Platinum(IV) Complexes with Chelating N-Heterocyclic Carbene Ligands

Relative stabilities of M/NHC complexes (M = Ni, Pd, Pt) against R-NHC, X-NHC and X-X couplings in M(0)/M(ii) and M(ii)/M(iv) catalytic cycles: a theoretical study

The complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via proposed typical M⁰/M^II catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, M^II/M^IV catalytic cycles have been proposed for a number of reactions. The catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling and the side reductive elimination of the NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R]⁺ cations. In this study, the relative stabilities of (NHC)M^II(R)(X)L and (NHC)M^IV(R)(X)₃L intermediates (X = Cl, Br, I; L = NHC, pyridine) against R-NHC coupling and other decomposition pathways via reductive elimination reactions were evaluated theoretically. The study revealed that the R-NHC coupling represents the most favorable decomposition pathway for both types of intermediates (M^II and M^IV), while it is thermodynamically and kinetically more facile for the M^IV complexes. The relative effects of the metal M (Ni, Pd, Pt) and ligands L and X on the R-NHC coupling for the M^IV complexes were significantly stronger than that for the M^II complexes. In particular, for the (NHC)₂M^IV(Ph)(Br)₃ complexes, Ph-NHC coupling was facilitated dramatically from Pt (ΔG = -36.9 kcal mol^-1, ΔG^≠ = 37.5 kcal mol^-1) to Pd (ΔG = -61.5 kcal mol^-1, ΔG^≠ = 18.3 kcal mol^-1) and Ni (ΔG = -80.2 kcal mol^-1, ΔG^≠ = 4.7 kcal mol^-1). For the M^II oxidation state of the metal, the bis-NHC complexes (L = NHC) were slightly more kinetically and thermodynamically stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). An inverse relation was observed for the M^IV oxidation state of the metal as the (NHC)₂M^IV(R)(X)₃ complexes were kinetically (4.3-15.9 kcal mol^-1) and thermodynamically (8.0-23.2 kcal mol^-1) significantly less stable than the (NHC)M^IV(R)(X)₃L (L = pyridine) complexes. For the Ni^IV and Pd^IV complexes, additional decomposition pathways via the reductive elimination of the NHC and X ligands to give the [NHC-X]⁺ cation (X-NHC coupling) or reductive elimination of the X-X molecule were found to be thermodynamically and kinetically probable. Overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not additionally stabilized by chelation) and high probability to initiate "NHC-free" catalysis in the reactions comprising M^IV intermediates.

Synthesis, structures and cytotoxic effects in vitro of cis- and trans-[PtIVCl4(NHC)2] complexes and their PtII precursors

Four new bis(N,N-dialkylbenzimidazol-2-ylidene)dichlorido platinum(ii) complexes 2 featuring N-alkyl substituents of increasing size (a: Me, b: Et, c: n-butyl, d: n-octyl) were synthesised and oxidised with PhICl₂ to give the corresponding [Pt^IVCl₄(N,N-dialkylbenzimidazol-2-ylidene)₂] complexes 4 as potential anticancer prodrugs. The known bis(N,N-dibenzylimidazol-2-ylidene)dichlorido platinum(ii) complex 1 was likewise oxidised to [Pt^IVCl₄(N,N-dibenzylimidazol-2-ylidene)₂] 3. In contrast, oxidation of complexes 1 and 2 with H₂O₂ or hypochlorites, or exchange of chlorido for hydroxo ligands in tetrachlorido complexes 4 failed to give isolable complexes of type [Pt^IVCl_4-n(OH)_n(NHC)₂]. In MTT assays the [Pt^IICl₂(NHC)₂]/[Pt^IVCl₄(NHC)₂] complex couples 1/3, 2c/4c, and trans-2c/trans-4c, bearing either N-benzyl or N-butyl substituents, each showed similar single-digit micromolar IC₅₀ values against at least three out of five human cancer cell lines, presumably due to an intracellular reduction of the Pt^IV complexes to their active Pt^II congeners. Unlike cisplatin, whose anticancer effect requires functional p53, each of them was active both in wildtype and in p53-negative HCT116 colon carcinoma cells. In ethidium bromide saturation assays with isolated DNA, cis-(bis-NHC)Pt^II complexes such as 1 caused morphological DNA changes more pronounced than those initiated by cisplatin, while the corresponding cis-(bis-NHC)Pt^IV complexes such as 3 interacted with DNA in a less structure-modifying way.

Platinum(iv) N-heterocyclic carbene complexes: their synthesis, characterisation and cytotoxic activity

Platinum(ii) N-heterocyclic carbene complexes have been oxidized by bromine or iodobenzene dichloride to provide the fully characterised corresponding platinum(iv) NHC complexes. Antiproliferative activities of Pt(iv) NHC complexes were assayed against several cancer cell lines and the results were correlated with respect to their stability. Mechanistic investigations revealed that mitochondrial dysfunction and ROS production were associated with the cytotoxic process induced by these compounds.

Alkane C-H functionalization and oxidation with molecular oxygen

The application of environmentally benign, cheap, and economically viable oxidation procedures is a key challenge of homogeneous, oxidative alkane functionalization. The typically harsh reaction conditions and the propensity of dioxygen for radical reactivity call for extraordinary robust catalysts. Mainly three strategies have been applied. These are (1) the combination of a catalyst responsible for C-H activation with a cocatalyst responsible for dioxygen activation, (2) transition-metal catalysts, which react with both hydrocarbons and molecular oxygen, and (3) the introduction of very robust main-group element catalysts for C-H functionalization chemistry. Herein, these three approaches will be assessed and exemplified by the reactivity of chelated palladium (N-heterocyclic carbene) catalysts in combination with a vanadium cocatalyst, the methane functionalization by cobalt catalysts, and the reaction of group XVII compounds with alkanes.

Palladium(ii) complexes with electron-poor, 4,5-disubstituted diimidazol-2-ylidene ligands: synthesis, characterization and catalytic activity

Palladium(ii) complexes with N-heterocyclic dicarbene ligands featuring electron-withdrawing substituents have been prepared and tested as catalysts in alkyne hydroarylations.

Propane activation by palladium complexes with chelating bis(NHC) ligands and aerobic cooxidation

The development of efficient aerobic oxidation methods remains a challenge for the selective functionalization of C-H bonds in alkanes. Herein we report the development of a C-H functionalization procedure for propane by using a palladium catalyst with chelating bis(N-heterocyclic carbene) ligands in trifluoroacetic acid together with a vanadium co-catalyst. Halides play a decisive role in the reaction. The experimental results are presented together with supporting kinetic data and an isotope effect. The reaction can be run with dioxygen as the oxidant if vanadium salts and halides are present in the reaction mixture. Experimental as well as computational results favor a mechanism involving C-H activation by palladium(II), followed by oxidation to palladium(IV) by bromine.

π-back-bonding interaction depending on the bridging chain lengths of chelated N-heterocyclic carbene platinum units in heterometallic trinuclear complexes affecting their electrochemical property

Newly synthesized heterometallic trinuclear M2Pt complexes (M = Rh, Ir) containing a platinum moiety having a chelated bis-N-heterocyclic carbene (bisNHC) ligand with a variety of alkylene chain lengths of the bridging part showed two reversible reduction waves in cyclic voltammetry. Only the second reduction potentials were affected by the alkyl chain lengths, which afforded different dihedral angles between the imidazolylidene rings and the platinum coordination plane resulting in the variation of π-back-donation from the platinum center to the carbene carbon atoms.