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.

Spin-crossover (SCO) behavior of a series of [Fe(X-pybox) 2 ](ClO 4 ) 2 was investigated, where X-pybox stands for 4-X-substituted 2,6-bis(oxazolin-2-yl)pyridine with X = H, Cl, Ph, CH 3 O, and CH 3 S. We confirmed that the mother compound [Fe(H-pybox) 2 ](ClO 4 ) 2 underwent SCO above room temperature. After X was introduced, the SCO temperatures (T 1/2 ) were modulated as 310, 230, and 330 K for X = Cl, Ph, and CH 3 S, respectively. The CH 3 O derivative possessed the high-spin state down to 2 K. Crystallographic analysis for X = H, Cl, CH 3 O, and CH 3 S was successful, being consistent with the results of the magnetic study. Distorted coordination structures stabilize the HS (high-spin) state, and the highest degree of the coordination structure distortion is found in the CH 3 O derivative. A plot of T 1/2 against the Hammett substituent constant σ p showed a positive relation. Solution susceptometry was also performed to remove intermolecular interaction and rigid crystal lattice effects, and the T 1/2 's were determined as 260, 270, 240, 170, and 210 K for X = H, Cl, Ph, CH 3 O, and CH 3 S, respectively, in acetone. The substituent effect on T 1/2 became very distinct, and it is clarified that electron-donating groups stabilize the HS state.

Substitution reactions of propargylic amines proceed in the presence of copper(I) catalysts. Mechanistic studies showed that C(sp)-C(sp(3)) bond cleavage assisted by nitrogen lone-pair electrons is essential for the reaction, and the resulting iminium intermediates undergo amine exchange, aldehyde exchange, and alkyne addition reactions. Because iminium intermediates are key to aldehyde-alkyne-amine (A(3)) coupling reactions, this transformation is effective not only for reconstruction of propargylic amines but also for chiral induction of racemic compounds in the presence of chiral catalysts.

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[Fe II (LH iPr)2(NCS)2] (LH iPr = N-2-pyridylmethylene-4-isopropylaniline) showed an abrupt spin-crossover (SCO) at T1/2↓ = 154 K on cooling and at T1/2↑ = 167 K on heating. The thermal hysteresis with a width of 13 K is related with the structural solid-state phase transition. The space group was unchanged as P21/n with Z = 8, and there are two crystallographically independent molecules in a unit cell at 130 and 180 K. The two iron (II) sites synchronously underwent the SCO. The most drastic structural change across the SCO was found in the conformation isomerization of an isopropyl group. Namely, rotation around the C(sp 2)-C(sp 3) bond by ca. 120° takes place during the SCO. There is no structural disorder in the high-temperature phase. The thermal hysteresis probably originates in the bulk isomerization requiring considerable activation energy in the crystalline solid.

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Substitution reactions of propargylic amines using various secondary amines proceed in the presence of a copper(I) catalyst.