COORDINATION-INDUCED SWITCH BETWEEN THE SINGLY OCCUPIED AND THE LOWEST UNOCCUPIED MOLECULAR-ORBITALS IN 2 METHYLVIOLOGEN-DERIVED CHROMOPHORES

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 u noccupied molecular orbitals (MOs) with very different character, viz. , a methylviologen-like dagger MO (b(1u)) and a quinonoid (dap) or tet razine-localized MO (4-bptz) of a(u) symmetry. Calculations (HMO, semi empirical NDO type, ab initio) suggest that the orbital crossing betwe en 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 elect rophiles, either alkyl cations, 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 t wo neutral W(CO)(5) fragments to dap is sufficient to cause a change f rom singly occupied a(u) (in dap(.-)) to singly occupied b(1u) in {dap [W(CO)(5)](2)}(.-). In contrast, the 4-bptz(.-) radical system require s coordination of two electrophiles R(+) (R = CH3, C2H5) in order to m ove b(1u) below a(u) while the ditungsten(0) radical anion complex is still a (2)A(u) species.