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Rhodium and Iridium β‐Diiminate Complexes – Olefin Hydrogenation Step by Step
Author(s) -
Budzelaar Peter H. M.,
Moonen Nicolle N. P.,
Gelder René de,
Smits Jan M. M.,
Gal Anton W.
Publication year - 2000
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(200004)2000:4<753::aid-ejic753>3.0.co;2-v
Subject(s) - chemistry , rhodium , iridium , isomerization , cyclooctene , hydride , olefin fiber , intramolecular force , degree of unsaturation , medicinal chemistry , ligand (biochemistry) , catalytic cycle , photochemistry , stereochemistry , catalysis , organic chemistry , hydrogen , biochemistry , receptor
The bulky β‐diiminate ligands [(2,6‐C 6 H 3 X 2 )NC(Me)CHC(Me)N(2,6‐C 6 H 3 X 2 )] – (X = Me, L Me ; X = Cl, L Cl ) have been found to be effective in stabilizing low coordination numbers (CN) in Rh and Ir complexes. The 14‐ complex L Me Rh(COE) (COE = cyclooctene) has a three‐coordinate T‐shaped Rh environment and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N 2 , MeCN, olefins), by the intramolecular coordination of a chlorine atom in L Cl Rh(COE), or by an agostic interaction in L Me Rh(norbornene). In solution at room temperature, L Me Rh(COE) undergoes rapid isomerization according to the allyl hydride mechanism; the corresponding 2,3‐dimethylbutene complex actually prefers the allyl hydride structure. Rhodium(I) complexes of L Me and L Cl catalyze olefin hydrogenation; hydrogenation of 2,3‐dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky β‐diiminate ligands allow direct observation of a number of reactive intermediates or their iridium analogues, including an olefin–dihydrogen complex (with Rh) and an olefin dihydride (with Ir). These observations, together with calculations on simple model systems, provide us with snapshots of a plausible hydrogenation cycle. Remarkably, hydrogenation according to this cycle appears to follow a 14‐ e /16‐ e path, in contrast to the more usual 16‐ e /18‐ e paths.