Synthesis and crystal structure of one new Pb(II) complex constructed by 1,10-phenanthroline and two carboxylate ligands


 The complex [Pb(phen)(4-NB)(CH3COO)] of lead(II) was prepared and characterized by means of elemental analysis, FT-IR, and single crystal X-ray analysis, where phen = 1,10-phenanthroline, 4-NB = 4-nitrobenzoate. The single crystal X-ray analysis indicates that the complex is a monomeric species, including two carboxylate ligands, and adopts a hemidirected structure. It is further extended by intermolecular C−H⋯O hydrogen bonds, π–π interactions and secondary Pb⋯O interactions to form two-dimensional supramolecular architecture.

2006; Fan and Zhu, 2006;Jin et al., 2010;Li et al., 2012). Due to the presence of the 6s lone-pair electrons, which may or may not be stereochemically active, Pb II complexes display either hemidirected or holodirected structures. The lone pair electron on lead(II) is stereochemically inactive and ligand-to-metal bonds are distributed evenly around the lead(II) ion is termed holodirected, otherwise it is called hemidirected (Janiak et al., 2000;Pellissier et al., 2007;Shimoni-Livny et al., 1998). Secondary bonds, as a kind of non-covalent interaction, usually existed in the hemidirected lead(II) complexes and formed by the stereochemically active lone pair, such as Pb···O, Pb···N, Pb···C, Pb···Cl, Pb···S, and Pb···π, which have some effect on the structures and properties of the lead(II) complexes (Catalano et al., 2015;Chen et al., 2015;Cheng et al., 2014b;Huang et al., 2017;Kowalik et al., 2020;Meng et al., 2009;Miao and Zhu, 2008;Mirdya et al., 2019Mirdya et al., , 2020Yang et al., 2012;Zhao et al., 2007). Thus, Pb 2+ ion is an ideal candidate for the construction of novel complexes (including monomeric compounds and metal-organic frameworks). In the reported lead(II) complexes, a number of compounds were tested for fluorescence properties, and the presence or absence of stereochemically active lone-pair Pb(II) has possible effect on luminescence of coordination compounds (Chen et al., 2015;Katz et al., 2008;Meng et al., 2009;Nugent et al., 2015). Therefore, some Pb(II) complexes exhibiting good luminescent properties have been observed (Alizadeh and Amani, 2011;Ding et al., 2008;Kowalik et al., 2020;Li, 2019;Ma et al., 2018;Sahu et al., 2013;Song et al., 2018;Xiong et al., 2012;Yang et al., 2016;Zhang et al., 2011;Zhao et al., 2007). Lead(II) compounds having catalytic and antimicrobial activities, fabricating organic light-emitting diode and nano materials have also been occasionally reported. For example, Wang et al. (2014) reported eleven germanium(II), tin(II), and lead(II) amino-(ether)-phenolate monomeric complexes and nine of them in the catalysis of the immortal ring-opening polymerisation (iROP) of L -lactide ( L -LA) or racemic-lactide (rac-LA) upon addition of iPrOH were tested, it is surprised to find the catalytic activity increases with metal size according to Ge II « Sn II » Pb II . Singh et al. (2016) reported lead(II) complexes with semicarbazone and thiosemicarbazones derived from (2-hydroxyphenyl) (pyrrolidin-1-yl)methanone assessed as antibacterial and antifungal agents and the results interestingly showed the lead(II) complexes have higher activity than the free ligands. Najafi et al. (2013) synthesized a new complex, [Pb(µ-8-Quin)I] 2 , and fabricated OLEDs by the prepared complex. The experiment results demonstrated the complex dye can serve as an electron transport layer and light emission layer along with the PVK as a hole transport layer in organic light-emitting diodes. Mirtamizdoust introduced one method of preparing PbO nanoparticles via thermolysis of lead(II) metal-organic coordination polymer (Mirtamizdoust, 2017). Most of the above compounds are constituted with N and O donor ligands. Which may be why Pb(II) compounds are increasingly synthesized and tested in some certain properties despite lead is a heavy toxic metal. 4-Nitrobenzoic acid has two functional groups, carboxyl (-COOH) and nitryl (-NO 2 ), whose carboxylate group can serve as good candidates to tune structures because of their versatile coordination properties, which can bind to metal ions with monodentate, bridging, and chelating modes (Kar et al., 2011;Lv et al., 2019;Ren et al., 2006;Sarma and Baruah, 2009;Tsaryuk et al., 2012;Zhu et al., 2012). In the complexes formed by 4-nitrobenzoate, the nitryl group is generally uncoordinated but its oxygen atom can participate in generating the hydrogen bonds as H-acceptor, which often acts as a contribution in the construction of interesting supramolecular architectures (Ren et al., 2006;Saha et al., 2019;Srinivasan et al., 2009;Xu et al., 2012). Moreover, the introduction of the neutral ligands can adjust the coordination modes of 4-nitrobenzoate with metal ions resulting in diverse structures. 1,10-Phenanthroline, as an excellent N-donor bidentate chelating ligand, not only was often used to construct novel fascinating molecular architectures and build supramolecular structures through weak interactions (Liu et al., 2004;Wan et al., 2003), but also to produce compounds of various properties (Erxleben, 2018;Lemercier et al., 2018;Sammes and Yahioglu, 1994). Based on the reasons above mentioned, 4-nitrobenzoic acid was chosen for the construction of novel lead(II) complex containing the ligand 1,10-phenanthroline. Herein we report the preparation and crystal structure of [Pb(phen)(4-NB)(CH 3 COO)] (Scheme 1), obtained by reactions of lead(II) acetate, 1,10-phenanthroline, and 4-nitrobenzoic acid.
The molecular structure of the complex determined by single crystal X-ray diffraction method and the result shows that compound crystallizes in the monoclinic P21/n space group, with four formula units in the unit cell, and exhibits monomeric species. As shown in Figure 1a, Pb(II) ion adopts a six-coordinated geometry completed by two nitrogen donors from one 1,10-phenanthroline molecule, two oxygen atoms from one 4-NB ligand, and two oxygen atoms from one acetate ion to generate a N 2 O 4 environment with a deformed pentagonal pyramid, which is like an umbrella. The Pb(II) atom is located in at the top of the umbrella (Figure 1b) and the bond Pb1-O5 is umbrella handle. The Pb1−O bond lengths fall in the range of 2.359(4)-2.656(4) Å (Table 1), which can be well comparable with those in the reported compounds (Lv et al., 2019;Mahjoub and Morsali, 2002;Morsali et al., 2003;Sarma and Baruah, 2009;Xu et al., 2012;Zhao et al., 2013). The Pb1-N bond distances [2.586(5) and 2.658(5) Å] (Table 1)  bonds in previously published Pb II -complexes containing 1,10-phenanthroline ligand (Cheng et al., 2014a;Fan and Zhu, 2006;Mahjoub and Morsali, 2002;Morsali et al., 2003;Xu et al., 2012). IR spectra of the complex were analyzed on the basis of the crystal structure. The band 3049 cm -1 can be assigned to the C−H stretching of phenyl ring. According to IR spectra of the reported complexes [Pb(phen) 2 (4-NBA)] 2 ·2(NO 3 )·H 2 O  and [Pb(8-OQ)(4-NB)] (Lv et al., 2019), the ν as (COO -) and ν s (COO -) at 1560 and 1385 cm -1 are attributed to the carboxylate group of 4-nitrobenzoate, the ν as (COO -) and ν s (COO -) at 1616 and 1427 cm -1 are attributed to the carboxylate group of acetate, respectively. The differences between ν as (COO -) and ν s (COO -) in the two carboxylate groups are approximately 180 cm -1 , which is close to Δν [ν as (COO -) -ν s (COO -)] values in [Pb(phen) 2 (4-NBA)] 2 ·2(NO 3 )·H 2 O and [Pb(8-OQ)(4-NB)] whose COOgroups adopted chelating mode, indicating the COOgroups from 4-nitrobenzoate and acetate in this complex all chelated Pb(II) ion. The characteristic absorption peaks at 1509 and 1340 cm -1 are assigned to the asymmetric stretching vibration peaks and the symmetric stretching vibration peaks of the NO 2 group, respectively. The characteristic peaks of the 1,10-phenanthroline are at about 1404, 832, and 724 cm -1 .
The Pb(II) complex based on 1,10-phenanthroline and 4-nitrobenzoic acid with the Pb(CH 3 COO) 2 was obtained and structurally characterized. The compound was a monomeric species involving hemidirected Pb(II) atom, and displayed 2-D supramolecular architecture constructed via intermolecular hydrogen bonds, π-π interactions and secondary Pb···O interactions. In this complex, the coordinate bonds around Pb(II) ion and non-covalent interactions probably affect the energy transfer and improve the luminescence properties of the compound compared with those of the free ligands.

X-ray crystallography
Single-crystal X-ray diffraction data was obtained using Agilent SuperNova diffractometer equipped with an Atlas CCD detector (Agilent Technologies Inc, Santa Clara, CA, USA) at 145(1) K with graphite monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods using SHELXT-2014 (Sheldrick, 2015a) and refined by full matrix least squares with SHELXL-2014 (Sheldrick, 2015b), refining on F 2 . Selected bonds and angles are given in Table 1. Detailed crystal data and structure refinement are listed in Table 4. Crystallographic data has been deposited with the Cambridge Crystallographic Centre as supplementary publication number CCDC-2052415. Author contributions: Lu-Lin Zhang: experimental design, complex preparation, data collection, data interpretation, and writing. Shi-Li Tang: complex preparation, data collection, data interpretation, and writing. De-Jun Li: complex preparation, data collection, and writing. Yuan-Zheng Cheng: conceptualization, data interpretation, writing, funding acquisition, and supervision. Li-Ping Zhang: conceptualization, data interpretation, writing, funding acquisition, and supervision.

Conflict of interest:
The authors declared that they had no conflicts of interests in their authorship and publication of this contribution.