Ditungsten siloxide hydrides, [(silox)(2)WHn](2) (n=1,2; silox equals (t)BuSiO), and related complexes

The addition of 4.0 equiv of Na(silox) to Na[W2Cl7(THF)(5)] afforded (silox)(2)ClW=WCl(silox)(2) (1, 65%). Treatment of 1 with 2.0 equiv of McMgBr in Et(2)O provided (silox)(2)MeW=WMe(silox)(2) (2, 81%). In the presence of 1 atm of H-2, reduction of 1 with 2.0 equiv of Na/Hg in DME provided (silox)(2)HW=WH(silox)(2) (3, 70%), characterized by a hydride resonance at delta 19.69 (J(WH) = 325 Hz, H-1 NMR). Exposure of 2 to 1 atm of H-2 yielded 3 and CH4 via (silox)(2)HW=WMe(silox)(2) (4); use of D-2 led to [(silox)(2)WD](2) (3-d(2)). Exposure of 3 to ethylene (similar to 1 atm, 25 degrees C) in hexanes generated (silox)(2)EtW=WEt(silox)(2) (5), but solutions of 5 reverted to 3 and free C2H4 upon standing. NMR spectral data are consistent with a sterically locked, gauche, C-2 symmetry for 1-5. Thermolysis of 3 at 100 degrees C (4 h) resulted in partial conversion to (silox)(2)HW=W(OSi(t)Bu(2)CMe(2)CH(2))(silox) (6a, similar to 60%) and free H-2, while extended thermolysis with degassing (5 d, 70 degrees C) produced a second cyclometalated rotational isomer, 6b (6a:6b similar to 3:1). When left at 25 degrees C (4 h) in sealed NMR tubes, 6 and free H-2 regenerated 3. Reduction of 1 with 2.0 equiv of Na/Hg in DME also afforded 6a (25%). When 3 was exposed to similar to 3 atm of H-2, equilibrium amounts of [(silox)(2)WH2](2) (7) were observed by H-1 NMR spectroscopy (3 + H-2 reversible arrow 7; 25.9-88.7 degrees C, Delta H = -9.6(4) kcal/mol, Delta S = -21(2) eu). Benzene solutions of 3 and 1-3 atm of D-2 revealed incorporation of deuterium into the silox ligands, presumably via intermediate 6. In sealed tubes containing [(silox)(2)WCl](2) (1) and dihydrogen (1-3 atm), H-1 NMR spectral evidence for [(silox)(2)WCl](2)(mu-H)(2) (8) was obtained, suggesting that formation of 3 from 1 proceeded via reduction of 8. Alternatively, 3 may be formed from direct reduction of 1 to give [(silox)(2)W](2) (9), followed by H-2 addition. Hydride chemical shifts for 7 are temperature dependent, varying from delta 1.39 (-70 degrees C, toluene-d(8)), to delta 3.68 (90 degrees C). Si-29{H-1} NMR spectra revealed a similar temperature dependence of the silox (delta 12.43, -60 degrees C, to delta 13.64, 45 degrees C) resonances. These effects may arise from thermal population of a low-lying, delta delta*, paramagnetic excited state of D-2d [(silox)(2)W](2)(mu-H)(4) (Delta E similar to 2.1 kcal/mol, (chi)(7a*) similar to 0.03), an explanation favored over thermal equilibration with an energetically similar but structurally distinct isomer (e.g., [(silox)(2)WH2](2)(mu-H)(2), Delta G degrees similar to 0.69 kcal/mol, (chi)(7b) similar to 0.25) on the basis of spectral arguments. Extended Huckel and ab initio molecular orbital calculations on model complexes [(H3SiO)(2)W](2)(mu-H)(4) (staggered bridged 7a', EHMO), [(H3SiO)(2)WH2](2) (all-terminal 7b', EHMO), [(H3SiO)(2)W](2) (9', EHMO), (HO)(4)W-2(H-4) (staggered-bridged 7'', ab initio), and (HO)(4)W-2(H-4) (bent-terminal 7*, ab initio) generally support the explanation of a thermally accessible excited state and assign 7* a geometry intermediate between the all-terminal and staggered-bridged forms.