nitrosonium complexes of triptycene

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 9, pp. 1344–1348. © Pleiades Publishing, Ltd., 2009. Original Russian Text © G.I. Borodkin, I.R. Elanov, R.V. Andreev, V.G. Shubin, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 9, pp. 1360–1363.

1344

Nitrosonium Complexes of Triptycene*

G. I. Borodkin, I. R. Elanov, R. V. Andreev, and V. G. Shubin

Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences, pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia

e-mail: [email protected]

Received October 17, 2008

Abstract—According to the 1H and 13C NMR data, triptycene reacts with nitrosonium tetrachloroaluminate to form positively charged π-complexes with one and two nitrosonium ions, which are rapidly (on the NMR time scale) converted into each other. DFT quantum-chemical calculations with Λ02 basis set indicated higher stability of singly charged complexes relative to doubly charged. Addition of three nitrosonium ions to triptycene molecule is unfavorable from the energy viewpoint.

* For preliminary communication, see [1].

Up to now, the structure and reactivity of numerous complexes formed by aromatic compounds and nitro-sonium cation (NO+) have been studied [1–13]. How-ever, published data on nitrosonium complexes with bridged aromatic compounds are very limited, though such complexes attract interest as molecular sensors (absorbents) for nitrosonium ion [12, 13] whose reduc-

tion gives nitrogen(II) oxide, a biologically important molecule [2, 14]. The goal of the present work was to study the structure and dynamics of triptycene com-plexes with nitrosonium ion by theoretical and experi-mental methods. Triptycene complexes are specific due to the possibility for binding of nitrosonium ion by two aromatic rings simultaneously (cf. [5, 12, 13]), as

DOI: 10.1134/S1070428009090061

I

NO+

–NO+

II

NO+

II'NO+

II''

NO+

NO+–NO+

III

NO+NO+ NO+

–NO+

IV

NO+NO+

NO+

1

2

344a5

1010a6

78 8a

11

16

15

14

13

12

9 9a

Scheme 1.

NITROSONIUM COMPLEXES OF TRIPTYCENE

RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 9 2009

1345

Table 1. 1H and 13C NMR spectra of triptycene (I) and its nitrosonium complexes II and III in SO2–CD2Cl2 at –70°C

Comp. no.

Ratio ArH–NO+

AlCl4–

1H, δ, ppm 13C, δC, ppm ΔΣδC qπ+ a

9-H, 10-H 1-H–8-H, 13-H–16-H C9, C10 C1–C8, C13–C16 C4a, C8a, C9a, C10a, C11, C12

Ib – 5.59 7.44, 6.98 52.18 125.20, 124.29 145.33 – – II, III 1 : 1 5.86 7.71, 7.26 51.94 128.62, 127.88 145.33 42.06 0.26 II, III 1 : 2 6.00 7.84, 7.39 51.84 130.05, 129.31 146.00 63.24 0.40 II, III 1 : 3 6.10 7.93, 7.48 51.79 130.91, 130.14 146.72 77.70 0.49 II, III 1 : 4 6.14 7.98, 7.51 51.78 131.34, 130.55 147.07 84.84 0.53 II, III 1 : 5 6.16 7.99, 7.54 51.78 131.58, 130.76 147.25 88.62 0.55 II, III 1 : 6 6.18 8.00, 7.55 51.73 131.73, 130.91 147.37 91.14 0.57

a Calculated assuming 160 ppm per unit π-charge [7, 15]. b Cf. [16].

Fig. 1. Plots of differences in the 13C chemical shifts versus [NO+

AlCl4–]/[I] ratio in the formation of triptycene (I) com-

plexes with nitrosonium ion.

7

6

5

4

3

2

1

0

–1 0 1 2 3 4 5 6

ΔδC, p

pm

[NO+ AlCl4

–]/[I]

C4a, C8a, C9a, C10a, C11, C12

C9, C10

C1, C4, C5, C8, C13, C16

C2, C3, C6, C7, C14, C15

well as for migration of NO+ from one benzene ring to another (cf. [7, 9]).

Triptycene reacted with nitrosonium tetrachloro-aluminate in SO2–CD2Cl2 at –60°C to give singly and doubly charged nitrosonium complexes, presumably via π-coordination. These complexes are involved in fast (on the NMR time scale) interconversions as shown in Scheme 1 (cf. [9]). Addition of an equimolar amount of NO+

AlCl4– to a solution of triptycene (I)

leads to fairly small downfield shift of the averaged signals from aromatic protons and carbon nuclei in the 1H and 13C NMR spectra (Table. 1, Fig. 1; cf. [6–9]). It is known that 13C chemical shifts of charged π-electron systems show a correlation with π-electron densities on the corresponding atoms [15]. The calculated total

positive π-charges on aromatic carbon atoms in π-com-plexes II and III (Table 1) are fairly similar to those found for nitrosonium π-complexes with naphthalene, methylnaphthalenes, and [2.2]paracyclophane (qπ

+ = 0.34–0.64 [7, 9] but are considerably smaller than in arenium ions (qπ+ ≈ 1) [15]. Raising the NO+

AlCl4–-to-I

molar ratio from 1 to 6 induces further downfield shift of the aromatic proton and carbon signals. Presumably, this is the result of equilibrium formation of doubly charged 2 : 1 π-complex III (Scheme 1, Fig. 1; cf. [9]).

Quantum-chemical calculations in terms of the density functional theory (DFT) using Λ02 basis set [17–20] indicated higher thermodynamic stability of the π-complexes as compared to σ-complexes. Singly charged π-complex A1 was localized as a minimum on the potential energy surface; the distance from the nitrogen atom to the nearest carbon atoms C4a and C11 in neighboring benzene rings was 2.42 Å, and the distance to remote carbon atoms in the same rings was 2.7–3.8 Å (Table 2, Fig. 2). The NO group in A1 is oriented at an angle of 52° with respect to the normals drawn to the planes of the nearest aromatic rings (Fig. 2). The structure of complex A1 is analogous to the structures of π-complexes formed by NO+ with hexamethylbenzene [21, 22] and other arenes [2, 5, 13] (Table 2), which were determined by X-ray analysis. Such complexes are characterized by angular orienta-tion of the NO group to the normal drawn to the benzene ring, sp2-hybridization of carbon atoms in the aromatic ring, and appreciably extended C–N distances between the nitrogen atom in the NO group and nearest carbon atoms of the aromatic ring (2.0–2.5 Å).

Doubly charged π-complex A2 was also localized as a minimum on the potential energy surface by


BORODKIN et al. 1346

Table 2. Calculated (DFT/Λ02) energies of formation (E), affinities of NO+ for triptycene (ANO+), charges on the NO group (qNO+), and some geometric parameters of nitrosonium complexes

Complex E,a a.u. ANO+, kJ/mol d(N–O), Å d(Ci–N),b Å d,c Å φ,d deg qNO+ e

A1 0–899.43797 291.8 1.126 2.419 (C4a), 2.420 (C11) 2.399 52 0.34 A2 –1028.90427 293.1 1.118 2.483 (C4a), 2.513 (C11) 2.447, 2.479 57 0.42 1.116 2.424 (C6), 2.393 (C7) 2.202 36 0.43 NO+

ArHf – – 1.09 ± 0.02 2.0–2.5 2.12 ± 0.07 36 ± 15 – a 1 a.u. = 2625.5 kJ/mol. b Distances between the nitrogen atoms and the nearest carbon atoms. c Distance between the nitrogen atom and aromatic ring plane. d Angle between the N–O bond and normal to the aromatic ring plane. e Mulliken charges. f Averaged X-ray diffraction data for π-complexes formed by NO+ with toluene, o-xylene, p-xylene, mesitylene, durene, pentamethylben- zene, and hexamethylbenzene [10].

Fig. 2. Optimized (DFT) structures of 1 : 1 and 2 : 1 complexes of nitrosonium ion with triptycene (I).

NN

N

O

O

O52

52

36

57

57

O

O

O

O

O

17

O

A1 A2

DFT/Λ02 calculations (Fig. 2). The formation of singly charged π-complex A1 from triptycene (I) and NO+ is a strongly exothermic process, while the addi-tion of the second NO+ cation to A1 with formation of complex A2 gives a very small energy gain (Table 2). The geometric parameters of doubly charged complex A2 are similar to those of A1 (Fig. 2, Table 2). One NO group in A2 is oriented in the same mode as in A1, while the other NO group is “attached” to the C6–C7 bond. As might be expected, each NO group in doubly charged π-complex A2 bear a larger positive charge than in singly charged π-complex. Our attempt to lo-calize triply charged π-complex of triptycene by addi-tion of NO+ to A2 was unsuccessful; such complex turned out to be unstable, and it readily lost the extra nitrosonium ion.

It is interesting that the NO group in triptycene nitrosonium complexes is capable of migrating from one aromatic ring to another; this migration is likely to follow intermolecular mechanism (Scheme 1) (cf. [9]). Intramolecular migration of the NO group is hardly probable; in this case the NO group should pass through the nodal plane of π-orbitals of the benzene ring. As noted above, the 13C NMR spectrum of II even at –70°C displays averaged signals, indicating that migration of the NO group is fast on the NMR time scale. The rearrangement cannot be “frozen” even at –100°C (SO2–SO2ClF–CD2Cl2).

Thus triptycene is capable of forming dynamic π-complexes with nitrosonium ion, and ring-to-ring migration of the NO group in these complexes is fast even at very low temperature.

NITROSONIUM COMPLEXES OF TRIPTYCENE


1347

EXPERIMENTAL

The 1H and 13C NMR spectra were recorded on a Bruker AC-200 spectrometer at 200 and 50.3 MHz, respectively, using the residual proton and carbon signals of deuterated methylene chloride as references (CHDCl2, δ 5.33 ppm; CD2Cl2; δC 53.6 ppm). Deuter-ated methylene chloride containing 99 mol % of deuterium was distilled over phosphoric anhydride. Triptycene was synthesized according to the procedure reported in [23]; SO2 [24], SO2FCl [25, and NO+

AlCl4–

[26] were prepared by known methods. Solution of nitrosonium complexes II and III for

recording 1H and 13C NMR spectra were prepared by adding nitrosonium tetrachloroaluminate to a solution of triptycene in SO2–CD2Cl2, cooled to –60°C (c = 0.1 M); the SO2–CD2Cl2 volume ratio was 2.5 : 1. Portions of NO+

AlCl4– were then added to the solu-

tion, and its NMR spectra were recorded. The molar ratios NO+

AlCl4––triptycene are given in Table 1. The

NMR spectra of complexes II and III at –100°C were recorded in the system SO2–SO2ClF–CD2Cl2 (1.5 : 2 : 1 by volume).

Quantum-chemical calculations (DFT, PBE func-tional) were performed using PRIRODA software package [18–20]. The geometric parameters were opti-mized using Λ02 basis set {(12s8p4d2f)/[4s3p2d1f] for C, O, and N atoms and (8s4p2d)/[3s2p1d] for hydro-gen atoms}. Critical points on the potential energy sur-face were identified by calculating Hesse matrix [27].

This study was performed under financial support by the Russian Foundation for Basic Research (project no. 06-03-32 406) and by the Chemistry and Material Science Department of the Russian Academy of Sciences (program no. 5.1.9).

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nitrosonium complexes of triptycene

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