Copper(II) Complexes with Bulky N-Substituted Diethanolamines: High-Field Electron Paramagnetic Resonance, Magnetic, and Catalytic Studies in Oxidative Cyclohexane Amidation

The novel coordination compounds [Cu-2-(H(t)BuDea)(2)(OAc)(2)] (1) and [Cu-2(H(n)BuDea)(2)Cl-2]center dot nH(2)O (2) have been prepared through the reaction of the respective copper(II) salts with N-tert-butyldiethanolamine (H(2)(t)BuDea, for 1) or N-butyldiethanolamine (H(2)(n)BuDea, for 2) in methanol solution. Crystallographic analysis reveals that, in spite of the common binuclear {Cu-2(mu-O)(2)} core, the supramolecular structures of the complexes are drastically different. In 1 binuclear molecules are linked together by H-bonds into 1D chains, while in 2 the neighboring pairs of binuclear molecules are H-bonded, forming tetranuclear aggregates. Variable-temperature (1.8-300 K) magnetic susceptibility measurements of 1 and 2 show a dominant antiferromagnetic behavior. Both complexes are also studied by HF-EPR spectroscopy. While the interaction between Cu(II) centers in 1 can be described by a single coupling constant J = 130.1(3) cm(-1) (using H = JS(1)S(2)), the crystallographically different {Cu-2(mu-O)(2)} pairs in 2 are expected exchange from ferro- to antiferromagnetic behavior (with J ranging from -32 to 110 cm(-1), according to DFT calculations). Complexes 1 and 2 act as catalysts in the amidation of cyclohexane with benzamide, employing (BuOOBu)-Bu-t-Bu-t as oxidant. The maximum achieved conversion of benzamide (20%, after 24 h reaction time) was observed in the 1/(BuOOBu)-Bu-t-Bu-t system. In the cases of (BuOO)-Bu-t(O)CPh or (BuOOH)-Bu-t oxidants, no significant amidation product was observed, while for (BuOO)-Bu-t(O)CPh, the oxidative dehydrogenation of cyclohexane occurred, giving cyclohexene, to afford the allylic ester (cyclohex-2-en-1-yl benzoate) as the main reaction product.