Ruthenium Complexes of Triazole-Based Scorpionate Ligands Transfer Hydrogen to Substrates under Base-Free Conditions

Abstract

The first ruthenium complexes of bulky tris­(triazolyl)­borate (Ttz) ligands were synthesized, fully characterized, and studied as transfer hydrogenation catalysts. The structures of the complexes were (η6-arene)­RuCl­(N,N), where in each case N,N is a κ2-Ttz or bis­(triazolyl)­borate (Btz) ligand (arene = p-cymene (1, 3, 5, 6), benzene (2), C6Me6 (4); N,N = TtzPh,Me* (1, 2), TtzMe,Me (3, 4), Ttz (5), Btz (6)). All but 5 were crystallographically characterized, and notably for 1 and 2 a rearranged ligand structure is observed (as indicated by an asterisk). These complexes were all effective catalysts for transfer hydrogenation of aryl ketones in isopropyl alcohol with base co-catalyst, with rates that were accelerated by moisture-free conditions. Complexes 1 and 2 are also effective catalysts for base-free transfer hydrogenation, and with 1 hydrogenation of several base-sensitive substrates was demonstrated. The ability of 1 to serve as a hydrogenation catalyst without base is attributed primarily to steric bulk, and a preliminary mechanism for formation of that active catalyst is proposed.

Heteroleptic 1,4‐Diazabutadiene Complexes of Ruthenium: Synthesis, Characterization and Utilization in Catalytic Transfer Hydrogenation

“…2‐hydroxyacetophenone ( S 11 ) and 2‐acetylpyridine ( S 12 ), were tried as substrates, the expected reduction did not take place at all, probably due to catalyst deactivation via coordination of these substrates. In view of the relative difficulty in reduction of ketone compared to that of aldehyde, [ 20 ] the average turn‐over number (1.6 × 10 2 ) observed in the present study is decent. The catalytic efficiency of our complexes in transfer hydrogenation of aldehydes and ketones is better than many of the reported ruthenium‐catalysts.…”

“…The catalytic efficiency of our complexes in transfer hydrogenation of aldehydes and ketones is better than many of the reported ruthenium‐catalysts. [ 1b,1e,2a,2c,2h,2i,20b,20c ] For comparison of the catalytic efficiency of our complexes, we prepared the well‐known [Ru(trpy)(bpy)Cl]ClO 4 complex and tested it as catalyst precursor (0.00002 mol‐%) under the same experimental condition for the transfer hydrogenation of 4‐chlorobenzaldehyde. The expected reduced product, viz.…”

The hydridotris(3-nitro-1,2,4-triazol-1-yl)borate, a new nitro-substituted electron withdrawing polydentate “scorpionate”-type ligand and related copper and silver phosphane complexes

“…Poly(triazolyl)borates [19,[62][63][64][65][66][67][68][69][70] are an attractive class of scorpionate ligands due to two important features: first, they are more electron-withdrawing than their poly(pyrazolyl)borate counterparts [71] and second, the remote exo-ring-nitrogen atoms, in the fourth position on the triazole rings, represent an additional binding site for metal ions resulting in new structures. In particular, they may bridge between metal centers and create a coordination polymer or may take part in hydrogen-bond interactions, assisting the formation of two-dimensional water intercalate or water-layer clathrates [72], thus leading to water soluble species [66,73,74].…”

Hydroelementation of Unsaturated C–C Bonds Catalyzed by Metal Scorpionate Complexes

“…Although the rich coordination chemistry of scorpionate ligands has been explored in detail, it is only in the past two decades that their utility in catalytic transformations has truly begun to flourish. Complexes bearing scorpionate ligands have been reported in olefin polymerization, olefin aziridination, lactone polymerization, ring‐opening metathesis polymerization, and transfer hydrogenation . Notably scorpionate complexes have made a significant impact in the hydroelementation of unsaturated carbon–carbon bonds.…”