BORABENZENE DERIVATIVES .20. (BORATABENZENE)(HEXAMETHYLBENZENE)IRON COMPLEXES - SYNTHESIS, STRUCTURE, AND REACTIVITY

The 19e complex CpFe(HMB) (HMB = hexamethylbenzene) (1a) reacts with dihaloboranes RBX2 (R = Me, Ph, NEt2; X = Cl, Br) to give paramagnetic 19e (boratabenzene)-(hexamethylbenzene) iron complexes (C5H5BR)Fe(HMB) (R = Me, Ph, NEt2) (2a-c). Oxidation with [FeCp2]PF6 affords the corresponding diamagnetic salts [(C5H5BR)Fe(HMB)]PF6 (2a-c.PF6), while oxidation of the diethylamino derivative 2c with FeCl3 in Et2O/H2O produces the hydroxo compound [(C5H5BOH)Fe(HMB)]PF6 (2d.PF6). The structures of both the neutral methyl compound 2a and the corresponding salt 2a.PF6 have been determined by X-ray crystallography. A comparison of the two structures demonstrates the labilizing effect of the excess electron. Both structures show distortions of the ring ligands which can be explained by qualitative MO considerations. The arene ligand possesses a boat conformation in 2a and an inverted boat conformation in 2a+. Cyclovoltammetric investigation of the cations [(C5H5BR)Fe(HMB)]+ (2a-c+) shows a reversible (+/0) and a quasireversible (0/-) reduction wave; compared to CpFe(HMB) these waves are shifted anodically. The phenyl derivative 2b reacts with MeI to form the iodide 2b.I as well as diamagnetic methyl addition products (C5H5BPh)-Fe(C6Me7) (6a) and three isomers (C5H5MeBPh)Fe(HMB) (3a-5a) with the methyl group added to the borabenzene ring. The cation 2b+ undergoes hydride addition with NaBH4 to produce (C5H5BPh)Fe(1,2,3,4,5,6-endo-C6HMe6) (6b) and two isomers (C5H6BPh)Fe(HMB) (3b, 4b) with bora-2,4- and bora-2,5-cyclohexadiene ligands.