Hydro­gen-bonded adducts of ferrocene-1,1'-diyl­bis­(di­phenyl­methanol): a finite cyclic 1:1 adduct with 2,2'-dipyridyl­amine

Ferrocene-1,1'-diylbis(diphenylmethanol), [Fe(C₅H₄CPh₂OH)₂], forms hydrogen-bonded adducts with a wide range of amines, particularly heteroaromatic amines and diamines. Thus with pyridine, this diol forms an adduct of stoichiometry (diol).(amine)₂ (Ferguson et al., 1993), in which the number of hydrogen bond donor hydroxyl groups is equal to the number of amine N acceptors: despite this, the molecular aggregate contains disordered O—H···O and O—H···N hydrogen bonds indicative of mobile hydroxyl H atoms.

Similarly, in the 1: 1 adduct formed by the diol with 4,4'-bipyridyl (Glidewell et al., 1994), the number of hydroxyl groups is equal to the number of N atoms but only half of the bipyridyl molecules are involved in the supramolecular aggregation, which takes the form of the centrosymmetric three-component aggregate (diol)-(diamine)-(diol), while the remainder of the bipyridyl molecules simply occupy isolated sites in the structure (Glidewell et al., 1994). By contrast, in each of the similar 1:1 adducts formed with 1,2-bis(4'-pyridyl)ethane (Zakaria et al., 2001) and 1,2-bis(4'-pyridyl)ethene (Zakaria et al., 2002), there is a finite two-component aggregate containing one intramolecular O—H···O hydrogen bond and one intermolecular O—H···N hydrogen bond, while the second N plays no part in the hydrogen bonding. Continuing this theme, we have now synthesized and characterized a 1:1 adduct [C₃₆H₃₀FeO₂]·[C₁₀H₉N₃], (I), formed between the ferrocenediol and 2,2'-dipyridylamine, which was selected in order to increase the ratio of hydrogen-bond donors to acceptors in the expectation of forcing both pyridyl N atoms to participate in the supramolecular aggregation.

Compound (I) forms a finite 1:1 aggregate (Fig. 1) and, in addition to the intramolecular O—H···O hydrogen bond so commonly found in adducts involving this diol (Ferguson et al., 1993; 1995; Glidewell et al., 1994; Zakaria et al., 2001, 2002), there are two other hydrogen bonds, one each of O—H···N and N—H···O types, linking the two molecular components by forming an R³₃(8) ring (Table 2, Fig. 1). It is striking that both of the hydroxyl O atoms act as both hydrogen-bond donors and acceptors, while pyridyl N3 forms no intermolecular hydrogen bonds. Moreover the conformation of the nearly planar diamine (Table 1) is such that N2 is on the same edge of the diamine as the N11—H1 bond, while N3 is on the opposite edge. Atom N3 does, however, participate in a short intramolecular C—H···N contact (Table 2). In order to investigate whether this contact makes any significant contribution to the preferred conformation of the amine, we have undertaken SCF-MO calculations using the AM1 technique (Dewar et al., 1985; Stewart, 1990), for the three possible near-planar conformers (A) - (C) (Scheme 1). Conformers (A) and (C) were both calculated to have C₂ symmetry, with torsional angles N—C—N—C of 155.0° and -24.3° respectively, suggestive of repulsion between the ortho C—H bonds in (A) and between the pyridyl N lone pairs in (C), and with enthalpies of formation respectively 1.5 kJ mol^-1 and 2.2 kJ mol^-1 higher than that of the planar conformer (B). Clearly these energy differences are insufficient to determine the overall conformation.

However, it is interesting to note that pure 2,2'-dipyridylamine crystallizes in three polymorphs: orthorhombic Pccn with Z' = 1 (Johnson & Jacobson, 1973); triclinic P-1 with Z' = 2 (Pyrka & Pinkerton, 1992); and monoclinic P2₁/c with Z' = 2 (Schödel et al., 1996), and that in each of these forms, the molecules adopt the same overall conformation as that observed here in (I). In the orthorhombic and triclinic forms, the molecules are linked into dimeric units by means of paired N—H···N hydrogen bonds forming an R²₂(8) motif, with no involvement in the supramolecular aggregation of the N on the opposite edge of each molecule. On the other hand, in the monoclinic polymorph where the molecules are linked into tetrameric aggregates, half of the molecules utilize all of their N atoms in the hydrogen bonding, acting as single donors and double acceptors, while the other half act as single donors only, leaving both pyridyl N atoms in these molecules uninvolved in the hydrogen bonding.