Study of the dative bond in 2-aminoethoxydiphenyl borate at various levels of theory: another poor performance of the B3LYP method for B-N dative bonds.

Aminoethoxydiphenyl borate (2-APB), 1, is a potent inhibitor of store-operated calcium entry channels (SOCCs). Other SOCC inhibitors are being investigated as promising pharmacological agents for a variety of conditions. Though toxic, 2-APB could be useful in the development of additional inhibitors, but its preferred binding structure must first be determined. Thus, we performed ab initio calculations to study the conformers and the strength of the dative bond of 2-APB. As a first step, we performed a series of computations at various levels of theory. We obtained vastly different dissociation energies for the dative bond depending on whether we used MP2 or B3LYP (7-10 kcal/mol different). This discrepancy has previously been observed for other B-N dative bonds by Gilbert, who found that the MP2 values were in much better agreement with experimental values (Gilbert, T. M. J. Phys. Chem. A 2004, 108, 2550-2554). Since we lacked experimental data for comparison, we performed CCSD(T) calculations and found them to have similar results to those from MP2. Thus, we conclude that MP2 is more accurate for 2-APB. The dissociation free energy at the MP2 level is 7 kcal/mol and indicates that the dative bond conformer will be the predominant structure in the gas phase. The dissociation energy is comparatively low due to the electron donation from the oxygen atom to the boron atom and due to the ring strain in the dative bond conformer.