Cp(Pri2MeP)FeH2SiR3: Nonclassical iron silyl dihydride

Article abstract of DOI:10.1021/ja800983n

Reactions of a new borohydride complex 2 with hydrosilanes afford half-sandwich dihydride silyl complexes 3a-f. According to X-ray and DFT evidence complexes 3 have unprecedented double H·...H interligand interactions. Copyright

Full text of DOI:10.1021/ja800983n

Published on Web 03/04/2008
Cp(Pri
2 2 3
MeP)FeH SiR : Nonclassical Iron Silyl Dihydride
†
‡
§
,|
Dmitry V. Gutsulyak, Lyudmila G. Kuzmina, Judith A. K. Howard, Sergei F. Vyboishchikov,* and
,
†
Georgii I. Nikonov*
Chemistry Department, Brock UniVersity, 500 Glenridge AVenue, St. Catharines, ON, L2S 3A1, Canada, N. S.
KurnakoV Institute of General and Inorganic Chemistry, 31 Leninskii Prospect, Moscow, 119991, Russia,
Department of Chemistry, UniVersity of Durham, South Road, Durham, DH1 3LE, United Kingdom, and Institut de
Qu ´ı mica Computacional, Campus de MontiliVi, UniVersitat de Girona, 17071 Girona, Catalonia, Spain
Received November 19, 2007; E-mail: gnikonov@brocku.ca
The skyrocketing price of precious metals and their recognized
toxicity pose a question of finding cheaper and environmentally
benign alternatives for applications in catalysis.1 The abundant
and nontoxic iron is particularly attractive to serve as a “cheap metal
Scheme 1. Preparation of Dihydrido Silyl Complexes 3a-f
,2
2
3
for a noble task”, such as hydrosilation. Though Cp(OC)
was the first transition metal silyl complex and some iron
2
FeSiMe
3
4
1
,3a,b
hydrosilation catalysts are available,
relatively little is known
5
-7
about iron hydrido silyl complexes. All Fe(II) derivatives appear
to be classical silyl hydrides,5 whereas formally Fe(I), Fe(III),
,6
8
9
2
10,11
3
and many Fe(IV) complexes are in fact η -HSiR
3
2
or η -H SiR
2
silane σ-complexes.1,12 Although X-ray study of formally Fe(IV)
complexes (arene)FeH (SiX (X ) F, Cl) suggested their classical
structures, a recent DFT study of the isoelectronic rhodium deriva-
tive CpRhH (SiMe revealed the presence of significant Si-H
interactions.13 The related half-sandwiches Cp(OC)FeH(SiX
2
3 2
)
7
7b
14b
(SiCl )
3 2
(2.220(2) Å) and Cp(OC)FeH(SiCl
)
3 2
(2.252(3) Å) and
2
3 2
)
is comparable to the FedSi double bond in the silylene derivative
14
3 2
) ,
2
1
Cp*(OC)Fe(dSiMes
2 3
)(SiMes ) (2.154(1) Å). A similar situation
15
long believed to be classical Fe(IV) species, actually have highly
3
i
has been previously observed for the η -silane complex [PhB(CH
2
P -
SiMePh), in which a short Fe-Si bond of 2.1280-
delocalized Si‚‚‚H‚‚‚Si bonding.1
1d,16
Assuming that electron donat-
3
12
Pr
2
)
3
]FeH(η -H
2
ing ligands could stabilize an Fe(IV) center, we targeted the
preparation of the so far unknown class of dihydride silyl complexes
(
7) Å is an inevitable consequence of two Si-H bonds being
simultaneously coordinated to the iron center.
2
2
Cp(R′
for a novel H‚‚‚SiR
silane Si-H activation on the Cp(R′
The treatment of [Cp(Pri
NaBH
Fe(BH
Reactions of the freshly prepared 2 with silanes in the presence of
NEt afford the dihydride silyl derivatives 3a-f unavailable
previously (Scheme 1). Complex 1 catalyzes hydrosilation of
benzaldehyde by H SiPh (5% load, 3h, 22 °C), whereas catalysis
by 3b requires heating at 50 °C (H SiMePh, 5% load of 3b, 12 h).
3 2 3
P)FeH (SiR ). Their X-ray and DFT studies provide evidence
Additional evidence for the occurrence of two interligand Si‚‚‚
H interactions in 3f comes from the observation of two sets of Si-
Cl bonds: those two lying trans to the hydrides (the H-Si-Cl
bond angle of 153.2°) are noticeably longer than the unique cis
3
‚‚‚H bonding and reveal an unusual mode of
5
3
P)FeH moiety.
+
]- (1) with
2
MeP)Fe(NCCH
in THF affords a highly unstable complex Cp(Pr
3
)
2
] [BF
4
i
4
2
MeP)-
2
3
Si-Cl bond (2.1125(9) vs 2.0835(14) Å, ∆ ) 0.0290(17) Å). In
) (2), which is a rare example of an iron borohydride.1
7,18
4
6
contrast, in related complexes Cp(OC)FeH(SiCl
FeH (SiCl both Si-Cl bonds are short and almost identical
2.048-2.061(4) Å and 2.072-2.079(2) Å). Finally, two hydrides,
strongly tilted toward the silyl group to make a H-Si distance of
.88(3) Å, were observed. Compared with the accurate ND values
3 2 6 6
) and (η -C H )-
2
3 2
)
3
(
3
1
2
24
for terminal Fe-H bonds (1.526(12)-1.609(2) Å), the Fe-H
distance of 1.35(3) Å is obviously subject to a systematic
foreshortening, pertinent to the X-ray diffraction method. How-
In contrast, bulkier complex 3a is inactive.
The compounds 3 were isolated in the form of yellow oils (3b,
c) or crystals (3a, d-f) and studied by spectroscopic methods and
2
5
ever, estimation shows that elongation of the Fe-H bond to 1.6 Å
1
X-ray analysis of 3e and 3f. In the H NMR spectrum of 3f, the
would result in an increase of the Si-H distance only to 1.93 Å.
equivalent hydrides give rise to a doublet at -13.93 ppm (J(P-H)
3
DFT calculations revealed unusual features of silane H -SiMe
3
25 Hz) flanked by 29Si satellites. The Si NMR displays a doublet
29
)
2
3
addition to Cp(Me P)FeH. Activation proceeds without barrier and
of triplets at -13.8 ppm, (J(P-Si) ) 14.5 Hz, J(H-Si) ) 18.9
Hz), a pattern that does not change upon cooling to -80 °C. This
can be consistent, in principle, both with a static structure and with
a very fast degenerate exchange between two forms of a silane
starts at an Fe-Si separation of about 3.0 Å by the formation of
1
3
an η -silane σ-complex characterized by the Fe-H bond of 1.692
3
Å (Mayer index (MI) of 0.37) and the Si-H bond of 1.641 Å
3
(
2
J(H -Si) ) -115.9 Hz, MI ) 0.52). At an Fe-Si distance of
R
2
â
2
R
hydride complex: Cp(L)FeH (η -H SiR
3
) a Cp(L)Fe(η -H SiR
3
19
)-
3
3
.65 Å, the Si-H bond is significantly weakened (1.930 Å, J(H -
â
H . Such an exchange was originally postulated for related Rh
and Ru20 complexes but was later ruled out for the former on the
basis of DFT calculations.13
2
Si) ) -51.7 Hz, MI ) 0.39) but the Si‚‚‚H interaction starts to
2
26
build up. At about this point, the sign of J(H -Si) turns negative
-0.7 Hz) and a significant value of the MI is achieved (0.11).
(
The X-ray structures of 3e and 3f (Figure 1) suggest the presence
of double Si‚‚‚H interactions. Complex 3f has a remarkably short
The equilibrium structure 4 shows the presence of two simultaneous
and equiValent Si‚‚‚H interactions characterized by the Si-H
distances of 2 Å and increased MIs of 0.18 (Table 1).
Fe-Si bond of 2.168(1) Å, which is much shorter than the Fe-Si
6
distances in the closely related compounds (η -C
6
5
H Me)FeH
2
-
1
1e
Double Si-H bonding has precedents in the SISHA interac-
†
2
Brock University.
tions between a hydride and η -silane ligands, in which two Si-H
‡
N. S. Kurnakov Institute of General and Inorganic Chemistry.
University of Durham.
§
interactions are not equivalent. The most remarkable new feature
|
Universitat de Girona.
of the equiValent Si-H interactions in complexes 3 is that the extent
3732
9
J. AM. CHEM. SOC. 2008, 130, 3732-3733
10.1021/ja800983n CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. This work was supported by grants from
NSERC to G.I.N., the Spanish Ministry of Education and Science
(Ram o´ n y Cajal Program) to S.F.V., by RFBR (grant to L.G.K.)
and by the Royal Society (London) grant to J.A.K.H.
Supporting Information Available: Experimental and computa-
tional details. This material is available free of charge via the Internet
at http://pubs.acs.org
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3
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(
(
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(
(
(
Table 1. Calculated Bond Lengths (in Å)/Mayer Bond Indices and
Si-H Coupling Constants (in Hz) for the Most Stable Isomers of
4-7
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bond
4a
5b
6c
7 a
Fe-Si
2.320/0.74
1.513/0.70
1.511/0.70
2.000/0.18
1.984/0.18
-
2.262/0.79
1.507/0.70
1.507/0.70
2.007/0.16
2.007/0.16
-
2.171/0.82
-22.9
-22.9
2.222/0.81
1.508/0.69
1.514/0.70
1.994/0.18
1.991/0.17
2.152/0.84
2.146/0.83
-42.4
2.194/0.89
1.512/0.70
1.512/0.70
1.995/0.17
1.995/0.17
2.122/0.87
2.126/0.85
-10.1
(
(
8) Simons, R. S.; Tessier, C. A. Organometallics 1996, 15, 2604.
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Fe-H
Fe-H
3
2
Si-H
Si-H
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1994, 480, 241.
Si-Cl(t)
Si-Cl(c)
(
11) (a) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer
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interactions between silicon and hydrogen atoms (SISHA), see: Lachaize,
S.; Sabo-Etienne, S. Eur. J. Inorg. Chem. 2006, 2115.
-
2
J(Si-H ) -23.5
3
J(Si-H ) -24.3
+1.3
-10.1
d
Jave
-23.9 (21.6) -20.3 (21.9) -17.3(19.2) -10.1(18.9)
(
12) Thomas, C. M.; Peters, J. C. Angew. Chem., Int. Ed. 2006, 45, 776.
a
Cs structure. b Cs structure with the Cl group cis to hydrides; Cl(c) is
c
(13) Vyboishchikov, S. F.; Nikonov, G. I. Organometallics 2007, 26, 4160.
(14) (a) Jetz, W.; Graham, W. A. G. Inorg. Chem. 1971, 10, 1159. (b)
Manojlovi c´ -Muir, L.; Muir, K. W.; Ibers, J. A. Inorg. Chem. 1970, 9,
3
d
cis to hydrides, Cl(t) is trans to H . Weighted according to the Boltzmann
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4
47. (c) Smith, R. A.; Bennett, M. J. Acta Crystallogr. 1977, B33, 1118.
(
d) Simpson, K. A. Ph.D. Thesis, University of Alberta, 1973.
of bonding does not depend on the substitution at Si (Table 1) and
(15) Schubert, U. AdV. Organomet. Chem. 1990, 30, 151.
27
(16) Vyboishchikov, S. F.; Nikonov, G. I. Chem.sEur. J. 2006, 12, 8518.
the orientation of the silyl group. In the related silane σ-com-
(
17) Schaeffer, G. W.; Roscoe, J. S.; Stewart, A. C. J. Am. Chem. Soc. 1956,
5,11
plexes and complexes with interligand hypervalent interactions
78, 729.
IHI),1 electron-withdrawing groups on Si tend to weaken the
Si-H bonding, which is not the case for model complexes
Cp(Me P)FeH (SiMe3-nCl
1d
(18) A few stable borohydride complexes of Fe are known: (a) Mehn, M. P.;
Brown, S. D.; Paine, T. K.; Brennessel, W. W.; Cramer, C J.; Peters, J.
C.; Que, L., Jr. Dalton Trans. 2006, 1347. (b) Guilera, G.; McGrady, G.
S.; Steed, J. W.; Kaltsoyannis, N. New J. Chem. 2004, 28, 444. (c) Hillier,
A. C.; Jacobsen, H.; Gusev, D.; Schmalle, H. W.; Berke, H. Inorg. Chem.
(
3
2
n
) (n ) 0-4, 4-7). The IHI additionally
requires the presence of an accepting group trans to the hydride,
2
001, 40, 6334. (d) Ghilardi, C. A.; Innocenti, P.; Midollini, S.; Orlandini,
but the data of Table 1 show that such a stereochemical condition
A. J. Chem. Soc., Dalton Trans. 1985, 605.
does not hold for 4-7. And in fact rotamers 5a,b27 have the longest
(19) Taw, F. L.; Bergman, R. G.; Brookhart, M. Organometallics 2004, 23, 886.
(
(
20) Ng, S. M.; Lau, C. P.; Fan, M.-F.; Lin, Z. Organometallics 1999, 18, 2484.
21) Tobita, H.; Matsuda, Hashimoto, H.; Ueno, K.; Ogino, H. Angew. Chem.,
Int. Ed. 2004, 43, 221.
Si-H bond to the hydride that is trans to the Cl substituent.
Calculation of significant attractive values of Mayer diatomic
energies28 (range from -40.4 to -44.6 kcal‚mol-1) unambiguously
(22) For the same reason, very short Ru-Si and Ru-B distances were observed
3
3
in some η -silane and η -borane complexes of Ru: (a) Atheaux, I.;
Donnadieu, B.; Rodriguez, V.; Sabo-Etienne, S.; Chaudret, B.; Hussein,
K.; Barthelat, J.-C. J. Am. Chem. Soc. 2000, 122, 5664. (b) Alcaraz, G.;
Clot, E.; Helmstedt, U.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc.
proves the existence of Si‚‚‚H interactions in 4-7. A large propor-
tion of this energy results from the interatomic exchange, indicating
that this Si‚‚‚H attraction has a substantial covalent character.
Another unusual feature of the multicentral bonding in 3-7 is
that substitution at silicon does not affect the value of J(Si-H).
The experimental coupling constants are close to 20 Hz and decrease
only slightly in magnitude from 3a to 3f (Table 1), as do averaged
calculated values. The J(H-Si) for individual Si-H pairs vary
largely in the range 0-40 Hz and are negative, suggesting a direct
Si-H bonding.26 The largest magnitude of J(H-Si) is observed
2007, 129, 8704.
(23) In 3e, both chlorides are trans to hydrides.
(
(
24) Bau, R.; Drabnis, M. H. Inorg. Chim. Acta 1997, 259, 27.
25) Hirshfeld, F. L. Cryst. ReV. 1991, 2, 169.
(26) (a) Ignatov, S. K.; Rees, N. H.; Tyrrell, B. R.; Dubberley, S. R.; Razuvaev,
A. G.; Mountford, P.; Nikonov, G. I. Chem.sEur. J. 2004, 10, 4991. (b)
3
The small positive value of 1.3 Hz for J(Si-H ) in 6 is within the error
of our calculations.
3 2 n
(27) For each complex Cp(Me P)FeH (SiMe3-nCl ) (n ) 0-4) we calculated
three silyl rotamers (a, b, and c). See Supporting Information for details.
28) (a) Mayer, I. Chem. Phys. Lett. 2003, 382, 265. (b) DFT-based formal-
ism: Vyboishchikov, S. F.; Salvador, P.; Duran, M. J. Chem. Phys. 2005,
122, 244110. (c) Correlated formalism: Vyboishchikov, S. F.; Salvador,
P. Chem. Phys. Lett. 2006, 430, 204.
(
2
9
for the hydride located cis to the chloride group on silicon.
Tentatively, we describe these unique30,31 multicentral Si‚‚‚H
interactions in terms of adduct formation between the hypervalent
(
29) This difference comes mainly from the Fermi contact contribution. Pre-
sently we have no rationale for this stereochemical control of J(H-Si).
30) Multiple Si-H interactions have been previously observed for some
complexes of ruthenium, but the effect of substituents has not been clearly
delineated: (a) H u¨ bler, K.; H u¨ bler, U.; Roper, W. R.; Schwerdtfeger, P.;
Write, L. J. Chem.sEur. J. 1997, 3, 1608. (b) Yardy, N. M.; Lemke, F.
R.; Brammer, L. Organometallics 2001, 20, 5670.
-
+
3
16
2
anion [H SiR
3
]
and [Cp(L)Fe] , i.e., as Cp(L)Fe(η -H
2
SiR
3
).
(
i
In conclusion, silane Si-H activation on Cp(Pr
2
MeP)FeH
proceeds differently from the conventionally postulated Si-H
oxidative addition or silane σ-complexation. It starts primarily from
the Si-H f Fe donation, but at a later stage it also involves
significant interaction between the silyl and metal-bound hydride,
(31) Double IHI has been described: Osipov, A. L.; Gerdov, S. M.; Kuzmina,
L. G.; Howard, J. A. K.; Nikonov, G. I. Organometallics 2005, 24, 587.
resulting in a structure with novel H‚‚‚SiR
3
‚‚‚H bonding.
JA800983N
J. AM. CHEM. SOC.
9
VOL. 130, NO. 12, 2008 3733