Abstract
Aromaticity is a fundamental chemical concept of ever-increasing diversity. According to Hückel’s and Baird’s rules, cyclic conjugated species with 4n+2 π-electrons are aromatic in the singlet electronic ground state (S0) and antiaromatic in the lowest triplet state (T1), and vice-versa. Thus, species with aromaticity in both states have not yet been reported. Here we carry out density functional theory calculations on recently synthesized organometallics, namely osmapentalyne and osmapentalenes, and demonstrate the first example (16-electron osmapentalene) of aromaticity in both S0 and T1 states, which we term adaptive aromaticity. Further electronic structure analysis reveals that the excitation pattern for the formation of the T1 state plays a crucial role in the achievement of adaptive aromaticity. Our findings highlight the role of a transition metal in unorthodox excitation behavior, and may aid the design of adaptive aromatics for photochemical and molecular magnetism applications.
Highlights
Aromaticity is a fundamental chemical concept of ever-increasing diversity
Through density functional theory (DFT) calculations, we revealed that these organometallics exhibit Craig-type Möbius aromaticity resulting from eight-center eight-electron (8c–8e) dπ-pπ conjugation
To assure that the simplification of phosphorus ligands does not alter the nature of adaptive aromaticity in 2, we examined two bulky ligands—trimethyl phosphine (PMe3) and triphenyl phosphine (PPh3), which are commonly used in experiments
Summary
Aromaticity is a fundamental chemical concept of ever-increasing diversity. According to Hückel’s and Baird’s rules, cyclic conjugated species with 4n+2 π-electrons are aromatic in the singlet electronic ground state (S0) and antiaromatic in the lowest triplet state (T1), and vice-versa. We carry out density functional theory calculations on recently synthesized organometallics, namely osmapentalyne and osmapentalenes, and demonstrate the first example (16-electron osmapentalene) of aromaticity in both S0 and T1 states, which we term adaptive aromaticity. Through density functional theory (DFT) calculations, we revealed that these organometallics exhibit Craig-type Möbius aromaticity resulting from eight-center eight-electron (8c–8e) dπ-pπ conjugation. Note that such Craig-type Möbius aromaticity was computationally demonstrated in 4nπ planar metallacycles early in 2010 by Mauksch and Tsogoeva[33] and in osmasilapentalyne[34] and dimetalla[10] annulenes by us[35]. This is supported by DFT calculations on simplified model complexes (verified in our previous studies) to examine aromaticity in their T1 states
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