Solvated-Ion-Pairing-Sensitive Molecular Bistability Based on Copper(I)-Coordinated Pyrimidine Ring Rotation

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper­(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both <b>2</b>·BF₄ {<b>2</b>⁺ = [Cu­(Mepypm)­(dppp)]⁺, where Mepypm = 4-methyl-2-(2′-pyridyl)­pyrimidine and dppp = 1,3-bis­(diphenylphosphino)­propane} and previously reported <b>1</b>·BF₄, which possesses a bulky diphosphine ligand (<b>1</b>⁺ = [Cu­(Mepypm)­(DPEphos)]⁺, where DPEphos = bis­[2-(diphenylphosphino)­phenyl] ether). Two rotational isomers of <b>2</b>⁺ were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of <b>2</b>·BF₄ in CDCl₃ (Δ<i>H</i> = 6 kJ mol^–1; Δ<i>S</i> = 25 J K^–1 mol^–1) were more positive than that tested under other conditions, such as in more polar solvents CD₂Cl₂, acetone-<i>d</i>₆, and CD₃CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of <b>2</b>⁺ with a bulky counterion, such as B­(C₆F₅)₄^–. The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions

Formation of a Hexacarbonyl Diiron Complex Having a Naphthalene-1,8-bis(phenylphosphido) Bridge and the Electrochemical Behavior of Its Derivatives

The P–P bond of the <i>cis</i>-<b>1</b> ligand in (μ-<i>cis</i>-<b>1</b>)­[Fe­(CO)₄]₂ (<i>cis</i>-<b>1</b> = naphthalene-1,8-diphenyldiphosphine) was cleaved by the two iron centers after CO dissociation from the iron centers, although the P–P bond of <i>cis</i>-<b>1</b> was stereochemically stabilized with a robust naphthalene group, unlike the usual diphosphines, which lack such support. The resulting (μ-nabip)­[Fe­(CO)₃]₂ (<b>3</b>; nabip = naphthalene-1,8-bis­(phenylphosphido)) had the diiron core linked by the bisphosphido bridge. Since the trans isomer (μ-<i>trans</i>-<b>1</b>)­[Fe­(CO)₄]₂ was stable under ambient conditions, the <i>cis</i> disposition of the two Fe­(CO)₄ fragments was responsible for the cleavage of the P–P bond. The one or two terminal CO ligands of <b>3</b> can be replaced by MeCN and a range of phosphine ligands: i.e., PMe₃, PPh₃, <i>cis</i>-<b>1</b>, and <i>trans</i>-<b>1</b>. Interestingly, it was found that the diphosphine <i>cis</i>-<b>1</b> could coordinate the iron center in an unusual κ² fashion to form a three-membered ring, which was confirmed by NMR spectra as well as X-ray analysis. These diiron complexes can be protonated with the strong acid TfOH in CH₂Cl₂ to form cationic complexes having a μ-H bridge between the two iron centers. The parent hexacarbonyl complex <b>3</b> could act as a proton reduction catalyst at −2.0 V in the presence of TsOH as the proton source in CH₂Cl₂. When protonated complexes having MeCN or phosphine ligands were used, the proton reduction potentials catalyzed by these complexes were shifted to a more positive range of around −1.77 to −1.37 V, depending on the terminal ligand

Solvated-Ion-Pairing-Sensitive Molecular Bistability Based on Copper(I)-Coordinated Pyrimidine Ring Rotation

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper­(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both <b>2</b>·BF₄ {<b>2</b>⁺ = [Cu­(Mepypm)­(dppp)]⁺, where Mepypm = 4-methyl-2-(2′-pyridyl)­pyrimidine and dppp = 1,3-bis­(diphenylphosphino)­propane} and previously reported <b>1</b>·BF₄, which possesses a bulky diphosphine ligand (<b>1</b>⁺ = [Cu­(Mepypm)­(DPEphos)]⁺, where DPEphos = bis­[2-(diphenylphosphino)­phenyl] ether). Two rotational isomers of <b>2</b>⁺ were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of <b>2</b>·BF₄ in CDCl₃ (Δ<i>H</i> = 6 kJ mol^–1; Δ<i>S</i> = 25 J K^–1 mol^–1) were more positive than that tested under other conditions, such as in more polar solvents CD₂Cl₂, acetone-<i>d</i>₆, and CD₃CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of <b>2</b>⁺ with a bulky counterion, such as B­(C₆F₅)₄^–. The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions

Reversible Copper(II)/(I) Electrochemical Potential Switching Driven by Visible Light-Induced Coordinated Ring Rotation

We here describe the first metal complex system in which electronic signals can be repeatedly extracted by converting bistable states related to an intramolecular ligand rotational motion, which is fueled by visible light. The molecular structure for relating an electron transfer and a motion consists of a copper center and a coordinated unsymmetrically substituted pyrimidine derivative, whose rotational isomerization causes an electrochemical potential shift. To harness light energy effectively through metal-to-ligand charge transfer (MLCT) excitation, we prepared a simple copper­(I) complex coordinated by a 4-methyl-2-(6′-methyl-2′-pyridyl)­pyrimidine and a bulky diimine. The thermodynamic and kinetic parameters of redox and rotational reactions were analyzed by cyclic voltammograms at variable temperatures, by considering four stable isomers related to copper­(II)/(I) states and rotational isomeric states. The key feature of this compound is that the rotation is frozen in the copper­(I) state (rate constant for the rotation, <i>k</i>_{I<i>i→o</i>} = 10^–4 s^–1) but is active in the copper­(II) state (<i>k</i>_{II<i>i→o</i>} = 10^–1 s^–1) at 203 K. The compound makes a bypass route to the isomeric metastable copper­(I) state, via a tentative copper­(II) state formed by photoelectron transfer (PET) in the presence of a redox mediator, decamethylferrocenium ion (DMFc⁺), or upon a partial oxidation of the complex. Light- and heat-driven rotation in the copper­(I) state with a potential shift (Δ<i>E</i>°′ = 0.14 V) was analyzed by electrochemical measurements of the complex in the solution state. The rotor could be reset to the initial state by heating, thereby completing the cycle and enabling repeated operation fueled by light energy. A significant redox potential shift associated with the copper­(II)/(I) transition accompanied the rotation, thereby providing a new type of molecular signaling system

Regulation of the Rate of Dinucleation of a Monocopper(I) Complex Containing Bipyrimidine Rotary Units by Restricted Double Pyrimidine Rotation

New copper­(I) complexes with coordinated 2-(4′-methyl)­pyrimidinyl moieties were fabricated, and the isomerism of their pyrimidine ring linkage was investigated. The ligands bis­[2-(diphenylphosphino)­phenyl] ether (DPEPhos) and 4,4′-dimethyl-2,2′-bipyrimidine (dmbpm) were used to synthesize a heteroleptic copper­(I) complex, [Cu^I(DPEPhos)­(dmbpm)]·BF₄ (<b>1</b>·BF₄), and a dinuclear copper­(I) complex, [(Cu^I)₂(DPEPhos)₂(μ-dmbmp)]­(BF₄)₂ [<b>2</b>·(BF₄)₂]. The X-ray crystallographic structures, UV–vis absorption spectra, and luminescence properties of the complexes were analyzed. The thermodynamic and kinetic aspects of the isomerism of <b>1</b>·BF₄ were examined by variable-temperature NMR. Double pyrimidine ring rotation was found to be restricted sterically by the bulky DPEPhos ligands. This limited the number of the possible isomers: <b>1</b>·BF₄ showed only isomers with either one (<i>io</i> isomer) or both (<i>oo</i> isomer) of the two methyl groups positioned away from the copper center, while dinuclear <b>2</b>·(BF₄)₂ was only found as a symmetric (<i>io</i>–<i>io</i>) isomer, with each of the two methyl groups positioned toward different copper centers. The addition of [Cu­(MeCN)₂(DPEPhos)] (<b>3</b>·BF₄) allowed both isomers of <b>1</b>·BF₄ to form <b>2</b>·(BF₄)₂, although at different rates and via different pathways, which were analyzed using time-dependent UV–vis spectroscopy. The <i>io</i> isomer dinucleated more quickly than the <i>oo</i> isomer owing to it being able to form <b>2</b>·(BF₄)₂ (i) without bond dissociation and (ii) without a sterically congested <i>ii</i> configuration around the copper center. In contrast, <i>oo</i>-<b>1</b>·BF₄ required (i) recombination of the bipyrimidine coordination bonds or (ii) formation of a product with higher thermodynamic energy, unsymmetric (<i>ii</i>–<i>oo</i>) <b>2</b>·(BF₄)₂. These findings are interpreted as demonstrating a novel kinetic property: a conversion rate determined by pyrimidine ring inversion

Ferrocene–Dithiolene Hybrids: Control of Strong Donor–Acceptor Electronic Communication to Reverse the Charge Transfer Direction

We prepared a novel class of ferrocene–dithiolene hybrid molecules, FcS₄dt­(Me)₂ and FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)] (where FcS₄dt indicates 2-(1,3-dithia[3]­ferrocenophane-2-ylidene)-1,3-dithiole-4,5-dithiolate and ^<i>t</i>Bu₂bpy indicates 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine), in which the ferrocene moiety was bound to the planar conjugated dithiolene skeleton via two sulfur atoms such that the cyclopentadienyl rings were perpendicular to the dithiolene backbone. The physical properties and electronic structures of the complexes and their oxidized species [FcS₄dt­(Me)₂]^•+ and [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]^•+ were investigated by means of single-crystal X-ray diffraction (XRD) analysis, cyclic voltammetry, electron paramagnetic resonance (EPR), and UV–vis near infrared (UV–vis–NIR) spectroscopy. The electron density distributions of the highest occupied molecular orbitals (HOMOs) of FcS₄dt­(Me)₂ and FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)] differed remarkably in that the HOMO of the former was ferrocene-based whereas that of the latter was dithiolene-based. The differences in the HOMO distributions originated from the energy level of the dithiolene-based π-orbital in each of the complexes, which was controlled by changing R in FcS₄dt­(R)₂ (R = Me for FcS₄dt­(Me)₂; 2R = Pt­(^<i>t</i>Bu₂bpy) for FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]). We succeeded in analyzing the crystal structure of [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]­(F₄TCNQ)·C₆H₁₄·CH₂Cl₂ (where F₄TCNQ indicates 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), which provided a rare example of the crystal structure of a [Pt­(diimine)­(dithiolate)]^•+ ion-based complex. A comparison of the bond lengths in FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)] and [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]^•+ suggested that the latter complex displayed a conjugated dithiolene-based π-radical character. These considerations agreed well with the electronic structures calculated using density functional theory (DFT) and time-dependent­(TD)-DFT methods. Significant electronic communication between the ferrocene and dithiolene moieties was detected for both [FcS₄dt­(Me)₂]^•+ and [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]^•+ in the appearance of an intramolecular charge transfer band, which was hardly observed for previously reported ferrocene–dithiolene hybrid molecules. The charge transfer direction was reversed between the two cations. The electron coupling parameter <i>H</i>_AB and the potential energy curves of the oxidized complexes were estimated based on the classical two-state Marcus–Hush theory. These results suggest that FcS₄dt-based metalladithiolenes can exhibit controllable electronic structures expressed as double-minimum potential energy curves

Ferrocene–Dithiolene Hybrids: Control of Strong Donor–Acceptor Electronic Communication to Reverse the Charge Transfer Direction

We prepared a novel class of ferrocene–dithiolene hybrid molecules, FcS₄dt­(Me)₂ and FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)] (where FcS₄dt indicates 2-(1,3-dithia[3]­ferrocenophane-2-ylidene)-1,3-dithiole-4,5-dithiolate and ^<i>t</i>Bu₂bpy indicates 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine), in which the ferrocene moiety was bound to the planar conjugated dithiolene skeleton via two sulfur atoms such that the cyclopentadienyl rings were perpendicular to the dithiolene backbone. The physical properties and electronic structures of the complexes and their oxidized species [FcS₄dt­(Me)₂]^•+ and [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]^•+ were investigated by means of single-crystal X-ray diffraction (XRD) analysis, cyclic voltammetry, electron paramagnetic resonance (EPR), and UV–vis near infrared (UV–vis–NIR) spectroscopy. The electron density distributions of the highest occupied molecular orbitals (HOMOs) of FcS₄dt­(Me)₂ and FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)] differed remarkably in that the HOMO of the former was ferrocene-based whereas that of the latter was dithiolene-based. The differences in the HOMO distributions originated from the energy level of the dithiolene-based π-orbital in each of the complexes, which was controlled by changing R in FcS₄dt­(R)₂ (R = Me for FcS₄dt­(Me)₂; 2R = Pt­(^<i>t</i>Bu₂bpy) for FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]). We succeeded in analyzing the crystal structure of [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]­(F₄TCNQ)·C₆H₁₄·CH₂Cl₂ (where F₄TCNQ indicates 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), which provided a rare example of the crystal structure of a [Pt­(diimine)­(dithiolate)]^•+ ion-based complex. A comparison of the bond lengths in FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)] and [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]^•+ suggested that the latter complex displayed a conjugated dithiolene-based π-radical character. These considerations agreed well with the electronic structures calculated using density functional theory (DFT) and time-dependent­(TD)-DFT methods. Significant electronic communication between the ferrocene and dithiolene moieties was detected for both [FcS₄dt­(Me)₂]^•+ and [FcS₄dt­[Pt­(^<i>t</i>Bu₂bpy)]]^•+ in the appearance of an intramolecular charge transfer band, which was hardly observed for previously reported ferrocene–dithiolene hybrid molecules. The charge transfer direction was reversed between the two cations. The electron coupling parameter <i>H</i>_AB and the potential energy curves of the oxidized complexes were estimated based on the classical two-state Marcus–Hush theory. These results suggest that FcS₄dt-based metalladithiolenes can exhibit controllable electronic structures expressed as double-minimum potential energy curves

Carbon(0)-Bridged Pt/Ag Dinuclear and Tetranuclear Complexes Based on a Cyclometalated Pincer Carbodiphosphorane Platform

A carbon(0)-bridged Pt₂Ag₂ cluster was synthesized from the reaction of a cyclometalated pincer carbodiphosphorane platinum complex with AgOTf, by forming Pt­(II)←C(0)→Ag­(I) dative bonds along with Pt­(II)–Ag­(I) and Ag­(I)–Ag­(I) metal–metal interactions. X-ray diffraction analysis reveals that the cluster adopts an antiparallel sandwich structure with a ladder-shaped PtC/AgAg/CPt core. The coordination plane of the platinum unit is highly distorted due to the in-plane steric repulsion between the PEt₃ ligand on the platinum and the nearest proton on each of the two cyclometalated phenyl rings in the pincer carbodiphosphorane framework. The cluster is very labile and displays different reactivity patterns toward trivalent phosphorus ligands. In the reaction with bulky PPh₃, a dinuclear complex was formed because of coordination of PPh₃ to the silver atom upon cleavage of the tetranuclear core. In contrast, replacement of the PEt₃ on the platinum center by P­(OPh)₃, which is sterically less demanding, led to a dinuclear complex where the eliminated PEt₃ ligand recoordinated to the silver atom

Carbon(0)-Bridged Pt/Ag Dinuclear and Tetranuclear Complexes Based on a Cyclometalated Pincer Carbodiphosphorane Platform

A carbon(0)-bridged Pt₂Ag₂ cluster was synthesized from the reaction of a cyclometalated pincer carbodiphosphorane platinum complex with AgOTf, by forming Pt­(II)←C(0)→Ag­(I) dative bonds along with Pt­(II)–Ag­(I) and Ag­(I)–Ag­(I) metal–metal interactions. X-ray diffraction analysis reveals that the cluster adopts an antiparallel sandwich structure with a ladder-shaped PtC/AgAg/CPt core. The coordination plane of the platinum unit is highly distorted due to the in-plane steric repulsion between the PEt₃ ligand on the platinum and the nearest proton on each of the two cyclometalated phenyl rings in the pincer carbodiphosphorane framework. The cluster is very labile and displays different reactivity patterns toward trivalent phosphorus ligands. In the reaction with bulky PPh₃, a dinuclear complex was formed because of coordination of PPh₃ to the silver atom upon cleavage of the tetranuclear core. In contrast, replacement of the PEt₃ on the platinum center by P­(OPh)₃, which is sterically less demanding, led to a dinuclear complex where the eliminated PEt₃ ligand recoordinated to the silver atom

Structural Modification on Copper(I)-pyridylpyrimidine Complexes for Modulation of Rotational Dynamics, Redox Properties, and Phototriggered Isomerization

The redox properties of copper pyridylpyrimidine complexes, which undergo linkage isomerism based on pyrimidine ring rotation, were compared under different coordination environments. A newly synthesized compound, [Cu­(Mepypm)­(L_Mes)]­BF₄ (<b>1</b>·BF₄, Mepypm = 4-methyl-2-(2′-pyridyl)­pyrimidine, L_Mes = 2,9-dimesityl-1,10-phenanthroline) was compared with previously reported complexes of [Cu­(MepmMepy)­(L_Mes)]­BF₄ (<b>2</b>·BF₄, MepmMepy = 4-methyl-2-(6′-methyl-2′-pyridyl)­pyrimidine), Cu­(Mepypm)­(DPEphos)]­BF₄ (<b>3</b>·BF₄, DPEphos = bis­[2-(diphenylphosphino)­phenyl]­ether), [Cu­(Mepypm)­(L_Anth)]­BF₄ (<b>4</b>·BF₄, L_Anth = 2,9-bis­(9-anthryl)-1,10-phenanthroline), and [Cu­(Mepypm)­(L_Macro)]­BF₄ (<b>5</b>·BF₄). Isomer ratios, isomerization dynamics, redox properties, and photoelectron conversion functions varied with the coordination structure. Methyl substituents on the 6-position of the pyridine moiety increased steric repulsion and contributed to quicker rotation, enhanced photoluminescence, and increased photodriven rotational isomerization