XII PhD-Chem Day
Bari, March 27-28, 2015
Synthesis of novel Ru N-Heterocyclic carbene complexes for
bifunctional metal-ligand catalysis
Cristiana Cesari, Rita Mazzoni, Martin Albrecht, Valerio Zanotti
Dipartimento di Chimica Industriale “Toso Montanari”, viale Risorgimento, 4 40136 Bologna - Italy
[email protected]
N-heterocyclic carbenes (NHC) are efficient ancillary ligands due to their strong coordination ability and
their tuneable character which allows the control of the steric and electronic properties of the metal centre [1]
Herein we report the synthesis, characterization and catalytic applications of new carbonylic ruthenium
complexes that combine tetraphenylcyclopentadienone and NHC ligands. Coordination of less bulky
carbenes occurred through a straightforward transmetallation from the corresponding silver carbene
intermediate to the dimeric precursor dicarbonyl(η4-3,4-bis-(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4dienone), leading to the formation of a series of mononuclear complexes of type 1 (Fig 1). Conversely,
sterically demanding imidazolium salts yielded the dinuclear Ru(II)-Ag(I) complexes of type 2, in which the
carbene remains coordinated to silver (Fig. 1) [2]. Furthermore, complexes of type 3 were obtained by
replacing imidazolylidenes with triazolylidenes (Fig. 1).
Figure 1. Carbonylic ruthenium complexes of type 1, 2 and 3.
1 R', R'' = alkyl; 2 R', R'' = aryl or t-Bu; 3a R = Ph, R' = R'' = CH3; 3b R = R' = Bu, R'' = CH3
3c R = R' = R'' = Bu; 3d R = R'' = CH3, R' = Ph
Triazolylidenes are known to be stronger donors than imidazolylidenes, so they further destabilize the
ruthenium(0) oxidation state in which will be beneficial in catalytic applications [3], thus complex 3a was
evaluated as catalyst precursor in transfer hydrogenation of 4-fluoroacetophenone and in the acceptorless
dehydrogenation of benzyl alcohol (Fig. 2).
Figure 2. Catalytic transfer hydrogenation of 4-fluoroacetophenone (left) and oxidation of benzyl alcohol (right)
Reference
[1]M.N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature, 2014, 510, 485-496.
[2]C. Cesari, S. Conti, S. Zacchini, V. Zanotti, M. C. Cassani, R. Mazzoni, Dalton Trans., 2014, 43, 17240-17243.
[3]K.F. Donnelly, A. Petronilho and M. Albrecht, Chem. Commun. 2013, 49, 1145–1159.
*Corresponding author: Cristina Cesari e-mail address: [email protected]