Taming 2,2'-biimidazole ligands in trivalent chromium complexes

Complete or partial replacement of well-known five-membered chelating 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands with analogous didentate 2,2'-biimidazole (H2biim) provides novel perspectives for exploiting the latter pH-tuneable bridging unit for connecting inert trivalent chromium with cationic partners. The most simple homoleptic complex [Cr(H2biim)3]3+ and its stepwise deprotonated analogues are only poorly soluble in most solvents and their characterization is limited to some solid-state structures, in which the pseudo-octahedral [CrN6] units are found to be intermolecularly connected via peripheral N-H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N-H vibrations drastically quench the targeted near infrared (NIR) CrIII-based phosphorescence, which makes these homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies. Restricting the number of bound 2,2'-biimidazole ligands to a single unit in the challenging heteroleptic [Cr(phen)2(Hxbiim)](1+x)+ (x = 2-0) complexes overcomes the latter limitations and allows (i) the synthesis and characterization of these [CrN6] chromophores in the solid state and in solution, (ii) the stepwise and controlled deprotonation of the bound 2,2'-biimidazole ligand and (iii) the implementation of Cr-centered phosphorescence with energies, lifetimes and quantum yields adapted for using the latter chromophores as sensitizers in promising 'complex-as-ligand' strategies.

Synchronized Collective Proton-Assisted Electron Transfer in Solid State by Hydrogen-Bonding Ru(II)/Ru(III) Mixed-Valence Molecular Crystals

A proton-coupled electron transfer (PCET) reaction was widely studied with isolated organic molecules and metal complexes in solution in view of the biological catalytic reaction, while studying this reaction in the crystalline or solid-state phase, which has a novel example, would give insight into the rather internal environment of proteins without solvation and a creation of new molecular materials. We tried to crystallize a hydrogen-bonded (H-bonded) coordination polymer with one-dimensional nanoporous channels, formed from redox-active Ru^III complexes, [Ru^III(Hbim)₃] (Hbim^- = 2,2'-biimidazolate monoanion). As a result, a synchronized collective PCET phenomenon was observed for the molecular nanoporous crystal by novel solid-state cyclic voltammetry (CV), which could be measured by only setting some crystals on the electrode surface. The nanoporous crystals, {[Ru^III(Hbim)₃]}_n (1), are simultaneously induced to a synchronized collective Ru^IIRu^III mixed-valence state, {Ru^IIRu^III}_n, with alternating arrays of Ru^II and Ru^III complexes by PCET in a way of the reductive state of {Ru^IIRu^II}_n. Further, a new crystal with {Ru^IIRu^III}_n, {[Ru^II(H₂bim)(Hbim)₂][Ru^III(bim) (Hbim)₂][K(MeOBz)₆]}_n (2), was also prepared, and the solid-state CV revealed the same electrochemical behavior of {Ru^IIRu^III}_n with 1. The single crystal with {Ru^IIRu^III}_n of 2 was unusually a semiconductor with 5.12 × 10^-6 S/cm conductivity at 298 K by an impedance method (8.01 × 10^-6 S/cm by a direct-current method at 277 K). Thus, an unprecedented electron-hopping conductor driven by a low-barrier proton transfer through a PCET mechanism (E_a = 0.30 eV) was realized in the H-bonding molecular crystal with {Ru^IIRu^III}_n. Such studies on a PCET reaction in the crystalline state is not only worthwhile as a model of essential biological reactions without solvation, but also proposed to a new design of molecular materials to occur an electron transfer by using an intermolecular H-bond.

Fast synthesis of a tris(N,N-diimine)chromium(iii) complex by a microwave-assisted approach

This article describes a quick, easy and eco-friendly procedure to synthesize [Cr(biim)₃](NO₃)₃ by employing microwave irradiation.

Metallotectons: using enantiopure tris(dipyrrinato)cobalt(iii) complexes to build chiral molecular materials

Resolution of a tricarboxylic acid derived from a chiral tris(dipyrrinato)cobalt(III) complex provides a series of enantiopure metallotectons that crystallise as porous hydrogen-bonded networks.