S. Saha et al. / Polyhedron 29 (2010) 3342–3348 3343ing the synthesis, X-ray crystal <strong>structure</strong> <strong>and</strong> magnetic study of anovel tetranuclear 1D <strong>polymer</strong>, [Cu 4 (N 3 ) 8 (en) 2 ] n (1).2. Experimental2.1. Materials <strong>and</strong> physical techniquesAll reagents <strong>and</strong> solvents for the synthesis <strong>and</strong> analysis werecommercially available <strong>and</strong> used as received without further purification.IR spectra were recorded as KBr pellets within the range4000–400 cm 1 on a Perkin–Elmer Spectrum RXI FTIR Spectrometer.Elemental analyses were carried out using a Heraeus CHN–O-Rapid elemental analyzer. Variable-temperature magnetic susceptibilitymeasurements were carried out on a microcrystalline samplein the 2–300 K temperature range using a Quantum DesignMPMS SQUID susceptometer operating under a magnetic field of0.1 T. Diamagnetic corrections for the complexes were estimatedfrom Pascal’s constants. The magnetic susceptibility for 1 was computedby exact calculation of the energy levels associated with thespin Hamiltonian through diagonalization of the full matrix withthe enhanced version of a general program for axial symmetry[48]. Least-squares fittings were accomplished with an adaptedversion of the function-minimization program MINUIT [49]. The error-factorR is defined as R ¼ P ðv exp v calc Þ 2, where N is the numberNv 2 expof experimental points.2.2. <strong>Synthesis</strong> of [Cu 4 (N 3 ) 8 (en) 2 ] n (1)Caution! Azido complexes of metal ions in the presence of organiclig<strong>and</strong>s are potentially explosive. Only a small amount ofmaterial should be prepared, <strong>and</strong> it should be h<strong>and</strong>led with care.To a 10 mL methanolic solution of Cu(NO 3 ) 2 3H 2 O (1.00 mmol,241.6 mg) <strong>and</strong> en (0.50 mmol, 30 mg) an aqueous solution ofNaN 3 (2.00 mmol, 130 mg) dissolved in minimum water was addedslowly. The mixture was stirred for 30 min <strong>and</strong> filtered. Rectangularblack crystals of 1 were obtained in two days, (yield: 70%). Anal.Calc. for C 2 H 8 Cu 2 N 14 : C, 6.75; H, 2.25; N, 55.16. Found: C, 7.12; H,2.02; N, 56.1%.2.3. X-ray crystallographyA diffraction quality single crystal of 1 was mounted on a BrukerSmart Apex <strong>II</strong> CCD Area Detector equipped with a graphite monochromatorMo Ka radiation (k = 0.71073 Å). Data Collection wascarried out using BRUKER APEX2 software [50]. Multiscan absorptioncorrection was applied using SADABS [51]. The <strong>structure</strong> of the complexwas solved by direct methods with SHELXS, <strong>and</strong> refined by fullmatrixleast squares <strong>based</strong> on F 2 with SHELXL [52]. All non-hydrogenatoms were refined anisotropically. The C-bound H atoms wereconstrained to ideal geometry <strong>and</strong> were included in the refinementin the riding model approximation. Further crystallographic analysis<strong>and</strong> figure production were carried out using PLATON99 [53] <strong>and</strong>ORTEP [54] programs. Details of the data collection parameters <strong>and</strong>crystallographic information for the complex are summarized inTable 1.3. Results <strong>and</strong> discussion3.1. <strong>Synthesis</strong> <strong>and</strong> IR SpectroscopyInspection of the Cu <strong>II</strong> /en/N 3ratios (Table 2) in the startingmaterials <strong>and</strong> the stoichiometry of our final product reveals excellentconformity. However, this has not always been the case for allthe related complexes I–IV; except <strong>II</strong> where these ratios were identical.We also observe that a parameter which seems crucial inTable 1Summary of crystal data <strong>and</strong> refinement details of 1.Empirical formula C 2 H 8 Cu 2 N 14Formula weight 355.30Crystal dimension (mm) 0.25 0.22 0.15Crystal systemtriclinicSpace group P1a (Å) 7.8047(14)b (Å) 8.2059(15)c (Å) 9.1786(16)a (°) 86.838(3)b (°) 84.493(4)c (°) 81.720(4)V (Å 3 ) 578.52(18)Z 2T (K) 150(2)D calc (g cm 3 ) 2.040l (mm 1 ) 3.686F (0 0 0) 352h (°) 2.23–25.00Total data 2029Unique data 1728R a 0.0511bR w 0.1275Goodness-of-fit (GOF) on F 2 , S 1.028R int 0.0446Dq max (e Å 3 ) 1.149Dq min (e Å 3 ) 0.808a R ¼ P ðjF o F cjÞ= P jF oj:b Rw ¼fR½wðjF o F cjÞ 2 Š=R½wjF oj 2 Šg 1=2 .defining the nuclearity of the <strong>polymer</strong>’s repeating unit is theen:N 3ratio; smaller such ratios (e.g., 1:10 for the octanucler IV)seem to favor the formation of high-nuclearity <strong>polymer</strong>ic products.However, these apply to ‘‘clean” preparations, i.e., those forwhich only one product is derived. Mondal <strong>and</strong> Mukherjee observed[44] that the reaction leading to <strong>II</strong> could also afford <strong>II</strong>I ifthe initial product was left in its mother liquor. This suggests thatthere is a kinetic factor defining the final product, possibly relatingto the relative solubilities of the different species in solution. It isconceivable that these solubilities relate to the dimensionality<strong>and</strong> nuclearity of each product.These tentative conclusions propose possible ways of affectingthe nuclearity <strong>and</strong> dimensionality of the products of this system,either through the increase of the <strong>azide</strong> ratio (for increased nuclearities),or through the solution concentration <strong>and</strong> reaction time, tocontrol dimensionality. However, these tentative conclusionsshould be tested through further synthetic studies.The IR (cm 1 ) spectra of the complex showed absorption peaksin the range of 3300–2950 cm 1 which are assigned to m as (N–H) ofthe ethylenediamine. Peaks at 2087 <strong>and</strong> 2096 cm 1 are assigned tothe v(N 3 ) vibration of the bridging <strong>azide</strong>s. Multiple peaks are indicativeof multiple bridging modes of the azido lig<strong>and</strong>.3.2. Description of the <strong>structure</strong>The single crystal X-ray <strong>structure</strong> of complex 1 reveals doublyend-to-end azido-bridged 1D chains of Cu 4 clusters (Fig. 1). Selectedbond distances <strong>and</strong> angles are listed in Table 3. The centrosymmeticCu 4 cluster consists of two symmetry-independent Cu <strong>II</strong>ions (Cu1 <strong>and</strong> Cu2) <strong>and</strong> their symmetry related counterparts. Cu1<strong>and</strong> Cu2 are bridged end-on by the N7 <strong>and</strong> N10 nitrogen atomsof two <strong>azide</strong> ions.Whereas Cu2 posseses an octahedral coordination environment,Cu1 possesses a distorted square-pyramidal coordination environment.Two amine nitrogens (N13 <strong>and</strong> N14) of a chelating ethylenediamine<strong>and</strong> nitrogen atoms N7 <strong>and</strong> N10 of the l 1,1 -<strong>azide</strong>s formthe equatorial plane of the coordination sphere of Cu2. AtomsN1 ii <strong>and</strong> N6 iii of two <strong>azide</strong>s occupy the axial sites of this octahe-
3344 S. Saha et al. / Polyhedron 29 (2010) 3342–3348Table 2Summarization of synthetic procedure for all <strong>copper</strong>-azido cluster <strong>based</strong> <strong>polymer</strong>s with ethylene diamine as blocking lig<strong>and</strong>.Cu(NO 3 ) 2 3H 2 O(mg/mmol)Ethylenediamine(mg/mmol)NaN 3(mg/mmol)Cu/en/N 3(startingmaterials)Cu/en/N 3(product)Solvent(s)T (°C)Requiredtime(h; days)Compounds Dimensionality Refs.I 302/1.25 15.0/0.25 162/ 5:1:10 5:2:10 MeOH/ Stirred 15 min 12 h [Cu 5 (N 3 ) 10 (en) 2 ] n 3 [44]2.50H 2 O at 60 °C<strong>II</strong> 241.6/1.0 40/0.66 130/2.0 3:2:6 3:2:6 MeOH/ RT 2 days [Cu 6 (N 3 ) 12 (en) 4 ] n 3 [44]H 2 O<strong>II</strong>I idem idem idem idem 1:1:2 MeOH/ RT 6 days [Cu 4 (N 3 ) 8 (en) 4 ] n 2H 2 O<strong>II</strong>I 483.2/2.0 130/2.0 130/2.0 1:1:1 1:1:2 MeOH/ RT 10 days [Cu 4 (N 3 ) 8 (en) 4 ] n 2 [44]H 2 OIV 241.6/1.0 60/1.0 650/ 1:1:10 2:1:4 MeOH Kept at 5 °C 15 days [Cu 8 (N 3 ) 16 (en) 4 ] n H 2 O 3 [45]10.01 241.6/1.0 30/0.5 130/2.0 2:1:4 2:1:4 MeOH/H 2 ORT 2 days [Cu 4 (N 3 ) 8 (en) 2 ] n 1 OurstudyFig. 1. The ORTEP diagram (30% ellipsoidal probability) of the azido bridged <strong>polymer</strong>ic chains of Cu4 clusters in 1 (i = 1 + x, y, z;ii=2 x, y, 1 z; iii = 1 x, y, 1 z).dron. Atoms N1, N7 <strong>and</strong> N10 <strong>and</strong> of three end-on <strong>azide</strong>s, along withN4 of an end-to-end <strong>azide</strong> ions form the basal plane of the squarepyramidal coordination environment of Cu1. N10 ii occupies theapex of this square pyramid.Cu1 <strong>and</strong> Cu2 form a pair, sharing a common edge, <strong>and</strong> thesepairs form the centrosymmetric Cu 4 cluster through end-on <strong>azide</strong>bridges, linking apical-equatorial <strong>and</strong> basal-axial positions of theoctahedral-square pyramidal pairs. This combination of coordinationof the bridges affects significantly the magnetic couplingswithin the cluster. The Cu 4 units are finally bridged by two endto-end<strong>azide</strong>s, each connecting a basal coordination position ofCu1 with an axial position of the symmetry counterpart of Cu2.The axial bond lengths of Cu1–N10_b, Cu2–N6_a <strong>and</strong> Cu2–N1_bare 2.437(4), 2.465(6) <strong>and</strong> 2.744(5) Å, respectively, which are significantlylonger than those for their corresponding equatorialCu–N bonds [Cu1–N1, Cu1–N4, Cu1–N7, Cu1–N10 ranging from1.972(5) to 2.030(4) Å <strong>and</strong> Cu2–N7, Cu2–N10, Cu2–N13, Cu2–N14 ranging from 1.987(5) to 2.018(4) Å], indicating the presenceof a strong Jahn-Teller effect [47]. The Cu–N en distances [Cu2–N13 2.002(4) Å <strong>and</strong> Cu2–N14 1.997(5) Å] are consistent with thosefor other ethylene diamine-<strong>copper</strong> complexes [44]. The Cu–N az –Cubond angles between Cu1 <strong>and</strong> Cu2 [Cu1–N7–Cu2 <strong>and</strong> Cu1–N10–Cu2] are 102.1(3) <strong>and</strong> 99.70(17)°, respectively.Azido bridged <strong>polymer</strong>ic chains of Cu 4 units run along crystallographica-axis <strong>and</strong> are aligned parallely in the ac-plane (Fig. 2). Thisparallel alignment is facilitated by hydrogen bonding between the–NH 2 donor <strong>and</strong> <strong>azide</strong> acceptors (Fig. 3 <strong>and</strong> Table 4). As shown inFig. 3, a cyclic hydrogen bonding pattern (R 2 2 (8) in Etter’s graph
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