Ancillary Ligation Research Articles

Synthetically Tunable White-, Green-, and Yellow-Green-Light Emission in Dual-Luminescent Gold(I) Complexes Bearing a Diphenylamino-2,7-fluorenyl Moiety.

The syntheses and photophysical characterization of five new gold(I) complexes bearing diphenylamine-substituted fluorenyl moieties are reported; four are characterized by X-ray diffraction crystallography. Ancillary ligation on gold(I) is provided by organophosphine and N-heterocyclic carbene ligands. Two complexes, Au-DPA0 and Au-DPA1, are σ-aryls, two, Au-ADPA0 and Au-ADPA1, are σ-alkynyls, and one, Au-TDPA1, is a σ-triazolyl bound through carbon. All complexes show vibronically structured absorption and luminescence bands that are assignable to π-π* transitions localized on the diphenylamine-substituted fluorenyl π system. The excited-state dynamics of all five chromophores are governed by selection of the ancillary ligand and σ attachment of the diphenylamine-substituted fluorenyl moiety. All of these chromophores are dual luminescent in a toluene solution at 298 K. The luminescence from the aryl derivatives, Au-ADPA0 and Au-DPA1, appears green. The alkynyl derivative containing a phosphine ancillary ligand, Au-ADPA0, is a white-light emitter, while the alkynyl derivative containing an N-heterocyclic carbene ancillary ligand, Au-ADPA1, is a yellow-light emitter. The luminescence from the triazolyl-linked chromophore, Au-TDPA1, appears as yellow-green. Spin-restricted density functional theory calculations support the assignments of ligand-centric optical transitions but with contributions of ligand-to-metal charge transfer involving the vacant Au 6p orbital.

Nickel‐Catalyzed Cross‐Coupling of Sulfonamides With (Hetero)aryl Chlorides

The development of Ni-catalyzed C-N cross-couplings of sulfonamides with (hetero)aryl chlorides is reported. These transformations, which were previously achievable only with Pd catalysis, are enabled by use of air-stable (L)NiCl(o-tol) pre-catalysts (L=PhPAd-DalPhos and PAd2-DalPhos), without photocatalysis. The collective scope of (pseudo)halide electrophiles (X=Cl, Br, I, OTs, and OC(O)NEt2 ) demonstrated herein is unprecedented for any reported catalyst system for sulfonamide C-N cross-coupling (Pd, Cu, Ni, or other). Preliminary competition experiments and relevant coordination chemistry studies are also presented.

Exploiting Ancillary Ligation To Enable Nickel-Catalyzed C-O Cross-Couplings of Aryl Electrophiles with Aliphatic Alcohols.

The use of (L)Ni( o-tolyl)Cl precatalysts (L = PAd-DalPhos or CyPAd-DalPhos) enables the C( sp2)-O cross-coupling of primary, secondary, or tertiary aliphatic alcohols with (hetero)aryl electrophiles, including unprecedented examples of such nickel-catalyzed transformations employing (hetero)aryl chlorides, sulfonates, and pivalates. In addition to offering a competitive alternative to palladium catalysis, this work establishes the feasibility of utilizing ancillary ligation as a complementary means of promoting challenging nickel-catalyzed C( sp2)-O cross-couplings, without recourse to precious-metal photoredox catalytic methods.

Density Functional Theory Calculations of Ti−TEMPO Complexes: Influence of Ancillary Ligation on the Strength of the Ti−O Bond

A series of mono(cyclopentadienyl)titanium TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) complexes were investigated by density functional theory (DFT) calculations to study the influence of the ligands on the Ti−O bond energies. Mono-Cp complexes with pendant amino groups were found to possess significantly weaker Ti−O bonds than those lacking pendant donor ligands. On the basis of DFT predictions, a novel mono-Cp pendant dimethylamino complex, CpNTiCl2(TEMPO) (12; CpN = C5H4CH2CH2N(CH3)2), was identified as a mono-Cp titanium TEMPO complex with a weak Ti−O bond (25 kcal/mol). The calculated bond energy of this complex is comparable to that of Cp2TiCl(TEMPO) (3; 17 kcal/mol). Thermolysis of 12 in the presence of CCl4 at 100 °C resulted in the formation of CpNTiCl3, consistent with the predicted low Ti−O bond energy.

Metal and Ligand K-edge XAS of Titanium−TEMPO Complexes: Determination of Oxidation States and Insights into Ti−O Bond Homolysis

Ti-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) provides a means for generating Ti(III) complexes by homolysis of the Ti-O bond. It has been determined that bis-Cp-Ti-TEMPO complexes readily undergo homolytic cleavage while the mono-Cp-Ti-TEMPO complexes do not. Here Ti K- and Cl K-edge XAS are applied to directly determine the oxidation state of TiCl3TEMPO, TiCpCl2TEMPO, and TiCp2ClTEMPO, with reference to Ti(III) and Ti(IV) complexes of known oxidation state. The Ti K-edge data show that Ti(III) complexes exhibit a pre-edge feature approximately 1 eV lower than any of the Ti(IV) complexes; while the Cl K-edges show that Ti(III) complexes have a Cl K- pre-edge feature to approximately 1 eV higher energy than any of the Ti(IV) complexes. Taken together, the Ti and Cl K-edge data indicate that the Ti-TEMPO complexes are best described as Ti(IV)-TEMPO anions (rather than Ti(III)-nitroxyl radicals). In addition, the Cl K-edges indicate that replacement of Cl by Cp weakens the bonding with the remaining ligands, with the Cl 3p covalency decreasing from 25% to 21% to 17% on going from TiCl3TEMPO to TiCpCl2TEMPO to TiCp2ClTEMPO. DFT calculations also show that the electronic structures of the Ti-TEMPO complexes are modulated by the replacement of chloride by Cp. The effect of the Cp on the ancillary ligation is one factor that contributes to facile Ti-O bond homolysis in TiCp2ClTEMPO. However, the results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.

Homolysis of Weak Ti−O Bonds: Experimental and Theoretical Studies of Titanium Oxygen Bonds Derived from Stable Nitroxyl Radicals

Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti-O bond homolysis in Ti-TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 9.0(+/- 3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti-O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti-TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies reveal that the strength of the Ti-O bond decreases dramatically with the number of cyclopentadienyl groups on titanium. The calculated Ti-O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal that the strength of these Ti-O bonds can be tuned over an interesting and experimentally accessible temperature range by appropriate ligation on titanium.

Formation of Neutral and Cationic Methyl Derivatives of Titanium Containing Cyclopentadienyl and Aryloxide Ancillary Ligation

The series of dimethyltitanium compounds [CpTi(OAr)Me2], ligated by one cyclopentadienyl (Cp) and one 2,6-disubstituted aryloxide (OAr), have been prepared by the reaction of [CpTi(OAr)Cl2] with 2 equiv of LiMe or by the addition of parent phenol (HOAr) to a cold ether solution of [CpTiMe3]. The compounds are stable, except for those containing less bulky o-methyl substituents; the compounds [CpTi(OC6H2Me2-2,6-X-4)Me2] (X = H (19), Br (20)) undergo ligand exchange to produce [CpTi(OC6H2Me2-2,6-X-4)2Me] (X = H (22), Br (23)) and [CpTiMe3]. X-ray crystal structures have been obtained for the dimethyl compounds [CpTi(OC6H2Np-2-But2-4,6)Me2] (13), [CpTi(OC6H2{C10H9}-2-But2-4,6)Me2] (14), [Cp*Ti(OC6HPh2-2,6-But2-3,5)Me2] (15), [CpTi(OC6Ph4-2,3,5,6-Br-4)Me2] (18), [CpTi(OC6H2Me2-2,6-Br-4)Me2] (20), [CpTi(OC6H3Pri2-2,6)Me2] (21), and the monomethyl species 23. Reaction of the dimethyl compounds with [B(C6F5)3] generates the corresponding cationic methyl species [CpTi(OAr)Me][MeB(C6F5)3]. The compound [CpTi(OC6HP...

Intramolecular hydroamination/cyclization of conjugated aminodienes catalyzed by organolanthanide complexes. Scope, diastereo- and enantioselectivity, and reaction mechanism.

Organolanthanide complexes of the general type Cp'(2)LnCH(TMS)(2) (Cp' = eta(5)-Me(5)C(5); Ln = La, Sm, Y; TMS = SiMe(3)) and CGCSmN(TMS)(2) (CGC = Me(2)Si(eta(5)-Me(4)C(5))((t)()BuN)) serve as effective precatalysts for the rapid, regioselective, and highly diastereoselective intramolecular hydroamination/cyclization of primary and secondary amines tethered to conjugated dienes. The rates of aminodiene cyclizations are significantly more rapid than those of the corresponding aminoalkenes. This dienyl group rate enhancement as well as substituent group (R) effects on turnover frequencies is consistent with proposed transition state electronic demands. Kinetic and mechanistic data parallel monosubstituted aminoalkene hydroamination/cyclization, with turnover-limiting C=C insertion into the Ln-N bond to presumably form an Ln-eta(3) allyl intermediate, followed by rapid protonolysis of the resulting Ln-C linkage. The rate law is first-order in [catalyst] and zero-order in [aminodiene]. However, depending on the particular substrate and catalyst combination, deviations from zero-order kinetic behavior reflect competitive product inhibition or self-inhibition by substrate. Lanthanide ionic radius effects and ancillary ligation effects on turnover frequencies suggest a sterically more demanding Ln-N insertion step than in aminoalkene cyclohydroamination, while a substantially more negative DeltaS( double dagger ) implies a more highly organized transition state. Good to excellent diastereoselectivity is obtained in the synthesis of 2,5-trans-disubstituted pyrrolidines (80% de) and 2,6-cis-disubstituted piperidines (99% de). Formation of 2-(prop-1-enyl)piperidine using the chiral C(1)-symmetric precatalyst (S)-Me(2)Si(OHF)(CpR)SmN(TMS)(2) (OHF = eta(5)-octahydrofluorenyl; Cp = eta(5)-C(5)H(3); R = (-)-menthyl) proceeds with up to 71% ee. The highly stereoselective feature of aminodiene cyclization is demonstrated by concise syntheses of naturally occurring alkaloids, (+/-)-pinidine and (+)-coniine from simple diene precursors.

Low-Valent Tungsten Aryloxide Compounds with Pyridine Ancillary Ligation: Intramolecular Reduction of Heterocyclic and Aromatic Rings Including Formation of an η5-Cyclohexadienyl Group

The sodium amalgam reduction (2 Na per W) of hydrocarbon solutions of [W(OC6HPh4-2,3,5,6)2Cl4] in the presence of pyridine or 4-tert-butylpyridine leads to the W(IV) compounds [W(OC6HPh4-2,3,5,6)2Cl2(py)2] (1) and [W(OC6HPh4-2,3,5,6)2Cl2(4-tert-butylpyridine)2] (2) in good yields. The ambient-temperature 1H NMR spectra of both 1 and 2 were found to be sharp and to exhibit extensive contact shifts due to the paramagnetic tungsten center. The addition of 2,2‘-bipyridine to an emerald green solution of [W(OC6HPh3-η6-C6H5)(OC6HPh4-2,3,5,6)(PEt3)] produced [W(OC6HPh4-2,3,5,6)2(bipy)2] (3), containing two bipy rings and two terminal aryloxide ligands coordinated mutually cis. The W−N(bipy) distances of 2.092(2), 2.093(2), 2.096(2), and 2.115(2) A in 3 are among the shortest so far reported for bipy ligands bound to either molybdenum or tungsten. The solid state and solution data for 3 indicate that reduction of both bipy ligands has occurred, but whether this involves four-electron reduction to yield d0-W(VI) i...

Photochemistry of platinum phosphine complexes. Perspective in CH bond activation

The photochemistry of the diphosphino Pt(II) hydrides [LPtH 2] (L=(t-Bu) 2P(CH 2) 2P(t-Bu) 2 ( 7); L=(t-Bu) 2P(CH 2) 3P(t-Bu) 2 ( 8);L=(t-Bu)(Ph)P(CH 2) 2P(Ph)(t-Bu) ( 9)) is reported. The primary photoevent is the dissociation of H 2 and formation of the 14-e [LPt] species. These coordinatively unsaturated intermediates provide a versatile entry point into the CH bond activation of hydrocarbons. [LPt] reacts with benzene in an oxidative addition reaction to yield [LPt(H)(C 6H 5)] complexes. The importance of the metal centre and ancillary ligation in the CH bond activation is discussed.

Studies of the alkylation of niobium and tantalum halides containing 2,6-diarylphenoxide ancillary ligation

The mixed halo(aryloxide) compounds [Nb{OC6H(C6H4Me-4)2-2,6-Ph2-3,5}2Cl3], [Ta(OC6HPh4-2,3,5,6)2Br3] and [Nb(OC6HPh4-2,3,5,6)2I3] have been isolated and structurally characterized. Although the chloride and bromide compounds are square pyramidal with an axial aryloxide, the tris(iodide) compound was determined to adopt a trigonal bipyramidal geometry in the solid state with trans, axial aryloxides. The reaction of the tantalum tris(chlorides) [Ta(OC6H3Ph2-2,6)2Cl3] or [Ta(OC6HPh4-2,3,5,6)2Cl3] with RMgCl (R = CH2Ph or CH2SiMe3) forms the corresponding mono-, bis- and tris-(alkyls), all shown to adopt a trigonal bipyramidal geometry in the solid state with trans, axial aryloxides. In contrast the mono(cycloalkyl) compounds [Ta(OC6H3Ph2-2,6)2(CnH2n+1)Cl] (n = 5, 6) were found to adopt square pyramidal structures in the solid state with the alkyl group axial (metal pseudo-equatorial) and mutually cis aryloxides. The alkylation of the corresponding niobium substrate [Nb(OC6HPh4-2,3,5,6)2Cl3] with Me3SiCH2MgCl produced a complex mixture of products. Careful analysis including the use of 29Si NMR identified the d0-alkyl products [Nb(OC6HPh4-2,3,5,6)2Cl2(CH2SiMe3)], [Nb(OC6HPh4-2,3,5,6)2Cl(CH2SiMe3)2] and [Nb(OC6HPh4-2,3,5,6)2(CH2SiMe3)3], the alkylidene compounds [Nb(OC6HPh4-2,3,5,6)2Cl(=CHSiMe3)] and [Nb(OC6HPh4-2,3,5,6)2(CH2SiMe3)(=CHSiMe3)] and the d1-alkyl [Nb(OC6HPh4-2,3,5,6)2(CH2SiMe3)2]. Treating a benzene solution of [Nb(OC6HPh4-2,3,5,6)2(CH2SiMe3)2]/[Nb(OC6HPh4-2,3,5,6)2(CH2SiMe3)(=CHSiMe3)] and PMe3 at 85 °C under 1200 psi of H2 produced deep-green crystals of the d1-di(hydride) [Nb(OC6HCy2-2,6-Ph2-3,5)2(H)2(PMe3)2].

Effect of Ancillary Ligation on the Relative Bond Disruption Enthalpies of Ru−H and Ru−Cl Bonds in Cp(PR3)2RuX (PR3= PMe3, PMe2Ph, PMePh2, PPh3; X = H, Cl)

The ruthenium chloride and hydride complexes Cp(PR3)(2)RuH {X = Cl; PR3 = PMe3 (1), PMe2Ph (2), PMePh2 (3), PPh3 (4); X = H; PR3 = PMe3 (5), PMe2Ph (6), PMePh2 (7), PPh3 (8)} were studied by spectroscopy and solution calorimetry. The structures of 2 and 3 are reported and complete the structural characterization of the series 1-4. In this series, the Ru-Cl distance (2.449 +/- 0.007 Angstrom) remains constant, while the Ru-PR3 distance increases in the order 1 6 > 7 > 8. Enthalpies of reaction for the hydride/chloride metatheses of 5-8 with chlorocarbons (CHCl3 or CCl4) were studied by solution calorimetry and allowed for the determination of Ru-Cl bond disruption enthalpies (BDEs) relative to the corresponding Ru-H BDEs. [BDE(Ru-Cl) - BDE(Ru-H)] values covered a 6 kcal/mol range with an average of 21.4 +/- 2.5 kcal/mol. The magnitude of [BDE(Ru-Cl) - BDE(Ru-H)] correlated with the number of methyl groups on the phosphine and was dependent on the electronic and steric properties of the phosphine.

Reactions of Alkynes and Olefins with Tantalum Hydrides Containing Aryloxide Ancillary Ligation: Relevance to Catalytic Hydrogenation

The reactivity of the three hydride compounds [Ta(OC6H3Ph2-2,6)2(H)2Cl(PMe3)2] (1), [Ta(OC6H3Pri2-2,6)2(H)2Cl(PMe2Ph)2] (2), and [Ta(OC6H3But2-2,6)2(H)2Cl(PMePh2)] (3) toward olefins and alkynes has been investigated. The reactivity observed is highly dependent on the nature of the ancillary aryloxide ligands. The 2,6-diphenylphenoxide 1 reacts with styrene to produce 1 equiv of ethylbenzene and the styrene adduct [Ta(OC6H3Ph2-2,6)2(η2-CH2CHPh)Cl(PMe3)] (5). In contrast, 1 reacts with 3-hexyne to eliminate H2 along with formation of the analogous alkyne complex 6. Structural studies of 5 and 6 show a square-pyramidal geometry with an axial olefin (alkyne) unit lying along the Cl−Ta−P axis. Structural parameters support a tantalacyclopropane (tantalacyclopropene) bonding picture for these molecules. Compound 5 is converted back into 1 under H2 along with formation of PhEt. The dihydride 2 reacts with styrene to form 1 equiv of PhEt, H2, and the dehydrogenation product [Ta(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)Cl(PMe2Ph)2] (7). The related adduct [Ta(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)Cl(PEt3)2] (9) was isolated by treatment of [Ta(OC6H3Pri2-2,6)2Cl3] with PEt3/Bu3SnH and was structurally characterized. Labeling studies show that the H2 generated comes exclusively from the aryloxide o-Pri group which was dehydrogenated. Both hydrides initially attached to the metal are transferred to the olefin substrate. In the case of the 2,6-di-tert-butylphenoxide compound 3, reaction with styrene generates the mono-cyclometalated compound [Ta(OC6H3ButCMe2CH2)(OC6H3But2-2,6)(CH2CH2Ph)Cl] (9). Structural studies of 9 confirm the presence of a phenethyl group. The related trans-phenylvinyl compound 10 is produced when 3 is reacted with phenylacetylene. Addition of 2,6-dimethylphenyl isocyanide (xyNC) to 10 produces the bis(iminoacyl) derivative 11, in which xyNC has inserted into the cyclometalated carbon as well as the Ta−CHCHPh bond in 10. Structural studies of 11 confirmed the trans arrangement of the hydrogen atoms in the phenylvinyl group. Mechanistic studies of the formation of 10 and 11 show the presence of two competing pathways. The first involves direct elimination of H2 from the dihydride and formation of an intermediate olefin/alkyne adduct. The product then arises by CH bond activation of the aryloxide with the hydrogen transferring to a carbon atom of the tantalacyclopropane (tantalacyclopropene) ring. The second pathway involves insertion of olefin/alkyne into a Ta−H bond followed by CH bond activation by the remaining hydride.

Synthetic, Thermochemical, and Catalytic Studies Involving Novel R2P(ORf) [R = Alkyl or Aryl; Rf = CH2CH2(CF2)5CF3] Ligands

A series of mixed alkyl (or aryl) phosphinite compounds have been prepared from the alcohol CF3(CF2)5CH2CH2OH (RfOH) and an appropriate ClPR2 (R = ORf, Ph, iPr, Cy) in the presence of base. A bidentate analog, (RfO)2PCH2CH2P(ORf)2, was synthesized in a similar manner. The ligands react with [Rh(CO)2Cl]2 yielding the complexes RhCl(CO)(PR3)2. The structure of RhCl(CO)(PPh2ORf)2 (6) is reported. Infrared studies of the carbonyl complexes yield a relative ligand donor strength for this series. Solution calorimetry was performed on the rhodium reaction in order to quantify this donor strength scale. Both approaches lead to the same donor strength scale: PCy2ORf > PiPr2ORf > PPh2ORf > P(ORf)3. These phosphinite ligands can be used as ancillary ligation in rhodium-mediated hydrogenation. A cationic rhodium complex of the chelating ligand displays selected solubility in fluorous media and biphasic catalysis can be performed.

Recent developments in the surface and catalytic chemistry of supported organoactinides

The adsorption of organoactinide complexes on dehydroxylated alumina (DA) yields coordinatively unsaturated, highly active catalysts for several demanding chemical transformations. Thus, in the case of Th(η 3-allyl) 4 supported on dehydroxylated γ-alumina (DA), facile arene hydrogenation and alkane activation processes can be effected in slurry reactions at 90°C and P H 2 = 30–180 psi. Benzene reduction occurs at a turnover frequency of ≈ 25000 h −1 per active site and cyclohexane CH exchange with D 2 at ≈ 1300 h −1 per active site. Active site characterization using D 2O poisoning, hydrogenolysis, and CH 3Cl dosing indicates ⩽ 8 ± 1 % of the Th surface sites are catalytically significant. EPR and XPS spectroscopy provide no evidence for Th oxidation states less than +4. The mechanisms for the above processes are discussed in terms of electrophilic “four-center” activation processes in which the active sites appear to bear sterically significant, nonhydridic ancillary ligation.

Organolanthanide-catalyzed hydroamination. A kinetic, mechanistic, and diastereoselectivity study of the cyclization of N-unprotected amino olefins

This contribution reports the efficient, regiospecific Cp{sup {prime}}{sub 2}LnR (Cp{sup {prime}} = {eta}{sup 5}-Me{sub 5}C{sub 5};R = H, CH(TMS){sub 2}, {eta}{sup 3}-C{sub 3}H{sub 5}, N(TMS){sub 2}; Ln-La,Nd,Sm,Y,Lu)-catalyzed hydroamination/cyclization of the amino olefins H{sub 2}NCHR{sup 1}R{sup 2}CH=CH{sub 2} to yield heterocycles. Kinetic isotope effects are observed for some of the cyclizations. The complexes were synthesized to model species in the catalytic cycle. Two independent molecules crystallize per unit cell with average La-NHCH{sub 3} and La{le}NH{sub 2}CH{sub 3} bond distances of 2.31 (1) and 2.70 (1) {Angstrom}, respectively. The amine-amido complexes undergo rapid intramolecular proton transfer between some of the amine and amido ligands. Intermolecular exchange with free amine is rapid on the NMR time scale at -80 {degrees}C. The ordering of precatalyst activities accords with known olefin insertion reactivities. Diastereoselection in H{sub 2}NCH(CH{sub 3})(CH{sub 2}){sub 2}CH=CH{sub 2}(5) cyclization depends on both lanthanide and ancillary ligation. Mechanistic evidence suggests that olefin insertion into the Ln-N bond of the amine-amido complexes is turnover-limiting and is followed by a rapid protonolysis of the resulting Ln-C bond. The proposed catalytic mechanism invokes parallel manifolds, with one manifold populated at high amine concentrations exhibiting high diastereoselectivity in the cyclization of 5, and with the second, favoredmore » at low substrate concentrations, exhibiting lower diastereoselectivity. The catalyst at high amine concentrations is postulated to be a Ln(amido)(amine){sub 2} complex. 93 refs., 15 figs., 8 tabs.« less

Manipulation of organoactinide coordinative unsaturation. Synthesis, structures, and reactivity of thorium hydrocarbyls and hydrides with chelating bis(tetramethylcyclopentadienyl) ancillary ligands

In an effort to open the actinide coordination sphere while preserving the frontier orbitals and other advantages of Cp'/sub 2/An ancillary ligation, the authors have synthesized and explored the actinide coordination chemistry of the first chelating bis(permethylcyclopentadienyl) ligand (A, Me/sub 2/SiCp''/sub 2/). In this contribution, they present an account of thorium hydrocarbyl and hydride chemistry based upon this ligand. The discussion includes both ligand synthetic as well as organometallic synthetic and homogeneous catalytic details. It is seen that Me/sub 2/SiCp''/sub 2/Th ligation can effect substantial changes in molecular structure and catalytic reactivity over Cp'/sub 2/Th analogues. They discuss in detail the crystal structures of a mononuclear thorium bis(hydrocarbyl) and a dimeric thorium tetrahydride. The latter result features a very short Th-Th distance.

A new approach to measuring absolute metal—ligand bond disruption enthalpies in organometallic compounds. The [(CH 3) 3SiC 5H 4] 3 U-system

Absolute uranium—ligand bond disruption enthalpies in the series Cp 3UR(Cp = η 5-Me 3SiC 5H 4) have been measured by halogenolytic isoperibol titration calorimetry of Cp 3U/Cp 3UI/Cp 3UR ensembles. Derived D(Cp 3UR) values in toluene solution are (kcal mol −1, R): 62.4(0.4), I; 28.9(1.7), n-Bu; 25.6(3.1), Bz; 39.3(2.3), CH 2SiMe 3; 44.8(1.1), Me; 48.5(2.2), vinyl; 86.7, CCPh. D(Cp 3U ← L) values (kcal mol −1, 95% confidence limits) in toluene solution were also determined for L = CO [10.3(0.2)] and THF [9.8(0.2)]. The magnitudes of the D(UR) values appear to reflect a combination of steric and electronic factors, and suggest that D(UI) is less sensitive to ancillary ligation (more transferable) then D(U-alkoxide). The complex Cp 3U(vinyl) crystallizes in the triclinic space group P 1 with two molecules in a unit cell of dimensions a = 11.298(1), b = 13.997(2), c = 9.460(2) Å, and α = 97.00(1), β = 105.98(1) and γ = 96.94(1)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.0196 for 4373 reflections having 2θ Mo-K α < 50° and I > 3σ( I). The molecule possesses a conventional pseudotetrahedral Cp 3MX geometry with UC(ring) = 2.759(4) Å (average), UC(α, vinyl) = 2.436(4) Å, < UC(α, vinyl)C(β, vinyl) = 137.7(3)°, < CgUCg = 116.4, 117.2, 120.0 and < CgUC(α, vinyl) = 95.1, 100.0, 100.1°. All hydrogen atoms were located and UH(Cα, vinyl) = 2.93(4) Å. The metrical parameters evidence severe non-bonded repulsions between the vinyl ligand and the Cp̋ ligands, as well as between different Cp̋ ligands. The quantity D(MI)—D(MCH 3) is proposed as a gauge of metal—ligand bonding.