Synthesis and properties of Re(I) tricarbonyl complexes of 6,6′-disubstituted-4,4′-bipyrimidines with high energy excited states suitable for incorporation into polynuclear arrays

A series of complexes [(N–N)Re(CO)3X]n (N–N = 6,6′-diaryl-4,4′-bipyrimidine, axial ligand X = Cl, MeCN, 4-phenyl-pyridine, or t-Bu-pyridine) have been synthesized and characterized. The substituent aryl on the bipyrimidine, as well as the axial ligand X, has important effects on the properties of these complexes. The co-planarity of the 4,4′-bipyrimidine core and its substituents contributes to the extent of π-electron delocalization and, hence, to the redox and spectroscopic properties of the complexes. The complexes exhibit Re-to-bpm metal-to-ligand charge transfer absorptions in the range of 379–464 nm, which are red-shifted with the increase in the delocalization in the substituted bpm ligand and the increase in the donor character of the axial ligand. The electrochemical data support metal-based oxidations (from +1.07 to +1.40 V) and ligand-based reductions (from −0.62 to −0.75 V) and correlate well with those obtained by UV–Vis spectroscopy. The X-ray crystal structures of two of the complexes have also been investigated.