Hydrogen bonding hexacoordinate

Sc(03SCF3)3 in the presence of a BINOL ligand.113 Optimized conditions involved use of 5-20 mol % of the catalyst along with a hindered amine such as cis-1,2,6-trimethylpiperidine. A hexacoordinate TS in which the amine is hydrogen bonded to the BINOL has been proposed. 

A zwitterionic hexacoordinate pentafluorosilicate (175) was recently reported by Tacke193.175 was prepared in the same manner as pentacoordinate zwitterionic complexes (cf Section II.A.3), as shown in equation 45. The crystal structure of 175 showed that one water molecule is present for each molecule, and that the structure is dominated by N—H-F and O—H-F hydrogen bonds. The Si—C bond length (1.943-1.946 A) is distinctly longer than in the corresponding pentacoordinate zwitterion (9), while the Si—F bonds (1.685-1.740 A) are comparable to those in the pentacoordinate analog. The solid state 29 Si NMR of 175 is also compatible with hexacoordination, with 529Si= -177.2 ppm. 

All of the complexes 30-38 remain neutral and hexacoordinate in apolar solvents toluene, carbon tetrachloride. However, most of these compounds undergo reversible ionization when dissolved in hydrogen-bond donor solvents CD2C12, CDC13, CHFC12 (Eq. 17) 65 67... 

The whole concept of P-donor interaction in the presence of a H-bonding network and its relevance to the active site of phosphoryl transfer enzyme mechanisms was also discussed in two companion papers which reached the conclusions that P-O donor interactions leading to hexacoordinate states, hydrogen bonding, and conformational distortions due to van der Waals forces, are all important in structuring the active site. 

Recent studies on hydrazide-based hypercoordinate silicon complexes demonstrated the unusual flexibility of these compounds their tendency to reversibly transform between penta- and hexacoordinate compounds [1, 2] on the one hand and to irreversibly rearrange to more stable complexes [3] on the other. Thus, neutral hexacoordinate bis-chelate complexes (1) undergo reversible ionization in solution (Eq. 1), which is strongly dependent on a variety of factors temperature (ionization is enhanced at low temperatures), solvent (ionization takes place in hydrogen-bond donor solvents such as CHCI3, CH2CI2, and CHFCI2), the nature of the anion, the... 

We determined that vicinal dihydroxy organic reagents stabilize the soluble forms of silica. The stability of monosilicic acid solution is determined by the structure of the stabilizer ethylene glycol and humic acids do not stabilize soluble forms of silica enough, but glycerin and catechol stabilize the silica solution when they are added at 5 - 7% to the solution. This fact is connected with the formation of hydrogen bonds and stable penta- and hexacoordinated compounds, preventing the processes of polycondensation of silica in solution. 

Three doubly deprotonated xylitol molecules act as 1,2-diolato ligands in the hexacoordinate silicate [Si(Xylt2,3H 2)3] 27 (O Fig. 6) which is the hydroxyunethyl derivative of the thre-itolato complex 19 (O Fig. 4) and, as a consequence, shows the same stabilizing intramolecular hydrogen bonds [33]. 

In 34 (O Fig. 9), the silicon center is hexacoordinated through three central, f/ireo-conflgured diol groups. The adjacent hydroxyl functions at C2 and C5 form stabilizing intramolecular hydrogen bonds similar to 19 (O Fig. 4) and 27 (O Fig. 6) [33]. 

From the kinetic and model studies it was postulated that the catalytic site of the P. chrysosporium MnP is located at or near the heme edge of 5-meso-carbon [87]. When the crystal structure of this MnP was elucidated, the active site was found to consist of a His ligand hydrogen bonded to an Asn residue, and a distal side peroxide binding pocket formed by a catalytic His and Arg [88]. Mn(II) binding site was shown to be at the propionate ends of the heme, Mn(II) being hexacoordinated by an Asp, two Glu residues, a heme propionate and two water molecules (Figure 14.4). 

Fig. 13.30. Active site of cytochrome P450j3 with the inhibitors a metyrapone b 1-phenyl c 4-phenyl-, d 2-phenyl-imidazole. In all cases Fe is hexacoordinated. In a, b, and c a nitrogen atom of the inhibitor occupies the sixth coordination site at Fe, but in d a water molecule remains at this position and the inhibitor is fixed in the active site through a hydrogen-bonding network, partially mediated by an additional water molecule to Asp251...

Moreover, two equivalent S P bonds are quite strong [2.334(1)A] and are comparable to that in the chloro precursor (15) [2.317(1)A] but shorter than other hexacoordinate phosphorus compounds with only one S- P coordination. The presence of hydrogen bonded chloride (to pyrazole NH) ion also enhances the stability of hexacoordinated phosphate (17). 

Carboxyl-terminated terphenylthiolate molecules form a bilayer on Au surface owing to hydrogen bonding between the protonated carboxylates [18]. Exposure of the bilayer to a K" " solution resulted in the intercalation of K" " ions between the layers without removal of the second layer. A stoichiometric ratio of K/O = 0.53 determined by XPS is consistent with hexacoordinated K" " ions in the bilayer. On the other hand, dipping in a Cu + solution led to binding of the metal ions and removal of the second organic layer (Fig. la). The measured ratio of Cu/O = 0.21 is close to the expected value of 0.25 for Cu " " ions coordinated with two - COO groups. It should be noted that intercalation in self-assembled multilayers was first demonstrated with amphiphilic silane bhayers [40]. 

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