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Volumn 10, Issue 5, 2004, Pages 1205-1214

Determination of Absolute Configuration of Acyclic 1,2-Diols with [Mo 2(OAc)4], Part 2: New Structural toward a Rationale of the Method: What Remains of [Mo2(OAc)4] in DMSO Solution?

Author keywords

Circular dichroism; Diols; Molybdenum; NMR spectroscopy; Structure elucidation

Indexed keywords

ACETIC ACID; CHELATION; CHEMICAL BONDS; COMPLEXATION; DISSOCIATION; DISSOLUTION; HYDRATION; MAGNETIC ANISOTROPY; MOLYBDENUM COMPOUNDS; STOICHIOMETRY;

EID: 1642353742     PISSN: 09476539     EISSN: None     Source Type: Journal    
DOI: 10.1002/chem.200304883     Document Type: Article
Times cited : (12)

References (69)
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    • (Eds.: J. C. Lindon, G. E. Tranter, J. L. Holmes), Academic Press, London
    • The induced CD terminology (see, for example: K. Vickery, B. Nordén, in Encyclopedia of Spectroscopy and Spectrometry (Eds.: J. C. Lindon, G. E. Tranter, J. L. Holmes), Academic Press, London, 2000, pp. 869-874) is used to emphasize the fact that the chiral diol moiety, which may be virtually transparent, induces a nonvanishing CD over the electronic transitions allied to the achiral dimolybdenum core.
    • (2000) Encyclopedia of Spectroscopy and Spectrometry , pp. 869-874
    • Vickery, K.1    Nordén, B.2
  • 9
    • 1642393661 scopus 로고    scopus 로고
    • note
    • In general, the empirical rule may be formulated as follows, making use of the steric descriptors "bS" and "bR". [1] For each alcoholic chiral carbon, the "bS"/"bR" configuration is assigned: the first priority is given to the hydroxyl, the second to the adjacent -C-OH, and the remaining two substituents are ordered according to their bulkiness (Ph/H for 1; Me/H for 2). In most cases "bS" corresponds to S and "bR" to R. The rule states that a "bR" or "bR,bR" 1,2-diol always gives rise to a negative CD band IV.[1,2a]
  • 15
    • 1642385670 scopus 로고    scopus 로고
    • pp. 647-649
    • b) pp. 647-649;
  • 16
    • 1642314115 scopus 로고    scopus 로고
    • pp. 682-704
    • c) pp. 682-704;
  • 17
    • 1642382433 scopus 로고    scopus 로고
    • pp. 656-670
    • d) pp. 656-670;
  • 18
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    • pp. 707-711
    • e) pp. 707-711.
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    • note
    • A fourth signal is always present in our spectra as a small singlet at δ = 1.40 ppm, which apparently exchanges (saturation transfer and EXSY experiments) with the methyl signal of acetic acid at δ = 1.90 ppm. Its position and intensity are entirely independent of measurement conditions (water content in DMSO, sample aging and degradation); its origin is uncertain.
  • 31
    • 85088718828 scopus 로고    scopus 로고
    • note
    • 3.[15a] Equation (1) is strictly valid within the dipolar approximation for magnetic moments, and if the Mo-Mo bond is axially symmetric.
  • 32
    • 85088719290 scopus 로고    scopus 로고
    • note
    • -1.
  • 35
    • 1642417888 scopus 로고    scopus 로고
    • note
    • 4].
  • 36
    • 1642297750 scopus 로고    scopus 로고
    • note
    • The other aromatic protons for the bound diol 1 resonate at δ = 7.70 and 7.61 (para and meta for the major species), and 7.35 and 7.24 ppm (minor species).
  • 37
    • 85088720646 scopus 로고    scopus 로고
    • note
    • 4] (where the acetates are in a "parallel" position).
  • 38
    • 1642411425 scopus 로고    scopus 로고
    • note
    • 4] (Figures 5 and 6) and of its diol complexes, in that an excess of water makes the CD collapse. Moreover, when a tenfold more concentrated sample (28.2 mM with a 0.01 cm cell pathlength) and a thoroughly dried flask and cell (in order to minimize the relative quantity of water) are used, a twofold more intense CD is obtained, with an exceptionally slow time evolution (stationary conditions are reached two to three times more slowly).
  • 39
    • 1642346541 scopus 로고    scopus 로고
    • note
    • The two minor species of the bound diol 2 have hardly detectable doublets for the methyl protons at δ = 1.06 and 1.19 ppm, with integrals smaller than 10% with respect to the major species (less than 1% with respect to the free diol). These minor species are not present as regularly as the main complex, so that in some conditions (depending on the water content and the solution aging) they are not detected at all. This may explain the slight variability in the CD spectra noted before for 2 and other nonhindered 1,2-diols.[1]
  • 40
    • 85088718668 scopus 로고    scopus 로고
    • note
    • 4] of different ages, we recorded a set of CD spectra whose normalized intensities varied, showing a dispersion above 50% and a standard variation around 30%.
  • 42
    • 85088723521 scopus 로고    scopus 로고
    • note
    • 6]DMSO) increase the effect of dissolved water, whose proportion with respect to the dimolybdenum complexes may vary considerably for the CD and NMR samples of the same diol. This cautions against a straight-forward quantitative comparison between the data obtained with the two spectroscopies.
  • 43
    • 0002186755 scopus 로고    scopus 로고
    • The only reported substitution of acetate ligands with alcohols requires use of a strong base for forming the alkoxide;[12] according to one report, however, trifluoroacetate is dissociated and protonated by the action of even an amine, though one with acidic properties (M.-C. Suen, Y.-Y. Wu, J.-D. Chen, T.-C. Keng, J.-C. Wang, Inorg. Chim. Acta 1999, 288, 82-89).
    • (1999) Inorg. Chim. Acta , vol.288 , pp. 82-89
    • Suen, M.-C.1    Wu, Y.-Y.2    Chen, J.-D.3    Keng, T.-C.4    Wang, J.-C.5
  • 49
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    • 2 core are well known (ref. [30], and: F. A. Cotton, B. A. Frenz, E. Pedersen, T. R. Webb, Inorg. Chem. 1975, 14, 391-398). This may explain the residual spectral differences between the two species.
    • (1975) Inorg. Chem. , vol.14 , pp. 391-398
    • Cotton, F.A.1    Frenz, B.A.2    Pedersen, E.3    Webb, T.R.4
  • 50
    • 1642379173 scopus 로고    scopus 로고
    • note
    • We cannot account for the small Δδ found for the methine protons of 2; they lie, however, quite out of the O-Mo-Mo-O plane (Figure 4), which corresponds approximately to one nodal plane of the δ orbital, and the anisotropy exerted by the quadruple bond might deviate from the expected cylindrical symmetry.
  • 57
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    • G. Snatzke, Angew. Chem. 1979, 91, 380-393; Angew. Chem. Int. Ed. Engl. 1979, 18, 363-377.
    • (1979) Angew. Chem. , vol.91 , pp. 380-393
    • Snatzke, G.1
  • 58
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    • G. Snatzke, Angew. Chem. 1979, 91, 380-393; Angew. Chem. Int. Ed. Engl. 1979, 18, 363-377.
    • (1979) Angew. Chem. Int. Ed. Engl. , vol.18 , pp. 363-377
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    • 1642367771 scopus 로고    scopus 로고
    • note
    • This demonstrates that the calculated shieldings are largely dependent on the overall structure (that is, parallel/perpendicular and/or with axial/equatorial substituents), and a small variation in structural parameters such as bond lengths and angles, within reasonable ranges, has only a limited effect.


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