Alex E. Carpenter

Zwitterionic Stabilization of a Reactive Cobalt Tris‐Isocyanide Monoanion by Cation Coordination

As a result of their metal-based nucleophilicity and ability to mediate a wide range of transformations involving maingroup and carbon-based electrophiles, organometallic metallates have received considerable attention. The homoleptic carbonyl metallates [Fe(CO)4] 2 and [Co(CO)4] serve as the prototypical examples of this molecular class and are stabilized in part by strong p-back-bonding interactions between the highly reduced metal centers and the CO ligands. In addition, several metallates containing other pacidic ligands, including olefins, arenes, isocyanides, and PF3, have been reported. h] However, in the vast majority of these cases the reduced metal centers possess coordinative and/or electronic saturation (i.e. 18e configurations), which adds to their stability. Accordingly, the chemistry of metallates featuring coordinatively unsaturated metal centers is significantly underdeveloped, despite the promise of coupling metal-based nucleophilic character with enhanced reactivity toward Lewis basic substrates. In an effort to stabilize organometallic complexes featuring both highly reduced metal centers and coordinative unsaturation, we have surveyed the ligation properties of the encumbering and p-acidic m-terphenyl isocyanide ligands CNAr and CNAr (Ar = 2,6-(2,4,6-Me3C6H2)C6H3); Ar = 2,6-(2,6-(iPr)2C6H3)C6H3). [4] Herein, we report the use of CNAr for the isolation of a zwitterionic complex that functions as a highly reactive source of the coordinatively unsaturated, tris-isocyanide cobalt monoanion [Co(CNAr)3] toward electrophilic reagents. In this respect, [Co(CNAr)3] serves as an isocyano mimic of the unstable tricarbonyl monoanion, [Co(CO)3] , which has been observed exclusively in the gas phase. Furthermore, we detail a unique method for the stabilization of the [Co(CNAr)3] anion that relies on h-arene coordination of the well-known and traditionally non-interacting bis-(triphenylphosphine)iminium cation, [PhP3=N=PPh3] + ([PPN]). Previously we reported the [PPN] salt, [PPN][Co(CNAr)4], [7] as a unique example of a discrete cobalt tetra-isocyanometallate unperturbed by anion–cation interactions.Prolonged stirring (30 h) of this salt in n-hexane solution results in CNAr ligand dissociation and the precipitation of the black, zwitterionic tris-isocyanide complex [(h-PPN)Co(CNAr)3, 1; Figures 1 and 2]. Crystallographic characterization of (h-PPN)Co(CNAr)3 (1)

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.

Chloro- and Trifluoromethyl-Substituted Flanking-Ring m-Terphenyl Isocyanides: η6-Arene Binding to Zero-Valent Molybdenum Centers and Comparison to Alkyl-Substituted Derivatives

Presented herein are synthetic and structural studies exploring the propensity of m-terphenyl isocyanide ligands to provide flanking-ring η(6)-arene interactions to zerovalent molybdenum centers. The alkyl-substituted m-terphenyl isocyanides CNAr(Mes2) and CNAr(Dipp2) (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3; Ar(Dipp2) = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) react with Mo(η(6)-napthalene)2 in a 3:1 ratio to form tris-isocyanide η(6)-arene Mo complexes, in which a flanking mesityl or 2,6-diisopropylphenyl group, respectively, of one isocyanide ligand is bound to the zerovalent molybdenum center. Thermal stability and reactivity studies show that these flanking ring η(6)-arene interactions are particularly robust. To weaken or prevent formation of a flanking-ring η(6)-arene interaction, and to potentially provide access to the coordinatively unsaturated [Mo(CNAr(R))3] fragment, the new halo-substituted m-terphenyl isocyanides CNAr(Clips2) and CNAr(DArF2) (Ar(Clips) = 2,6-(2,6-Cl2C6H3)2(4-t-Bu)C6H2; Ar(DArF2) = 2,6-(3,5-(CF3)2C6H3)2C6H3) have been prepared. Relative to their alkyl-substituted counterparts, synthetic and structural studies demonstrate that the flanking aryl rings of CNAr(Clips2) and CNAr(DArF2) display a lower tendency toward η(6)-binding. In the case of CNAr(DArF2), it is shown that an η(6)-bound 3,5-bis(trifluoromethyl)phenyl group can be displaced from a zerovalent molybdenum center by three molecules of acetonitrile. This observation suggests that the CNAr(DArF2) ligand effectively masks low-valent metal centers in a fashion that provides access to low-coordinate isocyano targets such as [Mo(CNAr(R))3]. A series of Mo(CO)3(CNAr(R))3 complexes were also prepared to compare the relative π-acidities of CNAr(Mes2), CNAr(Clips2), and CNAr(DArF2). It is found that CNAr(DArF2) shows increased π-acidity relative to CNAr(Mes2) and CNAr(Clips2), despite the fact that its electron-withdrawing CF3 groups are fairly distal to the terminal isocyano unit.

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.