Formation of a Dicopper Platform Based Polyrotaxane Whose “<i>String</i>” and “<i>Bead</i>” Are Constructed from the Same Components

The combination of the dicopper platform [Cu₂(<b>L</b>)₂(THF)₂] (<b>1</b>·2THF), where H_<b>2</b><b>L</b> is 1,1′-(1,3-phenylene)-bis-4,4-dimethylpentane-1,3-dione, and 1,4-bis­(4-pyridyl)­piperazine (bpp), afforded the first example of a one-dimensional polyrotaxane {[(<b>1</b>)­(μ₂-bpp)]­[(<b>1</b>)₂(bpp)₂]}_<i>n</i> whose “<i>string</i>” and “<i>bead</i>” are constructed from the same components. The <i>bead</i> of stoichiometry [(<b>1</b>)₂(bpp)₂] has a large rectangular cavity of dimensions 7.40 × 15.64 Å and is threaded onto a stair-like <i>string</i> of composition [(<b>1</b>)­(μ₂-bpp)]_<i>n</i>. The formation of the polyrotaxane is driven by π–π stacking between the <i>string</i> and the <i>beads</i> with precise electronic and steric complementarity between these components. A pathway for the formation of the polyrotaxane is proposed

Employment Discrimination: Recent Developments in the Supreme Court (Symposium: The Supreme Court and Local Government Law: The 1992-93 Term)

At a symposium entitled, “The Supreme Court and Local Government Law; The 1992/93 Term”, Professor Eileen Kaufman spoke about the cases involving employment discrimination that were decided during that particular Term, Hazen Paper Company v. Biggins and St. Mary\u27s Honor Center v. Hicks. While Hazen is an age discrimination case and St. Mary\u27s is a Title VII case, they can be viewed as companion cases which serve to explain what an employment discrimination plaintiff must now establish when attempting to prove disparate treatment by indirect evidence. By way of preview, suffice it to say that plaintiff\u27s task has been made more difficult as a result of these decisions

Post-Assembly Covalent Di- and Tetracapping of a Dinuclear [Fe₂L₃]⁴⁺ Triple Helicate and Two [Fe₄L₆]⁸⁺ Tetrahedra Using Sequential Reductive Aminations

The use of a highly efficient reductive amination procedure for the postsynthetic end-capping of metal-templated helicate and tetrahedral supramolecular structures bearing terminal aldehyde groups is reported. Metal template formation of a [Fe₂L₃]⁴⁺ dinuclear helicate and two [Fe₄L₆]⁸⁺ tetrahedra (where L is a linear ligand incorporating two bipyridine domains separated by one or two 1,4-(2,5-dimethoxyaryl) linkers and terminated by salicylaldehyde functions is described. Postassembly reaction of each of these “open” di- and tetranuclear species with excess ammonium acetate (as a source of ammonia) and sodium cyanoborohydride results in a remarkable reaction sequence whereby the three aldehyde groups terminating each end of the helicate, or each of the four vertices of the respective tetrahedra, react with ammonia then undergo successive reductive amination to yield corresponding fully tertiary-amine capped cryptate and tetrahedral covalent cages

Cation- and Anion-Exchanges Induce Multiple Distinct Rearrangements within Metallosupramolecular Architectures

Different anionic templates act to give rise to four distinct Cd^II-based architectures: a Cd₂L₃ helicate, a Cd₈L₁₂ distorted cuboid, a Cd₁₀L₁₅ pentagonal prism, and a Cd₁₂L₁₈ hexagonal prism, which respond to both anionic and cationic components. Interconversions between architectures are driven by the addition of anions that bind more strongly within a given product framework. The addition of Fe^II prompted metal exchange and transformation to a Fe₄L₆ tetrahedron or a Fe₁₀L₁₅ pentagonal prism, depending on the anionic templates present. The equilibrium between the Cd₁₂L₁₈ prism and the Cd₂L₃ triple helicate displayed concentration dependence, with higher concentrations favoring the prism. The Cd₁₂L₁₈ structure serves as an intermediate en route to a hexafluoroarsenate-templated Cd₁₀L₁₅ complex, whereby the structural features of the hexagonal prism preorganize the system to form the structurally related pentagonal prism. In addition to the interconversion pathways investigated, we also report the single-crystal X-ray structure of bifluoride encapsulated within a Cd₁₀L₁₅ complex and report solution state data for <i>J</i>-coupling through a CH···F^– hydrogen bond indicating the strength of these interactions in solution

Five Discrete Multinuclear Metal-Organic Assemblies from One Ligand: Deciphering the Effects of Different Templates

A rigid organic ligand, formed through the <i>subcomponent self-assembly</i> of <i>p</i>-toluidine and 6,6′-diformyl-3,3′-bipyridine, was employed in a systematic investigation into the synergistic and competing effects of metal and anion templation. A range of discrete and polymeric metal-organic complexes were formed, many of which represent structure types that have not previously been observed and whose formation would not be predicted on taking into account solely geometric considerations. These complex structures, capable of binding multiple guests within individual binding pockets, were characterized by NMR, ESI-MS, and single-crystal X-ray diffraction. The factors that stabilize individual complexes and lead to the formation of one over another are discussed

Chain-Reaction Anion Exchange between Metal–Organic Cages

Differential binding affinities for a set of anions were observed between larger (<b>1</b>) and smaller (<b>2</b>) tetrahedral metal–organic capsules in solution. A chemical network could thus be designed wherein the addition of hexafluorophosphate could cause perchlorate to shift from capsule <b>2</b> to capsule <b>1</b> and triflimide to be ejected from capsule <b>1</b> into solution

Silver(I) Coordination Polymers Incorporating Neutral γ-Carbon Bound <i>N</i>,<i>N</i>′-Bis(acetylacetone)alkanediimine Units

Five new silver(I) coordination polymers of types [Ag₂L¹(NO₃)₂]_<i>n</i> (<b>1</b>), [Ag₂L²(NO₃)₂]_<i>n</i> (<b>2</b>), [Ag₂L³(NO₃)₂]_<i>n</i> (<b>3</b>), [Ag₂L⁴(NO₃)₂]_<i>n</i> (<b>4</b>), and [AgL⁵(NO₃)]_<i>n</i> (<b>5</b>) have been synthesized, where L¹–L⁵ are the Schiff base ligands formed from 2:1 condensation of acetylacetone with the symmetrical diamines H₂N(CH₂)_<i>n</i>NH₂, with <i>n</i> = 2 and 6 to give L¹ and L², 1,2-bis(2-aminophenoxy)ethane to give L³, 1,4-bis(2-aminophenoxy)butane to give L⁴, and 1,6-bis(4-aminophenoxy)butane to give L⁵. The single crystal X-ray structures of the Ag(I) complexes <b>1</b>–<b>5</b> have been determined. Both <b>1</b> and <b>2</b> adopt three-dimensional polymeric network structures, while <b>3</b>–<b>5</b> adopt two-dimensional-layered framework structures. In <b>1</b> and <b>2</b>, the terminal O and γ-C atoms of two acacH-imine domains of the same ligand bind to different silver ions, while in each of <b>3</b>–<b>5</b> a γ-C from one or both acacH-imine domains binds to a silver ion, with the remaining coordination sites occupied by bridging monodentate or bidentate nitrato groups and/or a Schiff base ligand oxygen donor

Chain-Reaction Anion Exchange between Metal–Organic Cages

Differential binding affinities for a set of anions were observed between larger (<b>1</b>) and smaller (<b>2</b>) tetrahedral metal–organic capsules in solution. A chemical network could thus be designed wherein the addition of hexafluorophosphate could cause perchlorate to shift from capsule <b>2</b> to capsule <b>1</b> and triflimide to be ejected from capsule <b>1</b> into solution

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS–LS], [LS–HS], and [HS–HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{FeIII(tpa)}2spiro](PF6)2 (1), [{FeIII(tpa)}2Br4spiro](PF6)2 (2), and [{FeIII(tpa)}2thea](PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-5,5′,6,6′-tetraol, Br4spiroH4 = 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS–HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS–HS] state. In solution, all complexes undergo SCO from [HS–HS] at room temperature, via [LS–HS] to mixtures including [LS–LS] at 77 K, with the extent of SCO increasing in the order 1 2 3. Gas phase density functional theory calculations suggest a [LS–LS] ground state for all complexes, with the [LS–HS] and [HS–HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4– 4spiro4– 4–, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS–HS] state in relation to the midpoint energy between [LS–LS] and [HS–HS]. The relative stability of the [LS–HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 2 3. The bromo substituents of Br4spiro4– increase the ligand field strength relative to spiro4–, while the stronger ligand field provided by thea4– arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS–LS], [LS–HS], and [HS–HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{FeIII(tpa)}2spiro](PF6)2 (1), [{FeIII(tpa)}2Br4spiro](PF6)2 (2), and [{FeIII(tpa)}2thea](PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-5,5′,6,6′-tetraol, Br4spiroH4 = 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS–HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS–HS] state. In solution, all complexes undergo SCO from [HS–HS] at room temperature, via [LS–HS] to mixtures including [LS–LS] at 77 K, with the extent of SCO increasing in the order 1 2 3. Gas phase density functional theory calculations suggest a [LS–LS] ground state for all complexes, with the [LS–HS] and [HS–HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4– 4spiro4– 4–, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS–HS] state in relation to the midpoint energy between [LS–LS] and [HS–HS]. The relative stability of the [LS–HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 2 3. The bromo substituents of Br4spiro4– increase the ligand field strength relative to spiro4–, while the stronger ligand field provided by thea4– arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states