Spin States in Biochemistry and Inorganic Chemistry

Influence on Structure and Reactivity

Marcel Swart, Miquel Costas

• Publisher: Wiley
• Year: 2015
• ISBN:
  9781118898284
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

It has long been recognized that metal spin states play a central role in the reactivity of important biomolecules, in industrial catalysis and in spin crossover compounds. As the fields of inorganic chemistry and catalysis move towards the use of cheap, non-toxic first row transition metals, it is essential to understand the important role of spin states in influencing molecular structure, bonding and reactivity.

Spin States in Biochemistry and Inorganic Chemistry provides a complete picture on the importance of spin states for reactivity in biochemistry and inorganic chemistry, presenting both theoretical and experimental perspectives. The successes and pitfalls of theoretical methods such as DFT, ligand-field theory and coupled cluster theory are discussed, and these methods are applied in studies throughout the book. Important spectroscopic techniques to determine spin states in transition metal complexes and proteins are explained, and the use of NMR for the analysis of spin densities is described.

Topics covered include:

• DFT and ab initio wavefunction approaches to spin states
• Experimental techniques for determining spin states
• Molecular discovery in spin crossover
• Multiple spin state scenarios in organometallic reactivity and gas phase reactions
• Transition-metal complexes involving redox non-innocent ligands
• Polynuclear iron sulfur clusters
• Molecular magnetism
• NMR analysis of spin densities

This book is a valuable reference for researchers working in bioinorganic and inorganic chemistry, computational chemistry, organometallic chemistry, catalysis, spin-crossover materials, materials science, biophysics and pharmaceutical chemistry.

1
General Introduction to Spin States

Marcel Swart^1,2 and Miquel Costas¹

¹Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Spain

²Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain

1.1 Introduction

Spin is a fundamental property of all elements and molecules, which originates from their unpaired electrons. Spin states have a major role in defining the structure, reactivity, magnetic and spectroscopic properties of a molecule. Furthermore it is possible that more than one spin state is energetically accessible for a given molecule. In such cases, the molecule can accumulate multiple spectroscopic, magnetic and reactivity patterns arising from the different accessible spin states. The ground spin state of most organic molecules is a singlet, that is, they have a closed-shell electronic structure, and other states are energetically not accessible under standard conditions. Important exceptions are carbenes, which can exist as singlet and triplet spin states, and the molecule of dioxygen, whose triplet nature poses kinetic barriers to its thermodynamically favorable reaction with organic matter. The situation is completely reversed when transition metals are present, which makes that different spin states are accessible for the majority of transition metal complexes. This primarily results from the particular nature of d-orbitals of the metals (see Figure 1.1) that are close in energy and which can be occupied in different ways depending on the metal oxidation state, its ligands and its coordination geometry (see Figure 1.1). This picture can be further complicated when ligands are not redox innocent and can have a spin that can also engage in ferro or anti-ferromagnetic interactions with the spin of the metal center.

Figure 1.1 Transition metal d-orbitals shape (left) and orbital-level diagram (right).

Spin states play an important role [1, 2] in metalloenzymatic reactions (e.g. cytochrome P450cam), in metal-oxo complexes, in spin-crossover compounds and even in catalysis processes mediated by organometallic compounds where different reactions take place via different spin states [3, 4]. However, computational studies have shown that a correct description of the spin state is not trivial [1, 5, 6], and a combination of different density functionals (DFT) and/or ab initio methods may be needed. Experimental studies on biomimetic model complexes, enzymes or spin-crossover compounds have added to the complexity, making the spin state a challenging property that is poorly understood [1]. This was the origin for a CECAM/ESF Workshop organized in Zaragoza in September 2012 [7], leading subsequently to a COST Action (CM1305, ECOSTBio).

1.2 Experimental Chemistry: Reactivity, Synthesis and Spectroscopy

Spin states constitute a fundamental aspect of the electronic structure of molecules, and as such spin determines their electronic properties, magnetism and reactivity. Therefore, rationalization of the latter properties in paramagnetic molecules most often requires determination of their spin state. The most important spectroscopic techniques employed to determine spin states in transition metal complexes and proteins have been discussed in Chapter 4, and the use of nuclear magnetic resonance spectroscopy as a tool to shed information on the electronic structure of paramagnetic metal centers, especially those of metalloenzymes, is described in Chapter 16.

Compounds that can exist in multiple spin states open exciting possibilities in a number of fields. An interesting, widely explored case is transition metal centers in octahedral coordination environments with d-electron configurations d⁴ to d⁷, which can exist as high spin (HS) and low spin (LS) (see Chapters 5 and 12). Low-spin complexes favor pairing of electrons in t_2g orbitals rather than population of e_g orbitals, and the opposite happens for high-spin complexes. The energy difference between both states can be small, and with certain stimulus (light, heat or pressure) one can switch the predominant population of the two states in a reversible manner. In the solid state, cooperative intermolecular interactions may install kinetic barriers to spin interconversion, leading to hysteresis effects. In these cases, the system exhibits a bistability, a property that can potentially find use as memory units in electronic devices. Ongoing and exciting efforts in this field target the manipulation of the electronic spin by taking advantage of the quantum mechanical properties at molecular scale (quantum coherence and entanglement) as the key element for realizing quantum computing.

An important consequence of different spin states for a transition metal complex is that because of the change in occupation from non-bonding (d_xy, d_xz, d_yz) to anti-bonding orbitals (d_z², d_x²-y²), dramatic changes in spectroscopic properties and the metal–ligand bond distances are observed. For instance, typical Fe^II–N distances in low (S=0) or intermediate (S=1) are of the order of 1.98–2.09 Å, while for the high-spin state (S=2) distances of 2.15–2.25 Å are observed [8]. When comparing the geometries of low- and high-spin states for one and the same metal–ligand system, one finds usually mainly a lengthening of the metal–ligand distances. However, a recent study showed [9] that if the ligand is flexible enough with a large number of possible ligating atoms, severe changes in the coordination around the metal can be observed for different spin states. This feature is often observed for different oxidation states of a metal (e.g. Cu^I vs Cu^II), but is not so common for different spin states of the same metal in the same oxidation state. Translation of spin crossover phenomena in changes in the first coordination sphere of transition metal complexes may allow taking advantage of this property in solution state [10, 11].

The influence of spin states on reactivity can manifest itself in many ways. For example, it is at the basis of the reactions that sustain aerobic life. Spin-forbidden reactions, of, e.g. triplet dioxygen with singlet organic molecules to give singlet-only products, tend to be sluggish, despite being thermodynamically favorable processes. This is altered dramatically by the intermediacy of first-row transition metal ions in low oxidation states (Fe^II, Cu^I), which reduce the dioxygen molecule and form peroxide species that can oxidize organic functionalities (non-heme iron oxygenases, and models for the oxidizing species that form in their reactions are discussed in Chapters 10 and 15). Intermediacy of transition metals with multiple spin states in close energetic proximity is also used by nature extensively in order to open reaction paths to catalyze many otherwise unfeasible elementary processes. The interplay of multiple spin states in the oxidation reactivity of P450 is recognized as the origin of its chameleonic reactivity nature [12].

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Table of contents

• Cover
• Title page
• Copyright
• Dedication
• About the Editors
• List of Contributors
• Foreword
• Acknowledgments
• 1 General Introduction to Spin States
• 2 Application of Density Functional and Density Functional Based Ligand Field Theory to Spin States
• 3 Ab Initio Wavefunction Approaches to Spin States
• 4 Experimental Techniques for Determining Spin States
• 5 Molecular Discovery in Spin Crossover
• 6 Multiple Spin-State Scenarios in Organometallic Reactivity
• 7 Principles and Prospects of Spin-States Reactivity in Chemistry and Bioinorganic Chemistry
• 8 Multiple Spin-State Scenarios in Gas-Phase Reactions
• 9 Catalytic Function and Mechanism of Heme and Nonheme Iron(IV)–Oxo Complexes in Nature
• 10 Terminal Metal–Oxo Species with Unusual Spin States
• 11 Multiple Spin Scenarios in Transition-Metal Complexes Involving Redox Non-Innocent Ligands
• 12 Molecular Magnetism
• 13 Electronic Structure, Bonding, Spin Coupling, and Energetics of Polynuclear Iron–Sulfur Clusters – A Broken Symmetry Density Functional Theory Perspective
• 14 Environment Effects on Spin States, Properties, and Dynamics from Multi-Level QM/MM Studies
• 15 High-Spin and Low-Spin States in {FeNO}7, FeIV=O, and FeIII–OOH Complexes and Their Correlations to Reactivity
• 16 NMR Analysis of Spin Densities
• 17 Summary and Outlook
• Index
• EULA