Michael J. Ingleson

Framework functionalisation triggers metal complex binding

Post-synthetic derivatisation of a porous material produces a functionalized material that binds the metal complex V(O)acac2, in contrast to the unfunctionalized precursor, which is inactive for complex binding.

N-Heterocyclic carbene chemistry of iron: fundamentals and applications

The use of N-heterocyclic carbenes (NHCs) in the chemistry of iron is stimulating important new applications of one of the most ubiquitous ligand types in modern organometallic chemistry. A series of reports has shown how the flexible and modifiable stereo-electronic properties of NHC ligands can be combined with iron in a range of oxidation states to create opportunities for studying unique structures, bonding and reactivity. Of particular interest are the roles of iron NHC complexes in: the stabilization of unusual oxidation states and coordination environments; the activation of small molecules; homogeneous catalysis; and bio-mimetic chemistry. Our feature article summarizes the key developments in the field.

Generation of a solid Brønsted acid site in a chiral framework

Protonation of chiral porous materials introduces a Brønsted acid centre, the structure of which is unique to the heterogeneous phase requiring pore wall confinement for stable isolation.

Mechanistic studies into amine-mediated electrophilic arene borylation and its application in MIDA boronate synthesis.

Direct electrophilic borylation using Y(2)BCl (Y(2) = Cl(2) or o-catecholato) with equimolar AlCl(3) and a tertiary amine has been applied to a wide range of arenes and heteroarenes. In situ functionalization of the ArBCl(2) products is possible with TMS(2)MIDA, to afford bench-stable and easily isolable MIDA-boronates in moderate to good yields. According to a combined experimental and computational study, the borylation of activated arenes at 20 °C proceeds through an S(E)Ar mechanism with borenium cations, [Y(2)B(amine)](+), the key electrophiles. For catecholato-borocations, two amine dependent reaction pathways were identified: (i) With [CatB(NEt(3))](+), an additional base is necessary to accomplish rapid borylation by deprotonation of the borylated arenium cation (σ complex), which otherwise would rather decompose to the starting materials than liberate the free amine to effect deprotonation. Apart from amines, the additional base may also be the arene itself when it is sufficiently basic (e.g., N-Me-indole). (ii) When the amine component of the borocation is less nucleophilic (e.g., 2,6-lutidine), no additional base is required due to more facile amine dissociation from the boron center in the borylated arenium cation intermediate. Borenium cations do not borylate poorly activated arenes (e.g., toluene) even at high temperatures; instead, the key electrophile in this case involves the product from interaction of AlCl(3) with Y(2)BCl. When an extremely bulky amine is used, borylation again does not proceed via a borenium cation; instead, a number of mechanisms are feasible including via a boron electrophile generated by coordination of AlCl(3) to Y(2)BCl, or by initial (heteroarene)AlCl(3) adduct formation followed by deprotonation and transmetalation.

Nitric Oxide Chemisorption in a Postsynthetically Modified Metal−Organic Framework

Postsynthetic metal-organic framework (MOF) derivatization introduces accessible secondary amine functionalities that react with nitric oxide (NO) to form N-diazenium diolates. This is in contrast to the parent MOF that binds NO essentially irreversibly at open metal coordination sites.

The Hydride‐Ion Affinity of Borenium Cations and Their Propensity to Activate H2 in Frustrated Lewis Pairs

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu(3))](+) has a lower hydride-ion affinity (HIA) than B(C(6)F(5))(3). This resulted in H(2) activation being energetically unfavourable in a FLP with the strong base PtBu(3). However, ligand disproportionation of CatBH(PtBu(3)) at 100 °C enabled trapping of H(2) activation products. DFT calculations at the M06-2X/6-311G(d,p)/PCM (CH(2)Cl(2)) level revealed that replacing catechol with chlorides significantly increases the chloride-ion affinity (CIA) and HIA. Dichloro-borenium cations, [Cl(2)B(amine)](+), were calculated to have considerably greater HIA than B(C(6)F(5))(3). Control reactions confirmed that the HIA calculations can be used to successfully predict hydride-transfer reactivity between borenium cations and neutral boranes. The borenium cations [Y(Cl)B(2,6-lutidine)](+) (Y = Cl or Ph) form FLPs with P(mesityl)(3) that undergo slow deprotonation of an ortho-methyl of lutidine at 20 °C to form the four-membered boracycles [(CH(2){NC(5)H(3)Me})B(Cl)Y] and [HPMes(3)](+). When equimolar [Y(Cl)B(2,6-lutidine)](+)/P(mesityl)(3) was heated under H(2) (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation.

Simple inexpensive boron electrophiles for direct arene borylation

Electrophilic direct borylation is facilitated, and arene substrate scope enhanced, by using electrophiles derived from inexpensive reagents; specifically an amine, BCl(3) and AlCl(3).

Fused polycyclic aromatics incorporating boron in the core: fundamentals and applications

The incorporation of boron into the core structure of fused polycyclic aromatics generates compounds with highly attractive properties that have recently received significant attention. Embedding boron into the backbone of ladder or 2D poly aromatic hydrocarbons is an underexplored approach to modulate optoelectronic properties, with tricoordinate boron representing a novel acceptor moiety for organic optoelectronic applications. Furthermore, the incorporation of boron into polycyclics containing other heteroatoms (e.g., chalcogens or pnictogens) leads to more extensive structural diversity and considerable ability to modify the frontier orbital energies and character, often in a controlled manner, to fine tune material properties for specific applications. This feature article summarizes the recent key developments in this field.

Haloboration of Internal Alkynes with Boronium and Borenium Cations as a Route to Tetrasubstituted Alkenes

Hail boration! 2-Dimethylaminopyridine-ligated dihaloborocations [X2B(2-DMAP)](+) with a strained four-membered boracycle were used for the haloboration of terminal and dialkyl internal alkynes (see scheme). Esterification then provided vinyl boronate esters as useful precursors to tetrasubstituted alkenes. Following mechanistic studies, the scope of the haloboration was expanded simply by variation of the amine. Pin = 2,3-dimethyl-2,3-butanedioxy.

N-Methylacridinium Salts: Carbon Lewis Acids in Frustrated Lewis Pairs for σ-Bond Activation and Catalytic Reductions†

N-methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4-(N,N-dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6-lutidine which activates H2, even in the presence of H2O. Anion effects dominate reactivity, with both solubility and rate of H2 cleavage showing marked anion dependency. With the optimal anion, a N-methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine-boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over-reduction to the alkanes.