High-frequency and -field electron paramagnetic resonance of high-spin manganese(III) in axially symmetric coordination complexes

Abstract

High-frequency and -field electron paramagnetic resonance (HFEPR) has been used to study several complexes of high-spin manganese(III) (3d4, S = 2): [Mn(Me2dbm)X] and [Mn(OEP)X] (X = Cl-, Br -), where Meidbm-is the anion of 4,4'-dimethyldibenzoylmethane and OEP 2-is the dianion of 2,3,7,8,12,13,17,18-octaethylporphine. These non-Kramers (integer spin) systems are not EPR-active with conventional magnetic fields and microwave frequencies. However, use of fields up to 15 T in combination with multiple frequencies in the range of 95-550 GHz allows observation of richly detailed EPR spectra. Analysis of the field-and frequency-dependent HFEPR data allows accurate determination of the following spin Hamiltonian parameters for these complexes: [Mn(Me,dbm)Cl], D = -2.45(3) cm'; [Mn(Me2dbm)Br], D = -1.40(2) cm'; [Mn(OEP)Cl], D = -2.40(1) cm'; [Mn(OEP)Br], D = -1.07 (1) cm -' (E 0, and g 2.0 in all cases). Comparison of structural data with the electronic parameters for these and related complexes shows quantitatively the effects of axial and equatorial ligation on the electronic structure of Mn(III). These high-spin complexes can be employed as building blocks in the construction of single-molecule magnets. Thus the accurate determination and understanding of the electronic properties, best obtainable by HFEPR, of these monomeric units is important in understanding and improving the properties of the polynuclear single-molecule magnets which can be formed from them. IntroductoinPolynuclear transition metal clusters that possess large ground-state spins (S > 4) are of significant interest for use as magnetic materials. In particular, high-spin molecules with large-magnitude, negative zero-field splitting (zfs) (D < 0) exhibit magnetic hysteresis effects resembling nanomagnets and are termed single-molecule magnets (SMMs) [1][2][3][4][5][6][7][8][9]. The resultant zfs of a polynuclear complex reflects * Present address:

Advanced Paramagnetic Resonance Studies on Manganese and Iron Corroles with a Formal d4 Electron Count

“… a The error bars for all spin Hamiltonian parameters obtained by HFEPR in this work (row 1) are standard deviations in the least-squares fits to the 2D field/frequency maps such as the one shown in Figure . b In the case where all three g matrix components were determined, these are given in order g x , g y , g z (or g min , g mid , g max ); if two values are quoted, they are shown as g ⊥ and g || , and if only one value is given, then only an isotropic g value was determined. c dehmc is 8,12-diethyl-2,3,7,13,17,18-hexamethylcorrole trianion. d tpp is 5,10,15,20-tetraphenylporphyrin dianion. e oep is 2,3,7,8,12,13,17,18-octaethylporphryin dianion. The | E | value reported for Mn­(oep)Cl in frozen solution is an upper limit based on line widths f Me 2 pyNO is 3,5-dimethylpyridine N -oxide, which is bisaxially coordinated.…”

“…In the case of Mn­(tpfc), these include the two Mn III corrole complexes studied previously by HFEPR , and representative Mn III porphyrin complexes. A recent, extensive tabulation of spin Hamiltonian parameters in Mn III complexes is given by Tadyszak et al The zfs of high-spin 3d ions (e.g., Cr II and Mn III ) in tetragonally distorted octahedral geometries (including both square pyramidal–one axial ligand at infinite distance, and square planar–both axial ligands at infinite distances) is relatively well described by simple ligand-field theory (LFT), and has been discussed at length elsewhere. ,,− In-depth computational studies of zfs in Mn III species have also been reported. ,, Briefly, a negative axial zfs, D < 0, corresponds to a tetragonal elongation (“hole” in the d x 2 – y 2 orbital; 5 B 1 ground state), while a positive axial zfs, D > 0, corresponds to a compression, which also corresponds to the situation for trigonal bipyramidal geometry (“hole” in the d z 2 orbital; 5 A 1 ground state). , The former situation applies to most 6-coordinate, − 5-coordinate (square pyramidal), and 4-coordinate (square planar) Mn III complexes. − , …”

“…Accordingly, the Mn­(tpfc) zfs results act as a “control” for comparison with the zfs determined for the two different formally Fe IV corrole complexes. The zfs of Mn­(tpfc) is also of interest in its own right as a large number of porphyrin (i.e., formally dianionic) complexes of Mn III have been investigated by HFEPR, ,− even in a protein environment, as well as by other techniques, − but such studies on corroles are relatively few. , Moreover, of the two lone prior investigations of Mn III corroles by HFEPR, that by Licoccia and coworkers involved a differently substituted corrole macrocycle (dehmc = 8,12-diethyl-2,3,7,13,17,18-hexamethylcorrole), and the study by Bendix et al, although on a tpfc complex, comprised a Mn­(III) center bearing an axial triphenylphosphine oxide ligand, i.e., Mn­(tpfc)­(OPPh 3 ), and thus exhibited approximate square pyramidal geometry rather than square planar. Furthermore, Licoccia and coworkers investigated the HFEPR of their corrole complex in a highly concentrated (∼0.2 M) frozen solution of pyridine, where axial coordination by solvent to give monopyridine or perhaps even bis-pyridine adducts is likely.…”

A mononuclear MnIII/‘bis-tris’ complex and its conversion to a mixed-valence MnII/III5cluster

“…In fact, the value for 1 is in the range normally found for five-coordinate Mn III complexes with square pyramidal geometries. 43 We therefore suspected that the smaller than expected D value was an artifact due to the large distortion of complex 1 from octahedral geometry, leading to a significant rhombic zero-field splitting parameter E. Thus, we included both D and E in the magnetization fit for 1; shown in Fig 7 is the error surface for this D vs. E fit with a fixed g = 2.0. The best fit was with D = -3.25 cm -1 and |E| = 0.32 cm -1 .…”

Chiral biomolecule based dodecanuclear dysprosium(iii)–copper(ii) clusters: structural analyses and magnetic properties

“…[1][2][3][4][5][6] Among these, coordination architectures with excellent symmetric features have been the most investigated ones owing to their captivating coordination environments, giving rise to utilitarian attributes as a consequence of their symmetry-aided structure-property correlation. [7][8][9][10][11][12][13] The inherent symmetry in such complexes makes the structure-property correlation much easier to predict, leading to a strategic rationale based design principle for attaining the target properties and an insight thereinto. Over the decades, employment of a binary or ternary combination of linkers for yielding such complexes has proved quite an efficient protocol.…”