We report highly active iridium precatalysts, [Cp*Ir(N,N)Cl]Cl (1-4), for water oxidation that are supported by recently designed dihydroxybipyridine (dhbp) ligands. These ligands can readily be deprotonated in situ to alter the electronic properties at the metal; thus, these catalyst precursors have switchable properties that are pH-dependent. The pKa values in water of the iridium complexes are 4.6(1) and 4.4(2) with (N,N) = 6,6'-dhbp and 4,4'-dhbp, respectively, as measured by UV-vis spectroscopy. For homogeneous water oxidation catalysis, the sacrificial oxidant NaIO4 was found to be superior (relative to CAN) and allowed for catalysis to occur at higher pH values. With NaIO4 as the oxidant at pH 5.6, water oxidation occurred most rapidly with (N,N) = 4,4'-dhbp, and activity decreased in the order 4,4'-dhbp (3) > 6,6'-dhbp (2) ≫ 4,4'-dimethoxybipyridine (4) > bipy (1). Furthermore, initial rate studies at pH 3-6 showed that the rate enhancement with dhbp complexes at high pH is due to ligand deprotonation rather than the pH alone accelerating water oxidation. Thus, the protic groups in dhbp improve the catalytic activity by tuning the complexes' electronic properties upon deprotonation. Mechanistic studies show that the rate law is first-order in an iridium precatalyst, and dynamic light scattering studies indicate that catalysis appears to be homogeneous. It appears that a higher pH facilitates oxidation of precatalysts 2 and 3 and their [B(Ar(F))4](-) salt analogues 5 and 6. Both 2 and 5 were crystallographically characterized.

N-heterocyclic carbene (NHC) based ruthenium complexes were studied as catalysts for the transfer hydrogenation of ketones. Variations in the catalyst structure were investigated for their impact on hydrogenation and catalyst stability. Catalyst attributes included bis-or mono-NHC ligands, pendant ether groups in some cases, and arene ligands of varied bulk and donor strength. Ruthenium complexes were synthesized and fully characterized, including complexes with a monodentate NHC ligand containing a tethered ether N substituent (Im Et,CH2CH2OEt RuCl 2 (η 6 -arene); arene = benzene (4), p-cymene (5), hexamethylbenzene (6)), a complex with a monodentate NHC ligand with solely alkyl N substituents (Im Et,Pentyl RuCl 2 (η 6 -p-cymene) ( 8)), and a complex with a bis-NHC ligand ([RuCl(methylenebis(Im Et ) 2 )(η 6 -p-cymene)]PF 6 ( 7)) (Im = imidazole-derived NHC; superscripts indicate N substituents). X-ray crystal structures were obtained for 4, 5, 7, and 8. All of the ruthenium complexes were tested and found to be active transfer hydrogenation catalysts for the reduction of acetophenone to 1-phenylethanol in basic 2-propanol. Precatalyst 4, which contains a tethered ether group and benzene ligand, was found to be the most active catalyst. Variabletemperature 1 H NMR studies of complexes 4−6 show that arene lability increases in the order C 6 Me 6 < cymene < benzene, and this lability is directly correlated with catalytic activity. The catalysis appears to be homogeneous, and a mechanism invoking arene loss is proposed. Precatalyst 4 reduced electron-deficient ketones most easily, and 4′-nitroacetophenone was reduced under base-free conditions. The highest TOF (turnover frequency) and TON (turnover number) values obtained were 3003 h −1 and 845, respectively, for ketone reduction with catalyst 4.

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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.

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