Activation of B–H, Si–H, and C–F Bonds with Tp′Rh(PMe₃) Complexes: Kinetics, Mechanism, and Selectivity

Abstract

The photochemical reactions of Tp'Rh(PMe3)H2 (1) and thermal reactions of Tp'Rh(PMe3)(CH3)H (1a, Tp' = tris(3,5-dimethylpyrazolyl)borate) with substrates containing B-H, Si-H, C-F, and C-H bonds are reported. Complexes 1 and 1a are known activators of C-H bonds, including those of alkanes. Kinetic studies of reactions with HBpin and PhSiH3 show that photodissociation of H2 from 1 occurs prior to substrate attack, whereas thermal reaction of 1a proceeds by bimolecular reaction with the substrate. Complete intramolecular selectivity for B-H over C-H activation of HBpin (pin = pinacolate) leading to Tp'Rh(PMe3)(Bpin)H is observed. Similarly, the reaction with Et2SiH2 shows a strong preference for Si-H over C-H activation, generating Tp'Rh(PMe3)(SiEt2H)H. The Rh(Bpin)H and Rh(SiEt2H)H products were stable to heating in benzene in accord with DFT calculations that showed that reaction with benzene is endoergic. The intramolecular competition with PhSiH3 yields a ∼1:4 mixture of Tp'Rh(PMe3)(C6H4SiH3)H and Tp'Rh(PMe3)(SiPhH2)H, respectively. Reaction with pentafluoropyridine generates Tp'Rh(PMe3)(C5NF4)F, while reaction with 2,3,5,6-tetrafluoropyridine yields a mixture of C-H and C-F activated products. Hexafluorobenzene proves unreactive. Crystal structures are reported for B-H, Si-H, and C-F activated products, but in the latter case a bifluoride complex Tp'Rh(PMe3)(C5NF4)(FHF) was crystallized. Intermolecular competition reactions were studied by photoreaction of 1 in C6F6 with benzene and another substrate (HBpin, PhSiH3, or pentafluoropyridine) employing in situ laser photolysis in the NMR probe, resulting in a wide-ranging map of kinetic selectivities. The mechanisms of intramolecular and intermolecular selection are analyzed.

Rhodium and Iridium Fluorido and Bifluorido Complexes Derived from Peroxido Precursors

“…It has been demonstrated that a variety of rhodium and iridium fluorido complexes are accessible. [1d], Examples include the rhodium congener of Wilkinson's catalyst [Rh(F)(PPh 3 ) 3 ], which was reported by Macgregor and Grushin and co‐workers, and the square‐pyramidal iridium complex trans ‐[Ir(4‐C 5 F 4 N)(H)(F)(P i Pr 3 ) 2 ]. [2s], A very common method for synthesizing these fluorido complexes consists of the nucleophilic replacement of a metal‐bound halide (halogen = Cl, Br, I) by fluoride.…”

Selectivity of C–H Activation and Competition between C–H and C–F Bond Activation at Fluorocarbons

“…The relative Rh-C bond energies and their significance were discussed in Section 3.2. 29,101 Unlike many related complexes, hexafluorobenzene is unreactive toward Tp Rh(Ar F )(H)(PMe 3 ) under photochemical conditions and can even be used as an inert solvent.Pentafluoropyridine does undergo C-F oxidative addition, however, and 2,3,5,6-tetrafluoropyridine forms a mixture of C-H activation (80%) and C-F activation (20%) products 178.…”

N−H Bond Formation at a Diiron Bridging Nitride

“…110 kcal mol À1 . [22] As with the parent amide [Fe 2 NH 2 ] + , the crystal structures of [Fe 2 NHSi] + and [Fe 2 NHB] + revealed folded ligand geometries (Figure 1), with apparent FeÀFe bonding interactions (d Fe-Fe = ca. 2.58 , see Supporting Information) and sp 3 -hybridized m-NHE residues.…”