Adelino M. Galvão

Synthesis, bonding and dynamic behavior of fac-[Mo(II)(CO)2(η3-allyl)] derivatives

Cationic complexes [Mo(η 3 -allyl)(CO) 2 (L–L)L′]PF 6 , (L–L=C 6 H 5 SCH 2 CH 2 SC 6 H 5 , L′=NCCH 3 ( 1 ); bipy, NCCH 3 ( 2 ); py, (NCCH 3 ) 2 ( 3 ); (NCCH 3 ) 3 ( 4 ); dppe, NCCH 3 ( 5 ) and the neutral analogues [Mo(η 3 -allyl)(CO) 2 (L–L)X] (L–L=phen ( 6 ); bipy ( 7 ); X=Br) were synthesized. Complexes 2 , 5 , 6 and 7 were characterized by single crystal X-ray diffraction. Depending on the chelating ligand, these pseudo-octahedral complexes undergo different dynamic processes in solution and NMR spectroscopic evidence was provided for those studies. The structural trends of the limiting structures depicted by these complexes as well as the pathways to their inter-conversion were analyzed by ab initio theoretical calculations. Both NMR data and the calculations showed that for complex 2 the equatorial species predominates at room temperature but that two forms differing only by the conformation of the allyl coexist. Lowering the temperature leads to the appearance of the equatorial–axial isomer.

Cyanide and methylisocyanide complexes of rhenium(l) [NBu4][ReX(CN)(dppe)2] (X Cl or CN; dppe Ph2PCH2CH2PPh2) and trans-[ReX(CNMe)(dppe)2] (X H, F, Cl or CN): crystal structures of trans-[ReX(CNMe)(dppe)2] (X H or Cl)

Abstract Reaction of trans-[ReCI(N2)(dppe)2] (dppe  Ph2PCH2CH2PPh2) with [NBu4]CN gives [NBu4][ReCI(CN)(dppe)2] (1) which, upon further reaction with [NBu4]CN, forms [NBu4][trans-Re(CN)2(dppe)2](2) which is readily oxidized to [Re(CN)2(dppe)2] (3). Treatment of 1 with Me3SiCF3SO3 affords [ReH(CNMe)(dppe)2] (4) which is also obtained by reaction of trans-[ReCI(CNMe)(dppe)2] (5) with Li[BEt3H]. Compound 5 reacts with [NBu4]CN (in the presence of Tl[BF4) or [NBu4]F to give trans-[Re(CN)(CNMe)(dppe)2] (6) or trans-[ReF(CNMe)(dppe)2] (7) respectively. The crystal structures of 4 and 5 have been determined by X-ray diffraction analyses which indicate very bent isocyanide ligands (CNC angles of 147.7(7)° and 139.4(10)° respectively) and rather short ReC bond lengths (1.947(6) A and 1.861(12) A respectively). Cyclic voltammetry shows that in aprotic solvent these complexes undergo two successive single-electron reversible oxidations. The oxidation potential of the first oxidation, for 7, allows the estimation of the electrochemical parameter PL of fluoride as −1.3 V.

Benzene ring assembly promoted by a camphor derived palladium complex

Abstract Trans-[PdCl2L2] (1, L=3-NNMe2C10H14O), under mild reaction conditions, acts as a catalyst for the cyclic trimerization of alkynes. The best performance is achieved for the reaction with PhCCMe that affords 1,3,5-trimethyl-2,4,6-triphenyl benzene with high activity and selectivity (ca. 99%). As a general trend the catalytic activity is higher for internal (PhCCMe, PhCCPh) than for terminal alkynes (HCCPh, HCCtBu, HCCCO2Me). Under more drastic experimental conditions the reaction of 1 with PhCCPh yields trans-[PdCl2(PhCCPh)2] and no catalytic activity is observed. The molecular structure of 1,3,5-trimethyl-2,4,6-triphenyl benzene was confirmed by X-ray diffraction analysis. The molecules were characterized by 1H- and 13C-NMR spectroscopies, FAB-MS and, in some cases, elemental analyses.

Nucleophilic and electrophilic reactions of C5 cyclo-polyenes coordinated to the [CpMoL2]n+ fragment (n = 1,2; L = 1/2dppe, PMe3, P(OMe)3, CO)

Abstract Reaction of several nucleophiles (R−) with the dications [Cp2MoL2]2+ (L = CO, PMe3, dppe) produces the cyclopentadiene complexes [CpMo(η4-C5H5R)L2]+ (L = dppe, R = H, CH3, CH2CN, CH2PPh3, SMe; L = CO, R = H, CH3, SPh, PMe3; L = PMe3, R = H). Excess nucleophiles only produces regio and stereospecific double addition to one Cp ring in the case of H− forming CpMo(η3-C5H7)L2 (L = CO, PMe3, dppe). [CpMo(η4-C5H6)L2]+ reacts with LiCuMe2 to give CpMo(η3-C5H6Me)L2 (L = dppe, CO) and [Cp′Mo(η4-C5H6)(CO)2]+ reacts with NaSPh and PMe3 to give Cp′Mo(η3-C5H6SPh)(CO)2 (Cp′ = Cp, indenyl) and [CpMo(η3-C5H6PMe3)(CO)2]BF4 respectively. The structure of the exclusively formed conformers endo-[CpMo(η4-C5H6)(dppe)]PF6 and endo-CpMo(η3-C5H7)(dppe) was determined by NMR and X-ray crystallography and analyzed by EHMO calculations. The reverse H− abstractions from [CpMo(η4-C5H6)L2]+ and CpMo(η3-C5H7)L2 with Ph3C+ are specific in all cases except for CpMo(η3C5H7)(dppe) which gives oxidative decomposition to [Cp2Mo(dppe)]2− and [CpMo(dppe)2]2+. All the complexes [CpMo(η4-C5H5R)L2]+ and CpMo(η3-C5H7)L2 (L = CO1, PMe3, dppe) as well as their C6 ring congeners [CpMo(η4-C6H8)(CO)2]+ and CpMo(η3-C6H9)(CO)2 have irreversible cyclovoltammograms. Nucleophilic attacks of Me3NO/NCMe and PMe3 to [CpMo(η4-C6H8)(CO)2]+ gave [CpMo(η4-C6H8)(NCMe)2]BF4 and [CpMo(η3-C6H8PMe3)(CO)2]BF4 respectively. Both were crystallographically characterized.

Structural and electronic comparison of 15- to 17-electron dichloro-complexes of molybdenum and rhenium: electrochemical behaviour and crystal structures of trans-[ReCl2(dppe)2]A (A = Cl or BF4; dppe = Ph2PCH2CH2PPh2), trans-[ReCl2(dppe)2] and [NBun4]2[trans-MoCl2(dppe)2][BF4]3

The molecular structures of the complexes trans-[ReCl2(dppe)2]A 1(A = Cl or BF4, dppe = Ph2PCH2CH2PPh2), trans-[ReCl2(dppe)2]2 and [NBun4]2[trans-MoCl2(dppe)2][BF4]33 have been determined by X-ray crystallography. The metal–phosphorus and –chloride bond lengths are sensitive to the number of metal d electrons. The M–P (M = Mo or Re) distances contact by ca. 0.1 A for each electron added. In contrast, the Re–Cl distances increase by ca. 0.1 A upon addition of one electron (d5 to d6) but the Mo–Cl distances increase by less than 0.02 A on addition of an electron (d3 to d4). These results are interpreted mainly in terms of π effects. Reduction of these complexes leads to cleavage of the metal–chloride bond which has been tracked by ramp-clamp voltammetry.

HYDROCARBYL DERIVATIVES OF UCL2HB(PZ)32 : SYNTHESIS, CHARACTERIZATION AND REACTIVITY STUDIES TOWARDS PROTIC SUBSTRATES AND KETONES

Abstract The reaction of [UCl 2 HB(pz) 3 2 ] ( 1 ) with lithium alkyls LiR (R = Me, CH 2 SiMe 3 , C 6 H 4 - o -CH 2 NMe 2 ) in the 1:1 or 1:2 molar ratio affords the compounds [UCIRHB(pz) 3 2 ] (pz = C 3 H 3 N 2 , R = Me ( 2 ), CH 2 SiMe 3 ( 3 ), C 6 H 4 - o -CH 2 NMe 2 ( 4 )) and [UR 2 HB(pz) 3 2 ] (R = Me ( 5 ), CH 2 SiMe 3 ( 6 )) respectively in 60–80% yield. Complex 2 can also be obtained (60% yield) by redistribution at room temperature between the complexes 1 and 5 . Compounds 2, 3 and 4 react with pzH providing [UCl(pz)HB(pz) 3 2 ] ( 7 ) in almost quantitative yield and 5 and 6 react also with pzH leading to [U(pz) 2 HB(pz) 3 2 ] ( 8 ). The alkoxide [U(OC 6 H 4 - o -OMe) 2 HB(pz) 3 2 ] ( 9 ) was synthesized by reacting 5 or 6 with guaiacol. By reacting the chlorohydrocarbyls 2, 3 or 4 with excess of acetone the aldolate [UCl(OCMe 2 CH 2 (C=O)Me)HB(pz) 3 2 ] ( 11 ) was obtained, due to the activation of α -CH bond of acetone; however, for 3 the reaction is not clean and a mixture of 11 and [UCl(OCMe 2 CH 2 SiMe 3 )HB(pz) 3 2 ] ( 12 ) is always obtained. Stoichiometric amounts of acetone insert into the metal-carbon bonds of 2 and 5 yielding [UCl(O t Bu)HB(pz) 3 2 ] and [U(O t Bu) 2 HB(pz) 3 2 ] respectively, while the insertion product [U(OCMe 2 CH 2 SiMe 3 ) 2 HB(pz) 3 2 ] ( 10 ) can only be obtained when 6 reacts with excess of this substrate. 9 crystallizes from toluene/hexane in the triclinic space group P 1¯ with unit cell dimensions a = 12.295(2) A, b = 12.640(2) A, c = 13.994(2) A, α = 76.10(1)°, β = 72.50(1)°, γ = 80.71(1)°, V = 2004(2) A 3 and Z = 2. Recrystallization of a mixture containing [UCl(O t Bu)HB(pz) 32 ] and 11 led to a decomposition product which has been characterized by X-ray structural analysis as [UCl(Hpz)(O t Bu)(μ-O)B(μ-pz)(pz) 2 ] 2 ( 13 ): monoclinic P 2 1 / n , a = 13.701(3) A, b = 11.337(2) A, c = 14.857(4) A, β = 104.65(2)°, V = 2233(1) A 3 and Z = 2. Extended Huckel molecular orbital calculations provided some information on the bonding capabilities of the [UHB(pz) 3 2 ] fragment compared to [U(C 5 Me 5 ) 2 ] and on the vulnerability of the poly(pyrazolyl)borate ligands to nucleophilic attack.

Synthesis, characterization and reactivity of lantahnide(II) poly(pyrazol-1-yl)borates (LnSm, Eu and Yb); fluorescence studies of [EuL2(THF)2] [LB(pz)4, HB(pz)3]; X-ray crystal structures of [Eu{B(pz)4}2(THF)2] and ]Yb{B(pz)4}3]·(2H5OH

Abstract The reaction [LnI2(THF)x] (Ln - Sm, Eu, Yb) with 2 equiv. of K[B(pz)4] (pz = pyrazolyl) in THF resulted in the formation of [Ln{B(pz)4}2(THF)2] complexes. The molecular structure of [Eu{B(pz)4}2(THF)2] has been determined by single-crystal X-ray diffraction analysis. The [Sm{B(pz)4}2(THF)2] and [Yb{B(pz)4}2(THF)2] complexes are fluxional in solution, as indicated by the equivalence of the pyrazolyl rings in the 1H NMR spectra at room temperature. A static spectrum could be obtained for the Sm compound at -68°C with a pattern that is in accordance with the geometry found for the Eu complex, in the solid state. The complexes [{LnHB(pz)3}2(THF)2] (Ln - Sm, Eu, Yb) have been prepared by the procedure used to synthesize the [Ln{B)pz)4}2(THF)2] complexes. The THF molecules can be replaced by 1,2-dimethoxyethane yielding the compounds [{LnHB(pz)3}2(DME)] (Ln = Sm, Yb. [Sm[B)pz)42(THF2] and [Yb{B)pz)4}2(THF)2] react readily with alkyl halides, alcohols or alkynes to yield LnIII complexes that disproportionate to the [Ln{B(pz)4}3] complexes. The crystal structure of the compound [Yb{B(pz)4}3]·C2H5OH obtained in the reaction of [Yb{B(pz)4}2(THF)2] with ethanol was determined by X-ray diffraction analysis. Fluorescence studies on the Eu compounds are also reported.

Kinetic study of the alkylation of cyanide at [NBu4][trans-Re(CN)2(dppe)2]. Crystal structures of [NBu4][trans-Re(CN)2(dppe)2] and trans-[Re(CN)2(dppe)2]

Abstract The mechanism of alkylation of [NBu 4 ][ trans -Re(CN) 2 (dppe) 2 ] ( 1 ) by alkyl iodides (RI; R=Me, Et or Pr) or EtBr was studied by stopped-flow techniques, which indicate that it involves a fast first alkylation to give trans -[Re(CN)(CNR)(dppe) 2 ] that undergoes a subsequent relatively slow alkylation at the cyano-ligand with a rate constant that decreases with the increase of the carbon chain length of the R group and with the replacement of iodide by bromide in the organohalide. Sodium iodide inhibits the rates of alkylation, probably by forming ion pairs with trans -[Re(CN) 2 (dppe) 2 ] − as confirmed by the formation of the adducts [Re(CN)(CNM)(dppe) 2 ] (M=Li, Na, Tl or Ag) by reaction of 1 with convenient metal salts and by the kinetics of the reaction between MeI and [Re(CN)(CNNa)(dppe) 2 ]. The X-ray molecular structures of [NBu 4 ][ trans -Re(CN) 2 (dppe) 2 ] and trans -[Re(CN) 2 (dppe) 2 ] confirm they have pseudo octahedral geometries and indicate that the former crystallizes in the triclinic P 1 space group with a =17.938(2), b =18.473(3), c =20.061(3) A and the latter in the monoclinic space group P 2 1 / c with a =11.673(2), b =13.302(3), c =17.166(4) A.

Synthesis, characterization and bonding of fulvalene dimolybdenum(III) and ditungsten(III) cations with one thiolate bridging ligand. Crystal structure of [Mo2(μ-η5: η5-C10H8)(μ-SC6H5)(μ5-C5H5) 2][Re2(μ-SC6H5)3(CO)6]

Abstract New bridged fulvalene (Fv = μ-η 5 : η 5 -C 10 )H 8 ) binuclear molybdenum(III) and tungsten(III) compounds were obtained from the reaction of [M(η 5 -C 5 H 5 ) 2 (SC 6 H 5 ) 2 ] (M = Mo or W) with dirhenium decacarbonyl. The complex [Mo 2 Fv(μ-SC 6 H 5 )Cp 2 ][Re 2 (μ-SPh) 3 (CP) 6 ] was characterized by single crystal X-ray diffraction. The cation [Mo 2 Fv(μ-SC 6 H 5 )Cp 2 ] + showed a relatively rigid [Mo 2 FvCp 2 ] unit with an Mo-S-Mo angle of 83.88(7)°, M-S distances 2.442(2) and 2.450(2) A and a Mo⋯Mo distance of 3.256(1) A. In the [Re 2 (μ-SPh) 3 (CO) 6 ] − anion, each metal atom has an octahedral environment (three carbonyl and three thiolate groups) with the two octahedra sharing the face containing the sulphur donor atoms (Re-S-Re angles from 86.63(8) to 86.83(8)°). The Re⋯Re distance of 3.457(1) A is long. Extended Huckel molecular orbital calculations were done in order to understand the nature of the metal-metal interaction in the cation and some related complexes. Although the Mo⋯Mo distance is relatively long, there is a strong interaction between the two d 3 metal atoms, assigned to a μ bond, which leads to diamagnetic behaviour (the anion with two d 6 Re I centres is also diamagnetic).

Excited-State Proton Transfer in Indigo

Excited-state proton transfer (ESPT) in Indigo and its monohexyl-substituted derivative (Ind and NHxInd, respectively) in solution was investigated experimentally as a function of solvent viscosity, polarity, and temperature, and theoretically by time-dependent density functional theory (TDDFT) calculations. Although a single emission band is observed, the fluorescence decays (collected at different wavelengths along the emission band using time-correlated single photon counting (TCSPC)) are biexponential, with two identical decay times but different pre-exponential factors, which is consistent with the existence of excited-state keto and enol species. The femtosecond (fs)-transient absorption data show that two similar decay components are present, in addition to a shorter (<3 ps) component associated with vibrational relaxation. From TDDFT calculations it was shown that with both Ind and NHxInd, the reaction proceeds through a single ESPT mechanism driven by an Arrhenius-type activation through a saddle point, which is enhanced by tunneling through the barrier. From the temperature dependence of the steady-state and time-resolved fluorescence data, the activation energy for the process was found to be ∼11 kJ mol-1 for Ind and ∼5 kJ mol-1 for NHxInd, in close agreement with the values calculated by TDDFT: 12.3 kJ mol-1 (Ind) and 3.1 kJ mol-1 (NHxInd). From time-resolved data, the rate constants for the ESPT process in dimethyl sulfoxide were found to be 9.24 × 1010 s-1 (Ind) and 7.12 × 1010 s-1 (NHxInd). The proximity between the two values suggests that the proton transfer mechanism in indigo is very similar to that found in NHxInd, where a single proton is involved. In addition, with NHxInd, the TDDFT calculations, together with the viscosity dependence of the fast component, and differences in the activation energy values between the steady-state and time-resolved data indicate that an additional nonradiative process is involved, which competes with ESPT. This is attributed to rotation about the central carbon-carbon bond, which brings the system close to a conical intersection (CI). The CI is of the sloped type, where the seam is reached through an OH stretching vibration.