Hemilabile

More success has been had with Ir complexes incorporating permelhylcyclopentadiene and NHC ligands. Complexes 18-20 (Fig. 4.7) were evalnated for norbomene polymerisation following activation with MAO [22]. Complex 19 was the most active, giving a TOF of 12 220 h over 10 min, followed by 18 (TOF = 3 220 h" ), while 20 was inactive, indicating that a hemilabile pendant group seems essential. Analysis (NMR) of the polymers formed with 18 and 19 shows that polymerisation proceeds via an addition (coordination-insertion) mechanism. 

Various bifunctional potentially hemilabile ligands (72) bearing phosphorus groups were prepared and their coordination to rhodium was studied. Their effect on the hydroformylation of styrene was assessed.237... 

The hemilabile phosphino-ester and phosphino-thiophene ligands (139)-(142) behave like monodentate phosphine under catalytic conditions.476... 

B(3,5-(CF3)2C6H3)4-.512 Palladium complexes with a hemilabile terdentate carbene ligand, 1,3-bis(pyl)imidazol-2-ylidene, were active toward the catalytic polymerization of CO/norbornylene.513 Palladium complexes of cz s-bidentate C4-bridged diphosphines cis- and trans- 1,2-bis [(diphenylphosphino)methyl]cyclohexane, e fl o,e fl o-2,3-bis[(diphenylphosphino)methyl] norbornane,... 

Figure 1 Chelating and hemilabile ligands used in palladium-catalyzed amination of aryl halides.

Scheme 12 Concept of hemilabile axial donor a-diimine ligand...

Scheme 13 Hemilabile axial pyridine donor a-diimine ligand and complexes...

The bidentate ligand (dmpe) is believed to act in a hemilabile fashion, enabling a crucial intramolecular C-H activation to take place (Equation (37)). 

Neutral mixed chelate ligands containing both phosphorus- and nitrogenbinding sites often show a hemilabile character (they are able to bind via one or two atoms to the metal Fig. 20.2), which allows for the temporary protection and easy generation of reactive sites in the complexes. 

Mechanistic details of this reaction are scarce, but Aratani (14) mentions that the catalyst needs to be activated by heating in the presence of the diazo compound at 75-80°C until nitrogen evolution is observed and the color of the complex changes from green to brown. Reduction of the cupric precatalyst with a substituted hydrazine results in a yellow cuprous complex capable of inducing an instantaneous decomposition of diazoacetate at ambient temperature. Aratani proposes that the active catalyst is tetrahedral Cu(I), 26 in Scheme 2. Reaction with the diazoester from the less hindered face forms the Cu carbenoid having one hemilabile ligand (al-... 

The coordination of styrene with the pyrazole detached resulted in the formation of a stable n-complex (vide infra). Therefore, throughout the catalytic cycle the pyrazole plays the role of a hemilabile ligand [55], This is an important result because experimental studies [11] suggest that olefin coordination is slow and reversible. Moreover, the tuning of the steric bulk of the pyrazole substituents could be used to enhance the activity of the... 

The presence of water in the crystals can be explained by humid crystalization conditions and is proven by elemental analysis. The water adduct formation shows, that k 0, 0 -carboxylato ligands are hemilabile, chelating ligands, which can be easily displaced from one coordination site by a solvent. Thus, the carboxylato complexes 37a and 37b are equivalent to 16 VE fragments stabilized by the hemilabile carboxylato complexes. 

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