Dihydroacepentalene complexes

Cross-conjugated dihydroacepentalene complexes 156 (NR2 = NEt2, piperidino, 3,5-dimethylpiperidino, morpholino) and 159 were prepared by de Meijere and Butenschon [179, 180] by treatment of the ligands with either Fe2(CO)9 or CpCo(H2CCH2)2 in yields up to 70%. When complex 156 was reduced with sodium, the persistent radical anion was detected with spin density on the iron atom as indicated by electron spin resonance (ESR) spectroscopy. Further reduction with sodium afforded the ferrate(-2) 158, which is diamagnetic and could be characterized by NMR investigations (Scheme 10.56). 

Pentalene (135) and acepentalene (137) are antiaromatic cross-conjugated compounds with two or three fulvene subunits. According to theoretical calculations [142-147], 135 and 137 are unstable species [147, 148] but can be ligands in transition metal complexes. The respective anions, the planar 136 [149, 150] and the slightly bowl-shaped 138 [151, 152], can be generated by deprotonation of a dihydropentalene or by treatment of4,7-bis(trimethylstannyl)-4,7-dihydroacepentalene with methyllithium at -60 °C (Scheme 10.48). 

While some complexes of dihydroacepentalene derivatives have been reported some time ago [178-180], metal complexes of the unstable acepentalene (137), although they were the subject of DFT calculations [181], are still elusive. The acepentalene dianion (138) was obtained as its lithium salt in pure form by treatment of triquinacene (147) with BuLi/KOtBu/TMEDA giving 138 as its impure potassium salt, which was trapped with MegSnCl. The l,7-bis(trimethylstaimyl)-4,7-dihydroacepentalene (148) obtained was then treated with methyllithium, affording pure dilithium acepentalenediide (138-2Li+) (Scheme 10.52) [152]. 

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