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Inorg. Chem. 2004, 43, 2114−2124
Coordination Complexes of Molybdenum with 3,6-Di-tert-butylcatechol.
Addition Products of DMSO, Pyridine N-oxide, and Triphenylarsine
Oxide to the Putative [MoVIO(3,6-DBCat)2] Monomer and Self-Assembly
of the Chiral [{MoVIO(3,6-DBCat)2}4] Square
Cai-Ming Liu† and Ebbe Nordlander*
Inorganic Chemistry, Center for Chemistry and Chemical Engineering, Lund UniVersity,P.O. Box 124, Lund SE 221 00, Sweden Derek Schmeh, Richard Shoemaker, and Cortlandt G. Pierpont*
Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309 Received December 11, 2003 Molybdenum complexes of 3,6-di-tert-butylcatechol have been prepared from the reaction between [Mo(CO)6] and3,6-di-tert-butyl-1,2-benzoquinone. A putative "[MoO(3,6-DBCat)2]" monomer is assumed to form initially by reactionwith trace quantities of oxygen. Condensation of the reaction mixture leads to the formation of oligomeric products,including the [{MoO(3,6-DBCat) } 2 4] chiral square isolated by chromatographic separation. Molybdenum centers at the corner of the square are bridged by oxo ligands centered along edges. Four-fold and inversion crystallographicsymmetry gives tetramers as either ΛΛΛΛ or ∆∆∆∆ isomers, and the crystal structure consists of parallelcolumns of squares with the same chirality. Addition of O-Subst (O-Subst ) dmso, pyridine N-oxide, triphenylarsineoxide) ligands to [MoO(3,6-DBCat)2] occurs selectively to give cis-[MoO(O-Subst)(3,6-DBCat)2] products. All threeaddition complexes are fluxional in solution. The temperature-dependent stereodymanic behavior of [MoO(dmso)-(3,6-DBCat)2] has been shown to occur via a trigonal prismatic intermediate (Bailar twist) that conserves the cisdisposition of oxo and dmso ligands. Electrochemical and chemical reduction reactions have been investigated for[MoO(dmso)(3,6-DBCat)2] with interest in displacement of SMe2 with formation of cis-[MoO2(3,6-DBCat)2]2-. Cyclicvoltammetry shows an irreversible two-electron reduction for the complex at −0.852 V (vs Fc/Fc+). Chemical reductionusing CoCp2 was observed to give a product with an electronic spectrum that is generally associated with cis-[MoO2(Cat)2]2- complexes. Structural characterization revealed that the product was [CoCp2][MoO(3,6-DBCat)2],possibly formed as the product of dmso displacement upon one-electron reduction of [MoO(dmso)(3,6-DBCat)2].
The coordination chemistry of molybdenum with ligands derived from o-benzoquinones has been of interest in studieson oxo coordination to complexes of Mo(VI).1 Oxo andcatecholate ligands both stabilize high oxidation state metalions through strong σ and π donation to vacant metal orbitals,and both ligands are subject to displacement upon protonation ligands are relevant to oxo transfer chemistry (eq 2) in pro- (eq 1). Equilibria between coordinated oxo and catecholate cesses that are influenced by coligand and catechol substitu-ent effects, that may place the benzoquinone reduction * Authors to whom correspondence should be addressed. E-mail: [email protected] (E.N.); [email protected] (C.G.P.).
potential on either side of the O2 reduction potential (eq 3).
† Present address: Organic Solids Laboratory, Center for Molecular The reaction between Mo(CO)6 and 9,10-phenanthrene- Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing100080, China.
quinone (PhenBQ) was found to give the mixed catecholate/ 2114 Inorganic Chemistry, Vol. 43, No. 6, 2004
10.1021/ic0354264 CCC: $27.50 2004 American Chemical Society Published on Web 02/17/2004 different chemical properties from the products obtained with3,5-DBCat. In this report we describe the structural, chemical,and electrochemical properties of the molybdenum com- semiquinonate complex [MoV(PhenCat)2(PhenSQ)].2 Hy- plexes of 3,6-DBCat.10 With the potential oxido lability of drolysis under acidic conditions gave [MoVI2O5(PhenSQ)2].3 a transient [MoVIO2(3,6-DBCat)2]2- dianion, coupled oxo and The reaction between [Mo(CO)6] and 3,5-di-tert-butyl-1,2- electron transfer reactions will be of interest in the study of benzoquinone (3,5-DBBQ) gave [MoVI2O2(3,5-DBCat)4], oxo coordination to the molybdenum complexes of 3,6- which could also be obtained by treating [MoO2(acac)2] with H2-3,5-DBCat.4,5 These reactions illustrate the forward andreverse directions of reaction 1. The formation of [MoVI2O2- (3,5-DBCat)4] from [Mo(CO)6] and 3,5-DBBQ was foundto proceed by initial formation of [MoVI(3,5-DBCat) reacted with trace quantities of dioxygen in the reaction (3,6-DBBQ) was prepared by literature methods.11 Other reagents medium (eq 3) to give the oxo product.6 It was of interest to and materials were purchased from commercial sources and wereused as received. Reactions and solution studies were performed find that acidic hydrolysis of [MoVI2O2(3,5-DBCat)4] gave under nitrogen using deoxygenated solvents and standard Schlenk [MoVI2O5(3,5-DBCat)2]2- rather than the more common cis- techniques. Solid-state infrared spectra were recorded on a Nicolet dioxomolybdenum(VI) species [MoVIO2(3,5-DBCat)2]2- that Avatar 360 FT-IR spectrometer with samples prepared as KBr disks.
is known to form with a variety of other catecholate ligands.7 1H NMR spectra were recorded using Varian Unity 300 MHz and In fact, we have been unable to observe the [MoVIO2(3,5- Inova(r) 500 MHz spectrometers. Fast atom bombardment (FAB) DBCat)2]2- dianion in any synthetic procedure, suggesting mass spectra were obtained on a JEOL SX-102 mass spectrometer that the electron releasing property of the tert-butyl substit- using 3-nitrobenzyl alcohol as a matrix. UV-visible spectra were uents of 3,5-DBCat destabilizes terminal oxo ligands coor- recorded on a Cary-Varian 2290 spectrophotometer, and cyclic dinated to Mo(VI).
voltammetric measurements were made inside a nitrogen filled The coordination chemistry of molybdenum with the glovebox using a PAR 263 potentiostat/galvanostat analyzer.
symmetrical 3,6-di-tert-butylcatecholate ligand has been Solutions of the complexes were dissolved in CH2Cl2 containing(NBu described briefly, but none of the products of this chemistry 4)(PF6) (ca. 0.1 M) as supporting electrolyte. Platinum wire working and counter electrodes were used with a Ag/AgNO have been characterized structurally.8 Complexes formed reference electrode. The Fc/Fc+ couple appeared at +0.464 V (∆E with the 3,5-DBCat ligand are commonly dimeric or oligo- ) 62 mV) vs SCE with this experimental arrangement, and the meric with adjacent metal ions bridged by the oxygen atom ferrocene couple was used as an internal reference.
at the 1-ring position. The 3,6-DBCat ligand appears unable 2 4]. 3,6-DBBQ (0.132 g, 0.6 mmol) and
to bridge metal ions due to the blocking effect of bulky [Mo(CO)6] (0.079 g, 0.3 mmol) were refluxed in toluene (50 mL) substituents adjacent to both oxygen atoms.9 Consequently, under N2 for 20 h. During this time the color of the solution turned complexes of Mo(VI) formed with the 3,6-DBCat ligand from the green color of 3,6-DBBQ to deep blue-violet. The solution should have different structural features and, potentially, was reduced in volume, and the products were separated by TLCon silica plates using hexane/CHCl3 (3:1) as eluent. At high Rf a (1) (a) Larson, M. L.; Moore, F. W. Inorg. Chem. 1966, 5, 801. (b)
deep blue product was obtained in 55% yield, which was subse- Cleland, W. E., Jr.; Barnhart, K. M.; Yamanouchi, K.; Collison, D.; quently identified as [{MoO(3,6-DBCat) } 2 4]. Other fractions ob- Mabbs, F. E.; Ortega, R. B.; Enemark, J. H. Inorg. Chem. 1987, 26,
tained included unreacted 3,6-DBBQ and [Mo(CO) 1017. (c) Chisholm, M. H.; Clark, D. L.; Errington, R. J.; Folting, K.; Huffman, J. C. Inorg. Chem. 1988, 27, 2059. (d) El-Hendawy, A. M.;
blue fraction that appeared to contain (from mass spectral analysis) Griffith, W. P.; O'Mahoney, C. A.; Williams, D. J. Polyhedron 1989,
{MoO(3,6-DBCat) } 2 n oligomers and [Mo(3,6-DBCat)3].
8, 519. (e) Kabanos, T. A.; Slawin, A. M. Z.; Williams, D. J.
Polyhedron 1992, 11, 995. (f) Albrecht, M.; Franklin, S. J.; Raymond,
[{MoO(3,6-DBCat) } 2 4]: 1H NMR (300 MHz, C6D6, ppm) 6.493 K. N. Inorg. Chem. 1994, 33, 5785. (g) Sinclair, L.; Mondal, J. U.;
(s, 2Hring), 1.290 (s, 9HBu), 1.206 (s, 9HBu); UV-vis (CHCl3, Uhrhammer, D.; Schultz, F. A. Inorg. Chim. Acta 1998, 278, 1.
λmax(nm)) 278 ( ) 10.7 × 103 M-1 cm-1), 320 (9.4 × 103 M-1 (2) Pierpont, C. G.; Buchanan, R. M. J. Am. Chem. Soc. 1975, 97, 4912.
(3) Pierpont, C. G.; Buchanan, R. M. J. Am. Chem. Soc. 1975, 97, 6450.
× 103 M-1 cm-1); IR (KBr, cm-1): 1729 (m), (4) Buchanan, R. M.; Pierpont, C. G. Inorg. Chem. 1979, 18, 1616.
1626 (m), 1581 (m), 1482 (m), 1462 (m), 1361 (w), 1339 (w), 1262 (5) (a) Wilshire, J. P.; Leon, L.; Bosserman, P.; Sawyer, D. T. J. Am. (vs), 1200 (m), 1096 (vs), 1024 (vs), 990 (m), 948 (w), 802 (vs), Chem. Soc. 1979, 101, 3379. (b) Wilshire, J. P.; Leon, L.; Bosserman,
757 (s); FAB+-MS (m/z) 2210 [{MoO(3,6-DBCat) } + P.; Sawyer, D. T.; Buchanan, R. M.; Pierpont, C. G. In ThirdInternational Conference on the Chemistry and Uses of Molybdenum; [MoO(3,6-DBCat) + Barry, H. F., Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann cis-[Mo(O-iPr)2(3,6-DBCat)2]. A sample of [{MoO(3,6-DB-
Arbor, 1979; p 264. (c) Wilshire, J. P.; Leon, L.; Bosserman, P.; 2 4] (250 mg) was dissolved in hexane (25 mL), and i-propanol Sawyer, D. T. In Molybdenum Chemistry of Biological Significance;Newton, W. E., Otsuka, S., Eds.; Plenum Press: New York, 1980; p (∼5 mL) was layered on top of the solution. As the i-propanol 327. (d) Lim, M.-C.; Sawyer, D. T. Inorg. Chem. 1982, 21, 2839. (e)
diffused through the solution, the color changed from the dark violet Sawyer, D. T.; Tsuchiya, T.; Po, H. N.; Pham, K. Q. In Fourth color of the tetramer to deep purple. The solution was reduced in International Conference on the Chemistry and Uses of Molybdenum;Barry, H. F., Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: volume under a flow of N2, and dark purple crystals of [Mo(O-i Golden, CO, 1982; p 107.
Pr)2(3,6-DBCat)2] formed in 75% yield.
(6) Cass, M. E.; Pierpont, C. G. Inorg, Chem. 1986, 25, 122.
(7) Pierpont, C. G.; Buchanan, R. M. Inorg. Chem. 1982, 21, 652.
(8) Prokof'ev, A. I.; Vol'eva, V. B.; Prokof'eva, T. I.; Bubnov, N. N.;
(10) Part of this research appeared earlier in the form of a communication.
Solodovnikov, S. P.; Erahov, V. V.; Kabachnik, M. I. Dokl. Akad. Liu, C.-M.; Restorp, P.; Nordlander, E.; Pierpont, C. G. Chem. Nauk SSSR (Engl.) 1988, 300, 179.
Commun. 2001, 2686.
(9) Lange, C. W.; Conklin, B. J.; Pierpont, C. G. Inorg. Chem. 1994, 33,
(11) Belostotskaya, I. S.; Komissarova, N. L.; Dzhuaryan, E. V.; Ershov, V. V. IsV. Akad. Nauk SSSR 1984, 1610.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2115
Liu et al.
[Mo(O-iPr)2(3,6-DBCat)2]: 1H NMR (300 MHz, CDCl3, ppm) a flow of N2, and crystals of [MoO(py-O)(3,6-DBCat)2] were 6.590 (s, 2Hring), 5.733 (p, 1HiPrO), 1.474 (s, 9HBu), 1.454 (s, 9HBu), obtained in 86% yield.
1.199 (m, 6HiPrO); UV-vis (CHCl3, λmax(nm)) 273 (17.9 × 103 [MoO(py-O)(3,6-DBCat)2]: 1H NMR (300 MHz, CDCl3, 20 °C, M-1cm-1), 443 (8.52 × 103), 616 (6.33 × 103); IR (KBr, cm-1): ppm) 8.766(d, 1HPy), 8.226(s, 1HPy), 7.955(t, 1HPy), 7.689(t, 1HPy), 1591 (w), 1487 (m), 1441 (w), 1385 (s), 1357 (m), 1331 (w),1316 7.285(s, 1HPy), 6.499 (s, 4Hring), 1.166 (s, 36HBu); UV-vis (CHCl3, (m), 1287 (w), 1202 (w), 1165 (w), 1127 (w), 1109 (s), 1100 (s), λmax(nm)) 275 (15.7 × 103 M-1 cm-1), 513 (2.0 × 103); IR (KBr, 1031 (w), 977 (vs), 964 (vs), 856 (s), 814 (m), 807 (m), 705 (s); cm-1) 1475 (s), 1392 (m), 1379 (w), 1358 (w), 1341 (w), 1242 (s), FAB+-MS (m/z) 656 [Mo(i-PrO) 2(3,6-DBCat)2 ], 554 [MoO(3,6- 1201 (m), 1172 (s), 983 (s), 929 (s), 814 (w), 804 (4), 723 (s), 715 (s); FAB+-MS (m/z) 649 [MoO(PyO)(3,6-DBCat)2 ], 554 [MoO- (3,6-DBCat) ].
2]. Method 1. 3,6-DBBQ (0.132
g, 0.6 mmol) and [Mo(CO) 6] (0.079 g, 0.3 mmol) were refluxed in 2][MoO(3,6-DBCat)2]. Cobaltocene (76 mg, 0.4 mmol)
toluene (50 mL) under N was dissolved in 50 mL of CH 2 for 20 h. The solution was cooled to 2Cl2, and added to [MoO(dmso)- room temperature and reduced in volume to approximately 30 mL, (3,6-DBCat)2] (142 mg, 0.2 mmol) dissolved in 30 mL CH2Cl2 and DMSO (2-3 mL) was added under N 2. The volume of the 2. The solution immediately turned red-orange. The solvent solution was reduced further under a flow of N was removed under a flow of N 2, and dark crystals 2, giving a dark brown precipitate.
of [MoO(dmso)(3,6-DBCat) The precipitate was redissolved in a minimum quantity of CH 2] formed in 72% yield. Crystals of the complex obtained by this procedure were found to be unsolvated.
and condensed in volume, and 5 mL of acetonitrile was layered on However, when obtained from a solution containing excess DMSO, the solution to promote crystallization. Orange crystals of [CoCp2]- crystals of the complex formed as the DMSO solvate, [MoO(dmso)- [MoO(3,6-DBCat)2] were obtained in 95% yield as a mixed Method 2. [{MoO(3,6-DBCat) }
2 4] (250 mg) was dissolved in EPR (CH2Cl2) 〈giso toluene (30 mL), and 1 mL of DMSO was added to the solution.
a(95,97Mo) ) 44 × 10-4 cm-1; UV-vis (CH2Cl2, λmax(nm)) 406 The solution was heated at reflux for 3 h, and reduced in volume.
(5.8 × 104 M-1 cm-1); IR (KBr, cm-1) 1625 (bw), 1492 (s), 1452 Crystals of [MoO(dmso)(3,6-DBCat) (m), 1411 (vs), 1401 (vs), 1384 (vs), 1365 (m), 1268 (m), 1248 2] were obtained in approxi- mately quantitative yield. However, if the solution containing (m), 1203 (w), 1142 (w), 1028 (w), 1011 (w), 988 (m), 954 (m), [{MoO(3,6-DBCat) } 926 (s), 863 (m), 812 (w), 793 (w), 712 (m); FAB--MS (m/z) 554 2 4] and DMSO was maintained at room tem- perature without heating, the reaction time for complete conversion [MoO(3,6-DBCat)2 ].
to [MoO(dmso)(3,6-DBCat) Crystal Structure Determinations. Intensity measurements
2] was greater than 1 month.
were made using either a Siemens P4 diffractometer ([{MoO(3,6- [MoO(dmso)(3,6-DBCat)2]: 1H NMR (300 MHz, CDCl3, 18 °C, 2 4], [MoO(dmso)(3,6-DBCat)2]‚DMSO, [CoCp2][MoO(3,6- ring), 2.925 (s, 6HDMSO), 1.215 (s, 36HBu); 1H NMR or a Siemens SMART CCD 2Cl2, -98 °C, ppm) 6.62 (1, Hring), 6.59 (1, Hring), ring), 6.45 (1, Hring), 3.05 (3, HDMSO), 2.81 (3, HDMSO), 2], [MoO(Ph3AsO)(3,6-DBCat)2]‚1.5C7H8). Descriptions of the Bu), 0.99 (9, HBu), 0.83 (9, HBu); UV-vis (CHCl3, procedures used for data collection, structure solution, and refine- λmax(nm)) 274 (12.8 × 103 M-1 cm-1), 456 (6.5 × 103); IR (KBr, ment are contained in CIFs, and a summary of crystallographic cm-1) 1630 (m), 1575 (m), 1482 (m), 1467 (w), 1392 (m), 1358 parameters is given in Table 1. Structure solution and refinement (m), 1273 (w), 1241 (w), 1200 (s), 1179 (w), 1125 (m), 1028 (s), was routine for all six structure determinations. All data were 984 (s), 952 (m), 925 (s), 812 (w), 802 (w), 716 (s); FAB+-MS collected using Mo KR radiation with a wavelength of 0.71073 Å.
(m/z) 632 [MoO(dmso)(3,6-DBCat) + 2 ], 554 [MoO(3,6-DBCat)2 ].
Discrepancy indices are defined as R ) ∑ Fo - Fc /∑ Fo and cis-[MoO(OAsPh3)(3,6-DBCat)2]. 3,6-DBBQ (0.132 g, 0.6
[∑w( Fo - Fc )2/∑w(Fo)2]1/2. In cases where there were mmol) and [Mo(CO)6] (0.079 g, 0.3 mmol) were refluxed in toluene solvate molecules, solvent atom locations were obtained from a (50 mL) under N2 for 20 h. The solution was cooled to room difference Fourier map calculated with phases from the complete temperature, and triphenylarsine oxide (97 mg, 0.3 mmol) dissolved complex molecule. All calculations were carried out using programs in 20 mL of toluene was added under N2. The color of the solution contained in the SHELXTL library of crystallographic computer slowly turned to a deep red-brown. The solvent was evaporated under a flow of N2, and crystals of [MoO(OAsPh3)(3,6-DBCat)2]were obtained in 80% yield as a toluene solvate.
[MoO(OAsPh3)(3,6-DBCat)2]: 1H NMR (300 MHz, CDCl3, 20 °C, ppm) 7.845(s, 3H The coordination chemistry of metals of the first transition Ph3AsO), 7.820(s, 3HPh3AsO), 7.654(t, 3HPh3AsO), series with quinone ligands has been frequently concerned Ph3AsO), 7.355(s, 3HPh3AsO), 6.499 (s, 4Hring), 1.166 (s, 36HBu); UV-vis (CHCl3, λmax(nm)) 271 (15.5 × 103 M-1 cm-1), with complexes containing radical o-semiquinonate ligands.12 352 (4.6 × 103), 480 (6.1 × 103); IR (KBr, cm-1) 1574 (w), 1483 In contrast, related complexes prepared with metals of the (m), 1463 (w), 1441 (s), 1384 (vs), 1358 (s), 1343 (m), 1311 (w), second and third transition series contain metal ions in high 1272 (w), 1245 (m), 1201 (m), 1181 (w), 1125 (w), 1087 (m), 1028 oxidation states coordinated by catecholate dianions.13 Com- (w), 997 (w), 985 (s), 916 (vs), 829 (s), 811 (s), 799 (m), 740 (s), plexes of the Cr, Mo, W triad were among the first group of 714 (s); FAB+-MS (m/z) 876 [MoO(OAsPh 3)(3,6-DBCat)2 ], 554 metals to show this pattern in charge distribution with ligands [MoO(3,6-DBCat) + derived from tetrachloro-1,2-benzoquinone (Cl4BQ). The cis-[MoO(py-O)(3,6-DBCat)2]. 3,6-DBBQ (0.132 g, 0.6 mmol)
neutral complex of chromium obtained by reacting Cl4BQ and [Mo(CO)6] (0.079 g, 0.3 mmol) were refluxed in toluene (50 with Cr(CO)6 has been shown to be [CrIII(Cl4SQ)3], and the mL) under N2 for 20 h. The solution was cooled to roomtemperature, and pyridine N-oxide (29 mg, 0.3 mmol) dissolved in (12) Pierpont, C. G.; Attia, A. S. Collect. Czech. Chem. Commun. 2001,
10 mL of toluene was added under N2. The color of the solution 66, 33.
slowly turned to a deep purple. The solvent was evaporated under (13) Pierpont, C. G. Coord. Chem. ReV. 2001, 219-221, 415.
2116 Inorganic Chemistry, Vol. 43, No. 6, 2004
Table 1. Crystallographic Data for [{Mo(µ-O)(3,6-DBCat) }
0.5CH2Cl2 0.5CH3CN (3,6-DBCat)2 1.5C7H8 R (Rw) temperature-dependent magnetic properties of the complexshow the effects of antiferromagnetic coupling between theS ) 3/2 metal ion and the three radical ligands.14 Relatedreactions carried out with [Mo(CO)6] or [W(CO)6] werefound to give the [{MVI(Cl 4Cat)3 2] (M ) Mo, W) dimers.15 Differences in the structural features of the quinone ligandsof [CrIII(Cl 4SQ)3] and the [{MVI(Cl4Cat)3 2] dimers pointed to the difference in charge distribution, and the differencesin the features of catecholate and semiquinonate ligands haveproven to be a reliable method of assigning charge distribu-tion in subsequent complexes prepared with quinone ligands.13The complexes of molybdenum were among the earliest well-characterized examples of catecholate coordination,16 andreactions between [Mo(CO)6] and benzoquinone in a hydro-carbon solvent have been found to be useful in providingcontrol of reaction conditions. However, the reaction carriedout with 3,5-di-tert-butyl-1,2-benzoquinone was found to Figure 1. View of the [{MoO(3,5-DBCat) }
2 2] dimer showing bridging show an unusual sensitivity to trace quantities of dioxygen interactions for the catecholate oxygens at the 1-ring position of 3,5-di-tert-butylcatechol (ref 4).
in the reaction medium.6 Reactions carried out underscrupulously oxygen-free conditions were found to form 1).4 This is the oxygen atom adjacent to a ring carbon atom initially [MoVI(3,5-DBCat)3], which reacted further with that has a C-H bond, rather than a bulky tert-butyl group, dioxygen to give the [{MoO(3,5-DBCat) } and this oxygen has been found to commonly bridge metal product has also been obtained by treating MoO3 or [MoO2- ions in its coordination to metals with the ligand in the form (acac)2] with 3,5-di-tert-butylcatechol, and the 3,5-DBCat of either a catecholate or a semiquinonate. To block bridging ligand has shown no tendency to give the [MoO2(Cat)2]2- interactions, and to eliminate the possibility of structural dianion formed commonly with catecholate ligands contain- isomers that may form with 3,5-DBCat, we have used 3,6- ing substituents that are less strongly electron releasing than di-tert-butyl-1,2-benzoquinone (3,6-DBBQ) in recent studies the tertiary butyl group.5 The extreme oxygen sensitivity of on Cat and SQ coordination.9,10 [Mo(3,5-DBCat)3] is in sharp contrast with the resistance of Reaction between [Mo(CO)6] and 3,6-Di-tert-butyl-1,2-
[{MoO(3,5-DBCat) } 2 2] toward the addition of a second oxo benzoquinone. The presence of tert-butyl substituents at ring
carbon atoms adjacent to both quinone oxygen atoms Structural characterization on [{MoO(3,5-DBCat) } eliminates the possibility of forming a dimer that is similar shown that the adjacent metal ions are bridged by oxygen in structure to [{MoO(3,5-DBCat) } 2 2]. Instead, it was thought atoms at the 1-ring position of the catecholate ligands (Figure that a 5-coordinate monomeric product, [MoO(3,6-DBCat)2],might form as the exclusive product. This monomer might (14) (a) Pierpont, C. G.; Downs, H. H. J. Am. Chem. Soc. 1976, 98, 4834.
(b) Buchanan, R. M.; Kessel, S. L.; Downs, H. H.; Pierpont, C. G.; be similar in structure to trigonal bipyramidal [MoO{OC- Hendrickson, D. N. J. Am. Chem. Soc. 1978, 100, 7894.
3)3 4] with an equatorial oxo ligand.17 1H NMR spectra (15) (a) Pierpont, C. G.; Downs, H. H. J. Am. Chem. Soc. 1975, 97, 2123.
(b) deLearie, L. A.; Haltiwanger, R. C.; Pierpont, C. G. Inorg. Chem.
1988, 27, 3842.
(17) Johnson, D. A.; Taylor, J. C.; Waugh, A. B. J. Inorg. Nucl. Chem.
(16) Tkachev, V. V.; Atovmyan, L. O. Koord. Khim. 1975, 1, 845.
1980, 42, 1271.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2117
Liu et al.
recorded on the crude dark blue-violet product obtained fromthe reaction between [Mo(CO)6] and 3,6-DBBQ showedmultiple resonances in the tert-butyl region indicating amixture of products. Chromatographic separation gave twodistinct fractions in roughly equal proportions. The fractionobtained at lower Rf appeared to consist of a mixture ofproducts (from 1H NMR). Mass spectral analysis (EI) of thisfraction provided Mo isotope patterns for two complex ions.
Peaks in the 552-554 mass region fit the pattern expectedfor the [MoO(3,6-DBCat)2]+ ion that might result fromfragmentation of [{MoO(3,6-DBCat) } 2 n] oligomers. A second major component of this fraction gave peaks in the 756-758 region that could be modeled as the [Mo(3,6-DBCat)3]+ion. From experience with related 3,5-DBBQ reactions, it isnot unreasonable to expect that [Mo(3,6-DBCat)3] would beformed as a product of the [Mo(CO)6]/3,6-DBBQ reaction.
1H NMR analysis on the fraction obtained at higher Rf gave a less complicated result. Two resonances appeared in Figure 2. View of the ΛΛΛΛ-[{Mo(µ-O)(3,6-DBCat)2 4] chiral square
down the tetragonal 4-fold axis.
the tert-butyl region at 0.947, 0.997 ppm, and one resonanceappeared in the aromatic region at 6.45 ppm, and mass Table 2. Bond Lengths (Å) and Angles (deg) for
spectral analysis also gave a pattern for the [MoO(3,6- [{Mo(µ-O)(3,6-DBCat)2 4], cis-[Mo(O-iPr)2(3,6-DBCat)2], and the[MoO(3,6-DBCat)2]- Aniona DBCat)2]+ ion. Slow evaporation of the solvent combinationused in the chromatographic separation gave single crystals of the complex obtained in this fraction.
2 4]. Preliminary investigation of crys-
tals obtained from the high R f fraction indicated that they formed in the tetragonal crystal system. Solution of the structure revealed that the complex molecule was located atthe intersection of 2-fold axes at a position of 4-fold symmetry in space group P4/nnc. The structure consists of one independent 3,6-DBCat ligand chelated to a Mo atom a Lengths and angles to O1 and O2 in the table are averaged values to located on one 2-fold axis. A crystallographic 2-fold axis O1, O4 and O2, O3 of {[MoO(3,6-DBCat)2]-} in Figure 10. Values to O3 passing through the metal generates a second 3,6-DBCat in the table are to apical oxygen O5 in Figure 10.
ligand chelated to the Mo. Crystallographic 4-fold symmetrygenerates three additional [Mo(3,6-DBCat) possible that adjacent Mo centers of opposite chirality may 2] fragments, with the four Mo centers forming corners of a perfect square.
link through oxo bridges, and this may lead to open Bridging oxo ligands are located on 2-fold axes directed oligomers obtained in the low Rf fraction from the reaction between the locations of two Mo atoms, linking metal atoms mixture. Condensation to form either the square tetramer or along sides of the square. A view of the tetramer is given in a linear oligomer points to the formation of the [MoO(3,6- Figure 2, and bond distances and angles are listed in Table DBCat)2] monomer as the initial product of the reaction between [Mo(CO)6] and 3,6-DBBQ in the presence of trace Each of the Mo centers, of local C quantities of dioxygen.
2 symmetry, is optically active, and the 4-fold symmetry of the square requires the same optical symmetry at each of the metal atoms of the zation of a crude sample of [{MoO(3,6-DBCat)2 4] from tetramer. Translational symmetry along the 4-fold axis hexane was observed to change color upon addition of a generates columns of squares of the same chirality in the small quantity of i-propanol to promote crystallization. The crystal structure. Crystallographic inversion centers between dark blue-violet hexane solution of [{MoO(3,6-DBCat)2 4] columns generate adjacent columns of opposite chirality.
became dark purple. Slow evaporation of the solvent gave Consequently, the crystal structure consists of parallel crystals of the purple product. Crystallographic characteriza- columns of ∆∆∆∆ and ΛΛΛΛ isomers of [{MoO(3,6- tion indicated that it was the bis(iso-propoxo) complex cis- [MoVI(O-iPr)2(3,6-DBCat)2]. A view of the molecule is 2 4] as shown in Figure 3 formed as the product of enantioselective self-assembly.18 The disposition of tert-butyl shown in Figure 4; bond distances and angles are given in groups about the upper surface of the square (Figure 2) Table 2. The coordination geometry and bond lengths to the appears to make adjacent centers of opposite chirality metal are similar to those of [{MoO(3,6-DBCat)2 4], but with sterically unfavorable in the closed structure. It is certainly propyl groups bonded to the oxygens that would bridge toadjacent metals in the tetramer. The formation of this productfrom a reaction that must take place by hydrolysis of the (18) Ziegler, M.; Davis, A. V.; Johnson, D. W.; Raymond, K. N. Angew. Chem., Int. Ed. 2003, 42, 665.
oxo ligand of [MoO(3,6-DBCat)2] is quite surprising. It 2118 Inorganic Chemistry, Vol. 43, No. 6, 2004
Figure 3. Parallel columns of ΛΛΛΛ-[{Mo(µ-O)(3,6-DBCat) }
2 4] and ∆∆∆∆-[{Mo(µ-O)(3,6-DBCat)2 4] tetramers in the crystal structure of [{Mo(µ- Studies on the Addition of Oxygen-Atom Donors to
[MoO(3,6-DBCat)2]. Catalytic oxygen-atom transfer reac-
tions involving Mo(VI) are associated with processes of
biological and industrial importance.19,20 Focusing specifically
on bioinorganic reactions catalyzed by molybdoenzymes of
the DMSO reductase family, it has been of interest to
investigate the coordination of substrate ligands that may
potentially serve as oxygen-atom donors to [MoO(3,6-
DBCat)2]. The Mo center of the dmso reductase class of
enzymes is coordinated by two bidentate pyranopterin
dithiolene ligands (S-S), and, at the outset of this project,
the metal was thought to oscillate between [MoVIO2(S-S)2]
and [MoIVO(S-S)2] species with oxidation (or reduction) of
substrate.21 Structural characterization on a reduced form of
the enzyme containing a dmso substrate molecule coordinated
Figure 4. View of cis-[Mo(O-iPr)2(3,6-DBCat)2]. The molecule is located
along a crystallographic 2-fold axis that bisects the angle formed by the
to the metal showed that addition occurred at a coordination isopropoxide ligands at the Mo.
site cis to the oxo ligand of the reduced metal.21a More recentstructural characterization has concluded that the oxo ligand implies that the oxo ligand coordinated to the Mo(VI) center thought to be present in the [MoIVO(S-S)2] species is an is a stronger base than the isopropoxide ion. This must occur artifact resulting from structural disorder of the Mo atom, as a consequence of strong σ and π donation from thecatecholate oxygen atoms. 1H NMR spectra recorded on the (19) Hille, R. Chem. ReV. 1996, 96, 2757.
(20) Sheldon, R. A. In Applied Homogeneous Catalysis with Organometallic
complex show two resonances in the tertiary butyl region, Compounds; Cornils, B., Hermann, W. A., Eds.; VCH Publishers: consistent with the cis structure of the molecule, indicating Weinheim, 1996; p 411.
(21) (a) McAlpine, A. S.; McEwan, A. G.; Bailey, S. J. Mol. Biol. 1998,
that the complex is stereochemically rigid in solution at room 275, 613. (b) George, G. N.; Hilton, J.; Temple, C.; Prince, R. C.; Rajagopalan, K. V. J. Am. Chem. Soc. 1999, 121, 1256.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2119
Liu et al.
and that changes at the active site involve [MoVIO(S-S)2(O-ser)] and [MoIV(S-S)2(O-ser)] species.22 The results ofoxidized substrate addition reactions described below, nev-ertheless, remain pertinent to interests in oxo transferchemistry to Mo(VI).
With the potential stability of the [MoVIO(3,6-DBCat)2] monomer in dilute solutions, it became of interest to see ifreductively induced oxo transfer to the metal might beobserved in a process that does not require a change inoxidation state at the Mo (eq 4). The product of the reaction MoVIO(3,6-DBCat) + O-Subst + 2e [cis-MoVIO (3,6-DBCat) ]2- + Subst (4) would be a cis-dioxomolybdenum(VI) species of the type Figure 5. View showing the cis coordination of dmso in cis-[MoO(dmso)-
found commonly in the coordination chemistry of molyb- denum with catecholate ligands that do not contain two tert- Table 3. Selected Bond Lengths (Å) and Angles (deg) for the Products
butyl substituents.23 of Substrate Oxide Addition to [MoO(3,6-DBCat)2] cis-[MoO(dmso)(3,6-DBCat)2]. The addition of dmso to
a dilute, freshly prepared solution of [MoO(3,6-DBCat)2] gave an immediate color change to that of the addition 2]. Addition of dmso to a more concentrated solution containing a mixture of {MoO- 2 n oligomers, including the [{MoO(3,6-DB- 2 4] tetramer, required hours for complete conversion to the addition product. When isolated in crystalline form from toluene or hexane solutions, the complex is unsolvated.
However, samples obtained from a solution containing excess dmso form as the solvate, [MoO(dmso)(3,6-DBCat)2]‚DMSO. Crystalline samples of the complex in both the solvated and unsolvated forms have been investigated crystallographically.10 In both structure determinations fea- tures of the complex molecule are the same. The resultobtained for the dmso solvate is of slightly higher precision ring and tert-butyl protons of the 3,6-DBCat ligands and a and will be described herein. A view of the dmso addition third for the dmso ligand. The stereodynamic effect respon- product is shown in Figure 5. The coordinated dmso sible for time-averaging proton resonances will be described molecule is bonded to the Mo at a site cis to the oxo ligand.
Selected bond lengths and angles are given for [MoO(dmso)- cis-[MoO(py-O)(3,6-DBCat)2]. The addition of pyridine
N-oxide to [MoO(3,6-DBCat) 2]‚DMSO in Table 3.
2] is accompanied by a change It is perhaps surprising that, as a relatively weak donor, in color of the solution to a deep purple. Slow evaporation the dmso ligand bonds at a site adjacent to the oxo ligand, of a toluene solution of the addition product gave crystals with a catecholate oxygen occupying the coordination site suitable for crystallographic analysis. A view of the [MoO- trans to the oxo oxygen. The catecholate oxygens are strong (py-O)(3,6-DBCat)2] molecule is shown in Figure 6, and σ and π donors, as is the oxo oxygen. The structure is selected bond lengths are given in Table 3. As in the case significant in placing the dmso oxygen at a site poised to of [MoO(dmso)(3,6-DBCat)2], the pyridine N-oxide ligand give a cis-dioxomolybdenum(VI) species with release of is coordinated at a site cis to the terminal oxo ligand. Metrical dimethyl sulfide upon two-electron reduction (eq 4).24 At values to the metal are similar to the dmso addition product, room temperature the 1H NMR spectrum of [MoO(dmso)- and general structural features of the inner coordination sphere are similar for the two compounds. 1H NMR spectra 2] consists of three resonances: one each for the recorded at room temperature show five separate resonances (22) (a) Li, H.-K.; Temple, C.; Rajagopalan, K. V.; Schindelin, H. J. Am. for the ring protons of the pyridine ring, but the catecholate Chem. Soc. 2000, 122, 7673. (b) Bray, R. C.; Adams, B.; Smith, A.
ring and tert-butyl protons appear as single, time averaged T.; Benett, B.; Bailey, S. Biochemistry 2000, 39, 11258.
(23) (a) Kustin, K.; Liu, S.-T. J. Am. Chem. Soc. 1973, 95, 2487. (b)
peaks at 6.50 and 1.166 ppm as the result of a dynamic Charney, L. M.; Finklea, H. O.; Schultz, F. A. Inorg. Chem. 1982,
exchange process.
21, 549. (c) Griffith, W. P.; Nogueira, H. I. S.; Parkin, B. C.; Sheppard, R. N.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans.
3)(3,6-DBCat)2]. Addition of either tri-
1995, 1775. (d) Duhme, A.-K.; Dauter, Z.; Hider, R. C.; Pohl, S. Inorg.
phenylphosphine oxide or triphenylarsine oxide to a solution Chem. 1996, 35, 3059. (e) Duhme, A.-K.; Davies, S. C.; Hughes, D.
containing [MoO(3,6-DBCat)2] resulted in a color change L. Inorg. Chem. 1998, 37, 5380.
(24) Thapper, A.; Deeth, R. J.; Nordlander, E. Inorg. Chem. 2002, 41, 6695.
to dark red-brown as an indication of immediate adduct 2120 Inorganic Chemistry, Vol. 43, No. 6, 2004
to 90° with O1. It is further surprising that in all threestructures the O-Subst ligand bonds selectively at a site cisto the oxo ligand. Since monooxomolybdenum(VI) speciesare unusual, and there appears to be no structural charac-terization on addition products to five-coordinate [MoVIO-(L)4] complexes, there are no relevant examples for com-parison. Seven-coordinate peroxo(monooxo)molybdenum(VI)complexes are common, and the peroxo group is generallycoordinated at sites cis to the oxo ligand. Five-coordinate[MoVOL4] species have distorted square pyramidal geom-etries, and addition of weakly coordinating ligands occursat the site trans to the ModO bond. Examples characterizedstructurally include trans-[MoVOCl4(H2O)]-, trans-[MoV-OCl4(CH3CN)]-, and trans-[MoVO(Cl4Cat)2(thf)]-.25,26 Forthe d0 metals of the [MoO(O-subst)(3,6-DBCat)2] series,electrostatic repulsions resulting from the strong concentra-tion of π electron density in the Mo-OOxo and Mo-OCat Figure 6. View showing the cis coordination of pyridine N-oxide in cis-
bonds contribute to making the cis geometry preferred over the trans structure. Rotation of one catecholate ligand out ofthe tetragonal plane of the trans isomer moves the pπ-orbitalsof the catecholate oxygens away from the Mo-OOxo bond.
As a consequence, the O1-Mo-O6 angles in Table 3 arecloser to 90° than the angles to O3 and O4. Further, the Modxy orbital of the cis isomer is in position to participate inπ-bonding with catecholate oxygen O6 as a contributingelectronic interaction.
Subtle features of the three complex molecules deserve comment. Structural characterization on the dmso reductaseMo center with the dmso molecule coordinated cis to themonooxomolybdenum(IV) center has shown a relatively longS-O bond length of 1.7 Å for the bound dmso.21a The S-Olength for [MoO(dmso)(3,6-DBCat)2] is more normal witha value of 1.565(2) Å.
In previous structure determinations on complexes con- taining quinone ligands the ligand C-O length has been Figure 7. View showing the cis coordination of triphenylarsine oxide in
viewed as diagnostic of charge, with a value of 1.34 Å considered to be normal for a catecholate ligand. In fact, formation. Slow evaporation of solvent gave crystals of values for catecholate ligands vary within the range of lengths from 1.32 to 1.37 Å.27 This has been assumed to be random, 3)(3,6-DBCat)2] that were suitable for crystal- lographic characterization. As in the case of the dmso and but the pattern of C-O lengths found for the 3,6-DBCat pyridine N-oxide addition products the OAsPh ligands of the [MoO(O-subst)(3,6-DBCat) 2] series reveals a found to be located cis to the oxo ligand. A view of cis- pattern. The shortest ligand C-O length in all three structures (Table 3) appears for the oxygen (O5) trans to the oxo ligand, is given in Figure 7, and selected bond lengths and angles are given in Table 3 with and the longest ligand C-O lengths are found for the corresponding values for the other addition products. At room oxygens (O3, O4) of the catecholate chelated in the plane temperature, the ring and tert-butyl proton resonances appear cis to the oxo ligand. It is of interest that the shortest ligand as broadened single resonances at 6.499 and 1.166 ppm, C-O length is found for the most weakly coordinated ligand.
This is contrary to the view that strong π donation by Structural Features of the O-Substrate Addition Prod-
catecholate oxygens leads to short C-O bond lengths, and ucts. Metrical values for the dmso, O-py, and OAsPh
weak donation leads to lengthened C-O bonds.28 In the addition products are given in Table 3. It is generally foundthat the concentration of charge in the multiple molybdenum- (25) (a) Garner, C. D.; Hill, L. H.; Mabbs, F. E.; McFadden, D. L.; McPhail, A. T. J. Chem. Soc., Dalton Trans. 1977, 1202. (b) Weller, F.; Muller,
oxo bond results in angles to cis ligands that are greater than U.; Weiher, U.; Dehnicke, K. Z. Anorg. Allg. Chem. 1980, 460, 191.
90° and typically greater than 100°. This is found to be true (26) Manuscript in preparation.
(27) (a) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermu¨ller, for the bond angles between oxo O1 in Figures 5-7 and the T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213. (b) Carugo,
oxygen atoms of the catecholate ligand chelated at cis O.; Castellani, C. B.; Djinovic, K.; Rizzi, M. J. Chem. Soc., Dalton
Trans. 1992, 837.
coordination sites (O3, O4). However, the oxygen (O2) of (28) Pattison, D. I.; Levina, A.; Davies, M. J.; Lay, P. A. Inorg. Chem.
the O-Subst ligand in all three structures forms an angle close 2001, 40, 214.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2121
Liu et al.
Figure 8. Temperature dependence of the 1H NMR spectrum of cis-[MoO(dmso)(3,6-DBCat)2]‚DMSO recorded in CD2Cl2 solution.
present case it appears that the short C-O length is a pattern at -30 °C has been confirmed with the aid of a consequence of the weak σ bond.
COSY spectrum and with spectrum simulation. At 18 °C Stereodynamic Property of [MoO(dmso)(3,6-DBCat)
the two components of the AB spectrum are observed to 1H NMR spectra recorded on all three O-Subst addition have coalesced to give a single resonance at 6.56 ppm. The products at room temperature gave single resonances for both four resonances observed for the tert-butyl protons have the ring and tert-butyl protons of the 3,6-DBCat ligands. This coalesced to give two peaks at -30 °C at 1.13 and 1.20 ppm, might be interpreted as indicating a trans orientation for the but at 18 °C these resonances appear as a single peak at O-Subst ligands, but the crystallographic results clearly 1.17 ppm. The dmso resonances coalesce to give a single indicate otherwise. Spectra recorded over the temperature peak at the temperature (-30 °C) where the AB pattern range between +18 °C and -98 °C for [MoO(dmso)(3,6- appears for the ring protons. Spectra were recorded on the sample of [MoO(dmso)(3,6-DBCat) 2] at 500 MHz, shown in Figure 8, reveal that the 2] isolated as the dmso apparent equivalence of catecholate ring and tert-butyl solvate used for crystallographic characterization. Conse- protons is the result of dynamic site exchange. The spectrum quently, a sharp dmso resonance for free dmso in solution at -98 °C consists of four resonances for the ring protons was observed unchanged throughout the temperature range at 6.44, 6.54, and two, slightly overlapped resonances, at 6.62, and 6.64 ppm. All four ring proton resonances are split The dynamic process that averages proton resonances is by coupling of approximately 7.5 Hz. tert-Butyl proton nondissociative since there is no indication of dmso exchange resonances appear at -98 °C as two well-separated peaks with the free solvent. Further, changes in the three spectral at 0.85 and 1.02 ppm, and two overlapped resonances at 1.32 regions (ring, tert-butyl, dmso) occur around the position of ppm. Proton resonances for the dmso ligand appear as a the coalesced resonance at 18 °C. This points to a simple single peak at 18 °C (2.94 ppm), but split to give two exchange process that interchanges environmental regions resonances (2.82, 3.05 ppm) at -98 °C.
of the [MoO(dmso)(3,6-DBCat)2] molecule with a rate that The spectrum obtained at -98 °C is consistent with the increases with increasing temperature. The intermediacy of geometry of [MoO(dmso)(3,6-DBCat) a trans isomer for oxo and dmso ligands would result in a 2] obtained from the solid state structure. Overlapped resonances arise from a completely different chemical shift for equivalent ring and similarity in magnetic environment, and, from the geometry tert-butyl protons, and an intermediate structure of this type of the molecule shown in Figure 5, this likely occurs for the can be eliminated. The AB spectrum of the intermediate regions of the catecholate ligands associated with the trans eliminates the intermediacy of a trigonal prismatic structure catecholate oxygens O4 and O6. As the temperature of the with catecholate ligands chelated along trigonal faces, as the solution is increased from -98 °C, peak coalescence takes product of a Ray-Dutt twist. The appearance of an AB place. Restricted rotation results in broadened tert-butyl spectrum for the ring protons at -30 °C points uniquely to resonances, but the pattern of coalescence remains clear for the trigonal prismatic intermediate (I) that results from a
peaks of the ring protons. The two well-separated resonances trigonal (Bailar) twist.29 Constraints on the chelated catecho- at 6.44 and 6.54 ppm merge to give one half of a classic late ligands retain the cis disposition of oxo and dmso AB pattern at -30 °C; the other half of the pattern is ligands. In the equilibrium, catecholate oxygens are distrib- associated with merged resonances of the protons that were uted equally over sites trans to the oxo and dmso ligands slightly overlapped at -98 °C (Figure 8). The AB coupling (29) Rodger, A.; Johnson, B. F. G. Inorg. Chem. 1988, 27, 3062.
2122 Inorganic Chemistry, Vol. 43, No. 6, 2004
dianion that is generally obtained as a hydrolysis product.30This color is associated with an intense transition in the 400-410 nm region. The product of cobaltocene reduction wasobserved to have a strong transition at 406 nm with anextinction coefficient of 5.8 × 104 M-1 cm-1. Crystalssuitable for crystallographic characterization were obtainedby slow evaporation of a dichloromethane/acetonitrile solu- and over sites containing catecholate oxygens trans to one tion of the crude reduction product. The compound obtained another. At higher temperature the rotation rate about the by this procedure was found to form as a mixed CH2Cl2/ trigonal axis is increased, increasing the rate of racemization, CH3CN solvate of (CoCp2)[MoVO(3,6-DBCat)2]. A view of and increasing the exchange rate of ring and tert-butyl the complex anion is shown in Figure 10; selected bond protons among the different coordination positions.
lengths and angles are listed in Table 2. The coordination Electrochemical Characterization on MoO(dmso)(3,6-
geometry is square pyramidal with a clear distortion of the DBCat)2 and the Potential for Coordinated O-Subst
cis catecholate oxygens away from the apical oxo ligand to Reduction. Structural features of the cis-[MoO(O-Subst)-
give an average O1-Mo-OCat bond angle of 110°. The (3,6-DBCat)2] series place the oxygen atom of the oxidized reduction reaction has been followed using EPR. A signal O-Subst ligand at the site that would give a cis-dioxomo- characteristic of the [MoVO(3,6-DBCat)2]- anion was ob- lybdenum(VI) species upon reduction with two electrons and served to appear upon addition of cobaltocene to a CH2Cl2 release of a reduced Subst species.
solution of [MoO(dmso)(3,6-DBCat)2]. It tentatively appearsthat one-electron reduction of [MoO(dmso)(3,6-DBCat)2]leads to dissociation of dmso with formation of [MoVO(3,6-DBCat)2]-. Repeated scans over the potential range +1.0 Vto -1.5 V led to the appearance of a reduction peak at -0.4 The result of a full cyclic voltammetric scan over the V that is associated with the [MoVO(3,6-DBCat)2]- anion.
potential range +1.0 to -2.0 V, referenced to the Fc/Fc+ Holm and co-workers have studied the addition of [SiCl(t- potential in dichloromethane, is shown in Figure 9 for [MoO- Bu)(Ph)2] to cis-[MoVIO2(bdt)2]2- in a reaction that is related (dmso)(3,6-DBCat)2]. Two prominent redox processes appear (in the reverse direction) to the reaction that we had hoped with a number of low-current events that are associated with to observe upon reduction of [MoO(dmso)(3,6-DBCat)2].31 products of irreversible oxidation. Coupled oxidation and Electrochemical characterization on cis-[MoVIO(OSi(t-Bu)- reduction processes appear centered at +0.455 V (vs Fc/ Ph2)(bdt)2] was observed to result in formation of [MoVO- Fc+) with ∆E of 123 mV. Since the catecholate ligands are (bdt)2]- in repeated scans. The decomposition reaction was the only reduced centers of the neutral molecule, this couple attributed to proton sources in the solution, but there is is most reasonably a Cat/SQ redox process. It appears at an similarity to the formation of [MoVO(3,6-DBCat)2]- from unusually positive potential as the result of strong π donation to the d0 metal ion. The second prominent electrochemicalfeature that appears in the scan is an irreversible process at -0.852 V (vs Fc/Fc+). Current measurements have shown The reaction between [Mo(CO)6] and 3,6-di-tert-butyl-1,2- that this is a two-electron process. The oxidized center of benzoquinone, carried out in the presence of trace quantities the neutral molecule is the Mo(VI) ion, and it is reasonable of dioxygen, gives oligomers of {MoVIO(3,6-DBCat) } to assign this reduction as two-electron reduction of the is presumed that the product formed initially is monomeric metal. The irreversibility of the process is of interest as it [MoO(3,6-DBCat)2] with a TBP geometry similar to [MoO- may be associated with the release of dimethyl sulfide from 3)3 4]. Condensation of monomers through bridging the coordinated dmso ligand. The Mo complex that would oxo ligands gives linear oligomers with adjacent Mo centers be obtained from this process would be the cis-[MoVIO2- of mixed optical stereochemistry, or the cyclic chiral [{Mo- (3,6-DBCat)2]2- dianion, an ion that is well-known to form 2 4] square with four Mo centers of the with a variety of other catecholate ligands.22 To investigate same chirality. Condensation through the oxo ligands, this possibility, chemical reduction of [MoO(dmso)(3,6- together with the observed solvolysis reaction with i-PrOH DBCat)2] has been carried out using cobaltocene as a to give cis-[Mo(O-iPr)2(3,6-DBCat)2], is a consequence of a reducing agent.
weakened ModO bond due to strong π donation by the Reduction of [MoO(dmso)(3,6-DBCat)2] with Cobal-
catecholate oxygens. Addition of oxidized oxo-substrate tocene. A dichloromethane solution of cobaltocene was
ligands to [MoO(3,6-DBCat)2] occurs selectively at a coor- added to a solution containing [MoO(dmso)(3,6-DBCat)2] dination site cis to the oxo ligand. Characterization of the under an atmosphere of dry N2. The color of the solution stereodynamic behavior of cis-[MoO(dmso)(3,6-DBCat)2] in rapidly turned from the deep violet color of the dmsocomplex to bright orange. Catechols have been used as (30) (a) Page, W. J.; von Tigerstrom, M. J. Bacteriol. 1982, 151, 237. (b)
analytical indicators for the presence of molybdenum due Duhme, A.-K.; Hider, R. C.; Naldrett, M. J.; Pau, R. N. J. Biol. Inorg.
Chem. 1998, 3, 520.
to their ability to hydrolyze strongly bound oxo ligands (eq (31) Donahue, J. P.; Goldsmith, C. R.; Nadiminti, U.; Holm, R. H. J. Am. 1), and to the orange-red color of the cis-[MoO2(Cat)2]2- Chem. Soc. 1998, 120, 12869.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2123
Liu et al.
Figure 9. Cyclic voltammogram on [MoO(dmso)(3,6-DBCat)2] measured in CH2Cl2 solution at a scan rate of 100 mV/s.
from Mo(VI) to Mo(IV). Two electrons would be requiredfor the formation of a cis-[MoVIO2(3,6-DBCat)2]2- dianionfrom [MoVIO(O-SubstOx)(3,6-DBCat)2] (O-Subst ) dmso,Opy, OAsPh3). Experimentally, either O-SubstOx dissociationfrom the Mo(V) species formed upon one-electron reductionor reactivity of oxo ligands of the cis-dioxomolybdenum-(VI) product formed by two-electron reduction has precludedobservation of SubstRed displacement.
Acknowledgment. Research at Lund University was
supported by the Swedish Research Council (VR). Researchat the University of Colorado was supported by the National Figure 10. View of the [MoO(3,6-DBCat)2]- anion obtained by one-
electron reduction of [MoO(dmso)(3,6-DBCat)2] with cobaltocene.
Science Foundation. C.G.P. would like to thank The SwedishFoundation for International Cooperation in Research (STINT) dichloromethane showed that site exchange proceeds through for a research fellowship.
a trigonal prismatic intermediate, with no evidence for astructure with dmso coordinated trans to the oxo ligand. With Supporting Information Available: X-ray crystallographic files
dmso and the other O-subst ligands coordinated cis to the in CIF format for the structure determinations of the 6 compounds oxo ligand, the cis-[MoO(O-SubstOx)] unit is poised to form in Table 1. This material is available free of charge via the Internet 2 species with displacement of SubstRed. Enzy- matic Mo centers release SubstRed with reduction of the metal 2124 Inorganic Chemistry, Vol. 43, No. 6, 2004

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