Square-Planar Heteroleptic Complexes of α-Diimine-NiII-Catecholate Type: Intramolecular Ligand-to-Ligand Charge Transfer

Abstract

Two heteroleptic NiII complexes combined the redox-active catecholate and 2,2′- bipyridine ligand platforms were synthesized to observe a photoinduced intramolecular ligand-to-ligand charge transfer (LL’CT, HOMOcatecholate → LUMOα-diimine). A molecular design of compound [NiII(3,6-Cat)(bipy)]∙CH3CN (1) on the base of bulky 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) was an annelation of the ligand with an electron donor glycol fragment, producing derivative [NiII(3,6-Catgly)(bipy)]∙CH2Cl2 (2), in order to influence the energy of LL’CT transition. A substantial longwave shift of the absorption peak was observed in the UV-Vis-NIR spectra of 2 compared with those in 1. In addition, the studied NiII derivatives demonstrated a pronounced negative solvatochromism, which was established using a broad set of solvents. The molecular geometry of both compounds can be ascribed as an insignificantly distorted square-planar type, and the π–π intermolecular stacking of the neighboring α-diimines is realized in a crystal packing. There is a lamellar crystal structure for complex 1, whereas the perpendicular T-motifs with the inter-stacks attractive π–π interactions form the packing of complex 2. The redox-active nature of ligand systems was clearly shown through the electrochemical study: a quasi-reversible one-electron reduction of 2,2′-bipyridine and two reversible successive one-electron oxidative conversations (“catecholate dianion—o-benzosemiquinonato radical anion—neutral o-benzoquinone”) were detected.

Charge Transfer Chromophores Derived from 3d-Row Transition Metal Complexes

“…The vast class of CT colorants is presented by transition metal complexes, the metal-to-ligand charge transfer (MLCT) of which is facilitated by non-planar molecular geometry [ 30 , 31 ] with high coordination numbers, and driven by a pronounced back donation phenomenon [ 32 ]. Remarkably, the molecular design of highly effective MLCT dyes derived from Mo [ 27 ], Os [ 4 , 6 ], Re [ 4 , 6 , 33 ], and mainly Ru [ 4 , 6 , 27 , 34 , 35 ] has been named an absolute priority, which can be explained by their use as photosensitizers (otherwise “redox mediators” [ 36 ]) in photovoltaic (PV) devices of Grätzel cell’s type [ 37 , 38 ].…”

“…In this regard, CT chromophores of D-M II -A type (often this is dye with ligand-to-ligand charge transfer (LL’CT) and less coordination numbers) can provide a competitive alternative towards MLCT colorants due to noteworthy aspects: (1) pronounced charge separation on a molecular level with a planar mutual arrangement of interacting HOMO (donor organic part) and LUMO (acceptor organic part) [ 32 ]; (2) higher intense absorptivity; (3) an ability to fine-tune CT energy in the course of molecular design, varying electronic properties of ligands by functionalization; (4) lower energy of effective intramolecular CT in some cases, leading to an absorption/emission even in the NIR region [ 29 ] of an undeniable practical importance. It should be noted that a diminution in CT energy up to NIR absorption/emission is one of the priority tasks in the colorant chemistry.…”

“…Since a planar molecular geometry (most frequent in CT chromophores of D-M II -A type) is a favorable and necessary condition for realization of effective intramolecular LL’CT [ 32 , 41 ], at present the overwhelming majority of LL’CT chromophores comprises the derivatives of 10 group metals (Ni, Pd, Pt), which prefer a square-planar coordination environment [ 42 ]. Against 10 group metal CT dyes, very scant attention is paid to compounds of other transition elements, and a range of such chromophores is vastly limited in number and nature: thus, sporadic sources reported on 3d-row metal (Co [ 43 ] and Cu [ 44 , 45 , 46 , 47 ]) CT complexes.…”

“…As another serious omission in dye chemistry of CT chromophores of D-M II -A type, many available works take the main aim to perform a synthesis and routine analysis of general physic-chemical properties. On the other side, there is a scattered and non-systematized information concerning important CT characteristics, such as: (1) lifetime of excited state (all data for Pt II chromophores) [ 10 , 16 , 17 , 60 , 61 , 62 , 63 , 64 , 65 , 66 ]; (2) CT energy in terms of its dependence on the nature of metal center [ 67 , 68 , 69 , 70 , 71 ]/electronic properties of ligands [ 11 , 17 , 28 , 32 , 41 , 42 , 60 , 61 , 62 , 63 , 64 , 68 , 70 , 71 , 72 , 73 , 74 , 75 , 76 ]/solvent polarity [ 32 , 42 , 60 , 61 , 62 , 63 , 64 , 67 , 76 , 77 ]. However, knowledge about these key parameters helps turn colorant into “functional dye”.…”

Crystallographic Principal Modulation and Metal Core – Crystal Structure Exchangeable: Computational And Molecular Docking Simulation

“…Examining the geometrical structure of most Pd-complexes, it was found that 4-bonded coordinated structure locate between tetrahedral and square planar geometrical pathway depending on the electronic con guration of the metal. Also, a mutual arrangement around the planar environment basically depends the interacting molecular orbitals resulting the implementation of optimal planar geometry of the compound causing ligand-ligand charge transfer reactions [15][16][17] To view of mentioned above, researches move towards the molecular design study of many metal complexes preferring the square-planar structure.…”