Keith S. Kramer

Mo2(O)(OCH2tBu)4(PMe3)4. Preparation and characterization of a partial hydrolysis product of Mo2(OCH2tBu)6

Abstract Partial hydrolysis of Mo 2 (OCH 2 t Bu) 6 in the presence of excess trimethylphosphine produced the novel compound Mo 2 (PMe 3 )( μ -O)( μ -OCH 2 t Bu) (OCH 2 t Bu) 3 ( 1 ). The solid-state molecular structure of 1 as determined by single crystal X-ray diffraction revealed an MoMo double bond [2.4931(9) A] between the metal centers which are in local octahedral and trigonal bi-pyramidal coordination geometries, respectively. The complex possesses an asymmetric oxo bridge [1.996(4) A and 1.874(4) A] as well as a bridging alkoxide ligand. Based on the coordination geometries, as well as the metal-metal separation, it is proposed to contain metal centers with formal oxidation states Mo(4+) ( d 2 ) and Mo(2+) ( d 4 ). In solution 2 is evidently non-fluxional on the NMR time-scale and maintains a geometry akin to that seen in the solid state. Compound 1 reacted with carbon monoxide (1 equiv.) giving a 1:1 adduct, 2 , wherein the d 4 -Mo(2+) center is ligated by CO. Thermolysis of 2 does not yield oxidation products, i.e. Me 3 PO or CO 2 , but does show evidence for the formation of (PMe 3 ) 4 Mo(CO) 2 along with other uncharacterized molybdenum alkoxide products. While 2 may be formally derived from the oxidative cleavage of CO 2 by reaction with Mo 2 (OCH 2 t Bu) 4 (PMe 3 ) 4 , the latter inserts CO 2 into the MoOR bonds, as does Mo 2 (O i Pr) 4 (dmpe) 2 , where dmpe = Me 2 PCH 2 CH 2 PMe 2 .

Cationic d3-d3 dinuclear compounds of tungsten: [W2(O2CBut)5]+X−, where X−=BF4− and CF3SO3−

Abstract Ditungsten hexapivalate, W 2 (O 2 CBu t ) 6 , and triethyloxonium tetrafluoroborate, Et 3 O + BF 4 − , react in CH 2 Cl 2 at room temperature to yield the 1:1 complex [W 2 (O 2 CBu t ) 5 ][BF 4 ] ( 1 ), as a yellow crystalline solid. A similar reaction involving trimethylsilyltrifluoromethane-sulfonate, TMS-OTf, yields [W 2 (O 2 CBu t ) 5 ][OTf] ( 2 ). Compounds 1 and 2 are 1:1 electrolytes in CH 3 CN but show ion pairing in THF and CH 2 Cl 2 solutions with the degree of ion pairing being greater for 2 than for 1 . Only 1 is soluble in toluene. 1 H NMR spectroscopy shows there are 3 types of O 2 CBu t ligands in the integral ratio 1:2:2 which is consistent with the structure found in the solid state for 1 which has a (WW) 6+ unit, WW=2.28(1) A supported by three bridging pivalates and two chelating η 2 -O 2 CBu t ligands. The local geometry about each tungsten is a distorted pentagonal pyramid, WWO 5 , with relatively weak axial interactions ( trans to the WW bond) to the BF 4 − counter anions, WF=2.50(1) A (av.). This leads to an infinite polymeric structure in the solid state. Crystal data for (W 2 (O 2 CBu t ) 5 ][BF 4 ] at −171 °C: a =29.44(1), b =36.05(1), c =20.80(1) A, β=133.30(1)°, Z =16, D calc =1.82 g cm −3 and space group C 2/ c .

[W4(O)2)(O-i-Pr)8]2 an unusual octanuclear oxo-alkoxide cluster of tungsten

From the reaction between W4(p-tolyl)2(O-i-Pr)10 and H2, in hydrocarbon solvents, the compound [W4(O)2(O-i-Pr)8]2, 1, has been isolated in ca. 25% yield. Formation of 1 is traced to the decomposition of a compound of formula [W4O-i-Pr)10]n, which has been characterized only by1H NMR spectroscopy. The latter is formed by the stepwise hydrogenolysis of W4(p-tolyl)2(O-i-Pr)10 + H2 → W4(H)(p-tolyl)(O-i-Pr)10 + toluene; W4(H)(p-tolyl)(O-i-Pr)10 → [W4(O-i-Pr)10]n + toluene. In the presence of a small amount of H2O2 the previously characterized compound W4(O)(O-i-Pr)10, 2, is formed. The structure of 1 consists of two pseudo W4 tetrahedra linked through the agency of a pair of alkoxide bridges that bind two tungsten atoms that in turn cap triangles, W3(O)2(O-i-Pr)7, of tungsten atoms. At −176°C the cell dimensions werea=12.600(3),b=14.722(3),c=12.585(2),β=119.77(1),Z=1, anddcalcd = 2.240 g cm−3 in the space group P1¯. The W-W distances within these triangles are ca. 2.9 Å whereas the capping W-W atom distances are ca. 2.5 Å. The1H NMR spectrum of 1 in benzene-d6 is consistent with the maintenance of the solid-state structure in solution.

Site-selective hydrogenolysis, hydrogenation and alcoholysis involving the homometallic cluster [W6H5(CPri)(OPri)12]

The octahedral W6 cluster [W6(µ-H)4H(µ-CPri(µ-OPri)5] 1 reacts with D2 to give D for H exchange, reacts reversibly with ethene, and in the presence of H2 yields ethane by exclusive use of the terminal W–H bond; 1 also reacts with (CD3)2CDOD to give selective alcoholysis of one terminal OR group.