Synthesis, Structure, and UV–Vis Characterization of Antimony(III) Phthalocyanine: [(SbPc)2(Sb2I8)(SbBr3)]2

Abstract

A new antimony(III)–phthalocyanine complex with the formula of [(SbPc)₂(Sb₂I₈)(SbBr₃)]₂ has been obtained in the reaction of pure antimony powder with phthalonitrile under the oxidation conditions by iodine monobromide vapors. The complex crystallizes in the centrosymmetric space group of the triclinic system. Both independent (SbPc)⁺ units exhibit non-planar conformation, since the Sb(III) is larger than the equilibrium cavity size of the ring and cannot be accommodated without its expansion; thus, the metal protrudes out of the cavity, forming a saucer shape. The centrosymmetric anionic unit of the crystal consists of two (Sb₂I₈)²⁻ interacted anionic units forming (Sb₄I₁₆)⁴⁻ anionic complex that interacts with two SbBr₃ molecules to form [Sb₆I₁₆Br₆]⁴⁻ anionic aggregate. Each [Sb₆I₁₆Br₆]⁴⁻ anionic aggregate is surrounded by four (SbPc)⁺ cations forming a supramolecular centrosymmetric (SbPc)₄[Sb₆I₁₆Br₆] complex. Translationally related (SbPc)₄[Sb₆I₁₆Br₆] molecules form a stacking structure along the [100] and [011] directions with N₄–N₄ distances of 3.55 and 3.53 Å, respectively, between the back-to-back-oriented saucer-shaped (SbPc)⁺ units. The interaction between the building units of the crystal was analyzed using the Hirshfeld surface and the analysis of the 2D fingerprint plots. The UV–Vis absorption spectra of crystal 1 were taken in CH₂Cl₂ and toluene solutions in the concentration range from 10⁻⁵ to 10⁻⁶ mol/L. No significant changes related to aggregation in solutions were observed. The Q-band in toluene solution is red shifted by ~15 nm in comparison to that in CH₂Cl₂ solution. Oxidation of (SbPc)₄[Sb₆I₁₆Br₆] yields Sb^VPc derivative. Both Sb^III and Sb^V phthalocyanine derivatives absorb near infrared light (600–900 nm), which should be intriguing from the point of view of potential use as photosensitizers for PDT and as an infrared cut filter for plasma display and silicon photodiodes.

1. Introduction

Phthalocyanine and its metal complexes are still very intensively studied due to their various technological applications. The initial interest in them resulted from their deep and intense color; hence, they were used as pigments in the plastic industry and as dyes in the textile industry [1,2,3,4,5,6,7,8,9]. The dyeing properties of metallophthalocyanines result from their structure and very strong characteristic optical absorption in the visible region, the so-called Q band, which is characteristic for a macrocyclic tetrapyrrole ring with an 18π-electron-conjugated tetrapyrrole macrocycles (Scheme 1) [10]. The molar extinction coefficient of the Q-band of metal-free porphyrin, is 1.71 × 10⁵ dm³·mol⁻¹·cm⁻¹ at 407 nm, and for metal-free phthalocyanine it is 2.766 × 10⁵ dm³·mol⁻¹·cm⁻¹ in the visible spectral region (669 nm), and they may vary significantly depending on the solvents [11]. For metalloporphyrins and metallophthalocyanines, the molar extinction coefficient varies depending on the central metal as well as on the solvent in the wide range, for example, for the zinc porphyrin and zinc phthalocyanine the molar extinction coefficient is 5.74 × 10⁵ dm³·mol⁻¹·cm⁻¹ at 422.8 nm and 2.818 × 10⁵ dm³·mol⁻¹·cm⁻¹ at 674 nm, respectively [12,13]. The molar extinction coefficients of several metal(II) phthalocyanines in various solvents derived from the intensities of the Q-band were recently published [14].

For antimony(III)–phthalocyanine derivatives (Scheme 2) in ethanol solution, the molar extinction coefficient is 1.059 × 10⁵ and 1.717 × 10⁵ dm³·mol⁻¹·cm⁻¹ at ~730 nm for unsubstituted Sb(III) phthalocyanine and 1.06 × 10⁵ dm³·mol⁻¹·cm⁻¹ at ~740 nm for octa-substituted Sb(III) phthalocyanine, whereas for Sb(V) phthalocyanine, the molar extinction coefficient is 1.908 × 10⁵ dm³·mol⁻¹·cm⁻¹ at 699 nm [15,16].

Further interest of metallophthalocyanines results from their interesting properties, such as electronic, photonic, and photovoltaic that have already found industrial applications in many instruments or chemical processes, including catalysis, gas sensors, solar cells, charge-generating materials, optical data storage, infrared cut-off filters, active matrix displays, light organic-emitting diodes (OLED), organic photovoltaics (OPVs), etc. [17,18,19,20,21,22,23,24,25,26,27]. Strong and intense absorption of light by metal phthalocyanines in the so-called therapeutic window, namely in the near-infrared (NIR) region of visible light (600–900 nm), proved to be promising for photodynamic cancer therapy (PDT) in which they can be used as photosensitizers [28,29,30,31,32,33,34,35].

The physical and chemical properties of metallophthalocyanines are varied by the structure of the macrocyclic ligand with the extended π-electron delocalization system as well as by the central metal. Planar metallophthalocyanines with 18π electrons conjugate form a stacking structure in the solid state stabilized by π–π interactions, and this limits their solubility in the most commonly used organic solvents and undergo molecular aggregation [36]. Aggregation phenomena are common in chemistry of phthalocyanines and are known to give rise to significant changes in their physical properties [37,38,39] and may arouse interest from the point of view of supramolecular chemistry [40]. Metallophthalocyanines containing heavy atoms with too large radii to fit into the phthalocyanine core are nonplanar and lead to the reduction of π–π interactions and improve the intersystem crossing [41], resulting in the improvement in photophysical and photochemical properties as well as improve their solubility due to the reduction of molecular aggregation, resulting from changes in the electronic structure of the molecule as well as due to the dipole moment of the non-planar molecule [42].

A large number of metallophthalocyanine derivatives have so far been studied; however, the metallophthalocyanines with group 15 elements, unlike porphyrins analogues [43,44,45,46,47], were ignored for a long time until the first derivatives of bismuth and antimony were identified [48,49,50,51], and their structural characterizations are still less explored [52,53,54,55,56,57,58,59,60,61,62,63,64]. The variable valency of these metals depending on the reaction conditions (III or V) can lead to the formation of various MPc derivatives. SbPc derivatives can be promising candidates for new drugs because they are stable, powerful electron donors/acceptors [65,66], and some derivatives undergo facile Sb^III/Sb^V conversion under mild conditions [67,68]. Their use as drugs requires further research on their biocompatibility and effectiveness as well as their toxicity.

In this work, we report synthesis and X-ray structural characterization of a novel antimony(III) phthalocyanine, [(Sb^IIIPc)₂(Sb₂I₈)(SbBr₃)], obtained under an iodine monobromide vapor atmosphere. The Sb^III cation with a radius of about 0.90 Å [69] is larger than the size of the ring cavity and cannot be accommodated without ring expansion; therefore, the formation of non-planar derivatives of Sb^IIIPc can be expected. The antimony(III)–phthalocyanine complexes can be oxidized to Sb^VPc derivatives. In addition, the obtained antimony(III) phthalocyanine and its oxidized Sb^VPc derivative, as described below, exhibits absorption of a wide range of visible light. Therefore, this should be of interest to those who work not only with dyes and pigments but also with solar cells and charge-generating material. It should be noted that antimony-phthalocyanine derivatives due to strong absorption of light in the therapeutic window may be attractive from the viewpoint of their potential application as a photosensitizer for PDT [70,71,72,73,74].

2. Experimental Section

2.1. General Procedure

Phthalonitrile (98%), iodine monobromide (98%), and antimony (99.999%) were obtained from Merck (Darmstadt, Germany) and used as received. The composition of the obtained crystals was checked with energy dispersive spectroscopy (EDS) (FEI company, Thermo Fisher Scientific, Waltham, MA, USA). EDS spectra were acquired and analyzed using an EDAX Pegasus XM4 spectrometer with an SDD Apollo 4D detector mounted on a FEI Nova NanoSEM 230 microscope. In addition, the elemental analysis was also carried out with a PerkinElmer 240 elemental analyzer (Waltham, MA, USA). The Fourier transform infrared spectra were recorded between 4000 and 450 cm⁻¹ on a Bruker IFS 113 V FTIR (Bruker Optik GmbH, Leipzig, Germany) in nujol mull (Figure S1, in Supplementary Materials). Measurements of the UV–Vis spectra were carried out at room temperature using an Agilent UV–Vis/NIR Cary 5000 spectrometer (Agilent Technologies, Inc., Santa Clara, CA, USA). The UV–Vis spectra were recorded in CH₂Cl₂ and toluene solutions with concentrations ranging from 1 × 10⁻⁵ to 1 × 10⁻⁶ mol/L.

2.2. Synthesis [(Sb^IIIPc)₂(Sb₂I₈)(SbBr₃)]₂

The crystals of the [(Sb^IIIPc)₂(Sb₂I₈)(SbBr₃)]₂ (1) were isolated as a product of the reaction of Sb (0.1218 g, 1.0 mmol) with phthalonitrile (0.51252 g, 4 mmol) under iodine monobromide vapor according to a slightly modified method described elsewhere [75]. The powder Sb and phthalonitrile (in a molar proportion of 1:4) were mixed with iodine monobromide (2.068 g, 10.0 mmol) and pressed into the pellets. The pellets were inserted into an evacuated glass ampoule and sealed. The ampoule was heated at 240 °C for one day, and next, it was cooled to room temperature. After such a process, good quality single crystals of [(Sb^IIIPc)₂(Sb₂I₈)(SbBr₃)] suitable for the X-ray analysis were formed. Anal. Found: C, 26.51; Br, 8.26; I, 35.21; N, 7.72, and Sb, 21.20%. Calc. for C₆₄H₃₂Br₃I₈N₁₆Sb₅: C, 26.61; Br, 8.30; I, 35.14; N, 7.76; Sb, 21.07; and H, 1.12%.

2.3. X-ray Crystallography

X-ray intensity data collection for the [(Sb^IIIPc)₂(Sb₂I₈)(SbBr₃)]—(1) crystal was collected using graphite monochromatic MoKα radiation on a four-circle κ geometry Xcalibur diffractometer with Sapphire2 area CCD detector. Data collections were made using the CrysAlis CCD program [76]. Integration, scaling of the reflections, correction for Lorenz and polarization effects, and absorption corrections were performed using the CrysAlis Red program [76]. The structures were solved by the direct methods using SHELXT-2014/7 [77] and refined using the SHELXL-2018/3 program [78]. The hydrogen atoms joined to aromatic carbon atoms were introduced in their geometrical positions and treated as rigid. The final difference Fourier maps showed no peaks of chemical significance. Details of the data collection parameters, crystallographic data, and final agreement parameters are collected in Table 1. Visualizations of the structures were made with the Diamond 3.0 [79].

2.4. Hirshfeld Surface Analysis

Hirshfeld surface analysis and 2D fingerprint plots as well as percentage contributions for various intermolecular contacts in the investigated crystals, were calculated using the Crystal Explorer Ver. 3.1 program package [80].

2.5. Theoretical Calculations

The optimized geometry of the (SbPc)⁺ unit was performed based on the experimental X-ray geometry using the DFT and PBEPBE functional and LAN2MB basis sets using the Gaussian 2016 program package [81].

3. Results and Discussion

3.1. Synthesis and Initial Characterization

The preparation method leads directly to good-quality [(Sb^IIIPc)₂(Sb₂I₈)(SbBr₃)]—(1) single crystals. During prolonged heating, the liquid phthalonitrile undergoes catalytic tetramerization forming the phthalocyanine units while accepting the two electrons from the antimony present in the reaction forming (Sb^IIIPc)⁺ cationic units (Scheme 3). Simultaneously, the iodine monobromide accepted the third electron as well as oxidized the excess of antimony present in the reaction forming the centrosymmetric {[(Sb₂I₈)(SbBr₃)]₂}⁴⁻ anionic complex. These oppositely charged units interact with each other, leading to nucleation and, over time, to crystallization and growth of good-quality single crystals suitable for X-ray diffraction.

It should be noted that the heating time should not be extended too long, as it leads to partial decomposition of the phthalocyanine complex formed and to the formation of the phthalonitrile trimer [82]. Elemental analysis of the obtained crystals gave a satisfactory result, which is close to the calculated one. The sharp peak for the C≡N stretching vibrations in the IR spectrum of phthalonitrile at ~2200 cm⁻¹ disappeared after tetramerization into phthalocyanine moiety [83,84]. The IR spectrum of (SbPc)(I₃) ½(I₂) (Figure S1 in Supplementary Materials) showed vibration peaks between 700 cm⁻¹ and 1500 cm⁻¹, which can be attributed to the skeletal vibration of the phthalocyanine macrocycle [85]. The obtained [(Sb^IIIPc)₂(Sb₂I₈)(SbBr₃)] crystals have limited solubility in methanol and ethanol and dissolve much better in pyridine, DMF, DMSO, toluene, benzene, and other aromatic solvents. Contact with concentrated inorganic acids, such as H₂SO₄ or HNO_3, or their mixture or mixture with concentrated HCl leads to demetallation, resulting in the formation of metal-free phthalocyanine in the α-form [86] and appropriate salt.

3.2. Structural Characterization

The obtained antimony-phthalocyanine complex crystallizes in the centrosymmetric space group of the triclinic system. Refinement of the structure of the title compound (1) shows that both ionic parts of the crystal building blocks, i.e., in (SbPc)⁺ as well as in the [(Sb₂I₈)(SbBr₃)]²⁻, contain the antimony in the same oxidation state (Sb³⁺). The molecular structure of the asymmetric unit is illustrated in Figure 1.

Both independent (SbPc)⁺ units exhibit similar non-planar conformation, since the Sb³⁺ (ionic radius of about 0.90 Å [69]) is larger than the equilibrium cavity size of the ring and cannot be accommodated without ring expansion; thus, the metal protrudes out of the Pc-cavity, forming a domed shape. Each of the positively charged atoms, Sb4 and Sb4B, of the cations (SbPc)⁺ is bound to four isoindole N atoms of Pc macrocycles with distances in the range from 2.146(8) to 2.206(8) Å (Table 2) and lies outside the plane defined by these N atoms, i.e., 0.992(1) and 0.994(1) Å for Sb4 and Sb4B, respectively. The optimized geometry of the (SbPc)⁺ unit performed by DFT calculations also show its non-planar conformation with the Sb atom displaced by 0.886 Å out of N₄-isoindole plane (See Table S1 in Supplementary Materials). The greater displacement of Sb(III) from the N₄-plane in the crystal is caused by the interaction of the positively polarized Sb³⁺ center of (SbPc)⁺ with the (Sb₆I₁₆Br₆)⁴⁻ counterion.

The anion of the complex consists of four distorted SbI₆ octahedra lined together by μ₂ and μ₃ bridging I atoms into a (Sb₄I₁₆)⁴⁻ anion that weakly interacts with two SbBr₃ molecules forming the centrosymmetric (Sb₆I₁₆Br₆)⁴⁻ counterion complex (Figure 2). Generally, the Sb–I bond lengths fall into two groups, namely shorter Sb–I bonds with terminal I atoms and longer Sb–I bonds involving the bridging I atoms (Table 2). However, in the (Sb₆I₁₆Br₆)⁴⁻ anion, two different bridging I atoms exist. Atoms I1 and I2 bridge three Sb atoms, while atoms I3, I4, and I5 bridge only two Sb atoms. The Sb–Br bonds with distances ranging from 2.5659(14) to 2.6888(12) Å are shorter, as expected, than the Sb–I bonds (Table 2) and correlate with their covalent or ionic radii of I and Br [87,88]. Looking in more detail at the differences between the Sb–I and Sb–Br bond lengths and at the coordination geometry around atoms Sb1, Sb2, and Sb3, it is clear the Sb1 and Sb2 atoms are coordinated to the six I atoms through forces of different strength, whereas Sb3 coordinates to three Br and the three I atoms. Atom Sb1 links two I atoms with relatively short Sb–I bonds, two I atoms with intermediate Sb–I values, and two I atoms with relatively long Sb–I bonds, the longest being the Sb1ⁱ–I1 bond 3.4281(12) Å (Table 2).

The Sb2 atom in the coordination environment has one terminal I atom with the short Sb–I bond (2.7374(11) Å), two μ₂-bridged, and three μ₃-bridged I atoms with intermediate and longest Sb–I bonds (Table 2). The Sb3 links three Br atoms, all Br are terminal, with relatively short Sb–Br bond lengths, and the two μ₂-bridged and one μ₃-bridged I atoms with relatively long Sb–I bonds above 3.3 Å. Thus, the [Sb₆I₁₆Br₆]⁴⁻ ion can be regarded as being composed of three symmetrically equivalent pairs of units, viz. [SbI₅]²⁻, SbI_3, and SbBr₃ units. However, the mutual orientation of the [SbI₅]²⁻, SbI_3, and SbBr₃ units related by an inversion center leads to the formation of an [Sb₆I₁₆Br₆]⁴⁻ counter-ion in which all Sb atoms have distorted octahedral coordination environments (Figure 2).

A similar pattern concerning the Sb–I bond lengths is also observed in the [Sb₄I₁₆]⁴⁻ [59], [Sb₄I₁₄]²⁻ [62], and [Sb₆I₂₂]⁴⁻ [63] counter-ions of antimony(III)–phthalocyanine complexes. In the first complex, the [Sb₄I₁₆]⁴⁻ ion consists of four distorted SbI₆ octahedra, in the second, the [Sb₄I₁₄]²⁻ ion consists of two pairs of deformed SbI₆ octahedra and distorted square pyramidal SbI₅ polyhedra, while in the third, the [Sb₆I₂₂]⁴⁻ ion consists of six distorted SbI₆ octahedra similar as in the present compound in which the Br atoms have been replaced by the I atoms. A similar metal-polyheterohalide anion can be found in the bismuth(III)–phthalocyanine derivative whose [Bi₆Cl₁₁I₁₁]⁴⁻ anion consists of edge-shared distorted octahedra BiI₃Cl₃ [64].

In the crystal, there seems to be significant ionic attraction between the (SbPc)⁺ cations and [Sb₆I₁₆Br₆]⁴⁻ complex counterions. The influence of the ionic attraction between the oppositely charged (SbPc)⁺ and [Sb₆I₁₆Br₆]⁴⁻ ions is clearly manifested in the Sb–N(isoindole) coordination, leading to the molecular symmetry of the (SbPc)⁺ unit being close to C_s, rather than the D_4v symmetry that characterizes the conformation in solution. The basic packing unit includes two pairs of (SbPc)⁺ macrocycles associated by an inversion centre and an anionic [Sb₆I₁₆Br₆]⁴⁻ complex; thus, each anion is surrounded by four (SbPc)⁺ cations (Figure 3).

Each of the two independent (SbPc)⁺ cations interact with the [Sb₆I₁₆Br₆]⁴⁻ counterion in a slightly different manner. The Sb4 of (SbPc)+ interacts with three iodine atoms (I1, I2 and I3) while the Sb4B of (SbPc)+ interacts with two iodines (I5 and I7) and one bromine (Br3) with the contacts Sb–I and Sb–Br being much longer than the sum of the covalent radii of Sb (1.39 Å), I (1.40 Å) and Br (1.20 Å) [87] but shorter than the sum of the van der Waals radii Sb and I of ~4.3 Å or Sb and Br of ~4.1 Å [88]. The average Sb–I distance of 3.578 Å and the Sb–Br distance of 3.5941(18) Å between the Sb of (SbPc)⁺ units and I or Br atoms of [Sb₆I₁₆Br₆]⁴⁻ complex anion point that the interaction between (SbPc)⁺ units and [Sb₆I₁₆Br₆]⁴⁻ anion is intermediate between ionic and covalent. The centrosymmetric [(SbPc)₄(Sb₆I₁₆Br₆)] aggregates related by translation in crystal form a stacking structure along the [100] and [111] directions with back-to-back oriented (SbPc)⁺ pairs (Figure 4).

The interplanar N_4-isoindole–N_4-isoindole distance within the stack is ~3.5 Å, which indicates an interaction between the non-planar (SbPc)+ macrocycles with an intermediate force. The π-π interaction and overlapping of the π clouds of (SbPc)⁺ macrocycles in the present crystal 1 is significantly lower compared to that found in the normal planar M(II)Pc structures. Decreasing of π–π interactions together with the ionic character of the (SbPc)⁺ and (Sb₆I₁₆Br₆)⁴⁻ units result in improvement to its solubility as well as affects its optical spectroscopic properties.

3.3. Hirshfeld Surface Analysis

To better understand the nature of the interactions between the components constituting the crystal 1, the Hirshfeld surface analysis and the analysis of the 2D fingerprint plots were performed. The Hirshfeld surface (HS) analysis and the analysis of 2D fingerprint plots are good tools that not only allow for qualitative analysis of the intermolecular interactions in the crystals [89,90,91,92], but also allow for quantitative analysis, i.e., allow the percentage contribution to the HS surface resulting from particular types of interactions between building blocks of the crystal 1 to be calculated. The 3D Hirshfeld surfaces are mapped via the normalized contact distance (d_norm) relative to both (d_e) and (d_i) and the van der Waals radii of the atoms, where d_e is the distance from a point on the surface to the nearest nucleus outside the surface, and d_i is distance from a point on the surface to the nearest nucleus inside the surface and 2D-fingerprint plots of each units of crystal, i.e., both independent (SbPc)⁺ units and the [Sb₆I₁₆Br₆]⁴⁻ counterion, and are shown in Figure 5. Where the atoms make intermolecular contacts closer than the sum of van der Waals radii, the contacts are highlighted in red on the HS mapped with the d_norm; longer contacts are blue and the contacts around the sum of van der Waals radii are white. As can be seen from Figure 5, the HS analysis of both independent (SbPc)⁺ units in the crystal reveals that the (SbPc)⁺ units interact differently. The HS of both (SbPc)⁺ units exhibit that the dominant contacts are H–H interactions (dispersive forces) with a contribution of 36.7% and 45.0% in the HS of (SbPc)⁺ units containing the Sb4 and Sb4B, respectively. The interaction between the (SbPc)⁺ units with the (Sb₆I₁₆Br₆)⁴⁻ counterion is manifested in the HS, and for the 2D-fingerprint plots that show the H–I contacts for the (SbPc)⁺ unit containing Sb4, the H–I contact contribution is greater (9.0%) than for the Sb4B unit (3.4%). However, in the HS of the second (SbPc)⁺ unit, the H–Br interactions with a contribution of ~4.5% are observed, but interactions of this type (H–Br) are absent in the HS for the first (SbPc) unit containing Sb4. Similar correlations as for H–I and H–Br are observed in the C–I and C–Br interactions in the HS of the (SbPc)⁺ units. The H–C/C–H and H–N/N–H interactions in the HS with contributions of 17.3% and 5.9%, and 16.1% and 5.4%, respectively, are found in the SbPc units with Sb4 and Sb4B atoms, respectively. The contribution of the π–π interactions between the back-to-back-oriented pair of (SbPc)⁺ units, which correlate well with the C–C and C–N/N–C contacts in the HS of approximately 8.5% and 10.4% for (SbPc)₂ unit pairs containing Sb4 and Sb4B atoms, respectively, and is 2 ÷ 2.5 times smaller than in normal and planar metallophthalocyanine crystals in which these interactions reach about 22% [93].

The HS of the [Sb₆I₁₆Br₆]⁴⁻ unit shows small light-red areas resulting from the interaction of I–Sb and Br–Sb with the surrounding units (SbPc)⁺ with contributions of 6.8% and 1.4% in the HS. However, the main contribution to the HS results from the interactions of I–H and Br–H, which account for more than 50% of the HS area. The contribution in area of the HS of the (Sb₄I₁₆Br₆)⁴⁻ anion resulting from the I–C, Br–C, I–N, and Br–N interactions with a total share is about 40%. The deconvolution of the 2D-fingerprint plot illustrating the respective interactions for both independent (SbPc)⁺ units and for (Sb₄I₁₆Br₆)⁴⁻ counterion is shown in Figure S2 (in Supplementary Materials).

3.4. UV–Vis Spectrosopy

To characterize the optical properties of the obtained antimony(III) phthalocyanine complex, [(SbPc)₄(Sb₆I₁₆Br₆)], the absorption UV–Vis electronic spectra were made in CH₂Cl₂ and toluene solutions (Figure 6). The ground state electronic absorption spectra of [(SbPc)₄(Sb₆I₁₆Br₆)] in CH₂Cl₂ and in toluene solutions show relatively sharp Q bands typical of unaggregated MPc complexes, as has been reported before for other antimony(III)–phthalocyanine derivatives [94,95]. The Q band in the spectrum of the [(SbPc)₄(Sb₆I₁₆Br₆)] complex in both solutions is broadened, and the half-width of this band is almost twice as wide as compared to the spectra of normal/flat M(II) Pc complexes [96,97,98], since the Sb(III) is larger than the equilibrium cavity size of the ring and cannot be accommodated without ring expansion; thus, the metal protrudes out of the cavity, forming a non-planar Sb(III) phthalocyanine complex. Thus, the symmetry of the (SbPc)⁺ complex unit in solution becomes D_4v symmetry that is lower than D_4h as for normal/planar MPc complexes in solutions [99]. Lowering of the symmetry of the (SbPc)⁺ allows for some so far forbidden transitions, and consequently, spectral splitting as well as a broadening is observed. The Q-band, an electronic transition from HOMO to LUMO of the phthalocyanine ligand in character, in the spectrum of the SbPc complex 1 in the CH₂Cl₂ solution is observed at ~725 nm (logε = 5.33), and the Q band in the spectrum in toluene solution is red shifted to ~740 nm (logε = 5.35), whereas for normal, planar and with D_4h symmetry of metal(II) phthalocyanines, the Q band is observed at almost the same wavelength (~670 nm) [100].

The aggregation behavior of the investigated antimony(III)–phthalocyanine complex (1), [(SbPc)₄(Sb₆I₁₆Br₆)], was examined by varying the concentrations in both used solvents within the limits of the Beer–Lambert’s law. The aggregation of phthalocyanines, especially the planar metallophthalocyanines occurs in solution through a coplanar association that makes the conversion of a monomer to higher order complexes. However, this antimony(III) phthalocyanine complex, due to the non-planarity of the (SbPc)⁺ unit and the partially ionic character of the complex, is better soluble in the most organic common solvents. The aggregation behavior of the [(SbPc)₄(Sb₆I₁₆Br₆)] complex was investigated in CH₂Cl₂ and toluene solutions with concentrations between 8 × 10⁻⁶ mol/L and 10⁻⁶ mol/L. Both solvents are characterized by a non-zero dipole moment, μ = 1.60 D and μ = 0.36 D for CH₂Cl₂ and toluene, respectively [101]. The antimony(III) phthalocyanine complex (1) has minimum aggregation in both solvents, as illustrated in Figure 7. With an increase in concentration, the intensity of absorption of the Q-band increased linearly, with no new band and wavelength shifts observed, which clearly evidenced that the aggregate species was not observed.

The ability of [(SbPc)₄(Sb₆I₁₆Br₆)] to oxidize by H₂O₂ in a methanol solution was investigated. Hydrogen peroxide as an oxidant of metallophthlocyanines was often used. Therefore, for oxidation of [(SbPc)₄(Sb₆I₁₆Br₆)] dissolved in methanol (concentration of 5 × 10⁻⁶ mol/L) to Sb^VPc derivative the H₂O₂ was also used. The changes in the absorption spectrum in the range of the Q band (500–900 nm) during the oxidation process were monitored over time (Figure 8).

The Q band characteristic of Sb^IIIPc at 724 nm begins to decrease with the simultaneous appearance and increase in intensity of the Q band characteristic for Sb^VPc derivatives. Complete disappearance of the Q band from (Sb^IIIPc)⁺ occurred after approximately 2 h. Oxidation of Sb³⁺ to Sb⁵⁺ is associated with a reduction of the ionic radius from 0.90 to 0.64 Å [69], with the result that the too large Sb³⁺ ion protruding from the ring plane in (Sb^IIIPc)⁺ after oxidation to Sb⁵⁺ adapts to the size of the ring cavity. Thus, the Sb⁵⁺ lies in the plane of the Pc ring. The smaller size and higher electronegativity lead to increasing the conjugation of the Pc’s HOMO with the metal 5 pz orbital [102], which results in lower charge density and energy of the Pc’s HOMO. The effect of this is the observed blue shifting of spectral Q band positions (Figure 8). In the solution after oxidation of (Sb^IIIPc)⁺ with D_4v symmetry by H₂O_2, the [Sb^VPc(OH)₂]⁺ complex with OH groups in the trans positions with approximately D_4h symmetry is formed. A quite similar correlation between the Q bands was observed during the oxidation of the complex (SbPc)(I₃)∙½I₂ [68]. This Sb(V)–phthalocyanine complex, [Sb^VPc(OH)₂]⁺, was also suggested by Knӧr [15], who studied the oxidation of (Sb^IIIPc)F by hydrogen peroxide in an ethanol solution.

4. Conclusions

A new, low valence antimony–phthalocyanine complex under iodine vapor atmosphere, [(SbPc)₄(Sb₆I₁₆Br₆)]—(1), was obtained in the crystalline form. In the crystal, the (SbPc)⁺ unit is non-planar, since the Sb(III) is larger than the cavity size of the ring; thus, the metal protrudes out of the cavity, forming a saucer shape. The Hirshfeld surface analysis and the analysis of two-dimensional fingerprint plots show approximately 2.5 times lower π–π interaction between the partially overlapping and saucer-shaped phthalocyanine rings of (SbPc)⁺ units compared to normal, planar MPc’s. This improves its solubility. In the CH₂Cl₂ solution, the Q band is observed at 725 nm, whereas in toluene solution this band is red shifted to 740 nm. Since the [(SbPc)₄(Sb₆I₁₆Br₆)] as a low valent metal of 15 main group phthalocyanine complex is similar to other Sb^IIIPc derivatives, it does not fluorescence; therefore, it can be used as precursor in obtaining (after oxidation) the high valent Sb^VPc complex that is similar to other Sb^V–phthalocyanine derivatives, showing the fluorescence excitation spectral. Oxidation of the complex (Sb^IIIPc)⁺ with D_4v symmetry yields the [Sb^VPc(OH)₂]⁺ complex with OH groups in the trans positions with approximately D_4h symmetry. It should also be noted that the [(SbPc)₄(Sb₆I₁₆Br₆)] before and after the oxidation to the Sb^VPc derivative show a characteristic UV–Vis absorption in the near-infrared range of 600–900 nm (i.e., in the therapeutic window), which should be intriguing from the point of view of potential use as an infrared cut-off filter for plasma displays, silicon photodiodes, solar cells, and charge-generating material as well as photosensitizers for semiconductor lasers or for PDT.

Author Contributions

Conceptualization, R.K. and J.J.; formal analysis, R.K. and J.J.; investigation, R.K. and J.J.; X-ray analysis, J.J.; writing—original draft preparation, R.K. and J.J.; writing—review and editing, R.K. and J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

The authors would like to thank for the opportunity of making calculations with the Gaussian2016 program package in the Wrocław Center for Network and Supercomputing (http://www.wcss.wroc.pl (accessed on 2 January 2022)).

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Schemes and Tables

Scheme 1. Metalloporphyrins (a) and metallophthalocyanines (b) showing the 18π-electron tetrapyrrole macrocycle (in red). For metal-free porphyrin or metal-free phthalocyanine, M is two hydrogens.

Scheme 2. Examples of some Sb(III) and Sb(V) phthalocyanines (-OR = C6H5O- or tert-bythylO-).

Scheme 3. Synthesis of antimony phthalocyanine under oxidation condition of iodine monobromide vapor.

Figure 1. Asymmetric unit of 1 showing two independent (SbPc)+ units and (Sb3I8Br3)2−.

Figure 2. View of the anionic unit of 1.

Figure 3. View of the anionic [Sb6I16Br6]4− complex surrounded by four (SbPc)+ in the crystal of 1.

Figure 4. Packing of [(SbPc)4(Sb6I16Br6)] showing a stacking structure in 1.

Figure 5. Hirshfeld surface (a) and the 2D-fingerprint plots (b) for each building units of the crystal 1 and (c) percentage contributions of the respective interactions in the HS.

Figure 6. Electronic absorption spectra of antimony(III) phthalocyanine (1), [(SbPc)4(Sb6I16Br6)], in toluene (black) and CH2Cl2 (red) solutions.

Figure 7. Optical absorption spectra of SbPc-complex 1 in CH2Cl2 (a) and in toluene (b) at various concentrations (8 × 10−6–1 × 10−6 mol/L).

Figure 8. Spectral changes during the oxidation by H2O2 of 5 × 10−6 mol [(SbPc)4(Sb6I16Br6)] in methanol. The Q bands at 724 nm and 697 nm are characteristic for the SbIII and for SbV phthalocyanines, respectively.

Table 1

Crystal data and final refinement parameters for 1.

	[(SbIIIPc)2(Sb2I8)(SbBr3)]—(1)
Formula	C64H32Br3I8N16Sb5
Molecular weight	2888.73
Temperature (K)	295(2)
Crystal system	Triclinic
Space group	$P\overline{1}$
a, b, c (Å)	14.399(2), 15.344(3), 18.992(3)
α, β, γ (°)	95.64(2), 96.89(2), 113.93(2)
V (Å3)	3757.4(12)
Z	2
Dcal (gcm−3)	2.553
Crystal size (mm3)	0.250 × 0.125 × 0.080
μ (mm−1)	6.707
Absorption correction	multi-scan
Tmax./Tmin.	0.4754/1.000
Total/Unique/Obs refls	23,647/14,738/7721
R int	0.0271
R, [F2 > 2σ(F2)] 1	0.0476
wR [F2 all refls] 2	0.0957
S	1.001
Δρmax, Δρmin (e·Å−3)	2.072, −1.286

¹ R = Σ ||F_o| − |F_c||/ΣF_o; ² wR = {Σ [w(F_o² − F_c²)²]/ΣwF_o⁴}^½; w⁻¹ = σ²(F_o²) + (aP)² where P = (F_o² + 2F_c²)/3. The a parameter is 0.0263.

Table 2

Selected bond lengths and angles (Å, °) in crystal of 1.

(SbPc)+ Cations
Sb4–N2		2.191(8)	Sb4B–N2B		2.202(7)
Sb4–N4		2.178(8)	Sb4B–N4B		2.181(7)
Sb4–N6		2.190(9)	Sb4B–N6B		2.206(8)
Sb4–N8		2.206(8)	Sb4B–N8B		2.146(8)
N2–Sb4–N4		78.0(3)	N2B–Sb4B–N4B		77.6(3)
N4–Sb4–N6		79.4(3)	N4B–Sb4B–N6B		77.7(3)
N6–Sb4–N8		77.5(3)	N6B–Sb4B–N8B		78.5(3)
N2–Sb4–N8		77.7(3)	N2B–Sb4B–N8B		78.4(3)
Deviation of Sb from N4-plane		0.992(1)	Deviation of Sb from N4-plane		0.994(1)
(Sb6I16Br6)4− Anion
Terminal Sb–I
Sb1–I7	2.7668(13)	Sb1–I8	2.6965(12)	Sb2–I6	2.7374(11)
Bridged μ2
Sb1–I3	3.2329(11)	Sb2–I3	2.9833(13)	Sb1–I5	2.8744(11)
Sb3–I5 i	3.4481(10)	Sb2–I4	2.8302(10)	Sb3–I4	3.31901(10)
Bridged μ3
Sb1–I1	3.1765(14)	Sb2–I1	3.1856(11)	Sb2i–I1	3.3610(10)
Sb2–I2	3.0061(13)	Sb1i–I2	3.4281(12)	Sb3–I2	3.3531(11)
Terminal Sb–Br
Sb3–Br1	2.6888(12)	Sb3–Br2	2.5659(14)	Sb3–Br3	2.6321(15)
I8–Sb1–I7	96.78(4)	I8–Sb1–I5	90.48(4)	I7–Sb1–I5	98.42(4)
I8–Sb1–I1	93.88(4)	I7–Sb1–I1	166.21(3)	I5–Sb1–I1	90.20(3)
I8–Sb1–I3	91.83(3)	I7–Sb1–I3	85.88(4)	I5–Sb1–I3	174.85(3)
I1–Sb1–I3	85.05(3)	I6–Sb2–I4	97.06(3)	I6–Sb2–I3	90.66(4)
I4–Sb2–I3	88.68(3)	I6–Sb2–I2	91.49(3)	I4–Sb2–I2	93.83(4)
I3–Sb2–I2	176.48(3)	I6–Sb2–I1	90.88(3)	I4–Sb2–I1	171.80(3)
I3–Sb2–I1	89.16(3)	I2–Sb2–I1	88.02(3)	Br2–Sb3–Br3	92.11(5)
Br2–Sb3–Br1	97.60(5)	Br3–Sb3–Br1	96.8 (5)	Sb1–I1–Sb2	91.46(3)
Sb2–I3–Sb1	94.17(3)
Short Contacts between (SbPc)+ Units and (Sb6I16Br6)4− Anion
Sb4–I1 i	3.5083(13)	Sb4–I2i	3.6471(13)	Sb4–I3	3.6115(14)
Sb4B–I5	3.6283(14)	Sb4B–I7	3.4969(11)	Sb4B–Br3i	3.5941(18)

Symmetry Code: (i) −x, −y, −z + 1.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27061839/s1. Additional material contains the experimental IR spectrum of 1, full optimized parameters for (SbPc)⁺ unit and figures illustrating the deconvolution of the 2D-figerprint plots for individual types of interactions in 1. CCDC No. 2131217 contains the supplementary crystallographic data for 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 2 January 2022) or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: [phone]; or email: [email protected].

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