DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 2973 Theoretical and experimental studies of the protonated terpyridine cation. Ab initio quantum mechanics calculations, and crystal structures of two diVerent ion pairs formed between protonated terpyridine cations and nitratolanthanate(III) anions † Michael G. B. Drew,*a Michael J. Hudson,a Peter B. Iveson,a Mark L. Russell,a Jan-Olov Liljenzin,b Mats Skålberg,b Lena Spjuth b and Charles Madic c a Department of Chemistry, University of Reading, PO Box 224, Whiteknights, Reading, UK RG6 6AD.E-Mail: m.g.b.drew@reading.ac.uk b Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden c Commissariat à l’Energie Atomique, Direction du Cycle du Combustible, B.P. 171, 30207 Bagnols-sur-Cèze, Cedex, France Ab initio quantum mechanics calculations have been carried out on all possible conformations of the terpyridine ligand and its mono- and di-protonated forms.Results show that the lowest energy form of the ligand is when the N]C]C]N torsion angles are trans but that in the protonated forms, the cis arrangement is prevalent being stabilised by intramolecular N]H? ? ? N hydrogen bonds. Results are consistent with the experimental crystal structure data found in the literature and also with two crystal structures reported here in which two diVerent ion pairs formed between protonated terpyridine cations and lanthanate(III) nitrate anions have been prepared and analysed structurally.Compound 1 consists of discrete diprotonated terpyridine cations and hexanitratolanthanate anions, namely 3[H2terpy]212[La(NO3)6]32?3H2O. Water molecules are present as hydrogen bond acceptors in the diprotonated terpyridyl cavities. Each lanthanum atom is 12-co-ordinate and the La]O bond lengths vary between 2.609(11) and 2.765(10) Å. Compound 2 consists of a diprotonated [H2terpy]21 cation together with a [Sm(terpy)(NO3)4]2 anion, and a NO3 2 anion which is present as a hydrogen bond acceptor in the diprotonated terpyridyl cavity.The samarium atom is 11-co-ordinate, the Sm]O bond lengths vary between 2.494(5) and 2.742(5) Å while the Sm]N bond lengths vary between 2.626(4) and 2.650(5) Å. Introduction There is much current interest in the separation of lanthanide( III) and actinide(III) complexes by solvent extraction routes.1 Various oligoamines have been shown to co-extract lanthanides and actinides from nitric acid solutions into an organic phase.Among these are the oligoamine ligands 2,4,6-tris(4-tert-butyl- 2-pyridyl)-1,3,5-triazine 2 ligand L1 and 2,29:69,20-terpyridine L2. It is thought that the species extracted at low levels of acidity are simple metal co-ordination complexes, for which there is plenty of structural evidence.3 It is clear that the lanthanides (and by implication the similarly sized actinides) can fit into the tridentate cavity of the terpyridyl moiety.Thus we have previously reported the structure of Ce(NO3)4L1 (ref. 2) and in addition complexes of L2 with lanthanides are well known [e.g. LnCl3(L2)?xH2O (Ln = La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y)].4 For Ln = La–Nd, x = 5, for Ln = Tb–Lu, x = 4 and for Ln = Sm or Gd there are between four and five water molecules in the co-ordination sphere. Also isolated was a dimeric species in which the Sm is co-ordinated to three N atoms from L2, two bridging and two terminal chloride anions and one water molecule.Although direct co-ordination of the metal ions to the polyamine ligands is thought to be necessary for the selective extraction of actinides over the lanthanides, the need still remains to establish the nature of the species which may be formed at higher acid concentrations, especially those in which the lanthanide or actinide ion exists as an anion such as [Ln(NO3)6]32 or the [LnL(NO3)4]2 ion-pair. It has been demonstrated that terpyridine-type ligands in synergistic combination † Non-SI unit employed: cal = 4.184 J.with 2-bromodecanoic acid show some selectivity for An(III) over Ln(III) ions.5 The species involved in these extractions are not known with any certainty. We have adopted a theoretical and experimental approach to the identification of these species. We are using quantum mechanical methods to investigate the conformational preferences of the ligand and also the likely structures of the protonated forms.In addition we are trying to prepare solid complexes in order to provide evidence for the kind of species which is involved in this type of extraction. In this paper we report the results from our quantum mechanical calculations and also the crystal structures of two diVerent ion pairs of diprotonated terpyridine and anionic lanthanide nitrates (1 and 2) both of which contain free diprotonated terpyridine moieties unco-ordinated to a metal ion.The asymmetric unit of 1 contains three diprotonated terpyridyl cations, two hexanitratolanthanate(III) anions and three water molecules, 3[H2L2]212[La(NO3)6]32?3H2O while the asymmetric unit of 2 contains an [(H2L2)(NO3)]1 cation together with a [SmL2- (NO3)4]2 anion. Experimental The compounds Ln(NO3)3?6H2O (99.99%), Sm(NO3)3?6H2O (99.99%) and 2,29:69,20-terpyridine (L2) (98%) were used as received from Aldrich. 2-Bromodecanoic acid (98%) was purchased from Fluka and used without further purification.Preparation of complex 1 This compound was prepared from [H2L2][NO3]2 which was initially prepared as a solid: L2 (0.25 g, 0.001 mol) was dissolved2974 J. Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 Table 1 Crystal data and structure refinement for compounds 1 and 2 Formula Empirical formula MT /K l/Å Crystal system Space group a/Å b/Å c/Å b/8 U/Å3 Z, Dc/g cm23 m/mm21 F(000) Size/mm q Range/8 hkl Range Reflections observed, Rint Unique reflections Weighting scheme a, b * Reflections, parameters Final indices [I > 2s(I)] R1, wR2 All data R1, wR2 Largest diVerence peak, hole/e Å23 1 3[H2L2]212[La(NO3)6]32?3H2O C45H45La2N21O39 1781.84 293(2) 0.710 73 Monoclinic P21 10.826(9) 30.00(2) 11.140(9) 104.76(1) 3498(5) 2, 1.692 1.316 1776 0.2 × 0.2 × 0.5 2.82 to 25.07 0 < h < 11, 235 < k < 35, 213 < l < 12 8750, 0.0508 8570 0.144, 35.48 8570, 965 0.0582, 0.1779 0.0657, 0.1901 0.928, 21.615 2 [(H2L2)(NO3)]1[Sm(L2)(NO3)4]2 C30H24SmN11O15 928.95 293(2) 0.710 73 Monoclinic P21/n 17.29(2) 10.230(9) 20.31(2) 106.53(1) 3445(6) 4, 1.791 1.796 1852 0.25 × 0.25 × 0.20 1.83 to 25.94 0 < h < 21, 212 < k < 12, 224 < l < 23 9872, 0.0289 5838 0.181, 25.94 5838, 515 0.0352, 0.0971 0.0514, 0.1080 1.116, 20.984 * Weighting scheme w = 1/[2(Fo 2) 1 (aP)2 1 bP], where P = (Fo 2 1 2Fc 2)/3.in methanol (1 ml). The addition of 69% AnalaR nitric acid (0.14 ml, 0.0022 mol) to a stirred solution of the ligand resulted in the formation of a precipitate (yield 82%).6 After the addition of methanol (2 ml), the solid was filtered oV and dried under vacuum over calcium chloride. Then [H2L2][NO3]2 (0.05 g) was stirred for 10 min in acetonitrile (20 ml), heated to reflux and another solution containing La(NO3)3? 6H2O (0.0498 g, 0.0001 mol) dissolved in acetonitrile (15 ml) at ca. 40 8C was added dropwise to the stirred ligand solution. After the addition, the solution was stirred for 10 min and then allowed to stand at ambient temperature.Crystals suitable for X-ray analysis were deposited overnight at room temperature, 3[H2L2]212[La(NO3)6]32?3H2O (Found: C, 30.1; H, 2.7; N, 16.2. Calc. for C45H45La2N21O39: C, 30.3; H, 2.5; N, 16.5%). Preparation of complex 2 A solution of Sm(NO3)3?6H2O (0.077 g, 0.0002 mol) in acetonitrile (5 ml) was heated to around 40 8C and then added dropwise to a stirred solution of L2 (0.051 g, 0.0002 mol) and 2-bromodecanoic acid (0.087 g, 0.000 35 mol) in acetonitrile (5 ml), also at around 40 8C.A small amount of precipitate appeared on mixing and a further four quantities of acetonitrile (5 ml) were added to dissolve the solid. After a few minutes of heating the solution was allowed to cool down slowly in an oil bath and crystals were formed on standing overnight, [(H2L2)- (NO3)]1[Sm(L2)(NO3)4]2 (Found: C, 38.5; H, 2.55; N, 16.3. C30H24N11O15Sm requires C, 38.8; H, 2.6; N, 16.6%).Crystallography Crystal data for complexes 1 and 2 are given in Table 1, together with refinement details. Data for both crystals were collected with Mo-Ka radiation using the MARresearch Image Plate System. The crystals were positioned at 75 mm for 1 and 70 mm for 2 from the Image Plate. 95 Frames were measured at 28 intervals with a counting time of 2 min. Data analysis was carried out with the XDS program.7 The structures were solved using direct methods with the SHELXS program.8 In both structures the non-hydrogen atoms were refined with anisotropic thermal parameters.The locations of the nitrogen atoms in the rings were selected via thermal parameters and confirmed from the structure refinement by comparison with other assignments which gave higher R values and unreasonable thermal parameters. The hydrogen atoms bonded to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached.An empirical absorption correction was made for both structures using the DIFABS program.9 In 2, the two extra protons in the cation were readily observed in a Fourierdi Verence map bonded to N(11) and N(31) and successfully included in the refinement with no constraints. By contrast in 1 these hydrogen atoms bonded to nitrogen (and also those bonded to the water oxygen atoms) could not be located definitively although positive areas of electron density were located in appropriate positions. In order to establish whether the data were of suYcient quality so that the hydrogen atoms bonded to nitrogen should be locatable (and that therefore we should draw the conclusion from their absence that they were not positioned on the nitrogen atom) we looked at the Fourierdi Verence map for the hydrogen atoms bonded to carbon in these cations.In the first 200 peaks, only 12 of the 33 hydrogen atoms could be located, i.e.peaks were within 0.5 Å of calculated positions. We conclude that the data are not of suYcient quality so that we would necessarily expect to find definitive positions for the hydrogen atoms bonded to nitrogen and we therefore conclude that their likely positions are equivalent to those in 2 [bonded to atoms N(11) and N(31)], which is consistent with the formation of hydrogen bonds to the water molecule. Therefore the hydrogens in the cations A, B and C were included in calculated positions bonded to these outer nitrogen atoms.The assignment of absolute structure in 1 was carried out by comparisons of two structures with opposite signs for y coordinates. The structure with the lowest R value was chosen. Both structures were refined on F2 till convergence using SHELXL.10 All calculations were carried out on a Silicon Graphics R4000 Workstation at the University of Reading. Relevant bond lengths in each structure are shown in Table 2. The hydrogen bonds are shown in Table 3.CCDC reference number 186/1074. See http://www.rsc.org/suppdata/dt/1998/2973/ for crystallographic files in .cif format.J. Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 2975 Results and Discussion Structure of 3[H2L2]212[La(NO3)6]32?3H2O 1 The asymmetric unit of 1 contains three diprotonated terpyridyl cations [H2L2]21, two [La(NO3)6]32 anions and three water molecules. Thus the protonated terpyridine ligand is not coordinated to the lanthanate(III) nitrate anion.Instead the two terminal pyridine nitrogen atoms are protonated. Each protonated terpyridyl cation encapsulates a water molecule with which it forms two strong hydrogen bonds. Two parts of the structure are shown in Figs. 1 and 2. Fig. 1 shows the environment of one protonated terpyridyl cation and Fig. 2 of one lanthanate(III) anion. It is noteworthy that the environments of the other cations and anions are almost identical such that there are three cations in the asymmetric unit together with just three water molecules each of which is similarly encapsulated.There are no additional water molecules in the unit cell, indicating that the role of the water molecules in the structure is to stabilise the cations by forming hydrogen bonds and also to stabilise the packing by additional hydrogen bonds to the anions. While it is not impossible to imagine a structure in which the lanthanate nitrate anions are hydrogen bonded directly to the diprotonated cation, clearly the present arrangement in which the interaction between cation and anion is mediated by the water molecules is more favoured.Table 2 Bond lengths (Å) for compounds 1 and 2 Compound 1 La(1)]O(13) La(1)]O(32) La(1)]O(22) La(1)]O(14) La(1)]O(37) La(1)]O(33) La(1)]O(26) La(1)]O(27) La(1)]O(18) La(1)]O(24) La(1)]O(36) La(1)]O(16) 2.609(11) 2.649(11) 2.663(11) 2.665(13) 2.672(12) 2.674(10) 2.683(11) 2.702(13) 2.726(11) 2.734(12) 2.741(12) 2.758(14) La(2)]O(45) La(2)]O(56) La(2)]O(52) La(2)]O(58) La(2)]O(66) La(2)]O(41) La(2)]O(47) La(2)]O(53) La(2)]O(43) La(2)]O(63) La(2)]O(68) La(2)]O(64) 2.635(11) 2.653(12) 2.666(10) 2.669(11) 2.692(11) 2.702(10) 2.706(11) 2.702(9) 2.733(12) 2.725(11) 2.762(12) 2.765(10) Compound 2 Sm(1)]O(72) Sm(1)]O(62) Sm(1)]O(51) Sm(1)]O(53) Sm(1)]O(83) Sm(1)]N(21B) 2.494(5) 2.537(5) 2.535(4) 2.554(4) 2.559(4) 2.626(4) Sm(1)]O(82) Sm(1)]N(11B) Sm(1)]N(31B) Sm(1)]O(74) Sm(1)]O(63) 2.627(5) 2.635(5) 2.650(5) 2.690(5) 2.742(5) Table 3 Hydrogen bond distances (Å) in compounds 1 and 2 Compound 1 O(1A) ? ? ? O(63) O(1A) ? ? ? N(31AIII) O(1A) ? ? ? N(11AIII) O(1B) ? ? ? O(66II) O(1B) ? ? ? N(11B) O(1B) ? ? ? N(31B) O(1B) ? ? ? O(16II) O(1C) ? ? ? O(38II) O(1C) ? ? ? N(31CIV) O(1C) ? ? ? N(11CIV) O(1C) ? ? ? O(58) O(1C) ? ? ? O(47) 2.812(18) 2.814(19) 2.823(20) 2.800(18) 2.861(16) 2.800(16) 3.018(19) 2.799(18) 2.827(18) 2.860(21) 3.035(19) 3.142(18) O(1A) ? ? ? O(16I) N(11A) ? ? ? N(21A) N(21A) ? ? ? N(31A) O(1B) ? ? ? O(17I) O(1B) ? ? ? O(67) N(11B) ? ? ? N(21B) N(21B) ? ? ? N(31B) O(1C) ? ? ? O(36II) O(1C) ? ? ? O(41) N(11C) ? ? ? N(21C) N(21C) ? ? ? N(31C) 3.005(19) 2.691(17) 2.691(19) 2.965(22) 3.213(20) 2.702(18) 2.712(18) 3.210(18) 3.255(19) 2.660(21) 2.651(20) Compound 2 N(11A) ? ? ? O(303) N(11A) ? ? ? O(302) N(21A) ? ? ? O(303) 2.750(14) 3.095(16) 3.101(14) N(31A) ? ? ? O(303) N(21A) ? ? ? N(11A) N(21A) ? ? ? N(31A) 2.767(14) 2.677(13) 2.648(13) Symmetry elements: I 1 1 x, y, z; II 1 2 x, 0.5 1 y, 2z; III x, y, 1 1 z; IV x 2 1, y, z 2 1.While there are literally hundreds of examples of L2 complexed to metals recorded in the Cambridge Crystallographic Database (CCDS), there is only one previously reported example of the [H2L2]21 cation, namely in the salt 2[H2L2]21- [Tb(OH2)8]?7Cl2.11 The nitrogen atoms in the outer pyridine rings are protonated and a chloride anion was located in the diprotonated cavity.This structure containing [H2L2]21 together with those established in the four structures presented in our previous work2 for [HnL1]n1 suggest that the diprotonated terpyridyl species always attracts hydrogen bonding species into the tridentate cavity. In 1 in addition to the above three [H2L2]21?H2O moieties the asymmetric unit contains two [Ln(NO3)6]32 anions. The Ln]O distances in this structure vary between 2.609(11) and 2.765(10) Å. Although there are numerous structures in the CCDS with Ln]O (nitrate) bonds, there are only three previously reported structures with [Ln(NO3)6]32 anions.For these anions and the two in the present work, the average Ln]O distance is 2.67 Å [n = 60, s(n 2 1) = 0.044 Å]. It has been suggested previously that an analysis of the metal co-ordination sphere for this type Fig. 1 The structure of compound 1 showing the environment of one [H2L2]21 cation with a water encapsulated within the cavity and forming N]H? ? ? O hydrogen bonds.In addition, the water molecule forms hydrogen bonds with oxygen atoms from the nitrates in the [La(NO3)6]32 anions. All hydrogen bonds are shown as dotted lines. The environments of the other two [H2L2]21 cations in the asymmetric unit are similar. Lanthanum yellow, oxygen red, carbon green, hydrogen yellow, nitrogen purple. Fig. 2 The structure of compound 1 showing the environment of one [La(NO3)6]32 anion forming hydrogen bonds to the three water molecules which are each encapsulated within [H2L2]21 cations.All hydrogen bonds are shown as dotted lines. The environment of the second [La(NO3)6]32 anion in the unit cell is similar. Lanthanum yellow, oxygen red, carbon green, hydrogen yellow, nitrogen purple.2976 J. Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 of structure can be simplified by considering a bidentate ligand of small ‘bite’ such as nitrate to occupy only one site of a coordination polyhedron rather than two.The new metal coordination sphere would have a lower co-ordination number which is easier to analyse.12 Thus, for [Ln(NO3)6]32 the arrangement of nitrogen atoms around Ln should be close to an octahedral geometry. This is indeed the case for the environment around Ln(2) in which the N]Ln]N angles only vary between 82(1) and 97(1)8 and indeed there are six O]Ln(2)]O angles within 88 of 1808. There are larger deviations, however, around Ln(1), the N]Ln]N angles varying between 71(1) and 117(1)8 and while there are three O]Ln]O angles within 58 of 1808, the other potentially trans angles are ca. 1608. It is not clear why there should be such big geometrical diVerences between the two anions though it seems likely that the geometry could well be aVected by the significant numbers of hydrogen bonds found in the unit cell. Non-bonded nitrate oxygen atoms and bonded nitrate oxygen atoms in both anions are involved to approximately the same extent in hydrogen bonding with the water molecules in the terpyridyl cavity.This has the eVect of weakening the corresponding Ln]O bonds, for example, there are 10 bonds longer than 2.7 Å in the anions; however it is not clear why the geometry of one anion should be so diVerent from that of the other. The hydrogen bond dimensions are shown in Table 3. The distances between each water atom and the two protonated nitrogen atoms with which hydrogen bonds are formed are all between 2.80 and 2.87 Å.In addition, each water molecule forms several hydrogen bonds to the nitrate oxygen atoms, one strong at a distance of ca. 2.80 Å and several weaker (at distances of 3.0–3.3 Å). In Fig. 1 we show the arrangement of one water molecule which forms hydrogen bonds to two diVerent anions. The cations are approximately planar with N]C]C]N torsion angles of 20.1(2), 3.4(2)8 in ligand A, 1.7(1), 20.9(1)8 in ligand B and 20.5(2), 26.4(2)8 in ligand C, respectively. It is diYcult to establish the exact hydrogen bond pattern around each oxygen as there are so many short contacts [four for O(1A), six for O(1B), seven for O(1C)] less than 3.26 Å.It seems likely that the positions of the hydrogen atoms are fluxional and that weak interactions can occur to any of the close nitrate oxygen atoms. In Fig. 2 the environment around one anion which forms hydrogen bonds to all three [H2L2]21?H2O moieties is shown. It is interesting that one of the nitrate oxygen atoms forms hydrogen bonds to two diVerent water molecules.The distances between the protonated N(11) and N(31) atoms and the central unprotonated N(21) atom vary between 2.65(2) and 2.71(2) Å while the corresponding N]H? ? ? N angles are all between 106 and 1088. These angles are similar to those found in the structure of 2,29:69,20-terpyridinium trifluoromethanesulfonate (Fig. 7) which contains a monoprotonated terpyridyl cation with an N]N distance of 2.65 Å and an N]H? ? ? N angle of 1038, dimensions which are considered to be indicative of weak intramolecular hydrogen bonds.13 The corresponding N]N distances in the salt 2[H2L2]21[Tb(OH2)8]31?7Cl2 are also very similar, varying between 2.61 and 2.67 Å.11 Structure of [H2L2]21[SmL2(NO3)4]2[NO3]2 2 The structure of 2 is shown in Fig. 3.The asymmetric unit contains a [Sm(L2)(NO3)4]2 anion and an [H2L2]21 cation with an encapsulated NO3 2 anion. The stoichiometry and indeed the structure of the [Sm(L2)(NO3)4]2 anion is similar to that recently observed in the structure of [La(L2)2(NO3)2]1[La(L2)- (NO3)4].14 In both structures the metal atoms in the anion are 11-co-ordinate.There are nine other structures in the Cambridge Database containing Sm]O (nitrate) bonds. However, the Sm(III) ion is co-ordinated only to oxygen atoms in the surveyed structures but to both oxygen and nitrogen atoms in our structure. Six of these previously reported structures contain a 10-co-ordinate Sm(III) ion.Only one of the reported structures contains Sm(III) in both the anion and the cation and in both ions the metal is 11-co-ordinate.15 The Sm]O bond lengths in our structure vary between 2.494(5) and 2.742(5) Å. The average Sm]O (nitrate) distance from all the surveyed structures is 2.56 Å [n = 56, s(n 2 1) = 0.093 Å]. It is diYcult to explain why there is such a broad range in Sm]O distances in our structure. It is interesting to note that the oxygen atom involved in the longest Sm]O bond is trans to the shortest Sm]N bond which involves the nitrogen atom in the central pyridine ring.The three Sm]N bond lengths are all longer than those observed in the two previously published structures containing L2 with Sm.4 In the first of these structures the Sm(III) is either eight- or nine-co-ordinate depending on the cation. Thus, the formula can be described as Sm(L2)(Cl)(H2O)4,5. In the second structure the Sm(III) is eight-co-ordinate, the metal being co-ordinated to three L2 N atoms, four Cl atoms and one water molecule.The Sm]N bond lengths vary between 2.56 and 2.59 Å in both of these structures, while in 2 the Sm(III) ion is 11-co-ordinate and the bond lengths vary between 2.62 and 2.65 Å. The increased Sm]N distances in 2 are clearly a consequence of the higher co-ordination number compared to the other structures. The terpyridine co-ordinated to the Sm(III) ion is almost planar, the N]C]C]N dihedral angles are 5.6(1) and 3.7(1)8.The terpyridyl cation shows more distortion from planarity with corresponding dihedral angles of 23.6(1) and 11.0(1)8. The nitrate ion in the terpyridyl cavity is hydrogen bonded to the N]H protons through one oxygen only. As with the diprotonated terpyridine in 1, it is also possible to describe the N]H? ? ? N interactions as weak intramolecular hydrogen bonds. Theoretical structural analysis of L2, [HL2]1 and [H2L2]21 The terpyridyl ligand L2. There are three possible conformations for L2 which can be characterised by the N]C]C]N torsion angles as tt (trans,trans), ct (cis,trans) and cc (cis,cis). Fig. 3 The structure of compound 2 showing both the [H2L2]21 cation hydrogen bonded to a nitrate anion together with the [Sm(NO3)4L2] anion. Samarium brown, oxygen red, carbon green, hydrogen yellow, nitrogen purple.J. Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 2977 Table 4 Results from quantum mechanics calculations on terpy, [Hterpy]1 and [H2terpy]21.Geometry optimisation was carried out using the 6-31G** basis set. Energies in au (= 627.509 kcal mol21) Protonated Conformation Compound L2 [HL2]1 [H2L2]21 nitrogen N(11) N(21) N(31) N(11), N(21) N(11), N(31) N(21), N(31) cis,cis 2737.798 (cc) 2738.202 (H-cc1) 2738.213 (H-cc2) a 2738.457 (2H-cc1) 2738.486 (2H-cc2) c cis,trans 2737.807 (ct) 2738.203 (H-ct1) 2738.206 (H-ct2) 2738.189 (H-ct3) 2738.443 (2H-ct1) b 2738.484 (2H-ct2) 2738.443 (2H-ct3) b trans,trans 2737.818 (tt) 2738.195 (H-tt1) 2738.195 (H-tt2) a 2738.443 (2H-tt1) 2738.479 (2H-tt2) c a Structures with N(31) protonated are equivalent to structures with N(11) protonated.b Structures unstable to geometry optimisation. The 2H-tt1 structure was obtained. c Structures with N(21) and N(31) protonated are equivalent to structures with N(21) and N(11) protonated. We have analysed these conformations for L2, [HL2]1 and [H2L2]21 using the GAUSSIAN 94 program.16 Starting models were built using the CERIUS2 software 17 and the three rings were made approximately coplanar but no symmetry was imposed.Structures were then optimised using the 6-31G* basis set.18 Results are summarised in Table 4. For the neutral ligand L2, it was found that the order of energies was tt < ct < cc and this is consistent with the fact that the tt conformation is observed in the crystal structure of L2 19 and also the 49-phenyl,20 4-aniline 21 and 49-NMe2 22 derivatives. A structure with a boron cage in the 49 position viz 49-(closo-ocarboranyl) terpyridine also has this tt conformation.23 This trans conformation of pyridine rings is also found in quaterpyridine 24 and a para substituted sexipyridine.25 The reasons for this order of conformational preference is clear from the geometry of the optimised structures (Fig. 4). In the tt form, the N]C]C]N torsion angles are both 180.08. However in the ct form, while the trans torsion angle at 2176.18 shows that the rings are close to being coplanar, the cis torsion angle at 243.08 shows that the rings are very much twisted away from planarity. This twist is caused by the repulsion between the two ortho hydrogen atoms and possibly also from electron– electron repulsion between the lone pairs on the nitrogen atoms.This pattern of conformational change is also observed in the cc form where the torsion angles are 247.9, 47.98. Clearly, the cis arrangement reduces conjugation between the two rings and is particularly destabilised by repulsions between the ortho hydrogen atoms.These calculations were carried out on the free ligands but it is possible that the conformational preferences may well change in the presence of hydrogen bond donors or acceptors. It is interesting that the tt form is the lowest energy conformation of neutral terpyridyl which is unsuitable for tridentate complexation with a metal atom. However it is possible that this conformation could be stabilised in polar solvents by the formation of intermolecular hydrogen bonds.Thus a water molecule could enter the cavity with the same arrangement as found for the [H2L2]21?H2O cation found in 1 but with donor O]H? ? ? N instead of acceptor O ? ? ?H]N hydrogen bonds [see Fig. 5(a)]. A precedence for this proposed structure is found for N(21) N(31) N(11) N(21) N(31) N(11) N(21) N(31) N(11) cis, cis cis, trans trans, trans a crystal structure of terpyridyl co-crystallised with [SnPh3- (NCS)(H2O)].26 This is the only crystal structure where an oligopyridine contains adjacent pyridine ligands in the cis conformation, but the conformation is stabilised by the formation of two hydrogen bonds to the water molecule which is situated in the terpyridine cavity as well as being bonded to the tin. In order to calculate the eVect of the formation of this L2?H2O complex, a model structure was built using CERIUS2 [Fig. 5(a)] and subjected to optimisation in GAUSSIAN 94.Results show that the energy of the complex was 11.29 kcal mol21, lower than that of the L2 and H2O separated at infinity. Fig. 4 The three conformations of L2, cis,cis (cc), cis,trans (ct) and trans,trans (tt). Energies after geometry optimisation (au) and torsion angles (8) were for cc 2737.798, 247.9, 47.9; for ct 2737.807, 243.0, 2176.1 and for tt 2737.818, 180, 180.2978 J. Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 Water adducts built with the ct and tt structures proved not to be stable to geometry optimisation, no doubt because of the repulsions from adjacent C]H groups.In the cc L?H2O complex, the structure maintains Cs symmetry, though this was not imposed. The N]C]C]N torsion angles at 42.0, 242.08 are only slightly reduced (by 5.98) from values in the unhydrated form. Fig. 5 The stabilisation of the cis,cis conformation by intramolecular hydrogen bonds with water molecules. Geometry optimisation via GAUSSIAN 94.(a) The cc form accepting hydrogen bonds from a water molecule; N]C]C]N torsion angles are 242.0, 41.98. (b) The 2Hcc2 form donating hydrogen bonds to a water molecule; N]C]C]N torsion angles are 218.7, 18.78. Hydrogen bonds shown as dotted lines. Carbon green, hydrogen yellow, nitrogen purple, oxygen red. The N ? ? ? H and O ? ? ? H distances are 2.2, 3.15 Å respectively with an O]H? ? ? N angle of 177.38. This contrasts with torsion angles of 29.0, 218.08 and N ? ? ? O distances of 2.80, 2.76 Å in ref. 26. Monoprotonated [HL2]1. When L2 is monoprotonated, then there are seven possible structures (Fig. 6) depending on the conformation and which nitrogen atom is protonated. There are two cis,cis structures; H-cc1, H-cc2 depending on whether N(11) or N(21) is protonated [N(11) is equivalent to N(31)]; three cis,trans structures H-ct1, H-ct2, H-ct3 dependent upon whether N(11), N(21) or N(31) is protonated; and two trans, trans structures H-tt1, H-tt2 dependent upon whether N(11) or N(21) is protonated.These seven structures were built with CERIUS2 and geometry optimised with GAUSSIAN 94 and the resulting structures are shown in Fig. 6. As with terpyridyl L2, the energies of [HL2]1 can be correlated with two dominant structural features, one favourable and one unfavourable. The lowest energy structures contain at least one pair of mutually cis nitrogen atoms of which one is protonated so that an energetically favourable intramolecular hydrogen bond interaction can be formed.Thus H-cc2 has the lowest energy with two such interactions, next lowest are H-cc1, H-ct1 and H-ct2 with one such interaction and the three highest energies are H-ct3, H-tt1 and H-tt2 with no such interactions. The unfavourable interaction occurs when the N]H is cis to one or two C]H bonds leading to H ? ? ? H repulsion. Two of these interactions are found in H-tt2, and one in H-tt1, H-ct3 and H-ct2.It is interesting that this latter H-ct2 structure also has a favourable hydrogen bond interaction which more than compensates for this repulsion as the energy is one of the lowest found. Unfavourable ortho C]H interactions are also found in several of the structures. The geometry in the structures (Fig. 6) follow a regular logical pattern. For the cis N]C]C]N torsions, the angle is close to zero when a hydrogen bond is formed (e.g. 5.1 in H-cc1, 20.1 in H-cc2, 6.8 in H-ct1, 20.38 in H-ct2) but otherwise is twisted significantly to relieve steric strain (e.g. 228.6 in H-cc1, 237.28 in H-ct3). For the trans N]C]C]N torsions, the angle is only close to 1808 in H-tt1 where there are no ortho repulsions but in all the other structures the absolute value of the angle ranges from 148.8 to 155.88. There are other geometric changes concomitant with the formation of the intramolecular N]H? ? ? N hydrogen bonds. Thus in H-ct2 the Fig. 6 The seven structures of [HL2]1 which are characterised by the conformation and the nitrogen that is protonated.Energies after geometry optimisation (au) and torsion angles (8) were for H-cc1 2738.202, 228.6, 5.1; for H-cc2 2738.215, 20.1, 20.1; for H-ct1 2738.203, 6.8, 2163.8; for H-ct2 2738.206, 20.3, 155.8; for H-ct3 2738.189, 237.2, 2154.8; for H-tt1 2738.195, 2174.0, 2148.8 and for H-tt2 2738.195, 2150.0, 2150.0. Hydrogen bonds shown as dotted lines. Carbon green, hydrogen yellow, nitrogen purple.J.Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 2979 two C]C]N angles to the central pyridine atom are 115.18 to ring 1 to facilitate the hydrogen bond and 119.18 to ring 3 where there is no such hydrogen bond. The H ? ? ? N distances are 2.07 Å in H-cc1, but 2.16 Å in H-ct2 and 2.12 Å in H-ct1, 2.10 Å in H-ct2. The N]H? ? ? N angles range from 105 to 1088, rather small compared to the usual angle for a hydrogen bond, but the calculations presented here clearly indicate that the hydrogen bonds are significant. There is a significant change in the N]C bond lengths when the nitrogen is protonated and increases of ca. 0.01 Å are observed. Thus in H-cc2, the C]N distances are 1.338 Å compared to 1.326 and 1.315 Å where the N is unprotonated. There is very little experimental evidence on [HL2]1 that can be correlated with these calculations. There is only one crystal structure containing [HL2]1, viz the salt with [CF3SO3]213 and this has the H-ct1 structure.However the structure of diproton- Fig. 7 The structure of [HL2]1[CF3SO3]2 showing the intermolecular hydrogen bond between N]H and an oxygen atom in the anion.13 Sulfur brown, oxygen red, carbon green, fluorine light green, nitrogen purple, hydrogen yellow. ated quinquepyridinium which has the cis,trans,trans,cis conformations shows the outer two nitrogens to be protonated so that the structure contains two moieties equivalent to the H-ct1 structure.27 As is apparent from the energy values in Table 4, this H-ct1 structure is not the lowest energy structure, but unlike the more favourable H-cc2 and H-ct2 structures, it retains the possibility of being able to form intermolecular hydrogen bonds.Indeed in the two crystal structures, this is precisely what is found, that the protonated nitrogen atom forms an intramolecular hydrogen bond but also an intermolecular hydrogen bond to the anion in the crystal. The anion is situated well away from the cavity (Fig. 7) and calculations show that this anion (or indeed any other) could not form stable hydrogen bonds to the protonated central nitrogen atom as occurs in H-cc2 and H-ct2. We conclude, therefore, that in the presence of solvents and/or anions which can accept hydrogen bonds that the H-ct1 structure is more likely to be found in preference to H-cc2 and H-ct2 despite these having lower energies in the gas phase. Diprotonated [H2L2]21. For [H2L2]21 there are also seven different structures.There are two with the cis,cis conformation called 2H-cc1 [with N(11) and N(21) protonated], and 2H-cc2 [with N(11) and N(31) protonated]; three with the cis,trans conformation; 2H-ct1 [N(11), N(21) protonated], 2H-ct2 [N(11), N(31) protonated] and 2H-ct3 [N(21), N(31) protonated] and two with the trans,trans conformation; 2H-tt1 [N(11), N(21) protonated] and 2H-tt2 [N(11), N(31) protonated]. Models were built with CERIUS2 and then geometry optimised with GAUSSIAN 94 and the results are given in Table 4 and illustrated in Fig. 8. The same two structural features are crucial to determining the relative energy values, thus lower energy structures contain intramolecular hydrogen bonds and higher energy structures N]H? ? ?C]H repulsions. There are more of these latter types of repulsion in the diprotonated structures than in the monoprotonated structures and there is also the possibility of N]H? ? ?N]H repulsions. The C]H? ? ? C]H repulsions are present in all structures but seem to be less important, possibly because the electrostatic repulsions between hydrogen atoms is less.Because of these ortho–ortho repulsions only three structures contain intramolecular hydrogen bonds 2H-cc2, 2H-ct2 and 2H-cc1 and these together with 2H-tt2 have the lowest energies. The lowest energy conformation (2H-cc2) is also observed in the two crystal structures reported above. In this structure the Fig. 8 The seven structures of [H2L2]21 which are characterised by the conformation and the nitrogen that is protonated.Energies after geometry optimisation (au) and torsion angles (8) were for 2H-cc1 2738.456, 0.6, 58.7; for 2H-cc2 2738.486, 228.3, 228.3; for 2H-ct1 unstable; 2H-ct2 2738.484, 12.8, 2139.1; for 2H-ct3 unstable; for 2H-tt1 2738.444, 2130.5, 2155.1 and for H-tt2 2738.479, 2145.0, 2145.0. Hydrogen bonds shown as dotted lines. Carbon green, hydrogen yellow, nitrogen purple.2980 J.Chem. Soc., Dalton Trans., 1998, Pages 2973–2980 N]C]C]N torsion angles are 228.38 and the N]H? ? ? N distance is 2.43 Å. This compares with torsion angles of 0.4, 6.0; 23.3, 0.5; 21.9, 0.18 in 1 and 10.8, 23.78 in 2 and distances of 2.28–2.34 Å in the two structures (though it must be borne in mind that these are calculated hydrogen positions from the crystal structure determinations) and indeed to the 2.16 Å observed in the 1H-cc1 structure. The distance in 2H-ct2 is also large by comparison at 2.37 Å.It seems likely that the ligand in the crystal structures is more planar because of the intermolecular hydrogen bonds formed to the nitrate. In order to test this proposition, we carried out a GAUSSIAN 94 geometry optimisation of a water molecule contained within the cavity of the 2H-cc2 structure [Fig. 5(b)]. On optimisation the two torsion angles had decreased to 18.7 and 218.78 and the two H ? ? ?N distances reduced to 2.13, 2.13 Å. Results show that the energy of the complex was 18.72 kcal mol21, lower than that of [H2L2]21 and H2O separated at infinity.We conclude that in the case of the diprotonated [H2L2]21 the gas phase preference for the 2H-cc2 structure is enhanced in solution and that this is likely to be the only diprotonated species present. The conformation found in the 2H-cc1 structure is particularly interesting as it contains a N]H? ? ?N]H repulsion which is alleviated with a torsion angle of 58.78 while the second cis interaction contains a N]H? ? ? N attraction and the torsion angle remains close to zero at 0.68.Of the other structures 2Hct1 and 2H-ct3 proved not to be stable to geometry optimisation and reverted to the 2H-tt1 structure. The 2H-tt2 structure contains no hydrogen bonds but has a relatively low energy because there are no significant ortho repulsions. The 2H-tt1 structure has the highest energy of all because of significant repulsions between N]H and C]H in the cavity.Conclusion Theoretical calculations have shown that the lowest energy conformation of the terpyridyl ligand is the trans,trans form. However cis forms are stabilised on protonation by the formation of weak intramolecular N]H? ? ? N hydrogen bonds. It is likely that the lowest energy forms in solution for the protonated and diprotonated forms are the H-ct1 and 2H-cc2. These results are consistent with evidence from crystal structure determinations presented here and from the literature.It is suggested that any theory of extraction based on the ion-pair mechanism must be consistent with the presence of these cations both of which will be stabilised by the formation of intermolecular hydrogen bonds to solvent and/or accompanying anions. Acknowledgements We are grateful for the financial support by the EC Research Programme Contract NEWPART (FI41-CT-96-0010) and the Swedish Nuclear and Waste Management Co., SKB. We would also like to thank the EPSRC and the University of Reading for funding of the image-plate system.The use of the Origin 2000 at the High Performance Computer Centre at the University of Reading is gratefully acknowledged. References 1 K. L. Nash, Solvent Extraction and Ion Exchange, 1993, 11, 729. 2 G. Y. S. 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