Structural, Magnetic, and Redox Diversity of First-Row Transition Metal Complexes of a Pyridine-Based Macrocycle: Well-Marked Trends Supported by Theoretical DFT Calculations

Abstract

A series of first-row transition metal complexes with 15-membered pyridine-based macrocycle (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene = L) was prepared ([M(II)(L)Cl2], where M = Mn, Co, Ni, Zn (1, 3, 4, 6); [Fe(III)(L)Cl2]Cl (2), [Cu(II)(L)Cl]Cl (5)) and thoroughly characterized. Depending on the complexated metal atom, the coordination number varies from 7 (Mn, Fe, Co), through 5 + 2 for Ni and 4 + 1 for Cu, to 5 for Zn accompanied by changes in the coordination geometry from the pentagonal bipyramid (1-4) to the square pyramid (5 and 6). Along the series, the metal-oxygen distances were prolonged in such manner that their bonding character was investigated, apart from X-ray structural analysis, also by ab initio calculations (Mayer's bond order, electron localization function), which confirmed that, in 4 and 5, two and one oxygen donor atoms are semicoordinated, respectively, and one and two oxygen atoms are uncoordinated in 5, and 6, respectively. On the basis of the temperature variable magnetic susceptibility measurements, 1 and 2 behave as expected for 3d(5) high-spin configuration with negligible zero-field splitting (ZFS). On the other hand, a large axial ZFS (D(Co) ≈ 40 cm(-1), D(Ni) ≈ -6.0 cm(-1)) was found for 3 and 4, and rhombic ZFS (E/D ≈ 0.15) for 4. Antiferromagnetic exchange coupling was observed for 4 and 5 (J(Ni) = -0.48 cm(-1), and J(Cu) = -2.43 cm(-1), respectively). The obtained results correlate well with ab initio calculations of ZFS parameters as well as J-values, which indicate that the antiferromagnetic exchange is mediated by hydrogen bonds. The complexes were also investigated by cyclic voltammetry in water or acetonitrile. A quasi-reversible couple Mn(II)/Mn(III) at 1.13/0.97 V, an almost reversible couple Fe(II)/Fe(III) at 0.51/0.25 V, and a one-step/multistep reduction/oxidation of Cu(II) complex 5 at -0.33 V/0.06-0.61 V were detected.

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 ).…”

“…geometry , . Nevertheless, several examples of linear (H 2 dabph) or macrocyclic ligands (15‐pydienN 3 O 2 , 15‐pydienN 5 , 15‐pyN 3 O 2 , tdmmb, or more recent TODA) (Figure S1) provided seven‐coordinate pentagonal bipyramidal complexes with a pentadentate ligand in the equatorial plane, which show a large easy‐axis magnetic anisotropy ( D < 0) in the case of Fe II and Ni II complexes, and on the other side, a large easy‐plane magnetic anisotropy ( D > 0) in the case of Co II complexes. Slow relaxation of magnetization was observed for [Fe(H 2 dabph)Cl 2 ] and [Fe(H 2 dabph‐NH 2 )Cl 2 ] ( τ 0 = 5 × 10 –9 s and U eff = 53 cm –1 ), for [{Ni(H 2 dabph)} 3 {W(CN) 8 } 2 ] (below the blocking temperature T N = 3.6 K, τ 0 = 1.6 × 10 –9 s, U eff = 30 K), and for many different Co II complexes , , , .…”

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

“…36 In the cyclic For 4, a quasi-reversible couple Ni(II)/Ni(I) was found with the reduction peak at −1.16 V and less intense oxidation peak at −0.70 V. The quasi-reversibility of this redox process could be a result of low solubility of the Ni(I) product or its fast consequent decomposition. On the one hand, the much higher redox potentials in comparison with the previously studied similar complexes clearly demonstrate the ability of L to stabilize reduced species (lower oxidation states), which is a consequence of the modified structure of L containing two tertiary amino groups and two pyridine pendant arms, because it was shown previously on many transition complexes with different linear or macrocyclic polyamines, for example, Ni(II) complexes of cyclen derivatives (Supporting Information, Table S13), 17,18 that when their primary/secondary amino groups were substituted by the tertiary ones 67 and/or the pyridine group with π-acceptor ability were implemented into their structure, the lower oxidation states were stabilized, and the value of E 1/2 increased.…”

“…Recently, the field-induced SMM behavior was also observed/confirmed in various seven-coordinate Co(II) complexes [Co (15-pydienN In our previous study, the complexes of a 15-pyN 3 O 2 ( Figure 1) with the selected first-row transition metals were investigated in detail. 36 The coordination number of 7 was observed in [M (15- …”