Group 7, half-sandwich complexes

In the context of pyridin synthesis from alkynes and nitriles homogeneously catalyzed by (cyclopentadienyl)cobalt complexes (Eq. 13), it was found that electron-withdrawing groups on the cyclopentadienyl ring significantly increase the activity [63]. During the screening of various cobalt half-sandwich complexes... 

Dihydro-lH-l,2-azaboroles derive from cyclopentadiene by the isoelectronic replacement of a C=C group by a BN moiety ". The neutral rings react in xs boiling Fe(CO)j without any other solvent to form red-brown iron-dihydro-1,2-azaborolyldi-carbonyl dimers as their cis and transisomers in 56% yield, or with CojfCOg in petroleum ether at 60-80°C to the half-sandwich complex (dihydro-1,2-azaborolyl)Co(CO)2 (40%). The CO group can easily be substituted by olefins ... 

The group of Salzer has recently reported phospholanes 54-56 based on chiral half-sandwich complexes [79]. These were obtained by treatment of the appropriately substituted complex with a secondary phospholane, itself accessed via the cyclic sulfate or dimesylate and PH3. These were tested against a range of substrates with C=C, C=0 and C=N bonds, with variable results [80]. 

The controlled synthesis of carbons adjacent nido- and arachno-carborane anions can be achieved by the reduction of C (cage)-C (cage)-linked o-carboranes with group 1 metals.234-237 Typical examples are illustrated in Scheme 22. These have been found to undergo metallation reactions similar to their carbons apart analogs, forming full- and half-sandwich complexes of lanthanide metals.238... 

The conformational orientation adopted by the allenylidene group =C=C= CR R also merits comment. Thus, in half-sandwich complexes, mainly derived from [M(77 -CxHy)L2] (M = Fe, Ru, Os) metal fragments, a marked preference of the allenylidene group to adopt a vertical orientation in which the ipso carbon atoms of the R /R substituents are contained in the molecular plane (pseudo mirror plane bisecting the half-sandwich metal fragment) is observed. Preference for this conformation arises from the dominant metal ( xy-Cp back donation of the metal-HOMO into the allenylidene-LUMO n orbital (see Fig. 13). In contrast to this general trend, an unusual horizontal orientation of the allenylidene group was... 

Thus, with anion 26 under similar reaction conditions only one diastereomeric half-sandwich complex, 27a (R = Me), was observed in the crude product, isolated in high yield after chromatographic workup, and characterized by X-ray diffraction. The relative configuration is such that the two bulky groups - the PPh3 ligand and the Cp Fe moiety - avoid steric interference (Fig. 1.5.6) [24]. 

Access to polyolefins with a wider choice of properties has more recently been provided by various homogeneously soluble organometallic catalysts. Some of these catalysts, in particular those based on sandwich and half-sandwich complexes of zirconium and titanium and on nitrogen-containing complexes of group 4 and of some of the group 8-10 metals (see Section 7.4), are thus likely to be increasingly used for the production of polyolefins for special-purpose applications, which require properties not easily accessible otherwise. 

The effect of cyclopentadienyl-ring substituents on the reactivity of the group VII half-sandwich complexes (t7 -C5R5)M(CO)2L[M = Mn and Re R = H, Me, and Et (Mn only) L = Kr and Xe] toward CO in supercritical fluid solution at room temperature has been investigated (70). The reactivity of the corresponding alkane complexes ( 7 -C5R5)Mn(CO)2(n-heptane) (R = H, Me, and Et) toward small molecules such as CO, N2, and H2 in n-heptane solution steadily increased in the order H < Me < Et (71). These results indicated that steric rather... 

Some of the most extensively explored univalent group 13 compounds are the various cyclopentadienyl half-sandwich complexes of the general formula Cp M, where Cp is any cyclopentadienyl ligand and M is Al, Ga, In, and Tl. For Al and Ga, these compounds are typically prepared by the reduction of suitable dihalogenated M precursors, as outlined in Scheme 4, and they may also be prepared from available univalent halide precursors. For Cp ligands bearing relatively small substituents, Cp M compounds tend to form polyhedral (Cp M) clusters in the solid state and dissociate into monomers in both solution and the gas phase a depiction... 

Half-sandwich complexes of the group 6 and 8 transition metals carrying one a-ferrocenyl ligand are presented in Table 5-3. 

Table 5-3. Half-sandwich complexes of group 6 and 8 transition elements with a o-ferrocenyl (Fc) ligand...

A wide variety of substituents at the cyano group is tolerated by the cobalt catalyst. For example, monomeric cyanamide reacts with acetylene in the presence of 7 -borininato cobalt half-sandwich complexes ([Co-B]) to give 2-aminopyridine [5e] (eq. (11)). 

---