The bis(ortho-)chelated platinum complex [PtCl(NCN)], 1 (NCN = [C6H3(CH2NMe2)2-2,6]-), has been used as a novel metal precursor for C−H bond activation and subsequent cyclometalation of the potentially terdentate coordinating pincer ligand PCHP (PCP = [C6H3(CH2PPh2)2-2,6]-). Depending on the reaction temperature various intermediates, each with unique structural features, could be isolated and identified during the formation of the final, bis(ortho-)cyclometalated product [PtCl(PCP)], 2. These results provide valuable insight into the intimate steps of this reaction for which we like to introduce the term transcyclometalation process (see note). At room temperature, a sparingly soluble dimetallic complex is formed, which comprises a η1-monodentate C-bound NCN ligand and a bridging, P,P η2-ε2-coordinating PCHP ligand. Solution spectroscopy and X-ray analysis of this platinum dimer established a C−H bond which is interacting by intramolecular noncovalent H···Cl hydrogen bonding with the Pt−Cl motif. As a further intermediate, the formation of a trans-bis(phenyl) complex [Pt(η3-PCP)(η1-HNCN)]Cl, 5, has been identified, which is characterized by a bis(ortho-)cyclometalated PCP ligand which is η3-P,C,P‘ coordinated to the platinum center as well as an η1-monodentate C-bound NCN ligand. The equivalent HCl, which is formally released during the formation of 5, is intramolecularly trapped by one of the basic amine groups, as is apparent from the identification of precursors of 5, i.e., compounds in which either one ([3]Cl) or two ([4]X 2 ) of the NMe2 groups have become protonated (see Scheme ). Remarkably, the proton in [4]X 2 is not only bound to the nitrogen lone pair but also interacts with the filled d z 2 orbital of the nucleophilic platinum(II) center, thus disclosing complex 5 as an organometallic proton sponge that is able to sequester protons due to Pt,N-bidentate chelating properties. Prolonged exposure of the reaction mixture at 80 °C or performing the reaction at 110 °C afforded the transcyclometalated complex 2 and 1 equiv of the diaminoarene NCHN as the two ultimate products.

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The synthesis and characterization of new, five-coordinate, diamagnetic, 16-electron arylruthenium(II) complexes [RuIIX{C6H2(CH2PPh2)2-2,6-R-4}(PPh3)] (1, X = Cl, R = H; 2, X = Cl, R = Ph; 3, X = OSO2CF3, R = H; 4, X = I, R = H) are described. These coordinatively unsaturated complexes contain a stable Caryl−Ru σ-bond resulting from pseudomeridional terdentate P,C,P‘-bonding of the monoanionic {C6H2(CH2PPh2)2-2,6-R-4}- ligand (general abbreviation PCP-R-4; for R = H and R = Ph, abbreviated as PCP and PCP-Ph, respectively) that also provides two P → Ru bonds. This bonding mode of the PCP-R-4 ligands adopted in complexes 1−4 means that these species are structurally closely related to the complexes [RuCl{C6H2(CH2NMe2)2-2,6-R-4}(PPh3)] (R = H, Ph) in which there is terdentate N,C,N‘-coordination. Complexes 1 and 2 were synthesized via cyclometalation reactions of the respective neutral diphosphine compounds C6H3(CH2PPh2)2-2,6-R-4 (general abbreviation PC(H)P-R-4; for R = H and R = Ph, abbreviated as PC(H)P and PC(H)P-Ph, respectively) with [RuIICl2(PPh3)4]. The complex [RuII(OTf)(PCP)(PPh3)], 3, prepared by reaction of 1 with AgOTf (OTf = OSO2CF3 = triflate), has the triflate anion bound to ruthenium in noncoordinating solvents. On the NMR time scale complex 3 in solution exhibits temperature-dependent fluxionality that is associated with reversible changes of the complex stereochemistry. The triflate PCP complex 3 reacts cleanly with the neutral N-donor ligand 2,2‘:6‘,2‘‘-terpyridine (terpy) to afford [RuII(PCP)(terpy)][OTf], 5. However, whereas the reaction of terpy with the PCP-Ph complex 2 affords [RuII(PCP-Ph)(terpy)]Cl, 6, its reaction with PCP chloro complex 1 generates a mixture of products. These reactivities of PCP and PCP-Ph complexes (1−3) toward terpy are compared and contrasted with those of related ruthenium complexes containing the monoanionic aryldiamine ligand {C6H2(CH2NMe2)2-2,6-R-4}- (abbreviated as NCN-R-4).

The Ru II complexes [RuCl(R-PCP)(PPh 3 )] (R-PCP ) [C 6 H 3 {CH(Me)PPh 2 } 2 -2,6] -(7a), [C 6 H 3 {CH(Et)PPh 2 } 2 -2,6] -(7b), or [C 6 H 2 {CH 2 PPh 2 } 2 -2,6-SiMe 3 -4] -(9)) were synthesized using distinct synthetic routes. One of these routes consists of the reaction of the parent arene compound R-PCHP (i.e., C 6 H 4 {CH(Me)PPh 2 } 2 -1,3 (6a), C 6 H 4 {CH(Et)PPh 2 } 2 -1,3 (6b), or C 6 H 3 (CH 2 PPh 2 ) 2 -2,6-SiMe 3 -4 (8)) with [RuCl 2 (PPh 3 ) 3 ] in 1,2-dichloroethane. Following this procedure, complexes 7a and 7b were obtained. However, desilylation of 8 was observed with concomitant formation of complex [RuCl(C 6 H 3 {CH 2 PPh 2 } 2 -2,6)(PPh 3 )] (4). A second synthetic approach has been developed and applied in the high-yield synthesis of complexes 7a, 7b, and 9. This method, a transcyclometalation (TCM) reaction, involves the interconversion of one cyclometalated ligand metal complex, M(C,E), into another complex M(C,E′) with the concomitant consumption and formation of the corresponding arenes HC,E′ and HC,E, respectively. Using this method, previously observed problems such as nontolerance to the presence of a SiMe 3 group, contamination of the final product by uncoordinated PPh 3 , or the formation of paramagnetic impurities were overcome. Investigations in solution of the formation of these square-pyramidal Ru complexes using two-dimensional NMR techniques ( 1 H-1 H COSY and 31 P-1 H COSY) have shown that both synthetic routes were stereoselective, yielding only three of the four expected stereoisomers of complexes 7a and 7b.