Tetrachlorido[N 2,N 2′-(dimethylsilanediyl)bis(N-tert-butyl-3-methylbenzimidamidato)-κ2 N 2,N 2′]hafnium(IV)

The symmetric title molecule, [Hf(C26H40N4Si)Cl4], lies about a twofold rotation axis. The HfIV and Si atoms lie on the rotation axis with all other atoms being in general positions. The HfIV atom is six-coordinated by two N atoms from the N 2,N 2′-(dimethylsilanediyl)bis(N-tert-butyl-3-methylbenzimidamidate) ligand and four Cl− ions in a slightly distorted octahedral geometry. The two amidinate moieties are connected through the central Si atom with Si—N bond length of 1.762 (3) Å, generating the characteristic N—C—N—Si—N—C—N skeleton of a silyl-linked ansa-bis(amidine) species.

The symmetric title molecule, [Hf(C 26 H 40 N 4 Si)Cl 4 ], lies about a twofold rotation axis. The Hf IV and Si atoms lie on the rotation axis with all other atoms being in general positions. The Hf IV atom is six-coordinated by two N atoms from the N 2 ,N 2 0 -(dimethylsilanediyl)bis(N-tert-butyl-3-methylbenzimidamidate) ligand and four Cl À ions in a slightly distorted octahedral geometry. The two amidinate moieties are connected through the central Si atom with Si-N bond length of 1.762 (3) Å , generating the characteristic N-C-N-Si-N-C-N skeleton of a silyl-linked ansa-bis(amidine) species.

Comment
Anionic N,N-chelating amidinate ligands, have been widely used in the synthesis of organometallic complexes of the s-, p-, d-, and f-block metals for a number of years (Edelmann, 2012;Münch et al., 2008). Their steric and electronic properties can easily be modified by a simple variation of the substitution pattern (Liu et al., 2013;Qian et al., 2010). In the search for ancillary ligands to replace cyclopentadienyls to create non-metallocene species, amidinate anions have found many applications in coordination chemistry, and also as ancillary ligands to form metal complexes which act as catalysts in organic transformations and ethylene polymerizations (Lei et al., 2011).
Linked bis(amidinate) ligands are a very special branch of this class of compound and their chemistry has been developed in recent years. We explored a class of silyl linked bis(amidinate) ligands, and applied them to the synthesis of metal complexes. They imposed a close contact between the two amidinate moieties and had the advantage of affording binuclear complexes analogous to an "ansa-metallocene" (Bai et al., 2013). Here, the synthesis and characterization of the Hf(IV) complex SiMe 2 [NC(m-MePh)N(Bu t )H] 2 HfCl 4 bearing the silyl-linked ansa-bis(amidine) ligands will be described.
Treating the ansa-bis(amidine) SiMe 2 [NC(m-MePh)NH( t Bu)] 2 with HfCl 4 in CH 2 Cl 2 gave the title compound. Crystals suitable for X-ray investigation were obtained by recrystallization from toluene and its molecular structure is presented in Fig. 1. It is a symmetric molecule lying about a 2-fold rotation axis (e in Wyckoff notation). The Hf1 and Si1 atoms lie on this axis with all other atoms on general positions. The two amidinate moieties connect the central Si atom with Si-N2 distances 1.762 (3) Å, which matched our original proposal of forming a dianionic N-C-N-Si-N-C-N framework. The structure shows that all the substituents and the silyl bridge are on the same side of the N-C-N skeletons, resulting in two E-anti forms of the amidinate units. The two inner nitrogen atoms bind to Hf1 at a distance of 2.233 (3) Å, and the N2-Hf2-N2 i angle is 69.28 (11)° (i = -x + 1, y, -z + 1/2). The Hf center also exhibits a slightly distorted octahedral geometry.

Experimental
A solution of LiBu n (2.2 M, 2.27 ml, 5.0 mmol) in hexane was added to a stirred solution of t BuNH 2 (0.53 ml, 5.0 mmol) in THF (ca 30 ml) by syringe at 273 K. The reaction mixture was warmed to room temperature and kept stirring for 4 h and then m-MePhCN (0.59 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. Then SiMe 2 Cl 2 (0.3 ml, 2.55 mmol) was added by syringe at 273 K. After stirring at room temperature for 4 h, it was dried in vacuum to remove all volatiles. The residue was extracted with CH 2 Cl 2 (30 ml) and then HfCl 4 (0.812 g, 2.5 mmol) was added to this stirred solution at 273 K. The reaction mixture was warmed to room temperature, after stirring for 4 h the solution was dried in vacuum to remove all volatiles. The residue was

Refinement
The methyl H atoms were constrained to an ideal geometry, with C-H distances of 0.96 Å and U iso (H) = 1.5U eq (C), but each group was allowed to rotate freely about its C-C and C-Si bonds. The amino H atoms were constrained with N-H distances of 0.86 Å and U iso (H) = 1.2U eq (N). The phenyl H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.93 Å and U iso (H) = 1.2U eq (C).

Figure 1
The molecular structure, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Crystal data
[Hf (C 26  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.