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Reaction of acidified (pH ≈ 7) sodium tungstate solutions with transition metal cations (Fe3+, Ni2+, Zn2+, Co2+) leads to the formation of transition-metal-disubstituted Keggin-type heteropolytungs...

First, catalyst variation and solvent optimization studies are reported for polyoxoanion-supported transition-metal precatalysts, novel complexes such as [(1,5-COD)Ir I ·P 2 W 15 Nb 3 O 62 ] 8− , [(C 6 H 6 )Ru II ·P 2 W 15 Nb 3 O 62 ] 7− and [(OC) 3 Re I ·P 2 W 15 Nb 3 O 62 ] 8− , catalyzing cyclohexene oxidation at low (≤25%) conversion with O 2 to, predominatly, four main autoxidation products: 2-cyclohexen-1-yl hydroperoxide, 2-cyclohexen-1-one, 2-cyclohexen-1-ol, and cyclohexene oxide, all at a mass balance of ≥80–92% under the low conversion conditions. Next, radical-chain initiator, inhibitor, and other kinetic (rate law) evidence for the reaction behaving, at lower conversions, as classical Haber–Weiss autoxidation was obtained. Those studies lead, in turn, to the study of cyclohexene autoxidation at higher conversions, notably the identification of ca. 70 gas chromatography (GC)-detectable products. This was followed by identification of 27 of those ca. 70 products, the first reported detailed identification of more than 8–10 products of cyclohexene autoxidation. These results led, in turn, to the five main findings of this study: (i) product and kinetic—overall compelling—evidence that the main reaction is free-radical-chain autoxidation; (ii) the first detection of ca. 70 cyclohexene autoxidation products, followed by the identification of 27 of those products—this is not trivial given that complete product studies are the required first step of rigorous mechanistic work; (iii) plausible arrow-pushing mechanisms to many of the observed products using known radical chemistry—previously unavailable schemes; (iv) the observation of chlorinated hydrocarbons among the products, results which require the precedented participation in the oxidation catalysis by CH 2 Cl 2 solvent-derived, CHCl 2 radicals, and perhaps most significantly (v) the development of a relatively simple and quick, yet definitive, GC and GC–MS fingerprint method for detecting autoxidation catalysis using the prototype olefin, cyclohexene. Such product studies should prove to be a useful tool in the continuing problem of detecting, or ruling out, classical autoxidation in attempts to develop new oxidation chemistry using O 2 as a highly desirable terminal oxidant.

An improved, one-step synthesis of [RuII(1,5-COD)(CH3CN)4]2+ as the BF4− salt has been accomplished in 51% yield, an approximately 75% higher yield than the three-step literature synthesis of the corresponding PF6− salt. The improved synthesis consists of (i) grinding the insoluble [RuCl2(1,5-COD)]x precursor to increase the reaction rate and yield, (ii) treating the resultant [RuCl2(1,5-COD)]x with 2Ag+BF4− in refluxing acetonitrile with excess 1,5-COD present to inhibit 1,5-COD loss in the product and, most importantly, (iii) following the reaction directly by 1H-NMR spectrometry which revealed that the substitution reaction of the Ru(II), d6 precursor is, as expected, quite slow and requires ca. 120 h. The [Ru(1,5-COD)(CH3CN)4][BF4]2 product was characterized by 1H, 13C, and 19F-NMR, elemental analysis, and single-crystal X-ray crystallography. Problems in commercial Ru and F analyses are also addressed since this issue has been inadequately treated in the existing literature.

The preparation and crystallization of a tetrameric form of the tri-Ti IV -substituted Wells–Dawson-substructure (i.e. [P 2 W 18 O 62 ] 6− -type) heteropolyoxoanion, [(P 2 W 15 Ti 3 O 60.5 ) 4 ] 36− , as its water-soluble potassium salt, are described. This compound has been prepared by a direct reaction of Na 12 [P 2 W 15 O 56 ]·18H 2 O with 30% aqueous Ti IV (SO 4 ) 2 solution. The compound with a formula of {K 28 H 8 [P 2 W 15 Ti 3 O 60.5 ] 4 }·0.8KCl·56H 2 O ( 1 ) was obtained in 38% yield as an analytically pure, colorless, powdered solid. The crystallization of 1 was achieved in the presence of KCl in pH 2.0 water to form {K 28 H 8 [P 2 W 15 Ti 3 O 60.5 ] 4 }·46H 2 O ( 2 ). The KCl significantly aids the crystallization of 1 , but does not enter into the final composition of crystalline 2 . The mixed tetrabutylammonium plus potassium salt {(Bu 4 N) 17 H 11 K 8 [P 2 W 15 Ti 3 O 60.5 ] 4 } ( 3 ) was successfully prepared as an analytically pure white powder from the reaction of 1 in pH 2.0 water with solid (Bu 4 N)Br; the resultant 3 is acetonitrile- and DMSO-soluble, but water-insoluble. Compositional and structural characterization of 1 – 3 was accomplished by complete elemental analyses, TG/DTA, FT-IR, UV–Vis spectroscopy, and solution 31 P and 183 W NMR spectroscopies, including solid-state GHD/MAS 31 P NMR spectroscopy in the case of 2 . We also report a scaled-up, 10–12 g preparation of the trimeric {(Bu 4 N) 15 H 17 [P 2 W 15 Ti 3 O 61.3 ] 3 } ( 4 ) (i.e. the tri-TiOTi anhydride-bridged {[P 2 W 15 Ti 3 O 61 ]O[P 2 W 15 Ti 3 O 60 ]O[P 2 W 15 Ti 3 O 61 ]} 32− ) and its partial deprotonation and partial TiOTi cleavage with Bu 4 N + OH − to the primarily dimeric {(Bu 4 N) 9 H 3 [P 2 W 15 Ti 3 O ∼62 ] n } ( 5 ) ( n =1–2) (i.e. the primarily mono-TiOTi anhydride-bridged {[P 2 W 15 Ti 3 O 61 ]O[P 2 W 15 Ti 3 O 61 ]} 22− ). Most importantly, each of the tri-titanium-substituted Wells–Dawson-type polyoxoanions 1 – 5 are shown, by solution molecular weight experiments plus appropriate control experiments, to exist in aggregated, TiOTi anhydride forms, with the parent compounds 1 , 2 and 3 all existing as tetramers, [(P 2 W 15 Ti 3 O 60.5 ) 4 ] 36− . This finding of only TiOTi-bridged, anhydride forms of tri-Ti(IV) substituted Wells–Dawson polyoxoanions is important in that it corrects several previous, erroneous reports in the literature claiming, without molecular weight evidence, that the monomeric ‘[P 2 W 15 Ti 3 O 62 ] 12− ’ exists. Monomeric ‘[P 2 W 15 Ti 3 O 62 ] 12− ’ is, however, still unknown.

Following a critical analysis of the dioxygenase literature and injection of the insights therein into the development of new dioxygenase catalysts, two new, of four total exemplary, polyoxoanion precatalysts have been synthesized, characterized, and then discovered to exhibit record catalytic lifetime 3,5-di-tert-butylcatechol (DTBC; 1) dioxygenase activity using molecular oxygen as the terminal oxidant. A total of 24 additional polyoxoanion and other precatalysts have also been surveyed for their DTBC dioxygenase activity. The four exemplary precatalyst complexes are the trivanadium(V)-containing, orange-red parent polyoxoanions (n-Bu4N)7[SiW9V3O40], I, and (n-Bu4N)9[P2W15V3O62], II, and their previously unknown polyoxoanion-supported, dark green iron complexes (n-Bu4N)5[(CH3CN)xFe·SiW9V3O40], III, and (n-Bu4N)5[(CH3CN)xFe·P2W15V3O62], IV. Careful, high (95 ± 5%) mass balance studies are reported, studies rare in the dioxygenase literature, but studies made possible in the case of I−IV by their high act...

The stoichiometric oxygenation of 3,5-di-tert-butylcatechol (DTBC) with molecular oxygen at 25°C and using the novel polyoxoanion-supported FeII complex, (n-Bu4N)7[(CH3CN)xFeII·P2W15Nb3O62] (1), is reported. This is the first report of an iron(II) plus polyoxoanion-based catechol dioxygenase. The catechol oxygenation reaction exhibits a clean, 1:1 DTBC:1 stoichiometry to form 1·DTBC; this complex then exhibits a substrate-triggered binding of 1.0 equiv. of O2 to form a 1:1:1 complex 1·DTBC·O2, thereby proving the 1:1 catechol to O2 stoichiometry which defines a dioxygenase reaction. The overall oxygenation of DTBC proceeds through three distinct reaction stages (A→B→C→D), each stage being slower than the previous stage and of a different absorption maximum. Each intermediate can, therefore, be observed directly, another valuable feature of the present system, and a feature which should allow characterization of reaction intermediates and kinetic and mechanistic studies in a bona fide FeII, non-enzymatic dioxygenase system. The observed catechol cleavage products are extradiol ring-cleavage products characteristic of FeII extradiol enzymes; the non-ambiguity about the FeII oxidation state, and, hence, the ability to correlate unambiguously the FeII initial oxidation state with the formation of only extradiol products in a synthetic dioxygenase system is another important feature of the present system. The choice of the all-inorganic, oxidation resistant polyoxoanion ligand system is significant as well en route to the goal of achieving a long-lived, catalytic dioxygenase system without the need to worry about catalyst lifetime-limiting, ligand-oxidation side reactions.

Eleven new polyoxoanion-supported transition metal acetonitrile compositions have been synthesized in 1:1, 1:2, and even a few 1:3 Mn+ to polyoxoanion ratios; these new precatalysts were then tested for their catalytic efficacy for norbornene and cyclohexene oxygenation using [PhIO]n as the oxidant. The catalytic results identify {(CH3CN)xMn2+/P2W15Nb3O629-} (1) and {2(CH3CN)xMn2+/P2W15Nb3O629-} (2) as the best precatalysts within this new subclass of polyoxoanion-supported catalysts. The results reveal a modest ca. 14-fold rate increase for either 1 or 2 in norbornene or cyclohexene epoxidation compared to the polyoxoanion-free [MnII(CH3CN)4](BF4)2 solvate, kinetic results which require the presence of the P2W15Nb3O629- polyoxoanion in the rate-determining step of the active catalyst. Catalyst reisolation, then IR, UV−visible, and ion-exchange resin studies (and in comparison to authentic 2 that has not undergone catalysis) provide compelling evidence that 2 is, in fact, the true catalyst. Also compared ...

The Dawson-type P(2)W(15)Nb(3)O(62)(9)(-) polyoxoanion-supported Re(CO)(3)(+) complex, [Re(CO)(3).P(2)W(15)Nb(3)O(62)](8)(-) (1), has been synthesized and characterized in two different counter-cation compositions. The [(n-C(4)H(9))(4)N](8)(8+) complex provides a highly soluble compound which exists as a single isomer in solution. The carbonyl stretching infrared frequencies suggest that the P(2)W(15)Nb(3)O(62)(9)(-) ligand serves as a strong electron donor to the Re(CO)(3)(+) fragment. The P(2)W(15)Nb(3)O(62)(9)(-) polyoxoanion-supported Ir(CO)(2)(+) complex [Ir(CO)(2).P(2)W(15)Nb(3)O(62)](8)(-) (2) has also been synthesized and characterized as its octakis(tetrabutylammonium), [(n-C(4)H(9))(4)N](8)(8+), salt. This compound was characterized by NMR and IR, results which demonstrate that 2 also exists as a single isomer in solution. The [Ir(CO)(2).P(2)W(15)Nb(3)O(62)](8)(-) complex is stable in the absence of water, but decomposes quickly in the presence of even 1 equiv of water. Attempted preparation of the analogous P(2)W(15)Nb(3)O(62)(9)(-) -supported Rh(CO)(2)(+) complex (3), while monitoring by (31)P NMR, revealed that this compound is unstable in solution at room temperature. In addition, we have discovered that added Na(+) can induce the formation of non-C(3)(v)() symmetry isomers of supported Re(CO)(3)(+) and Ir(CO)(2)(+) and, by inference, supported Ir(1,5-COD)(+). When Na(+) is removed from these systems by addition of Kryptofix[2.2.2], the non-C(3)(v)() isomers convert back to the single, C(3)(v)() isomer with heating, thereby providing a model system for the little studied mobility of M(CO)(n)()(+) cations across a soluble-oxide surface. When [Rh(CO)(2).P(2)W(15)Nb(3)O(62)](8)(-) is irradiated in the presence of hydrogen and cyclohexene a novel polyoxoanion-stabilized Rh(0)(n)() nanocluster is formed, results that bear a strong analogy to Yates' work studying atomically-dispersed Rh(CO)(2)(+) on solid Al(2)O(3).(10e) Yates and co-workers observe that Rh(CO)(+).Al(2)O(3) loses a CO upon photolysis, and that the resultant Rh(CO)(1)(+).Al(2)O(3) is reduced under H(2) to form Rh(0), which in turn yields Rh(0)(n)() clusters on Al(2)O(3)-a process that, intriguingly, is largely reversible if CO is readded. Also briefly discussed is other relevant literature of solid-oxide-supported Re(CO)(3)(+) and M(CO)(2)(+) (M = Ir, Rh), literature that makes apparent the potential significance of these complexes as EXAFS and other spectroscopic models of solid-oxide-supported M(CO)(n)()(+).

To reveal the underlying problems and misleading results that can be obtained from uninitiated O 2 /aldehyde/olefin co-oxidative epoxidations that proceed by a radical-chain mechanism, a series of nearly identical experiments were set up at the University of Tokyo and Colorado State University. The main catalyst precursor studied is the oxidatively resistant, polyoxoanion-supported organometallic complex [( n -C 4 H 9 ) 4 N] 5 Na 3 [(1,5-COD)Ir · P 2 W 15 Nb 3 O 62 ], 1 . Three other L n M · P 2 W 15 Nb 3 O n− 62 complexes were also examined (L n M = Re(CO) + 3 , Ru(C 6 H 6 ) 2+ , and Rh(1,5-COD) + ], as well as the framework-incorporated cobalt(II) complex, [( n -C 4 H 9 ) 4 N] 4 H[PW 11 CO II O 3− 39 ]. Several types of important but often omitted control reactions were also performed, again to expose the pitfalls in studies of O 2 /aldehyde/olefin co-oxidative epoxidations, specifically: (i) the control of leaving out the catalyst completely, (ii) controls for O 2 -mass-transfer limitations, (iii) controls examining a range of different solvents (CH 3 CN, CH 2 Cl 2 , ClCH 2 CH 2 Cl, and Cl 2 CHCHCl 2 ), and (iv) controls comparing uninitiated versus deliberately peroxide-initiated reactions. The resultant reproducibilities, product conversions, selectivities, and yields are presented and discussed, as are stereochemical results using cis - and trans -stilbene. Several important insights are generated for the area of co-oxidative epoxidations proceeding by a radical-chain mechanism, most notably that the catalyzed results are inferior to ROOH-initiated, uncatalyzed co-oxidative epoxidations examined under otherwise identical conditions.

An improved synthesis of the prototype polyoxoanion−organometallic-support system, P2W15Nb3O629-, as its organic solvent-soluble all-[(n-C4H9)4N]+ salt is described. The improvements result in a 30% faster procedure which also provides a 60% increase in yield of [(n-C4H9)4N]9P2W15Nb3O62, 2. One key improvement in this synthesis is a pH titration of protonated [(n-C4H9)4N]5H4P2W15Nb3O62, 1, using [(n-C4H9)4N]OH and phenolpthalein indicator, which leads to fully deprotonated 2the key synthon required for clean support of organometallic cations onto the P2W15Nb3O629- polyoxoanion's “Nb3O93-” cap. The material obtained from this procedure is of the highest purity (≥95%) seen to date as judged by 31P NMR spectroscopy. [(n-C4H9)4N]9P2W15Nb3O62 has been further characterized by 1H, 13C, 17O, and 183W NMR spectroscopy as well as IR spectroscopy and complete elemental analysis. Also reported are control experiments that (i) confirm the identical reactivity of 2, obtained by either the procedure reported herein or ...

Reaction of acidified (pH approximately 7) sodium tungstate solutions with transition metal cations (Fe(3+), Ni(2+), Zn(2+), Co(2+)) leads to the formation of transition-metal-disubstituted Keggin-type heteropolytungstates with 3d-metal ions distributed over three different positions. A detailed investigation of the synthesis conditions confirmed that the complexes could equally be obtained using aqueous solutions of either Na(2)WO(4).2H(2)O (sodium monotungstate) at pH approximately 7, Na(6)[W(7)O(24)]. approximately 14H(2)O (sodium paratungstate A), or Na(10)[H(2)W(12)O(42)].27H(2)O (sodium paratungstate B) as starting materials. Three complexes, (NH(4))(6)Ni(II)(0.5)[alpha-Fe(III)O(4)W(11)O(30)Ni(II)O(5)(OH(2))].18H(2)O, (NH(4))(7)Zn(0.5)[alpha-ZnO(4)W(11)O(30) ZnO(5)(OH(2))].18H(2)O, and (NH(4))(7)Ni(II)(0.5)[alpha-ZnO(4)W(11)O(30)Ni(II)O(5)(OH(2))].18H(2)O were isolated in crystalline form. X-ray single-crystal structure analysis revealed that the solid-state structures of the three compounds consist of four main structural fragments, namely [MO(4)W(11)O(30)M'O(5)(OH(2))](n-) (Keggin-type, alpha-isomer) heteropolytungstates, hexaquo metal cations, [M''(OH(2))(6)](2+), ammonium-water cluster ions, [(NH(4)(+))(8)(OH(2))(12)], and additional ammonium cations and water molecules. The 3d metals occupy the central (tetrahedral, M) and the peripheral (octahedral, M') positions of the Keggin anion, as well as cationic sites (M'') outside of the polyoxotungstate framework. UV-vis spectroscopy, solution ((1)H, (183)W) and solid-state ((1)H) NMR, and also chemical analysis data provided evidence that the 3d-metal-disubstituted Keggin anions do not exist in solution but are being formed only during the crystallization process. Investigations in the solid state and in solution were completed by ESR, IR, and Raman measurements.