Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups / 0423 477

10. Hydrogen bonding and complex formation

433

Many crystal structures are determined in order to study the hydrogen bond geometry. In the literature there are attempts to rationalize the classification of the geometrical properties of X HÐ Ð ÐY (X, Y D N, O) of hydrogen bonds (involving amide groups73 too) by using statistical analyses of intermolecular hydrogen bonds in organic crystals (employing crystal data from the Cambridge Structural Data base).

The dependence of hydrogen bond distances (in linear hydrogen bonds) on the nature of the donor and acceptor atoms was investigated for N HÐ Ð ÐODC interactions (ammonium salts, amines/carbonyl, carboxylate groups etc.); statistical investigation of the linearity of the N HÐ Ð ÐODC bond shows74 that the mean of the distribution of the angles is 161°. (mean on 1352 examples) for intermolecular hydrogen bonds, while intramolecular hydrogen bonds may be less linear (the mean on 152 considered examples is 132°).

N HÐ Ð ÐODC hydrogen bonds show a tendency to occur in the directions of the oxygen sp2 lone pairs, probably because of an inherent preference for hydrogen bonding in a lone-pair direction and involving also steric factors75,76.

About 300 (of the 1500 N HÐ Ð ÐODC bonds considered) are three-centre bonds77. ‘Three-centre bond’ (as shown in 15) is considered a better definition than ‘bifurcated bond’. The three-centre bond is a situation where the proton interacts with two hydrogen bond acceptor atoms; both bonds are shorter than the sum of the van der Waals radii of the atoms involved. The two hydrogen bond acceptors may be different (Y D6 Z in 15).

	..Y		O ..
..			..
.				.
N H....		N	....H			N
	. Z		O .
(15)			(16)

For p-nitroanilines, the intermolecular hydrogen bond between the amino group and the nitro group is an effective tool for organizing molecules in solid state. Even if several other hydrogen bonding interactions between nitro and amino groups are well codified, the three-centre hydrogen bond interaction between hydrogens of the amino groups and the two inside lone pairs of electrons of a single nitro group78 is the more frequent picture of this interaction in the solid state as shown in 16.

The crystal and molecular structure79 of a carbazole/tetracyano ethylene 1:2 complex reveals the presence of a hydrogen bond between the amino group of the carbazole and a nitrile nitrogen of tetracyano ethylene in the solid state, which resembles the stronger hydrogen bond observed between amines and the cyano group.

Application of the solvatochromic comparison method (Kamlet and coworkers80) reveals a competition between intermolecular and intramolecular hydrogen bonding in molecules bearing both donor and acceptor groups81 such as 17.

H	(CH2 )n NH2
	N

NO2

n = 2, 3, 4

(17)

434	Luciano Forlani

N-(p-Nitrophenyl)alkylenediamines 18 form intramolecular hydrogen bonds between the two N H groups (aromatic and aliphatic) as reported in equilibrium 9. When hydrogen bond acceptor solvents are used, intramolecular hydrogen bonds are formed between 18 and solvents, but when the nitro group is in the ortho position, the hydrogen bond is formed between the aromatic amino and nitro groups.

H	(CH2 )n NH2		(CH2 )n
	N	N	H NH2

(9)

NO2	NO2
(18)	(19)

C. Self-association of Amines

Self-association of substances with acidic hydrogens82, including amines (to form, in a first approximate model, dimers mainly by hydrogen bonding interactions), is a long known problem. There are numerous reports in the literature on the thermodynamic, spectroscopic and structural aspects of self-association and dimer formation of water, alcohols, ammonia, amines and hydrogen sulphide83.

The dimerization energies in the vapor phase are of 2 3 kcal mol 184 . Dimerization of aromatic or aliphatic amines85 is the first step of the formation of aggregates and it is the usual model in investigating the properties of compounds containing the NH2 group, when it is at the same time both a hydrogen donor and acceptor (20). Nevertheless, the formation of non-cyclic trimers is reported to be the best stoichiometry for the self-association of butylamines in cyclohexane86.

H

..

H N H..... :.N R

R H

(20)

Dimer presence in apolar solvents is indicated as being responsible for some particular kinetic features when protic amines are nucleophilic reagents87,88. Aliphatic amines are slightly more associated than aromatic amines89 91.

Aniline may complex (as a proton donor) not only with tertiary amines (proton acceptors) such as N,N-dimethylaniline, pyridine or N,N-diethylcyclohexylamine, but also with apparently neutral molecules such as CCl492, benzene93 or chloroform, which acts as proton donor toward amines94.

Usually, the influence of the solvent on the proton donor and acceptor interactions is relevant95. The importance of the amine/solvent hydrogen bonding interaction depends on the strength of the amine/amine hydrogen bonding interaction96.

10. Hydrogen bonding and complex formation

435

AM1 calculations on dimers of nitroanilines are of interest in investigating the intermolecular forces which orientate the individual molecules in crystal chains. Dimerization in solid crystal may be considered responsible for the differences in molecular geometry between solid and gas phases97.

Self-association of heterocycles containing NH groups (such as imidazoles98, triazoles99 etc.) produces chains of planar molecules connected by N HÐ Ð ÐN hydrogen bonds. The effects of intermolecular hydrogen bonding on the molecular structures are confirmed also by ab initio molecular orbital calculations100.

D. Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding is important in the explanation of both the physical and chemical properties of molecules, with particular attention to their spectroscopic properties.

When two amino groups are in a rigid geometry as in the case of 1,8-diamino- naphthalene (21), some interesting properties (e.g. the anomalous strong basicity) of the amino groups can be understood by intramolecular hydrogen bonds101. Usually, if heteronuclear hydrogen bonds are formed, hydrogen bonds are weaker than homonuclear hydrogen bonds owing to lack of symmetry. This is the case of the ‘proton sponge’.

.. H ..

NR2

NR2

..

+

..

N

N

+

N .... H ....N

R = H

R = Me

(21)

(22)

(23)

(24)

In 21, the hydrogen bond is strong due to the short N

N distance in 23 (2.54

˚

101

2.65 A)

or in the 1,6-diaza[4,4,4]tetradecane cation 24 (2.53

˚

102

2.60 A)

. In addition, in cation

23 the -delocalization reinforces the strength of the hydrogen bond103. At same time 23 presents a very low rate of proton transfer101,104 to hydroxide ion in water [2 ð

105 (s 1 mol 1 dm3)], while deprotonation of the usual ammonium ions is a diffusion controlled process [1010 (s 1 mol 1 dm3 ]105.

The term ‘proton sponges’ indicates a series of compounds with unusually high basicity, compared to the basicity of isomers. A recent review106 collects and discusses crystallographic results of the literature on this subject.

The hydrofluoride of a ‘proton sponge’ is used in MeCN as fluoride ion donor: benzoyl chloride is transformed into benzoyl fluoride107 (reaction 10).

COCl	proton sponge.HF	(10)
	COF

A number of deprotonation equilibria between 1,8-bis(dimethylamino)naphthalene and several acids, including phenols108, thiols109, N H acids110 and carboxylic acids111, have been investigated. 1,2-Bis(dialkylaminomethyl)benzenes 25 are also strong proton acceptors112.

436 Luciano Forlani

NRMe	NMe2 NMe2

NRMe	NMe2 NMe2
R = Me, Et, Pr, Bu
(25)	(26)

1,4,5,8-Tetra(dimethylamino)naphthalene 26 is a ‘double proton sponge’113 which may be diprotonated by strong acids (HBr). The pKa values are 9.8 and 4.9 for the first and second protonation, respectively, on the dimethylsulphoxide pKa scale while the pKa of 22 is 7.5. 26 is a stronger base than 22 not only because of the electron donating effects of the amino groups in the para positions, but also because of the indirect steric effect in the other peri positions 4 and 5 of the naphthalene.

The complex between 20 and maleic acid reveals114 (by X-ray diffraction analyses of single crystals) the presence of two intermolecular hydrogen bonds between the NH3C group and the anion.

The protonated form of 22 shows the proton located in a cavity formed by four methyl groups. This proton exchanges slowly on the NMR time scale. The presence of proton in this cavity explains the dramatic high basicity of 22 (pKa D 12.1) with respect to 21 (pKa D 4.61)115.

Nitrogen anions, such as amide anions, may be stabilized by hydrogen bonding in a similar way to that of oxyanions116. In fact, 1,8-diaminonaphthalene is more acid (KC DMSYL/DMSO) than 1,5-diaminonaphthalene because of the stabilization of the anion by an intramolecular hydrogen bond as shown in 27117.

H H H

N N

−K+

(27)

Generally, relative populations of conformers are strongly affected by the presence of internal hydrogen bonding interactions. Gauche forms are more populated forms than anti forms of conformational conformers of substituted ethanes when internal hydrogen bonding (between vicinal groups) intervenes to stabilize the gauche form with respect to the anti form. Electron-diffraction and ab initio investigations of the conformational composition of ethylenediamine show that the two gauche conformations are the more populated118. This geometrical situation implies an internal hydrogen bond which is a relevant parameter to rotamer stability in the vapour phase.

A particular effect of an intramolecular hydrogen bond occurs in N-salicylideneaniline derivatives119 and anils of hydroxynaphthaldehydes120, and their tautomerism (shown

10. Hydrogen bonding and complex formation

437

in equilibrium 11) is clearly affected by the intramolecular hydrogen bonding. The position of the hydrogen (the NÐ Ð ÐH O hydrogen bond is short) is sensitive to the substituent variation121 on both rings, but substituent changes in the salicylidene moiety affect hydrogen bonding more than variations in the amine moiety. The differences in the proton transfer behaviour of derivatives of the system indicated by equilibrium 11 may be ascribed to intermolecular charge transfer interactions122. The intramolecular hydrogen bond O HÐ Ð ÐN (and the tautomeric equilibrium 28 29 in equation 12) is sensitive to the presence of charge transfer complexes123 as observed in the case of some (solid) complexes between N-(2-hydroxy-1-naphthylmethylene)-1-pyreneamine and 7,7,8,8-tetracyanoquinodimethane derivatives 30.

OH	O H
N	N

(11)

OH	O H
N	N

(12)

(28)

(29)

The process of rotation around the exocyclic CDC double bond in 2- aminomethylenedimedone 31 (investigated by the dynamic 1H NMR method) is affected by intramolecular hydrogen bonds and by the electronic effects of substituents on the nitrogen atom124. When R D H, the O HÐ Ð ÐN hydrogen bond prevents the rotation around the CDC double bond.

Me Me

R = H; R1 = H, Me, Ph

	O	O
(CN)2 C	C(CN)2	N H
	R	R1
	(30)	(31)

438	Luciano Forlani

The internal hydrogen bond (N H N) is responsible for suppression of cis trans isomerization in the singlet excited state of cis-1-(2-indolyl)-2-(2-pyridyl)ethene (32) of reaction 13. Clearly, the reverse isomerization is possible125. On the contrary, irradiation of cis-1-(2-pyrrolyl)-2-(2-quinolyl)ethene126 (35) induces the isomerization to 34. The isomerization (cis trans) is possible because the excitation allows the tautomerization of 35 to 36 (equation 14).

		N
N	hν
H	X	H
N	hν	N	(13)

(32)

(33)

		N	N
N	H		H
	hν		hν
		H
	N
		N	N
	hν
(34)		(35)	(36)

(14)

Strong intramolecular hydrogen bonds are observed in 2-fluorobenzamide127 (37). 15N, 19F, 1H spin spin couplings between fluorine (bonded to aromatic ring) and the nitrogen and the carbon of the amide group of 2-fluorobenzamide (37) and 2-fluoro-N- methylbenzamide (38) indicate that the spin information (F N) is transmitted mainly through the hydrogen bond. A typical internal hydrogen bond occurs between an ortho nitro group and an amino (or hydroxy) group as shown128 in 39.

	... H	R
F	..
	N
	C	R = H,	(37)
		O
		R = CH3	(38)

10. Hydrogen bonding and complex formation

439

The geometry of the nitro group gives a simple and generally accepted explanation of the supposed loss of electron-withdrawing effects by resonance in the nitro group when it is twisted from the plane of the phenyl ring: steric hindrance129 of groups in ortho positions to the nitro groups exerts steric hindrance to the resonance. In fact, in situations like 39, the presence of internal hydrogen bonding may also play a role in hindrance to mesomeric interactions between the nitro group and the phenyl ring.

R		H
O	N	O −
+ N		N +
− O		O

(39)

The idea of ‘steric hindrance to resonance’ starts from the principle that the mesomeric electron-withdrawing effect is larger when the involved orbitals present parallel directions than when they are twisted. Section II.E.4 reports a different point of view on the mechanism of the mesomeric electron-withdrawing effect of substituents.

Nitroanilines were extensively investigated by infrared, Raman, UV/VIS and 1H NMR spectroscopic methods. In some cases the comparison with similar spectroscopic data of the corresponding N,N-dimethylanilines provides simple and consistent conclusions involving geometrical properties of molecules caused by intramolecular hydrogen bonding.

Similar conclusions are obtained from AM1 calculations on dimers of nitroanilines, which are of interest in investigating the intermolecular forces to orientate the individual molecules in crystal chains. Dimerization in solid crystals may be considered responsible for the differences in molecular geometry between solid and gas phases130.

E. Electron Donor Acceptor Complexes

1. Introduction

According to the valence bond theory, if a total charge-transfer occurs between neutral starting partners, all acceptor molecules will have a negative charge and all donor molecules a positive charge131.

Aliphatic amines are simple lone-pair n-donors, similarly to other n-donors like phosphines, ethers, alcohols, iodides etc. Aromatic amines may be both n- and -donors132. Typical -donors are polycyclic aromatic derivatives; -acceptors are polynitro aromatic derivatives, quinones etc.

Weak nucleophile electrophile interactions (and the donor acceptor complexes) are considered precursors in aromatic electrophilic substitutions133 and in additions of electrophiles to CDC double bond of olefins: the first step (the addition of the electrophile to an electron-rich substrate) is probably the same for both reactions.

Some complications arise from the presence of proton donor acceptor interactions134 when the donor is a protic amine. The separate evaluation of the two kinds of interactions may be a difficult problem. Similarly, if the electron acceptor is also a proton donor, the overlapping of salification and complexation processes makes the separate investigation of the interactions very difficult. This is the case in the complexes between amines and picric acid or other related phenols. For complexes of 2,4,6-trinitro-3-hydroxypyridine135 and

440	Luciano Forlani

of some dinitro-2-hydroxypyridines136 the main interaction is an intermolecular hydrogen bond, with some possibility of ion pair formation.

Complexes between amines and simple phenols (in variable stoichiometric ratios) were also isolated55. Solid stable complexes between picric acid and N-alkyl- N,N-dialkyl- amides show variable amide picric acid ratios137. Probably in the solid state these complexes originate from hydrogen bonding interactions, while in solutions charge transfer (and proton transfer) complexes are involved. The major hydrogen bonding interaction between the CDO group of amides and the OH group of phenols is shown in 40.

		O2 N
	R
	C O ..... H O	NO2
	N
R	R	O2 N
	(40)

2. Intermolecular electron donor acceptor complexes

In principle, the behaviour of any molecular species in forming donor acceptor complexes depends on its ionization potential, electron affinity and polarizability. However, the donor (or acceptor) ability of a substance depends strongly on the requirements and properties of its partners. The same compound may act as a donor towards strong acceptor compounds or as an acceptor towards donor compounds. This is the case of the-amphoteric p-tricyanovinyl-N,N-dimethylaniline (41) which is a donor towards 2,4,7- trinitrofluorenone and an acceptor towards N,N-dimethylaniline138.

Charge transfer complexes between amines and discharged substances were investigated extensively (mainly by spectroscopic methods). The choice of more simple models excludes the presence of proton donor acceptor interactions which complicate the investigations of other interactions by overlapping different interactions.

Interactions between aliphatic amines (n-donors) and benzonitrile139 or dicyanobenzenes140,141 ( -acceptors), in n-hexane, are mainly electron donor acceptor interactions. It is reasonable to assume that the lone-pair of the donor is perpendicular to the plane of the acceptor as reported in 42 for 1,4-dicyanobenzene.

		N
(CN)2 C C	N(CH3 )2 NC	CN
CN
	(41)	(42)

The long known132 electron donor acceptor complexes between tertiary amines and carbon tetrahalides are simple systems. Thus, 1,4-diaza[2,2,2]bicyclooctane (DABCO) or quinuclidine afford solid complexes with carbon tetrabromide142.

UV/VIS spectroscopic analysis, as well as IR and 1H and 13C NMR in apolar solvents (iso-octane, chloroform) agrees with a stoichiometric ratio DABCO/CBr4 1:1 K D

10. Hydrogen bonding and complex formation

441

3.2 mol 1 dm3 for the equilibrium in equation 15:

DABCO C CBr4	K	DABCOÐCBr4	15

X-ray crystal analysis indicates that the DABCO/CBr4 complex consists of alternating planes of the diamine and carbon tetrabromide in which each acceptor is bound to two donor units. The quinuclidine/CBr4 complex consists of pairs of donor acceptor systems in which every quinuclidine molecule is bound to only a single molecule of carbon tetrabromide.

N,N-Dimethylaniline too is a useful electron-donor partner which interacts with electron-accepting substrates132 such as acyl halides and sulphonyl halides143.

The formation of charge transfer complexes between N,N-dimethylaniline or N,N- diethylaniline and C60 (43) or C70144 are recent subjects of spectroscopic studies, also in view of their potential optical and electronic applications. Even if the spectroscopic properties of C60, C70 are complicated by the presence of aggregates in room temperature solutions, the emissions from the excited state charge transfer complexes between fullerenes and N,N-dialkylanilines are strongly solvent-dependent: exciplet emissions are observed in hexane, but in toluene they are absent145.

(43)

Polynitro-fluorenones 44 and 9-dicyanomethylenenitrofluorenes 45 (as acceptors) form molecular complexes with substituted anilines in various solvents146. The constant of

R2

R1		R3
	X
X = O (44)	X = C(CN)2	(45)
R1, R2 , R3 = H, NO2

442 Luciano Forlani

stability of the complex between 45 (when R1 D R2 D R3 D NO2) and N,N-dimethyl aniline is increased on increasing the polarity of solvents. This fact may be explained by the polar character of the ground state of the considered complexes.

On the basis of X-ray crystallographic data (and of MNDO, MINDO/3 calculations), an interesting charge-transfer interaction is shown to be responsible for colour in solid (4-nitrophenacyl) anilines 46 and related compounds. Crystal packing of this compound reveals an alternating ‘head-to-tail’ arrangement: the aniline moiety is the electron donor part and the nitrophenacyl moiety is the acceptor147. The possible other tautomeric form of 46 cannot be responsible for the observed colours.

R

NHCH2 C

NO2

O

(46)

3. Complexes between amines and halogens

Complexes between halogen molecules and nitrogen atoms of amines have been investigated by several methods because of their importance as stable sources of active halogen. Charge-transfer complexes with 1:1 amine/halogen ratio have been found to possess a linear arrangement such as that shown in 47 for the cationic polymer of 1,4- diaza[2,2,2]bicyclooctane (DABCO) bromine complex148, where Br3 is the counter ion. Also 48 is a reasonable structure of DABCO/2Br2 complex149. The complex between quinuclidine and bromine150 has the structure 49 as determined by single-crystal X ray diffraction, in agreement with structure 47.

+	+	+		Br Br.....N	N.....Br Br
Br N	N Br N	N Br
	(47)			(48)
		+			−
		N Br	N	BF4

(49)

The electron donor acceptor complex trimethylamine/dibromine shows a structure151 (by electron diffraction in the gas phase) with an N Br Br angle of 112°. On the contrary, the structures of solid152 (CH3N)3/I2 [or (CH3N)3/ICl153] show linear N I I (or N I Cl) axes.

Complexes between tertiary aliphatic amines (triethylamine, tribenzylamine) and bromine are of interest also for their high reactivity. The Et3N/Br2 complex cannot be isolated because of the very fast oxidation reduction process of equation 16.

+ +

C2H5 3N Br2 C C2H5 3N ! (C2H5)2 NDCHCH3Br C C2H5 3 N HBr (16)