Carbene Ligands DOI: 10.1002/anie.201007937 A Fischer Carbene within an Arduengo Carbene** Javier Ruiz,* Lucía García, BernabØF. Perandones, and Marilín Vivanco The chemistry of N-heterocyclic carbenes (NHCs) has been an intense area of research in the last several years. [1] The pioneering work by Arduengo et al. [2] and Herrmann [3] on free ligand isolation and catalytic applications of NHC metal complexes, respectively, has significantly contributed to the development of this field. There is currently a widespread interest in developing new strategies for NHC ligand design involving the replacement of the carbon atom backbone with heteroatoms. [4] Apart from the well-known triazol-2-yli- denes, [5] NHCs containing boron [4a–d] and phosphorus [4f] as heteroatoms in the N-heterocyclic skeleton have been described. However, NHCs featuring a metal atom within the heterocyclic framework, derived from the standard imidazole cycle, are still unknown. [6, 7] Replacement of the carbon atom at C4 of an Arduengo carbene (Scheme 1, A) with a metal atom belonging to a transition-metal complex would lead to the generation of a carbene complex within the NHC skeleton (B), which, upon coordination of a new metallic center would give unprecedented heterometallic dicarbene derivatives (C). [8] This goal has been achieved as part of this study. Herein, we describe the first synthesis of an Arduengo carbene complex featuring a metal atom within the heterocyclic skeleton, which has been accomplished through an experimental approach implying transformation of an acyclic diaminocarbene complex into a NHC. The treatment of a solution of the cationic diaminocar- bene complex fac-[Mn{C(NHPh)(NHMe)}(CO) 3 (bipy)] + 1 [9] in dichloromethane with a stoichiometric amount of [AuCl- (PPh 3 )], in the presence of an excess of KOH, afforded the heterometallic Mn I /Au I neutral complex 3a, which was isolated as a black solid (Scheme 2). The reaction was monitored by IR spectroscopy, thus allowing observation of the formamidinyl complex 2 as a reaction intermediate. In an independent experiment we have proved that complex 2 does not react with [AuCl(PPh 3 )] unless KOH is present. Com- pound 3a was quantitatively transformed into its isomer 3b, corresponding to the location of the NMe and NPh groups in interchanged positions, by heating a solution of the complex in THF at reflux for 20 minutes. The phosphorus spectrum of both isomers showed a singlet signal near to d = 40 ppm, which is typical for the [Au(PPh 3 )] + fragment bonded to nucleophilic carbenes. [8b] This observation suggested that, apart from substitution of the remaining NÀH proton in 2 by the isolobal [Au(PPh 3 )] + fragment (intermediate I, Scheme 2), [10] a translocation process of the Mn I and Au I ions occurred (intermediate II), thus leading to the nitrogen atom being bonded to manganese and the carbene carbon atom being bonded to gold. The soft acid character of the Au I ion, which prefers to bind a soft carbon atom instead of a hard nitrogen atom, could be the driving force for this trans- formation. In the n(CO) region of the IR spectra of 3a and 3b is revealed a pattern of bands characteristic for cis-dicarbonyl complexes, consisting of two strong bands at much lower frequencies than the starting complex 1. This fact, together with the presence of a n(C =O) band at 1580 cm À1 , supports the existence of a carbamoyl group in the complex, very likely Scheme 1. Arduengo carbene (A), and unprecedented carbene complex within an NHC skeleton (B) and heterometallic dicarbene derivatives (C). [M], [M’] = transition-metal complexes. Scheme 2. Formation of 3a and 3b through the proposed intermedia- tes I and II. [*] Dr. J. Ruiz, L. García, Dr. B.F. Perandones, Dr. M. Vivanco Departamento de Química Orgµnica e Inorgµnica Universidad de Oviedo, Facultad de Química 33006 Oviedo (Spain) Fax: (+ 34) 985-103-446 E-mail: jruiz@uniovi.es [**] This work was supported by the Spanish Ministerio de Ciencia e Innovación (PGE and FEDER funding, Project CTQ2009-11457) and the Principado de Asturias (Project IB09-029). B.F.P. thanks the MICINN for a FPU grant (PGE and European Social Fund). Supporting information for this article, including experimental details of the synthesis and characterization of 3a, 3b, 4a, and 4b, is available on the WWW under http://dx.doi.org/10.1002/anie. 201007937. Communications 3010  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 3010 –3012