The elimination of molecular chlorine catalyzed by BCl₃: an aspect of chloroboron subphthalocyanine formation

Abstract

This study is focused on the final stage of the formation mechanism of chloroboron subphthalocyanine from boron trichloride and phthalonitrile, in the context of recent work on the earlier stages of this reaction. The results of extensive quantum-chemical energetic, structural, and charge-distribution computations show that a catalytic effect of boron trichloride is most probably essential to explain the fact that molecular chlorine is liberated in the formation of chloroboron subphthalocyanine (reaction 2). This process turns out to be closely concerted (reactions 2a and 2b).

Electronic Structure, Bonding, and Stability of Boron Subphthalocyanine Halides and Pseudohalides

“…This compares well with experimental findings in the solid state. [23][24][25][39][40][41][42]44] Likewise, for all pseudohalides the B-O bond length is found to be ≈1.43 Å, whereas for H-BsubPc it is found to be 1.21 Å , also in excellent agreement with experimental findings in the solid state (see Section S1, Supporting Information). [15,43,55] We further compute selected bond lengths and "bowl depths" for the various systems.…”

“…As mentioned above, the tetrameric bond coordination of the boron atom in BsubPcs with axial substituents is rather unusual and counter-intuitive from a naive chemical point of view, given that boron has three valence electrons available for (covalent) bond formation. As previously mentioned, two different ways to formally describe the axial B-X bond can be found in the literature [24,45] (cf. Figure 1).…”

“…Owing to the weak inter-molecular interaction, all DFT calculations shown in this work were performed on isolated BSubPc molecules. [23][24][25] Natural population analysis (NPA) [52] has been performed using the JANPA interface, [53] an approach specifically suited for use with diffuse basis sets. [54]…”

“…[20][21][22] Progress has also been made regarding the reaction mechanism: Claessens, Torres, et al isolated an intermediate of the BsubPc formation, which lead to a hypothesized BsubPc formation mechanism based on a reaction of a phthalonitrile with boron trichloride. [23] A computational model approach has then been adopted [24] in order to propose a more detailed theoretical mechanism, [25] which awaits detailed confirmation.…”

“…One issue is that axial substitution leads to a tetrameric coordination of the boron atom (see Figure 1), with two different ways to formally describe the B-X bond found across the literature. [24,45] Some describe the boron as formally carrying a positive charge (Figure 1a) whereas others describe it as formally carrying a negative charge (Figure 1b). While both descriptions can be rationalized chemically, they On the plot, the referenced single crystal data are available through the CCDC data base: PHTLBC; [12] PHTLBC01; [56] PHTLBC02-PHTLBC06.…”