Metal Vapor Reactions

The metal vapor method also has played an important role in the development of organometallic lanthanide chemistry (6-10). This high energy technique demonstrated that the lanthanide metals had a much greater range of organometallic chemistry than had been assumed previously. The metal vapor technique applied to lanthanides identified reasonable new research goals which could subsequently be pursued by solution techniques. Not only the metal vapor reactions. 

Metal vapor chemistry showed that the lanthanides had quite an extensive chemistry with unsaturated hydrocarbons. Some of the early surveys of metal vapor reactions with unsaturated hydrocarbons included some lanthanide metals and showed that reactivity was present for these metals (14-18). Subsequent synthetic studies in which the products were isolated and characterized led to some of the most unusual organolanthanide complexes currently known (19-28). 

The metal vapor reaction products differed from traditional organolanthanide complexes in many ways. First, the observed stoichiometries had low ligand to metal ratios. For example, the ytterbium and samarium 3-hexyne products (Reaction 4) had formal ligand to metal ratios of one, whereas most organolanthanides are commonly nine or ten coordinate (6-10). Second, the stoichiometries varied in an... 

Analysis of possible structures and reaction pathways in reactions 1-4 led to various model structures for these complexes (9t25). Some of these involved C-H activation of the substituents attached to the unsaturated carbon atoms. To test the validity of these models, two additional types of metal vapor reactions were examined. In one case, reactions with simpler unsubstituted hydrocarbons were examined. In another case, substrates ideally set up for oxidative addition of C-H to the metal center were examined. As described in the following paragraphs, both of these approaches expanded the horizons of organolanthanide chemistry. 

The simple hydrocarbon substrates included ethene, 1,2-propa-diene, propene and cyclopropane (22). Their reactivity with Sm, Yb and Er was surveyed. In contrast to the reactions discussed above, lanthanide metal vapor reactions with these smaller hydrocarbons did not provide soluble products (with the exception of the erbium propene product, Er(C H ) ). Information on reaction pathways had to be obtained primarily by analyzing the products of hydrolysis of the metal vapor reaction product. 

The (C Me ) Sm(THF) metal vapor product provided the first opportunity ta see if Smdl) complexes (y =3.5—3.8 Ufi) could be characterized by H NMR spectroscopy (24). Fortunately, the paramagnetism doesn t cause large shifting and broadening of the resonances and hence samarium provides the only Ln(III)/Ln(II) couple in which both partners are NMR accessible. Once the existence and identity of (C Mej- SmdHF) was known, a solution synthesis was developed from KC Me and Sml THF) (44). This system is the preferred preparative route and also provides another soluble organosamarium(II) complex, [(C Me )Sm(THF)2(u-I)]2, under appropriate conditions. This is another xample of how solution studies subsequently catch up to the research targets often identified first in metal vapor reactions. 

Cobalt-arene complexes can also be synthesized from metal vapor reactions. For example, ( -C6H5Me)Co(C6F5)2 is made at low temperature and low pressure conditions (equation 55). 

A remarkable cobalt-arene complex was also isolated from the metal vapor reaction of cobalt atoms with toluene (Scheme 13) and subsequent addition of an aUcyne (acetylene, butyne, or BTSA). According to EPR spectra, compounds of the type (toluene)Co(alkyne) are 19-electron complexes of near-axial symmetry where the aUcyne acts as a four-electron ligand on the cobalt atom. The methyl derivative has been structurally characterized with Co-Caikyne distances of 1.88 and 1.90 A, and a C C distance of 1.254 A (Figure 11). 

Lanthanide metal vapor reactions with unsaturated hydrocarbons such as CsHft (75) or CgHg (76), which are readily reduced by electropositive metals to common anionic ligands, give products arising from the reduction of the ligand and oxidation of the metal [Eqs. (24)-(26)]. The products of... 

In contrast to the above systems, lanthanide metal vapor reactions with unsaturated substrates not readily convertible to stable organic anions common as ligands generate some of the most unusual organolanthanide... 

Later, a similar metal vapor reaction with cobalt produced 122, among other products, which was stmcturally established using the X-ray technique the yields in such processes are low, typically <10% (Scheme 26) <2001JOM212>. 

Macroscale metal vapor reactions with alkanes have led to complex pseudo-organometallic composites, where surface H, alkyl, C, CH, CHj, etc. are bound to metal cluster surfaces Thus, C-H and C-C cleavage occurs on metal atoms or metal clusters in the matrix as it warms. For Ni only clusters react in this way, whereas for Zr, atoms and clusters react ... 

CpJSm (Evans et al. 1984b) was obtained by metal vapor reaction of samarium with CsMesH as a green complex, or from the red 2THF solvated species by desolvation and... 

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