1 Introduction

Studies of the noncovalent interactions play a vital role in fundamental research and numerous fields, such as supermolecular chemistry, crystal engineering, material science, and biochemistry[1-4]. It is generally accepted that hydrogen bond is one of the most extensively studied intermolecular interactions due to its widespread existence in various physical and chemical processes. With the development of the experimental and theoretical approaches, the role of other Lewis acid-Lewis base interactions has been discovered and investigated[5]. Amongst them, pnicogen bonds (ZBs), has been recognized as a new and important type of intermolecular interaction, where atoms from group V of the periodic table (N, P, As, Sb) act as Lewis acid centers[6]. It has been demonstrated that the pnicogen bonds can be more attractive than even several strong hydrogen bonds, which can determine the structures of the large complexes or aggregates. Owing to their diverse and potential uses in various areas, such as organocatalysis[7] and crystal materials [8], the pnicogen bonds have recently received more and more attention in the literature concerned with its fundamentals and its applications[9-16].
According to the σ-hole concept proposed by Politzer et al .[17], the electrostatic component of pnicogen bonds has been explained by the regions of the positive electrostatic potential (positive σ-hole) interacting attractively with the negative sites on the Lewis bases. Besides, there also exist the pnicogen bond labeled as π-hole bonding interaction in which the region of depletion of the electron density perpendicular to portions of a molecular framework plays the role of the Lewis acid center[18]. Such pnicogen bonded complexes of PO2X (X = F, Cl) with nitrogen bases have been extensively studied by Alkorta and coworkers[19], including the structures, binding energies and electronic properties, and it is revealed that complexation of PO2X with the strongest bases leads to P···N bonds with a significant degree of covalency. To evaluate the preference for the establishment of σ-hole versus π-hole interactions, a series of complexes involving 3rd-and 4th-row atoms of Groups IV–VI have been studied by Frontera et al .[20, 21]. For the complexes between O2YBr (Y=N, P, As) and NH3, H2O, and HF acting as Lewis bases, it is revealed that the complexes in which the lone pair of the Lewis base interacts with the π-hole are more favorable than those with σ-hole[20]. For the π-hole bonding complex between IF and ZO2F (Z=P, As), it is found that the largest binding energies were obtained when the fluorine atom acts as the electron donor[21].
Although several systems with pnicogen bonding interaction have been investigated extensively, some problems are still worth studying. One of the important aspects is tuning the interaction strength of the pnicogen bond for future modelling of novel materials. One way called “covalent way” is by changing different substituents on the Lewis acid or the Lewis base [22]. In several chemical cases, the electron-donor may be bonded to different sorts of carbon chains. The effect of the length and type of the chain on the electron-accepting P atom’s ability engaging in a P…N bond have been investigated by Scheiner and coworkers[23]. It is found that the incorporation of C=C double bond in the chain tends to strengthen the P…N pnicogen bond in the RH2P…NH3 complexes, and the addition of several conjugated double bonds such as aromatic phenyl ring has only a marginal further change. However, the substitution effects considered were mainly bonded to the electron-accepting P-atom. This lead to the natural question as to how might addition of various groups to the electron donor tune the pnicogen bond.
Another way to modulate the strength of the pnicogen bond is by introducing another intermolecular interaction, and the interplay between the two interactions can be referred to as “cooperative effect” [24, 25]. Understanding the relationships of these noncovalent interactions coexisting in the complexes is significant for the design of novel materials or supramolecular systems. The interplay between the pnicogen bond and regium bond in the ternary complexes HN3…FH2X…MCN (X=P, As; M=Cu, Ag, Au) has been studied by Li et al.[26], and it is found that the pnicogen bond is strengthened through the cooperative effect with the regium bond. According to the research by Bene and coworkers [27], the presence of the P…Y halogen bond makes PH3 a better electron-pair acceptor in the XY…PH3…N-base ternary complexes, which is caused by the synergistic effect between the pnicogen-bond and the halogen bond. A detailed computational analysis of the interplay between the σ-hole pnicogen-bonding and the aerogen-bonding interaction has been reported by Esrafili et al .[15], in which favorable cooperativity is observed in Y···PH2CN···ZO3 (Y = NH3, N2 and Z=Ar, Kr, Xe) complexes. Very recent calculations have noted that negative cooperative effect exists in the pnicogen bonded trimers formed by ZF3(Z=P, As, Sb, Bi) and a number of nucleophiles[28]. However, to our knowledge, little focus has been given to how might the π-hole pnicogen bond be effected by the other intermolecular interaction through cooperative effect. Herein, we focus on two typical intermolecular interactions, σ-hole halogen[29] and π-hole triel bonding interactions[30], wherein the group VII and III atoms act as the Lewis acid center.
According to the previous molecular electrostatic potential analysis, PO2F is a good candidate to act as the electrophile in forming π-hole pnicogen interactions. To study the bridging role of π-electrons of aromatic molecule in tuning the pnicogen bonds, we chose substituted benzonitrile (R-Ph-CN) as the Lewis bases. To study the effect of another interaction (halogen/triel bond) on the pnicogen bond, CN-Ph-CN and Br-Ph-CN are chosen as the center molecule in complexes X…CN-Ph-CN…PO2F and Y…Br-Ph-CN…PO2F for X=F2, Cl2, Br2, FCl, FBr, BrCl, FCN, ClCN, BrCN, BH3, BF3, and Y=NH3, NH2CH3, NHCH2, HCN. In the present paper, the binary complexes R-Ph-CN…PO2F (X=H, F, Cl, Br, CH3, NH2, CN), and a series of ternary complexes with pnicogen bond and halogen/triel bond are studied to deepen the understanding of the substitution effect and cooperative effect on the pnicogen bonding interaction. The quantum theory of atoms in molecules (QAIM), the molecular electrostatic potential (ESP), and natural bond orbitals (NBO) approaches were employed to investigate the structural and bonding properties, as well as the evolution of the pnicogen bonding interaction formed between the above complexes.