Synthesis, Crystal Structure, Thermal, Magnetic Properties and DFT Computations of a Ytterbium(III) Complex Derived from Pyridine-2,6-Dicarboxylic Acid

A new ytterbium(III) complex, (DMAH 2 ) 3 [Yb(Pydc) 3 ].4H 2 O (1) {Pydc = Pyridine-2,6-dicarboxylate anion, DMAH = Dimethylamine} has been prepared under mild solvothermal conditions and characterized by elemental analysis, IR spectroscopy, thermal analysis and single crystal X-ray diffraction. The DMAH molecules in 1, generated in situ from hydrolysis of N,N-dimethylformamide are responsible to assemble 2D coordination polymers through N-H ∙∙∙ O and O-H ∙∙∙ O hydrogen bonding. Magnetic susceptibility measurements indicate that the complex (1) obeys the Curie Weiss law and the overall magnetic behavior is typical for the presence of weak antiferromagnetic exchange coupling interactions. Theoretical data for geometrical parameters of complex 1 agree well with the experimental data. Large HOMO-LUMO energy gap of 4.33 eV has provided kinetic stability to the complex 1. NBO analysis reects that intramolecular charge transfer occurred between ligand and metal orbitals with the highest stabilization energy of 1024.04 kcal/mol. The negative electrostatic potential at the nitrogen and dianionic pyridine-2,6-dicarboxylate regions conrms that these are dynamic locations for Yb(III) binding.

Considering the possible role of lanthanide complexes for material applications, we have earlier reported the crystal structures of several lanthanide complexes with pyridine-2,6-dicarboxylic acid (H 2 Pydc) ligand [9,[20][21][22][23]. In these complexes the lanthanide atoms exhibit a nine-coordination environment, while H 2 Pydc coordinates in neutral, monoanionic and dianionic forms. To further explore the coordination chemistry of lanthanide-Pydc complexes to generate supramolecular networks through H-bonding. We report here the synthesis, structural characterization, thermal and magnetic properties as well as comprehensive DFT computations of a new anionic ytterbium(III) complex stabilized by dimethylammonium ions.

Materials and Measurements
All reagents were purchased commercially and were used without further puri cation. The percentage detection of H, C and N was performed on the Elemental Analyzer, Vario Micro Cube, Elementar, Germany. IR spectra were recorded on Perkin-Elmer FT-IR 180 spectrophotometer using KBr pellets over the range of 4000-400 cm −1 . Thermal analysis (25-1000°C) was performed under continuous nitrogen ow, with a ramp-rate of 10 °C min −1 using a SDT Q600 instrument (TA Instruments, USA). The temperature dependence magnetic susceptibility measurements were made in the temperature range of 5-300 K using

X-ray Structure Determination
Suitable crystals of complex 1 were selected for data collection, which was performed on a Bruker KAPA APEX II CCD diffractometer equipped with a graphite-monochromatic Mo-K α radiation at 296 K. The structure was solved by direct methods using SHELXS-97 and CRYSTALS [27,28], and re ned by fullmatrix least-squares methods on F 2 using SHELXL-97 and CRYSTALS [27,28] within the WINGX [29] suite of software. All non-hydrogen atoms were re ned with anisotropic parameters. Water H atoms were located in a difference map and re ned subject to a DFIX restraint of O-H = 0.83(2) Å. All other H atoms were located from different maps and then treated as riding atoms with C-H distances of 0.93-0.96Å and N-H distances of 0.90Å. Molecular diagrams were created using MERCURY [30]. Supramolecular analyses were made and the diagrams were prepared with the aid of PLATON and CrystalMaker® [31,32]. Details of data collection and crystal structure determinations are given in Table 1. All the DFT calculations of synthesized complex 1 are performed by using Gaussian 16 program Package [33] and are visualized by using GaussView 6.1.1 software [34]. Geometry optimization of complex 1 is carried out at B3LYP/SDD and M06-2X/SDD levels of theory to compare results of both methods with experimental data. Frequency calculations are also performed at the respective levels of theory to assure local minima. The B3LYP/SDD level of theory is most reliable and has been widely used for the ytterbium complexes [35]. The B3LYP/SDD employs three-parameter Becke (B3) exchange functional [36], Lee-Yang-Parr (LYP) nonlocal correctional functional [37], and Stuttgart SDD basis set [38]. Frontier molecular orbital (FMO) analysis, reactivity parameters, natural bond orbital (NBO) analysis and molecular electrostatic potential (MEP) analysis of complex 1 are also performed at the same level of theory.
Visualization of FMOs and MEP surface is carried out by using Multiwfn [39] and VMD [40] softwares. Furthermore, Hirshfeld surface analysis of complex 1 is performed by using Crystal Explorer software [41].

IR and Thermal Studies
The

Crystal structure of Complex 1
The molecular structure of complex 1, (DMAH 2 ) 3 [Yb(Pydc) 3 ].4H 2 O with the atom labeling is shown in Figure 2. The selected bond lengths and angles are given in Table S1. The complex 1 exists as a monomeric ionic species consisting of an ionic complex [Yb(Pydc) 3 ] 3− , three dimethylammonium counter ions and four non-coordinated water molecules ( Figure 2). The DMAH molecules are generated in situ from hydrolysis of N,N-dimethylformamide [21]. The Yb(III) ion in 1 is coordinated by three dianionic pyridine-2,6-dicarboxylate ligands (Pydc 2− ) and attains a distorted tricapped trigonal prismatic YbN 3 O 6 coordination geometry with the N atoms serving as the caps protruding through the prismatic side-faces ( Figure S1). The N-Yb-N bond angles are closer to 120° (116.28 (8) Figure S2).

Magnetic Measurement
The magnetic behavior of complex 1 is represented in the forms of χ m , χ m −1 and χ m T vs. T plots (χ m = molar magnetic susceptibility) shown in Figure S3. The susceptibility can be well described by the Curie-Weiss law above 40 K with a Curie constant C = 3.11 ± 0.0022 cm 3

Geometrical parameters
Molecular geometry of complex 1 is optimized by taking its X-ray crystallographic CIF le. The results of geometrical parameters obtained at two different methods B3LYP and M06-2X are compared with experimental data and are listed in Table 2. ). Some theoretical bond angles are bigger than the experimental ones such as O11-Yb1-O3 (92.35°) and N1-Yb1-N2 (122.08°). The reason behind the fact is that DFT calculations are performed in gas phase while experimental data is obtained in solid phase. The optimized structure of complex 1 is displayed in Figure  3.

Frontier molecular orbital analysis
Electronic characteristics and reactivity of any complex can be estimated via frontier molecular orbital analysis. Molecular orbital energies are computed at the B3LYP method along with SDD basis set and pictorial representation of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) electron densities along their energy gap is provided in Figure 4. The electron density of HOMO is localized at the carboxylate group of dianionic pyridine-2,6-dicarboxylate ligand (Pydc 2− ) while the LUMO orbital electron density is localized at the pyridinic moiety of complex 1. Energies of HOMO and LUMO are -6.12 eV and -1.78 eV, respectively. Complex 1 has a 4.33 eV HOMO-LUMO energy gap (Eg) indicating that it has excellent kinetic stability and low chemical reactivity.
Some reactivity parameters including ionization potential (I), electron a nity (A), chemical potential (µ), chemical hardness (η), chemical softness (S) and electrophilicity index (ω) are also calculated for the complex 1 and listed in Table 3. The chemical potential of -3.95 eV shows that the complex 1 is thermodynamically stable. It is an essential parameter for calculation of other electronic parameters. A larger chemical hardness value (2.16 eV) than that of chemical softness (1.08 eV) re ects that the complex 1 is thermodynamically stable and less reactive. A lower electrophilicity index (ω) value of complex 1 supports its nucleophilic nature.

Natural bond orbital (NBO) analysis
NBO analysis of the complex 1 is performed at the B3LYP/SDD level of theory by using built-in Gaussian NBO Version 3.1. Natural bond orbital analysis provides a valuable understanding of the intermolecular and intra-molecular interactions, hydrogen bonding and charge transfer between atoms of any molecular structure [47,48]. It also depicts electrical charge displacement and conjugative interactions. The loss of occupancy from a localized NBO (donor) of Lewis structure to an empty non-Lewis NBO (acceptor) causes interactions. Second order perturbation theory analysis of Fock matrix has been carried out to observe the donor-acceptor NBO transitions for complex 1. The stabilization energy for donor (i) to acceptor (j) delocalization can be calculated as follows: Where q i is occupancy of donor orbital, ε j , ε i are diagonal elements and F (i,j) is off-diagonal Fock matrix element. Larger value of stabilization energy E (2) between electron donor and acceptor orbitals increases the stability of the synthesized complex. Some major donor to acceptor NBO transitions starting from the highest E (2) are listed in Table S4.
These results re ect that many NBO transitions have occurred between different energy levels of complex

Molecular electrostatic potential (MEP) analysis
Molecular electrostatic potential analysis is the best tool to analyze the charge distribution of a molecular structure [49]. The electrophilic and nucleophilic sites in a molecule are described by MEP which is associated with electron density. In complex 1, more prominent red color patches at dianionic pyridine-2,6-dicarboxylate portion re ect the nucleophilic region with more negative potential. The blue color patches at dimethylammonium ions show the electrophilic nature. The molecular electrostatic potential surface of complex 1 is shown in Figure 5.

Hirshfeld surface analysis
Hirshfeld surface analysis is a useful tool for describing the space occupied by molecules in a crystal for partitioning of crystals electron density into molecular fragments [50]. It has a key contribution in de ning the surface properties of molecules and provides information more about the intermolecular interactions of molecular crystals. Hirshfeld surface investigations of complex 1 is performed by using Crystal Explorer program from the X-ray crystallographic CIF le. Intermolecular interactions of complex 1 are best quanti ed by using Hirshfeld surfaces and their corresponding two-dimensional ngerprint plots.
The d norm mapped surface of complex 1 is shown in Figure 6.
The red patches in the surface map of complex 1 correspond to the hydrogen bonding between O-H … O and C-H … O atoms. The white regions indicate weak van der Waals interaction and blue patches covering a large area are for the longer than van der Waals interactions. Furthermore, 2D ngerprint plots for the intermolecular percentage contribution of atom to atom in complex 1 is shown in Figure S4.

Conclusions
In this paper, a new zero dimensional ytterbium(III) complex with Pydc ligand has been synthesized under mild solvothermal condition. Extensive N-H···O and O-H···O bonding due to dimethylamine and water molecules are responsible for two two-dimensional framework. Solvent molecules and in situ generation of molecules like dimethylamine can be tuned for preferred structural topologies and cross-linking in to 2D and 3D coordination networks. Density functional theory simulations have been carried out for computing the geometrical parameters of complex 1 and the results are compared with the experimental data for reliability. Larger values of HOMO-LUMO energy gap (4.33 eV) and chemical hardness (2.16 eV) re ected that the complex is stable and less reactive in nature. NBO analysis has provided information about the nature of interactions and intermolecular charge transfer transitions from donor to acceptor orbitals. MEP analysis of complex 1 reveals that more electron density with negative electrostatic potential at the nitrogen and dianionic pyridine-2,6-dicarboxylate regions is responsible for ligand strong bonding with Yb(III) metal. Furthermore, dnorm Hirshfeld surface map indicates the nature of intermolecular interactions of molecular crystal. Its 2D ngerprint plots have the highest %age contribution of molecular surface from H…H (45.5%) followed by O…H (20.7%).