N 1,N 2-Bis(2,6-dimethylphenyl)-N 1-hydroxyformamidine N,N′-bis(2,6-dimethylphenyl)-N-oxidoformamidinium dichloromethane solvate

The title compound, 2C17H20N2O·CH2Cl2, was obtained by N-oxidation of the parent formamidine with m-chloro-peroxybenzoic acid (m-CPBA). This is the first use of the above-mentioned synthetic route for the preparation of hydroxyamidines. The title compound crystallizes as a cyclic dimer resulting from the presence of O—H⋯O and N—H⋯N hydrogen bonds.


S1. Comment
Hydroxyamidines have long been known to act as bidentate ligands that form stable 5-membered chelate rings with metal ions, and have been extensively studied as sequestrating agents for metals and in pharmacology (Kharsan & Mishra, 1980;Briggs et al., 1976). However, their role as ligands for coordination and supramolecular chemistry has so far received scarce attention (Krajete et al., 2004). This is somewhat surprising as they show good electronic delocalization and interesting design possibilities involving both the coordination geometry and the functionalization of the backbone.
These properties make hydroxyamidines and their complexes interesting candidates for incorporation into supramolecular assemblies. In this paper we studied the behaviour of the title compound, comprising the ligand N-hydroxy-N, N′-bis(2,6dimethylphenyl)formamidine in the solid state. As exemplified by Fig. 1, amidines present two sites for H-bonding interaction: the hydroxy group and the N sp 2 as H-bond donor and acceptor, respectively. It is therefore plausible to expect that the molecule will dimerize in a cyclic self-complementary H-bonded O-H···N fashion.
The asymmetric unit of compound 1 presents two inequivalent molecules of 1 and one molecule of dichloromethane.
The two molecules of the ligand unexpectedly form a cyclic dimer through a pair of H-bonding interactions O-H···O and one N-H···N. In the dimer, one formamidine is present in its neutral form, N=C-N-OH, while the second appears as the zwitterionic form, with the negative charge on the oxygen and the positive charge delocalized between the two nitrogen atoms, leading to two resonance forms: NH + =C-N-Oand NH-C-N + =O -(see scheme). Hence, the resulting bridges can be best described as O-H···Oand N-H···N H-bonds. The O-H···Ointeraction has been previously reported in the analogous crystal structure of a benzamidine system by Krajete et al. (2004). In that case, however, the ligand dimerizes Structural and photophysical studies of the coordination compounds of the formamidine here described are currently in progress and will be subject of a future publication.

S2. Experimental
The title compound was obtained by N-oxidation with m-chloro-peroxybenzoic acid (m-CPBA) of the parent formamidine; the latter was prepared according to the procedure of Krahulic et al. (2005). To a solution of N,N′-bis(2,6dimethylphenyl)-formamidine (1.0 g, 3.96 mmol) in 20 ml of dichloromethane, was added dropwise a solution of m-CPBA (0.9 g, 3.96 mmol) in 20 ml of the same solvent. The reaction mixture was stirred for 30 minutes at room temperature and successively washed with an aqueous solution of K 2 CO 3 (5%) (2 x 25 ml) and of saturated NaHCO 3 (2 x 25 ml). The combined organic fractions were dried over anhydrous Na 2 SO 4 and filtered. The solvent was removed by evaporation, to afford a crude off white product. Recrystallization in DCM/ hexane (1:1) at -10°C yielded colourless Xray quality crystals. Yield 92%.

S3. Refinement
N-bound and O-bound H atoms were located in a difference Fourier map and refined. All other H atoms were placed in calculated positions, with C-H = 0.93-0.99 Å, and refined using a riding model, with U iso (H) = 1.2 or 1.5 U eq (C).

Figure 1
A schematic view of the N-hydroxy-N,N′-bis(2,6-dimethylphenyl)formamidine ligand.  The molecular structure of the title compound, 1. Displacement ellipsoids are shown at 30% probability levels. CH 2 Cl 2 is not shown for clarity.

Special details
Experimental. X-ray crystallographic data for 1 were collected from a single-crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker smart diffractometer equiped with an APEX II CCD Detector, a graphite monochromator. The crystal-to-detector distance was 5.0 cm, and the data collection was carried out in 512 x 512 pixel mode. The initial unit-cell parameters were determined by a least-squares fit of the angular setting of strong reflections, collected by a 10.0 degree scan in 33 frames over four different parts of the reciprocal space (132 frames total). 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.