Stepwise coordination

Table 1. Thermodynamics of the stepwise coordination of hard ligands at 25 °C to... 

On the basis of DFT calculations, a catalytic cycle involving a copper vinylidene intermediate has been proposed (Scheme 9.22) [44]. The reaction is initiated by copper acetylide (138) formation. Sharpless and coworkers next invoke an unusual [3 + 3]-cycloaddition that would be forbidden by orbital symmetry, were it not stepwise. Coordination of an azide to complex 138 generates a zwitterionic complex (139). Internal nucleophilic attack of the acetylide moiety of 139 on the electrophilic... 

A proposed mechanistic sequence involves stepwise coordination of two molecules of C02 to CpFe(CO)2, followed by formation of Na2C03 and transient formation of CpFe(CO)3+ which quickly reacts with more of the anion to form the dimer. Support for this mechanism comes from the observation that reaction with labeled C02 results in extensive and rapid incorporation of labeled CO in the product dimer. 

FIGURE 2. Chemical structures of the ligands and stepwise coordination methods to prepare linear and branched oligomer and polymer wires on an Au/mica or Au/ITO plate. (Reprinted with permission from Ref. 13.)... 

We describe here that the redox oligomer wires fabricated with the stepwise coordination method show characteristic electron transport behavior distinct from conventional redox polymers. Redox polymers are representative electron-conducting substances in which redox species are connected to form a polymer wire.21-25 The electron transport was treated according to the concept of redox conduction, based on the dilfusional motion of collective electron transfer pathways, composed of electron hopping terms and/or physical diffusion.17,18,26-30 In the characterization of redox conduction, the Cottrell equation can be applied to the initial current—time curve after the potential step in potential step chronoamperometry (PSCA), which causes the redox reaction of the redox polymer film ... 

While molecular assembly has proven to be effective for a photoelectric conversion system, coordination reactions are possibly a simple approach for connecting such functional molecules, as presented in the previous section. We applied the stepwise coordination method to prepare a photoelectric conversion system. Since the molecular wire exhibits redox conduction through the wire,11,13 efficient photo-electron transport through the redox sites in the wire is also expected. In this section, we demonstrate the fabrication of a photoelectric conversion system using ITO electrodes modified with M(tpy)2 (M = Co, Fe, Zn) complex wires with a terminal porphyrin moiety as a photosensitizer. The behavior of photo-electron transfer from porphyrin to ITO through the molecular wire was investigated by changing the metal element in the M(tpy)2 moieties.14... 

FIGURE 11. Chemical structure of the ligands used in this chapter, and stepwise coordination methods for the preparation of modified ITO electrodes (i) immobilization of tpy-BzA, (ii) complexation with a metal ion, and (iii) complexation with tpy-ZnTPP. (Reprinted with permission from Ref. 14.)... 

This section described the fabrication of ITO electrodes modified with porphyrin-terminated M(tpy)2 complex wires by the stepwise coordination method, and it is demonstrated that the electronic nature of the molecular wire is critical to the photoelectron transfer from the porphyrin to ITO. These results suggest that the new facile fabrication method of molecular assemblies is effective in the construction of photoelectron transfer systems. The system could be upgraded by extending the wire length, embedding the redox potential step in the wire, increasing the photoreceptors in the wire, and/or incorporating donors and acceptors. 

Stepwise coordination by phosphinoalkynes has also been used to build up butterfly clusters. For example, reaction of [Co2(/t,-MeC=CMe)(CO)6] and Ph2PC=CPh affords [Co2(/r.-MeC=CMe)(T71-Ph2PC=CPh)(CO)5] (41), which reacts with [Co2(CO)8] to afford [Co2(/i.-MeC=CMe)(CO)5(/x3-7j3-Ph2PC=CPh)Co2(CO)6] (42), which upon thermolysis rearranges to give the butterfly tetracobalt cluster 43 illustrated in Scheme 16.157... 

Figure 9.13 Stepwise coordination method to prepare metal complex molecular wire on Au.

The step-by-step process of chelation described above is believed to be the usual way all polydentate ligands go about coordinating to metal ions in general - knitting themselves onto the metal ion stitch by stitch (or, more correctly, undergoing stepwise coordination). It also provides a way whereby compounds can achieve only partial coordination of a potential donor set, when there are insufficient coordination sites for the full suite of potential donor groups available on a ligand. 

A potentiometric titration of PHJ with AgF in a basic anhydrous HF solution showed an initial uptake of Ag+ with an end point at [Ag]/[P] = 0.5 (formation of Ag(PH3)J) followed by a further uptake of Ag+ forming AgPHJ. The complex-formation constants for the stepwise coordination were derived [63]. 

The stepwise coordination of a ligand L to a metal M to form a complex ML , can... 

Fig.18 Proposed mechanism for the stepwise coordination of NO3 anion to Eu -48 complex. Reprinted with permission from [126], (2002), American Chemical Society...

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