Metal atom reactions with arenes

Metal atoms are oxidized on interaction with many compounds containing polar bonds R—X. This effect has already been noted in Section III in reactions of metal atoms with arenes containing electronegative substituents. The products of oxidation that can be isolated are of three types, namely, RMX, RM, and R2 + MX unstable organometallic products can be sometimes stabilized by addition of a ligand, L, at low temperatures to give RM(Ln)X or RML . 

The free or polymer-bound bis(arene)metal complex can also react with metal atoms. Francis et al. (44) first published evidence that the siloxane-bound ic-complexes are converted to dimers and higher nuclearity clusters by additional metal atoms. Their experiments were conducted on quiescent thin liquid films of polymer applied to the optical window of a cryotip (see above, Small Scale Syntheses). Low nuclearity polymer-encapsulated molecules of Tin, Vn, Crn and Mon (n = 2-5) were inferred from quantitative studies of the metal atom aggregation process. The initial reaction appears to occur as follows ... 

Co-condensation of transition metal atoms with arenes such as benzene and toluene is well known to yield bis-arene-metal compounds. However, in many cases the yields based on the metal atoms are less than 40%. Evidence that competing reactions such as carbon-hydrogen activation can occur is provided by the isolation of non-metal-containing products such as biaryl derivatives (2JL). ... 

Trifluorophosphine is a very convenient ligand in metal atom chemistry to use along with other ligands, e.g., in the stabilization of metal arene complexes (Section III,B). Reaction of a mixture of PF3 and PH3 with nickel vapor yields Ni(PF3)3PH3 and Ni(PF3)2(PH3)2 but no Ni(PH3)4. Attempts to make Ni(PH3)4 lead to hydrogen evolution from the ligand during or after condensation with the nickel vapor (128). 

The only route to dibenzenetitanium so far described is the reaction of titanium atoms with benzene the reductive routes that give access to arene complexes of Group V and VI metals fail for titanium. Although yields of about 30% are reported for the preparation of dibenzene-, ditoluene-, and dimesitylenetitanium, the reactions are more sensitive than most to the effect of excess metal. Unless the ligand-to-titanium ratio is high and the rate of deposition of titanium vapor kept low, the products seem to be catalytically decomposed by finely divided Ti metal 4a, 7). 

Diarenemanganese cations have not been formed by the metal atom route, but condensation of arenes, cyclopentadiene, and manganese gives low yields of (arene) Mn(C5H5). As cyclopentadiene does not react with manganese atoms, a primary manganese-arene reaction is postulated (131, 140) (see Section III,A,3). 

Kinetics of the preparative reaction between arene and metal carbonyl to form the tricarbonylarene metals are complicated (81), but the reaction has recently been shown (139a) to be first order in metal carbonyl which is consistent with the simple SN2 mechanism with inversion of carbonyl groups about the metal atom (739). It is to be hoped that, with the establishment of the mechanisms of these reactions, a correlation with bond theory of the tricarbonylarene metals may emerge. 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Aluminium-based catalysts with tetraphenylporphinato [32,35,38,81], Schiff s base [37-40] and calix[4]arene ligands [41] are characterised by the appearance of a non-associated, isolated pentacoordinate metal atom forming an active bond Mt X with nucleophilic substituent X. As stated above, epoxide polymerisation with these catalysts involves the rearward attack of the nucleophilic substituent on the coordinating epoxide molecule [scheme (1)]. In order to explain this, a mechanism involving the simultaneous participation of two catalyst molecules in the initiation and propagation reaction has been proposed [40,41,62,82]. According to this mechanism, the rearward attack of the nucleophilic substituent on the coordinating monomer is carried out by the six-coordinate aluminium species. These species can appear as neutral epoxide... 

Laboratory in Oxford, and Geoffrey Ozin at the University of Toronto in the early 1970s. With the metal atom cocondensation technique (which as described in Chaps. 6 and 7 was also used to prepare a series of zerovalent arene and olefin metal complexes), they reported simultaneously that the elusive palladium and platinum tetracarbonyls, Pd(CO)4 and Pt(CO)4, as well as the coordinatively unsaturated fragments M(CO)3, M(CO)2, and M(CO) (M = Pd, Pt) were formed by cocondensation reactions of Pd and Pt atoms with CO in inert gas matrices at 4-10 K [119-122]. The comparison of the CO bond stretching force constants for Pd(CO)ra and Pt(CO)ra (n - 1-4) revealed that, in analogy to Ni(CO) , the most stable compounds were the tetracarbonyls. In a xenon matrix, Pd(CO)4 existed up to about 80 K [120]. Ozin s group as well as others... 

The Fischer-Hafner synthesis of sandwich compounds (33) does not permit functional groups to be incorporated into the arenes because of side reactions with the Lewis acid catalyst (Friedel-Crafts reducing conditions). This is not the case when metal atoms are used directly. Many metal-arene complexes have been identified that contain F, Cl, CH30, R2N and C02R substituents. It is reasonable to assume that polymer-bound phenyl substituents containing these functional groups will yield similar sandwich complexes. 

Apart from the metal atom aggregation reactions described below, bis(arene)metal complexes of the early transition metals are resistant to ligand displacement The rings on the corresponding bis(naphthalene)metal species (41) are by, contrast, labile. Polymer-supported analogs of these naphthalene compounds with vanadium and chromium are known (42), but Ti atoms attack the polymer at the silicon ether linkage. These and other hybrid polymers can be further modified once the metal atom is incorporated. Thus a-methyl naphthalene is displaced from the hybrid organometallic polymer shown in Scheme 7 (43). 

Oxidation of M(CO)6 with halogens affords M2(CO)gXj anions, but in the presence of PMe3, neutral mononuclear species MX(CO)3(PMe3)3 are formed. The former contain three bridging halide atoms, which can be substituted by MeO or CH3C02 by reaction with methanol or acetic acid. The metal-metal distance of over 3.5 A precludes direct interaction. The dinuclear chloro anions are converted to CpM(CO)4 by reaction with LiCp and to (i76-arene)M(CO)4+ in the presence of arene and AlBr3. 

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). 

The reaction of RhCls with an excess of the anion of TIPT in MeCN also generated a dinuclear complex in high yield, in which the bridging aromatic thiolate also binds via an arene substituent, but in this case the two rhodium atoms are not equivalent (76). A representation of the overall crystal and molecular structure is shown in Fig. 18. A simplified view of the coordination sphere of one rhodium atom is shown in Fig. 19. The structure comprises two distinct metal atoms, with... 

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