Dual Reactivity of 3a,6a-Diaza-1,4-diphosphapentalene: π-Donor versus n-Donor

Abstract

Herein, we present the synthesis, single-crystal X-ray structures, and spectroscopic properties for the 1:1 donor−acceptor complexes of 1,2,4,5-tetracyanobenzene (TCNB) with annelated 3a,6a-diaza-1,4-diphosphapentalenes (DDPs) based on cyclohexanone azine (2) and tetralone azine (4). These are the first complexes of an organic π-acceptor with donor phosphorus heterocycles. According to the X-ray study, the DDPs and TCNB molecules are alternately stacked with interplanar distances of 3.335 and 3.404 Å for 2 and 4, respectively, which are suitable for intermolecular π•••π interactions. The bond lengths and angles in the component molecules agree with values for neutral species, and the infrared spectra indicate a very slight degree of ionicity. The estimated HOMO−LUMO gap from the onset of optical absorption (1.40 eV) is in agreement with the band gap estimated from the density functional theory calculations for 2 (1.47 eV). By contrast, in a reaction with the related electron acceptor, tetrachloroterephthalonitrile, the DDPs proved to be donors of lone electron pairs in a nucleophilic aromatic substitution reaction of chlorine atoms demonstrating the duality of their electronic nature.

A New Class of Two‐Deck Dicationic Heteropentalenes Existing by the Balance between Interdeck Aromaticity and Covalent P−P Bonding

“…The maxima of the long-wave absorption bands in the electronic spectrum of free diazadiphosphapentalenes are between 430 and 440 nm. [18,34] As expected for two-deck ditriflates, these maxima are shifted towards the short-wavelength region. For solutions of (MeMeDDP) 2 (OTf) 2 ( 2), (Et-MeDDP) 2 (OTf) 2 (3), (cHexDDP) 2 (OTf) 2 (4) in DCM, these values are 320, 355, and 376 nm, respectively (Figures S18-S20).…”

“…Their important feature is that these compounds, unlike related aromatic P-containing heterocycles (azaphospholes, diazaphospholes, phosphinines), are π-donor heterocycles capable of forming charge-transfer complexes with organic π-acceptors. [18,19] The reason for such a striking difference is the high energy of the HOMO of diazadiphosphapentalenes localized on the π-system. A characteristic feature of DDPs is the dualism of their electronic structure, which makes it possible to consider them both as heterocycles and as stabilized (masked) singlet phosphinidenes [20] (Scheme 2).…”

3 a,6 a‐Diaza‐1,4‐Diphosphapentalenes: Synthesis and Complexation with the Organic π‐Acceptor 1,2,4,5‐Tetracyanobenzene

“…The distance between the planes is 3.39 Å. This value is close to that of the TCNB complex with cyclohexyl‐annelated DDP ( 1 ⋅TCNB, composition 1 : 1, 3.34 Å) [3] . A noticeable difference in the interplanar distances of the donor and acceptor in complexes 7 and 8 indicates a significant effect of substituents on complex formation.…”

“…Small‐band gap complexes (typically having a band gap smaller than 1.6 eV) are particularly attractive in organic electronics [28,31,32] . It should be noted that the HOMO‐LUMO gap values obtained for complexes 7 and 8 were lower than those obtained for previously studied TCNB complexes with DDP 1 and 2 based on azines of cyclohexanone and 1‐tetralone studied earlier (∼1.47 eV) [3] …”

“…Reactions of diazadiphosphapentalenes 1 , 2 (Scheme 2) with 1,2,4,5‐tetracyanobenzene (TCNB) gave rise to the first charge‐transfer complexes of organophosphorus heterocycles with an organic π‐acceptor [3] …”