A Strategy for the Synthesis of Metal Bis(2,2‘:6‘,2‘ ‘-terpyridine)-Terminated Molecular Dyads Having Controlled Torsion Angles at the Central Biphenyl Linker

Abstract

The synthesis of a series of binuclear complexes comprising bis(2,2':6',2' '-terpyridine)ruthenium(II) and -osmium(II) centers connected via a geometrically constrained 4,4'-biphenyl bridge is described. These compounds have been prepared by a "synthesis-at-metal" approach as well as by the conventional method of synthesizing the ligand and subsequently attaching the metal center. A computational investigation into the behavior of the biphenyl-based bridges has been used to provide lowest-energy conformations and to estimate the degree of internal fluctuation about the mean torsion angle. It is shown that the length of the constraining strap determines both the torsion angle and the internal flexibility, with longer straps twisting the biphenyl group so as to relax stereochemical interactions between the linking oxygen atoms. Longer straps can be formed from poly(ethylene glycol) residues that provide an additional binding site for small cations. Electrospray mass spectrometry carried out on solutions of these crown ether-like bridges confirmed that Li+, Na+, and K+ ions bind in the form of 1:1 complexes. This range of compounds should permit rational examination of how the torsion angle affects the rate of through-bond electron transfer, electron exchange, and charge shift.

Electron Exchange in Conformationally Restricted Donor–Spacer–Acceptor Dyads: Angle Dependence and Involvement of Upper‐Lying Excited States

“…The symmetrical Ru-Ru complexes are used as reference compounds with which to examine the rates of intramolecular electron exchange in the Ru-Os mixed-metal complexes. Synthesis, A C H T U N G T R E N N U N G purification and full characterization are reported elsewhere, [14] as is a description of the MDS studies. The strap length imposes a preferred torsion angle (f) for the biphenyl spacer unit and this property varies from 378 for the shortest strap to 1308 for the longest analogue (Table 1).…”

“…[14] The compounds were purified by extensive silica gel chromatography (CH 3 CN/H 2 O/KNO 3 (aq)) and identified by a number of analytical techniques including electrospray and MALDI mass spectrometry, 1 H NMR spectroscopy, and elemental analysis. Prior to making the spectroscopic studies, each complex was passed down a preparative thin-layer chromatography (tlc) plate using spectroscopic grade solvents to avoid the introduction of luminescent impurities.…”

“…In the following work, we investigate if the internal flexibility of the spacer, this having been confirmed by molecular dynamics simulations (MDS), [14] affects the rate of electron exchange in semi-rigid D-Sp-A dyads. The spacer is composed of a strapped 2,2'-dialkoxybiphenyl unit in which the length of the strap controls the torsion angle between the two phenyl rings.…”

“…The spacer is composed of a strapped 2,2'-dialkoxybiphenyl unit in which the length of the strap controls the torsion angle between the two phenyl rings. [14] Although these straps collapse into the lowestenergy conformation in a low temperature glass, thereby A C H T U N G T R E N N U N G facilitating a correlation between torsion angle and rate of electron exchange, [15] a wider range of torsion angles is to be expected in fluid solution. The main objective of this study is to establish if the global rate constant for electron exchange in such dyads still remembers the mean torsion angle pertaining to the lowest-energy conformer.…”

“…Quantum chemical calculations were used to obtain dihedral angles and molecular dynamics simulations provided the variation in angle around the mean value. [14] The instrumentation used for the spectroscopic studies has been described before. [17,19] The sample was dissolved in butyronitrile and thoroughly deoxygenated.…”