Vanadium macrocycles

Vanadium macrocycles could serve as geochemical tracers that link geo- and biosystems. Vanadyl porphyrins have been identified both in Precambrian shale as well as in extraterrestrial meteorites. Although there are reports of the presence of either vanadium or porphyrin-like molecules in many types of recent and ancient sediments, the nature of vanadium bonding in all these samples is not known. Once this knowledge is acquired, an insight into the structure of these natural resources will facilitate their intelligent applications in energy conversion and environmental control. 

The general procedure for the preparation of vanadium borates consists in heating a concentrated H2O solution of boric acid and vanadium oxide in an autoclave at 170 °C for several days [143]. Two different vanadium borate clusters 105 and 106 are obtained, one with two polyborate chains coordinated to a contorted vanadium oxide ring (105) and another one with a macrocyclic Bi8036(0H)6 ring (106). The latter ring is composed of six B306(0H) units and has a chair-like conformation (Fig. 27) [143]. 

Fig. 27. In vanadium borate 106 the macrocyclic borate ring (left) is sandwiched by two smaller (VO)60i2 rings (right) of triangular form via six axial B-( 3-0)-V bonds to form a cage-like cluster anion...

Macrocyclic ligands 1,4,7-triazacyclonona-iV-acetate 221 and iV-(2-hydroxybenzyl)-l,4,7-cyclononane 225 have been prepared from l,4,7-triazacyclio[5.2.1.04 10]decane 40 and were subsequently used for the synthesis of a series of mono- and dinuclear complexes of vanadium(rv) and (v) (Scheme 35) <1995ICA(240)217>. 

Asphaltenes may contain both porphyrin and nonporphyrin metals, depending upon the origin of the crude oil. Yen et al. (1969) characterized the vanadium complexes in a petroleum asphaltene by mass spectroscopy, optical spectroscopy, and ESR. Porphyrins (Etio and DPEP), acid-resistant porphyrin macrocycles of increased aromaticity (Rhodo), and nonporphyrins with mixed donor complexes were identified. Baker (1966) and Baker et al. (1967) extracted porphyrins from Boscan crude oil asphaltenes and also found Etio and DPEP as the two major porphyrin series. These homologous series range in molecular weight by 7 to 18 methylene groups. Gallegos (1967) observed by mass spectroscopy that asphaltenes and maltenes from a Boscan crude oil had nearly identical porphyrins in terms of mass distribution. 

Model compound studies have shown the importance of porphyrin macrocycle basicity, resulting from electron-withdrawing substituents and metal ligands, on the reducibility and susceptibility of the central metal to reaction. Similar insight into the differences in relative basicity of vanadium- and nickel-containing complexes found in petroleum may therefore be valuable in rationalizing the observed effects and predicting demetallation activity. 

D.R. Williams and co-workers accomplished the first total synthesis of marine dolabellane diterpene (+)-4,5-deoxyneodolabelline. The Type I carbon-Ferrier reaction was utilized to assemble the key frans-2,6-disubstituted dihydropyran with complete stereoselectivity (a-anomer). The macrocyclization was carried out with a vanadium-based pinacoi coupiing. 

Vanadium compounds, mainly square pyramidal complexes of the type [0=V(0—R—0)2], act as insulin mimetics with potential in diabetes control if toxicity issues can be overcome one is in use as an orally-administered agent in animals. A pentagonal bipyramidal complex of manganese(II), CMnC A ], where N5 is a pentaaza macrocycle with four amine nitrogen donors and one pyridine nitrogen donor, acts as a... 

H4(fsa)2(en)=A, -(2-hydroxy-3-carboxybenzilidene) ethy-lenediamine (see Figure 5). The copper (II) and vanadium (IV) sites, the oxygen atom of the vanadyl group and the methanol oxygen atom weakly bonded to the copper atom (not shown) are very close to forming a mirror plane perpendicular to the plane of the macrocycle. The unpaired copper electron therefore has close to a pure xy-type wavefunc-tion transforming as a" while the unpaired electron around the vanadium is described by a magnetic orbital... 

N-donor macrocyclic ligands, based on 1,4,7-trithiacyc-lononane and carrying three pendant arms bearing alkoxides give nonoxo vanadium (IV) complexes. ... 

All known X-ray diffraction data indicate that in penta-coordinated pyramidal vanadium chelates the vanadium atom lies out of the basal plane of the ligand macrocycle, i,e.j the plane of the four pyrrole nitrogen atoms. The distance from vanadium... 

The parent oxacalix[3] arenas show little ability to bind alkali metals,however, a range of quaternary ammonium cations are attracted to the symmetric cavity. Deprotonation of the phenol moieties allows them to bind to transition metals (scandium, titanium, vanadium, rhodium, molybdenum, gold, etc.), lanthanides (lutetium. yttrium, and lanthanum), and actinides (uranium as uranyl). Oxacalix[3]arenes derivatized on the lower rim can complex gallium, mercury, and alkali metals, including sodium, in a manner reminiscent of natural transmembrane cation filters. One major use was to purify crude samples of fullerenes. The pseudo-Cs symmetry of the macrocyclic cavity is complementary to threefold symmetry elements of Cgo. which binds preferentially in... 

The transmetallation process is accompanied by reduction of to V . This indicates the stabilising effect of the H2L798 for vanadium(lll). Two vana-dium(III) ions can also be incorporated within the macrocyclic cavity by direct template condensation of the corresponding ligsons in the presence of oxovana-dium(rV) [181],... 

Titanium, Zirconium and Hafnium A new type of macrocyclic sandwich complex (1) has been isolated from the reaction of the parent macrocyclic complex with NaCp. The same product has been characterised from the reaction when Ti(trataa)Cl2 or Ti(tmtaa)Cl is used as the substrate complex. The analagous vanadium complex CpV(tmtaa) has been prepared and demonstrated to contain 2 unpaired electrons compared with the single unpaired electron in the titanium complex.A more conventional complex CpTi(n -02C6H4)C1 is generated from the reaction of Ti(02C6H4)Cl2 with TlCp. ... 

A review of recent advances in transition-metal-catalysed oxidations by molecular oxygen has highlighted the scope and limitations, as well as the meehanisms of these reactions. " " An overview of the fundamental studies on a new method of synthesis of nicotinic acid by the gas-phase catalytic oxidation of -picoline by oxygen has been presented. The reactivity of vanadium species has been considered in order to discover the nature of the active catalyst. Kinetic equations for -picoline oxidation on vanadia-titania catalysts have been discussed. " The effect of quaternary ammonium salts or macrocyclic ethers on the autoxidation of ethylbenzene or the decomposition of the a-phenylethyl hydroperoxide intermediate catalysed by Ni(II) or Fe(III) acetylacetonates has been reviewed. ... 

Irradiation (Hg-source) of a mixture of CpV(CO)4 and dppa reportedly gives, besides other products, [(CpV(CO)2)2(dppa)2] (93), that was characterized by elemental analysis and IR spectroscopy [98]. In line with these analyses, a macrocycle with rra/i5 -dispositioned dppa moieties around the vanadium center was suggested (Fig. 2.34a), but not only would this conformation be severely strained, it would also contrast all other binuclear doubly dppa-bridged macrocycles. Instead, the insolubility in virtually all solvents would suggest a polymeric structure with vanadium centers singly-bridged by trans-positioncd dppa ligands (Fig. 2.34b). 

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