Zn···Zn interactions at nickel and palladium centers.

The analogy between ZnR fragments and the hydrogen radical represents a fruitful concept in organometallic synthesis. The organozinc(ii) and -zinc(i) sources ZnMe (Me = methyl) and [ZnCp*] (Cp* = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp*), which can be trapped by transition metal complexes [L M], yielding [L (ZnR) ]. The addition of the dizinc compound [ZnCp*] to coordinatively unsaturated [L M] by the homolytic cleavage of the Zn-Zn bond can be compared to the classic oxidative addition reaction of H, forming dihydride complexes [L M(H)]. 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 [ZnCp*] and [ZnCuCp*] may be regarded as the first indication of this so far unknown, side-on coordination mode of [ZnCp*]. With this background in mind the question arises if a series of complexes featuring the ZnM structural motif can be prepared exhibiting a (more or less) intact Zn-Zn interaction, 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)(PMe)] (), [Ni(ZnCp*)(ZnMe)(PMe)] (), [{Ni(CN Bu)(μ-ZnCp*)(μ-ZnMe)}] (), [Pd(ZnCp*)(CN Bu)] () and [PdZn(PCy)(Cp*)] (). 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.