Elsevier

Inorganica Chimica Acta

Volume 299, Issue 2, 15 March 2000, Pages 263-267
Inorganica Chimica Acta

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On an unusual shortening of the C(sp3)–C(sp3) bonds in n-propylcobaloximes

https://doi.org/10.1016/S0020-1693(99)00476-4Get rights and content

Abstract

The structures of two n-propyl cobaloximes, [(n-Pr)Co(Hdmg)2(L)]·H2O where L=H2O (6) and NH2Ph (7), and Hdmg=monoanion of dimethylglyoxime, are described. The large static disorder undergone by the n-propyl group is attributed to different conformations of n-Pr, differing in the torsional angle around the axial Co–L bond. This result contrasts with those reported for the anhydrous [(n-Pr)Co(Hdmg)2(H2O)] (1) derivative and its inclusion compound with α-cyclodextrin (2), where a significant lengthening of 0.16 Å in the Cα–Cβ bond of n-Pr in 2, with respect to an incredibly short bond in 1, was attributed to the interaction of the cobaloxime with α-cyclodextrin.

Introduction

Recently, a paper describing the preparation and structure of the cobaloxime (n-Pr)Co(Hdmg)2(H2O) (1), where Hdmg=monoanion of dimethylglyoxime, and of its inclusion compound with α-cyclodextrin (α-cd) (2) has been reported [1]. Interestingly, the structural analysis showed that the compound 2 is formed by the inclusion of the cobaloxime n-Pr group into the hydrophobic cavity of α-cd. The authors have found by comparison with the geometry of 1 with that of the cobaloxime moiety in 2, some obvious conformational changes: (a) the displacement of Co out of the four N donor plane towards the alkyl group is 0.008 Å in 1 and 0.038 Å in 2; (b) the two equatorial Hdmg units make an interplanar angle of 1° in 1 and of 10° in 2; (c) in 2 the axial Co–OH2 bond is lengthened by 0.02 Å, whereas the C–Co–O angle is narrowed by 3.5°; (d) the n-Pr Cα–Cβ distance, which is unexpectedly short for a C(sp3)–C(sp3) single bond (1.237(13) Å) in 1, is found to be lengthened by 0.16 Å in 2, while the Co–C–C angle is widened by 5° in 2. However, no significant change was observed in the Co–C distances. More recently, Luo et al. [2] have reported also the structure of (i-But)Co(Hdmg)2(H2O) (3) and of its inclusion compounds with (α-cd) (4) and (β-cd) (5). Distortions similar to (a)–(c) were observed, but the i-Bu Cα–Cβ distances in 35 are quite normal, whereas the Cβ–Me distances in 5 were found to be 1.34(1) and 1.24(1) Å, respectively. The authors ascribed [1] the observed differences (a)–(d) in 2, to the hydrophobic interaction between the n-Pr group and the α-cd cavity and to the steric interaction between the α-cd rim and the cobaloxime equatorial moiety. That such interactions are responsible for the geometrical changes (a)–(c) is convincing, but that they can provoke the dramatic change of the Cα–Cβ distance from 1.237(13) Å in 1 to 1.394(7) Å in 2 is highly questionable. In fact, recent molecular mechanics calculations [3] reproduced satisfactorily bond lengths and angles in 1 and 2, except for the Cα–Cβ distance calculated to be 1.536 Å. The analysis of the anisotropic thermal factors of the atoms in the two structures shows high values (about four times those of the other atoms) of U22 in 1 and of U11 in 2 for the β-carbon atom, and an orientation of the corresponding axis of the thermal ellipsoids nearly parallel to the coordination plane. This observation suggests that the unusual geometry of n-Pr in 1 and 2 might be the result of a mathematical artefact, due to the refinement, carried out without taking into account a possible static disorder of the alkyl moiety. In spite of the large number of structurally characterized alkylcobaloximes [4], to our knowledge no structural data for an n-propylcobaloxime are available for comparison. Therefore, we tried to check the static disorder hypothesis, redoing the structural analysis of 1. Unfortunately, we were not able to obtain any single crystal following the method described in Ref. [1]. However, good quality single crystals of the mono-hydrated form of 1, [(n-Pr)Co(Hdmg)2(H2O)]·H2O (6), were obtained. The structures of 6 and that of the [(n-Pr)Co(Hdmg)2(PhNH2)]·H2O analogue (7), were determined.

Section snippets

[(n-Pr)Co(Hdmg)2(H2O)]·H2O (6)

The complex (n-Pr)Co(Hdmg)2(Py), prepared as previously reported [5], was eluted through a column of acidic resin to obtain the corresponding aquo complex 6. Red–brown crystals, suitable for structure determination were obtained by evaporation of a water–methanol solution in the dark. (Found: C, 35.4; H, 6.90; N, 15.1. Calc. for C11H25N4O6Co: C, 35.8; H, 6.84; N, 15.2%.)

[(n-Pr)Co(Hdmg)2(PhNH2)]·H2O (7)

A water–methanol solution of 6, obtained as above, was treated with an excess of aniline under stirring at room temperature

Results and discussion

The ortep drawings, obtained by anisotropic refinement of all the non-H atomic species of 6 and 7, all with occupancy factor 1, are given in Fig. 1. The refined geometry of the Co–n-Pr moiety in 6 (R1=0.057) and 7 (R1=0.052) is very close to that of 1 and 2 (Table 2) as well as the trend of the thermal factors. The U22 value for Cβ in 6 and those of almost all the Uii of Cβ and Cγ in 7 are dramatically larger than the others, as shown also by the Cβ thermal ellipsoids of Fig. 1, whose longer

Acknowledgements

We thank the Ministero della Ricerca Scientifica e Tecnologica (Rome) and the Consiglio Nazionale delle Ricerche (CNR) for financial support.

References (8)

  • S. Geremia et al.

    J. Organometal. Chem.

    (1992)
  • L.B. Luo, H.L. Chen, W.X. Tang, Z.Y. Zhang, T.C.W. Mak, J. Chem. Soc., Dalton Trans. (1996)...
  • L.B. Luo et al.

    Inorg. Chem.

    (1998)
  • H.M. Marques et al.

    Inorg. Chem.

    (1998)
There are more references available in the full text version of this article.

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