Crystal structures of the Lewis acid–base adducts {[(C6H11)2N]3Ti–LB}+[MeB(C6F5)3]−·1.5C7H8; LB = (C6H5)3PO (1), p-F—C6H4CN (2)

In the molecular structures of the two title compounds, the central titanium(IV) atoms are coordinated in a distorted tetrahedral fashion. Thereby, the titanium atom is stabilized by the Lewis bases triphenylphosphine oxide (1) and p-fluorobenzonitrile (pFBN) (2) with dative O—Ti and N—Ti bonds, respectively.

The reaction of [(C 6 H 11 ) 2 N] 3 Ti-CH 3 with the Lewis acid B(C 6 F 5 ) 3 , followed by addition of the Lewis bases (C 6 H 5 ) 3 PO and p-F-C 6 H 4 CN led to the complex salts tris(dicyclohexylamido)(triphenylphosphine oxide)titanium methyltris-(pentafluorophenyl)borate toluene sesquisolvate, [Ti(C 12 (2), both crystallizing with 1.5 equivalents of toluene solvent molecules. The Lewis acid-base adducts (1) and (2) can be described by dative donor bonds. The packing of the complex cations, anions and solvent molecules in the crystal structure is consolidated by an intricate three-dimensional network of non-classical C-HÁ Á ÁF interactions. Disorder of some of the cyclohexyl groups and the toluene solvent molecules is observed.

Chemical context
Highly electrophilic d 0 cations of group 4 metals are known as strong Lewis acids (Bochmann, 2010). Among various other applications, Lewis acids are used in reactions to activate small molecules or in the catalysis of chemical reactions (Corma & García, 2003). The use of a Lewis acid in a catalytic reaction requires knowledge of its strength (Greb, 2018). There are now many different ways of determining the Lewis acidity of a compound. The best known is the Gutmann-Beckett method (Mayer et al., 1975). Another possibility is the use of parafluorobenzonitrile (Kü nzler et al., 2019). In this context, we report here syntheses and crystal structures of the Lewis acidbase adducts derived from Ti IV as the Lewis acid, {[(C 6 H 11 ) 2 N] 3 TiOP(C 6 H 5 ) 3 } + [H 3 CB(C 6 F 5 ) 3 ] À Á1.5C 7 H 8 (1) and {[(C 6 H 11 ) 2 N] 3 TiNC 7 H 4 F} + [H 3 CB(C 6 F 5 ) 3 ] À Á1.5C 7 H 8 (2).

Structural commentary
In compound (1), the titanium(IV) atom is coordinated by three N atoms and one O atom in a slightly distorted tetrahedral fashion ( Fig. 1) with bond angles around titanium(IV) ranging from 105.94 (4) to 112.06 (4) . The Ti-O bond length [1.9782 (8) Å ] is in the range of a single bond (1.99 Å ), similar to titanium-triphenylphosphane adducts (Pyykkö & Atsumi, 2009;Brock et al., 1985). The Ti-O-P angle amounts to 170.99 (5) and thus indicates a bonding situation deviating only slightly from linearity. As a result of the coordination of the triphenylphosphine oxide ligand to the central titanium atom, the O-P bond [1.5269 (8) Å ] is widened compared to the O-P bond in the free phosphine oxide [1.491 Å ;Brock et al., 1985). The angular sums around the nitrogen atoms reveal a trigonal-planar environment in each case, with values of 359.9 for N1, 359.7 for N2 and 360.0 for N3 (using only the major component of the disordered ligand for calculation).
In compound (2), the titanium(IV) atom is coordinated by four N atoms in a likewise slightly distorted tetrahedral fashion ( Fig. 2) with bond angles around titanium(IV) ranging from 104.66 (4) to 113.62 (4) . The Ti1-N4 bond length of 2.1608 (9) Å to the N C group of the para-fluorobenzonitrile ligand is in the range of a dative bond (Pyykkö & Atsumi, 2009), and by far greater than the other three Ti-N bonds (' 1.89 Å ), which are in the range of shortened single bonds. The  ] and the N4-C37-C38 angle [179.25 (12) ] indicate a nearly linear arrangement of the Ti-N C-C fragment. The trigonal-planar environment of the three nitrogen atoms of the amido ligands is described by the angle sums N1 = 359.3 (using only the major component of the disordered ligand for calculation), N2 = 359.9 and N3 = 360.0 .

Synthesis and crystallization
All reactions were carried out under a dry nitrogen atmosphere using Schlenk techniques or in a glove box. Solvents were dried according to standard procedures over Na/K alloy with benzophenone as an indicator and distilled under a nitrogen atmosphere. The cationic titanium complex was synthesized by reacting tris-(dicyclohexylamido)methyl-  Table 3 Experimental details. (1) Crystal data Chemical formula [Ti(C 12

Figure 3
A view along the a axis showing the packing of individual molecules in the crystal of (1), with C-HÁ Á ÁÁF bonds shown as dashed lines. Both components of the disorder are shown. Colour code: C dark grey, H light grey, N blue, Ti teal, B dark yellow, F bright green, P purple, O red.

Figure 4
A view along the a axis showing the packing of individual molecules in the crystal of (2) 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.