Vanadocenes

It is evident that the cyclopentadienyl rings assume a mutual staggered conformation. 

This shows that vanadocene maintains its molecular structure essentially unaltered on passing to 14 valence electrons. One might expect vanadocene to have a high predisposition to accept electrons in order to achieve the stability of 18 valence electrons. Nevertheless, it displays only a single, chemically reversible, reduction process at very negative potential values (E° = -2.74 V). 

Analogous to ferrocene, vanadocene can be permethylated to decamethylvanadocene (the X-ray structure of which is known75). In contrast with decamethylferrocene, which is reversibly oxidized more easily than ferrocene, decamethylvanadocene does not seem able to undergo reversible oxidation.76 

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Since the 21.9 kK. band probably represents a number of closely juxtaposed d-d excitations the observed extinction coefficient is not unreasonable, but since only one feature can be attributed to d-d transitions it is clearly not possible to determine the fitting parameters therefrom. If, however, one assumes a similar DtjB ratio (ca. 4.8) as found for vanadocene, 42 (a S2), (which differs only by a single 6 electron in its ground... 

B = 408 cm-1, (J3 = 0.44), and C/B = 8.84. The C/B ratio deduced is markedly greater than that customarily used, and it was partly upon this interpretation of the ferrocene spectrum that Cerny, Pavlik, and Maxova (44, 45) based their use of C/B values greater than 4.0 in interpreting the spectra of vanadocene and nickelocene. 

By an exactly similar procedure the values may be calculated for the d3, 42 (ct 82) system, which constitutes the ground state of vanadocene. One findsg, = 2.00, and... 

For vanadocene second-order perturbation calculations of the spin-orbit contribution to the zero field splitting, DLS, showed the II ( 2) level to lie below A ( - ), but the estimated value, 1.4 to 1.5 cm-1, constitutes only about half of the observed result. How-... 

The antitumor activity of titanocene dichloride 21 was first recognized in 1979 (150), and since then the activity of several other metallocenes (V, Nb, Mo, Fe, Ge, and Sn) has been reported (151). Most interest has centered on titanocene dichloride and on vanadocene dichloride 22, which are active against a diverse range of human carcinomas, including gastrointestinal and breast carcinomas, but not against head and neck cancers. There appears to be a lack of crossresistance between titanocene dichloride and cisplatin. 

The low temperature molecular structure of vanadocene, [V(f/5-C5H5)2], is shown in Figure 41.73... 

Despite not being electronically saturated, vanadocene is able to lose two further electrons through two distinct oxidation processes corresponding to Vn/Vm ( o/ = -0.55 V, vs. SCE) and Vm/Viv (Ep = + 0.59 V), the first of which is chemically reversible, Figure 42.74... 

The cyclopentadienyl rings of chromocene, as for vanadocene, assume a staggered conformation. 

In agreement with the presence of an additional electron in the bonding orbital, compared to vanadocene (chromocene 16 valence electrons, terminal electronic configuration e a ), the metal-carbon distance is shorter (2.15 A compared to 2.27 A of vanadocene). 

Like ferrocene and vanadocene, chromocene can be permethylated to decamethylchromocene (Cr-Cp (centroid)= 1-78 A79). 

The first study of ring-exchange reactions involving metallocenes has shown that vanadocene is quite inert to exchange with both Li(C5D5) and Ni(jt- 503)2. ... 

Electrochemical studies on V(II) complexes have been relatively few compared to the quantity of work on higher oxidation states. The recent discovery of a general synthesis has improved the availability of data on derivatives of vanadocene [15, 16]. For example, Ef values of L2V decrease in the order L = indenyl (—2.38 V versus Cp2Fe/TFlF)> Cp (—2.72 V versus... 

A bimetallic vanadocene analog (Fig. 2) where a cyclooctatetraendiyl ligand is coordinated rj - to each V atom was studied at — 38°C and shown to have a reversible reduction at very negative potentials [18]. If the Cp ligands are bound together by a short bridging group as shown in Fig. 2, then there is a reversible oxidation at mild potentials, otherwise the oxidation is irreversible. There is also a second, irreversible oxidation. The electrons involved in the oxidation reactions come from the V—V bond which explains the destabilization of these structures upon oxidation. 

A variety of vanadocene-derived organometallic species has been studied by electrochemical means. CP2VCI2 has been studied under a variety of conditions [57]. In MeCN, the oxidation appears well behaved by CV, but rotating-disk and channel-electrode voltammetry studies show that the oxidized complex undergoes follow-up chemistry shown in Eqs (1-3) ... 

In THE, CP2VCI2 has two irreversible reductions that each lead to chloride loss Ef" =—1.15 and —2.20 V versus Cp2Ee/THE), which ultimately lead to the observation of the reversible couple for vanadocene at —3.27 V versus Cp2Fe/THE [58], In acetonitrile, a series of substituted vanadocene dichlorides ((C5H4R)2VCl2, R = Me, Pr, Bu)) [59] is... 

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