Effect of Ligand Substituents on Spectroscopic and Catalytic Properties of Water-Compatible Cp*Ir-(pyridinylmethyl)sulfonamide-Based Transfer Hydrogenation Catalysts

Abstract

Transition-metal-based hydrogenation catalysts have applications ranging from high-value chemical synthesis to medicinal chemistry. A series of (pyridinylmethyl)sulfonamide ligands substituted with electron-withdrawing and -donating groups were synthesized to study the influence of the electronic contribution of the bidentate ligand in Cp*Ir piano-stool complexes. A variable-temperature NMR investigation revealed a strong correlation between the electron-donating ability of the substituent and the rate of stereoinversion of the complexes. This correlation was partially reflected in the catalytic activity of the corresponding catalysts. Complexes with electron-withdrawing substituents followed the trend observed in the variable-temperature NMR study, thereby confirming the rate-determining step to be donation of the hydride ligand. Strongly electron-donating groups, on the other hand, caused a change in the rate-determining step in the formation of the iridium-hydride species. These results demonstrate that the activity of these catalysts can be tuned systematically via changes in the electronic contribution of the bidentate (pyridinylmethyl)sulfonamide ligands.

Figure 1

Structure of the Ru(II)-TsDPEN precatalyst reported by Noyori and coworkers.

Figure 2

Mechanism for the transfer hydrogenation of protonated imines by Noyori–Ikariya-type catalysts (M = Rh and Ir). Step A in the cycle is the formation of the metal-hydride species by β-hydride elimination and step B is hydride donation.

Figure 3

Examples of related piano-stool iridium complexes used to investigate the effect of the electronic contribution of the bidentate ligand in transfer hydrogenation catalysts (a,b), study of the “hydricity” of the metal complex (c), and catalysts for water oxidation (d) and CO₂ hydrogenation (e,f).

Figure 4

Structure of the complexes (3x) investigated in this study, composed of ligands a–g.

Figure 5

¹H NMR spectra at 323 K in dimethylformamide-d₇ (DMF-d₇) of complexes 3a–f are shown in the 4.0–5.2 ppm region to illustrate how the peak shape of the resonances assigned to the two diastereotopic CH₂ protons varies with the substituent on the pyridine ring.

Figure 6

¹H NMR spectra in DMF-d₇ for complex 3c acquired in the temperature range of 293–373 K.

Figure 7

Plot of ln(k/T) vs 1/T to determine ΔH^⧧ and ΔS^⧧ labeled with the substituent, R, at the meta-position.

Figure 8

Plot of ΔG^⧧ (at 313 K) for each of the iridium complexes 3a–3g against the parameter ΔV_c, a theoretical measure of the electron-donating or electron-withdrawing ability of the, respective, substituent shown.

Figure 9

(a) Catalytic transformation investigated in this study and (b) example HPLC traces with peaks corresponding to the substrate and the two chiral products, retention times t_S = 9.7 min and t_R = 10.6 min, increasing over time during catalysis and the signal for the substrate, t_sub = 13.0 min, decreasing over time.

Figure 10

Plot of the first-order rate constants for each catalyst 3a–3g against the parameter ΔV_c, a theoretical measure of the electron-donating or -withdrawing ability of the pyridine substituent. The dashed line indicates a linear fit of the points, excluding m-Cl and m-NMe₂.

Figure 11

Adapted catalytic cycle for the transfer hydrogenation of imines by [Cp*Ir((pyridinylmethyl)sulfonamide)Cl]-derived catalysts under acidic aqueous conditions.