Two modes of peri-interaction between an aldehyde group and a carboxylate anion in naphthalaldehydate salts †

Crystal structures of the salts of 1,8-naphthalaldehydic acid (8-formyl-1-naphthoic acid) show one of two types of interaction between the functional groups. In the more commonly observed case, a carboxylate oxygen lies close to the aldehyde carbonyl carbon atom (O⋯C: 2.445–2.630 Å) and makes an n–pi* interaction. However, in two other cases the carboxylate group has rotated so that the aldehyde now directs its hydrogen atom at the face of the carboxylate group and forms a surprisingly short contact with the carbon atom (H⋯C: 2.29 and 2.42 Å). This interaction is likely to be electrostatic in nature.


Introduction
Carbonyl/carbonyl interactions of type CO⋯CO, in which a sp 2 oxygen lone pair impinges on a carbonyl carbon, can be regarded as n-π* interactions and have been proposed to stabilise the conformations of proteins, 1 polymers 2 and small molecules such as substituted N-acetyl-proline methyl esters 1 (Scheme 1). 3,4The interaction leads to a small pyramidalisation of the carbonyl carbon involved.Interestingly, it has been proposed that such interactions can provide a link in the relay of chirality in a protein, 5 and there is evidence that the formation of such an interaction modifies the properties of the groups involved. 4Furthermore, in the crystal structure of aspirin 2 a similar interaction is observed but between the hydroxyl oxygen of the carboxylic acid and the ester carbonyl group (Scheme 1).Calculations on aspirin and its anion support the existence of such stabilising interactions, which are larger for the anion, and which may be responsible for the high acidity of aspirin, as well as its efficacy as an acyl transfer agent in biology. 6The O⋯C distances from neutral oxygen observed in these interactions are typically >2.8 Å and calculations suggest stabilisations varying from 0.9 kcal mol −1 in aspirin to about 0.27 kcal mol −1 between two amide groups.
We have been investigating the interactions and reactions of the more nucleophilic dimethylamino group with various electrophilic organic groups placed in the peri positions of the naphthalene ring system.][10] This approach has been extended to the biphenyl and triptycene scaffolds. 11,12Together with several compounds containing short transannular N⋯C interactions 13 a set of structures with N⋯C separations over the range 2.764 to 1.612 Å has been built up which represent different stages in the formation of the N-C bond.The shorter N⋯C distances in this range correspond to long covalent N-C bonds.The longer N⋯C separations observed for interactions with esters and amides can be considered as n-π* interactions corresponding to early stages in bond formation and are associated with small pyramidalisations of the carbonyl carbon towards the nucleophile.Such an interaction may be the initiation for further chemical reactions.Thus, the aldehyde 3 reacts readily on oxygen with a proton or an acyl group at 0 °C with concomitant formation of a N-C bond between peri-substituents to give 7 10 and 8, 14 and similar protonations have been observed with some analogous ketones. 15In contrast, when the dimethylamino group is replaced by the much less nucleophilic methoxy group 948 | CrystEngComm, 2016, 18, 948-961 there is very little change in the MeO⋯C separations in a similar range of peri-naphthalenes 9-12 (2.550-2.611Å), though there are small pyramidalisations of the adjacent ketone or amide groups towards the methoxy group (0.039-0.044Å) (Scheme 3). 16Outside the peri-naphthalene series we have also observed similar contacts between an ether oxygen and a ketone, 12 and between an ester carbonyl oxygen and a nitrile, leading to small distortions of the electrophilic functional group. 17iven the weak nature of interactions involving neutral oxygen, we decided to investigate the electrophile/nucleophile interactions involving the more nucleophilic negatively charged oxygen, and here we report the structures of a series of salts of 1,8-naphthalaldehydic acid (8-formyl-1-naphthoic acid) 13 in whose anion 14 a negatively charged carboxylate group could make a close approach to an aldehyde group.Such interactions would be the anionic equivalent of the n-π* interactions between carbonyl groups described above.The parent acid adopts a cyclic structure 15 in the solid and solution state, so that in this case the groups have interacted fully and formed a bond (Scheme 3).

Discussion
1,8-Naphthalaldehydic acidcyclic form 15 The 13 C NMR spectrum of naphthalaldehydic acid dissolved in (CD 3 ) 2 CO was consistent with the cyclic structure 15 where the carboxylic acid group has added to the aldehyde, showing shifts of δ H : 6.94 and δ c : 97.1 for the methine group attached to two oxygen atoms.Similar results were obtained from solution in CD 3 OD.To investigate the solid state structure of 1,8naphthalaldehydic acid, crystals were grown from acetonitrile and the structure determined by X-ray diffraction at 100 K.The crystal structure, which is monoclinic and in space group P2 1 /c, confirmed that the cyclic structure is also present in the solid state (Fig. 1).Thus, a bond has been formed between the carboxylic acid and aldehyde groups (1.477(2) Å).
This journal is © The Royal Society of Chemistry 2016 This bond experiences a significant anomeric effect from the hydroxyl group causing it to lengthen from the typical value for a six-ring lactone (ca.1.34 Å) 18 and shortening the C-OH bond from a typical value of ca.1.43 Å 18 to 1.387(2) Å.The molecules are stacked along the b axis with O-H⋯OC hydrogen bonds between molecules in adjacent stacks related by a two-fold axis (O3-H3A: 0.94(2), H3A⋯O1′: 1.84(2), O3⋯O1′ 2.785(4) Å, O3-H3A⋯O1′: 176.5Ĳ17)°).In contrast, the crystal structures of the salts of naphthalaldehydic acid show that the anion does not adopt a cyclic structure which would have to contain an alkoxy anion, but consists of a naphthalene system with the more stable carboxylate anion and aldehyde group as peri substituents.

Group 1A salts of the naphthalaldehydate anion 14
Group 1A salts of 1,8-naphthalaldehydic acid were prepared by reaction with the corresponding metal hydroxides to give the lithium, potassium and cesium salts and crystals were grown from various alcohols.The potassium and cesium salts crystallise as monohydrates and are isostructural with orthorhombic crystal structures in space group Pna2 1 .The metal ions are coordinated by eight oxygen atoms: two sets of carboxylate groups, two aldehyde groups and two water molecules.Each carboxylate oxygen atom and the aldehyde oxygen atom are all coordinated to two metal ions and the water molecule bridges two metal ions.The functional groups are oriented so that their planes lie at 32°to each other and one carboxylate oxygen O(2) approaches the aldehyde carbon atom, and the aldehyde hydrogen atom lies nearly over the carboxylate group.The crystal structure of the potassium salt is shown in Fig. 2, and the interactions involving the anion are shown in Fig. 3.For the potassium salt, the carboxylate group is unsymmetrical with carbon-oxygen bonds of 1.241 (10) (to O1) and 1.280(9) Å (to O2) suggesting that more negative charge is concentrated on O2.Apart from the interaction with the aldehyde, this oxygen also makes two short coordinations to potassium ions (2.810(6) and 2.863(6) Å) while for O(1) one of its two coordinations to potassium ions is longer (2.872(6) and 3.140(7) Å) which will also influence the C-O bond lengths.The carboxylate and aldehyde groups are rotated out of the naphthalene plane in the same sense by similar amounts (33.3 and 29.2°respectively), splayed apart in-plane and displaced to opposite sides of the naphthalene's best plane so that the distance between the more negatively charged carboxylate oxygen atom O(2) and the aldehyde carbon atom C(12) is 2.522(10) Å.The OĲ2)⋯C(H)O angle is 111.9Ĳ5)°similar to the "Bürgi-Dunitz" angle and consistent with the alignment of electron density from oxygen with the anti-bonding pi orbital of the carbonyl group.The shortest contact from the aldehyde's electron deficient hydrogen atom to the carboxylate group is to oxygen O(2) (2.32 Å) and this hydrogen lies 2.55 Å from the carboxylate carbon atom.Selected molecular geometry for the anion is given in Table 1.Both carboxylate oxygen atoms coordinate the same potassium ion, as well as each making a coordination to a second potassium ion.The aldehyde oxygen atom coordinates two potassium ionsone coordination roughly aligned with an oxygen sp 2 lone pair (2.778(7) Å) and the other, rather longer, approximately perpendicular to the plane of the aldehyde group (3.341(7) Å).The water molecule bridges two potassium ions (O⋯K: 2.760(7) and 2.842(7) Å) and is involved in hydrogen bonds to carboxylate oxygen atoms (OĲ4)-HĲ41)⋯OĲ1): 2.09 Å, angle at H(41) 162.1°andOĲ4)-HĲ42)⋯OĲ2): 1.94 Å, angle at H(42) 166.9°).The cesium salt shows the same coordination pattern as for the potassium salt and the same asymmetry in the carboxylate group (1.248(12) and 1.270(13) Å).The carboxylate and aldehyde groups adopt similar positions with an O⋯C interaction distance of 2.502(12) Å, and a similar angle of approach to the carbonyl group (110.9Ĳ7)°)(Table 1).The aldehyde's hydrogen atom lies 2.31 Å from the carboxylate's oxygen O(2) and 2.56 Å from the carboxylate carbon atom.The eight cesium-oxygen coordination bonds lie in the range 3.052Ĳ8)-3.391Ĳ8)Å.
The crystals of the lithium salt of anion 14 crystallise in the monoclinic crystal system with space group P2 1 /c and are twinned.The structure was refined with two twin components related by a 2-fold axis rotation about the [100] reciprocal lattice direction.The atomic connectivity is confirmed, but due to poor data quality and twinning the atomic positions and derived geometry are not very accurate.
The molecular structure and crystal packing for the lithium salt are shown in Fig. 4. The lithium ions are fourcoordinate and there is no water contained in the crystal lattice.The conformation of the anion is similar to that in the potassium and cesium salts, and the carboxylate/aldehyde interaction has a OĲ2)⋯C( 12) contact of 2.445(15) Å, and a OĲ2)⋯C(12)OĲ3) angle of 107.4Ĳ8)°(Table 1).The aldehyde hydrogen lies 2.33 Å from carboxylate oxygen O(2), and 2.57 Å from the carboxylate carbon, similar to the potassium and cesium salts.The bond lengths of the carboxylate group are very similar (1.253(13) and 1.261(14) Å).The carboxylate oxygen atom remote from the aldehyde, O(1), coordinates to two lithium ions (OĲ1)-LiĲ1): 1.91(2) and 1.97(2) Å), while the second oxygen, O(2), coordinates just to one (OĲ2)-LiĲ1): 1.87(2) Å) as does the aldehyde oxygen (OĲ3)-LiĲ1): 1.95(2) Å).The angles at lithium between oxygen ligands lie in the range 97.9Ĳ10)-119.7Ĳ11)°.salts with the naphthalaldehydic acid, while the reaction with 4-dimethylaminopyridine gave a 1 : 1 molecular compound between the expected DMAP salt of anion 14 and the cyclic form of naphthalaldehydic acid 15.The X-ray crystal structures of these materials were measured at low temperature.Secondary amines such as morpholine react and substitute the OH group of 15 rather than form a salt. 19he salt with sym-triphenylguanidine crystallises from methanol in the orthorhombic crystal system in space group Pbca.The molecular structures of the cation and anion and the hydrogen bonding between them are shown in Fig. 5, and their crystal packing arrangement in Fig. 6.The triphenylguanidinium cation, rather than adopting a conformation with three fold symmetry, aligns two of its N-H bonds towards the anion.The torsion between these two N-H bonds is 44.7°, and the two hydrogen atoms make hydrogen bonds to the two oxygen atoms of the anion's carboxylate group: HĲ1′)⋯OĲ1): 1.87 Å, HĲ3′)⋯OĲ2): 1.85 Å with angles of 157.3°and 166.9°atHĲ1′) and HĲ3′) respectively.This is a well-known motif between carboxylate and guanidinium cations. 20There is a further hydrogen bond between the anion from carboxylate oxygen atom O(2) and the third guanidinium (N)-H atom from a second cation: HĲ2′)⋯OĲ2): 1.89 Å, with an angle of 162.2°at hydrogen.The anion's conformation is similar to that observed in its group 1A metal salts, and the carboxylate oxygen O(2) lies at 2.6303( 14) Å from the aldehyde carbon atom, and the OĲ2)⋯C(12)OĲ3) angle is 111.17Ĳ8)°(Table 1).Thus, the carboxylate oxygen atom O( 2) is involved in two hydrogen bonds and a O⋯C interaction, and the vectors of these three features lie roughly mutually perpendicular, with the hydrogen bonds lying closer to the axes of the oxygen atoms two sp 2 lone pairs.The OĲ2)⋯C (12) interaction is slightly longer than in the metal salts, due to the greater rotation of the carboxylate anion out of the naphthalene plane presumably to optimise the formation of hydrogen bonds.The distance of the aldehyde hydrogen to the nearer carboxylate oxygen is also longer than in the group 1A salts (2.44 v. 2.31-2.33Å), and its contact to the carboxylate carbon atom is shorter (2.40 v. 2.55-2.57Å). 1 H and 13 C NMR spectra of the salt in acetone suggests that it has converted to a 1 : 1 mixture of triphenylguanidine and naphthalaldehydic acid in solution.
In contrast the crystal structures of the remaining two naphthalaldehydate salts showed a quite different interaction between the peri functional groups, in which the carboxylate group is strongly twisted out of the naphthalene plane and the aldehyde group directs its hydrogen atom in towards the carboxylate group.In the refinements of these two crystal structures, the positions and isotropic displacement parameters of the aldehyde's hydrogen atom and the hydrogens attached to nitrogen in the cation were refined, and not just placed in calculated positions.For the salt with N,N,N′N′tetramethylguanidine measured at 110 K the structures of the anion and cation are shown in Fig. 7 and the hydrogen bonding arrangement and crystal packing are shown in Fig. 8.The crystal system is monoclinic and the space group P2 1 /n.The plane of the aldehyde group lies at 21Ĳ2)°to the best plane of the naphthalene system, with the hydrogen atom directed towards the carboxylate group whose plane lies at 68.71 (14) to  This is © The Royal Society of Chemistry 2016 the naphthalene system.The planes of the two functional groups lie at 58.8Ĳ5)°.There is a very short contact between the groups, from the electron deficient aldehyde hydrogen atom to the carbon of the carboxylate anion, (2.29(2) Å), and this hydrogen atom is at somewhat longer distances from both carboxylate oxygen atoms (OĲ1)⋯HĲ12): 2.54(2), OĲ2)⋯HĲ12): 2.65(2) Å).This interaction may be best viewed as an interaction of the electron deficient aldehyde hydrogen with the face of the carboxylate anion, with the hydrogen atom positioned at similar distances to the carboxylates oxygens which carry more of the net negative charge.The two functional groups are splayed apart both in the aromatic plane and perpendicular to it (Table 2).In the crystal packing arrangement hydrogen bonds connect two cations and two anions together in a "square" motif involving both hydrogens of each cation's amino group and the O(2) atom of the carboxylate group of each anion (H⋯O: 1.92(2) and 1.92(2) Å).The carboxylate group is asymmetric with the longer bond to the doubly hydrogen bonded oxygen atom (1.263(2) vs. 1.239(2)Å).The tetrasubstituted guanidinium cation is twisted to reduce steric interactions between two methyl groups, so that the N1-C15 and N2-C16 bonds make a torsion of 55.2°.The steric pressure is reduced by small pyramidalisations of the two nitrogen atom geometries, where the sums of bond angles are 358°, and the widening of bond angles C15-N1-C13 and C16-N2-C13 to 122.4-122.6°with consequent contraction of the CH 3 -N-CH 3 angles to 114.7-115.2°.
In the anion, the aldehyde and carboxylate groups lie at 45.6Ĳ5)°, a smaller angle than in the tetramethylguanidium salt (60.5Ĳ2)°).The aldehyde hydrogen is directed towards the carboxylate carbon with a H⋯CO 2 − separation of 2.42(2) Å, and distances of 2.76(2) and 2.60(2) Å to the two oxygen atoms.The smaller angle between the functional group planes means that there is also a rather long contact from a carboxylate oxygen to the aldehyde carbon (OĲ2)⋯CHO: 2.920(3) Å) with a OĲ2)⋯CO angle of 119.77Ĳ19)°.The structures of the anion in the salts with tetramethylguanidine and DMAP are compared in Fig. 12 along with those for the potassium and triphenylguanidinium salts.The greater angle between the functional group planes in the tetramethylguanidium salt means that the H⋯CO 2 − contact is shorter (2.29(2) Å), and the O⋯CHO contact longer (3.043(2)Å).There are no similar short contacts between an aldehyde and   the carbon of a carboxylate group reported in the Cambridge Structural Database (2015 release).In the structures of the two reported salts of the 2-formylbenzoate anion the main interaction between the functional groups is an intramolecular H⋯O contact (2.26 and 2.47 Å) between the aldehyde hydrogen and a carboxylate oxygen. 21Furthermore, there are short intramolecular H⋯O contacts between aldehyde groups in benzene-1,2-dicarbaldehyde 16 and naphthalene-2,3-dicarbaldehyde 17 (2.30-2.36Å), but in benzene-1,3dicarbaldehyde the intermolecular H⋯O contacts involving aldehyde groups are longer (2.64-2.66Å). 22 Two similar contacts with an ester carbonyl oxygen (2.43 Å) have been reported. 23The aldehyde group is known to make short contacts from the hydrogen atom to an ether oxygen of ca.2.3 Å, for example in 18-19 (Scheme 4). 24   This journal is © The Royal Society of Chemistry 2016

Conclusion
Two types of interaction between the carboxylate and aldehyde groups have been observed in a peri-naphthalene system.In one mode a negatively charged oxygen impinges on an aldehyde carbonyl for which there are only a few examples, such as in salts of aspirin with organic cations but for which the O⋯C contacts are longer (>2.75 Å). 25 Compared to the peri-interactions between methoxy groups and various electrophilic groups, the use of a negatively charged oxygen as the nucleophile has led to slightly shorter O⋯C interactions down to 2.45 Å while maintaining the angle of approach of the oxygen to the carbonyl group in the 107-112°r ange.This is achieved by in-plane and out of plane splayings of the groups.In the neutral parent acid a covalent O-C bond is formed between the two groups (1.477(2) Å).In the second mode the aldehyde has directed its hydrogen atom at the face of the carboxylate anion so that it makes a particularly short contact with the carbon atom (2.29-2.42Å), for which there is no example in the Cambridge Structural Database.The interactions in four of the salts are compared in Fig. 12.As the carboxylate group rotates further out of the aromatic plane, the O⋯CO contact gets longer, and the H⋯CO 2 − contact gets shorter.These interaction modes will be studied further using cations which make the minimal interaction with the anion, and the interactions investigated in more detail by conducting charge density measurements.

Experimental
General Solution NMR spectra were measured on a Jeol ECLIPSE 400 spectrometer at 400 MHz for 1 H and at 100.6 MHz for 13 C using CDCl 3 as solvent and tetramethylsilane (TMS) as standard unless otherwise stated, and measured in p.p.m. downfield from TMS with coupling constants reported in Hz.IR spectra were recorded on a Perkin Elmer Spectrum 100 FT-IR Spectrometer using Attenuated Total Reflection sampling unless otherwise stated, and are reported in cm −1 .Chemical analysis data were obtained from Mr. Stephen Boyer, London Metropolitan University.

Preparation of the potassium salt of 13
Naphthalaldehydic acid (0.50 g, 2.49 mmol) 13 was added to an aqueous solution of potassium hydroxide (2 ml, 1.25 M, 2.50 mmol) and the mixture was stirred and then left to stand.After 1 day, the solvent was evaporated and dissolved in warm isopropanol and left to cool and evaporate.The crystals that formed were collected, and washed with isopropanol and diethyl ether to give hydrated potassium naphthalaldehydate (0. Reaction of 13 with 4-dimethylaminopyridine 4-Dimethylaminopyridine (0.31 g, 2.53 mmol) was added to a stirred solution of 1,8-naphthalaldehydic acid 13 (0.50 g, 2.49 mmol) in hot methanol (6 ml) and the reaction mixture was stirred for 1 h and left to cool.The volume of methanol was reduced to 1ml by evaporation and ether added and the mixture left to crystallise.The product was filtered off and washed with ether to give a molecular complex between 4-dimethylaminopyridinium naphthalaldehydate and naphthalaldehydic acid, DMAPH.14

X-ray crystallography
Low temperature (100-150 K) X-ray diffraction data (Mo Kα) for compounds 15, K•14, Cs•14, and (PhNH) 3 •14 were collected at the UK National Crystallography Service, 26 University of Southampton on a Rigaku AFC12 diffractometer equipped with enhanced sensitivity (HG) Saturn724+ CCD detector mounted at the window of an FR-E+ SuperBright rotating anode generator (Mo Kα, λ = 0.71075 Å) with VHF Varimax optics (70 μm focus) using Crystal Clear software 27 for data collection and reduction.For compounds Li•14, DMAPH•14•15 and (MeNH) 2 CNH 2 •14 data were measured on an Agilent Xcalibur diffractometer equipped with a Sapphire detector and an 700 series Cryostream low temperature system at Nottingham Trent University using the CrysAlis-Pro software package. 28Structures were solved are refined using the SHELXS and SHELXL suite of programs 29 using the XSEED interface. 30Molecular illustrations were made with Mercury. 31Data are deposited at the Cambridge Crystallographic Data Centre with code numbers CCDC 1435389-1435396.
Crystal data for 15.

Fig. 3
Fig.3(a) The structure of anion 14 in its hydrated potassium salt with associated potassium ions and water molecules; (b) the structure of anion 14 showing the interaction between the two functional groups.

Fig. 4
Fig. 4 (a) The molecular structure of the naphthalaldehydate anion in Li•14, showing the interaction between the two functional groups and the coordinations to the lithium ions; (b) the crystal packing arrangement of Li•14.

Fig. 5
Fig. 5 (a) The interactions between the naphthalaldehydate anion 14 with two triphenylguanidinium cations in (PhNH) 3 C•14; (b) the structure of a single anion of 14 in (PhNH) 3 C•14; (c) the conformation of the cation in the crystal structure of (PhNH) 3 C•14; the cation is refined as two very similar conformations, only one of which is shown, differing slightly in the positions of two of the phenyl groups.

Fig. 6
Fig. 6 Crystal packing arrangement (viewed along the c-axis) for (PhNH) 3 C•14 with the cations shown in blue.The anions are packed in face to face pairs.

Fig. 7
Fig. 7 (a) The structure of the naphthaldehydate anion in (Me 2 N) 2 CNH 2 •14 showing the interaction between the two functional groups; (b) the structure of the cation in (Me 2 N) 2 CNH 2 •14.

Fig. 9
Fig. 9 Molecular structures of the three components of the co-crystal between 4-dimethylaminopyridinium naphthalaldehydate and naphthalaldehydic acid.

Fig. 10
Fig.10Crystal packing of the co-crystal between 4-dimethylpyridinium naphthalaldehydate and naphthalaldehydic acid, viewed down the a axis with the naphthalaldehydate anions shown in magenta.AA′BB′ stacks of the other components lie along the a axis.

Fig. 12
Fig.12Comparison of the interactions between the functional groups in the anion 14 for four of its salts, showing just the dominant interaction.The O⋯C distance (blue) increases down the series from 2.522 to 3.043 Å, and the H⋯CO 2 − (magenta) distance decreases from 2.55 to 2.29 Å down the series.