Double-decker phthalocyanine complexes with Tb3+ or Dy3+ showed slow magnetization relaxation as a single-molecular property. The temperature ranges in which the behavior was observed were far higher than that of the transition-metal-cluster single-molecule magnets (SMMs). The significant temperature rise results from a mechanism in the relaxation process different from that in the transition-metal-cluster SMMs. The effective energy barrier for reversal of the magnetic moment is determined by the ligand field around a lanthanide ion, which gives the lowest degenerate substate a large |Jz| value and large energy separations from the rest of the substates in the ground-state multiplets.

Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level

ion phase that leads to an alkyl radical coordinated to the iron-hydroxo complex by a weak OH---C hydrogen bond, labeled as CI; (ii) an alkyl (or OH) rotation phase whereby the alkyl group achieves a favorable orientation for rebound; and (iii) a rebound phase that leads to C-O bond making and the ferric-alcohol complexes, 4,2P. The two profiles remain close in energy throughout the first two phases and then bifurcate. Whereas the HS state exhibits a significant barrier and a genuine TS for rebound, in the LS state, once the right orientation of the alkyl group is achieved, the LS rebound proceeds in a virtually barrier-free fashion to the alcohol. As such, alkane hydroxylation proceeds by TSR,70-72,120 in which the HS mechanism is truly stepwise with a finite lifetime for the radical intermediate, whereas the LS mechanism is effectively concerted with an ultrashort lifetime for the radical intermediate. Subsequent studies of ethane and camphor hydroxylation by the Yoshizawa group117,181-183 arrived at basically the same conclusion, that the mechanism is typified by TSR. The differences between the results of Shaik et al.130,173,177-180 and Yoshizawa et al.117,181-183 were rationalized recently71,72 and shown to arise owing to technical problems and the choice of the mercaptide ligand,117,181-183 which is a powerful electron donor and is too far from the representation of cysteine in the protein environment. The most recent study of camphor hydroxylation, which was done at a higher quality,117 converged to the picture reported by Shaik et al.130,173,177-180 and shows a stepwise HS process with a barrier of more than 3 kcal/mol for C-O bond formation by rebound of the camphoryl radical vis-à-vis an effectively concerted LS process for which this barrier is 0.7 kcal mol-1 and is the rotational barrier for reaching the rebound position. By referring to Figure 21, it is possible to rationalize the clock data of Newcomb in a simple manner. The apparent lifetimes are based on the assumption that there is a single state that leads to the reaction, such that the radical lifetime can be quantitated from the rate constant of free radical rearrangement and the ratio of rearranged to unrearranged alcohol product. However, in TSR, the rearranged (R) product is formed only/mainly on the HS surface, while the unrearranged (U) product is formed mainly on Figure 20. Formal descriptions of iron(III)-peroxo, iron(III)-hydroperoxo, and iron(V)-oxo species with indication of the negative charges. The roles “electrophile” or “nucleophile” are assigned according to the charge type. Reprinted with permission from ref 7. Copyright 2000 Springer-Verlag Heidelberg. 3964 Chemical Reviews, 2004, Vol. 104, No. 9 Meunier et al.

Mechanism of oxidation reactions catalyzed by cytochrome p450 enzymes.

The widespread use of molecular machines in biology has long suggested that great rewards could come from bridging the gap between synthetic molecular systems and the machines of the macroscopic world. In the last two decades, it has proved possible to design synthetic molecular systems with architectures where triggered large amplitude positional changes of submolecular components occur. Perhaps the best way to appreciate the technological potential of controlled molecular-level motion is to recognize that nanomotors and molecular-level machines lie at the heart of every significant biological process. Over billions of years of evolution, nature has not repeatedly chosen this solution for performing complex tasks without good reason. When mankind learns how to build artificial structures that can control and exploit molecular level motion and interface their effects directly with other molecular-level substructures and the outside world, it will potentially impact on every aspect of functional molecule and materials design. An improved understanding of physics and biology will surely follow.

The first steps on the long path to the invention of artificial molecular machines were arguably taken in 1827 when the Scottish botanist Robert Brown observed the haphazard motion of tiny particles under his microscope.1,2 The explanation for Brownian motion, that it is caused by bombardment of the particles by molecules as a consequence of the kinetic theory of matter, was later provided by Einstein, followed by experimental verification by Perrin.3,4 The random thermal motion of molecules and its implications for the laws of thermodynamics in turn inspired Gedankenexperiments (“thought experiments”) that explored the interplay (and apparent paradoxes) of Brownian motion and the Second Law of Thermodynamics. Richard Feynman’s famous 1959 lecture “There’s plenty of room at the bottom” outlined some of the promise that manmade molecular machines might hold.5,6 However, Feynman’s talk came at a time before chemists had the necessary synthetic and analytical tools to make molecular machines. While interest among synthetic chemists began to grow in the 1970s and 1980s, progress accelerated in the 1990s, particularly with the invention of methods to make mechanically interlocked molecular systems (catenanes and rotaxanes) and control and switch the relative positions of their components.7−24

Here, we review triggered large-amplitude motions in molecular structures and the changes in properties these can produce. We concentrate on conformational and configurational changes in wholly covalently bonded molecules and on catenanes and rotaxanes in which switching is brought about by various stimuli (light, electrochemistry, pH, heat, solvent polarity, cation or anion binding, allosteric effects, temperature, reversible covalent bond formation, etc.). Finally, we discuss the latest generations of sophisticated synthetic molecular machine systems in which the controlled motion of subcomponents is used to perform complex tasks, paving the way to applications and the realization of a new era of “molecular nanotechnology”.

1.1. The Language Used To Describe Molecular Machines
Terminology needs to be properly and appropriately defined and these meanings used consistently to effectively convey scientific concepts. Nowhere is the need for accurate scientific language more apparent than in the field of molecular machines. Much of the terminology used to describe molecular-level machines has its origins in observations made by biologists and physicists, and their findings and descriptions have often been misinterpreted and misunderstood by chemists. In 2007 we formalized definitions of some common terms used in the field (e.g., “machine”, “switch”, “motor”, “ratchet”, etc.) so that chemists could use them in a manner consistent with the meanings understood by biologists and physicists who study molecular-level machines.14

The word “machine” implies a mechanical movement that accomplishes a useful task. This Review concentrates on systems where a stimulus triggers the controlled, relatively large amplitude (or directional) motion of one molecular or submolecular component relative to another that can potentially result in a net task being performed. Molecular machines can be further categorized into various classes such as “motors” and “switches” whose behavior differs significantly.14 For example, in a rotaxane-based “switch”, the change in position of a macrocycle on the thread of the rotaxane influences the system only as a function of state. Returning the components of a molecular switch to their original position undoes any work done, and so a switch cannot be used repetitively and progressively to do work. A “motor”, on the other hand, influences a system as a function of trajectory, meaning that when the components of a molecular motor return to their original positions, for example, after a 360° directional rotation, any work that has been done is not undone unless the motor is subsequently rotated by 360° in the reverse direction. This difference in behavior is significant; no “switch-based” molecular machine can be used to progressively perform work in the way that biological motors can, such as those from the kinesin, myosin, and dynein superfamilies, unless the switch is part of a larger ratchet mechanism.14

Artificial Molecular Machines

Homogeneous catalysis by transition metal oxygen anion clusters

The Design of Molecular Hosts, Guests, and Their Complexes (Nobel Lecture)†

The bis(phthalocyaninato)lutetium(III) derivative LuPc/sub 2/ x CH/sub 2/Cl/sub 2/ (1, LuC/sub 65/H/sub 34/N/sub 16/Cl/sub 2/) and the (phthalocyaninato)lutetium(III) complex (LuPc(OAc)(H/sub 2/O)/sub 2/) x H/sub 2/O x 2CH/sub 3/OH (2, LuC/sub 36/H/sub 33/N/sub 8/O/sub 7/) were synthesized in boiling 1-hexanol with use of lutetium(III) acetate and 1,2-dicyanobenzene in the presence of 1,8-diazabicyclo(5.4.0)undec-7-ene and purified by chromatography. The visible spectroscopic properties are reported for both complexes. 1 crystallizes in the orthorhombic system with a = 28.242 (9) A, b = 22.877 (8) A, c = 8.050 (4) A, Z = 4, and space group Pnma. 2 belongs to the monoclinic system with a = 12.033 (3) A, b = 20.807 (6) A, c = 14.011 (3) A, ..beta.. = 95.34 (2)/sup 0/, space group P2/sub 1//n, and Z = 4. 1 is a sandwich type complex in which the lutetium atom is eightfold coordinated by the isoindole nitrogens (N/sub iso/) of two staggered phthalocyanine rings. 28 references, 5 figures, 6 tables.

Synthesis, structure, and spectroscopic and magnetic properties of lutetium(III) phthalocyanine derivatives: LuPc2.CH2Cl2 and [LuPc(OAc)(H2O)2].H2O.2CH3OH

The X-ray structures of the β-substituted-pyrrole, its nickel(II) complex (NiTMOBP), and the nickel(II) derivative are reported. The synthesis of the free base H 2 TPFPOBP is also described. All these molecules are nonplanar, displaying saddle-shaped conformations

.beta.-Halogenated-pyrrole porphyrins. Molecular structures of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetramesitylporphyrin, nickel(II) 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetramesitylporphyrin, and nickel(II) 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin

Tervalent-Metal Porphyrin-Phthalocyanine Heteroleptic Sandwich-Type Complexes. Synthesis, Structure, and Spectroscopic Characterization of Their Neutral, Singly-Oxidized, and Singly-Reduced States

Oxoferryl porphyrin π cation radical species have been generated by the oxidative action of m-chloroperoxybenzoic acid on the triflato complexes of (tetrakis(2,6-dichlorophenyl)porphyrinato]- and (tetrakis(2,4,6-trimethoxyphenyl)porphyrinato]-iron(III) in methylene chloride/methanol-d 4 . The new high-valent complexes have been characterized by a variety of spectroscopic techniques.

Models for peroxidase compound I: generation and spectroscopic characterization of new oxoferryl porphyrin .pi. cation radical species

A systematic electrochemical, redox, optical absorption, and resonance Raman spectroscopic study of 22 gadolinium and cerium double- and triple-decker tetraphenylporphyrin-phthalocyanine (TPP-Pc) complexes is reported. The work is aimed at elucidating the extent of the localization or delocalization among the chromophore ligands in the mixed complexes, where there exists a charge imbalance between the metal centers and the dianion Pc/TPP ligands, as well as in different oxidation states for these types of complexes. For mixed double- and triple-decker complexes, [vert bar]Pc(M)TPP[vert bar] and [vert bar]TPP(M)TPP[vert bar] (where M = Gd[sup III], Ce[sup IV]), in which the metal cannot be oxidized, it appears that the ligand preferentially oxidized is the Pc moiety. This perfentially localized hole in the phthalocyanine results in new near-infrared transitions which show maxima around 1200-1300 nm for the double-decker complexes and shifts to 1800-2300 nm for the triple-decker derivatives. In this case, the removal of an electron from a complex redox orbital which is rich in Pc character is easily identified by the accompanying changes in the optical absorption characteristic of a Pc[sup [sm bullet][minus]] species and is also seen in FT Raman spectra (excited at 1064 nm) by a significant (>15 cm[sup [minus]1]) upshift of a redox-sensitivemore » Raman band at ca. 1500 cm[sup [minus]1]. 33 refs., 13 figs., 2 tabs.« less

Raymond Weiss

Papers

Synthesis, structure, and spectroscopic and magnetic properties of lutetium(III) phthalocyanine derivatives: LuPc2.CH2Cl2 and [LuPc(OAc)(H2O)2].H2O.2CH3OH

.beta.-Halogenated-pyrrole porphyrins. Molecular structures of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetramesitylporphyrin, nickel(II) 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetramesitylporphyrin, and nickel(II) 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin

Tervalent-Metal Porphyrin-Phthalocyanine Heteroleptic Sandwich-Type Complexes. Synthesis, Structure, and Spectroscopic Characterization of Their Neutral, Singly-Oxidized, and Singly-Reduced States

Models for peroxidase compound I: generation and spectroscopic characterization of new oxoferryl porphyrin .pi. cation radical species

Hole localization or delocalization ? an optical, raman, and redox study of lanthanide porphyrin-phthalocyanine sandwich-type heterocomplexes