Dipolar Sesquifulvalene Compounds with (Tetraaryl‑η4‑cyclobutadiene)(η5- cyclopentadienediyl)cobalt(I) Complex Units as Electron Donors

Mononuclear monohydro sequifulvalene complexes of cobalt and their vinylogous derivatives [(Ar4CBD)­Co­(Cp-Z-C7H7)] (Ar4CBD = C4Ar4; Ar = p-X-C6H4; X = H, Z = – (4); X = H, −C2H2– (9a); X = Cl, Z = −C2H2– (9b); X = Me, Z = −C2H2– (9c); X = OMe, Z = −C2H2– (9d); X = NMe2, Z = −C2H2– (9e); X = H, Z = −(C2H2)2– (10); X = H, Z = −(C2H2)3– (11)) were synthesized and transformed by hydride abstraction to the corresponding dipolar sesquifulvalene complexes of [(Ar4CBD)­Co­(Cp-Z-C7H6)]+ (Ar4CBD = C4Ar4; Ar = p-X-C6H4; X = H, Z = – (5); X = H, Z = −C2H2– (12a); X = Cl, Z = −C2H2– (12b); X = Me, Z = −C2H2– (12c); X = H, Z = −(C2H2)2– (13)) featuring a (tetraaryl-η4-cyclobutadiene)­(η5-cyclopentadienediyl)­cobalt­(I) sandwich complex as an electron donor and the cationic tropylium part as an electron acceptor. Hydride abstraction to form the complexes [(Ar4-η4-CBD)­Co­{(η5-C5H4)­Z­(C7H6)]+ (Ar = p-C6H4X; X = OMe, NMe2; Z = −C2H2−) and [(Ar4-η4-CBD)­Co­{(η5-C5H4)­Z­(C7H6)}]+ (Ar = C6H5; Z = −(C2H2)3– (14)) failed and led to decomposition. The mononuclear archetypal sesquifulvalene complex 5 was obtained by nucleophilic addition of the monolithiated complex (Ar4CBD)­Co­(CpLi) (3) to tropylium cation and subsequent hydride abstraction from the seven-membered ring. The vinylogous derivatives 9a–e, 10, and 11 were prepared by Horner–Wadsworth–Emmons (HWE) reactions of the suitable carbaldehydes (Ar4CBD)­Co­(Cp-Z-CHO) (Z = −(C2H2)n−) with the HWE reagent cyclohepta-1′,3′,5′-trien-1′-ylmethylphosphonate diethyl ester in a phase transfer catalyzed reaction. The donor–acceptor interaction was studied by means of 1H and 13C NMR and UV–vis spectroscopy, cyclic voltammetry, X-ray structure determination, and hyper-Rayleigh scattering (HRS) measurements. For analysis of electronic excitations DFT calculations were performed on the complexes, revealing the cyclobutadiene metal complex fragment to be the donor and the metal complex Cp-bridge-cycloheptatrienylium fragment to be the acceptor.