Dihapto binding

The oxophilic character of the early transition metals may provide such stabilization in the form of dihapto binding (I). This unusual donor behavior was first... 

In view of the fact that early transition metal alkyls insert CO under very mild conditions (2, 5.) we chose to examine the reactions of electron-rich metal hydrides ( ) with the resultant dihapto acyl complexes. Such acyls obviously benefit from reduction of the CO bond order from three (in OO) to two. More significantly, the dihapto binding mode will significantly enhance the electrophilic character of the acyl carbon. 

C-NMR determinations of the relaxation and correlation times of supermolecules (25) have indicated that complementary receptor-substrate pairs display similar molecular motions for both partners [19]. This dynamic fit may be considered to result from the combination of steric fit with dihapto binding. Thus, complementarity between components of a supramolecular species expresses itself in both its structural and dynamic properties. 

The pentaammineosmium(II) metal center also binds in a dihapto fashion to several other classes of aromatic molecules, including benzenes, naphthalenes, phenols, anisoles, anilines, furans, and thiophenes, activating them toward further functionalization. The reactivity and synthetic utility of these complexes are the subject of a recent review Harman, W. D. Chem. Rev. 1997, 97,1953. 

Little is known about the chemical nature of the recently isolated carbon clusters (C o> C70, Cg4, and so forth). One potential application of these materials is as highly dispersed supports for metal catalysts, and therefore the question of how metal atoms bind to C40 is of interest. Reaction of C o with organometallic ruthenium and platinum re nts has shown that metals can be attached directly to the carbon framework. Ihe native geometry of transition metal, and an x-ray difi action analysis of the platinum complex [(CgHg)3P]2Pt( () -C6o) C4HgO revealed a structure similar to that known for [(C4Hs)3P]2Pt( n -ethylene). The reactivity of C40 is not like that of relatively electron-rich planar aromatic molecules su( as benzene. The carbon-carbon double bonds of C40 react like those of very electron-deficient arenes and alkcnes. 

The reactivity of Qo comparable to that of electron deficient conjugated olefins is nicely reflected by reactions with transition metal complexes. A variety of single crystal structures and spectroscopic studies show that the complexation of transition metals to the fullerene core proceeds in a dihapto manner or as hydrometalation reactions rather than in rf- or ] -binding mode. This was elegantly demonstrated by the reaction of Cgg with ruthenium complexes (Scheme 8) [144]. A variety of iridium complexes ( ] -Cgo)Ir(CO)Cl(PR R R )2 were synthesized by allowing Cgg to react with different Vaska-type complexes Ir(CO)Cl(PR R R )2 [145]. ] -Complex formation was also observed upon reaction of Cgo with other Ir [146] as well as Rh [147] complexes. Hydro-metallation was obtained with Cp2Zr(H)Cl [140]. 

Cylindrical macrotricyclic molecules (19) are ditopic coreceptors constructed on two binding subunits, two macrocycles, linked by two bridges. When the macrocycles chosen are able to bind -NHJ groups, diammonium substrates may be expected to form mononuclear dihapto cryptates of type (20) by inclusion into the central molecular cavity of the tricyclic receptor. 

Dioxygen O2. reversibly binds hemoglobin, myoglobin and their models (bent Fe-0-0) dihapto side-on (M-t -02) peroxide complexes, resulting from oxidative addition of O2, are also known. Superoxo- and peroxo ligands bridging two metals (p ) can be r or rf. 

TiCp 2], generated as indicated above, binds molecular nitrogen giving two forms, monohapto and dihapto of a mononuclear complex that can be protonated to hydrazine (a rare case of transition-metal-induced reduction of dinitrogen). 

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