Coordination chemistry and photoswitching of dinuclear macrocyclic cadmium-, nickel-, and zinc complexes containing azobenzene carboxylato co-ligands

Introduction

The macrocyclic N₆S₂ donor ligand H₂L is an effective dinucleating ligand that supports a large number of mixed ligand complexes of the type [M₂L(μ-O₂CR)]⁺ [1], where M is a divalent or trivalent metal ion (M = Mn²⁺, Fe²⁺, Co²⁺, Co³⁺, Ni²⁺, Zn²⁺ and Cd²⁺) and μ-O₂CR is a μ_1,3-bridging carboxylato ligand (Figure 1). The encapsulation of a functional carboxylate ion by the bowl-shaped [M₂L] fragment can drastically alter its structural properties and reactivities. Prominent examples are the cis-bromination of α,β-unsaturated carboxylato ligands [2], regioselective Diels–Alder reactions of encapsulated dienoate ligands [3], and the stabilization of unusual co-ligand conformations, respectively [4]. The physicochemical properties of the bound guest molecules are also greatly affected. Thus, complexation of naphthalene diimide carboxylato ligands leads to a substantial (>95%) quenching of the diimide fluorescence [5], and incorporation of a Fe(CpCO₂H)₂ unit leads to a significant anodic shift of the metallocene’s redox potential [6].

The facile formation of such compounds and their unusual properties led us to synthesize derivatives bearing azobenzene carboxylates as bridging co-ligands. A series of mixed-ligand complexes of the type [M₂L(μ-L')]⁺, where M = Zn(II), Ni(II), or Cd(II), L' = p-azobenzene carboxylate (azo-H), p'-hydroxy-p-azobenzene carboxylate (azo-OH), p'-dimethylamine-p-azobenzene carboxylate (azo-NMe₂), and p'-(methoxycarbonyl)azobenzene-p-oxymethylcarboxylic acid (azo-CO₂Me) were synthesized (Figure 2). Their preparation and structural characterization by NMR, IR, UV–vis spectroscopy, SQUID magnetometry and X-ray crystallography is reported herein. Preliminary results concerning the photoisomerization of compound 1 are reported as well.

Results and Discussion

Synthesis of ligands and metal complexes

Scheme 1 shows the synthetic procedures. The yellow-orange colored zinc complexes (3, 5) were obtained directly from stoichiometric complexation reactions between H₂L·6HCl, Zn(OAc)₂·2H₂O, and the corresponding azobenzene carboxylate ion (prepared in situ from the free acid by deprotonation with NEt₃ as a base) in methanol. The green-brown nickel (2, 4, 7 and 9) and red-orange colored cadmium complexes 1, 6 and 8, on the other hand, were prepared by substitution reactions involving the known chlorido-bridged [Ni₂L(μ-Cl)]⁺ or [Cd₂L(μ-Cl)]⁺ precursors and the respective azobenzene carboxylates. All complexes were isolated as ClO₄⁻ salts with yields between 76% and 91%. The perchlorate salts are stable in air both in solution and in the solid state, and are very soluble in a range of common polar organic solvents (CH₃CN, EtOH, MeOH). The new compounds gave satisfactory elemental analyses and were characterized by mass spectrometry, spectroscopic methods (IR, UV–vis, ¹H and ¹³C NMR spectroscopy) and in case of 3' and 5–8 also by X-ray crystal structure analysis.

Characterization of complexes

IR and NMR spectroscopy

Table 1 lists selected analytical data for the synthesized compounds. The infrared spectra of 1–9 were recorded in the 4000–400 cm⁻¹ spectral range. The IR spectra display in each case two strong bands, one between 1620–1590 cm⁻¹ and the other in the 1419–1395 cm⁻¹ range. These bands can be assigned to the asymmetric (ν_as(RCO₂⁻)) and symmetric stretching vibrations (ν_s(RCO₂⁻)) of the carboxylato co-ligands [1]. Coordination is implied by the fact that the ν_as(RCO₂⁻) frequencies are significantly red-shifted by 78–103 cm⁻¹ to lower wavenumbers. A bathochromic shift of 14–32 cm⁻¹ is also clearly observed for the ν_s(RCO₂^–) frequencies. Notice that Δ = ν_as(RCO₂⁻) − ν_s(RCO₂⁻) is invariably larger than 180 cm⁻¹, a clear indication that the co-ligand is in the bridging mode [7-9]. Another prominent feature is the intense band around 1100 cm^–1, which can be readily assigned to a vibration of the ClO₄⁻ ion (ν₃F₄). An unambiguous assignment of the N=N vibration of the azo group was not possible due to the large number of overlapping bands in the fingerprint region.

To determine their structures in solution the diamagnetic zinc and cadmium complexes were subjected to ¹H and ¹³C NMR spectroscopic studies. Table 2, and Tables S4 and S5 (Supporting Information File 1) list the ¹H and ¹³C NMR spectroscopic data for complexes 1, 3, 5, 6 and 8. The data for [Zn₂L(μ-OAc)]⁺ and [Cd₂L(μ-OAc)]⁺ have been reported previously and are included for comparative purposes [10,11]. Figure 3 displays the ¹H NMR spectrum of [Cd₂L(μ-azo-NMe₂)]⁺ (6) in CD₃CN which is representative for all complexes. Only one set of signals is observed, showing that all compounds exist as single isomers in solution. Thus, the six N-methyl groups give rise to two singlets (one for the methyl protons on the benzylic nitrogen atoms (NBzCH₃) and one for the methyl protons on the central amine nitrogen of the linking diethylenetriamine units (NCH₃)). Note that the four aromatic protons (ArH) and the tert-butyl protons [C(CH₃)₃] appear as singlets. The remaining six signals can be assigned to the methylene protons of the linking diethylenetriamine chains (two doublets for the benzylic CH₂ and four multiplets for linking CH₂ groups). The ¹H NMR data are indicative of a C_2v symmetric structure of the [Cd₂L]²⁺ fragment in solution, as in [Cd₂L(μ-Cl)]⁺ and other carboxylato-bridged Cd₂ complexes supported by this macrocycle [10,12]. The remaining four signals (two doublets and one singlet for aromatic and one singlet for aliphatic CH₃ protons) can be attributed to the azobenzene co-ligand. These were readily assigned with the aid of APT, HSQC, HMBC and COSY spectra. The ¹³C NMR spectrum of complex 6 is also in agreement with C_2v symmetry revealing only 11 signals (seven for the aliphatic and four for the aromatic carbon atoms) for the 38 carbon atoms of the [Cd₂L]²⁺ fragment.

X-ray crystallography

Further confirmation regarding the composition and structures of the complexes were obtained by single crystal X-ray diffraction studies. Single crystals could be obtained for the tetraphenylborates 3' and 8 and perchlorate salts 5–7. The molecular structures of the complexes are displayed in Figures 4–8. Selected bond lengths are given in Table S6, Supporting Information File 1.

Description of the crystal structures

[Zn₂L(μ-azo-OH)][Zn₂L(μ-azo-O)]·BPh₄·4MeCN·3H₂O (3'·4MeCN·3H₂O): Suitable crystals of [Zn₂L(μ-azo-OH)]ClO₄ (3) could not be obtained. However, the addition of NaBPh₄ to a solution of 3 in MeCN led to crystallization of [Zn₂L(μ-azo-OH)][Zn₂L(μ-azo-O)]·BPh₄·4MeCN·3H₂O (3'·4MeCN·3H₂O). The asymmetric unit contains one [Zn₂L(μ-azo-OH)]⁺ cation (Figure 4), one [Zn₂L(μ-azo-O)] neutral complex (comprising a doubly deprotonated co-ligand), one BPh₄⁻ anion, and MeCN and H₂O solvate molecules. Both complexes are structurally very similar and the corresponding bond lengths lie within narrow ranges (Table S6 in Supporting Information File 1). The difference between the two molecules concerns the twisting angle between the carboxylate group and the azobenzene ring. Thus, in the [Zn₂L(μ-azo-OH)]⁺ cation the carboxylato group is twisted out of the azobenzene plane by an angle of 38.0°. In the neutral [Zn₂L(μ-azo-O)] complex this angle is only 5.8°. Note that the azobenzene co-ligands are not planar. They are twisted by 8–10° about the C–N=N–C linkages, as in other structures. The Zn–carboxylato bond lengths reveal no anomalies and are similar to those in [Zn₂L(μ-OAc)]⁺. There are no π–π stacking interactions between the azobenzene moieties. However, the [Zn₂L(μ-azo-OH)]⁺ and [Zn₂L(μ-azo-O)] complexes are connected by a OH···O hydrogen bond of length 2.46 Å (O3a···O3b, not shown in Figure 4).

[Zn₂L(μ-azo-NMe₂)]ClO₄·1.5MeCN (5·1.5MeCN): Crystals of [Zn₂L(μ-azo-NMe₂)]ClO₄·1.5MeCN (5·1.5MeCN) grown by recrystallization from MeCN are triclinic, space group P. The asymmetric unit comprises a [Zn₂L(μ-azo-NMe₂)]⁺ cation, a ClO₄^– anion, and MeCN solvate molecules. The [Zn₂L]²⁺ unit is isostructural with 3'. The co-ligand binds again via its carboxylato function in a μ_1,3-bridging mode to give a Zn···Zn distance of 3.440 Å, which is typical for carboxylato-bridged Zn complexes supported by the macrocyclic N₆S₂ donor ligand. The average Zn–S, Zn–N and Zn–O distances are at 2.536 Å, 2.305 Å, and 2.049 Å, respectively. These values compare well with those in 3' and other [Zn₂L(μ-carboxylato)]⁺ complexes. The [Zn₂L(μ-azo-NMe₂)]⁺ complexes in 5 assemble in pairs (Figure 5) most likely via π···π stacking interactions, as manifested by the distance of 3.34 Å between the planes through the azobenzene moieties.

[Cd₂L(μ-Azo-NMe₂)]ClO₄·0.5MeOH (6·0.5MeOH): Crystals of the title compound were grown from MeOH. The asymmetric unit comprises a [Cd₂L(μ-azo-NMe₂)]⁺ cation (with the carboxylate co-ligand disordered over two positions), a ClO₄⁻ anion, and MeOH solvate molecules. The [Cd₂L]²⁺ unit is isostructural with that in [Cd₂L(μ-OAc)]⁺ and the compounds above, with the typical bowl-shaped structure of the [Cd₂L]²⁺ fragment and a bridging carboxylate function (Figure 6). The average Cd–S, Cd–N and Cd–O distances at 2.663 Å, 2.428 Å, and 2.258 Å are longer than in 3', as one might expect from the larger ionic radius of Cd²⁺. The Cd···Cd distance is at 3.399 Å. Virtually the same values are observed in [Cd₂L(μ-OAc)]⁺ [10]. As in 5 π–π stacking of the azo-carboxylato co-ligands occurs (Figure 7). The shortest distance between two carbon atoms of adjacent benzene rings is at 3.41 Å.

[Ni₂L(μ-Azo-NMe₂)]ClO₄·xEtOH (7·ClO₄·xEtOH; x ≈ 4): Crystals of [Ni₂L(μ-azo-NMe₂)]ClO₄^.xEtOH obtained from a mixed ethanol/acetonitrile solvent system are triclinic, space group P. The asymmetric unit comprises two crystallographically independent [Ni₂L(μ-azo-NMe₂)]⁺ cations, two ClO₄^– ions and 8 EtOH solvate molecules. The latter were found to be highly disordered and were therefore removed by the SQUEEZE procedure implemented in PLATON. Removing the EtOH molecules led to a total solvent accessible void of 1500 ų, in good agreement with the space needed by ca. eight ethanol solvate molecules. The two [Ni₂L(μ-azo-NMe₂)]⁺ cations are structurally very similar as illustrated in Figure 8. In cation A, the co-ligand is nearly flat and coplanar with the Ni₂ carboxylato plane. In cation B, the benzene rings are tilted about the azo group and the Ni₂carboxlyato plane by angles of ≈32.3° and 37.5°, respectively. Interestingly, no π–π stacking occurs in this structure.

[Cd₂L(μ-azo-CO₂Me)]BPh₄^.MeCN (8·MeCN): The crystal structure determination of 8·MeCN confirms that azo-CO₂Me is also attached in the bridging mode. Figure 9 shows an ORTEP representation of the structure of the [Cd₂L(μ-azo-CO₂Me)]⁺ cation in 8. The [Cd₂L]²⁺ fragment is isostructural with that in 6, but the co-ligand protrudes laterally out of the binding pocket of the [Cd₂L]²⁺ fragment, most likely due to steric constraints exerted by the surrounding NMe₂ groups. We have observed similar effects in a stearato bridged complex, where the surrounding alkyl groups dictate the coordination mode of the carboxylato ligand [13]. The average Cd–S, Cd–N and Cd–O distances at 2.670, 2.429, and 2.258 Å, respectively, are all very similar to those in 6. The Cd···Cd distance is at 3.405 Å. Again, π–π stacking of the azo-carboxylato co-ligand occurs, the shortest distance between the two planes through the co-ligands amounts to 3.45 Å.

Magnetic properties of nickel complexes

The paramagnetic nickel complexes 2, 4, and 7 were further investigated by temperature dependent magnetic susceptibility measurements between 2 and 300 K in an applied external field of 0.5 T (or 1 T) by using a MPMS 7XL SQUID magnetometer (Quantum Design). Figure 10 shows the susceptibility data in the form of μ_eff versus T plots for the three complexes.

For complex 2, the effective magnetic moment increases from 4.86 μ_B (per dinuclear complex) at 300 K to a maximum value of 5.42 μ_B at 20 K, and then drops to 4.93 μ_B at 2 K. The complexes 4 and 7 behave in a similar fashion. At 300 K, μ_eff is at ≈4.87 per dinuclear complex which increases to maximum values of 5.34 μ_B (4) and 5.44 μ_B (7) at 20 K. On further lowering of the temperature, these values decrease again to 4.54 μ_B and 4.90 μ_B at 2 K. This behavior implies that the electron spins on the two Ni^II (S = 1) ions are coupled by an intramolecular ferromagnetic exchange interaction. This leads to an S_total = 2 ground state, in agreement with other carboxylato-bridged compounds supported by L²⁻ [14-16]. The ferromagnetic exchange interaction can be rationalized in terms of the Goodenough–Kanamori rules for superexchange [17], and recent DFT calculations, which revealed that a dominant ferromagnetic exchange interaction is propagated via the thiolato bridges [13].

The magnitude of the exchange interactions was determined by least-squares fitting of the experimental magnetic susceptibility data to the appropriate spin Hamiltonian (Equation 1) [18-20] including the isotropic HDvV (Heisenberg Dirac van Vleck) exchange interaction, the single-ion zero-field splitting of Ni(II) and the single-ion Zeeman interactions using a full-matrix diagonalization approach [21]. The experimental susceptibility data were fitted to Equation 1 over the temperature range 2–300 K, assuming identical D and g values for the two Ni(II) ions in each one of the complexes, but it should be mentioned that temperature-dependent magnetic susceptibility measurements do not allow a concise determination of the magnitude and sign of D [22].

By taking into account zero-field splitting and temperature-independent paramagnetism (TIP), reasonable fits of the experimental data were possible, yielding J = +22.6 cm⁻¹, g = 2.21 and D = |4.18| cm⁻¹ for 2. Fitting the experimental data of 4 and 7 led to very similar values, namely J = +21.79 cm^–1, g = 2.17, D = |3.64| cm^–1 and J = +21.70 cm^–1, g = 2.22 and D = |4.67| cm^–1, respectively. In each case the low-temperature fit was significantly improved by the inclusion of the D parameter, but as stated above, the D values should be taken rather indicative than definite [23]. The magnetic parameters for the azo-carboxylate complexes are in good agreement with those of nickel(II)-carboxylate complexes supported by L²⁻. For [Ni₂L(μ-O₂C(CH₂)₁₀SH)](ClO₄), for example, values of J = +23.0 cm⁻¹, g = 2.20 and D = |2.59| cm⁻¹ were determined [13].

UV–vis spectroscopy

All new compounds were further investigated by electronic absorption spectroscopy. The electronic absorption spectra were recorded in acetonitrile (1, 2 and 5–9) or dimethyl sulfoxide (3, 4) solution in the 190 to 1200 nm spectral range. Figure 11 shows a representative set of spectra recorded for Hazo-H and the corresponding Cd(II) complex [Cd₂(L)(μ-azo-H)]⁺ (1). The spectrum of the chlorido-bridged complex [Cd₂L(μ-Cl)]⁺ is also shown for comparison. Table 3 lists the corresponding data. A comparison reveals that the electronic absorption spectrum of [Cd₂L(μ-azo-H)]⁺ can (at least in the 260–450 nm range) in first order be traced to a superposition of the absorptions of the Cd-bound amino-thiophenolato ligand (absorbing at 260 and 300 nm) and the azobenzene co-ligand (absorbing at 325 and 441 nm). Note, that the band at 322 nm for free Hazo-H appears as a shoulder around 325 nm in 1. The slight red-shift may be a consequence of deprotonation and coordination to the Cd²⁺ ions. The spectrum of the deprotonated azoH^– ion, for comparison, absorbs at 329 nm. The absorption bands above 310 nm can thus be attributed to the π–π* and n–π* transitions of the trans-configured azobenzene chromophore, consistent with literature reports [24,25]. The absorptions below 310 nm (λ = 260 and 300 nm) are attributable to the π–π* transitions within the thiophenolate units of the [Cd₂L]²⁺ fragment, which may be further differentiated as p- and α-bands of the parent benzene chromophore utilizing Clar’s notation [26], assuming that the electron-donating alkyl and thiol substituents exert a bathochromic effect.

The spectral properties of the nickel complexes differ from those of the zinc and cadmium counterparts in that they exhibit two additional Laporte-forbidden but spin-allowed d–d transitions around 645 and 1120 nm, typical for carboxylato-bridged Ni₂ complexes supported by H₂L (Figure 12) [5]. They are assigned as ³A_2g(F)→ T_1g(F) (ν₂) and ³A_2g(F) → ³T_1g(P) (ν₁) of an octahedral nickel(II) (S = 1) ion (in pure O_h symmetry) [5]. The third spin-allowed ³A_2g(F) → ³T_2g(F) (ν₃) transition is expected around 440 nm, but is obscured by the stronger RS → Ni²⁺ charge transfer and π→π* transitions in this region.

We carried out orienting irradiation experiments with regard to a possible trans to cis photoisomerization of the bound azobenzene-carboxylate co-ligands. The diamagnetic Cd₂ complex [Cd₂L(μ-azoH)]ClO₄ (1) was selected for these studies. Note that this complex (with the azobenzene moiety in the trans form) is distinguished by a strong absorption band (shoulder) around 325 nm (ε = 23000 M^–1cm^–1) that originates from the π→π* transition in trans azo-H. The much weaker n→π* band appears in the visible at 450 nm. Dilute anhydrous acetonitrile solutions of complex 1 were placed in a standard quartz cuvette and irradiated at 365 nm for varying amounts of time (1 to 30 s) with a 14 W/cm² UV LED lamp and thereafter immediately placed in the chamber of a UV–vis spectrometer. Figure 13 shows the corresponding UV–vis spectra, which were recorded directly after sample preparation.

As can be seen, the intensity of the π→π* band at 325 nm for the trans-azobenzene carboxylate decreases with increasing irradiation time, while the intensity of the n→π* band increases with increasing irradiation time. A blue shift of the n–π* band by ≈10 nm is evident. The π→π* transitions for the thiophenolate group at 260 and 300 nm are also affected, being blue-shifted to 256 and 296 nm, respectively, and significantly reduced (≈260 nm) or increased (≈300 nm) in intensity. Isosbestic points at 220, 240, 280 and 380 nm are also clearly discernible indicative of the presence of a single equilibrium. On the basis of these data, we conclude that the trans-form of the bound co-ligand converts to its cis-form as indicated in Scheme 2. Further studies show, that the meta-stable cis-form relaxes back thermally to the more stable trans from (Figures S50 and S51 in Supporting Information File 1). Thus, the cis–trans isomerization appears to be not constrained by the [Cd₂L]²⁺ fragment, as revealed by the similar half-lives (τ_1/2 cis–trans free deprotonated azobenzene carboxylate: ≈80 h; τ_1/2 cis–trans Cd complex ≈73 h. In view of the fact that the azobenzene carboxylate is only partially buried in the binding pocket of the [Cd₂L]²⁺ complex this is not surprising.

Conclusion

A series of dinuclear macrocyclic [M₂L(μ-L')]⁺ complexes co-ligated by various azobenzene-carboxylates has been synthesized and characterized in solution and solid state. The diamagnetic Cd(II) and Zn(II) and the paramagnetic Ni(II) complexes were found to be isostructural, with the co-ligands bound via bridging carboxylate functions, a bowl-shaped structure of the supporting [M₂L]²⁺ entity, and trans-configured azo groups. NMR and UV–vis studies clearly show that the complexes retain their mixed-ligand nature in the solution state. Irradiation of the Cd complex 1 with a LED UV lamp leads to a trans-to-cis isomerization which appears to proceed within the coordination sphere of the [Cd₂L]²⁺ fragment. The switch from the trans to the cis form induces significant change of the π–π transitions of the supporting N₆S₂ macrocycle, which may be indicative of some increased π–π- (or charge transfer) interactions between the aromatic rings of the electron rich amino thiophenolato macrocycle and the electron poor azobenzene-carboxylato ligands. There are little, if any, differences in the time-scales for thermal relaxation of the free and Cd-bound azobenzene-carboxylate systems, suggesting that the cavity of [Cd₂L]²⁺ does not sterically constrain the photoisomerization process.

Experimental

Unless otherwise noted all preparations were carried out under a protective atmosphere of nitrogen by using standard Schlenk techniques [27]. Compounds H₂L [28], [Ni₂L(μ-Cl)]ClO₄ [29], [Cd₂L(μ-Cl)]ClO₄, and p-hydroxyazobenzene-4-carboxylic acid [30] and p'-carbomethoxy-azobenzene-p-oxymethylcarboxylic acid [31-33] were prepared as described in the literature. All other reagents were purchased from commercial vendors and used without further purification. Melting points were determined in open glass capillaries and are uncorrected. The IR spectra were recorded as KBr disks using a Bruker Tensor 27 FTIR spectrometer. UV–vis spectra were recorded on a Jasco V-670 UV–vis/near-IR spectrophotometer. Elemental analyses were carried out with a Vario EL – elemental analyzer. NMR spectra were recorded on a Bruker Fourier 300 spectrometer or Avance DRX 400 at 298 K. ¹H and ¹³C chemical shifts refer to solvent signals. ESIMS spectra were recorded on a Bruker Daltronics Esquire 3000 Plus spectrometer. Temperature-dependent magnetic susceptibility measurements on powdered solid samples were carried out using a MPMS 7XL Squid magnetometer (Quantum Design) over a temperature range of 2–300 K at an applied magnetic field of 0.5 and 1.0 Tesla. The observed susceptibility data were corrected for underlying diamagnetism. The synthesis of the complexes are shown in Supporting Information File 1.

X-ray crystallography. Single crystals of 3', 5–8 suitable for X-ray crystallography were selected and mounted on the tip of a glass fiber. The data sets were collected at 213(2) K or 200 K (7) using a STOE IPDS-2T diffractometer equipped with graphite monochromated Mo Kα radiation (0.71073 Å). Table S7 in Supporting Information File 1 lists selected crystallographic data. The intensity data were processed with the program STOE X-AREA [34] Structures were solved by direct methods [35] and refined by full-matrix least-squares on the basis of all data against F² using SHELXL-97 [36]. PLATON was used to search for higher symmetry [37]. H atoms were placed in calculated positions and treated isotropically using the 1.2-fold U_iso value of the parent atom except methyl protons, which were assigned the 1.5-fold U_iso value of the parent C atoms. Unless otherwise noted, all non-hydrogen atoms were refined anisotropically. ORTEP-3 was used for the artwork of the structures [38]. All non-hydrogen atoms were refined anisotropically.

In the crystal structure of 3'·4MeCN·3H₂O one MeCN molecule was found to be disordered over two positions (at half occupancy). It was not possible to locate H atoms for the H₂O solvate molecules, and so no H₂O hydrogen atoms were included in this structure. In the crystal structures of 5 and 6 the co-ligands were found to be disordered over two positions. This disorder could be successfully modelled by using appropriate DFIX, DANG and AFIX instructions implemented in SHELXL. The site occupancy factors of the two orientations were initially refined, but were fixed at 0.65 and 0.35 (or 0.50 and 0.50 for 3') in the final refinement cycle. The occupancy factor of a MeCN (for 5) and a MeOH solvate molecule (for 6) was set at 0.5 to keep the U_eq values reasonable. For 7, additional solvent molecules occupy interstitial spaces that are generated by packing, and were found to be highly disordered. The routine SQUEEZE was applied to remove diffuse electron density [39].

CCDC-1892644 (3'), -1892646 (5) -1892647 (6), -1892648 (8) and -1892649 (7) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

Irradiation experiments. Irradiation experiments were performed on dry acetonitrile solutions of complex 1 and azo-H (concentration = 10⁻⁵ M) in a quartz cuvette at room temperature. An OmniCure LX500 system and a 14 W/cm² UV LED lamp (365 nm) from Excelitas Technologies was used. After irradiation, the solutions were immediately placed in the chamber of the JASCO V-670 UV–vis/near-IR spectrophotometer for subsequent spectroscopic investigations.