Dinuclear Lanthanide(III) Complexes from the Use of Methyl 2-Pyridyl Ketoxime: Synthetic, Structural, and Physical Studies

Abstract

The first use of methyl 2-pyridyl ketoxime (mepaoH) in homometallic lanthanide(III) [Ln(III)] chemistry is described. The 1:2 reactions of Ln(NO₃)₃·nH₂O (Ln = Nd, Eu, Gd, Tb, Dy; n = 5, 6) and mepaoH in MeCN have provided access to complexes [Ln₂(O₂CMe)₄(NO₃)₂(mepaoH)₂] (Ln = Nd, 1; Ln = Eu, 2; Ln = Gd, 3; Ln = Tb, 4; Ln = Dy, 5); the acetato ligands derive from the Ln^III-mediated hydrolysis of MeCN. The 1:1 and 1:2 reactions between Dy(O₂CMe)₃·4H₂O and mepaoH in MeOH/MeCN led to the all-acetato complex [Dy₂(O₂CMe)₆(mepaoH)₂] (6). Treatment of 6 with one equivalent of HNO₃ gave 5. The structures of 1, 5, and 6 were solved by single-crystal X-ray crystallography. Elemental analyses and IR spectroscopy provide strong evidence that 2-4 display similar structural characteristics with 1 and 5. The structures of 1-5 consist of dinuclear molecules in which the two Ln^III centers are bridged by two bidentate bridging (η¹:η¹:μ₂) and two chelating-bridging (η¹:η²:μ₂) acetate groups. The Ln^III atoms are each chelated by a N,N'-bidentate mepaoH ligand and a near-symmetrical bidentate nitrato group. The molecular structure of 6 is similar to that of 5, the main difference being the presence of two chelating acetato groups in the former instead of the two chelating nitrato groups in the latter. The geometry of the 9-coordinate Ln^III centers in 1, 5 and 6 can be best described as a muffin-type (MFF-9). The 3D lattices of the isomorphous 1 and 5 are built through H-bonding, π⋯π stacking and C-H⋯π interactions, while the 3D architecture of 6 is stabilized by H bonds. The IR spectra of the complexes are discussed in terms of the coordination modes of the organic and inorganic ligands involved. The Eu(III) complex 2 displays a red, metal-ion centered emission in the solid state; the Tb^III atom in solid 4 emits light in the same region with the ligand. Magnetic susceptibility studies in the 2.0-300 K range reveal weak antiferromagnetic intramolecular Gd^III…Gd^III exchange interactions in 3; the J value is -0.09(1) cm^-1 based on the spin Hamiltonian Ĥ = -J(Ŝ_Gd1·Ŝ_Gd2).

Keywords: coordination chemistry; dinuclear lanthanide(III) complexes; magnetic properties of gadolinium(III) complexes; metal complexes of methyl 2-pyridyl ketoxime; photoluminescence studies; single-crystal X-ray structures.

Figure 1

General structural formula of simple 2-pyridyl oximes, in which R is a non-donor group; the ligand used in the present work is methyl 2-pyridyl ketoxime (R = Me; mepaoH).

Figure 2

The hydrolysis of MeCN to generate MeCO₂H.

Figure 3

Partially labeled plot of the structure of the dinuclear molecule [Nd₂(O₂CMe)₄(NO₃)₂(mepaoH)₂] that is present in the crystal of 1. Symmetry operation: (‘) = –x+2, −y+1, –z+2.

Figure 4

Partially labeled plot of the structure of the dinuclear molecule [Dy₂(O₂CMe)₄(NO₃)₂(mepaoH)₂] that is present in the crystal of 5. Symmetry operation: (‘) = –x+2, −y+1, –z+2.

Figure 5

Partially labeled plot of the structure of the dinuclear molecule [Dy₂(O₂CMe)₆(mepaoH)₂] that is present in the crystal of 6. Symmetry operation: (‘) = −x, −y, −z.

Figure 6

(a) 2D arrangement of the isomorphous complexes 1 and 5 parallel to the (100) plane. Thick dashed yellow lines indicate the C4-H(C4)⋯O7 (2.49 Å), C9-H_B(C_g)⋯O4 (2.43 Å) and C11-H_B(C11)⋯O2 (2.60 Å) H bonds, where C4 is an aromatic carbon atom of the mepaoH ligand and C9, C11 are the methyl carbon atoms of the crystallographically independent acetato groups. Thick dashed light green and dark green lines represent the C-H⋯π and π⋯π interactions, respectively, described in the text; (b) stacking of layers parallel to the a crystallographic axis for 1 and 5. Thick dashed violet lines represent the C1-H_A(C1)⋯O1 (2.52 Å) and C1-H_B(C1)⋯O4 (2.59 Å) H bonds, where C1 is the methyl carbon atom of the mepaoH ligand. All the distance and angle parameters for the H bonds, as well as the symmetry operations are listed in Table S3.

Figure 7

Intermolecular interactions in the crystal structure of complex 6. (a) 2D arrangement of the dinuclear molecules parallel to the (100) plane. Thick dashed yellow lines indicate the C16-Hc(C16)⋯O2 (2.46 Å) H bond, where C16 is the methyl carbon atom of the crystallographically unique η¹:η²:μ₂ (2.12) acetato group; (b) stacking of layers parallel to the α crystallographic axis. Thick dashed red-violet lines represent the C4-H(C4)⋯O3 (2.35 Å), C6-H(C6)⋯O2 (2.44 Å) and C14-H_C(C14)⋯O7 (2.58 Å) H bonds; C4 and C6 are aromatic carbon atoms of the mepaoH ligand. The distances and angles for the H bonds, as well as the symmetry operations are listed in Table S4.

Figure 8

Solid-state, room-temperature excitation (left) and emission (right) spectra of mepaoH (black curves) and complex 2 (red curves). The emission spectra were detected at 397 nm for both compounds. The excitation spectra were detected with maximum emission at 545 nm for mepaoH and 618 nm for 2.

Figure 9

Solid-state, room-temperature excitation (left, emission, at 545) and emission (right, excitation at 397 nm) of mepaoH (black curves) and complex 4 (green curves).

Figure 10

Temperature dependence of the χ_ΜΤ product of complex 3 at 0.1 T. The solid red line is the fit of the data to the theoretical Heisenberg model for a Gd^III₂ complex; see the text for details.