Zeolites, cobalt complexes

The reversible formation of a low-spin [Co (III) (NHS) n02 ]2+ complex within a Co (II) Y zeolite has been demonstrated by EPR spectroscopy. In this complex n is probably equal to five. A maximum of one cobalt complex per large cavity was farmed. The cobalt hyperfine structure shows that the unpaired electron is only 8% on the metal ion. Experiments utilizing 170 indicate that 02 enters the coordination sphere of the Co2+ ions and that the unpaired electron is largely associated with the oxygen molecule. The oxygen-17 hyperfine structure reveals that the two oxygen atoms are not equivalent hence, it is concluded that the oxygen is bonded as a peroxy-type superoxide ion. 

The Direct Enantioselective Synthesis of Diols from Olefins using Hybrid Catalysts of Chiral Salen Cobalt Complexes Immobilized on MCM-41 and Titanium-containing Mesoporous Zeolite... 

For transition metals, such migrations are accompanied by ligand replacement. The chemistry and catalysis of transition metal complexes in zeolites have been recently reviewed by Lunsford (156). In these catalysts, it can be desirable to replace, for example, ammonia by pyridine. Zeolite-encaged cobalt complexes, for example, exhibit high potential for oxygen activation. 

Additionally, acetylene may be also cocyclized with nitriles to form substituted pyridines using Co - zeolites as with soluble cobaltous complexes. Only Co is active in this reaction which needs an insertion of the nitrile into the M pentacycle. Apparently this is the element that enables competition of the nitrile with acetylene. It also appeared that the cobalt centers had to swing between the trivalent and monovalent states (45). The use of substituted acetylenes showed that the 2-, 5-substituted pyridines are significantly favoured by comparison with homogeneous media. 

The Co-Na-MOR and Co-H-MOR were prepared by ion exchange of Na-MOR and H-MOR with Co(acetate)2 solutions at 350 K [10P2]. In the Co-Na-MOR and Co-H-MOR samples exchanged to various extents with cobalt (Co/Al = 0.1 0.5), isolated Co was the most abundant species, whereas the residual cobalt was present as [Co-O-Co]. The FTIR results demonstrated that in all Co-MOR samples, irrespective of the cobalt content, cobalt is almost exclusively located inside the mordenite chaimels. In the presence of monovalent cobalt complex with EDMA, zeolites have been synthesized from aluminosilicate gels in hydrothermal conditions [llKlj. Three kinds of zeolites, i.e., MOR-, MFI-, and ANA-types were obtained. The cobalt atoms have been shown to be located in the MOR-framework, through Co-atom substitution and not in micropores as a complex. 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

Other Compounds. Adsorption of MeNC into a cobalt(ii) zeolite at — 196"C was followed by e.s.r. spectroscopy, and the presence of low-spin [Co-(CNMe)j] and [Co(CNMe) ] complex cations demonstrated.This work represents one of the few successful attempts to produce well-characterized transition-metal complexes in a zeolite framework. Addition of NaCp to CoClj and [(BgC2H,Q)CoCp], previously reduced with sodium naphthalide gives a new bimetallic complex, for which structure (94) is proposed. ... 

Winscom et al. (316) have studied the influence of hydration and oxidation on bis(dimethylglyoximato)-Co(II) complexes in a cobalt-exchanged NaX zeolite. Evidence for both six- and fourfold complexes is presented. They conclude that increasing water coordination destabilizes the half-filled dj2 orbital with respect to dxz and d, and eventually its energy will exceed that of the unoccupied n molecular orbital of 02 to make the formation of the Co(III)-Oj adduct energetically favorable. 

The attachment and encapsulation of metals and metal complexes in the cavities of zeolites is an active area of research and provides a versatile method for the modification of these molecular sieves (39). Because of the enforced dispersion of the metal complexes in the zeolite, systems not readily observable in solution can be investigated in zeolites. For example, the mononuclear superoxo adduct of the cobalt(HI)-ammine system, [Co(NH3 )6(00-)]2+, which would be expected to dimerize in solution, could be observed entrapped in zeolite Y (40). 

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

A more complete article (58b) further details the properties of faujasite EMT-entrapped cobalt(II)-Schiff base complexes as compared to the same complexes entrapped in zeolite Y. The Schiff bases used to... 

Formation and Structure of a Monomeric Oxygen Adduct of a Cobalt(II)-Ammonia Complex in a Co(II)Y Zeolite... 

In this work, 1 1 oxygenated low-spin cobalt(II)-ammonia complexes were synthesized within the zeolite framework by the adsorption of NH3 and 02 in Co(II)Y zeolites with differing cobalt(II) content. Spin densities and the nature of the superoxide anion (02 ) were estimated by introducing oxygen-17 in the ammoniated Co(II) zeolites. Questions concerning the equivalence of the two oxygen atoms have arisen in studies on oxygen adducts of Co (II) Schiff base compounds (7), and it was of interest to study this problem in cobalt(II) ammonia complexes. 

Upon adsorption of excess ammonia in a Co(II)Y zeolite a white, high-spin cobalt(Il)-ammonia complex with a spin configuration of (fe )5-(eg)2 is formed. According to studies of cobalt (II) complexes in solutions, salts, and in zeolites, a hexacoordinate Co(II)-ammonia complex is the most likely form when an excess of ammonia is present (3, 4> ) Indeed,... 

As shown in Table I, the magnetic parameters of the oxygenated Co (Il)-ammonia complex in the zeolite are comparable with other mononuclear Co(II)-02 complexes, regardless of the nature of the cobalt (II) ligands. The very similar spectrum observed by Fujiwara et ah (9) for... 

We may conclude that the divalent cobalt ions move out into the large cavities upon adsorption of NH3 to form a hexacoordinate cobalt(II)-ammonia complex. Following adsorption of 02 in the ammoniated Co(II)Y zeolites, oxygen enters the coordination sphere of the Co2+ ions. This is accompanied by a charge-transfer process to form a [Co(III) (NH3)502 ]2+ complex. The general intermolecular redox process can be approximated by the reactions... 

Tricarbonyl(naphthalene)chromium, 19 Trimethylsilyl chlorochromate, 327 Cobalt Compounds Acylcobaltate complexes, 101 [Bis(salicylidene-y-iminopropyl)methyl-amine]cobalt(II), 41 Cobalt(II) chloride, 249 Cobalt zeolites, 296 Dicarbonylcyclopentadienylcobalt, 96 Di- x-carbonylhexacarbonyldicobalt,... 

Hydrotreating catalysts are composed of cobalt or nickel molybdate or nickel tungstate on an alumina or zeolite support. The materials are sulfided with hydrogen sulfide (H2S) before use, but the final catalysts may retain some oxide and be of complex composition. 

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