Model systems for probing metal cation hydration: the V+(H2O) and ArV+(H2O) complexes

In support of mass-selected infrared photodissociation (IRPD) spectroscopy experiments, coupled-cluster methods including all single and double excitations (CCSD) and a perturbative contribution from connected triple excitations [CCSD(T)] have been used to study the V+(H2O) and ArV+(H2O) complexes. Equilibrium geometries, harmonic vibrational frequencies, and dissociation energies were computed for the four lowest-lying quintet states (5A1, 5A2, 5B1, and 5B2), all of which appear within a 6 kcal mol(-1) energy range. Moreover, anharmonic vibrational analyses with complete quartic force fields were executed for the 5A1 states of V+(H2O) and ArV+(H2O). Two different basis sets were used: a Wachters+f V[8s6p4d1f] basis with triple-zeta plus polarization (TZP) for O, H, and Ar; and an Ahlrichs QZVPP V[11s6p5d3f2g] and Ar[9s6p4d2f1g] basis with aug-cc-pVQZ for O and H. The ground state is predicted to be 5A1 for V+(H2O), but argon tagging changes the lowest-lying state to 5B1 for ArV+(H2O). Our computations show an opening of 2 degrees -3 degrees in the equilibrium bond angle of H2O due to its interaction with the metal ion. Zero-point vibrational averaging increases the effective bond angle further by 2.0 degrees -2.5 degrees, mostly because of off-axis motion of the heavy vanadium atom rather than changes in the water bending potential. The total theoretical shift in the bond angle of about +4 degrees is significantly less than the widening near 9 degrees deduced from IRPD experiments. The binding energies (D0) for the successive addition of H2O and Ar to the vanadium cation are 36.2 and 9.4 kcal mol(-1), respectively.     

Photodissociation spectroscopy of Zn+(H2O) and Zn+(D2O)

We report on a study of the photodissociation spectroscopy of weakly bound Zn+(H2O) and Zn+(D2O) complexes. The work is supported by ab initio electronic structure calculations of the ground and low-lying excited energy surfaces. We assign two molecular absorption bands in the near UV correlating to Zn+ (4s-4p)-based transitions, and identify vibrational progressions associated with both intermolecular and intramolecular vibrational modes of the cluster. Partially resolved rotational structure is consistent with a C(2V) equilibrium complex geometry. Experimental spectroscopic constants are in very good agreement with ab initio theoretical predictions. Results are compared with previous work on main group and transition metal ion-H2O clusters.     

Infrared spectroscopy of Cu+(H2O)(n) and Ag+(H2O)(n): coordination and solvation of noble-metal ions

M(+)(H(2)O)(n) and M(+)(H(2)O)(n)Ar ions (M=Cu and Ag) are studied for exploring coordination and solvation structures of noble-metal ions. These species are produced in a laser-vaporization cluster source and probed with infrared (IR) photodissociation spectroscopy in the OH-stretch region using a triple quadrupole mass spectrometer. Density functional theory calculations are also carried out for analyzing the experimental IR spectra. Partially resolved rotational structure observed in the spectrum of Ag(+)(H(2)O)(1) x Ar indicates that the complex is quasilinear in an Ar-Ag(+)-O configuration with the H atoms symmetrically displaced off axis. The spectra of the Ar-tagged M(+)(H(2)O)(2) are consistent with twofold coordination with a linear O-M(+)-O arrangement for these ions, which is stabilized by the s-d hybridization in M(+). Hydrogen bonding between H(2)O molecules is absent in Ag(+)(H(2)O)(3) x Ar but detected in Cu(+)(H(2)O)(3) x Ar through characteristic changes in the position and intensity of the OH-stretch transitions. The third H(2)O attaches directly to Ag(+) in a tricoordinated form, while it occupies a hydrogen-bonding site in the second shell of the dicoordinated Cu(+). The preference of the tricoordination is attributable to the inefficient 5s-4d hybridization in Ag(+), in contrast to the extensive 4s-3d hybridization in Cu(+) which retains the dicoordination. This is most likely because the s-d energy gap of Ag(+) is much larger than that of Cu(+). The fourth H(2)O occupies the second shells of the tricoordinated Ag(+) and the dicoordinated Cu(+), as extensive hydrogen bonding is observed in M(+)(H(2)O)(4) x Ar. Interestingly, the Ag(+)(H(2)O)(4) x Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag(+)(H(2)O)(3) x Ar but revived at n=4. Size dependent variations in the spectra of Cu(+)(H(2)O)(n) for n=5-7 provide evidence for the completion of the second shell at n=6, where the dicoordinated Cu(+)(H(2)O)(2) subunit is surrounded by four H(2)O molecules. The gas-phase coordination number of Cu(+) is 2 and the resulting linearly coordinated structure acts as the core of further solvation processes.     

Thermochemical stabilities and structures of the cluster ions OCS+, S2+, H+(OCS), and C2H5+ with OCS molecules in the gas phase

The gas-phase clustering reactions of OCS+, S2+, H+(OCS), and C2H5+ ions with carbonyl sulfide (OCS) molecules were studied using a pulsed electron-beam high-pressure mass spectrometer and applying density functional theory (DFT) calculations. In the cluster ions OCS+(OCS)(n) and H+(OCS)(OCS)(n), a moderately strong, here referred to as "semi-covalent", bond was formed with n = 1. However, the nature of bonding changed from semi-covalent to electrostatic with n = 1 --> 2. The bond energy of S2(+)(OCS) was determined experimentally to be 12.9 +/- 1 kcal/mol, which is significantly smaller than that of the isovalent S2(+)(CS2) complex (30.9 +/- 1.5 kcal/mol). DFT based calculations predicted the presence of several isomeric structures for H+(OCS)(OCS)(n) complexes. The bond energies in the C2H5+(OCS)(n) clusters showed an irregular decrease for n = 1 --> 2 and 7 --> 8. The nonclassical bridge structure for the free C2H5+ isomerized to form a semi-covalent bond with one OCS ligand, [H3CCH2...SCO]+, i.e., reverted to classical structure. However, the nonclassical bridge structure of C2H5+ was preserved in the cluster ions C2H5+(OCS)(n) below 140 K attributable to the lack of thermal energy for the isomerization. DFT calculations revealed that stability orders of the geometric isomers of H+(OCS)(OCS)(n) and C2H5+(OCS)(n) changed with increasing n values.     

Infrared photodissociation spectroscopy of V+(CO2)n and V+(CO2)nAr complexes

V+(CO2)n and V+(CO2)nAr complexes are generated by laser vaporization in a pulsed supersonic expansion. The complexes are mass-selected within a reflectron time-of-flight mass spectrometer and studied by infrared resonance-enhanced (IR-REPD) photodissociation spectroscopy. Photofragmentation proceeds exclusively through loss of intact CO2 molecules from V+(CO2)n complexes or by elimination of Ar from V+(CO2)nAr mixed complexes. Vibrational resonances are identified and assigned in the region of the asymmetric stretch of free CO2 at 2349 cm(-1). A linear geometry is confirmed for V+(CO2). Small complexes have resonances that are blueshifted from the asymmetric stretch of free CO2, consistent with structures in which all ligands are bound directly to the metal ion. Fragmentation of the larger clusters terminates at the size of n=4, and a new vibrational band at 2350 cm(-1) assigned to external ligands is observed for V+(CO2)5 and larger cluster sizes. These combined observations indicate that the coordination number for CO2 molecules around V+ is exactly four. Fourfold coordination contrasts with that seen in condensed phase complexes, where a coordination number of six is typical for V+. The spectra of larger complexes provide evidence for an intracluster insertion reaction that produces a metal oxide-carbonyl species.     

Reactions of Cu+(1S, 3D) with CH3Cl, CH2ClF, CHClF2, and CClF3

The reactions of gas-phase Cu+(1S) and Cu+(3D) with CH3Cl, CH2ClF, CHClF2, and CClF3 are examined using the drift cell technique at 3.5 Torr. State-specific product channels and overall bimolecular rate constants for depletion of the two Cu+ states are determined using electronic state chromatography. Cu+(1S) participates exclusively in association with all four neutrals, whereas Cl abstraction is the dominant product channel for Cu+(3D). The resulting CuCl+ product subsequently abstracts Cl- in a secondary process. Tertiary reactions are also observed, which include both hydride abstraction (with CH3Cl) and fluoride abstraction (with the fluorinated neutrals). All product channels can be understood in terms of the known thermochemical and quantum mechanical (i.e., spin) requirements. Cu+(1S) is depleted by all four neutrals at 30% to 40% of the ADO rate under these conditions, whereas Cu+(3D) is observed to react at approximately 80% of the ADO rate with CH3Cl, CH2ClF, and CHClF2. Reaction of excited state Cu+ with CClF3 occurs at only 7% of the ADO rate. The behavior of Cu+(3D) is consistent with a mechanism in which formation of CuCl+ occurs exclusively on the triplet surface via a mechanism in which the metal ion must interact exclusively with Cl.     

Unsupported gold(I)-copper(I) interactions through eta1 Au-[Au(C6F5)2]- coordination to Cu+ Lewis acid sites

The reaction of the complex [Au2Ag2(C6F5)4)N[triple bond]CCH3)2]n (1) with 1 equiv of CuCl in the presence of 1 equiv of pyrimidine ligand leads to the formation of the heteronuclear Au(I)-Cu(I) organometallic polymer [Cu{Au(C6F5)2}(N[triple bond]CCH3)(mu2-C4H4N2)]n (2) through a transmetalation reaction. Complex 2 displays unprecedented unsupported Au(I)...Cu(I) interactions of [Au(C6F5)2]- units with the acid Cu(I) sites in a [Cu(N[triple bond]CCH3)(mu2-pyrimidine)]n+(n) polymeric chain. Complex 2 has a rich photophysics in solution and in the solid state.     

A large protonated water cluster H+(H2O)27 in a 3D metal-organic framework

A large protonated water cluster, H+(H2O)27, has been trapped and stabilized within the well-modulated cavity of a 3D metal-organic framework formed by cobalt(II) and 4,4'-bipyridine-N,N'-dioxide with a globular Keggin structure [PW12O4]3- anion as template. The structurally characterized protonated water cluster might comprise a (H2O)26 shell with Oh symmetry and a monowater core within the center which is suggested to be a hydronium ion with the Eigen model.     

Infrared spectrum of NH4+(H2O): evidence for mode specific fragmentation

The gas phase infrared spectrum (3250-3810 cm-1) of the singly hydrated ammonium ion, NH4+(H2O), has been recorded by action spectroscopy of mass selected and isolated ions. The four bands obtained are assigned to N-H stretching modes and to O-H stretching modes. The N-H stretching modes observed are blueshifted with respect to the corresponding modes of the free NH4+ ion, whereas a redshift is observed with respect to the modes of the free NH3 molecule. The O-H stretching modes observed are redshifted when compared to the free H2O molecule. The asymmetric stretching modes give rise to rotationally resolved perpendicular transitions. The K-type equidistant rotational spacings of 11.1(2) cm-1 (NH4+) and 29(3) cm-1 (H2O) deviate systematically from the corresponding values of the free molecules, a fact which is rationalized in terms of a symmetric top analysis. The relative band intensities recorded compare favorably with predictions of high level ab initio calculations, except on the nu3(H2O) band for which the observed value is about 20 times weaker than the calculated one. The nu3(H2O)/nu1(H2O) intensity ratios from other published action spectra in other cationic complexes vary such that the nu3(H2O) intensities become smaller the stronger the complexes are bound. The recorded ratios vary, in particular, among the data collected from action spectra that were recorded with and without rare gas tagging. The calculated anharmonic coupling constants in NH4+(H2O) further suggest that the coupling of the nu3(H2O) and nu1(H2O) modes to other cluster modes indeed varies by orders of magnitude. These findings together render a picture of a mode specific fragmentation dynamic that modulates band intensities in action spectra with respect to absorption spectra. Additional high level electronic structure calculations at the coupled-cluster singles and doubles with a perturbative treatment of triple excitations [CCSD(T)] level of theory with large basis sets allow for the determination of an accurate binding energy and enthalpy of the NH4+(H2O) cluster. The authors' extrapolated values at the CCSD(T) complete basis set limit are De [NH4+-(H2O)]=-85.40(+/-0.24) kJ/mol and DeltaH(298 K) [NH4+-(H2O)]=-78.3(+/-0.3) kJ/mol (CC2), in which double standard deviations are indicated in parentheses.     

Characterization of hydrated Na+(phenol) and K+(phenol) complexes using infrared spectroscopy

Hydrated alkali metal ion-phenol complexes were studied to model these species in aqueous solution for M=Na and K. IR predissociation spectroscopy in the O-H stretch region was used to analyze the structures of M+(Phenol)(H2O)n cluster ions, for n = 1-4. The onset of hydrogen bonding was observed to occur at n=4. Ab initio calculations were used to qualitatively explore the types of hydrogen-bonded structures of the M+(Phenol)(H2O)4 isomers. By combining the ab initio calculations and IR spectra, several different structures were identified for each metal ion. In contrast to benzene, detailed in a previous study of Na+(Benzene)n(H2O)m [J. Chem. Phys. 110, 8429 (1999)], phenol is able to bind directly to Na+ even in the presence of four waters. This is likely the result of the sigma-type interaction between the phenol oxygen and the ion. With K+, the dominant isomers are those in which the phenol O-H group is involved in a hydrogen bond with the water molecules, while with Na+, the dominant isomers are those in which the phenol O-H group is free and the water molecules are hydrogen-bonded to each other. Spectra and ab initio calculations for the M+(Phenol)Ar cluster ions for M=Na and K are reported to characterize the free phenol O-H stretch in the M+(Phenol) complex. While pi-type configurations were observed for binary M+(Phenol) complexes, sigma-type configurations appear to dominate the hydrated cluster ions.     

Photofragment coincidence imaging of small I-(H2O)n clusters excited to the charge-transfer-to-solvent state

The photodissociation dynamics of small I-(H2O)n(n=2-5) clusters excited to their charge-transfer-to-solvent (CTTS) states have been studied using photofragment coincidence imaging. Upon excitation to the CTTS state, two photodissociation channels were observed. The major channel (approximately 90%) is a two-body process forming neutral I+(H2O)n photofragments, and the minor channel is a three-body process forming I+(H2O)n-1+H2O fragments. Both processes display translational energy [P(ET)] distributions peaking at ET=0 with little available energy partitioned into translation. Clusters excited to the detachment continuum rather than to the CTTS state display the same two channels with similar P(ET) distributions. The observation of similar P(ET) distributions from the two sets of experiments suggests that in the CTTS experiments, I atom loss occurs after autodetachment of the excited [I(H2O)n-]* cluster or, less probably, that the presence of the excess electron has little effect on the departing I atom.     

A Sandmeyer type reaction for bromination of 2-mercapto-1-methyl-imidazoline (N2C4H6S) into 2-bromo-1-methyl-imidazole (N2C4H5Br) in presence of copper(I) bromide

2-Mercapto-1-methyl-imidazoline (N(2)C(4)H(6)S) is converted at room temperature into 2-bromo-1-methyl-imidazole (N(2)C(4)H(5)Br) in presence of copper(I) bromide in acetonitrile-chloroform mixture via extrusion of sulfur as sulfate and oxidation of Cu(I) into Cu(II). 2-Bromo-1-methyl-imidazole was isolated as its self assembled tetranuclear Cu(II) cluster, [Cu(4)(η(1)-N-(N(2)C(4)H(5)Br)(4)(μ(4)-O)(μ-Br)(6)] 1 {η(1)-N-(N(2)C(4)H(5)Br) = 2-bromo-1-methyl-imidazole}.     

Versatility of heterocyclic thioamides in the construction of a trinuclear cluster [Cu3I3(dppe)3 (SC5H4NH)] and Cu(I) linear polymers {Cu6(SC5H4NH)6I6}n x 2nCH3CN and {Cu(I)6 (SC3H6N2)6X6}n (X = Br, I)

Reaction of copper(I) iodide with pyridine-2-thione (2-SC5H4NH) and 1,2-bis(diphenylphosphino)ethane (dppe) in a CH3CN-CHCl3 mixture yielded a triangular cluster, [Cu3I3(mu2-P,P-dppe)3 (eta1-SC5H4NH)], 1. Similar reaction with 2-SC5H4NH and a series of diphosphanes, Ph2P-X-Ph2P {X = -CH2- (dppm), -(CH2)3- (dppp), -(CH2)4- (dppb), -CH=CH- (dppen)}, gave a novel iodo-bridged hexanuclear Cu(I) linear polymer,{Cu6(mu3-SC5H4NH)4 (mu2-SC5H4NH)2 (I4)(mu-I)2-}n x 2nCH3CN, 2. Reactions of copper(I) iodide/copper(I) bromide with 1,3-imidazolidine-2-thione (SC3H6N2) in a CH3CN-CHCl3 mixture yielded hexanuclear Cu(I) linear chain polymers, [{Cu6(mu3-SC3H6N2)2 (mu2-SC3H6N2)4X2 (mu-X)4}n] (X = Br, 4; I, 5). In compound 1, two iodide atoms and one dppe form the dinuclear Cu(mu2-I)2 (mu2-dppe)Cu core, and two dppe ligands bridge this core with the third Cu(I) center coordinated to 2-SC5H4NH via the S atom. The chain polymer 2 has a centrosymmetric hexanuclear central core, Cu6S6I4 (mu-I)2--, formed by dimerization of six-membered trinuclear motifs, Cu3(mu2-SC3H6N2)3I3 via (mu3-S) bonding modes of the thione ligand, and has four terminal and two bridging iodine atoms in trans-orientations. Linear chains are separated by the nonbonded acetonitrile molecules. In 4 and 5, three copper(I) bromide or copper(I) iodide moieties and three SC3H6N2 ligands combined via bridging S donor atoms to form the six-membered trinuclear Cu3(mu2-SC3H6N2)3I3 cores which polymerized via S and X atoms in a side-on fashion to form linear chain polymers, [{Cu6(mu3-SC3H6N2)2 (mu2-SC3H6N2)4X2(mu-X)4}n]. The (mu3-S) modes of bonding of neutral heterocyclic thioamides are first examples, as are trinuclear cluster and linear polymers rare examples in copper chemistry.     

Glycine and Histidine in Regulation of Free-Radical Dephosphorylation of Glycerol Phosphate in the Presence of Cu2+(Fe2+) Ions

Russian Journal of General Chemistry - The effect of Gly and His on homolytic fragmentation of glycerol phosphate with the cleavage of the phosphoester bond, induced by the Cu2+(Fe2+)–H2O2...     

Construction of a novel copper(I) chain polymer of hexanuclear Cu6(2-SC5H4NH)6I6 cores with a new (mu 3-S) mode of bonding of pyridine-2-thione and of an unusual triangular Cu3I3(dppe)3(2-SC5H4NH) cluster

The reactions of copper(I) iodide with pyridine-2-thione (2-SC(5)H(4)NH) in the presence of a series of diphosphane ligands, Ph(2)P[bond]X[bond]Ph(2)P [X = [bond](CH(2))(m)[bond], m = 1(dppm), 2 (dppe), 3 (dppp), 4 (dppb); [bond]CH[double bond]CH[bond] (dppen)], yielded an iodo-bridged hexanuclear Cu(I) linear polymer, [Cu(6)(mu(3)-SC(5)H(4)NH)(4)(mu(2)-SC(5)H(4)NH)(2)(I(4))(mu-I)(2)-](n).2nCH(3)CN (1). A similar reaction with 1,2-bis(diphenylphosphino)ethane (dppe) and 2-SC(5)H(4)NH yielded a triangular cluster, Cu(3)I(3)(dppe)(3)(2-SC(5)H(4)NH), 2. In the chain polymer 1, three Cu(I) iodide and three 2-SC(5)H(4)NH ligands combined via bridging S donor atoms to form a boat-shaped trinuclear Cu(3)S(3)I(3) core, and two such cores combined in an inverse manner via four S-donor atoms (mu(3)-S) to form a centrosymmetric hexanuclear repeat unit, Cu(6)S(6)I(4)(mu-I)(2-), which finally formed the iodo-bridged infinite linear chain polymer 1. Linear chains are separated by the nonbonded acetonitrile molecules. Polymer 1 is the first such example of a linear chain formed by the hexanuclear Cu(6)S(6)I(6) core in copper chemistry as well as in metal-heterocyclic thioamide chemistry. In addition, it has the first mu(3)-S mode of neutral pyridine-2-thione ever reported. In the moiety Cu(3)I(3)(dppe)(3) of 2, two copper(I) centers are bridged by the iodide ligands forming a Cu(mu-I)(2)Cu core, while a third copper(I) center is terminally bonded to another iodide ligand. Polymer 2 is also rare, and the first triangular cluster of Cu(I) with an heterocyclic thioamide.     

Synthesis of 1D {Cu6(mu3-SC3H6N2)4(mu-SC3H6N2)2(mu-I)2I4}n and 3D {Cu2(mu-SC3H6N2)2(mu-SCN)2}n polymers with 1,3-imidazolidine-2-thione: bond isomerism in polymers

The reaction of copper(I) iodide with 1, 3-imidazolidine-2-thione (SC3H6N2) in a 1:2 molar ratio (M/L) has formed unusual 1D polymers, {Cu6(mu3-SC3H6N2)4(mu-SC3H6N2)2(mu-I)2I4}n (1) and {Cu6(mu3-SC3H6N2)2(mu-SC3H6N2)4(mu-I)4I2}n (1a). A similar reaction with copper(I) bromide has formed a polymer {Cu6(mu3-SC3H6N2)2(mu-SC3H6N2)4(mu-Br)4Br2}n (3a), similar to 1a, along with a dimer, {Cu2(mu-SC3H6N2)2(eta1-SC3H6N2)2Br2} (3). Copper(I) chloride behaved differently, and only an unsymmetrical dimer, {Cu2(mu-SC3H6N2)(eta1-SC3H6N2)3Cl2} (4), was formed. Finally, reactions of copper(I) thiocyanate in 1:1 or 1:2 molar ratios yielded a 3D polymer, {Cu2(mu-SC3H6N2)2(mu-SCN)2}n (2). Crystal data: 1, C9H18Cu3I3N6S3, triclinic, P, a = 9.6646(11) A, b = 10.5520(13) A, c = 12.6177(15) A, alpha = 107.239(2) degrees , beta = 99.844(2) degrees , gamma = 113.682(2) degrees , V = 1061.8(2) A(3), Z = 2, R = 0.0333; 2, C(4)H(6)CuN(3)S(2), monoclinic, P2(1)/c, a = 7.864(3) A, b = 14.328(6) A, c = 6.737(2) A, beta = 100.07(3) degrees , V = 747.4(5), Z = 4, R = 0.0363; 3, C12H24Br2Cu2N8S4, monoclinic, C2/c, a = 19.420(7) A, b = 7.686(3) A, c = 16.706(6) A, beta = 115.844(6) degrees , V = 2244.1(14) A(3), Z = 4, R = 0.0228; 4, C12H24Cl2Cu2N8S4, monoclinic, P2(1)/c, a = 7.4500(6) A, b = 18.4965(15) A, c = 16.2131(14) A, beta = 95.036(2) degrees , V = 2225.5(3) A(3), Z = 4, R = 0.0392. The 3D polymer 2 exhibits 20-membered metallacyclic rings in its structure, while synthesis of linear polymers, 1 and 1a, represents an unusual example of I (1a)-S (1) bond isomerism.     

Electric-field-induced g/u mixing of the E0g+(3P2) and D0u+(3P2) ion-pair states of jet-cooled I2 observed using optical triple resonance

Electric-field-induced electronic state g/u mixing of nearly isoenergetic rovibrational levels of the E0g+(3P2) and D0u+(3P2) ion-pair states of I2 has been observed using optical triple resonance combined with resonance ionization. Detectable mixing with applied fields of 1 kV/cm occurs over a range of energy level separations of < or = 0.3 cm(-1).     

A 2-D Copper Coordination Polymer [Cu(4,4''''-bpy)(H_2O)_4](C_4H_2O_4)(H_2O)_4 Integrated by Hydrogen Bonds and VDW Interactions

1 INTRODUCTION A great deal of interest in transition metal com- plex assembly has recently been devoted to the development of rational synthetic routes to novel one-, two- and three-dimensional crystal frameworks, due to their potential applications in…     

Synthesis and Structure of a New Copper(II) Complex Cu(C_(13)H_9N_3O_2Br)_2.H_2O

1 INTRODUCTION Owing to their biological activities and chemical/industrial versalities, considerable attention has been continually attracted to metal-hydrazine complexes. For instance, Schiff base hydrazine and its analogues have been well studied in order to better understand the action mechanism of Vitamin B6-containing enzymes[1~3]. Six-coordinated copper(Ⅱ) complexes are of considerable interest, mainly due to their coordination geometry and spectroscopic properties[4]. Due to th…     

Formation of M2+(O2)(C3H8) species in alkaline-earth-exchanged Y zeolite during propane selective oxidation

The adsorption of oxygen and d2-propane (CH3CD2CH3) on a series of alkaline-earth-exchanged Y zeolite at room temperature was studied with in situ infrared spectroscopy. Surprisingly at room temperature, oxygen adsorption led to the formation of supercage M2+(O2) species. Further, at low propane coverage, propane was found to adsorb linearly on Mg2+ cations, but a ring-adsorption structure was observed for propane adsorbing on Ca2+, Sr2+, and Ba2+ cations. It is demonstrated that O2 and propane can simultaneously attach to one active center (M2+) to form a M2+(O2)(C3H8) species, which is proposed to be the precursor in thermal propane selective oxidation. Selectivity to acetone in the propane oxidation reaction decreases with increasing temperature and cation size due to the formation of 2-propanol and carboxylate ions. An extended reaction scheme for the selective oxidation of propane over alkaline earth exchanged Y zeolites is proposed.