Bhaskaran Shankar

Luminescent Dirhenium(I)-Double-Heterostranded Helicate and Mesocate

The semirigid ligands 1,4-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)benzene (H2-pBC) and 1,3-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (H2-mBC), containing two hydroxyphenylbenzimidazolyl units as bis-chelating (or bis(bidentate)) N∩OH donor, were synthesized and were used to assemble neutral, luminescent heteroleptic, unsaturated double-hetero-stranded, rhenium(I)-based helicate (1) and mesocate (2) with the flexible bis(monodentate) nitrogen donor (1,4-bis(benzimidazol-1-ylmethyl)benzene/1,3-bis(benzimidazol-1-ylmethyl)benzene), and Re2(CO)10. The photophysical properties of the complexes were studied. Both complexes 1 and 2 exhibit dual emissions in both solution and solid state. In solution, these complexes show both fluorescence and phosphorescence. Complex 1 undergoes a predominantly ligand-centered oxidation, resulting in the generation of phenoxyl radicals.

New class of phosphine oxide donor-based supramolecular coordination complexes from an in situ phosphine oxidation reaction or phosphine oxide ligands.

A one-pot, multicomponent, coordination-driven self-assembly approach was used to synthesize the first examples of neutral bridging phosphine oxide donor-based supramolecular coordination complexes. The complexes were self-assembled from a fac-Re(CO)3 acceptor, an anionic bridging O donor, and a neutral soft phosphine or hard phosphine oxide donor.

Fe-terpyridyl complex based multiple switches for application in molecular logic gates and circuits

Molecular logic gates and circuits are constructed using the optical and electrochemical addressable-reversible-multiple switching event of the Fe(II)-4′-pyridyl terpyridyl complex (1) using multiple analytes. The process involves oxidation–reduction of Fe2+ as well as successive quaternization–dequaternization of the free pendant pyridyl group monitored optically. The whole switching process could also be visualized by the naked eye as colour changes upon switching are quite apparent and instant.

Synthesis and crystal structure of a wheel-shaped supramolecular coordination complex

AbstractSupramolecular coordination complex (SCC) possessing spatially arranged three anthraquinone dimers in a slipped-cofacial orientation was achieved by the treatment of Re2(CO)10, 2-hydroxymethylanthraquinone and tritopic N-donor via fac-Re(CO)3-directed one pot approach. The off-set π-stacking and C≡O···H bonding interactions stabilize the ring structure. Graphical AbstractSupramolecular coordination complex (SCC) possessing spatially arranged three anthraquinone dimers in a slipped-cofacial orientation was achieved by the treatment of Re2(CO)10, 2-hydroxymethylanthraquinone and tritopic N-donor via fac-Re(CO)3-directed one pot approach. The offset π-stacking and C≡O···H bonding interactions stabilize the ring structure.

Steric group enforced aromatic cyclic trimer conformer in tripodal molecules

A family of tripodal molecules (1–6) with/without steric ethyl groups at the central benzene scaffold and with furan/thiophene/pyridyl group at the 2-position of the benzimidazolyl unit was synthesised. Compounds 1–6 were characterized by elemental analysis and NMR spectroscopy. Compounds 1, 3, and 5 were further characterized by single crystal X-ray diffraction analysis. The molecular structures of 1 and 4 were optimized using density functional theory (DFT) calculations. X-ray and 1H NMR studies reveal that the introduction of three ethyl groups into a central benzene scaffold of furan/thiophene/pyridyl substituted benzimidazolyl based tripodal molecules enhances the edge-to-face C–H⋯π interactions, thereby favouring the aromatic cyclic trimer motif, in solution and the solid state. The unsubstituted central benzene scaffold allows the furan/thiophene substituted benzimidazolyl units in the tripodal molecules to move freely thereby weakening the edge-to-face C–H⋯π interactions between the aromatic cyclic trimer motif. Molecular modelling calculations indicate that the energy minimized structures of the tripodal molecules adopt a symmetric cyclic aromatic motif conformation.

Perfect symmetrical cyclic aromatic trimer motif in tripodal molecule

A noncovalent perfect symmetrical cyclic aromatic trimer motif stabilized through edge-to-face C–H⋯π interactions was found in substituted benzimidazolyl-based tripodal molecule. The distance 4.8 A, centroid to centroid, and interplanar angle 60° between any two aromatic units in the trimer motif are in excellent agreement with the theoretically predicted symmetrical cyclic benzene trimer cluster.

Heteroleptic Cu(II)–polypyridyl complexes as photonucleases

A series of heteroleptic Cu(II)–polypyridyl complexes with terpyridine (3N)/imidazole (2N) backbones and appended with pyridyl, 2-naphthyl, 9-anthryl and 1-pyrenyl groups are synthesized and evaluated for their photonuclease activity. An array of techniques viz. UV-vis, fluorescence, circular dichroism and thermal denaturation established strong DNA binding affinity (Kb = ∼104–106 M−1) and the binding modes were correlated with molecular docking studies. Photonuclease efficiency exceeded 90% for all the complexes under identical conditions. Interestingly, DNA binding propensity and photonuclease efficiency followed the increasing size, planarity, aromaticity, π-Stacking ability and hydrophobicity of the peripheral moiety.

Rhenium(I)-Based Monocyclic and Bicyclic Phosphine Oxide-Coordinated Supramolecular Complexes

Neutral, flexible ditopic phosphine (P–P) or phosphine oxide (O=P–P=O) donors, rigid anionic bis-chelating oxygen donors, and Re2(CO)10 were used to assemble ten phosphine oxide (P=O)-donor-based neutral monocyclic M2LL′-, bicyclic M4L2L″-, and bicyclic M4LL′2-type supramolecular coordination complexes (SCCs). A soft ditopic phosphine donor was transformed into a hard ditopic phosphine oxide donor, during the formation of the cyclic complexes 1–3, 5–6, and 9–10. Complexes 4, 7, and 8 were obtained using a hard P=O donor ligand. These SCCs were characterized using elemental analysis, FTIR, NMR, and single-crystal X-ray diffraction analysis. The absorption properties of 1–8 were studied using absorption UV–vis spectroscopic methods, and the results were analyzed using theoretical calculations. The results revealed that the neutral P=O donor significantly influenced the photophysical properties by enhancing the absorption coefficient in the visible region.

Sulfate Donor Based Dinuclear Heteroleptic Triple-Stranded Helicates from Sulfite and Ditopic Nitrogen Donor Ligands and Their Transformation to Dinuclear Homoleptic Double-Stranded Mesocates

Sulfate donor based supramolecular coordination complexes [{ fac-Re(CO)3}(μ-SO4)(L n)2{ fac-Re(CO)3}] (1-3) were obtained using ditopic N donors (L n; n = 1-3), NaHSO3, and Re2(CO)10 in a one-pot, multicomponent, coordination-driven self-assembly approach, in which SO32- becomes oxidized to SO42- during the reaction and acts as a building framework. Complexes 1-3 were characterized using IR, ESI-TOF-MS, and 1H NMR spectroscopy. The structures of complexes 1-3 were confirmed using single-crystal X-ray diffraction analysis. The transformation of the dinuclear heteroleptic triple-stranded helicate to the dinuclear homoleptic double-stranded mesocate [{Re(CO)3Cl}2(L n)2] (L n = L1, L2, L3; 4a-6a) was achieved by the addition of BaCl2. The direct treatment of Re(CO)5X (X = Cl, Br) with L1/L2/L3 yielded the dinuclear homoleptic double-stranded helicates [{Re(CO)3X}2 (L n)2] (4b-6b and 7-9).

Crystal structure of the cis and trans polymorphs of bis­[μ-2-(1,3-benzo­thia­zol-2-yl)phenolato]-κ3N,O:O;κ3O:N,O-bis­[fac-tri­carbonyl­rhenium(I)]

The molecular structure of the title compounds may be visualized as two octahedral metal coordinated units, fused through μ-oxide bridges across symmetry centres, leading to edge-sharing dimers.