Ditungsten Siloxide Hydrides, [(silox)2WHn]2(n= 1, 2; silox =tBuSiO), and Related Complexes

Publisher Website
Google Scholar
Abstract
The addition of 4.0 equiv of Na(silox) to Na[W2Cl7(THF)5] afforded (silox)2ClW⋮WCl(silox)2 (1, 65%). Treatment of 1 with 2.0 equiv of MeMgBr in Et2O provided (silox)2MeW⋮WMe(silox)2 (2, 81%). In the presence of 1 atm of H2, reduction of 1 with 2.0 equiv of Na/Hg in DME provided (silox)2HW⋮WH(silox)2 (3, 70%), characterized by a hydride resonance at δ 19.69 (JWH = 325 Hz, 1H NMR). Exposure of 2 to 1 atm of H2 yielded 3 and CH4 via (silox)2HW⋮WMe(silox)2 (4); use of D2 led to [(silox)2WD]2 (3-d2). Exposure of 3 to ethylene (∼1 atm, 25 °C) in hexanes generated (silox)2EtW⋮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, C2 symmetry for 1−5. Thermolysis of 3 at 100 °C (4 h) resulted in partial conversion to (silox)2HW⋮W(OSitBu2CMe2CH2)(silox) (6a, ∼60%) and free H2, while extended thermolysis with degassing (5 d, 70 °C) produced a second cyclometalated rotational isomer, 6b (6a:6b ∼ 3:1). When left at 25 °C (4 h) in sealed NMR tubes, 6 and free H2 regenerated 3. Reduction of 1 with 2.0 equiv of Na/Hg in DME also afforded 6a (25%). When 3 was exposed to ∼3 atm of H2, equilibrium amounts of [(silox)2WH2]2 (7) were observed by 1H NMR spectroscopy (3 + H2 ⇌ 7; 25.9−88.7 °C, ΔH = −9.6(4) kcal/mol, ΔS = −21(2) eu). Benzene solutions of 3 and 1−3 atm of D2 revealed incorporation of deuterium into the silox ligands, presumably via intermediate 6. In sealed tubes containing [(silox)2WCl]2 (1) and dihydrogen (1−3 atm), 1H NMR spectral evidence for [(silox)2WCl]2(μ-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)2W]2 (9), followed by H2 addition. Hydride chemical shifts for 7 are temperature dependent, varying from δ 1.39 (−70 °C, toluene-d8), to δ 3.68 (90 °C). 29Si{1H} NMR spectra revealed a similar temperature dependence of the silox (δ 12.43, −60 °C, to δ 13.64, 45 °C) resonances. These effects may arise from thermal population of a low-lying, δδ*, paramagnetic excited state of D2d [(silox)2W]2(μ-H)4 (ΔE ∼ 2.1 kcal/mol, χ(7a*) ∼ 0.03), an explanation favored over thermal equilibration with an energetically similar but structurally distinct isomer (e.g., [(silox)2WH2]2(μ-H)2, ΔG° ∼ 0.69 kcal/mol, χ(7b) ∼ 0.25) on the basis of spectral arguments. Extended Hückel and ab initio molecular orbital calculations on model complexes [(H3SiO)2W]2(μ-H)4 (staggered bridged 7a‘, EHMO), [(H3SiO)2WH2]2 (all-terminal 7b‘, EHMO), [(H3SiO)2W]2 (9‘, EHMO), (HO)4W2(H4) (staggered-bridged 7‘‘, ab initio), and (HO)4W2(H4) (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.

This publication has 57 references indexed in Scilit: