Formation and Structure of the [(1,2‐C6H4P2Sb)2]4− Ion: Implications for an Extended Family of Isoelectronic Main‐Group Radicals

Abstract

recognized the behavior of this type of mixed-metal system as superbases. This effect was first apparent in the reaction of MesPH2 (Mes= 2,4,6-Me3C6H2) with Sn(NMe2)2/PhCH2Na, which gives the unusual stannate ion [Sn{P(2-CH2-4,6Me2C6H2)}{PMes}] 3 [3] as a result of deprotonation of a PH2 group and an ortho-CH3 group. Remarkably, quadruple deprotonation of the PH2 and NH2 groups occurs in 1-NH22-PH2-C6H4 (LH4) using a similar Sn(NMe2)2/nBuLi mixture, giving the paramagnetic complex [({(L)Sn(NMe2)Li·THF}{(L)SnLi·3THF}Sn)2] containing the L 4 tetraanion and LC radical. Herein we report the reaction of 1,2-(PH2)2C6H4 with Sb(NMe2)3/nBuLi, which results in the complete deprotonation of the PH2 groups, giving the 6p-aromatic anion [1,2C6H4P2Sb] . Upon one-electron reduction, the [1,2C6H4P2Sb]2 4 tetraanion is formed. As established from DFT calculations, this highly charged, Sb Sb-bonded distibane is best described as a dimer of two 7p-[1,2-C6H4P2Sb]C 2 radicals (Scheme 2a) and is valence-isoelectronic with the important class of sulfur/nitrogen-based thiazolyl radicals (Scheme 2b). Lithiation of 1,2-(PH2)2C6H4 (1 equiv) with nBuLi (2 equiv) in tmeda (tmeda=Me2NCH2CH2NMe2) followed by reaction with Sb(NMe2)3 (0.67 equiv) gives [Li(tmeda)2] [1,2-C6H4P2Sb] (1) after crystallization from THF/tmeda. However, black crystals of [{1,2-C6H4P2Sb}2{Li(tmeda)}4] (2) [6] are obtained if this reaction is undertaken in toluene and the product crystallized from toluene/tmeda. Compound 2 presumably arises from one-electron reduction of the 6p-aromatic anion of 1 (Scheme 3). The diamagnetic nature of 2 persists in the solid state and in dilute solutions in toluene as a result of spin-pairing of the electrons within the Sb Sb-bonded dimer, as shown by the structural characterization (see below). This situation contrasts with the behavior of valence-isoelectronic dithiazolyl dimers, for which a dissociation energy of about 0 kJmol 1 results in extensive dissociation in solution. A few other related 6p-aromatic Group 15 anions have been reported; notably, the coordination chemistry of the 6pbenzatriazolyl ion, [1,2-C6H4N3] , which is valence-isoelectronic with the [1,2-C6H4P2Sb] ion of 1, has been investigated extensively. However, the only representative of this type for the heavier Group 15 elements is the 6p-diaraarzoyl anion [1,2-C6H4N2As] , which is obtained by the unusual reaction of the lithiate of 1,2-C6H4(NH2)2 with (Me2N)2AsCH2As(NMe2)2. [8] To our knowledge, however, one-electron reduction of this type of species has never been detected previously for Group 15 or elsewhere in the p block, except in sulfur/ nitrogen chemistry. The low-temperature solid-state structures of 1 and 2were both obtained. After many attempts, only one very limited data set was obtained for 1. Although the structural parameters involved should be treated with caution, the nature of the complex has been determined unequivocally, being composed of [Li(tmeda)2] + and 6p-aromatic [1,2-C6H4P2Sb] ions (Figure 1). The atoms within each of the four independent C2P2Sb rings are almost exactly coplanar (maximum deviation 0.020 B), as expected for the conjugation of all of the p orbitals throughout this ring. The small angle at antimony [mean 93.78] is consistent with the lone pair on Scheme 1. Stepwise deprotonation of amines (E=N) and phosphines (E=P). M=As, Sb, Bi, n=3; Sn n=2; R=aliphatic or aromatic group. Scheme 2. Comparison of a) the [1,2-C6H4P2Sb]C 2 radical and b) thiazolyl radical