A 1:1 adduct of hexa­methylene­tetramine and 4-hydroxy-3-methoxy­benz­aldehyde

Phenols usually crystallize with hexamethylenetetramine (HMT) to form O—H···N hydrogen-bonded adducts, in which the HMT acts as a mono-, bis- or tris-acceptor of hydrogen bonds (Jordan & Mak, 1970; Mak et al., 1977, 1978; Mahmoud & Wallwork, 1979; Coupar, Ferguson et al., 1997; Coupar, Glidewell & Ferguson, 1997). In only a few examples has HMT been found to act as an N—H···O hydrogen-bond donor, i.e. in the adducts of HMT with 2,4-dinitrophenol–water and with 2,4,6-trinitrophenol; in these, the nitrophenol molecules transfer the hydroxy H atom to the HMT moiety, thus forming an ion pair (Usman et al., 2001, 2002). This unusual behaviour of HMT in the solid state is due to these two nitrophenols being stronger acids compared with the other substituted phenols investigated. In order to further confirm this behaviour of HMT in the solid state, we have selected a very weak acid, 4-hydroxy-3-methoxybenzaldehyde (HMBA), to co-crystallize with HMT.

As expected, the H-atom transfer process was not observed in the title adduct, (I). The bond lengths and angles in (I) have normal values (Allen et al., 1987). The average values of the N—C bond lengths, and the C—-N–C and N—C—N bond angles in the HMT moiety are 1.470 (4) Å, 107.9 (2)° and 112.5 (2)°, respectively, and are comparable with those of uncomplexed HMT obtained by neutron diffraction at 130 K [average C—N 1.469 (2) Å, N—C—N 107.88 (9)° and C—N—C 112.58 (8)°; Kampermann et al., 1994] or with those of HMT in the adduct with 1,1,1-tris(hydroxyphenyl)ethane [average C—N is 1.467 (5) Å; Coupar, Ferguson et al., 1997]. The bond lengths and angles within the HMBA moiety agree with those of free HMBA (Velavan et al., 1995), with the maximum difference being a deviation of 0.015 Å in the length of the C13—O2 bond.

In the title adduct, the HMBA molecule is nearly planar, with the O2 atom of the carboxylic acid group deviating by 0.145 (3) Å from the plane of the HMBA ring. The four six-membered N—C—N—C—N—C rings of the HMT moiety adopt chair conformations whose puckering parameters (Cremer & Pople, 1975) are listed in Table 3, and the absolute value of the mean deviation of all atoms from their mean-ring planes is 0.236 (3) Å.

In the title adduct, the two components are connected by an intermolecular O1—H1···N4 hydrogen bond, with the HMT moiety acting as a mono-acceptor and the HMBA molecule acting as a mono-donor in the conventional intermolecular hydrogen bond between the hydroxy group of the HMBA molecule and one of the amine N atoms of the HMT moiety, as was also observed in the HMT–phenol adducts studied previously (Wallwork, 1962; Mahmoud & Wallwork, 1979; Coupar, Ferguson et al., 1997; Coupar, Glidewell & Ferguson, 1997). The N···O distance in the title adduct is slightly shorter than the average value (2.78 Å) found for O—H···N hydrogen bonds in other HMT–phenol adducts (Wallwork, 1962; Mahmoud & Wallwork, 1979; Coupar, Ferguson et al., 1997; Coupar, Glidewell & Ferguson, 1997). The O1 atom actually facilitates a bifurcated hydrogen-bonding system, as it is also involved in an intramolecular O1—H1···O3 interaction which forms a closed five-membered H1—O1—C7—C12—O3 ring.

In the crystal structure of the title adduct, there are two intermolecular C—H···O interactions which link the HMBA moieties into molecular sheets that extend in the b and c directions. The HMT molecules are linked by O—H···N hydrogen-bonding interactions to these molecular sheets of HMBA and are alternately stacked along the a axis. Fig. 2 shows the packing diagram of the title adduct viewed down the b axis, and indicates the intermolecular interactions. Three intermolecular C—H···π interactions involving the centroid of the aromatic ring of the HMBA were also observed (C_g is the centroid of ring C7—C12). All these interactions (see Table 2) stabilize the acentric packing in the title adduct.