The reactions of 1,2-bis(phenylthiomethyl)benzene(L1) and 1,2-bis(phenylselenomethyl)benzene(L2) with [(η 5 -Cp*)MCl(μ-Cl)] 2 (M = Rh or Ir) at room temperature, followed by treatment with NH 4 PF 6 have resulted in air and moisture insensitive half-sandwich complexes of composition [(η 5 -Cp*)M(L)Cl]-[PF 6 ] (Rh, 1−2; Ir, 3−4; L = L1 or L2). Their HR-MS, 1 H, 13 C{ 1 H}, and 77 Se{ 1 H} NMR spectra were found to be characteristic. The single crystal structures of 1−4 have been established by X-ray crystallography. The complexes 1−4 have been found efficient for catalytic transfer hydrogenation (TH) of aldehydes and ketones in glycerol, which acts as a solvent and hydrogen source. Complexes 1−2 are the first examples of Rh species explored for TH in glycerol. The catalysis appears to be homogeneous. The complexes of the (Se, Se) ligand are marginally efficient than the corresponding complexes of the (S, S) ligand. The reactivity of Rh complexes in comparison to those of Ir also appears to be somewhat more. The results of DFT calculations appear to be generally consistent with experimental catalytic efficiencies and bond lengths/angles.

1H-Benzoimidazole on subjection to a sequence of reactions with benzyl bromide, PhECH2Cl (E = S, Se), and elemental S or Se results in 1-benzyl-3-phenylchalcogenylmethyl-1,3-dihydrobenzoimidazole-2-chalcogenones (L1–L4), which are unsymmetrical bidentate chalcogen ligands having a unique combination of chalcogenoether and chalcogenone donor sites. Half sandwich complexes, [(η6-C6H6)Ru(L)Cl][PF6] (1–4), have been synthesized by reactions of [(η6-C6H6)RuCl(μ-Cl)]2 with the appropriate L at room temperature followed by treatment with NH4PF6. L1–L4 and their complexes 1–4 have been authenticated with HR-MS and 1H, 13C{1H}, and 77Se{1H} NMR spectra. The single-crystal structures of 1–4 have been determined by X-ray crystallography. Each L acts as an unsymmetric (E,E) or (E,E′) bidentate ligand. The Ru atom in 1–4 has pseudo-octahedral half-sandwich “piano-stool” geometry. The Ru–S and Ru–Se bond distances (Å) respectively are 2.358(3)/2.3563(18) and 2.4606(11)/2.4737(10) (thio- and selenoether), and 2.4534(17)/2.435(3) and 2.5434(9)/2.5431(10) (thione and selone). Catalytic activation with complexes 1–4 has been explored for the transfer hydrogenation (TH) of aldehydes and ketones using various sources of hydrogen. 2-Propanol and glycerol have been compared and found most suitable among the sources screened. The catalytic efficiency of other sources explored, viz. formic, citric, and ascorbic acid, is dependent on the pH of reaction medium and is not promising. A comparative study of 2-propanol and glycerol as hydrogen sources for catalytic activation of TH with 1–4 has revealed that with glycerol (for comparable conversion in the same time) more amount of catalyst is needed in comparison to that of 2-propanol. The catalytic process is more efficient with 3 (where Ru is bonded with selone), followed by 1 ≈ 4, and 2 showing the least activity among all four complexes. The transfer hydrogenation involves an intermediate containing a Ru–H bond and follows a conventional alkoxide intermediate based mechanism. The results of DFT calculations appear to be generally consistent with experimental catalytic efficiencies and bond lengths/angles.

The half sandwich complexes [(η(6)-C6H6)Ru(L)Cl][PF6] (1-5) have been synthesized by the reactions of (2-arylchalcogenomethyl)pyridine [L = L1-L3] and bis(2-pyridylmethyl)chalcogenide [L = L4-L5] (chalcogen = S, Se; Ar = Ph/2-pyridyl for S, Ph for Se) with [(η(6)-C6H6)RuCl2]2, at room temperature followed by treatment with NH4PF6. Their HR-MS, (1)H, (13)C{(1)H} and (77)Se{(1)H} NMR spectra have been found characteristic. The single crystal structures of 1-5 have been established by X-ray crystallography. The Ru has pseudo-octahedral half sandwich "piano-stool" geometry. The complexes 1-5 have been found efficient for catalytic oxidation of alcohols with N-methylmorpholine-N-oxide (NMO) and transfer hydrogenation of ketones with 2-propanol (at moderate temperature 80 °C) as TON values are up to 9.9 × 10(3) and 9.8 × 10(3) respectively for the two catalytic reactions. On comparing the required catalyst loading for good conversions and reaction time for the present complexes with those reported in literature for other transfer hydrogenation/oxidation catalysts, it becomes apparent that 1-5 have good promise. The complexes of Se ligands have been found more efficient than their sulphur analogues. The complexes of bidentate ligands are more efficient than those of terdentate, due to difficult bond cleavage in the case of latter. These orders of efficiency are supported by DFT calculations. The calculated bond lengths/angles by DFT are generally consistent with the experimental ones.

The reactions of 2-(pyridine-2-ylmethylsulfanyl)-benzoic acid (L) with [(η 5 -Cp*/η 6 -benzene)MCl(μ-Cl)] 2 , (benzene, M = Ru; Cp*, M = Rh, Ir) at room temperature followed by treatment with NH 4 PF 6 result in a new class of water-soluble halfsandwich complexes [(η 5 Cp*/η 6 -benzene)M(L)Cl][PF 6 ] (1−3, respectively, for M = Ru, Rh, Ir). Their characteristic HR-MS and 1 H and 13 C{ 1 H} NMR spectra have been found. The single-crystal structures of 1−3 have been established with X-ray crystallography. The Ru−S, Rh−S, and Ir−S bond lengths are 2.4079(6), 2.3989(10), and 2.3637(14) Å, respectively. Complexes 1−3 have been found to be efficient for catalytic transfer hydrogenation (TH) of carbonyl compounds in water with glycerol as a hydrogen donor. Glycerol has been explored for TH in water for the first time. The efficiency in water of other hydrogen sources, viz. HCOOH, citric acid, ascorbic acid, and 2-propanol, is less and/or is pH dependent. Catalysis with glycerol as a hydrogen source is pH independent and appears to be homogeneous. Higher reactivity for the Rh complex in comparison to the Ru and Ir species has been observed. DFT calculations are generally consistent with the experimental values of bond lengths and angles and catalytic reactivity order.

Ionizable residues in the hydrophobic interior of certain proteins are known to play important roles in life processes like energy transduction and enzyme catalysis. These internal ionizable residues show experimental apparent pK a values having large shifts as compared to their values in solution.In the present work, we study the pH-dependent conformational changes undergone by two variants of staphylococcal nuclease (SNase), L25K and L125K, using pH replica exchange molecular dynamics (pH-REMD) in explicit solvent. Our results show that the observed pK a of Lys25 and Lys125 are significantly different than their pK a in solution. We observed that the internal lysine residues prefer to be water-exposed when protonated at low pH, but they remain buried within the hydrophobic pocket when deprotonated at high pH. Using thermodynamic laws, we estimate the microscopic conformation-specific pK a of the water-exposed and buried conformations of the internal lysine residues and explain their relation to the macroscopic observed pK a values. We present the differences in the microscopic mechanisms that lead to similar experimentally observed apparent pK a of Lys25 and Lys125, and explain the need of thermodynamic models of different complexities to account for our calculations. We see that L25K displays pH-dependent fluctuations throughout the entire β barrel and the α1 helix. In contrast, pH-independent fluctuations are observed in L125K, primarily limited to the α3 helix. The present computational study offers a detailed atomistic understanding of the determinants of the observed anomalous pK a of internal ionizable residues, bolstering the experimental findings.