Elsevier

Polyhedron

Volume 187, 1 September 2020, 114581
Polyhedron

Zirconium alkyls with ancillary cyclooctatetraenyl ligands. X-ray crystal structures of [Li(tmed)][(C8H8)Zr(p-C6H4Me)Cl2] and (C8H8)Zr[CH(SiMe3)2]2

Dedicated to John Bercaw on the occasion of his 75th birthday.
https://doi.org/10.1016/j.poly.2020.114581Get rights and content

Abstract

Several new group 4 alkyl and aryl compounds have been synthesized that possess ancillary cyclooctatetraene ligands. Most of the complexes isolated are anionic and have stoichiometries of [Li(tmed)2][(C8H8)MRCl2], [Li(tmed)x][(C8H8)MR2Cl], or [Li(tmed)x][(C8H8)MR3], where M = Zr or Hf, and R = Me, Ph, p-Tol, or CH(SiMe3)2. Two neutral 14-electron compounds have also been prepared, (C8H8)M[CH(SiMe3)2]2, where R = Zr or Hf. The compound (C8H8)Zr[CH(SiMe3)2]2 appears to react reversibly with 7 atm of carbon monoxide but does not react with dihydrogen even at 70 atm. IR, 1H and 13C NMR data are reported, along with the crystal structures of two of the new compounds.

Graphical abstract

Several new Zr and Hf cyclooctatetraene compounds have been prepared: [Li(tmed)2][(C8H8)MRxCl3-x], where R = Me, Ph, p-Tol, or CH2(SiMe3)2, and (C8H8)M[CH(SiMe3)2]2. Crystal structures of two of the compounds, and reactions with CO and H2, are described.

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Introduction

The chemistry of early transition metals coordinated to cyclooctatetraene ligands is rich and fascinating. One unusual aspect of such compounds is the versatility of the C8H8 ligand, which can bind to metals in several different ways. The most common binding modes are η8 and η4, and in some instances both of these modes occur in the same compound, as in bis(cyclooctatetraene)zirconium(IV): the solution 1H and 13C NMR spectra of this compound, which feature only one resonance for the two C8H8 rings, indicate that intramolecular exchange of the two rings is rapid on the NMR time scale [1], [2]. Cyclooctatetraenyl ligands bound through an odd number of carbon atoms are unusual; an η3 ring and an η4 ring are present in (C5H4Me)Nb(C8H8)2, and two rings exchange on the NMR time scale [3].

In most cases, the binding mode that the C8H8 ligand adopts can be explained by the steric and electronic requirements of the metal center. For example, in the cationic complex, [(C5H5)Mo(C8H8)+], the C8H8 ligand is bound in an η8 fashion. Addition of a 2-electron donor, such as a tertiary phosphine or carbon monoxide, results in a change in hapticity, in which the C8H8 ligand adopts an η6 bonding mode. Similarly, the addition of a 4-electron donor causes the C8H8 ring to become η4 [4].

In some cases, dianionic C8H8 ligand can also bridge two metal centers. An example of an unusual binding mode is found in the triple decker sandwich compound, Ti2(C8H8)3, in which the central C8H8 ligand is twisted in an unusual fashion and is bound η4 each metal center. Two of the carbon atoms in the ring interact with both metal centers, whereas two other carbon atoms are not within bonding distance of either metal [5]. Perhaps one of the most intriguing types of metal-C8H8 bonding is found in Cr2(C8H8)3 and its Mo and W analogs, in which the shared C8H8 ring adopts a folded “butterfly” conformation beneath the two quadruply-bonded chromium atoms. The metal atoms are each bound to an additional C8H8 ring in η4 fashion [6], [7].

Several years ago, Wilke reported the synthesis of compounds of the type (C8H8)ZrR2 and (C8H8)HfR2 (R = Me, Et), but the compounds were poorly characterized and their reactivity was not explored [8]. Since then, the aryl complex (C8H8)Zr(mesityl)2 has been prepared and shown to react with carbon monoxide to afford acyl products [9]. Carbon-hydrogen bond activation has been reported for zirconium(IV) compounds containing C8H8 ligands: thermolysis of (C8H8)ZrCp*R results in the formation of a bridging cyclooctatrienyne species [10]. Zirconium(IV) alkyls containing derivatized C8H8 ligands have exhibited similar activity [11]. Several other classes of group 4 cyclooctatetraene complexes are known, such as those containing amido, imido, borohydride, pyrazolylborate, and cyclopentadienyl co-ligands [12], [13], [14], [15], [16], [17], [18].

We now describe the syntheses and characterizations of several new cyclooctatetraenyl organozirconium and -hafnium compounds containing a variety of alkyl and aryl ligands, and a preliminary assessment of their reactivity toward small molecules CO and H2.

Section snippets

Results and discussion

In 1972, Lehmkuhl reported that the electrolytic reduction of a mixture of ZrCl4 and C8H8 in tetrahydrofuran affords (C8H8)ZrCl2(thf), but this route is not particularly convenient and few details were provided [19]. In 1973, Wilke reported that (C8H8)ZrCl2(thf) and its Hf analog could be made by a two-step route beginning with the synthesis of (C8H8)2M via treatment of M(OR)4 with 2 equiv. of C8H8 and 4 equiv. of AlEt3 at 90 °C; subsequent treatment of (C8H8)2M with HCl in thf affords (C8H8)MCl

Conclusions

Treatment of the zirconium and hafnium cyclooctatetraenyl starting materials (C8H8)MCl2·2KCl·thf with alkyl- and aryl-lithium reagents affords new organometallic compounds of stoichiometries of [Li(tmed)2][(C8H8)MRCl2], [Li(tmed)x][(C8H8)MR2Cl], or [Li(tmed)x][(C8H8)MR3]. Evidently, with smaller alkyl groups, there is room for three anionic ligands in addition to the η8-C8H8 ligand. With very bulky alkyl groups, however, the neutral 14-electron 7-coordinate compounds (C8H8)M[CH(SiMe3)2]2 can

Experimental section

All operations were carried out under vacuum or under argon using standard Schlenk techniques. Solvents were dried by distillation from sodium/benzophenone immediately before use. ZrCl4 (Cerac), HfCl4 (Cerac), and solutions of halide-free methyllithium (Aldrich) and phenyllithium (Aldrich) were used as received. Cyclooctatetraene (BASF) was purified by distillation. Bis(trimethylsilyl)methyllithium was prepared by a literature route [31]. Infrared spectra were recorded on a Perkin-Elmer 599B

CRediT authorship contribution statement

Melissa J. Nelsen: Investigation, Writing - original draft. Gregory S. Girolami: Conceptualization, Supervision, Project administration, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Dr. Scott Wilson and Ms. Teresa Prussak-Wieckowska for collecting the X-ray crystallographic data, and the National Science Foundation for support of this work, most recently through grant CHE 16-65191.

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