Zn⋯Zn interactions at nickel and palladium centers

Abstract

The analogy between ZnR fragments and the hydrogen radical represents a fruitful concept in organometallic synthesis. The organozinc(ii) and -zinc(i) sources ZnMe2 (Me = methyl) and [Zn2Cp∗2] (Cp∗ = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp∗), which can be trapped by transition metal complexes [LaM], yielding [Lb(ZnR)n]. The addition of the dizinc compound [Zn2Cp∗2] to coordinatively unsaturated [LaM] by the homolytic cleavage of the Zn-Zn bond can be compared to the classic oxidative addition reaction of H2, forming dihydride complexes [LaM(H)2]. It has also been widely shown that dihydrogen coordinates under preservation of the H-H bond in the case of certain electronic properties of the transition metal fragment. The σ-aromatic triangular clusters [Zn3Cp∗3]+ and [Zn2CuCp∗3] may be regarded as the first indication of this so far unknown, side-on coordination mode of [Zn2Cp∗2]. With this background in mind the question arises if a series of complexes featuring the Zn2M structural motif can be prepared exhibiting a (more or less) intact Zn-Zn interaction, i.e. di-zinc complexes which are analogous to non-classical dihydrogen complexes of the Kubas type. In order to probe this idea, a series of interrelated organozinc nickel and palladium complexes and clusters were synthesized and characterized as model compounds: [Ni(ZnCp∗)(ZnMe)(PMe3)3] (1), [Ni(ZnCp∗)2(ZnMe)2(PMe3)2] (2), [{Ni(CNtBu)2(μ2-ZnCp∗)(μ2-ZnMe)}2] (3), [Pd(ZnCp∗)4(CNtBu)2] (4) and [Pd3Zn6(PCy3)2(Cp∗)4] (5). The dependence of Zn⋯Zn interactions as a function of the ligand environments and the metal centers was studied. Experimental X-ray crystallographic structural data and DFT calculations support the analogy between dihydrogen and dizinc transition metal complexes.