Comparison of the solid-state structures of [UCp4] and [ZrCp4] provides a textbook example of the effect of metal ion size on coordination geometries.1,2 While both metals are in the +4 oxidation state, U is significantly larger than Zr with covalent radii of 1.96(7) and 1.75(7) Å respectively.3 As a result, although U4+ is large enough for all the Cp rings to bind eta-5, zirconium is not, and one ring slips to the eta-1 coordination mode. During our work to develop new methods to break carbon–oxygen bonds in ethers we applied an indenyl (Ind = C9H7) analogue of a zirconocene complex as a catalyst. [Zr(η5-Ind)2Cl2] was used in combination with an aluminium reagent for the C–O bond functionalization of benzofuran.4 Here we present the crystal structure of [Zr(η5-Ind)2(η1-Ind)2Cl] (1), which was serendipitously isolated during preparations of [Zr(η5-Ind)2Cl2] (Figure 1).
Synthesis of 1 has previously been reported by Alt and co-workers,5 but crystallographic analysis was not reported. "Sterically-overloaded" 1 and its Hf analogue (1-Hf) were characterised by 1H NMR spectroscopy, showing η1-η5 fluxionality of the indenyl ligand in solution. 1-Hf has been characterised by single crystal diffraction experiment but the quality of the data only allowed conclusion on the general topology. This connectivity is analogous to our structure for 1. Quality crystal growth has also been difficult with the smaller Cp analogue.6,7 However, X-ray data has been reported once, in the PhD thesis of Strittmatter,8 whose results show a [Zr(η5-Cp)3Cl] structure. Thus, we infer from comparison with 1 that the Zr centre is sensitive to the steric demands of the L2X-type ligand in these complexes.
Selected bond lengths for 1 are presented above and as primary data through the CCDC (1541881). The hydrogen-atom attached to C1 was found in the Fourier difference density map. Just as with [MCp4] (M = U, Zr) it appears the Zr centre is too crowded to accommodate all three η5-ligands, and one indenyl group slips to an η1 configuration. The crystal structure of 1-U reiterates this point: the U4+ centre is able to bear three indenyl groups as η5-ligands, in contrast to 1.9 However, changing the X-type ligand on M4+ from a chloride to a hydride results in a different molecular structure (2), as shown by Takahashi and colleagues.10 An effect of this kind for Cp3MX complexes (M = Zr, Hf) was predicted computationally by Bursten & Palmer in 2006,11 based upon π-donor ability of the X-type ligand. However, the study considered only η1 or η5 coordination isomers: Takahashi and co-workers' report of 2 found all three indenyl ligands as η3-coordinated.10 The results of the X-ray structures for 1, 1-U and 2 show that both metal centre and ancillary ligands have a telling effect on the hapticity of the indenyl moiety. Crystal packing effects may also be in play.
In the crystal structure of 1, a short Zr–C1 distance of 2.363(2) Å is seen. The hitherto shortest (η1-Cp) bond length - 2.396(5) Å in [Zr(η1-CpR)(η5-CpR)2Cl] (3: R = SiMe3).12,13 In other the Zr–C(η1) containing complexes of these ligands the distances are in excess of 2.447 Å.13,14
Finally, we can compare complexes 1-3 to the interesting case of 4 (Figure 2), which contains Ind in an unusual η2 configuration.15 The latter complex bears a mixed Cp/Ind ligand system, created by Green and co-workers by attaching an indenyl to a cyclopentadienyl moiety via a C1 pendant arm.16 The bidentate ligand is dianionic, but ligation through the indenyl fragment appears to be a frustrated compromise between η1- and η2 - the Zr–C bond distances being much longer than those in 1-3. In summary, analysis of the structure of 1 in the context of similar structures has borne testament to the known effects that the metal identity, ligand sterics and electronics can have on the hapticity of L2X ligands like Cp and Ind.
Along with our studies on heterobimetallic complexes derived from magnesium, zinc and aluminium hydrides we have also been interested in calcium. A few molecular calcium hydrides are known,1 but the isolation of a terminal calcium hydride still remains a goal. The beta-diketiminate calcium hydride P is dimeric in the solid state and synthesized from reaction of the corresponding amide A and PhSiH3 (B). A side-reaction occurs though and along with the formation of P and the silazane, C, both SiH4 (D) and Ph2SiH2 (E) are observed. The fragility of PhSiH3 to redistribution is well established,2 and it appears that the calcium reagent promotes this reaction.
Figure 1. (top) 1H NMR data for the reaction of A and B in C6D6. (bottom) 1H-29Si HMQC NMR data at the same time point.
The side-reaction does not interfer with the synthesis though. The procedure reported by Harder and coworkers works well. Along with Mike Hill's group, we have found that the product could be crystallised directly from toluene as large colourless blocks by using initial higher concentrations, performing the reaction at 25 oC in toluene with an excess of PhSiH3 .
Reaction carried out under dinitrogen or argon atmosphere. Solvents were dried with an SPS system (Grubbs design) followed by storage over 3 Angstrom molecular sieves, A was prepared according to the literature,3 phenylsilane was purchased from Sigma-Aldrich and used as received. All glassware was dried at 120 oC for 24 h prior to use.
In a glovebox, A (0.8 g, 1.16 mmol) was transferred to a small ampoule and dissolved in toluene (5 mL, 0.2 - 0.25 M solution). Phenyl silane (432 microL, 3.5 mmol, 3 equiv.) was added by micropipette. The reaction mixture was agitated to ensure thorough mixing of the reagents and then left to stand at 25 oC for 24 h. Large colourless crystalline blocks formed over this period and were isolated by cannula filtration, then washed with n-hexane (2 x 5 mL) to yield P (0.52g, 0.49 mmol, 85 %). Data match that reported in the literature.1
Figure 2. Crystals of P in toluene.
Over the past few years there has been increasing interest in reactions in which gases are potential reaction intermediates. For example, in hydrogen-shuttling catalysis1 H2 is released from a substrate and following further reactions is ultimately consumed to produce the product(s).
Though there is the potential for the entire process to occur within the coordination sphere of a catalyst, release and consumption of dihydrogen gas into solution is a likely pathway. Despite the limited solubility of dihydrogen in many common solvents, the headspace of the glassware is not something that immediately comes to mind as an important variable in developing hydrogen-shuttling methods.
While studying hydrogen-shuttling catalysis we became concerned about the impact of the volume of the head space of our glassware used for our reactions. We hypothesised that for reactions carried out in sealed systems that H2 may be occupying the head space volume of the vessel, and that this may be effecting the efficiency of catalysis.
To control the head space of the vessel we used a tube within a tube (thanks to Bob Bergman for suggesting the design). By inserting a solid tapered borosilicate glass rod into a J Youngs NMR we could carry out reactions in a sealed system with very limited head space and effectively control this variable during catalytic screening.
The insert differs slightly from other coaxial inserts, such as those supplied by Sigma-Aldrich often used to include an internal standard,2 or the elegant system proposed for in situ mixing of reagents,3 in that it is solid and displaces a large volume of the J Youngs tube allowing us to limit the headspace volume.
Warning Note:The NMR tubes become quite heavy with the insert so if you intend to try this out get friendly with your NMR research officer before submitting the samples as there is a chance that they may not eject as easily as normal tubes from the instrument.