Near-IR absorbing donor-acceptor ligand-to-ligand charge-transfer complexes of nickel()

A new series of square-planar nickel( ii ) donor-acceptor complexes exhibiting ligand-to-ligand charge-transfer (LL'CT) transitions have been prepared. Whereas the use of a catecholate donor ligand in conjunction with a bipyridyl acceptor ligand affords a complex that absorbs throughout the visible region, the use of a azanidophenolate donor ligands in conjunction with a bipyridyl acceptor ligand affords complexes that absorbs well into the near-IR region of the solar spectrum. Three new complexes, (cat)Ni(bpy t Bu 2 ) ( 1 ; (cat) 2− = 3,5-di- tert -butyl-1,2-catecholate; bpy t Bu 2 = 4,4′-di- tert -butyl-2,2′-bipyridine), (ap)Ni(bpy t Bu 2 ) ( 2 ; (ap) 2− = 4,6-di- tert -butyl-2-(2,6-diisopropylphenylazanido)phenolate), and (ap Ph )Ni(bpy t Bu 2 ) ( 3 ; (ap Ph ) 2− = 10-(2,6-diisopropylphenylazanido)-9-phenanthrolate), have been prepared and characterized by structural, electrochemical, and spectroscopic methods. Whereas all three square-planar complexes show multiple reversible one-electron redox-processes and strong LL'CT absorption bands, in azanidophenolate complexes 2 and 3 , the LL'CT absorption covers the near-IR region from 700-1200 nm. The electronic absorption spectra and ground state electrochemical data for 2 and 3 provide an estimate of their excited-state reduction potentials, E +/ *, of −1.3 V vs. SCE, making them as potent as the singlet excited state of [Ru(bpy) 3 ] 2+ . Ligand-to-ligand charge-transfer complexes of nickel( ii ) incorporating azanidophenolate donor ligands absorb near-IR light to access highly reducing excited states.