Synthesis and Reactivity of Dihaloboryl Complexes

Transition metal dihaloboryl complexes of the types [(5-C5R5)Fe(CO)2BX2] (R = H, Me; X = F, Cl, Br), [(5-C5H4Me)Ru(CO)2BX2] (X = Cl), and [(OC)5MnBX2] (X = Cl, Br) were prepared via salt elimination reactions. Comparison of the CO stretching frequencies indicates a higher degree of Fe-B d-p back-bonding in the dibromoboryl complexes [(5-C5R5)Fe(CO)2BBr2] with respect to their -BCl2 and -BF2 analogues. This finding is constistent with the p-p back-bonding abilities of the halides (F > Cl > Br). Reactions of [(5-C5Me5)Fe(CO)2BX2] (X = Cl, Br) with 4-methylpyridine were performed, leading to the formation of the corresponding Lewis-base adducts and, with base in excess, to the first metal-substituted boronium cation [(5-C5Me5)Fe(CO)2BBr(NC5H4-4-Me)2]+.