Coordination of CO to low-valent phosphorus centres and other related P–C bonding situations. A theoretical case study

Abstract

The multi-faceted bonding of CO in molecular phosphorus compounds is described using calculated P–C bond strengths as a criterion. Full compliance matrices at coupled cluster level of HPCO (1a), singlet oxaphosphirane-3-ylidene HP(η²-CO)), the dimer (HPCO)₂ as well as PCH, HPCH₂ and H₂P–CH₃ were calculated to obtain quantifiable data and enable comparison. The quest for CO coordination and activation was examined for phosphaketenes 1a–f: the P–C compliance constants (in Å mdyn⁻¹) reveal a clear trend that shows a weakening of the P–CO bond strength from 1a to mono-ligation as in [(OC)₅W{P(CO)Me}] (1c) (0.301), in H₃BP(CO)Me (1b) (0.322), to bis-ligation as in [{(OC)₅W}₂P(CO)R] (1f) (0.488) to (H₃BP)₂(CO)Me (1d) (0.649). Availability of p-type electron density at phosphorus drastically strengthens the P–CO bond and weakens the C–O bond via π–back-donation, bis complexes are better described as weak CO (C→P) adducts to phosphorus. In complexes [(OC)₅W{P(CO)R}] the CO activation by phosphorus equals that of CO activation through tungsten in pentacarbonyltungsten complexes. A comparative study of various CO bonding motifs in molecular compounds indicates that acyclic (2) or cyclic diphospha-urea derivatives (2–5) or isomers (6) display P–CO bond strengths (compliance constants range 0.502–0.640) well below that of the P–C bond of H₂P–CH₃ (0.364), thus providing insight into the bonding and the ease of CO extrusion, experimentally known for some cases. A highly unusual adduct of CO was obtained in silico through two-fold P-ligation in diphosphiren-3-ones 2a–d, the parent compound of which was found to be properly described as a side-on (PP)→(CO) complex, in contrast to its aza-analogue 2a^N. A drastic weakening of the P–CO bond strength is observed from P₂CO (2a) (0.502) to the C₂-symmetrical (H₃BP)₂CO (2b) (0.913); the latter represents an extreme case of a weakly bound CO. Furthermore, calculated ³¹P NMR shifts and scalar ¹J(P,E) couplings were correlated with P–CO and PC–O compliance constants as a tool for experimentalists.