Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes.
The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhCandtbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in...
| मुख्य लेखकों: | , , , , |
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| स्वरूप: | Journal article |
| भाषा: | English |
| प्रकाशित: |
2001
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| _version_ | 1826285209203507200 |
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| author | Jaffart, J Etienne, M Maseras, F McGrady, J Eisenstein, O |
| author_facet | Jaffart, J Etienne, M Maseras, F McGrady, J Eisenstein, O |
| author_sort | Jaffart, J |
| collection | OXFORD |
| description | The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhCandtbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2beta, and a minor alpha-agostic rotamer 2alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R(2)C=CMe) [Tp' = Tp(Me2), R(2) = Me (3); Tp' = Tp(Me2,4Cl), R(2) = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta-agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta-agostic methylene complex 5AR-CSbeta and a minor alpha-agostic species 5AR-CSalpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2beta/2alpha = 4.0 +/- 0.1 at 193 K, DeltaG(o)(193) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-)(1), and DeltaS(o) = -27 +/- 1 J K(-)(1) mol(-)(1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG(2)= 47.5 +/- 2.5, DeltaH= 58.8 +/- 2.5 kJ mol(-)(1), and DeltaS = 59.0 +/- 10 J K(-)(1) mol(-)(1)). Upon selective deuteration of the beta-methyl protons in Tp(Me2)NbCl[CH(CD(3))(2)](PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the alpha-agostic rotamer is observed (K = 2-d(6)beta/2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(o)(193) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-)(1) and DeltaS(o)= -34 +/- 2 J K(-)(1) mol(-)(1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl(i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta-agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH(2)Me)(HC=CMe) is alpha-agostic only and that Tp(5)(-)(Me)NbCl(CH(2)Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a beta-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction. |
| first_indexed | 2024-03-07T01:25:25Z |
| format | Journal article |
| id | oxford-uuid:91cff300-27ea-477d-9cdd-11873ddc1b10 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T01:25:25Z |
| publishDate | 2001 |
| record_format | dspace |
| spelling | oxford-uuid:91cff300-27ea-477d-9cdd-11873ddc1b102022-03-26T23:21:12ZEquilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes.Journal articlehttp://purl.org/coar/resource_type/c_dcae04bcuuid:91cff300-27ea-477d-9cdd-11873ddc1b10EnglishSymplectic Elements at Oxford2001Jaffart, JEtienne, MMaseras, FMcGrady, JEisenstein, OThe isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhCandtbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2beta, and a minor alpha-agostic rotamer 2alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R(2)C=CMe) [Tp' = Tp(Me2), R(2) = Me (3); Tp' = Tp(Me2,4Cl), R(2) = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta-agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta-agostic methylene complex 5AR-CSbeta and a minor alpha-agostic species 5AR-CSalpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2beta/2alpha = 4.0 +/- 0.1 at 193 K, DeltaG(o)(193) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-)(1), and DeltaS(o) = -27 +/- 1 J K(-)(1) mol(-)(1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG(2)= 47.5 +/- 2.5, DeltaH= 58.8 +/- 2.5 kJ mol(-)(1), and DeltaS = 59.0 +/- 10 J K(-)(1) mol(-)(1)). Upon selective deuteration of the beta-methyl protons in Tp(Me2)NbCl[CH(CD(3))(2)](PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the alpha-agostic rotamer is observed (K = 2-d(6)beta/2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(o)(193) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-)(1) and DeltaS(o)= -34 +/- 2 J K(-)(1) mol(-)(1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl(i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta-agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH(2)Me)(HC=CMe) is alpha-agostic only and that Tp(5)(-)(Me)NbCl(CH(2)Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a beta-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction. |
| spellingShingle | Jaffart, J Etienne, M Maseras, F McGrady, J Eisenstein, O Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes. |
| title | Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes. |
| title_full | Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes. |
| title_fullStr | Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes. |
| title_full_unstemmed | Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes. |
| title_short | Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes. |
| title_sort | equilibria between alpha and beta agostic stabilized rotamers of secondary alkyl niobium complexes |
| work_keys_str_mv | AT jaffartj equilibriabetweenalphaandbetaagosticstabilizedrotamersofsecondaryalkylniobiumcomplexes AT etiennem equilibriabetweenalphaandbetaagosticstabilizedrotamersofsecondaryalkylniobiumcomplexes AT maserasf equilibriabetweenalphaandbetaagosticstabilizedrotamersofsecondaryalkylniobiumcomplexes AT mcgradyj equilibriabetweenalphaandbetaagosticstabilizedrotamersofsecondaryalkylniobiumcomplexes AT eisensteino equilibriabetweenalphaandbetaagosticstabilizedrotamersofsecondaryalkylniobiumcomplexes |