Pradip Munshi

Paramagnetic ruthenium(III) ortho-metallated complexes. Synthesis, spectroscopic and redox properties

Abstract The reaction of (CS)Cl(PPh 3 ) 2 Ru II (μ-Cl) 2 Ru II (PPh 3 ) 2 Cl(CS), A with the phenolic Schiff base ligands o- (OH)C 6 H 4 C(H)N-C 6 H 4 (R), (R= p -OMe, Me, H, Cl, NO 2 ; H 2 L 1 –H 2 L 5 ) in methanol under aerobic conditions afforded the complexes Ru III (HL) 2 (PPh 3 )Cl, 1 and Ru III (L)(PPh 3 )(CH 3 OH)Cl, 2 . In complexes 1 both the ligands (HL − ) are bound to the metal center at the deprotonated phenolic oxygen and azomethine nitrogen and in the complexes 2 the L 2− is in tridentate C,N,O mode where cyclometallation takes place from the ortho carbon atom of the amine fragment of H 2 L. During the reaction the metal ion is oxidized from the starting Ru II in A to Ru III in the products 1 and 2 . The complexes ( 1 and 2 ) are nonconducting and behave as one-electron paramagnets. Complexes display rhombic EPR spectra that have been analyzed to furnish values of axial (▵) and rhombic (V) distortion parameters as well as energies of the two expected ligand field transitions ( ν 1 and ν 2 ) within the t 2 shell. One of the transitions ( ν 2 ) has been observed in the predicted region. The complexes exhibit moderately strong ligand-to-metal charge-transfer transition in the visible region and intraligand transitions in the UV region. The complexes are electroactive and show ruthenium (IV)–ruthenium(III) ( E 1/2 , 0.75–0.88 V vs. Ag/AgCl) and ruthenium(III)–ruthenium(II) ( E 1/2 , −0.42 to −0.59 V) couples. The E 1/2 values vary linearly with the Hammett constant of the substituents R. The role of coordination of phenolato function in stabilizing the unusual paramagnetic ruthenium(III) oxidation state in the complexes 2 is noted.

Effect of liophilicity of catalyst in cyclic carbonate formation by transesterification of polyhydric alcohols

The effect of catalyst liophilicity is shown in cyclic carbonate formation by transesterification. 1,3-Dichlorodistannoxanes as liophilic transesterification catalysts facilitated cyclic carbonate formation from corresponding 1,2-diols and diethyl carbonate in continuous fashion without isolation of catalyst. Thus 0.5 mol% of catalyst could produce 1,2-glycerol carbonate quantitatively in 2 h with multiple recyclability. The product formed during the reaction was almost quantitative and did not require further purification. Isolation of catalyst at any stage showed retention of its activity and identity.

Dinuclear ruthenium(II) bipyridine complexes having non-symmetric α,α′-diimine based neutral bridging ligands.: Synthesis, spectroscopic and electrochemical properties

Abstract A group of three dinuclear ruthenium(II) complexes of the type [(bpy)2RuII(L)RuII(bpy)2](ClO4)4·2H2O, 1a–1c [bpy=2,2′-bipyridine, L=bridging ligand, NpC5H4CH=Ni–(R)–Ni=CHC5H4Np; R=none, 1a; R=–C6H4–, 1b; R=–CH2–C6H4–CH2–, 1c] have been synthesized and characterized. The complexes are essentially diamagnetic and behave as 1:4 electrolytes in acetonitrile solution. The mass of the molecular ion for the complex 1a and the geometry of the complexes 1 in solution have been assessed by fast atom bombardment (FAB) mass spectrometry and 1H/13C NMR spectroscopy, respectively. Complexes 1 display three metal-to-ligand charge-transfer (MLCT) transitions in the visible region, where the lowest energy MLCT transition is considered to be a dπ(RuII)→π*(L) transition. The other two higher energy MLCT transitions are believed to be dπ(RuII)→π*(bpy) transitions. Highly intense ligand-based π→π* transitions are observed in the UV region. In acetonitrile solvent, complexes 1 show one quasi-reversible two-electron oxidation process near 1.5 V vs. Ag/AgCl, due to simultaneous one-electron oxidations [ruthenium (III)⇌ruthenium(II)] of both of the ruthenium centers in 1 and multiple reductions in the range −0.5–−2.7 V vs. Ag/AgCl, due to successive reductions of the coordinated bridging ligand, L, as well as bipyridine. The chemically and electrochemically generated oxidized trivalent congeners of 1 are unstable at room temperature.

Ruthenium(II/III) bipyridine complexes incorporating thiol-based imine functions: Synthesis, spectroscopic and redox properties

Abstract A group of five new ruthenium(II) bipyridine heterochelates of the type [Ru II (bpy) 2 L] + 1a – 1e have been synthesized (bpy=2,2′-bipyridine; L=anionic form of the thiol-based imine ligands, HS–C 6 H 4 NC(H)C 6 H 4 (R) (R=OMe, Me, H, Cl, NO 2 ). The complexes 1a − 1e are 1:1 conducting and diamagnetic. The complexes 1a − 1e exhibit strong MLCT transitions in the visible region and intra-ligand transitions in the UV region. In acetonitrile solvent complexes show a reversible ruthenium(III)–ruthenium(II) couple in the range 0.2–0.4 V and irreversible ruthenium(III)→ruthenium(IV) oxidation in the range 1.15–1.73 V vs. SCE. Two successive bipyridine reductions are observed in the ranges −1.43 to −1.57 and −1.67 to −1.78 V vs. SCE. The complexes are susceptible to undergo stereoretentive oxidations to the trivalent ruthenium(III) congeners. The isolated one-electron paramagnetic ruthenium(III) complex, 1c + exhibits weak rhombic EPR spectrum at 77 K ( g 1 =2.106, g 2 =2.093, g 3 =1.966) in 1:1 chloroform–toluene. The EPR spectrum of 1c + has been analyzed to furnish values of distortion parameters ( Δ =8988 cm −1 ; V =0.8833 cm −1 ) and energy of the expected ligand field transitions ( ν 1 =1028 nm and ν 2 =1186 nm) within the t 2 shell. One of the ligand field transitions has been experimentally observed at 1265 nm.

Asymmetric hydrogenation of methyl pyruvate using platinum carbonyl cluster supported on an anion exchanger as the catalyst

Anionic platinum carbonyl clusters supported on quaternary amine functionalized cross-linked polystyrene, i.e., anion exchangers, are effective precatalysts for the hydrogenation of methyl pyruvate to methyl lactate. Kinetic data has been obtained for anion exchangers with different quaternary groups. Highest observed rate constant and enantioselectivity are obtained with cinchonine funtionalized resin. The kinetic data also indicates saturation kinetics. The decarbonylated used catalyst could be recarbonylated to give a material spectroscopically (infrared) equivalent to freshly anchored cluster.

A new class of sulfur bridged ruthenium–molybdenum complexes, (L)2RuII(μ-S)2MoIV(OH)2 [L=NC5H4N=NC6H4(R), R=H, o-Me/Cl, m-Me/Cl]. Synthesis, spectroscopic and electron-transfer properties

Abstract The reaction of (NH 4 ) 2 Mo VI S 4 with the complexes ctc -Ru II (L) 2 Cl 2 (1a–1e) [L=NC 5 H 4 N=NC 6 H 4 (R), R=H, o -Me/Cl, m -Me/Cl; ctc = cis–trans–cis with respect to chlorides, pyridine and azo nitrogens respectively] in MeOH–H 2 O (1:1) resulted in a group of stable sulfur bridged ruthenium–molybdenum complexes of the type (L) 2 Ru II (μ-S) 2 Mo IV (OH) 2 (2a–2e). In complexes 2 the terminal MoS bonds of the Mo VI S 4 2− unit get hydroxylated and the molybdenum ion is reduced from the starting Mo VI in MoS 4 2− to Mo IV in the final product 2. The cis–trans–cis (with respect to sulfurs, pyridine and azo nitrogens respectively) configuration of the RuL 2 S 2 fragment in 2 has been established by the 1 H NMR spectroscopy. In dichloromethane solution the complexes 2 exhibit a strong dπ(Ru II )→Lπ* MLCT transition near 550 nm, a strong sulfur to molybdenum LMCT transition near 500 nm and intra ligand π–π* transition in the UV region. In dichloromethane solution the complexes display reversible Ru II ⇌Ru III oxidation couples in the range 1.15–1.39 V, irreversible Mo IV →Mo V oxidations in the range 1.68–1.71 V vs SCE. Four successive reversible ligand (–NN–) reductions are observed for each complex in the ranges −0.37→−0.67 V (one-electron), −0.81→−1.02 V (one-electron) and −1.48→−1.76 V (simultaneous two-electron reduction) vs SCE respectively. The presence of trivalent ruthenium in the oxidized solutions 2 + is evidenced by the rhombic EPR spectra. The EPR spectra of the coulometrically oxidized species 2 + have been analyzed to furnish values of axial (Δ=4590–5132 cm −1 ) and rhombic ( ν =1776–2498 cm −1 ) distortion parameters as well as energies of the two expected ligand field transitions ( γ 1 =3798–4022 cm −1 ) and ( γ 2 =5752–6614 cm −1 ) within the t 2 shell. One of the ligand field transitions has been observed experimentally at 6173 cm −1 and 6289 cm −1 for the complexes 2b + and 2d + respectively by near-IR spectra which are close to the computed γ 2 values.

Ruthenium(II/III)–bipyridine complexes with four-membered sulphur donor co-ligands: synthesis, metal valence preference, spectroscopic and electron-transfer properties

Abstract A group of stable ruthenium(II) and (III) mixed-ligand tris-chelated complexes of the type [Run+(bpy)(L)2]z+ (1–8, n=2, Z=0; 9, n=3, Z=1) have been synthesized and characterized (bpy=2,2′-bipyridine; L=anionic form of the ligands, ROC(S)SK, (R=Me, Et, n Pr, i Pr, n Bu, i Bu, –CH2–Ph) or (EtO)2P(S)SNH4 or (Et)2NC(S)SNa). The complexes 1–8 are diamagnetic and electrically neutral and the complex 9 is one-electron paramagnetic and behaves as 1:1 electrolyte in acetonitrile solvent. The complexes 1–8 and 9 display two MLCT transitions near 530, 370 nm and 663, 438 nm respectively. Intra-ligand bipyridine based π–π∗ transition is observed near 300 nm. The complexes 1–8 exhibit room-temperature emission from the highest energy MLCT band (∼370 nm). At room temperature the lifetime of the excited states for the complexes 2 and 8 are found to be 90 and 95 ns respectively. In acetonitrile solution the complexes 1–9 show a reversible ruthenium(III)–ruthenium(II) couple in the range −0.08 → 0.40 V and irreversible ruthenium(III)–ruthenium(IV) oxidation in the range 1.19–1.45 V vs Ag/AgCl. One reversible bipyridine reduction is observed for each complex in the range −1.70 → −1.85 V vs Ag/AgCl. The presence of trivalent ruthenium in the oxidized solution for one complex 1 is evidenced by the axial EPR spectrum at 77 K. The isolated trivalent complex 9 also exhibits an axial EPR spectrum at 77 K. The EPR spectra of the trivalent ruthenium complexes (1+ and 9) have been analyzed to furnish values of distortion parameters (Δ(cm−1)→1+, 3689; 9, 3699) and energies of the two expected ligand field transitions (ν1(cm−1)→1+, 3489; 9, 3497 and ν2(cm−1)→1+, 4339; 9, 4348) within the t2 shell. One of the ligand field transitions has been experimentally observed at 4673 cm−1 for complex 9 and which close to the computed ν2 value (4348 cm−1).

Hydrogenation of α-acetamidocinnamic acid with polystyrene-supported rhodium catalysts

Abstract Divinylbenzene cross-linked chloromethylated polystyrene has been functionalised with cinchonine, ephedrine, 3 S ,4 S - N -benzylpyrrolidinediol and four achiral amines. The resins have been used as supports for anchoring [Rh(CO) 2 Cl 2 ] − . The polymer-supported complex has been tested as a catalyst precursor for the hydrogenation of α-acetamidocinnamic acid. Highest rate and modest enantioselectivity are obtained with cinchonine functionalized polymer-supported complex. This complex also undergoes reversible decarbonylation.

A facile and preferential synthesis of the complexes, cis-trans-cis-RuII [NC5H4NNC6H4(R)]2 (R = H, o-Me/Cl, m-Me/Cl, p-Me/Cl) Synthesis, spectroscopic characterization and electron-transfer properties

Abstract A group of seven complexes of the type ctc-RuIIL2Cl2 (2a−2g) (L = NC5H4-NN-C6H5(R), R = H, o-Me/Cl, m-Me/Cl, p-Me/Cl and ctc = cic-traps-cis with respect to chlorides, pyridine and azo nitrogens, respectively) have been synthesized and characterized. The complexes are diamagnetic (RuII, t2g6, S = 0) and electrically neutral. The molecular geometry of the complexes (2) in solution has been established by 1H NMR spectroscopy. They exhibit a strong metal to ligand charge-transfer band in the range 582–603 nm and intra ligand π-π∗ transition near 320 nm. In acetonitrile solution the complexes display reversible ruthenium (II) ruthenium(III) oxidation couples in the range 1.02 → 1.36 V vs SCE. Two successive quasi-reversible ligand reductions are observed for each complex in the ranges −0.42 → −0.60 V and −0.68 → −0.85 V vs SCE, respectively. The complexes 2a and 2e have been oxidized to the corresponding trivalent species (3a and 3e) by using HNO3 as oxidizing agent and isolated in solid state as perchlorate salts. Complexes are one-electron paramagnets (RuIII, t2g5, S = 1 2 ) and show 1 : 1 conductivity in acetonitrile solution. The presence of perchlorate counter ion in the complexes has been evidenced by the strong infrared bands near 1100 and 600 cm−1. Complexes exhibit a strong ligand to metal charge-transfer band near 530 nm and intra ligand π-π∗ transition near 370 nm. In glassy condition (77 K) complexes display rhombic EPR spectra corresponding to the distorted octahedral geometry.

Tuning catalyst solubility in CO2 by changing molar volume

Abstract Poor-solvating property of supercritical carbon dioxide (scCO2) has been a great challenge, which limits the use of CO2 as a common “green” solvent. The present report describes that by increasing molar volume (v) and lowering the melting temperature, which lowers cohesive energy density or solubility parameter (δ), it is possible to increase the solubility of metal-based catalysts in scCO2 without using costly fluorinated or tailor-made CO2-philic modifications. We have studied various chlorodistannoxanes (1) and Cu–β-diketonates (2) to support our views. The study of bio-diesel production and transesterification of hindered esters using 1 in scCO2 shows a 2–8-folds rate enhancement coupled with an easier catalyst and product separation than that in organic solvents. The methodology, which works at least within the range of Van der Waals sphere of interactions, can be useful to solubilizing the molecules in scCO2 and carries great opportunity in catalysis as well as in separation science.