CO₂ Binding and Splitting by Boron–Boron Multiple Bonds

Abstract

[3] whose combination of nucleophilic and electrophilic sites are well-suited to combine with the carbon and oxygen atoms of CO2, respectively ( Figure 1A). Another common mechanism of main-group-based CO2 activation is the (initial) 2+2 cycloaddition of one C=O bond of CO2 with another E-E multiple bond, e.g. P=N (i.e. the Aza-Wittig reaction), Si=O, Si=N, Ge=O, Sn=O, and B=N bonds ( Figure 1B). [4] To our knowledge, no CO2 fixation or activation has been observed by solely utilizing a nonpolar multiple bond, despite the fact that a range of highly reactive compounds with E-E multiple bonds are known.[5] However, in 2011 Kato and Baceiredo reported [6] the reaction of CO2 with a disilyne bisphosphine adduct, a compound thought to possess some multiple bonding character between its two silicon atoms despite the clearly non-planar geometry around the silicon atoms. Herein we present fixation and splitting reactions of CO2 through its interaction with distinctly non-polar multiple bonds of two significantly different diboron species:[5c,d,e] a doubly basestabilized diborene [5,7] with tricoordinate boron atoms and a B=B double bond, and a linear diboryne species bearing strongly π-acidic cyclic (alkyl)(amino)carbene (CAACs) donors, [8] effectively a diboracumulene [9] species with a B-B bond order between two and three. Interestingly, in the reaction of CO2, we were also able to isolate the thermally unstable 2+2 (C=O + B=B) cycloaddition product, which slowly undergoes cleavage of one C=O bond. The apparently facile reaction of CO2 with B-B multiply bound species is attributed to the high reactivity of the latter, which is able to overcome the lack of polarity in the bond and effect the initial cycloaddition step. Dibromodiborenes (L(Br)B=B(Br)L), very few of which exist in the literature, [10,11] were chosen as candidates for CO2 binding due to their sterically unhindered B=B bonds and thus presumed high reactivity. Upon treatment with one atmosphere of CO2 at room temperature, after 7 min the 11 B NMR spectroscopic signal of diborene 1 [10] (dB 20) was found to have completely disappeared, replaced by two broad signals (dB ca. 0, -10). Removal of the solvent from this mixture and extraction of the residue into hexane provided a solution from which orange crystals (2) were grown. The solid-state structure of 2 (Figure 2, middle) confirms the combination of the diborene 1 with CO2 to form a dibora-b-lactone structure in which the two boron atoms form a slightly puckered four-membered B-B-C-O ring with one carbon and one oxygen atom of the CO2 unit. The remaining oxygen atom is part of a carbonyl group with a short C-O distance of 1.20(1) Å but a relatively wide O-C-B angle (136.2(8)º). Interestingly, the endocyclic B-B-C angle is strongly acute (73.7(8)º). The NHC and Br groups are each oriented in a trans fashion with respect to the ring.

CO2Fixation and Catalytic Reduction by a Neutral Aluminum Double Bond

“…[14,16] Retention of the carbonyl moiety exocyclic to the ring was confirmed by IR spectroscopy (1640 cm À1 ), which is in good agreement with the calculated value (1658 cm À1 ). [14,16] Retention of the carbonyl moiety exocyclic to the ring was confirmed by IR spectroscopy (1640 cm À1 ), which is in good agreement with the calculated value (1658 cm À1 ).…”

“…[14,16] Retention of the carbonyl moiety exocyclic to the ring was confirmed by IR spectroscopy (1640 cm À1 ), which is in good agreement with the calculated value (1658 cm À1 ). [14] Rep-etition of this reaction at 50 8 8C(Scheme 1) resulted in adark red solution within 1h,a fter which subsequent work up and crystallization provided 3 in 20 %y ield. [24] Compound 2 is relatively stable in the solid state (m.p.…”

“…[14,31,32] Ther etention of the carbonyl group is confirmed upon comparison of structural data. Compound 3 represents the first dialuminacarbonyl compound and ar are example of ac entral M(m-CO)(m-O)M bridging motif.…”

“…[4][5][6] Whilst transition metals have dominated this field, the development of main group complexes that can act as transition metals [7,8] has begun to emerge as ac heaper and more environmentally benign alternative for CO 2 activation. [12,13] In terms of non-polar multiply bonded main-group compounds,C O 2 activation has been shown to proceed by an initial [2+ +2]-cycloaddition for diborenes [14] and disilenes. [12,13] In terms of non-polar multiply bonded main-group compounds,C O 2 activation has been shown to proceed by an initial [2+ +2]-cycloaddition for diborenes [14] and disilenes.…”

Selective CO2 Splitting by Doubly Reduced Aryl Boranes to Give CO and [CO3]2−

“…[7][8][9][10] Since the advent of frustrated Lewis pair (FLP) chemistry, increasing efforts have been devoted to the replacement of transition-metal catalysts by appropriate combinations of main-group Lewis acids and bases.I nt he context of CO 2 activation, ap lethora of capture products such as R 3 P-C(O)-O-BR' 3 are nowadays known. [18] As athermally unstable primary intermediate,they observed the dibora-b-lactone H, which rearranged with CO 2 splitting to form the isomeric 2,4diboraoxetan-3-one I. [11] Despite these impressive achievements, examples of catalytic CO 2 reductions with FLPs are still scarce and often require drastic reaction conditions,extended periods of time,a sw ell as rather expensive hydride donors (e.g.,H B(OR) 2 ,H SiR 3 ).…”