NH/π Interaction in a Spin-Crossover Complex [FeII(HLMe)2](BPh4)2·2CH3CN (BPh4- = Tetraphenylborate; HLMe = 2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine)

Analysis of Ion Pairing in Solid State and Solution in p-Cymene Ruthenium Complexes

The importance of ion pairing in different fields of chemistry is widely recognized. In this work, we have synthesized a set of cationic p-cymene ruthenium complexes of general formula [(p-cym)Ru(L')(κ²-O^N-L)]X (p-cym = p-cymene; L' = N-methylimidazole (MeIm), N-ethylpiperidylimidazole (EpipIm), 1,3,5-triaza-7-phosphaadamantane (PTA); L = 2-(1H-benzimidazol-2-yl)phenolato (L1), 2-(1,3-benzothiazol-2-yl)phenolato (L2); X = Cl^-, BF₄^-, OTf^-, BPh₄^-). X-ray diffraction studies on selected complexes revealed relatively strong anion-cation interactions in the solid state mainly based on N-H···X (X = Cl, F, O) and C-H···π interactions, also observed in the DFT-modeled complexes in the gas phase. Moreover, NMR studies showed that they exist as intimate ion pairs in solution and, remarkably, as head-to-tail quadruples in the particular case of the cation [(p-cym)Ru(MeIm)(κ²- O^N-L1)]⁺ ([1]⁺) with Cl^- and BPh₄^- as counteranions. Furthermore, a value of ΔG = -2.9 kcal mol^-1 at 299 K has been estimated for the equilibrium {[1]BPh₄···[1]BPh₄} ⇆ 2{[1]⁺···BPh₄^-} in concentrated CDCl₃ solutions. In addition, preliminary studies concerning the cytotoxic properties against HeLa cell lines of the derivatives suggested a positive effect derived from the presence of the lipophilic BPh₄^- anion and also from the NH group of the benzimidazolyl fragment.

Spin switching in tris(8-aminoquinoline)iron(ii)(BPh4)2: quantitative guest-losing dependent spin crossover properties and single-crystal-to-single-crystal transformation

As a derivative of 2-picolylamine, which contains rich protons favouring hydrogen bond formation to assemble a variety of valuable spin crossover (SCO) compounds, 8-aminoquinoline (aqin) will be a good candidate for constructing new mononuclear bistable state compounds. With the guidance of this view, two solvated compounds [Fe(aqin)3](BPh4)2·2(CH3CN) (1·2CH3CN) and [Fe(aqin)3](BPh4)2·1.5(CH3COCH3) (2·1.5CH3COCH3) were synthesized. The structural characterization and magnetic studies demonstrate that this strategy has been successful. Single-crystal diffraction reveals that both the mononuclear compounds have facial (fac-)-configuration cations, which form hydrogen bonds using -NH2 groups with solvent molecules (acetonitrile or acetone). Subsequent magnetic measurement shows the highly sensitive solvent-dependent occurrence of a spin transition above room temperature for both compounds. Interestingly, for compound 1·2CH3CN, in the successively repeated heating and cooling process, by monitoring the loss of solvent molecules by TGA, the shifting of the spin transition curve is found to be linearly dependent on the fraction of the residual solvent content. Additionally, the desolvated sample can re-solvate with CH3CN and recover the magnetic response reproducibly. Furthermore, after losing the acetonitrile molecules, the single-crystal-to-single-crystal transformation occurred to give 1.

Synthesis, structure, and spin crossover above room temperature of a mononuclear and related dinuclear double helicate iron(ii) complexes

A mononuclear iron(ii) complex, [Fe^II(L1^Me)₂](PF₆)₂ (1), and a related dinuclear one, [Fe(L2^C2)₂](PF₆)₄·5H₂O·MeCN (2), were prepared, where L1^Me is a tridentate N₃ coordinating Schiff-base ligand, 1-methyl-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and L2^C2 is a dimeric form ligand of L1^Me, 1,1'-(1,2-ethanediyl)bis-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and their magnetostructural relationships were investigated. Mononuclear 1 shows a gradual one-step spin crossover (SCO) from the low-spin (LS) state (S = 0) at room temperature (RT) to the mostly high-spin (HS) state (S = 2) at 470 K spanning a temperature range of more than 170 K, while dinuclear 2 shows an anomalous two-step SCO with a gradual transition in a first step from the LS-LS state at RT to around 432 K and then an abrupt spin transition centred at 432 K with 11 K wide hysteresis in a second step (T_1/2↑ = 437 K and T_1/2↓ = 426 K) reaching the mostly HS-HS state. Single crystal X-ray structure analyses revealed that the iron(ii) centre of mononuclear 1 has an N6 octahedral coordination environment with two tridentate L1^Me ligands, while the complex-cation of 2 has a dinuclear double helicate architecture as though a dimeric form of two complex-cations of 1 replaced their methyl group with a bridging ethylene chain. Both complexes have a cationic two-dimensional (2D) structure in which the 2D layer of mononuclear 1 is constructed of intermolecular CHN hydrogen bonds between adjacent complex-cations, while that of dinuclear 2 is constructed of both intermolecular π-π and CH/π interactions between neighboring helicates.

Two-Dimensional Iron(II) Spin Crossover Complex Constructed of Bifurcated NH···O- Hydrogen Bonds and π−π Interactions: [FeII(HLH,Me)2](ClO4)2·1.5MeCN (HLH,Me = Imidazol-4-yl-methylidene-8-amino-2-methylquinoline)

A 2D iron(II) spin crossover complex, [FeII(HLH,Me)2](ClO4)2.1.5MeCN (1), was synthesized, where HLH,Me = imidazol-4-yl-methylidene-8-amino-2-methylquinoline. 1 showed a gradual spin transition between the HS (S = 2) and LS (S = 0) states from 180 to 325 K within the first warming run from 5 to 350 K, in which 1.5MeCN is removed, and there was an abrupt spin transition at T1/2 downward arrow = 174 K in the first cooling run from 350 to 5 K. Following the first cycle, the compound showed an abrupt spin transition at T1/2 upward arrow = 185 K and T1/2 downward arrow = 174 K with 11 K wide hysteresis in the second cycle. The crystal structures of 1 were determined at 296 (an intermediate between the HS and LS states) and 150 K (LS state). The structure consists of a 2D extended structure constructed of both the bifurcated NH...O- hydrogen bonds between two ClO4- ions and two neighboring imidazole NH groups of the [FeII(HLH,Me)2]2+ cations and the pi-pi interactions between the two quinolyl rings of the two adjacent cations. Thermogravimetric analysis showed that solvent molecules are gradually eliminated even at room temperature and completely removed at 369 K. Desolvated complex 1' showed an abrupt spin transition at T1/2 upward arrow = 180 K and T1/2 downward arrow = 174 K with 6 K wide hysteresis.

Spin Crossover in a Series of Iron(II) Complexes of 2-(2-Alkyl-2H-tetrazol-5-yl)-1,10-phenanthroline: Effects of Alkyl Side Chain, Solvent, and Anion

2-(2H-Tetrazol-5-yl)-1,10-phenanthroline (HL0), its alkyl-substituted derivatives (Ln, where n = 1-8, 10, 12, 14, and 16, denoting the carbon atom number of the alkyl chain) at the 2H position of the tetrazole ring, and their iron(II) complexes (a for [Fe(L0)2], na for [Fe(Ln)2](ClO4)2, and nb for [Fe(Ln)2](BF4)2) were synthesized and characterized. The crystal structures of a, a.CH3OH, 1a.CH3OH, 1b.CH3OH.CH3CN, 2a.H2O, 2b.H2O, 4b.CH3OH, 5a.H2O, 5b.H2O, 6a, 6b, 7a, 7b, and 16a are described, along with thermal analyses. a undergoes an abrupt spin crossover (SCO) at 255 K with a hysteresis loop of 6 K. a.CH3OH, 2a.H2O, and 2b.H2O exhibit irreversible SCO behaviors due to the loss of solvent molecules upon heating. 3a, 3b, 4a, and 5a.H2O show simple spin transitions above 350 K. The desolvated samples of 4b.CH3OH and 5b.H2O undergo two-step spin transitions. 16a exhibits a two-step SCO behavior between 100 and 300 K, corresponding to sequential phase transitions from the low-spin (LS) phase to the intermediate phase and then to the high-spin phase, respectively, proved by crystal structure analysis and 57Fe Mössbauer spectroscopy. 1a.CH3OH, 10a, 10b, 12a, 12b, 14a, 14b, and 16b show gradual and incomplete SCO behaviors after cooling down from 400 K. 1b.CH3OH.CH3CN, 6a, 6b, 7a, 7b, 8a, and 8b remain in the LS state even at 400 K. This proves that the alkyl side chains, together with the solvent molecules and anions, play a crucial role in the complicated SCO behaviors in this system.

Structural−Electronic Correlation in the First-Order Phase Transition of [FeH2L2-Me](ClO4)2 (H2L2-Me = Bis[((2-methylimidazol-4-yl)methylidene)-3- aminopropyl]ethylenediamine)

The synthesis and detailed study of the new mononuclear spin crossover complex [Fe(II)H2L(2-Me)](ClO4)2 (where H2L(2-Me) = bis[((2-methylimidazol-4-yl)methylidene)-3-aminopropyl]ethylenediamine) are reported. Variable-temperature magnetic susceptibility measurements show the occurrence of a steep spin crossover centered at 171.5 K with a hysteresis loop of ca. 5 K width (T(/2)(increasing) = 174 K and T(1/2)(decreasing) = 169 K, for increasing and decreasing temperatures, respectively). The crystal structure has been resolved for the high-spin (HS) and low-spin (LS) states at 200 and 123 K, respectively, revealing a crystallographic phase transition that occurs concomitantly to the spin crossover: at 200 K, the complex crystallizes in the monoclinic system, space group P2(1)/n, while the space group is P2(1) at 123 K. The mean Fe-N distances are shortened by 0.2 A, but the thermal spin crossover is accompanied by significant structural changes: the rearrangement of the central atom C12 of a six-membered chelate ring of [Fe(II)H2L(2-Me)]2+ to two positions (C12A and C12B) and, consequently, the lack of an inversion center at 123 K (P2(1) space group). Both HS and LS supramolecular structures involve all possible hydrogen bonds between imidazole and amine NH functions, and perchlorate anions; however, the HS supramolecular structure is a one-dimensional (1D) network, and the LS phase may better be described as a two-dimensional (2D) extended structure of A and B molecules. The structural phase transition of [FeH2L(2-Me)](ClO4)2 seems to trigger the steep and hysteretic spin crossover. Discontinuities in the temperature dependence of the Mössbauer parameters (isomer shift and quadrupole splitting) at the spin crossover temperature confirmed the occurrence of a structural phase transition. The experimental enthalpy and entropy variations were determined by differential scanning calorimetry (DSC) as 7.5 +/- 0.4 kJ/mol and 45 +/- 3 J K(-1) mol(-1), respectively. The regular solution theory was applied to the experimental data, yielding an interaction parameter of Gamma = 3.36 kJ/mol, which is larger than 2RT(1/2), which fulfills the condition for observing hysteresis.

Enantioselective Friedel−Crafts Alkylation of Indoles with Nitroalkenes Catalyzed by Bifunctional Tridentate Bis(oxazoline)−Zn(II) Complex

[reaction: see text] A more practical and efficient catalytic asymmetric Friedel-Crafts alkylation of indoles with nitroalkenes using bifunctional tridentate bis(oxazoline)-Zn(OTf)(2) as catalyst has been developed. Various types of the nitroalkylated indoles were obtained in excellent yields (85-99%) and high enantioselectivities (up to 98% ee).