Dimerization of Aromatic C-Nitroso ... - ACS Publications

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Dimerization of Aromatic C‑Nitroso Compounds† Daniel Beaudoin* and James D. Wuest* Département de Chimie, Université de Montréal, Montréal, Québec H3C 3J7, Canada ABSTRACT: Aromatic C-nitroso compounds (Ar−NO) and related species have a rich chemical history, and they continue to interest researchers in many fields. Among the most distinctive and puzzling properties of these compounds is their ability to dimerize reversibly to form azodioxy compounds. The present review subjects this intriguing phenomenon to comprehensive analysis. All aspects of the subject are examined in detail, including the structures of monomeric and dimeric forms, the mechanism of dimerization, features that favor or disfavor dimerization, thermodynamic and kinetic factors, dimerization under specific conditions (including in solution, in the solid state, and on surfaces), and the special associative behavior of dinitroso and polynitroso compounds. By summarizing the current state of knowledge, the review promises to spur further advances in the evergreen field of C-nitroso chemistry, including the discovery of new ways to exploit the reversible dimerization of nitrosoarenes.

CONTENTS 1. Introduction 2. Structures of Nitrosoarenes and Their Azodioxy Dimers 2.1. Historical Perspective 2.2. Calculated and Experimental Geometries of Aromatic C-Nitroso Compounds 2.3. Calculated and Experimentally Determined Structures of cis-Azodioxy Dimers 2.4. Calculated and Experimentally Determined Structures of trans-Azodioxy Dimers 3. Mechanism of Dimerization 3.1. Cis−Trans Isomerization of Azodioxides 4. Effect of Structure on the Occurrence of Dimers in the Solid State 5. Criteria for Dimerization 5.1. Barrier of Rotation 5.2. Nitrogen and Oxygen NMR Spectroscopy 6. Monomer−Dimer Equilibria in Solution 6.1. Thermodynamic Parameters 6.2. Effect of Solvent on Monomer−Dimer Equilibria in Solution 6.3. Effect of Pressure on Monomer−Dimer Equilibria in Solution 6.4. Effect of Steric Hindrance on Monomer− Dimer Equilibria in Solution 7. Crossed Dimerization of Nitrosoarenes 8. Dimerization in the Solid State 9. Dimerization on Surfaces 10. Aromatic Dinitroso and Polynitroso Compounds 10.1. Intramolecular Association Leading to Cyclic Structures 10.2. Intermolecular Association Leading to Polymers 10.3. Intermolecular Association Leading to Covalent Organic Networks © 2016 American Chemical Society

11. Conclusion Author Information Corresponding Authors Notes Biographies Acknowledgments Dedication References

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279 279 279 279 279 279 279 280

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1. INTRODUCTION Organic compounds incorporating the nitroso group (R−NO) have a rich history in chemistry and biology, and their distinctive properties continue to captivate new generations of researchers. Aromatic C-nitroso compounds, where R = aryl, form a particularly important subset of nitroso compounds, yet full understanding of their complex and subtle behavior is still elusive, even after 140 years of investigation.1 Many aromatic Cnitroso compounds form cis- and trans-azodioxy dimers in solution and in the solid state (eq 1), and this fascinating

260 262 264 265 268 268 270 270 270 272 273

property has been a source of both opportunities and obstacles in the pursuit of new applications. For example, the importance of this monomer−dimer equilibrium in synthesis has been underscored by the following statement: “...the careful control of this equilibrium is an essential prerequisite for use of nitroso compounds in organic synthesis.”2 In recent years, interest in the chemistry of nitrosoarenes has resurged. This renewed attention has been fueled in part by their many applications in synthetic organic chemistry,1−5 their role as

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Received: September 16, 2014 Published: January 5, 2016

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reactive metabolites,6−9 and their use as spin traps.10−12 Various aspects of C-nitroso compounds have been reviewed, including their synthesis,13−15 reactivity,16 and interactions with metals,17,18 but their dimerization has not been the subject of a comprehensive review. General monographs on C-nitroso compounds have been published by Vančik, Feuer, Gowenlock, and Lüttke, but association is not treated in depth.1,19,20 In light of the recent reblooming of interest in the chemistry of nitrosoarenes and the central importance of their dimerization, a focused review of this subject offers a timely complement to existing general monographs. Although C-nitroso compounds with heteroaryl substituents are less well-studied than nitrosoarenes,21,22 they show closely related behavior and, therefore, are included within the scope of the present review.

azodioxy dimers (eq 1), as well as the expected absence of a dipole moment in the centrosymmetric trans isomer, led them to ultimately reject the correct cis- and trans-azodioxy structures 4 and 5 (Figure 1). The first crystallographic studies of dimers were reported independently in 1950 by Fenimore37 and by Darwin and Crowfoot Hodgkin.38 Their studies established that the dimers of 2,4,6-tribromonitrosobenzene, 4-bromonitrosobenzene, and 4chloronitrosobenzene adopt trans-azodioxy structures and that the lengths of the nitrogen−nitrogen bonds in these compounds are in the range 1.3−1.4 Å, close to the value expected for a normal NN double bond. Among the three canonical forms that can be used to represent a trans-azodioxide (Figure 2), the

2. STRUCTURES OF NITROSOARENES AND THEIR AZODIOXY DIMERS Figure 2. Canonical forms representing the structure of the transazodioxide formed by the dimerization of C-nitroso compound R−NO.

2.1. Historical Perspective23

The first nitrosoarenes were reported by Baeyer in 1874.24−26 It was quickly noted by early researchers that simple C-nitroso compounds exhibit an unusual property: in solution they are normally highly colored (typically green or blue), whereas in the solid state they are often colorless or pale yellow. This initially mysterious phenomenon was partially explained in 1891 when Behrend and König reported the isolation of a colorless solid in an attempt to prepare (nitrosomethyl)benzene (PhCH2NO).27 A cryoscopic study of the product revealed that it was in fact a dimer of the expected compound. In part on the basis of these results, Piloty proposed in 1898 that C-nitroso compounds can generally exist in two distinct forms in equilibrium: dimeric (usually colorless or yellow) or monomeric (normally green or blue), as shown in eq 2.28

structure with a NN double bond thus makes a dominant contribution. Previous measurement of a significant dipole moment for the trans dimer of nitrosomesitylene by Hammick and co-workers may have been based on unwarranted assumptions,39 and a nonzero value could also arise from the lack of a center of inversion in the structure, a possibility overlooked by the earlier researchers.37 A consequence of the formation of azodioxy dimers with N N bonds is the possibility of geometric isomerism.40 In 1955, Gowenlock and Trotman reported that nitrosomethane gives a cis-azodioxy dimer,41 and Lüttke subsequently provided evidence based on IR spectroscopy for the presence of cis-azodioxy dimers in solutions of aromatic C-nitroso compounds.42,43 Although the earlier crystallographic studies of Fenimore, Darwin, and Crowfoot Hodgkin had revealed trans-azodioxy structures,37,38 certain nitrosoarenes favor the formation of cis dimers in the solid state. For example, the crystal structure of cis-azodioxybenzene was first reported in 1970.44 It is important to note that some nitrosoarenes, such as 4-nitrosophenol and its derivatives, do not typically form dimers, in part because they commonly engage in tautomerism to give other structures, as illustrated by the formation of the monoxime of 1,4-benzoquinone in eq 3.45−51

At the time, the exact structure of the dimers was not known, and erroneous cyclic head-to-tail structure 129−31 and head-tohead alternative 232−34 were commonly used to represent the dimeric forms of C-nitroso compounds (Figure 1). Only at the

Figure 1. Various representations used to describe the dimers of Cnitroso compound R−NO.

Because the behavior of nitrosoarenes is complex, almost a century passed between the first syntheses of these compounds and the acquisition of a basic understanding of their structures, dimerization, and other characteristic properties. Recent advances in spectroscopy, crystallography, computational chemistry, and other areas have greatly increased our knowledge of nitrosoarenes and azodioxides, and exciting new opportunities for using these compounds are emerging. As a result, a contemporary review of aromatic C-nitroso compounds and their dimerization is timely.

beginning of the 1930s was the connectivity of bonding in the dimers of nitrosarenes properly established. At this time, Hammick and co-workers measured the dipole moments of various dimers; in particular, they obtained a value of ∼1.6 D for the moment of the dimer of nitrosomesitylene.35,36 On the basis of these observations and other data, they ruled out cyclic structures 1 and 2 for the dimers and instead postulated the formation of a singly-bonded dimer that can be represented by structure 3. The anticipated high strength of NN bonds in 259

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2.2. Calculated and Experimental Geometries of Aromatic C-Nitroso Compounds

nitroso groups are approximately coplanar with the aromatic rings, making mesomeric effects significant.

The nature of the aromatic ring greatly influences the behavior of aromatic C-nitroso compounds, both in solution and in the solid state. As a result, the properties of nitrosoarenes differ significantly from those of nitrosoalkanes. The aryl group in nitrosoarenes expresses its influence in two primary ways: (1) steric effects are imposed when the nitroso group is adjacent to other substituents, which influences the orientation of the group relative to the plane of the aromatic ring, and (2) significant electronic effects can arise when electron-donating substituents are located ortho or para with respect to the electronwithdrawing nitroso group.42 The typical impact of electrondonating groups is a result of resonance, which translates into additional electronic delocalization represented by quinoidal mesomeric forms (Figure 3).

2.3. Calculated and Experimentally Determined Structures of cis-Azodioxy Dimers

Although the cis isomers of azodioxy dimers often predominate in solution (vide infra), they are more rarely observed in the solid state. Table 2 summarizes structural data for the few cisazodioxides that have been crystallized and studied by X-ray diffraction, as well as calculated structures provided for comparison. Because steric repulsion between the aromatic rings in cis-azodioxides derived from nitrosoarenes disfavors coplanarity and extended conjugation, aryl substituents have almost no effect on the C−N, N−N, and N−O bond lengths. For example, computed distances are nearly identical for the cis dimers of 4-nitronitrosobenzene and 4-nitrosotoluene. Similarly, no substantial difference is observed in distances determined by X-ray diffraction for the cis dimers of perfluoronitrosobenzene and 2-nitrosoanisole. 2.4. Calculated and Experimentally Determined Structures of trans-Azodioxy Dimers

Azodioxy dimers of trans geometry are frequently observed in the solid state and have been the subject of numerous crystallographic studies. Table 3 summarizes structural data for transazodioxides that have been studied by computational approaches and/or by X-ray diffraction. As in the case of cis-azodioxides, substitution of the aromatic rings has almost no electronic effect on the C−N, N−N, and N−O bond lengths. Indeed, there is little difference between the structural parameters of dimers substituted by electron-donating groups and analogues substituted by electron-withdrawing groups. Except for the singular case of the structure of trans-azobenzenedioxide (which was resolved in a cocrystal), angles between the aromatic rings and the planes defined by the azodioxy groups lie in the range 52.78− 87.02°, which severely limits the opportunity for resonance. Important variations in the lengths of N−N bonds (1.128−1.39 Å) and N−O bonds (1.181−1.31 Å) are nevertheless observed in the crystal structures of trans azodioxides, which indicates a significant degree of structural flexibility in response to differences in molecular packing in the solid state. Collectively, the extensive collection of data in Tables 1−3 establishes the constitution of aromatic C-nitroso compounds and their azodioxy dimers, and little uncertainty remains about their basic structures. Moreover, bonding in nitroso groups and azodioxides can be seen to depend on substituents in logical ways, determined mainly by conformation and the presence or absence of mesomeric effects. Although these aspects of the chemistry of aromatic C-nitroso compounds and their dimers have now been understood for some time, other facets of the behavior of these compounds are less easily explained, and identifying the origin of many characteristic features has required extensive further study. For example, the data in Tables 1−3 raise the question of why certain nitrosoarenes dimerize spontaneously whereas others remain monomeric, even in the solid state. In addition, the structures of the dimers do not readily account for their facile dissociation, nor is the mechanism of dimerization and dissociation immediately apparent. Furthermore, the data indicate that bond distances and other characteristic structural parameters can be altered by the way dimers pack in the solid state, suggesting that their NN bonding has unusual features.

Figure 3. Additional electronic delocalization represented by quinoidal mesomeric forms arising when electron-donating substituents (D) are located ortho or para with respect to the electron-withdrawing nitroso group.

Two consequences of the presence of electron-donating groups conjugated with the nitroso group in nitrosobenzenes are (1) shortening of the C−N bond between the aromatic ring and the nitroso group and (2) lengthening of the N−O bond.52 Crystallographic studies of substituted nitrosobenzenes and calculations both confirm the generality of this phenomenon (Table 1). The table omits structures with significant disorder and R factors greater than 6%, because their low precision does not allow detailed analysis of geometric features.53−68 Moreover, structures of electron-rich heteroaromatic nitroso compounds are also omitted, because their key features resemble those of nitrosobenzene derivatives and they add little to the basic understanding of how substituents interact with the nitroso group. Specific electron-rich heteroaromatic nitroso compounds for which structural data are available include derivatives of benzothiazole,69 benzothiophene,70 imidazopyrimidine,71 indolizine,72 and pyrazole.73−77 In addition, the structures of over 60 related derivatives of 5-nitrosopyrimidine have been published in the last two decades.78−84 The marked interest in this family of compounds mainly stems from their biological activity85,86 and their use as intermediates in the synthesis of biologically relevant heterocycles.87−89 Despite the general coherence of crystallographic analyses and computational results, the data in Table 1 show that distances measured in studies of single crystals by X-ray diffraction do not always agree with the calculated values, which provides an indication that packing in the crystalline state can have a significant effect on the structure of nitrosoarenes. In addition, it is noteworthy that, in all structures presented in Table 1, the 260

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Table 1. Characteristic Structural Features in Nitrosobenzenes, As Determined by Calculations and X-ray Diffraction (XRD)

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Table 1. continued

a

In aqueous solution. bCo-crystal with cholic acid. cTrihydrate. dHexahydrate. eUnit cell contains two independent molecules; data shown for both. Co-crystal with 2-[4-(dimethylamino)phenylimino]-3-oxo-N-phenylbutanamide. gCo-crystal with deoxycholic acid. hCo-crystal with 1,1′-(1,4butanediyl)bis(imidazole).

f

Table 2. Characteristic Structural Features in cis-Azodioxy Dimers, As Determined by Calculations and X-ray Diffraction (XRD)

a

Unit cell contains two independent molecules; data shown for both.

3. MECHANISM OF DIMERIZATION

simultaneous formation of two NO bonds (Figure 5). This analysis of the energetics of association is instructive because it highlights the unusual behavior of C-nitroso compounds and helps explain how Hammick and other early researchers were understandably, albeit mistakenly, led to reject azodioxides as the products of dimerization, based in part on the anticipated properties of the NN bond. The analysis offered by Lüttke is insightful, but it oversimplifies the transformation by supposing that the potential energy depends only on dNN. In fact, the relative spatial orientation of the nitroso fragments has a dramatic effect on the energetics of the reaction. As shown by Hoffmann, Gleiter, and Mallory in 1970,122 the electronic reorganization occurring in the dimerization of C-nitroso compounds is forbidden by symmetry if the nitroso groups approach one another in the same plane. Moreover, this least-motion approach would have the undesirable effect of forcing together the two lone pairs of the interacting

In 1959, Lüttke offered a theoretical explanation for the ease of dissociation of azodioxides.120 In his view, electronic reorganization taking place during the dissociation accounts for the low energy of activation. A proper starting point for understanding the nature of the electronic reorganization is a detailed analysis of the molecular orbitals involved and a correlation of their energy levels, as shown in Figure 4 for the representative case of HNO. This analysis reveals a clear analogy between azodioxides and various isoelectronic species, including enediolate dianions, 1,3butadienyl dianions, and the dianion of the dimer of NO, which all have a total of six electrons in a set of four characteristic bonding and antibonding π molecular orbitals. The key to understanding the facility of the dissociation of azodioxides is to recognize that energy is required to lengthen the NN bond in the course of its rupture, but compensation is provided by the 262

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Table 3. Characteristic Structural Features in trans-Azodioxy Dimers, As Determined by Calculations and X-ray Diffraction (XRD)

a

Co-crystal with PhN(O·)C6H3Mes2.

atoms of nitrogen (Figure 6). As a result, such a process would be expected to have a high energy of activation. Using extended Hückel methodology,123 Hoffmann and co-workers optimized the trajectory of two reacting molecules of HNO that approach one another in orientations resulting in the formation of either cis- or trans-azodioxy dimers. At N−N distances of 2.50−3.50 Å, the calculations showed a preference for an approximately perpendicular (104−106°) orientation of the nitrogen atom of one HNO molecule relative to the plane of the other HNO molecule. This allows the lone pair of the nitrogen atom in one nitroso group to interact with the π* orbital of the other, much like nucleophilic attack on a carbonyl group (Figure 6).124 In addition, Hoffmann and co-workers concluded that there was no enthalpic barrier for either the cis or trans approach, which shared almost identical energy profiles.

After the emergence of new computational methods in quantum chemistry, researchers revisited the subject of the dimerization of HNO. In 1994, Lüttke et al. reported calculations at two levels of theory (MP4(SDTQ)//MP2/6-31G* and QCISD(T)//QCISD/6-31G*),125 which partly confirmed the earlier conclusions of Hoffmann and his colleagues. In particular, Lüttke et al. estimated an activation energy of >600 kJ/mol when the reacting molecules approach in the same plane. However, they also calculated nonzero activation energies (45−46 kJ/mol) when the trajectory is optimal. This conclusion was later supported by Uggerud and co-workers, who calculated a value of 44 kJ/mol at the MP4/6-311+G(2df,2dp)//MP2/6-31G(d,p) level of theory.126 In 2011, Fehling and Friedrichs reported subsequent calculations at the B3LYP/aug-cc-pVTZ level of theory that once again revealed no energetic barrier for the 263

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Figure 6. (a) Least-motion approach of two HNO molecules. (b) Optimal approach of two HNO molecules at large separation. In all cases, orbitals were calculated using density functional theory (generalized gradient approximation (GGA) with Perdew−Burke− Ernzerhof (PBE) parametrization)121 and are drawn at an isodensity of 0.03 e/Å.

highly ordered transition state, without any enthalpic barrier. The transition states for the gas-phase trans dimerization of HNO calculated in all of these studies are remarkably similar. In particular, the transition states have Ci symmetry, with the reacting molecules of HNO lying offset in parallel planes (Figure 7). This orientation allows for energetic stabilization to occur by mutual n-π* orbital interactions.

Figure 4. Diagram showing the correlation of energy levels in the dimerization of two HNO monomers to give trans-(HNO)2. Relevant molecular orbitals of monomeric HNO and of trans-(HNO)2 are shown. In all cases, orbitals were calculated using density functional theory (generalized gradient approximation (GGA) with Perdew−Burke− Ernzerhof (PBE) parametrization)121 and are drawn at an isodensity of 0.03 e/Å.

Figure 7. Transition states for the gas-phase trans dimerization of HNO as calculated (a) by Lüttke et al.,125 (b) by Uggerud and co-workers,126 and (c) from the data of Fehling and Friedrichs.127

3.1. Cis−Trans Isomerization of Azodioxides

Few studies have examined the mechanism of the cis−trans isomerization of azodioxides, and most of the reported work has focused on the isomerization of dimers of nitrosoalkanes. Nevertheless, conclusions drawn for this class of compounds have also proven to be valid for nitrosoarenes. A priori, two distinct processes can be advanced to account for the cis−trans isomerization of azodioxides: (1) unimolecular rotation around the NN bond and (2) dissociation to form monomers, followed by reassociation (Scheme 1). The first kinetic study of the isomerization of dimers of nitrosoalkanes was published by Chaudhry and Gowenlock in 1968, but interpretation of their results was complicated by the competing formation of oximes by tautomerism.128 In 1972, Wajer and de Boer reported that the formation of oximes could be suppressed in acetonitrile, allowing them to study the cis− trans isomerization of the dimers of nitrosomethane and nitrosocyclohexane.129 Their kinetic data proved to be consistent with the mechanism of dissociation−association. Although this study demonstrated that the isomerization of azodioxides occurs preferentially by dissociation into nitroso monomers, the basis for this preference remained unclear. An explanation was

Figure 5. Diagram showing changes in potential energy during the conversion of a trans-azodioxy dimer into two monomeric C-nitroso compounds, as described by Lüttke.120 In the diagram, dNN represents the N−N distance, EAdiss is the energy of activation for dissociation, EAass is the energy of activation for association, Ediss(theory) is the energy of dissociation neglecting the effect of electronic reorganization, and Ediss(observed) is the energy of dissociation measured experimentally.

dimerization of HNO.127 Because the experimentally determined temperature dependence of the rate constant for the gas-phase trans dimerization of HNO suggests a barrier of only 4−14 kJ/ mol, Fehling and Friedrichs proposed that the primary obstacle to dimerization is the entropy of activation resulting from a 264

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with the tendency to form dimers in the solid state. This metaanalysis shows that simple nitrosoarenes generally prefer dimeric structures in the solid state (Table 4). However, the introduction of substituents in the para position with respect to the nitroso group, particularly electron donors such as amines (entries 3 and 4), amides (entry 7), and ethers (entry 10), can reverse the normal preference and lead to monomeric solids. Substitution in the ortho position favors dimeric solids except in the case of amines (entry 2) and amides (entry 5), whereas substitution in the meta position yields dimeric solids even when an amide is present (entry 6). The marked influence of amines and amides in the ortho and para positions of nitrosoarenes reveals that resonance plays a decisive role in determining whether dimerization will occur. As described in previous sections, stabilization by resonance is largely lost when nitrosoarenes dimerize to form azodioxides. The importance of resonance is mirrored by the large Hammett constant of the nitroso group (σpara = 0.91).158,159 As a result, it is not surprising that the presence of strongly electron-donating substituents in nitrosoarenes disfavors the formation of azodioxides. This expectation is fully consistent with an analysis of the molecular orbitals of nitrosoarenes. Electron-donating substituents will raise the energy of the empty π*NO molecular orbital relative to its position in nitrosobenzene, thereby reducing the strength of its stabilizing interaction with the filled nonbonding nNO orbital of a second molecule in the course of dimerization. Conversely, nitrosoarenes substituted by electronwithdrawing groups systematically form dimers in the solid state because the substituents diminish the ability of the aromatic ring to contribute electron density to the nitroso group. It is important to point out that certain nitrosoarenes are known to be polymorphic and can be obtained in both monomeric and dimeric forms in the solid state. In such cases, however, the monomeric forms are typically prepared by rapid evaporation or sublimation,52,61,112 and they are normally

Scheme 1. Possible Routes for the Cis−Trans Isomerization of Azodioxides

provided in 1982 by ab initio calculations reported by Minato, Yamabe, and Oda.130 Their analysis of the molecular orbitals of azodioxides showed that unimolecular cis−trans isomerization is symmetry-forbidden. In addition, the occupied molecular orbitals π1 and π3 depicted in Figure 4 both contribute to giving the azodioxy system a strong NN π-bonding character, which needs to be disrupted for unimolecular isomerization to take place. In 1987, an NMR study of nitrosobenzene carried out by Orrell and co-workers confirmed the generality of the mechanistic conclusions of Wajer and de Boer.131 In particular, a 2D-EXSY experiment established that the rate of unimolecular conversion of cis-azodioxybenzene into the trans isomer is essentially zero.

4. EFFECT OF STRUCTURE ON THE OCCURRENCE OF DIMERS IN THE SOLID STATE In studies of C-nitroso compounds, the presence or absence of characteristic colors makes it relatively easy to determine whether the preferred form in the solid state is monomeric or dimeric. Because many nitrosoarenes have now been characterized, it has become possible to survey an extensive body of data and use it to correlate the structures of the monomeric forms

Table 4. Preferred Form (Monomer or Dimer) Adopted by Simple Monosubstituted Nitrosobenzenes in the Solid State, As Determined Qualitatively by Their Reported Colorsa

Entry

R

Form

Ref

Entry

R

Form

Ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

H 2-NHRb 4-NHRb 4-NR2b 2-NHAc 3-NHAc 4-NHAc 2-OMe 3-OMe 4-OMe 2-Me 3-Me 4-Me 2-I 3-I 4-I 2-Br

dimer monomer monomer monomer monomer dimer monomer dimer dimer monomer dimer dimer dimer dimer dimer dimer dimer

26 104,132 134−136 24,138,139 141 142 142 145 147 145 150 150 150 155 155 112 137

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

3-Br 4-Br 2-Cl 3-Cl 4-Cl 2-F 3-F 4-F 2-CHO 3-CHO 4-CHO 2-CO2Rc 3-CO2Rc 4-CO2Rc 2-NO2 3-NO2 4-NO2

dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer dimer

140 133 137 140 140 143 144 146 148 149 151 152,153 154,153 154,153 156 157 156

a c

Phenols and primary anilines have been omitted from the table because of their instability and tendency to undergo tautomerism. bR = alkyl, aryl. R = H, Me. 265

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Table 5. Thermodynamic Parameters Related to Rotation around the Aryl C−NO Bond of Aromatic and Heteroaromatic CNitroso Compounds, As Measured by NMR Spectroscopy

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Table 5. continued

a An affirmation of dimerization indicates that evidence for the formation of dimers has been reported in solution and/or in the solid state. bMajor rotamer → minor rotamer where relevant. cR = Me, Et. dR = H, Me, Et. eNR2 = dimethylamino, pyrrolidino, piperidino, or morpholino.

metastable, so only one form is included in Table 4. Furthermore,

solids undergo spontaneous transformation into dimeric forms,

it is not uncommon to find that such metastable monomeric

even in the solid state (vide infra). 267

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Table 6. Chemical Shifts Measured for Monomeric Aromatic and Heteroaromatic C-Nitroso Compounds by 14,15N and 17O NMR Spectroscopy in Solution and Their Correlation with the Ability To Form Dimers at Near-Ambient Conditions of Temperature and Pressure

a An affirmation of dimerization indicates that evidence for the formation of dimers has been reported in solution and/or in the solid state. bChemical shift relative to CH3NO2. cChemical shift relative to H2O.

5. CRITERIA FOR DIMERIZATION

measuring the barrier for rotation around the aryl C−NO bond, which increases in tandem with the importance of the quinoidal form.160 Table 5 compiles relevant thermodynamic parameters measured by NMR spectroscopy in various published studies of nitrosobenzenes and related heteroaromatic analogues. These data show that, when ΔG‡ for rotation around the aryl C− NO bond is greater than ∼38 kJ/mol at 25 °C, aromatic Cnitroso compounds favor monomeric structures, both in solution

5.1. Barrier of Rotation

Resonance stabilization in substituted nitrosoarenes reflects the contribution of quinoidal mesomeric forms to the overall structure, as shown in Figure 3. In turn, significant stabilization can be expected to reduce the tendency to dimerize. The contribution of mesomeric structures can be assessed by 268

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Table 7. Thermodynamic Parameters for the Dimer−Monomer Equilibria of Aromatic and Heteroaromatic C-Nitroso Compounds in Solution, As Measured by NMR Spectroscopy or UV−Vis Spectroscopy

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Table 7. continued

a Proportion of this dimer in solution was too small to be measured. bCalculated from the reported values of ΔH° and ΔS°. cCalculated from the reported values of Kd at various temperatures.

6. MONOMER−DIMER EQUILIBRIA IN SOLUTION

and in the solid state, whereas lower values are correlated with the observable formation of dimers.161

6.1. Thermodynamic Parameters

5.2. Nitrogen and Oxygen NMR Spectroscopy

For many decades, cryoscopy and ebullioscopy have been primary tools for studying oligomeric compounds. Using these techniques, Bamberger and co-workers carried out a series of studies, published between 1897 and 1901, in which they measured the proportion of nitrosoarene dimers in solution at the freezing point of benzene and at the boiling point of acetone.31,186,187 In the 1930s, Hammick and collaborators reported work that standardized and extended these early measurements.35,188 On the basis of these papers, it appears that the introduction of substituents ortho to the nitroso group strongly favors dimerization. However, despite the general reliability of qualitative conclusions emerging from this early body of work, certain quantitative data appear dubious and have been questioned.189 UV−vis spectroscopy has been widely used to measure equilibrium constants for nitrosoareness and their dimers, but relatively little work has been carried out at variable temperatures to allow thermodynamic parameters to be extracted.190−192 In addition, a major shortcoming of UV−vis spectroscopy is that it does not allow equilibrium-related parameters for cis- and transazodioxides to be measured individually. The advent of advanced NMR spectroscopy eventually permitted these data to be determined in solution. Because nitrosobenzene and many derivatives have now been studied extensively by 1H and 13C NMR spectroscopy, it has become relatively easy to assign the configuration of azodioxides based on their characteristic chemical shifts.193−199 Table 7 reports thermodynamic parameters for the dissociation of nitrosobenzene dimers and

In 1994, Dahn et al. reported that chemical shifts measured by 14 N, 15N, and 17O NMR spectroscopy in solution can be used to predict the predisposition of C-nitroso compounds to dimerize.173 In contrast to 1H and 13C chemical shifts, those measured for 14N, 15N, and 17O are highly sensitive to the electronic delocalization in the nitroso group, with measured chemical shifts increasing with the extent of resonance. Table 6 shows that aromatic and heteroaromatic C-nitroso compounds with values of δ (14,15N) below ∼500 ppm are found predominately as monomers in all conditions, whereas higher values are associated with an ability to dimerize at near-ambient conditions of temperature and pressure, either in solution or in the solid state. Far fewer 17O NMR shifts have been recorded; however, it appears that a value of δ (17O) close to 1400 ppm separates compounds with a tendency to dimerize from those that prefer to exist as monomers. Because a strong linear correlation between 14,15N and 17O chemical shifts has been established in aromatic C-nitroso compounds, both techniques appear to offer similar reliability as criteria for dimerization.173 Although the lengths of NO bonds in monomeric C-nitroso compounds and their associated IR-active stretching frequencies have also been used as criteria to predict the tendency to dimerize,112,174 these two parameters were ultimately found to be unreliable indicators, especially for the behavior of borderline compounds.161 270

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Table 8. Activation Parameters for Dissociation of the Dimers of Aromatic and Heteroaromatic C-Nitroso Compounds in Solution, As Measured by NMR Spectroscopy or UV−Vis Spectroscopy

a

Calculated from the reported values of ΔH‡ and ΔS‡.

nitrosobenzenes can be attributed to diminished stabilization of the monomer by resonance. Specifically, steric hindrance imposed by double ortho substitution forces the nitroso group well out of the aromatic plane, thereby decreasing conjugation. Table 7 also reveals that ΔH° and ΔS° for the dissociation of cis dimers are both systematically greater than those measured for the corresponding trans dimers. In many systems, the resulting compensation of enthalpy and entropy leads to the formation of significant amounts of both cis and trans dimers at room temperature, although often with a small preference for the cis form. The larger values of ΔS° for the dissociation of cisazodioxides can be attributed to increased freedom of rotation around C−N bonds, which is more restricted in cis dimers than

heteroaromatic analogues in solution, as measured in various independent studies. Table 7 shows that ΔH° and ΔS° for the dissociation of azodioxides in solution are both positive, as expected for a process in which a bond is broken and a molecule is cleaved. The enthalpic driving force for dimerization can be considered to result in part from the transfer of electron density from nitrogen to oxygen, a more electronegative element.209 Despite this, dissociation is thermodynamically favored at room temperature, except in the cases of 2-nitrosopyridines and ortho-disubstituted nitrosobenzenes. Dimerization of 2-nitrosopyridines is presumably favored by their electron-deficient nature, whereas the increased proportion of the dimeric form of ortho-disubstituted 271

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in their trans isomers. The larger values of ΔH° for the dissociation of cis dimers are a likely consequence of using polar solvents in these studies, which results in a greater enthalpy of solvation for cis dimers relative to trans dimers (vide infra). When nitrosoarenes are doubly ortho-substituted, however, the formation of trans dimers is favored by a large margin, except in the single anomalous case of 4-methoxy-2,6-dimethylnitrosobenzene (entry 17). This pattern is a consequence of pronounced steric repulsion between proximal bulky aryl substituents that exists in cis-azodioxy dimers of doubly orthosubstituted nitrosoarenes. In addition to providing values of ΔH° and ΔS° for equilibria between nitrosoarenes and their cis and trans dimers, NMR spectroscopy and UV−vis spectroscopy have also been used in kinetic studies to measure activation parameters for the dissociation of azodioxides. In this way, it is possible to examine both thermodynamic and kinetic selectivity in the formation of cis and trans isomers. Unfortunately, less effort has been devoted to the evaluation of activation parameters, and relatively few measurements have been made. Table 8 provides data obtained for the dimers of nitrosobenzene, as well as for various substituted derivatives and heteroaromatic analogues. In the series of compounds examined, ΔG‡ for the dissociation of cis dimers to give monomers is almost always lower than the value for trans dimers. In studies carried out in chloroform, much of the difference in ΔG‡ can be attributed to variations in ΔH‡, as the values of ΔS‡ are typically similar for both isomers. Significantly lower values of ΔS‡ have been measured in more polar solvents (water and acetonitrile). This observation suggests that order in the transition state for dissociation is increased by dipolar interactions of solvent molecules with emerging nitroso monomers, leading to a marked effect of polar media on the values of ΔS‡.207,210−212 For two systems, nitrosobenzene and 2-methylnitrosobenzene, relative thermodynamic parameters have been measured in solution for all relevant species, including the nitrosoarene monomers, cis dimers, trans dimers, and the transition states for their interconversion. It is possible, therefore, to construct detailed diagrams showing changes in free energy occurring during the transformation of the two nitrosoarenes into the corresponding cis- and trans-azodioxides at 25 °C (Figure 8). This diagram establishes that the cis dimers of nitrosobenzene and 2-methylnitrosobenzene are favored both thermodynami-

cally and kinetically under these conditions. Crystallization of 2methylnitrosobenzene is known to produce the trans-azodioxide, so it is clear that the preferred behavior in solution does not dictate the outcome of crystallization, which is controlled by unique kinetic and thermodynamic factors that intervene when a separate solid phase is formed. Figure 8 helps illustrate three characteristic properties of prototypical nitrosoarenes: (1) in solution at room temperature, nitrosoarene monomers are more stable than the dimeric forms; (2) cis dimers are normally preferred in solution relative to their trans isomers, both kinetically and thermodynamically; and (3) ortho substitution tends to destabilize nitrosoarene monomers relative to dimers. In agreement with the Hammond postulate,213 which states that the structure of a transition state for the interconversion of two species should more closely resemble the geometry of the species nearest in energy, the rates of dimerization of nitrosobenzene reflect the relative stabilities of the cis and trans products. However, the transformation of 2methylnitrosobenzene into the trans dimer is especially slow, presumably because the transition state is destabilized by steric interactions caused by ortho substitution. 6.2. Effect of Solvent on Monomer−Dimer Equilibria in Solution

It has long been recognized that solvents have a major influence on the position of the equilibrium between the monomeric and dimeric forms of aromatic C-nitroso compounds. Unfortunately, no general study of this important phenomenon has yet been reported. Representative examples of the effect of solvents on equilibrium constants measured near room temperature are compiled in Table 9. These data exemplify how polar solvents favor dimerization, especially when water or other solvents that can donate hydrogen bonds are present. cis-Azodioxy dimers typically have a large net dipole moment, and even trans-azodioxy dimers of aromatic C-nitroso compounds may retain a weak net dipole moment, as noted earlier in the case of the trans dimer of nitrosomesitylene. As a result, dipole−dipole interactions can make an important contribution to the stabilization of azodioxy dimers in solution, especially in the case of cis dimers. In particular, calculations suggest that the cis and trans dimers of HNO differ significantly in their enthalpies of solvation, and this difference accounts for the fact that the trans dimer is thermodynamically favored in the gas phase whereas the cis dimer is preferred in aqueous solution.127 In addition, computational studies of the dimerization of HNO and MeNO suggest that further solvent-related stabilization of cis- and trans-azodioxy dimers arises because they are both more polar and polarizable than the corresponding nitroso monomers, due to a shift of electron density along the N−O bond toward oxygen in the dimers.209 Deeper understanding of the effect of solvents on the equilibrium can be gained by examining the properties of a pair of cyclic cis-azodioxides derived from bis(o-nitrosobenzyl) and a related ether. These cyclic azodioxides display behavior closely similar to that of acyclic intermolecular dimers of simple nitrosoarenes.215,216 In particular, they are in rapid equilibrium with the corresponding dinitroso compounds at room temperature, and these equilibria have been shown by NMR spectroscopy and UV−vis spectroscopy to be markedly solvent-dependent (Table 10). The data in Table 10 confirm that highly polar environments stabilize azodioxy dimers more than nitroso monomers. A manifest increase in ΔH° for dissociation is observed as the

Figure 8. Diagrams showing changes in free energy occurring during dimerization of nitrosobenzene (PhNO) and 2-methylnitrosobenzene (o-TolNO) in solution. Data for PhNO (black profile) and o-TolNO (gray profile) were obtained in CDCl3 and CD3CN, respectively. The two nitrosoarenes (ArNO) have been arbitrarily placed at the same energy to facilitate comparison. 272

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Table 9. Effect of Solvents on the Equilibrium Constants for Dissociation of the Dimers of Aromatic and Heteroaromatic CNitroso Compounds in Solution, As Measured by NMR Spectroscopy

a

Calculated from the reported monomer−dimer ratios. bValues of Kd given at 308 K for refs 175 and 200 and at 300 K for ref 214.

Table 10. Thermodynamic Parameters for the Dissociation of Cyclic Azodioxides in Solution to Give Dinitroso Compounds

Entry

X

Method

Solvent

ΔH° (kJ/mol)

ΔS° (JK/mol)

ΔG° at 298.15 K (kJ/mol)

Ref

1 2 3 4 5 6 7 8 9 10

CH2CH2 CH2CH2 CH2CH2 CH2CH2 CH2OCH2 CH2OCH2 CH2OCH2 CH2OCH2 CH2OCH2 CH2OCH2

NMR UV−vis UV−vis NMR NMR NMR NMR NMR NMR NMR

THF-d8 dioxane PhNO2 CDCl3 THF-d8 C6D6 C6D5NO2 CDCl3 DMSO-d6 CD3OD

31.9 29.7 ± 2.0 28.5 ± 1.2 49.2 32.0 35.6 39.6 50.6 ± 0.6 50.8 46.4

101 85.4 ± 8.8 75.7 ± 5.4 132 104 116 115 141 ± 15 131 115

1.79 4.24 5.93 9.84 0.99 1.01 5.31 8.56 11.7 12.1

216 215 215 216 216 216 216 216 216 216

Table 11. Equilibrium Constants for the Dimerization of Substituted Nitrosobenzenes at 25 °C in CCl4 at Various Pressures, As Measured by UV−Vis Spectroscopy217

polarity rises, whereas ΔS° changes little. As a result, selective

6.3. Effect of Pressure on Monomer−Dimer Equilibria in Solution

stabilization of the dimer form presumably reflects dipolar

The dimerization of nitrosoarenes can be regarded as a prototypical example of a reaction in which a single molecule results from the creation of bonds between two identical

interactions between solvent molecules and the azodioxy linkages. 273

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Scheme 2. Photoinduced Rearrangement of 2,4,6-Tri-tert-butylnitrosobenzene (6) To Give Nitrosoalkane 7, and the Reaction of Compounds 6 and 7 To Form Azodioxide 9

The rearrangement was found to produce nitrosoalkane 7, which was isolated in the form of its azodioxy dimer (8). When two molar equivalents of 2,4,6-tri-tert-butylnitrosobenzene (6) were independently mixed with azodioxide 8 in benzene-d6, the appearance of the new azodioxide 9 was observed, and after 24 h the molar ratio of azodioxides 9 and 8 reached equilibrium at ∼5:1. This high ratio reflects the intrinsic instability of nitrosoarenes with bulky ortho substituents and their heightened tendency to react to form azodioxides with less-hindered nitroso partners, which are less apt to dimerize by themselves.

reactants. For this reason, it offers a privileged opportunity to probe the general effect of pressure on chemical reactions. In a study of this type,217,218 Yoshimura and Nakahara used UV−vis spectroscopy to examine the equilibria between several nitrosoarenes and their dimers at various pressures (Table 11). Unsurprisingly, high pressures proved to favor the formation of azodioxides at the expense of nitroso monomers, and for each nitroso compound the enhanced dimerization was found to correspond to an approximately threefold increase in the equilibrium constant from atmospheric pressure to 122.6 MPa. From these data, volume changes associated with dimerization could be assessed and were shown to agree with theoretical predictions, although the significantly different behavior of diand trisubstituted nitrosobenzenes could not be quantitatively explained.

7. CROSSED DIMERIZATION OF NITROSOARENES The presence of multiple C-nitroso compounds in solution typically leads to competitive formation of various azodioxides, both symmetric and asymmetric. Combinations of two different nitroso compounds, which can be called crossed dimerizations, create interesting opportunities. To simplify discussion of these processes, azodioxides derived from a single nitroso compound will be called homodimers, and azodioxides formed from two different nitroso compounds will be called heterodimers. When geometric isomerism is taken into account, it follows that six unique azodioxides can result from the combination of two different nitrosoarenes (eq 5). Obviously, detailed studies of crossed dimerization are challenging, especially when the processes do not show high selectivity.201,203 However, as described above, crossed dimerization can produce heterodimers selectively when steric hindrance inhibits formation of the homodimers of one of the two partners. This phenomenon is not limited to the combination of nitrosoarenes and nitrosoalkanes, as in the case illustrated earlier. In fact, a series of heterodimers of nitrosoarenes have been prepared by exploiting the effect of steric hindrance introduced by ortho

6.4. Effect of Steric Hindrance on Monomer−Dimer Equilibria in Solution

In general, ortho substituents in nitrosoarenes greatly favor the formation of azodioxides. Nevertheless, when very bulky groups occupy one or more ortho positions in nitrosoarenes, steric effects can become so important that dimerization is suppressed. For example, no dimerization has been observed in solution or in the solid state when one or two tert-butyl groups are present at the ortho position,196,219,220 whereas 2,6-diisopropylnitrosobenzene crystallizes as a dimer despite important steric hindrance.115 However, it is interesting to note that 2-tert-butylnitrosoarenes can react readily with other nitrosoarenes to form azodioxides, at least when the partners are not sterically hindered themselves. This phenomenon was first reported in 1978 by Barclay and coworkers,221 who studied the photoinduced rearrangement of 2,4,6-tri-tert-butylnitrosobenzene (6) and other reactions summarized in Scheme 2. 274

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Figure 9. Representations of the crystal structures of (a) trans-4,4′-dibromoazodioxybenzene and (b) 4-bromonitrosobenzene, as obtained by singlecrystal-to-single-crystal photodissociation of the crystalline azodioxy dimer at 100 K.53 Shown are atoms of carbon in gray, atoms of hydrogen in white, atoms of nitrogen in blue, atoms of oxygen in red, and atoms of bromine in carmine. C−H···O interactions are shown by yellow lines, and close Br···Br contacts are marked by red lines. Reprinted with permission from ref 53. Copyright 2005 American Chemical Society.

substituents.222 Recently, Vančik and co-workers have studied the influence of electronic effects on the crossed dimerization of nitrosoarenes.114,223,224 Although the processes typically show only modest selectivity, a possible preference for crossed dimerization emerges when the nitrosoarenes involved are electronically complementary. Theoretical calculations suggest that trans heterodimers are more stable than the corresponding homodimers when electronic complementarity is important.

crystalline form with a structure closely related to that of the initial dimer (Figure 9). According to the interpretation of the authors, NN double bonds in the azodioxy groups of the crystalline dimeric solid are thereby transformed into short nonbonded N···N distances between individual nitroso groups in the metastable monomeric phase, accompanied by significant reorientation of the aromatic rings. Warming the photodissociated crystals from 100 K to room temperature regenerated the crystalline dimer. In contrast, sublimation of 4-bromonitrosobenzene produces the monomer in a metastable green crystalline form that is unrelated to the crystal structure of the corresponding trans dimer.61 Thermally induced solid-state dimerization also occurs in crystals of the metastable monomer produced by sublimation, but the reaction is much slower than it is in crystals made by photodissociation of the dimer, and it proceeds via a nucleation− growth process (Figure 10). Optical microscopy provided evidence that nucleation is initiated at defects that first propagate to the surface of the crystals before the bulk reacts. In the studies of Vančik and co-workers, metastable monomeric solid forms of 4-chloronitrosobenzene and 4-nitronitrosobenzene were also shown to undergo spontaneous dimerization by a nucleation− growth process.

8. DIMERIZATION IN THE SOLID STATE As noted above, nitrosoarenes that are normally found as dimers in the solid state can sometimes be isolated as metastable solids composed of the monomeric form. This phenomenon makes nitrosoarenes interesting subjects for studies of thermally induced solid-state reactions. In recent years, Vančik and coworkers have reported investigations of the solid-state dimerization of simple nitrosoarenes.53,61,98,225−227 They relied on two methods to prepare the initial metastable monomeric phases: (1) sublimation and (2) irradiation of dimeric solids at low temperatures to promote photodissociation, in some cases by single-crystal-to-single-crystal transformations. For example, photodissociation of the colorless crystalline dimer of 4bromonitrosobenzene at 100 K was observed to occur topotactically to generate the monomer in a metastable green 275

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similar nitroso compounds to the surface of the particles, and the propensity of nitrosoarenes to dimerize leads to subsequent aggregation.

10. AROMATIC DINITROSO AND POLYNITROSO COMPOUNDS The complex chemistry of ostensibly simple nitrosoarenes and their dimers suggests that researchers studying more elaborate compounds with multiple nitrosoaryl groups can expect to confront many obstacles, both conceptual and practical. Such compounds fall into three distinct classes: (1) compounds in which the nitroso groups do not interact; (2) compounds in which the nitroso groups interact intramolecularly to form cyclic azodioxides or benzofuroxans; and (3) compounds in which the nitroso groups interact intermolecularly to form oligomeric or polymeric azodioxides.230 The first class is composed almost exclusively of compounds that are stabilized electronically relative to their dimeric form by an important mesomeric effect. As a result, these compounds solely exist in monomeric form and therefore are not included in the present review.231−234

Figure 10. Nucleation and growth at kinks and edges in the solid-state dimerization that occurs in green crystals of 4-bromonitrosobenzene produced by sublimation.61 The location and degree of dimerization can be tracked by the change of color from green (monomer) to colorless (dimer). Arrows highlight small crystals, which undergo complete reaction faster than bigger crystals. Crystals are shown after (a) 2 min, (b) 12 min, (c) 30 min, and (d) 105 min at room temperature. Reprinted with permission from ref 61. Copyright 2011 Royal Society of Chemistry.

10.1. Intramolecular Association Leading to Cyclic Structures

A common intramolecular reaction of compounds with multiple nitroso groups is the formation of cyclic cis-azodioxides. No cyclic trans-azodioxides have yet been reported.230 Representative compounds include cyclic cis-azodioxides arising from the intramolecular association of adjacent nitroso groups in bis(onitrosobenzyl) and related compounds,215,216,235 such as those featured in Table 10. In addition, another common group of cyclic azodioxides can be considered to be derived from bis(2nitrosophenyl)methane.236 For example, the structure of heterocyclic cis-azodioxide 11 was reported in 1989 by Banks and his colleagues (Figure 12). Finally, benzo[c]cinnoline-5,6-

9. DIMERIZATION ON SURFACES Only recently has the dimerization of nitrosoarenes been studied on surfaces.228,229 Studies of molecular adsorption have typically focused on interactions with the underlying surface or within the adlayer itself, but the adsorption of nitrosoarenes introduces the additional possibility of creating self-assembled bilayers by allowing adsorbed nitroso compounds to react with others that are free in solution. For example, Vančik and co-workers used Au···S interactions to bind sulfur-containing nitrosoarenes on Au(111), and they were able to observe both monomeric and dimeric forms of the adsorbed compounds by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). In particular, thiocyanate 10 was shown by STM to produce monolayer and bilayer domains (Figure 11). An interesting related property of sulfur-containing nitrosoarenes is their ability to induce the aggregation of Au nanoparticles. Formation of strong Au···S interactions promotes binding of thiocyanate 10 or

Figure 12. Representation of the structure of heterocyclic cis-azodioxide 11, as determined by X-ray diffraction.236 Shown are atoms of carbon in gray, atoms of hydrogen in white, atoms of nitrogen in blue, atoms of oxygen in red, and atoms of fluorine in green.

dioxide, which can be considered to be derived from an intramolecular reaction of 2,2′-dinitrosobiphenyl, is an example of another well-known family of cis-azodioxides. The properties of all these cyclic compounds are different from those of azodioxides derived intermolecularly from simple mononitrosoarenes. In particular, the cyclic structures dissociate much less easily, partly because the associated entropy of dissociation is greatly reduced and the preferred orthogonal trajectory is not easily accessible; in addition, the azodioxy unit is incorporated within an aromatic ring in benzo[c]cinnoline-5,6dioxide and related cyclic structures.237 Although numerous studies have reported cyclic azodioxides, few have probed the special nature of the cis-azodioxy unit in cyclic compounds.238−240 However, a crystallographic study of the structure of 1,10-dimethylbenzo[c]cinnoline-5,6-dioxide (12),

Figure 11. High-resolution STM images of monolayer and bilayer domains created when thiocyanate 10 is adsorbed on the (111) surface of Au.228 (a) Image showing an area of 19 × 19 nm2 (scale bar = 3.8 nm). (b) Image showing an area of 16 × 16 nm2 (scale bar = 3.2 nm). The white arrow in (a) marks individual molecules of dimers and their nucleation to form 2D crystals. The numbers 1, 2, and 2′ in (a) correspond to domains composed of a monolayer, bilayer, and bilayer on Au-adatoms, respectively. Reprinted with permission from ref 228. Copyright 2011 American Chemical Society. 276

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(19), the first polymer derived from a dinitrosobenzene (eq 7).247

published by Whittleton and Dunitz,241 reveals that the cisazodioxy unit is strongly resilient (Figure 13). In this structure,

Gentle thermolysis of the polymer (50−80 °C) generates vapors of 1,4-dinitrosobenzene, which can be condensed at low temperature (32−200 K) to produce a green monomeric solid. This reversible transformation allows the process of polymerization to be followed by spectroscopic methods.247,248 These studies have concluded that poly(trans-1,4-phenyleneazo-N,N′dioxide) has a high molecular weight because characteristic signatures of terminal unreacted nitroso groups cannot be detected by normal methods, including IR, Raman, UV−vis, and NMR spectroscopy. Nevertheless, these groups may still be present, and Gowenlock and Richter-Addo have suggested that further polymerization could be induced by adding more monomer, so that the system behaves as a living polymer.230 The facile reversibility of the polymerization of 1,4dinitrosobenzene has practical applications. In particular, poly(trans-1,4-phenyleneazo-N,N′-dioxide) has been used as a photosensitive resin249 and is widely employed as a vulcanizing agent in the cross-linking of rubber by nitroso−ene reactions.250 The generally accepted mechanism for cross-linking first involves thermolysis of the polymer to regenerate 1,4-dinitrosobenzene, followed by a nitroso−ene reaction251 and final dehydration of the resulting hydroxylamine.252−254 Many derivatives of 1,4-dinitrosobenzene have been prepared and shown to share the behavior of the parent compound, including (1) the formation of insoluble polymers incorporating trans-azodioxy groups and (2) facile thermal depolymerization to regenerate the monomeric form.255−263 An exception is 2,5-ditert-butyl-1,4-dinitrosobenzene, which does not polymerize because of important steric hindrance created by the presence of two ortho-tert-butyl groups.255 1,3-Dinitrosobenzene (20), first reported in 1905 by Alway and Gortner,264 polymerizes spontaneously at room temperature and yields poly(trans-1,3phenyleneazo-N,N′-dioxide) (21) (eq 8).

Figure 13. Representations of the crystalline structures of 1,10dimethylbenzo[c]cinnoline-N,N′-dioxide (12)241 and cis-azodioxide 13.242 Shown are atoms of carbon in gray, atoms of hydrogen in white, atoms of nitrogen in blue, and atoms of oxygen in red.

significant nonplanarity of the fused aromatic rings is tolerated to minimize steric repulsion of the methyl groups. Despite significant deformation, the molecule nevertheless prefers to exist (at least in the crystalline state) as a cyclic azodioxide rather than as a dinitroso compound. Similar behavior is shown by cyclic azodioxide 13, which can be considered to be derived from 1,8-dinitrosonaphthalene; again, the azodioxy group is part of an aromatic system and does not dissociate easily. The aromatic core of the molecule suffers significant distortion in the solid state to accommodate the azodioxy group (Figure 13).242 Another option for the intramolecular interaction of nitroso groups is the formation of benzofuroxans. This behavior is exhibited only by 1,2-dinitrosobenzenes such as the parent compound 14, which cyclizes reversibly to benzofuroxan (15) when heated (eq 6).243 Related compounds resulting from

1,3-Dinitrosobenzene has been prepared from unstable Narylhydroxylamine intermediates, rendering its synthesis and purification difficult.248 As a result, little information about related compounds is available,230 although 1,3,5-trinitrosobenzene is known to form an amorphous polymer that has been used effectively in the vulcanization of rubber.265,266 As in the case of poly(trans-1,4-phenyleneazo-N,N′-dioxide) (19), thermolysis of poly(trans-1,3-phenyleneazo-N,N′-dioxide) (21) regenerates the monomer as green vapors that can be condensed at low temperature and repolymerized upon warming.

multiple cyclizations have been prepared, including dibenzofuroxan 16 (derived from 1,2,3,4-tetranitrosobenzene)244 and tribenzofuroxan 17 (derived from hexanitrosobenzene).245 10.2. Intermolecular Association Leading to Polymers

1,4-Dinitrosobenzene (18) is the archetype of dinitrosoarenes able to polymerize by the intermolecular formation of azodioxy linkages. First reported in 1887 by Nietzki and Kehrmann,246 1,4dinitrosobenzene is a green compound that reacts spontaneously at room temperature to produce a yellow solid. The solid has been shown to incorporate trans-azodioxy units and has been characterized as poly(trans-1,4-phenyleneazo-N,N′-dioxide)

10.3. Intermolecular Association Leading to Covalent Organic Networks

In 2005, Gowenlock and Richter-Addo concluded their review of dinitroso and polynitroso compounds as follows: “...a need exists for synthetic routes to tri- and polynitroso compounds due to the 277

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Scheme 3. Polymerization of Tetranitrosoarenes 22−24 To Give Covalently Bonded Nitroso Polymer Networks NPN-1, NPN-2, and NPN-3

Figure 14. Representations of the structures of crystals of covalent organic networks NPN-1, NPN-2, and NPN-3. (a) NPN-1, showing part of the diamondoid framework (ball-and-stick image at top left), the degree of interpenetration (grayscale image at top right), and the cross sections of parallel channels viewed along the c-axis (space-filling image at bottom). Shown are atoms of carbon in gray, nitrogen in blue, oxygen in red, and silicon in yellow. Guests are disordered and omitted for clarity. (b) Analogous representation of NPN-2. (c) Analogous representation of NPN-3.

potential uses of these compounds in new materials.”230 Undeniably, the characteristic ability of many aromatic C-nitroso compounds to undergo reversible dimerization under mild conditions sets them in a special realm in science, midway between supramolecular chemistry, where molecules associate by forming weak interactions, and polymer science, where monomers are linked by strong covalent bonds that are essentially permanent. The polymerization of suitable aromatic C-nitroso compounds thereby offers a way to create new covalently bonded materials with highly ordered structures, under conditions where errors of assembly can be corrected reversibly. However, attempts to attain this goal must confront two obstacles: (1) compounds with multiple nitrosoaryl groups are not easily made, and (2) the preparation of polymers of high molecular weight requires the use of rigorously purified monomers to prevent interruption of sustained growth.

New synthetic methodology reported in 2011 allowed Beaudoin, Maris, and Wuest to prepare monomeric tetranitrosoarenes 22−24, with tetrahedrally oriented nitroso groups.267,268 Dilute solutions of compounds 22−24 in suitable solvent mixtures were found to undergo spontaneous polymerization, leading to the formation of relatively large single crystals of the corresponding covalently bonded nitroso polymer networks (Scheme 3). Analysis of the crystals by X-ray diffraction allowed detailed structural characterization and confirmed that the tetrahedral molecular geometry, in conjunction with the formation of transazodioxy linkages, led predictably to the construction of diamondoid networks (Figure 14). The large distances between the tetrahedral centers of adjacent nodes in these networks allow multiple networks to interpenetrate to fill part of the available volume. Nevertheless, the resulting structures all retain a 278

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Biographies

significant degree of openness, and the percentages of volume available for the inclusion of small molecular guests lie in the range 35−39%. These new nitroso polymers are an important addition to the growing number of porous structures called covalent organic frameworks (COFs),269,270 which are beginning to find applications in molecular storage, separation, catalysis, and other areas. Previously studied COFs have typically been made by condensation reactions that release other products, under conditions that do not readily allow reversible assembly. As a result, conventional COFs have only been available as microcrystalline powders, and their structural characterization has

Daniel Beaudoin received his B.Sc. and M.Sc. from the Université de Montréal, where he studied under the supervision of Professor André B. Charette. After a brief period spent teaching chemistry at the college level, he joined the research group of Professor James D. Wuest in 2009 for his Ph.D. degree. He is now a postdoctoral fellow at the RuprechtKarls-Universität Heidelberg, working with Professor Michael Mastalerz. His research focuses on the development of novel molecular materials.

been almost exclusively based on analysis by X-ray powder diffraction. In contrast, the materials derived from nitrosoarenes 22−24 result from addition polymerizations under conditions that favor reversible assembly and lead to the formation of large crystals. Future work of this type, using nitroso monomers with varied geometries, is likely to provide an expanded set of crystalline polymers as well as additional information about the detailed mechanism of assembly.

11. CONCLUSION Nitrosoarenes have been known for over 140 years, and various aspects of their chemistry have been summarized in previous reviews. Reversible dimerization, one of the most characteristic and puzzling properties of these compounds and their heteroaromatic analogues, remained mysterious for decades, and only recently has a full understanding of the process finally begun to emerge. The conclusions of recent studies of Jim Wuest received his A.B. in 1969 from Cornell University and his Ph.D. in 1973 from Harvard University, where he was a student of R. B. Woodward. After serving as an assistant professor of chemistry at Harvard, he moved to the Université de Montréal in 1981. His research uses tools from diverse areas of science and technology, including synthesis, structural analysis, surface science, and fabrication of optoelectronic devices, in an effort to learn how to make valuable new molecular materials by design.

dimerization of aromatic C-nitroso compounds are inherently fascinating, and improved knowledge of the behavior of these compounds is beginning to shed valuable light on the study of many other areas of chemistry. At the same time, new ways to exploit the reversible dimerization of nitrosoarenes are being discovered. The present review summarizes the current state of knowledge and will help spur further advances in the evergreen field of C-nitroso chemistry. The unique features of C-nitroso

ACKNOWLEDGMENTS The authors are grateful to the Natural Sciences and Engineering Research Council of Canada, the Ministère de l’Éducation du Québec, the Canada Research Chairs Program, NanoQuébec, and the Université de Montréal for financial support. In addition, the authors thank Prof. Matthias Ernzerhoz for helpful insight, and they are grateful to Prof. Eric J. Toone and Prof. Akira Murakami for providing access to relevant publications.

compounds have given them a rich past, promise an exciting future, and guarantee that researchers in the field will encounter many stimulating challenges.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

DEDICATION † Dedicated with respect to Prof. Brian G. Gowenlock, whose studies of nitroso compounds have greatly enriched the field and stimulated our interest in it.

Notes

The authors declare no competing financial interest. 279

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