Series of Near-IR-Absorbing Transition Metal Complexes with Redox Active Ligands

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Series of Near-IR-Absorbing Transition Metal Complexes with Redox Active Ligands © 2020 the Author(s) Published version Salojärvi, Esko; Peuronen, Anssi; Lahtinen, Manu; Huhtinen, Hannu; Vlasenko, Leonid S.; Lastusaari, Mika; Lehtonen, Ari Salojärvi, E., Peuronen, A., Lahtinen, M., Huhtinen, H., Vlasenko, L. S., Lastusaari, M., & Lehtonen, A. (2020). Series of Near-IR-Absorbing Transition Metal Complexes with Redox Active Ligands. Molecules, 25(11), Article 2531. https://doi.org/10.3390/molecules25112531 2020 molecules Article Series of Near-IR-Absorbing Transition Metal Complexes with Redox Active Ligands Esko Salojärvi 1, Anssi Peuronen 1, Manu Lahtinen 2, Hannu Huhtinen 3, Leonid S. Vlasenko 3,4, Mika Lastusaari 1and Ari Lehtonen 1,* 1Inorganic Materials Chemistry research group, Department of Chemistry, University of Turku, FI-20014 Turku, Finland; [email protected] (E.S.); anssi.peur[email protected] (A.P.); [email protected] (M.L.) 2Department of Chemistry, P.O. Box 35, University of Jyvaskyla, FI-40014 Jyvaskyla, Finland; [email protected] 3Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland; [email protected] (H.H.); [email protected] (L.S.V.) 4Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia *Correspondence: [email protected] Academic Editor: Andrey I. Poddel’sky Received: 28 April 2020; Accepted: 27 May 2020; Published: 29 May 2020   Abstract: New soluble and intensely near-IR-absorbing transition metal (Ti, Zr, V, Ni) complexes were synthesized using a redox non-innocent N,N’-bis(3,5-di-tertbutyl-2-hydroxy-phenyl) -1,2-phenyle nediamine (H 4 L) as a ligand precursor. In all the complexes, ([Ti(L ox ) 2 , [Zr(L ox ) 2 ], [V(L sq1 )(HL ox )] and [Ni(HL ox ) 2 ], two organic molecules coordinate to the metal center as tri- or tetradentate ligands. The solid-state structures of the complexes were determined using single crystal XRD, and the compounds were further characterized with Electrospray Ionisation Mass Spectrometry (ESI-MS). Thermoanalytical measurements indicated the thermal stabilities of the complexes. All compounds absorb strongly in the near-IR region and show very interesting magnetic and electrochemical properties. Moreover, it was shown that the V and Ni complexes can also convert absorbed near-IR photons to (un)paired electrons, which indicates great promise in photovoltaic applications. Keywords: metal organic complex; redox-active ligand; non-innocent ligands 1. Introduction In coordination chemistry, a ligand is referred as ‘non-innocent’ when it forms complexes where the oxidation state of the central atom cannot be absolutely defined [ 1 ]. For example, o-aminophenol N,N 0 -bis(3,5-di-tertbutyl-2-hydroxy-phenyl)-1,2-phenylenediamine H 4 L (the oxidation states of L, Scheme 1) has a rich redox-chemistry and its complexing abilities towards transition metals, i.e., Ti, Zr, Hf, Mo, W, Mn, Co, Cu and Zn, have attracted substantial interest over the past few years [ 2 – 8 ]. Earlier studies show that this proligand can form complexes with various transition metals in 1:1 and 2:1 stoichiometry. Moreover, the galore electrochemical behavior of H 4 L (from hereon L corresponds to the ligand in any oxidation state) allows five different oxidation states (Scheme 1). The foremost motivation for research on such redox-active ligands has been in the development of new homogenous catalysts, which could utilize these ligands as electron reservoirs during the catalytic cycle. On the other hand, the redox activity of the ligand may allow the corresponding metal complexes to have multiple low-energy charge transfer processes within, thus rendering the complex able to absorb electromagnetic radiation at the visible and the near-IR regions intensely. There are many metal-organic transition metal complexes with redox-active ligands which are studied for their potential applicability for strongly near-infrared (NIR)-absorbing dyes [ 9 – 19 ] or as single molecule magnets [ 20 – 23 ] (SMMs). NIR absorption is important for dye-sensitized solar Molecules 2020,25, 2531; doi:10.3390/molecules25112531 www.mdpi.com/journal/molecules Molecules 2020,25, 2531 2 of 17 cells [ 20 – 23 ], camouflaging, optical NIR filtering and for NIR photodetectors. Metal organic NIR dyes are also studied for their high thermal durability and lightfastness compared with organic NIR dyes [ 24 ]. As H 4 L is a redox-active ligand that can have, in certain protonation states, delocalized π -conjugated systems, it can form stable radicals upon complexation with transition metal ions [ 8 ]. Stable radicals, i.e., compounds carrying unpaired electrons, may have strong interactions with photons and therefore exhibit strong absorption at UV-Vis-NIR range. In this study, we have used H 4 L as a ligand with the aim of preparing new NIR-absorbing transition metal complexes and have studied their molecular structures as well as their magnetic properties. Molecules 2020, 25, x 2 of 17 single molecule magnets [20–23] (SMMs). NIR absorption is important for dye-sensitized solar cells [20–23], camouflaging, optical NIR filtering and for NIR photodetectors. Metal organic NIR dyes are also studied for their high thermal durability and lightfastness compared with organic NIR dyes [24]. As H4L is a redox-active ligand that can have, in certain protonation states, delocalized π-conjugated systems, it can form stable radicals upon complexation with transition metal ions.[8] Stable radicals, i.e., compounds carrying unpaired electrons, may have strong interactions with photons and therefore exhibit strong absorption at UV-Vis-NIR range. In this study, we have used H4L as a ligand with the aim of preparing new NIR-absorbing transition metal complexes and have studied their molecular structures as well as their magnetic properties. Scheme 1. The different oxidation states of the deprotonated ligand L (adapted from the reference [8]). 2. Results and Discussion 2.1. Synthesis of Complexes The reaction between metal precursors and H4L in basic methanol solutions under ambient atmosphere afforded the dark crystals of mononuclear complexes (Scheme 2). All the reactions yielded mononuclear complexes with a 2:1 ligand-to-metal ratio regardless of the equivalent ratios of the starting compounds. The complexes were isolated as dark crystals either directly from the reaction mixture or with subsequent crystallization. The synthesis of [V(Lsq1)(HLox)] was repeated using vanadyl acetylacetonate as a metal precursor to yield a chemically identical product, which crystallized in an orthorhombic crystal system. No further analyses were run utilizing the orthorhombic form. Powder X-ray diffraction (PXRD) was used to establish that the material used for all analyses corresponds to the monoclinic (C2/c) structure (see Supplementary Materials for PXRD analysis and the crystal structure of the orthorhombic polymorph). All complexes are air- and moisture-stable at room temperature. In the DSC-TGA (Differential Scanning Calorimetry- ThermoGravimetric Analysis) measurements, which were carried out with a heating rate of 5 K/min for all complexes in nitrogen atmosphere, it was found that the Ti complex loses ca. 8% of the sample’s weight, which could be attributed to the loss of solvate molecules from the crystal lattice (XRD measurements, see below). The weight then remains stable until ca. 270 °C, where it starts to rapidly decline, probably due to the decomposition of the molecule. The other complexes were found to be stable up to 340 °C, 250 °C and 230 °C, for Zr, V and Ni complexes, respectively, until decomposition occurred. Scheme 1. The different oxidation states of the deprotonated ligand L (adapted from the reference [ 8 ]). 2. Results and Discussion 2.1. Synthesis of Complexes The reaction between metal precursors and H 4 L in basic methanol solutions under ambient atmosphere afforded the dark crystals of mononuclear complexes (Scheme 2). All the reactions yielded mononuclearcomplexeswitha2:1ligand-to-metalratioregardlessoftheequivalentratiosofthestarting compounds. The complexes were isolated as dark crystals either directly from the reaction mixture or with subsequent crystallization. The synthesis of [V(L sq1 )(HL ox )] was repeated using vanadyl acetylacetonate as a metal precursor to yield a chemically identical product, which crystallized in an orthorhombic crystal system. No further analyses were run utilizing the orthorhombic form. Powder X-ray diffraction (PXRD) was used to establish that the material used for all analyses corresponds to the monoclinic (C2/c) structure (see Supplementary Materials for PXRD analysis and the crystal structure of the orthorhombic polymorph). All complexes are air- and moisture-stable at room temperature. In the DSC-TGA (Differential Scanning Calorimetry-ThermoGravimetric Analysis) measurements, which were carried out with a heating rate of 5 K/min for all complexes in nitrogen atmosphere, it was found that the Ti complex loses ca. 8% of the sample’s weight, which could be attributed to the loss of solvate molecules from the crystal lattice (XRD measurements, see below). The weight then remains stable until ca. 270 ◦ C, where it starts to rapidly decline, probably due to the decomposition of the molecule. The other complexes were found to be stable up to 340 ◦ C, 250 ◦ C and 230 ◦ C, for Zr, V and Ni complexes, respectively, until decomposition occurred. Molecules 2020,25, 2531 3 of 17 Molecules 2020, 25, x 3 of 17 Scheme 2. The syntheses of the complexes. The tert-butyl groups are omitted for clarity. 2.2. Structural Studies The single crystal X-ray diffraction studies (see Supplementary Materials, Table S1) showed that both Group IV complexes crystallize in a triclinic unit cell with two distinct complex molecules in the asymmetric unit. Ti complex crystallizes as an acetonitrile solvate, whereas Zr complex has a large, ca. 1000 Å3, void space in the crystal lattice with unresolved electron density. This void could potentially be large enough to accommodate an additional uncoordinated ligand. This is, however, not supported by 1H-NMR spectroscopic evidence which shows a methanol solvent peak instead. Therefore, the electron density in the void was treated as solvent using Olex2 solvent mask feature. In both cases, the complexes are formed of neutral molecules in which two nearly identical organic ligands are fully deprotonated (N-H and O-H protons) and coordinated to the metal center as tetradentate ONNO-donors. The coordination geometry around the central atom is best described as a distorted square antiprism with phenolic oxygens coordinated to the central atom in a mer form. The C-N and C-C interatomic distances in the central phenylene rings of each distinct ligand suggest a localized, i.e., a ‘cyclohexadiene diimine’-like, structure rather than a delocalized system (bond parameters are presented in Table 1). The four Ti-N bonds (Table 1) display significant elongation compared to the previously published [Ti(Lred)(py)2] complex but are very similar to the [Ti(Lox)(Cl)2] complex[4]. Although the long Ti-N distances can be caused by ligand-ligand steric effects, the discussed interatomic distances strongly suggest that the ligands in the presented [Ti(L)2] complex adopt the [Lox]2− form. Furthermore, according to Brown, it is possible to use the metrical oxidation state (MOS) to estimate the formal oxidation states of the metal-coordinated o-aminophenol moieties based on their geometrical parameters, i.e., the bond lengths of O-C and N-N bonds and the C-C bonds of the phenyl ring [25] or [Ti(L)2], these calculations gave the MOS values of −1.09(11), −1.22(6), −1.22(9) and −1.37(8), which support the idea that both ligands adopt the partially oxidized [Lox]2− form. The overall structure of the [Zr(L)2] complex is very similar to that of its Ti analogue (Figure 1), and the bonding parameters (Table 1) likewise indicate that the organic ligands adopt the [Lox]2− Scheme 2. The syntheses of the complexes. The tert-butyl groups are omitted for clarity. 2.2. Structural Studies The single crystal X-ray diffraction studies (see Supplementary Materials, Table S1) showed that both Group IV complexes crystallize in a triclinic unit cell with two distinct complex molecules in the asymmetric unit. Ti complex crystallizes as an acetonitrile solvate, whereas Zr complex has a large, ca. 1000 Å 3 , void space in the crystal lattice with unresolved electron density. This void could potentially be large enough to accommodate an additional uncoordinated ligand. This is, however, not supported by 1 H-NMR spectroscopic evidence which shows a methanol solvent peak instead. Therefore, the electron density in the void was treated as solvent using Olex 2 solvent mask feature. In both cases, the complexes are formed of neutral molecules in which two nearly identical organic ligands are fully deprotonated (N-H and O-H protons) and coordinated to the metal center as tetradentate ONNO-donors. The coordination geometry around the central atom is best described as a distorted square antiprism with phenolic oxygens coordinated to the central atom in a mer form. The C-N and C-C interatomic distances in the central phenylene rings of each distinct ligand suggest a localized, i.e., a ‘cyclohexadiene diimine’-like, structure rather than a delocalized system (bond parameters are presented in Table 1). The four Ti-N bonds (Table 1) display significant elongation compared to the previously published [Ti(L red )(py) 2 ] complex but are very similar to the [Ti(L ox )(Cl) 2 ] complex [ 4 ]. Although the long Ti-N distances can be caused by ligand-ligand steric effects, the discussed interatomic distances strongly suggest that the ligands in the presented [Ti(L) 2 ] complex adopt the [L ox ] 2− form. Furthermore, according to Brown, it is possible to use the metrical oxidation state (MOS) to estimate the formal oxidation states of the metal-coordinated o-aminophenol moieties based on their geometrical parameters, i.e., the bond lengths of O-C and N-N bonds and the C-C bonds of the phenyl ring [ 25 ] or [Ti(L) 2 ], these calculations gave the MOS values of − 1.09(11), − 1.22(6), − 1.22(9) and − 1.37(8), which support the idea that both ligands adopt the partially oxidized [L ox ] 2− form. The overall Molecules 2020,25, 2531 4 of 17 structure of the [Zr(L) 2 ] complex is very similar to that of its Ti analogue (Figure 1), and the bonding parameters (Table 1) likewise indicate that the organic ligands adopt the [L ox ] 2− oxidation state. Hence, both complexes can be described as [M(L ox ) 2 ], although the MOS values of the o-aminophenol moieties in [Zr(Lox)2] are slightly higher [−1.21(14), −1.39(18), −1.40(14) and −1.41(12)] compared to [Ti(Lox)2]. However, it is known that complexes of high oxidation state metals may have strong π -donation from the high-lying π orbitals of amidophenoxide ligands, which may cause the metrical oxidation state to differ from the theoretical value [ 25 ]. It therefore seems obvious that the formal oxidation state of both Ti and Zr centres should be assigned as +4. Table 1. Selected bond lengths (Å) for [Ti(Lox)2] and [Zr(Lox)2]. Ti1A-O1A 1.977(2) Zr1A-O1A 2.113(4) Ti1A-O2A 2.019(2) Zr1A-O2A 2.131(4) Ti1A-O3A 2.016(2) Zr1A-O3A 2.096(3) Ti1A-O4A 2.022(2) Zr1A-O4A 2.134(3) Ti1A-N7A 2.318(2) Zr1A-N7A 2.382(4) Ti1A-N14A 2.198(3) Zr1A-N14A 2.325(4) Ti1A-N27A 2.282(3) Zr1A-N27A 2.367(4) Ti1A-N34A 2.228(3) Zr1A-N34A 2.319(4) N7A-C6A 1.379(4) N7A-C6A 1.377(7) N7A-C8A 1.333(4) N7A-C8A 1.333(6) N14A-C13A 1.327(4) N14A-C13A 1.346(6) N14A-C15A 1.377(4) N14A-C15A 1.379(6) N27A-C26A 1.361(4) N27A-C26C 1.399(13) N27A-C28A 1.350(4) N27A-C28A 1.342(6) N34A-C33A 1.340(4) N34A-C33A 1.340(6) N34A-C35A 1.365(4) N34A-C35A 1.387(6) Molecules 2020, 25, x 4 of 17 oxidation state. Hence, both complexes can be described as [M(L ox ) 2 ], although the MOS values of the o-aminophenol moieties in [Zr(L ox ) 2 ] are slightly higher [−1.21(14), −1.39(18), −1.40(14) and −1.41(12)] compared to [Ti(L ox ) 2 ]. However, it is known that complexes of high oxidation state metals may have strong π-donation from the high-lying π orbitals of amidophenoxide ligands, which may cause the metrical oxidation state to differ from the theoretical value [25]. It therefore seems obvious that the formal oxidation state of both Ti and Zr centres should be assigned as +4. Table 1. Selected bond lengths (Å) for [Ti(L ox )2] and [Zr(L ox )2]. Ti1A-O1A 1.977(2) Zr1A-O1A 2.113(4) Ti1A-O2A 2.019(2) Zr1A-O2A 2.131(4) Ti1A-O3A 2.016(2) Zr1A-O3A 2.096(3) Ti1A-O4A 2.022(2) Zr1A-O4A 2.134(3) Ti1A-N7A 2.318(2) Zr1A-N7A 2.382(4) Ti1A-N14A 2.198(3) Zr1A-N14A 2.325(4) Ti1A-N27A 2.282(3) Zr1A-N27A 2.367(4) Ti1A-N34A 2.228(3) Zr1A-N34A 2.319(4) N7A-C6A 1.379(4) N7A-C6A 1.377(7) N7A-C8A 1.333(4) N7A-C8A 1.333(6) N14A-C13A 1.327(4) N14A-C13A 1.346(6) N14A-C15A 1.377(4) N14A-C15A 1.379(6) N27A-C26A 1.361(4) N27A-C26C 1.399(13) N27A-C28A 1.350(4) N27A-C28A 1.342(6) N34A-C33A 1.340(4) N34A-C33A 1.340(6) N34A-C35A 1.365(4) N34A-C35A 1.387(6) Figure 1. The crystal structures of [Ti(L ox ) 2 ] and [Zr(L ox ) 2 ]. The hydrogen atoms and tert-butyl groups are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. According to the single crystal XRD analysis, the vanadium complex is formally a neutral species, where two ligands show different coordination modes and protonation states, i.e., [V(L)(HL)]. One of the two distinct ligands is coordinated to the central metal ion as a fully deprotonated tetradentate ONNO-donor, whereas the second ligand is coordinated as a partially deprotonated tridentate ONN-donor. The sole phenolic hydroxyl group that remains protonated and uncoordinated forms a hydrogen bond to the phenolate oxygen of the four-dentate ligand [d(O1A···O4A = 2.967 Å)]. The phenolic oxygens are aligned in a mer-fashion and the geometry of the central V atom is best described as a distorted pentagonal bipyramid with the tridentate ligand in a planar geometry (dihedral V-N-N-O angle is −4.6°) and the tetradentate clearly twisted (dihedral O- N-N-O angle is 37.0°). Interestingly, the crystal packing of [V(L)(HL)] is isostructural with the known Figure 1. The crystal structures of [Ti(L ox ) 2 ] and [Zr(L ox ) 2 ]. The hydrogen atoms and tert-butyl groups are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. According to the single crystal XRD analysis, the vanadium complex is formally a neutral species, where two ligands show different coordination modes and protonation states, i.e., [V(L)(HL)]. One of the two distinct ligands is coordinated to the central metal ion as a fully deprotonated tetradentate ONNO-donor, whereas the second ligand is coordinated as a partially deprotonated tridentate ONN-donor. The sole phenolic hydroxyl group that remains protonated and uncoordinated forms a hydrogen bond to the phenolate oxygen of the four-dentate ligand [d(O1A···O4A =2.967 Å)] . The phenolic oxygens are aligned in a mer-fashion and the geometry of the central V atom is best described as a distorted pentagonal bipyramid with the tridentate ligand in a planar geometry (dihedral V-N-N-O angle is − 4.6 ◦ ) and the tetradentate clearly twisted (dihedral O-N-N-O angle is 37.0 ◦ ). Interestingly, the crystal packing of [V(L)(HL)] is isostructural with the known Mo 6+ and W 6+ Molecules 2020,25, 2531 5 of 17 complexes which have the formula [M(L) red (HL) sq1 ] [ 5 , 6 ], even if the formal oxidation state in the vanadium complex is apparently lower. Like the isostructural Mo and W complexes, [V(L)(HL)] consists of two distinct molecular units in the asymmetric unit. The intramolecular bond parameters between these units are very similar and thus the complex V1A is taken as an example while discussing the bonding characteristics of the complex. The intramolecular distances are shown in (Table 2, Figure 2). The C-C bond distances in the central six-membered C 6 rings indicate that the tridentate ligand bears a cyclohexadiene backbone whereas the tetradentate ligand shows a more delocalized C 6 system (Table 2). This is also reflected in the C-N bond distances, which for the tridentate ligand are ca. 0.05–0.07 Å shorter compared with the tetradentate ligand and are closer to values that would be expected for an imine. MOS calculations for the two distinct o-aminophenols of the tetradentate ONNO ligand gave the values of − 1.65(12) and − 1.79(11), thus yielding an approximate formal oxidation state of − 3 for the entire ligand. The fully coordinated o-aminophenol moiety of the trisdentate ONN ligand, on the other hand, yields a MOS value of − 1.10(6). This information, combined with the magnetic measurements (vide infra), indicates that the complex is expected to have one tetradentate and triply anionic [L sq1 ] 3− ligand, with an unpaired electron, as well as one trisdentate, partly protonated and singly anionic [HL ox ] − ligand and V 4+ d 1 central metal ion. Hence, although [V(L sq1 )(HL ox )] shows remarkable structural resemblance to its formerly synthesized Mo [ 7 ] and W [ 6 ] analogues, the oxidation states of both chelating ligands are different to the group VI metal complexes reported earlier. This reveals that the overall structure of the ligand is not entirely dependent on the oxidation state. Molecules 2020, 25, x 5 of 17 Mo 6+ and W 6+ complexes which have the formula [M(L) red (HL) sq1 ] [5,6], even if the formal oxidation state in the vanadium complex is apparently lower. Like the isostructural Mo and W complexes, [V(L)(HL)] consists of two distinct molecular units in the asymmetric unit. The intramolecular bond parameters between these units are very similar and thus the complex V1A is taken as an example while discussing the bonding characteristics of the complex. The intramolecular distances are shown in (Table 2, Figure 2). The C-C bond distances in the central six-membered C 6 rings indicate that the tridentate ligand bears a cyclohexadiene backbone whereas the tetradentate ligand shows a more delocalized C 6 system (Table 2). This is also reflected in the C-N bond distances, which for the tridentate ligand are ca. 0.05–0.07 Å shorter compared with the tetradentate ligand and are closer to values that would be expected for an imine. MOS calculations for the two distinct o-aminophenols of the tetradentate ONNO ligand gave the values of −1.65(12) and −1.79(11), thus yielding an approximate formal oxidation state of −3 for the entire ligand. The fully coordinated o-aminophenol moiety of the trisdentate ONN ligand, on the other hand, yields a MOS value of −1.10(6). This information, combined with the magnetic measurements (vide infra), indicates that the complex is expected to have one tetradentate and triply anionic [L sq1 ] 3− ligand, with an unpaired electron, as well as one trisdentate, partly protonated and singly anionic [HL ox ] − ligand and V 4+ d 1 central metal ion. Hence, although [V(L sq1 )(HL ox )] shows remarkable structural resemblance to its formerly synthesized Mo[7] and W[6] analogues, the oxidation states of both chelating ligands are different to the group VI metal complexes reported earlier. This reveals that the overall structure of the ligand is not entirely dependent on the oxidation state. Figure 2. The crystal structures of [V(L sq1 )(HL ox )] and [Ni(HL ox ) 2 ] with the intramolecular hydrogen bonds drawn with dashed lines. The C–H hydrogen atoms and tert-butyl groups are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Table 2. Selected bond lengths (Å) for [V(L sq1 )(HL ox )] and [Ni(HL ox )2]. V1A-O1A 3.571(2) Ni1-O1 2.0616(16) V1A-O2A 1.9712(19) Ni1-O2 3.548(2) V1A-O3A 1.9496(19) Ni1-O3 2.0315(14) V1A-O4A 2.0544(19) Ni1-O4 3.476(2) V1A-N7A 2.071(2) Ni1-N7 1.9924(16) V1A-N14A 2.155(2) Ni1-N14 2.1021(17) V1A-N27A 2.052(2) Ni1-N27 1.9882(16) V1A-N34A 1.989(2) Ni1-N34 2.0824(16) N7A-C6A 1.425(3) N7-C6 1.375(3) N7A-C8A 1.322(4) N7-C8 1.335(3) N14A-C13A 1.323(4) N14-C13 1.323(2) N14A-C15A 1.362(4) N14-C15 1.414(3) N27A-C26A 1.397(4) N27-C26 1.363(3) Figure 2. The crystal structures of [V(L sq1 )(HL ox )] and [Ni(HL ox ) 2 ] with the intramolecular hydrogen bonds drawn with dashed lines. The C–H hydrogen atoms and tert-butyl groups are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Table 2. Selected bond lengths (Å) for [V(Lsq1)(HLox)] and [Ni(HLox)2]. V1A-O1A 3.571(2) Ni1-O1 2.0616(16) V1A-O2A 1.9712(19) Ni1-O2 3.548(2) V1A-O3A 1.9496(19) Ni1-O3 2.0315(14) V1A-O4A 2.0544(19) Ni1-O4 3.476(2) V1A-N7A 2.071(2) Ni1-N7 1.9924(16) V1A-N14A 2.155(2) Ni1-N14 2.1021(17) V1A-N27A 2.052(2) Ni1-N27 1.9882(16) V1A-N34A 1.989(2) Ni1-N34 2.0824(16) N7A-C6A 1.425(3) N7-C6 1.375(3) N7A-C8A 1.322(4) N7-C8 1.335(3) N14A-C13A 1.323(4) N14-C13 1.323(2) N14A-C15A 1.362(4) N14-C15 1.414(3) N27A-C26A 1.397(4) N27-C26 1.363(3) N27A-C28A 1.392(4) N27-C28 1.308(3) N34A-C33A 1.371(4) N34-C33 1.310(2) N34A-C35A 1.402(3) N34-C35 1.415(2) Molecules 2020,25, 2531 6 of 17 [Ni(HL) 2 ] is formed of neutral molecules, where the two organic ligands are coordinated to the metal centre as three-dentate ONN-donors in a nearly symmetric fashion. In both ligands, the shortest C–N bonds (1.308, 1.310, 1.323 and 1.335 Å) are towards the central phenylene ring of the ligand corresponding to imine double bonds (see Table 2). In general, the bonding parameters indicate that the organic ligands are in a partly deprotonated [HL ox ] − oxidation state with each ligand having one dangling phenolic hydroxyl group which engages in intramolecular hydrogen bonding with the coordinated O-atoms. These two coordinated phenolic oxygens (O1 and O3) are aligned in a cis form in respect to the central atom. The coordination geometry around the central atom is best described as a distorted octahedron, which matches with the proposed structure for the high-spin Ni 2+ complex. The calculated MOS values are − 0.92(14) and − 1.15(12) for the coordinated o-aminophenol moieties. These findings support strongly the idea of two identical [Lox]2−ligands within the complex. 2.3. NMR and MS Spectroscopic Studies The 1 H-NMR spectra of Ti and Zr complexes display two distinct singlets (ca. 1.2 and 0.9 ppm) for the tert-butyl groups and four multiplets (ca. 6.95–7.55 ppm) for aromatic protons demonstrating the high symmetry of the molecules in the solutions. Although the ligand H 4 L can form stable radicals upon coordination and intramolecular redox reactions, the well-resolved NMR spectra (see Supplementary Information) verify the diamagnetic nature of the Ti and Zr complexes at room temperature in solution. The protonation states of the coordinated ligands were also studied using electrospray ionization (ESI-MS) mass spectroscopy, and the molecular ions [M +H] + were detected at m/z=1073.62 for [Ti(L ox ) 2 ] and at m/z=1115.62 for [Zr(L ox ) 2 ], which indicates that the ligands are fully deprotonated upon coordination. The NMR studies for the V and Ni complexes provided little information on the molecular structure in solution, indicating the paramagnetic nature of the complex. Therefore, the Evans NMR method [26] for measuring the magnetic susceptibilities in solutions was applied to get µeff=0.66 µB for the V complex. This effectively equals to less than half spins on the molecule, thus suggesting that the electronic ground state of the complex is a mixture of singlet and triplet/pentet states, which is also supported by SQUID (Superconducting Quantum Interference Device) results (see below). Although we cannot entirely rule out the possibility of paramagnetic impurity being the source of the non-zero magnetic moment in solution, such impurities are not present in the ESI-MS spectra, which gave m/z values of 1077.6760 (ESI(+)) and 1077.6251 (ESI( − )), with the highest intensity. This would thus indicate the detection of [M +H] + and [M +H] − ions, while the calculated exact mass for the complex based on the X-ray studies is 1076.6323. The MS patterns were also rather atypical for a vanadium complex (see Supplementary Information), which may very well be the manifestation of the redox behavior of the complex. In addition, the positive ionization mode gives traces of [M +Na] + and [M +K] + ions that are detected at m/z=1099.6508 and 1115.6145, respectively. [Ni(HL ox ) 2 ] proved to be paramagnetic in the NMR measurements, so the Evans method was applied to measure µeff =2.84 µB at room temperature. The magnetic properties were further studied in the solid state using SQUID experiments (see below). The protonation state of the coordinated ligands was also verified using ESI-MS, while the molecular ion [M +H]+was detected at m/z=1085.6731. 2.4. DFT Studies for the Vanadium Complex Density functional theory (DFT) was employed to investigate the electronic structure of the [V(L)(HL)] complex. DFT calculations were carried out using PBE0 functional [ 27 – 29 ] and def2-TZVP basis sets [ 30 ] (with def2/J auxiliary basis sets [ 31 ]) on a modified structure of [V(L)(HL)] with tert-butyl groups replaced with H atoms. Geometry optimizations were carried out within the Orca program (version 4.2.0) [ 32 ] for singlet, broken symmetry singlet, triplet and pentet electronic states, using the modified single crystal X-ray structure as a starting point. The optimized structures were then subjected to vibrational analyses to establish that they each correspond to a minimum on the potential energy surface. Molecules 2020,25, 2531 7 of 17 From the studied systems, a broken symmetry (BS) singlet (S=0) was found to correspond to the ground state, with the triplet (S=1) only 6 kJ/mol higher in energy, followed by the closed shell singlet (S=0; 22 kJ/mol higher than BS singlet). The optimized structure of the pentet (S=2) was found to be considerably less favorable (51 kJ/mol higher in energy). The coordination sphere bond lengths (Table S2) of the ground state broken symmetry solution fall within ± 0.03 Å from the corresponding experimental values (complex A in the asymmetric unit). The triplet structure shows a slightly larger deviation, whereas, interestingly, the closed shell singlet solution shows the best fit for experimental metal-ligand bond lengths. Furthermore, the asymmetry of the V-N bonds is best reproduced by the closed shell singlet optimized geometry. The xyz coordinates of the optimized structures are given in Supplementary Materials, Table S4. Analysis of the spin density of the broken symmetry singlet shows the localization of the opposite spins to the vanadium center and, respectively, to the tetradentate ligand with only a minute contribution from the tridentate ligand (Figure 3). The visual information together with the analysis of the Löwdin atomic spin populations strongly point to an antiferromagnetically coupled radical ligand and vanadium(4+) center as the ground state structure. The low-lying excited triplet state shows the spin localized, to a great extent, to the vanadium ion, but also delocalized along the ligand backbones. According to the Löwdin atomic spin populations the triplet contains a vanadium(III) ion, and thus, the ligands should exist at either [L ox ] 2− and [HL ox ] − or [L sq1 ] 3− and [HL sq2 ] (or [HL sq1 ] 2− and [L sq2 ] − ) oxidation states, respectively. From these formal oxidation states, the former has no unpaired electrons, whereas the latter oxidation states would require the assignment of half a spin to each of the ligands. As there is significant spin density on both ligands, the results indicate that the ligands of the excited triplet state structure can be assigned as [Lsq1]3−and [HLsq2] (or [HLsq1]2−and [Lsq2]−). Molecules 2020, 25, x 7 of 17 using the modified single crystal X-ray structure as a starting point. The optimized structures were then subjected to vibrational analyses to establish that they each correspond to a minimum on the potential energy surface. From the studied systems, a broken symmetry (BS) singlet (S = 0) was found to correspond to the ground state, with the triplet (S = 1) only 6 kJ/mol higher in energy, followed by the closed shell singlet (S = 0; 22 kJ/mol higher than BS singlet). The optimized structure of the pentet (S = 2) was found to be considerably less favorable (51 kJ/mol higher in energy). The coordination sphere bond lengths (Table S2) of the ground state broken symmetry solution fall within ±0.03 Å from the corresponding experimental values (complex A in the asymmetric unit). The triplet structure shows a slightly larger deviation, whereas, interestingly, the closed shell singlet solution shows the best fit for experimental metal-ligand bond lengths. Furthermore, the asymmetry of the V-N bonds is best reproduced by the closed shell singlet optimized geometry. The xyz coordinates of the optimized structures are given in Supplementary Materials, Table S4. Analysis of the spin density of the broken symmetry singlet shows the localization of the opposite spins to the vanadium center and, respectively, to the tetradentate ligand with only a minute contribution from the tridentate ligand (Figure 3). The visual information together with the analysis of the Löwdin atomic spin populations strongly point to an antiferromagnetically coupled radical ligand and vanadium(4+) center as the ground state structure. The low-lying excited triplet state shows the spin localized, to a great extent, to the vanadium ion, but also delocalized along the ligand backbones. According to the Löwdin atomic spin populations the triplet contains a vanadium(III) ion, and thus, the ligands should exist at either [L ox ] 2− and [HL ox ] − or [L sq1 ] 3− and [HL sq2 ] (or [HL sq1 ] 2− and [L sq2 ] − ) oxidation states, respectively. From these formal oxidation states, the former has no unpaired electrons, whereas the latter oxidation states would require the assignment of half a spin to each of the ligands. As there is significant spin density on both ligands, the results indicate that the ligands of the excited triplet state structure can be assigned as [L sq1 ] 3− and [HL sq2 ] (or [HL sq1 ] 2− and [L sq2 ] − ). Figure 3. Spin density plots for broken symmetry singlet and excited triplet states of modified structure of [V(L)(HL)]. 2.5. Electrochemical Studies Cyclic voltammetry was used to study the redox stability and the characteristic redox behavior of [Ti(L ox ) 2 ] and [Zr(L ox ) 2 ] by scanning the potential range from +1.3 to −2.3 V vs Fc + /Fc. A different range, from +1.0 to −2.0 V vs Fc + /Fc, was applied for [V(L sq1 )(HL ox )] and [Ni(HL ox ) 2 ]. The reversible electron transfer processes for Ti and Zr and Ni complexes are probably only ligand based [5,6,8], as similar waves can be observed in the voltammograms of each of the compounds. Some of the redox processes are at overlapping potentials, or faint, thus rendering the voltammograms somewhat Figure 3. Spin density plots for broken symmetry singlet and excited triplet states of modified structure of [V(L)(HL)]. 2.5. Electrochemical Studies Cyclic voltammetry was used to study the redox stability and the characteristic redox behavior of [Ti(L ox ) 2 ] and [Zr(L ox ) 2 ] by scanning the potential range from +1.3 to − 2.3 V vs Fc + /Fc. A different range, from +1.0 to − 2.0 V vs Fc + /Fc, was applied for [V(L sq1 )(HL ox )] and [Ni(HL ox ) 2 ]. The reversible electron transfer processes for Ti and Zr and Ni complexes are probably only ligand based [ 5 , 6 , 8 ], as similar waves can be observed in the voltammograms of each of the compounds. Some of the redox processes are at overlapping potentials, or faint, thus rendering the voltammograms somewhat difficult to interpret. The voltammogram of the V complex, on the other hand, is clearly different. Molecules 2020,25, 2531 8 of 17 (Figure 4, Table 3, Supplementary Materials Figures S7–S10) The voltammogram exhibits two reversible one-electron processes (+0.01 and +0.54 V) and several irreversible processes in the mix. It is therefore difficult to draw definite conclusions regarding which redox event takes place at each of the redox waves of the voltammogram. Table 3. Measured redox potentials for studied complexes (vs Fc+/Fc). Complex Reversible One-Electron Redox Waves (V) [Ti(Lox)2]−1.64 −1.19 −0.17 +0.24 +1.06 [Zr(Lox)2]−1.95 −1.49 −1.09 −0.02 +0.41 +0.74 [V(Lsq1)(HLox)] +0.01 +0.54 [Ni(HLox)2]−1.47 −1.07 −0.92 +0.04 +0.36 Molecules 2020, 25, x 8 of 17 difficult to interpret. The voltammogram of the V complex, on the other hand, is clearly different. (Figure 4, Table 3, Supplementary Materials Figures S7–S10) The voltammogram exhibits two reversible one-electron processes (+0.01 and +0.54 V) and several irreversible processes in the mix. It is therefore difficult to draw definite conclusions regarding which redox event takes place at each of the redox waves of the voltammogram. Table 3. Measured redox potentials for studied complexes (vs Fc+/Fc). Complex Reversible One-Electron Redox Waves (V) [Ti(Lox)2] −1.64 −1.19 −0.17 +0.24 +1.06 [Zr(Lox)2] −1.95 −1.49 −1.09 −0.02 +0.41 +0.74 [V(Lsq1)(HLox)] +0.01 +0.54 [Ni(HLox)2] −1.47 −1.07 −0.92 +0.04 +0.36 Figure 4. Cyclic voltammogram of [V(Lsq1)(HLox)]. Cyclic voltammetry is performed at RT vs Fc/Fc+, in CH2Cl2 with 100 mVs−1 scan rate and the rotation of the measurement is clockwise. 2.6. Optical Absorbtion Studies One of our motivations for the synthesis of the presented compounds was their potential applicability in photovoltaics as dyes, and thus, their optical absorption properties were studied initially using UV-vis spectroscopy. Accordingly, the UV-vis-NIR spectra in CH2Cl2 present distinct absorption peaks for each complex in the vis/NIR range (Table 4, Figure 5). The absorption coefficients of the different complexes range from low to significantly high when compared with the coefficient of the standard ruthenium sensitizer dye (ε ≈ 14 × 103 M−1cm−1, λ = 538 nm) [33]. The absorption spectra are rather similar in shape in the solid state, as well in CH2Cl2 solution for all complexes (see Supplementary Materials). The absorption coefficients of the different complexes range from low to significantly high when compared with the coefficient of the standard ruthenium sensitizer dye (ε ≈ 14 × 103 M−1cm−1, λ = 538 nm) [33]. The absorption spectra are rather similar in shape in the solid state, as well in CH2Cl2 solution for all complexes (see Supplementary Materials). The group IV complexes have similar, ligand-based, π→π* UV absorptions as the ones that have been previously reported [4]. The strong NIR absorptions above 1000 nm are most probably ligand-based transitions, i.e., ligand-to-ligand charge transfer (LLCT) in character, as reported for the related Co complex [7]. The LLCT processes are of both the intra and inter ligand type [34]. As these metal centers are high oxidation state species with empty d-orbitals, there may be some ligand-to-metal charge transfer (LMCT) involved. [V(Lsq1)(HLox)] has a broad absorption at λmax = 680 nm, which reaches the NIR range. This absorption may originate from LLCT with some LMCT or metal-to- ligand CT (MLCT) mixing. As the central metal is not d0, also a d-d transition is possible. In addition, Figure 4. Cyclic voltammogram of [V(L sq1 )(HL ox )]. Cyclic voltammetry is performed at RT vs Fc/Fc + , in CH2Cl2with 100 mVs−1scan rate and the rotation of the measurement is clockwise. 2.6. Optical Absorbtion Studies One of our motivations for the synthesis of the presented compounds was their potential applicability in photovoltaics as dyes, and thus, their optical absorption properties were studied initially using UV-vis spectroscopy. Accordingly, the UV-vis-NIR spectra in CH 2 Cl 2 present distinct absorption peaks for each complex in the vis/NIR range (Table 4, Figure 5). The absorption coefficients of the different complexes range from low to significantly high when compared with the coefficient of the standard ruthenium sensitizer dye ( ε≈ 14 × 10 3 M −1 cm −1 , λ =538 nm) [ 33 ]. The absorption spectra are rather similar in shape in the solid state, as well in CH 2 Cl 2 solution for all complexes (see Supplementary Materials). The absorption coefficients of the different complexes range from low to significantly high when compared with the coefficient of the standard ruthenium sensitizer dye ( ε≈ 14 × 10 3 M −1 cm −1 , λ =538 nm) [ 33 ]. The absorption spectra are rather similar in shape in the solid state, as well in CH 2 Cl 2 solution for all complexes (see Supplementary Materials). The group IV complexes have similar, ligand-based, π→π * UV absorptions as the ones that have been previously reported [ 4 ]. The strong NIR absorptions above 1000 nm are most probably ligand-based transitions, i.e., ligand-to-ligand charge transfer (LLCT) in character, as reported for the related Co complex [ 7 ]. The LLCT processes are of both the intra and inter ligand type [ 34 ]. As these metal centers are high oxidation state species with empty d-orbitals, there may be some ligand-to-metal charge transfer (LMCT) involved. [V(L sq1 )(HL ox )] has a broad absorption at λmax =680 nm, which reaches the NIR range. This absorption may originate from LLCT with some LMCT or metal-to-ligand CT (MLCT) mixing. As the central metal is not d 0 , also a d-d transition is possible. In addition, the broad and intense absorption at 850 nm for [Ni(HLox)2] is assignable as a LLCT band and/or to the Ni d-d absorptions. Molecules 2020,25, 2531 15 of 17 +K] + calcd. m/z1115.60). ESI( − )-MS: m/z1077.64 ([M] − calcd. m/z1076.63). m.p. 250 ◦ C (decomp.). Evans method, µeff=0.66 µB. 3.4. Preparation of [Ni(HLox)2] [Ni(acac) 2· 2H 2 O] (123 mg, 0.420 mmol) and H 4 L (446 mg, 0.863 mmol) were stirred in methanol (10 mL) for 21 h. Precipitated dark solid was filtered and washed with methanol, fractioned with column chromatography using CH 2 Cl 2 as an eluent and finally recrystallized from the Et 2 O/MeOH mixture to obtain XRD quality crystals. The isolated solid is insoluble in alcohols or acetonitrile, but dissolves in halogenated organic solvents, diethyl ether and hydrocarbon solvents. Yield 109 mg (23 %). IR: 2954 (m), 1442 (m), 1358 (s), 1253 (vs), 1193 (m), 1166 (s), 1131 (s), 1113 (s) 1022 (m), 985 (m), 908 (m), 868 (w), 833 (w), 756 (m), 598 (m) 580 (m), 552 (m), 501 (s) cm−1.1H-NMR (CDCl3): δ0.7–2.1 ppm (72 H, several overlapping peaks). ESI(+)-MS: m/z1085.6731 ([M +H] + calcd. m/z1085.6349). m.p. 230 ◦C (decomp.). Evans method, µeff=2.84 µB. Supplementary Materials: The supplementary material is available online at http://www.mdpi.com/1420-3049/ 25/11/2531/s1. Table S1, S2: Summary of crystallographic data. Figure S1: Molecular structure of orthorhombic polymorph of [V(Lsq1)(HLox)]. Figure S2: The experimental and simulated PXRD pattern of [V(Lsq1)(HLox)]. Figures S3–S6: UV-Vis-NIR spectra. Figures S7–S10: Cyclic voltammograms. Figures S11-S16: ESI-MS mass spectra. Figures S17–S18: SQUID measurements. Figures S19–S22: TGA/DSC measurements. Figures S23–S28: NMR spectra. Figures S29–S30: Evans’ method NMR measurements. Table S2: Experimental and optimized (DFT) bond parameters. Table S3 Optimized Cartesian coordinates. Author Contributions: Conceptualization, E.S. and A.L.; software, A.P.; formal analysis, E.S., H.H., L.S.V., A.P., M.L. (Manu Lahtinen) and M.L. (Mika Lastusaari); investigation, E.S.; writing—original draft preparation, E.S. and A.P.; writing—review and editing, A.L., A.P. and M.L. (Mika Lastusaari); visualization, E.S., A.P.; supervision, A.L., M.L. (Mika Lastusaari); project administration, E.S.; funding acquisition, E.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Walther Ahlström foundation and the Magnus Ehrnrooth foundation. A.P. gratefully acknowledges the funding from the Academy of Finland (no. 315911). We gratefully acknowledge the computational resources provided by CSC-IT Center for Science in Finland. Conflicts of Interest: The authors declare no conflicts of interest. References 1. Jørgensen, C.K. 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