Computational Investigation of the Metal and Ligand Substitution Effects on the Structure and Electronic States of the Phosphoranimide Tetramer Complexes of Cu(I), Ni(I), Co(I), and Fe(I).

The structurally unique saddle-shaped paramagnetic tetrametallic clusters of Co(I) and Ni(I) with phosphoranimide ligands have been synthesized and proposed as catalyst precursors. The analogous Cu(I) nanocluster is planar and diamagnetic. These notable variations in geometry and ground electronic states indicate that the effect of metal and ligand substituents on the structure and electronic properties of these complexes requires investigation. We present a computational study of a series of these novel homoleptic complexes containing Co(I), Ni(I), and Cu(I) as well as Fe(I) coordinated to phosphoranimides with electron-donating and withdrawing substituents, conducted at the relativistic density functional theory level using ZORA-PBE/TZP. The optimized structures of the saddle-shaped Co(I) and Ni(I) and planar Cu(I) tetramers with linear N-M-N coordination are validated with respect to X-ray diffraction determinations. The ground-state analysis indicates that Cu(I) complexes are diamagnetic, whereas Ni(I) and Co(I) complexes are in high-spin states, in agreement with magnetic susceptibility measurements. The computational results show that Fe(I) complexes are saddle shaped and high spin. The Co(I) complex is stabilized by a metal macrocycle distortion from square to diamond, as elucidated from its Walsh diagram. The effects of metals and ligand substituents on the ground electronic state, metal center coordination environment, and energy of the complexes are investigated. The bulky tertiary butyl substituent causes the largest saddle-shape distortion of the tetramer marcocycle, which partially offsets its electron-donating effect. Macrocycle distortions with N-M-N site angles ranging from obtuse to alternating obtuse reflex are correlated with the increasing number of unpaired electrons. The phenyl-substituted complexes are expected to have the highest reactivity toward electrophiles. Understanding the interplay between structural and electronic parameters is intended to guide the development of synthetic cooperative systems for multielectron redox reactions, models of biological systems, and molecular magnets.

Model Asphaltenes Adsorbed onto Methyl- and COOH-Terminated SAMs on Gold.

Petroleum asphaltenes are surface-active compounds found in crude oils, and their interactions with surfaces and interfaces have huge implications for many facets of reservoir exploitation, including production, transportation, and oil-water separation. The asphaltene fraction in oil, found in the highest boiling-point range, is composed of many different molecules that vary in size, functionality, and polarity. Studies done on asphaltene fractions have suggested that they interact via polyaromatic and heteroaromatic ring structures and functional groups containing nitrogen, sulfur, and oxygen. However, isolating a single pure chemical structure of asphaltene in abundance is challenging and often not possible, which impairs the molecular-level study of asphaltenes of various architectures on surfaces. Thus, to further the molecular fundamental understanding, we chose to use functionalized model asphaltenes (AcChol-Th, AcChol-Ph, and 1,6-DiEtPy[Bu-Carb]) and model self-assembled monolayer (SAM) surfaces with precisely known chemical structures, whereby the hydrophobicity of the model surface is controlled. We applied solutions of asphaltenes to these SAM surfaces and then analyzed them with surface-sensitive techniques of near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). We observe no adsorption of asphaltenes to the hydrophobic surface. On the hydrophilic surface, AcChol-Ph penetrates into the SAM with a preferential orientation parallel to the surface; AcChol-Th adsorbs in a similar manner, and 1,6-DiEtPy[Bu-Carb] binds the surface with a bent binding geometry. Overall, this study demonstrates the need for studying pure and fractionated asphaltenes at the molecular level, as even within a family of asphaltene congeners, very different surface interactions can occur.

Deciphering structure and aggregation in asphaltenes: hypothesis-driven design and development of synthetic model compounds.

Asphaltenes comprise the heaviest and least understood fraction of crude petroleum. The asphaltenes are a diverse and complex mixture of organic and organometallic molecules in which most of the molecular constituents are tightly aggregated into more complicated suprastructures. The bulk properties of asphaltenes arise from a broad range of polycyclic aromatics, heteroatoms, and polar functional groups. Despite much analytical effort, the precise molecular architectures of the material remain unresolved. To understand asphaltene characteristics and reactivity, the field has turned to synthetic model compounds that mirror asphaltene structure, aggregation behavior, and thermal chemistry, including the nucleation of coke. Historically, molecular asphaltene modeling was limited to commercial compounds, offering little illumination and few opportunities for hypothesis-driven research. More recently, however, rational molecular design and modern organic synthesis have started to impact this area. This review provides an overview of commercially available model compounds but is principally focused on the design and synthesis of structurally advanced and appropriately functionalized compounds to mimic the physical and chemical behavior of asphaltenes. Efforts to model asphaltene aggregation are briefly discussed, and a prognosis for the field is offered. A referenced tabulation of the synthetic compounds reported to date is provided.

Unexpected Michael Additions on the Way to 2.3,8.9-Dibenzanthanthrenes with Interesting Structural Properties.

The synthesis and properties of a new polycyclic aromatic hydrocarbon containing eight annulated rings and based on the anthanthrene core is described. An unexpected, nucleophile-dependent Michael addition to a dibenzanthanthrene-1,7-dione is found, giving a product with three triisopropylsilylacetylene units and a remarkable solid-state structure (as determined by X-ray crystallography).

Synthesis and Aggregation Behavior of Chiral Naphthoquinoline Petroporphyrin Asphaltene Model Compounds.

The synthesis of structurally relevant compounds that model the chemical behavior and supramolecular aggregation of the asphaltenes, the most polar and metal-rich fraction of heavy petroleum, has been extended to include fusions of important petroleum biomarkers. The synthetic protocol features a multicomponent reaction to form a dyad composed of a fused steroidal naphthoquinoline, followed by a pyrrole cyclocondensation reaction to incorporate the dyad into a chiral triad containing a NiII -porphyrin substituent. This synthetic protocol has been used to prepare large molecules that represent both "continental" and "archipelago" models of asphaltene composition. The steroid-naphthoquinoline-porphyrin triads have been studied by UV/Vis and circular dichroism (CD) spectroscopies, and the results suggest that the naphthoquinoline core, a tetrahydro[4]helicene, adopts a helical conformation, producing a CD signal electronically related to the characteristic Soret absorption band of the porphyrin subunit. Finally, supramolecular aspects of asphaltene aggregation have been examined on a molecular level through analysis of axial coordination of pyridine to the Ni-porphyrin. The relative affinity of pyridine for binding to the Ni center of the porphyrin is evaluated by comparing binding propensities in a series of sterically differentiated substituted porphyrins.

Steroid-Derived Naphthoquinoline Asphaltene Model Compounds: Hydriodic Acid Is the Active Catalyst in I2-Promoted Multicomponent Cyclocondensation Reactions.

A multicomponent cyclocondensation reaction between 2-aminoanthracene, aromatic aldehydes, and 5-α-cholestan-3-one has been used to synthesize model asphaltene compounds. The active catalyst for this reaction has been identified as hydriodic acid, which is formed in situ from the reaction of iodine with water, while iodine is not a catalyst under anhydrous conditions. The products, which contain a tetrahydro[4]helicene moiety, are optically active, and the stereochemical characteristics have been examined by VT-NMR and VT-CD spectroscopies, as well as X-ray crystallography.

Aggregation of asphaltene model compounds using a porphyrin tethered to a carboxylic acid.

A Ni(II) porphyrin functionalized with an alkyl carboxylic acid (3) has been synthesized to model the chemical behavior of the heaviest portion of petroleum, the asphaltenes. Specifically, porphyrin 3 is used in spectroscopic studies to probe aggregation with a second asphaltene model compound containing basic nitrogen (4), designed to mimic asphaltene behavior. NMR spectroscopy documents self-association of the porphyrin and aggregation with the second model compound in solution, and a Job's plot suggests a 1 : 2 stoichiometry for compounds 3 and 4.

Scalable, chromatography-free synthesis of alkyl-tethered pyrene-based materials. Application to first-generation "archipelago model" asphaltene compounds.

In this paper, we report a highly efficient, scalable approach to the total synthesis of conformationally unrestricted, electronically isolated arrays of alkyl-tethered polycyclic aromatic chromophores. This new class of modular molecules consists of polycyclic aromatic "islands" comprising significant structural fragments present in unrefined heavy petroleum, tethered together by short saturated alkyl chains, as represented in the "archipelago model" of asphaltene structure. The most highly branched archipelago compounds reported here share an architecture with first-generation dendrimeric constructs, making the convergent, chromatography-free synthesis described herein particularly attractive for further extensions in scope and applications to materials chemistry. The syntheses are efficient, selective, and readily adaptable to a multigram scale, requiring only inexpensive, "earth-abundant" transition-metal catalysts for cross-coupling reactions and extraction and fractional crystallization for purification. This approach avoids typical limitations in cost, scale, and operational practicality. All of the archipelago compounds and synthetic intermediates have been fully characterized spectroscopically and analytically. The solid-state structure of one archipelago model compound has been determined by X-ray crystallography.

Hydrocarbon-soluble nanocatalysts with no bulk phase: coplanar, two-coordinate arrays of the base metals.

A structurally unique class of hydrocarbon-soluble, ancillary-ligand-free, tetrametallic Co(I) and Ni(I) clusters is reported. The highly unsaturated complexes are supported by simple, sterically bulky phosphoranimide ligands, one per metal. The electron-rich nitrogen centers are strongly bridging but sterically limited to bimetallic interactions. The hydrocarbon-soluble clusters consist of four coplanar metal centers, mutually bridged by single nitrogen atoms. Each metal center is monovalent, rigorously linear, and two-coordinate. The clusters are in essence two-dimensional atomic-scale "molecular squares," a structural motif adapted from supramolecular chemistry. Both clusters exhibit high solution-phase magnetic susceptibility at room temperature, suggesting the potential for applications in molecular electronics. Designed to be catalyst precursors, both clusters exhibit high activity for catalytic hydrogenation of unsaturated hydrocarbons at low pressure and temperature.

A density functional theory investigation of the cobalt-mediated η5-pentadienyl/alkyne [5+2] cycloaddition reaction: mechanistic insight and substituent effects.

Alkyl-substituted η(5)-pentadienyl half-sandwich complexes of cobalt have been reported to undergo [5+2] cycloaddition reactions with alkynes to provide η(2),η(3)-cycloheptadienyl complexes under kinetic control. DFT studies have been used to elucidate the mechanism of the cyclization reaction as well as that of the subsequent isomerization to the final η(5)-cycloheptadienyl product. The initial cyclization is a stepwise process of olefin decoordination/alkyne capture, C-C bond formation, olefin arm capture, and a second C-C bond formation; the initial decoordination/capture step is rate-limiting. Once the η(2),η(3)-cycloheptadienyl complex has been formed, isomerization to η(5)-cycloheptadienyl again involves several steps: olefin decoordination, ß-hydride elimination, reinsertion, and olefin coordination; also here the initial decoordination step is rate limiting. Substituents strongly affect the ease of reaction. Pentadienyl substituents in the 1- and 5-positions assist pentadienyl opening and hence accelerate the reaction, while substituents at the 3-position have a strongly retarding effect on the same step. Substituents at the alkyne (2-butyne vs. ethyne) result in much faster isomerization due to easier olefin decoordination. Paths involving triplet states do not appear to be competitive.

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.

Quasi-planar homopolymetallic and heteropolymetallic coordination arrays. Surface-like molecular clusters of magnesium and aluminum.

The sterically isolated preorganized tetradentate ligand systems, tetrakis(2-hydroxy-3-n-propylphenyl)ethene and tetrakis(5-tert-butyl-2-hydroxy-3-trimethylsilylphenyl)ethene, nucleate the formation of quasi-planar raft-like polymetallic coordination complexes with high selectivity, providing topologically consistent structural models for metal coordination to the "oxo-surface" of silica- and alumina-supported heterogeneous catalysts. The coordination of magnesium salts to these systems yields trimetallic magnesium halide and alkyl complexes arrayed on the oxygen "surface" of the ligand, regardless of the steric profile of the ortho-substituents. The magnesium complexes, characterized in the solid state by X-ray crystallography, contain two chemically distinct metal environments, a relatively inert central magnesium bis(alkoxide) and two more labile pseudotetrahedral "wing" magnesium atoms. The central metal coordination is pseudo-octahedral; crystallography strongly suggests the presence of an unprecedented dative magnesium-olefin bonding interaction from the metal to ethene bridge of the ligand. Consistent with the chemistry proposed for typical magnesium-treated catalyst supports, the labile wing magnesium centers can be cleanly and sequentially exchanged for aluminum with retention of the surface-like coordination array. Thus, treatment with diethylaluminum chloride provides heterotrimetallic magnesium-aluminum complexes containing one aluminum and two magnesium sites or two aluminum and one magnesium site, respectively. All four heteropolymetallic complexes have been characterized by X-ray crystallography.

Ortho-silylated derivatives of tetrakis(2-hydroxyphenyl)ethene: a sterically isolated structural model for oxo-surface binding domains.

The introduction of sterically isolating ortho-trialkylsilyl, -aryldialkylsilyl, and -diarylalkylsilyl substituents onto the structurally preorganized tetrakis(2-hydroxyphenyl)ethene ligand framework has been accomplished by a 4-fold retro-Brook rearrangement. Installation of the most sterically demanding silyl substituents required the development of an iterative procedure, involving successive double silylation/metalation/migration sequences without the isolation of intermediates. This system was designed to function as a soluble structural model for the planar binding domains of heterogeneous "oxo-surfaces" of silica and alumina supports.

Selective mono- and 1,2-difunctionalisation of cyclopentene derivatives via Mg and Cu intermediates.

A single Br/Mg exchange of 1,2-dibromocyclopentene with iPrMgCl LiCl provides the corresponding beta-bromocyclopentenylmagnesium reagent, which can then be reacted with various electrophiles (yields: 65-82 %). In the presence of a secondary alkylmagnesium halide and Li2CuCl4 (2 mol %), these 2-bromoalkenylmagnesium compounds undergo bromine substitution and can then further react with electrophiles to give 1,2-difunctionalised cyclopentenes (63-79 %). The mechanism of this process is discussed.

Synthesis of sterically hindered ortho-substituted tetraphenylethenes. Electronic effects in the McMurry olefination reaction.

[reaction: see text] Contrary to literature consensus, the McMurry olefination reaction can be extended to the direct synthesis of sterically encumbered tetrakis(2-substituted) tetraphenylethenes from the corresponding 2,2'-disubstituted benzophenones. The reaction exploits previously unrecognized substrate-based electronic effects that dominate over otherwise controlling steric considerations and provides highly efficient access to derivatives of tetrakis(2-hydroxyphenyl)ethene, a novel preorganized ligand system for polymetallic coordination chemistry and catalysis.

Unprecedented coordination modes and demetalation pathways for unbridged polyenyl ligands. Ruthenium eta1,eta4-cycloheptadienyl complexes from allyl/alkyne cycloaddition.

Cationic (eta6-hexamethylbenzene)ruthenium(II) mediates the [3 + 2 + 2] cycloaddition of allyl and alkyne ligands, leading to the unexpected isolation of eta1,eta4-cycloheptadienyl complexes, an unprecedented coordination mode for transition metal complexes of simple organic rings. The nonconjugated, eta1,eta4-coordinated complex is obtained as the kinetic reaction product from treatment of the unsubstituted allyl complex with excess ethyne; this complex rearranges slowly at 80 degrees C to the thermodynamically more stable conjugated eta5-cycloheptadienyl isomer. The eta1,eta4-coordinated isomer is fluxional at room temperature, undergoing rapid and reversible equilibration with a cycloheptatriene hydride intermediate via facile beta-hydride elimination/reinsertion. The reinsertion process is remarkably regioselective, returning the nonconjugated eta1,eta4-cycloheptadienyl isomer exclusively at room temperature. For reactions incorporating dimethylacetylene dicarboxylate (DMAD) as one or both of the alkyne components, eta1,eta4-coordination appears to be both kinetically and thermodynamically favored, despite undergoing equilibration among all possible eta1,eta4-cycloheptadienyl and cycloheptatriene hydride isomers prior to arriving at one observed eta1,eta4-isomer. For this series, no isomerization to eta5-coordination is observed even upon prolonged heating. In contrast, the cyclization incorporating both DMAD and phenylacetylene proceeds directly to the eta5-cycloheptadienyl isomer at or below room temperature, indicating that eta5-coordination remains energetically accessible to this system. The DMAD-based cyclization reactions produce structurally diverse minor byproducts, including both eta1,eta4-methanocyclohexadiene and acyclic eta3,eta2-heptadienyl isomers, which have been isolated and rigorously characterized. The unusual eta1,eta4-coordination of the seven-membered ring leads to unique new organic products upon oxidative demetalation by iodinolysis. Thus, reactions with excess iodine afford bridged tricyclic cyclopropane-containing lactones or substituted cycloheptatrienes in good but sometimes variable yields, depending on the substrate and specific reaction conditions. The ruthenium in these reactions is returned in high yield as the interesting cationic mu-triiodo pseudodimer of (eta6-hexamethylbenzene)ruthenium, which is obtained as a triiodide salt. This Ru(III) complex, along with several representative Ru(II) cyclization products, has been characterized in the solid state by X-ray crystallography.

Selective syntheses of partially etherified derivatives of tetrakis(2-hydroxyphenyl)ethene. An alternative to the calix[4]arene ligand system.

Stereoselective syntheses of (E)- and (Z)-1,2-bis(2'-hydroxyphenyl)-bis(2'-methoxyphenyl)ethene have been developed, the former by convergent coupling of an orthogonally protected 2,2'-benzophenone derivative and the latter by selective partial dealkylation of tetrakis(2-methoxyphenyl)ethene. Selective single demethylation has also been demonstrated in the 5-tert-butyl series. Thus, divalent and monovalent derivatives of the preorganized tetrakis(2-hydroxyphenyl)ethene ligand system are now available for use in coordination chemistry, analogous to corresponding calix[4]arene systems. [structure: see text]

Cobalt-mediated two-carbon ring expansion of five-membered rings. Electrophilic carbon-carbon bond activation in the synthesis of seven-membered rings.

Electrophilic cobalt(III) mediates an unprecedented two-carbon ring expansion of coordinated five-membered rings, leading to a remarkably general new strategy for the synthesis of seven-membered carbocycles from readily available five-membered ring substrates. The reaction, a metal-mediated [5 + 2] cyclopentenyl/alkyne cycloaddition, proceeds via initial protonation of a cobalt(I) cyclopentadiene complex, followed by rearrangement to an agostic eta3-cyclopentenyl intermediate. The cyclic eta3-allyl residue then undergoes migratory coupling with alkyne followed by carbon-carbon bond activation of the unstrained five-membered ring and recyclization to the ring expanded product, although the order of events and intimate mechanism has not been conclusively established. The reaction is highly selective with respect to which five-membered ring ligand undergoes activation, presumably a consequence of rapid cobalt-mediated interannular hydride transfer and kinetic preference for alkyne insertion into the less substituted cyclopentenyl ring. The alkyne insertion is itself highly regioselective, proceeding via migration to the sterically smaller end of the alkyne. The reaction is sensitive to both the cobalt counterion and the ancillary eta5-cyclopentadienyl substituent but proceeds for a considerable range of alkyl-, aryl-, and trialkylsilyl-substituted terminal and internal alkynes.