The 2‐Arsaethynolate Anion: Synthesis and Reactivity Towards Heteroallenes

Abstract The synthesis and isolation of the 2‐arsaethynolate anion, AsCO−, and its subsequent reactivity towards heteroallenes is reported. Reactions with ketenes and carbodiimides afford four‐membered anionic heterocycles in formal [2+2] cycloaddition reactions. By contrast, reaction with an isocyanate yielded a 1,4,2‐diazaarsolidine‐3,5‐dionide anion and the unprecedented cluster anions As10 2− and As12 4−. These preliminary reactivity studies hint at the enormous potential synthetic utility of this novel anion, which may be employed as an arsenide (As−) source.

Heavier group 15 analogues of PCO À are unknown. Early attempts by Hübler and Becker to generate the 2-arsaethynolate anion, AsCO À ,y ielded Li 3 As 7 . [19] Theoretical studies have nevertheless indicated the stability of the heavier members of the homologous ECO À series (E = As,S b, Bi). [20] These studies prompted us to address the question posed by Lu, Schaefer, and co-workers:" These successes in transitioning from NCO À to PCO À raise the obvious question: can heavier congeners of PCO À be made?". Herein we report our efforts to reinvestigate the synthesis of the AsCO À ,which until now has only been isolated in matrix isolation experiments. [21] AsCO À can be prepared by following am odified version of the synthetic procedure reported by Grützmacher for PCO À (Scheme 1). [6] Reaction of elemental sodium and arsenic in a3 :1 ratio in dimethoxyethane (DME) with catalytic amounts of naphthalene affords ab lack suspension containing Na 3 As.C ritically,t his step requires prolonged reaction times and stirring for up to three days at 708 8C.
Protonation using as terically encumbered alcohol (tBuOH) followed by carbonylation using diethylcarbonate affords as olution of AsCO À (1). To aid solubility and facilitate the crystallographic characterization of the anion 1,4,7,10,13,16hexaoxacyclooctadecane (18-crown-6) can be added to the mixture in the final stage.The product can be precipitated out of solution by addition of 1,4-dioxane and isolated as ayellow powder.T his compound can subsequently be recrystallized from an umber of different solvent/anti-solvent mixtures, affording ac olorless crystalline material in moderate to high yields (approx. 60 %).
Thes tructure of the anion was determined by singlecrystal X-ray diffraction. Three different solvates were isolated by varying the solvent of crystallization and/or reaction conditions:[ Na (18- Of the three,t he latter two display positional disorder of the anion, and are therefore not suitable for the discussion of bond metric data (the structures are provided in the Supporting Information). [Na(18-crown-6)][1]·(C 4 H 8 O 2 ) 0.5 contains as ingle anion in the asymmetric unit accompanied by [Na(18-crown-6)] + and half am olecule of 1,4-dioxane ( Figure 1). Thestructure clearly shows alinear anion with an As1-C1-O1 angle of 176.6(2)8 8.T he AsÀCa nd CÀOb ond lengths are 1.707(3) and 1.197(3) , respectively.T he AsÀC distance lies between the values expected for ad ouble (1.81 ) and triple bond (1.66 ), [22] and is somewhat shorter than previously predicted computationally for 1 (1.740 ). [20] This difference may be due to the relatively short contact arising between Na1 and O1 [2.290(2) ],which would have the effect of increasing the triple-bond character of the AsÀC bond. However,i ti sa lso worth noting that the structure of 1 was computed in the gas phase,a nd that therefore no solvation effects and/or Coloumbic interactions were taken into consideration.
As for its lighter homologues,t wo main resonance forms can be proposed for 1,either with localization of the negative charge on the oxygen atom and aformal AsCtriple bond [1 a as depicted in Scheme 2],o ras tructure with af ormal As = C bond and an egative charge on arsenic [1 b ]. Natural bond orbital (NBO) calculations revealed Wiberg bond indices (WBI) of 2.14 and 1.70 for the AsÀCa nd CÀOb onds, respectively,a nd atomic charges of À0.50 and À0.60 for the arsenic and oxygen atoms,r espectively. [20] These data are Abstract: The synthesis and isolation of the 2-arsaethynolate anion, AsCO À ,a nd its subsequent reactivity towards heteroallenes is reported. Reactions with ketenes and carbodiimides affordf our-membered anionic heterocycles in formal [2+ +2] cycloaddition reactions.B yc ontrast, reaction with an isocyanate yielded a1 ,4,2-diazaarsolidine-3,5-dionide anion and the unprecedented cluster anions As 10 2À and As 12

4À
.These preliminary reactivity studies hint at the enormous potential synthetic utility of this novel anion, whichm ay be employed as an arsenide (As À )source. similar to those obtained for PCO À ,a nd reveal that both resonance forms contribute significantly to bonding,t hus allowing for an effective delocalization of negative charge along the anion. An atural resonance theory (NRT) analysis weighs the relative contributions of the ethynolate (AsC-O À )a nd ketenide ( À As=C=O) resonance forms as 35 and 48 %, respectively.
Thevibrational spectra obtained for 1 are consistent with values reported from matrix isolation studies. [21] TheR aman spectrum of [Na(18-crown-6)][1]·(C 4 H 8 O 2 ) 0.5 exhibits three bands at 444, 630, and 1746 cm À1 ,w hich arise from the corresponding bending,s ymmetric,a nd asymmetric stretching modes (cf.444, 609, and 1756 cm À1 as reported by Zhang et al.). Interestingly,wewere able to observe variations in the data depending on whether or not the anion interacts with the charge-balancing cation, thus Raman bands for [Na(18crown-6) 1.5 ] [1], where the anion shows no close contacts with Na + ,w ere observed at 442, 612, and 1758 cm À1 ,a nd are an even better match with previously reported data. Having developed al arge-scale synthesis for AsCO À we were prompted to explore its reactivity in an effort to establish comparisons with the chemistry of its lighter homologues NCO À and PCO À .O ur initial studies show that AsCO À is highly susceptible to oxidation, readily affording As 7 3À (as observed in early experiments by Becker and coworkers) or elemental arsenic when exposed to trace amounts of air or mild oxidants.T his heightened reactivity makes it much less convenient to work with than its phosphoruscontaining analogue.Ofthe reactions we have explored thus far, the most successful involve the reaction of AsCO À with unsaturated organic substrates such as heteroallenes.T hus, reaction of AsCO À with selected ketenes (O=C=CPh 2 )a nd carbodiimides (DippN=C=NDipp;D ipp = 2,6-diisopropylphenyl) afford the anionic [2+ +2] cycloaddition products As[C(O)] 2 CPh 2 À (2)a nd AsC(O)(CNDipp)NDipp À (3), respectively (Scheme 2). Ther eactions were monitored by 13 CNMR spectroscopy and were found to proceed to completion by the time the NMR data were collected. Analogous reactivity has been reported by our research group for the lighter PCO À analogue,w hich exhibits similar shifts for the resonances of the quartenary carbon atoms in the small heterocycles

Angewandte Chemie
Communications 8538 www.angewandte.org ring system with acute C1-As1-C2 bond angles [71.12 (14) and 67.43(9)8 8,for 2 and 3,respectively] which are similar to those recorded for the analogous phosphorus-containing compounds.T he As À Cb ond lengths [2:1 .925(4), 1.941(4) ; 3: 1.961(2) and 1.976 (2) ],a re logically longer than the corresponding PÀCb onds in the lighter homologues,a nd notably shorter for 2 when compared to 3.T his difference is the result of negative-charge delocalization over the two carbonyl moieties,and confers greater double-bond character to the As À Cbonds.Incontrast the backbone imino functionality (NDipp) present in 3 has apair of electrons in a p-type orbital orthogonal to the four-membered ring which can also be delocalized into the adjacent carbonyl groups,thus lengthening the AsÀCb onds. , Cp*C(NR) 2 As[W(CO) 5 ] 2 (R = isopropyl or cyclohexyl, Cp* = pentamethylcyclopentadienyl) and EAs(NAr) 2 (E = N, P, As;Ar= 2,6-dimesitylphenyl). [23][24][25] By contrast with the aforementioned studies,r eactions between 1 and isocyanates (O=C=NDipp) proceed less cleanly and their outcome is strongly dependent on the stoichiometric loading of reagents.F rom such reaction mixtures we were able to isolate the five-membered cyclic compound As[C(O)] 2 [NDipp] 2 À (4;F igure 3) which can be obtained as ac ompositionally pure species by fractional crystallization. Analogous phosphorus-containing ring systems have previously been reported from similar reactions between PCO À and O = C = NDipp. [10] 4 can be considered as the result of the cyclization of one molecule of 1 with two equivalents of isocyanate accompanied by loss of carbon monoxide (Scheme 3). 13 CNMR spectroscopic studies on these reaction mixtures revealed that 4 is the only product formed incorporating organic functionalities.D ecarbonylation occurs readily for reactions involving 1,a nd when al ow stoichiometric loading of O=C=NDipp was employed, ar ed crystalline solid could also be obtained and consisted of as olid solution of three different arsenic clusters As 10 2À (pseudo D 2h )and As 12 4À (the latter occurring as two different isomers: C 2h and D 4h ). These cluster anions can be thought of as intermediates in the oxidative decarbonylation of AsCO À and, as with the well-known As 7 3À cluster,a nd represent snapshots of ap rocess which ultimately affords elemental arsenic and carbon monoxide.
Thes ingle-crystal X-ray structure of 4 reveals as ingle five-membered ring system in the asymmetric unit accompanied by ac harge-balancing [Na(18-crown-6)] + cation. It crystallizes in the space group P2 1 /n and is isomorphous with its phosphorus-containing analogue [K(18-crown-6)][P-[C(O)] 2 (NDipp) 2 ]. [10] As expected, the structural differences between both anions are largely insignificant. TheN -As-C bond angle is 85.21(8)8 8,which is slightly more acute than the corresponding angle in the phosphorus-containing species [90.7(1) 8 8], and is in line with the longer As À Ca nd As À N bonds [1.898(2) 8, 195.8 ppm]. [10] Ared crystalline solid could be obtained as aside product from reactions involving isocyanates and AsCO À .Asinglecrystal X-ray diffraction analysis of this sample revealed three [Na (18-crown-6)] + cations in the asymmetric unit. Thep acking of such cations leaves space for as ingle cluster species. Close inspection of the heavily disordered cluster site revealed dianionic As 10 2À ,a nd at etraanionic As 12 4À species, both of which occupy the site in a1:1 ratio (hence giving rise

Angewandte
Chemie to an et 3À charge). Thel atter species exhibits two different topologies (Figure 4b and 4c) with C 2h and D 4h symmetry (with relative occupancies of 36 and 14 %, respectively). All three cluster anions are electron-precise species in which the arsenic atoms that are bonded to three adjacent vertices are neutral, and those that are only bonded to two adjacent atoms carry formal negative charges.
TheA s 10 2À geometry is related to that of the well-known As 7 3À .T he latter species adopts an orbornadiene-like geometry when coordinated to metal atoms in clusters such as [As 7 Cr(CO) 3 ] 3À . [26,27] As 10 2À can be thought of as two such norbornadiene-like clusters fused through the four atoms through which they characteristically would bond to am etal center. As such the cluster exhibits adistorted D 2h geometry. Theoptimized geometry obtained by theoretical calculations at the density functional level of theory (DFT) are consistent with such ageometry.The As 12 4À cluster adopts two different geometries,alow-symmetry C 2h isomer and ahigher symmetry,b ut thermodynamically less stable, D 4h geometry.D FT calculations showed the latter species to be 73 kJ mol À1 higher in energy,a nd may account for its lower abundance in the crystal lattice.Itisinteresting to note that the combination of As 10 2À and As 12 4À is energetically not favorable compared to the known As 11 3À cluster anion by 437 kJ mol À1 per ion pair (see Table S5 in the Supporting Information). [28] Thes tructures exhibited by these anions are without precedent for the elements of group 15 and indicate that, upon oxidation, AsCO À may be employed as as ource of metastable homoatomic arsenic clusters.A sm entioned above,e arly studies by Becker and co-workers aimed at isolating AsCO À resulted in the isolation of the Zintl ion As 7 3À ,arelated electron-precise cluster.
In summary,w eh ave developed as ynthetic route to AsCO À by carbonylation of NaAsH 2 .T his method afforded AsCO À (1)ingood yields,thus allowing subsequent reactivity studies which have demonstrated that cyclizations between the AsÀCb ond in 1 and unsaturated substrates are possible and yield novel heterocyclic species.S tudies are currently underway further exploring the reactivity of 1 towards alibrary of compounds.