Hajime Kameo

Recent Developments in the Coordination Chemistry of Multidentate Ligands Featuring a Boron Moiety

Synergistic effects between a transition metal and an appropriate ligand are required to promote a desired catalytic reaction. Ancillary ligands, provided by the versatile functionality of certain elements, give rise to an almost infinite potential for catalytic applications. Recently, the study of the synergistic effect between transition metals and boron has become easy on account of the development of various rigid multidentate frameworks. In this Review, we mainly focus on the chemistry of σ-acceptor (Z-type) borane ligands, particularly the key achievements of their unique reactivity and catalytic applications. Conceptually, the unique character of σ-acceptor borane ligands provides a new strategy for developing remarkable reactivity and novel catalytic applications. This study discusses recent developments in the field in this context. The chemistry of boron-based multidentate ligands that involve a covalent M-B bond such as the boryl ligand (-BR2), in which a boron moiety serves as a strong electron-donating ligand, is also rapidly developing. The effect of the boryl ligand on a metal center is totally different from that of the borane (-BR3) ligand, and different boron-based functionalities confer opposing electronic properties to the metal center. The interesting character of boryl-based chelating ligands augments their unique coordination chemistry, which is also summarized in this context.

Transition‐Metal‐Mediated Cleavage of Fluoro‐Silanes under Mild Conditions

Si-F bond cleavage of fluoro-silanes was achieved by transition-metal complexes under mild and neutral conditions. The Iridium-hydride complex [Ir(H)(CO)(PPh3 )3 ] was found to readily break the Si-F bond of the diphosphine- difluorosilane {(o-Ph2 P)C6 H4 }2 Si(F)2 to afford a silyl complex [{[o-(iPh2 P)C6 H4 ]2 (F)Si}Ir(CO)(PPh3 )] and HF. Density functional theory calculations disclose a reaction mechanism in which a hypervalent silicon species with a dative Ir→Si interaction plays a crucial role. The Ir→Si interaction changes the character of the H on the Ir from hydridic to protic, and makes the F on Si more anionic, leading to the formation of H(δ+) ⋅⋅⋅F(δ-) interaction. Then the Si-F and Ir-H bonds are readily broken to afford the silyl complex and HF through σ-bond metathesis. Furthermore, the analogous rhodium complex [Rh(H)(CO)(PPh3 )3 ] was found to promote the cleavage of the Si-F bond of the triphosphine-monofluorosilane {(o-Ph2 P)C6 H4 }3 Si(F) even at ambient temperature.

Activation of Strong Boron–Fluorine and Silicon–Fluorine σ-Bonds: Theoretical Understanding and Prediction

The oxidative addition of BF3 to a platinum(0) bis(phosphine) complex [Pt(PMe3)2] (1) was investigated by density functional calculations. Both the cis and trans pathways for the oxidative addition of BF3 to 1 are endergonic (ΔG°=26.8 and 35.7 kcal mol(-1), respectively) and require large Gibbs activation energies (ΔG°(≠)=56.3 and 38.9 kcal mol(-1), respectively). A second borane plays crucial roles in accelerating the activation; the trans oxidative addition of BF3 to 1 in the presence of a second BF3 molecule occurs with ΔG°(≠) and ΔG° values of 10.1 and -4.7 kcal mol(-1), respectively. ΔG°(≠) becomes very small and ΔG° becomes negative. A charge transfer (CT), F→BF3, occurs from the dissociating fluoride to the second non-coordinated BF3. This CT interaction stabilizes both the transition state and the product. The B-F σ-bond cleavage of BF2Ar(F) (Ar(F)=3,5-bis(trifluoromethyl)phenyl) and the B-Cl σ-bond cleavage of BCl3 by 1 are accelerated by the participation of the second borane. The calculations predict that trans oxidative addition of SiF4 to 1 easily occurs in the presence of a second SiF4 molecule via the formation of a hypervalent Si species.

Cooperative Catalysis of Combined Systems of Transition-Metal Complexes with Lewis Acids: Theoretical Understanding.

The combination of transition-metal complexes and Lewis acids has been recently applied to several catalytic reactions, in which the Lewis acid plays a crucial role as a non-innocent additive to accelerate the reaction. In this review article, the reasons for the acceleration by the Lewis acid are discussed based on our recent theoretical studies. In the H-H σ-bond activation of a dihydrogen molecule by a nickel(0)-borane complex, the empty p orbital of the borane moiety interacts with the H-H σ bonding MO to form charge transfer (CT) from the dihydrogen molecule to the borane moiety to accelerate the reaction. In the B-F σ-bond activation of BF3 by a platinum(0)-bisphosphine complex, the second BF3 molecule interacts with the F atom that is dissociating from the B atom to stabilize the transition state and product by the CT from the F atom to the second BF3 . In this reaction, the substrate BF3 plays a crucial role as the Lewis acid to accelerate the activation of the B-F σ bond. In the nickel-catalyzed decyanative coupling of arylcarboxybenzonitriles with acetylenes, two molecules of the aluminum Lewis acid interact with the cyano N atom and the carbonyl O atom of the substrate to stabilize the transition state and intermediate. In the nickel-catalyzed alkylation of aromatic amides with alkenes, the Lewis acid enhances the para regioselectivity of alkylation by interacting with the carbonyl O atom. In the nickel-catalyzed carboxylation of sp3 carbon and sp carbon atoms with carbon dioxide, not the σ-bond activation but the insertion reaction of carbon dioxide into the metal-carbon bond is accelerated by the Lewis acid by interacting with the O atom of carbon dioxide, because the CT from the metal-carbon bond to carbon dioxide is enhanced by the interaction. This theoretical knowledge suggests that the combination of transition-metal complex and Lewis acid can broaden the application range of transition-metal complex as catalyst.

Saturated Heavier Group 14 Compounds as σ-Electron-Acceptor (Z-Type) Ligands.

This review article describes the chemistry of transition-metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ-electron-acceptor (Z-type) ligands and discusses the characteristics of bonds between the transition metal and Z-type ligand. Moreover, we review the iridium hydride mediated cleavage of E-X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.