Synthesis, Electrochemistry, and Single-Molecule Conductance of Bimetallic 2,3,5,6-Tetra(pyridine-2-yl)pyrazine-Based Complexes

Abstract

The ligands 4'-(4-(methylthio)phenyl)-2,2':6',2″-terpyridine (L(1)), 4'-((4-(methylthio)phenyl)ethynyl)- 2,2':6',2″-terpyridine (L(2)), and bis(tridentate) bridging ligand 2,3,5,6-tetra(pyridine-2-yl)pyrazine (tpp) were used to prepare the complexes [Ru(L(1))2][PF6]2 ([1][PF6]2, [Ru(L(2))2][PF6]2 ([2][PF6]2), [{(L(1))Ru}(μ-tpp){Ru(L(1))}][PF6]4 ([3][PF6]4), and [{(L(2))Ru}(μ-tpp){Ru(L(2))}][PF6]4 ([4][PF6]4). Crystallographically determined structures give S···S distances of up to 32.0 Å in [4](4+). On the basis of electrochemical estimates, the highest occupied molecular orbitals of these complexes fall between -5.55 and -5.85 eV, close to the work function of clean gold (5.1-5.3 eV). The decay of conductance with molecular length across this series of molecules is approximately exponential, giving rise to a decay constant (pseudo β-value) of 1.5 nm(-1), falling between decay factors for oligoynes and oligophenylenes. The results are consistent with a tunnelling mechanism for the single-molecule conductance behavior.

Synthesis and Single‐Molecule Conductance Study of Redox‐Active Ruthenium Complexes with Pyridyl and Dihydrobenzo[b]thiophene Anchoring Groups

“…The analysis of the most probable conductance values reveals an exponential dependence on the molecular length with decay constants of β PY =2.07±0.1 nm −1 and β BT =2.16±0.1 nm −1 . Decay constant values observed in this study are similar to the values reported by Davidson et al . for ruthenium complexes with thiomethyl anchoring groups ( β SMe =1.5 nm −1 ).…”

“…The large spike peak at E pa =−1.71 V versus Fc + /Fc arises from the desorption of the complex 2 adsorbed on the electrode after the reductions . Complex 3 exhibits two successive one‐electron oxidation waves at 1.06 and 1.39 V versus Fc + /Fc corresponding to (Ru II –Ru II )/(Ru II –Ru III ) and (Ru II –Ru III )/(Ru III –Ru III ) processes, and two one‐electron reduction waves at −0.74 and −1.23 V versus Fc + /Fc derived from tppz/tppz − and tppz − /tppz 2− processes, respectively. An additional reduction wave arising from the reduction of ancillary L1 ligands was observed at −1.75 V versus Fc + /Fc as a two‐electron process, associated with a large desorption anodic spike peak at −1.58 V. Complex 4 exhibits two oxidation waves at 1.03 and 1.35 V versus Fc + /Fc, in which the former wave has a large shoulder peak arising from irreversible oxidation of two ancillary ligands L2 at 0.94 V. Further, three reduction waves were observed at −0.76, −1.25, and −1.75 V Fc + /Fc, each of which are assigned to the successive reductions of tppz and the reductions of ancillary ligands L2 as a two‐electron process.…”

“…To model the effect of an electrochemical environment, we used two hexafluorophosphate [PF 6 ] − counter ions for structures 1 and 2 , and four counter ions for structures 3 , 4 , 5 and 6 . In this study, simulations were carried out with six different optimal distances χ between the fluorine atoms of counter ions and nitrogen atoms of the backbone; see the Supporting Information for all electrochemical computational details…”

“…reported the synthesis of a series of ruthenium complexes with different molecular lengths and functionalized with thiomethyl and trimethylsilylethynyl anchoring groups . STM‐based single molecule conductance measurements of ruthenium complexes with thiomethyl anchoring groups showed the exponential decay of conductance as a function of molecular length with a decay parameter of 1.5 nm −1 . The choice of the linker group determines the range of relative orientations of the molecule with respect to the electrode.…”

Energy‐Level Alignment for Single‐Molecule Conductance of Extended Metal‐Atom Chains

“…[6][7][8][9] Most EC-STM BJ studies were performed at fixed E wk values and unable to monitor the system upon E F approaching E FMO .EC studies of single-molecule conductance focus mostly on organic redox moieties. [5,[7][8][9][10] Examples of organometallic compounds are rare and limited to those with one [11][12][13] or two [14,15] metal centers,s uch as ferrocenedicarboxylate, [13] ferrocenedithiol, [11,12] and osmium [11] or ruthenium [14] complexes ligated by bis(terpyridine). Extended metal-atom chains (EMACs,F igure 1a) [16][17][18] represent aunique category of stable,electroactive,and one-dimensional wires with metal atoms robustly interwoven by metal-metal and metal-ligand bonds.…”