Mapping Active Site Geometry to Activity in Immobilized Frustrated Lewis Pair Catalysts

Abstract

The immobilization of molecular catalysts imposes spatial constraints on their active site. We reveal that in bifunctional catalysis such constraints can also be utilized as an appealing handle to boost intrinsic activity through judicious control of the active site geometry. To demonstrate this, we develop a pragmatic approach, based on nonlinear scaling relationships, to map the spatial arrangements of the acid-base components of frustrated Lewis pairs (FLPs) to their performance in the catalytic hydrogenation of CO 2 . The resulting activity map shows that fixing the donoracceptor centers at specific distances and locking them into appropriate orientations leads to an unforeseen many-fold increase in the catalytic activity of FLPs compared to their unconstrained counterparts.

Engineering Frustrated Lewis Pair Active Sites in Porous Organic Scaffolds for Catalytic CO2 Hydrogenation

“…The catalytic behavior of an LPAS is determined by its chemical and geometric composition as exemplified by our recent works. 23,24 While the former is associated with the local chemical environment of the Lewis pairs, the latter aspect refers to the spatial arrangements of the donor-acceptor centers in the active site. Thus, the next step is to devise a suitable screening procedure for identifying highly active LPAS considering both the chemistry and geometry of the candidates.…”

“…Recently, some of us proposed a set of straightforward intuitive chemical and geometrical descriptors which enable a relative comparison of CTFH activity of immobilized B-N FLPs based on minimal active site information (see Figure 2). 23,24 To measure the influence of LPAS geometry on activity, we invoke two descriptors: relative distance (d) and orientation (angle, Φ) of the donor-acceptor units assumed during the catalytic cycle. 24 While d is trivially calculated from the distance between the B and N centers, Φ is estimated from the angle between their open coordination sites for substrate binding.…”

“…23,24 To measure the influence of LPAS geometry on activity, we invoke two descriptors: relative distance (d) and orientation (angle, Φ) of the donor-acceptor units assumed during the catalytic cycle. 24 While d is trivially calculated from the distance between the B and N centers, Φ is estimated from the angle between their open coordination sites for substrate binding. As previously reported by us, the geometric criteria for high CTFH activity stipulate that the LPASs featuring B-N distances between 2.4-3.2 Å and relative orientations between 70-140 • are substantially more reactive than other geometric arrangements.…”

“…However, this is no trivial task due to the requirement of placing both the acid and base centers in an appropriate geometric position while maintaining the necessary chemical environment of the Lewis pairs to perform a given catalytic reaction of interest. 23,24 The identification of the most promising active site architectures with appropriate chemical and geometrical compositions would thus be invaluable as a starting point for constructing such hybrid active sites. Although the chemical and geometric composition of the FLPs has been individually linked to their performance, 23,24 a clear connection between the full composition of active sites and resulting catalytic performance is currently lacking.…”

“…23,24 The identification of the most promising active site architectures with appropriate chemical and geometrical compositions would thus be invaluable as a starting point for constructing such hybrid active sites. Although the chemical and geometric composition of the FLPs has been individually linked to their performance, 23,24 a clear connection between the full composition of active sites and resulting catalytic performance is currently lacking. Analysis of a broader set of highly active FLP environments will guide the design of catalytic sites and realize the potential of a fully-immobilized catalyst design approach.…”