Theoretical study of isomers XBEY, BEXY, and XYBE (E = N, P, and As, and X, Y = F and Cl): Substituent effect

Highlights

• High-accuracy quantum chemical studies on substituent effect.

• Boron-containing molecules prefer B-substitution much more than E-substitution.

• A more X and Y electronegativity, a larger E atomic radius, and a more B-substitution result in greater structure stabilization.

Abstract

In order to obtain significant insights into effects of the substituents on characters of the novel boron-containing molecules, the optimized structures, energetics, and potential energy surfaces of the three series isomers, XBEY, BEXY, and XYBE (E = N, P, and As, and X, Y = F and Cl), have been examined in detail using high-accuracy quantum chemical calculations, in which the corresponding values are obtained at the M06-2X, MP2, and QCISD(T) levels of theory, respectively. Our findings demonstrate that the BEXY isomers possess less stable with respect to the XYBE derivatives, implying the boron-containing molecules prefer B-substitution much more than E-substitution. Moreover, stability of the XBEY is comparable to that of the XYBE, however, the corresponding difference can be identified by substituent effect as well, that is, a more X and Y electronegativity, a larger E atomic radius, and a more B-substitution result in greater structure stabilization.

Introduction

Owing to the delocalization of the group 15 element lone pair into a p orbital on the group 13 element [1], [2], [3], one can imply strong π bonding in the rare species featuring well-defined B-N, B-P, and B-As bonds. In 1987 Paetzold [4] has reported the iminoboranes derivatives, RB$\equiv$NR (R = substituent), which well established that B-N bonding involves a dative bond from the nitrogen lone pair to the low-lying empty p orbital of boron. In general, the efficient delocalization requires planar geometry at both atoms and the alignment of their coordination planes, however, these conditions become increasingly difficult to achieve because of the preference for pyramidal structure in the heavier elements [5], [6], [7]. Since then, in contrast to the work performed on the iminoboranes derived from HB$\equiv$NH (the analog of acetylene), the simplest boranylidenephosphane HBPH has also received considerable attention from many theoretical chemists [8], [9], proposing existence of the B = P double bonding. Based on the discovery of the type of bonds, in 2006 Power and coworkers have successfully prepared the doubly bonded As = B containing compounds by making use of the donor stabilization strategy [10]. With particular attentions, a large quantity of experimental and theoretical studies have been devoted to identify the chemical and physical properties of the boron-containing derivatives [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], in which the interest is curious about the ability of boron to behave as a nucleophile. More recently, Vondung and coworkers have reported the case of borylenes (L₂HB:) stabilized by phosphines [21], as an example of good electron-donor from boron. Obviously, the boron containing molecules detected in the gas phase derived from the redistribution of their electron densities.

It is very clear that, for the neutral boron compound to act as an electron donor, a certain accumulation of charges are needed to foster its basic character. During this period of research, one B → As σ and two B ← As π bonds have been identified theoretically [22]. Although those studies afford sufficient insights into the so-called double bond rule [23], [24], [25], [26], [27], some questions unsolved have been left. For example, what differences between boron-substituted and -unsubstituted isomers may emerge in geometries, as well as in stabilities? Although boron-substituted derivatives, XBNH, are more stable than the corresponding nitrogen-substituted isomers HBNX [28], this attempt—substituent effects on the relative stabilities and the nature of bonding in BNHX and HXBN systems—was less available so far [29], which may also provide clues for designing new synthetic routes to produce BN containing compounds. Moreover, Mo et al. have predicted that the iminoboranes energy differences, in which they increase as the electron-withdrawing character of the substituents increasing [28]. However, in the iminoboranes, if the N atom is replaced by the much heavier same group atoms, P and As, as an insight for regarding periodic trends, there is a question of how and to what extent the bonded substituents effect on the corresponding geometries and stabilities, representing a continuation of our interest in boron chemistry. Therefore, as an intriguing class, we believe that quantum chemical calculations are very helpful in solving these and related problems. Since the electronic structure of boron can in principle be modulated by the nature of the ligands surrounding it [30], in this paper we have investigated the effect of different substituents with increasing electron-withdrawing character on the strength and characteristics of the boron bonding. More recently, the group of Su [31], [32] has suggested that only bulkier substituents in the containing-boron compounds dramatically influence their stabilities, however, no systematic analysis of the substituent effect, such as boron-unsubstituted cases. With the above exposures, in the present study we have considered the three possible isomers: XBEY, BEXY, and XYBE, for E = N, P, and As, and X, Y = F and Cl, and then a comparative investigation is carried out using quantum chemical methods. Hence, the aim of this paper may be expected to result in a better understanding of the effects of the substituent as a consequence on the stabilities of the boron-bonds species.