Effective control of two elimination processes from a hydridobis(silyl)iron(IV) complex

Article abstract of DOI:10.1246/cl.2001.854

A novel hydridobis(silyl)iron(IV) complex Cp*(CO)Fe(H)-(SiMe₂O(2-C₅H₄N)}₂ (1) (Cp=η⁵-C₅Me₅) was synthesized in high yield. Two elimination processes from 1 were effectively controlled by the reaction conditions: HSiMe₂O(Z-C₅H₄N) is eliminated on heating or UV-irradiation, whereas HO(2-C₅H₄N) is eliminated in the presence of Et₃Al. The latter reaction was applied to a silylene introduction toward a silyl complex producing a silyl(silylene) complex.

Full text of DOI:10.1246/cl.2001.854

854
Chemistry Letters 2001
Effective Control of Two Elimination Processes from a Hydridobis(silyl)iron(IV) Complex
Takahiro Sato, Hiromi Tobita,* and Hiroshi Ogino†
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578
†Miyagi Study Center, The University of the Air, Sendai 980-8577
(Received June 8, 2001; CL-010539)
A novel hydridobis(silyl)iron(IV) complex Cp*(CO)Fe(H)-
was 2.58(3) Å, which is slightly shorter than the sum of the van
der Waals radii of the two atoms, 2.70 Å.6 There may be weak
hydrogen bonding between a polarized Feδ––Hδ+ bond and a
negatively polarized nitrogen, though we could not find any
examples of the hydrogen bonding of the type M–H···N in the
database of the Cambridge Crystallographic Data Centre.
{SiMe2O(2-C5H4N)}2 (1) (Cp*=η5-C5Me5) was synthesized in
high yield. Two elimination processes from 1 were effectively
controlled by the reaction conditions: HSiMe2O(2-C5H4N) is
eliminated on heating or UV-irradiation, whereas HO(2-
C5H4N) is eliminated in the presence of Et3Al. The latter reac-
tion was applied to a “silylene introduction” toward a silyl com-
plex producing a silyl(silylene) complex.
The reactivity of transition metal complexes containing
metal–silicon bonds is attracting much attention. One of the
reasons is that it could give grounds for the mechanisms of vari-
ous transition-metal-mediated transformation of organosilicon
compounds.1 Among them, hydridobis(silyl) complex
LnMH(SiR2X)2 is considered as an important species because it
is easily formed by the oxidative addition of hydrosilanes to a
coordinatively unsaturated silyl complex, and is regarded as a
key intermediate of the transition-metal-catalyzed polymerization
of hydrosilanes.2 Several different reaction patterns have been
reported separately for this species, but, to our knowledge, there
is no report on the attempt to control the reaction pathway by
changing the reaction conditions. We report here a quantitative
preparation of a stable hydridobis(2-pyridyloxysilyl)iron(IV)
complex from a 2-pyridyloxy-bridged bis(silylene)iron complex,3
and effective control of two elimination processes from this
species, reductive elimination of hydrosilane and 1,2-elimination
of 2-hydroxypyridine.
Complex 1 is stable at room temperature in hydrocarbon
solvents, while it undergoes reductive elimination of hydrosi-
lane HSiMe2O(2-C5H4N) (3)7 on heating or UV-irradiation to
afford a chelate complex 4 (eq 2).8
A hydridobis(2-pyridyloxysilyl)iron(IV) complex 14 was
prepared quantitatively by addition of 2-hydroxypyridine to an
internal-base-stabilized bis(silylene)iron complex 23 (eq 1).
The addition appears to occur selectively to the Fe–Si bond on
the N-side, although another route; i.e., addition to the Fe–Si
bond on the O-side followed by 1,3-shift of Si from N to O,
cannot be ruled out.
The reductive elimination in eq 2 proceeds irreversibly: 1
was not observed even when 4 was heated or irradiated in the
presence of excess hydrosilane 3. This means that the equilibri-
um of the reaction in eq 2 lies far to the right and the irre-
versibility is apparently due to the chelate effect. The detailed
structure and reactivity of 4 will be reported elsewhere.
When 1 was treated with a small excess of Et3Al (2 equiv)
at room temperature, a bis(silylene) complex 2 was formed
slowly as a main product by formal elimination of 2-hydroxy-
The 29Si NMR spectrum of 1 shows only one signal at δ =
64.2 ppm, which is consistent with the geometry in which two
silyl groups are mutually trans. It was further confirmed by X-
ray crystal structure analysis (Figure 1).5
The lengths of two Fe–Si bonds are 2.3206(6) Å and
2.3178(6) Å, which are both in the range of normal single
bonds (2.32–2.37 Å).1a The distance between a hydride
attached to Fe and a nitrogen atom of one of pyridyloxy groups
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
855
pyridine together with a small amount of reductive elimination
product 4 (eq 3).
elimination of 2-hydroxypyridine from 1 occurs in the presence
of Et3Al to afford 2. This sequence is a novel route to introduce
a silylene fragment to a metal center by means of a
hydro(alkoxy)silane, although Roper et al. reported a similar
reaction with use of aminohydrosilanes.9 This “oxidative addi-
tion–1,2-elimination” sequence utilizing alkoxy-functionalized
hydrosilane could be applied to the preparation of a wide vari-
ety of alkoxy-stabilized silylene complexes.
References and Notes
1
a) T. D. Tilley, “Transition-metal silyl derivatives,” in “The
Chemistry of Organic Silicon Compounds,” ed. by S. Patai and Z.
Rappoport, Wiley, New York (1989), Vol. 2, Chap. 24. b) J. Y.
Corey and J. Braddock-Wilking, Chem. Rev., 99, 175 (1999).
a) W. Jetz and W. A. G. Graham, Inorg. Chem., 10, 1159 (1971).
b) C. L. Randolph and M. S. Wrighton, J. Am. Chem. Soc., 108,
3366 (1986). c) D. A. Straus, C. Zhang, G. E. Quimbita, S. D.
Grumbine, R. H. Heyn, T. D. Tilley, A. L. Rheingold, and S. J.
Geib, J. Am. Chem. Soc., 112, 2673 (1990). d) Y. Kawano, H.
Tobita, and H. Ogino, J. Organomet. Chem., 428, 125 (1992). e)
M. A. Esteruelas, F. J. Lahoz, M. Oliván, E. Oñate, and L. A. Oro,
Organometallics, 13, 4246 (1994). f) H. Wada, H. Tobita, and H.
Ogino, Organometallics, 16, 2200 (1997).
2
Generation of a significant amount of ethane was observed
by 1H NMR during the reaction. Therefore, the actual counter-
parts of the elimination reaction giving 2 in eq 3 are probably
ethane and Et2AlO(2-C5H4N), although the latter has not been
identified yet.
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4
H. Tobita, T. Sato, M. Okazaki, and H. Ogino, J. Organomet.
Chem., 611, 314 (2000).
There are at least two possible roles conceivable for Et3Al.
The one is to enhance the ability of the 2-pyridyloxy group as a
leaving group by coordination to either N or O. This may
accelerate the 1,2-elimination of 2-hydroxypyridine. The other
is to change the elimination product 2-hydroxypyridine, which
has been already demonstrated to be very reactive toward 2 to
give 1 as shown in eq 1, to inactive Et2AlO(2-C5H4N) and
ethane. This makes the 1,2-elimination irreversible. The 1,2-
elimination reaction also proceeds more slowly by treatment of
1 with Ph3B (2 equiv) instead of Et3Al to give 2 in 82% on con-
version of 89% of 1 after 2 weeks at room temperature.
Formation of a reasonable amount of benzene was observed in
this reaction.
When 4 was combined with 3 (3 equiv) and Et3Al (1
equiv), and heated up to 120 °C, slow formation of 2 was
observed. After heating at 150 °C for 4 days, 4 mostly disap-
peared and 2 was obtained in 45% yield. A possible mecha-
nism of this “silylene introduction” reaction is illustrated in
Scheme 1. First, from 4 the unsaturated metal center is ther-
mally generated above 120 °C by the cleavage of the Fe–N
coordinate bond. Second, hydrosilane 3 undergoes oxidative
addition to the unsaturated Fe center to give 1. Finally, 1,2-
1: pale yellow crystals. mp 88 °C. Anal. Found: C, 56.87; H, 6.78;
N, 5.35%. Calcd for C25H36FeN2O3Si2: C, 57.24; H, 6.92; N,
5.34%. 1H NMR (300 MHz, C6D6) δ –11.15 (s, 1H, FeH), 1.11 (s,
6H, SiMe), 1.15 (s, 6H, SiMe), 1.56 (s, 15H, Cp*), 6.40–6.44,
6.64–6.67, 7.05–7.09, 8.11–8.13 (m, aromatic protons). 13C{1H}
NMR (75.5 MHz, C6D6) δ 9.8 (C5Me5), 10.3 (SiMe), 10.5 (SiMe),
95.3 (C5Me5), 113.4, 116.3, 138.6, 147.9, 163.9 (pyridine ring),
214.7 (FeCO). 29Si{1H} NMR (59.6 MHz, C6D6) δ 64.2. IR
(hexane solution) 1977 (w, sh, νFeH), 1934 (vs, νCO) cm–1. UV
(hexane solution) λmax 266 (14000), 311 nm (sh, 2000 mol–1 dm3
cm–1). MS (EI, 70 eV) m/z 523 (M+ – H, 0.2).
Crystal data for 1: C25H36FeN2O3Si2; Mr = 524.59, monoclinic, a =
11.5362(3), b = 13.7135(2), c = 17.1380(3) Å, β = 104.132(1)°, V
= 2629.2(1) Å3, T = 150(1) K, space group P21/c (No.14), Z = 4,
µ(Mo Kα) = 6.93 cm–1, Dcalc = 1.325 g cm–3, GOF = 1.19, R1 =
0.038 for unique 5306 reflections with I > 2σ(I).
5
6
7
A. J. Gordon and R. A. Ford, “The Chemist’s Companion: A
Handbook of Practical Data, Techniques, and References,” Wiley,
New York (1972).
3: colorless liquid. bp 90 °C/24 Torr. Anal. Found: C, 54.62; H,
7.33; N, 9.10%. Calcd for C7H11NOSi: C, 54.86; H, 7.23; N,
9.14%. 1H NMR (300 MHz, C6D6) δ 0.39 (d, 6H, SiMe, 3JHH = 2.8
3
Hz), 5.34 (septet, 1H, SiH, JHH = 2.8 Hz), 6.36–6.41, 6.56–6.60,
6.97–7.03, 7.97–8.00 (m, aromatic protons). 13C{1H} NMR (75.5
MHz, C6D6) δ –1.2 (SiMe), 112.5, 117.1, 139.1, 147.5, 163.1 (pyri-
dine ring). 29Si{1H} NMR (59.6 MHz, C6D6) δ 3.6. IR (neat,
NaCl plate) 2156 (νSiH) cm–1. MS (EI, 70 eV) m/z 153 (M+, 40).
4: red crystals. mp 112 °C (dec.). Anal. Found: C, 57.78; H, 6.77;
N, 3.73%. Calcd for C18H25FeNO2Si: C, 58.22; H, 6.79; N, 3.77%.
1H NMR (300 MHz, C6D6) δ 0.94 (s, 3H, SiMe), 1.04 (s, 3H,
SiMe), 1.54 (s, 15H, Cp*), 5.86–5.91, 6.36–6.39, 6.57–6.62,
7.76–7.78 (m, aromatic protons). 13C{1H} NMR (75.5 MHz,
C6D6) δ 7.3 (SiMe), 9.9 (SiMe), 10.5 (C5Me5), 89.8 (C5Me5),
110.5, 114.6, 137.6, 154.5, 170.6 (pyridine ring), 224.4 (FeCO).
29Si{1H} NMR (59.6 MHz, C6D6) δ 114.4. IR (KBr pellet) 1886
(νCO) cm–1. UV (hexane solution) λmax 246 (6000), 323 (2000),
405 nm (800 mol–1 dm3 cm–1). MS (EI, 70 eV) m/z 371 (M+, 12).
a) S. M. Maddock, C. E. F. Rickard, W. R. Roper, D. M. Salter,
and L. J. Wright, New Zealand Institute of Chemistry Conference,
Auckland, December 1993, Abstr., No. INO-OR 01. b) W. Roper,
W.-H. Kwok, M. Mohlen, C. Rickard, A. Williamson, T.
Woodman, and L. Wright, 2000 International Chemical Congress
of Pacific Basin Societies, Hawaii, December 2000, Abstr., No.
INOR 1095.
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