Crystal structures of two forms of a 2:1 cyclomaltohexaose (α-cyclodextrin)/4,4′-biphenyldicarboxylic acid inclusion complex

The inclusion complex of cyclomaltohexaose (α-cyclodextrin, α-CD)/4,4′-biphenyldicarboxylic acid (BPDC) was crystallized into two crystal forms; tetragonal, space group P42212, a=b=19.609(8) Å, c=32.257(8) Å (Form 1), and orthorhombic, space group P21212, a=35.397(8) Å, b=24.577(5) Å, c=27.969(9) Å (Form 2). Both crystal structures were determined by the X-ray diffraction method. The results show that a guest BPDC was included in two α-CDs in the tail-to-tail orientation to form the 2:1 α-CD/BPDC inclusion complex, and that a BPDC interacts with another BPDC by guest–water–guest hydrogen bonds. In Form 1, the α-CDs are arranged in a continuous one-dimensional column structure, whereas in Form 2 they are arranged in a discontinuous array.