Designing dinuclear iron(ii) spin crossover complexes. Structure and magnetism of dinitrile-, dicyanamido-, tricyanomethanide-, bipyrimidine- and tetrazine-bridged compounds

Abstract

In order to expand the few known examples of dinuclear iron(II) compounds displaying (weak) intradinuclear exchange coupling and spin-crossover on one or both of the iron(II) centres, various dinuclear compounds have been synthesised and assessed for their spin-crossover and exchange coupling behaviour. The key aim of the work was to prepare and structurally characterise ‘weakly linked’ and ‘covalently bridged’ systems incorporating bridging ligands such as alkyldinitriles (e.g.NC(CH₂)₄CN), bipyrimidine (bpym), dicyanamide (dca⁻), tricyanomethanide (tcm⁻), 3,6-bis(2-pyridyl)tetrazine (bptz) and 3,6-bis(2-pyridyl)2,5-dihydrotetrazine (H₂bptz). The ‘end groups’, which complete the Fe(II)N₆ chromophores, include tris(2-pyridylmethyl)amine (tpa), di(2-pyridylethyl)(2-pyridylmethyl)amine (tpa′), 3-(2-pyridyl)pyrazole (pypzH), 1,10-phenanthroline (1,10-phen), tris(pyrazolyl)methane (tpm) and NCX⁻ (X = S, Se). It was quite difficult to achieve the spin-crossover condition, many ligand combinations yielding high-spin/high-spin (HS–HS) Fe(II)Fe(II) spin states at all temperatures (300–2 K) with very weak antiferromagnetic coupling (J < −1 cm⁻¹), two such being the crystallographically characterised [(dca)(tpm)Fe(μ_1,5-dca)₂Fe(tpm)(dca)], 5, and [(tpa′)Fe(μ_1,5-tcm)₂Fe(tpa′)](tcm)₂(H₂O)₂, 6. In contrast, a strong field was created around the Fe(II) centres in [(tpa)Fe(μ-(NC(CH₂)₄CN))₂Fe(tpa)](ClO₄)₄·NC(CH₂)₄CN, 1, and the Fe–N bond distances, at 173 K, reflected this. This weakly-linked dinitrile example showed a spin-crossover beginning above 300 K. ‘Half crossover’ examples, yielding HS–LS states below the spin transition, similar to those noted by Real and coworkers in some μ-bpym systems, were noted for [(1,10-phen)(NCS)₂Fe(μ-bpym)Fe(NCS)₂(1,10-phen)], 2, [(pypzH)(NCSe)₂Fe(μ-bpym)Fe(NCSe)₂(pypzH)], 4, and [(tpa)Fe(μ-H₂bptz)Fe(tpa)](ClO₄)₄, 8. Interestingly, the μ-bptz analogue, 7, remained LS–LS at all temperatures with the start of a broad spin crossover evident above 300 K. No thermal hysteresis was evident in the spin transitions of these new dinuclear crossover species indicating a lack of intra- or interdinuclear cooperativity.