Frank E. Mabbs

Electron paramagnetic resonance of heterogeneous chromium catalysts

The X- and Q-band EPR spectra of Cr5+ and Cr3+ ions in heterogeneous Cr catalysts have been investigated as a function of the support type and composition (silica, silica–alumina, alumina, mordenite and AlPO-5). A spin-Hamiltonian operator for each of the observed types of resonances is proposed and discussed, while the extensive use of computer-simulated spectra provides detailed information on the nature of supported Cr ions. Mononuclear Cr5+ ions (γ-signal), with g-values ca. 2, are axial on alumina and silica–alumina, and rhombic on silica and mordenite. Two different X-band EPR signals of Cr3+ ions are present on amorphous and crystalline oxidic surfaces: a broad signal with g around 2 and zero field parameters D and E equal to 0, due to clustered Cr3+(β-signal) or isolated octahedral Cr3+ complexes, and a signal with a positive lobe in the region g= 3.5–5.5 (δ-signal). The latter signal is characterized by high values of D and E and due to the presence of a highly distorted Cr3+ octahedron. The data so obtained allow us to develop a general model of supported Cr5+ and Cr3+ ions on oxidic surfaces.

Cascade complexation of pseudo-halide by dicopper cryptates: a linear Cu–NNN–Cu unit

An X-ray crystallographic structure determination of a µ-azido azacryptate has confirmed the existence of collinear Cu–NNN–Cu geometry, earlier cited as the origin of unusual spectroscopy in dicopper µ-azido cryptates; the isoelectronic µ-cyanato cryptate is also isostructural.

Electrochemical and spectroelectrochemical studies on platinum complexes containing 2,2′-bipyridine

The electrochemical and spectroelectrochemical (UV/VIS/NIR and EPR) properties of a series of complexes of general formula [Pt(bipy)L2]n+(L = Cl– or CN–, n= 0; L = NH3, py, PMe3 or L2= en, n= 2; bipy = 2,2′-bipyridine, py = pyridine, en = ethylenediamine) have been investigated. The complexes undergo a reversible one-electron reduction process at potentials of ca.–1 V vs. Ag–AgCl. In each case UV/VIS/NIR spectra of the one-electron reduction products were consistent with co-ordinated bipyridyl anion-radical species rather than d9 metal centres. Electron paramagnetic resonance spectra of the chemically or electrochemically generated 17-electron species, in conjunction with the results of extended-Huckel molecular-orbital calculations, show a significant admixture of metal 5dyz and/or 6pz orbitals in the singly-occupied molecular orbital. The complex [Pt(bipy)(PMe3)2][BF4]2 has been prepared.

Single and double quantum transitions in the multi-frequency continuous wave electron paramagnetic resonance (cwEPR) of three six co-ordinate nickel(II) complexes: [Ni(EtL)2(Me5dien)] and [Ni(5-methylpyrazole)6]X2, X = (ClO4)− or (BF4)−. The single crystal X-ray structure at room temperature of [Ni(5-methylpyrazole)6](ClO4)2

Multi-frequency, variable temperature cw EPR powder spectra of [Ni(EtL)2(Me5dien)] (3) and [Ni(5-methylpyrazole)6]X2, X = (ClO4)− (1) or (BF4)− (2) are reported. The spectra of 1 and 2 show a temperature variation which is consistent with an increase in ∣D∣ as the temperature decreases. The changes in the spectra of 1 are irreversible. The low temperature spectra for each compound exhibit a double quantum transition whose powder dependence is the same as that for the single quantum transitions. The reasons for this behaviour, as well as the simulations of the whole spectra at various frequencies, is discussed. The single crystal X-ray structure of 1 at room temperature is reported, whilst it was found that these crystals disintegrated on cooling. This behaviour is consistent with the change in the EPR behaviour of this compound on cooling.

Crystal structure of tris(tetra-n-butylammonium) tri-µ-benzenethiolato-bis{tri-µ-sulphido-[µ3-sulphido-tris(benzenethiolatoiron)]molybdenum}[Bun4N]3[{(PhSFe)3MoS4}2(SPh)3]; an Fe3MoS4 cubic cluster dimer

An X-ray crystallographic study shows that the anion [{(PhSFe)3MoS4}2(SPh)3]3– contains two Fe3MoS4‘cubes’ linked across their molybdenum centres by three µ2-benzenethiolato-groups.

The electronic structure of CrO3+ and MoO3+ complexes

Abstract The results of ab initio molecular orbital calculations for [CrOfs]2− and polarised single crystal electronic spectra of [MoOCl3(Op(NMe2)3)2] and Ph4As[MoOCl4(H2O)] are presented. These data are consistent with the electronic transitions of the MO3+ moieties, O2pπ → Mdxy and Mdxy → Mddxy,dyz being the lowest energy transitions in the spectra of their respective complexes, both these transitions being of low intensity.

A molybdenum derivative of a four-iron ferredoxin type centre

The biological importance of four-iron ferredoxin centres is well recognised [l] and the detection and investigation of such units has been aided considerably by the synthesis and characterisation of their chemical analogues [2]. In contrast, much has still to be defined concerning the environment about, and function of, molybdenum in the various enzymes for which it is an essential constituent [3]. Certain of these molybdoenzymes, the nitrogenases, xanthine and aldehyde oxidases, and some nitrate reductases contain ferredoxin centres. The concept of a molybdenum-containing co-factor, common to different molybdoenzymes, derives from genetic arguments [4] and receives considerable support from the biochemical studies of Nason and Ketchum [5, 61. The first definitive isolation of such a co-factor was achieved by Shah and Brill [7] who obtained an iron-molybdenum co-factor from the Fe-MO protein of the nitrogenase of Azotobacter vinelandii. Analyses of this unit suggested that it contains iron, molybdenum and acid-labile sulphur in the ratios 8: 1:6. EXAFS studies [8] have suggested that the molybdenum is attached to several sulphur atoms in this co-factor and in nitrogenase: a conclusion which is compatible with the e.s.r. signals observed for the molybdenum centres of other molybdoenzymes [3]. Therefore, we have been interested in developing the chemistry of molybdenumiron-sulphur compounds. Here we report one such species, [FeBMo& (SC,H,),] 3--, which has been prepared and isolated as its [(n-CaH&N] + salt. Anal. : Calcd. for C102H153 N3Fe6M02S1,: C49.1;H 6.2;N 1.7;Fe 13S;Mo 7.7; S 21.9%. Found C 49.1; H 6.2; N 1.7; Fe 13.7; MO 7.6; S 22.0%. This air-sensitive, microcrystalline material, m.p. 216-217 “C (dec.) resembles [Fe,&(SC6Hs)4]2salts [2] in appearance and the uv/vis comprises intense absorptions above 275 nm, ‘tailing’ to lower energy, upon which features at 350 and 450 nm are apparent. The room temperature magnetic moment per Fe6M02 unit is 4.1 f 0.1 BM, which is the same as that reported for [Fe4S4(SCH2C6H&] 2[9] . No e.s.r. activity is manifest at room temperature.

New molecular superconductor containing paramagnetic chromium(iii) ions

The molecular charge-transfer salt β″-(bedt-ttf) 4 [(H 2 O)Cr(C 2 O 4 ) 3 ]·PhCN [bedt-ttf = bis(ethylenedithio)tetrathiafulvalene], containing paramagnetic Cr III ions (S = 3/2), is a superconductor with T c = 6.0(5) K.

Synthesis and characterization of 17-electron species of nitrosyl[tris-3,5-dimethylpyrazolylborato]molybdenum

Abstract The reaction of [Mo(NO){HB(Me 2 pz 3 }Cl 2 ] with various reducing agents in the presence of NHEt 3 led to the isolation of [NHEt 3 ][Mo(NO){HB(Me 2 pz) 3 }Cl 2 ], a 17-electron species. [NHEt 3 ]Mo(NO){HB(Me 2 pz) 3 }Cl 2 ] reacts with pyridine (py) to form [Mo(NO){HB(Me 2 pz) 3 }Cl(py)]. The spectroscopic properties of these two novel species are discussed and comparisons are drawn with [Mo(NO){HB(Me 2 pz) 3 }(py) 2 ][PF 6 ].

The EPR spectra and electronic structures of low-symmetry oxo-molybdenum(V) complexes

Abstract EPR spectra of fluid and frozen toluene solutions of MoOLX 2 , where L = hydrotris(3,5-dimethylpyrazolyl(borate), and X = Cl, SEt, OMe or NCS, at both X - and Q -band frequencies, have been simulated on the basic of C s -symmetry for the molecules. The non-coincidence of the g -and A -tensor axes are discussed in terms of the probable d -orbital mixings caused by the low-symmetry ligand fields in these molecules.