Reactions of Cp2M (M = Ni, V) with dilithium diamido-aryl reagents; retention and oxidation of the transition metal ions

The reactions of dilithium 1,2-diamidobenzene, [1,2-(HN)2C6H4]Li2 (L(1)H2)Li2, and dilithium 1,8-diamidonaphthalene, [1,8-(NH)2C10H6]Li2 (L(2)H2)Li2, with Cp2Ni and Cp2V have been used to obtain the new complexes (L(2)H2)2Ni{Li(THF)2}2 (3), (L(2)H2)3V{Li(THF)2}3 (4) and (L(1)H2)6Ni6·{[(L(1)H2)3(L(1)H)3Ni6Li(THF)](2-)·2[Li(THF)4](+)} (5), in which retention or oxidation of the initial metal(ii) centre is observed. Whereas 3 and 4 contain one transition metal ion within ion-paired structures, 5 has a complicated co-crystalline composition which contains octahedral Ni6-cages constructed from six square-planar (16e) Ni(II) centres.

Synthesis and structural analysis of half-titanocenes containing eta(2)-pyrazolato ligands, and their use in catalysis for ethylene polymerization

Cp*TiCl(2)(L) [L = C(3)H(3)N(2) (1), 3,5-Me(2)C(3)HN(2) (2) and 3,5-(i)Pr(2)C(3)HN(2) (3)], and Cp*Ti(C(3)H(3)N(2))(3) (4) were prepared in moderate yields by treating Cp*TiCl(3) with the pyrazoles in the presence of Et(3)N or with their corresponding lithium salts in Et(2)O. The structures of 1 and 2 determined by X-ray crystallography indicate that these complexes fold a distorted tetrahedral geometry around Ti, and the pyrazolato ligands coordinate to Ti with eta(2)-N,N'-coordination mode. In contrast, one of the pyrazolato ligand in 4 coordinates to Ti with eta(1)-N-bonding, whereas the other two ligands were bound to Ti with eta(2)-N,N'-fashion. The Cp analogues, CpTiCl(2)(L) [L = C(3)H(3)N(2) (5), 3,5-Me(2)C(3)HN(2) (6), 3,5-(i)Pr(2)C(3)HN(2) (7), and 3,5-Ph(2)C(3)HN(2) (8)], were also prepared by the reaction of CpTiCl(3) with the corresponding lithium salts in Et(2)O or n-hexane. The crystallographic analyses of 5 and 6 revealed that the pyrazolato ligands coordinate to Ti with eta(2)-N,N'-coordination mode. These complexes (1-3, 5-8) exhibited moderate catalytic activities for ethylene polymerization in the presence of methylaluminoxane (MAO), and the activities were highly affected by the substituent on the pyrazolato ligand employed. Complex 1 exhibited the highest activity affording polymer with uniform molecular weight distribution, suggesting that the polymerization proceeded with uniform catalytically active species. An increase in the steric bulk in the pyrazolato ligand led to a slight decrease in the activity by the Cp* analogues, whereas the activity by the Cp analogues increased upon increasing the steric bulk in the pyrazolato ligand employed.

Pyrazoles and pyrazolides-flexible synthons in self-assembly

This Perspective summarises the chemistry of the pyrazole ring, and reviews the metal coordination modes adopted by 1H-pyrazoles and their anions. Pyrazolide anions are probably the most versatile ligands in coordination chemistry, with 20 different terminal or bridging coordination modes having been identified so far. Metal cluster compounds supported by pyrazolido ligation are surveyed, concentrating on those reported during the past ten years. Highlights include the wide structural diversity in apparently simple main group pyrazolides; luminescence and charge-transfer complexes in coinage metal pyrazolide clusters; the use of robust metal pyrazolide clusters to construct liquid crystals, supramolecular materials and metal-organic frameworks; and supramolecular complexes formed by pyrazolide-supported metallacrowns.

An improved method for the computation of ligand steric effects based on solid angles

An improved algorithm has been designed to characterize ligand interactions in organometallic and coordination complexes in terms of the percentage of the metal coordination sphere shielded by a given ligand. The computations for ligand solid angles are performed numerically and employ introduced atomic radii that are larger than covalent but smaller than van der Waals radii. This approach enables facile evaluation of steric congestion in the metal coordination sphere, quantification of unfavorable interligand contacts, and in some cases prediction of the complex composition or ligand coordination on purely geometrical grounds.

Synthesis, structural characterization, and properties of chromium(iii) complexes containing amidinato ligands and η2-pyrazolato, η2-1,2,4-triazolato, or η1-tetrazolato ligands

Treatment of anhydrous chromium(III) chloride with 2 or 3 equivalents of 1,3-di-tert-butylacetamidinatolithium or 1,3-diisopropylacetamidinatolithium in tetrahydrofuran at ambient temperature afforded Cr(tBuNC(CH3)NtBu)2(Cl)(THF) and Cr(iPrNC(CH3)NiPr)3 in 78% and 65% yields, respectively. Treatment of Cr(tBuNC(CH3)NtBu)2(Cl)(THF) with the potassium salts derived from pyrazoles and 1,2,4-triazoles afforded Cr(tBuNC(CH3)NtBu)2(X), where X=3,5-disubstituted pyrazolato or 3,5-disubstituted 1,2,4-triazolato ligands, in 65-70% yields. X-Ray crystal structure analyses of Cr(tBuNC(CH3)NtBu)2(Me2pz) (Me2pz=3,5-dimethylpyrazolato) and Cr(tBuNC(CH3)NtBu)2(Me2trz) (Me2trz=3,5-dimethyl-1,2,4-triazolato) revealed eta2-coordination of the Me2pz and Me2trz ligands. Treatment of Cr(tBuNC(CH3)NtBu)2(Cl)(THF) with trifluoromethyltetrazolatosodium (NaCF3tetz) in the presence of 4-tert-butylpyridine afforded Cr(tBuNC(CH3)NtBu)2(CF3tetz)(4-tBupy) in 30% yield. An X-ray crystal structure determination showed eta1-coordination of the tetrazolato ligand through the 2-nitrogen atom. The complexes Cr(iPrNC(CH3)NiPr)3 and Cr(tBuNC(CH3)NtBu)2(X) are volatile and sublime with <1% residue between 120 and 165 degrees C at 0.05 Torr. In addition, these complexes are thermally stable at >300 degrees C under an inert atmosphere such as nitrogen or argon. Due to the good volatility and high thermal stability, these new compounds are promising precursors for the growth of chromium-containing thin films using atomic layer deposition.

Rhenium oxo compounds containing eta2-pyrazolate ligands

Reaction of potassium salts of sterically demanding pyrazolates (pz = bis-3,5-tert-butylpyrazolate, pz= bis-3,5-tert-butyl-4-methylpyrazolate) with Re2O7 affords soluble eta2-pyrazolate complexes of the type [(eta2-pz)ReO3(THF)n](1: pz, n= 1 and 2: pz, n= 0). They were characterized by spectroscopic methods and by X-ray crystallography confirming the eta2-coordinate ligands. Complex 1 employing the ligand with a proton in the 4-position retains one molecule of THF, whereas the additional methyl group in 2 leads to the base-free compound 2. Compound 1 reacts with pyridine and 3,5-dimethylpyridine to form Lewis base adducts of the type [(eta2-pz)ReO3(L)](3: L = py; 4: L = 3,5-Me2py). The pronounced sensitivity towards water of these complexes is demonstrated by the reaction of 1 with one equivalent of water forming the corresponding pyrazolium perrhenate [ReO4][pzH2](5). Its solid state structure shows a hydrogen bonded dimeric assembly. Catalytic activity of 1 is established in oxygen atom transfer-reactions (OAT) from dimethylsulfoxide to triphenylphosphine, and in epoxidations of cyclooctene employing bis(trimethylsilyl) peroxide (BTSP).