Mononuclear and dinuclear complexes of manganese(III) and Iron(III) supported by 2-salicyloylhydrazono-1,3-dithiane ligand: synthesis, characterization and magnetic properties

Abstract

The coordination chemistry of 2-salicyloylhydrazono-1,3-dithiane (H₂L) was studied with manganese and iron ions. The following complexes have been isolated as crystalline materials, and their crystal structures have been determined by single crystal X-Ray crystallography: Mn^III(acac)(HL)₂ (1) (acac = acetylacetonate), Mn^III(HL)₃·CH₂Cl₂ (2), Fe^III(HL)₃·2CHCl₃ (3), Fe^II(H₂L)₂Cl₂·2CH₃OH (4), Fe^III₂(μ-OMe)₂(HL)₄·0.5CH₃OH (5), Fe^III₂(μ-O) (HL)₄·3CH₂Cl₂ (6). All attempts to synthesize the dinuclear μ-methoxo complex [Mn^III₂(μ-OMe)₂(HL)₄] have so far failed, even when the procedure used in the case of 2-salicyloylhydrazono-1,3-dithiolane (H₂L′) ligand, which worked very efficiently, was employed. The new iron dinuclear μ-methoxo complex (5) presented in this study shows antiferromagnetic intramolecular coupling (J = −21.1 cm⁻¹), which is in agreement with the theoretical study proposed previously for its manganese analogue.

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Introduction

The control of magnetic coupling in addition to the possibility of inducing a spin-state modification at the molecular scale is among the most challenging themes in the field of molecular magnetism. Considering a variety of magnetically interesting molecules,¹ special attention has been paid to exchange coupled multinuclear paramagnetic complexes.^2,3 Systems presenting lower nuclearity are of interest as, among others, versatile building blocks for cooperatively associated magnetic systems.⁴ For instance they can be incorporated in crystal lattices along with conducting molecules, which may lead to the formation of innovative magnetic/conducting bifunctional materials.⁵ Due to the dependence of coupling pathways on the structure and symmetry of the organic ligands, dinuclear molecular systems offer interesting possibilities to tune metal-to-metal interactions by subtle structural changes within the organic periphery. We have reported an interesting dinuclear Mn^III complex Mn^III₂ (μ-OMe)₂(HL′)₄, which was found to exhibit the largest J value (J = + 19.7 cm⁻¹) reported so far for a Mn^III–Mn^III interaction. The peculiarity of the Mn^III₂(μ-OMe)₂(HL′)₄ solid-state structure arises from an unsymmetrical arrangement of the ligands leading to orthogonality of the magnetic orbitals and consequently the ferromagnetic exchange coupling. This situation is certainly related to intramolecular non-classical H-bonds occurring between hydrogen atoms of the dithiolane rings and the centroids of the phenol groups.⁶ In addition to the Mn^III chemistry, we have also studied the coordination chemistry of the 2-salicyloylhydrazono-1,3-dithiolane ligand to other metal ions such as iron,^7,8cobalt⁹ and chromium¹⁰ to form a series of mononuclear and dinuclear complexes (Scheme 1). These complexes have shown interesting chemical and physical properties. For example, an unusual spontaneous reduction from Fe^III to Fe^II of the mononuclear complexes was observed.⁷ This redox reaction was subsequently fully investigated by electrochemistry, EPR studies, magnetic measurements and solid-state molecular structure determination. Such a phenomenon opens up new research opportunities, as recently been pointed out by Wernsdorfer et al.¹¹ and forms the basis of a patent.¹²

Scheme 1 Transition metal complexes of 2-salicyloylhydrazono-1,3-dithiolane ligand.

The asymmetric Fe^III₂(μ-OMe)₂(HL′)₄ complex has also been synthesized and characterized.⁷ This dinuclear Fe^III compound shows antiferromagnetic intramolecular coupling, which is in agreement with the previously proposed theoretical model for the related Mn^III complex.⁶ As predicted by DFT calculations, the Cr^III μ-methoxo dinuclear complex featuring the same ligand also exhibited a strong antiferromagnetic coupling.¹⁰

Via minor modifications of the chelating ligand, it is expected to control the structural geometry of the complexes obtained, as well as their physical properties. We have thus started to explore the coordination chemistry of 2-salicyloylhydrazono-1,3-dithiane (H₂L), which is closely related to H₂L′ where it has a 1,3-dithiane group in place of the 1,3-dithiolane moiety.^9,10 A dinuclear μ-methoxo Cr^III₂(μ-OMe)₂(HL)₄ complex, chelated by HL exhibiting a strong antiferromagnetic coupling, was successfully synthesized by the reaction of CrCl₃ with this ligand in the presence of zinc.¹⁰ In addition, one novel Co^II mononuclear complex and one diamagnetic μ-hydroxo dinuclear Co^III complex [Co^III₂(μ-OH)₂(HL)₄] supported by the ligand HL were also stabilized and characterized.⁹

The hypothesis that the similarity between the structures of H₂L and H₂L′ would allow HL to be a suitable ligand to stabilize a dinuclear μ-methoxo Mn^III complex, which is expected to exhibit very strong intramolecular ferromagnetic interaction, prompted us to extend the coordination chemistry of ligand H₂L to Mn^III. In this article, we report our attempts to synthesize the desired dinuclear μ-methoxo Mn^III complex stabilized by 2-salicyloylhydrazono-1,3-dithiane. In addition, following a systematic approach and due to their similar coordination behaviour, we also describe the synthesis and characterization of four iron mononuclear and dinuclear complexes supported by HL and H₂L, and the synthesis and magnetic property measurement of μ-methoxo dinuclear iron complex.

Results and discussion

Synthesis and characterization of manganese complexes Mn^III(acac)(HL)₂ (1) and Mn^III(HL)₃ (2)

Scheme 2 Synthesis of complexes Mn^III(acac)(HL)₂(1) and Mn^III(HL)₃ (2).

In an attempt to synthesize the dinuclear complex, the first trials were those applying the conditions used previously in our laboratory (Scheme 2).^6,13 This procedure resulted in two new mononuclear complexes described here after. The complex Mn^III(acac)(HL)₂ (1) obtained in a 67% yield by reacting 2-salicyloylhydrazono-1,3-dithiane (H₂L) with manganese acetylacetonate in THF. Good quality crystals were obtained by slow diffusion of methanol to the complex solution in chloroform. An ORTEP view with selected distances and angles is given in Fig. 1. The dark red complex 1 crystallizes in the monoclinic space groupP 2₁/c and consists of a mononuclear Mn^III species chelated by two ligands in their basic forms HL⁻, and the coordination sphere of the Mn^III atom being completed by one acetylacetonate molecule. The Mn^III atom adopts a slightly distorted octahedral geometry with the O5–Mn–O4, N2–Mn–N4 and O6–Mn–O2 bond angles of 176.59(8)°, 158.37(8)° and 179.63(8)°, respectively, which are very similar to those observed in Fe^III complex supported by 2-salicyloylhydrazono-1,3-dithiolane ligand.⁸ No specific intramolecular CH–π interaction has been detected, but two classical hydrogen bonds between the OH groups of the phenol rings and the hydrazine groups were detected as shown in Fig. 1 (dashed line). Only two examples of a similar coordination mode for Mn^III acetylacetonate compounds containing binaphthyl Schiff base¹⁴ and 1, 3-bis(salicy1ideneiminato)propane¹⁵ ligands can be found in the Cambridge Data base.

Fig. 1 ORTEP view of the complex 1 with partial labelling scheme. The ellipsoids enclose 50% of the electronic density. Selected distances (Å) and angles (°): Mn–N2 2.261(2), Mn–N4 2.280(2), Mn–O2 1.922(2), Mn–O4 1.921(2), Mn–O5 1.912(3), Mn–O6 1.908(2); O5–Mn–O4 176.59(8), N2–Mn–N4 158.37(8), O6–Mn–O2 179.63(8). Dashed lines indicate intramolecular hydrogen bonds.

The complex Mn^III(HL)₃·CH₂Cl₂ (2) was obtained in 31% yield by reacting three equivalents of H₂L with manganese acetate dihydrate in methanol. Good quality crystals (but very sensitive to desolvation) were obtained by slow diffusion of pentane to a solution of the complex in dichloromethane (Scheme 1). In contrast to the case of 2-salicyloylhydrazono-1,3-dithiolane,⁶ and in spite of addition of excess base (NaOAc, NEt₃etc.) we were unable to generate the μ-methoxo dinuclear complex Mn^III₂(μ-OMe)₂(HL)₄.

Fig. 2 ORTEP view of the complex 2·CH₂Cl₂ with partial labelling scheme. The ellipsoids enclose 50% of the electronic density. The solvent CH₂Cl₂ was omitted for clarity. Selected distances (Å) and angles (°): Mn–N2 2.294(3), Mn–N4 2.088(4), Mn–N6 2.259(3), Mn–O2 1.922(3), Mn–O4 1.892(3), Mn–O6 1.894(3); O4–Mn–O6 177.3(2), O2–Mn–N4 167.3(1), N2–Mn–N6 161.5(1).

The dark red complex 2·CH₂Cl₂ crystallizes in the triclinic centrosymmetric space group P1̄ . The crystal structure (Fig. 2) shows that the Mn^III ion is surrounded by three anionic bidentate HL⁻ ligands, forming a distorted octahedral geometry of the metal centre. All distances are in agreement and very similar, to those measured in complex 1 and with other manganese complexes containing 2-salicyloylhydrazono-1,3-dithiolane ligand.¹³ As already observed in complex 1, the intramolecular O–H⋯N hydrogen bonds are established between OH groups of the phenol rings and the hydrazine groups. Besides the previously reported manganese complex supported by three 2-salicyloylhydrazono-1,3-dithiolane ligands,⁶ another very similar structure was observed in Mn^III complex bearing three N′-benzylidenesalicylhydrazide ligands.¹⁶

Attempts to synthesize Mn^III₂(μ-OMe)₂(HL)₄. Based on the observation that the μ-methoxo dinuclear complex^7,8 is usually unstable in solution and probably dissociates into mononuclear entities, which in the solid state again tend to assemble into the well-organized μ-methoxo dinuclear structure, we initially focused our efforts on crystallization conditions which could promote the condensation of two mononuclear entities to form the dinuclear complex. The synthetic methods (as shown in Scheme 2) of the mononuclear entities are the same that those used in the case of ligand H₂L′, where successful generation of the desired dinuclear μ-methoxo complex was observed.⁶ However, in the current case with H₂L ligand, only single crystals of 2 were obtained by slow diffusion of pentane into a solution of CH₂Cl₂. Combinations of all solvents and non-solvents listed above (Table 1) have been tested on different proportions using slow liquid or vapour diffusion, but all failed to obtain a crystal of the desired dinuclear complex.

Table 1 Solvents and non-solvents tested for the crystallization of dinuclear μ-methoxo complex with ligand H₂L

Solvents	Non-solvents
CHCl3	MeOH
CH2Cl2	EtOH
ClCH2CH2Cl	^i PrOH
DMF	Diethyl ether
THF	H2O

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The second attempt to synthesize the desired dinuclear complex was based on the assumption that there were little differences between the structures of the complex Mn^III₂(μ-OMe)₂(HL′)₄ and the desired structure Mn^III₂(μ-OMe)₂(HL)₄. Therefore, some tests have been set up to force the crystalline growth of Mn^III₂(μ-OMe)₂(HL)₄ instead of the complex 2. To do this, diverse proportions of powder from the reaction between two equivalents of HL′ with manganese(III) acetate dihydrate were added to a chloroform solution of the powder obtained from the same reactions with HL, followed by slow diffusion of methanol into this mixed solution. However, unfortunately the nucleation of the complex Mn^III₂(μ-OMe)₂(HL′)₄ did not seem to encourage the growth of Mn^III₂(μ-OMe)₂(HL)₄.

Synthesis and characterization of iron complexes

Following a systematic procedure and taking in consideration some shared characteristics of iron and manganese, we synthesized in parallel, with the same ligand four iron complexes using commercial crystalline FeCl₃ and Fe(acac)₃ as metal sources.

The aim was to examine the possibility to obtain well-defined molecular complexes of Fe^III bearing a H₂L ligand and to compare their respective reactivity towards the same ligand.

Synthesis of mononuclear complexes. The synthesis and crystallization methods of all the iron complexes are shown in Scheme 3. The direct reaction of three equivalents of the ligand H₂L with one equivalent of FeCl₃ in methanol in the absence of base affords a green powder. Recrystallization of the latter solid in CHCl₃ led to the formation of red rectangular crystals of the mononuclear Fe^III complex 3·2CHCl₃ in a good yield. The reaction of ligand H₂L (2 equivalents) with iron chloride in DMF as solvent affords a dark blue solution. A slow diffusion of MeOH in a sealed glass tube under a N₂ atmosphere led to a colour change from dark blue to yellow after one day and yellow single crystals of 4 (suitable for X-ray studies) were found to be formed at the bottom of tube after 3 or 4 days. Similar to our previous observation with ligand H₂L′,⁷ it was assumed that this dark solution consists of a mononuclear Fe^III complex with two chelating HL⁻ ligands, DMF and Cl⁻ in the coordination sphere of the iron (the analogue of III in Scheme 1). Unfortunately, we did not succeed in crystallizing this dark blue complex.

Scheme 3 Synthetic methods for the iron complexes 3, 4, 5 and 6.

Synthesis of dinuclear species. By using our previously reported procedure,⁷ complex 5 was synthesized by the reaction of the ligand H₂L with Fe(acac)₃ in DMF, followed by slow diffusion of methanol into a THF solution of the product. Similarly, it was hypothesized that the formation of complex 5 occurs via a non isolated mononuclear species [Fe^III(L)₂L₁L₂], which possess two deprotonated 2-salicyloylhydrazono-1,3-dithiane ligands and two additional coordination moieties (L₁ and L₂). L₁ and L₂ are probably remaining acetylacetonate ligand from the metal precursor or the methoxy group and a molecule of solvent coordinated.⁷ The reaction of 2 equivalents of the ligand with Fe(acac)₃ in methanol generated black precipitates, which can be crystallized in a mixture of CH₂Cl₂–Et₂O to yield dark green crystals of 6 in a relatively low yield. Dinuclear iron complexes with oxo bridges have been extensively studied mainly due to their special physical properties and their relevance as intermediates in catalytic reactions, and a large number of μ-oxo-bridged diiron complexes have been structurally characterized. Similar structures to complex 6, reported in literature, mainly bear ligands of salen,^17–21salicylaldiminato²² and N,N′-o-phenylenebis(oxamate)²³ type. In many cases, combining a less bulky ligand with Fe^III precursors and an excess of base in methanol as solvent, was reported to yield μ-oxo bridged diiron^III complexes where the O bridge is probably originated from the methanol solvent.^{17–19,21,23,24} The oxidation of Fe^II to Fe^III by air dioxygen is an alternative method to synthesize μ-oxo bridged diiron^III complexes.^17,20,22,23 In addition to methanol, water can also act as the source of the bridging O.^21,25–27 Based on the above analysis of the possible oxygen bridge origins reported in the literature, we assumed that the O bridge in complex 6 could probably come from some residual water present in the methanol solvent employed in the reaction.

Crystal structures description. The structures of the four iron complexes (3–6) have been completely characterized by single-crystal X-ray diffraction (see Table 2).

Complex 3·2CHCl₃ crystallizes in the triclinic space group P1̄. The Fe^III centre has a [N₃O₃] coordination sphere with a pseudo-octahedral geometry. The ligands HL⁻ are coordinated to the iron as bidentate chelating agents via the nitrogen atoms (N2, N4, N6) and the carboxyl oxygen atoms (O2, O4, O6) of the hydrazones in a propeller configuration fashion. The arrangement of the 2-salicyloylhydrazono-1,3-dithiane ligands in this iron complex is similar to the analogous manganese complex 2. The Fe–N distances [2.123(3), 2.146(3) and 2.243(3) Å] and Fe–O distances [1.984(2), 1.957(2) and 1.952(2) Å] are similar to those found in the previously reported Fe^III complex supported by 2-salicyloylhydrazono-1,3-dithiolane ligand.⁷ As shown in Fig. 3, the hydrogen bonding is donated from phenol O–H to a neighbouring nitrogen atom of the hydrazine group in each ligand. Several Fe^III complexes containing an [N₃O₃] coordination sphere and similar geometrical features have been reported in the literature with o-iminobenzosemiquinonato,²⁸o-aminophenol²⁹ and 8-quinolinato.³⁰

Fig. 3 ORTEP view of the complex 3·2CHCl₃ with partial labelling scheme. The ellipsoids enclose 50% of the electronic density. The solvent molecules (CHCl₃) were omitted for clarity. Selected distances (Å) and angles (°): Fe–N2 2.123(3), Fe–N4 2.146(3), Fe–N6 2.243(3), Fe–O2 1.984(2), Fe–O4 1.957(2), Fe–O6 1.952(2); O6–Fe–O4 165.7(1), N2–Fe–N4 158.5(2), O2–Fe–N6 165.3(1), O2–Fe–N2 76.7(1), O6–Fe–N6 74.7(1), O4–Fe–N4 76.6(1).

Complex 4·2CH₃OH crystallizes in the monoclinic centrosymmetric space groupP 2₁/c. As shown in Fig. 4, the compound consists of a mononuclear complex of Fe^II. The presence of hydrogen atoms on the N1 and N1′ nitrogen atoms (clearly visible by Fourier differences and by the special conformation of the hydroxyl groups) indicates that both bidentate ligands are neutral. Considering both chloride atoms (−I oxidation state) complete the octahedral geometry, we consider that the iron cation, exhibit a +II oxidation state. In 4, the two bidendate H₂L ligands are in a trans configuration, and the overall structure appears to be very similar to its analogues bearing benzoic hydrazide derivative ligands recently reported.^7,31 In addition, the intramolecular H-bonds in each ligand (dashed lines in Fig. 4) were also detected and they are probably responsible for the quasi-planar orientation of the ligands. Complex 4 constitutes then a new example of iron(II) compound obtained via spontaneous reduction, which was already observed with the ligand H₂L′ and related ligands as described in ref. 7 and 31.

Fig. 4 ORTEP view of the complex 4·2CH₃OH with partial labelling scheme. The ellipsoids enclose 50% of the electronic density. Selected distances (Å) and angles (°): Fe–N2 2.292(2), Fe–O2 2.134(1), Fe–Cl 2.3995(5); O2–Fe–N2 73.15(4), Cl–Fe–Cl′ 180, O2–Fe–O2′ 180. Symmetry operator for equivalent positions: ′ = −x + 2, −y, −z + 2.

Single crystals of complex 5·1/2CH₃OH suitable for X-ray diffraction analysis were obtained by slow diffusion of methanol into a THF solution of the solid. Complex 5·1/2CH₃OH crystallize in the orthorhombic space groupA b a 2. Exhibiting very similar structural features as its Fe^III,⁷Cr^III,¹⁰ and Mn^III6 analogues, complexes 5 crystallizes as a neutral asymmetric dinuclear Fe^III complex in which each iron metal centre is chelated by two HL⁻ bidentate ligands and connected to each other via two μ-OMe groups (Fig. 5). The two metal ions Fe1 and Fe2 (which both lie on a crystallographic twofold axis) are in slightly distorted octahedral geometries but with different environments. Around Fe1, the oxygen atoms O2–O2′ are in a trans configuration and N2–N2′ in a cis configuration, while around Fe2, the oxygen atoms O4–O4′ are cis to one another and N4–N4′ in a trans configuration. As previously pointed out, this situation is most likely related to two intramolecular CH–π interactions arising from one 1,3-dithiane CH moiety and the centroid of the neighbouring phenol group, as indicated in Fig. 5. Finally, considering our systematic research on such dinuclear μ-OMe metal complexes, a complete magnetic study of 5 was carried out, and the results are described below.

Fig. 5 ORTEP view of the complex 5·1/2CH₃OH with partial labelling scheme. H atoms and solvent molecules (CH₃OH) are omitted for clarity. The ellipsoids enclose 50% of the electronic density. The dashed lines show representative intramolecular hydrogen bonds and the arrows represent the CH–π interactions. Selected distances (Å) and angles (°): Fe1–N2 2.196(3), Fe1–O2 1.972(2), Fe1–O5 1.975(2), Fe2–O5 1.994(2), Fe2–O4 1.977(2), Fe2–N4 2.154(3); O2–Fe1–O2′ 174.1(2), N2–Fe1–N2′ 95.2(2), N4–Fe2–N4′ 160.3(2), O4–Fe2–O4′ 98.7(2), Fe1–O5–Fe2 102.44(9). Symmetry operator for equivalent positions: ′ = −x + 1, −y, z.

Complex 6·3CH₂Cl₂ crystallizes in the monoclinic space groupP 2₁/c (Fig. 6). The molecule contains a Fe–O–Fe core and the coordination geometry around each iron atom is described as distorted square pyramidal with the two hydrazine nitrogen atoms and the two carboxyl oxygen atoms of each ligand constituting the corners of the square plane and the μ-oxo oxygen atom occupying the axial position. No residual peak is detected in the Fourier difference map indicating unambiguously the absence of the hydrogen atom on the oxo oxygen atom. Also, the FeN₂O₂ coordination plane in each iron^III is oriented trans to the other relative to the oxo bridge, having a Fe–O–Fe bond angle of 167.1(2)°, which is in the normal range (135–180°) found in other closely related μ-oxo monobridged diiron complexes, where this angle is usually dominated by the steric parameters of the chelated ligands.^19,32–35 The Fe⋯Fe distance in 6 (3.475 Å) is in the same range as those already reported for complexes with Fe–O–Fe cores (3.39–3.56 Å).^17,24,33,36 The Fe–(μ-O) bond lengths observed [Fe1–O9 1.747(3), Fe2–O9 1.750(3) Å] are in the range found for other (μ-oxo)diiron^III complexes (1.75–1.80 Å)^{17,20,33,37–39} but significantly shorter than that found in Fe–(μ-CH₃O) of complex 5 (1.975(2) Å). Similar to complex 5, each iron atom is bound to two bidendate ligands in their basic forms and the geometry of the hydrazine chelating is comparable to those observed in either the mononuclear 3 or dinuclear μ-methoxo complex 5. Both the Fe–O(carboxyl) and Fe–N(hydrazine) bond distances are shorter than their counterparts in complex 5, probably due to the lower steric pressures at the metal centres in complex 6 than those in 5. No specific intramolecular CH–π interaction has been detected and only classical H–bonds occur between the OH groups of the phenol rings and the hydrazine groups.

Fig. 6 ORTEP view of the complex 6·3CH₂Cl₂ with partial labelling scheme. H atoms and solvent molecules (CH₂Cl₂) were omitted for clarity. The ellipsoids enclose 50% of the electronic density. The dashed lines show representative intramolecular hydrogen bonds and the arrows represent the CH–π interactions. Selected distances (Å) and angles (°): Fe1–N2 2.135(3), Fe1–O2 1.948(3), Fe1–N4 2.131(3), Fe1–O4 1.946(3), Fe1–O9 1.747(3), Fe2–O9 1.750(3), Fe2–N6 2.126(3), Fe2–O6 1.960(3), Fe2–N8 2.120(3), Fe2–O8 1.954(3); N4–Fe1–N2 157.6(2), O4–Fe1–O2 119.2(1), Fe1–O9–Fe2 167.1(2), N8–Fe2–N6 161.0(2), O8–Fe2–O6 119.8(2).

Table 2 Crystal data and X-ray structure refinement parameters at 173 K for all complexes

Compound reference	1	2·CH2Cl2	3·2(CHCl3)	4·2(CH3OH)	5·1/2(CH3OH)	6·3(CH2Cl2)
Chemical formula	C27H29MnN4O6S4	C33H33MnN6O6S6·CH2Cl2	C33H33FeN6O6S6·2(CHCl3)	C22H24Cl2FeN4O4S4·2(CH40)	C46H50Fe2N8O10S8·1/2(CH3O)	C44H44Fe2N8O9S8·3(CH2Cl2)
Formula Mass	688.72	941.88	1096.60	727.53	1258.64	1451.83
Crystal system	Monoclinic	Triclinic	Triclinic	Monoclinic	Orthorhombic	Monoclinic
a/[Å]	11.712(5)	10.429(2)	11.227(3)	10.394(4)	23.038(2)	12.8070(2)
b [Å]	12.670(5)	11.778(2)	13.922(7)	9.405(4)	10.461(6)	32.3360(8)
c [Å]	20.965(5)	17.263(3)	15.914(6)	17.332(3)	22.989(6)	15.2010(3)
α [°]	90.00	105.39(5)	69.021(2)	90.00	90.00	90.00
β [°]	98.49(1)	91.37(5)	89.488(2)	114.676(19)	90.00	105.1750(11)
γ [°]	90.00	90.35(5)	77.920(2)	90.00	90.00	90.00
V [Å^3]	3076.9(19)	2043.6(6)	2265.1(15)	1539.58(95)	5540(4)	6075.6(2)
Space group	P21/c	P1̄	P1̄	P21/c	Aba2	P21/c
Z	4	2	2	2	4	4
μ (Mo-Kα) [mm^−1]	0.748	0.812	1.014	0.980	0.888	1.075
F(000)	1424	968	1118	752	2602	2968
Crystal size [mm]	0.13 × 0.11 × 0.10	0.20 × 0.14 × 0.12	0.15 × 0.12 × 0.10	0.10 × 0.10 × 0.10	0.10 × 0.10 × 0.10	0.14 × 0.12 × 0.10
θ min–θmax	1.76–30.03	1.22–30.04	1.69–27.51	2.16–30.01	1.77–30.03	1.26–27.48
Data set [h, k, l]	−15/16, −15/17, −23/29	−14/14, −16/16, −21/24	−14/14, −18/16, −20/20	−12/14, −13/11, −24/24	−32/31, −14/14, −29/32	−16/11, −38/41, −19/19
Total, unique data, Rint	23 981, 8992, 0.0503	24 068, 11 916, 0.0375	19566, 10304, 0.0420	14 463, 4496, 0.0773	25 825, 7431, 0.0592	37 050, 13 388, 0.0656
Observed data [I > 2σ(I)]	5256	8810	7683	3713	5424	8532
No. reflections, no. parameters	8992, 379	11 916, 490	10 304, 541	4496, 191	7431, 350	13 388, 721
Final R1 values (I > 2σ(I))	0.0535	0.0847	0.0545	0.0395	0.0451	0.0576
Final wR(F^2) values (I > 2σ(I))	0.1308	0.1906	0.1156	0.1004	0.1106	0.1645
Final R2 values (all data)	0.1079	0.1139	0.0817	0.0504	0.0736	0.1034
Final wR(F^2) values (all data)	0.1543	0.2064	0.1336	0.1070	0.1250	0.2001
Goodness of fit on F^2	1.007	1.068	1.048	1.070	1.007	1.082
Max. and av. shift/error	0.000/0.000	0.001/0.000	0.001/0.001	0.002/0.000	0.001/0.000	0.003/0.000
Min, max residual density [e Å^−3]	−0.711/0.603	−0.692/2.169	−1.611/1.787	−0.916/0.573	−0.532/0.588	−2.018/1.971

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Magnetic measurements of the μ-methoxo dinuclear iron complex 5

The magnetic property of complex 5 was investigated in the solid state in the 1.8–300 K temperature range and an applied field of 5 kOe (Fig. 7). The χT value at room temperature (6.62 emu K mol⁻¹) is smaller than the expected value for two high spin Fe^III (8.75 emu K mol⁻¹ assuming g = 2). Upon cooling, the χT product increases to a maximum then decreases continuously to almost 0 (0.027 emu K mol⁻¹ at 1.8 K). This indicates the occurrence of a relatively strong antiferromagnetic intramolecular interaction between the two Fe^III ions. There is a small contribution of an impurity that gives the sharp rise at the lowest end of temperatures. The data were fit using the following spin Hamiltonian where all parameters have their usual meaning and the spin operator S is defined as S = S_Fe1 + S_Fe2:

H = −JS_Fe1S_Fe2 + gβHS

To reproduce the data satisfactorily over the whole temperature region, including the small increase at the lowest temperatures, we consider a certain amount (ρ) of paramagnetic impurity (S_impure = 5/2). The fit leads the following values: J = −21.1(1) cm⁻¹, g = 2.06(1) and ρ = 1.38(5)% with a good agreement factor R = 3.15 × 10⁻⁵ where R = (χ_exp − χ_calc)²/χ_exp².

Fig. 7 χ (circles) as a function of temperature for 5 and dark line corresponds to the fit of the χ = f(T) curve (see text).

Conclusions

In this work, we have demonstrated that the reactions of 2-salicyloylhydrazono-1,3-dithiane (H₂L) with different iron and manganese metal precursors under various conditions resulted in a series of new mononuclear or dinuclear complexes in good yields. All new complexes have been fully characterized by single crystal X-ray diffraction. The asymmetric dinuclear Fe^III₂(μ-OMe)₂(HL)₄ complex shows strong antiferromagnetic intramolecular coupling, which agrees with the previously proposed theoretical model for the related Mn^III complex.⁶

Following the previous findings that dinuclear μ-methoxo Mn^III₂(μ-OMe)₂(HL′)₄ presents one of the strongest intramolecular ferromagnetic interactions, our aim was the modulation of the magnetic interaction by small modifications of the peripheral ligand justifying the use of the 2-salicyloylhydrazono-1,3-dithiane ligand (H₂L). The similarity between the structures of H₂L and H₂L′would allow H₂L to be a suitable ligand to stabilize a dinuclear μ-methoxo Mn^III complex, which is expected to also exhibit a very strong intramolecular ferromagnetic interaction. However, we failed in the synthesis of such desired complex although many attempts have been made. We are a bit puzzled as we have successfully synthesized and characterized the dinuclear μ-methoxo Fe^III, Mn^III and Cr^III complexes chelated by HL′⁻ and in addition, dinuclear Fe^III and Cr^III complexes supported by HL⁻ were also successfully produced. Attempts are currently made in our laboratory, applying different synthetic methods and with the support of theoretical calculations, to achieve a better understanding of the different reactivity of the manganese when compared with other metals.

Experimental

General procedures

All manipulations were performed under aerobic conditions, using reagents and solvents as received. IR spectra were recorded in the region 4000–400 cm⁻¹ on a Nicolet 6700 FT-IR spectrometer (ATR mode, diamond crystal). Elemental analysis was performed by the “Service de microanalyses”, Université de Strasbourg. The ligand 2-salicyloylhydrazono-1,3-dithiane was prepared according to the reported procedure.¹⁰

Synthesis of Mn^III(acac)(HL)₂ (1)

To a solution of 2-salicyloylhydrazono-1,3-dithiane (0.20 g, 0.74 mmol) in THF (20 mL) was added a solution of manganese acetylacetonate (0.13 g, 0.36 mmol) under stirring. The dark solution was further stirred at room temperature for 24 h, then the solvent was removed by vacuum and the resulting solid was washed with methanol and dried (yield: 0.17 g, 67%). Dark red single crystals suitable for X-ray analysis were obtained by slow diffusion of methanol in a chloroform solution of the complex. IR (pure, orbit diamond, cm⁻¹): 2361, 1517. Anal. Calcd. for C₂₇H₂₉MnN₄O₆S₄: C, 47.08; H, 4.24; N, 8.13. Found: C, 46.78; H, 4.17; N 7.79.

Synthesis of Mn^III(HL)₃·CH₂Cl₂ (2)

To a solution of manganese acetate dihydrate (0.10 g, 0.36 mmol) in methanol (20 mL) was added 2-salicyloylhydrazono-1,3-dithiane (0.29 g, 1.08 mmol). The dark solution was further stirred at room temperature for 24 h before the solvent was removed under vacuum to afford a yellow solid (yield: 0.10 g, 31%). Red rectangular crystals suitable for X-ray analysis were obtained by slow diffusion of methanol through a CH₂Cl₂ solution of the complex. IR (pure, orbit diamond, cm⁻¹): 1588, 1515. Anal. Calcd. for C₃₃H₃₃MnN₆O₆S₆·CH₂Cl₂: C, 43.35; H, 3.75; N, 8.92. Found: C, 43.06; H, 3.68; N, 8.81.

Synthesis of Fe^III(HL)₃·2CHCl₃ (3)

FeCl₃ (0.013 g, 0.08 mmol) was added to the solution of H₂L (0.067 g, 0.25 mmol) dissolved in methanol (20 ml), and then the mixture was further stirred at room temperature for 24 h. After reaction the solvent was removed under vacuum to obtain a green powder. Red rectangular crystals of 3·2CHCl₃ were obtained by slow diffusion of diethyl ether into the CHCl₃ solution of the complex. (Yield: 0.075 g, 85%). IR (pure, orbit diamond, cm⁻¹): 1518, 1357, 1251, 754. Anal. Calc. for C₃₃H₃₃FeN₆O₆S₆·2CHCl₃: C, 38.33; H, 3.22; N, 7.66. Found: C, 38.51; H, 3.33; N 7.92.

Synthesis of Fe^II(H₂L)₂Cl₂·2CH₃OH (4)

A yellow solution of FeCl₃ (0.019 g, 0.12 mmol) in DMF (1 mL) was added to a well-stirred colourless solution of the ligand (0.064 g, 0.24 mmol) in DMF (2 mL). The colour changes immediately from yellow to dark blue. The homogeneous mixture was left under stirring overnight at room temperature. A yellow crystalline powder of the complex was obtained after a slow diffusion (3 to 4 days) of MeOH into the crude reaction mixture in a sealed glass tube (Yield: 0.052 g, 65%). Anal. Calc. for C₂₂H₂₄Cl₂FeN₄O₄S₄·2CH₃OH: C, 39.62; H, 4.43; N, 7.70. Found: C, 39.71; H, 4,04; N 7.67.

Synthesis of Fe^III₂(μ-OMe)₂(HL)₄·1/2CH₃OH (5)

A solution of iron(III) acetylacetonate (0.035 g, 0.1 mmol) in DMF (2 mL) was added to a well-stirred solution of H₂L (0.054 g, 0.2 mmol) in DMF (2 mL), and the resulting mixture colour changed quickly from colourless to dark brown. The mixture was further stirred at room temperature for 24 h. DMF was removed under vacuum and the obtained product of this step (typically the mononuclear species) was dissolved in THF to yield a dark solution, into which slow diffusion of methanol (during 6 to 12 h) affords well-formed orange crystals of 5·1/2CH₃OH in good yield (0.045 g, 72%). IR (pure, orbit diamond, cm⁻¹): 3513, 2814, 2921, 1619, 1588, 1520, 1487, 1364, 1252. Anal. Calc. for C₄₆H₅₀Fe₂N₈O₁₀S₈·1/2CH₃OH: C, 44.37; H, 4.12; N, 8.90. Found: C, 43.98; H, 3.94; N 8.71.

Synthesis of Fe^III₂(μ-O)(HL)₄·3CH₂Cl₂ (6)

A mixture of iron(III) acetylacetonate (0.035 g, 0.1 mmol) and H₂L (0.054 g, 0.2 mmol) in MeOH (20 mL) was stirred at room temperature in the dark and N₂ for 24 h. The black precipitate observed at the end of the reaction was filtered. The obtained black powder was washed with MeOH (2 × 5 mL) and dried under vacuum. Dark green crystals suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether into the CH₂Cl₂ solution of the black powder (yield: 0.015 g, 20%). IR (pure, orbit diamond, cm⁻¹): 1482, 1359, 1303,1237, 979, 749. Anal. Calc. for C₄₄H₄₄Fe₂N₈O₉S₈·3CH₂Cl₂: C, 38.88; H, 3.47; N, 7.72. Found: C, 38.69; H, 3.54; N, 7.49. After the initial synthesis above, we were unable to reproduce this reaction again for further characterization.

Magnetic measurements

Magnetic measurements were performed at the Institut de Physique et Chimie des matériaux de Strasbourg (UMR CNRS-ULP 7504) using a Quantum Design MPMS-XL SQUID magnetometer. The susceptibility measurement was performed in the 300–1.8 K temperature range and an applied field of 5 kOe. Isothermal field dependent magnetization measurement at room temperature confirms the absence of ferromagnetic impurities. Data were corrected for the sample holder and diamagnetism of the content estimated from Pascal constants.

Crystal structure determinations

Suitable crystals for the X-ray analyses of all compounds were obtained as described above. Single crystals of 1–6 were mounted on a Nonius Kappa-CCD area detector diffractometer (Mo-Kα, λ = 0.71073 Å). The complete conditions of data collection (Denzo software⁴⁰) and structure refinements are given in supporting materials. The cell parameters were determined from reflections taken from one set of 10 frames (1.0° steps in phi angle), each at 20 s exposure. The structures were solved using direct methods (SHELXS-97) and refined against F² using the SHELXL-97 and CRYSTALBUILDER softwares.^41,42 The absorption was not corrected. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated according to stereochemistry and refined using a riding model in SHELXL-97.

Acknowledgements

We thank the CNRS, the Ministère de la Recherche (Paris) and Fundação para a Ciência e Tecnologia, (FCT), for a postdoctoral fellowship (SFRH/BPD/44262/2008) to Dr Vitor Rosa. We also thank Dr Guillaume Rogez (IPCMS – Strasbourg) for his help with magnetic measurements.

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Footnote

† CCDC reference numbers 844919–844924. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra01316a

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