Fluxionality ( Stereochemical Nonrigidity )

1. Fluxionality(StereochemicalNonrigidity) 무기소재연구실 201450101 김 용 태

2. Contents • Fluxionality (Stereochemical non-rigidity) • PF5 - Berry pseudorotationmechanism • M-CO complexes - Co2(CO)8 • M-CO complexes - Fe2(CO)4(η5-C5H5)2 • M-CO complexes – Os3(CO)11P • M-allylcomplexes • Ring whizzer 무기 소재 연구실 김 용 태

3. Fluxionality(Stereochemical non-rigidity) • The atoms of the molecule are continuously involved in approximately harmonic vibrations about their equilibrium positions, but in other respects the structure may be considered rigid. • Rotation of one part a molecule with respect to another part gives different forms of the same molecule in which atoms are arranged differently in space. = Intramolecular rearrangement 무기 소재 연구실 김 용 태

4. Fluxionality(Stereochemical non-rigidity) • In some cases, the non-equivalence of two or more structures results, in the existence of molecules with different instantaneous identities. For example, ‘keto-enol equilibrium’ is known in the case of organic molecules. = [tautomeric type] • In other cases, the two or more configurations are equivalent (in structure, bonding and energy content). = [fluxional type] • Lately, the term ‘stereochemical non-rigidity’ be reserved as a general term for molecules of both the above types. • NMR spectrum is a useful data in observing the ‘fluxionality’. 무기 소재 연구실 김 용 태

5. Berry pseudorotation mechanism • Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation. It most commonly occurs in trigonalbipyramidalmolecules, such as PF5 or Fe(CO)5, though it can also occur in molecules with a square pyramidal geometry. 무기 소재 연구실 김 용 태

6. Berry pseudorotationmechanism • Ligands 2 and 3 move from axial to equatorial positions in the trigonalbipyramidwhile ligands 4 and 5 move from equatorial to axial positions. Ligand 1 doesn’t move and acts as a pivot. At the middle picture (transition state), ligands 2,3,4,5 are equivalent, forming the base of a square pyramid. The motion is equivalent to a 90°rotation about the M-L1 axis. 무기 소재 연구실 김 용 태

7. Berry pseudorotation mechanism • The two originally equatorial ligands then open out until they are 180°apart, becoming axial groups perpendicular to where the axial groups were before the pseudorotation. 무기 소재 연구실 김 용 태

8. Berry pseudorotation mechanism • The energy difference (~3.6 kcal/mol) is small in this process, so it is possible to change flexibly. 무기 소재 연구실 김 용 태

9. Berry pseudorotation mechanism • NMR (Low temperature) • NMR (High temperature) • At low temperature, the pseudorotation of PF5 works well and NMR peaks at high temperature are to be ‘one’ peak. 무기 소재 연구실 김 용 태

10. M-CO complexes • Carbonyl ligands on a M-CO complexes are able to undergo structural rearrangement , and this fluxional phenomenon is called ‘carbonyl scrambling’. • We can observe a wide range of different analytical and spectroscopic methods. (ex: 13C NMR , X-ray crystallography etc.) 무기 소재 연구실 김 용 태

11. M-CO complexes - Co2(CO)8 • Co2(CO)8 is good example to explain the carbonyl scrambling. • Co2(CO)8 has two types of structures; Terminal CO and Bridging CO. 무기 소재 연구실 김 용 태

12. M-CO complexes - Co2(CO)8 • It occurs whenever molecule has a number of closely related structures accessible by relatively low energy barriers. • This process is able to proceed a bit of energy. Both terminal CO complex(D3d) and bridging CO complex(C2v) are more stable than transition state(D2d). • Usually the terminal CO complexes have lower energy levels than the bridging CO complexes. 무기 소재 연구실 김 용 태

13. M-CO complexes - Fe2(CO)4(η5-C5H5)2 • Fe2(CO)4(η5-C5H5)2 exists in three isomeric forms: cis, trans, and unbridged.  • Fluxional process is so fast that only averaged single signal is observed in 1H NMR spectrum at room temperature. = Unbridged form • However, the fluxional process is not fast enough at low temperature like Co2(CO)8. It has two peaks(or more) in 1H NMR spectrum at 200K. = cis & trans forms 무기 소재 연구실 김 용 태

14. M-CO complexes - Fe2(CO)4(η5-C5H5)2 • Also, the fluxional process is not fast enough for IR spectrum. Thus, three absorptions are seen for each isomer. The νco bands for bridging CO ligands are around 1780 cm-1 whereas νco bands for terminal CO ligands are about 1980 cm-1. 무기 소재 연구실 김 용 태

15. M-CO complexes – Os3(CO)11P • Triosmiumundecacarbonylphosphite is another example to observe fluxional process. • The carbonyl ligands are fluxional due to the labilizing influence of the phosphine ligand. • This mechanism also occurs because of low energy barrier of TS. 무기 소재 연구실 김 용 태

16. M-CO complexes – Os3(CO)11P • The carbonyl exchange takes place via a bridging the OsA-OsB edge and perpendicular to triosmium plane. • The opening of the double bridged μ-CO intermediate leads to scrambling of the ‘six co-planar’ carbonyl ligands. 무기 소재 연구실 김 용 태

17. M-allylcomplexes • The allyl ligand functions as a trihapto ligand, using delocalized π-orbitals, or as a monohapto ligand, primarily σ-bonded to a metal. 무기 소재 연구실 김 용 태

18. M-allylcomplexes • η3-η1-η3 process proceeds with a bit of energy. • H2 and H3 environment average process slow at low temperature. So H2-H3 NMR peak will be divided into some peaks. η3 η1 η3 η1 무기 소재 연구실 김 용 태

19. Ring whizzer 1,2-shift • A ring whizzer is a fluxional molecule frequently encountered in organometallic chemistry in which rapid rearrangements occur by migrations about unsaturated organic rings. •  Ring whizzing (=ring walking) is an intramolecular isomerization that features fluxional process. It results in the change of the relative positions of the substituent groups on a ring. In a typical ring whizzing process, the hapticity of the migrational substituent group doesn’t change. 1,3-shift 무기 소재 연구실 김 용 태

20. Ring whizzer • At room temperature NMR peak looks like only ‘one’. The reason for this is that a series of successive [1,2] or [1,3] hydrogen shifts is occurring so quickly that only the 'averaged' proton position is shown in the NMR spectrum.  • Slowing down the exchange by cooling to about -100℃produces the expected spectrum described at the left. Many NMR spectra are affected by these “ring whizzing” processes, or so called 'degenerate' pericyclic processes. 무기 소재 연구실 김 용 태

21. Reference • Transition Metal Carbonyl Cluster Chemistry - Paul J. Dyson, J. Scott McIndoe (2000) • Synthesis and Characterization of Diphosphine Ligands and Diphosphine Substituted Osmium and Ruthenium Clusters - SrikanthKandala (2007) • Fluxional Organometallic and Coordination Compounds – Marcel Gielen, Rudolph Willem, Bernd Wrackmeyer (2004) • Organometallic Chemistry – R. C. Mehrotra (2007) 무기 소재 연구실 김 용 태

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