Relationships Between Defectivity and Porosity in High Surface Area Porous Aromatic Frameworks.

Porous aromatic framework (PAF) microporosity is known to be strongly dependent on synthetic approach but little is known about why certain reactions yield significantly and consistently more porous PAFs. This article explores the connections between synthetic pathway, PAF defectivity, and microporosity. Using a network disassembly strategy, we show that defectivity is highly dependent on synthetic approach and that more defective PAFs are associated with lower surface areas and pore volumes. This empirical association is corroborated through systematic introduction of defects to a modelPAF, which results in significant reduction of apparent surface area and pore volumes. Taken together, these data suggest that only highly efficient coupling reactions should be targeted for the synthesis of ultra-high surface area porous aromatic frameworks.

Scrutinizing formally NiIV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory.

Nickel K- and L2,3-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L2,3-edge XAS reveals that the physical d-counts of the formally NiIV compounds measured lie well above the d6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF62- is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d6 NiIV center. The reactivity of NiIV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.

A Ni(COD)2-free approach for the synthesis of high surface area porous aromatic frameworks.

Porous aromatic frameworks (PAFs) are attractive materials for applications where high surface area and material stability govern performance. Most of the highest surface area PAFs are synthesized using poorly scalable and costly methods involving super-stoichiometric bis(1,5-cyclooctadiene)Nickel(0) (Ni(COD)2). This communication describes a general approach for the synthesis of high surface area PAFs that does not use isolated Ni(COD)2. The method is general to at least seven microporous polymers and can be conducted on gram scales without the use of an inert atmosphere glovebox. This work is expected to improve the synthetic accessibility of these materials.

Bioinspired chemistry at MOF secondary building units.

The secondary building units (SBUs) in metal-organic frameworks (MOFs) support metal ions in well-defined and site-isolated coordination environments with ligand fields similar to those found in metalloenzymes. This burgeoning class of materials has accordingly been recognized as an attractive platform for metalloenzyme active site mimicry and biomimetic catalysis. Early progress in this area was slowed by challenges such as a limited range of hydrolytic stability and a relatively poor diversity of redox-active metals that could be incorporated into SBUs. However, recent progress with water-stable MOFs and the development of more sophisticated synthetic routes such as postsynthetic cation exchange have largely addressed these challenges. MOF SBUs are being leveraged to interrogate traditionally unstable intermediates and catalytic processes involving small gaseous molecules. This perspective describes recent advances in the use of metal centers within SBUs for biomimetic chemistry and discusses key future developments in this area.

Catalytically Relevant Intermediates in the Ni-Catalyzed C(sp2)-H and C(sp3)-H Functionalization of Aminoquinoline Substrates.

This Article describes the synthesis and characterization of cyclometalated aminoquinoline NiII σ-aryl and σ-alkyl complexes that have been proposed as key intermediates in Ni-catalyzed C-H functionalization reactions. These NiII complexes serve as competent catalysts for the C-H functionalization of aminoquinoline derivatives with I2. They also react stoichiometrically with I2 to form either aryl iodides or ß-lactams within minutes at room temperature. Furthermore, they react with AgI salts at -30 °C to afford isolable five-coordinate NiIII species. The NiIII σ-aryl complexes proved inert toward C(sp2)-I bond-forming reductive elimination under all conditions examined (up to 140 °C in DMF). In contrast, a NiIII σ-alkyl analogue underwent C(sp3)-N bond-forming reductive elimination at 140 °C in DMF to afford a ß-lactam product. However, despite the ability of this latter NiIII species to participate in stoichiometric product formation, the complex was not a competent catalyst for ß-lactam formation. Overall, these results suggest against the intermediacy of NiIII species in these C-H functionalization reactions.

Mechanism and Scope of Nickel-Catalyzed Decarbonylative Borylation of Carboxylic Acid Fluorides.

This Article describes the development of a base-free, nickel-catalyzed decarbonylative coupling of carboxylic acid fluorides with diboron reagents to selectively afford aryl boronate ester products. Detailed studies were conducted to assess the relative rates of direct transmetalation between aryl boronate esters and diboron reagents and a bisphosphine nickel(aryl)(fluoride) intermediate. These investigations revealed that diboron reagents undergo transmetalation with this Ni(aryl)(fluoride) intermediate at rates significantly faster than their aryl boronate ester congeners. Furthermore, the reactivity of both boron reagents toward transmetalation is enhanced with increasing electrophilicity of the boron center. These mechanistic insights were leveraged to develop a catalytic decarbonylative borylation of acid fluorides that proved applicable to a variety of (hetero)aryl carboxylic acid fluorides as well as diverse diboron reagents. The acid fluorides can be generated in situ directly from carboxylic acids. Furthermore, the mechanistic studies directed the identification of various air-stable Ni pre-catalysts for this transformation.

Connecting Organometallic Ni(III) and Ni(IV): Reactions of Carbon-Centered Radicals with High-Valent Organonickel Complexes.

This paper describes the one-electron interconversions of isolable NiIII and NiIV complexes through their reactions with carbon-centered radicals (R•). First, model NiIII complexes are shown to react with alkyl and aryl radicals to afford NiIV products. Preliminary mechanistic studies implicate a pathway involving direct addition of a carbon-centered radical to the NiIII center. This is directly analogous to the known reactivity of NiII complexes with R•, a step that is commonly implicated in catalysis. Second, a NiIV-CH3 complex is shown to react with aryl and alkyl radicals to afford C-C bonds via a proposed SH2-type mechanism. This pathway is leveraged to enable challenging H3C-CF3 bond formation under mild conditions. Overall, these investigations suggest that NiII/III/IV sequences may be viable redox pathways in high-oxidation-state nickel catalysis.

Palladium-Catalyzed Difluoromethylation of Aryl Chlorides and Bromides with TMSCF2H.

A palladium-catalyzed cross-coupling of aryl chlorides/bromides with TMSCF2H is described. Two different catalysts, Pd(dba)2/BrettPhos and Pd(P tBu3)2, are demonstrated and provide a variety of difluoromethylated arenes in good yields.

Base-free nickel-catalysed decarbonylative Suzuki-Miyaura coupling of acid fluorides.

The Suzuki-Miyaura cross-coupling of organoboron nucleophiles with aryl halide electrophiles is one of the most widely used carbon-carbon bond-forming reactions in organic and medicinal chemistry1,2. A key challenge associated with these transformations is that they generally require the addition of an exogenous base, the role of which is to enable transmetallation between the organoboron nucleophile and the metal catalyst3. This requirement limits the substrate scope of the reaction because the added base promotes competitive decomposition of many organoboron substrates3-5. As such, considerable research has focused on strategies for mitigating base-mediated side reactions6-12. Previous efforts have primarily focused either on designing strategically masked organoboron reagents (to slow base-mediated decomposition)6-8 or on developing highly active palladium precatalysts (to accelerate cross-coupling relative to base-mediated decomposition pathways)10-12. An attractive alternative approach involves identifying combinations of catalyst and electrophile that enable Suzuki-Miyaura-type reactions to proceed without an exogenous base12-14. Here we use this approach to develop a nickel-catalysed coupling of aryl boronic acids with acid fluorides15-17, which are formed in situ from readily available carboxylic acids18-22. This combination of catalyst and electrophile enables a mechanistic manifold in which a 'transmetallation-active' aryl nickel fluoride intermediate is generated directly in the catalytic cycle13,16. As such, this transformation does not require an exogenous base and is applicable to a wide range of base-sensitive boronic acids and biologically active carboxylic acids.

Stoichiometric and Catalytic Aryl-Perfluoroalkyl Coupling at Tri-tert-butylphosphine Palladium(II) Complexes.

This Communication describes studies of Ph-RF (RF = CF3 or CF2CF3) coupling at Pd complexes of general structure (PtBu3)PdII(Ph)(RF). The CF3 analogue participates in fast Ph-CF3 coupling (<5 min at 80 °C). However, the formation of side products limits the yield of this transformation as well as its translation to catalysis. DFT and experimental studies suggest that the side products derive from facile α-fluoride elimination at the 3-coordinate PdII complex. Furthermore, they show that this undesired pathway can be circumvented by changing from a CF3 to a CF2CF3 ligand. Ultimately, the insights gained from stoichiometric studies enabled the identification of Pd(PtBu3)2 as a catalyst for the Pd-catalyzed cross-coupling of aryl bromides with TMSCF2CF3 to afford pentafluoroethylated arenes.

Carbon-Carbon Bond-Forming Reductive Elimination from Isolated Nickel(III) Complexes.

This manuscript describes the design, synthesis, characterization, and reactivity studies of organometallic NiIII complexes of general structure TpNiIII(R)(R1) (Tp = tris(pyrazolyl)borate). With appropriate selection of the R and R1 ligands, the complexes are stable at room temperature and can be characterized by cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. Upon heating, many of these NiIII compounds undergo C(sp2)-C(sp2) or C(sp3)-C(sp2) bond-forming reactions that are challenging at lower oxidation states of nickel.

Oxidation of Ni(II) to Ni(IV) with Aryl Electrophiles Enables Ni-Mediated Aryl-CF3 Coupling.

This communication describes the synthesis and reactivity of Ni(IV)(aryl)(CF3)2 complexes supported by trispyrazolylborate and 4,4'-di-tert-butylbipyridine ligands. We demonstrate that isolable Ni(IV) complexes can be accessed under mild conditions via the oxidation of Ni(II) precursors with S-(trifluoromethyl)dibenzothiophenium triflate as well as with diaryliodonium and aryl diazonium reagents. The Ni(IV) intermediates undergo high yielding aryl-CF3 bond-forming reductive elimination. These studies support the potential viability of Ni(IV) intermediates in nickel-catalyzed coupling reactions involving diaryliodonium and aryldiazonium electrophiles.

Steric and electronic effects influencing ß-aryl elimination in the Pd-catalyzed carbon-carbon single bond activation of triarylmethanols.

An analysis of the palladium-catalyzed activation of carbon-carbon single bonds within triarylmethanols has led to a greater understanding of factors influencing the ß-aryl elimination process responsible for C-C bond cleavage. A series of competition reactions were utilized to determine that ß-aryl elimination of aryl substituents containing ortho-substitution proceeds with significant preference to unsubstituted phenyl rings. Further experiments indicate that substrates containing either strongly donating or withdrawing substituents are cleaved from triarylmethanols more readily than relatively neutral species.