Platinum(II) Diimine Complexes with Catecholate Ligands Bearing Imide Electron-Acceptor Groups: Synthesis, Crystal Structures, (Spectro)Electrochemical and EPR studies, and Electronic Structure

Abstract

A series of catechols with attached imide functionality (imide = phthalimide PHT, 1,8-naphthalimide NAP, 1,4,5,8-naphthalenediimide NDI, and NAP-NDI) has been synthesized and coordinated to the Pt (II)(bpy*) moiety, yielding Pt(bpy*)(cat-imide) complexes (bpy* = 4,4'-di- tert-butyl-2,2'-bipyridine). X-ray crystal structures of PHT and NAP complexes show a distorted square-planar arrangement of ligands around the Pt center. Both complexes form "head-to-tail" dimers in the solid state through remarkably short unsupported Pt...Pt contacts of 3.208 (PHT) and 3.378 A (NAP). The Pt(bpy*)(cat-imide) complexes are shown to combine optical (absorption) and electrochemical properties of the catecholate (electron-donor) and imide (electron-acceptor) groups. The complexes show a series of reversible reduction processes in the range from -0.5 to -1.9 V vs Fc (+)/Fc, which are centered on either bpy* or imide groups, and a reversible oxidation process at +0.07 to +0.14 V, which is centered on the catecholate moiety. A combination of UV-vis absorption spectroscopy, cyclic voltammetry, UV-vis spectroelectrochemistry, and EPR spectroscopy has allowed assignment of the nature of frontier orbitals in Pt(bpy*)(cat-imide) complexes. The HOMO in Pt(bpy*)(cat-imide) is centered on the catechol ligand, while the LUMO is localized either on bpy* or on the imide group, depending on the nature of the imide group involved. Despite the variations in the nature of the LUMO, the lowest-detectable electronic transition in all Pt(bpy*)(cat-imide) complexes has predominantly ligand-to-ligand (catechol-to-diimine) charge-transfer nature (LLCT) and involves a bpy*-based unoccupied molecular orbital in all cases. The LLCT transition in all Pt(bpy*)(cat-imide) complexes appears at 530 nm in CH2Cl2 and is strongly negatively solvatochromic. The energy of this transition is remarkably insensitive to the imide group present, indicating lack of electronic communication between the imide and the catechol moieties within the cat-imide ligand. The high extinction coefficient, approximately 6 x 10(3) L mol(-1) cm(-1) of this predominantly LLCT transition is the result of the Pt orbital contribution, as revealed by EPR spectroscopy of the complexes in various redox states. The CV profile of the oxidation process of Pt(bpy*)(cat-imide) in CH2Cl2 and DMF is concentration dependent, as was shown for NDI and PHT complexes as typical examples. Oxidation appears as a simple diffusion-limited process at low concentrations, with an increasing anodic-to-cathodic peak separation eventually resolving as two independent consecutive waves as the concentration of the complex increases. It is suggested that aggregation of the complexes in the diffusion layer in the course of oxidation is responsible for the observed concentration dependence. Overall, the Pt(bpy*)(cat-imide) complexes are electrochromic compounds in which a series of stepwise reversible redox processes in the potential range from 0.2 to -2 V (vs Fc (+)/Fc) leads to tuneable absorbencies between 300 and 850 nm.