Synthesis and Structure of a 1,3‐Diphosphacyclobutadienediide: An Aniomesolytic Fragmentation of a 1,3‐Diphosphetane‐2,4‐diyl in Solution

Abstract

on the occasion of his 65th birthday 1,3-diphosphetane-2,4-diyls [1] have a biradicaloid electronic structure with a small singlet-triplet gap, [2] and a low excitation energy. [3, 4] Their stability significantly depends on the substitution pattern. Thermally they undergo ring opening with generation of phosphanylcarbenes.[5] As was demonstrated recently, 1 a (R = H) isomerizes easily in a photochemical reaction to gauche-2,4-diphosphabicyclo[1.1.0]butane 2 (Scheme 1).[6] The formation of the bicyclic compound, however, can be suppressed by introducing sterically demanding substituents at both ring carbon atoms (1 b; R = SiMe 3 ) thus resulting in a homolytic cleavage of one PÀC aryl bond, with generation of 1,3-diphosphetane-2-yl 3.[7]The cleavage of the PÀC aryl bond in the photochemical reaction indicates that a radical anion formed from 1, through reaction with alkali metals, will undergo aniomesolytic cleavage, [8] thus finally giving the 1,3-diphosphacyclobutadienediide 4 (Scheme 2). Herein we report the synthesis of 4, the crystal structure of the dilithium complex 4 [Li(dme)] 2 and the mechanism of its formation.The 1,3-diphosphetane-2,4-diyl 1 a [6] is easily converted into the bis(trimethylsilyl) derivative 1 b. The reaction of 1 b with two equivalents of lithium proceeds with a color change from orange to red (Scheme 2). Analysis of the NMR spectroscopic data of the reaction mixture indicates the formation of two new products, in a ratio of 2:1, and identified as 1,3,5-tri-tert-butylbenzene and the dilithium salt 4 [Li(dme)] 2 . The analogous reaction of 1 b with potassium led to the dipotassium species 4 [K(dme) 2 ] 2 . Both species were isolated as highly air and moisture sensitive yellow solids.The molecular structure [9] of 4 [Li(dme)] 2 ( Figure 1) shows a monomer consisting of a planar, C i -symmetric four-membered P 2 C 2 -ring (a(PCP) = 83.8(1)8, a(CPC) = 96.2(1)8). The two lithium ions located 206.6(2) pm above and below the ring center are each coordinated by one molecule of DME. The PÀC bonds are equivalent within the experimental error (179.7(1), 180.0(1) pm) and are comparable to those in similar complexes of d-and p-block elements. [10,11] The silicon atoms of the SiMe 3 substituents lie in the four-membered ring plane and the methyl groups of the two SiMe 3 units adopt a staggered conformation. The SiÀC(P 2 ) bonds (181.9(1) pm) are comparable to the SiÀC bond lengths in silyl substituted cyclobutadiene dianions (183-186 pm).[12] These relatively short bond lengths have been attributed [12] to an effective p(C)!s*(Si) electron transfer ("negative hyperconjugation").The constitution of complex 4 [Li(dme)] 2 in solution is unequivocally assigned on the basis of multinuclear NMR spectroscopic investigations. The characteristic deshielding of the atoms in the central P 2 C 2 unit indicates a PC p-bonding system (d 31 P = 200.3, d 13 C Ring = 126.0 ppm (t, 2 J C,P = 59.2 Hz)). These resonance signals lie in the range typical for cyclic pdelocalized phospha-aromatics, such as P 2 (tBuC) 3 ·[...

Preparation and Reactions of 1,3‐Diphosphacyclobutane‐2,4‐diyls That Feature an Amino Substituent and/or a Carbonyl Group

“…[6] On the other hand, Bertrand and co-workers reported the synthesis of a boron-centered biradical 2, [7,8] and recently, interest has been shown with regard to the latest results of its radical-type reactivity [9] and the structural conversion of the B 2 P 2 biradical. [10] These attempts to obtain stable biradicals have had an impact on both main group chemistry, as well as the chemistry of reaction intermediates.…”

Binuclear cyclopentadienylcobalt sulfur and phosphinidene complexes Cp2Co2E2 (E=S, PX): Comparison with their Iron carbonyl analogues

“…7). If these electrons are paired, then the symmetrical butterfly structures are singlet diradicals similar to the singlet diradicals isolated by Bertrand and coworkers [24] as well as closely related (RPCR 0 ) 2 and (RNGeR 0 ) 2 species [25,26]. However, single electrons on the phosphorus atoms are avoided in the Table 2 The singlet structures of the Cp 2 Co 2 (PX) 2 (X = H, Cl, OH, OMe, NH 2 or NMe 2 ) complexes at the BP86/6-311G(d) level of theory.…”

Phosphines and Related Tervalent Phosphorus Systems

“…727,728 The reactions of the phosphaalkyne Mes*CRP with organolithium reagents, followed by alkylation with iodomethane, have given a series of stable 1,3-diphosphacyclobutane-2,4-diyl systems, e.g., (252), 729,730 and the reactivity of such diradicaloid rings towards the addition of electrons has also been investigated. 731 …”