Transcyclometalation processes with late transition metals: C-aryl-H bond activation via noncovalent C-H center dot center dot center dot interactions

The bis(ortho-)chelated platinum complex [PtCl(NCN)], 1 (NCN = [C6H3(CH2NMe 2)(2)-2 6](-)), has been used as a novel metal precursor for C-H bond activ ation and subsequent cyclometalation of the potentially terdentate coordina ting pincer ligand PCHP (PCP = [C6H3(CH2PPh2)(2)-2,6](-)); Depending on the :reaction temperature various intermediates, each with unique structural fe atures, could be isolated and identified during the formation of the final, bis(ortho-)cyclometalated product [PtCl(PCP)], 2. These results provide va luable:: insight into the intimate steps of this reaction for which we like to introduce the term transcyclometalation process (see note). Ar room tem perature, a sparingly soluble dimetallic complex is formed, which comprises a eta (1)-monodentate C-bound NCN ligand:and a bridging, P,P eta (2)-epsil on (2)-coordinating PCHP ligand.:Solution spectroscopy and X-ray analysis o f this platinum dimer established a C-H bond which is interacting by intram olecular noncovalent H . . . Cl hydrogen bonding with the Pt-CI motif. As a further intermediate, the formation of a trans-bis(phenyl) complex [Pt(eta (3)-PCP)(eta (1)-HNCN)]Cl, 5, has been identified, which is characterized by a bis(ortho-)cyclometalated PCP ligand which is eta (3)-P, C, P' coordin ated to the platinum center as well as an eta (1)-monodentate C-bound NCN l igand. The equivalent HCl, which is formally released during the formation of 5, is intramolecularly trapped by one of the basic amine groups, as is a pparent from the identification of precursors of 5, i.e., compounds in whic h either one ([3]Cl) or two ([4]X-2) of the NMe2 groups have become protona ted (see Scheme:5): Remarkably, the proton in [4]X-2 is not only bound to t he nitrogen lone pair but also interacts with the filled d(z2) orbital of t he nucleophilic platinum(II) center, thus disclosing complex 5 as an organo metallic proton sponge that is able to sequester protons due to Pt,N-bident ate chelating properties. Prolonged exposure of the reaction mixture at 80 degreesC or performing the reaction at 110 degreesC afforded the transcyclo metalated complex 2 and 1 equiv of the diaminoarene NCHN as the two ultimat e products.