A Self-Assembling Ligand Switch That Involves Hydroxide Addition to an sp² Hybridised Phosphorus Atom – A System Allowing OH^– Mediated Uptake of [MCl₂] (M = Pd, Pt) Centres

The coordination chemistry of the 1,1′-diphosphacobaltocenium salt [Co(η⁵-2-TBS-3,4-Me₂-PC₄H)₂]⁺ BF₄^– (rac-1) and its derivatives with respect to [MCl₂] (M = Pd, Pt) centres is presented. Rac-1, showing two sp² hybridised phosphorus atoms and a positive charge, is stable towards strong electrophiles (PhCH₂Br, MeI), but undergoes attack by hydroxide at phosphorus with subsequent rearrangement to form the neutral conjugate base [Co{η⁵-(2-TBS-3,4-Me₂-PC₄H)}{η⁴-(1-O,1-H-2-TBS-3,4-Me₂-PC₄H)}] (rac-2). Rac-1 undergoes reaction with [MCl₂(PhCN)₂] (M=Pd, Pt) in damp chloroform to form the P_sp2–P_sp3 chelating sec-phosphinite complexes [MCl₂{Co{η⁵-(2-TBS-3,4-Me₂-PC₄H)}{η⁴-(1-OH-2-TBS-3,4-Me₂-PC₄H)}}] 4 (M = Pd, Pt), which are also accessible directly, but under more forcing conditions, from rac-2. Computational analysis (M06/def2-TZVP) indicates that the conjugate base, in the form of its sec-phosphinite tautomer rac-3, shows significantly stronger bonding to the metal centre than does the parent 1,1′-diphosphacobaltocenium salt rac-1. The uptake and subsequent release of a [PtCl₂] centre as a function of the pH of the reaction medium hints at the potential for employing the rac-1/rac-2 ligand acid-base couple as a pH-driven coordination switch.