Reasons behind the Relative Abundances of Heptacoordinate Complexes along the Late First-Row Transition Metal Series

Abstract

A series of transition metal complexes [ML(1)] (H2L(1) = 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N'-diacetic acid, M = Co, Ni, Cu, or Zn) have been prepared and characterized. The X-ray structures of the [CoL(1)] and [CuL(1)] complexes reveal that the metal ions are seven-coordinated with a distorted pentagonal bipyramidal coordination. The five donor atoms of the macrocycle define the pentagonal plane of the bipyramid, while two oxygen atoms of the carboxylate groups coordinate apically. The [NiL(1)] complex presents a very distorted structure with long Ni-O distances involving two oxygen atoms of the crown moiety [2.544(3) Å]. This distortion is related to the Jahn-Teller effect that is expected to operate in d(8) pentagonal bipyramidal complexes. The spectroscopic characterization of the [ZnL(1)] and [CuL(1)] complexes using NMR and EPR and the theoretical calculation of the (13)C NMR shifts and g- and A-tensors using DFT confirm that these complexes retain the pentagonal bipyramidal coordination in aqueous solution. The stability trend of the [ML(1)] complexes (Co(2+) > Ni(2+) < Cu(2+) > Zn(2+)), which is in contradiction with the Irving-Williams order, has been analyzed using DFT calculations (TPSSh functional). The free energy values calculated in the gas phase for [CoL(1)](g) + [M(H2O)6](2+)(g) → [ML(1)](g) + [Co(H2O)6](2+)(g) (M = Ni, Cu, Zn) reproduce fairly well the stability trend observed experimentally, the agreement being improved significantly upon inclusion of solvent effects. Our results indicate that the pentagonal bipyramidal coordination is particularly unfavorable for Ni(2+), and thus preorganized ligands that favor this geometry such as L(1) are selective for Co(2+) over Ni(2+) cations.

1,4,7‐Triazacyclononane‐Based Bifunctional Picolinate Ligands for Efficient Copper Complexation

“…For instance, calculations performed with the BHLYP functional (50 % exact exchange) improve dramatically the agreement between the experimental and calculated g z values in comparison with the results obtained with the TPSSh functional (10 % exchange). However, the BHLYP functional overestimated the A z by a factor of 1.5, whereas TPSSh provided calculated values in very good agreement with the experimental values . Thus, we calculated the g and A tensors with the TPSS0 functional, which is a 25 % exchange version of TPSSh.…”

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

“…This observed trend could be explained by Jahn−Teller effect operating in seven-coordinate Ni(II) and five-coordinated Cu(II) complexes. 12 Indeed, the complexes 1−3 revealed coordination number of 7 with the N 5 O 2 donor atom set, Ni(II) complex 4 with one semicoordinated oxygen atom revealed coordination number of 6 + 1 (the N 5 O 2 donor atom set), while the molecular structure of Cu(II) complex is completely different (coordination number of 5, the N 4 O 1 donor atom set). To support our conclusions, the geometries of coordination polyhedra of 1−5 were analyzed by program Shape 2.1 providing continuous shape measurements (deviation of the real geometry from an ideal polyhedron).…”

“…Seven-coordination is more abundant for Mn, Fe, and Co complexes (4.5%, 1.5%, and 0.8%, respectively, of the total number of structures for given metals found in the Cambridge Structural Database (CSD)), while it is rather rare for Cu and Zn (0.12%, and 0.35%, respectively). And it is the least common in case of Ni complexes (0.08%), 12 because the Jahn−Teller theorem predicts a large distortion and a low stability of regular stereochemistry for sevencoordinate Ni(II) complexes. 13 −15 In the enormous group of polyaza and polyoxo-aza macrocyclic ligands, our attention has been focused on those with 2-pyridylmethyl pendant arm(s) or those providing sevencoordination in their metal complexes ( Figure 1).…”

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

“…In order to significantly increase the magnetic anisotropy in transition metal complexes, which is the crucial tuneable parameter for SMMs improvement, the modification of the coordination environment, provided by higher coordination numbers (seven or eight), and/or axial symmetry were employed . The coordination number of seven is not common for first‐row transition metals and its abundance along the series is irregular , . It is most abundant for Mn, Fe, and Co complexes, while in the case of Ni complexes is very rare due to the Jahn–Teller effect, which cause strong deformation of coordination sphere and low stability of these type of.…”