Aryl ligands, bulkiness

In spite of the attention focused on these bulky aryl substituents, especially in main-group chemistry, aryl ligands in which crowding is caused by the presence of ortho-aryl substituents formerly received little attention. In contrast, related terphenyl-substituted phenolate derivatives (e.g., -OC6H3-2,6-Ph2) of the early transition metals have been studied for many years, and zinc terphenolate derivatives have attracted attention (vide infra). A small number of similar aryl thiolate derivatives of transition metals (e.g., Mo, Fe, and Rh) have been characterized and published (vide infra). In addition, the related ligand -C6H2-2,4,6-Ph3 (Triph) had been employed as a ring substituent in bulky porphyrins.24... 

Recent successes are partly based on the use of aryl or bulky aryl substituents at the nitrogen atoms of the imidazolium rings. As ligands they are strong o-donors (stronger than t-BujP) and hardly rr-acccptors, they are sometimes called singlet carbenes . 

The suitably bulky aryl ligands cause steric congestion at the bismuth center and dramatically accelerate the second step [81]. 

Dibismuthenes of the general formula LBi=BiL can be synthesized when L is a bulky aryl ligand. The LBi=BiL molecule is centrosym-metric and therefore exists in the trans configuration. For L = 2,4, 6-tris[bis(trimethylsilyl)methyl]phenyl, X-ray analysis of the dibismuthene... 

We have found that the late metal congeners of this important compound class are easily synthesized and can be derivatized to yield aryl monocyclo-octatetraene lanthanide complexes if the metal is small (Er—Lu) and the ligand bulky and chelating. The synthesis of these compounds clears the way for comparative reactivity studies (with cyclopentadienyl lanthanide complexes) and allows investigations of fundamental chemical reactivity to be conducted for this ligand system. 

Whereas complexes of unsubstituted and substituted cyclopentadienyl ligands represent the vast majority of all published compounds in organolanthanide chemistry, examples of isolated and fully characterized (including X-ray structural analyses) compounds containing only cr-bonded alkyl and aryl ligands are still fairly rare. The first structurally characterized homoleptic lanthanide alkyls became available through the use of bulky mono-, bis-, and tris(trimethyl-silyl)-substituted methyl ligands. Simple unsolvated alkyls of the rare earth elements have not yet been synthesized. 

M. Beller has also developed a similar bulky mono-dentate phosphine ligand, di( 1 -adamantyl)- -butyIphosphine, for the coupling of aryl chlorides. 56 Although (Ad)2P(w-Bu) allows for the arylation of bulky anilines and secondary amines, as well as f-BuNH2, the reaction temperatures are significantly higher (100-120 °C). In addition, electron neutral or rich aryl chlorides couple best, whereas electron deficient aryl chlorides were not extensively reported. 

Similar Ni and Pd catalysts developed by Keim [4] and others [5,6] which do not posses the bulky hgand systems have been used to produce dimers or extremely low molecular weight oUgomers. Brookhart has suggested [ 1 ] that the bulky aryl ligands act to preferentially block the axial sites of the metal center as illustrated by Fig. 3. This feature in the catalyst system must in some... 

It is commonly recognized that electron-rich phosphines facilitate oxidative addition of aryl chlorides and bromides, and bulky phosphines accelerate reductive elimination. From these considerations, P(r-Bu)3 is one of the best ligands. Bulky and electron-rich are key words in discovering good ligands for active catalysts. 

The transformation of cr-aryl to 77-arene chromium complexes does not occur readily with bulky aryl ligands such as mesityl and 1-naphthyl 166). 

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