Tris(η4-naphthalene)- and tris(1-4-η4-anthracene)tantalate(1-): First homoleptic arene complexes of anionic tantalum

Reduction of tantalum pentachloride by 6 equiv of sodium naphthalene in 1,2-dimethoxyethane provided, after recrystallization from tetrahydrofuran, 50-55% yields of yellow, pyrophoric [Na(THF)][Ta(C₁₀H₈)₃]. The product was shown by ¹H and ¹³C NMR spectra and an X-ray study (on the corresponding [Na(crypt 2.2.2)]salt) to be tris(η⁴-naphthalene)tantalate(1-), 1, the first homoleptic naphthalene complex of a third row (5d) transition metal. Salts of 1 react under mild conditions with excess CO (1 atm pressure, -60° to +20 °C) and 3 equiv of anthracene, C₁₄H₁₀ (20 °C), to give 99 and 52% yields of yellow [Ta(CO)₆]- and orange [Ta(C₁₄H₁₀)₃]-, (2), respectively. The latter is the first homoleptic anthracene complex of a group 5 element and only the third one known, the others being Cr(η⁶-C₁₄H₁₀)₂ and [Co(η⁴-C₁₄H₁₀)₂]-. NMR spectra and X-ray structural characterization, as the [Na(crypt 2.2.2)] salt, established 2 to be [Ta(1-4-η⁴-C₁₄H₁₀)₃]- and is very similar to 1 in solution and in the solid state. Salts of 2 also undergo facile ligand substitution reactions. For example, it reacts with 1,3,5,7-cyclooctatetraene, COT, at 20° in THF to give high yields of the previously known [Ta(COT)₃]-, which was structurally characterized as the Na(crypt 2.2.2)salt. One particularly important feature concerning 1 and 2 is that they are the first available synthons for "naked" atomic Ta- and promise to be useful reagents for the general exploration of low-valent tantalum chemistry. Also, 1 and 2 represent the first homoleptic arene tantalum complexes to have been prepared by conventional syntheses. The only previously known substance of this class is the neutral bis(benzene)tantalum(0), which was accessed by the co-condensation of atomic tantalum and benzene vapor in a sophisticated (electron-gun furnace equipped) metal atom reactor.