Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet

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Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet Tribromophenyl Nitrenes Denis V. Korchagin,*,† Alexander V. Akimov,† Anton Savitsky,‡ Sergei V. Chapyshev,† Sergey M. Aldoshin,† and Eugenii Ya. Misochko*,† †

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Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region Russian Federation ‡ Faculty of Physics, Technical University Dortmund, Otto-Hahn-Strasse 4a, D-44227 Dortmund, Germany S Supporting Information *

ABSTRACT: Previously unknown the steric heavy atom effect on magnetic anisotropy parameters of triplet phenyl nitrenes is reported. The heavy bromine atom effect is revealed by W-band EPR and theoretical investigations of triplet 2,4,6-tribromophenyl nitrenes bearing different substituents in positions 3 and 5 of the phenyl ring (1a, H/H; 1b, CN/CN; 1c, N3/F; 1d, N3/N3; 1e, Cl/Cl; 1f, Br/Br). The zero-field splitting parameters of nitrenes 1a (D = 0.9930 cm−1, E = 0.0261 cm−1), 1c (D = 1.244 cm−1, E = 0.030 cm−1), and 1d (D = 1.369 cm−1, E = 0.093 cm−1), generated by the photolysis of the corresponding azides in frozen methylcyclohexane solution at 5 K, were determined from the W-band EPR spectra. To clarify the origin of considerable differences in the experimental D values of nitrenes 1a, 1c, and 1d, extensive DFT and CASSCF calculations of these nitrenes as well as of model nitrenes 1b, 1e, and 1f were performed. The calculations show that all nitrenes have nearly the same magnitudes of the spin−spin interactions (DSS ∼ 1 cm−1), but drastically differ in the spin−orbit coupling parameter (from DSOC = 0.087 cm−1 for 1a to DSOC = 0.765 cm−1 for 1f). Comprehensive analysis of various computational data showed that the magnitude of DSOC of nitrenes 1a−f is the function of the N···Br distance between the nitrene nitrogen and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a−1f, the smaller the N--Br distance and the larger DSOC. These features indicate that the heavy atom ef fect on magnetic anisotropy of triplet phenyl nitrenes originates from the through-space rather than through-bond electronic interactions between the bromine atoms and the nitrene unit.

1. INTRODUCTION

spin density on the nitrene unit can only moderately increase the D values of triplet nitrenes. The second term of magnetic anisotropy of high-spin molecules arises due to the anisotropic spin−orbit coupling (SOC). The impact of the SOC term on the D values of highspin nitrenes is often ignored, although it was reported16,17 that SOC dominates for diatomic nitrenes containing heavy halogen atoms (e.g., the D values in NF, NCl and NBr are equal to 2.4, 4, and ∼17 cm−1, respectively). SOC can become the dominant contributor to D for organic species with unpaired electrons centered on heavy elements (e.g., P, Si, and S), as has been recently observed for triplet phosphinidene by EPR spectroscopy.18 The so-called internal heavy atom ef fect is also a well-known phenomenon in photochemistry of aromatic compounds, in the molecules of which various heavy atoms substantially enhance SOC between the lowest singlet and triplet states.19 The first attempt to investigate the heavy atom effect on the D values of triplet nitrenes has been undertaken

High-spin nitrenes are the molecular ferromagnetic domains with the large zero-field splitting (ZFS) parameters D. Owing to these features, high-spin nitrenes are of considerable interest as model systems for investigations of magnetism in organic polyradicals.1 In the last two decades, a great variety of highspin nitrenes have been studied, using various spectroscopic techniques and computational methods.2−15 Most of these studies were focused on investigations of the dipolar spin−spin (SS) interaction between unpaired electrons on the nitrene units, providing the dominant contribution to the D values of nitrenes composed of the light atoms (H, C, N, O, and F). The highest parameter D = 1.8 cm−1 was recorded for triplet nitrene NH, in which both unpaired electrons are localized on the nitrene unit (spin density at the nitrene unit ρN = 2). In comparison with NH, triplet aryl nitrenes show much lower values of D, due to delocalization of spin density from the nitrene unit on the aromatic ring. The linear correlation between the experimental D values and natural spin densities on the nitrene unit for various triplet nitrenes is shown in Figure 1. This correlation demonstrates that localization of © XXXX American Chemical Society

Received: September 14, 2018 Revised: October 23, 2018 Published: October 25, 2018 A

DOI: 10.1021/acs.jpca.8b09014 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

1), using the W-band EPR spectroscopy and quantum chemical calculations. Chart 1. Model Triplet Nitrenes Used in the Present Study

2. METHODS 2.1. Starting Materials. Starting 2,4,6-tribromophenyl azide (A1) and 1,3-diazido-2,4,6-tribromo-4-fluorophenyl azide (A2) were synthesized according to the literature procedure.29,30 The solvent methylcyclohexane (MCH) was obtained from Sigma-Aldrich in their purest commercially available form and used without further purification. 2.2. EPR Measurements. W-band EPR spectra of triplet nitrenes 1a and 1c were recorded after the photolysis of azides A1 and A2 in methylcyclohexane glass. In a typical procedure, A1 and A2 were dissolved in freshly distilled MCH with the ratio of A1:MCH (or A2:MCH) = 1:1000. The pulsed EPR experiments were performed on a modified commercial W-band EPR spectrometer (Bruker Elexsys E680) operating at about 94 GHz.31 The deoxygenated sample solutions were placed in a quartz capillary (0.6 mm i.d.) and transferred to the precooled EPR probehead. The samples were illuminated at 5 K using a Hg arc lamp (Mercury Spectral Line Lamp, Lot-Oriel) equipped with narrow band filter transparent 297 nm and guided to the sample through a quartz fiber of 0.4 mm diameter. The field-swept echo-detected EPR spectra were acquired at 5 K using the Hahn-echo sequence (tp)−τ−(2tp)−τ−echo with tp = 20 ns and interpulse distance of τ = 180 ns. At each field point 1024 echo responses were averaged after subsequent pulse sequences repeated 8 × 103 times per second. The spectral simulations were performed using the EasySpin program package,32 operating with an exact numerical matrix diagonalization of the spin Hamiltonian:

Figure 1. Correlation between the D values and natural spin densities on the nitrene unit (ρN) for various triplet nitrenes: open squares (collected data in ref 13). The D values of 3,5-diazido-2,4,6tribromophenyl, D = 1.369 cm−1 and ρN = 1.525 (ref 27 and this study), 3-azido-5-fluoro-2,4,6-tribromophenyl, D = 1.244 cm−1 and ρN = 1.522 (this study), and 2,4,6-tribromophenyl, D = 0.993 cm−1 and ρN = 1.523 (ref 28) nitrenes are shown with red, green, and blue asterisks, respectively.

in 1978.20 It was found that the D values of para-halogenated phenyl nitrenes decrease in the order of D(I) < D(Br) < D(Cl) < D(F), thus indicating the absence of the positive heavy atom effect for these nitrenes. Recent high-level multiconfigurational (CASSCF) calculations21 revealed that contribution of the SOC term to the total D values of para-halogenated phenyl nitrenes does not exceed 10%, and the DSOC values gradually decrease in the row p-FC6H4N < p-ClC6H4N < p-BrC6H4N < p-IC6H4N. No heavy atom effect on the D values was also detected for meta-halogenated nitrenes m-BrC6H4N22 and mIC6H4N.12 These studies have shown that heavy atoms in paraand meta-positions of phenyl nitrenes did not promote the heavy atom effect on the D values of these triplet molecules. At the same time, the heavy atom effect on the SOC term of some high-spin molecules was predicted in a number of theoretical works.23,24 Experimentally, this effect on the ZFS parameters of triplet (S = 1), quintet (S = 2), and septet (S = 3) nitrenes, formed during the photolysis of 2,4,6-triazido-3,5-dibromopyridine, 1,3,5-triazido-2-bromo-4,6-dichlorobenzene, and 1,3,5triazido-2,4,6-tribromobenzene, has been confirmed only recently.25−27 In all these nitrenes, the bromine atoms were located in ortho-positions to the nitrene units, and the SOC terms were dominant and governed both the magnitude and the sign of magnetic anisotropy. Moreover, triplet 3,5diazidotribromophenyl nitrene (1d), obtained by the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene, showed the highest value of D = 1.369 cm−1 among all known to date triplet phenyl nitrenes.27 This value did not fit the linear correlation, shown in Figure 1, due to the large (∼30%) DSOC part. On the basis of these data, it was reasonable to expect that all 2,6dibromophenyl nitrenes should show the large DSOC values. However, recent X-band EPR and computational studies have revealed that the D SOC value of triplet 2,4,6tribromophenyl nitrene (1a) is surprisingly very small despite the presence of two bromine atoms in ortho-positions to the nitrene unit, see Figure 1.28 To determine the factors affecting the heavy atom effect in triplet phenyl nitrenes, herein we have studied the ZFS parameters of model nitrenes 1a−1f (Chart

H = gβHS + DSz 2 + E(Sx 2 − Sy 2)

(1)

(where D and E denote the ZFS parameters) for randomly oriented molecules with total spin S = 1. The ZFS parameters of mononitrenes 1a and 1c were determined by comparison of the computer simulated and experimentally recorded spectra following the procedure described in detail in refs 3 and 25. 2.3. Computational Methods. The geometries of the molecules under consideration were optimized at B3LYP/def2TZVP level of theory by DFT method. The nature of the stationary points was assessed by means of vibrational frequency analysis. The magnetic parameters of triplet mononitrenes 1, including both the spin−spin and the spin− orbit couplings, were obtained by DFT and CASSCF based (ab initio) methods. In DFT calculations, the direct spin−spin coupling (DSS) and spin−orbit coupling (DSOC) components of the ZFS parameters were obtained by using the equation of McWeeny−Mizuno33,34 and Pederson−Khanna approach,35 B


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The Journal of Physical Chemistry A respectively. ZFS parameters were calculated with the PBE36 functional and the Ahlrichs-DZ basis set37 provided the most accurate data for high-spin nitrenes.7 In CASSCF calculations, the values of the D tensor for C2v and C2 symmetrical mononitrenes 1a and 1d were obtained employing the multiconfigurational formalism with 3 and 10 excited states, respectively, for each irreducible representation of the spintriplet and singlet roots on the top of the state-average CASSCF wave function. The CASSCF active space consists of 14 orbitals and 20 electrons and 15 orbitals and 22 electrons for mononitrenes 1a and 1d, respectively. The CASSCF active spaces of 1a and 1d are shown in Figures 2 and 3. The

component. All calculations were performed with the ORCA program package (version 3.0.3).39

3. RESULTS AND DISCUSSION 3.1. W-Band EPR Spectrum of Mononitrene 1a. In our previous work,28 we have reported on the X-band (9.5 GHz) EPR spectrum and ZFS parameters of triplet nitrene 1a obtained by the photolysis of azide A1 in glassy MCH (Scheme 1). Scheme 1. Photochemical Generation of Triplet Nitrene 1a

Similarly to other triplet molecules with D ≈ 1 cm−1, nitrene 1a exhibits only two EPR lines in the powder X-band EPR spectrum, which correspond to two allowed perpendicular transitions. The spectral simulations of experimental EPR spectrum yield the ZFS parameters of nitrene 1a as D = 0.989 cm−1 and E = 0.0274 cm−1, under an assumption that isotropic g-value equals to 2.003 and the D-value is positive. In order to determine the exact values of the g-tensor and the sign of the D-value for nitrene 1a, in the present work we have utilized the W-band (94 GHz) EPR spectroscopy, in which a quantum of microwave radiation ∼3.1 cm−1 substantially exceeds the magnitude of D ≈ 1 cm−1. Such a study is of considerable interest for investigations of the heavy atom effect on the ZFS parameters of nitrenes 1a and 1d, the latter of which was recently characterized by W-band EPR spectroscopy under similar conditions.27 Figure 4 depicts the W-band EPR spectrum recorded after the photolysis of azide A1 in glassy MCH at 5 K.

Figure 2. CASSCF active space and the orbital occupancies in the main triplet configuration of mononitrene 1a.

Figure 3. CASSCF active space and the orbital occupancies in the main triplet configuration of mononitrene 1d.

Figure 4. (a) W-band field-swept echo-detected EPR spectrum recorded after photolysis of A1 in glassy MCH at 5 K and microwave frequency 93.99 GHz. (b) Calculated EPR spectrum of triplet mononitrene 1a using D = +0.9930 cm−1, E = 0.0261 cm−1, and g = [2.0091 2.0051 2.0070]. The dotted lines mark the magnetic field positions according to the nomenclature given in the text.

spectroscopy-oriented CI (SORCI)38 on top of the CAS reference states were carried out for calculation of the SOC C


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almost the same as the D-values of triplet phenyl nitrenes containing only light atoms (see Figure 1). 3.2. W-Band EPR Spectrum of Mononitrene 1c. The photolysis of diazide A2 proceeds according to Scheme 2. At the initial stages of the photolysis of this diazide, the formation of triplet nitrene 1c is dominant. Figure 6 shows the W-band

It reveals a series of intense lines typical for EPR spectra of triplet molecules in the high-field limit (hν > D). The calculated Zeeman energy levels and allowed EPR transitions in Figure 5 display two parallel (z1 and z2) and four

Figure 6. (a) W-band field-swept echo-detected EPR spectrum recorded after short photolysis of A2 in glassy MCH at 5 K and microwave frequency 93.89 GHz. EPR lines of quintet dinitrene Q are labeled with Q. The line labeled with R is ascribed to the site product radical (S = 1/2) with unresolved hyperfine structure. (b) Calculated EPR spectrum of triplet mononitrene 1c with D = +1.2435 cm−1, E = 0.030 cm−1, and g = [2.0112 2.0086 2.0047]. Figure 5. Calculated Zeeman energy levels and allowed transitions in canonical ZFS orientations as well as calculated W-band EPR spectrum of triplet mononitrene 1a with D = +0.9930 cm−1 and E = 0.0261 cm−1.

EPR spectrum recorded after short photolysis of A2. The spectrum consists of typical triplet EPR spectrum assigned to nitrene 1c and several weak EPR lines assigned to dinitrene Q.40 The analysis of the triplet spectrum yields the principal values of g-tensor: gxx = 2.0091, gyy = 2.0051, and gzz = 2.0070, and the ZFS parameters: D = 1.2435 cm−1 and E = 0.030 cm−1 for nitrene 1c. Thus, the D-values of tribromophenyl nitrenes 1a, 1c, and 1d are increased in the row D(1a) < D(1c) < D(1d). 3.3. Quantum Chemical Calculations. The DSS tensors of spin−spin contributions and the DSOC tensors of spin−orbit couplings for triplet nitrenes 1 were calculated separately. The total ZFS tensor DTot was obtained by summing DSS and DSOC. The ZFS parameters DSS, DSOC, and DTot were calculated by diagonalization of the tensors DSS, DSOC, and DTot, respectively. The scalar ZFS parameters D and E were calculated with conventional notations:

perpendicular (x1, x2, and y1, y2) EPR transitions, corresponding to six experimentally observable EPR lines marked in Figure 4. The ZFS parameters of triplet nitrene 1a were determined by comparison of the calculated and experimental EPR spectra. The fitting procedure yields the principal values of g-tensor: gxx = 2.0091, gyy = 2.0051, and gzz = 2.0070, and the ZFS parameters: D = 0.9930 cm−1 and E = 0.0261 cm−1. The ZFS parameters obtained are very close to those obtained from the analysis of the X-band EPR spectrum of triplet nitrene 1a.28 It is important to note that a good agreement between relative intensities of the peaks in the experimental and theoretical W-band EPR spectra of nitrene 1a unambiguously proves the positive sign of the D-value for this nitrene. The data obtained completely confirm our preliminary observations28 that the D-value of triplet nitrene 1a is much lower in magnitude than that of nitrene 1d and

D=

Dxx − Dyy 3 Dzz , E = 2 2

(2)

where Dxx, Dyy, and Dzz are the eigenvalues of the tensor D.

Scheme 2. Photochemical Generation of Mononitrene 1c

D


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Table 2. Selected Excited Electronic States of 1aa (with DSOC Contributions >0.01 cm−1)

Triplet nitrenes 1c and 1d may exist in the form of several stable conformers, as shown in Chart 2. The equilibrium Chart 2. Stable Conformers of Triplet Nitrenes 1c and 1d

state

characterb

ΔE, cm−1

DSOC, cm−1

1

12b1 → 23b2 (SOMO → SOMO) 23b2 → 12b1(SOMO → SOMO) 8b1 → 23b2 (π → SOMO) 8b1 → 23b2 (π → SOMO) 22b2 → 12b1(lone pair → SOMO) 22b2 → 12b1(lone pair → SOMO) 21b2 → 12b1(lone pair → SOMO) 31a1 → 12b1(lone pair → SOMO) 31a1 → 12b1(lone pair → SOMO)

14812 21826 43923 51263 51831 52780 55121 55199 55241

0.019 0.121 0.023 −0.028 −0.381 0.372 −0.025 0.250 −0.223

1 2 2 1 2 3 2 3 3

geometries of these conformers were optimized using DFT calculations at B3LYP/def2-TZVP level of theory. These calculations show that the most stable conformers 1d′(C2) and 1c(C1) lie ∼0.5−1 kcal/mol below in energy relatively other conformers, and all of them have nearly the same D-values. Therefore, we give below the D-values only for the most stable conformers. In accordance with our previous DFT calculations,28 the tensors DSS and DSOC coincide, and the “easy axes” Z lies along the C−N bond. Therefore, the total D value is the scalar sum of the SS and SOC contributions, DTot = DSS + DSOC. The results of our ab initio and DFT calculations of various parameters D and spin densities on selected atoms for three experimentally studied nitrenes 1a, 1c and 1d as well as for triplet phenyl nitrene (PhN) as a reference species are presented in Table 1. The calculated values of DTot for nitrenes 1a, 1c, and 1d agree well with the experimental data. In accord with our experimental data, both theoretical methods predict the growth of DTot in the order of D(1a) < D(1c) < D(1d). All nitrenes 1a, 1c, and 1d bear almost the same spin densities on the nitrene unit and only slightly differ in the DSS values. The growth of the parameter DTot in row D(1a) < D(1c) < D(1d) mainly arises due to an increase in DSOC. At the same time, relatively small DSOC for triplet nitrene 1a indicates that the presence of three bromine atoms in the phenyl ring of nitrenes is not yet prerequisite for the appearance of the large heavy atom effect. Therefore, both triplet nitrenes PhN and 1a show practically the same DExp values. Recent theoretical studies21 have shown that the SOC part of high-spin nitrenes containing light atoms, e.g. PhN, arises due to the spin-flip excitations between two singly occupied molecular orbitals (SOMO → SOMO). Our calculations reveal that contribution of the spin-flip SOMO → SOMO excitations to the SOC terms is also dominant for tribromophenyl nitrenes 1a and 1d (see Tables 2 and 3). The effect of bromine atoms on excited electronic states of both these nitrenes is only limited by the appearance of five the lone pair → SOMO excitations with large magnitudes of DSOC that completely compensate each other and, as the result, contribute nothing to the total SOC terms. Thus, analysis of the excited electronic states of nitrenes 1a and 1d does not

A1 A1 3 B2 1 B2 3 A1 1 A1 1 B2 3 B2 1 B2 1

a The active space consists of 14 orbitals: 31a1, 6a2−8a2, 8b1−14b1, and 21b2−23b2. bExcitation configurations from the main configuration of the 1a 3A2 ground state.

Table 3. Selected Excited Electronic States of 1da (with DSOC Contributions >0.01 cm−1) state

characterb

ΔE, cm−1

1

47b → 46b (SOMO → SOMO) 46b → 47b (SOMO → SOMO) 51a → 47b (π → SOMO) 43b → 46b (lone pair → SOMO) 43b → 46b (lone pair → SOMO) 42b → 46b (lone pair → SOMO) 51a → 47b (π → SOMO) 42b → 46b (lone pair → SOMO) 44b → 46b (lone pair → SOMO)

22990

0.254

43474 46476 47507 48356 48464 48965 58725

0.026 −0.427 0.381 −0.322 −0.019 0.383 −0.045

2 A 5 6 6 4 5 6 7

3

B A 1 A 1 B 1 B 3 B 1 B 3

DSOC, cm−1

a

The active space consists of 15 orbitals: 49a-52a, 39b-49b. Excitation configurations from the main configuration of the 1d 3 A2 ground triplet state. b

provide any clue to understanding the large difference in the SOC parts of these molecules. The SOC parts for both nitrenes arise due to the spin-flip excitations between two SOMO orbitals, and the difference in these excitations consists just in slightly different energy gaps between the interacting SOMO orbitals. To obtain more information about the heavy atom effect on the D values of tribromophenyl nitrenes, model triplet nitrenes 1b, 1e, and 1f have been theoretically studied by use of DFT calculations. Selected results of DFT calculations of nitrenes 1a−1f are summarized in Table 4. All nitrenes under study have very similar in magnitude the DSS values, but drastically differ in the DTot and DSOC values correlating well with the N···Br distance between the nitrene unit and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a−f, the smaller the N···Br distance (r) and the larger DSOC and DTot (see Figure 7). These features indicate that the heavy atom ef fect on magnetic anisotropy of triplet phenyl nitrenes originates from

Table 1. D Values (DExp, DTot, DSS, and DSOC) and Spin Densities on the Nitrene Units (ρN) and Bromine Atoms (ρ(o-Br/pBr)) for Phenyl Nitrene (PhN) and Nitrenes 1a, 1c, and 1d ab initio PhN 1a 1c 1d

DFT

DExp

DTot

DSS

DSOC

DTot

DSS

DSOC

ρN

ρ(o-Br/p-Br)

0.998 0.993 1.244 1.369

1.066 1.116 1.144 1.253

0.969 0.990 0.978 1.022

0.097 0.126 0.166 0.231

1.107 1.079 1.352 1.413

1.062 0.992 0.988 0.992

0.045 0.087 0.364 0.421

1.589 1.523 1.522 1.525

− 0.023/0.030 0.026/0.039 0.028/0.035

E


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Table 4. D Values (DExp, DTot, DSS, and DSOC), Spin Densities on the Nitrene Units (ρN) and Bromine Atoms (ρo‑Br), and the N···Br Distance between the Nitrene Unit and the Neighboring Bromine Atoms r(o-Br···N:) for Triplet Nitrenes 1a−1f Y1, Y2

DExpt

DTot

DSS

DSOC

ρN

ρo‑Br

r(o-Br···N:), Å

H (1a) CN (1b) F, N3 (1c) N3 (1d) Cl (1e) Br (1f)

0.993 − 1.244 1.369 − −

1.080 1.280 1.352 1.413 1.595 1.740

0.992 1.011 0.988 0.992 0.985 0.976

0.087 0.273 0.364 0.421 0.610 0.765

1.523 1.539 1.522 1.525 1.520 1.515

0.023 0.030 0.026 0.028 0.030 0.031

3.089 3.041 3.032 3.029 3.019 2.998

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Cartesian coordinates of molecules under consideration (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(E.Y.M.) E-mail: [email protected]. *(D.V.K.) E-mail: [email protected]. ORCID

Denis V. Korchagin: 0000-0002-0199-1382 Eugenii Ya. Misochko: 0000-0001-7903-8949 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Ministry of Science and Higher Education of the Russian Federation (Projects 00892014-0014, 0089-2014-0009, and 0089-2014-0005). D.V.K. thanks the Russian Foundation for Basic Research (RFBR, Grant 16-33-00353 mol_a) for support of quantum chemical calculations.

Figure 7. Experimental and DFT calculated parameters D for a series of triplet nitrenes 1a−1f with different substituents Y1 and Y2 vs the N···Br distance between the nitrene unit and the neighboring bromine atoms r(o-Br···N:) The spin−spin (DSS) and spin−orbit (DSOC) contributions to the total parameter D are shown with green and red rows, respectively.

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the through-space rather than through-bond electronic interactions between the bromine atoms and the nitrene unit. This finding explains well the absence of the heavy atom ef fect on magnetic anisotropy of para- and meta-halogenated triplet phenyl nitrenes, in the molecules of which heavy atoms are located far away from the nitrene units.

4. CONCLUSIONS Our experimental and theoretical studies of triplet 2,4,6tribromophenyl nitrenes 1a−1f show that the heavy atom effect on magnetic anisotropy of these magnetic molecules is strong only when heavy atoms are located spatially close (r < 3 Å) to the nitrene units. Otherwise, the heavy atom effect is not observed. The effect of heavy atoms on magnetic anisotropy of triplet phenyl nitrenes results in the growth of the D values due to the increase of the SOC contribution. The latter is the function of the through-space N···Br distance between the nitrene nitrogen and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a−f, the smaller the N···Br distance and the larger DSOC. We term this new effect as “steric heavy atom effect”. The understanding of this effect opens up new paths to the rational design of organic magnetic molecules possessing a large magnetic anisotropy.

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REFERENCES

(1) Nimura, S.; Yabe, A. In Molecular Magnetism in Organic-Based Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, NY, 1999; pp 127 ff. (2) Chapyshev, S. V.; Walton, R.; Sanborn, J. A.; Lahti, P. M. Quintet and Septet State Systems Based on Pyridylnitrenes: Effects of Substitution on Open-Shell High-Spin States. J. Am. Chem. Soc. 2000, 122, 1580−1588. (3) Misochko, E. Ya.; Akimov, A. V.; Chapyshev, S. V. High Resolution Electron Paramagnetic Resonance Spectroscopy of Quintet Pyridyl-2,6-Dinitrene in Solid Argon: Magnetic Properties and Molecular Structure. J. Chem. Phys. 2008, 128, 124504. (4) Misochko, E. Ya.; Akimov, A. V.; Chapyshev, S. V. High Resolution Electron Paramagnetic Resonance Spectroscopy of Septet Pyridyl-2,4,6-Trinitrene in Solid Argon: Fine-Structure Parameters of Six Electron-Spin Cluster. J. Chem. Phys. 2008, 129, 174510. (5) Koto, T.; Sato, K.; Shiomi, D.; Toyota, K.; Itoh, K.; Wasserman, E.; Takui, T. Random-Orientation High-Spin Electron Spin Resonance Spectroscopy and Comprehensive Spectral Analyses of the Quintet Dicarbene and Dinitrene with meta-Topological Linkers: Origins of Peculiar Line-Broadening in Fine-Structure ESR Spectra in Organic Rigid Glasses. J. Phys. Chem. A 2009, 113, 9521−9526. (6) Koto, T.; Sugisaki, K.; Sato, K.; Shiomi, D.; Toyota, K.; Itoh, K.; Wasserman, E.; Lahti, P. M.; Takui, T. High-Spin Nitrene FineStructure ESR Spectroscopy in Frozen Rigid Glasses: Exact Analytical Expressions for the Canonical Peaks and A D-Tensor Gradient Method for Line Broadening. Appl. Magn. Reson. 2010, 37, 703−736. (7) Misochko, E. Ya.; Korchagin, D. V.; Bozhenko, K. V.; Chapyshev, S. V.; Aldoshin, S. M. A Density Functional Theory Study of the Zero-Field Splitting in High-Spin Nitrenes. J. Chem. Phys. 2010, 133, 064101. (8) Sugisaki, K.; Toyota, K.; Sato, K.; Shiomi, D.; Kitagawa, M.; Takui, T. Spin−Orbit Contributions in High-Spin Nitrenes/

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The Journal of Physical Chemistry A Carbenes: A Hybrid CASSCF/MRMP2 Study of Zero-Field Splitting Tensors. ChemPhysChem 2010, 11, 3146−3151. (9) Sugisaki, K.; Toyota, K.; Sato, K.; Shiomi, D.; Kitagawa, M.; Takui, T. Ab Initio and DFT Studies of the Spin−Orbit and Spin− Spin Contributions to the Zero-Field Splitting Tensors of Triplet Nitrenes with Aryl Scaffolds. Phys. Chem. Chem. Phys. 2011, 13, 6970−6980. (10) Misochko, E. Ya.; Akimov, A. V.; Masitov, A. A.; Korchagin, D. V.; Aldoshin, S. M.; Chapyshev, S. V. Matrix Isolation ESR Spectroscopy and Magnetic Anisotropy of D3h Symmetric Septet Trinitrenes. J. Chem. Phys. 2013, 138, 204317. (11) Sugisaki, K.; Toyota, K.; Sato, K.; Shiomi, D.; Kitagawa, M.; Takui, T. Quantum Chemical Calculations of the Zero-Field Splitting Tensors for Organic Spin Multiplets. Prog. Theor. Chem. Phys. 2013, 24, 363−392. (12) Grote, D.; Sander, W. Nitreno Radicals. Nitrene and Nitrenium Ions 2013, 317−346. (13) Kvaskoff, D.; Bednarek, P.; George, L.; Waich, K.; Wentrup, C. Nitrenes, Diradicals, and Ylides. Ring Expansion and Ring Opening in 2-Quinazolylnitrenes. J. Org. Chem. 2006, 71, 4049−4058. (14) Wentrup, C. Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles. Chem. Rev. 2017, 117, 4562−4623. (15) Wentrup, C. Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry. Angew. Chem., Int. Ed. 2018, 57, 11508− 11521. (16) Sinnecker, S.; Neese, F. Spin-Spin Contributions to the ZeroField Splitting Tensor in Organic Triplets, Carbenes and Biradicals. A Density Functional and Ab Initio Study. J. Phys. Chem. A 2006, 110, 12267−12275. (17) Neese, F. Calculation of the Zero-Field Splitting Tensor on the Basis of Hybrid Density Functional and Hartree-Fock Theory. J. Chem. Phys. 2007, 127, 164112. (18) Akimov, A. V.; Ganushevich, Y. S.; Korchagin, D. V.; Miluykov, V. A.; Misochko, E.Ya. The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment. Angew. Chem., Int. Ed. 2017, 56, 7944−7947. (19) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1969; Chapter 7. (20) Hall, J. H.; Fargher, J. M.; Gisler, M. R. Substituent Effects on Spin Delocalization in Triplet Phenylnitrenes. 1. Para-Substituted Phenylnitrenes. J. Am. Chem. Soc. 1978, 100, 2029−2034. (21) Sugisaki, K.; Toyota, K.; Sato, K.; Shiomi, D.; Kitagawa, M.; Takui, T. An ab initio MO Study of Heavy Atom Effects on the ZeroField Splitting Tensors of High-Spin Nitrenes: How the Spin−Orbit Contributions are Affected. Phys. Chem. Chem. Phys. 2014, 16, 9171− 9181. (22) Wasserman, E. Electron Spin Resonance of Nitrenes. Prog. Phys. Org. Chem. 2007, 8, 319−336. (23) Havlas, Z.; Michl, J. Spin-Orbit Coupling in Biradicals. 3. Heavy Atom Effects in Carbenes. Collect. Czech. Chem. Commun. 1998, 63, 1485−1497. (24) Havlas, Z.; Kyvala, M.; Michl, J. Spin-Orbit Coupling in Biradicals. 4. Zero-Field Splitting in Triplet Nitrenes, Phosphinidenes, and Arsinidenes. Collect. Czech. Chem. Commun. 2003, 68, 2335− 2343. (25) Misochko, E. Ya.; Akimov, A. V.; Masitov, A. A.; Korchagin, D. V.; Yakushchenko, I. K.; Chapyshev, S. V. High-Spin Organic Molecules with Dominant Spin-Orbit Contribution and Unprecedentedly Large Magnetic Anisotropy. J. Chem. Phys. 2012, 137, 064308. (26) Misochko, E. Ya.; Masitov, A. A.; Akimov, A. V.; Korchagin, D. V.; Chapyshev, S. V. Heavy Atom Effect on Magnetic Anisotropy of Matrix-Isolated Monobromine Substituted Septet Trinitrene. J. Phys. Chem. A 2015, 119, 2413−2419. (27) Akimov, A.; Masitov, A.; Korchagin, D.; Chapyshev, S.; Misochko, E.; Savitsky, A. W-band EPR Studies of High-Spin Nitrenes with Large Spin-Orbit Contribution to Zero-Field Splitting. J. Chem. Phys. 2015, 143, 084313.

(28) Korchagin, D. V.; Akimov, A. V.; Yureva, E. A.; Aldoshin, S. M.; Misochko, E. Ya. Unexpected Effect of Substituents on the Zero-Field Splitting of Triplet Phenyl Nitrenes. Chem. Phys. Lett. 2016, 659, 234−236. (29) Adam, D. The Synthesis and Characterization of Halogen and Nitro Phenyl Azide Derivatives as Highly Energetic Materials; Dissertation. Ludwig-Maximilans-Universität: München, Germany, 2001. (30) Chapyshev, S. V.; Chernyak, A. V. Triazidation of 2,4,6Trifluorobenzenes. J. Fluorine Chem. 2013, 156, 303−306. (31) Nalepa, A.; Möbius, K.; Lubitz, W.; Savitsky, A. High-Field ELDOR-detected NMR Study of a Nitroxide Radical in Disordered Solids: Towards Characterization of Heterogeneity of Microenvironments in Spin-Labeled Systems. J. Magn. Reson. 2014, 242, 203−213. (32) Stoll, S.; Schweiger, A. J. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42−55. (33) McWeeny, R.; Mizuno, Y. The Density Matrix in ManyElectron Quantum Mechanics. II. Separation of Space and Spin Variables; Spin Coupling Problems. Proc. R. Soc. London, Ser. A 1961, 259, 554−577. (34) Petrenko, T. T.; Petrenko, T. L.; Bratus, V. Ya. The Carbon < 100> Split Interstitial in SiC. J. Phys.: Condens. Matter 2002, 14, 12433−12440. (35) Pederson, M. R.; Khanna, S. N. Magnetic Anisotropy Barrier for Spin Tunneling in Mn12O12 Molecules. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 9566−9572. (36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (37) Schafer, A.; Horn, H.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets for Atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571−2577. (38) Neese, F. A Spectroscopy Oriented Configuration Interaction Procedure. J. Chem. Phys. 2003, 119, 9428−9443. (39) Neese, F. The ORCA Program System. Wiley interdisciplinary Reviews - Computational Molecular Science 2012, 2, 73−120. (40) The EPR spectrum and the ZFS parameters of quintet dinitrene Q will be considered in a separate paper.

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Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet

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