Binuclear metal carbonyl DAB complexes X. Activation of h2-C=N coordinated DAB ligands towards C-C bond formation with alkynes. The X-ray structure of {2-phenyl-3-(tert-butylamino)-4-(tertbutyl-imino)-1-butene-1-yl}Ru2(CO)5. Application to the catalytic cyclotrimerization of alkynes

Abstract

Ru,(CO),(DAB) (DAB = 1,4-diazabutadiene) complexes react with alkynes forming RU,(CO)~(AIB) complexes (AIB = 3-amino-4-imino-1-buten-1-ylI)n. these products the DAB ligand and the alkyne are coupled via a C-C bond. The molecular structure of these complexes has been determined by a single-crystal X-ray study. Cell data are a = 12.41 1 (2) A, b = 16.137 (3) A, c = 12.487 (2) A, and p = 92.78 (2)’; the space group is P2’/n, and Z = 4. The refinement converged to R = 4.4%. The molecular structure of [2-phenyl-3-(tert-butylamino)-4-(tert-butylimino)-l-buten-l-yl]diruthenium pentacarbonyl established a C< single bond of 1.546 (10) b; between the original DAB and the phenylacetylene fragments. The RU,(CO)~u nit contains four terminal and one bridging CO groups. The Ru-Ru bond distance is 2.711 (1) b;, which is in accordance with a single Ru-Ru bond. The coordination around the ruthenium atoms is pseudooctahedral, disregarding the metal-metal interaction. Taking into account the Ru-Ru bond, we find that both ruthenium atoms are seven-coordinated. Ru2(CO),(AIB) complexes react with alkynes forming Ru2(CO),(AIB)(alkyne) and subsequently Ru2(CO),(AIB)(alkyne) complexes. According to the spectroscopic data, the alkyne in the former complex acts as a 2e-donor ligand and in the latter complex as a 4e-donor ligand, bridging the Ru2(C0), unit. Ru2(CO),(DAB), Ru,(CO),(AIB), Ru,(CO),(AIB)(alkyne), and R~~(C0)~(AIB)(alkyncaen) all be used as a catalyst for the cyclotrimerization of acetylenes. The reaction has to be carried out at 110 OC and is terminated after a maximum of 380 cycles. The catalytic conversions have been monitored by high-pressure liquid chromatography, and it could be shown that the actual catalyst is formed out of Ru,(CO),(AIB)(alkyne). Experiments with deuterated alkynes in the complexes evidenced that these coordinated alkynes are not involved in the (substituted) benzene formation. The cyclotrimerization of methyl acetylenecarboxylate results in the formation of 1,3,5-tris(methoxycarbonyl)benzene exclusively. This regioselectivity is an uncommon feature in catalytic alkyne trimerization.