Pyrazine-bridged polymetallic copper–iridium clusters

The title molecule is centrosymmetric, with a pyrazine ligand bridging two {Cu10Ir3} cluster units that are arranged in an unusual shape containing 13 vertices, 22 faces, and 32 sides.


Chemical context
Polynuclear metallic clusters, particularly those featuring organic ligands, are highly important as they can appear as intermediates or decomposition products in many transitionmetal-catalysed reactions.Metallic clusters can also exhibit properties between monometallic transition-metal complexes and higher order aggregates and nanoparticles (Tang & Zhao, 2020).Therefore, their synthesis, preparation, and analysis is highly important to advance current understanding on how such species can play a role in catalysis.Metal clusters based on Cu are particularly exciting as a wide range of Cu x X y L z clusters have been reported, where X is typically a halide or hydride, and L is a thioester, phosphine, or N-heterocycle (Harvey & Knorr, 2016;Dhayal et al., 2016;Graham et al., 2000;Liu & Astruc, 2018;Troyano et al., 2021).There are also examples of heterometallic clusters containing Cu atoms mixed with a range of other transition metals such as Re, Fe, Ir, Os, Co, Mo, W, Ag, and Au (Sculfort & Braunstein, 2011;Croizat et al., 2016;Hau et al., 2016;Yip et al., 2007;Gao et al., 2024;Zhang et al., 2023a).These mixed metal clusters provide a unique example to explore metalophilic interactions (Sculfort & Braunstein, 2011) and often have novel spectroscopic properties (Yip et al., 2007;Zhang et al., 2023a) or catalytic activity (Gao et al., 2024;Zhang et al., 2023a), particularly as Cu complexes find many uses in carbon-carbon and carbonheteroatom bond formation.To this end, we were able to grow single crystals of a novel heterometallic cluster compound containing two {Cu 10 Ir 3 } units bridged by a pyrazine ligand, which was examined using X-ray diffraction studies.

Structural commentary
The solvated molecular title compound [({Cu 10 Ir 3 }Cl 4 (IMes) 3 -(pyrazine)) 2 (pyrazine)]•3.18CH 3 OH (where IMes is 1,3-bis-(2,4,6-trimethyl-phenyl)imidazol-2-ylidene) is centrosymmetric and contains two tridecametallic {Cu 10 Ir 3 } clusters, stabilised by four Cl ligands, three N-heterocyclic carbene (IMes) ligands, and a pyrazine ligand, with a bridging pyrazine molecule linking two of these [{Cu 10 Ir 3 }Cl 4 (IMes) 3 (pyrazine)] units (Fig. 1).The {Cu 10 Ir 3 } cores are arranged in a geometry containing 13 vertices, 22 faces, and 32 sides with the atoms arranged in four planes with 2, 4, 4 and 3 metals in each plane (Fig. 2).The majority of the core consists of Cu atoms, with two existing as naked atoms with only interactions to adjacent Cu and Ir atoms.Of the remaining eight Cu sites, four are bonded to Cl ligands that bridge two Cu atoms across different atomic planes within the metallic core.Two of the three Cu atoms in a peripheral plane are bonded to terminal Cl ligands, with the third ligated to a terminal pyrazine molecule.Interestingly, a bridging pyrazine ligand is bonded to a Cu atom in a tetrametallic plane and provides a link to another [{Cu 10 Ir 3 }-Cl 4 (NHC) 3 (pyrazine)] unit, with the whole molecule having a centre of inversion in the middle of the bridging pyrazine ring.
Ir atoms are located in alternate planes with an Ir atom featuring in the peripheral bimetallic plane, and two Ir atoms featuring on opposite sides of the non-adjacent tetrametallic plane.This arrangement is likely a consequence of the bulky carbene ligand attached to Ir.All 18 Cu-Cu distances range from 2.4916 (18) to 3.0417 (18) A ˚.All but three of these distances are shorter than the sum of the van der Waals radii of Cu (2.80A ˚), and most are close to the sum of the Cu atomic radii (2.556A ˚), which suggests strong metalophilic interactions within the cluster (Sculfort & Braunstein, 2011).There appear to be no significant differences between the Cu-Cu and Ir-Cu bond lengths in the structure (2.66 � 0.13 A ˚, n = 18 and 2.62 � 0.07 A ˚, n = 16).

Supramolecular features
The methanol solvent molecules clearly fill voids left by the packing of [({Cu 10 Ir 3 }Cl 4 (IMes) 3 (pyrazine)) 2 (pyrazine)] as the shortest interactions are between methanol and the three terminal CH 3 groups of the IMes ligand.Long-range interactions between the molecules of [({Cu 10 Ir 3 }Cl 4 (IMes) 3 -(pyrazine)) 2 (pyrazine)] involve the non-bridging pyrazine ligands on adjacent molecules, with the shortest 2.327 A interaction between the two H65 atoms, and a 2.483 A ˚interaction between the free pyrazine N4 and the H65 atom of a non-bridging pyrazine ligand on a neighbouring molecule.This suggests that the pyrazine ligand is important in both linking the two tridecametallic cores, and also packing the   crystals together, which is unsurprising given its role in the formation of higher order polymers and metal-organic frameworks (Silva et al., 2023;Zhang et al., 2023b;Kawamura et al., 2017).Long-range interactions between IMes ligands of different molecules are also important with distances of 2.377 A ˚and 2.383 A ˚between pairs of ortho CH 3 and para CH 3 groups on the mesityl rings of adjacent molecules (H19B/ H41C and H20B and H42B).The crystal packing is shown in Fig. 3.The hydroxyl hydrogen atom (H2A) of the partially occupied methanol solvent molecule is hydrogen-bonded to the oxygen atom of the other disordered methanol molecule (Table 1).It should be noted, however, that the hydrogen atom is placed using a riding model as allowing free refinement of its coordinates gave an unfeasible result.The hydroxyl H atoms of the other methanol molecules are likely to be hydrogen-bonded to other highly disordered solvent molecules that were not modelled using the solvent mask (see Refinement).

Database survey
A search of the Cambridge Structure Database (CSD, Version 5.45, update November 2023; Groom et al., 2016) did not reveal any comparable compounds with tridecametallic polymetallic clusters.A few crystal structures for pentatomic Cu-Ir clusters have been reported, but these contain cores with a trigonal-bipyramidal shape with either Cu 3 Ir 2 L x (Rhodes et al., 1985) or Ir 4 CuL x (Adams et al., 2013) arrangements.Reported Cu-Ir distances are between 2.663 and 2.79 A ˚, which are generally longer than those in the cluster presented here [2.5227 (15) to 2.7478 (13) A ˚].The short Cu-Ir distances suggest strong metal-metal interactions, and could indicate Cu Ir bonds (Rhodes et al., 1985).There are many more examples of homometallic Cu clusters in the database, an analysis of 35 of these Cu-Cu bond lengths revealed an average intermetallic distance of 2.95 � 0.25 A ˚(mean � 1 standard deviation), which is consistent with the inter Cu distances in the structure reported here (2.66 � 0.13 A ˚), albeit slightly longer (Johnsson et al., 2000;Rao et al., 1983;Baumgartner et al., 1990).Similar to short Cu-Ir distances, this suggests that the Cu-Cu interactions are also strong and metalophillic.
After being left for 1 h at room temperature the solution was cooled to 278 K in a refrigerator for several weeks to form single crystals, which were found by X-ray diffraction to be the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.All hydrogen atoms were placed using a riding model.The crystal contains disordered methanol solvent molecules.One methanol molecule was modelled over two sets of sites (C70, C72) with a common oxygen site (O1) in a refined ratio of 0.60:0.40(3).Another methanol molecule (C71, O2) was modelled as partially occupied with a refined occupancy of 0.59 (2).There was additional solvent present, but its associated electron density was difficult to model by using discrete atoms.Therefore the SQUEEZE routine (Spek, 2015) in PLATON (Spek, 2020) was used to remove the contribution of the electron density in the corresponding solvent region from the intensity data.A void with a volume of 430 A ˚3 was predicted containing 66 electrons per unit cell.This would be equivalent to 3.67 methanol molecules.The given chemical formula and other crystal data do not take into account the unmodelled methanol solvent molecule(s).The final structure model contains high residual electron density due to unresolved effects of the crystal having a minor twin present.Attempts to model this as two non-merohedral components were unsuccessful.

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.Refinement.The high residual density is due to unresolved effects of the crystal having a minor twin present.Attempts to model this as two non-merohedral components were unsuccessful.The solvent was disordered and modelled as follows: One methanol was modelled in two positions with a common oxygen site in a refined ratio of 0.60:0.40(3).Another methanol was partially occupied with a refined occupancy of 0.59 (2).There was additional solvent which was too disordered to model using discrete atoms.Therefore a solvent mask was used which predicted a void with a volume of 430 cubic angstroms containing 66 electrons per unit cell.This would be equivalent to 3.67 methanols.

Figure 2
Figure 2The tridecametallic core of [(Cu 10 Ir 3 Cl 4 (IMes) 3 (pyrazine)) 2 (pyrazine)]•-3.18CH3 OH, with displacement ellipsoids drawn at the 50% probability level.Note that only the donor atoms of the ligands attached to the polyatomic core are shown.The core is shown in two different orientations, rotated by 90 � around the Ir1, Cu3, Cu4, Ir2, Ir3, Cu8, Cu9, Cu8, Cu9 plane.Atom labels marked in grey correspond to atoms hidden from view.The centrosymmetric compound contains two of these cores linked by a bridging pyrazine and therefore the two tridecametallic units are equivalent by symmetry.

Table 2
Experimental details.