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Organometallic compounds related to alkene polymerization catalysts: Cationic nickel(II) alkyls and hafnium(III) dimers
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Late transition metal complexes as precursors to metallic thin films and as promoters of carbon-silicon and carbon-phosphorus bond activation processes
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Chemical vapor deposition of copper, copper(I) oxide, and silver from metal-organic precursors
No full textMetal-organic chemical vapor deposition (MOCVD) from the tetrameric precursor copper(I) tert-butoxide, (Cu(O-t-Bu)) \sb4, results in the deposition of pure copper(I) oxide whiskers at 510 K and of copper metal with 2% oxygen contamination at 670 K. Quantitative analyses of the gaseous byproducts generated during the deposition, electron energy loss spectroscopy, and temperature programmed desorption experiments indicate that copper(I) oxide is formed by an elimination mechanism and copper metal is formed by deoxygenation of an initially deposited copper(I) oxide phase.New volatile monomeric Cu\sp{\rm II} alkoxides have been synthesized with the general formula Cu(OR)\sb2L, where OR is OCH(CF\sb3)\sb2 or OC(CH\sb3)(CF\sb3)\sb2 and L is a bidentate amine. These compounds were prepared by the reaction of Cu(OMe)\sb2 with HOR and the amine in diethyl ether. The degree of distortion from square planar geometry for these compounds was measured by EPR spectroscopy, UV-vis spectroscopy, and X-ray crystallography. At 570 K, these compounds are MOCVD precursors for the deposition of pure copper metal.The surface chemistry of copper(I) and copper(II) -diketonate complexes has been examined under ultrahigh vacuum conditions on copper single crystals by temperature programmed desorption studies, electron energy loss spectroscopy, infrared spectroscopy, and Auger spectroscopy. Above 200 K, the -diketonate ligands migrate from the adsorbed copper compound to the copper surface. At 375 K, the ligands begin to fragment to give trifluoromethyl and ketenylidene surface species. Decarbonylation of the ketenylidene groups at 525 K leads to a carbon overlayer.Silver films have been prepared by MOCVD from (CF\sb3CF = C(CF\sb3)Ag) \sb4 at 550 K. Studies of the deposition mechanism reveal that (CF\sb3CF = C(CF\sb3)Ag) \sb4 initially deposits AgF by an elimination reaction and AgF then loses fluorine to produce silver metal. The crystal structure of (CF\sb3CF = C(CF\sb3)Ag) \sb4 was determined and shows that the compound is a tetramer that consists of a square plane of silver atoms in which each edge is bridged by a perfluorobutenyl ligand.The ruthenium alkyl complexes (Li(tmed)) \sb2((\eta\sp4-C\sb 7H\sb8)RuMe\sb4) and (Li(tmed)) \sb2(\eta\sp5-C\sb8H\sb{11})RuMe\sb4) have also been prepared and characterized.Made available in DSpace on 2011-05-07T13:41:42Z (GMT). No. of bitstreams: 2
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Studies of carbon-hydrogen and silicon-hydrogen bond activation by early transition metal complexes
No full textCompounds that contain early transition metals particularly titanium, vanadium, and chromium, are used as catalysts for olefin polymerization in the Ziegler-Natta and Phillips processes. The key intermediates in these catalysts are thought to be six-coordinate metal alkyl/olefin complexes. The synthesis and study of early transition metal alkyl complexes as models of these catalytic centers can provide insight into the mechanism of polymerization processes.The hexamethyl metallates of zirconium and hafnium have been synthesized and the molecular structures of (Li(tmed)) \sb2 (ZrMe\sb6) (tmed = N,N,N\sp\prime,N\sp\prime-tetramethylethylenediamine) and (Li(diox)) \sb2 (ZrMe\sb6) (diox = dioxane) show that the zirconium is in a trigonal prismatic environment. Theoretical calculations on the geometric preferences of d\sp0 six-coordinate metal complexes suggest that non-octahedral geometries are favored due to mixing of metal-ligand -orbitals with metal d orbitals.The reaction of ethyllithium with HfCl\sb4 yields the anionic ethylene complex (Li(tmed)) \sb2 (HfEt\sb4(C\sb2H\sb4)). A C-H coupling constant of 119 Hz and a C-C distance of 1.50 A are observed for the ethylene ligand. Gas chromatograph/mass spectrometry studies show that this complex is formed by -hydride elimination from an ethyl group and subsequent loss of ethane. Anionic homoleptic alkyl complexes have also been synthesized with phenyl, (MPh\sb6\sp{2-}), and trimethylsilylmethyl ligands, (M(CH\sb2SiMe\sb3)\sb5\sp-) (M = Zr, Hf).Six-coordinate adducts of stoichiometry MMe\sb4(P-P) (M = T, Zr, Hf; P-P = bidentate phosphines) have been made with dmpe (1,2-bis(dimethylphosphino)ethane), diphos (1,2-bis(dimethylphosphino)benzene), dippe (1,2-bis(diisopropylphosphino)-ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane). Solution and solid-state \sp1H and \sp{13}C NMR studies of these complexes indicate that the energy barriers for exchange of the methyl groups are very low (9-13 kcal mol\sp{-1}). Crystal structure determinations of TiMe\sb4(dmpe), TiMe\sb4(diphos), and HfMe\sb4(dcpe) show that these complexes adopt significantly distorted cis-octahedral geometries.Homoleptic alkyl complexes of vanadium(III) and chromium(II) have been prepared with the -stabilized alkyl groups trimethylsilylmethyl and neopentyl. The low temperature \sp1H NMR spectrum of (Li(thf)\sb2) \sb2 (Cr\sb2(CH\sb2SiMe\sb3)\sb6) suggests that agostic interactions may be present in the molecule. The addition of tmed yields the interesting bis-metallacycle (Li(tmed)) \sb2 (Cr(\kappa\sb2-CH\sb2SiMe\sb2CH\sb2)\sb2).In addition, silicon-hydrogen bond activation is observed for the group VI arene compounds (\eta\sp6-C\sb6H\sb3Me\sb3)M(CO)\sb3 (M = Cr and Mo). The complex Mo(N\sb2)\sb2(dippe)\sb2 reacts with aryl silanes or hydrogen to form MoH\sb4(dippe)\sb2. The reactivity of these complexes provide insight into hydrosilation processes.Made available in DSpace on 2011-05-07T13:18:10Z (GMT). No. of bitstreams: 2
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The chemistry of manganese alkyls and aryls and of divalent titanium phenoxides
No full textThe organometallic chemistry of divalent, trivalent, and tetravalent manganese has received little attention despite intensive interest in the organometallic chemistry of the transition metals over the last 25 years. Similarly, the development of divalent titanium and vanadium chemistry has been quite limited. The study of organomanganese complexes and low-valent early transition metal complexes can provide insight into processes such as organic oxidations by manganese-containing complexes and Ziegler-Natta olefin polymerizations, respectively.The reaction of MnCl\sb2 with 4 equiv of an organolithium reagent followed by the addition of N,N,N\sp\prime,N\sp\prime-tetramethylethylenediamine (tmed) gives a series of tetraalkyl manganate(II) complexes of stoichiometry (Li(tmed)) \sb2(MnR\sb4) (R = Me, Et, CH\sb2CH\sb2-t-Bu, n-Bu, CH\sb2SiMe\sb3, and Ph) in good yields. The molecular structures of the methyl, ethyl, and neohexyl complexes are the first transition metal peralkyls wich contain -hydrogen atoms to be structurally characterized.The addition of 2 equiv. of methyllithium to MnMe\sb4(dmpe) followed by treatment with tmed gives the first tetravalent peralkyl manganate, (Li(tmed)) \sb2(MnMe\sb6), in good yield. This tetravalent species reacts with the divalent species (Li(tmed)) \sb2(MnMe\sb4) in diethyl ether to give the trivalent permethyl manganate (Li(tmed)) \sb2(MnMe\sb5) in a comproportionation reaction. If the comproportionation is performed in toluene, the product is the tetramethyl manganate(II) species (Li(tmed)\sb2) (MnMe\sb4).The interaction of manganous halides with 1/2 equiv. of the diarylmagnesium reagent MgMes\sb2(thf)\sb2 and 2 equiv. or trimethylphosphine in diethyl ether followed the addition of dry dioxygen leads to the isolation of a series of trivalent arylmanganese(III) species of stoichiometry Mn(Mes)X\sb2-(PMe\sb3)\sb2 (X = Cl, Br, and I). The molecular structure of Mn(Mes)-Br\sb2(PMe\sb3)\sb2 is a slightly distorted trigonalbipyramid with the phosphine ligands occupying the axial positions. Treatment of Mn(acac)\sb3 with 5 equiv. of LiMe leads to the isolation of the first oxoalkyl of manganese which has the stoichiometry Li\sb2(MnOMe\sb3) 2Li\sb2(OCMe=CHCMe\sb2O) tmed\}\sb2. The molecular geometry around the Mn\sp{\rm III} center is also a slightly distorted square-plane.Treatment of Ti(BH\sb4)\sb2(dmpe)\sb2 with 2 equiv. of sodium phenoxide in tetrahydrofuran gives Ti(OPh)\sb2(dmpe)\sb2 which is the first divalent titanium aryloxide or alkoxide of any kind. Along with TiMe\sb2(dmpe)\sb2, these three compounds are the only octahedral d\sp2 metal centers of the first-row transition series to exhibit spin-pairing within the t\sb{\rm 2g} manifold; the conplexes are diamagnetic. The spin-pairing of the two d-electrons in TiMe\sb2(dmpe)\sb2 is the result of -bonding from the reduced titanium center to the triaklylphosphines; the spin-pairing in the phenoxide complexes is due to -donation from the phenoxide groups.Made available in DSpace on 2011-05-07T14:26:56Z (GMT). No. of bitstreams: 2
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Part I. Synthesis, characterization, and reactivity of low valent organotitanium complexes. Part II. Chemical vapor deposition of rhodium metal thin films
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Previous issue date: 1996Titanium(II) complex CpTiCl(dmpe)\sb2 is prepared by reducing (CpTiCl\sb2)\sb{\rm x} with n-butyllithium in the presence of 1,2-bis(dimethylphosphino)ethane (dmpe). Subsequent treatment with methyllithium or n-butyllithium affords CpTiMe(dmpe)\sb2 and CpTiH(dmpe)\sb2, respectively. The crystal structures of CpTiX(dmpe)\sb2 (X = Cl, Me, H) show unusually long metal ligand distances. Analogous treatment of (Cp\sp*Ti(BH\sb4)Cl) \sb2 with n-butyllithium affords Cp\sp*Ti(BH\sb4)(PP), where PP is dmpe or (t-butyl)tris(dimethylphosphino-methyl)silane. All of these titanium(II) complexes catalyze oligomerization of ethylene to 1-butene, 2-ethyl-1-butene, and 3-methyl-1-pentene, probably via metallacyclopentane intermediates.Oxidations of the neutral titanium(II) complexes with 1,1\sp\prime-dimethylferrocinium salts afford the first examples of cationic alkyltitanium(III) complexes: (CpTiX(dmpe)\sb2\rbrack BAr\sb4, where Ar = Ph or 3,5-(CF\sb3)\sb2C\sb6H\sb3 (FPB). Two other complexes, (TiMe\rm \sb2(dmpe)\sb2\rbrack FPB and (Cp\sp*Ti(BH\sb4)(dmpe)) FPB, have been prepared by oxidation of the corresponding titanium(II) species. Crystallographic studies of (CpTiH(dmpe)\sb2) FPB, (TiMe\sb2(dmpe)\sb2) FPB, and (Cp\sp*Ti(BH\sb4)(dmpe)) FPB reveal that the titanium(III) cations show a lengthening of the Ti-P bond distances owing to a decrease in metal-ligand -back-bonding. The cationic titanium(III) alkyls neither oligomerize nor polymerize ethylene.Treatment of Rh(hfac)(CH\sb2=CH\sb2)\sb2 (hfac = hexafluoroacetylacetonate) with vinyl-trimethylsilane, 1,2-bis(trimethylsilyl)acetylene, Cu(hfac)(COT), or PMe\sb3 affords a series of new complexes. High purity rhodium films have been deposited at 200-300 \sp\circC using Rh(hfac)(CH\sb2 = CH\sb2)\sb2 as a CVD precursor. The deposition occurs via the disproportionation reaction 3 Rh(hfac)(alkene)\sb2 2 Rh + Rh(hfac)\sb3 + 6 alkene. Ultra high vacuum studies show that Rh(hfac)(CH\sb2 = CH\sb2)\sb2 adsorbs molecularly on copper surfaces up to 130 K. The hfac groups are oriented perpendicular to the surface at 220 K. Reduction of rhodium(I) occurs at approximately 300 K. Decomposition of surface-bound hfac groups occurs at higher temperatures. This process does not occur under CVD conditions owing to the higher coverages characteristic of this process, which favor bimolecular reactions that lead to the assembly of the observed Rh(hfac)\sb3 product.Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T15:02:56Z
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Zirconium and hafnium organometallic compounds: Molecular geometry for five-, and seven-coordinate species
No full textAlkylation of ZrCl\sb4 with phenyllithium affords the new d\sp0 complex \rm\lbrack Li(Et\sb2O)\rbrack\sb2\lbrack ZrPh\sb6\rbrack ; the \rm\lbrack ZrPh\sb6\sp{2-}) anion adopts a trigonal prismatic geometry. Despite the presence of orbitals on the phenyl ligands that are of the right symmetry to serve as -donors, the trigonal prismatic geometry clearly shows that the phenyl groups are not acting as -donors. The lithium cations form weak interactions with the ipso carbons of the phenyl rings.In an attempt to prepare six-coordinate d\sp0 species that lack Li\sp\cdotsC interactions, the synthesis of zirconium pentafluorophenyl complexes was investigated. The compounds (Li(12-crown-4)\rm\sb2\rbrack\sb2\lbrack ZrCl\sb2(C\sb6F\sb5)\sb4) and \rm\lbrack Li(tmed)\sb2\rbrack\lbrack Li(tmed)\rbrack\sb2\lbrack ZrF\sb2(C\sb6F\sb5)\sb5) were isolated; the first adopts an octahedral structure owing to the -donor character of the chloride ligands, while the latter adopts a regular pentagonal bipyramidal structure. The latter compound also shows that C-F bond activation has occuned.Alkylatidn of the phosphine complexes ZrCl\sb4(PP) and HfCl\sb4(PP) complexes with \rm LiCH\sb2SiMe\sb3 affords unusual lithium salts of five-coordinate zirconium auryls; one of these salts, \rm\lbrack Li(dcype)\sb2\rbrack\lbrack Zr(CH\sb2SiMe\sb3)\sb5) where dcype is 1,2-bis(dicyclohexylphosphino)ethane, was structurally characterized. The structure of the anion is best described as a distorted square pyramid; the structure is consistent with molecular orbital calculations which predict that square pyramidal structures should be adopted for such d\sp0 species. Equally interesting is the structure of the cation, which consists of a lithium atom surrounded by a distorted tetrahedral array of two diphosphine ligands. These results bear on important issues such as whether five-coordinate d\sp0 alkyls are susceptible to Jahn-Teller distortions, and whether trialkylphosphine ligands should be classified as hard or soft Lewis bases.Treatment of \rm (C\sb8H\sb8)ZrCl\sb2(thf)\sp{\cdot}2KCl with one equivalent of p-tolyllithium, followed by the addition of \rm N,N,N\sp\prime ,N\sp\prime-tetramethylethylenediamine (tmed), yields red crystals of the mono(p-tolyl) complex \rm\lbrack Li(tmed)\rbrack\lbrack (C\sb8H\sb8)Zr(p-\rm C\sb6H\sb4Me)Cl\sb2); this compound has been crystallographically characterized. Treatment of \rm (C\sb8H\sb8)MCl\sb2{\cdot}2KCl (M = Zr, Hf) with three equivalents of methyl-, phenyl-, or p-tolyllithium in tetrahydrofuran or diethyl ether, followed by addition of \rm N,N,N\sp\prime ,N\sp\prime-tetramethyl-ethylenediamine, yields several new organozirconium and -hafnium compounds of stoichiometry \rm\lbrack Li(tmed)\sb2\rbrack\lbrack (C\sb8H\sb8)MR\sb3) where R = Me, Ph, or p-\rm C\sb6H\sb4Me. With larger alkyl groups, electrically neutral organozirconium and -hafnium complexes can be isolated: treatment of \rm (C\sb8H\sb8)MCl\sb2(thf)\sp{\cdot}2KCl with 2 equiv. of LiCH(SiMe\sb3)\sb2 gives products of stoichiometry \rm (C\sb8H\sb8)M\lbrack CH(SiMe\sb3)\sb2\rbrack\sb2. The structure of this latter complex has been determined. Interestingly, this latter 14-electron molecule reacts reversibly with carbon monoxide but is unaffected by dihydrogen even at elevated pressures.Made available in DSpace on 2011-05-07T13:59:56Z (GMT). No. of bitstreams: 2
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Molecular-based magnets constructed from cyanometalate building blocks
No full textSolids related to Prussian blue (FeII,3lFen(CN)6]2,JcH20) are shown to be ideal
candidates for the preparation of molecular-based magnets with high magnetic ordering
temperatures: they can be easily prepared from well-characterized cyanometalate building
blocks, the metal centers are linked covalently into a 3D network, and a wide range of metals
with different spin states and oxidation states can be substituted into the lattice.
The manganese compounds K2Mnn[Mnn(CN)6l, CsMnn[Mnm(CN)6]«V2H20, and
Mnn3[Mnn(CN)6]»12H20 crystallize in face-centered cubic lattices with cell constants of 10.15,
10.69, and 10.62 A, respectively. Susceptibility and magnetization measurements show that
these compounds are ferrimagnets at low temperatures: the Neel points of these compounds are
41,31, and 37 K, respectively. K2Mnn[Mn!I( CN)6] exhibits magnetic hysteresis at 24 K with a
coercive field of ca. 370 Gauss and a remnant magnetization of 4.1 x 103 Gauss cm3 mol'1.
CsMnII[Mnin(CN)6],1/2H20 also exhibits magnetic hysteresis at 4.5 K, with a coercive field of
ca. 1100 Gauss and a remnant magnetization of 8.4 x 103 Gauss cm3 mol'1. A superexchange
mechanism is presented which qualitatively accounts for the relative magnitude of the TN'S. The
magnetic ordering temperature of CsMnn[Mnin(CN)6],1/2H20 is about 40 % lower than
expected; this is attributed to the effects of cyanide linkage isomerism, which is evident in the
IR spectrum of this compound but not the other two, and which causes a type of spin frustration.
The compounds Ni%[Mnra(CN)6]2«12H20 and CsNin[Mnra(CN)6]«H20 crystallize in
face-centered cubic lattices with cell constants of 10.29 and 10.42 A, respectively. Low field
magnetization measurements reveal that these compounds exhibit ferromagnetic transitions at Tc
= 30 and 42 K, respectively. Nin3[Mnra(CN)6]2*12H20 and CsNin[Mnra(CN)6>H20 both
exhibit hysteresis below their magnetic phase transition temperatures. Coercive fields of He = 48
G and He « 39 G and remnant magnetizations of Mr = 760 G cm3 mol*1 and Mr ~ 630 G cm3
mol*1 are observed at 20 and 34 K for these compounds, respectively. The local ferromagnetic
interactions are accounted for on the basis of an orbital symmetry model and the relative
magnetic ordering temperatures are interpreted in terms of mean field theory: vacancies in the
lattice reduce the magnetic ordering temperature.
As the antiferromagnetic exchange interactions in Prussian blue analogues propagate
principally through the empty it* orbitals of the cyanide ligands, higher magnetic ordering
temperatures should result upon substituting into the structure metals that have high-energy (and
more radially expanded) t2g orbitals, viz., early transition metals in lower oxidation states.
Accordingly, we have prepared the first vanadium-substituted Prussian blue analogues
Cs2Mnn[Vn(CN)6] and [NEu]o.4Mnn[Vn(CN)4Oo.5]o.8*l-2H20. Whereas the cesium salt
crystallizes in a face-centered cubic lattice (a = 10.66 A), the NEu salt crystallizes in a non-cubic
space group. These salts are ferrimagnets with Neel temperature of 125 and 230 K; only two
other molecular magnets have higher magnetic ordering temperatures. Both compounds exhibit
hysteresis loops characteristic of soft magnets at low temperatures, but the cesium salt shows
anomalous variable-temperature magnetization behavior owing to the very small magnitude of
Hc above 80 K. The magnetic properties of the rubidium salt Rbi.4MnII[VII(CN)6]o.85,l.lH20
and the potassium salt Ki.4MnH[vn(CN)6]o.85,l-3H20 resemble those of Cs2Mnn[Vn(CN)6]:
their Neel temperatures are 151K and 162 K, respectively.
The reaction of NEuCN with TiCMeCNMOgSCFs)] yields orange prisms of the first
authentic cyanotitanate: [NE%]3[Tini(CN)6]. Crystallographic studies reveal that the anion in
this salt adopts a nearly ideal octahedral geometry in the solid state (Ti-C = 2.202 A). This solid
has been characterized by infrared (VCN = 2071 enr1) and UV-vis spectroscopy (AQ = 22800
cm'*), and its variable-temperature EPR spectra and magnetic susceptibility have been studiedMade available in DSpace on 2011-05-07T12:36:29Z (GMT). No. of bitstreams: 2
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Observation of the first titanium alkyl/alkyl complexes: The key intermediate in the Ziegler-Natta mechanism for the polymerization of alkenes
No full textThe complexes trans-TiX\sb2(dmpe)\sb2 (X = BH\sb4, Br, Cl, Me, and OPh) react with alkenes to give two diamagnetic products: the mononuclear and dinuclear complexes trans-TiX\sb2 (alkene)(dmpe)(\eta\sp1-dmpe) and trans- (TiX\sb2(alkene)(dmpe)) \sb2(\mu-dmpe). A crystallographic study of (TiCl\sb2(C\sb2H\sb4)(dmpe)) \sb2(\mu-dmpe) confirms the trans geometry and shows that the ethylene ligand resides in the plane described by the phosphine ligands. The complexes TiMe\sb2(C\sb2H\sb4)(dmpe)(\eta\sp1-dmpe) and (TiMe\sb2(C\sb2H\sb4)(dmpe)) \sb2(\mu-dmpe) are the first structural models of the key titanium alkyl/alkene intermediate in the Cossee mechanism for Ziegler-Natta catalysis. Above 20\sp\circC, the TiX\sb2(dmpe)\sb2 complexes catalyze the dimerization of ethylene to 1-butene with turnover rates of up to 440 hr\sp{-1}; the rates of ethylene dimerization vary according to the nature of the X group (Me BH\sb4 > Cl OPh). Most of the evidence favors a mechanism involving the oxidative coupling of two coordinated ethylenes followed by -hydrogen abstraction and reductive elimination. Interestingly, there is no insertion of ethylene into the Ti-CH\sb3 bonds. This behavior is probably a general feature of d\sp2 metal centers.Addition of butadiene or 1,4-diphenyl-1,3-butadiene to TiMe\sb2(dmpe)\sb2 affords complexes of stoichiometry TiMe\sb2(\eta\sp4-C\sb4H\sb4R\sb2)(dmpe) (R = H or Ph); the latter has been isolated and characterized crystallographically. At higher temperatures, the reaction of TiMe\sb2(dmpe)\sb2 with 1,3-butadiene proceeds further to form TiMe\sb2(C\sb8H\sb{12})(dmpe); the C\sb8H\sb{12} ligand is formed by oxidative coupling of two butadienes and allylic rearrangement to expand the ring. The reaction of TiMe\sb2(dmpe)\sb2 with styrene gives the titanium(0) complex, Ti(\eta\sp2-styrene)(\eta\sp4-C\sb4H\sb4Ph\sb2)(dmpe), via the same oxidative coupling mechanism except that -hydrogen elimination is followed by reductive elimination of methane rather than of diphenylbutene.The reactions of TiX\sb2(dmpe)\sb2 with carbon monoxide lead to the formation of the eight-coordinate complexes TiX\sb2(CO)\sb2(dmpe)\sb2 for X = Cl or Br, which have been characterized by NMR spectroscopy.The reaction of TiMe\sb2(dmpe)\sb2 with t-butylsilane yields the zerovalent complex Ti(dmpe)\sb3, which has been crystallographically characterized. The reaction of TiMe\sb2 (dmpe)\sb2 with phenylsilane yields two complexes in succession that appear to be titanium silyl complexes of stoichiometry Ti(Si\sb2Ph\sb2H\sb4)Me\sb2(dmpe)\sb2 and Ti(Si\sb3H\sb5Ph\sb3)(dmpe)\sb2.Variable temperature NMR studies of the dinuclear chromium(II) alkyl anion (Li(thf)\sb2\rbrack\sb2\lbrackCr\sb2(CH\sb2SiMe\sb3)\sb6) show that the two bridging CH\sb2SiMe\sb3 groups engage in agostic CrH-C interactions that are static on the NMR timescale at 80\sp\circC. At higher temperatures, the molecule undergoes two different dynamic processes. One process, which has activation parameters of H = 10.6 0.5 kcal mol\sp{-1} and S = 4 2 eu, is ascribed to exchange between the terminal and bridging CH\sb2SiMe\sb3 ligands; the other process, which has activation parameters of H = 14.1 0.6 kcal mol\sp{-1} and S = 17 3 eu, is ascribed to rotation of the bridging CH\sb2SiMe\sb3 ligands about their Cr-C bonds. The latter values give an estimate of the strength of an agostic CrH bond.Made available in DSpace on 2011-05-07T12:51:53Z (GMT). No. of bitstreams: 2
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Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Zirconium and hafnium organometallic compounds: Molecular geometry for five-, and seven-coordinate species
No full textAlkylation of ZrCl\sb4 with phenyllithium affords the new d\sp0 complex \rm\lbrack Li(Et\sb2O)\rbrack\sb2\lbrack ZrPh\sb6\rbrack ; the \rm\lbrack ZrPh\sb6\sp{2-}) anion adopts a trigonal prismatic geometry. Despite the presence of orbitals on the phenyl ligands that are of the right symmetry to serve as -donors, the trigonal prismatic geometry clearly shows that the phenyl groups are not acting as -donors. The lithium cations form weak interactions with the ipso carbons of the phenyl rings.In an attempt to prepare six-coordinate d\sp0 species that lack Li\sp\cdotsC interactions, the synthesis of zirconium pentafluorophenyl complexes was investigated. The compounds (Li(12-crown-4)\rm\sb2\rbrack\sb2\lbrack ZrCl\sb2(C\sb6F\sb5)\sb4) and \rm\lbrack Li(tmed)\sb2\rbrack\lbrack Li(tmed)\rbrack\sb2\lbrack ZrF\sb2(C\sb6F\sb5)\sb5) were isolated; the first adopts an octahedral structure owing to the -donor character of the chloride ligands, while the latter adopts a regular pentagonal bipyramidal structure. The latter compound also shows that C-F bond activation has occuned.Alkylatidn of the phosphine complexes ZrCl\sb4(PP) and HfCl\sb4(PP) complexes with \rm LiCH\sb2SiMe\sb3 affords unusual lithium salts of five-coordinate zirconium auryls; one of these salts, \rm\lbrack Li(dcype)\sb2\rbrack\lbrack Zr(CH\sb2SiMe\sb3)\sb5) where dcype is 1,2-bis(dicyclohexylphosphino)ethane, was structurally characterized. The structure of the anion is best described as a distorted square pyramid; the structure is consistent with molecular orbital calculations which predict that square pyramidal structures should be adopted for such d\sp0 species. Equally interesting is the structure of the cation, which consists of a lithium atom surrounded by a distorted tetrahedral array of two diphosphine ligands. These results bear on important issues such as whether five-coordinate d\sp0 alkyls are susceptible to Jahn-Teller distortions, and whether trialkylphosphine ligands should be classified as hard or soft Lewis bases.Treatment of \rm (C\sb8H\sb8)ZrCl\sb2(thf)\sp{\cdot}2KCl with one equivalent of p-tolyllithium, followed by the addition of \rm N,N,N\sp\prime ,N\sp\prime-tetramethylethylenediamine (tmed), yields red crystals of the mono(p-tolyl) complex \rm\lbrack Li(tmed)\rbrack\lbrack (C\sb8H\sb8)Zr(p-\rm C\sb6H\sb4Me)Cl\sb2); this compound has been crystallographically characterized. Treatment of \rm (C\sb8H\sb8)MCl\sb2{\cdot}2KCl (M = Zr, Hf) with three equivalents of methyl-, phenyl-, or p-tolyllithium in tetrahydrofuran or diethyl ether, followed by addition of \rm N,N,N\sp\prime ,N\sp\prime-tetramethyl-ethylenediamine, yields several new organozirconium and -hafnium compounds of stoichiometry \rm\lbrack Li(tmed)\sb2\rbrack\lbrack (C\sb8H\sb8)MR\sb3) where R = Me, Ph, or p-\rm C\sb6H\sb4Me. With larger alkyl groups, electrically neutral organozirconium and -hafnium complexes can be isolated: treatment of \rm (C\sb8H\sb8)MCl\sb2(thf)\sp{\cdot}2KCl with 2 equiv. of LiCH(SiMe\sb3)\sb2 gives products of stoichiometry \rm (C\sb8H\sb8)M\lbrack CH(SiMe\sb3)\sb2\rbrack\sb2. The structure of this latter complex has been determined. Interestingly, this latter 14-electron molecule reacts reversibly with carbon monoxide but is unaffected by dihydrogen even at elevated pressures.U of I OnlyETDs are only available to UIUC Users without author permissio
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