Donor-Acceptor Activation of Carbon Dioxide.

The activation and functionalization of carbon dioxide entails great interest related to its abundance, low toxicity and associated environmental problems. However, the inertness of CO2 has posed a challenge towards its efficient conversion to added-value products. In this review we discuss one of the strategies that have been widely used to capture and activate carbon dioxide, namely the use of donor-acceptor interactions by partnering a Lewis acidic and a Lewis basic fragment. This type of CO2 activation resembles that found in metalloenzymes, whose outstanding performance in catalytically transforming carbon dioxide encourages further bioinspired research. We have divided this review into three general sections based on the nature of the active sites: metal-free examples (mainly formed by frustrated Lewis pairs), main group-transition metal combinations, and transition metal heterobimetallic complexes. Overall, we discuss one hundred compounds that cooperatively activate carbon dioxide by donor-acceptor interactions, revealing a wide range of structural motifs.

Supported σ-Complexes of Li-C Bonds from Coordination of Monomeric Molecules of LiCH3 , LiCH2 CH3 and LiC6 H5 to Mo≣Mo Bonds.

LiCH3 and LiCH2 CH3 react with the complex [Mo2 (H)2 (µ-AdDipp2 )2 (thf)2 ] (1⋅thf) with coordination of two molecules of LiCH2 R (R=H, CH3 ) and formation of complexes [Mo2 {µ-HLi(thf)CH2 R}2 (AdDipp2 )2 ], 5⋅LiCH3 and 5⋅LiCH2 CH3 , respectively (AdDipp2 =HC(NDipp)2 ; Dipp=2,6-i Pr2 C6 H3 ; thf=C4 H8 O). Due to steric hindrance, only one molecule of LiC6 H5 adds to 1⋅thf generating the complex [Mo2 (H){µ-HLi(thf)C6 H5 }(µ-AdDipp2 )2 ], (4⋅LiC6 H5 ). Computational studies disclose the existence of five-center six-electron bonding within the H-Mo≣Mo-C-Li metallacycles, with a mostly covalent H-Mo≣Mo-C group and predominantly ionic Li-C and Li-H interactions. However, the latter bonds exhibit non-negligible covalency, as indicated by X-ray, computational data and the large one-bond 6,7 Li,1 H and 6,7 Li,13 C NMR coupling constants found for the three-atom H-Li-C chains. By contrast, the phenyl group in 4⋅LiC6 H5 coordinates in an η2 fashion to the lithium atom through the ipso and one of the ortho carbon atoms.

Divergent CO2 Activation by Tuning the Lewis Acid in Iron-Based Bimetallic Systems.

Bimetallic motifs mediate the selective activation and functionalization of CO2 in metalloenzymes and some recent synthetic systems. In this work, we build on the nascent concept of bimetallic frustrated Lewis pairs (FLPs) to investigate the activation and reduction of CO2 . Using the Fe0 fragment [(depe)2 Fe] (depe=1,2-bis(diethylphosphino)ethane) as base, we modify the nature of the partner Lewis acid to accomplish a divergent and highly chemoselective reactivity towards CO2 . [Au(PMe2 Ar)]+ irreversibly dissociates CO2 , Zn(C6 F5 )2 and B(C6 F5 )3 yield different CO2 adducts stabilized by push-pull interactions, while Al(C6 F5 )3 leads to a rare heterobimetallic C-O bond cleavage, and thus to contrasting reduced products after exposure to dihydrogen. Computational investigations provide a rationale for the divergent reactivity, while Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method substantiates the heterobimetallic bonding situation.

Coordination of LiH Molecules to Mo≣Mo Bonds: Experimental and Computational Studies on Mo2LiH2, Mo2Li2H4, and Mo6Li9H18 Clusters.

The reactions of LiAlH4 as the source of LiH with complexes that contain (H)Mo≣Mo and (H)Mo≣Mo(H) cores stabilized by the coordination of bulky AdDipp2 ligands result in the respective coordination of one and two molecules of (thf)LiH, with the generation of complexes exhibiting one and two HLi(thf)H ligands extending across the Mo≣Mo bond (AdDipp2 = HC(NDipp)2; Dipp = 2,6-iPr2C6H3; thf = tetrahydrofuran, C4H8O). A theoretical study reveals the formation of Mo-H-Li three-center-two-electron bonds, supplemented by the coordination of the Mo≣Mo bond to the Li ion. Attempts to construct a [Mo2{HLi(thf)H}3(AdDipp2)] molecular architecture led to spontaneous trimerization and the formation of a chiral, hydride-rich Mo6Li9H18 supramolecular organization that is robust enough to withstand the substitution of lithium-solvating molecules of tetrahydrofuran by pyridine or 4-dimethylaminopyridine.

Experimental and Computational Studies on Quadruply Bonded Dimolybdenum Complexes with Terminal and Bridging Hydride Ligands.

This contribution focuses on complex [Mo2 (H)2 (µ-AdDipp2 )2 ] (1) and tetrahydrofuran and pyridine adducts [Mo2 (H)2 (µ-AdDipp2 )2 (L)2 ] (1⋅thf and 1⋅py), which contain a trans-(H)Mo≣Mo(H) core (AdDipp2 =HC(NDipp2 )2 ; Dipp=2,6-iPr2 C6 H3 ). Computational studies provide insights into the coordination and electronic characteristics of the central trans-Mo2 H2 unit of 1, with four-coordinate, fourteen-electron Mo atoms and ϵ-agostic interactions with Dipp methyl groups. Small size C- and N-donors give rise to related complexes 1⋅L but only one molecule of P-donors, for example, PMe3 , can bind to 1, causing one of the hydrides to form a three-centered, two-electron (3c-2e) Mo-H→Mo bond (2⋅PMe3 ). A DFT analysis of the terminal and bridging hydride coordination to the Mo≣Mo bond is also reported, along with reactivity studies of the Mo-H bonds of these complexes. Reactions investigated include oxidation of 1⋅thf by silver triflimidate, AgNTf2 , to afford a monohydride [Mo2 (µ-H)(µ-NTf2 )(µ-AdDipp2 )2 ] (4), with an O,O'-bridging triflimidate ligand.

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems.

Invited for the cover of this issue is the group of Joaquín López-Serrano and Jesús Campos at the Consejo Superior de Investigaciones Científicas and the University of Sevilla. The image depicts the importance of balancing the degree of frustration/interaction in the splitting of H2 by AuI /Pt0 . Read the full text of the article at 10.1002/chem.201905793.

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems.

A joint experimental/computational effort to elucidate the mechanism of dihydrogen activation by a gold(I)/platinum(0) metal-only frustrated Lewis pair (FLP) is described herein. The drastic effects on H2 activation derived from subtle ligand modifications have also been investigated. The importance of the balance between bimetallic adduct formation and complete frustration has been interrogated, providing for the first time evidence for genuine metal-only FLP reactivity in solution. The origin of a strong inverse kinetic isotopic effect has also been clarified, offering further support for the proposed bimetallic FLP-type cleavage of dihydrogen.

Photochemical H2 activation by an Zn-Fe heterometallic: a mechanistic investigation.

Addition of H2 to a Zn-Fe complex was observed to occur under photochemical conditions (390 or 428 nm LED) and leads to the formation of a heterometallic dihydride complex. The reaction does not occur under thermal conditions and DFT calculations suggest this is an endergonic, light driven process. Through a combined experimental and computational approach, the plausible mechanisms for H2 activation were investigated. Inhibition experiments, double-label cross-over experiments, radical trapping experiments, EPR spectroscopy and DFT calculations were used to gain insight into this system. The combined data are consistent with two plausible mechanisms, the first involving ligand dissociation followed by oxidative addition of H2 at the Fe centre, the second involving homolytic fragmentation of the Zn-Fe heterometallic and formation of radical intermediates.

Small molecule activation with bimetallic systems: a landscape of cooperative reactivity.

There is growing interest in the design of bimetallic cooperative complexes, which have emerged due to their potential for bond activation and catalysis, a feature widely exploited by nature in metalloenzymes, and also in the field of heterogeneous catalysis. Herein, we discuss the widespread opportunities derived from combining two metals in close proximity, ranging from systems containing multiple M-M bonds to others in which bimetallic cooperation occurs even in the absence of M⋯M interactions. The choice of metal pairs is crucial for the reactivity of the resulting complexes. In this context, we describe the prospects of combining not only transition metals but also those of the main group series, which offer additional avenues for cooperative pathways and reaction discovery. Emphasis is given to mechanisms by which bond activation occurs across bimetallic structures, which is ascribed to the precise synergy between the two metal atoms. The results discussed herein indicate a future landscape full of possibilities within our reach, where we anticipate that bimetallic synergism will have an important impact in the design of more efficient catalytic processes and the discovery of new catalytic transformations.

Unmasking the constitution and bonding of the proposed lithium nickelate "Li3NiPh3(solv)3": revealing the hidden C6H4 ligand.

More than four decades ago, a complex identified as the planar homoleptic lithium nickelate "Li3NiPh3(solv)3" was reported by Taube and co-workers. This and subsequent reports involving this complex have lain dormant since; however, the absence of an X-ray diffraction structure leaves questions as to the nature of the Ni-PhLi bonding and the coordination geometry at Ni. By systematically evaluating the reactivity of Ni(COD)2 with PhLi under different conditions, we have found that this classical molecule is instead a unique octanuclear complex, [{Li3(solv)2Ph3Ni}2(µ-η2:η2-C6H4)] (5). This is supported by X-ray crystallography and solution-state NMR studies. A theoretical bonding analysis from NBO, QTAIM, and ELI perspectives reveals extreme back-bonding to the bridging C6H4 ligand resulting in dimetallabicyclobutane character, the lack of a Ni-Ni bond, and pronounced σ-bonding between the phenyl carbanions and nickel, including a weak σC-Li → sNi interaction with the C-Li bond acting as a σ-donor. Employing PhNa led to the isolation of [Na2(solv)3Ph2NiCOD]2 (7) and [Na2(solv)3Ph2(NaC8H11)Ni(COD)]2 (8), which lack the benzyne-derived ligand. These findings provide new insights into the synthesis, structure, bonding and reactivity of heterobimetallic nickelates, whose prevalence in organonickel chemistry and catalysis is likely greater than previously believed.

Tuning Activity and Selectivity during Alkyne Activation by Gold(I)/Platinum(0) Frustrated Lewis Pairs.

Introducing transition metals into frustrated Lewis pair systems has attracted considerable attention in recent years. Here we report a selection of three metal-only frustrated systems based on Au(I)/Pt(0) combinations and their reactivity toward alkynes. We have inspected the activation of acetylene and phenylacetylene. The gold(I) fragments are stabilized by three bulky phosphines bearing terphenyl groups. We have observed that subtle modifications on the substituents of these ligands proved critical in controlling the regioselectivity of acetylene activation and the product distribution resulting from C(sp)-H cleavage of phenylacetylene. A mechanistic picture based on experimental observations and computational analysis is provided. As a result of the cooperative action of the two metals of the frustrated pairs, several uncommon heterobimetallic structures have been characterized.

Reactivity of a trans-[H-Mo[quadruple bond, length as m-dash]Mo-H] unit towards alkenes and alkynes: bimetallic migratory insertion, H-elimination and other reactions.

Complex [Mo2(H)2{µ-HC(NDipp)2}2(THF)2], (1·THF), reacts with C2H4 and PhCH[double bond, length as m-dash]CH2 to afford hydrido-hydrocarbyl and bis(hydrocarbyl) derivatives of the Mo[quadruple bond, length as m-dash]Mo bond. Reversible migratory insertion and ß-hydrogen elimination, as well as reductive elimination and other reactions, have been uncovered. PhC[triple bond, length as m-dash]CH behaves instead as a Brönsted-Lowry acid towards the strongly basic Mo-H bonds of 1·THF.