C-Mo distances, and the boron atom is displaced out of the ring plane away from Mo by 0.05 A. Thus, complexation causes this distortion of the planar free ligand. Similar distortions are found in the structures of 3c and 4 and indeed in most other complexed boron heterocycles." 6 , ' ' I
In addition, the formal double bonds (C1 -C2, C3-C3a) lengthen on complexation, while the formal single bonds (BI -C1, C2ZC3) remain unchanged. This suggests that the Mo atom removes electron density primarily from the electron-rich regions of the borepin ring which are trans to the CO ligands. The B-C1 unit is thereby eclipsed by one CO ligand. The conformational preferences of similar complexes have been discussed in detail by using an EH-MO model.["] The effect of complexation is to reduce the C-C bond alternation from * 0.058 8, in 1 b to * 0.023 8, in 3 b. Therefore the delocalized n bonding is enhanced in 3 b.
Experimental Procedure
3b: A solution of I b (140mg. 1.1 mmol) in pentane (2mL) was added to a suspension of [(C,H,N),Mo(CO),] (480 mg, 1.1 mmol) in ether (20 mL) at 25 'T Then BF, . OEt, (0.84 mL. 6.8 mmol) was added dropwise with stirring over 2 mins. After 3 h the solvent was removed under vacuum, and the residue was extracted with hexane. Crystallization at -2 0 ' C afforded 220 mg (0.7 mmol) of crude crystals. Recrystallization from hexane gave orange cubes,