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).
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.
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.
Reaction of the uranyl β-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.
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.
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