Experimental and theoretical insights into the photomagnetic effects in trinuclear and ionic Cu(ii)–Mo(iv) systems

Abstract

The crystal engineering of octacyanidomolybdate(IV) ions and copper(II) complexes with two types of chelating ligands: multidentate blocking ligand, tren = (tris(2 aminoethyl)amine), and bidentate ligand capable of forming unusual organic bridges, tn = 1,3-diaminopropane, resulted in the formation of {[Cu(tren)]₂(μ-tn)}·[Mo(CN)₈]·7.5H₂O (1) and {[Cu(tren)]₂(μ-tn)}·[Mo(CN)₈]·{[Cu(tren)]₂[Mo(CN)₈]}·9H₂O (2). The crystal structure of 1 comprises isolated anions [Mo(CN)₈]⁴⁻ wrapped with charge-compensating units {[Cu(tren)]₂(μ-tn)}⁴⁺ stabilised by extensive H-bond networks. Compound 2 forms alternating layers consisting of the same ions as 1 and neutral cyanido-bridged V-shaped trinuclear molecules {[Cu(tren)]₂[Mo(CN)₈]}. Additionally, two previously reported V-shaped trinuclear reference systems were synthesised: [Cu(tn)₂]₂[Mo(CN)₈]·2H₂O (3) and [Cu(tren)]₂[Mo(CN)₈]·5.25H₂O (4). UV–Vis–NIR absorption spectroscopy, supported by quantum chemical calculations (DFT and TD-DFT), confirmed the presence of Mo(IV) to Cu(II) charge transfer bands centred on trinuclear molecules (2–4) and their absence for the ionic sample 1 without CN⁻-bridges. Additionally, detailed descriptions of the energy level diagrams of 1–4 with the frontier molecular orbitals and possible optical transitions were made. Magnetic studies indicated paramagnetic behaviour with weak antiferromagnetic interactions at low temperature. Finally, photomagnetic studies of 1–3 showed the increase of magnetization after irradiation with 473 and 410 nm light at 10 K. Comparative analysis of all data suggests that the photomagnetic effect in 1 is well described by the light-induced excited spin-state trapping (LIESST) effect in the Mo(IV) centre, while other cyanido-bridged compounds prefer the metal-to-metal charge transfer with minor contribution of LIESST effects.