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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

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journal contribution
posted on 08.12.2008, 00:00 authored by Theodor Agapie, Michael W. Day, John E. Bercaw
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.