Evidence for the formation of a tantalum(IV) arene species from arene-alkyl complexes of tantalum(III)

Abstract

Alkylation of tantalum(III) arene complexes affords stable dialkyl and monoalkyl halide species ( 6-C6Me6)Ta(DIPP)R2 (DIPP = 2,6-diisopropylphenoxkte) and (776-C6Mee)Ta(DI PP)RX. The alkyl hydride complexes (776-C6Me6)Ta(D I PP)R(H) are also prepared from { 6-C6Me6)Ta(DIPP)RX and UBEt3H. The arene ring in ( 6-C6Me6)Ta(DIPP)Et2 exhibits a structure consistent with a diene-diyl distortion. The first evidence for the formation of a d1 arene species is presented in cyclic voltammetry experiments on these compounds.

Amidate Complexes of Tantalum and Niobium for the Hydroaminoalkylation of Unactivated Alkenes

“…To date, the possibility of a mechanism for the hydroaminoalkylation with early transition metals involving one-electron radical processes has not been explored in the literature. However, the redox couple is accessible between d 0 Ta­(V) and d 1 Ta­(IV) and has been reported to occur between 0.31 and 0.74 V in traditional organic solvents . Synthetically, Ta­(V) centers can be efficiently reduced to d 1 species with nucleophilic reagents such as n -butyllithium .…”

“…However, the redox couple is accessible between d 0 Ta(V) and d 1 Ta(IV) and has been reported to occur between 0.31 and 0.74 V in traditional organic solvents. 35 Synthetically, Ta(V) centers can be efficiently reduced to d 1 species with nucleophilic reagents such as n-butyllithium. 36 On the basis of this ease of reduction, the possibility of radical participation from a Ta(IV) center during catalysis is plausible and should be considered.…”

Niobium and Tantalum

“…Thus, stable T a 1 1 complexes (r|-C 6 R 6 )Ta(OAr) 2 ( R = Me, Et) are prepared f r o m the one-electron reduction o f (T|-C 6 R 6 )Ta(OAr) 2 Cl 596 ' 597 and electrochemical evidence has been presented for the existence of unstable [(r|-C 6 Me 6 )Ta(OAr)R 2 ] + complexes (R = Me or Ph) 598. …”

Synthetic, Structural, and Redox Studies of Arene Alkyl Complexes of Tantalum(III) Supported by Aryloxide and Arenethiolate Ligands

“…Several successful forays have been made into niobium and tantalum arene chemistry in recent years as a result of improved methods for introducing an arene ligand to these metals . The initial coordination of an η 6 -arene is typically accomplished by one of the following approaches: (1) by reduction of a metal halide in the presence of the arene, − including Al/AlX 3 reduction under Fischer−Hafner conditions; (2) by substitution reactions using a halide acceptor, in which reduction of the metal does not occur; − (3) by metal vapor synthesis procedures; − (4) by alkyne cyclotrimerization chemistry; − or more recently, (5) by arene exchange reactions . These synthetic strategies have permitted access to a range of oxidation states in niobium and tantalum arenes, from d 6 M(−I) to d 1 M(IV)…”

“…These complexes have proven especially valuable since rare Ta(II) and Ta(IV) arenes are both accessible from redox reactions of their Ta(III) counterparts. Thus, stable Ta(II) complexes (η 6 -C 6 R 6 )Ta(OAr) 2 (R = Me, Et; Ar = 2,6-C 6 H 3 i Pr 2 ) are prepared from the one-electron reduction of (η 6 -C 6 R 6 )Ta(OAr) 2 Cl, , and electrochemical evidence has been presented for the existence of labile [(η 6 -C 6 Me 6 )Ta(OAr)R 2 ] + complexes, thereby affording the first evidence for d 1 arene species.…”