Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes

Abstract

Chelate coordination of non-symmetrical didentate pyrazine-benzimidazole (L1) or pyridine-benzimidazole (L2) N-donor ligands around divalent iron in acetonitrile produces stable homoleptic triple-helical spin crossover [Fe(Lk) ] complexes existing as mixtures of meridional (C -symmetry) and facial (C -symmetry) isomers in slow exchange on the NMR timescale. The speciation deviates from the expected statistical ratio mer/fac=3:1, a trend assigned to the thermodynamic trans-influence, combined with solvation effects. Consequently, the observed spin state Fe ↔Fe equilibria occurring in [Fe(Lk) ] refer to mixtures of complexes in solution, an issue usually not considered in this field, but which limits rational structure-properties correlations. Taking advantage of the selective and quantitative formation of isostructural facial isomers in non-constrained related spin crossover d-f helicates (HHH)-[LnFe(Lk) ] (Ln is a trivalent lanthanide, Lk=L5, L6), we propose a novel strategy for assigning pertinent thermodynamic driving forces to each spin crossover triple-helical isomer. The different enthalpic contributions to the spin state equilibrium found in mer-[Fe(Lk) ] and fac-[Fe(Lk) ] reflect the Fe-N bond strengths dictated by the trans-influence, whereas a concomitant solvent-based entropic contribution reinforces the latter effect and results in systematic shifts of the spin crossover transitions toward higher temperature in the facial isomers.

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

“…A detailed analysis of a series of Ni II complexes led Hancock and coworkers to propose three empirical Equations (14)-( 16) to obtain more reliable ligand field strengths ∆ oct and Racah parameters B and C in cm −1 units (ε 1 and ε 2 are the extinction coefficients at the observed frequencies of the 1 E← 3 A 2 transition and 3 T 2 ← 3 A 2 transition, respectively) [76]. The analysis of the experimental absorption spectra using this model gives the corrected parameters gathered in Table 2 (Table 2) almost exactly matched those previously reported for the unconstrained [Ni(L2) 3 ] 2+ complex, for which the related [Fe(L2) 3 ] 2+ complex displayed spin-crossover behavior above room temperature (T 1/2~4 00 K in the solid state, T 1/2~3 50 K in acetonitrile solution [35]). Moreover ∆ oct ([Ni(L5) 3 ] 2+ ) = 11,630 cm− 1 is compatible with the ligand field range 11,200 ≤ ∆ oct (Ni II ) ≤ 12,400 cm −1 established by Busch and coworkers [78] as a reliable and useful benchmark for predicting and rationalizing the spin-crossover of the related Fe II complexes [40].…”

“…The didentate ligands L1 and L2 match the latter criteria and the associated pseudo-octahedral complexes [Fe(Lk) 3 ] 2+ indeed exhibit spin-crossover behaviors in acetonitrile solutions, the transition temperatures of which reveal the stronger Fe-N bonds induced by the strong-accepting pyrazine units in [Fe(L2) 3 ] 2+ (T 1/2~3 50 K) compared with pyridine units in [Fe(L1) 3 ] 2+ (T 1/2~3 10 K, Scheme 2) in the absence of sterical constraints [35].  (red areas in Scheme 1b).…”

“…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].…”

“…[35,36]. [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].…”

Cerium(iii) complexes with azolyl-substituted thiophenolate ligands: synthesis, structure and red luminescence

“…are also in the range of values characteristic for lanthanum complexes. [40][41][42] Despite the large number of aromatic systems in the SSN, NSN and OSN ligands, intermolecular p/p interactions in all structurally characterized complexes are not detected. It is interesting to note that the molecules of Ce(NSN) 3 (DME) form a dimeric pair in a crystal (Fig.…”