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 Cs-fashion, the furan-linked system in a C2v-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 kH/kD = 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; kH/kD = 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 C2v-, pseudo-Cs-, pseudo-C2-, and C1-symmetric structures.