Mechanistic investigations via DFT support the cooperative heterobimetallic C-H and O-H bond activation across Ta[double bond, length as m-dash]Ir multiple bonds

Site-Isolated Rhodium(II) Metalloradicals Catalyze Olefin Hydrofunctionalization

Rh(II) porphyrin complexes display pronounced metal-centered radical character and the ability to activate small molecules under mild conditions, but catalysis with Rh(II) porphyrins is extremely rare. In addition to facile dimerization, Rh(II) porphyrins readily engage in kinetically and thermodynamically facile reactions involving two Rh(II) centers to generate stable Rh(III)-X intermediates that obstruct turnover in thermal catalysis. Here we report site isolation of Rh(II) metalloradicals in a MOF host, which not only protects Rh(II) metalloradicals against dimerization, but also allows them to participate in thermal catalysis. Access to PCN-224 or PCN-222 in which the porphyrin linkers are fully metalated by Rh(II) in the absence of any accompanying Rh(0) nanoparticles was achieved via the first direct MOF synthesis with a linker containing a transition-metal alkyl moiety, followed by Rh(III)-C bond photolysis.

Highly Selective and Efficient Perdeuteration of n-Pentane via H/D Exchange Catalyzed by a Silica-Supported Hafnium-Iridium Bimetallic Complex

A Surface OrganoMetallic Chemistry (SOMC) approach is used to prepare a novel hafnium-iridium catalyst immobilized on silica, HfIr/SiO2, featuring well-defined [≡SiOHf(CH2 tBu)2(μ-H)3IrCp*] surface sites. Unlike the monometallic analogous materials Hf/SiO2 and Ir/SiO2, which promote n-pentane deuterogenolysis through C-C bond scission, we demonstrate that under the same experimental conditions (1 bar D2, 250 °C, 3 h, 0.5 mol %), the heterobimetallic catalyst HfIr/SiO2 is highly efficient and selective for the perdeuteration of alkanes with D2, exemplified on n-pentane, without substantial deuterogenolysis (<2 % at 95 % conversion). Furthermore this HfIr/SiO2 catalyst is robust and can be re-used several times without evidence of decomposition. This represents substantial advance in catalytic H/D isotope exchange (HIE) reactions of C(sp3)-H bonds.

Valence Tautomerism Induced Proton Coupled Electron Transfer:X-H Bond Oxidation with a Dinuclear Au(II) Hydroxide Complex

We report the preparation and characterization of the dinuclear AuII hydroxide complex AuII 2(L)2(OH)2 (L=N,N'-bis (2,6-dimethyl) phenylformamidinate) and study its reactivity towards weak X-H bonds. Through the interplay of kinetic analysis and computational studies, we demonstrate that the oxidation of cyclohexadiene follows a concerted proton-coupled electron transfer (cPCET) mechanism, a rare type of reactivity for Au complexes. We find that the Au-Au σ-bond undergoes polarization in the PCET event leading to an adjustment of oxidation levels for both Au centers prior to C(sp3)-H bond cleavage. We thus describe the oxidation event as a valence tautomerism-induced PCET where the basicity of one reduced Au-OH unit provides a proton acceptor and the second more oxidized Au center serves as an electron acceptor. The coordination of these events allows for unprecedented radical-type reactivity by a closed shell AuII complex.

Cooperative activation of carbon-hydrogen bonds by heterobimetallic systems

The direct activation of C-H bonds has been a rich and active field of organometallic chemistry for many years. Recently, incredible progress has been made and important mechanistic insights have accelerated research. In particular, the use of heterobimetallic complexes to heterolytically activate C-H bonds across the two metal centers has seen a recent surge in interest. This perspective article aims to orient the reader in this fast moving field, highlight recent progress, give design considerations for further research and provide an optimistic outlook on the future of catalytic C-H functionalization with heterobimetallic complexes.

On the use of 17O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes

¹⁷O-labelled tungsten siloxide complexes [WOCl₂(OSitBu₃)₂] (1-Cl) and [WOMe₂(OSitBu₃)₂] (1-Me) were prepared and characterized by ¹⁷O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking ¹⁷O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO_2-700 afforded material 2, with surface species [(SiO)WOMe₂(OSitBu₃)] as shown by elemental analysis, IR and ¹H and ¹³C MAS NMR. The DFT calculations of the grafting mechanism are in line with the observed reactivity. They indicate the occurrence of several isomeric species of close energy for the grafted W centers, precluding efficient ¹⁷O MAS NMR studies. The lack of catalytic activity in olefin metathesis and ring-opening olefin metathesis polymerization indicates that initiation by α-H elimination is not operative in 2, contrary to related tungsten surface species, which illustrates the crucial influence of the nature of the metal coordination sphere.

A versatile strategy for the formation of hydride-bridged actinide-iridium multimetallics

Reaction of the potassium pentamethylcyclopentadienyl iridate tris-hydride K[IrCp*H₃] with UCl₄ and ThCl₄(DME)₂ led to the complete replacement of the halide ligands to generate multimetallic complexes U{(μ-H)₃IrCp*}₄ (1) and Th{[(μ-H₂)(H)IrCp*]₂[(μ-H)₃IrCp*]₂} (2), respectively. These analogues feature a significant discrepancy in hydride bonding modes; 1 contains twelve bridging hydrides while 2 contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(iii) starting material, UI₃(1,4-dioxane)_1.5, resulted in the octanuclear complex {U[(μ²-H₃)IrCp*]₂[(μ³-H₂)IrCp*]}₂ (3). Computational studies indicate significant bonding character between U/Th and Ir in 1 and 2, with f-orbital involvement in the singly-occupied molecular orbitals of the uranium species 1. In addition, these studies attribute the variation in hydride bonding between 1 and 2 to differences in dispersion effects.

Highly Dispersed Silica-Supported Iridium and Iridium-Aluminium Catalysts for Methane Activation Prepared via Surface Organometallic Chemistry

The grafting of an iridium-aluminium precursor onto silica followed by thermal treatment under H2 yields small (<2 nm), narrowly distributed nanoparticles used as catalysts for methane H/D exchange. This Ir-Al/SiO2...

Strongly Polarized Iridiumδ--Aluminumδ+ Pairs: Unconventional Reactivity Patterns Including CO2 Cooperative Reductive Cleavage

The iridium tetrahydride complex Cp*IrH₄ reacts with a range of isobutylaluminum derivatives of general formula Al(iBu)_x(OAr)_3-x (x = 1, 2) to give the unusual iridium aluminum species [Cp*IrH₃Al(ⁱBu)(OAr)] (1) via a reductive elimination route. The Lewis acidity of the Al atom in complex 1 is confirmed by the coordination of pyridine, leading to the adduct [Cp*IrH₃Al(ⁱBu)(OAr)(Py)] (2). Spectroscopic, crystallographic, and computational data support the description of these heterobimetallic complexes 1 and 2 as featuring strongly polarized Al(III)^δ+-Ir(III)^δ- interactions. Reactivity studies demonstrate that the binding of a Lewis base to Al does not quench the reactivity of the Ir-Al motif and that both species 1 and 2 promote the cooperative reductive cleavage of a range of heteroallenes. Specifically, complex 2 promotes the decarbonylation of CO₂ and AdNCO, leading to CO (trapped as Cp*IrH₂(CO)) and the alkylaluminum oxo ([(iBu)(OAr)Al(Py)]₂(μ-O) (3)) and ureate ({Al(OAr)(ⁱBu)[κ²-(N,O)AdNC(O)NHAd]} (4)) species, respectively. The bridged amidinate species Cp*IrH₂(μ-CyNC(H)NCy)Al(ⁱBu)(OAr) (5) is formed in the reaction of 2 with dicyclohexylcarbodiimine. Mechanistic investigations via DFT support cooperative heterobimetallic bond activation processes.