Silica-supported,single-site titanium catalysts for olefin epoxidation. A molecular precursor strategy for control of catalyst structure

A molecular precursor approach involving simple grafting procedures was used to produce site-isolated titanium-supported epoxidation catalysts of high activity and selectivity. The tris(tert-butoxy)siloxy titanium complexes Ti[OSi(O(t)Bu)(3)](4) (TiSi4), ((i)PrO)Ti[OSi(O(t)Bu)(3)](3) (TiSi3), and ((t)BuO)(3)TiOSi(O(t)Bu)(3) (TiSi) react with the hydroxyl groups of amorphous Aerosil, mesoporous MCM-41, and SBA-15 via loss of HO(t)Bu and/or HOSi(O(t)Bu)(3) and introduction of titanium species onto the silica surface. Powder X-ray diffraction, nitrogen adsorption/desorption, infrared, and diffuse reflectance ultraviolet spectroscopies were used to investigate the structures and chemical natures of the surface-bound titanium species. The titanium species exist mainly in isolated, tetrahedral coordination environments. Increasing the number of siloxide ligands in the molecular precursor decreases the amount of titanium that can be introduced this way, but also enhances the catalytic activity and selectivity for the epoxidation of cyclohexene with cumene hydroperoxide as oxidant. In addition, the high surface area mesoporous silicas (MCM-41 and SBA-15) are more effective than amorphous silica as supports for these catalysts. Supporting TiSi3 on the SBA-15 affords highly active cyclohexene epoxidation catalysts (0.25-1.77 wt % Ti loading) that provide turnover frequencies (TOFs) of 500-1500 h(-1) after 1 h (TOFs are reduced by about half after calcination). These results demonstrate that oxygen-rich siloxide complexes of titanium are useful as precursors to supported epoxidation catalysts.     

Tris(tert-butoxy)siloxy derivatives of boron,including the boronous acid HOB[OSi(O(t)Bu)(3)](2) and the metal (siloxy)boryloxide complex Cp(2)Zr(Me)OB[OSi(O(t)Bu)(3)](2): a remarkable crystal structure with 18 independent molecules in its asymmetric unit

Silanolysis of B(O(t)Bu)(3) with 2 and 3 equiv of HOSi(O(t)Bu)(3) led to the formation of (t)BuOB[OSi(O(t)Bu)(3)](2) (1) and B[OSi(O(t)Bu)(3)](3) (2), respectively. Compounds 1 and 2 are efficient single-source molecular precursors to B/Si/O materials via thermolytic routes in nonpolar media, as demonstrated by the generation of BO(1.5).2SiO(2) (BOSi2(xg)) and BO(1.5).3SiO(2) (BOSi3(xg)) xerogels, respectively. Use of a block copolymer template provided B/Si/O materials (BOSi2(epe) and BOSi3(epe)) with a broad distribution of mesopores (by N(2) porosimetry) and smaller, more uniform particle sizes (by TEM) as compared to the nontemplated materials. Hydrolyses of 1 and 2 with excess H(2)O resulted in formation of the expected amounts of (t)BuOH and HOSi(O(t)Bu)(3); however, reaction of 1 with 1 equiv of H(2)O led to isolation of the new boronous acid HOB[OSi(O(t)Bu)(3)](2) (3). This ligand precursor is well suited for the synthesis of new metal (siloxy)boryloxide complexes via proton-transfer reactions involving the BOH group. The reaction of 3 with Cp(2)ZrMe(2) resulted in formation of Cp(2)Zr(Me)OB[OSi(O(t)Bu)(3)](2) (4) in high yield. This rare example of a transition metal boryloxide complex crystallizes in the triclinic space group Ponemacr; and exhibits a crystal structure with an unprecedented number of independent molecules in its asymmetric unit (i.e., Z' = 18 and Z = 36). This unusual crystal structure presented an opportunity to perform statistical analyses of the metric parameters for the 18 crystallographically independent molecules. Complex 4 readily converts to Cp(2)Zr[OSi(O(t)Bu)(3)](2) (5) upon thermolysis or upon dissolution in Et(2)O at room temperature.     

Lithiation of (tBuNH)3PNSiMe3 and formation of tetraimidophosphate complexes containing M3O3 rings (M = Li, K): X-ray structure of the stable radical [(Me3SiN)P(mu3-N(t)Bu)3[mu3-Li(THF)]3(OtBu))

The reaction of ((t)BuNH)(3)PNSiMe(3) (1) with 1 equiv of (n)BuLi results in the formation of Li[P(NH(t)Bu)(2)(N(t)Bu)(NSiMe(3))] (2); treatment of 2 with a second equivalent of (n)BuLi produces the dilithium salt Li(2)[P(NH(t)Bu)(N(t)Bu)(2)(NSiMe(3))] (3). Similarly, the reaction of 1 and (n)BuLi in a 1:3 stoichiometry produces the trilithiated species Li(3)[P(N(t)Bu)(3)(NSiMe(3))] (4). These three complexes represent imido analogues of dihydrogen phosphate [H(2)PO(4)](-), hydrogen phosphate [HPO(4)](2)(-), and orthophosphate [PO(4)](3)(-), respectively. Reaction of 4 with alkali metal alkoxides MOR (M = Li, R = SiMe(3); M = K, R = (t)Bu) generates the imido-alkoxy complexes [Li(3)[P(N(t)Bu)(3)(NSiMe(3))](MOR)(3)] (8, M = Li; 9, M = K). These compounds were characterized by multinuclear ((1)H, (7)Li, (13)C, and (31)P) NMR spectroscopy and, in the cases of 2, 8, and 9.3THF, by X-ray crystallography. In the solid state, 2 exists as a dimer with Li-N contacts serving to link the two Li[P(NH(t)Bu)(2)(N(t)Bu)(NSiMe(3))] units. The monomeric compounds 8 and 9.3THF consist of a rare M(3)O(3) ring coordinated to the (LiN)(3) unit of 4. The unexpected formation of the stable radical [(Me(3)SiN)P(mu(3)-N(t)Bu)(3)[mu(3)-Li(THF)](3)(O(t)Bu)] (10) is also reported. X-ray crystallography indicated that 10 has a distorted cubic structure consisting of the radical dianion [P(N(t)Bu)(3)(NSiMe(3))](.2)(-), two lithium cations, and a molecule of LiO(t)Bu in the solid state. In dilute THF solution, the cube is disrupted to give the radical monoanion [(Me(3)SiN)((t)BuN)P(mu-N(t)Bu)(2)Li(THF)(2)](.-), which was identified by EPR spectroscopy.     

EPR, 1H and 2H NMR, and reactivity studies of the iron-oxygen intermediates in bioinspired catalyst systems

Complexes [(BPMEN)Fe(II)(CH(3)CN)(2)](ClO(4))(2) (1, BPMEN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) and [(TPA)Fe(II)(CH(3)CN)(2)](ClO(4))(2) (2, TPA = tris(2-pyridylmethyl)amine) are among the best nonheme iron-based catalysts for bioinspired oxidation of hydrocarbons. Using EPR and (1)H and (2)H NMR spectroscopy, the iron-oxygen intermediates formed in the catalyst systems 1,2/H(2)O(2); 1,2/H(2)O(2)/CH(3)COOH; 1,2/CH(3)CO(3)H; 1,2/m-CPBA; 1,2/PhIO; 1,2/(t)BuOOH; and 1,2/(t)BuOOH/CH(3)COOH have been studied (m-CPBA is m-chloroperbenzoic acid). The following intermediates have been observed: [(L)Fe(III)(OOR)(S)](2+), [(L)Fe(IV)═O(S)](2+) (L = BPMEN or TPA, R = H or (t)Bu, S = CH(3)CN or H(2)O), and the iron-oxygen species 1c (L = BPMEN) and 2c (L = TPA). It has been shown that 1c and 2c directly react with cyclohexene to yield cyclohexene oxide, whereas [(L)Fe(IV)═O(S)](2+) react with cyclohexene to yield mainly products of allylic oxidation. [(L)Fe(III)(OOR)(S)](2+) are inert in this reaction. The analysis of EPR and reactivity data shows that only those catalyst systems which display EPR spectra of 1c and 2c are able to selectively epoxidize cyclohexene, thus bearing strong evidence in favor of the key role of 1c and 2c in selective epoxidation. 1c and 2c were tentatively assigned to the oxoiron(V) intermediates.     

Iron silylamide-grafted periodic mesoporous silica

The surface chemistry of a series of well-defined metalorganic ferrous and ferric iron complexes on periodic mesoporous silica (PMS) was investigated. In addition to literature known Fe(II)[N(SiMe(3))(2)](2)(THF), Fe(II)[N(SiPh(2)Me(2))(2)](2), and Fe(III)[N(SiMe(3))(2)](2)Cl(THF), the new complexes [Fe(II){N(SiHMe(2))(2)}(2)](2) and Fe(III)[N(SiHMe(2))(2)](3)(μ-Cl)Li(THF)(3) were employed as grafting precursors. Selection criteria for the molecular precursors were the molecular size (monoiron versus diiron species), the oxidation state of the iron center (II versus III), and the functionality of the silylamido ligand (e.g., built-in spectroscopic probes). Hexagonal channel-like MCM-41 and cubic cage-like SBA-1 were chosen as two distinct PMS materials. The highest iron load (12.8 wt %) was obtained for hybrid material [Fe(II){N(SiHMe(2))(2)}(2)](2)@MCM-41 upon stirring the reaction mixture iron silylamide/PMS/n-hexane for 18 h at ambient temperature. Size-selective grafting and concomitantly extensive surface silylation were found to be prominent for cage-like SBA-1. Here, the surface metalation is governed by the type of iron precursor, the pore size, the reaction time, and the solvent. The formation of surface-attached iron-ligand species is discussed on the basis of diffuse reflectance infrared Fourier transform (DRIFT) and electron paramagnetic resonance (EPR) spectroscopy, nitrogen physisorption, and elemental analysis.     

Long-range electronic coupling of MM quadruple bonds (M = Mo or W) via a 2,6-azulenedicarboxylate bridge

The preparation of 2,6-azulenedicarboxylic acid (I) from its diester, 2-CO(2)(t)Bu-6-CO(2)-C(10)H(6) (II), is reported together with the crystal and molecular structure of the ester, II. From the reactions between the dicarboxylic acid I and the MM quadruply bonded complexes M(2)(O(2)C(t)Bu)(4), where M = Mo or W, the azulenedicarboxylate bridged complexes [M(2)(O(2)C(t)Bu)(3)](2)(mu-2,6-(CO(2))(2)-C(10)H(6)) have been isolated, III (M = Mo) and IV (M = W). The latter compounds provide examples of electronically coupled M(2) centers via a polar bridge. The compounds show intense electronic absorptions due to metal-to-bridge charge transfer. This occurs in the visible region of the spectrum for III (M = Mo) but in the near-IR for IV (M = W). One electron oxidation with Ag(+)PF(6)(-) in THF generates the radical cations III(+) and IV(+). By both UV-vis-NIR and EPR spectroscopy the molybdenum ion III(+) is shown to be valence trapped or Class II on the Robin and Day classification scheme. Electrochemical, UV-vis-NIR, and EPR spectroscopic data indicate that, in the tungsten complex ion IV(+), the single electron is delocalized over the two W(2) centers that are separated by a distance of ca. 13.6 A. Furthermore, from the hyperfine coupling to (183)W (I = (1)/(2)), the singly occupied highest molecular orbital is seen to be polarized toward one W(2) center in relationship to the other. Electronic structure calculations employing density functional theory indicate that the HOMO in compounds III and IV is an admixture of the two M(2) delta orbitals that is largely centered on the M(2) unit having proximity to the C(5) ring of the azulenedicarboxylate bridge. The energy of the highest occupied orbital of the bridge lies very close in energy to the M(2) delta orbitals. However, this orbital does not participate in electronic coupling by a hole transfer superexchange mechanism, and the electronic coupling in the radical cations of III and IV occurs by electron transfer through the bridge pi system.     

Bismuth pyrostannate, Bi2Sn2O7, from the first structurally characterized heterobimetallic Bi:Sn alkoxides

The first heterobimetallic Bi:Sn alkoxide complexes [Bi(2)SnO(OCH(CF(3))(2))(5)(O(t)Bu)(3)(THF)] (1) and [BiSnO(OCH(CF(3))(2))(3)(O(t)Bu)(2)](2) (2) are described. The complexes were obtained through mixing and heating equimolar quantities of the component alkoxides, Bi(OCH(CF(3))(2))(3) and Sn(O(t)Bu)(4), under solvent-free conditions (1) and in THF (2). The solid-state structures were determined by single crystal X-ray diffraction showing ligand redistribution from Bi(III) to Sn(IV) in the two molecular species. Compound 2 behaves as a single-source precursor for the thermolytic formation of bismuth pyrostannate, Bi(2)Sn(2)O(7).     

Ground-state singlet L3Fe-(mu-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral Fe(II) centers

Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP(3)]Fe(mu-1,3-N(3))}(2) (2) ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP(3)]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135 degrees , and the short Fe-N bond distances (Fe-N(av) approximately equal to 1.70 A) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP(3)]Fe(II)(triple bond)N(1-Ad)}{(n)()Bu(4)N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP(3)]Fe(III)(triple bond)N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.     

An iron-peroxo porphyrin complex: new synthesis and reactivity toward a Cu(II) complex giving a heme-peroxo-copper adduct

Side-on eta2-peroxo-iron porphyrins are strong nucleophiles. In cytochrome P450-like aromatase and other enzymes, such species are postulated as the active oxidants. In cytochrome c oxidase, hemea3-peroxo, hemea3-hydroperoxo, or hemea3-(mu-peroxo)-copper species are proposed as transient intermediates forming prior to O-O bond cleavage. In this report, we describe (1) a facile method for reduction of a heme-O2 species [(F8TPP)FeIII(O2-)(S)] (2), generating the ferric peroxo porphyrin complex [(F8TPP)FeIII(O22-)]- (3) (UV-vis, THF: lambdamax = 435 (Soret), 540(sh), 561; EPR: g = 8.7, 4.2), and (2) that this can be subsequently reacted with a ligand-copper(II) complex, [CuII(TMPA)-(CH3CN)](ClO4)2 (4), affording a heme-peroxo-copper heterobinuclear compound, [(F8TPP)FeII(O22-)-CuII(TMPA)](ClO4) (5). Generation of [(F8TPP)FeIII(O22-)]- (3) using cobaltocene as a one-electron reductant was monitored by UV-vis, EPR, and 1H NMR spectroscopies. Reaction between 3 and 4 was followed by UV-vis spectroscopy, and the product 5 could be precipitated and characterized. Coordination by copper(II) in 5 makes possible further reduction of the mu-peroxo complex by cobaltocene yielding the mu-oxo analogue, [(F8TPP)FeIII(O2-)-CuII(TMPA)](ClO4) (6).     

Lithiation of tris(alkyl- and arylamido)orthophosphates EP[N(H)R]3 (E = O,S, Se): imido substituent effects and P=E bond cleavage

In the solid state, OP[N(H)Me](3) (1a) and OP[N(H)(t)Bu](3) (1b) have hydrogen-bonded structures that exhibit three-dimensional and one-dimensional arrays, respectively. The lithiation of 1b with 1 equiv of (n)BuLi generates the trimeric monolithiated complex (THF)[LiOP(N(t)Bu)[N(H)(t)Bu](2)](3) (4), whereas reaction with an excess of (n)BuLi produces the dimeric dilithium complex [(THF)(2)Li(2)OP(N(t)Bu)(2)[N(H)(t)Bu]](2) (5). Complex 4 contains a Li(2)O(2) ring in an open-ladder structure, whereas 5 embraces a central Li(2)O(2) ring in a closed-ladder arrangement. Investigations of the lithiation of tris(alkyl or arylamido)thiophosphates, SP[N(H)R](3) (2a, R = (i)Pr; 2b, R = (t)Bu; 2c, R = p-tol) with (n)BuLi reveal interesting imido substituent effects. For the alkyl derivatives, only mono- or dilithiation is observed. In the case of R = (t)Bu, lithiation is accompanied by P-S bond cleavage to give the dilithiated cyclodiphosph(V/V)azane [(THF)(2)Li(2)[((t)BuN)(2)P(micro-N(t)Bu)(2)P(N(t)Bu)(2)]] (9). Trilithiation occurs for the triaryl derivatives EP[N(H)Ar](3) (E = S, Ar = p-tolyl; E = Se, Ar = Ph), as demonstrated by the preparation of [(THF)(4)Li(3)[SP(Np-tol)(3)]](2) (10) and [(THF)(4)Li(3)[SeP(NPh)(3)]](2) (11), which are accompanied by the formation of small amounts of 10.[LiOH(THF)](2) and 11.Li(2)Se(2)(THF)(2), respectively.     

Synthesis and structures of aluminum and magnesium complexes of tetraimidophosphates and trisamidothiophosphates: EPR and DFT investigations of the persistent neutral radicals {Me2Al[(mu-NR)(mu-NtBu)P(mu-NtBu)2]Li(THF)2}(*) (R = SiMe3, tBu)

Reactions of (RNH)(3)PNSiMe(3) (3a, R = (t)()Bu; 3b, R = Cy) with trimethylaluminum result in the formation of {Me(2)Al(mu-N(t)Bu)(mu-NSiMe(3))P(NH(t)()Bu)(2)]} (4) and the dimeric trisimidometaphosphate {Me(2)Al[(mu-NCy)(mu-NSiMe(3))P(mu-NCy)(2)P(mu-NCy)(mu-NSiMe(3))]AlMe(2)} (5a), respectively. The reaction of SP(NH(t)Bu)(3) (2a) with 1 or 2 equiv of AlMe(3) yields {Me(2)Al[(mu-S)(mu-N(t)Bu)P(NH(t)()Bu)(2)]} (7) and {Me(2)Al[(mu-S)(mu-N(t)()Bu)P(mu-NH(t)Bu)(mu-N(t)Bu)]AlMe(2)} (8), respectively. Metalation of 4 with (n)()BuLi produces the heterobimetallic species {Me(2)Al[(mu-N(t)Bu)(mu-NSiMe(3))P(mu-NH(t)()Bu)(mu-N(t)()Bu)]Li(THF)(2)} (9a) and {[Me(2)Al][Li](2)[P(N(t)Bu)(3)(NSiMe(3))]} (10) sequentially; in THF solutions, solvation of 10 yields an ion pair containing a spirocyclic tetraimidophosphate monoanion. Similarly, the reaction of ((t)BuNH)(3)PN(t)()Bu with AlMe(3) followed by 2 equiv of (n)BuLi generates {Me(2)Al[(mu-N(t)Bu)(2)P(mu(2)-N(t)Bu)(2)(mu(2)-THF)[Li(THF)](2)} (11a). Stoichiometric oxidations of 10 and 11a with iodine yield the neutral spirocyclic radicals {Me(2)Al[(mu-NR)(mu-N(t)Bu)P(mu-N(t)Bu)(2)]Li(THF)(2)}(*) (13a, R = SiMe(3); 14a, R = (t)Bu), which have been characterized by electron paramagnetic resonance spectroscopy. Density functional theory calculations confirm the retention of the spirocyclic structure and indicate that the spin density in these radicals is concentrated on the nitrogen atoms of the PN(2)Li ring. When 3a or 3b is treated with 0.5 equiv of dibutylmagnesium, the complexes {Mg[(mu-N(t)()Bu)(mu-NH(t)()Bu)P(NH(t)Bu)(NSiMe(3))](2)} (15) and {Mg[(mu-NCy)(mu-NSiMe(3))P(NHCy)(2)](2)} (16) are obtained, respectively. The addition of 0.5 equiv of MgBu(2) to 2a results in the formation of {Mg[(mu-S)(mu-N(t)()Bu)P(NH(t)Bu)(2)](2)} (17), which produces the hexameric species {[MgOH][(mu-S)(mu-N(t)()Bu)P(NH(t)Bu)(2)]}(6) (18) upon hydrolysis. Compounds 4, 5a, 7-11a, and 15-17 have been characterized by multinuclear ((1)H, (13)C, and (31)P) NMR spectroscopy and, in the case of 5a, 9a.2THF, 11a, and 18, by X-ray crystallography.     

A general and modular synthesis of monoimidouranium(IV) dihalides

The conproportionation reaction between the dimeric diimidouranium(V) species [U(N(t)Bu)(2)(I)((t)Bu(2)bpy)](2) ((t)Bu(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridyl) and UI(3)(THF)(4) in the presence of additional (t)Bu(2)bpy yields U(N(t)Bu)(I)(2)((t)Bu(2)bpy)(THF)(2) (2), an unprecedented example of a monoimidouranium(IV) dihalide complex. The general synthesis of this family of uranium(IV) derivatives can be achieved more readily by adding 2 equiv of MN(H)R (M = Li, K; R = (t)Bu, 2,6-(i)PrC(6)H(3), 2-(t)BuC(6)H(4)) to UX(4) in the presence of coordinating Lewis bases to give complexes with the general formula U(NR)(X)(2)(L)(n) (X = Cl, I; L = (t)Bu(2)bpy, n = 1; L = THF, n = 2). The complexes were characterized by (1)H NMR spectroscopy and single-crystal X-ray diffraction analysis of compounds 2 and {U[N(2,6-(i)PrC(6)H(3))](Cl)(2)(THF)(2)}(2) (4). (The X-ray structures of 5 and 6 are reported in the Supporting Information.)     

A synthetic analogue of the active site of Fe-containing nitrile hydratase with carboxamido N and thiolato S as donors: synthesis, structure, and reactivities.

As part of our work on models of the iron(III) site of Fe-containing nitrile hydratase, a designed ligand PyPSH(4) with two carboxamide and two thiolate donor groups has been synthesized. Reaction of (Et(4)N)[FeCl(4)] with the deprotonated form of the ligand in DMF affords the mononuclear iron(III) complex (Et(4)N)[Fe(III)(PyPS)] (1) in high yield. The iron(III) center is in a trigonal bipyramidal geometry with two deprotonated carboxamido nitrogens, one pyridine nitrogen, and two thiolato sulfurs as donors. Complex 1 is stable in water and binds a variety of Lewis bases at the sixth site at low temperature to afford green solutions with a band around 700 nm. The iron(III) centers in these six-coordinate species are low-spin and exhibit EPR spectra much like the enzyme. The pK(a) of the water molecule in [Fe(III)(PyPS)(H(2)O)](-) is 6.3 +/- 0.4. The iron(III) site in 1 with ligated carboxamido nitrogens and thiolato sulfurs does not show any affinity toward nitriles. It thus appears that at physiological pH, a metal-bound hydroxide promotes hydration of nitriles nested in close proximity of the iron center in the enzyme. Redox measurements demonstrate that the carboxamido nitrogens prefer Fe(III) to Fe(II) centers. This fact explains the absence of any redox behavior at the iron site in nitrile hydratase. Upon exposure to limited amount of dioxygen, 1 is converted to the bis-sulfinic species. The structure of the more stable O-bonded sulfinato complex (Et(4)N)[Fe(III)(PyP[SO(2)](2))] (2) has been determined. Six-coordinated low-spin cyanide adducts of the S-bonded and the O-bonded sulfinato complexes, namely, Na(2)[Fe(III)(PyP[SO(2)](2))(CN)] (4) and (Et(4)N)(2)[Fe(III)(PyP[SO(2)](2))(CN)] (5), afford green solutions in water and other solvents. The iron(II) complex (Et(4)N)(2)[Fe(II)(PyPS)] (3) has also been isolated and structurally characterized.     

Electronic structure of square planar bis(benzene-1,2-dithiolato)metal complexes [M(L)(2)](z) (z = 2-, 1-, 0; M = Ni, Pd, Pt, Cu, Au): an experimental, density functional, and correlated ab initio study

The three diamagnetic square planar complexes of nickel(II), palladium(II), and platinum(II) containing two S,S-coordinated 3,5-di-tert-butylbenzene-1,2-dithiolate ligands, (L(Bu))(2-), namely [M(II)(L(Bu))(2)](2-), have been synthesized. The corresponding paramagnetic monoanions [M(II)(L(Bu))(L(Bu)(*))](-) (S = (1)/(2)) and the neutral diamagnetic species [M(II)(L(Bu)(*))(2)] (M = Ni, Pd, Pt) have also been generated in solution or in the solid state as [N(n-Bu)(4)][M(II)(L(Bu))(L(Bu)(*))] salts. The corresponding complex [Cu(III)(L(Bu))(2)](-) has also been investigated. The complexes have been studied by UV-vis, IR, and EPR spectroscopy and by X-ray crystallography; their electro- and magnetochemistry is reported. The electron-transfer series [M(L(Bu))(2)](2-,-,0) is shown to be ligand based involving formally one (L(Bu)(*))(-) pi radical in the monoanion or two in the neutral species [M(II)(L(Bu)(*))(2)] (M = Ni, Pd, Pt). Geometry optimizations using all-electron density functional theory with scalar relativistic corrections at the second-order Douglas-Kroll-Hess (DKH2) and zeroth-order regular approximation (ZORA) levels result in excellent agreement with the experimentally determined structures and electronic spectra. For the three neutral species a detailed analysis of the orbital structures reveals that the species may best be described as containing two strongly antiferromagnetically interacting ligand radicals. Furthermore, multiconfigurational ab initio calculations using the spectroscopy oriented configuration interaction (SORCI) approach including the ZORA correction were carried out. The calculations predict the position of the intervalence charge-transfer band well. Chemical trends in the diradical characters deduced from the multiconfigurational singlet ground-state wave function along a series of metals and ligands were discussed.     

Methanolysis and phenolysis routes to Fe6, Fe8, and Fe1) complexes and their magnetic properties: a new type of Fe8 ferric wheel

Alcoholysis of preformed tetranuclear and hexanuclear iron(III) clusters has been employed for the synthesis of four higher-nuclearity clusters. Treatment of [Fe(4)O(2)(O(2)CMe)(7)(bpy)(2)](ClO(4)) with phenol affords the hexanuclear cluster [Fe(6)O(3)(O(2)CMe)(9)(OPh)(2)(bpy)(2)](ClO(4)) (1). Reaction of [Fe(6)O(2)(OH)(2)(O(2)CR)(10)(hep)(2)] (R = Bu(t) or Ph) with PhOH affords the new "ferric wheel" complexes [Fe(8)(OH)(4)(OPh)(8)(O(2)CR)(12)] [R = Bu(t) (2) or Ph (3)]. Complexes 2 and 3 exhibit the same structure, which is an unprecedented type for Fe(III). In contrast, treatment of [Fe(6)O(2)(OH)(2)(O(2)CBu(t))(10)(hep)(2)] with MeOH leads to the formation of [Fe(10)(OMe)(20)(O(2)CBu(t))(10)] (4), which exhibits the more common type of ferric wheel seen in analogous complexes with other carboxylate groups. Solid-state variable-temperature magnetic susceptibility measurements indicate spin-singlet ground states for complexes 2 and 4. The recently developed semiempirical method ZILSH was used to estimate the pairwise exchange parameters (J(AB)) and the average spin couplings S(A)[empty set].S(B)[empty set] between the Fe(III) centers, providing a clear depiction of the overall magnetic behavior of the molecules. All exchange interactions between adjacent Fe(III) atoms are antiferromagnetic.     

Synthesis and X-ray crystal structure of a novel organometallic (μ3-oxido)(μ3-imido) trinuclear iridium complex

Reaction of the organometallic aqua ion [Cp*Ir(H(2)O)(3)](2+) with tert-butyl(trimethylsilyl)amine in acetone yielded a novel trinuclear (μ(3)-oxido)(μ(3)-imido)pentamethylcyclopentadienyliridium(III) complex, [(Cp*Ir)(3)(O)(N(t)Bu)](2+). Single crystal structure analyses show the complex can be isolated both in the double salt ((t)BuNH(3))[(Cp*Ir)(3)(O)(N(t)Bu)](CF(3)SO(3))(3) (1) and in the simple triflate [(Cp*Ir)(3)(O)(N(t)Bu)](CF(3)SO(3))(2) (2). The double salt is stabilized by hydrogen bonding between the tert-butylammonium ion and the three triflate anions. It is the first time that a trinuclear (μ(3)-oxido)(μ(3)-imido) transition metal complex has been structurally characterized.     

Catalytic activity of MCM-48-, SBA-15-, MCF-, and MSU-type mesoporous silicas modified with Fe3+ species in the oxidative dehydrogenation of ethylbenzene in the presence of N2O

MCM-48, SBA-15, MCF, and MSU mesoporous silicas were used as supports for a deposition of Fe oxide species. Iron was introduced using two different methods: the wetness impregnation and the molecular designed dispersion (MDD). The obtained catalysts were characterized with respect to their textural parameters (BET), chemical composition (electron microprobe analysis), and reducibility (TPR). The coordination environment of Fe was determined using EPR and UV-vis/DRS. The samples were tested as catalysts in the oxidative dehydrogenation of ethylbenzene to styrene in the presence of N(2)O. An influence of Fe dispersion and reducibility on the catalytic activity was discussed. Isolated Fe(3+) species appeared to be more selective in the styrene formation, whereas iron oxide clusters showed a higher selectivity in total oxidation of aromatic hydrocarbons. The reaction system was well described by the Mars- van Krevellen mechanism.     

Synthesis and X-ray structures of dilithium complexes of the phosphonate anions [PhP(E)(N(t)Bu)(2)](2-) (E = O,S, Se,Te) and dimethylaluminum derivatives of [PhP(E)(N(t)Bu)(NH(t)Bu)](-) (E = S,Se)

The dilithium salts of the phosphonate dianions [PhP(E)(N(t)Bu)(2)](2-) (E = O, S, Se) are generated by the lithiation of [PhP(E)(NH(t)Bu)(2)] with n-butyllithium. The formation of the corresponding telluride (E = Te) is achieved by oxidation of [Li(2)[PhP(N(t)Bu)(2)]] with tellurium. X-ray structural determinations revealed dimeric structures [Li(THF)(2)[PhP(E)(N(t)Bu)(2)]](2) in which the monomeric units are linked by Li-E bonds. In the case of E = Se or Te, but not for E = S, transannular Li-E interactions are also observed, resulting in a six-rung ladder. By contrast, for E = O, this synthetic approach yields the Li(2)O-templated tetramer [(THF)Li(2)[PhP(O)(N(t)Bu)(2)]](4).Li(2)O in THF or the tetramer [(Et(2)O)(0.5)Li(2)[PhP(O)(N(t)Bu)(2)]](4) in diethyl ether. The reaction of trimethylaluminum with PhP(E)(NH(t)Bu)(2) produces the complexes Me(2)Al[PhP(E)(N(t)Bu)(NH(t)Bu)] (E = S, Se), which were shown by X-ray crystallography to be N,E-chelated monomers.     

Investigations on the interaction of dichloroaluminum carboxylates with Lewis bases and water: an efficient road toward oxo- and hydroxoaluminum carboxylate complexes

A series of dichloroaluminum carboxylates [Cl(2)Al(O(2)CR)](2) (were R = Ph (1a), (t)Bu (1b), CHCH(2) (1c) and C(11)H(23) (1d)) were prepared and extended investigations on their structure and reactivity toward various Lewis bases and H(2)O performed. Compounds [Cl(2)Al(O(2)CR)](2) and their adducts with Lewis bases show a large structural variety, featuring both molecular and ionic forms with different coordination numbers of the metal center and various coordination modes of the carboxylate ligand. Upon addition of a Lewis base of moderate strength the molecular form [Cl(2)Al(O(2)CR)](2) equilibrates with new ionic forms. In the presences of 4-methylpyridine the six-coordinate Lewis acid-base adducts [Cl(2)Al(λ(2)-O(2)CR)(py-Me)(2)] [R = Ph (3a), (t)Bu (3b)] with a chelating carboxylate ligand were formed. The reactions of 1a, 1b, and 1d with 0.33 equiv of H(2)O in THF-toluene solution lead to oxo carboxylates [(Al(3)O)(O(2)CR)(6)(THF)(3)] [AlCl(4)] [where R = Ph (4a(THF)), (t)Bu (4b(THF)), and C(11)H(23) (4d(THF))] in high yield. The similar reaction of 1c in tetrahydrofuran (THF) afforded the chloro(hydroxo)aluminum acrylate [(ClAl)(2)(OH)(O(2)CC(2)H(3))(2) (THF)(4)][AlCl(4)] (5), while the hydrolysis of 1b in MeCN lead to the hydroxoaluminum carboxylate [Al(2)(OH)(O(2)C(t)Bu)(2)(MeCN)(6)][AlCl(4))(3)] (6). All compounds were characterized by elemental analysis, (1)H, (27)Al NMR, and IR spectroscopy, and the molecular structure of 1a, 3a, 3b, 4a(THF), 4b(THF), 4b(py-Me'), 5, and 6 were determined by single-crystal X-ray diffraction. The study provides a platform for testing transformations of secondary building units in Al-Metal-Organic Frameworks toward H(2)O and neutral donor ligands.