Tunable Electrochemical and Catalytic Features of BIAN- and BIAO-Derived Ruthenium Complexes

Chemical and Electrochemical Reductions of Monoiminoacenaphthenes

Redox properties of monoiminoacenaphthenes (MIANs) were studied using various electrochemical techniques. The potential values obtained were used for calculating the electrochemical gap value and corresponding frontier orbital difference energy. The first-peak-potential reduction of the MIANs was performed. As a result of controlled potential electrolysis, two-electron one-proton addition products were obtained. Additionally, the MIANs were exposed to one-electron chemical reduction by sodium and NaBH₄. Structures of three new sodium complexes, three products of electrochemical reduction, and one product of the reduction by NaBH₄ were studied using single-crystal X-ray diffraction. The MIANs reduced electrochemically by NaBH₄ represent salts, in which the protonated MIAN skeleton acts as an anion and Bu₄N⁺ or Na⁺ as a cation. In the case of sodium complexes, the anion radicals of MIANs are coordinated with sodium cations into tetranuclear complexes. The photophysical and electrochemical properties of all reduced MIAN products, as well as neutral forms, were studied both experimentally and quantum-chemically.

Novel Oxidovanadium Complexes with Redox-Active R-Mian and R-Bian Ligands: Synthesis, Structure, Redox and Catalytic Properties

A new monoiminoacenaphthenone 3,5-(CF₃)₂C₆H₃-mian (complex 2) was synthesized and further exploited, along with the already known monoiminoacenaphthenone dpp-mian, to obtain oxidovanadium(IV) complexes [VOCl₂(dpp-mian)(CH₃CN)] (3) and [VOCl(3,5-(CF₃)₂C₆H₃-bian)(H₂O)][VOCl₃(3,5-(CF₃)₂C₆H₃-bian)]·2.85DME (4) from [VOCl₂(CH₃CN)₂(H₂O)] (1) or [VCl₃(THF)₃]. The structure of all compounds was determined using X-ray structural analysis. The vanadium atom in these structures has an octahedral coordination environment. Complex 4 has an unexpected structure. Firstly, it contains 3,5-(CF₃)₂C₆H₃-bian instead of 3,5-(CF₃)₂C₆H₃-mian. Secondly, it has a binuclear structure, in contrast to 3, in which two oxovanadium parts are linked to each other through V=O···V interaction. This interaction is non-covalent in origin, according to DFT calculations. In structures 2 and 3, non-covalent π-π staking interactions between acenaphthene moieties of the neighboring molecules (distances are 3.36–3.40 Å) with an estimated energy of 3 kcal/mol were also found. The redox properties of the obtained compounds were studied using cyclic voltammetry in solution. In all cases, the reduction processes initiated by the redox-active nature of the mian or bian ligand were identified. The paramagnetic nature of complexes 3 and 4 has been proven by EPR spectroscopy. Complexes 3 and 4 exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The yields of products of cyclohexane oxidation were 43% (complex 3) and 27% (complex 4). Based on the data regarding the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play the most crucial role in the reaction. The initial products in the reactions with alkanes are alkyl hydroperoxides, which are easily reduced to their corresponding alcohols by the action of triphenylphosphine (PPh₃). According to the DFT calculations, the difference in the catalytic activity of 3 and 4 is most likely associated with a different mechanism for the generation of ^●OH radicals. For complex 4 with electron-withdrawing CF₃ substituents at the diimine ligand, an alternative mechanism, different from Fenton’s and involving a redox-active ligand, is assumed.

Ru(ii) water oxidation catalysts with 2,3-bis(2-pyridyl)pyrazine and tris(pyrazolyl)methane ligands: assembly of photo-active and catalytically active subunits in a dinuclear structure

Two mononuclear Ru(ii) complexes, i.e. [RuCl(κ³N-terpy)(κ²N-dpp)]PF₆ ([1]PF₆; terpy = 2,2':6',2''-terpyridine; dpp = 2,3-bis(2'-pyridyl-pyrazine) and [RuCl(κ³N-tpm)(κ²N-dpp)]Cl ([2]Cl; tpm = tris(1-pyrazolyl)methane), and one dinuclear complex, i.e. [Ru₂Cl(κ³N-tpm)(μ-κ²N:κ²N-dpp)Ru(κ²N-bpy)₂][PF₆]₃ ([3][PF₆]₃; bpy = 2,2'-bipyridine), have been synthesized and their water oxidation catalytic properties have been investigated. A combined DFT and experimental (³⁵Cl NMR and conductivity measurements) study aimed to elucidate the nature of [1]⁺ and [2]⁺ in aqueous solution has also been performed, indicating that one water molecule is allowed to enter the first coordination sphere of [2]⁺ in the ground state, replacing one tpm nitrogen. Conversely, in the case of [1]⁺, water coordination, assumed to be needed for the water oxidation process, presumably occurs following the oxidation of the metal. For all complexes, a catalytic wave has been detected in acetonitrile/water 1 : 1 (v/v) solution in the range 1.4-1.7 V vs. SCE. In all cases, water oxidation (investigated at pH < 8) takes place initially via a proton-coupled two-electron, two-proton process with the formation of an Ru(iv)[double bond, length as m-dash]O moiety, followed by one electron oxidation and water nucleophilic attack. The TON and TOF values (within the range of 16-33 and 1.3-2.2 h^-1, respectively) of the complexes are higher than those of the benchmark [Ru(LLL)(LL)(OH₂)]²⁺-type species (LLL and LL are tridentate and bidentate polypyridine ligands, respectively), which is [Ru(terpy)(bpm)(OH₂)]²⁺.

One-Electron Reduction of 2-Mono(2,6-diisopropylphenylimino)acenaphthene-1-one (dpp-mian)

The electrochemical characteristics of 2-mono(2,6-diisopropylphenylimino)acenaphthene-1-one (dpp-mian) have been investigated. One-electron reduction of dpp-mian involves the iminoketone fragment, which is revealed by the EPR spectrum obtained after the electrolysis of the dpp-mian solution in tetrahydrofuran (THF). The reduction of dpp-mian with one equivalent of metallic potassium leads to a similar EPR spectrum. The sodium complex [(dpp-mian)Na(dme)]₂ (1) produces an EPR signal with hyperfine coupling on the nitrogen atom of the iminoketone fragment of the dpp-mian ligand. Dpp-mian can also be reduced in a one-electron process by SnCl₂ ×(dioxane). In this case, complex (dpp-mian)₂ SnCl₂ (2) is formed, with the tin atom displaying an oxidation state of +4. Tin(II) chloride dihydrate, SnCl₂ ×2(H₂ O), also reduces dpp-mian, but the two ligands bound to tin in the product form a new carbon-carbon bond between the ketone moieties of the dpp-mian monoanions to form complex (bis-dpp-mian)HSnCl₃ (3). Metallic tin reduces dpp-mian to form the (bis-dpp-mian)₂ Sn (4) species. Compounds 1-4 were characterized by X-ray diffraction.

Ligand controlled switchable selectivity in ruthenium catalyzed aerobic oxidation of primary amines

A ligand controlled catalytic system for the aerobic oxidation of 1° amines to nitriles and imines has been developed where the varying π-acidic feature of BIAN versus phen in the frameworks of ruthenium catalysts facilitates switchable selectivity.

Impact of {Os(pap)2} in fine-tuning the binding modes and non-innocent potential of deprotonated 2,2'-bipyridine-3,3'-diol

The reaction of ctc-Os(II)(pap)2Cl2 (pap = 2-phenylazopyridine, ctc = cis-trans-cis with respect to chlorides and pyridine/azo nitrogens of pap, respectively) and ambidentate 2,2'-bipyridine-3,3'-diol (H2L) leads to the simultaneous formation of isomeric [Os(II)(pap)2(HL(-))](+) ((2+)/(3+)), seven-membered chelate containing Os(II)(pap)2(L(2-)) (4) and diastereomeric [{Os(II)(pap)2}2(μ-L(2-))](2+) (5a(2+) (meso, ΔΛ)/5b(2+) (rac, ΔΔ/ΛΛ)). The reaction of 2,2'-biphenol (H2L') and ctc-Os(II)(pap)2Cl2 yields Os(II)(pap)2(L'(2-)) (6), an analogue of 4. The identities of the newly designed complexes have been established by different analytical, spectroscopic and X-ray diffraction techniques. (1)H-NMR spectra of the complexes and single crystal X-ray structures of selective derivatives [2]ClO4, [3]ClO4, [5a](ClO4)2, and 6 establish the retention of the tc-configuration of the precursor {Os(pap)2}. In isomeric 2(+) and 3(+), monodeprotonated HL(-) is linked to the {Os(II)(pap)2} fragment through N,N and N,O(-) donors, resulting in nearly planar five- and six-membered chelates with O-HO(-) and O-HN hydrogen bonds at its back face, respectively. The O(-),O(-) donating L'(2-) extends a severely twisted seven-membered chelate with the {Os(pap)2} unit in 6. The N,O(-)/O(-),N donors of deprotonated L(2-) bridge the two {Os(II)(pap)2} units in a symmetric fashion in 5a(2+), forming two moderately twisted six-membered chelates. Though the deprotonation of the O-HN hydrogen bond in (+) by another unit of {Os(II)(pap)2} generates a diastereomeric mixture of 5a(2+) and 5b(2+), attempts to deprotonate the relatively stronger O-H···O(-) hydrogen bond in 2(+) have failed. The isomeric 2(+)/3(+), seven-membered chelate containing 4/6 and diastereomeric 5a(2+)/5b(2+) exhibit distinctive (1)H-NMR and absorption spectra as well as electrochemical responses. The pap (N[double bond, length as m-dash]N) based two successive reductions and the participation of HL(-), L(2-), L'(2-) in the oxidation processes of the respective complexes have been revealed using EPR and DFT calculated MOs and Mulliken spin density plots at the intermediate paramagnetic states.