Aromatic diosmatricyclic nitrogen-containing compounds

Article abstract of DOI:10.1021/ja8048892

Aromatic diosmatricyclic nitrogen-containing compounds are prepared from Os(VI) complex OsH₆(PⁱPr₃) by double 1,3-C-H bond activation of aromatic six-membered cycles with imino substituents meta disposed. Copyright

Full text of DOI:10.1021/ja8048892

Published on Web 08/12/2008
Aromatic Diosmatricyclic Nitrogen-Containing Compounds
Miguel A. Esteruelas,* Ana B. Masamunt, Montserrat Oliva´n, Enrique On˜ate, and Marta Valencia
Departamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n, UniVersidad de Zaragoza,
CSIC, 50009 Zaragoza, Spain
Received June 26, 2008; E-mail: maester@unizar.es
Aromatic metallacycles are transition metal containing ring
systems that exhibit aromatic properties.1 While much effort has
been focused on metallabenzenes2 and related metallacarbocycles,3
important progress is also being made in synthesizing and studying
the properties of heteroatom-containing species.4
Higher π-electron metallacyclic compounds are very rare. The
isolation of the first metallanaphthalene by Paneque and co-workers5
has extended the aromatic metallacycles to fused bicyclic com-
pounds.6 Evidence for the formation at -78 °C of a ruthenaphenan-
Figure 1. X-ray structure of complex 2 (isopropyl groups have been omitted
threne oxide has been also reported,7 and the intermediacy of
for clarity).
renaphenanthrenes in reactions of 2,2′-dilithiobiphenyl with
ReBr(CO)4(PPh3) has been suggested.8
Bis(imino)pyridines are emergent tridentate ligands, which are
receiving a lot of attention due to their metal complexation
chemistry.9 They are facilitating the entry into new families of
transition metal complexes, which are being investigated for their
potential use in a variety of catalytic metal-mediated organic
Figure 2. Plot of the valence 12b1 and 11a2 π orbitals of 2. The value of
the outermost contour line is 0.035.
transformations.10
The C-H bond activation reactions are promoted by low-valent
metal complexes.11 The use of high-valent metal compounds is rare,
in particular hydride derivatives. Despite this, we have reported
that the osmium(VI) complex OsH6(PiPr3)2 (1) activates C-H bonds
of amines,12 ketones,13 aldehydes,14 and imidazolium salts.15 Now,
we show that it is also able to produce the unprecedented activation
of both meta C-H bonds of the pyridine ring of bis(imino)pyridines
and related molecules, to afford aromatic dimetallatricyclic hetero-
atom-containing systems.
Treatment of toluene solutions of 1 with 0.5 equiv of 2,6-bis{1-
[(4-methylphenyl)imino]ethyl}pyridine under reflux leads to a dark
brown solution, from which the 1,7-diosma-2,4,6-triaza-s-indacene
complex 2 is isolated as dark red crystals by addition of methanol
and subsequent cooling of the mixture at 5 °C (eq 1).
(1.320(7) Å) bond length of the pyridinic ring and slightly shorter
than the N(1)-C(5) (1.346(6) Å) distance. All C-C bond lengths
are statistically identical and lie between 1.454(6) and 1.420(7) Å.
The main characteristic of the orbital interactions in aromatic
metallacyclic molecules is the involvement in the π bonding of
two filled metal d orbitals, dxz and dxy, instead p orbitals of main
group elements.16 The orbital interactions in 2 are fully consistent
with this. Figure 2 shows the 12b1 (π symmetry) and 11a2 (δ
symmetry) interactions between the metal dxz and dxy orbitals and
the 3b1 and 2a2 organic fragment orbitals.
The coordination geometry around each osmium atom can be
rationalized as a distorted pentagonal bipyramid with axial phos-
phines (P(1)-Os(1)-P(2) ) 160.11(5)°, P(3)-Os(2)-P(4) )
160.69(5)°) and the hydride ligands lying in the equatorial plane
between the metallated atoms of the organic fragment. In agreement
with this phosphine disposition, the 31P{1H} NMR spectrum in
toluene-d8 contains a singlet at 23.8 ppm, which is temperature-
invariant between 363 and 203 K. In contrast to the 31P{1H} NMR
spectrum, the 1H NMR spectrum is temperature-dependent. At
temperatures higher than 363 K, it shows a single hydride resonance
at -10.1 ppm, which is consistent with the operation of two
thermally activated site exchange processes on each osmium atom,
in agreement with the behavior of other trihydride derivatives.17
At ∼353 K, the first decoalescence occurs, and at ∼258 K, the
second one takes place. Thus, between 233 and 203 K, three
resonances at -5.84 (H(01) and H(04)), -11.79 (H(02) and H(05)),
and -12.42 (H(03) and H(06)) ppm are observed. These chemical
shifts show no significant temperature dependence. However, the
magnitude of the observed H(01)-H(02) (H(04)-H(05)) coupling
constant is sensitive to the temperature, decreasing from 84 to 48
Hz as the temperature decreases from 233 to 203 K. This indicates
the operation of quantum exchange coupling18 between H(01) and
H(02) and between H(04) and H(05), in addition to the thermally
The X-ray structure of 2 (Figure 1) proves the formation of the
dimetallatricycle, which is planar (maximum deviation 0.1267(34)
Å, N(2)). In agreement with a significant electron delocalization
through the system, the bond lengths in the cycles are between
those expected for single and double bonds. According to this, the
Os(1)-N(2) (2.179(4) Å) and Os(2)-N(3) (2.183(4) Å) distances
agree well with the Os-N bond lengths reported for osmapyrrole
derivatives,4i whereas the Os(1)-C(1) (2.100(5) Å) and Os(2)-C(3)
(2.100(6) Å) distances are similar to the Os-C bond lengths
reported for both osmafuran4f,k and osmapyrrole complexes.4i The
N(2)-C(8) (1.313(6) Å) and N(3)-C(6) (1.306(7) Å) distances of
the five-membered rings are statistically identical with the N(1)-C(4)
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11612 J. AM. CHEM. SOC. 2008, 130, 11612–11613
10.1021/ja8048892 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
activated site exchanges. The value of ∆H‡ for the processes
involving H(01) and H(02) and H(04) and H(05) is 11 kcal ·mol-1
while the H(02)/H(03) and H(05)/H(06) exchanges cost 15
kcal·mol-1
The trend of 1 to activate aromatic C-H bonds allows prepara-
tion of other dimetallatricyclic heteroatom-containing systems by
means of the same strategy, involving a double 1,3-C-H bond
activation of aromatic rings with imino substituents meta disposed.
Thus, the treatment of toluene solutions of 1 with 0.5 equiv of 1,3-
bis{1-[(4-methylphenyl)imino]ethyl}benzene under reflux affords
the 1,7-diosma-pyrrolo[3,4,f]isoindole complex 3 as dark red
crystals (eq 1).
The X-ray structure of 3 (see Supporting Information) is
consistent with that shown in Figure 1. The 31P{1H} and 1H NMR
spectra also agree well with those of 2. The 31P{1H} NMR spectrum
in toluene-d8 contains a singlet at 24.0 ppm. In agreement with the
X-ray structure, at 213 K, the 1H NMR spectrum shows three
hydride resonances at -6.04, -11.91, and -12.22 ppm. The
hydride ligands of each osmium atom also are involved in two
thermally activated site exchange processes. In this case the values
of ∆H‡ are 12 and 15 kcal ·mol-1. Furthermore the central hydrides
and those cisoid disposed to the nitrogen atoms undergo quantum
exchange coupling increasing the H-H coupling constant from 32
to 60 Hz as the temperature increases from 213 to 243 K.
The abnormal behavior of the 2,6-bis(imino)pyridine molecule
giving rise to the formation of 2 is a consequence of the high trend
of 1 to activate aromatic C-H bonds. In fact, in contrast to the
hexahydride, the known complex OsH2Cl2(PiPr3)219 (4) reacts with
2,6-bis{1-[(4-methylphenyl)imino]ethyl}pyridine to give the purple
compound 5 in high yield, which contains a usual tridentate ligand
(eq 2). Complex 5 has been also characterized by X-ray diffraction
analysis (see Supporting Information).
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In conclusion tricyclic aromatic systems formed by a central six-
membered ring fused with nitrogen-containing osma-five-membered
rings can be easily prepared from the osmium(VI) hexahydride
complex OsH6(PiPr3)2, by double 1,3-C-H bond activation of
aromatic six-membered cycles with imino substituents meta disposed.
Acknowledgment. Financial support from the Spanish MEC
(Projects CTQ2005-00656 and Consolider Ingenio 2010 (CSD2007-
00006)) and Diputacio´n General de Arago´n (E35).
Supporting Information Available: Experimental details for the
synthesis, characterization and crystallographic data for 2, 3, and 5, as
well for the computational studies. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA8048892
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