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This article describes the synthesis and reactivity studies of three cobalt complexes bearing aminophenol-derived ligands without nitrogen substitution: Co(II)((tBu2)APH)(2)((tBu2)AP)(2) (1), Co(2)(III)((tBu2)APH)(2)((tBu2)AP)(2)(μ-(tBu2)BAP)(2) (2), and Co(III)((tBu2)AP)(3) (3) ((tBu2)APH = 2-amino-4,6-di-tert-butylphenol, (tBu2)AP = 2-amino-4,6-di-tert-butylphenolate, μ- (tBu2)BAP = bridging 2-amido-4,6-di-tert-butylphenolate). Stoichiometric reactivity studies of these well-defined complexes demonstrate the catalytic competency of both Co(II) and Co(III) complexes in the aerobic oxidative cyclization of (tBu2)APH with tert-butyl isonitrile. Reactions with O(2) reveal the aerobic oxidation of Co(II) complex 1 to generate the Co(III) species 2 and 3. UV-visible time-course studies and EPR spectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent the first example of well-defined cobalt-aminophenol complexes that participate in catalytic aerobic oxidation reactions and highlight a key role for a ligand radical in the oxidation sequence.

Pincer ligands continue to generate interest owing to the unusual properties they impart on metal centers and consequently on the complexes they are involved in. These ligands and their metal complexes have aided in the understanding of a number of processes mediated by organometallic and non-organometallic systems. In homogenous catalytic transformations, pincer ligands provide enhanced chemical and thermal stability which serve to minimize the leaching of the metal during the catalytic cycle. These ligands also offer the ability to fine tune the electronic and the steric properties about the metal center, thereby increasing the scope of their applications. This review discusses some of the general synthetic protocols of the main classes of pincer ligands with a focus on some of their more popular applications.

A ruthenium(II) complex of 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (pbcta), of the formula [Ru(pbcta)Cl2(dmf)] (1, where DMF = dimethyl formamide) was prepared from RuCl3•xH2O and pbcta in DMF at reflux under argon atmosphere. The identity of 1 was confirmed from its elemental analysis, ESI MS, and a series of spectroscopic measurements. Voltammetric measurements on 1 in DMF and DFT studies on the structure optimized in the gas phase revealed predominantly ligand based electron transfer processes under argon. In the presence of a proton source, proton coupled electron transfer to the ligand occurs. Under a carbon dioxide atmosphere, voltammetric studies revealed that 1 is inactive for CO2 reduction, and the redox responses observed in the presence of the proton source and/or CO2 are ligand based leading to reactions with the coordinated pbcta. Transfer hydrogenation (TH) of aryl ketones was efficiently carried out in 2-propanol using 1 at reflux. TH of the aryl ketone substrates proceeded in air with almost quantitative conversions at 0.2–1.0 mol% catalyst.

Procedures for the preparation of the nucleophilic diphosphine ligands (R)-(4,4',6,6'-tetramethoxybiphenyl-2,2'-diyl)bis(diphenylphosphine) ((R)-Ph-Garphos, 2a) and (S)-(4,4',6,6'-tetramethoxybiphenyl-2,2'-diyl)bis(diphenylphosphine) ((S)-Ph-Garphos, 2b) were described. The ligands were used to prepare the ruthenium(II) Ph-Garphos complexes, chloro(p-cymene)(R)-(4,4',6,6'-tetraamethoxybiphenyl-2,2'-diyl)bis(diphenylphosphine)ruthenium(II) chloride ([RuCl(p-cymene)(R)-Ph-Garphos]Cl (3)) and chloro(p-cymene)(S)-(4,4',6,6'-tetraamethoxybiphenyl-2,2'-diyl)bis(diphenylphosphine)ruthenium(II) chloride ([RuCl(p-cymene)(S)-Ph-Garphos]Cl (4)). In the presence of the chiral diamine co-ligands (1R,2R)-1,2-diphenylethane-1,2-diamine (R,R-DPEN) and (1S,2S)-1,2-diphenylethane-1,2-diamine (S,S-DPEN), complexes 3 and 4 were found to be active catalyst precursors for the enantioselective reduction of aryl ketones under mild conditions (room temperature and 3-4 atm of H2). The chiral alcohols were isolated in moderate to good yields and with enantioselectivities of up to 93%. The ruthenium complexes, chloro(p-cymene)(R)-(4,4',6,6'-tetramethoxybiphenyl-2,2'-diyl)bis(bis(3,5-dimethylphenyl)-phosphine)ruthenium(II) chloride ([RuCl(p-cymene)(R)-Xyl-Garphos]Cl (5)) and chloro(p-cymene)(S)-(4,4',6,6'-tetramethoxybiphenyl-2,2'-diyl)bis(bis(3,5-dimethylphenyl)-phosphine)ruthenium(II) chloride ([RuCl(p-cymene)(S)-Xyl-Garphos]Cl (6)), were also prepared and used as catalyst precursors for the hydrogenation of aryl ketones in the presence of (R,R)-DPEN and (S,S)-DPEN. Significant improvements in the enantioselectivities of the alcohols (up to 98% ee.) were afforded. A combination of 6 and (S,S)-DPEN afforded (R)-1-(3-methoxyphenyl)ethanol in 89% yield and with 95% ee which was shown to be a suitable precursor for the preparation of (S)-rivastigmine.

Hydrazones and azo compounds have been actively studied for more than a century, with thousands of papers presented in the literature. Both functional groups allow for facile structural isomerization, leading to their applications as molecular switches and also as detectors for various environmental stimuli. With each decade, new applications are explored, however the fundamentals of the redox properties drive the ultimate usefulness of both moieties in applications where there is the possibility of a redox reaction occurring. This review highlights the general synthetic routes, some useful applications of hydrazones and azo compounds, as well as their general voltammetric properties under reductive and oxidative conditions, with an emphasis on non-aqueous media.