Natalie E. Pridmore

Enhanced photocatalytic hydrogen generation using polymorphic macroporous TaON.

Macroporous TaON (mac-TaON) is prepared using polymer sphere templating and controlled ammonolysis. In contrast to typical powder synthesis, which gives the β polymorph, mac-TaON is a mixture of β and γ polymorphs. mac-TaON shows twice the activity for photocatalytic hydrogen generation in comparison to mac-TaON when normalised for surface area.

Manganese(I)‐Catalyzed C−H Activation: The Key Role of a 7‐Membered Manganacycle in H‐Transfer and Reductive Elimination

Abstract Manganese‐catalyzed C−H bond activation chemistry is emerging as a powerful and complementary method for molecular functionalization. A highly reactive seven‐membered MnI intermediate is detected and characterized that is effective for H‐transfer or reductive elimination to deliver alkenylated or pyridinium products, respectively. The two pathways are determined at MnI by judicious choice of an electron‐deficient 2‐pyrone substrate containing a 2‐pyridyl directing group, which undergoes regioselective C−H bond activation, serving as a valuable system for probing the mechanistic features of Mn C−H bond activation chemistry.

Iridium(III) hydrido N-heterocyclic carbene-phosphine complexes as catalysts in magnetization transfer reactions.

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.

Halogen- and Hydrogen-Bonded Salts and Co-crystals Formed from 4-Halo-2,3,5,6-tetrafluorophenol and Cyclic Secondary and Tertiary Amines: Orthogonal and Non-orthogonal Halogen and Hydrogen Bonding, and Synthetic Analogues of Halogen-Bonded Biological Systems

Co-crystallisation of, in particular, 4-iodotetrafluorophenol with a series of secondary and tertiary cyclic amines results in deprotonation of the phenol and formation of the corresponding ammonium phenate. Careful examination of the X-ray single-crystal structures shows that the phenate anion develops a C=O double bond and that the C–C bond lengths in the ring suggest a Meissenheimer-like delocalisation. This delocalisation is supported by the geometry of the phenate anion optimised at the MP2(Full) level of theory within the aug-cc-pVDZ basis (aug-cc-pVDZ-PP on I) and by natural bond orbital (NBO) analyses. With sp2 hybridisation at the phenate oxygen atom, there is strong preference for the formation of two non-covalent interactions with the oxygen sp2 lone pairs and, in the case of secondary amines, this occurs through hydrogen bonding to the ammonium hydrogen atoms. However, where tertiary amines are concerned, there are insufficient hydrogen atoms available and so an electrophilic iodine atom from a neighbouring 4-iodotetrafluorophenate group forms an I⋅⋅⋅O halogen bond to give the second interaction. However, in some co-crystals with secondary amines, it is also found that in addition to the two hydrogen bonds forming with the phenate oxygen sp2 lone pairs, there is an additional intermolecular I⋅⋅⋅O halogen bond in which the electrophilic iodine atom interacts with the C=O π-system. All attempts to reproduce this behaviour with 4-bromotetrafluorophenol were unsuccessful. These structural motifs are significant as they reproduce extremely well, in low-molar-mass synthetic systems, motifs found by Ho and co-workers when examining halogen-bonding interactions in biological systems. The analogy is cemented through the structures of co-crystals of 1,4-diiodotetrafluorobenzene with acetamide and with N-methylbenzamide, which, as designed models, demonstrate the orthogonality of hydrogen and halogen bonding proposed in Ho’s biological study.

Competition and cooperation: hydrogen and halogen bonding in co-crystals involving 4-iodotetrafluorobenzoic acid, 4-iodotetrafluorophenol and 4-bromotetrafluorophenol

Co-crystallisation of 4-iodotetrafluorobenzoic acid with 1,4-dithiane leads to a polymer in which the benzoic acid dimer remains intact, forming halogen bonds with the dithiane, showing that considering both pKb and the iodine basicity scale (pKBI2) can direct the structure formed. Crystallisation of 4-halotetrafluorophenols with 1,4-dithiane or tetrahydrothiophene was always accompanied by oxidation to the S-oxide or S,S′-dioxide and in two cases the co-crystals formed involved hydrogen bonding (4-bromotetrafluorophenol) and both hydrogen and halogen bonding (4-iodotetrafluorophenol). Co-crystallisation of 4-iodotetrafluorophenol with 4,4′-bipyridine leads to a linear polymer with both hydrogen and halogen bonding.

Outer-Sphere Electrophilic Fluorination of Organometallic Complexes.

Organofluorine chemistry plays a key role in materials science, pharmaceuticals, agrochemicals, and medical imaging. However, the formation of new carbon-fluorine bonds with controlled regiochemistry and functional group tolerance is synthetically challenging. The use of metal complexes to promote fluorination reactions is of great current interest, but even state-of-the-art approaches are limited in their substrate scope, often require activated substrates, or do not allow access to desirable functionality, such as alkenyl C(sp(2))-F or chiral C(sp(3))-F centers. Here, we report the formation of new alkenyl and alkyl C-F bonds in the coordination sphere of ruthenium via an unprecedented outer-sphere electrophilic fluorination mechanism. The organometallic species involved are derived from nonactivated substrates (pyridine and terminal alkynes), and C-F bond formation occurs with full regio- and diastereoselectivity. The fluorinated ligands that are formed are retained at the metal, which allows subsequent metal-mediated reactivity.

Addition of a Cyclophosphine to Nitriles: An Inorganic Click Reaction Featuring Protio, Organo, and Main‐Group Catalysis

The addition of a cyclotriphosphine to a broad range of nitriles gives access to the first examples of free 1-aza-2,3,4-triphospholenes in a rapid, ambient temperature, one-pot, high-yield protocol. The reaction produces electron-rich heterocycles (four lone pairs) and features homoatomic σ-bond heterolysis, thereby combining the key features of the 1,3-dipolar cycloaddition chemistry of azides and cyclopropanes. Also reported is the first catalytic addition of P-P bonds to the C≡N bond. The coordination chemistry of the new heterocycles is explored.

Influence of Ring Strain and Bond Polarization on the Ring Expansion of Phosphorus Homocycles

Heterolytic cleavage of homoatomic bonds is a challenge, as it requires separation of opposite charges. Even highly strained homoatomic rings (e.g., cyclopropane and cyclobutane) are kinetically stable and do not react with nucleophiles or electrophiles. In contrast, cycloalkanes bearing electron-donating/withdrawing substituents on adjacent carbons have polarized C-C bonds and undergo numerous heterolytic ring-opening and expansion reactions. Here we show that upon electrophile activation phosphorus homocycles exhibit analogous reactivity, which is modulated by the amount of ring strain and extent of bond polarization. Neutral rings (tBuP)3, 1, or (tBuP)4, 2, show no reactivity toward nitriles, but the cyclo-phosphinophosphonium derivative [(tBuP)3Me]+, [3Me]+, undergoes addition to nitriles giving five-membered P3CN heterocycles. Because of its lower ring strain, the analogous four-membered ring, [(tBuP)4Me]+, [4Me]+, is thermodynamically stable with respect to cycloaddition with nitriles, despite similar P-P bond polarization. We also report the first example of isonitrile insertion into cyclophosphines, which is facile for polarized derivatives [3Me]+ and [4Me]+, but does not proceed for neutral 1 or 2, despite the calculated exothermicity of the process. Finally, we assessed the reactions of [4R]+ R = H, Cl, F toward 4-dimethylaminopyridine (dmap), which suggest that the site of nucleophilic attack varies with the extent of P-P bond polarization. These results deconvolute the influence of ring strain and bond polarization on the chemistry of inorganic homocycles and unlock new synthetic possibilities.

A Structurally Characterized Fluoroalkyne

The facile synthesis of a stable and isolable compound with a fluoroalkynyl group, M-C≡CF, is reported. Reaction of [Ru(C≡CH)(η5 -C5 Me5 )(dppe)] with an electrophilic fluorinating agent (NFSI) results in the formation of the fluorovinylidene complex [Ru(=C=CHF)(η5 -C5 Me5 )(dppe)][N(SO2 Ph)2 ]. Subsequent deprotonation with LiN(SiMe3 )2 affords the fluoroalkynyl complex [Ru(C≡CF)(η5 -C5 Me5 )(dppe)]. In marked contrast to the rare and highly reactive examples of fluoroalkynes that have been reported previously, this compound can be readily isolated and structurally characterized. This has allowed the structure and bonding in the CCF motif to be explored. Further electrophilic fluorination of this species yields the difluorovinylidene complex [Ru(C=CF2 )(η5 -C5 Me5 )(dppe)][N(SO2 Ph)2 ].

Lamotrigine ethanol monosolvate

The main motif within the structure is a lamotrigine dimer stabilized by two ethanol molecules. Here the lamotrigine dimer forms using amines in the ortho position of the triazine group.