Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 & Magnetic Anisotropy | Hot Paper| Highly Oxidized States of Phthalocyaninato Terbium(III) Multiple-Decker Complexes Showing Structural Deformations, Biradical Properties and Decreases in Magnetic Anisotropy Yoji Horii+,*[a] Marko Damjanović+,*[b, g] M. R. Ajayakumar,[c] Keiichi Katoh,*[a] Yasutaka Kitagawa,[d] Liviu Chibotaru,[e] Liviu Ungur,[f] Marta Mas-Torrent,[c] Wolfgang Wernsdorfer,[g] Brian K. Breedlove,[a] Markus Enders,*[b] Jaume Veciana,*[c] and Masahiro Yamashita*[a, h, i] Dedicated to Prof. Dr. Walter Siebert Abstract: Presented here is a comprehensive study of highly ruple and quintuple-decker complexes, ligand oxidation oxidized multiple-decker complexes composed of TbIII and leads to a decrease in the magnetic anisotropy, as predicted CdII ions and two to five phthalocyaninato ligands, which are from theoretical calculations. Unusual paramagnetic shifts stabilized by electron-donating n-butoxy groups. From X-ray were observed in the spectra of the +2e charged quadruple structural analyses, all the complexes become axially com- and quintuple-decker complexes, indicating that those two pressed upon ligand oxidation, resulting in bowl-shaped dis- species are actually unexpected triplet biradicals. Magnetic tortions of the ligands. In addition, unusual coexistence of measurements revealed that the series of complexes show square antiprism and square prism geometries around metal single-molecule magnet properties, which are controlled by ions was observed in +4e charged species. From paramag- the multi-step redox induced structural changes. netic 1H NMR studies on the resulting series of triple, quad- Introduction ty, necessary for solar cells,[2] organic light-emitting diodes,[3] electro-batteries[4, 5] and so on. There are two kinds of p-exten- Extended p-systems[1] have versatile physical properties, such sions: in-plane extension, and p–p stacking.[6] The bottom up as redox, light absorption and emission, and electroconductivi- synthesis of graphene nano-ribbons[7, 8] and the construction of [a] Dr. Y. Horii,+ Prof. Dr. K. Katoh, Prof. Dr. B. K. Breedlove, [g] Dr. M. Damjanović,+ Prof. Dr. W. Wernsdorfer Prof. Dr. M. Yamashita Institute of Nanotechnology (INT) Department of Chemistry, Graduate School of Science Karlsruhe Institute of Technology (KIT) Tohoku University Hermann-von-Helmholtz-Platz 1 6-3 Aramaki-Aza-Aoba Aoba-ku, Sendai, Miyagi, 980-8578 (Japan) 76344 Eggenstein-Leopoldshafen (Germany) E-mail: yoji.horii.c5@tohoku.ac.jp E-mail: marko.damjanovic@kit.edu keiichi.katoh.b3@tohoku.ac.jp [h] Prof. Dr. M. Yamashita [b] Dr. M. Damjanović,+ Prof. Dr. M. Enders School of Materials Science and Engineering, Nankai University Institute of Inorganic Chemistry, Heidelberg University Tianjin 300350 (P. R. China) Im Neuenheimer Feld 270, 69120 Heidelberg (Germany) [i] Prof. Dr. M. Yamashita E-mail: markus.enders@uni-heidelberg.de WPI-Advanced Institute for Materials Research, Tohoku University [c] Dr. M. R. Ajayakumar, Prof. Dr. M. Mas-Torrent, Prof. Dr. J. Veciana 2-1-1 Katahira, Sendai 980-8577 (Japan) Department of Molecular Nanoscience and Organic Materials E-mail: masahiro.yamashita.c5@tohoku.ac.jp Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC)/CIBER-BBN [+] These authors contributed equally to this work. 08193 Bellaterra (Spain) Supporting information and the ORCID identification number(s) for the au- E-mail: vecianaj@icmab.es thor(s) of this article can be found under: [d] Prof. Dr. Y. Kitagawa https://doi.org/10.1002/chem.202001365. Department of Materials Engineering Science T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. Graduate School of Engineering Science, Osaka University This is an open access article under the terms of Creative Commons Attri- 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-8531 (Japan) bution NonCommercial License, which permits use, distribution and repro- [e] Prof. Dr. L. Chibotaru duction in any medium, provided the original work is properly cited and is Theory of Nanomaterials Group, Katholieke Universiteit Leuven not used for commercial purposes. 3001 Leuven (Belgium) [f] Dr. L. Ungur Department of Chemistry, National University of Singapore 3 Science Drive 3, 117543 Singapore (Singapore) Chem. Eur. J. 2020, 26, 8621 – 8630 8621 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 porphyrin tapes[9] are representative examples with in-plane molecule memory using double-decker complexes has been extended p-systems, in which the precisely designed monomer achieved.[35] Moreover, double-decker complexes with TbIII ions units are connected regularly with sp2 bonds, making it possi- are outstanding SMMs[36] and have been used in spintronic de- ble to control the morphology and the physical properties of vices, wherein the interactions between the spins of TbIII ions, the resulting products. Longitudinal infinite stacking of p-con- conductive electrons and phonons of nanocarbon materials jugated molecules is often seen in crystals of molecular con- are utilized.[37, 38] Ishikawa et al. have reported a significant en- ductors[10–13] and polymeric chains in which phthalocyanine hancement of the SMM properties of the TbIII-phthalocyaninato (Pc) and/or porphyrin (Por) ligands are connected by silox- double-decker complexes via the axial compression induced anes,[14] ligand–metal coordination bonds,[15] weak intermolecu- by the ligand oxidation.[39–41] In comparison with double-decker lar interactions[16] and so on. In addition to infinite stacks, the complexes, the redox, structural and magnetic properties of development of Pc and Por chemistry has enabled the con- the other multiple-decker complexes have not been investigat- struction of stable oligomeric molecules with stacked p-sys- ed as extensively presumably because they have short histories tems.[17, 18] and low-profiles, are difficult to synthesize and purify and have A series of phthalocyaninato-lanthanide-cadmium multiple- low crystallinity. However, the more extended p-systems, as decker complexes[19–21] is a rare example of such oligomers compared with double-decker complexes, are promising for (Figure 1). Lanthanoid-phthalocyaninato double- and triple- bringing about valuable and unprecedented functionalities. decker complexes were first prepared by Kirin et al. in 1965[22] For instance, Fukuda et al. have reported that the oxidized and in 1967,[23, 24] respectively, and were followed by the syn- form of the quadruple-decker complex absorbs light in the thesis of their porphyrinato analogues reported by Buchler longer wavelength regions, which is useful for light-harvesting et al.[25] The first discrete phthalocyaninato quadruple-decker materials in dye-sensitized solar cells.[42] In addition, non-linear complex, in which two LuIII-Pc double-decker complexes are optical properties of multiple-decker complexes have been in- glued by CdII ions, resulting in the tetrameric Pc stack connect- vestigated by Jiang et al.[21] From the point of view of molecu- ed by Lu-Cd-Lu metal array, was reported by Fukuda et al. in lar magnetism, various groups have utilized the multiple- 2011.[26] Jiang et al. has further extended this strategy and re- decker complexes with two TbIII and DyIII centers for analyzing ported quintuple- and sextuple-decker complexes in a short the effects of intramolecular magnetic interactions which affect period of time.[20, 21] the SMM properties.[26, 43–46] Herein we present an extensive study of oxidized p-extend- ed multiple-decker oligomers, which were obtained through electrochemical and chemical oxidation reactions, utilizing single crystal X-ray diffraction (SCXRD), ESR, NMR, magnetic measurements and theoretical calculations. By using bulk elec- trolysis and UV/Vis/NIR measurements, we isolated and charac- terized the charged states of multiple-decker complexes. For concise naming, we use the number of 2,3,9,10,16,17,23,24-oc- Figure 1. Schematic representation of structures and naming convention of tabutoxy phthalocyaninato (obPc) ligands contained in the the phthalocyaninato multiple-decker complexes in this work. obPc ligands complex as [n], i.e., double- [2], triple [3], quadruple- [4] and can be distinguished as either inner (obPci and, for the central ligand in [5], quintuple-decker [5] complexes, and the charge of the com- obPcii) and outer (obPco). The chemical group annotations used for the solu- plex (m) is expressed as [n]m + , e.g., + 2e charged triple-decker tion NMR analysis are also summarized. complex is named [3]2 + . From the crystal structures of [3]2 + , [4]2 + , [4]4 + and [5]4 + , it was shown that both molecular and Among multiple-decker complexes, double-decker com- packing structures were controlled by the size of the mole- plexes are the most thoroughly studied, and several versatile cules and the numbers of counter ions. In addition, unusual functionalities related to future applications have been report- coexistence of square prism (SP) and square antiprism (SAP) ed. For instance, redox induced structural changes of Por geometries were observed in + 4e charged species. Paramag- double-decker complexes have been utilized for rotating units netic NMR analysis was used to characterize the structures and of various molecular machines.[27–30] More recently, Yamashita magnetic properties of the oxidized complexes in solution, and et al. have observed structural changes in Por double-decker we showed that magnetic anisotropy decreased with an in- complexes induced by ligand oxidation using single-crystal X- crease in the charges on the obPc ligands. Furthermore, al- ray (SXRD) measurements and have shown that ligand oxida- though [4]2 + and [5]2 + have even positive charges, the exis- tion induces axial compression, which has been supported by tence of p-radicals was proven by NMR and ESR spectroscopies theoretical calculations.[31] Moreover, the axial compression in and supported by DFT calculations. In addition, we report the Pc double-decker complexes induced by oxidation has been magnetic properties of the multiple-decker complexes which investigated by using solution NMR spectroscopy.[32] The con- are shown to be influenced by the structural changes induced formational changes, especially ligand rotation, in double- by oxidation of the ligand. decker complexes have been directly observed[33, 34] and con- trolled using a scanning electron microscope (STM) and scan- ning electron spectroscopy (STS). In other words, bits of single- Chem. Eur. J. 2020, 26, 8621 – 8630 www.chemeurj.org 8622 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 Results and Discussion angle between the obPc ligands (qobPc) sandwiching the TbIII ions, and the TbIII to neighboring metal ion distance (RTb–M : M = Tb, Cd) from X-ray and DFT optimized structures are sum- Isolation and characterization of oxidized multiple-decker marized in Figure 3 d. In [3]0 and [3]2 + , the two TbIII ions are complexes crystallographically equivalent and are connected by an inver- The multiple-decker complexes were stepwise oxidized by sion center (Figure 3 a). In [3]0, d, RTb–Tb and qobPc were deter- using cyclic voltammetry and chemical methods using phenox- mined to be 6.095 a, 3.517 a and 31.778, respectively. Oxida- athiin hexachloroantimonate (Ox·SbCl6). UV/Vis/NIR spectroe- tion to [3]2 + , which corresponds to the removal of two elec- lectrochemistry was used to follow the oxidations. In the spec- trons from the antibonding HOMO of [3]0 (Figure 2 c), causes d tra of the oxidized species presented in this article, isosbestic and RTb–Tb (5.959 a and 3.435 a, respectively) to shorten, result- points were observed (Figure 2 d), indicating that the oxidized ing in a square antiprism (SAP) geometry with a wide qobPc complexes are stable. The redox potentials became narrower (42.218) in order to avoid steric repulsion between the phenyl (Figure 2 b), and the number of stable oxidized species in- rings of the obPc ligands. Due to the large interligand steric ef- creased when the number of obPc ligands was increased be- fects induced by longitudinal compression, the outer obPcs cause the p-system extension decreased the electronic repul- (obPco) of [3]2 + had bowl-shaped distortions. The distortion sion between the positively charged holes. Since TbIII and CdII and changes in the geometrical parameters (qobPc, d and RTb–Tb) ions do not participate in the redox reactions, the oxidation re- are similar to those predicted from geometry optimization, as actions occurred in the extended ligand p-system. The delocal- is summarized in Figure 3 d. ization of holes is expected due to the highly delocalized mo- In the case of series [4], the space groups of [4]2 + and [4]4 + lecular orbitals (Figure 2 c). The agreement between the calcu- were determined to be P4/ncc and P4/n, respectively, and lated oscillator strengths and absorption spectra support the there are four-fold axes along the Tb-Cd-Tb axes (c-axes). Since validity of our calculations. The HOMO of the neutral species the positional disorder in two obPcs of [4]4 + gave identical ge- (Figure 2 c) has nodes between the obPc ligands, indicating ometries in each disordered state, we only show one represen- that ligand oxidation (i.e., removal of electrons from the tative geometry on the right side of Figure 3 b. Similar to the HOMO) results in a decrease in the interligand distances. DFT other multiple-decker series, in [4], the obPco ligands have optimized structures (Figures S246, S254, and S265) and crystal bowl-shaped distortions induced by oxidation. In series [4], al- structures of oxidized species followed this trend as discussed though the two TbIII ions are crystallographically inequivalent, later. the qobPc values of Tb1 and Tb2 (left side of Figure 3 b) of [4]0 are similar to each other (22.328 and 23.658). In [4]2 + , qobPc of Tb1 and Tb2 are 42.288and 38.728, respectively. The differences X-ray structural analysis in qobPc are greater in [4]4 + , in which there are two qobPc (44.948 We obtained new crystal structures of [3]2 + , [4]2 + , [4]4 + and for Tb1 and 16.438 for Tb2). In other words, SAP and square- [5]4 + , the crystals of which were grown by using chemical oxi- prism (SP) geometries are present in one [4]4 + unit. Since dations and a solvent diffusion method (see Supporting Infor- there are a small number of peaks in the 1H NMR spectra of mation). The X-ray structures, including the previously reported [4]4 + dissolved in CD2Cl2 due to its symmetric structure, the structures of the neutral species ([3]0, [4]0 and [5]0)[43, 44, 47] are asymmetric crystal structure of [4]4 + seems to be induced by shown in Figures 3 a–c.[48] The molecular length (d), stacking steric effects from the solvent molecules and SbCl6@ incorpo- Figure 2. (a) Cyclic voltammograms,[44] (b) redox potentials, (c) HOMO (isovalue = 0.01) of neutral species and (d) electrochemical UV/Vis/NIR spectra and oscil- lator strength (black bars) for multiple-decker complexes. Chem. Eur. J. 2020, 26, 8621 – 8630 www.chemeurj.org 8623 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 Figure 3. Crystal structures of (a) [3], (b) [4] and (c) [5]. The n-butoxy chains were omitted for clarity. (d) Charge dependence of the molecular length d, Tb- metal ion (Tb or Cd) distances RTb-M and stacking angle around metal ions qobPc. (e) Packing structures of multiple-decker complexes. Red, green and blue parts represent multiple-decker units, counter anions (SbCl6) and solvent molecules, respectively. rated into the crystal. The geometry optimization for [4]4 + , by SP. Moreover, the solvent molecules (benzene) located in the which the structure in the gas phase can be estimated, afford- grooves formed by the ligands stabilize the SP geometry. ed the symmetric structure, further supporting the importance Stacking of alternating [5]4 + and a disordered SbCl6@ generates of intermolecular forces in the crystals to stabilize the asym- 1D column packing along the c-axis (Figure 3 e). At the same metric conformation observed by using SCXRD. We deter- time, we determined the structure of [5]4 + containing toluene mined a preliminary crystal structure of [4]4 + crystalized from molecules obtained by slow diffusion using CH2Cl2 and toluene toluene, showing that the geometry of [4]4 + is independent of (Figures S302 and S303). Units of [5] with SAP and SP geome- the conditions (SAP and SP geometries coexist as shown in tries are aligned into 1D column packing along the c-axis. Fur- Figure S310). Moreover, the SP conformations are stabilized by thermore, the SP geometry is stabilized by toluene molecules toluene molecules incorporated into the grooves of the obPc incorporated to the groove of SP geometries (right side of Fig- ligands. ure 3 c). In other words, aromatic solvent molecules cause the In case of the structure of [5]4 + , the obPco ligands of [5]4 + coexistence of SAP and SP geometries in one multiple-decker show bowl-shaped distortions due to longitudinal compression unit. In all cases, the oxidation reactions resulted in compres- effects induced by the oxidation in contrast to the wave-like sions along the C4 axes, followed by decreases in the steric hin- distortions in [5]0. Although the TbIII ions in [5]0 are crystallo- drance due to bowl-shaped distortions of the obPco ligands. graphically equivalent with each other, those in [5]4 + had dif- The qobPc values increase when the complexes are oxidized ferent geometries, one of which was SAP and the other was from neutral (qobPc = 228–328) to 2 + (qobPc = 388–448) because Chem. Eur. J. 2020, 26, 8621 – 8630 www.chemeurj.org 8624 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 the wider qobPc decreases the steric hindrance between the co- to double-decker complexes ([2]).[32] Moreover, the increases in facial ligands. In the cases of [4]4 + and [5]4 + , there are two dif- the magnetic anisotropy via ligand oxidation are supported by ferent TbIII ions per molecular unit with qobPc & 458 (SAP) and 08 the ac magnetic properties of the cationic double-decker com- (SP). Although the SP arrangement is unfavorable from the plexes in the solid state reported by Ishikawa et al.[39–41] How- viewpoint of steric hindrance, aromatic solvent (benzene) mol- ever, in contrast to dMPC for [2], that for the series of complexes ecules in the crystal packing of [5]4 + stabilizes it. The structure [3]–[5] in the present research did not increase simply with an of [4]4 + with a narrow qobPc crystallized from toluene was also increase in the oxidation state of the ligand. stabilized (Figure S305), indicating that both SAP and SP geo- The 1H NMR signals were fully assigned using a combination metries generally occur in the crystal packing of highly oxi- of 1H NMR and 1H-1H COSY spectra and an estimation of dMPC dized species. Not only the molecular structures but also the using DFT optimized structures. As shown in Figure 4, the 1H crystal packings are controlled by oxidation. As shown in Fig- signals for CHar changed complicatedly upon oxidation. In con- ure 3 e, the neutral species were packed in slipped column ar- trast, the paramagnetic shift of most of the signals for CH2a, rangements because of intermolecular p–p stacking. Moreover, CH2b, CH2g and CH3 decreased with an increase in the oxida- [3]2 + was packed in a slipped column arrangement because of tion number, which is indicative of a decrease in cax.dobs for the its shorter d, which does not prevent the formation of p–p CHar protons heavily depended on the ligand contribution to stacking. However, the oxidized forms of [4] and [5] were dFC (dLFC) because these protons were close to the p-radicals. packed in 1D columns along the c-axis because of their larger Thus, the chemical shift of CHar (dobsar) is simplified as the sum dimensions along the z-axis and the bowl-shaped distortion of of dMPC, dLFC and less charge dependent dorb, as shown below. the obPco, both of which prevent the formation of intermolec- [Eq. (3)] ular p–p stacks. dar obs & dorb þ dMPC þ dLFC ð3Þ Solution NMR measurements dLFC for CHar was estimated using the spin densities at mea- sured nuclei obtained from DFT calculations.[32] The spin densi- Solution NMR is a useful tool to determine the molecular struc- ties on the CHar of obPco tend to be high compared to those tures and magnetic properties of lanthanoid complexes.[49–50] of obPci, and thus, the magnitude relation of dobsar for CHaro The chemical shift of an NMR active nucleus in a paramagnetic and CHari changes upon oxidation of the ligand, as seen in the complex can be expressed as the sum of the orbital term (dorb) 1 H NMR spectra of [4] and [5] beyond @55 ppm. In contrast to and hyperfine term (dHF). dHF is mainly composed of the Fermi CHar, the protons of the butoxy chains are less affected by the contact (dFC) and pseudocontact (dPC) shifts, as shown in p-radicals and charges on the ligands because these protons Eq. (1):[51–52] are far from the p-plane, making it possible to treat dFC as being zero.[32] Using the equation above and the DFT-opti- dobs ¼ dorb þ dHF ¼ dorb þ dFC þ dPC ð1Þ mized structures of the oxidized complexes to which n-butoxy chains were added (Figure S247, S255 and S266), the cax values dFC involves the interactions between the electron spin on were determined by least-squares fitting as to minimize the the nucleus and the nuclear spin, whereas dPC originates from difference in the observed and expected chemical shifts (dobs the through-space magnetic dipole–dipole interactions be- and dcalcd, respectively). The cax values of the multiple-decker tween the electron magnetic moment and the nuclear mag- complexes are shown in Figure 5 b. Except the increase in the netic moment.[53] Both metal ions and ligand-centered p-radi- cax value upon oxidation from [3]2 + to [3]3 + , cax values of the cals can contribute to dHF. In lanthanoid complexes, dPC due to multiple-decker complexes decreased with an increasing the the metal ion is the dominant term when the anisotropy is number of charges on the ligands, indicating that the magnet- high. The TbIII ions in the present complexes are affected by ic anisotropy decreased due to changes induced by chemical the ligand field of the obPc ligands, and therefore, the TbIII oxidation. Another point to be mentioned is the paramagnetic ions have strong axial magnetic anisotropies with the magnet- shift for the CHaro protons in [4]2 + and [5]2 + (centre of Fig- ic easy axes lying along the C4 axes of the multiple-decker ure 5 a). Since they have even charges, the ligand p-radical and complexes.[54] In this case, thedPC due to the metal ion (dMPC)[49] resulting dLFC are expected to be absent, and only dMPC is the is the dominant term and can be expressed as [Eq. (2)]: source of the paramagnetic shift. However, the observed cax chemical shifts for adding dLFC obtained from DFT calculations dMPC ¼ ð3 cos2 q @ 1Þ ð2Þ for triplet states of [4]2 + and [5]2 + to dobsar, affording shifts of 12pr 3 @102.9 ppm and @105.9 ppm for [4]2 + and [5]2 + , respectively, in which cax is the axial component of the magnetic suscepti- which are close to the experimental values. The remaining dif- bility of the lanthanoid center, r is the norm of the vector (r) ferences of about @10 ppm are due to the contribution of sin- connecting the lanthanoid center and the nucleus, q is the glet states and the pseudocontact shift contributions of the or- angle between the magnetic easy axis and the r (Figure S29). ganic radicals, as has been shown by more elaborate analy- Previously, we have concluded that the decrease in the molec- sis.[54, 55] ular length (d) and the increase in cax value contribute to the larger dMPC induced by ligand oxidation in TbIII-phthalocyanina- Chem. Eur. J. 2020, 26, 8621 – 8630 www.chemeurj.org 8625 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 Figure 4. 1H NMR spectra for oxidized and neutral species of (a) [3], (b) [4] and (c) [5] with the assignment of the signals at 295.0 K. Figure 5. (a) Relationship between the dobs and dcalcd, expressed with the agreement factor written under the abbreviation of the complex (with consideration of singlet and triplet states, where appropriate). Numbers are agreement factors (AFs) using AF = 1–[S(dcalcd–dobs)2/dobs2]1/2. (b) cax values for the series [3], [4] and [5] derived from 1H NMR analysis at 295 K. Chem. Eur. J. 2020, 26, 8621 – 8630 www.chemeurj.org 8626 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 Biradical properties of [4]2 + and [5]2 + [5]2 + were @34 cm@1 and @3 cm@1, respectively, the magni- tudes of which are small enough for the triplet (ferromagnetic) From the NMR analysis, in [4]2 + and [5]2 + , the p-radicals are states to be thermally accessible at liquid nitrogen tempera- on the ligands, especially on obPco. However, the relatively tures. These results are consistent with the triplet biradical sharp 1H NMR signals in the spectra of [4]2 + and [5]2 + com- states observed using ESR measurements at 80 K. pared to those of other radical complexes indicates that there So far, biradical compounds with Pc and Por have been syn- are contributions from singlet (p-radical silent) states as well. thesized by connecting two neutral double-decker monoradi- To support the presence of p-radicals, we performed electron cal complexes.[54, 60–63] Among them, a triplet biradical state has spin resonance (ESR) spectroscopy on Y(III) analogues of the been reported for a K + ion-induced supramolecular tetramer oxidized complexes, which were prepared via bulk electrolysis composed of crown-substituted double-decker complexes, in CH2Cl2 and characterized by comparing their absorption [K4{Lu(CR4Pc)Pc}2]4 + , reported by Ishikawa et al., where the four spectra with those of TbIII complexes (Figure S319). Although phthalocyaninato ligands align along the C4 axis to make a Y(III) analogues of [4]2 + and [5]2 + have even charges, these purely longitudinally extended p-system.[61] From empirical cal- complexes are ESR active at room temperature, and their g- culations, [K4{Lu(CR4Pc)Pc}2]4 + has a small negative Jex value values are close to 2, which is common for organic radicals. (@0.35 cm@1). In contrast, connecting the two double-decker After cooling to 80 K, in the ESR spectra of [4]2 + and [5]2 + , complexes via an in-plane p-extended ligand, such as fused- complicated signals, which were characteristic of a triplet bir- phthalocyanine FPc[54] or fused-porphyrin FPor,[62, 63] results in adical with an axial zero-field splitting parameter D, were ob- the formation of a singlet biradical species showing strong served (Figure 6 b, S316, S317). The simulated j D j value for antiferromagnetic interactions between S = 1/2 spins [4]2 + (0.0032 cm@1) is slightly larger than that for [5]2 + (@207.4 cm@1 and @487 cm@1, respectively). Weak Jex values for (0.0029 cm@1), indicating that [4]2 + has stronger dipole-dipole [4]2 + and [5]2 + in the present study are similar to that for interactions between unpaired electrons than [5]2 + does due [K4{Lu(CR4Pc)Pc}2]4 + . Jex tends to be weak when the biradicals to the smaller p-extended system in the former compound. Fo- occur in p–p stacks (Figure 6 d). These results clearly reflect the cusing on the shape and energy of the molecular orbitals in differences in orthogonality of interligand orbital overlap, the even-charged complexes, HOMO–LUMO gaps (DELUMO–HOMO which is retained in the longitudinally extended p-system with as shown Figure 6 c) for [4]2 + and [5]2 + (0.20 and 0.14 eV, re- wide interligand stacking angles q. spectively) are considerably smaller than those for others (0.47 to 1.21 eV) because the HOMO and LUMO of [4]2 + and [5]2 + Correlations between the magnetic anisotropy and the are non-bonding orbitals whose shapes are similar with each redox-induced structural changes other (S259, S270). The small DELUMO–HOMO for [4]2 + and [5]2 + allows the electrons to occupy the LUMO with low energy, re- From NMR analyses of the oxidized species, the magnetic sulting in the formation of biradical states. To better under- anisotropy (cax) on the TbIII ion decreased upon oxidation. To stand the biradical states, we performed natural orbital (NO) determine the correlation between the structural deformation analysis on antiferromagnetic (AFM) states using (U)B3LYP/6- caused by ligand oxidation and the magnetic anisotropy, we 31G*[56–58] with Stuttgart RSC 1997 ECP[59] for Y and Cd ions performed point-charge (PC) calculations. Although PC calcula- (see Supporting Information). The occupancies of the lowest tions are less accurate than ab initio methods are, its low com- unoccupied natural orbitals (LUNO) for the singlet states of putation cost makes it possible to estimate the ligand field (LF) [4]2 + and [5]2 + were 0.922 and 0.961, respectively. In addition, splitting of the present lanthanoid complexes having large diradical character y values for [4]2 + and [5]2 + were 0.845 and numbers of atoms and metal centers. Since Tb@N and Cd@N 0.922, respectively, indicating that there are contributions from bonds have covalent character, the negative charges on the triplet ferromagnetic (FM) states. The calculated exchange in- coordinating N atoms should extend in Tb(Cd)@N direction. teractions (Jex) between the two S = 1/2 spins of [4]2 + and Using PC calculations with the displacement parameters sug- Figure 6. (a) Spin densities (isovalue = 0.004) of triplet biradical species. (b) EPR spectra of Y(III) analogues at RT (experimental) and at 80 K (experimental and simulated). (c) Molecular orbital energies and HOMO–LUMO gaps of singlet species. (d) Comparison of Jex values between the dimer of double-decker com- plexes connected by a phenyl linker and the double-decker complexes connected by p–p stacking. Chem. Eur. J. 2020, 26, 8621 – 8630 www.chemeurj.org 8627 T 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202001365 Figure 7. (a) Crystal field splitting and structural model of neutral species based on point-charge calculations done on the DFT-optimized structures. (b) Ligand field splitting of multiple-decker complexes by CASSCF[64] calculations using the crystal coordination fragment around TbIII ions. Two kinds of energy levels are shown in cases when crystallographically inequivalent TbIII ions are present. Dotted lines represent experimental DE values estimated by ac magnetic susceptibility measurements in the presence of a dc bias magnetic field. (c) Selected ac magnetic susceptibilities at 10 K with a 2 kOe bias dc field. (d) Mechanism for the decrease in DE upon oxidation. gested by Baldov& et al.,[65] the effects of the covalent bonds ure 7 c). Among the studied complexes, cM’’ of [3] clearly shift- between the phthalocyaninato ligands and metal ions can be ed toward the higher n region as the oxidation state increased. considered. As is shown in Figures 7 a and S308, we construct- In contrast, the peak top of cM’’ for the other species are less ed the structural models for the PC calculations on the bases dependent on their oxidation states. In other words, the mag- of the DFT-optimized structures of the oxidized complexes and netic relaxation time (t) of [3] strongly correlates with the oxi- the reported displacement parameters,[65] which shift the posi- dation state. The activation energies for spin reversal (DE), tions of the negative charge of the coordinated N atoms closer which correspond to the energy difference between the highly to the metal ions. Since the obPco ligand is tetradentate with anisotropic ground doublet j : 6 > and the first excited LF a@2e charge, a@0.5e charge was assigned per coordinated N levels j : 5 > , were determined by fitting t vs. T@1 plots using atom. In a similar manner, a@0.25e charge was assigned to Arrhenius equations (magnetic section in Supporting Informa- each metal@N bond since each N atom of obPci coordinates to tion). In addition, ab initio calculations using the crystal struc- two metal ions. In addition, CdII ions and the TbIII ions on the tures around the TbIII ions were performed to determine the LF opposite side of were treated as + 2e and + 3e point charges, splitting (Figure 7 b and the details of ab initio calculations in respectively. From PC calculations using these models, the Supporting Information). We found that there was agreement ground states of the multiple-decker complexes were com- between the experimental DE values (dotted lines in Fig- posed of j Jz > = j : 6 > similar to the well-studied phthalocya- ure 7 b) and theoretical LF splitting. In particular, the DE values ninato TbIII double-decker complexes.[36] Moreover, the energy for series [3] decreased as the complexes were oxidized from separation between the LF sublevels (magnitude of the LF [3]0 to [3]2 + , the trend of which is consistent with the decrease splitting) successively decreased due to the structural deforma- in cax values obtained from NMR analyses and PC calculations. tion induced by oxidation (Figure 7 a), which consistent with We think that the approach of the TbIII ions (decrease in RTb–M the decrease in the cax values observed from NMR analysis. In shown in Figure 3 d) via axial compressions induced by ligand addition, the LF splitting is enhanced by the structural changes oxidation decrease the effects of the negative charges on the upon oxidation from [3]2 + to [3]3 (Figure 5 b). These simple cal- inner ligands (obPci), and therefore, the axial components of culations indicate that the cax values obtained from the NMR the LF parameters around the TbIII ions decrease (Figure 7 d). In measurements depend on the LF splitting of the TbIII ions and comparison to [3], the other complexes with CdII ions do not are modulated by redox-induced structural changes. In particu- show large changes in the ac magnetic properties and LF split- lar, the negative charges of three obPc ligands must be includ- ting upon ligand oxidation owing to the approach of the less ed to reproduce the increase in the LF splitting upon oxidation positively charged CdII ion to the TbIII ions. from [3]2 + to [3]3 + . The decrease in the LF splitting upon oxi- dation is more pronounced for [3] than it is for [4] and [5]. To Conclusions further understand the behavior of the cax values and LF split- ting, we conducted magnetic measurements on the oxidized We showed the versatile electrochemical, magnetic and struc- species characterized by X-ray analysis. Since the TbIII ions have tural properties of highly oxidized multiple-decker complexes. strong magnetic anisotropy (large cax value), the multiple- The ligand oxidation state and the magnetic properties of the decker complexes underwent slow magnetic relaxations char- lanthanoid center couple with each other due to structural de- acteristics of SMMs, as shown by the imaginary part of the ac formations. Stepwise oxidation, possible in the present series magnetic susceptibility (cM’’) vs. ac frequency (n) plots (Fig- of complexes, could lead to multistep molecular switches, Chem. Eur. 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