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)

Abstract

Variable-temperature 13C NMR studies on Os3Pt(μ-H)2(CO)10(PCy3) (1) show three distinct carbonyl exchange processes. The lowest energy process (ΔG‡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 PtL2 fragment, coupled with facial hydride migration, accounts for both this latter process and the simultaneous mutual exchange of the hydride ligands (for which ΔG‡295 = 63.1 (5) kJ mol−1, ΔH‡ = 55 (1) kJ mol−1, ΔS‡ = −27 (3) J mol−1 K−1). Above 333 K a final mechanism results in total CO scrambling. Protonation of 1 with HBF4 affords the trihydrido salt [Os3Pt(μ-H)3(CO)10(PCy3)]+BF4− (2) in high yield. Crystal data for 2: monoclinic, space group Pn, a = 9.925 (2) A, b = 11.747 (3) A, c = 16.186 (4) A, β = 89.75 (2)°, V = 1887.1 (8) A3, Z = 2, final R (Rw) values of 0.032 (0.034) for 4087 independent observed data, I ≥ 3σ(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 (ΔG‡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)(PR3) moiety about the Os3 triangle. The butterfly adducts Os3Pt(μ-H)2(CO)10(PCy3)(L) (3, L = CO; 4, L = PCy3) 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 13C 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. 1J(187Os–13C) coupling constants in the range 98–117 Hz are reported for complexes 1–3.