Borylcyclopentadienides

The cyclopentadienylboranes C5H5BR2 1−4 (R = NMe2, 1/2(OCMe2)2, Pri, Me) are synthesized from alkali metal cyclopentadienides MCp (M = Li, Na) and BCl(NMe2)2, ClB (OCMe2)2, BClPri2, and BBrMe2, respectively. Compound 1 reacts with lithium dialkylamides (LiNMe2, LDA, LiTMP) in THF or with elemental sodium to give the salts M[C5H5B(NMe2)2] [M(5) with M = Li, Na]. Compounds 2 and 3 are metalated by LiTMP, and compounds 3 and 4 by cyclopentadienides MCp (M = Li, Na) to give borylcyclopentadienides or boratafulvenes M[C5H5BR2] M(6)−M(8) (R = 1/2(OCMe2)2, Pri, Me). Estimated (NMR) room temperature equilibrium constants for the systems 1−4/LiCp in THF are 0.5, 4, >300, and >300, respectively; they show that dialkylboryl substituents stabilize cyclopentadienide ions markedly. The triclinic THF solvate Na(THF)(5) (≡10) forms an approximately trigonal helix with alternating Na(THF)+ and anion units, while the tetragonal tetrahydropyran solvate Na(THP)(5) (≡11) forms a tetragonal helix. The monoclinic solvate Li(12-crown-4)(6) (≡12) is molecular, and the orthorhombic Na(THP)2(8) (≡13) displays a chain structure. The B−C1 distances [154.5(2) for 11, 148.8(8) for 12, and 150.8(4) pm for 13], lengthened C1−C2/C1−C5 distances (av) (142.5 for 11, 142.1 for 12, and 143.6 pm for 13), slightly lengthened C3−C4 distances (140.0 for 11, 141.1 for 12, and 143.5 pm for 13), and shortened C2−C3/C4−C5 distances (av) (140.3 for 11, 137.3 for 12, and 137.9 pm for 13) are caused by π interactions (11 < 12, 13) between the C5 ring and the boryl function. These interactions are weak in the case of the B(NMe2)2 group and quite marked for the B(OCMe2)2 and BMe2 groups.