Intermetallic Communication through Carbon Wires in Heterobinuclear Cationic Allenylidene Complexes of Chromium

The reaction of [(CO)5M(THF)] (M = Cr, W) with lithiated 2-ethynylquinoline followed by alkylation
of the resulting alkynylpentacarbonylmetalate with [R3O]BF4 (R = Me, Et) gives allenylidene complexes
in which the terminal carbon atom of the allenylidene chain is part of an N-alkylated quinoline ring. The
reaction of [(CO)5M(THF)] (M = Cr, W) with lithiated 2-ethynylpyridine derivatives, Li[C⋮CC5H4BrN], and [Et3O]BF4 affords allenylidene complexes that contain a terminal six-membered N-heterocycle
brominated at the 5- or 6-position. Various alkynyl groups can be introduced into the 5-position of the
ring through [PdCl2(PPh3)2]-catalyzed coupling of the 5-bromo-substituted allenylidene complexes with
the terminal alkynes HC⋮CR‘ (R‘ = TMS, Ph, C10H21, 4-C6H4-C⋮CPh, 4-C6H4-C⋮CH, Fc (Fc = (C5H4)FeCp), 4-C6H4-C⋮CFc, 4-C6H4-C⋮CC6H4C⋮CFc). The analogous replacement reaction of the 6-bromo-substituted chromium complex with HC⋮CFc yields the corresponding 6-ferrocenylalkynyl-substituted
complex. Desilylation of (6a) gives (15a). CuI-catalyzed coupling of 15a with {M}−Br ({M} = Ru(CO)2Cp, Fe(CO)2Cp*) affords the binuclear complexes The
symmetrical binuclear complex is formed by oxidative coupling of 15a with [Cu(OAc)2]. The attachment
of a ferrocenyl group to the chromium center via PPh2 to give
is achieved via displacement of a cis-CO ligand in by PPh2Fc. On addition
of Co2(CO)8 to a Co2(CO)6 unit adds to the C⋮C bond to
form a trinuclear complex. The ferrocenyl unit in (R = Fc,
C6H4C⋮CFc, C6H4C⋮CC6H4C⋮CFc) is readily oxidized. Spectroelectrochemical studies (IR, UV/vis)
confirm that in the oxidized form there is strong electronic communication of the ferrocenyl group with
the (CO)5Cr unit.