Isonicotinamide ligands

Aakeroy et al. were also able to prepare building blocks based upon Ag(I) centres coordinated in a trigonal planar manner by three isonicotinamide ligands. The isostructural perchlorate and tetrafluoroborate salts of these cationic complexes involve amide-amide hydrogen-bonded links from each cation to six others, resulting in 3D interpenetrated networks that resemble the network found in a-ThSi [53]. 

An attempt to demonstrate the direct outer-sphere reduction of HOC1 was recently published, in which the reductant was selected to be [Ru(NH3)5isn]2+ because of its well-known behavior as an outer-sphere reductant and its relatively low standard potential (10). In this reagent the ligand isn is isonicotinamide. Further efforts to promote reaction via HOC1 entailed the use of chloride-free preparations of HOC1. 

In order to obtain further information on the magnitude of the overall reaction volume and the location of the transition state along the reaction coordinate, a series of intermolecular electron-transfer reactions of cytochrome c with pentaammineruthenium complexes were studied, where the sixth ligand on the ruthenium complex was selected in such a way that the overall driving force was low enough so that the reaction kinetics could be studied in both directions (153, 154). The selected substituents were isonicotinamide (isn), 4-ethylpyr-idine (etpy), pyridine (py), and 3,5-lutidine (lut). The overall reaction can be formulated as... 

Complexation equilibria are treated in the same manner as acid-base equilibria. The slightly different look of applicable equations is simply the result of the direction of the equilibria in acid-base and complex formation reactions. The former is conventionally written as proton dissociation, and the latter as metal-ligand association, as exemplified by the reaction of Run(NH3)5isn2 + (isn = isonicotinamide) with iodine in acidic aqueous solutions, Equation 8.108. 

A series of predominantly ID assemblies have been prepared by Aakeroy and co-workers using silver(I) ions [50], which have a tendency to adopt a linear two-coordinate geometry (i.e. AgLj building blocks, cf. VII, XVII, XVIII). These assemblies are obtained with ligands L = pyridine-4-carboxamide (isonicotinamide) [50a], pyridine-3-aldoxime [50b] and pyridine-3-acetoxime [50b], when using... 

Figure 16 Nicotinamide (pyridine-3-carboxamide) and isonicotinamide (pyridine-4-carbox-amide) ligands. Curved arrows indicated torsional flexibility at positions a-8. Reprinted from L. Brammer, J. C. Mareque Rivas, R. Atencio, S. Fang and F. C. Pigge, J. Chem. Soc., Dalton Trans., 3855-67 (2000). Reproduced by permission of the Royal Society of Chemistry.

Irradiation of an aqueous solution of [Os(NH3)5(N2)] by electrons or y-rays gives [Os(NH3)4(NH2XN2)] by action of OH radicals. Complexes involving heterocyclic ligands [Os(NH3)4(N2)L] (L = pyrazine, JV-methylpyrazium, isonicotinamide, and isonicotinic acid) have been synthesized by displacement of N2 from [Os(NH3)4(N2)2] under Ar. Aerial oxidation of these complexes in the presence of HX (L = pyr or isonicotinic acid) leads to the reversible formation of [Os(NH3)4X(L)]X2 (X = Cl or Br) pK, measurements indicate marked back bonding from osmium to the heterocycle. 

Reactions of the type shown in Eq. (1.11), where L = isonicotinamide, 4-ethyl-pyridine, 3,5-lutidine, or pyridine, all exhibited volumes of activation for the forward reaction of between +3 and +7 cm mol compared to overall reaction volumes of between +19 and +26 cm mol . This indicates that electron transfer from Fe to Ru is characterized by an early transition state in terms of volume changes along the reaction coordinate (see Fig. 1.21). The overall volume changes could be accoimted for in terms of electrostriction effects centered around the ammine ligands on the ruthenium center. A number of possible explanations in terms of the effect of pressure on electronic and nuclear factors were offered to account for the asymmetrical nature of the volume profile 67]. 

Rate constants, k, are for electron transfer from the donor to acceptor groups in ruthenated electron transfer proteins. Ruthenium ligands are abbreviated as follows NH3, a isonicotinamide, isn pyridine, py. MP are metallosubstituted porphyrins (M = Fe, Zn, Pd, Pt, Mg, Cd, and H), MP and MP- + are the photoexcited and oxidized radical cation forms of MP, respectively. PDE is porphyrin diester, d is the closest edge-to-edge distance between cofactor atoms. 

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