Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Recently accumulated experimental evidence for aurophilic interactions in and between molecular gold(I) compounds and the results of pertinent theoretical calculations are reviewed for the period from 2007 to mid-2011. The influence of the intra- and intermolecular bonding contacts between the closed-shell metal centres, Au–Au, on the molecular and crystal structures, and the consequences of these effects for the chemical and physical properties of gold compounds are summarized for the various classes of mono- and polynuclear systems. The literature survey builds on the contents of previous reviews and relates new experimental and theoretical findings to earlier observations (353 references).

Aurophilic interactions as a subject of current research: an up-date

ion of the chloride ligands, compound 2 was further reacted with MeMgCl, yielding the bimetallic compound 3 in which the biscarbene ligand is bridging two Cr(II) atoms, also bridged by two methyl ligands. Recent studies by Arnold and co-workers showed that the incorporation of an anionic functional group (alkoxide, amido, or amino) in an NHC unit allows the preparation of tridentate ligands capable of coordinating to d0 early transition metals such as yttrium, titanium, and zirconium.87 The lithium aminodicarbene chloride complex 5 reacts with Y[N(SiMe3)2]3 affording the yttrium(III) complex 6 (Scheme 10). Hexacoordinated titanium (7) and zirconium (8) complexes were synthesized by reaction of 4 with M(NEt2)4 (M ) Ti, Zr) (Scheme 10). Complexes 7 and 8 constituted the first examples of group 3 and 4 metal complexes with monoanionic biscarbene ligands. Another interesting example of early transition metal NHC-based complexes was described by Smith and coworkers.88 By the reaction shown in Scheme 11, the tris(NHC)borate 989,90 afforded a tricarbonyl complex of Mn(I) with a tripodal tris-NHC ligand, 10. The analysis of the ν(CO) bands on the IR spectrum of this complex showed that 9 is the most electron-donating tripodal ligand compared with all others that have been bound to the same Mntricarbonyl fragment. Compound 10 is air-sensitive and is easily oxidized to a homoleptic Mn(IV) complex (11, Scheme 11), which is the first example of a Mn(IV)-NHC complex reported to-date. 4.2. Catalytic Applications Although many catalytic applications of late transition metal complexes bearing poly-NHC ligands have been described, those of early transition metals are restricted to a few examples. In 2003, Gibson and co-workers reported Cr(III)-based ethylene oligomerization precatalysts that incorporate a CNC-pincer carbene ligand.91,92 More recently, McGuinness shed some light on the mechanism of the reaction with the mentioned pincer complexes in combination with MAO.93 The Cr(II) and Cr(III) complexes described by Theopold were tested for polymerization of ethylene with a MAO cocatalyst, showing low activities and a broad weight distribution of the polymers.85 In particular, Cr(II) compounds such as 2 (Scheme 9) were unreactive to ethylene when exposed to MAO, showing that the more Lewis acidic Cr(III) performs better in this reaction. The authors pointed out the ease with which the Cr(III) complexes are reduced to Cr(II), arguing that very strong σ-donating but soft NHC ligands may have a stronger affinity for the softer Cr(II) rather than the harder Cr(III). Thus, they concluded that this ligand system may be better suited for lower oxidation state chromium chemistry and that, due to the inactivity of the Cr(II) compounds tested, it would not lead to successful results in ethylene polymerization. 5. Poly-NHC Ligands in Complexes of Group 8 Metals

Complexes with poly(N-heterocyclic carbene) ligands: structural features and catalytic applications.

Redox reactions play important roles in almost all biological processes, including photosynthesis and respiration, which are two essential energy processes that sustain all life on earth. It is thus not surprising that biology employs redox-active metal ions in these processes. It is largely the redox activity that makes metal ions uniquely qualified as biological cofactors and makes bioinorganic enzymology both fun to explore and challenging to study.

Even though most metal ions are redox active, biology employs a surprisingly limited number of them for electron transfer (ET) processes. Prominent members of redox centers involved in ET processes include cytochromes, iron–sulfur clusters, and cupredoxins. Together these centers cover the whole range of reduction potentials in biology (Figure ​(Figure1).1). Because of their importance, general reviews about redox centers1−77 and specific reviews about cytochromes,8,24,78−90 iron–sulfur proteins,91−93 and cupredoxins94−104 have appeared in the literature. In this review, we provide both classification and description of each member of the above redox centers, including both native and designed proteins, as well as those proteins that contain a combination of these redox centers. Through this review, we examine structural features responsible for their redox properties, including knowledge gained from recent progress in fine-tuning the redox centers. Computational studies such as DFT calculations become more and more important in understanding the structure–function relationship and facilitating the fine-tuning of the ET properties and reduction potentials of metallocofactors in proteins. Since this aspect has been reviewed extensively before,105−110 and by other reviews in this thematic issue,2000,2001,2002 it will not be covered here.






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Figure 1


Reduction potential range of redox centers in electron transfer processes.

Metalloproteins containing cytochrome, iron-sulfur, or copper redox centers.

Chapter 3 – Advances in the Coordination Chemistry of Amidinate and Guanidinate Ligands ☆

Reduction of a diazadiene chromium halide complex, [(HLiPr)Cr(μ-Cl)]2 (1, HLiPr = bis(2,6-diisopropylphenyl)diazadiene), with KC8 gave a diamagnetic, bimetallic complex, (HLiPr)2Cr2 (2). Complex 2 has been structurally characterized by X-ray crystallography and consists of a Cr2 unit spanned by two diazadiene ligands. The very short Cr−Cr distance (1.8028(9) A) and low formal oxidation state of the Cr atoms suggest that the Cr−Cr bond order may be greater than 4. Spin-restricted and spin-unrestricted DFT calculations on a model complex both converge on a singlet spin state and accurately reproduce the metric parameters of 2. The calculations show high-order metal−metal bonding along with extensive delocalization over the diazadiene ligands. Natural resonance theory analysis shows population of resonance structures that contain quintuple bonds, and natural bond order analysis gives a bond order of 4.28 for the Cr−Cr bond.

The shortest metal-metal bond yet: molecular and electronic structure of a dinuclear chromium diazadiene complex.

A Chelating N-Heterocyclic Carbene Ligand in Organochromium Chemistry

We have prepared and structurally characterized several complexes of chromium coordinated by diimine (or 1,4-diazadiene) ligands, that is, Ar-N=C(R)-(R)C=N-Ar (RL(Ar)) (where Ar = 2,6-diisopropylphenyl ("iPr") or 2,6-dimethylphenyl ("Me") and R = H or Me). The reaction of CrCl2 with HLiPr gave dinuclear [(HLiPr)Cr]2(mu-Cl)3(Cl)(THF) when isolated from Et2O; in THF solution, however, the product exists as mononuclear (HLiPr)CrCl2(THF)2. Two isostructural derivatives, (MeLMe)CrCl2(THF)2 and (HL(Me))CrCl2(THF)2, have also been prepared. Furthermore, the bis-ligand complex, (HLiPr)2Cr, has been prepared along with its reduction product, Li(THF)4[(HLiPr)2Cr]. We have also synthesized the tetracarbonyl complex, (HLiPr)Cr(CO)4, by addition of HLiPr to Cr(CO)4(NCCH3)2. The structure and variable temperature magnetic susceptibility of the previously reported Cr halide dimer, [(HLiPr)Cr(mu-Cl)]2, is also discussed in detail. All of the diimine complexes have been characterized structurally, spectroscopically, and magnetically, and their electronic structures are discussed with the aid of density-functional theory calculations.

Synthesis, characterization, and electronic structure of diimine complexes of chromium.

The first tris(mercaptoimidazolyl)borate complexes of gold, Au(TmtBu) and (TmtBu)Au(PPh3), have been prepared and structurally characterized. Together with their copper and silver analogues M(TmtBu) and (TmtBu)M(PPh3)
				(M = Cu, Ag), these compounds constitute the first two complete series of TmR derivatives to be isolated for the coinage metals. In order to evaluate the steric and electronic effects of the bulky tert-butyl substituents in these species, comparative structural analyses with the known methyl-substituted analogue Ag(TmMe) and various (TmMe)M(PR3) derivatives (M = Cu, Ag) are also presented.

Tris(mercaptoimidazolyl)borate complexes of the coinage metals: syntheses and molecular structures of the first gold compounds and related copper and silver derivatives.

http://tirith.chem.udel.edu/ActaE/Poly(2007)26,4758.pdf

Nickel nitrosyl complexes in a sulfur-rich environment : The first poly(mercaptoimidazolyl)borate derivatives

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