Late First-Row Transition-Metal Complexes Containing a 2-Pyridylmethyl Pendant-Armed 15-Membered Macrocyclic Ligand. Field-Induced Slow Magnetic Relaxation in a Seven-Coordinate Cobalt(II) Compound

Abstract

The 2-pyridylmethyl N-pendant-armed heptadentate macrocyclic ligand {3,12-bis(2-methylpyridine)-3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1,14,16-triene = L} and [M(L)](ClO4)2 complexes, where M = Mn(II) (1), Fe(II) (2), Co(II) (3), Ni(II) (4), and Cu(II) (5), were prepared and thoroughly characterized, including elucidation of X-ray structures of all the compounds studied. The complexes 1-5 crystallize in non-centrosymmetric Sohncke space groups as racemic compounds. The coordination numbers of 7, 6 + 1, and 5 were found in complexes 1-3, 4, and 5, respectively, with a distorted pentagonal bipyramidal (1-4) or square pyramidal (5) geometry. On the basis of the magnetic susceptibility experiments, a large axial zero-field splitting (ZFS) was found for 2, 3, and 4 (D(Fe) = -7.4(2) cm(-1), D(Co) = 34(1) cm(-1), and D(Ni) = -12.8(1) cm(-1), respectively) together with a rhombic ZFS (E/D = 0.136(3)) for 4. Despite the easy plane anisotropy (D > 0, E/D = 0) in 3, the slow relaxation of the magnetization below 8 K was observed and analyzed either with Orbach relaxation mechanism (the relaxation time τ0 = 9.90 × 10(-10) s and spin reversal barrier Ueff = 24.3 K (16.9 cm(-1))) or with Raman relaxation mechanism (C = 2.12 × 10(-5) and n = 2.84). Therefore, compound 3 enlarges the small family of field-induced single-molecule magnets with pentagonal-bipyramidal chromophore. The cyclic voltammetry in acetonitrile revealed reversible redox processes in 1-3 and 5, except for the Ni(II) complex 4, where a quasi-reversible process was dominantly observed. Presence of the two 2-pyridylmethyl pendant arms in L with a stronger σ-donor/π-acceptor ability had a great impact on the properties of all the complexes (1-5), concretely: (i) strong pyridine-metal bonds provided slight axial compression of the coordination sphere, (ii) substantial changes in magnetic anisotropy, and (iii) stabilization of lower oxidation states.

Structure and Magnetism of Seven‐Coordinate FeIII, FeII, CoII and NiII Complexes Containing a Heptadentate 15‐Membered Pyridine‐Based Macrocyclic Ligand

“…It is more elongated than the other bond lengths of such type, because the mean value of the Ni–O bond length is 2.055 Å, and 90 % of all the observed Ni–O bonds have lengths in the range of 1.964–2.114 Å. Similar elongation of the Ni–O bonds was observed in other Ni II complexes with macrocyclic ligands (Figure and S1), [Ni(Ac 2 ‐15N 2 O 3 )](ClO 4 ) 2 , [Ni(15‐pyN 3 O 2 )Cl 2 ] or [Ni(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 , with the Ni–O bond lengths of 2.554(3), 2.506(3) and 2.663(3), and 2.653(2) Å, respectively.…”

“…The axial positions are occupied by two oxygen atoms (O pend ) from the carboxylate groups of the pendant arms. The M–O pend bonds are the shortest coordination bonds in 2 and 3 (Table ), and at the same time the Co–O pend bonds are the shortest axial coordination bonds in series of [Co(15‐pyN 3 O 2 )Cl 2 ] and [Co(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 (Table ).…”

“…Variations in the functional groups of the pendant arms can provide fine tuning of the ligand field for the complexed metal ions. Such approach was successfully employed in our previous study concerning the seven‐coordinate Mn II , Fe II , Co II , and Ni II complexes of py 2 ‐15‐pyN 3 O 2 (Figure ) with two 2‐pyridylmethyl pendant arms . Magnetic measurements revealed noticeable values of magnetic anisotropy for [Fe(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 ( D = –7.4 cm –1 ), [Co(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 ( D = 34 cm –1 ), and [Ni(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 ( D = –12.8 cm –1 , E/D = 0.136).…”

“…Magnetic measurements revealed noticeable values of magnetic anisotropy for [Fe(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 ( D = –7.4 cm –1 ), [Co(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 ( D = 34 cm –1 ), and [Ni(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 ( D = –12.8 cm –1 , E/D = 0.136). Moreover, the slow relaxation of the magnetization was observed [ τ 0 = 9.90 × 10 –10 s, U eff = 24.3 K (16.9 cm –1 )] for [Co(py 2 ‐15‐pyN 3 O 2 )](ClO 4 ) 2 …”

Magnetic Anisotropy and Field‐induced Slow Relaxation of Magnetization in Tetracoordinate CoII Compound [Co(CH3‐im)2Cl2]

“…The reason why U eff derived from the AC susceptibility data is smaller than theoretically predicted is that other relaxation processes than the Orbach one could also be present. However, the discrepancy can also be caused by the fact that with the Equation (5) only data having the maxima on the imaginary part of susceptibility are analyzed ( T ≤ 3.1 K), and the non-zero values are already observed at ≈ 4 K. Therefore, we analysed these high temperature data for the highest frequencies with a simplified model derived under the assumption that the adiabatic susceptibility is usually approaching zero in the single-molecule magnets [29] $\ln \left({\chi }^{″}/{\chi }^{\prime }\right)=\ln \left(2\pi f{\tau }_0\right)+U/kT$ …”