Nature of the observed ν(NO) band shift and splitting during the 3D to 2D structural change in transition metal nitroprussides

The electronic structure and related optical properties of the nitroprusside ion, [Fe(CN)₅NO]^2-, are dominated by the nitrosyl (NO⁺) group. The light-induced 2b₂(xy) → 7e(π*NO) (Fe → NO) and related appearance of meta-stable isomers for the NO group supports many of the functional properties of metal nitroprussides. In recent studies on the preparation of 2D transition metal nitroprussides from their 3D analogs, we have observed that such a structural change is concomitant with a decrease for the ν(NO) frequency and certain band splitting (unfolding). The 3D to 2D structural modification leads to an increase in the electron density at the iron atom. This results in an enhancement for the π*-back donation, which increases the electron population at the π*_2px and π*_2py orbitals. These last orbitals are doubly degenerated, and the increase for the π*-back bonding induces the removal of that degeneration via the Jahn-Teller effect. This intuitive mechanism explains the mentioned band unfolding related to the structural change. Such explanation was herein supported by a model based on the normal modes for the FeNO fragment derived from the small oscillation formalism around the equilibrium positions for the involved atoms. This model shows that there is a very close relationship between the deformation of the nitrosyl group (NO⁺) and the splitting of the IR bands. That is, the less linear the nitrosyl group, the greater the shift and splitting of the mentioned IR band. An excellent coincidence between the frequency values calculated using the model and the experimental ones was observed. The largest difference between the theoretically calculated ν(NO) frequencies and the experimental ones was only 6 cm^-1 and 5 cm^-1 in representative samples of 2D and 3D transition metal nitroprussides, respectively. The understanding of such an effect could be relevant for the functional properties of these 2D coordination polymers.

Crystal Engineering of Schiff Base Zn(II) and Cd(II) Homo- and Zn(II)M(II) (M = Mn or Cd) Heterometallic Coordination Polymers and Their Ability to Accommodate Solvent Guest Molecules

Based on solvothermal synthesis, self-assembly of the heptadentate 2,6-diacetylpyridine bis(nicotinoylhydrazone) Schiff base ligand (H₂L) and Zn(II) and/or Cd(II) salts has led to the formation of three homometallic [CdL]_n (1), {[CdL]∙0.5dmf∙H₂O}_n (2) and {[ZnL]∙0.5dmf∙1.5H₂O}_n (3), as well as two heterometallic {[Zn_0.75Cd_1.25L₂]∙dmf∙0.5H₂O}_n (4) and {[MnZnL₂]∙dmf∙3H₂O}_n coordination polymers. Compound 1 represents a 1D chain, whereas 2-5 are isostructural and isomorphous two-dimensional structures. The entire series was characterized by IR spectroscopy, thermogravimetric analysis, single-crystal X-ray diffraction and emission measurements. 2D coordination polymers accommodate water and dmf molecules in their cage-shaped interlayer spaces, which are released when the samples are heated. Thus, three solvated crystals were degassed at two temperatures and their photoluminescent and adsorption-desorption properties were recorded in order to validate this assumption. Solvent-free samples reveal an increase in volume pore, adsorption specific surface area and photoluminescence with regard to synthesized crystals.

Cyano-bridged polynuclear coordination compounds derived from paramagnetic [Mn(H2daptsc)]2+ and photochromic [Fe(CN)5NO]2− building blocks

Metal complexes with H₂daptsc ligand were first used as building blocks to create polynuclear multifunctional materials.

The impact of counterion on the metastable state properties of nitrosyl ruthenium complexes

Complexes of the trans-[RuNO(NH₃)₄F]²⁺ cation with noble-metal anions [PtCl₆]²⁻, [PdCl₄]²⁻, and [PtCl₄]²⁻, and perchlorate ClO₄⁻ were synthesized, the photo-generated linkage isomers were studied by spectroscopic and calorimetric methods.

Slow Magnetic Relaxation, Antiferromagnetic Ordering, and Metamagnetism in MnII (H2 dapsc)-FeIII (CN)6 Chain Complex with Highly Anisotropic Fe-CN-Mn Spin Coupling

Reactions of [Mn(H₂ dapsc)Cl₂ ]⋅H₂ O (dapsc=2,6- diacetylpyridine bis(semicarbazone)) with K₃ [Fe(CN)₆ ] and (PPh₄ )₃ [Fe(CN)₆ ] lead to the formation of the chain polymeric complex {[Mn(H₂ dapsc)][Fe(CN)₆ ][K(H₂ O)_3.5 ]}_n ⋅1.5n H₂ O (1) and the discrete pentanuclear complex {[Mn(H₂ dapsc)]₃ [Fe(CN)₆ ]₂ (H₂ O)₂ }⋅4 CH₃ OH⋅3.4 H₂ O (2), respectively. In the crystal structure of 1 the high-spin [Mn^II (H₂ dapsc)]²⁺ cations and low-spin hexacyanoferrate(III) anions are assembled into alternating heterometallic cyano-bridged chains. The K⁺ ions are located between the chains and are coordinated by oxygen atoms of the H₂ dapsc ligand and water molecules. The magnetic structure of 1 is built from ferrimagnetic chains, which are antiferromagnetically coupled. The complex exhibits metamagnetism and frequency-dependent ac magnetic susceptibility, indicating single-chain magnetic behavior with a Mydosh-parameter φ=0.12 and an effective energy barrier (U_eff /k_B ) of 36.0 K with τ₀ =2.34×10^-11  s for the spin relaxation. Detailed theoretical analysis showed highly anisotropic intra-chain spin coupling between [Fe^III (CN)₆ ]^3- and [Mn^II (H₂ dapsc)]²⁺ units resulting from orbital degeneracy and unquenched orbital momentum of [Fe^III (CN)₆ ]^3- complexes. The origin of the metamagnetic transition is discussed in terms of strong magnetic anisotropy and weak AF interchain spin coupling.

A Series of Field-Induced Single-Ion Magnets Based on the Seven-Coordinate Co(II) Complexes with the Pentadentate (N3O2) H2dapsc Ligand

A series of five new mononuclear pentagonal bipyramidal Co(II) complexes with the equatorial 2,6-diacetylpyridine bis(semicarbazone) ligand (H2dapsc) and various axial pseudohalide ligands (SCN, SeCN, N(CN)2, C(CN)3, and N3) was prepared and structurally characterizated: [Co(H2dapsc)(SCN)2]∙0.5C2H5OH (1), [Co(H2dapsc)(SeCN)2]∙0.5C2H5OH (2), [Co(H2dapsc)(N(CN)2)2]∙2H2O (3), [Co(H2dapsc)(C(CN)3)(H2O)](NO3)∙1.16H2O (4), and {[Co(H2dapsc)(H2O)(N3)][Co(H2dapsc)(N3)2]}N3∙4H2O (5). The combined analyses of the experimental DС and AC magnetic data of the complexes (1–5) and two other earlier described those of this family [Co(H2dapsc)(H2O)2)](NO3)2∙2H2O (6) and [Co(H2dapsc)(Cl)(H2O)]Cl∙2H2O (7), their theoretical description and the ab initio CASSCF/NEVPT2 calculations reveal large easy-plane magnetic anisotropies for all complexes (D = + 35 − 40 cm‒1). All complexes under consideration demonstrate slow magnetic relaxation with dominant Raman and direct spin–phonon processes at static magnetic field and so they belong to the class of field-induced single-ion magnets (SIMs).

Slow magnetic relaxation in mononuclear complexes of Tb, Dy, Ho and Er with the pentadentate (N3O2) Schiff-base dapsc ligand

AC measurements revealed field-induced single-ion magnet behavior of Dy and Er Kramers ion complexes. The properties of complexes were rationalized by superposition CF model.

Synthesis, Structure, and Magnetic Properties of 1D {[MnIII(CN)6][MnII(dapsc)]}n Coordination Polymers: Origin of Unconventional Single-Chain Magnet Behavior

Two one-dimensional cyano-bridged coordination polymers, namely, {[Mn^II(dapsc)][Mn^III(CN)₆][K(H₂O)_2.75(MeOH)_0.5]}_n·0.5n(H₂O) (I) and {[Mn^II(dapsc)][Mn^III(CN)₆][K(H₂O)₂(MeOH)₂]}_n (II), based on alternating high-spin Mn^II(dapsc) (dapsc = 2,6-diacetylpyridine bis(semicarbazone)) complexes and low-spin orbitally degenerate hexacyanomanganate(III) complexes were synthesized and characterized structurally and magnetically. Static and dynamic magnetic measurements reveal a single-chain magnet (SCM) behavior of I with an energy barrier of U_eff ≈ 40 K. Magnetic properties of I are analyzed in detail in terms of a microscopic theory. It is shown that compound I refers to a peculiar case of SCM that does not fall into the usual Ising and Heisenberg limits due to unconventional character of the Mn^III-CN-Mn^II spin coupling resulting from a nonmagnetic singlet ground state of orbitally degenerate complexes [Mn^III(CN)₆]^3-. The prospects of [Mn^III(CN)₆]^3- complex as magnetically anisotropic molecular building block for engineering molecular magnets are critically analyzed.