Spin-Crossover Temperature Predictable from DFT Calculation for Iron(II) Complexes with 4-Substituted Pybox and Related Heteroaromatic Ligands

Abstract

Spin-crossover (SCO) is a reversible transition between low and high spin states by external stimuli such as heat. The SCO behavior and transition temperature ( T 1/2 ) of a series of [Fe II (X-pybox) 2 ](ClO 4 ) 2 were studied to establish a methodology for ligand-field engineering, where X-pybox stands for 2,6-bis(oxazolin-2-yl)pyridine substituted with X at the 4-position of the pyridine ring. We utilized X = MeO, Me, 3-thienyl, Ph, H, MeS, 2-thienyl, N 3 , Cl, Br, 3-pyridyl, and 4-pyridyl. The solution susceptometry on five new derivatives with X = Me, 2-thienyl, N 3 , Br, and 3-pyridyl was performed in acetone, giving the SCO temperatures of 220, 260, 215, 280, and 270 K, respectively. The density-functional-theory molecular orbital (MO) calculation was performed on the ligands with geometry optimization. The atomic charge on the pyridine nitrogen atom [ρ(N py )] was extracted from the natural orbital population analysis. Positive correlation appeared in the T 1/2 versus ρ(N py ) plot with R 2 = 0.734, being consistent with the analysis using the Hammett substituent constants (σ p and σ p + ). This finding well agrees with the mechanism proposed: the rich electron density lifts the t 2g energy level through the dπ–pπ interaction, resulting in a narrow t 2g –e g energy gap and favoring the high-spin state and low T 1/2 . The MO method was successfully applied to the known SCO-active iron(II) compounds involving 4-substituted 2,6-bis(pyrazol-1-yl)pyridines. A distinct positive correlation appeared in the T 1/2 versus ρ(N py ) plot. The comparison of correlation coefficients indicates that ρ(N py ) is a more reliable parameter than σ p or σ p + to predict a shift of T 1/2 . Furthermore, this method can be more generalized by application to another known SCO family having 3-azinyl-4- p -tolyl-5-phenyl-1,2,4-triazole ligand series, where azinyl stands for a 2-azaaromatic ring. A good linear correlation was found in the T 1/2 versus ρ(N A ) plot (N A is the ligating nitrogen atom in the azaaromatic ring). Finally, we will state a reason why the present treatment is competent to predict the SCO equilibrium position only by consideration on the electronic perturbation.

New Spin‐Crossover Complexes of Substituted 2,6‐Bis(pyrazol‐3‐yl)pyridines

“…Despite providing slightly different HS state populations (the differences are, however, small given experimental errors of the Evans method and approximations used in the other two approaches[11d], [11e], ), they all agree on an essentially the same sluggish SCO occurring in the solutions of [Fe( L1 ) 2 ](BF 4 ) 2 and [Fe( L2 ) 2 ](ClO 4 ) 2 , as hinted by the molecular geometries of these complexes. The p ‐toluoyloxy and p ‐fluorophenyl groups in their ligands have high electron‐withdrawing ability, quantified by their Hammett constants and the calculated charges of coordinating nitrogen atoms in the free ligands L1 and L2 (–0.24 and –0.23 e, respectively) relative to the unsubstituted 3‐bpp (–0.30 e). [11e] The latter produces an SCO‐active complex [Fe(3‐bpp) 2 ](BF 4 ) 2 with T 1/2 of 244 K in a solution (Evans method).…”

Towards the Molecular Design of Spin‐Crossover Complexes of 2,6‐Bis(pyrazol‐3‐yl)pyridines

“…As N,N’‐disubstituted 3‐bpp complexes have no pendant NH groups that make their spin state extremely sensitive to the environment, the proposed ligand design, which may be also applicable to isomeric 1‐bpp, or other families of popular bi‐, tri‐, and higher denticity ligands, opens the way for their molecular design as spin‐crossover compounds for future breakthrough applications …”

“…This spin‐crossover (SCO) phenomenon is often observed for the iron(II) ion in octahedral N 6 coordination environments, where the switching occurs between differently coloured diamagnetic low‐spin (LS, S =0) and paramagnetic high‐spin (HS, S =2) states . Tailoring it for the above applications relies on proper chemical modifications of the ligands identified mostly through systematic screening of solutions (by NMR spectroscopy that is behind the popular Evans method) of closely related iron(II) complexes with bi‐ and tridentate or higher denticity ligands. The resulting structure‐function relationships, which are behind a successful ‘truly molecular’ design of SCO compounds, established the stabilization of the HS state by bulky groups close to the donor nitrogen atoms, with a less clear‐cut role of subtle electronic or other effects of more remote substituents .…”

“…Tailoring it for the above applications relies on proper chemical modifications of the ligands identified mostly through systematic screening of solutions (by NMR spectroscopy that is behind the popular Evans method) of closely related iron(II) complexes with bi‐ and tridentate or higher denticity ligands. The resulting structure‐function relationships, which are behind a successful ‘truly molecular’ design of SCO compounds, established the stabilization of the HS state by bulky groups close to the donor nitrogen atoms, with a less clear‐cut role of subtle electronic or other effects of more remote substituents . The contradictory electronic effects have been recently reconciled in a solution‐state study of 2,6‐bis(pyrazol‐1‐yl)pyridines (1‐bpp; Scheme ), which are among the most popular ligands to induce an SCO behaviour in iron(II) complexes, and to control it with a judicious choice of substituents .…”

Intramolecular Spin State Locking in Iron(II) 2,6-Di(pyrazol-3-yl)pyridine Complexes by Phenyl Groups: An Experimental Study

“…To get an experimental insight into this HS locking (which may be due to the HS state being thermodynamically more stable than the LS state, as suggested in a computational study for an iron(II) complex 1,3-bpp [17]), we reproduced the ligand L1 (the resulting complex Fe(L1) 2 OTf 2 was obtained as another crystallosolvate [23] but with no significant changes in [Fe(L1) 2 ] 2+ structure) and synthesized a new one (L2) with acetoxy groups instead of OH groups (Scheme 2). The two groups have different electron donating/withdrawal ability (as gauged by their Hammett constants [9] and calculated charges [24] of coordinating nitrogen atoms in the free ligands L1 and L2) and therefore exert different electronic effects on the HS/LS stabilization. For iron(II) complexes of 1-bpp, a more electron-withdrawing group at this position of the ligand favors the HS state (mostly through inductive effects [9]).…”