cyclo-Tetra-μ-fluorido-1:2κ2 F;2:3κ2 F;3:4κ2 F;1:4κ2 F-octanitrato-1κ8 O,O′;3κ8 O,O′-tetrakis(1,10-phenanthroline)-2κ4 N,N′;4κ4 N,N′-2,4-dichromium(III)-1,3-dineodymium(III) methanol tetrasolvate monohydrate

In the title compound, [Cr2Nd2F4(NO2)8(C12H8N2)4]·4CH3OH·H2O, two cis-difluoridobis(1,10-phenanthroline)chromium(III) fragments containing octahedrally coordinated chromium(III) bridge via fluoride ions to two tetranitratoneodymate(III) fragments, forming an uncharged tetranuclear square-like core. The fluoride bridges are fairly linear, with Cr—F—Nd angles of 168.74 (8)°. Cr—F bond lengths are 1.8815 (15) Å, slightly elongated compared to those of the parent chromium(III) complex, which has bond lengths ranging from 1.8444 (10) to 1.8621 (10) Å. The tetranuclear complex is centered at a fourfold rotoinversion axis, with the Cr and Nd atoms situated on two perpendicular twofold rotation axes. The uncoordinated water molecule resides on a fourfold rotation axis. The four methanol solvent molecules are located around this axis, forming a cyclic hydrogen-bonded arrangement. The title compound is the first structurally characterized example of unsupported fluoride bridges between lanthanide and transition metal ions.

In the title compound, [Cr 2 Nd 2 F 4 (NO 2 ) 8 (C 12 H 8 N 2 ) 4 ]Á-4CH 3 OHÁH 2 O, two cis-difluoridobis(1,10-phenanthroline)chromium(III) fragments containing octahedrally coordinated chromium(III) bridge via fluoride ions to two tetranitratoneodymate(III) fragments, forming an uncharged tetranuclear square-like core. The fluoride bridges are fairly linear, with Cr-F-Nd angles of 168.74 (8) . Cr-F bond lengths are 1.8815 (15) Å , slightly elongated compared to those of the parent chromium(III) complex, which has bond lengths ranging from 1.8444 (10) to 1.8621 (10) Å . The tetranuclear complex is centered at a fourfold rotoinversion axis, with the Cr and Nd atoms situated on two perpendicular twofold rotation axes. The uncoordinated water molecule resides on a fourfold rotation axis. The four methanol solvent molecules are located around this axis, forming a cyclic hydrogen-bonded arrangement. The title compound is the first structurally characterized example of unsupported fluoride bridges between lanthanide and transition metal ions.

Comment
The magnetic properties of polynuclear, mixed lanthanoide transition metal complexes have received much attention (Sessoli & Powell, 2009). Since early suggestions by Kahn (1985Kahn ( , 1987 that exchange interactions involving d-and f-electrons were likely to lead to ferromagnetic coupling, due to vanishing orbital overlaps, many such systems have been synthesized and studied structurally and magnetically. Despite the high activity in this field, there are still simple types of bridging ligands, which have not been studied in this context. Thus, fluoride, which is known to bind strongly to lanthanoides has not been known as a bridging ligand between paramagnetic transition metal ions and lanthanoide ions until the very recent introduction of fluoride in heterometallic wheels by McRobbie et al. (2011). However, in those systems, fluoride bridges are always supported by carboxylate groups connecting the same metal ions. Based on those systems it is very difficult or impossible to make deductions concerning the geometric preferences of fluoride as a bridging ion and concerning magnetic exchange over fluoride bridges. This problem is remedied by a system such as the title compound, which is the first example of unsupported fluoride bridges between 3d and 4f metals.
In the title compound the solvate water molecule is located on a proper fourfold axis, whereas the tetranuclear Cr 2 Nd 2 F 4 fragment is centered on a fourfold rotoinversion axes. Consequently, all the metal ions are required to lie in the same plane perpendicular to the tetragonal axes ( Fig. 1). The complexation of the neodymium atom induces a slight elongation of the Cr-F bonds by ca 0.03 Å in comparison with the parent compound (Birk et al., 2008). The neodymium atom is 10-coordinated with its coordination sphere completed by bidentate nitrate ions coordinating with unexceptional bond lengths and bite angles. The uncoordinated water molecule is located on a fourfold axis and has no direct partner for hydrogen bonding (the next nearest atom is C5 in a distance of 3.816 (3) Å), which explains the high thermal displacement parameters for its oxygen atom. Around the same fourfold axis, the methanol solvate molecules form a cyclic tetrameric arrangement held together by hydrogen bonds (Table 1  were all used as received. The synthesis of cis-[Cr(phen) 2 F 2 ]NO 3 proceeds in many ways analogous to the method described by Glerup et al. (1970) for the synthesis of cis-[Cr(phen) 2 F 2 ]ClO 4 . As a result of a significant difference in solubility of the two salts, some modification with respect to solvent volume and isolation procedure has been introduced. It should also be noted that the nitrate can be crystallized with a variable number of crystal water and that this number can change depending on whether the substance is stored in dry or moist air. Elemental analysis for C, H and N was performed with an CE Instrument: FLASH 1112 series EA, at the microanalytic laboratory, University of Copenhagen. Electrospray (ES) mass spectra were recorded on a Micromass Q-TOF apparatus with positive ion detection.
Yield of raw product: 31.7 g (79.0% of theoretical based on Cr III The title compound was prepared by reaction of a methanolic solution of cis-[Cr(phen) 2 F 2 ](NO 3 ) (210 mg, 0.41 mmol in 10 ml) with a methanolic solution of Nd(NO 3 ) 3 . 6H 2 O (175 mg, 0.40 mmol in 5 ml). Before combination, both solutions were filtered through filters with pore size 0.45 µm. Crystals formed over a period of 2-12 h. The yield was 284 mg (82% based on Nd). Crystals suitable for single-crystal X-ray diffraction were obtained directly using the concentrations given above. Upon drying, the crystals loose solvent and deteriorate. For the diffraction experiment, a crystal was taken from the mother liquor, covered with paraffin oil and cooled directly.

Refinement
H atoms were found in a difference Fourier map and were included in the refinement as constrained idealized protons riding the parent atom, with X-H = 0.84 Å (OH); 0.95 Å (aromatic CH); 0.98 Å (CH 3 ) with U iso equal to 1.2×U eq of the parent C atom (1.5×U eq of the parent atom in MeOH). No resonable assignment of the H atoms of the water of crystallization could be obtained. Consequently, these H atoms were excluded from the refinement. The maximum residual electron density is found at 1.04 Å from O20, the minimum residual electron density is at 0.37 Å from the same atom.
supplementary materials sup-3 Figures   Fig. 1. A view of the tetranuclear molecular structure of the title compound with the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level. Solvent methanol and water molecules were omitted.

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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.