Degradation and modification of metallaboranes

In the reaction between the osmahexaborane nido-(PPh3)2(CO)OsB5H9 (1) and the bidentate phosphines, [1,2-(PPh2)2(CH2)2] and [1,3-(PPh2)2(CH2)3] (abbreviated as dppe and dppp, respectively), two products are observed in each case. The major product, (4a), [2,2,2-(PPh3)2(CO)-nido-2-OsB4H7-3-(BH2·dppe)] (or 4b –BH2·dppp) is an osmapentaborane cluster with a BH2·dppp group attached to the cage at a basal B(3) position and the distal end of the pendent bidentate phosphine donor is uncoordinated. In the other (5a), [2,2-(PPh3)2(CO)-nido-2-OsB4H7-η2-3,2-(BH2·dppe)] (or 5b –BH2·dppp) the uncoordinated phosphine present in 4a (or 4b) has replaced a PPh3 group on the osmium center. In an effort to avoid intramolecular substitution, bidentate phosphines of the type PPh2XPPh2 {X=1,4-C6H4, (arphos), 1,4-CH2C6H4CH2 (dppx), and Fe(C5H4)2 (dppf)}were used to preclude the formation of species 5. However three products were obtained instead of just one. These were [2,2,2-(PPh3)2(CO)-nido-2-OsB4H7-3-BH2·PPh2XPPh2] (6), a linked metallaborane system of the of the type [{2,2,2-(PPh3)2(CO)-nido-2-OsB4H7-3-BH2·PPh2}2X] (7) and a bis-borane adduct of the type [2,2,2-(PPh3)2(CO)-nido-2-OsB4H7-3-BH2·PPh2XPPh2·BH3] (8). This chemistry, which is seen for all the rigid backbone bases studied, is similar to that observed in reactions of phosphine bases with 1 which were described previously and is explained in terms of chemistry previously observed in studies of 1.