Chelation bonding

By transformation of modified PAN fibres of type 2 into iV-hydroxyamide derivatives of type 3 (Scheme 1), it is possible to form intermolecular chelate bonds by interaction with Fe+3 ... 

Fig. 6 EGF-containing chimeric proteins anchored to the Ni-chelated surface through coordination. Bold lines in the molecular structures represent chelate bonding. TEG-thiol triethylene glycol-containing alkanethiol. Reproduced from Nakaji-Hirabayashi et al. [87] with permission from American Chemical Society, copyright 2009...

These iniferter sites containing an N-H group can be easily transformed into the corresponding thiol which leads to disulfide by oxidative coupling and can form chelation with metal ions (Eq. 47) [171,172]. Poly(St) prepared for polymerization with 44 and 45 was applied to the chain-extension reaction by the S-S bond or chelation bond formations. 

A variety of derivatives of bis-[Bi(tr)2X, X = Cl or N03], tris-(Bitr3), and tetra-([Bi(tr)4X][Na]) tropolonate complexes of bismuth (178, 179) have been prepared and spectroscopically characterized (176, 177, 180). Solid-state structures for examples of bis-(tropolonate) derivatives confirm the chelate interaction (171) and in the case of the nitrate derivative, reveal intermolecular alkoxide-bismuth [Bi-0 2.688 and 2.666 A] dimer contacts 52, which are slightly longer than the chelate bonds [Bi-0 2.130-2.323 A],... 

Mode IV represents the well known chelating bonding mode, one donor atom being the ylidic C (kC) and the other a heteroatom (kE), while mode V presents the... 

The distortion in [Lu(DPM)3(NCeH7)] stems from the fact that the Lu—N bond (2.492 A) is 0.25 A longer than the average Lu—O distance of 2.238 A, while the average chelate bite remains at 2.74 A Hke other /S-diketonate complexes. Such distortion is not uncommon. Even when all seven atoms are oxygens, a long M—O distance contributes to the distortion of the coordination polyhedron as in the case of [Dy(DPM)3(OH2)] (57), where the Dy—O (water) is longer by 0.11 A compared to the Dy—O (chelate) bond. 

I propose that the reason a chelate bond is so much more difficult to break is that the minimum energy path for the dissociation of a ligand from a metal is a linear displacement rather than an angular displacement. 

The central core of 26 is a flat four-membered O-Mg-O-Mg ring. One of the f-Bu groups is located above, and the other below this plane. To each of the oxygen atoms a sodium atom is bonded [Na-0 2.533(5) A] while the four butyl groups are bridge-bonded between the sodium and magnesium atoms in a rather asymmetric way [C-Mg 2.190(6) and C-Na 2.852(7) A]. Penta-coordination at each sodium atom is reached by the additional A, A -chelate bonding of a TMEDA molecule. 

The strucmre in the solid state of the l,4-bis(trimethylsilyl)-2-butene-l,4-diyhnagnesium TMEDA complex (139) shows similarities with that of 138 (Figure 66) . Also, in 139 the butene skeleton has a ) -interaction with magnesium with shorter Mg-C(l) and Mg-C(4) bonds [2.200(9) and 2.191(9) A, respectively] and longer Mg-C(2) and Mg-C(3) bonds [2.381(8) and 2.399(8) A, respectively]. Instead of the three coordinating TUF molecules in 138, in 139 a Af,iV -chelate bonded TMEDA molecule is present. 

In a similar way Cp(Me)Mg(OEt2) is capable of deprotonating iV,Ai -bis(2,4,6-tri-methylphenyl)(ierr-butyl)amidine to form the corresponding cyclopentadienylmagnesium amidinate complex (226) (equation 17). An X-ray crystal-structure determination of 226, of which the structure is shown schematically (equation 17), showed that this compound also exists as a monomer in the solid state. Like in 225 the cyclopentadienyl group is /7 -bonded to magnesium while the amidinate anion is Ai,A -chelate bonded with almost equal Mg—N bond distances [Mg-N 2.090(2) and 2.097(2) A]. Furthermore, an additional... 

The structures of the monoorganomagnesium / -diketiminates 233 , 234 , 235 and 236 are comparable. The Af,Af -chelate bonding of the / -diketiminate anion with almost equal Mg—N bond distances to magnesium results in a six-membered MgNaCs ring with all atoms located in one plane. The Mg—C bond of the cr-bonded alkyl group also lies in this plane. As a representative example the structure of 233 is shown (Figure 95). 

The cyclopentadienyl /3-diketiminate 249 and its 4-terf-butylpyridine adduct 250 have been prepared and structurally characterized (equation 19). An X-ray crystal-structure determination of 249 showed that the cyclopentadienyl group is /j -bonded to magnesium. On the basis of the observed bonding parameters of magnesium with the -diketiminate skeleton [Mg-N 2.006(2) and 2.021(2) A, Mg-C 2.729(3) and 2.826(3) A and Mg-C 2.689(3) A] this bonding is described in terms of a jr-interaction. However, in 250 the /3-diketiminate is iV,iV -chelate bonded to magnesium. 

The hybrid boroamidinate/amidinate ligand as present in the methylmagnesium complex 251 (Figure 97) is isoelectronic with the -diketiminate skeleton. The X-ray crystal-structure determination of 251 shows that the boroamidinate/amidinate anion adopts a similar iV,iV -chelate bonding as observed in organomagnesium /3-diketiminates. The structure of 251 is shown schematically (Figure 97). 

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