Pendant borane complexes

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. 

Hereafter are described the few complexes featuring pendant borane moeities that have been obtained as rearranged- or side-products starting from phosphine-borates. 

The structure of 73 was substantiated spectroscopically (5 nB = 82.5 ppm) and confirmed crystallographically. A centrosymmetric chloro-bridged dimeric structure with tricoordinate copper centers was observed. The pendant character of the borane is apparent from the rather long the B- Cu (3.05 A) and B- Cl (4.06 A) distances associated with a trigonal planar geometry around boron. Compared to related mononuclear palladium and rhodium complexes (see Section IV.B), the presence of the chloro-bridge disfavors the formation of Cl-B interactions (coordination mode F) in 73. 

According to X-ray diffraction data, in the solid-state complex 87 adopts a six-membered metallacyclic structure, as the result of P-Pd-Cl-B bridging coordination. The interaction of the borane moiety with the chlorine atom is apparent from the short Cl-B distance (2.165(2) A) and the pyramidalization of the boron environment (EBa = 349.1°). The bridging coordination of the Pd-Cl bond by the PB 7b contrasts with the B-pendant coordination mode adopted by complex 72, deriving from the related ligand 7e featuring mesityl groups at boron (see Section IV.A). This illustrates the role of steric factors in the participation, or not, of the Lewis acid in the coordination assembly. 

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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