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

Ewing, P., Farrugia, L. J. and Rycroft, D. S. (1988) Dynamic NMR studies on hydride and carbonyl fluxionality in triosmium–platinum cluster compounds. X-ray crystal structure of [Os₃Pt(μ-H)₃(CO)₁₀(PCy₃)]⁺BF₄⁻. Organometallics, 7(4), pp. 859-870. (doi: 10.1021/om00094a012)

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 P_n, a = 9.925 (2) Å, b = 11.747 (3) Å, c = 16.186 (4) Å, β = 89.75 (2)°, V = 1887.1 (8) ų, Z = 2, final R (R_w) 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) Å) and the Os(1)–Os(3) and Os(1)–Os(2) edges (2.891 (1) and 2.747 (1) Å, 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) Å. 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.

• Item Type: Articles
• Keywords: Organometallic chemistry, NMR spectroscopy.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Rycroft, Dr David and Farrugia, Dr Louis
• Authors: Ewing, P., Farrugia, L. J., and Rycroft, D. S.
• Subjects: Q Science > QD Chemistry
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Organometallics
• Publisher: American Chemical Society
• ISSN: 0276-7333
• ISSN (Online): 1520-6041