On choosing a reference redox system for electrochemical measurements: a cautionary tale

The potential of a quasi-reference electrode can be determined by introducing an internal reference redox system (IRRS) which comprises either the oxidizable or reducible form of a reversible (and, ideally, outer-sphere) redox couple and then observing the cyclic voltammetric responses. The objective is to choose the IRRS so that the cyclic voltammetric response for the simultaneously present electroactive analyte system (ANS) can be observed independently of the IRRS response. We identify three fundamental paradigms describing the relative positioning of the IRRS and ANS on the potential scale, the operative redox components for the IRRS and ANS, and the starting potential (E _start), reversing potential (E _rev), and ending potential (E _end) for the cyclic voltammetric scan as follows: paradigm A, an optimal paradigm which can produce completely independent cyclic voltammetric responses for the IRRS or for ANS; paradigm B, a less-than-optimal paradigm which can produce an independent cyclic voltammetry (CV) response for the ANS or a mixed response for the IRRS with that response on top of the ANS response; paradigm C, a problematic paradigm that can produce an independent CV response for the IRRS or a mixed response for the ANS with that response on top of the IRRS response; and any mixed response produces a thermodynamically favored redox cross-reaction which couples the IRRS and ANS systems and which can complicate the analysis of the ANS and IRRS responses. The conclusion is that paradigm C is to be avoided.     

Uncovering the Hidden Landscape of Tris(4-pyridyl) Ligands: Topological Complexity Derived from the Bridgehead

Supramolecular main group chemistry is a developing field which parallels the conventional domain of metallo-organic chemistry. Little explored building blocks in this area are main group metal-based ligands which have the appropriate donor symmetry to build desired molecular or extended arrangements. Tris(pyridyl) main group ligands (E(py)₃, E=main group metal) are potentially highly versatile building blocks since shifting the N-donor arms from the 2- to the 3-positions and 4-positions provides a very simple way of changing the ligand character from mononuclear/chelating to multidentate/metal-bridging. Here, the coordination behaviour of the first main group metal tris(4-pyridyl) ligands, E(4-py)₃ (E=Sb, Bi, Ph−Sn) is explored, as well as their ability to build metal-organic frameworks (MOFs). The complicated topology of these MOFs shows a marked influence on the counter anion and on the ability of the E(4-py)₃ ligands to switch coordination mode, depending on the steric and donor character of the bridgehead. This structure-directing influence of the bridgehead provides a potential building strategy for future molecular and MOF design in this area.     

A Convenient Synthesis of Cholest-2-En-6-ONE

Cholest-2-on-6-one, 3, is an important synthetic intermediate in the preparation of ecdysone and related insect moulting hormones.^1–3 We report here a convenient two-step synthesis of this compound from cholesterol which combines in one step the elimination of a 36 borate ester and hydroboration-oxidation at C-6.     

New relationship of displacement signal at quadrant photodiode: Control signal analysis and simulation of a laser tracker

New relationship of displacement signal using opposite sectors on a quadrant photodiode is derived. Standard and new displacement signals are analyzed in details. Through MATLAB^® laser tracking simulation models, based on common and suggested approaches, detailed analysis is performed, and it is shown that better results for the new relationship signal processing are obtained. Within new relationship of displacement signal, the sensitivity of the system to the displacement of the spot increases and, hence, provides better accuracy in positioning up to 30%.