Structure and reactivity of dinuclear complexes of iridium

Abstract

Reaction of $\rm Cp\sp{\*}Ir(CO)H\sb2\ (Cp\sp{\*}=\eta\sp5$-C$\rm\sb5Me\sb5)$ with triflic acid (HOSO$\rm\sb2CF\sb3)$ generates hydrogen and the hydride bridged dimer $\rm\lbrack Cp\sp{\*}Ir(CO)H\rbrack\sb2(\mu$-H)$\sp+.$ Variable temperature (VT) $\sp1$H NMR spectroscopy reveals that the bridging and terminal hydride ligands rapidly exchange at room temperature. Deprotonation of the trihydride cation dimer affords $\rm\lbrack Cp\sp{\*}Ir(CO)H\rbrack\sb2.$ The hydride ligands rapidly permute between the iridium atoms at room temperature, as demonstrated by VT $\sp1$H NMR of the isotopically labelled $\rm\lbrack Cp\sp{\*}Ir(\spCO)H\rbrack\sb2.$ VT $\sp1$H NMR of the lower symmetry analogue, $\rm\lbrack Cp\sp\wedge Ir(CO)H\rbrack\sb2\ (Cp\sp\wedge=\eta\sp5$-$\rm C\sb5Me\sb4Et),$ indicates that the chiral iridium centers are configurationally unstable. The dihydride dimers exist as unequal mixtures of two isomers, which rapidly interconvert at room temperature. X-ray structure determinations of $\rm\lbrack Cp\sp{\*}Ir(CO)H\rbrack\sb2$ and $\rm\lbrack Cp\sp{\*}Ir(CO)H\rbrack\sb2(\mu$-H)$\sp+$ indicate that the Ir-Ir bond length increases by 0.198 A upon protonation.The monomers $\rm Cp\sp{\*}Ir(CO)R\sb2\ (R=H,\ Me)$ both exhibit substantial thermal stability, though the dihydride slowly incorporates deuterium from $\rm C\sb6D\sb6$ into its Cp$\sp{\*}$ methyl groups. Thermolysis of $\rm\lbrack Cp\sp{\*}Ir(CO)H\rbrack\sb2$ in the solid state or in methylcyclohexane-$d\sb$ leads to extrusion of H$\sb2$ and exclusive formation of $\rm\lbrack Cp\sp{\*}Ir(\mu$-CO)) $\sb2,$ whereas thermolysis in $\rm C\sb6D\sb6$ or $\rm CD\sb2Cl\sb2$ is complicated by the co-generation of $\rm Cp\sp{\*}Ir(CO)H\sb2.$ Photolysis of $\rm\lbrack Cp\sp{\*}Ir(CO)H\rbrack\sb2$ leads to clean fragmentation of the dimer to form $\rm Cp\sp{\*}Ir(CO)H\sb2$ and $\rm\lbrack Cp\sp{\*}Ir(\mu$-CO)$\rbrack\sb2$ in a 2:1 ratio. Chemical oxidation of both the dihydride monomer and dimer induces proton transfer reactions.Reaction of $\rm\lbrack Cp\sp{\*}Ir(\mu$-CO)) $\sb2$ with one equivalent of HBAr$\rm\sp\prime\sb4\cdot(Et\sb2O)\sb2\ (Ar\sp\prime=3,5$-$\rm(CF\sb3)\sb2C\sb6H\sb3)$ generates $\rm\lbrack Cp\sp{\*}Ir(CO)\rbrack\sb2(\mu$-H)$\sp+.$ The mono-protonated dimer reacts instantaneously at room temperature with a variety of reagents, including H$\sb2$ and CO. Addition of a second equivalent of acid yields the dication $\rm\lbrack Cp\sp{\*}Ir(CO)\rbrack\sb2(\mu$-H)$\sb2\sp{2+}.$The dimethyl dimer, $\rm\lbrack Cp\sp{\*}Ir(\mu$-CO)Me) $\sb2,$ was generated by two different routes, the most successful of which involved reduction with Na/K alloy followed by alkylation with MeCl. Efforts to separate $\rm\lbrack Cp\sp{\*}Ir(\mu$-CO)Me) $\sb2$ from the persistent byproduct, $\rm\lbrack Cp\sp{\*}Ir(\mu$-CO)) $\sb2,$ have found limited success.