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

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.