Bidyut Kumar Santra

Paramagnetic ruthenium(III) cyclometallated complex. Synthesis, spectroscopic studies and electron-transfer properties

Abstract The reaction of Ru II (PPh 3 ) 3 X 2 (X = Cl, Br) with o -(OH)C 6 H 4 C(H)=N-CH 2 C 6 H 5 (HL) under aerobic conditions affords Ru II (L) 2 (PPh 3 ) 2 , 1 , in which both the ligands (L) are bound to the metal center at the phenolic oxygen (deprotonated) and azomethine nitrogen and Ru III (L 1 )(L 2 )(PPh 3 ), 2 , in which one L is in bidentate N,O form like in complex 1 and the other ligand is in tridentate C,N,O mode where cyclometallation takes place from the ortho carbon atom (deprotonated) of the benzyl amine fragment. The complex 1 is unstable in solution, and undergoes spontaneous oxidative internal transformation to complex 2 . In solid state upon heating, 1 initially converts to 2 quantitatively and further heating causes the rearrangement of complex 2 to the stable RuL 3 complex. The presence of symmetry in the diamagnetic, electrically neutral complex 1 is confirmed by 1 H and 31 P NMR spectroscopy. It exhibits an Ru II → L, MLCT transition at 460 nm and a ligand based transition at 340 nm. The complex 1 undergoes quasi-reversible ruthenium(II)—ruthenium(III) oxidation at 1.27V vs. SCE. The one-electron paramagnetic cyclometallated ruthenium(III) complex 2 displays an L → Ru III , LMCT transition at 658 nm. The ligand based transition is observed to take place at 343 nm. The complex 2 shows reversible ruthenium(III)—ruthenium(IV) oxidation at 0.875V and irreversible ruthenium(III)—ruthenium(II) reduction at −0.68V vs. SCE. It exhibits a rhombic EPR spectrum, that has been analysed to furnish values of axial (6560 cm −1 ) and rhombic (5630 cm −1 ) distortion parameters as well as the energies of the two expected ligand field transitions (3877 cm −1 and 9540 cm −1 ) within the t 2 shell. One of the transitions has been experimentally observed in the predicted region (9090 cm −1 ). The first order rate constants at different temperatures and the activation parameter Δ H # /Δ S # values of the conversion process of 1 → 2 have been determined spectrophotometrically in chloroform solution.

Ruthenium(II) bipyridine complexes with modified phenolic Schiff base ligands. Synthesis, spectroscopic characterization and Redox properties

Abstract A group of stable new ruthenium(II) mixed-ligand tris-chelated complexes of the type [Ru(bpy) 2 L]ClO 4 ( 1–6 ), (bpy = 2,2′-bipyridine; L = deprotonated form of the HL ligands, o -(HO)C 6 H 3 (R)C(R′)NCH 2 C 6 H 5 or o -(HO)C 6 H 3 (R)C(R′)NNHC 6 H 5 ; where R = H, p -NO 2 and R′ = H, CH 3 ) have been synthesized and characterized. The complexes are essentially diamagnetic and behave as 1 : 1 electrolytes in acetonitrile solution. They display two metal-to-ligand-charge-transfer (MLCT) transitions near 500 and 400 nm respectively and intra ligand π-π ∗ transitions in the UV-region. In acetonitrile solution the complexes exhibit weak emission from the lowest energy MLCT band at room-temperature. The quantum yields of the complexes are found to be in the range 0.0004–0.01. In acetonitrile solution the complexes show quasi-reversible ruthenium(II)-ruthenium(III) oxidation couples in the range 0.33 a 0.70 V and irreversible ruthenium(III)-ruthenium(IV) oxidations in the range 1.53 a 1.95 V vs SCE . Two successive reversible bipyridine reductions are observed for each complex in the ranges −1.4 a −1.62 V and −1.59 a −1.85 V vs SCE respectively. The presence of trivalent ruthenium in the oxidized solution for one complex 5 is evidenced by the rhombic EPR spectrum with g values, g 1 = 2.389, g 2 = 2.081 and g 3 = 1.810. The EPR spectrum of the coulometrically oxidized species, 5 + has been analyzed to furnish values of axial ( Δ = 4745 cm −1 ) and rhombic (V = 3692 cm −1 ) distortion parameters as well as the energies of the two expected ligand field transitions ( ν 1 = 3071 cm −1 and ν 2 = 6819 cm −1 ) within the t 2 shell. One of the ligand field transitions has been observed experimentally at 6578 cm −1 by near-IR spectrum which is close to the computed ν 2 value.

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).

Ruthenium-mediated selective activation of a C–H bond. Directaromatic thiolation in the complexes[RuII{o-SC6H3(R)NNC5H4N}2] (R = H, Me orCl)

The reaction of the complexes ctc-[Ru II L 2 Cl 2 ] (L = arylazopyridine, RC 6 H 4 NNC 5 H 4 N, where R = H, m-Me, p-Me or p-Cl; ctc = cis-trans-cis with respect to chlorides, pyridine and azo nitrogens respectively) with KS 2 COR′ (R′ = Me, Et, Pr n , Bu n or CH 2 Ph) in boiling dimethylformamide afforded [Ru II {o-SC 6 H 3 (R)NNC 5 H 4 N} 2 ] where the o-carbon atom of the pendant phenyl ring of both ligands L has been selectively and directly thiolated. The newly formed tridentate thiolated ligands are bound in a meridional fashion. When one methyl group is present at the meta position of the pendant phenyl ring of L the reaction resulted in two isomeric complexes due to free rotation of the singly bonded meta-substituted phenyl ring with respect to the azo group. The molecular geometry of the complexes in solution has been determined by 1 H NMR spectroscopy. This revealed the presence of an intimate mixture of the two isomers in solution in a 2∶1 ratio. In the visible region the complexes exhibit two metal-to-ligand charge-transfer transitions at ≈700 and ≈560 nm respectively and in the UV region intraligand (π–π*, n–π*) transitions. In acetonitrile solution the complexes exhibit one reversible ruthenium(II) ruthenium(III) oxidation couple near 0.4 V and an irreversible oxidative response near 1 V due to oxidation of the co-ordinated thiol group. Reduction of the co-ordinated azo groups occurs at ca. -0.8 and -1.4 V respectively. Coulometric oxidation of the complexes [Ru II {o-SC 6 H 3 (R)NNC 5 H 4 N} 2 ] at 0.6 V versus the saturated calomel electrode in dichloromethane produced unstable ruthenium(III) congeners. When R = p-Me, the presence of trivalent ruthenium in the oxidised solution was evidenced by a rhombic EPR spectrum having g 1 = 2.359, g 2 = 2.300 and g 3 = 1.952.

Synthesis, spectroscopic characterisation, electron-transfer propertiesand crystal structure of[RuII(bipy)2(2-SC5H4N)]ClO4 (bipy = 2,2′-bipyridine)

Two new ruthenium(II) mixed-ligand tris-chelated complexes of the type [Ru(bipy) 2 L]ClO 4 , (bipy = 2,2′-bipyridine; L = pyridine-2-thiolate 1 or pyridin-2-olate 2) have been synthesized. The complexes are essentially diamagnetic and behave as 1∶1 electrolytes in acetonitrile solution. They display two metal-to-ligand charge-transfer (m.l.c.t.) transitions near 500 and 340 nm respectively along with intraligand transitions in the UV region. Both exhibit room-temperature emission from the highest-energy (m.l.c.t.) band. At room temperature the lifetime of the excited states for the thiolato (1) and phenolato (2) complexes are 100 and 90 ns respectively. The geometry of the complexes in solution has been assessed by high-resolution 1 H NMR spectroscopy. The molecular structure of complex 1 in the solid state has been determined by single-crystal X-ray diffraction. It shows the expected pseudo-octahedral geometry with considerable strain due to the presence of the sterically hindered ligand L 1 . In acetonitrile solution the complexes show quasi-reversible ruthenium(II)–ruthenium(III) oxidation couples at 0.54 and 0.64 V versus saturated calomel electrode and quasi-reversible ruthenium(III)–ruthenium(IV) oxidations at 1.41 and 1.03 V respectively. Two reversible reductions are observed near -1.6 and -1.9 V for each complex due to electron transfer to the co-ordinated bipy units. The trivalent analogues of 1 and 2 are unstable at room temperature but can be generated in solution by coulometric oxidation at 263 K as evidenced by EPR spectroscopy.

Cobalt-mediated direct and selective aromatic thiolation in the complex[CoIII(o-SC6H4NNC5H4N)2]ClO4. Synthesis,spectroscopic characterisation and electron-transfer properties

The reaction of K[SC(S)OR] (R = Me, Et, Pr n , Pr i , Bu n , Bu i or CH 2 Ph) with the complex [Co II L 3 ][ClO 4 ] 2 ·H 2 O 1 [L = phenyl(2-pyridyl)diazene, C 6 H 5 NNC 5 H 4 N] in boiling dimethylformamide resulted in [Co III L′ 2 ]ClO 4 2 (L′ = o-SC 6 H 4 N NC 5 H 4 N). In complex 2 the o-carbon–hydrogen bond of the pendant phenyl ring of both the parent ligands L has been selectively and directly thiolated via carbon–sulfur bond cleavage of the dithiocarbonate. During the thiolation the metal ion is oxidised from the starting Co II in 1 to Co III in the final product 2. The reaction is highly sensitive to the nature of the solvent used, taking place only in those having high boiling points and relative permittivities. Its rate is dependent on the nature of the R group present in the thiolating agent, following the order Me ≈ Et > Pr n > Bu n > Pr i > Bu i benzyl. A meridional configuration (cis-trans-cis with respect to the sulfur, azo and pyridine nitrogens respectively) has been established by 1 H and 13 C NMR spectroscopy. The complex exhibits reversible Co III ⇌ Co II reduction at -0.135 V and four ligand-based azo (NN) reductions at -0.51 (one electron) and at -1.175 V (simultaneous three-electron reduction) respectively versus saturated calomel electrode. The oxidation of the co-ordinated thiol group occurs at 0.90 V.

Metal ion-mediated selective activations of C-H and C-Cl bonds. Direct aromatic thiolation reactions via C-S bond cleavage of dithioacids

The reactions of potassium salt of dithiocarbonate, R′OCS2K,4 (R′=Me, Et,nPr,nBu,iPr,iBu, -CH2Ph) with the low-spinctc-RuII(L)2Cl21,ctc-OsII(L)2Br22 andmer-[CoII(L)3](ClO4)2·H2O3 [L=2-(arylazo)pyridine, NC5H4-N=N-C6H4(R), R=H,o-Me/Cl,m-Me/Cl,p-Me/Cl;ctc: cis-trans-cis with respect to halides, pyridine and azo nitrogens respectively) in boiling dimethylformamide solvent resulted in low-spin diamagnetic RuII(L′)2,5, OsII(L′)26 and [CoIII(L′)2]ClO47 respectively (L′=o-S-C6H3(R)N=NC5H4N). In the complexes5, 6 and7 ortho carbon-hydrogen bond of the pendant phenyl ring of the ligands (L′) has been selectively and directly thiolated via the carbon—sulphur bond cleavage of4. The newly formed tridenate thiolated ligands (L′) are bound to the metal ion in a meridional fashion. In the case of cobalt complex (7), during the activation process the bivalent cobalt ion in the starting complex3 has been oxidised to the trivalent CoIII state. The reactions are highly sensitive to the nature and the location of the substituents present in the active phenyl ring. The presence of electron donating Me group at the ortho and para positions of the pendant phenyl ring with respect to the activation points can only facilitate the thiolation process. The complexes1c, 2c and3c) having chloride group at the ortho position of the active phenyl ring underwent the thiolation reaction selectively via the carbon—chloride bond activation process. The rate of carbon—chloride activation process has been found to be much faster compared to the C-H bond activation. The reactions are sensitive to the nature of the solvent used, taking place only in those having high boiling and polar solvents. The rate of the reactions is also dependent on the nature of the R′ group present in4, following the order: Me∼ Et>nPr>nBu>iPr>iBu≫-CH2Ph. The molecular geometry of the complexes in solution has been established by1H and13C NMR spectroscopy. The thiolated complexes (5, 6, 7) exhibit metal to ligand charge-transfer transitions in the visible region and intraligandπ-π* andn-π* transitions in the UV region. In acetonitrile solution the complexes display reversible MIII⇄MII reductions at 0.43 V for Ru (5a), 0.36 V for Os (6a) and −0.13 V for Co (7a) vs saturated calomel electrode (SCE).