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Role of the Metal in the Bonding and Properties of Bimetallic Complexes Involving Manganese, Iron, and Cobalt

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posted on 05.02.2014 by Stephen J. Tereniak, Rebecca K. Carlson, Laura J. Clouston, Victor G. Young, Eckhard Bill, Rémi Maurice, Yu-Sheng Chen, Hyun Jung Kim, Laura Gagliardi, Connie C. Lu
A multidentate ligand platform is introduced that enables the isolation of both homo- and heterobimetallic complexes of divalent first-row transition metal ions such as Mn­(II), Fe­(II), and Co­(II). By means of a two-step metalation strategy, five bimetallic coordination complexes were synthesized with the general formula M1M2Cl­(py3tren), where py3tren is the triply deprotonated form of N,N,N-tris­(2-(2-pyridylamino)­ethyl)­amine. The metal–metal pairings include dicobalt (1), cobalt–iron (2), cobalt–manganese (3), diiron (4), and iron–manganese (5). The bimetallic complexes have been investigated by X-ray diffraction and X-ray anomalous scattering studies, cyclic voltammetry, magnetometry, Mössbauer spectroscopy, UV–vis–NIR spectroscopy, NMR spectroscopy, combustion analyses, inductively coupled plasma optical emission spectrometry, and ab initio quantum chemical methods. Only the diiron chloride complex in this series contains a metal–metal single bond (2.29 Å). The others show weak metal–metal interactions (2.49 to 2.53 Å). The diiron complex is also distinct with a septet ground state, while the other bimetallic species have much lower spin states from S = 0 to S = 1. We propose that the diiron system has delocalized metal–metal bonding electrons, which seems to correlate with a short metal–metal bond and a higher spin state. Multiconfigurational wave function calculations revealed that, indeed, the metal–metal bonding orbitals in the diiron complex are much more delocalized than those of the dicobalt analogue.

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