Bis(boryl)metallocenes.: 2.: Syntheses of 1,1′-bis(boryl)cobaltocenium complexes

Bis(boryl)cobaltocenes Co(C5H4BR2)(2) (1) can be made from CoBr2(DME) and alkali metal borylcyclopentadienides M(C5H4BR2)(M= Li, Na) (2). The two dialkylamino compounds Ic(R = NMe2) and Id (R = NEt2) can be obtained in this way. Oxidation with C2Cl6 provides the ionic cobaltocenium chlorides (Ic)Cl and(ld)Cl. Further cobaltocenium compounds can be synthesized by modification of the substituents at boron. Treatment of(ld)Cl with excess BCl3 affords the highly reactive chloride Co(C5H4BCl2)(C5H4BCl3) (5). Pinacolysis of 5 then affords the monosubstitution product Co[C5H4B(OCMe2)(2)](C5H4BCl3) (9) and the disubstitution product [Co(C5H4B(OCMe2)2)2]Cl[(lh)CU, respectively, depending on stoichiometry and reaction conditions. Reaction of 5 with tetramethyltin replaces two chlorine atoms with methyl groups to give Co(C5H4BMe2)(C(5)H(4)BCk(3)) (10), while the more reactive trimethylaluminum replaces four chlorine substituents to give the salt [Co(C5H4BMe2)(2)]AlCl4 [(1b)AlCl4] and, after metathesis with NBu4PF6 in CH2Cl2, the more convenient hexafluorophosphate (Ib)PF6. The corresponding cobaltocene Ib is then accessible via conventional amalgam reduction of(1b)AlCl4. Reaction of with commercial AsF3 affords the robust inverse chelate Co(C5H4BF2)(2)(mu-OH) (11). Three structural types are encountered for the cobaltocenium derivatives: (i) ionic compounds (type A) such as (1c,d,h)Cl, (Ib)AlCl4, and (Ib)PF6; (ii) zwitterionic or semiquaternized compounds (type B) with one trigonaI and one tetrahedral boron center such as 5, 9, and 10; of these, 5 is fluxional in solution with two effectively equivalent ligands while 9 and 10 display static structures; and (iii) the inverse chelate structure of 11 (type C) which is found in the crystal and in solution.