NMR fluxionality

Dynamic exchange between chemically non-equivalent phosphorus donor sites is noted in the variable temperature spectra of [Pt2(/r-H)(/r-CO)(dppf-P,P )2] 

The temperature-dependent P NMR spectrum of [Rh(dppf-P,P )2][BPh4] reveals a different kind of solution fluxionality [65], which invokes changes in both 

A similar fluxional process is noted in Re2(/t-OMe)2(CO)6( -dppl), in which dppf is in its quasi-closo bridging state [74]. The rocking motion of the diphosphine about the Re. .. Re axis (Fig. 1-22) is a direct result of the opposing rotation of the 

In the solid state, the Cp rings of l- (dppf-P, F PdlBjH, 8 are in a near-staggered conformation with each of the Cp protons in its own unique chemical environment 

A similar dissociation process is displayed in another homoleptic complex [Cu2(dppf-P,F )2(f dppl)][ F4]2 1 [30]. A single broad resonance at 300 K is resolved into an AB2 multiplet at 233 K. An exchange mechanism involving limited dissociation of the dimeric complex to two monomeric species via bridge splitting has been suggested. The monomers could then undergo facile mutual interconversion. 

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Simple thermal substitution of CO in the reaetion of Ru3(CO)i2 with hexa-2,4-diyne-l,6-diol leads to Ru3(CO)9(jU-CO) (HO)CH2C=CC=CCH2(OH) with a parallel alkyne. The acetylide complexes (/i-H)Ru3(GO)9 C=CCR(OH)R (R = Me, R = Me, Ph) and their dppm (dppm = bis(diphenylphosphino)-methane) derivatives are also obtained (in four optical isomers) via thermal substitution H and C NMR fluxion-ality and hydration-dehydration reactions have been studied. Dehydration leads to the vinylacetylene derivatives (/i-H)Ru3(CO)7(dppm)(C CC=CH2Ph), formed by loss of the OH group and of one hydrogen of the methyl substituent on the same carbon atom. ... 

In solution, the phenyl derivative 12.29 (R = Ph) is fluxional. The mechanism of the fluxional process has been shown by an N NMR investigation of a partially N-labelled sample, Le., PhCN2 N3S3 ( N = 99% N), to involve a series of 1,3-nitrogen shifts (Section 4.8.4). Thermolysis or photolysis of 12.29 generates the corresponding 1,2,3,5-dithiadiazolyl radicals [RCNaSa]". ... 

AI(BH4)3 was the first fluxional compound to be recognized as such (1955) and its thermal decomposition led to a new compound which was the first to be discovered and structurally characterized by means of nmr ... 

Fe(CO)s is a highly toxic substance discovered in 1891, the only previously known metal carbonyl being Ni(CO)4. Like its thermally unstable Ru and Os analogues, its structure is trigonal bipyramidal (Fig. 25.10a) but its C nmr spectrum indicates that all 5 carbon atoms are equivalent and this is explained by the molecules fluxional behaviour (p. 914). 

It has 6-coordination with a chelating acetate [106] and may be converted (reversibly) into Ru(OAc)2(PPh3)3, which has the/ac-configuration with one monodentate and one bidentate acetate. It is fluxional at room temperature but at —70°C the phosphines are non-equivalent on the NMR timescale [107],... 

P NMR spectra indicate the T-shape geometry is retained in solution at -30°C but that the molecule is fluxional at room temperature. Rh(PPh3)3 undergoes a range of addition reactions with Lewis bases (CO, PF3, NH3) to afford various 16- and 18-electron species (Figure 2.14). 

NMR spectra show the ethene molecules to undergo a propeller type rotation about the metal-alkene axis the fluxionality is removed on cooling such rotation is not observed with coordinated C2F4, indicating a higher barrier to rotation, in keeping with the stronger Rh—C bonds [66]. 

There is evidently a flat potential energy surface for these clusters as different skeletal isomers can actually be isolated in the solid state for Au9[P(C6H40Me)3]8(N03)3 a brown form has a D4d crown structure and a green form has a D2h structure. In solution, however, they share a common structure with identical31P NMR spectra either the solution cluster has a regular arrangement of phosphines, or it is fluxional [186]. 

The only other C5(i-Pr)5 complex, Mo(Pr5Cp)(CO)3CH3 (30) also shows fluxional iso-propyl groups at 100 °C, but below the coalescence temperature two isomers in the ratio 4 3 are obeserved according to, 3C NMR spectra. They are... 

Cotton, F. A., Frenz, B. A., Deganello, G. Shaver, A. (1973) Fluxional organometallic molecules. XXXIV. The structure and temperature-dependent NMR spectrum of cis-(l,2,6-trihap/o- 3,4,5-fri/iapto-bicyclo[6.2.0]deca-l,3,5-lriene)hexacarbonyldi iron(Fe—Fe)," J. Organomet. Chem. 50,227-240. 

Reactions leading to a series of remarkable dizirconium alkyls and hydrides are outlined in Scheme 213. In solution, the tetrahydride undergoes a rapid fluxional process (observed by NMR spectroscopy) that exchanges the four hydrides. At low temperature three distinct hydrides can be observed, which is consistent with either a dibridged structure, or a tri-bridged species undergoing... 

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