Dynamic NMR studies on hydride and carbonyl fluxionality in triosmium-platinum cluster compounds. X-ray crystal structure of [Os3Pt(.mu.-H)3(CO)10(PCy3)]+BF4- (Cy = cyclohexyl)

Variable-temperature 13 C NMR studies on Os 3 Pt( μ -H) 2 (CO) 10 (PCy 3 ) ( 1 ) show three distinct carbonyl exchange processes. The lowest energy process (Δ G ‡ 233 = 49.7 (5) kJ mol −1 ) involves localized scrambling in two Os(CO) 3 groups, while a second process permutes all nine Os-bound CO ligands. A proposed mechanism involving rotation of the PtL 2 fragment, coupled with facial hydride migration, accounts for both this latter process and the simultaneous mutual exchange of the hydride ligands (for which Δ G ‡ 295 = 63.1 (5) kJ mol −1 , Δ H ‡ = 55 (1) kJ mol −1 , Δ S ‡ = −27 (3) J mol −1 K −1 ). Above 333 K a final mechanism results in total CO scrambling. Protonation of 1 with HBF 4 affords the trihydrido salt [Os 3 Pt( μ -H) 3 (CO) 10 (PCy 3 )] + BF 4 − ( 2 ) in high yield. Crystal data for 2 : monoclinic, space group P n , a = 9.925 (2) A, b = 11.747 (3) A, c = 16.186 (4) A, β = 89.75 (2)°, V = 1887.1 (8) A 3 , Z = 2, final R ( R w ) values of 0.032 (0.034) for 4087 independent observed data, I ≥ 3 σ ( I ). 2 has a tetrahedral framework with the hydrido ligands bridging the Pt–Os(3) edge (2.845 (1) A) and the Os(1)–Os(3) and Os(1)–Os(2) edges (2.891 (1) and 2.747 (1) A, respectively), with the other metal-metal edges Pt–Os(1) = 2.812 (1), Pt–Os(2) = 2.819 (1), and Os(2)–Os(3) = 2.790 (1) A. The two Os( μ -H)Os hydrides, unambiguously assigned by using NOE difference spectra, are involved in mutual exchange (Δ G ‡ 293 = 62.7 (5) kJ mol −1 ). The lower energy carbonyl exchange process, observed in magnetization transfer studies using DANTE, is compatible with the hydride fluxionality, and a higher energy carbonyl exchange is consistent with rotation of the PtH(CO)(PR 3 ) moiety about the Os 3 triangle. The butterfly adducts Os 3 Pt( μ -H) 2 (CO) 10 (PCy 3 )(L) ( 3 , L = CO; 4 , L = PCy 3 ) show strikingly different fluxional behavior. A highly unusual and very low-energy six-site Pt–Os carbonyl exchange, involving a proposed closo transition state, is observed for 3 . In contrast magnetization transfer and 13 C 2D exchange correlation NOESY studies on complex 4 show only high-energy CO exchange between Pt and Os and suggest a novel "flipping" of the butterfly framework through a planar transition state. In both compounds the hydrides are nonfluxional at ambient temperature. 1 J ( 187 Os– 13 C) coupling constants in the range 98–117 Hz are reported for complexes 1 – 3 .