Marcia L. Scudder

SUPRAMOLECULAR MOTIFS : CONCERTED MULTIPLE PHENYL EMBRACES BETWEEN PH4P+ CATIONS ARE ATTRACTIVE AND UBIQUITOUS

Examination of the Cambridge Structural Database reveals that Ph4P+ cations in crystals associate through phenyl-phenyl nonbonded interactions which are attractive, concerted, and widespread. Intermolecular phenyl-phenyl conformations, which are offset-face-to-face (off), edge-to-face (ef) or vertex-to face (vf), combine in five classes of supramolecular motifs for {Ph4P+}2 pairs, namely the sextuple phenyl embrace (SPE) with (ef)6 and offset sextuple phenyl embrace (OSPE) containing (off)1 (ef)2(ef/vf)2, the translational quadruple phenyl embrace (TQPE) with (ef)4, the parallel quadruple phenyl embrace (PQPE) with (off)1(vf)2, and the double phenyl embrace (DPE) with (off)1. Typical intermolecular attractive energies (kJ per mol of {Ph4P+}2) for these motifs are SPE 85, OSPE 57, TQPE 70, PQPE 41, DPE 34. There is strong interpenetration of the cations in these motifs: 489/770 structures in the CSD have P P≤7 A (spherical Ph4P+ has a van der Waals diameter of 13.6 A). Of the 812 instances of P P ≤7 A, 86% are SPE, 10% are OSPE, 2% are TQPE, and only 2% are unclassified, Average P P separations in the PQPE and DPE are 8.3 A. Centrosymmetry is prevalent in all except the TQPE, which has implications for the engineering of noncentric crystals.

Crystal supramolecular motifs: two-dimensional grids of terpy embraces in [ML2]z complexes (L=terpy or aromatic N3-tridentate ligand)

By analysis of crystal packing we have identified a crystal supramolecular motif that is a two-dimensional net of terpy embraces formed by metal complexes [M(terpy)2]2+ (terpy=2,2′:6′,2″-terpyridyl) and similar meridional [M(N3-tridentate)2] complexes. The terpy embrace involves two complexes attracted by one offset-face-to-face (off) and two edge-to-face (ef) interactions by the outer pyridyl rings of the ligand. In many crystals containing small monoanions there is a two-dimensional net of these embraces, in which each complex forms eight ef and four off interactions with its neighbours. The principal axes of the complexes are normal to the layer, which is exactly or approximately planar, and can occur with high (tetragonal) or low crystal symmetry. Grooves that occur on the layer surfaces, formed between parallel central pyridyl rings of the ligands, run in orthogonal directions on the two surfaces of each layer. Anions and solvent molecules in the crystals are usually disordered, in or near the grooves. The net attractive energy of the terpy embrace for a pair of [M(terpy)2]2+ is calculated to be ca. 15 kJ mol-1: in the two-dimensional net the attractive cation···cation energy per cation is ca. 29 kJ mol-1. Inclusion of the anions associated with one layer increases the attractive energy per [M(terpy)2]2+ to the order of 130 kJ mol-1. A variety of ligands, which are minor or major modifications of terpy, also form this supramolecular motif. Hydrogen bonding involving NH functions of these ligands, solvent, and/or anions, does not in general disrupt the motif. In one instance where the [M(N3-tridentate)2] complex is uncharged there is mutual interpenetration of contiguous layers. These infinite two-dimensional nets of octahedral metal complex sites formed as crystal supramolecules are analogous to the two-dimensional gridlike supermolecules formed by extended oligo-chelating ligands. Opportunities for crystal engineering are discussed.

Molecules embracing in crystals

We review the main types of intermolecular embrace motifs adopted by molecules with arylated surfaces, and draw attention to some recent interesting developments.

Electronic and Structural Properties of the Spin Crossover Systems Bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) Thiocyanate and Selenocyanate

Essentially high-spin [Fe(bpp)2][NCS]2·2H2O and [Fe(bpp)2][NCSe]2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) were isolated from an aqueous reaction mixture. Both salts undergo an abrupt transition to low spin below room temperature, that for the thiocyanate occurring in two steps and the high-spin AE low-spin AE high-spin cycle being accompanied by hysteresis in both steps. Recrystallization of the salts from nitromethane yielded a mixture from which bright yellow crystals were separated for structure determination. In addition, from the recrystallized selenocyanate, deep red-brown crystals of composition [Fe(bpp)2][NCSe]2·H2O·0.25 CH3NO2 were obtained. Recrystallized [Fe(bpp)2][NCS]2·2H2O and [Fe(bpp)2][NCSe]2 were identified as high spin with average Fe–N distances of 2.16 and 2.17 A, respectively. In the unit cell of [Fe(bpp)2][NCSe]2·H2O·0.25 CH3NO2, there are four independent iron atoms, three identified as low spin and the fourth as high spin. All salts crystallize in a layer-type array involving edge-to-face and face-to-face aryl–aryl-type interactions. Hydrogen bonding between pyrazole >NH groups, anions and solvate molecules is observed. The structure of the uncoordinated ligand was also determined, the molecule being found in a planar arrangement with thecis–cis configuration for the pyrazolyl groups relative to the central pyridyl and the >NH group being at the N 2 atom. Hydrogen bonding involving the >NH groups leads to stepped stacks of molecules. The principal difference in the geometry of coordinated and free bpp molecules is a contraction in the angles about the interannular bridges in the chelate rings. [Fe(bpp)2][NCS]2·2H2O: triclinic, space group P1–, a 8.302(6), b 8.446(6), c 21.531(13) A, a 78.78(5), b 82.80(5), g 89.85(4)˚, Z 2. [Fe(bpp)2][NCSe]2: triclinic, space group P1–, a 8.354(4), b 8.409(4), c 19.918(9) A, a 87.02(3), b 83.15(3), g 88.86(3)˚, Z 2. [Fe(bpp)2][NCSe]2·H2O·0.25 CH3NO2: monoclinic, space group Pn, a 16.425(12), b 20.774(9), c 16.933(14) A, b 90.91(4)˚, Z 8. Uncoordinated bpp: orthorhombic, space group Pna21, a 8.075(3), b 22.479(9), c 5.525(1) A, b Z 4.

Contrasting crystal supramolecularity for [Fe(phen)3]I8 and [Mn(phen)3]I8: complementary orthogonality and complementary helicity

The crystallisation, crystal structures, and analyses of crystal supramolecularity for [Fe(phen)3]I8 and [Mn(phen)3]I8 are reported. Crystalline [Fe(phen)3]I8 contains the first example of one-dimensionally extended sixfold aryl embraces (6AE) between [M(phen)3]2+ complexes. The interchain domains contain twisted I82– ions, which are translated along the axis of the domain to form polyiodide helices. Sections of the polyiodide helix slot into the grooves between the phen ligands in each [Fe(phen)3]2+ complex: these grooves are canted in helical fashion in each complex, even though the extended 6AE chain is not helical, and there is good local registry and geometrical complementarity between the polyiodide helices and the cation chains. The polyiodide helices are buttressed further by concerted C–H⋯I interactions. The crystal approaches trigonal symmetry. In contrast, crystalline [Mn(phen)3] (I3)(I5) contains neither 6AE nor the parallel fourfold aryl embraces (P4AE) characteristic of [M(phen)3]z complexes, but does display a relatively rare instance of a homo-chiral P4AE. The association of [Mn(phen)3]2+ complexes and the I3– and I5– ions is effective and close, and manifests well the complementary orthogonality of [M(phen)3]z complexes and polyiodides. There are two formula units in the asymmetric unit of [Mn(phen)3](I3)(I5), and the high-spin [Mn(phen)3]2+ cations are distorted from the octahedral stereochemistry of low-spin [Fe(phen)3]2+. The issues of whether these two structures could be regarded as substitutional polymorphs, and which is the more stable, and whether each could crystallise with the structure of the other, are discussed.

Analysis of pi–halogen dimer interactions present in a family of staircase inclusion compounds

The family of staircase inclusion compounds formed by the tetrahalo aryl hosts 1 and 2 are re-evaluated in terms of the pi–halogen dimer interactions present in their structures.

Pi–halogen dimer interactions and the inclusion chemistry of a new tetrahalo aryl host

The preparation of 1,4,8,11-tetrabromo-5b[small alpha],6,12b[small alpha],13-tetrahydropentaleno[1,2-b:4,5-b[prime or minute]]diquinoline is described. This is a further member of the tetrahalo aryl host family, and forms crystalline lattice inclusion compounds with many guests. The X-ray structures of the allyl cyanide, 1,2,3-trichloropropane, chlorobenzene, toluene, benzene-water, methyl chloroform and carbon tetrachloride inclusion compounds are described, and compared with that of the solvent-free apohost. Although several different structural types are produced, the recently reported pi-halogen dimer (PHD) interaction plays an important role in all of these, except for that of pure (where the packing energy is the least favourable of the series).

Hydrogen bonding influences on the properties of heavily hydrated chloride salts of iron(II) and ruthenium(II) complexes of 2,6-bis(pyrazol-3-yl)pyridine, 2,6-bis(1,2,4-triazol-3-yl)pyridine and 2,2′∶6′,2″-terpyridine

The structures of a series of hydrated complex chlorides [Fe(btp)2]Cl2·6H2O, [M(bpp)2]Cl2·6.5H2O (M = Fe, Ru) and [Ru(terpy)2]Cl2·6H2O are described (btp = 2,6-bis(1,2,4-triazol-3-yl)pyridine; bpp = 2,6-bis(pyrazol-3-yl)pyridine; terpy = 2,2′∶6′,2″-terpyridine). The ligands are all of the terimine type. [Fe(btp)2]Cl2·6H2O is high spin while [Fe(bpp)2]Cl2·6.5H2O is low spin, the difference in the average Fe–N distance in the two complexes being 0.24 A. In all four complex salts there is extensive hydrogen bonding between the water and chloride ions. In addition, for the pyrazolyl and triazolyl ligands this involves the uncoordinated >NH groups. The arrangement of the cations in the four lattices is discussed in terms of the previously described “terpy embrace” adopted widely by bis(terimine)metal systems.

Crystal supramolecularity: elaborate six-, eight- and twelve-fold concerted phenyl embraces in compounds [M(PPh3)3]z and [M(PPh3)4]z

Multiple phenyl embraces are concerted supramolecular motifs based on intermolecular phenyl–phenyl attractions, mainly in the edge-to-face (ef) geometry. Variations of the six-fold phenyl embrace (6PE) and higher order multiple phenyl embraces involving eight or twelve phenyl groups are described. They occur in crystals of metal complexes with three or four Ph3 ligands. The enlarged 6PE (E6PE) involves phenyl groups from three Ph3X ligands in each of two molecules, and is formulated generally as (XPh)3···(PhX)3: the H6PE, or (XPh)3···(Ph3X), is a hybrid of the E6PE and the 6PE [which is (XPh3)···(Ph3X)]. The 8PE uses {M(XPh2)2}···{(Ph2X)2M} and occurs in two variants according to whether the XMX planes on the two molecules are approximately parallel (P8PE) or orthogonal (O8PE). Molecules [M(PPh3)3]z can form a 12PE, which uses {M(XPh2)3}···{(Ph2X)3M} and includes ten intermolecular ef interactions. The crystal packing in various compounds containing [M(PPh3)3]z or [M(PPh3)4]z is described, and involves either multiple instances of one embrace type in high symmetry crystal lattices or combinations of various multiple phenyl embraces in lower symmetry crystal lattices. The current hierarchy of multiple phenyl embraces is summarised.

Syntheses and X-ray structures of molecular metal polyselenide complexes [M(Se4)2]2− M = Zn, Cd, Hg, Ni, Pb

Abstract Bis-tetraselenide complexes [M(Se4)2]2− M = Zn (1), Cd (2), Hg (3), Ni (4) and Pb (5) are readily prepared by the reaction of Se, Na and MCl2 in DMF, and crystallized with Ph4P+. Crystal structure determinations reveal pseudo-tetrahedral coordination in 1, 2 and 3, slightly distorted square-planar coordination in 4, and in 5 a four coordinate stereochemistry interpreted as primary trigonal orthogonal, plus one secondary bond. There is no evidence of variable size in the metallapolyselenane chelate rings, in contrast with data for comparable polysulphide complexes.