Probing the Origin of Adaptive Aromaticity in 16‐Valence‐Electron Metallapentalenes

Abstract

Species with adaptive aromaticity are aromatic in the ground and lowest-lying triplet excited states and they have normallyi ntermediate singlet-triplet gaps. Few examples of compounds with adaptive aromaticity are known to date, including1 6-valence-electron (16e) metallapentalenes. As weeping search could be conducted to discover new members of this group, but efficient designs with an explicit strategy would facilitatet he quest forn ew members of this elusive family. Densityfunctionaltheory calculations and aro-maticitye valuationsh ave been performed to reveal the natureo ft riplet-state aromaticity in 16e metallapentalenes. Our results show that coordination of strong so rp-donor ligandsh elps achieving adaptive aromaticity of 16e metallapentalenes by meanso faspin delocalization mechanism. These resultsh avei mportant implicationsf or understanding the unusual properties of the organometallic adaptive aromatics, leading the wayt oe fficient design of new compounds with tunable singlet-triplet gaps.

Adaptive Aromaticity in Metallasilapentalynes

“…In contrast, the delocalized electrons of metallapentalyne in T 1 (3.656e of π-EDDB 8MR in Os.C.T 1 ) is Ligands effect has been investigated in 16e metallapentalenes, indicating that σand π-donor ligands can be useful in achieving adaptive aromaticity. 16 Here, we examine the effect of a series of identical ligands in 18e osmasilapentalynes, keeping two PH 3 ligands at the axial positions. As shown in Figure 10 With medium σ donors or strong π acceptors (H 2 O, NH 3 , PMe 3 , CO, and PF 3 ), it is aromatic in the S 0 state and nonaromatic in the T 1 state.…”

“…15 Spin density analyses of Os.T 1 , Ru.T 1 , and Fe.T 1 are shown in Figure 7. The spin density is localized, mainly populating on the transition metal centers and silicon atom, consistent with the reported adaptive aromatics 13,14,16 and in contrast to metallapentalyne within delocalized spin density in the ring. 13 Electron density of delocalized bonds (EDDBs) can commendably quantify and visualize delocalized electrons.…”

“…As shown in Figure 10 With medium σ donors or strong π acceptors (H 2 O, NH 3 , PMe 3 , CO, and PF 3 ), it is aromatic in the S 0 state and nonaromatic in the T 1 state. Such a result agrees well with the previous report 16 except hydride. Spin density analyses (shown in Figure S13) suggest that spin distribution is mainly populated on the ligand, mental center, and silicon atom with donor ligands.…”

Prediction of Spin Density, Baird‐Antiaromaticity, and Singlet–Triplet Energy Gap in Triplet‐State Polybenzenoid Systems from Simple Structural Motifs

“…Additionally, new functional materials have been designed by using the concept of excited‐state aromaticity and antiaromaticity as a strategy to modulate the singlet–triplet (S–T) energy gap, [20, 31, 32] which is relevant to organic electronics, [32–34] photoswitching, [35, 36] photoluminescence, [37] and singlet‐fission [33, 38–40] capable materials. One possible avenue to modulate the S–T gap is through the use of compounds with “adaptive aromaticity”, which are aromatic in both the ground state and the excited state, such as metallopentalenes [41, 42] or aromatic chameleons [43–45] …”

Effect on the aromaticity of heterocyclic ligands by coordination with ruthenium electron‐withdrawing metal centers

“…This photophysical behavior has been recently explored using Baird's rule for the excited-state aromaticity to manipulate the singlettriplet energy gap, proposing a new family of derivatives of indolonaphthyridine thiophenes [52] and dipyrrolonaphthyridinedione compounds [53] , able to produce SF. The possibility to undergo an aromaticity in both ground and excited states (adaptive aromaticity) is an exclusive behavior demonstrated in a reduced number of molecules [53][54][55] . In this regard, the adaptive aromaticity has also been theoretically studied in a series of osmapyridine and osmapyridinium complexes to understand their triplet ground-state character, using different aromaticity indices, including HOMA, ELFπ, and MCI, among others [56] .…”