Crystal structure of [Th3(Cp*)3(O)(OH)3]2Cl2(N3)6: a discrete molecular capsule built from multinuclear organothorium cluster cations

The reported molecule contains a number of unusual features, the most notable being a finite yet exceptionally long cyclic metal-azido chain. These rare features are the consequence of both sterically protecting Cp* ligands and highly bridging oxide and hydroxide ligands in the same system and illustrate the interesting new possibilities that can arise from combining organometallic and solvothermal f-block element chemistry.


Chemical context
Pentamethylcyclopentadienyl (Cp*) ligands have become almost ubiquitous in the organometallic chemistry of the fblock elements (Evans & Davis, 2002). These ligands protect the reactive metal center and allow the solubilization and recrystallization of metal complexes in non-coordinating organic solvents. The actinides in particular have unique chemical properties owing to the participation of f-orbital electrons in chemical bonding (Neidig et al., 2013) and the breakdown of periodic trends in the elements due to relativistic electron motion (Cary et al., 2015), making organoactinide chemistry an important frontier in fundamental chemistry. However, the general instability of f-element organometallic complexes towards air, moisture, and protic solvents has prevented them from being applied in other areas where f-elements have been successfully applied, such as the formation of unique extended structures driven by their unusual coordination polyhedra (Burns & Nyman, 2018, Li et al., 2017Rocha et al., 2011).
Compared to uranium and the lanthanides, the coordination chemistry of thorium has been surprisingly under-investigated. Of the 55,423 entries in the Cambridge Structural Database [Version 2020.3.0 (November 2020); Groom et al., 2016] containing an f-element, only 1,241 contain thorium, and over two-thirds of these have only been reported since 2010. The increased number of Th-containing structures coincides with a renewed interest in actinide chemistry in general, and these ISSN 2056-9890 studies have revealed interesting structural features unique to Th-containing compounds such as a strong tendency of Th 4+ to form high-nuclearity yet discrete molecular complexes and ions (Wilson et al., 2007;Knope et al., 2011;Wacker et al., 2019.) The title compound of this study was isolated during research using {Th(Cp*) 2 }-based complexes to study novel organic transformations (Tarlton et al., 2020;Tarlton, Fajen et al., 2021) and actinide-main-group bonding involving Th 4+ (Tarlton, Yang et al. 2021;Rungthanaphatsophon et al., 2018;Vilanova et al., 2017). It represents an unprecedented case of overlap between organothorium chemistry and the formation of polynuclear oxo-bridged clusters. Spontaneous cluster formation in other, oxygen-free complexes of Th 4+ with tetrel group elements is explored in a second publication as part of this joint special issue .

Structural commentary
The molecule is a charge-neutral polynuclear metal complex of unusual size and complexity. It can be conceived as being built from two polyatomic cations with the formula [Th 3 (Cp*) 3 (O)(OH) 3 ] 4+ , each of which is sandwiched between two terminal Cl À ions at either end of the molecule and a ring of 6 N 3 À anions in the center (Fig. 1). The structure crystallizes in the monoclinic space group C2/m with Z = 2. The molecule resides on a crystallographic mirror plane perpendicular to b, a crystallographic twofold proper axis parallel to b, and the inversion center where the axis and plane coincide, giving the entire molecule exact C 2h symmetry. All Th 4+ centers in the structure are chemically equivalent, and the {[Th 3 (Cp*) 3 (O)(OH) 3 ]Cl} units have approximate C 3v symmetry. The symmetry of the overall molecule is lowered by the arrangements of the N 3 À ions, which tilt to differing degrees relative to the twofold axis.
There are no published structures containing a moiety exactly analogous to the [Th 3 (Cp*) 3 (O)(OH) 3 ] 4+ cluster, and published Th-O distances vary extremely widely due to the highly variable coordination geometry of Th 4+ . Another neutral hexanuclear Th 4+ complex with the formula Th 6 (O) 4 (OH) 4 (CHO 2 ) 12 (OH 2 ) 12 has been reported and shows comparable Th-O 2À distances, although the OH À ligands in this structure bridge three metal centers and have significantly longer bond distances (Takao et al., 2009). A polyatomic anion with the formula [Th 3 Cl 10 (OH) 5 (OH 2 ) 2 ] 3À is reported, which has the same six-membered cycle of Th 4+ and 2 -OH À ligands (but no O 2À ligands), and the Th-O distances for these ligands are very similar to those in [Th 3 (Cp*) 3 (O)(OH) 3 ] 4+ (Wacker et al., 2019).
The six Th 4+ atoms and six azido ligands are bridged into what is essentially a linear chain that has cyclized to form a 24-membered ring, which is the longest non-polymeric metal-azido chain reported. However, cycles with three or four repeat units are quite common, and one example is known for Th 4+ and has Th-N and azide N-N distances that overlap with those in this structure (Du et al. 2019). It is clear that while most of the individual building blocks within this structure have been observed previously, the unusual features arise from the termination of growth of a Th 4+ cluster ion by Cp* ligands, leading to very intricate interconnectivity between the metal centers.

Supramolecular features
The molecules pack through a herringbone arrangement of the Cp* ligands so that the methyl groups of one Cp* point towards the aromatic ring plane of the neighboring molecules, leading to infinite two-dimensional layers parallel to the ab face (Fig. 2, left). These layers stack along c such that the molecules in each layer reside over holes in the neighboring layer, analogous to cubic close packing of spheres (Fig. 2, right); this arrangement most likely reduces repulsion between the like-charged anionic groups at either end of the molecule. For each molecule of the main moiety there are two solvent molecules of crystallization, which are located in the holes in each layer on the crystallographic mirror planes. These solvent molecules were found to be either tetrahydrofuran (THF) or diethyl ether (Et 2 O) and are substitutionally disordered across the same site; their relative occupancies refined to 72%:28% THF:Et 2 O. Both molecules are positioned such that the ether oxygen atom accepts a hydrogen bond from one of the bridging OH À ions (Table 1, Fig. 1).

Synthesis and crystallization
The title compound was the byproduct of the reaction of (C 5 Me 5 ) 2 Th(CH 3 )[P(Mes)(SiMe 3 )], Mes = 2,4,6-Me 3 C 6 H 2 , (Rungthanaphatsophon et al., 2018) with two equivalents of Me 3 SiN 3 in dimethoxyethane (DME) at room temperature. After stirring overnight, the resulting solution was allowed to crystallize inside an N 2 -filled glove box at ambient temperature ($3 days). Crystals suitable for SCXRD were obtained by recrystallization of these solids from diethyl ether/tetrahydrofuran. The chloride is presumably due to the starting material, (C 5 Me 5 ) 2 Th(CH 3 )(Cl), which is used to make (C 5 Me 5 ) 2 Th(CH 3 )[P(Mes)(SiMe 3 )], while the oxo-and hydroxide ligands are due to an adventitious source of oxygen present in the solvent or glove box.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal structure was solved by an iterative dual space approach as implemented in SHELXT (Sheldrick, 2015a). All atoms could be refined anisotropically. The residual difference map contained large, chemically nonreasonable peaks near the Th atoms which could not be modeled but could be reduced by truncating some of the highangle data during reduction. The disordered THF and Et 2 O molecules were located from the difference map. For the THF molecule, the oxygen atom and carbon atom C1S had their y coordinates fixed to reside on the crystallographic mirror plane; the other atoms were refined as additionally disordered across both positions related by the mirror plane. All nonhydrogen atoms of the Et 2 O molecule had their y coordinates fixed to lie on the mirror plane. The chemical occupancy of the THF molecule was fixed to a free variable, which refined to Table 1 Hydrogen-bond geometry (Å , ). (10)  72 (1)%, and the chemical occupancies of the THF and Et 2 O molecules were constrained to sum to 100%. The fractional occupancies of all atoms in both solvent molecules were set to 50% of the chemical occupancies due to their residence on or disorder across a crystallographic mirror plane. Both solvent molecules were also refined with C-C distances restrained to 1.54 (2)  Jerry P. Jasinski tribute Data collection: APEX3 (Bruker, 2017); cell refinement: APEX3 and SAINT (Bruker, 2017); data reduction: APEX3 and SAINT (Bruker, 2017); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.