Control of Complex Formation through Peripheral Substituents in Click

Article pubs.acs.org/IC

Control of Complex Formation through Peripheral Substituents in Click-Tripodal Ligands: Structural Diversity in Homo- and Heterodinuclear Cobalt-Azido Complexes Michael G. Sommer,† Raphael Marx,‡ David Schweinfurth,† Yvonne Rechkemmer,‡ Petr Neugebauer,‡ Margarethe van der Meer,† Stephan Hohloch,† Serhiy Demeshko,§ Franc Meyer,§ Joris van Slageren,*,‡ and Biprajit Sarkar*,† †

Institut für Chemie und Biochemie, Anorganische Chemie, Fabeckstraße 34-36, D-14195, Berlin, Germany Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569, Stuttgart, Germany § Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammanstraße 4, D-37077, Göttingen, Germany ‡

S Supporting Information *

ABSTRACT: The azide anion is widely used as a ligand in coordination chemistry. Despite its ubiquitous presence, controlled synthesis of azido complexes remains a challenging task. Making use of click-derived tripodal ligands, we present here various coordination motifs of the azido ligands, the formation of which appears to be controlled by the peripheral substituents on the tripodal ligands with otherwise identical structure of the coordination moieties. Thus, the flexible benzyl substituents on the tripodal ligand TBTA led to the formation of the first example of an unsupported and solely μ1,1-azido-bridged dicobalt(II) complex. The more rigid phenyl substituents on the TPTA ligand deliver an unsupported and solely μ1,3-azido-bridged dicobalt(II) complex. Bulky diisopropylphenyl substituents on the TDTA ligand deliver a doubly μ1,1-azido-bridged dicobalt(II) complex. Intriguingly, the mononuclear copper(II) complex [Cu(TBTA)N3]+ is an excellent synthon for generating mixed dinuclear complexes of the form [(TBTA)Co(μ1,1-N3)Cu(TBTA)]3+ or [(TBTA)Cu(μ1,1-N3)Cu(TPTA)]3+, both of which contain a single unsupported μ1,1-N3 as a bridge. To the best of our knowledge, these are also the first examples of mixed dinuclear complexes with a μ1,1-N3 monoazido bridge. All complexes were crystallographically characterized, and selected examples were probed via magnetometry and high-field EPR spectroscopy to elucidate the electronic structures of these complexes and the nature of magnetic coupling in the various azido-bridged complexes. These results thus prove the power of click-tripodal ligands in generating hitherto unknown chemical structures and properties.

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INTRODUCTION

Azides are ubiquitous ligands in coordination chemistry.1−3 The most widely used form to date has been the unsubstituted inorganic azide, N3−.2,4−9 Apart from a basic interest in the bonding of this curious molecule in metal complexes, interest in azide-containing complexes has been largely due to their intriguing magnetic properties,1−16 and the possible use of metal bound azides for transferring an “N” atom to a substrate.17−19 Magneto-chemists have developed various rules for magnetic exchange coupling between azido-bridged paramagnetic metal centers, depending on the binding mode of the bridging azido ligands (selected possible binding modes are shown in Figure 1).1−3,7,20−22 Azide in the μ1,1 end-on coordination mode has been most frequently used as a bridge in combination with metal ions such as Mn(II),23−25 Ni(II),26−28 and Cu(II).29−33 In comparison, metal ions such as Co(II)3 and Fe(II)34−36 have been less investigated in that context (both end-on and end-to-end), © XXXX American Chemical Society

Figure 1. Possible bridging modes of azide in dinuclear complexes.

possibly because of challenging synthetic routes with those metal ions. Examples of dinuclear metal complexes bridged by only a single, unsupported μ-1,1 (end-on) azide ligand are extremely rare: To the best of our knowledge, the only known examples of such transition-metal complexes are with Cu(II),32,33 Zn(II),37 Rh(I),38 and Ir(I).38 Examples of dinuclear Received: September 27, 2016

A

DOI: 10.1021/acs.inorgchem.6b02330 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Co(II) complexes that are bridged by only a single μ-1,1 (endon) azide ligand and no additional bridges are not known, even though these metal ions play a prominent role in the investigation of magnetic phenomena including single molecule magnetism.39 Furthermore, the existence of various coordination modes of the azido ligand in a series of metal complexes containing similar coligands is rare.5 One common problem with the synthesis of azido complexes is the lack of synthetic control.5 Thus, targeted syntheses of a series of related metal complexes with a particular number and binding mode of azide ligands are difficult. Additionally, the conversion of a mononuclear metal-azido complex into a targeted azido-bridged homo- or heterodinuclear complex remains a challenging task. Substituted 1,2,3-triazoles derived from the Cu(I) catalyzed “click” reaction40 between azides and alkynes41−44 have been extensively used as ligands in coordination chemistry in recent years.45−73 In that context, we have made use of the tripodal ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBTA (Figure 2), and reported on its Co(II) and Fe(II)

on the ligand backbone on the type and formation of azidobridged metal complexes. Herein, we present the targeted synthesis and structural characterization of the mononuclear complexes [Co(TPTA)(N3)](ClO4), 1, and [Cu(TBTA)N3](ClO4), 2 (this compound has been published recently75), and the homodinuclear complexes [(TBTA)Co(μ 1,1 -N 3 )Co(TBTA)](ClO 4 ) 3 , 3, [(TPTA)Co(μ1,3-N3)Co(TPTA)](ClO4)3, 4, and [(TDTA)Co(μ1,1-N3)2Co(TDTA)](ClO4)2, 5. Furthermore, we also present the mixed dinuclear complexes [(TBTA)Cu(μ1,1N3)Co(TBTA)](ClO4)3, 6, and [(TBTA)Cu(μ1,1-N3)Cu(TPTA)](ClO4)3, 7. The mononuclear cobalt(II) complex [Co(TBTA)(N3)](ClO4) was reported by us previously.66 The influence of the substituents on the tripodal ligands on the formation and type of the azido complexes is investigated. Furthermore, magnetic properties and multifrequency EPR studies on selected metal complexes are presented, with a focus on the type of magnetic exchange coupling mediated by azido bridges in isolated dinuclear complexes.

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RESULTS AND DISCUSSION Syntheses and Crystal Structures. The mononuclear complexes 1 and 2 were synthesized in a one-pot reaction of a 1:1:1 mixture of the respective metal salt, the tripodal ligand, and NaN3 in acetonitrile (see the Experimental Section). The synthesis and the structural characterization of complex 2 have been reported elsewhere,75 and this compound was resynthesized and is discussed here for comparison. Slow diffusion of diethyl ether into the acetonitrile solution resulted in the formation of purple single crystals of 1. In the IR spectra of 1 and 2, the stretching vibration of the azide group was observed at 2057 and 2035 cm−1, respectively (Table 1). For comparison, for the complex [Co(TBTA)(N3)]ClO4, the corresponding azide stretch was observed at 2077 cm−1, showing the influence of the peripheral substituents of the tripodal ligands and packing effects on the stretching frequency of the azide group. On using only half an equivalent of NaN3 in the one-pot reaction with a 1:1 mixture of [Co(H2O)6](ClO4)2 and either TBTA or TPTA, the azido-bridged dinuclear complexes 3 and 4 were isolated in very good yields. The azide stretch appeared at 2091 and 2121 cm−1 for 3 and 4, respectively (Table 1). When using TDTA as the tripodal ligand, product formation was found to be independent of the stoichiometry of the individual components, and the only product that was isolated was the doubly μ1,1-N3− bridged, hexacoordinated (in contrast to pentacoordinated cobalt(II) centers for all other complexes) dicobalt complex 5. The yield of 5 was found to be independent of the stoichiometry of the reactants, thus indicating that a mononuclear complex does not form with the sterically bulky TDTA coligand. As discussed below, a possible explanation for this lies in the operation of noncovalent interactions within the TDTA ligands in 5. The stretching of the azide groups in 5 appeared at 2070 cm−1

Figure 2. Ligands TBTA, TPTA, and TDTA.

complexes that display magnetic bistability,45,64 show metal oxidation state dependent coordination changes,70 and are precatalysts for ethylene polymerization.65 Mononuclear [Co(TBTA)(N3)]+ was reported as well and its electronic structure, particularly with a focus on the ligand field of the N3− ligand, was investigated.66 Additionally, metal complexes of TBTA and related tripodal ligands have been studied for their intriguing electronic structures,74 their photochemical and electrochemical reactivities,46,48 and as oxidation catalysts.67 TBTA has also been used for increasing the efficiency of the copper(I)-catalyzed azide−alkyne cycloaddition reaction.51,55 Varying the benzyl substituents on TBTA and installing the more rigid phenyl substituents such as in tris[(1-phenyl-1H1,2,3-triazol-4-yl)methyl]amine, TPTA, or the more sterically demanding substituents such as 2,6-(diisopropyl)phenyl in tris[(1-(2,6-diisopropylphenyl)-1H-1,2,3-triazol-4-yl)methyl]amine, TDTA (Figure 2) is fairly straightforward. Thus, this series of ligands TBTA, TPTA, and TDTA provides the perfect background for investigating the influence of the substituents

Table 1. Summary of Azide Bond Lengths, Bridging Modes, and Vibration Energies 1 bridging mode d(N2−N3)/Å d(N3−N4)/Å ṽ/cm−1 a

1.164(4) 1.160(4) 2057

2a

3

4

5

6

7

1.204(3) 1.142(3) 2035

μ1,1 1.256(5) 1.169(6) 2091

μ1,3 1.172(4) 1.172(4) 2121

double μ1,1 1.178(3) 1.149(4) 2070

μ1,1 1.230(5) 1.155(6) 2087

μ1,1 1.230(3) 1.138(4) 2087

From ref 75. B


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Inorganic Chemistry

cobalt(II) center in that complex. The Co1−N2−N3 angle between the cobalt ion and the N3− ligand is 124.1(2)°, reflecting the bent coordination of the azido ligand. The coordination environment around the copper(II) center in 2 and the trends in the metal ligand bond lengths are similar to those observed in 1. However, the Cu1−N1 bond distance of 2.120(2) Å to the amine-N donor of the TBTA ligand in complex 2 is shorter than the corresponding distance in complex 1. The displacement of the Cu1 ion from the trigonal plane formed by the triazole-N donors is 0.376(1) Å. The Cu1−N2−N3 angle in 2 is 120.1(2)°, reflecting the bent coordination of the azido ligand to the copper(II) center. The N2−N3 distances (note that the numbering refers to the numbering in the molecular structure in the crystal, and not to the nomenclature used in numbering the azido ligand) within the azido ligand in 2 (1.204(3) Å) are longer than the corresponding distances in 1 (1.164(4) Å, Table 2), and this trend fits with the data on the azide stretching frequency obtained from IR spectroscopy, as a longer N2−N3 distance leads to lower stretching frequency (Table 1, see above). The dicobalt(II) complexes 3 and 4 crystallize in the monoclinic space groups P21/c and C2/c, respectively. In both complexes, the cobalt(II) centers are pentacoordinated and the coordination geometry is best described as trigonal bipyramidal (Figure 4). In both cases, the cobalt(II) ions are bridged by a single unsupported azido bridge. However, the binding mode of the azido bridge is different in the two complexes. While the azido ligand binds in a μ1,1-N3− fashion to the cobalt(II) centers in 3, its bridging mode is μ1,3-N3− in complex 4 (Figure 4). Similar to the mononuclear complex 1, in the dicobalt(II) complexes 3 and 4, the trigonal plane around the cobalt(II) centers is occupied by the three triazole-N donors, with the amine- and the azido-N donors taking up the axial positions. The metal ligand bond lengths in 3 and 4 and their trends are similar to what has been described for complex 1 (Tables 2, S5−S8) above. The Co1 and Co2 centers in 3 are displaced from the trigonal plane formed by the triazole-N donors by 0.517 and 0.490 Å, respectively. The corresponding values in 4 are 0.501 and 0.515 Å, respectively. The metal−ligand bond lengths in both 3 and 4 point to the existence of two HS cobalt(II) centers in each of these complexes. The Co1−Co2 distances in 3 and 4 are 3.489 and 5.698 Å, respectively, with the μ1,1-N3− coordination of the azido bridge in 3 allowing the cobalt(II) centers to approach each other more closely. The N2−N3 distance within the azido ligand is 1.256(5) and 1.172(2) Å in 3 and 4, respectively (Table 2). These data thus show a higher activation of the azido ligand (weakening of the N2−N3 bond) in the μ1,1-N3− bridging mode in 3 in comparison to the μ1,3-N3− bridging mode in 4, and corroborate the observations made based on azide stretching frequency from IR spectroscopy (see above). 3 and 4 thus represent rare examples of dicobalt(II) complexes where the cobalt centers are bridged solely by an unsupported azido ligand. Furthermore, to the best of our knowledge, 3 represents the first example of a dicobalt(II) complex bridged by a single, unsupported azido ligand in the μ1,1-N3− bridging mode. It is also remarkable that the peripheral substituents on the tripodal ligands TBTA and TPTA, which have otherwise identical geometries at the coordinating moieties, dictate the bridging mode of the azido bridge in these dinuclear complexes. The dicobalt(II) complex 5 crystallizes in the triclinic P1̅ space group. In contrast to 3 and 4, in complex 5, which contains the TDTA ligand with bulky substituents, the

(Table 1). From the values of the azide stretching frequency observed for complexes 1−5, it is tempting to correlate bond strengths within the azide group with its binding to one or two metal centers. However, finding a trend in that direction is difficult as the azide stretching frequency in mononuclear complexes is also dependent on the angle between the metal center and the azido ligand. Within the dinuclear complexes, it can be clearly seen that the μ1,1-N3− bridging mode leads to a higher activation (weakening of N2−N3 bond) of the azide group in comparison to the μ1,3-N3− bridging mode. Intriguingly, the mononuclear copper(II) complex 2 turned out to be an excellent precursor for the generation of azidobridged dinuclear complexes. Thus, the reaction of 2, [Co(H2O)6](ClO4)2, and TBTA in a 1:1:1 ratio delivered the unsupported and solely μ1,1-N3− bridged Cu/Co heterodinuclear complex 6. On the other hand, the reaction of 2, [Cu(H2O)6](ClO4)2, and TPTA also in a 1:1:1 ratio delivered the μ1,1-N3− bridged dicopper(II) complex 7 where the copper(II) centers are each coordinated to a different tripodal coligand. 7 was synthesized to further support that 2 is indeed a good precursor for generating mixed dinuclear complexes where the metal centers are bridged solely by an unsupported μ1,1-N3− bridge (see discussion on crystal structures below). For both 6 and 7, the azide stretching frequency is at 2087 cm−1, a value that is in the same range as for 3 (Table 1). The complexes 1−7 were characterized by mass spectrometry, and their purity was checked through elemental analyses (see the Experimental Section). Additionally, all the complexes were characterized by single-crystal X-ray diffraction. The mononuclear complex 1 crystallizes in the monoclinic space group P21/c. The cobalt(II) ion in 1 and the copper(II) ion in 2 are pentacoordinated, and the coordination geometry is best described as trigonal bipyramidal (Figure 3). The

Figure 3. ORTEP of 1. Ellipsoids are shown at 50% probability. Hydrogen atoms, counteranions, and solvent molecules have been omitted for clarity.

structural parameter for penta-coordinate complexes,76 τ, is 1.03 for 1 and 0.91 for 2. The triazole-N donors from the tripodal ligand make up the trigonal plane, with the amine-N donor from the tripodal ligand, and the azido-N donor taking up the axial positions. For 1, the Co1−N2 bond distance to the azido ligand is 1.995(3) Å and is the shortest metal ligand bond in that complex (Table 2). The Co1−N1 bond distance to the amine donor of the TPTA ligand is 2.433(5) Å and is the longest metal ligand bond length. The distances between the cobalt center and the triazole-N donor are all close to about 2 Å (Table 2 and the S1). The Co1 center is displaced by a distance of 0.573(1) Å from the trigonal plane formed by the triazole-N donors in the direction of the azido ligand. The metal ligand bond distances in 1 point to the existence of a high-spin (HS) C


Article

Inorganic Chemistry Table 2. Selected Bond Lengths in Å

a

atoms

1 (M = Co1)

2a (M = Cu1)

3 (M = Co1/Co2)

4 (M = Co1/Co2)

5 (M = Co1)

M−N1 M−N2 M−N10 M−N20 M−N30 N2−N3 N3−N4 M−N2/N4 M−N40 M−N50 M−N60 M−N5 metal−metal

2.433(5) 1.995(3) 2.037(3) 2.011(3) 2.029(3) 1.164(4) 1.160(4)

2.120(2) 1.935(2) 2.037(2) 2.045(2) 2.088(2) 1.204(3) 1.142(3)

2.355(3) 1.996(3) 2.033(3) 2.023(3) 2.024(3) 1.256(5) 1.169(6) 2.030(3) 2.031(3) 2.017(3) 2.047(3) 2.298(3) 3.489

2.366(3) 1.997(3) 2.037(3) 2.015(3) 2.019(3) 1.172(4) 1.172(4) 1.990(3) 2.033(3) 2.033(3) 2.029(3) 2.339(3) 5.698(1)

2.296(2) 2.081(3)/2.118(2) 2.091(2) 2.083(2) 2.125(2) 1.178(3) 1.149(4)

3.168(2)

6 (M = Co1 or Cu1) 7 (M = Cu1 or Cu2) 2.164(3) 1.996(3) 2.075(3) 2.039(3) 2.026(3) 1.230(5) 1.155(6) 1.991(3) 1.998(3) 2.039(3) 2.021(3) 2.245(3) 3.458(1)

2.114(2) 1.968(2) 2.025(2) 2.103(2) 1.978(2) 1.230(3) 1.138(4) 1.957(2) 1.990(2) 2.028(2) 2.111(2) 2.116(2) 3.453(1)

From ref 75.

Figure 5. ORTEP of 5 (top). Ellipsoids are shown at 50% probability. Hydrogen atoms and counterions have been omitted for clarity. Indication of weak intramolecular interactions (bottom).

other coordination positions are taken up by two μ1,1-N3− bridging azido ligands. Additionally, the third triazole arm of each of the TDTA ligands coordinates to a different cobalt center, with the TDTA ligands bridging the two cobalt ions in complex 5 (Figure 5). A similar bridging mode of a clickderived tripodal ligand was previously observed in a dicopper(I) complex.55 The trends within the metal−ligand bond lengths in 5 are mostly similar to those observed for 3 and 4 (Tables 2 and S9). The only significant difference is in the cobalt−N(azido) bond distance. In 5, the Co1−N2(azido) bond lengths are 2.118(2) Å and 2.081(3) Å, and thus longer than the corresponding bond lengths observed in 3 and 4 (Tables 2, S5, and S7). The double μ1,1-N3− bridging mode forces the cobalt(II) centers to go apart from the azido ligand and is a likely explanation for the longer Co−N(azido) bond lengths in 5. In addition, the higher coordination number of 6

Figure 4. ORTEPs of 3 (top) and 4 (bottom). Ellipsoids are shown at 50% probability. Hydrogen atoms, counterions, and solvent molecules have been omitted for clarity.

cobalt(II) centers are hexacoordinated with the coordination geometry best described as distorted octahedral (Figure 5). The molecular structure observed in the crystal is centrosymmetric. The cobalt ions are each coordinated via two triazole-N donors and an amine-N donor from a particular TDTA ligand. Two D


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Inorganic Chemistry in 5 can also explain the longer bond lengths. The Co−Co distance in 5 is 3.168(2) Å. While the steric bulk of the peripheral substituents could be a possible explanation for the different bridging modes observed in 3 and 5, the same factors do not explain the observance of hexacoordinated cobalt(II) centers in 5 as well as the difference in the bridging mode of the azido ligand in 4 and 5. A likely explanation for the two aforementioned differences lies in the observation of weak intramolecular noncovalent interactions that are observed in complex 5 (Figure 5). The 6-fold coordination, which is achieved through TDTA acting as a bridge between the cobalt ions, makes C−H···π interactions between the methyl groups of one TDTA ligand and the phenyl groups of the second TDTA ligand possible. Both the double μ1,1-N3− bridging mode and the functioning of TDTA as a bridge are factors that would make such dispersive interactions favorable. Hence, it is seen that a change in the peripheral substituents on the tripodal ligands can influence both the coordination number at the cobalt(II) centers, and the mode as well as number of azido bridges in these complexes. The heterodinuclear complex 6 crystallizes in the monoclinic P21/c space group. Both the cobalt(II) and the copper(II) ions in 6 are pentacoordinated, and the coordination geometry around each of the ions is best described as trigonal bipyramidal (Figure 6). In fact, the coordination around the metal centers is very similar to those observed in the mononuclear complexes 1 and 2 as well as the dicobalt(II) complex 3. The trends in the metal−ligand bond lengths are similar to those observed in the aforementioned complexes (Tables 2 and S11). The Cu1− N1(amine) bond length is 2.164(3) Å, and the Co1− N5(amine) distance is 2.245(3) Å. The longer bond distance between the cobalt(II) ion and the amine-N donor as compared to the corresponding distance between the copper(II) ion and the amine-N donor fits well with the trends observed in the mononuclear complexes 1 and 2 and provides evidence for the heterodinuclear nature of complex 6. The cobalt(II) and copper(II) centers in 6 are bridged by a single unsupported μ1,1-N3− bridge. To the best of our knowledge, this is the first example of such an azido-bridged heterodinculear complex. It should be noted that a tetranuclear complex has been reported in which a Cu(II) and a Mn(II) center are bridged by an unsupported μ1,1-N3− bridge.77 The N2−N3 distance within the azido bridge in 6 is 1.230(5) Å and is in a similar range to that observed in complex 3 (Table 2). While the structural data presented above and magnetic studies on 6 (see below) do strongly indicate the heterodinuclear nature of complex 6, distinguishing between cobalt(II) and copper(II) crystallographically is a challenging issue. In order to display that 2 is indeed a useful synthon for generating μ1,1-N3− bridged mixed dinuclear complexes, compound 7 containing two different tripodal ligands on the two copper(II) centers was synthesized (see above). 7 crystallizes in the monoclinic P21/n space group. The copper(II) ions in 7 are both pentacoordinated, and the copper−ligand bond distances are similar to those observed in complexes 2 and 6 (Figure 6 and Tables 2, S13). The coordination of TPTA to Cu1 and TBTA to Cu2 in complex 7 is unequivocally established through this structural characterization, thus proving the usefulness of complex 2 to act as a synthon for generating mixed dinuclear complexes where the metal centers are bridged by a single, unsupported μ1,1-N3− azido ligand. The N2−N3 bond length within the azido bridge in 7 is 1.230(3) Å and is in

Figure 6. ORTEP of 6 (top) and 7 (bottom). Ellipsoids are shown at 50% probability. Hydrogen atoms, counterions, and solvent molecules have been omitted for clarity.

a similar range to those observed for complexes 3 and 6 (Table 2). Magnetic Measurements and High-Frequency EPR Studies. In order to elucidate the magnetic properties and the electronic structures, particularly for the cobalt(II)-containing complexes, magnetic and EPR spectroscopic measurements were carried out. Complexes 1, 4, and 5 delivered reasonable high-frequency EPR (HFEPR) spectroscopic data, and hence their temperature-dependent magnetic behavior was simulated with the help of parameters obtained from the high-field EPR measurements. For the cobalt(II)-containing complexes 3 and 6, we did not obtain usable HFEPR data, and we just present the magnetic susceptibility in these complexes as a function of temperature. All simulations are based on the spin Hamiltonian Ĥ = −2J S1̂ Ŝ 2 +

∑ DiSẑ ,i

2

2 2 + Ei(Sx̂ , i − Sŷ , i ) + μB B·gi·Sî

i

(1)

where the first term that describes the isotropic exchange interaction is absent in the case of mononuclear complexes for E


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Inorganic Chemistry obvious reasons. The D and E terms represent the axial and rhombic second rank zero-field splitting terms, respectively. The last term describes the Zeeman interaction with g the gtensor. In case only one set of parameters is reported for dinuclear complexes, satisfactory fits could be obtained for equal parameters for the two ions. We estimate errors to be 1% in D and E and 0.01 in g. The room-temperature χT value for 1 that contains a pentacoordinate cobalt(II) ion is 2.21 cm3 mol−1 K (Figure 7).

Figure 7. Experimental (red circles) and simulated (D = −9.55 cm−1, E/D = 0.137, gav = 2.16, black line) temperature dependence of χT of 1.

This value matches reasonably well that for an S = 3/2 system, as expected for a HS cobalt(II) ion.78 We have previously shown that cobalt(II) in low-symmetry surroundings has a fully quenched orbital angular momentum.79 With decreasing temperature, the χT value remains constant until about 50 K, below which it decreases. We attribute the apparent slight increase to small imperfections in the diamagnetic correction or effects not considered in the spin Hamiltonian. At 1.8 K, a value of 1.37 cm3 mol−1 K is reached. The decrease at low temperatures is likely related to the effect of zero-field splitting. This was corroborated by HFEPR measurements (see below). Multifrequency EPR data were collected on 1 at 220, 300, and 380 GHz (Figure 8). The simulation of these data was only possible by considering a 1:1 mixture of two different S = 3/2 systems where the gx and gy values are interchanged in the two cases. The measurement of a powder diffractogram on samples of 1 showed the existence of only one species. However, it should be noted that compound 1 actually crystallizes as a racemic mixture, with the cobalt being the stereocenter. The two different species required for the simulation of the EPR data might be related to the existence of this racemic mixture. The EPR spectra at multiple frequencies could be best fitted with the parameters: D = −9.55 cm−1, E/D = 0.137, gx = 2.22, gy = 2.04, and gz = 2.23 (Table 3). These parameters were also used for the simulation of the magnetic susceptibility data. There is a slight discrepancy in the χT simulation between experiment and simulation (ca. 2%), which we attribute to uncertainties in the measurement and diamagnetic correction of the data. The axial zero-field splitting parameter obtained for 1 is similar to what was reported for the related complex [Co(TBTA)N3]ClO4.66 For EPR spectra recorded in the Xband, see the Supporting Information. For 4 that contains two cobalt(II) centers bridged by an azido ligand in the μ1,3-N3− bridging mode, a room-temperature χT value of 4.2 cm3 mol−1 K is observed, which is close to the value expected for two HS cobalt(II) centers each with S = 3/2 (Figure 9). On decreasing the temperature, this value decreases to reach 0.09 cm3 mol−1 K at 1.8 K. The decrease at lower temperature is attributed to a mixture of zero-field splitting and

Figure 8. Experimental (black) and simulated EPR spectra of 1 at 220, 300, and 380 GHz at 20 K.

antiferromagnetic exchange coupling. For complex 4, no EPR signals were observed at the X-band even down to very low temperatures. Thus, EPR data were collected at higher frequencies (Figure 10). The data at the various frequencies as well as the magnetic susceptibility data can be fitted by using the parameters D = 1.0 cm−1, E/D = 0.080, gx = 2.40, gy = 2.15, gz = 2.15, J = −6.5 cm−1 (Table 3). Thus, it is seen that the μ1,3N3− bridge induces antiferromagnetic exchange coupling between the two cobalt(II) centers. In the case of complex 5 that contains two hexacoordinated cobalt(II) ions that are bridged by two μ1,1-N3− ligands, the room-temperature χT value is 6.5 cm3 mol−1 K (Figure 11) and is hence larger than what is observed for 4 that contains pentacoordinate cobalt(II) centers. On lowering the temperature, the χT value first increases, reaching a maximum of 7.5 cm3 mol−1 K at 55 K and then decreases again to reach a value of 2.0 cm3 mol−1 K at 1.8 K. The above pattern indicates the operation of ferromagnetic coupling and zero-field splitting in complex 5. The magnetic susceptibility data could be simulated using the parameters D = 64.7 cm−1, E/D = 0.086 (Co1), D = 66.7 cm−1, E/D = 0.071 (Co2), gav = 2.473, J = 8.5 cm−1 (Table 3). Here, the average g values were calculated from the EPR simulations; see below. EPR spectra of complex 5 were recorded at various frequencies (Figure 12 and Figures S9 and S10). All the spectra could be simulated using the D = 64.7 cm−1, E/D = 0.086 (Co1), D = 66.7 cm−1, E/D = 0.071 (Co2), gx = 2.62, gy = 2.65, gz = 2.15, J = 8.5 cm−1. The same parameters were used for the simulation of both the magnetic data and the EPR spectra. Interestingly, the use of two slightly different parameter sets for the two ions in the molecule improved the fits, in spite of the two ions being related by a crystallographic inversion center. This may indicate a crystallographic symmetry lowering below 140 K or a higher sensitivity of HFEPR compared to XRD for small structural deviations. F


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Inorganic Chemistry Table 3. Summary of Spin Hamiltonian Parameters Extracted from Magnetic and EPR Measurements

Figure 9. Experimental (red circles) and simulated (D = 1.0 cm−1, E/ D = 0.080, gav = 2.23, J = −6.5 cm−1, 2.2% uncoupled S = 3/2 with g = 2, black line) temperature dependence of χT of 4.

Note that the elongated thermal ellipsoids of the bridging azide groups (Figure 5) may point to the latter explanation. Thus, it is seen that the zero-field splitting observed for the hexacoordinated cobalt(II) centers in 5 is much higher than the corresponding values obtained for the pentacoordinated cobalt(II) centers in 1 and 4. Additionally, the double μ1,1-N3− bridging in 5 induces a ferromagnetic exchange coupling between the two cobalt(II) centers as opposed to the antiferromagnetic coupling observed for the singly μ1,3-N3− bridged dicobalt(II) complex 4. This is in agreement with general rules for the magnetic exchange coupling mediated by azido bridges.20 For complexes 3 and 6, the room-temperature χT values of 4.26 and 2.10 cm3 mol−1 K, respectively (Figures S1 and S2), are in agreement with the existence two S = 3/2 systems in 3 and the existence of one S = 3/2 and one S = 1/2 system in 6. Even though we were not able to extract all the parameters from a reasonable simulation of these magnetic susceptibility data, the decrease of the χT values at lower temperature for both complexes indicates the operation of antiferromagnetic coupling and zero-field splitting, as has been discussed above for complex 4. Additionally, the room-temperature χT value of 6, which is close to the sum of the spin-only values of an S = 1/2 and an S = 3/2 system of 2.25 cm3 mol−1 K, provides

Figure 10. Experimental (black) and simulated (red) EPR spectra of 4 at 270, 300, and 340 GHz at 5 K.

further indication of the presence of a heterodinuclear cobalt(II)/copper(II) unit in that complex.

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CONCLUSION Summarizing, we have presented here the syntheses of cobaltazido complexes, where the nuclearity of the complex and the type of the bridge can be controlled by the stoichiometry of the reactants as well as by the peripheral substituents on the clicktripodal ligands. Using such a strategy, it has been possible to G


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Inorganic Chemistry

field splitting in these complexes by influencing the structure. The strategy presented here for the generation of the singly μ1,1-N3− bridged mixed dinuclear complexes is an innovative one and is expected to be operative for other metal centers as well. Furthermore, the single μ1,1-N3− bridging mode might be an interesting motif for generating photo- and thermal activation of the metal bound azido unit. Such approaches might offer access to metal-nitrido species or be helpful in Ntransfer reactions to organic substrates. Future work in our laboratories will be dedicated to those fields.

Figure 11. Experimental (red circles) and simulated (D = 64.7 cm−1, E/D = 0.086 (Co1), D = 66.7 cm−1, E/D = 0.071 (Co2), gav = 2.473, J = 8.5 cm−1, black line) temperature dependence of χT of 5.

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EXPERIMENTAL SECTION

Caution! Complexes containing perchlorates and azides are potentially explosive. Although we never experienced any problems during synthesis or analysis, all compounds should be synthesized only in small quantities and handled with great care! General Considerations. Starting materials obtained from commercial sources were used as received without further purification. Solvents were dried, distilled, and degassed using common techniques. For crystallization, “condensation and diffusion of Et2O” means that a test tube, filled to half with the respective solution, was placed in a saturated atmosphere of Et2O for several days. Elemental analysis (CHN) was measured with an Elementar Vario EL III and a PerkinElmer Analyzer 240. FTIR-ATR spectra were recorded with a Thermo Scientific Nicolet iS10 FT-IR spectrometer equipped with a smart orbit unit. Mass spectrometry was run on an Agilent 6210 ESITOF, Agilent Technologies, Santa Clara, CA, USA. Single-Crystal X-ray Diffraction. X-ray data were collected on a Bruker Smart AXS or Bruker D8 Venture systems or on a Stoe IPDS II between 100(2) and 140(2) K, using graphite-monochromated Mo Kα radiation (λα = 0.7107 Å). The strategy for the data collection was evaluated by using the Smart software. The data were collected by the standard omega scan or omega + phi scan techniques, and were scaled and reduced using the Saint+ and SADABS software. The structures were solved by direct methods using SHELXS-97 or SHELXL-2014/7 and refined by full-matrix least-squares, refining on F2. Non-hydrogen atoms were refined anisotropically.80−85 CCDC 966776, 972215, 972216, 1453200, 1453201, and 911113 contain the cif files of this work. Magnetic Measurements. Temperature-dependent magnetic susceptibility measurements of compound 3 were carried out with a Quantum-Design MPMS-XL-5 SQUID magnetometer equipped with a 5 T magnet in the range from 295 to 2.0 K at a magnetic field of 0.5 T. The powdered sample was contained in a gel bucket and fixed in a nonmagnetic sample holder. Each raw data file for the measured magnetic moment was corrected for the diamagnetic contribution of the sample holder and the gel bucket. Temperature-independent paramagnetism (TIP) and paramagnetic impurities (S = 3/2) were included according to χcalc = (1 − PI)·χ + PI·χmono + TIP; TIP = 610 × 10−6 cm3 mol−1, PI = 1% (fixed). Before simulation, the experimental data were corrected for TIP. The molar susceptibility data were corrected for the diamagnetic contribution. Magnetic properties were simulated using the julX program (E. Bill, Max Planck Institute for Chemical Energy Conversion, Mülheim/Ruhr, Germany). All other compounds were measured as pressed Teflon-wrapped powder pellets by means of a Quantum Design MPMS-XL-7 SQUID magnetometer. The experimental data were corrected for diamagnetism of the compound with the half of the molecular weight prior to the simulation. EPR Measurements. X-band EPR spectra at 9.47 GHz were recorded using a Bruker EMX spectrometer equipped with a continuous flow cryostat. HFEPR spectra were measured with a home-built spectrometer. For the HFEPR measurements, the samples were pressed into pure pellets to prevent orientation in the magnetic field. Simulations. Simulations of EPR spectra and SQUID measurements were done using the Easyspin 5.0.1286 toolbox for MATLAB unless otherwise stated. Line widths in HFEPR were modeled as D-

Figure 12. Experimental (black) and simulated EPR spectra of 5 at 270, 340, and 380 GHz, at 2 K, # marks an artifact.

present the first examples of a single and unsupported μ1,1-N3− bridged dicobalt(II) complex as well as a heterodinuclear cobalt(II)/copper(II) complex. Additionally, we have presented a rare example of a single and unsupported μ1,3-N3− bridged dicobalt(II) complex and a doubly μ1,1-N3− bridged hexacoordinated dicobalt(II) complex. It is remarkable that the peripheral substituents on the click-tripodal ligands, which otherwise possess identical donor atoms, have such a strong influence on the type of the complex that is formed. Temperaturedependent magnetic measurements showed the existence of antiferromagnetic coupling for the singly μ1,1-N3 and singly μ1,3N3 bridged complexes and the existence of ferromagnetic coupling in the doubly μ1,1-N3 bridged hexacoordinated dicobalt(II) complex. High-frequency EPR spectroscopy indicated that the axial zero-field splitting is small for the pentacoordinated cobalt(II) complexes and rather large for the hexacoordinated cobalt(II) complex. Thus, it is seen that the peripheral substituents on the click-tripodal ligands influence the nature and magnitude of exchange coupling as well as zeroH


Article

Inorganic Chemistry

the product. After filtration, the crystals rapidly lost solvent molecules. By thorough removal of the solvent in high vacuum, the product was collected in a good yield (216.0 mg, 0.115 mmol, 76%). IR (solid): ṽ = 2070 cm−1 (azide-stretch). MS: m/z (%): 841.4360 (100) [Co(TDTA)(N3)]+, 842.4380 (54) [Co(TDTA)(N3)]+, 843.4397 (15) [Co(TDTA)(N3)]+. Elemental analysis calcd (%) for C90H120Cl2Co2N26O8: C 57.41, H 6.42, N 19.34; found: C 57.01, H 6.46, N 18.85. Synthesis of [(TBTA)Cu(μ-1,1-N3)Co(TBTA)](ClO4)3·C7H8 (6). Complex 2 (108.4 mg, 0.136 mmol), Co(ClO4)2·6H2O (49.6 mg, 0.136 mmol), and TBTA (72.1 mg, 0.136 mmol) were dissolved in dry, degassed CH3CN (10 mL) and stirred under an inert atmosphere of N2 at room temperature for 16 h. The resulting solution was filtered to remove traces of solids, and dry, degassed toluene (1.2 mL) was added to the filtrate. Slow condensation and diffusion of dry, degassed Et2O into the resulting solution led to the formation of brown/green crystals, suitable for X-ray diffraction, of the product in moderate yield (101.0 mg, 0.063 mmol, 46%). IR (solid): ṽ = 2087 cm−1 (azide-stretch). MS: m/z (%): 593.1967 (100) [Cu(TBTA)]+, 594.1985 (35) [Cu(TBTA)]+, 595.1955 (50) [Cu(TBTA)]+, 596.1976 (15) [Cu(TBTA)]+, 624.1695 (90) [Co(TBTA)Cl]+, 625.1705 (40) [Co(TBTA)Cl]+, 626.1691 (40) [Co(TBTA)Cl]+, 627.1992 (12) [Co(TBTA)Cl]+. Elemental analysis calcd (%) for C67H68N23CoCuCl3O12: C 49.79, H 4.24, N 19.93; found: C 49.86, H 4.43, N 19.33. Synthesis of [(TBTA)Cu(μ-1,1-N3)Cu(TPTA)](ClO4)3·CH3CN (7). Complex 2 (93.3 mg, 0.117 mmol), Cu(ClO4)2·6H2O (43.3 mg, 0.117 mmol), and TPTA (57.1 mg, 0.117 mmol) were dissolved in CH3CN (11 mL) and stirred at room temperature for 16 h. Upon slow condensation and diffusion of Et2O into the resulting green solution green crystals, suitable for X-ray diffraction, of the product formed in a good yield (128.0 mg, 0.084 mmol, 72%). IR (solid): ṽ = 2087 cm−1 (azide-stretch). MS: m/z (%): 551.1491 (100) [Cu(TPTA)]+, 552.1511 (35) [Cu(TPTA)]+, 553.1477 (50) [Cu(TPTA)]+, 554.1491 (15) [Cu(TPTA)]+, 593.1957 (100) [Cu(TBTA)]+, 594.1977 (35) [Cu(TBTA)]+, 595.1943 (50) [Cu(TBTA)]+, 596.1955 (15) [Cu(TBTA)]+. Elemental analysis calcd (%) for C59H57N24Cu2Cl3O12: C 46.39, H 3.76, N 22.00; found: C 46.49, H 3.81, N 21.85.

strain (fwhm 700 MHz) and E-strain (fwhm 2500 MHz) for 1, and as unresolved hyperfine couplings for 4 (fwhm 30 000 for all orientations), and (fwhm 30 000, 30 000, 10 000 MHz for x,y,zorientations, respectively) for 5 (fwhm 30 000, 30 000, 10 000 MHz for x,y,z-orientations, respectively). Line widths for the X-band were modeled as unresolved hyperfine (1000 MHz fwhm isotropic for 4 and 1500 MHz fwhm isotropic for 1 and 5). Synthesis. The ligands TBTA, TPTA, and TDTA were synthesized according to a procedure described elsewhere.87 Synthesis of [Co(TPTA)(N3)]ClO4 (1). Co(ClO4)2·6H2O (133.9 mg, 0.366 mmol), TPTA (178.8 mg, 0.366 mmol), and NaN3 (23.8 mg, 0.366 mmol) were dissolved in dry, degassed CH3CN (10 mL) and stirred under an inert atmosphere of N2 at room temperature for 16 h. To the resulting purple solution dry, degassed Et2O (10 mL) was added. Upon refrigeration (ca. 0 °C) for 24 h, deep purple crystals, suitable for X-ray diffraction, of the product could be collected in a good yield (205.1 mg, 0.298 mmol, 81%). IR (solid): ṽ = 2057 cm−1 (azide-stretch). MS: m/z (%): 589.1513 (100) [Co(TPTA)(N3)]+, 590.1536 (30) [Co(TPTA)(N3)]+. Elemental analysis calcd (%) for C27H24N13CoClO4: C 47.07, H 3.51, N 26.43; found: C 47.29, H 3.54, N 26.65. Synthesis of [(TBTA)Cu(N3)]ClO4·1.5CH3CN (2). This compound was reported elsewhere75 and was resynthesized here for generating mixed complexes. Cu(ClO4)2·6H2O (210.0 mg, 0.566 mmol), TBTA (300.0 mg, 0.566 mmol), and NaN3 (36.8 mg, 0.566 mmol) were dissolved in CH3CN (15 mL) and stirred at room temperature for 16 h. Slow condensation and diffusion of Et2O into the resulting, green solution led to the formation of green crystals, suitable for X-ray diffraction, of the product in a moderate yield (170.5 mg, 0.213 mmol, 38%). IR (solid): ṽ = 2035 cm−1 (azide-stretch). MS: m/z (%): 593.1973 (100) [Cu(TBTA)]+, 594.1991 (36) [Cu(TBTA)]+, 595.1962 (50) [Cu(TBTA)]+, 596.1982 (17) [Cu(TBTA)]+. Elemental analysis calcd (%) for C33H34.5ClCuN14.5O4: C 49.72, H 4.36, N 25.48; found C 49.54, H 4.38, N 25.34. Synthesis of [(TBTA)Co(μ-1,1-N3)Co(TBTA)](ClO4)3 (3). TBTA (200 mg; 0.377 mmol) and Co(ClO4)2·6 H2O (138 mg; 0.377 mmol) were dissolved in CH3CN (6 mL) under an nitrogen atmosphere. After 30 min, NaN3 (12.2 mg, 0.188 mmol) was added and the solution was stirred overnight. After addition of ether (8 mL), a purple solid crystallized, which could be collected in good yields (229 mg; 80%). Single crystals could be grown by slow condensation of ether on top of an acetonitrile solution. IR (solid): ṽ = 2091 cm−1 (azide-stretch) MS: no molecule peak was observed due to decomposition of complex upon ionization. Elemental analysis calcd (%) for C60H60Cl3Co2N23O12: C, 47.43; H, 3.98; N, 21.20; found C, 47.93; H, 4.27; N, 20.30. Synthesis of [(TPTA)Co(μ-1,3-N3)Co(TPTA)](ClO4)3·1.5CH3CN (4). Co(ClO4)2·6H2O (133.9 mg, 0.366 mmol), TPTA (178.8 mg, 0.336 mmol), and NaN3 (11.9 mg, 0.183 mmol) were dissolved in dry, degassed CH3CN (10 mL) and stirred under an inert atmosphere of N2 at room temperature for 16 h. Slow condensation and diffusion of dry, degassed Et2O into the deep purple solution led to the formation of purple crystals, suitable for X-ray diffraction, of the product in a good yield (199.0 mg, 0.133 mmol, 73%). IR (solid): ṽ = 2121 cm−1 (azide-stretch) MS: no molecule peak was observed due to decomposition of complex upon ionization. Elemental analysis calcd (%) for C54H48Cl3Co2N23O12 (solvent molecules were lost in high vacuum): C 45.19, H 3.37, N 22.44; found: C 44.81, H 3.53, N 21.97. Synthesis of [(TDTA)Co(μ-1,1-N3)]2(ClO4)2 (5). Co(ClO4)2·6H2O (110.0 mg, 0.301 mmol), TDTA (223.0 mg, 0.301 mmol), and NaN3 (19.5 mg, 0.301 mmol) were dissolved in dry, degassed CH3CN (8 mL) and stirred under an inert atmosphere of N2 at room temperature for 16 h. Slow condensation and diffusion of dry, degassed Et2O into the deep purple solution led to the formation of pink crystals, suitable for X-ray diffraction, of a Et2O (2 equiv), CH3CN (1 equiv) solvate of

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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.6b02330. Crystallographic details and magnetic data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.S.). *E-mail: [email protected] (J.v.S.). ORCID

Franc Meyer: 0000-0002-8613-7862 Biprajit Sarkar: 0000-0003-4887-7277 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Fond der chemischen Industrie (FCI, ChemiefondsStipendium for M.G.S. and D.S.) and the Deutsche Forschungsgemeinschaft (DFG, SFB 765, SL104/2-1, SA1580/5-1, INST 41/863-1) are kindly acknowledged for I


Article

Inorganic Chemistry financial support of this work. We are thankful to Prof. Dr. R. Niewa, Universität Stuttgart, for recording a powder diffractogram of 1. Dr. A. Hagenbach is kindly acknowledged for measuring the crystals of 3.

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