Coordination-induced switch between the singly occupied and the lowest unoccupied molecular orbitals in two methylviologen-derived chromophores

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Abstract
The aromatic heterocycles 2,7-diazapyrene (dap) and 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (4-bptz) each exhibit two very close-lying lowest unoccupied molecular orbitals (MOs) with very different character viz. a methylviologen-like MO (b1u) and a quinonoid (dap) or tetrazine-localized MO (4-bptz) of au symmetry. Calculations (HMO semiempirical NDO type ab initio) suggest that the orbital crossing between b1u and au and thus the character of the singly occupied MO in one-electron reduced compounds can be effected by removal of electron density from the potentially coordinating pyridyl nitrogen centres. We have explored this suggestion experimentally by coordination of electrophiles either alkyl cation R+ or neutral complex fragments W(CO)5, to both pyridine N centres. Experimental data from UV–VIS–NIR and especially EPR spectroelectrochemistry show that coordination of two neutral W(CO)5 fragments to dap is sufficient to cause a change from singly occupied au(in dap˙–) to singly occupied b1u in {dap[W(CO)5]2}˙–. In contrast the 4-bptz˙– radical system requires coordination of two electrophiles R+(R = CH3, C2H5) in order to move b1u below au while the ditungsten(0) radical anion complex is still a 2Au species.

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