Redox-Neutral Transformations of Carbon Dioxide Using Coordinatively Unsaturated Late Metal Silyl Amide Complexes.

Two-coordinate silylamido complexes of nickel and copper rapidly react with CO to selectively form a new cyanate ligand along with hexamethyldisiloxane byproducts. Mechanistic insight into these reactions was obtained from the synthesis of proposed intermediates, several silyl- and phenyl- substituted amido analogues, and their subsequent reactivity with CO. These studies suggest that a unique intramolecular double silyl transfer step facilitates CO deoxygenation, which likely contributes to the rapid rates of reaction.

Heterolytic H-H and H-B Bond Cleavage Reactions of {(IPr)Ni(μ-S)}.

Kinetic and DFT computational studies reveal that the reaction of {(IPr)Ni(μ-S)} (1, IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazolin-2-ylidene) with dihydrogen to produce {(IPr)Ni(μ-SH)} (2) proceeds by rate-limiting heterolytic addition of H across a Ni-S bond of intact dinuclear 1, followed by cis/trans isomerization at Ni and subsequent H migration from Ni to S, to produce the bis-hydrosulfide product 2. Complex 1 reacts in a similar manner with pinacolborane to produce {(IPr)Ni}(μ-SH)(μ-SBPin) (3), showing that heterolytic activation by this nickel μ-sulfide complex can be generalized to other H-E bonds.

Crystal structure of the inverse crown ether tetra-kis-[μ2-bis-(tri-methyl-sil-yl)amido]-μ4-oxido-dicobalt(II)disodium, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O).

The title compound, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O), (I), represents a new entry in the class of inverse crown ethers. In the mol-ecule, each Co atom is formally in the oxidation state +II. The structure contains one half of a unique mol-ecule per asymmetric unit with the central μ4-oxido ligand residing on an inversion center, leading to a planar coordination to the Na and Co atoms.

Synthesis, structure, and reactions of a copper-sulfido cluster comprised of the parent CuS unit: {(NHC)Cu}(μ-S).

The synthesis of the first CuI2(μ-S) complex, {(IPr*)Cu}(μ-S) (IPr* = 1,3-bis(2,6-(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene; ), has been accomplished three synthetic routes: (1) salt metathesis between (IPr*)CuCl and NaS; (2) silyl-deprotection reaction between (IPr*)Cu(SSiMe) and (IPr*)CuF; and (3) acid-base reaction between (IPr*)Cu(SH) and (IPr*)Cu(O Bu). The X-ray crystal structure of exhibits two two-coordinate copper centers connected by a bent Cu-S-Cu linkage. Application of these synthetic routes to analogous precursors containing the sterically smaller ligand IPr (1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene), in place of IPr*, resulted in the formation of a transient product proposed as {(IPr)Cu}(μ-S) (), which decomposes quickly in solution.

Protonolysis and amide exchange reactions of a three-coordinate cobalt amide complex supported by an N-heterocyclic carbene ligand.

A three-coordinate cobalt species, IPrCoCl{N(SiMe3)2} [1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene], was synthesized by the reaction of {IPrCoCl2}2 with NaN(SiMe3)2. Compound 1 is a useful starting material for low-coordinate (IPr)Co species. 1 reacts with 2,6-di-tert-butyl-4-methylphenol (BHT-H) via aminolysis of the Co-N bond to generate a three-coordinate phenoxide complex, IPrCoCl(O-2,6-(t)Bu2-4-MeC6H2) (2).

Synthesis and reactivity of NHC-supported Ni2(μ(2)-η(2),η(2)-S2)-bridging disulfide and Ni2(μ-S)2-bridging sulfide complexes.

The (IPr)Ni scaffold stabilizes low-coordinate, mononuclear and dinuclear complexes with a diverse range of sulfur ligands, including μ(2)-η(2),η(2)-S2, η(2)-S2, μ-S, and μ-SH motifs. The reaction of {(IPr)Ni}2(μ-Cl)2 (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with S8 yields the bridging disulfide species {(IPr)ClNi}2(μ(2)-η(2),η(2)-S2) (2). Complex 2 reacts with 2 equiv of AdNC (Ad = adamantyl) to yield a 1:1 mixture of the terminal disulfide compound (IPr)(AdNC)Ni(η(2)-S2) (3a) and trans-(IPr)(AdNC)NiCl2 (4a).

Carbon-hydrogen bond activation, C-N bond coupling, and cycloaddition reactivity of a three-coordinate nickel complex featuring a terminal imido ligand.

The three-coordinate imidos (dtbpe)Ni═NR (dtbpe = (t)Bu2PCH2CH2P(t)Bu2, R = 2,6-(i)Pr2C6H3, 2,4,6-Me3C6H2 (Mes), and 1-adamantyl (Ad)), which contain a legitimate Ni-N double bond as well as basic imido nitrogen based on theoretical analysis, readily deprotonate HC≡CPh to form the amide acetylide species (dtbpe)Ni{NH(Ar)}(C≡CPh). In the case of R = 2,6-(i)Pr2C6H3, reductive carbonylation results in formation of the (dtbpe)Ni(CO)2 along with the N-C coupled product keteneimine PhCH═C═N(2,6- (i)Pr2C6H3). Given the ability of the Ni═N bond to have biradical character as suggested by theoretical analysis, H atom abstraction can also occur in (dtbpe)Ni═N{2,6-(i)Pr2C6H3} when this species is treated with HSn((n)Bu)3.

Functionalization of Complexed NO in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium.

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp*Zr(N(O)NCPhCPh), generated from nitrous oxide insertion into the Zr-C bond of Cp*Zr(η-PhCCPh) at -78 °C, to afford the salt [Cp*Zr(N(O)N(Me)CPhCPh)][OSCF] () in 48% isolated yield. A single-crystal X-ray structure of features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr(N(O)NCPhCPh) scaffold. In addition to , the methoxy-triflato complex Cp*Zr(OMe)(OSCF) () was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp*Zr=O(NCH) with MeOSCF.

Three-coordinate nickel carbene complexes and their one-electron oxidation products.

The synthesis and characterization of two new carbene complexes, (dtbpe)Ni═CH(dmp) (1; dtbpe = 1,2-bis(di-tert-butylphosphino)ethane; dmp = 2,6-dimesitylphenyl) and (dippn)Ni═CH(dmp) (2; dippn = 1,8-bis(di-iso-propylphosphino)naphthalene), are described. Complexes 1 and 2 were isolated by photolysis of the corresponding side-bound diazoalkane complexes, exemplified by (dtbpe)Ni{η(2)-N2CH(dmp)} (3). The carbene complexes feature Ni-C distances that are short and Ni-C-C angles at the carbene carbon that are intermediate between 120° and 180° (155.

Synthesis and reactivity of two-coordinate Ni(I) alkyl and aryl complexes.

Reaction of [(IPr)Ni(μ-Cl)]2 (1-Cl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with ClMg{CH(SiMe3)2}·Et2O affords (IPr)Ni{CH(SiMe3)2} (2), a two-coordinate Ni(I) alkyl complex. An analogous two-coordinate aryl derivative, (IPr)Ni(dmp) (dmp = 2,6-dimesitylphenyl), can be similarly prepared from Li(dmp) and 1-Cl. Reaction of 2 with alkyl bromides gives the three-coordinate Ni(II) alkyl halide complex (IPr)Ni{CH(SiMe3)2}Br.

Synthesis and characterization of three-coordinate Ni(III)-imide complexes.

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl.

A two-coordinate nickel imido complex that effects C−H amination.

An exceptionally low coordinate nickel imido complex, (IPr*)Ni═N(dmp) (2) (dmp = 2,6-dimesitylphenyl), has been prepared by the elimination of N2 from a bulky aryl azide in its reaction with (IPr*)Ni(η6-C7H8) (1). The solid-state structure of 2 features two-coordinate nickel with a linear C−Ni−N core and a short Ni−N distance, both indicative of multiple-bond character. Computational studies using density functional theory showed a Ni═N bond dominated by Ni(dπ)−N(pπ) interactions, resulting in two nearly degenerate singly occupied molecular orbitals (SOMOs) that are Ni−N π* in character.

Hydrogen-atom abstraction from Ni(I) phosphido and amido complexes gives phosphinidene and imide ligands.

Hydrogen-atom abstraction from M-E(H) to generate M═E-containing complexes (E = PR, NR) is not well studied because only a few complexes are known to undergo such reactions. Hydrogen-atom abstraction from nickel(I) phosphide and amide complexes led to the corresponding phosphinidene and imide compounds. These reactions are unparalleled in the organometallic chemistry of nickel and feature an unusual example of a transition-metal phosphinidene synthesized by hydrogen-atom abstraction.

Reactions of CO(2) and CS(2) with 1,2-bis(di-tert-butylphosphino)ethane complexes of nickel(0) and nickel(I).

Reaction of CS(2) with [(dtbpe)Ni](2)(η(2),μ-C(6)H(6)) (1; dtbpe =1,2-bis(di-tert-butylphosphino)ethane) in toluene gives the carbon disulfide complex (dtbpe)Ni(η(2)-CS(2)) (2), characterized by standard spectroscopic methods and X-ray crystallography. Reaction of CS(2) with the Ni(I) complex (dtbpe)Ni(OSO(2)CF(3)) gives the diamagnetic, trimetallic cluster [{(dtbpe)Ni(κ(1),η(2)-CS(2))}(2)(dtbpe)Ni][SO(3)CF(3)](2) (3-OTf). The solid-state structure of 3-OTf reveals that the two CS(2) ligands bind η(2) to two (dtbpe)Ni centers and κ(1) to the third, unique (dtbpe)Ni in the complex dication, and NMR spectroscopic data indicate that this structure is maintained in solution.

Arrested 1,2-hydrogen migration from silicon to nickel upon oxidation of a three-coordinate Ni(I) silyl complex.

Reaction of the dimeric Ni(I) chloride complex [(dtbpe)NiCl](2) (1) with dimesitylsilyl potassium affords the three-coordinate Ni(I) silyl complex (dtbpe)Ni(SiHMes(2)) (2). Alternatively, 2 can be prepared by an oxidative-addition reaction of Mes(2)Si(H)OTf (Tf = CF(3)SO(3)) with the nickel(0) complex [(dtbpe)Ni](2)(mu-C(6)H(6)) (3), with (dtbpe)Ni(OTf) (4) formed as an easily separable byproduct. The one-electron oxidation of 2 by ferrocenium affords diamagnetic [(dtbpe)Ni(mu-H)SiMes(2)][BAr(F)(4)] (5), a Ni(II) complex formed by partial 1,2-H migration from silicon to nickel and featuring an unusual 3-center, 2-electron bonding motif between Ni, Si, and the bridging H.

Monomeric and dimeric disulfide complexes of nickel(II).

Elemental sulfur reacts with a bulky bis(phosphine)nickel(0) complex to give a monomeric nickel(II) eta(2)-disulfido complex, oxidation of which results in the elimination of sulfur with dimerization to give an eta(2),eta(2)-disulfidodinickel(II) derivative in which the S-S bond can be reductively cleaved in a redox-reversible fashion.

Group-transfer reactions of nickel-carbene and -nitrene complexes with organoazides and nitrous oxide that form new C=N, C=O, and N=N bonds.

1-Adamantyl- and mesitylazide react with (dtbpe)Ni=CPh(2) (1; dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) at ambient temperature to give the ketimines RN=CPh(2) (2a, R = Mes; 2b, R = Ad) in high yield. Kinetic studies for the reaction of 1 with N(3)Ad yield activation parameters of DeltaH(double dagger) = +8(+/-1) kcal/mol and DeltaS(double dagger) = -44(+/-3) cal/(mol.K).

Two-coordinate d9 complexes. synthesis and oxidation of NHC nickel(I) amides.

Reaction of the d9-d9 Ni(I) monochloride dimer, [(IPr)Ni(mu-Cl)]2 (1), with NaN(SiMe3)2 and LiNHAr (Ar = 2,6-diisopropylphenyl) gives the novel monomeric, 2-coordinate Ni(I) complexes (IPr)Ni{N(SiMe3)2} (2) and (IPr)Ni(NHAr) (3). Reaction of 2 with Cp2Fe+ results in its 1-e- oxidation followed by beta-Me elimination to give a base-stabilized iminosilane complex [(IPr)Ni(CH3){kappa1-N(SiMe3)=SiMe2.Et2O}][BArF4] (6).

Eta2-organoazide complexes of nickel and their conversion to terminal imido complexes via dinitrogen extrusion.

1-Adamantyl- and mesitylazide react with [(dtbpe)Ni]2(eta2-mu-C6H6) to give the eta2 organic azide adducts (dtbpe)Ni(eta2-N3R) (R = Ad, 3a; Mes, 3b) that have been isolated in good yields and crystallographically characterized. These azide adducts are intermediates in the formation of the corresponding terminal imido complexes (dtbpe)NiNR (R = Ad, 4a; Mes, 4b), undergoing intramolecular loss of dinitrogen upon mild thermolysis.

Synthesis and characterization of side-bound aryldiazo and end-bound nitrosyl complexes of nickel.

Structural characterization of a nickel aryldiazo complex supported by the bulky 1,2-bis(di-tert-butylphosphino)ethane ligand reveals square-planar Ni geometry and an unusual side-on coordination mode for the N2R moiety, while the related nitrosyl complex displays trigonal-planar geometry at Ni and end-on coordination of the NO ligand.

Preparation of stable alkyl complexes of Ni(I) and their one-electron oxidation to Ni(II) complex cations.

The three-coordinate nickel(I) alkyl complexes (dtbpe)Ni(CH2CMe3) (2), (dtbpe)Ni(CH2SiMe3) (3), and (dtbpe)Ni(CH2CMe2Ph) (4) have been prepared by treatment of [(dtbpe)NiCl]2 with alkyllithium reagents. While thermally robust, they each undergo mild one-electron oxidation to give the corresponding Ni(II) complex cations [(dtbpe)Ni(CH2CMe3)+] (5), [(dtbpe)Ni(CH2SiMe3)+] (6), and [(dtbpe)Ni(CH2CMe2Ph)+] (7) as red-brown [PF6-] or [BArF4-] salts. In contrast to cationic amido and phosphido analogues that undergo alpha-deprotonation to afford imido and phosphinidene derivatives, deprotonation of 5-7 occurs at a gamma-CH3 group to give metallacyclobutane products (dtbpe)Ni(CH2CMe2CH2) (8), (dtbpe)Ni(CH2SiMe2CH2) (9), and (dtbpe)Ni(CH2CPhMeCH2) (10), not (dtbpe)Ni=CHR.

Group transfer from nickel imido, phosphinidene, and carbene complexes to ethylene with formation of aziridine, phosphirane, and cyclopropane products.

The reactions of nickel complexes bearing terminal imido, phosphinidene, and carbene ligands with ethylene are reported. In all three cases, corresponding three-membered rings, aziridine, phosphirane, and cyclopropane, were produced in moderate to excellent yields. NMR spectra of the phosphinidene (dtbpe)Ni=P(dmp) reaction with ethylene show a [2+2] cycloaddition adduct before phosphirane formation.

A new route to coordination complexes of nitroxyl (HN=O) via insertion reactions of nitrosonium triflate with transition-metal hydrides.

Nitrosonium triflate reacts with cold methylene chloride solutions of mer,trans-ReH(CO)3(PPh3)2 (1) with 1,1-insertion of NO+ into the Re-H bond to give the orange nitroxyl complex [mer,trans-Re(NH=O)(CO)3(PPh3)2][SO3CF3] (3) in 86% isolated yield. Use of [NO][PF6] or [NO][BF4] gives analogous insertion products at low temperature, which decompose on warning to ambient temperature to the fluoride complex mer,trans-ReF(CO)3(PPh3)2 (4). A related 1,1-insertion is observed in the reaction of 1 with [PhN2][PF6] in acetone that affords the yellow-orange phenyldiazene salt [mer,trans-Re(NH=NPh)(CO)3(PPh3)2][PF6] (2), which has been characterized by X-ray crystallographic methods.

Isocyanate and carbodiimide synthesis by nitrene-group-transfer from a nickel(II) imido complex.

The imido complex (dtbpe)Ni(N(2,6-(CHMe2)2C6H3)) reacts with CO and CNCH2Ph with addition at the Ni-N bond to give (dtbpe)Ni(C,N:eta 2-C(O)N(2,6-(CHMe2)2C6H3)) and (dtbpe)Ni(C,N:eta 2-C(NCH2Ph)N(2,6-(CHMe2)2C6H3)); both complexes react further with CO to liberate the isocyanate and carbodiimide ligands with formation of (dtbpe)Ni(CO)2.

Synthesis, structure, and reactions of a three-coordinate nickel-carbene complex, [1,2-Bis(di-tert-butylphosphino)ethane]Ni=CPh(2).

The three-coordinate nickel-carbene complex (dtbpe)Ni=CPh2 (3) was prepared from the thermolysis of the diphenyldiazoalkane complex (dtbpe)Ni(N,N':eta2-N2CPh2) (2) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane). Complex 3 was structurally characterized by single-crystal X-ray diffraction methods (Ni-C = 1.836(2) A).