ARTICLE pubs.acs.org/JPCC
StructureReactivity Relationships of Metalloporphyrin Modified by Ionic Liquid and Its Analogue Xingbang Hu,* Chaoying Liu, Youting Wu, and Zhibing Zhang School of Chemistry and Chemical Engineering and National Engineering Research Center for Organic Pollution Control and Resource, Nanjing University, Nanjing 210093, P. R. China
bS Supporting Information ABSTRACT: The influence of ionic liquid (IL) modification on the reactivity of metalloporphyrins was carefully investigated to design a recoverable and highly effective metalloporphyrin catalyst. On the basis of the research, which included a total of 34 different catalysts, the following modification methods make metalloporphyrins more powerful: (1) increasing the spin density (SDO), charge (QO), or isotropic fermi contact couplings (IFCCO) of the O atom in the FedO part and (2) decreasing the spin density carried by the Fe atom (SDFe) or the HOMOLUMO gap between a catalyst and reactant (LUMOCHOMOR). The order of the correlation between the structure parameters and the reactivity is SDo > Qo > IFCCO ≈ SDFe ≈ LUMOCHOMOR. Compared with changing the cation of metalloporphyrins, changing the anion is a more effective way to increase the reactivity. The order is AlBr4 > AlCl4 > BCl4 > PF6 > AsF6 > SbF6 > BF4 > Tf2N > AlF4 > HSO4 > CF3SO3 > CF3CO2 > Cl. However, the long distance between the IL part and the catalytic active center or electron-donating substituent on the N atom weaken the influence induced by the IL modification. These structurereactivity relationships could be used in designing a catalyst.
1. INTRODUCTION As a model compound of Cytochrome P450 enzyme, metalloporphyrin has been widely used in CH activation and oxidation reactions.13 Much effort was exerted to expand the metalloporphyrin family, especially in the field of water-soluble porphyrin415 and porphyrin catalysts with higher selectivity or reactivity.1627 Catalyst recycling and the aqueous catalysis process are required to develop water-soluble porphyrins.415 Recycling metalloporphyrins is one of the most important steps for the large-scale practical application of the related catalytic process due to the high cost of this catalyst. However, recycling traditional metalloporphyrins (such as meso-tetraphenylporphyrin iron and its analogue) is difficult because their solubility in water is quite poor. Modifying metalloporphyrins using a functional group including a cationanion pair can make this catalyst soluble in water.415 The solubility properties of these modified catalysts change with the difference of the cations and anions, which makes the phase separation from less polar organic solvents and aqueous media possible. Another purpose in developing a watersoluble porphyrin is aqueous catalysis. This process is based on the requirement of green chemistry, which uses water to replace the traditional organic solvent. Recently, some homogeneous catalysts have been modified by an ionic liquid (IL) or its analogue for the purpose of recycling these expensive or poisonous catalysts.2833 Our recent studies on acetylacetone catalysts have revealed that IL analogue modification can make the catalyst recoverable and obviously enhance the reactivity.3133 Hence, two objectives can be obtained using a r 2011 American Chemical Society
single move by using an IL analogue to modify metalloporphyrin. Some pioneers have synthesized similar metalloporphyrins, such as tetrakis (N-methyl-4-pyridinium) porphyrinato manganese(III),6,11 5,10,15,20-tetrakis (tetrafluoro-N,N,N-trimethyl anilinium) porphyrinato iron(III) or manganese(III),7,9,15 and tetrakis-5,10,15,20-(N,N-dime-thylimidazolium) porphyrinato manganese(III).14 However, these reported modifications still can not remarkably enhance the reactivity of metalloporphyrins. To date, it is still not easy to predict the influence of the IL modifications on the catalytic reactivity quantitatively or even qualitatively. If a general relationship between the structure and reactivity is built for metalloporphyrins modified by IL or its analogue, it will be beneficial to design water-soluble metalloporphyrin with more powerful reactivity. In the present study, based on a detailed theoretic investigation of the structure parameters and reactivity of a series of metalloporphyrins, some useful relationships were summarized. The CH activation processes of methane, regarded as touchstone reactions of catalytic reactivity, were chosen to test the reactivity of the metalloporphyrin catalyst modified by IL or its analogue. Although many inorganic3439 and organometallic32,4045 catalysts have been reported for the activation of methane, a better understanding of the roles of IL analogue modification is helpful in improving the reactivity of these existing CH activation catalysts. Received: September 1, 2011 Revised: October 28, 2011 Published: October 31, 2011 23913
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set has been used elsewhere and has been proven to work well compared with other methods.32,44,45 Geometric configuration optimization, energy calculation, frequency calculation, and zeropoint energy correction were performed using the same basis set. The computed stationary points have been characterized as minima or transition states by diagonalizing the Hessian matrix and analyzing the vibrational normal modes. In this way, the stationary points can be classified as minima if no imaginary frequencies are shown or as transition states if only one imaginary frequency is obtained. The particular nature of the transition states has been determined by analyzing the motion described by the eigenvector associated with the imaginary frequency. The CH activation barrier was obtained by comparing the energies of the transition state and reactant (optimized complex of catalyst and methane). All calculations were performed with the Gaussian03 suite of programs.57
Figure 1. Structures of the NIL- and IL-catalyst with different cations and anions.
2. COMPUTATIONAL METHODS For metalloporphyrins, the B3LYP method based on density functional theory can build a good model4146 and generate results good enough to predict experimental phenomenon.4751 Hence, all the structures were optimized using the B3LYP method in the present manuscript. In the methane oxidation catalyzed by the metal organic catalysts, the 6-31G basis set was widely used for these atoms, except for transition metals, with satisfying results.32,41,42,44,45,5154 Although the 3-21G basis set is still acceptable for the metalloporphyrin system,48,55,56 the 6-31G basis set is more popular in current studies. This basis set is good enough to describe these atoms, except for the transition metals and heavy elements, through a detailed comparison between the 6-31G and higher basis sets (such as 6-311+G* and 6-311++G**).32,44,45,50,51 Considering the relativistic effect of the transition metals and heavy elements is necessary.41,42,44,45,50,51 The present research investigates 34 different metalloporphyrins (including high- and low-spin states) and their reaction processes. Because the amount of the investigated catalysts is large and some of the modifications are quite complicated, the following combination of basis sets was used in the current study: the LANL2DZ basis set for Fe/As/Sb/Br atoms and the 6-31G basis set for the rest of the atoms. This combined basis
3. RESULTS AND DISCUSSION In the present article, a series of metalloporphyrins modified by IL or its analogue (IL-catalyst) and their unmodified counterparts (NIL-catalyst) with a Cl atom as the axial ligand were investigated (Figure 1). A total of 34 different catalysts were investigated, including 9 IL-catalysts with different cations (ammonium, pyridinium, and imidazolium salts), 12 IL-catalysts with different anions, 11 NIL-catalysts, and 2 catalysts with a hydrogen bond. The IL-catalysts with different cations are abbreviated as [IL-n][Cl] (n = 19); the IL-catalysts with different anions are abbreviated as [IL-1][X] (X = BF4, PF6, AsF6, SbF6, Tf2N, CF3SO3, CF3CO2, HSO4, AlF4, AlCl4, AlBr4, and BCl4); and the NIL-catalysts are abbreviated as NIL-n (n = 111). NIL 3 3 3 HCl and NIL 3 3 3 HNO3 are NILcatalysts forming a hydrogen bond with an acid (HF and HNO3). Transition metal catalysts have low- and high-spin states,58 both of which were taken into consideration. For the specific catalyst investigated, the low-spin state is a doublet, and the highspin state is a quartet. For metalloporphyrins with Cl atom as the axial ligand, the high-spin state reaction pathway is more energyfavorable, and the high-spin state catalysts are slightly more stable than the low-spin state one (see refs 44 and 45 and the Supporting Information), so only the high-spin state catalysts were discussed in the main text. 3.1. Structural Changes of Metalloporphyrins Induced through Modification. Some representative structural para-
meters of the optimized catalysts were listed in Tables 1 and 2. For the IL-catalysts, the negative charges carried by the anion range from 0.515 to 0.786. This value obviously depends on the variety of cation and anion. The minimum distances between the anion and cation change from 1.594 to 2.675 Å. The charge and distance values are typical for the IL32,58 and indicate the ionic characteristic of these modified metalloporphyrins. The FeO bond length ranges from 1.659 to 1.663 Å for different IL- and NIL-catalysts, which are close to that of Compound I of P450 (B3LYP method; basis set used: the LACVP for Fe and 6-31G for the rest of the atoms).42,43,51,52 The spin density and charge carried by the Fe and O atoms of the NIL- and IL-catalysts are also reasonable compared with those of Compound I.42,43,51,52 The FeO part is the active center of both the IL- and NILcatalysts. The IL modification exerted a remarkable effect on the following parameters: the distance between Fe and O (RFeO), 23914
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Table 1. Structure Parameters of Different Metalloporphyrins RFeO a
a
RAC b
SDFe c
SDO c
Q Fe d
QOd
Q anion d
IFCCO (a.u.) e
[IL-1][Cl]
1.660
1.703
1.013
1.014
0.496
0.318
0.515
0.1693
[IL-2][Cl]
1.659
1.761
1.027
0.995
0.500
0.324
0.586
0.1680
[IL-3][Cl]
1.660
1.734
1.028
0.991
0.488
0.328
0.590
0.1680
[IL-4][Cl]
1.660
1.705
1.019
1.001
0.492
0.323
0.565
0.1678
[IL-5][Cl]
1.660
2.585
1.030
0.990
0.489
0.329
0.624
0.1670
[IL-6][Cl]
1.660
2.601
1.030
0.990
0.490
0.329
0.632
0.1670
[IL-7][Cl]
1.661
2.675
1.060
0.985
0.494
0.332
0.599
0.1667
[IL-8][Cl] [IL-9][Cl]
1.660 1.660
1.859 2.082
1.034 1.040
1.001 0.980
0.493 0.495
0.321 0.334
0.529 0.742
0.1657 0.1669 0.1698
[IL-1][BF4]
1.660
1.636
1.000
1.024
0.494
0.307
0.752
[IL-1][PF6]
1.660
1.717
1.001
1.027
0.501
0.304
0.786
0.1700
[IL-1][AsF6]
1.660
1.699
1.002
1.027
0.501
0.305
0.780
0.1699
[IL-1][SbF6]
1.660
1.680
1.003
1.025
0.501
0.306
0.764
0.1698
[IL-1][Tf2N]
1.660
1.711
0.998
1.024
0.491
0.306
0.774
0.1698
[IL-1][CF3SO3]
1.660
1.653
1.006
1.018
0.494
0.311
0.749
0.1695
[IL-1][CF3CO2] [IL-1][HSO4]
1.660 1.660
1.594 1.664
1.009 1.008
1.016 1.019
0.496 0.499
0.312 0.310
0.703 0.748
0.1694 0.1695
[IL-1][AlF4]
1.660
1.633
1.003
1.021
0.495
0.309
0.731
0.1696
[IL-1][AlCl4]
1.659
2.223
0.989
1.032
0.496
0.300
0.785
0.1702
[IL-1][AlBr4]
1.659
2.391
0.986
1.033
0.494
0.299
0.745
0.1703
[IL-1][BCl4]
1.660
2.198
0.990
1.032
0.496
0.301
0.746
0.1703
NIL-1
1.660
1.029
0.987
0.497
0.330
0.1670
NIL-2
1.663
1.050
0.980
0.484
0.343
0.1679
NIL-3 NIL-4
1.662 1.661
1.050 1.036
0.9778 0.985
0.475 0.495
0.342 0.334
0.1669 0.1672
NIL-5
1.661
1.034
0.987
0.489
0.332
0.1676
NIL-6
1.660
1.028
0.990
0.491
0.330
0.1675
NIL-7
1.661
1.021
0.999
0.484
0.324
0.1681
NIL-8
1.661
1.028
0.992
0.480
0.329
0.1678
NIL-9
1.661
1.035
0.985
0.491
0.334
0.1672
NIL-10
1.660
1.018
1.000
0.484
0.324
0.1681
NIL-11 NIL 3 3 3 HNO3 NIL 3 3 3 HCl
1.660 1.662
1.012 1.027
1.006 1.001
0.486 0.488
0.319 0.326
0.1683 0.1690
1.662
1.024
1.004
0.491
0.325
0.1680
Distance between Fe and O (in Å). b The minimum distance between the anion and cation (in Å). c The spin density carried by Fe and O atoms. d The charge carried by Fe, O, and the anion. e The isotropic fermi contact couplings of the O atom.
the spin density carried by Fe and O (SDFe and SDO), the charge carried by Fe and O (Q Fe and Q O), the isotropic fermi contact couplings of O (IFCCO), the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO). Some relationships among the parameters were found (Figure 2). Although a meso-substituent has the ability to influence the FeO bond length and can result in the change of reactivity, this effect is not distinct for the IL- and NIL-catalysts. There are only five FeO bonds with obviously different lengths among the 34 catalysts investigated in the present study. Hence, the FeO bond length is not appropriate to be used as a structure parameter for predicting the reactivity of metalloporphyrins. The SDFe values range from 0.986 to 1.009 for the IL-catalysts with different anions and from 1.012 to 1.050 for the NIL-catalysts. The SDFe of all IL-catalysts with different anions are smaller than those of the NIL-catalysts. The SDO values are also well regulated. The SDO of all IL-catalysts with different anions are larger than those of the NIL-catalyst. For the charge carried by the Fe and O
atoms, no rule was found in the Q Fe values, whereas Q O values show a similar rule to the SDFe values (Figure 2 and Table 1). The O atoms of most of the IL-catalysts carry a less negative charge compared with those of the NIL-catalysts, especially for the O atoms of the [IL-1][X] catalysts. The IL-modification also has a remarkable influence on the IFCC of the O atoms. The IFCCO shows a similar trend with the SDO and Q O. Although the influence of IL-modification on the HOMO is quite obvious, no rule can be found out for the HOMOLUMO gap (Figure 2). On the basis of the previously mentioned results and discussion, the O atom becomes electron-deficient after the IL-modification, thus influencing the reactivity of the metalloporphyrins. Some rules among the parameters (such as SDFe, SDO, Q O, IFCCO, and HOMO) indicate the possibility of using these parameters to predict the reactivity. In an effort to ascertain the structurereactivity relationship, the IL- and NIL-catalysts during the CH activation processes of methane were investigated. 23915
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Table 2. Parts of the Orbital Energy of Different Metalloporphyrins gap
HOMO
(eV)a (Hartree)
a
gap
HOMO
(eV)a (Hartree)
[IL-1][Cl] [IL-2][Cl]
0.0824 0.2392 [IL-1][AlF4] 0.1915 0.2203 [IL-1][AlCl4]
0.2698 0.2634 0.2999 0.2778
[IL-3][Cl]
0.1242 0.2136 [IL-1][AlBr4]
0.2980 0.2781
[IL-4][Cl]
0.0006 0.2153 [IL-1][BCl4]
0.2880 0.2741
[IL-5][Cl]
0.0530 0.2101 NIL-1
0.2574 0.2171
[IL-6][Cl]
0.0434 0.2076 NIL-2
0.6878 0.1989
[IL-7][Cl]
0.0424 0.2149 NIL-3
0.4361 0.1956
[IL-8][Cl]
0.0002 0.2323 NIL-4
0.0903 0.2073
[IL-9][Cl] [IL-1][BF4]
0.1238 0.1836 NIL-5 0.2708 0.2653 NIL-6
0.0824 0.2035 0.1910 0.2207
[IL-1][PF6]
0.2942 0.2688 NIL-7
0.1671 0.2281
[IL-1][AsF6]
0.2948 0.2673 NIL-8
0.0798 0.2137
[IL-1][SbF6]
0.2949 0.2663 NIL-9
0.1368 0.2075
[IL-1][Tf2N]
0.2717 0.2680 NIL-10
0.2558 0.2201
[IL-1][CF3SO3] 0.2642 0.2550 NIL-11
0.2858 0.2327
[IL-1][CF3CO2] 0.2341 0.2514 NIL-1 3 3 3 HNO3 0.0551 0.2265 0.2678 0.2521 NIL-2 3 3 3 HCl 0.2721 0.2234 [IL-1][HSO4]
Figure 2. Variation of the structure parameter induced by the IL modifications (squares, NIL-catalyst; stars, [IL-n][Cl]; snowflakes, [IL-1][X]).
The HOMOLUMO gap of the catalyst.
3.2. Reactivity of Metalloporphyrins with and without IL Modification. 3.2.1. NIL-Catalysts. The porphyrin iron(III) with
an H atom at the meso-position is the core structure of the porphyrin family, such as Cytochrome P450, heme, and tetraphenylporphyrin iron(III).13 In academic and industrial catalysis, Cl is usually used as the axial ligand.1720,26,59 Hence, NIL-1 was used as a basal model in the present article (Figure 1). The CH activation barrier catalyzed by NIL-1 is 117.5 kJ/mol (Figure 3), which is quite closed to the data obtained with other basis sets.45 NIL-1, NIL-4, and NIL-9 yield almost the same CH activation barrier, which suggests that the electron-donating effect of CH3 or C6H5 is quite weak for metalloporphyrins. Indeed, the porphyrin with an H atom at the meso-position has been widely used as the model compound of Cytochrome P450 and tetraphenylporphyrin iron(III).32,4145,51 When the -R group is replaced by an electron-withdrawing group (such as fuorophenyl), the reactivity is obviously enhanced. The CH activation barrier catalyzed by NIL-1 is 9.2 and 13.9 kJ/ mol higher than those catalyzed by NIL-10 and NIL-11, respectively. This finding agrees with previous experimental results.8,17,18,59 For example, among a series of metalloporphyrins with a fluorine and chlorine substituent at different positions, the most electronegative porphyrin (F20TPP)FeCl was found to be the most active catalyst for the epoxidation of cyclooctene.17,18 When the -R group is replaced by NH2 or N(CH3)2, the Q O values become more negative due to the electron-donating effect. As a result, the CH activation barriers with NIL-2 and NIL-3 as catalysts are 3.9 and 3.5 kJ/mol higher than that of NIL-1. 3.2.2. IL-Catalysts with Different Cations. The NH2 or N(CH3)2 groups forming hydrogen bonds with some acids (such as HCl or HNO3) can reduce the electron-donating effect and lower the CH activation barriers (NIL 3 3 3 HCl vs NIL-2) (Figure 3). This finding enlightens us to explore some effective and simple methods to further reduce the electron-donating effect of the N atom. IL modification is an excellent choice because the
nitrogenous group of IL is electrophilic, and many simple methods are available to synthesize this compound.2830 Previous experimental studies have revealed that some IL modifications can enhance the catalyst reactivity.12,31,33,6062 For example, the reactivity of the Salen catalyst could be obviously enhanced through IL-modification.33 However, some studies suggest that IL-modification weakens the reactivity.6,63 Furthermore, experimental studies have also shown that there were no obvious differences between the reactivity of an IL-modified catalyst and its unmodified counterpart.64,65 The influence of IL-modification on the reactivity seems quite complicated. The following section will explain the reason for the complication. Our studies show that IL-modification can reduce the reaction barriers under several restrictions. First, the cation center must connect with the active center directly. For example, the CH activation barriers with [IL-1][Cl] as a catalyst is 11.7 kJ/mol lower than that catalyzed by NIL-1 (Figure 4). Although any kind of cations among ammonium, pyridinium, and imidazolium salts can enhance the reactivity of the catalyst, ammonium salt ([IL-1][Cl]) is more powerful than pyridinium and imidazolium salts ([IL-4][Cl] and [IL-8][Cl]). When the cation center connects with the active center indirectly, IL-modification has almost no influence on the catalytic reactivity. For example, the cation part and the active center of [IL-2][Cl] are connected by the CH2CH2 group, and those of [IL-3][Cl] are connected by the CH2CH2CH2CH2 group. The reactivities of [IL-2][Cl] and [IL-3][Cl] are almost the same as that of NIL-1. Many of the previous studies have adopted a similar indirect connection, and its influence on catalytic reactivity was found to be neglectable.64,65 Second, the electron-donating substituent on the N atom should be avoided. For example, the reactivity of [IL-4][Cl] is 5.8 and 5.1 kJ/mol higher than those of [IL-5][Cl] and [IL-6][Cl], respectively. The two IL-modified catalysts ([IL-7][Cl] and [IL-9][Cl]) investigated in the present study show lower reactivity compared with NIL-1. The common attribute of these two catalysts is that 23916
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Figure 3. Optimized transition states catalyzed by different NIL-catalysts. The values in parentheses are the reaction barriers in kJ/mol. The other values denote bond lengths in Å.
their anions interact with the porphyrin ring, which results in an increase of electrons on the O atom (Table 1). 3.2.3. IL-Catalysts with Different Anions. The anion of IL has a remarkable influence on its property, and replacing the anion through ion exchange is quite easy.2830 A total of 13 different anions were used in the present article. These anions include BF4, PF6, AsF6, SbF6, Tf2N, CF3SO3, CF3CO2, HSO4, AlF4, AlCl4, AlBr4, BCl4, and Cl. As mentioned above, all [IL-1][X] catalysts have bigger SDO and smaller SDFe values than the NIL-catalysts. Correspondingly, the CH activation barrier with any [IL-1][X] as catalysts is smaller than that catalyzed by any NIL-catalysts (Figure 5). The reactivity of the catalysts is strongly affected by the counterion and follows the order AlBr4 > AlCl4 > BCl4 > PF6 > AsF6 > SbF6 > BF4 > Tf2N > AlF4 > HSO4 > CF3SO3 > CF3CO2 > Cl. In the asymmetric hydrogenation of unfunctionalized olefins with cationic iridiumPHOX catalysts, BF4, PF6, and CF3SO3 have been used as the counterions. The experimental results revealed that the reactivity of the catalysts is PF6 > BF4 > CF3SO3, which shows the same order as our calculated results.66 In our previous experimental research, the catalytic activities of the IL-modified Salen-catalysts with different counteranions followed the order PF6 > BF4 > CF3CO2 . NIL-catalyst.33
A total of nine different cations were tested, and the barrier ranges from 105.8 to 120.3 kJ/mol. Thirteen different anions were investigated, and the barrier ranges from 92.6 to 105.8 kJ/mol. The anion is more powerful in increasing the reactivity of the IL-catalyst than the cation. Fortunately, changing the anion is easier than replacing the cation from the synthesis viewpoint.2830 What kind of parameters of the anions are responsible for the reactivity of the IL-catalyst is an interesting question. A series of parameters of the anions were investigated, including the ionization energy, electron affinity (EA) of corresponding radical, charge of the anion in the IL-catalyst, and volume. It was found that the EA values were dominant to control the reactivity (Figure 6). The following relationship was found: Barrier ¼ 0:0559EA þ 120:98 ðR 2 ¼ 0:816Þ Electron transfer between the anion and cation happens during the binding process. If the electron transfer number (N) was taken into consideration, a new relationship was found as follows: Barrier ¼ 0:0542EA N þ 114:35 ðR 2 ¼ 0:838Þ There are three data points obviously deviating from the others (Figure 6). These three data points belong to AlBr4, AlCl4, and 23917
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Figure 4. Optimized transition states catalyzed by IL-catalysts with different cations. The values in parentheses are the reaction barriers in kJ/mol. The other values denote bond lengths in Å.
BCl4, which are quite different from other anions because of the existence of four Cl or Br atoms. Maybe there are some other parameters of the anion to control the reactivity, which can not be found out in the present study. 3.3. StructureReactivity Relationship of IL- and NIL-Catalysts. Predicting the reactivity of a catalyst based on its structure is the aim of catalyst design. Each catalyst investigated in the present article has its unique attributes. On the basis of the above investigation, some relationships can be established between the reactivity and the values of the SD, Q , IFCC, and HOMOLUMO gap. 3.3.1. Relationship between SD and Reactivity. The reaction barriers were used as the y-axis, and the SD carried by Fe and O were used as the x-axis (Figure 7). A linear or almost linear relationship was found between the SDFe (or SDO) and the reactivity. The correlation coefficients are 0.857 and 0.949 for SDFe and SDO, respectively. Barrier ¼ 509:17SDFe 410:63
modification leading to smaller SDFe and larger SDO will yield a more powerful catalyst. According to a previous report, (N4Py)FeIVdO is more reactive than Compound I of P450.43 It is interesting to find that the SDFe value of the low-spin state (N4Py)FeIVdO is 1.06, whereas the same value for Compound I of P450 is 1.17. At the same time, the SDO value of (N4Py)FeIVdO is 0.98, whereas the same value for Compound I of P450 is 0.92.42,43 3.3.2. Relationship between Q and Reactivity. The reaction barriers were used as the y-axis and the Q values carried by Fe and O were used as the x-axis (Figure 8). No linear relationship was found between Q Fe and the reactivity. However, Q O values show an almost linear relationship with the reactivity of the catalyst. This finding suggests that the more negative charge the O atom carries, the less powerful the catalyst is. Barrier ¼ 783:69Q O 142:96
ðR 2 ¼ 0:857Þ
Barrier ¼ 555:35SDO þ 666:18
ðR 2 ¼ 0:949Þ
Designing a more powerful metalloporphyrin catalyst is much easier using these formulas. The calculated results suggest that
ðR 2 ¼ 0:923Þ
3.3.3. Relationship between IFCC and Reactivity. IFCC relates to the hyperfine splitting constant of the corresponding EPR spectrogram, which is a useful property for understanding the structure of a compound with unpaired electrons. A relationship between the IFCCO and the reactivity was found (Figure 9). That is, 23918
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Figure 5. Optimized transition states catalyzed by IL-catalysts with different anions. The values in parentheses are the reaction barriers in kJ/mol. The other values denote bond lengths in Å.
Figure 6. Relationships between the reaction barriers and the EA/ EA*N values. The R2 values in the parentheses were obtained without the three data points in the ring.
modification that leads to larger IFCCO yields a more powerful catalyst. Barrier ¼ 7834:3IFCCO þ 1428:2
ðR 2 ¼ 0:854Þ
3.3.4. Relationship between HOMOLUMO Gap and Reactivity. The HOMOLUMO gap is usually used to predict the reactivity of catalysts. Two kinds of HOMOLUMO gaps were investigated in the present study as follows: the HOMO LUMO gap of the catalyst (HOMOCLUMOC) and the HOMO LUMO gap between the catalyst and reactant (LUMOC HOMOR) (Figure 10). On the basis of the analysis of several different catalysts, it was found that the HOMOLUMO gap of the catalyst did not correlate with the reactivity. This result does not agree with some previous studies that suggested a smaller HOMOLUMO gap of the catalyst lead to a higher reactivity. This relativity may be
Figure 7. Relationships between the reaction barriers and the SDFe/ SDO values.
occasional because only three or four catalysts were tested.67,68 Indeed, the HOMOLUMO gap of the catalyst describes the ability of the electron transfer from the HOMO to the LUMO, which is not always related to the catalytic reactivity. For the CH activation investigated here, the electron transfers from the reactant to the catalyst during the reaction. The HOMOLUMO gaps between the catalyst and reactant were carefully investigated, and these gaps were correlated with their reactivity (Figure 10). A relationship was found to exist. This finding suggests that the smaller the HOMOLUMO gap between the catalyst and reactant is, the more powerful the catalyst is. Barrier ¼ 13:30Gap þ 49:01
ðR 2 ¼ 0:885Þ
The following structural parameters have been correlated with the reactivity: SDFe, SDO, Q Fe, Q O, IFCCO, LUMOCHOMOC, and LUMOCHOMOR. The order of the correlation coefficient is R2 23919
dx.doi.org/10.1021/jp208441j |J. Phys. Chem. C 2011, 115, 23913–23921
The Journal of Physical Chemistry C
Figure 8. Relationships between the reaction barriers and the Q Fe/Q O values.
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BF4 > Tf2N > AlF4 > HSO4 > CF3SO3 > CF3CO2 > Cl. This order is correlative with the EA values of the corresponding radical of the anion. For the cations, the ammonium salt is more powerful in altering the reactivity than pyridinium and imidazolium salts. (3) The following factors can weaken the influence induced by IL-modification: long distance between the IL part and the catalytic active center and electron-donating substituent on the N atom. Previous experimental studies have revealed that an electronegative substituent can enhance the reactivity of metalloporphyrin.8,17,18,59 IL-modification was found to increase,12,31,33,6062 decrease,6,63 or have no influence on the reactivity of the catalyst.64,65 Our theoretic studies can well explain these experimental results and predict more powerful metalloporphyrin catalysts. The current IL-modification methods reported in the references are used mainly for making the catalyst reusable.6,7,9,1115,2830,6065 The structurereactivity relationships proposed in the present study are expected to be used in designing a recyclable metalloporphyrin catalyst with more powerful reactivity.
’ ASSOCIATED CONTENT Figure 9. Relationship between the reaction barriers and the IFCCO.
bS
Supporting Information. Data of the low-spin state metalloporphyrin and full citation of ref 57. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (no. 20803062 and 21176110) and Jiangsu Province Natural Science Foundation (SBK201021038). Figure 10. Relationship between the reaction barriers and HOMOC LUMOC/ LUMOCHOMOR.
(SDo) > R2 (Q o) > R2 (IFCCO) ≈ R2 (SDFe) ≈ R2 (LUMOC HOMOR) . R2 (QFe) ≈ R2 (LUMOCHOMOC) ≈ 0.
4. CONCLUSIONS The structures and reactivities of 34 different IL- and NILmodified metalloporphyrin catalysts were investigated. On the basis of the results obtained from our investigations, the following can be deduced: (1) IL-modification can influence the following parameters: SDFe, SDO, Q Fe, Q O, IFCCO, R(FeO), and LUMO HOMO. The order of their correlation coefficient to the reactivity is R2 (SDO) > R2 (Q O) > 0.9 > R2 (IFCCO) ≈ R2 (SDFe) ≈ R2 (LUMOCHOMOR) > 0.85 . R2 (Q Fe) ≈ R2 (LUMOCHOMOC) ≈ 0. A modification that increases the SDO, Q O, and IFCCO or decreases SDFe and LUMOCHOMOR yields a more powerful catalyst. (2) Changing the anion is a more effective way to increase the reactivity compared with changing the cation. For the anions, the order of the ability to affect the reactivity is AlBr4 > AlCl4 > BCl4 > PF6 > AsF6 > SbF6 >
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