Fine‐Tuning of Lewis Acidity: The Case of Borenium Hydride Complexes Derived from Bis(phosphinimino)amide Boron Precursors

Abstract

Reactions of bis(phosphinimino)amines LH and L'H with Me2 S⋅BH2 Cl afforded chloroborane complexes LBHCl (1) and L'BHCl (2), and the reaction of L'H with BH3 ⋅Me2 S gave a dihydridoborane complex L'BH2 (3) (LH=[{(2,4,6-Me3 C6 H2 N)P(Ph2 )}2 N]H and L'H=[{(2,6-iPr2 C6 H3 N)P(Ph2 )}2 N]H). Furthermore, abstraction of a hydride ion from L'BH2 (3) and LBH2 (4) mediated by Lewis acid B(C6 F5 )3 or the weakly coordinating ion pair [Ph3 C][B(C6 F5 )4 ] smoothly yielded a series of borenium hydride cations: [L'BH](+) [HB(C6 F5 )3 ](-) (5), [L'BH](+) [B(C6 F5 )4 ](-) (6), [LBH](+) [HB(C6 F5 )3 ](-) (7), and [LBH](+) [B(C6 F5 )4 ](-) (8). Synthesis of a chloroborenium species [LBCl](+) [BCl4 ](-) (9) without involvement of a weakly coordinating anion was also demonstrated from a reaction of LBH2 (4) with three equivalents of BCl3 . It is clear from this study that the sterically bulky strong donor bis(phosphinimino)amide ligand plays a crucial role in facilitating the synthesis and stabilization of these three-coordinated cationic species of boron. Therefore, the present synthetic approach is not dependent on the requirement of weakly coordinating anions; even simple BCl4 (-) can act as a counteranion with borenium cations. The high Lewis acidity of the boron atom in complex 8 enables the formation of an adduct with 4-dimethylaminopyridine (DMAP), [LBH⋅(DMAP)](+) [B(C6 F5 )4 ](-) (10). The solid-state structures of complexes 1, 5, and 9 were investigated by means of single-crystal X-ray structural analysis.

Hydrosilylation of Carbonyls Catalyzed by Hydridoborenium Borate Salts: Lewis Acid Activation and Anion Mediated Pathways

“…The hydridoborenium borate ion pairs [ L BH] + [HB­(C 6 F 5 ) 3 ] − ( 1 ) and [ L BH] + [B­(C 6 F 5 ) 4 ] − ( 2 ) were generated by hydride abstraction from L BH 2 with B­(C 6 F 5 ) 3 and [Ph 3 C] + [B­(C 6 F 5 ) 4 ] − , respectively ( L H = [{(2,4,6-Me 3 C 6 H 2 N)­P­(Ph 2 )} 2 N]­H) . Compounds 1 and 2 proved to be efficient catalysts for the hydrosilylation of carbonyl compounds (aliphatic and aromatic aldehydes and ketones) in CHCl 3 medium and showed good catalytic activity at 40 and 70 °C, respectively.…”

“…With clear spectroscopic evidence of involvement of the anion of [ L BH] + [HB­(C 6 F 5 ) 3 ] − ( 1 ) (Figure ; vide supra ) as the hydride source in the hydrosilylation of carbonyls with the cation possibly assisting in the Lewis acid activation of the >CO group, it was worth investigating the same reaction using a similar hydridoborenium cation but with the counteranion devoid of any hydride. Therefore, the complex [ L BH] + [B­(C 6 F 5 ) 4 ] − ( 2 ) was an ideal candidate for such investigations . The stoichiometric reaction of [ L BH] + [B­(C 6 F 5 ) 4 ] − ( 2 ) with PhCHO at 70 °C was followed by heteronuclear NMR spectroscopy ( 1 H, 13 C­{ 1 H}, 11 B, 19 F, and 31 P­{ 1 H}).…”

“…Borenium cations with a hydride functionality have been considered important reagents with potential applications as metal-free catalysts in processes such as hydrogenation, , imine hydrogenation, dihydrogen activation, and alkyne 1,2-carboboration . At present, a few well-characterized cationic mononuclear borenium hydride species are known. − Previously, we have synthesized the borenium hydride cations [ L BH] + [HB­(C 6 F 5 ) 3 ] − ( 1 ) and [ L BH] + [B­(C 6 F 5 ) 4 ] − ( 2 ) (Figure ) supported by a bis­(phosphinimino)­amide ligand and demonstrated their Lewis acid behavior as well. , In a continuation of our efforts to exploit the Lewis acidity of these ionic complexes and to discover new catalysts from main-group compounds, herein we employed 1 and 2 for the hydrosilylation of aldehydes and ketones (Figure ). Currently, only two reports are available that utilize [9-BBN·(2,6-lutidine)] + NTf 2 – and {Al­[OC­(CF 3 ) 3 ] 4 ] − salts of a ferrocene-based planar-chiral borenium cation to catalyze the hydrosilylation of ketones. , Mechanistic investigations of these catalysts suggest the borenium ion activation of Et 3 SiH as the key step, similar to that outlined for B­(C 6 F 5 ) 3 -catalyzed hydrosilylation of carbonyls by Piers and co-workers .…”

Mononuclear Neutral Boron Hydrides Affordable as [N,N’] Chelates of Iminophosphonamides

“…Complex 2 is a rare example of a chloroborane complex which is structurally characterized. The only published well characterized chloroborane complex is [{(2,4,6‐Me 3 C 6 H 2 N)P(Ph 2 )} 2 N]BHCl . The IR spectrum of complex 2 showed B−H stretch at 2413 cm −1 .…”

“…In view of these facts, it is important to undertake the syntheses and structural elucidation of boron hydrides in a systematic manner. Till date, the structurally characterized mononuclear covalent boron hydrides are limited only to subporphyrinato boron hydride ( A ), bis(phosphinimino)amido chloroborane ( B ), bis(phosphinimino)amido borondihydride ( C ), and cyclopentadienyl substituted boron hydride ( D ) complexes, as shown in Scheme . The dinuclear covalent boron hydrides; [BH 2 (hpp)] 2 , [BH(hpp)] 2 , and bis(diisopropylamino)diborane reported by Himmel and co‐workers are also remarkable molecules in boron hydride chemistry…”