Excited states of Ru(II) and Re(I) bipyridyl complexes attached to cyclotriphosphazenes: a synthetic, spectroscopic, and computational study.

A series of new cyclotriphosphazene ligands substituted with pendant 2,2'-bipyridyl moieties, namely, bis(1,1'-biphenyl)-2,2'-dioxycyclotriphosphazene (L(1)), bis[(1,1'-biphenyl)-2,2'-dioxy][bis{4-(2,2'-bipyridin)-4-yl-phenyoxy}]cyclotriphosphazene (L(2)), (pentaphenoxy)[4-(2,2'-bipyridin)-4-yl-phenyoxy]cyclotriphosphazene (L(3)), and (pentaphenoxy) [4-{6-phenyl(2,2'-bipyridin)-4-yl}-phenoxy]cyclotriphosphazene (L(4)), has been used to synthesize the ruthenium(II) and rhenium(I) complexes, (L)Ru(bpy)(2)(4) (L = L(1) or L(3)), (L(2)) {Ru(bpy)(2)}(2)(4), [(L)Re(CO)(3)Cl] (L = L(1), L(3) or L(4)), and [(L(2)) {Re(CO)(3)Cl}(2)]. Single crystal X-ray structures of [(L(1))Re(CO)(3)Cl] and [(L(4))Re(CO)(3)Cl] show the bipyridyl component of the cyclotriphosphazene substituted ligands is bound to the Re(I) giving a distorted octahedral "N(2)C(3)Cl" coordination sphere in both cases. Density functional theory (DFT) methods were employed to model the ground-state vibrational properties of the molecules, and their accuracies verified using vibrational spectroscopy. Electronic transitions were identified using UV-visible and resonance Raman spectroscopic techniques, aided by time-dependent (TD) DFT methods. Transient resonance Raman spectra of the excited states of the compounds were acquired and found to be comparable to those reported for studied metal bipyridyl units lacking the cyclotriphosphazene substituents. The cyclotriphosphazene unit has little effect on the properties of the metal bipyridyl chromophore.