A novel Zn(II) complex of N-nicotinyl phosphoramide: Combined experimental and computational studies

Journal of Molecular Structure 1092 (2015) 130–136

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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

A novel Zn(II) complex of N-nicotinyl phosphoramide: Combined experimental and computational studies Khodayar Gholivand a,⇑, Foroogh Molaei a, Jérôme Thibonnet b a b

Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran Université François Rabelais, Département de Chimie, 32 Avenue Monge, 37200 Tours, France

h i g h l i g h t s  Synthesis and characterization of a novel Zn(II) complex of N-nicotinyl phosphoramide.  O- and N-coordination led to a 1D coordination polymer.  Solid and gas phase structures have been compared.  Electronic properties have been studied by NBO and QTAIM analyses.

a r t i c l e

i n f o

Article history: Received 30 December 2014 Received in revised form 11 March 2015 Accepted 12 March 2015 Available online 19 March 2015 Keywords: Phosphoric triamide Zn(II) complex Crystal structure DFT NBO QTAIM

a b s t r a c t A novel Zn(II) coordination polymer of N-nicotinyl phosphoric triamide ligand, {[Zn(L)2(H2O)2](NO3)2}n C1 (L = 3-NC5H4C(O)NHP(O)(NC6H12)2), has been synthesized and characterized by IR and 1H, 13C, 31P NMR spectroscopy. Crystal structure analysis of C1 demonstrates a distorted octahedral geometry for Zn(II) ions. The oxygen atom of phosphoryl group (Ophosphoryl) and the nitrogen atom of pyridine ring (Npyridine) of ligand take part in coordination to Zn(II) centers in a bidentate bridging mode. This coordination pattern results in infinite 1D polymeric chains along c axis, which are composed of metal shared 16membered puckered rings. Structural data show that despite binding through Ophosphoryl, the P@O bond distance unexpectedly shortens in C1 when compared to that of the ligand, conceivably due to the steric factors in the solid state. This is confirmed by comparing the structural and electronic properties of C1 and L in the gas phase by using density functional theory (DFT) calculations. Natural bond orbital (NBO) analysis reveals the metal–ligand interaction for C1 as donor–acceptor type delocalizations (charge transfer). Besides, on the basis of the quantum theory of atoms in molecules (QTAIM) analysis, the nature of ZnAO and ZnAN bondings is found to be mainly electrostatic with a small amount of covalent character. Ó 2015 Elsevier B.V. All rights reserved.

Introduction Complexes of zinc with ligands containing oxygen and/or nitrogen donor atoms are still receiving much attention for a number of biological actions, such as anticancer [1], antibacterial [2], and antifungal activities [3]. In addition, the design and synthesis of transition metal coordination polymers are under progress owing to the variety of architectures and topologies, and photoluminescent, catalysis, molecular adsorption and nonlinear optical (NLO) properties [4]. On the other hand, the ongoing research interest in phosphoramide chemistry has been driven by their great potential ⇑ Corresponding author. Tel.: +98 21 82883443; fax: +98 21 82883455. E-mail address: [email protected] (K. Gholivand). http://dx.doi.org/10.1016/j.molstruc.2015.03.020 0022-2860/Ó 2015 Elsevier B.V. All rights reserved.

applications, especially in the field of biology and medicine [5]. Besides, their effective coordinating capabilities are well-accepted. In this regard, we have extensively studied the phosphoramide adducts with organotin(IV) compounds [6,7], and lanthanide(III) complexes [8–10], delineating versatile coordination chemistry, intermolecular interactions and packing modes. Since polyfunctional ligands have garnered major attention, a new class of phosphoramides containing nicotinamide, isonicotinamide and pyridine derivatives, with an extra donor site (Npyridine), has been introduced by our research group. The aim was to illuminate their versatile coordination manner, once interacting with different metals. It is shown that in some cases there is a competition between donor sites, in favor of Ophosphoryl (oxygen atom of phosphoryl) [11] or Npyridine (nitrogen atom of pyridine) [11,12]. Also binding through Ophosphoryl and Npyridine in a bridging bidentate

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mode has been observed, which results in 1D [13] and 3D [14] coordination polymers with diverse architectures and supramolecular assemblies. Although, it might be considered that in carbacylamidophosphate family with APONHCOA fragment, the probability of coordination from C@O group is reduced due to its less polarizability in comparison with P@O [15]. Thus, Ophosphoryl and Npyridine can be considered as dominant binding sites to metal. Totally, it is concluded that the coordination pattern of this type of ligands is critically dependent on metal, and substituents on phosphorus atom and accordingly the steric and electronic properties of ligand. The position of nitrogen atom in pyridine ring, and participation of donor atoms in intermolecular interactions specifically hydrogen bonding, when compete with metal–ligand interaction are further important [11–16]. As a part of our continuing research, in this work we present synthesis and spectroscopic characterization of the first Zn(II) complex of N-nicotinyl phosphoric triamide ligand with the formula {Zn[3-NC5H4C(O)NHP(O)(NC6H12)2]2(H2O)2(NO3)2}n, C1. Crystal structure of C1 has been established by X-ray analysis, and compared with the ligand. By using DFT calculations and NBO and QTAIM analyses, we tried to get insight into the electronic properties of C1, structural and electronic perturbations occurred on ligand upon coordination, and the nature of Zn–ligand interaction. The synthesis pathway of C1 is demonstrated in Scheme 1.

Yield 54%. m.p. 156 °C. IR (KBr, cm1): m = 3541 (br, OAH,), 3114 (br, NAH), 2928 (s), 2861 (m), 1679 (s, C@O), 1602 (w), 1451 (vs), 1390 (s), 1305 (s), 1235 (m), 1194 (s, P@O), 1113 (m), 1062 (m), 945 (m, PAN), 835 (m), 704 (m), 539 (w, ZnAO), 469 (w, ZnAN). 1 H NMR (300.13 MHz, MeOD): d = 1.66–1.71 (m, 16H, CH2), 3.17– 3.30 (m, 8H, NACH2), 7.58 (dd, 3JHH = 7.9 Hz, 3JHH = 5.0 Hz, 1H, Hpy), 8.30 (d, 3JHH = 7.7 Hz, 1H, Hpy), 8.73 (d, 3JHH = 4.8 Hz, 1H, Hpy), 9.01 (s, 1H, Hpy) ppm. 13C NMR (75.47 MHz, MeOD): d = 27.9 (s), 31.2 (d, 3JPC = 4.1 Hz), 48.5 (d, 2JPC = 4.5 Hz), 125.4 (s), 131.9 (s), 138.3 (s), 149.8 (s), 153.4 (s), 170.1 (s, C@O) ppm. 31 1 P{ H} NMR (121.49 MHz, MeOD): d = 14.05 ppm. Experimental data of 3-NC5H4C(O)NHP(O)(NC6H12)2 Yield 92%. m.p. 149 °C. IR (KBr, cm1): m = 3103 (br, NAH), 2924 (s), 2855 (m), 1672 (s, C@O), 1589 (w), 1454 (vs), 1412 (m), 1278 (m), 1181 (vs P@O), 1109 (m), 1064 (m), 909 (m, PAN), 734 (m), 553 (w), 498 (w). 1H NMR (300.13 MHz, MeOD): d = 1.66–1.72 (m, 16H, CH2), 3.20–3.30 (m, 8H, NACH2), 7.56 (dd, 3JHH = 8.0 Hz, 3 JHH = 4.9 Hz, 1H, Hpy), 8.27 (d, 3JHH = 8.0 Hz, 1H, Hpy), 8.72 (dd, 3 JHH = 4.9 Hz, 1H, Hpy), 9.00 (d, 1H, Hpy) ppm. 13C NMR (75.47 MHz, MeOD): d = 27.9 (s), 31.3 (d, 3JPC = 4.0 Hz), 47.4 (d, 2 JPC = 4.5 Hz), 125.2 (s), 131.9 (d, 3JPC = 9.0 Hz), 137.7 (s), 149.8 (s), 153.4 (s), 169.2 (s, C@O) ppm. 31P{1H} NMR (121.49 MHz, MeOD): d = 13.90 ppm.

Experimental section Crystal structure determination Materials and methods All chemicals and solvents were purchased from Sigma–Aldrich or Merck and used as received. Melting points were obtained with an Electrothermal instrument. IR spectra was recorded on a Nicolet 510P spectrophotometer using KBr disk. NMR spectra were recorded on a Bruker Avance 300 spectrometer. 1H and 13C chemical shifts were measured relative to internal TMS and 31P chemical shift was determined relative to 85% H3PO4 as external standard, at room temperature. Synthesis of {Zn[3-NC5H4C(O)NHP(O)(NC6H12)2]2(H2O)2(NO3)2}n C1 For synthesis of 3-NC5H4C(O)NHP(O)(NC6H12)2 (L), a solution of 4 mmol hexamethyleneimine (0.40 g) was added dropwise to a solution of 1 mmol 3-NC5H4C(O)NHP(O)Cl2 (0.24 g) [11] in 20 ml dry acetonitrile at 0 °C. After 8 h stirring, the solvent was evaporated and the residue was washed with distilled water [17]. To a solution of 1 mmol L (0.36 g) in hot ethanol, a solution of 0.5 mmol Zn(NO3)26H2O (0.15 g) was added and stirred for 4 h. Suitable single crystals of C1 were obtained by recrystallization from ethanol/ acetonitrile solution (2:1) at room temperature.

Suitable single crystals of C1 were mounted on a glass fiber. Data collections were carried out at room temperature on a Bruker-Nonius Kappa-CCD-diffractometer equipped with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). Cell parameters were retrieved and refined using DENZO-SMN software on all reflections [18]. Data reductions were performed with the DENZO-SMN software [18]. An empirical absorption correction based on the symmetry-equivalent reflections was applied to each data set using the SORTAV program [19]. The structure was solved by SIR2002 [20] and refined with the SHELXL-97 [21]. Hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms. Crystal data and experimental details of the structure determination for C1 are listed in Table 1. Computational details DFT calculations with three-parameters hybrid functional B3LYP [22] were carried out using GAUSSIAN 03 software package [23]. A fragment relating one Zn(II) center was extracted from Xray atomic coordinates and fully optimized using the relativistic

O N

2

Zn(NO3)2.6H2O

O

NH

P

Hot ethanol

P N

N H

Zn

O

N

N

(C1) Scheme 1. Preparation pathway of C1.

N

O

O C

(L)

Zn

H2O

O

O

C

N

OH2

N

C

N

N H

P N

N

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Table 1 Crystal data and structure refinements for C1. Compound

C1

Empirical formula Formula weight Temperature (K) Crystal system, space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z, Dcalc (Mg m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h range for data collection (°) Index ranges

C18H31N5O6PZn0.5 477.13 293(2) Monoclinic, C2/c 27.4121(4) 10.4858(2) 17.9730(3) 90.00 121.949(1) 90.00 4383.55(13) 8, 1.446 0.704 2016 0.4  0.2  0.14 2.29–29.48 37 6 h 6 35 14 6 k 6 13 20 6 l 6 23 23569 5805 (Rint = 0.0553) 25.00 (99.7) 0.8 and 0.795 5805/1/342 1.052 R1 = 0.0473, wR2 = 0.1087 R1 = 0.081, wR2 = 0.1181 0.513 and 0.397

Reflections collected Independent reflections Completeness to h (%) Max. and min. transmission Data/restraints/parameters Goodness-of-fit on F2 Final R indices R indices (all data) Largest diff. peak and hole (eÅ3)

effective core potential standard basis set LANL2DZ [24] for zinc and 6-31G(d) for other atoms. The topological analysis of the electron charge density performed for the title complex was carried out using Bader’s quantum theory of atoms in molecules (QTAIM) [25]. The QTAIM analysis was performed with the help of AIM 2000 software [26] using the wave functions generated at the B3LYP/LANL2DZ/6-31+G(d) level. Natural bond orbital (NBO) analysis [27] was performed at the same level of theory. The structure of ligand was fully optimized at the B3LYP/6-31G(d) level and QTAIM and NBO analyses were conducted at the B3LYP/6-31+G(d).

Results and discussion Spectroscopic studies The spectroscopic characterization of C1 has been performed using IR and NMR techniques. In the IR spectrum of C1, the stretching bands appeared at 1194 and 1679 cm1 with appreciable intensity, are respectively assigned to phosphoryl and carbonyl groups, Table 2. The corresponding modes in the IR spectrum of the free ligand are observed at 1181 and 1672 cm1, respectively. It should be pointed out that in the case of carbacylamidophosphate ligands, binding through phsophoryl and carbonyl functional groups to metal is generally accompanied by a shift of the m(P@O) and m(C@O) to lower frequency regions [9,28]. However, comparing those vibrational frequencies in L and C1 reveals 13 cm1 increase

in value for m(P@O) in complex, along with a minor blue shift for m(C@O). This apparently shows that these bonds are not participated in coordination to metal (despite the X-ray results which prove the coordination of phosphoryl group to Zn(II) ion). Additionally, the specific band related to the stretching of pyridine ring shifts to higher wavenumber in C1 (1602 cm1) comparing to L (1589 cm1). Since certain vibrational modes of pyridine derivatives increase in value upon coordination from the ring nitrogen [29], this observation makes evident of the coordination from Npyridine to Zn(II) center (agreeing with the X-ray data). The NAH stretching vibration in ligand (3103 cm1) shifts to higher energy in complex (broad band at 3114 cm1), which based on the X-ray results can be attributed to the presence of weaker hydrogen bond in complex (NAH  Onitrate, DAA = 3.028(5) Å, \DHA = 174°) in comparison with the free ligand (NAH  Ophosphoryl, DAA = 2.8313(16) Å, \DHA = 173°) [9]. The absorption bands at 539 and 469 cm1 are assigned to the ZnAO and ZnAN stretching modes, respectively. Moreover, a broad band at 3541 cm1 can be correlated to the coordinated water molecules. 1 H and 13C NMR spectra of C1 in MeOD indicates the characteristic peaks relating to organic moieties. In comparison with the NMR data for ligand, no remarkable changes are occurred in chemical shifts and coupling constants by coordination to Zn(II), except that the splitting of carbon atom at d 13C = 131.9 ppm has been diminished in complex. Besides, an insignificant downfield shift for phosphorus atom from 13.90 ppm for L to 14.05 ppm for C1 has been detected. X-ray crystallography investigation Single crystals of C1 have been provided by slow evaporation of ethanol/acetonitrile solution at room temperature. The complex crystallizes in monoclinic C2/c space group with eight discrete molecules in the unit cell. Molecular structure is depicted in Fig. 1, and selected bond lengths and angles are listed in Table 3. The Zn(II) ion adopts a slightly distorted octahedral geometry with N2O4 donor set, comprising four phosphoramide ligands plus two water molecules. The ligands act as bridges between Zn(II) ions through simultaneous binding from Ophosphoryl and Npyridine donor atoms. This coordination pattern leads to the formation of infinite polymeric chains along c axis, in which the intrachain distance between metal centers is 8.986 Å, Fig. 2a. Each linear chain contains metal shared 16-membered rings, being composed of Zn, P, N, O and C atoms. The donor atoms around Zn(II) centers are located in trans positions with 180° trans angles, while cis NAZnAO and OAZnAO angles display deviation from 90° in the range of 84.94(6)–95.06(6)°. The ZnAO and ZnAN bond lengths are 2.0690(14) and 2.1716(16) Å, respectively, in accordance with the reported values for Zn(II) complexes with N-heterocyclic and carboxylic ligands [4d,30]. Some of the carbons atoms of the seven-membered amine rings (C3, C4, C8 and C11) are disordered over two positions with 0.60/0.40 occupancies. N5, O5 and O6 atoms of nitrate group also are disordered over two positions with 0.73/0.27 occupancies. The adjacent 1D chains are further linked to each other through CH  HC non-covalent weak interactions to

Table 2 Selected spectroscopic and X-ray parameters of C1 and L. Comp.

m (cm1)

d

NAH

P@O

C@O

Strpy

L

3103

1181

1672

1589

C1

3114

1194

1679

1602

31

P (ppm)

d (Å)

OAPANAC (°)

Ref.

1.2246(17)

170.07(12)

[17]

1.223(2)

52.5(2)

This work

P@O

PANamine

PANamide

C@O

13.90

1.4863(11)

1.6331(13) 1.6384(13)

1.6937(12)

14.05

1.4775(15)

1.6242(19) 1.6236(19)

1.6858(17)

K. Gholivand et al. / Journal of Molecular Structure 1092 (2015) 130–136

133

Fig. 1. General view of C1 with the presentation of non-hydrogen atoms by probability ellipsoids of thermal vibrations (p = 50%).

Table 3 Selected geometrical parameters of C1 for solid state and gas phase structures, calculated at the B3LYP/LANL2DZ/6-31G(d). C1

X-ray

Calcd.

Bond lengths (Å) Zn(1)AO(1) Zn(1)AO(2) Zn(1)AN(3) P(1)A(O1) P(1)AN(1) P(1)AN(2) P(1)AN(4) C(18)AO(3)

2.0690(14) 2.1036(17) 2.1716(16) 1.4775(15) 1.6242(19) 1.6236(19) 1.6858(17) 1.223(2)

2.129/2.133 2.135/2.131 2.201/2.198 1.514 1.666 1.678 1.694 1.260

Bond angles (°) O(1)AZn(1)AO(1) O(2)AZn(1)AO(2) N(3)AZn(1)AN(3) O(1)AZn(1)AN(3) O(2)AZn(1)AN(3) O(1)AZn(1)AO(2) O(1)AP(1)AN(1) O(1)AP(1)AN(2) N(1)AP(1)AN(2) N(4)AP(1)AN(1)

180.00(5) 180.00(7) 180.00(7) 88.14(6)/91.86(6) 84.94(6)/95.06(6) 88.89(7)/91.11(7) 114.75(10) 110.44(10) 110.90(10) 105.45(10)

179.04 179.17 179.92 86.55–93.52 86.32–90.81 88.23–91.95 111.34 111.34 110.56 106.73

construct the supramolecular 3D structure. A view of linear chains projected along c direction is depicted in Fig. 2b. There is also a hydrogen bonding between amidic NAH and oxygen atom of uncoordinated nitrate with DAA = 3.028(5) Å and \DHA = 174°. This is the first polymeric structure reported for N-nicotinyl phosphoric triamide complexes, since the previous compounds are composed of molecular building blocks. In ligands with (N)3AP@O scaffold, binding to metal from Ophosphoryl is generally accompanied by lengthening of P@O and shortening of PAN bonds [7,9,31]. However, comparing the structural data of C1 and L [17] shows the reverse trend for phosphoryl group: the P@O bond length shortens from 1.4863(11) Å for L to 1.4775(15) Å for C1, and as expected the PAN bond distances decrease from 1.6331(13)–1.6937(12) Å for L to 1.6236(19)– 1.6858(17) Å for C1 (Table 2). In case of pyridine ring, coordination from Npyridine is associated with almost no variation for CAN bond distances (1.343(2) Å for L and 1.338(3) Å for C1). The distance of carbonyl group shows no significant changing from L

(1.2246(17) Å) to C1 (1.223(2) Å), due to its non-participation in coordination. In addition, the orientation of phosphoryl and carbonyl functionals alters from anti for L to syn for C1 with the values of 170.07(12) and 52.5(2)° for OAPANACcarbonyl torsion angles, respectively. Quantum chemical calculations Since the usual behavior in structural changes of phosphoramide ligands upon coordination to metal through Ophosphoryl (lengthening of P@O bond) has not been followed in the case of C1, we used DFT calculations to find out about the structural and electronic aspects and plausible behavior of C1 in the gas phase. Hence, a single 6-coordinate Zn(II) center was extracted from the X-ray structure and fully optimized (Fig. 3). Then, the geometrical and electronic parameters (obtained by NBO and QTAIM analyses) have been compared with those for the fully optimized structure of the ligand. It is obvious that in this case the effect of neighboring molecules, which is a determinant factor in the solid state structure and crystal packing, has been neglected to have a net comparison between individual ligand and complex molecules. The optimized values for selected geometrical parameters of C1 are presented in Table 3, which show good agreement with the experimental results. Interestingly, the calculated P@O bond length in C1 (1.514 Å) is longer than that of the free ligand (1.495 Å) for their optimized structures, or even in comparison with the X-ray determined P@O distance for L. Decreasing the calculated PAN bond lengths in C1 comparing to those for L is also observed. Natural bond orbital analysis Natural bond orbital (NBO) analysis has been performed on C1 and L, in order to have an in depth insight into the electronic characteristics of the optimized structures. It is expected that upon coordination of ligand to metal, the negative electronic charges on donor atoms increase in value, because under the electrostatic field of the metal ion, charge density transfers from adjacent atoms to donor ones [10]. Based on the NBO results, calculated natural charges on Ophosphoryl and Npyridine are 1.11 and 0.45 for L, which with a significant increase for Npyridine negative charge, change to 1.13 and 0.56 for C1, respectively. Accordingly, the positive charge on phosphorus atom alters from +2.45 for L to +2.50 for C1. The changes in electronic density of donor atoms are mainly

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Fig. 2. (a) One-dimensional polymeric chain of C1 extended along c axis, and (b) a view of 1D chains in three dimensional network (projection along c crystal axis).

Fig. 3. Selected fragment of C1 optimized at the B3LYP/LANL2DZ/6-31G(d) level (hydrogen atoms are removed for clarity).

accompanied by more natural population on 2p orbitals, plus more p-character in the hybridization of their lone pairs (Table 4). For instance, the natural electronic configuration (NEC) of Ophosphoryl

is [core]2s1.81 2p5.28 3d0.02 for L which changes to [core]2s1.75 2p5.36 3p0.01 for C1, and the hybridization of the lone pair of phosphoryl oxygen, LP(Ophosphoryl), varies from sp0.55 for L to sp0.96 for C1 (Table 4). In NBO method, the strength of delocalization of electron density between occupied Lewis-type orbitals and formally unoccupied non-Lewis orbitals (antibonding or Rydberg) can be estimated using the second order perturbation theory [27]. The interaction of donor atoms of ligand with Zn(II) can be illustrated as donation of electron density from lone pairs of nitrogen and oxygen atoms to acceptor valence Zn(II) orbitals. The stabilization energies, E(2), for LP(Ophosphoryl) ? 4p(Zn) and LP(Npyridine) ? 4s(Zn) delocalizations are calculated to be 34.57 and 37.88 kcal/mol, respectively, indicating stronger interaction in the case of Npyridine, Table 5. The occupancy of lone pairs of donor atoms which are less than natural two electrons (LP(Ophosphoryl): 1.89e, LP(Npyridine): 1.82e, Table 5), and the amount of positive charge on Zn(II) ion (+1.22) are in agreement with the presence of the stabilizing donor–acceptor interactions for C1. On the other hand, the electronic back donation occurs from donor 4s(Zn) orbital to Ry⁄ (Ophosphoryl) and Ry⁄ (Npyridine) extra-valence acceptor orbitals, with E(2) values of 25.25 and 26.16 kcal/mol, respectively. Overall, in the framework of NBO analysis, the interaction between Zn(II) ion and ligand in complex C1, is found to be stabilizing donor–acceptor delocalizations.

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K. Gholivand et al. / Journal of Molecular Structure 1092 (2015) 130–136 Table 4 Calculated natural charges and natural electron configuration (NEC) for selected atoms and hybridization of donor lone pairs. Comp.

Atomic charge

NEC

Hybridization

Ophosphoryl

Npyridine

P

Zn

Ophosphoryl

Npyridine

Zn

LP(Ophosphoryl)

LP(Npyridine)

L

1.11

0.45

2.45



[core]2s1.81 2p5.28 3d0.02

[core] 2s1.38 2p4.04 3d0.01 4p0.01

–

sp0.55

sp2.40d0.01

C1

1.13

0.56

2.50

1.22

[core]2s1.75 2p5.36 3p0.01

[core]2s1.34 2p4.19 3s0.01 3p0.02

[core]4s0.34 3d9.99 4p0.43 4d0.01 5d0.01

sp0.95

sp2.77

Table 5 Second order stabilization energies between donor and acceptor orbitals (E(2), kcal/mol) in NBO basis for C1. Donor orbital

Acceptor orbital

E(2) (kcal/mol)

LP(Ophosphoryl) LP(Npyridine) 4s(Zn) 4s(Zn)

4p(Zn) 4s(Zn) Ry⁄ (Ophosphoryl) Ry⁄ (Npyridine)

34.57 37.88 25.25 26.16

Topological analysis of electron density In quantum theory of atoms in molecules (QTAIM) analysis, the nature and strength of bonding is investigated by means of topological analysis of the electron density. Table 6 lists the values of electron density (q), Laplacian (r2q) and total electronic energy density (H(r)) computed at selected bond critical points. In QTAIM analysis, the large values of q, r2q < 0 and H(r) < 0 refer to the shared interactions (covalent bonds), small values of q, r2q > 0 and H(r) > 0 are calculated for closed-shell interactions (ionic, hydrogen bonds and van der Waals interactions), and r2q > 0 and H(r) < 0 are assigned to the intermediate ones [25]. The electronic energy density is obtained from the equation H(r) = G(r) + V(r), where G(r) and V(r) are the local kinetic and potential energy densities, respectively. The estimated charge density values at the ZnAO and ZnAN bond critical points of C1 are calculated to be 0.053 and 0.058 au, respectively. Based on these small values of q (less than 0.1), r2q > 0 and H(r) < 0, the nature of the ZnAL interaction are found to be mainly electrostatic with partial amount of covalent character. Additionally, Espinosa et al. have suggested that closed-shell, intermediate and shared interactions are respectively associated with |V(r)|/G(r) 6 1, 1 < |V(r)|/ G(r) < 2, and |V(r)|/G(r) > 2 ratio [32]. Hence, the |V(r)|/G(r) ratio of 1.1332 for Zn  Ophosphoryl and 1.2452 for Zn  Npyridine further confirm the intermediate nature with dominance of ionic character for ZnAL interaction. Comparing the topological properties of the free ligand with those of C1 reveals that the q value at P@O bond critical point decreases from 0.223 for L to 0.211 au for C1, and the |H(r)| value also reduces upon complexation (Table 6). This is in accordance with a decrease in the covalent contribution of the P@O bond and its elongation upon coordination to metal. The structural changes and electronic perturbations occurred in L by complexation to Zn(II), when comparing the optimized structures of L and an individual fragment of C1 with octahedral configuration, are entirely in agreement with the common trend

Fig. 4. The relative stability of anti and syn configuration of ligand L for optimized structures (hydrogen atoms are eliminated for clarity).

observed in phosphoramide complexes [7,9]. Therefore, the unusual shortening of the P@O bond length observed in the X-ray determined structure of C1, might be a result of the 16-membered ring strain and the steric requirement imposed on the ligand to form such a puckered ring. The effect of neighboring units in the corresponding polymeric structure might also be considered. Besides, as is shown in Fig. 4, the optimized structure of the free ligand with anti P@O and C@O configuration, is 4.10 kcal/mol more stable than the syn-oriented structure, due to the electronic dipole–dipole repulsion [7,14]. Therefore the syn orientation observed for P@O and C@O groups in C1 is another consequence of the mentioned solid state requisites. In contrast, for our previously studied complexes of N-(iso)nicotinyl phosphoramide ligands with discrete molecular [11,12] or even 1D and 3D polymeric structures [13,14], the anti position of phosphoryl and carbonyl functionals have been preserved. Conclusion In this work, synthesis and spectral characterization of a novel Zn(II) complex (C1) containing N-nicotinyl derivative of phosphoramide ligand have been reported. Structural analysis shows that the ligand acts as a bridge between two metal centers by binding through Ophosphoryl and Npyridine. This creates a distorted octahedral geometry for Zn(II) ions, along with two coordinated water molecules. The binding pattern leads to one-dimensional polymeric chains extended along c axis. Based on X-ray diffraction studies,

Table 6 Calculated QTAIM parameters (electron density, q, its Laplacian, r2q, and total electronic energy density, H(r)) at selected critical points. Comp.

L C1

ZnAOphosphoryl

ZnANpyridine 2

P@O 2

q

r q

H(r)

q

r q

H(r)

q

r2q

H(r)

– 0.053

– 0.219

– 0.008

– 0.058

– 0.173

– 0.014

0.223 0.211

1.512 1.361

0.152 0.139

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the length of phosphoryl bond shortens in C1 in comparison with that of the ligand, while this behavior is not observed when comparing their optimized structures in the gas phase. Hence, this shortening of P@O, as well as changing the orientation of phosphoryl and carbonyl groups in C1 can be attributed to the steric demands in the solid state and effect of neighboring units in polymeric structure of the complex. To elucidate the electronic properties and nature of binding interactions, QTAIM and NBO analyses have been performed. In the framework of NBO program, the metal–ligand interaction is determined as charge transfer between donor atoms of ligand and Zn(II) ion. In addition, the small values of q, r2q > 0, H(r) < 0, and also the |V(r)|/G(r) values obtained by QTAIM analysis, indicate that the Zn–ligand interaction is not merely ionic, and there is a partial amount of covalent character (mixing of metal and ligand orbitals). Supplementary data CCDC 989381 contains the supplementary crystallographic data for C1. This data can be obtained free of charge via http://www. ccdc.cam.ac.uk/data_request/cif. Acknowledgements Financial support of this work by Tarbiat Modares University is gratefully acknowledged. We also would like to thank Michel Giorgi of Aix-Marseille University for X-ray experiment. References ˇ obeljic´, A. Pevec, I. Turel, M. Swart, D. Mitic´, M. Milenkovic´, I. Markovic´, [1] (a) B. C M. Jovanovic´, D. Sladic´, M. Jeremic´, K. Andelkovic´, Inorg. Chim. Acta 404 (2013) 5–12; (b) S. Bjelogrlic´, T. Todorovic´, A. Bacchi, M. Zec, D. Sladic´, T. Srdic´-Rajic´, D. ´ , J. Inorg. Biochem. 104 (2010) Radanovic´, S. Radulovic´, G. Pelizzi, K. Andelkovic 673–682. [2] (a) M. Patel, M. Chhasatia, P. Parmar, Eur. J. Med. Chem. 45 (2010) 439–446; (b) E. Viñuelas-Zahínos, F. Luna-Giles, P. Torres-García, M.C. FernándezCalderón, Eur. J. Med. Chem. 46 (2011) 150–159. [3] K. Singh, Y. Kumar, P. Puri, M. Kumar, C. Sharma, Eur. J. Med. Chem. 52 (2012) 313–321. [4] (a) X. Zheng, S.Q. Guo, X.Y. Yu, J.K. Hu, Y.H. Luo, H. Zhang, X. Chen, Inorg. Chem. Commun. 18 (2012) 29–33; (b) O.R. Evans, W.B. Lin, Acc. Chem. Res. 35 (2002) 511–522; (c) M. Fujita, Y.J. Kwon, S. Washizu, K. Ogura, J. Am. Chem. Soc. 116 (1994) 1151–1152; (d) Y. Huang, B. Yan, M. Shao, J. Mol. Struct. 919 (2009) 185–188. [5] (a) M.J. Dominguez, C. Sanmartin, M. Font, J.A. Palop, S.S. Francisco, O. Urrutia, F. Houdusse, J.M. Garsia-Mina, J. Agric. Food Chem. 56 (2008) 3721–3731; (b) J.C. Bollinger, J. Levy-serpier, J. Debord, B. Penicaut, J. Enzym. Inhib. Med. Chem. 3 (1990) 211–217; (c) P.S. Schwartz, D.J. Waxman, Mol. Pharmacol. 60 (2001) 1268–1279.

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