Synthesis and Reactivity of Tantalum Complexes Supported by Bidentate X₂ and Tridentate LX₂ Ligands with Two Phenolates Linked to Pyridine, Thiophene, Furan, and Benzene Connectors: Mechanistic Studies of the Formation of a Tantalum Benzylidene and Insertion Chemistry for Tantalum−Carbon Bonds

Using either alkane elimination or salt metathesis methods, tantalum complexes have been prepared with new ligand systems with tridentate bis(phenolate)donor (donor = pyridine, furan, and thiophene) or bidentate bis(phenolate)benzene arrangements. The ligand framework has two X-type phenolates connected to the flat heterocyclic L-type donor at the 2,6- or 2,5- positions or to the 2,6- positions of benzene via direct ring−ring (sp²−sp²) linkages. Solid-state structures of these complexes show that in all cases the ligands bind in a mer fashion, but with different geometries of the LX₂ frameworks. The pyridine-linked system binds in a C_s-fashion, the furan-linked system in a C_2v-fashion, and the thiophene-linked system in a C₁-fashion. A bis(phenolate)pyridine tantalum tribenzyl species (7), upon heating in the presence of dimethylphenylphosphine, generates a stable benzylidene complex by α-hydrogen abstraction with loss of toluene and PMe₂Ph trapping. This process was found to be independent of PMe₂Ph concentration with ΔH^⧧ = 31.3 ± 0.6 kcal·mol⁻¹ and ΔS^⧧ = 3 ± 2 cal·mol⁻¹·K⁻¹, and the kinetic isotope effect k_H/k_D = 4.9 ± 0.4, consistent with a mechanism involving rate determining α-hydrogen abstraction with loss of toluene, followed by fast phosphine coordination to the resulting benzylidene species. An X-ray structure determination reveals that the benzylidene π-bond is oriented perpendicular to the oxygen−oxygen vector, in accord with the prediction of DFT calculations. Tantalum alkyl complexes with the benzene-linked bis(phenolate) ligand (Ta(CH₃)₂[(OC₆H₂-tBu₂)₂C₆H₃] (16), Ta(CH₂Ph)₂[(OC₆H₂-tBu₂)₂C₆H₃] (17), and TaCl₂CH₃[(OC₆H₂-tBu₂)₂C₆H₄] (18)) are obtained with (to afford pincer complexes) or without cyclometalation at the ipso-position. Deuterium labeling of the phenol hydrogens and of the linking 1,3-benzene-diyl ring reveals an unexpected mechanism for the metalation of bis(phenol)benzene with TaCl₂(CH₃)₃ to generate 18. This process involves protonolysis of a methyl group, followed by C-H/Ta-CH₃ σ bond metathesis leading to cyclometalation of the linking ring, and finally protonation of the cyclometallated group by the pendant phenol. TaCl₂CH₃[(OC₆H₂-tBu₂)₂C₆H₄] was found to undergo σ bond metathesis at temperatures over 90 °C to give the pincer complex TaCl₂[(OC₆H₂-tBu₂)₂C₆H₃] (19) and methane (ΔH^⧧ = 27.1 ± 0.9 kcal·mol⁻¹; ΔS^⧧ = −2 ± 2 cal·mol⁻¹·K⁻¹; k_H/k_D = 1.6 ± 0.2 at 125 °C). Ta(CH₃)₂[(OC₆H₂-tBu₂)₂C₆H₃] (16) was found to react with tBuNC to insert into the Ta-CH₃ bonds and generate an imino-acyl species (23). Reaction of 16 with Ph₂CO or PhCN leads to insertion into the Ta-Ph bond to give 21 and 22. Complexes 6, 7, 10, 11-P, 12, 13, 17, 18, 19-OEt₂, 21, 22, and 23 have been structurally characterized by single crystal X-ray diffraction, and all show a mer binding mode of the diphenolate ligands, but the ligand geometry varies leading to C_2v-, pseudo-C_s-, pseudo-C₂-, and C₁-symmetric structures.