Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η2-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η2-C8H13)-1,2-μ-(ortho-xylylene)-3,1,2-IrC2B9H9

Abstract

Agostic (C−H···M) complexes [closo-3,3-(σ,η2-C8H13)-1,2-μ-(ortho-xylylene)-3,1,2-IrC2B9H9] (5) and [closo-3,3-(σ,η2-C8H13)-1,2-μ-(ortho-xylylene)-3,1,2-RhC2B9H9] (9), stable in the solid state, have been prepared via the reaction of [M(η4-COD)Cl]2 (M = Ir, Rh) with the K+ salt of the [nido-7,8-μ-(ortho-xylylene)-7,8-C2B9H10]- anion and characterized by a combination of analytical (in the case of 5) and multinuclear NMR data, including a single-crystal X-ray diffraction study of 5. The crystallographic study confirmed the agostic structure of 5 and revealed that the orientation of the σ,η2-cyclooctenyl moiety relative to the carborane ligand is substantially influenced by the specific intramolecular C−H···π interaction between the agostic hydrogen and the π-system of the cage aromatic substituent. In solution, 5 exhibited both “side-to-side” agostic hydrogen migration and reversible interconversion with [closo-3-(η3-C8H13)-1,2-μ-(ortho-xylylene)-3,1,2-IrC2B9H9] (8). The agostic rhodium complex (9), in contrast, converts irreversibly both in the solid state and in solution to its η3-cyclooctenyl isomer [closo-3-(η3-C8H13)-1,2-μ-(ortho-xylylene)-3,1,2-RhC2B9H9] (11), which thus could be obtained as a pure solid. In solution, complex 11 is fluxional and shows an agostic C−H···Rh interaction. The fluxional process involves the exchange between the endo hydrogen atoms, on one hand, and the exo and allyl hydrogens of the C8-ring, on the other hand, confirmed by 2D [1H−1H]-EXSY spectroscopy. Solution structures of the agostic complexes obtained are discussed in detail on the basis of normal and low-temperature 1H and 13C/13C{1H} NMR spectroscopic data.