Acid-base interaction between transition-metal hydrides: Dihydrogen bonding and dihydrogen evolution

Article abstract of DOI:10.1002/anie.201005274

Reaction of the acidic tungsten(II) hydride 2 with the nickel(II) pincer complex 1 in either THF or toluene after an initial dihydrogen bonding (DHB) interaction led to the formation of the Ni-W bimetallic species 3 (see picture). The first example of DHB between two metal hydrides with opposite polarity was analyzed by NMR and IR spectroscopy, X-ray crystallography, and DFT calculations.

Full text of DOI:10.1002/anie.201005274

DOI: 10.1002/anie.201005274
Transition-Metal Hydrides
Acid–Base Interaction between Transition-Metal Hydrides:
Dihydrogen Bonding and Dihydrogen Evolution**
Vladislava A. Levina, Andrea Rossin,* Natalia V. Belkova,* Michele R. Chierotti,
Lina M. Epstein, Oleg A. Filippov, Roberto Gobetto, Luca Gonsalvi, Agustꢀ Lledꢁs,
Elena S. Shubina,* Fabrizio Zanobini, and Maurizio Peruzzini*
Dedicated to Professor Ivano Bertini on the occasion of his 70th birthday
Unconventional hydrogen bonding that involves transition-
metal complexes has attracted considerable attention and
several efforts have been made to unravel and rationalize the
many conceivable interactions.[1] Core transition metals are
able to accept a hydrogen bond through their d-electron lone
pairs, thus forming M···HA hydrogen bonds,[2] whereas
hydride ligands that possess a partial negative charge are
unusual proton-accepting sites that form a dihydrogen bond
(DHB), M-Hdꢀ···d+HA.[1] The simultaneous presence of both
types of bonds was also observed.[3] On the other hand, the
reactivity of transition-metal hydrides as proton source is also
well appreciated,[4] and hydrogen bonds of the M-Hd+···B type
(B = organic base) were recently pointed out for neutral
transition-metal hydrides.[5] However, DHB interactions
where transition metal hydride complexes serve as both
proton acceptor and proton donor in a hydrogen bond have
not been described to date, although dihydrogen evolution
was observed when “hydridic” and “acidic” hydride com-
plexes such as [OsH2(PMePh2)4]/[CpM(H)(CO)3] system
(Cp = h5-C5H5ꢀ, M = Mo, W) were allowed to react.[6]
To further investigate this unconventional acid–base
interaction, we report herein on the reaction of the stable
electron-rich nickel(II) pincer hydride [(tBuPCP)Ni(H)] (1)
(
tBuPCP = 2,6-C6H3(CH2PtBu2)2)[7] with the acidic tungste-
n(II) complex [CpW(H)(CO)3] (2).[5,8] The proton-accepting
properties of 1 were first assessed by studying its interaction
with excess CF3CH2OH (TFE) in [D8]toluene by variable
temperature (VT) IR and NMR measurements. The experi-
ments clearly highlighted the dihydrogen bond formation,[9]
with a reaction enthalpy DH8 of ꢀ2.5 kcalmolꢀ1 and a basicity
factor Ej = 1.06.[2a,b] Evolution of H2 was observed with a
strong acid such as HBF4.
[*] Dr. A. Rossin, Dr. L. Gonsalvi, Dipl.-Chem. F. Zanobini,
Dr. M. Peruzzini
Consiglio Nazionale delle Ricerche (CNR)
Istituto di Chimica dei Composti Organometallici (ICCOM)
via Madonna del Piano 10, 50019 Sesto Fiorentino (Italy)
Fax: (+39)0555-225-203
E-mail: andrea.rossin@iccom.cnr.it
maurizio.peruzzini@iccom.cnr.it
Homepage: http://www.iccom.cnr.it
Mixing equimolar amounts of 1 and 2 in carefully
degassed THF at 273 K under a nitrogen atmosphere led to
a dark-orange solution, from which a reddish-orange crystal-
line material precipitated on slowly raising the temperature to
298 K, accompanied by H2 evolution. Replacing 1 with
[(tBuPCP)Ni(D)] or 2 with [CpW(D)(CO)3] led to HD
Prof. A. Lledꢀs
1
formation (dH = 4.56 ppm, t, JH-D = 42.5 Hz). Single crystal
Universitat Autꢁnoma de Barcelona (UAB)
Departament de Quꢂmica, Edifici Cn
Bellaterra (Barcelona) (Spain)
X-ray diffraction analysis revealed that the final product is the
bimetallic ion pair [CpW(CO)2(m-k,C:k,O-CO)···Ni(tBuPCP)]
(3, Figure 1 and Tables S1,S2 in the Supporting Informa-
tion).[10]
The most relevant structural feature of 3 is the presence of
one carbonyl ligand that bridges the two metal centers in a
rather unconventional “isocarbonylic” mode.[11] The slightly
V. A. Levina, Dr. N. V. Belkova, Prof. L. M. Epstein, Dr. O. A. Filippov,
Prof. E. S. Shubina
A.N. Nesmeyanov Institute of Organoelement Compounds Russian
Academy of Sciences, Vavilov str. 28
119991 Moscow (Russia)
E-mail: nataliabelk@ineos.ac.ru
shu@ineos.ac.ru
Dr. M. R. Chierotti, Prof. R. Gobetto
Dipartimento di Chimica I.F.M., Universitꢃ di Torino, Torino (Italy)
[**] The authors thank “Firenze Hydrolab”, a project sponsored by Ente
Cassa di Risparmio di Firenze (ECRF), the PIRODE project by
MATTM (Rome), the Russian Foundation for Basic Research (RFBR,
project 11-03-01210), and the CNR-RAS bilateral agreement for
supporting this research. A.R. would like to thank the Centre de
Supercomputaciꢀ de Catalunya (CESCA) for providing additional
computational resources. A.L. thanks the Spanish MICINN (proj-
ects CTQ2008-06866-C02-01 and CSD2007-00006). O.F. acknowl-
edges the Russian Federation President grant (MK-314.2010.3)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005274.
Figure 1. X-ray crystal structure of 3. Hydrogen atoms on the ligands
are omitted for clarity. Thermal ellipsoids set at 40% probability.
Angew. Chem. Int. Ed. 2011, 50, 1367 –1370
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1367

Communications
order rate constant kobs = (6.5 ꢁ 0.1)10ꢀ3 m sꢀ1 which leads to
DG° = 20.4 kcalmolꢀ1 in THF. The reaction mechanism
resembles that found for the protonation of basic tungsten
hydrides,[14] with the reaction rate expressed by Equa-
tion (1)[13] and the observed rate constant kobs = K1k2, where
ꢀ
longer C O bond with respect to the terminal bonds (1.198(2)
versus 1.153(2) ꢀ, respectively), evidences simultaneous
electron back-donation from both metal ions. The terminal
ꢀ
W CO bonds (1.955(1) and 1.918(2) ꢀ, respectively) are
longer than the bridging bond (1.905(1) ꢀ), thus suggesting a
significant dipole from W to Ni. The Ni···O distance
(1.972(9) ꢀ) is much longer than that reported for other
[(iPrPCP)Ni-OR] (R = H, Me) complexes.[12] The almost linear
arrangement of the W-C-O-Ni atoms is evidenced by the bond
angle of 1608 at the oxygen atom. In keeping with the solid-
state structure, three distinct resonances for the CO group at
d = 224.0, 229.0, and 239.4 ppm appear in the 13C CPMAS
NMR spectrum (see Figure S1 in the Supporting Informa-
tion), whereas the 13C NMR spectrum of 3 in [D3]CH3NO2
shows only one carbonyl signal at d = 224.10 ppm.
ꢀd½1ꢂ
dt
k2k3
kꢀ2 þ k
K1½2ꢂ
¼ k0obs½1ꢂ ¼
½1ꢂ ꢃ k2K1½2ꢂ½1ꢂ
ð1Þ
3 1 þ K1½2ꢂ
k2 is the rate constant for the proton-transfer step (trans-
formation 1···2 to 5).
Complex 3 dissociates in more polar acetonitrile to give a
light yellow solution of the saltlike adduct [(tBuPCP)Ni-
(MeCN)]+[CpW(CO)3]ꢀ (4) that comprises a square-planar
nickel(II) cation binding one MeCN molecule and the piano
stool tungsten(II) anion (Figure 2 and Tables S4,S5).
By monitoring the process by multinuclear VT NMR and
VT IR spectroscopy in the 190–298 K temperature range,
strong experimental evidence for the formation of a 1···2
adduct preceding the H2 elimination that yields 3 is provided.
Thus, in the presence of a nearly equimolar amount of 2, IR
spectra in THF at 190 K showed a slight increase of the nNiH
band intensity (see Figure S2) as expected for dihydrogen
bond formation. The appearance of a new high-frequency
band at 1750 cmꢀ1 was observed in the presence of a 4–10-fold
excess of 2 in toluene (see Figure S3). The dNiH signal shifts
toward lower frequencies by only 0.04 ppm in the presence of
2 (4 equivalents, [D8]toluene). VT proton spin–lattice relax-
ation times T1 of 1 and 1···2 hydride resonances were recorded
in [D8]THF (Figure S4). In the measured temperature range,
T1 values for the 1···2 adduct are shorter than those observed
for 1 (see Table S3), which is due to the presence of the
additional dipolar contribution consistent with the formation
of a NiH···HW unconventional hydrogen bond. At higher
temperature, the resonances caused by 1 and 2 started
disappearing while 3 formed together with molecular hydro-
gen (Scheme 1).
The reaction profile of the H2 evolution has been
examined by DFT calculations at the M06 level carried out
on the real system and by evaluating the total energies in THF
(polarizable continuum model (PCM), Figure 3). An initial
intermediate (1···2) was found, in which the two complexes
interact through their hydrides and one cyclopentadienyl
Figure 2. X-ray crystal structure of 4. Hydrogen atoms on the ligands
are omitted for clarity. Thermal ellipsoids set at 40% probability.
Scheme 1. Reaction between metal hydrides 1 and 2.
IR spectra recorded for a mixture of 1 and a 1.3-fold
excess of 2 in both THF and toluene at 190 K (Figure S5–S6)
clearly indicated the presence of yet another intermediate
with CO absorption signals at 1889 and 1770 cmꢀ1, similar to
the nCO shown by the [CpW(CO)3]ꢀ anion.[13] The same bands
were observed in the experiment with [D1]-1. These bands are
tentatively ascribed to the proton-transfer product 5 (see
below) and disappear gradually upon warming. New nCO
bands appear already at 230 K (at 1666, 1824, and a shoulder
at 1906 cmꢀ1; Figure S5–S7), thus matching those found in the
solid state (KBr) spectrum of 3. Monitoring the reaction
kinetics at room temperature (Figure S7) gave the second
Figure 3. Energy versus reaction coordinate profile for the transforma-
tion 1+2!3+H2 (THF, PCM). (For the optimized geometries of the
various complexes, see Figure S8–S12 and Table S7 in the Supporting
Information.)
1368
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1367 –1370

ꢀ
approximately 10 mL. At this point, a crystalline orange solid started
to precipitate from the solution. The Schlenk tube was then closed
and left to stand under nitrogen over 3 days. Subsequently, the
supernatant was filtered off and the needlelike orange crystals were
washed with small portions of THF (2 ꢂ 5 mL) and dried under
C H bond (Figure S8). An atoms-in-molecules (AIM) anal-
ꢀ
ysis revealed two (3, ꢀ1) critical points between the Ni H and
ꢀ
ꢀ
both C H and W H bonds of 2 (Figure S9, Table S6). The
interaction energies calculated for each H···H contact are
ꢀ3.5 and ꢀ1.7 kcalmolꢀ1, respectively, which is consistent
~
nitrogen. Yield: 90%. IR (KBr): n(CO) = 1906 (s), 1824 (s),
1666 cmꢀ1 (s); 1H NMR (300 MHz, CD3NO2, 298 K): d = 7.26 (m,
3H; ArH), 5.16 (s, 5H; Cp), 3.87 (s (br), 4H; ArCH2), 1.52 ppm
(virtual triplet (vt), *JH-P = 6.6 Hz, 36H; CH3); 13C NMR (100 MHz,
CD3NO2, 298 K): d = 224.1 (CO), 163.7 (t, 2JC-P = 14.9 Hz; Car-ipso),
ꢀ
with the observed shorter contact to the C H compared to
ꢀ
ꢀ
the W H proton. Despite the stronger Ni H interaction with
the C H proton, which is most likely caused by the steric
hindrance of the bulky tert-butyl groups, the most productive
interaction in terms of subsequent proton transfer is that with
ꢀ
2
3
151.4 (t, JC-P = 9.7 Hz; Car-o), 127.2 (s; Car-p), 120.5 (t, JC-P = 9.7 Hz;
C
ar-m), 82.3 (s; Cp), 34.1 (vt, *JC-P = 9.0 Hz; PCH2), 32.1 (vt, *JC-P =
ꢀ
W H. This result is due to the easier polarization and
14.2 Hz; PC(CH3)3), 26.0 ppm (s; CH3); 31P NMR (121.49 MHz,
CD3NO2, 298 K): d = 105.7 ppm; elemental analysis calcd for
C32H48NiO3P2W (785.20): C 48.95, H 6.16; found: C 48.86, H 6.20.
[(tBuPCP)Ni(MeCN)]+[CpW(CO)3]ꢀ 4: 3 (0.1 g) was dissolved in
ꢀ
ꢀ
heterolytic splitting of M H bonds in comparison to C H
bonds.[15] A transition state (TS) was found, at 12.0 kcalmolꢀ1
from the starting geometry (Figure S10). This structure can
also be viewed as containing a “bridging nonclassical dihy-
drogen ligand”, judging from the calculated Ni–H, H–H, and
H–W distances (1.71, 1.03, and 2.14 ꢀ, respectively). The
(elongated) H2 molecule connects the two metal centers in a
quite unusual m,h1:1 end-on mode.[16] Interestingly, a similar
structure has been recently calculated in the transition state
for the heterolytic H2 splitting by frustrated Lewis pairs.[17]
The late transition-state structure evolves along the intrinsic
reaction coordinate (IRC) path towards a “true” proton-
transfer product 5, which is very close in energy (DE(TS-5) =
3.1 kcalmolꢀ1) and has a very similar geometry to the TS
(Figure S11). The experimental difficulty in the isolation of
such an elusive dihydrogen intermediate is probably due to
the rather flat energy surface around the TS and 5. From 5,
facile H2 loss produces 3 as the final thermodynamically stable
species (Figure S12, DE(1···2jj3+H2) = ꢀ5.1 kcalmolꢀ1). Cal-
culations with Grimmeꢁs B97D functional gave the same
pathway for the 1 + 2!3 + H2 transformation, with very
similar structures along the reaction coordinate (see Fig-
ure S13–S17).
acetonitrile (5 mL), and
a pale yellow solution was obtained.
Addition of acetone (ca. 15 mL) followed by concentration of the
resulting mixture by evaporation of the solvent with a nitrogen stream
led to precipitation of 4 as a pale yellow solid. The supernatant was
filtered off, the residue was washed with fresh n-pentane (2 ꢂ 5 mL)
and finally dried in vacuo for 30 min. Crystals suitable for X-ray
diffraction analysis were obtained from crystallization of the crude
solid from a diluted acetone/n-hexane solution. Yield: 92%. IR
(KBr): n(CO) = 1920 (m), 1887 (s), 1780 cmꢀ1 (s); 1H NMR
~
(300 MHz, [D6]acetone, 298 K): d = 6.92 (m, 3H; ArH), 5.09 (s, 5H;
Cp), 3.35 (vt, *JH-P = 4.2 Hz, 4H; ArCH2), 1.75 (s, 3H; CH3CN),
1.39 ppm (vt, *JH-P = 6.9 Hz, 36H; CH3); 13C NMR (75 MHz,
2
[D6]acetone, 298 K): d = 225.7 (s, CO), 153.4 (vt, JC-P = 11.4 Hz; Car-
ipso), 135.2 (s, Car-m), 126.9 (s; Car-p), 122.4 (vt, 2JC-P = 9.0 Hz; Car-o), 83.8
(s; Cp), 68.3 (CH3CN, s), 53.8 (CH3CN, t, 3JC-P = 19.4 Hz), 34.9 (PCH2,
vt, *JC-P = 7.6 Hz), 32.2 ppm (PC(CH3)3, vt, *JC-P = 13.5 Hz);
31P{1H} NMR (121.49 MHz, [D6]acetone, 298 K): d = 80.0 ppm; ele-
mental analysis calcd for C34H51NNiO3P2W (826.26): C 49.42, H 6.22,
N 1.70; found: C 49.30, H 6.36, N 1.61.
General experimental information, details of the synthesis of
[D1]-1, and of the crystal-structure determinations of 3 and 4 are
provided in the Supporting Information, which also includes
13C CPMAS NMR and VT IR spectra, as well as the computational
methodology that describes the DFT functional/basis set used and the
optimized Cartesian coordinates of the M06-calculated molecules.
In conclusion, on the basis of combined crystallographic,
spectroscopic (solid state and solution VT NMR and VT IR),
and DFT analysis we have collected a large body of evidence
for the first example of a DHB adduct between two
transition-metal hydrides with opposite polarities; the
adduct precedes proton transfer and H2 evolution. The
proton-transfer process proceeds via several intermediates
that have been detected and/or computed. Among them, the
most remarkable structure is the m,h1:1-H2 species 5, which
features an end-on coordination mode between the two
transition metals. Further studies on the mutual reactivity of
acidic and basic transition-metal hydrides are currently
underway to highlight the possible role of these unusual
interactions in either molecular recognition or bimetallic
catalysis involving transition-metal hydrides.
Received: August 24, 2010
Revised: November 18, 2010
Published online: December 29, 2010
Keywords: dihydrogen bonding · hydrogen · nickel ·
.
transition-metal hydrides · tungsten
[1] a) M. Besora, A. Lledꢃs, F. Maseras, Chem. Soc. Rev. 2009, 38,
957 – 966; b) N. V. Belkova, E. S. Shubina, L. M. Epstein, Acc.
Chem. Res. 2005, 38, 624 – 631; c) M. Peruzzini, R. Poli, Recent
Advances in Hydride Chemistry, Elsevier SA, Amsterdam, NL,
2001.
[2] a) L. M. Epstein, E. S. Shubina, Coord. Chem. Rev. 2002, 231,
165 – 181; b) E. S. Shubina, N. V. Belkova, L. M. Epstein,
J. Organomet. Chem. 1997, 536–537, 17 – 29; c) L. Brammer,
Dalton Trans. 2003, 3145 – 3157.
[3] a) P. A. Dub, O. A. Filippov, G. A. Silantyev, N. V. Belkova, J.-C.
Daran, L. M. Epstein, R. Poli, E. S. Shubina, Eur. J. Inorg. Chem.
2010, 1489 – 1500; b) N. V. Belkova, M. Besora, M. Baya, P. A.
Dub, L. M. Epstein, A. Lledꢃs, R. Poli, P. O. Revin, E. S.
Shubina, Chem. Eur. J. 2008, 14, 9921 – 9934; c) N. V. Belkova,
P. O. Revin, M. Besora, M. Baya, L. M. Epstein, A. Lledꢃs, R.
Poli, E. S. Shubina, E. V. Vorontsov, Eur. J. Inorg. Chem. 2006,
2192 – 2209; d) N. V. Belkova, E. Collange, P. Dub, L. M.
Experimental Section
[CpW(CO)2(m-k,C:k,O-CO)···Ni(tBuPCP)] 3: In a typical experiment,
1 (0.15 g, 0.33 mmol) was dissolved in THF (20 mL). The resulting
light-yellow solution was cooled to 273 K in an ice bath. 2 (0.16 g,
0.5 mmol, 1.5 equiv) was then added; no color change was observed
immediately after the addition. The mixture was stirred for 20 min,
and the color slowly turned orange. Subsequently, the temperature
was raised to 298 K, and the resulting dark-orange solution was
concentrated by evaporation of the solvent under a nitrogen stream to
Angew. Chem. Int. Ed. 2011, 50, 1367 –1370
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1369

Communications
Epstein, D. A. Lemenovskii, A. Lledꢃs, O. Maresca, F. Maseras,
R. Poli, P. O. Revin, E. S. Shubina, E. V. Vorontsov, Chem. Eur. J.
2005, 11, 873 – 888.
J. Organomet. Chem. 2009, 694, 1581 – 1585 (Y-Cr); b) P. V.
Poplaukhin, X. Chen, E. A. Meyers, S. G. Shore, Inorg. Chem.
2006, 45, 10115 – 10125 (Yb–Mn); c) J. A. Kovacs, R. G. Berg-
man, J. Am. Chem. Soc. 1989, 111, 1131 – 1133 (W-Zr). No
reference to Ni–W systems was found.
[4] S. S. Kristjansdottir, J. R. Norton in Transition Metal Hydrides
(Ed.: A. Dedieu), VCH, New York, 1991, pp. 309 – 357.
[5] N. V. Belkova, E. I. Gutsul, O. A. Filippov, V. Levina, D. A.
Valyaev, L. M. Epstein, A. Lledꢃs, E. S. Shubina, J. Am. Chem.
Soc. 2006, 128, 3486 – 3487.
[6] K. Abdur-Rashid, T. P. Fong, B. Greaves, D. G. Gusev, J. G.
Hinman, S. E. Landau, A. J. Lough, R. H. Morris, J. Am. Chem.
Soc. 2000, 122, 9155 – 9171.
[7] a) B. J. Boro, E. N. Duesler, K. I. Goldberg, R. A. Kemp, Inorg.
Chem. 2009, 48, 5081 – 5087; b) C. J. Moulton, B. L. Shaw,
J. Chem. Soc. Dalton Trans. 1976, 1020 – 1024.
[8] R. P. L. Burchell, P. Sirsch, A. Decken, G. S. McGrady, Dalton
Trans. 2009, 5851 – 5857.
[9] In the presence of a fivefold TFE excess the NiH resonance of 1
(d = ꢀ9.77 ppm at 220 K) shifts to higher field (Dd = ꢀ0.25) and
its relaxation time T1min shortens from 583 ms (210 K) to 244 ms
(220 K). IR spectra exhibit a lower frequency shift of the nOH
band of the alcohol, thus evidencing the hydrogen bond
formation. The values of (ꢀ2.5 ꢁ 0.2) kcalmol-1 for DH8 and
(ꢀ4.3 ꢁ 0.8) e.u. for DS8 were calculated from the temperature
dependence of the DHB formation constant.[1b,2a] The intensity
of the nNiH band at 1739 cmꢀ1 decreases in the presence of TFE
while a high-frequency shoulder appears at 1770 cmꢀ1 at 190 K.
[10] The reaction is not reversible: when a CD3NO2 solution of 3 was
pressurized with H2 at 30 bar for 18 hours at room temperature,
no reaction was observed; subsequent heating at 608C for
2 hours generated unknown decomposition products. No trace of
either 1 or 2 was detected in the sample.
[12] a) [(iPrPCP)Ni(OH)]: J. Cꢄmpora, P. Palma, D. del Rio, E.
ꢅlvarez, Organometallics 2004, 23, 1652 – 1655 (d(Ni-O) =
1.865(2)); b) [iPrPCP)Ni(OMe)]: J. Cꢄmpora, P. Palma, D.
del Rio, M. M. Conejo, E. ꢅlvarez, Organometallics 2004, 23,
5653 – 5655 (d(Ni-O) = 1.855(2)).
[13] V. A. Levina, O. A. Filippov, E. I. Gutsul, N. V. Belkova, L. M.
Epstein, A. Lledꢃs, E. S. Shubina, J. Am. Chem. Soc. 2010, 132,
11234 – 11246.
[14] a) N. V. Belkova, L. M. Epstein, A. I. Krylova, E. G. Faerstein,
E. S. Shubina, Russ. Chem. Bull. 2007, 56, 870 – 874; b) N.
Avramovic, J. Hock, O. Blacque, T. Fox, H. W. Schmalle, H.
Berke, J. Organomet. Chem. 2010, 695, 382 – 391.
[15] N. V. Belkova, L. M. Epstein, E. S. Shubina, Eur. J. Inorg. Chem.
2010, 3555 – 3565.
[16] For an example of a side-on bridging H2 ligand, see: J. P.
Collman, J. E. Hutchinson, P. S. Wagenknecht, N. S. Lewis, M. A.
Lopez, R. Guilard, J. Am. Chem. Soc. 1990, 112, 8206 – 8208. For
another curious example of a side-on m3-H2 coordination to a Ru3
triangle, see: G. Sꢆss-Fink, L. Plasseraud, A. Maisse-Franꢇois, H.
Stoeckli-Evans, H. Berke, T. Fox, R. Gautier, J.-Y. Saillard,
J. Organomet. Chem. 2000, 609, 196 – 203. No examples of
structures that contain an end-on bridging coordination mode
were found.
[17] a) D. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50 – 81;
Angew. Chem. Int. Ed. 2010, 49, 46 – 76; b) S. Grimme, H. Kruse,
L. Goerigk, G. Erker, Angew. Chem. 2010, 122, 1444 – 1447;
Angew. Chem. Int. Ed. 2010, 49, 1402 – 1405; c) T. A. Rokob, A.
Hamza, A. Stirling, T. I. Soꢃs, I. Pꢄpai, Angew. Chem. 2008, 120,
2469 – 2472; Angew. Chem. Int. Ed. 2008, 47, 2435 – 2438.
[11] Few other crystallographically characterized examples of “iso-
carbonylic” (m-k,C:k,O-CO) bridges between two metal centres
are available; for examples, see: a) S. T. Liddle, B. M. Gardner,
1370
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1367 –1370