Synthesis and Reactivity of [(silox)2Mo=NR]2Hg (R = ~tBu, ~tAmyl; silox = OSi~tBu3): Unusual Thermal Stability and Ready Nucleophilic Cleavage Rationalized by Electronic Factors

摘要

Treatment of (DME)Cl2Mo(=NR)2 (R = ~tBu, (1-~tBu), ~tAmyl (1-~tAmyl)) with 2 equiv of ~tBu3SiOH (siloxH) and 1 equiv of HCl produced (silox)2Cl2Mo=NR (R = ~tBu, (3-~tBu), ~tAmyl (3-~tAmyl)); subsequent reduction by Na/Hg afforded the 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)2L2-Mo=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 reveal that all possess a closed O-Mo-O angle when compared to the N=Mo-O angles. A molecular orbital rationale and supporting calculations suggest that this is a manifestation of the greater pi-donating ability of the imido relative to 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 DELTA G~(not=) = 31.9(1) kcal/mol. In the presence of strong (L = PMe, pyridine, S_8) 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-Hg bond 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 prepared via substitution of 6-~tBu. According to calculations and a molecular orbital rationale, dissociation of the Mo-Hg bond in (5-R)2Hg (R =~tBu, ~tAmyl) is orbitally forbidden, and the addition of a nucleophile to the terminus of the Mo-Hg-Mo linkage mitigates the symmetry requirements. The mechanism of thermal degradation was studied with mixed success. NMR spectroscopy revealed imido exchange between (5-~tBu)2Hg and (5-~tAmyl)2Hg during an initial induction period and a subsequent rapid exchange period that implicated free 5-R (R = ~tBu, ~tAmyl). Further crossover studies revealed siloxide exchange as an additional complication.