Crystal structures of potassium trifluorido(4-methoxyphenyl)borate and potassium trifluorido(4-fluorophenyl)borate

Despite their different compositions and space groups, the irregular KF8 coordination polyhedra of the potassium cations in these structures are almost identical. The layer stacking sequences are AAA… in the p-methoxy compound and ABAB… in the p-fluoro compound.


Chemical context
The phenyltrifluoridoborate anion is an interesting intermediate species between the well-known tetrafluoridoborate (BF 4 À ) and tetraphenylborate [B(C 6 H 5 ) 4 À ] ions (Conole et al., 1995) and may serve as a bulky charge-balancing anion (Quach et al., 2001;Fei et al., 2010). As part of our studies in this area, we now describe the syntheses and structures of the para-substituted phenyltrifluoridoborate salts K + C 7 H 7 BF 3 O À (I) and K + C 6 H 4 BF 4 À (II).

Structural commentary
Compound (I) comprises one cation and one anion in the asymmetric unit (Fig. 1). In the anion, the C7 atom of the methoxy group is close to coplanar with the benzene ring [displacement = 0.048 (2) Å ]. The B atom adopts its expected tetrahedral BF 3 C geometry (Conole et al., 1995) and the C1-B1 bond length of 1.5987 (18) Å is consistent with previous data (Quach et al., 2001). One of the B-F bonds (to F1) in (I) is notably longer than the other two, which might reflect the different modes of coordination of the fluorine atoms to the potassium ions. The F-B-F bond angles (mean = 105.7 ) are significantly smaller than the C-B-F angles (mean = 113.0 ). F1 is displaced by À1.427 (2) Å from the plane of the benzene ring and F2 and F3 are displaced in the opposite sense, by 0.715 (2) and 0.252 (2) Å , respectively. The potassium ion in (I) is coordinated by eight fluorine atoms, with one of the K-F bonds substantially longer than the others (Table 1): the next-nearest F atom is over 4 Å distant. The coordination geometry of the K + ion, which arises from one tridentate, one bidentate and three monodentate BF 3 À groups, is irregular and highly asymmetric (Fig. 2), with five of the F atoms forming an approximate plane and the other three (arising from one BF 3 group) lying to one side. The metal ion is displaced by 1.00 Å from the geometric centroid of the eight F atoms. In terms of the F atoms in (I), F1 bonds to three different metal ions (mean K-F = 2.734 Å ), generating a distorted FBK 3 tetrahedron, whereas F2 bonds to two K + ions (mean K-F = 2.755 Å ) in an FBK 2 distorted T-shape. If the geometry around F3 is not merely deemed to be irregular, it could be described as an FBK 3 trigonal-based pyramid, with the long K-F bond (Table 1) as the apex (mean K-F = 2.963 Å ). The extended structure in (I) consists of (010) sheets in which the KF 8 polyhedra share faces in the [100] direction and edges in [001]: the shortest KÁ Á ÁK separation is 4.4523 (4) Å .
The asymmetric unit of compound (II) also consists of an ion-pair (Fig. 3). The geometry of the anion in (II) is very similar to that of the equivalent species in (I): the C1-B1 bond length is 1.590 (2) Å and the mean F-B-F and C-B-F bond angles are 105.5 and 113.2 , respectively. The displacements of F1, F2 and F3 from the benzene-ring plane are À1.386 (2), 0.813 (3) and 0.131 (3) Å , respectively. As seen for (I), the B1-F1 bond in (II) is noticeably longer than the B1-F2 and B1-F3 bonds.

Figure 1
The asymmetric unit of (I) showing 50% displacement ellipsoids.
ion is displaced by 0.98 Å from the geometric centroid of the fluorine atoms. The extended structure of (II) consists of (001) sheets [rather than (010) sheets, as seen in (I)] of face-and edgesharing KF 8 groups with the same topology as in (I): the shortest KÁ Á ÁK separation is 4.4255 (5) Å .

Supramolecular features
In (I) the methoxyphenyl groups lie roughly normal to (010). When the packing is viewed along [101] (Fig. 5), it may be seen that adjacent benzene ring planes are rotated by 90 , which facilitates the formation of a weak edge-to-face intra-sheet C-HÁ Á Á interaction (Table 3). An intra-sheet C2-H2Á Á ÁF2 hydrogen bond also occurs. The only possible inter-sheet interaction in (I) is an extremely weak C-HÁ Á ÁO hydrogen bond with an HÁ Á ÁO separation essentially the same as the van der Waals separation of these species. The layer-stacking sequence for (I) is AAA . . . .
When the crystal structure of (II) is viewed down [110] (Fig. 6), adjacent aromatic rings show the same 90 rotation as they do in (I), but the only directional interaction identified is an intralayer weak C-HÁ Á ÁF hydrogen bond (Table 4) and there are no C-HÁ Á Á interactions. There are no identified inter-layer interactions and the stacking sequence is ABAB . . . . 70 Harrison and Wardell K + ÁC 7 H 7 BF 3 O À and K + ÁC 6 H 4 BF 4 À research communications Table 4 Hydrogen-bond geometry (Å , ) for (II).

Figure 4
The coordination of the potassium ion in (II). See Table 2 for symmetry codes.

Figure 6
The unit-cell packing in (II) viewed approximately down [110].

Figure 5
The unit-cell packing in (I) viewed approximately down [101].

Database survey
Compound (I) is closely related to K +. C 6 H 5 BF 3 À (Conole et al., 1995), (III). Compounds (I) and (III) have the same space group and a similar unit cell, extended in the b-axis direction for (I) to accommodate the methoxy group. The potassium ion in (III) has almost the same KF 8 coordination geometry as the equivalent species in (I) and (II) described above. In (III), weak edge-to-face C-HÁ Á Á interactions occur between approximately perpendicular aromatic rings, as they do in (I). As already noted, the C 6 H 5 BF 3 À anion has found use as a bulky charge-balancing species (Quach et al., 2001;Fei et al., 2010).

Synthesis and crystallization
(I) and (II) were received as commercial samples from Aldrich and recrystallized from ethanol solution, yielding colourless blocks.

Refinement
The H atoms were placed in idealized positions (C-H = 0.95-0.98 Å ) and refined as riding atoms with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). The methyl group in (I) was allowed to rotate, but not to tip, to best fit the electron density.. Experimental details are given in Table 5.  (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(F 2 ) is used only for calculating R-factors(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.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(F 2 ) is used only for calculating R-factors(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.