Synthesis and properties of new tris(cyanoethyl)phosphine complexes of platinum (0,II), palladium (0,II), iridium (I) and rhodium (I).: Conformational analysis of tris(cyanoethyl)phosphine ligands

Abstract

The tris(cyanoethyl)phosphine (tcep) complexes trans-[PtCl 2(tcep) 2], cis-[PtMe 2(tcep) 2], and trans-[PtMeCl(tcep) 2] are prepared by treatment of the corresponding [PtXY(cod)] (cod=1,5-cyclooctadiene) with tcep. Reduction of trans-[PtCl 2(tcep) 2] with NaBH 4 gives trans-[PtHCl(tcep) 2] which, in the presence of tcep and NEt 3, gives the coordinatively unsaturated platinum(0) complex [Pt(tcep) 3]. This coordinatively unsaturated species is also formed when [Pt(norbornene) 3] reacts with tcep. [Pt(tcep) 3] is very unreactive compared to its PEt 3 analogue: it is air-stable and does not react with further tcep to form an 18-electron species. It is protonated by HBF 4·OEt 2 to form [PtH(tcep) 3]BF 4. The complex trans-[PdCl 2(tcep) 2] is made from [PdCl 2(NCPh) 2] and tcep and the derivatives trans-[PdX 2(tcep) 2] (X=Br or I) are made by metathesis of the dichloro complex. Reduction of trans-[PdCl 2(tcep) 2] with LiOMe in the presence of tcep gave the palladium(0) complex [Pd(tcep) 3] which, like its platinum(0) analogue, undergoes exchange with free tcep on the NMR timescale. The palladium complex reacts with dibenzylideneacetone (dba) to form [Pd( η 2-dba)(tcep) 2]; the same product is formed in the reaction of [Pd( η 2-dba) 2] and tcep. Reaction of [Pd 2Cl 2( η 3-C 3H 3) 2] and tcep gives [PdCl(tcep)( η 3-C 3H 3)] or [Pd(tcep) 2( η 3-C 3H 3)]Cl depending on stoichiometry. The rhodium(I) and iridium(I) complexes trans-[MCl(CO)(tcep) 2], [MCl(tcep)(cod)] and [MCl(tcep) 3] are all readily made from tcep and an appropriate precursor. All new compounds have been fully characterised by a combination of elemental analysis, IR, 31P, 13C, 1H and 195Pt NMR spectroscopy. The crystal structure of [IrCl(tcep) 3] as a MeCN solvate shows a distorted square planar coordination geometry ( trans angles at Ir(I) ca. 164°, cis P–Ir–P av. 96°, cis P–Ir–Cl av. 85°). Analysis of the conformations of tcep ligands in this and other published tcep complexes shows there is a preference for conformations in which aaa, aag or g +g − (a=anti, g=gauche) arrangements of the three M–P–C–C chains are avoided.