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(CH(2))(4)CN), bipyrimidine (bpym), dicyanamide (dca(-)), tricyanomethanide (tcm(-)), 3,6-bis(2-pyridyl)tetrazine (bptz) and 3,6-bis(2-pyridyl)2,5-dihydrotetrazine (H(2)bptz). The 'end groups', which complete the Fe(ii)N(6) 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(mu(1,5)-dca)(2)Fe(tpm)(dca)], 5, and [(tpa')Fe(mu(1,5)-tcm)(2)Fe(tpa')](tcm)(2)(H(2)O)(2), 6. In contrast, a strong field was created around the Fe(II) centres in [(tpa)Fe(mu-(NC(CH(2))(4)CN))(2)Fe(tpa)](ClO(4))(4).NC(CH(2))(4)CN, 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 mu-bpym systems, were noted for [(1,10-phen)(NCS)(2)Fe(mu-bpym)Fe(NCS)(2)(1,10-phen)], 2, [(pypzH)(NCSe)(2)Fe(mu-bpym)Fe(NCSe)(2)(pypzH)], 4, and [(tpa)Fe(mu-H(2)bptz)Fe(tpa)](ClO(4))(4), 8. Interestingly, the mu-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.

First Dicyanamide‐Bridged Spin‐Crossover Coordination Polymer: Synthesis, Structural, Magnetic, and Spectroscopic Studies

“…The first one is the mononuclear compound [FeA C H T U N G T R E N N U N G (abpt) 2 A C H T U N G T R E N N U N G (dca) 2 ] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole), which has an incomplete spin transition with T 1/2 = 86 K. [32] The dinuclear complex {[FeA C H T U N G T R E N N U N G (bztpen) 2 ]A C H T U N G T R E N N U N G (m-dca)}A C H T U N G T R E N N U N G (PF 6 ) 3 (bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine) shows a complete two-step spin crossover, with transitions approximately centred at 350 K and 200 K. [33] Interestingly, the 2 } (tpm = tris(pyrazolyl)methane) dimer remains in the HS state. [52] The magnetic measurements on 2 reveal a complete twostep spin transition. Such spin transitions have been reported for mononuclear, [35,[53][54][55] binuclear, [56] and polymeric [57][58][59] iron(II) compounds.…”

Spin‐Crossover Studies on a Series of 1D Chain and Dinuclear Iron(II) Triazine‐Dipyridylamine Compounds

“…A range of structural designs of the bridging geometry and of the chemical nature of the bridging ligands has been employed in order to attempt to answer some of these questions. [17] The ligand common to both studies is 2,4,6-tris(dipyrid-2-ylamino)-1,3,5-triazine (labelled dpyatriz by Gamez et al [1][2] and tdt herein, Figure 1) while the other ligand is 2-chloro-4,6-bis(dipyridin-2-ylamino)-1,3,5-triazine (labelled cddt, Figure 1). …”

Solid‐ and Solution‐State Studies of the Novel μ‐Dicyanamide‐Bridged Dinuclear Spin‐Crossover System {[(Fe(bztpen)]2[μ‐N(CN)2]}(PF6)3⋅n H2O

“…Similarly, recent work aimed at synthesizing dca-bridged dinuclear Fe II SCO compounds led to double dca bridges and HS-HS behavior. [22] Two-step SCO behavior: Two-step SCO transitions are scarce and only a few well-documented examples have been reported. The mononuclear complex [Fe(2-pic) 3 ]Cl 2 ·EtOH (2-pic = 2-picolylamine) was the first example of a two-step SCO and is generally considered prototypical.…”