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(2) (Me = methyl) and [Zn(2)Cp*(2)] (Cp* = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp*), which can be trapped by transition metal complexes [L( a )M], yielding [L( b )(ZnR)( n )]. The addition of the dizinc compound [Zn(2)Cp*(2)] to coordinatively unsaturated [L( a )M] by the homolytic cleavage of the Zn–Zn bond can be compared to the classic oxidative addition reaction of H(2), forming dihydride complexes [L( a )M(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 [Zn(3)Cp*(3)](+) and [Zn(2)CuCp*(3)] may be regarded as the first indication of this so far unknown, side-on coordination mode of [Zn(2)Cp*(2)]. With this background in mind the question arises if a series of complexes featuring the Zn(2)M 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)(PMe(3))(3)] (1), [Ni(ZnCp*)(2)(ZnMe)(2)(PMe(3))(2)] (2), [{Ni(CN( t )Bu)(2)(μ(2)-ZnCp*)(μ(2)-ZnMe)}(2)] (3), [Pd(ZnCp*)(4)(CN( t )Bu)(2)] (4) and [Pd(3)Zn(6)(PCy(3))(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.