Ketimide Complexes Research Articles

A homoleptic Fe(iv) ketimide complex with a low-lying excited state.

The reaction of 4 equiv. of Li(N[double bond, length as m-dash]C( t Bu)Ph) with FeIICl2 results in isolation of [Li(Et2O)]2[FeII(N[double bond, length as m-dash]C( t Bu)Ph)4] (1), in good yields. The reaction of 1 with 1 equiv. of I2 leads to formation of [FeIV(N[double bond, length as m-dash]C( t Bu)Ph)4] (2), in moderate yields. 57Fe Mössbauer spectroscopy confirms the Fe(iv) oxidation state of 2, and X-ray crystallography reveals that 2 has a square planar coordination geometry along with several intramolecular H⋯C interactions. Furthermore, SQUID magnetometry indicates a small magnetic moment at room temperature, suggestive of an accessible S = 1 state. Both density functional theory and multiconfigurational calculations were done to elucidate the nature of the ground state. Consistent with the experimental results, the ground state was found to be an S = 0 state with an S = 1 excited state close in energy.

Reductive Coupling of a Diazoalkane Derivative Promoted by a Potassium Aluminyl and Elimination of Dinitrogen to Generate a Reactive Aluminium Ketimide.

The reaction of 9-diazo-9H-fluorene (fluN2 ) with the potassium aluminyl K[Al(NON)] ([NON]2- =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) affords K[Al(NON)(κN1 ,N3 -{(fluN2 )2 })] (1). Structural analysis shows a near planar 1,4-di(9H-fluoren-9-ylidene)tetraazadiide ligand that chelates to the aluminium. The thermally induced elimination of dinitrogen from 1 affords the neutral aluminium ketimide complex, Al(NON)(N=flu)(THF) (2) and the 1,2-di(9H-fluoren-9-yl)diazene dianion as the potassium salt, [K2 (THF)3 ][fluN=Nflu] (3). The reaction of 2 with N,N'-diisopropylcarbodiimide (iPrN=C=NiPr) affords the aluminium guanidinate complex, Al(NON){N(iPr)C(N=CMe2 )N(CHflu)} (4), showing a rare example of reactivity at a metal ketimide ligand. Density functional theory (DFT) calculations have been used to examine the bonding in the newly formed [(fluN2 )2 ]2- ligand in 1 and the ketimide bonding in 2. The mechanism leading to the formation of 4 has also been studied using this technique.

A Ketimide-Stabilized Palladium Nanocluster with a Hexagonal Aromatic Pd7 Core.

Herein, we report the synthesis and characterization of the mixed-valent, ketimide-stabilized Pd7 nanosheet, [Pd7(N═CtBu2)6] (1), via reaction of PdCl2(PhCN)2 and Li(N═CtBu2). tBuCN, isobutylene, and isobutane are also formed in the reaction. The presence of these products suggests that Li(N═CtBu2) acts as a reducing agent in the transformation, converting the Pd(II) starting material into the mixed-valent Pd(I)/Pd(0) product. Complex 1 features a hexagonal planar [Pd7]6+ core stabilized by six ketimide ligands, which surround the [Pd7]6+ center in an alternating up/down fashion. In situ NMR spectroscopic studies, as well as density functional theory (DFT) calculations, suggest that 1 is formed via the intermediacy of the bimetallic Pd(II) ketimide complex, [(tBu2C═N)Pd(μ-N,C-N═C(tBu)C(Me)2CH2)Pd(N═CtBu2)] (2). DFT calculations also reveal that 1 is a rare example of an all-metal aromatic nanocluster with hexagonal symmetry, sustaining a net diatropic ring-current of 10.6 nA/T, which is similar to that of benzene (11.8 nA/T) or other well-established transition-metal aromatic systems. Finally, we have found that 1 reacts with Ph3P, cleanly forming the tris-ligated 16-electron Pd(0) phosphine complex, Pd(PPh3)3 (3), suggesting that 1 could be a useful precatalyst for a variety of cross-coupling reactions.

Synthesis and Characterization of a Linear, Two-Coordinate Pt(II) Ketimide Complex.

Herein we report the synthesis and characterization of a linear, two-coordinate Pt(II) ketimide complex, Pt(N═CtBu2)2 (1), formed via the reaction of PtCl2(1,5-COD) with 2 equiv of Li(N═CtBu2). Also generated in the reaction is the bimetallic complex [(tBu2C═N)Pt(μ-N,C-N═C(tBu)C(Me)2CH2)Pt(N═CtBu2)] (2). Both complexes 1 and 2 have been characterized by NMR spectroscopy and X-ray crystallography. Notably, complex 1 exhibits short Pt-N distances (av. Pt-N = 1.817 Å) and an unusually deshielded 195Pt chemical shift (δPt = -629 ppm) with a large 1J(195Pt,14N) coupling constant (537 Hz). These data, in combination with a detailed density functional theory electronic structure analysis, reveal the presence of highly covalent Pt═N multiple bonds formed by a combination of σ-donation, π-donation, and π-backdonation.

Divergent Pathways Involving 1,3-Dipolar Addition and N-N Bond Splitting of an Organic Azide across a Zirconium Methylidene.

The zirconium methylidene (PNP)Zr=CH2 (OAr) (1) reacts with N3 Ad to give two products (PNP)Zr=NAd(OAr) (2) and (PNP)Zr(η2 -N=NAd)(N=CH2 )(OAr) (3), both resulting from a common cycloaddition intermediate (PNP)Zr(CH2 N3 Ad)(OAr) (A). Using a series of control experiments in combination with DFT calculations, it was found that 2 results from a nitrene by a carbene metathesis reaction in which N2 acts as a delivery vehicle and forms N2 CH2 as a side product. In the case of 3, N-N bond splitting of the azide at the α-position allowed the isolation of a rare example of a parent ketimide complex of zirconium. Isotopic labeling studies and solid-state X-ray analysis are presented for 2 and 3, in addition to an independent synthesis for the former.

Divergent Pathways Involving 1,3‐Dipolar Addition and N−N Bond Splitting of an Organic Azide across a Zirconium Methylidene

AbstractThe zirconium methylidene (PNP)Zr=CH2(OAr) (1) reacts with N3Ad to give two products (PNP)Zr=NAd(OAr) (2) and (PNP)Zr(η2‐N=NAd)(N=CH2)(OAr) (3), both resulting from a common cycloaddition intermediate (PNP)Zr(CH2N3Ad)(OAr) (A). Using a series of control experiments in combination with DFT calculations, it was found that 2 results from a nitrene by a carbene metathesis reaction in which N2 acts as a delivery vehicle and forms N2CH2 as a side product. In the case of 3, N−N bond splitting of the azide at the α‐position allowed the isolation of a rare example of a parent ketimide complex of zirconium. Isotopic labeling studies and solid‐state X‐ray analysis are presented for 2 and 3, in addition to an independent synthesis for the former.

Promoting oxo functionalization in the uranyl ion by ligation to ketimides

Reaction of [UO2Cl2(THF)2]2 with 8 equiv of Li(N=CtBuPh) or 6 equiv of Li(N=CtBu2) affords the uranyl ketimide complexes, [Li(THF)(Et2O)]2[UO2(N=CtBuPh)4] (1) and [Li(THF)(TMEDA)][UO2(N=CtBu2)3] (2), respectively. Complexes 1 and 2 have been fully characterized, including analysis by X-ray crystallography. Most importantly, both complexes feature dative interactions between their lithium counterions and the uranyl oxo ligands. These interactions are likely a consequence of their weaker UO bonds, which are caused by electrostatic repulsion between the anionic N donor atom of the strongly electron donating ketimide ligand and the oxo ligand of the uranyl fragment.

Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(N═C(t)Bu2)4 (M = V, Nb, Ta).

Addition of 4 equiv of Li(N═C(t)Bu2) to VCl3 in THF, followed by addition of 0.5 equiv of I2, generates the homoleptic V(IV) ketimide complex, V(N═C(t)Bu2)4 (1), in 42% yield. Similarly, reaction of 4 equiv of Li(N═C(t)Bu2) with NbCl4(THF)2 in THF affords the homoleptic Nb(IV) ketimide complex, Nb(N═C(t)Bu2)4 (2), in 55% yield. Seeking to extend the series to the tantalum congener, a new Ta(IV) starting material, TaCl4(TMEDA) (3), was prepared via reduction of TaCl5 with Et3SiH, followed by addition of TMEDA. Reaction of 3 with 4 equiv of Li(N═C(t)Bu2) in THF results in the isolation of a Ta(V) ketimide complex, Ta(Cl)(N═C(t)Bu2)4 (5), which can be isolated in 32% yield. Reaction of 5 with Tl(OTf) yields Ta(OTf)(N═C(t)Bu2)4 (6) in 44% yield. Subsequent reduction of 6 with Cp*2Co in toluene generates the homoleptic Ta(IV) congener Ta(N═C(t)Bu2)4 (7), although the yields are poor. All three homoleptic group 5 ketimide complexes exhibit squashed tetrahedral geometries in the solid state, as determined by X-ray crystallography. This geometry leads to a d(x(2)-y(2))(1) ((2)B1 in D(2d)) ground state, as supported by DFT calculations. EPR spectroscopic analysis of 1 and 2, performed at X- and Q-band frequencies (∼9 and 35 GHz, respectively), further supports the (2)B1 ground-state assignment, whereas comparison of 1, 2, and 7 with related group 5 tetra(aryl), tetra(amido), and tetra(alkoxo) complexes shows a higher M-L covalency in the ketimide-metal interaction. In addition, a ligand field analysis of 1 and 2 demonstrates that the ketimide ligand is both a strong π-donor and strong π-acceptor, an unusual combination found in very few organometallic ligands.

Cobalt(II) Complex of a Diazoalkane Radical Anion

β-Diketiminate cobalt(I) precursors react with diphenyldiazomethane to give a compound that is shown by computational studies to be a diazoalkane radical anion antiferromagnetically coupled to a high-spin cobalt(II) ion. Thermolysis of this complex results in formal N-N cleavage to give a cobalt(II) ketimide complex. Experimental evaluation of the potential steps in the mechanism suggests that free azine is a likely intermediate in this reaction.

The ketimide ligand is not just an inert spectator: heteroallene insertion reactivity of an actinide-ketimide linkage in a thorium carbene amide ketimide complex.

The ketimide anion R2C—N− is an important class of chemically robust ligand that binds strongly to metal ions and is considered ideal for supporting reactive metal fragments due to its inert spectator nature; this contrasts with R2N− amides that exhibit a wide range of reactivities. Here, we report the synthesis and characterization of a rare example of an actinide ketimide complex [Th(BIPMTMS){N(SiMe3)2}(N—CPh2)] [2, BIPMTMS=C(PPh2NSiMe3)2]. Complex 2 contains Th—Ccarbene, Th—Namide and Th—Nketimide linkages, thereby presenting the opportunity to probe the preferential reactivity of these linkages. Importantly, reactivity studies of 2 with unsaturated substrates shows that insertion reactions occur preferentially at the Th—Nketimide bond rather than at the Th—Ccarbene or Th—Namide bonds. This overturns the established view that metal-ketimide linkages are purely inert spectators.

The Ketimide Ligand is Not Just an Inert Spectator: Heteroallene Insertion Reactivity of an Actinide–Ketimide Linkage in a Thorium Carbene Amide Ketimide Complex

AbstractThe ketimide anion R2CN− is an important class of chemically robust ligand that binds strongly to metal ions and is considered ideal for supporting reactive metal fragments due to its inert spectator nature; this contrasts with R2N− amides that exhibit a wide range of reactivities. Here, we report the synthesis and characterization of a rare example of an actinide ketimide complex [Th(BIPMTMS){N(SiMe3)2}(NCPh2)] [2, BIPMTMS=C(PPh2NSiMe3)2]. Complex 2 contains ThCcarbene, ThNamide and ThNketimide linkages, thereby presenting the opportunity to probe the preferential reactivity of these linkages. Importantly, reactivity studies of 2 with unsaturated substrates shows that insertion reactions occur preferentially at the ThNketimide bond rather than at the ThCcarbene or ThNamide bonds. This overturns the established view that metal‐ketimide linkages are purely inert spectators.

Reactivity and Mössbauer Spectroscopic Characterization of an Fe(IV) Ketimide Complex and Reinvestigation of an Fe(IV) Norbornyl Complex

Thermolysis of Fe(N═C(t)Bu2)4 (1) for 8 h at 50 °C generates the mixed valent Fe(III)/Fe(II) bimetallic complex Fe2(N═C(t)Bu2)5 (2) in moderate yield. Also formed in this reaction are tert-butyl cyanide, isobutane, and isobutylene, the products of ketimide oxidation by the Fe(4+) center. Reaction of 1 with 1 equiv of acetylacetone affords the Fe(III) complex, Fe(N═C(t)Bu2)2(acac) (3), concomitant with formation of bis(tert-butyl)ketimine, tert-butyl cyanide, isobutane, and isobutylene. In addition, the Mössbauer spectra of 1 and its lower-valent analogues [Li(12-crown-4)2][Fe(N═C(t)Bu2)4] (5) and [Li(THF)]2[Fe(N═C(t)Bu2)4] (6) were recorded. We also revisited the chemistry of Fe(1-norbornyl)4 (4) to elucidate its solid-state molecular structure and determine its Mössbauer spectrum, for comparison with that recorded for 1.

Synthesis and Characterization of [M2(NCtBu2)5]− (M=Mn, Fe, Co): Metal Ketimide Complexes with Strong Metal–Metal Interactions

Synthesis and Characterization of [M₂(NC<i>t</i>Bu₂)₅]⁻ (M=Mn, Fe, Co): Metal Ketimide Complexes with Strong Metal–Metal Interactions

Synthesis and Characterization of [M2(NCtBu2)5]− (M=Mn, Fe, Co): Metal Ketimide Complexes with Strong Metal–Metal Interactions

Hilfreiche Unterstützung: Die Zugabe von 2.5 Äquiv. Li(NCtBu2) zu MCl2 (M=Mn, Fe, Co) und der anschließende Zusatz von [12]Krone-4 führen zur Bildung von [Li([12]Krone-4)2][M2(NCtBu2)5] in guten Ausbeuten. Diese Komplexe weisen kleine M-M-Abstände und eine starke magnetische Wechselwirkung zwischen den Metallzentren auf, was belegt, dass Ketimide geeignete Hilfsliganden für den Aufbau von Metall-Metall-Bindungen sind.

Probing the 5f Orbital Contribution to the Bonding in a U(V) Ketimide Complex

Reaction of UCl(4) with 5 equiv of Li(N═C(t)BuPh) generates the homoleptic U(IV) ketimide complex [Li(THF)(2)][U(N═C(t)BuPh)(5)] (1) in 71% yield. Similarly, reaction of UCl(4) with 5 equiv of Li(N═C(t)Bu(2)) affords [Li(THF)][U(N═C(t)Bu(2))(5)] (2) in 67% yield. Oxidation of 2 with 0.5 equiv of I(2) results in the formation of the neutral U(V) complex U(N═C(t)Bu(2))(5) (3). In contrast, oxidation of 1 with 0.5 equiv of I(2), followed by addition of 1 equiv of Li(N═C(t)BuPh), generates the octahedral U(V) ketimide complex [Li][U(N═C(t)BuPh)(6)] (4) in 68% yield. Complex 4 can be further oxidized to the U(VI) ketimide complex U(N═C(t)BuPh)(6) (5). Complexes 1-5 were characterized by X-ray crystallography, while SQUID magnetometry, EPR spectroscopy, and UV-vis-NIR spectroscopy measurements were also preformed on complex 4. Using this data, the crystal field splitting parameters of the f orbitals were determined, allowing us to estimate the amount of f orbital participation in the bonding of 4.

Stabilizing High-Valent Metal Ions with a Ketimide Ligand Set: Synthesis of Mn(N═CtBu2)4

Reaction of MnCl(2) with 4 equiv of Li(N=C(t)Bu(2)) generates [Li(THF)](2)[Mn(N=C(t)Bu(2))(4)] (1) in 80% yield. Oxidation of 1 with 0.5 equiv of I(2) produces [Li][Mn(N=C(t)Bu(2))(4)] (2) in 88% yield. Both complexes 1 and 2 exhibit tetrahedral structures about the Mn center in the solid-state, as determined by X-ray crystallography. Reaction of 2 with 12-crown-4 generates [Li(12-crown-4)(2)][Mn(N=C(t)Bu(2))(4)] (3) in 94% yield. Interestingly, in the solid-state, complex 3 exhibits a squashed tetrahedral structure about Mn. Addition of 1 equiv of I(2) to 1 generates the Mn(IV) ketimide, Mn(N=C(t)Bu(2))(4) (4), in 75% yield. Complex 4 was fully characterized, including analysis by X-ray crystallography and cyclic voltammetry. Like 3, complex 4 also exhibits a squashed tetrahedral structure in the solid-state. Interestingly, thermolysis of complex 4 at 50 °C for 6 h results in the formation of Mn(3)(N=C(t)Bu(2))(6) (6), which can be isolated in 49% yield. Also observed in the reaction mixture is pivalonitrile, isobutylene, and isobutene, the products of ketimide ligand oxidation. We have also synthesized the homoleptic Cr(IV) ketimide complex, Cr(N=C(t)Bu(2))(4) (5), and have analyzed its electrochemical properties with cyclic voltammetry.

Synthesis and reactivity of cationic niobium and tantalum methyl complexes supported by imido and β-diketiminato ligands

The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes [(BDI)MeM(N(t)Bu)][X] (BDI = [Ar]NC(CH(3))CHC(CH(3))N[Ar], Ar = 2,6-(i)Pr(2)C(6)H(3); M = Nb, Ta; X = MeB(C(6)F(5))(3), B(C(6)F(5))(4)] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but formed phosphine-trapped acyl complexes [(BDI)(R(3)PC(O)Me)M(N(t)Bu)][B(C(6)F(5))(4)] (R = Et, Cy) in the presence of a combination of trialkylphosphines and CO. Treatment of the monoalkyl cationic Nb complex with XylNC (Xyl = 2,6-Me(2)-C(6)H(3)) resulted in irreversible formation of the iminoacyl complex [(BDI)(XylN[double bond, length as m-dash]C(Me))Nb(N(t)Bu)][B(C(6)F(5))(4)], which did not bind phosphines but would add a methide group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe(2))Nb(N(t)Bu). Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; ii) protonolysis reactions with Ph(3)SiOH to form thermally robust cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the cationic Nb methyl complex.

Synthesis and Characterization of an Iron(IV) Ketimide Complex

Addition of 4 equiv of LiN═C(t)Bu(2) to FeCl(2) in Et(2)O/THF results in the formation of [Li(THF)](2)[Fe(N═C(t)Bu(2))(4)] (1). Oxidation of 1 with 0.5 equiv of I(2) in Et(2)O/DME yields [Li(DME)][Fe(N═C(t)Bu(2))(4)] (2) in moderate yield. Both 1 and 2 are high spin and exhibit tetrahedral geometries in the solid state. Oxidation of 1 with 1 equiv of I(2) in Et(2)O yields Fe(N═C(t)Bu(2))(4) (3) in good yield. Surprisingly, complex 3 exhibits a diamagnetic ground state and a nearly square planar geometry about the Fe center.

Synthesis of Copper(I) Complexes with Ketimide and Hydrazide Ligands

Copper(I) chloride reacted with lithium ketimides to form tetrameric homoleptic copper(I) ketimide complexes, [Cu(N=CR2)]4, where R = t-Bu or Ph. Mesityl copper reacted with excess 1-trimethylsilylmethyl-2,2,-dimethylhydrazine to give the mixed ligand complex (2,4,6-C6Me3H2)Cu4[N(SiMe3)NMe2]3. Single-crystal X-ray crystallographic studies revealed that the three complexes have eight-member ring structures in which the ring has a hinged or butterfly shape. Although an eight-member ring structure is common for copper(I) amido, alkyl, and aryl clusters, the structure of [Cu(N=C-t-Bu2)]4 is unusual because the hinge angle is significantly smaller than is common, resulting in short Cu···Cu contacts compared to related complexes.

Probing the Chemistry, Electronic Structure and Redox Energetics in Organometallic Pentavalent Uranium Complexes

A series of organometallic pentavalent uranium complexes of the general formula (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(Y) (Y = monoanionic, non-halide ligand) have been prepared using a variety of routes. Utilizing the direct oxidation of (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(THF) (2) with the appropriate copper(I) salt yielded the triflate (Y = OTf (OSO(2)CF(3)), 11), thiolate (Y = SPh, 12), and acetylide (Y = C[triple bond]CPh, 13) complexes, while a salt metathesis route between the U(V)-imido (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(I) (10) and various alkali salts gave the diphenylamide (Y = NPh(2), 14), aryloxide (Y = OPh, 15), alkyl (Y = Me, 16), and aryl (Y = Ph, 17) complexes. Paired with 13, the isolation of 16 and 17 shows that U(V) can support the full range of carbon anions (sp, sp(2), and sp(3)), and these are, to the best of our knowledge, the first examples of pentavalent uranium complexes with anionic carbon moieties other than carbocyclic (C(5)R(5), C(7)H(7), C(8)H(8)) ligands. Finally, both protonolysis and insertion pathways afforded the U(V)-imido ketimide complex (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(N=CPh(2)) (18). The complexes have been isolated in good yield and characterized using various combinations of (1)H NMR spectroscopy, elemental analysis, mass spectrometry, single crystal X-ray diffraction, cyclic voltammetry, UV-visible-NIR absorption spectroscopy, and magnetic susceptibility measurements. All (C(5)Me(5))(2)U(=N-Ar)(X) (X = F, Cl, Br, I) and (C(5)Me(5))(2)U(=N-Ar)(Y) complexes exhibit U(VI)/U(V) and U(V)/U(IV) redox couples by voltammetry. The potential separation between these couples remains essentially constant at approximately 1.50 V, but both processes shift in tandem in potential by approximately 700 mV across the series of X/Y ligands. No significant differences between mu(eff) values or temperature dependencies in the magnetic susceptibility were observed for these complexes regardless of the identity of the ancillary X/Y ligand. However, an excellent linear correlation was observed between the chemical shift values of C(5)Me(5) ligand protons in the (1)H NMR spectra and the oxidation potentials of (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(X/Y), suggesting that there is a common origin, overall sigma-/pi-donation from the ancillary X/Y ligand to the metal, contributing to both observables. Combined, these data confer the following trend in increasing sigma/pi-donating ability of the X/Y ligand to the U(V) metal center: OTf < I < Br < Cl < SPh < C[triple bond]CPh < F < [OPh approximately Me approximately Ph] << NPh(2) < N=CPh(2). These (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(X/Y) complexes also show distinct hallmarks of a covalent bonding interaction between the metal and the imide ligand that is modulated to varying degrees by the interaction between the X/Y ancillary ligand and the U(V) metal center. These signatures of covalency include stabilization of multiple metal oxidations states [U(VI), U(V), and U(IV)] and enhanced intensities in the intraconfiguration (f-f) transitions. Of particular note in this regard is the more than 20-fold enhancement in the f-f intensities observed for Y = C[triple bond]CPh and N=CPh(2), which is a clear reflection of the covalent metal-ligand bonding interactions sustained by the acetylide and ketimide ligands in these pentavalent systems.