Structure of the borosilicate zeolite catalyst SSZ-82 solved using 2D-XPD charge flipping

The structure of the calcined borosilicate zeolite catalyst SSZ-82 ([Si(61.3)B(4.7)O(132)], Pmmn, a = 24.2783(4), b = 11.4665(2), and c = 14.1127(3) ?) has been solved from X-ray powder diffraction (XPD) data using the recently developed 2D-XPD charge flipping approach. The electron density maps generated with the more conventional powder charge flipping (pCF) algorithm could not be interpreted easily, so this new method, which begins by phasing low-resolution, 2D subsets of the data, was applied. Crystallographic phases were derived for the three main projections ([100], [010], and [001]) by using just the corresponding subsets of reflections (0kl, h0l, and hk0, respectively) from the full set of 3039 extracted intensities. These phases were then imposed on the (otherwise random) starting phases in the application of the pCF algorithm to the full data set. The framework structure, with 11 Si/B atoms in the asymmetric unit and a novel 12-/10-ring 2D channel system, could be seen clearly in the resulting electron density map. This is the first application of the 2D-XPD method to data collected on a material of unknown structure. Rietveld refinement of the structure revealed the positions of the B atoms in the framework and indicated that some water had been readsorbed in the pores.     

High‐Silica Zeolite SSZ‐61 with Dumbbell‐Shaped Extra‐Large‐Pore Channels

The synthesis of the high‐silica zeolite SSZ‐61 using a particularly bulky polycyclic structure‐directing agent and the subsequent elucidation of its unusual framework structure with extra‐large dumbbell‐shaped pore openings are described. By using information derived from a variety of X‐ray powder diffraction and electron microscopy techniques, the complex framework structure, with 20 Si atoms in the asymmetric unit, could be determined and the full structure refined. The Si atoms at the waist of the dumbbell are only three‐connected and are bonded to terminal O atoms pointing into the channel. Unlike the six previously reported extra‐large‐pore zeolites, SSZ‐61 contains no heteroatoms in the framework and can be calcined easily. This, coupled with the possibility of inserting a catalytically active center in the channel between the terminal O atoms in place of H⁺, afford SSZ‐61 intriguing potential for catalytic applications.     

Distinguishing between Dexter and rapid sequential electron transfer in covalently linked donor-acceptor assemblies

The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M2(L)(mcb)(Ru(4,4'-(X)2-bpy)2)](PF6)3 (where M = Mn(II) or Zn(II), X = CH3 or CF3, mcb is 4'-methyl-4-carboxy-2,2'-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a Ru(II) polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the 3MLCT excited state of the Ru(II) chromophore but with a lifetime (tau(obs) = 5.0 +/- 0.1 ns) and radiative quantum yield (Phi(r) approximately 7 x 10(-4)) that are significantly attenuated relative to the Zn(II) model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) (tau(obs) = 730 +/- 30 ns and Phi(r) = 0.024, respectively). Quenching of the 3MLCT excited state is even more extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (tau(obs) = 45 +/- 5 ps) is >10(4) shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (tau(obs) = 1.31 +/- 0.05 micros). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form k(nr) = k0 + A x exp{-DeltaE/kB T} with k0 = 1.065 +/- 0.05 x 10(7) s(-1), A = 3.7 +/- 0.5 x 10(10) s(-1), and DeltaE = 1230 +/- 30 cm(-1). Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of lambda approximately 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn(II)2 core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (DeltaG0(ET) = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be approximately 10 cm(-1) and approximately 0.15 cm(-1) in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) results in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude. In addition to providing insight into the influence of donor/acceptor proximity on exchange energy transfer, this study underscores the utility of variable-temperature measurements in cases where Dexter and electron-transfer mechanisms can lead to indistinguishable spectroscopic observables.     

Tetranuclear and Pentanuclear Vanadium(IV/V) Carboxylate Complexes: [V(4)O(8)(NO(3))(O(2)CR)(4)](2-) and [V(5)O(9)X(O(2)CR)(4)](2-) (X = Cl(-), Br(-)) Salts

The syntheses and properties of tetra- and pentanuclear vanadium(IV,V) carboxylate complexes are reported. Reaction of (NBzEt(3))(2)[VOCl(4)] (1a) with NaO(2)CPh and atmospheric H(2)O/O(2) in MeCN leads to formation of (NBzEt(3))(2)[V(5)O(9)Cl(O(2)CPh)(4)] 4a; a similar reaction employing (NEt(4))(2)[VOCl(4)] (1b) gives (NEt(4))(2)[V(5)O(9)Cl(O(2)CPh)(4)] (4b). Complex 4a.MeCN crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -148 degrees C: a = 13.863(13) ?, b = 34.009(43) ?, c = 12.773(11) ?, and Z = 4. The reaction between (NEt(4))(2)[VOBr(4)] (2a) and NaO(2)CPh under similar conditions gives (NEt(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6a), and the use of (PPh(4))(2)[VOBr(4)] (2b) likewise gives (PPh(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6b). Complex 6b crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -139 degrees C: a = 18.638(3) ?, b = 23.557(4) ?, c = 12.731(2) ?, and Z = 4. The anions of 4a and 6b consist of a V(5) square pyramid with each vertical face bridged by a &mgr;(3)-O(2)(-) ion, the basal face bridged by a &mgr;(4)-X(-) (X = Cl, Br) ion, and a terminal, multiply-bonded O(2)(-) ion on each metal. The RCO(2)(-) groups bridge each basal edge to give C(4)(v)() virtual symmetry. The apical and basal metals are V(V) and V(IV), respectively (i.e., the anions are trapped-valence). The reaction of 1b with AgNO(3) and Na(tca) (tca = thiophene-2-carboxylate) in MeCN under anaerobic conditions gives (NEt(4))(2)[V(4)O(8)(NO(3))(tca)(4)] (7). Complex 7.H(2)O crystallizes in space group C2/c with the following unit cell dimensions at -170 degrees C: a = 23.606(4) ?, b = 15.211(3) ?, c = 23.999(5) ?, and Z = 4. The anion of 7 is similar to those of 4a and 6b except that the apical [VO] unit is absent, leaving a V(4) square unit, and the &mgr;(4)-X(-) ion is replaced with a &mgr;(4),eta(1)-NO(3)(-) ion. The four metal centers are now at the V(IV), 3V(V) oxidation level, but the structure indicates four equivalent V centers, suggesting an electronically delocalized system. Variable-temperature magnetic susceptibility data were collected on powdered samples of 4b, 6a, and 7 in the 2.00-300 K range in a 10 kG applied field. 4b and 6a both show a slow increase in effective magnetic moment (&mgr;(eff)) from approximately 3.6-3.7 &mgr;(B) at 320 K to approximately 4.5-4.6 &mgr;(B) at 11.0 K and then a slight decrease to approximately 4.2 &mgr;(B) at 2.00 K. The data were fit to the theoretical expression for a V(IV)(4) square with two exchange parameters J = J(cis)() and J' = J(trans)() (H = -2JS(i)()S(j)()): fitting of the data gave, in the format 4b/6a, J= +39.7/+46.4 cm(-)(1), J' = -11.1/-18.2 cm(-)(1) and g = 1.83/1.90, with the complexes possessing S(T) = 2 ground states. The latter were confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.500-50.0 kG ranges: fitting of the data gave S(T) = 2 and D = 0.00 cm(-)(1) for both complexes, where D is the axial zero-field splitting parameter. Complex 7 shows a nearly temperature-independent &mgr;(eff) (1.6-2.0 &mgr;(B)) consistent with a single d electron per V(4) unit. The (1)H NMR spectra of 4b and 6a in CD(3)CN are consistent with retention of their pentanuclear structure on dissolution. The EPR spectrum of 7 in a toluene/MeCN (1:2) solution at approximately 25 degrees C yields an isotropic signal with a 29-line hyperfine pattern assignable to hyperfine interactions with four equivalent I = (7)/(2) (51)V nuclei.