Sachinath Bera

Radical and Non-Radical States of the [Os(PIQ)] Core (PIQ = 9,10-Phenanthreneiminoquinone): Iminosemiquinone to Iminoquinone Conversion Promoted o-Metalation Reaction

The coordination and redox chemistry of 9,10-phenanthreneiminoquinone (PIQ) with osmium ion authenticating the [Os(II)(PIQ(•-))], [Os(III)(PIQ(•-))], [Os(III)(C,N-PIQ)], [Os(III)(PIQ)], and [Os(III)(PIQ(2-)) ] states of the [Os(PIQ)] core in the complexes of types trans-[Os(II)(PIQ(•-))(PPh3)2(CO)Br] (1), trans-[Os(III)(PIQ(•-))(PPh3)2Br2] (2), trans-[Os(III)(C,N-PIQ)(PPh3)2Br2]·2CH2Cl2 (3·2CH2Cl2), trans-[Os(III)(C,N-PIQ(Br))(PPh3)2Br2]·2CH2Cl2 (4·2CH2Cl2), trans-[Os(III)(C,N-PIQ(Cl2))(PPh3)2Br2] (6), trans-[Os(III)(PIQ(•-))(PPh3)2Br2](+)1/2I3(-)1/2Br(-) (1(+)1/2I3(-)1/2Br(-)), [Os(III)(PIQ)(PPh3)2Br2](+) (2(+)), and [Os(III)(PIQ(2-))(PPh3)2Br2](-) (2(-)) are reported (PIQ(•-) = 9,10-phenanthreneiminosemiquinonate anion radical; C,N-PIQ = ortho-metalated PIQ, C,N-PIQ(Br) = ortho-metalated 4-bromo PIQ, and C,N-PIQ(Cl2) = ortho-metalated 3,4-dichloro PIQ). Reduction of PIQ by [Os(II)(PPh3)3(H)(CO)Br] affords 1, while the reaction of PIQ with [Os(II)(PPh3)3Br2] furnishes 2. Oxidation of 1 with I2 affords 1(+)1/2I3(-)1/2Br(-), while the similar reactions of 2 with X2 (X = I, Br, Cl) produce the ortho-metalated derivatives 3·2CH2Cl2, 4·2CH2Cl2, and 6. PIQ and PIQ(2-) complexes of osmium(III), 2(+) and 2(-), are generated by constant-potential electrolysis. However, 2(+) ion is unstable in solution and slowly converts to 3 and partially hydrolyzes to trans-[Os(III)(PQ(•-))(PPh3)2Br2] (2PQ), a PQ(•-) analogue of 2. Conversion of 2(+) → 3 in solution excludes the formation of aryl halide as an intermediate for this unique ortho-metalation reaction at 295 K, where PIQ acts as a redox-noninnocent ambidentate ligand. In the complexes, the PIQ(•-) state where the atomic spin is more localized on the nitrogen atom is stable and is more abundant. The reaction of 2PQ, with I2 does not promote any ortho-metalation reaction and yields a PQ complex of type trans-[Os(III)(PQ)(PPh3)2Br2](+)I5(-)·2CH2Cl2 (5(+)I5(-)·2CH2Cl2). The molecular and electronic structures of 1-4, 6, 1(+), and 5(+) were established by different spectra, single-crystal X-ray bond parameters, cyclic voltammetry, and DFT calculations.

Molecular and Electronic Structures of Ruthenium Complexes Containing an ONS-Coordinated Open-Shell π Radical and an Oxidative Aromatic Ring Cleavage Reaction

The coordination chemistry of 2,4-di-tert-butyl-6-[(2-mercaptophenyl)amino]phenol (LONSH3), which was isolated as a diaryl disulfide form, (LONSH2)2, with a Ru ion is disclosed. It was established that the trianionic LONS3- is redox-noninnocent and undergoes oxidation to either a closed-shell singlet (CSS), LONS-, or an open-shell π-radical state, LONS•2-, and the reactivities of the [RuII(LONS•2-)] and [RuII(LONS-)] states are different. The reaction of (LONSH2)2 with [Ru(PPh3)3Cl2] in toluene in the presence of PPh3 affords a ruthenium complex of the type trans-[Ru(LONS)(PPh3)2Cl] (1), while the similar reaction with [Ru(PPh3)3(H)(CO)Cl] yields a LONS•2- complex of ruthenium(II) of the type trans-[RuII(LONS•2-)(PPh3)2(CO)] (2). 1 is a resonance hybrid of the [RuII(LONS-)Cl] and [RuIII(LONS•2-)Cl] states. It is established that 2 incorporating an open-shell π-radical state, [RuII(LONS•2-)(CO)], reacts with an in situ generated superoxide ion and promotes an oxidative aromatic ring cleavage reaction, yielding a α-N-arylimino-ω-ketocarboxylate (LNS2-) complex of the type [RuII(LNS2-)(PPh3)(CO)]2 (4), while 1 having a CSS state, [RuII(LONS-)Cl], is inert in similar conditions. Notably, 2 does not react with O2 molecule but reacts with KO2 in the presence of excess PPh3, affording 4. The redox reaction of (LONSH2)2 with [Ru(PPh3)3Cl2] in ethanol in air is different, leading to the oxidation of LONS to a quinone sulfoxide derivative (LONSO0) as in cis-[RuII(LONSO0)(PPh3)Cl2] (3), via 1 as an intermediate. The molecular and electronic structures of 1-4 were established by single-crystal X-ray crystallography, electron paramagnetic resonance spectroscopy, electrochemical measurements, and density functional theory calculations. 1+ is a resonance hybrid of [RuIII(LONS-)(PPh3)2Cl ↔ RuIV(LONS•2-)(PPh3)2Cl]+ states, 2- is a LONS3- complex of ruthenium(II), [RuII(LONS3-)(PPh3)2(CO)]-, and 2+ is a ruthenium(II) complex of LONS- of the type [RuII(LONS-)(PPh3)2(CO)]+, where 35% diradical character of the LONS- ligand was predicted.

Coordination of o-benzosemiquinonate, o-iminobenzosemiquinonate, 4,4′-di-tert-butyl-2,2′-bipyridine and 1,10-phenanthroline anion radicals to oxidovanadium(IV)

This article reports on the stabilization of organic radical anions promoted by the oxidovanadium(IV) ion. A 3,5-di-tert-butylcatecholate (tBucatH−) complex of oxidovanadium(V) of the type [(LaONO2−)(VO3+)(tBucatH−)] (1) was isolated using tridentate (E)-N′-((3-hydroxynaphthalen-2-yl)methylene)benzohydrazide (LaONOH2) as a coligand, whereas o-benzosemiquinonate (sq˙−) and p-nitro-o-iminobenzosemiquinonate (NO2isq˙−) radical anion complexes of oxidovanadium(IV) of the types [(LNNO−)(VO2+)(sq˙−)] (2) and [(LNNO−)(VO2+)(NO2isq˙−)] (3) were successfully isolated using (1Z,N′E)-N′-(phenyl(pyridin-2-yl)methylene)benzohydrazonic acid (LNNOH) as a coligand. Oxidovanadium(IV) complexes of the types [(LbONO2−)(VO2+)(phen)] (4) and [(LbONO2−)(VO2+)(tBubpy)] (5), which undergo reversible reduction to the 4,4′-di-tert-butyl-2,2′-bipyridine radical anion (tBubpy˙−) and the 1,10-phenanthroline radical anion (phen˙−) to afford the coupled states [(LbONO2−)(VO2+)(phen˙−)]− (4−) and [(LbONO2−)(VO2+)(tBubpy˙−)]− (5−), respectively, were isolated (LbONOH2 = (E)-N′-(2-hydroxybenzylidene)benzohydrazide). The molecular geometries of the complexes were confirmed by the single-crystal X-ray structure determinations of 1, 3 and 4. In 1, the V–Ophenolato length cis to VO is 1.879(2) A and the dissimilar V–O and V–OH lengths corresponding to the tBucatH− ligand are 1.832(2) and 2.312(2) A, respectively. In 1, the average C–O lengths in tBucatH− are 1.351(3) A, whereas in 3 the average C–O and C–N lengths in NO2isq˙− are 1.293(4) and 1.355(5) A, respectively. In 4, the V–Ophenolato length cis to VO (1.937(3) A) is relatively longer. The 51V NMR spectrum of 1 displays a signal at −337.2 ppm, whereas the signals for 2 and 3 are deshielded to +382.4 and +71.8 ppm, respectively. The closed-shell singlet (CSS) solutions of 3 and 5− at the B3LYP/DFT level are unstable and the open-shell singlet (OSS) solutions are 0.5 and 7.3 kcal mol−1 lower in energy, respectively, than the CSS solutions. In 3 and 5− the alpha spin (100%) is localized on the vanadium ion, whereas the beta spin is delocalized across the aminophenol and bipyridine fragments. 2 and 3 exhibit lower-energy absorption bands at 785 and 585 nm, which are defined as CSS → OSS perturbation transitions.

Zinc(II), iron(II/III) and ruthenium(II) complexes of o-phenylenediamine derivatives: oxidative dehydrogenation and photoluminescence

Reactions of benzoyl pyridine, o-phenylenediamine and anhydrous ZnX2 in methanol afford imine complexes [Zn(L1)X2] (X = Cl, 1; X = Br, 2) in good yields (L1 = (E)-N(1)-(phenyl(pyridin-2-yl)methylene)benzene-1,2-diamine). The reduction of 1 with NaBH4 affords (E)-N(1)-(phenyl(pyridine-2-yl)methylene)benzene-1,2-diamine (L2H). The reaction of L2H with [Ru(II)(PPh3)3Cl2] results in the oxidative dehydrogenation to L1 generating cis-[Ru(II)(L1)(PPh3)Cl2] (3). The reaction of L2H with salicylaldehyde affords (E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol (L3H2). The reaction of L3H2 with anhydrous FeCl3 in CH3OH affords cis-[Fe(III)(L3H(-))Cl2] (4). Reaction of L3H2 with [Ru(II)(PPh3)3Cl2] results in the oxidative dehydrogenation to diimine, L4H, affording trans-[Ru(II)(L4(-))(PPh3)2](+), which is isolated as trans-[Ru(II)(L4(-))(PPh3)2]PF6 (5(+)PF6(-)) (L4H = 2-((E)-(2-((E)-phenyl(pyridin-2-yl)methyleneamino)phenylimino)methyl)phenol). The reduction of L3H2 with NaBH4 produces 2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)amino)methyl)phenol (L5H3). With iron(III) L5H3 undergoes oxidative dehydrogenation to L3H2 affording 4, while with [Ru(II)(PPh3)3Cl2], L5H3 undergoes 4e + 4H(+) transfer giving 5(+). A fluid solution of L3H2 at 298 K exhibits an emission band at 470 nm (λ(ex) = 330 nm, τ1 = 3.70 ns) and a weaker band at 525 nm (λ(ex) = 330, 390 nm, τ1 = 1.1 ns) at higher concentrations due to molecular aggregation, which are temperature dependent. 4 is brightly emissive (λ(ex) = 330 nm, λ(em) = 450 nm, Φ = 0.586, τ1 = 3.70 ns). Time resolved emission spectra (TRES) and lifetime measurements confirm that the lower energy absorption band of L3H2 at 390 nm, which is absent in complex 4, has a larger non-radiative rate constant (k(nr)). The redox innocent Al(III) adduct of L3H2 is fluorescent (λ(ex) = 330 nm, λ(em) = 450 nm, τ1 = 3.70 ns). On the contrary, the cis-[Fe(II)(L3H(-))Cl2](-) and cis-[Co(L3H(-))Cl2](-) analogues are non emissive. Density function theory (DFT) calculations, redox potentials and the near infra-red (NIR) absorption data prove that 4 is emissive due to the stable [Fe(III)(L3H(-)*)] state, while 3, 5(+), cis-[Fe(II)(L3H(-))Cl2](-) and cis-[Co(L3H(-))Cl2](-) are non-emissive due to transformations of the [M(II)(L*)] to [M(III)(L˙(-)*)] states.

Oxidovanadium(IV), oxidomolybdenum(VI) and cobalt(III) complexes of o-phenylenediamine derivatives: oxidative dehydrogenation and photoluminescence

Reactions of o-phenylenediamine derivatives (L3H2) incorporating a (Ph)(Py)(H)C–N(H)– function with the oxidovanadium(IV) and oxidomolybdenum(VI) ions afford amide complexes of types [VIVO(L32−)] (3), [VIVO(L3t-Bu 2−)] (4) and cis-[MoVIO2(L32−)] (5) (L3H2 = ((E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol); L3t-BuH = ((E)-2,4-di-tert-butyl-6-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol)), while the similar reaction of L3H2 with the anhydrous CoCl2 in air results in oxidative dehydrogenation (OD) of the (Ph)(Py)(H)C–N(H)– function, affording a cobalt(III) diimine complex, trans-[CoIII(L4−)Cl2] (6) (L4H = 2-((E)-(2-((E)-phenyl(pyridin-2-yl)methyleneamino)phenylimino) methyl)phenol), contradicting the participation of the higher oxidation states of the metal ions in OD reaction of amines. 3–6 are characterized by elemental analyses and mass, IR, 1H NMR and EPR spectra. The molecular geometries of 4·CH3OH, 5 and 6 were confirmed by single crystal X-ray structure determinations. The VIV–Ophenolatocis to the VO bond and the VIVO lengths in 4·CH3OH are 1.925(2) and 1.612(2) A. Two cis MoO lengths are 1.710(2) A and 1.720(2) A in 5. The aliphatic –C–N– lengths in 4·CH3OH and 5 are 1.448(3) and 1.479(2) A, while the same is 1.285(4) A in 6. DFT calculations on 3 and 6 inferred a significant mixing among dM and NN-ligand backbone favoring a t26 state of the metal ion for the OD of the amine fragment to have stronger dM → πketimine* back-bonding. The πNHPh → πaldimine* transition of L3H2 is red shifted in 3 and 4 quenching the emissive πPhenolato → πaldimine* transitions, elucidated by the TD DFT calculations on 3 (and 3+). The πNPh → πaldimine* transitions are blue shifted in the oxidovanadium(V) analogues, [VVO(L32−)]+ (3+) and [VVO(L3t-Bu 2−)]+ (4+), which are fluorescent (3+, λex = 331, λem = 444 nm; 4+, λex = 339, λem = 490 nm) recorded by the fluorescence-spectroelectrochemical measurements in CH2Cl2. 5 and 6 emit weakly at 466 and 473 nm (5, λex = 336 nm, ϕ = 0.003; 6, λex = 324 nm, ϕ = 0.027).

Orthometalated N-(Benzophenoxazine)-o-aminophenol: Phenolato versus Phenoxyl States

The molecular and electronic structures of the orthometalated ruthenium(III) and osmium(III) complexes of N-(benzophenoxazine)-o-aminophenol (OXLH2) that exhibits versatile redox activities are reported. The redox chemistry of OXLH2 is remarkably different from that of N-(aryl)-o-aminophenol (APLH2). The study established that OXLH2 is redox noninnocent and is a precursor of a phenoxyl radical. One of the C–H bonds of OXLH2 is activated by ions, and OXLH2 reveals three different redox states as dianionic phenolato (OXL2–), monoanionic phenoxyl radical (OXL•–), and zwitterionic phenoxium cation (OXL±) states. The reaction of OXLH2 with [RuII(PPh3)3Cl2] in boiling toluene in air affords an orthometalated OXL2– complex of ruthenium(III), trans-[(OXL2–)RuIII(PPh3)2(Cl)] (1), whereas the similar reaction with [OsII(PPh3)3Br2] yields an orthometalated OXL•– complex, cis-[(OXL•–)OsIII(PPh3)Br2] (2). 1 and 2 exhibit ligand-based reversible redox waves due to OXL•–/OXL2–, OXL±/OXL•–, and MIII/MII couples. The 1+ ion is a OXL•– complex of ruthenium(III). 2– exhibits temperature-dependent valence tautomerism due to [OsII(OXL•–) ↔ OsIII(OXL2–)] equilibrium. 22– is a OXL2– complex of osmium(II), while 12+ and 2+ are OXL± complexes of metal(III). 2 is an oxidant and effective catalyst for oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone, and the turnover number is 119.7 h–1. The UV–vis–NIR absorption spectrum of 1 displays an NIR band at 800 nm due to an intra-ligand-charge-transfer transition, which is absent in 2 incorporating a OXL•– radical. The molecular and electronic structures of 1 and 2 and their oxidized and reduced analogues were confirmed by single-crystal X-ray crystallography, variable-temperature electron paramagnetic resonance spectroscopy, spectroelectrochemical measurements, and density functional theory calculations.

Coordination of o-benzosemiquinonate, o-iminobenzosemiquinonate and aldimine anion radicals to oxidovanadium(iv)

The study focuses on the stabilization of organic radicals by the oxidovanadium(IV) ion and it proves to be significant in exploring the bioactivity of vanadium. In addition to the o-benzosemiquinonate and o-iminobenzosemiquinonate anion radicals, the existence of reactive aldimine anion radicals coordinated to oxidovanadium(IV) ion was detected. Radical and non-radical oxidovanadium(IV) complexes of the types [(L1−)(VIVO)(acac)] (1), [(L2−)(VIVO)(acac)] (2), [(L1−)(VIVO)(sq˙−)] (3), [(L1−)(VIVO)(t-Busq˙−)] (4), [(L1−)(VIVO)(NO2isq˙−)] (5) and [(L1−)2(VIVO)2(SO4)]·½CH2Cl2 (6·½CH2Cl2) containing redox non-innocent tridentate NNO-donor aldimines (L1H and L2H) as coligands are reported (acac = acetylacetonato, sq˙− = o-benzosemiquinonate, t-Busq˙− = 3,5-di-tert-butyl-o-benzosemiquinonate and NO2isq˙− = p-nitro-o-iminobenzosemiquinonate anion radicals). The sq˙−, t-Busq˙− and NO2isq˙− states in complexes were established by X-ray crystallography, EPR spectroscopy and solid state cross polarization magic angle spinning (CP/MAS) 51V NMR spectroscopy, where the 51V nuclei in 3–5 were deshielded in a range from −100.3 to +608.7 ppm. The cathodic waves of 1 and 2 due to L1−/L1˙2− and L2−/L2˙2− redox couples are reversible. Density functional theory (DFT) calculations authenticated that 1− and 2− are open shell pi radical (L1˙2− and L2˙2−) complexes of oxidovanadium(IV). The energies of the open shell singlet (OSS) and triplet solutions of 1− and 2− are lower than the corresponding closed shell singlet (CSS) solutions. In 1− and 2− ions, 35–39% beta spin is localized on the πaldimine* function. The UV-vis-NIR absorption spectra of the complexes were analyzed by spectroelectrochemical measurements and time dependent (TD) DFT calculations.

Cobalt Ion Promoted Redox Cascade: A Route to Spiro Oxazine-Oxazepine Derivatives and a Dinuclear Cobalt(III) Complex of an N-(1,4-Naphthoquinone)-o-aminophenol Derivative

The study discloses that the redox activity of N-(1,4-naphthoquinone)-o-aminophenol derivatives (LRH2) containing a (phenol)-NH-(1,4-naphthoquinone) fragment is notably different from that of a (phenol)-NH-(phenol) precursor. The former is a platform for a redox cascade. LRH2 is redox noninnocent and exists in Cat-N-(1,4-naphthoquinone)(2-) (LR 2-) and SQ-N-(1,4-naphthoquinone) (LR •-) states in the complexes. Reactions of LRH2 with cobalt(II) salts in MeOH in air promote a cascade affording spiro oxazine-oxazepine derivatives (OXLR) in good yields, when R = H, Me, tBu. Spiro oxazine-oxazepine derivatives are bioactive, and such a molecule has so far not been isolated by a schematic route. In this context this cascade is significant. Dimerization of LRH2 → OXLR in MeOH is a (6H+ + 6e) oxidation reaction and is composed of formations of four covalent bonds and 6-exo-trig and 7-endo-trig cyclization based on C-O coupling reactions, where MeOH is the source of a proton and the ester function. It was established that the active cascade precursor is [(LMe •-)CoIIICl2] (A). Notably, formation of a spiro derivative was not detected in CH3CN and the reaction ends up furnishing A. The route of the reaction is tunable by R, when R = NO2, it is a (2e + 4H+) oxidation reaction affording a dinuclear LR 2- complex of cobalt(III) of the type [(LNO2 2-)2CoIII2(OMe)2(H2O)2] (1) in good yields. No cascade occurs with zinc(II) ion even in MeOH and produces a LMe •- complex of type [(LMe •-)ZnIICl2] (2). The intermediate A and 2 exhibit strong EPR signals at g = 2.008 and 1.999, confrming the existence of LMe •- coordinated to low-spin cobalt(III) and zinc(II) ions. The intermediates of LRH2 → OXLR conversion were analyzed by ESI mass spectrometry. The molecular geometries of OXLR and 1 were confirmed by X-ray crystallography, and the spectral features were elucidated by TD DFT calculations.

Arylamino radical complexes of ruthenium and osmium: dual radical counter in a molecule

Radical and non-radical ruthenium and osmium complexes of 1-amino-9,10-anthraquinone (AqNH2), which is defined as a molecule of dual radical counter, are disclosed. 1-Amido-9,10-anthraquinone (AqNH-) complexes of the types trans-[RuII(AqNH-)(PPh3)2(CO)Cl] (1), trans-[OsII(AqNH-)(PPh3)2(CO)Br] (2) and trans-[RuIII(AqNH-)(PPh3)2Cl2] (3) were isolated. AqNH- of 1-3 is redox active and undergoes oxidation reversibly at +(0.05-0.35) V to the 1-amino-9,10-anthraquinone radical (AqNH˙) and reduction at -(0.86-1.60) V to the 1-amido-9,10-anthrasemiquinonate anion radical (AqNHSQ˙2-). The reaction of 2 with I2 in CH2Cl2 afforded a crystalline AqNH˙ complex of the type trans-[OsII(AqNH˙)(PPh3)2(CO)Br]+I5-·½I2 (2+I5-·½I2). AqNH˙ and AqNHSQ˙2- complexes of the types trans-[RuII(AqNH˙)(PPh3)2(CO)Cl]+ (1+), trans-[RuIII(AqNH˙)(PPh3)2Cl2]+ (3+), trans-[RuII(AqNHSQ˙2-)(PPh3)2(CO)Cl]- (1-) and trans-[OsII(AqNHSQ˙2-)(PPh3)2(CO)Br]- (2-) were generated chemically/electrochemically in solution. The electronic states of the complexes were authenticated by single crystal X-ray structure determinations of 1, 2·5/4 toluene, 3 and 2+I5-·½I2, EPR spectroscopy and density functional theory (DFT) calculations. AqNH˙ instigates a 2c-3e pπ-dπ interaction and the length in 2+I5-·½I2, 1.978(5) Å, is relatively shorter than the OsII-NHAq- length, 2.037(2) Å, while the Aq-NH˙ bond, 1.365(8) Å, is longer than the Aq-NH- bond, 1.328(3) Å. DFT calculations predicted that the atomic spin is delocalized over the ligand backbone (1+, 56%) particularly in one of the p-orbitals of the nitrogen and the metal atoms of the 1+ and 2+ ions, while the spin is dominantly localized on the anthraquinone fragment of the 1- and 2- ions. TD DFT calculations were employed to elucidate the origins of the lower energy absorption bands of the neutral complexes. Hypsochromic shifts of the UV-vis-NIR absorption maximum during 1→1+, 2→2+ and 3→3+ conversions were recorded by spectroelectrochemical measurements.