Pendant borane complexes phosphines coordination

The rhenium complex 76 related to 74b was also prepared recently by Labinger and Bercaw using another synthetic strategy. 2 In this case, the pendant borane moieties were introduced by hydroboration of unsaturated phosphines in the coordination sphere of the metal. The cationic rhenium complex 75 featuring two diphenyl(vinyl)phosphines was readily converted into the corresponding bis(phosphine-borane) complex 76 (Scheme 45). The coordination mode of 76 was substantiated spectroscopically (5 nB = 87.7 ppm) and crystallographically. 

More recently, complexes featuring pendant Lewis acids were prepared from C -bridged phosphine-boranes (Figure 6). As mentioned above, the orf/io-phenylene linker usually favors the participation of the Lewis acid in coordination, but in the Pd complex 13, this effect is counterbalanced by the bulky Mes which prevents interaction of the boron center with the metal fragment. Despite smaller substituents at boron, the Ru complex 14 adopts the same coordination mode. = The flexibility of the CH CH linker probably comes into play here (the phosphine-borane ligand adopts antiperiplanar conformation). 

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