Synthesis, Crystal Structures, and Magnetic and Electrochemical

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18Hexaazatrinaphthylene (HATNPh6)‑Bridged Titanium Complexes Pia Sander,† Aleksandra Markovic,† Marc Schmidtmann,† Oliver Janka,‡ Gunther Wittstock,*,† and Rüdiger Beckhaus*,† †

Institut für Chemie, Carl von Ossietzky Universität Oldenburg, D-26111 Oldenburg, Germany Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 28/30, 48149 Münster, Germany

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‡

S Supporting Information *

ABSTRACT: Trinuclear titanium complexes coordinated by one ligand with three coordination sites have shown properties of mixed valency and a high number of reversible redox steps. Herein we report on the hexaphenyl-substituted derivative (Cp2Ti)3(μ3-HATNPh6) (2). On reaction of 2 with the ferrocenium salt [Cp2Fe]BF4, the cationic complexes [(Cp2Ti)3(μ3-HATNPh6)]n+ (n = 1−3; 3−5) become available in a selective way. Cyclic voltammograms show 10 reversible redox states of the trinuclear species 2 without decomposition. In order to classify the degree of electronic communication between the titanium centers, comproportionation constants Kc, IVCT bands in NIR spectra, and magnetic measurements were analyzed. These parameters show strong coupling effects between the titanium centers but no full delocalization. In addition, single-crystal X-ray analysis of the neutral complex 2 and its oxidation products (1+ (3), 2+ (4), and 3+ (5)) revealed the geometric structure of the molecule in the solid state. For the cationic species anion−π interactions between the electron-deficient central ring of the HATNPh6 ligand and BF4− counterions were found.

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INTRODUCTION

bidentate ligands lead to well-defined complexes of type A with different cavity sizes that depend in particular on the size of the bridging ligand. For the formation of structure type B, a broad range of examples is known employing 3-fold chelating ligands, for which trinuclear complexes derived from cyanuric acid and melamine are prominent examples.1,12 In addition, complexes of type B can also be obtained by C−C coupling reactions of different dinitriles in the coordination sphere of titanocene.13,14 Hexaazatriphenylene (HAT) ligands constitute another famous class of ligands for the formation of type B structures. Because of their planar aromatic discotic system, D 3h symmetry, and electron-deficient π system, HAT ligands are popular building blocks for multinuclear complexes,15−19 whose charge transfer, fluorescence, and magnetic behaviors are of particular interest. The HAT derivatives can be synthesized by (i) condensation of hexaaminobenzene with 1,2-diketones or (ii) condensation of hexaketocyclohexane and ethylenediamine or 1,2-diaminobenzene derivatives. The HAT ligand itself is also obtained by a hexadecarboxylation of HAT(COOH)6.20 Route i involves the explosive triaminotrinitrobenzene as an intermediate.15,21 Therefore, the second route is more commonly used, which provides safe access to

Trinuclear transition-metal complexes can be built in different ways, as shown in Scheme 1. The main possibilities employ either three bridging bidentate ligands (A)1−3 or one ligand with three metal coordination sites (B).1,4,5 Multinuclear coordination complexes such as A are known for late and early transition metals and a great variety of bridging ligands: for example, bis-azines such as pyrazine or larger homologues (e.g. 4,4′-bipyridine),6−8 dicarboxylates such as benzenedicarboxylic acids,9 and cyanides.1,10,11 Selfassembly reactions of metal-containing units and suitable Scheme 1. Selected Coordination Modes A (with Three Bridging Bidentate Ligands) and B (with One Ligand with Three Coordination Sites) for Trinuclear Complexes

Received: June 22, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.8b01728 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

hexaazatrinaphthylene derivative HATNPh6, in which the methyl groups of IIIb are replaced by phenyl substituents that render the compounds soluble (toluene, THF). While the synthesis of the isolated organic HATNPh6 ligand is known,26 the application of HATNPh6 in the synthesis of titanium complexes became possible by an improved ligand synthesis.25 Additionally, oxidation products of (Cp2Ti)3(μ3-HATNPh6) (2) have been synthesized by stepwise oxidation with ferrocenium salts. The cationic structures were identified and characterized by means of single-crystal X-ray diffraction and magnetic measurements.

hexaazatrinaphtylene derivatives (HATN) with 1,2-diaminobenzene derivatives. Several HATN (I) and HAT (II) type ligands are known so far15 (Scheme 2). Scheme 2. Selected Examples of HATN (I) and HAT (II) Derivatives

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EXPERIMENTAL SECTION

All operations were performed under a nitrogen or argon atmosphere with rigorous exclusion of oxygen and moisture using glovebox and Schlenk techniques. Solvents were thoroughly dried and saturated with nitrogen. 4,5-Diphenyl-1,2-diamine,26 HATNPh6,27 Cp2Ti(η2C2(SiMe3)2),28 and [Cp2Fe]BF429 were prepared according to the literature procedures. Elemental analyses were performed with a EuroEA 3000 elemental analyzer. IR spectra were recorded on a Bruker Tensor 27 spectrometer using an attenuated total reflection (ATR) method. Melting points were determined using a “Mel-Temp” apparatus by Laboratory Devices. ESI mass spectra were taken on a Waters Q-TOF Premier mass spectrometer. Cyclic Voltammetry. Cyclic voltammetry was performed in an Ar-filled glovebox with a potentiostat (Compactstat, Ivium Technologies, Eindhoven, The Netherlands). The Au disk as the working electrode (WE) had a diameter of 2 mm. A Pt plate as auxiliary electrode (Aux, area 1 cm2) and a Ag/Ag+ reference electrode (ref) filled with 0.01 M AgClO4 and 0.1 M [nBu4N]ClO4 in THF completed the electrochemical three-electrode cell. Experiments were run at a scan rate of 0.5 V s−1 in dry and degassed THF solutions of the analyte (∼0.3 mM) and supporting electrolyte (0.2 M [nBu4N]PF6). Cyclic voltammograms were referenced relative to the ferrocene/ferrocenium redox couple. X-ray Diffraction. X-ray data were measured on a Bruker Apex II CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) (for details see the Supporting Information). Spectroscopy. NIR spectra were measured with a fiber-coupled Matrix-F FT-NIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) in a 10 × 10 mm quartz cuvette (0.3 mM in dry and degassed THF). UV−vis spectra were taken with a GetSpec 2048 CCD array spectrometer (GetSpec, Sofia, Bulgaria) in a 10 × 1 mm quartz cuvette (0.3 mM in dry and degassed THF). Magnetic Measurements. Powdered samples of 2−5 were packed in PE capsules in an argon-filled glovebox and attached to the sample holder rod of a vibrating sample magnetometer unit (VSM) to measure the magnetization M(T) with a Quantum Design PhysicalProperty-Measurement-System (PPMS). The samples were investigated in the temperature range of 2.5−300 K with external magnetic fields up to 80 kOe. (Cp2Ti)3(μ3-HATNPh6) (2). Method A. Cp2Ti(η2-C2(SiMe3)2) (50 mg, 0.14 mmol) and 6,7-diphenylquinoxaline (41 mg, 0.14 mmol) were suspended in 5 mL of toluene and stored without stirring at 60 °C for 48 h. A nearly black, dark red solid was formed. The solid was isolated by decanting, washed with small portions of n-hexane (2 × 3 mL), and subsequently dried in vacuo to yield a dark red solid (46 mg, 72%). Method B. HATNPh6 (445 mg, 1.28 mmol) and Cp2Ti(η2C2(SiMe3)2) (310 mg, 0.37 mmol) were suspended in 5 mL of toluene and stored without stirring at 60 °C for 24 h. Nearly black, dark red crystals were formed. The solid was isolated by decanting, washed with small portions of n-hexane (2 × 3 mL), and subsequently dried in vacuo to yield dark red crystals (438 mg, 86%). Mp: >250 °C. IR (ATR): ν̃ 3077 (w), 3054 (w), 3022 (w), 1598 (m), 1525 (w), 1494 (m), 1456 (m), 1408 (w), 1345 (m), 1310 (m), 1292 (w), 1264 (m), 1210 (m), 1177 (m), 1092 (w), 1072 (w), 1049 (w), 1014 (m), 961 (m), 788 (s), 766 (s), 740 (m), 728 (m), 694 (s),

HAT or HATN systems can be classified as small 2D Ncontaining polyheterocyclic aromatic systems.15 Due to their excellent electron-acceptor properties, they have attracted tremendous interest in organic electronics: for example, for applications in semiconductors22 or as promising materials for future nanoelectronics.23 A variety of metal complexes containing these types of ligands are known,16 among them complexes with the transition metal titanium (Scheme 3).4,5,24 By the unexpected Scheme 3. Trinuclear Titanocene Complexes III and IV, Formed by Dehydrocoupling of Quinoxaline (IIIa) (or 6,7Dimethylquinoxaline (IIIb)) or C−C Coupling of Pyrazine (IV)

titanium-initiated trimerization of three pyrazine molecules, a 4a,4b,8a,8b,12a,12b-hexahydrodipyrazino[2,3-f:2′,3′-h]quinoxaline ligand (HATH6) is formed, which can only be isolated in the form of a (Cp*2Ti)3HATH6 complex.4,25 In comparison to other trinuclear complexes employing ligand I or II, the HATH6 complex IV is characterized by a central cyclohexane ring.4,25 A further kind of trimerization takes place by dehydrocoupling of quinoxaline derivatives, where hexaazatrinaphthylene derivatives (HATN, III) ligand systems are formed in the presence of titanocene fragments (Scheme 3).5 Electrochemical investigation of the titanocene HATNMe6 complex IIIb indicates a mixed-valent system and a noninnocent character of the HATN ligand.5,24 Because of its low solubility, further investigations failed. For a deeper understanding of mixed-valent titanium complexes, compounds of type III are required that allow spectroscopic characterization in solution. To this end, we report on the synthesis, structure, and electrochemical properties of the new titanocene B


Article

Inorganic Chemistry

Scheme 4. : Synthesis of 2 by Dehydrogenative Coupling (Method A) or Direct Coordination on the Ligand (Method B)

667 (m), 641 (s), 603 (m), 574 (s) cm−1. MS (ESI, toluene, positive scan) m/z (%): 1374.8 (100) [2]+, 1298.1 (23) [2 − C6H5]+, 1196.3 (8) [2 − Cp2Ti]+, 740.27 (6). HRMS: calculated for [C90H66N6Ti3]+, 1374.3787; found, 1374.3805. Anal. Calcd for C90H66N6Ti3: C, 78.61; H, 4.84; N, 6.11. Found: C, 78.48; H, 4.94; N, 5.63. [(Cp2Ti)3(μ3-HATNPh6)]BF4 (3). 2 (150 mg, 0.11 mmol) and [Cp2Fe]BF4 (30 mg, 0.11 mmol) were suspended in n-hexane (5 mL). The suspension was stirred overnight at room temperature. During the course of the reaction, the formation of a brown-red microcrystalline solid was observed. The solid was isolated by decantation, washed with n-hexane (5 mL), and subsequently dried in vacuo to yield a brownish red powder (157 mg, 98%). Mp: >250 °C. IR (ATR): ν̃ 3055 (w), 3022 (w), 1599 (w), 1578 (w), 1535 (m), 1458 (w), 1439 (s), 1413 (w), 1298 (m), 1215 (m), 1177 (m), 1128 (m), 1097 (m), 1071 (m), 1049 (m), 1015 (m), 801 (s), 766 (m), 700 (s), 665 (m), 643 (s), 575 (s) cm−1. MS (ESI, acetone, positive scan) m/z (%): 1374.1 (36) [3]+, 1196.2 (100) [3 − Cp2Ti]+, 1019.2 (24) [3 − 2 × Cp2Ti]+, 687.6 (63) [3]2+, 598.5 (17) [3 − Cp2Ti]2+ HRMS: calculated for [C90H66N6Ti3]+, 1374.3787; found, 1374.3804. [(Cp2Ti)3(μ3-HATNPh6)][BF4]2 (4). 2 (150 mg, 0.11 mmol) and [Cp2Fe]BF4 (60 mg, 0.22 mmol) were suspended in a mixture of nhexane (5 mL) and tetrahydrofuran (THF) (2 mL). The suspension was stirred overnight at room temperature. During the course of the reaction the formation of a red-brown precipitate was observed. The solid was isolated by decantation, washed with n-hexane (5 mL), and subsequently dried in vacuo to yield a reddish powder (157 mg, 93%). Mp: >250 °C. IR (ATR): ν̃ 1532 (m), 1478 (m), 1438 (m), 1359 (m), 1303 (m), 1262 (s), 1175 (s), 1125 (m), 1105 (m), 1047 (m), 1009 (s), 914 (m), 802 (s), 766 (s), 748 (m), 695 (s), 663 (m), 634 (s), 587 (m), 573 (s) cm−1. MS (ESI, acetone, positive scan) m/z (%): 1374.4 (5) [4]+, 1196.4 (11), [4 − Cp2Ti]+, 1018.4 (14) [4 − 2 × Cp2Ti]+, 687.2 (100) [4]2+, 598.2 (20) [4 − Cp2Ti]2+ HRMS: calculated for [C90H66N6Ti3]2+, 687.1888; found, 687.1882. [(Cp2Ti)3(μ3-HATNPh6)][BF4]3 (5). 2 (150 mg, 0.11 mmol) and [Cp2Fe][BF4] (90 mg, 0.33 mmol) were suspended in a mixture of nhexane (2 mL) and THF (5 mL). The suspension was stirred overnight at room temperature. During the course of the reaction the formation of a dark red precipitate was observed. The solid was isolated by decantation, washed with n-hexane (5 mL), and subsequently dried in vacuo to yield a red powder (178 mg, 99%). Mp: >250 °C. IR (ATR): ν̃ 3101 (w), 1538 (m), 1504 (w), 1472 (m), 1440 (m), 1362 (m), 1310 (m), 1262 (m), 1182 (m), 1051 (s), 1017 (s), 957 (w), 815 (s), 770 (m), 700 (s), 665 (w), 639 (m), 575 (m), 555 (w) cm−1. MS (ESI, acetone, positive scan) m/z (%): 1196.2 (16) [5 − Cp2Ti+]+, 1018.2 (15), [5 − Cp2Ti+ − Cp2Ti]+, 687.6 (100) [5]2+, 598.1 (79) [5 − 2 × Cp2Ti]2+.

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derivative Cp2Ti(η2-C2(SiMe3)2),30 or the direct coordination of the titanocene(II) fragment to the HATNPh6 ligand 1 leads to the highly reactive and sensitive complex (Cp2Ti)3(μ3HATNPh6) (2) (Scheme 4). Compound 2 is very sensitive to air and moisture. It shows good solubility in aromatic solvents, which was the desired feature in comparison to the previously known but insoluble complex IIIb.5 Because of the increased solubility, further characterization of this mixed-valent complex is possible. Crystals suitable for X-ray diffraction were only obtained directly from the reaction mixture of method B. The proof that methods A and B lead to the identical complex 2 was obtained by ESI mass spectrometry, in which the reaction products of methods A and B exhibit a molecular peak at m/z 1374.8 with a typical fragmentation by an abstraction of one phenyl substituent (m/z 1298) or one titanocene unit (m/z 1196 [(2 − Cp2Ti)]+).31 2 crystallizes in the orthorhombic space group C2221 with five molecules of toluene. The molecular structure is shown in Figure 1. The HATN ligand of 2 is nearly planar with only a slight deviation of the outer annulated benzene rings from the mean plane (3.79 and 3.66°). As is known from IIIb,5 the Nheterocycle is reduced, yielding a low valent N,N′-chelating titanium complex. The titanium centers are lying in plane with the ligand. The bond lengths in the central aromatic ring are shortened and more balanced (1.412(3)−1.434(5) Å) than in the free HATNPh6 ligand 1 (conjugated bonds average: 1.430 and 1.474 Å).27 The two mesomeric forms C and D are conceivable (Scheme 5). Elongated C−N distances in 2 (1.349(3)−1.389(3) Å) in comparison to those in the free ligand 1 (1.323−1.359 Å)27 indicate contributions from the mesomeric form C. The Ti−N distances (2.173(4)-2.204(3) Å) lie in the expected range for titanium-coordinated Nheterocycles las in IIIa,5 indicating contributions from the mesomeric form D. Synthesis of (Cp2Ti)3(μ3-HATNPh6)n+ (n = 1−3; 3−5). As expected from the analogous HATNMe6 titanium complex (IIIb),24 oxidation of 2 should be possible by ferrocenium salts (Scheme 6). Ferrocenium salts are in this case the best oneelectron oxidizing agents because the consumed reagent (ferrocene) is easily separated. The color does not change significantly during the reaction of complex 2 with [Cp2Fe]BF4 in n-hexane or n-hexane/THF mixtures. The UV−vis spectra of 2 and its oxidation products 3−5 are very similar, being distinguished only by a progressive shift of the wavelength of maximum absorbance to lower wavenumbers (2, 540 nm; 3, 500 nm; 4, 480 nm; 5, 440 nm). The cation 3 is hardly soluble

RESULTS AND DISCUSSION

Synthesis of (Cp2Ti)3(μ3-HATNPh6). The dehydrogenative coupling of 6,7-diphenylquinoxaline in the presence of highly reactive and reductive titanocene(II) fragments generated from the corresponding bis(trimethylsilyl)acetylene C


Article

Inorganic Chemistry

Scheme 6. : Chemical Oxidation of Complex 2 to the Oxidation Products 3−5

Figure 1. Molecular structure of 2 (ORTEP-like plot with thermal ellipsoids at the 50% probability level, hydrogen atoms omitted for clarity). Selected bond lengths (Å): Ti1−N1 2.182(2), Ti2−N2 2.173(2), Ti2−N3 2.203(2), N1−C34 1.349(3), N1−C16 1.389(3), N2−C35 1.362(3), N2−C21 1.382(3), N3−C36 1.360(3), N3−C37 1.388(3), C34−C35 1.412(3), C34−C34′ 1.434(5) C35−C36 1.417(3), C36−C36 1.418(5), C37−C37 1.428(5), C37−C38 1.407(3), C38−C39 1.390(3), C39−C39 1.422(5), C16−C21 1.424(3), C17−C18 1.393(4), C18−C19 1.418(4), C19−C20 1.392(3), C20−C21 1.404(3).

Reacting 2 with 3 equiv of [Cp2Fe]BF4 finally yields the third oxidation product [(Cp2Ti)3(μ3-HATNPh6)]3+[BF4−]3 (5) as a red powder which is hardly soluble in THF. No mass peak can be identified in ESI-MS. Crystals for single-crystal Xray diffraction were obtained by diffusion of n-hexane into a solution of 5 in acetone. 5 crystallizes with one molecule acetone in the monoclinic space group P21/n (Figure 4). On comparison of the solid-state structures of the neutral complex 2 and its oxidation products, some crystallographic parameters change upon oxidation while the positions of the titanium centers with respect to the ligand do not change and are still in plane with the ligand. Because of a higher charge on the titanium centers and a resulting weaker back-bonding, the average Ti−N distance increases slightly with increasing oxidation (from 2.186 Å for 2 to 2.223 Å for 5; Table 1). On the other hand, the distance Ti−Cpcentroid distance decreases upon oxidation (from 2.074 Å for 2 to 2.048 Å for 5), indicating a stronger bonding of the Ti centers to the Cp ligands. The C−C distances in the central ring decrease and equalize when complexation occurs in complex 2 due to the delocalization of an extra electron. This renders the central ring a more conjugated system than in the free ligand 1.27 In the oxidized product 3 the central ring shows three short (average 1.411 Å) and three long (average 1.438 Å) C−C bonds. They are still shorter than in the free ligand 1 (average 1.430 and 1.474 Å).27 The oxidation products 4 and 5 show similar distances in the central ring (4, 1.410 and 1.444 Å; 5, 1.416 and 1.446 Å). In comparison to the known complexes IIIa5 and IIIb,24 all discussed bond lengths of 2−5 lie in the range of the values reported previously. Only the Ti−N distances differ between 2 and IIIb upon oxidation. The greatest difference is found between the third oxidation products of 2 and IIIb (2.223 Å for 5 and 2.256 Å for the third oxidation product of IIIb24). Another interesting feature of the molecular structures of 3− 5 is the position of the counteranions BF4− in the solid state. They are located above and/or below the central ring of the HATNPh6 ligand (Figure 5). The distance between the boron atom of the anion and the ring centroid of the central ring

Scheme 5. : Mesomeric forms C and D of (Cp2Ti)3(μ3HATNPh6) (2)

in aliphatic or aromatic solvents but moderately soluble in THF. It seems to be stable against air and moisture and has a melting point above 250 °C. Also here a mass peak and fragmentation is found via ESI-MS. Single crystals were obtained by diffusion of n-hexane into an acetone solution of 3. The cation crystallizes with three molecules of acetone in the monoclinic space group P21/n. The molecular structure is shown in Figure 2. The doubly oxidized product [(Cp2Ti)3(μ3HATNPh6)]2+[BF4−]2 (4) is obtained when 2 is reacted with 2 equiv of [Cp2Fe]BF4. The solubility of 4 is lower than that of the singly charged cation 3. In ESI-MS spectra the molecular peak and characteristic fragmentation is observed (MS m/z: 1374.4 [4]+, 1196.4 [4 − Cp2Ti]+, 1018.4 [4 − 2 × Cp2Ti]+, 687.2 [4]2+, 598.2 [4 − Cp2Ti]2+). Dark red crystals of 4 can be obtained by diffusion of n-hexane into a THF solution of 4. 4 crystallizes with six molecules of THF in the orthorhombic space group Fdd2 (Figure 3) with disordered BF4 anions. D


Article

Inorganic Chemistry

Figure 2. Molecular structure of 3 (ORTEP-like plot with thermal ellipsoids at the 50% probability level, hydrogen atoms and cyclopentadienyl rings omitted for clarity). Selected bond lengths (Å): Ti1−N6 2.199(2), Ti1−N1 2.205(2), Ti2−N2 2.205(2), Ti2−N3 2.225(2), Ti3−N4 2.191(2), Ti3−N5 2.192(2), N1−C31 1.342(3), N1−C37 1.391(3), N2−C32 1.346(3), N2−C42 1.394(3), N3−C33 1.350(3), N3−C55 1.398(3), N4−C34 1.339(3), N4−C60 1.387(3), N5−C35 1.347(3), N5−C73 1.389(3), N6−C36 1.339(3), N6−C78 1.395(3), C31−C32 1.413(3), C32−C33 1.435(3), C33−C34 1.410(3), C34−C35 1.439(3), C35−C36 1.409(3), C31−C36 1.439(3), C37−C38 1.397(3), C38−C39 1.389(3), C39−C40 1.414(3), C40−C41 1.395(3), C41−C42 1.394(3), C42−C37 1.416(3), C55−C56 1.399(3), C56−C57 1.394(3), C57−C58 1.420(3), C58−C59 1.381(3), C59−C60 1.399(3), C60−C55 1.416(3), C73−C74 1.403(3), C74−C75 1.390(3), C75−C76 1.416(3), C76−C77 1.392(3), C78−C73 1.419(3). The shortest contact between BF4− and the centroid of the central π-ring system is 3.842 Å.

well separated, while the couples 4+/5+ and 5+/6+ overlap slightly. In analogy to the reduction steps, also the half-wave potentials are lower in comparison to those of IIIb.24 2 is therefore more stable than IIIb. To the best of our knowledge, the nine reversible redox steps of 2 are unprecedented. For mixed-valent systems the comproportionation constant Kc provides information about the electronic interaction between the metals centers.39 The Kc values,31 calculated according to eq 2 in the Supporting Information, for the mixed-valent redox states −, 0, +, 2+, 4+, and 5+ are shown in Table 3. The oxidation and reduction products of 2 have comproportionation constants Kc in the range of 1.25 × 103 to 1.41 × 1010 (Table 3). In order to classify the different oxidation states according to the strength of the electronic communication between the metal centers, the classification of Robin and Day is used.40 For 1−, 0 and 1+, high Kc values of 10 9 −10 10 can be calculated. This indicates a strong delocalization between the metal centers corresponding to Robin−Day class III. For 2+, 4+, and 5+, the delocalization is lower (Kc value between 106 and 103) indicating Robin−Day class II. In contrast to IIIb, no oxidation state involves localized charges on metal centers without electronic communication between them.24 In the case of the trinuclear ruthenium HATN complexes, Kc values between 105 and 108 were found.41,42 Further confirmation of this Robin−Day classification is presented in the following.

decreases upon oxidation from 3.842 Å for 3 to 3.432 Å for 5. Three of the four fluorine atoms are involved in the interaction, where the shortest distance between a fluorine atom and the ligand decreased (3.002 Å for 3, 2.909 Å for 4, and 2.722 Å for 5). The interaction between anion and πacidic aromatic systems are known as “anion−π interactions”.32,33 The distances between a fluorine atom and the aromatic central π systems are in the expected range known from the literature34,35 and agree with the values reported for the HATNMe6 titanium complex.24 Electrochemistry. Complex 2 shows multiple electron transfer reactions, as is known for other complexes with HATN ligands17,24,36 and expected from the chemical synthesis of 3− 5. The cyclic voltammogram (CV) of 2 in THF with 0.2 M [nBu4N]PF6 as supporting electrolyte shows three reversible reduction waves and six reversible oxidation waves (Figure 6). The half-wave potentials extracted from these voltammograms are shown in Table 2 and are compared to those of the HATNMe6 complex IIIb.24 The three reduction waves can be described as well-separated one-electron-reduction steps. In comparison to IIIb,24 one additional redox step is observed and the half-wave potential is lower (0.19 V for 0/1+ and 0.13 V lower for 0/1−). A third reduction step is not so unusual in this field of HAT or HATN complexes: for instance, the HAT(CN)6 ligand37 or the trinuclear ruthenium complex with a HAT ligand38 also show a third reduction step. The six oxidation waves of 2 are all reversible, one-electronoxidation steps. Until the 3+/4+ pair, the oxidation waves are E


Article

Inorganic Chemistry

Figure 3. Molecular structure of 4 (ORTEP-like plot with thermal ellipsoids at the 50% probability level, hydrogen atoms and cyclopentadienyl rings omitted for clarity). Selected bond lengths (Å): Ti1−N1 2.225(3), Ti2−N2 2.219(3), Ti2−N3 2.210(3), N1−C16 1.329(4), N1−C19 1.379(5), N2−C17 1.332(5), N2−C24 1.385(4), N3−C18 1.337(4), N3−C37 1.385(4), C16−C17 1.411(5), C17−C18 1.443(5), C16−C16′ 1.446(7), C18−C18′ 1.407(6), C19−C20 1.400(5), C20−C21 1.372 (6), C21−C22 1.416(6), C22−C23 1.383(5), C23−C24 1.402(5), C19− C24 1.419(5), C37−C38 1.394(5), C38−C39 1.380(5), C37−C37′ 1.418(7), C39−C39′ 1.433(6). The shortest contact between BF4− and the centroid of the central π-ring system is 3.462 Å.

Figure 4. Molecular structure of 5 (ORTEP-like plot with thermal ellipsoids at the 50% probability level, hydrogen atoms and cyclopentadienyl rings omitted for clarity). Selected bond lengths (Å): Ti1−N1 2.235(2), Ti1−N6 2.227(3), Ti2−N2 2.233(3), Ti2−N3 2.213(3), Ti3−N4 2.206(3), Ti3−N5 2.226(3), N1−C31 1.328(4), N1−C37 1.380(4), N2−C32 1.339(4), N2−C42 1.378(4), N3−C33 1.338(4), N3−C55 1.373(4), C31−C36 1.451(4), C31−C32 1.414(4), C32−C33 1.439(4), C33−C34 1.419(4), C34−C35 1.450(4), C35−C36 1.414 (5), C37−C38 1.401(5), C38−C39 1.381(4), C39−C40 1.426(4), C40−C41 1.376(5), C41−C42 1.410(4), C37−C42 1.416(4), C55−C56 1.403(4), C56−C57 1.377(5), C57−C58 1.423(5), C58−C59 1.377(4), C59−C60 1.403(5), C60−C55 1.421(4), C73−C74 1.409(4), C74−C75 1.376(4), C75−C76 1.427(5), C76−C77 1.376(5), C77−C78 1.401(4), C78−C73 1.419(5). The shortest contact between BF4− and the centroid of the central π-ring system is 3.432 Å.

F


Article

Inorganic Chemistry Table 1. Selected Average Bond Lengths of 2−5 Ti−N (Å) B- - -Ccentral,centroid (Å)a Ti−Cpcentroid (Å) C−C distances central ring (Å)

2

3

4

5

2.074 1.412−1.434

2.203 3.842 2.068 1.410−1.440

2.218 3.462 2.056 1.408−1.446

2.223 3.501/3.432 2.048 1.414−1.449

2.186

a

Figure 5. Anion−π interaction of BF4− and the HATNPh6 ligand in 4.

Table 2. Electrochemical Data for Complexes 2 and IIIba E1/2 value (V)

a

redox couple

2

IIIb24

3−/2− 2−/1− 1−/0 0/1+ 1+/2+ 2+/3+ 3+/4+ 4+/5+ 5+/6+

−2.66 −2.19 −1.61 −1.08 −0.49 −0.12 0.06 0.24 0.49

−2.21 −1.74 −1.27 −0.54 −0.30 0.11 0.18 0.40

Solvent: THF, 0.3 mM.

Table 3. Comproportionation constants Kc of the mixed valent states of 2

Figure 6. CV of 0.3 mM 2 at a gold electrode with a scan rate of 0.5 V/s in 0.2 M solutions of [nBu4N]PF6 in THF.

Spectroscopy. Typical for mixed-valent systems are electron transitions from the metal in the lower oxidation state to the metal in the higher oxidation state. This intervalence charge transfer (IVCT) takes place in the nearinfrared (NIR) range and assists in classification.39 The NIR spectra of 2−5 are shown in Figure 7. They confirm the assumption for a radical anionic ligand in the neutral complex 2 and agree with the corresponding results for IIIb.24 The

charge

ΔE1/2 (V)

− 0 + 2+ 4+ 5+

0.58 0.53 0.59 0.37 0.18 0.25

Kc 9.47 1.31 1.41 2.31 1.25 2.00

× × × × × ×

109 109 1010 106 103 104

π−π* transition of this radical gives rise to an absorption band at around 1000 nm for 2 appearing like a doublet that has already been observed for the HAT(CN)6 radical anion.37 By formation of a radical anionic ligand, a mixed-valent system with two TiII centers and one TiIII center is created. This is in accordance with the formation of (Cp2Ti)-2,2′-bipy G


Article

Inorganic Chemistry

Scheme 7. : Schematic Representation of the Electronic States of the Trinuclear Complexes 2−5

trivalent cation is formed. The first oxidation, however, is the most interesting one, since either a diamagnetic TiII can be oxidized to form a paramagnetic TiIII cation or the ligand can be oxidized, hence losing the radical electron. This oxidation step can be easily traced because losing the radical electron results in a reduced effective magnetic moment while oxidizing the TiII would result in an increased effective magnetic moment. Figure 8 depicts the temperature dependence of the effective magnetic moments μeff for 2−5. For 2 the μeff value increases

Figure 7. NIR spectra of 2−5 in THF, concentration 0.1 mM.

complexes.43,44 The typical IVCT band for 2 is found at 2100 nm, comparable also to that of the trinuclear ruthenium HAT complexes.41 The small absorption bands around 1700 and 1400 nm are metal to ligand charge transfer (MLCT) bands. After the first oxidation, measurements of complex 3 show a new doublet at around 1300 nm while the typical band for organic radicals at around 1000 nm disappeared. Most likely the first band at 1200 nm results from a MLCT and the second at around 1400 nm from an IVCT. For the second oxidation product 4, a broad IVCT band is observed between 1500 and 2400 nm. As expected, complex 5 contains three TiIII centers and no IVCT band or π−π* transitions are found. The shape of the IVCT band serves as an indicator for classification of delocalization. With increasing degree of delocalization, the IVCT bands become sharper with increased peak intensities.45,46 For further consideration only complex 2 is of interest, as the resolution of the IVCT bands for complexes 3 and 4 is insufficient. The Hush formula for trinuclear systems47 was used for the calculation of the coupling constant HTi(II)−Ti(III) (shown in eq 3 in Supporting Information).31 Table 4 summarizes the spectral parameters calculated from the experimental NIR spectra.

Figure 8. Temperature dependence of the effective magnetic moments of compounds 2−5.

with increasing temperature and reaches a value of μeff = 3.02 μB at 300 K. The plateau at low temperatures indicates weak antiferromagnetic couplings between the anionic radical and the TiIII center, in line with the value of μeff, which is close to 2.83 μB for a system with two coupled electrons. This behavior is also shown for other N-heterocycle-bridged titanium complexes, e.g. with 2,2′-bipyridine, such as (Cp2Ti)(2,2′bipy)43,44,48 and (Cp*Ti(μ-Cl)(2,2′-bipy))2.49 Upon oxidation to 3, the μeff value significantly drops to μeff = 1.66 μB at 300 K, almost perfectly in line with one unpaired electron (1.73 μB); hence, the ligand is oxidized. 4 and 5 exhibit two (2 × TiIII, 1 × TiII) and three (3 × TiIII) unpaired electrons. Therefore, the suggested coupling found in the NIR spectra (IVCT, Robin− Day class II) is not observed in the magnetic measurements. Subsequently, the expected effective magnetic moments are μcalc = 2.45 μB for 4 and μcalc = 3.00 μB for 5. The experimental moments at 300 K are μeff = 2.21 μB (4) and μeff = 2.82 μB (5) and are therefore perfectly in line with the expected values. Scheme 7 summarizes the resulting electronic structures of 2− 5.

Table 4. NIR Spectroscopic Data of Complex 2 νmax (cm−1)

ν1/2 (cm−1)

εmax (dm3 mol−1 cm−1)

dTi−Ti (Å)

HTi(II)−Ti(III) (cm−1)

4771

1151

3111

7.248

261

Because 0 > 2HTi(II)−Ti(III) > νmax, a delocalization between the metal centers but no full delocalization is found, which confirms the assignment as Robin−Day class II. Magnetism. In order to investigate the oxidation processes in more detail, 2−5 were investigated via magnetic susceptibility measurements. The magnetic susceptibilities were measured with an external field of H = 1 T in the temperature range of 3−300 K (Figures S9−S12, top panel). Furthermore, the compounds were investigated by means of zero-field-cooled/field-cooled routines (ZFC/FC) between 2.5 and 100 K (Figures S9−S11, middle panel) and by magnetization isotherms recorded at 3, 10, and 50 K (Figures S9 and S10, bottom panel). From the obtained magnetic data, the temperature-dependent, effective magnetic moments were calculated. As shown in Scheme 7, 2 can be oxidized until the

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CONCLUSION We investigated the electronic structure and redox behavior of (Cp2Ti)3(μ3-HATNPh6) and its cationic titanocene complexes. The complex (Cp2Ti)3(μ3-HATNPh6) can be synthesized either by dehydrogenative coupling of diphenylquinoxaH


Article

Inorganic Chemistry

(4) Kraft, S.; Beckhaus, R.; Haase, D.; Saak, W. Directed Reduction of Six-Membered Nitrogen HeterocyclesSelective Formation of Polynuclear Titanium Complexes. Angew. Chem., Int. Ed. 2004, 43, 1583−1587. (5) Piglosiewicz, I. M.; Beckhaus, R.; Saak, W.; Haase, D. Dehydroaromatization of Quinoxalines: One-Step Syntheses of Trinuclear 1,6,7,12,13,18-Hexaazatrinaphthylene Titanium Complexes. J. Am. Chem. Soc. 2005, 127, 14190−14191. (6) Chakrabarty, R.; Mukherjee, P. S.; Stang, P. J. Supramolecular Coordination: Self-Assembly of Finite Two- and Three-Dimensional Ensembles. Chem. Rev. 2011, 111, 6810−6918. (7) Cook, T. R.; Zheng, Y.-R.; Stang, P. J. Metal−Organic Frameworks and Self-Assembled Supramolecular Coordination Complexes: Comparing and Contrasting the Design, Synthesis, and Functionality of Metal−Organic Materials. Chem. Rev. 2013, 113, 734−777. (8) Kraft, S.; Hanuschek, E.; Beckhaus, R.; Haase, D.; Saak, W. Titanium-Based Molecular Squares and Rectangles: Syntheses by SelfAssembly Reactions of Titanocene Fragments and Aromatic NHeterocycles. Chem. - Eur. J. 2005, 11, 969−978. (9) Albrecht, C.; Krüger, T.; Wagner, C.; Rüffer, T.; Lang, H.; Steinborn, D. Zur Reaktivität von Titanocenkomplexen [Ti(Cp′)2(η2Me3SiC≡CSiMe3)] (Cp′ = Cp, Cp*) gegenüber Benzoldicarbonsäuren. Z. Anorg. Allg. Chem. 2008, 634, 2495−2503. (10) Schinnerling, P.; Thewalt, U. Die Struktur von [Cp2TiIII(CN)]14. J. Organomet. Chem. 1992, 431, 41−45. (11) Haehnel, M.; Ruhmann, M.; Theilmann, O.; Roy, S.; Beweries, T.; Arndt, P.; Spannenberg, A.; Villinger, A.; Jemmis, E. D.; Schulz, A.; Rosenthal, U. Reactions of Titanocene Bis(trimethylsilyl)acetylene Complexes with Carbodiimides: An Experimental and Theoretical Study of Complexation versus C−N Bond Activation. J. Am. Chem. Soc. 2012, 134, 15979−15991. (12) Comba, P.; Lampeka, Y. D.; Lötzbeyer, L.; Prikhod’ko, A. I. Macrocyclic Melamine-Based Ligand Complexes as Building Blocks for the Metal-Directed Synthesis of Heterometallic Di- and Trinuclear Compounds. Eur. J. Inorg. Chem. 2003, 2003, 34−37. (13) Becker, L.; Arndt, P.; Spannenberg, A.; Jiao, H.; Rosenthal, U. Formation of Tri- and Tetranuclear Titanacycles through Decamethyltitanocene-Mediated Intermolecular C−C Coupling of Dinitriles. Angew. Chem., Int. Ed. 2015, 54, 5523−5526. (14) Tomaschun, G.; Altenburger, K.; Reiß, F.; Becker, L.; Spannenberg, A.; Arndt, P.; Jiao, H.; Rosenthal, U. Group 4 Metallocene Complexes and Cyanopyridines: Coordination or Coupling to Metallacycles. Eur. J. Inorg. Chem. 2016, 2016, 272−280. (15) Segura, J. L.; Juarez, R.; Ramos, M.; Seoane, C. Hexaazatriphenylene (HAT) derivatives: from synthesis to molecular design, self-organization and device applications. Chem. Soc. Rev. 2015, 44, 6850−6885. (16) Kitagawa, S.; Masaoka, S. Metal complexes of hexaazatriphenylene (hat) and its derivativesfrom oligonuclear complexes to coordination polymers. Coord. Chem. Rev. 2003, 246, 73−88. (17) Moilanen, J. O.; Chilton, N. F.; Day, B. M.; Pugh, T.; Layfield, R. A. Strong Exchange Coupling in a Trimetallic Radical-Bridged Cobalt(II)-Hexaazatrinaphthylene Complex. Angew. Chem., Int. Ed. 2016, 55, 5521−5525. (18) Moilanen, J. O.; Day, B. M.; Pugh, T.; Layfield, R. A. Openshell doublet character in a hexaazatrinaphthylene trianion complex. Chem. Commun. 2015, 51, 11478−11481. (19) Roy, S.; Kubiak, C. P. Tricarbonylrhenium(i) complexes of highly symmetric hexaazatrinaphthylene ligands (HATN): structural, electrochemical and spectroscopic properties. Dalton Trans. 2010, 39, 10937−10943. (20) Sarma, M. S. P.; Czarnik, A. W. Hexadecarboxylative Synthesis of Hexaazatriphenylene. Synthesis 1988, 1988, 72−73. (21) Rogers, D. Z. Improved synthesis of 1,4,5,8,9,12-hexaazatriphenylene. J. Org. Chem. 1986, 51, 3904−3905. (22) Jiang, W.; Li, Y.; Wang, Z. Heteroarenes as high performance organic semiconductors. Chem. Soc. Rev. 2013, 42, 6113−6127.

line and titanocene precursors or by direct coordination of titanocene units to the HATNPh6 ligand. Selective oxidation with ferrocenium salt leads to the oxidation products (Cp2Ti)3(μ3-HATNPh6)n+ (n = 1−3). Single-crystal X-ray diffraction reveals systematic small structural modification upon oxidation. Elongation of Ti−N distances goes along with contraction of Ti−C bonds. The cationic complexes show anion−π interactions between the fluorine atoms of the counterion BF4− and the π-acidic central ring of HATNPh6. Cyclic voltammetry allowed observation of six oxidation and three reduction steps. Calculations of the comproportionation constants Kc show strong electronic metal−metal interactions. Spectroscopic investigations in the near-infrared region confirm IVCT bands characteristic for mixed-valent systems. The calculated coupling constant HTi(II)−Ti(III) supports a classification according to Robin−Day class II.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01728. X-ray diffraction data collection and refinement details, ESI-MS spectra, UV−vis spectra, and magnetic susceptibilities (PDF) Accession Codes

CCDC 1834504−1834507 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*G.W.: e-mail, [email protected]. *R.B.: e-mail, [email protected]; web, https://www.uni-oldenburg.de/ac-beckhaus/. ORCID

Oliver Janka: 0000-0002-9480-3888 Gunther Wittstock: 0000-0002-6884-5515 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M. and P.S. gratefully acknowledge support by stipends through the Graduate Programme “Nanoenergy” of the Ministry for Science and Culture of the State of Lower Saxony.

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REFERENCES

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