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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

Agapie, Theodor and Day, Michael W. and Bercaw, John E. (2008) 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. Organometallics, 27 (23). pp. 6123-6142. ISSN 0276-7333. https://resolver.caltech.edu/CaltechAUTHORS:AGAo08a

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Abstract

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 (sp_2−sp_2) 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 LX_2 frameworks. The pyridine-linked system binds in a Cs-fashion, the furan-linked system in a C2_v-fashion, and the thiophene-linked system in a C_1-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 PMe_2Ph trapping. This process was found to be independent of PMe_2Ph 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(CH_3)2[(OC_6H_2-tBu_2)2C_6H_3] (16), Ta(CH_2Ph)2[(OC_6H_2-tBu_2)2C_6H_3] (17), and TaCl_2CH_3[(OC_6H_2-tBu_2)2C_6H_4] (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 TaC_l2(CH_3)3 to generate 18. This process involves protonolysis of a methyl group, followed by C-H/Ta-CH_3 σ bond metathesis leading to cyclometalation of the linking ring, and finally protonation of the cyclometallated group by the pendant phenol. TaCl_2CH_3[(OC_6H_2-tBu_2)2C_6H_4] was found to undergo σ bond metathesis at temperatures over 90 °C to give the pincer complex TaCl_2[(OC_6H_2-tBu_2)2C_6H_3] (1_9) 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(CH_3)_2[(OC_6H_2-tBu_2)_2C_6H_3] (16) was found to react with tBuNC to insert into the Ta-CH_3 bonds and generate an imino-acyl species (23). Reaction of 16 with Ph_2CO 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-OEt_2, 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-C_2-, and C_1-symmetric structures.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/om8002653DOIArticle
http://pubs.acs.org/doi/abs/10.1021/om8002653PublisherArticle
ORCID:
AuthorORCID
Agapie, Theodor0000-0002-9692-7614
Additional Information:© 2008 American Chemical Society. Publication Date (Web): November 6, 2008. We thank Lawrence M. Henling (Caltech) for assistance with single crystal X-ray crystallographic studies. This work has been supported by USDOE Office of Basic Energy Sciences (Grant No. DE-FG03-85ER13431).
Funders:
Funding AgencyGrant Number
CaltechUNSPECIFIED
Department of Energy (DOE)DE-FG03-85ER13431
Subject Keywords:X-RAY-STRUCTURE; ALKYLIDENE-CENTERED REACTIVITY; ARYL OXIDE LIGATION; CRYSTAL-STRUCTURE; LIVING POLYMERIZATION; ZIRCONIUM COMPLEXES; ARYLDIAMINE LIGAND; PINCER COMPLEXES; PYRIDINE-2,6-BIS(ARYLOXIDE) LIGANDS; NEOPENTYLIDENE COMPLEXES.
Issue or Number:23
Record Number:CaltechAUTHORS:AGAo08a
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:AGAo08a
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13413
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:14 May 2009 22:48
Last Modified:03 Oct 2019 00:37

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