Dibenzylzirconium Complexes of Chelating Aminodiolates. Synthesis, Structural Studies, Thermal Stability, and Insertion Chemistry

Organometallic Zirconium Compounds in an Oxygen-Rich Coordination Environment: Synthesis and Structural Characterization of Tris(oxoimidazolyl)hydroboratozirconium Compounds

A series of tris(oxoimidazolyl)hydroborato ligands, which serve as L₂X [O₃] donors, have been employed to obtain organometallic zirconium compounds in an uncommon oxygen-rich coordination environment. For example, Cp[To^MeBenz]ZrCl₂ has been synthesized via the reaction of [To^MeBenz]Na with CpZrCl₃ and bears a structural resemblance to the bent metallocene dichloride derivative Cp₂ZrCl₂. In addition, the half-sandwich counterparts [To^MeBenz]ZrCl₃ and [To^But]ZrCl₃ have been obtained by metathesis of ZrCl₄ with [To^MeBenz]Na and [To^But]Na, respectively. The structurally related zirconium benzyl compounds [To^RBenz]Zr(CH₂Ph)₃ (R = Me, Bu^t, 1-Ad) have also been synthesized via the reactions of [To^RBenz]Tl with Zr(CH₂Ph)₄, and X-ray diffraction studies demonstrate that the benzyl ligands in these compounds are conformationally flexible and exhibit a large range of Zr-CH₂-Ph bond angles (94.7-131.7°). Protolytic cleavage of one of the benzyl ligands of [To^RBenz]Zr(CH₂Ph)₃ (R = Bu^t, 1-Ad) may be achieved by treatment with [PhNHMe₂][B(C₆F₅)₄] to generate {[To^RBenz]Zr(CH₂Ph)₂}[B(C₆F₅)₄], which are catalysts for the polymerization of ethylene. The molecular structure of the ether adduct, {[To^ButBenz]Zr(CH₂Ph)₂(OEt₂)}[B(C₆F₅)₄], has been determined by X-ray diffraction. In addition to the use of tris(oxoimidazolyl)hydroborato ligands, bis(oxoimidazolyl)hydroborato ligands have also been used to obtain zirconium benzyl compounds in oxygen-rich environments, namely, [Bo^MeBenz]₂Zr(CH₂Ph)₂ and [Bo^AdBenz]₂Zr(CH₂Ph)₂.

Chelating σ-Aryl Post-Metallocenes: Probing Intramolecular [C-H···F-C] Interactions and Unusual Reaction Pathways

Our interest in chelating σ-aryl ancillary ligands was motivated by their potential to impart unusual reactivity, since we envisioned that σ-donors with minimal π-donation would create a catalytic center with enhanced electrophilicity. We developed a family of Group 4 post-metallocene catalysts supported by pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands bearing a fluorinated moiety in the vicinity of the metal. Notable features of these meta-substituted tris(hetero)aryl frameworks include their coordination geometry and inherent rigidity. For the first time, the elusive C-H···F-C interaction was manifested as NMR-discernible (1)H-(19)F coupling in solution and characterized by a neutron diffraction study. Their existence carries implications for catalyst design and in the context of weak attractive ligand-polymer interactions (WALPI), since they substantiate the practical viability of the ortho-F···H(β) ligand-polymer interactions proposed for living Group 4 fluorinated bis(phenoxyimine) catalysts. In metal-catalyzed olefin polymerization reactions, the notion of noncovalent interactions between an active ancillary ligand and the growing polymer chain is new. These interactions must be fragile and transient in nature, otherwise the intrinsic chain propagation process would be disrupted, and inherently tunable attractive forces such as hydrogen bonds are ideally suited to this role. The nature, relevance, and usability of extremely weak hydrogen bonds such as C-H···F-C has been a topical yet controversial area of research. We subsequently prepared a series of Group 4 complexes supported by fluorinated (σ-aryl)-2-phenolate-6-pyridyl [O,C,N] ligands. [(1)H,(19)F]-HMBC NMR experiments were conducted to probe the observed (1)H-(19)F coupling, and specifically separate contributions from scalar (J) coupling and cross-correlation (CR) interference. For the first time, a significant scalar component was confirmed for the (1)H-(19)F coupling in Ti-[O,C,N] and [O,N,C] complexes, which occurs with chemical connectivity across intramolecular C-H···F-C interactions. This result is important because the applicability of weak attractive ligand-polymer interactions in catalysis is feasible only if the observed coupling and hence the noncovalent interaction is genuine. The verified intramolecular C-H···F-C contacts in these complexes can therefore be considered as synthetic models for ligand-polymer interactions in olefin polymerization processes. Significantly, reports concerning late transition metal systems have appeared that hint at the generality of the WALPI concept for modulating polymerization reactions. We evaluated the olefin polymerization reactivity of Ti-[O,N,C] catalysts through judicious substitution. DFT calculations, which revealed diverse kinetically competitive reaction pathways and active sites (including unusual ethylene-assimilated species) in addition to normal chain propagation, were also employed to rationalize polymerization efficiencies. Further developments in catalytic applications of multidentate σ-aryl ligand systems and novel reactivity of the corresponding complexes can be envisaged.

Reactions of heteroallenes with cyclam-based Zr(IV) complexes

This work describes reactions of heteroallenes with diamido-diamine cyclam-based Zr(iv) complexes of the general formula (Bn2Cyclam)ZrX2 (X = O(t)Bu, , O(i)Pr, , SPh, , NH(t)Bu, ) as well as the di-orthometallated species ((C6H4CH2)2Cyclam)Zr, . The reactions of isocyanates or isothiocyanates with , or resulted in the formation of N-bonded ureate or thioureate cyclam complexes upon [2 + 2] cycloaddition of the Zr-Namido bonds of the cyclam to the heteroallene (). DFT calculations showed that κ(2)-N,N'-ureate bonding is favoured over κ(2)-N,O-ureates, which in turn may be formed in reactions with bulky isocyanates as 1-naphthyl isocyanate (NpN[double bond, length as m-dash]C[double bond, length as m-dash]O). The reactions of with N,N'-cyclohexylcarbodiimide (CyN[double bond, length as m-dash]C[double bond, length as m-dash]NCy) and carbon disulfide afforded guanidinate and dithiocarbamate fragments, respectively, appended to one of the nitrogen atoms of the cyclam ligand. These reactions represent a reliable method for the synthesis of asymmetrically N-functionalized cyclams giving rise to C1 symmetry Zr(iv) species by addition of one equivalent of heteroallenes. The reaction of (Bn2Cyclam)Zr(NH(t)Bu)2, , with one equivalent of mesityl isocyanate (MesN[double bond, length as m-dash]C[double bond, length as m-dash]O) also proceeds through insertion, involving one Zr-NH(t)Bu bond. However, it was observed that the reaction of (Bn2Cyclam)Zr(NH(t)Bu)2, , with MesN[double bond, length as m-dash]C[double bond, length as m-dash]O follows a different path if the reaction is carried out at 60 °C. In this case the reaction leads to [2 + 2] addition of the Zr-Ncyclam bond to the isocyanate, with a concomitant occurrence of orthometallation of the one benzyl pending group of the cyclam ring. The reaction of (t)BuN[double bond, length as m-dash]C[double bond, length as m-dash]O with the di-orthometallated complex ((C6H4CH2)2Cyclam)Zr, , also gave a κ(2)-N,N'-ureate fragment, by isocyanate addition to the macrocycle. DFT calculations on these systems were conducted in an attempt to rationalise the reactivity patterns observed.

Chelating Dialkoxide Titanium Complex: A Versatile Building Block for the Construction of Heterometallic Derivatives

The heterometallic complex [TiCp*(O(2)Bz)(2)AlMe(2)] (2) has been synthesised by reaction of [TiCp*(O(2)Bz)(OBzOH)] (1) with AlMe(3) (Cp*=eta(5)-C(5)Me(5); Bz=benzyl). Complex 1 reacts with HOTf to yield the cationic derivative [TiCp*(OBzOH)(2)]OTf (3) (HOTf=HSO(3)CF(3)). Compound 3 reacts with [{M(mu-OH)(cod)}(2)] (M=Rh, Ir; cod=cyclooctadiene) to render the early-late heterometallic complexes [TiCp*(O(2)Bz)(2){M(cod)}(2)]OTf (M=Rh (4); Ir (5)). The molecular structure of complex 4 has been established by single-crystal X-ray diffraction studies.

Dynamic stereochemical rearrangements in chiral organometallic complexes

Organometallic complexes have long been known to display a wide variety of dynamic stereochemical processes. Classic examples of such processes include the exchange of axial and equatorial environments in trigonal bipyramidal complexes, such as Fe(CO)(5), and the migration of the metal moiety round the periphery of the cyclopentadiene ring in eta(1)-bound Cp complexes. The systematic study of fluxional processes is of interest because it can not only help provide a detailed, quantitative 'picture' of the bonding between the metals and ligands involved, but it can also help to rationalise chemical reactivity patterns. The introduction of chirality into organometallic complexes, usually in the form of a non-racemic chiral ligand, has led to an explosion in the importance such species, particularly with regard to their applications in organic functional group transformations. The presence of a chiral centre can also provide an excellent spectroscopic handle on the complex in question, enabling both novel fluxional processes to be observed and new light to be shed on old (unresolved) problems. In this critical review (101 references) the literature on metal-centred fluxional rearrangements in chiral transition and main group organometallic complexes is reviewed, complementing the recent review by Faller (see reference 8).

A new titanium building block for early–late heterometallic complexes; preparation of a new tetrameric metallomacrocycle by self assembly

The new titanium dicarboxylate complex Cp*TiMe(OOC)2py (2) [Cp*=eta5-C5Me5; (OOC)2py = 2,6-pyridinedicarboxylate] has been synthesized. The reaction of complex 2 with water renders [Cp*Ti(OOC)2py]2O (3). The molecular structure of 3 has been studied by X-ray diffraction methods. Complex 2 reacts with isocyanides to yield the respective iminoacyl derivatives Cp*Ti(eta2-MeCNR)(OOC)2py [R=tBu (4), 2,6-dimethylphenyl (xylyl) (5)]. The molecular structure of complex4 has been established by X-ray diffraction. Compound 2 has been employed as a new building block for the preparation of new early-late heterometallic compounds; it reacts with [M(mu-OH)(COD)]2 (M = Rh, Ir) to give the corresponding tetranuclear metallomacrocycle derivatives [Cp*Ti{(OOC)(2)py}(mu-O)M(COD)]2 [M = Rh (6); Ir (7)]. The molecular structure of 6 has been established by X-ray diffraction.

Early transition metal complexes containing 1,2,4-triazolato and tetrazolato ligands: synthesis, structure, and molecular orbital studies

Several early transition metal complexes bearing 1,2,4-triazolato and tetrazolato ligands have been prepared by reaction of the pyrazolato complexes Ti(tBu(2)pz)(4-x)Cl(x) (tBu(2)pz = 3,5-di-tert-butylpyrazolato; x = 1, 2) and M(tBu(2)pz)(5-x)Cl(x) (M = Nb, Ta: x = 2, 3) with the sodium or potassium salts derived from 1,2,4-triazoles and tetrazoles. The X-ray structure analysis of Ti(tBu(2)pz)(2)(Me(2)C(2)N(3))(2) shows eta(2)-coordination of the 1,2,4-triazolato ligands, while in Ti(tBu(2)pz)(3)(C(2)H(2)N(3)) and Nb(tBu(2)pz)(3)(Me(2)C(2)N(3))(2) the analogous groups are joined in a eta(1)-fashion in the solid-state structure. Solution NMR studies at different temperatures suggest transition states involving eta(2)-1,2,4-triazolato ligands for the complexes containing eta(1)-1,2,4-triazolato ligands in the solid state. X-ray crystal structures of analogous tetrazolato complexes Ti(tBu(2)pz)(3)(PhCN(4)) and Nb(tBu(2)pz)(3)(PhCN(4))(2) show eta(1)-coordination of the 2-nitrogen atoms of the tetrazolato ligands. Molecular orbital calculations have been carried out on several model titanium complexes and provide detailed insight into the bonding between early transition metal centers and 1,2,4-triazolato and tetrazolato ligands. The eta(2)-coordination mode of 1,2,4-triazolato and tetrazolato ligands is predicted to be more stable than the eta(1)-coordination mode by 13.8-5.2 kcal/mol.

Coordination chemistry of amine bis(phenolate) titanium complexes: tuning complex type and structure by ligand modification

The coordination chemistry of titanium(IV) complexes of amine bis(phenolate) ligands was investigated by synthesizing various types of complexes and analyzing them specroscopically and structurally. Steric effects of tridentate [ONO]- and tetradentate [ONNO]-type ligands were studied by reacting the ligand precursors with titanium tetra(isopropoxide). [ONNO]-type ligands featuring an amine donor located on a pendant arm led to octahedral bis(isopropoxide) complexes, regardless of the steric bulk around the metal. Several such complexes having varying steric crowding were thus synthesized. On the other hand, steric effects were found to play a major role in determining the complex constitution when [ONO]-type ligands, featuring no side donor, were involved. Relatively sterically undemanding ligands led to octahedral bis(homoleptic) complexes, whereas increased steric bulk resulted in the formation of pentacoordinate bis(isopropoxide) complexes. These pentacoordinate complexes readily lead to bis(heteroleptic) complexes by reaction with nonsterically demanding [ONO]- and [ONNO]-type ligand precursors. In the latter case the sidearm nitrogen remains uncoordinated to the metal. The bis(isopropoxide) complexes of the [ONNO]-type ligands may also lead to bis(heteroleptic) complexes, however, these reactions are much slower.