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Статья Paper
Spectral Characteristics and Solvation of Tetrakis(3,5-di-ferf-butylphenyl)porphine and its Complexes with Some d Metals
Elena V. Antina,a@ Elena V. Balantseva,b and Mikhail B. Berezina
aInstitute of Solution Chemistry of Russian Academy of Sciences, 153045 Ivanovo, Russia bIvanovo State University of Chemistry and Technology, 153000 Ivanovo, Russia @Corresponding author E-mail: eva@isc-ras.ru
An influence of structural factors and surrounding features on both spectral and enthalpy characteristics of the dissolution and solvation processes and the thermodynamic stability of tetrakis(3,5-di-tert-butylphenyl)porphine (Hp) and its coordination compounds with a number of d-metals have been studied in different solvents using spectroscopic and calorimetric methods. The contribution of n-n-interaction of H2P ([MP]) - benzene to the solvation process was estimated. It have been shown that n-n-stacking interaction between aromatic systems of [MP] and benzene molecules is in the following sequence: [PdP]< [CdP]< [NiP]< [ZnP]< [CuP]< [AgP]< [CoP]. The stability of [MP] increases in the row: [HgP] < [CdP] < [ZnP] < [AgP] < [CuP] < [NiP] ~ [CoP] < [PdP], which is in agreement with spectral criterion of strength. The coordination unsaturation of metal porphyrins follows the sequence: [(Ac)FeP] < [NiP] < [PdP] < [CoP] << [CdP] <[ZnP] < [CuP] < [AgP] < [HgP] < [(Ac)MnP].
Keywords: Tetrakis(3,5-di-tert-butylphenyl)porphine, solution calorimetry, spectral characteristics, complexes with d metals, solvation process.
Introduction
Very often it is difficult to interprete the results obtained for porphyrins, their metal complexes and processes of axial coordination in metalloporphyrins, because the data on specific interaction features between the compounds of this type and solvate molecules are absent. The special emphasis under investigation of the solvation processes is devoted to a correct choice of reference system (porphyrin - "standard" solvent). The main requirement to this system is a solvation of solute only by universal interactions forces. As a rule, benzene or its alkyl derivatives, for instance, a toluene and xylene are used as a "standard" solvent in calorimetric and spectroscopic methods. However, the results of X-ray diffraction and thermogravimetric analysis obtained for the porphyrin crystals which were precipitated from saturated "standard" solvents show that such "standard" is not always "inactive" and in some cases can form stable solvate structures with tetrapyrrole macrocycles.[1-4] Such behaviour of a "standard" solvent leads to the incorrect estimations of properties, for example, inaccuracy may occur in case of evaluating the metalloporphyrin coordination unsaturation in compliance with the axial coordination enthalpies Aa.c.H° obtained via thermochemical approach.[5]
It seems more desirable to use paraffin hydrocarbons like a "standard" medium surroundings. However, the majority of macrocyclic tetrapyrroles have poor solubility in these solvents. Increase of solubility of porphyrins in nonpolar solvents by introduction of alkyl bulky substituents should not significantly distort[5] the general pattern of "structure - property" relationship. Therefore, here we have selected the tetrakis(3,5-di-feri-butylphenyl)
porphine (H2P) and its coordination compounds as objects of our work. Due to their high solubility in the media of different nature we were able to use cyclohexane as a reference solvent having universal solvating properties.[6-8]
Experimental
The synthesis, methods of purification and the spectral properties (XH NMR, EPR) of some H2P and [MP] (M=Con, Nin, Cu11, Zn11, Pdn, Ag11, Cd11, Hg11) as well as [(Ac)MP] (M=Fem, Mn111) were previously published by our working group.[7,9]
The crystal samples of metalloporphyrins were dried under low pressure at 353-363 K during 24 h or more up to a moment when the weight of sample was constant. The solvents (pyridine, benzene, chloroform, and cyclohexane) used in experiments were the highly purified commercial products. All the solvents were treated additionally to remove the impurity and a rest of water using the known methods.[10] The water content of the solvent was determined by Karl Fischer titration and did not exceed 0.02 %.
The data of solution enthalpies (AsolH0) of H2P and [MP] have been obtained on isoperibolic calorimeter at 298.15 K. It should be noted that there are no data on the enthalpy of crystal lattice and enthalpy of conformational changes, which can appear in transfer of substance from solid to liquid phase. Taking this fact into account to discuss the results obtained in calorimetric experiments we used the values of transfer enthalpy from the reference solvent (cyclohexane) to the studied solvent which were calculated using the following Equation (1):
AtrH0 = AsolH0. - AsolH0st (1),
where AsolH0, and AsolH0st are the changes of enthalpy during dissolution of the compound in the studied and reference solvent, correspondingly. Analysis of coordination properties of [MP] toward electron donating pyridine molecules have been carried out
using the enthalpies of axial coordination (Aa.c.H0) estimated from the Equation (2):
Aa.c.H0 = AtrHMp - AtrHH2L (2),
where AtrHMP and AtrHH2L are the transfer enthalpies of the complex and the ligand in the studied solvent, correspondingly.
The visible spectra of samples have been measured on Specord M 40 spectrophotometer within 350 - 700 nm region.
Results and Discussion
The spectral characteristics of H 2P and its coordination compounds with d metals in some organic solvents are presented in Tables 1 and 2. It is known[11] that there are few main types of porphyrin spectra which are determined by the number and intensity of spectral bands. As it is possible to see from the Tables 1,2 and Figure 1 H2P has ethio-type of spectrum (Figure 1a) in all studied solvents.
Table 1. Values (X, nm) and logarithm of molar absorptivity (lge) of spectral bands for the tetrakis(3,5-di-ferf-butylphenyl)porphine and its metal complexes in organic solvents.
X, lge
Solvent -
H2P [CoP] [NiP] [CuP] [ZnP] [PdP] [AgP] [CdP] [HgP]
651 3.12
595 3.14 567 sh 618 3.37 570 sh 620 sh 601 3.45 583 sh 616 3.69 599 sh
C6H12 548 514 3.35 3.55 527 5.19 529 4.10 539 5.59 546 4.19 523 4.32 543 4.19 548 5.14 551 3.68
415 4.51 412 6.13 418 4,54 416 6.25 421 4.67 418 4.52 427 4.52 419 6.31 428 4.60
633 3.97
586 4.06 576 sh 608 3.62 560 sh 581 3.90 597 3.87 557 sh 602 3.97 618 4.06
C6H6 545 511 417 4.14 4.35 5.35 523 412 4.17 5.18 522 415 4.39 5.33 533 415 4.42 5.44 545 420 4.41 5.28 520 416 4.50 5.38 536 423 4.30 4.98 562 433 4.06 4.76 573 431 4.09 5.08
639 4.01 - -
586 3.97 572 sh 610 3.59 558 sh 581 4.01 597 3.97 562 sh 603 4.04
CHCl3 547 513 4.16 4.37 523 4.62 524 4.38 535 4.30 545 4.43 520 4.69 538 4.51 562 4.11
416 5.46 411 5.72 416 5.40 415 5.28 419 5.43 412 5.27 422 5.07 432 5.19
639 3.86
586 3.77 597 sh 608 3.70 570 sh 597 4.27 597 3.77 561 sh 605 4.10 622 4.04
C5H5N 547 513 3.99 4.23 529 3.85 524 4.35 538 4.26 559 4.39 519 4.43 538 4.37 562 4.19 578 4.13
417 5.31 411 5.06 417 5.36 419 5.31 427 5.34 416 5.20 425 5.22 431 5.37 437 4.97
Table 2. Values (X, nm) and logarithm of molar absorptivity (lge) of spectral bands for the complexes of tetrakis(3,5-di-tert-butylphenyl)-porphine with ions of rf-metals in organic solvents.
X, lge X, lge Solvent-Solvent-
[(Ac)MnP] [(Ac)FeP] [(Cl)FeP] [(FeP)2O] [(Ac)MnP] [(Ac)FeP] [(Cl)FeP] [(FeP)2O]
C6H12 614 4.95 690 3.44 683 3.50 628 3.37 CHCl3 618 4.98 710 sh 687 sh
577 4.95 650 sh. 675 sh 573 4.10 581 4.96 590 sh 667
523 4.76 575 3.59 585 sh 528 4.82 569 sh 587 sh
477 5.85 514 3.90 506 3.88 414 4.54 466 5.85 516 3.50 514
412 5.45 421 4.20 422 5.15 410 sh 419 5.16 419
383 5.60 386 3.94 365 3.83 386 5.60 364 sh 385
C6H6 613 4.97 700 sh 680 3.52 615 4.18 C5H5N 636 4.91 710 sh 716 627 4.02
577 4.95 660 sh 675 sh 573 4.34 615 4.90 590 sh 668 sh 573 4.81
526 4.80 568 3.70 585 sh 514 sh 579 4.76 564 sh 606 sh 514 sh
472 5.85 510 sh 504 3.92 417 5.45 525 4.30 532 3.50 513 421 5.23
388 5.40 415 5.35 419 5.18 476 5.85 427 5.16 422
374 5.60 364 sh 372 3.85 406 sh 364 sh
386 4.40
Solvation of Tetrakis(3,5-di-tert-butylphenyl)porphines
350 450 550
wavelength, nm
650
1.6
M
c 1 2 <u 1 u
o 0,8
Q.
o
0,4
350 450 550
wavelength, nm
650
Figure 1. Absorption spectra of some compounds in benzene: a) H2P; b) [ZnP]; c) [AgP]; d) [(Ac)MnP].
This spectral type is characteric for the most of porphyrin derivatives.111"121 Hypsohromic shift (-) of the first band during transfer of H 2P from cyclohexane to the other solvents (AX1 =X1 -X1 „ „ ) is distinctly higher in benzene (-18
v max max max, CgH^7 jo v
nm) in comparison to pyridine and chloroform (-12 nm).
Among the investigated coordination compounds with d metals the complexes of palladium(II), nickel(II), copper(II), cobalt(II), and silver(II) show the spectra of "hypso-type", the complexes of zinc(II), mercury(II), and cadmium(II) show "normal-type", whereas [(Ac)FeP] and [(Ac)MnP] demonstrate the "hyper-type" spectra.
It was revealed that the transfer of Zn11, Cd", and Hg" complexes to cyclohexane from the other solvents was accompanied by a mostly intensive hypsohromic effect of the long-wave band (XImax). This leads to conversion of "normal-type" spectrum to "hypso-type" spectrum (Figure 1b,c) and could be caused by a lesser auxo influence on n-system of the chromophore by the metal ions with electronic configuration d10 as compared to the metals with incomplete 3d sub-shell. We have to remind that ions with configuration (n-1)d10ns-° are able to react with porphyrin ligands only through c-type coordinating interaction, whereas ions with incomplete (n-1)d sub-shell can coordinate porphyrin ligand by both c- and ± n-dative interactions. This effect of electronic structure of metal ion is difficult to detect in aromatic (benzene, pyridine) and proton-donating (chloroform) solvents. These difficulties arise from the solvatochromic effects due to the specific solvation interactions what ultimately reflects in changes of the spectral characteristics of compounds.
Transfer of complexes from the cyclohexane to the polar solvents with more distinct donor-acceptor properties is accompanied by solvatochromic effect with considerable changes of X1 position. The general behavior of AX1 =
0 max r ° max
X1max i - X1max C(sHi2 changes in the spectra of [M11P] and [(X)
M111P] solutions is presented in Table 3; X1
and X1
J A 7 max, CgH12 max, i
are the positions of the long-wave band in cyclohexane and in the investigated solvent, correspondingly.
The sequences are in a good agreement with the case of transfer processes from benzene to chloroform, however for the transfer to pyridine the significant realignment in a position sequence of [MP] is observed. Transfer of [MIIP] complexes, with the exception of [HgP] and [CoP], are accompanied by hypsochromic shift. Its value is decreased when benzene is replaced by chloroform and by pyridine. The minimal solvatochromic effects are displayed for [PdP] and [NiP]. In the case of [HgP], [CoP], and [(X)MmP] complexes the reverse behaviour was revealed. Increase of value of bathochromic (+AXI ) or hypsochromic (-AXI ) shift is
max max
an evidence of change in chromophore activity of ligand consisting of coordinating compounds. That is caused by amplification or weakening of polarization of chromophore n-system. In general, we can assume that the reason of the observed changes are the solvation contributions from: 1) n-n-interactions between the aromatic systems of solvent and [MP] (during transfer processes to benzene and pyridine); 2) donor-acceptor interactions of [MP] with the molecules of more polar solvents. Note that changes in the absorption spectra of some complexes in pyridine and chloroform in comparison
CH are due to the sum of two main solvation effects:
6 12
Table 3. The long-wave band positions in the UV-vis spectra of [MP] and [(X)M111P] in various solvents.
C6H6: [HgP] < [(Ac)FeP] < [CoP] < [(Ac)MnP] < [PdP] < [NiP] = [CuP] < [CdP] < [AgP] < [ZnP],
AX1 max +19 +10 +9 +1 -4 -10 -10 -14 -26 -39
CHCl3: - [(Ac)FeP] < [CoP] < [(Ac)MnP] < [PdP] < [NiP] < [CuP] < [CdP] < [AgP] < [ZnP],
AX1 max + 20 +5 +4 -4 -8 -12 -13 -21 -39
Py: [CoP] < [HgP] < [(Ac)FeP] < [(Ac)MnP] < [CuP] < [PdP] < [NiP] < [CdP] < [AgP] < [ZnP]
AX1 max +30 +23 +22 +1 0 -4 -10 -11 -22 -23
Notice: symbols (+) and (-) correspond to the bathochromic or hypsochromic shift of X1 during the transfer from cyclohexane to the investigated solvent.
AX1 = AX1 ± AX1 . When these effects have the same direction there is a rise of chromophore activity of n-system, otherwise it is decreased in the case of different directions of the effects.
It is known[12] that position of the first absorption band in the spectra of the same type of covalent porphyrin complexes varies symbatically with the stability of the complex supposing that the role of inverse dative n-bonds is slight and we can neglect the distorting factor of the ligand structure. In investigated solvents for all complexes [MIIP] the value of the shift of the first absorption band is negative as compared to that of the initial ligand (AX1 = XIMp - XIH P) (Table 4). For instance, in the case of [PdP] in cyclohexane the AX1 value will be calculated as 523 - 651 = -128. It should be noted that for the calculation we used only the well-defined peaks while the shoulders were ignored. According to the spectral criteria of stability AX1 in cyclohexane, benzene, and chloroform (the solvents which are not able to solvate a complex-forming atom specifically[13]) the [MIIP] complexes form the next row of stability: [HgP] < [CdP] < [ZnP] < [AgP] < [CuP] < < [NiP] ~ [CoP] < [PdP], whereas the stability of complexes of triple-charged metals is decreased upon transfer from [(Ac)MnP] to [(Ac)FeP]. Earlier the similar behavior for some tetraphenylporphyrinates [MTPP] (M = Pd11, Ni11, Cu11, and Zn11) was established on a basis of analysis done for the data on spectral criteria.[1214] An exception is [AgP] complex, which is less stable then [AgTPP] probably due to the reducing of inverse dative n-effect of Ag —^ N coordination owing to the strong +I-induction effect of substituents.
Thereby, an introduction of tert-butyl substituents does not have a distinct influence on general character of dependence between the [MTPP] stability and chromophore structure.
The low stability of the mercury(II) complex compared with other complexes of H2P is caused by large ionic radius of Hg2+. That leads to the weakening of effective coordination interactions between metal and porphyrin resulting from the displacement of metal ion out of macrocycle plane.
The obtained data point out that solvation factors have an essential influence on the state of investigated compounds in solutions. Therefore, it was imortant to study the peculiarities of solvation by the solvents of different nature using the dissolution calorimetry.
Hypersolubility of the investigated compounds allows to study the thermochemical characteristics of dissolution and solvation by calorimetric method more thoroughly. adue to that we were succeeded in measuring of dissolution enthalpies for H2P and some of its coordination compounds in cyclohexane, which displays a predominantly universal type of solvation.
Enthalpy characteristics of these solution processes (AsolH0), transfer (AtrH0) and axial coordination (Aa.c.H0st, C H ) of the studied compounds in comparison with the available literature data for H2TPP and some [MTPP] are collected in Tables 5 and 6. As can be seen the solvation processes of H2P and [MIIP] in chloroform, pyridine, and benzene are more exothermic that that for the unsubstituted analogues.[4] It is evident that this phenomenon is induced by the increasing contributions from the universal solvation and weakening of molecular crystal lattice which are caused by the effects of alkyl substitution.[815]
The dissolution processes of H2P and majority of [MP] (except [CoP], [(Ac)MnP], and [(Ac)FeP]) are the most endothermic in cyclohexane. Observable relative difference in AsolH0 values is a consequence of different rigidity of the crystal lattice of individual substances.
The solvation contribution from n-n-stacking interactions H2P - solvent that was estimated assuming that AtrH0 from cyclohexane to benzene and pyridine is identical for the both aromatic solvents and equals -37.8 kJ/mol. Thereby, it is possible to conclude that in pyridine there is no donor-acceptor interactions of the nitrogen atom of Py and proton of the HN< groups of the macrocyclic ligand. It is in agreement with the findings of V.V Aleksandriiskii and coworkers.[16] The comparison of results with the data obtained for H2TPP, which forms pyridinium salts, allows to conclude that the observed effect may be associated with the decrease of acidity of the H2P ligand due to the +I-electronic effects of the alkyl substituents.
For the of most coordination compounds (excepting [CoP] and [(Ac)MIIIP]) the AtrH0 values to benzene have been found to be negative and their absolute values are increased in the order:
[(Ac)MnP] < [CoP] < [(Ac)FeP] < [HgP] < [CdP] < [ZnP] < [H2P] < [NiP] < [PdP] << [CuP] < [AgP] (I).
This sequence demonstrates the improving of conditions for the n-n-type solvation process for [CuP] and [AgP] complexes. This inference is in the good agreement with the results of thermogravimetric analysis which demonstrates that [CuP] and [AgP] form the most stable crystal solvates with two C6H6 molecules, whereas [(Ac)MnP] and [HgP] do not reveal any pronounced ability to form crystal solvates with benzene.[917]
It was found that exothermicity of the transfer processes of compounds from cyclohexane to chloroform is higher than that from cyclohexane to benzene. This can be attributed to the two reasons: 1) improvement of the solvation conditions due to the growth of a polarity and dielectric constant of the solvent; 2) contribution from the donor-acceptor (>N—HCCl3) interactions. The latter is strongly displayed in the case of
Table 4. Spectral criterion of strength AX1 for [MP] (nm).
Solvent AX1, nm
PdP CoP NiP CuP AgP ZnP CdP HgP (Ac)FeP (Ac)MnP
C6H12 -128 -124 -122 -112 -108 -105 -103 -100 39 -37
C6H6 -113 -110 -111 -100 -97 -88 -71 -60 67 -20
CHCl3 -119 -116 -115 -104 -101 -94 -77 - 71 -21
Solvation of Tetrakis(3,5-di-fer/-butylphenyl)porphines
Table 5. Enthalpy characteristics of the processes of solution (AsolH0), transfer from cyclohexane (AtrH0) and pyridine axial coordination (Aa.c.H0) obtained for H2P and [МР] (kJ/mol, 298.15 К).
Compound C6H12 C6H6 CHCl3 C5H5N
AsolH0 AsolH0 AtrH0 AsolH0 AtrH0 AsolH0 AtrH0
H2P 19.7±0.5 -18.1±1.2 -37.8 -35.4±1.2 -55.1 -18.1±1.8 -37.8
[ZnP] 26.4±1.2 -6.3±0.5 -32.7 -19.1±1.2 -45.5 -26.4±1.9 -52.8 -15.0
[CoP] -6.5±1.4 11.2±0.4 17.7 -2.6±0.5 3.9 -46.0±1.0 -39.5 -1.7
[CdP] 43.7±0.8 16.6±0.7 -27.1 -2.5±0.3 -46.2 -3.6±1.0 -47.3 -9.5
[CuP] 28.0±1.1 -25.2±1.7 -53.2 -25.7±1.4 -53.7 -29.3±1.9 -57.3 -19.5
[NiP] 12.2±1.0 -26.2±0.8 -38.4 -44.3±1.9 -56.5 -19.3±1.5 -31.5 6.3
[AgP] 50.2±0.9 -4.0±0.5 -54.2 -21.2±0.8 -71.4 -8.9±0.4 -59.1 -21.3
[PdP] 15.5±1.5 -24.2±1.2 -39.7 -37.1±1.7 -52.6 -21.7±1.6 -37.2 0.6
[HgP] 10.7±0.6 -9.6±0.5 -20.3 - - -56.0±1.9 -66.7 -28.9
[(Ac)MnP] 31.0±0.8 50.7±1.8 19.7 30.9±2.0 -0.1 -37.6±1.8 -68.6 -30.8
[(Ac)FeP] 11.9±1.0 27.1±1.0 15.2 -20.7±1.5 -32.6 -16.4±0.7 -28.3 9.5
H2P,[5] that is common for the synthetic ligands (including H2TPP) and especially for the natural porphyrins.[18] In the case of [MP] the contribution from such interactions to the total solvation can increase by even a weak distortion of the planar structure of the coordination center (N4) caused by the steric effect of the bulky tert-butyl substituents.
On the basis of the data presented in Table 5 we have determined the following sequence of complexes in compliance with increasing of the negative values of axial enthalpy Aa.c.H0 of pyridine calculated using the AtrH0 from cyclohexane:
([(Ac)FeP] < [NiP] < [PdP] < [CoP]) << [CdP] < [ZnP] < < [CuP] < [AgP] < [HgP] < [(Ac)MnP] (II).
Analysis of Aa.c.H0 values demonstrates a low ability to the additional coordination of the electron-donating molecules among [(Ac)FeP], [NiP], [PdP], [CoP] and an amplification in a coordinating unsaturation for the next complexes of this series. By reason of a low solubility of [MTPP] in cyclohexane the corresponding data for AsolH0 can not be obtained. Therefore, in Table 6 we present the values of axial coordination enthalpies for the investigated metalloporphyrinates and their analogues which were calculated using the transfer enthalpies from benzene and were termed as (Aa.c.H0 ).
st, C6Hj/
In agreement with Aa.c.H01
from the specific solvation of metalloporphyrins by benzene.
Table 6. The axial coordination enthalpies (Aa.c.H0st^H) of pyridine by metallocomplexes of H2P, H2TPP,[4] H2(4-T3uPh)4P[4] (298.15 K, kJ/mol).
Compound Aa.c.H0 tCH st,C6H6 Compound Aa.c.H0 stC H st,C6H6
[ZnP] -20.1 [NiP] 6.9
[ZnTPP][4] -47.3 [NiTPP][4] -
[Zn(4-'BuPh)4P][4] -36.1 [AgP] -4.9
[CoP] -57.2 [PdP] 2.5
[CoTPP][4] -31.6 [HgP] -46.4
[CdP] -20.2 [(Ac)MnP] -88.3
[CdTPP][4] -30.0 [(Ac)MnTPP][4] -27.2
[CuP] -4.1 [(Cl)MnTPP][4] -29.7
[CuTPP][4] 1.3 [(Ac)FeP] -43.5
[(Ac)FeTPP][4] -19.2
values the capability complexes (row III).
For instance, the high contribution from the interaction of [AgP] and [CuP] with benzene (Table 5) almost completely compensates the effect of axial coordination of pyridine, that explains the small values of Aa.c.H0st for given
of [МР] to axial coordination grows up in the following sequence:
Conclusions
([NiP]<[PdP]<[CuP]<[AgP]) <[CdP] ~ [ZnP] < [(Ac)FeP] < [HgP] < [CoP] < [(Ac)MnP] (III).
It is easy to see from the comparison of the rows (II) and (III) that discrepancies in their sequence are strikingly disclosed for compounds which are susceptible to specific molecular n-n-complex forming with benzene. The reason of mismatches is that the value of Aa.c.H01 „ „ in such
st, C6H12
cases along with a contribution from axial coordination takes into account the compensation energy contribution
Thus, due to a higher solubility of the studied compounds we were able to use cyclohexane[19] as a standard solvent, which allowed us to identify the ability of [CuP] and [AgP] complexes to the additional coordination of pyridine molecules. This ability would be impossible to discover using the data obtained from analysis of Aa.c.H0 values. Nevertheless, we would like to un-
st.benzene '
derline that in the case of low solubility of porphyrins in aliphatic solvents for the reliable understanding it would be rational to combine the calorimetric data analysis with
thermogravimetric and/or X-ray structure analysis of crystal solvates.
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Received 30.04.2010 Accepted 20.10.2010
