Journal of Molecular Structure 1039 (2013) 107–112

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Journal of Molecular Structure

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High-valent isocyanide complexes. Coexistence of strong p-donor and formalp-acceptor ligands in chromium(V) nitride complexes

Thorbjørn J. Morsing ⇑, Jesper Bendix ⇑Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark

h i g h l i g h t s

" First examples of mixed nitride–isocyanide complexes." Lack of p-backbonding in high-valent isocyanide systems." Pseudo-linear electronic structure of Cr(V)–nitrides.

a r t i c l e i n f o

Article history:Received 9 January 2013Received in revised form 2 February 2013Accepted 2 February 2013Available online 13 February 2013

Keywords:High-valentIsocyanideNitridoEPRLigand-field

0022-2860/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.molstruc.2013.02.002

⇑ Corresponding authors. Tel.: +45 35320101.E-mail addresses: morsing@kiku.dk (T.J. Morsing),

a b s t r a c t

The first examples of a metal complexes combining isocyanide ligands with the nitride ligand,namely trichloro(methylisocyanide)nitridochromate(V) (1) and dichlorobis(methylisocyanide)nitrido–chromium(V) (2) are reported. These complexes are prepared by ligand substitution on [Cr(N)Cl4]2�

and the former has been structurally characterized as its tetraethylammonium salt. In 1, the CrBNdistance at 1.5510(9)/1.5497(10) Å is within the normal range for Cr(V)BN bonds. The chromium–isocyanide bond length at 2.063(1) Å is shorter than CrACN bond lengths in cyanide complexes of nitridochromium(V), but significantly longer than CrAC bond lengths in homoleptic isocyanides of Cr(II) andCr(0). The r-donation of methylisocyanide coordinated to the nitride–Cr(V) moiety is comparable to thatof amine ligands with DCH3NC � 20,800 cm�1. UV–vis spectroscopy further allows for the quantification ofthe excited state CrBN stretching frequency to be ca. 820 cm�1 in the 2E(dxz, dyz, C4v) excited state of both1 and 2. Reversible binding of the second methylisocyanide ligand is observed spectroscopically andfound to be quite weak with K2 = 6 � 10�3 for the equilibrium [Cr(N)Cl3(CNCH3)]� + CNCH3 = [Cr(N)Cl3

(CNCH3)2] + Cl�. Lability of the isocyanide ligands in 1 and 2 is demonstrated by EPR spectroscopyemploying competition with trimethylphosphite.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Monodentate as well as polydentate isocyanide ligands areubiquitous in the organometallic coordination chemistry of thetransition metals [1]. In parallel with the coordination chemistryof carbonmonoxide, isocyanide ligation is largely confined tolow-valent complexes obeying the 18-electron rule. Conversely,very few examples of high-valent complexes with co-existing oxi-do and isocyanide ligands have been reported. These are confinedto oxido complexes with Mo(IV) [2], Re(V) [3], and notably Fe(IV)[4] as central ions. Besides oxido ligands, nitrido ligands are wellknown for stabilization of high oxidation states, but here theapparent incommensurable requirements by the nitride and isocy-anide is even more pronounced as no such mixed complexes have

ll rights reserved.

bendix@kiku.dk (J. Bendix).

been reported to date. However, since mixed cyano-nitrido com-plexes have been reported and studied for several different metalcenters: Cr [5], Mn [5,6], Tc [7], Re [8], Os [9], it appeared thatmixed isocyanide–nitrido complexes should also be accessible.Two approaches dominate in the synthesis of isocyanide com-plexes: for relatively robust metal centers, N-alkylation of cyanocomplexes is an efficient route circumventing isolation and storageof the malodorous and unstable free isocyanide ligands [10]. At-tempts at preparing and isolating methylisocyanide complexes ofMn(V) and Cr(V) by this route however, proved unsuccessful de-spite detectable production of isocyanide. The other importantroute to isocyanide complexes is by ligand metathesis using (ex-cess) free isocyanide ligand. The apparent quite low affinity ofMn(V) and Cr(V) for isocyanide ligands, demonstrated by the freeisocyanide detected in attempts at alkylations of the respective cy-ano complexes suggested that the preferred nitrido precursorshould feature as labile ligands as possible. We have previously

Table 1Crystallographic data for (Et4N)[Cr(N)Cl3(CNCH3)].

Compound (Et4N)[Cr(N)Cl3(CNCH3)]

108 T.J. Morsing, J. Bendix / Journal of Molecular Structure 1039 (2013) 107–112

established [Cr(N)Cl4]2� as a versatile precursor for the preparationof nitrido chromium(V) complexes [11] and we report here on thereactivity of this complex towards excess of methyl isocyanide.

Formula C10H23Cl3CrN3

M (g mole�1) 343.71Crystal size (mm3) 0.30 � 0.25 � 0.20Crystal system MonoclinicSpace group P21/ca (Å) 11.7610(7)b (Å) 25.1480(15)c (Å) 11.055(4)a (�) 90b (�) 98.249(8)c (�) 90U (Å3) 3235.9(11)

2. Experimental section

2.1. Materials

Reagent grade chemicals were obtained from commercialsources and used as received. The used solvents were HPLC grade,but not dried further before use. (Et4N)2[Cr(N)Cl4] and methyliso-cyanide was synthesized according to literature methods [12,13].

Z 8Dx (g cm�3) 1.411T (K) 123(2)l 1.197No. measured/observed reflections 82393/13576Parameters refined 307Final R1 0.0279wR2 [I > 2r(I)] 0.0539R1, wR2 (all data) 0.0368, 0.0572S (GoF) 1.158

2.2. Physical measurements

Elemental analysis for C, H and N was performed with a CEInstrument: FLASH 1112 series EA, at the microanalytic laboratory,University of Copenhagen. Mid-range FTIR spectra were recordedon powdered samples on a Bruker ALPHA-P spectrometer in ATRmode. The EPR spectra were recorded on a Bruker Elexsys E500equipped with a Bruker ER 4116 DM dual mode cavity, an EIP538B frequency counter and a ER035M NMR Gauss-meter. Theelectronic absorption spectra were recorded on a Perkin–ElmerLamda 2 UV–VIS spectrometer.

Fig. 1. ORTEP plot of the asymmetric unit in the crystal structure of 1 showing 50%probability displacement ellipsoids. The H atoms on the tetraethylammoniummoieties have been removed for clarity.

2.3. Preparation of (Et4N)[Cr(N)Cl3(CNCH3)] (1)

(Et4N)2[Cr(N)Cl4] (50 mg, 0.11 mmol) was suspended in 0.5 mldichloromethane and methylisocyanide (0.07–0.09 ml, 1.4–1.8 mmol) was added by syringe upon which the yellow suspen-sion turned into a lush green solution. Diethyl ether was thenadded dropwise (about 0.15 ml) and the solution stirred betweenadditions, until a lasting precipitation was almost achieved (slightprecipitation which redissolved). Subsequently, vapor diffusion ofdiethyl ether into the solution over a period of several hours affor-ded large green crystals, leaving the mother liquor almost color-less. The mother liquor was removed by syringe and the crystalswere washed with a 1:3 mixture of diethyl ether and dichloro-methane and dried under a nitrogen flow. Isolated yieldswere >90% (>36 mg). The product is stable in a dry atmosphere,but quite hygroscopic.

Anal. Calcd for C10H23N3Cl3Cr: C, 34.93; H, 6.75; N, 12.23.Found: C, 34.58; H, 6.88; N, 12.10. IR (powder, m cm�1) m(CrBN):1014, m(CAN): 2260. UV–VIS (kmax/nm e/M�1 cm�1)) 669 (29.78),425 (50.15), 293 (1918), 247 (�1830). EPR: g = 1.9846.

2.4. Crystal structure determination

A crystal of 1 suitable for X-ray crystallography was mountedand intensity data was collected at 123(2) K on an Enraf-NoniusKappaCCD area detector using x and h scans with a scan widthof 1.0� and 60 s exposure times using the program COLLECT [14].The crystal-to-detector distance was 35.0 mm. The program eval-ccd was used for data reduction and the data was corrected forabsorption by integration [15]. The structure was solved with di-rect methods utilizing SHELXS and refined by least-squares methodsusing SHELXL97 [16]. All non-hydrogen atoms were refined usinganisotropic displacement parameter. All hydrogen atoms were in-cluded and refined with isotropic factor of 1.2Ueq of the parentatom. Crystallographic data is given in Table 1. Data are depositedas CCDC 911090.

3. Results and discussion

3.1. Crystal structure

Compound 1 crystallizes in the monoclinic space group P21/cand the asymmetric unit contains two formula moieties. An ORTEPplot of the asymmetric unit is given in Fig. 1 and selected bondlengths and bond angles are shown in Table 2.

Corresponding bond distances in the two complexes of theasymmetric unit are very similar, as seen from Table 2, where alsoa slightly larger variation in corresponding bond angles can beobserved. The chromium nitrido distances of 1.5510(9) and1.5497(10) Å in the two independent complex anions are similarto those determined for other Cr(V)-nitrido complexes [17]. Ascommonly found for five-coordinated complexes with a strongp-donor ligand, the geometry around the metal center is squarepyramidal with the metal center raised above the plane of theequatorial ligator atoms. In 1, the chromium centers are located

Table 2Selected bond lengths and angles for 1.

Bond length Length (Å) Bond angle Angle (�)

Cr1AN15 1.5520(9) N15ACr1AC20 94.93(5)Cr2AN17 1.5497(10) N17ACr2AC27 97.21(5)Cr1ACl7 2.2909(3) N15ACr1ACl7 108.67(4)Cr1ACl3 2.3280(3) N15ACr1ACl3 102.42(4)Cr1ACl38 2.2894(4) N15ACr1ACl38 108.27(4)Cr2ACl8 2.3144(5) N17ACr2ACl8 106.10(4)Cr2ACl4 2.3086(3) N17ACr2ACl4 105.49(4)Cr2ACl6 2.2973(3) N17ACr2ACl6 104.35(4)Cr1AC20 2.0634(10) Cl3ACr1AC20 162.63(3)Cr2AC27 2.0646(10) Cl4ACr2AC27 157.22(3)C20AN23 1.1475(13) Cr1AC20AN23 176.56(8)C27AN14 1.1476(13) Cr2AC27AN14 177.28(9)N23AC22 1.4324(13) C20AN23AC22 177.50(10)N14AC31 1.4335(13) C27AN14AC31 178.41(10)

Fig. 2. Solution, X-band EPR of 1 in CHCl3. Cf Table 3 for simulation parameters.

T.J. Morsing, J. Bendix / Journal of Molecular Structure 1039 (2013) 107–112 109

0.5235(3) Å out of the plane spanned by the chloride and isocya-nide ligands, the pyramidalization is, thus, slightly larger than inthe parent complex, [Cr(N)Cl4]2�, for which an out-of-plane dis-tance of 0.4466(4) Å was found [12]. The average C„N bond lengthfor the two independent anions in 1 is 1.148(1) Å. For chromium, aseries of homoleptic hexakis phenylisocyanide complexes havebeen structurally characterized for chromium in the oxidationstates 0, . . . , +3 [18]. Along this series, the average C„N bond lengthvaries as expected for a p-backbonding ligand: Cr(0):1.176(4) Å;Cr(I):1.159(2) Å; Cr(II):1.158(7) Å; Cr(III):1.14(1) Å. By comparisonwith this series, backbonding to methylisocyanide in 1 is judged tobe at most slight, despite the possible synergy between the extremep-donor, nitride, and the isocyanide, both overlapping efficiently witha common chromium d-orbital of p-symmetry. The behavior of the{CrV(N)}2+ as very comparable to CrIII in terms of charge density isin line with previous spectroscopic observations [11]a.

3.2. IR-spectroscopy

The powder IR-spectrum of 1 exhibits a sharp peak at1014 cm�1 characteristic of the chromium nitrido stretching mode[5]a,[19], and a peak at 2260 cm�1 from the CAN stretch in methy-lisocyanide, which is significantly higher than the value for freemethylisocyanide of 2165/2166 cm�1 [20]. The spectrum is shownin the Supplementary material. Again, comparison with lower va-lent chromium phenylisocyanide complexes with mCN of2150 cm�1 for Cr(II), 2060 cm�1 for Cr(I), and 1975 cm�1 for Cr(0)[21], suggest that methyl isocyanide does not exhibit any notableback donation properties in the mixed nitrido-isocyanidechromate(V).

3.3. EPR spectra

The EPR spectrum of 1 is shown in Fig. 2 together with its sim-ulation. The spectrum resembles that of the starting material,

Table 3EPR parameters for nitrido-isocyanide and nitrido-phosphite complexes.

Complex Solvent gisoa A(53Cr) (10�4 cm�1)

[Cr(N)Cl4]2�b CH3CN 1.978 28.0[Cr(N)(CNCH3)Cl3]� CHCl3 1.9846 26.5[Cr(N)(CNCH3)2Cl2] CHCl3 1.9880 25.3[Cr(N)(P(OCH3)3)Cl3]� CHCl3 1.9859 25.3[Cr(N)(P(OCH3)3)Cl3]�c CH2Cl2 1.9867 25.3[Cr(N)(P(OCH3)3)2Cl2] CHCl3 1.9903 24.9

a All spectra were recorded at the X-band (9.635 GHz) at 296 K using a microwave pob From Ref. [12].c Prepared directly from [Cr(N)Cl4]2�.

[Cr(N)Cl4]2�, exhibiting a featureless main signal without discern-ible super hyperfine coupling to nitrogen, but with four distinctsatellites arising from hyperfine coupling to chromium (9.5%, 53Crwith I = 3/2). The value of giso for 1 is somewhat larger than thatfor [Cr(N)Cl4]2� (cf. Table 3), in agreement with ligand-field-theorybased perturbation expressions and the fact that isocyanide pro-vides a stronger ligand field than chloride when coordinated tothe CrV(N) moiety (cf. Section 3.4).

When a suspension of (Et4N)2[Cr(N)Cl4] in chloroform is treatedwith a moderate excess of methylisocyanide, the solution turns abrownish green and gives an EPR spectrum displayed in Fig. 3 tracea. The spectrum is that of a mixture of species: still dominated by 1,but with an added distinct signal with a higher g-value. Furtheraddition of isocyanide effects complete conversion of 1 into thespecies with giso = 1.9880 (cf. Fig. 3 traces b and c).

The generated species is the neutral bis(methylisocyanide)complex [Cr(N)(CNCH3)2Cl2] (2). This assignment is corroboratedby UV–vis spectroscopy (cf. Section 3.4) and by competition withphosphite ligands, which have ligand properties similar to thoseof methylisocyanide. To verify the identity of 2, the followingexperiment was conducted: a drop trimethylphosphite was addedto a suspension of (Et4N)2[Cr(N)Cl4] in chloroform, and the solutionfiltered. The resulting yellow–green solution contains exclusivelythe neutral species [Cr(N)(P(OCH3)3)2Cl2], as demonstrated by thecharacteristic EPR spectrum (se Fig. 4) showing a coupling to twoequivalent phosphorous nuclei (I = ½) where the resultant tripletis further split by a weak coupling to the nitride (I = 1) producinga pattern of a triplet (1:2:1) of triplets (1:1:1).

Upon addition of methylisocyanide to this solution, the cou-pling to phosphorous disappears, and one obtains a spectrumwhich is identical to that assigned to 2 in Fig. 3. The reverse substi-tution, replacing isocyanide with phosphite can also be achieved: If

A(14N) (10�4 cm�1) A(31P) (10�4 cm�1) Lorentzian band width (G)

(2.5) – 3.2(2.4) – 2.71(2.4) – 2.682.5 46 2.732.5 46 2.732.38 41 1.80

wer in the range 0.6–10 mW.

Fig. 3. Conversion of 1 to 2.

Fig. 4. EPR spectrum of [Cr(N)(P(OCH3)3)2Cl2] generated directly from [Cr(N)Cl4]2�

in CHCl3. Cf. Table 3 for simulation parameters.

Fig. 5. EPR spectrum of [Cr(N)(P(OCH3)3)Cl3]� generated directly from [Cr(N)Cl4]2�.Cf. Table 3 for simulation parameters.

Fig. 6. The electronic absorption spectra of 1 (in CH2Cl2), 2 (in CH2Cl2), and[Cr(N)Cl4]2� (in CH3CN).

110 T.J. Morsing, J. Bendix / Journal of Molecular Structure 1039 (2013) 107–112

a solution of 1 in CHCl3 is treated with an excess of phosphite, 1 istransformed via the mono-phosphite complex [Cr(N)(P(OCH3)3)-Cl3]� to yield the bis-phosphite complex: [Cr(N)(P(OCH3)3)2Cl2].EPR data illustrating this transformation are included in the Sup-plementary material (Fig. S3). In principle, the transformation of1 to [Cr(N)(P(OCH3)3)2Cl2] could also be envisaged to proceed via[Cr(N)(P(OCH3)3)(CNCH3)Cl2]. However, a clean preparation of theintermediate [Cr(N)(P(OCH3)3)Cl3]� could be achieved by heteroge-neous reaction of (Et4N)2[Cr(N)Cl4], essentially insoluble in CH2Cl2,with a deficit of P(OCH3)3 in CH2Cl2 yielding the spectrum shownin Fig. 5. Parameterization of the intermediate spectra obtainedin the conversion of 1 to [Cr(N)(P(OCH3)3)2Cl2] yielded a g-valueclose (giso = 1.9859; cf. Supplementary material, Fig. S4) to that ob-tained from the [Cr(N)(P(OCH3)3)Cl3]� in the different solvent(giso = 1.9867; cf. Fig. 5 and Table 3), but significantly different fromthat expected (giso = 1.9992) for hypothetical [Cr(N)(P(OCH3)3)(CNCH3)Cl2] by averaging of the experimental g-values for[Cr(N)(P(OCH3)3)2Cl2] and 2.

3.4. Electronic absorption spectra

The electronic absorption spectra of 1, 2 and [Cr(N)Cl4]2� areshown in Fig. 6. For all three complexes two d–d transitions are ob-served. At lowest energy, ranging from 12900 cm�1 ([Cr(N)Cl4]2�)

to 16800 cm�1 (2), the transition with dx2�y2 dxy orbital characteris observed. Systems with a weak equatorial ligand field and thisorbital ordering with the d-d orbitals lowest in energy, have beenclassified as electronically pseudo-linear and 1 and 2, thus belongsto this class [11]b. In agreement with the approximate C4v symme-try and the resulting symmetry forbidden character, the lowest en-ergy transition has low intensity in all three complexes. It isnotable, however, that it is almost an order of magnitude more in-tense in terms of oscillator strength in 1 as compared to the moresymmetrical complexes [Cr(N)Cl4]2� and 2 (with the spectra of 1and 2 both measured in CH2Cl2). This implies a relatively large con-tribution to the mixing of orbitals of different parity by the staticligand field rather than vibronic contributions to the intensities.The systematic increase in energy of the dx2�y2 dxy transitionupon substitution of methylisocyanide for chloride yields a differ-ence in AOM parameters for the two ligands of 3=4ðeCH3NC

r � eClp Þ�

ðeCH3NCp � eCl

p Þ = 1980 cm�1. Extrapolation of the position of the firstband would place it at 20800 cm�1 for the hypothetical[Cr(N)(CNCH3)4]2+ complex. For orthoaxial placement of the liga-tors this would be identical to the spectrochemical parameterDCH3NC, which places methyl isocyanide as having a ligand fieldstrength comparable to saturated amines e.g. cyclam (Dcyclam =21900 cm�1) [19]a, in these high-valent systems.

T.J. Morsing, J. Bendix / Journal of Molecular Structure 1039 (2013) 107–112 111

The second d–d band with a maximum between 22500 cm�1

and 25000 cm�1 is assigned to the symmetry allowed transition(s){dxz, dyz} dxy in idealized C4v symmetry. This transition is ob-served in all known chromium(V) nitride complexes and invariablyfound in a quite narrow energy range around 400 nm its positiondominated by the strong p-donation of the nitride ligand. In thecomplexes [Cr(N)Cl4]2�, 1, and 2, this transition shifts to higher en-ergy upon substitution of methylisocyanide for chloride. In termsof AOM parameters, this transition has the energy eN

p � 2eeqp for

an orthoaxial complex. Thus p-accepting equatorial ligands willshift this transition towards higher energy. For the actual coordina-tion geometry of 1, the AOM expressions for the energies of thetwo split components of this band are given in Eq. (1), where theisocyanide ligand lies in the xz-plane:

EðdxzÞ � EðdxyÞ ¼ eNp � 1:92eCl

p þ 0:19eClr þ 0:03eCH3NC

r � 0:03eCH3NCp

EðdyzÞ � EðdxyÞ ¼ eNp � 1:24eCl

p þ 0:38eClr � 0:99eCH3NC

p

ð1Þ

And their difference is given by:

d ¼ EðdxzÞ � EðdyzÞ¼ �0:19eCl

r � 0:68eClp þ 0:03eCH3NC

r þ 0:96eCH3NCp ð2Þ

The relative intensities of the 400 nm band in spectra of the threecomplexes could suggest that spectral intensity has shifted out ofthis band in the spectra of 1 and 2 as compared to [Cr(N)Cl4]2�. How-ever, in order for the transition to the higher lying component (dxz)not to overlap with that to the lower lying component, an energyseparation, d, of more than 5000 cm�1 is required. The chloride con-tribution to d (the first two factors) can be estimated to�1500 cm�1

based on the spectral data for [Cr(N)Cl4]2�. This would then require aquite significant back-bonding contribution from methyl isocyanide(eCH3NC

p < �3500 cm�1) for the higher lying component not to beencompassed by the 400 nm band, but shifted into the CT region.Based on the IR and structural data we consider a value foreCH3NCp � 0 cm�1 more probable and the observed slight shift to be

due to an increase in donor strength of the dominating nitride ligandwith diminishing negative charge of the auxiliary ligand sphere. Thiswould leave the transitions to both dxz and dyz in the observed band.

The band at 400–450 nm shows vibronic coupling to the CrBNstretch for both 1 and 2 with a spacing giving the frequency of thisvibration in the excited state. A section of the first derivative of theabsorption spectra is given in the insert of Fig. 6 and shows moreclearly the fine structure of the vibronic coupling. Though notcompletely equally spaced, the energy differences between thepeaks in the vibronic fine structure give an approximate value ofthe CrBN-stretch in the excited state of 820 cm�1 for both com-plexes. Such vibronic coupling have previously been observed inchromium(V) nitrido complexes and the value obtained for theexcited state stretching frequency, is close to the values previouslypublished (855 cm�1 and 880 cm�1 in [PPh4]2[Cr(N)(CN)4(py)]�H2O�py and [PPh4]2[Cr(N)(CN)4]�2H2O, respectively) [5]a. Thereduction in the frequency compared to the ground state value of1014 cm�1 is in accordance with the assignment of the transitionas {dxz, dyz} dxy, which populates the p⁄ antibonding orbitalsand lowers the bond order of the CrAN bond.

Stepwise conversion of 1 to 2 was achieved by addition ofincreasing quantities of neat methylisocyanide to a solution of 1 indichloromethane and followed spectroscopically. Analysis of theresulting spectra of the mixtures yielded an estimate of the forma-tion constant K2 = 6 � 10�3 for chloride substitution by methyliso-cyanide according to the equllibrium [Cr(N)Cl3(CNCH3)]� +CNCH3 = [Cr(N)Cl3(CNCH3)2] + Cl� (cf. Supp. Material), illustratingthe quite weak binding of the isocyanide to the high-valent metalcenter.

4. Conclusion

The first example of complexes combining the extreme p-donor nitride with the normally strong p-backbondingisocyanide ligands have been characterized. Structural and spec-troscopic evidence suggests that methyl isocyanide exhibitslittle, if any p-backbonding properties in the high-valent mixednitrido-isocyanide complexes described here. EPR spectroscopyclearly identifies the complexes involved, and competition withexcess isocyanide and phosphite ligand shows the binding ofthe formal p-acceptor ligands to be quite weak in agreementwith their normal preference for low-valent metal centers.However, lack of p-backbonding does not prevent the isocyanideligand from substituting ligands such as chloride ortrimethylphosphite in non-coordinating solvents, with the isocy-anide providing a relatively large ligand field, solely throughr-interaction with the high-valent Cr(V) center.

Acknowledgments

J.B. acknowledges financial support from the Danish ResearchCouncils (12-125226) and (21-04-0477). T.J.M. acknowledges re-ceipt of a Carlsberg Scholar Stipend.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.molstruc.2013.02.002.

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