Variable-temperature one- and two-dimensional 13C CP/MAS NMR studies of the dynamics of monohaptocyclopentadienyl rings of hafnium and titanium tetracyclopentadienyl in the solid state

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Abstract

Organometallic compounds with cyclopentadienyl ligands (C5H5) can undergo fluxional motion. We have studied the Group IVB transition metal complexes of the formula M(η5-C5H5)2(η1-C5H5)2, where M=Ti and Hf, in the solid state using variable-temperature 13C CP/MAS NMR spectroscopy. We present data which indicate that intramolecular rearrangement proceeds via a sigmatropic shift. Using two-dimensional (2D) exchange NMR, we can follow the rearrangement of the single bonded cyclopentadienyl ligands.

Introduction

Fluxional motion in stereochemically nonrigid organometallic molecules has interested inorganic chemists ever since its discovery in (η1-C5H5)(η5-C5H5)Fe(CO)2 by Piper and Wilkinson in 1956 [1]. Considerable effort has been invested in studying the fluxional motion of cyclopentadienyl rings σ-bonded to metal centers in solution [2] and in the solid state [3], [4]. In these compounds, the ring has been observed to undergo either a ring flip or a sigmatropic rearrangement, which shifts the identity of the carbon atom attached to the metal center [2]. This rearrangement may be well defined, such as a [1,2] or a [1,3] shift, or more random, such that either shift may occur with equal probability. Solution-state 1H-NMR spectroscopy has been used extensively to determine the mechanism of exchange for monohaptocyclopentadienyl-containing compounds [2]. In the solid state, however, the problem is much more difficult, as strong dipole–dipole interactions make high-resolution 1H-NMR extremely difficult and sites that are crystallographically inequivalent complicate peak assignments. Many of the NMR studies of fluxional motion of cyclopentadienyl groups in the solid state have primarily focused on determining the activation energy and not in determining the mechanism of reorientation [3]. Recently, two-dimensional (2D) exchange spectroscopy has been used to study fluxional motion in organometallic compounds in the solid state [5]. Two-dimensional experiments are desirable because they give information about all exchanging sites in the molecule simultaneously, in contrast to other magnetization transfer experiments which only provide information about exchange occurring from a specific site to another site [6].

In this communication we report variable-temperature (VT) 1D and 2D CP/MAS NMR spectra of Hf(η5-C5H5)2(η1-C5H5)2 (1) and Ti(η5-C5H5)2(η1-C5H5)2 (2). The structures of 1 (M=Hf) and 2 (M=Ti) are shown in Fig. 1 [7], [8]. The 1D spectra of 2 have been reported previously [4]. One-dimensional NMR spectra showed sharp resonances for carbons in the monohapto groups in 1 and 2 at temperatures below ∼190 K. At 298 K, no signals were observed from the monohapto groups, probably due to molecular motion interfering with the 1H high power decoupling. This phenomenon is common in systems that undergo motion on the order of the decoupler frequency [9]. Two-dimensional exchange spectra of both compounds at showed that the rings underwent a sigmatropic rearrangement rather than ring flips, and that there were only two possible assignments for the carbon resonances. The final determination of the sigmatropic rearrangement mechanism will be possible when definitive peak assignments can be made.

Section snippets

Experimental

All syntheses were performed under an inert atmosphere using standard Schlenk-line and glove-box techniques. Sodium cyclopentadienide (Aldrich) was used as received in a supersealed bottle under argon. All solvents were distilled from Na/benzophenone under a dry argon atmosphere. Hf(η5-C5H5)2(η1-C5H5)2 was prepared from the reaction of Hf(η5-C5H5)2Cl2 (Aldrich) and Na(C5H5) by the method of Rogers et al. [7]. Similarly, Ti(η5-C5H5)2(η1-C5H5)2 was prepared from the reaction of Ti(η5-C5H5)2Cl2

Results and discussion

The variable-temperature 13C CP/MAS NMR spectra of Hf(η5-C5H5)2(η1-C5H5)2 (1) are shown in Fig. 2. From Fig. 1, it is apparent that the carbon site pairs in the two η1-C5H5 rings are magnetically equivalent. Therefore, C1 and C1′ will now collectively be referred to as the C1 site. At 173 K, five peaks are evident. The largest peak at 113.0 ppm is due to carbons in the η5-C5H5 rings. The four resonances at 90.0, 126.5, 127.9, and 130.7 ppm are assigned to the η1-C5H5 rings. Because the four

Acknowledgements

We are very grateful to Professor Alexander Pines for his advice and encouragement of this work. We would also like to thank Professor Jerry L. Atwood for X-ray crystal structure data, and Drs Matthias Ernst, Jonathan Jones, Andrew Kolbert, Russell Larsen, Klaus Schmidt-Rohr, and Jay Shore for helpful suggestions. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division of the US Department of Energy under contract no.

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Cited by (0)

1

Present address: Max-Planck-Institut fuer Polymerforschung, Postfach 3148, D-55021, Germany.

2

Present address: Clorox Technical Center, 7200 Johnson Drive, Pleasanton, CA 94588, USA.

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