Jason A. Widegren

A review of the problem of distinguishing true homogeneous catalysis from soluble or other metal-particle heterogeneous catalysis under reducing conditions

This review considers cases in which a discrete transition-metal complex is used as a precatalyst for reductive catalysis; it focuses on the problem of determining if the true catalyst is a metal-complex homogeneous catalyst or if it is a soluble or other metal-particle heterogeneous catalyst. The various experiments that have been used to distinguish homogeneous and heterogeneous catalysis are outlined and critiqued. A more general method for making this distinction is then discussed. Next, the circumstances that make heterogeneous catalysis probable, and the telltale signs that a heterogeneous catalyst has formed, are outlined. Finally, catalytic systems requiring further study to determine if they are homogeneous or heterogeneous are listed. The major findings of this review are: (i) the in situ reduction of transition-metal complexes to form soluble-metal-particle heterogeneous catalysts is common; (ii) the formation of such a catalyst is easy to miss because colloidal solutions often appear homogeneous to the naked eye; (iii) a variety of experiments have been used to distinguish homogeneous catalysis from heterogeneous catalysis, but there is no single definitive experiment for making this distinction; (iv) experiments that provide kinetic information are key to the correct identification of the true catalyst; and (v) a more general approach for distinguishing homogeneous catalysis from heterogeneous catalysis has been developed. Additionally, (vi) the conditions under which a heterogeneous catalyst is likely to form include: (a) when easily reduced transition-metal complexes are used as precatalysts; (b) when forcing reaction conditions are employed; (c) when nanocluster stabilizers are present; and (d) when monocyclic arene hydrogenation is observed. Finally, (vii) the telltale signs of heterogeneous catalysis include the formation of dark reaction solutions, metallic precipitates, and the observation of induction periods and sigmoidal kinetics.

A review of soluble transition-metal nanoclusters as arene hydrogenation catalysts

A critical review of the use of soluble transition-metal nanoclusters for the hydrogenation of monocyclic aromatic compounds is presented. The review begins with a brief introduction to arene hydrogenation and to nanocluster science. The introductory material is followed by a detailed discussion of the approximately 20 papers in the literature that deal with the use of soluble transition-metal nanoclusters for the hydrogenation of monocyclic aromatic compounds. Metal particle catalysts on solid supports are not reviewed herein, and are considered only as far as they serve to compare and contrast with soluble transition-metal nanoclusters. The major findings of this review are: (i) soluble nanocluster catalysts are implicated as the true catalysts in many putatively “homogeneous” arene hydrogenations; (ii) with few exceptions, nanocluster catalysts used for arene hydrogenation are poorly characterized; (iii) soluble nanocluster catalysts for arene hydrogenation have modest activity and lifetime; (iv) Rh and Ru are used almost exclusively as the active metals; (v) two catalyst systems, one developed by Roucoux and co-workers and the other by our own research group, stand out from the rest in terms of activity and lifetime; (vi) selective arene hydrogenation, especially for the synthesis of the all-cis diastereomer of substituted cyclohexanes, has received considerable attention and is a promising area for future study and, perhaps, fine chemical applications (selectivities >90% for the all-cis diastereomer have been achieved by several groups).

Synthesis and characterization of the tetrameric, tri-titanium(IV)-substituted Wells–Dawson-substructure polyoxotungstate, [(P2W15Ti3O60.5)4]36−: the significance of ultracentrifugation molecular weight measurements in detecting aggregated, anhydride forms of polyoxoanions

Abstract 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.

Synthesis and pH-variable ultracentrifugation molecular weight measurements of the dimeric, Ti–O–Ti bridged anhydride form of a novel di-TiIV-1,2-substituted α-Keggin polyoxotungstate. Molecular structure of the [(α-1,2-PW10Ti2O39)2]10− polyoxoanion

The preparation and characterization of a Keggin-type, novel di-TiIV-1,2-substituted polyoxotungstate are described. The dimeric, Ti–O–Ti bridged anhydride form of the di-TiIV-1,2-substituted α-Keggin polyoxotungstate, K10[α,α-P2W20Ti4O78]·12H2O 1, was unexpectedly found in the varied molar-ratio reactions of tri-lacunary precursor Na9[A-PW9O34]·19H2O with Ti(SO4)2 in aqueous solution. Although this compound was first found as a minor product in the preparation of the dimeric, tri-TiIV-1,2,3-substituted species, K10H2[α,α-P2W18Ti6O77]·17H2O 3, it was successfully prepared as a main product in this work and structurally characterized. Compound 1, as analytically pure, homogeneous colorless needle crystals, was obtained as a major product in 29.2% yield (2.7 g scale) from recrystallization under acidic conditions (at pH 2.2) of the 1 ∶ 2 molar-ratio reaction product. X-Ray structure analysis revealed that the molecular structure of 1 consisted of a dimeric anhydride formed by two Ti–O–Ti bonds linking two [α-1,2-PW10Ti2O40]7− Keggin units. Interestingly, ultracentrifugation molecular weight (MW) measurements of this compound in aqueous solution showed the pH-dependent interconversion between monomer [α-1,2-PW10Ti2O40]7−2 and dimer [α,α-P2W20Ti4O78]10−1; this compound was present as the monomer under less acidic conditions (pH 7.8), while it was in dimeric form under more acidic conditions (pH 1.0 and 2.2). Characterization of 1 was also accomplished by complete elemental analyses, TG/DTA, FTIR and solution (31P and 183W) NMR spectroscopy.

Synthesis and characterization of tri-titanium(IV)-1,2,3-substituted α-Keggin polyoxotungstates with heteroatoms P and Si. Crystal structure of the dimeric, Ti–O–Ti bridged anhydride form K10H2[α,α-P2W18Ti6O77]·17H2O and confirmation of dimeric forms in aqueous solution by ultracentrifugation molecular weight measurements

The preparation and characterization of the α-Keggin-type, novel tri-TiIV-1,2,3-substituted polyoxotungstates with heteroatoms P and Si are described. The molecular structure of K10H2[α,α-P2W18Ti6O77]·17H2O 1, obtained as a main product (30.4% yield) from the 1 ∶ 3 molar ratio reaction of tri-lacunary precursor Na9[A-PW9O34]·19H2O with Ti(SO4)2·4H2O in aqueous solution, was determined by X-ray structure analysis. It had the dimeric, anhydride structure formed by three Ti–O–Ti bonds linking two Keggin [α-1,2,3-PW9Ti3O40]9− units. From the filtrate after 1 was filtered off, a different compound 2 was obtained as a minor product. A related α-Keggin tri-TiIV-1,2,3-substituted polyoxotungstate with heteroatom Si, the dimeric anhydride form K10H4[α,α-Si2W18Ti6O77]·15H2O 3, was obtained as a main product (31.1% yield) from a 1 ∶ 3 molar ratio reaction of Na10[A-α-SiW9O34]·15H2O with Ti(SO4)2. Ultracentrifugation molecular weight measurements under less acidic conditions revealed that 1 and 3 were present as dimers in aqueous solution. Characterization of 1 and 3 was also accomplished by complete elemental analyses, TG/DTA, FTIR and solution (31P and 183W) NMR spectroscopies.

Improved synthesis and crystal structure of tetrakis(acetonitrile)(η4-1,5-cyclooctadiene)ruthenium(II) bis[tetrafluoroborate(1−)]

Abstract 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.