Charles C. Mokhtarzadeh

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).

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.

Direct observation of β-chloride elimination from an isolable β-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 β-chloroalkyl in a system relevant to coordination-insertion polymerization of polar olefins. Kinetic analysis of NiCl(CCl2CCl3)(CNAr(Mes2))2 decomposition indicates that β-chloride (β-Cl) elimination proceeds by a unimolecular mechanism that does not require initial dissociation of a CNAr(Mes2) ligand. The results suggest that a direct β-Cl elimination pathway is available to four-coordinate, Group 10 metal vinyl chloride polymerization systems.

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.

Nickel bis-m-terphenylisocyanide dihalide complexes formed from 1,2-alkyl dihalides: probing for isolable β-haloalkyl complexes of square planar nickel

Abstract The pentachloroethyl complex NiCl(CCl2CCl3)(CNArMes2)2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) was recently shown to be a unique example of an isolable β-chloroalkyl complex of square planar nickel. In an effort to generate additional β-haloalkyl complexes of square planar nickel, the reactivity of the precursor complex Ni(COD)(CNArMes2)2 with alkyl halides has been probed. Treatment of Ni(COD)(CNArMes2)2 with 1,2-dichloroethane results in the exclusive formation of the dichloride complex NiCl2(CNArMes2)2 without the buildup of detectable intermediates. Similarly, Ni(COD)(CNArMes2)2 reacts with either 1,2-dibromoethane or 1,2-diiodoethane to produce the dibromide, NiBr2(CNArMes2)2, and diiodide, NiI2(CNArMes2)2, species, respectively. Observable intermediates were also not detected in these latter reactions, indicating that either β-halo elimination is rapid or 1e− halogen-atom abstraction pathways are accessible to Ni(COD)(CNArMes2)2. The sterically and electronically modified m-terphenyl isocyanides CNArDipp2 and CNArClips2 (ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2C6H3; ArClips2 = 2,6-(2,6-(Cl)2C6H3)2-4-(t-Bu)C6H2) were also investigated as ancillary ligands for stabilization of a β-chloroalkyl complex of square planar nickel. Treatment of the zero-valent precursors Ni(COD)(CNArDipp2)2 and Ni(COD)(CNArClips2)2 with either 1,2-dichloroethane or hexachloroethane resulted in rapid formation of the dichlorides NiCl2(CNArDipp2)2 and NiCl2(CNArClips2)2 as exclusive products. These results highlight the unique combination of steric and electronic properties that lead to the stability of the parent β-chloroalkyl complex NiCl(CCl2CCl3)(CNArMes2)2.

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.