Search for aromatic anions in the P₂E₃⁻ (E = N, P, As, Sb, Bi) series

Abstract

We report a systematic computational study focused on the global minimum and low-lying isomer search for the P₂E₃⁻ (E = N, P, As, Sb, Bi) series of anions. We found nine planar five-membered rings and proved their aromatic character by various quantum chemical techniques. The possible use of two different P₂N₃⁻ aromatic anions as ligands in a number of sandwich complexes was also studied.

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Aromatic species with delocalized (4n + 2)π electrons have increased stability compared to their non-aromatic analogues.^1,2 Along with the well-known aromatic carbocyclic compounds (benzene (n = 1), naphthalene (n = 2), etc.), aromaticity was also detected in a number of species containing metals and main group elements.^3,4 Special attention to this field leads to the novel aromatic compounds being intensively sought both experimentally and theoretically.^5–8 Planar five-membered all-pnictogen cycles with 6π electrons are of particular interest among the main group aromatics because they are isovalent with the cyclopentadienyl anion (C₅H₅⁻) and can be used as inorganic building blocks.^9–11 Specifically, P₅⁻ and As₅⁻ fragments were stabilized in several sandwich complexes,¹² including a carbon-free metallocene [(η⁵-P₅)₂Ti]²⁻.¹³

Recently, Velian and Cummins reported the synthesis of P₂N₃⁻ anion by an original “click” reaction between an anthracene-based source of P₂, P₂(C₁₄H₁₀)₂,¹⁴ and the azide anion N₃⁻ in tetrahydrofuran solution and computationally confirmed the aromatic properties of P₂N₃⁻.¹⁵ The proposed P₂ transfer reaction opens new opportunities for synthetic realization of unusual phosphorus-containing compounds. After that, Mandal et al. have predicted a number of all-pnictogen aromatic species with X₂Y₃⁻ and X₃Y₂⁻ formulae and explored the possibility of using a P₃N₂⁻ anion as an η⁵-ligand in metallocenes by density functional theory calculations.¹⁶ Inspired by these two studies, here we report a systematic ab initio search for possible isomers with P₂E₃⁻ (E = N, P, As, Sb, Bi) stoichiometry in order to characterize expected aromatic anions.

An unbiased quantum-chemical search¹⁷ for P₂E₃⁻ species performed at the B3LYP-D3(BJ)/def2-SVP level of theory revealed that the global minimum for each E is a planar five-membered ring with a C_2v-symmetrical (E = N, As, Sb, Bi) or D_5h-symmetrical (E = P) structure (Fig. 1).¹⁸ It is important to note that two different kinds of such rings were found for P₂E₃⁻ (E = N, As, Sb, Bi), both of them having increased stability over the rest of the corresponding isomers (Fig. 1 and Fig. S1–S5, ESI†). One structural motif has a pnictogen atom between two phosphorus centers (I.A-like), whereas another has no atoms between them (I.B-like). Bond lengths optimized at B3LYP-D3(BJ)/def2-QZVPD and CCSD/aug-cc-pVTZ(-PP) levels of theory (results obtained agree with each other within 0.06 Å) as well as natural population analysis (NPA)¹⁹ partial charges are presented in Fig. 1. Rather short P–P, E–P, and E–E interatomic distances suggest the presence of multiple bonding.

Fig. 1 Representative isomers of (I) P₂N₃⁻, (II) P₅⁻, (III) P₂As₃⁻, (IV) P₂Sb₃⁻, and (V) P₂Bi₃⁻, their point group symmetries, spectroscopic states, bond lengths (Å), NPA partial charges (|e|, in italics), and relative energies (kcal mol⁻¹). The energies are given at the CCSD(T)/aug-cc-pVTZ(-PP)//B3LYP-D3(BJ)/def2-QZVPD + ΔSR (E = N, P, As) + ΔSO + ΔZPE level of theory. Bond lengths are given at B3LYP-D3(BJ)/def2-QZVPD and CCSD/aug-cc-pVTZ(-PP) (in bold) theoretical levels.

It should be stressed that, in contrast to the recently reported anion I.B,¹⁵ anionic heterocycle I.A has been unknown to date. Moreover, I.A is a global minimum structure, which lies 16.1 kcal mol⁻¹ below I.B in energy (here and elsewhere results on the total energy differences for P₂E₃⁻ are given at the CCSD(T)/aug-cc-pVTZ(-PP)//B3LYP-D3(BJ)/def2-QZVPD + ΔSR (E = N, P, As) + ΔSO + ΔZPE theoretical level; ΔSR, ΔSO, and ΔZPE denote scalar relativistic, spin–orbit, and zero-point energy corrections, respectively).¹⁸ However, isomers with I.B-like structures are the most preferred ones in the P₂E₃⁻ (E = As, Sb, Bi) series so that the relative energy of the second lowest-lying isomer (I.A-like structures) changes from 1.1 kcal mol⁻¹ (P₂As₃⁻) and 3.6 kcal mol⁻¹ (P₂Sb₃⁻) to 10.5 kcal mol⁻¹ (P₂Bi₃⁻), as our calculations predict (Fig. 1). The energy difference between the first two lowest-lying isomers was calculated to be 7.0 kcal mol⁻¹ and 32.6 kcal mol⁻¹ in the case of P₂N₃⁻ and P₅⁻, respectively (Fig. S1 and S2, ESI†).

Thus, our computational findings suggest that P₂E₃⁻ (E = N, As, Sb, Bi) anions with I.A-like and I.B-like structures, most of which were characterized for the first time (see Tables S1 and S2, ESI,† for predicted IR and Raman spectra and ³¹P chemical shifts), possess increased thermodynamic stability and can be considered as potential targets for chemical synthesis.

To check whether these P₂E₃⁻ anions exhibit aromatic properties, we performed nucleus-independent chemical shift (NICS),²⁰ natural resonance theory (NRT),²¹ and adaptive natural density partitioning (AdNDP)²² calculations.¹⁸

All NICS_zz profiles calculated for both the planar P₂E₃⁻ and C₅H₅⁻ anions consist of negative NICS values and show a minimum (Fig. 2, Fig. S6, S7 and Table S3, ESI†), which is typical for aromatic compounds.²³ A decrease of the maximum absolute NICS_zz value is accompanied by the displacement of the profile’s minimum to higher distances above the ring critical point at the electron density gradient field as the atomic weight of E is increased, indicating weakening of the π-aromaticity. It is worth noting that absolute NICS_zz values of I.B-like structures are higher than those of I.A-like ones.

Fig. 2 NICS_zz profiles for P₂E₃⁻ anions with (a) I.A-like and (b) I.B-like structures computed at the B3LYP/def2-QZVPD level of theory. Results for C₅H₅⁻ anions are also presented for comparison.

NRT analysis reveals five canonical resonance structures with the contribution ranging from 11% to 17% for each system (Fig. S8 and S9, ESI†), reflecting electron delocalization.

In contrast to NRT analysis, the AdNDP method allows one to describe electron delocalization, being an important feature of aromatic compounds, via multicenter bonds without invoking the concept of resonance. As the picture of chemical bonding in P₂E₃⁻ species of similar structure slightly depends on the pnictogen E, here we discuss in detail the results of AdNDP calculations only for the I.A and I.B anions with E = N (Fig. 3). The complete AdNDP results are given in Fig. S10 and S11 (ESI†).

Fig. 3 Results of AdNDP analysis for the (a) I.A and (b) I.B structures.

The chemical bonding pattern in the global minimum of P₂N₃⁻ (I.A, Fig. 3a) represents five s-type lone pairs (two P lone pairs with occupation numbers (ON) equal to 2.0 |e| and three N lone pairs with ON = 1.9 |e|), five covalent two-center two-electron (2c-2e) σ-bonds with ON = 2.0 |e| (four P–N and one N–N bonds), and three 5c-2e π-bonds (ON = 2.0 |e|) between all atoms of the ring. The same picture of chemical bonding is observed in structure I.B (Fig. 3b), except for the presence of three different types of 2c-2e σ-bonds (two P–N, two N–N, and one P–P bonds with ON = 2.0 |e|).²⁴ It is important to note that the set of three 5c-2e π-bonds found in P₂E₃⁻ species is also the key element of the chemical bonding pattern for C₅H₅⁻ (Fig. S12, ESI†), which is a well-known 6π-electron aromatic system.

Additionally, vertical detachment energies (VDEs) from the highest occupied molecular orbital range from 3.1 eV (V.B) to 4.3 eV (I.B), as calculated by the OVGF/aug-cc-pVTZ(-PP) method (Table S4, ESI†). The VDEs are similar to those for E₅⁻ (E = P, As, Sb, Bi) aromatic cluster anions reported by Zhai and coworkers (2.9–4.1 eV),⁴ but they are higher than VDE for C₅H₅⁻ (2.0 eV, Table S5, ESI†).

Thus, one can conclude that all P₂E₃⁻ anions shown in Fig. 1 are aromatic.

Taking into account the resembling aromatic properties of C₅H₅⁻ and P₂N₃⁻ anions, we also studied hypothetical M(η⁵-P₂N₃)₂, M(η⁵-C₅H₅)(η⁵-P₂N₃), and M(η⁵-C₅Me₅)(η⁵-P₂N₃) sandwich complexes (M = Fe, Ru, Os; Me = CH₃) to draw a parallel with classical metallocenes M(η⁵-C₅H₅)₂. Compounds with one P₂N₃⁻ (I.B) moiety were found to be ca. 20 kcal mol⁻¹ higher in energy (M06-L/def2-TZVP level of theory) than their P₂N₃⁻ (I.A)-substituted isomers (Fig. 4, Fig. S14 and S15, ESI†), which is in line with the results obtained for P₂N₃⁻ anions (Fig. 1). Dissociation energies of the P₂N₃-containing complexes into the M²⁺ and two negatively charged ligands ( = 610–740 kcal mol⁻¹) are comparable with those of M(η⁵-C₅H₅)₂ ( = 720–780 kcal mol⁻¹), indicating their thermodynamic stability (Tables S6 and S7, ESI†).

Fig. 4 Representative isomers of (I) Fe(η⁵-P₂N₃)₂, (II) Ru(η⁵-P₂N₃)₂, and (III) Os(η⁵-P₂N₃)₂ complexes with P₂N₃⁻ (I.A and I.B) ligands, their point group symmetries, spectroscopic states and relative energies (kcal mol⁻¹). The energies are given at the M06-L/def2-TZVP//M06-L/def2-TZVP + ΔZPE level of theory.

According to an energy decomposition analysis (EDA; Tables S8 and S9, ESI†), the energy of electrostatic interactions (ΔE_elstat, 50–55%) between ML⁺ and P₂N₃⁻ fragments gives slightly higher contribution to the total attractive interactions compared to the orbital interaction energy (ΔE_orb, 45–50%). EDA results for M(η⁵-C₅H₅)₂ compounds show that M(η⁵-C₅H₅)⁺ and C₅H₅⁻ fragments interact more strongly than ML⁺ and P₂N₃⁻ ones (ΔΔE_int = 24–58 kcal mol⁻¹) because of the gain in electrostatics (ΔE_elstat, 57–58%).

In summary, exploring the potential energy surfaces of P₂E₃⁻ (E = N, P, As, Sb, Bi) anions revealed planar five-membered cyclic structures for each E under study. An increased stability of the structures over the corresponding isomers can be explained by their aromatic properties. Two different structural motifs were found for aromatic anions (E = N, As, Sb, and Bi), one of them being theoretically characterized for the first time. Remarkably, the recently synthesized P₂N₃⁻ aromatic anion turned out to lie ca. 16 kcal mol⁻¹ higher in energy than the global minimum, which is the aromatic anion predicted in this work. Substitution of the C₅H₅⁻ or C₅Me₅⁻ ligand by the P₂N₃⁻ moiety can lead to sandwich complexes with bonding properties, similar to the classical metallocene ones. Recent progress in the synthesis of aromatic pnictogen-containing heterocycles^15,25–27 allows us to expect that the present contribution will stimulate further development of this research field.

The Siberian Supercomputer Center is gratefully acknowledged for providing computational resources.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Computational details, complete results of calculations performed, including IR and Raman spectra and ³¹P chemical shifts for aromatic P₂E₃⁻ anions, and Cartesian coordinates of the most relevant species. See DOI: 10.1039/c6cp02241c

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