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Hydrogen-Atom Transfer in Open-Shell Organometallic Chemistry: The Reactivity of RhII(cod) and IrII(cod) Radicals

A series of new metalloradical rhodium and iridium complexes [MII(cod)(N‐ligand)]2+ in the uncommon oxidation state +II were synthesized by one‐electron oxidation of their [MI(cod)(N‐ligand)]+ precursors (M=Rh, Ir; cod=(Z,Z)‐1,5‐cyclooctadiene; and N‐ligand is a podal bis(pyridyl)amine ligand: N,N‐b...

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Published in:Chemistry : a European journal 2007-04, Vol.13 (12), p.3386-3405
Main Authors: Hetterscheid, Dennis G. H., Klop, Martijn, Kicken, Reinout J. N. A. M., Smits, Jan M. M., Reijerse, Eduard J., de Bruin, Bas
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Language:English
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Summary:A series of new metalloradical rhodium and iridium complexes [MII(cod)(N‐ligand)]2+ in the uncommon oxidation state +II were synthesized by one‐electron oxidation of their [MI(cod)(N‐ligand)]+ precursors (M=Rh, Ir; cod=(Z,Z)‐1,5‐cyclooctadiene; and N‐ligand is a podal bis(pyridyl)amine ligand: N,N‐bis(2‐pyridylmethyl)amine (dpa), N‐(2‐pyridylmethyl)‐N‐(6‐methyl‐2‐pyridylmethyl)amine (pla), or N‐benzyl‐N,N‐bis(6‐methyl‐2‐pyridylmethyl)amine (Bn‐dla). EPR spectroscopy, X‐ray diffraction, and DFT calculations reveal that each of these [MII(cod)(N‐ligand)]2+ species adopts a square‐pyramidal geometry with the two cod double bonds and the two pyridine fragments in the basal plane and the Namine donor at the apical position. The unpaired electron of these species mainly resides at the metal center, but the apical Namine donor also carries a considerable fraction 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 MII(olefin) complexes are quite stable as solids, in solution they spontaneously transform into a 1:1 mixture of MIII(allyl) species and protonated MI(olefin) complexes (in the forms [MI(olefin)(protonated N‐ligand)]2+ for M=Rh and [MIII(H)(olefin)(N‐ligand)]2+ for M=Ir). Similar reactions were observed for the related propene complex [MII(propene)(Me2tpa)]2+ (Me2tpa=N,N,N‐tris(6‐methyl‐2‐pyridylmethyl)amine). The decomposition rate of the [MII(cod)(N‐ligand)]2+ species decreases with increasing N‐ligand bulk in the following order: dpa>pla>Bn‐dla. Decomposition of the most hindered [MII(cod)(Bn‐dla)]2+ complexes proceeds by a second‐order process. The kinetic rate expression v=kobs[MII]2 in acetone with kobs=k′[H+][S], where [S] is the concentration of additional coordinating reagents (MeCN), is in agreement with ligand‐assisted dissociation of one of the pyridine donors. Solvent coordination results in formation of more open, reactive species. Protonation of the noncoordinating pyridyl group increases 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 MII(cod) to another MII species (ΔH≠=11.5±2 kcal mol−1, ΔS≠=−27±10 cal K−1 mol−1, and Δ${G{{{\ne}\hfill \atop 298\>{\rm K}\hfill}}}$=19.5±5 kcal mol−1). Allylic CH activation of paramagnetic [MII(cod)(N‐ligand)]2+ species (M=Rh, Ir; cod=1,5‐cyclooctadiene; N‐ligand is a bis
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.200600711