Marcus R. Bond

The square-planar to flattened-tetrahedral CuX42− (X = Cl, Br) structural phase transition in 1,2,6-trimethylpyridinium salts

The abrupt green-to-yellow thermochromism observed for certain A2CuCl4 compounds has long been attributed to the presence of strong N-H hydrogen bonding in the low-temperature phase that favors square-planar CuCl4(2-) (green in color) through reduction of ligand-ligand repulsion. Weakening of the hydrogen bonding at higher temperature results in the transformation to (far more common) flattened-tetrahedral geometry - the expected geometry for CuBr4(2-) complexes due to their greater ligand-ligand repulsion. The square-planar to flattened-tetrahedral transitions in (1,2,6-trimethylpyridinium)2CuX4 provide the first examples of this phase transformation in the absence of N-H hydrogen bonding and for a CuBr4(2-) complex. These results suggest that the square-planar to flattened-tetrahedral transformation in CuX4(2-) systems may be more common than previously thought.

Nine compounds containing high-nuclearity [CunX2n+2]2− (n = 4, 5 or 7; X = Cl or Br) quasi-planar oligomers

The structures of seven A(2)Cu(4)X(10) compounds containing quasi-planar oligomers are reported: bis(1,2,4-trimethylpyridinium) hexa-μ-chlorido-tetrachloridotetracuprate(II), (C(8)H(12)N)(2)[Cu(4)Cl(10)], (I), and the hexa-μ-bromido-tetrabromidotetracuprate(II) salts of 1,2,4-trimethylpyridinium, (C(8)H(12)N)(2)[Cu(4)Br(10)], (II), 3,4-dimethylpyridinium, (C(7)H(10)N)(2)[Cu(4)Br(10)], (III), 2,3-dimethylpyridinium, (C(7)H(10)N)(2)[Cu(4)Br(10)], (IV), 1-methylpyridinium, (C(6)H(8)N)(2)[Cu(4)Br(10)], (V), trimethylphenylammonium, (C(9)H(14)N)(2)[Cu(4)Br(10)], (VI), and 2,4-dimethylpyridinium, (C(7)H(10)N)(2)[Cu(4)Br(10)], (VII). The first four are isomorphous and contain stacks of tetracopper oligomers aggregated through semicoordinate Cu···X bond formation in a 4(5/2,1/2) stacking pattern. The 1-methylpyridinium salt also contains oligomers stacked in a 4(5/2,1/2) pattern, but is isomorphous with the known chloride analog instead. The trimethylphenylammonium salt contains stacks of oligomers arranged in a 4(3/2,1/2) stacking pattern similar to the tetramethylphosphonium analog. These six structures feature inversion-related organic cation pairs and hybrid oligomer/organic cation layers derived from the parent CuX(2) structure. The 2,4-dimethylpyridinium salt is isomorphous with the known (2-amino-4-methylpyridinium)(2)Cu(4)Cl(10) structure, in which isolated stacks of organic cations and of oligomers in a 4(1/2,1/2) pattern are found. In bis(3-chloro-1-methylpyridinium) octa-μ-bromido-tetrabromidopentacuprate(II), (C(6)H(7)ClN)[Cu(5)Br(12)], (VIII), containing the first reported fully halogenated quasi-planar pentacopper oligomer, the oligomers stack in a 5(3/2,1/2) stacking pattern as the highest nuclearity [Cu(n)X(2n+2)](2-) oligomer compound known with isolated stacking. Bis(2-chloro-1-methylpyridinium) dodeca-μ-bromido-tetrabromidoheptacuprate(II), (C(6)H(7)ClN)(2)[Cu(7)Br(16)], (IX), contains the second heptacopper oligomer reported and consists of layers of interleaved oligomer stacks with a 7[(7/2,1/2)][(-9/2,-1/2)] pattern isomorphous with that of the known 1,2-dimethylpyridinium analog. All the oligomers reported here are inversion symmetric.

Octakis(dimethylammonium) hexa-μ2-chlorido-hexachloridotrinickelate(II) dichloride: a linear trinickel complex with asymmetric bridging

The title compound, (C2H8N)8[Ni3Cl12]Cl2, crystallizes as linear [Ni3Cl12]6- complex anions with inversion symmetry, separated from one another by dimethylammonium cations and noncoordinated chloride ions. The gross structural arrangement of the trinickel complex is as a segment of face-sharing NiCl6 octahedra similar to the (NiCl3)n chains of CsNiCl3-type compounds. On closer inspection, the regular coordination geometry of the complex consists of octahedral NiCl6 in the center linked by two symmetrically bridging chloride ions to square-pyramidal NiCl5 on each end. A long semicoordinate bond is formed by each of the terminal Ni(II) cations, to give a 5+1 coordination geometry and form an asymmetric bridge to the central Ni(II) cation. The dimethylammonium cations surround the complex with an extensive hydrogen-bonding network, linking the complex to the noncoordinated chloride ions. Asymmetric bridging in the complex arises from short hydrogen bonds from the same dimethylammonium cation to the apical and asymmetric bridging chloride ions, causing the complex to scissor outward.

[CuCl3(H2O)](-) complexes aggregated to form hydrate columns in methyl-substituted pyridinium or piperidinium salts.

1,2,3-Trimethylpyridinium aquatrichloridocuprate(II), (C(8)H(12)N)[CuCl(3)(H(2)O)], (I), 3,4-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (II), and 2,3-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (III), exhibit the same fundamental structure, with (I) and (II) isomorphous and with the unit-cell constants of (III) similar to the reduced unit-cell constants of (I) and (II). The distorted square-planar [CuCl(3)(H(2)O)](-) complex [mirror symmetric in (I) and (II)] forms two semicoordinate Cu···Cl bonds to a neighboring complex to produce a dimer with 2/m symmetry [only inversion symmetry in (III)]. The semicoordinate Cu...Cl bond length of the dimer shows significant elongation at 295 K compared with that at 100 K, while the coordinate Cu-Cl bond lengths are slightly contracted at 295 K compared with those at 100 K. The inorganic dimers are linked by eight hydrogen bonds to four neighboring dimers to establish a checkerboard network layer in the ab plane, with voids between the dimers that accommodate, on both sides, inversion-related organic cation pairs. The organic cations are required by mirror-plane symmetry to be disordered in (I) and (II). The organic cations and [CuCl(3)(H(2)O)](-) complexes are nearly coplanar and tilted out of the layer plane to establish a hybrid organic-inorganic layer structure parallel to (202) [(11-2) in (III)], with hydrate columns (defined by water molecules) and hydrophobic columns (defined by methyl groups) parallel to each other [and along the 2(1) axes in (I) and (II)]. In 1,1-dimethylpiperidinium aquatrichloridocuprate(II), (C(7)H(16)N)[CuCl(3)(H(2)O)], (IV), the bulkier organic cation prevents semicoordinate bonding between complexes, which are hydrogen bonded side-to-side in zigzag chains that place water molecules in columns along half of the 2(1) axes.

catena-Poly[bis(trimethylphenylammonium) [hexa-mu-chlorido-dichloridotricuprate(II)]]: an alternating zigzag chain of CuCl(4) and Cu(2)Cl(6) complexes.

The title compound, {(C(9)H(14)N)(2)[Cu(3)Cl(8)]}(n), consists of parallel chains of alternating quasiplanar Cu(2)Cl(6) and planar CuCl(4) complexes separated by trimethylphenylammonium cations. Both inorganic complexes possess inversion symmetry. Pairs of neighboring chloride ions of the CuCl(4) complex each form a symmetric bridge and an asymmetric bridge to Cu(2)Cl(6) complexes on either side. The Cu(2)Cl(6) complex contains two symmetric chloride bridges between the copper cations with a terminal chloride bound to each five-coordinated Cu(II) ion. The CuCl(4) complex completes its coordination environment by forming two long semicoordinate contacts to the bridging chloride ions of neighboring Cu(2)Cl(6) complexes. The use of the bridging rather than the terminal chloride ions to form semicoordinate contacts generates a new zigzag chain structure that differs from the straight chain structures found for other A(2)Cu(3)Cl(8) compounds. The zigzag chain structure is adopted so as to conform to the shorter repeat distance dictated by stacking of the organic cations.

1,2-Diiodo-4,5-dimethyl­benzene

The structure of the title compound, C8H8I2, conforms closely to the mm2 symmetry expected for the free molecule and is the first reported structure of a diiododimethylbenzene. Repulsion by neighboring I atoms and the neighboring methyl groups opposite to them results in a slight elongation of the molecule along the approximate twofold rotation axis that bisects the ring between the two I atoms. In the extended structure, the molecules form inversion-related pairs which are organized in approximately hexagonal close-packed layers and the layers then stacked so that molecules in neighboring layers abut head-to-tail in a manner that optimizes dipole–dipole interactions.

(1E)-2-(Di­acetyl­amino)-1-methyl­prop-­1-enyl acetate

The analysis of the title compound, C10H15NO4, firmly establishes the configuration of the double bond as E, a stereochemistry that had been assigned tentatively by other methods. The diacetylamine and acetate substituents are approximately coplanar to one another, but approximately perpendicular to the planar ethene core. H atoms of the ethene methyl substituents are found within the ethene plane, indicating that hyperconjugation does not play an important role in stabilizing the double bond.

Bis(1-methyl­piperidinium) tetra­chlorido­cuprate(II)

The structure of the title compound, (C6H14N)2[CuCl4], consists of two inequivalent 1-methylpiperidinium cations and a flattened tetrahedral [CuCl4]2− anion. Each organic cation exhibits a chair conformation with the methyl group in the equatorial position. They are segregated into alternating layers parallel to (100) and stacked along [100]. The first cation is arranged in parallel stacks in a herringbone pattern with rows of [CuCl4]2− anions fitting between the stacks and with a Cl− ion directed into the interior of the layer. The second organic cation forms distorted hcp layers that separate the other organic cation/[CuCl4]2− slabs. N—H⋯Cl hydrogen bonding between the cations and the anions consolidates the crystal packing.

Poly[tetrakis(n-propylammonium) [octa-μ-chlorido-dichloridotrinickelate(II)]]: a hybrid organic-inorganic layer compound in the Cs4Mg3F10 structure type.

The title compound, {(C(3)H(10)N)(4)[Ni(3)Cl(10)]}(n), contains zigzag layers of tri-μ-chlorido-bridged linear 2/m-symmetric Ni(3)Cl(12) segments, linked by mono-μ-chlorido quasi-linear bridges to two other segments at each end. These inorganic layers are interleaved with interdigitated bilayers of mirror-symmetric n-propylammonium cations, the ammonium head groups of which are directed into the inorganic layers to form multiple N-H···Cl hydrogen bonds, while the propyl tail groups pack together in a tongue-and-groove manner in the center of the bilayer. The propyl groups are ordered at 100 K but disordered with opposite conformations on the mirror plane at 240 K.

Bis(1,3,4-trimethylpyridinium) tetrachloridocuprate(II) and bis(1,3,4-trimethylpyridinium) tetrabromidocuprate(II): an examination of the A2CuX4 Fdd2 structure type.

The title bis(1,3,4-trimethylpyridinium) tetrahalidocuprate(II) structures, (C8H12N)2[CuCl4], (I), and (C8H12N)2[CuBr4], (II), respectively, consist of flattened [CuX4]2- tetrahedral complex anions and planar 1,3,4-trimethylpyridinium cations. Chloride compound (I) is a rare example of an A2CuCl4 structure with an elongated unit cell in the polar space group Fdd2. The [CuCl4]2- anions have twofold rotational symmetry and are arranged in distorted hexagonal close-packed (hcp) layers, which are interleaved with layers of cations, each in a four-layer repeat sequence, to generate the elongated axis. The organic cations stack along [101] or [101] in alternating layers. The methyl groups meta on the cation ring and the larger of the trans Cl-Cu-Cl angles both face the same direction along the polar axis and are the most prominent features determining the polarity of the structure. Bromide compound (II) crystallizes in a centrosymmetric structure with a similar layer structure but with only a two-layer repeat sequence. Here, symmetry-inequivalent cations are segregated into alternating layers with cations, forming hcp layers of inversion-related cation pairs in one layer and parallel stacks of cations in the other. The change in space group when the larger Br(-) ion is present suggests that the 1,3,4-trimethylpyridinium ion has a minimal size to allow the Fdd2 A2CuX4 structure type.