Electronic Coupling in a Highly Preorganized Bimetallic Complex Comprising Pyrazolate-Bridged CpMn(CO)2 Moieties

Abstract

By means of a multistep synthetic procedure a dimanganese complex has been prepared, in which a N,N′-bridging pyrazolate ligand spans two CpMn(CO)2 subunits in a highly preorganized chelate arrangement. The Xray crystallographic analyses of the MnIMnI complex K+1− and of its non-chelate precursor complex elucidate details of the molecular structure, in particular an unusual pyrazolate binding mode in the solid state and intertwining of the CO ligands in the crowded bimetallic array 1−. The MnIMnI compound (1−), the mixed-valent MnIMnII (1), and the oxidized MnIIMnII form (1+) have been characterized by various analytical and spectrosopic methods, such as electrochemistry, variable-temperature EPR spectroscopy, IR spectroelectrochemistry, and UV/Vis/NIR spectroelectrochemistry as well as by DFT and TD-DFT calculations. Strong electronic coupling in the mixed-valent complex is observed, but time- (and temperature-) dependent valence detrapping occurs, thus placing 1 in class II according to the Robin and Day assignment, close to the class II/III transition. From variable-temperature EPR spectroscopy a rough estimate of the activation energy and rate for thermal electron transfer can be deduced, with Eth≠=13.6 kJ mol−1 and kth=2.6×1010 s−1 at 298 K. Unexpectedly, no intervalence CT transition for 1 is detected in solution, but one appears in the optical spectrum of solid 1. The conclusions drawn from experiments are fully supported by DFT calculations that were carried out for all three forms of the dimanganese complex. A broken symmetry treatment for mixed-valent 1 reveals almost perfect localization of both spin and charge on one Mn center. According to TD-DFT the first excited states of 1 give rise to the IT processes in the NIR-energy region, as observed in the solid-state spectrum. The HOMOs are located at the Mn ions and are favorably arranged for π interactions with the bridging pyrazolate.