Spin delocalization in electron-rich iron(III) piano-stool σ-acetylides.: An experimental (NMR) and theoretical (DFT) investigation

Several paramagnetic electron-rich Fe(III) mononuclear arylacetylide complexes of formula [(eta(2)-dppe)(eta(5)-C5Me5)Fe(CC-Ar)](+) in which Ar represents a functional aryl group were studied by means of multinuclear NMR. All signals detected for the various nuclei were assigned. Hyperfine coupling constants for selected nuclei of the arylacetylide ligand were derived from H-1 or F-19 NMR contact shifts. These NMR data are diagnostic of a metal-centered unpaired electron partly residing in a pi molecular orbital on the arylacetylide ligand, in line with DFT computations. We show here that the H-1 NMR paramagnetic shifts of the ortho (H-1) and meta (H-2) arylacetylide protons convey decisive information on the charge distribution in the aryl ring. Estimates of the relaxation rates of the unpaired electron were also derived from half-widths of the H-1 NMR signals. Finally, line-broadening studies of Fe(II)/Fe(III) mixtures allowed extracting the self-exchange rates for several redox couples among these complexes. The self-exchange rates appear slightly substituent dependent and are apparently larger for compounds with electron-withdrawing substituents on the aryl ring. Reorganization energies of ca. 4000 cm(-1) could be derived for these outer-sphere electron-transfer processes.