C–H Activation of Pyrazolyl Ligands by Ru(II)

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 [(κ3-(N,C5,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 (η6-p-cymene)­Ru­(P­(OCH2)3­CEt)­(Br)­Ph was heated in the presence of
C­(pz)4 to yield [(κ3-N,C5,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.