Crystal structures of two new six-coordinate iron(III) complexes with 1,2-bis(diphenylphosphane) ligands

Two new structures from a very small group to date of six-coordinate monocationic iron(III) complexes containing two bidentate phosphane and two halido ligands are presented as dichloromethane solvates. The Fe—P and Fe—Cl bond lengths are longer and shorter, respectively, than those previously reported for cations in this group.


Chemical context
Bidentate phosphanes (bisphosphanes) are versatile supporting ligands in coordination chemistry because of the accessibility of various electronic and steric properties through manipulation of their backbone structures and phosphorus substituents. While these ligands are commonly used to stabilize low-valent metal complexes due to their function as both -donor and -acceptor ligands, bisphosphane ligands have also been observed to support metal centers in higher oxidation states. For example, there exist a few structurally characterized examples of iron(III) complexes in which two bisphosphane ligands are coordinated to a single metal center, resulting in axial coordination of halido (X) ligands. These complex cations have been shown to be accessible through a variety of synthetic routes Field et al., 1990Field et al., , 2000Evans et al., 1992;Miller et al., 2002;Hoffert et al., 2011). A review of the literature finds that investigations into these complexes date back almost sixty years to the work of Chatt and Hayter, in which three distinct iron(III) bisphosphane complexes, formulated as complex salts with the molecular structures [(PP) 2 FeCl 2 ][FeCl 4 ] [PP = 1,2-bis(diethylphosphano)benzene (debz), 1,2-bis(diethylphosphano)ethane (depe), and 1,2-bis(dimethylphosphano)ethane (dmpe)], were prepared through the reaction of iron(III) chloride with one shoichiometric equivalent of bisphosphane (Chatt & Hayter, 1961; for later reports of various preparative methods of similar compounds, see: Levason et al., 1975;Gargano et al., 1975;Warren et al., 1976;. Structural confirmation for this general molecular formula was achieved through the crystallographic characterization of [(dmpe) 2 FeCl 2 ] [FeCl 4 ], although this synthesis employed photolytic oxidation of the iron(II) complex (dmpe) 2 FeCl 2 and not direct reaction of an iron(III) precursor with bisphosphane (Field et al., 1990). At the time of this report, the only other known molecular structure for a sixcoordinate iron(III) complex cation bearing a (PP) 2 X 2 ligand set was [(o-C 6 F 4 (PMe 2 ) 2 ) 2 FeCl 2 ] [BF 4 ] . This particular species was synthesized through metathesis of the original tetrachloridoferrate(III) anion with HBF 4 . The initial salt, [(o-C 6 F 4 (PMe 2 ) 2 ) 2 FeCl 2 ] [FeCl 4 ], prepared via a nearly 1:1 stoichiometric reaction of iron(III) chloride with o-C 6 F 4 (PMe 2 ) 2 , was not structurally characterized.
Our group is interested in the application of bisphosphanes as supporting ligands within iron-catalyzed cross-coupling reactions. Numerous literature protocols for iron-catalyzed cross-coupling methods involve use of bisphosphanes as substoichiometric additives in conjunction with iron(II) or iron(III) salts, promoting the formation of the active catalyst in situ. Methods development in our laboratory using the dppen ligand in conjunction with iron(III) chloride resulted in the formation of [(dppen) 2 FeCl 2 ][FeCl 4 ] (1) from reaction mixtures and its subsequent structural characterization. As reported herein, 1 was then independently prepared via the method of Chatt & Hayter (1961). While we have not observed this compound to exhibit catalytic effectiveness in cross-coupling, a literature search indicated that this ionic complex was first synthesized in the 1970s using the same reaction stoichiometry (Levason et al., 1975). This report lacked structural characterization of the complex, but its formulation as a complex salt was supported by magnetic susceptibility, Mö ssbauer, and IR absorption measurements. Upon confirming the structure of 1, we sought to examine an analogous species, [(dpbz) 2 FeCl 2 ][FeCl 4 ] (2), by taking advantage of the same steric substitution at phosphorus, but with a slightly varied backbone character (ortho-phenylene in place of the C 2 H 2 of dppen). Such studies are important as they expand the coordination chemistry library of iron(III) complexes bearing bisphosphane ligands. In addition, 1 and 2 join only two other structurally characterized examples of coordinatively saturated iron(III) monocations with a (PP) 2 X 2 ligand set that have been synthesized without using oxidative methods (Miller et al., 2002.

Structural commentary
Both 1 and 2 are characterized as six-coordinate complex cations in which the iron(III) center is ligated by two bisphosphane ligands (dppen in 1, dpbz in 2) in a trans fashion (see Scheme). The two retained chlorido ligands are thus coordinated axially, and the displaced chlorido ligand results in generation of a single tetrachloridoferrate(III) anion in both cases. Compound 1 (Fig. 1) crystallizes in the centrosymmetric space group C2/c. The iron atom of the cation is located at a crystallographic inversion center, resulting in Cl-Fe-Cl and trans P-Fe-P angles of 180 . Deviation from ideal octahedral geometry is due to the 80.92 (2) bite angles of the P-Fe-P chelate rings (Table 1). The Fe-P distances are considerably longer than those of the other structurally characterized iron(III) cations with a (PP) 2 X 2 donor set (range 2.29-2.34 Å ; Groom et al., 2016, see Database survey below), but with shorter Fe-Cl distances than those of the other reported X = Cl compounds (range 2.23-2.25 Å ). The ethylene backbones of each dppen ligand in the cation of 1 are bent out of the equatorial plane by 24.86 (8) . The tetrachloridoferrate(III) anion lies along a crystallographic twofold axis that includes the metal center. Phenyl group C3-C8 (and thus its symmetry equivalent, Fig. 1 Displacement ellipsoid plot of 1 drawn at the 50% probability level with hydrogen atoms omitted. The full cation of the title formula is generated by a crystallographic inversion center (1 À x, 1 À y, 1 À z) at atom Fe1. The full anion is generated by a crystallographic twofold axis (Àx, y, 3 2 À z), which includes atom Fe2. The symmetry-equivalent atoms of the dichloromethane solvent molecule are generated by a crystallographic twofold axis (1 À x, y, 3 2 À z) that contains atom C27.
The asymmetric unit of 2 contains the cation, anion, and solvent molecule in general positions. The solvent molecule was modeled as disordered over three positions  (3)]. Despite the structural similarity of the backbone linkers and steric periphery of dppen and dpbz, the space group assignment and crystallographic symmetry of 2 contrasted from 1. Metrically, however, 1 and 2 are quite similar. The axial chlorido ligands within the cation of 2 are located at Fe-Cl distances very close to that found in the cation of 1 and the Cl-Fe-Cl and trans P-Fe-P angles are very nearly linear (Fig. 2, Table 2). Additionally, the bite angles in the cation of 2 as well as Fe-Cl distances and Cl-Fe-Cl angles of its tetrachloridoferrate(III) anion are very similar to those of 1. As observed for the ethylene backbones of the dppen ligands of 1, the ortho-phenylene backbones of the dpbz ligands in 2 are also canted out the equatorial plane by 21.9 (1) and 22.9 (1) . The crystal of 2 studied was an inversion twin, whose component mass ratio refined to 0.76 (3):0.24 (3).
Both 1 and 2 are dichloromethane solvates under the common crystallization procedure used (see below). In 1, the solvent molecule is located along a crystallographic twofold axis that includes the carbon atom. Crystal desolvation is suspected, since its occupancy only refined to 0.592 (4). In contrast, 2 was found to possess full occupation of co-crystallized dichloromethane, modeled as disordered over three general positions [0.740 (3)

Supramolecular features
There are no significant supramolecular features beyond a few very weak C-HÁ Á ÁCl and C-HÁ Á Á interactions.

Synthesis and crystallization
Anhydrous FeCl 3 (98%, Alfa Aesar), cis-1,2-bis(diphenylphosphanel)ethylene (dppen, 98%, Strem), and 1,2-bis(di-     Once isolated and dried, solid 1 and 2 were found to be indefinitely stable outside of an inert atmosphere (greater than one year). Both complexes were crystallized by layering toluene over a concentrated dichloromethane solution of the complex and allowing the layers to diffuse at room temperature (anhydrous solvents were not used during crystallizations). Red-green dichroic single crystals suitable for X-ray diffraction studies were generally observed to crystallize within 24 h. Out of a large number of polar and non-polar common organic solvents examined, only dichloromethane, chloroform, acetone, and nitromethane appreciably solubilized 1 and 2. During preparation of crystallizations, dichloromethane solutions of 1 and 2 were observed under incandescent light to be green at low concentrations and red at high concentrations.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.
Phenyl ring C3-C8 of 1 was modeled as disordered over two general positions [0.561 (6):0.439 (6)]. Analogous bond lengths and angles between the two positions and in both directions around the rings were restrained to be similar. Additionally the P1-C3 and P1-C3 0 bond lengths were restrained to be similar. Anisotropic displacement parameters for pairs of proximal atoms were constrained to be equivalent. The occupancy of the cocrystallized dichloromethane solvent molecule refined to 0.592 (4), which is consistent with crystal desolvation.
2 was refined as an inversion twin in P1 whose twin component mass ratio refined to 0.76 (3):0.24 (3). Because of significant parameter correlation, anisotropic displacement parameters for pseudosymmetrically related atom pairs were constrained to be equivalent. The co-crystallized dichloro-   (3)]. Analogous bond lengths and angles among the three positions of the disordered dichloromethane solvent molecule were restrained to be similar. Anisotropic displacement parameters for proximal and pseudosymmetrically related atoms were constrained to be equivalent. A solution and refinement of 2 in centrosymmetric space group P1 caused an increase in the R1 residual (strong data) from 0.071 to 0.118, which was not unexpected given the uneven twin component mass ratio when refined in P1. In the centrosymmetric model, the anion and solvent were modeled pairwise as disordered over a crystallographic inversion center.
For 1 the maximum residual peak of 0.74 e À Å À3 and the deepest hole of À0.67 e À Å À3 are found 0.72 and 0.82 Å , respectively, from atom CL4.

trans-Bis[1,2-bis(diphenylphosphane)ethylene]dichloridoiron(III) tetrachloridoferrate(III) dichloromethane 0.59solvate (1)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.74 e Å −3 Δρ min = −0.67 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Phenyl ring C3-C8 is modeled as disordered over two positions (56:44). Analogous bond lengths and angles between the two positions were restrained to be similar. Anisotropic displacement parameters for pairs of proximal atoms were constrained to be equivalent. The occupancy of the cocrystallized dichloromethane solvent molecule refined to 0.592 (4).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. The structure was modeled as an inversion twin whose component mass ratio refined to 0.76 (3):0.24 (3). A solution and refinement in centrosymmetric space group P-1 caused an increase in the R1 residual (strong data) from 0.071 to 0.118. The cocrystallized dichloromethane solvent molecule is modeled as disordered over three positions  (3)). Analogous bond lengths and angles among the three positions of the disordered dichloromethane solvent molecule were restrained to be similar. Anisotropic displacement parameters for proximal and pseudosymmetrically related atoms were constrained to be equivalent.