DOI: 10.1002/zaac.200700451

Synthesis and Structure of the Cerium Schiff-Base Complexes [Ce(Salen�)2] and[(THF)2KCe(Salen�)2]

Jochen Gottfriedsen*, Marlies Spoida, and Steffen Blaurock

Magdeburg, Chemisches Institut der Otto-von-Guericke-Universität

Received September 3rd, 2007.

Dedicated to Professor Dr. Richard A. Andersen on the Occasion of his 65th Birthday

Abstract. The synthesis of [Ce(Salen�)2] (1) (H2Salen� � N,N�-bis(3,5-di-tert-butylsalicylidene)ethylenediamine) was performedusing two different approaches. CeCl3 reacts with two equivalentsof K2Salen� in THF under the formation of [(THF)2KCe(Salen�)2](2). Complex 2 could be converted to the CeIV complex 1 via oxi-dation with p-benzoquinone and air, respectively. The reversible re-

Introduction

Tetravalent cerium compounds received much attentionlately due to their high oxidation potential and multiple ap-plications [1]. They are well established as precursors in dif-ferent MOCVD (metal organic chemical vapor deposition)or ALE (atomic layer epitaxy) processes to deposit CeO2

and mixed oxides [2, 3]. Materials containing cerium oxidesare of great current interest due to their importance in con-version catalysts [4], as solid oxide fuel cells [5], solar cells[6], buffer layers for YBCO high-temperature superconduc-tors [7], gates for metal-oxide semiconductor devices, andphosphors [8]. Furthermore, the application of CeIV com-pounds in oxidation processes in organic synthesis is wellestablished and further developments in this field are ofcurrent interest [9]. Moreover, studies of the behaviour ofCeIV complexes in biological systems were performed show-ing their high reactivity [10]. Thus, there is a demand fornew, stable CeIV compounds, in order to deepen this fieldof chemistry and extend the cognitions of CeIV complexes.The synthesis of cerium complexes was often performedby oxidation reactions of a CeIII precursor with theappropriate oxidizing reagent: e.g. I2 in case of[CeI{N(SiMe2

tBu)CH2CH2}3N] [11] and TeCl4 andPBr2Ph3, respectively, for the synthesis of the heterolepticcomplexes [Ce{N(SiMe3)2}3X] (X � Cl or Br) [12]. Thehomoleptic amide complex [Ce(NCy2)4] was synthesized viaoxidation with dry air [13]. Alternatively, p-benzoquinone

* Dr. J. GottfriedsenChemisches Institut der Otto-von-Guericke-UniversitätUniversitätsplatz 2D-39106 Magdeburg, GermanyFax: Int. �49-391-6712933e-mail: jochen.gottfriedsen@vst.uni-magdeburg.de

514 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2008, 634, 514�518

duction process was realized using elemental potassium in boilingTHF. Furthermore, the reaction of the CeIV starting material[(tBuO)4Ce(THF)2] with the “free” ligand H2Salen� in boiling tolu-ene lead in the formation of 1 as well.

Keywords: Cerium; Schiff-base complexes

was used for the oxidation of the CeIII complexesCe(OCtBu3)3 and [Ce(COT)2Li(THF)2] [14].

Herein, we present our studies of homoleptic Ce SALENcomplexes using N,N�-bis(3,5-di-tert-butylsalicylidene)-ethylenediamine (H2Salen�). The chemistry of SALEN-typeligands with transition metals and their application inhomogeneous catalysis has been intensively explored [15].Also, main group element [16] and lanthanide [17] com-plexes with SALEN-type ligands were studied recently. Inorder to study stable homoleptic cerium complexes in theoxidation state �4 we turned to Schiff-base ligands, namelySALEN-type systems, since this type of ligand was onlysparely studied for complexes of CeIV and its possibility tostabilize the oxidation state �4. A homoleptic CeIV com-plex of the type [Ce(Salophen)2] (H2Salophen � N,N�-dis-alicylidene-1,2-phenylenediamine was already synthesizedfrom cerium ammonium nitrate and H2Salophen [18]. Fur-thermore, Archer et al. reported the synthesis of linear CeIV

Schiff-base coordination polymers based on bis-SALEN-type ligands [19]. We report the synthesis of the two novelcomplexes [Ce(Salen�)2] (1) and [(THF)2KCe(Salen�)2] (2)which could be converted into one another by oxidationand reduction processes.

Experimental Section

General Comments. The reactions were carried out in an inert at-mosphere of dry nitrogen using standard dry box and Schlenk tech-niques. All organic solvents were freshly distilled from sodiumbenzophenone ketyl immediately prior to use. The starting materi-als H2Ssalen� and [(tBuO)4Ce(THF)2] were synthesized accordingto literature procedures [20]. All other starting materials were pur-chased from Aldrich Chemical Co. and used as received. The melt-ing points were determined in a sealed capillary without correction.NMR spectra were recorded on a Bruker DPX 400 or AVANCE600 NMR spectrometer. All chemical shifts are reported in ppm.

Cerium Schiff-Base Complexes

Chemical shifts are referenced to tetramethylsilane as internalstandard. 1H and 13C NMR chemical shifts of the of complex 2were assigned and detected using HSQC and HMBC techniques.The mass spectra (EI, 70 eV) were obtained using a Finnigan SSQ7000. Only characteristic fragments containing the isotopes of thehighest abundance are listed. The elemental analysis were per-formed on a LECO CHNS932 apparatus. The single crystal X-raydiffraction studies were performed on a Stoe IPDS diffractometer.The structures were solved by Patterson methods (SHELXS-97)[21]. Refinements were carried out using full-matrix least-squarestechniques on F2 using the program SHELXL-97 [22]. Crystallo-graphic data for the structures reported in this paper [23] have beendeposited with the Cambridge Crystallographic Data Centre as asupplementary publication CCDC 652363 (1) and 652362 (2). Cop-ies of the data can be obtained free of charge on application to theCCDC via www.ccdc.cam.ac.uk/datarequest/cif.

[Ce(Salen�)2] (1): Method A: Portions of 0.683 g (1.19 mmol) [(tBu-O)4Ce(THF)2] and 1.168 g (2.38 mmol) H2Salen� were dissolved in50 mL toluene and heated under reflux conditions for 12 h. Sub-sequently, the solvent was completely removed and the residuedried under vacuum. The resulting purple solid was dissolved in15 mL pentane and re-crystallized at 5 °C. The product could beisolated as cubic formed purple crystals in 80 % (1.068 g) yield.

Method B: To a solution of 0.17 g (0.13 mmol) [(THF)2KCe(S-alen�)2] (2) in 20 mL toluene 0.016 g (0.148 mmol) benzoquinonewere added. The color of the solution changed immediately fromorange to purple. The reaction mixture was stirred for anotherhour. Filtration and removal of the solvent gave a purple powderwhich was re-crystallized from 5 mL pentane at �20 °C and iso-lated in 40 % (0.058 g) yield.

Method C: According method A for the preparation of complex 1, aportion of 0.195 g (4.86 mmol) was reacted with 1.2 g (2.11 mmol)H2Salen� followed by the addition of 0.3 g (1.2 mmol) CeCl3 in80 mL THF. Subsequently, air was bubbled through the reactionmixture for 1h inducing a color change to deep purple. The reactionmixture was filtered and the filtrate evaporated to dryness. Theresulting dark brown solid was dissolved in 40 ml of pentane, fil-tered, the volume of the solution reduced to 10 mL and stored at5 °C. Crystallization after 5 days gave the product in 65 % (0.77 g)yield. M.p. 238 °C (decomposition). Elemental analysis forC64H92N4O4Ce (1121.56): C 68.21 (calc. 68.54), H 8.05 (8.27), N4.66 (5.00) %.1H-NMR (C6D6, 25 °C, 400 MHz): δ � 8.52 (s, 4H, CH2-N�CH-Ar), 7.52(d(4J(1H,1H)� 2,4 Hz), 4H, Ar-H), 7.09 (d(4J(1H,1H)� 2,4 Hz), 4H, Ar-H),4.60 (s, 8H, (-CH2-N�CH-Ar)2), 1.40 (s, 36H, Ar-C(CH3)3), 1.28 (s, 36H,Ar-C(CH3)3). 13C-NMR ({H}, C6D6, 25 °C, 100 MHz): δ � 168.5 (CH2-N�CH-C(Ar)), 166.9 (C-O), 138.2 (C(Ar)-C(CH3)3), 135.9 (C(Ar)-C(CH3)3),130.1 (C(Ar)-H), 129.4 (C(Ar)-H), 124.5 (CH2-N�CH-C(Ar)), 64.5 (CH2-N�CH), 35.9 (C(CH3)3), 33.7 (C(CH3)3), 31.7 (C(CH3)3), 30.5 (C(CH3)3).MS (EI, 140Ce): m/z 1120.6 (M� · , 100 %), 629 (M� � Salen�, 65 %).

(THF)2KCe(Salen�)2 (2): Method A: An amount of 0.56 g(13.96 mmol) of potassium hydride was suspended in 50 mL THFand a THF solution (50 mL) of 3.4 g (6.98 mmol) H2Salen� wasadded at room temperature. The reaction mixture was stirred for6 h followed by the addition of 0.86 g (3.49 mmol) CeCl3 and sub-sequent stirring for another 12 h. Filtration of the reaction mixturegave an orange filtrate which was reduced to 15 mL under vacuum.Crystallization at 6 °C gave the product as orange colored crystal-line needles in 45 % (2.05 g) yield.

Method B: A solution of 0.5 g (0.44 mmol) [Ce(Salen�)2] (1) in50 mL THF was treated with a threefold excess elemental potass-

Z. Anorg. Allg. Chem. 2008, 514�518 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 515

ium (0.05 g, 1.29 mmol) and the reaction mixture heated to refluxfor 6 h. The reaction mixture was filtered of the non-reacted potass-ium and the filtrate reduced to a 5 mL volume under vacuum.Crystallization at �20 °C gave the product as an orange microcrys-talline powder in 55 % (0.32 g) yield. M.p. 386 °C (decomposition).Elemental analysis for C72H106N4O6CeK (1302.85): C 65.89 (calc.66.38), H 7.96 (8.20), N 4.12 (4.30) %.1H-NMR (d8-THF, 25 °C, 400 MHz): δ � 9.75 (s, 4H, Ar-H), 7.74 (sbr, 4H,CH2-N�CH-Ar), 7.16 (sbr, 4H, Ar-H), 1.98 (sbr, 72H, Ar-C(CH3)3). 13C-NMR ({H}, d8-THF, 25 °C, 100 MHz): δ � 187.1 (C-O), 160.4 (CH2-N�CH-C(Ar)), 150.5 (CH2-N�CH-C(Ar)), 136.5 (C(Ar)-C(CH3)3), 131.9(C(Ar)-H), 130.7 (C(Ar)-H), 38.2 (C(CH3)3), 35.2 (C(CH3)3), 34.7(C(CH3)3), 33.2 (C(CH3)3), A signal for (CH2-N�CH) was not found. MS(EI, 70 eV, 140Ce): m/z 1160 (M� · � 2 THF, 5 %), 1120 (M� � 2 THF �K, 30 %), 630 ([Salen�Ce]�, 100 %).

Results and Discussion

While our studies of different synthetic pathways of syn-thesizing homoleptic and heteroleptic CeIV Schiff-base com-plexes, we developed two different approaches to generatethe bisSALEN CeIV complex [Ce(Salen�)2] (1). The directsynthesis of complex 1 was performed using [(tBuO)4-

Ce(THF)2] as a CeIV starting material according toScheme 1.

Scheme 1 Synthesis of complexes 1 and 2.

The reaction of the “free” Salen� ligand with [(tBuO)4-

Ce(THF)2] in a two to one ratio gave [Ce(Salen�)2] (1) inhigh yields. Complex 1 is highly soluble in aliphatic andaromatic solvents like pentane, hexane, benzene, and tolu-ene, as well as in diethyl ether and chloroform. The 1H and13C NMR data are in the expected region for a diamagneticcompound and are only slightly different from the ones ofthe “free” ligand N,N�-bis(3,5-di-tert-butylsalicylidene)-ethylenediamine. The second possible approach to generate[Ce(Salen�)2] (1) is the oxidation of the appropriate CeIII

complex. According to the synthesis of the corresponding

J. Gottfriedsen, M. Spoida, S. Blaurock

yttrium complex [17a], the treatment of CeCl3 withK2Salen� (made in situ by deprotonation of N,N�-bis(3,5-di-tert-butylsalicylidene)ethylenediamine with potassium hy-dride) in a molar ratio of 1:2 gave the CeIII complex[(THF)2KCe(Salen�)2] (2) as orange crystals (Scheme 1).Complex 2 is non-soluble in aliphatic solvents, poorly sol-uble in aromatic solvents in contrast to the equivalent yt-trium complex [17a] and only moderately in THF andDME. Due to the expected paramagnetism of complex 2the chemical shifts of the 1H and 13C NMR spectra differfrom the ones found for [Ce(Salen�)2] (1). The 1H NMRsignal attributed to the iminic hydrogen atom (N�CH�Ar)is shifted to high field by 0.78 ppm in complex 2 (8.52 ppmin complex 1 and 7.74 ppm in complex 2). Furthermore,only one broad singlet is found for the hydrogen atoms at-tributed to the tert-butyl groups in complex 2, whereas twosignals were found for the related groups in complex 1. Asignal which can be assigned to the CH2-groups of the C2

bridge of the Salen� ligand is not clearly detectable for com-plex 2.

Fig. 1 Molecular Structure of complex 1. Displacement ellipsoidsare drawn at the 50 % level. Hydrogen atoms have been omitted forclarity. Selected bond lengths/A and angles/°:

Ce(1)�O(2) 2.237(3), Ce(1)�O(4) 2.239(3), Ce(1)�O(3) 2.244(3),Ce(1)�O(1) 2.258(4), Ce(1)�N(2) 2.591(4), Ce(1)�N(3) 2.592(4),Ce(1)�N(4) 2.600(4), Ce(1)�N(1) 2.605(4), O(2)�Ce(1)�O(4) 96.18(1),O(2)�Ce(1)�O(3) 88.84(1), O(4)�Ce(1)�O(3) 155.43(1), O(2)�Ce(1)�O(1)154.68(1), O(4)�Ce(1)�O(1) 88.06(1), O(3)�Ce(1)�O(1) 97.63(1),O(2)�Ce(1)�N(2) 70.18(1), O(4)�Ce(1)�N(2) 77.88(1), O(3)�Ce(1)�N(2)81.30(1), O(1)�Ce(1)�N(2) 134.92(1), O(2)�Ce(1)�N(3) 81.57(1),O(4)�Ce(1)�N(3) 134.74(1), O(3)�Ce(1)�N(3) 69.76(1),O(1)�Ce(1)�N(3) 77.85(1), N(2)�Ce(1)�N(3) 139.75(1),O(2)�Ce(1)�N(4) 77.41(1), O(4)�Ce(1)�N(4) 69.48(1), O(3)�Ce(1)�N(4)134.96(1), O(1)�Ce(1)�N(4) 80.81(1), N(2)�Ce(1)�N(4) 130.54(1),N(3)�Ce(1)�N(4) 65.91(1), O(2)�Ce(1)�N(1) 135.17(1),O(4)�Ce(1)�N(1) 81.86(1), O(3)�Ce(1)�N(1) 77.73(1), O(1)�Ce(1)�N(1)70.12(1), N(2)�Ce(1)�N(1) 65.62(1), N(3)�Ce(1)�N(1) 130.20(1),N(4)�Ce(1)�N(1) 139.65(1).

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The oxidation of complex 2 under formation of the CeIV

complex 1 was carried out with p-benzoquinone and air,respectively (Scheme 1). On the other hand, compound 1with the cerium in a tetravalent state could be re-convertedinto the CeIII complex 2 using elemental potassium in boil-ing THF (Scheme 1).

X-ray structure determination

Single crystal X-ray determinations of complexes 1 and 2revealed coordination environments around the Ce atomswhich differs from the “sandwich” type structure that wasfound for [Ce(Salophen)2] [18]. The cerium atom in bothcomplexes is surrounded by distorted eight-coordinatesquare antiprismatic coordination (Fig. 1 and 2).

Fig. 2 Molecular Structure of complex 2. Hydrogen atoms havebeen omitted for clarity. Selected bond lengths/A and angles/°:

Ce(1)�O(4) 2.357(3), Ce(1)�O(1) 2.369(3), Ce(1)�O(2) 2.457(3),Ce(1)�O(3) 2.462(3), Ce(1)�N(1) 2.687(3), Ce(1)�N(4) 2.695(3),Ce(1)�N(3) 2.714(3), Ce(1)�N(2) 2.730(3), K(1)�O(5) 2.667(6), K(1)�O(6)2.705(8), K(1)�O(3) 2.740(3), K(1)�O(2) 2.751(3), K(1)�C(16) 3.200(4),K(1)�C(33) 3.220(4), K(1)�C(21) 3.504(4), O(4)�Ce(1)�O(1) 101.20(11),O(4)�Ce(1)�O(2) 87.25(1), O(1)�Ce(1)�O(2) 162.05(8), O(4)�Ce(1)�O(3)162.31(9), O(1)�Ce(1)�O(3) 86.23(1), O(2)�Ce(1)�O(3) 90.31(9),O(4)�Ce(1)�N(1) 81.32(1), O(1)�Ce(1)�N(1) 68.56(1), O(2)�Ce(1)�N(1)128.87(9), O(3)�Ce(1)�N(1) 86.64(1), O(4)�Ce(1)�N(4) 68.10(9),O(1)�Ce(1)�N(4) 79.53(1), O(2)�Ce(1)�N(4) 89.20(1), O(3)�Ce(1)�N(4)129.41(9), N(1)�Ce(1)�N(4) 130.08(1), O(4)�Ce(1)�N(3) 129.38(9),O(1)�Ce(1)�N(3) 86.19(1), O(2)�Ce(1)�N(3) 76.29(9), O(3)�Ce(1)�N(3)66.62(9), N(1)�Ce(1)�N(3) 144.58(1), N(4)�Ce(1)�N(3) 64.21(1),O(4)�Ce(1)�N(2) 88.97(1), O(1)�Ce(1)�N(2) 129.22(9),O(2)�Ce(1)�N(2) 66.08(9), O(3)�Ce(1)�N(2) 74.07(9), N(1)�Ce(1)�N(2)64.06(1), N(4)�Ce(1)�N(2) 147.46(1), N(3)�Ce(1)�N(2) 124.45(1).

The angle between the planes containing N1-Ce-N2 andN3-Ce-N4, respectively, is 78.76(2)° in 1 and 66.51(1)° in 2displaying the influence of the potassium ion coordinatedto one oxygen atom of each Salen� ligand in complex 2.

Cerium Schiff-Base Complexes

The dihedral angles between the arene rings of the Salen�ligands also show the influence of the coordinated potass-ium ions in 2 on the structural features of the two solidstate structures. While the dihedral angles in [Ce(Salen�)2](1) are 6.23(1)° and 2.69(3)° and therefore the Salen�ligands are nearly planer regarding the phenyl rings, the di-hedral angles in [(THF)2KCe(Salen�)2] (2) are significantlyhigher with values of 16.28(2)° and 20.13(2)°. As expected,the bond lengths of the Ce�O bonds in the tetravalent com-plex 1 (2.237(3) A to 2.258(4) A) are shorter than the onesin complex 2 (2.357(3) A to 2.462(3) A) due to the differenteffective ionic radii [24]. The bond lengths in complex 1 areslightly longer than those in CeIV alkoxy and CeIV calix[4]-arene complexes [25], but are in good agreement with theones found for [Ce(Salophen)2] [18] and tripodale CeIV

Schiff-base complexes [26]. The longer Ce�O distances incomplex 2 correspond well with known CeIII�O distances[27]. The Ce�N bond distances of complexes 1 (2.591(4) to2.605(4) A) and 2 (2.687(3) to 2.730(3) A) show the samepicture with shorter distances in complex 1. Although, thestructural type of [Ce(Salophen)2] is different from the onefound for complex 1, the Ce�N bond distances are in thesame range.

Conclusion

In conclusion, we showed the synthesis of the two new com-plexes [Ce(Salen�)2] (1) and [(THF)2KCe(Salen�)2] (2) usingdifferent synthetic pathways. Complex 1 could be generatedvia oxidation of complex 2 with O2. Examples for the suc-cessful oxidation from CeIII precursors to CeIV complexesis comparatively rare and was described for the synthesis of[Ce(NCy2)4] [13] and β-diketonate complexes [1]. Further-more we generated the CeIV complex 1 in a straightforwardreaction with p-benzoquinone and presented the reversiblereduction with elemental potassium.

Acknowledgements. The authors thank Prof. Krautscheid (Univer-sität Leipzig, Germany) for providing the single crystal X-ray facili-ties, and Prof. Frank T. Edelmann for helpful discussions. This workwas financially supported by the Otto-von-Guericke-UniversitätMagdeburg. Furthermore, we thank BAYER Industry Services forproviding solvents and starting materials.

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J. Gottfriedsen, M. Spoida, S. Blaurock

[20] a) D. J. Darensbourg, R. M. Meckiewicz, J. L. Rodgers, C. C.Fang, D. R. Billodeaux, J. H. Reibenspies, Inorg. Chem. 2004,43, 6024�6034; b) Herrmann/Brauer, Synthetic Methods of

Organometallic and Inorganic Chemistry, Vol. 6, Georg ThiemeVerlag, Stuttgart 1997, p. 42.

[21] G. M. Sheldrick, SHELXS-97, Program of Crystal Structure

Solution; University of Göttingen: Göttingen, Germany, 1997.[22] G. M. Sheldrick, SHELXL-97, Program of Crystal Structure

Refinement; University of Göttingen: Göttingen, Germany,1997.

[23] Crystal data for 1: C64H92CeN4O4, M � 1121.54, monoclinic,space group P21/c, a � 13.369(3) A, b � 18.906(4) A, c �

27.711(6) A, α � 90 °, β � 97.49(3)°, V � 6944(2) A3, Z � 4,T � 210(2) K, λ � 0.71073 A, absorption coefficient0.697 mm�1, F(000) � 2376, crystal size 0.40 x 0.20 x0.10 mm, θ � 2.09 to 25.92°, limiting indices: �16�h�16,�21�k�23, �31�l�33, completeness to θ � 25.92 / 99.1 %,absorption correction: none, data collection using Stoe IPDS1, 39128 collected reflections, 13421 unique reflections (Rint �

0.1482), final R indices [I>2σ(I)]: R � 0.0655, wR2 � 0.1685,R indices (all data): R1 � 0.0868, wR2 � 0.1754, refinedagainst full-matrix least-squares on F2, GOF 0.940.

www.zaac.wiley-vch.de © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2008, 514�518518

Crystal data for 2: C76H116CeKN4O7, M � 1376.95, monoclinic,space group P21/n, a � 16.464(3) A, b � 26.273(5) A, c �

19.253(4) A, β � 98.09(3)°, V � 8245(3) A3, Z � 4, T �

210(2) K, λ � 0.71073 A, absorption coefficient 0.650 mm�1,F(000) � 2932, crystal size 0.50 x 0.30 x 0.30 mm, θ � 2.27 to28.02°, limiting indices: �21�h�21, �34�k�34, �25�l�24,completeness to θ � 28.02 / 98.7 %, absorption correction:XRED, data collection using Stoe IPDS 1, 78679 collectedreflections, 19696 unique reflections (Rint � 0.0391), final Rindices [I>2σ(I)]: R � 0.0472, wR2 � 0.1582, R indices (alldata): R1 � 0.0707, wR2 � 0.1657, refined against full-matrixleast-squares on F2, GOF 0.978.

[24] R. D. Shannon, Acta Crystallogr. 1976, A32, 751�767.[25] a) S. Daniele, L. G. Hubert-Pfalzgraf, M. Perrin, Polyhedron

2002, 21, 1985�1990; b) B. A. Vaartstra, J. C. Huffman, P. S.Gradeff, L. G. Hubert-Pfalzgraf, J. C. Daran, S. Parraud, K.Yunlu, K. G. Caulton, Inorg. Chem. 1990, 29, 3126�3131; c)J. Gottfriedsen, D. Dorokhin, Z. Anorg. Allg. Chem. 2005,631, 2928�2930.

[26] P. Dröse, J. Gottfriedsen, Z. Anorg. Allg. Chem. 2008, 634, 87.[27] H. Flemig, I. Pantenburg, G. Meyer, Z. Anorg. Allg. Chem.

2006, 632, 2205�2208.