One-Pot Synthesis of Dihalo(porphyrinato)osmium(IV) Complexes. Evidence for Monohalo(carbonyl)osmium(III) Intermediates † Zeev Gross* and Atif Mahammed Department of Chemistry, TechnionsIsrael Institute of Technology, Haifa 32000, Israel ReceiVed April 3, 1996 X trans-Dichloro-, trans-dibromo-, and trans-diiodoosmium(IV) tetraarylporphyrins were obtained by extremely facile synthetic routes directly from the reactions of the corresponding (carbonyl)osmium(II) complexes with CCl 4 , CBr 4 , and CI 4 , respectively. At short reaction times, appreciable amounts of intermediatessone for each reactionswere observed by spectroscopic investigations. These intermediates were shown to be (carbonyl)- (halo)(porphyrinato)osmium(III) complexes by independent preparation of an authentic (carbonyl)(bromo)- (porphyrinato)osmium(III) complex, which was identical to the reaction intermediate in the reaction of CBr 4 and very similar to those of the other reactions. This provided strong evidence for the reaction mechanism, two stepwise one-electron oxidations of the metal ion. The relatively strong binding of carbon monoxide to osmium(III) is proposed to be an important factor in avoiding dimerization of the reaction intermediates. Introduction Ongoing interest in the chemistry of ruthenium(IV) and osmium(IV) porphyrins derives from their rich coordination chemistry, 1-5 including very interesting organometallic complexes. 3,4a The most important precursors of organoruthe- nium and organoosmium porphyrins are trans-dihalometal(IV) complexes, [M IV (por)X 2 ], with M ) Ru, Os, and X ) halide anion. It was also recently proposed that [Ru IV (por)X 2 ] complexes are important intermediates in the highly efficient oxygenation of hydrocarbons by aromatic N-oxides under catalysis of ruthenium porphyrins in the presence of mineral acids. 6 Practically all ruthenium and osmium porphyrin com- plexes are prepared from the corresponding metal carbonyls, [M II (por)(CO)]. The carbonyl group in these complexes is considered chemically inert, and most existing synthetic routes for preparation of more reactive derivatives rely on its removal by either oxidative or photochemical methods. The full series of dihalo(porphyrinato)ruthenium complexess[Ru IV (por)X 2 ], with X ) F, Cl, Br, Iscan be prepared from [Ru(por)(CO)] in three distinct steps. The first two steps, leading to the dinuclear complexes [Ru(por)] 2 , require however photochemical, high temperatures, and ultrahigh-vacuum procedures. 3 Such lengthy and experimentally demanding procedures were avoided in the recently reported two-step syntheses of the first two dihalo- (porphyrinato)osmium(IV) complexes by three independent groups. In all three cases, the carbonyl complexes [Os(por)- (CO)] were first oxidized to the corresponding trans-dioxo- osmium(VI) porphyrins. Subsequent reduction of [Os VI (oep)- (O) 2 ] with Br 2 afforded [Os IV (oep)Br 2 ] in 40% yield, 2c and [Os IV (ttp)Cl 2 ] was obtained in yields of 80% and 51% from the reaction of [Os VI (ttp)(O) 2 ] with SOCl 2 or SnCl 2 , respec- tively. 4 In line with the desire for simple methods for the preparation of [M IV (por)X 2 ] derivatives, we have very recently discovered an extremely simple alternative one-pot synthetic route to [Ru IV (por)X 2 ] complexes. Thus, [Ru IV (tdmpp)Cl 2 ] and [Ru IV - (tdmpp)Br 2 ] were prepared directly from [Ru(tdmpp)(CO)] in > 80% yields by simply heating it with CCl 4 or CBr 4 . 5 For non sterically hindered porphyrins, such as [Ru(ttp)(CO)], dimeric derivatives were however formed. We now report our investigation of that reaction for osmium porphyrins, which shows that it has even a larger scope for Os than for Ru. Thus, the dichloro-, dibromo-, and diiodoosmium(IV) complexes of both ttp and tmp were prepared in very simple one-pot procedures directly from the corresponding (carbonyl)os- mium(II) porphyrins (Scheme 1). The in situ observation of [Os III (por)(CO)(X)] intermediates during these reactions sidentified by comparison to the independently prepared and fully char- acterized [Os III (por)(CO)(Br)] complexsclearly suggest that the transformations of [Os II (por)(CO)] to [Os IV (por)X 2 ] proceed by two distinct one-electron oxidation steps. In addition, the characterization of the intermediates as (carbonyl)osmium(III) complexes also provided a clue about the different reaction pathways of [Os(por)(CO)] and [Ru(por)(CO)]. Results and Discussion The reactions were investigated for two porphyrin derivatives, one which can form dimeric productss[Os(ttp)(CO)]sand one which cannots[Os(tmp)(CO)]. In the reactions with CCl 4 the reagent served as solvent as well and the reactions with CBr 4 and CI 4 were performed in benzene at reflux. Since CI 4 releases I 2 very easily, the last reactions were also studied with I 2 . The yields were very high for both CI 4 and I 2 , and accordingly the syntheses were much more conveniently performed with I 2 . The † Abbreviations used: por ) unspecified porphyrin dianion; oep ) 2,3,7,8,12,13,17,18-octaethylporphyrin dianion; tpp ) 5,10,15,20-tetra- phenylporphyrin dianion; ttp )5,10,15,20-tetra-p-tolylporphyrin dianion; tmp ) 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphyrin dianion; tdmpp ) 5,10,15,20-tetrakis(2,6-dimethylphenyl)porphyrin dianion. X Abstract published in AdVance ACS Abstracts, November 1, 1996. (1) Groves, J. T.; Scott, J. S. J. Am. Chem. Soc. 1995, 117, 5594. Cheng, S. Y. S.; Rajapakse, N.; Rettig, S. J.; James, B. R. J. Chem. Soc., Chem. Commun. 1994, 2669. Huang, J.-S.; Che, C.-M.; Li, Z.-Y.; Poon, C.-K. Inorg. Chem. 1992, 31, 1313. (2) (a) Collman, J. P.; Bohle, D. S.; Powell, A. K. Inorg. Chem. 1993, 32, 4004. (b) Che, C. M.; Huang, J. S.; Li, Z. Y.; Poon, C. K.; Tong, W. F.; Lai, T. F.; Cheng, M. C.; Wang, C. C.; Wang, Y. Inorg. Chem. 1992, 31, 5220. (c) Che, C. M.; Leung, W. H.; Chung, W. C. Inorg. Chem. 1990, 29, 1841. 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