Ruthenium porphyrins with axial π-conjugated arylamide and arylimide ligands

Article abstract of DOI:10.1002/chem.201305084

A series of ruthenium porphyrins [Ru^IV(por)(NHY)₂] and [Ru^VI(por)(NY)₂] bearing axially coordinated π-conjugated arylamide and arylimide ligands, respectively, have been synthesized. The crystal structures of [Ru^IV(tmp)(NHY)₂] (tmp=5,10,15,20-tetramesitylporphyrinato(2-)) with Y=4-methoxy-biphenyl-4-yl (Ar-Ar-p-OMe), 4-chloro-biphenyl-4-yl (Ar-Ar-p-Cl), and 9,9-dibutyl-fluoren-2-yl (Ar^Ar) show axial Ru-N(arylamide) distances of 1.978(4), 1.971(6), and 1.985(13) A, respectively. [Ru^IV(tmp)(NH{Ar^Ar})₂] is an example of metalloporphyrins that bind an arylamide ligand featuring a co-planar biphenyl unit. The [Ru^IV(por)(NHY)₂] complexes show a quasi-reversible reduction couple or irreversible reduction wave attributed to Ru^IV→Ru^III with E_pc from -1.06 to -1.40 V versus Cp₂Fe^+/0 and an irreversible oxidation wave with E_pa from -0.04 to 0.19 V versus Cp ₂Fe^+/0. Reaction of the [Ru^IV(por)(NHY) ₂] with bromine afforded [Ru^IV(por)(NHY)Br]. PhI(OAc) ₂ oxidation of the [Ru^IV(por)(NHY)₂] gave [Ru^VI(por)(NY)₂]; the latter can be prepared from reaction of [Ru^II(por)(CO)] with aryl azides N₃Y. The crystal structure of [Ru^VI(tmp)(N{Ar-Ar-p-OMe})₂] features Ru-N(arylimide) distances of 1.824(5) and 1.829(5) A. Alkene aziridination and C-H amination catalyzed by [Ru^II(tmp)(CO)]+π-conjugated aryl azides , or mediated by [Ru^VI(por)(NY)₂] with Y=biphenyl-4-yl (Ar-Ar) and Ar-Ar-p-Cl, gave aziridines and amines in moderate yields. The electronic structure of [Ru^VI(por)(NY)₂] was examined by DFT calculations.

Full text of DOI:10.1002/chem.201305084

DOI: 10.1002/chem.201305084
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&
Metal–Arylamides/Aylimides
Ruthenium Porphyrins with Axial p-Conjugated Arylamide and
Arylimide Ligands
Siu-Man Law,[a] Daqing Chen,[a] Sharon Lai-Fung Chan,[c] Xiangguo Guan,[a] Wai-Man Tsui,[a]
Jie-Sheng Huang,[a] Nianyong Zhu,[a] and Chi-Ming Che*[a, b]
Abstract: A series of ruthenium porphyrins [RuIV(por)(NHY)2]
and [RuVI(por)(NY)2] bearing axially coordinated p-conjugated
arylamide and arylimide ligands, respectively, have been syn-
thesized. The crystal structures of [RuIV(tmp)(NHY)2] (tmp=
5,10,15,20-tetramesitylporphyrinato(2ꢀ)) with Y=4’-me-
thoxy-biphenyl-4-yl (ArꢀAr-p-OMe), 4’-chloro-biphenyl-4-yl
(ArꢀAr-p-Cl), and 9,9-dibutyl-fluoren-2-yl (Ar^Ar) show axial
RuꢀN(arylamide) distances of 1.978(4), 1.971(6), and
1.985(13) ꢀ, respectively. [RuIV(tmp)(NH{Ar^Ar})2] is an exam-
ple of metalloporphyrins that bind an arylamide ligand fea-
turing a co-planar biphenyl unit. The [RuIV(por)(NHY)2] com-
plexes show a quasi-reversible reduction couple or irreversi-
ble reduction wave attributed to RuIV!RuIII with Epc from
ꢀ1.06 to ꢀ1.40 V versus Cp2Fe+/0 and an irreversible oxida-
tion wave with Epa from ꢀ0.04 to 0.19 V versus Cp2Fe+/0. Re-
action of the [RuIV(por)(NHY)2] with bromine afforded [RuIV-
(por)(NHY)Br]. PhI(OAc)2 oxidation of the [RuIV(por)(NHY)2]
gave [RuVI(por)(NY)2]; the latter can be prepared from reac-
tion of [RuII(por)(CO)] with aryl azides N3Y. The crystal struc-
ture of [RuVI(tmp)(N{ArꢀAr-p-OMe})2] features RuꢀN(aryli-
mide) distances of 1.824(5) and 1.829(5) ꢀ. Alkene aziridina-
tion and CꢀH amination catalyzed by “[RuII(tmp)(CO)]+p-
conjugated aryl azides”, or mediated by [RuVI(por)(NY)2] with
Y=biphenyl-4-yl (ArꢀAr) and ArꢀAr-p-Cl, gave aziridines
and amines in moderate yields. The electronic structure of
[RuVI(por)(NY)2] was examined by DFT calculations.
complexes[4] or metal–mono(imide) complexes (A and B in
Figure 1).[5] Despite the reports of a number of metalloporphyr-
in arylamide[6–8] and arylimide[5b,8–13] complexes, we are aware
of one such complex, [VIV(ttp)(N{ArꢀAr})] (ArꢀAr=biphenyl-4-
yl),[3,5b] that bears mutually conjugated phenyl groups in the
Introduction
Studies on metal-to-ligand p-back-bonding interactions in low-
valent metal complexes containing p-acid ligands prevail in in-
organic and organometallic chemistry.[1] Of equal importance
are metal-ligand p-bonding interactions that are prevelant in
high-valent metal complexes.[2] In our endeavor to exploit
metalꢀnitrogen p-bonding interaction as a means for electron-
ic communication between redox active metal ions and
remote functional groups over a long distance, we are interest-
ed in metalloporphyrins[3] axially coordinated with p-conjugat-
ed arylamide and arylimide ligands that possess mutually con-
jugated phenyl groups. These types of p-conjugated aryl-
amide/arylimide ligands coordinated to transition metals are
sparsely seen in the literature; we noted a few such examples,
which belong to metal–mono(amide) or cis-metal–bis(amide)
[a] S.-M. Law, Dr. D. Chen, Dr. X. Guan, Dr. W.-M. Tsui, Dr. J.-S. Huang,
Dr. N. Zhu, Prof. Dr. C.-M. Che
Department of Chemistry and State Key Laboratory of Synthetic Chemistry
The University of Hong Kong, Pokfulam Road, Hong Kong (P. R. China)
E-mail: cmche@hku.hk
[b] Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (P. R. China)
[c] Dr. S. L.-F. Chan
Department of Applied Biology and Chemical Technology
The Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong (P. R. China)
Figure 1. Types of metal complexes with p-conjugated imides, and the cor-
responding catalytic and stoichiometric nitrogen group transfer/insertion re-
actions, reported in this work. Inset: literature-reported metal complexes
with p-conjugated imides.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201305084.
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axial ligand. Cenini, Gallo, and co-workers reported the synthe-
sis of [RuVI(tpp)(N{Ar-3,5-CF3})2] (Ar-3,5-CF3 =3,5-di(trifluoro-
methyl)phenyl),[3,8] which has been characterized by X-ray crys-
tal structure analysis. It should be noted that there are exam-
ples of reactive metal–arylimide non-porphyrin complexes
such as the ones reported by Hillhouse,[14a,b] Betley,[14c] and co-
workers.
aerobic ethanol or dichloromethane for 8 h afforded the corre-
sponding [RuIV(tmp)(NHY)2] (1, Scheme 1 and entries 1–7 in
Table 1) in 54–73% yields. Complexes [RuIV(por)(NH{ArꢀAr})2]
(por=ttp, 3,4,5-OMe-tpp) and [RuIV(por)(NH{Ar^Ar})2] (por=
ttp, 4-OMe-tpp, 4-Cl-tpp) were prepared in 52–76% yields from
similar reactions (entries 8–12, Table 1). We also treated [RuVI-
(ttp)O2] with NH2(ArꢀCꢁCꢀAr) and NH2(ArꢀCꢁCꢀArꢀCꢁCꢀ
Ar); these reactions did not give the corresponding [RuIV(ttp)-
(NHY)2]. When [RuVI(F20-tpp)O2] was treated with NH2{Ar^Ar},
and also NH2(Ar-p-X) (X=Cl, NHPh), under similar reaction con-
ditions, bis(arylamine)ruthenium(II) porphyrins [RuII(F20-tpp)-
(NH2Y)2] (2, Scheme 1 and entries 16–18 in Table 1) were ob-
tained in 20–50% yields. In contrast, [RuII(por)(NH2{Ar^Ar})2]
(por=tmp, ttp, 4-OMe-tpp, 4-Cl-tpp, 3,4,5-OMe-tpp) were not
detected in the final reaction mixtures of the corresponding
[RuVI(por)O2] with NH2(Ar^Ar) as revealed by UV/Vis and
1H NMR spectroscopy. Upon stirring a solution of [RuII(F20-tpp)-
(NH2{Ar^Ar})2] in aerobic dichloromethane for 5 days, [RuIV(F20-
tpp)(NH{Ar^Ar})2] was obtained in 95% yield.
We previously prepared [RuIV(por)(NPh2)2] (por=3,4,5-OMe-
tpp; 3,5-Cl-tpp)[6b,d] and [RuIV(por)(NH{Ar-p-X})2] (por=ttp, 4-Cl-
tpp; X=Cl, NO2)[6d] from the reactions of [RuVI(por)O2] with
NHPh2 and NH2(Ar-p-X), respectively. Similarly, [RuIV(por)(NH{Ar-
p-Cl})2] (por=tmp, 4-OMe-tpp) were prepared in 78–88%
yields in this work (entries 14 and 15, Table 1). The reactions of
[RuVI(por)O2] with alkylamines[6d,22,23] and amino esters[24] were
reported to afford bis(amine)ruthenium(II) porphyrins. Other
related reactivity of [RuVI(por)O2] include the reaction with ben-
zophenone imine to give [RuIV(por)(N=CPh2)2][22b] or [RuII(por)-
(NH=CPh2)2],[25] and the reaction with 1,1-diphenylhydrazine to
give [RuIV(por)(NHNPh2)2].[26]
The formation of [RuIV(por)(NHY)2] from reaction of [RuVI-
(por)O2] with excess amounts of NH2Y in aerobic solutions is
likely to proceed through intermediates [RuII(por)(NH2Y)2]; the
latter underwent oxidative deprotonation by air to give the ar-
ylamide complexes.[6d] Indeed, in the course of the synthesis of
[RuIV(3,4,5-OMe-tpp)(NH{ArꢀAr})2], the UV/Vis absorption spec-
trum of the reaction mixture first showed Soret and b bands at
approximately 418 and 505 nm, respectively (reaction time:
~30 min), which are characteristic of bis(amine)ruthenium(II)
porphyrins.[6d] Further evidence is the above-mentioned isola-
tion of [RuII(F20-tpp)(NH2Y)2], instead of [RuIV(F20-tpp)(NHY)2], for
the reactions with NH2(Ar^Ar) and NH2(Ar-p-X) (X=Cl, NHPh)
at a reaction time of 8 h, coupled with the nearly quantitative
conversion of [RuII(F20-tpp)(NH2{Ar^Ar})2] to [RuIV(F20-tpp)(NH-
{Ar^Ar})2] by autoxidation for 5 days. The high stability of
[RuII(F20-tpp)(NH2Y)2] can be attributed to the relatively elec-
tron-deficient F20-tpp ligand, which renders the RuII complexes
less prone to undergo oxidative deprotonation. For compari-
son, [RuII(por)(NH2{Ar-p-Cl})2] (por=ttp, 4-Cl-tpp) rapidly under-
go autooxidation to [RuIV(por)(NH{Ar-p-Cl})2] in aerobic solu-
tions within 0.5–1.5 h.[6d]
Metalloporphyrin complexes of p-conjugated arylimides/aryl-
nitrenes are also of importance in CꢀN bond formation via
metal-catalyzed nitrogen group transfer/insertion reactions,
like sulfonylimide and simple arylimide complexes of rutheni-
um porphyrins [RuVI(por)(NSO2Y)2][15] and [RuVI(tpp)(N{Ar-3,5-
CF3})2][3,8] both of which are reactive toward CꢀH amination of
hydrocarbons. Examples of p-conjugated arylnitrenes generat-
ed by photolysis of the corresponding aryl azides have been
documented,[16,17] and the photolytically generated biphenyl-2-
yl nitrene undergoes intramolecular CꢀH insertion reaction.[18]
There are several reports on metal-catalyzed CꢀN bond form-
ing reactions[19–21] which possibly involve reactive metal–mon-
o(imide/nitrene) intermediates including Rh or Fe complexes
of biphenyl-4-yl imides/nitrenes[19,20] and Rh or Ru complexes
of biphenyl-2-yl imides/nitrenes.[21] But the transfer/insertion of
p-conjugated arylimides/arylnitrenes from their well-character-
ized complexes of transition metals to hydrocarbons has been
little (if at all) explored.
Herein we report the syntheses, characterization, and reac-
tivity of a series of trans-bis(p-conjugated arylamide) and
trans-bis(p-conjugated arylimide) complexes (C and
D in
Figure 1) of ruthenium porphyrins: [RuIV(por)(NHY)2] (Scheme 1)
and [RuVI(por)(NY)2] (Scheme 2), respectively. Nitrogen group
transfer/insertion reactions of these [RuVI(por)(NY)2] complexes
with alkenes/CꢀH bonds have been observed to give aziridina-
tion/amination products; the corresponding catalytic intermo-
lecular aziridination/amination of alkenes/CꢀH bonds has been
achieved using a “[RuII(por)(CO)]+p-conjugated aryl azide”
protocol (Figure 1). Also reported herein are the electrochemi-
cal studies on [RuIV(por)(NHY)2] and the reactions of [RuIV(por)-
(NHY)2] with Br2 and PhI(OAc)2 to give [RuIV(por)(NHY)Br] and
[RuVI(por)(NY)2], respectively, together with the studies of the
electronic structure of [RuVI(por)(NY)2] by density functional
theory (DFT) calculations.
Results and Discussion
Synthesis
The ruthenium porphyrin complexes synthesized in this work
and the abbreviations for various p-conjugated aryl groups (in
bold face) including Ar^Ar that bears a co-planar biphenyl
unit, along with those for simple aryl groups (in plain face), are
listed in Table 1. The abbreviations for the porphyrin ligands
are included in ref. [3] (see also the inset of Scheme 1).
Bis(p-conjugated arylamide) complexes: Treatment of the
dioxoruthenium(VI) porphyrin [RuVI(tmp)O2] with excess
The fate of arylamines upon oxidation by [RuVI(por)O2] was
examined using NH2(ArꢀAr) and NH2(Ar-p-Cl) as examples,
amounts of p-conjugated arylamines NH2Y including NH2(Arꢀ which were found to be oxidized to YꢀN=NꢀY (Y=ArꢀAr, Ar-
Ar), NH2(ArꢀAr-p-OMe), NH2(ArꢀAr-p-Cl), NH2(Ar^Ar), NH2(Arꢀ p-Cl) in 55 and 48% yields, respectively. Such YꢀN=NꢀY com-
ArꢀAr), NH2(ArꢀCꢁCꢀAr), and NH2(ArꢀCꢁCꢀArꢀCꢁCꢀAr), in
pounds were previously formed through, for example, amine
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Scheme 1. Syntheses of [RuIV(por)(NHY)2] (1) from reaction of [RuVI(por)O2] with NH2Y or from oxidative deprotonation of [RuII(por)(NH2Y)2] (2). Inset: porphyri-
nato(2ꢀ) ligands employed in this work.
oxidation with KMnO4 catalyzed by FeSO4·7H2O[27] or CuSO4
and Al2O3,[28] by treating the amine with HgO and I2 in di-
chloromethane,[29] or by [cobalt(II)-salen]-catalyzed oxidation
using oxygen as oxidant.[30] A possible mechanism for the for-
mation of YꢀN=NꢀY from the oxidation of NH2Y by [RuVI-
(por)O2] involves generation of reactive [RuVI(por)(NY)2] inter-
mediate, which subsequently reacts with the arylamine and/or
undergo coupling of the coordinated arylimide ligand. In this
regard, we attempted to develop oxidation of NH2(ArꢀAr)
using [RuII(por)(CO)] as a catalyst, 2,6-dichloropyridine N-oxide
(2,6-Cl2pyNO) as a terminal oxidant, and dichloromethane as
solvent. However, ArꢀArꢀN=NꢀArꢀAr was not formed in the
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Scheme 2. Syntheses of [RuVI(por)(NY)2] (3) from reaction of [RuII(por)(CO)] with N3Y.
1
reaction mixture as indicated by H NMR spectroscopy analysis,
and >90% of the starting NH2(ArꢀAr) was recovered.
Besides oxidative deprotonation of metal–arylamine porphy-
rin complexes/intermediates to give [RuIV(por)(NHY)2] and [OsIV-
(por)(NHY)2],[6a,c] other methods have also been reported for
preparing/generating metal–arylamide complexes of porphyr-
ins. [TiIV(ttp)(NPh2)2] was prepared from reaction of [TiIV(ttp)Cl2]
with LiNPh2;[7a] a similar reaction of [HfIV(ttp)Cl2] with LiNH(Ar-p-
Me) afforded [HfIV(ttp)(NH{Ar-p-Me})2] (Ar-p-Me=4-tolyl).[7b]
Treatment of [OsVI(por)O2] with hydrazine gave [OsIV(por)-
(NPh2)(OH)].[6e] The reaction of imide complexes [MIV(ttp)(N{Ar-
2,6-iPr})] (M=Zr, Hf; Ar-2,6-iPr=2,6-diisopropylphenyl) with
tBuC(O)Me produced [MIV(ttp)(NH{Ar-2,6-iPr})(OC(tBu)=CH2)].[7c]
Bis(p-conjugated arylimide) complexes: Reaction of car-
bonylruthenium(II) porphyrin [RuII(tmp)(CO)] with p-conjugated
aryl azides N3Y (3 equiv; Y=ArꢀAr, ArꢀAr-p-OMe, ArꢀAr-p-Cl,
ArꢀArꢀAr, ArꢀCꢁCꢀAr) in refluxing benzene for 2 h afforded
[RuVI(tmp)(NY)2] (3, Scheme 2; entries 19–23 in Table 1) in 60–
65% yields. The unreacted aryl azides were washed away with
hexane/diethyl ether, giving the [RuVI(tmp)(NY)2] in >95%
purity (by 1H NMR analysis).[31]
Complexes [RuVI(tmp)(NY)2] (Y=ArꢀAr, ArꢀAr-p-OMe, ArꢀAr-
p-Cl) can also be generated by oxidative deprotonation reac-
tions of the corresponding [RuIV(tmp)(NHY)2] with PhI(OAc)2
(see below). In view of the formation of [RuVI(ttp)(NtBu)2] and
[RuVI(ttp)(NtBu)O] from treatment of [RuVI(ttp)O2] with tert-bu-
tylamine in aerobic hexane,[6d] we examined the reaction of
[RuVI(tmp)O2] with NH2(ArꢀAr) under similar conditions. But
the latter reaction in aerobic hexane gave the same product
[RuIV(tmp)(NH{ArꢀAr})2] as that in ethanol solvent; no ruthe-
nium(VI)–arylimide species was detected.
Table 1. Ruthenium porphyrin complexes synthesized in this work.
Entry
Complex
[RuIV(por)(NHY)2] (1)
1
2
3
[RuIV(tmp)(NH{ArꢀAr})2] (1a)[a]
[RuIV(tmp)(NH{ArꢀAr-p-OMe})2] (1b)[b]
[RuIV(tmp)(NH{ArꢀAr-p-Cl})2] (1c)[c]
[RuIV(tmp)(NH{Ar^Ar})2] (1d)[d]
4
5
6
7
8
[RuIV(tmp)(NH{ArꢀArꢀAr})2] (1e)[e]
[RuIV(tmp)(NH{ArꢀCꢁCꢀAr})2] (1 f)[f]
[RuIV(tmp)(NH{ArꢀCꢁCꢀArꢀCꢁCꢀAr})2] (1g)[g]
[RuIV(ttp)(NH{ArꢀAr})2] (1h)[a]
9
[RuIV(3,4,5-OMe-tpp)(NH{ArꢀAr})2] (1i)[a]
10
11
12
13
14
15
[RuIV(ttp)(NH{Ar^Ar})2] (1j)[d]
[RuIV(4-OMe-tpp)(NH{Ar^Ar})2] (1k)[d]
[RuIV(4-Cl-tpp)(NH{Ar^Ar})2] (1l)[d]
[RuIV(F20-tpp)(NH{Ar^Ar})2] (1m)[d]
[RuIV(tmp)(NH{Ar-p-Cl})2] (1n)[h]
[RuIV(4-OMe-tpp)(NH{Ar-p-Cl})2] (1o)[h]
[RuII(por)(NH2Y)2] (2)
16
17
18
[RuII(F20-tpp)(NH2{Ar^Ar})2] (2a)[d]
[RuII(F20-tpp)(NH2{Ar-p-Cl})2] (2b)[h]
[RuII(F20-tpp)(NH2{Ar-p-(NHPh)})2] (2c)[i]
[RuVI(por)(NY)2] (3)
19
20
21
22
23
24
25
[RuVI(tmp)(N{ArꢀAr})2] (3a)[a]
[RuVI(tmp)(N{ArꢀAr-p-OMe})2] (3b)[b]
[RuVI(tmp)(N{ArꢀAr-p-Cl})2] (3c)[c]
[RuVI(tmp)(N{Ar-ꢀArꢀAr})2] (3d)[e]
[RuVI(tmp)(N{ArꢀCꢁCꢀAr})2] (3e)[f]
[RuVI(tmp)(N{Ar-p-NO2})2] (3 f)[j]
[RuVI(ttp)(N{Ar-p-NO2})2] (3g)[j]
[RuIV(por)(NHY)Br] (4)
26
27
28
[RuIV(tmp)(NH{ArꢀAr})Br] (4a)[a]
[RuIV(tmp)(NH{Ar^Ar})Br] (4b)[d]
[RuIV(tmp)(NH{ArꢀArꢀAr})Br] (4c)[e]
The use of aryl azides for synthesizing metal–arylimide por-
phyrin complexes can be dated back to 1987, in which year
the synthesis of [CrIV(tpp)(N{Ar-p-Me})] from N3(Ar-p-Me) and
[CrII(tpp)] was reported.[9a] Subsequently, [OsVI(por)(N{Ar-p-
NO2})2] (por=ttp, 4-Cl-tpp) were synthesized from [OsII(ttp)]2
and N3(Ar-p-NO2), and were both structurally characterized by
X-ray crystal analysis;[13] a similar reaction for [RuII(ttp)]2 gave
[RuVI(tpp)(N{Ar-p-NO2})2].[13b] Recently, Cenini, Gallo, and their
[a] ArꢀAr=biphenyl-4-yl.
[b] ArꢀAr-p-OMe=4’-methoxy-biphenyl-4-yl.
[c] ArꢀAr-p-Cl=4’-chloro-biphenyl-4-yl. [d] Ar^Ar=9,9-dibutyl-fluoren-2-
yl. [e] ArꢀArꢀAr=1,1’:4’,1“-terphenyl-4-yl. [f] ArꢀCꢁCꢀAr=4-(phenyle-
thynyl)phenyl. [g] ArꢀCꢁCꢀArꢀCꢁCꢀAr=4-((4-(phenylethynyl)phenyl)e-
thynyl)phenyl. [h] Ar-p-Cl=4-chlorophenyl. [i] Ar-p-(NHPh)=4-(phenylami-
no)phenyl. [j] Ar-p-NO2 =4-nitrophenyl.
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co-workers isolated [RuVI(tpp)(N{Ar-3,5-CF3})2] by treatment of
[RuII(tpp)(CO)] with N3(Ar-3,5-CF3) and determined the X-ray
crystal structure of the ruthenium–bis(arylimide) complex.[8]
Similar reactions of [RuII(por)(CO)] (por=tmp, ttp) with N3(Ar-p-
NO2) in this work gave [RuVI(por)(N{Ar-p-NO2})2] in 60–70%
yields (entries 24 and 25, Table 1). These bis(arylimide) com-
plexes of metalloporphyrins all feature strongly electron-with-
drawing substituents on the arylimide ligands, which would
render the arylimides less susceptible to hydrolysis and thus
enhance the stability of the complexes.
Notably, the bis(p-conjugated arylimide) complexes [RuVI-
(tmp)(NY)2] (Y=ArꢀAr, ArꢀAr-p-OMe, ArꢀArꢀAr, ArꢀCꢁCꢀAr)
prepared in this work are devoid of strongly electron-withdraw-
ing groups on the arylimide ligands. Probably, the sterically en-
cumbered porphyrin ligand (tmp) and the electron delocaliza-
tion over the p-conjugated phenyl groups play a significant
role in stabilizing these bis(arylimide) complexes.
Other examples of previously reported metal–arylimide por-
phyrins are mono(arylimide) complexes, including [VIV(ttp)(N-
{Ar-p-X})] (X=H, Me, OMe, Cl, tBu) reported in 1985,[5b] and the
subsequently reported complexes [CrIV(por)(N{Ar-p-X})] (por=
tpp, X=Cl, I, OMe; por=ttp, X=Me),[9] [OsVI(por)(N{Ar-p-F})(O)]
(por=tpp, 3,4,5-OMe-tpp),[6a,c] [TiIV(ttp)(N{Ar-p-X})] (X=H,
Me),[7a,10a,b] [RuIV(por)(N{Ar-p-X})] (por=tbpp, X=H, Me, Cl, I;
por=ttp, X=Me),[11] [MIV(ttp)(N{Ar-2,6-iPr})] (M=Zr, Hf),[7b]
[MoIV(ttp)(NY)] (Y=Ph, Ar-p-Me, Ar-2,4,6-Me, Ar-2,6-iPr),[10c] and
[VIV(oep)(NPh)].[12] These complexes were prepared by reactions
of MꢀCl (M=V, Ti, Ru) complexes with NH2Y[5b,11,12] or LiNHY,[7a,-
reactions of [CrII(por)] with aryl azides,[9] or autoxidation
b,10a,c]
Figure 2. 1H NMR spectra (300 MHz) of: a) [RuIV(tmp)(NH{ArꢀAr})2] (1a), and
b) [RuVI(tmp)(N{ArꢀAr})2] (3a) in CDCl3. The tmp signals are labeled by Hb
(pyrrolic protons), Hm (m-H of the 5,10,15,20-mesityl groups), o-CH3, and p-
CH3. The arylamide/arylimide Ho’ and Hm’ signals were assigned by compari-
of OsꢀNHY complexes.[6a,c]
Stability
1
son with the H NMR spectra of [RuIV(ttp)(NH{Ar-p-X})2][6d] and [RuVI(tpp)(N{Ar-
p-NO2})2].[13b]
The complexes [RuIV(por)(NHY)2] and [RuVI(por)(NY)2] prepared
in this work (Table 1) are soluble in chloroform and dichloro-
methane. Of these complexes, [RuIV(por)(NHY)2] are stable
toward air and moisture for more than one month in the solid
state and for about one week in aerobic chloroform or di-
chloromethane solutions; [RuVI(por)(NY)2] are stable for serveral
hours in chloroform solutions and for about 1 day in the solid
state (all at room temperature). Upon standing the solutions
for days, [RuVI(por)(NY)2] was reduced to [RuIV(por)(NHY)2] and
decomposed to some unknown species.
is only a single set of ArꢀAr signals, indicating that the two ax-
ially coordinated arylamide or arylimide ligands are equivalent;
3) for the ArꢀAr group, the phenyl moiety closer to the por-
phyrin ring gives more separated and upfield-shifted signals
(Ho’ and Hm’ in Figure 2 at dꢂ3 and 6 ppm, respectively) due
to a larger effect of the porphyrin ring current; the other
phenyl moiety exhibits much less separated signals at d
ꢂ7 ppm (Ho’’, Hm’’, Hp’’ in Figure 2). Compared with the ArꢀAr
signals of [RuIV(tmp)(NH{ArꢀAr})2] (d=7.14–6.99, 6.03,
3.28 ppm), those of [RuVI(tmp)(N{ArꢀAr})2] (d=7.08–6.82, 5.94,
2.86 ppm) are upfield-shifted, in agreement with the shorter
RuꢀN(arylimide) than RuꢀN(arylamide) distances (see below).
Complexes [RuIV(tmp)(NHY)2] (Y=ArꢀAr, ArꢀAr-p-OMe, Arꢀ
Ar-p-Cl, Ar^Ar, ArꢀArꢀAr, ArꢀCꢁCꢀAr, ArꢀCꢁCꢀArꢀCꢁCꢀ
Ar, Ar-p-Cl) exhibit Hb signals with similar chemical shifts (d=
8.16–8.19 ppm). The Hb chemical shifts of [RuVI(tmp)(NY)2] (Y=
ArꢀAr, ArꢀAr-p-OMe, ArꢀAr-p-Cl, ArꢀArꢀAr, ArꢀCꢁCꢀAr) are
also similar (d=8.53–8.56 ppm), though slightly smaller than
Spectroscopy
1H NMR spectroscopy: The [RuIV(por)(NHY)2], [RuVI(por)(NY)2],
and [RuII(por)(NH2Y)2] (Table 1) all show diamagnetic 1H NMR
spectra. The signals of porphyrin pyrrolic protons, Hb, appear
at d=8.16–8.57 ppm for [RuIV(por)(NHY)2] (Table S1 in the Sup-
porting Information; NH signals not located)[6d] and at d=
8.53–8.84 ppm for [RuVI(por)(NY)2] (Table S2 in the Supporting
Information).
The spectra of [RuIV(tmp)(NH{ArꢀAr})2] and [RuVI(tmp)(N{Arꢀ that of [RuVI(tmp)(N{Ar-p-NO2})2] (d=8.64 ppm). This shows
Ar})2], as examples, are shown in Figure 2. Both spectra have
the following features: 1) the intensity ratio of signals corre-
sponds to a 1:2 molar ratio of tmp versus axial ligand; 2) there
that the various p-conjugated aryl groups have a similar effect
on the pyrrolic proton resonances despite the different extent
of p conjugation.
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More significant variation of Hb chemical shifts was observed
among different types of porphyrin ligands, with d=8.17–
8.43 ppm for [RuIV(por)(NH{Ar^Ar})2] (por=tmp, ttp, 4-OMe-
tpp, 4-Cl-tpp, F20-tpp), d=8.17–8.57 ppm for [RuIV(por)(NH{Arꢀ
Ar})2] (por=tmp, ttp, 3,4,5-OMe-tpp), and d=8.64–8.84 ppm
for [RuVI(por)(N{Ar-p-NO2})2] (por=tmp, ttp).
Comparison of the Hb chemical shifts of [RuIV(tmp)(NH{Arꢀ
Ar})2] (d=8.17 ppm) and [RuVI(tmp)(N{ArꢀAr})2] (d=8.55 ppm)
indicates that the Hb protons of the latter are more greatly de-
shielded, consistent with the higher oxidation state of Ru in
[RuVI(por)(NY)2] than in [RuIV(por)(NHY)2].[32] However, compared
with [RuVI(tmp)O2], which shows Hb signal at d=8.81 ppm,[33]
[RuVI(tmp)(N{ArꢀAr})2] exhibits less deshielded Hb protons, sug-
gesting that the arylimide ligand is a better p donor than the
oxo ligand.
As expected, the Hb signals of [RuII(F20-tpp)(NH2Y)2] (Y=
Ar^Ar, Ar-p-Cl, Ar-p-(NHPh)) (d=8.03–8.07 ppm) are considera-
bly upfield from that of [RuIV(F20-tpp)(NH{Ar^Ar})2] (d=
8.41 ppm). It is worth noting that NH2(Ar-p-(NHPh)) has two
amino groups that can be deprotonated, unlike the other aryl-
amines used in this work. While [RuII(F20-tpp)(NH2{Ar-
p-(NHPh)})2] was obtained from the reaction of [RuVI(F20-tpp)O2]
with NH2(Ar-p-(NHPh)), change of [RuVI(F20-tpp)O2] to [RuVI-
(por)O2] (por=tmp, 4-Cl-tpp) for this reaction resulted in the
formation of a mixture of products featuring three Hb signals:
d=8.15, 8.13, 8.11 ppm (intensity ratio: 1.2:2.2:1.0) for por=
tmp; d=8.37, 8.35, 8.33 ppm (intensity ratio: 1.8:2.6:1.0) for
por=4-Cl-tpp, which can be attributed to complexes [RuIV-
(por)(NH{Ar-p-(NHPh)})2], [RuIV(por)(N(Ph){Ar-p-NH2})2], and [RuIV-
(por)(NH{Ar-p-(NHPh)})(N(Ph){Ar-p-NH2})] (Figure S1 in the Sup-
porting Information).[34]
3262–3403 cmꢀ1. Similar n(NH) bands are not present in the IR
spectra of the [RuVI(por)(NY)2] complexes.
X-ray crystal structures
Diffraction-quality crystals were obtained for [RuIV(tmp)(NHY)2]
with Y=ArꢀAr-p-OMe, ArꢀAr-p-Cl, and Ar^Ar and for [RuVI-
(tmp)(N{ArꢀAr-p-OMe})2]. The structures of these complexes
determined by X-ray crystallography[35] are depicted in Fig-
ures 3–5 and Figure S2 in the Supporting Information, and the
crystallographic data and selected bond lengths and angles
are listed in Tables S3–S6 in the Supporting Information. For
previously reported [VIV(ttp)(N{ArꢀAr})][5b] mentioned in the In-
troduction section, its crystal structure remains to be deter-
mined, although [VIV(oep)(NPh)] has been structurally charac-
terized recently.[12]
UV/Vis and IR: Complexes [RuIV(por)(NHY)2] in Table 1 exhibit
UV/Vis spectra which feature the Soret bands at 412–419 nm
and b bands at 522–533 nm, similar to those (Soret: 413–
417 nm, b: 527–529 nm) reported for [RuIV(por)(NH{Ar-p-X})2]
(por=ttp, 4-Cl-tpp; X=Cl, NO2).[6d] [RuII(F20-tpp)(NH2Y)2] (Y=
Ar^Ar, Ar-p-Cl, Ar-p-(NHPh)) gave Soret and b bands at 401–
403 and 501–502 nm, respectively; these bands are markedly
blue-shifted from the corresponding bands of [RuIV(F20-
tpp)(NH{Ar^Ar})2] (Soret: 412 nm, b: 522 nm).
Figure 3. ORTEP drawing for [RuIV(tmp)(NH{ArꢀAr-p-OMe})2] (1b, thermal el-
lipsoids at 30% probability; hydrogen atoms are omitted). Ru1ꢀN3
1.978(4) ꢀ, Ru1-N3-C29 141.9(3)8.
[RuVI(por)(NY)2] in Table 1 exhibit Soret bands at 417–423 nm
and b bands at 526–530 nm. Conversion of [RuII(tmp)(CO)] to,
for example, [RuVI(tmp)(N{Ar^Ar})2] resulted in a red shift of
the Soret band from 412 to 423 nm; the latter is also red-shift-
ed from that of [RuIV(tmp)(NH{ArꢀAr})2] (Soret: 418 nm).
For [RuVI(tmp)(NY)2] bearing various axial arylimide ligands,
the effects of aryl groups of the arylimide ligands on the Soret/
b bands of these complexes are similar (Soret: 421–423 nm, b:
526–529 nm). A similar phenomenon was observed for the
effect of aryl groups of the axial arylamide ligands in [RuIV-
(tmp)(NHY)2] on their Soret (416–419 nm) and b (526–528 nm)
bands.
The IR spectra of the [RuIV(por)(NHY)2] in Table 1 show
a n(NH) band at 3254–3269 cmꢀ1, similar to those reported for
[RuIV(por)(NH{Ar-p-X})2] (por=ttp, 4-Cl-tpp; X=Cl, NO2: n(NH)
at 3264–3267 cmꢀ1).[6d] [RuII(F20-tpp)(NH2Y)] (Y=Ar^Ar, Ar-p-Cl,
Ar-p-(NHPh)) each shows two n(NH) bands in the region of
Complexes [RuIV(tmp)(NHY)2] show RuꢀN(arylamide) distan-
ces of 1.978(4), 1.971(6), and 1.985(13) ꢀ for Y=ArꢀAr-p-OMe,
ArꢀAr-p-Cl, and Ar^Ar, with axial Ru-N-C angles of 141.9(3)8,
139.3(6)8, and 136.1(9)8, respectively, which are comparable to
the corresponding geometrical parameters (1.956(7) ꢀ and
135.8(6)8) reported for [RuIV(ttp)(NH{Ar-p-Cl}2].[6d] These Ruꢀ
N(arylamide) distances (1.956(7)–1.985(13) ꢀ) are substantially
shorter than the RuꢀN(amine) distance in [RuII(tmp)-
(NH2CH2Ph)2] (2.129 ꢀ),[23] revealing the multiple-bonding char-
acter of the RuꢀN(arylamide) bonds in the [RuIV(por)(NHY)2]
complexes.
[RuIV(tmp)(NH{Ar^Ar})2] features a unique arylamide ligand
in which the two mutually conjugated phenyl rings are co-
planar, thus maximizing the through resonance between these
phenyl groups (Figure 4). Changing the aryl group from Ar^Ar
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Figure 4. ORTEP drawing for [RuIV(tmp)(NH{Ar^Ar})2] (1d, thermal ellipsoids
at 30% probability; hydrogen atoms are omitted). Ru1ꢀN3 1.985(13) ꢀ, Ru1-
N3-C29 136.9(3)8.
Figure 5. ORTEP drawing for [RuVI(tmp)(N{ArꢀAr-p-OMe})2] (3b, thermal ellip-
soids at 30% probability; hydrogen atoms are omitted). Ru1ꢀN5 1.829(5),
Ru1ꢀN6 1.824(5) ꢀ; Ru1-N5-C57 136.9(3)8, Ru1-N6-C70 153.5(4)8, N5-Ru1-N6
172.8(2)8.
to ArꢀAr-p-OMe and ArꢀAr-p-Cl results in appreciable distor-
tion of the two phenyl groups from co-planarity. For example,
a dihedral angle of 24.58 is observed for the two phenyl rings
in each ArꢀAr-p-OMe group of [RuIV(tmp)(NH{ArꢀAr-p-OMe})2].
Such distortion does not significantly alter the RuꢀN(arylamide)
distances in the complex. In the crystal structure of non-por-
phyrin
cis-bis(arylamide)
complex
[Lu(tBu3{2’-
Me3SiCH2}tpy)(NH{ArꢀAr-3,5-Ph})2] (tpy=2,2’:6’,2’’-terpyridine;
ArꢀAr-3,5-Ph=3,5-diphenyl-biphenyl-4-yl),[4b] the two phenyl
rings of the biphenyl moiety of ArꢀAr-3,5-Ph make larger dihe-
dral angles of 35.38 and 43.78.
For [RuVI(tmp)(N{ArꢀAr-p-OMe})2], its RuꢀN(arylimide) distan-
ces of 1.824(5), 1.829(5) ꢀ and axial Ru-N-C angles of 136.9(3)8,
153.5(4)8 are comparable to those reported for [RuVI(tpp)(N{Ar-
3,5-CF3})2] (1.808(4), 1.806(4) ꢀ; 139.8(3)8, 143.7(4)8; Figure 5).[8]
These two [RuVI(por)(NY)2] complexes show similar orientation
of the axial arylimide ligands with respect to the correspond-
ing porphyrin ring. Their RuꢀN(imide) distances are compara-
ble to, and the axial imide ligands are more bent than, those
of the sulfonylimide analogue [RuVI(tmp)(NSO2{Ar-p-OMe})2]
(RuꢀN(imide) 1.79(3) ꢀ, axial Ru-N-S: 162.5(3)8).[36]
Comparison of the structures between [RuVI(tmp)(N{ArꢀAr-p-
OMe})2] and [RuIV(tmp)(NH{ArꢀAr-p-OMe})2] revealed the fol-
lowing differences. 1) The RuꢀN(arylimide) distances (1.824(5),
1.829(5) ꢀ) are considerably shorter than the RuꢀN(arylamide)
distance (1.978(4) ꢀ), in line with the larger character of RuꢀN
multiple bonding anticipated for Ru–imide than Ru–amide
complex. 2) The dihedral angle between the two phenyl rings
of the ArꢀAr-p-OMe group in the arylimide complex (16.38,
16.98) is smaller than that in the arylamide complex (24.58),
suggesting a higher extent of p conjugation in the former
case. 3) The axial N-Ru-N axis is slightly bent (with an angle of
172.8(2)8) in the arylimide complex, but is linear (with an angle
Figure 6. Top-views of: a) [RuIV(tmp)(NH{ArꢀAr-p-OMe})2] (1b, along N3···N3’
axis), and b) [RuVI(tmp)(N{ArꢀAr-p-OMe})2] (3b, along N5···N6 axis). The axial
CꢀNꢀRuꢀNꢀC moiety is in ball-and-stick representation, whereas the ArꢀAr-
p-OMe group behind the porphyrin plane is in wire frame representation.
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of 1808) in the arylamide counterpart. 4) The two axial aryli-
mide ligands adopt a syn configuration, in contrast to the anti
configuration of the two axial arylamide ligands (Figure 6).
The quasi-reversible/irreversible reduction waves I have Epc
values in the range of ꢀ1.06 to ꢀ1.40 V and the Epa values of
the irreversible oxidation waves II are in the range of ꢀ0.044
to 0.19 V (vs. Cp2Fe+/0, Table 2). We assign these reduction and
oxidation processes to RuIV!RuIII and RuIV!RuV, respectively,
owing to the following features of the Epc (wave I) and Epa
(wave II) values: 1) they are substantially different from those
of the porphyrin-centered reduction/oxidation;[39] 2) they are
less sensitive to the substituent on the aryl group of the coor-
Electrochemistry
The cyclic voltammograms of several [RuIV(tmp)(NHY)2] com-
plexes in dichloromethane were measured. The cyclic voltam-
mogram of [RuIV(tmp)(NH{ArꢀCꢁCꢀArꢀCꢁCꢀAr})2] as an ex-
ample, in which a total of six waves, I–IV and I’/II’, were ob-
served, is shown in Figure 7. Both the reduction wave III and
dinated arylamide ligand. For example, the Epc(RuIV/III)/Epa(RuV/IV
)
values of [RuIV(tmp)(NHY)2] are ꢀ1.32/+0.048 V for Y=ArꢀAr-
p-OMe and ꢀ1.23/+0.15V for Y=ArꢀAr-p-Cl. By changing the
aryl group of [RuIV(tmp)(NHY)2] from Ar^Ar to ArꢀCꢁCꢀArꢀ
CꢁCꢀAr, the Epc(RuIV/III) and Epa(RuV/IV) can be tuned by 340
and 234 mV, respectively.
DFT calculations: electronic structure of [RuVI(por)(NY)2]
We studied the electronic structure of [RuVI(por)(NY)2] by densi-
ty functional theory (DFT) calculations, using model complex
[RuVI(tmp)(N{ArꢀAr-p-OH})2]. The geometrical parameters of
the DFT-optimized structure of this model complex (Figure 8)
agree well with those of the crystal structure of [RuVI-
(tmp)(N{ArꢀAr-p-OMe})2] (Table 3).
The computed molecular orbital (MO) diagram is depicted in
Figure 9. HOMO and HOMOꢀ2 are nonbonding orbitals of
N(px,py) of the arylimide ligand, whereas LUMO and LUMO+
1 are antibonding orbitals involving N(imide) and Ru dyz or dxz
orbitals, respectively. HOMOꢀ1 and HOMOꢀ3 are porphyrin-
based orbitals; HOMOꢀ5 is nonbonding Ru dxy orbital.
HOMOꢀ29 is the p-bonding oribtal between p oribtals of
Figure 7. Cyclic voltammogram of [RuIV(tmp)(NH{ArꢀCꢁCꢀArꢀCꢁCꢀAr})2]
(1g) in CH2Cl2 at 258C. Scan rate: 100 mVsꢀ1
.
the oxidation wave IV are reversible; the reduction wave I is
quasi-reversible, and the other waves are irreversible. The irre-
versible waves I’ and II’ are attributed to the species generated
by reduction I and oxidation II, respectively, since neither wave
I’ is present prior to reduction I, nor is wave II’ before oxida-
tion II. A collection of the potentials of waves I–IV for the [RuIV-
(tmp)(NHY)2] complexes examined are listed in Table 2.
The potentials of the reversible oxidation couples IV (E1/2
=
0.68–0.78 V vs. Cp2Fe+/0, Table 2) are comparable to those of
the porphyrin-centered oxidation reported for [RuII(tmp)(CO)]
(0.86 V),[37] [RuVI(tmp)(NSO2{Ar-p-X})2] (X=OMe, Me, H, Cl, NO2:
0.61–0.75 V),[15d] and [RuVI(tmp)O2] (0.69 V),[38] revealing a por-
phyrin-centered oxidation. For the reversible reduction couples
III at E1/2 =ꢀ1.54 to ꢀ1.78 V, their potentials are comparable to
those of the porphyrin-centered reduction of the above RuVI
complexes (E1/2 =ꢀ1.46 to ꢀ1.61 V).[15d]
Table 2. Redox potentials (E vs. Cp2Fe+/0) of [RuIV(tmp)(NHY)2] (1).
[RuIV(tmp)(NHY)2] Y
III E1/2 [V] I Epc [V] II Epa [V] IV E1/2 [V]
Ar^Ar (1d)
–
–
ꢀ1.40
ꢀ1.32
ꢀ1.25
ꢀ1.24
ꢀ1.23
ꢀ1.11
ꢀ1.06
ꢀ0.044
0.048
0.094
0.12
0.15
0.19
0.68
–
0.73
–
0.78
–
0.78
ArꢀAr-p-OMe (1b)
ArꢀArꢀAr (1e)
ArꢀAr (1a)
ꢀ1.59
–
ꢀ1.78
–
ArꢀAr-p-Cl (1c)
ArꢀCꢁCꢀAr (1 f)
Figure 8. DFT-optimized structure of [RuVI(tmp)(N{ArꢀAr-p-OH})2] (hydrogen
ArꢀCꢁCꢀArꢀCꢁCꢀAr (1g) ꢀ1.54
0.19
atoms are not shown).
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The computed C-Nimide-Nmide-C torsion angle of ꢀ0.38 (C21-
N5-N6-C33, Table 3) is close to 08 and is comparable to those
in the crystal structures of [RuVI(tmp)(N{ArꢀAr-p-OMe})2]
(ꢀ14.68) and previously reported [RuVI(tpp)(N{Ar-3,5-CF3})2][8]
(5.78), [OsVI(ttp)(N{Ar-p-NO2})2][13a] (ꢀ2.58) and [OsVI(4-Cl-tpp)(N-
{Ar-p-NO2})2][13b] (ꢀ5.98). In contrast, a large C-Namide-Namide-C tor-
sion angle of 1808 has been found in the DFT-optimized struc-
ture of [RuIV(por0)(NHPh)2] reported previously[40] and in the
crystal structure of [RuIV(tmp)(NH{ArꢀAr-p-OMe})2]. The C-
Table 3. Selected bond distances [ꢀ] and angles [o] obtained by DFT cal-
culations and experimental studies.
[RuVI(tmp)(N{ArꢀAr-p-OH})2] [RuVI(tmp)(N{ArꢀAr-p-OMe})2]
(calcd)
(3b, exptl)[a]
Ru1-N5
Ru1-N6
Ru1-N1
Ru1-N2
Ru1-N3
Ru1-N4
N5-C21[b]
N6-C33[b]
1.831
1.832
2.063
2.097
2.101
2.075
1.340
1.340
1.829(5)
1.824(5)
2.061(4)
2.066(3)
2.059(4)
2.055(4)
1.373(7)
1.341(7)
145.5(4)
153.5(4)
172.8(2)
ꢀ14.6(7)
N
amide-Namide-C torsion angle for the structurally characterized
[RuIV(ttp)(NH{Ar-p-Cl})2] (~1368)[6d] is also large though consider-
ably deviated from 1808 due possibly to crystal packing effect.
The computed C-Namide-Namide-C[40] and C-Nimide-Nmide-C torsion
angles are in accord with the anti configuration of [RuIV(por)-
(NHY)2] but the syn configuration of [RuVI(por)(NY)2] revealed
by X-ray crystal structure determinations (see, for example,
Figure 6).
Ru1-N5-C21[b] 145.6
Ru1-N6-C33[b] 145.5
N5-Ru1-N6
DA[b,c]
169.0
ꢀ0.3
[a] Data from the X-ray crystal structure. [b] The C21 or C33 atom here for
[RuVI(tmp)(N{ArꢀAr-p-OMe})2] corresponds to the C57 or C70 atom, re-
spectively, in Figure 5. [c] DA: C21-N5-N6-C33 torsion angle.
The Nimide-Ru-Nimide axis is appreciably bent in both the DFT-
optimized structure of [RuVI(tmp)(N{ArꢀAr-p-OH})2] and the
crystal structure of [RuVI(tmp)(N{ArꢀAr-p-OMe})2] (N5-Ru1-N6
angle: 169.08 and 172.8(2)8, respectively; Table 3), with the two
trans arylimide ligands bending to the same side to give the
syn configuration. A similar syn configuration with an apprecia-
bly bent Nimide-M-Nimide axis has also been found for the previ-
ously reported crystal structures of several d2 metal–bis(aryli-
mide) porphyrins including [RuVI(tpp)(N{Ar-3,5-CF3})2],[8] [OsVI-
(ttp)(N{Ar-p-NO2})2],[13a] and [OsVI(4-Cl-tpp)(N{Ar-p-NO2})2][13b]
(Nimide-M-Nimide angles: 165.1(2)8–167.97(18)8). This is different
from the linear Namide-Ru-Namide axis (with an angle of 1808) in
the DFT-optimized structure of
N(imide) and dp orbitals of Ru. This MO diagram indicates a sin-
glet ground state consistent with a d2 RuVI formulation, which
accounts for the related spectral features (such as the diamag-
1
netic H NMR spectra of [RuVI(por)(NY)2] with larger Hb chemical
shifts than those of [RuIV(por)(NHY)2]) described above.
As revealed from the frontier MOs (Figure 9), there is exten-
sive delocalization of the electron density over the whole YN=
Ru=NY axial moiety, and the delocalization becomes more
prominent in the HOMO.
[RuIV(por0)(NHPh)2][40] and the
crystal
structure
of
[RuIV-
(tmp)(NH{ArꢀAr-p-OMe})2] both
with anti configuration.
For
[RuVI(tmp)(N{ArꢀAr-p-
OH})2], the N5/N6 orbital used
for the p-bonding of NimideꢀRuꢀ
Nimide (N5ꢀRu1/N6ꢀRu1) has
4.3% s orbital and 95.7% p orbi-
tal characters (from NBO analy-
sis). We further performed DFT
calculations on the structurally
characterized [RuVI(tpp)(N{Ar-3,5-
CF3})2],[8]
[OsVI(ttp)(N{Ar-p-
NO2})2],[13a] and [OsVI(4-Cl-tpp)(N-
{Ar-p-NO2})2],[13b] which gave
geometrical parameters in good
agreement with those in their
crystal structures (Table S7 in the
Supporting Information) and re-
vealed similar s orbital characters
of 6.6, 7.5, and 7.5%, respective-
ly, in the corresponding N(imide)
(N5/N6) orbital used for the
NimideꢀMꢀNimide p-bonding. We
suggest that, for these metal–bi-
Figure 9. MO diagram of [RuVI(tmp)(N-ArꢀAr-p-OH)2] (isovalue=0.04 a.u.). H=HOMO, L=LUMO.
s(arylimide)
complexes,
the
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slight mixing of the 2s and 2p orbitals of N(imide) atom leads
to an appreciably bent Nimide-M-Nimide (M=Ru, Os) axis that
gives the syn configuration allowing for a better p-overlap be-
tween Ru/Os and N(imide) (Figure 10) and/or a better p-bond-
ing along CꢀNimideꢀMꢀNimideꢀC (such as C21ꢀN5ꢀRu1ꢀN6ꢀC33
in Figure 8).
Scheme 3. Reactivity of [RuIV(por)(NHY)2] (1) with PhI(OAc)2.
identified on the basis of their Hb chemical shifts (Table S8 in
the Supporting Information). For a certain por ligand, replace-
ment of the imide ligand(s) of [RuVI(por)(NY)2] by one and two
oxo ligand(s) increases the Hb chemical shift by 0.12–0.13 and
0.26–0.28 ppm, respectively (calculated from the data in
Table S8 in the Supporting Information), similar to the corre-
sponding increase of Hb chemical shifts by 0.18 and 0.37 ppm,
respectively, in the case of [RuVI(ttp)(NtBu)2] (Hb 8.73),[6d] [RuVI-
(ttp)(NtBu)O] (Hb 8.91),[6d] and [RuVI(ttp)O2] (Hb 9.1).[42]
The 1H NMR spectra of the reaction mixture of [RuIV-
(tmp)(NH{ArꢀAr-p-Cl})2] with PhI(OAc)2 at various reaction time
of t=5–70 min are depicted in Figure S4 (in the Supporting In-
formation) as examples. At t=5 min, two new Hb signals with
d=8.54 and 8.67 ppm appeared, which were assigned to [RuVI-
(tmp)(N{ArꢀAr-p-Cl})2] and [RuVI(tmp)(N{ArꢀAr-p-Cl})O], respec-
tively, with the former in larger amount. As the reaction time
increased to t=15 min, the amount of the former substantially
decreased whereas the amount of the latter increased accord-
ingly (the starting complex was completely consumed), and
another new Hb signal with d=8.81 ppm assigned to [RuVI-
(tmp)O2][33] began to be discernible. Further increase of the re-
action time to t=70 min led to disappearance of [RuVI-
(tmp)(N{ArꢀAr-p-Cl})2] and [RuVI(tmp)(N{ArꢀAr-p-Cl})O], with
[RuVI(tmp)O2] formed as the dominant complex in 87% yield
(determined by 1H NMR spectroscopy).
Figure 10. Schematic diagram showing: a) the N5ꢀRu/OsꢀN6 p-interaction
with mixture of pp orbitals and s oribtals, b) the N5ꢀRu/OsꢀN6 p-interaction
with pure pp orbitals.
Reactivity of [RuIV(por)(NHY)2]
Reaction with bromine: Treatment of [RuIV(tmp)(NHY)2] (Y=
ArꢀAr, Ar^Ar, ArꢀArꢀAr) with a solution of bromine in di-
chloromethane caused the b band to be red-shifted from ap-
proximately 527 to about 541 nm within 7 min; meanwhile,
the Soret band blue-shifted slightly from approximately 418 to
about 416 nm, as revealed by UV/Vis measurements. After
work up, the corresponding complexes [RuIV(tmp)(NHY)Br] (4)
were isolated in 51–69% yields, together with the N,N-cou-
pling products YꢀN=NꢀY in 60–92% yields. The oxidative de-
protonation products [RuVI(tmp)(NY)2] or [RuVI(tmp)(NY)O] were
not obtained from these reactions. Previously, bromine had
been employed for the oxidation of [RuII(por)(NH2tBu)2] to
[RuVI(por)(NtBu)O].[6d,22a] Related oxidation of [RuII(por)(NH3)2] by
N-bromosuccinimide was found to give [RuVI(por)(N)Br].[41]
1
The H NMR spectra of the isolated [RuIV(tmp)(NHY)Br] (Fig-
ure S3, for Y=ArꢀAr, in the Supporting Information) feature
sharp signals at normal fields and show upfield-shifted Hb sig-
nals (d=7.58–7.70 ppm) and downfield-shifted Ho’ and Hm’ sig-
nals compared with those of their [RuIV(tmp)(NHY)2] counter-
parts. For example, the Hb/Ho’/Hm’ signals are shifted from d=
8.17/3.28/6.03 ppm for [RuIV(tmp)(NH{ArꢀAr})2] to d=7.69/
4.20/6.10 ppm for [RuIV(tmp)(NH{ArꢀAr})Br]. In agreement with
the unsymmetrical coordination at the axial sites in [RuIV(tmp)-
(NHY)Br], the two ortho-methyl moieties of each meso-mesityl
group in the tmp ligand give two well-separated signals.
The time required for the complete consumption of [RuIV-
(tmp)(NHY)2] increased along Y=ArꢀAr-p-Cl (15 min)<ArꢀAr
(30 min)<ArꢀAr-p-OMe (40 min; Table S9 in the Supporting In-
formation). This indicates that the electron-donating para-sub-
stituent on the arylamide ligand retards the reaction, in line
with the increased basicity of the arylamide ligand which ren-
dered it more difficult to be deprotonated.
Nitrogen group transfer reactions with hydrocarbons
Reaction with PhI(OAc)2: We treated [RuIV(por)(NHY)2] (por=
tmp, Y=ArꢀAr, ArꢀAr-p-OMe, ArꢀAr-p-Cl; por=ttp, Y=Ar-p-
NO2) with 4.5 equiv of PhI(OAc)2 in CDCl3 at 08C, and the reac-
Stoichiometric reactions of [RuVI(tmp)(NY)2]: The reactivity of
bis(p-conjugated arylimide) complexes toward hydrocarbons
was examined using [RuVI(tmp)(NY)2] (3, Y=ArꢀAr, ArꢀAr-p-Cl),
together with cyclooctene and 1,2,3,4-tetrahydronaphthalene,
and the results are depicted in Scheme 4 and Table 4. Reac-
tions of the two arylimide complexes with cyclooctene in re-
fluxing benzene for 6 h afforded the N-(p-conjugated aryl) azir-
idines 5a and 5b in 56–58% yields (determined by 1H NMR
spectroscopy); similar reactions for 1,2,3,4-tetrahydronaphtha-
1
tion mixtures were monitored by H NMR spectroscopy. These
reactions resulted in oxidative deprotonation to give [RuVI-
(por)(NY)2], which were subsequently converted to [RuVI-
(por)(NY)O] and [RuVI(por)O2] possibly by hydrolysis in the pres-
ence of trace amounts of water in the solvent and/or imide-
oxo exchange with PhI=O generated from PhI(OAc)2 and
water (Scheme 3). The three types of RuVI complexes were
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Scheme 4. Nitrogen group transfer/insertion reactions of [RuVI(tmp)(NY)2] (3).
lene gave benzylic CꢀH amination products 6a and 6b in 55–
60% yields. Comparable yields of amination products were re-
ported previously for the reactions of [RuVI(tpp)(N{Ar-3,5-CF3})2]
with hydrocarbons such as isobutyl benzene, which afforded
the benzylic CꢀH amination product in 57% yield.[8a] Stoichio-
metric nitrogen group transfer/insertion reactions of [RuVI-
(por)(NSO2{Ar-p-X})2] (X=OMe, Me, H, Cl, NO2) with various al-
kenes and alkanes have also been reported, with yields of up
to 88% being obtained for the resulting N-sulfonyl aziridines/
amines.[15]
Catalytic reactions with “[RuII(tmp)(CO)]+N3Y”: We then
examined the catalytic nitrogen group transfer/insertion reac-
tions using [RuII(tmp)(CO)] as the catalyst, in view of its reac-
tion with the p-conjugated aryl azides N3Y (Y=ArꢀAr, ArꢀAr-
p-OMe, ArꢀAr-p-Cl, ArꢀArꢀAr) to give [RuVI(tmp)(NY)2] (3).
Treatment of cyclooctene and cycloheptene in refluxing ben-
zene with N3Y (Y=ArꢀAr, ArꢀAr-p-Cl) in the presence of [RuII-
(tmp)(CO)] (2 mol%) for 13–15 h gave the corresponding aziri-
Scheme 5. Nitrogen group transfer/insertion reactions catalyzed by
“[RuII(tmp)(CO)]+N3Y”.
dines 5a–5c in 48–55% isolated yields (Scheme 5; entries 1–3,
Table S10 in the Supporting Information). For benzylic hydro-
carbons including 1,2,3,4-tetrahydronaphthalene, indan, diphe-
nylmethane, and ethylbenzene, their reactions with N3Y (Y=
ArꢀAr, ArꢀAr-p-OMe, ArꢀAr-p-Cl, ArꢀArꢀAr) catalyzed by [RuII-
(tmp)(CO)] (2 mol%) under similar conditions for 24–30 h af-
forded benzylic CꢀH amination products 6a–6k in 30–45%
yields (Scheme 5; entries 4–14, Table S10 in the Supporting
Information).
The moderate product yields using “[RuII(tmp)(CO)]+N3Y”
protocol could partially be attributed to the sterically encum-
bered porphyrin ligand in catalyst [RuII(tmp)(CO)] rendering the
ruthenium–arylimide/nitrene intermediate to be less reactive.
[RuIV(por)Cl2] were previously reported to be effective catalysts
for CꢀH bond oxidation and alkene epoxidation with
2,6-Cl2pyNO,[43] and aldehyde CꢀH amination with
phosphoryl azides.[44] Upon changing the catalyst
from [RuII(tmp)(CO)] to [RuIV(ttp)Cl2], the yield of the
Table 4. Stoichiometric reaction of [RuVI(tmp)(NY)2] (3) with hydrocarbons.[a]
Y
Hydrocarbon
Product
Yield
[%][b]
N-(p-conjugated aryl) aziridine 5a increased from 50
to 65% for the aziridination of cyclooctene with
N3(ArꢀAr), and the yields of N-(p-conjugated aryl)
ArꢀAr (3a)
56
amines 6c and 6j increased from 40 to 51% and
55% for the benzylic CꢀH amination of 1,2,3,4-tetra-
hydronaphthalene and ethylbenzene, respectively,
ArꢀAr-p-Cl (3c)
58
with N3(ArꢀAr-p-OMe). Diazene compounds, which
probably came from coupling reaction of the rutheni-
um arylimide/nitrene intermediates, were the major
ArꢀAr (3a)
60
55
byproducts in these nitrogen group transfer/insertion
reactions.
General remarks
The development of reactive high-valent ruthenium–
imide complexes supported by porphyrin ligands is
instrumental in elucidating the mechanisms and ex-
panding the scope of ruthenium porphyrin-mediated
nitrogen group transfer/insertion reactions. To date,
the following types of ruthenium–imide porphyrin
complexes have been isolated: 1) RuVI–oxo(alkyli-
ArꢀAr-p-Cl (3c)
[a] Reaction conditions: [RuVI(tmp)(NY)2] (0.1 mmol), hydrocarbons (2 mL), benzene
(1 mL), refluxing under N2, 6 h. [b] 1H NMR yield using 1,1-diphenylethylene as the in-
ternal standard.
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mide) complexes [RuVI(por)(NtBu)O], together with their
osmium analogues [OsVI(por)(NtBu)O] and [OsVI(por)(NtBu)2],
prepared in our group in 1992[22a] from oxidative deprotona-
tion of [MII(por)(NH2tBu)2] (M=Ru or Os) with bromine or air;
2) RuIV–mono(arylimide) complexes [RuIV(por)(N{Ar-p-X})] pre-
pared by Leung and co-workers[11] in 1997 from reaction of
[RuIV(por)Cl2] with NH2(Ar-p-X); 3) RuVI–bis(sulfonylimide) com-
plexes [RuVI(por)(NSO2{Ar-p-X})2] synthesized by Che and co-
workers in 1999[15b] and 2005[15d] by treatment of [RuII(por)(CO)]
with PhI=NSO2(Ar-p-X); 4) RuVI–bis(arylimide) complexes [RuVI-
(tpp)(N{Ar-p-NO2})2] prepared by Smieja and co-workers in
2002 from the reaction of [RuII(ttp)]2 with N3(Ar-p-NO2),[13b] and
[RuVI(tpp)(N{Ar-3,5-CF3})2] prepared by Cenini, Gallo, and co-
workers in 2009 from the reaction of [RuII(por)(CO)] with N3(Ar-
3,5-CF3).[8a] Of these ruthenium–imide porphyrin complexes,
[RuVI(tpp)(N{Ar-3,5-CF3})2][8a] and [RuVI(tmp)(NSO2{Ar-p-OMe})2][36]
have been structurally characterized by X-ray crystal analysis.
Nitrogen group transfer/insertion reactions of isolated ruthe-
nium–imide porphyrin complexes are hitherto focused on
alkene aziridination and CꢀH amination by [RuVI(por)(NSO2{Ar-
p-X})2],[15] together with CꢀH amination by [RuVI(tpp)(N{Ar-3,5-
CF3})2].[8a] The osmium(VI) analogues [OsVI(por)(NY)2][13] and
[OsVI(por)(NSO2Y)2][45] have not been found to exhibit such reac-
tivities. In general, metal–imides that are reactive toward
alkene aziridination and/or CꢀH amination remain sparse.[8,14,15]
One of the well-documented approaches to metal–imide/ni-
trene complexes is the use of aryl azides as the imide/nitrene
source.[8,9,13] Organic azides are an attractive source of imide/ni-
trene owing to the nonhazardous nature of the byproduct N2.
There are various types of organic azides known in the litera-
ture. Besides aryl azides, typical organic azides employed as ni-
trene source in catalysis include sulfonyl azides, alkoxycarbonyl
azides, and phosphoryl azides.[46] On the basis of previous syn-
thesis of [CrIV(por)(NY)][9] and [MVI(por)(NY)2] (M=Ru, Os)[8,13]
from aryl azides, organic azides would be useful imide/nitrene
source for preparation of metal–imide/nitrene porphyrin
complexes.
Conclusion
[RuIV(por)(NHY)2] and [RuVI(por)(NY)2] complexes bearing p-con-
jugated arylamide and p-conjugated arylimide ligands, respec-
tively, have been prepared and characterized by spectroscopic
means and X-ray crystal structure determinations. A combina-
tion of p-conjugated arylimide and sterically encumbered por-
phyrin ligands allows the preparation of ruthenium–bis(aryli-
mide) complexes without the need to introduce a strongly
electron-withdrawing group onto the arylimide ligand. The
extent of p-conjugation in the arylamide and arylimide ligands
slightly affects the spectral properties of the complexes and
the RuꢀN(arylamide/arylimide) distances. Electrochemical stud-
ies on [RuIV(por)(NHY)2] revealed considerable effect of the p-
conjugated arylamide ligands on the metal-centered reduction
and oxidation potentials. DFT calculations on [RuVI(tmp)(N{Arꢀ
Ar-p-OH})2] lend support for the d2 RuVI formulation of [RuVI-
(por)(NY)2] and provide insight into their syn configuration re-
vealed by X-ray crystal structure determination.
The [RuIV(por)(NHY)2] and [RuVI(por)(NY)2] complexes undergo
interesting redox and/or CꢀN bond formation reactions. [RuIV-
(por)(NHY)Br] are formed in the reaction of [RuIV(por)(NHY)2]
with bromine. Oxidation of [RuIV(por)(NHY)2] using PhI(OAc)2 af-
fords [RuVI(por)(NY)2], which demonstrates the generation of
metal–imide complexes by oxidative deprotonation with
a PhI(OR)2 oxidant. [RuVI(tmp)(NY)2] undergo stoichiometric
alkene aziridination and CꢀH amination reactions to give N-(p-
conjugated aryl) aziridines and amines, respectively, in moder-
ate yields. The corresponding catalytic nitrogen group transfer/
insertion reactions with comparable product yields can be ach-
ieved using
protocol.
a
“[RuII(tmp)(CO)]+p-conjugated aryl azide”
Acknowledgements
We gratefully acknowledge The University of Hong Kong, the
Hong Kong Research Grants Council (HKU, 700813), and the
National Natural Science Foundation of China (NSFC,
21272197) for financial support.
Incorporation of p-conjugated aryl groups into the imide li-
gands has the potential to generate metal–imides that feature
extensive delocalization involving metal–nitrogen multiple
bonds and/or allow the study of electronic communication be-
tween redox active high-valent metal ions and remote func-
tional groups, as well as undergo nitrogen group transfer/in-
sertion reactions to produce aziridines/amines bearing p-con-
jugated aryl groups. The present work has expanded the reac-
tive metal–imide complexes to the p-conjugated arylimides
and has demonstrated the formation of N-(p-conjugated aryl)
aziridines/amines by nitrogen-group transfer/insertion reac-
tions of well-defined metal–imide complexes. [RuIV(tmp)(NHY)2]
(Y=ArꢀAr-p-OMe, ArꢀAr-p-Cl, and Ar^Ar) and [RuVI-
(tmp)(N{ArꢀAr-p-OMe})2] are unique examples of structurally
characterized trans-metal–bis(p-conjugated arylamide) and
trans-metal–bis(p-conjugated arylimide) complexes, respective-
ly; the latter also contributes a structurally characterized metal-
loporphyrin with p-conjugated arylimide ligand(s).
Keywords: metal–amide complexes · metal–imide complexes ·
N ligands · porphyrinoids · ruthenium
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1
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Received: December 31, 2013
Revised: May 30, 2014
Published online on July 25, 2014
Chem. Eur. J. 2014, 20, 11035 – 11047
www.chemeurj.org
11047
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