Niobium, and Tantalum Complexes

Bisbenzene vanadium and bismesitylene vanadium have been synthesized by the Fischer-Hafner method 53, 133). The reaction is thought to proceed via formation of the bisarene vanadium cation, e.g.. 

The cation [(l,3,5-Me3CgH3)2V]+ has been isolated as the iodide salt by hydrolysis of the crude reaction mixture at 0°C in the presence of Lil (5.3). The cation may also be prepared from the neutral complex by oxidation with V(CO)s [Eq. (10)] (55). The same compound is obtained if (l,3,5-Me3C6H3)2V is treated with CO under pressure [Eq. (11)] 54a). 

Compound (l,3,5-Me3CgH3)V(CO)3 was proposed as an intermediate in the reaction shown in Eq. (11) but was not isolated. Treatment of compound [(l,3,S-Me3CgH3)2V][V(CO)g] with Lil in tetrahydrofuran (THE) precipitated the cation as the iodide salt. Both the neutral and cationic bisarene vanadium complexes are paramagnetic having 1 and 2 unpaired electrons, respectively 131). 

The observation of the ion (CgHa)V in the mass spectrum of (7r-C7H7)(7r-C5Ha)V 296, 354) has led to the proposal that the parent ion (CeHe)2V is initially formed by rearrangement of the ion (77-07117)-(t7-C5H5)V in a vibrationally or electronically excited state (354). 

Vanadium hexacarbonyl reacts with various arenes yielding complexes [(arene)V(CO)4] [V(CO)6] from which hexafluorphosphate or tetra-phenylborate salts can be prepared by metathesis reaction (50, 52). The cation [(CeHe)V(CO)4] reacts with NaBH4 to give the neutral cyclo-hexadienyl compound (CeH7)V(CO)4 (51). Similar results are observed with methyl-substituted derivatives. The addition of the hydride ion was shown to occur preferentially at unsubstituted positions of the aromatic ring. 

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Kuterev, Stoyanov, Bagreev and Zolotov [461] reported on the investigation, using IR spectroscopy, of the composition of niobium and tantalum complexes extracted by amines from fluoride solutions. It was shown that in solutions that contain hydrofluoric acid in concentrations ranging from 1M to 12M, both niobium and tantalum were extracted in the form of NbF6 and TaF6 ... 

L.A. Kamenskaya Phys.-Chem. investigations of interactions between niobium and tantalum complex fluorides and oxyfluorides and potassium and sodium fluorides and chlorides in molten conditions. - Abstracts of Dissertation, Kiev, 1976 (in Russian). 

Investigations into the use of niobium and tantalum complexes as Lewis acids [99]... 

There are a number of ways that nitriles may be converted to alkylimido ligands. Low valent niobium and tantalum complexes have been shown to reductively couple nitriles to give bridging... 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

Niobium and Tantalum Complexes in Catalysis and Metal-Mediated Organic Synthesis 19... 

AUcyl ligands in niobium and tantalum complexes are susceptible to attack by electrophiles (see Electrophilic Reaction). Hydrogenation (see Hydrogenation) of niobium or tantalum M R bonds to provide the metal hydrides is an important reaction of synthetic utility. Insertion reactions of unsaturated reagents into Nb- or Ta bonds are common. The unsaturated reagents include aUcenes, aUcynes, CO, NO, RN=C=NR, CNR, and others. 

NIOBIUM AND TANTALUM COMPLEXES IN CATALYSIS AND METAL-MEDIATED ORGANIC SYNTHESIS... 

We found also that (77-C5H,)V(CO)3PPh3 as well as its niobium analog, when irradiated, underwent substitution of one PPh3 and one CO by tolan. Hence, niobium (and tantalum) complexes of the type are able to attach a third molecule of tolan and transform this to hexaphenylbenzene, unlike the vanadium analog, which transforms just two tolans to tetraphenyl-cyclobutadiene or, if irradiated, combines them with carbon monoxide and produces tetraphenylcyclone. 

Niobium and tantalum complexes of neutral unidentate Schiff base ligands have been prepared. MCI5 (M = Nb or Ta) reacts with Sal-NR in alcohol to give... 

We have recently reported similar comparisons from the photoelectron data of niobium and tantalum complexes.(36) From comparison of the spectra of... 

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