Studies into the mechanism of CO-induced N2 cleavage promoted by an ansa-hafnocene complex and C-C bond formation from an observed intermediate

Carbonylation of the hafnocene dinitrogen complex, [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-(t)Bu)Hf](2)(μ(2), η(2), η(2)-N(2)), yields the corresponding hafnocene oxamidide compound, arising from N(2) cleavage with concomitant C-C and C-N bond formation. Monitoring the addition of 4 atm of CO by NMR spectroscopy allowed observation of an intermediate hafnocene complex with terminal and bridging isocyanates and a terminal carbonyl. (13)C labeling studies revealed that the carbonyl is the most substitutionally labile ligand in the intermediate and that N-C bond formation in the bridging isocyanate is reversible. No exchange was observed with the terminal isocyanate. Kinetic data established that the conversion of the intermediate to the hafnocene oxamidide was not appreciably inhibited by carbon monoxide and support a pathway involving rate-determining C-C coupling of the isocyanate ligands. Addition of methyl iodide to the intermediate hafnocene resulted in additional carbon-carbon bond formation arising from CO homologation following nitrogen methylation. Similar reactivity with (t)BuNCO was observed where C-C coupling occurred upon cycloaddition of the heterocumulene. By contrast, treatment of the intermediate hafnocene with CO(2) resulted in formation of a μ-oxo hafnocene with two terminal isocyanate ligands.     

Transition metal ion complexes of theS-methyldithiocarbazate prepared from 2-acetylpyridine

Summary Metal ion complexes of 2-acetylpyridineS-methyldithiocarbazate, HNNS, have been prepared and spectrally characterised. Preparations in EtOH yield complexes in which the deprotonated ligand, NNS, is complexedvia its pyridyl nitrogen, azomethine nitrogen, and thione sulphur. The stoichiometries are: [M(NNS)₂]X (M=Fe³⁺, Co³⁺ and X=ClO ₄ ^– , [FeCl₄]^–, BF ₄ ^– , 1/2 [CoCl₄]^2– and 1/2 [CoBr₄]^2–), [M(NNS)X] (M=Ni²⁺, Cu²⁺ and X=Cl^–, Br^–), [Cu(NNS)H₂O]BF₄ and Ni(HNNS)(NNS)F(EtOH)]BF₄. The spectral (i.e., i.r., u.v.-vis.-n.i.r. and e.s.r.) and physical properties of these complexes are compared to those of theS-methyldithiocarbazates of 2-formylpyridine and 2-acetylpyridineN-oxide, as well as the related thiosemicarbazones prepared from 2-acetylpyridine. Thermal studies of the nickel(II) complexes indicate that the nature of thermal decomposition of coordinated NNS is different from that of HNNS.     

Transition metal ion complexes of theS-methyldithiocarbazate prepared from 2-acetylpyridineN-oxide

Summary Metal ion complexes of theS-methyldithiocarbazate of 2-acetylpyridineN-oxide, HONS, have been prepared and characterised by spectral and physical methods. Preparative ethanolic solutions on reflux yield complexes in which the deprotonated form of the ligand, ONS, is complexedvia itsN-oxide oxygen, azomethine nitrogen, and thione sulphur. Representative stoichiometries are as follows: [M(ONS)₂]X (M=Fe³⁺, and X=ClO ₄ ^– , FeCl ₄ ^– or M=Co³⁺ and X=BF ₄ ^– ) and [M(ONS)X] (M=Co²⁺, Ni²⁺ and Cu²⁺ and X=Cl^–, Br^–). The i.r., electronic, and electron spin resonance spectra of the complexes are compared to analogous complexes of theX-methyldithiocarbazate of 2-acetylpyridine as well as the thiosemicarbazones of 2-acetylpyridineN-oxide.     

Nickel(II) complexes of a thiosemicarbazone prepared from 2-Acetylpyridine

Summary The ligand 3-azabicyclo[3.2.2]nonane-3-thiocarboxylic acid 2-[1-(2-pyridinyl)ethylidene]hydrazide (HL), which is observed in an unusual tautomeric form in the solid state, and its selenium analogue (HLSe) have been used to prepare a series of nickel(II) complexes. Compounds of the general formula [NiLX] (X=Cl, Br, NCS, N₃, NO₂ or NCSe) as well as [Ni(LSe)Cl] have been found to be diamagnetic, planar complexes. A single crystal study of [NiL(NCS)] shows the deprotonated ligand bound in a tridentate mannervia its pyridyl nitrogen, imine nitrogen and the thione sulphur atom with the nitrogen atom of the thiocyanato-ligand occupying the fourth coordination position. The solids prepared from the nickel(II) salts having tetrafluoroborate, nitrate and iodide ions approximate to octahedral symmetry and have neutral HL ligands coordinated in a bidentate fashionvia the pyridine and imine nitrogens with the remaining coordination sites being occupied by the anions or water molecules. The [NiL₂] solid is also octahedral with the two deprotonated ligands bonding as tridentate groupsvia the same atoms as in the [NiLX] complexes.     

Guanidinate-stabilized monomeric hafnium amide complexes as promising precursors for MOCVD of HfO2

Novel guanidinato complexes of hafnium [Hf{eta2-(iPrN)2CNR2}2(NR2)2] (R2 = Et2, 1; Et, Me, 2; Me2, 3), synthesized by insertion reactions of N,N'-diisopropylcarbodiimide into the M-N bonds of homologous hafnium amide complexes 1-3 and {[mu2-NC(NMe2)2][NC(NMe2)2]2HfCl}2 (4) using a salt metathesis reaction, are reported. Single-crystal X-ray diffraction analysis revealed that compounds 1-3 were monomers, while compound 4 was found to be a dimer. The observed fluxional behavior of compounds 1-3 was studied in detail using variable-temperature and two-dimensional NMR techniques. The thermal characteristics of compounds 1-3 seem promising for HfO2 thin films by vapor deposition techniques. Metal-organic chemical vapor deposition experiments with compound 2 as the precursor resulted in smooth, uniform, and stoichiometric HfO2 thin films at relatively low deposition temperatures. The basic properties of HfO2 thin films were characterized in some detail.     

2-methylaminopyridine N-oxide complexes prepared from various cobalt(II) salts

2-Methylaminopyridine N-oxide (HL) complexes have been formed with CoX₂ (X = ClO₄, BF₄, NO₃ and Cl) using both 6:1 and 2:1 ligand-to-metal ion mole ratios. From the former preparative mixtures species with four ligands per Co(II) except with Co(BF₄)₂ which had six, were isolated while two ligands per Co(II) were found for the solids isolated from the latter mixtures. Characterization of the eight solids was accomplished by a study of their IR and electronic spectra as well as partial elemental analyses, molar conductivities and thermal measurements. Of note is that HL rarely functions as a bidentate ligand in these complexes.     

Studies on DNA cleavage and antimicrobial screening of transition metal complexes of 4-aminoantipyrine derivatives of N2O2 type

A new series of transition metal complexes of Cu(II), Ni(II), Zn(II) and VO(IV), were synthesized from the Schiff base (L) derived from 4-aminoantipyrine, 3-hydroxy-4-nitrobenzaldehyde and acetylacetone. The structural features were arrived from their elemental analyses, magnetic susceptibility, molar conductance, Mass, IR, UV-Vis., ¹H NMR and ESR spectral studies. The data show that the complexes have composition of [ML]X type. The UV-Vis., magnetic susceptibility and ESR spectral data of the complexes suggest a square-planar geometry around the central metal ion except for VO(IV) complex which has square-pyramidal geometry. The redox behavior of copper and vanadyl complexes were studied by cyclic voltammetry. The antimicrobial screening tests were also recorded and gave good results in the presence of metal ion in the ligand system. The nuclease activity of the above metal complexes shows that the copper and nickel complexes cleave DNA through redox chemistry, whereas other complexes are not effective.     

2-dimethyl- and 2-diethylaminopyridine N-oxide complexes prepared from various cobalt(II) salts

2-dimethyl- and 2-diethylaminopyridine N-oxide (DM and DE, respectively) complexes have been formed with CoX₂ (X=ClO₄, BF₄, NO₃ and Cl). Although preparative ligand: Co(II) mole ratios were varied, only one unique solid was isolated for each ligand and cobalt salt combination. Characterization of the eight solids was accomplished primarily by partial elemental analyses and infrared and electronic spectra. Compared to 2-methylaminopyridine N-oxide both of these dialkyl ligands tend to bond as bidentate ligands although some of the complexes appear to have the N-oxide bound in both a monodentate and bidentate fashion.     

N2 activation by iron-sulfur complexes

Inspired by the determination of the structure of the nitrogenase enzyme cofactor by Rees et al., the binding of an N₂ molecule to some model iron-sulfur compounds was investigated usingab initio calculations. Side-on and end-on coordination to one two and four iron centers were investigated. In most cases, the N₂ is loosely bound and retains its internal triple bond, but a few examples are found where the N₂ is “activated” and has a longer N-N bond length.     

Dynamic transformations of cyclopentadienyl chelate complexes of zirconium and hafnium

Russian Chemical Bulletin -     

Addition and metathesis reactions of zirconium and hafnium imido complexes

The zirconium and hafnium imido metalloporphyrin complexes (TTP)M = NArtPr (TTP = meso-5,10,15,20-tetra-p-tolylporphyrinato dianion; M = Zr (1), Hf; AriPr = 2,6-diisopropylphenyl) were used to mediate addition reactions of carbonyl species and metathesis of nitroso compounds. The imido complexes react in a stepwise manner in the presence of 2 equiv of pinacolone to form the enediolate products (TTP)M[OC(tBu)CHC(tBu)(Me)O] (M = Zr (2), Hf (3)), with elimination of H2NAriPr. The bis(mu-oxo) complex [(TTP)ZrO]2 (4) is formed upon reaction of (TTP)Zr = NAriPr with PhNO. Treatment of compound 4 with water or treatment of compound 2 with acetone produced the (mu-oxo)bis(mu-hydroxo)-bridged dimer [(TTP)Zr]2(mu-O)(mu-OH)2 (5). Compounds 2, 4, and 5 were structurally characterized by single-crystal X-ray diffraction.     

Atomic layer deposition of hafnium oxide from hafnium chloride and water

Hafnium oxide (HfO2) is a leading candidate to replace silicon oxide as the gate dielectric for future generation metal-oxide-semiconductor based nanoelectronic devices. Atomic layer deposition (ALD) has recently gained interest because of its suitability for fabrication of conformal films with thicknesses in the nanometer range. This study uses periodic density functional theory (DFT) to investigate the mechanisms of both half-reactions occurring on the growing surface during the ALD of HfO2 using HfCl4 and water as precursors. We find that the adsorption energy and the preferred site of adsorption of the metal precursor are strong functions of the water coverage. As water coverage increases, the metal precursor preferentially interacts with multiple surface adsorption sites. During the water pulse the removal of Cl can be facilitated by either a ligand exchange reaction or the dissociation of Cl upon increase in coordination of the Hf atom of the precursor. Our predicted potential energy surface indicates that a more likely mechanism is hydration of the adsorbed Hf complex up to a coordination number of 7, followed by the dissociation of a chloride ion that is stabilized by solvation. Born-Oppenheimer molecular dynamics (BOMD) simulations of an adsorbed metal precursor in the presence of a multilayer of water shows that Cl dissociation is facile if sufficient water molecules are present to solvate the Cl(-) anions. Hence, solvation plays a crucial role during the water pulse and provides an alternative explanation for why ALD growth rates for this system decrease at high temperatures.     

Fluorinated diarylamido complexes of lithium, zirconium, and hafnium

Deprotonation of N-(2-fluorophenyl)-2,6-diisopropylaniline (H[ (i) PrAr-NF]) with 1 equiv of n-BuLi in toluene at -35 degrees C produced cleanly [ (i) PrAr-NF]Li. Subsequent recrystallization of [ (i) PrAr-NF]Li in diethyl ether generated the bis(ether) adduct [ (i) PrAr-NF]Li(OEt 2) 2. An X-ray study of [ (i) PrAr-NF]Li(OEt 2) 2 showed it to be a four-coordinate species with the coordination of the fluorine atom to the lithium center. The reactions of [ (i) PrAr-NF]Li with MCl 4(THF) 2 (M = Zr, Hf), regardless of the stoichiometry employed, afforded the corresponding dichloride complexes [ (i) PrAr-NF] 2MCl 2 (M = Zr, Hf). Alkylation of [ (i) PrAr-NF] 2MCl 2 with a variety of Grignard reagents generated [ (i) PrAr-NF] 2MR 2 (M = Zr, Hf; R = Me, i-Bu, CH 2Ph). The X-ray structures of [ (i) PrAr-NF] 2ZrCl 2, [ (i) PrAr-NF] 2HfCl 2, [ (i) PrAr-NF] 2ZrMe 2, [ (i) PrAr-NF] 2Zr( i-Bu) 2, and [ (i) PrAr-NF] 2Hf(CH 2Ph) 2 are all indicative of the coordination of the fluorine atoms to these group 4 metals, leading to a C 2-symmetric, distorted octahedral geometry for these molecules.     

New high-selectivity hydrogenation catalysts prepared from bimetallic acetate complexes

The catalytic properties of new Pd-Zn/Al₂O₃ catalysts in selective acetylene hydrogenation in an acetylene-ethylene mixture at 30–120°C and atmospheric pressure are reported. The catalysts prepared from the bimetallic complex Pd-Zn(OOCMe)₄(OH₂) are much more selective than the catalysts prepared by simultaneously supporting the homonuclear complexes Pd₃(OOCMe)₆ and Zn(OOCMe)₂ · 2H₂O. It is demonstrated by diffuse reflectance IR spectroscopy of adsorbed CO that the heat treatment of the supported bimetallic complex at 250°C in flowing H₂ yields a Pd-Zn alloy on the surface. It is this alloy that ensures the high selectivity of the Pd-Zn/Al₂O₃ catalysts.     

Functionalization of polymers prepared by ATRP using radical addition reactions

Low molecular weight linear poly(methyl acrylate), star and hyperbranched polymers were synthesized using atom transfer radical polymerization (ATRP) and end‐functionalized using radical addition reactions. By adding allyltri‐n‐butylstannane at the end of the polymerization of poly(methyl acrylate), the polymer was terminated by allyl groups. When at high conversions of the acrylate monomer, allyl alcohol or 1,2‐epoxy‐5‐hexene, monomers which are not polymerizable by ATRP, were added, alcohol and epoxy functionalities respectively were incorporated at the polymer chain end. Functionalization by radical addition reactions was demonstrated to be applicable to multi‐functional polymers such as hyperbranched and star polymers.     

Coupling of coordinated 2-iminophosphorano-1-phosphaallyl leading to bridged iminophosphoranato complexes of zirconium and hafnium

Reaction of MCl4 (M = Zr, Hf) with 2 equiv of 2-iminophosphorano-1-phosphaallyl lithium [Li[P(Ph)C(=CHPh)P(Me)2=NSiMe3](THF)1.5] (1) affords ligand coupling complexes 3 and 4, respectively, while similar treatment of ZrCl4 with [Li[P(Ph)C(=C(SiMe2Bu(t))Ph)P(Me)2=NSiMe3](THF)2] (2) yields ligand transfer complex 5.