Magnetic exchange interaction via HF2−-bridges? Structure and magnetism of pipzH2[MnF4(HF2)]

Solid State Sciences 2 (2000) 373 – 376

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Magnetic exchange interaction via HF− 2 -bridges? Structure and magnetism of pipzH2[MnF4(HF2)] U. Bentrup a, K. Harms b, W. Massa b,*, J. Pebler b b

a Institut fu¨r Angewandte Chemie Berlin-Adlershof, 12484 Berlin, Germany Fachbereich Chemie der Philipps-Uni6ersita¨t und Wiss. Zentrum fu¨r Materialwissenschaften, Hans-Meerwein-Straße, D-35043 Marburg, Germany

Received 25 January 2000; accepted 6 March 2000

Abstract pipzH2[MnF4(HF2)] (pipz=piperazine) has been prepared from an aqueous hydrofluoric acid solution of Mn(III) acetate and its crystal structure was determined by single-crystal X-ray analysis: triclinic, space group P1( , Z = 1, a =5.636(1), b=6.151(1), c=6.498(1) A, , a=99.52(1), b= 90.62(1), g = 107.43(1)°, wR2 = 0.063, R = 0.023. The structure consists of hitherto unique anionic chains where [MnF4] units are bridged by HF− 2 anions. The resulting [MnF6] octahedra are strongly elongated, mainly due to the Jahn–Teller effect (ŽMnFeq 1.84 A, , MnFax 2.19 A, ). The geometry of the centrosymmetrical bifluoride anion is close to that of KHF2 (HF 1.14 A, ), the angles at the angular bridge MnFH are 111.7°. Surprisingly, magnetic investigations revealed pure paramagnetic behavior. Thus, in contrast to several examples where antiferromagnetic coupling was observed via OH···F hydrogen bonds, the very strong hydrogen bonds in the symmetric F···H···F− anion are obviously not able to mediate magnetic exchange interactions. © 2000 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved. Keywords: Magnetic exchange interaction; Crystal structure; Jahn – Teller effect

1. Introduction Exchange interactions via hydrogen bonds are of actual interest due to their important role in biological processes, like the proton-coupled electron transfer (PCET) reactions during photosynthesis [1]. One possible quantitative approach to exchange interactions between transition metals is the determination of magnetic exchange energies from temperature-dependent measurements of magnetic susceptibilities in

* Corresponding author. Tel. + 49-6421-2825525; fax: +496421-2828917. E-mail address: [email protected] (W. Massa)

low-dimensional systems [2,3]. Chain structures with such 1D magnetic properties are especially favored by Jahn–Teller stabilization in Mn(III) fluorine compounds with high-spin d4 configuration [4–6]. While the usual structure element in alkali fluoromanganates(III) A2MnF5·H2O is an octahedral chain anion [MnF5]2 − built from trans-connected [MnF6] octahedra, the use of the large piperazinium cation leads, in concentrated HF solution, to the formation of a trans-chain anion with bridging HF− 2 anions instead of F− ions. This unusual chain compound pipzH2[MnF4(HF2)] is therefore a good model for studying magnetic interactions mediated by strong hydrogen bonds. The results of a single-crystal structure determination and the magnetic properties are reported here.

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U. Bentrup et al. / Solid State Sciences 2 (2000) 373–376

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Table 2 Atomic coordinates and (equivalent) isotropic displacement parameters (A, 2) for pipzH2[MnF4(HF2)]

2. Experimental

2.1. Synthesis

Atom

Red– brown crystalline pipzH2[MnF4(HF2)] is formed after adding 20 ml of a 10% solution of piperaziniumfluoride in 20% aqueous HF to a solution of 2.4 g manganese(III) acetate dihydrate in 10 ml 40% HF and reducing the volume on a water bath. After crystallization at room temperature, the sample is filtered and dried in air.

Table 1 Crystal data, measuring pipzH2[MnF4(HF2)] Crystal data Formula Mr Space group Z Lattice constants a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) Dcalc. (g cm−3) Absorption m (mm−1) Data collection Diffractometer Radiation Temperature (K) Scan mode Measuring range Reflections (total) Refinement Reflections unique/observed [I\2s(I)] Parameters Absorption correction Tmax, Tmin Extinction correction o [8] Residuals R, wR2 (all refl.) R, wR2 (observed refl.) Goodness of fit S Residual electron density max./min. (e A, −3)

and

refinement

conditions

Mn F1 H1 F2 F3 N H1N H2N C1 H11 H12 C2 H21 H22

x 0 0.3287(2) 0.5 0.0878(2) 0.1954(2) 0.4537(2) 0.558(4) 0.362(4) 0.5863(3) 0.703(4) 0.468(4) 0.2775(3) 0.194(4) 0.146(3)

y

z

Ueq/Uiso

0 −0.0986(2) 0 −0.0148(2) 0.3059(2) 0.3804(2) 0.471(4) 0.256(4) 0.3005(3) 0.231(3) 0.193(4) 0.4975(3) 0.547(4) 0.381(3)

0 0.0758(2) 0 −0.2707(1) 0.0555(2) 0.6743(2) 0.771(4) 0.726(3) 0.4942(3) 0.549(3) 0.403(4) 0.6077(3) 0.722(4) 0.511(3)

0.0160(2) 0.0323(2) 0.087(14) 0.0277(2) 0.0278(2) 0.0212(3) 0.040(6) 0.035(6) 0.0232(3) 0.026(5) 0.037(6) 0.0234(3) 0.037(6) 0.022(5)

for

2.2. Structure determination pipzH2[MnF4(HF2)] 258.10 P1( 1 5.636(1) 6.151(1) 6.498(1) 90.52(1) 90.62(1) 107.43(1) 2.026 1.618 Four-circle diffractometer CAD4 (Nonius) Mo–Ka, graphite monochromator 293 v-scans max u =30.4°; 9h, 9k,+l 1378 1275 (Rint = 0.019)/1169 87 Semi-empirical with c scans 0.556/0.503 0.019(7) 0.0297, 0.0585 0.0233, 0.0568 1.237 0.42/–0.49

The single-crystal X-ray investigation was performed on a four-circle diffractometer (CAD4, Nonius) using Mo–Ka radiation (graphite monochromator). The intensities were corrected for Lorentz and polarization effects, and a semi-empirical absorption correction was applied based on Cscans. Table 1 reports the crystallographic characteristics and the experimental conditions of data collection and refinement. All calculations were performed using SHELX programs [7,8]. Atomic scattering factors and anomalous dispersion corrections were taken from the ‘International Tables for X-Ray Crystallography’ [9]. The structure was solved in the space group P1( by direct methods and refined against the F 2 data to residuals wR2 = 0.0585 (for all 1275 reflections), R= 0.0233 (for 1169 reflections\ 2s(I)) with anisotropic displacement parameters for all non-H atoms. All H atoms were located in a difference Fourier synthesis and could be refined with individual isotropic displacement factors. Table 2 presents the final results for the atomic positions. Characteristic distances and angles are given in Table 3.

2.3. Magnetic in6estigation Susceptibility measurements were performed on a powder sample in the temperature range of 1.7–293 K on a SQUID magnetometer (Quantum Design).

U. Bentrup et al. / Solid State Sciences 2 (2000) 373–376

3. Results and discussion

3.1. Crystal structure The structure consists of anionic chains parallel to a built from planar [MnF4] units with MnF distances of 1.837 A, , on average. These are further connected by bridging bifluoride anions (Fig. 1). The Table 3 Bond lengths (A, ) and angles (°) for pipzH2[MnF4(HF2)] MnF1 (2×) MnF2 (2×) MnF3 (2×) H1F1 F1···F1% NC1 NC2 NH CH

2.1900(10) 1.8274(9) 1.8465(9) 1.1405(10) 2.281(2) 1.490(2) 1.488(2) 0.85(3)–0.90(2) 0.91(2)–0.99(2)

F1MnF2 F1MnF3 F2MnF3 MnF1H1

88.44(4) 88.55(4) 88.95(4) 111.67(6)

C1NC2 NC1C2% NC2C1%

111.2(1) 110.1(1) 110.5(1)

Fig. 1. Anionic chain in the structure of pipzH2[MnF4(HF2)]. Displacement ellipsoids at the 50% probability level. H atoms with arbitrary radius.

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,) MnF distances towards the HF− 2 ligands (2.190 A are strongly elongated, mainly due to the Jahn– Teller effect [10]. They are longer than those in MnFMn bridges of trans-chain A2MnF5·H2O compounds [6] which are between 2.08 and 2.14 A, , probably because of the reduced charge at the F atoms. The HF− 2 ions lie on centers of symmetry, the F···F distance (2.281 A, ) is close to that in alkali bifluorides. In a difference Fourier map based on a model with omitted H atoms, no double maximum is indicated. Thus, the bifluoride anions are probably symmetric, though for a clear proof a neutron diffraction experiment would be necessary. The bridges are strongly angular with an angle between the MnF1 bond and a F···H···F axis of 111.7°. The anionic chains, running along the a axis, form a distorted tetragonal rod packing. The piperazinium(2+ ) cations are positioned in a CsClanalogous way between eight [MnF6] octahedra (Fig. 2). One of the acid H atoms at nitrogen (H1N) forms a strong H bond towards an equatorial F3 atom (N···F 2.690 A, , NH···F 170°), the second one (H2N) forms a weak bond (N···F 2.755 A, , NH···F 156°) towards the second equatorial F2 ligand in an adjoining chain. In the possible exchange pathway MnF3···H1NNH2N···F2Mn, an alternative to the MnF···H···FMn chain, one H bond is weak, and the respective orbitals at N are orthogonal. Thus, no contribution to a possible magnetic exchange interaction is expected, especially as at the equatorial MnF bonds empty d orbitals are involved, which, after Babel’s rules [11], are less effective in magnetic exchange than the half-occupied dz 2 orbitals oriented towards the FHF bridges. Any possible magnetic exchange interaction encountered must be therefore due to the HF− 2 bridges,.

3.2. Magnetic properties

Fig. 2. Unit cell of pipzH2[MnF4(HF2)] with NH···F H bonds.

Measurements of the magnetization at fields up to 15 kG do not show any field dependence of the magnetic susceptibility down to 1.8 K. Therefore, the magnetic measurements at 0.5 kG and 5 kG reveal, for the magnetic powder susceptibilities, a Curie– Weiss behavior (Curie constant of C =2.90(3) cm3 K/mol and Curie–Weiss temperature Up :0 K) with a magnetic moment for the Mn3 + ion of m=4.82(2) mB, which almost represents the spin-only value of 4.90 mB. From the figure of magnetic susceptibilities (Fig. 3) one must conclude that there is no evidence

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U. Bentrup et al. / Solid State Sciences 2 (2000) 373–376

magnetic centers to a chain. Another reason may be the special electronic situation of a symmetrical H bond. The hitherto found relevant magnetic interactions all involve asymmetric hydrogen bonds. 4. Supplementary material Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publication no. CCDC124964. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: +44(1223)336-033; e-mail: [email protected]]. Acknowledgements Fig. 3. Magnetic susceptibilities and reciprocal susceptibilities as a function of temperature. Lines fitted assuming Curie‘s law with m = 4.82(2) mB.

of long or short range correlations via bridging HF− 2 ions. Previous measurements on an older sample, mentioned in Ref. [5], suggested 1D antiferromagnetic behavior. This effect was probably caused by partial decomposition of the [MnF4(HF2)]2 − anionic chain to a [MnF5]2 − chain (with well known 1D antiferromagnetic properties [4 – 6]) and HF and could not be verified on a freshly prepared sample. This paramagnetic behavior is in remarkable contrast to the magnetic properties of a series of aquafluorometallates(III), for instance K2[FeF5(H2O)] [12], pdaH2[MF5(H2O)] (pda =oand m-phenylenediamine, M= Cr(III), Fe(III) [13,14]) or enH2[FeF5(H2O)] [14]. These compounds show antiferromagnetic properties and their structures are built by quasi-isolated [MF5(H2O)]2 − anions connected via H bonds OH···F, the only plausible pathway for the observed magnetic exchange interactions. A possible explanation for the lack of magnetic exchange interactions in the case of the MnF···H···FMn-bridges above 1.8 K can be seen in the very low MnF···H angle of 112°. As could be shown experimentally at chain compounds A2MnF5·H2O [4 – 6], antiferromagnetic s-superexchange interactions decrease with cos2 of the bridge angle MFM and tend to 0 at an angle of 90°. In K2[FeF5(H2O)], the corresponding angle FeF···O is 135.6°, and there are double H bonds connecting the

The financial support of Deutsche Forschungsgemeinschaft, Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technik, and Fonds der Chemischen Industrie is gratefully acknowledged. References [1] M. Ward, Chem. Soc. Rev. 26 (1997) 365. [2] R.D. Willett, G. Gatteschi, O. Kahn (Eds.), Magneto-Structural Correlations in Exchange Coupled Systems, NATO ASI Series C, Vol. 140, Reidel, Dordrecht, 1983. [3] P. Delhaes, M. Drillon (Eds.), Organic and Inorganic Low-Dimensional Crystalline Materials, NATO ASI Series B, Vol. 168, Plenum, New York, London, 1987. [4] J. Pebler, W. Massa, H. Lass, B. Ziegler, J. Solid State Chem. 71 (1987) 87. [5] W. Massa, M. Molinier, J. Pebler, J. Fluorine Chem. 72 (1995) 171. [6] W. Massa, Rev. Inorg. Chem. 19 (1999) 117. [7] G.M. Sheldrick, SHELXS-86, Program for the solution of crystal structures, Universita¨t Go¨ttingen, 1986. [8] G.M. Sheldrick, SHELXL-97, Program for the refinement of crystal structures from diffraction data, Universita¨t Go¨ttingen, 1997. [9] T. Hahn (Ed.), International Tables of Crystallography, Vol. A, fourth ed., Kluwer, Dordrecht, 1995. [10] H.A. Jahn, E. Teller, Proc. Roy. Soc. A 161 (1937) 220. [11] D. Babel, Comments. Inorg. Chem. 5 (1986) 285. [12] R.L. Carlin, R. Burriel, J.A. Rojo, F. Palacio, Inorg. Chem. 23 (1984) 2213. [13] L. Schro¨der, G. Frenzen, W. Massa, D.-H. Menz, Z. Anorg. Allg. Chem. 619 (1993) 1307. [14] L. Schro¨der, Thesis, Univ. Marburg, Cuvillier-Verlag, Go¨ttingen, 1995.