Organozinc hydroxylamides: on the bulk-dependent interplay of nuclearity, structure and dynamics

Abstract

The reactions of zinc dialkyls, R₂Zn (1a–d; R = Me (a), Et (b), iPr (c) and tBu (d)), with N,N-dialkylhydroxylamines, HO–NR′₂ (2a–c; R′ = Me (a), Et (b) and iPr (c)), afford organozinc hydroxylamides under alkane extrusion. Species of different nuclearity are observed, depending on the hydroxylamine 2 employed. The smaller 2a and 2b give pentanuclear complexes of the general formula Zn(RZn)₄O–NR′₂)₆ (R = Me, Et, iPr and tBu; R′ = Me and Et), whereas the derivatives of 2c are tetramers of the general formula (RZn)₄(O–NR′₂)₄ (R = Me, Et and iPr; R′ = iPr) as governed by bulk issues about the N-donor. Due to the ability of the double-donor unit O–NR₂ to change its bridging mode, two coordination isomers exist for both types of compounds. The pentanuclear species crystallise either in a heterofenestrane or an octahedroid motif. For these species, the central Zn atom exhibits either coordination number 4 or 6; in solution, a rapid change between coordination isomers is observed. Due to the absence of a central Zn atom in the tetranuclear species, these aggregate in heterocubane geometries or such derived thereof. They display the O–N units in either κ³O or κ²O;κ¹N mode. The tetranuclear species are also yielded with the less sterically encumbered precursors under thermodynamic conditions (i.e. reflux), as exemplified by the reaction of Me₂Zn (1a) with HO–NEt₂ (2b). They are non-dynamic in solution, showing that a central cation is mandatory for the fluxional behaviour observed for the pentanuclear derivatives. DFT studies on the O–NMe₂ series reveal that the relative energies of the pentazinc isomers become more similar with increasing RZn group size; possible conversions of these to their tetrazinc counterparts were also scrutinised. Two κ³O-bridged degradation products of hydroxylamide complexes could be structurally characterised. They were formed either by partial product hydrolysis, or by in situ oxygenation of the starting zinc dialkyl.