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

Abstract

A series of copper­(I) complexes bearing electron-deficient β-diketiminate ligands have been prepared. The study includes [{{ArNC­(CR3)}2CH}­Cu­(η2-toluene)n] (Ar = Mes, R = F, n = 0.5, [1 2 ·tol]; Ar = C6F5, R = Me, n = 1, [2·tol]; Ar = 2,6-Cl2C6H3, R = H, n = 0.5, [3 2 ·tol]). Reactions of [1–3 n ·tol] with boranes, alanes, a zinc hydride, a magnesium hydride, and a calcium hydride generate the corresponding σ complexes ([1–3·B], [3·B′], [3·Al], [3·Al′], [1–3·Zn], [1·Mg], and [1·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 σ-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 σ-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 ΔH° and negative values of ΔS° 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–Chatt–Duncanson model. Density functional theory calculations show that the donation of σ-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 σ*-M–H (or E–H) orbital.

Selective Reduction of CO2 to a Formate Equivalent with Heterobimetallic Gold‐ ‐ ‐Copper Hydride Complexes

“…[21,44,45] Ther eversible dissociation of 2·Au was confirmed by reaction with PCy 3 ,which resulted in the trapping of the copper(I) fragment as 2·PCy 3 (see the Supporting Information, Figure S4). Va riable temperature (VT) NMR analysis of 1·Au and 2·Au resulted in the observation of both sides of the equilibrium at low temperature (1·Au:2 13 K; 2·Au:2 53 K), and allowed vantH off analysis with 2·Au to give the thermodynamic parameters DH8 8 = À0.1 kcal mol À1 , DS8 8 = À9.3 cal K À1 mol À1 , and DG8 8 298 =+2.7 kcal mol À1 .T hese values are consistent with the weak and reversible binding previously reported for s-complexes of copper(I).…”

“…[12,24] Theh ydroboration of CO 2 (1 atm) with HBpin can be catalyzed by 10 mol %o f1·Au or 2·Au in C 6 D 6 to produce predominantly the formoxyborane,H CO 2 Bpin. Thefrontier molecular orbitals of 1 and 2 [21] are isolobal with bent {ML 2 }d 10 fragments (M = Ni, Pd). [22,[26][27][28][29] Fore xample,p alladium complexes supported by silyl pincer ligands have recently been reported for the selective hydroboration of CO 2 with turnover numbers (TONs) > 60 000, and copper NHC complexes show similar selectivities but with TONs < 10.…”

“…[15][16][17][18] Fore xample,b orane-capped copper(I) hydrides ( Figure 1) have been prepared by addition of boranes to at erminal copper hydride. These species are prepared by the simple addition of am onomeric gold(I) hydride to ab ent d 10 {CuL 2 }f ragment [3,21] and are selective catalysts for the reduction of CO 2 with HBPin (HBPin = pinacolborane) into af ormate equivalent. These species are prepared by the simple addition of am onomeric gold(I) hydride to ab ent d 10 {CuL 2 }f ragment [3,21] and are selective catalysts for the reduction of CO 2 with HBPin (HBPin = pinacolborane) into af ormate equivalent.…”

“…[25] Thea bsolute activities and selectivities of the heterobimetallic catalysts are greatly inferior to those of an umber of transitionmetal systems. [21,37] Comparing the product distributions from the Group 10 precatalysts revealed that the heterobimetallic effect was also observed for the Pd/Auadmixtures (Figure 3). Importantly,i nt he current case,the monomeric complexes 1·tol, 2·tol,and [Au(H)(IPr)] show negligible activity for CO 2 reduction, and neither complex produces significant amounts of HCO 2 Bpin in the absence of its coinage metal counterpart.…”

Tunable Binding of Dinitrogen to a Series of Heterobimetallic Hydride Complexes

“…that support bimetallic dissociation into the monometallic components. 19,20 The dinitrogen ligand was found to display a strong nNºN stretching frequency (Al, 2160 cm -1 ; Zn, adducts in which the key interaction between the two fragments is via the bridging hydride ligands.…”