Photocrystallographic Studies on Transition Metal Nitrito Metastable Linkage Isomers: Manipulating the Metastable State

Supramolecular Chemistry in Metal-Organic Framework Materials

Far from being simply rigid, benign architectures, metal-organic frameworks (MOFs) exhibit diverse interactions with their interior environment. From developing crystal sponges to studying reactions in framework materials, the role of both supramolecular chemistry and framework structure is evident. We explore the role of supramolecular chemistry in determining framework…guest interactions and attempts to understand the dynamic behavior in MOFs, including attempts to control pore behavior through the incorporation of mechanically-interlocked molecules. Appreciating and understanding the role of supramolecular interactions and dynamic behavior in metal-organic frameworks emerge as important directions for the field.

Electronic structure of light-induced nitrosyl linkage isomers revealed by X-ray absorption spectroscopy at Ru L3,2-edges

X-ray absorption spectroscopy (XAS) is a powerful tool for examining changes of the electronic and molecular structure following light-induced excitation of a molecule. Specifically, this method can be applied to investigate the ground (GS, RuNO) and metastable states (MS1, RuON and MS2, Ruη2(NO)) of the nitrosyl ligand (NO), which differ in their coordination mode to the metal. In this work, we report for the first time experimental and theoretical (DFT) Ru L3,2-edge XA spectra for the octahedral complex trans-[RuNOPy4F](ClO4)2 (1, Py = pyridine) in both ground and metastable states. The transition from GS to MS1 using 420 nm light excitation leads to a significant downshift of the 2p → LUMO(+1) peaks by about 0.5-0.8 eV, attributed to the destabilisation of 2p orbitals and stabilization of LUMO(+1). Subsequent irradiation of MS1 at 920 nm produces isomer MS2, for which even greater stabilization of LUMO occurs, though without a significant change in 2p energy. The change in 2p energy is attributed to a variation in the charge on the Ru atom after NO isomerization, while LUMO(+1) stabilization is related to changes in the Ru(NO) bond length and the composition of this orbital.

Photoinduced Isomerization of [N2]2- in a Bimetallic Lutetium Complex

The first lanthanide dinitrogen photoswitch [(C5Me4H)2(THF)Lu]2(μ-η2:η2-N2), 1, is reported. 1 is a unique example of controlled isomerization between side-on and end-on coordination modes of [N2]2- in a bimetallic lutetium dinitrogen complex that results in photochromism. Near-infrared light (NIR) was used to promote this effect, as evidenced by single X-ray diffraction (XRD) connectivity and Raman data, generating the [N2]2- end-on bound isomer, [(C5Me4H)2(THF)Lu]2(μ-η1:η1-N2), 2. Although different ligands and coordinating solvents were studied to replicate and control the optical properties in 1/2, only the original configuration with C5Me4H ligands and THF as the coordinating solvent worked. Supported by the first-principles calculations, the electronic structures along with the mechanistic details of the side-on to end-on isomerization were unraveled. Preliminary reactivity studies show that 2 formed with NIR light reacts with anthracene, generating dihydroanthracene and anthracene dimers, indicating new redox reaction pathways.

Methods in molecular photocrystallography

Over the last three decades, the technology that makes it possible to follow chemical processes in the solid state in real time has grown enormously. These studies have important implications for the design of new functional materials for applications in optoelectronics and sensors. Light-matter interactions are of particular importance, and photocrystallography has proved to be an important tool for studying these interactions. In this technique, the three-dimensional structures of light-activated molecules, in their excited states, are determined using single-crystal X-ray crystallography. With advances in the design of high-power lasers, pulsed LEDs and time-gated X-ray detectors, the increased availability of synchrotron facilities, and most recently, the development of XFELs, it is now possible to determine the structures of molecules with lifetimes ranging from minutes down to picoseconds, within a single crystal, using the photocrystallographic technique. This review discusses the procedures for conducting successful photocrystallographic studies and outlines the different methodologies that have been developed to study structures with specific lifetime ranges. The complexity of the methods required increases considerably as the lifetime of the excited state shortens. The discussion is supported by examples of successful photocrystallographic studies across a range of timescales and emphasises the importance of the use of complementary analytical techniques in order to understand the solid-state processes fully.

Deciphering Iron-Catalyzed C-H Amination with Organic Azides: N2 Cleavage from a Stable Organoazide Complex

Catalytic C-N bond formation by direct activation of C-H bonds offers wide synthetic potential. En route to C-H amination, complexes with organic azides are critical precursors towards the reactive nitrene intermediate. Despite their relevance, α-N coordinated organoazide complexes are scarce in general, and elusive with iron, although iron complexes are by far the most active catalysts for C-H amination with organoazides. Herein, we report the synthesis of a stable iron α-N coordinated organoazide complex from [Fe(N(SiMe3 )2 )2 ] and AdN3 (Ad=1-adamantyl) and its crystallographic, IR, NMR and zero-field 57 Fe Mössbauer spectroscopic characterization. These analyses revealed that the organoazide is in fast equilibrium between the free and coordinated state (Keq =62). Photo-crystallography experiments showed gradual dissociation of N2 , which imparted an Fe-N bond shortening and correspond to structural snapshots of the formation of an iron imido/nitrene complex. Reactivity of the organoazide complex in solution showed complete loss of N2 , and subsequent formation of a C-H aminated product via nitrene insertion into a C-H bond of the N(SiMe3 )2 ligand. Monitoring this reaction by 1 H NMR spectroscopy indicates the transient formation of the imido/nitrene intermediate, which was supported by Mössbauer spectroscopy in frozen solution.

Interrelations between Linkage Isomers of an Efficient Square-planar Nickel(II) Nitrite Photoswitch in the Solid State

An efficient nitrite nickel(II) photoswitch, with the 1-phenyl-2-hydroxyimino-3-[(2'-dimethylamino)ethyl]imino-1-propanone moiety used as the ancillary ligand, is reported. In the ground-state ('dark') crystal structure, the studied compound exists predominantly as the nitro-(η1 -N(O)2 ) isomer, however, traces of the exo- and endo-nitrito-(η1 -ONO) forms are detected both at 100 K (4-5 % each) and under ambient conditions (~9 % each). When excited with the 405-530 nm LED light, the nitro-to-nitrito isomerization takes place. The total conversion exceeds 90 %. The exo-nitrito linkage isomer constitutes the dominant photo-generated form, whereas the relative population of both nitrito species depends on temperature. The reaction is fully reversible and reproducible. The photo-products are stable up to 200 K. The system constitutes a good model case for the reaction mechanism studies. Thus, experimental and theoretical investigations on the photo-isomerism were conducted and are presented in detail. Eventually, the nitro→exo-nitrito→endo-nitrito reaction pathway is proposed.

Uncovering the role of non-covalent interactions in solid-state photoswitches by non-spherical structure refinements with NoSpherA2

We present a charge density study of two linkage isomer photoswitches, [Pd(Bu4dien)(NO2)]BPh4·THF (1) and [Ni(Et4dien)(NO2)2] (2) using Hirshfeld Atom Refinement (HAR) methods implemented via the NoSpherA2 interface in Olex2. HAR is used to explore the electron density distribution in the photoswitchable molecules of 1 and 2, to gain an in-depth understanding of key bonding features and their influence on the single-crystal-to-single-crystal reaction. HAR analysis is also combined with ab initio calculations to explore the non-covalent interactions that influence physical properties of the photoswitches, such as the stability of the excited state nitrito-(η1-ONO) isomer. This insight can be fed back into the crystal engineering process to develop new and improved photoswitches that can be optimised towards specific applications.

Regulating photosalient behavior in dynamic metal-organic crystals

Dynamic photoactuating crystals have become a sensation due to their potential applications in developing smart medical devices, molecular machines, artificial muscles, flexible electronics actuators, probes and microrobots. Here we report the synthesis of two iso-structural metal-organic crystals, [Zn(4-ohbz)₂(4-nvp)₂] (1) and [Cd(4-ohbz)₂(4-nvp)₂] (2) {H4-ohbz = 4-hydroxy benzoic acid; 4-nvp = 4-(1-naphthylvinyl)pyridine} which undergo topochemical [2 + 2] cycloaddition under UV irradiation as well as sunlight to generate a dimerized product of discrete metal-complex [Zn(4-ohbz)₂(rctt-4-pncb)] {rctt-4-pncb = 1,3-bis(4'-pyridyl)-2,4-bis(naphthyl)cyclobutane} (1') and one-dimensional coordination polymer (1D CP) [Cd(4-ohbz)₂(rctt-4-pncb)] (2') respectively, in a single-crystal-to-single-crystal (SCSC) process. The Zn-based compound demonstrates photosalient behaviour, wherein crystals show jumping, splitting, rolling, and swelling upon UV irradiation. However, the Cd-based crystals do not show such behaviour maintaining the initial supramolecular packing and space group. Thus the photomechanical behaviour can be induced by choosing a suitable metal ion. The above findings are thoroughly validated by quantitative density functional theory (DFT) calculations which show that the Zn-based crystal shifts towards an orthorhombic structure to resolve the anisotropic UV-induced mechanical strain. Furthermore, the mechano-structure-property relationship has been established by complimentary nanoindentation measurements, which are in-line with the DFT-predicted single crystal values.

Photogeneration of Several Linkage Isomers and Investigation of Forward and Backward Nitro-Nitrito Isomerization Processes in a Palladium Complex

Photoinduced linkage isomers (PLIs) of the nitro-ligand were generated and comprehensively characterized in a square planar unit [Pd(NH₃)₃NO₂]⁺ of the complex salts [Pd(NH₃)₄][Pd(NH₃)₃NO₂][MOx₃]·yH₂O (M = Cr (Cr), Rh (Rh), Co (Co), Ox = oxalate). Structural (XRD) and spectroscopic (IR, UV-vis) investigations at 10 and 150 K allowed determining the structures of several photoinduced linkage isomers, endo-ONO (PLI1, 2) and exo-ONO (PLI3, 4) isomers generated by irradiation with 365 nm from the initial NO₂ (GS), along with the assignment of the infrared (IR) bands to each structural isomer. Based on a combination of these methods, the photo- and thermally induced interplay of PLIs was investigated. Irradiation in the temperature range of 10-80 K induces the formation of both endo- and exo-ONO isomers, while increasing the temperature up to 150 K results in the formation of only endo-ONO isomers. The structural arrangement of the endo-ONO and exo-ONO PLI is strongly influenced by intermolecular interactions due to the partial occupation of a neighboring site by water molecules. The investigation of thermal dynamics of PLIs revealed that the thermal decay of the exo-ONO isomer occurs via two steps exo-ONO → endo-ONO → NO₂. The kinetic parameters (E_a, k₀) of both decay processes were determined together with the characteristic decay temperatures (T_d) by IR spectroscopy. According to the photoinduced dynamics measured by IR spectroscopy, the mechanism of PLI formation in [Pd(NH₃)₃NO₂]⁺ could be described as NO₂ → endo-ONO → exo-ONO.

Bismuth and antimony halometalates containing photoswitchable ruthenium nitrosyl complexes

The first examples of Bi(III) and Sb(III) halide compounds combined with a photoswitchable ruthenium nitrosyl unit are reported. The structures of [RuNOPy₄Br]₄[Sb₂Br₈][Sb₃Br₁₂]₂ (1) and (H₃O)[RuNOPy₄Br]₄[Bi₂Br₉]₃·3H₂O (2) were determined by X-ray diffraction, and exhibit three different structural types of group 15 halometalates. Low-temperature IR-spectroscopy measurements reveal that the irradiation of 1 at 365 nm switches a stable Ru-NO (GS) unit to a metastable Ru-ON (MS1) linkage. Moreover, the light excitation of 2 at 365 or 405 nm induces the additional formation of a side-bond isomer Ru-η²-(NO) (MS2). The reverse reactions MS1/MS2 → GS can be induced by red-infrared light irradiation or by heating at temperatures >200 K. The obtained synthetic and spectroscopic data open the way for the preparation of hybrid halide complexes with a variety of photoswitchable complexes (NO₂, SO₂, N₂, etc.), and give an insight into the behavior of light-induced species embedded in polynuclear halides.

An optically reversible room-temperature solid-state cobalt(III) photoswitch based on nitro-to-nitrito linkage isomerism

A simple trinitro cobalt complex [Co(3,3'-diamino-N-methylpropanediamine)(NO₂)₃] was proven to be photoswitchable at room temperature as the Pca2₁ polymorph with the maximum nitro-to-nitrito conversion reaching ca. 55%. Solid-state IR, UV-vis and XRD indicate that the transformation can be triggered optically in both ways via 470 nm and 570-660 nm LED light, respectively.

Structural Capture of η1-OSO to η2-(OS)O Coordination Isomerism in a New Ruthenium-Based SO2-Linkage Photoisomer That Exhibits Single-Crystal Optical Actuation

Recent discoveries of a range of single-crystal optical actuators are feeding a new form of materials chemistry, given their broad range of potential applications, from light-induced molecular motors to light sensors and optical-memory media. A series of ruthenium-based coordination complexes that exhibit sulfur dioxide linkage photoisomerization is of particular interest because they exhibit single-crystal optical actuation via either optical switching or nano-optomechanical transduction processes. We report the discovery of a new complex in this series of chemicals, [Ru(SO₂)(NH₃)₄(3-fluoropyridine)]tosylate₂ (1), which forms an η¹-OSO photoisomer with 70% photoconversion upon the application of 505 nm light. The uncoordinated oxygen atom in this η¹-OSO photoisomer impinges on one of the arene rings in a neighboring tosylate counter ion of 1 just enough that incipient nano-optomechanical transduction is observed. The structure and optical properties of this actuator are characterized via in situ light-induced single-crystal X-ray diffraction (photocrystallography), single-crystal optical absorption spectroscopy and microscopy, as well as single-crystal Raman spectroscopy. These materials-characterization methods were also used to track thermally induced reverse isomerization processes in 1. One of these processes involves an η¹-OSO to η²-(OS)O transition, which was found to proceed sufficiently slowly at 110 K that its structural mechanism could be determined via a time sequence of photocrystallography experiments. The resulting data allowed us to structurally capture the transition, which was shown to occur via a form of coordination isomerism. Our newfound knowledge about this structural mechanism will aid the molecular design of new [RuSO₂] complexes with functional applications.

Relating Excited States to the Dynamics of Macroscopic Strain in Photoresponsive Crystals

Exposure of a photoreactive single crystal to light with a wavelength offset from its absorption maximum can have two distinct effects. The first is the "direct" effect, wherein the excited state generated in individual chemical species is influenced. The second is the "indirect" effect, which describes the penetration of light into the crystal and hence the spatial propagation and completeness of transformation. We illustrate using the nitro-nitrito isomerization of [Co(NH₃)₅NO₂]Cl(NO₃) as an example that the direct and indirect effects can be independently determined. This is achieved by comparing the dynamics of macroscopic crystal deformation (bending curvature and crystal elongation) induced by the photochemical reaction when irradiating a crystal at the absorption maximum and at different band edges (above or below the maximum) of the same band. Quantitative description of the macroscopic strain dynamics in comparison with experiments allowed us to suggest that irradiation at different tails of the same absorption band causes isomerization to proceed via different excited states and an additional photochemical reaction (presumably, reverse nitrito-nitro isomerization) can occur on irradiation at the ligand-field band edges.

Exploring the influence of polymorphism and chromophore co-ligands on linkage isomer photoswitching in [Pd(bpy4dca)(NO2)2]

The polymorphic Pd(II)-nitrite complex [Pd(bpy4dca)(NO2)2] (1) (bpy4dca = 2,2’-bipyridine-4,4’-dicarboxylic acid methyl ester) is shown to undergo photoinduced nitro → nitrito linkage isomer switching in two crystal forms, to varying excited...

Solvent dielectric delimited nitro–nitrito photorearrangement in a perylenediimide derivative

The discovery of vibrant excited-state dynamics and distinctive photochemistry has established nitrated polycyclic aromatic hydrocarbons as an exhilarating class of organic compounds. Herein, we report the atypical photorearrangement of nitro-perylenediimide (NO₂-PDI) to nitrito-perylenediimide (ONO-PDI), triggered by visible-light excitation and giving rise to linkage isomers in the polar aprotic solvent acetonitrile. ONO-PDI has been isolated and unambiguously characterized using standard spectroscopic, spectrometric, and elemental composition techniques. Although nitritoaromatic compounds are conventionally considered to be crucial intermediates in the photodissociation of nitroaromatics, experimental evidence for this has not been observed heretofore. Ultrafast transient absorption spectroscopy combined with computational investigations revealed the prominence of a conformationally relaxed singlet excited-state (S^CR₁) of NO₂-PDI in the photoisomerization pathway. Theoretical transition state (TS) analysis indicated the presence of a six-membered cyclic TS, which is pivotal in connecting the S^CR₁ state to the photoproduct state. This article addresses prevailing knowledge gaps in the field of organic linkage isomers and provides a comprehensive understanding of the unprecedented photoisomerization mechanism operating in the case of NO₂-PDI.

The unprecedented photorearrangement of nitro-perylenediimide (NO₂-PDI) to nitrito-perylenediimide (ONO-PDI) is shown to occur through a cyclic six-membered transition state triggered by visible-light excitation.

Slow 3MLCT Formation Prior to Isomerization in Ruthenium Carbene Sulfoxide Complexes

A series of photochromic complexes with general formulas of [Ru(bpy)₂(NHC-SR)]²⁺ and [Ru(bpy)₂(NHC-S(O)R)]²⁺ were prepared and investigated by X-ray crystallography, electrochemistry, and ultrafast transient absorption spectroscopy {where bpy is 2,2'-bipyridine and NHC-SR and NHC-S(O)R are chelating thioether (-SR) and chelating sulfoxide [-S(O)R] N-heterocyclic carbene (NHC) ligands}. The only differences between these complexes are the nature of the R group on the sulfur (Me vs Ph), the identity of the carbene (imidazole vs benzimidazole), and the number of linker atoms in the chelate (CH₂ vs C₂H₄). A total of 13 structures are presented {four [Ru(bpy)₂(NHC-SR)]²⁺ complexes, four [Ru(bpy)₂(NHC-S(O)R)]²⁺ complexes, and five uncomplexed ligands}, and these reveal the expected coordination geometry as predicted from other spectroscopy data. The data do not provide insight into the photochemical reactivity of these compounds. These carbene ligands do impart stability with respect to ground state and excited state ligand substitution reactions. Bulk photolysis reveals that these complexes undergo efficient S → O isomerization, with quantum yields ranging from 0.24 to 0.87. The excited state reaction occurs with a time constant ranging from 570 ps to 1.9 ns. Electrochemical studies reveal an electron transfer-triggered isomerization, and voltammograms are consistent with an ECEC (electrochemical-chemical electrochemical-chemical) reaction mechanism. The carbene facilitates an unusually slow S → O isomerization and an unusally fast O → S isomerization. Temperature studies reveal a small and negative entropy of activation for the O → S isomerization, suggesting an associative transition state in which the sulfoxide simply slides along the S-O bond during isomerization. Ultrafast studies provide evidence of an active role of the carbene in the excited state dynamics of these complexes.

Nanooptomechanical Transduction in a Single Crystal with 100% Photoconversion

Materials that exhibit nanooptomechanical transduction in their single-crystal form have prospective use in light-driven molecular machinery, nanotechnology, and quantum computing. Linkage photoisomerization is typically the source of such transduction in coordination complexes, although the isomers tend to undergo only partial photoconversion. We present a nanooptomechanical transducer, trans-[Ru(SO2)(NH3)4(3-bromopyridine)]tosylate2, whose S-bound η1-SO2 isomer fully converts into an O-bound η1-OSO photoisomer that is metastable while kept at 100 K. Its 100% photoconversion is confirmed structurally via photocrystallography, while single-crystal optical absorption and Raman spectroscopies reveal its metal-to-ligand charge-transfer and temperature-dependent characteristics. This perfect optical switching affords the material good prospects for nanooptomechanical transduction with single-photon control.

A Future Perspective on Phototriggered Isomerizations of Transition Metal Sulfoxides and Related Complexes

Photochromic molecules are examples of light-activated bistable molecules. We highlight the design criteria for a class of ruthenium and osmium sulfoxide complexes that undergo phototriggered isomerization of the bound sulfoxide. The mode of action in these complexes is an excited-state isomerization of the sulfoxide from S-bonded to O-bonded. We discuss the basic mechanism for this transformation and highlight specific examples that demonstrate the effectiveness and efficiency of the isomerization. We subsequently discuss future research directions within the field of phototriggered sulfoxide isomerizations on transition metal polypyridine complexes. These efforts involve new synthetic directions, including the choice of metal as well as new ambidentate ligands for isomerization.

Low-energy optical switching of SO2 linkage isomerisation in single crystals of a ruthenium-based coordination complex

Single crystals that behave as optical switches are desirable for a wide range of applications, from optical sensors to read–write memory media. A series of ruthenium-based complexes that exhibit optical switching in their single-crystal form via SO₂ linkage photoisomerisation are of prospective interest for these technologies. This study explores the optical switching behaviour in one such complex, trans-[Ru(SO₂)(NH₃)₄(H₂O)]tosylate₂ (1), in terms of its dark and photoinduced crystal structure, as well as its light and thermal decay characteristics, which are deduced by photocrystallography, single-crystal optical absorption spectroscopy and microscopy. Photocrystallography results reveal that a photoisomerisation level of 21.5(5)% is achievable in 1. Biphasic photochromic crystals of 1 were generated by applying green and then red light to switch on and off the η²-(OS)O photoisomer in different regions of a crystal. Heat is a known alternative to its thermal decay, whereby a method is demonstrated that employs optical absorption spectra to determine its activation energy of 30 kJ mol⁻¹. This low-energy barrier to optical switching agrees well with computational studies on 1, as well as being comparable to activation energies in ruthenium-based nitrosyl linkage photoisomers that also display solid-state optical switching.

Single crystals that behave as optical switches are desirable for a wide range of applications, from optical sensors to read–write memory media.

Photocrystallographic and spectroscopic studies of a model (N,N,O)-donor square-planar nickel(II) nitro complex: in search of high-conversion and stable photoswitchable materials

A new, cheap, easy-to-synthesize and air-stable photoswitchable nickel(II) complex, QTNiNO2, is reported. The metal centre in QTNiNO2 is coordinated by a nitro group and a [2-methyl-8-amino-quinoline]-1-tetralone ligand. The compound crystallizes in the tetragonal space group I41/a with one complex molecule comprising the asymmetric unit, and the crystals are stable under ambient conditions. Irradiation of the solid-state form of QTNiNO2 with 530-660 nm LED light at 160 K converts the ambidentate nitro moiety fully to the nitrito linkage isomer which is stable up to around 230 K, as indicated by IR spectroscopy measurements. The structures of all species present in the examined crystals and their thermal stability were confirmed via X-ray multi-temperature and photocrystallographic experiments. The impact of temperature on the (photo)isomerization reaction taking place in a single crystal was additionally investigated. The experimental results are supported by computational analyses of crystal packing and intermolecular interactions that influence the isomerization process studied.

Isolating reactive metal-based species in Metal-Organic Frameworks - viable strategies and opportunities

Structural insight into reactive species can be achieved via strategies such as matrix isolation in frozen glasses, whereby species are kinetically trapped, or by confinement within the cavities of host molecules. More recently, Metal-Organic Frameworks (MOFs) have been used as molecular scaffolds to isolate reactive metal-based species within their ordered pore networks. These studies have uncovered new reactivity, allowed observation of novel metal-based complexes and clusters, and elucidated the nature of metal-centred reactions responsible for catalysis. This perspective considers strategies by which metal species can be introduced into MOFs and highlights some of the advantages and limitations of each approach. Furthermore, the growing body of work whereby reactive species can be isolated and structurally characterised within a MOF matrix will be reviewed, including discussion of salient examples and the provision of useful guidelines for the design of new systems. Novel approaches that facilitate detailed structural analysis of reactive chemical moieties are of considerable interest as the knowledge garnered underpins our understanding of reactivity and thus guides the synthesis of materials with unprecedented functionality.

The impact of counterion on the metastable state properties of nitrosyl ruthenium complexes

Complexes of the trans-[RuNO(NH₃)₄F]²⁺ cation with noble-metal anions [PtCl₆]²⁻, [PdCl₄]²⁻, and [PtCl₄]²⁻, and perchlorate ClO₄⁻ were synthesized, the photo-generated linkage isomers were studied by spectroscopic and calorimetric methods.

Reversible Encapsulation of Xenon and CH2 Cl2 in a Solid-State Molecular Organometallic Framework (Guest@SMOM)

Reversible encapsulation of CH2 Cl2 or Xe in a non-porous solid-state molecular organometallic framework of [Rh(Cy2 PCH2 PCy2 )(NBD)][BArF4 ] occurs in single-crystal to single-crystal transformations. These processes are probed by solid-state NMR spectroscopy, including 129 Xe SSNMR. Non-covalent interactions with the -CF3 groups, and hydrophobic channels formed, of [BArF4 ]- anions are shown to be important, and thus have similarity to the transport of substrates and products to and from the active site in metalloenzymes.