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Twisting versus Delocalization in CAAC- and NHC-Stabilized Boron-Based Biradicals: The Roles of Sterics and Electronics.

ABSTRACT: Twisted boron-based biradicals featuring unsaturated C₂ R₂ (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground-state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C₂ R₂ -bridged systems featuring N-heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals.

SUBMITTER: Schmid P

PROVIDER: S-EPMC8048672 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Recently, the NHC?B 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 B?NHC molecule 1 has been published in Science where it is described as a stabilized B2 molecule in its 1? excited state (B2*) (NHC = N-heterocyclic carbene C3N2H2(C6H3Pri2-2,6)2). The bonding of 1 based on sophisticated calculations and the BB distances of the solid compound was discussed as the first example of a B2 triple bond in a stable molecule. Now we present an only experimentally based interpretation of 1via detailed thermodynamic considerations, including its fragmentation to B2 molecules. Furthermore, from the vibrational spectrum force constants (fBB for the BB bond and fBC for the BC bond) were extracted, which are classical examples to indicate single, double and triple CC bonds in organic chemistry. The consequence of both properties of 1 (?E and f) generates a new interpretation which is in contrast to the triple bond donor-acceptor description visualized by arrows and which casts a critical light on the interpretation of any NHC "stabilized" molecule.

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Twisting versus Delocalization in CAAC- and NHC-Stabilized Boron-Based Biradicals: The Roles of Sterics and Electronics.

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