Wurster's crowns: a comparative study of ortho- and para-phenylenediamine-containing macrocyclic receptors.

Two series of isomeric, redox-responsive azacrown ethers based on ortho- and para-phenylenediamine (Wurster's crowns) have been synthesized and their properties explored through 13C NMR spectroscopy, electrospray ionization mass spectrometry, cyclic voltammetry, and X-ray crystallography. These crowns display strong affinity for alkali metal cations while maintaining comparable selectivity profiles to the parent crown ethers from which they are derived. Like Wurster's reagent (N,N,N',N'-tetramethyl-p-phenylenediamine or para-TMPD), the para-Wurster's crowns undergo two reversible one-electron oxidations. The integrity of the alkali metal ion complexes is maintained in the neutral and singly oxidized ligand states but not after removal of two electrons. In contrast, the oxidation of ortho-Wurster's crowns is scan rate dependent, occurring at potentials substantially higher than their para counterparts, with their complexes oxidizing irreversibly. X-ray crystal structures of representative complexes show, in all cases, participation of the redox-active phenylenediamine subunits in complex formation via direct bonding to the guest cation.

A bridge to coordination isomer selection in lanthanide(III) DOTA-tetraamide complexes.

Interest in macrocyclic lanthanide complexes such as DOTA is driven largely through interest in their use as contrast agents for MRI. The lanthanide tetraamide derivatives of DOTA have shown considerable promise as PARACEST agents, taking advantage of the slow water exchange kinetics of this class of complex. We postulated that water exchange in these tetraamide complexes could be slowed even further by introducing a group to sterically encumber the space above the water coordination site, thereby hindering the departure and approach of water molecules to the complex. The ligand 8O2-bridged DOTAM was synthesized in a 34% yield from cyclen. It was found that the lanthanide complexes of this ligand did not possess a water molecule in the inner coordination sphere of the bound lanthanide. The crystal structure of the ytterbium complex revealed that distortions to the coordination sphere were induced by the steric constraints imposed on the complex by the bridging unit. The extent of the distortion was found to increase with increasing ionic radius of the lanthanide ion, eventually resulting in a complete loss of symmetry in the complex. Because this ligand system is bicyclic, the conformation of each ring in the system is constrained by that of the other; in consequence, inclusion of the bridging unit in the complexes means only a twisted square, antiprismatic coordination geometry is observed for lanthanide complexes of 8O2-bridged DOTAM.

An efficient synthesis of the constrained peptidomimetic 2-oxo-3-(N-9-fluorenyloxycarbonylamino)-1-azabicyclo[4.3.0]nonane-9-carboxylic acid from pyroglutamic acid.

[reaction: see text] Azabicyclo[X.Y.0]alkane amino acids are rigid dipeptide mimetics that are useful tools for structure-activity studies in peptide-based drug discovery. Herein, we report an efficient synthesis of three diastereomers of 9-tert-butoxycarbonyl-2-oxo-3-(N-tert-butoxycarbonylamino)-1-azabicyclo[4.3.0]nonane (3S,6S,9S, 3S,6R,9R, and 3S,6R,9S). Methyl N-Boc-pyroglutamate is cleaved with vinylmagnesium bromide to produce an acyclic gamma-vinyl ketone. Michael addition of N-diphenylmethyleneglycine tert-butyl ester produces the N-Boc-delta-oxo-alpha,omega-diaminoazelate intermediate, which, on hydrogenloysis, gives the fused ring system. Acidolytic deprotection followed by Fmoc-protection provided building blocks suitable for solid-phase synthesis.

Aluminium alkyl and aryloxide complexes of pyrazine and bipyridines: synthesis and structure.

The reaction of AlMe(3) and [((t)Bu)(2)Al(micro-OPh)](2) with pyrazine (pyz), 4,4'-bipyridine (4-4'-bipy), 1,2-bis(4-pyridyl)ethane (bpetha) and 1,2-bis(4-pyridyl)ethylene (bpethe) yields (Me(3)Al)(2)(micro-pyz)(1), (Me(3)Al)(2)(micro-4,4'-bipy)(2), (Me(3)Al)(2)(micro-bpetha)(3), (Me(3)Al)(2)(micro-bipethe)(4), Al((t)Bu)(2)(OPh)(pyz)(5), [((t)Bu)(2)Al(OPh)](2)(micro-4,4-bipy)(6a), [((t)Bu)(2)Al(OPh)](2)(micro-bpetha)(7a), [((t)Bu)(2)Al(OPh)](2)(micro-bipethe)(8a). Compounds 1-4, 6a and 7a have been confirmed by X-ray crystallography. In solution compounds 1-4 undergo a rapid ligand-dissociation equilibrium resulting in a time-average spectrum in the (1)H NMR. In contrast, the solution equilibria for compounds 5-8a are sufficiently slow such that the mono-aluminium compounds may be observed by (1)H NMR spectroscopy: Al((t)Bu)(2)(OPh)(4,4-bipy)(6b), Al((t)Bu)(2)(OPh)(bpetha)(7b) and Al((t)Bu)(2)(OPh)(bpethe)(8b). The inability to isolate [((t)Bu)(2)Al(OPh)](2)(micro-pyz) and the relative stability of each complex is discussed with respect to the steric interactions across the bridging ligand (L) and the electronic effect on one Lewis acid-base interaction by the second Lewis acid-base interaction on the same ligand.

1,4-dioxobenzene compounds of gallium: reversible binding of pyridines to [[((t)Bu)(2)Ga](2)(mu-OC(6)H(4)O)](n) in the solid state.

The gallium aryloxide polymer, [[((t)Bu)(2)Ga](2)(mu-OC(6)H(4)O)](n)(1) is synthesized by the addition of Ga((t)()Bu)(3) with hydroquinone in a noncoordinating solvent, and reacts with pyridines to yield the yellow compound [((t)()Bu)(2)Ga(L)](2)(mu-OC(6)H(4)O) [L = py (2), 4-Mepy (3), and 3,5-Me(2)py (4)] via cleavage of the Ga(2)O(2) dimeric core. The analogous formation of Ga((t)()Bu)(2)(OPh)(py) (5) occurs by dissolution of [((t)Bu)(2)Ga(mu-OPh)](2) in pyridine. In solution, 2-4 undergo dissociation of one of the pyridine ligands to yield [((t)()Bu)(2)Ga(L)(mu-OC(6)H(4)O)Ga((t)Bu)(2)](2), for which the DeltaH and DeltaS have been determined. Thermolysis of compounds 2-4 in the solid-state results in the loss of the Lewis base and the formation of 1. The reaction of 1 or [((t)Bu)(2)Ga(mu-OPh)](2) with the vapor of the appropriate ligand results in the solid state formation of 2-4 or 5, respectively. The deltaH and deltaS for both ligand dissociation and association for the solid-vapor reactions have been determined. The interconversion of 1 into 2-4, as well as [((t)Bu)(2)Ga(mu-OPh)](2) into 5, and their reverse reactions, have been followed by (13)C CPMAS NMR spectroscopy, TG/DTA, SEM, EDX, and powder XRD. Insight into this solid-state polycondensation polymerization reaction may be gained from the single-crystal X-ray crystallographic packing diagrams of 2-5. The crystal packing for compounds 2, 3, and 5 involve a head-to-head arrangement that is maintained through repeated ligand dissociation and association cycles. In contrast, when compound 4 is crystallized from solution a head-to-tail packing arrangement is formed, but during reintroduction of 3,5-Me(2)py in the solid state-vapor reaction of compound 1, a head-to-head polymorph is postulated to account for the alteration in the deltaH of subsequent ligand dissociation reactions. Thus, the deltaH for the condensation polymerization reaction is dependent on the crystal packing; however, the subsequent reversibility of the reaction is dependent on the polymorph.

Transition-metal complexes of a bifunctional tetradentate gallium alkoxide ligand.

The mixed gallium transition-metal complexes [FeCl[Ga(2)((t)Bu)(4)(neol)(2)]] (1) and [M[Ga(2)((t)Bu)(4)(neol)(2)]], M = Co (2), Ni (3), Cu (4), have been prepared by the reaction of [Ga(2)((t)Bu)(4)(neol-H)(2)] (neol-H(2) = 2,2-dimethyl-propane-1,3-diol) with the appropriate metal halide and Proton Sponge. Compounds 1-4 have been characterized by NMR (3), UV/vis, and IR spectroscopy and magnetic susceptibility (solution and solid state), and their molecular structures have been confirmed by X-ray crystallography. The molecular structure of compounds 1-4 consists of a tetracyclic core formed from two four-membered and two six-membered rings. The central metal atom adopts a square pyramidal (1) or square planar (2-4) geometry. The magnetic susceptibilities for 1, 2, and 4 are as expected for strong ligand field environments. On the basis of spectroscopic and structural data, the [Ga(2)((t)Bu)(4)(neol)(2)](2-) ligand appears to be more flexible than other chelating ligands; this is proposed to be due to the flexibility in the O-Ga-O bond angle.

Crystal Structure of [Ru(terpy)(2)](0): A New Crystalline Material from the Reductive Electrocrystallization of [Ru(terpy)(2)](2+).

Reductive electrocrystallization of [Ru(terpy)(2)](PF(6))(2) (where terpy = 2,2':6',2' '-terpyridine) from an acetonitrile solution containing 100 mM TBAPF(6) results in the formation of black crystals. Crystal data: [Ru(terpy)(2)].(PF(6))(2)[(CH(3))(2)CO], monoclinic, space group P2(1)/c with a = 20.801(2) Å, b = 8.943(1) Å, c = 19.453(2) Å, beta = 92.524(9) degrees, and Z = 4; [Ru(terpy)(2)] (1), orthorhombic, Fdd2 with a = 39.757(4) Å, b = 56.464(6) Å, c = 8.507(1) Å, and Z = 32. X-ray analysis reveals that the crystals consist exclusively of [Ru(terpy)(2)](0) (1), with no solvent or counteranion present in the lattice. [Ru(terpy)(2)](0) units are structurally very similar to the parent [Ru(terpy)(2)](2+), with nearly perfect octahedral symmetry around the metal center and with two terpy ligands that are basically planar. Analysis of the crystal packing shows that [Ru(terpy)(2)](2+) crystals have close intermolecular distances, while [Ru(terpy)(2)](0) crystals show only intermolecular interactions along the c axis with contacts that are less than 3.5 Å. Analysis of molecular volumes and empty spaces reveals the presence of cavities, which could contain substantial electron density.

Synthesis of Indium Amide Compounds.

Indium trichloride reacts with 3 equiv of lithium amide in diethyl ether to give In(NRR')(3) (R = Ph or t-Bu, R' = SiMe(3); R = t-Bu, R' = SiHMe(2)) and with 3 or 4 equiv of LiNMe(SiMe(3)) to yield Li[In{NMe(SiMe(3))}(4)]. The chloride also reacts with LiNPh(2) in THF to give the salt Li[In(NPh(2))(3)Cl] and with LiNRR' in pyridine to yield the neutral adduct In(NRR')(3)(py) (R = R' = Ph; R = Me, R' = SiMe(3)). The volatile liquids In[N(t-Bu)(SiHMe(2))](3) and In[NMe(SiMe(3))](3)(py) react with p-Me(2)Npy to form the solid compounds In[N(t-Bu)(SiHMe(2))](3)(p-Me(2)Npy) and In[NMe(SiMe(3))](3)(p-Me(2)Npy), respectively. X-ray crystallographic studies show that In(NPh(2))(3)(py), In[N(t-Bu)(SiHMe(2))](3)(p-Me(2)Npy), and the ether adduct of In[NPh(SiMe(3))](3) contain nearly planar In(amide)(3) fragments. Crystallographic studies also show that the anion in the salt [Li(THF)(4)][In(NPh(2))(3)Cl] is nearly tetrahedral and in [Li(p-Me(2)Npy)][In{NMe(SiMe(3))}(4)] the tetrahedral-like anion is bound to the Li cation via two amide nitrogens. The Li in the latter structure is also bonded to p-Me(2)Npy, resulting in a planar three-coordinate geometry for Li. Crystal data are the following. C(31)H(52)N(3)OSi(3)In at -50 degrees C: P2(1)/n (monoclinic), a = 11.003(2) Å, b = 18.678(3) Å, c = 17.618(3) Å, beta = 95.42(1) degrees, and Z = 4. C(41)H(35)N(4)In.C(7)H(8) at -50 degrees C: P&onemacr; (triclinic), a = 10.112(2) Å, b = 12.786(3) Å, c = 15.870(5) Å, alpha = 87.42(2) degrees, beta = 74.95(2) degrees, gamma = 78.15(2) degrees, and Z = 2. C(25)H(58)N(5)Si(3)In at -50 degrees C: P2(1)/c (monoclinic), a = 9.797(3) Å, b = 18.203(6) Å, c = 19.592(5) Å, beta = 100.27(2) degrees, and Z = 4. C(52)H(62)ClInLiN(3)O(4) at 23 degrees C: P2(1)/n (monoclinic), a = 16.076(2) Å, b = 17.185(2) Å, c = 18.447(3) Å, beta = 97.41(1) degrees, and Z = 4. C(23)H(58)InLiN(6)Si(4) at 23 degrees C: P&onemacr; (triclinic), a = 15.792(3) Å, b = 16.345(3) Å, c = 16.678(3) Å, alpha = 62.69(1) degrees, beta = 81.00(1) degrees, gamma = 86.94(1) degrees, and Z = 4.

Formation of Ruthenium(II) Complexes with Unsymmetrical Terdentate Ligands.

Three unsymmetrical terdentate ligands, 2-(2'-quinolyl)-1,10-phenanthroline (4), 2-(1'-isoquinolyl)-1,10-phenanthroline (5), and 3,3'-dimethylene-2-(2'-quinolyl)-1,10-phenanthroline (6), have been prepared by application of the Friedländer condensation. Ligand 4 forms predominantly a pentaaza-coordinated (N5) complex with Ru(II), [Ru(4-N,N',N")(4-N,N')Cl](PF(6)), whose structure was determined by X-ray analysis (C(42)H(26)ClF(6)N(6)PRu: monoclinic, C2/c, a = 29.308(3) Å, b = 15.205(2) Å, c = 19.505(2) Å, beta = 107.312(8) degrees, V = 8298(1) Å(3), Z = 8). Ligand 5 forms predominantly a hexaaza-coordinated (N6) complex, [Ru(5-N,N',N")(2)](PF(6))(2). The bridged species 6 shows intermediate behavior, and the rate for interconversion of the N5 to N6 complex has been measured. The stereochemical features of these two binding modes are examined, and both pi-stacking and steric effects are invoked to explain the observed behavior, leading to the proposal of a backside displacement mechanism for their interconversion.

Synthesis and Structure of [Tp(Bu)()t()2]In, a Highly Twisted [Tris(3,5-di-tert-butylpyrazolyl)hydroborato]indium(I) Complex: Comparison with the Re-Evaluated Ordered Structure of [Tp(Bu)()t]In.

The indium(I) complex [Tp(Bu)()t()2]In ([Tp(Bu)()t()2] = tris(3,5-di-tert-butylpyrazolyl)hydroborato), synthesized by the reaction of [Tp(Bu)()t()2]Na with InCl, exhibits a structure in which the [Tp(Bu)()t()2] ligand adopts a highly twisted configuration due to steric interactions of the tert-butyl substituents in the 5-positions of the pyrazolyl groups. In contrast, the absence of 5-tert-butyl substituents allows the pyrazolyl groups in [Tp(Bu)()t]In to be coplanar with their respective In-N-N-B planes. The structure of [Tp(Bu)()t]In has been previously reported but was noted to exhibit an unusual type of disorder in which a nitrogen atom of one molecule was coincident with the boron atom of its disordered configuration [Dias, H. V. R.; Huai, L.; Jin, W.; Bott, S. G. Inorg. Chem. 1995, 34, 1973-1974]. In view of the unusual nature of the disorder, which involved both a 2-fold rotation and a canting of the molecule, the disordered structure of [Tp(Bu)()t]In was re-evaluated. Significantly, an ordered structure of [Tp(Bu)()t]In was obtained. The disorder present in the previously reported structure is a consequence of adopting a space group with unnecessarily high symmetry. Thus, [Tp(Bu)()t]In provides an example where the structure is much better described as ordered in a noncentrosymmetric space group, rather than disordered in the centrosymmetric alternative. [Tp(Bu)()t()2]In is monoclinic, of space group P2(1)/c (No. 14), with a = 18.781(9) Å, b = 10.380(2) Å, c = 20.849(6) Å, beta = 112.76(3) degrees, and Z = 4. [Tp(Bu)()t]In is orthorhombic, of space group Cmc2(1) (No. 36), with a = 16.193(3) Å, b = 15.214(3) Å, c = 9.963(3) Å, and Z = 4.