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

Abstract

Spin multiplicities and coordination structures of dinitrogen-bridged hetero-dinuclear complexes of 3d metals, (μ-N₂)[M¹(AIP)][M²(AIP)] (AIPH = (Z)-1-amino-3-imino-prop-1-ene; M¹, M² = V(I) to Co(I)), were investigated using the CASPT2 method. (μ-N₂)[V(AIP)][Cr(AIP)] has a low spin doublet (²B₂) ground state with an η²-side-on dinitrogen coordination structure but (μ-N₂)[Mn(AIP)][Fe(AIP)] has a high spin octet (⁸A₂) ground state with an η¹-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 (¹⁰B₁) ground state with an η²-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 ¹⁰B₁ state. The optimized structures and the molecular orbitals indicate that the Cr atom strongly interacts with the N₂ moiety in the ¹⁰B₁ state but the Mn atom strongly interacts with the N₂ moiety in the ²A₂ state. The Cr–Fe complex has a high spin nonet (⁹B₁) ground state with an η²-side-on structure like the Cr–Mn complex, but only the Cr–Co complex has a medium spin quartet ⁴A₂ ground state with an η²-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.