Dealing with Spin States in Computational Organometallic Catalysis
The present chapter gives an overview of the intriguing effects that spin states have on catalysis and how this can (and cannot) be understood at present. For instance, highly reactive transition-metal complexes are often too fast to be trapped for charac
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Dealing with Spin States in Computational Organometallic Catalysis Marcel Swart
Contents 1 General Introduction 2 Spin States 2.1 Where Do They Come From, Where Do They Go? 3 Quantum Chemistry and Spin States 3.1 Density Functional Theory 3.2 Wavefunction Theory 3.3 Combining Wavefunctions and Density Functionals: MC-PDFT and DMRG-PDFT 3.4 Methods Put to the Test: Recent Benchmark Studies 4 Multi-state Reactivity 4.1 Exchange-Enhanced Reactivity 5 Catalysis 5.1 C-H Activation 5.2 Intradiol vs. Extradiol Selectivity 5.3 Catalase 6 Conclusions and Perspectives References
Abstract The present chapter gives an overview of the intriguing effects that spin states have on catalysis and how this can (and cannot) be understood at present. For instance, highly reactive transition-metal complexes are often too fast to be trapped for characterization by spectroscopy and/or crystallography. While significant advances have been made in theory with improved density functional approximations and more efficient wavefunction methods, these have not yet progressed to the point of being robust general-purpose chemical tools. Recent developments in the application of spectroscopy and theory on catalytically (in)active transition-metal complexes are discussed together with future perspectives.
M. Swart (*) IQCC and Department of Chemistry, Universitat de Girona, Girona, Spain ICREA, Barcelona, Spain e-mail: [email protected]
M. Swart
Keywords Catalysis · Density functional approximations · High-valent metals · Oxidation chemistry · Spin states · Transition metals
Abbreviations DFAs DFT IPEA MECP SCO
Density functional approximations Density functional theory Ionization potential, electron affinity Minimum energy crossing point Spin cross-over
1 General Introduction Chemistry can (or should) be defined as the discipline of transformation, where molecules meet, interact, and then depart completely reshaped; these processes can be enhanced or made more selective through the implication of (transition) metals. The first-row transition metals (Sc-Cu) play a special role in this, in the sense that they are earth-abundant (allowing for sustainable, or green, catalysis) and in general show a larger sensitivity to how the electrons are distributed over the d-orbitals than the corresponding transition metals in higher rows of the periodic table. The possible distributions of electrons over the d-orbitals obviously depend on the number of electrons; which metal is involved; how many coordinating ligands are present; and of which type. For instance, for a d6 system, the six electrons can occupy the three non-bonding orbitals, (xy)2(xz)2(yz)2, and hence lead to a low-spin state; however, if the anti-bonding orbitals (x2-y2, z2) are sufficiently low to overcome the pairing energy by increased exchange interactions, the high-spin state (xy)2(xz)1(yz)1(x2-y2)1(z2)1 might be lower in energy. It is well established that electrons, despite the electron-electron repulsion, have the tendency to pair up, with an orbital being occupied by one spin-up
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