Benzyne ligand

A paramagnetic vanadium-benzyne complex (112) has been synthesized by thermolysis of CpVPh2(PMe3)2 (111) at 50°C tia = 3-4 h) [Eq. (18)].64 An X-ray crystal structure was reported for 112. Bond lengths for the benzyne ligand are given in Table III. The C1-C2 distance [1.368(5) A] is similar to those in the other structurally characterized benzyne complexes. A band in the IR spectrum of 112 at 1547 cm"1 was assigned to the coordinated triple bond. 

Electrons donated by the benzyne ligand, followed by the electron count for the complexes in parentheses. 

Reductive elimination of biaryls is a common decomposition route Benzyne ligands might also be (Figure 4.9a), as illustrated by the osmium example (Figure 4.9b) in which described as phenyiene-1,2-diyls the biaryl remains hexahapto coordinated. Alternatively, benzyne com-J plexes may result from p-C-H or P-C-Br abstraction processes (see below). 

Migration of the benzyne ligand in Os3(CO)7(CgH4)(EMe2)2 (E = As, Ph) is also necessary to explain the temperature dependence of these complexes proton NMR spectra (45). Four methyl resonances would be expected from the unsymmetrical structure (93), but only two are... 

FIGURE 42 Structure of Os3(CO)7(PPh2)2(C6H4) containing a benzyne ligand interacting with the OS3 core. 

Some [MX]+ ions enter into reactions in which the ligand X and the reacting molecule become chemically bonded. Polymerization processes have been observed involving the [MC4H4]+ ions (147). The butadiene complex ions [MC4H4]+ of Co and Ni are unreactive to ethyne but the Fe, Ru, and Rh ions react to yield benzene and the bare metal ion. The [MC4H4]+ complex ions of Os+, Ir+, and Pt+ react with ethyne to form the MC4I I4 + ions that probably correspond to the benzyne complexes previously observed for platinum (126). 

A tantalum-benzyne complex possessing a mdo-carborane as a ligand (130) has been prepared66 by reaction of 128 with phenylmagnesium bromide followed by refluxing in toluene in the presence of PMe3 (Scheme 16). An X-ray crystal structure was reported for 130. Bond lengths for the benzyne unit are recorded in Table III. 

The tantalum-benzyne complex (130) is much less reactive than other early transition-metal aryne complexes. It shows no reaction with acetone, benzophenone, benzaldehyde, acetonitrile, 3-hexyne, or methanol. The lack of reactivity of 130 was attributed to nonlability of the PMe3 ligand. Indeed, no phosphine exchange was observed when 130 was mixed with an excess of PMe3-d9. Refluxing 129 in a mixture of methanol and toluene (3 10 v/v) leads to clean formation of 131. This presumably results from reaction of a 16-electron benzyne complex with the alcohol. 

The /3-H elimination may involve phenyl ligands, as in the decomposition of Cp(PMe3)2VPh2 to the benzyne complex Cp(PMe3)2V(T/2-QH4),lir7 or metal amide ligands to give ketimines,108... 

Metal compounds are often used to stabilize reactive organic fragments via complexation and intriguing alkyne ligand in this classification is benzyne. A series of iridium benzyne complexes has been made with compound (44) as an example and the reaction chemistry of coordinated benzyne has been explored. ... 

AUcynylphosphines have also been shown to undergo an interesting double insertion with Ni(0)-benzyne complexes. Thus, two equivalents of Ph2P-CC-R insert into the Ni-benzyne bond in (24) to form a series of novel naphthalene-based bis(phosphine) ligands coordinated to the Ni center (25,... 

---