Impact of counteranions on N-heterocyclic carbene gold(i)-catalyzed cyclization of propargylic amide.

N-Heterocyclic carbene (NHC) Au(i)-catalyzed organic synthesis has recently been receiving increasing attention, especially with the activation of alkynes. In contrast, counteranions, being widely problematic in Au(i)-catalyzed transformations, are commonly considered as innocent partners and are not respectably included in a computational model. Herein, we report density functional theory (DFT) investigations of the Au(i)-catalyzed cyclization of propargylic amides to exploit the mechanistic effect of several counteranions to shed some light for further future developments. Among the counteranions used in this study, NTf2 -, ClO4 -, TsO-, TFA-, TfO-, MsO-, and SbF6 -, both the cyclization and protodeauration step favor the 5-exo-dig product over the 6-endo-dig product when the alkyne moiety is terminated with hydrogen. These anions reveal a crucial influence on the energy profile through lowering the barriers of the reaction. Mechanistically, the results obtained from all counteranions show that the protodeauration is slower than the cyclization. By using an energetic span model, the results clearly indicate that the rate-determining state is the protodeauration step for all counteranions, and thus protodeauration is the turnover-limiting step. The turnover frequency (TOF) results for the formation of the 5-exo-dig product show cyclization reactivity in the order of MsO- > TFA- > ClO4 - > NTf2 - > TfO- > TsO- ≫ SbF6 -, whereas an order of TFA- > MsO- > NTf2 - > TfO- ≈ ClO4 - > SbF6 - ⋙ TsO- is calculated for the protodeauration, suggesting that SbF6 - and TsO- are disfavored due to their slow protodeauration. In this regard, and for the 6-endo-dig pathway, our conclusions demonstrate an order of TfO- > TFA- > MsO- > NTf2 - > ClO4 - > TsO- ⋙ SbF6 - for the cyclization and TFA- > TsO- > MsO- > TfO- > NTf2 - > ClO4 - ⋙ SbF6 - for the protodeauration, advocating that the anions SbF6 -, NTf2 - and ClO4 - are unlikely partners for the 6-endo-dig pathway because of their slow protodeauration. Finally, the findings here advise that any engineering of the counteranion to increase the efficiency of catalytic system would be more effective on the protodeauration step rather than the cyclization step.

The nature of metal-metal bonding in Re-, Ru- and Os-corrole dimers.

Studies of multiple bonding between transition metal complexes offer fundamental insight into the nature of bonding between metal ions and facilitate predictions of the physical properties and the reactivities of metal complexes containing metal-metal multiple bonds. Here we report a computational interrogation on the nature of the metal-metal bonding for neutral, oxidized, and reduced forms of dinuclear rhenium and osmium corrole complexes, [{Re[TpXPC]}2]0/1+/1- and [{Os[TpXPC]}2]0/1+/1-, using a complete active space self-consistent (CASSCF) methodology and density functional theory (DFT) calculations. For [{Re[TpXPC]}2]0, [{Ru[TpXPC]}2]0, and [{Os[TpXPC]}2]0, CASSCF calculations shows that the effective bond order is 3.29, 2.63, and 2.73, respectively. On their oxidized forms, [{Re[TpXPC]}2]1+, [{Ru[TpXPC]}2]1+, and [{Os[TpXPC]}2]1+ molecules, the results indicate an electron removal from a ligand-based orbital, where [{Re[TpXPC]}2]1+ gives slightly different geometry from its neutral form due to populating the δ* orbital. In this regard, the CASSCF calculations give an effective bond order of 3.25 which is slightly lower than in the [{Re[TpXPC]}2]0. On their reduced forms, the electron addition appears to be in the metal-based orbital for [{Re[TpXPC]}2]1- and [{Ru[TpXPC]}2]1- whereas in the ligand-based orbital for the Os-analogue which has no effect on the Os-Os bonding, an effective bond order of 3.18 and 2.17 is presented for the [{Re[TpXPC]}2]1- and [{Ru[TpXPC]}2]1-, respectively, within the CASSCF simulations. These results will further encourage theoreticians and experimentalists to design metalloporphyrin dimers with distinct metal-metal bonding.

Structural, spectroscopic and electronic properties of a family of face-shared bi-octahedral Ru25+/6+ complexes with a bridging 2,5-di(2-pyridyl)pyrrolide ligand.

A family of Ru2 dimers, [Ru2(µ-κ2N,N':κ2N',N''-dpp)2(µ-X)(X)2]q+ (X = Cl, Br, q = 0 and X = I, q = 1) is synthesized from a [Ru2(OAc)4Cl] paddlewheel starting material. The neutral products are mixed-valence Ru25+ dimers with a Ru-Ru bond order of 0.5, while the cationic iodide is a Ru26+ dimer with formal bond order of 1.0. The Ru-Ru distance is strikingly independent of the identity of the halide and the oxidation state of ruthenium, most likely a consequence of the small bridging nitrogen which constrains the geometry. The spectroscopic properties (EPR, UV/Vis) of the Br complex are consistent with a large σ-σ* splitting in [Ru2(µ-κ2N,N':κ2N',N''-dpp)2(µ-Br)(Br)2].

Redox-Dependent Metal-Metal Bonding in Trinuclear Metal Chains: Probing the Transition from Covalent Bonding to Exchange Coupling.

The synthesis and physical properties of two new cationic tri-metallic chains, [(PEt3 )3 RuCl3 M'Cl3 Ru(PEt3 )3 ]1+ , M'=Rh and Ir are reported. These are isostructural with a previously reported 17-electron all-ruthenium analogue, but replacing a d5 RuIII ion in the central position with d6 RhIII /IrIII has a significant impact on the nature of the metal-metal interactions. All three materials have been characterized electrochemically at the 18-, 17- and 16-electron levels. X-ray crystallography and spectroelectrochemistry, complemented by electronic structure analysis at the DFT and CASPT2 levels, indicate that whilst the presence of a RuIII ion in the center of the chain allows multi-center covalent bonding to develop, a closed-shell RhIII /IrIII ion pushes the system towards the exchange-coupled limit, where the outer Ru centers are only weakly interacting. This family of three isostructural compounds reveals how changes in metal composition can have subtle effects on physical properties of systems that lie close to the localized/delocalized borderline.