Silylenes with iridium

Recently, the first base-stabilized iridium silylene complexes, IrH2 Si(OTf)Ph2) (TFBXPR3) (TFB = tetrafluorobenzobarrelene), were prepared and characterized following the reaction sequence of Scheme 27. As expected, in the complex with PR3 = P(Pr-z)3, the Ir—Si distance of 2.337(2) A is significantly shorter than those determined previously for the six-coordinate silyliridium(III) complexes (Table 11). The silylene character was also supported by the relatively long Si—O bond of 1.790(5) A compared with the normal range of 1.63-1.66 A186. 

In addition to ruthenium, Tilley and coworkers also reported that cationic iridium silylenoid complexes were efficient olefin hydrosilation catalysts [reaction (7.6)].56 This silylene complex catalyzes the hydrosilation of unhindered mono- or disubsti-tuted olefins with primary silanes to produce secondary silanes with anti Markovni-kov selectivity. Iridium catalyst 32 exhibited reactivity patterns similar to those of ruthenium 30 only primary silanes were allowed as substrates. In contrast to 30, cationic iridium 32 catalyzed the redistribution of silanes. Exposing phenylsilane to 5 mol% of 32 in the absence of olefin produced diphenylsilane, phenylsilane, and silane. 

A careful study of the thermal reactivity of the related [IrMe(H) -/ ,3 /-MesSi(H)(CH2)2PPh2KPMe3)2] 327 with MeOH strongly supports the occurrence of a 1,2-H shift from the silyl silicon atom to the iridium center to generate a hydrido(silylene)iridium(l) intermediate species, and then points to validate mechanism B for these classes of iridium-mediated C-Si bond-forming processes. 

Silylene-based pincer ligands offer exciting reactivities in terms of transition metal complex formation and their applications in catalytic systems. The pincer complex [SiCSi)Ni(II) can be synthesized by oxidative addition of C—H bond of the corresponding [SiC(H)Si] ligand. [SiCSi]Ni(II) complex has been employed as catalyst for Ni-catalyzed Sonogashira reactions (8). Moreover bis(silylene) pincer complexes of iridium and rhodium reveal strong 5-donating ability of divalent silicon and have demonstrated selectivity in catalytic C—H borylation reactions with arenes (9). 

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