Asymmetric spin crossover behaviour and evidence of light-induced excited spin state trapping in a dinuclear iron(II) helicatew

Currently there is an ever increasing interest in spin crossover compounds, partly motivated by their potential application as molecular memory or visual display devices, which has led to a better understanding of the parameters (temperature, pressure, light, structure etc.) that influence this fascinating phenomenon. Spin crossover is best exemplified by the low spin Fe (S = 0) to high spin Fe (S = 2) conversion. Nevertheless, scant literature deals with dinuclear Fe spin crossover helicate complexes. In this respect, Williams reported a dinuclear Fe helicate with thermally induced two-step spin crossover with negative cooperativity, whereas Tuna synthesised a series of Fe helicate species using a variety of counter anions that exhibited various spin crossover behaviours, which were later reinterpreted by Gütlich. More recently, Kojima reported two Fe helicate compounds presenting either no magnetic conversion or an abrupt [LS–HS] to [HS–HS] spin crossover (LS and HS stand for low spin and high spin, respectively). The synthesis and characterisation of helicate species has been closely associated with the development of metallosupramolecular chemistry. Recently we reported using metallohelicate complexes as molecular hosts and monitored anion binding within their intra-helical cavities through H NMR spectroscopy. Indeed, the inclusion of spin crossover within an Fe metallohelicate complex makes them very attractive targets for the recognition of guest molecules using the spin crossover magnetic signature. With the motivation of using metallohelicates in the formation of supramolecular devices, we have commenced a study into the nature of the spin crossover in dinuclear Fe helicate complexes and its dependence on the structure and functionality of the ligand. We wish to explore the possibility of generating one-step [HS–HS] to [LS–LS] spin crossover helicates since we believe that the larger associated structural and electronic changes are of advantage for device or sensor application. To this end we synthesised the new bis-bidentate ligand L which contains imidazolimine ‘head groups’ (to access spin crossover behaviour) linked via a conjugated and rigid oxydianiline bridge (to generate a helicate). The corresponding triple-stranded Fe helicate [Fe2(L)3](ClO4)4, 1, was synthesised and its ability to undergo a spin crossover upon temperature variation and light irradiation was studied by both magnetic susceptibility and reflectivity measurements. The bis-bidentate ligand L was synthesised by Schiff-base condensation in methanol using two equivalents of 1-methyl2-imidazolecarboxaldehyde and one equivalent of 4,4’-oxydianiline. The corresponding Fe dinuclear helicate, 1, was obtained in good yield by stirring three equivalents of L with two equivalents of Fe(ClO4)2 6H2O in methanol for 30 minutes.w The expected triple-stranded helical structure was highlighted by H NMR spectroscopy and confirmed by single crystal X-ray diffractionz for 1 2MeCN, Fig. 1. As anticipated, the structure of 1 2MeCN consists of a dinuclear triple helicate within which the Fe centres are in pseudo-octahedral environments bound to three imidazolimine units from three different ligands. Each ligand binds to two Fe centres, yielding an intra-helical metal–metal separation of 11.35 Å.