C-H activation of pyrazolyl ligands by Ru(II)

Inorg Chem. 2014 Jun 16;53(12):6270-9. doi: 10.1021/ic500811n. Epub 2014 May 27.

Abstract

Previously, hydridotris(pyrazolyl)borate (Tp) Ru(II) alkyl and aryl complexes of the type TpRu(L)(NCMe)R (R = methyl or aryl; L = charge-neutral two-electron donating ligand) were demonstrated to activate aromatic C-H bonds. To determine the impact of replacing the anionic Tp ligand with charge-neutral poly(pyrazolyl)alkane ligands, [(C(pz)4)Ru(P(OCH2)3CEt)(NCMe)Me][BAr'4] (pz = pyrazolyl, BAr'4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was prepared. Heating a C6D6 solution of [(C(pz)4)Ru(P(OCH2)3CEt)(NCMe)Me][BAr'4] with 1 equiv of NCMe resulted in C-H activation of the 5-position of a pyrazolyl ring to yield [(κ(3)-(N,C(5),N)C(pz)4)Ru(P(OCH2)3CEt)(NCMe)2][BAr'4] and CH4. Intramolecular C-H activation of the 5-position of a pyrazolyl ring also occurred when (η(6)-p-cymene)Ru(P(OCH2)3CEt)(Br)Ph was heated in the presence of C(pz)4 to yield [(κ(3)-N,C(5),N)C(pz)4]Ru(P(OCH2)3CEt)(NCMe)Br and C6H6. Density functional theory calculations revealed that the different reactivities of TpRu(P(OCH2)3CEt)(NCMe)R and [(C(pz)4)Ru(P(OCH2)3CEt)(NCMe)Me][BAr'4] result from the stronger binding of the Tp pyrazolyl rings to Ru(II) compared to that of C(pz)4.