.50 g of
LWO2Cl). The purple color of these solutions may reflect the
presence of a mixture of orange and blue products. Having
failed to isolate putative LWOCl(py) on workup, attempts were
made to trap this complex. Reflux of the maroon solution
obtained by reacting LWO2Cl with PPh3 in pyridine with either
Me3SiS2CNEt2 (3.5 h) or NaO2PPh2/18-crown-6 (2.5 h) pro-
duced no LWO(S2CNEt2) or LWO(O2PPh2). Reflux of the
maroon solution with anhydrous NaS2CNEt2/18-crown-6 for 1
day produced only a 1.4% yield of LWO(S2CNEt2);21 the major
product of this reaction was again [LWO2]2(µ-O).34 The
formation of OPPh3 was confirmed by infrared spectroscopy
of the filtrate residue. While it is assumed that LWO(S2CNEt2)
is formed in this reaction from transient “LWOCl(py)”, it is
conceivable that this complex instead forms by reaction of
(77) (a) Holm, R. H.; Berg, J. M. Pure. Appl. Chem. 1984, 56, 1645. (b)
Berg. J. M.; Holm, R. H. J. Am. Chem. Soc. 1985, 107, 917. (c) Berg,
J. M.; Holm, R. H. J. Am. Chem. Soc. 1985, 107, 925. (d) Holm, R.
H.; Berg, J. M. Acc. Chem. Res. 1986, 19, 363. (e) Harlan, E. W.;
Berg. J. M.; Holm, R. H. J. Am. Chem. Soc. 1986, 108, 6992. (f)
Caradonna, J. P.; Harlan, E. W.; Holm, R. H. J. Am. Chem. Soc. 1986,
108, 7856. (g) Caradonna, J. P.; Reddy, P. R.; Holm, R. H. J. Am.
Chem. Soc. 1988, 110, 2139. (h) Craig, J. A.; Holm, R. H. J. Am.
Chem. Soc. 1989, 111, 2111.
(78) (a) Gheller, S. F.; Schultz, B. E.; Scott, M. J.; Holm, R. H. J. Am.
Chem. Soc. 1992, 114, 6934. (b) Schultz, B. E.; Gheller, S. F.;
Muetterties, M. C.; Scott, M. J.; Holm, R. H. J. Am. Chem. Soc. 1993,
115, 2714.
Reactivity Studies. OAT and CEPT reactions feature in
mechanistic interpretations of the molybdoenzymes.22,64 Many
oxomolybdenum compounds,22,64,75 and the dimethyl sulfoxide
reductase from Rhodobacter sphaeroides,76 catalyze OAT
reactions, typically from donors such as Me2SO to acceptors
such as phosphines. Moreover, a number of catalytic OAT
systems, some of which closely mimic enzyme systems, have
been developed.24,27-29,77-81 These are based on the intercon-
(79) Das, S. K.; Chaudhury, P. K.; Biswas, D.; Sarkar, S. J. Am. Chem.
Soc. 1994, 116, 9061.
(74) da Silva, J. J. R.; Williams, R. J. P. The Biological Chemistry of the
Elements; Clarendon: Oxford, U.K., 1994; pp 90, 328.
(75) (a) Holm, R. H. Chem. ReV. 1987, 87, 1401. (b) Holm, R. H. Coord.
Chem. ReV. 1990, 100, 183.
(76) Schultz, B. E.; Hille, R.; Holm, R. H. J. Am. Chem. Soc. 1995, 117,
827.
(80) Yoshinaga, N.; Ueyama, N.; Okamura, T.; Nakamura, A. Chem. Lett.
1990, 1655.
(81) Arzoumanian, H.; Lopez, R.; Agrifoglio, G. Inorg. Chem. 1994, 33,
3177.
(82) Reference 8b, pp 32-35.
(83) Hall, K. A.; Mayer, J. M. J. Am. Chem. Soc. 1992, 114, 10402.

6322 Inorganic Chemistry, Vol. 36, No. 27, 1997
Eagle et al.
NaS2CNEt2 with LWOCl2 or the orange oxo products of these
reactions, casting further doubt on the intermediacy of LWOCl-
(py). As LWO(S2PPh2) is more robust than LWO(S2CNEt2),21
the reaction of these LWOCl(py) solutions with NH4S2PPh2 was
attempted. After 6 h of reflux, thin layer chromatography
indicated the formation of the same products observed in the
previous reactions with Me3SiS2CNEt2. After 4 days, new red
and yellow products had formed but no LWO(S2PPh2) was
detected at any stage of the reaction. The yellow product was
identified as LWS(µ-S)2WS(S2PPh2).21
refluxing benzene. These reactions remained colorless at all
stages, and unchanged products were isolated.
In contrast to the disappointing outcomes of the reactions
described above, the LWO2X complexes react readily with boron
sulfide to generate the corresponding oxo-thio- and bis(thio)-
W(VI) complexes, LWOSX and LWS2X.19-21 Similarly, Yu
and Holm17 have reported the in situ generation of oxo-thio-
and bis(thio)-W(VI) Schiff base complexes from their dioxo-
W(VI) analogues.
Summary
Reaction of LWO2(S2PPh2) with PMe2Ph in refluxing toluene
led to the formation of seven species after 18 h, as judged by
TLC (silica, CH2Cl2). There was no trace of the expected
product LWO(S2PPh2). However, a light orange product was
isolated by chromatography; its infrared spectrum exhibited the
strong ν(WO2) (940, 905 cm-1) and ν(PPh) (1430 cm-1) bands
of the starting material but lacked the strong ν(PS2) stretch at
An extensive series of dioxo-W(VI) complexes of the type
LWO2X has been prepared and characterized, and the first X-ray
crystal structure of a (selenolate)oxotungsten complex has been
determined. A variety of synthetic methodologies have been
employed to generate cis-LWO2X complexes containing hard
halide and O- and N-donor ligands (X ) Cl-, NCS-, OMe-,
OPh-, and O2CH-), soft mono- and ambidentate S-donor
ligands (X ) SPh- and S2PPh2-S-), and soft Se-donor ligands
(SePh-). The complexes have been thoroughly characterized,
both spectroscopically and structurally. Electrochemical studies
show that the W(VI)/W(V) reduction potentials of the LWO2X
complexes are extremely cathodic, being some 560-620 mV
more negative than the corresponding potentials of analogous
Mo complexes. The complexes are very stable and fail to
participate in clean OAT and CEPT reactions. This behavior
underscores the very considerable chemical and redox stability
of the [WO2]2+ moiety.
565 cm-1
. This product was tentatively identified as the
phosphinothiolate complex LWO2(SPPh2), consistent with ab-
straction of a sulfur atom from the S2PPh2- ligand in preference
to oxygen atom abstraction from the [WO2]2+ unit. The greater
strength of WdO (138-161 kcal‚mol-1) bonds over PdS (ca.
82-92 kcal‚mol-1) bonds83 is consistent with this observation.
Other examples of phosphinothiolate complexes include Cp2-
Ti(SPPh2)84 and CpW[SP(Ph){N(SiMe3)2}](CO)2.85
The R-hydroxyketone benzoin readily reduces [WO2(bdt)2]2-
and [WO2{O2CC(S)Ph2}2]2- to the corresponding oxotungsten-
(IV) complexes. Parallel reactions were not observed between
LWO2(OPh) and benzoin in refluxing pyridine (chosen to
stabilize transient five-coordinate “LWO(OPh)”) or between
LWO2(S2PPh2) and less bulky 3-hydroxybutanone (acetoin) in
Acknowledgment. We gratefully acknowledge the financial
support of the Australian Research Council.
Supporting Information Available: An X-ray crystallographic file
in CIF format is available on the Internet only. Access information is
given on any current masthead page.
(84) Gelmini, L.; Stephan, D. W. Organometallics 1987, 6, 1515.
(85) Reisacher, H.-U.; McNamara, W. F.; Duesler, E. N.; Paine, R. T.
Organometallics 1997, 16, 449.
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