Elusive triplet bismuthinidenes featuring unprecedented giant and positive zero field splittings were isolated in condensed phase
image: In the center, the near infrared spectrum shows a broad absorption at 9870 cm–1 with a band width of 570 cm–1, which likely reflects its formally spin-forbidden nature. The molecular structure is depicted in the upper right side.
Credit: ©Science China Press
This study is led by Dr. Gengwen Tan (Sun Yat-sen University) and Dr. Shengfa Ye (Dalian Institute of Chemical Physics, Chinese Academy of Sciences). It is a formidable task to isolate triplet pnictinidenes due to their intrinsic high reactivity, and bismuthinidenes are still elusive. Now, the research groups in China report two bismuthinidenes that were facilely synthesized by dechlorination of the bismuth dichloride precursors with two molar equivalents of potassium graphite. Single-crystal X-ray diffraction analysis reveals that the bismuth atoms connect the ligands with a Bi–C single σ bond. More importantly, the intermolecular Bi•••Bi distances substantially exceed the Bi–Bi single bond and the sum of the van der Waals radii of two Bi atoms, evidences that there exists no Bi–Bi bonding interaction. Therefore, they represent the first isolable examples of free bismuthinidenes containing one-coordinated bismuth atoms. “It is amazing that such reactive species could be synthesized in a simple approach”, said Tan.
The nuclear magnetic resonance (NMR) spectra of one of the bismuthinidenes exhibit abnormally upfield shifts for the 13C resonance of the carbon connected to the bismuth atom (δ –204.8 ppm) and that of the aromatic proton at the para-position to the bismuth atom (δ –1.06 ppm), while the corresponding signals of the precursor are observed at δ 210.3 and 7.58 ppm, respectively. These results indicate that the bismuthinidenes might have an S = 1 ground state. However, the variable-temperature (T) magnetic susceptibility (χ) measurements with a super conducting quantum interference device (SQUID) show that the χT value is essentially independent of T and is close to zero at room temperature, indictive of an S = 0 ground state. This seems to be incompatible with the ground-state spin multiplicity deduced from multinuclear NMR investigations.
To elucidate the electronic structure of the bismuthinidene, the researchers undertook detailed theoretical computations. Noncovalent interaction (NCI) analyses show noticeable van der Waals interactions between the bismuth center and the flanking fluorenyl functionalities. Theoretical results suggested that it possesses a triplet ground state that is 18.4 and 18.5 kcal/mol lower in energy than the open- and closed-shell singlet states, respectively. The dominant electron configuration of the triplet ground state is (Bi 6s)2(Ph π1,2,3 )6(Bi–C σz )2(Bi 6px)1(Bi 6py)1(Ph π*4,5,6)0(Bi–C σ*z)0 and accounts for 86% of the wavefunction. The computed Bi–C Mayer bond order is 0.77, and one hardly identifies any discernible π-bonding between Bi 6py and Ph C 2p orbitals because of the exceedingly large energy separation and radius difference between Bi 6p and C 2p atomic orbitals. In other words, the Bi 6px and 6py orbitals are nearly degenerate, and the bismuthinidene ought to have a triplet ground state on the grounds of Hund’s rule.
Furthermore, theoretical calculations predicted that bismuthinidene possesses an axial zero-field splitting (ZFS) D >4300 cm –1. Due to such a huge, positive ZFS, the system almost exclusively populates at the lowest-energy non-magnetic Ms = 0 level even at room temperature, whereas the populations of the excited magnetic Ms = ±1 levels are negligible. This unprecedented giant D value originates from the exceptionally strong spin-orbit coupling (SOC) between the triplet ground state and low-lying closed-shell singlet excited states, which gets accentuated by the effective SOC constant of Bi reaching as high as 12000 cm–1. As a consequence, the precise ground state of bismuthinidene cannot be readily determined by SQUID measurements up to 300 K. Consistent with the theoretical prediction, the near infrared spectrum registers a broad absorption at 9870 cm–1 with a band width of 570 cm–1. This feature has a low intensity (ε = 120 M–1 cm–1), which likely reflects its formally spin-forbidden nature; thus, we tentatively attributed it to Ms = 0 → Ms = ±1 transitions, an assignment that requires further experimental verification.
The unique electronic structures of the bismuthinidenes may lead to interesting reactivity, which will trigger more efforts to develop bismuth-based small molecule activation and catalysis.
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Triplet Bismuthinidenes Featuring Unprecedented Giant and Positive Zero Field Splittings