Preparations and electrochemical properties of pyrazine-bridged ruthenium-binuclear complexes exhibiting molecular hysteresis

Photocrystallography of [Ru(bpy)2(dmso)2]2+ reveals an O-bonded metastable state

We report the first instance of observing the phototriggered isomerization of dmso ligands on a bis sulfoxide complex, [Ru(bpy)₂(dmso)₂], in the crystalline solid state. The solid-state UV-vis spectrum of the crystal demonstrates an increase in optical density around 550 nm after irradiation, which is consistent with the solution isomerization results. Digital images of the crystal before and after irradiation display a notable color change (pale orange to red) and cleavage occurs along planes, (1̄01) and (100), during irradiation. Single crystal X-ray diffraction data also confirms that isomerization is occurring throughout the lattice and a structure that contains a mix of the S,S and O,O/S,O isomer was attained from a crystal irradiated ex situ. In situ irradiation XRD studies reveal that the percentage of the O-bonded isomer increases as a function of 405 nm exposure time.

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.

Mechanically interlocked materials. Rotaxanes and catenanes beyond the small molecule

An overview of the progress in mechanically interlocked materials is presented. In particular, we focus on polycatenanes, polyrotaxanes, metal–organic rotaxane frameworks (MORFs), and mechanically interlocked derivatives of carbon nanotubes (MINTs).

Redox-stimulated motion and bistability in metal complexes and organometallic compounds

SIGNIFICANCE

Control over reversible changes to molecular structure forms the basis for artificial molecular machines that could eventually lead to the development of molecule-based nanotechnology.

RECENT ADVANCES

Particular applications in information storage and processing could emerge where the structural rearrangements give rise to bistability and molecular hysteresis effects.

CRITICAL ISSUES

Oxidation-state-dependent coordination and bonding preferences in transition metal complexes and organometallic compounds provide a versatile approach to the control of molecular motions by redox input, but so far, few structural motifs have been applied in redox-actuated molecular machines.

FUTURE DIRECTIONS

Further progress toward molecule-based nanoscale devices might be accomplished with molecular components derived from a wider range of structural themes and forms of molecular motion. Examples of redox-stimulated rearrangements in metal complexes and organometallic compounds are described that have been employed in molecular machines or could be considered for the design of new functional molecules.

Redox-switchable intramolecular π-π-stacking of perylene bisimide dyes in a cyclophane

Molecular actuation by stepwise electrochemical reduction is demonstrated for a cyclophane that exhibits a pronounced conformational transition from a closed cavity with cofacially stacked PBIs in the neutral state to an expanded open cavity in the three- and fourfold reduced state.

Novel 2-mercaptopyridine-ruthenium complex exhibiting electrochemically induced linkage isomerization switched on/off by protolysis

A ruthenium complex [ruthenium bis(2,2'-bipyridine)(2-mercaptopyridine)(pyridine)](PF6)2 was crystallographically characterized from its deprotonated form and was electrochemically investigated. In the deprotonated complex, the 2-mercaptopyridine ligand coordinates to the Ru atom only by the S atom; therefore, the N atom of the 2-mercaptopyridine ligand can be protonated. In a CH3CN solution, the complex shows a reversible redox couple attributed to RuIII/II-S. The addition of a base to the CH3CN solution of the complex gives irreversible voltammograms, implying electrochemically induced linkage isomerization between RuII-S and RuIII-N. Analysis of the observed cyclic voltammograms gave the equilibrium and rate constants for linkage isomerization: KIINS = 1.2 x 1018, KIIINS = 0.64, kIINS = 5 x 10 s(-1), kIISN = 4 x 10-17 s(-1), kIIINS = 0.26 s(-1), and kIIISN = 0.40 s(-1).

Efficient Energy Conversion in Photochromic Ruthenium DMSO Complexes

The photochromic compounds trans- and cis-[Ru(tpy)(Mepic)(dmso)](OSO2CF3) (2 and 3, respectively; tpy is 2,2':6',2"-terpyridine; Mepic is 6-methyl-2-pyridinecarboxylate; dmso is dimethyl sulfoxide) and cis-[Ru(tpy)(Brpic)(dmso)](PF6) (4; Brpic is 6-bromo-2-pyridinecarboxylate) were prepared and characterized by single-crystal X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. The geometry labels refer to the relationship between the carboxylate oxygen of the picolinate ligand and dmso. Electrochemical studies reveal that only the trans isomer shows S-to-O isomerization following oxidation of Ru(II) and O-to-S isomerization following reduction of Ru(III). The cis isomers of both complexes feature reversible one-electron Ru(III/II) couples. All complexes undergo phototriggered S-to-O isomerization following MLCT (metal-to-ligand charge transfer) excitation with quantum yields (Phi(S-->O)) of 0.79 (2), 0.011 (3), and 0.014 (4). The methyl group in 2 promotes isomerization by hindering rotation of the dmso ligand about the Ru-S bond. Computational results support this role for the methyl group. Relative energy calculations show that the barrier to rotation is approximately 8 kcal mol(-1). These results suggest that rotation is an important vibration for isomerization in photochromic ruthenium-dmso complexes.

Designing Molecular Bistability in Ruthenium Dimethyl Sulfoxide Complexes

Compounds of the type [Ru(tpy)(L2)(dmso)](z+) (tpy is 2,2':6',2' '-terpyridine; L2 can be 2,2'-bipyridine (bpy), N,N,N',N'-tetramethylethylenediamine (tmen), 2-pyridine carboxylate (pic), acetylacetonate (acac), malonate (mal), or oxalate (ox)) have been studied by X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. When L2 is bpy, tmen, or pic, the dmso ligand can be intramolecularly isomerized either electrochemically or photochemically. Isomerization is not observed when L2 is acac, mal, or ox. Isomerization results in a drastic change in the absorption spectrum, as well as in the voltammetry. Absorption maxima shift by 3470 (419-490 nm), 4775 (421-527 nm), and 4440 cm(-)(1) (429-530 nm) for the bpy, pic, and tmen complexes, respectively. Reduction potentials for S-bonded and O-bonded complexes differ by 0.57, 0.75, and 0.62 V for the bpy, pic, and tmen complexes, respectively. Quantum yields of isomerization (phi(S)(-->)(O)) were determined for the bpy (0.024 +/- 1), pic (0.25 +/- 1), and tmen (0.007 +/- 1) complexes. In comparison of these data to photosubstitution quantum yields, it appears that the isomerization mechanism does not involve the ligand field states. This result is surprising given the importance of these states in the photochemistry of ruthenium and osmium polypyridine complexes. These results and details of the mechanism are discussed.

Rapid electrochemically induced linkage isomerism in a ruthenium(ii) polypyridyl complex

Rapid and complete switching between the N6 and the N5O donor set induced by changing the metal oxidation state has been observed for a new structural motif based on a ruthenium(II) polypyridyl complex.

[(NH3)5Ru(1,2,4,5-tetrazine)]2+: Synthesis and Experimental and Theoretical Study of Its Solvatochromism in the Visible Spectral Region

The title compound has been first synthesized and fully characterized as both tetraphenylborate and perchlorate salt. Its 300-900 nm absorption spectrum, recorded in nitromethane, water, and dimethyl sulfoxide, reveals the peculiar existence of two distinct bands whose intensities depend on the solvent donor number. This feature can be attributed to two separate metal-to-ligand charge-transfer transitions, in agreement with the theoretical predictions obtained by extensive configuration interaction calculations, which take into account the solvent effects. The calculation of the potential energy curves of the ground and excited states along the Ru-tetrazine coordinate allows the interpretation of the relative intensities of the observed bands, as well as the interpretation of their line-shape profiles.

Turning Off Phototriggered Linkage Isomerizations in Ruthenium Dimethyl Sulfoxide Complexes

We report on the spectroscopy, electrochemistry, and linkage isomerization in a family of [Ru(tpy)(L2)(dmso)](z)()(+) complexes (tpy is 2,2':6',2' '-terpyridine, dmso is dimethyl sulfoxide, and L2 is a variable ligand: 2,2'-bipyridine (bpy), 2-picolinate (pic), N,N,N',N'-tetramethylethylenediamine (tmen), acetylacetonate (acac), or malonate (mal)). The identity of this bidentate ligand serves to tune the absorption maxima (lambda(max) = 419-502 nm) and the reduction potential (E(1/2) = 1.67 to 0.82 V) of these complexes. Photochemical and electrochemical studies show that S-->O and O-->S linkage isomerization may be triggered through an electron transfer mechanism, resulting in dramatic shifts in both the absorption maxima and the reduction potential (for [Ru(tpy)(pic)(dmso)](+) S-bonded, 421 nm, 1.38 V vs Ag/AgCl; O-bonded, 527 nm, 1.38 V vs Ag/AgCl). Certain of these complexes [Ru(tpy)(acac)(dmso)](+) and [Ru(tpy)(mal)(dmso)] do not undergo isomerization. These results are discussed in the context of electron transfer triggered isomerization.

Room-temperature photochromism in cis- and trans-[Ru(bpy)2(dmso)2]2+

We report on phototriggered Ru-S --> Ru-O and thermal Ru-O --> Ru-S intramolecular linkage isomerizations in cis- and trans-[Ru(bpy)2(dmso)2]2+. The cis complex features only S-bonded sulfoxides (cis-[S,S]), whereas the trans isomer is characterized by S- and O-bonded dmso ligands. Both cis-[S,S] and trans-[S,O] exhibit photochromism at room temperature in dmso solution and ionic liquid (IL). Rates of reaction in IL were monitored by UV-visible spectroscopy and are similar to those reported in dmso solution (k(O-->S) ranges from approximately 10(-3) to 10(-4) s(-1)). Cyclic voltammetric measurements of cis-[S,S] and trans-[S,O] are consistent with an electrochemically triggered linkage isomerism mechanism. While both cis-[S,S] and trans-[S,O] are photochromic at room temperature, neither complex is emissive. However, upon cooling to 77 K, cis-[S,S] exhibits LMCT (ligand-to-metal charge transfer) emission typical of many ruthenium polypyridine complexes. In contrast to cis-[S,S], trans-[S,O] does not show any detectable emission even at 77 K.

Synthesis, structure, and spectroscopic, photochemical, redox, and catalytic properties of ruthenium(II) isomeric complexes containing dimethyl sulfoxide, chloro, and the dinucleating bis(2-pyridyl)pyrazole ligands

Two isomeric Ru(II) complexes containing the dinucleating Hbpp (3,5-bis(2-pyridyl)pyrazole) ligand together with Cl and dmso ligands have been prepared and their structural, spectroscopic, electrochemical, photochemical, and catalytic properties studied. The crystal structures of trans,cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2a, and cis(out),cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2b, have been solved by means of single-crystal X-ray diffraction analysis showing a distorted octahedral geometry for the metal center where the dmso ligands coordinate through their S atom. 1D and 2D NMR spectroscopy corroborates a similar structure in solution for both isomers. Exposure of either 2a or 2b in acetonitrile solution under UV light produces a substitution of one dmso ligand by a solvent molecule generating the same product namely, cis(out)-[Ru(II)Cl(2)(Hbpp)(MeCN)(dmso)], 4. While the 1 e(-) oxidation of 2b or cis(out),cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3b, generates a stable product, the same process for 2a or trans,cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3a, produces the interesting linkage isomerization phenomenon where the dmso ligand switches its bond from Ru-S to Ru-O (K(III)(O)(-->)(S) = 0.25 +/- 0.025, k(III)(O)(-->)(S) = 0.017 s(-1), and k(III)(S)(-->)(O) = 0.065 s(-1); K(II)(O)(-->)(S) = 6.45 x 10(9), k(II)(O)(-->)(S) = 0.132 s(-1), k(II)(S)(-->)(O) = 2.1 x 10(-11) s(-1)). Finally complex 3a presents a relatively high activity as hydrogen transfer catalyst, with regard to its ability to transform acetophenone into 2-phenylethyl alcohol using 2-propanol as the source of hydrogen atoms.

Trinuclear pyrazine-bridged ruthenium complexes: syntheses, electrochemistry, NIR-Vis spectra, and their interpretation in terms of a 5-orbital-3-parameter model

A study of absorption spectra in the near-infrared (NIR) and visible (vis) regions of trinuclear Ru complexes containing pyrazine (pyz) as bridging ligand, trans-[(Ru(NH(3))(5)pyz)(2)Ru(NH(3))(4)](m+)(m = 6-9), is reported. The spectra were recorded on aqueous solutions containing the described species formed in situ by stoichiometric additions of a standard solution of Ce(SO(4))(2). They were interpreted in terms of a simple 5-orbital-3-parameter model which includes the effects of d-pi interaction and electronic correlation. The model is shown to account for the observed NIR-vis spectra of the complex ions. The 6+ parent species was synthesized by an improved literature method and fully characterized. The novel 8+ complex was also prepared and characterized. The 9+ ion was established to be slowly reduced by water, with dioxygen formation. Electrochemical (CV and DPV) studies were performed on the trinuclear 6+ complex, as well as on its constituent fragments [Ru(NH(3))(5)(pyz)](2+) and trans-[Ru(NH(3))(4)(pyz)(2)](2+).

Electrochemical assembling/disassembling of helicates with hysteresis

A series of eight tetradentate, ditopic, bisimino bisheterocyclic ligands (1-8), and their complexes with Cu(I) and Cu(II), have been studied in CH(3)CN solution, by means of (1)H NMR, mass, and UV/vis spectroscopy, while the crystal and molecular structure of the Cu(II) complexes [Cu(3)](CF(3)SO(3))(2) and [Cu(4)](CF(3)SO(3))(2) and of the Cu(I) complexes [Cu(2)(4)(2)](ClO(4))(2) and [Cu(2)(5)(2)](ClO(4))(2) have been determined by X-ray diffraction methods. The Cu(II) complexes are monomeric, almost square-planar structures, both in solution and in the solid state, while the Cu(I) complexes are two-metal, two-ligand dimers which can be both helical and "box-like" in the solid, while they adopt a simple helical configuration in acetonitrile solution. The systems made of ligands 1-8 and copper are bistable, as under the same conditions either the Cu(I) helical dimers or the Cu(II) monomers can be obtained and are stable. The electrochemical behavior of the 16 copper complexes has been studied in acetonitrile solutions by cyclic voltammetry. One reduction and one oxidation wave were found in all cases, which display no return wave and are separated by a 500-1000 mV interval. Irreversibility is due to the fast self-assembling process that follows the reduction of [Cu(II)(L)](2+) and to the fast disassembling process that follows the oxidation of [Cu(I)(2)(L)(2)](2+) (L = 1-8). However, the overall [oxidation+disassembling] or [reduction+self-assembling] processes, i.e., [Cu(I)(2)(L)(2)](2+) = 2[Cu(II)(L)](2+) + 2e(-), are fully reversible. Moreover, CV profiles show that solutions containing copper and L undergo hysteresis on changing the applied electrochemical potential: in the same potential interval, the systems can exist in solution as either [Cu(I)(2)(L)(2)](2+) or [Cu(II)(L)](2+), depending on the electrochemical history of the solution. Moreover, by changing the structural or donor features of the ligands it is possible to modulate the potentials at which the system undergoes a transition from one to the other of its two possible states, in the hysteresis cycle. In addition, the spectral properties of the Cu(I) and Cu(II) complexes of the considered ligands make these systems good candidates for storing information in solution, which can be electrochemically written or erased and spectroscopically read.

Artificial Molecular Machines

The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.