Variation of the ground spin state in homo- and hetero-octanuclear copper(ii) and nickel(ii) double-star complexes with a meso-helicate-type metallacryptand core

Abstract

Homo- and heterometallic octanuclear complexes of formula Na₂{[Cu₂(mpba)₃][Cu(Me₅dien)]₆}(ClO₄)₆·12H₂O (1), Na₂{[Cu₂(Mempba)₃][Cu(Me₅dien)]₆}(ClO₄)₆·12H₂O (2), Na₂{[Ni₂(mpba)₃][Cu(Me₅dien)]₆}(ClO₄)₆·12H₂O (3), Na₂{[Ni₂(Mempba)₃][Cu(Me₅dien)]₆}(ClO₄)₆·9H₂O (4), {[Ni₂(mpba)₃][Ni(dipn)(H₂O)]₆}(ClO₄)₄·12.5H₂O (5), and {[Ni₂(Mempba)₃][Ni(dipn)(H₂O)]₆}(ClO₄)₄·12H₂O (6) [mpba = 1,3-phenylenebis(oxamate), Mempba = 4-methyl-1,3-phenylenebis(oxamate), Me₅dien = N,N,N′,N′′,N′′-pentamethyldiethylenetriamine, and dipn = dipropylenetriamine] have been synthesized through the “complex-as-ligand/complex-as-metal” strategy. Single-crystal X-ray diffraction analyses of 1, 3, and 5 show cationic M^II₂M′^II₆ entities (M, M′ = Cu and Ni) with an overall double-star architecture, which is made up of two oxamato-bridged M^IIM′^II₃ star units connected through three meta-phenylenediamidate bridges between the two central metal atoms leading to a binuclear metallacryptand core of the meso-helicate-type. Dc magnetic susceptibility data for 1–6 in the temperature range 2–300 K have been analyzed through a “dimer-of-tetramers” model [H = −J(S_1A·S_3A + S_1A·S_4A + S_1A·S_5A + S_2B·S_6B + S_2B·S_7B + S_2B·S_8B) −J′S_1A·S_2B, with S_1A = S_2B = S_M and S_3A = S_4A = S_5A = S_6B = S_7B = S_8B = S_M′]. The moderate to strong antiferromagnetic coupling between the M^II and M′^II ions through the oxamate bridge in 1–6 (−J_Cu–Cu = 52.0–57.0 cm⁻¹, −J_Ni–Cu = 39.1–44.7 cm⁻¹, and −J_Ni–Ni = 26.3–26.6 cm⁻¹) leads to a non-compensation of the ground spin state for the tetranuclear M^IIM′^II₃ star units [S_A = S_B = 3S_M′−S_M = 1 (1 and 2), 1/2 (3 and 4), and 2 (5 and 6)]. Within the binuclear M^II₂meso-helicate cores of 1–4, a moderate to weak antiferromagnetic coupling between the M^II ions (−J′_Cu–Cu = 28.0–48.0 cm⁻¹ and −J′_Ni–Ni = 0.16–0.97 cm⁻¹) is mediated by the triple m-phenylenediamidate bridge to give a ground spin singlet (S = S_A−S_B = 0) state for the octanuclear M^II₂Cu^II₆ molecule. Instead, a weak ferromagnetic coupling between the Ni^II ions (J′_Ni–Ni = 2.07–3.06 cm⁻¹) operates in the binuclear Ni^II₂meso-helicate core of 5 and 6 leading thus to a ground spin nonet (S = S_A + S_B = 4) state for the octanuclear Ni^II₈ molecule. Dc magnetization data for 5 reveal a small but non-negligible axial magnetic anisotropy (D = −0.23 cm⁻¹) of the S = 4 Ni^II₈ ground state with an estimated value of the energy barrier for magnetization reversal of 3.7 cm⁻¹ (U = −DS²). Ac magnetic susceptibility data for 5 show an unusual slow magnetic relaxation behaviour at low temperatures which is typical of “cluster glasses”. The temperature dependence of the relaxation time for 5 has been interpreted on the basis of the Vogel–Fulcher law for weakly interacting clusters, with values of 2.5 K, 1.4 × 10⁻⁶ s, and 4.0 cm⁻¹ for the intermolecular interaction parameter (T₀), the pre-exponential factor (τ₀), and the effective energy barrier (U_eff), respectively.