Nickel(II) carbonyl, ammonia, and acetonitrile complexes supported by a pyridine dipyrrolide pincer ligand

The synthesis, isolation and crystal structures of nickel(II) carbonyl, acetonitrile and ammonia complexes supported by a dianionic, pyridine dipyrrolide pincer ligand [pyrr2py]2−, namely, carbonyl[2,2′-(pyridine-2,6-diyl)bis(3,5-di-p-tolylpyrrolido-κN)]nickel(II), [Ni(C41H33N3)(CO)], ammine[2,2′-(pyridine-2,6-diyl)bis(3,5-di-p-tolylpyrrolido-κN)]nickel(II), [Ni(C41H33N3)(NH3)], and (acetonitrile-κN)[2,2′-(pyridine-2,6-diyl)bis(3,5-di-p-tolylpyrrolido-κN)]nickel(II), [Ni(C41H33N3)(CH3CN)], as well as the free ligand 2,6-bis(3,5-di-p-tolylpyrrol-2-yl)pyridine, C41H35N3 or [pyrr2py]H2 are reported.


Chemical context
Pincer ligands were first introduced by Moulton and Shaw in 1976 (Moulton & Shaw, 1976). They are utilized widely as auxiliary ligands to produce transition-metal complexes useful in a range of applications including catalysis (Alig et al., 2019;Peris & Crabtree, 2004Piccirilli et al., 2020;Albrecht & van Koten, 2001). There are several pincer-ligand varieties in the literature ranging from those featuring both symmetric and non-symmetric flanking arms, P-, N-, O-, S-and C-donor sites, as well as neutral, mono, di-and trianionic systems. Monoanionic, carbon-centered (e.g., from phenyl) ligands with P-or N-donors at the flanking arms are more common among pincers (Peris & Crabtree, 2018). These tridentate ligands are particularly interesting for their ability to preferentially occupy the meridional coordination sites on a metal ion.

Structural commentary
The free ligand [pyrr 2 Py]H 2 is monomeric and crystallizes with both pyrrole nitrogen atoms facing the center of the coordination pit, well situated for metal-ion coordination (Fig. 2). This is different from the structure observed with the t-butyl substituted pincer analog (VIWSOL; Komine et al., 2014) in which one pyrrole N-H bond is directed outward to form a hydrogen bond with a lattice acetonitrile molecule. The pyrrole and pyridine moieties are essentially coplanar. The nickel(II) carbonyl complex [pyrr 2 Py]Ni(CO) was synthesized from the in situ-generated potassium salt K 2 [pyrr 2 Py] and Ni(OTf) 2 in the presence of carbon monoxide. The important CO stretch of this molecule is observed at 2101 cm À1 , which is only slightly lower than that of free CO (2143 cm À1 ), indicating relatively weak Ni!CO -backbonding. The nickel(I) tris(pyrazolyl)borate complex [HB(3-Ph,5-MePz) 3 ]Ni(CO) for comparison displays its CO stretch at 2003 cm À1 (Abubekerov et al., 2016). The X-ray crystal structure shows that the pincer complex [pyrr 2 Py]Ni(CO) is a monomeric, square-planar complex (Fig. 3) Molecular structure of [pyrr 2 Py]H 2 with displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
Important intermolecular contacts and a packing diagram of [pyrr 2 Py]H 2 are shown in Fig. 6 and Fig. S1 in the supporting information. Neighboring molecules of [pyrr 2 Py]H 2 showcontacts between pyrrole and pyridine groups (the closest separation is 3.21 Å ) as well as C(arene)-HÁ Á Áarene contacts. The complex [pyrr 2 Py]Ni(CO) does not show extensive intermolecular interactions apart from NiCOÁ Á ÁH-C(arene) contacts between the carbonyl moieties and hydrogen atoms of neighboring arene as illustrated in Fig. 7 and Fig. S2. In the structure of [pyrr 2 Py]Ni(NH 3 ), the arene groups interact with neighboring molecules via the ammonia hydrogen atoms (see Molecular structure of [pyrr 2 Py]Ni(NH 3 ) with displacement ellipsoids drawn at the 50% probability level.

Figure 5
Molecular structure of [pyrr 2 Py]Ni(NCMe) with displacement ellipsoids drawn at the 50% probability level. A disordered hexane molecule has been omitted for clarity.

Figure 6
The crystal packing of [pyrr 2 Py]H 2 . between arenes and the hydrogen atoms of the acetonitrile moieties. The resulting packing diagram is shown in Fig. 9 and Fig. S4.

Database survey
A search of the Cambridge Structural Database for related pyridine dipyrrolide complexes involving transition-metal ions revealed 38 hits involving ligands with different alkyl or aryl substituents (CSD Version 5.41, Update 2, May 2020; Groom et al., 2016). No nickel pyridine dipyrrolide complexes have been reported thus far. Perhaps the most closely related compounds are the four-coordinate platinum (VIWSIF; Komine et al., 2014), palladium (XIKKIO, XIKKOU; Yadav et al., 2018) and zinc (VIWSIF; Komine et al., 2014) complexes featuring all-nitrogen coordination spheres at the metal. In addition, there are ten hits for related free ligands. Most of them, however, are different solvates of the same ligand system.

Synthesis and crystallization
All experiments were done under a purified nitrogen atmosphere with standard Schlenk techniques. Solvents were purchased from commercial sources and purified using an Innovative Technology SPS-400 PureSolv solvent-drying system or distilled over conventional drying agents and degassed by the freeze-pump-thaw method three times prior to use. All other chemicals needed were obtained from commercial vendors. Glassware was oven dried at 150 C overnight. The NMR spectra were recorded at 25 C on JEOL Eclipse 500 and 300 spectrometers ( 1 H: 500.16 MHz or 300.53 MHz). Proton chemical shifts are reported in ppm versus Me 4 Si. Infrared spectra were taken on a JASCO FT-IR 410 spectrometer.

Figure 8
The crystal packing of [pyrr 2 Py]Ni(NH 3 ). Hydrogen atoms except those on ammonia have been omitted for clarity.

Synthesis of [pyrr 2 Py]Ni(NH 3 ):
A solid sample of the ligand 2,6-bis(3,5-ditolyl-2-pyrrolyl)pyridine ([pyrr 2 Py]H 2 ) (0.10 g, 0.175 mmol) and KH (0.021 g, 0.525 mmol) were placed in a 50 mL Schlenk flask. THF (ca 10 mL) was added to the mixture at room temperature and then refluxed for 1.5 h. It was allowed to cool down to room temperature and filtered through a Celite pad, which was then washed with 5 mL of THF. The filtrate was added to Ni(OTf) 2 (0.062 g, 0.175 mmol) in 10 mL of THF and stirred overnight at room temperature. Then THF was removed and the residue was extracted into ether. Then anhydrous ammonia gas was passed through the ether solution for 20 minutes at 273 K. After stirring for 1 h, the solution was filtered, and the volume of the solution was decreased to 4 mL.  Table 1. Non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms, except for the N-H hydrogen atoms, were placed in calculated positions using riding models, and refined riding on their parent atoms with C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for aromatic hydrogen atoms, C-H = 0.99 Å and U iso (H) = 1.2U eq (C) for methylene hydrogen atoms (of hexane), and C-H = 0.98 Å with U iso (H) = 1.5U eq (C) for methyl hydrogen atoms. The N-H hydrogen atoms of [pyrr 2 Py]H 2 and [pyrr 2 Py]Ni(NH 3 ) were obtained from a difference-Fourier map and refined freely. The nickel carbonyl complex [pyrr 2 Py]Ni(CO) is located on a plane of symmetry containing the Ni-CO moiety but perpendicular to the [pyrr 2 Py] ligand plane, and consequently only a half is contained in the asymmetric unit. The complex [pyrr 2 Py]-Ni(NCCH 3 ) crystallizes with a molecule of hexane, which was disordered over two sites [with refined occupancy rates of 77.9 (5)% and 22.1 (5)%]. C-C bond distances were restrained to a target value of 1.53 (2) Å (DFIX restraint of SHELXL), 1,3 CÁ Á ÁC distances were restrained to be similar to each other (SADI restraint of SHELXL, esd = 0.04 Å ), and U ij components of ADPs were restrained to be similar for atoms closer to each other than two Å (SIMU restraint of SHELXL, esd = 0.02 Å 2 for terminal atoms and 0.01 Å 2 for all others).  For all structures, data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT Bruker, 2016); program(s) used to solve structure: ShelXT (Sheldrick, 2015b); program(s) used to refine structure: SHELXL (Sheldrick, 2015a); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). 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.