Organometallic Oligomer Resolved by Radial Distribution Function of

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Organometallic Oligomer Resolved by Radial Distribution Function of X-ray Diffraction Analysis Roberto Matassa,†,‡ Marilena Carbone,§ Ilaria Fratoddi,† and Ruggero Caminiti*,† Dipartimento di Chimica, UniVersita` di Roma “Sapienza”, P. le A. Moro 5, I-00185 Rome, Italy; CGA-UniNetLab, UniVersita` degli Studi di Palermo, Via F. Marini 14, 90128 Palermo, Italy; and Dipartimento di Scienze e Tecnologie Chimiche, UniVersita` di Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy ReceiVed: October 19, 2009; ReVised Manuscript ReceiVed: January 4, 2010

Platinum-organic oligomers are actively studied for their large physical and functional properties such as solubility, processability, color, luminescence, and optoelectronics related to the different metal groups and auxiliary coligands around the metal coordination spheres. Previous studies on nanotechnology devices have shown that the structural organization of handled metallopolymer generates several 2D or 3D nano-objects, but only based on trans polymorph chains. Here we report the first self-assembly of powder cis-Pt-DEBP oligomers that shows great self-assembling ability to form nanoscale supramolecular architectures. As a powder is obtained that shows a poor crystalline organization of the aggregates, the energy-dispersive X-ray diffraction is the nondestructive technique of choice to obtain short-range order structural parameters of a single nanoobject by radial distribution function analysis. The supramolecular architecture of 8-units-long chains reveals a self-assembling organization of 18 chains exhibiting an overall linear inverted open square structure. The ensemble of oligomer chains form a parallelepiped shape with small internal square cavities of ∼3.2 nm diameter capable of hosting smaller molecules, which opens up to all applications where sieving and sensing is important. This structural investigation of short-range order materials has provided a substantial additional impetus to the field by opening up the area of self-assembled supramolecular materials based on metallopolymers for technological applications. Introduction Scientific research has addressed in recent years the emerging field of synthesis of nanostructured materials for technological applications in different fields, from optoelectronics to nanomedicine.1 Rigid-rod polymers and model molecules containing transition metals in the backbone such as Pd(II) and Pt(II) centers exhibit liquid crystal ordering in solution, have interesting optical and photophysical properties,2-4 and can be prepared with nanostructured features because of their rigid coordination environment that drives the self-assembly.5 In addition, they offer potential and constitute a potentially new class of molecular wires6 based on enhanced onedimensional conductivity7 and self-assembling monolayers based on Pd(II) acetylide model molecules have been recently deposited on gold surfaces and NEXAFS studies allowed to evaluate the molecular orientation on the surface.8 Fine tuning of the desired physical and functional properties such as solubility, processability, color, luminescence, and optoelectronics in general is commonly achieved by appropriate structural variations of different metal groups, auxiliary coligands around the metal coordination spheres, or bridging spacers.9,10 Monodisperse Pt(II) acetylide oligomers were recently prepared in view of applications in organic solar cells and as nonlinear absorption materials,11 and bis(acetylide)Pt(II) complexes have recently been used as emissive layers in LED * To whom corespondence should be addressed. E-mail: r.caminiti@ caspur.it. † Universita` di Roma “Sapienza”. ‡ Universita` degli Studi di Palermo. § Universita` di Tor Vergata.

devices.12 In this framework, the synthesis and characterization of the metal polyyne poly[1,1′-bis(ethynyl)-4,4′-biphenyl-(bis(tributyl)phosphine)Pt(II)] (Pt-DEBP) has been developed due, for example, to its H2S sensing properties.13,14 The characterization of Pt-DEBP synthesized so far yielded a nanomaterial which self-assembles in fibrils of randomly oriented hollow molecular tubes, with an outer diameter of about 6-7 nm.15 Electron diffraction and (cryo-artifact-free) high-resolution imaging techniques by E. A. Dray and co-workers supported a structural organization in crystallites with a diameter of about 50 nm over large areas of the specimens and the tendency of the polymer to align along the stretching direction.16 Typical synthesis of Pt-DEBP polymer is performed by a dehydrogenation reaction involving [Pt(PBu3)2Cl2] complex and 4,4′-diethynylbiphenyl (DEBP) monomer in diethylamine as solvent.17 Regardless of the cis or trans conformation of the starting Pt(II) complex, trans isomer was generally observed,18 although a tetranuclear cyclic cis oligomer has been isolated in proper reaction conditions14 and in the literature examples of cyclic complexes are reported.19,20 Here, we present the synthesis and characterization of the cis-conformer of the Pt-DEBP oligomer. The synthesis is in every way similar to that of the trans oligomer, with the exception of a few crucial parameters, i.e., the absence of CuI catalyst, the temperature, which is kept at 25° instead of 60 °C, and reaction time of 2 h instead of 4-24 h.18 This polymorph is obtained selectively starting from the cis-Pt complex and a 8-units-long chain was isolated with proper purification. 31P NMR spectrum revealed a signal due to internal units at -2.43 ppm with a coupling constant JP-Pt equal to 2249 Hz, consistent with cis configuration.21,22 The characterization by EDXD technique is performed on the powder product of

10.1021/jp9099896  2010 American Chemical Society Published on Web 01/29/2010

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Figure 1. (A) Experimental structural function (SF) of Pt-DEBP obtained with energy dispersive X-ray diffraction, and (B) the corresponding experimental Diff(r).

the synthesis, without dissolution in any solvent, and hence without allowing possible structural relaxations in liquid environments. Here, we report our experimental X-ray diffraction pattern and the corresponding analysis23,24 that leads unequivocally to the conclusion that the synthesized polymorph is in cis-conformation, at variance with all characterizations performed so far. Experimental Section Materials and Sample Preparation. The oligomer was synthesized by the dehydrohalogenation reaction17,18 involving the reaction of cis-[Pt(PBu3)2Cl2] square planar complex with the 4,4′-diethynylbiphenyl (DEBP) monomer in the presence of diethylamine as the solvent. Cis geometry around the metal center was induced in the reaction product by slightly changing the experimental procedure, in particular by using a cis Pt(II) precursor isomer, in analogy with the formation of a cyclic tetranuclear Pt(II) based compound.14 The experimental conditions and main spectroscopic characterizations are here reported. A 0.50 g (0.75 mmol) sample of cis-[Pt(PBu3)2Cl2] and 0.15 g (0.75 mmol) of DEBP were dissolved in 30 mL of diethylamine, and the reaction was run at T ) 25 °C for 2 h without the use of Cu(I) catalysts. The product was extracted from the reaction mixture with CH2Cl2/H2O, and the organic phase was dried over Na2S2O3 and then purified by chromatography on SiO2 with a eluant mixture petroleum ether/CH2Cl2 7/3. The eluted fraction was precipitated from EtOH, dried, and characterized (0.21 g, yield 33%): UV(nm, CHCl3): 340.0; FTIR (cm-1, Nujol mulls): 2112 (ν CtC), 1602 (ν CdC); 1H NMR (δ ppm, CDCl3): 0.93 (t, -CH3), 1.44 (q, CH2-CH2-CH3), 1.60 (m, -CH2-CH2CH3), 2.01 (m, P-CH2), 2.03 (m, P-CH2), 7.43 (ArH), 7.29 (ArH); 31P-NMR (δ ppm, CDCl3): -2.43, (JP-Pt ) 2249 Hz, internal units), 7.52 (JP-Pt ) 2358 Hz, terminal units). GPC (CHCl3): Mw (weight-average molecular weight) ) 6880 u.m.a; Mn (number-average molecular weight) ) 5350 u.m.a; p ) Mw/ Mn ) 1.29 molecular weight distribution; n (average degree of polymerization) ) 8. Techniques. FTIR spectra in the range 400-4000 cm-1 were recorded with a Bruker Vertex 70 Fourier transform spectrometer as film deposited from CH2Cl2 solutions on ZSM5 cells. 1 H, 31P NMR spectra were recorded on a Bruker AC 300P spectrometer at 300 and 121 MHz, respectively, in appropriate solvents (CDCl3); the chemical shifts (ppm) were referenced to TMS for 1H NMR assigning the residual 1H impurity signal in the solvent at 7.24 ppm (CDCl3). 31P NMR chemical shifts are

relative to H3PO4 (85%) probe. UV-vis spectra were recorded at room temperature using quantitative solutions in CHCl3 on a Varian-Cary 100 spectrophotometer. Molecular weights were determined by gel permeation chromatography (GPC) on a Perkin-Elmer instrument equipped with a PL-gel column and UV detector. Measurements were performed in CHCl3 (HPLC grade), using monodisperse polystyrene standards at 25 °C, flow rate 1 mL/min. EDXD Data Collection and Processing. The characterization of compound was carried out by energy-dispersive X-ray diffraction technique.25 A finely ground sample of Pt-DEBP (50-70 mg) was loaded on the sample holder. X-ray diffraction data were recorded by a custom-built X-ray energy scanning diffractometer, consisting of a Seifert X-ray HV generator supplying a water-cooled tungsten X-ray source, with maximum power of 3.0 kW. The Bremsstrahlung (braking radiation) of the X-ray source was used. The operating conditions were the following: high voltage supplies 45 kV, and current intensity 35 mA. A germanium solidstate detector (SSD) was used for the diffraction spectra recording. The SSD was linked to a multichannel analyzer by an electronic chain. A set of collimating slits in front of and behind the sample, two step motors for moving arms on which the source and detector were mounted, and an adjustable sample holder placed in the optical center of the diffractometer completed the setup. The experimental structural scattering parameter range, q ) 0.5-170 nm-1, was obtained by merging several measurements performed in correspondence to a set of scattering angles θ ) 26.0°, 21.0°, 15.5°, 10.5°, 5.0°, 3.0°, 2.0°, 1.0°, and 0.5° using the relation q ) 4π sin ϑ/λ ) EC sin ϑ, where q is expressed in nm-1, λ in nm; the utilized energy range is Emin ) 13.5 keV and Emax ) 38.2 keV and the value of the constant C is 10.14 (keV nm)-1. The experimental data were corrected for the following effects: escape peak suppression, normalization to the incident radiation intensity, division by X-ray absorption and polarization coefficients, and subtraction of the contributions, due to inelastic scattering, from the observed intensities I(E,θ).26,27 Then the spectra were joined to reconstruct the whole diffraction pattern. Atomic scattering factors, fh(q), were taken from International Tables.28 The experimental static structure function SF, the experimental radial distribution D(r) and the form Diff(r) ) D(r) - 4πr2F0 were obtained by same previously work (Figure 1A,B).23 Theoretical peak shapes

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TABLE 1: Final Values of the Adjusted Rms σ for the Model Used Distance range (nm)

σ

0.00 < r e 0.25 0.25 < r e 0.41 0.41 < r e 0.78 0.78 < r e 1.05 1.05 < r e 1.56 1.56 < r

0.071 0.142 0.212 0.267 0.301 0.357

were calculated by Fourier transforming the theoretical structure function, calculated by the Debye equation for the pair interactions of the theoretical models proposed: imn(q) N 2 fm(q)fn(q)(sin(rmnq)/rmnq) exp(-1/2σmn q2), where rmn ) ∑m*n corresponds to the distance between m and n atoms and σmn is the root mean square (rms) in the interatomic distance. The number of parameters was reduced by taking the same σ value for distances falling within predefined ranges rmn, instead of using a different σmn value for each distance. The distance ranges of the interatomic interactions and the associated rms σ were determined and the values are available in Table 1. For calculating the best agreement between experimental data and theoretical peaks, we used the formula: RHamilton ) (∑mi)1|Fe(qi)| - |Fc(qi)|2/∑mi)1|Fc(qi)|2)1/2, where the ith index runs over the m experimental points and the e and c labels of F(qi) refer to the experimental and calculated structural functions. The final fitting procedure yielded a very good agreement between the experimental data and theoretical peaks with a RHamilton estimate error of 22.69%. Results and Discussion The experimental structure function (SF) qI(q)M(q) was collected in the reciprocal space range 2 < q < 170 nm-1 and shows the typical damped oscillations of the amorphous samples (Figure 1A). The polymer SF exhibits two partially overlapping peaks at 5.50 and 7.50 nm-1, one well-defined broad peak at 13.50 nm-1 and a few broad peaks of lower intensity at 20.00, 27.50, and 38.50 nm-1 followed by large oscillations at high q values, related to the intramonomer contacts. The corresponding Fourier transform, i.e., the radial distribution function in the Diff(r) form (Figure 1B in the range 0-14 nm) provides direct information in real space on the inter- and intrachain contacts. Three regions can be singled out in the Diff(r) function: the range 0.1 to ∼0.5 nm containing intramonomeric contributions, the range ∼1.00 to ∼1.64 nm where both intra- and interchain interactions are present and the range ∼1.64 to ∼4.00 nm with broad peaks due to the interchain interactions only. Beyond 4.0 nm the intensity of the peaks becomes very small, suggesting the absence, in this region, of conformational variations of the polymer chains’ repeating units. The polymer structure yielding the experimental Diff(r) was determined by making hypothesis on the overall structure and calculating the corresponding theoretical Diff(r). Subsequent refinements of the theoretical Diff(r) allowed an excellent agreement with the experimental one of an amorphous sample. The starting setup for the structural determination took several models into account of similarly formulated Pt(II) oligomers [(PtLn)mR]u (with R the spacer group, L the auxiliary ligand, and u number of repeated units). Our first choice of parameters included the data available on the corresponding binuclear oligomer trans-[(Pt(PBu3)2Cl)2DEBP],29 which shares with our sample the metal, the spacers and the ligands, though partly

differs for the preparation procedure. Additional structural information was retrieved from the binuclear complexes trans[(Pt(PBu3)2Cl)2DEB]30 and trans-[(Pt(PEt3)2Ph)2DEB].31 A first theoretical Diff(r) was obtained employing oligomer chains of 8 repeating units in trans conformation. Furthermore, a total of 18 chains in parallel and stacked configuration were packed together, 6 in the xy plane, repeated three times in the z-direction (in our arbitrary choice of coordinates the oligomers chain lies along the x axis, and the chains are piled up along the y axis with phenyl rings parallel to each other and the butyl substitutes of different chains interacting with each other along the z direction). An optimization procedure was iterated by shifting the chains with respect to each other along the three axes. The best fit of the theoretical curve is reported as red solid line in Figure 2A, compared to the experimental data (black dots). The match with the experimental curve is good at short distances (0.5-1.5 nm) proving that the parameters introduced for the single unit are correct. However, the peak at 1.64 nm corresponding to the Pt-Pt interaction is too intense and, most of all, peaks at regular distances which are multiple values of 1.64 nm and are clearly ascribable to the Pt-Pt interactions of different monomers, are completely absent in the experimental curve. Moreover, a peak is present at 0.46 nm related to the Pt-Pt distance which is too intense in comparison to the experimental data. These mismatches between theoretical and experimental curves brought us to consider a different conformation for the oligomers units. To recover the theoretical peak intensity, models of the oligomer with the building units in cis-configurations were simulated. For this purpose, we needed, as starting parameters, to combine information gathered from the previous structural studies of the square oligomer cis-[(Pt(PMe3)2Ph)2(OTf)2]4,32 and of the binuclear polymer cis-[(Pt(PMePh2)BPYTDZ]u.33 Furthermore, we employed for the oligomer in cis-configuration the same numbers of the repeating units of monomer and of chains as for the trans-configuration. We obtain a nanoaggregate of linear cis-polymeric zigzag chain that retains a too intense peak at 1.64 nm, but selectively suppresses the theoretical peaks at distances multiple of this value (Figure 2B). This fit is still not satisfactory, due to the too high intensity at 3.7 nm (related to the distance between the first and third Pt atom of the single chain) as well as for values multiple of 2.3 nm (excluding 2.3 nm itself, which is, instead well reproduced). A better refinement of the theoretical model to match the experimental Diff(r) is possible by taking into account that the cis-Pt-DEBP chains are free to rotate around the torsion angle R between the diethynyl groups of two monomers CtCsPtsC (Figure 3A), which may give rise to different overall cisconfigurations. Such a rotation does not have any real structural effect in the case of the trans-Pt-DEBP polymer, since any similar rotation always leaves the chain as linear rigid rodlike structure. The linear cis-polymeric zigzag chain was modified by applying a torsion angle R of about 180° every second monomer, hence providing an open square chain, similar to a square supramolecular self-assembly of cis-[(Pt(PMe3)2Ph)2(OTf)2]4 and of cyclo-[Pt(dppe)(µ-CtCC′C)]4 species.32,34 This way, the theoretical model largely improves. The peak at 1.64 nm decreases to a better fitting value, since the Pt-Pt distances of the open side of the square oligomer break down

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Figure 2. Experimental radial distribution Diff(r)s (black dots) and theoretical curves (red solid lines) calculated for a single nano-object of 18 assembled Pt-DEBP oligomers. The theoretical curves correspond to the following models: (A) linear rigid rodlike chains in trans-configuration, (B) linear zigzag chains in cis-configuration, (C) linear open square chains in cis-configuration, and (D) linear inverted open square in cis-configuration.

the spatial repetition order. The distances at multiple values of 2.3 nm as well as at 3.7 nm disappear though some too large intensities at 3.3 and 5.3 nm still remain (Figure 2C). A closed square-planar model was also tested,14 leading the theoretical peak at 1.64 nm too intense compared to the experimental one (Figure 1S of the Supporting Information). The high intensity of the peak is strictly related to the structural order of the Pt-Pt distance among the monomeric units in cis-configuration. In order to reduce these theoretical peaks, we adopted an open-square planar model where the torsion angle R is set to ∼180° every fourth monomer each chain. This model yields a very good fit of the theoretical with respect to the experimental Diff(r). A finer and final tuning of the model in cis conformation with the inversion of the torsion angle every fourth unit was performed by considering the chain slipping and tilting of the single nano-object. A reiterated procedure was necessary for determining the organization of the chain and of the butyl groups because of the theoretical model were affected by an unrealistically contact between butyl groups of adjacent units after refinement. By synchronizing all structural parameters, we determined the structure of the Pt-DEBP sample. To simplify the model description, the structural parameters are referred to the centers of mass of the single chain in Figure 3A, including the main structural parameters.15,35,36 In the inset the complete single chain of all atoms is shown. The atomic Cartesian coordinates corresponding to the cis-Pt-DEBP and the

trans-[Pt(PBu3)2Cl] tail/head of the oligomer are available in the Supporting Information, Tables 2S and 3S. The numerical values of the Cartesian coordinates of the Pt-DEBP centers of mass are reported in Table 2. The adjacent oligomeric chains, formed of 8 repeated units are shifted of one unit with respect to each other along the x axis with the phenyl rings lying in the x-z plane (Figure 3B). An unshifted chains model would produce too intense theoretical peaks at about 0.520 and at about 1.000 nm with respect to the experimental one. Six chains are separated by an alternate distance of 0.486 ( 0.011 nm and 0.545 ( 0.014 nm along the y-axis, and possess a moderate tilt angle of 1.00° ( 0.50° around the y-axis.37 The oligomeric chains are staked along the z-axis with a distance of 2.846 ( 0.081 nm between the centers of mass. The strong face-to-face stacking is induced by electrostatic intermolecular interactions between the biphenyl rings and by the butylphosphine groups that lock the neighboring chains so that they are not free of shifting along the x axis. This contributes to the channels’ assembly in the solid state. The parallel chains into xz plane are separated through noncovalent interactions of about 1.34 nm more than in the xy plane. These chains’ arrangements reflect significant sterical repulsive interactions between adjacent repeat units resulting from a configuration in which butyl groups adopt a preferential orthogonal orientation. Furthermore, the biphenyl groups of the conjugated spacer group are strictly disposed parallel to the xz

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Figure 3. Structural models of the cis-Pt-DEBP sample. (A) Oligomeric chain model of 8 repeating units lying in the zx plane. Butyl groups are omitted for clarity. The main values of the structural parameters are shown, i.e., the Pt-Pt distances, the C-Pt-C angles, and the positions where a change of torsional angle R is applied along the chain. The inset represents the chemical structure of Pt-DEBP monomeric unit. In the model, the platinum is red, phosphorus yellow, carbon black, and hydrogen gray. (B) 3D view of six chains model packed in parallel configuration along the y-axis, where the green selected chains are shifted by one unit with respect to the intercalated one. (C) 3D view of 18 chains.

TABLE 2: Cartesian Coordinates (nm) of the cis-Pt-DEBP Oligomer Centers of Mass N

x

y

z

N

x

y

z

N

x

y

z

1 2 3 4 5 6

0.000 0.008 0.041 0.106 0.188 0.008

0.000 -0.478 -1.012 -1.512 -2.067 -2.554

0.000 -0.145 0.107 -0.001 0.224 0.088

7 8 9 10 11 12

-0.110 -0.102 -0.069 -0.004 0.078 0.092

0.020 -0.458 -0.992 -1.492 -2.047 -2.534

2.871 2.642 3.061 2.744 3.091 2.841

13 14 15 16 17 18

-0.020 -0.012 0.023 0.086 0.168 0.183

-0.010 -0.488 -1.022 -1.522 -2.077 -2.564

5.787 5.454 5.935 5.465 6.022 5.757

plane30 which may be expected to be quite perpendicular to the coordination plane of the metal.18 The experimental and finale theoretical Diff(r)s are reported in Figure 2D and the final structure of the Pt-DEBP sample is displayed in Figure 3C.

corresponding trans oligomer. This structural study provides more informations on the structural variations of the fundamental 2D or 3D self-assembled of nanoscale supramolecular architectures for the development device in large area (e.g., LEDs, displays, photovoltaic cells, sensing, etc.).

Conclusions

Acknowledgment. We gratefully acknowledge Lorenzo Gontrani for software assistance and CASPUR (Consorzio Interuniversitario per le Applicazioni di Supercalcolo Per Universita` e Ricerca) for supplying the computing facilities.

We synthesized and characterized by EDXD the first cis-PtDEBP powder nano material. The cis-Pt-DEBP 8-units long chains exhibit an inversion of the torsion angle every fourth unit that imprints a linear inverted open square overall structure. The application of the torsional angle of 180° every fifth, sixth, or seventh unit is counterproductive since it yields a theoretical Diff(r) pattern similar to the cis-polymeric zigzag chain model of Figure 2B. X-ray amorphous analysis reveals a superstructural selforganization of 18 chains forming a parallelepiped shape of about 6.8 × 2.6 × 8.6 nm with small internal square cavities of approximately 1.0 × 2.6 × 0.8 nm. The results presented here show that we revealed the ability of the supramolecular self-assembly of cis-Pt-DEBP nanomaterial, as powder production of the synthesis, compared to the characterizations performed on Pt-DEBP in solutions yielding systematically the

Supporting Information Available: Closed square-planar theoretical model in the Diff(r) form compared with the experimental one (Figure 1S), theoretical data of the atomic cartesian coordinates (Å) of the Pt-DEBP unit and of the trans[Pt(PBu3)2Cl] head/tail of the oligomer. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ozin, G. A.; Arsenault, A. C.; Cademartori, L. Nanochemistry, 2nd ed.; RSC Publishing: London, 2009. (2) Whittell, G. R.; Manners, I. AdV. Mater. 2007, 19, 3439–3468. (3) Wong, W. Y. J. Inorg. Organomet. Polym. Mater. 2005, 15, 197– 219.

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