Catalytic Access to Diastereometrically Pure Four- and Five-Membered Silyl-Heterocycles Using Transborylation.

Silyl-heterocycles offer a unique handle to expand and explore chemical space, reactivity, and functionality. The shortage of catalytic methods for the preparation of diverse and functionalized silyl-heterocycles however limits widespread exploration and exploitation. Herein the borane-catalyzed intramolecular 1,1-carboboration of silyl-alkynes has been developed for the synthesis of 2,3-dihydrosilolyl and silylcyclobut-2-enyl boronic esters. Successful, catalytic carboboration has been achieved on a variety of functionally diverse silyl-alkynes, using a borane catalyst and transborylation-enabled turnover. Mechanistic studies, including 13C-labelling, computational studies, and single-turnover experiments, suggest a reaction pathway proceeding by 1,2-hydroboration, 1,1-carboboration, and transborylation to release the alkenyl boronic ester product and regenerate the borane catalyst.

Transition Metal-Free Catalytic C-H Zincation and Alumination.

C-H metalation is the most efficient method to prepare aryl-zinc and -aluminium complexes that are ubiquitous nucleophiles. Virtually all C-H metalation routes to form Al/Zn organometallics require stoichiometric, strong Brønsted bases with no base-catalyzed reactions reported. Herein we present a catalytic in amine/ammonium salt (Et3N/[(Et3N)H]+) C-H metalation process to form aryl-zinc and aryl-aluminium complexes. Key to this approach is coupling an endergonic C-H metalation step with a sufficiently exergonic dehydrocoupling step between the ammonium salt by-product of C-H metalation ([(Et3N)H]+) and a Zn-H or Al-Me containing complex. This step, forming H2/MeH, makes the overall cycle exergonic while generating more of the reactive metal electrophile. Mechanistic studies supported by DFT calculations revealed metal-specific dehydrocoupling pathways, with the divergent reactivity due to the different metal valency (which impacts the accessibility of amine-free cationic metal complexes) and steric environment. Notably, dehydrocoupling in the zinc system proceeds through a ligand-mediated pathway involving protonation of the ß-diketiminate Cγ position. Given this process is applicable to two disparate metals (Zn and Al), other main group metals and ligand sets are expected to be amenable to this transition metal-free, catalytic C-H metalation.

The Sensitivity of Structure to Ionic Radius and Reaction Stoichiometry: A Crystallographic Study of Metal Coordination and Hydrogen Bonding in Barbiturate Complexes of All Five Alkali Metals Li-Cs.

A systematic study has been conducted on barbiturate complexes of all five alkali metals, Li-Cs, prepared from metal carbonates or hydroxides in an aqueous solution without other potential ligands present, varying the stoichiometric ratio of metal ion to barbituric acid (BAH). Eight polymeric coordination compounds (two each for Na, K, and Rb and one each for Li and Cs) have been characterised by single-crystal X-ray diffraction. All contain some combination of barbiturate anion BA- (necessarily in a 1:1 ratio with the metal cation M+), barbituric acid, and water. All organic species and water molecules are coordinated to the metal centres via oxygen atoms as either terminal or bridging ligands. Coordination numbers range from 4 (for the Li complex) to 8 (for the Cs complex). Extensive hydrogen bonding plays a significant role in all the crystal structures, almost all of which include pairs of N-H···O hydrogen bonds linking BA- and/or BAH components into ribbons extending in one dimension. Factors influencing the structure adopted by each compound include cation size and reaction stoichiometry as well as hydrogen bonding.

Shape-Selective Supramolecular Capsules for Actinide Precipitation and Separation.

Improving actinide separations is key to reducing barriers to medical and industrial actinide isotope production and to addressing the challenges associated with the reprocessing of spent nuclear fuel. Here, we report the first example of a supramolecular anion recognition process that can achieve this goal. We have designed a preorganized triamidoarene receptor that induces quantitative precipitation of the early actinides Th(IV), Np(IV), and Pu(IV) from industrially relevant conditions through the formation of self-assembled hydrogen-bonded capsules. Selectivity over the later An(III) elements is shown through modulation of the nitric acid concentration, and no precipitation of actinyl or transition-metal ions occurs. The Np, Pu, and Am precipitates were characterized structurally by single-crystal X-ray diffraction and reveal shape specificity of the internal hydrogen-bonding array for the encapsulated hexanitratometalates. This work complements ion-exchange resins for 5f-element separations and illustrates the significant potential of supramolecular separation methods that target anionic actinide species.

Structure-Property Relationships for Potential Inversion From Electron Acceptors Based on Thiophene-Fused Triptycene Quinones, 1,4-Diketones and Their Malononitrile Adducts.

The synthesis and properties of a series of 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) inspired electron acceptors based on thiophene-fused quinone and triptycene motifs is presented. This has yielded insights into structure-property relationships for establishing and modulating simultaneous two-electron reduction processes in TCAQ analogues. These new compounds were synthesised using a Friedel-Crafts acylation between triptycene and thiophene-3,4-dicarbonyl chloride. Isomeric para-quinones featuring a [c]-fused thiophene on one side and a ß,ß- or α,ß-fused triptycene on the other were isolated alongside a thiophene-3,4-diketone which bears two triptycene fragments. Knoevenagel condensation of these products with malononitrile produced a quinoidal bis(dicyanomethylene), an oxo-dicyanomethylene and an acyclic bis(dicyanomethylene). This series of new electron accepting molecules has been studied using X-ray crystallography and the implications of their 3D structures on NMR and UV/vis absorbance spectroscopy and cyclic voltammetry results have been ascertained with conclusions underpinned by computational methods.

NacNac-zinc-pyridonate mediated ε-caprolactone ROP.

Herein we report the synthesis, isolation and polymerisation activity of two new zinc compounds based on a 2,6-diisopropylphenyl (Dipp) ß-diiminate (NacNac) ligand framework with zinc also ligated by an amidate (2-pyridonate or 6-methyl-2-pyridonate) unit. The compounds crystallised as either monomeric (6-Me-2-pyridonate derivative) or dimeric (2-pyridonate) species, although both were found to be monomeric in solution via1H DOSY NMR spectroscopy, which was supported by DFT calculations. These observations suggest that both complexes initiate ring-opening polymerisation (ROP) through a single-site monometallic mechanism. High molecular weight poly ε-caprolactone (PCL) was achieved via exogenous initiator-free ROP conditions with both catalysts. An increase in the 2-pyridonate initiator steric bulk (6-Me- vs. 6-H-) resulted in an improved catalytic activity, facilitating complete monomer conversion within 1 h at 60 °C. Pyridonate end-groups were observed by MALDI-ToF mass spectrometry, contrasting with previous observations for DippNacNac-Zn acetate complexes (where no acetate end groups are observed), instead this more closely resembles the reactivity of DippNacNac-Zn alkoxide complexes in ROP (where RO end groups are observed). Additional major signals in the MALDI-ToF spectra were consistent with cyclic PCL species, which are attributed to back-biting ring-closing termination steps occuring in a process facilitated by the pyridonate unit being an effective leaving group. To the best of our knowledge, these complexes represent the first examples of pyridonate, and indeed amidate, initated ROP.

Odd and Even Numbered Ferric Wheels.

The structurally related odd and even numbered wheels [FeIII 11 ZnII 4 (tea)10 (teaH)1 (OMe)Cl8 ] (1) and [FeIII 12 ZnII 4 (tea)12 Cl8 ] (2) can be synthesized under ambient conditions by reacting FeIII and ZnII salts with triethanolamine (teaH3 ), the change in nuclearity being dictated by the solvents employed. An antiferromagnetic exchange between nearest neighbors, J = -10.0 cm-1 for 1 and J = -12.0 cm-1 for 2, leads to a frustrated S = 1/2 ground state in the former and an S = 0 ground state in the latter.

Experimental Quantification of Halogen⋠⋠⋠Arene van der Waals Contacts.

Crystallographic and computational studies suggest the occurrence of favourable interactions between polarizable arenes and halogen atoms. However, the systematic experimental quantification of halogen⋅⋅⋅arene interactions in solution has been hindered by the large variance in the steric demands of the halogens. Here we have synthesized molecular balances to quantify halogen⋅⋅⋅arene contacts in 17 solvents and solvent mixtures using 1 H NMR spectroscopy. Calculations indicate that favourable halogen⋅⋅⋅arene interactions arise from London dispersion in the gas phase. In contrast, comparison of our experimental measurements with partitioned SAPT0 energies indicate that dispersion is sufficiently attenuated by the solvent that the halogen⋅⋅⋅arene interaction trend was instead aligned with increasing exchange repulsion as the halogen increased in size (ΔGX ⋅⋅⋅Ph =0 to +1.5 kJ mol-1 ). Halogen⋅⋅⋅arene contacts were slightly less disfavoured in solvents with higher solvophobicities and lower polarizabilities, but strikingly, were always less favoured than CH3 ⋅⋅⋅arene contacts (ΔGMe ⋅⋅⋅Ph =0 to -1.4 kJ mol-1 ).

Tetra-, tetradeca- and octadecametallic clusters of Mn.

Reaction of equimolar amounts of MnBr2·4H2O and HL1 ((3,5-dimethyl-1H-pyrazol-1-yl)methanol) in a basic MeCN solution leads to the formation of [MnII4(L1)4Br4(H2O)4] (1), whose metallic skeleton is a [MnII4] tetrahedron and cluster core a [MnII4(µ3-O)4] cubane. Replacing MnBr2·4H2O with Mn(O2CMe)2·4H2O affords [MnIII2MnII12O2(L1)4(OAc)22] (2) which is best described as a series of edge-sharing [Mn4] tetrahedra that have self-assembled into a linear array in which each [Mn2] pair is 'twisted' with respect to its neighbours in a corkscrew-like manner. Employment of the triangle [MnIII3O(OAc)6(py)3](ClO4) as a reactant instead of a MnII salt results in the formation of [MnIII14MnII4O14(L1)4(HL1)2(OAc)18(H2O)2] (3) whose core is comprised of three vertex-sharing [MnIII4] butterflies flanked on either side by one [MnIII4] cubane and one [MnIII2MnII2] tetrahedron. Dc magnetic susceptibility and magnetisation measurements of polycrystalline samples of 1-3 reveal the predominance of antiferromagnetic exchange interactions. For [Mn4] (1) this leads to a diamagnetic ground state, while for [Mn18] (3) competing exchange interactions result in Single-Molecule Magnet (SMM) behaviour with Ueff = 22 K.

Amides as modifiable directing groups in electrophilic borylation.

Amide directed C-H borylation using ≥two equiv. of BBr3 forms borenium cations containing a R2N(R')C[double bond, length as m-dash]O→B(Ar)Br unit which has significant Lewis acidity at the carbonyl carbon. This enables reduction of the amide unit to an amine using hydrosilanes. This approach can be applied sequentially in a one-pot electrophilic borylation-reduction process, which for phenyl-acetylamides generates ortho borylated compounds that can be directly oxidised to the 2-(2-aminoethyl)-phenol. Other substrates amenable to the C-H borylation-reduction sequence include mono and diamino-arenes and carbazoles. This represents a simple method to make borylated molecules that would be convoluted to access otherwise (e.g. N-octyl-1-BPin-carbazole). Substituent variation is tolerated at boron as well as in the amide unit, with diarylborenium cations also amenable to reduction. This enables a double C-H borylation-reduction-hydrolysis sequence to access B,N-polycyclic aromatic hydrocarbons (PAHs), including an example where both the boron and nitrogen centres contain functionalisable handles (N-H and B-OH). This method is therefore a useful addition to the metal-free borylation toolbox for accessing useful intermediates (ArylBPin) and novel B,N-PAHs.

Influence of alkali metal cations on the formation of the heterobimetallic actinide tert-butoxides [AnM3(OtBu)7] and [AnM2(OtBu)6] (AnIV = Th, U; MI = Li, Na, K, Rb, Cs).

Heterobimetallic tert-butoxides of alkali metal cations with tetravalent actinide centers exhibit two distinctive structural motifs, [AnM2(OtBu)6] and [AnM3(OtBu)7] (AnIV = Th, U and MI = Li, Na, K, Rb, Cs), evidently governed by the size of the alkali metal ions. Both [AnM3(OtBu)7] AnM3 (AnIV = U, MI = Li; AnIV = Th, MI = Li, Na) and [AnM2(OtBu)6] AnM2 (AnIV = U, MI = Na-Cs; AnIV = Th, MI = K-Cs) compounds are obtained in nearly quantitative yields by reacting actinide and alkali metal silyl amides with an excess of tert-butyl alcohol. The AnM3 complexes form a cubane-type coordination motif, whereas the AnM2 complexes display a geometry resembling two face-shared bipyramids. The sodium derivatives of thorium and uranium (ThNa3 and UNa2) allow the determination of the structural transition threshold as a function of the ratio of the ionic radii ri(AnIV)/ri(MI). The AnM3 complexes are formed for ratios above 0.92 and the AnM2 type is formed for ratios below 0.87. All compounds are unambiguously characterized in both solution and solid states by NMR and IR spectroscopic studies and single crystal X-ray diffraction analyses, respectively.

Utilizing Raman Spectroscopy as a Tool for Solid- and Solution-Phase Analysis of Metalloorganic Cage Host-Guest Complexes.

The host-guest chemistry of coordination cages continues to promote significant interest, not least because confinement effects can be exploited for a range of applications, such as drug delivery, sensing, and catalysis. Often a fundamental analysis of noncovalent encapsulation is required to provide the necessary insight into the design of better functional systems. In this paper, we demonstrate the use of various techniques to probe the host-guest chemistry of a novel Pd2L4 cage, which we show is preorganized to selectively bind dicyanoarene guests with high affinity through hydrogen-bonding and other weak interactions. In addition, we exemplify the use of Raman spectroscopy as a tool for analyzing coordination cages, exploiting alkyne and nitrile reporter functional groups that are contained within the host and guest, respectively.

Iron-catalysed alkene and heteroarene H/D exchange by reversible protonation of iron-hydride intermediates.

C-H functionalisation reactions offer a sustainable method for molecular construction and diversification. These reactions however remain dominated by precious metal catalysis. While significant interest in iron-catalysed C-H activation reactions has emerged, the isolation, characterisation and mechanistic understanding of these processes remain lacking. Herein the iron-catalysed C(sp2)-H bond hydrogen/deuterium exchange reaction using CD3OD is reported for both heterocycles and, for the first time, alkenes (38 examples). Isolation and characterisation, including by single-crystal X-ray diffraction, of the key iron-aryl and iron-alkenyl C-H metallation intermediates provided evidence for a reversible protonation of the active iron hydride catalyst. Good chemoselectivity was observed for both substrate classes. The developed procedure is orthogonal to previous iron-catalysed H/D exchange methods which used C6D6, D2, or D2O as the deuterium source, and uses only bench-stable reagents, including the iron(ii) pre-catalyst. Further, a new mechanism of iron-hydride formation is reported in which ß-hydride elimination from an alcohol generates the iron hydride. The ability to produce, isolate and characterise the organometallic products arising from C-H activation presents a basis for future discovery and development.

Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles.

Dimeric aminoboranes, [H2BNR2]2 (R = Me or CH2CH2) containing B2N2 cores, can be activated by I2, HNTf2 (NTf2 = [N(SO2CF3)2]), or [Ph3C][B(C6F5)4] to form isolable H2B(µ-NR2)2BHX (for X = I or NTf2). For X = [B(C6F5)4]- further reactivity, presumably between [H2B(µ-NMe2)2BH][B(C6F5)4] and aminoborane, forms a B3N3-based monocation containing a three-center two electron B-(µ-H)-B moiety. The structures of H2B(µ-NMe2)2BH(I) and [(µ-NMe2)BH(NTf2)]2 indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf2 binding. While the iodide congener reacted very slowly with alkynes, the NTf2 analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)2BNR2 species and R2NBH(NTf2) as the major products. Further studies indicated that the B2N2 core is maintained during the first hydroboration, and that it is during subsequent steps that B2N2 dissociation occurs. In the mono-boron systems, for example, i Pr2NBH(NTf2), NTf2 is N-bound; thus, they have less steric crowding around boron relative to the B2N2 systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B2N2-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B2N2 electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H2B(µ-N2C3H3)]2 analogues, making them less useful for the directed diborylation of a single substrate.

Boron-Catalyzed, Diastereo- and Enantioselective Allylation of Ketones with Allenes.

The diastereo- and enantioselective allylation of ketones remains a synthetic challenge, with transition metal catalysis offering the most applied methods. Here, a boron-catalyzed allylation of ketones with allenes is presented. Excellent yield, regioselectivity, and diastereoselectivity were found across functionalized substrates. The reaction was further developed to accommodate an enantioenriched boron catalyst and thus gave asymmetric ketone allylation in good yield, diastereoselectivity, and enantioselectivity. Mechanistic studies supported a hydroboration-allylation-transborylation pathway.

Diastereoselective, Catalytic Access to Cross-Aldol Products Directly from Esters and Lactones.

High oxidation-state carbonyl coupling partners including esters and lactones were reacted with enones to give aldol-type products directly using two-fold organoborane catalysis. This new retrosynthetic disconnection to aldol-type products is compatible with enolisable coupling partners, without self-condensation, and couples the high reactivity of secondary dialkylboranes with the stability of pinacolboronic esters. Excellent chemoselectivity, substrate scope (including those containing reducible functionalities and free alcohols) and diastereocontrol were achieved to access both the syn- and anti-aldol-type products. Mechanistic studies confirmed the two-fold catalytic role of the single secondary borane catalyst for boron enolate formation and formation of an aldehyde surrogate from the ester or lactone coupling partner.

A graceful break-up: serendipitous self-assembly of a ferromagnetically coupled [NiII14] wheel.

The complex [NiII14(HL2)12(HCOO)14Cl14(MeOH)(H2O)] describes an aesthetically pleasing wheel displaying ferromagnetic nearest neighbour exchange.

Dissecting Solvent Effects on Hydrogen Bonding.

The experimental isolation of H-bond energetics from the typically dominant influence of the solvent remains challenging. Here we use synthetic molecular balances to quantify amine/amide H-bonds in competitive solvents. Over 200 conformational free energy differences were determined using 24 H-bonding balances in 9 solvents spanning a wide polarity range. The correlations between experimental interaction energies and gas-phase computed energies exhibited wild solvent-dependent variation. However, excellent correlations were found between the same computed energies and the experimental data following empirical dissection of solvent effects using Hunter's α/ß solvation model. In addition to facilitating the direct comparison of experimental and computational data, changes in the fitted donor and acceptor constants reveal the energetics of secondary local interactions such as competing H-bonds.

Guest-induced magnetic exchange in paramagnetic [M2L4]4+ coordination cages.

Paramagnetic complexes that possess magnetically switchable properties show promise in a number of applications. A significantly underdeveloped approach is the use of metallocages, whose magnetic properties can be modulated through host-guest chemistry. Here we show such an example that utilises a simple [CuII2L4]4+ lantern complex. Magnetic susceptibility and magnetisation data shows an absence of exchange in the presence of the diamagnetic guest triflate. However, replacement of the bound triflate by ReBr62- switches on antiferomagnetic exchange between the Cu and Re ions, leading to an S = 1/2 ground state for the non-covalent complex [ReBr62-⊂CuII2L4]2+. Comparison of this complex to a "control" palladium-cage host-guest complex, [ReBr62-⊂PdII2L4]2+, shows that the encapsulated ReBr62- anions retain the same magnetic anisotropy as in the free salt. Theoretically calculated spin-Hamiltonian parameters are in close agreement with experiment. Spin density analysis shows the mode of interaction between the CuII and ReIV centres is through the Re-Br⋯Cu pathway, primarily mediated through the Cu(dx2-y2)|Brsp|Re(dyz) interaction. This is further supported by overlap integral calculations between singly occupied molecular orbitals (SOMOs) of the paramagnetic ions and natural bonding orbitals analysis where considerable donor-to-acceptor interactions are observed between hybrid 4s4p orbitals of the Br ions and the empty 4s and 4p orbitals of the Cu ions.

Enhanced N-directed electrophilic C-H borylation generates BN-[5]- and [6]helicenes with improved photophysical properties.

Helicenes are chiral polycyclic aromatic hydrocarbons (PAHs) of significant interest, e.g. in supramolecular chemistry, materials science and asymmetric catalysis. Herein an enhanced N-directed electrophilic C-H borylation methodology has been developed that provides access to azaborine containing helicenes (BN-helicenes). This borylation process proceeds via protonation of an aminoborane with bistriflimidic acid. DFT calculations reveal the borenium cation formed by protonation to be more electrophilic than the product derived from aminoborane activation with BBr3. The synthesised helicenes include BN-analogues of archetypal all carbon [5]- and [6]helicenes. The replacement of a CC with a BN unit (that has a longer bond) on the outer helix increases the strain in the BN congeners and the racemization half-life for a BN-[5]helicene relative to the all carbon [5]helicene. BN incorporation also increases the fluorescence efficiency of the helicenes, a direct effect of BN incorporation altering the distribution of the key frontier orbitals across the helical backbone relative to carbo-helicenes.