Reversible Coordination of Boron-, Aluminum-, Zinc-, Magnesium-, and Calcium-Hydrogen Bonds to Bent {CuL2} Fragments: Heavy sigma Complexes of the Lightest Coinage Metal

摘要

A series of copper(I) complexes bearing electron-deficient beta-diketiminate ligands have been prepared. The study includes [{{ArNC(CR3)}(2)CH)-Cu(eta(2)-toluene)(n)] (Ar = Mes, R = F, n = 0.5, [1(2)center dot tol]; Ar = C6F5, R = Me, n = 1, [2 center dot tol]; Ar = 2,6-Cl2C6H3, R = H, n = 0.5, [32 center dot tol]). Reactions of [1-3(n)center dot tol] with boranes, alanes, a zinc hydride, a magnesium hydride, and a calcium hydride generate the corresponding a complexes ([1-3 center dot B], [3 center dot B'], [3 center dot Al], [3.Al'], [1-3 center dot Zn], [1 center dot Mg], and [1 center dot Ca]). These species all form reversibly, being in equilibrium with the arene solvates in solution. With the exception of the calcium complex, the complexes have all been characterized by single-crystal X-ray diffraction studies. In solution, the sigma-hydride of the aluminum, zinc, magnesium, and calcium derivatives resonates between -0.12 and -1.77 ppm (C6D6 or toluene-d(8),193-298 K). For the sigma-borane complexes, the hydrides are observed as a single resonance between 2 and 3.5 ppm (C6D6, 298 K) and bridging and terminal hydrides rapidly exchange on the NMR time scale even at 193 K Quantification of the solution dynamics by van't Hoff analysis yields expectedly small values of Delta H degrees and negative values of AS consistent with weak binding and a reversible process that does not involve aggregation of the copper species. The donor acceptor complexes can be rationalized in terms of the Dewar-Cliatt-Duncanson model. Density functional theory calculations show that the donation of sigma-M H (or E-H) electrons into the 4s-based orbital (LUMO or LUMO+1) of the copper fragment is accompanied by weak back-donation from a d(xz)-based orbital (HOMO or HOMO-1) into the sigma*-M-H (or E-H) orbital.