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). Th e two dialkylamino compounds Ic(R = NMe2) and Id (R = NEt2) can be obt ained in this way. Oxidation with C2Cl6 provides the ionic cobaltoceni um 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(C5H4BC l2)(C5H4BCl3) (5). Pinacolysis of 5 then affords the monosubstitution product Co[C5H4B(OCMe2)(2)](C5H4BCl3) (9) and the disubstitution produ ct [Co(C5H4B(OCMe2)2)2]Cl[(lh)CU, respectively, depending on stoichiom etry and reaction conditions. Reaction of 5 with tetramethyltin replac es 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)AlC l4] 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 cobaltoc enium derivatives: (i) ionic compounds (type A) such as (1c,d,h)Cl, (I b)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 e quivalent ligands while 9 and 10 display static structures; and (iii) the inverse chelate structure of 11 (type C) which is found in the cry stal and in solution.