Protonation and Bromination of an Osmabenzyne: Reactions Leading to the Formation of New Metallabenzynes Ting Bin Wen, † Sze Ming Ng, † Wai Yiu Hung, † Zhong Yuan Zhou, ‡ Man Fung Lo, † Lai-Yung Shek, † Ian D. Williams, † Zhenyang Lin, † and Guochen Jia* ,† Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, and Department of Applied Biology and Chemical Technology, Hong Kong Polytechnic UniVersity, Kowloon, Hong Kong Received October 26, 2002 ; E-mail: chjiag@ust.hk We have recently reported the synthesis and characterization of the first metallabenzyne [Os(tCC(SiMe 3 )dC(CH 3 )C(SiMe 3 )dCH) Cl 2 (PPh 3 ) 2 ] (1). 1 The compound is interesting 2 because it is structurally related to metallabenzenes 3-5 and benzynes 6 which have attracted considerable attention of experimental and theoretical chemists. Unlike benzyne, which is thermally unstable, complex 1 can be isolated as a greenish-blue solid and can be conveniently characterized by NMR spectroscopy and X-ray diffraction. The X-ray diffraction study of 1 shows that the six-membered metal- lacycle has a planar delocalized structure. 1 The delocalized structural feature is similar to that of aromatic ring systems such as benzene and metallabenzenes. In these regards, it would be interesting to find out if metallabenzynes could also undergo typical reactions of aromatic compounds or benzynes. When a mixture of 1 and 2 equiv of HBF 4 in wet dichlo- romethane was stirred for 8 h, the cationic osmabenzyne 2 was produced (Scheme 1). 2 is presumably formed by protonation of one of the chloride ligands followed by trapping the intermediate with water present in the reaction medium. The structure of 2 has been confirmed by X-ray diffraction. 7 As shown in Figure 1, the complex contains an essentially planar six-membered metallacycle with a water molecule trans to the formally carbyne carbon. The Os-C(1) (1.756(5) Å) bond is slightly shorter than that (1.815(4) Å) of 1 but is still longer than OstC bonds in complexes such as OsHCl 2 (tCCH 2 Ph)(P(i-Pr) 3 ) 2 (1.711(4) Å), 8 OsHCl 2 (tCCHd CPh 2 )(PCy 3 ) 2 (1.715(4) Å) 9 and OsCl 3 (tCCH 2 CMe 3 )(PPh 3 ) 2 (1.728(3) Å). 10 The Os-C(5) bond (2.016(5) Å) is slightly longer than that of 1 (1.939(5) Å) and is close to the Os-CH bonds in osmabenzenes Os(C(SMe)CHCHC(X)CH)I(CO)(PPh 3 ) 2 (X ) NO 2 , 2.011(7) Å; X ) Br, 2.039(9) Å). 4a The Os-C(1)-C(2) angle is at 153.8(4)°, which is slightly larger than that of 1 (148.7(3)°). The solid-state structure is fully supported by the solution NMR spectroscopic data and elemental analysis. The most relevant compound to 2 is the cationic aquocarbyne osmium complex, [OsCl 2 (tCC 6 H 4 NMe 2 )(H 2 O)(PPh 3 ) 2 ] + . 11 When a mixture of 1 and 6 equiv of HBF 4 in dichloromethane was stirred for 20 h, the desilylated cationic osmabenzyne 3 was produced. Isolated 2 could also be slowly converted to 3 when treated with excess of HBF 4 . If the protonation reaction was carried out in the presence of NaCl and the resulting mixture was treated with water, the neutral desilylated osmabenzyne 4 can be isolated in 73%. Treatment of isolated 3 with NaCl also gave 4. The structure of complex 3 can be inferred from its NMR spectroscopy. The 31 P{ 1 H} NMR spectrum (in CD 2 Cl 2 ) showed a singlet at 10.5 ppm. The 13 C{ 1 H} NMR spectrum displayed the OstC signal at 309.3 ppm, the OsCH signal at 210.3 ppm, the CH signals at 125.2 and 107.7 ppm, and the CCH 3 signal at 176.6 ppm. In the 1 H NMR spectrum, the three CH signals were observed at 12.65 (d, J(HH) ) 8.2 Hz, OsCH), 6.33 (d, J(HH) ) 8.2 Hz, OsCHdCH), and 4.26 ppm (OstCCH). The tCCH signal ap- peared in relatively high field, presumably because the CH group is adjacent to the carbyne carbon. The structure of 4 has also been determined by X-ray diffraction (Figure 2). 7 The structural features associated with the metallacycle are very similar to those of 1. Consistent with the structure, the 1 H NMR spectrum showed three CH signals at 12.19 (OsCH), 5.66 (OsCHdCH), and 4.03 ppm (OsCCH). In the 13 C{ 1 H} NMR spectrum, the five carbon signals of the metallacycle were observed at 302.2 (OstC), 212.4 (OsCH), 175.9 (CCH 3 ), 125.2 (OsCHd CH), and 107.6 (OstCCH) ppm. The reactions of 1 with HBF 4 to give 3 and 4 are similar to the reactions of C 6 H 5 SiMe 3 with acids 12 in that the Me 3 Si groups of the six-membered ring are replaced with hydrogens. Like the reactions of C 6 H 5 SiMe 3 with acids, replacement of the Me 3 Si groups in the metallacycle with hydrogens in the protonation reactions most † The Hong Kong University of Science and Technology. ‡ The Hong Kong Polytechnic University. Figure 1. Molecular structure for the complex cation of 2. Scheme 1 Published on Web 01/03/2003 884 9 J. AM. CHEM. SOC. 2003, 125, 884-885 10.1021/ja029110r CCC: $25.00 © 2003 American Chemical Society