Organometallic Magnets

Table of Contents

1. Frontmatter

2. Hybrid Organic–Inorganic Cyanide-Bridged Networks

   Tomasz Korzeniak, Beata Nowicka, Barbara Sieklucka

   Abstract

   Hybrid organic–inorganic CN-bridged networks are an important and versatile group of molecular magnets. Cyanide ligands mediate relatively strong magnetic interactions and at the same time allow easy design of polynuclear assemblies via building block approach. Introduction of organic ligands allows effective manipulation of topology and dimensionality, enabling formation of discrete polynuclear structures, chains and layers as well as intricate 3D architectures. Organic molecules in hybrid systems can act as blocking or bridging ligands as well as guest molecules. Most importantly, apart from directing the structure formation, organic ligands can be used to induce additional desired properties. In this chapter, we present numerous examples of hybrid CN-bridged assemblies to illustrate their diverse functionalities. They include single molecule (SMMs) and single chain magnets (SCMs), magnetic sponges, multi-switchable spin-crossover (SCO) and charge-transfer systems as well as materials combining magnetic ordering with optical activity or luminescence. Current efforts in the research of CN-bridged systems concentrate on several topics connected with their potential applications, like search for materials with high critical temperature of magnetic ordering, development of bistable systems responsive to multiple stimuli, or surface deposition and formation of heterostructures.

3. Cobalt(II) Complexes as Single-Ion Magnets

   Shalini Tripathi, Atanu Dey, Maheswaran Shanmugam, Ramakirushnan Suriya Narayanan, Vadapalli Chandrasekhar

   Abstract

   This book chapter is mainly devoted to the recent findings about the cobalt(II) single-ion magnets (SIMs). Influence of various coordination numbers (from 2 to 8) around Co(II) in determining the spin Hamiltonian (SH) parameters of the complexes is reviewed. This chapter also discloses the importance of proper ligand design for stabilizing Ising or uniaxial anisotropy in mononuclear Co(II) complexes.

4. Cobalt(II)/(III)–Lanthanide(III) Complexes as Molecular Magnets

   Atanu Dey, Shalini Tripathi, Maheswaran Shanmugam, Ramakirushnan Suriya Narayanan, Vadapalli Chandrasekhar

   Abstract

   This chapter deals with single-molecule magnets (SMMs) obtained from heterometallic Co(II)/4f complexes. The design principles involved in building various types of heterometallic complexes are discussed along with their magnetic properties. A large group of hybrid Co(II)/4f complexes of varying nuclearity are discussed. Some examples of Co(III)/4f complexes are also presented.

5. Mannich Base Ligands as Versatile Platforms for SMMs

   Enrique Colacio

   Abstract

   Aminophenol Mannich base derivatives are versatile and flexible ligands for preparing a wide variety of homo- and heterometallic discrete coordination compounds, ranging from mononuclear to hexanuclear, which exhibit aesthetically pleasant structures with intricate topologies. These ligands are particularly adapted to obtain 3d/4f systems, where invariably the amino fragment is coordinated to the transition metal ion and the phenolate oxygen atoms bridge transition metal and lanthanide ions. Their coordination spheres are completed by donor atoms belonging either to methoxy and aldehyde groups of the Mannich base ligands or to terminal and bridging ancillary ligands. Moreover, robust 3d-4f dinuclear units can be assembled with either bridging ligands or complexes acting as bridging ligands to afford heterometallic complexes with increased nuclearity. The complexes containing one or two paramagnetic ions often exhibit appealing magnetic properties, alone or combined with other physical properties, that essentially arise from large local magnetic anisotropy and magnetic exchange coupling of the metal ions. This chapter provides an overview of recent results on single-molecule magnets (SMMs) based on aminophenol Mannich base ligands that illustrate the scope, state of the art and fruitful dynamism of this field of research.

6. Tetrathiafulvalene-Based Magnets of Lanthanides

   Olivier Cador, Fabrice Pointillart

   Abstract

   Tetrathiafulvalene (TTF)-based ligands and lanthanide ions have been intensively used for their electronic conductivity and optical properties, respectively. Their combination leads to a new class of coordination compounds that are able to display single-molecule magnet (SMM) behavior. Magnetic bistability resulting of such behavior could find potential applications in high-density data storage and quantum computing. In this chapter, a library of TTF-based magnets containing lanthanide ions is presented. Among this series, the influence of the coordination sphere and intra- and intermolecular interactions such as exchange, dipolar, supramolecular, and hyperfine interactions is probed through molecular engineering, magnetic dilutions, and isotopic enrichment.

7. Geometry and Magnetism of Lanthanide Compounds

   Zhenhua Zhu, Jinkui Tang

   Abstract

   Lanthanide single molecule magnets (Ln-SMMs) were still been considered as the exceptionally promising candidates in high-density data storage and quantum calculation although the single atom magnets with smaller size have been discovered. Recent developments that the intrinsic magnetic properties of Ln-SMMs can be preserved when deposited on the surface of substrates greatly inspired us to make more efforts in facilitating the above practical applications. It is well-known that the single molecule magnet (SMM) behavior is strongly dependent on the coordination environments experienced by the lanthanide ions. Here, we focus on the representative Ln-SMMs with different coordination geometries from the view of coordination numbers, discuss the methods of modulating ligand fields, highlight the importance of constructing predominant bonds, and explain the relationship between the geometry, crystal field, and molecular magnetisms.

8. Single-Ion Anisotropy: An Insight to Complicated Magnetic Molecules

   Shang-Da Jiang, Bing-Wu Wang, Song Gao

   Abstract

   Molecular magnetism has been developed by chemists for a few decades. The research focus shifted from magnetic frameworks and clusters to mononuclear molecules from the beginning of this century due to the poor understanding of single-ion anisotropy. In the past decades, huge triumphs on the mononuclear researches have been achieved, while we feel that it is the moment to move to more complicated molecules with the present knowledge. Based on the overview of the theoretical models for the molecular magnetism, some unconventional characterization methods to investigate the magnetic anisotropy are introduced. We discussed the strategies to control the magnetic anisotropy of spin carriers as a hint to understand the magnetic clusters’ behavior. More importantly, the researches on the complicated magnetic molecules based on the information of the single-ion anisotropy are summarized. To the end, a few conclusions are provided, as well as the perspectives for the further researches in molecular magnetism with respect to chemistry.

9. Lanthanide Organometallics as Single-Molecule Magnets

   María José Heras Ojea, Lewis C. H. Maddock, Richard A. Layfield

   Abstract

   Innovative synthetic chemistry has underpinned many important advances in molecular magnetism, particularly so with the development of single-molecule magnets (SMMs). Recently, the organometallic approach to SMMs has provided a series of eye-catching materials based on certain lanthanides that have re-energised a mature field of magnetism research. This chapter summarises the main highlights and shows that three lanthanides – terbium, dysprosium and erbium – and two ligands, cyclopentadienyl and cyclo-octatetraenyl, have played pivotal roles. The chapter considers the lanthanides in terms of conceptually simple models of 4f electronic structure and spin–orbit coupling and their relationship with the popular oblate and prolate depictions of electron density. For organisational purposes, the chapter is loosely divided by ligand hapticity, beginning with a review of η⁵-cyclopentadienyl compounds of dysprosium, from the discovery of the first organometallic SMM in 2010 to a series of cationic dysprosium metallocenes and radical-bridged SMMs that currently define the state of the art. Ingenious combinations of the η⁸-cyclo-octatetraenyl ligand with erbium, and the SMM properties of the ensuing compounds, are described. Less widely used organometallic ligands such as η⁶-arene and η⁷-cycloheptatrienyl are also considered, as are heteroaromatic ligands in which a carbon atom is replaced by an isolobal fragment based on, e.g., boron or phosphorus.

   Organometallic chemistry has provided a valuable approach to the design of lanthanide SMMs that complements the impressive achievements made with Werner-type coordination chemistry. Important challenges remain to be surmounted, and the main message is that if SMMs are to achieve their potential in the arena of device technology then there is a clear need for more research into this fascinating family of magnetic materials.

10. Role of Ab Initio Calculations in the Design and Development of Lanthanide Based Single Molecule Magnets

    Tulika Gupta, Mukesh Kumar Singh, Gopalan Rajaraman

    Abstract

    In this book chapter, we have reviewed recent trends in employing ab initio calculations based on complete active space self-consistent field (CASSCF)/restricted active space spin interaction with spin–orbit coupling (RASSI-SO) procedure to interpret, rationalize and predict suitable lanthanide based molecular magnets. We begin with the general introduction on the methods used followed by various pragmatic instances where ab initio calculations have been employed to understand the magnetic anisotropy in lanthanide based single-ion magnets (SIMs). While a detailed section is dedicated to the mononuclear Dy^III SIMs, we have also covered other lanthanide SIMs briefly. Particularly, we have classified various SIMs based on the observed crystal-field splitting between ground and first excited states and this likely to shed light on the most important issue of suitable geometries that could yield high blocking temperature SIMs.

11. Complete Active Space Wavefunction-Based Analysis of Magnetization and Electronic Structure

    Frédéric Gendron, Hélène Bolvin, Jochen Autschbach

    Abstract

    A theoretical framework for the generation of natural orbitals, natural spin orbitals, as well as orbital- and spin-magnetizations, from multi-configurational ab initio wavefunction calculations including spin-orbit coupling is presented. Selected case studies show how these computational orbital and magnetization tools can be used to interpret and rationalize the magnetic properties of complexes containing transition metals, lanthanides, and actinides.

12. Magnetism of Actinide Coordination Compounds

    Jan van Leusen, Manfred Speldrich, Paul Kögerler

    Abstract

    The magnetochemical interpretation of actinide ions in ligand environments represents a complex challenge, since the approximations and simplifications that are commonly employed in transition metal and lanthanide coordination compounds cannot be applied in the case of 5f ^N systems. We herein aim to deconvolute the various contributions to the magnetic characteristics of such systems, and we demonstrate how to construct appropriate microscopic model Hamiltonians. The approach to account for all relevant intrinsic effects is finally showcased by a number of examples.

13. Correction to: Cobalt(II) Complexes as Single-Ion Magnets

    Shalini Tripathi, Atanu Dey, Maheswaran Shanmugam, Ramakirushnan Suriya Narayanan, Vadapalli Chandrasekhar

14. Correction to: Cobalt(II)/(III)–Lanthanide(III) Complexes as Molecular Magnets

    Atanu Dey, Shalini Tripathi, Maheswaran Shanmugam, Ramakirushnan Suriya Narayanan, Vadapalli Chandrasekhar

15. Backmatter