Article pubs.acs.org/JPCA
Reactivity of Hydrated Monovalent First Row Transition Metal Ions M+(H2O)n, M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, toward Molecular Oxygen, Nitrous Oxide, and Carbon Dioxide Christian van der Linde, Sonja Hemmann, Robert F. Höckendorf, O. Petru Balaj, and Martin K. Beyer* Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstraße 40, 24098 Kiel, Germany S Supporting Information *
ABSTRACT: The reactions of hydrated monovalent transition metal ions M+(H2O)n, M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, toward molecular oxygen, nitrous oxide, and carbon dioxide were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Clusters containing monovalent chromium, cobalt, nickel, or zinc were reactive toward O2, while only hydrated cobalt was reactive toward N2O. A strongly size dependent reactivity was observed. Chromium and cobalt react very slowly with carbon dioxide. Nanocalorimetric analysis, 18O2 exchange, and collision induced dissociation (CID) experiments were done to learn more about the structure of the O2 products. The thermochemistry for cobalt, nickel, and zinc is comparable to the formation of O2− from hydrated electrons. These results suggest that cobalt, nickel, and zinc are forming M2+/O2− ion pairs in the cluster, while chromium rather forms a covalently bound dioxygen complex in large clusters, followed by an exothermic dioxide formation in clusters with n ≤ 5. The results show that hydrated singly charged transition metal ions exhibit highly specific reactivities toward O2, N2O, and CO2.
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INTRODUCTION Transition metals (M) play an important role in biological systems like active sites of enzymes,1 in industrial2 and technological applications,3−5 and in the atmosphere.6,7 They occur in a wide variety of oxidation states because of their partly filled d electron subshell. The oxidation state +I in bulk aqueous solution is rare, since disproportionation reactions are energetically favored. Other ions present in the bulk like H+ and OH− also promote the oxidation of M(I) cations. Some monovalent transition metal ions in aqueous solution are accessible by pulse radiolysis studies, especially Co+, Ni+, Zn+, and Cd+.8−12 For producing hydrated M(I) in the gas phase, a laser vaporization source is well suited. Singly charged ions of the type M+(H2O)n can often be produced, although the corresponding monovalent transition metal ions are usually unknown in bulk aqueous media.13−21 The M(I) cations are usually stable in a nanoscale aqueous environment of 20−50 water molecules. Decay mechanisms known from bulk aqueous media do not play a role in these small systems where disproportionation is not possible, since only a single metal ion is present in the cluster. These conditions allow studying the reactivity of the monovalent transition metals. Monovalent metal ions show interesting redox properties with a cluster-size dependent reactivity to hydrogen formation,22−24 for example, Mg+(H2O)n,14,15,25 Al+(H2O)n,16,17,26 and V+(H2O)n.18 Formation of atomic hydrogen is thermochemically forbidden for zinc, but becomes exothermic upon uptake of two hydrogen chloride molecules.27 The interaction of metal ions with water clusters has received considerable attention in the gas phase, as discussed in a number of review articles.23,24,28−31 Sequential binding energies of water to first row transition metal ions M+(H2O)n, n = 1−4, © 2012 American Chemical Society
M = Ti to Cu, were measured by Armentrout and co-workers via energy-resolved collision induced dissociation (CID).32 Monovalent iron and cobalt ions hydrated with up to 10 water molecules were investigated with photodissociation and CID by Mestdagh and co-workers.33−35 In addition to water loss, hydrogen elimination with formation of FeOH+(H2O)m was observed in photodissociation of Fe+(H2O)n. The electronic structure of monovalent transition metal ions with one water molecule attached was studied in detail for V+(H2O) and Ni+(H2O).36,37 Infrared spectroscopy by Duncan and co-workers revealed structural features of M+(H2O), M = V, Cr, Mn, Fe,38−41 as well as V2+(H2O).42 Larger clusters were studied for Ni+(H2O)n, n ≤ 25,43 indicating that all water molecules are involved in hydrogen bonding by n = 10. Infrared spectra by Ohashi and coworkers combined with density functional calculations are consistent with a linear 2-fold coordination of Cu+(H2O)n, n ≤ 7,44 and square planar configuration of V+(H2O)n, n ≤ 8.45 Interestingly, only three H2O seem to coordinate directly to the Co+ ion in Co+(H2O)n, n = 4−6,46 suggesting that Co+ has a reduced tendency toward coordinative saturation. According to the orbital orientation model with half-filled dx2 − y2 and dz2 orbitals,47,48 six H2O molecules are expected to directly coordinate to the metal center in coordinatively saturated Co+(H2O)n. Special Issue: Peter B. Armentrout Festschrift Received: March 2, 2012 Revised: April 16, 2012 Published: April 16, 2012 1011
dx.doi.org/10.1021/jp3020723 | J. Phys. Chem. A 2013, 117, 1011−1020
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Ni (99.9%), 63Cu (99.3%), 64Zn (99.4%) (STB Isotope Germany GmbH). Chromium was delivered as a sheet which was used directly. Iron, nickel, and copper, delivered as powder, were pressed and melted under vacuum conditions using inductive heating. The obtained nugget was cleaned with aqua regia and pressed to target size. Manganese and cobalt are naturally monoisotopic, and vanadium occurs as 51V with 99.75% purity. Cobalt and vanadium were purchased as metal foils, manganese as sputter target, and machined to target size.
Activation of single H2O molecules by neutral group IV and group V metal atoms49,50 as well as Fe atoms51 was studied by matrix isolation infrared spectroscopy and photolysis, supplemented by quantum chemical calculations, by Zhou and coworkers. The activation of H2O by monovalent iron Fe+ was shown to be a prototype for two-state-reactivity52 by Ugalde and co-workers,53 who also investigated all other first-row transition metal ions.54−57 The competition between a metal-oxide and the dihydroxide structure of [M,O2,H2]+ for M = Fe, Co, Ni was elucidated by CID studies by Schröder et al.58 While M(OH)2+ is favored for M = Fe, (H2O)MO+ is energetically preferred for M = Ni. Both structures are energetically close for M = Co. The thermochemistry of the metal−oxygen bond MO+, M = Ca to Zn, was measured by Armentrout and co-workers, analyzing the thresholds of endothermic reactions with O2 and N2O.59−62 Schwarz and co-workers have looked extensively into the reactivity of transition metal oxide ions, in particular with respect to C−H and C−C bond activation.63 Zhou and co-workers have studied the interaction of H2O with neutral tantalum oxide and dioxide64 as well as platinum dioxide65 by matrix isolation spectroscopy and density functional theory (DFT) calculations. The potential energy surface of rearrangement reactions involving V+, O2, and H2O was the target of a collaborative study of the Schwarz and Bohme groups.66 The very delicate activation of a dioxygen ligand at Cr+ upon collision with H2O was revealed by isotope labeling studies by Bondybey and co-workers.67 We have recently shown that Mg+(H2O)n, n ≈ 20−60, take up O2 and CO2.68 Quantum chemical calculations corroborate the interpretation that Mg+(H2O)n exhibit the chemistry of the hydrated electron in these reactions. Whether similar charge transfer processes are operative when Mg+ is replaced by a transition metal is so far unknown. We therefore investigate the reactivity of hydrated transition metal clusters M+(H2O)n, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with oxygen, nitrous oxide, and carbon dioxide in the gas phase by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry.
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RESULTS AND DISCUSSION Reactivity toward Oxygen. Kinetics and Size Dependence. Large clusters with chromium, cobalt, nickel, and zinc core ions take up one oxygen molecule, reaction 1, where [M,O2]+ denotes the elemental composition of the ionic core of the cluster without prejudice on its structure. It may even involve activated water, for example, M(OH)4+(H2O)n−2 or M(O)(OH)2+(H2O)n−1 are potential structures of [M,O2]+(H2O)n. M+(H 2O)n + O2 → [M,O2 ]+ (H 2O)n − x + x H 2O, M = Cr, Co, Ni, Zn
(1)
Figure 1 shows the mass spectrum of the reaction of Zn+(H2O)n with O2. After 20 s all Zn(H2O)n+ is converted to
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EXPERIMENTAL SECTION The experiments were performed on a modified FT-ICR Bruker/Spectrospin CMS47X mass spectrometer described in detail before.69−72 The instrument is equipped with an unshielded 4.7 T superconducting magnet, a Bruker infinity cell, an APEX III data station, a TOPPS ion optics power supply, and an ICC2 Infinity Cell Controller with BCH preamplifier. M+(H2O)n ions were generated in an external laser vaporization source73−75 by evaporation of a solid metal target and supersonic expansion of the hot plasma in a helium/water mixture (Helium, Linde 99.996%).16,76 The vaporization laser and frequency doubling crystal were heated by 20 laser shots to minimize changes in the initial size distribution, followed by another 20 laser shots at 10 Hz and 5 mJ pulse energy to generate the ions. The reaction delay is measured relative to the end of the fill cycle, which means that at nominal t = 0 s, some clusters may have resided up to 2 s in the cell. It is thus unavoidable that reaction products are observed at nominal 0 s reaction delay. 16O2 (Linde, 99.9991%), 18O2 (Sigma-Aldrich, 99 atom %), Nitrous Oxide (Sigma-Aldrich, 99.0%), and CO2 (Linde, 99.995%) were introduced into the ultra high vacuum region via a needle valve at constant pressures in the range of 5 × 10−8 to 5 × 10−7 mbar. Isotopically enriched targets were used where applicable,52Cr (99.9%), 56Fe (99.7%),
Figure 1. Mass spectra of the reaction of Zn+(H2O)n with O2 at a pressure of 1.9 × 10−7 mbar after 0 , 5, and 10 s. Quantitative formation of [Zn,O2](H2O)n−x+ was observed.
ZnO2(H2O)n−x+ at an O2 pressure of 1.9 × 10−7 mbar. No reaction with a second O2 molecule was observed. The cluster distribution is shrinking through absorbed blackbody radiation, followed by evaporative cooling, a process termed blackbody infrared radiative dissociation (BIRD).77−88 The kinetic plots for all reacting metals are shown in Figure 2. While Zn+(H2O)n show pseudo-first-order behavior for 20 s, Ni+(H2O)n, Co+(H2O)n, and Cr+(H2O)n show a more or less pronounced size dependent reactivity. For short reaction times chromium, cobalt, and nickel show approximately pseudo-firstorder behavior. At longer reaction delays as well as higher reactant gas pressure, the size dependence is clearly visible. Reaction rate of the reaction of chromium and nickel with O2 increases with shrinking cluster size. Cobalt shows a very strong 1012
dx.doi.org/10.1021/jp3020723 | J. Phys. Chem. A 2013, 117, 1011−1020
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Figure 2. Kinetics and average cluster size of the reaction of M+(H2O)n, M = Cr, Co, Ni, Zn with O2 at reactant pressures of p(Cr) = 8.9 × 10−8 mbar, p(Co) = 2.2 × 10−7 mbar, p(Ni) = 4.2 × 10−7 mbar, and p(Zn) = 1.9 × 10−7 mbar. Dashed lines indicate different ensembles from multiensemble fits to describe size-dependent reactivity. While cobalt shows a strong size dependence especially for small clusters, chromium and nickel are only slightly size dependent. Zinc shows a good pseudo-first-order fit.
rows leads to an order of reactivity of Zn+ ∼ Co+ ≥ Cd+ > Ni+. Table 1 indicates that this series is also valid in the gas phase.
size dependence. In the beginning the clusters are only shrinking by BIRD. Reactivity increases first with shrinking cluster size but decreases again for smaller clusters. It seems to stop when only four to five water molecules are left in the cluster. The size dependence is treated in the fits with different cluster size ensembles. Unreactive size ensembles are converted to reactive ensembles via BIRD. For cobalt there seems to be no reactivity at cluster sizes bigger than approximately 30 water molecules. The beginning of the nickel kinetics indicates a similar behavior. The reaction of nickel is an order of magnitude slower than for chromium, zinc, and cobalt, which are of comparable rate. This fits to earlier observed rate constants in solution phase obtained from pulse radiolysis studies. Meyerstein and Mulac derived an order of reactivity from studying reactions with good electron acceptors, like O2, N2O, and H2O2, using pulse radiolysis: Zn+ ≥ Cd+ ≥ Ni+. They attributed this different reactivity to the electronic structure of the monovalent metals.10,11 In zinc the additional electron is located in an s orbital while in Ni+ it may be located in the 3d shell.10,11 Later, Meyerstein completed the order of reactivity and got a reactivity row of Zn+ ∼ Co+ > Ni+ for reactions with oxidants,11 where this simple explanation breaks down. Combining both
Table 1. Absolute Rate Constants kabs of the Reaction of M+(H2O)n with O2a M
kabs/10−11 cm−1 s−1
V Cr Mn Fe Co Ni Cu Zn Mg
