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

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 (b_1u) and a quinonoid (dap) or tetrazine-localized MO (4-bptz) of a_u symmetry. Calculations (HMO semiempirical NDO type ab initio) suggest that the orbital crossing between b_1u and a_u 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 a_u(in dap˙^–) to singly occupied b_1u 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 b_1u below a_u while the ditungsten(0) radical anion complex is still a ²A_u species.