Syntheses and electronic structures of μ-nitrido bridged pyridine, diimine iridium complexes

Article abstract of DOI:10.1039/c4cc03624g

The syntheses and X-ray crystal structures of dinuclear μ-azido and μ-nitrido bridged iridium complexes bearing the pyridine, diimine ligand (PDI) are reported. Their electronic structures and formal oxidation states of the metal centers are analyzed by t

Full text of DOI:10.1039/c4cc03624g

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COMMUNICATION
Syntheses and electronic structures of l-nitrido
bridged pyridine, diimine iridium complexes†
Cite this: Chem. Commun., 2014,
50, 8735
Friedrich Angersbach-Bludau, Christopher Schulz, Julia Schoffel and Peter Burger*
¨
Received 14th May 2014,
Accepted 6th June 2014
DOI: 10.1039/c4cc03624g
www.rsc.org/chemcomm
The syntheses and X-ray crystal structures of dinuclear l-azido and bridged product Ir2N3+. The presence of the azido ligand was
l-nitrido bridged iridium complexes bearing the pyridine, diimine evidenced by an n(N3) IR band at 2143 cmꢀ1, its dinuclear
ligand (PDI) are reported. Their electronic structures and formal structure was established by ESI mass spectrometry. The 1H NMR
+
oxidation states of the metal centers are analyzed by theoretical spectrum of Ir2N3 revealed a highly symmetrical environment
and experimental methods, revealing the non-innocence of the PDI consistent with either D2d- or D2h-symmetry. X-ray crystallography
+
and nitrido ligands.
of Ir2N3 (see ESI†) revealed a perpendicular arrangement of the
individual sq.-pl. ligand frameworks (D2d).
Recently, new members beyond group 8 were added to the yet
small family of electron rich late transition metal nitrido complexes
with a terminal MRN unit.1–4 Their unprecedented reactivity was
first elucidated by our group in the hydrogenation of a formal d6-
configured sq.-pl. pyridine, diimine iridium(III) nitrido complex,
(PDI)IrRN, (IrN) to the amido congener.3 The electronic structure
of IrN was analyzed in experimental, theoretical and reactivity
studies3–7 and provided evidence for the non-innocence of the
PDI ligand, which is well established in the literature.8 Our recent
study on the Si–H activation in silanes by the nitrido complex IrN
revealed an ambiphilic propensity of the iridium nitrido moiety
relating to both a nucleo- (N3ꢀ) and an electro- (N+) philic center.7
This corresponds to a formal change in the oxidation state by 4
units in the (PDI)IrRN system. Therefore a non-innocent propen-
sity can be ascribed also to the nitrido ligand. Herein, we analyze
the oxidation state of the nitrido system by the theoretical localized
orbital bonding analysis (LOBA), which was introduced by Thom
et al.9 We will also apply it to a related dinuclear m-N-bridged
iridium system, [((PDI)Ir)2m-N]+1,0,ꢀ1, which displays variable elec-
tron configurations depending on its charge.
The syntheses of the previously unknown cationic m-azido
and m-nitrido bridged dinuclear complexes are summarized in
Scheme 1. The reaction of the terminal azido compound IrN3
with the THF complex IrTHF+ leads to the diamagnetic m-azido
¨
Institut fur Anorganische und Angewandte Chemie, FB Chemie,
¨
Universitat Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany.
E-mail: burger@chemie.uni-hamburg.de
† Electronic supplementary information (ESI) available: Details of the syntheses
and characterization, and X-ray crystallographic data and DFT calculations.
CCDC 1002262–1002265. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc03624g
Scheme 1 Synthetic access to m-nitrido bridged compounds.
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From the separation of the two redox waves DDE = 500 mV in
the cyclovoltammogram a comproportionation constant of Kc =
6.5 ꢁ 108 can be estimated from the previously established relation-
ship11 K = 10(DDE/59 mV) for the equilibrium according to eqn (3).
Due to the lack of single crystals of the open-shell, para-
magnetic complex Ir2N, its structural assignment has to be based
on X-band ESR-, NMR- and IR-spectroscopic, ESI-mass spectro-
metric and elemental analysis, which were fully consistent with our
proposal. Besides the electrochemical and mass spectrometrical
data further indirect support for a dinuclear structure was obtained
from the X-ray analysis of the anionic congener Ir2Nꢀ (Fig. 1), as the
m-nitrido bridged dinuclear structure is maintained in the anionic
complex Ir2Nꢀ. In comparison with the cationic congener Ir2N+ the
bonds of the iridium centers to the central nitrido nitrogen
atom are slightly elongated by 0.04 Å. The IrmN-Ir bridging unit
Fig. 1 Ortep representation of the m-nitrido bridged complexes Ir2N+ (left) and
Ir2Nꢀ (right, 50% probability level). The triflate counterion of Ir2N+, hydrogen
atoms and co-crystallized solvent molecules are omitted for clarity. Selected
bond distances in [Å] and angles in [1]; IrN+: Ir1–N4 1.8067(3); Ir1–N4–Ir1i
167.3(2). IrNꢀ: Ir1–N4 1.845(2); Ir2–N4 1.838(2); Ir1–N4–Ir2 156.3(1).
+
Both photolysis and thermolysis of Ir2N3 induce loss is slightly more bent than in the cationic complex Ir2N+ (167.31)
of dinitrogen and lead to the corresponding symmetrically and displays an angle of 156.3(1)1. This might be caused by the
m-nitrido bridged complex Ir2N+. The structural assignment sodium counterion, which is weakly coordinated to the nitrido
of the dinuclear paramagnetic compound Ir2N+ was assisted nitrogen atom N4. The coordination sphere of the sodium ion
by X-ray crystallography (Fig. 1). The bridging m-nitrido unit is is completed by a THF ligand and two aryl rings of the ketimine
slightly bent and displays an Ir1–N4–Ir1i angle of 167.3(2)1 and unit, which bind in a Z6-fashion.
an Ir1–N4 distance of 1.8067(3) Å. Related m-nitrido bridged
The assignment of the oxidation states, respectively, the study
systems were previously reported for Ru, Os and Ir complexes and of the non-innocence of the PDI and nitrido ligands was our
display M–Nm-nitrido bond lengths in the range of 1.68–1.87 Å.10 central focus and was addressed through a combined analysis of
The value for Ir2N+ is ca. 0.15 Å longer than the related IrRN the electronic, magnetic and structural properties of the m-nitrido
bond (1.646 Å) in the terminal nitrido complex IrN and 0.15 Å bridged systems Ir2N+, Ir2N and Ir2Nꢀ. The most important results
shorter than the Ir–Nazido bond in Ir2N3+ suggesting a bond order are summarized in Table 1, further details can be found in the
between 1 and 3. This is tentatively supported by the recorded ESI.† The anionic m-nitrido bridged complex Ir2Nꢀ is diamagnetic,
n(Ir–N) vibrational stretch of 852 cmꢀ1, which is red-shifted both of its neutral and cationic congeners Ir2N+ and Ir2N are
from the terminal n(IrRN) IR band at 958 cmꢀ1 observed in paramagnetic. For the open-shell system Ir2N, the X-band EPR
the mononuclear nitrido complex IrN.
signal at g = 1.95 and the Curie–Weiss behaviour of the vT
The cationic complex Ir2N+ is paramagnetic and displays
H NMR shifts were consistent with a S = ground state. The
1
12
1
2
1
broad H NMR resonances with a chemical shift range of +120 comproportionation constant Kc = 6.5 ꢁ 108 for eqn (3) corro-
to ꢀ90 ppm at 298 K. The 1H NMR data were consistent with a borated a strongly electronically delocalized structure according
time averaged D2d-symmetrical structure with a linear m-nitrido to a Robin-Day class III system for IrN.13 The paramagnetism of
bridge. The cyclic voltammogram of the t-butyl substituted the even-electron complex Ir2N+ on the other hand came as a
derivative tBuIr2N+ in THF revealed two reduction waves at ꢀ300 surprise. It manifested itself through a magnetic moment of
and ꢀ800 mV vs. the Cp2Fe/Cp2Fe+ redox couple. This corre- meff = 3.23 mB in THF solution, an X-band EPR-signal at g = 1.92
sponds to the electrochemical scheme shown below:
and the large dispersion and temperature dependence of the
1
1H NMR shifts. The vT H NMR data for Ir2N+ can be fitted to
þeꢀ
ꢀeꢀ
þeꢀ
ꢀeꢀ
tBuIr2Nþ
tBuIr2N
tBuIr2Nꢀ
ꢀ!
ꢀ
ꢀ!
ꢀ
the Van-Vleck equation14 for a S = 0 # S = 1 spin equilibrium
with a preference of ca. 4 kJ molꢀ1 for an open shell singlet
ground state (vide infra). It was supported by broken symmetry
DFT calculations for the unsubstituted D2d-symmetrical model
system, which revealed an essentially energetically degenerate
and sparked the idea to synthesize the singly and doubly reduced
neutral and anionic complexes Ir2N and Ir2Nꢀ. The reaction of
Ir2N+ with either TDAE, ((NMe2)2CQC(NMe2)2) or cobaltocene
provided indeed the neutral complex Ir2N in good yields (eqn (1)).
Ir2N+ + Cp2Co or TDAE - Ir2N
Ir2N + NaBEt3H or Na(N(SiMe3)2) - Ir2Nꢀ
Ir2N+ + Ir2Nꢀ # 2Ir2N
(1)
(2)
(3)
Table 1 Electronic properties of chlorido, azido and nitrido complexes
Parameter
IrCl
IrN
Ir2N3
Ir2N+
S = 0a S =
Ir2N
Ir2Nꢀ
+
1
2
Ground state
S = 0 S = 0 S = 0
S = 0
61.9
64.8
Ir(I)
XPS [eV]b Ir(4f7/2
,
)
5/2
62.0
65.0
Ir(I)
0
63.1
66.0
Ir(III)
ꢀ2
62.2
65.0
Ir(I)
0
62.5
65.3
Ir(I)
ꢀ1
61.9
64.8
Ir(I)
Further reduction of Ir2N with Na(N(SiMe3)2) and NaBEt3H
as unconventional reducing agents led to the anionic m-nitrido
bridged complex Ir2Nꢀ in 79–98% yield (eqn (2)). The neutral
complex Ir2N can also be obtained in the comproportionation
reaction of complexes Ir2N+ and Ir2Nꢀ according to eqn (3).
LOBA/ox. state
PDI charge
Nitrido ligand
ꢀ1.5 ꢀ2
—
Nꢀ
—
N+
N+ N+
a
b
Open shell singlet. XPS of the Ir(III) reference compound (see ESI).
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Fig. 3 LMOs (Pipek–Mezey) used in the LOBA of the IrCl model complex.
Top: ligand based; bottom: metal centered LMOs (MPA: dz2(98%), dyz(80%),
d
xy(98%), dxz(67%) left to right).
Fig. 2 Frontier orbitals of the open shell radical (PDI)Ir and qualitative
bonding scheme in Ir2N+1,0,ꢀ1 (diagram for Ir2N+).
There is also a set of 4 LMOs corresponding to the dz2, dyz, dxy
and dxz orbitals as anticipated for a d8-configured sq.-pl. system
(Fig. 3, bottom). In the LOBA for the terminal nitrido complex
situation for the S = 0 and S = 1 states. The bonding scheme and IrN the same set of four ligand based orbitals was derived (see
magnetic properties of the m-nitrido bridged systems can be ESI†). The situation for the metal centered orbitals is different,
explained with the aid of the MO diagram shown in Fig. 2.
however. While the dz2 and dyz orbitals can be unambiguously
The m-nitrido bridged system in Fig. 2 is assembled from the assigned to the metal center summing up to 4 electrons, the
central nitrido nitrogen atom and two open-shell [(PDI)Ir] situation for the dxy and dxz orbitals is severely more compli-
model fragments. For the latter a doublet ground state with a cated. The values of 62% and 57% for the Mulliken population
p-symmetrical SHOMO (15b2) was previously reported by us.3,4 analysis (MPA) of the LMOs suggest highly covalent character of
The spin density is essentially located on the non-innocent PDI the Ir–Nnitrido p-bonds.
ligand with the electronic structure being best described as d8-
Note that this view is consistent with the ambiphilic propen-
configured (PDIꢀ)Ir+(I). There are two further relevant orbitals sity of the nitrido unit observed in H–H, C–H and Si–H bond
in the frontier orbital region, the occupied dxy-orbital (6b1) and activation processes.3,6,7 Two further electrons in IrN are stored
the vacant metal centered s-acceptor orbital 23a1. These orbitals in the non-innocent PDI ligand. This was previously revealed
are combined to three sets of symmetry adapted orbitals of e and from a theoretical analysis and experimentally from the relatively
a1 symmetry on the left hand side of Fig. 2 and are essential for short exocyclic Cpy–Cimi bond of 1.424 Å between the pyridine
binding to the nitrido ligand. Inspection of Fig. 2 immediately ring and the imine unit,3,5 which was established by a number of
reveals that stable electronic situations can be obtained up to a authors as a clear diagnostic sign for electron transfer to the thus
total of 12 electrons. This holds for all the nitrido systems Ir2N+, non-innocent PDI ligand.8 Finally, it deserves a special mention
Ir2N and Ir2Nꢀ, which have 10, 11, and 12 electrons according to that the LOBA of the nitrido complex IrN also gives a clear hint to
this scheme. The open-shell character of the cationic complex the non-innocence of the (reduced) PDI ligand. This became
Ir2N+ becomes immediately apparent since the degenerate 2e aware through a PDI based LMO, which revealed MPA values of
orbital set is filled with 2eꢀ.
70% for the nitrogen atom of the pyridine group, 23% for iridium
The MO diagram presented in Fig. 2 also explains the observed and 7% for nitrido nitrogen atom (ESI†). Note that this type of
diamagnetism of the anionic complex, in which the degenerate 2e LMO is present in all other systems described herein, where the
orbital set is fully occupied with 4 electrons. The interesting non-innocence of the PDI-ligand comes into play. The electrons
question remains, how many electrons can be assigned to the of this unique LMO were therefore assigned to the non-innocent,
nitrido ligand, 4, 6, or 8? This will be addressed in our localized i.e. PDIꢀ2, ligand. This is in full agreement with previous
bond analysis (LOBA) below and will provide answers to both the complementary XAS and XPS measurements which supported
oxidation state of the iridium center and the electronic configu- a d6-configured Ir(III) center with an overall ‘‘(PDI2ꢀ)Ir(3+)(Nꢀ)’’
ration of the nitrido ligand, i.e. N+, Nꢀ or N3ꢀ
The LOBA was introduced for the theory assisted assignment of
.
electronic structure.3,5
For the m-nitrido bridged systems an analogous analysis was
oxidation states of metal centers in coordination compounds.9 The performed. The X-ray photoelectron spectroscopy (XPS) data for
procedure is straightforward for purely dative bonds with typical Ir2N+, Ir2N and Ir2Nꢀ display ionization energies (IEs) for the
ligand contributions of 470%, the situation is complicated for iridium 4f7/2 component in the narrow range of 61.9–62.5 eV,
covalent bonds, which approach a population of 50% for both the same holds for the 4f5/2 component (Table 1). Although the
bond partners. The latter situation is observed for the Ir–Nnitrido IE for the cationic system Ir2N+ is ca. 0.5 eV higher than
bonding in our iridium nitrido systems (vide infra). As a bench- the identical values for the neutral and anionic and nitrido
mark for the LOBA the d8-configured chlorido model complex complexes Ir2N+ and Ir2Nꢀ, the data for all nitrido compounds
(PDI)Ir–Cl (IrCl) was chosen. From a DFT calculation four distinct compare well with experimental IEs of the d8-configured chlorido
Pipek–Mezey localized molecular orbitals (LMOs) were identified and m-bridged azido Ir(I) congeners IrCl5 and Ir2N3+ (Table 1). For
(Fig. 3, top), which could be safely assigned to the ligands. the two d6-configured square-planar and octahedral PDI Ir(III)
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reference compounds significantly higher IEs of 63 eV for the
The assignment of 1.5 negative charges to both PDI ligands
Ir(4f7/2) components were recorded (Table 1). Considering a in Ir2N adheres to the strongly delocalized (Robin-Day class III)
typical increase of the IE of ca. 1 eV upon the increase of the electronic structure evidenced from the electrochemical data.
oxidation state by 1 unit, it can be assumed with some certainty
The nitrido iridium systems display strongly delocalized
that the oxidation states of the Ir centers in Ir2N+, Ir2N and electronic structures. The LOBA allowed the assignment of
Ir2Nꢀ are definitely lower than +3. Based on the comparison with the formal oxidation states of the Ir centers and revealed non-
the Ir(I) reference compounds IrCl and Ir2N3+ a d8-configuration innocent PDI ligands, N+ nitrido nitrogen atoms and electron-
is therefore suggested for the Ir centers in the dinuclear com- rich d8-configured Ir(I) centers in the m-nitrido bridged systems.
plexes Ir2N+, Ir2N and Ir2Nꢀ.
Their reactivity will be reported in due course.
The LOBA unveiled essentially identical situations for the
metal centers in all bridging m-nitrido systems. The electrons of
the LMOs corresponding to the dz2, dyz and dxy orbitals could
be safely assigned to the iridium centers leading to (at least)
d6-electron configurations. As noted for the terminal nitrido
complex IrN, the MPAs for the remaining LMOs (dxz)
also revealed lower values for the Ir centers (62–64%) with
contributions of 36–38% from the Nnitrido atom. This makes an
unambiguous assignment of the two bonding electrons to
either of the bonding partners difficult. If evenly shared, this
would result in d7-electron configurations for the Ir centers.
The LOBA also revealed the aforementioned PDI ligand based
LMOs, elucidating reduced, i.e. non-innocent PDI ligands in
the nitrido model systems for Ir2N+, IrN and IrNꢀ. This is
consistent with the MO diagram for the open shell system
Ir2N+ showing that additional electrons enter PDI based
orbitals, which is also reflected in the experimental and calcu-
lated bond distances and orders of the Cpy–Cimi and Cimi–Nimi
bonds for the model systems Ir2N+, Ir2N and Ir2Nꢀ (see ESI†).
This view is further corroborated from the inspection of the
M–Nnitrido bond distances and orders, which display only small
changes upon addition of electrons.
Notes and references
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Taking the XPS data into account, the following electron
configuration is anticipated for the cationic complex Ir2N+:
[(PDIꢀ1)(Ir+(d8))2-m-N+(s2p2)].
Note that in this assignment the electrons of the LMO
corresponding to the dxz-orbital (vide supra) are fully assigned
to the iridium centers. For the neutral and anionic congeners
the electron configurations shown below are proposed:
Ir2N: [(PDIꢀ1.5)(Ir+(d8))2-m-N+(s2p2)]
11 C. Creutz, Prog. Inorg. Chem., 1983, 30, 1.
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THF solution at RT, providing some arguments for a quartet state
(S = 3/2).
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14 L. W. Pignolet and W. D. Horrocks Jr., J. Am. Chem. Soc., 1969,
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Ir2Nꢀ: [(PDIꢀ2)(Ir+(d8))2-m-N+(s2p2)]
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