Collision induced dissociation (CID) to probe the outer sphere coordination chemistry of bis-salicylaldoximate complexes.

Ligand-ligand interactions in the outer coordination sphere make an important contribution to the effects of 3-substituents on the stabilities of anionic Cu(II) salicylaldoximato complexes [CuL(L-H)](-). When substituents contain a different number of bonds the interpretation of CID tandem mass spectrometry must take into account the ability of ions to redistribute energy acquired in collisions within different numbers of vibrational modes.

Binuclear cobalt complexes of Schiff-base calixpyrroles and their roles in the catalytic reduction of dioxygen.

The syntheses and characterization of a series of binuclear cobalt complexes of the octadentate Schiff-base calixpyrrole ligand L are described. The cobalt(II) complex [Co(2)(L)] was prepared by a transamination method and was found to adopt a wedged, Pac-man geometry in the solid state and in solution. Exposure of this compound to dioxygen resulted in the formation of a 90:10 mixture of the peroxo [Co(2)(O(2))(L)] and superoxo [Co(2)(O(2))(L)](+) complexes in which the peroxo ligand was found to bind in a Pauling mode in the binuclear cleft in pyridine and acetonitrile adducts in the solid state. The dioxygen compounds can also be prepared directly from Co(OAc)(2) and H(4)L under aerobic conditions in the presence of a base. The reduction of dioxygen catalyzed by this mixture of compounds was investigated using cyclic voltammetry and rotating ring disk electrochemistry and, in acidified ferrocene solutions, using UV-vis spectrophotometry, and although no formation of peroxide was seen, reaction rates were slow and had limited turnover. The deactivation of the catalyst material is thought to be due to a combination of the formation of stable hydroxy-bridged binuclear complexes, for example, [Co(2)(OH)(L)](+), an example of which was characterized structurally, and the catalytic resting point, the superoxo cation, is formed by a pathway independent of the major peroxo product. Collision-induced dissociation mass spectrometry experiments showed that, while [Co(2)(O(2))(L)]H(+) ions readily lose a single O atom, the resulting Co-O(H)-Co core remains resistant to further fragmentation. Furthermore, DFT calculations show that the O-O bond distance in the dioxygen complexes is not a good indicator of the degree of reduction of the O(2) unit and provide a reduction potential of ca. +0.40 V versus the normal hydrogen electrode for the [Co(2)(O(2))(L)](+/0) couple in dichloromethane solution.

Probing the self-assembly of inorganic cluster architectures in solution with cryospray mass spectrometry: growth of polyoxomolybdate clusters and polymers mediated by silver(I) ions.

Cryospray mass spectrometry (CSI-MS) has been used to probe the mechanism of self-assembly of polyoxometalate clusters in solution. By using CSI-MS and electronic absorbance spectroscopy it was possible to monitor in real-time the self-assembly of polymeric chains based on [Ag 2Mo 8O 26] (2-) n building blocks. The role of the Ag (I) ion in the solution state rearrangement of molybdenum Lindqvist ({Mo 6}) into the silver-linked beta-octamolybdate ({Mo 8}) structure (( n-C 4H 9) 4N) 2 n [Ag 2Mo 8O 26] n ( 1) is revealed in unprecedented detail. A monoanionic series, in particular [AgMo m O 3 m+1 ] (-) where m = 2 to 4, and series involving mixed oxidation state polyoxomolybdate species, which illustrate the in-solution formation of the (Ag{Mo 8}Ag) building blocks, have been observed. CSI-MS detection of species with increasing metal nuclearity concomitant with increasing organic cation contribution supports the hypothesis that the organic cations used in the synthesis play an important structure-directing role in polyoxometalate (POM) growth in solution. A real-time decrease in [{Mo 6}] and associated increase in [{Mo 8}] have been observed using CSI-MS and electronic absorbance spectroscopy, and the rate of {Mo 6} interconversion to {Mo 8} was found to decrease on increasing the size of the countercation. This result can be attributed to the steric bulk of larger organic groups hindering {Mo 6} to {Mo 8} rearrangement and hindering the contact between silver cations and molybdenum anions.

The solvation of Mg2+ with gas-phase clusters composed of alcohol molecules.

The dication Mg2+ has been clustered with a range of different alcohols to form [Mg(ROH)N]2+ complexes, where N lies in the range 2-10. Observations on the chemistry of the complexes reveal two separate patterns of behavior: (i) unimolecular metastable decay, where at small values of N the complexes undergo rapid charge separation via Coulomb explosion; and (ii) electron capture-induced decay, where collisional activation promotes bond-breaking processes via charge reduction. For the latter it has been possible to identify a generic set of reactions that are common to all of the different [Mg(ROH)N]2+ complexes; however, there are examples of reactions that are specific to individual alcohols and values of N. For metastable decay, it is shown that there is a clear correlation between the value of N at which a complex ceases to be metastable and the ionization energy of R, the radical that forms the complementary ion in the Coulomb explosion step. Metastable decay in two of the [Mg(ROH)N]2+ complexes follows a very different pathway that eventually results in proton abstraction. It is suggested that this difference is due to the precursor complexes adopting geometries that have at least one ROH molecule in a secondary solvation shell.

A gas-phase study of the preferential solvation of Mn(2+) in mixed water/methanol clusters.

The kinetic shift that exists between two competing unimolecular fragmentation processes has been used to establish whether or not gas-phase Mn(2+) exhibits preferential solvation when forming mixed clusters with water and methanol. Supported by molecular orbital calculations, these first results for a metal dication demonstrate that Mn(2+) prefers to be solvated by methanol in the primary solvation shell.

Enhanced acidity of Zn2+ in the presence of small numbers of water molecules.

Experiments performed on gas phase [Zn(H2O)N]2+ complexes, for N in the range 4-7, show the ions readily undergo unimolecular (metastable) decay with respect to proton release via the reaction [Zn(H2O)N]2+--> Zn(2+)OH(-)(H2O)M + H3O(+)(H2O)N-M-2. To account for these products, it is proposed that the larger complexes have a stable [Zn(H2O)4]2+ core to which additional molecules are retained in an outer shell through hydrogen bonding. At N = 7, this arrangement would make it possible for proton release to be associated with a chain of up to four water molecules, which equates with ideas proposed for the activity of Zn2+ in metalloenzymes.

The solvation of Cu2+ with gas-phase clusters of water and ammonia.

A detailed study has been undertaken of the gas-phase chemistry of [Cu(H2O)N]2+ and [Cu(NH3)N]2+ complexes. Ion intensity distributions and fragmentation pathways (unimolecular and collision-induced) have been recorded for both complexes out as far as N=20. Unimolecular fragmentation is dominated by Coulomb explosion (separation into two single charged units) on the part of the smaller ions, but switches to neutral molecule loss for N>7. In contrast, collisional activation promotes extensive electron capture from the collision gas, with the appearance of particular singly charged fragment ions being sensitive to the size and composition of the precursor. The results show clear evidence of the unit [Cu(X)8]2+ being of special significance, and it is proposed that the hydrogen-bonded structure associated with this ion is responsible for stabilizing the dipositive charge on Cu2+ in aqueous solution.

Fragmentation pathways of [Mg(NH3)n]2+ complexes: electron capture versus charge separation.

New experimental results are presented from a detailed study of gas-phase [Mg(NH(3))(n)](2+) complexes and their fragmentation pathways. The reactions examined range from those observed as metastable (unimolecular) decompositions through to collision-induced processes, which have been accessed using a variety of collision gases. Measurements of ion intensity distributions coupled with unimolecular decay studies show that [Mg(NH(3))(4)](2+) not only is the most intense species detected but also sits at a critical boundary between complexes that are unstable with respect to charge separation and those that are sufficiently solvated to be deemed stable on the time scale of the experiment. Metastable fragmentation patterns have been used to provide information on the evolution of solvent structure around the central dication. In addition to highlighting the particular significance of [Mg(NH(3))(4)](2+), these measurements show some evidence to suggest the buildup of structures via a hydrogen-bonded network to give conformers of the form (4+1) and (4+2), respectively. Collision-induced dissociation studies show the ions to exhibit several fragmentation pathways, including the loss of NH(3) and NH(3) + H, which are promoted primarily through electron capture dissociation (ECD). This picture contrasts with the conclusion from a number of earlier studies that collisional activation mainly promotes charge separation. From the results presented it is suggested that electron capture may play a more dominant role in the charge reduction of multiply charged metal-ligand species than had previously been appreciated.

Gas-phase study of the chemistry and coordination of lead(II) in the presence of oxygen-, nitrogen-, sulfur-, and phosphorus-donating ligands.

Using a pickup technique in association with high-energy electron impact ionization, complexes have been formed in the gas phase between Pb(2+) and a wide range of ligands. The coordinating atoms are oxygen, nitrogen, sulfur, and phosphorus, together with complexes consisting of benzene and argon in association with Pb(2+). Certain ligands are unable to stabilze the metal dication, the most obvious group being water and the lower alcohols, but CS(2) is also unable to form [Pb(CS(2))(N)](2+) complexes. Unlike many other metal dication complexes, those associated with lead appear to exhibit very little chemical reactivity following collisional activation. Such reactions are normally promoted via charge transfer and are initiated using the energy difference between M(2+) + e(-) --> M(+) and L --> L(+) + e(-), which is typically approximately 5 eV. In the case of Pb(2+), this energy difference usually leads to the appearance of L(+) and the loss of a significant fraction of the remaining ligands as neutral species. In many instances, Pb(+) appears as a charge-transfer product. The only group of ligands to consistently exhibit chemical reactivity are those containing sulfur, where a typical product might be PbS(+)(L)(M) or PbSCH(3)(+)(L)(M).