C-H Activation of Pyrazolyi Ligands by Ru(II)

Previously, hydridotris(pyrazoly)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)(3)CEt)(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)(3)CEt)(NCMe)Me][BAr'(4)] with 1 equiv of NCMe resulted in C-H activation of the 5-position of a pyrazolyl ring to yield [(kappa(3)-(N,C-5,N)C(pz)(4))Ru(P(OCH2)(3)CEt)(NCMe)(2)][BAr'(4)] and CH4. Intramolecular C-H activation of the 5-position of a pyrazolyl ring also occurred when (eta(6)-p-cymene)Ru(P(OCH(2))3CEt)(Br)Ph was heated in the presence of C(pz)(4) to yield [(kappa(3)-N,C-5,N)C(pz)(4))]Ru(P(OCH2)(3)CEt)(NCMe)Br and C6H6. Density functional theory calculations revealed that the different reactivities of TpRu(P(OCH2)(3)CEO(NCMe)R and [(C(pz)(4))Ru(P(OCH2)(3)CEt)(NCMe)Me][BAr'(4)] result from the stronger binding of the Tp pyrazolyl rings to Ru(II) compared to that of C(pz)(4).