Ditungsten Siloxide Hydrides, [(silox)(2)WH(n)()](2) (n = 1, 2; silox = (t)BuSiO), and Related Complexes

The addition of 4.0 equiv of Na(silox) to NaW(2)Cl(7)(THF)(5)] afforded (silox)(2)ClW&tbd1;WCl(silox)(2) (1, 65%). Treatment of 1 with 2.0 equiv of MeMgBr in Et(2)O provided (silox)(2)MeW&tbd1;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&tbd1;WH(silox)(2) (3, 70%), characterized by a hydride resonance at delta 19.69 (J(WH) = 325 Hz, (1)H NMR). Exposure of 2 to 1 atm of H(2) yielded 3 and CH(4) via (silox)(2)HW&tbd1;WMe(silox)(2) (4); use of D(2) led to (silox)(2)WD](2) (3-d(2)). Exposure of 3 to ethylene ( approximately 1 atm, 25 degrees C) in hexanes generated (silox)(2)EtW&tbd1;WEt(silox)(2) (5), but solutions of 5 reverted to 3 and free C(2)H(4) 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&tbd1;W(OSi(t)Bu(2)CMe(2)CH(2))(silox) (6a, approximately 60%) and free H(2), while extended thermolysis with degassing (5 d, 70 degrees C) produced a second cyclometalated rotational isomer, 6b (6a:6b approximately 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 approximately 3 atm of H(2), equilibrium amounts of (silox)(2)WH(2)](2) (7) were observed by (1)H NMR spectroscopy (3 + H(2) right harpoon over left harpoon 7; 25.9-88.7 degrees C, DeltaH = -9.6(4) kcal/mol, DeltaS = -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), (1)H NMR spectral evidence for (silox)(2)WCl](2)(&mgr;-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). (29)Si{(1)H} 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, deltadelta, paramagnetic excited state of D(2)(d)() (silox)(2)W](2)(&mgr;-H)(4) (DeltaE approximately 2.1 kcal/mol, chi(7a) approximately 0.03), an explanation favored over thermal equilibration with an energetically similar but structurally distinct isomer (e.g., (silox)(2)WH(2)](2)(&mgr;-H)(2), DeltaG degrees approximately 0.69 kcal/mol, chi(7b) approximately 0.25) on the basis of spectral arguments. Extended Hückel and ab initio molecular orbital calculations on model complexes (H(3)SiO)(2)W](2)(&mgr;-H)(4) (staggered bridged 7a', EHMO), (H(3)SiO)(2)WH(2)](2) (all-terminal 7b', EHMO), (H(3)SiO)(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.