Implications of CO₂ Activation by Frustrated Lewis Pairs in the Catalytic Hydroboration of CO₂: A View Using N/Si⁺ Frustrated Lewis Pairs

Abstract

A series of base-stabilized silylium species were synthesized and their reactivity toward CO 2 explored, yielding the characterization of a novel N/ Si + FLP-CO 2 adduct. These silicon species are active catalysts in the hydroboration of CO 2 to the methoxide level with 9-BBN, catecholborane (catBH), and pinacolborane (pinBH). Both experiments and DFT calculations highlight the role of the FLP-CO 2 adduct in the catalysis. Depending on the nature of the hydroborane reductant, two distinct mechanisms have been unveiled. While 9-BBN and catBH are able to reduce an intermediate FLP-CO 2 adduct, the hydroboration of CO 2 with pinBH follows a different and novel path where the B−H bond is activated by the silicon-based Lewis acid catalyst. In these mechanisms, the formation of a highly stabilized FLP-CO 2 adduct is found detrimental to the kinetics of the reaction.

Synthesis and characterization of metal (M = Al or Ga) 2-phosphino(phenolate/benzenethiolate) complexes and their electrochemical behavior in the presence of CO2

“…Diethyl ether, pentane and toluene (Aldrich) were further dried using an LC Technology Solutions SP-105 purification system and stored over 4Å molecular sieves in the glovebox prior to use. The Me 3 Al, Me 3 Ga (Aldrich), and B(C 6 F 5 ) 3 (TCI) reagents were used as-received without further purification.…”

“…The transformation of the dimethyl Ga complexes 3 and 7 to their bis(pentafluorophenyl) analogs was then targeted and was achieved by heating 3 and 7 in refluxing toluene overnight with an excess of B(C 6 F 5 ) 3 . The concept of Me/C 6 F 5 ligand exchange was first introduced by Klosin et al where it was shown that the reaction of B(C 6 F 5 ) 3 with Me 3 Al produced Al(C 6 F 5 ) 3 and Me 3 B when reacted in non-coordinating solvents [18].…”

“…The concept of Me/C 6 F 5 ligand exchange was first introduced by Klosin et al where it was shown that the reaction of B(C 6 F 5 ) 3 with Me 3 Al produced Al(C 6 F 5 ) 3 and Me 3 B when reacted in non-coordinating solvents [18]. When adopting this methodology, initial attempts were performed adding enough B(C 6 F 5 ) 3 such that the methyl and pentafluorophenyl groups were in a 1 : 1 stoichiometric ratio, or 0.66 equivalents of B(C 6 F 5 ) 3 per metal center. This resulted in an incomplete exchange as evidence by isolated crystals of a monosubstituted complex LGaMe(C 6 F 5 ) (where L = SC 6 H 4 -2-P t Bu 2 ).…”

“…The activation of CO 2 at transition metal centers has been thoroughly examined, which has resulted in a variety of metal-CO 2 adducts, most often with an electron-rich metal center interacting with the Lewis acidic carbon of CO 2 [2]. Far less examined is the activation of CO 2 by main-group metal complexes and to an even lesser extent, systems in which a secondary atom in the coordination sphere is involved in the activation of the CO 2 [3]. This is somewhat surprising as this is precisely how nature activates CO 2 during photosynthesis, a highly effective process for carbon fixation which results in an annual production of 10 11 tons of biomass [2c].…”

“…This synergistic behavior between a Lewis acid (main-group metal) and a Lewis base site (lone-pair base in ligand) towards CO 2 complexation has been detailed from the context of frustrated Lewis pairs (FLPs), [6] and there have been several reports of FLPs being used to activate CO 2 (See Fig. 1) [3,7]. With the expansion of FLPs to include Lewis pairs which are not sterically encumbered, [8] one could certainly consider our previous main-group complexes to satisfy the criteria required for FLP status.…”