Bruce R. McGarvey

Isotropic NMR shifts in transition metal complexes: The calculation of the fermi contact and pseudocontact terms☆

Abstract The Fermi contact and pseudocontact contribution to isotropic NMR shifts in paramagnetic complexes are considered for the following cases, for which formulae customarily used are not strictly applicable: (1) there is an appreciable contribution to the magnetic moment of the complex from unquenched orbital angular momentum; (2) there is appreciable orbital contribution (induced by spin-orbit effects) from spin density at the ligand nucleus; (3) there is appreciable mixing of the ground electronic state and thermally populated excited states by the applied magnetic field. An approximate density matrix treatment is developed to handle these situations. The following examples are discussed as illustrations: octahedral ( t 2 g ) 1 and ( t 2 g ) 5 complexes and complexes in which zero-field splitting occurs.

Structure, chemical and photochemical reactivity and biological activity of some ruthenium amine nitrosyl complexes

Abstract Through spectroscopic (X-ray, Infrared, 1H-NMR, EPR, UV–vis) and electrochemical (cyclic voltammetry, differential pulse polarography) data and quantum mechanical calculations, the formulation [Ru(II)NO+] was attributed to a series of new ruthenium(II) amine compounds. A remarkable stability of the Ru(II) relative to Ru(III) was observed upon coordination to NO. The presence of nitrosyl in the coordination sphere results in dramatic implications in the lability, acidity and redox properties of the ligand trans to NO. These effects are higher than expected just on the basis of one unity increment in the metal center charge. Based on molecular orbital (MO) analysis and on reduction product analysis, the site of the reduction [Ru(NO)]3++e−→[Ru(NO)]2+ was assigned to the NO ligand. The dissociation of the coordinated NO0 is dependent on the trans effect and trans influence of the trans ligand L. Irradiation of the nitrosyl complexes with 300–350 nm light results in NO aquation and formation of the corresponding aquaruthenium(III) complex, i.e. trans-[ Ru ( NO )( NH 3 ) 4 L ] 3+ → H 2 O , H + hν trans-[ Ru ( NH 3 ) 4 ( H 2 O ) L ] 3+ + NO 0 Irradiation in the visible region (400–500 nm) did not result in any observable reaction in solution; however, at low temperature and in the solid state, evidence in favor of the formation of linkage isomers has been obtained. The hypotensive properties of trans-[Ru(NO)(NH3)4(P(OC2H5)3)](PF6)3 and trans-[Ru(NO)Cl(cyclam)](ClO4)2 have been demonstrated in mice and rats.

Metals in Anticancer Therapy: Copper(II) Complexes as Inhibitors of the 20S Proteasome

Selective 20S proteasomal inhibition and apoptosis induction were observed when several lines of cancer cells were treated with a series of copper complexes described as [Cu(L(I))Cl] (1), [Cu(L(I))OAc] (2), and [Cu(HL(I))(L(I))]OAc (3), where HL(I) is the ligand 2,4-diiodo-6-((pyridine-2-ylmethylamino)methyl)phenol. These complexes were synthesized, characterized by means of ESI spectrometry, infrared, UV-visible and EPR spectroscopies, and X-ray diffraction when possible. After full characterization species 1-3 were evaluated for their ability to function as proteasome inhibitors and apoptosis inducers in C4-2B and PC-3 human prostate cancer cells and MCF-10A normal cells. With distinct stoichiometries and protonation states, this series suggests the assignment of species [CuL(I)](+) as the minimal pharmacophore needed for proteasomal chymotryspin-like activity inhibition and permits some initial inference of mechanistic information.

ESR and Optical Spectroscopy of CrO43− in Chlorospodiosite, Ca2PO4Cl

Studies of the spectral properties of CrO43− ion in Ca2PO4Cl (chlorospodiosite) single crystals confirm the structural results established by single‐crystal x‐ray structure analyses. Detailed electron spin resonance measurements of a single crystal of Ca2(PO4, CrO4)Cl are described and the appropriate spin‐Hamiltonian parameters determined. It is shown that the unpaired electron (3d1) in its ground state occupies a dz2 orbital, which is largely concentrated on the chromium atom. The visible and ultraviolet absorption spectrum of a single crystal of Ca2PO4Cl containing CrO43− is described and shown to be similar to the spectrum of MnO42−, which had been interpreted on the basis of a Ballhausen and Liehr molecular‐orbital level scheme. The infrared absorption spectra show more splitting of the vibrational bands of the chromate (V) compound (Ca2CrO4Cl) than of the phosphate (Ca2PO4Cl), in agreement with the structural results which indicated a greater distortion of the CrO43− than that of the PO43− tetrahedr...

Survey of ligand field parameters of strong field d5 complexes obtained from the g matrix

Abstract Using a newly proposed approach involving an internally consistent set of equations, the ligand field parameters Δ / ξ , V / ξ and k are obtained from literature values of the g matrix for strong field d 5 systems of various conformations in which | Δ / ξ |≤10. Qualitative analysis of the observed results is done using the Angular Overlap Model, AOM.

Bioinspired Five-Coordinate Iron(III) Complexes for Stabilization of Phenoxyl Radicals†

Considerable effort has been directed towards the integration of biomimetic principles into molecular materials that have customized and controllable properties. The notion of stimulus-triggered molecular switching between two or more ground states of comparable energy is particularly relevant because such switching leads to detectable electronic and structural changes. Coordination complexes that merge transition-metal ions with ligands that stabilize organic radicals are among the most promising candidates for redox-responsive switching processes. Among the electroactive ligands that have been well characterized, those that contain phenolate moieties are significant because of their synthetic versatility and redox accessibility. This importance has been highlighted by studies on metal–phenoxyl complexes that have several geometries. Iron(III) complexes that contain phenolates tend to favor an octahedral geometry and are electrochemically reversible, but usually do not withstand multiple redox cycles. Thus, an understanding of the alternative geometries of such complexes becomes a necessary strategy for the future development of redox switches. We are investigating bioinspired designs that incorporate the basic geometries that are present in redox-versatile enzymes, such as tyrosine hydroxylase and intradiol dioxygenase, in which five-coordinate iron(III) centers support radical-based mechanisms for generating l-3,4-dihydroxyphenylalanine (l-DOPA) and cleaving catechol-type rings, respectively. We have reported the behavior of high-spin iron(III) complexes that are confined to low-symmetry, pentadentate N2O3 environments. [6] In these complexes, the assignment of oxidation states becomes challenging because of the contributions of ligandand metal-centered orbitals to the same redox process, and the presence of five unpaired electrons. Nonetheless, we have shown that high oxidation states are unavailable to the metal ion, and that the ligand supports up to three consecutive oxidations, which leads to antiferromagnetic interactions. Relative to octahedral fields, these fivecoordinate environments are expected to yield low-degeneracy molecular orbitals (MOs) that are sensitive to subtle but noticeable structural changes in the ligands. These changes should lead to orbital rearrangements that modify the sequence by which phenolate oxidations occur. Herein, we investigate the behavior of the five-coordinate species [FeL] (1) and [FeL] (2, Scheme 1), in which a low-symmetry ligand field is purposefully enforced around the 3d metal ion by the N2O3 ligands. Ligands [L ] and [L] both contain N2O3 environments with three phenolate moieties, denoted A, A’, and B; phenolates A and A’ share the same amine group and are chemically equivalent, whereas phenolate B is attached to either an azomethine group in L or to a methylamine group in L. Both species have four accessible ground states: [FeL]/[FeL] , [FeLC], [FeLCC], and [FeLCCC]. The aim of this study is to determine the sequence in which each of the phenolate rings is oxidized in the presence of the azomethine and the methylamine groups, and to test the feasibility of consecutive, multielectronic oxidations by ion-pairing effects with the supporting electrolyte. This study is intended to contribute to the fundamental understanding of the redox and electronic behavior of high-spin 3d 5 ions in five-coordinate ligand fields, and provide significant insight into bioinspired redox cycling. Complexes 1 and 2 were synthesized as previously described and crystals that were suitable for analysis by [*] M. M. Allard, J. A. Sonk, Dr. M. J. Heeg, Prof. H. B. Schlegel, Prof. C. N. Verani Department of Chemistry, Wayne State University 5101 Cass Ave. Detroit, MI 48202 (USA) E-mail: cnverani@chem.wayne.edu Homepage: http://chem.wayne.edu/veranigroup/

Temperature dependence of the pseudocontact shift in lanthanide shift reagents

Abstract The method of calculating the pseudocontact shift of lanthanide shift reagents as a power series of T − n , the reciprocal of temperature, developed by Bleaney has been extended to the T −3 term. Using crystal field parameters reported in the literature, it is found that the T −3 term is no more than 10% of the T −2 term in magnitude at room temperature for all ions to which the theory is applicable. The calculations show that even though the pseudocontact shift cannot be fitted exactly to a T −2 temperature dependence, the simple equation for the T −2 term is adequate for estimating the actual shift at room temperature to an accuracy of 10 to 20%.

Theory of the Isotropic NMR Shifts in Trigonal Co(II) Complexes

The theory of both the contact and pseudocontact shift in Co(II) trigonal complexes is considered. The theory predicts a pseudocontact shift in Co(II)poly(1‐pyrazolyl)borate complexes, in good agreement with experimental results. The predicted contact shifts are found to be much different from that predicted by previously proposed theories. In the region of 200–400°K, the temperature dependence is found to deviate strongly from a 1 / T dependence for the pseudocontact shift but not for the contact portion of the shift. The assumption made in earlier theories, that the contact interaction is the same for all six Kramers doublets, is examined in detail.

NMR and EPR spectroscopies and electron density distribution in polyoxoanions

Abstract The results of NMR and EPR studies are considered for large polyoxoanions of molybdenum and tungsten with paramagnetic and some diamagnetic centers which produce delocalized electrons. For various polyoxoanions the possible mechanisms of electron density transfer involving π- and σ-bonding, depending on hybridization, bond lengths, and geometric configuration are analyzed. The different contributions to the observed chemical shifts including contact, pseudocontact (dipolar ligand centered), dipolar (metal centered) and spin polarization terms are considered and sometimes evaluated. Formation of the delocalized electron pair or bipolaron is proven by 17 O- and 183 W-NMR. The changes observed in the 183 W-NMR spectra of polyanions with non-equivalent tungsten atoms with the delocalized electron pair are believed to be due to the existence of the spin bipolaron. A model of the circulation of the electron density in Co 3+ W 12 O 40 5− due to the dynamic Jahn–Teller effect is developed. The possibility of spin-paired electron transfer from the 3d-cation into the ligand orbitals of the nucleus under investigation is proposed.

The ligand hyperfine interaction with rare earth ions. I. A revised covalent model

It is shown that a reformulation of the covalent model can explain the /sup 19/F hyperfine interactions observed for Yb/sup 3 +/ and Tm/sup 2 +/ in cubic and tetragonal sites of alkaline earth fluorides. The same model does not explain data for Ce/sup 3 +/, Gd/sup 3 +/, or Eu/sup 2 +/ indicating that the spin polarization model is the dominant mechanism in the first part of the rare earth series but that the covalent model becomes dominant in the latter half of the series. The analysis shows that the distortion from cubic symmetry in the tetragonal site is not as severe as an earlier NMR analysis suggested but is still large. It further suggests that the interstitial ion is closer to the rare earth ion than most observers have assumed. (AIP)