Spin Transitions in six-coordinate iron(II) complexes

The transition metal coordination chemistry of anion radicals

The spin-states and spin-transitions of mononuclear iron(II) complexes of nitrogen-donor ligands

It is known that the relaxed excited state of [Ru(bpy)3]2+ is best described as a metal to ligand charge transfer (MLCT) state having one formally reduced bipyridine and two neutral. Previous reports have suggested [Malone, R. et al. J. Chem. Phys. 1991, 95, 8970] that the electron “hops” from ligand to ligand in the MLCT state with a time constant of about 50 ps in acetonitrile. However, we have done transient absorption anisotropy measurements indicating that already after one picosecond the molecule has no memory of which bipyridine was initially photoselected, which suggests an ultrafast interligand randomization of the MLCT state.

Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast Interligand Randomization of the MLCT State

[Pt(tpy)Cl]^+ (tpy is 2,2‘:6‘,2‘‘-terpyridine) undergoes reversible one-electron reductions in 0.1 M TBAH/DMF (TBAH is tetrabutylammonium hexafluorophosphate) at E° ‘_(+/0) = −0.74 V and E° ‘_(0/-) = −1.30 V (vs AgCl (1.0 M KCl)/Ag). The first reduction couple is substantially positive of those observed for other M(II)−tpy complexes (for example, E° ‘_(0/-) = −1.36 V for [Zn(tpy)Cl_2]), a finding that suggests there is coupling between the empty 6p_z orbital of square planar Pt(II) and the π* orbital of tpy, stabilizing the (π*)^1 radical state. The dimerization constants of both [Pt(tpy)Cl]^+ and [Pt(tpy)Cl] in 0.1 M TBAH/DMF were determined spectroelectrochemically and found to be 8(1) × 10 and 10(4) × 10 M^(-1), respectively. On the basis of variable-concentration studies, a species observed at an intermediate level of reduction is formulated as the mixed-valence dimer [(Pt(tpy)Cl)_2]^+, with K_(mix) = [(Pt(tpy)Cl)_2^+]/([Pt(tpy)Cl^+][Pt(tpy)Cl]) = 18(4) × 10 M^(-1). Analysis of variable-temperature EPR spectra indicates that the first reduction is ligand-centered (^2B_2) with substantial contributions from Pt(II) 5d_(yz) (4−6%) and 6p_z (3−4%). The second reduction is tentatively assigned as metal-centered; 5d_(x^2 - y^2) is the likely acceptor orbital.

S. Kremer

Papers

The lower excited electronic states of singly and doubly reduced 2,2′-bipyridine

Temperature dependence of the 5T2 - 1A1 energy separation at the ground-state crossover in [FeIIN6] complexes