Synthesis, Structural Characterization, and Cyclometalation Chemistry of Tantalum Terphenyl Compounds

Abstract

The κ 1 -m-terphenyl complex of tantalum, [Ar Tol 2 ]Ta-(NMe 2 ) 3 Cl ([Ar Tol 2 ] = 2,6-di-p-tolylphenyl), has been synthesized by the reaction of [Ta(NMe 2 ) 3 Cl 2 ] 2 with two equivalents of [Ar Tol 2 ]Li.[Ar Tol 2 ]Ta(NMe 2 ) 3 Cl provides access to a variety of monoalkyl compounds, [Ar Tol 2 ]Ta(NMe 2 ) 3 R (R = Me, Et, Pr n , Bu n , and Np; Np = CH 2 Bu t ), via the reactions with the corresponding RLi. In addition, the reaction of [Ta(NMe 2 ) 3 Cl 2 ] 2 with excess [Ar Tol 2 ]Li affords the bis(terphenyl) complex, [Ar Tol 2 ] 2 Ta(NMe 2 ) 3 , while the reaction of [Ar Tol 2 ]Ta(NMe 2 ) 3 Cl with LiBH 4 gives the borohydride complex, [Ar Tol 2 ]Ta(NMe 2 ) 3 (κ 2 -BH 4 ). The dichloride compound, [Ar Tol 2 ]Ta(NMe 2 ) 2 Cl 2 , which is obtained via the reaction of [Ar Tol 2 ]Ta(NMe 2 ) 3 Cl with Me 3 SiCl, provides access to a series of dialkyl derivatives, [Ar Tol 2 ]Ta(NMe 2 ) 2 R 2 (R = Me, Et, Pr n , Bu n , and Np), via the reactions with the corresponding RLi. The κ 1 -mterphenyl ligands in these complexes are susceptible to metalation. Thus, [Ar Tol 2 ]Ta(NMe 2 ) 3 R eliminates RH to afford [κ 2 -Ar Tol,Tol′ ]Ta(NMe 2 ) 3 (Tol′ = C 6 H 3 Me), while [Ar Tol 2 ]Ta(NMe 2 ) 2 Np 2 eliminates NpH to form [κ 2 -Ar Tol,Tol′ ]Ta(NMe 2 ) 2 Np. ■ INTRODUCTIONAncillary ligands play a critical role in organometallic chemistry by allowing tunable properties of a metal center. 1 One such class that has received considerable attention is that of the mterphenyl ligand ([Ar Ar 2 ]) (Figure 1), which provides significant steric protection by virtue of the outer aryl rings. Two common examples are [Ar Mes 2 ] and [Ar Dipp 2 ] (Figure 1). 2 Indeed, structurally characterized [κ 1 -Ar Ar 2 ]M compounds are known for a substantial number of metals, including transition metals despite this interest in the use of terphenyl ligands, there are still many elements that have received little attention (Figure 2). In this regard, our recent use of the 2,6-di-p-tolylphenyl ligand ([Ar Tol 2 ]) to synthesize [Ar Tol 2 ]TaMe 3 Cl and [Ar Tol 2 ]TaMe 2 Cl 2 represents, to the best of our knowledge, the first application of terphenyl ligands to tantalum chemistry. 6 While many of the applications of m-terphenyl ligands have focused on the use of steric protection to allow for the isolation of uncommon and low coordination complexes, 2 our interest pertains to the potential of using such ligands to construct multidentate variants via cyclometalation. As an illustration, we recently employed this approach to obtain the first examples of transition metal compounds that feature a [CCC] X 3 -donor pincer ligand, 36 namely [κ 3 -Ar Tol′ 2 ]Ta(PMe 3 ) 2 MeCl, [κ 3 -Figure 1. Examples of [κ 1 -m-terphenyl] ligands.Figure 2. Elements for which κ 1 -m-terphenyl compounds have been structurally characterized by X-ray diffraction (blue), and those for which there are no structurally characterized examples listed in the Cambridge Structural Database (red).Article pubs.acs.org/Organometallics