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The presence of Pd⋯Cu and Pd⋯Pd interactions as well as the order of metal atoms in a chain held by a modular polynucleating ligand is controlled by the coordinating ability of the anions, leading to selective formation of bi- and tetranuclear Pd/Cu and Pd4 chains. Metal–metal cooperative reactivity in these complexes was tested in Ar–O bond formation and alkyne activation.

Cooperation between two different metals plays a crucial role in many synergistic catalytic reactions, such as the Sonogashira C-C cross-coupling reaction, where an interaction between the Pd and Cu centers is proposed in the transmetalation step. Although several heterobimetallic Pd/Cu complexes were proposed as structural models of the active species in Sonogashira coupling, the detailed understanding of the metal-metal cooperation in transmetalation is still lacking in current systems. In this work, we report a stepwise and systematic approach to building heteromultimetallic Pd/Cu assemblies as a tool to study metal-metal cooperativity. We obtained fully characterized Pd/Cu multimetallic assemblies that show reactivity in alkyne activation, formation of catalytically relevant aryl/acetylide species, and C-C elimination, serving as functional models for Sonogashira reaction intermediates. The combined experimental and DFT studies highlight the importance of ligand-controlled coordination geometry, metal-metal distances and dynamics of the multimetallic assembly for transmetalation step.

We report the selective formation of heterobimetallic PtII/CuI complexes that demonstrate how facile bond activation processes can be achieved by altering the reactivity of common organoplatinum compounds through their interaction with another metal center. The interaction of the Cu center with the Pt center and with a Pt-bound alkyl group increases the stability of PtMe2 towards undesired rollover cyclometalation. The presence of the CuI center also enables facile transmetalation from an electron-deficient tetraarylborate [B(ArF)4]− anion and mild C–H bond cleavage of a terminal alkyne, which was not observed in the absence of an electrophilic Cu center. The DFT study indicates that the Cu center acts as a binding site for the alkyne substrate, while activating its terminal C–H bond., The selective formation of heterobimetallic PtII/CuI complexes demonstrates how facile bond activation processes can be achieved by altering the reactivity of common organoplatinum compounds through their interaction with another metal center.

Efficient iron-catalyzed homocoupling of terminal alkynes and cross-dimerization of aryl acetylenes with trimethylsilylacetylene is reported. The complex [Fe(H)(BH4 )(iPr-PNP)] (1) catalyzed the (cross-)dimerization of alkynes at room temperature, with no need for a base or other additives, to give the corresponding dimerized products with Z selectivity in excellent yields (79-99 %).

A novel pincer ligand based on the pyrazine backbone (PNzP) has been synthesized, (2,6-bis(di(tert-butyl)phosphinomethyl)pyrazine), tBu-PNzP. It reacts with FeBr2 to yield [Fe(Br)2(tBu-PNzP)], 1. Treatment of 1 with NaBH4 in MeCN/MeOH gives the hydride complex [Fe(H)(MeCN)2(tBu-PNzP)][X] (X = Br, BH4), 2·X. Counterion exchange and exposure to CO atmosphere yields the complex cis-[Fe(H)(CO)(MeCN)(tBu-PNzP)][BPh4] 4·BPh4, which upon addition of Bu4NCl forms [Fe(H)(Cl)(CO)(tBu-PNzP)] 5. Complex 5, under basic conditions, catalyzes the hydrogenation of CO2 to formate salts at low H2 pressure. Treatment of complex 5 with a base leads to aggregates, presumably of dearomatized species B, stabilized by bridging to another metal center by coordination of the nitrogen at the backbone of the pyrazine pincer ligand. Upon dissolution of compound B in EtOH the crystallographically characterized complex 7 is formed, comprised of six iron units forming a 6-membered ring. The dearomatized species can activate CO2 and H2 by metal-ligand cooperation (MLC), leading to complex 8, trans-[Fe(PNzPtBu-COO)(H)(CO)], and complex 9, trans-[Fe(H)2(CO)(tBu-PNzP)], respectively. Our results point at a very likely mechanism for CO2 hydrogenation involving MLC.

Cationic five-coordinate Pt(IV) alkyl complexes stabilized by bulky N-heterocyclic carbenes have been isolated and fully characterized. Related species have been postulated as key intermediates in carbon-heteroatom coupling reactions and most particularly in Shilov-type chemistry. The alkyl groups exhibit a pronounced electrophilic character and can undergo nucleophilic addition of pyridine, bromide, or iodide to form new carbon-heteroatom bonds. Nevertheless, direct reductive coupling to form C-X bonds can be operative in the absence of an external nucleophile source

The reaction of the N-heterocyclic carbenes (NHCs) IiPr 2Me2 and IiPr2 with [PtMe 3I]4 does not lead to the clean formation of complexes trans-[PtMeI(NHC)2], as previously found for bulkier NHCs. However, these species can be prepared in high yields by an alternative procedure using the dimethyl species [PtMe2(NHC)2] and iodomethane. The halogenated complexes trans-[PtMeI(NHC)2] have been used as precursors for the formation of the coordinatively unsaturated Pt(II) derivatives [PtMe(NHC)2][SbF6] using AgSbF6 as a halogen abstractor, whereas NaBArF is not reactive enough to completely remove the iodide ligand, leading instead to the dinuclear species [Pt2(¿-I)(Me)2(NHC)4][BArF]. Alternatively, low-coordinate T-shaped complexes [PtMe(NHC)2] [BArF] can be prepared by protonation of the dimethyl species [PtMe2(NHC)2] with [H(OEt2)2] [BArF]. The complex [PtMe(IiPr2Me 2)2][BArF] undergoes a cyclometalation process under mild heating, leading to the derivative [Pt(IiPr 2Me2¿)(IiPr2Me 2)][BArF] (where the prime denotes the cyclometalated ligand). This result is in contrast with the fast cyclometalation observed for the related Pt(II) complex bearing tert-butyl-substituted NHC (ItBu). Different cyclometalation reaction mechanisms have been computationally addressed. DFT calculations indicate that the cyclometalation involving complexes with not very bulky NHCs ligands occurs via cis intermediates. Cyclometalation products bearing tert-butyl-substituted NHCs are kinetically and thermodynamically favored over those involving isopropyl groups. © 2014 American Chemical Society.

Efficient iron-catalyzed homocoupling of terminal alkynes and cross-dimerization of aryl acetylenes with trimethylsilylacetylene is reported. The complex [Fe(H)(BH4 )(iPr-PNP)] (1) catalyzed the (cross-)dimerization of alkynes at room temperature, with no need for a base or other additives, to give the corresponding dimerized products with Z selectivity in excellent yields (79-99 %).

Addition of Br(2) or I(2) to 14-electron, cationic Pt(II)-alkyl complexes led to the formation of the corresponding carbon-halogen Pt(II) coupling products. Low temperature experiments with Br(2) allowed us to isolate and characterize crystallographically a very unusual mononuclear, paramagnetic Pt(III)-alkyl intermediate with a seesaw structure that can be further oxidized to a transient Pt(IV) species before reductive carbon-halogen coupling reaction takes place.