Comparison of copper imine and amine podates : geometric consequences of podand size and donor type

The imine podands tris[(2-nitrobenzylidene)aminoethyl]amine and tris[(2-nitrobenzylidene)aminopropyl]amine both stabilize copper(I), forming {tris[(2-nitrobenzylidene)aminoethyl]amineN}copper(I) perchlorate acetonitrile disolvate, [Cu(C27H27N7O6)]ClO4 2CH3CN, (II), and {tris[(2-nitrobenzylidene)aminopropyl]amineN}copper(I) perchlorate, [Cu(C30H33N7O6)]ClO4, (VI), respectively. The larger propyl-based ligand is a poorer ®t for the Cu ion. The reduced amine podand tris[(2-nitrobenzyl)aminoethyl]amine binds Cu and the resulting compound, chloro{tris[(2-nitrobenzyl)aminoethyl]amineN}copper(II) chloride ethanol solvate, [Cu(C27H33N7O6)Cl]Cl C2H5OH, (IV), shows both intraand intermolecular hydrogen bonding, which gives rise to RRS or SSR conformations in the podand strands rather than the expected pseudo-threefold symmetry.


Comment
We have had a long-standing interest in the chemistry of both imine and amine cryptates derived from tris(aminoethyl)amine (tren) and tris(3-aminoisopropyl)amine (trpn) [see, for example, McKee et al. (2003) and Nelson et al. (1998)]. We have investigated some simple podate complexes derived from the same amines in order to clarify the geometric requirements associated with each (Coyle, 1999). A search of the Cambridge Structural Database (Version 5.27;Allen, 2002;Fletcher et al., 1996) showed that, although many tris(aminoethyl)amine/salicylate complexes have been investigated, surprisingly few simple podates with other substituted benzaldehyde derivatives have been structurally characterized to date. In this paper, we compare the structures of two Cu I podates, one derived from tris(aminoethyl)amine (tren) and one from tris(aminopropyl)amine (trpn), with the Cu II amine analogue of the smaller tren-based podate.
In podate and cryptate complexes with potential threefold symmetry, imine donors typically stabilize Cu I and are easily hydrolysed by Cu II (Harding et al., 1995;Arthurs et al., 2001). Reduction of the imine donors to the corresponding amines generates a site in which Cu I is activated to reaction with dioxygen, as shown elegantly by Suzuki, Schindler and their co-workers (Komiyama et al., 2004;Schatz et al., 2001). However, Cu II binds readily to the reduced ligands.
The structure of the imine podand tris[(2-nitrobenzylidene)aminoethyl]amine, (I), was reported recently (McKee et al., 2006). Reaction of (I) with Cu(CH 3 CN) 4 ClO 4 in acetonitrile gave the Cu I complex [Cu(I)]ClO 4 Á2CH 3 CN, (II), as darkbrown crystals (Fig. 1). The Cu I ion is coordinated to all four N atoms in an approximately trigonal±pyramidal geometry (Table 1), although the bonds to the imine N atoms [average 2.003 (2) A Ê ] are signi®cantly shorter than that to the bridgehead amine [Cu1ÐN1 = 2.196 (1) A Ê ], and the Cu I ion is 0.172 (1) A Ê out of the mean plane of the imine N atoms in the opposite direction to the bridgehead. The nitro groups are not involved in the coordination of the metal and the three strands are arranged fairly tightly about the approximate threefold axis. There are two important factors controlling this geometry, namely the essentially planar geometry at the imine N atoms [angle sums 359.9 (2), 359.9 (2) and 359.8 (2) for atoms N11, N21 and N31, respectively] and the steric demands imposed by coordination of all four N donors of the ligand. These result in the CÐN C plane being tilted with respect to the`default' orientation (parallel to the pseudo-threefold axis and perpendicular to the plane of the three sp 2 -hybridized imine donors); the interplanar angles are 71.6 (1), 73.5 (1) and 74.1 (1) for the N11, N21 and N31 strands, respectively. In other words, the orientation of the conjugated nitrobenzylidene strands is determined by the orientation of the imine lone pairs. It is therefore not surprising that this geometry is common for tren-based imine podands in the absence of additional intra-or intermolecular interactions. There are no signi®cant interactions between the cation and perchlorate anion or solvent molecules. The anion is disordered and was modelled with approximately 10% occupancy of the minor orientation (Fig. 1).
The amine podand, tris[(2-nitrobenzyl)aminoethyl]amine, (III), was obtained by reduction of (I) with NaBH 4 , which reduced the imine groups but not the nitro substituents. Reaction of ligand (III) with CuCl 2 in ethanol yielded the amine complex [Cu(III)Cl]ClÁC 2 H 5 OH, (IV), as green crystals. The formula unit of (IV) is shown in Fig. 2. The geometry at the Cu I ion is approximately trigonal±bipyramidal (Table 2), with the bridghead tertiary amine and the coordinated Cl À ion as apical donors. The coordination geometry is similar to that observed for the analogous Cu II podate derived from benzaldehyde [tris(benzylaminoethyl)amine; Komiyama et al., 2004;Schatz et al., 2001).
Two of the nitro groups of (IV) are hydrogen bonded to the adjacent secondary amines (Table 3), but the third strand is different, with the amine (N31) hydrogen bonded to the ethanol solvent molecule. Consequently, the con®guration at N31 is opposite to that at N11 and N21 (SRR in Fig. 2, although, since the structure is centrosymmetric, the RSS con®guration is also present). This difference breaks the pseudo-threefold symmetry of the cation. The non-coordinated Cl À ion Cl2 makes a relatively short hydrogen bond to the ethanol solvent molecule [3.105 (4) A Ê ] and shows further interactions with N21 and with N11 of an adjacent molecule. The latter two interactions are long for hydrogen bonds to Cl À , at 3.302 (4) and 3.474 (4) A Ê , respectively (Steiner, 2002). However, both are bifurcated and involve coordinated amines. The resulting hydrogen-bond pattern links the structure in chains running parallel to the b axis (Fig. 3)     interaction between these chains is a %±% interaction between the C24±C29 ring and its symmetry equivalent by inversion under (1 À x, Ày, 1 À z); the rings are necessarily parallel, the interplanar distance is 3.393 (4) A Ê and centroid-to-centroid distance is 3.710 (4) A Ê . Complex (VI), namely {tris[(2-nitrobenzylidene)aminopropyl]amine}copper(I) perchlorate, is analogous to complex (II), except that the longer tripodal amine tris(aminopropyl)amine (trpn) is used in place of tren. As for (II), the Cu ion is stabliized in the +1 state and has trigonal±pyramidal geometry ( Fig. 4 and Table 4). However, the Cu I ion is displaced from the imine plane by 0.167 (1) A Ê towards the bridgehead [i.e. in the opposite sense from complex (II)]. As observed for complex (II), the requirement to coordinate the Cu I ion to all four N-atom donors results in tilting of the CÐ N C planes relative to the plane of the three sp 2 -hybridized imine donors. In complex (VI), however, this effect is much more pronounced [interplanar angles 34.9 (2), 36.3 (2) and 39.4 (2) for atoms N11, N21 and N31, respectively].
The three-dimensional`podand bite' in the two Cu I complexes, (II) and (VI), can be compared by considering the dimensions of the trigonal pyramid formed by the four N-atom donors, with the tertiary amine (N1) at the apex and the imine atoms N11, N21 and N31 in the basal plane. As mentioned above, the Cu I ion is outside the pyramid in complex (II) and inside for (VI). However, the CuÐN1 distances are identical ]. An indication of steric strain in complex (VI) is given by the NÐCÐC and CÐ CÐC angles in the saturated chain between N1 and the imine N atoms; the average angle is 114.4 (3) , compared with 110.5 (2) for complex (II).
We have observed similar patterns in the geometry of Cu ions in cryptand hosts derived from tren and trpn [see, for example, Farrar et al. (1995) and Nelson et al. (1998)], supporting the suggestion that steric constraints mean that the larger podand has more dif®culty accommodating bonding between the Cu I ion and all four donors than the smaller analogue. These results also go some way to explaining the initially counterintuitive ®nding that, in the dinuclear iminocryptate series, the shortest internuclear distances between cationic guests are found for the larger hosts (Drew et al., 2000;Farrar et al., 1995;Nelson et al., 1998). In the case of the cryptand ligands, the twist imposed on each strand by the coordination of the imine donors shortens the distance between the two metal binding sites.

Experimental
For the preparation of [Cu I (I)]ClO 4 Á2CH 3 CN, (II), tris[(2-nitrobenzylidene)aminoethyl]amine, (I) (0.93 g, 1.7 mmol), was dissolved in dry deoxygenated acetonitrile (30 ml) and a solution of Cu(CH 3 CN) 4 ClO 4 (0.55 g, 1.7 mmol) in deoxygenated acetonitrile (20 ml) was added slowly with stirring. The red±brown solution was stirred for 30 min at 313 K and then cooled, during which time an orange crystalline product precipitated. This was ®ltered off and dried under nitrogen, losing the acetonitrile solvent in the process (yield 0.70 g, 52%). Analytical results (available in the archived CIF) are consistent with the stated composition for all compounds reported here.
The amine podand tris[(2-nitrobenzyl)aminoethyl]amine, (III), was prepared by reduction of the imine analogue (Liu et al., 1992). The imine (I) (2.15 g, 3.9 mmol) was dissolved in methanol (60 ml). Na 2 B 4 O 7 (0.81 g, 4.0 mmol) was added, followed by NaBH 4 (0.65 g, 17.2 mmol) in small portions over a period of 30 min. The solution was stirred for 2 h and then the solvent was removed on a rotary evaporator. NH 4 Cl (4 g, 76 mmol) in water (40 ml) was added and the mixture was extracted with CHCl 3 (3 Â 60 ml). The CHCl 3 solution was washed with water, dried over MgSO 4 and ®ltered. Finally, the solvent was removed under reduced pressure to yield the amine as a pale-yellow oil (yield ca 88%). The IR spectrum of the oil con®rmed that the ligand had been successfully reduced. The imine stretch at ca 1630 cm À1 was no longer present, but symmetric and antisymmetric stretches of the nitro group at 1347 and 1526 cm À1 , respectively, con®rmed that the substituent remained unchanged. The amine was used in the next step without further puri®cation.
For the preparation of [Cu II (III)Cl]ClÁC 2 H 5 OH, (IV), the amine ligand (III) (0.05 g, 0.09 mmol) was dissolved in ethanol (1.5 ml), forming a pale-orange solution. On addition of a solution containing CuCl 2 (0.013 g, 0.09 mmol) in ethanol (1 ml), a turquoise solution was formed. Green crystals of (IV) were obtained on allowing the solution to stand (yield 0.03 g, 48%).
Ligand (V) was prepared by the dropwise addition of tris(3aminoisopropyl)amine (0.32 g, 1.7 mmol) in methanol (20 ml) with stirring to nitrobenzaldehyde (0.77 g, 5.1 mmol) in methanol (20 ml). The resulting solution was stirred at 313 K for 30 min and the volume was then reduced to yield a yellow oil, viz. (V). The oil was dissolved in deoxygenated acetonitrile (30 ml) and Cu(CH 3 CN) 4 ÁClO 4 (0.55 g, 1.7 mmol) was added. A brown solution formed and dark-red crystals of [Cu I (V)]ClO 4 , (VI), were obtained on allowing the solution to stand (yield 0.69 g, 54%).  The structure of complex (VI), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability level and H atoms have been omitted for clarity.