Reactivity of mononuclear Pd(ii) and Pt(ii) complexes containing the primary phosphane (ferrocenylmethyl)phosphane towards metal chlorides and PPh3

Abstract

The reaction of the primary phosphane (ferrocenylmethyl)phosphane [PH₂CH₂Fc, 1] with an equimolar amount of [M(cod)Cl₂] (cod = 1,5-cyclooctadiene, M = Pd, Pt) gave the unusual monobridged M(II) dinuclear complexes of formula [(cod)ClM(μ-PHCH₂Fc)M(PH₂CH₂Fc)Cl₂] [M = Pt, 2; M = Pd, 3]. The mechanism leading to 2 and 3 involves the preliminary formation of cis-[M(PH₂CH₂Fc)₂Cl₂] which further reacts with free [M(cod)Cl₂] before undergoing dehydrochlorination. The reaction of 3 with PPh₃ led to the unexpected formation of the cyclic trinuclear complex [Pd₃(μ-PHCH₂Fc)₃(PPh₃)₂(PH₂CH₂Fc)Cl₃] (5). Further reaction of 5 with PPh₃ led to the substitution of the tertiary phosphane for the primary one with formation of two geometric isomers 4a and 4b of formula [Pd(μ-PHCH₂Fc)(PPh₃)Cl]₃ differing only for the position of the PPh₃ group replacing the PH₂CH₂Fc ligand. In complex 4b each μ-PHCH₂Fc group bridging two palladium atoms has a terminal PPh₃ ligand in trans position with respect to the first Pd (the cis position being occupied by a chlorine) and a chlorine ligand trans to the other Pd (the cis position being occupied by a terminal PPh₃ ligand) thus rendering chemically equivalent the three PPh₃ (and the three phosphides). In complex 4a the three terminal PPh₃ as well as the three bridging phosphido ligands are chemically inequivalent. Complexes 4a and 4b coexist in equilibrium in solution, but only 4a precipitates from CH₂Cl₂–n-hexane yielding crystals suitable for X-ray analyses. These crystals are water/n-hexane solvates and belong to the monoclinic space P2₁/c with a = 25.063(6) Å, b = 15.564(4) Å, c = 26.089(5) Å, β = 120.017(16)°. They contain two identical couples of enantiomers of 4a. The peculiar arrangement of three metals and three phosphorus atoms of the bridging ligands ascertained for trimers 4a, 4b and 5 has no precedent in the rich class of palladium and platinum phosphido chemistry.