Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Metal and Proton Relay-Controlled Hierarchical Multistep Switching Cascade

Transition metals play an important role in many biological processes including cellular regulation and signal transduction. Emulating such processes on the molecular level, while challenging, can help us learn how to manipulate intermolecular communication, an important requirement for the development of solution-based molecular machines. In this work, we demonstrate a transition metal-based artificial multistep switching cascade that exhibits intrinsic hierarchical level control. The process starts with Zn(II), which initiates a transition metal relay by displacing a macrocycle-encapsulated Pd(II). The latter then binds to a hydrazone switch leading to coordination-coupled deprotonation (CCD). Finally, the proton generated through CCD activates the E/Z isomerization of a second noncoordinating pH-sensitive hydrazone switch. This whole multistep process can be reset to the original state by removing the Pd(II) from the system.

Rapid Access to Encapsulated Molecular Rotors via Coordination-Driven Macrocycle Formation

Macrocycle formation that relies upon trans metal coordination of appropriately placed pyridine ligands within an arylene ethynylene construct provides rapid and reliable access to molecular rotators encapsulated within macrocyclic stators. Showing no significant close contacts to the central rotators, X-ray crystallography of AgI -coordinated macrocycles provides plausibility for unobstructed rotation or wobbling of rotators within the central cavity. Solid-state 13 C NMR of PdII -coordinated macrocycles supports the notion of unobstructed movement of simple arenes in the crystal lattice. Solution 1 H NMR studies indicate complete and immediate macrocycle formation upon the introduction of PdII to the pyridyl-based ligand at room temperature. Moreover, the formed macrocycle is stable in solution; a lack of significant changes in the 1 H NMR spectrum upon cooling to -50 °C is consistent with the absence of dynamic behavior. The synthetic route to these macrocycles is expedient and modular, providing access to rather complex constructs in four simple steps involving Sonogashira coupling and deprotection reactions.

Toward Molecular Cybernetics - the Art of Communicating Chemical Systems

The emerging field of molecular cybernetics has the potential to widely broaden our perception of chemistry. Chemistry will develop beyond its current focus that is mainly concerned with single transformations, pure compounds, and/or defined mixtures. On this way, chemistry will become autonomous, networked and smart through communicating molecules each of which serves a control engineering purpose, like the set of wheels in the machinery of life. The present personal account describes our latest developments in this field.

Chemical signal cascading in a supramolecular network

A chemically-triggered signalling cascade between cucurbituril host-guest complexes by means of multi-step competitive displacement is demonstrated. The inter-complex communication of chemical information yields the release of bio-relevant cargo, reminiscent of cellular signalling pathways.

Selective and reversible interconversion of nanosliders commanded by remote control via metal-ion signaling

A multi-device network mainly consisting of two two-component nanosliders was formed by self-sorting of six components. Addition/removal of zinc(ii) ions reversibly reorganized the network by chemical signaling involving the translocation of copper(i) from a relay station followed by the selective disassembly/assembly of one of both multi-component devices. The thus liberated machine parts served to erect a three-component nanoslider alongside the other unchanged two-component nanoslider.

Remote Control of the Synthesis of a [2]Rotaxane and its Shuttling via Metal-Ion Translocation

Remote control in an eight-component network commanded both the synthesis and shuttling of a [2]rotaxane via metal-ion translocation, the latter being easily monitored by distinct colorimetric and fluorimetric signals. Addition of zinc(II) ions to the red colored copper-ion relay station rapidly liberated copper(I) ions and afforded the corresponding zinc complex that was visualized by a bright sky blue fluorescence at 460 nm. In a mixture of all eight components of the network, the liberated copper(I) ions were translocated to a macrocycle that catalyzed formation of a rotaxane by a double-click reaction of acetylenic and diazide compounds. The shuttling frequency in the copper-loaded [2]rotaxane was determined to k 298=30 kHz (ΔH ≠=62.3±0.6 kJ mol-1, ΔS ≠=50.1±5.1 J mol-1 K-1, ΔG ≠ 298=47.4 kJ mol-1). Removal of zinc(II) ions from the mixture reversed the system back generating the metal-free rotaxane. Further alternate addition and removal of Zn2+ reversibly controlled the shuttling mode of the rotaxane in this eight-component network where the ion translocation status was monitored by the naked eye.

Three-state switching in a double-pole change-over nanoswitch controlled by redox-dependent self-sorting

The four-arm nanomechanical switch 1 with four different terminals exhibits two switching arms (contacts A and D) and two distinct stations for binding (contacts B and C). In switching State I, the azaterpyridine arm is intramolecularly coordinated to a zinc(ii) porphyrin station (connection A ↔ B) while contact D (a ferrocenylbipyridine unit) and contact C (phenanthroline) remain disconnected. After addition of copper(i) ions (State II) both connections A ↔ B and C ↔ D are established. Upon one-electron oxidation, double-pole change-over switching cleaves both connections A ↔ B & C ↔ D and establishes the new connection A ↔ C (State III). Fully reversible three-state switching (State I → State II → State III → State II → State I) was achieved by adding appropriate chemical and redox stimuli.

Heteroleptic copper phenanthroline complexes in motion: From stand-alone devices to multi-component machinery

Two and a half decades of copper phenanthroline-based switches, devices and machines have illustrated the rich dynamic nature of these metal complexes. With an emphasis on the metal-ligand dissociation as the rate-determining step the present review summarizes not only spectacular examples of machinery, but also highlights rate data collected during a variety of investigations. Copper-ligand exchange reactions are mostly triggered by redox processes, addition of metal ions or addition of ligands. While the rate data spread over >8 orders of magnitude, individual effects of solvent, steric bulk, flexibility, σ-basicity and the trajectory (intra- vs. intermolecular dissociation) have large impact. Unfortunately, in many cases the exact mechanism in the rate-determining step (nucleophile-induced vs. monomolecular metal-ligand dissociation) has not been determined, suggesting to invest further efforts in the physical (in)organic chemistry of such coordination-driven systems.

New molecular switch architectures

In this paper we elaborate on recently developed molecular switch architectures and how these new systems can help with the realization of new functions and advancement of artificial molecular machines. Progress in chemically and photoinduced switches and motors is summarized and contextualized such that the reader may gain an appreciation for the novel tools that have come about in the past decade. Many of these systems offer distinct advantages over commonly employed switches, including improved fidelity, addressability, and robustness. Thus, this paper serves as a jumping-off point for researchers seeking new switching motifs for specific applications, or ones that address the limitations of presently available systems.

Oscillating Emission of [2]Rotaxane Driven by Chemical Fuel

A molecular shuttle consisting of a dibenzo-24-crown-8 macrocycle and an axle with two degenerate peripheral triazolium stations, a central dibenzyl ammonium station, and two anthracenes stoppers was exposed to 2-cyano-2-phenylpropanoic acid as a chemical fuel. Protonation/deprotonation of the amine reversibly switches the rotaxane from a static and little emissive to a dynamic fluorescent shuttling device, the latter exhibiting rapid motion (15 kHz at 25 °C). Four fuel cycles were run.

Networking switches for smart functions using copper signaling and dynamic heteroleptic complexation

This personal frontier account describes our recent progress in networking nanoswitches to generate emergent functions, such as catalytic machinery, and identifies the key impediments in mastering the paradigm shift from pure compounds to smart mixtures.

A high-speed network of nanoswitches for on/off control of catalysis

Copper(i) ion translocation is the key for fast and reliable communication between networked devices in the catalytic machinery.

Catalytically active nanorotor reversibly self-assembled by chemical signaling within an eight-component network

As an example of advanced molecular cybernetics eight components work together through chemical signaling reversibly setting up multifunctional nanomachinery.

Fluorescent Quantum Dots Make Feasible Long-Range Transmission of Molecular Bits

The modeling and realization of an effective communication platform for long-range information transfer is reported. Messages are encrypted in molecular bits by concentration pulses of fluorescent carbon quantum dots having self-quenching emission that dynamically depends on the concentration pulses. Messages are transferred along longer paths when received and decoded by means of dynamical emission response with respect to the ones encoded by absorbance scaling linearly with messenger concentration. These results represent a significant breakthrough in view of the futuristic development of a nonspecific molecular communication platform to encode and transfer information in multiple fluid environments, ranging from physiological to industrial ones.

Silver(i)-organic frameworks constructed from silver(i) 3,6-pyrazyldiethynide and 3,8-1,10-phenanthrolyldiethynide complexes

Two diynes bearing functional groups with different binding modes, 3,6-diethynylpyrazine (H₂L1) and 3,8-diethynyl-1,10-phenanthroline (H₂L2), were utilized as ligands to synthesize two new organometallic units, Ag₂L1·3AgNO₃ (1) and Ag₂L2·6AgNO₃ (2), in order to investigate the effect of the bridging and chelating modes of the ligands on the structures of networks constructed from silver-ethynide compounds. Structural studies show that in 1, silver-ethynide cluster units aggregate to form chair-like organometallic slides through Ag-N coordination bonds. These slides are linked through argentophilic interaction to generate novel 2D ladder-like layers, and are further bridged by nitrate anions to afford a 3D network in the solid state. It is observed that all the Ag ions in one layer interact to afford a 2D silver network. However, in 2, the silver-ethynide cluster units only interact to generate unique sine wave-like organometallic chains through argentophilic interaction, which are further connected by nitrate anions to form a 3D network. In the solid state, both 1 and 2 are luminescent at room temperature.

Hydrazone switches and things in between

This feature article surveys the various ways by which a structurally simple hydrazone can be used in accessing different functional materials, mainly photo/chemically activated switches, fluorophores and sensors.

Hydrazone Switch-Based Negative Feedback Loop

A negative feedback loop that relies on the coordination-coupled deprotonation (CCD) of a hydrazone switch has been developed. Above a particular threshold of zinc(II), CCD releases enough protons to the environment to trigger a cascade of reactions that yield an imine. This imine sequesters the excess of zinc(II) from the hydrazone switch, hence lowering the effective amount of protons, and switching the cascade reactions "OFF", thus establishing the negative feedback loop.

Cooperative Switching in Nanofibers of Azobenzene Oligomers

Next-generation molecular devices and machines demand the integration of molecular switches into hierarchical assemblies to amplify the response of the system from the molecular level to the meso- or macro-scale. Here, we demonstrate that multi-azobenzene oligomers can assemble to form robust supramolecular nanofibers in which they can be switched repeatedly between the E- and Z-configuration. While in isolated oligomers the azobenzene units undergo reversible photoisomerization independently, in the nanofibers they are coupled via intermolecular interactions and switch cooperatively as evidenced by unusual thermal and kinetic behavior. We find that the photoisomerization rate from the Z-isomer to the E-isomer depends on the fraction of Z-azobenzene in the nanofibers, and is increased by more than a factor of 4 in Z-rich fibers when compared to E-rich fibers. This demonstrates the great potential of coupling individual photochromic units for increasing their quantum efficiency in the solid state with potential relevance for actuation and sensing.

Nanoarchitectonics for Dynamic Functional Materials from Atomic-/Molecular-Level Manipulation to Macroscopic Action

Objects in all dimensions are subject to translational dynamism and dynamic mutual interactions, and the ability to exert control over these events is one of the keys to the synthesis of functional materials. For the development of materials with truly dynamic functionalities, a paradigm shift from "nanotechnology" to "nanoarchitectonics" is proposed, with the aim of design and preparation of functional materials through dynamic harmonization of atomic-/molecular-level manipulation and control, chemical nanofabrication, self-organization, and field-controlled organization. Here, various examples of dynamic functional materials are presented from the atom/molecular-level to macroscopic dimensions. These systems, including atomic switches, molecular machines, molecular shuttles, motional crystals, metal-organic frameworks, layered assemblies, gels, supramolecular assemblies of biomaterials, DNA origami, hollow silica capsules, and mesoporous materials, are described according to their various dynamic functions, which include short-term plasticity, long-term potentiation, molecular manipulation, switchable catalysis, self-healing properties, supramolecular chirality, morphological control, drug storage and release, light-harvesting, mechanochemical transduction, molecular tuning molecular recognition, hand-operated nanotechnology.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.

Controlling Motion at the Nanoscale: Rise of the Molecular Machines

As our understanding and control of intra- and intermolecular interactions evolve, ever more complex molecular systems are synthesized and assembled that are capable of performing work or completing sophisticated tasks at the molecular scale. Commonly referred to as molecular machines, these dynamic systems comprise an astonishingly diverse class of motifs and are designed to respond to a plethora of actuation stimuli. In this Review, we outline the conditions that distinguish simple switches and rotors from machines and draw from a variety of fields to highlight some of the most exciting recent examples of opportunities for driven molecular mechanics. Emphasis is placed on the need for controllable and hierarchical assembly of these molecular components to display measurable effects at the micro-, meso-, and macroscales. As in Nature, this strategy will lead to dramatic amplification of the work performed via the collective action of many machines organized in linear chains, on functionalized surfaces, or in three-dimensional assemblies.

Guest-induced transformation of a porphyrin-edged Fe(II)4L6 capsule into a Cu(I)Fe(II)2L4 fullerene receptor

The combination of a bent diamino(nickel(II) porphyrin) with 2-formylpyridine and Fe(II) yielded an Fe(II) 4 L6 cage. Upon treatment with the fullerenes C60 or C70 , this cage was found to transform into a new host-guest complex incorporating three Fe(II) centers and four porphyrin ligands, in an arrangement that is hypothesized to maximize π interactions between the porphyrin units of the host and the fullerene guest bound within its central cavity. The new complex shows coordinative unsaturation at one of the Fe(II) centers as the result of the incommensurate metal-to-ligand ratio, which enabled the preparation of a heterometallic cone-shaped Cu(I) Fe(II) 2 L4 adduct of C60 or C70 .

Redox-dependent self-sorting toggles a rotary nanoswitch

The pyridine–pyrimidine (py–pym) arm as the moving part of the two-state nanomechanical rotary switch [Cu(1)]⁺ is toggled reversibly between two stations using one-electron oxidation/reduction.

Molecules with a sense of logic: a progress report

Ones and zeros can be handled by molecules through the input-control of their signaling features. The progress in this exciting field during the last five years is covered in this tutorial review.

From self-sorted coordination libraries to networking nanoswitches for catalysis

This feature article sketches our long way from the development of dynamic heteroleptic coordination motifs to the self-sorting of multi-component libraries and finally the design of a new family of triangular nanomechanical switches, which are useful for ON–OFF control of catalysis and in bidirectional communication.