Response to Comments on “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix”

Abstract

Scheschkewitz and Alabugin et al. suggest that photolysis under confinement in a crystalline matrix of 4,6-dimethyl-a-pyrone does not yield the crystal structure of 1,3-dimethylcyclobutadiene (Me 2 CBD) as we reported, but rather that of a 4,6-dimethyl-b-lactone intermediate. We provide arguments that the square-planar Me 2 CBD S /CO 2 complex and the rectangular-bent Me 2 CBD R molecule are stabilized under confinement by the guanidinium-sulfonate-calixarene host matrix used in our study. W e recently reported the single-crystal x-ray structure of 1,3-dimethylcyclobutadiene synthesized within a guanidinium-sulfonate-calixarene (G 4 C) crystalline matrix (1). Scheschkewitz (2) and Alabugin et al.(3) contend that our structures instead correspond with a bicyclic b-lactone intermediate. Our view on the importance of the quality of the x-ray data on structure resolution and the view of the Technical Comment authors coincide. In their comments, Scheschkewitz and Alabugin et al. consider our work as an unconvincing attempt, mainly due to the presumed poor quality of the experimental x-ray data. They fail, however, to place the crystallographic work in the particular context of highly advanced and challenging chemistry of unstable or reactive host-guest crystalline systems. A number of very comparable studies, albeit on other systems, have preceded our work (4-7), and the problems encountered are virtually the same as ours: resolved and unresolved disorder leading to results far from optimal data quality. Data and model quality are not expressed in a satisfactory way by just one or two numbers or parameters. Instead, a more or less complete set of parameters needs to be given to characterize the quality of data and model. Table S1 summarizes the different quality parameters for these very comparable studies (1, 4-7) in the field of reactive host-guest crystalline systems. Our data are no worse than these studies, if not better, and in all cases the conclusion would be that the data warrants the structural model described in the paper. Our opinion is that all previous important contributions related to unstable molecules sequestered in porous crystals can reach their full potential.The difficulties of experiments involving hostguest crystalline systems are related to the following factors: (i) in the case of nonreactive molecules (4, 5), the multiple orientations of the guest in a host cavity induce disorder; (ii) in the case of reactional processes, inconsistent diffusion of the reagents or unequal distribution of the irradiation energy over the whole volume of the crystal also leads to disorder (6, 7); and (iii) the preservation of the crystallinity of the hostmatrix may be affected during the reactional process by a loss of water or solvent molecules. All these factors lead to what is called "poor" data quality. Data quality can indeed lead to alternative interpretations, as illustrated by the comments of Scheschkewitz (2) and Alabugin et al. (3). However, we show below that our interpretation is more likely.Schesch...

Unprecedented Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution

“…Solid-state photosynthesis of CBD in a crystalline matrix under confined conditions: Inclusion of 4,6-dimethyl-apyrone, Me 2 1, within the G 4 C matrix [17][18][19] in an aqueous solution of either G 4 C/Me 2 1 or 4·G/ C/Me 2 1 components led to single crystals of G 4 CA C H T U N G T R E N N U N G {Me 2 1} (Figure 1).…”

“…All previous studies agree that the CO 2 /CBD interaction results "from the repulsion between CBD and CO 2 rather than an attraction" [4] and is mostly favored by the solid matrix constraints/effects. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Within this context, the 1 H NMR spectra of G 4 CA C H T U N G T R E N N U N G {Me 2 CBD} showed no further evolution after the saturation of the aqueous solution with gaseous CO 2 . These NMR results have been also confirmed by Raman spectroscopy investigations in aqueous solution (see Figure S3 in the Supporting Information for details).…”

“…Highquality X-ray crystallographic data show the high purity of the crystal formation. [18] Single-crystal photolysis experiments: In situ X-ray observation was performed by using single crystals placed on the goniometer, immersed in a N 2 flow to cool it down to 175 K. The crystal was then, under continuous phi-rotation, irradiated by using a Bluepoint 2.1-UV point source (l = 320-500 nm). Supplementary ample informative material is provided in the Supporting Information of this paper and in references [17] and [18c] and their Supporting Information.…”

“…The photolysis reaction of the 4,6-dimethyl-a-pyrone precursor Me 2 1 under confinement in a protective guanidinium-calixarene-sulfonate G 4 C matrix allows the preparation of the dimethyl-Dewar-b-lactone Me 2 2, the 1,3-cyclobutadiene-carboxyl zwitterion intermediate Me 2 3, as well as1,3-dimethylcyclobutadiene, Me 2 CBD, in the solid crystalline state and in aqueous solution (Scheme 1). [17,18] We report here a more detailed analysis of the formation of Me 2 CBD in the solid state, based on more advanced refinements of the original X-ray data and on new supporting FTIR, Raman, and NMR spectroscopic, and ESI mass spectrometric data. 1,3-Dimethylcyclobutadiene is stable in water in the presence of the G 4 C matrix at room Abstract: Cyclobutadiene (CBD), the smallest cyclic hydrocarbon bearing conjugated double bonds, has long intrigued chemists because of its chemical characteristics.…”

Multireference Configuration Interaction Methods – An Application to the Valence Isomerism in Cyclobutadieno-p-benzoquinone and its Diprotonated Form

“…[12,13] However, this finding initiated heated debate in literature on whether the measured crystal structure corresponds to 1,3-dimethylcyclobutadiene at al. [14][15][16][17] Valence bond isomerism of cyclobutadiene has also aroused considerable interest of theoretical chemists, to mention only the most recent high level computational studies by Balkova and Bartlett, [12] Sancho-Garcia et al, [18] Shen et al, [19] Lefrancois et al, [20] and our research group. [13] We have shown that the barrier for automerization reaction of the parent CBD molecule obtained at the MR-AQCC level of theory including extrapolations to the basis set limit is 6.3 kcal mol −1 .…”