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

Reactions of the tetrairidium anion [Ir₄(CO)₁₁(Ph)]^– (<b>1</b>) with [Cu­(NCMe)₄]­[BF₄] and Ag­[NO₃] have yielded the new iridium–copper and iridium–silver complexes Ir₄(CO)₁₁(μ-η¹-Ph)­[μ₃-Cu­(NCMe)] (<b>2</b>) and [Et₄N]­[{Ir₄(CO)₁₁Ph}₂(μ₄-Ag)] (<b>3</b>), respectively. Compound <b>2</b> consists of a tetrahedral Ir₄ cluster with a Cu­(NCMe) group bridging one of the Ir₃ triangular faces of the cluster and a semibridging η¹-phenyl ligand that is σ–π-coordinated as a bridge across one of the Ir–Cu bonds. The complex anion of <b>3</b> contains two Ir₄(CO)₁₁Ph 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 <b>3</b> reacts with a second equivalent of Ag­[NO₃] to yield the uncharged complex [Ir₄(CO)₁₁]₂(μ₄-Ag)­(μ-Ag)­(μ₃-Ph)­(μ-Ph) (<b>4</b>), which contains two Ir₄(CO)₁₁ clusters linked by a quadruply bridging silver atom and one triply bridging Ph ligand. The second Ag atom in <b>4</b> is an edge bridge on one of the Ir₄ clusters, and the second Ph ligand bridges an Ir–Ag bond to it. When it is dissolved in NCMe, compound <b>4</b> is split in two and adds 1 equiv of NCMe to the Ag atom in each half to form the compound Ir₄(CO)₁₁(η¹-Ph)­[μ₃-Ag­(NCMe)] (<b>5</b>; 73% yield). Unlike <b>2</b>, the phenyl ligand in <b>5</b> is terminally coordinated. The NCMe ligand is coordinated to the Ag atom. When <b>4</b> was treated with PPh₃, the complex Ir₄(CO)₁₁(μ-η¹-Ph)­[μ₃-Ag­(PPh₃)] (<b>6</b>) was obtained in 87% yield. The cluster of <b>6</b> is structurally similar to that of <b>5</b> 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 <b>2</b>. 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 <b>2</b> and <b>4</b> was analyzed by DFT computational methods