Mixed-terminal-ligand oxo-centered carboxylate-bridged trinuclear complexes: gas phase generation by means of electrospray ionization FT-ICR MS, condensed phase synthesis, and X-ray structure of K+[Cr3O(C6H5COO)6(F)2(H2O)]−·2(CH3)2CO

Paramagnetic 19F NMR and Electrospray Ionization Mass Spectrometric Studies of Substituted Pyridine Complexes of Chromium(III): Models for Potential Use of 19F NMR to Probe Cr(III)-Nucleotide Interaction

The synthesis and characterization of chromium basic carboxylate complexes, [Cr₃(O₂CR)₆L₃]⁺, containing trifluoroacetate, 3-fluoropyridine, 3-trifluoromethylpyridine, and 4-trifluoromethylpyridine are described. The substituted pyridine ligands are used as models of DNA bases to determine whether ¹⁹F NMR would be a potentially useful probe of the binding of Cr³⁺ to DNA. The ¹⁹F NMR resonances of the coordinated ligands, while broadened by delocalization of unpaired electron density from the S=3/2 chromic centers, are readily discernable, and the contact shifts are of sufficient magnitude that the signals from coordinated and free ligands can easily be differentiated. Thus, ¹⁹F NMR appears to be a potentially useful probe of the binding of Cr³⁺ to DNA containing F-labeled bases. Additionally, electrospray MS is shown to be a convenient method to establish the identity of chromium basic carboxylate assemblies.

Pressure versus temperature effects on intramolecular electron transfer in mixed-valence complexes

Mixed-valence trinuclear carboxylates, [M(3)O(O(2)CR)(6)L(3)] (M = metal, L = terminal ligand), have small differences in potential energy between the configurations M(II)M(III)M(III)⇔M(III)M(II)M(III)⇔M(III)M(III)M(II), which means that small external changes can have large structural effects, owing to the differences in coordination geometry between M(2+) and M(3+) sites (e.g., about 0.2 Å for Fe-O bond lengths). It is well-established that the electron transfer (ET) between the metal sites in these mixed-valence molecules is strongly dependent on temperature and on the specific crystal environment; however, herein, for the first time, we examine the effect of pressure on the electron transfer. Based on single-crystal X-ray diffraction data that were measured at 15, 90, 100, 110, 130, 160, and 298 K on three different crystals, we first unexpectedly found that our batch of Fe(3)O (O(2)CC(CH(3))(3))(6)(C(5)H(5)N)(3) (1) exhibited a different temperature dependence of the ET process than previous studies of compound 1 have shown. We observed a phase transition at around 130 K that was related to complete valence trapping and Hirshfeld surface analysis revealed that this phase transition was governed by a subtle competition between C-H⋅⋅⋅π and π⋅⋅⋅π intermolecular interactions. Subsequent high-pressure single-crystal X-ray diffraction at pressures of 0.15, 0.35, 0.45, 0.74, and 0.96 GPa revealed that it was not possible to trigger the phase transition (i.e., valence trapping) by a reduction of the unit-cell volume, owing to this external pressure. We conclude that modulation of the ET process requires anisotropic changes in the intermolecular interactions, which occur when various directional chemical bonds are affected differently by changes in temperature, but not by the application of pressure.

Electron induced dissociation: a mass spectrometry technique for the structural analysis of trinuclear oxo-centred carboxylate-bridged iron complexes

We report electron induced dissociation (EID) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry of the singly-charged cations [Fe(3)O(CH(3)COO)(6)](+) and [Fe(3)O(HCOO)(6)+H(2)O](+). Trinuclear oxo-centered carboxylate-bridged iron complexes of this type are of interest due to their electronic and magnetic properties, and because of their role as synthetic precursors of single molecule magnets. EID of these complexes is particularly efficient and provides detailed information about the triangular core, and the nature and number of ligands. EID behavior is in marked contrast to the collision induced dissociation (CID) of these species. Whereas EID allows virtually complete structural characterization, the structural information provided by CID is very limited. The results suggest that EID is particularly suitable for the structural analysis of singly-charged polynuclear metal complexes.

Preparation and characterization of novel oxo-centered basic p-chlorobenzoic bridging trinuclear complexes

Three new oxo-centered trinuclear complexes, one of them a mixed-valence complex [Mn3O(C7H4O2Cl)6(Py)3]Py (1) and the others, mixed-metal complexes of [Fe2MnO(C7H4O2Cl)6(Py)3]NO3 (2) and [Fe2CoO(C7H4O2Cl)6(Py)3] (3) were synthesized by the direct reaction between metal nitrates and p-chlorobenzoic acid. These complexes were characterized by elemental analyses (CHN), atomic absorption spectroscopy and spectral (IR, electronic) studies. These are new type of oxo-bridged mixed-metal complexes in which the carboxylate ligand is pchlorobenzoic acid. The UV spectra of the complexes exhibited a strong band in the region 42,500 cm-1 which is related to the (? ? ?*) transitions of the pyridine ligand. The IR spectra of these compounds showed two strong stretching vibrations bands, indicating a bridging coordination mode of the carboxylic group of the ligand in the complexes.

Mass spectrometric and spectroscopic studies of the nutritional supplement chromium(III) nicotinate

Despite chromium nicotinate's popular use as a chromium nutritional supplement, the structure and composition of chromium nicotinate have only been poorly described. As solid chromium nicotinate is intractable, being insoluble or unstable in common solvents, studies on the solid have been limited, and studies of the solution from which the "compound" precipitates have additionally provided little additional data. The results of mass spectrometric and spectroscopic investigations designed to further elucidate the structure and composition of chromium nicotinate are described. The results demonstrated that the three common methods for producing "chromium nicotinate" all yield different compounds, all of which are polymers of Cr(III), oxygen-bound nicotinate, hydroxide, and water. Implications for interpreting results of nutritional studies of "chromium nicotinate" are discussed.

Synthesis and characterization of novel oxo-bridged, trinuclear mixed-metal complexes of Cr(III) and Fe(III)

Two new heterotrinuclear p-chlorobenzoates, [Fe2CrO(C7H4O2Cl)6(py)3]NO3 (1) and [Cr2FeO(C7H4O2Cl)6(py)3]NO3 (2) were prepared as nitrate salts and characterized by elemental analyses (CHN), spectroscopic (infrared, electronic) studies and atomic absorption spectroscopy. These complexes are a new type of oxo-bridged mixed-metal complex in which the carboxylate ligand is p-chlorobenzoic acid. Bridging coordination modes for the carboxylates were indicated by the presence of ?asym (M2M'O) vibrations in the infrared spectra.

Heterocyclic Thionates as a New Class of Bridging Ligands in Oxo-Centered Triangular Cyclopentadienylchromium(III) Complexes

The interactions of the benzothiazolate complex, CpCr(CO)(2)(SCSN(C(6)H(4))) (2), and the tetrazole thiolate complex, CpCr(CO)(3)(eta(1)-SCN(4)Ph) (3), with controlled amounts of Me(3)OBF(4) and (MeO)(2)SO(2), respectively, produced the corresponding mu(3)-oxo trinuclear thionate-bridged complexes, [Cp(3)Cr(3)(mu(2)-OH)(mu(3)-O)(mu(2)-eta(2)-SCSN(C(6)H(4)))(2)](5)BF(4) (45%) and [Cp(3)Cr(3)(mu(2)-OH)(mu(3)-O)(mu(2)-eta(2)-SCN(4)Ph)(2)](9)(MeOSO(3)) (53%), together with their respective free dimethylated thiolate ligands, [MeSCSNMe(C(6)H(4))](4)BF(4) and (Me(2)SCN(4)Ph)(8)MeOSO(3). The reaction of 3 with Me(3)OBF(4) resulted in the isolation of a binuclear complex, [Cp(2)Cr(2)(mu-OH)(mu-eta(2)-SCN(4)Ph)(2)](7)BF(4) (43%), and (8)BF(4) (27%). The reaction of the thiopyridine complex, CpCr(CO)(2)(SPy) (4), with I(2) also produced a similar mu(3)-oxo complex 10 (31%), together with CpCrI(2)(THF) (11) and the disulfide (SPy)(2). Similar reactions with 2 and 3 and I(2) yielded species 5 and 7, together with 11 and disulfides derived from their respective ligands. Cyclic voltammograms recorded in solutions of 5 and 9 indicated that the compounds could be reduced and oxidized at very similar potentials. An EPR spectrum characteristic of a compound with axial symmetry was obtained for 9 at 7 K. Single-crystal X-ray diffraction analyses confirmed that species 7 is dinuclear, whereas 5 and 9 are structural trinuclear analogues, each containing a mu(3)-oxo central core.