Electron Density Analysis of the “O–O” Charge-Shift Bonding in Rubrene Endoperoxide

Abstract

Rubrene endoperoxide p-xylene (1) has been obtained in crystalline form from recrystallization and purification of the organic semiconductor, rubrene, and for the first time characterized by single-crystal X-ray diffraction methods. 1 is produced by reaction of rubrene with molecular oxygen to create rubrene endoperoxide, C42H28O2, in which an O2-bridge is joining the two phenyl-substituted C atoms opposite each other in the second of the four aromatic rings in tetracene thereby breaking the resonance along the tetracene moiety. The electron density distribution of 1 reveals that the intramolecular O-O bond is best characterized as charge-shift bonding with loss of electronic concentration in the interatomic region evidenced by the Laplacian. Likewise the ELI-D indicates little electron localization in this region. Furthermore, source function and ELI-D analysis of 1 clearly quantifies the lack of electronic delocalization across the six-membered ring that carries the peroxide-bridge.

Charge‐Shift Bonding: A New and Unique Form of Bonding

“…Thus,the three methods lead to the same classification of the homonuclear bonds into two families.E LF and QTAIM diagnose the attractive/repulsive nature of the covalent "shared densities", while VB brings about energetic components that highlight the dominant role of the RE CS energy in the CSB group.N ote that the quoted QTAIM properties for [1.1.1]propellane are derived from experimental densities, [52] and as such, lend experimental support to the characterization of the inverted C À Cb ond as CSB.Asimilar consensus was recently reported for the O À Ob ond of Rubrene endoperoxide, [54] for Mn À Mn and Mn À CO bonds in Mn 10 (CO) 10 , [55] as well as other bonds. [57][58][59][60][61][62][63]…”

“…[55] Thus,e lectron density determined by either experiment or theoretical calculations leads to ap ositive Laplacian, and to other criteria that suit CSBs. [54] We are confident that other experimental electron densities [53] will enable the analysis of av ariety of bonds by means of QTAIM, ELF.a nd EST and will provides the means to explore the CSB territory.…”

“…An egative Laplacian means that the bonding region is dominated by electron-sharing and lowering of the potential energy,w hile ap ositive Laplacian means that the shared density in the bonding region is typified by excess kinetic energy-density, G(r c ), and is hence repulsive.All the QTAIM parameters for bonds in amolecule can be either calculated or derived from experimental density determination, and are used by experimental chemists to characterize interactions within molecules. [52][53][54][55] As such, QTAIM is very useful and one might expect it to tag the presence of CSBs,i nt hose cases which are typified by excess Pauli repulsion in the bonding region (see Tables S1 and S2). Figure 11 highlights the distinction of the covalent and CSB families by the three theoretical approaches.I td epicts the ELF bonding basins for H 3 CÀCH 3 ,F ÀF, and the wing and Figure 11.…”

“…Ther ecent activity is indicative that the CSB concept is already stimulating chemists,t hrough articulations of the concept by experimental chemists, [78,81,82] and through QTAIM and ELF analyses of experimental electron densities, isodesmic reaction tests, [52,54,55,58,59,[87][88][89][90] and explorations of how protonation of heteroatoms change their bonds to CSBs, [91] inverted Si-Si CSBs in silabicyclo[1.1.0]butanes, [92] and predictions of novel helium bonds, [57] which reveal CSB characteristics.S ince the chemical bond is the heartland of chemistry, [1] we might expect this trend to continue.…”

Charge‐Shift Bonding: A New and Unique Form of Bonding

“…Note that the quoted QTAIM properties for [1.1.1]propellane are derived from experimental densities, and as such, lend experimental support to the characterization of the inverted C−C bond as CSB. A similar consensus was recently reported for the O−O bond of Rubrene endo ‐peroxide, for Mn−Mn and Mn−CO bonds in Mn 10 (CO) 10 , as well as other bonds …”