Synthesis, Characterization and Theoretical Calculations of Novel Five Membered Rings Containing Phosphorus and Arsenic Atoms

Journal of the Brazilian Chemical Society

Almeida, Wagner B. De¹  Duarte, Leopoldo W. M.¹  Rocha, Willian R.¹  Caliman, Vinicius¹  Universidade Federal de Juiz de Fora, UFJF, Juiz de Fora, Brazil¹  Universidade Federal de Minas Gerais, UFMG¹ 

关键词: arsadiphospholes;    synthesis;    31P NMR;    aromaticity;    ab initio calculations;   

DOI  :  10.1590/S0103-50532002000500009

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Five membered rings containing phosphorus atoms have been synthesized since 1959.1 However, nearly 75% of all known rings of this kind were prepared in the last ten years, showing the importance that chemists are giving to this type of compounds. Phosphole (Figure 1), is known to be a nonplanar, nonaromatic molecule.2-5 This behavior is attributed to the pyramidal character at phosphorus.6 This class of compounds is of particular interest in coordination chemistry due to its ability to act in several different coordination modes. For instance, phosphole can act as a two electron donor, when bonding occurs to a metal by donation of the phosphorus lone pair only; four electron donor, when the diene system is coordinated only and six electron donor when the diene system is bound to one metal and the phosphorus behaves as a two electron donor to a second metal. All these coordination modes have been reviewed in the literature.2-4   Theoretical calculations7-9 have shown that increasing the number of phosphorus atoms in the ring structure of phosphole leads to a decrease in the pyramidal character of the sp3 phosphorus, giving rise to a planar phosphole structure. These calculations also showed that the ring planarity is directly related to the electronic delocalization (or aromaticity) and a planar ring is predicted to be strongly aromatic by means of theoretical calculations. The synthesis of the first triphosphole, 1-[bis(trimethylsilyl)methyl]-3-5-di-tert-butyl-1,2,4 -triphosphole (1) (Figure 1) was reported10 by one of us in 1995. This molecule showed some planarity, with the sum of bond angles around the sp3 phosphorus atom equal to 342°, which is considerably greater than the analogous mono and diphospholes (302 and 320°, respectively). Using the same synthetic route, Cloke et al.11 recently published the synthesis of a triphosphole, 1-[bis(trimethylsilyl)methyl]-3,5-trimethylsilyl-1,2,4 -triphosphole (2) containing a planar ring structure, with Σangles around the σ3 phosphorus. Compound 2 is not only planar but also shows aromatic character. The Bird aromaticity index12 for 2 was found to be 84, which is significantly greater than 56 found for 1. In this work we present the synthesis of two new arsadiphospholes, which are five membered rings containing 2 phosphorus, 2 carbons and an arsenic atom forming the ring structure. Ab initio molecular orbital calculations were also performed to evaluate the relative stability, inversion barrier and aromaticity character of the five possible isomers of arsadiphosphole. The effect of bulky substituents on the aromaticity and energy barrier to pyramidal inversion is also discussed.   Results and Discussion The synthetic route used for the syntheses of both triphospholes reported so far (compounds 1 and 2), involved the coupling reaction between the anionic rings (P3C2But2 )- and [P3C2(SiMe3)2 ]- and a bulky R group [R = -CH(SiMe3)2]. The synthesis of the anionic rings 4-arsa-1,2-diphosphacyclopentadienyl (3) and 2-arsa-1,4-diphosphacyclopentadienyl (4) reported by Nixon and co-workers,13 prompted us to synthesize two new arsadiphospholes using the same bulky side group described above. The mixture containing both rings 3 and 4, which was obtained from SiMe3P=C(But)OSiMe3 and LiAs(SiMe3)2, reacts with BrCH(SiMe3)2 in dimethoxyethane as shown in Figure 2. The reaction of BrCH(SiMe3)2 with the anionic ring 3 may generate two isomers 5 and 7 (route A in Figure 2). The former is obtained through the bonding of the R group to the phosphorus atom and in the later, the R group bonds to the arsenic atom. The reaction of BrCH(SiMe3)2 with the anionic ring 4 in route B, may produce compounds 6, 8 and 9, generated from the bonding of the bulky side group to the two unequivalent phosphorus atoms and to the arsenic atom, respectively. In the synthetic procedure employed here (Figure 2), we found that the mixture containing both rings 3 and 4 reacts with BrCH(SiMe3)2 to give, exclusively, the two new arsadiphospholes 5 and 6, which have both been characterized by 31P NMR spectroscopy. As expected, in compound 5 the R group is bonded to the ring via one of the two phosphorus bonded together, which are the less hindered atoms. Compound 6 showed the R group attached to the ring via the phosphorus bonded directly to the arsenic atom.   Figure 3 shows a 31P{1H} NMR spectrum of a mixture containing compounds 1, 5 and 6. The three doublets of doublets at δ 111.2, 179.2 and 243.7 easily identify compound 1, which has been completely characterized.10 The proposed structure for compound 5 (Figure 2) is based on this spectrum which shows the two phosphorus resonances at δ 106.8 and 181.5, which are typical for sp3 and sp2 hybridized phosphorus, respectively. Each signal appears as a doublet with a coupling constant 1Jpp = 523.6 Hz, typical for a direct P-P bond. The spectrum in Figure 3 also shows the two phosphorus resonances for compound 6. These resonances appear at δ 132.2 and 240.6 which are typical for sp3 and sp2 hybridized phosphorus, respectively. The coupling constant between the two phosphorus is 16.8 Hz, and is typical for a two bonds coupling. These values, which are in complete agreement with the literature reports,14,15 support the proposed structure presented in Figure 2 where the sp3 phosphorus is bonded to the arsenic atom and the sp2 phosphorus lies between two carbon atoms. The synthesis of compounds 5 and 6 clearly shows that the R fragment bonds to the rings via the less stericaly hindered atoms, and it always occurs via one of two heteroatoms bonded together. The exclusive formation of compound 6 from the anionic ring 4 also shows the preference of the R group for the phosphorus atom. The relative stability of these isomers, generated from routes A and B, were investigated by ab initio calculations and also the inversion barrier about the sp3 heavy atom and the aromaticity character of each isomer will be discussed in the next sessions. Calculations The optimized MP2/6-31G(d) structural parameters for the five possible arsadiphosphole isomers generated in routes A and B (Figure 2) are shown in Figure 4 and the total energies, inversion barrier (ΔE#) and relative stability (ΔE) are shown in Table 1.   As can be seen in Table 1, for all the five possible arsadiphosphole isomers there exist two possible structures for each isomer, corresponding to a planar and a nonplanar structures. The former corresponds to first-order saddle points on the potential energy surface, with respect to the out-of-plane bending about the phosphorus in isomers A, B and C and out-of-plane bending about the arsenic in isomers D and E. The planar forms of isomers A, B, C, D and E have imaginary frequencies of 323i, 318i, 339i, 387i and 335i cm-1 respectively. The nonplanar structures correspond to real minima on the potential energy surface. Analyzing the relative energy for the minimum structures found in routes A and B, ΔE, quoted in Table 1, it can be seen that isomer A, analogous to 5, in which the phosphorus atom is in α-position, is the most stable structure found in route A. Analogously, isomer B, in which the arsenic atom is in α-position, is the most stable structure obtained in route B. Using the computed MP2/6-31G(d) + ZPE relative energy for the minimum structures, we found a thermodynamic isomer distribution of 100 % of isomer A in route A and 97.6% of isomer B, 2.0% of isomer C and 0.4% of isomer E in route B at 298.15 K. The other isomer is present on a negligible percentage. This isomer distribution predicted theoretically for routes A and B is in complete agreement with the experimental finding in which only isomers 5 and 6, analogous to A and B, respectively, were obtained in routes A and B. As we shall see later, the formation of isomers 5 and 6 is also connected with the aromatic character as well as with the energy barrier to pyramidal inversion around the phosphorus atom. The pyramidal inversion barrier about phosphorus and arsenic atoms can also be seen in Table 1. For isomers A and B, which are the most stable ones, found in routes A and B, the inversion barriers computed at the MP2/6-31G(d) level of theory are 3.2 kcal mol-1 (1 kcal = 4.184 kJ). The inversion barriers about arsenic are much higher, exhibiting the values of 11.1 and 8.6 kcal mol-1 for isomers D and E respectively. Previous theoretical work on triphospholes9 has shown that the inversion barriers about phosphorus are 2.0 and 1.7 kcal mol-1 for 1,2,5- and 1,2,4-triphospholes respectively, at the MP2/6-311G(2d)//MP2/6-31G(d) level and 3.7 and 4.4 kcal mol-1 respectively at the CCSD(T)/6-311G(2d)//MP2/6-31G(d) level of theory. This fact shows that the introduction of an arsenic atom on the five membered ring has little effect in the inversion barrier about the phosphorus, as compared with the analogous triphospholes. Similarly to the

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