Synthesis, crystal structure and magnetic properties of the complex [ReCl₃(tppz)]·MeCN

Abstract

The reaction of the starting materials [Re^IIICl₃(MeCN)(PPh₃)₂] or [Re^VOCl₃(PPh₃)₂] with 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) in acetonitrile yielded the Re(III) complex [ReCl₃(tppz)]·MeCN (1). This complex crystallizes in the monoclinic space group P2₁/n and its crystal structure consists of neutral mononuclear entities with meridional geometry of the chloride ligands, and the six-coordination of the Re(III) ion being completed by the tridentate tppz ligand. Each metal centre exhibits a highly distorted octahedral coordination with Re–Cl and Re–N_tppz bond lengths covering the ranges 2.3590(9)–2.3606(8) and 1.971(2)–2.096(2) Å, respectively. The magnetic properties of 1 have been investigated in the temperature range 1.9–290 K. They are characteristic of a six-coordinate Re(III) mononuclear complex with d⁴ low-spin (³T₁ ground state). The magnetic data of 1 are discussed through a deep analysis of the influence of the ligand-field, spin–orbit coupling, tetragonal distortion and covalency effects. The second-order Zeeman effect between the non-magnetic ground state (M_J = 0) and higher energy levels (M_J ≠ 0) determines the magnetic susceptibility of 1, the value of the temperature-independent paramagnetic susceptibility being 3378 × 10⁻⁶ cm³ mol⁻¹. This value compares well with those reported for other structurally characterized Re(III) complexes.

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Introduction

2,3,5,6-Tetrakis(2-pyridyl)pyrazine (tppz), synthesized by Goodwin and Lions in 1959,¹ is a remarkably versatile high-electron donor ligand offering a wide variety of binding modes for the construction of novel systems. It has evidenced that tppz can act as a bidentate (α, β and γ), tridentate, bis-bidentate, tris-bidentate and bis-tridentate ligand² as well as a counterion in its diprotonated form.³ Up to now, 206 compounds incorporating tppz have been reported. Among them, 59 complexes are mononuclear and 147 are multinuclear (including bi-, tri-, tetranuclear compounds).⁴

A scientific interest in tppz complexes can be attributed to rich photophysical and redox properties associated with these compounds making them useful for analytical purposes and probes for biologically relevant molecules such as DNA.⁵ More recently, tppz has been examined as fluorescence sensor for series of metal ions and off–off–on switching of fluorescence depending on stepwise complex formation with tppz has been reported.⁶ A particular emphasis has been paid to studies of the homo- and hetero-polynuclear ruthenium complexes which have shown that tppz is an effective mediator for intermetallic coupling nearly of the order of the Creutz–Taube ion.⁷

Another relevant aspect of the coordination chemistry of tppz concerns the ability of this polydentate organic ligand as a bridge to mediate magnetic interactions between paramagnetic centers separated by more than 6.4 Å. The compounds incorporating the [M₂(tppz)]⁴⁺ unit [M = Ni(II) and Cu(II)] were found to exhibit significant antiferromagnetic interactions between the paramagnetic metal ions involved.^2b,8 Interestingly, the relatively good efficiency of the tppz bridge in transmitting magnetic interactions between paramagnetic centers contrasts with the much poorer ability of the related bridging pyrazine molecule in its metal complexes, where negligible or weak antiferromagnetic interactions were observed (J values ranging from −7.4 to 0 cm⁻¹).⁹ Finally, mononuclear complexes of Co(II) of formula [Co(tppz)₂]²⁺ with the tppz ligands coordinated in a tridentate coordination mode deserve to be mentioned. These compounds attract scientific interest due to the thermally induced spin crossover behavior from a high-spin S_Co = 3/2 at high temperatures to a low-spin S_Co = 1/2 at lower temperatures.^2b,8b,10

The present contribution covers the synthesis and magneto-structural characterization of the new mononuclear rhenium(III) complex [ReCl₃(tppz)]·MeCN (1). The coordination chemistry of Re(III) is dominated by the diamagnetic Re₂⁶⁺ core.¹¹ Examples of mononuclear Re(III) complexes incorporating phosphorus,¹² sulphur¹³ and less frequently nitrogen¹⁴ or oxygen¹⁵ donor ligands have also been described but their magnetic behavior, with compounds exhibiting different degrees of paramagnetism and even diamagnetism,^14h,m,15d can still be considered as largely unexplored.

Results and discussion

Synthesis

The complex [ReCl₃(tppz)]·MeCN (1) was prepared by treating [ReOCl₃(PPh₃)₂] or [ReCl₃(MeCN)(PPh₃)₂] with 2,3,5,6-tetra-(2-pyridyl)pyrazine (tppz) in refluxing acetonitrile:

[Re^IIICl₃(MeCN)(PPh₃)₂] + tppz → [Re^IIICl₃(tppz)] + 2PPh₃ + MeCN (1)

[Re^VOCl₃(PPh₃)₂] + tppz → [Re^IIICl₃(tppz)] + OPPh₃ + PPh₃ (2)

A relatively high yield of [ReCl₃(tppz)] (ca. 70%) in the reaction (2) indicates that the tridentate nitrogen donor tppz facilitates the substitution of the phosphine ligands and then the abstraction of the oxo group and reduction to the [Re^IIICl₃] unit. A similar trend was confirmed by Zubieta et al. for bifunctional chelators based on pyridyl, imidazole- and carboxylate-derivatized amino acids.^14f In general, the use of additional PPh₃ is necessary to abstract the oxo group and reduce the [Re^V=O]³⁺ core of the [ReOCl₃(PPh₃)₂] complex to the [Re^IIICl₃] unit.^14m,16

Crystal structure

Compound 1 crystallizes in the monoclinic space group P2₁/n (Table 1) and its crystal structure consists of neutral [ReCl₃(tppz)] units and solvated acetonitrile molecules. A perspective view of the asymmetric unit of 1 together with the atom numbering is shown in Fig. 1. No intermolecular interactions with enough strength to govern the crystal packing or molecule conformation were found in the structure of 1 except for weak intra- and intermolecular C–H⋯N and C–H⋯Cl type interactions [C(6)–H(6)⋯N(4) with D⋯A distance = 2.570 Å and D–H⋯A angle = 114.0°, C(6)–H(6)⋯Cl(1)^a with D⋯A distance = 2.760 Å and D–H⋯A angle = 139.0° and C(22)–H(22)⋯N(99)^b with D⋯A distance = 2.560 Å and D–H⋯A angle = 134.0°; symmetry code: (a) = −x, 1 − y, −z and (b) = 1 − x, 2 − y, −z] and Re–Cl⋯π contacts [Re1–Cl1⋯Cg1^a (N4–C10–C11–C12–C13–C14) with a distance of 3.761(2) Å and an angle (between X(I) → Cg(J) vector and normal to plane J) of 10.99°] (Fig. 2).

Table 1 Crystal data and structure refinement for complex 1

	1
Empirical formula	C26H19Cl3N7Re
Formula weight (g mol^−1)	722.03
Temperature (K)	293(2)
Wavelength (Å)	0.71073
Crystal system	Monoclinic
Space group	P21/n
Unit cell dimensions (Å, °)	a = 14.9278(4)
	b = 10.2675(2)
	c = 17.3725(5)
	β = 99.442(3)
Volume (Å^3)	2626.63(12)
Z	4
ρcalcd (Mg m^−3)	1.826
Absorption coefficient (mm^−1)	4.962
F(000)	1400
Crystal size [mm]	0.051 × 0.088 × 0.104
θ range for data collection [°]	3.34 to 25.05
Index ranges	−17 ≤ h ≤ 17, −12 ≤ k ≤ 12, −20 ≤ l ≤ 20
Reflections collected	14 243
Independent reflections	4633 [Rint = 0.0296]
Completeness to 2θ = 25°	99.8%
Min. and max. transm.	0.617 and 1.000
Data/restraints/parameters	4633/0/335
R indices (all data)	R1 = 0.0272, wR2 = 0.0453
Largest diff. peak and hole (e Å^−3)	0.560 and −0.375

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Fig. 1 Perspective view of the molecular structure of [ReCl₃(tppz)]·MeCN (1) showing the atom numbering. Displacement ellipsoids are drawn at 50% probability level.

Fig. 2 View of the molecular packing of 1 with marked Re–Cl⋯π as well as C–H⋯N and C–H⋯Cl type interactions.

The Re(III) ion is six-coordinate in a highly distorted octahedral geometry defined by three nitrogen atoms of the tridentate coordinated tppz ligand and three chloride donors, meridionally arranged. The major angular distortions from the idealized octahedral geometry, reflected in the cisoid [77.45(9)–103.44(7)°] and transoid [154.98(10)–179.08(7)°] angles, are attributed to geometrical constraints issued from the occurrence of two fused five-member chelate rings of the tridentate tppz ligand, resulting in N(2)–Re(1)–N(1) and N(2)–Re(1)–N(3) angles of 77.45(9) and 77.58(9)°, respectively (Table 2). The tppz molecule as a whole is far from planarity. The dihedral angles between the mean planes of the two coordinated pyridyl rings and the pyrazine ring are 12.41(14) and 22.41(11)°, while the uncoordinated pyridyl rings are inclined to the pyrazine ring by 67.46(8) and 38.25(12)°. Low energy conformations of the free tppz are predicted to have each pyridyl ring twisted from coplanarity with the pyrazine by roughly 50°.^2f

Table 2 The selected bond lengths (Å) and angles (°) for 1

Bond lengths		Bond angles
Re(1)–N(1)	1.971(2)	N(1)–Re(1)–N(3)	77.58(9)
Re(1)–N(3)	2.096(2)	N(1)–Re(1)–N(5)	77.45(9)
Re(1)–N(5)	2.084(2)	N(1)–Re(1)–Cl(1)	91.44(7)
Re(1)–Cl(1)	2.3606(8)	N(1)–Re(1)–Cl(2)	179.08(7)
Re(1)–Cl(2)	2.3590(9)	N(1)–Re(1)–Cl(3)	90.93(7)
Re(1)–Cl(3)	2.3597(8)	N(3)–Re(1)–Cl(1)	90.69(7)
		N(3)–Re(1)–Cl(2)	101.53(7)
		N(3)–Re(1)–Cl(3)	90.97(7)
		N(5)–Re(1)–N(3)	154.98(10)
		N(5)–Re(1)–Cl(1)	88.19(7)
		N(5)–Re(1)–Cl(2)	103.44(7)
		N(5)–Re(1)–Cl(3)	91.17(7)
		Cl(2)–Re(1)–Cl(1)	88.82(3)
		Cl(2)–Re(1)–Cl(3)	88.83(3)
		Cl(3)–Re(1)–Cl(1)	177.36(3)

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The Re–Cl distances [2.3606(8), 2.3590(9) and 2.3597(8) Å] compare well with the values reported for the related Re(III) complexes with the six-coordinate metal centre in a distorted ReN₃Cl₃ octahedral geometry (Table S1†). The bond length of Re(III) to the middle tppz-nitrogen atom N(2) [1.971(2) Å] is shorter than the Re(1)–N(1) and Re(1)–N(3) distances [2.084(2) and 2.096(2) Å], a pattern which is usually observed in complexes incorporating tppz-like ligands.^14a,17 The shortness of Re–N_pyrazine distance within the tppz unit is attributed to a more efficient overlap of the t_2g metal orbitals with the π* orbitals of the central pyrazine ring compared to the distal pyridyl groups.

Magnetic properties

It is well known that the octahedral ligand field for 5d metal ions is always strong and so the d⁴ electronic configuration of the six-coordinate Re(III) ion is defined by a low-spin state ³T₁ term (t⁴₂), which, in the case of 1 is split by the spin–orbit coupling (SOC) and an axial ligand field as shown in Fig. 3 (hereafter the symbol of the parity will be omitted). SOC splits the ³T₁ term into three spin–orbit energy levels (J = 0, 1 and 2, as shown in the central part of Fig. 3), being the non-magnetic singlet, J = 0, the ground state for λ < 0 (λ is the spin–orbit coupling parameter). In the absence of SOC, a tetragonal distortion splits the ³T₁ term into an orbital singlet (³A₂) and an orbital doublet (²E) separated by an energy gap (Δ) which is defined as positive if the orbital singlet is the lowest (left of Fig. 3) or negative in the reverse situation (right of Fig. 3). Due to the strong spin–orbit coupling and ligand-field acting in the 5d metal ions, both perturbations have to be taken into account simultaneously.

Fig. 3 Splitting of the ³T₁ term of the t⁴₂ electronic configuration by spin–orbit coupling and a tetragonal ligand-field component.

Having these considerations in mind and in order to interpret the magnetic properties of 1, we used the Hamiltonian of eqn (3) (see Appendix)

Ĥ = −κλL̂Ŝ + Δ(L̂_z² − 2/3) + βH(−κL̂ + 2Ŝ) (3)

where the first term is the spin–orbit coupling, the second one accounts for the orbital distortion of the triplet T₁ (T₁ breaks its degeneracy giving an orbital doublet E and an orbital singlet A₂ under a C₄ symmetry group, which are separated by an energy gap Δ). κ is the orbital reduction due to covalency.

In the case of a strict octahedral symmetry (Δ = 0), an exact analytical equation for the magnetic susceptibility can be deduced [eqn (4)].

where x = λ/kT.¹⁸ When a lower symmetry is present, the dependence of the susceptibility on temperature is complicated and no analytical expression can be obtained. As shown in Fig. 3, nine non-degenerate energy levels appear (|J,M_J ≥ functions) under a tetragonal ligand-field distortion. The relevant determinants and the energy values for each level are given in the Appendix.¹⁹ These levels are plotted in Fig. 3 as a function of the Δ/λ ratio.

From Fig. 3 and for λ < 0, one can see that the singlet M_J = 0 is always the ground state for any value of Δ (positive or negative) and also that the energy gap between this non-magnetic ground state and the first magnetic excited state (M_J = ±1) is of the same order as |λ|. Due to the strong SOC (λ ≈ −1250 cm⁻¹),¹⁸ one can assume that the ground state is the only populated one at room temperature and below (kT < λ). Therefore, the magnetic susceptibility for 1 arises only from the second-order Zeeman effect between the non-magnetic ground state (M_J = 0) and higher magnetic levels (M_J ≠ 0), that is, a temperature-independent paramagnetic susceptibility (χ_TIP). In this respect, from eqn (4) and for |λ|/kT ≫ 1, one obtains eqn (5):

Then, the experimental thermal dependence of the χ_MT product of 1 below room temperature is expected to follow a straight line which tends to zero when the temperature vanishes (χ_MT = constant value × T).

Fig. 4 shows a family of χ_MT curves as a function of temperature for different values of the λ, Δ and κ parameters. These curves are obtained through the Hamiltonian of eqn (3) and using the VPMAG program.²⁰ In all these curves, one can see that the χ_MT values tend linearly to zero at low temperatures. It is interesting to note that ligand fields of low symmetry whose values are of the same order of magnitude as (or less than) the spin–orbit coupling constant (|Δ| ≤ |λ|) do not much affect the average paramagnetism of the t⁴₂ configuration, so that one may largely neglect them in dealing with regular octahedral complexes and use eqn (5) to evaluate the temperature independent paramagnetism. So, if we assume the free–ion parameter λ₀ ≈ 1250 cm⁻¹ and an orbital reduction parameter κ ≈ 0.75 (value commonly observed for 5d metal ions),^18,21 a value of the χ_TIP about 3000 × 10⁻⁶ cm³ mol⁻¹ for six-coordinate Re(III) complexes can be expected.

Fig. 4 Theoretical χ_MT vs. T plots as a function of (a) the spin–orbit parameter (λ), (b) reduction parameter (κ), (c), positive tetragonal field (Δ > 0) and (d) negative tetragonal field (Δ < 0).

The χ_MT versus T plot for 1 (χ_M is the magnetic susceptibility per mol of Re(III) ions] is shown in Fig. 5. A linear plot is observed, as expected. At room temperature, χ_MT is equal to 0.93 cm³ mol⁻¹ K and it decreases linearly with temperature, as predicted.

Fig. 5 Temperature dependence of the χ_MT product for 1: (o) experimental; (solid line) theoretical curve through Hamiltonian of eqn (3) and the best-fit parameters (see text); (dotted line) linear fit from equation χ_MT = A + BT (see text). The inset shows a detail of the low temperature domain.

However, it does not tend to zero when temperature vanishes, but to ca. 0.010 cm³ mol⁻¹ K. This feature may be attributed to the presence of some paramagnetic impurity, most likely due to an oxidation of a very small amount of Re(III) to Re(IV). A satisfactory match of the experimental χ_MT values is achieved through eqn (3) with the introduction of a paramagnetic impurity (ρ) in the form of eqn (6).

χT_exp = (1 − ρ)χT_Re(III) + ρχT_Re(IV) (6)

where (for S = 3/2 and g = 1.85).

Least-squares best-fit parameters through the VPMAG program²⁰ were: λ = −1190(20) cm⁻¹, Δ = 1010(25) cm⁻¹, κ = 0.720(5) and ρ = 0.00625(2) [that is ca. 0.63% of Re(IV) is present in 1]. The corresponding value of the χ_TIP is 3080 × 10⁻⁶ cm³ mol⁻¹. This value is very close to that calculated by eqn (5) and it is also similar to those observed in other Re(III) complexes.^18,22 However, in the temperature range explored (1.9–290 K), all these parameters (λ, Δ and κ) are very correlated and it is not possible to obtain unambiguous values for them. Even, the sign for Δ cannot be unambiguously determined. The values of the parameters given above are one of the more reliable set of the best fits. Moreover, because of the large correlation existing between diamagnetism of the ligands (χ_dia) and the χ_TIP, the experimental magnetic susceptibilities obtained for 1 were carefully corrected for the diamagnetism of the constituent atoms [χ_dia = −397 × 10⁻⁶ cm³ mol⁻¹] and also for the sample holder.

Finally, it is interesting to note that, for this type of Re(III) complexes, which have large values of λ (λ ≫ kT), the experimental thermal dependence of the χ_MT product is expected to follow a straight line, χ_MT = A + BT, where A is the value of χ_MT corresponding to the occurrence of a small paramagnetic impurity and B is the temperature-independent paramagnetism (χ_TIP) of Re(III) defined by eqn (5) (the octahedral distortion Δ being neglected). In the case of 1, a fit to a linear plot yields A = 0.00998 cm³ mol⁻¹ K [which corresponds to 0.63% of Re(IV) in 1] and B = 0.003110 cm³ mol⁻¹ (χ_TIP = 3110 × 10⁻⁶ cm³ mol⁻¹). These values are practically the same than those obtained through eqn (3).

Conclusions

The preparative routes of complex 1 whose crystal structure and magnetic properties are described herein demonstrate that tridentate nitrogen donors such as tppz make easy the substitution of the phosphine ligands and the release of the oxo group with reduction of the [Re^V=O]³⁺ core from the [ReOCl₃(PPh₃)₂] complex to the [ReCl₃] entity. A deep analysis of the variable-temperature magnetic data of 1 reveals that it is a d⁴ low-spin species (³T₁ ground state) whose magnetic behaviour is dictated by a large spin–orbit coupling [λ = −1190(20) cm⁻¹]. Therefore, the magnetic susceptibility of 1 arises only from the second-order Zeeman effect between the non-magnetic ground state and the higher magnetic levels, the value of the TIP being 3110× 10⁻⁶ cm³ mol⁻¹.

Experimental section

Materials

The reagents used to the synthesis were commercially available and they were used without further purification. The [ReCl₃(MeCN)(PPh₃)₂]²³ and [ReOCl₃(PPh₃)₂]²⁴ complexes were prepared according to the literature method.

Instrumentation

IR spectrum was recorded on a Nicolet iS5 spectrophotometer in the spectral range 4000–400 cm⁻¹ with the sample in the form of KBr pellets [Fig. S1 in the ESI†]. X-ray powder diffraction (XRPD) measurements [Fig. S2 in the ESI†] were performed on a PANalytical Empyrean X-ray diffractometer using Cu-Kα radiation (λ = 1.5418 Å), in which the X-ray tube was operated at 40 kV and 30 mA ranging from 5 to 50°. The ¹H NMR and ¹³C NMR spectra were recorded at 295 K on a Bruker Advance 500 NMR spectrometer with resonance frequencies of 500 MHz for ¹H NMR spectra and 125 MHz for ¹³C NMR spectra by using DMSO-d₆ as solvent and TMS as an internal solvent [Fig. S3 in the ESI†]. The X-ray diffraction data of 1 were collected on Oxford Diffraction four-circle diffractometer Gemini A Ultra with Atlas CCD detector using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Details concerning crystal data and refinement are given in Table 1. Lorentz, polarization and empirical absorption correction using spherical harmonics implemented in SCALE3 ABSPACK scaling algorithm²⁵ were applied. The structure was solved by the Patterson method using SHELXS97 and refined by full-matrix least-squares on F² using SHELXL97.²⁶ All the non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions refined using idealized geometries (riding model) and assigned fixed isotropic displacement parameters, d(C–H) = 0.93 Å, U_iso(H) = 1.2 U_eq(C) (for aromatic); and d(C–H) = 0.96 Å, U_iso(H) = 1.5 U_eq(C) (for methyl). The methyl groups were allowed to rotate about their local threefold axis.

CCDC-1431249 (complex 1) contain the supplementary crystallographic data for this paper.†

Synthesis of the rhenium(III) complex

Preparation of [ReCl₃(tppz)]·MeCN (1).
Method A. [ReCl₃(MeCN)(PPh₃)₂] (0.50 g, 0.60 mmol) was added to tppz (0.23 g, 0.60 mmol) in acetonitrile (100 mL) and the reaction mixture was refluxed for 4 h. The resulting solution was reduced in volume to 10 mL and allowed to cool at room temperature. A brown crystalline precipitate of 1 was filtered off and dried in the air. X-ray quality crystals of 1 as brown prisms were obtained by slow recrystallization from acetonitrile, and collected by filtration. Yield: ca. 85%.
Method B. [ReOCl₃(PPh₃)₂] (0.50 g, 0.60 mmol) was added to tppz (0.23 g, 0.60 mmol) in acetonitrile (100 mL) and the reaction mixture was refluxed for 4 h. The resulting solution was reduced in volume to 10 mL and allowed to cool at room temperature. A brown crystalline precipitate of 1 was filtered off and dried in the air. X-ray quality crystals of 1 were obtained by slow recrystallization from acetonitrile, and collected. Yield: ca. 70%.

Anal. calcd for C₂₁H₁₆ClN₂O₆Re (1): C, 41.08; H, 5.77; N, 4.56. Found: C, 40.94; H, 5.71; N 4.64%. ¹H NMR (500 MHz, DMSO-d₆): δ = 7.69–7.56 (m, 4H), 7.52 (d, J = 19.0 Hz, 2H), 7.43 (d, J = 21.3 Hz, 2H), 7.35 (s, 4H), 7.27 (s, 4H), 2.08 (s, 3H) ppm. ¹³C NMR (125 MHz, DMSO-d₆): δ = 133.7, 133.6, 132.9, 132.7, 132.5, 132.0, 131.9, 130.7, 129.3, 129.2, 118.6, 1.6 ppm. IR (KBr, cm⁻¹): 1598w, 1586w and 1562w [ν(C=N_tppz)] and ν(C=C_tppz)].

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Polish National Science Centre (Grant No. DEC-2012/05/N/ST5/00743 and DEC-2012/07/N/ST5/02213) and the Spanish MICINN (Project CTQ-2013-44844P). Joanna Palion-Gazda is grateful for a scholarship from the DoktoRIS project co-financed by the European Social Fund.

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Footnote

† Electronic supplementary information (ESI) available: Magneto-structural data for Re(III) complexes (Table S1), IR (Fig. S1), ¹H and ¹³C NMR spectra (Fig. S3), and XPRD (Fig. S2) for 1, Appendix [energy levels for six-coordinate Re(III)] and X-ray crystallographic tables (CIF for 1). CCDC 1431249. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra21466a

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