DOI: 10.1002/chem.200600711 Hydrogen-Atom Transfer in Open-Shell Organometallic Chemistry: The Reactivity of Rh II ACHTUNGTRENNUNG(cod) and Ir II ACHTUNGTRENNUNG(cod) Radicals Dennis G. H. Hetterscheid, [a] Martijn Klop, [a] Reinout J. N. A. M. Kicken, [a] Jan M. M. Smits, [a] Eduard J. Reijerse, [b] and Bas de Bruin* [c] Introduction The organometallic chemistry of open-shell d 7 Rh II and Ir II complexes is quite fascinating and often leads to unexpected reaction pathways. With few exceptions, rhodium(II) com- plexes have been stabilized by preventing close contact of reagents or other rhodium(II) centers to the rhodium(II) center through coordinative saturation or application of li- gands with sufficient steric bulk. [1] Less shielded rhodium(II) and iridium(II) species like [M II ACHTUNGTRENNUNG(por)] (M = Rh, Ir, por 2 = bulky meso-tetraarylporphyrinate) show remarkable reactiv- ity towards a variety of otherwise rather inert substrates. [2] Activation under mild conditions of H 2 , Si  H and Sn  H bonds, benzylic and allylic C  H bonds, and even methane have been reported. Apparently the [M II ACHTUNGTRENNUNG(por)] fragments Abstract: A series of new metalloradi- cal rhodium and iridium complexes [M II ACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(N-ligand)] 2 + in the uncom- mon oxidation state + II were synthe- sized by one-electron oxidation of their [M I ACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(N-ligand)] + precursors (M = Rh, Ir; cod = (Z,Z)-1,5-cyclooctadiene; and N-ligand is a podal bis- ACHTUNGTRENNUNG(pyridyl)amine ligand: N,N-bis(2-pyri- dylmethyl)amine (dpa), N-(2-pyridyl- methyl)-N-(6-methyl-2-pyridylmethyl)- amine (pla), or N-benzyl-N,N-bis(6- methyl-2-pyridylmethyl)amine (Bn- dla). EPR spectroscopy, X-ray diffrac- tion, and DFT calculations reveal that each of these [M II ACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(N-ligand)] 2 + species adopts a square-pyramidal ge- ometry with the two cod double bonds and the two pyridine fragments in the basal plane and the N amine donor at the apical position. The unpaired electron of these species mainly resides at the metal center, but the apical N amine donor also carries a considerable frac- tion of the total spin density (15–18%). Density functional calculations proved a valuable tool for the analysis and simulation of the experimental EPR spectra. Whereas the M II ACHTUNGTRENNUNG(olefin) com- plexes are quite stable as solids, in so- lution they spontaneously transform into a 1:1 mixture of M III ACHTUNGTRENNUNG(allyl) species and protonated M I ACHTUNGTRENNUNG(olefin) complexes (in the forms [M I ACHTUNGTRENNUNG(olefin)(protonated N-ligand)] 2 + for M = Rh and [M III (H)- ACHTUNGTRENNUNG(olefin)ACHTUNGTRENNUNG(N-ligand)] 2 + for M = Ir). Simi- lar reactions were observed for the re- lated propene complex [M II ACHTUNGTRENNUNG(propene)- ACHTUNGTRENNUNG(Me 2 tpa)] 2 + (Me 2 tpa = N,N,N-tris(6- methyl-2-pyridylmethyl)amine). The decomposition rate of the [M II ACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(N- ligand)] 2 + species decreases with in- creasing N-ligand bulk in the following order: dpa > pla > Bn-dla. Decomposi- tion of the most hindered [M II ACHTUNGTRENNUNG(cod)- ACHTUNGTRENNUNG(Bn-dla)] 2 + complexes proceeds by a second-order process. The kinetic rate expression v = k obs ACHTUNGTRENNUNG[M II ] 2 in acetone with k obs = k’[H + ][S], where [S] is the con- centration of additional coordinating reagents (MeCN), is in agreement with ligand-assisted dissociation of one of the pyridine donors. Solvent coordina- tion results in formation of more open, reactive species. Protonation of the noncoordinating pyridyl group increas- es the concentration of this species, and thus [H + ] appears in the kinetic rate expression. The kinetic data are in agreement with bimolecular hydrogen- atom transfer from M II ACHTUNGTRENNUNG(cod) to another M II species (DH ° = 11.5  2 kcalmol 1 , DS ° = 27  10calK 1 mol 1 , and DG ° 298 K = 19.5  5 kcalmol 1 ). Keywords: C  H activation · hydro- gen transfer · iridium · N ligands · rhodium [a] D. G. H. Hetterscheid, M. Klop, Dr. R. J. N. A. M. Kicken, J. M. M. Smits Radboud University Nijmegen Toernooiveld 1, 6525 ED Nijmegen (The Netherlands) [b] Dr. E. J. Reijerse Max-Planck-Institut für Bioanorganische Chemie Stiftstrasse 34–36, 45470 Mülheim an der Ruhr (Germany) [c] Dr. B. de Bruin University of Amsterdam van)t Hoff Institute for Molecular Sciences (HIMS) Department of Homogeneous and Supramolecular Catalysis Nieuwe Achtergracht 166, 1018 WV Amsterdam (The Netherlands) Fax: (+ 31)20-525-6422 E-mail: BdeBruin@science.uva.nl Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 3386–3405 3386