Dinuclear Di(N-heterocyclic carbene) iridium(III) complexes as catalysts in transfer hydrogenation

Article abstract of DOI:10.1002/ejic.201501027

Two novel di(N-heterocyclic carbene) complexes of formula (μ-PyrIm-CH₂-ImPyr)[IrCp?Cl]₂(PF₆)₂ (1) and μ-MeIm-CH₂(p-C₆H₂)CH₂-ImMe[IrCp? Cl]₂ (2) (Im = imidazol-2-ylidene) have been synthesised by transmetallation of the dicarbene ligand from the corresponding dicarbene silver complex, using [IrCp?(μ-Cl)Cl]₂ as an iridium precursor. The structure of complex 2 has been determined by X-ray diffraction and is characterized by a double ortho-metallation of the p-xylylene bridge between the carbene units. Both complexes show good activity in the transfer hydrogenation of ketones to alcohols in 2-propanol. Dinuclear iridium(III) complexes bearing a bridging di(NHC) ligand have been synthesised and tested as catalysts in transfer hydrogenation reactions.

Full text of DOI:10.1002/ejic.201501027

DOI: 10.1002/ejic.201501027
Full Paper
NHC Complexes
Dinuclear Di(N-heterocyclic carbene) Iridium(III) Complexes as
Catalysts in Transfer Hydrogenation
Andrea Volpe,[a] Salvatore Baldino,[b] Cristina Tubaro,*[a] Walter Baratta,*[b] Marino Basato,[a]
and Claudia Graiff[c]
Abstract: Two novel di(N-heterocyclic carbene) complexes of plex 2 has been determined by X-ray diffraction and is charac-
formula (μ-PyrIm-CH2-ImPyr)[IrCp*Cl]2(PF6)2 (1) and μ-MeIm-
CH2(p-C6H2)CH2-ImMe[IrCp*Cl]2 (2) (Im = imidazol-2-ylidene)
have been synthesised by transmetallation of the dicarbene li-
gand from the corresponding dicarbene silver complex, using
[IrCp*(μ-Cl)Cl]2 as an iridium precursor. The structure of com-
terized by a double ortho-metallation of the p-xylylene bridge
between the carbene units. Both complexes show good activity
in the transfer hydrogenation of ketones to alcohols in 2-prop-
anol.
Results and Discussion
Introduction
Reaction of [IrCp*(μ-Cl)Cl]2 with the silver dicarbene complexes
1a[18] or 2a[19] (Ir/diNHC 2:1 ratio) in acetonitrile or MeOH/DCM
at room temperature affords the novel dinuclear (diNHC)irid-
ium(III) complexes 1 and 2 (Scheme 1). In both complexes the
dicarbene ligand is coordinated in a bridging fashion between
the two metal fragments IrCp*Cl. In the case of the cationic
complex 1, the iridium coordination sphere is completed by the
nitrogen atom of the (imidazol-2-ylidene)-pyridine substituent,
while complex 2 is neutral on account of the double metalla-
tion of the phenylene ring, which acts as a linker between the
two NHC moieties.
N-Heterocyclic carbenes (NHCs) have emerged in the last two
decades as a new class of σ-donor ligands, alternative or com-
plementary to the classical ones based on phosphorus or nitro-
gen donor atoms. The applications of metal N-heterocyclic
carbene complexes span from catalysis[1,2] to bioinorganic
chemistry[3] and material science.[4] With regard to the first
topic, several NHC–metal complexes have been synthesized and
effectively employed as catalysts for example in olefin metathe-
sis (Ru complexes)[5] and C–C coupling (Pd complexes).[6] More-
over, ruthenium(II),[7–10] iridium(I)[11] and Cp* iridium(III)[12,13]
complexes bearing NHC ligands have also been successfully
employed in the transfer hydrogenation reaction of carbonyl
compounds.[14] Most of the examples reported in the literature
are complexes with carbene ligands bearing a second donating
group, such as pyridine,[12e,7] or phosphine.[8] The stability of
the complexes can be enhanced by using poly-NHC ligands, as
we have already demonstrated for palladium(II), copper(I) and
iridium(III) complexes in C–C coupling, nitrene transfer and wa-
ter oxidation reactions, respectively.[15–17] In this work we de-
scribe the synthesis and catalytic application in transfer hydro-
genation of two new iridium(III) dinuclear complexes bearing a
bridging dicarbene ligand.
1
Complexes 1 and 2 were characterized by H NMR and 13C
NMR spectroscopy, elemental analysis and positive mass spec-
1
trometry. The H NMR spectrum of 1 in [D3]acetonitrile shows
a singlet for the methylene protons of the dicarbene, suggest-
1
ing a bridging coordination of the diNHC ligand. The H NMR
spectroscopic data of 1 are consistent with the proposed struc-
ture, whereas the 31P NMR spectrum shows the typical heptet
–
for the PF6 counterion. Finally, the positive ESI mass spectra
present two peaks relative to the fragments [M – PF6]+ (m/z =
1173) and [M – 2PF6]2+ (m/z = 514), confirming both the dicat-
ionic and dinuclear nature of the compound. No suitable crys-
tals for an X-ray study were obtained. The 1H NMR spectrum
of 2 is consistent with the formation of a dinuclear species,
characterized by the double ortho-metallation of the phenylene
ring of the bridge connecting the two carbene units and afford-
ing an AB system for the methylene hydrogens. Conversely, in
the dinuclear dicarbene silver precursor 2a the methylene
hydrogens give rise to a singlet, on account of the less strained
and fluxional behaviour of the silver complex. The MALDI mass
spectra present a peak relative to the fragment [M – Cl]+ (m/z =
956). It is worth pointing out that the formation of 2 does not
require the addition of an external base (i.e. potassium acetate
or cesium carbonate), indicating that metallation is a straight-
[a] Dipartimento di Scienze Chimiche, Università di Padova,
via Marzolo 1, 35131 Padova, Italy
E-mail: cristina.tubaro@unipd.it
www.chimica.unipd.it
[b] Dipartimento di Chimica, Fisica e Ambiente, Università di Udine,
Via Cotonificio 108, 33100 Udine, Italy
E-mail: walter.baratta@uniud.it
[c] Dipartimento di Chimica, Università di Parma,
Viale delle Scienze 17/A, 43100 Parma, Italy
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201501027.
Eur. J. Inorg. Chem. 2016, 247–251
247
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Scheme 1. Synthesis of complexes 1 and 2.
forward process.[20] The 13C NMR spectra of both complexes 1 pentadienyl ligand completes the coordination at the metal.
and 2 show a unique signal for the carbene carbons at δ = The Ir–C1 and Ir–C8 bond lengths are 1.998(3) and 2.073(3) Å,
172.0 and 157.6, in the typical range of carbene carbons coordi- respectively, in good agreement with those found in the dinu-
nated to an iridium(III) center,[13,21] upfield shifted with respect clear IrIII complex containing a NHC donor and an ortho-metal-
to the corresponding silver complexes (δ ≈ 180 ppm).
lated phenylene ligand [1.987(4) and 1.989(4), 2.089(3) and
2.091(3) Å, respectively].[20a] The bite angle C1–Ir–C8 is
84.65(11)° and falls in the range [83.2–87.2°], which is observed
for CNHC–Ir–CPh angles in iridium complexes containing a six-
member chelate ring.[22] The six-membered ring, formed by Ir1,
C1, N2, C5, C6 and C8 atoms, is in a boat conformation. The Ir–
Cl and the Ir–CT bond lengths [2.4254(7) and 1.903(3) Å, respec-
tively] fall in the typical range for similar chloride cyclopentadi-
enyl iridium(III) compounds.
The iridium complexes 1 and 2 display catalytic activity in
the transfer hydrogenation (TH) of ketones in 2-propanol under
basic conditions (Scheme 2), using acetophenone as a model
substrate. The cationic pyridine-based complex 1 (0.5 mol-%)
catalyses the reduction of acetophenone to 1-phenylethanol
(97 % after 4 h) under reflux conditions and in the presence of
NaOiPr (3 mol-%) with a TOF of 38 h–1 (Table 1), while with the
ortho-metallated complex 2 only 57 % conversion was attained
in 14 h with a reduced rate (TOF = 6 h–1).
The molecular structure of compound 2 was established by
an X-ray diffraction study performed on a crystal obtained by
the diffusion of n-hexane into a dichloromethane solution of 2
(Figure 1). The structure is centrosymmetric, with the inversion
centre located in the middle of the phenylene ring. The coordi-
nation around the metal centre is described as a three-legged
piano stool, where the three legs are the chlorine atom and
the carbon atoms of the imidazol-2-ylidene and the metallated
phenylene units. The centroid (CT) of the pentamethylcyclo-
Scheme 2. TH of ketones catalysed by complexes 1 and 2.
Complexes 1 and 2 were tested in the TH of alkyl aryl, diaryl,
dialkyl and cyclic ketones. With catalyst 1, 3-methoxyaceto-
phenone is quantitatively transformed into the corresponding
alcohol (99 %) in 4 h with a TOF of 30 h–1, whereas complex 2
shows a higher rate at 50 % conversion (TOF = 50 h–1), but a
lower conversion (86 %) is achieved after 5 h (Table 1). A higher
rate for the methoxy derivative with respect to acetophenone
has also been found with some ruthenium complexes.[23]
Benzophenone is reduced to benzhydrol in 12 h (catalyst 1,
95 %) and in 16 h (catalyst 2, 99 %) with comparable TOF values
(16 and 13 h–1, respectively), showing a lower activity with re-
Figure 1. ORTEP diagram of complex 2. Ellipsoids are drawn at their 30 %
probability level. Hydrogen atoms and crystallization solvent (dichlorometh-
ane) have been omitted for clarity. Selected bond lengths [Å] and angles [°]:
C1–Ir 1.998(3), C8–Ir 2.073(3), Cl1–Ir 2.4254(7), CT–Ir 1.903(3); C8–Ir–Cl1
89.03(8), C1–Ir–Cl1 92.69(9), CT–Ir–Cl1 120.57(10), CT–Ir–C1 128.40(12), CT–Ir–
C8 129.53(12), C1–Ir–C8 84.65(11). Symmetry code ′: 1 – x, –y, 1 – z.
Eur. J. Inorg. Chem. 2016, 247–251
www.eurjic.org
248
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Table 1. Catalytic TH of ketones with complexes 1 and 2 (0.5 mol-%) in the
presence of NaOiPr (3 mol-%).
presence of additional alcohol, as described by Milstein.[26] The
rate of the transfer hydrogenation of 1 and of the Cp*Ir(NHC)
catalysts reported in the literature[12,13] is in the range 101–
102 h–1. However, a direct comparison is not possible since dif-
ferent catalytic conditions were employed (i.e. temperature,
base concentration, presence of silver salts).
Conclusions
In conclusion, we have reported the synthesis of two novel di-
nuclear iridium(III) complexes, both having a bridging di(N-
heterocyclic carbene) ligand. In particular complex 2 displays a
structurally interesting double ortho-metallated phenylene
bridge, as proven by an X-ray crystal structure determination.
These complexes catalyse the TH of carbonyl compounds,
where the cationic complex 1 is more active than the neutral
ortho-metallated complex 2. Further studies are underway to
extend this synthetic route to other iridium carbene complexes
and to characterize the species involved in the catalytic TH
process.
Experimental Section
[a] The conversion and TOF (mol of ketone converted into alcohol per mol
of catalyst per hour at 50 % conversion) were determined by GC analysis.
General: All manipulations were carried out using standard Schlenk
techniques under an atmosphere of argon. The reagents were pur-
chased as high-purity products and generally used as received. The
silver complexes 1a and 2a were prepared according to literature
procedures.[18,19] The NMR spectra were recorded with a Bruker Av-
ance 300 MHz instrument, chemical shifts are in ppm and are rela-
tive to the residual solvent signal or H3PO4 (85 % in D2O). The GC
analyses were performed with a Varian GP-3380 gas chromato-
graph.
Conditions: T = 82 °C, substrate 0.1
M in 2-propanol.
spect to the acetophenone substrates. Complete conversion of
cyclopentanone into cyclopentanol has been observed with 1
in 3 h (98 %, TOF = 50 h–1) and with 2 in 8 h (99 %, TOF =
9 h–1). Interestingly, complex 1 catalyses the quantitative reduc-
tion of cyclohexanone to cyclohexanol (100 %) in 40 min with
a TOF of 170 h–1, while 2 takes a longer time (4 h) to achieve
complete conversion (99 %, TOF = 61 h–1). Using complex 1,
the aliphatic ketones 2-nonanone and also 3-heptanone afford
the corresponding alcohols in 12 h (99 and 98 %, respectively),
whereas 2 leads to poor conversion. The unsaturated ketone 5-
hexen-2-one was chemoselectively transformed into 5-hexen-2-
ol in 6 h with catalyst 1 (96 %, TOF = 40 h–1) and in 10 h with
complex 2, the latter reaching a lower conversion (86 %) with
a lower rate (TOF = 17 h–1).
Synthesis of Complex 1: A solution of [IrCp*Cl2]2 (80 mg,
0.10 mmol) in acetonitrile (15 mL) was added to a solution of the
silver(I) complex 1a (73 mg, 0.05 mmol) in acetonitrile (15 mL) and
the suspension was stirred at room temperature in the dark for 4 h.
The mixture was then filtered through Celite and the filtrate was
concentrated under reduced pressure to about 2–3 mL. Addition of
diethyl ether (10 mL) afforded the product as a pale yellow solid,
which was filtered off, washed with diethyl ether (2 × 5 mL) and
dried under vacuum; yield 58 %. C37H44Cl2F12Ir2N6P2 (1318.1): calcd.
C 33.78, H 3.36, N 6.37; found C 33.57, H 3.69, N 5.96. 1H NMR
(300 MHz, CD3CN): δ = 1.85 (s, 30 H, CH3Cp*), 6.57 (s, 2 H, CH2), 7.60
(t, J = 6 Hz, 2 H, Hpyr), 7.92–8.00 (m, 6 H, Him and Hpyr), 8.21 (t, J =
6 Hz, 2 H, Hpyr), 8.69 (d, J = 6 Hz, 2 H, Hpyr) ppm. 1H NMR (300 MHz,
[D6]DMSO): δ = 1.87 (s, 30 H, CH3Cp*), 6.54 (s, 2 H, CH2), 7.67 (t, J =
6 Hz, 2 H, Hpyr), 7.87 (d, 2 H, Him), 8.30–8.35 (m, 4 H, Hpyr), 8.58 (d,
In the absence of base complexes 1 and 2 are not catalyti-
cally active, suggesting that NaOiPr is crucial for the formation
of an Ir hydride species.[24] A possible mechanism of the TH
with complexes 1 and 2 involves the formation of the Ir iso-
propoxide species, by substitution of the chloride, and succes-
sive β-hydrogen elimination, leading to the Ir hydride complex. 2 H, Him), 8.83 (d, J = 6 Hz, 2 H, Hpyr) ppm. 1H NMR (300 MHz, D2O):
δ = 1.71 (s, 30 H, CH3Cp*), 6.58 (s, 2 H, CH2), 7.49 (t, J = 6 Hz, 2 H,
Hpyr), 7.60 (d, 2 H, Him), 7.85 (m, 2 H, Hpyr), 7.93 (d, 2 H, Him), 8.08
(t, J = 6 Hz, 2 H, Hpyr), 8.63 (d, J = 6 Hz, 2 H, Hpyr) ppm. 13C NMR
(75 MHz, CD3CN): δ = 9.0 (CH3Cp*), 64.0 (CH2), 94.8 (CCp*), 112.5
(Cim), 120.0 (Cim), 123.5 (Cpyr), 125.1 (Cpyr), 142.9 (Cpyr), 152.0 (Cpyr),
152.9 (Cpyr), 172.0 (C-Ir) ppm. 13C1H NMR (75 MHz, DMSO): δ = 9.0
(CH3Cp*), 93.2 (CCp*), 112.8 (Cim), 119.9 (Cim), 122.7 (Cpyr), 125.4
(Cpyr), 142.8 (Cpyr), 151.5 (Cpyr), 152.4 (Cpyr), 168.9 (C-Ir) ppm. The
CH2 signal was not detected. 31P NMR (121 MHz, CD3CN): δ = –144.1
(heptet, PF6) ppm. ESI-MS (CH3CN): m/z (%) = 1173 [M – PF6]+, 514
Insertion of the ketone substrate into the Ir–H bond affords the
Ir alkoxide, which reacts with 2-propanol giving the alcohol and
the Ir isopropoxide that closes the catalytic cycle. The higher
activity of 1, with respect to 2, is likely due to the generation
of a cis vacant site by displacement of the pyridine ligand, al-
lowing an inner-sphere mechanism.[25] Although a direct hydro-
gen transfer[12a] and an inner-sphere mechanism through ring
slippage or ring displacement have been postulated for Cp*Ir
systems,[12c,14a] it is worth noting that 18-electron alkoxide IrIII
complexes may also undergo β-hydrogen elimination in the [M – 2PF6]2+
.
Eur. J. Inorg. Chem. 2016, 247–251
www.eurjic.org
249
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[3] a) A. Monney, M. Albrecht, Coord. Chem. Rev. 2013, 257, 2420; b) L. Oeh-
ninger, R. Rubbiani, I. Ott, Dalton Trans. 2013, 42, 3269; c) W. Liu, R. Gust,
Chem. Soc. Rev. 2013, 42, 775; d) K. M. Hindi, M. J. Panzner, C. A. Tessier,
C. L. Cannon, W. J. Youngs, Chem. Rev. 2009, 109, 3859.
Synthesis of Complex 2: The same procedure as described for
complex 1, but using a solution of [IrCp*Cl2]2 (80 mg, 0.10 mmol)
in dichloromethane (8 mL) and a solution of the silver(I) complex
2a (52 mg, 0.05 mmol) in MeOH/dichloromethane (1:1) (15 mL);
yield 64 %. C36H44Cl2Ir2N4 (988.1): calcd. C 42.71, H 4.48, N 5.67;
[4] L. Mercs, M. Albrecht, Chem. Soc. Rev. 2010, 39, 1903.
[5]
[6]
R. H. Grubbs, Angew. Chem. Int. Ed. 2006, 45, 3760; Angew. Chem. 2006,
118, 3845.
a) E. A. B. Kantchev, C. J. O'Brien, M. G. Organ, Angew. Chem. Int. Ed.
2007, 46, 2768; Angew. Chem. 2007, 119, 2824; b) G. C. Fortman, S. P.
Nolan, Chem. Soc. Rev. 2011, 40, 5151.
1
found C 42.74, H 4.50, N 5.49. H NMR (300 MHz, CDCl3): δ = 1.61
(s, 30 H, CH3Cp*), 3.97 (s, 6 H, CH3), 4.71 (AB system, 4 H, CH2), 6.92
(s, 2 H, Him), 6.95 (s, 2 H, Him), 7.10 (s, 2 H, CHxylyl) ppm. 13C NMR
(75 MHz, CDCl3): δ = 9.3 (CH3Cp*), 37.0 (NCH3), 56.7 (CH2), 89.8
(CCp*), 120.6 (CH), 121.2 (CH), 133.2 (C), 137.0 (CH), 138.0 (C), 157.6
(NCN) ppm. MALDI (CHCl3; sinapinic acid in 75 % MeCN, 0.1 % TFA):
m/z (%) = 955.9 [M – Cl]+.
[7]
Y. Cheng, J. Sun, H. Yang, H. Xu, Y. Li, X. Chen, Z. Xue, Organometallics
2009, 28, 819.
[8]
[9]
[10]
P. L. Chiu, H. M. Lee, Organometallics 2005, 24, 1692.
F. E. Fernández, M. C. Puerta, P. Valerga, Organometallics 2011, 30, 5793.
a) J. Witt, A. Pöthig, F. E. Kühn, W. Baratta, Organometallics 2013, 32,
4042; b) W. Baratta, J. Schütz, E. Herdtweck, W. A. Herrmann, P. Rigo, J.
Organomet. Chem. 2005, 690, 5570.
Typical Procedure for the Catalytic Transfer Hydrogenation of
Ketones: The iridium complex (2.5 μmol) was introduced into an
oven-dried Schlenk flask and the ketone (0.5 mmol) and 2-propanol
were added under argon (total volume 4.85 mL). The yellow mixture
was refluxed (90 °C bath temperature) under argon for 5 min and
[11]
a) M. V. Jiménez, J. Fernández-Tornos, J. J. Pérez-Torrente, F. J. Modrego,
P. Garcıá-Orduña, L. A. Oro, Organometallics 2015, 34, 926; b) K. Riener,
M. J. Bitzer, A. Pöthig, A. Raba, M. Cokoja, W. A. Herrmann, F. E. Kühn,
Inorg. Chem. 2014, 53, 12767; c) S. Gülcemal, A. Gürhan Gökçe, B. Çetink-
aya, Inorg. Chem. 2013, 52, 10601; d) M. V. Jiménez, J. Fernández-Tornos,
J. J. Pérez-Torrente, F. J. Modrego, S. Winterle, C. Cunchillos, F. J. Lahoz,
L. A. Oro, Organometallics 2011, 30, 5493; e) A. Binobaid, M. Iglesias, D.
Beetstra, A. Dervisi, I. Fallis, K. J. Cavell, Eur. J. Inorg. Chem. 2010, 5426;
f) H. Türkmen, T. Pape, F. E. Hahn, B. Çetinkaya, Eur. J. Inorg. Chem. 2008,
5418.
a solution of NaOiPr (150 μL, 0.1 M, 0.015 mmol) in 2-propanol was
added. The reaction started and the mixture changed its colour.
With complex 1 the solution gradually turned into deep red,
whereas with 2 it became deep yellow. The reaction was sampled
by removing an aliquot of the reaction mixture and diethyl ether
was added (1:1 in volume). The solution was filtered through a short
silica pad and the conversion was determined by GC analysis
(ketone 0.1 M, Ir 0.5 mol-%, NaOiPr 3 mol-%).
[12]
a) S. Sabater, M. Baya, J. A. Mata, Organometallics 2014, 33, 6830; b) U.
Hintermair, J. Campos, T. P. Brewster, L. M. Pratt, N. D. Schley, R. H. Crab-
tree, ACS Catal. 2014, 4, 99; c) J. Campos, U. Hintermair, T. P. Brewster,
M. K. Takase, R. H. Crabtree, ACS Catal. 2014, 4, 973; d) W. B. Cross, C. G.
Daly, Y. Boutadla, K. Singh, Dalton Trans. 2011, 40, 9722; e) D. Gnanam-
gari, E. L. O. Sauer, N. D. Schley, C. Butler, C. D. Incarvito, R. H. Crabtree,
Organometallics 2009, 28, 321; f) A. P. da Costa, M. Viciano, M. Sanaú, S.
Merino, J. Tejeda, E. Peris, B. Royo, Organometallics 2008, 27, 1305; g) R.
Corberán, E. Peris, Organometallics 2008, 27, 1954; h) R. Corberán, M.
Sanaú, E. Peris, Organometallics 2007, 26, 3492; i) A. Bartoszewicz, R.
Marcos, S. Sahoo, A. K. Inge, X. Zou, B. Martín-Matute, Chem. Eur. J. 2012,
18, 14510.
F. Hanasaka, K.-i. Fujita, R. Yamaguchi, Organometallics 2006, 25, 4643.
a) D. Wang, D. Astruc, Chem. Rev. 2015, 115, 6621; b) J. Ito, H. Nishiyama,
Tetrahedron Lett. 2014, 55, 3133; c) R. Malacea, R. Poli, E. Manoury, Coord.
Chem. Rev. 2010, 254, 729; d) R. H. Morris, Chem. Soc. Rev. 2009, 38,
2282; e) W. Baratta, P. Rigo, Eur. J. Inorg. Chem. 2008, 4041; f) C. Wang,
X. Wu, J. Xiao, Chem. Asian J. 2008, 3, 1750.
Solid-State Structure Determination of Compound 2: Data for
compound 2 were collected at 203 K with a Bruker APEX II single-
crystal diffractometer, using Mo-Kα graphite-monochromated radia-
tion (λ = 0.71073 Å) and equipped with an area detector.[27] Com-
pound 2 crystallizes in the monoclinic system, space group P21/c,
with a = 11.2125(6) Å, b = 14.7547(8) Å, c = 13.6141(7) Å, β =
111.2420(10)°, V = 2099.25(19) Å3, Z = 2, μ = 6.747 mm–1, ρ =
1.835 g cm–3. Unique reflections: 6173, (Rint = 0.0468), final R =
0.0245, Rw = 0.0557, GOF = 1.039. The structure was solved by
direct methods with SHELXS-97 and refined against F2 with SHELXL-
97, with anisotropic thermal parameters for all non-hydrogen at-
oms.[28] The hydrogen atoms were placed in the ideal geometrical
positions.
[13]
[14]
CCDC-999051 contains the supplementary crystallographic data for
compound 2. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[15]
[16]
[17]
A. Biffis, C. Tubaro, G. Buscemi, M. Basato, Adv. Synth. Catal. 2008, 350,
189.
C. Tubaro, A. Biffis, R. Gava, E. Scattolin, A. Volpe, M. Basato, M. M. Díaz-
Requejo, P. J. Perez, Eur. J. Org. Chem. 2012, 1367.
A. Volpe, A. Sartorel, C. Tubaro, L. Meneghini, M. Di Valentin, C. Graiff, M.
Bonchio, Eur. J. Inorg. Chem. 2014, 665.
Supporting Information (see footnote on the first page of this
article): NMR spectroscopic data for complexes 1 and 2.
[18]
[19]
Z. Xi, X. Zhang, W. Chen, S. Fu, D. Wang, Organometallics 2007, 26, 6636.
M. V. Baker, D. H. Brown, R. A. Haque, B. W. Skelton, A. H. White, J. Inclu-
sion Phenom. Macrocyclic Chem. 2009, 65, 97.
Acknowledgments
The University of Padova (HELIOS STPD08RCX) is acknowledged
for financial support.
[20]
a) R. Maity, A. Rit, C. Schulte to Brinke, C. G. Daniliuc, F. E. Hahn, Chem.
Commun. 2013, 49, 1011; b) R. Mainty, H. Koppetz, A. Hepp, F. E. Hahn,
J. Am. Chem. Soc. 2013, 135, 4966; c) R. Mainty, A. Rit, C.
Schulte to Brinke, J. Kösters, F. E. Hahn, Organometallics 2013, 32, 6174.
a) S. Sanz, A. Azua, E. Peris, Dalton Trans. 2010, 39, 6339; b) G. Su, X.-K.
Huo, G.-X. Jin, J. Organomet. Chem. 2011, 696, 533.
a) R. Zhong, Y.-N. Wang, X.-Q. Guo, Z.-X. Chen, X.-F. Hou, Chem. Eur. J.
2011, 17, 11041; b) R. Corberán, V. Lillo, J. A. Mata, E. Fernandez, E. Peris,
Organometallics 2007, 26, 4350; c) C.-F. Chang, Y.-M. Cheng, Y. Chi, Y.-C.
Chiu, C.-C. Lin, G.-H. Lee, P.-T. Chou, C.-C. Chen, C.-H. Chang, C.-C. Wu,
Angew. Chem. Int. Ed. 2008, 47, 4542; Angew. Chem. 2008, 120, 4618; d)
R. Corberán, M. Sanaú, E. Peris, J. Am. Chem. Soc. 2006, 128, 3974; e) A. P.
da Costa, M. Sanaú, E. Peris, B. Royo, Dalton Trans. 2009, 6960; f) R.
Corberán, M. Sanaú, E. Peris, Organometallics 2006, 25, 4002.
a) W. Baratta, M. Ballico, S. Baldino, G. Chelucci, E. Herdtweck, K. Siega,
S. Magnolia, P. Rigo, Chem. Eur. J. 2008, 14, 9148; b) W. Baratta, G. Chel-
Keywords: Iridium · Di(NHC) complexes · Hydrogenation ·
Metalation · Dinuclear complexes
[21]
[22]
[1] a) N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Syn-
thetic Tools (Ed.: S. Diez-Gonzalez), RSC Catalysis Series, RSC, Cambridge,
UK, 2010; b) N-Heterocyclic Carbenes in Transition Metal Catalysis and
Organocatalysis (Ed.: C. S. J. Cazin), in: Catalysis by Metal Complexes,
Springer, Heidelberg, Germany, 2010, vol. 32.
[2] See for example: a) R. Corberán, E. Mas-Marza, E. Peris, Eur. J. Inorg. Chem.
2009, 1700; b) S. P. Nolan, Acc. Chem. Res. 2011, 44, 91; c) S. Gaillard,
C. S. J. Cazin, S. P. Nolan, Acc. Chem. Res. 2012, 45, 778.
[23]
Eur. J. Inorg. Chem. 2016, 247–251
www.eurjic.org
250
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ucci, S. Gladiali, K. Siega, M. Toniutti, M. Zanette, E. Zangrando, P. Rigo,
Angew. Chem. Int. Ed. 2005, 44, 6214; Angew. Chem. 2005, 117, 6370.
[24] a) S. E. Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev. 2004, 248,
2201; b) P. Espinet, A. C. Albéniz A. C. in Fundamentals of Molecular Catal-
ysis, Current Methods in Inorganic Chemistry (Eds.: H. Kurosawa, A. Yama-
moto), Elsevier, Amsterdam, 2003, vol. 3, chap. 6, p. 328; c) Recent Advan-
ces in Hydride Chemistry (Eds.: M. Peruzzini, R. Poli), Elsevier, Amsterdam,
2001.
[27] SMART Software Users Guide, version 5.1, Bruker Analytical X-ray Sys-
tems, Madison, WI, 1999; SAINT Software UsersGuide, version 6.0, Bruker
Analytical X-ray Systems, Madison, WI, 1999; G. M. Sheldrick, SADABS,
Bruker Analytical X-ray Systems, Madison, WI, 1999. APEX II Software
User Guide, SAINT, version 7.06a, SADABS, version 2.01, Bruker AXS Inc.,
Madison, Wisconsin, USA, 2008.
[28] G. M. Sheldrick, SHELX-97, Programs for Crystal Structure Analysis, Re-
lease 97–2, Göttingen, Germany, 1997.
[25] a) J. S. M. Samec, J. E. Bäckvall, P. G. Andersson, P. Brandt, Chem. Soc. Rev.
2006, 35, 237; b) M. C. Warner, J.-E. Bäckvall, Acc. Chem. Res. 2013, 46,
2545.
Received: September 8, 2015
[26] O. Blum, D. Milstein, J. Organomet. Chem. 2000, 593, 479.
Published Online: December 2, 2015
Eur. J. Inorg. Chem. 2016, 247–251
www.eurjic.org
251
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim