Intrazeolite phototopotaxy. EXAFS analysis of precursor 8{W(CO)6}-Na56Y and photooxidation products 16(WO3)-Na56Y and 28(WO3)-Na56Y

The intrazeolite photooxidation chemistry of alpha-cage encapsulated hexacarbonyltungsten(0) in Na56Y with O2, denoted n{W(CO)6}-Na56Y/O2/hv, which has previously been shown to provide a novel synthetic pathway to alpha-cage located tungsten(VI) oxide, denoted n(WO3)-Na56Y, is now the subject of an extended X-ray absorption fine structure (EXAFS) analysis. The EXAFS data of a precursor 8{W(CO)6}Na56Y, which contains on average one W(CO)6 per alpha-cage shows that the W(CO)6 guest maintains its structural integrity with only minor observable perturbations of the skeletal WC and ligand CO bonds compared to those found for the same molecule in the free state. The EXAFS analysis results for the photoxidation products 16(WO3)-Na56Y and 28(WO3)-Na56Y are very similar and display the presence of two terminal tungsten-oxygen bonds (1.75-1.77 angstrom) and two bridging tungsten-oxide bonds (1.94-1.95 angstrom), together with a short distance to a second tungsten (3.24-3.30 angstrom). This bond length and coordination number information for n = 16 and 28 samples is best interpreted in terms of the formation of a single kind of tungsten trioxide dimer unit (WO3)2, most likely interacting with extraframework Na+ cations, denoted ZONa...O2W(mu-O)2WO2...NaOZ. In conjunction with earlier chemical and spectroscopic information on this system, the EXAFS data support the contention that 16(WO3)-Na56Y contains a uniform array of single size and shape tungsten (VI) oxide dimers (WO3)2 housed in the 13-angstrom supercages of the zeolite Y host. The sequential addition of WO3 units to the 16(WO3)-Na56Y sample appears to increase the (WO3)2 dimer population, causing a buildup of alpha-cage encapsulated dimers-of-dimers {(WO3)2}2 rather than further cluster growth to trimers (WO3)2 and/or tetramers (WO3)4

Intrazeolite metal carbonyl topotaxy. A comprehensive structural and spectroscopic study of intrazeolite Group VI metal hexacarbonyls and subcarbonyls

This paper focuses attention on the intrazeolite anchoring, thermal decarbonylation, ligand exchange, and addition chemistry of M(CO)6-M'56Y, where M = Cr, Mo, W; M' = H, Li, Na, K, Rb, Cs. The key points to emerge from this study include the following. (i) M(CO)6-M'56Y samples have the hexacarbonylmetal(O) molecule associated with two alpha-cage extraframework cations (or Bronsted protons), via the oxygen end of two trans bonded carbonyls with a saturation loading of 2M(CO)6/alpha-cage. (ii) M(CO)6-M'56Y samples have the hexacarbonylmetal(O) guest confined to the internal surface of the zeolite with a homogeneous distribution throughout the zeolite crystals. (iii) A Mo and Rb EXAFS structure analysis of 8{Mo(CO)6}-Rb56Y shows that the alpha-cage encapsulated Mo(CO)6 guest maintains its structural integrity, with some evidence for anchoring via extraframework Rb+ cations. (iv) A rapid C-13O intrazeolite ligand exchange occurs M(12CO)6-M '56Y to yield M(12CO)m(13CO)6-m-M'56Y, the extent of which depends on the 13CO loading. (v) M(CO)3-M'56Y can be cleanly generated via the mild vacuum thermal decarbonylation of M(CO)6-M56Y, the tricarbonyl stoichiometry of which is unequivocally established from its observed and calculated diagnostic M(12CO)n(13CO)3-n-M'56Y vibrational isotope pattern and from EXAFS structural data. (vi) Intrazeolite ractions of M(CO)3-M'56Y with large and small arenes, trienes, and phosphines cleanly yield the respective intrazeolite six-coordinate complexes (shown to be identical with the products of direct impregnation of the latter complexes), thereby supporting the tricarbonylmetal(0) assignment as well as pinpointing the location of the M(CO)3-M'56Y tricarbonylmetal(0) fragment on the internal surface of the zeolite. (vii) Cation effects in the mid/far-IR, EXAFS data, and optical reflectance spectra indicate that the supercage-confined M(CO)3-M'56Y moiety is anchored to an oxygen framework site rather than to an extrawork cation site via the metal or oxygens of the carbonyls. (viii) The tricarbonyl fragments show C(s) and C3-upsilon symmetry depending on the choice of M and M' which can be rationalized in terms of a second-order Jahn-Teller effect. (ix) EXAFS data for the mild thermal decomposition of Mo(CO)3-Rb56Y demonstrates the formation of molybdenum atoms statistically distributed in the zeolite lattice

Photoreactions of group 6 metal carbonyls with olef i ns

Abstract -A scheme is presented, based on preparative and mechanistic studies, concerning the principles which govern the course of multiple photosubstitution of group 6 metal carbonyls with olefins: (i) after initial (n -olefin)M(CO) formation, photcdetachment of CO in cis-position to the olefin is strongly favoured over frans-CO dissociation; (ii) a (n2-C=C) M subunit with frarrs-orfhogonal position of the olefins is distinctly more stable than other geometries. This is rationalized in terms of competitive demand of CO and olefin ligands for metal d electron density, whereby the single-faced n-acceptor character of the olefin plays a crucial role. -Sequential photosubstitution of M(C0)6 with olefins yields (n2-olefin)M(C0)5 and, ultimately, trans-(n -olefin) M(C0)4, as verified for all three group 6 metals. Quantum yield measurements (0.72 for W and 0.61 for Cr in the first step: ca. 0.5 for W and ca. 0.04 for Cr in the second step: at A = 302 nm) and studies revealing the role of cis-(n -olefin)2M(C0)4 were performed with E-cyclooctene, which exhibits exceptionally strong bonding to transition metals. In accord with the above principles, photosubstitution of (l14-norbornadiene)M(C0)4 complexes yields bornadiene)M(CO) . Implications of these findings with respect to photocatalytic processes are 3 briefly discussed