Synergistic Catalysis in Heterobimetallic Complexes for Homogeneous Carbon Dioxide Hydrogenation

Abstract

Two heterobimetallic Mo,M’ complexes (M’ = IrIII, RhIII) were synthesized and fully characterized. Their catalytic activity in homogeneous carbon dioxide hydrogenation to formate was studied. A pronounced synergistic effect between the two metals was found, most notably between Mo and Ir, leading to a fourfold increase in activity compared with a binary mixture of the two monometallic counterparts. This synergism can be attributed to spatial proximity of the two metals rather than electronic interactions. To further understand the nature of this interaction, the mechanism of the CO2 hydrogenation to formate by a monometallic IrIII catalyst was studied using computational and spectroscopic methods. The resting state of the reaction was found to be the metal-base adduct, whereas the rate-determining step is the inner-sphere hydride transfer to CO2. Based on these findings, the synergism in the heterobimetallic complex is beneficial in this key step, most likely by further activating the CO2.

Stronger Together! Mechanistic Investigation into Synergistic Effects during Homogeneous Carbon Dioxide Hydrogenation Using a Heterobimetallic Catalyst

“…This assumption was due to the absence of electronic interaction between the two metals (supported by cyclic voltammetric measurements) and the fact that M2 (Scheme 1) by itself is not an active catalyst. 19 This assumption is further corroborated by the HP NMR Scheme 1. Catalytic CO 2 Hydrogenation to Formate Using DBU as a Base and the Heterobimetallic Complex C1 and Its Monometallic Counterparts M1 and M2 as Catalysts 19 experiments discussed above, confirming that the Mo 0 center is stable under catalytic conditions, and no transformation apart from the formation of [Mo 0 (CO) 3 ;Ir III H] was observed even over extended periods of time.…”

“…As previously reported, the reaction is preceded by an activation/induction period, where a solution of C1 in acetonitrile (MeCN) is subjected to 30 bar of dihydrogen (H 2 ) pressure in the presence of a base; only after 30 min is CO 2 added. 19 Without this induction period, the catalytic activity dropped significantly (chapter 3.2). To understand the induction process, a solution of C1 in CD 3 CN was subjected to different bases in the presence of 1 atm of H 2 .…”

“…To further develop this system and apply it to different structural motifs and substrates, it is imperative to attain a thorough understanding of both the mechanism of this reaction and the synergistic interaction. 19 In this work, we present a detailed investigation of the mechanism employing NMR spectroscopic studies, kinetic experiments, and theoretical calculations. Furthermore, we studied how the synergistic interaction increases reactivity, using chromium (Cr) and tungsten (W) analogues of C1 and kinetic measurements of model reactions, supported by computational studies.…”

“…20 In our previous work, we elucidated the mechanism of CO 2 hydrogenation to formate salts with M1 as the catalyst and found that the base adduct of M1 was the overall resting state of the reaction when DBU or TMG was employed. 19 Only when a high-pressure (HP) NMR setup was used to increase the H 2 pressure to 30 bar and the temperature was raised to 60 °C could a very small amount of the hydride species [Mo 0 (CO) 3 ;Ir III H] be observed. Full conversion to the hydride species [Mo 0 (CO) 3 ;Ir III H] was achieved when the noncoordinating base tris(dimethylamino)-(phenylimino)phosphorane (PhPhos) was used (Scheme 2).…”

“…Therefore, the mechanisms of CO 2 hydrogenation by C1 and M1 are comparable, with the main difference of beneficial interaction in the heterobimetallic complex, particularly in the rate-determining step (RDS), leading to higher activity. 19 In general, the mechanism can be subdivided into four parts: first, equilibrium between the active catalyst and base adduct, second, H 2 activation, third, hydride transfer from the complex to CO 2 , and, fourth, formate release, coupled with ion-pair formation and regeneration of the active catalyst (Figure 2).…”