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

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 σ_C–Li → s_Ni 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.

We disclose the actual octanuclear nature of the major compound from reacting Ni(COD)₂ and PhLi, assigned for more than four decades as ‘Li₃NiPh₃(solv)₃’. We provide a thorough bonding analysis and discuss its potential implications in catalysis.

Supported σ-Complexes of Li-C Bonds from Coordination of Monomeric Molecules of LiCH3, LiCH2CH3 and LiC6H5 to MoMo Bonds

LiCH₃ and LiCH₂CH₃ react with the complex [Mo₂(H)₂(μ‐Ad^Dipp2)₂(thf)₂] (1⋅thf) with coordination of two molecules of LiCH₂R (R=H, CH₃) and formation of complexes [Mo₂{μ‐HLi(thf)CH₂R}₂(Ad^Dipp2)₂], 5⋅LiCH₃ and 5⋅LiCH₂CH₃, respectively (Ad^Dipp2=HC(NDipp)₂; Dipp=2,6‐ⁱPr₂C₆H₃; thf=C₄H₈O). Due to steric hindrance, only one molecule of LiC₆H₅ adds to 1⋅thf generating the complex [Mo₂(H){μ‐HLi(thf)C₆H₅}(μ‐Ad^Dipp2)₂], (4⋅LiC₆H₅). 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 ^6,7Li,¹H and ^6,7Li,¹³C NMR coupling constants found for the three‐atom H−Li−C chains. By contrast, the phenyl group in 4⋅LiC₆H₅ coordinates in an η² fashion to the lithium atom through the ipso and one of the ortho carbon atoms.