Spin Crossover in Double Salts Containing Six- and Four-Coordinate Cobalt(II) Ions.

The preparation and spectroscopic and structural characterization of three cobalt(II) complexes of formulas [Co(tppz) ₂ ](dca) ₂ (1), [Co(tppz) ₂ ][Co(NCS) ₄ ]·MeOH (2), and [Co(tppz) ₂ ][Co(NCO) ₄ ]·2H ₂ O (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide] are reported here. Compounds 1-3 have in common the presence of the cationic [Co(tppz) ₂ ] ²⁺ entity where each mer-tridentate tppz ligand coordinates to the cobalt(II) ion equatorially through two pyridyl donors and axially via the pyrazine, completing the six-coordination. The electroneutrality is achieved by the organic dca group (1) and the anionic tetrakis(thiocyanato-κN)cobaltate(II) (2) and tetrakis(cyanato-κN)cobaltate(II) (3) complexes. Direct current (dc) magnetic susceptibility measurements of 1 in the temperature range 1.9-400 K show the occurrence of a thermally induced spin crossover behavior of the [Co(tppz) ₂ ] ²⁺ unit from a high spin (S = 3/2) at higher temperatures to a low-spin (S = 1/2) at lower temperatures, with the low spin phase being reached at T ≤ 200 K. X-band electron paramagnetic resonance (EPR) measurements in solution at low temperatures were used to characterize the low spin state. An analytical expression based on the combination of the spin-orbit coupling and both first- and second-order Zeeman effects for a d ⁷ electronic configuration was used to fit the magnetic data of 1, the values of the best-fit parameters being C _vib = 0.1367(9), λ = -168(2) cm ^-1 , α = 1.12(1), Δ = 1626(15) cm ^-1 , and g _LS = 2.12(1). The magnetic behavior of the four-coordinate cobalt(II) ions [Co(NCS) ₄ ] ^2- (2) and [Co(NCO) ₄ ] ^2- (3) with a ⁴ A ₂ ground state overlaps with the spin crossover of the [Co(tppz) ₂ ] ₂₊ entity, the abrupt decrease of the χ _M T product below 15.0 K being due to zero-field splitting effects between the spin components |±1/2> and |±3/2>. The combined analysis of the dc magnetic data and the Q-band EPR spectra in the solid state of 2 and 3 led to the following sets of best-fit parameters: C _vib = 0.105(5), λ = -170(4) cm ^-1 , α = 1.10(2), Δ = 1700(25) cm ^-1 , g _LS = 2.10(1), g _HS = 2.27(1), and |D| = 3.80(2) cm ^-1 (2) and C _vib = 0.100(1), λ = -169(5) cm ^-1 , α = 1.10(3), Δ = 1500(30) cm ^-1 , g _LS = 2.10(1), g _HS = 2.28(1), and |D| = 4.30(2) cm ^-1 (3). Some evidence of slowing of the relaxation of the magnetization has been found in the out-of-phase ac signal at very low temperatures under applied dc fields of 0.1-0.4 T for 3, suggesting the occurrence of single-ion magnet behavior of its [Co(NCO) ₄ ] ^2- anionic entity.