Isolation and Chemistry of Tantalum(V) Compounds Containing Two Resolved 3,3‘-Disubstituted-1,1‘-bi-2,2‘-Naphthoxide Ligands

Reaction of dimeric [(Et2N)2Cl2Ta(μ-Cl)2TaCl2(NEt2)2] with (R)-[H2O2C20H10(R)2-3,3‘]
(R = SiMe3, SiMe2Ph, SiMePh2, or SiPh3) produces the 3,3‘-dialkyl-1,1‘-bi-2,2‘-naphthoxido
complexes (R,R)-[H2NEt2][Ta(O2C20H10{R}2-3,3‘)2Cl2] (R = SiMe3, (R,R)-1; SiMe2Ph, (R,R)-2; SiMePh2, (R,R)-3; SiPh3, (R,R)-4). Structural studies of the SiMe3 derivative (R,R)-1 show
a pseudo-octahedral anion with mutually cis chloride ligands. Although NMR studies indicate
that the anion of (R,R)-1 is stable in DMSO-d6, an equilibrium mixture is observed in C6D6
solution with the neutral complex (R,R)-[Ta{O2C20H10(SiMe3)2-3,3‘}2Cl], (R,R)-1a, and
[H2NEt2]Cl. The temperature dependence of the equilibrium has been studied. Addition of
[nBu4N]Cl to a mixture of (R,R)-1/(R,R)-1a in C6D6 is shown (1H NMR) to shift the equilibrium
toward (R,R)-1. Alkylation of (R,R)-1 with Grignard reagents RMgCl results in formation of
the five-coordinate (R,R)-[Ta(O2C20H10{SiMe3}2-3,3‘}2R‘] (R‘ = CH2SiMe3, (R,R)-5; CH2Ph,
(R,R)-6; c-C5H9, (R,R)-7).