Cyclometallation

Cyclometallation refers to a process in which unsaturated moieties form a metallacyclic compound. It is sometimes categorized under oxidative additions, but we prefer this separate listing. Examples of the process are presented in Fig. 4.31. Metal complexes which actually have displayed these reactions are M = L Ni for reaction a, M = Cp2Ti for reactions b and c, M = Ta for d, and M = (RO W for e. The latter examples involving metal-carbon multiple bonds, have only been observed for early transition metal complexes, the same ones mentioned under the a-elimination heading. 

Three mechanisms can be proposed for the intimate reaction mechanism for c-e, analogous to the organic 2+2 cycloadditions a pericyclic (concerted) mechanism, a diradical mechanism, and a diion mechanism. In view of the polarization of the metal(+) carbon(-) bond, an ionic intermediate maybe expected. The retention of stereochemistry, if sometimes only temporary, points to a concerted mechanism. 

The reverse reaction of a cyclometallation is of importance for the construction of catalytic cycles. The simple retrocyclometallations of a and b are not produc- 

Reaction a in Fig. 4.31 may be succeeded by various other reactions such as insertions, P-eliminations or regular reductive eliminations (see Fig. 4.33). 

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The reversible reaction of tri-n-butylstannylfuran with the cyclometallated palladium complex 24 yields the ti C) coordinated 2-furyl complex 25 (98JA11016). 

Phenylpyrazole upon reaction with [MeMn(CO)5] yields 42 (75AJC1259). This product is cyclometallated and contains a five-membered ring. 

The cyclometallated palladium and platinum derivatives of trimesityl phosphine or arsine react with pyrazole or 3,5-dimethylpyrazole to form metal chelates 243 (E = P, As M = Pd, Pt R = H, Me) having the trans configuration (81TMC24). 

Reaction of the cyclometallated complexes 244-246 with pyrazole and excess sodium hydride affords cyclic dimers 247 (990M3991), where C N denotes the corresponding cyclometallated ligand in accordance with structures 244-246. The [Pt2(thienylpyridine)2(/j.-pz)](C104)3] is known as well. 

Diphenylimidazole with palladium acetate forms the cyclometallated complex 80 (X = OAc) (97AOC491). The acetate group is replaced by chloride or bromide when 80 (X = OAc) reacts with sodium chloride or lithium bromide, respectively, to give 80 (X = C1, Br). Bromide with diethyl sulfide forms the mononuclear complex 81. Similar reactions are known for 1 -acetyl-2-phenylimidazole (96JOM(522)97). 1,5-Bis(A -methylimidazol-2-yl)pen-tane with palladium(II) acetate gives the cyclometallated complex 82 (OOJOM (607)194). 

Phenyl- and 2-naphthylbenzothiazole with iridium(III) chloride give cyclometallated derivatives of the type 54 containing two chloride bridging ligands (01IC1704). 

In the rhodium and iridium complexes, the C-coordination, carbene function, and cyclometallated cases prevail. Benzothiazole-2-thione was studied extensively as a ligand and various situations of the exocyclic S-monodentate coordination as well as N,S-combinations in the di-, tri-, and tetranuclear species were discovered. 

The cationic species [IrX2(CO)(PMe2Ph)3]+ (XVI, Q = CO) undergoes nucleophilic attack by methoxide forming the carboxylate complex (XXI). In the presence of very strong base, a cyclometallation of a methyl group occurs forming (XXIV) [165],... 

When the phosphonium ylide 81 is reacted with zinc amide, the corresponding a-zincated phosphorus yUde is formed. Thermally unstable, it evolves almost quantitatively to zincatacyclobutane 82 which in presence of pyridine leads to the formation of the zincataphosphoniaindane 83. In order to explain this unprecedented cyclometallation reaction, a mechanism is proposed involving a low coordinated zinc center. The new product, reacted with benzaldehyde leads to the diphenylallene 84 (Scheme 27) [106-108]. 

P-chiral dibenzophosphole oxide (52a) (Scheme 14) shows liquid crystalline behaviour [52], a property that is of interest in the area of electro-optical displays [53]. Chiral resolution of (52a) was achieved by column chromatographic separation of the diastereoisomers obtained following coordination of the o -benzophosphole (52b) to chiral cyclometallated palladium(II) complexes [52]. Notably, the presence of a stereogenic P-centre is sufficient to generate a chiral cholesteric phase. 

Rh(COE)2Cl]2 in the presence of IMes, MA -bis-[2,4,6-(trimethyl)phenyl]imi-dazol-2-ylidene, reacted with at room temperature to give the trigonal bipyrami-dal [Rh(H)2Cl(IMes)2] 6 (which is analogons to 3) via intermediate formation of the isolable cyclometallated 5 (Scheme 2.2) [3],... 

Scheme 2.2 The formation of the rhodium dihydride complex via a cyclometallated intermediate...

Scheme 2.1 Production of cyclometallated derivatives [Au(N,C)Cl2] from 2-benzylpyridines by refluxing in MeCN/H20.

As in the case of 2-substituted pyridine adducts, activation of an aromatic C—H bond is observed when the adducts of 6-thbipy and of 6-Bnbipy are heated under reflux in aqueous media. A cyclic dimer (1) with bridging N,C ligands is obtained in the first case [24], while cyclometallated derivatives [Au(N, N,C)C1] (2) are formed in... 

Bnpy-H) = cyclometalated 2-benzylpyridine, saccH = saccharine. (6-Bnbipy-H) = cyclometalated 6-(l,l-dimethylbenzyl)-2,2 -bipyridine. 

Treatment of the cyclometallated complexes [Au(N,N,C)Cl][PF6] [N,N,CH = 6-methylbenzyl- (a) or 6-(l,l-dimethylbenzyl)-2,2 -bipyridine (b)] [20] with KOH or Ag20 in aqueous media affords the hydroxo complexes [Au(N,N,C)(OH)][PF6] (36) in fairly good yields [45b, 101] these are air-stable white solids, quite soluble in water and in many organic solvents. When refluxed in anhydrous THF they condense to give the oxo-bridged complexes [Au2(N,N,C)2( J--0)] (37) (Equation 2.10 in Scheme 2.5) which, in turn, can be obtained by a different route [102] (see Section 3.2) the reaction can be reversed by refluxing the 0x0 complex in water. 

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