The monodeprotonation of [CH2(PPh2→BH3)­(PPh2E)] (E = S (6), O (7)) afforded [CH­(PPh2→BH3)­(PPh2E)]− (E = S (6 ‑ ), O (7 – )), whose structures were confirmed by X-ray crystallography. The kinetics of the second deprotonation appeared to be crucial in efficient synthesis of the corresponding dianions. Thus, the double deprotonation of 6 only led to 6 2– ; the analogous reaction with 7 was slower and resulted only in the partial formation of 7 2– . Double deprotonation of the compound [CH2(SiMe3)­(PPh2S)] (8) also resulted in the partial formation of [C­(SiMe3)­(PPh2S)]2‑ (8 2– ), whose structure was confirmed by X-ray crystallography. The rare monomeric Mg carbene compound [MgC­(PPh2→BH3)­(PPh2S)] (9) was obtained by the reaction of 6 with Mg­(nBu)2. The X-ray structure of 9 is presented.

Stable nucleophilic carbene compounds have recently been shown to be able to mimic in some instances the reactivity of metal fragments in the reaction of unactivated E-H bonds (E = H, R3Si, NH2, R2P). However, the insertion into a B-H bond of the strongly Lewis acidic BH3 molecule has never been observed at a single C atom or even at a metal fragment. Our results show that designed stable, highly electrophilic carbenoid fragments in compounds 4 and 6 can achieve this reactivity in a controlled manner. Density functional theory calculations corroborated the experimental results on the presently designed systems as well as the lack of reactivity on nucleophilic carbenes.

The treatment of Li2C(PPh2NMes)2 (1, Mes = 2,4,6-Me3C6H2) with hexachloroethane yielded the corresponding carbenoid 2 in good yields. The reactivity of 2 was studied with BH3·SMe2 to give a zwitterionic boronium species 4, also a stable carbenoid. Both carbenoid species were found to be excellent catalysts for the CO2 reduction by BH3·SMe2.

This study describes the first use of a bis(phosphoranyl)methanido aluminum hydride, [ClC(PPh 2 NMes) 2 AlH 2 ] (2, Mes = Me 3 C 6 H 2 ) for the catalytic hydroboration of CO 2 . Complex 2 was synthesized by the reaction of a lithium carbenoid [Li(Cl)C(PPh 2 NMes) 2 ] with two equivalents of AlH 3 •NEtMe 2 in toluene at -78 o C. 10 mol % of 2 was able to catalyze the reduction of CO 2 with HBpin in C 6 D 6 at 110 o C for 2 days to afford a mixture of methoxyborane [MeOBpin] (3a; yield: 78 %, TOF: 0.16 h -1 ) and bis(boryl)oxide [pinBOBpin] (3b). When more potent [BH 3 •SMe 2 ] was used instead of HBpin, the catalytic reaction was extremely pure, resulting in the formation of trimethyl borate [B(OMe) 3 ] (3e) [catalytic loading: 1 mol % (10 mol %); reaction time: 60 min (5 min); yield: 97.6 % (>99 %); TOF: 292.8 h -1 (356.4 h -1 )] and B 2 O 3 (3f). Mechanistic studies show that the Al-H bond in complex 2 activated CO 2 to form [ClC(PPh 2 NMes) 2 Al(H){OC(O)H}] (4), which was subsequently reacted with BH 3 •SMe 2 to form 3e and 3f, along with the regeneration of complex 2. Complex 2 also shows good catalytic activity towards hydroboration of carbonyl, nitrile and alkyne derivatives. ASSOCIATED CONTENTSupporting Information. The Supporting Information is available free of charge on the ACS Publications website.Experimental procedures (PDF), theoretical studies (XYZ) X-ray crystallographic data (CIF)