Christian Hering

Low-temperature isolation of an azidophosphenium cation.

The first two-coordinate phosphorus cations, observed in socalled phosphamethine cyanines, were reported by Dimroth and Hoffmann in 1964. The term phosphenium ion has been introduced to imply a high degree of positive charge accumulation at the two-coordinate phosphorus center with a formally vacant 3p orbital. Phosphenium ions have some resemblance to carbenes of the type X C Y except that P replaces the central carbon atom. As is well-known, carbenes are stabilized best when X and Y are atoms or groups such as NR2, which can serve as p-electron donors to the carbon atom. Parry et al. reported the first examples of acyclic phosphenium ions [(Me2N)2P] + and [(Me2N)(Cl)P] , which were obtained by chloride abstraction from the corresponding aminochlorophosphanes by employing Lewis acids, such as ECl3 (E = Fe, Al, Ga). [4] Structural data of acyclic aminophosphenium ions are still limited to only a few examples substituted by an amino group: [(iPr2N)2P]X (X = AlCl4 , GaCl4 , BPh4 ). 6] To the best of our knowledge, halogenor pseudohalogen-substituted phosphenium ions of the type [R2N P X] (X = halogen or pseudohalogen) have not been isolated and structurally characterized. Cyclo-diphosphadiazenium salts can also be regarded as phosphenium ions (Scheme 1, species B). Upon addition of a Lewis acid to the cyclo-diphosphadiazanes A, the corresponding cyclic cation B is formed, which can be regarded as binary P/N four-membered heterocyclic cation with twoand three-coordinate phosphorus atoms and a delocalized p bond within the NPN unit. Only recently, the synthesis and full characterization of a 1-azido-cyclo-1,3-diphospha2,4-diazenium salt was reported (Scheme 1, species C). As illustrated in Scheme 1, an equilibrium between a cyclodiphosphadiazane and its monomer, the corresponding iminophosphane, might be observed depending on the sterical strain of the bulky substituent R. For example, for R = terphenyl (Ter), only a stable dimer is found in the solid state and in solution but no monomeric iminophosphane R N=P Cl. Addition of GaCl3 results in the formation of cyclodiphosphadiazenium salt B, and in the presence of Me3SiN3, species C can be isolated. In contrast, if the bulky substituent R = supermesityl (Mes*), the monomeric species D is favored; thus upon addition of GaCl3, an iminophosphenium ion is formed, which reacts as dipolarophile in the presence of the 1,3-dipole molecule Me3SiN3 to the corresponding tetrazaphosphole (Scheme 1, species F) in a formal [3+2] cyclization. However, no isomeric species C is observed. Recently, it was shown that disguised dipolarophiles, that is, the amino-substituted dichlorophosphane Ter(Me3Si)N PCl2 can also be used, which releases Me3SiCl, forming the necessary highly reactive, bare dipolarophile Ter N=P Cl in situ. Again, addition of the 1,3-dipole Me3SiN3 in the presence of a Lewis acid yields the tetrazaphospole F. These synthetic concepts can also be applied to the heavier analogues, but although the isolation and comprehensive characterization of tetrazapnictoles of the type R NE4 (E = N, P, As, Sb) were achieved, there are still open questions with respect to the reaction mechanism. Theoretical studies, carried out to determine the mechanism, indicate that pnictenium ions are intermediates on the reaction path. Following our interest in the chemistry of compounds bearing binary NPn fragments (Pn = P, 8, 10] As, Sb, and Bi), we studied the reaction of the disguised dipolarophile (Me3Si)2N PCl2 (1) with the Lewis acid GaCl3 by means of low-temperature techniques. We report herein on the synthesis and full characterization of the hitherto unknown, highly labile amino(azido)phosphenium salt [(Me3Si)2N=P N3][GaCl4] (4) utilizing a pseudohalogen/chlorine exchange reaction in aminochlorophosphenium ion [(Me3Si)2N PCl] (2a, Scheme 2). The cation in the azide-substituted phosphenium salt 4 can formally be regarded as the first known phosphapentacenium ion [R2NPNNN] ; the parent pentacenium ion N5 + was described by Christe et al. in 1999. Scheme 1. Different reaction paths observed for cyclo-diphosphadiazanes (species A) depending on the sterical strain: path 1 for R = Mes*, path 2 for R = Ter.

Chlorine/methyl exchange reactions in silylated aminostibanes: a new route to stibinostibonium cations.

The Lewis acid assisted triflate/methyl, azide/methyl, and chlorine/methyl exchange reactions between silicon and antimony have been studied in the reaction of R(Me(3)Si)N-SbCl(2) (R = Ter) with AgOTf, AgN(3), KOtBu, GaCl(3), and Me(3)SiN(3)/GaCl(3), resulting in the formation of different methylantimony compounds. Furthermore, R(Me(3)Si)N-SbCl(2) (R = SiMe(3)) was reacted with GaCl(3) at low temperatures to yield a hitherto unreported amino(chloro)stibenium cation, the proposed intermediate in methyl exchange reactions. Tetrachloridogallate salts bearing different stibinostibonium cations such as [(Me(3)Sb)SbMe(2)](+) and [(Me(3)Sb)(2)SbMe](2+) along with the GaCl(3) adduct of SbMe(3) were isolated from such R(Me(3)Si)N-SbCl(2)/GaCl(3) mixtures (R = SiMe(3)) at ambient temperatures, depending on the reaction parameters.

Dimers and Trimers of Diphosphenes: A Wealth of Cyclo-Phosphanes

Various new P-based ring systems were synthesised by transferring established reaction routes from NP chemistry to the analogous PP compounds. Due to the different electronic situations of phosphorus and nitrogen with respect to s and p character of the lone pair, different reactivity of the phosphorus compounds was observed, especially with regard to the specificity of the reactions and the stability of the products. Whereas Mes*NPCl (Mes*=2,4,6-tri-tert-butylphenyl) is stable in the solid state and in solution, the formal phosphorus congener Mes*PPCl is highly reactive and could not be observed. Instead, several formal dimers and trimers of Mes*PPCl could be isolated, which constitute an intriguing variety of three- and four-membered ring systems.

On the synthesis and reactivity of highly labile pseudohalogen phosphenium ions.

The synthesis and characterization of salts bearing highly labile pseudohalogen-substituted aminophosphenium cations of the type [(Me3Si)2NPX][GaCl4] (X = NCO, NCS, O(SiMe3)) and their respective reactivity toward Lewis bases (4-dimethylaminopyridine, dmap) and dienes (2,3-dimethyl-1,3-butadiene, dmb; 1,3-cyclo-hexadiene, chd) are described. As π-acidic species, aminophosphenium cations react with dmap at low temperatures to yield adduct salts of the type [(Me3Si)2NP(dmap)X][GaCl4] (X = Cl, N3, NCO) which were fully characterized. In the reaction with dienes at -50 °C, salts bearing phospholenium cations were obtained that could be structurally characterized. The crystal structures of novel 7-phosphanorbornenium cations of the type [(Me3Si)2NP(C6H8)X][GaCl4] (X = Cl, N3, NCO) are reported. All compounds were further investigated by means of density functional theory, and the bonding situation was accessed by Natural Bond Orbital (NBO) analysis.

Structure and bonding of novel acyclic bisaminoarsenium cations.

The synthesis and characterization of a salt bearing a labile bisaminoarsenium cation of the type {[(Me3Si)2N]2As}(+) (9a) are described, which was obtained in the reaction of the chloroarsane [(Me3Si)2N]2AsCl (8) with GaCl3. Reacting 8 with AgOTf did not yield an arsenium salt, but the cyclo-diarsadiazane [(Me3Si)2NAs-μ-NSiMe3]2 (11) was obtained in excellent yields. Moreover, the reactivity of the analogous antimony species [(Me3Si)2N]2SbCl (12) was studied. In the reaction with GaCl3, the aminochlorostibenium salt [(Me3Si)2NSbCl](+)[(Me3Si)2N(GaCl3)2](-) (5) was isolated. In the reaction with AgOTf, substitution of the chlorine in 12 resulted in the formation of [(Me3Si)2N]2SbOTf (13), a compound with significant stibenium character. All new compounds have been fully characterized by means of X-ray, vibrational spectroscopy, CHN analysis, and NMR experiments. All compounds were further investigated by means of density functional theory and the bonding situation was accessed by natural bond orbital (NBO) analysis.

Diatomic PN – trapped in a cyclo-tetraphosphazene

(Me3Si)2NPCl2, which is formally capable of eliminating two equivalents of Me3SiCl, is shown to be a suitable starting material to prepare highly reactive PN species by successive elimination of Me3SiCl. Me3SiCl elimination can be triggered either thermally and/or by addition of a Lewis acid such as GaCl3, thus leading to the formation of a highly labile aminochlorophosphenium cation in [(Me3Si)2NPCl][GaCl4] and iminophosphenium salt [Me3Si–NP][GaCl4] upon warming to ambient temperatures. This work describes the synthesis and characterization of a cyclo-tetraphosphazane in [PN(dmb)]4 (5) (dmb = 2,3-dimethyl-1,3-butadiene) obtained by thermal elimination of Me3SiCl from (Me3Si)2NPCl2 at 120 °C in toluene solution. The reactive intermediate Me3SiNPCl was trapped with dmb to form the cyclic phosphazane Me3SiN(dmb)PCl, which eventually oligomerizes to give 5. In the presence of dmb or chd (chd = 1,3-cyclohexadiene) (Me3Si)2NP(OTf)2 reacts by eliminating Me3SiOTf to yield the spirocyclic phospholenium salts [Me3SiN(dmb)P(dmb)][OTf] (7) and [Me3SiN(chd)P(chd)][OTf] (8), of which the solid state structures were successfully determined. 7 decomposes when exposed to moisture to give an unprecedented cyclic ammonium phosphinoxide [P(O)H(dmb)2NH2][OTf] (9). Tetraphosphazane 5 is shown to be a versatile ligand in transition metal chemistry. It coordinates in an η3-fashion in {[PN(dmb)]4Mo(CO)3} (5·Mo) and is able to coordinate a second metal fragment, exemplified by the formation of the ditungsten complex{[PN(dmb)]4W2(CO)7} (5·W2) with a semi-bridging carbonyl ligand. All compounds were structurally characterized and the bonding situation was investigated by density functional theory and natural bond orbital analysis (NBO).

The elusive cyanoformate: an unusual cyanide shuttle.

CO2 makes up just 0.04% of the earth\9s atmosphere and is essential for all life on earth. Nevertheless, it is the major anthropogenic greenhouse gas. Consequently, significant scientific effort has been devoted to establishing CO2 as a cheap and readily available carbon feedstock. The electrophilic nature of CO2 is exploited in its catalytic reduction to formates by either enzymes, well-defined metal complexes, or metal oxides. Nanoporous materials such as amine-grafted metal– organic frameworks (MOFs) can reversibly bind CO2 and thus allow regeneration of the sorbent. It comes as a surprise that CO2 complexes with small nucleophilic anions, such as halides or pseudo-halides (e.g. OH , CN , OCN , SCN , and N3 ) have not been thoroughly studied to date. The only wellcharacterized halide/pseudo-halide CO2 species are hydrogen carbonate (old: bicarbonate) [HO-CO2] and fluorocarbonate [FCO2] . In this context, a recent report by Clybourne, Tuononen and co-workers on the preparation of the elusive cyanoformate anion warrants particular attention (Scheme 1). The synthesis of [NC-CO2] was realized by exposing a concentrated [PPh4][CN] solution in CH3CN to an atmosphere of CO2. [PPh4][NC-CO2] precipitates as a colorless crystalline solid and its formation was confirmed by combined crystallographic, spectroscopic, and computational methods. As expected for a Lewis acid–base adduct, the CCN–CCO2 distance is 1.480(9) , whereas the C–N distance with 1.06(1) is relatively short, in the typical range of a triple bond. This view is supported by computational studies revealing the C C bond to be predominantly a s-type donor–acceptor interaction and hence, cyanide acts as a two-electron donor to CO2. In contrast to CO2, ketones are known to interact readily with cyanides to form cyanohydrins; this was reported first in Strecker s pioneering work in 1850. However, the reaction of CO2 with CN is considerably less favorable for several reasons: 1) Distorting the linear CO2 molecule is energetically not favorable (310 kJmol ); 2) the p bonds in CO2 are considerably stronger than p bonds at sp-hybridized carbons; 3) the poor overall entropy is not compensated by the small enthalpic preference for cyanoformate formation. This situation is nicely reflected by the relatively large O-C-O angle of 1338, which indicates that CO2 wants to escape this unfavorable situation. The stability of [NC-CO2] increases with decreasing solvent polarity (toluene). Thus, fragmentation into CO2 and CN is favored in polar reaction media. However, kinetics protects the anion from immediate breakdown as C C bond scission has a relatively high activation barrier of ca. 40 kJ mol . The authors carried out decomposition studies utilizing in situ generated [Bu4N][NCCO2] in an ionic liquid. This mixture was added to anhydrous toluene, tetrahydrofuran, and acetonitrile, respectively, to monitor the decay of the nas(CO2) vibration in [NC-CO2] and determine the half-lives (toluene: 110 min; acetonitrile: 17 min). Traces of water led to immediate decomposition, as CN is an excellent nucleophile and hydrogen carbonate is formed. Besides hydrogen carbonate (A), fluorocarbonate is the only example of a stable halide/pseudo-halide adduct of CO2 (Figure 1). Seppelt et al. reported the facile synthesis of [FCO2] (B) by exposing anhydrous solutions of ammonium fluoride salts to CO2. [4] These salts are poorly soluble in common solvents and decompose when traces of water are present. The main hurdle for the preparation seems to be the low affinity of CO2 towards F , which amounts to just 133 kJmol . [FCO2] is isoelectronic with [NO3] ; consequently neutral [NC-NO2] (C) is the isoelectronic nitrogen congener of the cyanoformate anion. In a recent report Christe et al. described the challenging synthesis and comprehensive characterization of nitryl cyanide from NO2BF4 and tBuMe2SiCN in nitromethane at 30 8C. Fractional condensation yielded pure [NC-NO2] which melted at 85 8C (boiling point 7 8C) and complete decomposition was obScheme 1. Synthesis of the labile cyanoformate according to Clyburne et al. (top) and depiction of its breakdown into CO2 and CN depending on the polarity of the reaction medium (bottom).