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

Abstract

We have previously developed a strategy for the preparation of heterometallic main-group imido (RN 2À ) and phosphinidene (RP 2À ) compounds involving stepwise deprotonation of REH 2 (E = N,P) with alkali-metal organometallics (for example nBuLi), followed by reaction with M(NMe 2 ) n (M = As, Sb, Bi, n = 3; Sn n = 2; Scheme 1).[1] Recently, however, we have recognized the behavior of this type of mixed-metal system as superbases. [2] This effect was first apparent in the reaction of MesPH 2 (Mes = 2,4,6-Me 3 C 6 H 2 ) with Sn(NMe 2 ) 2 /PhCH 2 Na, which gives the unusual stannate ion [Sn{P(2-CH 2 -4,6-Me 2 C 6 H 2 )}{PMes}] 3À [3] as a result of deprotonation of a PH 2 group and an ortho-CH 3 group. Remarkably, quadruple deprotonation of the PH 2 and NH 2 groups occurs in 1-NH 2 -2-PH 2 -C 6 H 4 (LH 4 ) using a similar Sn(NMe 2 ) 2 /nBuLi mixture, givingcontaining the L 4À tetraanion and LC 3À radical.[4]Herein we report the reaction of 1,2-(PH 2 ) 2 C 6 H 4 with Sb(NMe 2 ) 3 /nBuLi, which results in the complete deprotonation of the PH 2 groups, giving the 6p-aromatic anion [1,2-C 6 H 4 P 2 Sb] À . Upon one-electron reduction, the [1,2-C 6 H 4 P 2 Sb] 2 4À tetraanion is formed. As established from DFT calculations, this highly charged, Sb À Sb-bonded distibane is best described as a dimer of two 7p-[1,2-C 6 H 4 P 2 Sb]C 2À radicals (Scheme 2 a) and is valence-isoelectronic with the important class of sulfur/nitrogen-based thiazolyl radicals (Scheme 2 b).[5]Lithiation of 1,2-(PH 2 ) 2 C 6 H 4 (1 equiv) with nBuLi (2 equiv) in tmeda (tmeda = Me 2 NCH 2 CH 2 NMe 2 ) followed by reaction with Sb(NMe 2 ) 3 (0.67 equiv) gives [Li-(tmeda) 2 ]+ [1,2-C 6 H 4 P 2 Sb] À (1) after crystallization from THF/tmeda. However, black crystals of [{1,2-C 6 H 4 P 2 Sb} 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 SbÀSb-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 kJ mol À1 results in extensive dissociation in solution.[5]

A Persistent Dipotassium 1,2,4‐Diazaphospholide Radical Complex: Synthesis, X‐Ray Structure, and Bonding

“…, [6] which DFT computations indicated might be a dimer of two 7pe À dianion radicals (A). Probably owing to their high reactivity, anion radicals derived by reduction of five-membered heterocyclic aromatic systems have never been characterized structurally [7] before the present work.…”