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Substrate Redox Non-Innocence Inducing Stepwise Oxidative Addition Reaction: Nitrosoarene C-N Bond Cleavage on Low-Coordinate Cobalt(0) Species Dongyang Wang, Xuebing Leng, Shengfa Ye, and Liang Deng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03726 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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Journal of the American Chemical Society

Substrate Redox Non-Innocence Inducing Stepwise Oxidative Addition Reaction: Nitrosoarene C-N Bond Cleavage on LowCoordinate Cobalt(0) Species Dongyang Wang,† Xuebing Leng,† Shengfa Ye,‡ and Liang Deng*† †State

Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China ‡Max-Planck-Institut

für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr D-45470, Germany

Supporting Information Placeholder ABSTRACT: The reactions of nitrosoarenes with transitionmetal species are fundamentally important for their relevance to metal-catalyzed transformations of organo-nitrogen compounds in organic synthesis and also the metabolization of nitroarenes and anilines in biology. In addition to the well-known reactivity of metal-mediated N-O bond activation and cleavage of nitrosoarenes, we present herein the first observation of a nitrosoarene C-N bond oxidative addition reaction on the interaction of a three-coordinate cobalt(0) species [(IPr)Co(vtms)2] with 2,4,6-tri(t-butyl)-1-nitroso-benzene (Ar*NO). The reaction produces a cobalt nitrosyl aryl complex [(IPr)Co(Ar*)(NO)] (1) with a bis(nitrosoarene)cobalt complex [(IPr)Co(η2-ONAr)(κ1-O-ONAr)] (2) as an intermediate. Spectroscopic characterizations, DFT calculations and kinetic studies revealed that the redox-noninnocence of nitrosoarene induces a stepwise pathway for the C-N bond oxidative addition reaction.

reactivities serve as the basis of the synthetic utility of nitrosoarenes as oxygen- and/or nitrogen-sources in transitionmetal-catalyzed reactions.

Chart 1. Modes of the Bond Cleavage Reaction of Nitrosoarene Mediated by Transition-Metals + [M] O

N=O cleavage

N

+ [M]

Ar

C-N cleavage

Cotton, Warren, et al.

Ar

[M]

N O

this study

Scheme 1. Reactions of [(IPr)Co(vtms)2] with Ar*NO Dipp N

Nitrosoarenes (ArNO) have found wide applications in modern synthetic chemistry and material science, e.g. as synthetic reagents in organic synthesis, as spin-traps in electron spin resonance spectroscopy, and as building-blocks for covalent organic networks.1-6 In biology, nitrosoarenes are the reactive metabolites of nitroarenes and anilines, and their chemistry toward biomolecules is among the great interests of investigation.2, 5, 7-8 Along with these interests is the fundamental question as to the reactions of nitrosoarenes with transition-metal species, which has been subjected to extensive exploration for its relevance to biology and catalysis.9-11 The known studies have established that nitrosoarenes can readily bind to transition-metal species to form nitrosoarene complexes via their nitroso groups in different coordination modes including κ1-O-ONAr, κ1-N-ONAr, or η2-ONAr.12-23 Nitrosoarene ligands proved redox-noninnocent, and can exist in different valence states, viz. [ArNO]0, [ArNO]•1or [ArNO]2-.13, 24-25 Interestingly, the interaction of nitrosoarenes with transition-metal complexes might also lead to the cleavage of their N-O bonds (Chart 1). Examples of the latter conversion include the isolation of a bridging oxo complex [W2(μ-O)(μOBut)2(OBut)4(NPh)2] from the reaction of W2(OBut)6 with PhNO,26 and the attainment of [((MeCN(C6H3-2’,6’Me2))2CH)2Co2(μ-O)(μ-NC6H3-2’,6’-Me2)] from the interaction of an β-diketiminato cobalt(I) complex [((MeCN(C6H3-2’,6’Me2))2CH)Co(η6-toluene)] with 2,6-Me2C6H3NO.27 These

[MO] + [M(NAr)]

C

N Dipp

Co0 Me3Si

Dipp N

+ 2 Ar*NO toluene, 80 oC - 2 vtms

C

N Dipp

CoII *Ar

SiMe3

N

O

1 Dipp N

+ 2 Ar*NO O n-hexane, r.t. N - 2 vtms *Ar

C

N Dipp

CoII 2

toluene O N

Ar*

80 oC - Ar*NO

Ideally, nitrosoarenes might react with transition-metal species to undergo C-N bond oxidative addition reaction, giving transition-metal nitrosyl complexes (Chart 1). This type of reaction resembles the S-N bond cleavage reactions of S-nitroso compounds with copper complexes,28-29 but is unprecedented for nitrosoarenes. Bergman’s alkene exchange reaction of a dinitroso cobalt complex CpCo(η2:η2-ONCRHCRHNO) with an alkene R’HC=CHR’ to afford a new dinitroso cobalt complex CpCo(η2:η2-ONCR’HCR’HNO) and RHC=CHR involves C-N bond-cleavage and -formation.30 However, theoretical study suggests a cyclo-addition mechanism for this reaction.31 In this regard, we reported herein the first observation of nitrosoarene CN bond oxidative addition reaction on the reaction of a threecoordinate cobalt(0) complex [(IPr)Co(vtms)2] (IPr = 1,3bis(2’,6’-diisopropylphenyl)imidazole-2-ylidene; vtms =

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vinyltrimethylsilane) with 2,4,6-tri(t-butyl)-1-nitroso-benzene (Ar*NO). The reaction produces the first three-coordinate cobalt nitrosyl complex [(IPr)Co(Ar*)(NO)] via a cobalt bis(nitrosoarene) intermediate [(IPr)Co(η2-ONAr*)(κ1-OONAr*)]. Associated with the redox-noninnocent nature of these NO-based ligands, spectroscopic and mechanistic studies revealed a stepwise mechanism for this C-N bond oxidative addition reaction. Treatment of [(IPr)Co(vtms)2]32 with the nitrosoarene 2,4,6tri(t-butyl)-1-nitroso-benzene (Ar*NO, two equiv.)33 in toluene at 80 oC gave a green solution. After workup and recrystallization, green crystals of a cobalt nitrosyl aryl complex [(IPr)Co(Ar*)(NO)] (1) were isolated in 46% yield (Scheme 1). Complex 1 has been characterized by various spectroscopic methods and its structure has been unambiguously established by single-crystal X-ray diffraction study (vide infra). In addition to 1, unreacted Ar*NO and a byproduct vinyltrimethylsilane were also evidenced in the resultant reaction mixture as indicated by 1H NMR analysis. The use of excess amount of Ar*NO seems crucial to ensure the good yield of 1 as the equimolar reaction of [(IPr)Co(vtms)2] with Ar*NO at 80 oC merely produce 1 in 10% yield. The low yield of 1 in the 1:1 stoichiometry reaction could be due to side reaction as unidentified paramagnetic species was detected by 1H NMR analysis on the reaction mixture.

structure of 1 established by XRD suggests that the S = 1/2 state lies 16.3 kcal/mol lower in energy than the S = 3/2 state. Corresponding orbital analyses44 on the doublet ground state indicated that the SOMO (singly occupied molecular orbital) and the two α-spin components of the antiferromagnetically coupled orbitals are essentially metal-based, thus defining a high-spin Co(II) ion (Figure S3). The two β-spin components of the antiferromagnetically coupled orbitals possess predominant NOπ* character, indicative of the SNO = 1 formulation of the NOligand. This Co-NO bonding interaction is also similar to that in [(Tp*)Co(NO)].38 Magnetic property study on the solid sample of 1 corroborate this assignment (Figure S25). As mentioned earlier, the interaction of nitrosoarenes with transition-metal species usually leads to the activation of the N-O bonds, and nitrosoarene C-N bond cleavage mediated by transition-metal species has remained unknown.2, 9, 11 The production of the cobalt(II) nitrosyl complex 1 from the reaction of [(IPr)Co(vtms)2] with Ar*NO thus represents the first example of the type. Noting that light irradiation can facilitate C-N bond cleavage reaction of nitrosoarenes,45-47 the control reaction of [(IPr)Co(vtms)2] with Ar*NO at 80 oC in dark was performed, and proved still to produce 1 in moderate yield. On the other hand, irradiating Ar*NO with a high-pressure mercury lamp or a fluorescent lamp for hours did not result in detectable decomposition of the nitrosoarene. These observations imply that light-irradiation does not play a role in the nitrosoarene C-N bond cleavage reaction for the production of 1, and the C-N bond cleavage step should be mediated by a low-coordinate cobalt(0) species. In accord with this presumption, a cobalt nitrosoarene complex [(IPr)Co(η2-ONAr*)(κ1-O-ONAr*)] (2) has been isolated from the reaction of [(IPr)Co(vtms)2] with Ar*NO (two equiv.) at room temperature, and its thermal decomposition reaction in toluene was found to furnish 1 in 72% yield along with Ar*NO (Scheme 1).

Figure 1. Molecular structure of [(IPr)Co(NO)(Ar*)] (1). Characterization data and theoretical studies suggest that 1 is a high-spin cobalt(II) (SCo = 3/2) species bearing an antiferromagnetically coupled triplet NO- anion (SNO = 1). As a rare example of metal nitrosyl complexes with an EnemarkFeltham notion of {M(NO)}9, 34-37 the molecular structure of 1 established by single-crystal X-ray diffraction study (Figure 1) shows a long N-O distance of 1.198(5) Å and a bent Co-N-O angle of 135.9(4) deg, which imply the anionic nature of the nitrosyl ligand ([NO]-).34, 38 The Co-C(aryl) and Co-C(carbene) distances (2.046(4) and 2.020(3) Å, respectively) of 1 are also found comparable to those of their counterparts in threecoordinate high-spin cobalt(II) aryl 39-40 and cobalt(II)-Nheterocyclic carbene (NHC) complexes.41-43 The X-band EPR spectrum of a toluene solution of 1 recorded at 298 K displays a well-resolved 59Co (I = 7/2) nuclear hyperfine splitting and can be nicely simulated as an Stotal = 1/2 system with the g-values of g1 = 2.53, g2 = 2.25, g3 = 1.95 and 59Co nuclear hyperfine constants of A_Co1 = 145 MHz, A_Co2 = 345 MHz, and A_Co3 = 51 MHz (Figure S12). The EPR spectrum is similar to that of [(Tp*)Co(NO)] (Tp* = hydro-tris(3,5-Me2-pyrazolyl)borate), which is also described as a Co(II) (SCo = 3/2) metal center antiferromagnetically coupled to a triplet NO- anion (SNO = 1).38 DFT calculations at the B3LYP level based on the molecular

Figure 2. Molecular structure of [(IPr)Co(η2-ONAr*)(κ1-OONAr*)] (2). Single-crystal X-ray diffraction study established the structure of 2 as a three-coordinate bis(nitrosoarene)cobalt complex, in which the two nitrosoarene ligands bond to the metal center in two different fashions, η2-ONAr* and k1-O-ONAr* (Figure 2). The N-O bonds of the Ar*NO ligands (1.366(3) and 1.314(2) Å for the η2-ONAr* and k1-O-ONAr* ligands, respectively) are longer than the N=O bonds of free C-nitroso compounds (ca. 1.23 Å).48 As compared to the known nitrosoarene complexes, the distances of the N-O bonds in 2 are shorter than those of the dianionic ligands [η2-ONR]2- in the metallaoxaaziridines [(PPh3)2Pt(η2-ONPh)] (1.410(7) Å) 23 and [Cp*Mo(η2ONCH2SiMe3)(CH2SiMe3)(O)] (1.447(2) Å),49 and are more close to the ones of the radical anionic ligands [η2-ONAr]•1- in [(MeCN(C6H3-2’,6’-Me2)2CH)Cu(η2-ONPh)] (1.333(4) Å) 16 and [((F3CCNDipp)2CH)Ni(η2-ONPh)] (1.325(2) Å).13 More intriguingly, structure distortion was observed on the η2-ONAr*


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Journal of the American Chemical Society ligand as its nitrogen atom (N2) and the vicinal carbon atom (C46) are sitting above the plane defined by the other five arene carbon atoms of Ar* by 0.55 and 0.20 Å, respectively. In contrast, the nitrogen and carbon atoms of the arene ring in the k1-OONAr* ligand are coplanar to each other. To further probe the electronic structure of 2, spectroscopic and theoretical studies have also been performed. The results revealed that 2 at its the ground state is best described as a low-spin cobalt(II) complex (SCo = 1/2) bearing two radical anions [Ar*NO]•1-. The X-band EPR spectrum of 2 (Figure S15) shows an isotropic signal centered at giso = 2.00 consisting of three lines arising from a hyperfine coupling to a single 14N nucleus (Aiso(14N) = 33.21 MHz). The spectrum is similar to that reported for [((F3CCNDipp)2CH)Ni(η2-ONPh)•1-],13 and is characteristic for a ligand-centered S = ½ radical. Being consistent with the signal observed by the EPR spectroscopy, DFT calculations in combination with corresponding orbital analysis (Figure S6) suggest that 2 can be considered as an S = 1/2 Co(II) ion antiferromagnetically to a [κ1-O-ONAr*]•1- ligand, and the remaining unpaired spin is located on the [η2-ONAr*]•1- ligand. For this Stotal = 1/2 state, the cobalt center, and the NO moieties of the k1-O-ONAr* and η2-ONAr* ligands bear the corresponding spin populations of +1.23, -0.88, and +0.61 (Figure S7). As a consequence, the net spin density (Stol = 1/2) is predominantly located on the radical anion ligand [η2-ONAr*]•1-. It’s should be noted that, while EPR and DFT studies points out a ground spinstate of S = 1/2 for 2, magnetic susceptibility study on the solid sample of 2 (Figure S26) implies that the complex might possess low lying excited states.

Scheme 2. A Proposed Stepwise Mechanism for the Nitrosoarene C-N Bond Oxidative Addition Reaction with Cobalt(0) Complex 0

[(IPr)Co (vtms)2]

+ 2 Ar*NO - 2 vtms

II

[(IPr)Co (ONAr

*.1-

)2] (2)

- Ar*NO [(IPr)CoII(NO1-)(Ar*1-)] (1)

-elimination

[(IPr)CoII(ONAr*2-)] (A)

As spectroscopy and theoretical studies revealed that both 1 and 2 are cobalt(II) complexes bearing radical anionic ligands ([NO]•1and [ONAr*]•1- in 1 and 2, respectively), the C-N bond cleavage step in the conversion of 2 to 1 should be described as αelimination reaction (Scheme 2). α-Elimination reaction of Cnitroso complexes is the reversed process of migratory insertion of coordinated nitrosyl into metal-carbon bond to form C-nitrosometal complexes.50-52 While the reversed reaction is welldocumented in literature, to our knowledge, α-elimination reaction of C-nitrosyl metal complex to form metal nitrosyl complex is unknown. In order to gain more insight on the process, kinetic study on the thermal decomposition of 2 in C6D6 has been performed. Monitoring the decrease of the concentration of 2 in the temperature range of 50-80 oC over time revealed pseudofirst-order kinetics (Figure S9) with the measured rate constants spanning in the range 0.014 ± 0.001 to 0.30 ± 0.01 min-1 (Figure S10 and Table S2). An Eyring plot analysis gave the activation parameters ΔHǂ = 22 ± 1 kcal mol-1 and ΔSǂ = 2 ± 2 cal mol-1 K-1 (Figure S11). The small activation entropy accords with that of the α-elimination reaction of acyl cobalt species,53 and implies the unimolecular nature of the rate-determining step. Accordingly, we reason that the thermal decomposition of 2 might initially lead to the dissociation of one Ar*NO ligand to form a two-coordinate cobalt species (IPr)Co(ONAr*) (A). The low-coordinate and formal cobalt(0) nature of A should result in a cobalt(II)-based metallaoxaaziridine nature of the complex.23,38,49 An α-elimination

reaction of A then leads to C-N bond cleavage and produce 1, which should be the rate determining step (Scheme 2). Taken all together, the conversions from [(IPr)Co(vtms)2] to 2 and 1 has indicated a unique stepwise pathway of electron-transfer followed by α-elimination for the formally cobalt-mediated nitrosoarene CN bond oxidative addition reaction. The reaction pathway differs from the classical mechanisms of SN2,54 one-electron-transfer,55 and concerted oxidative addition reactions,56 and the redox noninnocent nature of nitrosoarene is apparently among the key causes. Accordingly, one could speculate that similar stepwise mechanism might operate in the reported C-C and C-N bond oxidative addition reactions57-58 of nitriles,59-60 and isocyanides,61 as these unsaturated substrates could be redox-innocent when meeting low-valent metal species. In summary, we found that the reaction of three-coordinate cobalt(0) complex [(IPr)Co(vtms)2] with nitrosoarene Ar*NO can yield a nitrosoarene C-N bond oxidative addition product [(IPr)Co(Ar*)(NO)] (1). This C-N bond cleavage reaction represents a new reaction mode of nitrosoarenes with transitionmetal species. Mechanistic study on the reaction revealed the involvement of a bis(nitrosoarene)cobalt intermediate [(IPr)Co(η2ONAr*)(κ1-O-ONAr*)] (2) en route to final C-N bond cleavage product 1. As electronic structure study indicated the cobalt(II) nature of 1 and 2, the C-N bond cleavage step of this cobaltmediated C-N bond oxidative addition reaction was found αelimination in character. Such a reaction pathway differs from many of the reported transition-metal-mediated oxidative addition reactions, and signifies the unique reactivity of redox-noninnocent substrates. We are now exploring the effect of the steric nature of aryl groups of nitrosoarenes and reaction stoichiometry on the reaction outcome of cobalt(0)-NHC complexes with nitrosoarenes.

ASSOCIATED CONTENT Supporting Information Experimental section, characterization data, and computational details. This material is available free of charge via the internet at http://xxxxxxx.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Mr. Zhaobo Hu and Prof. You Song at Nanjing University for the help on magnetic property study, Dr. Chao Zheng, Dr. M. Tamizmani, and Mr. Qi Chen at SIOC for their valuble suggestions on calculation. D.L. thanks the finacial support from the National Natural Science Foundation of China (Nos. 21725104, 21690062, 21432001, and 21821002), the National Key Research and Development Program (2016YFA0202900), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB20000000), and the Program of Shanghai Academic Research Leader (19XD1424800). S.Y. gratefully acknowledges the financial support from the Max-Planck Society, in particular, the joint work space between MPI-CEC and MPI-KOFO.

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TOC Substrate Redox Non-Innocence Inducing Stepwise Oxidative Addition Reaction: Nitrosoarene C-N Bond Cleavage on Low-Coordinate Cobalt(0) Species


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Substrate Redox Non-Innocence Inducing Stepwise Oxidative

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