Small “Yaw” Angles, Large “Bite” Angles and an Electron‐Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride‐Transfer Activity

Abstract

Cyclometalated complexes are an important class of (pre)catalysts in many reactions including hydride transfer. The ring size of such complexes could therefore be a relevant aspect to consider while modulating their catalytic activity. However, any correlation between the cyclometalating ring size and the catalytic activity should be drawn by careful assessment of the pertinent geometrical parameters, and overall electronic effects thereof. In this study, we investigated the vital role of key stereoelectronic functions of two classes of iridacyclic complexes-five-membered and six-membered cycles-in manupulating the catalytic efficiency in a model hydride-transfer reaction. Our investigation revealed that there exists an interesting multidimensional synergy among all the relevant stereoelectronic factors-yaw angle, bite angle, and the electronic properties of both the ligand and the metal center-that governs the hydride donor ability (hydricity) of the complexes during catalysis. Thus the six-membered chelate complexes with small yaw and large bite angles, strong donor ligand, and electron-rich metal were found to be better catalysts than their five-membered analogues. A frontier molecular orbital analysis supported the significant role of the above stereoelectronic synergistic effect associated with the chelate ring to control the hydride donor ability of the complexes.

Emerging Implications of the Concept of Hydricity in Energy‐Relevant Catalytic Processes

“…In effect, the energyo ft his LUMO, therefore, dictates the stability of the resulting MÀHb ond. A stronger MÀHb ond, and hence, lower hydride-donor ability, originates from al ower-energy stabilized LUMO, whereas a high-energyL UMO resultsi naless stable hydride complex, making it ab etter hydride donor.S tudies by the group of DuBois on [M(diphosphine) 2 ] + (M = Ni, Pd, Pt) systems [14c, 15, 19, 21a, 22] and by the group of Choudhury on [Cp*Ir(C NHC ÀC aryl )] + systems [23] represent how the steric and electronic modulation of ligand backbone influence the electronic environment around the metal center,w hich further affects the hydricity of the L n MÀHc omplex.V arieties of other hydride complexes were reported to show tunable hydricities as af unction of such electronic and steric effects of the attached ligands. [2] Accordingly,M iller and co-workersd eterminedt he effective hydricity values of [Cp*Ir(R 2 -bpy)H] + complexes with variable substituents (R) from electron withdrawing to donating in nature.…”

Ruthenacycles and Iridacycles as Transfer Hydrogenation Catalysts

“…Choudhoury and coworkers synthesized a series of iridacycles based on 1-aryl- or 1-benzyl-substituted N -heterocyclic carbenes (NHCs) ( Scheme 34 ) [ 54 ]. Next, they investigated the yield of the TH of acetophenone as a function of the steric properties of the ligand expressed in bite-angles, as well as yaw-angles (the deviation from the ideal 180° angle between the Ir-C bond to the NHC and the angle of the NHC itself [ 55 ]).…”

Unusually large ‘yaw’ angle upon coordination of a new bulky unsymmetrical 3-hydroxyadamantyl-functionalized N-heterocyclic carbene ligand to rhodium(i)

“…18,19 Quite interestingly, in a recent contribution, Semwal et al have shown that the hydridetransfer activity of iridium-NHC complexes is correlated with the geometric arrangement (yaw and bite angles) of the chelating NHC ligands. 20 In this context, our group developed a N-heterocyclic carbene (NHC) ligand functionalized with an alkoxy-containing pendant arm and demonstrated the interest of this platform to prepare a range of monometallic and heterobimetallic complexes. [21][22][23] Following a similar synthetic strategy, we chose to extend this chemistry to the more bulky hydroxyl-adamantyl wingtip to tune the steric profiles of these NHC complexes.…”

“…Note however that drastically larger (up to 30°) yaw angles values have been observed in chelating bidentate NHC ligands as a result of ring strain. 20,[32][33][34] As shown in Table S2, θ values are not directly correlated with Rh-Ccarbene bond lengths, the latter being slightly elongated when bulky adamantyl N-substituents are used (Table S2, entries 1 and 9), disregarding of the yaw distortion.…”