Structure of a dinuclear cadmium complex with 2,2′-bipyridine, monodentate nitrate and 3-carboxy-6-methylpyridine-2-carboxylate ligands: intramolecular carbonyl(lone pair)⋯π(ring) and nitrate(π)⋯π(ring) interactions

A dinuclear Cd 3-carboxy-6-methylpyridine-2-carboxylate complex is reported, in which the anionic ligand displays an unusual μ2-κ3 coordination mode. The crystal structure consists of hydrogen-bonded planar arrays held by X—H⋯O (X = O,C) and X—O⋯π(ring) (X = N,O) interactions, leaving interstitial columnar voids.


Chemical context
Pyridinedicarboxylate ligands derived from pyridine-2,3-dicarboxylic acid (pydcH 2 ) have been extensively used in the construction of a large variety of structural motifs. The two deprotonated forms pydcH À and pydc 2À have been shown to adopt a wide range of coordination modes through their carboxylate oxygen and pyridyl nitrogen atoms (Wang et al., 2009). A search in the CSD (Version 5.3; Groom & Allen, 2014) disclosed ca 200 complexes displaying diverse topologies, viz. monomers (Gao et al., 2010;Drew et al., 1971), dimers (Shankar et al., 2013), oligomers (Yu et al., 2003) as well as one-dimensional (Semerci et al., 2014), two-dimensional (Ç olak et al., 2011) and three-dimensional (Kanoo et al., 2012) polymers. In the vast majority of cases the ligand adopts an N,O-chelating mode, although there are a few exceptions to this where the binding sites attach to different metal atoms (e.g. Wang et al., 2014). By contrast, when complexes ISSN 2056-9890 containing similar ligands but with methyl substituents in the 6-position were sought, namely those generated from 6-methylpyridine-2,3-dicarboxylic acid (mepydcH 2 ), only a single structure was found involving the monoanionic mepydcH À ligand similar to that reported here (Gurunatha & Maji, 2009). This unique structural motif appears in the form of three isostructural, monomeric M II (M = Fe, Co, Ni) complexes [M(bpee) 2 (mepydcH) 2 ] (bpee = 1,2-bis(4-pyridyl)ethylene) with octahedral geometry around M II . Both mepydcH À fragments act in a simple 2 N,O 2 -chelating mode binding to a single nucleus while the two N-bound bpee ligands are trans-monodentate. The formation of these mononuclear complexes is unusual considering the obvious bridging potential of the bpee ligands. Mixed-ligand complexes based on non-methylated 2,3-pyridinedicarboxylate and 4,4 0bipyridine-like ligands usually generate stable polymeric structures with the exo-bidentate ligands adopting a bridging role (Kanoo et al., 2012;Wang et al., 2009;Maji et al., 2005).
In an attempt to understand the coordination behaviour of this unusual monoanionic mepydcH À ligand better, we report the structure of the dinuclear complex [Cd 2 (2,2 0 -bipyridine) 2 -(mepydcH) 2 (NO 3 ) 2 ]ÁMeOH (I). The uncommon bridgingchelating 2 -( 3 N,O 2 :O 2 ) coordination behaviour and the fact that the ligand is only singly deprotonated has no counterpart in complexes of the non-methylated ligands and makes this a genuinely novel structure. The closest relatives with 2,2 0 -bipyridine as the auxiliary ligand are found with di-anionic pydc 2À ligands, but these are either mononuclear (Wang & Okabe, 2005) or form coordination polymers (Li et al., 2013;Yin & Liu, 2009;Zhang et al. 2013).

Structural commentary
The complex consists of a Cd II cation to which a singly protonated 3-carboxy-6-methylpyridine-2-carboxylate ion (mepydcH À ) chelates through the pyridine N and carboxylate O atoms. A chelating 2,2 0 -bipyridine that binds through both nitrogen atoms and a unidentate nitrate anion complete the coordination sphere; the asymmetric unit also contains a noncoordinating half-occupancy methanol solvate. This five coordinate Cd II unit, in turn, binds to its centrosymmetric image through the carboxylate oxygen atom of the mepydcH À ligand, forming a pair of Cd-O-Cd bridges. As a result, a dimeric unit forms ( Fig. 1) with each Cd II atom in a six-coordinate N 3 O 3 ligand environment. The Cd-X (X = N or O) distances are reasonable, spanning the range 2.304 (2)-2.332 (3) Å . However, the coordination angles vary widely [X-Cd-X ranges: cis 71.15 (10)-115.79 (9) ; trans 142.36 (8)-159.48 (9) ]; the result is a rather distorted octahedral geometry around Cd1. Selected geometric parameters are shown in Table 1; the bridging Cd-O distances are the shortest in the coordination sphere, 2.304 (2) and 2.310 (2) Å , resulting in a CdÁ Á ÁCd separation of 3.700 (3) Å . This value is slightly larger than the mean for similar environments found in the CSD (3.61 Å for 885 cases), though well within the sample standard deviation (0.22 Å ).

Supramolecular features
The crystal structure, made up of isolated dimers, is sustained by three different types of non-covalent interaction, viz., hydrogen bonds (Table 2) Displacement ellipsoid plot of (I) (with 40% probability ellipsoids), showing the dimeric unit with atom and ring labelling. Interactions within the dimeric unit are also shown, C-HÁ Á ÁO as dashed lines, C-OÁ Á Á(ring) as double-dashed lines. For symmetry codes see Tables 2 and  3; additional symmetry code: (i) 1 À x, 1 À y, 1 À z. contacts (Table 3). These interactions can be clearly differentiated according to the substructure that they support: a) Contacts #1 (Table 2) and #9, #10 (Table 3) are internal to the dinuclear motif, as shown in Fig. 1. The first one links the bipyridine C10A-H10A group with the coordinating nitrate oxygen O1C. Contact #9 is a typical lone pair-interaction with a dihedral angle of 72.19 between the carboxylate and the ring plane, and a C-OÁ Á ÁCg2 angle of 126.63 . These values are close to those for the ideal geometry (90 and 120 , respectively) when a lone pair provided by a carbonyl oxygen points toward the centroid of an aromatic ring (Egli & Sarkhel, 2007). By contrast, in contact #10 the orientation of the nitrate plane is more or less parallel to the ring plane (6.84 ), suggesting ainteraction with the -orbitals of the nitrate fragment interacting with those of the aromatic ring. A similar argument has already been applied by Frontera et al. (2011) and García-Raso et al. (2009) when nitrate anions interact with pyrimidinium rings. These carbonyl(lone pair)Á Á Á(ring) (#9) and nitrate()Á Á Á(ring) (#10) interactions in (I) fulfill a relevant function, serving to strengthen the dimeric unit ( Fig. 1).
b) Strong intermolecular O-HÁ Á ÁO contacts #2 (Table 2) involving the hydrogen atom of the free carboxylic acid group of the mepydcH À ligand with a non-bonded oxygen atom of a nitrate ligand, has the pivotal action of linking the dimers along a, forming chains parallel to [100] (Fig. 2). c) C-HÁ Á ÁO interactions #3, #4 and #5 (Table 2), in turn, serve to link the above chains laterally along b, to form 2D substructures parallel to (001) (Fig. 3a). These planes juxta- Table 2 Hydrogen-bonding interactions (Å , ) in (I).
Cg1 is the centroid of the N1A/C1A-C5A ring and Cg2 is the centroid of the N2A/C6A-C10A ring.

Figure 3
Two projections along [100], presenting (within square brakets) views of the two-dimensional substructures parallel to (001) pose along [001] with rather weak direct interactions. In the process, however, significant columnar voids parallel to the chains are formed (with a volume 13% of the total cell volume, Fig. 3b) in which the partial occupancy methanol solvate molecules reside. These are not free, but enter instead into a number of weak C-HÁ Á ÁO, O-HÁ Á ÁO and C-HÁ Á Á interactions (#6, #7 and #8 in Table 2) linking them to a framework of complex molecules, further stabilizing the structure.

ATR (attenuated total reflectance) FT-IR spectroscopy
The IR spectra of mepydcH 2 , 2.2 0 -bipyridine and (I) were recorded on an Agilent Cary 630 FT-IR spectrometer with Varian Resolutions Pro software, using a Diamond ATR accessory. The FT-IR spectrum of (I) (Fig. 4) was recorded in the 4000-600 cm À1 range, and confirms the structural data indicating the presence of the coordinating nitrate and mepydcH À anions. Bands due to the unidentate NO 3 À group were found at 1478 and 1298 cm À1 and appear due to the asym (ONO) and sym (ONO) vibrations, with a shoulder at 1010 cm À1 due to the (NO) stretching modes of nitrate groups (Nakamoto, 1997). The carboxylic acid group (COOH) of the mepydcH À ligand in complex (I) is identified by a weak band at 3083 cm À1 , (OH) stretching for a hydrogen-bonded system (Alisir et al., 2013), and a very strong band at 1738 cm À1 , (C O) stretch. The deprotonated carboxylate (COO À ) is characterized by the asymmetric and symmetric stretching modes as at 1593 cm À1 and s at 1322 cm À1 . This confirms the unidentate coordination of the carboxylate O atom, with the difference between these frequencies being > 200 cm À1 (Á = as À s = 271 cm À1 ) (Deacon & Phillips, 1980). Finally, around 1400 cm À1 , a set of three bands appears (1412, 1391 and 1369 cm À1 ) of almost equal intensity due to the (C C) + (C N) vibrations from the coordinating 2,2 0 -bipyridine ligand (Yan et al., 2011).

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.