Evaluating steady-state and time-resolved fluorescence as a tool to study the behavior of asphaltene in toluene.

A combination of steady-state fluorescence, fluorescence lifetime measurements and the determination of time-resolved emission spectra were employed to characterize asphaltene toluene solutions. Lifetime measurements were shown to be insensitive to the source of asphaltene or the alkane solvent from which asphaltene was precipitated. This insensitivity suggests that either the composition of Athabasca and Cold Lake asphaltene is very similar or that the fluorescence behavior is dominated by the same sub-set of fluorophores for the different samples. These results highlight the limitations in using fluorescence to characterize asphaltene solutions. Different dependencies were observed for the average lifetimes with the asphaltene concentration when measured at two different emission wavelengths (420 nm and 520 nm). This result suggests that different fluorophores underwent diverse interactions with other asphaltene molecules as the asphaltene concentration was raised, suggesting that models for asphaltene aggregation need to include molecular diversity.

Computational and experimental study of the structure, binding preferences, and spectroscopy of nickel(II) and vanadyl porphyrins in petroleum.

We present a computational exploration of five- and six-coordinate Ni(II) and vanadyl porphyrins, including prediction of UV-vis spectroscopic behavior and metalloporphyrin structure as well as determination of a binding energy threshold between strongly bound complexes that have been isolated as single crystals and weakly bound ones that we detect by visible absorption spectroscopy. The excited states are calculated using the tandem of the time-dependent density functional theory (TD-DFT) and the conductor-like polarizable continuum model (CPCM). The excited-state energies in chloroform solvent obtained by using two density functionals are found to correlate linearly with the experimental Soret and alpha-band energies for a known series of five-coordinate vanadyl porphyrins. The established linear correction allows simulation of the excited states for labile octahedral vanadyl porphyrins that have not been isolated and yields Soret and alpha-band bathochromic shifts that are in agreement with our UV-vis spectroscopic results. The PBE0 and PW91 functionals in combination with DNP basis set perform best for both structure and binding energy prediction. The reactivity preferences of Ni(II) and vanadyl porphyrins toward aromatic fragments of large petroleum molecules are explored by using the density functional theory (DFT). Analysis of electrostatic potentials and Fukui functions matching shows that axial coordination and hydrogen bonding are the preferred aggregation modes between vanadyl porphyrins and nitrogen-containing heterocycle fragments. This investigation improves our understanding on the cause for broadening of the Ni and V porphyrin Soret band in heavy oils. Our findings can be useful for the development of metals removal methods for heavy oil upgrading.

Autoxidation-product-initiated dioxygenases: vanadium-based, record catalytic lifetime catechol dioxygenase catalysis.

In recent work, it was shown that V-containing polyoxometalates such as (n-Bu4N)7SiW9V3O40 or (n-Bu4N)9P2W15V3O62, as well as eight other V-containing precatalysts tested, evolve to a high activity, long catalytic lifetime (> or = 30,000-100,000 total turnovers) 3,5-di-tert-butylcatechol dioxygenase, in which Pierpont's complex [VO(DBSQ)(DTBC)]2 (where DBSQ is 3,5-di-tert-butylsemiquinone and DTBC is the 3,5-di-tert-butylcatecholate dianion) was identified as a common catalyst or catalyst resting state (Yin, C.-X.; Finke, R. G. Vanadium-Based, Extended Catalytic Lifetime Catechol Dioxygenases: Evidence For a Common Catalyst. J. Am. Chem. Soc. 2005, 127 (25), 9003-9013). Herein, those findings are followed up by studies aimed at answering the following questions about this record catalytic lifetime 3,5-di-tert-butylcatechol dioxygenase catalyst: (i) What is the key to how V leaches from, for example, seemingly robust V-containing polyoxometalate precatalysts? (ii) What is the key to the sigmoidal, apparently autocatalytic kinetics observed? (iii) What can be learned about the underlying reactions that form [VO(DBSQ)(DTBC)]2? (iv) Finally, do the answers to (i-iii) lead to any broader insights or concepts? Key findings from the present work include the fact that the reaction involves a novel, autoxidation-product-induced dioxygenase, that is, one in which the undesired autoxidation of the 3,5-di-tert-butylcatechol substrate to the corresponding benzoquinone and H2O2 turns on the desired dioxygenase catalysis via a V-leaching process which eventually yields Pierpont's complex, [VO(DBSQ)(DTBC)]2. Plausible reactions en route to [VO(DBSQ)(DTBC)]2 consistent with the kinetic data, the role of H2O2, and the relevant literature are provided. The results provide a prototype example of the little observed but likely more general concept of an autoxidation-product-initiated reaction. The results also provide considerable simplification of, and insight into, the previously disparate literature of V-based 3,5-di-tert-butylcatechol dioxygenase catalysis.

Kinetic and mechanistic studies of vanadium-based, extended catalytic lifetime catechol dioxygenases.

Recently we showed that V-containing polyoxometalates such as (n-Bu4N)7SiW9V3O40 or (n-Bu4N)9P2W15V3O62, as well as eight other V-containing precatalysts tested, evolve to high-activity, long catalytic lifetime (> or = 30,000-100,000 total turnovers) 3,5-di-tert-butylcatechol (DTBC) dioxygenases in which Pierpont's complex [VO(DBSQ)(DTBC)]2 is apparently a common catalyst resting state [Yin, C.-X.; Finke, R. G. J. Am. Chem. Soc. 2005, 107, 9003-9013]. In a separate paper, autoxidation of DTBC to the corresponding benzoquinone and H2O2 was shown to be a key to the catalyst evolution process: the H2O2, DTBC, and O2 plus virtually any V-based precatalyst tested form [VO(DBSQ)(DTBC)]2 under the catalytic conditions, that catalyst formation process being autocatalytic in H2O2. The resulting novel concept is that of an autoxidation-product-initiated dioxygenase [Yin, C.-X.; Sasaki, Y.; Finke, R. G. Inorg Chem. 2005, in press]. Herein the following questions about this record catalytic lifetime 3,5-di-tert-butylcatechol dioxygenase catalyst are explored: (i) What is the rate law for 3,5-di-tert-butylcatechol dioxygenation when one begins with Pierpont's [VO(DBSQ)(DTBC)]2? (ii) Does it support the hypothesis that this complex is a catalyst resting state or, perhaps, even the true catalyst? (iii) Can a mechanism be written from that information and from the knowledge in the dioxygenase literature? The results answer each of these questions and provide considerable mechanistic insight into the most catalytically active and long-lived DTBC dioxygenase catalyst presently known.

Vanadium-based, extended catalytic lifetime catechol dioxygenases: evidence for a common catalyst.

In 1999, a catechol dioxygenase derived from a V-polyoxometalate was reported which was able to perform a record >100 000 total turnovers of 3,5-di-tert-butylcatechol oxygenation using O2 as the oxidant (Weiner, H.; Finke, R. G. J. Am. Chem. Soc. 1999, 121, 9831). An important goal is to better understand this and other vanadium-based catechol dioxygenases. Scrutiny of 11 literature reports of vanadium-based catechol dioxygenases yielded the insight that they all proceed with closely similar selectivities. This, in turn, led to a "common catalyst hypothesis" for the broad range of vanadium based catechol dioxygenase precatalysts presently known. The following three classes of V-based compounds, 10 complexes total, have been explored to test the common catalyst hypothesis: (i) six vanadium-based polyoxometalate precatalysts, (n-Bu4N)4H5PV14O42, (n-Bu4N)7SiW9V3O40, (n-Bu4N)5[(CH3CN)(x)Fe(II).SiW9V3O40], (n-Bu4N)9P2W15V3O62, (n-Bu4N)5Na2[(CH3CN)(x)Fe(II).P2W15V3O62], and (n-Bu4N)4H2-gamma-SiW10V2O40; (ii) three vanadium catecholate complexes, [V(V)O(DBSQ)(DTBC)]2, [Et3NH]2[V(IV)O(DBTC)2].2CH3OH, and [Na(CH3OH)2]2[V(V)(DTBC)3]2.4CH3OH (where DBSQ = 3,5-di-tert-butylsemiquinone anion and DTBC = 3,5-di-tert-butylcatecholate dianion), and (iii) simple VO(acac)2. Product selectivity studies, catalytic lifetime tests, electron paramagnetic resonance spectroscopy (EPR), negative ion mode electrospray ionization-mass spectrometry (negative ion ESI-MS), and kinetic studies provided compelling evidence for a common catalyst or catalyst resting state, namely, Pierpont's structurally characterized vanadyl semiquinone catecholate dimer complex, [VO(DBSQ)(DTBC)]2, formed from V-leaching from the precatalysts. The results provide a considerable simplification and unification of a previously disparate literature of V-based catechol dioxygenases.

Is it true dioxygenase or classic autoxidation catalysis? Re-investigation of a claimed dioxygenase catalyst based on a Ru(2)-incorporated, polyoxometalate precatalyst.

A 1997 Nature paper reported that a novel Ru(2)-incorporated sandwich-type polyoxometalate, {[WZnRu(III)(2)(OH)(H(2)O)](ZnW(9)O(34))(2)}(11)(-), is an all-inorganic dioxygenase catalyst for the hydroxylation of adamantane and the epoxidation of alkenes using molecular oxygen. Specifically, it was reported that the above Ru(2)-containing polyoxometalate catalyzes the following reaction by a non-radical-chain, dioxygenase mechanism: 2RH + O(2) --> 2ROH (R = adamantane). A re-investigation of the above claim has been performed, resulting in the following findings: (1) iodometric analysis detects trace peroxides (0.5% relative to adamantane), the products of free-radical-chain autoxidation, at the end of the adamantane hydroxylation reaction; (2) a non-dioxygenase product, H(2)(18)O, is observed at the end of an adamantane hydroxylation reaction performed using (18)O(2); (3) kinetic studies reveal a fractional rate law consistent with a classic radical-chain reaction; (4) a non-dioxygenase approximately 1:1 adamantane products/O(2) stoichiometry is observed in our hands (instead of the claimed 2:1 adamantane/O(2) dioxygenase stoichiometry); (5) adamantane hydroxylation is initiated by the free radical initiator, AIBN (2,2'-azobisisobutyronitrile), or the organic hydroperoxide, t-BuOOH; (6) four radical scavengers completely inhibit the reaction; and (7) {[WZnRu(III)(2)(OH)(H(2)O)](ZnW(9)O(34))(2)}(11)(-) is found to be an effective catalyst for cyclohexene free-radical-chain autoxidation. The above results are consistent with and strongly supportive of a free-radical-chain mechanism, not the previously claimed dioxygenase pathway.