Reactions of PhIX2 I(III) Oxidants with Heavy Triphenyl Pnictines

The reactions of [PhI(pyridine)2]2+, PhI(OAc)2 and PhI(OTf)(OAc) with Ph3As, Ph3Sb and Ph3Bi are described. The reactions of [PhI(pyridine)2]2+ with Ph3Sb and Ph3Bi affords dicationic Pn(V) complexes ligated by pyridine in one step. These were previously reported by Burford in multi step syntheses. Reactions with PhI(OAc)2, which were already known for Sb and Bi giving Pn(V) diacetates, was confirmed to give the same type of compound for As. Reactions with PhI(OAc)(OTf) were less clean, resulting in the isolation of iodonium cations [Ph-I-Ph]+ for As and Bi, while Ph3Sb gave an oxobridged di-antimony species characteristic of the decomposition of a high valent triflate bound species. ABSTRACT The reactions of [PhI(pyridine) 2 ] 2+ , PhI(OAc) 2 and PhI(OTf)(OAc) with Ph 3 As, Ph 3 Sb and Ph 3 Bi are described. The reactions of [PhI(pyridine) 2 ] 2+ with Ph 3 Sb and Ph 3 Bi affords dicationic Pn(V) complexes ligated by pyridine in one step. These were previously reported by Burford in multistep syntheses. Reactions with PhI(OAc) 2 , which were already known for Sb and Bi giving Pn(V) diacetates, was confirmed to give the same type of compound for As. Reactions with PhI(OAc)(OTf) were less clean, resulting in the isolation of iodonium cations [Ph-I-Ph] + for As and Bi, while Ph 3 Sb gave an oxobridged di-antimony species characteristic of the decomposition of a high valent triflate bound species.


Introduction
The synthesis of polycationic species of the heavier late p-block elements has seen little investigation as compared to the lighter elements due to reactivity of the intermediates and target complexes as well as the requirement of a main-group halide starting material which can be difficult to handle. 1 Much of the work utilises halide abstraction resulting in a triflate containing species which can used in further ligand exchange with other mono-, di-and tridentate ligands. 2 Work by Burford et al. highlights this as using the above method they was able to isolate the bistriflate bismuth(V) 1 followed by exchange with pyridine ligands to give the dicationic bismuth species 2 and monocationic species 3 (Scheme 1). 3 The corresponding chemistry for antimony was also reported by the Burford group. 4 Scheme 1. Isolation of bismuth (V) complexes 2 and 3 using triphenylbismuth.
This strategy is a general route to ligand stabilized polycations in groups 15 and 16. As shown in Scheme 2, the commonly used route in the literature is the abstraction of a halide with a species that introduces triflate or another good leaving group, with some examples requiring a specific halide for this reaction to proceed as well as the element in the correct oxidation state. 5 Addition of ligands to late main group halides can result in spontaneous reduction reactions, with elimination of elemental halogen as a possible by-product leading to deleterious effects. 6 The triflate adducts can be very sensitive to moisture and air making them relatively difficult to store and handle. These I(III) pyridyl compounds were first reported in 1994 by Weiss and later reinvestigated by Zhdankin. [8][9][10] Ritter also used the oxidant to access pyridine stabilized Pd(IV) complexes, where the pyridine could be displaced with 18 F labelled fluoride to generate PET labelling agents. 11 Wengryniuk has recently shown the efficacy of these oxidants in oxidative ring formation reactions. [12][13][14] In the main group we have explored the chemistry of these dicationic I(III) oxidants with aromatic group 16 rings, which largely resulted in electrophilic aromatic substitution type reactions on the ring or ring substitutents. 15,16 We also reported one reaction in group 15, the reaction of Ph3P with [PhI(4-DMAP)2] 2+ , which resulted in oxidation of phosphorus to P(V) and ligation of a 4-DMAP giving dicationic complex 6 (Scheme 4), 17 previously reported by Burford using the oxidation/halide abstraction method from the phosphine. 18   We initially viewed these reactions as indicating that I(III) oxidants are incompatible with phosphine ligands, although have since found they are compatible when the phosphine is bound to a metal in some cases. 19 In light of Burford's work on heavier pnictogen(V) dications described above, we wondered if these I(III) dications could be an effective reagent for their generation, circumventing the steps involving halogenation/halide abstraction.
In this study we examined the ability of iodine(III) oxidants with anionic and neutral ligands as potential reagents to isolate dicationic complexes of arsenic, antimony and bismuth using AsPh3, SbPh3 and BiPh3 as starting materials. reaction had peaks corresponding to PhI indicating that an oxidation has occurred, at least one other phenyl containing species and protonated pyridine as compared with an authentic sample. A colourless single crystal was grown from the solution left to stand at -30 o C for 24 hours and the X-ray crystallography revealed the product to be bis-As(V) cation (7) with a completely depronotated acetonitrile fragment bridging the two As(V) centres. This compound has been reported in literature by a different route. 20 We surmise in this case that acetonitrile coordinates to the oxidized arsenic centres, rendering the hydrogen atoms more acidic, resulting in deprotonation by pyridine and eventual rearrangement to the observed product. It should be noted here that the As analogue of 3 is not known and may not be stable as it is for Bi and Sb. Next, triphenylarsine was reacted with PhI(OAc)2 in CDCl3. The 1 H NMR spectrum of the colourless reaction mixture confirmed that reaction was driven to completion with no starting material after stirring for 24 hours. The spectrum had peaks corresponding to PhI and another species which indicates the possible formation of As(V) as the iodine became reduced. Colourless crystals were grown at -30 o C. The crystals were of poor quality, but X-ray crystallography allowed for structural confirmation of the formation of triphenylarsine diacetate (9, Scheme 7 ). Compound 9 is a known compound and has been synthesized by other routes. 23 Attempts to exchange the acetate groups in 9 for triflates as a better leaving group using TMS-OTf resulted in no reaction. In summary, reactions of these I(III) oxidants with triphenylarsine did not result in productive advances.

As
As OAc OAc PhI(OAc) 2 2 eqiv. TMS-OTf As OTf Colourless crystals were obtained and X-ray diffraction studies showed via unit cell analysis 27 the result was a diaryliodonium triflate salt 13 (Scheme 11) with 1 H NMR of the crystals also matching literature reports. 28 Compound 13 has been synthesised via numerous different methods one of which is known to involve a diarylstannane reagent. 29,30 The proposed mechanism for the formation of these diaryliodonium salts is thought to be by a ligand exchange reaction with a nucleophilic arylating reagent containing silicon, tin or mercury.
In some experiments the in situ mass spectrum of the reaction mixture showed a signal that could be attributed to diphenylbismuth acetate, indicating a possible aryl exchange process giving 13 but this could not be observed reliably.
The reaction of PhI(OAc)2 with Ph3Bi is known to give the triphenylbismuth(V) diacetate. 31 As with Sb, an attempted synthesis of a bis-triflate complex via metathesis with TMS-OTf from triphenylbismuth(V) diacetate resulted in no reaction.