Synthesis of Silyl Aluminum Reagents: Relevance Toward Atomic Layer Deposition of Metal Silicides and the Serendipitous Synthesis of a Novel Al-Hydride Cluster.

The novel mono-silyl [(R3Si)AlX2]2, di-silyl [(R3Si)2AlX]2, tri-silyl (R3Si)3Al·Et2O, and -ate-complex [(R3Si)4Al]-·Li+(Et2O)2 have been synthesized by reaction of AlX3 (X = Cl, Br) with silyl lithium reagents (tBuMe2SiLi, Et3SiLi) in Et2O. Treatment of these compounds with Me3N yields the corresponding amine-coordinated silyl aluminum complexes (R3Si)AlX2·NMe3, (R3Si)2AlX·NMe3, and (R3Si)3Al·NMe3. An intramolecular amine-coordinated mono-silyl aluminum complex Me2N(CH2)3(tBuMe2Si)2SiAlCl2 was prepared by the reaction of Me2N(CH2)3(tBuMe2Si)2SiLi with AlCl3 in Et2O. In addition, reaction of [(tBuMe2Si)2AlBr]2 with LiAlH4 yields the novel aluminum hydride cluster [(tBuMe2Si)2Al(µ-H)AlH3]6 which upon addition of TMEDA yields the ion pair [((tBuMe2Si)2AlH)2(µ-H)]-[AlH2(TMEDA)2]+. The amine-coordinated di- and tri-silyl aluminum complexes possess higher thermal stability than the analogous etherate complexes and are reasonably volatile (100-140 °C, 0.2 Torr). The materials presented herein were analyzed via thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) to assess their viability as potential ALD precursors.

Mechanism of Thermal Atomic Layer Etch of W Metal Using Sequential Oxidation and Chlorination: A First-Principles Study.

Thermal atomic layer etch (ALE) of W metal can be achieved by sequential self-limiting oxidation and chlorination reactions at elevated temperatures. In this paper, we analyze the reaction mechanisms of W ALE using the first-principles simulation. We show that oxidizing agents such as O2, O3, and N2O can be used to produce a WOx surface layer in the first step of an ALE process with ozone being the most reactive. While the oxidation pulse on clean W is very exergonic, our study suggests that runaway oxidation of W is not thermodynamically favorable. In the second ALE pulse, WCl6 and Cl2 remove the oxidized surface W atoms by the formation of volatile tungsten oxychloride (WxOyClz) species. In this pulse, each adsorbed WCl6 molecule was found to remove one surface W atom with a moderate energy cost. Our calculations further show that the desorption of the additional etch products is endothermic by up to 4.7 eV. Our findings are consistent with the high temperatures needed to produce ALE in experiments. In total, our quantum chemical calculations have identified the lowest energy pathways for ALE of tungsten metal along with the most likely etch products, and these findings may help guide the development of improved etch reagents.

Side-On Coordination of Nitrous Oxide to a Mononuclear Cobalt Center.

Despite its utility as an oxygen-atom transfer reagent for transition metals, nitrous oxide (N2O) is a notoriously poor ligand, and its coordination chemistry has been limited to a few terminal, end-on κ1-N complexes. Here, the synthesis of a mononuclear cobalt complex possessing a side-on-bound N2O molecule is reported. Structural characterization, IR spectroscopy, and DFT calculations support an η2-N,N binding mode for binding of N2O to the cobalt center.

Synthesis and Characterization of a Neutral U(II) Arene Sandwich Complex.

Reduction of IU(NHAriPr6)2 (AriPr6 = 2,6-(2,4,6-iPr3C6H2)2C6H3) results in a rare example of a U(II) complex, U(NHAriPr6)2, and the first example that is a neutral species. Here, we show spectroscopic and magnetic studies that suggest a 5f46d0 valence electronic configuration for uranium, along with characterization of related U(III) complexes.

A Highly-Reduced Cobalt Terminal Carbyne: Divergent Metal- and α-Carbon-Centered Reactivity.

Reported here is the isolation of a dianionic cobalt terminal carbyne derived from chemical reduction of an encumbering isocyanide ligand. Crystallographic, spectroscopic and computational data reveal that this carbyne possesses a low-valent cobalt center with an extensively filled d-orbital manifold. This electronic character renders the cobalt center the primary site of nucleophilicity upon reaction with protic substrates and silyl electrophiles. However, reactions with internal alkynes result in [2+2] cycloaddition with the carbyne carbon to form a new C-C bond.

Controlled Expansion of a Strong-Field Iron Nitride Cluster: Multi-Site Ligand Substitution as a Strategy for Activating Interstitial Nitride Nucleophilicity.

Multimetallic clusters have long been investigated as molecular surrogates for reactive sites on metal surfaces. In the case of the µ4 -nitrido cluster [Fe4 (µ4 -N)(CO)12 ]- , this analogy is limited owing to the electron-withdrawing effect of carbonyl ligands on the iron nitride core. Described here is the synthesis and reactivity of [Fe4 (µ4 -N)(CO)8 (CNArMes2 )4 ]- , an electron-rich analogue of [Fe4 (µ4 -N)(CO)12 ]- , where the interstitial nitride displays significant nucleophilicity. This characteristic enables rational expansion with main-group and transition-metal centers to yield unsaturated sites. The resulting clusters display surface-like reactivity through coordination-sphere-dependent atom rearrangement and metal-metal cooperativity.

Terminal Iron Carbyne Complexes Derived from Arrested CO2 Reductive Disproportionation.

The encumbered tetraisocyanide dianion Na2 [Fe(CNArMes2 )4 ] reacts with two molecules of CO2 to effect reductive disproportionation to CO and carbonate ([CO3 ]2- ). When the reaction is performed in the presence of silyl triflates, reductive disproportionation is arrested by silylative esterification of a mono-CO2 adduct. This results in the formation of four-coordinate terminal iron carbynes possessing an aryl carbamate substituent owing to the direct attachment of an C(O)OSiR3 group to an isocyanide nitrogen atom. Crystallographic, spectroscopic, and computational analyses of these iron-carbon multiply bonded species reveal electronic structure properties indicative of a conformationally locked iron carbyne unit.

Dinitrogen binding, P4-activation and aza-Büchner ring expansions mediated by an isocyano analogue of the CpCo(CO) fragment.

Synthetic studies targeting an m-terphenyl isocyanide analogue of the unstable 16e(-), S = 1 complex CpCo(CO) are reported (Cp = η(5)-C5H5). The m-terphenyl isocyanide CNAr(Dipp2) (Ar(Dipp2) = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) is shown to readily bind to both CpCoI2 and Cp*CoI2 fragments (Cp* = η(5)-C5Me5) and provide mono-isocyanide starting materials that are suitable for chemical reduction. Treatment of CpCoI2(CNAr(Dipp2)) with KC8 produces the bridging isocyanide dimer, [CpCo(µ-CNAr(Dipp2))]2, thereby indicating that the steric combination of Cp and CNAr(Dipp2) ligands does not allow for the production of mononuclear complexes. However, Cp*CoI2(CNAr(Dipp2)) with KC8 under an N2 atmosphere results in the formation of the complex, Cp*Co(N2)(CNAr(Dipp2)), which is a unique two-legged piano stool complex featuring a coordinated dinitrogen ligand. The N2 ligand in Cp*Co(N2)(CNAr(Dipp2)) is shown to be labile and, upon removal by application of vacuum, leads to the production of an η(4)-coordinated 1-azabenz[b]azulene complex by aza-Büchner cyclization of the CNAr(Dipp2) ligand. This cyclization reaction is rationalized via the intermediacy of the unobserved 16e(-) species [Cp*Co(CNAr(Dipp2))]. While this intramolecular aza-Büchner cyclization prevents isolation of [Cp*Co(CNAr(Dipp2))], the dinitrogen complex Cp*Co(N2)(CNAr(Dipp2)) is shown to serve as a reliable synthon for this 16e(-) species upon reaction with small molecule substrates. Both free CNAr(Dipp2) and diphenylacetylene react with Cp*Co(N2)(CNAr(Dipp2)) to form two-legged piano stool complexes. In addition, Cp*Co(N2)(CNAr(Dipp2)) reacts readily with 0.5 and 1.0 equivalents of P4 to produce poly-phosphorus products resulting from P-P single bond cleavage.

Synthesis and protonation of an encumbered iron tetraisocyanide dianion.

Reported here are synthetic studies probing highly reduced iron centers in an encumbering tetraisocyano ligand environment. Treatment of FeCl2 with sodium amalgam in the presence of 2 equiv of the m-terphenyl isocyanide CNAr(Mes2) (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3) produces the disodium tetraisocyanoferrate Na2[Fe(CNAr(Mes2))4]. Structural characterization of Na2[Fe(CNAr(Mes2))4] revealed a tight ion pair, with the Fe center adopting a tetrahedral coordination geometry consistent with a d(10) metal center. Attempts to disrupt the cation-anion contacts in Na2[Fe(CNAr(Mes2))4] with cation-sequestration reagents lead to decomposition, except for the case of 18-crown-6, where a mononuclear complex featuring a dianionic 1-azabenz[b]azulene ligand was isolated in low yield. Formation of this 1-azabenz[b]azulene is rationalized to proceed by an aza-Büchner ring expansion of a CNAr(Mes2) ligand mediated by a coordinatively unsaturated Fe center. Disodium tetraisocyanoferrate Na2[Fe(CNAr(Mes2))4] is readily protonated by trimethylsilanol (HOSiMe3) to produce the monohydride ferrate salt, Na[HFe(CNAr(Mes2))4], the anionic portion of which serves as an isocyano analogue of the hydrido-tetracarbonyl metalate [HFe(CO)4](-). Treatment of Na[HFe(CNAr(Mes2))4] with methyl triflate (MeOTf; OTf = [O3SCF3](-)) at low temperature in the presence of dinitrogen yields the five-coordinate Fe(0) complex Fe(N2)(CNAr(Mes2))4. The formation of Fe(N2)(CNAr(Mes2))4 in this reaction is consistent with the intermediacy of the neutral tetraisocyanide Fe(CNAr(Mes2))4. The decomposition of Fe(N2)(CNAr(Mes2))4 to the dimeric complex [Fe(η(6)-(Mes)-µ(2)-C-CNAr(Mes))]2 and a seven-membered cyclic imine derived from a CNAr(Mes2) ligand is presented and provides insight into the ability of CNAr(Mes2) and related m-terphenyl isocyanides to stabilize zerovalent four-coordinate iron complexes in a strongly π-acidic ligand field.

Comparative measure of the electronic influence of highly substituted aryl isocyanides.

To assess the relative electronic influence of highly substituted aryl isocyanides on transition metal centers, a series of C4v-symmetric Cr(CNR)(CO)5 complexes featuring various alkyl, aryl, and m-terphenyl substituents have been prepared. A correlation between carbonyl-ligand (13)C{(1)H} NMR chemical shift (δCO) and calculated Cotton-Kraihanzel (C-K) force constant (kCO) is presented for these complexes to determine the relative changes in isocyanide σ-donor/π-acid ratio as a function of substituent identity and pattern. For nonfluorinated aryl isocyanides possessing alkyl or aryl substitution, minimal variation in effective σ-donor/π-acid ratio is observed over the series. In addition, aryl isocyanides featuring strongly electron-releasing substituents display an electronic influence that nearly matches that of nonfluorinated alkyl isocyanides. Lower σ-donor/π-acid ratios are displayed by polyfluorinated aryl isocyanide ligands. However, the degree of this attenuation relative to nonfluorinated aryl isocyanides is not substantial and significantly higher σ-donor/π-acid ratios than CO are observed in all cases. Substituent patterns for polyfluorinated aryl isocyanides are identified that give rise to low relative σ-donor/π-acid ratios but offer synthetic convenience for coordination chemistry applications. In order to expand the range of available substitution patterns for comparison, the syntheses of the new m-terphenyl isocyanides CNAr(Tripp2), CNp-MeAr(Mes2), CNp-MeAr(DArF2), and CNp-FAr(DArF2) are also reported (Ar(Tripp2) = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3); p-MeAr(Mes2) = 2,6-(2,4,6-Me3C6H2)2-4-Me-C6H2); p-MeAr(DArF2) = 2,6-(3,5-(CF3)2C6H3)2-4-Me-C6H2); p-FAr(DArF2) = 2,6-(3,5-(CF3)2C6H3)2-4-F-C6H2).

Direct observation of ß-chloride elimination from an isolable ß-chloroalkyl complex of square-planar nickel.

Reported here are the isolation, structural characterization, and decomposition kinetics of the four-coordinate pentachloroethyl nickel complex, NiCl(CCl2CCl3)(CNAr(Mes2))2 (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3). This complex is a unique example of a kinetically persistent ß-chloroalkyl in a system relevant to coordination-insertion polymerization of polar olefins. Kinetic analysis of NiCl(CCl2CCl3)(CNAr(Mes2))2 decomposition indicates that ß-chloride (ß-Cl) elimination proceeds by a unimolecular mechanism that does not require initial dissociation of a CNAr(Mes2) ligand. The results suggest that a direct ß-Cl elimination pathway is available to four-coordinate, Group 10 metal vinyl chloride polymerization systems.

Facile reduction of a uranyl(VI) ß-ketoiminate complex to U(IV) upon oxo silylation.

Reaction of the uranyl ß-ketoiminate complex UO(2)((tBu)acnac)(2) (1) ((tBu)acnac = (t)BuNC(Ph)CHC(Ph)O) with Me(3)SiI, in the presence of Ph(3)P, results in the reductive silylation of the uranyl moiety and formation of the U(V) bis-silyloxide complex [Ph(3)PI][U(OSiMe(3))(2)I(4)] (2). Subsequent reaction of 2 with Lewis bases, such as 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), and tetrahydrofuran (THF), results in a further reduction of the metal center and isolation of the U(IV) complexes U(OSiMe(3))(2)I(2)(bipy)(2) (3), U(OSiMe(3))(2)I(2)(phen)(2) (4), and [U(OSiMe(3))(2)I(THF)(4)][I(3)] (5), respectively.