Porphyrazine Induced by Non-Covalent Interactions Combined with

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C: Physical Processes in Nanomaterials and Nanostructures

Spin Crossover in Iron(II) Porphyrazine Induced by Non-Covalent Interactions Combined with Hybridization of Iron(II) Porphyrazine and Ligand’s Orbitals : CASPT2 , CCSD(T) and DFT Studies Rabindranath Lo, Debashree Manna, Radek Zbo#il, Dana Nachtigallova, and Pavel Hobza J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05352 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Spin Crossover in Iron(II) Porphyrazine Induced by Non-Covalent Interactions Combined with Hybridization of Iron(II) Porphyrazine and Ligand’s Orbitals : CASPT2 , CCSD(T) and DFT Studies Rabindranath Lo,a,b† Debashree Manna,a,b† Radek Zbořil,a,b Dana Nachtigallová,a,b* and Pavel Hobza,a,b * a Institute

of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i.,

Flemingovo nám. 2, 16610 Prague 6, Czech Republic b

Regional Centre of Advanced Technologies and Materials, Palacký University, 77146 Olomouc,

Czech Republic †Both authors contributed equally to this work and can be considered as the first author Email: [email protected]; [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

Abstract

Spin crossover processes invoked by external stimuli e.g. temperature, pressure, irradiation with light, electric or magnetic field as well as formation of dative bond are well-known phenomena. The effect of weak noncovalent interactions on spin crossover is, however, much less explored. To improve understanding of the origin of spin crossover processes driven by noncovalent interactions the multiACS Paragon Plus Environment

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reference and DFT calculations are performed on iron(II) porphyrazine complexes with CO, NO, benzene and pyridinyl radical. The electron structure analyses are performed to discuss the differences in the nature of covalent and noncovalent bonding of CO and NO in axial and parallel orientations, respectively, as well as noncovalent bonding of benzene and pyridinyl radical with iron(II) porphyrazine. It is shown that the weak intermixing between the d-orbitals of metal center and suitably oriented p-orbitals of Ncontaining ligands are responsible for their stronger influence on the spin crossover compared to those CO and benzene ligands.

Introduction The ability of transition metal complexes to reflect environmental (ligand field) changes by the spin state transition at the central atom, the spin crossover (SCO), is the phenomenon first observed by Cambi et al.1 Since this discovery an enormous effort has been put to its rationalization and explanation.2-5 Among the SCO systems, Fe complexes are particularly interesting due to their potential applications in material science.4,6-10 The open shell characters of 3d-orbitals occupied by six (Fe(II)) and five (Fe(III)) electrons allow for rearrangements of the electron distribution to form singlet (closed- and open-shell), triplet and quintet spin-states of Fe(II) and doublet, quartet or sextet of spin-states of Fe(III), respectively. These spin-states are separated by energy gap sufficiently small to be overcome by a weak perturbation. Several studies have reported on SCO processes initiated by external stimuli, such as temperature, pressure, electric field, light absorption5,7,10-18 and covalent ligand-binding associated with changes in coordination number.19-21 As discussed relatively recently, non-covalent interactions have also emerged as an external stimulus to induce SCO process.22-30 It has been shown that the magnetic spin-transitions relate to changes of structure parameters, most notably modifications of the bond parameters in the metal-ligand bond31 and, thus, the character of ligands can be used to tune SCO process.21 Computational approach exploring moderate size of model systems feasible to be calculated by means of DFT and wave-function based ab Initio methods can help to reveal the electron-structure relation ACS Paragon Plus Environment

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operating during SCO.32-35 Despite the fact that DFT methods are the most frequently used to calculate these systems, a careful selection of a functional with respect to its reliability to describe open-shell system of the metal center is a challenging task. Wave-function based methods, in particular the multi-reference methods, can validate and test the performance of DFT methods. Radoń36 in his recent study has compared the performance of about thirty functionals and has found that only two, B2PLYP-D3 and OPBE in particular, has been able to provide the spin-state energetics of Fe-complexes with the error smaller than 5 kcal/mol with respect to experimental findings. The multi-reference approach, in particular the size of an active space needs to be carefully selected to describe correctly the ground spin-state and its selection still remains an open question even in the case of naked Fe-porphyrins-based systems.37-40 Description of binding of small ligands to Fe-porphyrines (FeP) and a consequent SCO represent a similar problem as already listed for a naked system.41 Despite a large criticism for the performance of DFT (see e.g. ref. 4143), to our opinion this method will remain mostly used for the systems discussed here and, when used with caution, can provide important information on the character of FeP…ligand binding. Concentrating on CO and NO ligands, both being able to tune the spin state of Fe,44-51 large number of calculations have been performed using both DFT and multi-reference methods.42,49,52-59 Computational studies devoted to the magnetic transitions of Fe-porphyrine-based systems invoked by non-covalent interactions are rather sparse. Among them, combined experimental and theoretical studies on tuning magnetic properties of complexes of transition metal with porphyrins adsorbed on carbon nanomaterials have been reported.60,61 Recently, in our laboratory we have studied the character of noncovalent interactions between Fe(II)-phthalocyanine (Fe(II)Pc) adsorbed on N-doped graphene.61,62 Sarmah and Hobza have performed dispersion-corrected DFT calculations to address the thermodynamic stabilities and monitor the electronic and magnetic properties of Fe(II)-phthalocyanine non-covalently bound to N-doped graphene. The role of N atom in the SCO process of FePc observed upon its adsorption on graphene systems using Atomic Force Microscopy experiment has been rationalized with the help of DFT and multi-reference MCSCF calculations61 and the crucial role in SCO has been assigned to singlyoccupied antibonding molecular orbital localized on nitrogen. ACS Paragon Plus Environment

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Motivated by our previous observation of SCO of Fe(II)Pc invoked by non-covalent interactions and the effect of N-atom perturbation,61 we have performed computational studies of model system Fe(II)porphyrazine (FePz), which is very similar to FePc (cf Figure 1). In this study, we focus on the determination of its ground state, which has not been definitely revealed, and its electronic changes induced by interaction with CO and NO ligands.63,64 The character of covalent bindings of CO and NO to FeP-based systems in axial positions with respect to porphyrine molecular plane have been already investigated and the most stable linear and bend orientations of CO and NO, respectively, have been found.49,55,65 To gain more insight into the role of non-covalent bonding in spin transition processes we study effects of CO and NO ligands bonded in the axial and parallel orientations with respect to FePz molecular plane to simulate and compare the electronic structure transitions of FePz upon covalent and non-covalent interactions. The latter is further investigated also on the FePz…benzene and FePz…pyridinyl radical complexes which model the electronic properties of FePz on graphene and Ndoped graphene surfaces. Experimental reports are available mentioning neutralization of pyridinium cation (C5H5NH+) via electro-oxidation to produce neutral pyridinyl radical (C5H6N).66-67 Since the magnetic properties of FePz subsystem are the main focus of our study we interpret our results in terms of singlet, triplet and quintet states, although for N-containing complexes the overall spin states are doublet, quartet and sextet, respectively, due to an unpaired electron on N atom.

Calculations All the structures (FePz and FePz…X with X = CO, NO axial; benzene; pyridinyl radical) have been fully optimized within C1 symmetry at the DFT-D3/B97D level of theory with the Grimme’s advanced dispersion-corrected approach68 (DFT-D3) using the TZVPP69,70 basis set. For parallel CO and NO complexes with FePz, structures have been obtained using distance scan technique with the optimized FePz…X geometries in axial orientation. In the optimized isolated neutral pyridinyl radical, the hydrogens are negligibly out-of-plane which make pyridnyl radical nearly planar system. Thus, it can reasonably

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mimic planar N-doped graphene surface. The optimized structures have been used to evaluate the complex interaction energies as 𝑬(𝒊𝒏𝒕𝒆𝒓) = 𝑬(𝒄𝒐𝒎𝒑𝒍𝒆𝒙) ― 𝑬(𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒆 𝟏) ― 𝑬(𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒆 𝟐)

(𝟏)

Wiberg Bond indexes (WBI) have been calculated from NBO calculations using the complex geometries at B97D/def2-TZVPP level. CCSD(T) calculations performed using the DFT optimized structures have been carried out with 631G**(0.25, 0.15) (exponents of d- and p-polarization functions on heavy atoms and hydrogens were equal to 0.25 and 0.15, respectively).71 Basis set superposition error has been included in the evaluation of CCSD(T) interaction energy calculations. Multi-configurational self-consistent field (MCSCF) calculations have been carried out using DFT optimized structures. The active space (12, 10) was considered for isolated FePz ; while for the complexes active space for the CASSCF has been constructed from Fe(3d), π-orbitals of the N of porphyrazine ring and relevant ligand orbitals, resulting in 14 active electrons in 14 active orbitals (CAS(14,14)). Due to the computational limits, the evaluation of higher order correlation effects at the CASPT2 level has been performed using smaller active space (CAS(8,10)). For radical complexes (NO and pyridnyl radical), CAS(13,14) active space is used for CASSCF calculations and CAS(9,11) is used for CASPT2 calculations. cc-pVDZ basis sets72,73 have been used in these calculations. DFT and CCSD(T) calculations have been performed using TURBOMOLE 6.6 suite of programs.74 The NBO calculations have been performed using NBO 3.1 program75 as implemented in Gaussian 09.76 Molpro 2010.1 version77 has been used to calculate multi-reference CASSCF and CASPT2 calculations.

Results and Discussion Isolated FePz Relative energies of singlet (S), triplet (T) and quintet (Q) spin states of isolated FePz calculated at the DFT level employing several DFT functionals, and using CASSCF and CASPT2 methods are summarized in Table 1. The results show similar trends as those observed for other porphyrine-based systems.37,38,40 ACS Paragon Plus Environment

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Most functionals predict the triplet ground state, followed by quintet and singlet. An exception has been found for double-hybrid B2-PLYP-D3 functional which, in agreement with multi-reference approaches, places the quintet below triplet and singlet with the energy gaps of about 10 and 50 kcal/mol, respectively. Notably, the quintet ground state has been predicted also for (FePc), system similar to FePz (see Figure 1), in CASPT2 and DMRG calculations and confirmed by Mössbauer spectroscopy performed in conditions which closely resemble gas phase.37

b)

a)

Figure 1. Geometries of a) Fe(II)-porphyrazine and b) Fe(II)-phthalocyanine (C- green, H- pink, Nblue, Fe- dark orange).

Table 1: Relative energies (kcal/mol) of FePz in different spin states calculated at various levels of theory. Quintet 18.7

Triplet 0.0

Singlet 30.4

BLYP-D3/TZVPP// BLYP-D3/TZVPP

22.8

0.0

24.3

BP86-D3/TZVPP// BP86-D3/TZVPP

22.5

0.0

27.1

PBE-D3/TZVPP// PBE-D3/TZVPP

22.2

0.0

27.1

TPSS-D3/TZVPP// TPSS-D3/TZVPP

25.2

0.0

29.0

B3LYP-D3/TZVPP// B3LYP-D3/TZVPP

17.8

0.0

34.6

BHLYP/TZVPP// BHLYP/TZVPP

4.1

0.0

35.7

wB97XD/def2-TZVPP// wB97XD/def2-TZVPP

21.3

0.0

27.1

M06-2X/def2-TZVPP// M06-2X/def2-TZVPP

24.7

0.0

35.5

M06-HF/def2-TZVPP// M06-HF/def2-TZVPP

29.3

0.0

41.9

B97-D3/TZVPP// B97-D3/TZVPP


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B2-PLYP-D3/TZVPP// B97-D3/TZVPP

0.0

12.4

47.9

B2-PLYP-D3/def2-QZVP// B97-D3/TZVPP

0.0

11.6

47.9

CASSCF (12/10) CASPT2 (12/10)

0.0 0.0

4.5 13.5

46.7 54.6

Ligand Binding The structures of CO and NO complexes optimized at the DFT-D3/B97D/TZVPP level considered in this study are shown in Figure 2a-2d.

a)

b)

1.700

c)

3.100

d)

2.113

1.712

f)

e)

3.667

3.943

Figure 2. Geometries of FePz complexes with various ligands in different orientations [a: CO, b: CO (parallel), c:NO, d: NO (parallel), e: benzene, f: pyridinyl radical] (C- green, H- pink, N- blue, Fe- dark ACS Paragon Plus Environment

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orange, O- red). a: Fe-C, b: Fe…C, c: Fe-N, d: Fe…N, e: Fe…C and f: Fe…N distances are given in Å. For complex f, the distance is measured between N atom of pyridinyl radical and Fe. For comparison purpose, the distance between the C at similar position (as in N in pyridinyl radical) and Fe was considered in complex e.

The results of relative energies of possible spin states, i.e. singlet, triplet or quintet of FePz… CO and doublet, quartet and sextet of FePz…NO complexes (the corresponding multiplicity of FePz is singlet, triplet and quintet, respectively) are presented in Table 2. CO binding in axial orientation Experimental observation, e.g. Mössbauer spectroscopy measurements78 and X-ray photoelectron spectroscopy44,49 are in agreement with the results of DFT calculations.44,49,79 It is generally accepted that the addition of CO to FePz leads to the formation of closed-shell singlet complex with diamagnetic character. CO-Fe bond is described as traditional σ-donation and π-back-donation,55 in which 5σ and 2π* molecular orbitals of CO hybridize with 3d orbitals of Fe. Due to the symmetry reasons of linear complex, 5σ(CO) MO hybridizes with 3dz2(Fe) orbital and 2π*(CO) MO hybridizes with 3dπ orbitals, respectively.53 Our DFT calculations (Table 2) show that the B97-D3 functional provides the singlet ground state of FePz…CO in axial orientation in agreement with previous findings, while the B2-PLYPD3 functional, which correctly predicts the quintet ground state for isolated FePz, gives the wrong state ordering and puts the singlet state by about 11 kcal/mol and 14 kcal/mol above the triplet and quintet states, respectively. In agreement with previous experimental and computational results also the multireference CASSCF(14,14) calculations predict the singlet ground state. CASSCF(8,10) and CASPT2(8,10) relative energies calculations, accounting for higher correlations do not reverse this ordering here. Both, B97-D3 and multi-reference calculations, thus, predict quenching of the spin and SCO transition from triplet to singlet and from quintet to singlet, respectively, upon CO binding. NO binding in bent orientation


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The electronic structure of FePz…NO complex have appeared more challenging to describe due to the strong electronic correlation in {FeNO}7, where the number 7 in superscript stands for six electrons distributed in Fe 3d-orbitals and one NO unpaired electron.58,80-83 As discussed by Boguslawski et al.42 NO complexes in general represent challenging systems for standard computational approaches to obtained correct spin density distribution and several studies have been devoted to its predictions for FeP…NO systems and the oxidation state of Fe.42,53,55 According to the CASSCF calculations, the wavefunction of the experimentally determined doublet spin state of the complex (singlet of FeP)79 is formed by two main electronic configurations, which correspond to Fe(II)-NO0 and Fe(III)-NO-.42 The same character has been found also for the quartet spin state. Reduced symmetry in FeP…NO results in a larger amount of hybridization, the electron donation from 5σ(NO) and back-donation to 2π*(NO) MO’s are, however, similar to FePz…CO, leaving unpaired electron in 3dz2(Fe) orbital.53 The results of calculations of the spin state relative energies of FePz…NO in axial orientation (Table 2) show a correct prediction of the ground state multiplicity, i.e. doublet, in calculations employing the B97-D3 functional, while the B2-PLYP-D3 functional fails to predict the correct ground state, similarly to FePz…CO complex. The ordering obtained with the B97-D3 functional is supported also by the multi-reference calculations. The inspection of the natural orbitals calculated at the CASSCF level (Figure 3) shows that the unpaired electron occupies the molecular orbital which results from the hybridization of 3d(Fe) with z-component and pz(NO) MOs. This hybridization is enabled by the fact that energies of π* MO of NO and iron d orbitals are close each other, which is not the case for CO ligand where 5σ(CO) MO and iron d orbitals are well separated. Binding with benzene or pyridinyl radical Both, the multireference and B97-D3 calculations predict the triplet ground state of FePz...benzene and, thus, SCO from quintet to triplet upon complexation is predicted with the former method, while the latter suggests no change of the spin state. Both methods reveal the doublet ground state of FePz… pyridinyl radical complex. Thus, similarly to NO, complexation of FePz with pyridinyl radical quenches partially the multiplicity from triplet to doublet and from quintet to doublet (singlet at FePz) as observed in the ACS Paragon Plus Environment

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B97-D3 and multi-reference calculations, respectively. The unpaired electron occupies the molecular orbital resulting from hybridization of Fe-3d orbitals with z-component and N(pz) orbital of pyridinyl radical. It is important to note that for isolated pyridinyl radical, the single electron is highly delocalized. However, upon placement of FePz on it, the single electron is highly localized on N (pz) orbital, which is then hybridized with the z-components of Fe d orbitals. This results reordering of d-orbitals of Fe, which promotes spin crossover. The energy gap between the most stable doublet complex and next lying quartet complex is, however, smaller than in the case of axially bound complex.

Table 2. Relative energies (kcal/mol) for various spin states of FePz…X complexes (X = closed- and open-shell ligands) Quintetb Tripletb Singletb FePz…CO (axial) B2-PLYP-D3 0.0 2.6 13.8 B97D3 28.9 16.7 0.0 a CASSCF 10.0 16.1 0.0 (13.8) (12.9) FePz…CO (parallel) B97D3 17.8 0.0 30.5 FePz…NO (axial) B2-PLYP-D3 17.7 0.0 10.6 B97D3 48.4 28.2 0.0 CASSCFa 94.2 4.18 0.0 (12.1) (39.5) FePz…NO (parallel) B97D3 28.7 10.5 0.0 FePz…benzene B97D3 15.2 0.0 20.3 a CASSCF 27.6 0.0 41.1 (15.9) (4.6) FePz…pyridinyl B97D3 25.2 7.4 0.0 a radical CASSCF -17.8 -5.3 0.0 (27.5) (26.0) aCalculated with CAS(14,14) The values in parentheses show the correction for higher level correlations, . (Erel(CASPT2) – Erel(CASSCF) at the CASSCF(8,10) level. bFor complexes with N-containing ligands singlet, triplet and quintet stands for doublet, quartet and sextet of the whole complex, respectively. Interaction energies Binding of ligands lifts Fe atom from the plane of FePz characterized by the out-of-plane distances. This parameter together with the results of interaction energies calculated at the DFT level employing various ACS Paragon Plus Environment

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functionals and CCSD(T) level of the complexes in their ground state multiplicities are listed in Table 3. Concerning the accuracy of the DFT methods, the hybrid B3LYP-D3 and PBE0-D3 functionals give a nice agreement between the interaction energies of FePz and NO/CO ligands in axial position obtained with the CCSD(T) method, with the error less than 1 kcal/mol. On the other hand, GGA functionals (B97D3, BLYP-D3 and PBE-D3) significantly overestimate the interaction energies; the overestimation ranges from 26% (using B97-D3) to 68% (using PBE-D3) and from 56% (using B97-D3) to 87% (using PBED3) for CO and NO axial ligands, respectively. This limits the use of DFT functionals for FePz type complexes. Stronger binding of NO compared to CO, in agreement with the results reported in ref 55, correlates with the larger displacement of Fe from FePz molecular plane. Despite the similarity of out-ofplane displacements of Fe (parameter d, Table 3) in parallel and axial ligand complexes, the former are much less stable and even unstable. The total interaction energies range from -7.1 to +2.2 and -14.4. to 5.8 kcal/mol for CO and NO ligands in parallel orientations, respectively. The situation is different for benzene and pyridinyl radical ligands which show negligible out-of-plane displacements. With the exception of the FePz…benzene PBE-D3 result, all calculated values agree within 2 kcal/mol. The contribution of the dispersion energy to the total interaction depends on the ligand orientation. Based on the calculations with B97-D3 functional, dispersion energy contribution amounts to 16% (FePz…CO) and 24% (FePz…NO) of the total interaction energy in the case of axial ligand orientation, while it contributes by 58% to the total interaction energy of parallel FePz…NO and it is the only stabilization factor of parallel FePz…CO, similarly to the complex with benzene. In contrast to unstable FePz…CO complex, the dispersion attraction is large enough in FePz…benzene to stabilize the complex. The dispersion energy is also the main factor which stabilizes FePz complex with pyridinyl radical, its contribution is, however, smaller.

Table 3. DFT interaction energies (kcal/mol) for FePz…X complexes (X = closed- and open-shell ligands); values in parentheses correpond to dipsersion energy


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d(Å)

B97D3/TZV PP

B3LYPD3/TZVPP

PBE0D3/TZVPP

BLYPD3/TZVPP

PBED3/TZVPP

CCSD(T) / 631G**(0. 25, 0.15)

Axial ligands FePz…CO

0.272 0.424

-19.4 [-4.4] -24.0 [-4.3]

-19.3 [-2.8] -23.0 [-2.8]

-27.9 [-5.3] -39.1 [-5.2]

-34.0 [-2.7] -43.5 [-2.8]

-19.6

FePz…NO

-24.6 [-5.9] -36.1 [-5.6]

Parallel ligands FePz…CO

0.330

-7.1 [-3.2]

-6.0 [-2.2]

1.4 [-3.8]

1.9 [-2.2]

FePz…NO

0.424

-6.5 [-4.8]

-5.8 [-3.1]

-12.2 [-5.8]

-14.4 [-3.0]

FePz…benzene

0.070

2.2 [-3.5] -10.9 [-6.3] -11.8 [-17.7] -30.1 [-23.0]

-11.8 [-15.2] -29.7 [-18.2]

-10.9 [-10.2] -28.8 [-11.7]

-11.3 [-16.2] -30.5 [-22.2]

-7.2 [-10.2] -29.1 [-11.5]

FePz…pyridinyl radical

0.007

-23.1

Differences of CO and NO axial and parallel orientations are revealed also in the values of the Wiberg Bond Indices (WBI, see Table 4) which nicely correlate with the calculated interaction energies. WBI values of axially oriented CO and NO ligands confirm the covalent character, in agreement with relatively large interaction energies (not considering dispersion contributions) of -18.7 and -30.5 kcal/mol obtained at the B97-D3 level, respectively. The overlap of CO-lone pair and Fe d-orbitals in the parallel complex does not allow formation of covalent dative bond and, consequently, there is a very low or no stabilization between subsystems (total interaction energy is repulsive). WBI index of 0.78 found in the in the case of parallel FePz…NO reflects a rather strong covalent character of Fe-NO bond formed between nitrogen single-occupied π*(NO) molecular orbital and vacant dz2 with a radical character (radical covalent dative bond) due to a single electron operating in this bond. Different situation is observed in the cases of FePz…benzene and FePz…pyridinyl radical complexes. Corresponding WBI indexes are negligible in both cases, the character of stabilization is, however, different. In the former complex the negligible value of WBI correlates with a weak interaction energy and only the dispersion energy contributes to stabilization. Despite very small WBI value in the latter complex, the covalent character, caused by the hybridization, is responsible for the sizable stabilization energy ranging from 7.1 to 17.6 kcal/mol, depending on the functional used. The ACS Paragon Plus Environment

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hybridization originates from the singly occupied π*( pyridinyl radical) molecular orbital, similarly as in the case of FePz…NO (parallel) complex, and formation of the radical dative bond. Although the character of the interaction is the same, the Fe…N distance in FePz…NO is significantly smaller than in FePz…pyridinyl radical (see Figure 2) which explains much larger WBI of the former.

Table 4. Wiberg Bond Index for FePz…X complexes (X = closed- and open-shell ligands); Wiberg Bond index (WBI) Axial ligands FePz…CO FePz…NO Parallel ligands FePz…CO FePz…NO FePz…benzene FePz…pyridinyl radical

1.291 1.358 0.024 0.782 0.005 0.04

Origin of SCO The formation of strong covalent bonds in axial FePz…CO and FePz…NO complexes and changes of the spin state of FePz upon ligand binding is well known and well documented in previous studies.42,44,49,53,55,58,78,79,80,82,83 Possibility of SCO induced by much weaker noncovalent interactions as found in FePz…NO with the ligand in the parallel orientation is, however, a rather new information. This finding matches with recent experimental observation of modulation of electronic structure of FePc by its noncovalent interactions with graphene and nitrogen doped graphene.61 The size of such systems does not allow investigation of their electronic structures to be performed at the multi-reference level. The FePz…benzene and FePz…pyridinyl radical complexes investigated in the current study allow for such descriptions and, at the same time, they can be used to simulate the situation observed for FePc on graphene and N-doped graphene.61 The ground states of isolated FePz differ at the DFT level (using B97D3 functional) and multi-reference levels, giving triplet and quintet spin states, respectively. The ground state of FePz…benzene complex is in triplet at both levels. MO diagram for FePz…benzene obtained at the MCSCF level (Figure 3) suggests that the complexation with benzene leads to rearrangements of ACS Paragon Plus Environment

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Fe(3d) orbitals; singly occupied and almost degenerate dx2-y2, dxz, dyz, dxy orbitals of the isolated quintet FePz split resulting in doubly occupied dxy and singly occupied dxz and dyz orbitals in the triplet FePz…benzene complex. dx2-y2 orbital is destabilized and becomes empty upon complexation and the originally doubly occupied dz2 orbital remains unchanged. Complexation with pyridinyl radical results in triplet and quintet to doublet transitions at the DFT and MCSCF levels, respectively. The investigation of MCSCF orbitals reveals the origin of SCO, i.e. hybridization of singly occupied π*(pyridinyl radical) molecular orbital with 3d(Fe) orbitals in the direction perpendicular to the FePz molecular plane leading to reordering of 3d(Fe) occupied and unoccupied orbitals (cf Figure 3). The weak intermixing between orbitals with z-component of pyridinyl radical (pz orbital of N) and of FePz (dxz, dyz, dz2) results in reshuffling of Fe d-orbitals energies and occupancies facilitated by energetically close lying of pz orbital of N molecular orbital of pyridinyl radical (see Figure 3).

Figure. 3 The orbital occupancy in isolated FePz and its complexes from MCSCF calculations. Conclusions The results of the multi-reference, CCSD(T) and DFT calculations are used to explain the effects of covalent dative and noncovalent interactions on the SCO process on the Fe center of FePz. The nature of previously explored spin crossover, induced by small ligands CO and NO which bound covalently to Fe (WBI values of about 1.3) in axial orientation, is compared to the character of weak noncovalent ACS Paragon Plus Environment

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interactions of the same ligands in parallel orientation, characterized by significantly smaller WBI values. It is shown that in parallel FePz…NO the singly occupied p orbital perpendicular to the FePz plane hybridizes with Fe d-orbitals of a suitable symmetry. The hybridization causes re-ordering of Fe d-orbitals and, consequently changes the orbital occupation and the spin state of FePz. The same mechanism is responsible for the SCO in the FePz…pyridinyl radical complex with the WBI smaller by several orders of magnitude. Importantly, such effects are less pronounced in the complexes with parallel CO and benzene. The described mechanism offers a possible way to control the spin state of a molecular system by simple positioning of the molecule onto a suitably functionalized substrate. Author Information Corresponding Author *E-mail: [email protected] (P.H.). [email protected] (D.N.) ORCID Pavel Hobza: 0000-0001-5292-6719 Rabindranath Lo: 0000-0002-4436-3618 Radek Zbořil: 0000-0002-3147-2196 Acknowledgements This work was part of Research Project RVO: 61388963 of the Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic. This work was supported by the Ministry of Education, Youth and Sports from the Large Infrastructures for Research, Experimental Development, and Innovations project “IT4 Innovations National Supercomputing Center LM2015070”. This work was also supported by the Czech Science Foundation, Project 19-27454X. Notes The authors declare no competing financial interest References


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Porphyrazine Induced by Non-Covalent Interactions Combined with

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