Bis{2-[(phenyl­imino)­eth­yl]-1H-pyrrol-1-ido-κ²N,N′}nickel(II): a supra­molecular structure formed by C—Hπ hydrogen bonds

Since α-di­imine ligands with bulky aryl substituents form a series of metal complexes (Johnson et al., 1995; Ittel et al., 2000) which present outstanding activities for α-olefin polymerization, investigations have focused on the exploration of nitro­gen-based polydentate ligands (Small et al., 1998; Gibson et al., 1998; Britovsek et al., 2003; Tenza et al., 2009). One of the explorations aims to promote the synthetic application of imino­pyrrolyl ligands for preparing many kinds of transition metal complexes (Mashima & Tsurugi, 2005). However, contrasting with the considerable research reported on symmetric bis­(imino)­pyrrole complexes, metal complexes of ligands containing both an imine and a pyrrolyl group are uncommon. To the best of our knowledge, only a limited number of mono(imino)­pyrrole compounds have been reported in the literature (Dawson et al., 2000; Anderson et al., 2006; Carabineiro et al., 2007; Pérez-Puente et al., 2008; Imhof, 2012, 2013), in most of which the pyrrole ring is unsubstituted. To prepare original structures incorporating bis­(imino)­pyridine ligands (Small et al., 1998; Gibson et al., 1998; Britovsek et al., 2003), we introduced a methyl side arm on the pyrrole ring (Su, Li et al., 2012; Su, Qin et al., 2012) to give the title complex, (II). It is notable that all previous reports of the synthesis of these ligands invariably relate to deprotonation (Tenza et al., 2009), whereas we provide here a simple synthetic route avoiding that process for this kind of complex.

N-[1-(1H-Pyrrol-2-yl)ethyl­idene]aniline (0.100 g, 0.543 mmol) was dissolved in methanol (10 ml) in a 50 ml flask, and a methanol solution of NiCl₂.6H₂O (0.129 g, 0.543 mmol) was added slowly dropwise. The mixture was stirred at room temperature for 3 h. After filtering and washing with hexane, the solvent was removed and a red powder was obtained. Selecting chloro­form and acetone (1:1 v/v) to dissolve the red powder (methanol and a small amount of water were poor solvents), red–brown [Brown given in CIF tables - please clarify] crystals of (I) suitable for X-ray diffraction analysis were obtained using the liquid-phase diffusion method (yield 69.2%). Analysis, calculated for C₂₄H₂₂N₄Ni: C 67.80, H 5.22, N 13.18%; found: C 68.05, H 5.43, N 12.99%. MS (EI): m/z 424 (M⁺). IR (KBr): ν(C═N) 1659 cm^-1.

Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were positioned geometrically and treated using a riding model, with C—H = 0.93 and 0.96 Å for aromatic and methyl H atoms, respectively, and with U_iso(H) = 1.5U_eq(C) for methyl H atoms and 1.2U_eq(C) otherwise.

The molecular structure of (II) and the atom-labelling scheme are shown in Fig. 1, and selected geometric parameters are listed in Table 2. Complex (II) crystallizes with the imino group of the ligand ortho to the pyrrolide N atom and has two inverted N,N'-bidentate chelating pyrrolide ligands. The Ni^II cation is located on a crystallographic inversion centre, tetra­coordinated by two imino N atoms and two pyrrolide N atoms, with the pyrrolide rings and the imine groups trans to each other (Fig. 1). The sum of the angles around the Ni^II centre is 360°, indicating that this atom is in an essentially square-planar conformation. The phenyl substituents on the

imine N atoms show a dihedral angle of 78.79 (9)° with respect to the NiN₄ square plane and are parallel to each other due to the imposed inversion centre. The five-membered pyrrolide (py) ring formed by atoms N2/C8–C11 is planar, with a maximum deviation from the plane of 0.004 (3) Å [For which atom?]. In addition, we note that the Ni—N_imine distance [1.939 (2) Å] is substanti­ally longer than the Ni—N_py bond [1.894 (3) Å], due to the anionic nature of the pyrrolide N atom and the steric hindrances of the phenyl-ring substituents. However, all Ni—N bond lengths in (II) are shorter than the normal values for typical Ni^II—N bonds (2.07 Å; Orpen et al., 1989), particularly the imine Ni—N distances. This may indicate a stronger σ-donor character of atom N1 induced by the methyl (C12) substituent of the iminic carbon (C7), which may also give rise to a higher degree of steric congestion around the Ni^II cation.

Comparison of the data for the free ligand, (I) (Su, Li et al., 2012), and its Ni^II complex, (II), also highlights some structural differences. The first feature to note is that the acetyl­imino­pyrrolyl ligand bite angles of (II) (N_imino—Ni—N_py) are very acute [83.13 (11)°], and this decreases the N1—C7—C8 and N2—C8—C7 angles in relation to those observed in the organic ligand precursor (I) [122.82 (16) and 118.71 (16)°, respectively]. Also, the angles at the pyrrole N atom (C—N_py—C) decrease upon coordination, which is compensated by increases in the angles at the C atoms bound to the pyrrole N atom. The bond lengths within the pyrrole ring appear to be significantly affected, and both the C—C and C—N distances, apart from that opposite the C—C bond of the N atom, appear to increase. The angle at imino atom N1 (C6—N1—C7) decreases, but the imine C7═N1 double bond and N1—C6 lengthen upon coordination, indicating that π-back-donation from the Ni^II centre to the imine fragment is relatively strong.

Evidently, an inter­esting phenomenon could be observed in several structures containing various metal complexes of pyrrole ligands (Anderson et al., 2006; Carabineiro et al., 2007; Pérez-Puente et al., 2008; Imhof, 2012, 2013). It is noted that the M—N bond lengths in these examples reveal that the M—N_imino bonds are ca 0.01–0.05 Å longer than the corresponding M—N_py bond within each metal bidentate-chelate unit. In addition, a significant negative correlation is found between the N_imino—M—N_py angle for the five-membered chelate rings of pyrrole complexes and the M—N_py bond length. For example, the M—N_py distance increases from 1.9061 (15) Å for [Ni(imino­pyrrole)₂] (Ar = 2,4,6-Me₃C₆H₂ [Where is Ar located? Ar-imino?]; Anderson et al., 2006) to 1.915 (2) Å for [Ni(imino­pyrrole)₂] (Ar = 2,6-Me₂C₆H₃; Pérez-Puente et al., 2008) to 1.9388 (13) Å for [Co(imino­pyrrole)₂] (Ar = 2,6-diiso­propyl­aniline; Carabineiro et al., 2007) to 2.0189 (19) Å for [Pd(imino­pyrrole)₂] (Ar = 2,4,6-Me₃C₆H₂; Imhof, 2013) to 2.022 (2) Å for [Pd(imino­pyrrole)₂] (Ar = C₆H₆, Imhof, 2012), while the corresponding inner bite angle decreases from 83.80 (6) to 83.50 (10) to 82.15 (6) to 80.91 (8) to 80.00 (9)°, respectively; the above values are all averages. However, complex (II) does not conform to the above law. The Ni—N_py bond length is 1.894 (3) Å. The N_imino—Ni—N_py angle should be greater than 83.80 (6)° according to the law, but this is not the case, since the angle is 83.13 (11)°. This special case may be due to packing effects.

Examination of the structure with PLATON (Spek, 2009) shows that there is no classical donor (N) present, whereas there are two C—H···π inter­actions (Table 3), thus saturating the hydrogen-bonding capability of the π-electron clouds. One is an inter­molecular inter­action, with phenyl atom C5 acting as the donor, while the other is intra­molecular (see Figs. 1 and 2), with pyrrolide atom C11 acting as the donor. The molecules are linked into simple chains by means of two C—H···π hydrogen bonds in which the molecules act as hydrogen-bond donors and acceptors, resulting in infinite chains.

The supra­molecular assembly of (II) takes the form of a one-dimensional hydrogen-bonded structure. The angles of approach of the H···Cg vector to the planes of the aromatic rings are about 77 and 71° [For which order of the rings?], respectively, and the perpendicular projections of the H atoms onto the pyrrolide ring planes are 0.604 (2) and 0.867 (7) Å, respectively, from the centroids of the rings. In both inter­actions, the H atom lies above the centre of the ring, with the C—H bond pointing towards a pyrrolide and phenyl ring C atom. This corresponds to a type III inter­action, according to the classification of Malone et al. (1997). Furthermore, it has been recognized that these weak C—H···π inter­actions can play an important role in the conformations of organic molecules (Umezawa et al., 1999).