A Novel Polymeric Lead(II)-Azido Compound: Synthesis, Structural Characterization, and DFT Calculations of [Pb(dmp)(N 3 ) 2 ] n

A Novel Polymeric Lead(II)-Azido Compound: Synthesis, Structural Characterization, and DFT Calculations of [Pb(dmp)(N 3 ) 2 ] n
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   ARTICLE DOI: 10.1002/zaac.200900288 A Novel Polymeric Lead(II)-Azido Compound: Synthesis, StructuralCharacterization, and DFT Calculations of [Pb(dmp)(N 3 ) 2 ] n Behrouz Shaabani,* [a] Babak Mirtamizdoust, [a] Muhammad Shadman, [b] andHoong-Kun Fun [c] Keywords:  Lead; Azides; Polymers; π–π Stacking; X-ray diffraction Abstract.  A novel 1D Pb II coordination polymer containing Pb 2 -(µ- N 3 ) 2  unit [Pb(dmp)(N 3 ) 2 ] n  (dmp = 2,9-dimethyl-1,10-phenanthroline)has been prepared and characterized. Single-crystal X-ray diffractionanalyses show that the coordination number for Pb II ions is six, PbN 6 ,with “stereochemically active” electron lone pairs and the coordination 1. Introduction  Non-transition-metal complexes take various coordinationnumbers and in fact show diverse structures. The diversity of structures in non-transition-metal complexes is ascribed to thefact that d atomic orbitals of the metal atom are fully occupied.However, non-transition-metal complexes have valence s and p electrons that can play an important role in determining their molecular structures. Transition metal complexes have beenstudied extensively both experimentally and theoretically, andmany properties of transition metal complexes related to partlyfilled d orbitals of the metal ion are well explained in terms of ligand field theory [1, 2]. However, relatively less theoreticalstudies on s and p block (non-transition) metal complexes have been reported to date [3].Divalent lead, with its electronic configuration [Xe] 4f  14 5d 10 6s 2 , is one of the post-transition metal elements that exhibitsthe so-called “inert-pair effect” and their compounds are inter-esting and frequently discussed in considering the “stereo-chemical activity” of valence shell electron lone pairs [4–8].The azido group N 3 –  is one of the most interesting in inor-ganic coordination compounds found so far. This topic at-tracted much attention according to two main reasons. At first, * Dr. B. ShaabaniFax: +98-411-3340191E-Mail:[a] Department of Inorganic ChemistryFaculty of ChemistryUniversity of TabrizTabriz, Iran[b] Department of Physical and Inorganic ChemistryFaculty of ChemistryUniversity of MazandaranBabulsar, Iran[c] X-ray Crystallography UnitSchool of Physics, University of Sains Malaysia11800 USM, Penang, Malaysia 2642  © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim  Z. Anorg. Allg. Chem.  2009 ,  635 , 2642–2647 sphere being hemidirected. The single-crystal X-ray data show thechains interact with each other through the π–π stacking interactions,which create a 3D framework. The structure of title complex has beenoptimized by density functional theory. Structural parameters and IR spectra for the complex are in agreement with the crystal structure. the azido anion can behave as a bidentate bridging ligand (end-to-on, µ 1,1 -N 3  “EO” [9–11], or end-to-end, µ 1,3 -N 3 , “EE”, bridging modes [12–15]), and as a tridentate bridging ligand(µ 1,1,1 -N 3  [11] or µ 1,1,3 -N 3  [16–19]). Both types of bidentatecoordination can mediate ferromagnetic and antiferromagneticinteractions between the metallic ions.The inorganic azido salts are an important class of solid com- pounds for a number of reasons. Some are of practical impor-tance as explosives; others as industrial chemicals, and evenothers as useful photographic materials at low temperature.Lead azide is a primary explosive and is widely used as a primer. The sodium and lithium azide systems are much more benign in behavior. Therefore, to gain an understanding of the behavior patterns observed [20], one should know the impor-tance of the fundamental electronic structure properties of thisclass of solid.The first lead(II)-azido coordination polymer has been syn-thesized in our research group [21] and recently we have syn-thesized some other lead(II)-azido coordination polymers [22,23]. As a continuation of the previous study, in this paper weextend these experimental studies to investigate the interac-tions of this universal bridging ligand (azide anion) withlead(II) ions in the presence of aromatic amine and this reportis the first theoretically novel study of lead(II)-azido com- plexes. The electronic structure has been determined by thedensity functional theory (DFT) method. Currently DFT iscommonly used to examine the electronic structure of metalcomplexes. It meets the requirements of being accurate, easyto use and fast enough to allow the study of relatively largemolecules of metal complexes [24]. DFT has shown to be suf-ficient for the arrangement optimization and calculation of spectral properties. It gives good agreement with the experi-mental data and its use is justified in the case of large molecu-les.  A Novel Polymeric Lead(II)-Azido Compound [Pb(dmp)(N 3 ) 2 ] n 2. Experimental Section 2.1 Materials and Physical Measurements All the reagents used for the syntheses were commercially availableand were used without further purification. IR spectra were recordedas nujol mulls with Perkin–Elmer 597 and Nicolet 510P spectropho-tometers. Microanalyses were carried out with a Heraeus CHN-O-Rapid analyzer. Melting points were measured with an Electrothermal9100 apparatus and are uncorrected.  1 H NMR and  13 C NMR spectrawere measured with a BRUKER DRX-500 AVANCE spectrometer at500 MHz, respectively. Caution : Although we have experienced no problem with the com- pounds reported in this work, sodium azide and azide complexes are potentially explosive and should be handled in small quantities andwith great caution. 2.2 Preparation of [Pb(dmp)(N  3  ) 2  ]  n  (1) The title complex was prepared by the branch tube [25] method: 2,9-Dimethyl-1,10-phenanthroline (0.20 g, 1 mmol) was placed in one armof a branched tube and lead(II) acetate (0.37 g, 1 mmol) and sodiumazide (0.13 g, 2 mmol) in the other. Methanol was then carefully addedto fill both arms, the tube sealed and the ligand-containing arm im-mersed in a bath at 60 °C, while the other was left at ambient tempera-ture. After 3 weeks, crystals (m.p. 283 °C) suitable for an X-ray struc-ture determination had deposited in the arm at ambient temperature.They were then filtered off, washed with acetone and ether, and air dried. Yield: 55 %. Anal. C 28 H 24  N 16 Pb 2 : calcd. C 33.66, H 2.42, N22.43 %; found C 33.50, H 2.10, N 22.20 %.  IR:  ν ˜ = 680  m , 1475  s ,1505, 1578  s , 2045 vs, 2053 s, 2990  m , 3056  w  cm  –1 .  1 H NMR  (DMSO):  δ  = 2.80 (s, 6 H, methyl-H), 7.63 (s, 2 H, py-H), 7.87 (d, 2H, py-H), 8.39 (d, 2 H, py-H).  13 C NMR   (DMSO):  δ  = 25.3 (methyl),124.4 (py), 126.1 (py), 127.4 (py), 137.4 (py), 145.1 (py), 159.1 (py). 2.3 Crystallography Crystallographic data were collected at 100 K with the Oxford Cyro-system Cobra low temperature attachment. The data were collectedwith a Bruker Apex2 CCD diffractometer with a graphite monochro-mated Mo-  K  α  radiation at a detector distance of 5 cm and with APEX2software [26]. The collected data were reduced using SAINT program[26], and the empirical absorption corrections were performed usingSADABS program [26]. The structures were solved by direct methodsand refined by least-squares using the SHELXTL software package[27]. Materials for publication were prepared using SHELXTL [27]and ORTEPIII [28].Crystallographic data for the structures reported in this paper has beendeposited with the Cambridge Crystallographic Data Centre as supple-mentary publication CCDC-698854 for [Pb(dmp)(N 3 ) 2 ] n  ( 1 ). Copies of the data can be obtained on application to CCDC, 12 Union Road,Cambridge CB2 1EZ, UK [Fax: +44-1223-336033; E-Mail: de-]. 2.4 Computational Details The geometry of the [Pb(dmp)(N 3 ) 2 ] n  complex has been optimized byusing B3LYP density functional model [29, 30]. In these calculations,we used the 3-21G* basis set for carbon and hydrogen atoms, whereasthe 6-31G* basis set was used for nitrogen atoms. For the lead atoms,  Z. Anorg. Allg. Chem.  2009 , 2642–2647 © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim  2643 the LanL2DZ valence and effective core potential functions were used[31, 32] All DFT calculations were performed using the Gaussian 98R-A.9 package [33]. X-ray structures were used as input geometrieswhen available. 3. Results and Discussion 3.1 Spectroscopic Studies Reaction between lead(II) acetate and mixtures of “dmp”(dmp = 2,9-dimethyl-1,10-phenanthroline) and sodium azidein methanol provided crystalline material with the formula[Pb(dmp)(N 3 ) 2 ] n  ( 1 ). IR spectra display characteristic absorp-tion bands for “dmp” ligands and azide anions. The selectedspectroscopic data and the obtained data from DFT calcula-tions are given in Table 1. The strong doublet absorption bandcentered at ca. 2045–2055 cm  –1 is assigned to the ν asy (N 3 –  ).This reflects the bonding mode of N 3 –  as end-to-on (µ 1,1 ) [34].The relatively weak band around 3056 cm  –1 is attributed to theabsorption of the aromatic CH hydrogen atoms and the bandaround 2990 cm  –1 is attributed to the absorption of the ali- phatic CH hydrogen atoms [34]. The  1 H NMR spectrum of theDMSO solution of   1  displays a distinct signal at 2.80 ppmassigned to the aliphatic protons of “dmp” ligand. The signalsat 7.63, 7.87, and 8.39 ppm are assigned to aromatic protonsof “dmp” ligand. The  13 C NMR spectrum of the DMSO solu-tion of the compound  1  displays six distinct signals assignedto the aromatic carbons of py groups of the “dmp” ligand andanother signal at 25.3 ppm assigned to carbon atoms of methylgroups of “dmp” ligand, respectively. Table 1.  Experimental FT-IR data a) for [Pb(dmp)(N 3 ) 2 ] n , comparedwith the theoretical IR data obtained from DFT calculations.Assignment Experimental Calculated ν asy  (N 3 –  ) 2045 vs, 2053 s 2203, 2228 ν(C–H)aliphatic 2990 m 3032 ν(C–H)aromatic 3056 w 3193 ν(C–H) 680 m 757 ν(C–C) 1475 s, 1505 s, 1578 s 1477, 1532, 1584 ν(Pb–N) 440a) Frequencies for IR in cm  –1 . m medium; w weak; vs very strong; sstrong. 3.2 Description of Crystal Structure The determination of the structure of [Pb(dmp)(N 3 ) 2 ] n  by X-ray crystallography [35] showed the complex crystallizes inthe triclinic system with space group of   P  1¯ and in the solidstate (Figure 1) to be 1D coordination polymer, which is a fea-ture of many structures [36–42]. The molecular structure of the asymmetric [Pb(dmp)(N 3 ) 2 ] n  unit with the atom labeling isshown in Figure 2. Table 2 shows the experimental value of  bond lengths and angles of complex and their obtained calcula-tions from DFT.  B. Shaabani et al.  ARTICLE Figure 1.  Fragment of the coordination polymer showing the 1D poly-mer. Figure 2.  The molecular structure of the asymmetric [Pb(dmp)(N 3 ) 2 ] n unit. Table 2.  Selected bond lengths /Å and angles /° for [Pb(dmp)(N 3 ) 2 ] n .Symmetry transformations used to generate equivalent atoms:  i  = –   x , –   y , –   z .Experimental CalculatedPb1 ··· Pb1 i–1 4.189 3.958 N1–Pb1 2.699(2) 2.663 N2–Pb1 2.673(2) 2.767 N3–Pb1 2.529(2) 2.755 N3 i  –Pb1 2.502(2) 2.530 N6–Pb1 2.803(2) 2.566 N6 i  –Pb1 2.841(2) 2.755Pb1 ··· Pb1 i–2 4.341 4.276 N3–Pb1–N6 i 81.19(7) 80.95Pb1–N3–Pb1 i 112.75(7) 108.56 N2–Pb1–N6 i 136.06(6) 136.16 N6–Pb1–N6 i 73.46(7) 73.04 N3–Pb1–N2 118.04(6) 118.45 N6–Pb1–N2 76.06(6) 76.31 N3–Pb1–N1 88.95(6) 88.45 N2–Pb1–N1 62.61(6) 62.59 N3–Pb1–N3 i 67.25(7) 67.69 N3–Pb1–N6 96.49(7) 93.48 The lead atoms are linked by two nitrogen atoms of “dmp”ligands with Pb–N distances of 2.699(2) and 2.673(1) Å, andfour nitrogen atoms of azide anions with Pb–N distance of 2.502(2), 2.529(2), 2.841(2), and 2.803(2) Å. Therefore the co-ordination number for lead is six (Figure 1). 2644  © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim  Z. Anorg. Allg. Chem.  2009 , 2642–2647 The arrangement of these ligands suggests a gap or hole inthe arrangement around the metal ions [N2–Pb1–N6 i =136.06(6)°], possibly occupied by a “stereoactive” lone pair of electrons on lead(II) [43]. The observed shortening of the Pb–  N bonds on the side of Pb II ion opposite to the putative lone pair [N3 i  –Pb1 = 2.502(2) Å compared with N6–Pb1 =2.803(2) Å adjacent to the lone pair] supports the presence of this possibility [44]. These distances and angles are compara- ble with our previously reported 1,10-phenanthroline and azidoadducts of lead(II) [N8 i  –Pb1–N3 i angle is 131.71(19)°,2.418(6) Å compared with 2.816(5) Å adjacent to the lone pair] [22].The arrangement of the nearest coordination environment of every lead atom is, therefore, likely caused by the geometricalconstraint of coordination “dmp” ligand, azide anions as wellas by the influence of a stereo-chemically active lone pair of electrons in hybrid orbital on the metal atom. Such an environ-ment leaves space for bonding of another atom of azide anion.To find any potential donor atom within this “vacancy”, weneed to extend the bonding limit to at least 3.6 Å [44]. It is possible to find azido nitrogen atoms approaching each lead(Pb1 ···  N5 i = 3.58 Å) (Figure 3), thus the coordination sphereis almost completed and the coordination number will be seven(PbN 7 ) with holodirected arrangement [43] (Symmetry code: i  = –   x , –   y , –   z ). Figure 3.  Schematic representation of Pb II environments. The lead(II) atoms are bridged by four azide ions in µ 1,1  end-to-on fashion, whose coordination modes are common [22,44]. The Pb ··· Pb distance through the EO azido bridges is4.189 and 4.341 Å. The packing diagram of   1  exhibits a 3Dsupramolecular architecture arising from lone pair activity andπ–π stacking interaction. (Figure 4). There are three differenttypes of noncovalent π–π stacking interactions [45, 46] be-tween the parallel aromatic rings belonging to adjacent chainsas shown in Figure 4 and Figure 5. The interplanar distancesof “dmp” ligands are 3.26, 3.55, and 3.47 Å, appreciablyshorter than the normal π–π stacking [47, 48]. Therefore it can be expected that the electron-poor methyl pyridyl rings willinteract with less electron-poor rings such as phenyl groups(Figure 5). These are very close to the corresponding reportedstructures [21–23].  A Novel Polymeric Lead(II)-Azido Compound [Pb(dmp)(N 3 ) 2 ] n Figure 4.  Packing of 1D chains to form 3D supramolecular layersthrough π–π stacking interactions. Figure 5.  Projection of the nearest neighbor pair π–π stacks of hete-roaromatic bases in [Pb(dmp)(N 3 ) 2 ] n . Therefore, two factors, lone pair activity and π–π stacking,may control the coordination sphere of this complex. The obvi-ous question, then, is whether the lone pair activity hasstretched coordinate bonds to result in ligand stacking or whether it is the stacking interaction, which has imposed a positioning of the donor atoms to form gap in the coordinationsphere. However, self-assembly of this complex is likely to becaused by both the lone pair activity and the π–π interactionof the aromatic rings. 3.3 DFT Calculations The calculated structural parameters are listed in Table 2.The significant figure is maintained to be four, which is suffi-  Z. Anorg. Allg. Chem.  2009 , 2642–2647 © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim  2645 cient for one bond length to be differentiated from the other.It should be noted that the experimental data belong to solid phase, whereas the calculated data correspond to the isolatedmolecule in gas-phase. However, the experimental and compu-tational data in Table 2 clearly show that both data onlyslightly differ from each other. For example, the largest differ-ence between experimental and calculated N6–Pb1 length isabout 0.237 Å, whereas the biggest deviation occurs in thePb1–N3–Pb1 i angle by ca. 4.19 °. As a result, the calculatedgeometrical parameters represent a good approximation.The computational IR frequencies are listed in Table 1 to-gether with experimentally determined frequencies. The calcu-lated azide frequency of 2203 cm  –1 and 2228 cm  –1 is wellcompared with the experimentally determined value of 2045 cm  –1 and 2053 cm  –1 . The assignment of the experimental ν(Pb–N) vibration is based on the theoretically calculated fre-quencies. Both calculated and experimental frequencies arefound to be in agreement with the frequency value 340 cm  –1 of a lead complex [Pb(dmp)(N 3 ) 2 ] n .The Mulliken charges of lead(II) and the coordinated atomswere also calculated. The positive charge of the lead(II) ionswas 1.003. The considerably low positive charge in the lead(II)ion of   1  may be explained by its contribution to the Pb ··· Pbinteractions. The charges of the nitrogen atoms of the “dmp”ligands were –0.687 and –0.631, respectively, whereas the ni-trogen atoms of both azide anions (N6 and N3) have similar charges: N6 = –0.539 and N3 = –0.530.The calculations indicate that complex  1  has 78 occupiedmolecular orbitals (MOs) for [Pb(dmp)(N 3 ) 2 ] unit. The valueof the energy separation between the highest occupied molecu-lar orbital (HOMO) and the lowest unoccupied molecular or- bital (LUMO) was calculated. Figure 6 shows the HOMO andLUMO for lead(II) complex. As will be seen in Figure 6, theHOMO of the title complex is principally localized among twonitrogen atoms of one azide anion, whereas the LUMO is delo- Figure 6.  Frontier molecular orbitals for [Pb(dmp)(N 3 ) 2 ] unit.  B. Shaabani et al.  ARTICLE calized approximately on all atoms of the “dmp” ligand includ-ing the lead(II) and azide anions. The calculated HOMO– LUMO gap is 0.061 a.u. (1.662 eV). Compared with lead azide[Pb(N 3 ) 2 ] gap (4.7 eV) [20], it was found that the lead azide bandgap was much wider than is the complex  1  gap. Therefore,the complex can be expected to be a primary explosive and to be very sensitive to shock. 4. Conclusions In this work, a novel 1D Pb II coordination polymer contain-ing Pb 2 -(µ-N 3 ) 2  unit, has been synthesized under presence of aromatic amine (2,9-dimethyl-1,10-phenanthroline), and itscrystal structure has been determined. The arrangement of li-gands suggests a gap in coordination arrangement around themetal ions possibly occupied by a “stereo-active” lone pair of electrons on lead(II). Coordination modes of azide ions areµ 1,1  end-to-on fashion. Two factors, lone pair activity and π–πstacking, may control the coordination sphere of this complex.This is the first theoretical study of lead(II)-azido complex thatexamined the electronic structure of title complex, which cor-responds well the experimental data. Low positive charge inthe lead(II) ion may be explained by its contribution to thePb ··· Pb interactions. The calculations indicate that the HOMOof the title complex is principally localized among two nitro-gen atoms of one azide anion, whereas the LUMO is delocal-ized approximately on all atoms of the “dmp” ligand includingthe lead(II) and azide anions. Acknowledgement Support of this investigation by University of Tabriz. H. K. F wouldlike to thank the  Malaysian Government   and  Universiti Sains Malaysia for Scientific Advancement Grant Allocation (SAGA) grant No. 304/PFIZIK/653003/A118. References [1] D. F. Schriver, P. W. Atkins,  Inorganic Chemistry , 3rd ed. OxfordUniversity Press, New York,  1999 .[2] C. 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