B(C6F5)3- vs Al(C6F5)3-derived metallocenium ion pairs. Structural, thermochemical, and structural dynamic divergences

The thermodynamic and structural characteristics of Al(C6F(5)3-derived vs B(C6F5)3-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)3 or 1.0 or 2.0 equiv of Al(C6F5)3 with rac-C2H4(eta5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+[H3CAl(C6F5)3](-)(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al-(CH3)(bridging) bond lengths of 1b (2.505(4) A and 2.026(4) A, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)3-derived ion pairs. Ion pair formation enthalpies (DeltaH(ipf)) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1(1) kcal mol(-1) (1b-->1c), indicating Al(C6F5)3 to have significantly less methide affinity than B(C6F5)3. Analogous experiments with Me2Si(eta5-Me4C5)(t-BuN)Ti(CH3)2 indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are DeltaH++(ce) = 22(1) kcal mol(-1), DeltaS++(ce) = 8.2(4) eu, DeltaH++(ae) = 14(2) kcal mol(-1), and DeltaS++(ae) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant ( approximately 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives DeltaH++(ce) approximately 22 kcal mol(-1)and DeltaH++(ae) approximately 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.     

Catalytic propene polymerization: determination of propagation,termination, and epimerization kinetics by direct NMR observation of the (EBI)Zr(MeB(C6F5)3)propenyl catalyst species

Treatment of (EBI)ZrMe2 with B(C6F5)3 and 100 equiv of 1-hexene in toluene-d8 at -40 degrees C for 1 h, followed by addition of >5 equiv of propene, generates a living polypropene catalyst (3) with a polyhexene "tail". Upon further addition of propene, speciation of the catalyst and propagation kinetics are measured directly by NMR, yielding a propagation rate law that is first order in [propene] and [living catalyst] with a rate constant of 2.6 M-1 s-1 at -40 degrees C and the activation parameters DeltaH = 3.4(0.2) kcal/mol and DeltaS = -42(2) cal/(mol K). In the absence of propene, elimination of vinylidene-terminated polymer is observed with a first-order rate constant of 3.3 (0.5) x 10-5 s-1 at -40 degrees C and DeltaH = 12.9(1.4) kcal/mol and DeltaS = -23(5) cal/(mol K). Use of 1-13C-propene and 1,1-2H-2,3-13C-propene and 1H, 19F, and 13C NMR spectroscopy provide rigorous characterization of the living catalyst and enable the first direct observation of both 1,3-isomerization and chain-end epimerization processes at a metallocene catalyst. Isomerization and epimerization occur with rates that are similar to that of vinylidene termination and are consistent with previous mechanistic hypotheses featuring isomerization via a tertiary alkyl intermediate and switching between enantiofaces of the coordinated alkene.     

Addition of Pt(PBu(t)(3)) groups to Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(5)-C). Synthesis, structures, and dynamical activity

The reaction of Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(5)-C), 7, with Pt(PBu(t)(3))(2) yielded two products Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(6)-C)[Pt(PBu(t)(3))], 8, and Ru(5)(CO)(12)(eta(6)-C(6)H(6))(mu(6)-C)[Pt(PBu(t)(3))](2), 9. Compound 8 contains a Ru(5)Pt metal core in an open octahedral structure. In solution, 8 exists as a mixture of two isomers that interconvert rapidly on the NMR time scale at 20 degrees C, DeltaH() = 7.1(1) kcal mol(-1), DeltaS() = -5.1(6) cal mol(-)(1) K(-)(1), and DeltaG(298)(#) = 8.6(3) kcal mol(-1). Compound 9 is structurally similar to 8, but has an additional Pt(PBu(t)(3)) group bridging an Ru-Ru edge of the cluster. The two Pt(PBu(t)(3)) groups in 9 rapidly exchange on the NMR time scale at 70 degrees C, DeltaH(#) = 9.2(3) kcal mol(-)(1), DeltaS(#) = -5(1) cal mol(-)(1) K(-)(1), and DeltaG(298)(#) = 10.7(7) kcal mol(-1). Compound 8 reacts with hydrogen to give the dihydrido complex Ru(5)(CO)(11)(eta(6)-C(6)H(6))(mu(6)-C)[Pt(PBu(t)(3))](mu-H)(2), 10, in 59% yield. This compound consists of a closed Ru(5)Pt octahedron with two hydride ligands bridging two of the four Pt-Ru bonds.     

Redox properties of C6S8(n-) and C3S5(n-) (n = 0, 1, 2): stable radicals and unusual structural properties for C-S-S-C bonds

The new anionic carbon sulfides C6S10(2-) and C12S16(2-) are described and crystallographically characterized. The C12S16(2-) anion consists of two C6S8 units connected by an exceptionally long (2.157(12) A) S-S bond. In solution, C12S16(2-) exists in equilibrium with the radical C6S8(-*). The equilibrium constant for radical formation (293 K, THF) is 1.2 x 10(-4) M, as determined by optical spectroscopy at varying concentrations. Radical formation occurs through scission of the S-S bond. On the basis of variable temperature EPR spectra, the thermodynamic parameters of this process are DeltaH = +51.5 +/- 0.5 kJ x mol(-1) and DeltaS = +110 +/- 3 J x mol(-1) x K(-1). C6S10(2-) is an oxidation product of C3S5(2-) and consists of two C3S5 units connected by an S-S bond. The S-S bond length (2.135(4) A) is long, and the CS-SC torsion angle is unusually acute (52.1 degrees ), which is attributed to an attractive interaction between C3S2 rings. The oxidation of (Me4N)2C3S5 occurs at -0.90 V vs Fc+/Fc in MeCN, being further oxidized at -0.22 V. The similarity of the cyclic voltammogram of (Me4N)2C6S10 to that of (Me4N)2C3S5 indicates that C6S10(2-) is the initial oxidation product of C3S5(2-).     

C6F5Xe]+ and [C6F5XeNCCH3]+ salts of the weakly coordinating borate anions, [BY4]- (Y = CN, CF3, or C6F5)

New examples of [C6F5Xe]+ salts of the weakly coordinating [BY4]- (Y = CN, CF3, or C6F5) anions were synthesized by metathesis of [C6F5Xe][BF4] with MI[BY4] (MI = K or Cs; Y = CN, CF3, or C6F5) in CH3CN at -40 degrees C, and were crystallized from CH2Cl2 or from a CH2Cl2/CH3CN solvent mixture. The low-temperature (-173 degrees C) X-ray crystal structures of the [C6F5Xe]+ cation and of the [C6F5XeNCCH3]+ adduct-cation are reported for [C6F5Xe][B(CF3)4], [C6F5XeNCCH3][B(CF3)4], [C6F5Xe][B(CN)4], and [C6F5XeNCCH3][B(C6F5)4]. The [C6F5Xe]+ cation, in each structure, interacts with either the anion or the solvent, with the weakest cation-anion interactions occurring for the [B(CF3)4]- anion. The solid-state Raman spectra of the [C6F5Xe]+ and [C6F5XeNCCH3]+ salts have been assigned with the aid of electronic structure calculations. Gas-phase thermodynamic calculations show that the donor-acceptor bond dissociation energy of [C6F5XeNCCH3]+ is approximately half that of [FXeNCCH3]+. Coordination of CH3CN to [C6F5Xe]+ is correlated with changes in the partial charges on mainly Xe, the ipso-C, and N, that is, the partial charge on Xe increases and those on the ipso-C and N decrease upon coordination, typifying a transition from a 2c-2e to a 3c-4e bond.     

Divergent pathways of C-H bond activation: reactions of (t-Bu(3)PN)(2)TiMe(2) with trimethylaluminum

The reaction of AlMe(3) with (t-Bu(3)PN)(2)TiMe(2) 1 proceeds via competitive reactions of metathesis and C-H activation leading ultimately to two Ti complexes: [(mu(2)-t-Bu(3)PN)Ti(mu-Me)(mu(4)-C)(AlMe(2))(2)](2) 2, [(t-Bu(3)PN)Ti(mu(2)-t-Bu(3)PN)(mu(3)-CH(2))(2)(AlMe(2))(2)(AlMe(3))] 3, and the byproduct (Me(2)Al)(2)(mu-CH(3))(mu-NP(t-Bu(3))) 4. X-ray structural data for 2 and 3 are reported. Compound 3 undergoes thermolysis to generate a new species [Ti(mu(2)-t-Bu(3)PN)(2)(mu(3)-CH(2))(mu(3)-CH)(AlMe(2))(3)] 5. Monitoring of the reaction of 1 with AlMe(3) by (31)P[(1)H] NMR spectroscopy revealed intermediates including (t-Bu(3)PN)TiMe(3) 6. Compound 6 was shown to react with AlMe(3) to give 2 exclusively. Kinetic studies revealed that the sequence of reactions from 6 to 2 involves an initial C-H activation that is a second-order reaction, dependent on the concentration of Ti and Al. The second-order rate constant k(1) was 3.9(5) x 10(-4) M(-1) s(-1) (DeltaH(#) = 63(2) kJ/mol, DeltaS(#) = -80(6) J/mol x K). The rate constants for the subsequent C-H activations leading to 2 were determined to be k(2) = 1.4(2) x 10(-3) s(-1) and k(3) = 7(1) x 10(-3) s(-1). Returning to the more complex reaction of 1, the rate constant for the ligand metathesis affording 4 and 6 was k(met) = 6.1(5) x 10(-5) s(-1) (DeltaH(#) = 37(3) kJ/mol, DeltaS(#) = -203(9) J/mol x K). The concurrent reaction of 1 leading to 3 was found to proceed with a rate constant of k(obs) of 6(1) x 10(-5) s(-1) (DeltaH(#) = 62(5) kJ/mol, DeltaS(#)= -118(17) J/mol x K). Using these kinetic data for these reactions, a stochastic kinetic model was used to compute the concentration profiles of the products and several intermediates with time for reactions using between 10 and 27 equivalents of AlMe(3). These models support the view that equilibrium between 1 x AlMe(3) and 1 x (AlMe(3))(2) accounts for varying product ratios with the concentration of AlMe(3). In a similar vein, similar equilibria account for the transient concentrations of 6 and an intermediate en route to 3. The implications of these reactions and kinetic and thermodynamic data for both C-H bond activation and deactivation pathways for Ti-phosphinimide olefin polymerization catalysts are considered and discussed.     

Guest exchange dynamics in an M4L6 tetrahedral host

Guest exchange in an M(4)L(6) supramolecular assembly was previously demonstrated to proceed through a nonrupture mechanism in which guests squeeze through apertures in the host structure and not through larger portals created by partial assembly dissociation. Focusing on the [Ga(4)L(6)](12-) assembly [L = 1,5-bis(2',3'-dihydroxybenzamido)naphthalene], the host-guest kinetic behavior of this supramolecular capsule is defined. Guest self-exchange rates at varied temperatures and pressures were measured to determine activation parameters, revealing negative DeltaS and positive DeltaV values [PEt(4)(+): DeltaH = 74(3) kJ mol(-1), DeltaS = -46(6) J mol(-1) K(-1), k(298) = 0.003 s(-)); NEt(4)(+): DeltaH = 69(2) kJ mol(-1), DeltaS = -52(5) J mol(-1) K(-1), k(298) = 0.009 s(-1); NMe(2)Pr(2)(+): DeltaH = 52(2) kJ mol(-1), DeltaS = -56(7) J mol(-1) K(-1), DeltaV = +13(1) cm(3) mol(-1), k(298) = 4.4 s(-1); NPr(4)(+): DeltaH = 42(1) kJ mol(-1), DeltaS = -102(4) J mol(-1) K(-1), DeltaV = +31(2) cm(3) mol(-1), k(298) = 1.4 s(-1)]. In PEt(4)(+) for NEt(4)(+) exchange reactions, egress of the initial guest (G1) is found to be rate determining, with increasing G1 and G2 (the displacing guest) concentrations inhibiting guest exchange. This inhibition is explained by the decreased flexibility of the host imparted by exterior, or exohedral, guest interactions by both the G1 and G2 guests. Blocking the exohedral host sites with high concentrations of the smaller NMe(4)(+) cation (a weak endohedral guest) enhances PEt(4)(+) for NEt(4)(+) guest exchange rates. Finally, guest displacement reactions also demonstrate the sensitivity of guest exchange to thermodynamic endohedral guest binding affinities. When the initial guest (G1) has a weaker affinity for the host, G2 concentration dependence is observed in addition to dependence on the G2 binding strength.     

Proton exchange kinetics from the bound waters on the oxo-centered rhodium(III) trimer [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+: a variable pH and temperature 1H NMR study

Proton exchange from the bound to the bulk waters on the oxo-centered rhodium(III) trimer, [Rh(3)(micro(3)-O)(micro-O(2)CCH(3))(6)(OH(2))(3)](+)(abbreviated as Rh(3)(+)), was investigated over the temperature range of 219.1-313.9 K using a (1)H NMR line-broadening technique. By solving the modified Bloch equations for a two-site chemical exchange, lifetimes (tau) for proton transfer at pH = 2.7, 3.6, and 7.0 ([Rh(3)(+)]= 26 mM, T= 298 K) were determined to be 0.3 (+/-.08) ms, 2 (+/-0.3) ms, and 0.2 (+/-0.2) ms, respectively. From the temperature dependence of the rate, the activation parameters were determined to be DeltaH(++)= 16.2 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 123 (+/-2) J mol(-1) K(-1), DeltaH(++)= 14.9 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 141 (+/-2) J mol(-1) K(-1), and DeltaH(++)= 45 (+/-2) kJ mol(-1) and DeltaS(++)=- 22 (+/-5) J mol(-1) K(-1) for pH = 2.7, 3.6 and 7.0, respectively. All results are reported for a mixed solvent system [acetone : 250 mM NaClO(4)(aq)(3:1)], which was necessary to depress the freezing point of the solution so that the (1)H NMR signal due to bound water could be observed. The pK(a) of Rh(3)(+) was measured to be 8.9 (+/-0.2) in the mixed solvent, which is near the pK(a) for an aqueous solution (8.3 (+/-0.2)). Surprisingly, the lifetimes for protons on Rh(3)(+) are close to those observed for the Rh(OH(2))(6)(3+) ion, in spite of the considerable difference in structure, Br?nsted acidity of the bound waters and average charge on the metal ion.     

Increased reactivity of the *Cr(CO)3(C5Me5) radical with thiones versus thiols: a theoretical and experimental investigation

2-pyridinethione (2-mercaptopyridine, H-2mp) undergoes rapid oxidative addition with 2 mol of the 17-electron organometallic radical *Cr(CO)3Cp (where Cp*=C5Me5), yielding hydride H-Cr(CO)3Cp* and thiolate (eta1-2mp)Cr(CO)3Cp*. In a slower secondary reaction, (eta1-2mp)Cr(CO)3Cp* loses CO generating the N,S-chelate complex (eta2-2mp)Cr(CO)2Cp* for which the crystal structure is reported. The rate of 2-pyridine thione oxidative addition with *Cr(CO)3Cp* (abbreviated *Cr) in toluene best fits rate=kobs[H-2mp][*Cr]; kobs(288 K)=22 +/- 4 M(-1) s(-1); DeltaH++=4 +/- 1 kcal/mol; DeltaS++=- 40 +/- 5 cal/mol K. The rate of reaction is the same under CO or Ar, and the reaction of deuterated 2-pyridine thione (D-2mp) shows a negligible (inverse) kinetic isotope effect (kD/kH=1.06 +/- 0.10). The rate of decarbonylation of (eta1-2mp)Cr(CO)3Cp* forming (eta2-2mp)Cr(CO)2Cp* obeys simple first-order kinetics with kobs (288 K)=3.1x10(-4) s(-1), DeltaH++=23 +/- 1 kcal/mol, and DeltaS++=+ 5.0 +/- 2 cal/mol K. Reaction of 4-pyridine thione (4-mercaptopyridine, H-4mp) with *Cr(CO)3Cp* in THF and CH2Cl2 also follows second-order kinetics and is approximately 2-5 times faster than H-2mp in the same solvents. The relatively rapid nature of the thione versus thiol reactions is attributed to differences in the proposed 19-electron intermediate complexes, [*(S=C5H4N-H)Cr(CO)3Cp*] versus [*(H-S-C6H5)Cr(CO)3Cp*]. In comparison, reactions of pyridyl disulfides occur by a mechanism similar to that followed by aryl disulfides involving direct attack of the sulfur-sulfur bond by the metal radical. Calorimetric data indicate Cr-SR bond strengths for aryl and pyridyl derivatives are similar. The experimental conclusions are supported by B3LYP/6-311+G(3df,2p) calculations, which also provide additional insight into the reaction pathways open to the thione/thiol tautomers. For example, the reaction between H* radical and the 2-pyridine thione S atom yielding a thionyl radical is exothermic by approximately 30 kcal/mol. In contrast, the thiuranyl radical formed from the addition of H* to the 2-pyridine thiol S atom is predicted to be unstable, eliminating either H* or HS* without barrier.     

Activation of arene C[bond]H bonds by a cationic hafnium silyl complex possessing an alpha-agostic Si[bond]H interaction

The reaction of Cp2Hf(SiMes2H)Me (1) with B(C6F5)3 produces zwitterionic Cp2Hf(eta2-SiHMes2)(mu-Me)B(C6F5)3 (2), which is stable for >8 h at -40 degrees C in toluene-d8. Spectroscopic data provide evidence for an unusual alpha-agostic Si-H interaction in 2. At room temperature, 2 reacts with the C-H bonds of aromatic hydrocarbons such as benzene and toluene to produce Cp2Hf(Ph)(mu-Me)B(C6F5)3 (3), isomers of Cp2Hf(C6H4Me)(mu-Me)B(C6F5)3 (4-6), and Cp2Hf(CH2Ph)(mu-Me)B(C6F5)3 (7), respectively. The reaction involving benzene is first-order in both 2 and benzene; rate = k[2][C6H6]. Mechanistic data including activation parameters (DeltaH = 19(1) kcal/mol; DeltaS = -17(3) eu), a large primary isotope effect of 6.9(7), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.     

{H(3-(t)Bupz)B(3-(t)Bupz)(2)-eta(2)}AlEt(2) and {H(3-(t)Bupz)B(3-(t)Bupz)(5-(t)Bupz)-eta(2)}AlEt(2). Structure, Dynamic Solution Behavior, and the 1,2-Borotropic Shift

HB(3-(t)Bupz)(3)Tl and AlEt(3) in benzene yield {H(3-(t)Bupz)B(3-(t)Bupz)(2)-eta(2)}AlEt(2), 1, as a hydrocarbon-soluble crystalline solid. Compound 1 is also obtained in a related reaction involving ClAlEt(2) via a preferential metathesis of the Al-Cl bond. Crystal data for 1 at -101 degrees C: a = 11.770(3) ?, b = 11.054(3) ?, c = 21.973(6) ?, beta = 95.57(1) degrees, Z = 4, space group P2(1)/a. In 1 the Al center is four-coordinate with Al-C = 1.97(1) ? and Al-N = 1.99(1) ? and with C-Al-C = 127 degrees and N-Al-N = 101 degrees being the largest and smallest angles, respectively. The average N-B-N angle is 109(1) degrees. In toluene-d(8) and tetrahydrofuran-d(8), 1 shows two types of 3-(t)Bupz groups in the integral ratio 2:1 and two distinct ethyl ligands. At low temperature there is a broadening of the 3-(t)Bupz singlet that is assigned to the eta(2)-(t)Bupz ligands. Up to +60 degrees C, compound 1 is nonfluxional on the NMR time scale but does isomerize to {H(3-(t)Bupz)B(3-(t)Bupz)(5-(t)Bupz)-eta(2)}AlEt(2), 2. Crystal data for 2 at -172 degrees C: a = 29.235(5) ?, b = 11.298(1) ?, c = 22.033(3) ?, beta = 129.66(1) degrees, Z = 8, space group = C2/c. In 2 there is a pseudotetrahedral Al center with Al-C = 1.97(1) ? (average) and Al-N = 1.95(1) ? (average) and with C-Al-C = 119 degrees and N-Al-N = 98 degrees as the largest and smallest angles, respectively. The average N-B-N angle is 108(1) degrees. In 2 the eta(2)-tris(alkylpyrazolyl)borate ligand isomerizes by a 1,2-borotropic shift to give one 5-(t)Bupz fragment that is part of the eta(2)-N,N' aluminum-bonded ligand. Variable-temperature (1)H NMR spectra of 2 in toluene-d(8) and THF-d(8) reveal temperature-dependent exchange involving the 3-(t)Bupz moieties, with more rapid site exchange in toluene-d(8) than in THF-d(8). At low temperature there are two ethyl signals, one of which indicates diastereotopic methylene protons, as well as three (t)Bu signals in the ratio 1:1:1. The dynamic behavior of 2 is consistent with an eta(2) right harpoon over left harpoon eta(3) exchange process as opposed to an eta(2) right harpoon over left harpoon eta(1) exchange wherein the Al center is transiently three-coordinate. The isomerization of 1 to 2 has been studied in benzene-d(6) (DeltaH() = 21.0(2) kcal/mol, DeltaS() = -15(1) eu) and THF-d(8) (DeltaH() = 18.3(4) kcal/mol, DeltaS() = -15(1) eu) and compared to a related isomerization involving {H(2)B(3-(t)Bupz)(2)-eta(2)}AlMe(2) reported by Parkin and Looney [Polyhedron 1990, 9, 265] in benzene-d(6) (DeltaH() = 34.5(8) kcal/mol, DeltaS() = 6(2) eu). It is proposed that the rate-determining 1,2-borotropic shift in the 1 --> 2 reaction occurs in a noncoordinating (t)Bupz group and that this is followed by a rapid associative interchange of pz groups wherein the sterically less demanding 5-(t)Bupz moiety remains bound to the metal.     

Spectroscopic studies on binding of 1-phenyl-3-(coumarin-6-yl)sulfonylurea to bovine serum albumin

The interaction of 1-phenyl-3-(coumarin-6-yl)sulfonylurea (SU22) with bovine serum albumin (BSA) has been investigated by fluorescence quenching spectroscopy combined with UV-absorption, circular dichroism (CD), Fourier transform infrared (FT-IR) spectroscopy techniques under simulative physiological conditions for the first time. Fluorescence data and UV-absorption spectra revealed that the quenching mechanism of fluorescence of BSA by SU22 was a static quenching process and the number of binding sites was about 0.8858; the thermodynamic parameters (DeltaG=-29.23 kJ mol(-1), DeltaH=-47.48 kJ mol(-1), and DeltaS=-61.24 J mol(-1)K(-1)) explained that hydrogen bond and Van der Waals interaction were the main binding force stabilizing the complex. The binding average distance between SU22 and BSA was obtained (3.20 nm) on the basis of the F?rster's theory. In addition, The CD spectra and FT-IR spectra have proved that BSA secondary structure changed in the presence of SU22 in aqueous solution.     

Variable-temperature 19F NMR and theoretical study of 1,9- and 1,7-C60F(CF3) and C(s-) and C1-C60F17(CF3): hindered CF3 rotation and through-space J(FF) coupling

Milligram amounts of the new compounds 1,9- and 1,7-C60F(CF3) (ca. 85:15 mixture of isomers) and C60F3(CF3) were isolated from a high-temperature C60/K2PtF6 reaction mixture and purified to 98 mol % compositional purity by two-dimensional high-performance liquid chromatography using Buckyprep and Buckyclutcher columns. The previously observed compounds C60F5(CF3) and C60F7(CF3) were also purified to 90+ mol % for the first time. Variable-temperature 19F NMR spectra of the mixture of C60F(CF3) isomers and the previously reported mixture of C(s)- and C1-C60F17(CF3) isomers demonstrate for the first time that fullerene(F)n(CF3)m derivatives with adjacent F and CF3 substituents exhibit slow-exchange limit hindered CF3 rotation spectra at -40 +/- 10 degrees C. The experimental and density functional theory (DFT) predicted deltaH++ values for CF3 rotation in 1,9-C60F(CF3) are 46.8(7) and 46 kJ mol(-1), respectively. The DFT-predicted deltaH++ values for 1,7-C60F(CF3), C(s)-C60F17(CF3), and C1-C60F17(CF3) are 20, 44, and 54 kJ mol(-1), respectively. The (> or = 4)J(FF) values from the slow-exchange-limit 19F spectra, which vary from ca. 0 to 48(1) Hz, show that the dominant nuclear spin-spin coupling mechanism is through-space coupling (i.e., direct overlap of fluorine atom lone-pair orbitals) rather than coupling through the sigma-bond framework. The 2J(FF) values within the CF3 groups vary from 107(1) to 126(1) Hz. Collectively, the NMR data provide an unambiguous set of (> or = 4)J(FF) values for three different compounds that can be correlated with DFT-predicted or X-ray diffraction derived distances and angles and an unambiguous set of 2J(FF) values that can serve as an internal standard for all future J(FF) calculations.     

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)).     

The solution structures and dynamics and the solid-state structures of substituted cyclopentadienyltitanium(IV) trifluorides

Organotitanium fluorides (C5Me4R)TiF3 (R = H, Me, Et) sublimate with formation of crystalline dimers. From solution, we obtained crystals of dimers and tetramers. The tetramer [{(C5Me5)TiF3}4] irreversibly dissociates in the solid state to dimers (DeltaH = 8.33 kcal mol(-1)). The variable-temperature (1)H and (19)F NMR spectroscopy measurements of the toluene-d(8) solution of [{(C5Me5)TiF3}2] revealed at 202 K one monomeric, two dimeric (with C2h and Cs symmetry), two tetrameric (with D2 and C2v symmetry), and two trimeric (both C2 symmetry) molecules. With the increase in temperature and dilution of the solution, the composition of the solution shifts to the smaller molecules. The thermodynamic and activation parameters for the reversible dissociation of dimers to monomers in the solution are DeltaH = 9.2 kcal mol(-1), DeltaS = 24.2 cal mol(-1) K(-1), DeltaH(double dagger) = 12.2 kcal mol(-1), DeltaS(double dagger) = 9.7 cal mol(-1) K(-1). The dissociation path with a weakly double-bridged transition-state dimer was proposed. The thermodynamic parameters for the reversible dissociation of the C2v tetramer to the dimers in solution are DeltaH = 7.9 kcal mol(-1) and DeltaS = 26.8 cal mol(-1) K(-1). From both tetramers, the D2 molecule is 0.34(5) kcal mol(-1) lower in enthalpy and 6.5(5) cal mol(-1) K(-1) lower in entropy than the C2v molecule. The structures of both trimers were proposed. The low-temperature 19F NMR spectra of the CDCl3 solution of [{(C5Me5)TiF3}2] are consistent with equilibria of a monomer, two dimers (with C2h and Cs symmetry), and a trimer. The vapor pressure osmometric molecular mass determination of CDCl3 solution of [{(C5Me5)TiF3}2] at 302 K is consistent with the equilibrium of the dimer and the monomer.     

Solution structures and dynamic properties of chelated d(0) metal olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) (R = H, Me): Models for the [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" catalysts.

To model the Ti-olefin interaction in the putative [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" Ti-catalyzed olefin polymerization, chelated alkoxide olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) have been investigated. The reaction of [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(2) (1a,b; R = H, Me) with HOCMe(2)CH(2)CH(2)CH=CH(2) yields mixtures of [eta(5)-C(5)R(4)SiMe(2)NH(t)Bu]TiMe(2)(OCMe(2)CH(2)CH(2)CH=CH(2)) (2a,b) and [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(OCMe(2)CH(2)CH(2)CH=CH(2)) (3a,b). The reaction of 2a/3a and 2b/3b mixtures with B(C(6)F(5))(3) yields the chelated olefin complexes [[eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][MeB(C(6)F(5))(3)] (4a,b; 71 and 89% NMR yield). The reaction of 2b/3b with [Ph(3)C][B(C(6)F(5))(4)] yields [[eta(5): eta(1)-C(5)Me(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][B(C(6)F(5))(4)] (5b, 88% NMR yield). NMR studies establish that 4a,b and 5b exist as mixtures of diastereomers (isomer ratios: 4a/4a', 62/38; 4b/4b', 75/25; 5b/5b', 75/25), which differ in the enantioface of the olefin that is coordinated. NMR data for these d(0) metal olefin complexes show that the olefin coordinates to Ti in an unsymmetrical fashion primarily through C(term) such that the C=C pi bond is polarized with positive charge buildup on C(int). Dynamic NMR studies show that 4b/4b' undergoes olefin face exchange by a dissociative mechanism which is accompanied by fast inversion of configuration at Ti ("O-shift") in the olefin-dissociated intermediate. The activation parameters for the conversion of 4b to 4b' (i.e., 4b/4b' face exchange) are: DeltaH = 17.2(8) kcal/mol; DeltaS = 8(1) eu. 4a/4a' also undergoes olefin face exchange but with a lower barrier (DeltaH = 12.2(9) kcal/mol; DeltaS = -2(3) eu), for the conversion of 4a to 4a'.     

Mononuclear and polynuclear copper(I) complexes with a new N,N',S-donor ligand and with structural analogies to the copper thionein core

The N,N',S-donor ligand 4-methoxy-3,5-dimethyl-2-((3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)methyl)pyridine (L) was prepared from 2-(chloromethyl)-4-methoxy-3,5-dimethylpyridine hydrochloride and 3-(2-(methylthio)phenyl)-1H-pyrazole. The Cu(I) complexes [Cu2(L)2CH3CN][Cu(L)CH3CN](BF4)3 (1), [Cu(L)PPh3]BF4 (2), and [Cu6(L)2(C6F5S)6] (3) were prepared and characterized by X-ray crystallography (PPh3=triphenylphosphine, C6F5S-=pentafluorothiophenolate). The unit cell of compound 1 consists of cocrystallized mononuclear and dinuclear entities in which all of the copper atoms exhibit distorted tetrahedral coordination. Compound 2 is monomeric with L bound in the kappa3-N,N',S mode and a PPh3 molecule that completes the coordination environment. Compound 2 presents a fluxional behavior in CDCl3 solution due to the boat inversion of the six-membered N,N' chelate ring (DeltaH=+43.6(3) kJ mol(-1), DeltaS=-16(1) J mol(-1) K(-1)). Crystallization of 3 in acetonitrile leads to a polynuclear structure that contains a CH3CN molecule coordinated to one of the copper atoms: [Cu6(L)2(C6F5S)6CH3CN] (3a). The core of 3a partially resembles a {Cu4S6} adamantane-like moiety, the only difference being that the Cu-NCCH3 interaction leads to the opening of the cluster by disrupting a Cu-Cu interaction. Part of this assembly is found in the yeast metallothionein copper(I)-cysteinate core whose crystal structure has recently been reported. Two additional [Cu(L)]+ peripheral moieties interact with the cluster by means of bridging thiolates. ESI-mass spectrometry, conductivity measurements, and 1H/19F pulsed gradient spin echo (PGSE) NMR experiments suggest that 3a dissociates in acetonitrile solution: 3a+CH3CN-->[Cu4(C6F5S)6]2-+2[Cu(L)CH3CN]+. The stability of the cluster with respect to the hypothetical mononuclear species, [Cu(L)(C6F5S)], is confirmed by DFT calculations (B3LYP), which illustrate the exergonic character of the reaction: 6[Cu(L')(C6H5S)]-->[Cu6(L')2(C6H5S)6]+4L' (DeltaG298=-58.6 kJ mol(-1), where L' and C6H5S- are simplified models for L and C6F5S-, respectively). The energetics pertinent to the ionic dissociation of the cluster in acetonitrile is computed using the polarizable continuum model (PCM) approach.     

Influence of terpyridine as pi-acceptor ligand on the kinetics and mechanism of the reaction of NO with ruthenium(III) complexes

The kinetics of the unusually fast reaction of cis- and trans-[Ru(terpy)(NH3)2Cl]2+ (with respect to NH3; terpy=2,2':6',2"-terpyridine) with NO was studied in acidic aqueous solution. The multistep reaction pathway observed for both isomers includes a rapid and reversible formation of an intermediate Ru(III)-NO complex in the first reaction step, for which the rate and activation parameters are in good agreement with an associative substitution behavior of the Ru(III) center (cis isomer, k1=618 +/- 2 M(-1) s(-1), DeltaH(++) = 38 +/- 3 kJ mol(-1), DeltaS(++) = -63 +/- 8 J K(-1) mol(-1), DeltaV(++) = -17.5 +/- 0.8 cm3 mol(-1); k -1 = 0.097 +/- 0.001 s(-1), DeltaH(++) = 27 +/- 8 kJ mol(-1), DeltaS(++) = -173 +/- 28 J K(-1) mol(-1), DeltaV(++) = -17.6 +/- 0.5 cm3 mol(-1); trans isomer, k1 = 1637 +/- 11 M(-1) s(-1), DeltaH(++) = 34 +/- 3 kJ mol(-1), DeltaS(++) = -69 +/-11 J K(-1) mol(-1), DeltaV(++) = -20 +/- 2 cm3 mol(-1); k(-1)=0.47 +/- 0.08 s(-1), DeltaH(++)=39 +/- 5 kJ mol(-1), DeltaS(++) = -121 +/-18 J K(-1) mol(-1), DeltaV(++) = -18.5 +/- 0.4 cm3 mol(-1) at 25 degrees C). The subsequent electron transfer step to form Ru(II)-NO+ occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis isomer the reduction of the Ru(III) center is induced by the coordination of an additional NO molecule (cis isomer, k2=51.3 +/- 0.3 M(-1) s(-1), DeltaH(++) = 46 +/- 2 kJ mol(-1), DeltaS(++) = -69 +/- 5 J K(-1) mol(-1), DeltaV(++) = -22.6 +/- 0.2 cm3 mol(-1) at 45 degrees C). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms of a dissociative substitution mechanism (cis isomer, DeltaV(++) = +23.5 +/- 1.2 cm3 mol(-1); trans isomer, DeltaV(++) = +20.9 +/- 0.4 cm3 mol(-1) at 55 degrees C) that produces two different reaction products, viz. [Ru(terpy)(NH3)(H2O)NO]3+ (product of the cis isomer) and trans-[Ru(terpy)(NH3)2(H2O)]2+. The pi-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern.     

Separating electrophilicity and Lewis acidity: the synthesis, characterization, and electrochemistry of the electron deficient tris(aryl)boranes B(C6F5)(3-n)(C6Cl5)n (n = 1-3)

A new family of electron-deficient tris(aryl)boranes, B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 1-3), has been synthesized, permitting an investigation into the steric and electronic effects resulting from the gradual replacement of C(6)F(5) with C(6)Cl(5) ligands. B(C(6)F(5))(2)(C(6)Cl(5)) (3) is accessed via C(6)Cl(5)BBr(2), itself prepared from donor-free Zn(C(6)Cl(5))(2) and BBr(3). Reaction of C(6)Cl(5)Li with BCl(3) in a Et(2)O/hexane slurry selectively produced B(C(6)Cl(5))(2)Cl, which undergoes B-Cl exchange with CuC(6)F(5) to afford B(C(6)F(5))(C(6)Cl(5))(2) (5). While 3 forms a complex with H(2)O, which can be rapidly removed under vacuum or in the presence of molecular sieves, B(C(6)Cl(5))(3) (6) is completely stable to refluxing toluene/H(2)O for several days. Compounds 3, 5, and 6 have been structurally characterized using single crystal X-ray diffraction and represent the first structure determinations for compounds featuring B-C(6)Cl(5) bonds; each exhibits a trigonal planar geometry about B, despite having different ligand sets. The spectroscopic characterization using (11)B, (19)F, and (13)C NMR indicates that the boron center becomes more electron-deficient as n increases. Optimized structures of B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 0-3) using density functional theory (B3LYP/TZVP) are all fully consistent with the experimental structural data. Computed (11)B shielding constants also replicate the experimental trend almost quantitatively, and the computed natural charges on the boron center increase in the order n = 0 (0.81) < n = 1 (0.89) < n = 2 (1.02) < n = 3 (1.16), supporting the hypothesis that electrophilicity increases concomitantly with substitution of C(6)F(5) for C(6)Cl(5). The direct solution cyclic voltammetry of B(C(6)F(5))(3) has been obtained for the first time and electrochemical measurements upon the entire series B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 0-3) corroborate the spectroscopic data, revealing C(6)Cl(5) to be a more electron-withdrawing group than C(6)F(5), with a ca. +200 mV shift observed in the reduction potential per C(6)F(5) group replaced. Conversely, use of the Guttmann-Beckett and Childs' methods to determine Lewis acidity on B(C(6)F(5))(3), 3, and 5 showed this property to diminish with increasing C(6)Cl(5) content, which is attributed to the steric effects of the bulky C(6)Cl(5) substituents. This conflict is ascribed to the minimal structural reorganization in the radical anions upon reduction during cyclic voltammetric experiments. Reduction of 6 using Na((s)) in THF results in a vivid blue paramagnetic solution of Na(+) [6](?-); the EPR signal of Na(+)[6](?-) is centered at g = 2.002 with a((11)B) 10G. Measurements of the exponential decay of the EPR signal (298 K) reveal [6](?-) to be considerably more stable than its perfluoro analogue.