Quinoneimido complexes of a metalloporphyrin: Isolation, X-ray crystal structures, and DFT calculations

Article abstract of DOI:10.1002/asia.200900666

Terminal quinoneimido complexes [Ru^IV (por) (NQu)(X)] (X=OEt: 1, OH: 2) were formed from the reaction of [Ru ^VI(por)O2] with 2,6-dimethylaniline. The X-ray crystal structures of 1 feature Ru-N(quinoneimido) bond lengths of 1.787(7)-1.80(2). Reaction of 2 with N-phenyl-benzene-1,4-diamine afforded bis(quinoneimido) complex [Ru(por) (NQu) (NQu')]. DFT calculations on 3 and 4 revealed a significant effect of 'Ru doping' on the oligoaniline structure.

Full text of DOI:10.1002/asia.200900666

DOI: 10.1002/asia.200900666
Quinoneimido Complexes of a Metalloporphyrin: Isolation, X-ray Crystal
Structures, and DFT Calculations
Wai-Man Tsui, Jie-Sheng Huang, Glenna So Ming Tong, Steven C. F. Kui,
Chi-Ming Che,* and Nianyong Zhu[a]
Quinoneimine and quinoneimido ligands (designated here
as QuNR and QuNꢀ, respectively) bear close relevance to
arylamido (ArNRꢀ) and arylimido (ArN2ꢀ) ligands
(Scheme 1).[1] The latter two have been subjected to im-
metal ions being +2 or less. Terminal quinoneimido com-
plexes of transition metals are extremely rare, which were
once considered (as hypothetical species) in the oxidation of
nickel o-phenylenediamine complexes[8] and were proposed
to be the key intermediates in dirhodium-catalyzed carba-
zole formation from biaryl azides.[9] To the best of our
knowledge, only one transition-metal complex bearing a ter-
minal quinoneimido ligand has been isolated and reported
in literature, that is, a mono(quinoneimido) nickel(II) com-
plex recently reported by Stephan and Bai.[6] Herein we
report the isolation, spectroscopy, and X-ray crystal struc-
tures of several terminal quinoneimido complexes of ruthe-
nium(IV), which constitute the first quinoneimido com-
plexes of a metalloporphyrin.
Scheme 1. Quinoneimine, quinoneimido, arylamido, and arylimido li-
gands.
Our approach to quinoneimido complexes came from an
unexpected reaction of high-valent metal-oxo complexes
ACTHNUTRGNEUNG
[RuVI(por)O2] (por=porphyrinato2ꢀ) with an aniline deriva-
mense studies in the field of metal–ligand multiple-bonded
complexes and in metal-catalyzed nitrogen/group-transfer
catalysis.[2] While there are a large number of metal com-
plexes of quinoneimines known in the literature,[3,4] quino-
neimido complexes of transition metals are sparse.[5–7] In the
early 1990s, Sharp and co-workers[5a,b] reported di- or tetra-
nuclear rhodium complexes analogous to bridging quinone-
imido complexes. Subsequently, bridging quinoneimido li-
gands were reported to exist in a few di- or trinuclear os-
mium[5c] and palladium/platinum[5d] complexes and hexanu-
clear osmium complexes,[5e] with the oxidation states of the
tive, in contrast to the formation of the mono(quinoneimi-
do) nickel(II) complex from reaction of a b-diketiminate
nickel(II) complex with an aryl azide.[6] We previously re-
ported that [RuVI
ACHTUNGTRENNUNG
A
a
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Me2C6H3)2]. Strikingly, the products obtained were rutheni-
um porphyrins bearing terminal quinoneimido groups.
[a] Dr. W.-M. Tsui, Dr. J.-S. Huang, Dr. G. S. M. Tong, Dr. S. C. F. Kui,
Prof. Dr. C.-M. Che, Dr. N. Zhu
ACTHNUTRGNEUNG
Treatment of [RuVI(por)O2] (por=ttp,[11a] 4-MeO-tpp,[11b]
4-Cl-tpp[11c]) with 20 equiv of 2,6-dimethylaniline in ethanol
for 8 h produced a red purple precipitate, which was found
Department of Chemistry and Open Laboratory of Chemical Biology
of the Institute of Molecular Technology for Drug Discovery and
Synthesis
The University of Hong Kong
Pokfulam Road, Hong Kong
to be a mixture of quinoneimido complexes [RuIV
ACHTUNGTRENNUNG
ACHUTNGREN(NUG NQu)ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Fax : (+852)2857-1586
4-Cl-tpp: 2c) (yieldꢁ65%). Complexes 2 probably resulted
from the hydrolysis of 1 owing to the presence of a trace
amount of water in the solvent. Indeed, stirring a mixture of
E-mail: cmche@hku.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900666.
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759

COMMUNICATION
Hd and Hdꢁ signals should stem from the prohibited rotation
of the 2,6-dimethylphenyl group about the directly attached
C=N bond (which also splits the Me2 and Me2ꢁ signals as ob-
served). Owing to the unsymmetrical coordination of the
axial sites, the Hb and Hbꢁ atoms of the meso-tolyl groups
give different signals. The Ha signal (d=8.71 ppm) of 2a ap-
pears at a lower field than those of [RuIV
ClC6H4)2] (d=8.41 ppm[10a]) and methyleneamido complex
[RuIV (N=CPh2)(OH)] (d=8.61 ppm[12]).
(ttp)
ACHTUNGTRENUN(NG ttp)(NH-p-
A
ACHTUNGTRENNUNG
UV/Vis spectra of 2a–c feature Soret and b bands at lmax
ꢁ420 and 530 nm, respectively (Figure S2 in the Supporting
Informaion), similar to those of [RuIV
ACHTNUGTRNE(UGN ttp)(NH-p-ClC6H4)2]
(416 and 529 nm)[10a] and [RuIV
ACTHUNTRGNENUG(ttp)ACHTUGNTREN(NUGN N=CPh2)(OH)] (419
and 527 nm).[12] The IR spectra of 2a–c exhibit oxidation
state marker bands[10a] at 1012–1015 cmꢀ1, consistent with a
RuIV formulation.[10a,12] In the electrospray mass spectra of
2a–c, intense cluster peaks attributable to [RuACTHNUGRTENNUG(por)ACHTUNGTRENNUNG
(NQu)]+
were observed (Figures S3 and S4 in the Supporting Infor-
mation).
The X-ray crystal structures of 1a·2EtOH and 1b·1.5H2O
were determined (Figure 2; see also Table S1 and Figures S5
and S6 in the Supporting Information).[13] Despite the disor-
Scheme 2. Formation of quinoneimido ruthenium(IV) complexes.
1 and 2 in dichloromethane/water (10:1 v/v) for 0.5 h com-
pletely converted 1 into 2.
1
Both 1 and 2 exhibit well-resolved diamagnetic H NMR
spectra (Figure 1, and Figure S1 in the Supporting Informa-
tion), featuring substantially upfield-shifted Hd and Hdꢁ sig-
nals (dꢁ4.3, 3.5 ppm) compared with the corresponding sig-
nals of arylamido complex [RuIV
ACHTNUTRGNEG(UN ttp)(NH-p-ClC6H4)2] (d=
5.84 ppm[10a]), which is consistent with the attachment of Hd
and Hdꢁ to a quinone-like ring.[5a,b] The large splitting of the
Figure 2. Structure of 1b·1.5H2O with omission of hydrogen atoms and
solvent molecules. Ellipsoids are represented at the 30% probability
level.
der of the peripheral 2,6-dimethylphenyl group of the quino-
neimido ligand in 1a·2EtOH and the ethyl group of EtOꢀ in
1b·1.5H2O, a combination of the structural information ob-
tained from the two crystals could well provide structural
features of a [RuIV
ACHTUNGTREN(NNUG por)ACHTNURTGENG(NUN NQu)ACTHNUGTRNEN(UGN OEt)] species.
Like the mono(quinoneimido) nickel(II) complex,[6] 1a,b
each contain a linear terminal quinoneimido group (Ru-N-
ꢀ
C: 179.8(4)8 for 1a and 178.1(7)8 for 1b), with a short Ru
N(quinoneimido) distance of 1.80(2) ꢂ for 1a and
1.787(7) ꢂ for 1b. These geometric parameters are compa-
rable to those of the linear arylimido complex [(h6-cyme-
ne)Ru=N-2,4,6-tBu3C6H2] (Ru-N-C: 177.8(4)8; Ru-N(aryli-
mido): 1.753(3) ꢂ)[14] and linear methyleneamido complex
Figure 1. 1H NMR spectrum of 2a in CDCl3.
760
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Quinoneimido Complexes of a Metalloporphyrin
[RuIV
(tpp)
E
(Ru-N-C:
175.0(4)8;
Ru-
N(methyleneamido): 1.808(4) ꢂ).[12] The bond lengths and a
angle shown in Figure 2 agree well with the quinoneimido
ꢀ
core. The Ru OEt distances of 1.92(3) (1a) and 1.914(7) ꢂ
(1b) fall within the range of 1.892(3)–1.947(11) ꢂ observed
[{RuIV
ACTHNGUTERNNU(G por)(O-p-
ꢀ
for
the
Ru OR
distances
in
MeC6H4)}2O][15a] and [RuIV
G
ACHTUNGTRENNUNG
Complexes 2 are air-stable both in the solid state and in
solution. Their quinoneimido groups are remarkably inert
toward attack by protons. For example, treatment of 1a or
2a with CF3COOH at room temperature for several hours
ꢀ
did not cleave the Ru N(quinoneimido) bond, as revealed
by H NMR spectroscopic analysis. These demonstrate the
1
remarkable ability of a terminal quinoneimido ligand to sta-
1
bilize a high-valent metal ion. H NMR measurements (Fig-
ures S7–S11 in the Supporting Information) also reveal that
the axial HOꢀ group in 2a is reactive toward a variety of
protic reagents including PhOH, Ph3SiOH, (dmpym)SH
(4,6-dimethylpyrimidine-2-thiol), p-ClC6H4NH2, and Ph2C=
NH to afford [RuIV (NQu)(X)] (Xꢀ =PhOꢀ: 3; Ph3SiOꢀ:
ACHTUNGTRENNUNG(ttp)ACHTUTGNRENNUGN
4; (dmpym)Sꢀ: 5; p-ClC6H4NHꢀ: 6; and Ph2C=Nꢀ: 7;
Scheme 3),[16] thus providing a unique access to rutheni-
um(IV) porphyrins that contain an alkoxide, arylamido
group, thiolate, or methyleneamido group trans to a quino-
neimido group.
Interestingly, reaction of 2a with N-phenyl-benzene-1,4-
diamine (C6H5NH-p-C6H4NH2) in dichloromethane for two
days resulted in the formation of bis(quinoneimido) com-
plex [RuIV
ACHTUNGTRENNUNG(ttp)ACHTUNGTRENNUNG(NQu)ACTHNUGTREN(GUNN NQu’)] (8, Scheme 3). Work-up gave
8 as a purple solid contaminated by ca. 10 mol% of 2a, as
1
revealed by its H NMR spectrum (Figure S12 in the Sup-
porting Information).[17] The mass spectrum of 8 shows
prominent cluster peaks at m/z 1188 [M]+ and 1007
[MꢀNQu’]+. To the best of our knowledge, complex 8 is the
first example of directly observed bis(quinoneimido) com-
plexes of a transition metal.
To examine the extent of electron delocalization over the
QuN-RuIV-OEt or QuN-RuIV-NQuꢁ moiety in 1 and 8, we
performed density functional theory (DFT) calculations on
model compounds 9 and 10 (Figure 3). As the QuN-RuIV-
NQu’ moiety in 8 could be viewed as a model for metal-
doped oligoanilines, we also carried out DFT calculations
on model compound 11 (Figure 3). The optimized structure
of 9 (see Table S3 in the Supporting Information) agrees
well with the X-ray crystallographic data of 1a,b (within
0.026 ꢂ root-mean-square deviations over all heavy atoms).
Replacing the MeOꢀ axial ligand in 9 with Qu’Nꢀ to form
Scheme 3. Reactions of 2a with various protic reagents.
occupied, with the antibonding counterparts being the
LUMOs (Figure 4). A comparison of the computed bond
lengths (Table S6 in the Supporting Information) and bond
angles (Figure 3) between 11 and the Qu’N-Ru-NQuꢁ
moiety in 10 reveals that the “Ru doping” (i.e. replacing the
middle phenyl group of 11 by a RuIV ion) considerably
shortens the nearest C=N bonds by 0.025 ꢂ (other bond
lengths are little changed) and markedly straightens the mo-
lecular chain. This can be attributed to the p-bonding inter-
action between the redox-active RuIV center and the N atom
of the quinoneimido groups.
ꢀ
10 lengthens the Ru N(quinoneimido) bond by ca. 0.057 ꢂ,
ꢀ
accompanied by a slight increase and decrease of C N and
C=N bond lengths, respectively (see Table S3 in the Sup-
porting Information). From the MO surfaces of the
HOMOs shown in Figure 4 (see also Tables S4 and S5 in the
Supporting Information), it can be seen that both 9 and 10
have the electron density delocalized over the whole Qu’N-
Ru-OMe or Qu’N-RuIV-NQu’ chain, and the delocalization
becomes more prominent in the case of 10. The p-bonding
orbitals between Ru and N (and also O in the case of 9) are
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761

C.-M. Che et al.
COMMUNICATION
chemical[19]/metal-promoted[3c,20] oxidative dimerization of
anilines or from nucleophilic attack of coordinated amido
groups by amines.[1c] Particularly intriguing are the trans
bis(quinoneimido) ruthenium complexes, as the redox be-
havior of the bis(quinoneimido) chains could provide useful
information about the effect of doping a redox-active metal
ion into oligoanilines.
Acknowledgements
This work was supported by The University of Hong Kong (University
Development Fund), the Hong Kong Research Grants Council (HKU 1/
CRF/08), and the University Grants Committee of the Hong Kong SAR
of China (Area of Excellence Scheme, AoE/P-10/01).
Figure 3. Optimized structures of 9–11 by DFT calculations. The key
bond lengths [ꢂ] and angles [8] are indicated.
Keywords: density functional calculations · N ligands ·
porphyrins · quinoneimido complexes · ruthenium
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generation of a quinoneimido ligand from the reaction of
ACHTUNGTRENNUNG
[RuVI(por)O2] with an arylamine[18] is a C=N bond-forma-
tion process mediated by ruthenium porphyrins, and pro-
vides useful insight into the oxidation chemistry of aryl
amines by oxometalloporphyrins. These reactions could be
related to the formation of quinoneimines from electro-
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Chem. Asian J. 2010, 5, 759 – 763

Quinoneimido Complexes of a Metalloporphyrin
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the proton resonances among [RuIV
S2 in the Supporting Information) reveals the following effect of Xꢀ
ACHUTNGTRENNUG(ttp)ACHTUNGTERN(NUGN NQu)(X)] (1–7, see Table
groups on the proton resonances of [RuIV (NQu)]: i) The Ha
ACTHNUTRGENNUG(ttp)ACHTUTGNRENNUGN
[11] a) ttp=meso-tetrakis
tetrakis(p-methoxyphenyl)porphyrinato2ꢀ
kis(p-chlorophenyl)porphyrinato2ꢀ
G
;
b) 4-MeO-tpp=meso-
signal for Xꢀ =HOꢀ is downfield from those for the other Xꢀ
;
c) 4-Cl-tpp=meso-tetra-
groups. ii) The splitting between the Hb and Hb’ signals is the largest
.
ACTHNGUTERNNUG
for X=Ph3SiOꢀ/(dmpym)Sꢀ/Ph2C=Nꢀ and the smallest for X=p-
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[13] CCDC 611331 and CCDC 611332 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
ClC6H4NHꢀ. iii) The Hd, Hd’, Me2, Me2’ signals of Ru-NQu shift
downfield along Xꢀ =Ph3SiOꢀ!HOꢀ, EtOꢀ, PhOꢀ, (dmpym)Sꢀ!p-
ClC6H4NHꢀ!Ph2C=Nꢀ.
[17] The Ru-NQu proton resonances of 8 are similar to those of 7 (see
Table S2 in the Supporting Information). Owing to the presence of
an equatorial pseudo-mirror plane in the molecule of 8, its Hb and
Hb’ signals almost coalesce. The large splitting between the Hg and
Hgꢁ signals, and between the Hh and Hh’ signals, precludes alternative
formulation of the Qu’Nꢀ group as the arylamido groups C6H5NH-
p-C6H4NHꢀ or [C6H5N-p-C6H4NH2]ꢀ.
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[18] Preliminary studies showed that [RuVI
ACTHNUTRGEN(UNG por)O2] could also react with
excess 2,6-dichloroaniline to afford the corresponding quinoneimido
complexes. However, no such complexes were obtained by reacting
[16] Coordination of the Xꢀ groups to [Ru
ACTHNUTRGNE(NUG ttp)] (in 3–7) is indicated by
the large upfield shifts of their proton resonances, relative to the
corresponding free XH, as a result of the porphyrin ring current
effect. The diamagnetic 1H NMR spectra of 3–7 preclude formula-
[RuVI
ACHTNGUTERNU(NG por)O2] with excess 2,4,6-trimethylaniline.
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N
G
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249.
Received: November 23, 2009
Published online: February 1, 2010
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