Spectroscopic Properties and Magnetic Behavior of [Cr(urea)₆][Cr₂O₇]Br·H₂O

The chromium(III) bromide hexahydrate was obtained from Aldrich Chemical Co. and used as supplied. All chemicals were reagent-grade materials and used without further purification. [Cr(urea)₆][Cr₂O₇]Br·H₂O was prepared as described previously.³ Before the spectra were recorded, the complex was purified from aqueous solution by recrystallization. Anal. Found: C, 10.32; H, 3.08; N, 23.38; Calc. for [Cr{CO(NH₂)₂}₆][Cr₂O₇]Br·H₂O: C, 9.92; H, 3.61; N, 23.14%. UV-visible data, λ_max in nm: 274, 291, 367 (sh), 380, 441 (sh), 605. IR data (KBr, cm^-1 ): 3634 (vs) (ν OH), 3450 (vs) (ν_as NH), 3352 (vs) (ν_s NH), 3352 (vs) (ν NH...H bonded), 1652 (s) (ν_s C=O + δ_as NH₂), 1638 (s) (ν_s CO + δ_s NH₂), 1151 (s) (ρ NH₂), 1035 (s) (ν_s C=N), 940 (vs) (νCr–O), 924 (s) (νCr–O), 881 (s) (νCr–O), 797 (s), 776 (m) (ω CO), 689 (w), 633 (s), 598 (m), 531 (sh), 465 (m), and 452 (w) [ν Cr-O(urea)].

Diffuse reflectance electronic spectra were measured on a JASCO V-570 UV/VIS/NIR spectrophotometer, equipped with an integrating sphere in the range of 200−1500 nm. The mid-infrared spectrum was obtained from a KBr pellet using a JASCO 460 plus series FT-IR spectrometer. The magnetic properties were investigated with a Quantum Design MPMS-XL superconducting quantum interference device SQUID magnetometer at an applied field of 0.5 T and a temperature range of 5-300 K. Powder samples were measured in a pharmaceutical cellulose capsule. Diamagnetic correction was applied with Pascal’s method. Analyses for C, H, and N were performed on a Perkin-Elmer 2400II CHNS/O analyzer at the Tokyo University of Science.

The FT-IR spectrum of [Cr(urea)₆][Cr₂O₇]Br·H₂O is presented in Fig. 2.

The FT-IR spectrum showed a very strong sharp absorption band at 3634 cm^-1 due to the O-H stretching mode of the hydrate water molecule. The symmetric and asymmetric modes of the noncoordinated NH₂ group appeared at 3450 and 3352 cm⁻¹, respectively, along with an additional less intense band at 3218 cm⁻¹. The absorption band observed near 1507 cm⁻¹ is assigned to CO + CN vibrations. This indicates the formation of a CrA-OA(urea) bond instead of the CrA-NA(urea) bond reported in literature⁴ for [Cr(urea)₆]Cl₃. The strong absorption bands at 1638 and 1151 cm⁻¹ can be assigned to NH₂ bending and rocking modes, respectively. However, for [Cr(urea)₆]Cl₃ the NH₂ rocking vibration value is 1175 cm⁻¹. The 24 cm⁻¹ lowering in the vibrational frequency indicates the weakening of the N-H bonds due to the presence of strong hydrogen bonding between the NH₂ groups and Cr₂O₇²⁻ (i.e., NA-H…OB). The strong absorption at 1035 cm⁻¹ and the medium absorption at 622 cm⁻¹ are assigned to ν_s(C=N) and δ(NCO), respectively. In compound (I), a sharp peak for the CrB-OB bond in Cr₂O₇²⁻ was observed at 881 cm⁻¹. This is lower than that in the free dichromate ion.⁵ Sharp peaks were observed at 940, 881, and 776 cm⁻¹ that are assigned to the asymmetric, symmetric stretch for Cr₂O₇²⁻, and the symmetric Cr1B-O1B-Cr2B stretching modes, respectively (Hilliard et al., 1982). The lowering in all these values is attributed to the weakening of the CrB-OB bond due to the formation of NA-H…OB hydrogen bonds.

The solid-state UV-visible spectrum of compound (I) is shown in Fig. 3.

Two bands corresponding to the⁴A_2g→⁴T_1g (⁴F) and⁴A_2g→⁴T_1g (⁴P) transitions in the complex cation [Cr(urea)₆]³⁺ are believed to be obscured by the intense bands of the Cr₂O₇²⁻ moiety. In order to have some point of reference for the splitting of the electron bands containing the dichromate anion, we have fitted the band profiles using six main Gaussian curves, as shown in Fig. 3. A deconvolution procedure on the experimental band pattern yielded maxima at 16530, 21505, 23475, 27100, 35090, and 38025 cm⁻¹ for [Cr(urea)₆][Cr₂O₇]Br·H₂O. Three electronic bands observed at 21505, 27100, and 38025 cm−1 can be assigned as the lowest-energy singlet transitions of the dichromate ion:¹A₁→¹E^a,¹A₁→¹A₁+¹E^b, and¹A₁→¹E^c because the UV–visible spectrum of K₂Cr₂O₇ having a Cr(VI) metal center shows absorption bands at 22320, 28010, and 38910 cm⁻¹, respectively.^7,8

The three lowest terms of the Cr(III) free ion 3d³ configuration, i.e.,⁴F,⁴P and²G (⁴F is the ground state) are reduced in an octahedral environment as follows:

In the case of chromium(III) complex with octahedral symmetry, several transitions due to spin-allowed and spin-forbidden are possible, as shown in Fig. 4.

In Fig. 3, two intense bands located at 16530 and 23475 cm⁻¹ correspond to the⁴A_2g→⁴T_2g (ν₁) and 4 A_2g→⁴T_1g (⁴F) (ν₂) transitions for the [Cr(urea)₆]³⁺ moiety. The electronic bands for the [Cr(urea)₆]³⁺ moiety are almost in agreement with those reported for [Cr(urea)₆]Cl₃.⁹

For octahedral d³ system, the formula between three spin-allowed electronic transition energies and ligand field parameters is as followings.¹⁰

The first spin allowed transition directly gives the value of 10Dq. For [Cr(urea)₆]³⁺ moiety, the crystal field splitting parameter, Dq and Racah interelectronic repulsion parameter, B were obtained as 1653 cm⁻¹ and 707 cm⁻¹, respectively. These parameters were calculated from the values of ν₁ and ν₂ by means of following eqs. (4) and (5).

The nephelauxetic parameter, β of 0.77 was calculated by the eq. (6).

where B (free ion) is 918 cm⁻¹ for chromium(III) ion.¹¹ The β value indicates that there is an appreciable covalent character in the metal–ligand σ bond.

The third spin allowed transition,⁴A_2g→⁴T_1g (⁴P) (ν₃) from eq. (3) is predicted to appear at 36715 cm⁻¹. Therefore, the remaining main band of 35090 cm⁻¹ may be assigned to the⁴A_2g→⁴T_1g (⁴P) (ν₃) transition.

The second derivative of a spectral peak generally results in a peak of greater intensity than the original, but inverted. In the second derivative spectrum, the minima correspond to the maxima in the original spectrum, and hence, the minima are described as the positions of the peaks.¹² The spinforbidden⁴A_2g→²E_g (R),²T_1g (T),²T_2g (J) bands were found at 14125, 14950, and 20750 cm⁻¹, respectively, from the second derivative of the solid-state absorption spectrum (Fig. 5), but could not be resolved into separate components.

The Racah parameter, C can be calculated from the position of the⁴A_2g²E_g absorption band and the eq. (7).¹³

The value of C is evaluated to be 2979 cm⁻¹, which is significantly reduced from the free ion value of Cr(III), C (free ion) = 4133 cm⁻¹. A comparsion of the two values reveals that C is decreased by 28 % from C (free ion).¹¹ This decrease is also due to effect of bond covalency. The values of Dq, B and C parameters obtained in here are comparable to those reported for [Cr(chxn)₃]³⁺ (Dq = 2175 cm⁻¹, B = 703 cm⁻¹ and C = 2953 cm⁻¹ ).¹⁴ The spin-orbit coupling can be calculated from the Cole and Garret empirical relation.¹⁵

Using B and Eq. (8) yields λ_eff = 69 cm⁻¹, which is in good agreement with the value expected for chromium(III) ion in crystals.¹⁶ The deviation of λ_eff from λ₀ (87 cm⁻¹ for free ion) is due to the bonding effects of urea ligands toward chromium(III) ion in the crystal.¹⁷

The magnetic susceptibility of [Cr(urea)₆](Cr₂O₇)Br·H₂O was measured in the temperature range of 5-300 K at 10 kOe. The plots of χ_MT vs. T(a), and χ_M⁻¹vs. T(b) are shown in Fig. 6. The value of χ_MT at 300 K is 21.9 cm³ K mol⁻¹. On decreasing the temperature, the χ_MT value slightly increases to a maximum of 24.0 cm³ K mol⁻¹ at 15 K and decreases again down to 23.8 cm³ K mol⁻¹ at 5 K. The effective magnetic moment values, μ_eff, were calculated from the equation:

where χ_M is the molar magnetic susceptibility (emu mol⁻¹ ) and T is the absolute temperature.

The observed effective magnetic moments are in the range 3.73 μ_B - 3.89 μ_B, as shown in Fig. 7. The chromium(III) ion (3d³ ) has three unpaired electrons in the 3d shell, therefore its compounds were considered to have magnetic a moments close to the spin-only value, 3.88 μ_B and consistent with S = 3/2 spin state of a mononuclear Cr(III) d³ center.¹⁷