Sequential protonation and methylation of a hydride-osmium complex containing a cyclopentadienyl ligand with a pendant amine group

Complex OsH{eta5-C5H4(CH2)2NMe2}(P(i)Pr3)2 (1) reacts with 1 equiv of trifluoromethanesulfonic acid (HOTf) and trifluoromethanesulfonic acid-d1 (DOTf) to produce the dihydride and hydride-deuteride complexes, [OsHE{eta5-C5H4(CH2)2NMe2}(P(i)Pr3)2]OTf (E = H (2), D (2-d1), respectively. Treatment of 2 and 2-d1 with a second equivalent of HOTf gives [OsHE{eta5-C5H4(CH2)2NHMe2}(P(i)Pr3)2][OTf]2 (E = H (3), D (3-d1) as a result of the protonation of the nitrogen atom. While the hydride and deuteride ligands of 2, 2-d1, 3, and 3-d1 do not undergo any H/D exchange process with the solvent, in acetone-d6, the NH proton of 3 and 3-d1 changes places with a deuterium atom of the solvent to yield [OsHE{eta5-C5H4(CH2)2NDMe2}(P(i)Pr3)2][OTf]2 (E = H (3-Nd1), D (3-d2)). Complex 3-Nd1 can also be obtained from the treatment of complex 2 with DOTf in dichloromethane. No exchange process between the hydride and the ND positions in 3-Nd1 or between the deuteride and NH positions in 3-d1 has been observed. Treatment of 3-Nd1 and 3-d1 with sodium methoxide results in a selective reaction of the base with the ammonium group to regenerate 2 and 2-d1, respectively. Complex 1 also reacts with methyl and methyl-d3 trifluoromethanesulfonate (CH3OTf and CD3OTf, respectively) to give [OsH{eta5-C5H4(CH2)2NMe2CE3}(P(i)Pr3)2]OTf (E = H (4), D (4-d3)) as a result of the addition of the CE3 (E = H, D) group to the nitrogen atom. Complex 4 has been characterized by an X-ray diffraction analysis. It reacts with a second molecule of CH3OTf or CD3OTf to produce [OsH{eta5-C5H4(CH2)2NMe3}{CH2CH(CH3)P(i)P2}(P(i)Pr3)[OTf]2 (5). Similarly, complex 4-d3 reacts with a second molecule of CH3OTf or CD3OTf to yield [OsH{eta5-C5H4(CH2)2NMe2CD3}{CH2CH(CH3)P(i)P2}(P(i)Pr3)[OTf]2 (5-d3). In acetonitrile, complex 5 evolves to an equilibrium mixture of the acetonitrile adducts [Os{eta5-C5H4(CH2)2NMe3}(NCCH3)(P(i)Pr3)2][OTf]2 (7) and [Os{eta5-C5H4(CH2)2NMe3}(NCCH3)2(P(i)Pr3)][OTf]2 (8). In methanol or methanol-d4, complex 4 is not stable and loses trimethylamine to give the vinylcyclopentadienyl derivatives [OsHE(eta5-C5H4CH=CH2)(P(i)Pr3)2]OTf (E = H (9), D (9-d1)) as a result of the protonation or deuteration of the metallic center and a subsequent Hofmann elimination. Protonation of 4 with HOTf gives the dihydride-trimethylammonium derivative [OsH2{eta5-C5H4(CH2)2NMe3}(P(i)Pr3)2][OTf]2 (10). Treatment of 9 with sodium methoxide produces OsH(eta5-C5H4CH=CH2)(P(i)Pr3)2 (11).     

From a paramagnetic, mononuclear supersulfidonickel(II) complex to a diamagnetic dimer with a four-sulfur two-electron bond

The first isolable "side-on" supersulfidonickel(II) complex 1(1) with the elusive S2-. ligand has been synthesized by facile oxidation of the corresponding nickel(I) precursor [LNi] with elemental sulfur. Remarkably, paramagnetic 1(1) associates voluntarily to give the diamagnetic dimer 1(2) with a four-sulfur two-electron bond as proven by X-ray diffraction analysis, spectroscopic measurements (1H NMR, EPR, SQUID), and DFT calculations. Gentle sulfur transfer of 1(1) to Ph3P or its reaction with [LNi] affords solely the genuine disulfide complex 3 having a Ni2(mu-mu2:mu2-S2) core.     

Cyclodextrin complexation of a stilbene and the self-assembly of a simple molecular device

(E)-4-tert-Butyl-4'-oxystilbene, 1(-), is thermally stable as the (E)-1(-) isomer but may be photoisomerized to the (Z)-1(-) isomer as shown by UV-vis and (1)H NMR studies in aqueous solution. When (E)-1(-) is complexed by alphaCD two inclusion isomers (includomers) form in which alphaCD assumes either of the two possible orientations about the axis of (E)-1(-) in alphaCD.(E)-1(-) for which (1)H NMR studies yield the parameters: k(1)(298 K)= 12.3 +/- 0.6 s(-1), DeltaH(1)(++)= 94.3 +/- 4.7 kJ mol(-1), DeltaS1(++)= 92.0 +/- 5.0 J K(-1) mol(-1), and k(2)(298 K)= 10.7 +/- 0.5 s(-1), DeltaH(2)(++)= 93.1 +/- 4.7 kJ mol(-1), DeltaS2(++)= 87.3 +/- 5.0 J K(-1) mol(-1) for the minor and major includomers, respectively. The betaCD.(E)-1(-) complex either forms a single includomer or its includomers interchange at the fast exchange limit of the (1)H NMR timescale. Complexation of 1(-) by N-(6(A)-deoxy- alpha-cyclodextrin-6(A)-yl)-N'-(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, results in the binary complexes 2.(E)-1(-) in which both CD component annuli are occupied by (E)-1(-) and which exists exclusively in darkness and 2.(Z)-1(-) in which only one CD component is occupied by (Z)-1(-) and exists exclusively in daylight at lambda > or = 300 nm. Irradiation of solutions of the binary complexes at 300 and 355 nm results in photostationary states dominated by 2.(E)-1(-) and 2.(Z)-1(-), respectively. In the presence of 4-methylbenzoate, 4(-), 2.(Z)-1(-) forms the ternary complex 2.(Z)-1(-).4(-) where 4(-) occupies the second CD annulus. Interconversion occurs between 2.(Z)-1(-).4(-) and 2.(E)-1(-)+4(-) under the same conditions as for the binary complexes alone. Similar interactions occur in the presence of 4-methylphenolate and 4-methylphenylsulfonate. The two isomers of each of these systems represent different states of a molecular device, as do the analogous binary complexes of N,N-bis(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, 3, [3.(E)-1(-) and 3.(Z)-1(-), where the latter also forms a ternary complex with 4(-).     

Selective and reversible base-induced elimination of a ruthenium dithioether yields a vinyl metallosulfonium complex

Chemical oxidation of tris(2-diphenylphosphinebenzenethiolato)ruthenate(II) [Ru-1](-) with ferrocenium hexafluorophosphate in the presence of ethylene yields [(2-diphenylphosphinebenzenethiolato)(ethane-1,2-diylbis(thio-2,1-phenylene)diphenylphosphine)ruthenium(II)] hexafluorophosphate, [Ru-1·C(2)H(4)]PF(6), from addition of the alkene across cis sulfur sites. The [Ru-1·C(2)H(4)](+) complex displays a single redox couple at +794 mV versus ferrocenium/ferrocene. (1)H NMR of [Ru-1·C(2)H(4)](+) displays ethylene resonances at δ = 1.29 (td, 1H), 1.59 (td, 1H), 2.78 (dd, 1H), and 3.03 (dd, 1H). In the presence of base [Ru-1·C(2)H(4)](+) is selectively deprotonated at the pseudoequatorial proton on the carbon α to the sulfur trans to phosphorus, yielding the vinyl metallosulfonium derivative [Ru-1·C(2)H(3)]. (1)H and (31)P NMR spectra of [Ru-1·C(2)H(3)] are temperature dependent, associated with inversion of the sulfur lone pair at the vinyl metallosulfonium. The activation energy for the fluxional process calculated using density functional theory (B3LYP/LANL2DZ+6-31g) of 14.36 kcal/mol is consistent with the experimentally determined value of 13.08 kcal/mol. The complex [Ru-1·C(2)H(3)] crystallizes as yellow blocks in the triclinic space group P-1 with unit cell dimensions of a = 11.2718(5) ?, b =12.0524(3) ?, c = 23.6075(10) ?, α = 101.715(3)°, β = 98.154(4)°, and γ = 105.209(3)°. Addition of hydrochloric acid to [Ru-1·C(2)H(3)] regenerates [Ru-1·C(2)H(4)](+). Addition of DCl confirms the selectivity of this reverse reaction.     

A large thermal hysteresis loop produced by a charge-transfer phase transition in a rubidium manganese hexacyanoferrate

In a rubidium manganese hexacyanoferrate, RbMn[Fe(CN)(6)], the magnetic susceptibility (chi(M)) decreased at 225 K (=T(1/2)decreasing) and abruptly increased at 300 K (=T(1/2)increasing) in the cooling and warming processes, respectively. X-ray photoelectron spectroscopy and infrared spectroscopy indicated that the high-temperature (HT) and low-temperature (LT) phases were composed of Mn(II)-NC-Fe(III) and Mn(III)-NC-Fe(II), respectively. A structural change from cubic (F43m, a = 10.533 A) to tetragonal (I4m2, a = b = 7.090 A, c = 10.520 A) accompanied the phase transition, and, on the basis of these results, the HT and LT phases were assigned to Mn(II)(t(2g)(3)e(g)(2), (6)A(1g); S = (5)/(2))-NC-Fe(III) (t(2g)(5), (2)T(2g); S = (1)/(2)) and Mn(III)(e(g)(2)b(2g)(1)a(1g)(1), (5)B(1g); S = 2)-NC-Fe(II) (b(2g)(2)e(g)(4), (1)A(1g); S = 0), respectively. This phenomenon is caused by a metal-to-metal charge transfer from Mn(II) to Fe(III) and a Jahn-Teller distortion of the produced Mn(III) ion. The reaction mechanism is discussed, considering the entropy difference between the HT and LT phases.     

Stabilization of a helical water chain in a metal-organic host of a trinuclear Schiff base complex

Three heterometallic trinuclear Schiff base complexes, [{CuL1(H2O)}2Ni(CN)4].4H2O (1), [{CuL2(H2O)}2Ni(CN)4] (2), and [{CuL3(H2O)}2Ni(CN)4] (3) (HL1=7-amino-4-methyl-5-azahept-3-en-2-one, HL2=7-methylamino-4-methyl-5-azahept-3-en-2-one, and HL3=7-dimethylamino-4-methyl-5-azahept-3-en-2-one), were synthesized. All three complexes were characterized by elemental analysis, IR and UV spectroscopies, and thermal analysis. Two of them (1 and 3) were also characterized by single crystal X-ray crystallography. Complex 1 forms a hydrogen-bonded one-dimensional metal-organic framework that stabilizes a helical water chain into its cavity, but when any of the amine hydrogen atoms of the Schiff base are replaced by methyl groups, as in L2 and L3, the water chain vanishes, showing explicitly the importance of the host-guest H-bonding interactions for the stabilization of a water cluster.     

Alkene oxidation by a platinum oxo complex and isolation of a platinaoxetane

Oxo complex [(1,5-COD)4Pt4(mu3-O)2Cl2](BF4)2 (1) reacts readily with ethylene and norbornylene. The ethylene reaction yields acetaldehyde and a 1:1 mixture of (1,5-COD)Pt(Cl)(CH2CH3) (2) and [(1,5-COD)Pt4(eta3-CH2CHCH(CH3))](BF4) (3), while the norbornylene reaction yields a platinaoxetane complex, the first metallaoxetane to be obtained from the reaction of an oxo complex and an alkene.     

Site-directed regiocontrolled synthesis of a 'head-to-head' photodimer via a single-crystal-to-single-crystal transformation involving a linear template

Co-crystallisation of 1,8-naphthalenedicarboxylic acid (1,8-nap) with trans-1-(3-pyridyl)-2-(4-pyridyl)ethylene (3,4-bpe) gives a discrete molecular solid-state assembly, 2(3,4-bpe).2(1,8-nap) 1, that is held together by four O-H...N hydrogen bonds wherein the diacid directs a regiocontrolled [2 + 2] photodimerization; the reaction occurs by way of a single-crystal-to-single-crystal transformation.     

Precipitation of a dioxouranium(VI) species from a room temperature ionic liquid medium

The novel complex 1-butyl-3-methylimidazolium mu(4)-(O,O,O',O'-ethane-1,2-dioato)-bis[bis(nitrato-O,O)dioxouranate(VI)] (1) has been precipitated from a room-temperature ionic liquid medium containing 1-butyl-3-methylimidazolium nitrate, nitric acid, and acetone. X-ray analysis of complex 1 shows the unit cell contains four [C(4)mim](+) cations and two independent [[UO(2))(NO(3))(2)](2)(mu(4)-C(2)O(4))](2-) moieties, both of which are located about inversion centers. The [C(4)mim](+) cations are arranged such that they produce large channels in which the anions are located. This arrangement of [[(UO(2))(NO(3))(2)](2)(mu(4)-C(2)O(4))](2-) groups is unique to this compound. Crystal data for compound 1: M = 1154.56, monoclinic, space group P2(1)/c, a = 15.452(2) A, b = 20.354(3) A, c = 10.822(4) A, beta = 106.84(2) degrees, U = 3258(1) A(-)(3), Z = 4, mu = 10.023 mm(-1), R(int) = 0.0788.     

Isocytosine as a hydrogen-bonding partner and as a ligand in metal complexes

Isocytosine (ICH; 1) exists in solution in an equilibrium of tautomers 1a and 1b with the N1 and N3 positions carrying the acidic proton, respectively. In the solid state, both tautomers coexist in a 1:1 ratio. As we show, the N3H tautomer 1b can selectively be crystallized in the presence of the model nucleobase 1-methylcytosine (1-MeC). The complex 1b x (1-MeC)2 x H2O (2) forms pairs through three hydrogen bonds between the components; hydrogen bonds between identical molecules are also formed, leading to an infinite tape structure. On the other hand, the N1H tautomer 1a co-crystallizes with protonated ICH to give [1a x ICH2]NO3 (3), again with three hydrogen bonds between the partners, yet the acidic proton is disordered over the two entities. With M(II)(dien) (M=Pt, Pd; dien=diethylenetriamine) preferential coordination of tautomer 1a through the N3 position is observed. DFT calculations, which were also extended to Pt(II)(tmeda) linkage isomers (tmeda=N,N,N',N'-tetramethylethylenediamine), suggest that intramolecular hydrogen bonding between the ICH tautomers and the co-ligands at M, while adding to the preference for N3 coordination, is not the major determining factor. Rather it is the inherently stronger Pt-N3 bond which favors complexation of 1a. With an excess of M(II)(dien), dinuclear species [M2(dien)2(IC-N1,N3)]3+ (M=Pd(II), 4 and Pt(II), 5) also form and were isolated as their ClO4(-) salts and structurally characterized. In strongly acidic medium 5 is converted to [Pt(dien)(ICH-N1)]2+ (6), that is, to the Pt(II) complex of tautomer 1b.     

Substitution and derivatization reactions of a water soluble iron(II) complex containing a self-assembled tetradentate phosphine ligand

Facile substitution reactions of the two water ligands in the hydrophilic tetradentate phosphine complex cis-[Fe{(HOCH2)P{CH2N(CH2P(CH2OH)2)CH2}2P(CH2OH)}(H2O)2](SO4) (abbreviated to [Fe(L1)(H2O)2](SO4), 1) take place upon addition of Cl-, NCS-, N3(-), CO3(2-) and CO to give [Fe(L1)X2] (2, X = Cl; 4, X = NCS; 5, X=N3), [Fe(L1)(kappa2-O(2)CO)], 6 and [Fe(L1)(CO)2](SO4), 7. The unsymmetrical mono-substituted intermediates [Fe(L1)(H2O)(CO)](SO(4)) and [Fe(L(1))(CO)(kappa(1)-OSO(3))] (8/9) have been identified spectroscopically en-route to 7. Treatment of 1 with acetic anhydride affords the acylated derivative [Fe{(AcOCH2)P{CH2N(CH2P(CH2OAc)2)CH2}2P(CH2OAc)}(kappa2-O(2)SO2)] (abbreviated to [Fe(L2)(kappa2-O(2)SO2)], 10), which has increased solubility over 1 in both organic solvents and water. Treatment of 1 with glycine does not lead to functionalisation of L1, but substitution of the aqua ligands occurs to form [Fe(L(1))(NH(2)CH(2)CO(2)-kappa(2)N,O)](HSO(4)), 11. Compound 10 reacts with chloride to form [Fe(L(2))Cl(2)] 12, and 12 reacts with CO in the presence of NaBPh4 to form [Fe(L2)Cl(CO)](BPh4) 13b. Both of the chlorides in 12 are substituted on reaction with NCS- and N3(-) to form [Fe(L2)(NCS)2] 14 and [Fe(L2)(N3)2] 15, respectively. Complexes 2.H2O, 4.2H2O, 5.0.812H2O, 6.1.7H2O, 7.H2O, 10.1.3CH3C(O)CH3, 12 and 15.0.5H2O have all been crystallographically characterised.     

Iriomoteolide-3a, a cytotoxic 15-membered macrolide from a marine dinoflagellate amphidinium species

A 15-membered macrolide, iriomoteolide-3a (1), with an allyl epoxide has been isolated from a marine benthic dinoflagellate Amphidinium sp. (strain HYA024), and the structure was assigned by detailed analyses of 2D NMR data. Relative and absolute configurations were elucidated on the basis of conformational studies of 1 and its acetonide (2) and modified Mosher's method of 1, respectively. Iriomoteolide-3a (1) and the acetonide (2) exhibited potently cytotoxic activity against antitumor cells.     

Dynamic equilibrium between cyclic oligomers. Thermodynamic and structural characterization of a square and a triangle

A dynamic equilibrium has been found in CDCl3 between a neutral molecular square, [cis-Mo2(DAniF)2]4(O2CC6F4CO2)4 (1) and triangle, [cis-Mo2(DAniF)2]3(O2CC6F4CO2)3 (2) (DAniF = the anion of N,N'-di-p-anisylformamidine). The two components have been crystallographically characterized and solution studies by 1H and 19F NMR spectra of the concentration- and the temperature-dependence of the equilibrium have been performed. The conversion of three moles of molecular squares 1 to four moles of molecular triangles 2 has an equilibrium constant of 1.98(7) x 10(-4) at 23.7 degrees C. At this temperature, the DeltaG(0) for the conversion of three moles of squares to four moles of triangles is 21.0 kJ mol(-1). The conversion is enthalpically disfavored (DeltaH(0) = 23.5 kJ mol(-1)), but entropically favored (DeltaS(0) = 8.2 J K(-1) mol(-1)).     

Reduction of nitrous acid with a macrocyclic rhodium complex that acts as a functional model of nitrite reductase

Nitrous acid reacts with L(2)(H(2)O)Rh(2+) (L(2) = meso-hexamethylcyclam) in acidic aqueous solutions to generate a strongly absorbing intermediate Int-1 (λ(max) 400 nm, ε = 1200 M(-1) cm(-1)). The reaction follows a mixed second order rate law with k = (6.9 ± 0.3) × 10(4) M(-1) s(-1), independent of [H(+)]. The lack of acid dependence shows that Int-1 is a rhodium(II) complex of HNO(2), most reasonably assigned as L(2)(H(2)O)Rh(HNO(2))(2+). This species is analogous to the early iron and copper intermediates in the reduction of nitrite by nitrite reductases and by deoxyhemoglobin. In the presence of excess L(2)(H(2)O)Rh(2+), the lifetime of Int-1 is about 1 min. It decays to a 1:1 mixture of L(2)(H(2)O)RhNO(2+) and L(2)Rh(H(2)O)(2)(3+) with kinetics that are largely independent of the concentration of excess L(2)(H(2)O)Rh(2+) and of [H(+)] at [H(+)] < 0.03 M. At [H(+)] > 0.03 M, an acid-catalyzed pathway becomes effective, suggesting protonation and dehydration of Int-1 to generate L(2)(H(2)O)RhNO(3+) (Int-2) followed by rapid reduction of Int-2 by excess L(2)(H(2)O)Rh(2+). Int-2, which was generated and characterized independently, is an analog of the electrophilic intermediates in the mechanism of biological reduction of nitrite to (?)NO. Excess nitrite greatly reduces the lifetime of Int-1, which under such conditions decomposes on a millisecond time scale by nitrite-catalyzed disproportionation to yield L(2)(H(2)O)RhNO(2+) and L(2)Rh(III). This reaction provides additional support for the designation of Int-1 as a Rh(II) species. The complex reaction mechanism and the detection of Int-1 demonstrate the ability of inorganic complexes to perform the fundamental chemistry believed to take place in the biological reduction of HNO(2) to NO catalyzed by nitrite reductases or deoxyhemoglobin.     

Borane-mediated silylation of a metal-oxo ligand

The addition of 1 equiv of HSiPh(3) to UO(2)((Ar)acnac)(2) ((Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)), in the presence of 1 equiv of B(C(6)F(5))(3), results in the formation of U(OSiPh(3))(OB{C(6)F(5)}(3))((Ar)acnac)(2) (1), via silylation of an oxo ligand and reduction of the uranium center. The addition of 1 equiv of Cp(2)Co to 1 results in a reduction to uranium(IV) and the formation of [Cp(2)Co][U(OSiPh(3))(OB{C(6)F(5)}(3))((Ar)acnac)(2)] (2) in 78% yield. Complexes 1 and 2 have been characterized by X-ray crystallography, while the solution-phase redox properties of 1 have been measured with cyclic voltammetry.     

1,2-Shift of a Carboxyl Group in a Wagner-Meerwein Rearrangement

On treatment with HSO₃F in SO₂C1F at 0°, 3-hydroxy-2,2-dimethyl-3-phenyl-propionic acid ( 1a ) is transformed into 2-phenyl-3-methyl-2-butenoic acid ( 2a ) (isolated yield: 40–44%). Using monolabelled [3-¹³C]- 1a ( 1a *) and doubly labelled [1,3-¹³C₂]- 1a ( 1a **), the migration of HOOC (or a mechanistically equivalent group) was proved; a cross experiment established the intramolecular character of the rearrangement. By following the reaction at low temperature in an NMR. spectrometer, the formation of intermediates and side products was demonstrated.     

Relaxation dynamics of a linear molecule in a random static medium: a scaling analysis

We present extensive molecular dynamics simulations of the motion of a single linear rigid molecule in a two-dimensional random array of fixed overlapping disklike obstacles. The diffusion constants for the center of mass translation, D(CM), and for rotation, D(R), are calculated for a wide range of the molecular length, L, and the density of obstacles, rho. The obtained results follow a master curve Drho(micro) approximately (L(2)rho)(-nu) with an exponent micro=-3/4 and 1/4 for D(R) and D(CM), respectively, that can be deduced from simple scaling and kinematic arguments. The nontrivial positive exponent nu shows an abrupt crossover at L(2)rho=zeta(1). For D(CM) we find a second crossover at L(2)rho=zeta(2). The values of zeta(1) and zeta(2) correspond to the average minor and major axis of the elliptic holes that characterize the random configuration of the obstacles. A violation of the Stokes-Einstein-Debye relation is observed for L(2)rho>zeta(1), in analogy with the phenomenon of enhanced translational diffusion observed in supercooled liquids close to the glass transition temperature.     

Fast photodriven electron spin coherence transfer: a quantum gate based on a spin exchange j-jump

Photoexcitation of the electron donor (D) within a linear, covalent donor-acceptor-acceptor molecule (D-A(1)-A(2)) in which A(1) = A(2) results in sub-nanosecond formation of a spin-coherent singlet radical ion pair state, (1)(D(+?)-A(1)(-?)-A(2)), for which the spin-spin exchange interaction is large: 2J = 79 ± 1 mT. Subsequent laser excitation of A(1)(-?) during the lifetime of (1)(D(+?)-A(1)(-?)-A(2)) rapidly produces (1)(D(+?)-A(1)-A(2)(-?)), which abruptly decreases 2J 3600-fold. Subsequent coherent spin evolution mixes (1)(D(+?)-A(1)-A(2)(-?)) with (3)(D(+?)-A(1)-A(2)(-?)), resulting in mixed states which display transient spin-polarized EPR transitions characteristic of a spin-correlated radical ion pair. These photodriven J-jump experiments show that it is possible to use fast laser pulses to transfer electron spin coherence between organic radical ion pairs and observe the results using an essentially background-free time-resolved EPR experiment.     

Snapshots of a metamorphosing Cu(II) ground state in a galactose oxidase-inspired complex

The novel ligand 2,6-bis[S-(3,5-di-tert-butyl-2-hydroxyphenyl)sulfanylmethyl]pyridine (H(2)L1) and its copper(II) complex Cu(L1), 1, were synthesized with the aim of constructing a model of the active site of the enzyme galactose oxidase (GOase). Cyclic voltammetry studies show that 1 undergoes ligand-based quasi-reversible oxidations (phenolate/phenoxyl) and reversible metal-based reduction [copper(II)/copper(I)] similar to those of GOase, but at potentials much higher and lower, respectively, than those found for the enzyme. At room temperature, spectrophotometric titrations show that 1 binds strongly to 1 equiv of pyridine. In frozen solutions (77 K), 1 quantitatively binds both pyridine and ethers (e.g., 1,4-dioxane) as assessed by X- and Q-band EPR spectroscopy. Profound shifts in the pattern of g values result, from rhombic (g(1) > g(2) > g(3)) in toluene to either inverted axial patterns (g(1) = g(2) > g(3)) in the presence of ethers or a near-axial pattern (g(1) > g(2) > g(3)) in the presence of pyridine. Crystallographic analyses of the parent complex 1.MeCN, the dioxane-bridged dimer [(Cu(L1))(2)((mu-1,4)-1,4-dioxane)].(Me(2)CO)(2) (2), and the pyridine complex [Cu(L1)(pyridine)] (3) show that the pyridine and ether ligands bond to copper at a sixth octahedral position left vacant by the pentadentate NO(2)S(2) coordination mode of L1(2-) and induce perturbations of its geometry. Hybrid DFT calculations based on the crystallographic coordinates combined with perturbation theory expressions for the g values of a d(9) system correlate the results from EPR spectroscopy to the proportions of d(x)(2)(-)(y)(2) and d(z)(2) character in the relevant copper-centered unoccupied molecular orbital. The combination of spectroscopic, structural, and computational results for this set of copper(II) complexes provides a demonstrative example of the physical phenomena underlying rhombic EPR spectra of d(9) systems.     

Synthesis of a bimetallic platinum-tungsten complex with a bridging micro-diboranyl-oxycarbyne moiety

The novel bimetallic micro-diboranyl-oxycarbyne bridged platinum-tungsten complex [W{eta(1),micro-CO-B(NMe(2))-B(NMe(2))-(eta(5)-C(5)H(4))}(CO)(2){Pt(PPh(3))(2)}] (W-Pt) () has been synthesised by a two-step reaction, starting from the dilithiated half-sandwich compound Li[W(eta(5)-C(5)H(4)Li)(CO)(3)] () via the ansa-diboranyl-oxycarbyne tungsten complex [W{eta(1)-CO-B(NMe(2))B(NMe(2))(eta(5)-C(5)H(4))}(OC)(2)] () by use of stoichiometric amounts of B(2)(NMe(2))(2)Br(2) and [Pt(eta(2)-C(2)H(4))(PPh(3))(2)], respectively.