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Dirhodium Complexes Bridged by Bis(diphenylphosphino)phthalazine (PNNPPh): Central Ring Size and Charge Effects As Compared with the Pyrazolate Derivative (PNNPPy)

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journal contribution
posted on 13.12.2010, 00:00 by Takafumi Yamaguchi, Takashi Koike, Munetaka Akita
A dirhodium carbonyl complex with 1,4-bis((diphenylphosphino)methyl)phthalazine (PNNPPh), [(μ-κ22-PNNPPh){Rh(CO)}2](BF4)2, has been prepared and its reactivity studied as compared with the previously reported 3,5-bis((diphenylphosphino)methyl)pyrazolate (PNNPPy) analogue [(μ-κ22-PNNPPy){Rh(CO)2}2]BF4. The two quadridentate ligands are different in the size of the central ring and the charge; six-membered ring/neutral (PNNPPh) vs five-membered ring/mononegative (PNNPPy). The reactivities of the two systems turn out to be very similar, as can be seen from formation of the analogous, unique tetranuclear μ4-acetylide ([(μ-PNNPPh)2{Rh(CO)}44-CC-p-tol)]BF4) and μ4-dicarbide complexes ([(μ-PNNPPh)2{Rh(CO)}44-C2)](BF4)2). However, the PNNPPh system exhibits the following features. (1) The enlargement of the central ring causes shortening of the metal−metal distance, frequently leading to bond formation between them. For more positively charged PNNPPh species, (2) back-donation decreases to facilitate CO dissociation and (3) the rhodium centers become more Lewis acidic. Another feature is that the PNNPPh complex undergoes oxidative addition upon treatment with internal alkynes to form stable adducts with unique coordination structures (e.g., 1,4-dimetallacyclohexa-2,5-diene).