Crystal structures of a novel NNN pincer ligand and its dinuclear titanium(IV) alkoxide pincer complex

The title compound is an LiBr-bridged TiIV alkoxide dimer, supported by a novel monoanionic [NNN] pincer-type ligand. The bis[2-(1-imino-2,2-dimethylpropyl)-4-methylphenyl]amine ligand is the first reported ligand that bears hydrogen atoms on its ketimine side arms.


Chemical context
Pincer ligands occupy the meridional coordination sites on a metal ion and were first introduced by Moulton and Shaw in 1976. In the original system, the pincer ligand 2,6-bis[(di-tbutylphosphino)methyl]phenyl binds to the late transition metals Ni, Pd, Pt, Rh, and Ir through the deprotonated aromatic carbon and the pendant -PR 2 side arms (R = t-butyl) (Moulton & Shaw, 1976). Under the HSAB theory, this particular arrangement can be viewed as a soft-hard-soft coordination mode. Since this discovery, the library of ISSN 2056-9890 tridentate ligands that exhibit this unique meridional coordination of a metal atom has been extended not only by additional monoanionic pincer and pincer-type ligands, but as well by numerous neutral, dianionic and trianionic pincer-type ligands (Van Koten, 2013;Gunanathan & Milstein, 2011;O'Reilly & Veige, 2014). Recent advances in the chemistry of metal complexes supported by trianionic pincer and pincertype ligands which exhibit a unique hard-hard-hard binding mode (Sarkar et al., 2008) highlight their potential for applications as catalysts in polymerizations (McGowan et al., 2013), alkene isomerizations (McGowan et al., 2011, and as catalytic group or atom-transfer reagents (O'Reilly et al., 2009).
Monoanionic diarylamino [NNN] ligands with imine functionality on the flanking side arms were reported in 1978 by Black and Rothnie (Black & Rothnie, 1978), and are gaining interest as evidenced by newly introduced systems in recent years. In the present work, we introduce a protocol for the synthesis and characterization of a novel [NNN]H 3 pincertype ligand that involves the addition of a nitrile to an aryllithium salt, a protocol described by Parham and coworkers (Parham et al., 1978).

Structural commentary
Ketimine ligands typically possess a bulky group (such as t Bu) on their N atoms. The ligand moiety of 1 ( Fig. 1) is unique in that it contains a proton in the terminal position. The complete molecule of 1 is located about a twofold rotation axis. The coordinated Li atoms exhibit an N2-Li2 bond length of 2.065 (7) Å and a N3-Li1 bond length of 2.065 (7) Å . The two lithium ions are both bridged by two bromides with an Li1-Br1 bond length of 2.504 (6) Å and an Li-Br1(Àx + 1, y, Àz + 1 2 ) bond length of 2.531 (7) Å . Furthermore, both coordinated lithium ions carry a bound Et 2 O solvent molecule, each with an Li-O bond length of 1.961 (7) Å . The short C N bond length of 1.277 (4) Å is comparable to reported C N bond lengths. For instance the C N bond length in furazan is 1.29 Å (Allen et al., 1987).
Similar to its solid-state structure, 1 exhibits C 2 symmetry in solution. The 1 H NMR spectrum in CDCl 3 (see Supporting information) exhibits a singlet at 2.26 ppm attributable to the methyl groups on the aryl backbone of the ligand framework. Another characteristic singlet that appears at 1.20 ppm has three times the intensity of the backbone CH 3 and is attributable to the tert-butyl CH 3 protons residing on each ligand arm. Furthermore, the 1 H NMR spectrum exhibits a quartet at 3.48 ppm and a triplet at 1.20 ppm, both signals can be assigned to the -CH 2 and -CH 3 groups of bound Et 2 O. The central backbone N-H resonates as a singlet at 5.32 ppm, and the ketimine N-H protons resonate at 9.42 ppm. 1 H-15 N gHMBC indirect detection demonstrates that the central nitrogen resonates at 77.00 ppm. In contrast, the chemical shifts of the ketimine nitrogen atoms are not observable. Furthermore, in a NOESY1D experiment the tert-butyl CH 3 groups show an nOe with the Et 2 O -CH 3 group when irradiated selectively at 1.28 ppm. From the occurrence of this nOe, it can be concluded that one Et 2 O molecule is bonded to every lithium atom.
In the solid state, complex 2 is located on an inversion center (Figs. 2 and 3) and the Ti IV core exhibits a slightly distorted octahedral environment. The N2-Ti bond length of 2.069 (2) Å confirms that the central pincer nitrogen atom is deprotonated. The slightly elongated N1-Ti and N3-Ti bonds of 2.136 (3) and 2.130 (3) Å are indicative of an L-type bonding of the ket-mine nitrogen atoms. The bond lengths and the fact that the titanium metal atom is coordinated by three isopropoxide ligands supports the claim that the [NNN] ligand within 2 must be monoanionic with both ketimine N-H protons still present. The Ti-O1, Ti-O2 and Ti-O3 bond lengths are 1.805 (2), 1.901 (2) and 1.934 (2) Å , respectively. The increase in bond length between Ti-O2 and Ti-O3 in comparison to Ti-O1 is attributed to the coordination of Li to O2 and O3. While the O3-Ti-O1 bond angle of 173.58 (9) deviates slightly from the optimal angle of 180 , the angle N1-Ti-N3 is 160.94 (11) . This distortion is due to the short bond length that can be found in a C N bond. The dinuclear complex also exhibits four disordered regions. The isopropyl groups on C25, C28, C31 and the tert-butyl group on C21 are all disordered and were refined in two parts. The bridging Br ligands are also disordered and were refined in two parts; namely Br1 and Br2.

Experimental
Unless specified otherwise, all manipulations were performed under an inert atmosphere using standard Schlenk or glovebox techniques. Glassware was pre-dried in an oven before use. Pentane, toluene, and diethyl ether (Et 2 O) were dried using a GlassContours drying column. Chlorofrom-d 1 (Cambridge Isotopes) was dried over anhydrous CaCl 2 ; vacuum transferred, passed over a plug of basic alumina, and stored over 4 Å molecular sieves. Di-p-tolylamine, n BuLi (2.5 M in hexanes), titanium(IV)isopropoxide, and HCl (1 M in Et 2 O) were purchased from Sigma Aldrich and used as received. Trimethylacetonitrile was vacuum distilled and freeze pump thawed prior to use. Bis(2-bromo-4-methylphenyl)amine was prepared by literature methods (Corey et al., 2010).

Synthesis and crystallization of title compound 1
In a nitrogen-filled glove-box, a glass vial was charged with bis(2-bromo-4-methylphenyl)amine (0.125 g, 0.35 mmol), 3.0 mL of Et 2 O. 3.1 eq. n BuLi (2.5 M in hexanes) (0.44 mL, 1.1 mmol) was added dropwise to a stirring solution of bis(2bromo-4-methylphenyl)amine. The reaction mixture color changed from colorless to yellow. After stirring for 120 min, pivalonitrile was added dropwise, resulting in a color change from yellow to orange. After an additional 180 min of stirring, excess HCl (1 M in Et 2 O) was added dropwise, resulting in a color change from orange to yellow and the formation of a white microcrystalline powder. The pale-yellow solution was filtered through Celite 2 . The volatiles in the resulting filtrate were removed in vacuo and the oily residue was triturated three times (3 Â 2 mL) with pentane. Single crystals were obtained by cooling a concentrated toluene solution of 1 to 238 K. Yield: 0.091 g (0.14 mmol, 41%). 1 H NMR (500 MHz, CDCl 3 , 298 K): = 1.20 (t, 12H, CH 3 (Et 2 O) 2 ), 1.28 (s, 18H,

Synthesis and crystallization of title compound 2
In a nitrogen-filled glove-box, a glass vial was charged with [NNN]H 3 (LiBr) 2 (1) (0.075 g, 0.118 mmol), and 3 mL of Et 2 O. 1.1 eq. Ti(O i Pr) 4 (38.5 mL, 0.130 mmol) was added dropwise to a stirring solution of 1. The reaction mixture changed color instantaneously from yellow to dark red. After stirring for 120 min, the dark-red solution was filtered through Celite 2 . The volatiles in the resulting filtrate were removed in vacuo and the resulting oily residue was washed three times (3 Â 2 mL) with pentane. Single crystals were obtained by preparing a concentrated solution of the oily complex 2 in toluene and cooling it for two weeks at 238 K. Yield: 0.107 g (0.081 mmol, 69%).

Refinement details complex 1
Crystal data, data collection and structure refinement details are summarized in Table 1. The non-H atoms were refined with anisotropic displacement parameters and all of the H atoms were calculated in idealized positions (C-H = 0.93/ 1.00 Å ) and refined riding on their parent atoms with U iso (H)= 1.2/1.5U eq (C), except for the -N-H hydrogen atoms which were obtained from a difference Fourier map and refined freely. The dimer complex is located on a twofold rotation axis of symmetry and thus only a half is contained in the asymmetric unit. One ethyl group of the Li-coordinating ether ligand is disordered and was refined in two parts (C15-C16/ C15 0 -C16 0 ). Their site-occupation factors dependently refined to 0.812 (8) and 0.188 (8), for the major and minor parts, respectively.

Refinement details complex 2
The non-H atoms were refined with anisotropic displacement parameters and all of the H atoms were calculated in idealized positions (C-H = 0.93/1.00 Å ) and refined riding on their parent atoms with U iso (H)= 1.2/1.5U eq (C), except for the -N-H hydrogen atoms which were obtained from a difference-Fourier map and refined freely. The Ti dimer is located on an inversion center and thus a half dimer is present in the asymmetric unit. One and a half toluene solvent molecules are also present in the asymmetric unit. The half toluene molecule is disordered around inversion symmetry while the one in a general position is disordered in two parts. The toluene molecules were significantly disordered and could not be modeled properly, thus SQUEEZE (Spek, 2015), a part of the PLATON (Spek, 2009) package of crystallographic software, was used to calculate the solvents' disorder areas and remove their contributions to the overall intensity data. The disordered solvents area is centered around the 0.0, 0.0, 0.0 position and showing an estimated total of 151 electrons and a void volume of 586 Å 3 . The dimer also exhibits four disordered regions. The isopropyl groups on C25, C28, C31 and the t-butyl group on C21 are all disordered and were refined in two parts with their site occupation factors fixed to 0.6/0.4 in the final refinement model. The bridging Br ligands are also disordered and refined in two parts, Br1 and Br2, to values of 0.674 (12) and 0.326 (12), respectively. The -N-H hydrogen atoms were obtained from a difference-Fourier map and refined freely. For both compounds, data collection: APEX2 (Bruker, 2008); cell refinement: APEX2 (Bruker, 2008); data reduction: SAINT (Bruker, 2008). Program(s) used to solve structure: SHELXT2013 (Sheldrick, 2015a) for (1); SHELXTL2014 (Sheldrick, 2008) for (2). Program(s) used to refine structure: SHELXL2013 (Sheldrick, 2015b) for (1); SHELXTL2014 (Sheldrick, 2008) for (2). Molecular graphics: DIAMOND (Brandenburg, 2014) for (1); SHELXTL2014 (Sheldrick, 2008) for (2). Software used to prepare material for publication: SHELXL2013 (Sheldrick, 2015b) for (1); SHELXTL2014 (Sheldrick, 2008) for (2). (

1) Di-µ-bromido-µ-{2-(2,2-dimethylpropanimidoyl)-N-[2-(2,2-dimethylpropanimidoyl)-4-methylphenyl]-4-methylaniline}-bis[(diethyl ether)lithium]
Crystal data  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. Refinement. All H atoms were positioned geometrically ( C-H = 0.93/1.00 Å) and allowed to ride with U iso (H)= 1.2/1.5U eq (C). Methyl ones were allowed to rotate around the corresponding C-C. The dimer complex is located on a 2-fold rotation axis of symmetry thus only a half is contained in the asymmetric unit. One ethyl group of the Li coordinated ether ligand is disordered and was refined in two parts (C15-C16/C15′-C16′). Their site occupation factors dependently refined to 0.812 (8) and 0.188 (8) for the major and minor parts, respectively. The nitrogen protons were obtained from a Difference Fourier map and refined freely.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.005 Δρ max = 0.72 e Å −3 Δρ min = −0.43 e Å −3 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. Refinement. All H atoms were positioned geometrically ( C-H = 0.93/1.00 Å) and allowed to ride with U iso (H)= 1.2/1.5U eq (C). Methyl ones were allowed to rotate around the corresponding C-C. The asymmetric unit consists of the Ti dimer and one and a half toluene solvent molecules. The half toluene molecule is disordered around inversion symmetry while the one in general position is disordered in two parts. The toluene molecules were disordered and could not be modelled properly, thus program SQUEEZE, a part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The dimer also exhibits four disordered regions. The isopropyl groups on C25, C28, C31 and the t-butyl group on C21 are all disordered and each was refined in two parts. In each disordered case, the site occupation factors of the major and minor parts were fixed (only in the final cycle of refinement) to 0.6 and 0.4, respectively. The bridging Br ligand is also disordered and was refined in two parts, Br1 and Br2, with their site occupation factors refining to 0.68 (1) and 0.32 (1), respectively.