A cyano-bridged bimetallic ferrimagnet: Synthesis, X-ray structure and magnetic study

A cyano-bridged bimetallic ferrimagnet: Synthesis, X-ray structure and magnetic study
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:  Author's personal copy A cyano-bridged bimetallic ferrimagnet: Synthesis, X-ray structureand magnetic study Rupam Sen a , Abir Bhattacharya b , Dasarath Mal a , Ashis Bhattacharjee c , Philipp Gütlich c ,Alok K. Mukherjee b , Massimo Solzi d , Chiara Pernechele d , Subratanath Koner a, * a Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India b Department of Physics, Jadavpur University, Jadavpur, Kolkata 700 032, India c Institut für Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg Universität, D-55099 Mainz, Germany d Departimento di Fisica, Universita di Parma, 7/A, I-43100 Parma, Italy a r t i c l e i n f o  Article history: Received 28 May 2010Accepted 25 June 2010Available online 1 July 2010 Keywords: Cyano-bridged complexFerrimagnetPowder X-ray structureRietveld methodAC susceptibilitySpin–glass system a b s t r a c t Mixing of   trans -[Mn(cyclam)Cl 2 ]Cl (cyclam=1,4,8,11-tetraazacyclotetradecane) and potassium hexacy-anochromate (K 3 [Cr(CN) 6 ]) aqueous solutions instantaneously yields a 1D infinite chain complex{[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ). The crystal structure of   1 , crystallizinginthemonoclinic systemwith space group  P  2 1 / n  has been solved from X-ray powder diffraction data following direct spaceapproach and refined by the Rietveld method. The structure analysis of   1  reveals alternating [Cr(CN) 6 ] 3  and [Mn(cyclam)] 3+ ions generating one-dimensional polymeric (–Cr–CN–Mn–NC–) n  chain propagatingalong the [001] direction. The coordination environment of both the metal ions, Mn(III) and Cr(III), isoctahedral. While a notable distortion in the coordination environment around Mn(III) centers wasobserved in complex  1 , Cr(III) centers have suffered no such distortion. A ferrimagnetic interactionbetweentheheterobimetallic centerswas evidencedthroughvariabletemperature magnetic susceptibil-ity measurements. The AC susceptibility measurement reveals that the compound  1  undergoes sponta-neous ferrimagnetic ordering. Ferrimagnetic ordering has been rarely observed among the cyano-bridged compounds in the previous studies.   2010 Elsevier Ltd. All rights reserved. 1. Introduction Inthelast fewyears, therehasbeenconsiderable interestinthepreparation and properties of molecule-based magnetic materials.Over the years new fascinating areas of research have opened upwhere combined efforts of physicists and chemists facilitate to ex-pand the basic understanding of these molecule-based materials[1–6]. Rational designing of these materials can even lead to theselective change in their magnetic properties. Preparation of mol-ecule-based magnet requires consideration of spin carriers andbridges. In the case of metal assembled systems spin carriers areusually transition metal ions while bridging ligands connect themetalcenters. Cyanidebeinganefficientbridgingligand, extensivestudies have been undertaken on cyano-bridged transition metalcomplexes in respect of their structural and magnetic properties[7–9]. Particularly, the cyano-bridged 3D bimetallic assemblies of Prussian Blue type [10–13], derived from [M(CN) 6 ] 3  (M=Cr(III)and Fe(III)] and transition metal ions [M 0 ] n + , had attracted greatattractionowingtothefactthatmanyofthemexhibitconsiderablyhigh critical (Curie) temperatures [10]. Difficulties in preparingsingle-crystals suitable for X-ray structure analysis for this classofmaterialsprecludemagneto-structuralcorrelation.Furthermore,the face-centered cubic systems (based on powder XRD results)usually possess low or no magnetic anisotropy, which would givelow or no magnetic coercivity [14–16]. To prevail over these prob-lems, many attempts have been made to synthesize hybrid Prus-sian Blue analogues by the reaction of coordinately unsaturatedtransition metal complexes [ML] m + (L=any polydentate ligand)with hexacyanometalate building blocks [M´ (CN) 6 ] 3  (M´ =Fe, Cror Mn) [17–27]. Accordingly, the coordination of organic ligandscould lower the symmetry of the lattice, and afford desirablemolecular structures. In this context, the preparation of metalassembled systems with infinite chain magnetic behavior haveattractedmuchattention[2–5].Alotofcyano-bridgedhomometal-lic or heterometallic ferromagnets have been prepared, ferrimag-netic chain systems have sparsely been reported so far [28–30].However, owing to the problem of obtaining X-ray quality single-crystals of many Prussian Blue analogues, the satisfactorymagneto-structural evolution to quench the usual interest of fun-damentals as well as applications of these compounds remainedelusive [31–34].In earlier attempts, we have succeeded in isolating Prussianblue analogues having interesting magnetic properties [12,13,25,27]. Furtherexplorationofcyano-bridgedtransitionmetal com-pounds containing cyclam ligand allowed us to structurally and 0277-5387/$ - see front matter    2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.poly.2010.06.022 *  Corresponding author. Tel.: +91 33 2414 6666x2778; fax: +91 33 2414 6414. E-mail address: (S. Koner).Polyhedron 29 (2010) 2762–2768 Contents lists available at ScienceDirect Polyhedron journal homepage:  Author's personal copy magneticallycharacterizeararevarietyofferrimagneticsystem.Inthe present study we have been able to get a satisfactory solutionof the molecular structure of the one-dimensional chaincompound, {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ) (cyclam=1,4,8,11-tetraazacyclotetradecane) using X-ray powder diffractiondata. 2. Experimental  2.1. Materials 1,4,8,11-Tetraazacyclotetradecane and potassium hexacyano-chromate were purchased formAldrichand were used as received. trans -[MnCl 2 (cyclam)]Cl was prepared following the literaturemethod[35].SolventswerepurchasedfromMerck(India)andtheywere distilled and dried before use.  2.2. Physical measurements Fourier transform infrared spectra were measured on a Shima-dzu S-8400 FTIR spectrometer in KBr pellet. The metal content of the sample was estimated on a Varian Techtron AA-ABQ atomicabsorption spectrometer. Thermogravimetric analysis (TGA) wascarried out using a Perkin–Elmer PYRES DIAMOND TG–DTA appa-ratus. Magnetic measurements (dc susceptibility) were carried outwith Quantum Design’s MPMS XL SQUID magnetometer. Diamag-netic corrections were estimated from the Pascal constants. TheAC susceptibility was measured on an MPMSXL-5 SQUID systemapplying a dc magnetic field of 4Oe oscillating at 1, 20, 300 and1000Hz.  2.3. Synthesis of {[Mn(cyclam)( l -CN)  2 Cr(CN) 4 ]  H   2 O} n  ( 1 ) Thephasepurecompoundwassynthesizedbythefollowingpro-cedure. Toasolutionof   trans -[MnCl 2 (cyclam)]Cl(0.18g, 0.5mmol)in methanol (30mL) was added a solution of K 3 [Cr(CN) 6 ] (0.30g,0.5mmol)inmilliQH 2 O(20mL)atroomtemperature.Theresultingsolution was stirred for 30min. The light violet precipitate thusformed was then collected by filtration, washed with methanol,anddriedinvacuo(yieldwascalculated85%basedonMn).  Anal. Calc.for {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n : Cr, 10.89; Mn, 11.51; C,40.36; H, 4.6; N, 29.40. Found: Cr, 10.87; Mn, 11.47; C, 40.26; H,4.65; N, 29.34%. Selected IR peaks (KBr disk, cm  1 ):  t (C „  N),2108cm  1 and3500–[ t (O–H)].Thermogravimetricanaly-sisshowedalossof3.7%inthetemperaturerange90–135  CwhichcorrespondstooneH 2 Omoleculeperformulaunit.  2.4. X-ray powder data collection The sample was ground to fine powder using agate pestle andmortar, andmountedinatop-loadedsampleholder. X-raypowderdiffractiondatawerecollectedinaBruker D8Advancepowder dif-fractometer using Cu K a  radiation ( k  =1.5418Å). The diffractionpattern at roomtemperature (22  C) was recorded over an angularrangeof 4–83   (2 h ) instep-scanmodewithastepsizeof 0.01   (2 h )and counting time of 40s per step using the Bragg-Brentanogeometry.  2.5. Indexing and space group determination The first 20 peaks of the powder diffraction pattern were fittedusing the program TOPAS  3.0 [36], and the refined 2 h  positions wereinput into the auto-indexing program  TREOR   [37]. The solution withthe highest figure of merit [( M  (20)=28,  F  (20)=43 (0.010,48)][38,39] indexed all peaks in the monoclinic system with  a  =19.657(13),  b  =12.066(8),  c   =10.275(7)Å,  b  =102.36(1)   and  V   =2381(5)Å 3 . The whole powder pattern decomposition was per-formed with EXPO2004 following the Le Bail algorithm [40] usingasplittypepseudo-Voigtpeakprofilefunction[41]. Analysisof thepowder pattern by the FINDSPACE module of EXPO2004 revealedpossible extinction symbol as  P  2 1 / n , which was used for subse-quent structure analysis.  2.6. Structure solution and refinement  Structure solution was carried out in direct space using theprogram  FOX  [42], which attempts to minimize the difference be-tween the observed and calculated powder profiles by a simu-lated annealing approach (in parallel tempering mode) thatmoves the constituent fragments defining the structure withinthe unit cell, varying their positions, orientations and whenappropriate, their conformation. Initial structural model inputfor global optimization in direct space was taken from Appeltand Vahrenkamp [43] and optimized using the energy minimiza-tion procedure incorporated in MOPAC 5.0 [44]. Lattice and pro-file parameters, zero-point error and interpolated backgroundcalculated from the previous powder pattern decompositionbased on Le Bail algorithm, were introduced into the programFOX. Bond lengths and bond angles were constrained within0.10Å and 5.0  , respectively, but the torsion angles were allowedto change.The atomic coordinates obtained from the simulated-annealingprocedure of FOX [ R wp  =0.0906] were taken as the starting modelfor Rietveldrefinement withthe program GSAS  [45]. The refinementwas carried out using the 2 h  angular ranges 4–83   with soft con-straints on bond lengths, bond angles and planar fragments inthemolecule.ThebackgroundwasdescribedbytheShiftedCheby-shevfunctionoffirstkindwith36pointsregularlydistributedoverthe entire 2 h  range. The peak profiles were fitted with the pseudo-Voigt functions using the Thompson–Cox–Hasting formalism [46].These functions take into account the experimental resolution andpeak broadening due to the size and strain effects. The latticeparameters, the background coefficients, the zero-point correctionand profile parameters were refined initially followed by therefinement of positional coordinates of all non-hydrogen atoms.Hydrogen atoms were placed in the calculated positions withC–H distances restrained. Final Rietveld refinement of 171 param-eters (87 coordinates, 4 lattice parameters, 36 background coeffi-cients, 1 scale factor, 8 profile parameters and 35 orientationdistribution function coefficients) converged to  R p  =0.0272,  R wp =0.0376,  R F2 ¼ 0 : 1239 and  v 2 =6.025. For the soft constraints,the mean-square deviations to the assigned values were 0.01Åfor bond lengths, 1.0   for bond angles and 0.01Å for planes. The fi-nal agreement between the observed and the calculated patternswas excellent (Fig. 1). Recently we have successfully solved thecrystal structure of [Cu(dca) 2 (dien)] complex (dien=diethelenetri-amine) independently using single-crystal and X-ray powder dif-fraction data following the same protocol [47]. Relevantcrystallographic data for compound  1  are summarized in Table 1. 3. Results and discussion  3.1. Structure of {[Mn(cyclam)( l -CN)  2 Cr(CN) 4 ]  H   2 O} n  ( 1 ) An ORTEP view of {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ) withatom numbering scheme is shown in Fig. 2, while the relevantbond distances and angles are listed in Table 2. The crystal struc-ture of   1  consists of polymeric chains of alternating [Cr(CN) 6 ] 3  and [Mn(cyclam)] 3+ ions running along the [001] direction. Each[Cr(CN) 6 ] 3  anion provides two apical  trans  cyanide groups to R. Sen et al./Polyhedron 29 (2010) 2762–2768  2763  Author's personal copy bridge the two [Mn(cyclam)] 3+ units generating a (–Cr–CN–Mn–NC–) n  chain, which is significantly bent from linearity; the Mn–N5–C11 bond angle is 154.11(4)  . Within each chain, neighboringMn  Cr and Mn/Cr  Mn/Cr distances are 5.22Å and 10.28Å,respectively. The Cr–C–N bond angles in the complex are approxi-mately linear [172.53(4)–176.23(4)  ]. The metal center in the[Cr(CN) 6 ] 3  unit adopts an octahedral environment; the Cr–C bonddistances do not differ significantly for bridging [2.050(1)–2.051(1)Å] and terminal [2.050(1)–2.051(1)Å] CN  groups. Thesedistances are similar to that observed in other complexes contain-ing a [Cr(CN) 6 ] 3  fragment [48,49]. The geometry around the Mnatom is also essentially octahedral with a MnN 6  chromophore.The axial positions are occupied by the nitrogen atoms of thebridging CN  groups with Mn–N distances of 1.998(1),2.058(6)Å, and the equatorial positions by the N 4  set of donoratoms of the cyclam ligand with Mn–N distances ranging between1.988(1) and 2.017(1)Å.Althoughthe H-atoms could not be located reliably fromthe X-ray powder structure analysis, the crystal packing of   1  exhibits afew weak C–H  N contacts (Table 3) linking the adjacent poly-meric chains (Fig. 3). The inter-chain metal–metal (next nearestneighbor) distances are Cr  Mn, 8.053Å; Cr  Cr, 7.659Å andMn  Mn, 7.655Å, respectively. Additional reinforcement betweensuccessive chains connected via C3–H3B  N9 hydrogen bonds isestablishedbypairsof C–H  O(water) andN–H  O(water) hydro-gen bonds forming  R 42 (12) rings [50].  3.2. Magnetic study The dc molar magnetic susceptibility ( v M ) of complex  1  hasbeen measured under 10kOe magnetic field as a function of temperature in the 4-300K range and is shown in Fig. 4a. Fig. 4b Fig. 1.  High-resolution powder diffraction data and Rietveld fit for the refinedstructure of {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ). The lower trace (blue colour)is the difference, measured (red line), calculated (black cross) and background (skyblue), onthesamescale. (Forinterpretationofthereferencestocolour inthisfigurelegend, the reader is referred to the web version of this article.)  Table 1 Crystal data and Rietveld refinement parameters for {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ). Empirical formula [MnC 10 N 4 H 20 ] [Cr(CN) 6 ]  H 2 OFormula weight 481.219 T   (K) 295(K)Crystal system monoclinicSpace group  P  2 1 / nUnit cell dimensions a (Å) 19.657(13) b  (Å) 12.066(8) c   (Å) 10.275(7) b  (  ) 102.36(1) V   (Å 3 ) 2381(5)  Z   4Density (calculated) (cm 3 ) 2.3032 h  range for data collection (  ) 4–83Step size 0.01  (2 h )Wavelength (Å) 1.5418No. of profile data steps 7870No. of variable parameters 171No. of background points refined 36 R p  0.0272 R wp  0.0376 R F2 0.1239 v 2 6.025 R F = P |( I  K (obs)) 1/2  ( I  K (calc)) 1/2 )|/ P ( I  K (obs) 1/2 , R P  = P |(  y i (obs)  (  y i (calc)|/ P (  y i (obs)and R wP ={ P w i |(  y i (obs)  (  y i (calc) 2 |/ P w i (  y i (obs) 2 } 1/2 . Fig. 2.  ORTEP drawing of {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ) showing theatom numbering scheme.  Table 2 Selected bond distances (Å) and bond angles (  ) for {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ). Bond distances (Å) Bond angles (  )MN1–N1 2.003(1) Mn1–N5–C11 154.11(4)MN1–N2 2.017(1) Cr1–C11 – N5 176.23(4)Mn1–N3 1.988(1) Cr1–C13 – N7 176.16(4)Mn1–N4 2.014(1) Cr1–C15–N9 172.53(4)Mn1–N5 1.998(1) Mn1–N6–C12 171.00(1)Mn1–N6 2.058(6) Cr1–C12–N6 173.66(4)Cr1–C11 2.050(1) Cr1–C14–N8 175.29(3)Cr1–C12 2.051(1) Cr1–C16–N10 174.51(4)Cr1–C13 2.051(1) N1–Mn1–N3 172.15(3)Cr1–C14 2.050(1) N2–Mn1–N4 159.95(2)Cr1–C15 2.050(1) N1–Mn1–N4 85.19(2)Cr1–C16 2.050(1) N2–Mn1–N3 89.76(2)2764  R. Sen et al./Polyhedron 29 (2010) 2762–2768  Author's personal copy represents the  v M  1 versus  T   plot for compound  1  which indicatesthat in the high temperature region  v M  for  1  follows the Curie–Weiss law [ v M  = C  /( T   h )]. The estimated Weiss constant  h  andCurie constant  C   are   2.5K and 4.57cm 3 mol  1 Oe  1 K, respec-tively. The estimated value of the  C   is comparable to the theoreti-cally expected value (4.87cm 3 mol  1 Oe  1 K) for one [Mn(III)–Cr(III)] unit. The variation of the corresponding  v M T   values per[Mn(III)–Cr(III)] unit as a function of temperature is shown inFig. 5. The  v M T   value per [Mn(III)–Cr(III)] unit of 3.76cm 3 mol  1 Kestimated at 300K for complex  1  is slightly higher than the valueexpected for magnetically uncoupled Mn(III) and Cr(III) ions( v M T   =3.54cm 3 mol  1 Kfor  g  Cr  =  g  M n  =2.0). Uponcooling, v M T   val-ues smoothly decrease, exhibiting a minimum at ca. 20K ( v M T  being 1.30cm 3 mol  1 K), and increase sharply to reach a value of 9.72cm 3 mol  1 K at 6.05K. This temperature dependence of   v M T  of   1  is typical for ferrimagnetic interaction arising out of the non-compensationof twoantiferromagneticallycoupledunequal spins.Upon further cooling the  v M T   values drop to ca. 8.86cm 3 mol  1 Kshowing a cusp at around 6K (Fig. 5; inset) indicating that thecompound  1  is a three-dimensional magnetic system [51–54].For cyano-bridged chain system containing Mn(III) ( t  2  g  3 e  g  1 ) andCr(III) ( t  2  g  3 e  g  0 ) both ferro- and antiferromagnetic couplings arepossible. However, the antiferromagnetic interaction is generallystrongerthantheferromagneticinteraction,andtendstodominatethe super-exchange in this type of competitive situation [55]. Infact, the negative value of   h  for the compound  1  is indicative of an antiferromagnetic interaction between Mn(III) and Cr(III) cen-  Table 3 Hydrogen bonding geometry (Å,   ) in {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ). D–H  A H  A D  A D–H  AC3  H3B  N9 a 2.30 2.998(1) 129.2C2  H2B  O1 b 2.13 3.001(1) 150.4C3  H3A  O1 c 2.40 3.153(2) 135.2N2  H2N  O1 c 2.10 3.030(2) 166.0C1  H1A  N9 d 2.41 3.298(2) 153.6C7  H7A  N1 e 2.24 3.287(2) 144.0 a   x ,   y  +2,    z  . b  x , +  y  +1, +  z  . c   x ,   y  +1,    z  . d  x , +  y , +  z   +1. e   x  +1/2, +  y  1/2,    z   +1/2. Fig. 3.  Drawing showing the crystal packing with the interlinked hydrogen bonds through  R 42 (12) rings forming a two-dimensional layer. Fig. 4.  (a) Temperature dependence of dc molar magnetic susceptibility ( v M ) of {[Mn(cyclam)( l -CN) 2 Cr(CN) 4 ]  H 2 O} n  ( 1 ); (b) drawing showing the  v M  1 versus  T  plot for compound  1 . R. Sen et al./Polyhedron 29 (2010) 2762–2768  2765
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