For example, the following "C NMR shifts were measured for ips0 cyclopentadienyl carbon atoms of compounds of type I : ER, = SiPh,: 31.0 (CDCI,); ER, = GePh,: 28.5 (CDCI,); ER, = PPh: 18.7 (C,D,); ER, = AsFh: 18.3 (C,D,); ER, = S-14.3 (C,D,). a) References [l], [8], and [lo]; b) D. Seyferth, H. P. Withers, J. Organomel. Chem. 1980, 185, Cl Crystallographic data for 4a (C,,H,,BFeNSi,):monoclinic, P2,/c, a=11.614(1), h=11.506(1), c=13.601(1)A, p=98.526(2)", V=1797A3. 2 = 4, p = 9.62 cm-', Mo,, ( j . = 0.71069 A), pca,cd = 1.31 gcm-', 100 K, Enraf-Nonius DIP2020 image plate diffractometer with graphite monochromator, dark red crystal (dimensions 0 21 x 0.26 x 0.31 mm3). Of 18806 reflections recorded (1<0<27", -1 3 1 h 5 1 3 , -1 4 5 k i 1 4 , -1 6 5 / 2 1 6 ) , 3605 were unique and 3186 were observed with I > 3 4 1 ) . The images were processed with the DENZO and SCALEPACK programs [23]. Solution by direct methods and refinement by the full-matrix least-squares method. Subsequent difference Fourier analysis revealed that one of the Si(CH,), groups was disordered over two sites with 80% and 20% fractional occupancy. In the final stages of the structure determination all methyl hydrogen atoms were added geometrically after each refinement cycle and included in the structure factor calculations: 235 refined parameters for 31 89 observations. Corrections for Lorentz and polarization effects, secondary extinction and anomalous scattering were applied. Refinement was completed by using Chebyshev weighting scheme with three parameters [24]. Refinement on F, R = 0.028, R, = 0.035, GOF = 1.056, = 0.46 e k ' , and ApmLn = 0.46 e k ' . All crystallographic calculations were carried out using the CRYSTALS package 1251. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publication no. CCDC-100378. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ (fax: Int code +(1223) 336-033; e-mail: deposit@chemcrys. cam .ac. u k) P. Paetzold, Adv. Inorg. Chem. 1987, 31, 123, see p 137.

A series of amide oligomers have been prepared from isophthalic acid and a bisaniline derivative. These compounds assemble into double-stranded zipper complexes in solution via hydrogen-bonding and edgeto-face aromatic interactions. The stability and structures of the complexes have been determined by 1 H NMR spectroscopy in chloroform solution. The stability of the complexes increases with increasing chain length, indicating cooperativity between the individual recognition sites in the oligomers. Oligomers which are complementary form more stable complexes than non-complementary systems with overhanging ends. Addition of polar solvents such as methanol destabilizes the complexes, because it competes for hydrogen-bonding interactions which appear to be the main driving force for binding in this system.

The formation of hydrogenbonded complexes between three different compounds has been investigated by 1 H NMR spectroscopy. Titration experiments for binary and ternary mixtures show that these compounds form a termolecular complex in chloroform. The complexation-induced changes in chemical shift indicate that the structure of the ternary complex is similar to the hydrogen-bonded structures found in the simple binary mixtures. However, the association constant for the formation of the ternary complex is significantly larger than that expected based on the stabilities of the binary complexes: the association constant increases by a factor of three, equivalent to a stabilisation of 1 ± 2 kJ mol À1. An explanation for this phenomenon is that the formation of a small hydrogen-bond network polarises the hydrogen-bonding groups and thereby increases the strengths of the individual hydrogenbonding interactions.

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