TUNGSTEN(VI) HEXAHYDRIDE COMPLEXES SUPPORTED BY CHELATING TRIPHOSPHINE LIGANDS - PROTONATION TO GIVE ETA-2-DIHYDROGEN COMPLEXES AND CATALYTIC DEHYDROGENATION OF CYCLOOCTANE TO CYCLOOCTENE

Reactions Of WCl4(PPh3)2.CH2Cl2 with the chelating triphosphine (triph os) ligands PPh(CH2CH2PPh2)2 (PP2), PPh(C6H4-o-PPh2)2 (TP), and MeC(CH 2PPh2)3 (P3) in refluxing benzene or toluene give WCl4(triPhos) (triph os = PP2 (1), TP (2), P3 (3)). Treatment of 1-3 with LiAlH4 in Et2O at room temperature followed by hydrolysis in THF at 0-degrees-C affords WH6(triphos) (triphos = PP2 (4), TP (5), P3 (6)), which are the first tungsten polyhydride complexes supported by a chelating triphosphine ligand. Variable-temperature H-1 NMR spectra and T1 data of 4-6 are co nsistent with the formulation of them as classical hexahydride complex es containing no eta2-H-2 ligands. Reaction of 4 with Ph3SiH in reflux ing THF gives the rare silyl polyhydide complex WH5(SiPh3)(PP2)(7). Pr otonation of 4-6 with HBF4-OEt2 in CD2Cl2 at 193 K affords the cationi c nonclassical eta2-H-2 complexes [WH7-2x(eta2-H-2)x(triphos)]+ (triph os = PP2 (8), TP (9), P3 (10); X = 1-3). Deprotonation of 8-10 with NE t3 regenerates the parent hexahydrides 4-6 quantitatively. The variabl e-temperature H-1 NMR T1 data for the hydride resonances of 8-10 are c onsistent with the nonclassical eta2-H-2 coordination. The alternative formulation of 8-10 as classical heptahydride complexes (i.e., [WH7(t riphos)]+) containing no eta2-H-2 ligand can be ruled out because it w ould exceed the maximum oxidation state and coordination number of tun gsten. In the presence of tert-butylethylene as a hydrogen acceptor, c omplexes 4 and ReH5(PP2) (11) are active catalysts for the thermal deh ydrogenation of cyclooctane to cyclooctene, whereas their analogues co ntaining monodentate phosphine ligands are inactive under similar cond itions.