Gregory L. Hillhouse

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. Reaction of 2 with CO resulted in nitrene-group transfer to form (dmp)NCO and (IPr*)Ni(CO)3 (3). Net C−H insertion was observed in the reaction of 2 with ethene, forming the vinylamine (dmp)NH(CH═CH2) (5) via an azanickelacyclobutane intermediate, (IPr*)Ni{N,C:κ2-N(dmp)CH2CH2} (4).

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). Treatment of 1 with N(2)O at low temperature results in clean conversion to the benzophenone complex (dtbpe)Ni(eta(2)-OCPh(2)) (5) upon elimination of N(2). The nickel-imido complexes (dtbpe)Ni=NR (4a, R = Mes; 4b, R = Ad) react with N(3)Mes and N(3)Ad at ambient temperature to give the diazenes RN=NR (6a, R = Mes; 6b, R = Ad) in good yield. B3LYP/6-311+G(d) calculations support a mechanism for all three reactions that features 1,3-dipolar cycloaddition to give five-membered ring (Huisgen) intermediates, followed by N(2) elimination to give the products. Calculated activation parameters for the reaction of (dhpe)Ni=CH(2) (dhpe = 1,2-bis(dihydridophosphino)ethane) with N(3)Me compare well with the experimental values.

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.

Oxygen-atom transfer from nitrous oxide (NNO) to nickel alkyls. Syntheses and reactions of nickel(II) alkoxides☆

Abstract The dialkyl nickel complexes (bipy)Ni(R) 2 )( 1 R Et; 2 , R  iso -Bu; bipy  2,2′-bpyridine) react with nitrous oxide (1 atm) to generate nickel(II) alkoxy alkyl complexes (bipy)Ni(OR)(R) ( 3 , R  Et; 4 , R  iso -Bu), which in turn undergo CO-insertion followed by reductive elimination under a carbon monoxide atmosphere to afford the corresponding esters (ethyl propionate from 3 ; 2-methylbutanoic acid-2-methylpropyl ester from 4 ). Nitrous oxide reacts with the metallacyclopentane complex (bipy)Ni{(CH 2 ) 4 } ( 5 ) to give the oxametallacycle (bipy)Ni{O(CH 2 ) 3 CH 2 } ( 6 ) and with (phen) Ni{(CH 2 ) 4 } ( 7 ; phen  1,10-phenanthroline) to give (phen) Ni{O(CH 2 ) 3 CH 2 } ( 8 ). Complex 6 undergoes elimination reactions with HCl to give 1-butanol, with I 2 to give tetrahydrofuran, and with CO to give δ-valerolactone. The α-methyl substituted metallacycle (bipy)Ni {(CH 2 ) 3 CHCH 3 } ( 9 ) reacts with N 2 O in a regioselective fashion (at the more highly substituted NiC bond) to give (bipy) Ni{OCH(CH 3 )CH 2 CH 2 CH 2 } ( 10 ) exclusively. Complex 10 undergoes elimination reactions with HCl to give 2-pentanol, with I 2 to give 2-methyl-tetrahydrofuran, and with CO to give δ-caprolactone. The β,β′-dimethyl substituted metallacycles, meso -(bipy)Ni{CH 2 CH(CH 3 )CH(CH 3 )CH 2 } ( 11 ) and d,l -(bipy)Ni{CH 2 CH (CH 3 )CH(CH 3 )CH 2 } ( 12 ) react with N 2 O to produce the oxametallacycles (bipy)Ni{ cis -OCH 2 CH(CH 3 )CH(CH 3 )CH 2 }( 13 ) and (bipy)Ni{ trans -OCH 2 CH(CH 3 )CH(CH 3 )CH 2 } ( 14 ), respectively. Complex 13 undergoes elimination reactions with HCl to give 2,3-dimethyl-l-butanol, with I 2 to give cis -3,4-dimethyl-tetrahydrofuran, and with CO to give tetrahydro- cis -4,5-dimethyl-2 H -pyran-2-one. The metallacycles (bipy)Ni{CH 2 C(CH 2 )C(CH 2 )CH 2 } ( 15 ) and (bipy)Ni{(CH 2 ) 2 C 6 H 4 } ( 16 ), both possessing sp 2 -hybridized ring carbons in the β- and β′-positions, show no reactivity toward N 2 O. However, 16 reacts with HCl to afford o -xylene, with CH 2 Br 2 to afford indan, and with maleic anhydride to afford tetrahydronaphthalene anhydride.

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. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

η2-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.

Reactions of CO2 and CS2 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. Oxidation of 2 by ferrocenium hexafluorophosphate affords the identical trimetallic complex dication as the PF(6)(-) salt, [{(dtbpe)Ni(κ(1),η(2)-CS(2))}(2)(dtbpe)Ni][PF(6)](2) (3-PF(6)). These results are consistent with the intermediacy of a Ni(I)-CS(2) complex, [(dtbpe)Ni(CS(2))(+)], that is unstable with respect to disproportionation. Reaction of 1 with one equivalent of CO(2) provides the carbon dioxide adduct (dtbpe)Ni(η(2)-CO(2)) (4), that was also crystallographically characterized. Thermolysis of 4 in benzene solution at 80 °C results in reduction of the CO(2) ligand to CO, trapped as (dtbpe)Ni(CO)(2), and partial oxidation of a dtbpe ligand to give O═P(tert-Bu)(2)CH(2)CH(2)P(tert-Bu)(2).

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). Oxidation of 3 gives [(IPr)Ni(eta3-NHAr)(THF)][BArF4] (4), which upon loss of THF affords dimeric [(IPr)Ni(N,eta3:NHC6iPr2H3)]2[BArF4]2 (5).

Isocyanate and carbodiimide synthesis by nitrene-group-transfer from a nickel(ii) imido complexElectronic supplementary information (ESI) available: complete experimental, spectroscopic, analytical, and crystallographic details. See http://www.rsc.org/suppdata/cc/b2/b204846a/

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 of 1,2-bis(di-tert-butylphosphino)ethane (dtbpe) complexes of nickel: radical coupling and reduction reactions promoted by the nickel(I) dimer [(dtbpe)NiCl]2

Abstract Tetrahydrofuran solutions of (dtbpe)NiCl2 (dtbpe=1,2-bis(di-tert-butylphosphino)ethane) are reduced by KC8 to afford the dimeric Ni(I) complex [(dtbpe)NiCl]2 (1) in 73% yield. Reaction of 1 with [FeCp2][PF6] effects a one-electron oxidation to give the mixed-valent Ni(I,II) binuclear species [{(dtbpe)NiCl}2][PF6], [1][PF6]. Complex 1 reacts with the radicals NO and TEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxyl) to give the diamagnetic Ni(0) nitrosyl (dtbpe)Ni(Cl)(NO) (2) and the Ni(II) complex (dtbpe)Ni(Cl)(O,N:η2-TEMPO) (3). Reduction of the SS bond of diphenyldisulfide by 1 results in formation of the Ni(II) arylthiolate complex (dtbpe)NiCl(SPh) (4). The radical anions NaOCPh2 and KN2Ph2 react cleanly with 1 to afford (dtbpe)Ni(η2-OCPh2) (5) and (dtbpe)Ni(η2-N2Ph2) (6). Phenylacetylide CC bond coupling is effected by reaction of 1 with LiCCPh to form [(dtbpe)Ni(C,C′:η2-CCPh)]2 (7). In addition to standard spectroscopic (IR, NMR, EPR) and magnetic measurements, complexes 1, [1][PF6], 3, and 5 were also characterized by single-crystal X-ray diffraction methods.