Benign synthesis of carboxamide ligands, H2Me2bqb and H2Me2bpb. Preparation, characterization and electrochemistry of Ni(II) complexes: The crystal structure of [NiII(Me2bqb)]

Ni(II) complexes of a new tetradentate NN'N''O picolinoyl-1,2-phenylenediamide-phenolate redox-active ligand at different redox levels

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H₃L² in its deprotonated form, at three redox levels, open-shell semiquinonate(1-) π radical, quinone(0) and closed-shell dianion of its 2-aminophenolate part, have been synthesized. The coordinated ligand provides N (pyridine) and N' and N'' (carboxamide and 1,2-phenylenediamide, respectively) and O (phenolate) donor sites. Cyclic voltammetry on the parent complex [Ni(L²)] 1 in CH₂Cl₂ established a three-membered electron-transfer series (oxidative response at E_1/2 = 0.57 V and reductive response at -0.32 V vs. SCE) consisting of neutral, monocationic and monoanionic [Ni(L²)]^z (z = 0, 1+ and 1-). Oxidation of 1 with AgSbF₆ affords [Ni(L²)](SbF₆) (2) and reduction of 1 with cobaltocene yields [Co(η⁵-C₅H₅)₂][Ni(L²)] (3). The molecular structures of 1·CH₃CN, 2·0.5CH₂Cl₂ and 3·C₆H₆ have been determined by X-ray crystallography at 100 K. Characterization by ¹H NMR, X-band EPR (g_av = 2.006 (solid); 2.008 (CH₂Cl₂-C₆H₅CH₃ glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [Ni^II{(L²)˙^2-}], [Ni^II{(L²)^-}]⁺/1⁺ and [Ni^II{(L²)^3-}]^-/1^- electronic states, respectively. Thus, the redox processes are ligand-centred. While 1 possesses paramagnetic S_t (total spin) = 1/2, 2 and 3 possess diamagnetic ground-state S_t = 0. Interestingly, the variable-temperature (2-300 K) magnetic measurement reveals that 1 with the S_t = 1/2 ground state attains the antiferromagnetic S_t = 0 state at a very low temperature, due to weak noncovalent interactions via π-π stacking. Density functional theory (DFT) electronic structural calculations at the B3LYP level of theory rationalized the experimental results. In the UV-VIS-NIR spectra, broad absorptions are recorded for 1 and 2 in the range of 800-1600 nm; however, such an absorption is absent for 3. Time-dependent (TD)-DFT calculations provide a very good fit with the experimental spectra and allow us to identify the observed electronic transitions.

Linked Picolinamide Nickel Complexes as Redox Carriers for Nonaqueous Flow Batteries

The use of nickel complexes utilizing non-innocent ligands based on picolinamide to function as redox carriers in flow batteries was explored. The picolinamide moiety was linked together with -CH₂ CH₂ - (bpen), -CH₂ CH₂ CH₂ - (bppn), and -C₆ H₄ - (bpb) moieties, resulting in two, three, and four quasi-reversible waves, respectively, for the nickel complexes and >3 V difference between the outermost positive and negative waves. The redox events were theoretically modelled for each complex, showing excellent agreement (<0.3 V difference) between the experimental and modelled potentials. Bulk cycling of the most soluble complex, Ni(bppn), indicated only one of the three waves was reversible. Therefore, Ni(bppn) has the ability to act as a negative charge redox carrier in flow cells.

Cyclometalated Ir(III) Complex as a Metalloligand and a Selective Cu(II) Sensor: Synthesis and Structural Characterization of a Heterometallic Tetranuclear Ir(III)/Cu(II) Complex

A cyclometalated Ir(III) complex, [Ir(ppy)₂(LH)] (2) [LH₂ = 1,2-bis(pyridine-2-carboxamido)benzene; ppyH = 2-phenylpyridine] was synthesized and structurally characterized. [Ir(ppy)₂(LH)] contains free donor sites and functions as a metalloligand. Accordingly, it interacts with a Cu(II) salt to afford the heterometallic tetranuclear complex, [{Ir(ppy)₂(L)}₂{Cu₂Cl₂}] (3). In the latter, a Cu₂Cl₂ dimer bridges the two cyclometalated Ir(III) units, resulting in a Ir^III-Cu^II-Cu^II-Ir^III motif. 2 also functions as a selective and reversible sensor for Cu²⁺, as revealed by quenching of its emission and subsequent revival of the luminescence signal upon addition of EDTA.

Low-spin [MII(L)2] and [MIII(L)2]+ (M = Fe and Co) complexes of tridentate azo-containing pyridine/pyrazine amide ligands: structures, properties and redox potential correlations

Using deprotonated forms of tridentate azo-containing pyridine-2-/pyrazine-2-carboxamide 2-[N-(2-phenylazo)carbamoyl]-pyridine/pyrazine, seven bis-ligand complexes of Fe^II/Co^II and Fe^III/Co^III have been synthesized. Molecular structures of six of them reveal that these six-coordinate complexes utilize all available donor sites of the ligands and assume M^II/IIIN₂(pyridine/pyrazine)N'₂(amide)N''₂(azo) coordination. Complexes of Fe^II and Co^III are diamagnetic and those of Fe^III and Co^II are paramagnetic (S = 1/2; room-temperature magnetic data and EPR spectra). Cyclic voltammetry experiments in CH₂Cl₂ reveal facile metal-centred Fe^III/Fe^II and Co^III/Co^II redox responses, and all complexes display quasireversible-to-irreversible ligand(azo)-centred redox processes. The E_1/2 values of M^III/M^II redox processes for Fe, Co and Ni (reported earlier) complexes of the pyridine amide ligand linearly correlate with those for six-coordinate [M^III(bpy)₃]³⁺/[M^II(bpy)₃]²⁺, [M^III(terpy)₂]³⁺/[M^II(terpy)₂]²⁺, [M^III(L)]⁺/[M^II(L)]⁰ or [M^III(L')₂]⁺/[M^II(L')₂]⁰ (bpy = 2,2'-bipyridine, terpy = 2,2':6',2''-terpyridine, hexadentate L(2-) = 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane and tridentate L'(-) = {2-[2-(arylimino)phenylazo]-pyridine}) couples. Density functional theory (DFT) at the B3LYP level and time-dependent (TD)-DFT calculations rationalize the electronic structure of the present complexes and throw light on the origin of observed electronic transitions.