φ-Aromaticity in prismatic {Bi6}-based clusters

Abstract

The occurrence of aromaticity in organic molecules is widely accepted, but its occurrence in purely metallic systems is less widespread. Molecules comprising only metal atoms (M) are known to be able to exhibit aromatic behaviour, sustaining ring currents inside an external magnetic field along M–M connection axes (σ-aromaticity) or above and below the plane (π-aromaticity) for cyclic or cage-type compounds. However, all-metal compounds provide an extension of the electrons’ mobility also in other directions. Here, we show that regular {Bi6} prisms exhibit a non-localizable molecular orbital of f-type symmetry and generate a strong ring current that leads to a behaviour referred to as φ-aromaticity. The experimentally observed heterometallic cluster [{CpRu}3Bi6]–, based on a regular prismatic {Bi6} unit, displays aromatic behaviour; according to quantum chemical calculations, the corresponding hypothetical Bi62− prism shows a similar behaviour. By contrast, [{(cod)Ir}3Bi6] features a distorted Bi6 moiety that inhibits φ-aromaticity.

A stable superatomic Cu6(SMPP)6 nanocluster with dual emission

“…The successful synthesis of such Cu NCs includes Cu 146 Se 73 (PPh 3 ) 30 , 5 Cu 72 Se 36 (PPh 3 ) 20 , 6 Cu 52 Se 26 (PPh 3 ) 16 , 6 Cu 32 Se 16 (PPh 3 ) 12 , 6 some small copper NCs such as Cu 4 NCs, 7,8 Cu 6 NCs, [9][10][11][12][13][14] Cu 8 NCs 14,15 Cu 12 NCs, 16 Cu 13 NCs, 16 Cu 14 NCs, 15,17 Cu 15 NCs, 18 as well as some other copper NCs involving hydrides. [19][20][21][22][23][24][25][26] Among them, the Cu 6 NCs stabilized by various ligands attract the most research interest, such as Cu 6 (MPP) 6 , 14 [Cu 6 (mna) 6 ] 6− , 12 Cu 6 (dmpymt) 6 , 10 Cu 6 (SC 7 H 4 NO) 6 , 13 and Cu 6 (SC 5 H 3 NSiMe 3 ) 6 , 9 allowing such sixatom metal NCs as model systems, [27][28][29][30][31][32][33][34][35] just as Au 25 [SR] 18 , [36][37][38] to fully understand the relationship between structure and property, and to explore stable ones for cluster-genetic materials. [39]…”

Does φ-Aromaticity exist in prismatic {Bi6}-based clusters?

“…2 Very recently in this journal Peerless et al isolated and investigated the heterometallic cluster [{CpRu}3Bi6] -(1 -) containing at its heart the Bi6 2cage of the elongated trigonal prism topology. 3 To explain its exceptionally high abundance and selective formation tendency (as compared to other Zintl clusters) as well as the unusual magnetic-response properties, the authors introduced the so-called φ-aromaticity. Here, we show that introducing a new type of aromaticity in this case seems unjustified and somewhat misleading.…”

“…In regular prisms none of the transannular antibonding MOs is occupied including the φ-type lowest unoccupied molecular orbital (LUMO) a2ˈ, which in turn becomes the highest occupied MO (HOMO) upon reduction. 9 Peerless et al 3 have overlooked the fact that the transannular antibonding φ-type orbital a2ˈ (HOMO) in Bi6 2interferes with the corresponding transannular bonding orbital a1ˈ (HOMO-9) giving rise to a pair of the resonating 6p 2 lone-pairs; the remaining occupied degenerated MOs eˈ represent the delocalized 6c-4e π*-π* transannular bonding. 9 The 6p 2 -6p 2 strong repulsion counteracts the weak 6c-4e transannular bonding and consequently Bi6 2dissociation into two Bi3rings is thermodynamically favorable (ΔG° = -124.5 kJ mol -1 ), while the dissociation of the elemental Bi6 into 2Bi3• is unfavorable (ΔG° = +206.1 kJ mol -1 ).…”

“…Second, Peerless et al 3 speculate that the isolated cluster 1is aromatic while the heterometallic cluster [{(cod)Ir}3Bi6] -(2 -) is not due to its lower symmetry. 3 In particular, the authors consider that the more regular {Bi6} prism in 1is better described as the Bi6 2-(allegedly featuring φaromaticity) whereas that of more structurally distorted 2by the non-aromatic Bi6 4-.…”

“…Second, Peerless et al 3 speculate that the isolated cluster 1is aromatic while the heterometallic cluster [{(cod)Ir}3Bi6] -(2 -) is not due to its lower symmetry. 3 In particular, the authors consider that the more regular {Bi6} prism in 1is better described as the Bi6 2-(allegedly featuring φaromaticity) whereas that of more structurally distorted 2by the non-aromatic Bi6 4-. The authors should be aware, however, that symmetry cannot be used as an aromaticity criterion per se, 2 and distorted aromatic systems are known to preserve their bonding patterns and aromatic character.…”