Molecular Magnetic Architectures: From Mononuclear to Polynuclear complexes

The broad field of molecular magnetic materials has potential applications in the storage and processing of the information. Despite of the enormous applications of these materials, till now, there are no suitable candidates ready for the real application. So, over the decades, plenty of research has been progressing to develop suitable candidates in the field of Spin-crossover (SCO) and single molecule magnetism (SMM). In SCO, there are several reports with large hysteresis but found to be not suitable for applications since they do not satisfy many other criteria, which include the stability of the hysteresis that should be around room temperature. At the same time, for the development of the molecular devices, the bottom-up fabrication of these functional molecules is needed. Even though most of the SCO complexes are mononuclear, significant progress has been seen towards polynuclear complexes due to their tunable properties. Molecular chirality is an additional concept that plays a key role in magnetism, particularly in spintronic applications. So introducing the concept of chirality into these polynuclear SCO complexes may result in novel magneto-optical hybrid complexes. On the other hand, in the field of SMMs, lanthanide complexes have risen as attractive materials due to extremely large anisotropy in lanthanide ions. However, most of these complexes show characteristic SMM behaviour at liquid helium temperatures; there is a need for designing newer SMMs with higher blocking temperatures. In the present thesis, in the first chapter, a brief introduction to fields - SCO and SMM, and their importance to the present technological world is provided. Moreover, the methods for characterization used for these complexes are presented. The second chapter deals with the studies of bis(pyrazolyl)pyridine (bpp) derivatives in bulk and surface. The ligand (bpp-COOH) was deposited on Ag(111) surface and was found to form a Kagome lattice structure on annealing, and the ligand showing two types of coordination modes with Fe on Ag(111). STM and XPS were used to study the self-assembled structures formed on the surface. On the other hand, the bulk SCO Fe(II) complexes were prepared using a derivative of bpp ligand by varying the counter anions. The structural and SCO properties of the complexes were investigated by various techniques like X-ray diffraction (XRD), Superconducting Quantum Interference Device (SQUID) magnetometry, and differential scanning calorimetry (DSC). Interestingly, our magnetic studies on the complex synthesized with perchlorate anion showed a stable hysteresis of 60 K around room temperature. The third chapter deals with the chiral resolution of tetra-nuclear Fe(II) SCO grid complexes. For this deconvolution of grid complexes, we designed and synthesized a novel chiral ligand. The complexation of these ligands with Fe(II) resulted in enantiomerically pure grid complexes which were elucidated by XRD and Circular Dichroism (CD) studies. These enantiomeric complexes showed gradual SCO and photo-induced SCO properties. The CD spectra calculated using TDDFT showed good agreement with experimental results obtained from Mössbauer spectroscopy and SQUID magnetometry. The fourth chapter deals with mononuclear, binuclear and tri-nuclear Tb-sandwich SMM complexes. We explored and characterized the series of complexes of mixed porphyrin and phthalocyanine mono Tb SMM sandwich complexes by tuning the periphery of porphyrin ligand and redox properties. Moreover, a series of binuclear complexes were synthesized by varying the length of the linker, and their magnetic properties were characterized. Besides, preliminary electron paramagnetic resonance (EPR) studies were performed in the neutral complexes to study the radical. In summary, various kinds of molecular magnetic architectures were synthesized and characterized in this thesis as below - (i) surface and bulk studies of bpp based Fe(II) complexes, (ii) tetranuclear enantiomeric Fe(II) SCO grid complexes, and (iii) mononuclear, binuclear and trinuclear Tb- SMM complexes based on porphyrins and phthalocyanine. Such design and studies of these magnetic materials are key for future devices based on a bottom-up approach

Ditopic Hexadentate Ligands with a Central Dihydrobenzo-diimidazole Unit Forming a [2x2] Zn4_{4} Grid Complex

Ditopic ligands are appropriate candidates for homo‐ and hetero‐metallic supramolecular complexes. Two ditopic hexadentate ligands, which are highly interconvertible, are separated from a single ligand L by blocking tautomerism. They form remarkable divergent tetra‐nuclear grid complexes with Zinc(II) showing luminescence properties. A family of ditopic hexadentate ligands based on the parent compound 2,6‐bis(6‐(pyrazol‐1‐yl)pyridin‐2‐yl)‐1,5‐dihydrobenzo[1,2‐d:4,5‐d’]diimidazole (L) was developed and synthesized by using a straightforward condensation reaction, which forms the interlinking central benzo[1,2‐d:4,5‐d’]diimidazole bridge in the ligand backbone. The two secondary amine groups of the benzodiimidazole unit tautomerize and allow the formation of two tauto‐conformers, which upon treatment with metal salts forms different isomeric coordination complexes. Here we report six new derivatives (1–6) that can tautomerize (varying the pyrazolylpyridine part) and 14 derivatives (7–13) with different alkyl and benzyl substitution on secondary amino groups (of L) that prevent the tautomerization. This way, it is possible to study the properties of isomeric coordination complexes and their intrinsic cooperativity by the example of [2x2] grid complexes in the future. A [2x2] Zn$_{4}$ complex of the ligand L was synthesized and structurally characterized

Investigations on the Spin States of Two Mononuclear Iron(II) Complexes Based on N-Donor Tridentate Schiff Base Ligands Derived from Pyridine-2,6-Dicarboxaldehyde

Iron(II)-Schiff base complexes are a well-studied class of spin-crossover (SCO) active species due to their ability to interconvert between a paramagnetic high spin-state (HS, S = 2, $^{5}$T$_{2}$) and a diamagnetic low spin-state (LS, S = 0, $^{1}$A$_{1}$) by external stimuli under an appropriate ligand field. We have synthesized two mononuclear FeII complexes, viz., [Fe(L$^{1}$)$_{2}$](ClO$_{4}$)$_{2}$.CH$_{3}$OH (1) and [Fe(L$^{2}$)$_{2}$](ClO$_{4}$)$_{2}$.2CH$_{3}$CN (2), from two N$_{6}$–coordinating tridentate Schiff bases derived from 2,6-bis[(benzylimino)methyl]pyridine. The complexes have been characterized by elemental analysis, electrospray ionization–mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR), solution state nuclear magnetic resonance spectroscopy, $^{1}$H and $_{13}$C NMR (both theoretically and experimentally), single-crystal diffraction and magnetic susceptibility studies. The structural, spectroscopic and magnetic investigations revealed that 1 and 2 are with Fe–N$_{6}$ distorted octahedral coordination geometry and remain locked in LS state throughout the measured temperature range from 5–350 K

Iron in a Cage: Fixation of a Fe(II)tpy2_{2} Complex by Fourfold Interlinking

The coordination sphere of the Fe(II) terpyridine complex 1 is rigidified by fourfold interlinking of both terpyridine ligands. Profiting from an octa‐aldehyde precursor complex, the ideal dimensions of the interlinking structures are determined by reversible Schiff‐base formation, before irreversible Wittig olefination provided the rigidified complex. Reversed‐phase HPLC enables the isolation of the all‐trans isomer of the Fe(II) terpyridine complex 1, which is fully characterized. While temperature independent low‐spin states were recorded with superconducting quantum interference device (SQUID) measurements for both, the open precursor 8 and the interlinked complex 1, evidence of the increased rigidity of the ligand sphere in 1 was provided by proton T$_{2}$ relaxation NMR experiments. The ligand sphere fixation in the macrocyclized complex 1 even reaches a level resisting substantial deformation upon deposition on an Au(111) surface, as demonstrated by its pristine form in a low temperature ultra‐high vacuum scanning tunneling microscope experiment

Structural Insights into Hysteretic Spin‐Crossover in a Set of Iron(II)‐2,6‐bis(1 H ‐Pyrazol‐1‐yl)Pyridine) Complexes

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT$_{1/2}$) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) (bpp) complexes – [Fe(bpp−COOEt)$_{2}$](X)$_{2}$⋅CH$_{3}$NO$_{2}$ (X=ClO$_{4}$, 1; X=BF$_{4}$, 2). Stable spin-state switching – T$_{1/2}$=288 K; ΔT$_{1/2}$=62 K – is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO – T$_{1/2}$=331 K; ΔT$_{1/2}$=43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations – from the Jahn-Teller-distorted HS state to the less distorted LS state – and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements

Chiral Resolution of Spin-Crossover Active Iron(II) [2x2] Grid Complexes

Chiral magnetic materials are proposed for applications in second-order non-linear optics, magneto-chiral dichroism, among others. Recently, we have reported a set of tetra-nuclear Fe(II) grid complex conformers with general formula C/S-[Fe4L4]$^{8+}$ (L: 2,6-bis(6-(pyrazol-1-yl)pyridin-2-yl)-1,5-dihydrobenzo[1,2-d : 4,5-d′]diimidazole). In the grid complexes, isomerism emerges from tautomerism and conformational isomerism of the ligand L, and the S-type grid complex is chiral, which originates from different non-centrosymmetric spatial organization of the trans type ligand around the Fe(II) center. However, the selective preparation of an enantiomerically pure grid complex in a controlled manner is difficult due to spontaneous self-assembly. To achieve the pre-synthesis programmable resolution of Fe(II) grid complexes, we designed and synthesized two novel intrinsically chiral ligands by appending chiral moieties to the parent ligand. The complexation of these chiral ligands with Fe(II) salt resulted in the formation of enantiomerically pure Fe(II) grid complexes, as unambiguously elucidated by CD and XRD studies. The enantiomeric complexes exhibited similar gradual and half-complete thermal and photo-induced SCO characteristics. The good agreement between the experimentally obtained and calculated CD spectra further supports the enantiomeric purity of the complexes and even the magnetic studies. The chiral resolution of Fe(II)- [2×2] grid complexes reported in this study, for the first time, might enable the fabrication of magneto-chiral molecular devices

Rotation in an Enantiospecific Self‐Assembled Array of Molecular Raffle Wheels

Tailored nano-spaces can control enantioselective adsorption and molecular motion. We report on the spontaneous assembly of a dynamic system—a rigid kagome network with each pore occupied by a guest molecule—employing solely 2,6-bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid on Ag(111). The network cavity snugly hosts the chemically modified guest, bestows enantiomorphic adsorption and allows selective rotational motions. Temperature-dependent scanning tunnelling microscopy studies revealed distinct anchoring orientations of the guest unit switching with a 0.95 eV thermal barrier. H-bonding between the guest and the host transiently stabilises the rotating guest, as the flapper on a raffle wheel. Density functional theory investigations unravel the detailed molecular pirouette of the guest and how the energy landscape is determined by H-bond formation and breakage. The origin of the guest\u27s enantiodirected, dynamic anchoring lies in the specific interplay of the kagome network and the silver surface

Bi-stable spin-crossover in charge-neutral [Fe(R-ptp)(2)] (ptp=2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine) complexes

Bi-stable charge-neutral iron(II) spin-crossover (SCO) complexes are a class of switchable molecular materials proposed for molecule-based switching and memory applications. In this study, we report on the SCO behavior of a series of iron(II) complexes composed of rationally designed 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine (ptp) ligands. The powder forms of [Fe2+(R-ptp(-))(2)](0) complexes tethered with less-bulky substituents-R = H (1), R = CH2OH (2), and R = COOCH3 (3; previously reported)-at the 4-position of the pyridine ring of the ptp skeleton showed abrupt and hysteretic SCO at or above room temperature (RT), whereas complex 5 featuring a bulky pyrene substituent showed incomplete and gradual SCO behavior. The role of intermolecular interactions, lattice solvent, and electronic nature of the chemical substituents (R) in tuning the SCO of the complexes is elucidated