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.