Structure dependent charge transfer in bipyrimidinium–octacyanotungstate ion pairs

Two solid state forms of the {(bpymH₂)²⁺,[W^IV(CN)₈]⁴⁻} charge-transfer ion pair have been obtained in the reaction between 2,2′-bipyrimidine (bpym) and octacyanotungstate(IV) in acidic solution and characterised in terms of structure and spectroscopic properties: (bpymH₂)₂[W^IV(CN)₈]·4H₂O (1) and (PPh₄)₂(bpymH₂)[W^IV(CN)₈]·10H₂O (2). In contrast to the yellow potassium salt, compound 1 is red and shows an outer-sphere charge-transfer (IPCT) band in the visible range of the electronic spectrum. In addition, due to thermal electron transfer ca. 0.3% of the diamagnetic W^IV centres is oxidised to paramagnetic W^V, as shown by the EPR spectra. In the yellow compound 2 the extent of the thermal charge separation increases to 16%, which can be rationalised by charge delocalisation through the π–π interactions between bpym and the phenyl rings of the additional PPh4+ cations.