CO2 cleavage by tantalum/M (M = iridium, osmium) heterobimetallic complexes.

A novel Ta/Os heterobimetallic complex, [Ta(CH2tBu)3(µ-H)3OsCp*], 2, is prepared by protonolysis of Ta(CHtBu)(CH2tBu)3 with Cp*OsH5. Treatment of 2 and its iridium analogue [Ta(CH2tBu)3(µ-H)2IrCp*], 1, with CO2 under mild conditions reveal the efficient cleavage of CO2, driven by the formation of a tantalum oxo species in conjunction with CO transfer to the osmium or iridium fragments, to form Cp*Ir(CO)H2 and Cp*Os(CO)H3, respectively. This bimetallic reactivity diverges from more classical CO2 insertion into metal-X (X = metal, hydride, alkyl) bonds.

Probing Basal and Prismatic Planes of Graphitic Materials for Metal Single Atom and Subnanometer Cluster Stabilization.

Supported metal single atom catalysis is a dynamic research area in catalysis science combining the advantages of homogeneous and heterogeneous catalysis. Understanding the interactions between metal single atoms and the support constitutes a challenge facing the development of such catalysts, since these interactions are essential in optimizing the catalytic performance. For conventional carbon supports, two types of surfaces can contribute to single atom stabilization: the basal planes and the prismatic surface; both of which can be decorated by defects and surface oxygen groups. To date, most studies on carbon-supported single atom catalysts focused on nitrogen-doped carbons, which, unlike classic carbon materials, have a fairly well-defined chemical environment. Herein we report the synthesis, characterization and modeling of rhodium single atom catalysts supported on carbon materials presenting distinct concentrations of surface oxygen groups and basal/prismatic surface area. The influence of these parameters on the speciation of the Rh species, their coordination and ultimately on their catalytic performance in hydrogenation and hydroformylation reactions is analyzed. The results obtained show that catalysis itself is an interesting tool for the fine characterization of these materials, for which the detection of small quantities of metal clusters remains a challenge, even when combining several cutting-edge analytical methods.

Photolysis-driven bond activation by thorium and uranium tetraosmate polyhydride complexes.

Transition metal multimetallic complexes have seen intense study due to their unique bonding and potential for cooperative reactivity, but actinide-transition metal (An-TM) species are far less understood. We have synthesized uranium- and thorium-osmium heterometallic polyhydride complexes in order to study An-Os bonding and investigate the reactivity of An-Os interactions. Computational studies suggest the presence of a significant bonding interaction between the actinide center and the four coordinated osmium centers supported by bridging hydrides. Upon photolysis, these complexes undergo intramolecular C-H activation with the formation of an Os-Os bond, while the thorium complex may activate an additional C-H bond of the benzene solvent, resulting in a µ-η1,η1 phenyl ligand across one Th-Os interaction.

π-Bonding of Group 11 Metals to a Tantalum Alkylidyne Alkyl Complex Promotes Unusual Tautomerism to Bis-alkylidene and CO2 to Ketenyl Transformation.

A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex [Li(THF)2][Ta(CtBu)(CH2tBu)3], 1, reacts with (IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species [Ta(CH2tBu)3(µ-CtBu)M(IPr)] 2-M. Interestingly, π-bonding of group 11 metals to the Ta─C moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with [Ta(CHtBu)(CH2tBu)2(µ-CHtBu)M(IPr)] 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO2 resulting in rare examples of coinage metal ketenyl species, (tBuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.

Oxidative Addition of E-H (E=C, N) Bonds to Transient Uranium(II) Centers.

Two-electron oxidative addition is one of the most important elementary reactions for d-block transition metals but it is uncommon for f-block elements. Here, we report the first examples of intermolecular oxidative addition of E-H (E=C, N) bonds to uranium(II) centers. The transient U(II) species was formed in-situ by reducing a heterometallic cluster featuring U(IV)-Pd(0) bonds with potassium-graphite (KC8). Oxidative addition of C-H or N-H bonds to the U(II) centers was observed when this transient U(II) species was treated with benzene, carbazole or 1-adamantylamine, respectively. The U(II) centers could also react with tetracene, biphenylene or N2O, leading to the formation of arene reduced U(IV) products and uranyl(VI) species via two- or four-electron processes. This study demonstrates that the intermolecular two-electron oxidative addition reactions are viable for actinide elements.

Planar Tetranuclear Uranium Hydride Cluster Supported by ansa-Bis(cyclopentadienyl) Ligands.

Polynuclear metal hydride clusters play important roles in various catalytic processes, with most of the reported polynuclear metal hydride clusters adopting a polyhedral three-dimensional structure. Herein, we report the first example of a planar tetranuclear uranium hydride cluster [(CpCMe2CMe2Cp)U]4(µ2-H)4(µ3-H)4 (U4H8). It was synthesized by reacting an ansa-bis(cyclopentadienyl) ligand-supported uranium chloride precursor [(CpCMe2CMe2Cp)U]3(µ2-Cl)3(µ3-Cl)2 with NaHBEt3. The presence of hydrides in U4H8 was confirmed by NMR spectroscopy and its reactivity with phenol and carbon tetrachloride. DFT calculations also facilitated the determination of the hydrides' positions in U4H8, featuring four bridging µ2-H ligands and four face-capping µ3-H ligands, with the four U centers arranged in a rhombic geometry. The U4H8 represents not only the first example of planar polynuclear actinide metal hydride cluster but also the uranium hydride cluster with the highest nuclearity reported to date.

Multimetallic Uranium Nitride Cubane Clusters from Dinitrogen Cleavage.

Dinitrogen cleavage provides an attractive but poorly studied route to the assembly of multimetallic nitride clusters. Here, we show that the monoelectron reduction of the dinitrogen complex [{U(OC6H2-But3-2,4,6)3}2(µ-η2:η2-N2)], 1, allows us to generate, for the first time, a uranium complex presenting a rare triply reduced N2 moiety ((µ-η2:η2-N2)•3-). Importantly, the bound dinitrogen can be further reduced, affording the U4N4 cubane cluster, 3, and the U6N6 edge-shared cubane cluster, 4, thus showing that (N2)•3- can be an intermediate in nitride formation. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide. These results show that dinitrogen reduction provides a versatile route for the assembly of large highly reactive nitride clusters, with U6N6 providing the first example of a molecular nitride of any metal formed from a complete cleavage of three N2 molecules.

A binuclear guanidinate yttrium carbyne complex: unique reactivity toward unsaturated C-N, C-O and C-S bonds.

A guanidinato-stabilized binuclear yttrium carbyne complex [(PhCH2)2NC(NC6H3iPr2-2,6)2]2Y2(µ2-Me)(AlMe3)2(µ4-CH) (1) was synthesized via C-H bond activation and its versatile reactivities were investigated. Complex 1 underwent σ-bond metathesis with PhSSPh and nucleophilic addition with PhCN to form the corresponding yttrium thiolate complex 3 and aza-allyl complex 4 respectively. Additionally, the rare yttrium carbide complex 5 was also prepared by treatment of complex 1 with S8. Interestingly, in the reaction with PhNCS, the C[double bond, length as m-dash]S double bond was cleaved, followed by C-H bond activation to give the yttrium sulfide complex 7 with a ketenimine dianion ligand. Unexpectedly, the reaction of complex 1 with CO (1 atm) resulted in deoxygenative coupling of CO, to afford mono- or dioxo-yttrium complexes at different temperatures. The mechanism of the possible formation processes of complexes 3 and 9 was elucidated by DFT calculations.

Monodentate Alkoxy/Aryloxy Anchored Yttrium Dihydrides; Anticipated Outcomes and Surprises.

The synthesis, characterization, and solid-state structure of bulky alkoxy- and aryloxy-supported yttrium polynuclear hydrides are reported. Hydrogenolysis of the supertrityl alkoxy anchored yttrium dialkyl, Y(OTr*)(CH2SiMe3)2(THF)2 (1) (Tr* = tris(3,5-di-tert-butylphenyl)methyl), resulted in the clean conversion to the tetranuclear dihydride, [Y(OTr*)H2(THF)]4 (1a). X-ray analysis revealed a highly symmetrical structure (4̅ site symmetry) with the four Y atoms located on the corners of a compressed tetrahedron, each bonded to an OTr* and tetrahydrofuran (THF) ligand and the cluster held together by four face-capping, µ3-H, and four edge-bridging, µ2-H, hydrides. DFT calculations on the full system with and without THF, but also on model systems, clearly show that the structural preference for complex 1a is controlled by the presence and coordination of THF molecules. Contrary to the exclusive formation of the tetranuclear dihydride, hydrogenolysis of the bulky aryloxy yttrium dialkyl, Y(OAr*)(CH2SiMe3)2(THF)2 (2) (Ar* = 3,5-di-tert-butylphenyl) gave a mixture of the analogous tetranuclear 2a and trinuclear, [Y3(OAr*)4H5(THF)4], polyhydride, 2b. Similar results, i.e., a mixture of tetra-/tri-nuclear products, were obtained from hydrogenolysis of the even bulkier Y(OArAd2,Me)(CH2SiMe3)2(THF)2 compound. Experimental conditions were established to optimize the production of either the tetra- or trinuclear products. X-ray structure of 2b revealed a triangular array of three yttrium atoms with two face-capping µ3-H and three edge-bridging µ2-H hydrides, with one yttrium bonded to two aryloxy ligands while the other two have a complement of one aryloxy and two THF ligands; the solid-state structure is close to being C2 symmetric, with the C2 axis running through the unique Y and unique µ2-H hydride. As opposed to 2a, which shows distinct 1H NMR resonances for µ3/ µ2-H (δ = 5.83/6.35 ppm, respectively), no hydride signals for 2b were observed at room temperature, indicating hydride exchange on the NMR time scale. Their presence and assignment were secured at -40 °C from 1H SST (spin saturation) experiment.

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2 Mn(N2 )].

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)2 Mn(N2 )] (1, Cp=η5 -cyclopentadienyl, C5 H5 ) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N2 )MnCOPh]Li (3) that is stable only below -40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above -20 °C with N2 loss to give a phenylate complex [Cp(CO)2 MnPh]Li (2). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)2 MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N2 ligand in 1. DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N2 remains to be demonstrated.

Synthesis and Reactivity of a Zinc Diazoalkyl Complex: [3+2] Cycloaddition Reaction with Carbon Monoxide.

This work reports the synthesis, characterization, and reactivity of the first example of a well-defined zinc α-diazoalkyl complex. Treatment of zinc(I)-zinc(I) bonded compound L2 Zn2 [L=CH3 C(2,6-i Pr2 C6 H3 N)CHC(CH3 )(NCH2 CH2 PPh2 )] or zinc(II) hydride LZnH with trimethylsilyldiazomethane affords zinc diazoalkyl complex LZnC(N2 )SiMe3 . This complex liberates N2 in the presence of a nickel catalyst to form an α-zincated phosphorus ylide by reacting with the pendant phosphine. It selectively undergoes formal [3+2] cycloaddition with CO2 or CO to form the corresponding product with a five-membered heterocyclic core. Notably, the use of CO in such a [3+2] cycloaddition reaction is unprecedented, reflecting a novel CO reaction mode.

1,2-Palladasilacyclobutene: The Missing Link in the Pd-Catalyzed Annulation of Alkynes for the Silirene-to-Silole Transformation.

The palladium-catalyzed annulation reaction of alkynes enables an attractive approach to siloles. Their access from silirenes and terminal alkynes proved rather general, involving reactive intermediates that have remained elusive to date. Starting from 1,2-bis(3-thienyl)silirene as a source of photochromic siloles, the mechanism of the annulation reaction has been revisited, and palladasilacyclobutenes resulting from the activation of the silirene could be isolated and thoroughly characterized (NMR, X-ray, and DFT). Their role as reactive intermediates and their fate in the course of the reaction were also studied in situ. In combination with in-depth DFT calculations, a clearer picture of the mechanism and the reactive key species is disclosed.

Magnesium(I) Reduction of Aluminum(III) Hydride Complexes: Generation of Mixed Valence Aluminum (AlI /Al0 ) Hydride Cluster Compounds, [Al6 H8 (NR3 )2 {Mg(ß-diketiminate)}4 ].

Reduction of a range of amido- and aryloxy-aluminum dihydride complexes, e.g. [AlH2 (NR3 ){N(SiMe3 )2 }] (NR3 =NMe3 or N-methylpiperidine (NMP)), with ß-diketiminato dimagnesium(I) reagents, [{(Ar Nacnac)Mg}2 ] (Ar Nacnac=[HC(MeCNAr)2 ]- , Ar=mesityl (Mes) or 2,6-xylyl (Xyl)), have afforded deep red mixed valence aluminum hydride cluster compounds, [Al6 H8 (NR3 )2 {Mg(Ar Nacnac)}4 ], which have an average Al oxidation state of +0.66, the lowest for any well-defined aluminum hydride compound. In the solid-state, the clusters are shown to have distorted octahedral Al6 cores, having zero-valent Al axial sites and mono-valent AlH2 - equatorial units. Several novel by-products were isolated from the reactions that gave the clusters, including the Mg-Al bonded magnesio-aluminate complexes, [(Ar Nacnac)(Me3 N)Mg-Al(µ-H)3 [{Mg(Ar Nacnac)}2 (µ-H)]]. Computational analyses of one aluminum hydride cluster revealed its Al6 core to be electronically delocalized, and to possess one unoccupied, and six occupied, skeletal molecular orbitals.

Correction: Isolation of C1 through C4 derivatives from CO using heteroleptic uranium(iii) metallocene aryloxide complexes.

[This corrects the article DOI: 10.1039/D2SC06375A.].

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.

Isolation of C1 through C4 derivatives from CO using heteroleptic uranium(iii) metallocene aryloxide complexes.

The conversion of C1 feedstock molecules such as CO into commodity chemicals is a desirable, but challenging, endeavour. When the U(iii) complex, [(C5Me5)2U(O-2,6- t Bu2-4-MeC6H2)], is exposed to 1 atm of CO, only coordination is observed by IR spectroscopy as well as X-ray crystallography, unveiling a rare structurally characterized f element carbonyl. However, using [(C5Me5)2(MesO)U (THF)], Mes = 2,4,6-Me3C6H2, reaction with CO forms the bridging ethynediolate species, [{(C5Me5)2(MesO)U}2(µ2-OCCO)]. While ethynediolate complexes are known, their reactivity has not been reported in much detail to afford further functionalization. For example, addition of more CO to the ethynediolate complex with heating forms a ketene carboxylate, [{(C5Me5)2(MesO)U}2(µ 2:κ 2:η 1-C3O3)], which can be further reacted with CO2 to yield a ketene dicarboxylate complex, [{(C5Me5)2(MesO)U}2(µ 2:κ 2:κ 2-C4O5)]. Since the ethynediolate showed reactivity with more CO, we explored its reactivity further. A [2 + 2] cycloaddition is observed with diphenylketene to yield [{(C5Me5)2U}2(OC(CPh2)C([double bond, length as m-dash]O)CO)] with concomitant formation of [(C5Me5)2U(OMes)2]. Surprisingly, reaction with SO2 shows rare S-O bond cleavage to yield the unusual [(O2CC(O)(SO)]2- bridging ligand between two U(iv) centres. All complexes have been characterized using spectroscopic and structural methods, and the reaction of the ethynediolate with CO to form the ketene carboxylate has been investigated computationally as well as the reaction with SO2.

Ruthenium nanoparticles canopied by heptagon-containing saddle-shaped nanographenes as efficient aromatic hydrogenation catalysts.

The search for new ligands capable of modifying the metal nanoparticle (MNP) catalytic behavior is of increasing interest. Herein we present the first example of RuNPs stabilized with non-planar heptagon-containing saddle-shaped nanographenes (Ru@1 and Ru@2). The resemblance to graphene-supported MNPs makes these non-planar nanographene-stabilized RuNPs very attractive systems to further investigate graphene-metal interactions. A combined theoretical/experimental study allowed us to explore the coordination modes and dynamics of these nanographenes at the Ru surface. The curvature of these saddle-shaped nanographenes makes them efficient MNP stabilizers. The resulting RuNPs were found to be highly active catalysts for the hydrogenation of aromatics, including platform molecules derived from biomass (i.e. HMF) or liquid organic hydrogen carriers (i.e. N-indole). A significant ligand effect was observed since a minor modification on the hept-HBC structure (C[double bond, length as m-dash]CH2 instead of C[double bond, length as m-dash]O) was reflected in a substantial increase in the MNP activity. Finally, the stability of these canopied RuNPs was investigated by multiple addition experiments, proving to be stable catalysts for at least 96 h.

Synthesis and Reactivity of Triangular Heterometallic Complexes Containing Zn-Zn Bond.

This work provides a facile access to a series of triangular [Zn2M] (M = group 10 and 11 metals) clusters. Treatment of Zn-Zn-bonded compounds [LZn-ZnL] (L = CH3C(2,6-iPr2C6H3N)CHC(CH3)(NCH2CH2PR2); R = Ph, iPr) with zero-valent transition-metal reagents selectively afforded the corresponding triangular clusters [Zn2M], where M = Ni(0), Pd(0), and Pt(0). Notably, the isoelectronic triangular clusters [Zn2M]+, where M = Ag(I) and Cu(I), could also be obtained by reactions of [LZn-ZnL] with AgOTf and CuOTf, respectively. The [Zn2Ag]+ complex containing elusive Zn-Ag bonds was investigated by density functional theory analysis, showing a 3c-2e bonding feature in the metallic ring. The electrochemical behaviors of [Zn2M] complexes were examined and revealed the donation of electron density from the Zn-Zn σ-bond to the metal centers. Reaction of the [Zn2Ni] complex with isocyanide gave heterometallic species by coordination of isocyanide to the nickel center, keeping the trimetallic ring core structure intact. In contrast, the Zn-Zn bond was rapidly cleaved upon treatment of the [Zn2Ni] complex with dihydrogen or phenyl acetylene, generating the hydride- or acetylide-bridged heterotrimetallic complex.

Supramolecular nanocapsules as two-fold stabilizers of outer-cavity sub-nanometric Ru NPs and inner-cavity ultra-small Ru clusters.

The synthesis of metallic nanoparticles (MNP) with high surface area and controlled shape is of paramount importance to increase their catalytic performance. The detailed growing process of NP is mostly unknown and understanding the specific steps would pave the way for a rational synthesis of the desired MNP. Here we take advantage of the stabilization properties exerted by the tetragonal prismatic supramolecular nanocapsule 8·(BArF)8 to develop a synthetic methodology for sub-nanometric RuNP (0.6-0.7 nm). The catalytic properties of these sub-nanometric nanoparticles were tested on the hydrogenation of styrene, obtaining excellent selectivity for the hydrogenation of the alkene moiety. In addition, the encapsulation of [Ru5] clusters inside the nanocapsule is strikingly observed in most of the experimental conditions, as ascertained by HR-MS. Moreover, a thorough DFT study enlightens the nature of the [Ru5] clusters as tb-Ru5H2(η6-PhH)2(η6-pyz)3 (2) trapped by two arene moieties of the clip, or as tb-Ru5H2(η1-pyz)6(η6-pyz)3 (3) trapped between the two Zn-porphyrin units of the nanocapsule. Both options fulfill the Wade-Mingos counting rules, i.e. 72 CVEs for the closotb. The trapped [Ru5] metallic clusters are proposed to be the first-grown seeds of subsequent formation of the subnanometric RuNP. Moreover, the double role of the nanocapsule in stabilising ∼0.7 nm NPs and also in hosting ultra-small Ru clusters, is unprecedented and may pave the way towards the synthesis of ultra-small metallic clusters for catalytic purposes.

Hydroboration of vinyl halides with mesitylborane: a direct access to (mesityl)(alkyl)haloboranes.

The direct access to (mesityl)(alkyl)haloboranes (Mes(Alk)BX) (X = Br, Cl) from mesitylborane dimer and vinyl halides is presented. The involved hydroboration reaction results in the transfer of the halogen atom from the carbon of the starting material to the boron in the final product. The reactivity of the obtained Mes(Alk)BX has been evaluated for the synthesis of the bipyridyl boronium cations and 2-arylpyridine derived boron N^C-chelates. The formation mechanism of Mes(Alk)BX is apprended by DFT-calculations which shows that their formation involves two concomitant pathways derived from the regioslectivity of the hydroboration reaction.