Control of the Electronic Structure of Manganese Nitrido Complexes by Para Ring Substituents:a Theoretical Study①

NING Tu-Rong SONG Jin-Shui, WEI Jing ZHANG Min-Yi, LU Qin-Qin, HUANG Jing LI Chun-Sen, ② (Stte Key Lortory of Structurl Chemistry, Fujin Institute of Reserch on the Structure of Mtter, Chinese Acdemy of Sciences, Fuzhou 350002, Chin) (Fujin Provincil Key Lortory of Theoreticl nd Computtionl Chemistry, Ximen 361005, Chin)

A density functional theory investigation was performed to find the relationship between various substituents on the ligand of manganese nitrido complexes and the electronic structures.The prediction of energy difference of two electronic structures by HOMO-LUMO gaps was illustrated by a valence-bond state correlation diagram.

1 INTRODUCTION

Transformation of dinitrogen to nitrogen-containing compounds is an important process in nature and it has attracted a great deal of attention in chemistry[1].In the past decades, high-valent metal nitrido complexes have been proposed as key intermediates in the biological enzymatic N2fixation[2,3]and the industrial Haber-Bosch process[4].Various transition metals are involved in these reactions such as molybdenum[5,6], ruthenium[7],iron[8-15], manganese[16-24], etc.For instance, ruthenium nitrido salen complex was found to be capable of oxidizing phenols into p-benzoquinone with fourelectron transfer[7].High-valent iron(V) nitrido complex with a neutral ligand has been spectroscopically and chemically characterized recently and proved to be a powerful two electron oxidant[8].The molybdenum nitrido complex was trapped from dinitrogen cleavage, which was considered as an intermediate of catalytic activation of dinitrogen.Particularly, manganese nitrido and imido complexes have been postulated as the active reagents responseble for nitrogen transfer[14,23,24]and dinitrogen cleavage reaction[5,6,25-27].

It was found by spectroscopic and theoretical investigations that manganese nitrido complexes changed the spin state, and then the electronic structure, when the ligand fields altered[17].Mayer and coworkers synthesized a series of manganese nitridos in different oxidation states III, IV, V, which perform the amination of silyl enol ethers in a nitrogen atom transfer process[24], and found that the electronic structures of manganese nitridos are dependent on ligand framework[18].Clarke and Storr investigated the oxidized manganese(V) nitrido salen complexes with different para ring substituents and revealed that nitrido activation is dictated by remote ligand electronics[16].As such, the electronic structure of metal nitrido complex may be tuned by the ligand electronics, thereby contributing to the reactivity.For instance, Stranger and coworkers calculated the mechanism of dinitrogen activation by transition metal nitrido from three-coordinated complexes and found that the metals with d3configuration were better than other transition metals.The substituents on the ligand also lead significant effects on the thermodynamics of intermediates and productions[27].It is therefore important to highlight the impact of the ligand electronics on the electronic structures of metal nitrido complex.

The general rules for electronic structures of manganese salen nitrido complexes with different ligands remain under-investigated.In this work,based on the research of Clarke and Storr[16], the effects of two electronic structures of complexes with different substituents on the ligand were investigated theoretically.Electronic configurations of the complexes and their geometries, orbital occupations,bond properties, and relative energies will be described.The relationship of HOMO-LUMO gap and energy difference of two types of complexes is figured out.This work may be helpful for designing new catalysis for nitrogen fixation by controlling the electronic structures of the metal nitrido complexes.

2 COMPUTATIONAL MODEL AND METHODS

Based on the manganese nitrido salen complex synthetized by Clarke and Storr, we designed a serial of analogues shown in Fig.1 with different substituents (R), ranging from electron-donating groups to electron-withdrawing groups on the para ring of ligand (R = H, CF3,tBu, NMe2, CH3, NH2, OH, F,SH, Cl, CH2NH2, CH2OH, CH=CH2, CN, COCH3,NMeF, OCH3and SCH3).Two electronic structures characterized by different positions of the radical are labeled as [MnV(SalenR?)N]+and [MnVI(SalenR)N]+in Fig.1, respectively.[MnV(SalenR?)N]+corresponds to the one whose radical resides on ligand (L-type),whereas [MnVI(SalenR)N]+has the radical residing on the center metal (M-type).In addition, the optimized geometries of M- and L-type species were labeled asmandl, correspondingly.To examine the geometries and electronic structures, the bond properties at these two geometries were calculated and compared.Relative energy (RE) of the M- and L-type manganese salen nitrido complexes was calculated by EL-EMfor one species to assign the ground state and the relaxed excited state.Moreover, the vertical excitation states that invert the M- and L-type electronic structures at their own geometries were also investigated.The vertical excitation energy was estimated by the gap of b HOMO and LUMO of each species approximately.The relationship between RE and gaps of two electronic structures was modeled by a valence-bond state correlation diagram (VBSCD)[28,29].

Fig.1.Model complex and substituents.(a) M-type electronic structure [MnV(SalenR?)N]+,(b) L-type electronic structure [MnVI(SalenR)N]+

All the calculations were carried out by using ORCA program package[30].Full geometry optimization and frequency calculations were performed by B3LYP functional[31,32]coupled with def2-SVP[33]basis set for all atoms.A larger basis set of def2-TZVPP[33]was employed for single point energy corrections.To improve computational efficiency,the RIJCOSX approximation[34-36]in combination with def2-SVP/J and def2-TZVPP/J[37]auxiliary basis sets was applied.Dispersion effects were computed by using the well-established dispersion corrections D3 with Becke-Johnson damping scheme[38,39].Mulliken population and Mayer bond order were calculated to investigate single electron location and bond character of the complexes.

3 RESULTS AND DISCUSSION

3.1 Geometries and electronic structure

For the complexes with various substituents on the ring ligand, themandlgeometries were found and all stationary points were located except themgeometry when R = NMe2.

To describe the electronic structure features of M-and L-type complexes, typical orbital occupations of the complex for R = CF3were chosen as example.As can be seen from Fig.2, the electronic structures of M-type complexes are mainly characterized by the singly occupied Mn-dxyorbital and the doubly occupied ligand ring p orbital.The corresponding complex has a d1configuration on the metal center.By contrast, the electronics of L-type complexes display a picture of singly occupied ligand ring porbital and doubly occupied Mn-dxyorbital.The corresponding complex has a low-spin d2configuration on the metal center.Our results of Mn(V) salen nitrido complex are in good consistence with Gray’s work[22].Compared with Mayer’s Mn(V) nitrido complex in which the metal center has a high-spin ground state with a different ligand[18], it seems that the electronic configurations of [Mn(SalenR)N]+complexes are sensitive to the coordination environment.

Fig.2.Electronic structure of the complex (R = CF3).(a) M-type; (b) L-type.R=CF3

Table 1 collects the bond lengths and properties of[Mn(SalenR)N]+complexes atmandlgeometry.Atmgeometry, the bond lengths of Mn–N(1) are about 1.50～1.51 ?, which are in good agreement with crystal structures[22].As such, the bond orders of Mn–N(1) for these complexes are also very close to each other.The bond orders around 2.5 suggest two and half bonds between two atoms.Moreover, the total spin densities of Mn–N(1) bond are about 0.90～0.93, indicating an unpaired electron resides on the orbital of the center metal.These similarities in bond properties of all complexes are attributed to the same M-type electronic structures at this geometry.

Atlgeometry, the distances of Mn–N(1) for these complexes have a slight variation of 1.47～1.48 ?,which are shorter than the respectivemgeometry.The calculated bond orders around 2.8 suggest a triple bond between Mn and N(1) atoms.Note that,at this geometry their population of spin density shows one electron resides on the ligand orbital but no unpaired electron on the Mn–N(1) moiety,indicating an L-type configuration.Moreover, the spin densities for various complexes listed in Table 1 are almost unchanged.Therefore, similar to the M-type complexes, the properties of L-type ones are also independent of ligand substituents.In Storr’s work their calculation showed the spin density for L-type complexes mainly localized on one of the ligating aromatic rings and the substituent[16].However, our results show the spin density is delocalized on the whole ligand.Considering that the two aromatic rings are nearly identical, the delocalized configuration from our calculations seems more reasonable.

Table 1.Selected Bond Lengths (?), Bond Orders and Spin Densities at m and l Geometry

Overall, the most remarkable geometric feature of M- and L-types is the length of Mn–N(1) bond,which has significant influence on the electronic structures and bond properties.For a given electronic structure, the properties of key bonds Mn–N(1)around the metal are nearly the same whether the substituents are electron-donating or electronwithdrawing.

3.2 Relationship of relative energies and gaps

The relative energy (RE) of the complexes for L-and M-type species are listed in Table 2.Complexes with electron-withdrawing groups CF3, F, Cl CN,COCH3and weak electron-donating groups H,tBu,CH3have positive RE values, indicating the M-type is the ground state.By contrast, the complexes with strong electron-donating groups NH2, OH, SH and their derivatives OCH3, SCH3, NMeF, NMe2prefer the L-type as the ground state.These results related to the electronics of substituents and ground states are in consistence with previous work[16].

How does the substituent change the ground state of manganese nitrido complexes? By comparing the two electronic structures in Fig.2, both a (spin up)orbitals of metal and ligand are occupied.However,the HOMO and LUMO of b (spin down) orbitals are different.Electron transfer between b orbitals of metal and ligand leads to the conversion of two electronic structures, because the electronics of substituent can change the energies of ligand ring orbital.Here we take M-type configuration as an example.At the beginning the metal and ligand orbital are LUMO and HOMO.If a substituent has a ligand orbital with high energy, the electron in bligand orbital prefers to move into the metal orbital,thus giving an L-type configuration.

Table 2.Collection of Relative Energies (RE, kcal/mol), b Orbital Energies(eM, a.u.) and Vertical Excitation Energies (ΔEV, kcal/mol) at m and l Geometry

Table 2 also collects the energies of b HOMO and LUMO (?) for this serial of complexes.Here em,M,em,L, el,Mand el,Lrepresent the orbital energies of ligand and metal orbital atmandlgeometry,respectively.Because the orbital energies of different complexes cannot be compared directly, here the gaps of HOMO and LUMO were used to estimate the vertical excitation energy (ΔEV) at two geometries.

The energy gaps of the two orbitals are also listed in Table 2.In themgeometry column, the energy gap represents the vertical excitation energy of electron transfer from ligand orbital to metal orbital.As such, large gaps correspond to the species with ground state in L-type, while small gaps give M-type ground state.By contrast forlgeometry, small energy gaps of two orbitals correspond to large RE values, indicating electron can easily transfer from metal to the ligand to form the L-type complex.Moreover, we found that half of the gap differences is quite close to RE.The relationship of RE and vertical excitation energy can be proposed as

To check the rationality of this equation, a linear fit of two columns was derived and shown in Fig.3.Interestingly, the R2of 0.986 shows a good correlation between RE and half of gap difference, with the intercept of 1 and a small error term of 1.43.These findings could help us to estimate the stability of different electronic structures by the orbital energies.Besides, the physical picture of mysterious C term is also worth to be uncovered.

Fig.3.Linear plot of RE and half of orbital gap difference

3.3 Explanation of the linear relationship

The linear relationship of RE and orbital gap difference could be explained from valence bond state correlated diagram, as shown in Fig.4.

Fig.4.Valence bond state-correlation diagram for the M- and L-type complexes

Define potential energy surfaces of two electronic structures EMand ELare in a parabolic curve approximately

where kMand kLrepresent the force constants of the potential surfaces and geometrical distance D from xmto xlis defined as

Thus, atmandlgeometries the vertical excitation energies can be read from the diagram as

When x = xmand x = xl, the potential energies in Eqs.4 and 5 could be written as

Therefore, subtracting Eq.8 from Eq.7 and combining Eq.9 and 10 one has

Compared with Eq.3, the origin of C term could be found as

Therefore, the linear relationship of RE and orbital gaps found from Eq.11 is an explanation of Eq.3 by the VBSCD.The C term in Eq.12 depends on the potential surface (kMand kL) and the geometrical change D, which relate to the electronic and geometrical difference of two conformers,respectively.Specially, if the two electronic structures are identical, the RE of two species should be 0.In this case, the profiles of two states should be in mirror symmetry in VBSCD and the same k value with C as 0.In general case the two electronic structures are not identical, resulting in a nonzero C value.Note that in our data sets the RE is in a range of –5.1～10.1 kcal/mol.One may concern how C changes if the RE value is out of this range.However,it should be noted that a huge RE will lead to an unstable relaxed exited state, which is not suitable to use Eq.12 to estimate RE.For example, in our calculations when R = NMe2onlylgeometry can be located because the M-type electronic structure has high energy, and thus no gap andmgeometry can be used to calculate RE.

4 CONCLUSION

In this work, we designed a serial of manganese nitrido complexes, [Mn(SalenR)N]+, with different R substituents.The geometries of M- and L-type electronic structures and bond properties of[Mn(SalenR)N]+complexes were investigated.The relative energies between these two electronic structures depend on the electronic properties of the substituents.Interestingly, for each type of complexes, the geometries of ligand bonds, bond orders and spin densities are nearly the same.A linear relationship between relative energy and orbital gap difference was found.The explanation from VBSCD shows this linear relationship is universal for these two electronic structures.The physical picture of intercept term is related to the electronic and geometrical difference of electronic structures.Further work of exploration on the N2activation by[Mn(SalenR)N]+complexes will be performed.

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