Structure and Magnetism of Seven‐Coordinate Fe^III, Fe^II, Co^II and Ni^II Complexes Containing a Heptadentate 15‐Membered Pyridine‐Based Macrocyclic Ligand

Abstract

A heptadentate macrocyclic ligand, H 2 L (3,12,18triaza-6,9-dioxabicyclo[12.3.1]octadeca-1,14,16-triene-3,12-diacetic acid) with two acetate pendant arms, and its complexes [Fe III L]ClO 4 (1), [Fe II L]·H 2 O (2), [Co II L]·H 2 O (3), and [Ni II L]·H 2 O (4) were synthesized. The complexes possess an axially compressed pentagonal bipyramidal geometry with the coordination numbers of 7 for 2 and 3, and 6 + 1 for 4. The magnetic susceptibility measurements revealed magnetic anisotropy for compounds 1-4 expressed by axial zero-field splitting (ZFS) parameters (D = [a] [{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), [22] and for many different Co II complexes. [10,18,23,24] Furthermore, the magnetic anisotropy has been tuned by playing with donor/acceptor properties of two axial coligands, as was shown on series of Co II complexes, e.g. [Co(tdmmb)(X) 2 ] 2+/0 , [20] where X = H 2 O, CN -, NCS -, SPh -, with the more pronounced anisotropy for the complex with SPh -(D = 39.7 cm -1 ), or [Co(15-pyN 3 O 2 )X 2 ], [23] where X = Cl -, Brand Iwith the highest D = 41 cm -1 for the dibromido complex, and also on Mn II complexes [Mn(15-pyN 3 O 2 )X], [25] where X = Br -, I -, N 3 -, NCS -, with a small influence of axial coligands on the magnetic anisotropy (|D| < 0.7 cm -1 ).The recent results suggest that also seven-coordinate lanthanide complexes with pentagonal bipyramidal geometry show remarkable properties, [26] concretely a very high magnetization reversal barrier was found for [Dy(OPCy 3 )(H 2 O) 5 ] 3+ , [27] [Dy(tBuPO(NHiPr) 2 ) 2 (H 2 O) 5 ] 3+ , [28] [Dy(bbpen)Cl], and [Dy-(bbpen)Br] [29] with U eff = 543, 735, 708, and 1025 K, respectively.A different approach how to tune desired structural and magnetic properties of complexes can be associated with modification of the original pentadentate macrocycle, e.g. 15-pyN 3 O 2 , with two pendant arms containing different functional groups providing heptadentate ligand and seven-coordinate complexes. 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 1) with two 2-pyridylmethyl pendant arms. [30] 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 . [30] Eur.

Structural, magnetic, redox and theoretical characterization of seven-coordinate first-row transition metal complexes with a macrocyclic ligand containing two benzimidazolyl N-pendant arms

“…When the obtained D-values are compared with those of previously studied complexes with L2 and L3 (Table 2), 12,38 they are very similar for Mn II and Fe II complexes 1 and 2, but they are much higher for Co II and Ni II complexes 3 and 4 revealing their larger magnetic anisotropy. The sign of the D-value for 2 is positive and thus different from previously studied Fe II complexes with L2 and L3 (Table 2), but it is in accordance with the CASSCF/NEVPT2 calculations for 2 ( Table 2) and also with the previously performed calculations (see Fig.…”

“…5, right) and this can be quantified by deviations from the PBPY-7 ideal symmetry obtained by the program SHAPE (1.112 and 1.201 for L, Table S2, † 1.237 for L2, 12 and 2.988 for L3). 38 As the change in the equatorial ligand field has a more dramatic impact on the value of D than on the variation of the axial ligand field in the Ni II complexes (see Fig. 12 later in the section, Theoretical calculations), the increase of negative D for the Ni II complexes (L3 → L) is more likely governed by the decreasing deviation from the ideal D 5h symmetry.…”

“…Furthermore, in the order going from Mn II to Ni II complex 1 → 4 all M-N distances decrease, which is in agreement with the decreasing ionic radius of complexed metal ions. On the other hand, the M-O distances remain the same for 1-3 and significantly increase in the case of Ni II complex 4 due to the Jahn-Teller effect, 71 which is typical of structurally similar seven-coordinate pentagonal bipyramidal Ni II complexes [NiL1Cl 2 ], 72 [NiL2](ClO 4 ) 2 , 12 [NiL3], 38 [NiL6](ClO 4 ) 2 35 for which the Ni-O distances often exceed 2.5 Å. Thus, the pentagonal bipyramidal geometry of the Ni II centre in 4 is the most distorted one (Ni-O is 2.413(2) and 2.416(2) Å) and this distortion for the second crystallographically independent molecules present in the asymmetric unit is even more pronounced (Ni-O is 2.357(2) and 2.486(2) Å, see Table 1 and Fig.…”

“…The second strategy is based on the structural modification of the pentadentate macrocyclic ligand (e.g. L1 or 1,10-diaza-15-crown-5) 35,36 with two pendant arms containing various functional groups with different coordination abilities (2-pyridylmethyl in L2, 12,37 acetate in L3, 38 2-aminobenzyl in L6 35 and 2-benzimidazolylmethyl in L7, Fig. 1).…”

A ferromagnetic Ni(ii)–Cr(iii) single-chain magnet based on pentagonal bipyramidal building units

“…2,3 For instance, Fe(II) and Ni(II) exhibit easy-axis anisotropy with zero-field splitting parameter D in the range of -10 to -20 cm -1 , whereas Co(II) has planar anisotropy with D values around 30 cm -1 . [4][5][6][7][8][9][10][11][12][13][14][15] Moreover, the structural robustness of this coordination geometry makes such complexes suitable building units for polynuclear compounds. 16 These characteristics provide a unique opportunity to design polynuclear nano-magnets taking advantage of the large magnetic anisotropy of the pentagonal bipyramid complexes, especially those with Ising-type anisotropy.…”