Redetermination of the crystal structure of endo-2-carboxy-bicyclo[2.2.1]- heptane, C8H12O2

Abstract

Source of materialThec ompoundw as obtained commercially (Frinton Laboratories). Crystals suitable for the diffraction study were obtained upon sublimation of the product at room temperature. Experimental detailsCarbon-bound Hatomswere placed in calculated positions (C-H 0.99 Åfor methylene groups and C-H 1.00 Åfor methine groups) and were included in the refinement in the riding model approximation, with U iso (H) setto1.2U eq (C). The Hatom of the carboxyl group was allowed to rotate with afixed angle around the C-O bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite [11]), with U iso (H) sett o1 .5U eq (O). Due to very low residual electron densities after refinement of givenstructuralmodel,nodisorder model could be set up. DiscussionThe bicycloheptane scaffold has been afocusofresearch in organic chemistry for many decades. Especially questions centered around classicala nd non-classicalc arbocationsf ocusedo nt his keystructure andonlyveryrecentlythe existenceofpenta-coordinate carboncould be proven experimentally on grounds of diffraction experiments based on single crystals [1,2].T he latter finding motivated us to include bicycloheptane-derived ligands into our research efforts in synthesizing novel coordination compounds of transition metals. As aconvenient starting point for our studies, the title compound was chosen as it is commercially available. The molecular and crystal structure of endonorbornane-2-carboxylic acid have been mentioned in the literature once, however, no 3D coordinates have been provided [3]. As the structures of envisioned target molecules shall be elucidated by means of diffraction experiments on single crystals, the structure of the title compound was redetermined. The title compound is the endo-configured carboxylic acid of norbornane. A puckering analysis according to Cremer &Pople [4]showed the five-membered rings in the norbornane scaffold to adopt an E 3 and an E 5 conformation, respectively [5]. The flap atom in both cases is carbon atom C7. For the six-membered ring, a B 2,5 (B C2,C5 )conformation was found [6]. The C-O bond lengths were measured at 1.2130(14) Åa nd 1.3206(14) Åf or the doublebonded and the single-bonded oxygen atom.The latter two values are in good agreement with other values reported for carboxylic acidsw hose metrical parameters have been deposited with the Cambridge Structural Database [7].Inthe crystal, classical hydrogen bondso ft he O-H×××Ot ype arep redominant. There are C-H×××Oc ontacts whose range falls below the sum of van-derWaals radii of the atomsparticipating in them.While the hydrogen bonds give rise to the formation of centrosymmetric dimers as often observed for carboxylic acids [8], the C-H×××Ocontacts extend thesed imeric unitst os trands along the crystallographic a axis. The latter contacts are supported by one of the hydrogen atomsatabridge-head position of the norbornane scaffold and employ the carbonylic oxygen atom as acceptor thus giving rise to two anti-dromic C-H×××Oc ontact chains. In terms...

Redetermination of the crystal structure of 3,3,3-triphenylpropanoic acid, C21H18O2 – Deposition of hydrogen atomic coordinates

“…While the first members of the homologous series of aliphatic carboxylic acids constitute a family of liquid substances at room temperature, the introduction of halogen atoms, additional hydroxyl groups or aromatic moieties in the backbone quickly results in a marked increase of melting points and subsequent tendency to form crystalline substances at ambient conditions which allows for the determination of metrical parameters on grounds of diffraction studies which enables comparative studies in case the acids are to be encountered as ligands in coordination compounds. In continuation of our interest in the structures of functionalized carboxylic acids [7][8][9][10][11][12][13][14] the crystal and molecular structure of the title compound was determined to gain an understanding for the effects the introduction of three bulky phenyl groups on the terminal carbon atom will have. Structural information is at hand for a co-crystallizate of the title compound and 2-aminopyrimidine [15].…”

The crystal structure of 3-chloropropionic acid, C3H5ClO2

“…An even stronger increase in melting point is observed for the introduction of the first terminal chlorine atom in propionic acid with a change from −21 to 41°C. In continuation of our interest in the structures of functionalized carboxylic acids [8][9][10][11][12][13][14][15], the crystal and molecular structure of the title compound was determined to gain an understanding for this drastic change in physical properties. The structure of propionic acid has been reported earlier [16].…”

The crystal structure of 2-(2-methoxyphenyl)acetic acid, C9H10O3

“…The introduction of the acetyl group has even been found to alter pharmacological properties such as in the case of aspirine [7,8] where acetylation improved on the agreeableness of the salicylate prescription. In continuation of our interest in the structural versatility of carboxylic acids [9][10][11][12][13][14][15][16] the molecular and crystal structure of the title compound were determined. According to the structure solution the compound is a derivative of acetic acid bearing a 2-methoxyphenyl substituent on its backbone.…”