3.1 IR and Thermal Studies
The reactions of YbCl3.6H2O; with H2Pydc in a 1:1 molar ratio in a mixture of water and DMF under solvothermal conditions resulted in formation of a crystalline complex, (DMAH2)3[Yb(Pydc)3].4H2O (1). The DMAH molecules in 1 were generated in situ from hydrolysis of N,N-dimethylformamide [21]. In the IR spectrum of complex 1, a broad peak in the region of 3419–3253 cm−1 was observed, which was assigned to the O–H and N–H stretching vibrations of water molecules and dimethylaminium ions respectively. The band at 1560 cm−1 is related to the N-H bending vibration of dimethylaminium ions. The characteristic absorptions for the asymmetric and symmetric stretches of carboxylate group are found at 1607 and 1447 cm−1 respectively, while in the IR spectrum of free ligand, these bands are observed at 1688 and 1375 cm−1 respectively. A significant shift of the asymmetric mode, νas(COO) towards the lower wavenumber upon coordination and that of the symmetric mode, νs(COO) towards the higher wavenumber indicates the binding of Pydc ligand to the metal through the carboxylate oxygen atoms [20].
The thermal decomposition of complex 1 is illustrated in Figure 1. At the first stage four non-coordinated water molecules are released at 90°C corresponding to the weight loss of 9.3 % (calculated 8.2 %). The loss of water is associated with an endothermic transition in DSC. The next weight loss of 39 % between 210°C and 350°C corresponds to the removal of two Pydc ligands (calculated 37.6 %). The difference indicates the removal of some other volatile component like ammonia or dimethylamine.The DSC curve exhibits an endothermic transition at 305 oC. The elimination of the third Pydc ligand takes place after 500°C (19.5 % wt loss against the theoretical value of 18.8 %) and is marked by an exothermic transition in DSC at 520°C. Beyond this point the remaining organic moieties are lost up to 950°C leaving behind a residue of 24 %, which is attributed to Yb2O3 (calcd. 22.5%). The thermal data agrees well the results obtained from the elemental analysis.
3.2 Crystal structure of Complex 1
The molecular structure of complex 1, (DMAH2)3[Yb(Pydc)3].4H2O 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 (Pydc2−) and attains a distorted tricapped trigonal prismatic YbN3O6 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)°-122.72(9)°), while the O-Yb-O bond angles ranged between 74.94(8)°-146.01(8)°. The distortion in tricapped trigonal prismatic geometry is attributed to the upper and lower distorted triangular faces with mean deviations of -1.802º and 6.426º from regular triangular faces respectively. Of the 14 triangular faces of YbN3O6 polyhedron, the dihedral angles between O3-O9-N3 and O9-O7-N2 faces are 59.902º and 63.507º, while between the relatively distorted triangular faces O5-O11-N3 and O5-O1-N2, they are 52.435º and 54.713º. The dihedral angles between N3-N2-N1, N2-N1-N3 and N1-N3-N2 triangular faces are 61.161º, 60.468º and 58.372º respectively. The Yb-N and Yb-O bond lengths fall in the ranges, 2.361(2)Å-2.386(2) Å and 2.440(2)-2.461(3) Å, respectively. These data are in agreement with the corresponding values of the similar reported Yb(III) complexes [42–45]. The pyridine ring mean planes are approximately planar, with the maximum deviations of 0.0102(22) Å for C(5) atom (C(8) and C(16) atoms are deviated by 0.0066(25) Å and 0.0052(26) Å respectively). The structural features of 1 are closely related to the other [Ln(Pydc)3]3− type complexes [6, 11, 21, 23].
We have earlier reported the crystal structures of a similar series of the Pydc-based coordination polymers with the formula [Ln(Pydc)3](DMAH2).H(DMAH)2 (Ln = Ce, Ho, Nd, Sm) [21]. Their structural analysis reveals that their metal coordination sphere is quite identical to that of compound 1. Pydc ligand in these complexes as well as in 1 adopts only one kind of coordination modes, where the oxygen atoms of the carboxylate groups and the nitrogen atom of a pyridine ring form a chelate with the metal atom.
Within the structure of 1, extensive hydrogen-bonding interactions take place between the carboxylate groups, water molecules and ammonium ions. The molecules of 1 are linked to each other by a combination of N-H∙∙∙O, O-H∙∙∙O and C-H∙∙∙O hydrogen bonds (Table S2). The amnonium nitrogen atom N(5) in the molecule at (x, y, z) acts as hydrogen-bond donor, via atoms H(5A) and H(5B), to carboxylate oxygen atoms O(6)ii and O(7), forming a C22(8) chain running parallel to the [100] direction. The water oxygen atom O(14) in the reference molecule at (x, y, z) acts as hydrogen-bond donor, via atoms H(14A) and H(14B), to atoms O(4)v and O(12)vi, forming a C22(10) chain running parallel to the [100] direction. Similarly, the carboxylate atoms O(6) and O(8)vii accept hydrogen bonds from H(16A) and H(16B) of water atom O(16) yielding a C22(10) chain running parallel to the [100] direction. Finally, dimethylamine and water molecules link neighboring polymeric chains via N-H∙∙∙O and O-H∙∙∙O hydrogen bonds into a two-dimensional framework parallel to the ac plane (Figure S2).
3.3 Magnetic Measurement
The magnetic behavior of complex 1 is represented in the forms of χm, χm−1 and χmT vs. T plots (χm = 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 cm3K mol−1 and Weiss constant θ = – 35.52 K. In the high temperature end (300 K), χmT = 2.69 cm3K mol−1 provides an effective magnetic moment µeff of 4.64 µB, which is slightly larger than the expected multiplet 2F7/2 value of 4.50 µB per formula for one Yb(III) ion of one uncoupled (gJ = 1.1) Yb(III) ion [46]. The product of χmT was found to decrease with decreasing temperature to reach a final value of 0.97 cm3Kmol−1 at 5 K with an effective magnetic moment of 2.79 µB. The overall behavior of χmT with temperature and the negative value of θ is typical for the presence of weak antiferromagnetic exchange coupling interactions.
3.4. DFT computations
3.4.1 Geometrical parameters
Molecular geometry of complex 1 is optimized by taking its X-ray crystallographic CIF file. 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.
Table 2
Experimental and theoretical bond lengths (Å) and bond angles (degree) of Complex 1
Complex 1 Bond Lengths
|
Bond Length
|
Experimental
|
Theoretical
|
Bond Length
|
Experimental
|
Theoretical
|
B3LYP
|
M06-2X
|
B3LYP
|
M06-2X
|
N1-Yb1
|
2.453
|
2.471
|
2.432
|
O7-Yb1
|
2.361
|
2.394
|
2.357
|
O1-Yb1
|
2.371
|
2.326
|
2.297
|
N3-Yb1
|
2.441
|
2.483
|
2.454
|
O3-Yb1
|
2.386
|
2.406
|
2.386
|
O9-Yb1
|
2.368
|
2.412
|
2.339
|
N2-Yb1
|
2.461
|
2.471
|
2.451
|
O11-Yb1
|
2.362
|
2.348
|
2.321
|
O5-Yb1
|
2.365
|
2.323
|
2.324
|
|
|
|
|
Complex 1 Bond Angles
|
Bond Angle
|
Experimental
|
B3LYP
|
M06-2X
|
Bond Angle
|
Experimental
|
B3LYP
|
M06-2X
|
O5-Yb1-O9
|
88.52
|
88.12
|
84.65
|
N3-Yb1-N2
|
120.99
|
120.58
|
123.21
|
O11-Yb1-O3
|
83.92
|
92.35
|
94.55
|
O9-Yb1-O3
|
80.39
|
73.01
|
72.15
|
N1-Yb1-N2
|
116.28
|
122.08
|
124.06
|
O5-Yb1-N3
|
73.63
|
70.89
|
71.43
|
O7-Yb1-O1
|
88.24
|
90.45
|
91.07
|
N3-Yb1-N1
|
122.72
|
117.32
|
112.72
|
O11-Yb1-N1
|
73.06
|
69.88
|
68.61
|
|
|
|
|
The results clearly depict that the bond lengths and bond angles computed at B3LYP method are in good agreement with the experimental data than M06-2X method. The theoretical bond lengths at B3LYP/SDD for N1-Yb1, O3-Yb1, N2-Yb1 and O11-Yb1 b match well with the experimental data and are 2.471 Å (Exp. 2.453 Å), 2.406 Å (Exp. 2.386 Å), 2.471 Å (Exp. 2.461 Å) and 2.348 Å (Exp. 2.362 Å). However, there is minor deviation less than 1 Å in some bond lengths between theoretical and experimental X-ray crystallographic data. The highest deviation (0.045 Å) in theoretical bond length is observed for O1-Yb1 bond from the experimental one. Similarly, the theoretical bond angles that correlate better with the experimental data are O5-Yb1-O9, O7-Yb1-O1, N3-Yb1-N2, and O5-Yb1-N3 and their values are 88.12° (Exp. 88.52°), 90.45° (Exp. 88.24°), 120.58° (Exp. 120.99°) and 70.89° (Exp. 73.63°). 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.
3.4.2 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 (Pydc2−) 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 affinity (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) reflects that the complex 1 is thermodynamically stable and less reactive. A lower electrophilicity index (ω) value of complex 1 supports its nucleophilic nature.
Table 3
HOMO, LUMO energies, energy gap Eg (eV) and their quantum reactivity parameters
Parameter
|
Energies (au)
|
Energies (eV)
|
HOMO
|
-0.22472
|
-6.12
|
LUMO
|
-0.06576
|
-1.78
|
Eg
|
0.15901
|
4.33
|
I
|
0.22472
|
6.12
|
A
|
0.06576
|
1.78
|
µ
|
-0.14524
|
-3.95
|
η
|
0.07948
|
2.16
|
S
|
0.03974
|
1.08
|
ω
|
0.132704
|
3.62
|
Other thermodynamic parameters of complex 1 have been computed theoretically to confirm the chemical stability. Total energy, heat capacity at constant volume, entropy and zero point vibrational energy of complex 1 are 483.38 kcal/mol, 206.43 cal/mol-kelvin, 332.109 cal/mol-kelvin and 446.67 kcal/mol. Similarly, rotational constants are also computed for complex 1 and are listed in Table S3.
3.4.3 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:
$${E}^{\left(2\right)}={q}_{i}\frac{{F}^{2} (i,j)}{{\epsilon }_{j}-{\epsilon }_{i}} \left(1\right)$$
Where qi is occupancy of donor orbital, εj, εi 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 reflect that many NBO transitions have occurred between different energy levels of complex 1. The highest stabilization energy value of 1024.04 kcal/mol is observed from O89-H91 donor to antibonding O80-H82 acceptor orbital. The second largest E(2) of 606.13 kcal/mol is obtained by the charge transfer from lone pair of O89 to antibonding O80-H82 orbital and so on. These stronger intramolecular interactions with larger stabilization energies might be responsible for the stability of complex 1.
3.4.4 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 reflect 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.
3.4.5 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 defining 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 file. Intermolecular interactions of complex 1 are best quantified by using Hirshfeld surfaces and their corresponding two-dimensional fingerprint plots. The dnorm 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 fingerprint plots for the intermolecular percentage contribution of atom to atom in complex 1 is shown in Figure S4. The results show that the highest intermolecular percentage contribution for complex 1 is from H…H atom up to 45.5%. The %age contributions from O…H and H…O atoms are 20.7% and 18%, respectively. Similarly, intermolecular %age contributions of C…H and H…C atoms are 6.4% and 5.6% and so on.