Diaqua(1,4,7,10,13-pentaoxacyclopentadecane)iron(II) bis(μ-cis-1,2-dicyano-1,2-ethylenedithiolato)bis[(cis-1,2-dicyano-1,2-ethylenedithiolato)ferrate(III)] 1,4,7,10,13-pentaoxacyclopentadecane disolvate

The title compound, [Fe(C10H20O5)(H2O)2][Fe2(C4N2S2)4]·2C10H20O5, consists of an [FeII(15-crown-5)(H2O)2]2+ cation, sandwiched between and O—H⋯O hydrogen bonded by two additional 15-crown-5 ether molecules and two independent [FeIII(mnt)2]− anions, where 15-crown-5 ether denotes 1,4,7,10,13-pentaoxacyclopentadecane and mnt denotes cis-1,2-dicyano-1,2-ethylenedithiolate. Each independent [FeIII(mnt)2]− unit forms a centrosymmetric dimer supported by two intermonomer FeIII—S bonds [Fe—S = 2.4715 (9) and 2.4452 (9) Å]. In the crystal structure, the dimers form one-dimensional π–π stacks along the a axis, with an interplanar separation of 3.38 (6) Å.


Comment
The Fe 2 S 2 clusters (i.e., H-clusters) in Fe-only hydrogenases (FeHases) are known to be highly active as catalysts towards hydrogen evolution reaction (HER) (Adams, 1990;Peters et al., 1998;Nicolet et al., 1999;Frey, 2002), in spite of the fact that metal iron itself exhibits much lower catalytic activity toward HER than platinum does (Trasatti, 1972;Sakata, 2000). A large variety of structural and functional models of FeHases have been developed and their H 2 -evolving activities have been evaluated so far (Gloaguen et al., 2001;Georgakaki et al., 2003;Liu et al., 2005;Sun et al., 2005). However, up to now, only two water-soluble models of FeHases have been ascertained to exhibit H 2 -evolving activity in aqueous media, even though their activities are still quite low (Na et al., 2006). On the other hand, an iron-dithiolene complex, [Fe II (1,2-benzenedithiolato-S,S) 2 ] 2-, considered as a bio-inspired model, was found to generate a half equivalent of H 2 in tetrahydrofurane in the presence of HCl (Sellmann et al., 1991). In order to develop the more highly effective models of FeHases, our recent interests concentrate on such iron-dithiolene complexes, which are both air-stable and water-soluble. Compound (I) reported herein has been developed to improve the water-solubility of (NBu 4 )[Fe III (mnt) 2 ] (Hamilton & Bernal, 1967). Although the sodium salt Na[Fe III (mnt) 2 ] (McCleverty et al., 1967) is soluble in water, the compound prepared by the literature method was found to involve a large amount of impurities. Thus, the improvement in the purity of the complex was another reason to develop a new water-soluble salt of this complex. The H 2 -evolving activity of (I) will be separately reported elsewhere (Yamaguchi et al., unpublished results and 3). The oxidation states of these iron centers can be unambiguously judged from the overall charge of each complex together with the neutralization principle applied to any salt. The validity of these assignments can also be discussed in terms of the Fe-O and Fe-S distances (see below).
The Fe II ion encapsulated within the central 15-crown-5 ether is ligated by five oxygen atoms of the ether and also by two oxygen atoms of axial aqua ligands (Fig. 1). The central [Fe II (H 2 O) 2 (15-crown-5)] 2+ unit is sandwiched by two additional 15-crown-5 ether molecules, where each association is stabilized with two hydrogen bonds formed between the axial aqua ligand and two oxygen atoms of 15-crown-5 ether (see Table 1 and Fig. 1 and axially ligated by a sulfur atom from the adjacent monomer with a longer Fe-S distance [2.4452 (9) and 2.4715 (9) Å]. Atom Fe1 is shifted out of the least-squares plane defined with four atoms S1-S4 by 0.3634 (4) Å, even though the four-atom r.m.s. deviation given in the calculation was 0.177 Å. In the same manner, atom Fe2 is shifted out of the pseudo plane defined with S5-S8 by 0.3858 (4) Å, where the four-atom r.m.s. deviation was 0.104 Å.
The stack of cations merely arise from the van der Waals interactions, while that of anions is stabilized with a relatively strong π-π stacking interactions formed between two adjacent mnt moieties, where only one independent stacking geometry can be found in the crystal. As shown in Figure 5, a set of atoms C1-C4/N1-N2 and that of C9 i , C11 i , N12 i , S6 i have a significant contribution to the π-π association at this geometry. The interplanar separation is calculated as 3.376 (55) Å based on the average shift of atoms C9 i , C11 i , N12 i and S6 i from the best plane defined by atoms C1-C4/N1-N2, and important short contacts at this geometry are C4-C11 i = 3.371 (3) and N2-C12 i = 3.324 (3) Å [Symmetry code for (i) 1 -x, 1 -y, -z].

Experimental
Compound (I) was prepared as follows. Na[Fe III (mnt) 2 ] . 3H 2 O was prepared as previously described (McCleverty et al., 1967). To a solution of Na[Fe III (mnt) 2 ] . 3H 2 O (0.108 g, 0.26 mmol) in ethanol (15 ml) was added 15-crown-5 ether (0.209 g, 0.95 mmol). The resulting dark-brown solution was stirred for 5 min and evaporated under reduced pressure until crystallization started. Standing of the solution at room temperature for 4 days afforded the black needles of (I), which were collected by filtration, washed with cold ethanol, and dried in vacuo. Yield: 0.072 g (39%). Since the starting material contains about sup-3

Refinement
H atoms except for those of water molecules were placed in idealized positions (methylene C-H = 0.99 Å), and included in the refinement in a riding-model approximation, with U iso (H) = 1.2Ueq (methylene C). H atoms of water molecules were refined isotropically. The hydrogen bonding geometries of these H atoms well support the validity of the positions determined by the least-squares calculations. In the final difference Fourier map, the highest peak was located 0.92 Å from atom Fe1. The deepest hole was located 0.72 Å from atom Fe1.    sup-4 Fig. 5. A view perpendicular to the plane defined by atoms C1-C4/N1-N2 which has a πstack to the plane defined by atoms C9 i , C11 i , N12 i and S6 i [Symmetry code for (i) 1 -x, 1y, -z]. Thermal ellipsoids are displayed at the 50% probability.