Structures and Magnetic Properties of Iron(iii) Complexes with Long Alkyl Chains

Iron(III) compounds with long alkyl chains, [Fe(Cn-pap) 2 ]ClO 4 (Cn-pap: alkoxy-2-(2-pyridylmethyleneamino)phenol, (5)) have been synthesized. The compounds were characterized by single crystal X-ray structure analysis and temperature dependent magnetic susceptibility in order to research the relationship between magnetic properties and the presence of long alkyl chains in soft molecules of the present type. The compounds 1, 2, 3 and 4 are in the high-spin (HS) state over the temperature range of 5 to 400 K. On the other hand, compound 5 is low-spin (LS) showing that the difference in magnetic properties depends on the length of the alkyl chain in the respective compounds.


Introduction
Metal proteins and enzymes consist of flexible structures typified by peptide links surrounding active centers giving rise to "flexible space" which may play an important role in the biochemical function of such systems [1].In soft materials such as liquid crystals, gels and polymers, the presence of "flexible space" can be an important influence controlling the properties of such materials [2][3][4][5][6][7][8][9][10].This motivated us to construct metal complexes with attached long alkyl chains that would result in flexible space in conjunction with the presence of metal complex centers.Positioning a spin-crossover (SCO) metal complex within the flexible space formed by the long alkyl chains is expected to result in altered properties based on synergic interactions occurring between the SCO and its environment reflecting external stimuli.A variety of d n (n = 4-7) transition metal compounds exhibiting SCO between the high-spin (HS) and low-spin (LS) states have been reported [10].Gradual or abrupt SCO may be observed in the solid state, depending on the degree of intermolecular cooperativity present.Flexibility in molecular assemblies based on molecular units has been shown to be an important factor in achieving synergy and resulting in various interesting physical properties among advanced materials [2][3][4][5][6][7][8][9][10]. ) exhibit abrupt SCO behavior [11][12][13][14][15].We suggested that the strong intermolecular interactions (π-π stacking, hydrogen bonding and so on) are important for cooperativity.Although the π-conjugated system clearly plays an important role, in some cases the presence of long alkyl chains may also influence the magnetic behavior through intermolecular interaction.We have attempted to add long alkyl chains to iron(III) compounds with pap ligands.In this paper, we report about the iron(III) compounds [Fe(Cn-pap) 2 ]ClO 4 (n = 8 (1), 10 (2), 12 (3), 14 (4), 16 (5)).

Results and Discussion
The preparation of the ligands L1-L5 and the complexes 1-5 were essentially carried out according to the literature with minor modifications (Scheme 1) [14].
The compounds 1-5 were prepared and characterized by elemental analysis.Single crystals of 4 suitable for X-ray structural analysis were obtained by slow recrystallization of the initial product from MeOH:CHCl 3 (1:1).Single-crystal X-ray analysis of 4 was successfully carried out at 143 K (Figure 1) [16].An ORTEP view of compound 4 is shown in Figure 1a.X-ray crystallographic data for 4 are given in Table 1.Complex 4 crystallizes in the space group P-1.The iron(III) ions are octahedrally coordinated by four nitrogen atoms and two oxygen atoms from two C14-pap ligands, i.e., an N 4 O 2 donor set.The Fe-N distances, the Fe-O distances, the N-Fe-N angles, the N-Fe-O angles and the O-Fe-O angles for 4 are shown in Table 1.The Fe(1)-N(1) and Fe(1)-N(3) axial bond distances, 2.109 (2)  The temperature dependences of the magnetic susceptibility for the compounds 1-5 were measured in the form of the χ m T versus T curve, where χ m is the molar magnetic susceptibility and T is the temperature (Figure 2).The compounds 1-4 are in the HS state in the temperature range of 5-400 K.The χ m T value for 1 increased abruptly from 1.64 cm 3 •Kmol −1 at 5 K to 3.00 cm 3n •Kmol −1 at 65 K and increased gradually to 3.89 cm 3 •Kmol −1 at 400 K.The value at 400 K is smaller than that in iron(III) HS (4.38 cm 3 •Kmol −1 ), and it is proposed that an LS specie is mixed even at 400 K.The χ m T value for 2 increased abruptly from 3.32 cm 3 •Kmol −1 at 5 K to 4.16 cm 3 •Kmol −1 at 24 K and increased gradually Temperature/ K to 4.69 cm 3 •Kmol −1 at 400 K.The χ m T value for 3 increased abruptly from 2.54 cm 3 •Kmol −1 at 5 K to 3.39 cm 3 •Kmol −1 at 27 K and increased gradually to 4.00 cm 3 •Kmol −1 at 400 K.The value also indicates the mixture of a small amount of LS specie.The χ m T value for 4 increased abruptly from 4.01 cm 3 •Kmol −1 at 5 K to 5.55 cm 3 •Kmol −1 at 41 K and decreased gradually to 4.21 cm 3 •Kmol −1 at 400 K. Compound 4 shows weak ferromagnetic interactions between the complexes, it is proposed that the ferromagnetic interaction results from interchain interactions.Furthermore, the decrease in the χ m T value below 50 K is due to zero field splitting.On the other hand, the χ m T value for 5 increased gradually from 0.57 cm 3 •Kmol −1 at 5 K to 1.32 cm 3 •Kmol −1 at 400 K, in accord with compound 5 being in the LS state at room temperature.This compound has the longest alkyl chain (C16), and hence the strongest interchain interaction can be expected, in accord with it adopting the LS state.

Experimental Section
All reagents were obtained commercially (Wako, Kumamoto, Japan) and were used in the form that they were received.The reactions were performed under N 2 .

Synthesis of 2-Pyridylmethanol Acetate
5-Hydroxy-2-methylpyridine N-oxide (6.60 g, 0.05 mol, 1 eq.) was added slowly to (CH 3 CO) 2 O (65 mL) at 110 °C.The resultant dark brown reaction mixture was stirred for 2 h at 140 °C.EtOH (150 mL) was added and the solution was concentrated in vacuo to yield a brown oil.CHCl 3 (40 mL) was added and the solution was neutralized with saturated NaHCO 3 .The organic layer was collected and washed with saturated NaHCO 3 (30 mL × 2), dried (MgSO 4 , q.s.) and filtered.The solvent was removed in vacuo to yield a brown oil, which was further dried under high vacuum for 2 h.Without purification the product was used for next reaction.

Synthesis of 5-Hydroxy-2-Hydroxymethylpyridine
A solution of 2-pyridylmethanol acetate (q.s.) in conc.HCl (10 mL) was heated under reflux for 1.5 h.The reaction mixture was cooled to room temperature and concentrated in vacuo.CHCl 3 (40 mL) was added and the solution neutralized carefully with saturated NaHCO 3 .The aqueous layers were collected through washings with saturated NaHCO 3 and concentrated in vacuo.The brown solid was triturated with MeOH and filtrates collected through filtration.The solvent was removed in vacuo yielding a brown solid (3.20 g). 1 H-NMR (CDCl 3 ): 7.85 (d 1H) 7.18 (d 1H) 7.01 (dd 1H) 4.51 (s 2H).

Synthesis of
5-(n-Alkoxy)pyridine-2-carbaldehyde (1 eq) and 2-aminophenol (1 eq) were stirred in MeOH.A solution color change from yellow to orange was observed.The solution was concentrated in vacuo and left in a refrigerator for several hours.The precipitate was collected by vacuum filtration.The magnetic properties were determined by temperature-dependent susceptibility measurement with a Superconducting Quantum Interference Device (SQUID) magnetometer at field strengths of 1 T.

Conclusions
[Fe(Cn-pap) 2 ]ClO 4 (n = 8, 10, 12, 14, 16) have been synthesized and characterized.The single crystal X-ray analysis for compound 4 showed that there is fastener effect between long alkyl chains of neighboring molecules.The compounds 1-4 are in the HS states and compound 5 is in the LS state.Furthermore 4 shows weak ferromagnetic interaction between individual complexes in accord with the presence of a cooperative interaction being an important factor for inducing such a magnetic interaction.

Table 1 .
Crystal parameter, and bond lengths and angles for compound 4.