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].2H2O (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide] are reported here. Compounds 13 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-kappa N)cobaltate(II) (2) and tetrakis(cyanato-kappa N)cobaltate(II) (3) complexes. Direct current (dc) magnetic susceptibility measurements of 1 in the temperature range 1.9400 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 spinorbit 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), lambda = -168(2) cm(1), alpha = 1.12(1), Delta = 1626(15) cm(1), and gLS = 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 chi T-M 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), lambda = -170(4) cm(1), alpha= 1.10(2), Delta = 1700(25) cm(1), g(LS) = 2.10(1), g(HS) = 2.27(1), and |D| = 3.80(2) cm1 (2) and C-vib = 0.100(1), lambda= -169(5) cm(1), alpha = 1.10(3), Delta = 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 0.10.4 T for 3, suggesting the occurrence of single-ion magnet behavior of its [Co(NCO)(4)](2) anionic entity.