Synthesis of heterocyclic carbene ligands via 1,2,3-diheterocyclization of allenylidene complexes with dinucleophiles Normen Szesni, Christiane Hohberger, Gehad Genidy Mohamed 1 , Nicolai Burzlaff 2 , Bernhard Weibert, Helmut Fischer * Fachbereich Chemie, Universita ¨ t Konstanz, Fach M727, D-78457 Konstanz, Germany Received 8 June 2006; received in revised form 18 July 2006; accepted 25 July 2006 Available online 19 September 2006 Dedicated to Professor Bogdan Marciniec on the occasion of his 65th birthday. Abstract Heterocyclic carbene complexes are accessible from p-donor-substituted allenylidene complexes, [(CO) 5 Cr@C@C@C(NMe 2 )Ph] (1) and [(CO) 5 Cr@C@C@C(O-endo-Bornyl)OEt] (4), and various dinucleophiles by 1,2,3-diheterocyclization. The reaction of 1 with 1,2- dimethylhydrazine gives the 1,2-dimethylpyrazolylidene complex [(CO) 5 Cr=C-C(H)=C(Ph)-NMe-NMe] (2) in high yield in addition to small amounts of the a,b-unsaturated carbene complex [(CO) 5 Cr@C(NMe 2 )–C(H)@C(NMe 2 )Ph] (3). The analogous reaction of 4 with 1,2-dimethylhydrazine affords the 1,2-dimethylpyrazolylidene complex [(CO) 5 Cr=C-C(H)=C(O- endo-Bornyl)-NMe-NMe] (5) and, via dis- placement of the C c -bound ethoxy substituent, the hydrazinoallenylidene complex [(CO) 5 Cr@C@C@C(O-endo-Bornyl){NMe-N(H)Me}] (6). Treatment of 6 with catalytic amounts of acids induces cyclization to 5. On addition of 1,1-dimethylhydrazine to 1 the zwitterionic pyrazolium-5-ylidene complex [(CO) 5 Cr-C=C(H)-C(Ph)=N-NMe 2 ] (7) is formed. The reaction of 1 with 1,2-diaminocyclohexane affords a octahydro-benzo[1,4]diazepinylidene complex (10) and, via intermolecular substitution, a binuclear bisallenylidene complex (11). Thia- zepinylidene complexes (12–14), containing 7-membered N/S-heterocyclic carbene ligands, are formed highly selectively in the reaction of 1 with 2-aminoethanethiol or related cysteine derivatives by a substitution/cyclization sequence. The analogous reaction of 1 with homo- cysteine methylester yields a thiazocanylidene complex (15). All new heterocyclic carbene ligands are strong donors exhibiting r-donor/ p-acceptor ratios similar to those of the known imidazolylidene complexes. On photolysis of 2 and 12 in the presence of triphenylphos- phine, the corresponding cis-carbene tetracarbonyl triphenylphosphine complexes (16 and 17) are formed. The solid state structure of complexes 2, 7, 14, 15, and 16 is established by X-ray structural analysis. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Allenylidene complexes; Chromium; Pyrazolylidene complexes; Cyclization; Heterocyclic carbene ligands 1. Introduction The first planned synthesis of transition metal carbene complexes was reported by Fischer and Maasbo ¨l in 1964 [1]. Since then, a series of syntheses has been developed [2]. A large number of experimental and theoretical studies gives insight into the structures and properties of this ver- satile class of organometallic compounds that has evolved into a very powerful tool in organic and organometallic synthesis [3]. The first complexes containing N-heterocy- clic ligands (imidazolylidene ligands) were described in 1968 by O ¨ fele (chromium complexes) [4] and Wanzlick (mercury complexes) [5]. The first complexes containing C–C saturated N-heterocyclic ligands (imidazolinylidene 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.07.049 * Corresponding author. Tel.: +49 7531 882783; fax: +49 7531 883136. E-mail address: helmut.fischer@uni-konstanz.de (H. Fischer). 1 Present address: Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt. 2 New address: Institut fu ¨r Anorganische Chemie, Universita ¨t Erlangen- Nu ¨ rnberg, Egerlandstrasse 1, 91058 Erlangen, Germany. www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 5753–5766