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

Abstract

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. ■ INTRODUCTIONη 1 -bridging aryl ligands A are commonly found in polynuclear metal complexes of the coinage metals: Cu, Ag, and Au. 1 Aryl copper compounds have been used for a variety of carbon−carbon bond forming cross-coupling reactions. 1a There are relatively few examples of η 1 -bridging aryl ligands in polynuclear transition-metal carbonyl complexes. 2 In most cases, η 1 -bridging aryl ligands bridge two similar metal atoms in a symmetrical fashion and the plane of the ring is approximately perpendicular to the metal−metal bond vector (A). 1,2 Although they are much less common, η 1 -aryl ligands bridging heteronuclear pairs of metal atoms are often coordinated asymmetrically (B). 1a,3 In these unsymmetrical cases, the plane of the aryl ring is usually not perpendicular to the M−M′ bond vector. In analogy to the well-known semibridging behavior of carbonyl ligands, 4 we will, hereafter, refer to these asymmetrical bridging phenyl ligands as semibridging ligands. Viewed without a charge, all η 1 -bridging aryl ligands serve as one-electron donors. η 2 -Bridging aryl ligands C are al...

Reductive Coupling of Bridging Diaryl Ligands in Half-Sandwich Cobalt(II) Dimers: Revisiting Triple-Decker Cobalt(I) Complexes

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