Conformational flexibility in amidophosphoesters: a CSD analysis completed with two new crystal structures of (CHO)P(O)X [X = NHCH and N(CHCH)].

The crystal structures of diphenyl (cycloheptylamido)phosphate, CHNOP or (CHO)P(O)(NHCH), (I), and diphenyl (dibenzylamido)phosphate, CHNOP or (CHO)P(O)[N(CHCH)], (II), are reported. The NHCH group in (I) provides two significant hydrogen-donor sites in N-H...O and C-H...O hydrogen bonds, needed for a one-dimensional hydrogen-bond pattern along [100] in the crystal, while (II), with a (CHCH)N moiety, lacks these hydrogen bonds, but its three-dimensional supramolecular structure is mediated by C-H...π interactions. The conformational behaviour of the phenyl rings in (I), (II) and analogous structures from the Cambridge Structural Database (CSD) were studied in terms of flexibility, volume of the other group attached to phosphorus and packing forces. From this study, synclinal (±sc), anticlinal (±ac) and antiperiplanar (±ap) conformations were found to occur. In the structure of (II), there is an intramolecular C-H...O interaction that imposes a +sc conformation for the phenyl ring involved. For the structures from the CSD, the +sc and ±ap conformations appear to be mainly imposed by similar C-H...O intramolecular interactions. The large contribution of the C...H/H...C contacts (32.3%) in the two-dimensional fingerprint plots of (II) is a result of the C-H...π interactions. The differential scanning calorimetry (DSC) analyses exhibit peak temperatures (T) at 109 and 81 °C for (I) and (II), respectively, which agree with the strengths of the intermolecular contacts and the melting points.