Pyridine Ring Opening at Room Temperature at a Rhenium Tricarbonyl Bipyridine Complex Miguel A. Huertos, Julio Pe ´ rez,* and Lucı ´a Riera* Departamento de Quı ´mica Orga ´nica e Inorga ´nica-IUQOEM, Facultad de Quı ´mica, UniVersidad de OViedo-CSIC, 33006 OViedo, Spain Received March 5, 2008; E-mail: japm@uniovi.es; lrm@fq.uniovi.es The cleavage of the C-N bond of pyridine derivatives is a challenging task, and the homogeneous, metal-mediated C-N bond cleavage of pyridines has been accomplished only with a few, highly reactive early transition metal complexes. 1 Bipy and phen have been known for more than one century. These ligands form very stable, five-membered chelate rings with transition metals. Complexes containing fac- Re(CO) 3 (N-N) (N-N ) bipy or phen) fragments are particu- larly stable and have been widely used in bioinorganic chem- istry, 2 supramolecular chemistry, 3 and catalysis of CO 2 reduction 4 among other areas. Activation of their bipy or phen ligands remains very rare, 5 and cleavage of their pyridine C-N bonds is unknown. Herein we disclose an activation of bipy and phen ligands at Re(CO) 3 (N-N) fragments which, for the bipy complex, ends in C-N cleavage of one of the pyridine rings. The compound [Re(CO) 3 (MeIm)(bipy)]OTf (1a) was pre- pared by reaction of [Re(OTf)(CO) 3 (bipy)] 6 with N-methylimi- dazole. When 1a was treated with KN(SiMe 3 ) 2 in THF at -78°, a neutral product was formed, as evidenced by shifts in the IR spectra to lower ν(CO) values, which was too unstable for isolation. The addition of MeOTf (excess) in CH 2 Cl 2 afforded a mixture from which the triflate salt 4 could be isolated. Compound 4 was fully characterized, including an X-ray structural determination (see Figure 1). The cationic complex consists of a fac-Re I (CO) 3 fragment bonded to the three nitrogen atoms of a tridentate ligand. The latter can be described as a central cyclopentadienyl bridgehead group and three different N-donor arms: an N-methylimidazole, a pyridine, and a dim- ethylamine. The most striking feature of this structure is the fact that, in the overall reaction, one of the pyridine rings of the bipy chelate underwent nitrogen extrusion to afford a 2,4- cyclopentadienyldimethylamine unit. The facts that no similar transformation was known, that pyridine C-N bond cleavage took place under very mild conditions, and that the starting metal complex is an easily available, very stable species prompted us to further study this reaction. Thus, when [Re(CO) 3 (MesIm)(bipy)]OTf (1c), the mesityl- imidazole analogue of 1a, was allowed to react with KN(SiMe 3 ) 2 (Scheme 1), the neutral product 2c was stable enough for isolation. The IR showed ν(CO) bands at 2005, 1893, and 1886 cm -1 , consistent with the formation of a neutral compound. The 1 H and 13 C NMR spectra of 2c featured distinct signals for every nucleus of the bipy moiety and for each of the three methyl groups of the mesityl substituent. The three carbonyl ligands appeared also as three singlets in the 13 C NMR spectrum. These features reflect the asymmetry of the molecule of 2c and therefore indicate that the reaction involved a transformation of the bipy ligand. The X-ray determination of the structure of 2c (Figure 2a) demonstrated the presence of a bond between the C(2) carbon of the imidazole moiety and C(3) of bipy. As a result, the latter has become an sp 3 -hybridized carbon, and the aromaticity of the pyridine ring was lost. In solution, this is reflected in a large upfield shift of three of the signals of the bipy in 1 H NMR (at 4.87, 5.52, and 6.34 ppm) and 13 C NMR (70.1 ppm for the sp 3 C). The nitrogen atom adjacent to the pyridine carbon that underwent the coupling is now part of an amido ligand. The phenanthroline analogue of 1c, [Re(CO) 3 (MesIm)(phen)]OTf (1d), reacted with KN(SiMe 3 ) 2 in a similar way, affording complex 2d, whose spectral features indicate isostructurality with 2c. The reactions of 2c,d with the equimolar amounts of MeOTf in CH 2 Cl 2 afforded stable products 3c,d (Scheme 1), which were spectroscopically char- acterized; in addition, the structure of 3d was determined by X-ray diffraction (Figure 2b). The results showed that methy- lation at nitrogen transformed the amido-like nitrogen of 2c,d into an amine ligand in the cationic complexes of 3c,d. Compounds 3c,d did not further react with MeOTf. In view of the results with the mesitylimidazole complexes discussed above, we returned our attention to the transformation from 1a to 4, trying to find out if species akin to 2c and 3c Figure 1. Molecular structure of the cation present in 4. Figure 2. Molecular structure of 2c (a) and the cation of 3d (b). Published on Web 04/09/2008 10.1021/ja801655d CCC: $40.75  2008 American Chemical Society 5662 9 J. AM. CHEM. SOC. 2008, 130, 5662–5663