Redox-Active-Ligand-Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Article abstract of DOI:10.1002/anie.201603659

Coordination of a redox-active pyridine aminophenol ligand to Ru^IIfollowed by aerobic oxidation generates two diamagnetic Ru^IIIspecies [1 a (cis) and 1 b (trans)] with ligand-centered radicals. The reaction of 1 a/1 b with excess NaN₃under inert atmosphere resulted in the formation of a rare bis(nitrido)-bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru-N-Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a/1 b supports the oxidation of a nitride (N^3?) to half an equivalent of N₂. The trinuclear omplex is reactive toward TEMPO-H, tin hydrides, thiols, and dihydrogen.

Full text of DOI:10.1002/anie.201603659

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International Edition: DOI: 10.1002/anie.201603659
German Edition: DOI: 10.1002/ange.201603659
Redox Chemistry
Redox-Active-Ligand-Mediated Formation of an Acyclic Trinuclear
Ruthenium Complex with Bridging Nitrido Ligands
Bidraha Bagh, Daniꢀl L. J. Broere, Maxime A. Siegler, and Jarl Ivar van der Vlugt*
Abstract: Coordination of a redox-active pyridine amino-
phenol ligand to RuII followed by aerobic oxidation generates
two diamagnetic RuIII species [1a (cis) and 1b (trans)] with
ligand-centered radicals. The reaction of 1a/1b with excess
NaN3 under inert atmosphere resulted in the formation of
a rare bis(nitrido)-bridged trinuclear ruthenium complex with
two nonlinear asymmetrical Ru-N-Ru fragments. The sponta-
neous reduction of the ligand centered radical in the parent 1a/
1b supports the oxidation of a nitride (N3ꢀ) to half an
equivalent of N2. The trinuclear omplex is reactive toward
TEMPO-H, tin hydrides, thiols, and dihydrogen.
T
he preparation and reactivity of late-transition-metal
nitrides has attracted much attention as metal–nitrogen
multiple bonds become increasingly more difficult to accom-
modate with high-electron-count d-metal ions.[1] Group 8
metal nitrides have enjoyed specific attention because of their
relevance as faithful nitrogenase model systems.[2] Both
terminal[3] (I–III; Figure 1) and bridging[4] (IV–VI) ruthenium
nitrido complexes have been reported for RuIV, RuV, and RuVI
oxidation states.[5] Terminal Ru N species are highly electro-
ꢁ
philic and reactive toward a variety of substrates, and involve
inter alia H2 activation and hydrogen atom transfer (HAT)
reactions.[6] These mechanisms allow, for example, hydro-
genolysis (to NH3) or radical-type bond functionalization. In
contrast, the reactivity of ruthenium complexes with bridging
nitrido ligands is unexplored. Furthermore, multinuclear
ruthenium complexes with nonlinear and asymmetrically
bridging nitrido ligands are extremely rare.[7]
Redox-active ligands have emerged as a very appealing
class of ligands to induce odd-electron chemistry on either
redox-inert or platinum-group metal complexes.[8] Amino-
phenol-derived architectures have been particularly forth-
coming in allowing new reactivity for a variety of transition
metals, for example, by either acting as an electron reservoir
to enable two-electron chemistry at d0 metal centers or
invoking radical reactivity in the coordination sphere of
noble metals.[9] Ruthenium complexes with redox-active
Figure 1. Top: Representative terminal (I–III) and bridging (IV–VI)
ruthenium-nitrido species. Bottom: Azide decomposition without
(upper) and with (lower) involvement of a redox-active ligand media-
ting oxidation state of a late-transition metal.
catecholato and o-amidophenolato systems are well-
known.[10] However, the potential for intramolecular redox
chemistry between ruthenium nitrido species and redox-
active ligands is uncharted. Such electron-transfer phenom-
ena might significantly impact the resulting reactivity (as well
as provide a potential route toward generation) of such
nitrido species. We herein address a synthetic strategy to
manipulate azido-to-nitrido conversion by introducing elec-
tronic interplay between a redox-active ligand motif and
a ruthenium center, which induces a higher metal oxidation
state. This strategy has allowed the isolation of a unique
trinuclear species bearing a highly asymmetric Ru-N-Ru-N-
Ru skeleton featuring one redox-active ligand per ruthenium.
Preliminary reactivity screening suggests that the resulting
bridging nitride fragments are susceptible to hydrogen-atom
transfer from TEMPO-H, tin hydrides, thiols, and activation
of dihydrogen.
[*] Dr. B. Bagh, D. L. J. Broere, Dr. Ir. J. I. van der Vlugt
Homogeneous, Bioinspired and Supramolecular Catalysis
Van’t Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904, 1098 XH, Amsterdam (The Netherlands)
E-mail: j.i.vandervlugt@uva.nl
Dr. M. A. Siegler
We previously reported the synthesis, electrochemistry,
and (catalytic) reactivity of square-planar palladium(II)
complexes bearing a redox-active pyridine aminophenol
ligand NNOH2 and close analogues thereof (Scheme 1).[11]
Coordination of NNOH2 to [RuCl2(PPh3)3] under inert
Small Molecule X-ray Crystallography Facility
Department of Chemistry, John Hopkins University
Baltimore, MD 21218 (USA)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603659.
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The reaction of 1a in THF with
excess NaN3 for two days under
inert atmosphere and ambient light
conditions resulted in the selective
formation of the dark-brown com-
plex 2 (Scheme 1). The IR spectrum
of this species does not contain any
stretch attributable to an azido frag-
ment. Exclusion of light had no
effect on the outcome of the reac-
tion and using 1b under identical
reaction conditions led to the same
result. Species 2 is diamagnetic and
shows C1 symmetry on the NMR
timescale. The 31P NMR spectrum
showed two resonances at d = 22.69
and 26.34 ppm in C6D6. Slow diffu-
sion of n-pentane into a benzene
solution of 2 afforded single crystals
Scheme 1. Synthesis of 1a, 1b, and 2: 1) 1 molar equiv [RuCl2(PPh3)3], C6H6, Ar, 24 h; 2) air, 48 h;
3) excess NaN3, THF, Ar, 48 h. THF=tetrahydrofuran.
suitable for X-ray structure determination (Figure 3). The
ꢀ
Ru O,
Figure 2. Displacement ellipsoid plots (50% probability level) of 1a
and 1b (H atoms and lattice solvent molecules (for 1b) omitted for
clarity).[23] Selected bond lengths [ꢀ] and angles [8]: For 1a: Ru1-O1
2.063(2), Ru1-N1 1.904(2), C1-O1 1.283(4), C6-N1 1.349(4), C1-C6
1.443(4); Cl1-Ru1-Cl2 88.01(3). For 1b: Ru1-O1 2.040(4), Ru1-N1
1.907(6), C1-O1 1.283(8), C6-N1 1.357(8), C1-C6 1.453(9); Cl1-Ru1-Cl2
170.93(7).
Figure 3. Left: displacement ellipsoid plots (50% probability level) of 2
(H atoms and lattice solvent molecules are omitted for clarity). Right:
View of core skeleton of 2.[23] Selected bond lengths [ꢀ] and angles [8]:
Ru1-O1a 2.018(3), Ru2-O1b 1.992(3), Ru3-O1c 2.017(3), Ru1-N1a
1.907(3), Ru2-N1b 1.945(2), Ru3-N1c 1.928(3), Ru1-N3a 1.694(3),
Ru3-N3b 1.695(3), Ru2-N3a 1.844(3), Ru2-N3b 1.871(3); Ru1-N3a-Ru2
162.14(2), Ru2-N3b-Ru3 166.74(2), N3a-Ru2-N3b 96.84(1), N2a-Ru1-
N3a 96.44(13), N3a-Ru1-N4a 113.92(15), P1-N4a-Ru1 137.0(2), P2-
N4c-Ru3 131.7(2).
atmosphere followed by aerobic oxidation resulted in two
species, identified as cis-[RuCl2(PPh3)(NNO)] (1a) and trans-
[RuCl2(PPh3)(NNO)] (1b), in a roughly equimolar ratio
based on multinuclear NMR spectroscopy. These isomers
were isolated as dark-violet (1a, 43%) and dark-blue (1b,
46%) diamagnetic crystalline solids. The NMR spectra of 1b
are consistent with overall Cs symmetry, while 1a exhibits
C1 symmetry on the NMR timescale. The molecular struc-
tures of 1a and 1b were confirmed by single-crystal X-ray
diffraction (Figure 2). The geometry around the ruthenium
centers in 1a and 1b is distorted octahedral, and the
ruthenium–ligand bond lengths are comparable.[12,13]
ꢀ
ꢀ
Ru N, and Ru Npyr bond lengths are similar at the three
=
ruthenium centers. Also both phosphiniminato (N PPh3)
ꢀ
moieties show comparable Ru NP bond lengths, but their
relative opposing orientation with respect to the NNO
ꢀ =
framework of the central ruthenium makes these Ru N
ꢀ
PPh3 units magnetically inequivalent. The Ru Nnitrido bonds
with the peripheral Ru centers [1.694(3) and 1.695(3) ꢀ] are
By using the metric oxidation state (MOS) method,[14]
values of ꢀ0.74(ꢂ 0.07) and ꢀ0.78(ꢂ 0.14) were obtained for
1a and 1b, respectively.[15] Based on the observed non-integer
MOS values and the interaction between the DFT calculated
ligand orbital with a metal p-type orbital (see the Supporting
Information), we prefer an intermediate description between
RuII/NNOIBQ and RuIII/NNOISQ rather than a pure RuIII/
NNOISQ situation for 1a and 1b, with antiferromagnetic
coupling between RuIII and NNOISQ likely accounting for the
overall diamagnetic character of these species (see the
Supporting Information).
much shorter than the Ru Nnitrido bonds with the central Ru
ꢀ
center [1.871(3) and 1.844(3) ꢀ]. The Ru-N-Ru fragments
deviate significantly from linearity [ffRu-N-Ru of 162.14(2)
and 166.74(2)] relative to known nitrido-bridged dinuclear
ruthenium complexes.[4] These combined data suggest that
= ꢀ ꢀ =
Ru N Ru N Ru is an appropriate description for the
bonding description in 2.
Metal nitrido species are commonly generated by photol-
ysis or thermolysis of the corresponding metal azido com-
plexes, although salt metathesis of ruthenium halides with, for
example, NaN3 sometimes leads to the spontaneous forma-
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tion of nitrido species without detection of the relevant azido
spectroscopy (d = 32.94)[19] and mass spectrometry [m/z 277
(M+)].
Based on the metric parameters obtained experimentally
intermediate.[3a,4d] We set out to investigate the mechanism of
the reaction of 1 with NaN3 to generate 2. Reaction of 1a with
excess NaN3 in [D8]THF was monitored by 1H and 31P NMR
spectroscopy (Figure 4; for 1b see the Supporting Informa-
tion).
from X-ray crystallography, all three NNO moieties are best
described as fully reduced amidophenolato NNOAP fragments
[MOS values of ꢀ1.98(ꢂ 0.10), ꢀ1.81(ꢂ 0.06) and ꢀ1.77(ꢂ
0.09)].[14,15] The metric parameters were well-reproduced by
the DFT optimized geometry. Analysis of the molecular
orbitals revealed highly delocalized orbitals over the Ru-N-
Ru-N-Ru skeleton, including the highest occupied molecular
orbital (HOMO; Figure 5). The spin density of the optimized
geometry in the triplet state, which is higher in energy by
16.0 kcalmolꢀ1, showed similar delocalization. The large
singlet–triplet gap is indicative of strong anti-ferromagnetic
coupling. Consequently, we propose to describe the electronic
structure of 2 as [(L)(NNOAP)RuV]-N3ꢀ-[RuIV(NNOAP)]-N3ꢀ-
[RuV(NNOAP)(L)] (L = N PPh3) with highly delocalized
=
anti-ferromagnetically coupled electrons over the Ru-N-Ru-
N-Ru framework.
Figure 4. Progress of the reaction of 1a with NaN3 in [D8]THF
monitored by 31P NMR spectroscopy (Ph3PO is indicated by *).
Based on the experimental data and supported by DFT
(see the Supporting Information), we propose the following
mechanism for the formation of 2 from 1a (Scheme 2). Initial
chloride substitution of 1a by sodium azide yields the
bis(azido) complex A within four hours, with a visible color
change from dark-violet to dark-green and appearance of
a strong IR band at 2027 cmꢀ1 [16,17]
After 6 hours, concom-
.
itant with a color change from dark-green to dark-blue, a new
major species was observed by 1H and 31P NMR spectroscopy
(see the Supporting Information). This species C is formu-
Figure 5. Left: DFT (BP86, def2-TZVP) calculated HOMO of the
optimized geometry of 2. Right: proposed predominant redox-isomer
of 2.
=
lated as a ruthenium nitrido complex, [Ru(N)(N PPh3)-
(NNO)], bearing a phosphiniminato moiety, with strong IR
ꢀ1
bands at 1041 (Ru N) and 1092 cm (N P).[18] Trapping of C
ꢁ
=
=
with PPh3 gave the [Ru(N PPh3)2(NNO)] 3, which was
spectroscopically characterized. Ultimately, three molecules
of C combine to form the trinuclear complex 2 within
24 hours, concomitant with the elimination of formally half an
equivalent of dinitrogen and one equivalent of iminophos-
The structural data of 2 suggest three dianionic amido-
phenolato NNOAP states, which supports intramolecular
substrate-to-ligand electron transfer, thus resulting in one-
electron reduction of each ligand and three-electron reduc-
tion overall, relative to the starting complex 1, upon reaction
with azide. This corresponds to the oxidation of a nitrido
ligand (N3ꢀ) to half an equivalent of N2 (Scheme 3). The
formation of N2 is supported by mass spectrometry when 1a
phorane (Ph3P NH), which was detected by 31P NMR
=
Scheme 2. Proposed pathway for reaction of 1a toward NaN3 with
formation of 2 via the intermediates A and C, and trapping of C to
generate 3.
Scheme 3. Isotopic-labelling experiment showing the formation of
three isotopomers of N2 and 31P NMR spectrum of isotopomers of
complex 2. * represents 14N isotopomer, 8 indicates 15N isotopomer.
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was reacted with Na15N14N2 (terminal 15N) in THF under
Keywords: azides · density functional calculations ·
redox chemistry · ruthenium · structure determination
argon for two days in a closed vessel. The nitride and
phosphiniminato of C contain both 14N (50%) and 15N (50%).
Dinitrogen formed from azide activation would give only
14N14N and 15N14N. Mass analysis of the headspace above the
reaction medium revealed the additional presence of 15N15N,
which supports the reductive coupling of two ruthenium
nitrides toward N2.[20] The corresponding 15N-labeled isoto-
pomer of 2 resulted in two new doublets in the 31P NMR
spectrum.
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When 2 is exposed to 5 bar of H2 at 508C, two new 31P
resonances at d = 17.65 (broad) and 38.00 ppm (sharp) were
obtained within 12 hours (see the Supporting Information for
details). A pure dark-brown diamagnetic solid, correlating to
the sharp singlet at d = 38.00 ppm, precipitated out of solution
upon cooling. Variable-temperature 1H NMR spectroscopy
showed resonances for three chemically inequivalent NNO
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=
ligands and one coordinated N PPh3 moiety. Mass analysis
(m/z 1626.54 [M]+) supported formulation of this species, 4,
as [(L)(NNO)Ru]-NH-[Ru(NNO)]-NH-[Ru(NNO)] (L =
ꢀ1
=
N PPh3). A weak, broad IR band was present at 3576 cm ,
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ꢀ
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The same spectroscopic features were obtained upon reaction
of 2 with 2 equivalents of TEMPO-H in [D8]THF, with the
TEMPOC radical being detected by EPR spectroscopy. The
reaction of 2 with 2 equivalents of either PhSH or BuSnH as
ꢀ
ꢀ
hydrogen-atom donors also led to 4, and PhS SPh and BuSn
SnBu, respectively. Although it is reasonable to assume that
both H2 activation and hydrogen atom transfer occur directly
at the bridging nitrido fragments, an alternative pathway
ꢀ
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involving short-lived Ru H species cannot be fully excluded.
Additional studies to elucidate the detailed mechanism for
these HAT reactions are ongoing.
In conclusion, ruthenium complexes 1a and 1b, which are
best described as ruthenium(III) species with a ligand-cen-
tered NNOISQ radical, undergo spontaneous azide decompo-
sition upon salt metathesis with NaN3 to generate a rare
trinuclear ruthenium complex (2) featuring two asymmetric
Ru-N-Ru fragments. The redox-active NNO ligands appear
to act as electron acceptors, thus facilitating the formation of
the trinuclear assembly and N2. Isotopic labeling supports
nitride coupling as the source of dinitrogen. The high extent
of delocalization over the Ru-N-Ru-N-Ru fragment is a pos-
sible explanation for the observed diamagnetism. The bridg-
ing nitride moieties in 2 displays hydrogen-atom transfer
reactivity towards TEMPO-H, tin hydride, and thiol. Sim-
ilarly 2 reacts with H2 and thus might be catalytically
competent for hydrogen-transfer reactions.[22]
Acknowledgments
Research funded by the European Research Council (ERC)
through Starting Grant 279097 to J.I.v.d.V. We thank Ed
Zuidinga for ESI-MS, Norbert Geels for MS analysis of N2
isotopomers and Professors Bas de Bruin and Joost Reek for
general discussions.
4
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Received: April 14, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Redox Chemistry
Three strikes aN2d you’re out! The spon-
taneous azide decomposition on mono-
nuclear ruthenium complexes bearing
a redox-active aminophenol-derived tri-
dentate NNO ligand selectively generates
a rare trinuclear complex with an unsym-
B. Bagh, D. L. J. Broere, M. A. Siegler,
J. I. van der Vlugt*
&&&& — &&&&
Redox-Active-Ligand-Mediated
Formation of an Acyclic Trinuclear
Ruthenium Complex with Bridging
Nitrido Ligands
= ꢀ ꢀ =
metric Ru N Ru N Ru skeleton, pro-
posedly by oxidative nitride coupling. DFT
and experimental data support a pivotal
role for the NNO ligand. The bridging
bis(nitrido) complex is reactive towards
H2 and hydrogen-atom donor species.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!