The molecular and electronic structure of dipiperidinosquaraine

Abstract

Dipiperidinosquaraine (9) crystallizes in the orthorhombic system, space group Pnab, with four molecules in a cell of dimensions a = 7.1798(9), b = 9.5362(8), and c = 18.788(2) A. The molecule is located over a center of symmetry, so the four-membered ring (C -C = 1.456(1) A av) is exactly planar; the nitrogen and oxygen atoms are nearly coplanar with the ring, with C -0 = 1.2396(7) and C -N = 1.3112(7) A. Both experimental and theoretical evidence indicates the presence of considerable C -N and C -0 n-bonding, but only a small amount of C -C n-bonding in the squaraine system. The calculations indicate the presence of strong 1,3-repulsive interactions within the four-membered ring. Ring puckering, which is a common mechanism to alleviate these repulsions, is prevented here by the extensive C -N x-bonding. Molekulare und elektronische Struktur von DipiperidinoquadratainDipiperidinoquadratain (9) kristallisiert im orthorhombischen Gitter, Raumgruppe Pnab, rnit 4 Molekulen in einer Elementarzelle mit den Abmessungen a = 7.1798(9), b = 9.5362(8) und c = 18.788(2) A. Das Molekul liegt auf einem Symmetriezentrum, d.h. der Vierring (C-C = 1.45611) A) ist genau planar; die Stickstoff-und Sauerstoffatome sind fast mit dem Ring coplanar, mit C -0 = 1.2396(7) und C -N = 1.31 12(7) A. Sowohl die experimentellen als auch die theoretischen Daten zeigen beachtliche C -N-und C -0-x-Bindungen, aber nur eine geringfugige C -C-n-Bindung im Quadratainsystem. Die Berechnungen ergeben starke 1,3-abstoRende Wechselwirkungen im Vierring. Ringfaltung, ein sonst ublicher Mechanismus bei der Vermeidung solcher AbstoRungen, wird hier durch die starke C -N-a-Bindung verhindert.Squaraines -1,3-substituted derivatives of squaric acid -have been known for 20 years l). Various formulations have been used to represent their structures, the most common being l a -c.

Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines

“…Support for this description can also be gleaned from work on amino squaraines. Both experimental and theoretical studies (9,10) on a related series of aminosquaraine compounds (7 and 8) indicated that these compounds experience considerable C-N a-bonding between the central four-membered ring and the amino substituents, supporting the MNDO evidence presented for aryl squaraines.…”

The First Structurally Characterized Aluminum Squaraine Complex: L2(AlMe2)4·2THF·2toluene [L = Bis(2,6‐diisopropylanilino)squaraine]

“…These bonding characteristics can be explained by the p‐π conjugation of the lone pairs on O(1) and N(2) and the carbon p orbital 10. Like previously reported squaraines, the C atoms in the four‐membered ring in 2 are all sp 2 hybridized 2b. Delocalization of the electrons is believed to exist, and this accounts for the longer C(1)−C(4A) double bond [1.428(3) Å].…”

“…There are some noticeable differences in the structure of the squaraine ligands in complex 2 compared to those reported previously 2b,2c,6. The C(2)−N(1) bond length of 1.281(3) Å is typical of a C=N double bond (1.273 Å in CH 2 =NH8).…”

“…Delocalization of the electrons is believed to exist, and this accounts for the longer C(1)−C(4A) double bond [1.428(3) Å]. The C(1)−C(2) [1.471(3) Å], C(3)−C(4A) [1.446(3) Å], and C(2)−C(3) bond lengths [1.483(3) Å] are in the range of a C−C single bond (1.466 Å) 2b. This C‐C bonding character of 2 is different from that of the reported bis(dimethylamino)squaraine6 and dipiperidinosquaraine2b in which the C‐C bond lengths are almost equal.…”

“…In fact, the versatility of squaraines implies that this class of compounds are ideal ligands. Only a few diamino derivatives of squaric acid have been structurally characterized 2b,2c,6. No complex of aluminum with squaraines as ligand has been isolated and structurally characterized so far.…”