A compound exhibiting a uranium–carbon triple bond represents the inaugural instance of a Fischer carbyne created using an f-element, with studies indicating that this ‘is far from your standard triple bond’.
<p Compounds featuring a metal–carbon triple bond are categorized as metal carbynes. There exist two varieties of metal carbynes: Schrock and Fischer, named in honor of Nobel laureates Richard Schrock and Ernst Fischer, who initially discovered metal carbenes (compounds with metal–carbon double bonds).
Schrock carbynes arise between a carbon atom possessing a triplet electron configuration and a metal. Conversely, Fischer carbynes demonstrate three bonds between a singlet carbon atom and a metal: one σ bond linking the carbon to the metal, and two π bonds resulting from the metal back-donating electrons to the carbon.
These metal carbynes commonly display distinct substituents on the carbon atom. Schrock carbynes prefer hydrogen atoms along with alkyl and silyl groups, while Fischer carbynes utilize amines or esters.
Regarding reactivity, Fischer carbynes tend to engage more slowly as they are stabilized by the amine or ester groups. Schrock carbynes face electrophilic attack at the carbyne carbon atom, whereas Fischer carbynes interact with nucleophiles at the same site. Such compounds are valuable for the synthesis of alkynes and nitriles.
Chemists achieved the synthesis of metal carbynes over half a century ago. The creation of such complexes with actinides, instead of transition metals, has since been a target to gain insights into how these heavy atoms establish bonds.
An international group of researchers has now achieved that objective by reacting a uranium triamide complex with a diazo phosphorus ylide, which eliminates a phosphorus compound during the process.
Single crystal x-ray crystallography revealed that the complex is linear, with two uranium cations flanking the diazo compound. The uranium–carbon bond length of 2.38Å is significantly shorter than that observed in uranium carbenes, for instance.
Computational assessments indicated that the carbon atom donates two of its electrons to create a σ bond, while the uranium engages in two back π bonds, each containing one electron. This is atypical, as back bonds generally involve two electrons. Steve Liddle at the University of Manchester, who spearheaded the research, emphasizes that ‘it is anything but your conventional triple bond’. Quantum crystallography experiments, which ascertain molecular and electronic structure, aided in affirming this peculiar bonding.
The complex interacted with alkyl, silyl, and tin halides by substituting the uranium–carbon triple bond and actively forming adducts with small molecules. Many of these reactions align with Fischer carbynes, yet the researchers observed that the reaction rates frequently tended to be ‘sluggish’, attributed to the stability of the uranium–carbon bond.