π-Face Donor Properties of N-Heterocyclic Carbenes in Grubbs II Complexes

• Published in: Chemistry : a European journal 2008-06, Vol.14 (18), p.5465-5481
• Main Authors: Leuthäußer, Steffen, Schmidts, Volker, Thiele, Christina M., Plenio, Herbert
• Format: Article
• Language: English
• Subjects: dynamic NMR spectroscopy; metathesis; N-heterocyclic carbenes; redox chemistry; ruthenium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200800139

The electron‐donating properties of eighteen N‐heterocyclic carbenes (N,N′‐bis(2,6‐dimethylphenyl)imidazol)‐2‐ylidene and the respective dihydro ligands) with 4,4′‐R substituted aryl rings (4,4′‐R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4‐R substituent has a strong influence on the RuII/III redox potentials ranging between ΔE1/2=+0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4‐R ≠ 4‐R′ were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)CRu bond (ΔG = 89 kJ mol−1 at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4‐R/4‐R′ substituted mesityl ring with respect to the RCHPh unit, gives rise to different redox potentials (4‐R = NEt2, 4‐R′ = Br: ΔE1/2=+0.232 and +0.451 V). In the oxidized Grubbs II complex (4‐R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the RuCHPh unit. This orientational effect provides clear evidence for strong π–π through‐space interactions in the RuIII complexes, assuming that the alternative through‐bond transfer of electron density is equally efficient in both isomers. The π‐face donation between a coplanar N‐aryl group and the RuCHPh unit represents a new quality of N‐heterocyclic carbenes in metal complexes (see scheme) and is described in detail herein.