Synthesis and characterization of a 2-chloro-1,3,2-diaza-phospholidine-4,5-diimine

An oxalamidine 10a was used as precursor for the synthesis of 2-chloro-1,3,2-diaza-phospholidine-4,5-diimine 7a . Both products were characterized by single-crystal X-ray diffraction studies which revealed for 7a the presence of an unusually short P–Cl bond distance. In contrast to other known 2-chloro-1,3,2-diazaphospholidines, 7a fails to react with gallium trichloride, trimethylsilyl trilate, or silver triflate via heterolytic P–Cl bond fission and formation of an N-heterocyclic phosphenium ion. A computational study suggests that this unusual behavior is due to the fact that the exocyclic imino-groups stabilize the P–Cl bond in 7a by diminishing the hyperconjugation between the lone-pairs of the ring nitrogen atoms and the  *(P–Cl) orbital and destabilize at the same time the hypothetical phosphenium cation.


Introduction
Five-membered phosphorus-nitrogen heterocycles like 1,3,2-diazaphospholidines 1 1 and 1,3,2diazaphospholenes 2 2,3,4 (Scheme 1) are of central importance for the synthesis of stable phosphenium ions 3, 4 with a cationic, two-coordinate phosphorus atom 5,6,7 which are isoelectronic analogues to neutral diaminocarbenes 5, 6, respectively.Whereas cations 3 with CC-saturated heterocyclic rings were among the first phosphenium ions ever prepared, 8 the interest in the CC-unsaturated N-heterocyclic phosphenium ions (NHP) 4 2,4 developed only much later in the wake of the rise of the chemistry of stable N-heterocyclic carbenes (NHC). 9he most widely applied synthetic approach to access both CC-saturated and unsaturated phosphenium ions involves reactions of neutral precursors 1, 2 with suitable electrophiles (e. g.AlCl3, GaCl3, AgOTf, Me3SiOTf etc.) under heterolytic cleavage of the phosphorus-chlorine bonds. 5,6,7Cations 4 have alternatively been accessed via a [1+4]-cycloaddition route starting from a diazabutadiene and a transient P(I) fragment generated via disproportionation of PI3, or via reduction of PCl3 with SnCl2, respectively. 10In view of this approach we became interested in the synthesis of derivatives 7 featuring both a five-membered phosphorus-nitrogen heterocycle and a diazabutadiene moiety.Molecules of this type have not only a large potential as bridging ligands in coordination chemistry, but offer also a prospect to generate functional phosphenium ions via P-Cl bond heterolysis, or prepare novel fused heterocyclic systems via subsequent [1+4]-cycloadditions. Herein we report on our first results on synthesis and characterization of a novel 2-chloro-1,3,2-diazaphospholidine of type 7, and its reactivity towards electrophiles.

Results and Discussion
The oxalamidine precursor 10a was prepared as shown in Scheme 2 following a published protocol. 11In a first step, oxalyl chloride was reacted with 2,6-diisopropyl aniline in the presence of triethylamine to give oxalic diamide 8a which was then converted to oxalimidoyl dichloride 9a by reaction with PCl5.Further condensation with 2,6-dimethyl aniline finally gave oxalamidine 10a which was isolated in reasonable yield after crystallization.A single-crystal Xray diffraction study (Fig. 1) revealed that the oxalamidine unit adopts a transoid, strongly twisted (twist angle 45°) C2-symmetrical conformation in which the amide protons are located at the 2,6-dimethylphenyl groups and the C=N double bonds feature E-configuration.The N1-C2 (1.284(3) Å) and C2-N3 distances (1.361(2) Å) represent typical double and single bonds, and their large difference indicates a quite low degree of conjugation within the amidine NCNmoiety. 14The observation of broadened resonances in room temperature 1 H NMR spectra suggests that in solution, as in the case of other oxalamidines, 11 a mixture of several rapidly equilibrating prototropic and conformational isomers is present.Attempts to prepare 2-chloro-1,3,2-diazaphospholidine 7a like other derivatives of the same type 12,13 via base-induced condensation from 10a and PCl3 using triethyl amine as acid scavenger failed and resulted only in isolation of the corresponding oxalamidinium salt.However, conversion of 10a to 7a was finally accomplished after deprotonation of 10a with two equivalents butyl lithium at -78°C and subsequent addition of PCl3.The product was isolated after work-up as a yellow, air and moisture sensitive powder and characterized by analytical and spectral data.The 31 P NMR chemical shift of 134 ppm is close to the lower end of the range of approx.138 -155 ppm reported for other 1,3-diaryl-1,3,2-diazaphospholidines. 12,13The occurrence of broadened signals in 1 H NMR spectrum can be explained as a consequence of dynamic processes associated with hindered rotation of N-aryl moieties and, possibly, inversion of imino-nitrogen atoms.A notable feature in the EI mass spectrum is the lack of an [M-Cl] + fragment peak which is a prominent attribute in the mass spectra of diamino chloro phosphines. 7l Cl Conditions and reagents: i) 2 equivs.R 2 NH2, NEt3, THF, 0°C; ii) PCl5, toluene, 16h reflux; iii) 2 equivs.R 1 NH2, NEt3, toluene, reflux; iv) 2 equivs, BuLi, THF, -78°C; v) 1 equiv PCl3.
Single crystals of 7a were obtained after crystallization from hexane at -20 C° and were characterized by single-crystal X-ray diffraction.A graphical representation of the molecular structure is shown in Fig. 2. Important bond distances and angles are summarized in Table 1.
Molecular structure of 7a.Thermal ellipsoids are drawn at 50% probability level, and hydrogen atoms were omitted for clarity.

Table 1. Selected bond distances (Å) and angles (°) for
As a consequence of the structural constraints imposed by formation of the heterocyclic ring, the oxalamidine unit in 7a displays a cisoid conformation.The twist angle between both amidine units is much smaller (5°) than in 10a and enforces a perceptibly non-planar conformation of the heterocyclic ring.The central C3-C4 distance is similar to that in 10a and suggests likewise the presence of a single bond.The difference between the distances of exocyclic C3-N3/C4-N4 (1.267(6)/1.253(6)Å) and endocyclic N2-C3/C4-N5 distances (1.390(6)/1.425(6)Å) is still more pronounced than in 10a and indicates an even lower degree of NCN-conjugation within the amidine moieties. 14In contrast to 10a, one of the C=N double bonds displays a Zstereochemistry which enables presumably a minimization of steric interactions between the bulky N-aryl moieties.The coordination geometry at the endocyclic nitrogen atoms exhibits no significant deviation from planarity (sum of bond angles: N2 359.5(11)°,N5 356.7(10)°), and the phosphorus atom features the expected pyramidal coordination (sum of bond angles 295.7(6)°).The P1-N2 and P1-N5 bond distances are longer than in other known 2-chloro-1,3,2diazaphospholidines (1.65-1.68Å). 13,15 In contrast, the exocyclic P-Cl bond (2.127(2) Å) matches the common distance in acyclic diaminochlorophosphines (2.13(6) Å 15 ) and is substantially shorter than the bond lengths in other 2-chloro-1,3,2-diazaphospholidines (2.17-2.25 Å 13,16 ) or 2-chloro-1,3,2-diazaphospholenes (2.24-2.76Å 15 ).Taken together, the observed structural features seem to indicate a bonding situation in which the electron deficient iminogroups compete with the PCl-moiety for mesomeric interaction with the lone-pair of the ring nitrogen atoms.As this effect is expected to weaken the hyperconjugation between the nitrogen lone-pairs and the antibonding *(P-Cl)-orbital, 15 less electron density is transferred from the nitrogen lone-pairs into the *(P-Cl)-orbital, and the P-Cl bond can thus retain a higher covalent bond order than in other N-heterocyclic chlorophosphines.
As a higher bond order impedes heterolytic cleavage of the P-Cl bond, it can be expected that 7a is less prone to interact with electrophiles under formation of a phosphenium cation than other N-heterocyclic chlorophosphines.In accord with this expectation, 7a showed no evidence to react with GaCl3 (1 -3 equivs.)or excess of Me3SiOTf under chloride abstraction and formation of a phosphenium ion; monitoring the reaction by 31 P NMR indicated that even after prolonged exposure to an excess of reagent at best unspecific decomposition could be observed.Reaction of a CH2Cl2 solution of 7a with one equivalent of AgOTf produced a precipitate of AgCl together with a solution that contained according to an 31 P NMR spectroscopic survey a single phosphorus containing product.Although all attempts toward isolation of this product remained unsuccessful and its constitution remains therefore still in the dark, tentative assignment as a phosphenium cation salt is, however, definitely ruled out by the observed 31 P NMR chemical shift of 109 ppm which appears at even higher field as the signal of 7a and lies far outside the expected range of 31 P NMR chemical shifts between 200 and 450 ppm for diaminophosphenium cations. 5,6,7he outcome of the reactions of 7a with electrophiles suggests that the exocyclic imino moieties have an adverse effect on the stability of a diazaphospholidinium cation and its formation is therefore disfavored.This hypothesis is further substantiated by calculated energies of the isodesmic chloride transfer reactions shown in Scheme 3 which allow to estimate the stabilities of phosphenium cations 3b, 4b, and 9b relative to the parent diaminophosphenium ion [(H2N)2P] + as reference.The calculation of negative values for E1 and E2 at all levels of theory indicates that Nheterocyclic cations 3b and 4b are increasingly more stable than 8b and confirms the results of earlier computational studies in which the same conclusion was derived from the energies of hydride transfer reactions. 4,17In contrast, E3 was calculated to be positive at all levels of theory.Comparison of the values of the differences (E1 -E2) and (E3 -E2) suggests that the destabilization imposed by formal replacement of the C2H4-backbone of 3b by two imino-groups (~ +20 kcal mol -1 ) is approximately twice as large as the extra stabilization brought by the presence of an additional double bond and ring aromatization in 4b (~ -10 kcal mol -1 ).Analysis of calculated bond distances, Wiberg Bond indices, and electron densities at the bond critical points () for the P-Cl-bonds of the model compounds 1b, 2b, and 7b (Table 2) reveals further that decreasing cation stability in the order 4b, 3b, 8b correlates with increasing P-Cl bond shortening in the neutral chlorophosphine precursors 2b, 1b, 7b, and confirms the interpretation of this effect as arising from an increased covalent bond order.The decrease in the NPA charges q(Cl) on the chlorine atom with increasing P-Cl bond shortening suggests in line with the results of previous studies 17 that this trend is also associated with a lower degree of hyperconjugative electron transfer from the nitrogen lone-pairs into the *(P-Cl) orbital.Altogether, the computational results allow thus to conclude that the imino-substituents at the heterocyclic ring destabilize the phosphenium cation and strengthen at the same time the P-Cl bond in the neutral chlorophosphine precursor.Cooperation of both effects provides a reasonable explanation for the experimentally observed reluctance of N-heterocyclic chlorophosphine 7b to undergo electrophile induced heterolytic P-Cl bond cleavage.Table 2. Bond distances (P-Cl), electron densities at bond critical points (), Wiberg bond indices (WBI) and atomic charges q(P), q(Cl) obtained from Natural Population Analysis (NPA) for the P-Cl bonds in the model compounds 1b, 2b, and 7b as calculated at the B3LYP/6-311+G**-level of theory P−Cl / Å  / a.u.WBI q(P) / a.u.q(Cl) / a.

Conclusions
A 2-chloro-1,3,2-diazaphospholidine-4,5-diimine was synthesized by reaction of a doubly metalated oxalamidine with PCl3.The product displays an extraordinarily short P-Cl bond and fails to undergo electrophile induced bond cleavage to give a phosphenium ion.A computational study allows to relate these features to the presence of the exocyclic imino-groups which destabilize the hypothetical phosphenium cation and exert at the same time a stabilizing effect on the P-Cl bond.

Experimental Section
General.All manipulations were carried out under an atmosphere of dry argon using standard vacuum line techniques.Solvents were dried by standard procedures.nd the program AIM 2000 19 was used for Bader analysis of electron densities.The reaction energies listed in Scheme 3 and further data listed in Table 2 were calculated at different levels of theory using molecular structures of model compounds that had previously been optimized at the same level of theory.
13All other were commercially available and used as received.NMR spectra were recorded on a Bruker Avance 250 ( 1 H, 250.1 MHz;13C, 62.8 MHz; 31 P, 101.2 MHz) spectrometer at 303 K; chemical shifts are referenced to external SiMe4 ( 1 H, 13 C) or 85% H3PO4 ( 31 P). Elmental analyses were determined on a Perkin-Elmer 24000 CHN/O Analyzer.Melting points were determined in sealed capillaries at a Büchi Melting Point B-545 apparatus.Computational studies were carried out with the Gaussian03 package,