As early as 1894 Michaelis and Schroeter isolated the first phosphorus(III)-nitrogen heterocycle from the reaction of aniline hydrochloride with an excess of PCl 3 (Scheme 1). [1] Interestingly, the authors assumed to have isolated the monomeric species, C 6 H 5 ÀN=PÀCl, which they called "phosphazobenzene chloride" but already speculated on the existence of the dimer. Today we know that the dimer is the stable form, and such four-membered rings of the type [XP(m-NR)] 2 , which contain alternating phosphorus(III) and nitrogen centers, are called cyclo-1,3-diphospha(III)-2,4-diazanes (X = halogen, R = organic group; old name: 1,3-diaza-2,4diphosphetidines). [2] They play a major role in preparative phosphorus-nitrogen chemistry because such species are good starting materials for polycyclic inorganic and organometallic compounds. [3, 4] Cyclo-1,3-diphospha(III)-2,4-diazanes (1) can exist as cis or trans isomers with trigonal-pyramidal P and trigonal-planar N atoms. [3] Both the N and the P atoms have one localized lone pair. Thus, formally eight electrons are found for this type of electron-rich heterocycles (Scheme 2). To the best of our knowledge, four-membered P 2 N 2 rings bearing 6 p electrons are unknown. As illustrated in Scheme 2, three possible target molecules (2, 3, and 4) with electronic structures that are related to those of aromatic hydrocarbons ([4 n + 2] p electrons), [5] can be considered. The most promising candidate for synthesis seemed to be the neutral cyclo-1,3-diphospha-2,4-diazane biradicaloid, [P(m-NR)] 2 (3; R= bulky group) which should formally be generated in a twoelectron reduction process upon chloride ion abstraction. Different reducing reagents, such as lithium metal, [Cp 2 Ti-(btmsa)] [6] (btmsa = bis(trimethylsilyl)acetylene, Me 3 SiÀC CÀSiMe 3 ) with the reactive {Cp 2 Ti II } fragment concealed in the h 2 -bounded btmsa complex, and [{Cp 2 Ti III Cl} 2 ] were utilized. Furthermore, it was of interest to study the effect of the bulky group on the reduction process. Thus, the terphenyl (Ter = 2,6-Mes 2 À C 6 H 3 , Mes = 2,4,6-Me 3 C 6 H 2 ) [7] and hypersilyl group (Hyp = (Me 3 Si) 3 Si) [8] were used for kinetic stabilization.Following our interest in Group 15 heterocycle chemistry, [9] we describe herein the synthesis, isolation and full characterization of a formal aromatic P 2 N 2 heterocycle of the type [P(m-NR)] 2 (R = Hyp, Ter) with an unusual biradicaloid bond situation.

The reactivity of biradicaloid [P(μ-NTer)]2 was employed to activate small molecules bearing single, double, and triple bonds. Addition of chalcogens (O2 , S8 , Sex and Tex ) led to the formation of dichalcogen-bridged P2 N2 heterocycles, except from the reaction with molecular oxygen, which gave a P2 N2 ring featuring a dicoordinated P(III) and a four-coordinated P(V) center. In formal [2πe+2πe] addition reactions, small unsaturated compounds such as ethylene, acetylene, acetone, acetonitrile, tolane, diphenylcarbodiimide, and bis(trimethylsilyl)sulfurdiimide are readily added to the P2 N2 heterocycle of the biradicaloid [P(μ-NTer)]2 , yielding novel heteroatom cage compounds. The synthesis, reactivity, and bonding of the biradicaloid [P(μ-NTer)]2 were studied in detail as well as the synthesis, properties, and structural features of all addition products.

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

Bereits 1894 isolierten Michaelis und Schroeter den ersten Phosphor(III)-Stickstoffheterocyclus aus der Reaktion von Anilin-hydrochlorid mit einem Überschuss an PCl 3 (Schema 1).[1] Interessanterweise nahmen die Autoren an, dass sie die monomere Spezies, C 6 H 5 ÀN=PÀCl, isoliert hatten, die sie "Phosphazobenzolchlorid" nannten. Sie spekulierten aber bereits über die Existenz des Dimers, von dem wir heute wissen, dass es die stabile Form darstellt. Viergliedrige Ringe des Typs [XP(m-NR)] 2 , die alternierend Phosphor(III) und Stickstoff enthalten, werden cyclo-1,3-Diphospha(III)-2,4-diazane genannt (X = Halogen, R = organischer Rest; alter Name: 1,3-Diaza-2,4-diphosphetidine).[2] Sie spielen eine bedeutende Rolle in der präparativen Phosphor-Stickstoff-Chemie, da sie gute Ausgangsstoffe für die Synthese von polycyclischen anorganischen und metallorganischen Verbindungen darstellen. [3, 4] Cyclo-1,3-Diphospha(III)-2,4-diazane (1) existieren als cis-oder trans-Isomere mit trigonal-pyramidal umgebenen Pund trigonal-planar umgebenen N-Atomen.[3] Sowohl die Nals auch die P-Atome haben ein lokalisiertes freies Elektronenpaar, womit sich formal acht Elektronen für diese elektronenreichen Heterocyclen ergeben (Schema 2). Unseres Wissens sind viergliedrige P 2 N 2 -Ringe mit 6 p-Elektronen unbekannt. Wie in Schema 2 dargestellt, gibt es drei Kandidaten (2, 3 und 4), die eine elektronische Struktur ähnlich der aromatischer Kohlenwasserstoffe mit [4 n + 2] p-Elektronen besitzen.[ . Des Weiteren sollte der Einfluss des sperrigen Rests auf den Reduktionsprozess untersucht werden. Daher wurden sowohl der Terphenyl-(kurz Ter = 2,6-Mes 2 C 6 H 3 , Mes = 2,4,6-Me 3 C 6 H 2 ) [7] als auch der Hypersilyl-Rest (kurz Hyp = Si(Me 3 Si) 3 ) [8] zur kinetischen Stabilisierung eingesetzt.Unserem Interesse an Heterocyclen mit Elementen der 15. Gruppe [9] folgend, beschreiben wie hier die Synthese und vollständige Charakterisierung eines formal aromatischen P 2 N 2 -Heterocyclus des Typs [P(m-NR)] 2 (R = Hyp, Ter) mit einer ungewçhnlichen diradikaloiden Bindungssituation.Diradikale sind Moleküle mit zwei ungepaarten Elektronen (in zwei (fast) entarteten nichtbindenden Molekülorbi-talen), die nahezu unabhängig voneinander agieren.[10] Spezies, in denen zwei Radikalzentren miteinander wechselwirken, werden oft als Diradikaloide bezeichnet. [11,12] Während radikalische Intermediate organischer Reaktionen in der Regel kurzlebig sind, konnten in den letzten zwanzig Jahren viele Diradikaloide der schweren Hauptgruppenelemente isoliert werden, die formal als Zwischenprodukte des s-Bindungsbildungsprozesses aufgefasst werden kçnnen.[12] Niecke et al. haben Pionierarbeit auf diesem Gebiet geleistet: Sie untersuchten [ClC(m-PMes*)] 2[13] und verschiedener Derivate, die als isolobale Analoga des intensiv untersuchten S 2 N 2 angesehen werden.[14] Zusätzlich zu diesen Kohlenstoff-baSchema 2. Viergliedrige Heterocyclen mit alternierenden N-und P IIIAtomen, die über 6 p-Elektronen verfügen, ausgehend von cyclo-1,3-Diphospha(III)-2,4-diazanen. Schema...