Molecular machines based on metal ion translocation

Hydrogen and copper ion-induced molecular reorganizations in scorpionand-like ligands. A potentiometric, mechanistic, and solid-state study

Two aza scorpionand-like macrocycles (L2 and L3) have been prepared. L2 consists of a tren amine with two of its arms cyclizized with a 2,6-bis(bromomethyl)pyridine. In L3, the remaining pendant arm has been further functionalized with a fluorophoric naphthalene group. X-ray data on the compounds [H(L3)]ClO4.H2O (1) and [H3(L3)](H2PO4)3.H2O (2) as well as solution studies (pH-metry, UV-vis, and fluorescence data) show the movement of the pendant arm as a result of the protonation degree of the macrocycles and of the formation of intramolecular hydrogen bonds. X-ray data on the complexes [Cu(L2)](ClO4)2]2.H2O (3) and [Cu(L3)](ClO4)2 (4) and solution studies on Cu2+ coordination show the implication of the nitrogen of the arm in the binding to the metal ion. Kinetic studies on the decomposition and formation of the Cu2+ complexes provide additional information about the pH-dependent molecular reorganizations. Moreover, the obtained information suggests that the kinetics of the tail on/off process is essentially independent of the lability of the metal center.

Redox-Driven Intramolecular Anion Translocation between a Metal Centre and a Hydrogen-Bond-Donating Compartment

Dicationic ligands incorporating two 2,2'-bipyridine units and two imidazolium moieties, [1](2+) and [2](2+), form stable chelate complexes with Cu(II) and Cu(I) in acetonitrile solution. Each Cu(II) complex binds two X(-) ions according to two stepwise equilibria, the first involving the Cu(II) centre and the second involving the bis-imidazolium compartment. Cu(I) complexes are able to host only one NO(3)(-) ion in the bis-imidazolium cavity, while other anions induce demetallation. Thus, in the presence of one equivalent of NO(3)(-), the Cu(II)/Cu(I) redox change makes the anion translocate quickly and reversibly from one binding site to the other within the [Cu(II,I)(1)](4+/3+) system, as demonstrated by cyclic voltammetry and controlled-potential electrolysis experiments.

Synthetic molecular motors and mechanical machines

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Controlling multiple fluorescent signal output in cyclic peptide-based supramolecular systems

A multicomponent equilibrium network based on self-assembling alpha,gamma-cyclic peptides with controlled fluorescence output is described. The network takes advantage of the large association constant of alpha,gamma-cyclic peptides and the controlled formation homo- and heterodimers, making use for the first time of excimer/FRET effects in conjunction for studying complex interaction networks. In addition, we study the Dapoxyl/pyrene FRET pair for the first time.

Digital logic circuit based on a single molecular system of salicylidene Schiff base

The salicylidene Schiff base N-3,5-dichloro-salicylidene-(S)-alpha-phenylethylamine (SPEA) has been synthesized and characterized. Stimulated by one optical input (UV light) and two chemical inputs (OH(-) and Zn(2+)), SPEA undergoes reactions of photochemistry, deprotonation, and complexation. Tailing these reactions by means of the UV-vis absorption spectra and fluorescence spectra, two obvious optical outputs, an absorption band at 323 nm and a fluorescent emission peak at 460 nm, have been obtained. On the basis of encoding binary digits in these inputs and outputs applying positive logic conventions, one monomolecular circuit, which integrates one OR, two NOT, and four AND gates, has been achieved.

A New Macrocyclic Cryptand with Squaramide Moieties: An Overstructured CuII Complex That Selectively Binds Halides: Synthesis, Acid/Base- and Ligational Behavior, and Crystal Structures

The synthesis and characterization of the novel 24,29-dimethyl-6,7,15,16-tetraoxotetracyclo-[19.5.5.0(5,8).0(14,17)]-1,4,9,13,18,21,24,29-octaazaenatriaconta-Delta(5,8),Delta(14,17)-diene (L) are reported. Molecule L incorporates two squaramide functions in a overstructured chain connecting two opposite nitrogen atoms of the Me(2)[12]aneN(4) polyaza macrocyclic base to obtain a cage topology. The basicity and binding properties of L towards Cu(II) were determined by means of potentiometric measurements in aqueous solution (298.1+/-0.1 K, I=0.15 mol dm(-3)). Molecule L behaves as a diprotic base under the experimental conditions employed and forms only mononuclear Cu(II) complexes in which the squaramide moieties are not involved in the stabilization of the metal ion that is stabilized by the amine functions of the polyaza base inside the three-dimensional cavity. The [CuL](2+) species was tested as a host for the series of halide anions. UV-visible spectrophotometric experiments permitted the determination of the addition constants of halides to the Cu(II)-complexed species. The [CuL](2+) species binds the anions F(-), Cl(-), and Br(-) by forming the [CuLX](+) species, but does not bind the biggest I(-) anion. A trend of selectivity as a function of the hydrogen-bonding capability as well as the dimensions of the anion were established; the maximum value of selectivity was for addition of the F(-) anion (log K=4.8). This selectivity is due to the presence of the overstructured chain containing the squaramide groups up to the Me(2)[12]aneN(4) macrocyclic base. The squaramide groups, by providing hydrogen-bond contacts, permit the [CuL](2+) species to selectively bind these anions through the formation of a hydrogen-bond network with F(-) and Cl(-). The crystal structures of the [CuLF](+) and [CuLCl](+) cations support the results obtained in aqueous solution.

A molecular gate based on a porphyrin and a silver lock

A Sn(IV) metallaporphyrin bearing a 4-pyridyl group on one meso position and a handle equipped also with a pyridyl unit functions in solution as a molecular gate in the presence of silver cation: the complexation-decomplexation of Ag(I) corresponds to the opening and closing motions of the gate.

Transition metal complexes as molecular machine prototypes

The field of molecular machines, i.e. multicomponent systems able to undergo large amplitude motions under the action of an external signal, has experienced a spectacular development since the beginning of the 1990s. Transition metal complexes have played an important role in this context, often as components of catenanes and rotaxanes. The present tutorial review will discuss a few systems of this type, taken from the contributions of our group or from others. The stimulus responsible for the controlled motion of the machine can be chemical, electrochemical, or photochemical. Examples of these three categories will be considered.

A Triethylenetetramine Bearing Anthracene and Benzophenone as a Fluorescent Molecular Logic Gate with Either−Or Switchable Dual Logic Functions

Fluorescence behaviors of a triethylenetetramine bearing anthracene (AN) and benzophenone (BP) fragments at the respective ends, L1, have been studied in water, where effects of pH (H+) and metal cations on the emission properties have been studied in detail. L1 behaves as a fluorescent molecular logic gate driven by H+ (Input1) and metal cations (Input2) as input chemicals. The most notable feature of L1 is that this molecule expresses the "either-or" switchable dual logic functions. Operation of L1 with Cu2+ as Input2 expresses the INHIBIT logic function, where a strong AN fluorescence appears only at pH 4 (with H+) without Cu2+ [Input1(1)-Input2(0)]. In contrast, operations of L1 with all other metal cations as Input2 express the TRANSFER logic function, where the presence of H+ allows strong AN fluorescence regardless of whether the metal cation exists or not [Input1(1)-Input2(0); Input1(1)-Input2(1)]. These emission switching behaviors of L1 are driven by the difference in the coordination stability between L1 and metal cations and the photoinduced intramolecular electron and energy transfer processes: (i) a pH-induced electron transfer from unprotonated nitrogen atoms of the polyamine chain to the photoexcited AN [ELT(N-->AN*)]; (ii) a pH- and metal coordination-induced electron transfer from the photoexcited AN to the ground-state BP [ELT(AN*-->BP)]; and (iii) a Cu2+ coordination-induced energy transfer from the photoexcited AN to Cu2+ [ENT(AN*-->Cu2+)].

Making molecular machines work

In this review we chart recent advances in what is at once an old and very new field of endeavour--the achievement of control of motion at the molecular level including solid-state and surface-mounted rotors, and its natural progression to the development of synthetic molecular machines. Besides a discussion of design principles used to control linear and rotary motion in such molecular systems, this review will address the advances towards the construction of synthetic machines that can perform useful functions. Approaches taken by several research groups to construct wholly synthetic molecular machines and devices are compared. This will be illustrated with molecular rotors, elevators, valves, transporters, muscles and other motor functions used to develop smart materials. The demonstration of molecular machinery is highlighted through recent examples of systems capable of effecting macroscopic movement through concerted molecular motion. Several approaches to illustrate how molecular motor systems have been used to accomplish work are discussed. We will conclude with prospects for future developments in this exciting field of nanotechnology.

Single and Double pH-Driven Cu2+ Translocation with Molecular Rearrangement in Alkyne-Functionalized Polyamino Polyamido Ligands

A new series of ligands, containing one (L1H(2)-L4H(2)) or two (L5H(4)-L6H(4)) 1,4,8,11-tetraaza-5,7-dione units and functionalized with a propargyl group on the C atom between the C=O moieties, has been synthesized. Protonation constants for the ligands and formation constants of their Cu(2+) complexes have been determined in water, and the coordination geometry of the complexes existing at various pH values has been investigated by coupled pH-metric and spectrophotometric titrations. Ligands capable of simple uptake of Cu(2+) with the formation of neutral, square-planar complexes containing the -2-charged diamino-diimido donor sets and ligands containing further coordinating groups (quinoline or pyridine) capable of single and double cation translocation have been investigated. The role of the substituents on the amino groups and the structural role played by the propargyl group have been examined as regards Cu(2+) complexation and translocation. In the double-translocating ligand L6H(4), when the two Cu(2+) ions move inside the diamino-diamido donor set, the slim propargyl group allows an unprecedented folding of the whole ligand with apical coordination of one pyridine to form a five-coordinate, square-pyramidal Cu(2+) ion. The crystal and molecular structures of this unusual [L6Cu(2)] complex have been determined by X-ray diffraction. Finally, oxidation of Cu(2+) to Cu(3+) has been studied by cyclic voltammetry in water, which revealed that the redox reaction occurs only when the copper cation is within the diamino-diimido compartment. Moreover, both functionalization of the primary amines with bulky substituents and apical coordination of Cu(2+) make access to the 3+ oxidation state more difficult and disrupt the reversibility of the electrochemical process.

An electrochiroptical molecular switch: mechanistic and kinetic studies

Previously, we reported the synthesis and preliminary characterization of Cu(I/II) complexes of N,N-bis(2-quinilylmethyl)-(l)-methionine (Zahn, S.; Canary, J. W. Science 2000, 288, 1404-7). The chemically oxidized and reduced forms of the complexes gave nearly mirror image circular dichroism (CD) spectra as a result of reorganization of the inner coordination sphere of the copper atom. The reorganization involved exchange of oxygen for sulfur in proceeding from the Cu(II) to Cu(I) oxidation state and corresponding ligand conformational changes required to accommodate this exchange. In this paper, we demonstrate that the complex can be triggered by electrochemical means. The electrochemical and stereochemical details of the redox-induced ligand reorganization were probed by independent synthesis of alternative chemical intermediates, CD spectroelectrochemistry, curve fitting of cyclic voltammograms, CD titration, and scanning electrochemical microscopy (SECM). A square-type mechanism was most consistent with the data: the Cu(II) complex is reduced followed by a ligand reorganization step to give the stable reduced form of the complex. The Cu(I) complex is then oxidized in a fast step followed by another ligand reorganization. A millisecond time scale rate was found by SECM for one of the key conformational conversion steps.

Supramolecular Self-Assembly of Mixed f−d Metal Ion Conjugates

[reaction: see text] The synthesis of the novel Eu(III) cyclen complex, Eu1, is described. In buffered pH 7.4 water, the Eu(III) emission was "switched on" upon excitation of the Terpy antenna. In the presence of various transition-metal ions (e.g., Fe(II) and Ni(II)), both the singlet-excited state and the Eu(III) emission were quenched ("switched off"), whereas in the ground state, an MLCT band was formed, signifying the formation of stable mixed linear trimetallic f-d supramolecular self-assemblies.

Synthesis and Photochemistry of a Two-Position Ru(terpy)(phen)(L)2+ Scorpionate Complex

A dissymmetric 1,10-phenanthroline chelate (N-phen-S) bearing two polyether chains terminated by two monodentate ligands of the benzonitrile (N) and dialkylesulfoxide (S) types was synthesized, characterized, and coordinated to ruthenium. The corresponding Ru(terpy)(N-phen-S)2+ complexes (terpy = 4'-(3,5-ditertiobutylphenyl)-2,2';6',2' '-terpyridine) were fully characterized as being two coordination isomers of the scorpionate type with one of the two tails occupying the sixth position on the coordination sphere. Photoexpulsion of the coordinated tail led to opening of the ruthena-macrocycle and subsequent rearrangement of the bidentate chelate. This rearrangement consisted of a 90 degrees rotation of the phenanthroline around the ruthenium atom. Selective irradiation of one isomer in a mixture of the two was undertaken using band-pass filters; this resulted in an enrichment of the nonirradiated isomer in the mixture. Thermal back-coordination of the tail was investigated in the dark. It took place quantitatively from the corresponding ruthenium chloride complex by trapping of the anion with silver salts.

Communicating Chemical Congregation: A Molecular AND Logic Gate with Three Chemical Inputs as a “Lab-on-a-Molecule” Prototype

We demonstrate the first three-input molecular AND logic gate based on three chemical inputs as a direct way of detecting congregations of chemical species. The AND gate operates in water and responds to Na+, H+, and Zn2+ inputs with an enhanced fluorescence signal when pre-set concentration thresholds are exceeded. Future "lab-on-a-molecule" devices could have application in medicine for rapid disease screening.

Potentiometric and spectrophotometric study of a new dipodal ligand N,N'-bis{2-[(2-hydroxybenzylidine)amino]ethyl}malonamide with Co(II), Ni(II), Cu(II) and Zn(II)

A new dipodal ligand, N,N'-bis{2-[(2-hydroxybenzylidine)amino]ethyl}malonamide (BHAEM) was synthesized by Schiff base condensation of N,N'-bis(2-aminoethyl)malonamide with two equivalent of salicylaldehyde and characterized on the basis of elemental analyses and various spectral (UV-vis, IR, (1)H NMR and (13)C NMR) data. The complexation reaction of the ligand with H(+), Co(II), Ni(II), Cu(II) and Zn(II) in solution was investigated by spectrophotometric and potentiometric method. Two protonation constants of BHAEM assigned for two hydroxyl groups of aromatic ring were determined and its hydrolysis mechanism was proposed through potentiometric result. In presence of metal ions, BHAEM shows different coordination properties. All metal ions form ML type complex where the ligand coordinates to the metal ion through two N-amine and two O-phenolate groups. In addition, Ni(II) and Cu(II) form additional complex species of the type MLH(-1) and MLH(-2), respectively due to ionization of amide protons. The molecular geometry of BHAEM was examined theoretically using the molecular mechanics MM3 force field followed by semi-empirical PM3 method and various spectral data UV-vis, IR and (1)H NMR were calculated from the energy-minimized structure applying semi-empirical ZINDO, PM3 and TNDO/2 method, respectively and compared with the experimental data. The probable structure of metal complexes in solution were proposed through calculated minimum strain energy by applying molecular mechanics MM(+) force field coupled with molecular dynamics simulation. Further the proposed structure of Cu(BHAEM) was refined through semi-empirical AM1/d method.

Membrane Media Create Small Nanospaces for Molecular Computation

Molecular computation is fashionable because of the small device sizes that should be possible. For instance, information processing in nanospaces is widespread in biology. However, experimental cases with computing molecules occupy rather large volumes until now (ca. 1012 nm3). Now we demonstrate small-scale computation using a designed logic gate in spheres of 3 nm radius (volume = ca. 102 nm3) which are produced by detergent micelles. The logic gate accommodated in the small nanospace fluoresces only when both H+ and Na+ ions are available.

Magnetic, electronic and electrochemical studies of mono and binuclear Cu(II) complexes using novel macrocyclic ligands

A series of new mono and binuclear copper (II) complexes [Cul]X(2)and [Cu(2)lX(2)] where 1 = L(1), L(2) and L(3) are the macrocyclic ligands. In mononuclear complexes the geometry of Cu(II) ion is distorted squareplanar and in binuclear complexes the geometry of Cu(II) is tetragonal. The synthesized complexes were characterized by spectroscopic (IR,UV-vis and ESR) techniques. Electrochemical studies of the complexes reveals that all the mononuclear Cu(II) complexes show a single quasireversible one-electron transfer reduction wave (E(pc) = -0.76 to -0.84V) and the binuclear complexes show two quasireversible one electron transfer reduction waves (E(pc)(1) = -0.86 to -1.01V, E(pc)(2) = -1.11 to -1.43V) in cathodic region. The ESR spectra of mononuclear complexes show four lines with nuclear hyperfine splittings with the observed g(11) values in the ranges 2.20-2.28, g( perpendicular) = 2.01-2.06 and A(11) = 125-273. The binuclear complexes show a broad ESR spectra with g = 2.10-2.11. The room temperature magnetic moment values for the mononuclear complexes are in the range [mu(eff) = 1.70-1.72BM] and for the binuclear complexes the range is [mu(eff) = 1.46-1.59BM].

Piezoluminescence at the air-water interface through dynamic molecular recognition driven by lateral pressure application

The steroid cyclophanes with a cyclic core consisting of a 1,6,20,25-tetraaza[6.1.6.1]paracyclophane connected to four steroid moieties (cholic acid or cholanic acid) through a flexible l-lysine spacer were spread on water as Langmuir monolayers. The pi-A isotherm of the cholic-type steroid cyclophane includes a transition to the condensed phase with a limiting area of approximately 2 nm(2). This value is close to the cross-sectional area of the steroid cyclophane with a standing-up conformation of the cholic acid moieties, strongly suggesting that the cavity converts from a two-dimensional cavity to a three-dimensional cavity upon compressing the monolayer. Surface-reflective fluorescence spectroscopy of the monolayer using an aqueous fluorescent probe (6-(p-toluidino)naphthalene-2-sulfonate (TNS)) showed an abrupt increase in the TNS fluorescence intensity at a molecular area of 2 nm(2). Efficient binding of the guest probe would occur upon the completion of the three-dimensional cavity. Repeated compression and expansion induces periodic changes in the fluorescence intensity. This indicates a piezoluminescence effect through the catch and release of the TNS guest upon dynamic cavity formation. Analyses of the binding behavior of TNS to the steroid cyclophane resulted in binding constants in the range of approximately (5-9) x 10(4) M(-1) which are similar to that observed in lipid bilayer media (K = 5.1 x 10(4) M(-1)). The fluorescence intensity within the condensed phase was significantly increased with increasing pressure, suggesting that suppression of the molecular motion of the bound TNS may retard the nonemission process. Similar monolayer experiments were carried out with the monolayer of the cholanic-type steroid cyclophane that cannot form an open conformation on water. Both the phase transition in the pi-A isotherm and the change in the fluorescence intensity were negligible, confirming that the dynamic characteristic of the cavity is indispensable for the efficient pressure-induced binding of the guest and the consequent luminescence.

Method for the Evaluation of the Reorganization Energy of Electron Transfer Reactions Produced under Restricted Geometry Conditions

The kinetics of the electron transfer reactions between S(2)O(8)(2-) and the complexes (Ru(NH(3))(5)L)(2+) (L = pyridine, pyrazine, and 4-cyanopyridine) have been studied in micellar (SDS) solutions. A method for the evaluation of the reorganization energy of these reactions, based on the comparison of their rate constants, is proposed. From the results obtained, we concluded that the observed rate constants go through a minimum for the surfactant concentration in which the reorganization energy goes through a maximum. The method can be applied to any kind of restricted geometry conditions.

Redox-Triggered Molecular Movement in a Multicomponent Metal Complex in Solution and in the Solid State

The Cu(I) and Cu(II) complexes of the new 1,8-diferro-cenylmethyl-4,11-dimethyl-1,4,8,11-tetraazacyclotetra-decane ligand (denoted L) have been isolated and characterized by X-ray structure determination and electrochemical studies. The Cu(I) complex presents an unprecedented stability toward dioxygen. The two complexes adopt two energetically distinct and stable geometries, which differ mainly by the relative positioning of the substituents above or below the cyclam plane. Triggered by a copper-centered electron transfer, a fast and reversible motion of the noncoordinating subunits is obtained in homogeneous solution and in the solid state.

Highly Selective Colorimetric Naked-Eye Cu(II) Detection Using an Azobenzene Chemosensor

Colorimetric azobenzene based chemosensors 1 and 2 were designed for detection of transition-metal ions such as Cu(II) under physiological pH conditions. The internal charge transfer (ICT) sensors are highly colored, absorbing in the green. For 1, the Cu(II) recognition gives rise to red-to-yellow color changes that are visible to the naked-eye and reversible upon addition of EDTA, whereas for 2, which lacks the aromatic o-methoxy chelating group, no such changes were observed.

Toward a Photochemical and Thermal Molecular Machine: Reversible Ligand Dissociation and Binding in a Ruthenium(II)-2,2‘-bipyridine Complex with Tris(2-pyridylmethyl)amine

A Ru(II) complex having tris(2-pyridylmethl)amine (TPA) and 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)]X(2) (X = ClO(4), PF(6)), exhibited a severe distortion of the coordination of the axial pyridine moiety of TPA due to steric hindrance. The complex showed interesting dissociation-binding behavior of the axial pyridine arm to form a solvent adduct with TPA ligation in a unique meridional tridentate fashion. The complex undergoes thermal dissociation to form solvent-coordinated species via an S(N)2-like mechanism with activation energy of 117 kJ/mol. In contrast, the complex showed reversible photochemical dissociation and rebinding via an S(N)1-like mechanism by MLCT irradiation. The photochemical dissociation was accelerated approximately 200-fold faster than the thermal process. The dissociation process involves selective binding behavior toward external ligands (solvents) with pi-acceptor character, which is indispensable, and no sigma-donating molecules could bind to the Ru(II) center. The guest molecule can be released upon photoirradiation after its thermal binding.

Stability and kinetics of the acid-promoted decomposition of Cu(ii) complexes with hexaazacyclophanes: kinetic studies as a probe to detect changes in the coordination mode of the macrocycles

The synthesis, protonation and Cu(II) coordination features of the novel azacyclophane type receptors 2,6,10,13,17,21-hexaza[22]-(2,6)-pyridinophane (L2), 2,6,9,12,15,19-hexaza[20]-(2,6)-pyridinophane (L5) and 2,6,9,12,15,19-hexaza[20]metacyclophane (L6) are presented. The protonation and Cu(II) constants are analysed and compared with the previously reported open-chain polyamines 4,8,11,15-tetrazaoctadecane-1,18-diamine (L1) and 4,7,10,13-tetraazahexadecane-1,16-diamine (L4) and of the cyclophane 2,6,10,13,17,21-hexaaza[22]paracyclophane (L3). All the systems form mono- and dinuclear complexes whose stability and pH range of existence depend on the type of hydrocarbon chains and molecular topology. The effects of the cyclic or open-chain nature and of the presence of the pyridine rings on the protonation and formation of mono- and dinuclear complexes are discussed. Stopped-flow kinetic measurements on the acid-promoted decomposition of the Cu(II) complexes have been carried out for the different systems. With respect to the decomposition of the dinuclear complexes, because the size of the macrocycles forces both metal ions to be close to each other, the release of the first ion occurs within the mixing time of the stopped-flow except for the dinuclear complexes of L2. However, the most interesting kinetic result is the observation of different kinetics of decomposition for the different mononuclear complexes formed by a given ligand. This effect is especially evident for L3 and L6 and indicates a change in the coordination mode of the ligand for the different mononuclear species. Therefore the Cu(II) ion performs a slippage motion through the macrocyclic cavity driven by pH changes. The stopped-flow experiments are an excellent tool to detect these slippage processes that may be present for the complexes with other macrocycles.

On, off and intermediate coordination of a bridgehead triarylamine donor in tripodal complexes: Towards the tuning of coordinative bond distance

A series of first row transition metal complexes of the tripodal ligand 2,2',2"-nitrilotribenzoic acid H3L has been prepared and characterised by X-ray crystallography: Mononuclear [M(L)]- species [Cu(H2O)4]3[Cu(L)(H2O)]6.25H2O (2), [Co(H2O)6][Co(L)(H2O)].8H2O (4), [Zn(H2O)6][Zn(L)(H2O)].8H2O (5) and a neutral [M(L)] complex [Fe(III)2(L)(H2O)3].5H2O (8) are formed as well as dimeric [M(L)]2 2- species (HNEt3)2[Cu(L)]2.2CH3CN (1), (HNEt3)3[Ni(L)]2(ClO4).H2O (3), (HNEt3)2[Fe(II)(L)]2.2CH3CN (6) and (HNEt3)2[Fe(III)2(L)2(mu-O)](7). The complexes display a unique variation in the M-N distance (2.09 A for Cu(II) to 3.29 A for Fe(III)) to the bridgehead triphenylamine donor and are classified into compounds with "On","Off" and "Intermediate" N-coordination. The trigonal-bipyramidal coordination polyhedron changes towards tetrahedral in the intermediate and octahedral in the Off-state. The M-N distance of individual complexes is reversibly tuned by external chemical input such as changes of metal ion oxidation state (Fe(II)/Fe(III)) or variation of the axial coligand as a consequence of solvent or pH variation. Possible reasons for the exceptional tolerance of the M-N bond to distance variations are discussed under consideration of gas phase DFT calculations of [Zn(L)]-.