Stereochemical nonrigidity complexes

The a-substituted tropolonates 2 which are tris chelate complexes with a M06 coordination core have received considerable study by DNMR.27, 4S 46) The complexes are of the M(A-B)3 and M(A-B )3 types and therefore the averaging sets of Eaton efa/.26, 27 in Table 1 are applicable. The metal ions and a-R-substituents used in these studies include M = Al(III), Ga(III), Co(III), V(III), Mn(III), Ru(III), Rh(III) and Ge(IV) R = isopropyl (C3H7) and isopropenyl (C3HS), however, only complexes of Al(III), Ga(III), and Co(III) have yielded definitive mechanistic information.27, 45 46> On the basis of line shape changes of the methyl resonances these complexes can be classed kinetically as follows stereochemically nonrigid complexes which attain the fast-exchange limit of inversion and/or isomerization... 

Stereochemical nonrigidity is ubiquitous with seven and higher coordinated complexes because the geometries associated with them are easily interconverted by relatively small atomic displacements. Intramolecular rearrangements are complex but their understanding is helped considerably, but not solved, by nmr techniques. ... 

Zirconium and hafnium complexes are highly fluxional. NMR studies indicate stereochemical nonrigidity, facile exchange of terminal and bridging ligands, and rapid intermolecular ligand exchange. 

The temperature-dependent PMR spectra of the M(R2Dtc)3 complexes [M = Ga(III), In(III)] show them to be stereochemically nonrigid. Kinetic parameters for the intramolecular metal-centered rearrangement (by a trigonal... 

Variable-temperature PMR studies on the stereochemically nonrigid, (Cl4-n)Ti(R2Dtc) (n = 2, 3, or 4) complexes show the metal-centered rearrangements to be fast on the PMR time scale at temperatures higher than —90°. Hindered rotation about the C-N bonds was observed for R = r -Pr, and activation parameters were determined for this process. 

The (T s-Cp)Zr(Me2Dtc)3 complex was obtained by the reaction of (jf-Cp)2ZrCl2 with NaMe2Dtc in dry CH2C12 under N2 (89). Evidence for stereochemical rigidity (four Me resonances of relative intensity 2 1 2 1 at 37°C), and the obvious contrast with the stereochemically nonrigid ClTi(Me2Dtc)3 complex prompted Bruder et al. (89) to undertake a structural determination of this... 

Similar mixed-ligand complexes of the type (R, R2Dtc)2(MNT)Fe have been synthesized. The complexes were obtained initially as dianions, [(RiR2-Dtc)2(MNT)Fe]J", and were subsequently oxidized either by air or Cu(II) ions in acetonitrile (510). They also exhibit the singlet- triplet equilibrium however, they show a higher population of the triplet state than is found for the TFD analogues. The complexes are stereochemically nonrigid and display the same type of kinetic processes as their TFD counterparts. Thermodynamic activation parameters for inversion of the two complexes (TFD versus MNT) do not differ within experimental error. 

The stereochemical nonrigidity of the Fe(MePhDtc)3 was the first to be recognized (493, 495). At —108°C the complex is frozen out and two isomers are present (cis and trans). The cis isomer is unaffected upon warming. However, two of the three sites of the trans isomer are interchanged. No geometric cis—trans isomerization is observed, and at higher temperatures S2C N bond rotation occurs. 

In (dtc)2Fe(mnt) and (dtc)2 Fe(tfd) (30) and their Ru analogs several novel observations were made 45 the complexes are stereochemically nonrigid, they show a temperature dependent, singlet-triplet equilibrium and they can be subjected to oxidation and reduction over a series of four steps (2— to 1 +), which, in the case of the Ru complexes, are fully reversible. 

The observant reader will undoubtedly have noted that no evidence of any stereochemical nonrigidity (on the NMR time scale) has been obtained for any of the mononuclear complexes derived from OFCOT. However, the dinuclear complex 88 does exhibit fluxional behavior (151). If the solid-state structure of 88 were rigidly maintained in solution, the 19F-NMR spectrum should show eight resonances due to symmetry inequivalent fluorines. However, only four broad resonances are present in the 19F NMR spectrum at room temperature, indicative of a dynamic process that interconverts the two enantiomorphs of 88. Two mechanisms have been proposed for such interconversions in COT analogues a glide and a twitch (Scheme 3). The... 

For rearrangement reactions of the kinetically fast variety, i.e., for complexes which are stereochemically nonrigid, DNMR techniques have in several cases permitted detailed mechanistic conclusions via the observation of site interchanges.s-8 The first experiments were carried out by Fay and Piper15 on tris(trifluoroacetylacetona-to) complexes of aluminum(III), gallium(III) and indium(III), 3, using 19 F DNMR. These complexes which are members of the class of octahedral tris chelates M(A—B)3... 

Complexes belonging to type 468 (M = Be, Zn, Cd, Hg, Pb) are very important subjects of study in the field of stereochemical nonrigid tetrahedral structures [851,852], Using the method of spin labels (a method in dynamic NMR), it is possible to determine the kinetic parameters, separately as intra- or intermolecular processes, for the stereoisomerization reactions in solution. 

Stereochemical nonrigidity, especially if it is fluxional, seems likely to be consistently characteristic of complexes with coordination numbers of 7 or greater. All 7-coordinate complexes so far investigated by nmr techniques have shown ligand-atom equivalence even though there is no plausible structure for a 7-coordinate complex that would give static or instantaneous equivalence. 

Aluminum /3-diketonates have been much studied by nmr methods because of their stereochemical nonrigidity. Aluminum(III), Ga111, and Inm form stable complexes with di-, tri-, and hexadentate chelating ligands in which the metal is octahe-drally coordinated. These complexes are stable under physiological conditions. Complexes of the radioisotopes 67Ga (y, t1/2 = 3.25 d), Ga (/3+, = 68 min), and... 

Measurements of the and Si chemical shifts (see Sect. 4.5.5) and IR absorptions of silatranes and 2.3-dioxa-6-aza-2-silacyclooctanes in various aggregate states have shown that the extent of Si -N coordination increases in passing from the gaseous to the crystalline state as well as with deacreasing the temperature of solution their polarity, polarizability and electrophilicity (see Sect. 4.5.5). The temperature dependence of Si chemical shifts of stereochemically nonrigid intramolecular silicon complexes has been explained in terms of the existence of equilibrium between tetra- and penta-coordinate forms 263.266,332) substituent position exchange in the trigonal bipyramidal stereoisomers 261-264)... 

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