Complexes with diamino-substituted unsaturated C₃ and C₅ ligands : First Group 6 Pentatetraenylidenes and New Allenylidene Complexes

Abstract

Sequential reaction of [(CO)5M(THF)] (M = Cr (1a), W (1b)) with Li[CCC(NMe2)3] and BF3·OEt2 afforded the first diaminoallenylidene complexes [(CO)5MCCC(NMe2)2] (M = Cr (2a), W (2b)). These compounds are inert toward nucleophilic attack of HNMe2 at room temperature. 2b was characterized by an X-ray structural analysis. Treatment of 1 with the cumulogous acetylide Li[CCCCC(NMe2)3] and then with BF3·OEt2 produced the first group 6 pentatetraenylidene complexes, [(CO)5MCCCCC(NMe2)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)5MC5 unit. Although the compounds 3 are stable at room temperature, they quantitatively add HNMe2 within 30 min to give the alkenyl(amino)allenylidene complexes [(CO)5MCCC(NMe2)C(H)C(NMe2)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.