Hydrogenation Mechanisms in (Boratacycle)tantalum Analogues of Dimethylzirconocene

Abstract

The hydrogenation of Cp*[C4H4B-N(i-Pr)2]TaMe2 (1) (Cp* = C5Me5) in the presence of PMe3 affords Cp*[C4H4B-N(i-Pr)2]Ta(H)2(PMe3) (2) in essentially quantitative yield. Similarly, the hydrogenation of Cp*[C4H4B-Me]TaMe2 (3) in the presence of PMe3 affords Cp*[C4H4B-Me]Ta(H)2(PMe3) (4). Hydrogenation of 1 and 3 is accompanied by the reversible formation of side products. The most important of these complexes, Cp*[C4H4B-N(i-Pr)2]Ta(PMe3)2 (5) and Cp*[C4H4B-Me]Ta(PMe3)2 (6), react slowly with dihydrogen forming 2 and 4, respectively. In the early stages of the hydrogenation of 1, the C−H activation product Cp*[C4H4B-N(i-Pr)2]Ta(H)(CH2PMe2) (7) is also present. Mechanistic details of the hydrogenation of 1 and 3 are discussed. Hydrogenation of [C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2 (8) in the presence of PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PMe3)2 (9) as the exclusive product. The use of a bulkier phosphine, P(i-Pr)3, gives [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2[P(i-Pr)3] (10). Changing the phosphine to one of intermediate bulk, PEt3, leads to the formation of trans-[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PEt3) (11t). The cis isomer (11c) is observable during early reaction times. 11c is a classical dihydride, perturbed by an unsymmetric three-center/two-electron interaction with the boron of the boratabenzene ligand. Isomerization of 11c to 11t proceeds via phosphine loss followed by kinetically detectable rearrangement of the unsaturated intermediate prior to phosphine recoordination. Treatment of 11c with excess PMe3 results in the formation of 9 via a mixed-phosphine intermediate, [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PEt3)(PMe3) (12). The addition of [H(OEt2)2][B(C6H3(CF3)2] to 11c results in the protonation of the nitrogen atom of the borollide ligand (H-11c + ). H-11c + is stable at room temperature for over a week. Treatment of 10 with excess PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PMe3) (13). Upon thermolysis in the presence of a large excess of PMe3, 13 is converted to 9. A mechanistic scheme for the hydrogenation of complexes such as 1 is proposed.

Intramolecular Nucleophilic Substitution on Coordinated Borabenzenes: A New Entry into Boratabenzene Complexes

“…Boratabenzene catalysts have appeared with reactivities that complement those observed with standard group 4 metallocenes . Furthermore, while both cyclopentadienyl (Cp) and boratabenzene are formally monoanionic 6π electron donors, boratabenzene is a weaker donor . Isostructural complexes containing boratabenzene instead of Cp therefore have a greater tendency for lower oxidation states and offer altered mechanistic pathways for elementary reactions …”

“…4 Furthermore, while both cyclopentadienyl (Cp) and boratabenzene are formally monoanionic 6π electron donors, boratabenzene is a weaker donor. 5 Isostructural complexes containing boratabenzene instead of Cp therefore have a greater tendency for lower oxidation states and offer altered mechanistic pathways for elementary reactions. 6 One of the main difficulties in advancing a broader use of boratabenzene complexes in industry 7 and in noncatalytic reactions useful for organic chemistry 8 is the multistep synthesis of the boratabenzene framework.…”

Transition Metal Polyhydride Complexes. 11. Mechanistic Studies of the Cis to Trans Isomerization of the Iridium(III) Dihydride Ir(H)2(CO)L (L = C6H3(CH2P(H)2)2)

“…Examples of the latter include several reports on the kinetics and thermodynamics of the dihydrogen/dihydride equilibrium in some tungsten and rhenium complexes. − However, if the cis-dihydride product, which is usually formed first in the H 2 addition process, is not thermodynamically stable, subsequent rearrangements may give another cis- or trans-dihydride complex. This concomitant process has not been thoroughly investigated either experimentally or theoretically. − To the best of our knowledge, the rearrangement of a kinetically formed dihydride isomer to the thermodynamically stable dihydride isomer has been investigated experimentally in only three cases: M(H) 2 (L) 4 (M = Fe, Ru; L = phosphorus donor), Ir(H) 2 Br(CO)(dppe), and [C 5 H 5 B−Ph][C 4 H 4 B−N(i-Pr) 2 ]Ta(H) 2 (PEt 3 ) . In the first case, an intramolecular nondissociative path, specifically the “tetrahedral jump”, was established by NMR line-shape analysis .…”

Phosphidoboratabenzene–rhodium(i) complexes as precatalysts for the hydrogenation of alkenes at room temperature and atmospheric pressure

“…12b It is precisely the hemilability of the DTBB and its donor capability as a bulky phosphine that makes it an attractive framework for catalytic applications. 15 Herein we report the synthesis and characterization of a rhodium di-tertbutylphosphidoboratabenzene complex and its catalytic activity towards alkene hydrogenation at room temperature and atmospheric pressure.…”