Ruthenium(II) Porphyrin Quinoid Carbene Complexes: Synthesis, Crystal Structure, and Reactivity toward Carbene Transfer and Hydrogen Atom Transfer Reactions

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Article

Ruthenium(II) Porphyrin Quinoid Carbene Complexes:

Synthesis, Crystal Structure and Reactivity Towards

Carbene Transfer and Hydrogen Atom Transfer Reactions

Hai-Xu Wang, Qingyun Wan, Kai Wu, Kam-Hung Low, Chen

Yang, Cong-Ying Zhou, Jie-Sheng Huang, and Chi-Ming Che

J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 08 May 2019

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Ruthenium(II) Porphyrin Quinoid Carbene Complexes: Synthesis,

Crystal Structure and Reactivity Towards Carbene Transfer and Hy-

drogen Atom Transfer Reactions

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Hai-Xu Wang,^†Qingyun Wan,^†Kai Wu,^†Kam-Hung Low,^†Chen Yang,^†Cong-Ying Zhou,^†Jie-Sheng

Huang,^†,^*and Chi-Ming Che^†,‡,

*

^†State Key Laboratory of Synthetic Chemistry and Department of Chemistry, The University of Hong Kong, Pokfulam

Road, Hong Kong SAR, China

^‡HKU Shenzhen Institute of Research & Innovation, Shenzhen, China

ABSTRACT: Reactivity study of novel metal carbene complexes can offer new opportunities in catalytic carbene transfer reactions as

well as in other synthetic protocols. Metal complexes with quinoid carbene (QC) ligands are assumed to be key intermediates in a vari-

ety of metal-catalyzed QC transfer reactions using diazo quinones, which demands development of the chemistry of QC transfer of

well characterized metal-QC complexes. Herein we report the isolation and QC transfer of ruthenium porphyrins [Ru(Por)(QC)] which

contribute the first examples of (i) structurally characterized metal-QC complex (by X-ray crystallography) and (ii) isolated metal-QC

complex that undergoes QC transfer reaction. The complexes [Ru(Por)(QC)] were prepared from reaction of [Ru(Por)(CO)] with diazo

quinones and exhibited dual reactivity, i.e. hydrogen atom transfer (HAT) as well as QC transfer. The stoichiometric QC transfer reac-

tions from these Ru-QC complexes to nitrosoarenes (ArNO) afforded nitrones in up to 90% yield, and the corresponding catalytic reac-

tions were also developed. Both the stoichiometric and catalytic reactions for a series of QC ligands bearing electron-donating and -

withdrawing substituents showed a reverse substituent effect on the QC transfer reactivity. Complexes [Ru(Por)(QC)] are also reactive

towards C–H and X–H (X = N, S) bonds and can catalyze aerobic oxidation of 1,4-cyclohexadiene; their stoichiometric HAT reactions

with unsaturated hydrocarbons gave product yields of up to 88%. The unique dual reactivity and electronic feature of [Ru(Por)(QC)]

were studied by spectroscopic means and density functional theory (DFT) calculations.

plexes may exhibit unique properties/reactivities compared

with other carbenes. Milstein and co-workers isolated and

■ INTRODUCTION

a

spectroscopically characterized a ruthenium-QC complex¹¹

Isolation and reactivity studies of metal carbene complexes

are fundamental to the rapid development and mechanism

and subsequently an iron counterpart,^11band describe the two

complexes as metallaquinones, without observation of QC

transfer to organic substrates (Figure 1a, inset). In addition, the

two metallaquinones, termed ruthenaquinone and ferraquinone,

were not generated from reactions with diazo quinones; for-

mation of both the ruthenaquinone and ferraquinone requires

chelating auxiliary groups and terminal CO co-ligands, and

reactivity studies were performed to the ferraquinone

revealing its reactions with H₂, Br₂, alcohols and acid to form

ferrahydroquinones.^11bOn the other hand, the proposed metal-

QC intermediates in the catalytic reactions^5,7-9have not been

isolated nor directly detected by spectroscopic means.¹²Thus,

challenges remain to address the following issues about metal-

QC complexes: (i) clear observation of their formation from

reaction with diazo quinones, (ii) examination of their intrinsic

stability or isolation of their examples devoid of chelating

auxiliary group(s), (iii) elucidation of their structure features

by X-ray crystallographic studies, and (iv) exploration of their

novel reactivities, particularly the possibility of QC transfer.

In the present paper, we aim to address these issues by isola-

tion of ruthenium porphyrin QC complexes [Ru(Por)(QC)]

from reaction of [Ru(Por)(CO)] (Por = porphyrinato dianion)

,

elucidation of metal-carbene-based methodologies^{1 2}such as

metal-catalyzed carbene transfer reactions which have enjoyed

wide applications in practical organic synthesis. A number of

isolated metal carbene complexes that undergo carbene trans-

,

fer reactions have been reported. ³These complexes bear

alkoxy, alkoxycarbonyl or aryl carbenes such as C(Ar)CO₂R

and CAr₂. Conjugated carbenes based on carbocyclic back-

bone have been attracting increasing attention in metal-

catalyzed carbene transfer reactions. Notable examples include

Rh-catalyzed alkene cyclopropanation recently reported by

Baran and co-worker⁵as well as a variety of catalytic process-

4

, , ,

es using diazo quinones (also called quinone diazides),^{6 7 8 9}the

key intermediates in which are generally proposed to be metal

complexes of the corresponding carbene, herein called quinoid

carbene (QC). These proposed M-QC species in the catalytic

processes were assumed to undergo QC transfer to various

organic targets including C–H bond,⁷alkene,^5,8O–H bond,^9a,f

anhydride,^9benol ether,^9ccyclic ether,^9dand halide^9e(Figure

1a). Quinoid carbenes (QCs), previously named oxocyclohex-

adienylidenes ¹⁰or cyclohexadienone carbenes,⁵are electro-

philic and feature delocalized -system;¹⁰their metal com-

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dialkylcarbenes ¹⁶catalyzed by metalloporphyrins. Also, we

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used the same type of QC source as that in the catalytic pro-

cesses,^5,7-9i.e. diazo quinones. Diazo compounds are well doc-

umented to coordinate with metal ions;¹⁷however, previous

reports on the stoichiometric reactions of metal complexes

with diazo quinones are rare,¹⁸which resulted in the formation

of diazo quinone complexes of rhodium(I)^18band iridium(I)^18a

without observation of the corresponding metal-QC complexes.

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Scheme 1. Syntheses of Ruthenium Porphyrin Quinoid

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Carbene Complexes 1–4

Figure 1. (a) Literature reported examples of “metal catalyst +

diazo quinone” systems for catalytic quinoid carbene (QC) transfer

to various organic groups/substrates via proposed metal-QC inter-

mediates.^59Inset: Spectroscopically observed metallaquinones

reported by Milstein and co-workers.¹¹(b) Structurally character-

ized ruthenium porphyrin QC complexes reported in this work,

which are formed from reaction with diazo quinones and exhibit

QC transfer and hydrogen atom transfer (HAT) dual reactivity.

Interestingly, treatment of [Ru(Por)(CO)] with diazo qui-

nones N₂QC^R(2,6-disubstituted 4-diazocyclohexa-2,5-

dienones) in CH₂Cl₂at 40 °C or room temperature afforded

Ru-QC complexes [Ru(Por)(QC^R)] (Por = TPP, 1a–d; TMP,

2a–d; TPFPP, 3a; TDCPP, 4) in 35–92% yields (reaction 1,

Scheme 1). While 1b–d were only stable in solutions for ~4 h

at room temperature, the other complexes generally exhibited

good stability and showed no sign of decomposition in solu-

tions at room temperature for around one week. Other types of

diazo quinones, such as 4-diazonaphthalen-1(4H)-one

(N₂QC^Naph), can also react with [Ru(Por)(CO)] to afford Ru-

QC complexes. For example, treatment of [Ru(TPFPP)(CO)]

with diazo quinones, along with structure characterization,

reactivity, catalytic, and mechanistic studies (Figure 1b), in-

cluding DFT calculations. The Ru-QC complexes isolated in

this work have been characterized by X-ray crystal structure

determination, as well as by spectroscopic means; strikingly,

they exhibit a dual reactivity, i.e. QC transfer as well as qui-

none-like hydrogen atom transfer (HAT), and their QC trans-

fer reactions with a nucleophile was retarded by electron-

withdrawing substituent (but accelerated by electron-donating

substituent) on the carbene ligand, which is an “inverted” elec-

trophilicity/reactivity order compared with that reported previ-

with N₂QC^Naphin CH₂Cl₂at 40 C gave [Ru(TPFPP)(QC^Naph)]

(3b, 32% yield, reaction 2 in Scheme 1), which is only stable

for ~1 d in solution.

X-ray Crystal Structures. Slow evaporation of a hexane-

AcOEt solution of 1a, layering MeOH over a CH₂Cl₂solution

o

ously for free QCs^10aand isolated metal complexes of electro-

,

philic carbenes.^4e,g,

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■ RESULTS

of 2d, or slow evaporation of

a

hexane-CH₂Cl₂-

ⁿBuOCH₂CH₂OH solution of 3a gave diffraction-quality crys-

tals of 1a·AcOEt, 2d·MeOH, or 3a·ⁿBuOCH₂CH₂OH, respec-

tively, the structures of which were determined by X-ray crys-

tallography (Figure 2). These Ru-QC complexes each adopt an

octahedral coordination geometry with a sixth O-donor ligand

(AcOEt, MeOH, or ⁿBuOCH₂CH₂OH for 1a, 2d, or 3a, respec-

tively).

Synthesis. In exploration of a synthetic route to a metal

complex with non-chelating QC ligand (unlike the pincer-type

chelating QC ligand in the metallaquinones reported by Mil-

stein and co-workers¹¹), we focused on porphyrin auxiliary

ligand systems in view of their ability to support isolable metal

complexes with other types of carbenes including C(Ar)CO₂R

4,

and CAr₂¹⁵along with transfer of such carbenes^1f,k,r,15and

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respectively) are shorter than that in 1a (2.358(10) Å), possi-

bly reflecting reduced trans influence of the carbene ligands in

2d and 3a. For all of 1a, 2d, and 3a, their Ru–C_carbenedistances

(1.841(9)–1.922(8) Å) are significantly shorter than the Ru–

C_aryldistance (2.121(6) Å) in the zwitterionic Ru-aryl counter-

part of the ruthenaquinone (Figure 3, inset C).^11a

The coordinated QC ligands in 1a, 2d, and 3a feature a cy-

clohexadienone moiety similar to that of related quinones, as

revealed by comparison with the X-ray crystal structures of

two di(tert-butyl)-substituted 1,4-benzoquinones²¹(Figure 3,

insets A and B). The quinoid structures of the QCs in 1a, 2d,

and 3a contrast with the benzenoid structure in the zwitterion-

ic Ru-aryl counterpart of the ruthenaquinone (Figure 3, inset

C).^11aWorthy of note is that the C=O group of each QC ligand

is co-planar with the carbene plane, unlike the cases of com-

mon α-carbonyl carbenes such as C(Ar)CO₂R in which the

C=O group tilts ~60° away from the carbene plane^20a,c(see, for

example, Figure 3, inset D, and also the results from DFT cal-

culations).

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Figure 3. Bond distances (Å) for the Ru-QC moieties in the crys-

tal structures of (a) 1a·AcOEt, (b) 2d·MeOH, and (c)

3a·ⁿBuOCH₂CH₂OH. Insets: bond distances (Å) in the X-ray

crystal structures of two related quinones (A and B; standard er-

rors not available for A; only the average C=O, C–C, C=C dis-

tances are reported for B)²¹and selected bond distances (Å) in the

X-ray crystal structures of zwitterionic form of the ruthenaqui-

none^11a(C) and [Ru(TPFPP)(C(Ph)CO₂Et)(MeOH)]^20c(D).

Figure 2. ORTEP diagrams of (a) 1a·AcOEt, (b) 2d·MeOH, and

(c) 3a·ⁿBuOCH₂CH₂OH with thermal ellipsoids at 30% probabil-

ity level. Hydrogen atoms are not shown.

The Ru–C_carbenedistance in [Ru(TTP)(QC^tBu)] (1a) is

1.841(9) Å, which is similar to the typical Ru–C_carbenedistances

previously reported for ruthenium porphyrins bearing

CHCO₂R, C(CO₂R)₂, C(COAr)₂, C(Ph)CO₂R, or CAr₂carbene

In the crystal structures of 1a, 2d and 3a, their QC carbene

planes make angles of ~29°, ~19°, and ~4°, respectively, with

the closest pair of diagonal Ru–N bonds (Figure S2; the corre-

sponding angles reported for ruthenium porphyrins bearing

other types of carbene ligands are ~2°–39° ^20c). The porphyrin

rings in 1a, 2d and 3a show slight saddle (1a) or ruffle (2d and

3a) distortions (Figure S2, inset) and the ruthenium atoms are

displaced from the mean porphyrin planes by 0.220, 0.140 and

0.165 Å, respectively.

,

ligands (1.806(3)–1.877(8) Å).^4b,f,

Compared with 1a,

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complexes [Ru(TMP)(QC^Br)] (2d) and [Ru(TPFPP)(QC^tBu)]

(3a) show longer Ru–C_carbenedistances of 1.922(8) and 1.915(3)

Å, respectively; this might be associated with the larger steric

hindrance of the TMP (see Figure S1 in the Supporting Infor-

mation) and TPFPP ligands and/or the electron-withdrawing

substituents in QC^Brand TPFPP (see Discussion section). The

Ru–O distances in 2d and 3a (2.264(6) and 2.2945(19) Å,

Spectral Features. Complexes 1–4 (in CDCl₃or CD₂Cl₂)

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V vs Fc^+/0for Por = TMP (2a), TTP (1a), and TPFPP (3a),

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exhibited well-resolved H NMR signals in the diamagnetic

region (see the Supporting Information). All the protons on

their QC ligands were shielded due to the ring current effect of

the porphyrin macrocycles (e.g. the signals of the C–H protons

 to carbene carbons appear at δ 0.72–2.05 ppm). The H_(pyr-

rolic protons) signals of their porphyrin ligands appear at δ

8.31–8.35 (TTP in 1a–d), 8.07–8.12 (TMP in 2a–d), 8.35–

8.42 (TPFPP in 3a,b), and 8.16 ppm (TDCPP in 4), similar to

those of other ruthenium porphyrin carbene complexes (e.g. δ

8.36, 8.13, 8.32, 8.27 ppm for [Ru(TTP)(CPh₂)],

[Ru(TMP)(CPh₂)], [Ru(TPFPP)(CPh₂)], [Ru(TDCPP)(CPh₂)],

respectively²⁰).

respectively, and [Ru(TMP)(QC^R)] gave E_O1values (V vs Fc^+/0

)

t

of 0.28 (R = Bu, 2a), 0.37 (R = Ph, 2b), 0.52 (R = F, 2c), and

0.53 (R = Br, 2d). These observations indicate increase of E_O1

values with weaker electron-donating, or stronger electron-

withdrawing, substituents on the porphyrin and/or QC ligands.

These E_O1values (0.28–0.54 V vs Fc^+/0) fall in the range of E_O1

values observed for the first oxidation waves of ruthenium por-

phyrin complexes bearing other types of carbene ligands (0.19–

0.77 V vs Fc^+/0).^20cFor [Ru(TTP)(QC^tBu)] (1a) and

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t

[Ru(TMP)(QC^R)] (R = Bu, Ph; 2a, 2b), their E_O1values are

similar to those measured for [Ru(TTP)(CO)] (0.50 V vs Fc^+/0

)

and [Ru(TMP)(CO)] (0.32 V vs Fc^+/0), respectively, under

similar conditions. Spectroelectrochemistry of 2b in CH₂Cl₂at

0.85 V vs Ag/AgCl (Figure 5) revealed that the first oxidation

caused a marked change of its UV-vis spectrum, with isosbestic

points at 354, 432, 486, and 612 nm, together with appearance

of a band in the 600–700 nm region characteristic of por-

phyrin-centered oxidation,²³resembling the first oxida-

The ¹³C NMR spectra were measured for 1a, 2a–d, and

3a,b (in CDCl₃or CD₂Cl₂; the other complexes were not

sufficiently stable (in solution) or soluble), each featuring

two downfield signals (δ 245.12–287.19 and 183.35–198.28

ppm) which could be assigned to Ru=C and C=O carbons,

respectively. The Ru=C chemical shifts of 1a, 2a,b, 3a, and

3b (δ 260.10–287.19 ppm) are comparable to those of por-

phyrin-supported Ru=C(CO₂R)₂(δ 271.36–280.49 ppm)^4b,20c

and Ru=CHCO₂Ar (δ 281.30 ppm, Ar = 2,6-^tBu-4-Me-

C₆H₂)¹⁹complexes but appreciably smaller than those of the

non-porphyrin-supported ruthenaquinone (δ 303.08 ppm)^11a

and porphyrin-supported Ru=CAr₂(δ 316.30–346.69 ppm)

counterparts.^20cComplexes 2c,d bearing electron-

withdrawing F and Br substituents, respectively, on their

QC ligands show appreciably smaller Ru=C chemical shifts

(δ 248.45, 245.12 ppm). For [Ru(Por)(QC^tBu)] (1a, 2a, 3a),

the Ru=C chemical shift follows an order 2a (Por = TMP, δ

271.06 ppm) < 1a (Por = TTP, δ 274.79 ppm) < 3a (Por =

TPFPP, δ 287.19 ppm).

In the UV-vis spectra of 1–4 in CH₂Cl₂(Figure S3 in the

Supporting Information), the Soret bands appear at λ_max407–

411 nm (1a–d), 405–410 nm (2a–d), 401–403 (3a,b), and 406

nm (4). No appreciable solvatochroism was observed by

changing the solvent to MeOH or acetone, unlike the ruthena-

quinone^11a(Figure 1, inset) which exists in benzene or THF

solution but, when dissolved in methanol or acetone, changes

color and transforms to the zwitterionic Ru-aryl form (Figure

3, inset C).

E_O2

E_O1

E_R1

E_R2

2b

2d

3a

-2

-1

0

1

2

E (V vs Ag/AgNO₃)

Figure 4. Cyclic voltammograms of 2b, 2d, and 3a in CH₂Cl₂(V

vs Ag/AgNO₃) with 0.1 M (ⁿBu₄N)PF₆as electrolyte and at a scan

rate of 0.1 V s^–1.

Electrochemistry. Coordinated QCs in metal-QC complex-

es constitute a unique type of redox noninnocent carbene lig-

ands in view of the ferraquinone-ferrahydroquinone couple

revealed by chemical oxidation/reduction reactions.^11bHowev-

er, the redox behaviors of metal-QC complexes, including

metallaquinones, under electrochemical conditions remain

unexplored.

Table 1. Redox Potentials V vs Ag/AgNO₃(Values in Pa-

a

renthesis: V vs Fc^+/0

)

complex

E_O1

E_O2

E_R1

E_R2

E(Fc^+/0

)

0.65

–1.00

–1.39

0.17

1a

(0.48)

(1.17) (1.56)

–1.10 –1.50

(1.26) (1.66)

–0.83 –1.33

(0.99) (1.49)

–0.59 –1.17

(0.75) (1.33)

–0.51 –1.15

(0.67) (1.31)

–0.78

–1.14^b

(0.94) (1.30)

We measured the cyclic voltammograms of [Ru(Por)(QC^R)]

complexes 1a, 2a–d and 3a in CH₂Cl₂(Figure 4; Figure S4 in

the Supporting Information); the redox potentials are compiled

in Table 1. These complexes generally exhibit three reversible

or quasi-reversible redox waves: two reduction waves (E_1/2

represented by E_R1and E_R2) and one oxidation wave (E_1/2rep-

resented by E_O1). An extra oxidation wave (quasi-reversible or

irreversible, E_1/2or E_parepresented by E_O2) was observed for

[Ru(TMP)(QC^R)] (2a–d) bearing electron-rich TMP ligand.

For comparison, we also measured the cyclic voltammograms

of [Ru(Por)(CO)] (Por = TTP, TMP) under similar conditions

(Figure S4 in the Supporting Information); these Ru-carbonyl

porphyrins have been studied in detail by cyclic voltamme-

try.²²

0.44

1.13

0.16

2a

2b

2c

2d

3a

(0.28) (0.97)

0.53 1.17

(0.37) (1.01)

0.68 1.18

(0.52) (1.02)

0.69

1.35^b

(0.53) (1.00)

0.70

(0.54)

^aE_1/2values. ^bIrreversible (E_pa).

The E_O1values of [Ru(Por)(QC^tBu)] are 0.28, 0.48, and 0.54

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2.0

1.5

1.0

0.5

0.0

transfer reactivity of metal complexes of non-chelating QCs

towards organic substrates. As metal-catalyzed QC transfer

reactions of diazo quinones^5-9are not known for Ru catalysts,

we intended to find a type of organic compounds to which a

Ru-mediated QC transfer can occur, and then to develop the

corresponding Ru-catalyzed QC transfer reactions.

Nitrosoarenes are the candidates of choice in this work,

based on the following considerations: In our reports on ruthe-

nium porphyrin-catalyzed three-component reactions for syn-

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thesis of isoxazolidines ²⁶or aziridines,^26b,cthe proposed

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mechanisms involve carbene transfer of porphyrin-supported

Ru=CHX (X = CO₂Et, C(O)Ar, P(O)(OMe)₂, C_nF_2n+1) inter-

mediates to nitrosoarenes to in situ generate nitrone intermedi-

ates,^26,but isolation of those nitrone intermediates (one of

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which was observed spectroscopically^26a) was hampered by

their relatively poor stability. In fact, we are aware of no pre-

vious examples of reaction of isolated metal-carbene com-

plexes with nitrosoarenes to give isolated nitrones. As nitrones

possessing QC moieties are quite stable and isolable,²⁸the QC

transfer from [Ru(Por)(QC)] to nitrosoarenes, if occurs, would

not only demonstrate the QC transfer reactivity of an isolated

metal-QC complex but also facilitate mechanistic studies on

the nitrone formation from a metal-carbene complex with ni-

trosoarenes.

Wavelength (nm)

Figure 5. Spectroelectrochemistry of 2b in CH₂Cl₂with 0.1 M

(ⁿBu₄N)PF₆as electrolyte at 0.85 V vs Ag/AgCl.

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tion of [Ru(TMP)(CO)] attributed to porphyrin-centered

process.^22b,c

The two reduction waves of [Ru(Por)(QC^R)] are reminiscent

of two sequential 1 e^–reductions of quinones such as 1,4-

benzoquinone in aprotic organic solvents,²⁴and occur at po-

tentials (E_R1and E_R2) which, particularly E_R1, are considerably

less cathodic than the potentials for the first reduction waves

of the corresponding [Ru(Por)(CO)] complexes (see Figure S4)

and also the porphyrin-supported ruthenium^20cor iron²⁵com-

plexes with other types of carbene ligands.^20cThese two reduc-

tions, at least the first reduction, of [Ru(Por)(QC^R)] could be

ascribed to be essentially the QC-ligand centered, consistent

with the finding that the E_R1values of [Ru(Por)(QC^R)] are

markedly less cathodic for the QC^Rligands bearing weaker

electron-donating, or stronger electron-withdrawing, substitu-

ents (cf. E_R1of [Ru(TMP)(QC^R)]: –1.26, –0.99, –0.75, –0.67 V

vs Fc^+/0for R = ^tBu (2a), Ph (2b), F (2c), Br (2d), respectively).

Reducing the electron density of the Por ligands in

[Ru(Por)(QC^tBu)] also leads to less cathodic E_R1values (–1.26,

–1.17, –0.94 V vs Fc^+/0for Por = TMP (2a), TTP (1a), and

TPFPP (3a), respectively).

Interestingly, treatment of 1a–d with excess nitrosoarenes

(PhNO and its para-substituted derivatives) at room tempera-

ture resulted in QC transfer to the nitrosoarenes, affording

nitrones 5a–h in high isolated yields (7590%, Table 2). A

similar treatment of 3b with PhNO gave nitrone 5i in 77%

yield (Scheme 2). All these nitrone products were character-

ized by NMR and HRMS, and for 5d its crystal structure was

determined by X-ray crystallography (Figure 6). The fate of

the Ru-QC complexes after QC transfer was exemplified by

the reactions of 1a–d with PhNO affording [Ru(TTP)(PhNO)₂],

a previously reported Ru(II)-nitrosoarene complex.²⁹

Binding Behavior towards Pyridines and Imidazoles.

[Ru(Por)(QC^R)] complexes could form adducts with N-donor

ligands such as pyridine and 1-methylimidazole. These ad-

ducts were found to be only stable for ~3 h in solution and

were susceptible to QC dissociation and degradation; for in-

stance, treatment of 2b with excess pyridine gave

[Ru(TMP)(Py)₂] (and 2,6-diphenyl-p-benzoquinone) after the

reaction mixture had been left standing overnight, analogous

to the formation of [Ru(TPP)(Py)₂] from treatment of

[Ru(TPP)(CHCO₂Ar)(THF)] with pyridine.¹⁹

The formation of adduct [Ru(TMP)(QC^Ph)(Py)] (2b·Py)

from [Ru(TMP)(QC^Ph)] (2b) led to a red shift of the Soret

band in the UV-vis spectrum (Figure S5), like the case of

[Ru(TPFPP)(CPh₂)] vs [Ru(TPFPP)(CPh₂)(Py)].^20cInteresting-

ly, complex 2b showed preference for the binding with pyri-

dines and 1-methylimidazole, as revealed by their large bind-

ing constants (K_L) with 2b (Table S5), while formation of six-

coordinate products with PPh₃or PhNO ligand could not be

detected by UV-vis and ¹H NMR monitoring.

Figure 6. ORTEP drawing of 5d (thermal ellipsoids at 50% prob-

ability level, hydrogen atoms not shown) and its bond distances.

1

We monitored the reaction between 1a and PhNO by H

NMR spectroscopy. The time course plot showed no apprecia-

ble induction period, and the nitrone 5a and [Ru(TTP)(PhNO)₂]

products were cleanly formed (Figure 7). Hammett studies

were then conducted for the reactions of 1a with nitrosoarenes

p-X-C₆H₄NO (X = Me, H, Cl, CO₂Et). UV-vis monitoring of

the reactions between 1a and large excess (50–200 equiv) of

these nitrosoarenes gave pseudo-first-order rate constants (k_obs),

Dual Reactivity. (i) Stoichiometric and Catalytic QC

Transfer Reactions with Nitrosoarenes. The isolation of

[Ru(Por)(QC)] complexes allows direct exploration of carbene

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2

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100

and the second-order rate constants (k₂) were determined by

plotting k_obsagainst [ArNO] (Figure 8). By correlating the log

(k_X/k_H) values with the _paravalues of the nitrosoarene

80

60

40

20

0

Table 2. Stoichiometric Reactions Between 1a–d and Ni-

trosoarenes

1a

5a

[Ru(TTP)(PhNO)₂]

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60

0

1

2

3

4

5

6

7

Time (h)

Figure 7. Time course plot of the stoichiometric reaction between

1a and PhNO.

0.012

Me

H

Cl

0.008

CO₂Et

0.004

0.000

0.00000

0.00025

0.00050

0.00075

0.00100

[p-X-C₆H₄NO] (M)

Figure 8. Plot of k_obsagainst [ArNO] for the reactions between 1a

and p-X-C₆H₄NO (X = Me, H, Cl and CO₂Et).

substituents,³⁰a Hammett plot with a negative  value of

2.14 was obtained (Figure 9, Table S6), indicating that nitro-

soarenes acted as nucleophiles in these reactions and suggest-

ing that the coordinated QC ligand of 1a is electrophilic.

To inspect the effect of QC substituents on the QC transfer

of [Ru(Por)(QC)], we examined the kinetics of the reactions

between 1a–d and PhNO; their second-order rate constants k₂

0.5

Me

 = -2.14

H

0.0

-0.5

-1.0

-1.5

R²= 0.951

Cl

CO₂Et

Scheme 2. Stoichiometric Reaction of 3b with PhNO

-0.2

0.0

0.2

0.4

_para

Figure 9. Hammett plot of log (k_X/k_H) against _paravalues of ni-

trosoarene substituents for the reactions between 1a and p-X-

C₆H₄NO (X = Me, H, Cl and CO₂Et).

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o

were determined similarly from k_obsvs [ArNO] plots (Figure

at 40 C, a series of nitrones 5ai were obtained in moderate

2

3

4

5

6

7

10). The resulting log k₂values are reasonably well correlated

with the Hammett constants (⁺_meta) of the substituents meta to

the carbene carbons,³⁰and a Hammett plot with, intriguingly, a

negative  value (–1.36) was obtained (Figure 11, Table

S7), indicating that increase of electron-withdrawing power of

the substituent on the electrophilic QC lowers its reaction rate.

yields (3057%) from the reactions of nitrosoarenes with di-

azo quinones catalyzed by [Ru(TTP)(CO)] (Table 3 and

Scheme 3).

Given the above-mentioned intriguing effect of QC substit-

uent on the rate of the stoichiometric reactions (Figure 11), we

examined the corresponding QC substituent effect in the cata-

lytic reactions. The [Ru(TTP)(CO)]-catalyzed reactions of

8

9

t

N₂QC^R(R = Bu, Ph, F and Br) with PhNO were traced by ¹H

0.008

NMR spectroscopy (time required for complete reaction: 10 h

for N₂QC^tBu, 36 h for N₂QC^Ph, 52 h for N₂QC^F, 60 h for

N₂QC^Br) and their initial rates (r_int) were also recorded. A good

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60

1a

1b

1c

0.006

1d

Table 3. Reactions of Diazo Quinones N₂QC^Rand Nitro-

soarenes Catalyzed by [Ru(TTP)(CO)]

0.004

0.002

0.000

0.00000

0.00025

0.00050

0.00075

0.00100

[PhNO] (M)

Figure 10. Plots of k_obsagainst [PhNO] for the reactions between

1ad and PhNO.

1.0

^tBu

0.8

 = -1.36

Ph

R²= 0.996

0.6

0.4

F

0.2

Br

0.0

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

⁺

meta

Figure 12. Hammett plot of log r_intvs ⁺_metavalues of QC substit-

uents for the [Ru(TTP)(CO)]-catalyzed reactions of N₂QC^R(R =

^tBu, Ph, F and Br) with PhNO.

10

11

PhSH

33

14

PhSSPh

PhNH₂

PhN=NPh

^aReaction conditions: 4 (0.01 mmol), substrate (0.2 mmol),

(ii) HAT Reactions with CH or X–H Substrates (X = S,

N) and Catalytic Aerobic Oxidation. Metal-catalyzed car-

bene insertion into C–H bonds is an appealing method of C–H

functionalization.¹The reactivity of [Ru(Por)(QC^R)] towards

C–H bonds was also explored. For example, treatment of

[Ru(TDCPP)(QC^Br)] (4) with 20-fold excess of 1,4-

cyclohexadiene (CHD) at room temperature for 24 h afforded

benzene in 88% yield (Table 4, entry 1), apparently by oxida-

tive CH activation of CHD; however, no QC^BrC–H insertion

product was detected. After the reaction, 4 was converted to a

species formulated as [Ru^III(TDCPP)(Ar^Br)] (6, Ar^Br= 3,5-Br₂-

4-OH-C₆H₂) based on its paramagnetic ¹H NMR spectrum (see

the Supporting Information) which features the H_signal at δ

–29.64 ppm (comparable to those of [Ru^III(Por)(Ar)]³¹) and

resembles the spectrum of [Ru^III(TDCPP)(Ph)].^31cAlso, the

ESI-MS spectrum of 6 shows a cluster peak at m/z = 1240.0

consistent with its formulation. Similarly, 1d and 2d could

react with CHD as well, affording benzene in 87% and 83%

yield, respectively.

Besides CHD, a number of other CH substrates could also

react with 4, including a substituted CHD, two natural prod-

ucts -terpinene and terpinolene, 1,4-dihydronaphthalene,

9,10-dihydroanthracene (DHA), xanthene, 2,5-dihydrofuran,

and isochroman (Table 4, entries 2–9). These reactions afford-

ed CH bond activated/functionalized products in moderate-

to-high yields (4383%). For treatment of 4 with relatively

inert substrates such as cyclohexene and ethylbenzene, no

reaction occurred even at elevated temperature (80 °C).

b

1

CH₂Cl₂or CDCl₃(0.5 mL), 24 h, under Ar. Determined by H

NMR or GC. Yield based on 4. ^dReaction time is 48 h. Oxygen

atom of C=O group might come from trace amount of air or mois-

ture in the reaction system.

c

e

We then examined the reactivity of 4 towards XH sub-

strates PhSH and PhNH₂; the reactions afforded PhSSPh and

PhN=NPh, respectively, albeit in lower yields (Table 4, entries

10 and 11). Alcohols such as ethanol and benzyl alcohol were

reported to react with ferraquinone to give aldehydes and fer-

rahydroquinone;^11bin contrast, treatment of such alcohols with

4 did not result in appreciable reaction. Aprotic molecules (e.g.

PhSMe) and hydride sources (e.g. Et₃SiH) were also not reac-

tive with 4.

Kinetic studies were performed for the reaction of 4 with

DHA. Under pseudo-first-order conditions, its second-order

rate constant (k₂) was determined to be 0.44 ± 0.02 M^-1s^-1at

298 K (Figure 13), close to that of MnO₄ion (0.48 M^-1s^-1).³²

–

Changing the porphyrin ligand from TDCPP of 4 to TTP (to

give 1d) and sterically encumbered TMP (to give 2d) slightly

increased the k₂value to 1.46 ± 0.05 and 2.21 ± 0.04 M^-1s^-1,

respectively. Eyring plot of the reaction between 4 and DHA

was obtained in the temperature range of 273–313 K, and the

activation parameters were determined as H^‡= 14.2 ± 0.5

kcal mol^-1and S^‡= –12.3 ± 1.6 cal mol^-1K^-1(Figure 14).

Addition of Lewis acid such as Sc(OTf)₃, FeCl₃or proton (tri-

fluoroacetic acid), which has been shown to improve the reac-

tivity of metal-oxo and related species,³³did not significantly

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accelerate the HAT step (Table S11), yet FeCl₃was found to

facilitate the anthracene formation. However, addition of BF₃,

Scheme 4. Conversion of CHD to Benzene Catalyzed by 4

2

3

4

5

6

7

8

a non-bulky and non-redox active Lewis acid, led to an ap-

proximately 4-fold increase of the reaction rate (Table S11).

With DHA-d₄as substrate, the k₂value of its reaction with 4 at

298 K is 0.067 ± 0.003 M^-1s^-1, from which a KIE value of 6.6

± 0.6 was obtained, indicating involvement of C–H bond

cleavage in the rate-determining step.

9

t

DFT/TD-DFT calculations on [Ru(TTP)(QC^R)] (R = Bu, 1a;

0.010

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Br, 1d). The DFT-optimized structure of 1a is consistent with

its crystal structure, with the discrepancy in bond distances of

the Ru-QC scaffold being ≤ 0.06 Å (Figure S13). TD-DFT

calculations were conducted to simulate the UV-vis spectrum

of 1a, and the simulated spectrum compares well with the

experimental one (Figure S15).

0.008

KIE = 6.6 ± 0.6

0.006

k_H= 0.44 ± 0.02 M^-1s^-1

According to the MO diagram of 1a, the bonding between

Ru ion and QC ligand is rather covalent in nature, as indicated

by the strong mixing in both of the corresponding  and *

orbitals (Figure 15). One of the most distinct features of QC

ligand is that, it binds to the metal center through its molecular

orbitals that are distributed all over the quinoid moiety, which

is associated with the co-planarity of the carbonyl group with

the carbene plane. This is in stark contrast to the common car-

bene ligands such as CPh₂and C(X)CO₂R which mainly inter-

act with metal centers through the p_atomic orbitals of the

carbene carbons.^3p,q,s,v,³⁴The electronic transition of 1a from

Ru-carbene  (HOMO-3) to * (LUMO) is predicted at 534

nm with moderate oscillator strength; such a transition can

also be observed in the experimental UV-vis spectrum and it is

mixed with the Q bands of porphyrin transitions (Figure S15).

The electronic structure of 1d is largely similar to that of 1a,

with the Ru-carbene  and * orbitals also residing at HOMO-

3 and LUMO, respectively, except that both the  and * or-

bitals of 1d are lower in energy than those of 1a (Figure 15).

0.004

k_D= 0.067 ± 0.003 M^-1s^-1

0.002

0.000

0.005

0.010

0.015

0.020

[DHA] (M)

Figure 13. Plots of k_obsagainst [DHA] for the reactions of 4 with

DHA and DHA-d₄at 298 K.

-5

y = -7123x + 17.55

R²= 0.993

-6

-7

-8

H^‡= 14.2 ± 0.5 kcal mol^-1

S^‡= -12.3 ± 1.6 cal mol^-1K^-1

-3

-9

0.0032 0.0033 0.0034 0.0035 0.0036 0.0037

Ru=C *

LUMO, –3.33 eV

1/T

Figure 14. Eyring plot for the reaction of 4 with DHA (273–313

Ru=C *

LUMO, –3.77 eV

-4

-5

-6

K).

Attempts have been made to develop a CH activation ca-

talysis using Ru-QC complex as catalyst. As treatment of 4

with CHD gave 6 and benzene, and 6 was found to be highly

air-sensitive and could be readily converted back to 4 under

aerobic conditions, the conversion of CHD to benzene could

be rendered catalytic using 4 as catalyst and air as terminal

oxidant. For example, treatment of CHD with catalyst 4 (0.05

o

Ru=C 

mol % based on CHD) at 60 C under aerobic conditions for

H-3, –5.77 eV

24 h gave benzene with a turnover number (TON) of 25

(Scheme 4; after the catalysis, the Ru complex(es) decom-

posed completely). Addition of FeCl₃to the reaction mixture

increased the TON to 48 (Scheme 4), but also led to a more

rapid deterioration of the catalytic system (12 h).

DFT and TD-DFT Calculations. To gain further

knowledge on the electronic structures of [Ru(Por)(QC)] in-

cluding the influence of QC substituents, we performed

Ru=C 

H-3, –6.09 eV

1a

1d

Figure 15. Comparison of calculated MO energy levels in 1a and

1d showing the  interactions between Ru and QC. Composition

of LUMO: Ru 23%, QC^tBu63%, TTP 14% for 1a; Ru 28%, QC^Br

58%, TTP 14% for 1d.

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The lower energy of the* orbital (LUMO) in 1d than in 1a is

parallel to the experimentally measured less cathodic potential

of the first reduction of 2d than that of 2a in the electrochemi-

cal studies (Table 1).

G

1.40 Å

1.19 Å

kcal mol^-1

5

6

7

8

The mechanism of the QC transfer reaction between 1d

and PhNO was investigated by DFT calculations. As shown

in the computed reaction pathway (Figure 16), PhNO prefers

to directly attack the QC carbon of the five-coordinate Ru-

QC complex. The reaction proceeds in a concerted manner,

and the QC group becomes bent and the Ru-carbene bond

starts to elongate as the nitrogen atom of PhNO approaches

the carbene carbon. The transition state TS1_Bris predicted to

be 14.2 kcal mol^1higher in energy than the reactants, which

= TDCPP

17.2

9

TS3

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0

-3.2

is comparable to the experimental value (G^‡

= 17.15 ±

6

expt

4 +

0.05 kcal mol^1from k₂= 1.66 ± 0.15 M^-1s^-1). At TS1_Br, the

Ru–C(carbene) bond is slightly lengthened to 2.02 Å (cf.

1.89 Å in 1d), whereas the distance between carbene carbon

and PhNO (1.94 Å) is much longer than the final C–N bond

in 5d (1.35 Å in both computed and crystal structures of 5d).

All these indicate a rather early transition state in this reac-

tion. After nitrone formation, the [Ru(TTP)] moiety binds

another two molecules of PhNO to afford the final

[Ru(TTP)(PhNO)₂] product. Although axial ligand has been

reported to activate metal-carbene porphyrins for carbene

transfer,^4e,fsuch a pathway, e.g. preformation of six-

coordinate [Ru(TTP)(QC^Br)(PhNO)] followed by attack on

its carbene carbon by another PhNO molecule, is less favor-

able according to DFT calculations (Figure S16). The six-

coordinate transition state TS2_Brwas calculated to lie at a

slightly higher energy level (15.1 kcal mol^1) than TS1_Br

(14.2 kcal mol^1). In addition, the kinetic studies revealed

that the rate of reaction of 1d with PhNO is only first-order

in [PhNO] (Figure 10), in disfavor of a mechanism involving

participation of more than one PhNO molecules at the rate-

determining step.

0.97 Å

1.40 Å

1.19 Å

6

TS3

Figure 17. Computed free energy surface for the HAT reaction of

4 with DHA (above) and the DFT-optimized structures of TS3

and 6 (bottom).

the incoming C–H bond is almost orthogonal to the QC plane,

thus maximizing its overlap with the LUMO orbital of 4. The

hydrogen atom being transferred is located closer to the recip-

ient oxygen (1.19 Å) than to the DHA molecule (1.40 Å). The

O–H bond distance in TS3 is 0.22 Å longer than that in 6,

while the reactive C–H bond in DHA is lengthened by ~0.3 Å.

The product anthracene was predicted to be formed via only

one transition state TS3 without other intermediates, with the

QC ligand being appreciably aromatized in TS3 and adopting

a structure approximately midway between the structures of

QC and Ar ligands in 4 and 6, respectively (Figure S18).

G

kcal mol^-1

2.02 Å

14.2

■ DISCUSSION

6.7

5d

TS1_Br

0.0

The widely proposed involvement of metal quinoid carbene

(QC) intermediates in metal-catalyzed synthetically useful

reactions with diazo quinones^5-9highlights the importance of

metal-QC complexes in the chemistry of QC transfer reactions.

The present work, to the best of our knowledge, first demon-

strates the isolation of metal-QC complexes from reactions of

metal complexes with diazo quinones and the QC transfer of

an isolated or directly detected metal-QC complex to organic

substrates, along with structure characterization of metal-QC

complexes by X-ray crystallography. Although a considerable

number of isolated and/or structurally characterized metal-

carbene complexes,^3,4including those supported by porphyrin

ligands,⁴can undergo carbene transfer reactions with organic

substrates, their carbene ligands (such as C(Ph)CO₂R and

CAr₂) are distinctly different from QCs. In the literature, ex-

amples of isolated or directly detected metal-QC complexes

are extremely rare,¹¹which all demonstrate a metallaquinone

feature of metal-QC complexes stabilized by chelating auxilia-

1d + PhNO

2 PhNO

+ 5d

-33.5

= TTP

Figure 16. Computed free energy surface for the QC transfer

reaction of 1d with PhNO.

For the HAT reaction between 4 and DHA, its transition-

state structure was investigated by DFT calculations (Figure

17; for other pathway, see Discussion section). The calculated

barrier (17.2 kcal mol^1) is in good agreement with the exper-

imental value (17.86 ± 0.97 kcal mol^1at 298 K) obtained

from the Eyring plot (Figure 14). At the transition state (TS3),

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ry groups on QCs with the chelating QC ligands being inte-

types of cycloaddition reactions with the catalytically-active

2

3

4

5

6

7

8

9

grated into a quinone-like core in disfavor of QC transfer.

Remarkably, the metal-QC complexes [Ru(Por)(QC)] iso-

lated in this work, which all feature catalytically related,

monodentate QC ligands without chelating auxiliary groups,

exhibit high stability yet show dual reactivity. On the basis of

the experimental and DFT calculation studies performed on

[Ru(Por)(QC)] in this work, the following aspects of these

metal-QC complexes are discussed here.

Electronic Structures. The [Ru(Por)(QC)] complexes (1–4)

can be described as d⁶Ru(II) species bearing neutral QC lig-

ands, based on their NMR spectra and X-ray crystal structures

and DFT calculations (see, for example, the calculated Ru d

orbitals of 1a depicted in Figure S14). Additional evidence

may come from the similarity between the first oxidations of

[Ru(Por)(QC)] and the well documented Ru(II)-carbonyl

complexes [Ru(Por)(CO)] revealed by electrochemical and

computational studies.³⁵We further measured the IR spectrum

species being too unstable to be trapped or well character-

ized.^1p,³⁹For ruthenium porphyrin carbene complexes, exam-

ples that undergo stoichiometric carbene transfer reaction with

organic substrates are scarce^4b,f,19and without showing a dual

carbene reactivity.

In this work, the isolated Ru(Por)(QC)] complexes dis-

play a unique substrate-dependent dual reactivity, i.e. QC

transfer to nitrosoarenes and quinone-like reactivity (HAT

of CH or XH compounds), and can be further developed

into catalytic versions. Moreover, characterization of the

complexes and mechanistic studies of their stoichiometric

reactivities have been made possible by tuning the ligand

environment. The observed reactivities of these Ru-QC

complexes are probably associated with their low-lying

carbene orbitals inferred from electrochemical and DFT

studies. It would be of interest to build a more detailed

connection between their unique reactivity pattern and their

intrinsic electronic nature.

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of [Ru(TTP)(QC^tBu)] (1a), which features an oxidation-state

36

marker band

at 1010 cm^1comparable to that of

(i) QC Transfer Reactions and Effect of QC Substituents.

Nitrosoarenes were found to be good substrates for the QC

transfer reactions of [Ru(Por)(QC)], and the reactions led to

the isolation of nitrones in up to 90% yield (Table 2). These

reactions contrast with previously reported stoichiometric re-

actions of metal-carbene complexes (CO)₅M=C(OMe)Ph (M =

[Ru(TTP)(CO)] (1008 cm^1), together with bands at 1653,

1593 and 1568 cm^1attributable to quinoid moieties.³⁷

The good stability of [Ru(Por)(QC)] in MeOH contrasts

with the facile conversion of the ruthenaquinone (a Ru(0)

species) to the zwitterionic Ru(II)-aryl complex in this sol-

vent;^11athe latter apparently originates from a 2 e^–transfer

from the highly reducing Ru(0) center to the chelating QC

ligand. Such intramolecular Ru-to-QC electron transfer

would be less favorable in the [Ru(Por)(QC)] system due to

the less reducing Ru(II) center or the higher Ru(IV/II) redox

potential. As to the higher stability of [Ru(TMP)(QC^R)] than

that of [Ru(TTP)(QC^R)], this probably originates from the

steric effect of TMP ligand, as shown by the space-filling

representation of the crystal structure of 2d (Figure S1) in

which two of the ortho-methyl groups on the meso-aryl sub-

stituents are poised in front of the QC plane, thereby pre-

venting the carbene * orbital from being attacked by incom-

ing molecules.

Some useful comparisons can be made between Ru-QC and

Ru^II-NHC complexes (NHC = N-heterocyclic carbene). The

isolated Ru-QC complexes, [Ru(Por)(QC)], exclusively bear

mono-QC ligand, and no bis-QC complexes [Ru(Por)(QC)₂]

were observed, unlike the isolation of bis-NHC complexes

[Ru^II(Por)(NHC)₂].³⁸Also, the first oxidation of [Ru(Por)(QC)]

(E_O1= 0.280.54 vs Fc^+/0, Table 1) is mainly porphyrin-

centered, whereas that of [Ru^II(Por)(NHC)₂] is metal-centered

and with a rather low potential of E_1/2(Ru^III/II) = –0.16 vs Fc^+/0

(Por = 4-F-TPP).³⁸Thus, unlike the highly electron-donating

NHC ligand which tends to stabilize Ru^IIIand with relatively

small trans effect, the QC ligands show large trans effect and

are strong -acceptors comparable to CO ligands. The strong

-accepting ability of the QCs could also be inferred from the

observation of reductions (by cyclic voltammetry, Figure 4)

ascribed to QC-centered process, which suggests low-lying

carbene orbitals.

b

Cr,⁴⁰W^40a) with nitrosoarenes (such as PhNO) to give me-

tathesis-like products (such as O=C(OMe)Ph and/or

PhN=C(OMe)Ph),⁴⁰rather than carbene transfer product

nitrones. There is also a previous report on the stoichiometric

reaction of iron-carbene complex [Cp(CO)₂Fe=CHAr]⁺(Ar =

p-C₆H₄OMe) with nitrosoarenes (such as PhNO), which gave

iron-nitrone complexes without resulting in the isolation of

nitrones.⁴¹

For isolated or directly detected metal-QC complexes, the

effects of QC substituents on the reactivity of their QC ligands

coordinated with the same metal ion have not been reported

previously. In fact, only two metal-QC complexes, a ruthena-

quinone and a ferraquinone, were reported before, each bear-

ing one chelating QC ligand.¹¹In the case of free QCs, which

adopt triplet ground state, introducing electron-withdrawing

substituents by halogenation is reported to increases the QC’s

electrophilicity.^10a,

42

Considering other types of carbene ligands, the effects of

carbene substituents on the reactivity of isolated metal-

carbene complexes are previously focused on the reactions

of M=C(X)Y (M = Cr, W; X = OR, SR, Ar; Y = Ar, OAr)

complexes with nucleophiles (such as RS^and RO^).¹³The-

se reactions of electrophilic carbenes, which do not lead to

carbene transfer, all feature Hammett plots with positive ρ

values¹³(except for the reaction of a W=C(SR)Ar complex

with HOCH₂CH₂SH^13adescribed below), indicating in-

crease of reactivity by stronger electron-withdrawing sub-

stituents and vice versa. DFT calculations on the reaction

mechanism of Cr=C(X)R (X = OMe, SMe; R = Me, Ph)

with nucleophile NH₃also revealed a higher reaction barri-

er for the stronger π-donor X group.⁴³In addition, previous

mechanistic investigations of reactive metal-carbene com-

plexes have been largely confined to variation of substrates

Dual Reactivity. Literature reports on dual reactivity of a

metal-carbene species, such as undergoing not only carbene

transfer but also other type of carbene reaction, are sparse.

Examples include some traditional Fischer carbene complexes

(especially Cr and W complexes) which can undergo stoichi-

ometric transformations other than carbene transfer reaction^1g

and the metalloenolcarbenes which can be engaged in various

and

electronic/steric

tuning

of

auxiliary

lig-

b

ands.^{3c,j,l,m,q,u,4c,e,34f,}

44,45

With regard to carbene transfer reactions of isolated metal-

carbene complexes with organic compounds, the effect of

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carbene substituents on the reactivity remains rarely stud-

on carbene carbon by a nucleophile. Of these effects, (b) and

ied,^4e,gdespite a few reports on related systems involving in

situ generated metal-carbene species in the reaction mix-

tures.^14,45aFor example, the reactions of N₂C(CO₂R)C₆H₄-p-

R′ with alkene, Si–H or C–H substrate catalyzed by

dirhodium complexes^14a,bor [Fe(Por)Cl]^14cproceeded faster

for stronger donor R′, with their Hammett plots featuring

negative ρ values^14b,cwhich could result from the formation

of metal-carbene intermediate as the rate-determining step.^14b

Another example is the cyclopropanation of Me-

OCH=CHOMe by Au(I)-carbene [IMesAu=CHC₆H₄-p-R]⁺

(IMes: an NHC ligand) generated in gas phase;^45athe reac-

tion gave a Hammett plot with positive ρ value, which was

rationalized by assuming the dissociation of the electron-rich

three-membered ring as the rate-determining step.⁴⁵Direct

use of isolated metal-carbene complexes for such studies on

carbene substituent effect would facilitate elucidation of

their carbene transfer mechanisms.

(c) would reduce, whereas the others can enhance, the car-

bene electrophilicity/reactivity. The net effect should depend

on the reaction systems including the types of metal-carbene

complexes, auxiliary ligands, and the nucleophiles.

Previously reported DFT calculations on iron porphyrin

complexes bearing different types of carbene ligands in-

cluding CPh₂and CCl₂revealed that stronger electron-

withdrawing carbene group causes considerably less posi-

tive charge on the carbene carbon (corresponding to mark-

edly smaller ¹³C NMR Fe=C chemical shifts), due to hin-

dering of the carbene electron donation ability by electron-

withdrawing groups.^34cFor 1a and 1d, which belong to the

same type of carbene ligand (QC), the calculated Mulliken

charges on their carbene carbons are rather similar (1a: –

0.069; 1d: –0.066; q_C< 0.005). From the ¹³C NMR spec-

tra of 2a–d, their Ru=C chemical shifts evidently decrease

with increasing electron-withdrawing ability of the QC

substituent, like the cases of the iron-carbene porphyrin

complexes^34cand three-coordinate Cu=C(C{O}R)C₆H₄-p-R′

complexes,^3qthough different from the cases of Fe/Ru-

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The QC substituent effect revealed by kinetic studies on

the QC transfer of [Ru(TTP)(QC^R)] (1ad) with PhNO,

which features a negative ρ value (Figure 11) suggestive of

lower electrophilicity for stronger electron-withdrawing sub-

stituent, was quite unexpected. This reversed reactivity order,

relative to that reported for free QCs^10,42and other isolated

metal-carbenes^4e,g,13mentioned above, parallels that found in

this work for the corresponding [Ru(TTP)(CO)]-catalyzed

QC transfer reactions (Figure 12). Previously, the stoichio-

metric reaction of (OC)₅W=C(SCH₂CH₂OH)C₆H₄-p-R with

HOCH₂CH₂SH in MeCN-H₂O (1:1 v/v) was also reported to

give a Hammett plot featuring a negative ρ value, which was

rationalized by participation of water molecule in the transi-

tion state causing partial positive charge on the thiol S at-

om.^13aIn addition, there have been reports for other unusual

substituent effects, such as nonlinear Hammett plots, in stoi-

carbene complexes [Cp(CO)(L)M=CHC₆H₄-p-R]⁺(M = Fe,

49

Ru; L = CO, PPh₃).

This could suggests that, for

[Ru(Por)(QC^R)], stronger electron-withdrawing substituent

R on QC lowers the QC electrophilicity, which is likely to

be associated with reduced QC-to-Ru donation and/or en-

hanced Ru-to-QC -backdonation. For example, the DFT

calculations suggest more pronounced Ru-to-carbene -

backdonation in 1d than in 1a by analyzing the component

of their Ru-carbene * orbitals (see the data in the caption

of Figure 15).

We speculate that the activation barrier of the QC trans-

fer reaction of 1a–d with PhNO is also related to the extent

of Ru-QC -interaction and/or degree of localization of the

LUMO orbital: electron-donating substituents on QC ren-

der the LUMO more localized on the carbene carbon,

which can possibly facilitate its interaction with PhNO. On

the other hand, in the presence of electron-withdrawing

groups on QC, the Ru-QC complexes may need to undergo

more reorganization processes to weaken the Ru-carbene 

bond and to localize the * orbital at the carbene carbon

before the reaction can take place, thus creating a relatively

higher kinetic barrier. For further comparison, besides the

transition state TS1_Brderived from 1d (with electron-

withdrawing Br on QC), the transition state TS1_tBuderived

chiometric atom/group transfer reactions,^44e,together with

46

reports on rare cases of inverted reactivity order in hydrogen

atom abstraction by Fe^IV=O complexes [Fe^IV(O)(TMC)(X)]ⁿ⁺

(TMC = tetramethylcyclam) (i.e. a higher reactivity for

stronger electron-donating axial X ligand, attributed to a

two-state reactivity of the Fe^IV=O complexes with the contri-

bution of the more reactive state being increased by such

axial ligand).⁴⁷

To rationalize the intriguing substituent effect in the QC

transfer of 1ad with PhNO, several factors/aspects can be

considered, with steric factor being minor or ignorable in

view of the large separation between the meta-substituents

and the reaction site in the DFT-calculated transition states

(see, for example, Figure 16), which is different from the

cases of [Fe(Por)(CXY)] (X = H, Ph; Y = CO₂Et, Ph) in pre-

viously reported studies about carbene substituent effect on

carbene transfer wherein a steric effect could be significant

as the carbene substituents X and Y are directly bonded to

the carbene carbon and thus close to the reaction site.^4e,g

Generally speaking, the electrophilicity of a coordinated

carbene ligand is associated with donation from carbene to

metal and insufficient -backdonation from the metal to car-

bene. ⁴⁸Increasing the electron-withdrawing ability of the

substituent on carbene may have several electronic effects,

including: (a) lowering the electron density of free carbene,

(b) reducing carbene donation to metal, (c) enhancing -

backdonation from metal to carbene, and (d) stabilizing the

negative charge built up in the transition state of the attack

t

from 1a (with electron-donating Bu on QC) was also cal-

culated by DFT method (Figure S17). The two transition

states are similar in terms of their QC structure and the

chemical bonds that are breaking/forming, except that the

bending angle of the quinoid plane in TS1_Bris ca. 10^olarg-

er than that in TS1_tBu(Figure 18). Possibly, as the lower-

lying LUMO in 1d results in a stronger Ru-carbene -

interaction, more bending of its QC^Brgroup is required for

the partial disruption of this stronger Ru=C  bond and for

the localization of LUMO to the carbene carbon; this also

possibly causes a higher kinetic barrier for the reaction

with 1d. The DFT-calculated reaction barrier (G^‡_calc) for

TS1_tBuis 12.9 kcal mol^1, which compares reasonably well

with the value derived from experimentally determined k₂

value (G^‡

= 16.28 ± 0.03 kcal mol^1from k₂= 7.14 ±

expt

0.36 M^-1s^-1); both the G^‡and G^‡

values for TS1_tBu

expt

calc

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6

(a)

(b)

the QC ligands in our case are monodentate, unlike the chelat-

ing QC ligands in the previously reported metallaquinones (in

which the metal-QC coordination is apparently assist-

ed/enhanced by chelating auxiliary groups on the QC lig-

ands),¹¹the present work could demonstrate an “intrinsic”

metallaquinone feature of a metal-QC complex.

C

N

O

Br

Ru

Redox properties of quinone compounds have been well

documented in the literature.⁵⁰For the Ru-QC complexes,

the reduction waves in their cyclic voltammograms (e.g.

Figure 4) attributable to QC-based process could account for

their HAT reactivity with C–H and X–H substrates. On the

basis of the reaction of 4 with CHD to give 6 and benzene,

together with the facile conversion of 6 to 4 by air oxidation,

a mechanism was proposed for the HAT reactivity of 4,

along with resemblance of the 4/6 redox couple to the qui-

none/semiquinone couple (Scheme 5). The Ru(III)-aryl

complex 6 is akin to the semiquinone (the phenoxy radical

corresponds to a metalloradical in 6), and it was possibly

formed via HAT from CHD to the QC oxygen followed by

an intramolecular metal-to-ligand redox-induced electron

transfer (RIET), analogous to RIET processes reported for

other metal complexes containing quinone-type ligands.⁵¹

The DFT calculations on the mechanism of HAT between 4

and DHA (Figure 17), and also the large KIE value (6.6 ±

0.6) determined experimentally, are in accord with the pro-

posed mechanism (Scheme 5). As the DFT calculations re-

vealed the formation of 6 via a single transition state (TS3)

without involving metal intermediate(s) (Figure 17), the

HAT and RIET might happen concomitantly, which could

also be suggested by the appreciably aromatized QC ligand

in TS3 (Figure S18).

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TS1_tBu

TS1_Br

G^ǂ= 12.9 kcal mol^-1

G^ǂ= 14.2 kcal mol^-1

calc

G^ǂ= 16.28 ± 0.03 kcal mol^-1

G^ǂ= 17.15 ± 0.05 kcal mol^-1

expt

(k = 7.14 ± 0.36 M^-1s^-1)

(k = 1.66 ± 0.15 M^-1s^-1)

Figure 18. Comparison of DFT-calculated structures for (a)

TS1_tBuand (b) TS1_Br.

are smaller than the corresponding ones for TS1_Br(Figure

18), consistent with the higher QC transfer reactivity ob-

served for 1a.

Interestingly, for the QC transfer reactions of 1a–d with

PhNO, a fair correlation can be established between the

experimentally obtained carbene chemical shifts and stoi-

chiometric/catalytic kinetics (Figure 19). This correlation

links the electronic properties (characterization data) of

isolated metal-QC complexes to their stoichiometric and

the corresponding catalytic carbene transfer reactivities

(kinetic data).

Scheme 5. Proposed Mechanism for HAT Reaction of 4

with CHD and the 4/6 Redox Couple

1.2

Stoichiometric Kinetics

^tBu

Catalytic Kinetics

0.8

Ph

R²= 0.985

F

0.4

Br

R²= 0.986

0.0

240

250

260

270

(Ru=C) (ppm)

Figure 19. Correlation of kinetic data from stoichiometric and

catalytic reactions with Ru=C ¹³C NMR chemical shifts.

(ii) HAT Reactivity. One of the well-known reactivities of

conventional metal-carbene complexes/intermediates is their

carbene transfer reactions with C–H and X–H (e.g. X = N, O,

S, Si) substrates, resulting in carbene insertion into these

bonds.^1,2Such carbene insertion reactions have not been ob-

served for the [Ru(Por)(QC)] (1–4) complexes, despite their

QC transfer reactions with nitrosoarenes. The observed inter-

esting quinone-like HAT reactivity of these Ru-QC complexes

with C–H and X–H substrates (Table 4) should stem from the

unique metallaquinone¹¹feature of the Ru-QC complexes. As

An alternative mechanistic pathway to be considered is that

the carbene carbon (rather than QC oxygen) abstracts a hydro-

gen atom⁵²from the C–H bond. For [Ru(Por)(QC)], this path-

way is not favored by experimental observations, as attack on

the carbene carbon would encounter much larger steric hin-

drance for 2d (bearing sterically encumbered porphyrin ligand

TMP) than for 1d (bearing simple porphyrin ligand TTP),

which is inconsistent with the observed higher reaction rate of

DHA with 2d than with 1d (k₂: 2.21 ± 0.04 vs 1.46 ± 0.05 M^-1

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s^-1). The higher HAT rate for 2d, which bears a more electron-

stability of the catalytic system during turnover.

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rich TMP ligand (than the TTP ligand of 1d) and would thus

be less oxidizing (see Table 1), is likely to be associated with

the higher basicity of the QC group of 2d, as HAT reactions of

metal complexes depend on both of their redox potentials and

the basicity of their reactive groups responsible for the HAT

reactivity (for relatively low redox potentials, the basicity

could play a dominant role).⁵³

The reactions of 4 with C–H and X–H substrates are unlike-

ly to proceed by a single-electron-transfer (SET) or hydride-

transfer mechanism, as aprotic molecule (e.g. PhSMe) and

hydride source (e.g. Et₃SiH) were not reactive with 4. In addi-

tion, the finding that no QC C–H or X–H insertion products

were detected in the reaction mixtures between 4 and C–H or

X–H substrates demonstrates a high chemoselectivity (in

terms of HAT vs carbene insertion) of these reactions.

Concerning the observed effect of Lewis acid additives such

as BF₃and FeCl₃on the reaction of 4 with DHA and/or CHD,

possible causes may be associated with the steric^33gor redox

properties of the additives, in view of the improved elementary

HAT step by BF₃(Table S11) and the facilitated anthracene

formation (from DHA) by FeCl₃, but not by bulky Sc(OTf)₃,

along with the improved product turnovers by FeCl₃in the 4-

catalyzed conversion of CHD to benzene (Scheme 4). A

mechanism which involves rate-limiting HAT transfer from

CHD to 4 to generate a radical intermediate and reaction of the

radical intermediate with air or FeCl₃to give benzene (FeCl₃

acting as a 1 e^oxidant), could be proposed (Scheme 6; BF₃

was not used as an additive for the aerobic catalysis owing to

its air-sensitive and non-redox active features).

■ CONCLUSION

We have isolated a series of terminal monodentate quinoid

carbene (QC) complexes of ruthenium, [Ru(Por)(QC)] (1–4),

which were prepared using diazo quinones N₂QC as QC

sources and exhibited dual reactivity including QC transfer

with organic substrates, lending evidence for the widely pro-

posed generation of metal-QC intermediates in various metal-

catalyzed QC transfer reactions of diazo quinones. These iso-

lated Ru-QC complexes have been characterized by spectro-

scopic means, electrochemical measurements, and X-ray crys-

tal structure determinations, and span various porphyrin lig-

ands as well as a panel of QC ligands bearing electron-

withdrawing and -donating substituents, which allows fine-

tuning of the energy level and reactivity of the carbene group

and facilitates examination of the effect of QC substituents.

DFT calculations provide useful insights into the reaction

mechanisms and the electronic properties of the Ru-QC com-

plexes, including their low-lying LUMO orbitals which are

spread over the whole quinoid rings. Such a highly conjugated

carbene * orbital is crucial for their dual reactivity feature, as

the incoming substrate can approach either the carbene carbon

or the carbonyl oxygen, depending on the nature of the sub-

strate.

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The stoichiometric QC transfer reactions of [Ru(Por)(QC)]

with nucleophilic nitrosoarenes afforded nitrone products in

75–90% yields; this reactivity, along with the preparation of

[Ru(Por)(QC)] from [Ru(Por)(CO)] and diazo quinones, led to

the development of a [Ru(Por)(CO)]-catalyzed nitrone for-

mation from nitrosoarenes and diazo quinones. An inverted

substituent effect on carbene reactivity, compared with that for

free electrophilic QCs, was revealed by kinetic studies on the

stoichiometric QC transfer reactions of complexes

Scheme 6. Proposed Mechanism for Conversion of CHD to

Benzene Mediated by 4

t

[Ru(TTP)(QC^R)] (1a–d; R = Bu, Ph, F, Br) with PhNO, and

also by the corresponding studies on the catalytic counterparts.

The inverted substituent effect of [Ru(TTP)(QC^R)] could be

rationalized by electronic factors and DFT-calculated carbene

orbitals and transition state structures. A structure-reactivity

relationship was established between ¹³C NMR carbene chem-

ical shifts and stoichiometric/catalytic kinetics.

It is worth noting that the transformation of Ru-QC complex

4 to “semiquinone-like” Ru-aryl complex 6 (Scheme 5) is a

net 1 e^process, unlike the 2 e^-process of the ferraquinone-

ferrahydroquinone couple^11bwhich resembles the quinone-

hydroquinone couple. Moreover, the high chemoselectivity of

4 towards HAT mechanism and much lower reactivity in other

pathways differ from the oxidation reactions with quinones

which have been known to proceed by various mechanisms

including HAT, SET and hydride transfer.⁵⁴In addition, the

rate constant (k₂) for the DHA oxidation by 4 (0.44 ± 0.02 M^-1

s^-1) is comparable to that by Ru^VI-oxo porphyrins [Ru-

^VI(TMP)O₂] (0.17 M^-1s^-1) and [Ru^VI(TPFPP)O₂] (1.74 M^-1s^-1),

though smaller than that by some other Ru-oxo complexes

Complexes [Ru(Por)(QC)] can also react with relatively

weak C–H and X–H (X = S, N) bonds and show strong prefer-

ence for

a

HAT mechanism. The reaction of

[Ru(TDCPP)(QC^Br)] (4) with the C–H bond of CHD gave a

semiquinone-like Ru(III)-aryl complex [Ru^III(TDCPP)(Ar^Br)]

(6) which can be readily converted back to 4 under aerobic

conditions. The 4/6 redox couple has been utilized to develop

a catalytic aerobic oxidation of CHD to benzene, demonstrat-

ing the potential application of Ru-QC complexes in metal-

catalyzed aerobic C–H activation/functionalization reactions.

Discovery of new reactivities of metal-carbene complexes

and elucidation of their structure-reactivity relationship are of

fundamental importance for the design of novel synthetic

methodologies based on metal-carbene functionalities.⁵⁶De-

spite promising synthetic utility of metal-QC complexes, there

is currently rather limited knowledge of such complexes or

related species in the literature.^11,⁵⁷The present work, by com-

bining isolation, characterization and stoichiometric/catalytic

reactivity study, and also DFT calculations, of the novel Ru-

QC porphyrin complexes, provides useful information for the

(22–125 M^-1s^-1).^31c,Different from these Ru-oxo systems

55

which, after HAT, can undergo radical rebound to transfer the

oxygen atoms to the C–H substrates,^31c,55the QC group in 4

was transformed to an aryl ligand which is rather inert towards

homolytic cleavage and radical rebound. This property of 4

renders it plausible to develop different types of metal-

catalyzed C–H activation/functionalization reactions (relative

to metal-oxo systems) such as the aerobic oxidation catalysis

depicted in Scheme 4. Current challenges are to enhance the

reactivity towards more inert C–H bonds and to improve the

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design of related catalytic systems as well as for the future

Hidano, T.; Sasagawa, H.; Fujiwara, M.; Miyazaki, S.; Nadano, R.;

2

3

Ichikawa, J. Regioselective syntheses of fluorinated cyclopentanone

derivatives: ring construction strategy using transition-metal-

difluorocarbene complexes and free difluorocarbene. Synthesis-

Stuttgart 2018, 50, 514-528. (r) Simões, M. M. Q.; Gonzaga, D. T. G.;

Cardoso, M. F. C.; Forezi, L. D. S. M.; Gomes, A. T. P. C.; da Silva,

F. D. C.; Ferreira, V. F.; Neves, M. G. P. M. S.; Cavaleiro, J. A. S.

Carbene transfer reactions catalysed by dyes of the metalloporphyrin

group. Molecules 2018, 23, 792/1-792/34. (s) Vaitla, J.; Bayer, A.

Sulfoxonium ylide derived metal carbenoids in organic synthesis.

Synthesis-Stuttgart 2019, 51, 612-628.

research in metal-QC chemistry.

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■ ASSOCIATED CONTENT

Supporting Information. Abbreviations, experimental proce-

dures, characterization of compounds, computational details, Ta-

bles S1S12, Figures S1S18, NMR spectra of compounds, Car-

tesian coordinates from DFT calculations, and CIF files for the

crystal structures of 1a, 2d, 3a, and 5d. This material is available

free of charge via the Internet at http://pubs.acs.org.

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■ AUTHOR INFORMATION

Corresponding Authors

*jshuang@hku.hk

*cmche@hku.hk

Notes

The authors declare no competing ﬁnancial interest.

■ ACKNOWLEDGMENT

This work was supported by Hong Kong Research Grants Council

(HKU 17303815) and Basic Research Program-Shenzhen Fund

(3) For examples bearing non-porphyrin ligands, see: (a) Dötz, K.

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(JCYJ20160229123546997,

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and

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mond, Dr. Ka-Pan Shing, and Dr. Xiting Zhang for fruitful dis-

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Article Doi

Ruthenium(II) Porphyrin Quinoid Carbene Complexes: Synthesis, Crystal Structure, and Reactivity toward Carbene Transfer and Hydrogen Atom Transfer Reactions

DOI: 10.1021/jacs.9b03357

Source and publish data:

Authors:

Article abstract of DOI:10.1021/jacs.9b03357

Full text of DOI:10.1021/jacs.9b03357

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