1-Benzyl-4-((phenylthio)-/(phenylseleno)methyl)-1H-1,2,3-triazole (L1/L2) and 4-phenyl-1-((phenylthio)-/(phenylseleno)methyl)-1H-1,2,3-triazole (L3/L4) synthesized using the click reaction have been reacted for the first time with [{(η6-C6H6)RuCl(μ-Cl)}2] and NH4PF6 to design the half-sandwich complexes [(η6-benzene)RuLCl]PF6 (1–4 for L = L1–L4), which have been characterized by single-crystal X-ray diffraction and explored for the catalytic oxidation of alcohols with N-methylmorpholine N-oxide (NMO) and transfer hydrogenation of ketones with 2-propanol. There is a pseudo-octahedral “piano-stool” disposition of donor atoms around Ru in 1–4. In 1 and 2, N(3) of the triazole skeleton coordinates with Ru, whereas in other complexes the nitrogen involved is N(2). The Ru–S and Ru–Se bond distances are 2.3847(11)/2.3893(10) and 2.497(5)/2.4859(9) Å, respectively. The catalytic processes are more efficient with 3 and 4 (compared to 1 and 2), in which N(2) of the triazole is involved in coordination with Ru. The nature of the chalcogen and steric factors together also appear to affect the efficiency of complexes. HOMO–LUMO energy gaps are lower for 3 and 4 than for 1 and 2. The formation of RuIVO species probably results in oxidation and transfer hydrogenation involves an intermediate containing Ru–H. Bond distances and angles based on DFT calculations are generally consistent with experimental values.

The reactions of 1-benzyl-4-((phenylthio/phenylseleno)methyl)-1H-1,2,3-triazole (L1/L2) and 4-phenyl-1-((phenylthio/phenylseleno)methyl)-1H-1,2,3-triazole (L3/L4), synthesized using the click reaction, with [(η5-Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μ-Cl)]2 at room temperature followed by treatment with NH4PF6 result in complexes of the type [[(η5-Cp*)M(L)Cl] (1–8). Their HR-MS and 1H, 13C{1H}, and 77Se{1H} NMR spectra have been found characteristic. The single-crystal structures of 2, 3, and 6 have been established by X-ray crystallography. There is a pseudo-octahedral “piano-stool” disposition of donor atoms around Rh/Ir. In 1, 2, 5, and 6 N(3) of the triazole skeleton coordinates with Rh/Ir, whereas in the other four complexes the nitrogen involved is N(2). These complexes have been explored as catalysts for N-methylmorpholine N-oxide (NMO) based and Oppenauer-type oxidation of alcohols and transfer hydrogenation (TH) of carbonyl compounds with 2-propanol. Oppenauer type oxidation is somewhat slower than that based on NMO. The homogeneous nature of TH is supported by a poisoning test. The catalytic processes are more efficient with Rh complexes than the corresponding Ir complexes. The complexes having N(2) coordinated with the metal have shown better activity than those in which N(3) is involved in ligation. The reactivity with respect to ligands is in the order Se > S. In TH the species formed with loss of Cp* appears to be involved in catalysis with Rh as well as Ir complexes. Such a loss is noticed in the case of Rh for the first time. Generally results of DFT calculations are consistent with the experimental results.

1-(2,6-Diisopropylphenyl)-4-(phenylthio/selenomethyl)-1H-1,2,3-triazole (L1/L2) was synthesized by a 'Click' reaction and treated with [Pd(CH3CN)2Cl2] for 5 h or [(η(6)-C6H6)RuCl(μ-Cl)]2 for 8 h (followed by reaction with NH4PF6) at room temperature, resulting in complexes [Pd(L)Cl2] (1 and 2) or [(η(6)-C6H6)Ru(L)Cl]PF6 (3 and 4) (L = L1 or L2), respectively. The four complexes (1-4) and ligands (L1 and L2) were characterized with (1)H, (13)C{(1)H} and (77)Se{(1)H} NMR spectroscopy and high resolution mass spectrometry. The single crystal structures of 1-4 were solved. The geometry of Pd in 1 and 2 is distorted square planar. The Pd-S and Pd-Se bond distances in 1 and 2 are 2.277(3) and 2.384(6) Å respectively. In 3 and 4, there is a pseudo-octahedral "piano-stool" type disposition of donor atoms around Ru. The Ru-S and Ru-Se bond lengths in 3 and 4 are 2.3728(12) and 2.4741(6) Å respectively. The catalytic activity of complexes 1 and 2 was explored for Suzuki-Miyaura coupling (SMC) in water and the Sonogashira coupling reaction. For various aryl bromides, including deactivated ones, complexes 1 and 2 were found to be efficient catalysts for both couplings. The optimum loading of 1 and 2 required to catalyze both coupling reactions is of the order of 0.001-2 mol% of Pd. For SMC, no additive or phase transfer catalyst was added. For catalysis of the transfer hydrogenation (TH) of aldehydes and ketones, the half-sandwich Ru(ii) complexes 3 and 4 were explored. Their optimum catalytic loading was found to be 0.1-0.4 mol% of Ru. For TH, both the water solvent and the glycerol hydrogen source are environmentally friendly. The catalytic efficiencies of 3 and 4 are comparable with those reported for other catalysts for TH carried out with 2-propanol or glycerol as a H-source. 1, with a sulfur ligand, is more efficient than 2 (Se analog) for both SMC and the Sonogashira coupling. The activities of 3 and 4 for TH are in the order Se > S.

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Organoselenium ligands are building blocks of several transition metal complexes which catalyze various organic reactions efficiently in solution. In this review the survey of developments pertaining to the designing of such complexes (along with their Se-ligands), and their uses as catalysts for various organic reactions has been presented with critical analysis of present status of the subject. Undoubtedly a new family of catalysts or their precursors has been generated by organoselenium ligands. The catalytic activity and selectivity of their metal complexes can sometimes be fine-tuned efficiently by changing the ligand's framework. They show good thermal and air-stability. This, coupled with ease in their handling and synthesis, may make them not only attractive but good alternatives to several popular catalysts.