Synthesis, spectra and crystal structure of (E)-(CO)2(NO)Cr[(η5-C5H4)-CH=CH(η5-C5H4)]Cr(CO)2(NO)

Compounds 1,2-bis[(eta(5)-cyclopentadienyl)dicarbonylnitrosylchromium]ethene (7) thereafter called 1,2-dicynichrodenylethene and 1,2-diferrocenylethene (10) were prepared from formylcynichrodene (3) and formylferrocene (9), respectively, via the McMurry's low-valent titanium coupling method. Compounds (eta(5)-vinylcyclopentadienyl)dicarbonylnitrosylchromium (6) and vinylferrocene (11) were obtained by the dehydration of the corresponding alcohols. The structure of 7 was solved by an X-ray diffraction study: space group, P2(1)/c; monoclinic; a = 6.379(5), b = 11.295(3) and c = 11.9352(24); Z = 2. It turns out that compound 7 adopts a transoid conformation at the ethenylene bridge and the two cyclopentadienyl rings are coplanar. The nitrosyl group in each cynichrodenyl moiety of 7 is located at the side towards the corresponding ethenylene carbon atom with a twist angle of 46.5 degrees. The chemical shifts of H(2)-H(5) protons and C(2)-C(5) carbon atoms of a series of vinyl derivatives of compounds bearing cyclopentadienyl rings have been assigned using two-dimensional HetCOR-NMR spectroscopy. For the derivatives of cynichrodene (1) and ferrocene, it was found that the shielding of C(2,5) and C(3,4) carbon atoms is parallel to the shielding of the ortho- and papa-carbon atoms of benzene derivatives. The electron density distribution in the cyclopentadienyl ring is discussed on the basis of C-13-NMR data. Surprisingly, the vinyl group donates electron density to the adjacent cynichrodene moieties rather than withdraws from them. (C) 1999 Elsevier Science S.A. All rights reserved.