While metal-to-ligand charge transfer (MLCT) has been extensively studied for numerous solar energy, photochemical, and photophysical applications, much less is known about the reverse - LMCT. The bicyclic guanidinates hpp- and hpp*- (anions of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, hppH, and 3,3,7,7-tetramethyl-1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, hpp*H) can stabilize metal centers by a resonance contribution that provides extra charge transfer from the bridgehead nitrogen lone pair (as shown by bridgehead nitrogen planarity in solid-state structures). Here we describe the syntheses and spectrofluorimetry of several tantalum and zirconium hpp- and hpp*- complexes that display LMCT, including TaV(hpp)Cl4, TaV(hpp)2Cl3, TaV(hpp)(hpp*)Cl3, TaV(hpp*)2Cl3, TaV(hpp*)Cl4, the pentamethylcyclopentadienyl (Cp*) compounds Cp*TaV(hpp)Cl3 and d1 Cp*TaIV(hpp)Cl2, ZrIV(hpp)Cl3, ZrIV(hpp)2Cl2, and Cp*ZrIV(hpp)Cl2. The results of spectrofluorometric studies on Zr(hpp)(bipy’)Cl3 complexes (bipy’ = bipy, (Me3C)2bipy, Me2bipy, (F3C)2bipy) will be described, as they exhibit unusual, rare ligand-to-ligand charge transfer (LLCT).

Ligand to metal charge transfer in early transition metal bicyclic guanidinate complexes

253rd National Meeting (2017)

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