Heterobimetallic complexes stabilized by the P2N2 macrocyclic ligand system: synthesis and reactivity of a rhodium-copper system that activates molecular hydrogen.

The synthesis and reactivity of the bimetallic rhodium-copper complex, Rh(COE)[P2N2]Cu, which is stabilized by the P2N2 macrocycle, is reported. In the solid state, the rhodium and copper centers are on opposite sides of the macrocyclic ring with the Cu(I) in a linear environment and the Rh(I) in a square planar array. However, in solution a very symmetrical structure is suggested on the basis of the 1H NMR data, which is consistent with at least two separate fluxional processes, rotation of the cyclooctene unit and movement of the Rh(I) unit between the two amido donors. Addition of H2 to Rh(COE)[P2N2]Cu results in the formation of ([P2N2H]RhH(µ-H)2Cu)2via hydrogenation of the coordinated cyclooctene unit, oxidative addition of H2 to the rhodium center and hydrogenolysis of the copper amido unit. Monitoring the reaction of H2 by NMR spectroscopy indicated the formation of a number of intermediates which suggests hydrogenolysis of the copper amido linkage occurs to generate CuH in some form, along with Rh(COE)[P2N2H], which is converted to Rh(H)2[P2N2H] by hydrogenation of the cyclooctene, which then recombines with the CuH present to generate the final product. Deuteration studies indicate that there is considerable H/D scrambling in the cyclooctane produced that we attribute to reversible beta-elimination, migratory insertion steps.

Forays into rhodium macrocyclic chemistry stabilized by a P2N2 donor set. Activation of dihydrogen and benzene.

The reaction of the dilithium diamido-diphosphine macrocycle, Li2[N(SiMe2CH2P(Ph)CH2SiMe2)2N] (Li2[P2N2]) with [Rh(COD)Cl]2 generates the dirhodium macrocyclic compound, [P2N2][Rh(COD)]2 (where COD = η4-1,5-cyclooctadiene), wherein both rhodium-COD units are syn to each other and have square planar geometries. While this dirhodium derivative does react with H2, no clean products could be isolated. Upon reaction of Li2[P2N2] with [Rh(COE)2Cl]2 (where COE is η2-cyclooctene), the dilithium-dihodium derivative ([Rh(COE)][P2N2]Li)2(dioxane) forms, which was characterized by single-crystal X-ray analysis and NMR spectroscopy. The cyclooctene derivative reacts with dihydrogen in benzene to generate the dilithium-dirhodium-dihydride complex ([Rh(H)2][P2N2]Li)2(dioxane); also formed is the dilithium-dirhodium-phenylhydride complex ([Rh(C6H5)H][P2N2]Li)2(dioxane) via oxidative addition of a C-H bond of the solvent. The phenyl-hydride is eventually converted to the dihydride derivative via further reaction with H2. This process is complicated by adventitious H2O, which leads to the isolation of the amine-dihydride, Rh[P2N2H](H)2; drying of the H2 eliminates this side product. Nevertheless, careful addition of H2O to ([Rh(COE)][P2N2]Li)2(dioxane) results in protonation of one of the amido units and the formation of the rhodium-amine cyclooctene derivative, Rh[P2N2H](COE), which upon reaction with H2 generates the aforementioned amine-dihydride, Rh[P2N2H](H)2. The mechanism by which dihydrogen and C-H bonds of benzene are activated likely involves initial dissociation of cyclooctene from the 18-electron centers in ([Rh(COE)][P2N2]Li)2(dioxane), followed by H-H and C-H bond activation. The ability of one of the amido units of the P2N2 macrocycle to be protonated is a potentially useful proton storage mechanism and is of interest in other bond activation processes.

Redox behaviour of ([fc(NPiPr2)2]Fe)2, formation of an iron-iron bond and cleavage of azobenzene.

The redox behaviour of the dimeric tetrairon complex, ([fc(NPiPr2)2]Fe)2 (where fc(NPiPr2)2 = 1,1'-(C5H4NPiPr2)2Fe) has been investigated. Upon reduction with KC8 an Fe-Fe bond is formed with the complex maintaining a high spin configuration and having the formula [K(THF)6]([fc(NPiPr2)2]Fe)2. In contrast, oxidation of the complex is ligand based; for example, addition of the 1,2-diiodoethane (I2 equivalent) results in the formation of the monomeric iron(ii) diiodide [fc(NiPr2I)2]FeI2 wherein the phosphine is oxidized. The dimeric tetrairon complex reacts photolytically with azobenzene, cleaving the N[double bond, length as m-dash]N double bond and forming the new monomeric bis(phosphoramidate) iron complex. [fc(NP(NPh)iPr2)2]Fe. Characterization of these paramagnetic complexes was accomplished by magnetic susceptibility studies and X-ray analyses.

Dinitrogen functionalization at a ditantalum center. Balancing N2 displacement and N2 functionalization in the reaction of coordinated N2 with CS2.

The reaction of carbon disulfide (CS2) with the side-on end-on dinitrogen complex ([NPNSi]Ta)2(µ-η1:η2-N2)(µ-H)2 (1) (where [NPNSi] = [PhP(CH2SiMe2NPh)2]) has been studied and shown to generate two products, the ratio of which depends on the concentration of added carbon disulfide. At high concentrations of CS2, N-N bond cleavage and functionalization occur to generate a ditantalum complex with an isothiocyanato ligand N-bound to Ta along with bridging sulfido and nitrido moieties. At lower concentrations of CS2, less dinitrogen functionalization is observed; instead, N2 is displaced and the CS2 molecule is completely disassembled to generate a ditantalum derivative with bridging methylene and two sulfide moieties. Kinetic and DFT studies have been performed and provide clues about the product ratio and mechanistic information and shed light on why N2 functionalization is challenging.

Synthesis and Reactivity of a Low-Coordinate Iron(II) Hydride Complex: Applications in Catalytic Hydrodefluorination.

A low-coordinate iron hydride complex bearing an unsymmetrical NpN (enamido-phosphinimine) ligand scaffold was synthesized and fully characterized. Insertion reactivity with azobenzene, 3-hexyne, and 1-azidoadamantane was explored, and the isolated products were analogous to previously reported ß-diketiminate iron hydride insertion products. Surprisingly, the NpN iron hydride displays unprecedented reactivity toward hexafluorobenzene, affording an NpN iron fluoride complex and pentafluorobenzene as products. The NpN iron hydride is a precatalyst for catalytic hydro-defluorination of perfluorinated aromatics in the presence of silane. Kinetic studies indicated that the rate-determining step during catalysis involved silane.

Variable coordination geometries via an amine-tethered-enamidophosphinimine ligand on cobalt.

The synthesis of two new amine-tethered enamidophosphinimine donor sets is described and their coordination chemistry with cobalt(ii) detailed. Deprotonation of the enamine tautomers of each system generates the lithium derivatives that are used in metathesis reactions with either anhydrous CoCl2 or CoCl2(THF)1.5 to generate tetrahedral κ3-(NNpNR)CoCl (where R = DIPP, 2,6-diisopropylphenyl; R = TMS, trimethylsilyl) or tetrahedral κ2-(NNpNTMS)2Co, the latter likely due to the insolubility of the CoCl2. For the R = DIPP derivative, replacement of the chloride proceeds smoothly by methathesis with LiCH2SiMe3 and KBEt3H to generate the bulky alkyl derivative κ3-(NNpNDIPP)CoCH2SiMe3, and the dihydride-bridged dimer [κ2-(NNpNDIPP)Co]2(µ-H)2, respectively; interestingly, the hydride dimer has the diethylaminoethyl arms uncoordinated. Reduction of κ3-(NNpNDIPP)CoCl with KC8 under dinitrogen generates the π-arene complex, κ2:η6-(NNpNDIPP)Co, and not the expected N2 complex. In an attempt to generate a dinitrogen complex, we investigated the metathesis and reduction chemistry of the N-trimethylsilyl substituted phosphinimine cobalt derivative, κ3-(NNpNTMS)CoCl. Although reaction with LiCH2SiMe3 produced the anticipated tetrahedral Co(ii) alkyl complex κ3-(NNpNTMS)Co(CH2SiMe3), reaction with KBEt3H and KC8 did not give the analogous products to that of the DIPP derivative; instead, the reaction with hydride was complicated and the only product isolated was the bis(ligand) derivative, κ2-(NNpNTMS)2Co, while reaction with KC8 under N2 generated the C-H activated complex κ4-(NNpNSiMe2CH2)Co. Many of these complexes have been characterized by single-crystal X-ray analysis, elemental analysis and solution magnetic susceptibility measurements. In addition, a number of these derivatives were screened for their ability to act as catalyst precursors for the hydrosilylation of 1-hexene.

Synthesis of a sterically bulky diphosphine synthon and Ru(ii) complexes of a cooperative tridentate enamide-diphosphine ligand platform.

In order to generate tridentate enamido diphosphine ligand platforms, we developed procedures for the preparation of tBu2PCH2CH2P(tBu)I, which involve low temperatures, pentane solvent and addition of 4 equiv. of tBuLi to Cl2PCH2CH2PCl2 or 2 equiv. of tBuLi to known Cl(tBu)PCH2CH2P(tBu)Cl also at low temperatures in pentane; an alternate method involves the inverse addition of Cl(tBu)PCH2CH2P(tBu)Cl to 2 equiv. of tBuLi in pentane at 0 °C; all of these methods generate good yields of the tetraphosphine dimer (tBu2PCH2CH2P(tBu))2 contaminated by small amounts of tBu2PCH2CH2PtBu2 (dtbpe), which can be conveniently separated by sublimation. Subsequent oxidative cleavage of the P-P bond with I2 or 1,2-diiodoethane results in the formation of the desired tBu2PCH2CH2P(tBu)I, which undergoes C-P bond formation when added to 1 equiv. of the lithium N-2,6-diisopropylphenylenamide of cyclopentylidene imine to generate the HNPP ligand precursor; this species exists as a tautomeric mixture of the corresponding enamine and imine, the ratio of which depends on workup conditions used. This enamine-imine mixture can be used directly to form Ru(ii) species either directly with heating to generate the five-coordinate (NPP)RuCl(CO) via loss of H2 or by inclusion of 1 equiv. of KOtBu to generate (NPP)RuH(CO). X-ray crystallographic studies confirm that the geometry in the solid state matches the solution spectroscopic data. Subsequent studies of (NPP)RuH(CO) indicate that it reacts with benzaldehyde, benzyl alcohol, and H2 in a cooperative manner to generate a series of hydride carbonyls that have been characterized fully by NMR spectroscopy and X-ray crystallography.

Low coordinate iron derivatives stabilized by a ß-diketiminate mimic. Synthesis and coordination chemistry of enamidophosphinimine scaffolds to generate diiron dinitrogen complexes.

In an effort to mimic N-diaryl-ß-diketiminate ligands (Nacnac), we have converted N-arylimine phosphine ligands to enamine-phosphinimines via the Staudinger reaction. By varying the aryl azide, one can access enamine-phosphinimine ligands with the same or different N-aryl substituents on both the enamine and phosphinimine units, which allows this intrinsically unsymmetrical bidentate donor set to present variable steric effects at the metal center. The enamine-phosphinimine was deprotonated and used in metathetical reactions with Fe(ii) bromide precursors to generate low coordinate complexes of the empirical formula [(CY5)NpN(Ar,Ar')]FeBr (where CY5 = cyclopentenyl; Ar,Ar' = 2,6-diisopropylphenyl, 2,6-dimethylphenyl, 2,4,6-trimethylphenyl). Depending on the substituents, these bromide derivatives can be monomeric or dimeric via bromide bridges. Reduction under dinitrogen using potassium graphite generates the dinitrogen complexes ([(CY5)NpN(Ar,Ar')]Fe)2(µ-N2) for Ar,Ar' = 2,6-diisopropylphenyl, and Ar = 2,6-dimethylphenyl, Ar' = 2,4,6-trimethylphenyl. However, for the former, a unsymmetrical side product can be isolated that has a bridging N-2,6-diisopropylphenylimide unit with one enamido-phosphine ligand bound to one iron and the other iron stabilized with an intact enamido-phosphinimine. When the steric bulk is reduced on both nitrogen donors, a complicated product mixture is obtained after reduction from which a small amount of [(CY5)NpN(Ar,Ar')]Fe[(CY5)NP(Ar)] (Ar,Ar' = 2,6-dimethylphenyl) could be isolated. All of these complexes are paramagnetic and have been characterized by elemental analysis, magnetic studies and X-ray crystallography.

Synthesis of Iron and Cobalt Complexes of a Ferrocene-Linked Diphosphinoamide Ligand and Characterization of a Weak Iron-Cobalt Interaction.

A ferrocene-based bis(phosphinoamine) fc(NHP(i)Pr2)2 has been deprotonated and used in salt metathesis reactions to form dimeric complexes ([fc(NP(i)Pr2)2]M)2 (M = Fe, Co). A novel coordination environment for Co(II) is observed including a weak but significant Fe-Co interaction, which was characterized using X-ray crystallography, Mössbauer spectroscopy, and VT-magnetometry. Density functional theory (DFT) calculations including natural bond order analysis provides further support for the interaction and suggests a combination of Fe → Co and Co → Fe interactions.

N2 activation by an iron complex with a strong electron-donating iminophosphorane ligand.

A new tridentate cyclopentane-bridged iminophosphorane ligand, N-(2-diisopropylphosphinophenyl)-P,P-diisopropyl-P-(2-(2,6-diisopropylphenylamido)cyclopent-1-enyl)phosphoranimine (NpNPiPr), was synthesized and used in the preparation of a diiron dinitrogen complex. The reaction of the iron complex FeBr(NpNPiPr) with KC8 under dinitrogen yielded the dinuclear dinitrogen Fe complex [Fe(NpNPiPr)]2(µ-N2), which was characterized by X-ray analysis and resonance Raman and NMR spectroscopies. The X-ray analysis revealed a diiron complex bridged by the dinitrogen molecule, with each metal center coordinated by an NpNPiPr ligand and dinitrogen in a trigonal-monopyramidal geometry. The N­N bond length is 1.184(6) Å, and resonance Raman spectra indicate that the N­N stretching mode ν(14N2/15N2) is 1755/1700 cm­1. The magnetic moment of [Fe(NpNPiPr)]2(µ-N2) in benzene-d6 solution, as measured by 1H NMR spectroscopy by the Evans method, is 6.91µB (S = 3). The Mössbauer spectrum at 78 K showed δ = 0.73 mm/s and ΔEQ = 1.83 mm/s. These findings suggest that the iron ions are divalent with a high-spin configuration and that the N2 molecule has (N═N)2­ character. Density functional theory calculations performed on [Fe(NpNPiPr)]2(µ-N2) also suggested that the iron is in a high-spin divalent state and that the coordinated dinitrogen molecule is effectively activated by π back-donation from the two iron ions (dπ) to the dinitrogen molecule (πx* and πy*). This is supported by cooperation between a large negative charge on the iminophosphorane ligand and strong electron donation and effective orbital overlap between the iron dπ orbitals and N2 π* orbitals supplied by the phosphine ligand.

Ruthenium Complexes Stabilized by Bidentate Enamido-Phosphine Ligands: Aspects of Cooperative H2 Activation.

Four bidentate, hybrid ligands ((R)(NP)(R')H) featuring imine-nitrogen and alkyl-phosphine donors linked by a cyclopentyl ring were synthesized. The ortho position of the aryl group attached to nitrogen is varied such that R is Me or Pr(i); additionally, the groups decorating phosphorus (R') are varied between Bu(t) or Pr(i). The addition of each ligand to RuHCl(PPr(i)3)2(CO) in the presence of KOBu(t) generates four enamido-phosphine complexes RuH{(R)(NP)(R')}(PPr(i)3)(CO) that were characterized by NMR spectroscopy, elemental analyses, and, in the case of R = Pr(i) and R' = Bu(t) or Pr(i), X-ray crystallography. Depending on R', the reaction of RuH{(R)(NP)(R')}(PPr(i)3)(CO) with H2 generates varying amounts of the imine-phosphine complex RuH2{(R)(NP)(R')H}(PPr(i)3)(CO). Insights into the mechanism of H2 activation by these enamido derivatives were explored using RuH{(Pr)(NP)(Pri)}(PPr(i)3)(CO), for which an intermediate was identified as the dihydrogen-dihydride complex, RuH2(H2){(Pri)(NP)(Pri)H}(PPr(i)3)(CO), on the basis of the T1,min value of 22 ms for the (1)H NMR resonance at δ -7.2 at 238 K (measured at 400 MHz). The N donor of the enamine tautomeric form of the ligand is protonated by H2 or D2 and dissociates from Ru. Tautomerization of the enamine to the imine form of the dissociated arm is involved in formation of the final product.

Anionic tantalum dihydride complexes: heterobimetallic coupling reactions and reactivity toward small-molecule activation.

The anionic dihydride complex [Cp2TaH2](-) was synthesized as a well-defined molecular species by deprotonation of Cp2TaH3 while different solubilizing agents, such as [2.2.2]cryptand and 18-crown-6, were applied to encapsulate the alkali-metal counterion. The ion pairs were characterized by multiple spectroscopic methods as well as X-ray crystallography, revealing varying degrees of interaction between the hydride ligands of the anion and the respective countercation in solution and in the solid state. The [Cp2TaH2](-) complex anion shows slow exchange of the hydride ligands when kept under a D2 atmosphere, but a very fast reaction is observed when [Cp2TaH2](-) is reacted with CO2, from which Cp2TaH(CO) is obtained as the tantalum-containing reaction product, along with inorganic salts. Furthermore, [Cp2TaH2](-) can act as a synthon in heterobimetallic coupling reactions with transition-metal halide complexes. Thus, the heterobimetallic complexes Cp2Ta(µ-H)2Rh(dippp) and Cp2Ta(µ-H)2Ru(H)(CO)(P(i)Pr3)2 were synthesized and characterized by various spectroscopies and via single-crystal X-ray diffraction. The new hydride bridged tantalum-rhodium heterobimetallic complex is cleaved under a CO atmosphere to yield mononuclear species and slowly exchanges protons and hydride ligands when exposed to D2 gas.

Cleavage of an aryl carbon-nitrogen bond of a phosphazido iron(II) complex promoted by hydride metathesis.

Upon reaction with KBEt3H, the pseudo tetrahedral Fe(II) complex with a bulky enamido-phosphazide ligand set undergoes elimination of N2 and 1,3-Me2C6H4 to generate the dinuclear Fe(II) derivative with bridging phosphinimido units. When the reaction is performed using KBEt3D, no deuterium is incorporated into the eliminated 1,3-Me2C6H4; all of the deuterium ends up as D2. When the reaction is performed in THF-d8, only 2-d-1,3-Me2C6H3D was detected by GCMS. These studies are consistent with a radical mechanism.

N2 coordination.

The impact of the report by Allen and Senoff in 1965 on the serendipitous discovery of the first complex with coordinated dinitrogen, [Ru(NH3)5N2](2+), is discussed in the context of what was known beforehand and the impact this work had on coordination chemistry.

Reduction of carbon dioxide promoted by a dinuclear tantalum tetrahydride complex.

The reaction of 1 equiv of carbon dioxide with the dinuclear tetrahydride complex ([NPN]Ta)(2)(µ-H)(4) [where NPN = PhP(CH(2)SiMe(2)NPh)(2)] results in the formation of ([NPN]Ta)(2)(µ-OCH(2)O)(µ-H)(2) via a combination of migratory insertion and reductive elimination. The identity of the ditantalum complex containing a methylene diolate fragment was confirmed by single-crystal X-ray analysis, NMR analysis, and isotopic labeling studies. Density functional theory calculations were performed to provide information on the structure of the initial adduct formed and likely transition states and intermediates for the process.

Oxygen extrusion from amidate ligands to generate terminal Ta=O units under reducing conditions. How to successfully use amidate ligands in dinitrogen coordination chemistry.

A series of mixed Cp* amidate tantalum complexes Cp*Ta(RNC(O)R')X(3) (where R = Me(2)C(6)H(3), (i)Pr, R' = (t)Bu, Ph, X = Cl, Me) have been prepared via salt metathesis and their fundamental reactivities under reducing conditions have been explored. Reaction of the tantalum chloro precursors with potassium graphite under N(2) or Ar leads to the stereoselective formation of the terminal tantalum oxo species, Cp*Ta=O(η(2)-RN=CR')Cl. This represents the formal extrusion of oxygen from the amidate ligand to the reduced tantalum center and is accompanied by the formation of the iminoacyl fragment bound to Ta(v). Amidate dinitrogen complexes, [Cp*TaCl(RNC(O)(t)Bu)](2)(µ-N(2)) (where R = Me(2)C(6)H(3), (i)Pr) were synthesized via salt metathesis from the known [Cp*TaCl(2)](2)(µ-N(2)) precursor, establishing that amidate ligands can support dinitrogen complexes, but not the reduction process often necessary for their synthesis.

Synthesis and characterization of organo-scandium and yttrium complexes stabilized by phosphinoamide ligands.

Addition of three equivalents of phosphinoamine, (ArNHP(i)Pr(2)) [Ar = 3,5-dimethylphenyl] to M(CH(2)SiMe(3))(3)(THF)(2) [M = Sc, Y] precursors gives complexes of the form (ArNP(i)Pr(2))(3)M(THF) [M = Sc, Y]. In the case of scandium, addition of Sc(CH(2)SiMe(3))(3)(THF)(2) to (ArNP(i)Pr(2))(3)Sc(THF) affords (ArNP(i)Pr(2))(2)Sc(CH(2)SiMe(3))(THF), which has been isolated and structurally characterized. In contrast, addition of Y(CH(2)SiMe(3))(3)(THF)(2) to (ArNP(i)Pr(2))(3)Y(THF) generates a distribution of phosphinoamide-containing products consistent with the formulations (ArNP(i)Pr(2))(2)Y(CH(2)SiMe(3))(THF) and (ArNP(i)Pr(2))Y(CH(2)SiMe(3))(2)(THF), as ascertained using NMR spectroscopy. Attempts to react the alkyl-containing phosphinoamide complexes with small molecules such as H(2) led to disproportionation type processes.

New cyclopentenyl-linked [NPN] ligands and their coordination chemistry with zirconium: synthesis of a dinuclear side-on-bound dinitrogen complex.

The synthesis and characterization of two 1,2-cyclopentyl-bridged diiminophosphine proligands, (CY5)[NPN](DMP)H(2) (CY5 = cyclopentylidene; DMP = 2,6-Me(2)C(6)H(3)) and (CY5)[NPN](DIPP)H(2) (DIPP = 2,6-(i)Pr(2)C(6)H(3)), are presented, and tautomerization to the corresponding 1,2-cyclopentenyl-bridged enamineimine phosphine precursors is reported. These two new proligands are obtained by deprotonation of N-DMP- or N-DIPP-cyclopentylideneimine (N-DMP, 2,6-dimethylphenyl; N-DIPP, 2,6-diisopropylphenyl) and the subsequent addition of 0.5 equiv of dichlorophenylphosphine. Each ligand precursor exists as a mixture of isomers that consist of the diimine, enamineimine, and dienamine tautomers and corresponding stereoisomers, each of which could be identified. The bis(dimethylamido)zirconium complexes (CY5)[NPN](DMP)Zr(NMe(2))(2) and (CY5)[NPN](DIPP)Zr(NMe(2))(2) were prepared directly from the neutral proligands and Zr(NMe(2))(4) via protonolysis. Exchange of the dimethylamido ligands in the latter complexes for chlorides and iodides takes place upon reaction with excess Me(3)SiCl and Me(3)SiI, respectively. A dinuclear zirconium-dinitrogen complex, {(CY5)[NPN](DMP)Zr(THF)}(2)(µ-η(2):η(2)-N(2)), was obtained via KC(8) reduction of (CY5)[NPN](DMP)ZrCl(2) under 4 atm of N(2). On the basis of single-crystal X-ray analysis, N(2) has been reduced to a side-on-bound hydrazido (µ-η(2):η(2)-N(2)(4-)) unit. This dinitrogen complex is thermally unstable and decomposes in solution.