The indenyl effect on d0 organotransition metal complex LMCT luminescence: Synthesis, structural characterization, cyclic voltammetry, spectroscopic studies, and TD-DFT computations on (η5-permethylindenyl)tetrachlorotantalum(V), and comparison to (η5-pentamethylcyclopentadienyl)tetrachlorotantalum(V)

The new organotantalum(V) chloride (eta(5)-permethylindenyl)tetrachlorotantalum, Ind*TaCl4, was prepared by addition of previously-unreported Ind*SnBu3 to TaCl5 in toluene at 70 degrees in 53 % isolated yield. The solid-state four-legged piano-stool structure, consistent with solution multinuclear NMR spectroscopies, demonstrated that tantalum is bonded to all carbons of the five-membered ring of the permethylindenyl ligand, with little deviation in the Ta-C distances. The permethylindenyl ring torsional angles for the three independent molecules in the unit cell vary from 2.46(17)degrees to 5.10(17)degrees. The cyclic voltammogram of Ind*TaCl4 in tetrahydrofuran showed one reversible reduction feature at -1.24 V [Ta(V)/Ta(IV)] and one quasi-reversible reduction feature at -1.65 V [Ta(IV)/Ta(III)] vs. Fc/Fc(+). The UV/vis LMCT absorption lambda(max) at 593 nm (epsilon = 2000 M-1 cm(-1)) in toluene is remarkably red-shifted from that of the known eta(5)-pentamethylcyclopentadienyl analog, luminescent-yellow Cp*TaCl4. TD-DFT calculations were used to explain the comparative UV/vis absorption spectra.