Variable coordination of a chiral diphosphine containing an amidinium/NHC group within its backbone: μ-P,P′, κ2-P,P′ and κ3-P,C,P′ coordination modes

Abstract

A diphosphine ligand (1·HPF₆6666), which is a potential precursor to a PC_NHCP pincer, with a backbone containing two phenylene groups and a central bicyclic 4-aza-2-azoniabicyclo[3.2.1]oct-2-ene unit has been synthesised and coordinated to Pd(II) and Pt(II) to give trans-[M(κ²-P,P′-1·H)Cl₂]PF₆ where M = Pd (2) or Pt (3a). Single-crystal structure determinations of 2 and 3a show the complexes to be isostructural with the diphosphine coordinated in a trans-spanning fashion and the amidinium unit being protonated and non-coordinated. 2 and 3a react with CH₃I to give the dimers trans-[Pd₂(μ-1·H)₂I₄](PF₆)₂, 6, and trans-[Pt₂(μ-1·H)₂I₄](PF₆)₂, 7, as the major products. This bridging mode of coordination of [1·H]⁺ is also seen in trans-[Rh₂(μ-1·H)(1,5-COD)₂Cl₂]PF₆, 4, and [Pt₂(μ-κ²-1·H)(dvdms)]PF₆, 5. Upon treatment with KO^tBu complexes 2 and 3a undergo deprotonation at the amidinium carbon to give trans-[M(κ³-P,C,P′-1)Cl]PF₆ where M = Pd (8), and Pt (9). The related trans-[Rh(κ³-P,C,P′-1)(CO)]PF₆ (10) is prepared directly from 1·HPF₆6666 and Rh(acac)(CO)₂: this and the palladium and platinum complexes 8 and 9 are isolated as isomeric mixtures as a consequence of a conformational isomerism. In situ deprotonation of 1·HPF₆6666 followed by addition of Ag(CF₃SO₃) gave S_Ag-[Ag(κ³-P,C,P′-1)(CF₃SO₃)], 11. Some preliminary studies of the reactivity of 2 and 8 in Suzuki-type reactions are reported and the Pt(0) system has been shown to be an active hydrosilylation catalyst.