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

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.