Solid State Analysis of Metal-Containing Polymers Employing Mössbauer Spectroscopy, Solid State NMR and F EI TOF MALDI MS

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Polymers in general and metal-containing polymers in particular are often sparingly soluble or insoluble, in contrast to small molecules. Thus, special significance is attached to characterization techniques that can be applied to the materials as

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  Solid State Analysis of Metal-Containing Polymers EmployingMo ¨ssbauer Spectroscopy, Solid State NMR and F EI TOFMALDI MS Charles E. Carraher Jr.  • Frank D. Blum  • Manikantan B. Nair  • Girish Barot  • Amitabh Battin  • Tiziana Fiore  • Claudia Pellerito  • Michelangelo Scopelliti  • Anna Zhao  • Michael R. Roner  • Lorenzo Pellerito Received: 18 December 2009/Accepted: 9 February 2010/Published online: 3 March 2010   Springer Science+Business Media, LLC 2010 Abstract  Polymers in general and metal-containingpolymers in particular are often sparingly soluble orinsoluble, in contrast to small molecules. Thus, specialsignificance is attached to characterization techniques thatcan be applied to the materials as solids. Here, threetechniques are discussed that give structural informationgained from the solid material. Mo¨ssbauer spectroscopy isa powerful technique that may give information on thestructure about the metal-containing moiety for about 44different nuclei. Its use in describing the structure of theproduct obtained from organotin dichlorides and theunsymmetrical ciprofloxacin is presented along with thereaction implications of the results. Solid state NMR is alsoa useful tool in describing the structure of metal-containingpolymers and its use is briefly described. Finally, MALDIMS can be used to gain structural information. For manymetals it is particularly useful because of the presence of different isotopes that allow the identification of unitsthrough comparison of these isotope abundances with ionfragment clusters. Each of these tools can provide impor-tant structural characterization information. Keywords  Mo¨ssbauer spectroscopy   Solid-state NMR  MALDI MS    F MALDI MS    Organotin polyethers   Antimony-containing polymers    Metallocene polymers 1 Introduction Polymer solubility is a general problem and is especiallytrue with metal-containing polymers [1–3]. Because of this, we and others have focused part of our effort on structural C. E. Carraher Jr. ( & )    G. Barot    A. Battin    A. ZhaoDepartment of Chemistry and Biochemistry, Florida AtlanticUniversity, Boca Raton, FL 33431, USAe-mail: carraher@fau.eduC. E. Carraher Jr.    G. Barot    A. Battin    A. ZhaoFlorida Center for Environmental Studies, Palm Beach Gardens,FL 33410, USAF. D. Blum    M. B. NairDepartment of Chemistry and Materials Research Center,Missouri University of Science and Technology, Rolla,MO 65409-0010, USAT. Fiore    C. Pellerito    M. Scopelliti    L. PelleritoDipartimento di Chimica Inorganica e Analitica ‘‘StanislaoCannizzaro’’, Universita` degli Studi di Palermo, Viale delleScienze, Ed. 17, 90128 Palermo, Italye-mail: tfiore@unipa.itC. Pelleritoe-mail: cpellerito@unipa.itM. Scopellitie-mail: mscopelliti@unipa.itL. Pelleritoe-mail: bioinorg@unipa.itA. ZhaoEverglades Research and Education Center, Universityof Florida, Belle Glade, FL 33430, USAM. R. RonerDepartment of Biology, University of Texas at Arlington,Arlington, TX 76010, USAe-mail: roner@uta.edu Present Address: F. D. BlumDepartment of Chemistry, Oklahoma State University,Stillwater, OK 74078, USA  1 3 J Inorg Organomet Polym (2010) 20:570–585DOI 10.1007/s10904-010-9336-y  analysis techniques that can be carried out on solid mate-rials. Here, we will briefly describe some of these tech-niques utilizing examples from our research. Thetechniques that will be covered are Mo¨ssbauer spectros-copy, MALDI MS, and solid state NMR. 2 Mo ¨ssbauer Spectroscopy Mo¨ssbauer spectroscopy is a powerful technique that givesinformation on the electronic distribution and on structuralenvironment of about 44 different nuclei [4–12]. The use of  Mo¨ssbauer spectroscopy to assist in the structural deter-mination of polymers has been practiced since the 1960swith an early review dated in 1971 [13]. Its use as apolymer analysis tool has been recently reviewed [14].Pittman helped pioneer this area for polymers employing itin describing a number of ferrocene-containing polymers,where portions of the pendant ferrocenes were converted toferricenium units [15–17]. Mo¨ssbauer spectroscopy allows the structural analysis of certain elements situated in complex structures. Briefly,Mo¨ssbauer spectroscopy is a resonant absorption spec-troscopy that is observed best in isotopes having long-lived, low-lying excited nuclear energy states. The largestrecoil-free resonant cross-section is found for iron 57.Recently, Mo¨ssbauer spectroscopy was used on Mars toidentify iron compounds that are present in the Martianlandscape. There are over 20,000 entries in SciFinder forMo¨ssbauer spectroscopy of which the two largest entriesare for iron and tin-containing compounds. Mo¨ssbauerspectroscopy is an extremely powerful structural charac-terization tool that has been greatly overlooked, probablybecause often Mo¨ssbauer spectrometers must be dedicatedto a single element and measurements generally take hoursto weeks to complete. Even so, it has been helpful in ourwork.2.1 Experimental  119 Sn Mo¨ssbauer SpectroscopyThe  119 Sn Mo¨ssbauer spectra were measured at liquidnitrogen temperature with a multichannel analyzer [TAKESMod. 269, Ponteranica, Bergamo (Italy)] and the followingWissenschaftliche Elektronik system [MWE, Mu¨nchen(Germany)]: a MR250 driving unit, a FG2 digital functiongenerator and a MA250 velocity transducer, moved at linearvelocity, constant acceleration, in a triangular waveform.The organotin(IV) samples were maintained at liquidnitrogen temperature in a model NDR-1258-MD Cryoliquid nitrogen cryostat (Cryo Industries of America, Inc.,Atkinson, NH, USA) with a Cryo sample holder. Thetemperature was controlled at 77.3  ±  0.1 K with a modelITC 502 temperature controller from Oxford Instruments(Oxford, England). The multichannel calibration was per-formed with an enriched iron foil [ 57 Fe  =  95.2%, thickness0.06 mm, Dupont, MA (USA)], at room temperature, byusing a  57 Co/Rh [10 mCi, Ritverc GmbH, St. Petersburg(Russia)], while the zero point of the Doppler velocity scalewas determined, at room temperature, through absorptionspectra of natural CaSnO 3  ( 119 Sn  =  0.5 mg/cm 2 ) and aCa 119 SnO 3  source [10 mCi, Ritverc GmbH, St. Petersburg(Russia)]. The obtained 5  9  10 5 count spectra were refinedto obtain the isomer shift,  d  (mm s - 1 ), and the nuclearquadrupole splitting, | D exp | (mm s - 1 ).2.2 Results Obtained with  119 Sn Mo¨ssbauerSpectroscopyWe have synthesized a number of organotin polymersbased on reactions between organotin dihalides and variousantibiotics [18–32]. These polymers have shown a good ability to inhibit a variety of cancer cell lines, bacteria, andviruses. One of these antibiotics is ciprofloxacin.Ciprofloxacin is a broad spectrum antibiotic used to treatbothgramnegativeandgrampositivebacterial infections.Asecondgenerationfluoroquinolone,itismarketedworldwideunder over 300 different brand names. It kills bacteria byinterfering with the enzymes that cause DNA to rewind afterbeing copied resulting in DNA and protein synthesis beingstopped. We have synthesized a variety of products from thereaction of ciprofloxacin with organotin dihalides, Fig. 1[28, 29]. These products show a good ability to inhibit a varietyofcancercelllinesandsomeabilitytoinhibitvariousviruses and a number of bacteria [30–32]. While the general repeat unit for R 2 Sn ciprofloxacinate isgiven in Fig. 1, there are several structural variations thatinvolvetheprecisestructureabouttheorganotinmoiety.Theorganotin moiety can be connected through two oxygenatoms, two nitrogen atoms, or one nitrogen atom and oneoxygen atom. The structures with two nitrogen and twooxygenatomsarereferredtoasthesymmetricstructures,andthe structure containing one oxygen atom and one nitrogen NNNFOOOSnRORONSnRRN Fig. 1  Proposed general structure of the R 2 Sn-ciprofloxacinatepolymerJ Inorg Organomet Polym (2010) 20:570–585 571  1 3  atom connected to the organotin is referred to as the asym-metric structure. These possibilities are shown below.  O  Sn  O   N  Sn  N   N  Sn  O  Further, the carbonyl group can be attached to theoxygen by what is referred to as bridging and non-bridging.The bridging structure forms a distorted octahedralarrangement about the organotin moiety, whereas the non-bridging structure forms a distorted tetrahedral structure.Infrared and Mo¨ssbauer spectroscopy can be used to assignthese structures. Further, the bridging structures can beeither  cis  or  trans . Mo¨ssbauer spectroscopy is capable of distinguishing between these possibilities.The analysis of the experimental spectra is given inFig. 2.The structures of six coordinate R 2 SnCh 2  species wereconsidered to be simple octahedral (HCh can be a varietyof bidentate, monoprotic chelating agents). Most earlystudies interpreted results on the basis of simple  trans  or  cis structures [33–39]. It is true that some structures are  trans in solution [40, 41] and the solid state [42], and others are certainly  cis  [43, 44]. In 1977, Kepert [45] reported that many octahedral organometallic complexes, includingseveral tin complexes, are of neither regular  trans  norregular  cis  geometry, but that an intermediate geometry,skew or trapezoidal bipyramidal, is more stable (skewstructures have C–Sn–C angles of 135–155   C).As a rule, R 2 SnCh 2  complexes prefer a  trans  arrange-ment where the ligand bite (distance between the twocoordinating atoms) is large and tend to become  cis  whenthe bite is small [36]. For example, acetylacetonate (acac)-type ligands form a six-member chelate-metal ring, and trans  configurations are expected. The calculated andexperimental C–Sn–C angles are 175–178   for thebenzoylacetonates and di benzoylmethanoates  trans  struc-tures [46]. Both picolinates and tropolonates (trop) havesmaller bites than the acac family ligands and the structuralassignment is described as skew or  cis -skew configurationsfor R 2 Sn(trop) 2  (119–143   for C–Sn–C angle) [46]. In thequinolinolate group of these complexes, the oxinates, withless steric crowding about the central atom, have structuresthat are nearly  cis  (109–120  ) [37]. SnOOOORRR 1 R 1 ( 1) Sn O O O O R RR 1 R 1 (2) The analysis of the Mo¨ssbauer spectra of the threediorganotin(IV) ciprofloxacinate complexes allowed thecalculation of the Mo¨ssbauer parameters, isomer shifts,  d ,and quadrupole splittings, | D exp |, reported in Table 1. Eachcomplex showed a two doublets spectrum characteristic of the occurrence of two different tin(IV) environments in theorganotin polymers. The values of Mo¨ssbauer parametersare in the range found for other organotin(IV) derivatives.   49.0 59.0 69.0 79.0 89.0 99.0 110.1 6-4-2-0246   r  e   l  a   t   i  v  e  a   b  s  o  r  p   t   i  o  n   (   %   ) mm/s Fig. 2  Mo¨ssbauer spectrum of the product of diethyltindichloride and ciprofloxacinwhere  ?  denotes experimentalpoints. The  bold line  is the fittedspectrum of the Et 2 Sn(IV)ciprofloxacinate which givestwo doublets, the firstattributable to N–Sn–Nenvironment ( long dashes ), thesecond to OCO–Sn–OCO ( short dashes )572 J Inorg Organomet Polym (2010) 20:570–585  1 3  Because of the electron-withdrawing property, the isomershift,  d , of the diphenyltin derivative is lower than that of the dialkyltin(IV) compounds [47–50] (see Table 1). The experimental | D 1exp | values ranged from 1.61 forPh 2 Sn(IV)ciprofloxacin to 2.03 mm s - 1 for Bu 2 Sn(IV)cip-rofloxacin, while | D 2exp | ranged from 2.04 to 2.66 mm s - 1 for these organotinciprofloxacinates (Table 1).The | D 1exp | values are consistent with a R 2 SnN 2  tetra-hedral configuration (Fig. 3). The | D 2exp | values are con-sistent with a tetrahedral environment around the tin(IV)atoms, presumably distorted towards a skew trapezoidal trans -R 2 SnO 4  configuration, with C–Sn–C angles   180  (Fig. 4).Infrared spectroscopy is an assumed solid state analysistechnique that can be carried out on solids, liquids andgases and so is not covered as a separate solid state analysistechnique. Even so, it is often employed in tandem withother analysis techniques as well as being used as a stand-alone tool. Here, we briefly describe its use in conjunctionwith Mo¨ssbauer spectroscopy for analyzing the structure of the ciprofloxacin polymers.The infrared spectra for the diorganotin(IV) ciprofloxa-cinate polymers are complicated by the presence of anadditional carbonyl, the ring ketone, assigned around1,623 cm - 1 .The strong peak at 1,708 cm - 1 in the ciprofloxacinspectrum, assigned to the carbonyl group of the carboxylicacid, is missing in the spectrum of the products. A newband is found in all of the polymer spectra at 1,578 cm - 1 .This band is assigned to the asymmetric stretching forbridged carboxylic groups. A new band is also found atabout 1,420 cm - 1 for all of the polymer products assignedto the symmetric carbonyl stretching. The ketone carbonylband is also present at about 1,620 cm - 1 for all of thepolymer products.The infrared spectrum is then consistent with theorganotin moiety present in a distorted octahedral structurein the symmetric –O–Sn–O structure. The data is alsoconsistent with the structural assignments given by theMo¨ssbauer findings.2.3 Reaction ImplicationsThe most reasonable way for the symmetrical structures toform about the organotin moiety is by the preferential initialaddition of one of the Lewis bases, either the nitrogen oroxygen. The organotinpolymer production occurs by meansof the coupling of these units and their subsequent growth.We believe that the initial growth step in the polymer-ization is the formation of the –O–Sn–O– group based onthe following. The reaction with model compounds (i.e.compounds that are structurally similar antibiotics butcontain only one functional group) enrofloxacin in partic-ular, occurs rapidly and in good yield. Here only theenrofloxacin–Sn–enrofloxacin compound can form,  3 . N O O N OO Sn N N N N F F CH 3 CH 3 O O R R (3) By comparison, reaction with the ester of ciprofloxacin,where only the amine reacts with the organotin chloride,gives a poor yield of the corresponding dimer,  4 . Table 1  Experimental Mo¨ssbauer parameters for diorganotin(IV)ciprofloxacinateCompound  d 1  | D 1exp |  D tet  d 2  | D 2exp |Et 2 Sncipro 2a 0.97 1.96  - 1.82 1.08 2.29Bu 2 Sncipro 2a 0.98 2.03  - 1.82 1.20 2.66Ph 2 Sncipro 2a 0.82 1.61  - 1.61 0.92 2.04Cipro  =  ciprofloxacinate, sample thickness ranged between 0.50 and0.60 mg  119 Sn cm - 2 ; isomer shift,  d  ±  0.03, mm s - 1 , with respect toBaSnO 3 ; nuclear quadrupole splittings, | D exp |  ±  0.02, mm s - 1a The partial quadrupole splittings, mm s - 1 , used in the calculationsare: {Alk}  = - 1.37; {Ph}  = - 1.26; {N}  = - 0.564   Fig. 3  Tetrahedral configuration of tin connected to two nitrogens of two ciprofloxacin-derived moietiesJ Inorg Organomet Polym (2010) 20:570–585 573  1 3  SnNN N N N N O O O O O OR RF F C H 3 CH 3 (4) Both products were formed employing reaction condi-tions similar to those employed for the polymer synthesis.As noted, the yield for the enrofloxacin was high whereasthe product yield for the ciprofloxacin ester was low, con-sistent with the initial formation of the –O–Sn–O– product.We are currently investigating the use of Mo¨ssbauerspectroscopy to help solve other structures. For instance,we have reported the synthesis of a number of metal-con-taining materials that form what we call anomalous fibers.This phenomenon has been known for about 40 years, butthe precise structures of the fibrous and non-fibrous prod-ucts have not been fully investigated [14, 51, 52]. Pre- liminary Mo¨ssbauer evidence indicates that the non-fibrousmaterials contain only tetrahedral organotin compounds,while the fibrous products are present in extremely strainedenvironments. 3 Solid State NMR Spectroscopy We have synthesized a wide variety of organotin polye-thers for the purpose of beginning to understand thestructure/property relationship with respect to their abilityto inhibit cancer growth [53–58]. One of these products was derived from the reaction of dibutyltin dichloride and poly(ethylene glycol) (PEG10,000; Fig. 5). This product shows a reasonable ability toretard the growth of a number of cancer cell lines [56]. Of specific interest is its ability to inhibit several pancreaticcancer cell lines offering very high ChemotherapeuticIndex values [58]. This product is soluble so structuralstudies and comparisons can be made that are not usuallyavailable with metal-containing polymers. Here, we brieflydescribe  13 C NMR spectra for this product.3.1 Experimental  13 C-Nuclear Magnetic Resonance(NMR)Liquid state  13 C NMR spectra were recorded on a VarianMercury 400 (100 MHz) spectrometer. Chemical shiftswere reported in ppm (from tetramethylsilane), with thesolvent resonance employed as the internal standard(CDCl 3  at 77.0 ppm) using a 5 mm tube. Samples wereprepared in the 5–20 mg/ml range in D 2 O and CDCl 3 .Broadband proton decoupling was used.The solid state NMR spectra were obtained on a TecmagDiscovery spectrometer equipped with a DotyXC4 mmCP-MAS probe (Doty Scientific, Columbia, SC) operatingat 100.7 MHz for  13 C. Proton cross-polarization was usedfollowed by proton decoupling. The samples were spun atapproximately 9.6 kHz. A cross polarization (CP) time of 2.0 ms at a field strength of 66 kHz, and a 1 s recycle delaywas used. The number of scans used depended on thesample, but ranged from 5,000 to 21,000. The chemicalshifts were set using an external standard of poly(methylmethacrylate) in a separate experiment just before mea-suring the samples of interest to this study.3.2 Results from  13 C NMRIn the  13 C NMR (100 MHz, CDCl 3 ) spectrum of Bu 2 SnCl 2 ,signals were found at  d  26.8, 26.5, and 26.3 ppm attribut-able to the three CH 2  groups next to tin metal, and at   Fig. 4  Octahedralconfiguration about tin574 J Inorg Organomet Polym (2010) 20:570–585  1 3
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