Joshua S. Figueroa

Triple-Bond Reactivity of Diphosphorus Molecules

We report a mild method for generating the diphosphorus molecule or its synthetic equivalent in homogeneous solution; the P2 allotrope of the element phosphorus is normally obtained only under extreme conditions (for example, from P4 at 1100 kelvin). Diphosphorus is extruded from a niobium complex designed for this purpose and can be trapped efficiently by two equivalents of an organic diene to produce an organodiphosphorus compound. Diphosphorus stabilized by coordination to tungsten pentacarbonyl can be generated similarly at 25°C, and in this stabilized form it still efficiently consumes two organic diene molecules for every diphosphorus unit.

Thallium(I) as a Coordination Site Protection Agent: Preparation of an Isolable Zero‐Valent Nickel Tris‐Isocyanide

Blocking the pass: Low-valent Ni centers readily bind Tl(I) ions in a synthetically reversible fashion. The Tl units, in turn, serve as coordination site protection agents for Ni with respect to incoming Lewis basic ligands. This synthetic sequence allows for the isolation of a reactive zero-valent Ni tris-isocyanide complex.

Isocyano Analogues of [Co(CO)4]n: A Tetraisocyanide of Cobalt Isolated in Three States of Charge

The encumbering m-terphenyl isocyanide ligand, CNAr(Mes2) (Mes = 2,4,6-Me(3)C(6)H(2)), is used to stabilize homoleptic tetraisocyanide complexes of cobalt in the 1-, 0, and 1+ charge state. Most importantly, these complexes serve as isolable analogues of the binary carbonyl complexes [Co(CO)(4)](-), Co(CO)(4), and [Co(CO)(4)](+). Sodium amalgam reduction of CoCl(2) in the presence of CNAr(Mes2) provides the salt Na[Co(CNAr(Mes2))(4)], which can be oxidized with 1 equiv of ferrocenium triflate (FcOTf) to the neutral complex, Co(CNAr(Mes2))(4). X-ray diffraction, FTIR spectroscopy, and low-temperature EPR spectroscopy reveal that Co(CNAr(Mes2))(4) modulates between D(2d)- and C(2v)-symmetric forms. DFT calculations are used to rationalize this structural modulation in terms of thermal access to low-energy b(2)-symmetric C-Co-C bending modes. Treatment of Na[Co(CNAr(Mes2))(4)] with 2 equiv of FcOTf, followed by addition of Na[BAr(F)(4)], provides the salt [Co(CNAr(Mes2))(4)]BAr(F)(4), which contains a diamagnetic, square planar monovalent cobalt center. The molecular and electronic structures of [Co(CNAr(Mes2))(4)]BAr(F)(4) are compared and contrasted to the reported properties of the carbonyl cation, [Co(CO)(4)](+).

Bond activation, substrate addition and catalysis by an isolable two-coordinate Pd(0) bis-isocyanide monomer.

Mg metal reduction of the divalent precursor PdCl(2)(CNAr(Dipp2))(2) (Dipp = 2,6-diisopropylphenyl) provides the isolable, two-coordinate Pd(0) bis-isocyanide, Pd(CNAr(Dipp2))(2), which is the first stable monomeric isocyanide complex of zerovalent palladium. Variable temperature (1)H NMR and FTIR studies on Pd(CNAr(Dipp2))(2) in the presence of added CNAr(Dipp2) revealed that free and coordinated isocyanide undergo rapid exchange, but the components do not form a stable tris-isocyanide complex. Bis-isocyanide Pd(CNAr(Dipp2))(2) is active for oxidative addition reactions and readily reacts with benzyl chloride and mesityl bromide to form Pd(Cl)(Bz)(CNAr(Dipp2))(2) and Pd(Br)(Mes)(CNAr(Dipp2))(2), respectively. Room-temperature Suzuki-Miyaura cross-coupling reactions are mediated by Pd(CNAr(Dipp2))(2). Coordinatively and electronically unsaturated substrates also react with Pd(CNAr(Dipp2))(2). Addition of thallium(I) triflate (TlOTf) to Pd(CNAr(Dipp2))(2) results in the salt [TlPd(CNAr(Dipp2))(2)]OTf, while addition of O(2) results in the peroxo complex (eta(2)-O)Pd(CNAr(Dipp2))(2). Most remarkably, 2 equiv of nitrosobenzene react with Pd(CNAr(Dipp2))(2) to form the square planar complex (kappa(1)-N-PhNO)(2) Pd(CNAr(Dipp2))(2), the geometry of which strongly suggests the formation of a divalent Pd center. With the aid of density functional theory calculations, this valence change is rationalized in terms of a formal reduction of the bond order in each NO unit to 1.5.

Solution behavior and structural properties of Cu(I) complexes featuring m-terphenyl isocyanides.

The synthesis of the m-terphenyl isocyanide ligand CNAr (Mes2) (Mes = 2,4,6-Me 3C 6H 2) is described. Isocyanide CNAr (Mes2) readily functions as a sterically encumbering supporting unit for several Cu(I) halide and pseudo halide fragments, fostering in some cases rare structural motifs. Combination of equimolar quantities of CNAr (Mes2) and CuX (X = Cl, Br and I) in tetrahydrofuran (THF) solution results in the formation of the bridging halide complexes (mu-X) 2[Cu(THF)(CNAr (Mes2))] 2. Addition of CNAr (Mes2) to cuprous chloride in a 2:1 molar ratio generates the complex ClCu(CNAr (Mes2)) 2 in a straightforward manner. Single-crystal X-ray diffraction has revealed ClCu(CNAr (Mes2)) 2 to exist as a three-coordinate monomer in the solid state. As determined by solution (1)H NMR and FTIR spectroscopic studies, monomer ClCu(CNAr (Mes2)) 2 resists tight binding of a third CNAr (Mes2) unit, resulting in rapid isocyanide exchange. Contrastingly, addition of 3 equiv of CNAr (Mes2) to cuprous iodide readily affords the tris-isocyanide species, ICu(CNAr (Mes2)) 3, as determined by X-ray diffraction. Similar coordination behavior is observed in the tris-isocyanide salt [(THF)Cu(CNAr (Mes2)) 3]OTf (OTf = O 3SCF 3), which is generated upon treatment of (C 6H 6)[Cu(OTf)] 2 with 6 equiv of CNAr (Mes2) in THF. The disparate coordination behavior of the [CuCl] fragment relative to both [CuI] and [CuOTf] is rationalized in terms of structure and Lewis acidity of the Cu-containing fragments. The putative triflate species [Cu(CNAr (Mes2)) 3]OTf itself serves as a good Lewis acid and is found to weakly bind C 6H 6 in an eta (1)- C manner in the solid-state. Density Functional Theory is used to describe the bonding and energetics of the eta (1)- C Cu-C 6H 6 interaction.

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)

Effective control of ligation and geometric isomerism: direct comparison of steric properties associated with bis-mesityl and bis-diisopropylphenyl m-terphenyl isocyanides.

A synthetic procedure for the sterically encumbered m-terphenyl isocyanide CNAr(Dipp2) (Dipp = 2,6-diisopropylphenyl) is presented. In comparison to the less encumbering m-terphenyl isocyanide ligand CNAr(Mes2), the steric attributes of the flanking Dipp groups effectively control the extent of CNAr(Dipp2) ligation to monovalent Cu and Ag centers and zero-valent Mo centers. Direct structural comparisons of Cu(I) and Ag(I) complexes of both CNAr(Dipp2) and CNAr(Mes2) are made. It was found that only two CNAr(Dipp2) ligands are accommodated by monovalent Cu and Ag centers, whereas three CNAr(Mes2) units can readily bind. As demonstrated by both (1)H NMR and FTIR spectroscopic studies, addition of a third equivalent of CNAr(Dipp2) to [(THF)(2)Cu(CNAr(Dipp2))(2)]OTf in C(6)D(6) solution results in slow isocyanide exchange. However, rapid isocyanide exchange is observed when an additional equivalent of CNAr(Dipp2) is added to (TfO)Ag(CNAr(Dipp2))(2). Three CNAr(Mes2) ligands react smoothly with fac-Mo(CO)(3)(NCMe)(3) to afford the octahedral complex fac-Mo(CO)(3)(CNAr(Mes2))(3), which can be converted irreversibly to the mer isomer upon heating in solution. Contrastingly, addition of CNAr(Dipp2) to fac-Mo(CO)(3)(NCMe)(3) results in a mixture of both the tetracarbonyl and the tricarbonyl complexes trans-Mo(CO)(4)(CNAr(Dipp2))(2) and trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2), respectively, in which the encumbering CNAr(Dipp2) ligands are in a trans-disposition. Ultraviolet irradiation of the preceding mixture in NCMe/Et(2)O under an argon flow provides exclusively the tricarbonyl complex trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2). Addition of free CNAr(Dipp2) to trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2) does not result in the binding of a third isocyanide unit by the Mo center as determined by (1)H NMR spectroscopy. Treatment of trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2) with the Lewis base pyridine (py) affords the complex fac,cis-Mo(py)(CO)(3)(CNAr(Dipp2))(2) as determined by X-ray diffraction. Notably, the encumbering nature of the CNAr(Dipp2) units forces a cis C(iso)-Mo-C(iso) angle of about 100 degrees.

Redox Noninnocence of Nitrosoarene Ligands in Transition Metal Complexes

Studies on the coordination of nitrosoarene (ArNO) ligands to late-transition metals are used to provide the first definition of the geometric, spectroscopic, and computational parameters associated with a PhNO electron-transfer series. Experimentally, the Pd complexes PdCl(2)(PhNO)(2), PdL(2)(PhNO)(2), and PdL(2)(TolNO) (L = CNAr(Dipp2); Ar(Dipp2) = 2,6-(2,6-(i)Pr(2)C(6)H(3))(2)-C(6)H(3)) are characterized as containing (PhNO)(0), (PhNO)(•1-), and (TolNO)(2-) ligands, respectively, and the structural and spectroscopic changes associated with this electron transfer series provide the basis for an extensive computational study of these and related ArNO-containing late-transition metal complexes. Most notable from the results is the unambiguous characterization of the ground state electronic structure of PdL(2)(PhNO)(2), found to be the first isolable, transition metal ion complex containing an η(1)-N-bound π-nitrosoarene radical anion. In addition to the electron transfer series, the synthesis and characterization of the Fe complex [Fe(TIM)(NCCH(3))(PhNO)][(PF(6))(2)] (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) allows for comparison of the geometric and spectroscopic features associated with metal-to-ligand π-backbonding as opposed to (PhNO)(•1-) formation. Throughout these series of complexes, the N-O, M-N, and C-N bond distances as well as the N-O stretching frequencies and the planarity of the ArNO ligands provided distinct parameters for each ligand oxidation state. Together, these data provide a delineation of the factors needed for evaluating the oxidation state of nitrosoarene ligands bound to transition metals in varying coordination modes.

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.