This is a follow up to my earlier post about C⩸N+, itself inspired by this ChemRxiv pre-print[cite]10.26434/chemrxiv.8009633.v1[/cite] which describes a chemical synthesis of singlet biradicaloid C2 and its proposed identification as such by chemical trapping.
First row diatomics based on the iso-electronic principle of eight valence electrons include both C⩸N+ and C⩸C, as well as species such as B⩸N, C⩸O2+ and even the unlikely N⩸O3+. The diatomic bond is represented here by ⩸ which carries the message of six electrons pairing to form a conventional triple bond and the remaining two valence electrons more weakly spin-pairing to form overall a singlet biradicaloid species with a quadruple bond. The “BDE” (bond dissociation energy) of the 4th pair is around 20 kcal/mol for C⩸C,[cite]10.1002/chem.201400356[/cite] which arguably entitles it to be called a weak bond.[cite]10.1002/chem.201600011[/cite]
Here I am going to explore –B⩸N+ and isoelectronic C⩸C via formation by radioactive decay of tritium into helium (Table, FAIR data DOI: 10.14469/hpc/5691).
| Entry | system | ΔΔG | ΔΔH |
|---|---|---|---|
| 1 | [Li-C≡C-T] → Li-C≡C-He+ + e → Li+ + C⩸C + He | -44.9 | -27.6 |
| 2 | [(-)C≡C-T] → (-)C≡C-He+ + e → C⩸C + He | -42.2 | -31.9 |
| 3 | [Li-N≡B-T] → Li-N≡B-He+ + e → Li+ + –B⩸N+ + He | -9.0 | +2.9 |
The thermochemistry includes a significant contribution from entropy, which favours the reaction. At its simplest, this involves the replacement of a X-He (X=C,B) bond by the 4th C⩸X bond. The BDEs (bond dissociation energies) of the X-He bond are very small (< 1 kcal/mol) and hence the reaction is driven largely by the enthalpy of forming the final C⩸X bond, together with entropy increase. Contrast this with the reaction reported above involving cleavage of a C–IPh bond,[cite]10.26434/chemrxiv.8009633.v1[/cite] where the C–I BDE is larger (~70-80 kcal/mol; > 20 kcal/mol). This makes the reported trapping of C2 from this reaction all the more intriguing.