Stepwise reduction of a phosphaalkyne PC bond to a phosphaalkene and a phosphine at the FeH(dppe)2 centre. Crystal and molecular structure of the η1-co-ordinated phosphaalkyne complex trans-[FeH(η1-PCBut)(dppe)2][BPh4]

Abstract

Treatment of trans-[FeH(Cl)(dppe)₂] 1 (dppe = Ph₂PCH₂CH₂PPh₂) in thf with PCBu^t, in the presence of Tl[BF₄], gave the η¹-co-ordinated phosphaalkyne complex trans-[FeH(η¹-PCBu^t)(dppe)₂][BF₄] 2a which forms trans-[FeH(η¹-PCBu^t)(dppe)₂][BPh₄] 2b on reaction with Na[BPh₄] or, upon reaction with HBF₄ in CH₂Cl₂, converts into the η¹-fluorophosphaalkene and the difluorophosphine complexes trans-[FeH(η¹-PFCHBu^t)(dppe)₂]A (A = BF₄ 3a or FeCl₂F₂ 3b) and trans-[FeH(PF₂CH₂Bu^t)(dppe)₂][BF₄] 4, respectively. The phosphine complex trans-[FeH(PH₃)(dppe)₂][BF₄] 5a was also formed in a reaction of 1 with PCBu^t in the presence of Tl[BF₄] and [NH₄][BF₄] and converts into trans-[FeH(PH₃)(dppe)₂][BPh₄] 5b upon treatment with Na[BPh₄]. A single crystal structural study of 2b showed that a shortening of the PC triple bond from 1.542(2) to 1.512(5) Å results upon η¹ co-ordination of the phosphaalkyne, which represents the shortest P–C bond so far reported. The electrochemical P_L ligand parameter has been estimated for η¹-PCBu^t and its net electron donor/acceptor ability is compared with those of related unsaturated ligands.