Synthesis and Reactivity of [(silox)2Mo=NR]2Hg (R = tBu, tAmyl; silox = OSitBu3): Unusual Thermal Stability and Ready Nucleophilic Cle

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• 作者：Devon C. Rosenfeld ; Peter T. Wolczanski ; Khaldoon A. Barakat ; Corneliu Buda ; Thomas R. Cundari ; Frank C. Schroeder ; Emil B. Lobkovsky
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：November 12, 2007
• 年：2007
• 卷：46
• 期：23
• 页码：9715 - 9735
• 全文大小：821K
• 年卷期：v.46,no.23(November 12, 2007)
• ISSN：1520-510X

文摘

Treatment of (DME)Cl₂Mo(=NR)₂ (R = ^tBu, (1-^tBu), ^tAmyl (1-^tAmyl)) with 2 equiv of ^tBu₃SiOH (siloxH) and 1 equivof HCl produced (silox)₂Cl₂Mo=NR (R = ^tBu, (3-^tBu), ^tAmyl (3-^tAmyl)); subsequent reduction by Na/Hg affordedthe Mo(V) chloride, (silox)₂ClMo=N^tBu (4-^tBu), and the Mo(IV) mercury derivatives, [(silox)₂Mo=NR]₂Hg (R = ^tBu((5-^tBu)₂Hg), ^tAmyl ((5-^tAmyl)₂Hg)). Reductions of 3-^tBu and 3-^tAmyl in the presence of L (L = PMe₃, pyridine,4-picoline) led to the isolation of adducts (silox)₂(Me₃P)Mo=NR (R = ^tBu (6-^tBu), ^tAmyl (6-^tAmyl)) and (silox)₂L₂Mo=N^tBu (L = py (7-py), 4-pic (7-4-pic)). Single-crystal X-ray structural investigations of pseudo-tetrahedral 4-^tBu,Hg-capped, pseudo-trigonal planar (5-^tBu)₂Hg, pseudo-tetrahedral 6-^tBu, and trigonal bipyramidal 7-4-pic revealthat all possess a closed O-Mo-O angle when compared to the N=Mo-O angles. A molecular orbital rationaleand supporting calculations suggest that this is a manifestation of the greater -donating ability of the imido relativeto that of the siloxides. While the D(Mo-Hg) of [(HO)₂Mo=NH]₂Hg ((5')₂Hg) was calculated to be 22.4 kcal/mol,(5-R)₂Hg (R =^tBu, ^tAmyl) are remarkably stable; (5-^tBu)₂Hg degraded in a first-order fashion with G = 31.9(1)kcal/mol. In the presence of strong (L = PMe, pyridine, S₈) or weak (L = 2-butyne, ethylene, N₂O, 1,4,7,10-tetrathiacyclododecane, 1,4,7,10,13,16-hexathiacyclooctadecane) nucleophiles, an enhanced rate of Mo-Hgbond cleavage was noted, with some of the former group generating adducts in <5 min; the products were 6-^tBu,7-py, (silox)₂(S)Mo=N^tBu (10-^tBu), (silox)₂Mo=N^tBu(C₂Me₂) (8-^tBu), (silox)₂(C₂H₄)Mo=N^tBu (11-^tBu), (silox)₂(O)Mo=N^tBu (9-^tBu), and a mixture of 10-^tBu and 11-^tBu, respectively. Some of these were independently preparedvia substitution of 6-^tBu. According to calculations and a molecular orbital rationale, dissociation of the Mo-Hgbond in (5-R)₂Hg (R =^tBu, ^tAmyl) is orbitally forbidden, and the addition of a nucleophile to the terminus of theMo-Hg-Mo linkage mitigates the symmetry requirements. The mechanism of thermal degradation was studiedwith mixed success. NMR spectroscopy revealed imido exchange between (5-^tBu)₂Hg and (5-^tAmyl)₂Hg during aninitial induction period and a subsequent rapid exchange period that implicated free 5-R (R = ^tBu, ^tAmyl). Furthercrossover studies revealed siloxide exchange as an additional complication.