Crystal structures and quantum chemical calculations of dichloro[4-(dimethylamino)phenyl]arsine and tris[4-(dimethylamino)phenyl]arsine

Abstract

Dichloro[4-(dimethylamino)phenyl]arsine (1) and tris[4-(dimethylamino)phenyl]arsine (2) were synthesized and characterized using single crystal X-ray diffraction studies, NMR spectroscopy, IR spectroscopy and elemental analyses techniques. The X-ray structure analysis of 1 (P1¯, triclinic, Z = 4; R1 = 0.0478) revealed two crystallographically independent molecules in the asymmetric part of the unit cell. The average AsCl bond (220.6 p.m.) is found to be slightly longer than that of arsenic(III) chloride (216.1 or 216.2 p.m.), but to be rather similar to that in the orthorhombic modification of chlorobis[2,4,6-tris(trifluoromethyl)phenyl]arsine (219.2 p.m.). Mean As–Caryl (191.7 p.m.) and Caryl–N bond lengths (135.9 p.m.) suggest extended electronic interactions between the dichloroarsanyl group, the π-electron system of the arene ring and the nitrogen lone pair. In both molecules the nitrogen atoms are found in a planar coordination sphere; the sums of bond angles vary only slightly between 359.1° for 1a and 359.9° for 1b. In contrast to these observations, sums of bond angles of 292.7° for 1a and 293.98° for 1b indicate a pyramidal coordination sphere at arsenic. As well, the X-ray structure analysis of 2 (P1¯, triclinic, Z = 2; R1 = 0.0347) reveals bond lengths and angles at arsenic (AsC 194.9 p.m., CAsC 99.3°, AsCC 121.3°) as to be expected and obtained for similar compounds such as triphenyl arsine or triphenyl arsine derivatives. The sums of angles at two of the nitrogen atoms amount to values of 353.6° and 356.1° and deviate significantly from the value of the third (348.1°). Hence, two of three dimethylamino groups are found to be almost planar, whereas the third group shows a coordination sphere which has to be classified as an intermediate between trigonal planar and trigonal pyramidal. The molecule shows a high degree of C3 pseudosymmetry; the sum of angles at arsenic amounts to 298.0°. The average Caryl–N bond length (139.0 p.m.) suggests an interaction of the nitrogen lone pair with the π-system of the arene ring but it turns out to be much weaker than in dichloro[4-(dimethylamino)phenyl]arsine (1). Additionally, quantum chemical calculations were performed on several para substituted dichlorophenyl arsine compounds in order to ascertain optimized structural data and to shed some light onto these phenomena. Indeed, the aforementioned shortening of the N–Cipso bond to 135.9 p.m. can only in part be attributed to the well-known electronic interaction between the lone pair at nitrogen and the antibonding π∗ orbitals of the adjacent Cipso–Cortho bonds – on an average, these two distances are significantly elongated by 1.7 p.m. with respect to the standard value (140 p.m.).