Reaction of [{RhCl(CO)2}2] with a phosphine/thioether/borane ligand (TXPB) gave [Rh(μ-Cl)(CO)(TXPB)]
(1), a rare example of a complex containing a M−Cl−BR3
bridging interaction. Reaction of 1 with K[CpFe(CO)2] gave
[(TXPB)Rh(μ-CO)2Fe(CO)Cp] (2), which contains an unprecedented η3-interaction between rhodium and the B−C
ipso
−C
ortho
unit of a triarylborane. DFT calculations suggest a bonding
description intermediate between that expected for an isolated
borane/alkene complex and a fully delocalized allyl-like
complex.
The incorporation of o-phenylene-linked diamidophosphine ligands onto the readily available alkyne complexes Ta(alkyne)Cl 3 (DME) (where alkyne = hex-3-yne or 1,2-bis(trimethylsilylacetylene); DME = 1,2-dimethoxyethane) results in the formation of a versatile set of starting materials of the general formula [ Ph NPN*]Ta(alkyne)Cl (where [ Ph NPN*] = PhP(2-(N-mesityl)-5-Me-C 6 H 3 ) 2 ).Upon reaction with KBEt 3 H, the synthesis of the corresponding hydride complexes [ Ph NPN*]Ta(alkyne)H can be achieved; these complexes feature extremely downfield (δ ∼21 ppm) doublet resonances ( 2 J HP = ∼35 Hz) in the respective 1 H NMR spectra that are assigned to the newly formed Ta−H moieties. Subsequent reaction of these Ta hydrides with 2,6-dimethylphenylisocyanide and phenylacetylene results in the insertion of these species into the Ta−H bond and the formation of the corresponding iminoformyl and phenylvinyl complexes, respectively. While the former intermediate cannot be detected, the latter was characterized by NMR spectroscopy. Both of these processes result in the further transformation to generate C−C coupled products by a reductive elimination sequence with the coordinated alkyne; in the case of the iminoformyl, an azadiene results, whereas with the phenylvinyl derivative a butadienyl fragment is generated. Single-crystal X-ray diffraction and a suite of NMR spectroscopic techniques were used to characterize these species. A discussion of the bonding of the products in the context of the process by which they form is presented. The rate of formation of the butadienyl moiety from the phenylvinyl intermediate results in the activation parameters of ΔH ⧧ = 22.2 ± 0.3 kcal/mol and ΔS ⧧ = −8.7 ± 0.2 cal/(mol)(K).
The synthesis, characterization, and reactivity with H 2 of a series of tantalum hydrocarbyl complexes are reported. The reaction of [NPN*]TaMe 3 (4, where NPN* = PhP(2-(Nmesityl)-5-Me-C 6 H 3 ) 2 ) with dihydrogen (H 2 , 4 atm) results in the formation of the dinuclear tetrahydride ([NPN*]Ta) 2 (μ-H) 4 (5), without the observation of intermediates. The preparations of two alkyne benzyl complexes of the formula [NPN*]Ta(alkyne)-(CH 2 Ph) (where alkyne = BTA = bis(trimethylsilyl)acetylene ( 6), 3-hexyne ( 7)) are reported starting from the respective chloroalkyne complexes [NPN*]Ta(alkyne)Cl, by addition of benzylpotassium. Hydrogenation of these two alkyne benzyl complexes ultimately results in the formation of the same dinuclear tetrahydride 5; however, using lower pressures of H 2 and shorter reaction times results in the isolation of an intermediate in each case. Hydrogenation of 6 generates the alkene hydride complex [NPN*]Ta(trans-1,2-C 2 H 2 (SiMe 3 ) 2 )H (8); addition of H 2 to 7 gives [NPN*]Ta(1-hexene)H ( 9), in which the 3-hexyne moiety has been partially hydrogenated and isomerized to the 1-hexene regioisomer. Both of these alkene hydride complexes can be converted to the dinuclear tetrahydride complex 5 by addition of H 2 . A mechanism is proposed for the formation of the intermediates that involves hydrogenolysis of the alkyne moiety prior to the benzyl ligand; the formation of the trans-alkene units is suggested to be a result of a zwitterionic alkylidene intermediate that allows free rotation of a C−C single bond.
In an effort to study the β-H abstraction/1,3addition mechanism for CH bond activation, the syntheses of tungsten organometallic complexes featuring a cyclopropyl ligand were undertaken. Attempts to prepare the chloro cyclopropyl complex Cp*W(NO)Cl(c-C 3 H 5 ) instead afforded the previously reported allyl complex Cp*W(NO)Cl(η 3 -CH 2 CHCH 2 ), due to a ring-opening rearrangement of the cyclopropyl ligand. The alkyl cyclopropyl complex Cp*W(NO)(CH 2 SiMe 3 )(c-C 3 H 5 ) (1 TMS ) was prepared and characterized in solution via NMR spectroscopy but also undergoes a ligand rearrangement process to afford the allyl complex Cp*W(NO)(CH 2 SiMe 3 )(η 3 -CH 2 CHCH 2 ) (2 TMS ), which was characterized by NMR spectroscopy and an X-ray diffraction study. Attempts to inhibit the ligand rearrangement or isolate 1 TMS in the solid state have been unsuccessful. In order to investigate the mechanism of the conversion of 1 TMS to 2 TMS , a kinetic study was undertaken, and some preliminary results are discussed.
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