Reaction of Nitromethane with an Iridium Pincer Complex. Multiple Binding Modes of the Nitromethanate Anion Xiawei Zhang, Thomas J. Emge, Rajshekhar Ghosh, Karsten Krogh-Jespersen,* and Alan S. Goldman* Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey, New Brunswick, New Jersey 08903 ReceiVed July 30, 2005 The reaction of nitromethane with (PCP)Ir (PCP ) κ 3 -2,6-( t Bu 2 PCH 2 ) 2 C 6 H 3 ) yields the bidentate O,O- ligated nitromethanate complex (PCP)Ir(H)(κ 2 -O,O-NO 2 CH 2 )(1). Reaction of 1 with CO affords a CO adduct with a mono-oxygen-ligated nitromethanate, 2, which represents the first characterized transition metal mono-oxygen-ligated nitromethanate complex. At elevated temperature, complex 2 isomerizes to give the carbon-bound nitromethyl complex 3. Complex 1 also undergoes addition of cyclohexylisocyanide (analogous to the reaction with CO) to form the mono-oxygen-ligated nitromethanate complex 4, which also isomerizes to form the corresponding nitromethyl complex, 5. The (PCP)Ir-(CH 3 NO 2 ) system is the first species known to display three binding modes with a nitromethanate anion. Results from density functional calculations illustrate the structures and energies of the minima and transition states on the potential energy surfaces. The calculations suggest that 1 is the thermodynamic product of (PCP)Ir reacting with nitromethane; a kinetic product, formed via oxidative addition of a nitromethane C-H bond, should readily rearrange to form 1. Introduction The inorganic and organometallic chemistry of nitroalkanes has long been of interest. In recent years, metal-mediated reactions of nitromethane have seen increasing applications in organic synthesis. Aldol-type reactions have made accessible a large number of new nitro alcohols and nitro glycols, which in turn can be transformed into valuable building blocks. 1-3 In the past several years, the development of catalysts for asym- metric nitroaldol (Henry) reactions has been a particularly active area. 4-6 Coordinated nitroalkanate ions, most commonly nitrometha- nate, are the putative active species in reactions such as the metal-catalyzed nitroaldol and related reactions. The nitrometha- nate anion can bind in several different coordination modes. Nitromethanate complexes with bidentate coordination through the two oxygen atoms 7-16 or monodentate coordination of one carbon atom 17-23 have been isolated and characterized with different transition metal systems. In most such cases, base was used to deprotonate the nitroalkane and to promote the coordination. The structure proposed for the key nitromethanate intermediates in the transition metal-catalyzed nitroaldol reac- tions is monodentate oxygen-bound. 4-6 However, no mono- dentate oxygen-bound complexes have been previously isolated and characterized. Pincer-ligated transition metal complexes are currently the subject of intense study. 24-26 The 14-electron species (PCP)Ir (PCP ) κ 3 -2,6-( t Bu 2 PCH 2 ) 2 C 6 H 3 ), generated from (PCP)IrH 2 , can readily cleave hydrocarbon C-H bonds, 27 activate the O-H bond in water, 28 and oxidatively add N-H bonds of anilines. 29 In this context, we have been investigating the reactivity of * To whom correspondence should be addressed. E-mail: agoldman@ rutchem.rutgers.edu; krogh@rutchem.rutgers.edu. (1) Luzio, F. A. Tetrahedron 2001, 57, 915. (2) The Nitro Group in Organic Synthesis; Ono, N., Ed.; Wiley-VCH: New York, 2001; 392 pp. (3) Seebach, D.; Beck, A. K.; Mukhopadhyay, T.; Thomas, E. HelV. Chim. Acta 1982, 65, 1101. 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