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We report the reactivity, structures and spectroscopic characterization of reactions of phosphine-based ligands (mono-, di- and tri-dentate) with iron-carbide carbonyl clusters. Historically, the archetype of this cluster class, namely [Fe 6 (μ 6 -C)(μ 2 -CO) 4 (CO) 12 ] 2- , can be prepared on a gram-scale but is resistant to simple ligand substitution reactions. This limitation has precluded the relevance of iron-carbide clusters relating to organometallics, catalysis and the nitrogenase active site cluster. Herein, we aimed to derive a simple and reliable method to accomplish CO → L (where L = phosphine or other general ligands) substitution reactions without harsh reagents or multi-step synthetic strategies. Ultimately, our goal was ligand-based chelation of an Fe n (μ n -C) core to achieve more synthetic control over multi-iron-carbide motifs relevant to the nitrogenase active site. We report that the key intermediate is the PSEPT-non-conforming cluster [Fe 6 (μ 6 -C)(CO) 16 ] (2: 84 electrons), which can be generated in situ by the outer-sphere oxidation of [Fe 6 (μ 6 -C)(CO) 16 ] 2- (1: closo , 86 electrons) with 2 equiv. of [Fc]PF 6 . The reaction of 2 with excess PPh 3 generates a singly substituted neutral cluster [Fe 5 (μ 5 -C)(CO) 14 PPh 3 ] (4), similar to the reported reactivity of the substitutionally active cluster [Fe 5 (μ 5 -C)(CO) 15 ] with monodentate phosphines (Cooke & Mays, 1990). In contrast, the reaction of 2 with flexible, bidentate phosphines (DPPE and DPPP) generates a wide range of unisolable products. However, the rigid bidentate phosphine bis(diphenylphosphino)benzene (bdpb) disproportionates the cluster into non-ligated Fe 3 -carbide anions paired with a bdpb-supported Fe(ii) cation, which co-crystallize in [Fe 3 (μ 3 -CH)(μ 3 -CO)(CO) 9 ] 2 [Fe(MeCN) 2 (bdpb) 2 ] (6). A successful reaction of 2 with the tripodal ligand Triphos generates the first multi-iron-chelated, authentic carbide cluster of the formula [Fe 4 (μ 4 -C)(κ3-Triphos)(CO) 10 ] (9). DFT analysis of the key (oxidized) intermediate 2 suggests that its (μ 6 -C)Fe 6 framework remains fully intact but is distorted into an axially compressed, 'ruffled' octahedron distinct from the parent closo cluster 1. Oxidation of the cluster in non-coordinating solvent allows for the isolation and crystallization of the CO-saturated, intact closo -analogue [Fe 6 (μ 6 -C)(CO) 17 ] (3), indicating that the intact (μ 6 -C)Fe 6 motif is retained during initial oxidation with [Fc]PF 6 . Overall, we demonstrate that redox modulation beneficially 'bends' Wade-Mingo's rules via the generation of electron-starved (non-PSEPT) intermediates, which are the key intermediates in promoting facile CO → L substitution reactions in iron-carbide-carbonyl clusters., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Developing efficient photocatalysts that perform multi electron redox reactions is critical to achieving solar energy conversion. One can reach this goal by developing systems which mimic natural photosynthesis and exploit strategies such as proton-coupled electron transfer (PCET) to achieve photochemical charge accumulation. We report herein a heteroleptic Cu(i)bis(phenanthroline) complex, Cu-AnQ, featuring a fused phenazine-anthraquinone moiety that photochemically accumulates two electrons in the anthraquinone unit via PCET. Full spectroscopic and electrochemical analyses allowed us to identify the reduced species and revealed that up to three electrons can be accumulated in the phenazine-anthraquinone ring system under electrochemical conditions. Continuous photolysis of Cu-AnQ in the presence of sacrificial electron donor produced doubly reduced monoprotonated photoproduct confirmed unambiguously by X-ray crystallography. Formation of this photoproduct indicates that a PCET process occurred during illumination and two electrons were accumulated in the system. The role of the heteroleptic Cu(i)bis(phenanthroline) moiety participating in the photochemical charge accumulation as a light absorber was evidenced by comparing the photolysis of Cu-AnQ and the free AnQ ligand with less reductive triethylamine as a sacrificial electron donor, in which photogenerated doubly reduced species was observed with Cu-AnQ, but not with the free ligand. The thermodynamic properties of Cu-AnQ were examined by DFT which mapped the probable reaction pathway for photochemical charge accumulation and the capacity for solar energy stored in the process. This study presents a unique system built on earth-abundant transition metal complex to store electrons, and tune the storage of solar energy by the degree of protonation of the electron acceptor., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We report a copolymeric fluorescent sensor that is selective for lithium chloride. The two constituent polymers comprise pendent triphenylethylene (TPE) moieties for aggregate induced emission (AIE) along with either strapped-calix[4]pyrrole or secondary ammonium groups that drive aggregation via self-assembly upon polymer mixing. Addition of LiCl in acetonitrile disrupts the strapped-calix[4]pyrrole/secondary ammonium chloride salt host-guest crosslinks leading to disaggregation of the polymer chains and a decrease in TPE emission. The lack of AIE perturbation upon addition of NaCl, KCl, MgCl 2 or CaCl 2 provides for high selectivity for LiCl relative to potential interferants. This supramolecular dual polymer approach could serve as a complement to more traditional sensor systems., Competing Interests: The authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)

A cyclo[2]carbazole[2]pyrrole (2) consisting of two carbazoles and two pyrroles has been synthesized by directly linking the carbazole 1- and 8-carbon atoms to the pyrrole α-carbon atoms. Macrocycle 2 is an extensively conjugated 16-membered macrocyclic ring that is fixed in a pseudo-1,3-alternate conformation. This provides a preorganized anion binding site consisting of two pyrrole subunits. 1 H NMR spectroscopic analysis revealed that only the two diagonally opposed pyrrole NH protons, as opposed to the carbazole protons, take part in anion binding. Nevertheless, cyclo[2]carbazole[2]pyrrole 2 binds representative anions with higher affinity in CD 2 Cl 2 than calix[4]pyrrole (1), a well-studied non-conjugated tetrapyrrole macrocycle that binds anions via four pyrrolic NH hydrogen bond interactions. On the basis of computational studies, the higher chloride anion affinity of receptor 2 relative to 1 is rationalized in terms of a larger binding energy and a lower host strain energy associated with anion complexation. In the presence of excess fluoride or bicarbonate anions, compound 2 loses two pyrrolic NH protons to produce a stable dianionic macrocycle [2-2H] 2- displaying a quenched fluorescence., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor. In comparison to other non-scaffolded acyl-containing complexes, the complex described herein retains molecularly well-defined chemistry upon addition of multiple equivalents of exogenous base. Clean deprotonation of the acyl(methylene) C-H bond with a phenolate base results in the formation of a dimeric motif that contains a new Fe-C(methine) bond resulting from coordination of the deprotonated methylene unit to an adjacent iron center. This effective second carbanion in the ligand framework was demonstrated to drive heterolytic H 2 activation across the Fe(ii) center. However, this process results in reductive elimination and liberation of the ligand to extrude a lower-valent Fe-carbonyl complex. Through a series of isotopic labelling experiments, structural characterization (XRD, XAS), and spectroscopic characterization (IR, NMR, EXAFS), a mechanistic pathway is presented for H 2 /hydride-induced loss of the organometallic acyl unit ( i.e. pyCH 2 -C[double bond, length as m-dash]O → pyCH 3 +C[triple bond, length as m-dash]O). The known reduced hydride species [HFe(CO) 4 ] - and [HFe 3 (CO) 11 ] - have been observed as products by 1 H/ 2 H NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO) 4 ]. The former species ( i.e. [HFe(CO) 4 ] - ) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate ([ Tol Im](BAr F ) = fluorinated imidazolium as hydride acceptor). This work provides mechanistic insight into the reasons for lack of functional biomimetic behavior (hydride transfer) in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Recent decades have witnessed the emergence of Au(i) bis-N-heterocyclic carbenes (NHCs) as potential anticancer agents. However, these systems exhibit little interaction with serum proteins ( e.g. , human serum albumin), which presumably impacts their pharmacokinetic profile and tumor exposure. Anticancer drugs bound to human serum albumin (HSA) often benefit from significant advantages, including longer circulatory half-lives, tumor targeted delivery, and easier administration relative to the drug alone. In this work, we present Au(i) bis-NHCs complexes, 7 and 9 , capable of binding to HSA. Complex 7 contains a reactive maleimide moiety for covalent protein conjugation, whereas its congener 9 contains a naphthalimide fluorophore for non-covalent binding. A similar drug motif was used in both cases. Complexes 7 and 9 were prepared from a carboxylic acid functionalized Au(i) bis-NHC (complex 2 ) using a newly developed post-synthetic amide functionalization protocol that allows coupling to both aliphatic and aromatic amines. Analytical, and in vitro techniques were used to confirm protein binding, as well as cellular uptake and antiproliferative activity in A549 human lung cancer cells. The present findings highlight a hitherto unexplored approach to modifying Au(i) bis-NHC drug candidates for protein ligation and serve to showcase the relative benefits of covalent and non-covalent HSA binding., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Two-phenoxy walled calix[4]pyrroles 1 and 2 strapped with small rigid linkers containing pyridine and benzene, respectively, have been synthesized. 1 H NMR spectroscopic analyses carried out in CDCl 3 revealed that both of receptors 1 and 2 recognize only F - and HCO 3 - among various test anions with high preference for HCO 3 - (as the tetraethylammonium, TEA + salt) relative to F - (as the TBA + salt). The bound HCO 3 - anion was completely released out of the receptors upon the addition of F - (as the tetrabutylammonium, TBA + salt) as a result of significantly enhanced affinities and selectivities of the receptors for F - once converted to the TEAHCO 3 complexes. Consequently, relatively stable TEAF complexes of receptors 1 and 2 were formed via anion metathesis occurring within the receptor cavities. By contrast, the direct addition of TEAF to receptors 1 and 2 produces different complexation products initially, although eventually the same TEAF complexes are produced as via sequential TEAHCO 3 and TBAF addition. These findings are rationalized in terms of the formation of different ion pair complexes involving interactions both inside and outside of the core receptor framework., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Calix[4]pyrrole (C4P)-based systems have been extensively explored as binding agents for anions and ion pairs. However, their capacity to act as molecular containers for neutral species remains underexplored. We report here the molecular recognition of pyrazine N , N '-dioxide ( PZDO ) using a series of aryl extended C4Ps including three α,α-diaryl substituted C4Ps (receptors 1-3 ), an α,β-diaryl substituted C4P (receptor 4 ) and an α,α,α,α-tetraaryl substituted C4P (receptor 5 ). Single crystal structural analyses of the 2 : 1 host-guest complexes between receptors 1-3 and PZDO revealed that the C4P subunits exist in an unusual partial cone conformation and that the PZDO guest is held within electron-rich cavities formed by the lower rims of the individual C4P macrocycle. In contrast, receptor 5 was seen to adopt the cone conformation in the solid state, allowing one PZDO molecule to be accommodated inside the upper-rim cavity. Evidence for guest-directed self-assembly is also seen in the solid state. Evidence for C4P- PZDO interactions in CD 3 CN/CD 3 OD solution came from 1 H NMR spectroscopic titrations. Electrostatic potential maps created by means of density functional theory calculations were constructed. Density functional theory calculations were also performed to analyse the energetics of various limiting binding modes. On the basis of these studies, it is inferred that interactions between the 'two-wall' C4P derivatives ( i.e. receptors 1-4 ) and PZDO involve a complex binding mode that differs from what has been seen in previous host-guest complexes formed between C4Ps and N -oxides. The present study thus paves the way for the further design of C4P-based receptors with novel recognition features., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A new mixed hexaphyrin, pyrihexaphyrin (0.1.0.0.1.0) ( 1 ), was prepared via an acid catalyzed cyclization between 5,5'-(pyridine-2,6-diyl)bis(pyrrole-2-carbaldehyde) ( 2 ) and terpyrrole ( 3 ). This expanded porphyrin undergoes a ring contraction upon metallation with uranyl silylamide [UO 2 [N(SiMe 3 ) 2 ] 2 ] under anaerobic conditions followed by purification over basic aluminum oxide exposed to air. The uranyl-contracted pyrihexaphyrin (0.0.0.0.1.0) complex ( 4 ) produced as a result contains a unique structural architecture and possesses a formally 22 π-electron globally aromatic periphery, as inferred from NMR spectroscopy, single crystal X-ray diffraction, and computational analyses. Support for the proposed contraction mechanism came from experimental data and DFT calculations. Proton NMR and mass spectroscopic analysis provided the first insight into expanded porphyrin-mediated activation of the uranyl dication (UO 2 2+ ).

New tetrakis - and hexakis (1 H -pyrrole-2-carbaldehyde) anion receptors are described. The anion binding properties of these receptors were studied in organic media and in the solid state. The receptors displayed good affinity for the dihydrogenphosphate and pyrophosphate anions (as the tetrabutylammonium salts) in chloroform even in the presence of a polar protic solvent, methanol. Solution phase spectroscopic analyses proved consistent with the binding mode seen in single crystal structural studies of the dihydrogenphosphate and pyrophosphate complexes and provided support for the contention that these receptors undergo conformational reorganization in order to accommodate the bound oxoanions both in chloroform solution and in the solid state.

Ferrocene containing N -heterocyclic carbene (NHC) ligated gold(I) complexes of the type [Au(NHC) 2 ] + were prepared and found to be capable of regulating the formation of reactive oxygen species (ROS) via multiple mechanisms. Single crystal X-ray analysis of bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)-gold(I) chloride ( 5 ) and bis(1,3-di(ferrocenylmethyl)imidazol-2-ylidene)-gold(I) chloride ( 6 ) revealed a quasi-liner geometry around the gold(I) centers, (i.e., the C - Au - C bond angle were measured to be ~177° and all the Au - C carbene bonds distances were in the range of 2.00 (7) - 2.03 (1) Å). A series of cell studies indicated that cell proliferation inhibition and ROS generation were directly proportional to amount of ferrocene contained within the [Au(NHC) 2 ] + complexes (IC 50 of 6 < 5 < bis(1-benzyl-3-mesitylimidazol-2-ylidene)-gold(I) chloride ( 4 )). Complexes 4 - 6 were also confirmed to inhibit thioredoxin reductase as inferred from lipoate reduction assays and increase chelatable intracellular zinc concentrations. RNA microarray gene expression assays revealed that 6 induces endoplasmic reticulum stress response pathways as a result of ROS increase.

In order to understand the still-poorly understood interplay between calix[4]arene conformations and cation and anion recognition in multicomponent systems, the ion pair receptors 1 and 2 were synthesized. In solution and in the solid state, the calix[4]arene subunit of receptor 1 adopts a cone conformation, while that of 2 interconverts between the cone and the partial cone conformation. These geometric features differ from previous systems where the calix[4]arene moiety was locked in the 1,3-alternate conformation. A combination of 1 H NMR spectroscopic analyses and single crystal X-ray diffraction studies reveal that receptor 1 binds the fluoride and the chloride anion via significantly different binding modes, displaying, for instance, 1 : 1 and 2 : 3 binding stoichiometries with CsF and CsCl, respectively. In the case of 2 , the conformation of the calix[4]arene constituent of 2 is highly dependent on the size and quantity of anions present. For example, upon treatment of 2 with the fluoride anion (as both the TBA + and Cs + salts), the calix[4]arene unit coexists as cone and partial cone conformers that are inter-convertible. In the presence of excess CsF, the aromatic rings of the calix[4]arene subunit becomes locked in the pinched cone conformation with the result that an ion pair-mediated coordination polymer is formed. In the presence of excess CsCl, the calix[4]arene unit of 2 adopts only the partial cone conformation stabilized by aryl CH-anion hydrogen bonding interactions. The present systems constitute a rare set of related receptors wherein the effects of conformational changes are so tightly coupled with ion recognition.