Spin state variability in Fe²⁺ complexes of substituted (2-(pyridin-2-yl)-1,10-phenanthroline) ligands as versatile terpyridine analogues

Abstract

Fe spin crossover complexes [Fe(L)] (L = 2-(6-R-pyridin-2-yl)-1,10-phenanthroline with R = H, methoxy, bromo, -(1H-pyrazol-1-yl) or L = 2-(3-methoxy-pyridin-2-yl)-1,10-phenanthroline) were prepared. These air stable and durable complexes show SCO behaviour with very different transition temperatures T ranging from 130 K to 600 K depending on the substitution pattern. The use of H NMR spectroscopy to elucidate the thermodynamics and kinetics of SCO in a solution of this series is described in detail. By introduction of an additional pyrazole donor (R) in the ortho-position to the pyridine, the N6 octahedral coordination sphere is expanded to N8 coordination with a trigonal dodecahedral structure. This leads to a strong stabilization of the high spin state and an increased longitudinal relaxation R of the proton spins. The larger R values were ascribed to different electronic structures with non-orbital degenerate quintet ground states and a larger energetic separation from the first excited state. These results are also supported by Mössbauer spectroscopy. The N8 coordination sphere stabilizes the complex in the high spin state and no indication for SCO was found. DFT calculations confirmed the experimentally obtained order of T and allowed the calculation of the complex structure in experimentally non-accessible spin states. Complexes of this series can be oxidized to the Fe complexes in a chemically reversible fashion. Interestingly, the lowest oxidation potential was observed for the N8 coordinated complex.

Tailoring of the Frontier Orbital Character in Co2+/3+ Complexes with Triarylamine Substituted Terpyridine Ligands

“…Different from C 2 v ‐symmetric ligands, there is additional “left‐right” anisotropy in [Co 2+ ( 3 ) 2 ] 2+ (XRD) and [Co 2+ ( 1 ) 2 ] 2+ (DFT) (Co1–N1/N5: 2.167/2.184 Å and Co1–N3/N7: 2.190/2.197 Å) with elongated Co–N bonds corresponding to ortho ‐substituted pyridine donors. These results echo observations made previously with related Fe 2+ complexes , . Overall the CoN 6 coordination sphere in [Co 2+ ( 1 / 3 ) 2 ] 2+ exhibits significant cis ‐angle distortion, which can be largely ascribed to the presence of the bulky substituents.…”

Spin Crossover in 3D Metal Centers Binding Halide-Containing Ligands: Magnetism, Structure and Computational Studies

“…Petzold and co-workers [148] synthesized a series of ligands of the family 2-(6-R 1pyridin-2-yl)-1,10-phenanthroline, one of them carrying a Br substituent (L46). Its Fe(II) complex, [Fe(L46) 2 ]X 2 (X = BF 4 − or ClO 4 − ), undergoes SCO to the LS state on lowering the temperature.…”

Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands

“…I.e., at lower temperatures, the two-site model systematically overestimates the linewidths. Such deviations have been observed previously [64,65]. They have been attributed to the breakdown of the fast-exchange approximation inherent to the two-site model, rendering Equation (2) invalid at lower temperatures.…”

“…As the factor (∆ω 2 /k obs ) decreases continuously with increasing temperature, the maximum linewidth LW max is located at T max < T 1/2 . Such bell-shaped LW versus T plots in the NMR spectra of bis-meridional coordinated iron (II) complexes have been recently reported by us together with a detailed analysis in terms of admixed SCO dynamics [63,64]. While, in principle, each of the parameters γ hs , ∆ω and k obs in Equations (2) and (3) must be considered structure-dependent, the coinciding SCO energies ∆ SCO E from DFT ( (Figure 5b), rule out significant ligand-structure imposed differences in γ hs and ∆ω.…”

“…[63][64][65], we have recorded field-dependent NMR line broadening, which could be quantitatively analyzed in terms of a two-sites chemical exchange model. In particular, the linewidths could be correlated with the kinetics of SCO (kobs = kL→H + kH→L).…”