Rhenium complexes carboxylates

A greater tendency of rhenium complexes (compared to technetium analogues) to expand their coordination numbers has been invoked to rationalize the stronger interaction of the perrhenate ion with carboxylate ligands. This association has been suggested as a possible cause of the different quantitative biodistribution and excretion characteristics of pertechnetate and perrhenate perrhenate is accumulated in thyroid to a lesser extent and renally excreted more rapidly than pertechnetate [6]. 

Although the structure of the CO2 adduct, which is an important intermediate in the photocatalysis of the rhenium complexes, has not yet been confirmed, several proposals have been made. As candidates, a C02-bridged binuclear complex (CO)3(dmb)Re—CO2—Re(dmb)(CO)3 (16, dmb = 4,4 -dimethyl-2,2 -bipyridine) and a rhenium carboxylate complex Re(bpy) (COlsCCOOH) (17) were synthesized. 

CO2 reduction at metallic electrodes is generally poorly selective [151]. Monoelec -tronic reduction of carbon dioxide may occur at a platinum cathode in non-aqueous solvents, but at very negative potentials. Catalytic activation of CO2 has been described (e.g. at a cathode modified by a rhenium complex in a hydroorganic solvent) the observed conversions did correspond to the formation of CO and formic acid. In organic synthesis, CO2 was mainly used as an electrophile (toward electrogenerated anions from jt -acceptors or electrogenerated nucleophiles when adequate transition metals ions were present in situ) for the purpose of carboxylation. 

The resulting reduced photoproduct acts as the catalytic intermediate for COj reduction (Figure 33). It has been suggested that carbon monoxide ligand dissociation, followed by formation of a rhenium-formate intermediate, leads to cyclic reduction of carbon dioxide to CO. Interestingly, a rhenium(I)-carboxylate complex, /ac-Re(0—C(=0)—H)(bpy)(CO)3, has been isolated as a by-product of the photosystem. The latter complex is inactive in COj reduction to CO, and hence a rhenium-formate intermediate formed by COj insertion into the hydride bond was suggested as the active intermediate for CO formation (Figure 33). 

Rhenium(VI) complexes, 4,194 alkoxides, 4,196 amides, 4,194 amines, 4,199 carboxylates, 4,199 dimethylformamide, 4,199 dioxane, 4,198 halides, 4,195,199 2-hydroxypyridine, 4,199 imides, 4,194 magnetic behavior, 1,271 mixed sulfur-nitrogen compounds, 4,196 N heterocycles, 4,199 nitrides, 4,194 oxide halides, 4, 195 oxoanions,4,196 pyridine, 4,199 sulfates, 4,198 sulfur compounds, 4,196 tellurates, 4,198... 

An pFl-dependent coordination isomerism has been observed for the tumor targeting rhenium(V) 0x0 complex [ReO(DMSA)2] (DMSA = 2,3-mercaptosuccinate). In solution the crystallo-graphically characterized syn,endo-isomQx (87a) slowly isomerizes into the anti- (87b) and the iyn,exo-isomers (87c) which has consequences for the biodistribution of the potential pharmaceutical (Scheme 10). The conversion rate decreases with increasing pH suggesting an acid catalyzed reaction. The syn,exo-comy> ex is favored in alkaline solutions (pH > 8.4) which can be understood in terms of the repulsions between the deprotonated carboxylic groups and the 0x0 ligand. 

Table 11 Acidities of rhenium Fischer carbene complexes and intrinsic rate constants for their deprotonation by amines and carboxylate ions in 50% MeCN-50% water (v/v) at 25°Ca,b...

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