Rhenium complexes, characteristics

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]. 

Photoactive Characteristics of Rhenium Complex Encapsulated in A1MCM-41 by Ion-exchange Method... 

Five different polymeric species containing rhenium(I) tricarbonyl complexes are shown in Fig. 17. Polymer A contained Re(CO)3Cl units attached to a dipyridyphenazine ligand [79]. The polymer components were varied from x = 0.1 and y = 0.9 to x = 1 and y = 0 with the latter variation providing the most evidence for the presence of the rhenium complex. A shoulder located at ca. 550-600 nm was attributed to the characteristic MLCT optical transition for the rhenium complex. The photoluminescence spectrum gave a continuous decrease of emission from the poly(p-phenylenevinylene) back-... 

The three rhenium complexes with anionic ligands, /be-[Re (bpylfCOlgfL)] (12 L=NCS, 8 CN , la Cl ), show similar photophysical and electrochemical characteristics (Table IX). 

Larive and coworkers recently used NMR diffusion measurements to characterize a series of ligands (32-36) and rhenium complexes that were used as building units in the construction of molecular squares that could not be characterized by mass spectrometry (Fig. 6.17, [43]). There, good correlation was found between the diffusion coefficient and the reciprocal of the estimated Stokes radii (l/tg), as shown in Fig. 6.17c. The authors also concluded, based on the NMR diffusion measurements, that the complexity of some of the spectra are intrinsic characteristics of the supramolecular systems prepared, rather than contamination from low molecular species. The data extracted from these NMR diffusion experiments is presented in Table 6.4. 

Technetium(v) and Rhenium(v) Complexes.—Re2Tc5 has been obtained by ampoule synthesis from the elements at 800 °C, and its thermodynamic constants and X-ray diffraction characteristics determined/ Re2S3Cl4 has been identified in the Re—S—Cl system. Chloride complexes of technetium(v), K2[TcOCl5] and K2[TcO(OH)Cl4], have been obtained from a solution ofKTc04in HC1. "... 

Complexes containing rhenium in the oxidation state +IV are comparatively rare. There is no extended chemistry in aqueous media and many rhenium(IV) complexes tend to hydrolyze when exposed to water. The stabilization of rhenium(IV) centers requires a well-balanced donor-acceptor behavior of the ligands and, thus, none of the classical rr-donor ligands such as O or TT-acceptors such as carbonyls or nitrosyls are characteristic for this oxidation state... 

Isolation of 3-cyclopropenyl metal compounds by this method has been achieved so far for iron and rhenium metals only. Thus, the reaction of Na[CpFe(CO)J (NaFp) with cyclopropenylium salts at -70 °C, in THF, gave 3-Fp-cyclopropene complexes (equation 194)2 267. The X-ray crystal structure of the most stable iron complex 3-Fp-C3Ph3 exhibits a regular cyclopropene C—C single and double bond distances (151 and 129 pm), and a characteristic distance of 208 pm for the Fe—C (T-bond267. The H NMR (CS2) spectrum of the 3-Fp-C3Ph,H complex displays a singlet at S = 2.63 ppm, of the cyclopropen yl proton at the 3-position. ... 

In addition to the bimetallic complexes of rhenium and alkaline metals formed as byproducts in the exchange reactions of rhenium halids with alkali alkoxides (such as, for example, LiReO(OPr )5 xLiCl(THF)2 [519]) there has been recently prepared a number ofbimetallic complexes ofrhenium and molybdenum, rhenium and tungsten, and rhenium and niobium [904, 1451]. The latter are formed either due to the formation of a metal-metal bond, arising due to combination of a free electron pair on rhenium (V) and a vacant orbital of molybdenum (VI) atom or via insertion of molybdenum or tungsten atoms into the molecular structure characteristic of rhenium (V and VI) oxoalkox-ides. The formation of the compounds with variable composition becomes possible in the latter case. 

Platinum nanoparticles, preparation, 12, 78 Platinum-nitrogen bonds, in platinacycles, 8, 508 Platinum-osmium carbonyl clusters, characteristics, 8, 420 Platinum-oxygen bonds, in platinacycles, 8, 505 Platinum particles, surface reactivity, 12, 542 Platinum-phosphorus bonds, in platinacycles, 8, 508 Platinum-rhenium carbonyl clusters, characteristics, 8, 420 Platinum(II)-ruthenium(II) binary complexes, preparation,... 

Fig. 6 Photocurrent/potential characteristics for Ti02 ( ), and rhenium ( ) and iron (A) cyanide complexed Ti02 electrodes. The Ti02 was illuminated by UV (380 nm), the others by visible light.

Understandably, there is an enormous richness in the photophysical and photochemical behavior of the excited states present in diimine rhenium tricarbonyl complexes. Indeed, this plethora of molecular photophysical characteristics has led to a wide range of interesting and important applications, including their use as catalysts [21-25], sensors [26-33], probes for photo-polymerization [10, 34, 35], optical switches [36 15], light-emitting materials [46-52], nonlinear optical materials [53-56], binding or photocleavage of DNA [57-61], and radiopharmaceuticals [62-66], Under the purview of this article our focus will be to cover photophysical and photochemical properties and hence other aspects, such as synthetic, catalytic, pharmaceutical, etc., will not be discussed. 

These bands show negative solvatochromism as revealed by band shifts to lower energy in less polar solvents [5, 7, 8, 12], The direction of the solvent dependence is associated with a reduced (and reversed) molecular dipole in their MLCT excited states. Emissions from these complexes are typically broad and structureless, and they also often exhibit a rigidochromic effect [7-12], Tables 1 and 2 summarize the luminescence characteristics and environmental effects on absorption and emission maxima for rhenium(I) tricarbonyl diimine complexes. 

These complexes also usually exhibit substantial photostability under visible light irradiation and, due to their relatively long-lived triplet excited-state characteristics, the emission lifetimes are easily quenched by bimolecular electron- and/or energy-transfer processes in solution [6, 76], The electronic structures of MLCT excited molecules of diimine rhenium(I) tricarbonyl complexes can be viewed as a charge-separated species, [LRen(CO)3(diimine ")], with an essentially oxidized... 

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