Definition of Magneto‐Structural Correlations for the Mn^II Ion

Abstract

The electronic properties of the high spin mononuclear MnII complexes [Mn(tpa)(NCS)2] (1) (tpa=tris-2-picolylamine), [Mn(tBu3-terpy)2](PF6)2 (2) (tBu3-terpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine) and [Mn(terpy)2](I)2 (3) (terpy=2,2':6',2''-terpyridine) with an N6 coordination sphere have been determined by multifrequency EPR spectroscopy. The X-ray structures of 1.CH3CN and 2.C4H10 O.0.5 C2H5OH.0.5 CH3OH reveal that the MnII ion lies at the center of a distorted octahedron. The D-values of 1-3 all fall in the narrow range of 0.041 to 0.105 cm(-1). The comparison of the results reported here and those found in the literature is consistent with the following observation: the D value is sensitive to the coordination number (6 or 5) of the MnII ion as long as the coordination sphere involves only nitrogen and/or oxygen based ligands. This magneto-structural correlation has been analyzed in this work though DFT model calculations. The zero-field splitting (zfs) parameters of 1-3 have been calculated and are in reasonable agreement with the experimental values. Hypothetical simplified models [Mn(NH3)x(OH2)y]2+ (x+y=5 or 6 and [Mn(NH3)5X]+ (X=OH, Cl)) have been constructed to investigate the origin of the zfs. This investigation reveals i) that D is sensitive to the coordination number (5 or 6) of the MnII ion, ii) for the five coordinate systems the major contribution to D is the spin-orbit coupling part, iii) for the six coordinate systems the major contribution to D is the spin-spin interaction and iv) the deprotonation of a water ligand leads to an increase of D, consistent with the relative ligand fields of OH(-) versus H2O.

Theory of chemical bonds in metalloenzymes. XV. Local singlet and triplet diradical mechanisms for radical coupling reactions in the oxygen evolution complex

“…The J ab , J bc , J cd , and J db integrals in the S 0 state have been determined to be −35 (−25), −40 (−50), −69 (−60), and −5 (−10) (cm −1 ), respectively; the J values in the parentheses are determined assuming another assumption for fitting the experimental ENDOR spectra. The corresponding J values for the S 2 state are −180 (−130), −60 (−45), −40 (−32), and −5 (−10) (cm −1 ), respectively31–33, 135–143. The sign of the experimental J values are negative though their magnitude is strongly dependent on the oxidation number of the manganese ion.…”

“…For example, Zheng and Dismukes32 have derived the HB spin Hamiltonian models for several manganese oxide clusters in Figure 1; their equations have been utilized for experimental determinations of the effective exchange integrals ( J ) between the nearest Mn ions. In fact, the J ‐values between the nearest neighbor Mn ions have been determined by the ENDOR experiments31–33, 135–143. The J ab , J bc , J cd , and J db integrals in the S 0 state have been determined to be −35 (−25), −40 (−50), −69 (−60), and −5 (−10) (cm −1 ), respectively; the J values in the parentheses are determined assuming another assumption for fitting the experimental ENDOR spectra.…”

“…The BS DFT (UB3LYP) calculations of the model complexes (Mn 4 O 4 and [L 6 Mn 4 O 4 ] 1+ (L 6 (Ph 2 PO 2 ) 6 )]) for CaMn 4 O 4 cluster in PSII are also performed to elucidate their electronic and spin structures; charge and spin populations are utilized for the purpose. Present computational results of manganese oxide clusters, together with those of other groups115–143, are discussed, emphasizing underlying basic concepts, and chemical pictures, explanations, and correspondences to experimental results4–75. Finally, implications of the comprehensive guiding principles for theoretical illumination of oxygen evolution are discussed in relation to oxygenation reactions in p450 and MMO.…”

“…In this series of articles1–3, we have performed theoretical studies of the nature of chemical bonds in metalloenzymes from the viewpoint of protein‐assisted confined states of magnetic inorganic complexes that have strongly correlated electron systems. Past decades structure and reactivity of manganese oxide enzymes have been accepted both experimental4–75 and theoretical interest1–3, 76–144. The nature of chemical bonds of these active sites has indeed been investigated with both experimental and theoretical methods: for example, X‐ray diffraction and various spectroscopic observations, and broken‐symmetry (BS) molecular orbitals (MO) and Kohn‐Sham (KS) density functional (DFT) calculations.…”

Zero-field splitting in nickel(II) complexes: A comparison of DFT and multi-configurational wavefunction calculations

“…13,14 We also reported a combination of molecular dynamics (MD) simulations and ZFS calculations yielding a time correlation function for the fluctuations of the ZFS in aqueous nickel(II) solution. 15,16 The development of the theoretical tools to calculate the ZFS parameters by quantum chemistry technique over the last decade has been quite impressive, with Frank Neese and his group as the leading team, 10,11,[17][18][19][20][21][22][23][24][25][26][27][28][29][30] but also with important contributions from other authors. [31][32][33][34][35][36][37] The development has followed two routes, making use of either efficient density functional theory (DFT) techniques or of high-end wavefunction methods based on multi-configurational SCF techniques (such as complete active space self-consistent field (CASSCF) and related extensions).…”