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 (η⁵-permethylindenyl)tetrachlorotantalum, Ind*TaCl₄, was prepared by addition of previously-unreported Ind*SnBu₃ to TaCl₅ in toluene at 70° 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)° to 5.10(17)°. 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 λ_max at 593 nm (ε = 2000 M⁻¹ cm⁻¹) in toluene is remarkably red-shifted from that of the known η⁵-pentamethylcyclopentadienyl analog, luminescent-yellow Cp*TaCl₄. TD-DFT calculations were used to explain the comparative UV/vis absorption spectra.