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Fusing Nickelocene and Cyclopentadiene by Two Silyl Bridges

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Fusing Nickelocene and Cyclopentadiene by Two Silyl Bridges Fusing Nickelocene and Cyclopentadiene by Two Silyl Bridges. Synthesis and *H, 13C, and 29Si NMR Investigation of a Paramagnetic Building Block for High-Nuclear Metallocenes Monika Fritz, Johann Hiermeier, Frank H. Köhler* Anorganisch-chemisches Institut, Technische Universität München, Lichtenbergstraße 4, D-85747 Garching Z. Naturforsch. 49b, 763-769 (1994); received March 16, 1994 Nickelocenes, Disilylcyclopentadiene. Lithium Cyclopentadienide, Paramagnetic NMR Spectra Two isomers of tetrahydro-4,4,8,8-tetramethyl-4,8-disila-s-indacene (LH2) were monode- protonated and treated with cyclopentadienyl anion and NiBr2(THF)i 5 to give a 72% yield of the mixed nickelocene CpNi(LH) where a cyclopentadiene is fused to a nickelocene. The analysis of the paramagnetic 'H, 13C, and 29Si NMR spectra demonstrated that the syn and anti isomer of CpNi(LH) formed in a ratio of 5/1. Both isomers could be deprotonated to yield the anion CpNi(L~). According to its 13C NMR spectrum the bridging ligand L is not planar. Introduction tadienyl (Cp) ligands it was realized recently by Stacking of organometallic fragments or mol­ formal condensation of two Cps to conjugated six- ecules is a general strategy to obtain coordination membered rings [4], In previous studies we have polymers. The repeat unit within these polymers addressed stepwise stacking by using a building block concept where a metallocene is linked with or oligomers usually contains one or two jz ligands leading to different types of stacking. For instance, Cp_ through two silyl groups as represented by A with boron-containing ring systems linear stacks or [5]. The reaction of A with metal halides should fragments of linear stacks were obtained [1] which lead to trimetallic model compounds which allow are also known as oligodecker complexes. Linear a convenient study of the interactions between dif­ stacks were also formed by the reaction of organic ferent metallocenes. An obvious extension would be the analogous reaction of B to give coordi­ jt acceptors with metallocenes [2] and, finally, metallocenes could be assembled in a face-to-face nation polymers. For the diamagnetic case M = Fe arrangement by bridging with naphthalene [3], we have studied A and B in depth [5 a], and pre­ liminary results demonstrate that the building block concept also works for M = Ni [5 b]. Here we report the details of the paramagnetic nickelocene building block. Results and Discussion A. Synthesis The isomeric silyl-bridged cyclopentadienes la and l b (Scheme 1) were converted to the monoanion 2 as described previously [6]. Further Stepwise stacking is an alternative approach reaction with an excess of Cp“ and with solvated which starts with bridged j z ligands. For cyclopen- nickel(II) bromide gave, after work-up, the mixed- ligand nickelocene 3 in 72% yield when di-^-bu- tylether was used as solvent. In THF the yield was * Reprint requests to Prof. Dr. F. H. Köhler. 53%. NMR spectroscopy showed that two diaster- eoisomers 3 a and 3 b were formed in the ratio 51 0932-0776/94/0600-0763 $06.00 © Verlag der Zeitschrift für Naturforschung, 1. This is in contrast to the iron analogue for which D-72072 Tübingen no anti isomer like 3b could be detected [5 a]. The 764 M. Fritz et al. ■ Fusing Nickelocene and Cyclopentadiene mass spectrum and the elemental analysis were in full accord with the anticipated formula. 3a and 3b are highly soluble in hexane and sensitive to oxygen; water splits off the cyclopentadiene part of the molecule. The propensity of nickelocenes to react with alkyl lithium [7] led us to use lithium piperidide for the deprotonation of 3a and 3b. At -78 °C the reaction proceeded slowly as could be seen from the formation of the anion 4 which appeared as a green precipitate. However, when the mixture was heated to room temperature before the reaction was finished, the mixture turned black above Fig. 1. 29Si NMR spectrum of a 5/1 mixture of 3a and 3b dissolved in THF at 310 K. The feature near -9 0 0 -40 °C. Impurities were removed from 4 by ex­ ppm is expanded in the insert. S = standard (hexa- traction with hexane and the product was iden­ methyldisiloxane); G = signal from the glas tube; scale tified by 13C NMR spectroscopy. in ppm. of ’H NMR signals yielding a ratio of 5/1. For the time being the bigger signals are assigned to 3 a because only the syn isomer was found for the cor­ responding ferrocene [5 a]. The paramagnetic sig­ nal shifts at 298 K are: -904 and -933 ppm for 3a and -889 and -911 ppm for 3 b. In order to establish the structure of 3 a and 3 b by *H and 13C NMR data series of temperature- dependent spectra had to be investigated because the assignment of the paramagnetically shifted sig­ nals proved to be non-trivial. Typical spectra are represented in Figs. 2 and 3 while the numerical / 3a data are given in the experimental part (Tables II and III). For each isomer fifteen 13C NMR signals were ex­ pected. The six nickelocene signals (C4a,5,6, 7,7a, Scheme 1. a: /i-BuLi; b: Cp , NiBr2(THF)! 5; c: C5H 10NLi. B. NMR Investigation o /3 a /b and 4 Analytically pure 3 gave a 29Si NMR spectrum (Fig. 1) which consisted of two sets of two signals near -900 ppm, a range which was known from (Me3SiCp)2Ni [8], The number and the pairwise different intensities of these signals indicated that the syn isomer 3 a and the anti isomer 3 b were Fig. 2. 13C NMR spectrum of a 5/1 mixture of 3a and 3b present. As the smaller signals appeared as shoul­ dissolved in THF at 210 K. S = solvent. The nuclei of 3a ders, integration was performed with several pairs are primed; scale in ppm. M. Fritz et al. • Fusing Nickelocene and Cyclopentadiene 765 and Cp) could not be localized owing to excessive /?-SiCH3. Molecular models suggest that the same broadening. Up to now corresponding signals were order applies for 3 b although the numerical values only found for nickelocenes which are more sym­ might change considerably. Thus the signals of a-/d- metric and/or more soluble [5 b, 9], Fig. 2 displays SiCH3 must have much larger shifts to high fre­ nine small signals for 3 b but only eight big ones for quency than those of y-//3-SiCH3. This is in good 3 a because two signals coincide accidentally near agreement with the experiment because only CH3 140 ppm. The coincidence could not be removed in groups are left for the assignment of the two big sig­ the temperature range 205 < T < 330 K, but the in­ nals near 800 ppm. Although further distinction of tegration leaves no doubt about the assignment. a- and (3-SiCH3 as well as y- and /3-SiCH3 is possible In the fully coupled spectrum multiplets due to by following 6 it must be regarded as tentative be­ one-bond CH couplings were observed in the ex­ cause the differences in 6 are now smaller. perimental shift range 250>d>-50. For 3 a the Another aid for the signal assignment is the peak with double intensity, and two more peaks extrapolation of the temperature-dependent signal gave doublets (C 1,2,3,3 a), two gave quartets (two shifts. When 1/T approaches zero the signal shifts SiCH3) and one remained a singlet (C8a). Similarly should be close to those of a diamagnetic analogue 3b gave three doublets (C 1,2,3), two quartets (two unless anomalies like antiferromagnetic exchange, SiCH3), and one singlet (C8a). The signals of the low-spin/high-spin equilibria, dynamic phenomena SiCH3 groups were further distinguished based on and the like are present. Thus when we apply the the dihedral angles 6 between the Si-CH3 bonds extrapolation to the above-mentioned three dou­ and the normals to the Cp planes. The normals are blets of 3a we obtain <3 = 145 ± 5, 134 ± 5, and approximately parallel to the spin-containing car­ 65 ± 10 which compares well with Cl/3, C2, and bon 2 pz orbitals of C 4 a/7 a which determine the hy- C3a of the iron analogue of 3a having ö = 140.7/ perconjugative spin transfer to the CH3 nuclei [10] 137.6, 132.4 and 57.7 [5 a], respectively. C 3 a can be and thus the signal shift <3para (<3para = d>o + Bcos20, identified independently by its one-bond CH coup­ d0 and B being constants). It was assumed that the ling of 130 ± 10 Hz which is smaller than that of the angles are similar to those determined for the iron vinyl-type carbon nuclei C 1-3 (160 ± 10 Hz) as ex­ analogue of 3a by X-ray crystallography [5 a] yield­ pected. No distinction is possible for Cl/3 although ing the following order of dihedral angles of the the coincidence of the signals is lifted owing to SiCH3 groups: a-SiCH3 < (3-SiCH3 << y-SiCH3< different reference shifts obtained from C 1 and Table I. Paramagnetic NMR data3 of 3a, 3b, and 4. Position 13C NMR ’H NMR <3('H) for lH NMR of nuc- T ---> o o C refe­ leusb 3a 3b 4 3a 3b 3a 3b rence0 1 0 -1 3 f -15.1 -2.4 -2.1 6.7 6.8 7.06 2 -18 63 16.8 -0.2g -0.4® 6.4® 6.2® 6.88 3 0 l f -15.1 1.1® 4.4® 6.1® 6.3® 6.88 3a -39 167 17.7 5.8 16.3 4.3 3.4 4.18 4 a/7 a e e 1525 5 - 7 e e 1525 -245h -244h j j 4.20 -242h -234h 4.39 -227h 4.20 8a -53 -35 17.7 a 685 545 509 18.2 16.2 -0.7 -0.4 -0.48 ß -17 89 31.9 -2.2' -2.7' o .r 0.2' 0.47 y 57 101 31.9 -o.s 1 -2.4' 0.2' 0.0' 0.45 ö 622 226 509 15.8 7.8 0.2 0.5 0.35 Cp e e 1525 -260 -260 j j 4.03 a In ppm relative to the data of the iron analogue of 3 a (cf Experimental), negative sign for shifts to low fre­ quency; see Scheme 1 for numbering; c extrapolated from the data given in Table III; all values ± 0.5 ppm; d data of the iron analogue of 3a in C6D<s at 20 °C [5a]; e not detected; f_1 interchange of assignment not excluded for position of nuclei f 1/3, ® 1/2, h 5 -7 ; 1 ß /y\] not determined.
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