Semibridging Phenyl Ligands in Iridium–Copper and Iridium–Silver Cluster Compounds: Synthesis, Structures, and Bonding

Reactions of the tetrairidium anion [Ir 4 (CO) 11 (Ph)] – ( 1 ) with [Cu(NCMe) 4 ][BF 4 ] and Ag[NO 3 ] have yielded the new iridium–copper and iridium–silver complexes Ir 4 (CO) 11 (μ-η 1 -Ph)[μ 3 -Cu(NCMe)] ( 2 ) and [Et 4 N][{Ir 4 (CO) 11 Ph} 2 (μ 4 -Ag)] ( 3 ), respectively. Compound 2 consists of a tetrahedral Ir 4 cluster with a Cu(NCMe) group bridging one of the Ir 3 triangular faces of the cluster and a semibridging η 1 -phenyl ligand that is σ–π-coordinated as a bridge across one of the Ir–Cu bonds. The complex anion of 3 contains two Ir 4 (CO) 11 Ph anions linked by a single quadruply bridging silver atom that has adopted a bow-tie geometry between the four iridium atoms. It contains two terminally coordinated σ-phenyl ligands. Compound 3 reacts with a second equivalent of Ag[NO 3 ] to yield the uncharged complex [Ir 4 (CO) 11 ] 2 (μ 4 -Ag)(μ-Ag)(μ 3 -Ph)(μ-Ph) ( 4 ), which contains two Ir 4 (CO) 11 clusters linked by a quadruply bridging silver atom and one triply bridging Ph ligand. The second Ag atom in 4 is an edge bridge on one of the Ir 4 clusters, and the second Ph ligand bridges an Ir–Ag bond to it. When it is dissolved in NCMe, compound 4 is split in two and adds 1 equiv of NCMe to the Ag atom in each half to form the compound Ir 4 (CO) 11 (η 1 -Ph)[μ 3 -Ag(NCMe)] ( 5; 73% yield). Unlike 2, the phenyl ligand in 5 is terminally coordinated. The NCMe ligand is coordinated to the Ag atom. When 4 was treated with PPh 3, the complex Ir 4 (CO) 11 (μ-η 1 -Ph)[μ 3 -Ag(PPh 3 )] ( 6 ) was obtained in 87% yield. The cluster of 6 is structurally similar to that of 5 except that the phenyl ligand has adopted a semibridging coordination to the silver atom, similar to that found between the phenyl ligand and the copper atom in 2 . All of the new products were characterized by single-crystal X-ray diffraction analyses. The bonding of the bridging phenyl ligands to the clusters in 2 and 4 was analyzed by DFT computational methods.