Interaction of meso-tetraarylporphyrins with formic acid: A variable temperature 1H NMR study

    Interaction of meso-tetraarylporphyrins with formic acid: A variable temperature 1 H NMR study Saeed Zakavi, Mahshad Najafi Ragheb PII: DOI: Reference:

S1387-7003(13)00325-0 doi: 10.1016/j.inoche.2013.08.014 INOCHE 5188

To appear in:

Inorganic Chemistry Communications

Received date: Accepted date:

22 May 2013 22 August 2013

Please cite this article as: Saeed Zakavi, Mahshad Najafi Ragheb, Interaction of mesotetraarylporphyrins with formic acid: A variable temperature 1 H NMR study, Inorganic Chemistry Communications (2013), doi: 10.1016/j.inoche.2013.08.014

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ACCEPTED MANUSCRIPT Interaction of meso-tetraarylporphyrins with formic acid: A variable temperature 1H NMR study

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Saeed Zakavi,* Mahshad Najafi ragheb

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Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan

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45137-66731, Iran

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* Corresponding author. Tel.: +98 241 4153202; Fax: +98 241 4153232. E-mail address: [email protected] (S. Zakavi).

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Abstract

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The dications of meso-tetraphenylporphyrin, H2TPP, meso-tetra(4-Cl)phenylporphyrin, H2T(4-

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Cl)PP and meso-tetra(4-OCH3phenyl)porphyrin, H2T(4-OCH3)PP with formic acid as a weak

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carboxylic acid have been studied by UV-vis and variable temperature 1H NMR spectroscopies. As would be expected in the presence of a strongly basic counter anion, the formation of very strong N-H-O hydrogen bonds between the diprotonated species and the formate anion leads to considerable broadening of the NH resonance of H2T(4-OCH3)PP(HCOOH)2, H2T(4Cl)PP(HCOOH)2 and H2TPP(HCOOH)2, so that the signal cannot be observed at or close to room temperatures, but appears at 0 °C for H2T(4-OCH3)PP(HCOOH)2 and at -30 °C for the latter dications. While the signal of the  protons of H2TPP(HCOOH)2 moves continuously upfield from 20 to -60 °C, the corresponding signal of H2T(4-OCH3)PP(HCOOH)2 and H2T(4Cl)PP(HCOOH)2 moves upfield at higher temperatures and downfield at lower temperatures, corresponding to a decrease followed by an increase in the porphyrin ring current. Keywords: Variable temperature

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H NMR; porphyrin diacids; formic acid; meso-

tetraarylporphyrins; hydrogen bond. 1

ACCEPTED MANUSCRIPT Out-of-plane deformations of porphyrins and their effects on spectral and chemical properties of the aromatic macrocycle and the metal complexes including oxidation potentials, basicity of the

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inner nitrogen atoms, and axial ligand binding affinity, have been the subjects of extensive studies over the past two decades. The biological function of porphyrin cofactors in proteins can

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be influenced by these modified chemical characteristics [1-4]. Substitution of the meso and/or  positions with either many moderately bulky groups or a few very bulky substituents is the most

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common strategy employed to induce nonplanar deformations of the porphyrin core [1]. Adduct formation between porphyrins and molecular σ and/ or π acids is accompanied with nonplanar

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distortions of the porphyrin macrocycle [5-11] and therefore may be used for this purpose. The pyrrolenine nitrogen atoms of porphyrins bearing lone pairs of electrons (pKb ~ 9) can be

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protonated with strong acids such as trifluoroacetic acid, HClO4, HCl and HBF4 [8-17]. Although spectrophotometric titration of H2tpp with CF3COOH in nitrobenzene has given an intermediate

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spectrum attributed to monoprotonated moiety, H3TPP+, there has been no evidence for a monoprotonated intermediate between the free base and diprotonated porphyrin in chloroform [18]. The optimization of direct or water-mediated hydrogen bond interactions between the counter anions and porphyrin diprotonated species, has been shown to be a crucial factor determining the actual degree of saddling of the porphyrin core of the diacids of mesotetraphenylporphyrins with HX (X = F, Cl, Br, I) [14] and HBF4 [17]. Although the intrinsic basicity of porphyrins as well as the strength of acids are important factors in the adduct formation of porphyrins with protic acids, hydrogen bond formation between the diprotonated porphyrins and the counter ions is a more determining factor influencing the stability of dications [19]. We have previously reported the interaction of porphyrins with weak and strong carboxylic acids such as acetic-, formic- and propionic acid studied by UV-vis spectroscopy and cyclic

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ACCEPTED MANUSCRIPT voltammetry [19-21]. In the present work, variable temperature 1H NMR spectroscopy has been used to study the interaction of meso-tetraarylporphyrins with formic acid as a weak carboxylic

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acid. To our knowledge, it is the first NMR study of porphyrin dications with a weak protic acid.

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Meso-tetraphenylporphyrin, H2TPP, meso-tetra(4-Cl)phenylporphyrin, H2T(4-Cl)PP and

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meso-tetra(4-OCH3phenyl)porphyrin, H2T(4-OCH3)PP were prepared and purified according to the literature methods [22,23]. Due to the absence of any bulky substituents at the ortho positions

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of the phenyl groups, the free base porphyrins may be considered to possess an essentially planar

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conformation in solution [10,24]. It should be noted that the ortho-positions are the most effective ones that affect the conformation of free base porphyrins in solution and the solid state

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[10].

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As in the case of strong acids [6,13-17,19], shifts of the absorption bands to higher

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wavelengths were observed upon interaction of meso-tetraarylporphyrins with formic acid (Table 1). UV-vis or 1H NMR titration has been usually used to confirm the 1:2 adduct formation between different porphyrins and strong acids. However, due to the equilibrium between the free base porphyrin and the protonated species [20,21] and the necessity of using excess amounts of acid to complete the reaction, the methods cannot be used to confirm or rule out the possibility of involvement of a monoprotonated species as an intermediate upon protonation of porphyrins with formic acid. As we have previously reported [19-21], successive addition of formic acid led to the disappearance of the bands of the free base porphyrin with no observable change in the position of the bands due to the protonated species (Fig. S1). Accordingly, the formation of a monoprotonated intermediate cannot be excluded in this study. On the other hand, the low temperature 1H NMR spectra demonstrate the exclusive formation of diprotonated species in the presence of excess amounts of formic acid (vide infra). Diprotonation of the porphyrins led to the 3

ACCEPTED MANUSCRIPT red shift of the Soret and Q(0,0) bands (Table 1). Shift of the Soret band of porphyrin dications to higher wavelengths is mainly due the out-of-plane deformation of porphyrin core [15,19-21].

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The greater shift of the Soret band of H2T(4-OMe)PP(HCOOH)2 relative to H2T(4-

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Cl)PP(HCOOH)2 and H2TPP(HCOOH)2 suggests a greater degree of distortion of the porphyrin core of the former with respect to the latter. Also, the shift of the Q(0,0) bands may be

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considered as a criterion for the π electron donor ability of the meso substituents [16], decreases

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in the order H2T(4-OMe)PP(HCOOH)2 >> H2T(4-Cl)PP(HCOOH)2 > H2TPP(HCOOH)2.

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Table 1.

H2TPP(HCOOH)2: 1H NMR spectra of H2TPP(HCOOH)2 in the range of -60 to 20 °C are shown

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in Fig. 1 and Fig. 2. Also, the spectral data are summarized in Table 2. There is no signal due to

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the NH protons down to -30 °C, probably due to the severe exchange broadening of the NH

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resonance; the formation of strong intermolecular hydrogen bonds between the protonated porphyrin core and the counter anion was revealed by x-ray crystallographic studies of porphyrin dications [8-11,17]. It is observed that in contrast to the dication of meso-tetraalkyl- and mesotetraarylporphyrins with trifluoroacetic acid [11,12,15], the NH resonance of the dication of porphyrins with H2C2O4 as a weak acid could not be observed at room temperature and low temperatures higher than ca. -30 °C. This observation is probably due to the formation of stronger hydrogen bonds between the formate anion compared to trifluoroacetate one; the carboxylate anions with electron-withdrawing groups such as trifluoroacetate form weaker hydrogen bonds in comparison with those with electron-donating substituents [25]. The broad NH resonance at -30 °C becomes sharper at -45 °C. At -60 °C, the signal is quite sharp and has an integral of four protons, consistence with the formation of a diprotonated species.

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ACCEPTED MANUSCRIPT Figure 1.

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Figure 2.

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Table 2.

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The rather broad singlet resonance of formic acid (-HC=O proton) at  7.85 (Fig. 2, 20 °C) gradually shifts to upfield and becomes broader with lowering temperature from 20 to -60 °C.

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Extensive line broadening leads the disappearance of the resonance at -60 °C. As was motioned above, the 1H NMR spectra have been recorded in the presence of excess amounts of formic

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acid. The 1H NMR spectrum of formic acid in CDCl3 at room temperature shows two resonances at δ 8.070 and 10.400 for the –CH and –OH protons, respectively. Also, x-ray crystallographic

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studies on the dication of meso-tetraalkylporphyrins with trifluoroacetic acid prepared in the presence of excess amounts of the acid showed that each of the trifluoroacetates of the dications

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in turn is hydrogen bonded to a trifluoroacetic acid molecule of solvation [11]. The lack of –OH protons of the free formic acid molecules in the 1H NMR spectra of H2TPP(HCOOH)2 confirms that the –OH groups are involved in hydrogen bonds. The variable temperature 1H NMR spectrum of formic acid shows no resonances in this region (see Fig. S2). Accordingly, the signal at  7.85 ppm may be assigned to the –C-H protons of formic acid molecules attached to the dications. The solubility of formic acid in chloroform as a non-polar solvent is expected to decrease gradually with lowering temperature from 20 to -60 °C. This, in turn, would increase the degree of aggregation of formic acid molecules and leads to the formation of strong intermolecular –C-H---O hydrogen bonds as well as the O-H---O ones and causes severe broadening of the signal.

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ACCEPTED MANUSCRIPT The H resonance shows upfield shift upon lowering temperature (Table 2 and Fig. 1 and Fig. 2). The upfield shift of  protons is usually attributed to the decrease in the porphyrin ring

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current as a result of the out-of-plane deformation of porphyrin core [4,5,12]. Accordingly, the

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increased upfield shift of the  protons from 20 to -60 °C may reveal the enhanced degree of

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saddling of the porphyrin core of H2TPP(HCOOH)2. On the other hand, saddling of porphyrin core is accompanied with enhanced coplanarity between the porphyrin ring system average plane

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and meso-phenyl groups. Therefore, the observed downfield shift of the protons of phenyl groups

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with lowering temperature may be a consequence of increased electron-donating from the phenyl substituents to the porphyrin core of H2TPP(HCOOH)2.

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H2T(4-Cl)PP(HCOOH)2: With the exception of the  protons, the pattern of shift of different

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protons (Table S1) is similar to that observed in the case of H2TPP(HCOOH)2. While the

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resonances of β protons of H2TPP(HCOOH)2 move continuously upfield upon decreasing temperature from 20 to -60 °C, in the case of H2T(4-Cl)PP(HCOOH)2 the β protons move upfield from 20 to -15 °C and then downfield from -15 to -60 °C. Also, the observation of a sharp NH signal with an integral of four protons at low temperatures is consistence with the exclusive formation of a diprotonated species. H2T(4-OCH3)PP(HCOOH)2: According to the data of Table S2, again the protons of aryl groups move continuously downfield from 20 to -60 °C. As was observed for H2T(4-Cl)PP(HCOOH)2, the β protons move upfield from 20 to 0 °C and shift downfield from 0 to -60 °C, so that the total result of lowering temperature from 20 to -60 °C is the downfield shift of the β protons. Here, the NH resonance may be observed at 0 °C. Due to the presence of strong electron donating substituents at the para positions of H2T(4-OCH3)PP, its basicity is greater than that of the other

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ACCEPTED MANUSCRIPT porphyrins [16]. Accordingly, there will be less positive charge on the N-H protons of H2T(4OCH3)PP (HCOOH)2 compared to the other dications which in turn leads to weaker hydrogen

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bond donor ability of the corresponding N-H bonds. In other words, the weaker N-H-O hydrogen

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bonds between the counter anions and the diprotonated moiety compared to the other dications result in the less broadening of the NH signal even at temperatures close to room temperature,

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i.e. 0 °C. Again, the observed integral of four protons confirms the formation of a diprotonated

formation

of

H2T(4-OCH3)PP(HCOOH)2,

H2T(4-Cl)PP(HCOOH)2

and

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The

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species.

H2TPP(HCOOH)2 upon protonation of the corresponding free base porphyrins with excess

H NMR spectroscopy in the range -60 to 20 °C. The NH signal of the dications was observed

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amounts of formic acid (beyond 1:2 molar ratio) in CDCl3 was evident by variable temperature

only at temperatures lower than 0 °C, which is in contrast to the dications of porphyrins with

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trifluoroacetic acid. On lowering the temperature, upfield shift of the β protons of H2T(4OCH3)PP(HCOOH)2 and H2T(4-Cl)PP(HCOOH)2 followed by the downfield shift of the signal was observed. It should be noted that while the out-of-plane deformation of porphyrin core has been shown to decrease the porphyrin ring current [26], the enhanced coplanarity of the meso aryl substituents at lower temperatures and the consequent better π electron donation from the meso aryl substituents to the porphyrin core [14,16,27,28] seems to compensate the loss of aromaticity of the macrocycle to some extent and result in the downfield shift of the signal due to the β protons relative to that at higher temperatures. The presence of equilibrium between the free base porphyrin and the protonated species necessitates the use of excess amounts of acid to complete the reaction and consequently spectrophotometric or NMR titration method cannot be used to determine the molar ratio of porphyrin to acid. However, the observation of a NH signal 7

ACCEPTED MANUSCRIPT with an integral of four protons at lower temperatures is consistent with the formation of diprotonated species. In the case of H2TPP(HCOOH)2, the β protons move continuously upfield

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in the temperature range from -60 to 20 °C. On the other hand, the signal due to the protons of

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aryl groups shift downfield in the temperature range. In the UV-vis spectra, shift of the Soret and Q(0,0) bands to longer wavelengths was observed for all the dications, although shift of the band

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of H2T(4-OCH3)PP(HCOOH)2 is significantly greater than that of the other dications.

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Acknowledgements

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Financial Support of this work by the Institute for Advanced Studies in Basic Sciences (IASBS) is acknowledged.

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Supplementary material

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Figure S1 shows the spectral changes upon addition of formic acid to a ca. 10-6 M

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solution of H2TPP in dichloromethane. Figure S2 demonstrates the variable temperature 1H NMR spectrum of formic acid in CDCl3 in the range -60 to 20 °C. Tables S1 and S2 summarize the chemical shifts of the protons of H2T(4-Cl)PP(HCOOH) and H2T(4-OMe)PP(HCOOH)2 in the temperature range -60 to 20 °C in CDCl3, respectively. References

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ACCEPTED MANUSCRIPT [21] S. Zakavi, M.N. Ragheb, M. Rafiee, Inorg. Chem. Commun. 22 (2012) 48-53. [22] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Goldmacher, J. Assour, L. Korsakoff, J. Org.

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Figure 1. Effect of temperature on the chemical shifts of the protons of H2TPP(HCOOH)2 in CDCl3; a, (b, c) and d show CHCl3, the solvent impurities and TMS, respectively.

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The signal at the signal.

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Figure 2. 1HNMR spectra of H2TPP(HCOOH)2 in the region

7.6 to 8.9 ppm in CDCI3.

7.85 ppm has been assigned to formic acid by using the integral and broadness of

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Q(0,0) band

 (nm)

 (nm)

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H2TPP(HCOOH)2

647

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 (cm-1)b

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OMe)PP(HCOOH)2

-304

452

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H2T(4-

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H2T(4-OMe)PP

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-1299

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H2TPP

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with HCOOH

Soret band

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Porphyrins and the dications

-1742

H2T(4-Cl)PP

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 (cm-1)

H2T(4-

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Table 1. UV-vis spectral data of the meso-tetraarylporphyrins and their dications with formic acid a

-852

421

651

444

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Cl)PP(HCOOH)2

-1230

-480

 (cm-1)

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The spectra of dications were prepared using excess amounts of HCOOH.

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b

 (cm-1) = 107(1/2-1/1)

ACCEPTED MANUSCRIPT Table 2. Chemical shifts of the protons of H2TPP(HCOOH)2 in the temperature range -60 to 20 °C in CDCl3.a,b δ (NH)

δ (Hm,p)

δ (Ho)

δ (Hβ)

20 0 15 30 45 60 -

c c c 0.541d 0.520 0.489

7.777 7.883 7.973 8.019 8.045 8.045

8.174 8.340 8.487 8.581 8.621 8.622

8.828 8.764 8.716 8.716 8.681 8.689

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a

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Temperature (°C)

In ppm relative to TMS as internal standard. b See the text and Fig. 1 for the spectral data of formic

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acid. c No signal was observed. d A very broad signal was detected.

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Graphical Abstract

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Interaction of meso-tetraarylporphyrins with formic acid: A variable temperature 1H NMR

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study

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Saeed Zakavi,* Mahshad Najafi ragheb

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Variable-temperature 1H NMR spectroscopy was performed in the range -60 to 20˚C to study the

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diprotonated species of meso-tetraarylporphyrins (aryl = phenyl, 4-methoxylphenyl and 4-chlorophenyl)

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with formic acid as a weak carboxylic acid.

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ACCEPTED MANUSCRIPT Highlights 

Adduct formation between meso-tetraarylporphyrins and formic acid as a weak carboxylic

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acid

Variable temperature 1H NMR spectra of porphyrin:formic acid molecular complexes

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Low temperature 1H NMR evidence for the formation of porphyrin diacids with a weak

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

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carboxylic acid

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