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Theoretical study of the effect of ligand topology on Fe(IV)O and Ru(IV)O complex reactivities
Inorganica Chimica Acta 443 (2016) 235–242
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Theoretical study of the effect of ligand topology on Fe(IV)O and Ru(IV)O complex reactivities Zhe Tang a, Yi Wang a,⇑, Xiaolei Cui a, Ying Yang a, Jing Tian a, Xu Fei a, Shuangjiang Lv b,⇑ a b
School of Biological Engineering, Dalian Polytechnic University, Dalian 116034, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China
a r t i c l e
i n f o
Article history: Received 5 November 2015 Received in revised form 29 December 2015 Accepted 2 January 2016 Available online 8 January 2016 Keywords: Non-heme Density functional theory calculation Hydrogen abstraction Ligand topology effect
a b s t r a c t Density functional theory calculations were carried out to clarify the effect of ligand topology on the stability and reactivity of cis-a-[FeIV(O)(BQCN)]2+ (Fe-2a), cis-b-[FeIV(O)(BQCN)]2+ (Fe-2b), cis-a-[RuIV(O) (BQCN)]2+ (Ru-2a) and cis-b-[RuIV(O)(BQCN)]2+ (Ru-2b) (BQCN = N,N0 -dimethyl-N,N0 -bis(8-quinolyl)cyclohexanediamine). All the iron and ruthenium isomers possess the triplet ground states. The relative stability between the two iron isomers follows the order of Fe-2a > Fe-2b, which is in agreement with the conclusions of Hong et al. Moreover, the trend of the relative stability of the two ruthenium isomers is Ru-2a > Ru-2b. The Density-of-States spectrums represent that the contribution of BQCN ligand to 3 Fe-2b is more than the corresponding contribution to 3Fe-2a. The iron isomers react with isopropylbenzene via a two-state reactivity pattern on competing triplet and quintet spin states, while the ruthenium isomers react with isopropylbenzene by a single-state mechanism, only on the triplet spin state. The H-abstraction is affected by the tunneling contribution, which can decrease the reaction barriers. By adding the ZPE correction, PCM model assessed by DFT-D3 (BJ) and the tunneling correction, the analysis of the trend of the hydrogen-abstraction reactions barriers shows that Fe-2a is the more reactive than Fe-2b with the higher FeIV/III redox potential. Moreover, for the ruthenium complexes, although Ru-2a is the more reactive than Ru-2b, only 0.6 kcal/mol lower. Above all, the ligand topology has little effect on the reactivities of the [RuIV(O)(BQCN)]2+ complexes. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Oxygen-activating enzymes with mononuclear non-heme iron active sites participate in many metabolically important reactions that have environmental, pharmaceutical, and medical significance [1]. Over the past 15 years, studies have demonstrated that the iron (IV)-oxo state is accessible synthetically in a non-heme ligand environment [2]. Since the first crystal structure of a synthetic mononuclear non-heme iron(IV)-oxo complex was reported in a chemical model of non-heme iron enzymes in 2003 [3], a variety of mononuclear non-heme iron(IV)-oxo complexes bearing tetradentate N4 and pentadentate N5 and N4S ligands have been synthesized and characterized using various techniques, such as Mössbauer resonance and Raman and X-ray absorption spectroscopy, and density functional theory calculations [3–45]. In the biomimetic field, the reactivity and stability of non-heme iron(IV)-oxo complexes are sensitive to the ligand structure [40,46]. A number of important factors control the reactivities of ⇑ Corresponding authors. Fax: +86 0411 86323646. E-mail addresses: [email protected] (Y. Wang), [email protected] (S. Lv). http://dx.doi.org/10.1016/j.ica.2016.01.004 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
non-heme iron complexes in oxidation reactions. One such factor that determines the reactivities of iron catalysts is the ligand structure around the iron center [47]. Linear tetradentate N4 ligands can form around an octahedral iron center in three different topologies: cis-a, cis-b, and trans forms [47]. The cis-labile exchangeable sites which could affect the mechanism of the oxidation in iron(V)-oxo complex have been studied by Prat I et al. [48]. Until 2011, the effect of topology on non-heme iron(IV)-oxo complex reactivity had not been explored. Mononuclear non-heme iron(IV)-oxo complexes with two different topologies have been synthesized and characterized using various spectroscopic methods by Hong et al. [47]. The complexes are cis-a-[FeIV(O)(BQCN)]2+ (Fe-2a) and cis-b-[FeIV(O)(BQCN)]2+ (Fe-2b), in which the N-methyl groups are anti-aligned and aligned, respectively, (BQCN = N,N0 -dimethyl-N,N0 -bis(8-quinolyl)cyclohexanediamine). The 1H NMR spectra of Fe-2a and Fe-2b exhibit distinct peaks, which indicates that they each retain their respective ligand topologies in solution [47]. The FeIV/III redox potential of Fe-2a is 0.11 V higher than Fe-2b, and both of them are more reactive than [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), [FeIV(O)
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(Bn-TPEN)]2+ (Bn-TPEN = N-benzyl-N,N0 ,N0 -tris(2-pyridylmethyl)1,2-diaminoethane), and [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11tetramethyl-1,4,8,11-tetraazacyclotetradecane) [3,12,13,46]. Ligand topology is therefore an important factor that affects oxidizing power of the non-heme iron (IV)O complexes. However, little is known about it to date, except the theoretically-reported reaction barrier of Fe-2a [24]. The fundamental reasons for the different stability and reactivity of Fe-2a relative to Fe-2b are of great significance for the better understanding of the topological effects of equatorial ligands. Nonetheless, the detailed theoretical study on the effect of ligand topology on the non-heme Fe(IV)O complex reactivity remains vacant until now. In order to determine the ligand structures around the ruthenium center, a hypothetical [RuIV(O)(BQCN)]2+ complex, which contains two isomers Ru-2a and Ru-2b, was theoretically designed. Therefore, we have performed a density functional theory (DFT) calculation of the geometric and electronic structures, the tunneling effect, the FeIV/III redox potentials, and the isopropylbenzene hydrogen abstraction by the iron and ruthenium complexes.
UTPSSh functional is shorter on the quintet spin state. As expected, the UOPBE/B2//UOPBE/B1, UTPSSh/B2//UTPSSh/B1, UOPBE/B2// UB3LPY/B1, and UTPSSh/B2//UB3LYP/B1 results are comparable with those obtained from UB3LYP/B2//UB3LYP/B1. Therefore, all functionals give the same ordering of species and we selected UB3LYP as the method of choice. The computational details are presented in the Supplementary electronic material (SI). 2.3. Tunneling corrections Eckart tunneling calculations were performed using TheRate Program [57]. Due to the tunneling, the transmission coefficient j, is calculated by integrating of the barrier ‘‘penetration” probability as a Boltzmann-averaged function of the energy [58]. The effect of the transmission coefficient on the barrier is calculated by the equation [59]:
DDE# tun ¼ RT ln jðTÞ
ð1Þ
2. Computational methods
where R denotes the universal gas constant and T is the absolute temperature. Generally, the experimental rate data were collected at 273 K.
2.1. Standard methods
2.4. Electronic structures of the Fe-2a and Fe-2b
We considered the triplet and quintet spin states for Fe-2a and Fe-2b; and the triplet spin states for Ru-2a and Ru-2b. DFT calculations were performed with the GAUSSIAN 09 suite of a quantum chemical package [49], and the spin-unrestricted Becke, threeparameter, Lee–Yang–Parr (B3LYP) functional [50] as the method of choice. The geometries for the four non-heme complexes were fully optimized without symmetry constraints. The Lanl2DZ basis set [51] was used for iron and ruthenium, moreover 6-311G⁄⁄ [51c,52] for the other atoms. To save computational resources, for the reaction pathways, we used the Lanl2DZ basis set for iron and ruthenium, the 6-311G⁄⁄ basis set for electron-rich O atom, and the 6-31G⁄⁄ basis set for C, H, and N atoms (B1 in brief). This basis set is used to optimize transition states and minima. All transition states were confirmed by one imaginary mode from vibrational analysis calculations and the local minima described here only have real frequencies. Single-point energy calculations were performed with a larger basis set TZVP [53] for all atoms (B2 in brief) of iron complexes. For ruthenium complexes, the def2-TZVP [53] on Ru and TZVP for other atoms were used for single-point energy calculation (B2 in brief). To evaluate the solvent effect, acetonitrile was used in the self-consistent reaction field calculations according to the polarizable continuum model (PCM), and the PCM model was assessed by DFT-D3 (BJ) [54] (with Grimme’s 2011 dispersion correction) and the SMD model. The mechanism of hydrogen abstraction reaction was determined using intrinsic reaction coordinate studies.
In order to understand the ligand topology effect better, we calculated the electronic structures, including Orbital population, Wiberg bond order, Hirshfeld charge, Density-of-States (DOS) and Overlap Population Density-of-States (OPDOS) Spectrums. All the calculations were performed using Multiwfn program [60].
2.2. Tests of the functionals It has been reported that the UB3LYP functional has verified the spin-state splitting of mononuclear non-heme iron complexes. However, in this work, the focus is on two iron complexes with triplet and quintet states, which are close in energy, so it is necessary to test the functionals. To determine the choice of UB3LYP, we used both of the UOPBE [55] and UTPSSh [56] functionals. For the two iron complexes, all three functionals predict that Fe-2a is the more stable than Fe-2b. Tests of the relative energy and geometric parameters of the transition states for the reactions of two iron isomers with isopropylbenzene were performed with the three functionals. Because the UTPSSh underestimates the relative energy of the quintet spin state, the O–H distance which is calculated by the
3. Results and discussion 3.1. Properties of the four complexes Our computational study produced geometric and electronic structures of the two iron and ruthenium isomers. The two possible iron and two hypothetical ruthenium isomers, which have the same BQCN ligand but different ligand wrapping around the metal center, will be discussed in detail. Structurally, the N-methyl groups of the BQCN ligand have different configurations in the two topologies: anti-aligned in the cis-a isomer (2a) and aligned in the cis-b isomer (2b) [47]. The two ruthenium isomers followed by metal center exchange of the iron by ruthenium. We set z to be along the Fe–O axis, whereas x and y are along the equatorial nitrogens. 3.1.1. Properties of the two iron isomers The geometric structures for the two iron isomers are shown in Fig. 1, with the corresponding energies of the singlet, triplet and quintet spin states relative to the triplet state at the higher level, UB3LYP/B2//UB3LYP/B1. The key geometric and electronic parameters in the UB3LYP/B1 optimized geometries of the corresponding isomers are listed in Table 1. The bond length and bond order reveal that the Fe–O bond typically has double bond character and the bond length does not change significantly from the triplet to the quintet spin state, because r⁄x2y2 is strongly Fe–N r-antibonding and is perpendicular to the Fe–O bond. The calculated Fe–O bond lengths for the two isomers are 1.630 Å and 1.626 Å, respectively, which are similar to that in crystal structures of some other non-heme iron(IV)-oxo complexes with an S = 1 spin state [3,8]. On the triplet/quintet spin state, the Wiberg bond orders between the Fe fragment and the O fragment are 1.640/1.671 for Fe-2a and 1.649/1.686 for Fe-2b, respectively. The reason that we have chosen the Wiberg bond order and the Hirshfeld charge analyses has been noted in SI.
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Fig. 1. The geometric structures of Fe-2a, Fe-2b, Ru-2a and Ru-2b. The B2 level relative energies (in kcal/mol) of the
1,3,5 Z KL
species.
Table 1 The geometric parameters, Wiberg bond order and Hirshfeld charge of O atom for the two iron isomers and two ruthenium isomers. Fe-2a
Bond length
Wiberg bond order Hirshfeld charge a
a
M=O M-NCMe M-Naxial M=O M-NCMe O
Ru-2a
Ru-2b
T
Q
Fe-2b T
Q
T
T
1.630 2.035 2.176 1.640 0.660 0.245
1.626 2.303 2.169 1.671 0.367 0.240
1.626 2.030 2.127 1.649 0.680 0.235
1.622 2.234 2.089 1.686 0.451 0.228
1.788 2.123 2.293 1.810 0.881 0.241
1.787 2.128 2.241 1.800 0.883 0.234
M = Fe/Ru.
The high- and low-lying virtual orbitals of the two isomers are dominated by the d-block orbitals. The triplet state has a d2pxz⁄1pyz⁄1 configuration, whereas the quintet state has a d1pxz⁄1pyz⁄1rx2y2⁄1 configuration. DFT calculations show that the triplet spin state is the ground state, which is favored energetically by 1.4 kcal/mol and 3.2 kcal/mol below the quintet state for Fe-2a and Fe-2b at the B2 level, respectively. Complexes Fe-2b is 3.6 kcal/mol less stable than Fe-2a (Fig. 1). These results agree with a previous study [47]. The equatorial ligand affects the energy of the orbitals lying in the plane, namely the d and r⁄x2y2 orbitals [61]. Because the quintet spin state arises from the triplet state by the promotion of one electron from the d to the r⁄x2y2 orbital, the triple-quintet energy gap agrees with the calculated d–r⁄x2y2 orbital energy gap [13,62,63]. As shown in Fig. 2, the d–r⁄x2–y2 orbital energy gap is 5.77 eV and 6.01 eV for 3Fe-2a and 3Fe-2b, respectively. For the same metal center, therefore, as the orbital energy decreases, the triplet-quintet energy gap decreases. It is important to note that on the triplet spin state, the b-LUMO (Lowest Unoccupied Molecular Orbital) is the p⁄xz orbital, which is
the best electron acceptor orbital. Figs 3 and 4 offer the contributions of the different ligands to the chemical bonding through the DOS (Density-of-States) and OPDOS (Overlap Population Density-of-States). In contrast to 3Fe-2b, the contribution of BQCN ligand to TDOS (Total Density-of-States) is less to 3Fe-2a on the b-LUMO. Furthermore, the negative OPDOS region represents the anti-bonding interaction. The anti-bonding interaction between the Fe and O fragments of 3Fe-2a is stronger than that of 3Fe-2b. 3.1.2. Properties of the two ruthenium isomers Fig. 1 also shows that the two ruthenium isomers have the triplet ground states, which is the same with the ruthenium-oxo porphyrin complexes, the ground state was found to be a low-spin [64], and the triplet spin state is favored energetically by 20.4 and 21.6 kcal/mol below the singlet spin state, and 60.9 and 60.1 kcal/mol below the quintet spin state, respectively. Due to the larger atomic radius of ruthenium, the splitting energy of the ruthenium complexes is larger than that of the iron complexes, thus, the interaction between the ruthenium center and equatorial
Fig. 2. Energy level diagram and iron d-based molecular orbitals of the two iron isomers. LUMO lowest unoccupied molecular orbital, HOMO highest occupied molecular orbital (UB3LYP/B2 level).
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Fig. 3. Density-of-States (DOS) and Overlap Population Density-of-States (OPDOS) spectrums of 3Fe-2a (UB3LYP/B2 calculation).
Fig. 4. Density-of-States (DOS) and Overlap Population Density-of-States (OPDOS) spectrums of 3Fe-2b (UB3LYP/B2 calculation).
ligand is larger. Comparing with the iron complexes, the ruthenium complexes are preferred in the triplet spin state. DFT calculations show that 3Ru-2b is 5.2 kcal/mol less stable than 3Ru-2a, which is in the same trend of the iron isomers in which 3Fe-2a is more stable. For the ruthenium isomers, the quintet spin state is too high to participate in the reaction, thus the reactions only involve the triplet spin state. As shown in Table 1, the main structures of the two isomers reveal that the Ru–O distance is 1.788 and 1.787 Å for 3Ru-2a and 3Ru-2b, respectively. Moreover, the Wiberg bond orders between the Ru and O fragments are 1.810 and 1.800 for 3Ru-2a and 3Ru-2b, respectively. Thus, the RuO bond also has the double bond character. 3.2. Theoretical calculations of C–H bond activation by the four isomers As proposed previously for hydrogen abstraction by non-heme iron species, in all mechanisms, the initial step involves the
formation of a reactant cluster followed by a transition state for hydrogen abstraction that leads to the product [13]. The energy profiles for hydrogen abstraction from isopropylbenzene by the two iron isomers are shown in Fig. 5 and by the two ruthenium isomers are shown in Fig. 7. For the two iron complexes, the reactions proceed via two closely located spin states, including triplet and quintet spin states, and for the two ruthenium isomers, the reactions proceed only via the triplet spin state, because the quintet spin states are approximately 60 kcal/mol higher than those on the triplet spin states. 3.2.1. Energy profiles for the reaction between Fe-2a and CH The hydrogen abstraction reaction of Fe-2a with isopropylbenzene is initiated on the triplet ground spin state surface (see Fig. 5). The initial stage of this reaction is the formation of a reactant complex, before the hydrogen atom of CH is abstracted by Fe-2a to yield the product via a transition state. At the B2//B1 level, the gas barrier (relative to the 3RC) is 14.2 kcal/mol for the triplet spin
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structure as a binding between the FeIV and O atoms. The two valence orbitals, which are denoted as dxz and dz2, are usually considered to be active on the triplet and quintet spin state, respectively. The CH attacks the oxo group from the side, and the triplet reaction pathway is the p-pathway [40]. The Fe–O–H angle is 129.20°. In the corresponding quintet spin state, the reaction pathway should be the r-pathway [40], the CH attacks the oxo group from the top to yield an Fe–O–H angle of 163.71°. To better explain the oxidation states and the electronic structures of Fe-2a, we show in Table 3 the spin densities and Mulliken charges. The FeO moiety possesses spin densities of 1.10/3.00 in iron and 0.98/0.78 in oxygen of 3,5RC, whereas the corresponding moiety of 3,5TSH possesses spin densities of 0.87/3.58 in iron and 0.75/0.46 in oxygen (see Table 3). On the reaction pathway, the spin densities and the geometric parameters are similar with the previous study [24] (Table S6).
Fig. 5. Energy profiles (kcal/mol) for hydrogen-abstraction reactions between the cumene and [FeIV(O)(BQCN)]2+ complexes in the triplet and quintet spin states. Relative energies are given for UB3LYP/B2 level and for the UB3LYP/B2 + ZPE + Esolv level.
state and 7.8 kcal/mol for the quintet spin state. By taking into account the environmental effects and the zero-point-energy correction, the barrier is found to be 8.9 kcal/mol for the triplet spin state and 5.9 kcal/mol for the quintet spin state (relative to the 3 RC). With the tunneling correction considered, the effective barri3 ers DE2# eff is reduced (see Table 2). The TSH possess an energy of 5 8.0 kcal/mol, while the TSH is 5.8 kcal/mol. Thus, Fe-2a is more reactive than some other non-heme iron (IV)-oxo complexes [3,12,13,38,46]. Because the iron active center is coordinately saturated, the C–H–O triad stays almost collinear and there is no direct interaction between the iron center and the carbon. As shown in Fig. 6, the C–H distance of 3TSH is longer than that of 5TSH, whereas the O–H distance of 3TSH is shorter than that of 5TSH. These differences imply that 5TSH will be formed ‘‘early”. The key difference in the transition state structures between the triplet and quintet spin states is the Fe–O–H angle, which results from a balance between the orbital interaction and the Pauli repulsion from the equatorial ligand [61]. A simple way to analyze the orbitals is to consider the
3.2.2. Energy profiles for the reaction between Fe-2b and CH The reactivity of Fe-2b is similar to that of Fe-2a toward CH (Fig. 5). The reaction barrier is 14.5 kcal/mol for the triplet spin state and 9.0 kcal/mol for the quintet spin state at the B2//B1 level (relative to 3RC). Inclusion of the solvation correction and the zeropoint correction decreases the barrier to 10.5 and 7.2 kcal/mol for the triplet and quintet spin states, respectively (relative to 3RC). The barriers decrease with the addition of tunneling effects, those become 9.5 kcal/mol on the triplet state and 7.1 kcal/mol on the quintet state, respectively (see Table 2). The reaction still follows the two-state reactivity (TSR) pattern [65–68]. The geometric C–H–O/Fe–O–H angles for 3TSH and 5TSH are 160.70/127.90° and 176.72/159.47°, respectively (see Fig. 6). As listed in Table 3, the calculated spin densities on the FeO moiety generated are 1.09/2.96 in iron and 0.98/0.78 in the O atom in 3,5RC. Furthermore, the corresponding moiety possesses spin densities of 0.86/3.56 in iron and 0.76/0.45 in the O atom in 3,5TSH. 3.2.3. Energy profiles for the reactions between the Ru isomers and CH To establish the oxidative capability of the Ru isomers, we studied the hydrogen-abstraction of isopropylbenzene by Ru complexes. Interestingly, as shown in Fig. 7, our calculations show that the Ru-isomers are more reactive than many other Ru complexes [23,69]. The small activation energy barriers are 18.8 and 19.6 kcal/mol for Ru-2a and Ru-2b, respectively. Adding zero-point energy correction and solvation correction, the reaction barriers have decreased. The tunneling effect correction leads to the decrease of the energy gap, DE2# eff become 12.1 kcal/mol and 12.7 kcal/mol for Ru-2a and Ru-2b, respectively (see Table 2). Moreover, the geometry of the two isomers is similar with each other (Fig. 8).
Fig. 6. Orbitals showing electronic delocalization and the geometric details (lengths in Å and angles in °) of the 3,5TSH species in the hydrogen-abstraction reactions between the cumene and the two iron isomers.
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Fig. 7. Energy profiles (kcal/mol) for hydrogen-abstraction reactions between the cumene and [RuIV(O)(BQCN)]2+ complexes in the triplet spin state. Relative energies are given for UB3LYP/B2 level and for the UB3LYP/B2 + ZPE + Esolv level.
Table 2 Transmission coefficients, tunneling corrections and the reaction barriers for the two iron isomers and two ruthenium isomers. Fe-2a
j (T)
DDE# tun DE1(B2//B1) DE2(B2//B1+Esol+ZPE) DE1# eff DE2# eff
Fe-2b
Ru-2a
Ru-2b
T
Q
T
Q
T
T
4.939 0.9 14.2 8.9 13.3 8.0
1.101 0.1 7.8 5.9 7.7 5.8
5.840 1.0 14.5 10.5 13.5 9.5
1.116 0.1 9.0 7.2 8.9 7.1
31.28 1.9 18.8 14.0 16.9 12.1
47.13 2.1 19.6 14.8 17.5 12.7
The tunneling corrections calculated at the UB3LYP/B1 level. The energies are in # # # kcal/mol. DE1# eff = DE1(B2//B1) + DDEtun; DE2eff = DE2(B2//B1+Esol+ZPE) + DDEtun.
Table 3 Calculated Mulliken charges and Mulliken spin densities for the Fe, O and H moieties in the model systems of the two iron isomers. Triplet state
2a RC TS Pro 2b RC TS Pro
Quintet state
Fe
O
H
Fe
O
H
1.10 0.53 0.87 0.55 0.94 0.48
0.98 0.33 0.75 0.50 0.16 0.56
0.00 0.09 0.01 0.25 0.00 0.32
3.00 0.66 3.58 0.72 3.75 0.66
0.78 0.33 0.46 0.50 0.02 0.55
0.00 0.09 0.02 0.22 0.00 0.33
1.09 0.51 0.86 0.56 0.91 0.56
0.98 0.32 0.76 0.50 0.18 0.51
0.00 0.10 0.01 0.25 0.01 0.31
2.96 0.64 3.56 0.73 3.74 0.67
0.78 0.32 0.45 0.50 0.02 0.59
0.00 0.10 0.02 0.22 0.00 0.34
Numbers in bold are Mulliken charge data. RC: reactant complex; TS: transition state; Pro: product.
Fig. 8. Orbitals showing electronic delocalization and the geometric details (lengths in Å and angles in °) of the 3TSH species in the hydrogen-abstraction reactions between the cumene and the two ruthenium isomers.
flip, the electron in the d orbital in the triplet spin state shifts to the r⁄x2y2 orbital, the SOC between the states would be maximized when both orbitals are localized on iron. As far as we know [13], the decreased iron weight of the r⁄x2y2 orbital will decrease the initial SOC. It can be seen from Fig. 9, the weight of the r⁄x2y2 orbital on iron are small and have followed the trend: Fe-2b < Fe-2a. Moreover, there are no experimental results [47] which have confirmed spin state transitions in BQCN. Thus, due to the low spin orbital coupling, the reactions mainly occur in the triplet spin state. Thus, we may conclude that the H-abstraction reactions pass via the S = 1 surface. To compare the relative reaction energies, the different topology of the equatorial ligand, this results in the different transition state barriers. On the triplet spin state, the CH attacks the oxo group from the side. After adding ZPE and solvation corrections, the activation energies are 8.9 kcal/mol for 3Fe-2a and 10.5 kcal/mol for 3Fe-2b, respectively, with that the higher FeIV/III redox potential, the higher reactivity is (see Table S8). As shown in Table 1, the Hirshfeld charges on the oxygen atom are 0.245 and 0.235 for 3Fe-2a and 3Fe-2b, respectively. As far as we know, the more negative charges on the atom, the more prone to occur the nucleophilic reaction. In order to establish the relative reactivities of the two iron isomers, we studied the effect of the tunneling on the reaction barrier of the H-abstraction. Adding the tunneling correction reduces the reaction barrier, as shown in Table 2. The influence of the tunneling effect on the triplet spin state is more than that on the quintet spin state. Along the reaction pathway, the DDE# tun is 0.9 kcal/mol for the 3Fe-2a, while it is 1.0 kcal/mol for 3Fe-2b. Thus, the different ligand topologies have little effect on the tunneling effect. At the B2//B1 + ZPE + Esolv + DDE# tun level, the triplet and quintet reaction barriers are 8.0/5.8 kcal/mol for Fe-2a, 9.5/7.1 kcal/mol for Fe-2b,
3.3. Discussions In our study, we have calculated the electronic structures, geometric structures, and the reaction pathways of the H-abstraction reactions of the four complexes. We now focus on the reasons and elucidate the effect of the ligand topology on the Fe(IV)O and Ru(IV)O complex reactivities. It has been reported that [24], for non-heme FeIVO species, the reactant ground state is a low-spin triplet state, whereas the lowest energy TS is the high spin quintet spin state. The spin–orbital coupling (SOC) interaction between the triplet and quintet states should affect the spin inversion [13,70,71]. As reported in the previous studies [70], during the spin
Fig. 9. r⁄x2y2 orbitals and the weight of these orbitals for two iron complexes on the quintet spin state.
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respectively. Thus, the reaction barrier difference is 1.5 kcal/mol for the two iron isomers. The potential energy profiles and the geometric structures of the hydrogen abstraction transition state on the triplet and quintet spin states for the two iron isomers are shown in Figs. 5 and 6, respectively. A comparison of the electronic and geometric structures between the different transition states is necessary. According to frontier molecular orbital theory, on the triplet state, when the fifth electron is added to the p⁄xz orbital, the d5 electrons will have a (d)2(p⁄xz)2(p⁄yz)1 configuration. In the quintet spin state, one electron transfers from the isopropylbenzene into the r⁄z2 orbital and the d5 electronic configuration is (d)1(p⁄xz)1(p⁄yz)1(rx2y2⁄ )1(r⁄z2)1. Therefore, the reaction pathway on the triplet state surface is the classical p-pathway, whereas on the quintet state surface it is the r-pathway [40]. Above all, Fe-2a is more reactive than Fe-2b isomer. It is good that the trend of the reaction barriers for Fe-2a and Fe-2b fully conforms to the experimental trends [47], whether they are in the gas phase or in the solvent. To understand the ligand topology effect on the relative reactivities of the two Ru isomers, we also tested the reactions of the two isomers with isopropylbenzene. Fig. 7 shows the reaction pathways for hydrogen abstraction and Fig. 8 shows the geometric parameters of the transition states of the two ruthenium complexes. At the B2//B1 + ZPE + Esol + DDE# tun level, the triplet reaction barrier is 12.1 kcal/mol and 12.7 kcal/mol for 3Ru-2a and 3Ru-2b, respectively. From the optimized transition state geometries, there is little difference between the two structures. Thus, for the ruthenium complexes, the reaction barriers and geometric parameters have been little affected by the ligand topology.
4. Conclusions Theoretical geometric structures and hydrogen-abstraction mechanisms for cis-a-[FeIV(O)(BQCN)]2+ (Fe-2a) and cis-b-[FeIV(O)(BQCN)]2+ (Fe-2b), cis-a-[RuIV(O)(BQCN)]2+ (Ru-2a) and cis-b-[RuIV(O)(BQCN)]2+ (Ru-2b) complexes have been outlined. UB3LYP calculations show that the two iron complexes and the two ruthenium complexes possess a triplet ground spin state. The relative stabilities of the two iron isomers follow the trend of Fe-2a > Fe-2b, which is in agreement with the conclusions of Hong et al. [47], while the trend of the relative stabilities of the two ruthenium isomers is Ru-2a > Ru-2b. Our study indicates that the triplet and quintet spin states are very close in cis-a-[FeIV(O)(BQCN)]2+ and cis-b-[FeIV(O)(BQCN)]2+, and the TSR mechanism is favored for hydrogen abstraction. An analysis of the trend in hydrogen-abstraction barriers shows that the reactivity of Fe-2a is greater than that of Fe-2b. However, our results also show that the different ligand topologies have little effect on the tunneling effect. A calculation of the relative stability and reactivity of hydrogen-abstraction by the [FeIV(O)(BQCN)]2+ complexes demonstrates that ligand topology is an important factor that contributes to the stability and reactivity of non-heme iron complexes. DFT calculation also determined the influences of the equatorial ligand wrapping around the Ru center on the relative reactivity of the two isomers. For the ruthenium complexes, the triplet spin state is energetically favored by 60 kcal/mol below the quintet spin state, and the mechanism of the hydrogen-abstraction reaction is a single-state reaction. The relative reactivity of Ru-2a is higher than that of Ru-2b and the reaction barrier is only 0.6 kcal/mol lower. Thus, except the relative stability, the ligand topology has little effect on the reactivities of the [RuIV(O)(BQCN)]2+ complexes.
Abbreviation: [FeIV(O)(BQCN)]2+
cis-a-[FeIV(O)(BQCN)]2+ cis-b-[FeIV(O)(BQCN)]2+
Fe-2a Fe-2b
[RuIV(O)(BQCN)]2+
cis-a-[RuIV(O)(BQCN)]2+ cis-b-[RuIV(O)(BQCN)]2+
Ru-2a Ru-2b
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Theoretical study of the effect of ligand topology on Fe(IV)O and Ru(IV)O complex reactivities Zhe Tang a, Yi Wang a,⇑, Xiaolei Cui a, Ying Yang a, Jing Tian a, Xu Fei a, Shuangjiang Lv b,⇑ a b
School of Biological Engineering, Dalian Polytechnic University, Dalian 116034, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China
a r t i c l e
i n f o
Article history: Received 5 November 2015 Received in revised form 29 December 2015 Accepted 2 January 2016 Available online 8 January 2016 Keywords: Non-heme Density functional theory calculation Hydrogen abstraction Ligand topology effect
a b s t r a c t Density functional theory calculations were carried out to clarify the effect of ligand topology on the stability and reactivity of cis-a-[FeIV(O)(BQCN)]2+ (Fe-2a), cis-b-[FeIV(O)(BQCN)]2+ (Fe-2b), cis-a-[RuIV(O) (BQCN)]2+ (Ru-2a) and cis-b-[RuIV(O)(BQCN)]2+ (Ru-2b) (BQCN = N,N0 -dimethyl-N,N0 -bis(8-quinolyl)cyclohexanediamine). All the iron and ruthenium isomers possess the triplet ground states. The relative stability between the two iron isomers follows the order of Fe-2a > Fe-2b, which is in agreement with the conclusions of Hong et al. Moreover, the trend of the relative stability of the two ruthenium isomers is Ru-2a > Ru-2b. The Density-of-States spectrums represent that the contribution of BQCN ligand to 3 Fe-2b is more than the corresponding contribution to 3Fe-2a. The iron isomers react with isopropylbenzene via a two-state reactivity pattern on competing triplet and quintet spin states, while the ruthenium isomers react with isopropylbenzene by a single-state mechanism, only on the triplet spin state. The H-abstraction is affected by the tunneling contribution, which can decrease the reaction barriers. By adding the ZPE correction, PCM model assessed by DFT-D3 (BJ) and the tunneling correction, the analysis of the trend of the hydrogen-abstraction reactions barriers shows that Fe-2a is the more reactive than Fe-2b with the higher FeIV/III redox potential. Moreover, for the ruthenium complexes, although Ru-2a is the more reactive than Ru-2b, only 0.6 kcal/mol lower. Above all, the ligand topology has little effect on the reactivities of the [RuIV(O)(BQCN)]2+ complexes. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Oxygen-activating enzymes with mononuclear non-heme iron active sites participate in many metabolically important reactions that have environmental, pharmaceutical, and medical significance [1]. Over the past 15 years, studies have demonstrated that the iron (IV)-oxo state is accessible synthetically in a non-heme ligand environment [2]. Since the first crystal structure of a synthetic mononuclear non-heme iron(IV)-oxo complex was reported in a chemical model of non-heme iron enzymes in 2003 [3], a variety of mononuclear non-heme iron(IV)-oxo complexes bearing tetradentate N4 and pentadentate N5 and N4S ligands have been synthesized and characterized using various techniques, such as Mössbauer resonance and Raman and X-ray absorption spectroscopy, and density functional theory calculations [3–45]. In the biomimetic field, the reactivity and stability of non-heme iron(IV)-oxo complexes are sensitive to the ligand structure [40,46]. A number of important factors control the reactivities of ⇑ Corresponding authors. Fax: +86 0411 86323646. E-mail addresses: [email protected] (Y. Wang), [email protected] (S. Lv). http://dx.doi.org/10.1016/j.ica.2016.01.004 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
non-heme iron complexes in oxidation reactions. One such factor that determines the reactivities of iron catalysts is the ligand structure around the iron center [47]. Linear tetradentate N4 ligands can form around an octahedral iron center in three different topologies: cis-a, cis-b, and trans forms [47]. The cis-labile exchangeable sites which could affect the mechanism of the oxidation in iron(V)-oxo complex have been studied by Prat I et al. [48]. Until 2011, the effect of topology on non-heme iron(IV)-oxo complex reactivity had not been explored. Mononuclear non-heme iron(IV)-oxo complexes with two different topologies have been synthesized and characterized using various spectroscopic methods by Hong et al. [47]. The complexes are cis-a-[FeIV(O)(BQCN)]2+ (Fe-2a) and cis-b-[FeIV(O)(BQCN)]2+ (Fe-2b), in which the N-methyl groups are anti-aligned and aligned, respectively, (BQCN = N,N0 -dimethyl-N,N0 -bis(8-quinolyl)cyclohexanediamine). The 1H NMR spectra of Fe-2a and Fe-2b exhibit distinct peaks, which indicates that they each retain their respective ligand topologies in solution [47]. The FeIV/III redox potential of Fe-2a is 0.11 V higher than Fe-2b, and both of them are more reactive than [FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), [FeIV(O)
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(Bn-TPEN)]2+ (Bn-TPEN = N-benzyl-N,N0 ,N0 -tris(2-pyridylmethyl)1,2-diaminoethane), and [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11tetramethyl-1,4,8,11-tetraazacyclotetradecane) [3,12,13,46]. Ligand topology is therefore an important factor that affects oxidizing power of the non-heme iron (IV)O complexes. However, little is known about it to date, except the theoretically-reported reaction barrier of Fe-2a [24]. The fundamental reasons for the different stability and reactivity of Fe-2a relative to Fe-2b are of great significance for the better understanding of the topological effects of equatorial ligands. Nonetheless, the detailed theoretical study on the effect of ligand topology on the non-heme Fe(IV)O complex reactivity remains vacant until now. In order to determine the ligand structures around the ruthenium center, a hypothetical [RuIV(O)(BQCN)]2+ complex, which contains two isomers Ru-2a and Ru-2b, was theoretically designed. Therefore, we have performed a density functional theory (DFT) calculation of the geometric and electronic structures, the tunneling effect, the FeIV/III redox potentials, and the isopropylbenzene hydrogen abstraction by the iron and ruthenium complexes.
UTPSSh functional is shorter on the quintet spin state. As expected, the UOPBE/B2//UOPBE/B1, UTPSSh/B2//UTPSSh/B1, UOPBE/B2// UB3LPY/B1, and UTPSSh/B2//UB3LYP/B1 results are comparable with those obtained from UB3LYP/B2//UB3LYP/B1. Therefore, all functionals give the same ordering of species and we selected UB3LYP as the method of choice. The computational details are presented in the Supplementary electronic material (SI). 2.3. Tunneling corrections Eckart tunneling calculations were performed using TheRate Program [57]. Due to the tunneling, the transmission coefficient j, is calculated by integrating of the barrier ‘‘penetration” probability as a Boltzmann-averaged function of the energy [58]. The effect of the transmission coefficient on the barrier is calculated by the equation [59]:
DDE# tun ¼ RT ln jðTÞ
ð1Þ
2. Computational methods
where R denotes the universal gas constant and T is the absolute temperature. Generally, the experimental rate data were collected at 273 K.
2.1. Standard methods
2.4. Electronic structures of the Fe-2a and Fe-2b
We considered the triplet and quintet spin states for Fe-2a and Fe-2b; and the triplet spin states for Ru-2a and Ru-2b. DFT calculations were performed with the GAUSSIAN 09 suite of a quantum chemical package [49], and the spin-unrestricted Becke, threeparameter, Lee–Yang–Parr (B3LYP) functional [50] as the method of choice. The geometries for the four non-heme complexes were fully optimized without symmetry constraints. The Lanl2DZ basis set [51] was used for iron and ruthenium, moreover 6-311G⁄⁄ [51c,52] for the other atoms. To save computational resources, for the reaction pathways, we used the Lanl2DZ basis set for iron and ruthenium, the 6-311G⁄⁄ basis set for electron-rich O atom, and the 6-31G⁄⁄ basis set for C, H, and N atoms (B1 in brief). This basis set is used to optimize transition states and minima. All transition states were confirmed by one imaginary mode from vibrational analysis calculations and the local minima described here only have real frequencies. Single-point energy calculations were performed with a larger basis set TZVP [53] for all atoms (B2 in brief) of iron complexes. For ruthenium complexes, the def2-TZVP [53] on Ru and TZVP for other atoms were used for single-point energy calculation (B2 in brief). To evaluate the solvent effect, acetonitrile was used in the self-consistent reaction field calculations according to the polarizable continuum model (PCM), and the PCM model was assessed by DFT-D3 (BJ) [54] (with Grimme’s 2011 dispersion correction) and the SMD model. The mechanism of hydrogen abstraction reaction was determined using intrinsic reaction coordinate studies.
In order to understand the ligand topology effect better, we calculated the electronic structures, including Orbital population, Wiberg bond order, Hirshfeld charge, Density-of-States (DOS) and Overlap Population Density-of-States (OPDOS) Spectrums. All the calculations were performed using Multiwfn program [60].
2.2. Tests of the functionals It has been reported that the UB3LYP functional has verified the spin-state splitting of mononuclear non-heme iron complexes. However, in this work, the focus is on two iron complexes with triplet and quintet states, which are close in energy, so it is necessary to test the functionals. To determine the choice of UB3LYP, we used both of the UOPBE [55] and UTPSSh [56] functionals. For the two iron complexes, all three functionals predict that Fe-2a is the more stable than Fe-2b. Tests of the relative energy and geometric parameters of the transition states for the reactions of two iron isomers with isopropylbenzene were performed with the three functionals. Because the UTPSSh underestimates the relative energy of the quintet spin state, the O–H distance which is calculated by the
3. Results and discussion 3.1. Properties of the four complexes Our computational study produced geometric and electronic structures of the two iron and ruthenium isomers. The two possible iron and two hypothetical ruthenium isomers, which have the same BQCN ligand but different ligand wrapping around the metal center, will be discussed in detail. Structurally, the N-methyl groups of the BQCN ligand have different configurations in the two topologies: anti-aligned in the cis-a isomer (2a) and aligned in the cis-b isomer (2b) [47]. The two ruthenium isomers followed by metal center exchange of the iron by ruthenium. We set z to be along the Fe–O axis, whereas x and y are along the equatorial nitrogens. 3.1.1. Properties of the two iron isomers The geometric structures for the two iron isomers are shown in Fig. 1, with the corresponding energies of the singlet, triplet and quintet spin states relative to the triplet state at the higher level, UB3LYP/B2//UB3LYP/B1. The key geometric and electronic parameters in the UB3LYP/B1 optimized geometries of the corresponding isomers are listed in Table 1. The bond length and bond order reveal that the Fe–O bond typically has double bond character and the bond length does not change significantly from the triplet to the quintet spin state, because r⁄x2y2 is strongly Fe–N r-antibonding and is perpendicular to the Fe–O bond. The calculated Fe–O bond lengths for the two isomers are 1.630 Å and 1.626 Å, respectively, which are similar to that in crystal structures of some other non-heme iron(IV)-oxo complexes with an S = 1 spin state [3,8]. On the triplet/quintet spin state, the Wiberg bond orders between the Fe fragment and the O fragment are 1.640/1.671 for Fe-2a and 1.649/1.686 for Fe-2b, respectively. The reason that we have chosen the Wiberg bond order and the Hirshfeld charge analyses has been noted in SI.
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Fig. 1. The geometric structures of Fe-2a, Fe-2b, Ru-2a and Ru-2b. The B2 level relative energies (in kcal/mol) of the
1,3,5 Z KL
species.
Table 1 The geometric parameters, Wiberg bond order and Hirshfeld charge of O atom for the two iron isomers and two ruthenium isomers. Fe-2a
Bond length
Wiberg bond order Hirshfeld charge a
a
M=O M-NCMe M-Naxial M=O M-NCMe O
Ru-2a
Ru-2b
T
Q
Fe-2b T
Q
T
T
1.630 2.035 2.176 1.640 0.660 0.245
1.626 2.303 2.169 1.671 0.367 0.240
1.626 2.030 2.127 1.649 0.680 0.235
1.622 2.234 2.089 1.686 0.451 0.228
1.788 2.123 2.293 1.810 0.881 0.241
1.787 2.128 2.241 1.800 0.883 0.234
M = Fe/Ru.
The high- and low-lying virtual orbitals of the two isomers are dominated by the d-block orbitals. The triplet state has a d2pxz⁄1pyz⁄1 configuration, whereas the quintet state has a d1pxz⁄1pyz⁄1rx2y2⁄1 configuration. DFT calculations show that the triplet spin state is the ground state, which is favored energetically by 1.4 kcal/mol and 3.2 kcal/mol below the quintet state for Fe-2a and Fe-2b at the B2 level, respectively. Complexes Fe-2b is 3.6 kcal/mol less stable than Fe-2a (Fig. 1). These results agree with a previous study [47]. The equatorial ligand affects the energy of the orbitals lying in the plane, namely the d and r⁄x2y2 orbitals [61]. Because the quintet spin state arises from the triplet state by the promotion of one electron from the d to the r⁄x2y2 orbital, the triple-quintet energy gap agrees with the calculated d–r⁄x2y2 orbital energy gap [13,62,63]. As shown in Fig. 2, the d–r⁄x2–y2 orbital energy gap is 5.77 eV and 6.01 eV for 3Fe-2a and 3Fe-2b, respectively. For the same metal center, therefore, as the orbital energy decreases, the triplet-quintet energy gap decreases. It is important to note that on the triplet spin state, the b-LUMO (Lowest Unoccupied Molecular Orbital) is the p⁄xz orbital, which is
the best electron acceptor orbital. Figs 3 and 4 offer the contributions of the different ligands to the chemical bonding through the DOS (Density-of-States) and OPDOS (Overlap Population Density-of-States). In contrast to 3Fe-2b, the contribution of BQCN ligand to TDOS (Total Density-of-States) is less to 3Fe-2a on the b-LUMO. Furthermore, the negative OPDOS region represents the anti-bonding interaction. The anti-bonding interaction between the Fe and O fragments of 3Fe-2a is stronger than that of 3Fe-2b. 3.1.2. Properties of the two ruthenium isomers Fig. 1 also shows that the two ruthenium isomers have the triplet ground states, which is the same with the ruthenium-oxo porphyrin complexes, the ground state was found to be a low-spin [64], and the triplet spin state is favored energetically by 20.4 and 21.6 kcal/mol below the singlet spin state, and 60.9 and 60.1 kcal/mol below the quintet spin state, respectively. Due to the larger atomic radius of ruthenium, the splitting energy of the ruthenium complexes is larger than that of the iron complexes, thus, the interaction between the ruthenium center and equatorial
Fig. 2. Energy level diagram and iron d-based molecular orbitals of the two iron isomers. LUMO lowest unoccupied molecular orbital, HOMO highest occupied molecular orbital (UB3LYP/B2 level).
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Fig. 3. Density-of-States (DOS) and Overlap Population Density-of-States (OPDOS) spectrums of 3Fe-2a (UB3LYP/B2 calculation).
Fig. 4. Density-of-States (DOS) and Overlap Population Density-of-States (OPDOS) spectrums of 3Fe-2b (UB3LYP/B2 calculation).
ligand is larger. Comparing with the iron complexes, the ruthenium complexes are preferred in the triplet spin state. DFT calculations show that 3Ru-2b is 5.2 kcal/mol less stable than 3Ru-2a, which is in the same trend of the iron isomers in which 3Fe-2a is more stable. For the ruthenium isomers, the quintet spin state is too high to participate in the reaction, thus the reactions only involve the triplet spin state. As shown in Table 1, the main structures of the two isomers reveal that the Ru–O distance is 1.788 and 1.787 Å for 3Ru-2a and 3Ru-2b, respectively. Moreover, the Wiberg bond orders between the Ru and O fragments are 1.810 and 1.800 for 3Ru-2a and 3Ru-2b, respectively. Thus, the RuO bond also has the double bond character. 3.2. Theoretical calculations of C–H bond activation by the four isomers As proposed previously for hydrogen abstraction by non-heme iron species, in all mechanisms, the initial step involves the
formation of a reactant cluster followed by a transition state for hydrogen abstraction that leads to the product [13]. The energy profiles for hydrogen abstraction from isopropylbenzene by the two iron isomers are shown in Fig. 5 and by the two ruthenium isomers are shown in Fig. 7. For the two iron complexes, the reactions proceed via two closely located spin states, including triplet and quintet spin states, and for the two ruthenium isomers, the reactions proceed only via the triplet spin state, because the quintet spin states are approximately 60 kcal/mol higher than those on the triplet spin states. 3.2.1. Energy profiles for the reaction between Fe-2a and CH The hydrogen abstraction reaction of Fe-2a with isopropylbenzene is initiated on the triplet ground spin state surface (see Fig. 5). The initial stage of this reaction is the formation of a reactant complex, before the hydrogen atom of CH is abstracted by Fe-2a to yield the product via a transition state. At the B2//B1 level, the gas barrier (relative to the 3RC) is 14.2 kcal/mol for the triplet spin
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structure as a binding between the FeIV and O atoms. The two valence orbitals, which are denoted as dxz and dz2, are usually considered to be active on the triplet and quintet spin state, respectively. The CH attacks the oxo group from the side, and the triplet reaction pathway is the p-pathway [40]. The Fe–O–H angle is 129.20°. In the corresponding quintet spin state, the reaction pathway should be the r-pathway [40], the CH attacks the oxo group from the top to yield an Fe–O–H angle of 163.71°. To better explain the oxidation states and the electronic structures of Fe-2a, we show in Table 3 the spin densities and Mulliken charges. The FeO moiety possesses spin densities of 1.10/3.00 in iron and 0.98/0.78 in oxygen of 3,5RC, whereas the corresponding moiety of 3,5TSH possesses spin densities of 0.87/3.58 in iron and 0.75/0.46 in oxygen (see Table 3). On the reaction pathway, the spin densities and the geometric parameters are similar with the previous study [24] (Table S6).
Fig. 5. Energy profiles (kcal/mol) for hydrogen-abstraction reactions between the cumene and [FeIV(O)(BQCN)]2+ complexes in the triplet and quintet spin states. Relative energies are given for UB3LYP/B2 level and for the UB3LYP/B2 + ZPE + Esolv level.
state and 7.8 kcal/mol for the quintet spin state. By taking into account the environmental effects and the zero-point-energy correction, the barrier is found to be 8.9 kcal/mol for the triplet spin state and 5.9 kcal/mol for the quintet spin state (relative to the 3 RC). With the tunneling correction considered, the effective barri3 ers DE2# eff is reduced (see Table 2). The TSH possess an energy of 5 8.0 kcal/mol, while the TSH is 5.8 kcal/mol. Thus, Fe-2a is more reactive than some other non-heme iron (IV)-oxo complexes [3,12,13,38,46]. Because the iron active center is coordinately saturated, the C–H–O triad stays almost collinear and there is no direct interaction between the iron center and the carbon. As shown in Fig. 6, the C–H distance of 3TSH is longer than that of 5TSH, whereas the O–H distance of 3TSH is shorter than that of 5TSH. These differences imply that 5TSH will be formed ‘‘early”. The key difference in the transition state structures between the triplet and quintet spin states is the Fe–O–H angle, which results from a balance between the orbital interaction and the Pauli repulsion from the equatorial ligand [61]. A simple way to analyze the orbitals is to consider the
3.2.2. Energy profiles for the reaction between Fe-2b and CH The reactivity of Fe-2b is similar to that of Fe-2a toward CH (Fig. 5). The reaction barrier is 14.5 kcal/mol for the triplet spin state and 9.0 kcal/mol for the quintet spin state at the B2//B1 level (relative to 3RC). Inclusion of the solvation correction and the zeropoint correction decreases the barrier to 10.5 and 7.2 kcal/mol for the triplet and quintet spin states, respectively (relative to 3RC). The barriers decrease with the addition of tunneling effects, those become 9.5 kcal/mol on the triplet state and 7.1 kcal/mol on the quintet state, respectively (see Table 2). The reaction still follows the two-state reactivity (TSR) pattern [65–68]. The geometric C–H–O/Fe–O–H angles for 3TSH and 5TSH are 160.70/127.90° and 176.72/159.47°, respectively (see Fig. 6). As listed in Table 3, the calculated spin densities on the FeO moiety generated are 1.09/2.96 in iron and 0.98/0.78 in the O atom in 3,5RC. Furthermore, the corresponding moiety possesses spin densities of 0.86/3.56 in iron and 0.76/0.45 in the O atom in 3,5TSH. 3.2.3. Energy profiles for the reactions between the Ru isomers and CH To establish the oxidative capability of the Ru isomers, we studied the hydrogen-abstraction of isopropylbenzene by Ru complexes. Interestingly, as shown in Fig. 7, our calculations show that the Ru-isomers are more reactive than many other Ru complexes [23,69]. The small activation energy barriers are 18.8 and 19.6 kcal/mol for Ru-2a and Ru-2b, respectively. Adding zero-point energy correction and solvation correction, the reaction barriers have decreased. The tunneling effect correction leads to the decrease of the energy gap, DE2# eff become 12.1 kcal/mol and 12.7 kcal/mol for Ru-2a and Ru-2b, respectively (see Table 2). Moreover, the geometry of the two isomers is similar with each other (Fig. 8).
Fig. 6. Orbitals showing electronic delocalization and the geometric details (lengths in Å and angles in °) of the 3,5TSH species in the hydrogen-abstraction reactions between the cumene and the two iron isomers.
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Fig. 7. Energy profiles (kcal/mol) for hydrogen-abstraction reactions between the cumene and [RuIV(O)(BQCN)]2+ complexes in the triplet spin state. Relative energies are given for UB3LYP/B2 level and for the UB3LYP/B2 + ZPE + Esolv level.
Table 2 Transmission coefficients, tunneling corrections and the reaction barriers for the two iron isomers and two ruthenium isomers. Fe-2a
j (T)
DDE# tun DE1(B2//B1) DE2(B2//B1+Esol+ZPE) DE1# eff DE2# eff
Fe-2b
Ru-2a
Ru-2b
T
Q
T
Q
T
T
4.939 0.9 14.2 8.9 13.3 8.0
1.101 0.1 7.8 5.9 7.7 5.8
5.840 1.0 14.5 10.5 13.5 9.5
1.116 0.1 9.0 7.2 8.9 7.1
31.28 1.9 18.8 14.0 16.9 12.1
47.13 2.1 19.6 14.8 17.5 12.7
The tunneling corrections calculated at the UB3LYP/B1 level. The energies are in # # # kcal/mol. DE1# eff = DE1(B2//B1) + DDEtun; DE2eff = DE2(B2//B1+Esol+ZPE) + DDEtun.
Table 3 Calculated Mulliken charges and Mulliken spin densities for the Fe, O and H moieties in the model systems of the two iron isomers. Triplet state
2a RC TS Pro 2b RC TS Pro
Quintet state
Fe
O
H
Fe
O
H
1.10 0.53 0.87 0.55 0.94 0.48
0.98 0.33 0.75 0.50 0.16 0.56
0.00 0.09 0.01 0.25 0.00 0.32
3.00 0.66 3.58 0.72 3.75 0.66
0.78 0.33 0.46 0.50 0.02 0.55
0.00 0.09 0.02 0.22 0.00 0.33
1.09 0.51 0.86 0.56 0.91 0.56
0.98 0.32 0.76 0.50 0.18 0.51
0.00 0.10 0.01 0.25 0.01 0.31
2.96 0.64 3.56 0.73 3.74 0.67
0.78 0.32 0.45 0.50 0.02 0.59
0.00 0.10 0.02 0.22 0.00 0.34
Numbers in bold are Mulliken charge data. RC: reactant complex; TS: transition state; Pro: product.
Fig. 8. Orbitals showing electronic delocalization and the geometric details (lengths in Å and angles in °) of the 3TSH species in the hydrogen-abstraction reactions between the cumene and the two ruthenium isomers.
flip, the electron in the d orbital in the triplet spin state shifts to the r⁄x2y2 orbital, the SOC between the states would be maximized when both orbitals are localized on iron. As far as we know [13], the decreased iron weight of the r⁄x2y2 orbital will decrease the initial SOC. It can be seen from Fig. 9, the weight of the r⁄x2y2 orbital on iron are small and have followed the trend: Fe-2b < Fe-2a. Moreover, there are no experimental results [47] which have confirmed spin state transitions in BQCN. Thus, due to the low spin orbital coupling, the reactions mainly occur in the triplet spin state. Thus, we may conclude that the H-abstraction reactions pass via the S = 1 surface. To compare the relative reaction energies, the different topology of the equatorial ligand, this results in the different transition state barriers. On the triplet spin state, the CH attacks the oxo group from the side. After adding ZPE and solvation corrections, the activation energies are 8.9 kcal/mol for 3Fe-2a and 10.5 kcal/mol for 3Fe-2b, respectively, with that the higher FeIV/III redox potential, the higher reactivity is (see Table S8). As shown in Table 1, the Hirshfeld charges on the oxygen atom are 0.245 and 0.235 for 3Fe-2a and 3Fe-2b, respectively. As far as we know, the more negative charges on the atom, the more prone to occur the nucleophilic reaction. In order to establish the relative reactivities of the two iron isomers, we studied the effect of the tunneling on the reaction barrier of the H-abstraction. Adding the tunneling correction reduces the reaction barrier, as shown in Table 2. The influence of the tunneling effect on the triplet spin state is more than that on the quintet spin state. Along the reaction pathway, the DDE# tun is 0.9 kcal/mol for the 3Fe-2a, while it is 1.0 kcal/mol for 3Fe-2b. Thus, the different ligand topologies have little effect on the tunneling effect. At the B2//B1 + ZPE + Esolv + DDE# tun level, the triplet and quintet reaction barriers are 8.0/5.8 kcal/mol for Fe-2a, 9.5/7.1 kcal/mol for Fe-2b,
3.3. Discussions In our study, we have calculated the electronic structures, geometric structures, and the reaction pathways of the H-abstraction reactions of the four complexes. We now focus on the reasons and elucidate the effect of the ligand topology on the Fe(IV)O and Ru(IV)O complex reactivities. It has been reported that [24], for non-heme FeIVO species, the reactant ground state is a low-spin triplet state, whereas the lowest energy TS is the high spin quintet spin state. The spin–orbital coupling (SOC) interaction between the triplet and quintet states should affect the spin inversion [13,70,71]. As reported in the previous studies [70], during the spin
Fig. 9. r⁄x2y2 orbitals and the weight of these orbitals for two iron complexes on the quintet spin state.
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respectively. Thus, the reaction barrier difference is 1.5 kcal/mol for the two iron isomers. The potential energy profiles and the geometric structures of the hydrogen abstraction transition state on the triplet and quintet spin states for the two iron isomers are shown in Figs. 5 and 6, respectively. A comparison of the electronic and geometric structures between the different transition states is necessary. According to frontier molecular orbital theory, on the triplet state, when the fifth electron is added to the p⁄xz orbital, the d5 electrons will have a (d)2(p⁄xz)2(p⁄yz)1 configuration. In the quintet spin state, one electron transfers from the isopropylbenzene into the r⁄z2 orbital and the d5 electronic configuration is (d)1(p⁄xz)1(p⁄yz)1(rx2y2⁄ )1(r⁄z2)1. Therefore, the reaction pathway on the triplet state surface is the classical p-pathway, whereas on the quintet state surface it is the r-pathway [40]. Above all, Fe-2a is more reactive than Fe-2b isomer. It is good that the trend of the reaction barriers for Fe-2a and Fe-2b fully conforms to the experimental trends [47], whether they are in the gas phase or in the solvent. To understand the ligand topology effect on the relative reactivities of the two Ru isomers, we also tested the reactions of the two isomers with isopropylbenzene. Fig. 7 shows the reaction pathways for hydrogen abstraction and Fig. 8 shows the geometric parameters of the transition states of the two ruthenium complexes. At the B2//B1 + ZPE + Esol + DDE# tun level, the triplet reaction barrier is 12.1 kcal/mol and 12.7 kcal/mol for 3Ru-2a and 3Ru-2b, respectively. From the optimized transition state geometries, there is little difference between the two structures. Thus, for the ruthenium complexes, the reaction barriers and geometric parameters have been little affected by the ligand topology.
4. Conclusions Theoretical geometric structures and hydrogen-abstraction mechanisms for cis-a-[FeIV(O)(BQCN)]2+ (Fe-2a) and cis-b-[FeIV(O)(BQCN)]2+ (Fe-2b), cis-a-[RuIV(O)(BQCN)]2+ (Ru-2a) and cis-b-[RuIV(O)(BQCN)]2+ (Ru-2b) complexes have been outlined. UB3LYP calculations show that the two iron complexes and the two ruthenium complexes possess a triplet ground spin state. The relative stabilities of the two iron isomers follow the trend of Fe-2a > Fe-2b, which is in agreement with the conclusions of Hong et al. [47], while the trend of the relative stabilities of the two ruthenium isomers is Ru-2a > Ru-2b. Our study indicates that the triplet and quintet spin states are very close in cis-a-[FeIV(O)(BQCN)]2+ and cis-b-[FeIV(O)(BQCN)]2+, and the TSR mechanism is favored for hydrogen abstraction. An analysis of the trend in hydrogen-abstraction barriers shows that the reactivity of Fe-2a is greater than that of Fe-2b. However, our results also show that the different ligand topologies have little effect on the tunneling effect. A calculation of the relative stability and reactivity of hydrogen-abstraction by the [FeIV(O)(BQCN)]2+ complexes demonstrates that ligand topology is an important factor that contributes to the stability and reactivity of non-heme iron complexes. DFT calculation also determined the influences of the equatorial ligand wrapping around the Ru center on the relative reactivity of the two isomers. For the ruthenium complexes, the triplet spin state is energetically favored by 60 kcal/mol below the quintet spin state, and the mechanism of the hydrogen-abstraction reaction is a single-state reaction. The relative reactivity of Ru-2a is higher than that of Ru-2b and the reaction barrier is only 0.6 kcal/mol lower. Thus, except the relative stability, the ligand topology has little effect on the reactivities of the [RuIV(O)(BQCN)]2+ complexes.
Abbreviation: [FeIV(O)(BQCN)]2+
cis-a-[FeIV(O)(BQCN)]2+ cis-b-[FeIV(O)(BQCN)]2+
Fe-2a Fe-2b
[RuIV(O)(BQCN)]2+
cis-a-[RuIV(O)(BQCN)]2+ cis-b-[RuIV(O)(BQCN)]2+
Ru-2a Ru-2b
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