Electrophilic abstractions on Os(H)(2)X(NO)L-2: selective access to new unsaturated cationic Os(II) hydrides

摘要

Nitrosylation of Os(H)(3)ClL2 (L = (PPr3)-Pr-i) affords the known Os(H)(2)Cl(NO)L-2 (2). Soft electrophiles (Ag+, Na+) react with complex 2 by chloride abstraction to ultimately yield truly 16-electron dihydride Os(H)(2)(NO)((PPr3)-Pr-i)(2)(+) (4a), characterized by variable-temperature NMR. Complex 4a reversibly binds H-2, forming Os(H)(2)(H-2)(NO) ((PPr3)-Pr-i)(+) with an unusually high barrier for intramolecular hydride exchange. Under kinetic conditions, protonation of 2 with strong acids follows the selectivity for chloride abstraction. Thermodynamically, protonation at the hydride is preferred, quantitatively producing cationic OsHCl(NO)L-2(+), isolated and characterized by X-ray diffraction as the BAr4F- salt (7) (Ar-F = 3,5-(CF3)(2)C6H3). Structures of isoelectronic OsHCl(NO)(PH3)(2)(+) and OsHCl(CO) (PH3)(2) were optimized with ab initio DFT (Becke3LYP) methods and compared to show the greater unsaturation of the metal in the cationic species. Both complexes, 4a and 7, are highly electrophilic and reversibly coordinate dichloromethane in solution. The observed reactivity patterns of the synthesized unsaturated hydrides are rationalized in terms of the determining influence of the 'push-pull' pi-stabilization of the metal center. (C) 1998 Elsevier Science S.A. All rights reserved. [References: 39]