Double-square scheme

Fig. 13. A double square scheme of redox behavior of the binuclear complexes...

We now turn our attention to molecular hysteresis which has two essential factors. One is that a system can be expressed a double square scheme diagram (or ladder scheme diagram ) [5], as shown in Fig. 17a. A, A, A", B, B, and B" are chemical species or states. These series of A and B vary reversibly with one another under an external perturbation such as potential, pH, ion concentration, light, etc. With A and B more stable than B" and A", respectively, A" and B" can be rapidly converted to B and A. Hence we will obtain a scheme as shown in Fig. 17b. The other important thing is that the conversion is slow between A and B. The slow rate produces a bistability, A and B, which depends on the direction of an external perturbation. This is molecular hysteresis. Some binuclear or multinuclear metal complexes with the double square scheme diagram have been reported [31]. However, because they were not designed to exhibit molecular hysteresis, their hysteresis behaviors in redox are insufficient. 

Fig. 17a, b. A double square scheme diagram and scheme of the hysteresis behavior... 

Figure 2.18 A square scheme illustrating the disassembling of the [Cu2(16)2]2 + double helicate complex, following Cu -to-Cu" oxidation, and the consequent assembling of two [Cun(16)]2+ mononuclear complexes, following the Cu"-to-Cu reduction. The process ultimately derives from the geometrical coordinative preferences of the two oxidation states Cu1 prefers a tetrahedral coordination, which can be achieved with the double helicate arrangement Cu11 prefers a square coordination geometry, which is fulfilled by the coordination of a single molecule of 16.

The first part of the underlying double square [46] scheme (10) contains a single electron transfer to the 7i(C=C) bonded tetracyanoethylene (TCNE) ligand in the blue complex with W(CO)5 which leads - in a relatively slow chemical reaction on the cyclovoltammetric time scale - to a yellow isomerized anion radical product in which TCNE coordinates via one nitrile nitrogen lone pair. The high resolution EPR, UVA is and infrared data of the singly reduced complex clearly illustrate the lowered symmetry and diminished n back donation in the secondary (TiO monoanionic form [46]. 

Fig. 6 The energy profile for a cyclic double hydrogen bond in acetic acid. Filled squares, total energy, open squares, atom-atom Coulombic energy, triangles. correct Coulombic energy. The result is insensitive to the choice of atomic point charges according to common schemes (e.g., Muliiken, ESP, etc.)...

In Table I we give the irreducible representations, in Mulliken s notation, contained in the central and attached orbitals for compounds of other types of symmetry. When 3 or 4 atoms are trigonally or tetragonally attached, we have supposed that the plane of these atoms is a plane of symmetry, as in (N03) or Ni(CN)4 When there is no such symmetry plane, as in NHg, the distinctions between u and g, or between primes and double primes, are to be aholished,9 and the symmetries degenerate to Csv, Civ instead of DSh, D h- When 6 atoms are attached in the scheme Z>3, or 8 in % they are arranged respectively at the corners of a trigonal and a square prism. 

Several metal-organic polymers assembled by dithioether ligands, which are functionalized by heteroelements such as O, N, or Si in the spacer unit, have been recently described and reacted with Cu(I) salts. For example, the 2D square-grid coordination polymer 37 incorporating the 16-membered dithiaoxa crown-ether L37 has been prepared by a self-assembly process involving the interaction of the dithiaoxa macrocycle with CuCN (Scheme 17) the parallel reaction of the dithiaoxa crown-ether with Cul afforded the ID double-stranded coordination polymer 37 . This example demonstrates, how the nature of the counterion can control the form of the network.157... 

Interstrand cross-links are unstable in conditions close to physiological conditions [57]. The bonds between platinum and the N(7) of guanine residues are cleaved spontaneously, with essentially one cleavage reaction per cross-linked duplex in either of both DNA strands (tl/2 for the cleavage reaction is about 29 h). As shown in the reaction scheme (Fig. 2), the cleavage generates monofunctional adducts which can react further to yield interstrand and intrastrand cross-links. The distorted local conformation could allow the formation of adducts which are not usually formed in double-stranded DNA containing a monofunctional adduct. An attractive hypothesis to explain the instability of the interstrand cross-links is that one of the two water molecules, in apical position with respect to the square of the platinum atom, labilises the G-Pt bond in solvolysis reaction. When the local... 

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