Swarup Chattopadhyay

A Series of Peroxomanganese(III) Complexes Supported by Tetradentate Aminopyridyl Ligands: Detailed Spectroscopic and Computational Studies

A set of four [Mn(II)(L(7)py(2)(R))](2+) complexes, supported by the tetradentate 1,4-bis(2-pyridylmethyl)-1,4-diazepane ligand and derivatives with pyridine substituents in the 5 (R = Br) and 6 positions (R = Me and MeO), are reported. X-ray crystal structures of these complexes all show the L(7)py(2)(R) ligands bound to give a trans complex. Treatment of these Mn(II) precursors with either H(2)O(2)/Et(3)N or KO(2) in MeCN at -40 degrees C results in the formation of peroxomanganese complexes [Mn(III)(O(2))(L(7)py(2)(R))](+) differing only in the identity of the pyridine ring substituent. The electronic structures of two of these complexes, [Mn(III)(O(2))(L(7)py(2)(H))](+) and [Mn(III)(O(2))(L(7)py(2)(Me))](+), were examined in detail using electronic absorption, low-temperature magnetic circular dichroism (MCD) and variable-temperature variable-field (VTVH) MCD spectroscopies to determine ground-state zero-field splitting (ZFS) parameters and electronic transition energies, intensities, and polarizations. DFT and TD-DFT computations were used to validate the structures of [Mn(III)(O(2))(L(7)py(2)(H))](+) and [Mn(III)(O(2))(L(7)py(2)(Me))](+), further corroborating their assignment as peroxomanganese(III) species. While these complexes exhibit similar ZFS parameters, their low-temperature MCD spectra reveal significant shifts in electronic transition energies that are correlated to differences in Mn-O(2) interactions among these complexes. Taken together, these results indicate that, while the [Mn(III)(O(2))(L(7)py(2)(H))](+) complex exhibits symmetric Mn-O(peroxo) bond lengths, consistent with a side-on bound peroxo ligand, the peroxo ligand of the [Mn(III)(O(2))(L(7)py(2)(Me))](+) complex is bound in a more end-on fashion, with asymmetric Mn-O(peroxo) distances. This difference in binding mode is rationalized in terms of the greater electron-donating abilities of the methyl-appended pyridines and suggests a simple way to modulate Mn(III)-O(2) bonding through ligand perturbations.

Harnessing Scorpionate Ligand Equilibria for Modeling Reduced Nickel Superoxide Dismutase Intermediates

Hydrotris(3-phenyl-5-methylpyrazoyl)boratonickel(II) complexes with organoxanthate or dithiocarbamate coligands equilibrate between kappa(2)- and kappa(3)-chelation modes of the scorpionate ligand in solution, connecting N2S2 square-planar and N3S2 pyramidal ligand fields and a spin crossover. The complexes also exhibit quasi-reversible oxidations at low anodic potentials, thus modeling the structure, dynamics, and redox reactivity of the reduced NiSOD active site.

Chemistry of a new family of aryl ruthenium species incorporating α-diimine chelation and a pendant imine-phenol function

Abstract The reaction of excess 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) with Ru(η2-RL)(PPh3)2(CO)Cl (1) has respectively furnished [Ru(η1-RL)(PPh3)2(CO)(bpy)](PF6) (2) and [Ru(η1-RL)(PPh3)2(CO)(phen)](PF6) (3) in very good yield (η2-RL is C6H2O-2-CHNHC6H4R(p)-3-Me-5, η1-RL is C6H2OH-2-CHNC6H4R(p)-3-Me-5 and R is H, Me, MeO, Cl). The chelation of bpy/phen is attended with the cleavage of RuO and RuCl bonds and iminium-phenolato→imine-phenol prototropic shift. The carbon monoxide ligand is located syn to the phenolic oxygen in 2 and 3 as opposed anti in 1. The crystal and molecular structures of [Ru(η1-HL)(PPh3)2(CO)(bpy)](PF6)·CH2Cl2 (2(H)·CH2Cl2) and [Ru(η1-ClL)(PPh3)2(CO)(phen)](PF6)·H2O (3(Cl)·H2O) are reported. In the lattice of 3(Cl)·H2O, the water molecules form dimers (O⋯O, 2.617(8) A). In the hydrogen bonded imine-phenol function the N⋯O distance is 2.550(9) A in 2(H)·CH2Cl2 and 2.581(7) A in 3(Cl)·H2O. The Rubpy fragment in 2(H)·CH2Cl2 along with the CO ligand and the metallated carbon atom define an equitorial plane from which the metallated aldimine fragment (pendant Ph excluded) is rotated by 39.1° due to interligand repulsion and the two RuP distances (2.384(3) and 2.456(3) A) become significantly different. The repulsion also counteracts the trans influence of the metallated carbanionic site, the RuN bond trans to the site being shorter. The behaviour of 3(Cl)·H2O is similar. In dichloromethane solution 2 and 3 display a quasireversible RuIII/RuII cyclic voltammetric response with E1/2 in the range 0.75–0.85 V versus SCE. The CO stretch in 2 and 3 is 30–40 cm−1 higher than that in 1 due to the presence of dπ(Ru)–pπ(bpy/phen) back-bonding. The corresponding MLCT absorption (λmax, ∼470 nm) gives rise to fluorescence (λmax, ∼540 nm). The reaction of 2-(2-pyridyl)benzthiazole (pbt) with 1 has furnished [Ru(η1-RL)(PPh3)2(CO)(pbt)](PF6) (8) in which pbt is (N,N) chelated. The properties of 8 are generally similar to those of 2 and 3.

Synthesis and structure of pyridine-2-thiolato ruthenium aryls bearing a pendant imine/phenol function

Abstract The reaction of Ru(η2-RL)(PPh3)2(CO)Cl (1) with sodium pyridine-2-thiolate (Napyt) has afforded the complexes of the type Ru(η1-RL)(PPh3)2(CO)(pyt) (3) in excellent yield (η2-RL is C6H2O-2-CHNC6H4R(p)-3-Me-5, η1-RL is C6H2OH-2-CHNC6H4R(p)-3-Me-5 and R is Me, MeO, Cl). The reaction is irreversible and is believed to proceed via cis attack by pyt− on an octahedral face. The type 3 species are thermodynamically more stable than the corresponding carboxylates, nitrites and nitrates. Structure determination of the solvate, 3(R=Cl)·C6H6·0.5C6H14 has revealed a distorted octahedral RuC2P2NS coordination sphere with the pairs (P, P), (C(aryl), N) and (C(carbonyl), S) defining the three trans directions. The η1-ClL ligand metallated ortho to the phenolic oxygen occurs in the imine–phenol tautomeric form (N⋯O, 2.639(8) A) which is consistent with IR and 1H NMR data. The transformation 1→3 is associated with changes in tautomeric form, hapticity and conformation (in relation to CO) of RL. The Ru(η1-ClL) fragment excluding the pendant C6H4Cl(p) group is nearly coplanar (dihedral angle 6.1°) with the Ru(pyt) plane. The RuS distance, 2.524(1) A, is longer than usual due to steric reasons. The type 3 complexes display a quasi-reversible Ru(III)/Ru(II) couple near 0.6 V versus SCE.

A family of thioxanthato ruthenium and osmium aryls

The title complexes of type M(RL 2 )(PPh 3 ) 2 (CO)(S 2 CSEt) (2a: M = Ru; 2b: M = Os) have been synthesized in excellent yields by reacting M(RL 1 )(PPh 3 ) 2 (CO)X (1a: M = Ru, × = Cl; 1b: M = Os, × = Br) with potassium ethyl thioxanthate and have been characterized with the help of spectral and electrochemical data. The RL 2 ligand in 2 is the imine-phenol tautomer of N-C 6 H 4 R(ρ)-4-methylsalicylaldimine (R = Me, MeO, Cl) coordinated at the carbanionic-C2 atom only while RL 1 in 1 is the iminium-phenolato tautomer chelated via carbanionic-C2 and phenolato-O atoms. The synthetic reaction is thus attended with tautomerization of the Schiff base ligand. It is also associated with a rotation of the ligand by ~180° around the M-C bond in order to exclude steric repulsion. These features have been revealed by structure determination of 2a (R = Me). The metallated aldimine ring is found to be highly noncoplanar (dihedral angle ~40°) with the thioxanthate chelate ring due to steric repulsion originating from the relatively large size of the sulfur atom. This phenomenon, which is absent in both the precursor 1 (R = Me) and in the carboxylate analogue Ru(MeL 2 )(PPh 3 ) 2 (CO)(O 2 CMe), 7, has distinctive effects on bond parameters of 2a (R = Me). Thus the two Ru-P bonds in 2a (R = Me) differ in length by as much as 0.06 A. The thioxanthate 2 is thermodynamically more stable than the precursor 1 as well as the carboxylate 7. Accordingly, both of these are irreversibly transformed to 2a (R = Me) upon treatment with thioxanthate.