Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

Abstract

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and non-invasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands L4-L6 were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III), Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triplestranded helical [LnM(Lk)3] 5+ cations, which can be crystallized with non-coordinating counteranions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)3] 5+ (strictly high-spin), [LnFe(L5)3] 5+ (spin-crossover (SCO) around room temperature) and [LnFe(L6)3] 5+ (SCO at very low temperature). The introduction of the 2 * * D,A D,A

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

“…Benefiting from the huge amount of experimental data collected during the last decades for [Fe(N ∩ N)3] 2+ complexes, where N ∩ N is an α,α'-diimine chelate ligand possessing two N-heterocyclic donor atoms, it was shown that the connection of a six-membered heterocycle to a five-membered heterocycle in N ∩ N provides favorable ligand-field strengths around Fe II for promoting spin-state equilibria (Equation 5) with transition temperatures T1/2 = ∆HSCO/∆SSCO (i.e., the temperature at which ∆GSCO = 0 and xhs = xls = 0.5) within the 30-500 K range [14,15,33]. [35,36]. [35].…”

“…[35]. Whereas the connection of methyl groups adjacent to the donor nitrogen atom in the bound 6-methyl-pyridine groups in [Fe(L4) 3 ] 2+ produces such large inter-strand interactions that the contraction accompanying the high-spin to low-spin transition cannot be envisioned [41], the situation with the remote 3-methyl substituted pyridine units in [Fe(L3) 3 ] 2+ is less clear and a sophisticated triple-stranded heterometallic LaFe helicate containing the facial [Fe(L3) 3 ] 2+ chromophore has been shown to display partial SCO behavior at low temperature (T 1/2~5 0 K) [36]. Taking into account that (i) the replacement of a pyridine with a pyrazine ring in going from [Fe L1 [36].…”

“…Whereas the connection of methyl groups adjacent to the donor nitrogen atom in the bound 6-methyl-pyridine groups in [Fe(L4) 3 ] 2+ produces such large inter-strand interactions that the contraction accompanying the high-spin to low-spin transition cannot be envisioned [41], the situation with the remote 3-methyl substituted pyridine units in [Fe(L3) 3 ] 2+ is less clear and a sophisticated triple-stranded heterometallic LaFe helicate containing the facial [Fe(L3) 3 ] 2+ chromophore has been shown to display partial SCO behavior at low temperature (T 1/2~5 0 K) [36]. Taking into account that (i) the replacement of a pyridine with a pyrazine ring in going from [Fe L1 [36]. Finally, since minor structural variations may induce large changes in ligand-field strength, the systematic exploration of unpredictable intermolecular packing interactions [42,43] operating in crystalline samples of [Fe(L5) 3 ]X 2 complexes (X -= monoanionic counter-ions) may contribute to the lucky search for some 'ideal' Fe II complexes, which additionally exhibit hysteretic behavior and bistability [17,44,45].…”

“…Finally, since minor structural variations may induce large changes in ligand-field strength, the systematic exploration of unpredictable intermolecular packing interactions [42,43] operating in crystalline samples of [Fe(L5) 3 ]X 2 complexes (X -= monoanionic counter-ions) may contribute to the lucky search for some 'ideal' Fe II complexes, which additionally exhibit hysteretic behavior and bistability [17,44,45]. e(L3)3] chromophore has been shown to display partial SCO behavior at low tempe K) [36]. Taking into account that (i) the replacement of a pyridine with a pyrazine r rom [Fe(L1)3] 2+ and [Fe(L2)3] 2+ stabilizes the low-spin state by T1/2 = 40 K (Scheme 2) an the methyl group from the 5-position in [Fe(L1)3] 2+ to the 3-position in fac-[Fe(L3)3] 2+ (as elated LaFe helicate) produces an opposite trend with the stabilization of the high-spin 50-310 = −260 K [36], we thus ingenuously explored the possibility of combining both a identate ligand L5 where the methyl group is now connected to the 3-position of a pyr cheme 3a) with the hope of pushing the transition temperature toward cryos atures for [Fe(L5)3] 2+ around T1/2 ~ T1/2 ([Fe(L2)3] 2+ ) − 260 = 350 − 260 = 90 K (3-methyl-pyra fort is justified by our long-term quest for designing a pseudo-octahedral spin-cros 3] 2+ unit that can modulate the luminescence of adjacent emissive lanthanid molecular assemblies via energy transfers within a temperature domain (77-150 K) acce al reading and addressing [36].…”

“…e(L3)3] chromophore has been shown to display partial SCO behavior at low tempe K) [36]. Taking into account that (i) the replacement of a pyridine with a pyrazine r rom [Fe(L1)3] 2+ and [Fe(L2)3] 2+ stabilizes the low-spin state by T1/2 = 40 K (Scheme 2) an the methyl group from the 5-position in [Fe(L1)3] 2+ to the 3-position in fac-[Fe(L3)3] 2+ (as elated LaFe helicate) produces an opposite trend with the stabilization of the high-spin 50-310 = −260 K [36], we thus ingenuously explored the possibility of combining both a identate ligand L5 where the methyl group is now connected to the 3-position of a pyr cheme 3a) with the hope of pushing the transition temperature toward cryos atures for [Fe(L5)3] 2+ around T1/2 ~ T1/2 ([Fe(L2)3] 2+ ) − 260 = 350 − 260 = 90 K (3-methyl-pyra fort is justified by our long-term quest for designing a pseudo-octahedral spin-cros 3] 2+ unit that can modulate the luminescence of adjacent emissive lanthanid molecular assemblies via energy transfers within a temperature domain (77-150 K) acce al reading and addressing [36]. Finally, since minor structural variations may induce s in ligand-field strength, the systematic exploration of unpredictable intermolecular pa ions [42,43] operating in crystalline samples of [Fe(L5)3]X2 complexes (X-= monoan -ions) may contribute to the lucky search for some 'ideal' Fe II complexes, which additio hysteretic behavior and bistability [17,44,45].…”