Copper(II) Complexes of 2,2′:6′,2″‐Terpyridine Derivatives for Catalytic Aerobic Alcohol Oxidations – Observation of Mixed‐Valence Cu^ICu^II Assembles

Abstract

The reactions of copper(II) salts with simple terpyridine (tpy) ligands gave mononuclear monoligand complexes 1-3, of which two new structures have been characterized by X-ray crystallography. These complexes were applied as catalysts for the oxidation of benzylic alcohol to benzaldehyde in air in the presence of the radical 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO). Copper complexes 1 and 2 are efficient catalysts for the reactions in water at 70°C with 4-dimethylaminopyridine (DMAP) as an extra base. The influence of TEMPO on the formation of new reactive intermediates during the catalytic reactions was tentatively investigated by introducing TEMPO into the reaction solutions of tpy deriva-

New copper(II) species from the copper/2,2′-bypyridine and copper/4-dimethylaminopyridine catalyzed aerobic alcohol oxidations

“…In continuation with our previous work on the identification of new copper species involved in the copper-catalyzed aerobic alcohol oxidation reactions [11][12][13][14][15] Yellow-brown crystals that were previously characterized were first observed within two days, which, however, disappeared upon elongated exposure to the air without isolation [11].…”

“…The continuing work revealed that a dinuclear Cu II complex displayed higher reactive activity than some analogues derived from simple monodentate nitrogen ligands structurally related to DMAP [12,13]. In addition, we also serendipitously isolated and characterized two novel mixed-valent Cu I Cu II assemblies (dimer or polymeric chain) based upon the chelating 2,2':6',2''-terpyridine ligands, in search of similar TEMPO-mediated aerobic oxidations of alcohols [14,15].…”

“…Recently, we were interested to investigate the influence of various organic ligands in the catalytic reactivity of Cu I -or Cu II -catalyzed aerobic alcohol oxidations, with a focus on the structural elucidation of possible Cu I or Cu II complexes formed during the catalytic reactions [10][11][12][13][14][15]. We have observed that tetranuclear copper clusters derived from N,O-Schiff base ligands catalyzed aerobic oxidations of benzylic alcohols under aqueous basic conditions, featuring higher activity than its dinuclear analogue [10].…”

Syntheses, structures and cytotoxicity of cobalt(II) complexes with 4′-chloro-2,2′:6′,2″-terpyridine

“…Both mono-and bis-ligand transition metal complexes M(4 0 -Cltpy)X 2 and M(4 0 -Cltpy) 2 (anion) 2 (M = Mn II , Fe II , Ni II , Zn II , Cd II , or Ru II ; X = halogen) were revealed in a few recent reports, with respect to their single-crystal structural analysis [12]. We have recently reported the synthesis, structures and catalytic properties of novel Cu II and mixed-valent Cu I Cu II complexes of 4 0 -Cltpy, with a focus on their structure-activity relationship in catalysis [13]. However, cobalt complexes of 4 0 -Cltpy were little explored and known examples include homoleptic complexes Co II (4 0 -Cltpy) 2 (anion) 2 and a cobalt(III) complex Co III (4 0 -Cltpy)Cl 3 that was structurally characterized [14].…”

“…Elemental Analysis was performed by Midwest Microlab LLC in Indianapolis. Cu(4 0 -Cltpy)Cl 2 (4) was prepared as reported previously [13].…”