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Open-shell transition-metal complexes, EPR/NMR, and catalysis

Significant parts of our extensive methodological developments are directed towards improved descriptions of transition-metal systems. These are of particular interest to metalloenzymes and bioinorganic chemistry described elsewhere. Here we rather focus on model complexes and aspects in homogeneous catalysis. Given that such open-shell transition-metal complexes are challenging targets for quantum-chemical calculations, they also serve as testing ground for improved DFT methods and inclusion of relativistic corrections.

Systematic validation studies for both hyperfine tensors and g-tensors of transition metal complexes have allowed tremendous insights into various factors that affect the achievable accuracy. A careful and systematic validation of DFT and coupled-cluster approaches for the calculation of metal hyperfine couplings [1] showed, e.g., the large dependence of spin-density distribution on exchange-correlation functional (in particular exact-exchange admixture). While often larger exact-exchange admixture improves the important core-shell spin polarization, in cases where the SOMO(s) is(are) metal-ligand antibonding, problems with spin contamination may arise from extensive valence-shell spin polarization. These aspects, and also the origin of core-shell spin polarization patterns have been studied in detail [2]. Perturbational treatments of spin-orbit corrections [4,5] to hyperfine couplings provide substantial contributions already for 3d metals [5]. More recently, two- and four-component relativistic calculations indicated even significant higher-order spin-orbit effects on g-tensors, already in 3d-systems and even more so in 4d- and 5d-complexes [6,7]. Furthermore, systematic studies of different exchange-correlation functionals in g-tensor calculations have provided a methodological basis [8-10].

Apart from various model complexes aimed at metalloenzyme sites, a further area of current interest have been transition metal complexes, in which the unpaired spin density is more on a (typically organic) ligand than on the metal center. Such ligand-centered radical complexes with redox-active ligands are of general interest in molecular magnetism and catalysis (and also for the understanding of radical enzymes like galactose oxidase). The prediction of their EPR parameters, in particular of their g-tensors, is far from trivial, as simple models developed either for organic radicals (e.g. Stone's theory) or for metal-centered spin density (e.g. ligand field theory) fail to capture spin delocalized over bridging or terminal organic ligands in transition metal complexes. Examples of our studies in this field are the interpretation of g-tensors in a series of dinuclear rhenium complexes [11] or the discovery of an unusual spin-polarization pattern in radical-ligand bridged dinuclear copper complexes [12]. In a series of ruthenium complexes with quinoid ligands, we could show that by a detailed and careful comparison of computational data, physical oxidation states could be assigned even in non-trivial situations [13]. Further application examples of EPR-parameter calculations for transition-metal complexes include novel cobaltocenophanes [14] and vanadoarenophanes [15].

New avenues are opened by the possibility [16] to calculate NMR chemical shifts for paramagnetic complexes, leading to various new applications in materials sciences [17] and catalysis.

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[1] A Critical Validation of Density Functional and Coupled-Cluster Approaches for the Calculation of EPR Hyperfine Coupling Constants in Transition Metal Complexes M. Munzarová, M. Kaupp J. Phys. Chem. A 1999, 103, 9966-9983.

[2] Mechanisms of EPR Hyperfine Coupling in Transition Metal Complexes M. L. Munzarová, P. Kubáček, M. Kaupp J. Am. Chem. Soc. 2000, 112, 11900-11913.

[3] Relativistic Spin-Orbit Effects on Hyperfine Coupling Tensors by Density Functional Theory A. V. Arbuznikov, J. Vaara, M. Kaupp J. Chem. Phys. 2004, 120, 2127-2139.

[4] Spin-Orbit Effects on Hyperfine Coupling Tensors in Transition Metal Complexes Using Hybrid Density Functionals and Accurate Spin-Orbit Operators C. Remenyi, A. V. Arbuznikov,. R. Reviakine, J. Vaara, M. Kaupp J. Phys. Chem. A 2004, 108, 5026-5033.

[5] Calculation of Electronic g-Tensors for Transition Metal Complexes Using Hybrid Density Functionals and Atomic Meanfield Spin-Orbit Operators M. Kaupp, R. Reviakine, O. L. Malkina, A. Arbuznikov, B. Schimmelpfennig, V. G. Malkin J. Comput. Chem. 2002, 23, 794-803.

[6] Relativistic Two-Component Calculations of Electronic g-Tensor for Oxo-Molybdenum(V) and Oxo-Tungsten(V) Complexes: The Important Role of Higher-Order Spin-Orbit Contributions P. Hrobárik, O. L. Malkina, V. G. Malkin, M. Kaupp Chem. Phys. 2009, 356, 229-235.

[7] Assessment of Higher-Order Spin-Orbit Effects on Electronic g-Tensors of d1 Transition-Metal Complexes by Relativistic Two- and Four-Component Methods P. Hrobárik, M. Repiský, S. Komorovský, V. Hrobáriková, M. Kaupp Theor. Chem. Acc. 2011, 129, 715-725.

[8] Validation study of meta-GGA functionals and of a model exchange-correlation potential in density functional calculations of EPR parameters A. V. Arbuznikov, M. Kaupp, V. G. Malkin, R. Reviakine, O. L. Malkina Phys. Chem. Chem. Phys. 2002, 4, 5467-5474.

[9] A Density Functional Study of EPR-Parameters for Vanadyl Complexes Containing Schiff Base Ligands M. L. Munzarová, M. Kaupp J. Phys. Chem. B 2001, 105, 12644-12652.

[10] A Comparative Density Functional Study of the EPR-Parameters of Amavadin C. Remenyi, M. L. Munzarová, M. Kaupp J. Phys. Chem. 2005, 109, 4227-4233.

[11] Multi-Frequency EPR Study and Density-Functional g-Tensor Calculations of Persistent Organorhenium Radical Complexes S. Frantz, H. Hartmann, N. Doslik, M. Wanner, W. Kaim, H.-J. Kümmerer, G. Denninger, A.-L. Barra, C. Duboc-Toia, J. Fiedler, I. Ciofini, C. Urban, M. Kaupp J. Am. Chem. Soc. 2002, 124, 10563-10571.

[12] Density functional study of EPR parameters and spin density distributions of dicopper(I) complexes with bridging azo and tetrazine radical anion ligands C. Remenyi, R. Reviakine, M. Kaupp J. Phys. Chem. A 2006, 110, 4021-4033.

[13] Where is the Spin? Understanding Electronic Structure and g-Tensors for Ruthenium Complexes with Redox-Active Quinonoid Ligands C. Remenyi, M. Kaupp J. Am. Chem. Soc. 2005, 127, 11399-11413.

[14] Synthesis, Crystal Structure, EPR and DFT studies and Redox Properties of [2]Tetramethyl-disilacobaltocenophane H. Braunschweig, F. Breher, M. Kaupp, M. Groß, T. Kupfer, D. Nied, K. Radacki, S. Schinzel Organometallics 2008, 27, 6427-6433.

[15] Synthesis, Reactivity and Electronic Structure of [n]Vanadoarenophanes: An Experimental and Theoretical Study H. Braunschweig, M. Kaupp, C. J. Adams, T. Kupfer, K. Radacki, S. Schinzel J. Am. Chem. Soc. 2008, 130, 11376-11393.

[16] Density Functional Calculations of NMR Chemical Shift Tensors for Paramagnetic Systems with Arbitrary Spin Multiplicity. Validation on 3d-Metallocenes P. Hrobárik, R. Reviakine, A. V. Arbuznikov, O. L. Malkina, V. G. Malkin, F. H. Köhler, M. Kaupp J. Chem. Phys. 2007, 126, 024107/1-19.

[17] Combining NMR spectroscopy and quantum chemistry as tools to quantify spin density distributions in molecular magnetic compounds M. Kaupp, F. H. Köhler Coord. Chem. Rev. 2009, 253, 2376-2386.

[18] Jacobsen’s Catalyst for Hydrolytic Kinetic Resolution: Structure Elucidation of Paramagnetic Co(III) Salen Complexes in Solution via Combined NMR and Quantum Chemical Studies S. Kemper, P. Hrobarik, M. Kaupp. N. E. Schloerer J. Am. Chem. Soc. 2009, 131, 4172-4173. Erratum: J. Am. Chem. Soc. 2009, 131, 6641-6641.

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