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 [L_aM], yielding [L_b(ZnR)_n]. The addition of the dizinc compound [Zn₂Cp*₂] to coordinatively unsaturated [L_aM] by the homolytic cleavage of the Zn–Zn bond can be compared to the classic oxidative addition reaction of H₂, forming dihydride complexes [L_aM(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(CN^tBu)₂(μ₂-ZnCp*)(μ₂-ZnMe)}₂] (3), [Pd(ZnCp*)₄(CN^tBu)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.