An Arsenic–Nitrogen Biradicaloid: Synthesis, Properties, and Reactivity

Abstract

Biradicals are molecules which contain two unpaired electrons in two nearly degenerate non-bonding molecular orbitals. [1] Both electrons can either be antiparallel forming an open-shell singlet state or parallel describing a triplet state. [2] As a result of the two unpaired electrons such biradicals are usually transient species during the process of bond breaking and making. Introduction of steric strain by bulky substituents to prevent bond formation or dimerization, delocalization, and substitution of carbon atoms by suitable main-group elements can lead to a considerable stabilization of such biradicals, however, at the expense of the biradical character, which decreases. Thus the designation of such stabilized species as biradicaloids seems to be more appropriate. [3, 4] Singlet biradicals commonly show a relatively small energy gap between their lowest energy singlet and triplet state. The stability of biradicals is increased by increasing the HOMO-LUMO gap leading to a larger singlet-triplet splitting, and lower occupation of the LUMO. [1] However, when the LUMO occupation reaches zero, a closed-shell singlet is finally obtained, and such species cannot be referred to as biradical or biradicaloid, respectively, anymore.Following our interest in the heterocyclic chemistry of Group 15 elements, [5] we studied the reaction of four-membered rings of the type [XE(m-NR)] 2 E = Group 15 element, X = halogen) containing alternating pnictogen(III) and nitrogen centers, with reducing agents such as [Cp 2 Ti(btmsa)] (Cp = h-C 5 H 5 , btmsa = bis(trimethylsilyl)acetylene, Me 3 Si-C C-SiMe 3 ), [6] [{Cp 2 TiCl} 2 ] or Mg. [7] Upon chloride abstraction and reduction (Scheme 1), such cyclo-1,3-dipnicta(III)-2,4diazanes [ClE(m-NR)] 2[8] with bulky substituents R (R = terphenyl = Ter = 2,6-Mes 2 C 6 H 3 , Mes = 2,4,6-Me 3 C 6 H 2 ) [9] should form remarkably tight ring structures of the type [E(m-NR)] 2 featuring two localized radical sites. The only known example, the [P(m-NR)] 2 biradicaloid, exhibits two radical centers in the

Experimental Observation of Thermally Excited Triplet States of Heavier Group 15 Element Centered Diradical Dianions

“…In comparison to the limited number of arsenic‐centered monoradicals, its diradicals are even scarcer. There have been only two stable examples of arsenic‐based diradicals ( C and D ) reported by Schulz and co‐workers. It is noteworthy that C and D only feature some diradical characters, and their triplet states cannot be thermally accessible under experimental conditions.…”

Cyclic Group 15 Radical Cations

“…[34][35][36] Since in the mass spectra of the biradicaloids [P(m-NTer) 2 P] (2P 2 ), [P(m-NTer) 2 As] (2PAs), and [As(m-NTer) 2 As] (2As 2 )t he corresponding molecular cations (3 + C)c ould be detected (m/z 716, 760, 804) with surprisingly high intensities,wewere intrigued by the idea to carry out the selective oxidation of the dipnictadiazane-1,3diyls 2 yielding the 5p electrons containing radical cations 3 + C as shown in Scheme 2. [34][35][36] Since in the mass spectra of the biradicaloids [P(m-NTer) 2 P] (2P 2 ), [P(m-NTer) 2 As] (2PAs), and [As(m-NTer) 2 As] (2As 2 )t he corresponding molecular cations (3 + C)c ould be detected (m/z 716, 760, 804) with surprisingly high intensities,wewere intrigued by the idea to carry out the selective oxidation of the dipnictadiazane-1,3diyls 2 yielding the 5p electrons containing radical cations 3 + C as shown in Scheme 2.…”

Stable Heterocyclopentane‐1,3‐diyls

“…[29] In contrast, housane 2' exhibits a three-membered ring annulated to an almost perpendicular lying four-membered heterocycle with a PÀP banana single bond (P1ÀP2 2.167 ), PÀN single bonds (N1ÀP1 1.757, N2ÀP1 1.797, N1ÀP2 1.792 ) and a localized C=O double bond. [18,19,[31][32][33][34] Also the natural orbital occupation numbers (HOMO: 1.44 and LUMO: 0.56 e) clearly indicate open shell singlet biradical character. NBO occupation of p atomic orbitals perpendicular to the five-membered ring amounts to 5.7 e (C 0.84, N1 1.70, P1 0.66, N2 1.60, P2 0.90).…”

“…[18][19][20] Further investigations will be concerned with the activation of isolobal analogues of CO, namely isonitriles RÀNC. By irradiation the housane-type isomerization product could be spectroscopically characterized and these findings were rationalized with the aid of computations.…”