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Nitrogen Lewis Acids Alla Pogoreltsev, Yuri Tulchinsky, Natalia Fridman, and Mark Gandelman* Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Technion City, Haifa 32000, Israel
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S Supporting Information *
ABSTRACT: Being a major conception of chemistry, Lewis acids have found countless applications throughout chemical enterprise. Although many chemical elements can serve as the central atom of Lewis acids, nitrogen is usually associated with Lewis bases. Here, we report on the first example of robust and modifiable Lewis acids centered on the nitrogen atom, which provide stable and well-characterized adducts with various Lewis bases. On the basis of the reactivity of nitrogen Lewis acids, we prepared, for the first time, cyclic triazanes, a class of cyclic organic compounds sequentially bearing three all-saturated nitrogen atoms (N−N−N motif). Reactivity abilities of these N-Lewis acids were explained by theoretical calculations. Properties and future applications of nitrogen Lewis acids are intriguing.
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INTRODUCTION Lewis acids (LA) and their interactions with Lewis bases (LB) are among the most fundamental facets of chemistry. An employment of LA in various chemical enterprises is indispensable. They have found a broad implication in molecular recognition including design of supramolecular architectures, censoring devices, and biological models.1 An ability of LA to form adducts with LB has established their crucial role in catalytic mediation of chemical reactions.2 The recent discovery of metal-free methods to activate strong chemical bonds by frustrated Lewis pairs3 as well as by conceptually related carbenes4,5 (serving as an acid and a base simultaneously) opened a new area of LA employment in chemistry and catalysis. Classical main group element Lewis acids, largely based on boron, aluminum, tin, silicon, phosphorus, or antimony, provide a free low-lying orbital for LB interactions.2,6−9 Conversely, nitrogen-centered species are usually considered as Lewis bases due to available lone pair of electrons of the relatively high energy. Certain nitrogen-based compounds, such as chloramines, nitrenes, diazocarboxylates, azides, and diazonium salts, possess a N-centered electrophilic behavior. However, their reaction with nucleophiles or bases leads to the formation of unstable intermediates, which undergo further irreversible chemical transformations rather than forming a stable adduct.10−13 The nitrosonium and nitronium salts (NO+X− and NO2+X−, respectively) were reported to form relatively stable donor−acceptor adducts with pyridine, although the crystallographic confirmation for such adducts is absent.14,15 Most importantly, nitrosonium and nitronium cations are oxidants (not compatible with many functional groups), very hygroscopic, and sterically and electronically nonmodifiable compounds. As such, these electrophilic Nbased species are hardly conceivable as functional LA for any of the aforementioned applications. The reactivity of N-heterocyclic carbenes (NHCs) and their main group congeners (e.g., silylenes, germylenes, stannylenes,16,17 and phosphenium18 and arsenium cations19) has © 2017 American Chemical Society
attracted much attention not only as ligands to metals, but also as free molecules.20 Because of the presence of both a lone pair of electrons in an sp2-type orbital and a vacant pπ orbital, these species possess an amphoteric Lewis character, thus enabling their employment in activation of small molecules, such as H2, NH3, and CO, among others.4,5,21,22 Contrary to these species, the triazolium salt 1 (Figure 1), which represents an electronic analogue of ubiquitous NHC, has been known for many years as a stable inert molecule and is used as ionic liquids, linkers, etc.23
Figure 1. Schematic presentation of triazolium ion 1 and its resonance forms.
Recently, it was demonstrated that nitrenium ion (one of the resonance forms of triazolium 1) can coordinate to various transition metals, thus exhibiting a Lewis basic behavior.24−26 Experimental and computational studies have revealed that, as a ligand for transition metals, nitrenium ion acts as a relatively weak σ-donor, but a considerable π-acceptor.24 The availability of vacant pπ-orbital of nitrenium species for metal−ligand πinteractions suggests that it also might be involved in σ-type interactions with various Lewis bases (Figure 2). However, while such Lewis acidic behavior of the heavier nitrenium analogues, phosphenium27,28 and arsenium,29,30 is established, the analogous reactivity of triazolium has not yet been observed. In this Article, we report on our discovery of novel class of Lewis acids centered on nitrogen atom. We present the first Received: December 5, 2016 Published: February 27, 2017 4062
DOI: 10.1021/jacs.6b12360 J. Am. Chem. Soc. 2017, 139, 4062−4067
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Journal of the American Chemical Society
formation of the heterocyclic six-membered ring (Figure 4A) with N−P bond lengths within the typical range of aminophosphonium salts (1.62−1.64 Å). Notably, in addition to 6b, formation of a small amount of another compound possessing a P−N bond (doublet at δ 12.6 ppm with J(31P−15N) = 39 Hz in 31P NMR) was detected in the early stages of the reaction. This assumingly could be attributed (by 1H, 31P, and 31P-HMBC NMR experiments) to the expected unstable symmetrical adduct 6a. Treatment of 2 with lithium di-tert-butylphosphide at room temperature provided similar results, yielding the rearranged compound 7b (31P NMR: δ 112.4 ppm, J(31P−15N) = 81.8 Hz). In this case, an even larger amount of the adduct 7a (δ 63.6 ppm, J(31P−15N) = 74.9 Hz, 7a:7b ratio of 1:5) could be observed by 31P NMR within ca. 5 min after the addition, a signal that completely disappears in 2 h. To the best of our knowledge, conversion of 2 to 6b or 7b represents a first example of ring opening and expansion of the triazolium salts by chemical transformation, and compounds possessing such P−N rings are unprecedented. On the basis of these results, we assumed that the initially formed adducts 6a and 7a rearranged to the thermodynamically more stable six-membered cyclic products 6b and 7b. It is likely that the lone pair of electrons on the phosphorus atom is essential for the triazole ring opening, as shown in the proposed mechanism in Scheme 1B. Therefore, we anticipated that “blocking” this lone pair of electrons might provide the desired stable adducts of nitrenium LA with bases. Gratifyingly, the addition of “protected” phosphides in the form of an oxide or a BH3-complex led to the formation of the stable adducts 8 and 9. Their molecular structures were unambiguously elucidated by multinuclear NMR techniques and confirmed by single-crystal X-ray analysis (Figure 4B,C). Remarkably, contrary to the planar nitrenium species, the central nitrogen atoms (N2) in both structures are nearly tetrahedral (Σ angles 327.96 and 333.40 for 8 and 9, respectively) and lie out of the benzotriazole plane. Such pyramidalization of central nitrogen is expected if the vacant porbital of sp2-hybridized N atom in nitrenium undergoes interaction with the LB, resulting in sp3-hybridized center. The N−P bonds are quite long for regular phosphoramides (1.731 Å in 8 and 1.7393 Å in 9 vs 1.610−1.634 Å in average).31 It suggests that the formed P−N bond has a considerable ionic character, placing these products as hybrids between covalent molecules (P−N) and Lewis pairs (N+P−). The nearly equivalent N−N bonds of the triazole ring are elongated, as compared to the starting aromatic benzotriazolium system (1.4062 Å for 8, and 1.4292 Å for 9, vs 1.310 Å in 2), suggesting a single N−N bond character. To elucidate the influence of conjugation on the Lewis acidity of nitrenium, the reactivity of 2 was compared to that of its hydrogenated analogue 3 and to naphthotriazolium 4 (Figure 3). Interestingly, triazolium species 3 proved entirely unreactive toward Lewis bases. Naphthotriazolium 4, in turn, reacted similarly to 2 with oxo-phosphides or phosphideboranes, yielding stable adducts 10 and 11 (Scheme 2A). Notably, reaction of 4 with “unprotected” diaryl- and dialkylphosphides did not result in the formation of the corresponding six-membered ylides. Instead, upon addition of R2PM to 4, a deep blue color was developed, and the relatively stable Ncentered radical species were clearly observed by EPR measurements (Figure S4). We identify these species as naphthotriazolium radicals, which show characteristics similar
Figure 2. Schematic presentation of the formation of Lewis acid−base adduct based on nitrenium species.
examples of reactivity of stable N-heterocyclic nitrenium ions toward various Lewis bases, whereas central nitrogen possesses typical p-type LA behavior. To the best of our knowledge, it represents a first example of robust, stable, and stereoelectronically modifiable nitrogen Lewis acids, which form well-defined adducts with Lewis bases. These adducts were fully characterized in solution by selective 15N labeling experiments, and in the solid phase by X-ray crystallography. This nitrogenbased family of compounds fulfills a “missing link” in the line of well-established Lewis acids based on boron, aluminum, phosphorus, tin, antimony, and other main group elements. On the basis of this reactivity, we, for the first time, have prepared and fully characterized cyclic triazanes, organic compounds sequentially bearing three all-saturated nitrogen atoms (N−N−N motif).
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RESULTS AND DISCUSSION We have envisioned that, in case of considerable contribution of the resonance form 1c (nitrenium, Figure 1) to the electronic structure of the triazolium salt, the free pπ-orbital on the central nitrogen might possess acidic Lewis properties. If positive, nitrenium structures would open a door to an unprecedented modifiable class of LA based on nitrogen. To explore Lewis acidity of nitrenium ions, several triazolium (2−4) and triazinium (5) salts have been prepared (Figure 3, see the
Figure 3. Selected nirenium species.
Supporting Information for synthetic details). To facilitate identification of interaction of the central nitrogen in 2−5 with Lewis bases, these ions were selectively labeled with 15N at the 2-position. Addition of an equivalent amount of potassium diphenylphosphide to 2 at room temperature resulted in the formation of a new product resonating as a doublet at δ 38.5 and δ −254.3 ppm in 31P{1H} and 15N{1H} NMR, respectively. The strong magnetic 31P−15N coupling (J = 69.5 Hz) clearly indicated the presence of a nitrogen−phosphorus bond. However, the observed 1H NMR pattern was incompatible with the expected symmetrical adduct 6a (Scheme 1A). Multinuclear NMR experiments suggested the formation of a cyclic ylide-like structure 6b. Addition of a strong acid HBF4 to this compound resulted in the formation of a salt 6c. The N−H pattern exhibits a doublet of doublets at 6.2 ppm with 1J(1H−15N) = 82 Hz and 2 1 J( H−31P) = 19 Hz in 1H NMR. The molecular structure of 6c was confirmed by X-ray analysis of its single crystals. The structural data revealed a slightly distorted chairlike con4063
DOI: 10.1021/jacs.6b12360 J. Am. Chem. Soc. 2017, 139, 4062−4067
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Scheme 1. (A) Reaction of 2 with Lewis Bases; and (B) Proposed Mechanism for the Formation of Six-Membered Ring Product
Scheme 2. (A) Reaction of 4 with Lewis Bases; and (B) Proposed Mechanism for the Formation of Naphthotriazolium Radical
Figure 4. ORTEP drawing of molecular structures of 6c, 8, and 9. Hydrogen atoms are omitted for clarity, except for the hydrogen of N1-amine. Selected bond lengths [Å] and angles [deg] of (A) 6c, N1− H1 0.8600, N1−N3 1.432(4), P1−N1 1.639(3), P1−N2 1.642(3), N1−P1−N2 105.67(15), N3−N1−P1 104.1(2), P1−N1−H1 127.9, N3−N1−H1 127.9; (B) 8, P1−N2 1.731(3), N1−N2 1.4063(10), N2−N3 1.4061(10), P1−O1 1.471(3), N3−N2−N1 111.5(2), N1− N2−P1 108.73(18), N3−N2−P1 107.73(18), O1−P1−N2 117.02(13); and (C) 9, P1−N2 1.7393(16), N1−N2 1.4302(11), N2−N3 1.4282(10), P1−B1 1.915(3), N3−N2−N1 110.58(13), N3− N2−P1 115.56(11), N1−N2−P1 107.26(11), N2−P1−B1 108.58(10).
to those of previously reported benzotriazolium-based radicals.32 After the workup, compound 4 was completely recovered, while the phosphide was almost quantitatively converted to the corresponding phosphine tBu2PH or its homocoupled (P−P) product Ph2P−PPh2 (when the reaction is performed with Ph2PK). These observations might suggest that the initially formed adducts readily undergo homolytic P− N bond cleavage, and the resulting radical phosphorus species are quenched by homocoupling (in case of diphenyl phosphide) or proton abstraction from the solvent (THF) or traces of water (in case of very bulky di-t-butyl phosphide) (Scheme 2B). A facile capability of naphthotriazolium systems to stabilize N-centered radicals is likely to be responsible for such reactivity (vide infra). An interesting and rich reactivity was observed for sixmembered cyclic nitrenium 5 (1,2,3-triazinium cation). Reaction of the dark purple solution of 5 in dimethylformamide with potassium diphenylphosphide resulted in the formation of a colorless product 12 (Scheme 3). Formation of an N−P bond in 12 and its symmetrical structure were unambiguously characterized by multinuclear NMR. Contrary to the triazolium-based products 6a and 7a, triazinium 5 furnishes a
stable adduct 12, which does not undergo further ring expansion rearrangement. Such a rearrangement, which would lead to a seven-membered ring, is, likely, thermodynamically unfavored. Compound 5 also reacted smoothly with various metal phosphides, such as diphenylphosphide oxide and boraneprotected phosphides, to afford the corresponding adducts 13− 15. Of special interest is an ability of triazinium cation to provide adducts with neutral phosphines. When 5 was reacted with stoichiometric amounts of tertiary phosphines (PR3, R = Me or Bu), the corresponding amino-phosphonium adducts 16 and 17 were obtained in quantitative yields. Remarkably, the coordination in these adducts is reversible: when compound 17 was reacted with a slight excess of trimethylphosphine at room temperature, a mixture of products 16 and 17 was obtained (Scheme 4). The bulkier tBu3P did not exhibit any appreciable 4064
DOI: 10.1021/jacs.6b12360 J. Am. Chem. Soc. 2017, 139, 4062−4067
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of the central nitrogen atom, as a result of the σ-donation from Lewis bases to a vacant pπ orbital of nitrenium Lewis acid. This interaction forces the nitrogen atom to adopt sp3 hybridization and, thus, to be shifted out of plane of the naphthalene ring and two adjacent nitrogen atoms. Moreover, a considerable elongation of N−N bonds, in comparison to those of the starting nitrenium ion 5, is observed in all adduct structures (av 1.42 Å vs 1.28 Å, respectively). It clearly indicates that a largely N−N double bond in the nitrenium 5 is converted to a single bond upon its reaction with LB. Notably, this effect was only slightly observed when the nitrenium species coordinated to transition metals, although the π-back-donation (dπ electron donation of the metal to the p(N) orbital) in that case was detected both experimentally and computationally.24 The formation of a new σ-bond due to the occupation of a free orbital by the electron pair of the base and pyramidalization of central motif is a typical behavior of lobe-LUMO acids.33 As such, it puts the nitrenium family on par with the classical LA centered on boron, aluminum, carbon, etc. The reactivity of 5 is not limited to phosphorus Lewis bases. It can selectively react with carbon bases of type RM (R = alkyl, aryl). For example, reaction of 5 with nBuLi or PhLi is completed within a few minutes at an ambient temperature, forming colorless new products 18 and 19. Molecular structures of these unique triazanes (1,3-diamino-2-R-piperidines; R = butyl, phenyl) were unambiguously determined by multinuclear NMR analysis. It should be emphasized that nitrenium salts not only proved to be a unique family of nitrogen Lewis acids, but their reactivity with LB allowed us to prepare and isolate an entirely new type of organic cyclic compounds, bearing a pattern of three all-saturated consecutive nitrogen atoms. As aforementioned, such cyclic triazanes are considerably stable both in solution and in the solid state. The nitrogen−nitrogen bond distances in the N−N−N motif are in the typical range of a single N−N bond as compared to hydrazines (e.g., N−N bond length in H2N−NH2 is 1.45 Å).
Scheme 3. Reaction of 5 with Various Lewis Bases
coordination to the N-center of 5, which places this combination of LA/LB as a potential frustrated Lewis pair. Scheme 4. Formation of Reversible Adducts of 5 with Bases
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The composition and identity of the triazinium-phosphide and phosphine adducts 13−17 were verified by multinuclear NMR studies. Molecular structures of 13, 14, and 16 were also confirmed by X-ray diffraction analysis (Figure 5A−C). The N−P bond lengths in these crystals are within the typical ranges for phosphoramides, aminophosphine boranes, and aminophosphonium salts (13, 14, and 16, respectively). The remarkable feature of these structures is a pyramidal geometry
THEORETICAL STUDIES OF NITRENIUM REACTIVITY To understand differences in properties and reactivity of nitrenium-based Lewis acids, we studied these systems by DFT. First, geometries of the adducts of model nitrenium species 1′ (R = Me), 2, 4, and 5 with Me2P− and PMe3 were optimized at the B3LYP/6-31g(d) level of theory. The optimized geometries were then used for the calculation of enthalpies (Table 1) and Table 1. Enthalpies of Formation for Nitrenium Adducts (ΔH, kcal/mol) 1′-PMe2 2-PMe2 4-PMe2 5-PMe2
Figure 5. ORTEP drawing of molecular structures of 13, 14, and 16. (A) 13: P1−N1 1.721(3), N1−N2 1.433(5), N1−N3 1.432(5), P1− O25 1.398(7), N3−N1−N2 116.2(3), N3−N1−P1 110.1(2), N2− N1−P1 111.8(3), O25−P1−N1 128.7(4); (B) 14: P(1)−N(2) 1.694(2), P(1)−B(1) 1.904(3), N(1)−N(2) 1.402(3), N(2)−N(3) 1.422(3), N(1)−N(2)−N(3) 118.60(19), N(1)−N(2)−P(1) 117.79(15), N(3)−N(2)−P(1) 117.03(17), N(2)−P(1)−B(1) 111.79(14); and (C) 16: P1−N2 1.669(3), N2−N3 1.416(4), N1− N2 1.408(5), N1−N2−N3 119.8(3), N1−N2−P1 112.2(2), N3−N2− P1 116.3(2).
DMSO
THF
−15.3 −26.5 −30.7 −41.3
−27.3 −37.9 41.9 −52.4
1′-PMe3+ 2-PMe3+ 4-PMe3+ 5-PMe3+
DMSO
THF
25.5 15.3 11.6 −0.3
23.9 15.0 11.6 −0.9
free energies (Table S1) of adduct formation in DMSO and THF applying the same functional and 6-311+g(d,p) basis set. As evident from Table 1, the adduct stability increases upon going from 1′ to 5, which is consistent with the experimentally observed higher reactivity of the more conjugated species. Notably, the formation of phosphine adducts is energetically unfavorable for all species except 5, for which it is roughly 4065
DOI: 10.1021/jacs.6b12360 J. Am. Chem. Soc. 2017, 139, 4062−4067
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Journal of the American Chemical Society energo-neutral. Indeed, the only stable adduct with R3P was experimentally obtained for nitrenium 5. Because both computational and experimental results suggest a strong correlation between Lewis acidity and the extent of πconjugation of nitrenium species, a closer look on their molecular orbitals is instructive (Figure 6). The HOMO and
Figure 7. Relation between ELUMO of nitrenium cations and relative stability of their adducts with different Lewis bases. N = 1′, 2, 4, or 5.
nitrenium species. The optimized geometries of nitreniumbased radicals Nrad are all planar (see the Supporting Information), while Lewis adduct formation involves a significant out-of-plane shift of the apical nitrogen atom. Therefore, the stability of the adduct with the nitrenium species 5 relative to its isomer 4 might be attributed to the stronger propensity of a six-membered ring to undergo an out-of-plane distortion, which is also confirmed by available crystal structures.
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CONCLUSIONS We disclosed a first example of stable, robust, and stereoelectronically modifiable nitrogen Lewis acids. This new class of Lewis acids is based on the well-known triazolium/nitrenium salts, which provide stable adducts with a series of Lewis bases. As such, these nitrogen compounds join a line of wellestablished Lewis acids based on boron, aluminum, tin, and other elements. In the course of efforts to uncover the reactivity of these systems, we have prepared cyclic triazanes, a new class of organic compounds bearing three consecutive all-saturated nitrogen (N−N−N) atoms. Properties of these new LA in catalysis and FLP chemistry, as well as the chemical and energetic potential of triazanes, are under study in our laboratories.
Figure 6. Frontier orbitals of nitrenium Lewis acids.
HOMO−1(−2) in these systems are mostly comprised from the bonding combinations of the aromatic pπ-orbitals, whereas the LUMO corresponds to the antibonding combination of the same orbitals. As expected, upon moving from 1′ to 5 and extending the degree of π-conjugation, the HOMO/LUMO gap decreases. It might be assumed that nitrenium cations owe their Lewis acidity to the relatively low energy of the LUMO possessing the nitrenium’s vacant p-orbital character. Indeed, there is an apparent correlation between ELUMO and the adduct formation energies ΔE. However, some discrepancies can be indicated. While nitrenium species 4 is almost identical to its isomer 5 in terms of ELUMO, it is considerably different in both reactivity (experimental) and in adduct formation energy ΔE (Table S2). Compound 4 possesses reactivity and energetic (ΔE) features similar to those of species 2. This is also in line with the experimental evidence: contrary to 5, compounds 4 and 2 are inert toward phosphines. It seems, therefore, that ELUMO alone is not sufficient to account for all reactivity differences between nitrenium species. To obtain further insight about the relation between ELUMO of nitrenium cations and the stability of their adducts, we considered a series of isodesmic reactions comparing the transfer of a phosphide (I), a phosphine (II), or an electron (III) between the model triazolium 1′ and other nitrenium species (in THF; Figure 7). Plotting the resulting ΔEr against ELUMO provides an interesting observation: the ΔEr for Me3P and Me2P− transfer reactions (I−II) exhibits a nearly linear dependence on ELUMO for the five-membered nitrenium species 1′, 2, and 4, but an abrupt increase in stability occurs for the corresponding adducts of 5. On the other hand, ΔEr for the electron transfer reaction (III) is nearly linear with respect to ELUMO for all four
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.6b12360. Experimental details, X-ray data, EPR data, and computational details including the Cartesian coordinates and electronic energies of all calculated structures; crystallographic information and CCDC numbers for compounds 4, 6c, 8, 9, 13, 14, and 16 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Mark Gandelman: 0000-0002-9651-2902 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the German-Israeli Project Cooperation (DIP) is acknowledged. 4066
DOI: 10.1021/jacs.6b12360 J. Am. Chem. Soc. 2017, 139, 4062−4067
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