Chromium(III) complexes with 1,2,4-diazaphospholide and 2,6-bis(N-1,2,4-diazaphosphol-1-yl) pyridine ligands: synthesis, X-ray structural characterization, EPR spectroscopy analysis, and magnetic susceptibility studies

2.1 Syntheses of complexes 1–3

No Cr(III) complexes with 1,2,4-diazaphospholide anions have previously been reported, although the Cr(0) carbonyl compound [(η¹(N):η¹(P)Hdp){Cr(CO)₅}₂], with a neutral 1H-1,2,4-diazaphosphole ligand (Hdp), was described several years ago [17]. It therefore appeared that an exploration of Cr(III) complexes with the 1,2,4-diazaphospholide/2,6-bis(N-2,4-diazaphosphol-1-yl)pyridine ligand was required. CrCl₃(THF)₃ was reacted with K[3,5-tBu₂dp] [20] at a ratio of 1:1 in toluene at 0°C. The reaction proceeded smoothly and gave [η²-3,5-tBu₂dp)Cr(THF)₂Cl₂] (1) as violet crystals in 75% yield (see Scheme 1). Reacting CrCl₃(THF)₃ with K[3,5-ph₂dp]·0.67THF [20] similarly gave the complex [(η²-3,5-ph₂dp)Cr(THF)₂Cl₂] (2) as blue-green crystals in a fair yield (45%) (see Scheme 1). Compounds 1 and 2 were soluble in n-hexane and very soluble in toluene and THF.

Scheme 1:

Preparation of complexes 1 and 2.

As mentioned above, 2,6-bis(N-2,4-diazaphosphol-1-yl)pyridine was synthesized very recently, and it has only been used to prepare a 2,6-bis(N-2,4-diazaphosphol-1-yl)pyridine zinc complex [29]. Encouraged by this, the complex {[2,6-bis(N-2,4-diazaphosphol-1-yl)pyridine]CrCl₃} (3) was prepared by reacting CrCl₃(THF)₃ with 2,6-bis(N-1,2,4-diazaphospholyl-1-yl)pyridine [29] at a ratio of 1:1 in anhydrous ethanol at room temperature (Scheme 2). Complex 3 was isolated as deep green crystals, and was soluble in ethanol, dimethylformamide, and dimethylsulfoxide. We found that complex 3 could also be obtained in a fair yield and under similar conditions by reacting CrCl₂ with 2,6-bis(N-1,2,4-diazaphospholyl-1-yl)pyridine in the presence of adventitious oxygen at room temperature.

Scheme 2:

Preparation of complex 3.

2.2 Descriptions of the structures of complexes 1–3

X-ray diffraction analyses of crystals of complexes 1 and 2 showed that in each complex the metal center is pseudo-octahedrally coordinated by the [3,5-R₂dp]^– ligand in a η² coordination mode, two chloro ligands, and two THF molecules. The structures of 1 and 2 are shown in Figs. 1 and 2, respectively [38]. Complexes 1 and 2 were actually found to be structurally very similar. Important crystallographic data for complexes 1 and 2 are given in Table 1, and selected bond lengths and bond angles are given in Table 2. The Cr–N bond lengths (2.013(3) Å in 1 and 2.014(19) Å in 2) and the N–Cr–N bond angles (39.1(14)° in 1 and 39.09(10)° in 2) were comparable to those previously found in [Cr(η²-3,5-tBu₂pz)₃] (pz = pyrazolato), in which the Cr–N bond length was 1.986(6) Å and the N–Cr(III)–N bond angle was 40.24(11)° [39].

Fig. 1:

Molecular structure of 1 in the crystal. Displacement ellipsoids are drawn at the 30% probability level; hydrogen atoms are omitted for clarity.

Fig. 2:

Molecular structure of 2 in the crystal. Displacement ellipsoids are drawn at the 30% probability level; hydrogen atoms are omitted for clarity.

Complex 3 was found to have slightly distorted octahedral geometry, with three basal imine N donors from the tridentate ligand and three chloro ligands in meridional geometry, as shown in Fig. 3. Important crystallographic data for complex 3 are provided in Table 1, and selected bond lengths and bond angles are given in Table 2. The Cr–N{pyridine} bond was found to be shorter (at 1.997(2) Å) than the Cr–N{dp} bonds, which were 2.053(2) Å long, as shown in Table 2. Similar bond lengths have been found in [Cr(pybox)(THF)₂] (pybox = 2,6-bis{(4S)-(–)-isopropyl-2-oxazolin-2-yl}pyridine), in which the Cr–N{pyridine} bond length was 2.017(6) Å and the Cr–N{oxazolino} bond length was 2.078(6) Å [40], and in the bis(imino)pyridyl Cr(III) complexes, in which the Cr–N{pyridine} bond length was 1.981(3) Å and the Cr–N{imino} bond length was 2.142(3) Å [41]. As expected, the most obvious structural change, compared with the structure of the free ligand in the solid state, occurred when 3 was formed, was that the terminal dp-yl rings moved from trans, trans to cis, cis geometry [29]. The structure of 3 was found to be somewhat comparable to the structure of [Cu(bdppz)(NCCH₃)]²⁺·2ClO₄⁻ [29].

Fig. 3:

Molecular structure of 3 in the crystal. Displacement ellipsoids are drawn at the 30% probability level; hydrogen atoms are omitted for clarity.

2.3 EPR analysis of complex 1

Crystals of complex 1 were subjected to EPR measurements, which were performed on a Bruker ESP-A300-10/12 spectrometer. The X-band and a high-sensitivity resonator were used. The microwave frequency was 9.5 GHz, the microwave power was 22.6 mW, the modulation amplitude was 18 G, the modulation frequency was 100 kHz, the receiver gain was 1.00 × 10³, the time constant was 164 ms, and the conversion time was 80 ms. The X-band EPR spectra of polycrystalline 1 at room temperature (298 K) (Fig. 4) and at 77 K (in liquid nitrogen) (Fig. 5), and the g-strain values are shown in Figs. 6 and 7. The quartet ⁴F (S = 3/2) is the fundamental term for the free Cr(III) ion (i.e. a d³ ion with spherical symmetry). In an octahedral environment, this term splits into two orbital triplets, ⁴T₂ and ⁴T₁, and an orbital singlet, ⁴A₂, which becomes the fundamental term. Thus, for an octahedral d³ complex having Kramer’s doublet lowest in energy, three transitions can sometimes be detected with small zero field splitting [42]. However, only one transition line will be found if the zero field parameter is large relative to the frequency of the spectrometer. We could not obtain an EPR spectrum of 1 dissolved in THF even at a low temperature (77 K). Fortunately, polycrystalline 1 gave EPR spectra that were consistent with the presence of high-spin octahedral metal ions [43]. The X-band spectrum of 1 at 298 K (see Fig. 4) contained three broad signals corresponding to three transitions, and it at 77 K (see Fig. 5) contained two broad signals corresponding to two transitions. Similar spectra were previously found for high-spin octahedral tris(2-trimethylamidopyridinato)chromium [39]. The effective spin therefore became Ŝ = 1/2, with the spin Hamiltonian shown in the equation

H=μB(g′xlxS^x+g′ylyS^y+g′zlzS^z)

Fig. 4:

X-band EPR spectrum of compound 1 in the crystalline state at 298 K.

Fig. 5:

X-band EPR spectrum of compound 1 in the crystalline state at 77 K.

Fig. 6:

EPR spectrum of compound 1 in the crystalline state at 298 K.

Fig. 7:

EPR spectrum of compound 1 in the crystalline state at 77 K.

In this equation, g′x = 4.3017,g′y = 3.2915, and g′z = 2.3540 at 298 K (see Fig. 6) and g′x = 4.1598 and g′y = 3.0125 at 77 K (see Fig. 7). These g_i values are comparable with those previously found for a six-coordinated octahedral tetraorganochromate(III) (g′x = 4.02,g′y = 3.55, and g′z = 1.95) [44].

Magnetic susceptibility measurements were made on a polycrystalline sample of 1 using a Quantum Design SQUID MPMSXL-7 magnetometer. The magnetic properties of complex 1 were determined from the magnetic susceptibility χ(T) measurements made at different temperatures (shown in Fig. 8). The experimental susceptibility data followed the Curie–Weiss law over the temperature range 25–300 K (shown in Fig. 9). This would be expected for magnetically isolated ions. The equation for the Curie–Weiss law is χ(T)=χ0+C/(T−θ), where C is the Curie constant (1.889 cm³ mol^–1 K), θ is the Curie paramagnetic temperature (–9.77 K), and χ₀ is the temperature-independent contribution (5.1 × 10⁻⁴ cm³ mol^–1) to χ(T). The molecular susceptibility increased weakly as the temperature decreased from 300 to 25 K; at that temperature the Cr spins became decoupled because of thermal randomization. A broad maximum corresponding to antiferromagnetic interactions was found at approximately 25 K, and the susceptibility increased rapidly below 20 K, indicating the presence of a non-diamagnetic ground state. The χ(T) value was slightly temperature dependent below 300 K, confirming that the system followed the Curie–Weiss law. The χ(T) values gradually decreased at temperatures below 25 K, and this should mainly be caused by crystal field effects and the presence of some paramagnetic impurities. The magnetic moment of 1 was found to be slightly temperature dependent, μ_eff being 3.80 μ_B at 250 K and 3.86 μ_B at 300 K, as is shown in Fig. 9. These magnetic moments were similar to the spin-only magnetic moment that would be expected for three unpaired electrons (3.87 μ_B) [35, 36, 41].

Fig. 8:

Experimental χ_m and χ_mT (χ_m is the magnetic susceptibility of a mononuclear unit) values plotted against T for complex 1.

Fig. 9:

Experimental 1/χ_m and μ_eff values plotted against T for complex 1.

3.1 Preparation of [(η²-3,5-tBu₂dp)Cr(THF)₂Cl₂] (1)

A mixture of CrCl₃(THF)₃ (0.375 g, 1.0 mmol) and K[3,5-tBu₂dp] (0.236 g, 1.0 mmol) was prepared at 0°C, and then toluene (30 mL) was added using a syringe. The mixture was stirred for 24 h, and then filtered through Celite (Imerys Filtration Minerals, San Jose, CA, USA). The filtrate was concentrated to about 10 mL, and cooling of the solution to –30°C gave 1 as blue-purple crystals. Yield 75% based on ligand. M.p. 179°C. – IR (KBr pellet, Nujol mull): ν = 2923, 2853, 1460, 1377, 1261, 1095, 1019, 799 cm^–1. – C₁₈H₃₄Cl₂CrN₂O₂P (464.3): calcd. C 46.52, H 7.32, N 6.03; found C 46.57, H 7.29, N 6.05%.

3.2 Preparation of [(η²-3,5-ph₂dp)Cr(THF)₂Cl₂] (2)

A mixture of CrCl₃(THF)₃ (0.375 g, 1.0 mmol) and K[3,5-ph₂dp]·0.67THF (0.330 g, 1.0 mmol) was prepared at 0°C, and then toluene (30 mL) was added using a syringe. The mixture was stirred for 48 h, and then filtered through Celite. The filtrate was concentrated to about 10 mL, and cooling of the solution to –30°C gave 2 as blue-green crystals. Yield 45% based on the ligand. M.p. 230°C. – IR (KBr pellet, Nujol mull): ν = 2923, 2853, 1454, 1406, 1376, 1261, 1093, 1015, 799, 693 cm^–1. – C₂₂H₂₆Cl₂CrN₂O₂P (504.3): calcd. C 52.34, H 5.16, N 5.55; found C 52.36, H 5.18, N 5.53%.

3.3 Preparation of {[2,6-bis(1,2,4-diazaphosphol-1-yl)pyridine]CrCl₃} (3)

A mixture of CrCl₃(THF)₃ (0.375 g, 1.0 mmol) and [2,6-bis(N-1,2,4-diazaphospholyl-1-yl)pyridine] (0.247 g, 1.0 mmol) was prepared, and then dry ethanol (50 mL) was added using a syringe. The mixture was stirred for 24 h, and then filtered through Celite. The filtrate was concentrated to about 15 mL, and cooling the solution to –30°C gave 3 as dark green crystals. Yield 70% based on ligand. M.p. 269°C. – IR (KBr pellet, Nujol mull): ν = 2959, 2923, 2853, 1609, 1577, 1461, 1376, 1260, 1087, 1021, 798, 731 cm^–1. – C₉H₇Cl₃CrN₅P₂ (405.5): calcd. C 26.63, H 1.39, N 17.26; found C 26.66, H 1.38, N 17.23%.

3.4 X-ray structure determinations

Suitable single crystals of complexes 1–3 were sealed under argon in thin-walled glass capillaries. X-ray diffraction data were collected on an Xcalibur CCD diffractometer (graphite-monochromatized MoK_α radiation, λ = 0.71073 Å, φ–ω scan technique). The intensity data were integrated by means of the program Saint [45]. Sadabs [46] was used to perform area-detector scaling and absorption corrections. The structures were solved by Direct Methods and were refined against F² using all unique reflections with the aid of the Shelxtl package [47–50]. All non-hydrogen atoms in 1–3 were refined anisotropically. The H atoms were included in calculated positions with isotropic displacement parameters related to those of the supporting carbon atoms, but were not included in the refinement.

CCDC 1437817, 1437818, and 1437819 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.