2-[1-(2,6-Dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridylcobalt(II) dichlorides: synthesis, characterization and ethylene polymerization behavior

A series of unsymmetrical 2,6-bis(imino)pyridylcobalt(II) complexes, {2-[2,6-(CH(C(6)H(5))(2))(2)-4-Me-C(6)H(2)N==C(CH(3))]-6-(2,6-R(1)(2)-4-R(2)-C(6)H(2)N==CCH(3))-C(5)H(3)NCoCl(2)} where R(1) = Me, Et or (i)Pr, R(2) = H or Me, together with the new symmetrical complex 2,6-[2,6-(CH(C(6)H(5))(2))(2)-4-Me-C(6)H(2)N==C(CH(3))](2)-C(5)H(3)NCoCl(2), were synthesized. All of the compounds were fully characterized by (1)H NMR and IR spectroscopy, as well as by elemental analysis. The molecular structures of Co1 (R(1) = Me, R(2) = H) and Co5 (R(1) = Et, R(2) = Me) were further confirmed by single crystal X-ray diffraction, which indicated that the cobalt centres were penta-coordinate with a pseudo square-pyramidal geometry. Upon treatment with MAO or MMAO, these cobalt pre-catalysts exhibited higher activities than any previously reported cobalt pre-catalysts, with values as high as 4.64 × 10(6) g PE mol(-1)(Co) h(-1) for ethylene polymerization at atmospheric pressure. The polyethylenes obtained were of high molecular weight and narrow molecular weight distribution.     

Half-Titanocene chlorides 2-(benzimidazol-2-yl)quinolin-8-olates: Synthesis, characterization and ethylene (co-)polymerization behavior

 A series of half-titanocene chloride 2-(benzimidazol-2-yl)quinolin-8-olates C1-C6 were synthesized by treating the lithium salts of the ligand with CpTiCl₃. All the complexes were characterized by ¹H-NMR, ¹³C-NMR and elemental analyses, and the crystal structure of C3 and C6 was measured by X-ray. These half-titanocene complexes showed moderate catalytic activities toward ethylene polymerization (up to 1840 kg·mol^-1(Ti)·h^-1) when activated with MMAO, affording the high molecular weight polymers. And they also exhibited good activity for copolymerization of ethylene and α-olefin with low content of co-monomer.     

Synthetic and structural studies of monocyclopentadienyl cyclometalated aryl tantalum(V) compounds

Cyclometalated aryl tetra- or trichlorido cyclopentadienyl tantalum complexes [TaXCl(3){C(6)H(4)(2-CH(2)NMe(2))-κ(2)C,N}] (X = Cl 1, η(5)-C(5)H(5)2, η(5)-C(5)H(4)(SiMe(3)) 3, η(5)-C(5)Me(5)4) containing a five-membered TaC(3)N chelate ring were synthesized by reaction of the TaXCl(4) (X = Cl, η(5)-C(5)H(5), η(5)-C(5)H(4)(SiMe(3)), η(5)-C(5)Me(5)) with the appropriate lithium aryl reagent [Li{C(6)H(4)(2-CH(2)NMe(2))}]. The reported complexes were studied by IR and NMR spectroscopy and the X-ray molecular structures of compounds 2, 3 and 4 were determined by diffraction methods. These compounds were theoretically analyzed by the DFT method and their structures were rationalized. The preferential coordination of the 2-{(dimethylamino)methyl}phenyl ligand was justified by an analysis of the molecular orbitals of the Ta(η(5)-C(5)H(5))Cl(3) and C(6)H(4)(2-CH(2)NMe(2)) fragments. In addition, the exchange pathways that account for the NMR equivalency of the Me(2)N- methyl groups and -CH(2)- hydrogen atoms of the coordinated C(6)H(4)(2-CH(2)NMe(2))-κ(2)C,N ligand were theoretically studied.     

Ligand-controlled synthesis of vanadium(I) β-diketiminates and their catalysis in cyclotrimerization of alkynes

Three dimeric vanadium(I) β-diketiminates [V{μ-(η(6)-ArN)C(Me)CHC(Me)C(N-Ar)}](2) (Ar = 2,6-Me(2)C(6)H(3) (2), 2,6-Et(2)C(6)H(3) (3), 9-anthracenyl (4)) were prepared and isolated upon reduction of their corresponding dichloro precursors VCl(2)(Nacnac). Compounds 2-4 all show a structure with each vanadium atom being η(2) bonded to the β-diketiminate framework and η(6) bonded to a flanking ring of a β-diketiminato ligand, attached to the other vanadium centre within the dimer. No metal-metal bonding interactions are observed in these dimers due to long vanadium-vanadium separations. Compounds 2-4 display an antiferromagnetic exchange between the two vanadium centres. An imido azabutadienyl complex (η(2)-PhCC(H)C(Ph)NC(6)H(3)-2,6-(i)Pr(2))VN(C(6)H(3)-2,6-(i)Pr(2))(OEt(2)) (5) was isolated from the reduction of VCl(2)(HC(C(Ph)NC(6)H(3)-2,6-(i)Pr(2))(2)) by KC(8). Compounds 2-4 and the inverted-sandwich divanadium complex (μ-η(6):η(6)-C(6)H(5)Me)[V(HC(C(Me)NC(6)H(3)-2,6-(i)Pr(2))(2))](2) (1) reduce Ph(2)S(2) to give two vanadium dithiolates V(SPh)(2)[(HC(C(Me)NC(6)H(3)-2,6-R(2))(2))] (R = Et (6), (i)Pr (7)) through an oxidative addition. Most notably, 1 and 3 catalyze the cyclotrimerization of alkynes, giving tri-substituted benzenes in good yields and a 1,3,5-triphenylbenzene coordinated intermediate 8 was isolated and characterized.     

Notable norbornene (NBE) incorporation in ethylene-NBE copolymerization catalysed by nonbridged half-titanocenes: better correlation between NBE incorporation and coordination energy

CpTiCl2(N=CtBu2) exhibits both remarkable catalytic activity and efficient norbornene (NBE) incorporation for ethylene-NBE copolymerization: the NBE incorporation by Cp'TiCl2(X) (X = N=CtBu2, O-2,6-iPr2C6H3; Cp' = Cp, C5Me5, indenyl) was related to the calculated coordination energy after ethylene insertion.     

Characterization of the sterically encumbered terphenyl-substituted species 2,6-Trip2H3C6Sn-Sn(Me)2C6H3-2,6-Trip2, an unsymmetric, group 14 element, methylmethylene, valence isomer of an alkene, its related lithium derivative 2,6-Trip2H3C6(Me)2Sn-Sn(Li)(Me)C6H3-2,6-Trip2, and the monomer Sn(t-Bu)C6H3-2,6-Trip2 (Trip = C6H2-2,4,6-i-Pr3)

The reaction of the recently reported sterically encumbered terphenyl tin(II) halide species Sn(Cl)C6H3-2,6-Trip2 (Trip = C6H2-2,4,6-i-Pr3), 1, with 1 equiv of MeLi or MeMgBr afforded 2,6-Trip2H3C6Sn-Sn(Me)2C6H3-2,6-Trip2, 2, which is the first stable group 14 element methylmethylene (i.e., CH3CH) analogue of ethylene (H2CCH2). Reaction of 1 with 1.5 equiv of MeLi yielded the stannylstannate species 2,6-Trip2H3C6(Me)2Sn-Sn(Li)(Me)-C6H3-2,6-Trip2, 3, whereas reaction of 1 with 1 equiv of t-BuLi gave the heteroleptic stannanediyl monomer Sn(t-Bu)C6H3-2,6-Trip2 (4). The compounds 2-4 were characterized by 1H, 13C (7Li, 3 only), and 119Sn NMR spectroscopy in solution and by UV-vis spectroscopy. The X-ray crystal structures of 2-4 were also determined. The formation of the stannylstannanediyl 2 instead of the expected symmetrical, valence isomer "distannene" form (Sn(Me)C6H3-2,6-Trip2)2, 6, is explained through the ready formation of LiSn(Me)2C6H3-2,6-Trip2, 5, which reacts rapidly with 1 to produce 2 which can then react with a further equivalent of MeLi to give 3. The stability of singly bonded 2 in relation to the formally doubly bonded 6 was rationalized on the basis of the difference in the strength of their tin-tin bonds. In contrast to the methyl derivatives, the reaction of 1 with t-BuLi proceeded smoothly to give the monomeric compound 4. Apparently, the formation of a t-Bu analogue of 5 was prevented by the more crowding t-Bu group. Compound 2 is also the first example of a stable molecule with bonding between a two-coordinate, bivalent tin and four-coordinate tetravalent tin. Both compounds 2 and 3 display large J 119Sn-119Sn couplings between their tin nuclei and the tin-tin bond lengths in 2 (2.8909(2) A) and 3 (2.8508(4) A) are relatively normal despite the presence of the sterically crowding terphenyl substituents.     

Iron-catalyzed intermolecular [2π+2π] cycloaddition

The bis(imino)pyridine iron dinitrogen compounds, ((iPr)PDI)Fe(N(2))(2) and [((Me)PDI)Fe(N(2))](2)(μ(2)-N(2)) ((R)PDI = 2,6-(2,6-R(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = (i)Pr, Me), promote the catalytic intermolecular [2π + 2π] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C-C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.     

柄型金属有机化合物(V)——锗桥连茚基及取代茚基锆化合物的合成及催化烯烃聚合

锗桥连茚及取代茚配体相继与丁基锂及ＺｒＣｌ４作用,生成锗桥连茚基及取代茚基锆化合物Ｍｅ２Ｇｅ(２－Ｒ１－４－Ｒ２－Ｉｎｄ)２ＺｒＣｌ２[Ｒ１＝Ｒ２＝Ｈ(１);Ｒ１＝Ｍｅ,Ｒ２＝Ｈ(２);Ｒ１＝Ｍｅ,Ｒ２＝Ｐｈ(３)]．化合物１－３均为内消旋和外消旋异构体的混合物,通过多次重结晶得到化合物１和２的纯外消旋异构体及化合物３的内消旋异构体．由元素分析和１Ｈ ＮＭＲ谱表征了化合物的分子结构．研究了在甲基铝氧烷(ＭＡＯ)的助催化下,化合物１－３对乙烯和丙烯聚合的催化性能．由锗桥连茚基化合物１－３得到的聚乙烯的分子量分布比一般茂金属催化剂略宽．内消旋和外消旋异构体的混合物(３)由于两个催化活性中心不等同而使得到的聚乙烯的分子量分布相当宽．外消旋异构体１和２催化丙烯聚合得到高等规聚丙烯．     

Synthesis and characterization of cyano-substituted carborane-based compounds. Molecular structure of [1-(4-C7H7)-12-(C5H3-3-(CN)-3,4-(CH3)2)-C2B10H10

The reaction of cyanogen chloride with [1-(4-C(7)H(7))-12-(C(5)H(3)-3,4-(CH(3))(2))-C(2)B(10)H(10)] (7) was found to yield two new C(5)-substituted carborane cluster-based compounds, [1-(4-C(7)H(7))-12-(C(5)H(2)-3-(CN)-3,4-(CH(3))(2))-C(2)B(10)H(10)] (8) and [1-(4-C(7)H(7))-12-(C(5)H-2,4-(CN)(2)-3,4-(CH(3))(2))-C(2)B(10)H(10)] (9). This cyano-substitution pattern is in contrast to the known substitution for the analogous organic quinarene[5.6.7] system. The observed unique cluster-based products may be understood by a combination of steric and electronic effects. Compounds 8 and 9 were characterized by complete multinuclear NMR, (1)H-(1)H COSY NMR, (1)H-(13)C HMQC NMR, FTIR, UV-Vis, IR, MS data and a single crystal analysis for 8 [X-ray data for 8: C(17)H(25)B(10)N, monoclinic, space group P2(1)/n with cell constants a = 8.6794(17) ?, b = 11.021(2) ?, c = 43.175(9) ?, β = 91.00(3)°, V = 4129.2(14) ?(3), Z = 8, R(1) = 0.0729, wR(2) = 0.1464].     

Reductive reactivity of the organolanthanide hydrides, [(C5Me5)2LnH]x, leads to ansa-allyl cyclopentadienyl (eta(5)-C5Me4CH2-C5Me4CH2-eta(3))2- and trianionic cyclooctatetraenyl (C8H7)3- ligands

The reductive reactivity of lanthanide hydride ligands in the [(C5Me5)2LnH]x complexes (Ln = Sm, La, Y) was examined to see if these hydride ligands would react like the actinide hydrides in [(C5Me5)2AnH2]2 (An = U, Th) and [(C5Me5)2UH]2. Each lanthanide hydride complex reduces PhSSPh to make [(C5Me5)2Ln(mu-SPh)]2 in approximately 90% yield. [(C5Me5)2SmH]2 reduces phenazine and anthracene to make [(C5Me5)2Sm]2(mu-eta(3):eta(3)-C12H8N2) and [(C5Me5)2Sm]2(mu-eta(3):eta(3)-C10H14), respectively, but the analogous [(C5Me5)2LaH]x and [(C5Me5)2YH]2 reactions are more complicated. All three lanthanide hydrides reduce C8H8 to make (C5Me5)Ln(C8H8) and (C5Me5)3Ln, a reaction that constitutes another synthetic route to (C5Me5)3Ln complexes. In the reaction of [(C5Me5)2YH]2 with C8H8, two unusual byproducts are obtained. In benzene, a (C5Me5)Y[(eta(5)-C5Me4CH2-C5Me4CH2-eta(3))] complex forms in which two (C5Me5)(1-) rings are linked to make a new type of ansa-allyl-cyclopentadienyl dianion that binds as a pentahapto-trihapto chelate. In cyclohexane, a (C5Me5)2Y(mu-eta(8):eta(1)-C8H7)Y(C5Me5) complex forms in which a (C8H8)(2-) ring is metalated to form a bridging (C8H7)(3-) trianion.     

Methylaluminium 8-quinolinolates: synthesis, characterization and use in ring-opening polymerization (ROP) of ε-caprolactone

The stoichiometric reactions of 2-(2,6-R-phenylimino)quinolin-8-ol (L1-L5, L1: R = Me, L2: R = Et, L3: R = (i)Pr, L4: R = Cl, L5: R = F) with Me(3)Al afforded the dimeric aluminium complexes [Me(2)AlL](2) (1-5) in good yields. By contrast, stoichiometric reactions of 2-(1-(2,6-R-phenylimino)propyl) quinolin-8-ol (L6-L10, L6: R = Me, L7: R = Et, L8: R = (i)Pr, L9: R = Cl, L10: R = F)) with Me(3)Al gave the mononuclear aluminium complexes Me(2)AlL (6-10) accompanied with by-products of the form Me(2)AlL·Me(3)Al (11-15). All methylaluminium complexes were characterized by NMR spectroscopy, elemental analysis, and the molecular structures of complexes 3, 6 and 8 were determined by single-crystal X-ray diffraction. Aluminium compounds 1-5 possessed negligible activity towards the ring-opening polymerization of ε-caprolactone either in the presence or absence of BnOH. In contrast, in the presence of BnOH, the mononuclear aluminium compounds 6-10 could efficiently initiate the ring-opening polymerization of ε-caprolactone; the polymerization proceeded in a living manner.     

Synthesis and structural characterization of Be(eta 5-C5Me5)(eta 1-C5Me4H). Evidence for ring-inversion leading to Be(eta 5-C5Me4H)(eta 1-C5Me5)

The mixed-ring beryllocene Be(C5Me5)(C5Me4H), that contains eta 5-C5Me5 and eta 1-C5Me4H rings, the latter bonded to the metal through the CH carbon atom (X-ray crystal structure) reacts at room temperature with CNXyl (Xyl = C6H3-2,6-Me2) to give an iminoacyl product, Be(eta 5-C5Me4H)[C(NXyl)C5Me5] derived from the inverted beryllocene structure Be (eta 5-C5Me4H)(eta 1-C5Me5).     

Chemical reactivity and electrochemistry of metal-metal-bonded zincocenes

Attempts to prepare mixed-ligand zinc-zinc-bonded compounds that contain bulky C(5)Me(5) and terphenyl groups, [Zn(2)(C(5)Me(5))(Ar')], lead to disproportionation. The resulting half-sandwich Zn(II) complexes [(η(5)-C(5)Me(5))ZnAr'] (Ar' = 2,6-(2,6-(i)Pr(2)C(6)H(3))(2)-C(6)H(3), 2; 2,6-(2,6-Me(2)C(6)H(3))(2)-C(6)H(3), 3) can also be obtained from the reaction of [Zn(C(5)Me(5))(2)] with the corresponding LiAr'. In the presence of pyr-py (4-pyrrolidinopyridine) or DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), [Zn(2)(η(5)-C(5)Me(5))(2)] reacts with C(5)Me(5)OH to afford the tetrametallic complexes [Zn(2)(η(5)-C(5)Me(5))L(μ-OC(5)Me(5))](2) (L = pyr-py, 6; DBU, 8), respectively. The bulkier terphenyloxide Ar(Mes)O(-) group (Ar(Mes) = 2,6-(2,4,6-Me(3)C(6)H(2))(2)-C(6)H(3)) gives instead the dimetallic compound [Zn(2)(η(5)-C(5)Me(5))(OAr(Mes))(pyr-py)(2)], 7, that features a terminal Zn-OAr(Mes) bond. DFT calculations on models of 6-8 and also on the Zn-Zn-bonded complexes [Zn(2)(η(5)-C(5)H(5))(OC(5)H(5))(py)(2)] and [(η(5)-C(5)H(5))ZnZn(py)(3)](+) have been performed and reveal the nonsymmetric nature of the Zn-Zn bond with lower charge and higher participation of the s orbital of the zinc atom coordinated to the cyclopentadienyl ligand with respect to the metal within the pseudo-ZnL(3) fragment. Cyclic voltammetric studies on [Zn(2)(η(5)-C(5)Me(5))(2)] have been also carried out and the results compared with the behavior of [Zn(C(5)Me(5))(2)] and related magnesium and calcium metallocenes.     

2-Aminopyrrolines: new chiral amidinate ligands with a rigid well-defined molecular structure and their coordination to Ti(IV)

The use of an amino-oxazolinate (NN(ox) = kappa2-2,6-dimethylphenylamido-4(S)-isopropyloxazoline) as a chiral analogue to amidinate ligands in the chemistry of titanium was found to lead to undesired side reactions. The reaction of 2,6-dimethylphenylamido-4(S)-isopropyloxazoline with [Ti(NMe2)4] afforded the bis(amidinato) complex [Ti(NN(ox))2(NMe2)2] (2) which was thermally converted to the ring-opened decomposition products [Ti(NN(ox)){kappa3-N(2,6-C6H3Me2)C(NMe2)NC(iPr)CH2O}(NMe2)] (3) and [Ti{kappa3-N(2,6-C6H3Me2)C(NMe2)-NC(iPr)CH2O}2] (4). The NMR spectra of 4 recorded at low temperature displayed two sets of resonances corresponding to two symmetric isomers in a 2:5 ratio, the probable geometries of which were established by ONIOM (QM/MM) simulations. To suppress ring opening of the oxazolines, their oxygen atom was formally replaced by a CH2 group in the synthesis of a series of amino-pyrroline protioligands 2-RN(H)(5-C4H5NR') (HN(R)N(R')). Their reaction with [Ti(NMe2)4] gave the thermally stable complexes [Ti(N(R)N(R'))2(NMe2)2], of which three derivatives were characterized by X-ray diffraction. They are stereochemically dynamic and undergo reversible ligand rearrangements in solution, for which the activation parameters were determined by variable-temperature (1)H NMR spectroscopy.     

Synthesis, structural characterization, reactivity, and thermal stability of [eta(5):sigma-Me2C(C5H4)(C2B10H10)]Ti(R)(NMe2)

Reaction of [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]TiCl(NMe2) (1) with 1 equiv of PhCH2K, MeMgBr, or Me3SiCH2Li gave corresponding organotitanium alkyl complexes [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(R)(NMe2) (R = CH2Ph (2), CH2SiMe3 (4), or Me (5)) in good yields. Treatment of 1 with 1 equiv of n-BuLi afforded the decomposition product {[eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti}2(mu-NMe)(mu:sigma-CH2NMe) (3). Complex 5 slowly decomposed to generate a mixed-valence dinuclear species {[eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti}2(mu-NMe2)(mu:sigma-CH2NMe) (6). Complex 1 reacted with 1 equiv of PhNCO or 2,6-Me2C6H3NC to afford the corresponding monoinsertion product [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(Cl)[eta(2)-OC(NMe2)NPh] (7) or [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(Cl)[eta(2)-C(NMe2)=N(2,6-Me2C6H3)] (8). Reaction of 4 or 5 with 1 equiv of R'NC gave the titanium eta(2)-iminoacyl complexes [eta:(5)sigma-Me2C(C5H4)(C2B10H10)]Ti(NMe2)[eta(2)-C(R)=N(R')] (R = CH2SiMe3, R' = 2,6-Me2C6H3 (9) or tBu (10); R = Me, R' = 2,6-Me2C6H3 (11) or tBu (12)). The results indicated that the unsaturated molecules inserted into the Ti-N bond only in the absence of the Ti-C(alkyl) bond and that the Ti-C(cage) bond remained intact. All complexes were fully characterized by various spectroscopic techniques and elemental analyses. Molecular structures of 2, 3, 6-8, and 10-12 were further confirmed by single-crystal X-ray analyses.     

Tuning the active species from syndiospecific styrene polymerisation to ethylene/styrene copolymerisation by (aryloxo)(cyclopentadienyl)titanium complexes-MAO catalysts

Exclusive formation of poly(ethylene-co-styrene)s were observed by introduction of ethylene into the solution of syndiospecific styrene polymerisation using Cp'TiCl(2)(O-2,6-(i)Pr(2)C(6)H(3)) (Cp' = 1,2,4-Me(3)C(5)H(2), tert-BuC(5)H(4))-MAO catalysts without by-production of syndiotactic polystyrene, whereas the styrene polymerisation did not proceed when ethylene was removed from the reaction mixture of ethylene/styrene copolymerisation.     

Synthesis and characterization of novel Pd(II) complexes with chelating and non-chelating heterocyclic iminocarbene ligands

The imidazolium salts [3-R1-1-{2-Ar-imino)-2-R2-ethyl}imidazolium] chloride (C-N; Ar = 2,6-iPr2C6H3; R1/R2 = Me/Me (a), Me/Ph (b), Ph/Me (c), 2,4,6-Me3C6H2 (d), 2,6-iPr2C6H3 (e)) react with Ag(2)O to give Ag(I) iminocarbene complexes (C-N)AgCl (4a-e) in which the iminocarbene ligand is bonded to Ag via the imidazoline-2-ylidene carbon atom. The solid-state structures of 4b and 4d were determined by X-ray crystallography and revealed the presence of monomeric (carbene)AgCl units with Z and E configurations at the imine C=N bonds, respectively. Carbene transfer to Pd occurs when compounds 4b-e are treated with (COD)PdCl2 to yield bis(carbene) complexes (C-N)2PdCl2 (6b-e) containing two kappa1-C bonded iminocarbene moieties. NMR spectroscopic data indicated a trans coordination geometry at Pd. This conclusion was supported by an X-ray structure determination of 6b which clearly demonstrated the non-chelating nature of the iminocarbene ligand system. EXSY 1H NMR spectroscopy suggests that the non-chelating structures undergo E/Z isomerization at the imine C[double bond, length as m-dash]N double bonds in solution. The preparative results contrast our earlier report that the reaction between 4a and (COD)PdCl2 results in a chelating kappa2-C,N bonded iminocarbene complex (C-N)PdCl2. The coordination mode and dynamic behavior of the iminocarbene ligand systems have been found to be dramatically affected by changes in the substitution pattern of the ligand system. Sterically unencumbered systems (a) favor the formation of kappa2-C,N chelate structures containing one iminocarbene moiety per metal upon coordination at Pd(II); these complexes were demonstrated to engage in reversible, solvent-mediated chelate ring-opening reactions. Sterically encumbered systems (b-e) form non-chelating kappa1-C iminocarbene Pd(II) complexes containing two iminocarbene ligands per metal. Transannular repulsions across the chelate ring are believed to be the origin of these structural differences.     

Kinetics and mechanism of N2 hydrogenation in bis(cyclopentadienyl) zirconium complexes and dinitrogen functionalization by 1,2-addition of a saturated C-H bond

The rates of hydrogenation of the N2 ligand in the side-on bound dinitrogen compounds, [(eta(5)-C5Me4H)2Zr]2(mu2,eta(2),eta(2)-N2) and [(eta(5)-C5Me5)(eta(5)-C5H2-1,2-Me2-4-R)Zr]2(mu2,eta(2),eta(2)-N2) (R = Me, Ph), to afford the corresponding hydrido zirconocene diazenido complexes have been measured by electronic spectroscopy. Determination of the rate law for the hydrogenation of [(eta(5)-C5Me5)(eta(5)-C5H2-1,2,4-Me3)Zr]2(mu2,eta(2),eta(2)-N2) establishes an overall second-order reaction, first order with respect to each reagent. These data, in combination with a normal, primary kinetic isotope effect of 2.2(1) for H2 versus D2 addition, establish the first H2 addition as the rate-determining step in N2 hydrogenation. Kinetic isotope effects of similar direction and magnitude have also been measured for hydrogenation (deuteration) of the two other zirconocene dinitrogen complexes. Measuring the rate constants for the hydrogenation of [(eta(5)-C5Me5)(eta(5)-C5H2-1,2,4-Me3)Zr]2(mu2,eta(2),eta(2)-N2) over a 40 degrees C temperature range provided activation parameters of deltaH(double dagger) = 8.4(8) kcal/mol and deltaS(double dagger) = -33(4) eu. The entropy of activation is consistent with an ordered four-centered transition structure, where H2 undergoes formal 1,2-addition to a zirconium-nitrogen bond with considerable multiple bond character. Support for this hypothesis stems from the observation of N2 functionalization by C-H activation of a cyclopentadienyl methyl substituent in the mixed ring dinitrogen complexes, [(eta(5)-C5Me5)(eta(5)-C5H2-1,2-Me2-4-R)Zr]2(mu2,eta(2),eta(2)-N2) (R = Me, Ph), to afford cyclometalated zirconocene diazenido derivatives.     

Organometallic uranium(V)-imido halide complexes: from synthesis to electronic structure and bonding

Reaction of (C5Me5)2U(=N-2,4,6-(t)Bu3-C6H2) or (C5Me5)2U(=N-2,6-(i)Pr2-C6H3)(THF) with 5 equiv of CuX(n) (n = 1, X = Cl, Br, I; n = 2, X = F) affords the corresponding uranium(V)-imido halide complexes, (C5Me5)2U(=N-Ar)(X) (where Ar = 2,4,6-(t)Bu3-C6H2 and X = F (3), Cl (4), Br (5), I (6); Ar = 2,6-(i)Pr2-C6H3 and X = F (7), Cl (8), Br (9), I (10)), in good isolated yields of 75-89%. These compounds have been characterized by a combination of single-crystal X-ray diffraction, (1)H NMR spectroscopy, elemental analysis, mass spectrometry, cyclic voltammetry, UV-visible-NIR absorption spectroscopy, and variable-temperature magnetic susceptibility. The uranium L(III)-edge X-ray absorption spectrum of (C5Me5)2U(=N-2,4,6-(t)Bu3-C6H2)(Cl) (4) was analyzed to obtain structural information, and the U=N imido (1.97(1) A), U-Cl (2.60(2) A), and U-C5Me5 (2.84(1) A) distances were consistent with those observed for compounds 3, 5, 6, 8-10, which were all characterized by single-crystal X-ray diffraction studies. All (C5Me5)2U(=N-Ar)(X) complexes exhibit U(V)/U(IV) and U(VI)/U(V) redox couples by voltammetry, with the potential separation between these metal-based couples remaining essentially constant at approximately 1.50 V. The electronic spectra are comprised of pi-->pi* and pi-->nb(5f) transitions involving electrons in the metal-imido bond, and metal-centered f-f bands illustrative of spin-orbit and crystal-field influences on the 5f(1) valence electron configuration. Two distinct sets of bands are attributed to transitions derived from this 5f(1) configuration, and the intensities in these bands increase dramatically over those found in spectra of classical 5f(1) actinide coordination complexes. Temperature-dependent magnetic susceptibilities are reported for all complexes with mu(eff) values ranging from 2.22 to 2.53 mu(B). The onset of quenching of orbital angular momentum by ligand fields is observed to occur at approximately 40 K in all cases. Density functional theory results for the model complexes (C5Me5)2U(=N-C6H5)(F) (11) and (C5Me5)2U(=N-C6H5)(I) (12) show good agreement with experimental structural and electrochemical data and provide a basis for assignment of spectroscopic bands. The bonding analysis describes multiple bonding between the uranium metal center and imido nitrogen which is comprised of one sigma and two pi interactions with variable participation of 5f and 6d orbitals from the uranium center.     

Synthetic, reactivity, and structural studies on half-sandwich (eta5-C5Me5)Be and related compounds: halide, alkyl, and iminoacyl derivatives

The half-sandwich compounds [(eta(5)-C(5)Me(5))BeX] (X=Cl, 1 a; Br, 1 b), readily prepared from the reaction of the halides BeX(2) and M[C(5)Me(5)] (M=Na or K), are useful synthons for other (eta(5)-C(5)Me(5))Be organometallic compounds, including the alkyl derivatives [(eta(5)-C(5)Me(5))BeR] (R=Me, 2 a; CMe(3), 2 b; CH(2)CMe(3), 2 c; CH(2)Ph, 2 d). The latter compounds can be obtained by metathetical exchange of the halides 1 with the corresponding lithium reagent and exhibit NMR signals and other properties in accord with the proposed formulation. Attempts to make [(eta(5)-C(5)Me(5))BeH] have proved fruitless, probably due to instability of the hydride toward disproportionation into [Be(C(5)Me(5))(2)] and BeH(2). The half-sandwich iminoacyl [(eta(5)-C(5)Me(5))Be(C(NXyl)Cp')] and [(eta(5)-C(5)Me(4)H)Be(C(NXyl)Cp')]3, 6 where Xyl=C(6)H(3)-2,6-Me(2) and Cp'=C(5)Me(5) or C(5)Me(4)H, are formed when the beryllocenes [Be(C(5)Me(5))(2)], [Be(C(5)Me(4)H)(2)], and [Be(C(5)Me(5))(C(5)Me(4)H)] are allowed to react with CNXyl. Isolation of three different iminoacyl isomers from the reaction of the mixed-ring beryllocene [(eta(5)-C(5)Me(5))Be(eta(1)-C(5)Me(4)H)] and CNXyl, namely compounds 5 a, 5 b, and 6, provides compelling evidence for the existence in solution of different beryllocene isomers, generated in the course of two very facile processes that explain the solution dynamics of these metallocenes, that is the 1,5-sigmatropic shift of the Be(eta(5)-Cp') unit around the periphery of the eta(1)-Cp' ring, and the molecular inversion rearrangement that exchanges the roles of the two rings.