Metal arenes

Transition Metal Arene jr-Complexes in Organic Synthesis and Catalysis... 

General Structural Features. The general structure of halfsandwich ruthenium(II)-arene complexes is shown in Fig. 12. The structural, stereochemical and electronic features of metal-arene complexes have been discussed (63). A typical piano-stool geometry consists of an rj6-arene occupying three coordination sites of the pseudo-octahedral complex, leaving the three legs X, Y, and Z available for coordination. The sites X and Y can be taken up by two monodentate ligands, but are more commonly... 

James et al. reported a case of product inhibition in the Rh-catalyzed enantioselective hydrogenation of N-phenyl benzaldehyde imine [37]. These authors were able to isolate the deactivated catalyst, and to obtain its X-ray structure, which showed, surprisingly, that it was a rhodium complex with the product bound through a rf-n-axene interaction (Scheme 44.5). More cases of inhibition via formation of metal arene complexes will be detailed in Section 44.5. 

The synthesis of 77 -complexes was accomplished by reacting zwitterionic phosphonio-benzophospholides such as 25 and 26 with metal-arene complexes [M(arene)(CO)3] (M=Cr, Mo, Mn ) in a non-coordinating solvent (Scheme 17). The reactions proceeded with high regioselectivity and gave no evidence for the formation of a-complexes involving phosphorus lone-pairs [32,42,43],... 

Scheme 9 Metal-arene interactions in biaryl ligands...

The molar conductance of MTHF solutions of alkali metal-arene adducts (M+A , M+2A ) was measured for A = anthracene, tetracene, pentacene, biphenyl, p-terphenyl... 

By coordinating to arenes, transition metals can facilitate ring lithiation by decreasing the electron density in the ring and acidifying the ring protons. We shall consider briefly the two most important metal-arene complexes in this regard—arenechromium tricarbonyls and ferrocenes. 

The element of planar chirality plays a pivotal role in many modern ligand systems. The particularly huge success of ferrocenyl ligands has not been matched by any other chiral backbone to date. Metallocene and metal-arene-based ligand backbones exhibit the common feature that they become planar chiral only upon addition of (at least) two substituents on one ring fragment. [2.2]Paracyclophanes, however, need only one substituent (Fig. 2.1.3.1) to be chiral. 

According to NMR data, reaction of lithium tetramethylpiperidine (TMP) and t-Bu2Zn, in a 1 1 molar ratio (Scheme 6) affords the heteroleptic zincate [f-Bu2Zn(TMP)]Li (36) . An interesting feature of this compound is that it is capable of metalate arenes that contain a directing metalating group (DMG) in ortho-posilion and thus allow further derivatization by reaction of the zincate intermediate with electrophiles (E+). 

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 synthetic potential of transition metal atoms in organometallic chemistry was first demonstrated by the formation of dibenzenechrom-ium (127). Apart from chromium, Ti, V, Nb, Mo, W, Mn, and Fe atoms each form well-defined complexes with arenes on condensation at low temperatures. Interaction has also been observed between arenes and the vapors of Co, Ni, and some lanthanides. Most important, the synthesis of metal-arene complexes from metal vapors has been successful with a wide range of substituted benzenes, providing routes to many compounds inaccessible by conventional reductive preparations of metal-arene compounds. 

Clearly, there is a delicate balance between metal-arene formation and dehalogenation of the arene. Among the aforementioned isolated compounds, those of chromium show greater resistance to air oxidation than those of vanadium for example, Cr[C8H4(CF3)2]2 is completely stable in air (32, 59, 79, 115). 

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