Unusually Stable Four-Membered Chelate Rings in Nickel(II), Palladium(II), and Platinum(II) Complexes with the Ligand 2,2-Bis(diphenylphosphino)propane (2,2-dppp). Crystal and Molecular Structure of [PdI₂(2,2-dppp)]·0.8CH₂Cl₂

The ligand Ph2PC(CH3)2PPh2 (2,2-dppp) reacts with Ni(II) salts to give robust square-planar chelate complexes [NiX2(2,2-dppp-P,P‘)] (X = Cl, 1; X = Br, 2) and the salt [Ni(2,2-dppp)2](ClO4)2 (3). The Pd(II) analogues [PdCl2(2,2-dppp)] (4), [PdI2(2,2-dppp)] (5), and [Pd(2,2-dppp)2](BF4)2 (6) have also been made. The crystal structure of 5 (obtained as 5·0.8CH2Cl2) has been determined by X-ray diffraction. The geminal C(CH3)2 group, when compared with related Ph2PCH2PPh2 (dppm) complexes, causes a small compression of the P−C−P chelate ring angle (to 91.3(3)° for 5). With [PtCl2(PhCN)2], 2,2-dppp affords [PtCl2(2,2-dppp)] (7), which undergoes metathesis with NaI to give [PtI2(2,2-dppp)] (8), and [Pt(2,2-dppp)2]Cl2 (9) was obtained by reaction of [PtCl2(PhCN)2] with 2 equiv of 2,2-dppp. Reactions were then attempted with 7 and 9, which are known to result in ring-opening reactions with the corresponding complexes of dppm, but in all cases, only 2,2-dppp chelate complexes could be isolated. Thus, treatment of 7 or 9 with excess MeLi gave exclusively [PtMe2(2,2-dppp)] (10) rather than cis,cis-[Pt2Me4(2,2-dppp)2]. Attempts to ring-open 10 with excess 2,2-dppp to give cis-[PtMe2(2,2-dppp−P)2] were unsuccessful. Treatment of 10 with 1 equiv of HCl or, better, treatment of [PtCl(Me)(1,5-cyclooctadiene)] with 2,2-dppp gave only mononuclear [PtCl(Me)(2,2-dppp)] (11) and no dimer of the type [MePt(μ-2,2-dppp)2(μ-Cl)PtMe]Cl. Treatment of 7 with 2 equiv of NaCN in EtOH gave the chelate complex [Pt(CN)2(2,2-dppp)] (12) rather than trans,trans-[Pt2(CN)4(μ-2,2-dppp)2], and treatment of 7 with LiC⋮CPh in thf or (better) H2NNH2·H2O/HC⋮CPh in EtOH gave [Pt(C⋮CPh)2(2,2-dppp)] (13), rather than trans,trans-[Pt2(C⋮CPh)4(μ-2,2-dppp)2]. Reaction of 7 with LiC⋮CBun in thf likewise gave [Pt(C⋮CBun)2(2,2-dppp)] (14) in low yield. The complexes were characterized by microanalyses (C, H, N and, in some cases, halide), infrared spectroscopy, 31P{1H} and 1H NMR spectroscopy, and fast atom bombardment mass spectrometry

The Impact of <i>E</i>−<i>Z</i> Photo-Isomerization on Single Molecular Conductance

The single molecule conductance of the E and Z isomers of 4,4′-(ethene-1,2-diyl)dibenzoic acid has been determined using two scanning tunneling microscopy (STM) methods for forming molecular break junctions [the I(s) (I = current and s is distance) method and the in situ break junction technique]. Isomerization leads to significant changes in the electrical conductance of these molecules, with the Z isomer exhibiting a higher conductance than the E isomer. Isomerization is achieved directly on the gold surface through photoirradiation, and the STM is used to determine conductance before and after irradiation; reversible switching between the two isomers could be achieved through irradiation of the surface bound species at different wavelengths. In addition, three groups of molecular conductance values [A (“low”), B (“medium”), and C (“high”)] have been measured for these carboxylate-terminated molecules. The origin of these conductance groups as well as the increase of the conductance for the Z isomer have been analyzed by comparing the length of the molecules extended in the gap, derived from molecular modeling, with the experimentally observed break-off distance for both isomers

Nucleophilic Addition to Complexes of (Ph₂P)₂CCH₂ as a Route to Functionalized, Redox-Active Ruthenium(II)−Diphosphine Complexes

The ligand (Ph2P)2CCH2 (dppen) reacts with [RuCl2(PPh3)3] in CH2Cl2 to give trans-[RuCl2(dppen)2], 1. Complex 1 has reversible Ru(II)/Ru(III) electrochemistry, and on treatment with NOBF4, 1 affords [RuCl2(dppen)2]BF4, 1+. Refluxing solutions of 1 in chlorobenzene affords cis-[RuCl2(dppen)2], 2. With excess RNH2 in chlorobenzene or toluene, 1 reacts to give functionalized diphosphine complexes of general formula trans-[RuCl2({Ph2P}2CHCH2NHR)2] (R:  PhCH2, 3a; [CH2]3NH2, 3b; n-octyl, 3c; R-α-CH(Me)Ph, 3d; [CH2]3Si(OEt)3, 3e) characterized principally by a small upfield shift in their 31P{1H} NMR spectra compared to that of 1 and by their distinctive 1H NMR spectra. Complex 3b·4MeOH crystallizes in the space group P1̄ with a = 11.448(6) Å, b = 13.10(1) Å, c = 11.178(7) Å, α = 93.16(6)°, β = 99.51(5)°, γ = 92.19(5)°, V = 1648(2) Å3, and Dcalcd = 1.250 g cm-3 for Z = 1. Complex 3e reacts with the surface hydroxyl groups of indium-doped tin oxide (ITO) electrodes, to give monolayers of anchored, redox-active Ru(II)−diphosphine complexes, characterized by cyclic voltammetry. The modified electrodes are stable to repetitive cycling over the Ru(II)/Ru(III) redox wave in acetonitrile−tetraethylammonium tetrafluoroborate and in aqueous buffer (pH 8). With anodized Pt electrodes, however, 3e reacts to form multilayers. Reaction of 1 with secondary amines is more sluggish than reaction with primary amines, and only adducts with pyrrolidine (3f) and morpholine (3g; impure) were isolated. With LiC⋮C(CH2)3CH3, 1 reacts to give trans-[Ru(C⋮CR)2({Ph2P}2CHCH2C⋮CR)2] (R = n-butyl, 5); acetylide nucleophiles displace the chloride ligands as well as adding to the dppen double bonds of 1. Other carbanions fail to react with 1. The cis complex 2 reacts with a limited range of primary amines, to afford cis-[RuCl2({Ph2P}2CHCH2NHR)2] (R:  n-hexyl, 4a; [CH2]3Si[OEt]3, 4b). However, 4b was too insoluble for electrode derivatization. The “half-sandwich” complex [CpRuCl(dppen)] (6; Cp = η5-C5H5) also reacts with RNH2 to give [CpRuCl({PPh2}2CHCH2NHR)] (R:  −[CH2]3NH2, 7a; −CH2Ph, 7b)

Nucleophilic Addition to Complexes of (Ph₂P)₂CCH₂ as a Route to Functionalized, Redox-Active Ruthenium(II)−Diphosphine Complexes

The ligand (Ph2P)2CCH2 (dppen) reacts with [RuCl2(PPh3)3] in CH2Cl2 to give trans-[RuCl2(dppen)2], 1. Complex 1 has reversible Ru(II)/Ru(III) electrochemistry, and on treatment with NOBF4, 1 affords [RuCl2(dppen)2]BF4, 1+. Refluxing solutions of 1 in chlorobenzene affords cis-[RuCl2(dppen)2], 2. With excess RNH2 in chlorobenzene or toluene, 1 reacts to give functionalized diphosphine complexes of general formula trans-[RuCl2({Ph2P}2CHCH2NHR)2] (R:  PhCH2, 3a; [CH2]3NH2, 3b; n-octyl, 3c; R-α-CH(Me)Ph, 3d; [CH2]3Si(OEt)3, 3e) characterized principally by a small upfield shift in their 31P{1H} NMR spectra compared to that of 1 and by their distinctive 1H NMR spectra. Complex 3b·4MeOH crystallizes in the space group P1̄ with a = 11.448(6) Å, b = 13.10(1) Å, c = 11.178(7) Å, α = 93.16(6)°, β = 99.51(5)°, γ = 92.19(5)°, V = 1648(2) Å3, and Dcalcd = 1.250 g cm-3 for Z = 1. Complex 3e reacts with the surface hydroxyl groups of indium-doped tin oxide (ITO) electrodes, to give monolayers of anchored, redox-active Ru(II)−diphosphine complexes, characterized by cyclic voltammetry. The modified electrodes are stable to repetitive cycling over the Ru(II)/Ru(III) redox wave in acetonitrile−tetraethylammonium tetrafluoroborate and in aqueous buffer (pH 8). With anodized Pt electrodes, however, 3e reacts to form multilayers. Reaction of 1 with secondary amines is more sluggish than reaction with primary amines, and only adducts with pyrrolidine (3f) and morpholine (3g; impure) were isolated. With LiC⋮C(CH2)3CH3, 1 reacts to give trans-[Ru(C⋮CR)2({Ph2P}2CHCH2C⋮CR)2] (R = n-butyl, 5); acetylide nucleophiles displace the chloride ligands as well as adding to the dppen double bonds of 1. Other carbanions fail to react with 1. The cis complex 2 reacts with a limited range of primary amines, to afford cis-[RuCl2({Ph2P}2CHCH2NHR)2] (R:  n-hexyl, 4a; [CH2]3Si[OEt]3, 4b). However, 4b was too insoluble for electrode derivatization. The “half-sandwich” complex [CpRuCl(dppen)] (6; Cp = η5-C5H5) also reacts with RNH2 to give [CpRuCl({PPh2}2CHCH2NHR)] (R:  −[CH2]3NH2, 7a; −CH2Ph, 7b)

Influence of Conformational Flexibility on Single-Molecule Conductance in Nano-Electrical Junctions

The temperature dependence of the single-molecule conductance of conformationally flexible alkanedithiol molecular bridges is compared to that of more rigid analogues which contain cyclohexane ring(s). Molecular conductance has been measured with a scanning tunneling microscope (STM) at fixed gap separation by observing the stochastic formation of molecule bridges between a gold STM tip and substrate (the so-called “I(t)” technique). Under these conditions, the junction can be populated by a wide distribution of conformers of alkanedithiol molecular bridges and a strong temperature dependence of the single-molecule conductance is observed. By contrast the rigid analogues that contain cyclohexane ring(s), which cannot form the thermally accessible gauche rich conformers open to the alkanedithiols, show no dependence of the single-molecule conductance on temperature. This comparison demonstrates that it is the conformational flexibility and access to thermally populated higher energy conformers of the linear polymethylene (alkane) bridges which leads to the temperature dependence. By removing this possibility in the cyclohexane ring-containing bridges, this conformational gating is excluded and the temperature dependence is then effectively suppressed

Ionic Liquid Based Approach for Single-Molecule Electronics with Cobalt Contacts

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane­(di)­thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions

Single-Molecule Electrochemical Gating in Ionic Liquids

The single-molecular conductance of a redox active molecular bridge has been studied in an electrochemical single-molecule transistor configuration in a room-temperature ionic liquid (RTIL). The redox active pyrrolo-tetrathiafulvalene (pTTF) moiety was attached to gold contacts at both ends through −(CH₂)₆S– groups, and gating of the redox state was achieved with the electrochemical potential. The water-free, room-temperature, ionic liquid environment enabled both the monocationic and the previously inaccessible dicationic redox states of the pTTF moiety to be studied in the in situ scanning tunneling microscopy (STM) molecular break junction configuration. As the electrode potential is swept to positive potentials through both redox transitions, an ideal switching behavior is observed in which the conductance increases and then decreases as the first redox wave is passed, and then increases and decreases again as the second redox process is passed. This is described as an “off–on–off–on–off” conductance switching behavior. This molecular conductance vs electrochemical potential relation could be modeled well as a sequential two-step charge transfer process with full or partial vibrational relaxation. Using this view, reorganization energies of ∼1.2 eV have been estimated for both the first and second redox transitions for the pTTF bridge in the 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIOTf) ionic liquid environment. By contrast, in aqueous environments, a much smaller reorganization energy of ∼0.4 eV has been obtained for the same molecular bridge. These differences are attributed to the large, outer-sphere reorganization energy for charge transfer across the molecular junction in the RTIL

Single-Molecule Junction Formation in Deep Eutectic Solvents with Highly Effective Gate Coupling

The environment surrounding a molecular junction affects its charge-transport properties and, therefore, must be chosen with care. In the case of measurements in liquid media, the solvent must provide good solvation, grant junction stability, and, in the case of electrolyte gating experiments, allow efficient electrical coupling to the gate electrodes through control of the electrical double layer. We evaluated in this study the deep eutectic solvent mixture (DES) ethaline, which is a mixture of choline chloride and ethylene glycol (1:2), for single-molecule junction fabrication with break-junction techniques. In ethaline, we were able to (i) measure challenging and poorly soluble molecular wires, exploiting the improved solvation capabilities offered by DESs, and (ii) efficiently apply an electrostatic gate able to modulate the conductance of the junction by approximately an order of magnitude within a ∼1 V potential window. The electrochemical gating results on a Au–VDP–Au junction follow exceptionally well the single-level modeling with strong gate coupling (where VDP is 1,2-di­(pyridine-4-yl)­ethene). Ethaline is also an ideal solvent for the measurement of very short molecular junctions, as it grants a greatly reduced snapback distance of the metallic electrodes upon point-contact rupture. Our work demonstrates that DESs are viable alternatives to often relatively expensive ionic liquids, offering good versatility for single-molecule electrical measurements

Impact of Junction Formation Method and Surface Roughness on Single Molecule Conductance

In recent years, several experimental studies have shown that different values of single molecule conductance can be observed for the same type of molecule. Although this observation has been tentatively attributed either to differing molecular conformations or to differing contact geometries, the reason for the different conductance groups remains still unclear. To elucidate this issue, a comparison of four different experimental methods to measure single molecule conductance is presented here for the case of alkanedithiols between gold electrodes, which is considered to be a model system. Three different fundamental conductance groups exhibiting low, medium, and high conductance, respectively, were observed for each molecule. The comparison of measurements performed on surface areas with different step densities reveals that the medium (high) conductance group can be attributed to the adsorption of one (two) contacting S atoms at step sites, whereas the low conductance group can be attributed to molecules adsorbed between flat surface regions. This finding is corroborated by a gap separation analysis for the different conduction groups, by matrix isolation measurements, and by a comparison of the results presented here with conductance measurements performed on self-assembled monolayers. The results presented here help to resolve apparent discrepancies in single molecule conductance measurements and are of general significance for molecular electronics and electrochemistry, since they show how molecular conductance is influenced by the contact morphology and, thus, by the atomic structure of the substrate surface

Redox State Dependence of Single Molecule Conductivity

Spontaneous formation of stable molecular wires between a gold scanning tunneling microscopy (STM) tip and substrate is observed when the sample has a low coverage of α,ω-dithiol molecules and the tunneling resistance is made sufficiently small. Current−distance curves taken under these conditions exhibit characteristic current plateaux at large tip−substrate separations from which the conductivity of a single molecule can be obtained. The versatility of this technique is demonstrated using redox-active molecules under potential control, where substantial reversible conductivity changes from 0.5 to 2.8 nS were observed when the molecule was electrochemically switched from the oxidized to the reduced state