Complexes with Diamino-Substituted Unsaturated C₃ and C₅ Ligands:  First Group 6 Pentatetraenylidenes and New Allenylidene Complexes

Abstract

Sequential reaction of [(CO) 5 M(THF)] (M = Cr ( 1a ), W ( 1b )) with Li[C⋮CC(NMe 2 ) 3 ] and BF 3 ·OEt 2 afforded the first diaminoallenylidene complexes [(CO) 5 M C C C(NMe 2 ) 2 ] (M = Cr ( 2a ), W ( 2b )). These compounds are inert toward nucleophilic attack of HNMe 2 at room temperature. 2b was characterized by an X-ray structural analysis. Treatment of 1 with the cumulogous acetylide Li[C⋮CC⋮CC(NMe 2 ) 3 ] and then with BF 3 ·OEt 2 produced the first group 6 pentatetraenylidene complexes, [(CO) 5 M C C C C C(NMe 2 ) 2 ] (M = Cr ( 3a ), W ( 3b )). The X-ray structural analysis of 3b reveals a strong alternation of the CC bond lengths within the (CO) 5 MC 5 unit. Although the compounds 3 are stable at room temperature, they quantitatively add HNMe 2 within 30 min to give the alkenyl(amino)allenylidene complexes [(CO) 5 M C C C(NMe 2 )C(H) C(NMe 2 ) 2 ] (M = Cr ( 4a ), W ( 4b )). Complex 4b was characterized by an X-ray structural analysis. 4b is the first example of an open chain metal alkenylallenylidene with an s-cis conformation. The complexes 4 are comparable in stability and inertness toward nucleophiles such as amines to the metal diaminoallenylidenes 2 . The spectroscopic data and the X-ray structural analyses of 2b, 3b, and 4b indicate that these heteroatom-substituted compounds are best described as hybrids of metallacumulene and strongly polarized alkynyl structures.