Hetero-dinuclear complexes of 3d metals with a bridging dinitrogen ligand: theoretical prediction of the characteristic features of geometry and spin multiplicity.

Spin multiplicities and coordination structures of dinitrogen-bridged hetero-dinuclear complexes of 3d metals, (μ-N2)[M(1)(AIP)][M(2)(AIP)] (AIPH = (Z)-1-amino-3-imino-prop-1-ene; M(1), M(2) = V(i) to Co(i)), were investigated using the CASPT2 method. (μ-N2)[V(AIP)][Cr(AIP)] has a low spin doublet ((2)B2) ground state with an η(2)-side-on dinitrogen coordination structure but (μ-N2)[Mn(AIP)][Fe(AIP)] has a high spin octet ((8)A2) ground state with an η(1)-end-on coordination structure. These results are similar to those of the homo-dinuclear Cr and Fe analogues, respectively. In (μ-N2)[Cr(AIP)][M(AIP)] (M = Mn(i), Fe(i), or Co(i)) consisting of an early 3d metal (Cr) and a late one (Mn to Co), on the other hand, we found characteristic features in the geometry and the ground state electronic structure which are different from those of homo-dinuclear analogues. The Cr-Mn complex has a high spin decet ((10)B1) ground state with an η(2)-side-on structure. This decet state has the highest spin multiplicity in the dinuclear transition metal complexes, to our knowledge. The A2 state with a doublet spin multiplicity is moderately less stable than the (10)B1 state. The optimized structures and the molecular orbitals indicate that the Cr atom strongly interacts with the N2 moiety in the (10)B1 state but the Mn atom strongly interacts with the N2 moiety in the (2)A2 state. The Cr-Fe complex has a high spin nonet ((9)B1) ground state with an η(2)-side-on structure like the Cr-Mn complex, but only the Cr-Co complex has a medium spin quartet (4)A2 ground state with an η(2)-side-on structure. The different ground electronic state of the Cr-Co complex arises from the presence of 3d orbitals at low energy. Based on these results, it is concluded that the geometry is determined by the Cr center but the electronic structure and the spin multiplicity are determined by the combination of early and late 3d metals in the dinitrogen-bridged hetero-dinuclear chelates of 3d metals.