Synthesis of a Rhodium Complex Featuring the Rh–H–B Linkage via a Hydride Migration from Rhodium to Borane: Study on the Electronic Deviation Induced by the Presence of the Boron Moiety

Abstract

The rhodium complex [{o-(Ph 2 P)C 6 H 4 } 2 (Ph)-BH]Rh(CO)(PPh 3 ) (3-Rh) ,featuring a B−H−Rh interaction, was synthesized via phosphine ligand exchange between tris(triphenylphosphine)carbonylrhodium(I) hydride, RhH-(CO)(PPh 3 ) 3 (2-Rh), and the diphosphine−borane {o-(Ph 2 P)C 6 H 4 } 2 BPh (5). In contrast, the reaction of 5 with the iridium analogue of 2-Rh, IrH(CO)(PPh 3 ) 3 (2-Ir), resulted in the formation of the iridium complex [{o-(Ph 2 P)C 6 H 4 } 3 B]Ir(H)(CO) (4-Ir), with a direct Ir→B interaction. These compounds 3-Rh and 4-Ir were fully characterized using 1 H, 11 B, and 31 P NMR spectroscopy, X-ray diffraction analysis, and elemental analysis. Density functional theory calculations of 3-Rh revealed that the boron accepts a significant amount of electron density from the Rh center through a three-center−two-electron Rh−H−B interaction and that the presence of the boron moiety considerably weakens the Rh−CO bond. The electronic variation induced by the boron moiety can promote the facile CO elimination of 3-Rh. The "boron enhancement effect" observed in the catalytic transfer hydrogenation of ethyl phenyl ketone is also reported. ■ INTRODUCTIONA multidentate ligand bearing a boron moiety at one end and an electron donor at another end is a useful platform for versatile bonding modes and structural motifs. 1 One interesting bonding mode is the three-center−two-electron M−H−B linkage (M = transition metal), which is a key intermediate at the B−H oxidative addition stage (Chart 1). Another interesting bonding mode is a dative metal−boron linkage, where a borane ligand accepts electrons directly from a transition metal through a σ-bonding interaction. These types of bonds are provided when the metal, boron atoms, and the donor moiety are placed at ideal positions by the chelate-type ligand. The mercapto-imidazolyl system and its derivatives are some of the most popular scaffolds for inducing such interactions. 2−4 The intensive studies of Hill 2 and Parkin 3 on this system augmented the chemistry of the boron-based ligand. For example, Hill et al. realized the synthesis of the rhodium complex bearing a Rh−H−B linkage as well as the iridium complex with a dative Ir→B interaction. 2n Further, they showed the interconversion between the M−H−B and H−M→B linkages via the migration of a hydride between the transition metal and boron atoms. 2m Recently, Owen et al. reported complexes with the same types of interactions using an azaindole-based system, 5 and they provided further examples of the conversions between M−H−B and H−M→B linkages,which are useful as the driving force to catalyze the hydrogenation of several olefins. 5e A phosphine−borane system developed by Bourissou is another important framework. 6,7 One of the fascinating properties of this scaffold enables the design of the complex having a structure similar to that of the related boron-free complex. Comparison between the phosphine−borane complex and the related boron-free complex can provide information concerning the properties of borane ligan...

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