Charge transfer complexes of metal-free phthalocyanine radical anions with decamethylmetallocenium cations: (Cp*₂Co⁺)(H₂Pc˙⁻)·solvent and (Cp*₂Cr⁺)(H₂Pc˙⁻)·4C₆H₄Cl₂

Abstract

Charge transfer complexes (Cp*₂Co⁺)(H₂Pc˙⁻)·0.5C₆H₄Cl₂·0.7C₆H₅CN·0.3C₆H₁₄ (1) and (Cp*₂Cr⁺)(H₂Pc˙⁻)·4C₆H₄Cl₂ (2) have been obtained as single crystals. Both complexes contain metal-free phthalocyanine (Pc) radical anions and decamethylmetallocenium cations. Reduction of the Pc macrocycle leads to the appearance of new bands at 1026–1030 nm in the NIR range and blue shifts of both Soret and Q-bands of H₂Pc in the spectra of 1 and 2. The geometry of the Pc macrocycles supports the formation of H₂Pc˙⁻ by the alternation of shorter and longer C–N(imine) bonds in the macrocycles in 2. Complex 1 contains pairs of H₂Pc˙⁻ having effective π–π interactions with two sandwiched Cp*₂Co⁺ cations, whereas complex 2 contains stacks composed of alternating Cp*₂Cr⁺ and H₂Pc˙⁻ ions. The magnetic moment of 1 is 1.64 μ_B at 300 K due to the contribution of the H₂Pc˙⁻ spins with the S = 1/2 state and diamagnetism of Cp*₂Co⁺. This is supported by the observation of a narrow EPR signal of 1 with g = 2.0032–2.0036 characteristic of H₂Pc˙⁻. Strong antiferromagnetic coupling of spins with a Weiss temperature of −23 K is observed between H₂Pc˙⁻ in 1. This coupling is probably mediated by the Cp*₂Co⁺ cations. The magnetic moment of 2 is 4.18 μ_B at 300 K indicating the contribution of both paramagnetic H₂Pc˙⁻ (S = 1/2) and Cp*₂Cr⁺ (S = 3/2) species. In spite of the presence of stacks of alternating ions in 2, only weak magnetic coupling is observed with a Weiss temperature of −4 K most probably due to ineffective π–π interactions between Cp*₂Cr⁺ and H₂Pc˙⁻. The EPR spectrum of 2 contains an asymmetric signal attributed to Cr^III (g₁ = 3.9059–3.9220) and a narrow Lorentzian signal from H₂Pc˙⁻ with g₂ = 1.9943–1.9961. In addition to these signals, a broad EPR signal grows in intensity below 80 K with g₄ = 2.1085–2.2438 which can be attributed to both paramagnetic Cp*₂Cr⁺ and H₂Pc˙⁻ species having exchange interactions.

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Introduction

Metal phthalocyanines can be promising components in the design of conducting and magnetic compounds.^1–6 Chemical or electrochemical oxidation of metal phthalocyanines or the {M^III(CN)₂Pc}⁻ anions yields partially oxidized phthalocyanine macrocycles which with appropriate packing can provide highly conducting properties including one-dimensional metal conductivity stable down to liquid helium temperatures.^1–4 Since metal phthalocyanines can contain paramagnetic metals, they can also be used as active components in the design of magnetic assemblies. For example, the oxidation of manganese(II) phthalocyanine or substituted naphthalocyanines by tetracyanoethylene yields polymeric compounds with the alternation of the (Mn^IIIPc)⁺ and TCNE˙⁻ ions. These compounds show ferrimagnetic ordering of spins.^5,6

Metal-free and metal-containing phthalocyanines have weak acceptor properties⁷ and can be reduced by strong donors like alkali metals, LiCp*, or sodium fluorenone ketyl to yield radical anion salts.^8–17 Decamethylchromocene is also a strong donor and forms a variety of charge transfer complexes with different planar π-acceptors most of which show promising magnetic properties.^18–20 The first reduction potentials of metal-free and metal-containing phthalocyanines are in the −0.4 – −0.8 V range.⁷ Since decamethylchromocene (Cp*₂Cr) has an E_ox of −1.04 V vs. SCE²¹ and decamethylcobaltocene (Cp*₂Co) has an E_ox of −1.44 V vs. SCE,²¹ they can reduce phthalocyanines to form charge transfer (CT) complexes. The first compound of such type was obtained with iron(II) phthalocyanine, Fe^IIPc. Cp*₂Cr reduces the Fe^IIPc forming CT complex (Cp*₂Cr⁺){Fe^I(Pc²⁻)}⁻·4C₆H₄Cl₂, which contains stacks of alternating [Fe^I(Pc²⁻)]⁻ and Cp*₂Cr⁺ ions. Alternation of these ions in the stacks having different spin states [Fe^I(Pc²⁻)]⁻ (Fe^I, S = 1/2) and Cp*₂Cr⁺ (Cr^III, S = 3/2) results in the ferrimagnetic ordering of spins below 5 K.²² However, until now no more CT complexes of phthalocyanines and decamethylmetallocenes have been obtained.

In this work, we have obtained and studied the first charge transfer (CT) complexes of metal-free phthalocyanine (H₂Pc) with decamethylchromocene and decamethylcobaltocene: (Cp*₂Co⁺)(H₂Pc˙⁻)·0.5C₆H₄Cl₂·0.7C₆H₅CN·0.3C₆H₁₄ (1) and (Cp*₂Cr⁺)(H₂Pc˙⁻)·4C₆H₄Cl₂ (2). These complexes were obtained in crystalline forms allowing us to study their crystal structures. The optical and magnetic properties of these complexes were also studied for polycrystalline samples. In contrast to previously studied (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ which contains metal-reduced [Fe^IPc(²⁻)]⁻ anions with spin density localized on the Fe^I centers,²² the negative charge and spin of H₂Pc˙⁻ are delocalized over the Pc macrocycle. The variation of magnetic properties of the CT complexes will be compared referring to the reduced moieties, Fe^I or H₂Pc˙⁻.

Results and discussion

Synthesis

The interaction of decamethylcobaltocene (Cp*₂Co) and decamethylchromocene (Cp*₂Cr) with metal-free phthalocyanine (H₂Pc) in o-dichlorobenzene under anaerobic conditions produced deep blue solutions characteristic of the reduced Pc macrocycle. In the case of Cp*₂Co, the CT complex partially precipitated after cooling the solution, and in this case 3 mL of benzonitrile was added to dissolve this complex. Synthesis with Cp*₂Cr was carried out in pure o-dichlorobenzene. Slow mixing of the obtained solutions with n-hexane for 2 months yielded crystals on the walls of the diffusion cell. The obtained crystals were isolated from the solution, washed with n-hexane and dried to give rhombic plates of 1 with copper luster (in 64% yield) and elongated parallelepipeds of 2 with copper luster (in 46% yield). The compositions of complexes were determined from X-ray diffraction studies on single crystals as (Cp*₂Co⁺)(H₂Pc˙⁻)·0.5C₆H₄Cl₂·0.7C₆H₅CN·0.3C₆H₁₄ (1) and (Cp*₂Cr⁺)(H₂Pc˙⁻)·4C₆H₄Cl₂ (2). Several crystals tested from each synthesis had the same unit cell parameters to show that only one crystal phase was formed for both complexes. Elemental analysis cannot be used to confirm the composition of the complexes due to their high air sensitivity.

Optical properties

The IR spectra of starting H₂Pc, 1 and 2 are shown in Fig. S1 and S2 and are listed in Table S1.† The IR spectra of 1 and 2 are a superposition of the absorption bands of H₂Pc, Cp*₂M and solvent molecules. However, some absorption bands of H₂Pc strongly decreased in intensity (bands at 1004 and 1501 cm⁻¹) or disappear (1118 cm⁻¹). An absorption band corresponding to the vibrations of the N–H bonds of H₂Pc is manifested at 3274 cm⁻¹. This band is shifted to larger wavenumbers during the reduction of the Pc macrocycle appearing at 3299 (1) and 3301 (2) cm⁻¹ (the shift is 25–27 cm⁻¹). Similar shifts were previously found for the H₂Pc˙⁻ radical anions in (cryptand[2,2,2][Na⁺])(H₂Pc˙⁻)·1.5C₆H₄Cl₂ and (TOA⁺)·(H₂Pc˙⁻)·C₆H₄Cl₂ (TOA⁺ is a tetraoctylammonium cation).¹⁵

The spectra of H₂Pc, 1 and 2 in the UV-visible-NIR range are shown in Fig. 1. The spectrum of starting H₂Pc shows the Soret band at 338 nm and the split Q-band at 640 and 698 nm. Reduction of the Pc macrocycle leads to noticeable changes in the spectrum of H₂Pc. The Soret band is blue shifted to 305 (1) and 309 (2) nm in the spectra of both complexes, whereas the Q-band is split and slightly blue shifted to 622 and 672 nm (1) and 592, 632, 672 nm (2) (Fig. 1). New bands observed in the NIR range are positioned at 1026 and 913 nm for 1 and 1030 and 932 nm for 2. These bands are attributed to H₂Pc˙⁻. The appearance of the NIR bands and the blue shift of the Soret and Q-bands are characteristic of radical anion salts with the reduced Pc macrocycle.^15–17

Fig. 1 UV-visible-NIR spectra of starting H₂Pc and salts 1 and 2 measued in KBr pellets prepared for 1 and 2 in anaerobic conditions.

Crystal structures

There are two independent H₂Pc and Cp*₂Co units and three sites for solvent molecules in 1. These sites are occupied by disordered C₆H₅CN, C₆H₄Cl₂ and C₆H₁₄ molecules to give the composition of 1 as (Cp*Co⁺)(H₂Pc˙⁻)·0.5C₆H₄Cl₂·0.7C₆H₅CN 0.3C₆H₁₄. The crystal structure of 1 is shown in Fig. 2. It contains blocks in which two Cp*₂Co⁺ cations are sandwiched between two H₂Pc˙⁻ radical anions (Fig. 2). The interplanar distance between the Pc macrocycles in these blocks is 10.373 Å. The block consists of a couple of Cp*₂Co and H₂Pc˙⁻ units, which are related by the inversion center. The interplanar distances among the Cp* planes of Cp*₂Co and the Pc planes of two oppositely located H₂Pc˙⁻ are not uniform and are equal to 3.42 and 3.53 Å. This results in the difference of van der Waals C⋯C contacts between Cp*₂Co and H₂Pc˙⁻, which are in the 3.42–3.44 and 3.55–3.67 Å ranges, respectively. Projection of two halves of Cp*₂Co⁺ cations on the plane of the Pc macrocycle in 1 is shown in Fig. 2b. Both halves are positioned over oppositely located isoindole units of the Pc macrocycle.

Fig. 2 (a) Crystal structure of 1 viewed along the a axis. Solvent molecules are not shown for clarity. The van der Waals C⋯C contacts (<3.56 Å) between H₂Pc˙⁻ and Cp*₂Co⁺ are shown by green dashed lines; (b) projection of two halves of Cp*₂Co⁺ (bonds are shown by green solid lines) on the Pc plane. Hydrogen atoms are not shown. The overlap integrals between H₂Pc˙⁻ and Cp*₂Co⁺ are shown as s1–s6.

There are halves of independent H₂Pc˙⁻ and Cp*₂Cr⁺ ions and two independent disordered C₆H₄Cl₂ molecules in 2 to yield the composition as (Cp*₂Cr⁺)(H₂Pc˙⁻)·4C₆H₄Cl₂. In spite of similar size and shape of the Cp*₂M⁺ cations, complex 2 has a completely different structural motif from that in complex 1 with stacks of alternating H₂Pc˙⁻ and Cp*₂Cr⁺ ions directed along the a axis (Fig. 3a and b). This structure is isostructural to that observed previously for the (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ CT complex showing ferrimagnetic ordering of spins below 5 K.²² The Cp*₂Cr⁺ cations are positioned closer to the center of the Pc ligand in comparison with 1 but not exactly over its center (Fig. 3c). The interplanar distance between the Pc macrocycles in the stacks of 10.639 Å is longer than those in 1 (10.373 Å) and (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ (10.471 Å).²² As a result, the interplanar distances between Cp* and H₂Pc˙⁻ are increased in 2 up to 3.54 Å, and there are no C(Cp*)⋯C(H₂Pc˙⁻) atomic contacts shorter than 3.56 Å. This situation is different from that in the [Fe^I(Pc²⁻)]⁻ complex which has the interplanar distances between the Cp* and [Fe^I(Pc²⁻)]⁻ planes of 3.46 Å and multiple van der Waals contacts between them in the 3.47–3.57 Å range.²² The solvent C₆H₄Cl₂ molecules are incorporated between the Cp*₂Cr⁺ ions belonging to the neighboring stacks of ions (Fig. 3a and b).

Fig. 3 Crystal structure of 2: side view on the stacks of alternating H₂Pc˙⁻ and Cp*₂Cr⁺ ions (a) and (b). The van der Waals C⋯C contacts (<3.56 Å) between the components are shown by green dashed lines; (c) projection of one half of Cp*₂Cr⁺ (bonds are shown by green solid lines) on the Pc plane. Hydrogen atoms are not shown. Types of overlap integrals between H₂Pc˙⁻ and Cp*₂Cr⁺ (s1) and between H₂Pc˙⁻ (s2 and s3) are shown.

The intermolecular overlap integrals in 1 and 2 were examined by the extended Hückel method^23,24 based on the results of X-ray single crystal structure analyses. The interactions between Cp*₂M⁺ and H₂Pc˙⁻ were evaluated from the overlap integrals among the frontier orbitals {highest occupied molecular orbital (HOMO) for Cp*₂M⁺ and singly occupied molecular orbital (SOMO) for H₂Pc˙⁻}. As we discussed in the Crystal structure section, the Cp*₂Co⁺ and H₂Pc˙⁻ ions form blocks in which two Cp*₂Co⁺ cations are sandwiched between two H₂Pc˙⁻. Since there are two independent Cp*₂Co⁺ and each Cp*₂Co⁺ approaches closer only to one of the two H₂Pc˙⁻, four different face-to-face overlap integrals s1–s4 are observed in 1 (Fig. 2a). There are also side-to-face contacts between the Cp* ligand of Cp*₂Co⁺ and two H₂Pc˙⁻ (overlap integrals s5 and s6, Fig. 2a). The overlap integrals s1, s2, s3 and s4 were estimated to be 0.0038, 0.0041, 0.0036 and 0.0010, respectively. As was expected, side-to-face overlap integrals s5 and s6 are essentially smaller to be 0.0002 and 0.0004, respectively. These data indicate rather effective face-to-face π–π interactions between the Cp* of Cp*₂Co⁺ and H₂Pc˙⁻ in 1. At the same time all overlap integrals between H₂Pc˙⁻ in 1 are in the 0.0001–0.0002 range to predict the band insulating state for the H₂Pc˙⁻ network.

Chains of alternating Cp*₂Cr⁺ and H₂Pc˙⁻ ions are formed in 2 (Fig. 3a and b). Since the Cp*₂Cr⁺ cations are positioned at an equal distance from the planes of both surrounding H₂Pc˙⁻, only one type of face-to-face overlap integral is observed between them (s1). The s1 overlap integral is equal to 0.0005 (several times smaller than those in 1) indicating ineffective π–π interactions between Cp*₂Cr⁺ and H₂Pc˙⁻ in the alternating stacks. This can be explained by the positioning of the Cp* ligand of Cp*₂Cr⁺ over an empty central part of the Pc macrocycle (Fig. 3c) and the absence of short van der Waals contacts between them. There are also weak side-by-side van der Waals contacts between H₂Pc˙⁻ along the b axis (Fig. 3a, overlap integral s2) and the [0, 1 −1] direction (Fig. 3b, overlap integral s3). In both cases the estimated overlap integrals did not exceed 0.0002 indicating weak π–π interactions between H₂Pc˙⁻ in 2.

The structure of (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ is isostructural to that of 2 with stacks of alternating Cp*₂Cr⁺ and [Fe^I(Pc²⁻)]⁻ ions along the a axis.²² However, in this case the overlap integral between the HOMOs of both [Fe^I(Pc²⁻)]⁻ and Cp*₂Cr⁺ was calculated to be 0.0013 showing a larger π–π interaction between the Cp* of Cp*₂Cr and [Fe^I(Pc²⁻)]⁻ in (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ than that in 2.

There are shorter C–N(imine) and longer C–N(pyrrole) bonds in the Pc macrocycle. Previous studies of the radical anions of metal-free and metal-containing phthalocyanines showed that the formation of a 19-π-electron system in Pc˙³⁻ from a more stable aromatic 18-π-electron system of Pc²⁻ generally results in the alternation of the C–N(imine) bonds due to the partial loss of aromaticity in the Pc ligand.^{11,12,15–17} Changes in the C–N(imine) bonds cannot be analyzed in 1 due to low bond length precision. However, this alternation is well pronounced in 2 to give the shorter and longer C–N(imine) bonds with 1.315(3) and 1.354(3) Å length, respectively, and are positioned in such a way that they belong to two oppositely located isoindole units. The geometry of two independent H₂Pc˙⁻ is slightly different in 1. One macrocycle has twisted conformation but another macrocycle is nearly planar with deviation of only one phenylene group from planarity (the dihedral angle between the 24-atom Pc plane and the plane of this phenylene group is 7.59°). The Pc macrocycle in 2 has a slightly non-planar shape with two phenylene groups located above the 24-atom Pc plane and two such groups located below this plane. These effects can be attributed to the packing effects with the Cp*₂M⁺ cations.

Both Cp*₂Co⁺ cations have unusual eclipsed conformation in 1 with methyl substituents of one Cp* ligand positioned nearly facing each other to the methyl substituents of another Cp* ligand. Most probably such unfavorable conformation is stabilized by specific interactions with two Pc macrocycles. The average length of the Co–C(Cp*) bonds for two independent Cp*₂Co⁺ cations in 1 is 2.051(9) and 2.048(9) Å. This length corresponds to the formation of the Cp*₂Co⁺ cations which have the average Co–C(Cp*) bond length of 2.04–2.05 Å,²⁵ whereas neutral Cp*₂Co has longer Co–C(Cp*) bonds of 2.101(3) Å.²⁶ The Cp*₂Cr⁺ cations have staggered conformation in 2 which is typical for decamethylmetallocenes. The average Cr–C(Cp*) bond length in 2 is 2.198(3) Å. This value is close to that for the Cp*₂Cr⁺ cations (the average Cr–C(Cp*) bond length is 2.176(3)–2.180(3) Å).^27,28 These bonds are longer than those for neutral Cp*₂Cr (2.152(4) Å (ref. 18)).

Magnetic properties

The magnetic properties of 1 and 2 were analyzed for polycrystalline samples by SQUID and EPR techniques. The effective magnetic moment of 1 is equal to 1.64 μ_B at 300 K (Fig. 4a) corresponding to the contribution of one non-interacting S = 1/2 spin per formula unit (the calculated value is 1.73 μ_B). Since it is well known that the Cp*₂Co⁺ cations containing Co^III are diamagnetic,²¹ the observed magnetic moment was attributed to the H₂Pc˙⁻ spins. Previously it was shown that these radical anions have the S = 1/2 spin state.¹⁵ The Weiss temperature determined from the linear temperature dependence of the reciprocal molar magnetic susceptibility of 1 in the 300–50 K range is −23 K (Fig. 4c) showing rather strong antiferromagnetic coupling of spins. The magnetic moment of 1 decreases below 120 K (Fig. 4a) due to the antiferromagnetic coupling of spins. However, long-range antiferromagnetic ordering is not observed down to 1.9 K (Fig. 4a and c). Magnetic exchange between H₂Pc˙⁻ can be mediated by diamagnetic Cp*₂Co⁺ cations. We can suppose that this is a rather effective route for magnetic coupling between H₂Pc˙⁻ due to effective π–π interactions between Cp*₂Co⁺ and H₂Pc˙⁻.

Fig. 4 Temperature dependencies of the effective magnetic moment of 1 (a) and 2 (b) and those of the reciprocal molar magnetic susceptibility for 1 (c) and 2 (d).

The presence of spin on H₂Pc˙⁻ is supported by the EPR data since only one narrow EPR signal is observed in the spectrum of 1 with a g-factor of 2.0036 and a linewidth (ΔH) of 0.54 mT at 297 K (Fig. 5a). The signal is even more narrowed with the temperature decrease (Fig. 5b) and has g = 2.0032 and ΔH = 0.28 mT at 4.2 K. Narrow EPR signals (ΔH = 0.1–0.2 mT) with similar g-factors were observed previously for the H₂Pc˙⁻ radical anions in the salts with organic cations.^15,16

Fig. 5 (a) EPR signal in complex 1 measured at 297 K; (b) temperature dependence of linewidth for the EPR signal of 1.

The effective magnetic moment of 2 is equal to 4.18 μ_B at 300 K (Fig. 4b). It is close to the value of 4.24 μ_B calculated for the system of two non-interacting S = 3/2 and S = 1/2 spins per formula unit. Therefore, in contrast to 1, complex 2 contains paramagnetic Cp*₂Cr⁺ cations with a high S = 3/2 spin state. This spin state was previously found for different salts of Cp*₂Cr⁺.^18,22,28,29 The H₂Pc˙⁻ radical anions have the S = 1/2 spin state as in 1. Therefore, uniform stacks of magnetic centers with different spin states {S = 3/2 (Cp*₂Cr⁺) and S = 1/2 (H₂Pc˙⁻)} are formed in 2. Similar stacks of the alternating spins with S = 3/2 (Cp*₂Cr⁺) and S = 1/2 [Fe^I(Pc²⁻)]⁻ spin states are formed in (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ which shows ferrimagnetic ordering of spins below 5 K.²² However, in contrast to this complex, only weak antiferromagnetic coupling of spins is observed in 2 with a Weiss temperature of −4 K determined in the 10–300 K range (Fig. 4d). This value is even smaller than that for 1 containing diamagnetic Cp*₂Co⁺. The difference in the magnetic behavior of 2 and (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ in spite of a similar structure and spin state of the components can be explained by different conditions for π–π interactions between the components. The center of the Cp* ligand of Cp*₂Cr⁺ is positioned exactly over the Fe^I atom in (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ leading to the formation of short Fe⋯C(Cp*) contacts of 3.40–3.56 Å. As a result, the Cp* ligand can effectively mediate magnetic coupling between the paramagnetic Fe^I and Cr^III centers. On the contrary, the Cp* ligand of Cp*₂Cr⁺ is positioned over the empty central part of the H₂Pc macrocycle in 2 (Fig. 3c). Short van der Waals contacts between Cp*₂Cr⁺ and H₂Pc˙⁻ are absent and, hence, the π–π interaction is relatively weak between them (due to small overlap integrals), while the additional electron of H₂Pc˙⁻ is delocalized over the Pc macrocycle. In this case the Cp* ligand cannot effectively mediate magnetic coupling between Cr^III and H₂Pc˙⁻ resulting in only weak antiferromagnetic interactions. Therefore, compounds with stacks of alternating paramagnetic metal atoms like Fe^I and Cr^III between which magnetic coupling is mediated through the Cp* ligands are more promising in the design of magnetic assemblies. In the case of compounds with stacks of alternating organic paramagnetic radical anions like H₂Pc˙⁻ and Cr^III, magnetic coupling depends strongly on the conditions for the π–π interaction between the components.

Complex 2 manifests a complicated EPR spectrum due to the presence of different paramagnetic species. An asymmetric signal with g₁ = 3.922 at 20 K can be attributed unambiguously to Cp*₂Cr⁺ with the S = 3/2 spin state (Fig. 6, spectrum at 20 K). Asymmetry of this signal can be due to polycrystallinity of the sample. A narrower intense signal containing two lines at 20 K with g₂ = 1.9943 and a linewidth (ΔH) of 1.8 mT (Fig. 6, green curve) and g₃ = 1.9831 and ΔH = 4.2 mT at 20 K (Fig. 6, red curve) can be ascribed to H₂Pc˙⁻. These signals are observed from 4.1 K up to room temperature. They have the following parameters at 4.1 K: g₁ = 3.9059, g₂ = 1.9961 (ΔH = 1.78 mT) and g₃ = 1.9832 (ΔH = 3.24 mT) (Fig. 6). A new signal appears below 80 K (Fig. 6, blue curve). It grows in intensity and broadens with the temperature decrease. The parameters of this signal are g₄ = 2.1085 and ΔH = 46.8 mT at 20 K and g₄ = 2.2438 and ΔH = 80 mT at 4.1 K. Most probably the broad signal originates from both paramagnetic H₂Pc˙⁻ and Cp*₂Cr⁺ species having exchange interactions since the g-factor of this signal is approximately intermediate between those characteristic of H₂Pc˙⁻ and Cp*₂Cr⁺. The g_⊥-value of the EPR signal from Cp*₂Cr⁺ is also close to 2.000.^28,30 However, generally this component has weaker intensity in the spectrum of Cp*₂Cr⁺ and it is not so broad.^28,30

Fig. 6 EPR spectrum of polycrystalline 2 at 20 and 4.1 K.

Experimental

Materials

Decamethylcobaltocene (Cp*₂Co) and metal-free phthalocyanine (H₂Pc, 98%) were purchased from Aldrich. Decamethylchromocene (Cp*₂Cr, >95%) was purchased from Strem. o-Dichlorobenzene (C₆H₄Cl₂, Acros) was distilled over CaH₂ under reduced pressure, n-hexane was distilled over Na/benzophenone, and benzonitrile (C₆H₅CN, Aldrich) was distilled over sodium under reduced pressure. Solvents were degassed and stored in an MBraun 150B-G glove box. Complexes 1 and 2 were synthesized and stored in the glove box under a controlled atmosphere containing less than 1 ppm of water and oxygen. KBr pellets used for the IR and UV-visible-NIR analyses were prepared in the glove box. EPR and SQUID measurements were performed on the polycrystalline samples of 1 and 2 sealed in 2 mm quartz tubes in the glove box under ambient pressure.

General

UV-visible-NIR spectra were recorded in KBr pellets on a PerkinElmer Lambda 1050 spectrometer in the 250–2500 nm range. FT-IR spectra were obtained in KBr pellets with a PerkinElmer spectrum 400 spectrometer (400–7800 cm⁻¹). EPR spectra were recorded for the polycrystalline samples of 1 and 2 with a JEOL JES-TE 200 X-band ESR spectrometer equipped with a JEOL ES-CT470 cryostat in the temperature range from 293 down to 4 K. A Quantum Design MPMS-XL SQUID magnetometer was used to measure the static magnetic susceptibility of 1 and 2 at 100 mT magnetic field under cooling and heating conditions in the 300–1.9 K range. A sample holder contribution and core temperature independent diamagnetic susceptibility (χ_d) were subtracted from the experimental values. The χ_d values were estimated by the extrapolation of the data in the high-temperature range by fitting the data with the expression: χ_M = C/(T − Θ) + χ_d, where C is the Curie constant and Θ is the Weiss temperature. The effective magnetic moment (μ_eff) was calculated with the following formula: μ_eff = (8·χ_M·T)^1/2.

Synthesis

Crystals of 1 and 2 were obtained by a diffusion technique. A reaction mixture was filtered into a 1.8 cm-diameter, 50 mL glass tube with a ground glass plug, and then 30 mL of n-hexane was layered over the solution. Slow mixing of the solutions resulted in the precipitation of crystals over 2 months. The solvent was then decanted from the crystals, and they were washed with n-hexane. The compositions of the obtained compounds were determined from X-ray diffraction analysis on a single crystal. Several crystals from one synthesis were found to consist of single crystalline phase. Due to the high air sensitivity of 1 and 2, elemental analysis could not be used to determine the composition because they reacted with oxygen in the air before the quantitative oxidation procedure could be performed.

(Cp*₂Co⁺)(H₂Pc˙⁻)·0.5C₆H₄Cl₂·0.7C₆H₅CN·0.3C₆H₁₄ (1) was obtained via the reduction of H₂Pc (21.5 mg, 0.042 mmol) by slight excess of Cp*₂Co (14 mg, 0.0425 mmol) in 15 mL of o-dichlorobenzene by stirring at 100 °C for 4 hours. After cooling down to room temperature, the complex partially precipitated from the solution and 3 mL of benzonitrile was added to dissolve the precipitate by stirring the mixture at 100 °C for 2 hours. This resulted in complete dissolution of the precipitate and the formation of deep blue solution characteristic of the reduced Pc macrocycle. The solution was cooled down to room temperature and filtered into the tube for diffusion. Rhombic plates of 1 with copper luster were obtained in 64% yield.

(Cp*₂Cr⁺)(H₂Pc˙⁻)·4C₆H₄Cl₂ (2) was obtained via the reduction of H₂Pc (21.5 mg, 0.042 mmol) by slight excess of Cp*₂Cr (14 mg, 0.0435 mmol) in 16 mL of o-dichlorobenzene by stirring at 100 °C for 4 hours. This resulted in complete dissolution of H₂Pc and the formation of deep blue solution which was cooled down to room temperature and filtered into the tube for diffusion. Elongated parallelepipeds of 2 with copper luster were obtained in 46% yield.

X-ray crystal structure determination

X-ray diffraction data‡ for 1 were collected on an Oxford diffraction “Gemini-R” CCD diffractometer with graphite monochromated MoK_α radiation using an Oxford Instrument Cryojet system. Raw data reduction to F² was carried out using CrysAlisPro, Oxford Diffraction Ltd. X-ray diffraction data for 2 were collected on a Bruker Smart Apex II CCD diffractometer with graphite monochromated MoK_α radiation using a Japan Thermal Engineering Co. cooling system DX-CS190LD. Raw data reduction to F² was carried out using Bruker SAINT.³² The structures were solved by direct methods and refined by the full-matrix least-squares method against F² using SHELX-2013.³³ Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated geometrically. The tiny crystal sample of 1 showed very low diffraction resulting in a low number of intense reflections. As a result, the observed/unique reflection ratio is low. Nevertheless the crystal structure solution and refinement were stable. The structure of 1 contains three positions of solvent molecules. In all these positions solvent molecules are disordered and the positions are shared by several types of molecules. The occupancies for the C₆H₄Cl₂ molecules are 0.221(3)/0.605(3)/0.114(5)/0.061(2), for the C₆H₅CN molecules are 0.886(5)/0.350(3)/0.173(3) and the C₆H₁₄ molecules have only one orientation with the 0.589(3) occupancy. The structure of 2 contains two positions of strongly disordered solvent C₆H₄Cl₂ molecules with the 0.510(4)/0.203(4)/0.121(3)/0.108(3)/0.058(3) and 0.355(13)/0.229(11)/0.227(4)/0.189(6) occupancies. To keep the anisotropic thermal parameters of the disordered atoms within the reasonable limits the displacement components were restrained using ISOR, SIMU and DELU SHELXL instructions. This resulted in 1173 and 1393 restraints used for the refinement of the crystal structures of 1 and 2, respectively.

Conclusions

Summarizing the studies here, new charge transfer complexes of metal-free phthalocyanine (H₂Pc) and decamethylmetallocenes (Cp*₂Co, Cp*₂Cr) were obtained. In both complexes, the nearly full transfer of an electron from decamethylmetallocenes to H₂Pc is realized forming the H₂Pc˙⁻ radical anions and the Cp*₂M⁺ cations. This can be explained by essentially more negative oxidation potentials of Cp*₂M (E_ox = −1.04 − −1.47 V vs. SCE in acetonitrile)²¹ in comparison with the first reduction potential of H₂Pc (E_red = −0.66 V vs. SCE in DMF).³¹ The formation of these ions is supported by their geometric parameters, the appearance of new bands of H₂Pc˙⁻ in the NIR range and blue shifts of both Soret and Q-bands of H₂Pc. Magnetic data also support the formation of diamagnetic Cp*₂Co⁺ (1), high-spin paramagnetic Cp*₂Cr⁺ with the S = 3/2 spin state (2), and paramagnetic H₂Pc˙⁻ with the S = 1/2 spin state. An effective antiferromagnetic coupling between the H₂Pc˙⁻ spins is attained in 1 which is most probably mediated through the diamagnetic Cp*₂Co⁺ cations. Complex 2 has stacks of alternating Cp*₂Cr⁺ and H₂Pc˙⁻ ions and is isostructural to the previously studied complex (Cp*₂Cr⁺)[Fe^I(Pc²⁻)]⁻·4C₆H₄Cl₂ which manifested ferrimagnetic ordering of spins below 5 K.²² However, complex 2 shows only weak antiferromagnetic coupling of spins. Since the additional electron in H₂Pc˙⁻ is delocalized over the Pc macrocycle, we suppose that the additional factor to modulate the relative orientation between the Cp* ligand of Cp*₂Cr⁺ and H₂Pc˙⁻ will improve the magnetic interactions, e.g. the incorporation of solvent molecules with different sizes and shapes than those in 2.

Acknowledgements

The work was supported by RSF Grant No. 17-13-01215, and by JSPS KAKENHI Grant Numbers JP23225005 and JP26288035.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: IR spectra of starting compounds and complexes 1 and 2. CCDC 1529825 and 1529823. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt00336f

‡ Crystal data of 1 at 150(1) K: C_61.70H_57.65ClCoN_8.70, M_r = 1015.51 g mol⁻¹, black plate, triclinic, P1̄, a = 14.339(2), b = 18.376(3), c = 20.764(3) Å, α = 90.130(13), β = 101.362(14), γ = 103.949(13)^°, V = 5198.5(15) ų, Z = 4, d_calc = 1.298 g cm⁻³, μ = 0.431 mm⁻¹, F(000) = 2134, 2θ_max = 59.538°, reflections measured 46 575, unique reflections 23 594, reflections with I > 2σ(I) = 5358, parameters refined 1555, restraints 1173, R₁ = 0.1182, wR₂ = 0.2503, G.O.F. = 0.976, CCDC 1529825.

Crystal data of 2 at 150(1) K: C₇₆H₆₄Cl₈CrN₈, M_r = 1424.95 g mol⁻¹, black parallelepiped, triclinic, P1̄, a = 10.7851(6), b = 12.5254(6), c = 15.0629(7) Å, α = 68.377(4), β = 70.048(5), γ = 71.112(5)°, V = 1731.85(17) ų, Z = 1, d_calc = 1.366 g cm⁻³, μ = 0.524 mm⁻¹, F(000) = 736, max. 2θ_max = 56.882°, reflections measured 21 878, unique reflections 8445, reflections with I > 2σ(I) = 6022, parameters refined 784, restraints 1393, R₁ = 0.0669, wR₂ = 0.1882, G.O.F. = 1.013, CCDC 1529823.

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