Crystal structure of bis(N-tert-butylbenzamidinium) hexachloridozirconate(IV) dichloromethane disolvate

In the crystal of the title complex salt, the amidinium cations and the centrosymmetric ZrIV complex anions are linked by N—H⋯Cl hydrogen bonds, forming a two-dimensional network extending along the b-axis direction.


Chemical context
Amidinates represent an important class in the array of Ncentered ligands comparable to the cyclopentadienyl system (Edelmann, 1994;Barker & Kilner, 1994;Collins, 2011). They are four-electron, monoanionic and N-donor bidentate chelates, demonstrating a great diversity by variation of substituents on the conjugated N-C-N backbone. Their steric and electronic properties are easily tunable to meet the requirements of different metal atoms. In the course of extending amidinate chemistry, we have explored a practical synthetic pathway to the alkyl-ended amidinate and ansabis(amidinate) ligands (Bai et al., 2013). They have been applied in the synthesis of Group 4 complexes, which are good catalysts for ethylene polymerization (Bai et al., 2010). Amidines are convenient precursors for both monoanionic amidinate ligands and bianionic ansa-bis(amidinate) ligands (Coles, 2006). Some amidines could be prepared by Yb complex-catalysed addition reactions of aromatic amines and nitriles (Wang et al., 2008). On the other hand, monoanionic amidinate could be used to prepare amidine and amidinium through acidolysis. Based on the same skeleton, the transformation from amidinate to amidinium will undergo an electrical inversion. Here, we report the synthesis and structural characterization of a bis(N-tert-butyl-benzamidinium) hexachloridozirconate complex derived from the monoanionic amidinate.

Structural commentary
The anion in the title compound, (I), is centrosymmetric with the Zr IV cation located on an inversion centre ( Fig. 1) (2) Å ] is a little shorter. In the amidinium moiety, the terminal tert-butyl group is disposed in the direction opposite to the phenyl group on the ipso-carbon of the N-C-N backbone, which could minimize steric hindrance between the two groups. The dihedral angle between the aromatic ring and [NCN] plane is 43.3 (4) . The two C-N bond lengths are equivalent [1.300 (8) and 1.299 (9) Å ], composing a typical conjugated N-C-N skeleton. The lengths of the C-N bonds in (I) are shorter than those reported for a similar amidinium cation (1.325 Å ; Centore et al., 2003).

Supramolecular features
The extended structure consists of amidinium cations forming an extended hydrogen-bonded network with the chlorine atoms of the hexachloridozirconate anions. The amidinium cations involving N1 and N2 all serve as hydrogen-bond donors while only the chlorine atoms in the equatorial plane of the hexachloridozirconate anions act as acceptors ( anions occupy the vertex positions and amidinium cations are on the edge. The corresponding motif obeys the operation of centrosymmetry and the inversion centre is the central point of the square. Moreover, the two-dimensional network extends along the b axis (Fig. 3). In other words, the layered network is parallel to (101) and perpendicular to (010). Besides the N-HÁ Á ÁCl hydrogen bonds, a C-HÁ Á ÁCl hydrogen bond can be observed between two centrosymmetrically related dichloromethane solvent molecules, leading to the formation of a [CHCl] 2 six-membered ring geometry. The angle of the C-HÁ Á ÁCl hydrogen bond is 171 , suggesting a closely linear arrangement of the related C, H and Cl atoms, also resulting in a long distance between donor and acceptor atoms [3.70 (2) Å ].

Database survey
There are 38 structures that incorporate the zirconate anions, including [ZrCl 6 ] 2À , [Zr 2 Cl 10 ] 2À and [Zr 2 Cl 9 ] À . Of those 38 structures, only one has amidinium as the counter-cation (Centore et al., 2003). Its [Zr 2 Cl 10 ] 2À anion has two bridging Cl atoms and its amidinium cation has three substituents attached on the two nitrogen atoms. In contrast to the title The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are presented as small spheres of arbitrary radius. [Symmetry code: (i) Àx + 2, Ày, Àz + 1.]
compound, no N-HÁ Á ÁCl hydrogen bond is observed due to the hindrance of the N-substituents and the lack of an Nbound hydrogen atom.

Synthesis and crystallization
General Procedure: All manipulations and reactions were performed under an inert atmosphere of nitrogen using standard Schlenk techniques. Solvents were pre-dried over sodium, distilled from sodium-benzophenone (diethyl ether and dioxane) and stored over molecular sieves (4 Å ). CH 2 Cl 2 was purified by distillation over CaH 2 . HCl was prepared by treating NaCl with concentrated H 2 SO 4 and dissolved in dioxane.
Synthesis of bis(N-tert-butyl-benzamidinium) hexachloridozirconate(IV): The title compound was prepared by a one-pot reaction of tert-butylamine, LiBu, PhCN, HCl (3.6 M in dioxane) and ZrCl 4 . A solution of LiBu n (2.2 M, 2.27 ml, 5.0 mmol) in hexane was slowly added into a solution of tertbutylamine (0.53 ml, 5.0 mmol) in Et 2 O (30 ml) by syringe at 273 K. The reaction mixture was warmed to room temperature and kept stirring for 3 h. Then benzonitrile (0.51 ml, 5.0 mmol) was added by syringe at 273 K. The reaction mixture was warmed to room temperature and kept stirring for 4 h. HCl (2.78 ml, 10.0 mmol, 3.6 M in dioxane) was added by syringe at 273 K. After stirring at room temperature for 4 h, ZrCl 4 (0.583 g, 2.5 mmol) was added at 273 K. The resulting mixture was stirred at room temperature overnight and all volatiles were removed in vacuo. The residue was extracted with dichloromethane and the filtrate was concentrated to give colorless crystals (yield 1.325 g, 64%). The intermediate process involved an addition reaction of lithium amide and nitrile to yield lithium monoamidinate. Crystals of (I) suitable for single-crystal X-ray investigation were obtained by recrystallization from CH 2 Cl 2 .

Catalytic activity for ethylene polymerization
The catalytic activity of the title compound for ethylene polymerization was examined. At normal pressure and in the presence of methylaluminoxane (MAO), it was found to be an inactive catalyst for ethylene polymerization at 303 K or higher temperature. The reaction was then performed at 10 atm in a 250 mL autoclave. However, only a trace to very small amount of polymer formation could be observed, even when heating the reaction system or changing the ratio of (I) to MAO. Therefore, a conclusion could be drawn that the title compound can not catalyse ethylene polymerization.

Bis(N-tert-butylbenzamidinium) hexachloridozirconate(IV) dichloromethane disolvate
Crystal data (C 11  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.73 e Å −3 Δρ min = −0.90 e Å −3 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.