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

Reactions
of the tetrairidium anion [Ir4(CO)11(Ph)]– (1) 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)] (2) and [Et4N]­[{Ir4(CO)11Ph}2(μ4-Ag)] (3), respectively. Compound 2 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 3 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 3 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) (4), 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 4 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 4 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)] (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 PPh3, the complex Ir4(CO)11(μ-η1-Ph)­[μ3-Ag­(PPh3)] (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.