Avinash Shankar Kumbhar

Serendipitous self-assembly is a potential approach for achieving structurally versatile and unexpected solids with chosen metal–ligand combinations. Cyclen (1,4,7,10-tetraazacyclododecane) is an interesting flexible molecule, well explored for biological and material applications of its metal complexes. The dihedral angle (θ) between four nitrogen atoms of free cyclen is close to 0°. Still, it may vary upon complexation, allowing flexibility to adopt any two conformers, viz., (i) planar and (ii) twisted. The present work establishes the crucial role of metal ions in controlling the conformation adopted by cyclen through metal complexation. SCXRD studies of the complexes of cyclen with Cu^II and Ni^II with halides and pseudohalides as counterions reveal the existence of a planar conformation (θ = ∼0–3°) for cyclen in Cu^II systems and a highly twisted conformation (hitherto unreported) in Ni^II systems (θ = ∼52–55°). Further, SQUID magnetic studies for the dimeric [Ni1] complex reveal antiferromagnetic interactions between the two Ni centres.

A self-assembled M₆L₈ type cage-connected 1D-coordination network of formula {[Ni₆(MeSi(³py)₃)₈Cl₉(H₂O)₂]Cl₃·16H₂O}_∞ (1) was obtained from a 3-pyridyl substituted silane ligand MeSi(³py)₃. This complex shows significantly high performance for the electrocatalytic and photocatalytic hydrogen evolution reaction (HER) in water. A maximum turnover number (TON) of 2824 has been observed for photocatalytic HER after 69 h.

Two new one-dimensional (1D) coordination polymers (CPs), namely catena-poly[[[aquacadmium(II)]-bis(μ-4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]phenanthroline-6,13-dione)] bis(perchlorate) dihydrate], {[Cd(C₁₄H₁₀N₆O₂)₂(H₂O)](ClO₄)₂·2H₂O} _n or {[Cd(BPG)₂(H₂O)](ClO₄)₂·2H₂O} _n , 1, and catena-poly[[lead(II)-bis(μ-4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]phenanthroline-6,13-dione)] bis(perchlorate) dihydrate], {[Pb(C₁₄H₁₀N₆O₂)₂](ClO₄)₂·2H₂O} _n or {[Pb(BPG)₂](ClO₄)₂·2H₂O} _n , 2, have been synthesized using bipyridine–glycoluril (BPG; systematic name: 4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]phenanthroline-6,13-dione), a urea-fused tecton, in a mixed-solvent system. The Cd^II ion in 1 is heptacoordinated and the Pb^II ion in 2 is hexacoordinated, with the Cd^II ion adopting a pentagonal bipyramidal geometry and the Pb^II ion adopting a distorted octahedral geometry. Both CPs form infinite linear chain structures which are hydrogen bonded to each other leading to the formation of three-dimensional supramolecular network structures. Topological analysis of CPs 1 and 2 reveals that the structures exhibit 1D chain-like arrangements in an AB–AB sequence and shows platonic uniform 2-connected uninodal topologies. Furthermore, a comparative analysis of a series of structures based on the BPG ligand indicates that the size of the metal ion and the types of counter-ions used have a great influence on the resulting frameworks and properties.

The coordination polymer {[Cu₂(sba)₂(bpg)₂(H₂O)₃]·5H₂O}_n encapsulates arrays of water molecules H-bonded to the framework displaying a high conductivity value of 0.94 × 10⁻² S cm⁻¹ with an activation energy of 0.64 eV.

The unidirectional proton coupled electron transfer (PCET) from the excited state of Ru(II) imidazole phenanthroline complex [Ru(bpy)₂ipH]²⁺ to 1,4‐benzoquinone, was studied by steady‐state (SS) and time‐resolved (TR) fluorescence and transient absorption (TA) measurements. The pK_a (9.7) and pK_a* (8.6) values of the complex suggest that it behaves as a photoacid on excitation. The difference in the quenching rates obtained from SS and TR fluorescence studies indicate participation of both dynamic quenching and static quenching involving the hydrogen bonded ipH ligand of [Ru(bpy)₂ipH]²⁺ with the 1,4‐benzoquinone quencher, formed in the ground state. Within the hydrogen bonded complex, the ruthenium centre acts as the electron donor, while the ipH ligand acts as the proton donor to the hydrogen bonded 1,4‐benzoquinone that acts simultaneously both as the electron and proton acceptor. It is proposed that the static quenching in the hydrogen bonded [Ru(bpy)₂ipH]²⁺‐1,4‐benzoquinone pairs occurs involving the PCET mechanism, while the dynamic quenching occurs through the simple ET mechanism, on diffusional encounter of the isolated 1,4‐benzoquinone with the excited [Ru(bpy)₂ipH]²⁺ complex. The occurrence of broad TA bands around 420–430 nm suggests formation of both 1,4‐benzoquinone radical anion as well as the 1,4‐benzosemiquinone radical by the interaction of excited [Ru(bpy)₂ipH]²⁺ with 1,4‐benzoquinone, thus supporting the ET process in the studied system.

Two heteronuclear ruthenium(II)–platinum(II) complexes [Ru(bpy)₂(BPIMBp)PtCl₂]²⁺ (3) and [Ru(phen)₂(BPIMBp)PtCl₂]²⁺ (4), where bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline, and BPIMBp = 1,4′-bis[(2-pyridin-2-yl)-1H-imidazol-1-ylmethyl]-1,1′-biphenyl, have been designed and synthesized from their mononuclear precursors [Ru(bpy)₂(BPIMBp)]²⁺ (1) and [Ru(phen)₂(BPIMBp)]²⁺ (2) as multitarget molecules for Alzheimer’s disease (AD). The inclusion of the cis-PtCl₂ moiety facilitates the covalent interaction of Ru(II) polypyridyl complexes with amyloid β (Aβ) peptide. These multifunctional complexes act as inhibitors of acetylcholinesterase (AChE), Aβ aggregation, and Cu-induced oxidative stress and protect neuronal cells against Aβ-toxicity. The study highlights the design of metal based anti-Alzheimer’s disease (AD) systems.

Proton conducting properties have been investigated in a new Ni(II)-based hydrogen-bonded porous framework synthesized using a urea-fused bipyridine-glycoluril (BPG) tecton. This hydrogen-bonded self-assembled structure encapsulates water molecules in the channels with hydrogen-bonding networks which exhibits a significant temperature dependent proton conductance of 1.5 × 10⁻⁴ S cm⁻¹ at 95 °C and 95% RH with a low activation energy (E_a) of 0.54 eV, implying a Grotthuss proton hopping mechanism mediated by hydrogen-bonded water molecules in the channels. In addition, this framework exhibited a very high water uptake under humid conditions. A continuous array of water molecules and chloride ions embedded in the highly hydrophilic porous channels of the hydrogen-bonded framework acts as the proton conducting medium.