Hydrogen-bridged bis(silylene) complexes of ruthenium and iron: Synthesis, structures and multi-centre bonding interactions at the M-Si-H-Si four-membered ring

Article abstract of DOI:10.1039/c7dt00505a

Hydrogen-bridged bis(silylene) complexes of ruthenium and iron were synthesized as the first examples of this type containing Group 8 metals and their rare four-membered ring structures and multi-center bonding interactions at the M-Si-H-Si moiety were clarified by NMR spectroscopy, X-ray crystallography and theoretical calculations.

Full text of DOI:10.1039/c7dt00505a

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Hydrogen-bridged bis(silylene) complexes of
ruthenium and iron: synthesis, structures and
multi-centre bonding interactions at the
M–Si–H–Si four-membered ring†
Cite this: DOI: 10.1039/c7dt00505a
Received 10th February 2017,
Accepted 6th March 2017
DOI: 10.1039/c7dt00505a
Hisako Hashimoto,* Katsuto Komura, Takatoshi Ishizaki, Yuto Odagiri and
Hiromi Tobita*
rsc.li/dalton
Hydrogen-bridged bis(silylene) complexes of ruthenium and iron (CO)2{Me2Si⋯H⋯SiMe(tBu)} (1),4 which is viewed as a snap-
were synthesized as the first examples of this type containing shot of a transition state for 1,3-hydrogen migration that
Group 8 metals and their rare four-membered ring structures and would be involved in redistribution reactions of hydrosilanes.5
multi-center bonding interactions at the M–Si–H–Si moiety were Detailed analysis including multiple NMR measurements and
clarified by NMR spectroscopy, X-ray crystallography and theore- DFT calculations revealed that 1 has an unprecedented four-
tical calculations.
centre bonding interaction at the WSi2H moiety: the WSi2H
moiety is regarded as a 3-electron ligand described by two
canonical forms shown in Chart 1. In our continued research
into this new type of multi-centre bonding interaction, we also
succeeded in synthesising ruthenium and iron complexes ana-
logous to 1. Here we report the results of our experimental and
theoretical studies on them as well as the comparison of their
bonding aspects with those of 1.
A multi-centre bonding interaction, exemplified by the three-
centre two-electron bond, is often observed in electron-
deficient systems i.e. diboranes, some borane compounds,1
some silylium cations2 and η2-E–H transition metal complexes
(E = H, C, Si, etc.)3 (Fig. 1). Multi-centre bonding interactions
are also supposed to be involved in intermediates and
transition states in various reactions and play key roles in
these processes. Recently, our group reported the synthesis
of a hydrogen-bridged bis(silylene)tungsten complex Cp*W
Initially we attempted to apply a photochemical method
that was successfully employed for the synthesis of tungsten
complex 1,4 but this method only gave a complex mixture
in the cases of Group 8 metal complexes. Eventually, a thermal
method developed for the synthesis of a ruthenium silylene
complex6 was successfully applied to obtain the expected com-
plexes. Namely, Cp*Ru(CO)(py)Me was treated with BPh3 in
t
t
the presence of H2 BuSiSiMe2 Bu to give Cp*Ru(CO){(tBu)
MeSi⋯H⋯SiMe(tBu)} (2) in 67% yield as a mixture of cis and
trans isomers with respect to the substituents of the silicon
atoms (eqn (1)). The cis : trans ratio of 2 was determined to be
3 : 1 at room temperature by 1H NMR spectroscopy.7
Fig. 1 Some examples of silicon compounds containing multi-centre-
bonding interactions.
ð1Þ
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku,
Sendai, 980-8578, Japan. E-mail: hhashimoto@m.tohoku.ac.jp,
tobita@m.tohoku.ac.jp; Fax: +81 22 795 6543; Tel: +81 22 795 6540
†Electronic supplementary information (ESI) available: Experimental details and
DFT calculations. CCDC 1530379 and 1530380. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c7dt00505a
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bonding interaction in 2. The existence of a CO ligand is con-
firmed by 13C{1H} NMR (cis: 210.7 ppm, trans: 210.4 ppm) and
IR (1911 cm−1 for both isomers).
For iron complex 3, all NMR spectra suggest the exclusive
formation of a cis isomer. The spectral patterns of 3 well
resemble those of cis-2. For example, the 1H NMR spectrum
Chart 1 Description of complex 1.
3
exhibits a doublet for two SiMe groups (0.47 ppm, J = 2.4 Hz),
a singlet for two SitBu groups (1.20 ppm), a septet for the brid-
ging hydrogen (3.65 ppm) and a singlet for the Cp* ligand
(1.75 ppm). The Si–H coupling constant (19.2 Hz) is close to
those of 1 (23 Hz) and 2 (21.6 Hz). The 29Si{1H} NMR spectrum
shows a signal at 102.7 ppm. The 13C{1H} NMR signal of the
CO ligand is observed at 223.0 ppm and the CO stretching
band is observed at 1895 cm−1 in the IR spectrum.
ð2Þ
The crystal structures of both cis-2 and 3 were determined
Similarly, treatment of Cp*Fe(CO)(py)Me with BPh3 in the unambiguously as shown in Fig. 2(a) and (b), respectively.
t
t
presence of H2 BuSiSiMe2 Bu gave an analogous iron complex Selected bond lengths and angles are listed in Table 1. The
Cp*Fe(CO){(tBu)MeSi⋯H⋯SiMe(tBu)} (3) only as a cis form in overall structures of cis-2 and 3 are almost identical to each
87% yield (eqn (2)). The same reaction of ruthenium and iron other and their characteristic features resemble those of 1
t
complexes using less sterically-hindered H2MeSiSiMe2 Bu except for the geometry around the metal centre (W: four-
t
t
(a precursor for the synthesis of 1) instead of H2 BuSiSiMe2 Bu legged piano-stool geometry). cis-2 and 3 take a three-legged
gave no isolable products in both cases. Complexes 2 and 3 piano-stool geometry composed of a Cp*, a CO and two silylene
were fully characterised based on elemental analysis, NMR ligands. These two silylene ligands are bridged by a hydrogen
and IR spectroscopy and X-ray crystallography.
In the H NMR spectrum (C6D6, 300 K) of 2, signals of two Ru–Si bond lengths (2.3219(9) and 2.3284(9) Å) are nearly the
SiMe and two SitBu groups for cis-2 appear equivalently at same and are slightly longer than the typical RuvSi double
atom, forming a M–Si–H–Si four-membered-ring. For cis-2, two
1
0.46 ppm and 1.20 ppm, respectively. The corresponding bond lengths, e.g. 2.220(2)
Å
of Cp*(OC)(H)RuvSi(H)
signals for trans-2 in contrast are observed inequivalently {C(SiMe3)3},6 but are close to those of base-stabilized bis(sily-
(SiMe: 0.56, 0.88 ppm; SitBu: 1.08, 1.13 ppm), reflecting its lene)ruthenium complexes (2.31–2.42 Å).11 The Si–H bond
asymmetric environment with respect to the upper and lower lengths (1.74(3) and 1.71(3) Å) are longer than the typical Si–H
spaces of the Ru–Si–H–Si four-membered ring. The signal of bond length (ca. 1.48 Å) and are close to those of 1 (1.71(8), 1.75
the bridging hydrogen atom is observed as a septet (3J = (9) Å). The Si–Si distance (2.4946(12) Å) is within the known Si–
2.8 Hz) at 3.34 ppm for cis-2 and as a pseudo-septet (3J = Si single bond lengths (2.34–2.70 Å)12 and is comparable to that
2.4 Hz) at 3.42 ppm for trans-2. These signals are shifted (2.455(3) Å) of 1. The sum of three bond angles at each Si atom
upfield compared with the SiH signal (3.70 ppm) of the precur- (two Ru–Si–C and one C–Si–C angles) is calculated to be
t
t
sor hydrosilane H2 BuSiSiMe2 Bu. A similar upfield shift was 354.09(16)° for Si1 and 353.90(16)° for Si2, which is close to
also observed for 1. The septet pattern of the SiHSi signals is 360°, suggesting the sp2-hybridisation of the two Si atoms. The
due to the coupling with two SiMe groups and is consistent dihedral angle between the Ru1–Si2–Si1 and Si2–Si1–H1 planes
with
the
symmetrically
hydrogen-bridged
structure. is 168(3)°. This value is slightly smaller than that (179(4)°) of 1
Importantly, by 1H-homodecoupling of the SiMe signal, the Si– but still indicates a fair planarity of the RuSi2H moiety.
H coupling constant J(Si–H) was determined to be 21.6 Hz
from the satellite signals that is close to the Schubert’s cri-
terion (20 Hz),3 which indicates the existence of weak Si–H
bonding interaction. The 29Si{1H} NMR spectrum (C6D6,
300 K) shows a signal at 91.4 ppm for cis-2 and two signals at
55.4 and 113.8 ppm for trans-2. These values are comparable
to those (43.6, 70.2 ppm) of 1 and those (59.8–108.7 ppm) of
base-stabilised
bis(silylene)ruthenium
complexes.8
Furthermore, the Si–Si coupling constant J(Si–Si) was available
for the trans-isomer in which two Si atoms are inequivalent
and was determined to be 33.7 Hz by an INEPT–INADEQUATE
29Si NMR measurement.9 This value is comparable to that
(28.2 Hz) of 1 4 and is far larger than those of 1,3-cyclodisilox-
anes (3.8–4.8 Hz) that have no direct Si–Si bond,9b though it is
much smaller than those of typical disilanes (80–90 Hz).10
Fig. 2 The ORTEP drawings of cis-2 (a) and 3 (b). The thermal ellipsoids
represent 50% probability. Hydrogen atoms are omitted for clarity
Therefore, this value suggests the existence of weak Si⋯Si except H1 and H34.
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Table 1 Selected bond lengths [Å] and angles (°) for 1,4 cis-2 and 3 and
their model complexes 1’, cis-2’ and 3’, and WBI and NPA charges for 1’,
cis-2’ and 3’ obtained by DFT calculations
(0.42) are comparable to that of 1 (0.41), which are roughly
half of the WBI for the Si–Si single bond of Me3Si–SiMe3
(0.92). The WBIs for Si–H (av. 0.42) of cis-2′ and 3′ are also
about half of that (0.96) for the Si–H bond in H3Si–SiH3. These
data support the existence of weak but significant Si⋯Si and
Si⋯H bonding interactions. The NPA charges of the bridging
H atoms for both cis-2′ and 3′ are slightly negative, while those
Bond lengths
[Å]/angles [°]
1
1′
cis-2
cis-2′
3
3′
M–Si1
M–Si2
2.489(2) 2.512 2.3219(9) 2.346 2.2352(5) 2.248
2.487(2) 2.528 2.3284(9) 2.350 2.2392(5) 2.250
Si1⋯Si2
2.455(3) 2.4845 2.4946(12) 2.499 2.4597(6) 2.514 of their two Si atoms are greatly positive. These data suggest
Si1–H
1.75(9) 1.70
1.71(8) 1.68
59.11(8) 59.4
1.74(3)
1.71(3)
64.88(3)
1.73 1.72(2)
1.71 1.72(2)
1.72
1.73
that the Si centres are highly electrophilic and the bridging
hydrogen is hydridic, as seen in 1′. The charge difference
between δ(Si) and δ(μ-H) is the largest (1.53) in cis-2′, implying
that the ruthenium complex should show the highest reactivity
toward polar substrates among these three types of complexes.15
In summary, we have succeeded in isolating hydrogen-
bridged bis(silylene) complexes of Group 8 metals, Ru (2) and
Fe (3). It was revealed that these complexes also have rare four-
membered-ring structures having a short diagonal S⋯Si bond
with multi-centre bonding interactions at the MSi2H moiety,
as has been reported for the tungsten analogue 1. Comparison
of three complexes 1–3 by means of DFT calculation predicted
the highest reactivity of the ruthenium complex.
Si2–H
Si1–M–Si2
M–Si1–H
M–Si2–H
65.9 66.87(2) 68.0
102.4 100.3(11) 98.8 100.0(6) 98.5
95.2 100.9(11) 95.7 100.1(6) 94.0
105(3)
106(3)
Wiberg bond indices
1′
cis-2′
3′
M–Si1
M–Si2
Si1⋯Si2
Si1–H
Si2–H
0.63
0.64
0.41
0.42
0.45
0.62
0.61
0.38
0.41
0.43
0.57
0.56
0.42
0.43
0.41
NPA charges
1′
cis-2′
3′
δ (M)
δ (Si1), (Si2)
δ (μ-H)
−0.48
+1.21, +1.26
−0.49
−0.44
+1.26, +1.25
+1.30, +1.30
This work was supported by the Ministry of Education,
Culture, Sports, Science and Technology, Japan (Grants-in-aid
for Scientific Research No. JP15H03782, JPK05444, JP24109011).
−0.19
−0.23
−0.23
Notably, there are only small differences (ca. 0.04 Å for
both) in the Si–Si and two Si–H bond lengths among the three
complexes cis-2, 3 and 1 (Table 1). The M–Si bond lengths of
ruthenium and iron complexes [cis-2: av. 2.3252(9), 3: av.
2.2372(5) Å] are fairly shorter than that of tungsten complex 1
[av. 2.488(2) Å] in tandem with the Si–M–Si bond angles of
cis-2 (64.88(3)°) and 3 (66.87(2)°) that are larger than that
(59.11(8)°) of tungsten complex 1 by about 6°.
For further understanding of the bonding situation, we per-
formed DFT calculations using model complexes of 1′,13 cis-2′
and 3′ where Cp* ligands are replaced by Cp ligands (Table 1).
Important bonding orbitals of the optimized structure of cis-2′
are illustrated in Fig. 3.14 The HOMO−5 represents the Ru–Si
σ-bonding interaction between the Ru dxz orbital and two sets
of Si sp2-hybridised orbitals. The HOMO−6 consists of the Ru
dx2−y2 orbital, two sets of Si sp2-hybridised orbitals and the H
1s orbital. The HOMO−9 is mainly composed of two Si p-orbi-
tals and the H 1s orbital, which corresponds to a Si2H 3c–2e
interaction. These three orbitals resemble those reported pre-
viously for the tungsten system,4 and constitute analogous
four-centre bonding interactions at the RuSi2H moiety. The
Wiberg bond indices (WBIs) for Si⋯Si in cis-2′ (0.38) and 3
Notes and references
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(b) R. H. Crabtree, Angew. Chem., Int. Ed. Engl., 1993, 32,
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4 H. Hashimoto, Y. Odagiri, Y. Yamada, N. Takagi, S. Sakaki
and H. Tobita, J. Am. Chem. Soc., 2015, 137, 158.
5 I. Ojima, S. Inaba, T. Kogure and Y. Nagai, J. Organomet.
Chem., 1973, 55, C7–C8.
6 M. Ochiai, H. Hashimoto and H. Tobita, Angew. Chem., Int.
Ed., 2007, 46, 8192.
7 Variable-temperature NMR studies suggest that 2 shows
fluxional behaviours accompanied by isomerisation
between the isomers. The details will be reported elsewhere.
8 (a) H. Tobita, H. Wada, K. Ueno and H. Ogino,
Organometallics, 1994, 13, 2545; (b) H. Tobita, H. Kurita
and H. Ogino, Organometallics, 1998, 17, 2844;
(c) T. N. Choo, W-H. Kwok, C. E. F. Rickard, W. R. Roper
and L. J. Wright, J. Organomet. Chem., 2002, 645, 235;
(d) J. J. G. Minglana, M. Okazaki, H. Tobita and H. Ogino,
Chem. Lett., 2002, 406; (e) M. Okazaki, J. J. G. Minglana,
N. Yamahira, H. Tobita and H. Ogino, Can. J. Chem., 2003,
81, 1350.
Fig. 3 Schematic drawings of important bonding orbitals of cis-2’.
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9 (a) O. W. Sørensen, R. Freeman, T. Frenkiel, 12 M. Kaftory, M. Kapon and M. Botashansky, in The
T. H. Mareci and R. Schuck, J. Magn. Reson., 1982, 46, 180;
(b) H. B. Yokelson, A. J. Millevolte, B. R. Adams and
Chemistry of Organic Silicon Compounds, ed. Z. Rappoport
and Y. Apeloig, Wiley, New York, 1998, part 1, ch. 5, vol. 2.
R. West, J. Am. Chem. Soc., 1987, 109, 4116; (c) K. Ueno, 13 We recalculated 1′ to compare the results with those for cis-
H. Tobita and H. Ogino, J. Organomet. Chem., 1992, 430,
93.
10 H. Marsmann, in NMR Basic Principles and Progress, ed.
2 and 3 under the same conditions, although 1′ has already
been calculated in ref. 4. There have been no essential
differences between the results of these two calculations.
P. Diehl, E. Fluck and K. R. Osfield, Springer-Verlag, Berlin, 14 See the ESI† for details.
New York, 1981, vol. 17, p. 128.
15 Preliminary research on the reactivity of these complexes
11 Based on a survey of the Cambridge Structural Data Base,
CSD version 5.37 (November, 2015).
supported this estimation, which will be reported in the
near future.
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