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Hydrogen‐Atom Transfer in Open‐Shell Organometallic Chemistry: The Reactivity of Rh II (cod) and Ir II (cod) Radicals
Author(s) -
Hetterscheid Dennis G. H.,
Klop Martijn,
Kicken Reinout J. N. A. M.,
Smits Jan M. M.,
Reijerse Eduard J.,
de Bruin Bas
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600711
Subject(s) - chemistry , ligand (biochemistry) , electron paramagnetic resonance , protonation , reactivity (psychology) , crystallography , unpaired electron , amine gas treating , medicinal chemistry , olefin fiber , square pyramidal molecular geometry , pyridine , stereochemistry , radical , crystal structure , organic chemistry , catalysis , medicine , ion , biochemistry , physics , receptor , alternative medicine , nuclear magnetic resonance , pathology
A series of new metalloradical rhodium and iridium complexes [M II (cod)(N‐ligand)] 2+ in the uncommon oxidation state +II were synthesized by one‐electron oxidation of their [M I (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 [M II (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 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 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 M II (olefin) complexes are quite stable as solids, in solution they spontaneously transform into a 1:1 mixture of M III (allyl) species and protonated M I (olefin) complexes (in the forms [M I (olefin)(protonated N‐ligand)] 2+ for M=Rh and [M III (H)(olefin)(N‐ligand)] 2+ for M=Ir). Similar reactions were observed for the related propene complex [M II (propene)(Me 2 tpa)] 2+ (Me 2 tpa= N , N , N ‐tris(6‐methyl‐2‐pyridylmethyl)amine). The decomposition rate of the [M II (cod)(N‐ligand)] 2+ species decreases with increasing N‐ligand bulk in the following order: dpa>pla>Bn‐dla. Decomposition of the most hindered [M II (cod)(Bn‐dla)] 2+ complexes proceeds by a second‐order process. The kinetic rate expression v= k obs [M II ] 2 in acetone with k obs = 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 M II (cod) to another M II species (Δ H ≠ =11.5±2 kcal mol −1 , Δ S ≠ =−27±10 cal K −1 mol −1 , and Δ ${G{{{\ne}\hfill \atop 298\>{\rm K}\hfill}}}$ =19.5±5 kcal mol −1 ).