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Ring-whizzing in polyene-PtL 2 complexes revisited

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ABSTRACT

Ring-whizzing was investigated by hybrid DFT methods in a number of polyene–Pt(diphosphinylethane) complexes. The polyenes included cyclopropenium+, cyclobutadiene, cyclopentadienyl+, hexafluorobenzene, cycloheptatrienyl+, cyclooctatetraene, octafluorooctatetraene, 6-radialene, pentalene, phenalenium+, naphthalene and octafluoronaphthalene. The HOMO of a d10 ML2 group (with b2 symmetry) interacting with the LUMO of the polyene was used as a model to explain the occurrence of minima and maxima on the potential energy surface.

No MeSH data available.


Two representations for the half-filled e2u set of π orbitals in cyclooctatetraene.
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Figure 8: Two representations for the half-filled e2u set of π orbitals in cyclooctatetraene.

Mentions: Cyclooctatetraene (COT) has been a favorite ligand since the dawn of organometallic chemistry [2]. Fig. 8 shows two representations for the half-filled e2u set of π orbitals in the flat D8h geometry. One can see from the representation in a) that an η2 or η4 conformation are possibilities. In b) one can envision η1 or η3 as potential structures. The optimized structures for C8F8–Pt(dpe) are illustrated in Fig. 9. To conserve space the groups around the phosphorus atoms have been removed. COT and C8F8 have a tub shaped structure with D2d symmetry [53–54]. As expected an η2 structure, 28, was found to be a minimum. A 1,4-diyl minimum was also found where there are two Pt–C σ bonds, 30. This structure has also been suggested by means of the low temperature 31P and 13C NMR of COT-Pt(R2PCH2CH2PR2), R = iPr [55]. The transition state that interconnects 28 to 30 is shown in 29. The coordination geometry around Pt is typical of that in η2 olefin complexes. What is novel is that the COT (and C8F8) ring is essentially flat with the uncoordinated portion of the polyene having alternating C–C bond lengths of ≈1.45 and 1.35 Å. This is in fact the structure of an analogous Ni complex as determined by X-ray crystallography [56]. The haptotropic rearrangement of 28 to 30 does not permute all of the carbon atoms in the COT ring. There is a mirror plane in the plane of the paper for all of the structures in Fig. 9. This equivalences the carbons on the front side of the paper with those on the back side. Compounds 28–30 do not have a mirror plane perpendicular to this and, therefore, C2 (see 28) does not become equivalent to C3, etc. As we shall see, a structure akin to 35 would accomplish this. In searching for another structure that accomplishes this we discovered tricyclic 32. The transition state that converts 28 into 32 is 31. For the C8F8 complex, 28, the Pt–C distances are 2.08 Å. In 31 the corresponding distances are 2.11 and 2.26 Å with the dashed green bond being formed measuring at 2.32 Å. In COT–Pt(dpe) the transition state 31 is akin to an η3 complex with the three Pt–C bond lengths calculated to be 2.22–2.26 Å. Since 32 has Cs symmetry (discounting the dpe ligand), it serves as a way-point for ring-whizzing. It is easy to see the electronic basis for ring folding and construction of the tricyclic molecule. Consider that in 28 the filled ML2 b2 orbital coordinates to the two lower p AOs in the upper component of e2u in Fig. 8. Then empty a1 interacts with the lower component in Fig. 8. As ML2 slips over the polyene in a clockwise motion the appropriate e2u representations become those in Fig. 8. The empty orbital at the top right in Fig. 8 interacts with the filled b2 ML2 orbital and a1 interacts with the filled e2u. This is explicitly drawn in 33 and 34, respectively, of Fig. 10. The important consequence of this motion is that the p AO on the opposite side of the ring in 34 has the correct phase to generate a C–C σ bond and this collapses to bicyclic 32.


Ring-whizzing in polyene-PtL 2 complexes revisited
Two representations for the half-filled e2u set of π orbitals in cyclooctatetraene.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4979650&req=5

Figure 8: Two representations for the half-filled e2u set of π orbitals in cyclooctatetraene.
Mentions: Cyclooctatetraene (COT) has been a favorite ligand since the dawn of organometallic chemistry [2]. Fig. 8 shows two representations for the half-filled e2u set of π orbitals in the flat D8h geometry. One can see from the representation in a) that an η2 or η4 conformation are possibilities. In b) one can envision η1 or η3 as potential structures. The optimized structures for C8F8–Pt(dpe) are illustrated in Fig. 9. To conserve space the groups around the phosphorus atoms have been removed. COT and C8F8 have a tub shaped structure with D2d symmetry [53–54]. As expected an η2 structure, 28, was found to be a minimum. A 1,4-diyl minimum was also found where there are two Pt–C σ bonds, 30. This structure has also been suggested by means of the low temperature 31P and 13C NMR of COT-Pt(R2PCH2CH2PR2), R = iPr [55]. The transition state that interconnects 28 to 30 is shown in 29. The coordination geometry around Pt is typical of that in η2 olefin complexes. What is novel is that the COT (and C8F8) ring is essentially flat with the uncoordinated portion of the polyene having alternating C–C bond lengths of ≈1.45 and 1.35 Å. This is in fact the structure of an analogous Ni complex as determined by X-ray crystallography [56]. The haptotropic rearrangement of 28 to 30 does not permute all of the carbon atoms in the COT ring. There is a mirror plane in the plane of the paper for all of the structures in Fig. 9. This equivalences the carbons on the front side of the paper with those on the back side. Compounds 28–30 do not have a mirror plane perpendicular to this and, therefore, C2 (see 28) does not become equivalent to C3, etc. As we shall see, a structure akin to 35 would accomplish this. In searching for another structure that accomplishes this we discovered tricyclic 32. The transition state that converts 28 into 32 is 31. For the C8F8 complex, 28, the Pt–C distances are 2.08 Å. In 31 the corresponding distances are 2.11 and 2.26 Å with the dashed green bond being formed measuring at 2.32 Å. In COT–Pt(dpe) the transition state 31 is akin to an η3 complex with the three Pt–C bond lengths calculated to be 2.22–2.26 Å. Since 32 has Cs symmetry (discounting the dpe ligand), it serves as a way-point for ring-whizzing. It is easy to see the electronic basis for ring folding and construction of the tricyclic molecule. Consider that in 28 the filled ML2 b2 orbital coordinates to the two lower p AOs in the upper component of e2u in Fig. 8. Then empty a1 interacts with the lower component in Fig. 8. As ML2 slips over the polyene in a clockwise motion the appropriate e2u representations become those in Fig. 8. The empty orbital at the top right in Fig. 8 interacts with the filled b2 ML2 orbital and a1 interacts with the filled e2u. This is explicitly drawn in 33 and 34, respectively, of Fig. 10. The important consequence of this motion is that the p AO on the opposite side of the ring in 34 has the correct phase to generate a C–C σ bond and this collapses to bicyclic 32.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

Ring-whizzing was investigated by hybrid DFT methods in a number of polyene–Pt(diphosphinylethane) complexes. The polyenes included cyclopropenium+, cyclobutadiene, cyclopentadienyl+, hexafluorobenzene, cycloheptatrienyl+, cyclooctatetraene, octafluorooctatetraene, 6-radialene, pentalene, phenalenium+, naphthalene and octafluoronaphthalene. The HOMO of a d10 ML2 group (with b2 symmetry) interacting with the LUMO of the polyene was used as a model to explain the occurrence of minima and maxima on the potential energy surface.

No MeSH data available.