Fluoride Ion as Ligand and Hydrogen Bond Acceptor: Crystal Structures of Two Dinuclear Cu Ii Complexes Built on a Diazecine Template

Two dinuclear Cu II complexes based on a diazecine ligand were characterized by X-ray diffraction, one of which includes the rare Cu II –F bond, resulting from dissociation of a BF 4 − ion. The F − ligands actively participate in the crystal structure, behaving as acceptors for hydrogen bonding.


Introduction
Relatively few coordination Cu II complexes with coordinated fluoride ion [1] are described in the literature, compared to the solid-state materials bearing a Cu II -F bond, which draw attention due to their particular magnetic properties [2,3].Several qualitatively different approaches have been described for obtaining them.While the most direct method is the use of sparingly soluble CuF 2 as starting material [4][5][6], in most cases, the fluoride ion has come from an anion such as BF 4 − or PF 6 − .The decomposition of BF 4 − to yield complexes, described by Reedijk et al. as early as 1974 [7], was later exploited in several groups for the preparation of a variety of coordination complexes with

−
. In several instances, complexes bearing a Cu II -F bond have resulted from exposure to oxygen of Cu I complexes with BF 4 − [22] or PF 6 − [23][24][25] as counterions.It also seems that presence of sterically crowded and electron rich ligands, pointed out by Straub as essential for the formation of low-coordinate Cu I species [23], could promote the dissociation of the fluoroanions and/or the stabilization of the coordinated fluoride.
In this context, we would like to contribute with the X-ray study of a couple of closely related dinuclear Cu II complexes, in which a BF 4 − ion is the F − source for the formation of Cu II -F bonds.The present study is of limited interest from a synthetic point of view, because of reproducibility and separation issues we were unable to address.However, this work clearly shows that X-ray diffraction allows the accurate assignment of F − sites in the coordination sphere, and that this ligand is distinguishable from other potential ligands having almost similar scattering power, OH − and H 2 O, providing that each ligand affords a specific hydrogen bond pattern in the resulting crystal structure.

Results and Discussion
Both complexes are based on the octahydro [4,5]diimidazo [1,6]diazecine cyclic framework, abbreviated dimeim hereafter.This ligand has proven excellent for yielding di-copper complexes with a variety of counterions [26][27][28].By reacting 2:1 equivalents of Cu(BF 4 ) 2 •6H 2 O and dimeim in methanol, we obtained the expected complex, [Cu 2 (H 2 O) 4 (dimeim)](BF 4 ) 4 (1, see Figure 1), a deep-blue compound initially intended for a study about the influence of the counterion on the magnetic properties of the cation.However, the very first crop of crystalline material contained light-blue small crystals, for which analytical data and X-ray crystallography gave a different formula, [Cu 2 (H 2 O) 2 (F) 2 (dimeim)](BF 4 ) 2 •2H 2 O (2, see Figure 1).The relative yields for 1 and 2 varied from synthesis attempts, and a satisfactory separation could not be achieved.However, on the basis of their colors, suitable single crystals for 1 and 2 were picked from the raw material, and their crystal structures determined.Numerous studies [26][27][28][29][30] carried out during the last ten years showed that the central 10-membered diazecine heterocycle of these compounds is systematically stabilized as a C 2h skewed boat-chair-boat conformer.Such a rigid, highly symmetrical conformation strongly favors the formation of centrosymmetric binuclear coordination compounds, using diazecine and imidazole N atoms as coordination sites [26][27][28][29][30].This rule is again observed for 1 and 2, two binuclear complexes with five-coordinate Cu II centers.

[Cu 2 (H 2 O) 4 (dimeim)](BF 4 ) 4 (1)
The major complex crystallizes with four BF 4 − anions and two independent half-cations in the asymmetric unit, each one completed to a full [Cu 2 (H 2 O) 4 (dimeim)] 4+ cation through inversion centers (Figure 2).The same arrangement in a similar triclinic cell for the ClO 4 − salt of the same cation has been recently characterized [30], while a monoclinic polymorph of this compound, with an asymmetric unit containing a single [Cu 2 (H 2 O) 4 (dimeim)](ClO 4 ) 4 formula, was previously reported [28].These three closely related structures share a virtually identical conformation for the cation and present identical coordination geometries.In the crystal of 1, a fit between independent cations gives a r.m.s.deviation which is less than 0.1 Å.The coordination sphere of the metal centers is closer to a square pyramidal (SP) geometry ( Cu1 = 0.16,  Cu2 = 0.05) with the apical positions occupied by water molecules.As expected, the angle restrictions induced by the chelate rings in the basal coordination planes (e.g., N3, N7, N10, O2 for Cu1; see Figure 2) push the metal from the base, by 0.246 Ǻ (Cu1) or 0.212 Ǻ (Cu2).In the crystal, cations and BF 4 − ions are held together by OH•••F hydrogen bonds involving coordinated water molecules as donor groups (Table 1 and Figure 2).The resulting supramolecular structure is dominated by the formation of a R 4 4 (16) ring motif [31], which, interestingly, was also present in both polymorphs of the ClO 4 − salt [28,30].Dashed bonds represent the strongest hydrogen bonds formed between the coordinated water molecules and the BF 4 − ions (see Table 1).

[Cu 2 (H
Complex 2 derives directly from 1, by substitution of two coordinated water molecules in the basal planes by two F − ions, affording cation [Cu 2 (H 2 O) 2 (F) 2 (dimeim)] 2+ .Substituted water molecules are still present in the crystal, but now as lattice water molecules.Because of the charge drop for the cationic species, the formula is completed by two BF 4 − anions.The propensity of the diazecine cycle to be centrosymmetric is reflected in the composition of the asymmetric unit of 2, limited to a half chemical formula (Figure 3).1).
Despite the substitution, the coordination of the metal center remains almost unchanged compared to 1, with a square pyramidal geometry characterized by  Cu = 0.03 and a deviation of Cu from the basal plane of 0.30 Å.The Cu-F bond length, 1.912(2) Å, is difficult to support, because few X-ray structures of Cu II coordination complexes including such a bond have been reported so far.This bond length is however consistent with those found in available SP complexes with a basal Cu-F bond, 1.888 Å [15], 1.916 Å [16], and 1.902-1.932Å [32].Moreover, the definite proof of the presence of a Cu-F bond should be sought rather in the bond trans to the Cu-F bond.The Cu-N7 distance, 2.111(3) Å, is slightly larger than the expected distance, 2.053 Å, as spotted by MOGUL [33] on the basis of 15 hits.The corresponding bond length in the non-fluorinated complex 1, is, for instance, Cu1-N7 = 2.088(4) Å and Cu2-N27 = 2.073(4) Å.The lengthening of the Cu-N7 bond in complex 2 should thus be regarded as a consequence of the difference in trans influence of the F − ligand compared to H 2 O.
As in 1, the crystal structure in 2 is dominated by hydrogen bonds involving water molecules and F − ions, although, in contrast to 1, the most efficient acceptor in 2 is the F − ligand coordinated to the metal center, F2 (see Table 1 3) Å [15].Based on these contacts, a 2D supramolecular structure is formed for compound 2. First, R 4 3 (10) ring motifs are formed between cations and free water molecules, and edge-sharing motifs propagate in the [010] direction.The second base vector for the building of the 2D network is [001], and involves the same constituents to form R 4 4 (24) large rings (Figure 3).The lattice water molecule O3 is common to both R motifs, and should thus be seen as an essential component in the stabilization of the observed crystal structure.

Experimental Section
Synthesis of the complexes.The ligand dimeim was prepared as previously described [26].To 50 mL of a methanolic solution containing two mmol of Cu(BF 4 ) X-ray diffraction.Data for complexes 1 and 2 were collected at room temperature on a Bruker P4 diffractometer [34] using the Mo-Kα radiation (Table 2).Raw data were corrected for absorption effects [35] and the structures refined with the SHELX programs [36].Although sample for 2 was a rather long crystal not uniformly irradiated during data collection, a reduction of the sample size was not intended, because very few single crystals were available.However, we assume that absorption correction for 2 is still reliable (R int = 0.028).Accurate determination of sites occupied by H atoms was critical for water molecules, in order to discriminate F − and water molecules (and possibly OH − groups, assuming that hydroxyl groups could be present in the reaction media).All water H atoms in both complexes were thus assigned from difference maps, and refined with free coordinates, although soft restraints were applied to O-H bond lengths and H•••H separations, 0.85(1) and 1.34(1) Å, respectively.Sensible orientations for water molecules are obtained in the final models, since all O-H groups are engaged in efficient hydrogen bonds.On the other hand, in the case of 2, final difference map is featureless in the vicinity of the coordinated fluoride site, corroborating that Cu-F bonds have been formed, rather than Cu-OH or Cu-OH 2 bonds.CIF data, including structure factors, are deposited as supplementary material.

Conclusions
In conclusion, substitution of basal water by F − ligand in complex 1, and concomitant release of water to form the hydrate 2, allows the stabilization of a crystal structure featuring strong intermolecular hydrogen bonds between coordinated and free water molecules and the fluoride ligands, which also act as acceptors for hydrogen bonding.

Figure 2 .
Figure 2. Left: ORTEP-like view of one cation and two anions in complex 1; Right: Part of the crystal structure of 1. Green and blue cations are based on Cu1 and Cu2, respectively.Dashed bonds represent the strongest hydrogen bonds formed between the coordinated water molecules and the BF 4− ions (see Table1).
). Corresponding hydrogen bonds use the coordinated water molecule (O1) as well as the lattice water molecule (O3) as donor groups, and may be considered as strong interactions, with non bonding H•••F distances of ca.1.80 Å, and O-H•••F angles close to 180°.It is worth noting that he upper limit for very strong H•••F interactions in coordination chemistry is around OH•••F = 1.74(

Table 1 .
Main hydrogen bonds in complexes 1 and 2 involving water molecules as donor groups and F − or lattice water as acceptor groups.Weak N-H•••F contacts present in the crystal structures are not displayed in the table and may be found in the CIF deposited as supplementary material.

Table 2 .
Crystal data for complexes 1 and 2.