Preparation and molecular and electronic structures of iron(0) dinitrogen and silane complexes and their application to catalytic hydrogenation and hydrosilation

Article abstract of DOI:10.1021/ja046753t

Reduction of the five-coordinate iron(II) dihalide complexes ( ^iPrPDI)FeX₂ (^iPrPDI = ((2,6-CHMe ₂)₂C₆H₃N=CMe)₂C ₅H₃N; X = Cl, Br) with sodium amalgam under 1 atm of dinitrogen afforded the square pyramidal, high spin iron(0) bis(dinitrogen) complex (^iPrPDI)Fe(N₂)₂. In solution, ( ^iPrPDI)Fe(N₂)₂ loses 1 equiv of N₂ to afford the mono(dinitrogen) adduct (^iPrPDI)Fe(N₂) ₂. Both dinitrogen compounds serve as effective precatalysts for the hydrogenation and hydrosilation of olefins and alkynes. Effecient catalytic reactions are observed with low catalyst loadings (≤0.3 mol %) at ambient temperature in nonpolar media. The catalytic hydrosilations are selective in forming the anti-Markovnikov product. Structural characterization of a high spin iron(0) alkyne and a bis(silane) σ-complex has also been accomplished and in combination with isotopic labeling studies provides insight into the mechanism of both catalytic C-H and catalytic C-Si bond formation.

Full text of DOI:10.1021/ja046753t

Published on Web 10/05/2004
Preparation and Molecular and Electronic Structures of Iron(0)
Dinitrogen and Silane Complexes and Their Application to
Catalytic Hydrogenation and Hydrosilation
Suzanne C. Bart, Emil Lobkovsky, and Paul J. Chirik*
Contribution from the Department of Chemistry and Chemical Biology,
Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received June 2, 2004; E-mail: pc92@cornell.edu
Abstract: Reduction of the five-coordinate iron(II) dihalide complexes (iPrPDI)FeX2 iPrPDI ) ((2,6-
(
CHMe2)2C6H3NdCMe)2C5H3N; X ) Cl, Br) with sodium amalgam under 1 atm of dinitrogen afforded the
square pyramidal, high spin iron(0) bis(dinitrogen) complex (iPrPDI)Fe(N2)2. In solution, (iPrPDI)Fe(N2)2 loses
1 equiv of N2 to afford the mono(dinitrogen) adduct (iPrPDI)Fe(N2). Both dinitrogen compounds serve as
effective precatalysts for the hydrogenation and hydrosilation of olefins and alkynes. Effecient catalytic
reactions are observed with low catalyst loadings (e0.3 mol %) at ambient temperature in nonpolar media.
The catalytic hydrosilations are selective in forming the anti-Markovnikov product. Structural characterization
of a high spin iron(0) alkyne and a bis(silane) σ-complex has also been accomplished and in combination
with isotopic labeling studies provides insight into the mechanism of both catalytic C-H and catalytic C-Si
bond formation.
Introduction
ficient C-C bond forming cross-coupling reactions, although
in some cases the oxidation state and identity of the catalytically
Catalytic bond forming reactions promoted by homogeneous
transition metal complexes have become an indispensable tool
in synthetic chemistry.1 Most syntheses of complex molecular
targets rely on one, if not many, key metal-mediated steps.2
While unprecedented levels of activity and selectivity have been
achieved,3 increasing pressures for atom-economical4 and
catalytic processes with reduced environmental impact5 fuel the
search for improved synthetic transformations that minimize
waste production, energy consumption, and generation of toxic
substances.6
One approach to accomplishing this goal is to replace toxic,
heavy metal catalysts with more benign iron compounds. For
an effective process, however, the new catalysts should provide
high activity at low loadings and high substrate concentrations
ideally without organic solvent.7,8 The low toxicity and high
terrestrial abundance9 of iron make it an attractive metal for
this purpose. Work from several laboratories has recently
focused on developing homogeneous iron catalysts as precious-
metal surrogates.10 Building on the seminal work of Kochi,11
Fu¨rstner12-14 and Hayashi15 have independently reported ef-
active iron species is not well understood. Iron catalysts have
also been developed for olefin polymerization and display levels
of activity and selectivity comparable to those of more traditional
group 4 metallocene catalysts.16,17
These results suggested to us that iron, when in an appropriate
coordination geometry and spin state, may be a reasonable
alternative to precious metals in a wide range of catalytic bond
forming reactions. Inspired by the success of iron catalysts in
olefin polymerization18 and reports on MAO-activated iron
catalysts for olefin hydrogenation,19,20 our initial focus was on
reactions that involved insertion of an unsaturated organic
molecule into an iron hydride or iron alkyl. Specifically, we
(10) Examples of iron-catalyzed reactions: (a) tom Dieck, H.; Dietrich, J. Chem.
Ber. 1984, 117, 694. (b) Corey, E. J.; Imai, N.; Zhang, H.-Y. J. Am. Chem.
Soc. 1991, 113, 728. (c) Takacs, J. M.; Weidner, J. J.; Takacs, B. E.
Tetrahedron Lett. 1993, 34, 6219. (d) Watahiki, T.; Akabane, Y.; Mori,
S.; Oriyama, T. Org. Lett. 2003, 5, 3045.
(11) Kochi, J. K. Acc. Chem. Res. 1974, 7, 351.
(12) Fu¨rstner, A.; Leitner, A.; Me´ndez, M.; Krause, H. J. Am. Chem. Soc. 2002,
124, 13856.
(13) Fu¨rstner, A.; De Souza, D.; Parra-Rapado, L.; Jensen, J. T. Angew. Chem.,
Int. Ed. 2003, 42, 5358.
(14) (a) Siedel, G.; Laurich, D.; Fu¨rstner, A. J. Org. Chem. 2004, 69, 3950. (b)
Scheiper, B.; Bonnekessel, M.; Krause, H.; Fu¨rstner, A. J. Org. Chem.
2004, 69, 3943.
(1) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley: New York,
1994.
(15) Nagano, T.; Hayashi, T. Org. Lett. 2004, 6, 1297.
(16) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998,
120, 4049.
(17) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox,
P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.;
Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999,
121, 8728.
(2) Cornils, B.; Herrmann, W. A. Applied Homogeneous Catalysis with
Organometallic Compounds; VCH: Weinheim, Germany, 1996.
(3) (a) Knowles, W. S. Angew. Chem., Int. Ed. 2002, 41, 1998. (b) Noyori, R.
Angew. Chem., Int. Ed. 2002, 41, 2008. (c) Sharpless, K. B. Angew. Chem.,
Int. Ed. 2002, 41, 2024.
(4) Trost, B. M. Acc. Chem. Res. 2002, 35, 695.
(5) Baker, R. T.; Tumas, W. Science 1999, 284, 1477.
(6) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686.
(7) DeSimone, J. M. Science 2002, 297, 799.
(18) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci, F.; Zanobini, F.; Laschi,
F.; Sommazzi, A. Eur. J. Inorg. Chem. 2003, 1620.
(19) Knijnenburg, Q.; Horton, A. D.; Van Der Heijden, H.; Gal, A. W.;
Budzelaar, P.; Henricus, M. PCT Int. Appl. WO 2003042131, 2003; 26
pp.
(8) Hoge, G.; Wu, H.-P.; Kissel, W. S.; Pflum, D. A.; Greene, D. J.; Bao, J.
J. Am. Chem. Soc. 2004, 126, 5966.
(9) Goldschmidt, V. W. Geochemistry; Oxford Press: London 1958; p 647.
(20) Bart, S. C.; Chirik, P. J. Unpublished results.
9
13794
J. AM. CHEM. SOC. 2004, 126, 13794-13807
10.1021/ja046753t CCC: $27.50 © 2004 American Chemical Society

Catalytic Hydrogenation and Hydrosilation
A R T I C L E S
are interested in developing highly active catalysts for both the
hydrogenation and hydrosilation of unsaturated organic mol-
ecules, given the widespread synthetic utility of these reactions
and their reliance on toxic, precious metals.21-23
We recently reported24 the synthesis and characterization of
a series of four-coordinate, and in some cases enantiopure, iron-
(II) dialkyl complexes that are reluctant to participate in olefin
and alkyne insertion reactions, possibly due to their high spin,
S ) 2 electronic configuration.25 On the basis of these results,
we began to explore alternative coordination geometries and
spin states and were encouraged by the reports of 1-hexene
hydrogenation by Fe(CO)5.26 However, the utility of these
reactions is hampered by the high temperatures and pressures,
typically in excess of 200 °C and 200-250 atm, required for
catalytic activity. Likewise, Wrighton has developed a photo-
catalytic method for both olefin hydrogenation and hydrosilation
using Fe(CO)5 that operates efficiently at 22 °C and 1 atm of
pressure, but continued irradiation with 366 nm light is required
for turnover.27 Bianchini and co-workers have reported a cationic
iron hydride complex that hydrogenates alkynes to alkenes under
mild conditions; however, further conversion of alkenes to
alkanes is not observed.28 Concurrent with our studies, Daida
and Peters have developed a family of tris(phosphino)borato-
ligated iron alkyl and hydride complexes that are competent
for olefin and alkyne hydrogenation at room temperature and 1
atm of H2.29
Figure 1. Molecular structure of 1-(N2)2 (30% probability ellipsoids).
Hydrogen atoms omitted for clarity.
thermal conditions. Tridentate pyridinediimine (PDI) ligands of
the general form [(2,6-ArNdC(Me))2C5H3N] (Ar ) substituted
aryl group) are attractive for this purpose given their relative
ease of synthesis, modularity, and well-documented success in
supporting catalytically active iron compounds.16-19 In addition,
the relative π-acidity of this class of molecule as compared to
σ-donating trialkylphosphine ligands may also aid in stabiliza-
tion of electron-rich Fe(0) complexes.
Here we describe the synthesis and characterization of an
unusual high spin, d8 square pyramidal iron(0) bis(dinitrogen)
complex that serves as an effective precatalyst for the hydro-
genation and selective hydrosilation of both olefins and alkynes.
In addition, two catalytically competent intermediates, an iron
alkyne compound and a bis(silane) σ-complex, have been
synthesized and crystallographically characterized. These mol-
ecules, in combination with isotopic labeling studies, have
provided insight into the mechanism of iron-catalyzed C-H and
C-Si bond forming reactions.
Reduction of the iron(II) dihalide complexes containing 2,6-
diisopropylphenyl substituents, 1-Cl2 or 1-Br2, with 0.5%
sodium amalgam under 1 atm of dinitrogen produced green
crystals identified as the five-coordinate iron(0) bis(dinitrogen)
complex 1-(N2)2 (eq 1).
Results and Discussion
Synthesis of the Well-Defined Catalyst Precursor 1-(N2)2.
Inspired by Wrighton’s findings that photogenerated Fe(CO)3
is the active species in Fe(CO)5-catalyzed olefin hydrogenation
and isomerization reactions,27 we targeted the synthesis of a
well-defined iron precursor that could provide access to a
reactive, 14-electron isolobal L3Fe(0) fragment under mild
The bis(dinitrogen) compound can also be readily synthesized
by addition of 2 equiv of NaBEt3H to 1-Br2 in toluene.
Presumably, generation of Fe(0) proceeds by facile reductive
elimination of dihydrogen from the putative iron(II) dihydride
1-H2. Dark green 1-(N2)2 was characterized by a combination
of IR and electronic spectroscopies, elemental analysis, and
X-ray diffraction. Coordination of two N2 ligands in the solid
state was also confirmed by a Toepler pump experiment. Unlike
ruthenium, where coordination of two N2 ligands has been
known for some time,30,31 to our knowledge synthesis of 1-(N2)2
provides the first example of an iron(0) bis(dinitrogen) complex.
Single crystals of 1-(N2)2 suitable for X-ray diffraction were
grown from a concentrated pentane/ether (∼10:1) solution. The
solid-state structure is shown in Figure 1, and selected bond
distances and angles are reported in Table 1. While several iron-
(0) dinitrogen complexes have been reported,32 only a limited
number have been crystallographically characterized.33 More
(21) Chaloner, P. A.; Esteruelas, M. A.; Joo´, F.; Oro, L. A. Homogeneous
Hydrogenation; Kluwer Academic Publishers: Dordrecht, The Netherlands,
1994.
(22) (a) Brook, M. A. Silicon in Organic, Organometallic, and Polymer
Chemistry; Wiley: New York, 2000. (b) Ojima, I.; Li, Z.; Zhu, J. The
Chemistry of Organic Silicon Compounds; Wiley: Avon, U.K., 1998;
Chapter 29.
(23) Early-transition-metal and lanthanide catalysts are also effective for
hydrogenation and hydrosilation. For representative examples see: (a)
Troutman, M. V.; Apella, D. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999,
121, 4916. (b) Conticello, V. P.; Brard, L.; Giardello, M. A.; Tsuji, Y.;
Sabat, M.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 2761.
(c) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J.
Am. Chem. Soc. 1985, 107, 8111.
(24) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.; Chirik, P.
J. Organometallics 2004, 23, 237.
(25) (a) Harvey, J. N.; Poli, R.; Smith, K. M. Coord. Chem. ReV. 2003, 238,
347-361. (b) Poli, R. Chem. ReV. 1996, 96, 2135.
(30) (a) Fergusson, J. E.; Love, J. L. Chem. Commun. 1969, 399. (b) Kane-
Maguire, L. A. P.; Sheridan, P. S.; Basolo, F.; Pearson, R. G. J. Am. Chem.
Soc. 1968, 90, 5295.
(31) (a) Sellmann, D. Angew. Chem., Int. Ed. Engl. 1974, 13, 639. (b) Allen,
A. D.; Harris, R. O.; Loescher, B. R.; Stevens, J. R.; Whiteley, R. N. Chem.
ReV. 1973, 73, 11.
(26) Harmon, R. E.; Gupta, S. K.; Brown, D. J. Chem. ReV. 1973, 73, 21.
(27) Schroeder, M. A.; Wrighton, M. S. J. Am. Chem. Soc. 1976, 98, 551.
(28) Bianchini, C.; Meli, A.; Peruzzini, M.; Frediani, P.; Bohanna, C.; Esteruelas,
M. A.; Oro, L. A. Organometallics 1992, 11, 138.
(29) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, in press.
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13795

A R T I C L E S
Bart et al.
Table 1. Selected Bond Distances and Angles for 1-(N2)2
1-(N2)2 are consistent with the π-accepting nature of the PDI
ligand relative to the phosphines and make the observed N2
stretching frequencies intermediate to the value of 2098 cm-1
reported for (PEt3)2(CO)2Fe(N2)33a and the range of 2060-2130
cm-1 typically found in iron(II) dinitrogen complexes.34
The solid-state magnetic susceptibility of 1-(N2)2 was inves-
tigated by SQUID magnetometry (see the Supporting Informa-
tion). Variable temperature data collected from 4 to 300 K are
consistent with a paramagnetic molecule that has an excellent
fit (R2 ) 0.995) to the Curie-Weiss law. The observed values
for the magnetic moment clearly establish an S ) 1 ground
state for 1-(N2)2.
distance
(Å)
angle
(deg)
N(1)-N(2)
N(3)-N(4)
Fe(1)-N(1)
Fe(1)-N(3)
Fe(1)-N(6)
N(5)-C(2)
N(7)-N(8)
1.090(2)
1.104(3)
1.8341(16)
1.8800(19)
1.8362(14)
1.332(2)
Fe(1)-N(1)-N(2)
Fe(1)-N(3)-N(4)
N(1)-Fe(1)-N(3)
N(1)-Fe(1)-N(5)
N(1)-Fe(1)-N(7)
N(5)-Fe(1)-N(6)
N(6)-Fe(1)-N(7)
178.40(19)
171.81(17)
98.02(8)
96.65(7)
97.41(7)
74.49(6)
79.90(6)
1.333(2)
deviation of Fe(1)
deviation of N(1)
0.297
0.072
Five-coordinate, iron(0) complexes have been known for over
a century, with Fe(CO)5 being the most familiar example.35
Since that time, several (CO)4FeL and (CO)3FeL2 (L )
phosphine) complexes have been prepared and a continuum of
geometries36,37 ranging between trigonal bipyramidal38,39,40 and
square pyramidal41 have been observed. In addition, pentakis-
(phosphine)iron(0) complexes have also been prepared and
crystallographically characterized and both limiting geometries
observed.42,43 In many cases, attempts to measure the barrier to
interconversion have been unsuccessful44,45 owing to facile
ligand site exchange by a classic Berry mechanism,46 although
a static A2B3-type spectrum has been observed for Fe(P(OMe)3)5
at -104 °C.47 Importantly, each of the aforementioned com-
plexes is diamagnetic, and thus, the triplet ground state observed
for 1-(N2)2 is, to our knowledge, the first example of a high
spin, d8 iron(0) complex.48 While unusual for iron, high spin,
d8 electronic configurations are well-known for isoelectronic
Ni(II) complexes. Many of these molecules have been charac-
terized by X-ray diffraction49-51 and their electronic structures
thoroughly investigated.52 Formally, 1-(N2)2 can also be viewed
as an intermediate spin iron(II) diamide complex where the PDI
ligand has been reduced by a two-electron transfer from the
iron center.
significantly, each of the previously structurally characterized
iron(0) dinitrogen complexes contains only one N2 ligand and
adopts a trigonal bipyramidal geometry.
The molecular geometry about the iron in 1-(N2)2 is best
described as a distorted square pyramid with one N2 ligand
completing the fourth site of the basal plane while the other
occupies the apical position. The basal N2 ligand lies 0.072 Å
below the plane defined by N(5)-N(6)-N(7) of the PDI ligand.
In contrast, the iron atom deviates more significantly, lying 0.297
Å above the plane defined by N(5)-N(6)-N(7). As with other
structurally characterized iron complexes containing the iPrPDI
ligand,16-18 the aryl substituents are oriented perpendicular to
the basal plane of the molecule. The geometric preference for
square pyramidal over pseudo trigonal bipyramidal is most likely
steric in origin. Several (PDI)FeII complexes that adopt a trigonal
bipyramidal geometry have been reported,18 but those that
incorporate the bulky 2,6-diisopropyl ligand seem to favor
square pyramidal coordination.16 In this present case, placing
two dinitrogen ligands axially would produce unfavorable steric
interactions with the aryl substituents perpendicular to the plane
of the PDI ligand.
Both the bond distances and vibrational stretching frequencies
of the N2 ligands are consistent with minimal activation by the
d8 iron center. The basal and apical N2 ligands have N-N bond
distances of 1.090(2) and 1.104(3) Å, respectively, and are
indistinguishable from each other and free N2 according to the
3σ criterion. Curiously, the apical N2 ligand is significantly bent
with an Fe(1)-N(3)-N(4) angle equal to 171.81(17)°. For
comparison, the basal N2 ligand is more linear with an Fe(1)-
N(1)-N(2) angle of 178.40(19)°. The observed N-N bond
distances are in good agreement with the N-N bond distance
of 1.142(7) Å reported for (depe)2Fe(N2) (depe ) diethylphos-
phinoethane)33b and the value of 1.078(30) Å found in
(Et3P)2(CO)2Fe(N2).33a
Attempts to characterize 1-(N2)2 in solution revealed interest-
ing dynamic behavior. Dissolving the molecule in either pentane
(34) Hirano, M.; Akita, M.; Morikita, T.; Kubo, H.; Fukuoka, A.; Komiya, S.
J. Chem. Soc., Dalton Trans. 1997, 3453.
(35) (a) Mond, L.; Quincke, F. J. Chem. Soc. 1891, 59, 604. (b) Berthelot, M.
C. R. Hebd. Seances Acad. Sci. 1891, 112, 1343.
(36) Battaglia, L. P.; Delledonne, D.; Nardelli, M.; Pelizzi, C.; Predieri, G.;
Chiusoli. G. P. J. Organomet. Chem. 1987, 330, 101.
(37) Cotton, F. A.; Hardcastle, K. I.; Rusholme, G. A. J. Coord. Chem. 1973,
2, 217.
(38) Cowley, A. H.; Davis, R. E.; Remadna, K. Inorg. Chem. 1981, 20, 2146.
(39) Allison, D. A.; Clardy, J.; Verkade, J. G. Inorg. Chem. 1972, 11, 2804.
(40) Howell, J. A. S.; Palin, M. G.; McArdle, P.; Cunningham, D.; Goldschmidt,
Z.; Gottlieb, H. E.; Hezroni-Langerman, D. Inorg. Chem. 1993, 32, 3493.
(41) Casey, C. P.; Whiteker, G. T.; Campana, C. F.; Powell, D. R. Inorg. Chem.
1990, 29, 3376.
The absence of significant π-back-bonding is further cor-
roborated by the observation of two strong N-N stretches
centered at 2053 and 2124 cm-1 in the solid-state (KBr pellet)
infrared spectrum. Most iron(0) dinitrogen complexes supported
by phosphine ligands have N2 stretching frequencies between
(42) Wong, W. K.; Chiu, K. W.; Wilkinson, G.; Howes, A. J.; Motevalli, M.;
Hursthouse, M. B. Polyhdedron 1985, 4, 603.
(43) Elschenbroich, C.; Nowotny, M.; Berhrendt, A.; Harms, K.; Wocadlo, S.;
Pebler, J. J. Am. Chem. Soc. 1994, 116, 6217.
(44) Brookhart, M.; Chandler, W. A.; Pfister, A. C.; Santini, C. C.; White, P.
S. Organometallics 1992, 11, 1263.
(45) Meakin, P.; Jesson, J. P. J. Am. Chem. Soc. 1973, 95, 7272.
(46) Meutterties, E. L. Acc. Chem. Res. 1970, 3, 266.
1950 and 2068 cm-1 32,33
The higher frequencies observed for
.
(47) Meakin, P.; English, A. D.; Ittel, S. D.; Jesson, J. P. J. Am. Chem. Soc.
1975, 97, 1254.
(32) (a) George, T. A.; Rose, D. J.; Chang, Y.; Chen, Q.; Zubieta, J. Inorg.
Chem. 1995, 34, 1295. (b) Cable, R. A.; Green, M.; Mackenzie, R. E.;
Timms, P. L.; Turney, T. W. Chem. Commun. 1976, 270. (c) Hills, A.;
Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J.; Rowley, A. T. J. Chem.
Soc., Dalton Trans. 1993, 3041.
(33) (a) Kandler, H.; Gauss, C.; Bidell, W.; Rosenberger, S.; Burgi, T.;
Eremenko, I. L.; Veghini, D.; Orama, O.; Burger, P.; Berke, H. Chem.s
Eur. J. 1995, 1, 541. (b) Perhuisot, P.; Jones, W. D. New J. Chem. 1994,
18, 621. (c) Komiya, S.; Akita, M.; Yoza, A.; Kasuga, N.; Fukuoka, A.;
Kai, Y. Chem. Commun. 1993, 787. (d) Berke, H.; Bankhardt, W.; Huttner,
G.; Vonseyrel, J.; Zsolnai, L. Chem. Ber. 1981, 114, 2754.
(48) Betley and Peters have recently reported an example of an S ) 1, Fe(0)
four-coordinate dinitrogen complex: Betley, T. A.; Peters, J. C. J. Am.
Chem. Soc. 2003, 125, 10782.
(49) Sacconi, L.; Orioli, P. L.; DiVaira, M. J. Am. Chem. Soc. 1965, 87, 2059.
(50) Legg, J. I.; Nielson, D. O.; Smith, D. L.; Larson, M. L. J. Am. Chem. Soc.
1968, 90, 5030.
(51) Sacconi, L.; Mani, F.; Bencini, A. In ComprehensiVe Coordination
Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.;
Pergamon: Oxford, 1987; Vol. 5.
(52) Furlani, C. Coord. Chem. ReV. 1968, 3, 141.
9
13796 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Catalytic Hydrogenation and Hydrosilation
A R T I C L E S
for two unpaired electrons. The low magnetic moment may be
due to spin-orbit coupling resulting in a quenching of the orbital
angular moment as is typically encountered in square planar
molecules.54 Despite the S ) 1 ground state, sharp, assignable
1H NMR spectra are observed (Figure 3). These spectral features
contrast those of related four-coordinate iron(II) complexes with
S ) 2 ground states where no H-H coupling is observed and
the chemical shift dispersion is over 500 ppm.16-18,24 The sharp
resonances observed for samples of 1-N2 prepared with rigorous
exclusion of N2 broaden upon exposure to dinitrogen, suggesting
a dynamic equilibrium between the four- and five-coordinate
compounds on the NMR time scale.
Synthesis and solid-state characterization of a high spin, d8
square pyramidal iron(0) complex such as 1-(N2)2 suggests that
addition of stronger field ligands may afford more familiar low
spin, diamagnetic, iron(0) derivatives. Exposure of 1-(N2)2 to 1
atm of CO resulted in substitution of the dinitrogen ligands and
produced the corresponding iron(0) dicarbonyl complex 1-(CO)2
(eq 3).
Olive green 1-(CO)2 was characterized by a combination of
elemental analysis and infrared, electronic, and multinuclear
NMR spectroscopies. The pentane solution infrared spectrum
of 1-(CO)2 exhibits two strong CO bands centered at 1914 and
1974 cm-1, demonstrating that, unlike 1-(N2)2, both ligands are
retained in solution. The infrared bands appear at lower
frequency relative to the CO stretching frequencies of 2002 and
2024 cm-1 observed for neat Fe(CO)5. This observation is
consistent with the weaker field of the iPrPDI ligand relative to
CO. As expected for a dynamic, five-coordinate Fe(0) complex,
Figure 2. In situ React-IR data in toluene demonstrating reversible N2
dissociation from 1-(N2)2.
or toluene affords a dark, olive green solution. Monitoring the
dissolution process with a Toepler pump experiment indicated
loss of 1 equiv of dinitrogen, consistent with formation of 1-N2
in solution (eq 2).
1
the H and 13C NMR spectra of 1-(CO)2 recorded in benzene-
d6 or toluene-d8 display the number of resonances expected for
a molecule with C2V symmetry. The origin of the ligand
symmetrization is likely a Berry pseudorotation46 where the
apical and basal CO ligands are equivalent by a rocking through
the plane of the iron.
Preparation of paramagnetic 1-(N2)2 and 1-N2 along with a
diamagnetic reference compound, 1-(CO)2, allows measurement
of the magnitude of the isotropic chemical shifts of the mixture
of dinitrogen compounds in solution. A detailed table of shifts
and their temperature dependencies are presented in the Sup-
porting Information. In general, the observed isotropic shifts
are consistent with a spin delocalization mechanism where the
hydrogens in the plane of the iron exhibit the most pronounced
chemical shift differences. For example, the chemical shift of
the imine methyl group for the dinitrogen complexes is centered
at 13.45 ppm at 20 °C, whereas in 1-(CO)2 this peak appears
at 2.12 ppm. Similar isotropic shifts are observed for the meta
and para pyridine peaks. In contrast, the resonances for the
orthogonal aryl groups have isotropic shifts approaching zero
(see the Supporting Information).
Further evidence for this equilibrium was also obtained by in
situ infrared spectroscopy. In toluene at 23 °C, a single band
centered at 2036 cm-1 is observed for 1-N2 (Figure 2). Cooling
the solution to -78 °C under an atmosphere of N2 produces
two additional bands centered at 2122 and 2058 cm-1, in
agreement with those observed for 1-(N2)2. This behavior is
reversible as repeated cooling and warming cycles produce the
bands assigned to 1-(N2)2 and 1-N2, respectively. It should be
noted that loss of N2 from 1-(N2)2 is faster in toluene, perhaps
owing to the greater solubility of dinitrogen in pentane.
The dynamic coordination and dissociation of dinitrogen from
1-(N2)2 was also investigated by NMR spectroscopy. At 25 °C,
a solution magnetic moment of 2.52 µB was measured by Evans
method53 for 1-N2 and is slightly lower than the value expected
(53) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(54) Gerloch, M.; Miller, J. R. Prog. Inorg. Chem. 1968, 10, 1.
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13797

A R T I C L E S
Bart et al.
Figure 3. 1H NMR spectrum of 1-(N2)2/1-N2 in benzene-d6 at 22 °C.
hydrogens. In contrast, the orthogonal aryl groups provide no
contribution to these orbitals, and as a result small isotropic
shifts are observed.
Catalytic Activity of 1-(N2)2 in Homogeneous Hydrogena-
tion. With a well-defined iron(0) complex containing labile N2
ligands in hand, the catalytic activity of 1-(N2)2 was evaluated
in carbon-hydrogen and carbon-silicon bond forming reac-
tions.55 Preliminary experiments revealed that 1-(N2)2 is an
active precatalyst for the hydrogenation of simple olefins, such
as 1-hexene and cyclohexene, to the corresponding alkanes at
ambient temperature and 1 atm of H2 (eq 4).
These results prompted a more systematic study of olefin
hydrogenation. Each catalytic reaction was conducted using a
1.25 M solution of olefin in toluene containing 0.3 mol %
1-(N2)2 with 4 atm of hydrogen at 22 °C. The reactions can
also be efficiently conducted at 1 atm of H2; 4 atm was used to
avoid potential complications in comparing relative rates at low
hydrogen pressures. The results of the catalytic hydrogenation
reactions are contained in Table 2. Terminal alkenes such as
1-hexene and styrene are hydrogenated to the corresponding
alkanes with high activity, proceeding to completion within
minutes under standard conditions. Internal and gem-disubsti-
tuted olefins are also effectively hydrogenated, although slightly
longer reaction times are required to reach complete conversion.
Figure 4. Selected molecular orbitals of 1-N2 (ADF2003.01, TZ2P, ZORA,
R-spin formalism).
The electronic structures of 1-(N2)2, 1-N2, and 1-(CO)2 were
investigated with density functional theory (ADF2003.01, TZ2P,
ZORA) and account for the spin delocalization phenomenon
and the observed isotropic shifts. Selected frontier molecular
orbitals (R-spin formalism) of 1-N2 are presented in Figure 4.
Those for 1-(N2)2 and 1-(CO)2 are contained in the Supporting
Information. For both 1-N2 and 1-(N2)2, the LUMOs and one
of the singly occupied orbitals (B) are principally the π-system
of the PDI ligand that provides a pathway for the unpaired spin
to delocalize from the iron center. Significant orbital coefficients
are observed on the para pyridine carbon as well as on CdN,
consistent with the larger isotropic shifts observed for these
(55) Although 1-(N2)2 loses 1 equiv of dinitrogen in solution, we will refer to
the precatalyst as 1-(N2)2 throughout the paper as this is the compound
that is isolated and charged into the catalytic reactions.
9
13798 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Catalytic Hydrogenation and Hydrosilation
A R T I C L E S
Table 2. Olefin Hydrogenation with 0.3 mol % 1-(N2)2
Table 3. Comparison of 1-(N2)2 with Traditional Precious Metal
Catalysts for the Hydrogenation of 1-Hexene
time
tof
time
tof
catalyst
(min)
(mol/h)
catalyst
(min)
(mol/h)
1-(N2)2
10% Pd/C
12
12
1814
366
(PPh3)3RhCl
[(COD)Ir(PCy3)py]PF6
12
12
10
75
alkynes such as trimethylsilylacetylene have not been successful,
producing a number of unidentified organic products.
The high activity of 1-(N2)2 for the catalytic hydrogenation
of olefins and alkynes prompted comparison with traditional
precious metal catalysts. Developing a meaningful, quantitative
comparison is difficult given that each catalyst operates under
a different set of optimized conditions. Each catalytic reaction
was carried out with 0.3 mol % metal complex in 4 mL of
toluene with 1-hexene under 4 atm of H2 at 22 °C for 12 min.
The results from each catalytic experiment are reported in Table
3. The catalytic hydrogenations with (Ph3P)3RhCl produced only
7% hexane along with considerable amounts of internal hexenes
arising from isomerization of the olefin. Crabtree’s [(COD)Ir-
(PCy3)py]PF659 and the ubiquitous heterogeneous catalyst 10%
Pd/C are not as effective as 1-(N2)2 under these conditions. As
stated previously, however, the precious metal catalysts are
known to operate most effectively in polar solvents.
Catalytic Activity of 1-(N2)2 in Homogeneous Hydrosila-
tion. The utility of 1-(N2)2 for catalytic hydrogenation prompted
investigation into other types of carbon-element bond forming
reactions involving insertion mechanisms. The hydrosilation of
carbon-carbon multiple bonds is a powerful, atom-economical
transformation that provides a flexible route to silanes and
alcohols after appropriate oxidation.60 While radical initiators61
and Lewis acids62 are known to promote hydrosilation, transition
metal catalysis63 has emerged as the preferred method due its
high activity, selectivity, and in some cases stereospecificity.64
Traditional hydrosilation catalysts contain precious metals such
as platinum65 or rhodium,66 although several highly active group
3 transition metal67 and lanthanide-based68 catalysts have also
been reported. In contrast, the development of well-defined
homogeneous iron catalysts for hydrosilation has been quite
limited. Iron carbonyl complexes such as Fe(CO)5 hydrosilate
simple olefins at high temperature, with significant dehydro-
genative hydrosilation accompanying saturated alkylsilane
formation.69 Since this seminal discovery, the selective dehy-
a Time required to reach >98% conversion as judged by GC. b Deter-
mined on the basis of the time required to reach completion. c The product
is (+)-p-menth-1-ene. d A 5 mol % concentration of 1-(N2)2.
Diolefins such as 1,5-hexadiene and (+)-(R)-limonene are
also rapidly hydrogenated under standard conditions. The
activity for 1,5-hexadiene is slightly reduced from 1-hexene,
requiring 60 min rather than 12 min for complete consumption
of the diolefin. Hydrogenation of (+)-(R)-limonene selectively
yields (+)-p-menth-1-ene arising from preferential hydrogena-
tion of the gem-disubstituted olefin over the trisubstituted alkene.
Functionalized olefins such as dimethyl itaconate are also
hydrogenated, although higher catalyst loadings of 5.0 mol %
are required for comparable activity. Attempts to hydrogenate
more hindered olefins such as 1-methylcyclohexene and tet-
ramethylethylene under standard conditions have been unsuc-
cessful, although conjugated dienes such as 1,3-butadiene,
hindered terminal olefins such as tert-butylethylene, and internal
olefins such as trans-2-hexene are also readily hydrogenated.
Preparative-scale olefin hydrogenation can also be carried out
in neat alkene with high activity. For example, addition of 4
atm of H2 to neat 1-hexene or cyclohexene with only 0.04 mol
% (40 ppm) 1-(N2)2 allowed quantitative isolation of hexane or
cyclohexane after 19 and 26 h, respectively. Importantly, the
product can be easily separated from the catalyst by vacuum
transfer. If nonvolatile, liquid substrates are used, the catalyst
can be simply removed by exposing the reaction mixture to air
and filtering away the oxidized iron byproduct. The high activity
of 1-(N2)2 in nonpolar solvents or pure substrates contrasts the
behavior of traditional precious-metal catalysts, which typically
require polar solvents for optimal activity.56,57 Hydrogenations
in neat alkene were studied because solvent-free reactions are
environmentally more attractive and obviate the need for
product-solvent separation.58
(59) Crabtree, R. H. Acc. Chem. Res. 1979, 12, 331.
(60) Hiyama, T.; Kusumoto, T. In ComprehensiVe Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991.
(61) Dohmaru, T.; Nagata, T.; Tsurugi, J. Chem. Lett. 1973, 1031.
(62) (a) Song, Y.-S.; Yoo, B. R.; Lee, G. H.; Jung, I. N. Organometallics 1999,
18, 3109. (b) Yamamoto, K.; Takemae, M. Synlett 1990, 259.
(63) Marciniec, B. ComprehensiVe Handbook on Hydrosilation; Pergamon:
Oxford, 1992.
Internal alkynes such as diphenylacetylene are also effectively
hydrogenated with 1-(N2)2. Monitoring the catalytic hydrogena-
1
tion reaction in situ by H NMR spectroscopy revealed initial,
selective formation of cis-stilbene followed by complete conver-
sion to bibenzyl (eq 5). Other internal alkynes such as 2-butyne
can also be hydrogenated, initially producing cis-2-butene and
ultimately yielding butane. Attempts to hydrogenate terminal
(64) Bosnich, B. Acc. Chem. Res. 1998, 31, 667.
(65) Speier, J. L. AdV. Organomet. Chem. 1977, 17, 407.
(66) Chalk, A. J. J. Organomet. Chem. 1970, 21, 207.
(67) For representative examples see: (a) Molander, G. A.; Dowdy, E. D.; Noll,
B. C. Organometallics 1998, 17, 3754. (b) Molander, G. A.; Knight, E. E.
J. Org. Chem. 1998, 63, 7009.
(68) Fu, P. F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 117,
7157.
(69) Nesmeyanov, A. N.; Freidlina, R. K.; Chukovskaya, E. C.; Petrova, R. G.;
Belyavsky, A. B. Tetrahedron 1962, 17, 61.
(56) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 3091.
(57) Betley and Peters have reported a class of rhodium complexes that maintain
their activity in nonpolar media: Betley, T. A.; Peters, J. C. Angew. Chem.,
Int. Ed. 2003, 42, 2385.
(58) Kimmich, B. F. M.; Fagan, P. J.; Hauptman, E.; Bullock, R. M. Chem.
Commun. 2004, 1014.
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13799

A R T I C L E S
Bart et al.
Table 4. Olefin Hydrosilation with Catalytic 1-(N2)2
a Time required to reach >98% conversion as judged by GC. b Deter-
mined on the basis of the time required to reach completion. c 25% of the
internal hydrosilation product was also obtained.
drogenative hydrosilation of styrene derivatives promoted by
Fe3(CO)12 has also been described.70
Reaction of 1-hexene with either PhSiH3 or Ph2SiH2 in the
presence of 0.3 mol % 1-(N2)2 produced rapid hydrosilation over
the course of minutes at 22 °C (eq 6). In both cases, the anti-
Markovnikov product was formed exclusively and the hydrosi-
lation with PhSiH3 proceeded much faster than the correspond-
ing reaction with Ph2SiH2.
Figure 5. (a) Molecular structure of 1-(PhCtCPh) (30% probability
ellipsoids). Hydrogen atoms omitted for clarity. (b) View of the core of the
molecule.
Synthesis and Characterization of Potential Catalytic
Intermediates. In an attempt to gain a better understanding of
the mechanisms of both catalytic hydrogenation and hydro-
silation, several stoichiometric reactions with 1-(N2)2 were
conducted. One observation was that the catalytic reactions
undergo a striking color change from green to red upon addition
of olefin or alkyne. Mixing 1 equiv of PhCtCPh and solid
1-(N2)2 followed by addition of pentane resulted in liberation
of 2 equiv of dinitrogen (confirmed by Toepler pump analysis)
and formation of a red solution. Recrystallization at -35 °C
allowed isolation of red crystals identified as the alkyne complex
1-(PhCtCPh) (eq 8). Significantly, independently prepared
1-(PhCtCPh) is catalytically competent for both olefin hy-
drogenation and hydrosilation with turnover frequencies identical
to those of 1-(N2)2.
On the basis of these results, the ambient temperature
hydrosilation of a series of olefins was examined using 1 equiv
of PhSiH3 in pentane solution in the presence of 0.3 mol %
1-(N2)2. The progress of each reaction was monitored by gas
chromatography and the identity of each hydrosilation product
determined by multinuclear NMR spectroscopy and by com-
parison of GC traces of authentic samples. The results of this
study are presented in Table 4. The relative rates of hydrosilation
follow the same trend as those of hydrogenation, where terminal
alkenes such as 1-hexene and styrene react most rapidly
followed by gem-disubstituted olefins and internal olefins. In
the case of trans-2-hexene, the terminally functionalized product
predominates, although significant quantities (∼25%) of internal
silanes are also obtained. Significantly, no products arising from
dehydrogenative hydrosilation were observed.
The hydrosilation of alkynes was also examined and proceeds
efficiently under mild conditions. Addition of PhSiH3 to Ph-
CtCPh produced the corresponding silylolefin in quantitative
yield (eq 7). Reaction with excess silane was not observed even
in the presence of large excesses (>5 equiv) due to steric
hindrance of the resulting silylalkene.
The solid-state structure of 1-(PhCtCPh) was determined
by single-crystal X-ray diffraction and is shown in Figure 5.
Selected bond distances and angles are presented in Table 5. In
viewing the structure of 1-(PhCtCPh), it is apparent that the
overall geometry of the molecule has changed dramatically from
that of 1-(N2)2. Both the iron atom and the carbon of the alkyne
ligand, C(11), are significantly displaced from the plane defined
(70) Kakiuchi, F.; Tanaka, Y.; Chatani, N.; Murai, S. J. Organomet. Chem. 1993,
456, 45.
9
13800 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Catalytic Hydrogenation and Hydrosilation
A R T I C L E S
Table 5. Selected Bond Distances and Angles for 1-(PhC≡CPh)
distance
angle
(Å)
(deg)
C(10)-C(11)
Fe(1)-C(10)
Fe(1)-C(11)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
N(1)-C(2)
N(3)-C(8)
1.283(6)
1.934(5)
1.952(5)
2.039(4)
1.908(4)
2.039(4)
1.326(6)
1.326(6)
C(10)-Fe(1)-C(11)
C(10)-C(11)-C(12)
C(11)-C(10)-C(18)
N(1)-Fe(1)-N(2)
N(2)-Fe(1)-N(3)
N(1)-Fe(1)-N(3)
38.55(19)
153.1(5)
147.2(5)
77.89(15)
78.06(15)
143.37(15)
deviation of Fe(1)a
deviation of C(11)a
0.587
1.794
a Deviation from the plane defined by N(1)-N(2)-N(3).
by the three nitrogens of the PDI ligand (Table 5). Applying
the Dewar-Chatt-Duncanson model for olefin coordination,71
1-(PhCtCPh) can be described either as a distorted five-
coordinate Fe(II) complex or as a pseudotetrahedral iron(0)
compound. The carbon-carbon bond length of the coordinated
alkyne is 1.283(6) Å, significantly elongated from the CtC
bond length of 1.210(3) Å reported for free PhCtCPh,72 and
is consistent with considerable π-back-bonding from the iron
center.
The molecular structure of 1-(PhCtCPh) also provides
insight into the nature of substrate-catalyst interaction. In
addition to the previously noted deviation in molecular geom-
etry, the phenyl ring attached to C(10) is oriented almost
perpendicular to the iron-iPrPDI ligand plane, most likely to
avoid unfavorable steric interactions with the isopropylaryl
groups flanking the iron atom. In contrast, the phenyl substituent
attached to C(11) lies nearly parallel to the ligand plane and is
significantly rotated with respect to the other phenyl substituent,
nestled between the two aryl rings of the PDI ligand.
Solution magnetic measurements also support the Fe(0)/
Fe(II) bonding description. The magnetic moment in benzene-
d6 at 22 °C was found to be 2.71 µB, in reasonable agreement
with the spin-only value for an S ) 1 molecule. Two unpaired
electrons are consistent with both a formally d8 tetrahedral
Fe(0) complex or an intermediate spin d6 Fe(II) compound.
Unlike 1-N2, the 1H NMR spectrum of 1-(PhCtCPh) is broad
and featureless and has not proven useful for characterization.
This behavior is most likely due to the displacement of the iron
atom from the ligand plane, reducing the overlap required for
conjugation with the pyridinimine π-system, thereby placing
the unpaired spins in iron-based d-orbitals.
Provided the activity of 1-(N2)2 for catalytic hydrogenation
and hydrosilation, the stoichiometric reactions of the iron bis-
(dinitrogen) complex with H2 and silanes were also investigated.
Stirring a pentane solution of 1-(N2)2 with excess PhSiH3
followed by recrystallization at -35 °C afforded a green
crystalline solid identified as the iron(0) bis(silane) complex
(iPrPDI)Fe(η2-SiH3Ph)2 (1-(Si)2) (eq 9). The solution magnetic
moment was determined in benzene-d6 at 22 °C and found to
Figure 6. (a) Molecular structure of 1-(Si)2 (30% probability ellipsoids).
Hydrogen atoms, except silane hydrogens, and cocrystallized pentane
molecule omitted for clarity. (b) View of the core of the molecule.
be 2.68 µB, consistent with an S ) 1 high spin, five-coordinate
Fe(0) complex. Isolated 1-(Si)2 is an effective precatalyst for
the hydrosilation of 1-hexene.
The solid-state structure of 1-(Si)2 was determined by X-ray
diffraction, and the molecular structure is shown in Figure 6.
Accompanying selected bond distances and angles are reported
in Table 6. The data were of sufficient quality such that all of
the hydrogen atoms were located and freely refined. The
geometry of the molecule is similar to that of 1-(N2)2 and is
best described as distorted square pyramidal with basal and
apical silane ligands. As with 1-(N2)2, the iron atom is nearly
coplanar with the three nitrogens of the iPrPDI ligand being
raised out of the plane by 0.260 Å. The basal silicon is raised
out of the plane by 0.209 Å, more so than the basal N2 ligand
in 1-(N2)2. This distortion is most likely a consequence of the
σ-coordination of the bulky silane as compared to linear N2.
The phenyl rings of the silane ligands adopt an orientation
similar to that observed in the alkyne adduct 1-(PhCtCPh).
The basal phenyl ring lies essentially in the pocket formed by
(71) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 3rd ed.; Prentice Hall:
Upper Saddle River, NJ, 2004; p 482.
(72) Zanin, I. E.; Antipin, M. Y.; Struchkov, Y. T. Kristallografiya 1991, 36,
411.
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13801

A R T I C L E S
Bart et al.
Table 6. Selected Bond Distances and Angles for 1-(Si)2
The resonance centered at -0.02 ppm exhibits cross-peaks with
the ligand isopropyl groups, while the resonance centered at
-7.02 ppm displays no through-space interactions with the aryl
rings. On the basis of these data, the peak at -0.02 ppm is
assigned to the apical silane, whereas the resonance at -7.02
ppm corresponds to the basal silane. The observation of Cs
ligand symmetry in combination with two distinct silane
environments clearly establishes a higher barrier for intra-
molecular dynamics than is observed for 1-(CO)2 and 1-Cl2.16
Applying the rocking mechanism suggested for 1-(CO)2, a
higher barrier for 1-(Si)2 would be anticipated due to the larger,
three-dimensional features of the silane ligands.
distance
(Å)
angle
(deg)
Fe(1)-H(1M)
Fe(1)-H(2M)
Fe(1)-Si(1)
Fe(1)-Si(2)
Si(1)-H2(M)
Si(2)-H1(M)
Si(1)-H1SA
Si(1)-H1SB
Si(2)-H2SA
Si(2)-H2SB
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
N(1)-C(1)
1.45(3)
1.51(3)
2.4733(7)
2.3266(8)
1.59(2)
1.82(3)
1.41(2)
1.39(3)
1.33(2)
Fe(1)-Si(1)-H(2M)
Fe(1)-Si(2)-H(1M)
H1SA-Si(1)-H1SB
H2SA-Si(2)-H2SB
H2M-Si(1)-C(34)
35.8(9)
38.5
111.3(14)
105.7(15)
95.8(9)
N(1)-Fe(1)-N(2)
N(2)-Fe(1)-N(3)
N(1)-Fe(1)-N(3)
N(2)-Fe(1)-Si(1)
N(2)-Fe(1)-Si(2)
79.61(9)
79.52(8)
155.62(8)
170.69(7)
80.97(6)
1.38(2)
1.9550(19)
1.8399(19)
1.9525(19)
1.334(3)
1.327(3)
1
A two-dimensional H-29Si HMBC experiment was also
conducted to correlate the η2-Si-H bonds with the appropriate
free Si-H resonances. The data clearly establish that the singlet
observed at 2.73 ppm corresponds to the uncomplexed Si-H
bonds from the apical silane at -0.02 ppm whereas the singlet
at 6.03 ppm corresponds to the basal silane resonating at -7.02
ppm. The remaining assignments for 1-(Si)2 can be found in
the Experimental Section.
N(3)-C(7)
deviation of Fe(1)a
deviation of Si(1)a
0.260
0.209
a Deviation from the plane defined by N(1)-N(2)-N(3).
the isopropylaryl groups, while the apical phenyl group is again
oriented to avoid steric interactions with the flanking isopropyl
groups and is slightly twisted from the plane defined by
Si(1)-Fe(1)-Si(2).
Additional information regarding the solution structure of
1-(Si)2 was obtained from one-dimensional 29Si NMR spec-
troscopy. The 29Si NMR spectrum was recorded at -80 °C and
exhibited two sets of resonances for the inequivalent silane
ligands (Figure 7). The first, centered at 50.23 ppm, displays
coupling to both the terminal Si-H bonds and the silicon-
hydrogen bond engaged in bonding to the iron center. From
The silicon-hydrogen distance of the basal σ-complexed
silane is slightly elongated to 1.59(2) Å in comparison to the
values of 1.39(3) and 1.41(2) Å for the free silicon hydrogens,
values in good agreement with those of free silanes.73 The apical
σ-complexed silicon-hydrogen bond shows much greater
elongation with a bond length of 1.82(3) Å in comparison to
free Si-H bond lengths of 1.38(2) and 1.33(2) Å. The longer
apical silicon-hydrogen bond may result from greater overlap
of the iron dxz and dyz orbitals with the Si-H σ*-orbital. The
iron-silicon bond lengths (Table 6), while typically not a
reliable measure of σ-complexation, are within the range
typically found in iron σ-complexes.74
The solution structure of 1-(Si)2 was elucidated by multi-
nuclear NMR spectroscopy. Unlike 1-(N2)2, both silane ligands
in 1-(Si)2 are retained in solution, demonstrating the higher
affinity of the L3Fe(0) fragment for silane σ-complexation over
coordination of N2. At 20 °C, benzene-d6 solutions of 1-(Si)2
exhibit broadened resonances that are a result of either
paramagnetism or ligand exchange dynamics that are on the
time scale of the NMR experiment.
Cooling a toluene-d8 sample to temperatures below -20 °C
produces sharp signals that display 3JHH coupling. Importantly,
the chemical shifts of these resonances do not change dramati-
cally as is observed for 1-N2, suggesting that the broadening
observed at room temperature is due to a dynamic process rather
than paramagnetism. Low temperature, static spectra of 1-(Si)2
display Cs molecular symmetry consistent with the solid-state
structure, which contains an idealized mirror plane of symmetry
bisecting the basal silane ligand. Complete assignment of each
iPrPDI ligand resonance was accomplished by gCOSY and
ROESY NMR experiments, the full details of which can be
found in the Experimental Section. Over all temperatures
studied, two resonances upfield of SiMe4 are observed at -0.02
and -7.02 ppm and assigned to the σ-complexed Si-H bonds.
A NOESY experiment, in conjunction with the ROESY data,
was used to definitively assign the apical and basal ligands.
1
this peak, JSiH coupling constants of 54 and 196 Hz were
measured for the η2-Si-H and free Si-H bonds, respectively.
The second 29Si resonance, centered at -44.58 ppm, yielded
larger coupling constants of 119 and 220 Hz for the bound and
1
free silicon hydrogens, respectively. The values of the JSiH
coupling constants are well within the range typically assigned
to silane σ-complexes74,75 and indicate that in solution 1-(Si)2
is best viewed as a high spin, d8 distorted square pyramidal
iron complex. This formulation is in agreement with both the
solid-state structure and the crystal structure observed for
1-(N2)2. Moreover, the larger 1JSiH coupling constant observed
for the basal silane ligand is consistent with the shorter Si-H
bond observed in the solid-state structure.
While several iron silane σ-complexes have been reported,
most are dinuclear.74 Thus, structural characterization of 1-(Si)2
is a rare example of a mononuclear iron bis(silane) complex.76
Monomeric phosphine complexes such as ((nBu)Ph2P)3FeH3-
77
SiMePh2 and (dppe)(CO)2Fe(H)SiR3 (dppe ) diphenylphos-
phinoethane; R ) OMe, OEt, Me)78 have been reported
previously and characterized in solution by NMR spectroscopy.
Variable-temperature NMR experiments demonstrated the flux-
ional behavior of these molecules.
The analogous reactivity of 1-(N2)2 with dihydrogen was also
explored. Exposure of a benzene-d6 solution of 1-(N2)2 to 1 atm
of H2 produced a brown solution. Analysis by 1H NMR
spectroscopy revealed complete consumption of the starting
dinitrogen complex with concomitant growth of a new C2V
(74) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175.
(75) Schubert, U. AdV. Organomet. Chem. 1990, 30, 151.
(76) Other bis(silane) complexes: (a) (Ru) Delpech, F.; Sabo-Etienne, S.;
Chaudret, B.; Daran, J.-C. J. Am. Chem. Soc. 1997, 119, 3167. (b) (Ir)
Luo, X.-L.; Crabtree, R. H. J. Am. Chem. Soc. 1989, 111, 2527.
(77) Schubert, U.; Gilbert, S.; Mock, S. Chem. Ber. 1992, 36, 2628.
(78) Gilbert, S.; Knorr, M.; Mock, S.; Schubert, U. J. Organomet. Chem. 1994,
480, 241.
(73) Kawachi, A.; Tanaka, K.; Tamao, K. Organometallics 1997, 16, 5102.
9
13802 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Catalytic Hydrogenation and Hydrosilation
A R T I C L E S
Figure 7. 29Si NMR spectrum of 1-(Si)2 at -80 °C in toluene-d8.
symmetric product with spectral features nearly identical to those
of 1-N2. On the basis of the NMR data, elemental analysis,
isotopic labeling, and Toepler pump analysis, the product is
assigned as the iron(0) dihydrogen complex 1-H2 (eq 10).
While all of the ligand resonances are observed and can be
readily assigned (Experimental Section), signals attributable to
the dihydrogen ligands have not been located. Attempts to
observe IR bands for the dihydrogen (or dideuterium) ligand
have also been unsuccessful. Addition of even trace amounts
of N2 to 1-H2 results in rapid displacement of the dihydrogen
molecule and quantitative regeneration of 1-N2. This extreme
nitrogen sensitivity makes handling and isolating pure 1-H2
challenging. Comparison of the relative stability of each iron-
(0) compound allows ordering of the ligand preferences for the
(iPrPDI)Fe(0) fragment as PhSiH3 > N2 > H2.
The solution magnetic susceptibility of 1-H2 was determined
in benzene-d6 and found to be 2.70 µB at 22 °C, consistent with
an S ) 1 ground state. This value, taken in conjunction with
the NMR data, suggests that the structure of 1-H2 is similar to
that of 1-N2, although an iron(II) dihydride cannot be definitively
excluded. Presumably the electron-withdrawing character of the
iPrPDI ligand renders the iron center sufficiently electron
deficient to inhibit oxidative addition of either H2 or silane.
Significantly, isolated samples of 1-H2 are competent for the
catalytic hydrogenation of olefins and alkynes with activities
similar to those of 1-(N2)2.
Preparation of the corresponding bis(deuterium) complex 1-D2
was accomplished by addition of 1 atm of D2 gas to 1-(N2)2.
Monitoring benzene solutions of the organometallic product by
2H NMR spectroscopy as a function of time did not result in
location of the η2-D2 ligands but was effective in demonstrating
selective deuterium incorporation into the isopropyl methyl
groups. The exchange process takes place over the course of
several days at ambient temperature and is much slower than
the time scale of a catalytic hydrogenation reaction. A proposed
mechanism that accounts for the experimental observations is
presented in the Supporting Information. Although many
pathways involving a range of intermediates are possible, we
favor loss of the η2-D2 ligands to form a five-coordinate, 16-
electron iron(II) cyclometalated hydride, in analogy to 1-Cl2.16,17
Exchange of the hydride for the deuteride by σ-bond metathesis
Figure 8. Electronic spectra of 1-N2, 1-(CO)2, 1-(Si)2, and 1-(PhCtCPh)
recorded in pentane at 22 °C.
Table 7. Absorption Maxima (nm) and Extinction Coefficients for
the Series of Fe(0) Compounds
1-N2
1-(N2)2
1-(CO)2
1-(Si)2
1-(PhC
t
CPh)
a
b
λmax
ꢀ
λmax
λmax
ꢀ
λmax
ꢀ
λmax
ꢀ
342
423
657
950
9000
6700
2000
790
458
634
883
379 3600 342 13,000
414
468
789
2800
2000
1000
452 3300 430
9900
3500
3300
790
817 1800 560
685
916
a Measured in nanometers. References: λmax
( -
iPrPDI) ) 354 nm, λmax
(2-Cl3) ) 370 nm. b Spectrum recorded at -78 °C.
or an oxidative addition-reductive elimination sequence pro-
duces isotopic exchange. Subsequent reductive elimination of
a C-D bond generates the observed product.
Electronic Spectra of (iPrPDI)Fe(0) Complexes. The elec-
tronic structure of the series of iron(0) compounds reported in
this work was investigated by UV-vis spectrophotometry. To
aid with spectral assignment, the synthesis of an iPrPDI metal
complex that would not be complicated by d-d transitions or
MLCT bands was targeted. For this reason, (iPrPDI)ScCl3 (2-
Cl3) was prepared and crystallographically characterized and
its electronic spectrum recorded (see the Supporting Informa-
tion).
Variable temperature electronic spectroscopy provided ad-
ditional evidence for the reversible dissociation and recoordi-
nation of dinitrogen from 1-(N2)2. The spectrum of 1-N2
recorded at 22 °C in pentane is presented in Figure 8 and
exhibits four bands (Table 7). Upon cooling to -78 °C, these
bands disappear at the expense of three new bands that are
assigned to the five-coordinate, bis(dinitrogen) complex 1-(N2)2.
This behavior is in agreement with the in situ solution IR data.
Band assignments, while tentative, are based on spectra recorded
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13803

A R T I C L E S
Bart et al.
for the free ligand, 2-Cl3, and literature data.17,18 For 1-N2,
charge-transfer bands centered at 342 and 423 nm are observed
along with strong intensity-stealing MLCT bands centered at
657 nm.79 A weaker d-d transition is observed at 950 nm.
Similar features are observed for 1-(CO)2 and 1-(Si)2 (Figure
8, Table 7). In these molecules, the d-d bands most likely arise
from the dipole- and spin-allowed 3B1 f 3E transition typically
observed for isoelectronic S ) 1, square pyramidal Ni(II) d8
complexes.51
hydrogenation reaction. The analogous experiment with cis-2-
butene also provided butane-d2; however, a 2:1 ratio of internal
to terminal deuterium was observed in this case. The migration
of the deuterium label from the internal to the terminal position
demonstrates the proclivity of internal olefins to participate in
alkyl migrations during catalytic hydrogenation. This behavior
is reminiscent of group 4 metallocene alkyl hydride complexes,
where internal alkyls undergo facile chain isomerization and
isotopic scrambling reactions, while terminal alkyls do not.81
The electronic spectrum of 1-(PhCtCPh) (Figure 8) reflects
the structural difference of the molecule as compared to 1-(CO)2
or 1-(Si)2. Recall that the iron atom in 1-(PhCtCPh) is
displaced significantly from the basal plane, resulting in a
pseduotetrahedral rather than a square pyramidal iron(0) center.
The intraligand-charge-transfer band is red shifted significantly
and has a much lower extinction coefficient than that of the
square pyramidal compounds. Likewise, weaker intensity MLCT
bands are also observed at 468 and 789 nm. No bands assignable
to d-d transitions were located.
Isotopic Labeling Studies and Mechanistic Insights. A
series of isotopic labeling studies was also performed with the
intent of providing additional mechanistic insight into iron-
catalyzed hydrogenation and hydrosilation. The stereochemistry
of initial hydrogen addition was established by addition of D2
to an olefin, where isotopic scrambling is prohibited by restricted
â-hydrogen elimination. Deuteriolysis of norbornene in the
presence of catalytic 1-(N2)2 afforded exo,exo-2,3-d2-norbornane
exclusively, consistent with syn addition of D2 (H2) (eq 11).80
This result is also in agreement with the observation of cis-
stilbene as the sole organic intermediate in the hydrogenation
of diphenylacetylene to bibenzyl.
As reported in an earlier section, treatment of 1-(N2)2 with
D2 gas resulted in isotopic exchange of the isopropyl methyl
substituents arising from reversible cyclometalation. To deter-
mine whether this process is competitive with olefin hydrogena-
tion, the catalytic deuteriolysis of 1-hexene was performed. After
the reaction was complete, the hexane-d2 was removed and the
catalyst solution was treated with 1 atm of CO to generate
2
diamagnetic 1-(CO)2. Analysis of this compound by H NMR
spectroscopy revealed no deuterium incorporation into the
ligand, demonstrating that cyclometalation is not competitive
with terminal olefin hydrogenation.
Addition of excess olefin to 1-(N2)2 without dihydrogen or
silane produced red solutions, most likely due to the formation
of olefin complexes in analogy to isolated 1-(PhCtCPh).
Allowing solutions containing 1-butene to stand at ambient
temperature in the presence of catalytic 1-(N2)2 resulted in net
alkene isomerization to yield an equilibrium mixture of cis- and
trans-2-butene. Similar results were also obtained for 1-hexene,
where a mixture of 2- and 3-hexenes is observed (eq 13).
Quantitation of the internal hexenes was not possible due to
1
overlapping resonances in the H NMR spectrum.
The addition of D2 to alkenes that could participate in facile
â-hydrogen elimination and subsequent “chain running” pro-
cesses was also investigated. Deuteriolysis of 1-butene with 5
mol % 1-(N2)2 rapidly afforded butane-d2 in quantitative yield
(eq 12).
Samples of pure internal olefins such as cis- and trans-2-
butene isomerize in the presence of catalytic amounts 1-(N2)2
to an equilibrium mixture of 2-butenes (eq 14).
It is important to note that the rate of terminal olefin isomer-
ization is much slower than that of catalytic hydrogenation or
hydrosilation. Internal olefins are not observed during the course
of the catalytic reaction. While a detailed mechanistic investiga-
tion of olefin isomerization promoted by 1-(N2)2 is currently
under investigation, we favor an allyl hydride intermediate
originally proposed by Casey and Cyr for similar reactions
promoted by Fe(CO)5.82
2
Analysis of the product by H NMR spectroscopy revealed a
1:1 ratio of the isotopic label in the terminal and internal
positions of the butane-d2, indicating that terminal olefins do
not undergo competitive chain running processes. This result
also demonstrates that the olefin inserts solely in a 1,2-fashion
and that isomerization of the iron n-alkyl hydride to the sec-
alkyl hydride is not competitive on the time scale of the catalytic
A mechanism consistent with the experimental data collected
is presented in Figure 9. Loss of 2 equiv of N2 from 1-(N2)2 (or
alternatively 1 equiv from 1-N2) affords the active L3Fe(0)
fragment. The next step, olefin coordination, is preferred over
oxidative addition of H2 given the extreme nitrogen sensitivity
of 1-H2 and observation and crystallographic characterization
(79) Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy: An
Introduction to Vibrational and Electronic Spectroscopy; Dover: New York,
1978; Chapter 5.
(81) Chirik, P. J.; Day, M. D.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1999, 121, 10308.
(82) Casey, C. P.; Cyr, C. R. J. Am. Chem. Soc. 1973, 95, 2248.
(80) Marchand, A. P.; Marchand, N. W. Tetrahedron Lett. 1971, 18, 1365.
9
13804 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Catalytic Hydrogenation and Hydrosilation
A R T I C L E S
Solvents for air- and moisture-sensitive manipulations were initially
dried and deoxygenated using literature procedures.83 Argon and
hydrogen gas were purchased from Airgas Inc. and passed through a
column containing manganese oxide supported on vermiculite and 4
Å molecular sieves before admission to the high-vacuum line. Benzene-
d6 and toluene-d8 were purchased from Cambridge Isotope Laboratories,
distilled from sodium metal under an atmosphere of argon, and stored
over 4 Å molecular sieves or sodium metal. 1-Hexene, cyclohexene,
1-methylcyclohexene, styrene, phenylsilane, and diphenylsilane were
purchased from Acros, dried over LiAlH4 and either vacuum transferred
or distilled before use. Norbornene was purchased from Acros and dried
over LiAlH4 by melting at 60 °C in an evacuated vessel and stirring
for 24 h. The olefin was separated from the drying agent by vacuum
transfer. (+)-(R)-Limonene and R-methylstyrene were purchased from
Aldrich, dried over LiAlH4, and vacuum distilled. Diphenylacetylene
was recrystallized from pentane, dried under high vacuum for 16 h,
and then recrystallized from dry pentane in the drybox. Carbon
monoxide was purchased from Aldrich and passed through a liquid
nitrogen cooled trap immediately before use. The ligands (2,6-
CHMe2C6H3)2NdCMe)2C5H3N, 1-Cl2,16 and 1-Br2 were prepared
according to literature procedures.
1H NMR spectra were recorded on Varian Mercury 300 and Inova
400 and 500 spectrometers operating at 299.763, 399.780, and 500.62
MHz, respectively. All chemical shifts are reported relative to the peak
for SiMe4 using 1H (residual) chemical shifts of the solvent as a
Figure 9. Proposed mechanism for catalytic hydrogenation with 1-(N2)2.
1
secondary standard. For paramagnetic molecules, the H NMR data
are reported with the chemical shift followed by the peak width at half-
height in hertz or multiplicity, the integration value, and, where possible,
the peak assignment. 29Si NMR spectra were recorded on an Inova
500 spectrometer operating at 99.320 MHz, and were referenced to
that for 50% SiMe4 in toluene.
of 1-(PhCtCPh). However, this anecdotal evidence does not
definitively exclude an iron(II) dihydride in the catalytic cycle.
Following olefin coordination, oxidative addition of H2
provides a formally 18-electron olefin dihydride intermediate.
Olefin insertion and subsequent alkane reductive elimination
afford the alkane product and regenerate the L3Fe(0) fragment.
Isotopic labeling studies establish chain running in internal
olefins that accompanies hydrogenation, suggesting competitive
â-hydrogen elimination of the intermediate iron sec-alkyl
complex to yield the terminal olefin dihydride intermediate. The
reverse process, â-hydrogen elimination of the primary alkyl
to yield the internal alkyl complex, does not compete with
catalytic turnover. In the absence of dihydrogen, olefin isomer-
ization is observed and most likely proceeds through allylic
C-H bond activation.82 This reaction is also not competitive
with olefin hydrogenation as no isomerized product is observed
during the hydrogenolysis of either cis- or trans-2-butene.
Catalytic hydrosilation most likely proceeds through a mech-
anism similar to that of hydrogenation. While definitive isotopic
labeling studies have not yet been performed, we believe an
intermediate iron olefin complex undergoes oxidative addition
of silane to yield an iron(II) olefin silyl hydride. Isolation of
the iron(0) bis(silane) complex argues against direct oxidative
addition to the L3Fe(0) fragment. The observed products are
most likely a result of ligand-induced oxidative addition
followed by Chalk-Harrod or modified Chalk-Harrod insertion
and reductive elimination pathways.70,72 More detailed kinetic,
isotopic labeling, and mechanistic studies are currently under
investigation, and the results of these studies will be reported
in due course.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox and were quickly transferred to the
goniometer head of a Siemens SMART CCD area detector system
equipped with a molybdenum X-ray tube (λ ) 0.71073 Å). Preliminary
data revealed the crystal system. A hemisphere routine was used for
data collection and determination of lattice constants. The space group
was identified, and the data were processed using the Bruker SAINT
program and corrected for absorption using SADABS. The structures
were solved using direct methods (SHELXS) completed by subsequent
Fourier synthesis and refined by full-matrix least-squares procedures.
All calculations were performed with the Amsterdam Density
Functional (ADF2003.01) suite of programs.84-86 Relativistic effects
were included using the zero-order regular approximation. The Vosko,
Wilk, and Nusair (VWM) local density approximation87 and Becke’s
exchange88 and Perdew’s correlation89 (BP86) were used. The cores of
the atoms were frozen up to 1s for C, N, and O and 2p for Fe.
Uncontracted Slater-type orbitals (STOs) of triple-ú quality with two
polarizations were employed. This basis set is denoted TZ2P in the
ADF program. Each geometry optimization was carried out without
(83) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518.
(84) te Velde, G.; Bickelahaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra,
C.; Barends, E. J.; Snijders, J. G.; Zielger, T. J. Comput. Chem. 2001, 22,
931.
(85) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerend, E. J. Theor.
Chem. Acc. 1998, 99, 391.
(86) Baerends, E. J.; Autschbach, J. A.; Berces, A.; Bo, C.; Boerrigter, P. M.;
Cavallo, L.; Chong, D. P.; Deng, L.; Dickson, R. M.; Ellis, D. E.; Fan, L.;
Fischer, T. H.; Guerra Fonseca, C.; van Gisbergen, S. J. A.; Groeneveld,
J. A.; Gritsenko, O. V.; Gru¨ning, M.; Harris, F. E.; van den Hoek, P.;
Jacobsen, H.; van Kessel, G.; Koostra, F.; van Lenthe, E.; Osinga, V. P.;
Patchkovshii, S.; Philipsen, P. H. T.; Post, D.; Pye, C. C.; Ravenek, W.;
Ros, P.; Schipper, P. R. T.; Schreckenbach, G.; Snijders, J. G.; Sola, M.;
Swart, M.; Swerhone, D.; te Velde, G.; Vernooijs, P.; Versluis, L.; Visser,
O.; van Wezenbeek, E.; Wiesenekker, G.; Wolff, S. K.; Woo, T. K.; Ziegler,
T. ADF2002.03, SCM, Theoretical Chemistry, Vrije Universiteit, Amster-
dam, The Netherlands; http://www.scm.com/.
Experimental Section
General Considerations. All air- and moisture-sensitive manipula-
tions were carried out using standard vacuum line, Schlenk, and cannula
techniques or in an M. Braun inert atmosphere drybox containing an
atmosphere of purified nitrogen. The M. Braun drybox was equipped
with a cold well designed for freezing samples in liquid nitrogen.
(87) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1990, 58, 1200.
(88) Becke, A. D. Phys. ReV. 1988, A38, 2398.
(89) Perdew, J. P. Phys. ReV. 1986, B33, 8822.
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13805

A R T I C L E S
Bart et al.
symmetry constraints. The orbital pictures shown in Figure 4 were
generated using MOLEKEL90 to process the ADF results.
CHMe2), 0.75 (sept, 16 Hz, 4H, CHMe2), 1.13 (d, 8 Hz, 12H, CHMe2),
2.58 (s, ∆ν1/2 ) 20 Hz, 1H, p-Pyr), 7.68 (m, 6H, m,p-aryl), 10.11 (d,
10 Hz, 2H, p-Pyr), 13.61 (s, ∆ν1/2 ) 56 Hz, 6H, NdC(Me)). IR (toluene,
Catalytic reactions were assayed by gas chromatography by com-
parison to authentic samples. Gas chromatography was performed on
a Shimadzu GC-2010 gas chromatograph equipped with an RTX-5
capillary column (15 m) with an injector temperature of 250 °C and a
detector temperature of 250 °C. The methods used for separating
hydrogenation substrates and products are as follows: cyclohexene/
cyclohexane, 1-hexene/n-hexane, 1,5-hexadiene/n-hexane, and 1-me-
thylcyclohexene/methylcyclohexane used an oven temperature of 27
°C for 12 min, with a ramp rate of 40 °C/min to a final temperature of
150 °C for 10 min, (+)-(R)-limonene/(+)-p-menth-1-ene and nor-
bornene/norbornane used an oven temperature of 50 °C for 30 min,
and styrene/ethylbenzene, R-methylstyrene/cumene, and R-ethylstyrene/
2-phenyl-1-butene used an oven temperature of 80 °C for 2 min, with
a ramp rate of 10 °C/min to a final temperature of 110 °C for 20 min.
The methods used for separating hydrosilation substrates and products
are as follows: cyclohexene, 1-hexene, and 1,5-hexadiene from the
products used an oven temperature of 80 °C for 2 min, with a ramp
rate of 10 °C/min to a final temperature of 110 °C for 20 min, (+)-
(R)-limonene, styrene, R-methylstyrene, and diphenylacetylene from
the products used an oven temperature of 120 °C for 1 min, with a
ramp rate of 20 °C/min to a final temperature of 240 °C for 15 min.
UV-vis spectra were recorded on a Hewlett-Packard 8543E spec-
trophotometer. Absorbance spectra were collected from 300 to 1100
nm. Stock solutions were prepared by dissolving 0.005 g of the desired
compound in pentane and diluting to 25.00 mL. Subsequent solutions
for Beer’s law plots were made with 2.00, 3.00, or 4.00 mL of stock
solution and dilution of the sample to 6.00 mL. Solution infrared spectra
were recorded with an in situ IR spectrometer fitted with a 30-bounce,
silicon-tipped probe optimized for sensitivity. The spectra were acquired
in 16 scans (30 s intervals) at a gain of 1 and a resolution of 4. A
representative reaction was carried out as follows: The IR probe was
inserted through a nylon adapter and O-ring seal into a flame-dried,
cylindrical flask fitted with a magnetic stir bar and T-joint. The T-joint
was capped by a septum for injections and a nitrogen line. Following
evacuations under full vacuum and flushing with nitrogen, the flask
was charged with toluene and a background was recorded at ambient
temperature and at -78 °C. The flask was then charged with a toluene
solution of 1-N2 to make the final reaction volume 10.0 mL with
approximately 0.005 M concentration. The samples were cooled using
an acetone/dry ice bath. The reactions were recorded between 20 and
40 intervals.
23 °C): νN ) 2036 cm-1. IR (pentane): νN ) 2046 cm-1
.
2
2
Preparation of 1-(N2)2: Method 2. A 50 mL round-bottomed flask
was charged with 0.102 g (0.15 mmol) of 1-Br2 and approximately 30
mL of diethyl ether. The resulting blue suspension was cooled to -35
°C in the drybox freezer. Via microsyringe, 0.293 mL (0.293 mmol)
of a 1.0 M solution of NaBEt3H in toluene was added to the reaction
flask. After being stirred under a N2 atmosphere at ambient temperature
for 3 h, the green-purple solution was filtered through Celite, and the
volatiles were removed from the filtrate in vacuo. Recrystallization of
the resulting residue from pentane at -35 °C afforded analytically and
spectroscopically pure 1-(N2)2.
Preparation of (iPrPDI)Fe(CO)2 (1-(CO)2). A thick-walled glass
vessel was charged with 0.030 g (0.051 mmol) of 1-(N2)2, approximately
10 mL of pentane, and a stir bar. On the high-vacuum line, 1 atm of
CO was added. Upon addition, the dark green solution turned green-
red. After the solution was stirred for 1 h, the excess CO and solvent
were removed in vacuo to yield 0.026 g (88%) of an olive green solid
identified as 1-(CO)2. Anal. Calcd for C35H43N3O2Fe: C, 70.82; H,
7.30; N, 7.08. Found: C, 70.60; H, 6.94; N, 6.89. 1H NMR (benzene-
d6): δ ) 0.96 (d, 8 Hz, 12H, CHMe2), 1.40 (d, 8 Hz, 12H, CHMe2),
2.08 (s, 6H, C(Me)), 2.77 (q, 16 Hz, 4H, CHMe2), 7.12-7.23 (m,p-
aryl, p-Pyr), 7.63 (d, 10 Hz, 2H, m-Pyr). 13C NMR (benzene-d6): δ )
16.21 (C(Me)), 24.57 (CHMe2), 25.00 (CHMe2), 27.70 (CHMe2),
117.525, 121.38, 123.75, 126.62, 140.30, 145.53, 149.78, 155.68 (aryl,
Pyr, imine). IR(pentane): νCO ) 1914, 1974 cm-1
.
Toepler Pump Experiment for Characterization of 1-(N2)2. A 100
mL round-bottomed flask was charged with 0.095 g (0.16 mmol) of
1-(N2)2 and 0.030 g (0.17 mmol) of diphenylacetylene. Vacuum-
transferred pentane was collected in the flask at -78 °C. The resulting
reaction mixture was stirred, forming a red solution and liberating
dinitrogen. The free dinitrogen was collected with the aid of a Toepler
pump into a volume of 71.7 mL. After approximately 2 h, 68 Torr
(82%) of dinitrogen was collected. Similar experiments were conducted
for the solution characterization of 1-N2 and 1-H2, where 86% and 82%
of the expected amount of gas was collected. In the case of 1-H2, the
gas that was collected was burned, confirming its identity as H2.
General Hydrogenation Procedure with 1-(N2)2. A thick-walled
glass vessel was charged with 0.008 g (0.02 mmol) of 1-(N2)2,
approximately 4 mL of toluene, and a stir bar. Approximately 4.90
mmol of substrate was added to the vessel. The vessel was submerged
in liquid nitrogen and evacuated. One atmosphere of hydrogen was
admitted and the reaction mixture warmed to ambient temperature and
stirred for the appropriate time. The progress of the reaction was assayed
periodically by gas chromatography.
Preparation of (iPrPDI)Fe(N2)2 (1-(N2)2): Method 1. A 100 mL
round-bottomed flask was charged with 9.96 g (49.8 mmol) of mercury
and approximately 15 mL of pentane. The resulting slurry was chilled
to -35 °C in the drybox freezer. Sodium (0.050 g, 2.2 mmol) was cut
into small pieces and added slowly to the cold mercury slurry with
stirring. Once all of the sodium was added, the resulting amalgam was
stirred in a N2 atmosphere for 20 min to ensure complete dissolution
of the metal. A pentane slurry containing 0.300 g (0.431 mmol) of
1-Br2 was added to the flask containing the amalgam. The reaction
was stirred vigorously, forming a dark green solution and white
precipitate. After 48 h, the reaction mixture was decanted away from
the amalgam, and the resulting green solution was filtered through Celite
to remove the NaBr. Removing the solvent in vacuo yielded a brown
solid which after recrystallization from an ether/pentane (∼1:1) mixture
afforded 0.160 g (63%) of a green crystalline solid identified as 1-(N2)2.
Anal. Calcd for C33H43N7Fe: C, 66.77; H, 7.30; N, 16.52. Found: C,
66.49; H, 7.52; N, 16.44. IR (KBr): νN2 2053, 2124 cm-1. IR (toluene,
-78 °C): νN ) 2058, 2122 cm-1. IR (pentane, 23 °C): νN ) 2132,
General Hydrogenation Procedure with 1-(N2)2: Neat Olefin. A
thick-walled glass vessel was charged with 0.004 g (0.008 mmol) of
1-(N2)2 and a stir bar. Approximately 2 mL of the alkene was added
by vacuum transfer into a calibrated tube followed by a second vacuum
transfer into the thick-walled glass vessel. The vessel was submerged
in liquid nitrogen and evacuated. One atmosphere of dihydrogen was
added to the vessel at -196 °C and the reaction warmed to ambient
temperature.
General Hydrosilation Procedure. A 6 mL vial fitted with a PTFE
silicon septum was charged with 0.002 g of 1-(N2)2, 1 mL of pentane,
and a stir bar. To the resulting green solution were added 1.23 mmol
of the desired olefin and silane (PhSiH3 or Ph2SiH2). Immediately upon
the addition of olefin, the solution changed color from green to red.
The resulting reaction mixture was stirred for the indicated amount of
time (Table 4), and periodic aliquots of the reaction mixture were
quenched with water-pentane mixtures and analyzed by GC and NMR
spectroscopy. The identity of each product was compared to literature
2
2
2073 cm-1. In solution the molecule loses 1 equiv of N2 over time to
afford 1-N2. 1H NMR (benzene-d6, 22 °C): δ ) -0.31 (d, 8 Hz, 12H,
(90) Flukiger, P.; Luthi, H. P.; Portmann, S.; Weber, J. MOLEKEL 4.3,
Swiss Center for Scientific Computing, Manno, Switzerland, 2000;
http://www.cscs.ch/molekel.
9
13806 J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004

Catalytic Hydrogenation and Hydrosilation
A R T I C L E S
values.91 One product, Ph(PhSiH2)CdC(H)Ph, has not been previously
reported and has been characterized as follows: 1H NMR (benzene-
d6): δ ) 4.97 (s, 2H, SiH), 6.8-7.14 (m, 14H, aryl and CH), 7.47 (d,
2H, aryl). 13C NMR (benzene-d6): δ ) 127.09 (CH), 128.51 (CH),
128.35 (CH), 128.69 (CH), 128.80 (CH), 129.55 (CH), 130.37 (CH),
130.57 (CH), 131.95 (C), 136.43 (CH), 137.60 (C), 139.31 (C), 142.20
(C), 143.28 (CH).
Preparation of (iPrPDI)Fe(PhCtCPh) (1-(PhCtCPh)). A 20 mL
scintillation vial was charged with 0.046 g (0.078 mmol) of 1-(N2)2
dissolved in approximately 5 mL of pentane, forming a dark green
solution. Diphenylacetylene (0.014 g, 0.078 mmol) was added with
stirring. Immediately upon addition, a dark red solution formed which
was filtered through a pad of Celite to remove any particulates. The
solvent was removed in vacuo, and the resulting red solid was
recrystallized from pentane at -35 °C to afford 0.050 g (91%) of
1-(PhCtCPh). Anal. Calcd for C47H53N3Fe: C, 78.86; H, 7.46; N,
5.87. Found: C, 78.35; H, 7.12; N, 5.47. Magnetic susceptibility
(benzene-d6): µeff ) 2.71 µB. 1H NMR (benzene-d6): δ ) -1.1 (∆ν1/2
) 33), -2.6 (∆ν1/2 ) 370), -5.6 (∆ν1/2 ) 320), -18.4 (∆ν1/2 ) 970),
-34.6 (∆ν1/2 ) 188), -40.4 (∆ν1/2 ) 106).
Preparation of (iPrPDI)Fe(η2-SiH3Ph)2 (1-(Si)2). A 20 mL scintil-
lation vial was charged with 0.050 g (0.084 mmol) of 1-(N2)2 dissolved
in approximately 5 mL of pentane. With stirring, 0.018 g (0.17 mmol)
of PhSiH3 was added, forming a brown-purple solution. The reaction
mixture was concentrated and cooled to -35 °C, forming a green
solution from which a green solid was deposited. Additional material
can be isolated by taking the mother liquor and repeating the above
procedure. Anal. Calcd for C45H59N3FeSi2: C, 71.68; H, 7.89; N, 5.57.
Found: C, 71.51; H, 8.17; N, 5.49. Magnetic susceptibility (benzene-
d6): µeff ) 2.68 µB. 1H NMR (benzene-d6, 23 °C): δ ) -6.69 (s, 1H,
η2-SiH (basal)), -0.04 (5.5 Hz, 12H, CHMe2), 0.92-1.09 (40.46,
CHMe2 and others), 1.851 (30.52, 3H), 2.82 (34.17, 4H), 3.44 (18.55,
1H, p-Pyr), 3.97 (6.38, 6H), 7.04-7.18 (m, includes toluene-d8), 7.39
(13.82, 2H), 7.57 (2.10, 4H), 7.87 (25.28, 1H), 8.39 (19.86, 1H), 9.70
(3.52, 2H), 11.46 (32.23, 6H, C(Me)). 1H NMR (toluene-d8, -40 °C):
δ ) -6.80 (s, 1H, η2-SiH (basal)), 0.05 (s, 1H, η2-SiH (apical)), 0.67
(d, 6.5 Hz, 6H, CHMe2), 0.75 (d, 6.5 Hz, 6H, CHMe2), 0.81 (d, 6.5
Hz, 6H, CHMe2), 0.95 (d, 6.5 Hz, 6H, CHMe2), 1.17 (d, 6 Hz, 9H),
1.84 (s, 6H, MeCdN), 1.90 (s, 6H), 2.55 (br s, 2H, CHMe2), 2.78 (s,
2H, SiH2 (apical), 3.11 (q, 6.5 Hz, 2H, CHMe2), 6.11 (s, 2H, SiH2
(basal)), 6.51 (d, 7 Hz, 2H, o-phenyl (basal)), 6.92 (d, 7.5 Hz, 2H,
m-phenyl (basal)), 6.99 (s, 2H, p-Pyr, p-phenyl, or m,p-aryl), 7.00-
7.22 (m, includes toluene-d8, p-Pyr, p-phenyl, or m,p-aryl), 7.23 (t, 7.5
Hz, p-Pyr, p-phenyl, or m,p-aryl), 7.30 (t, 7 Hz, p-Pyr, p-phenyl, or
m,p-aryl), 7.41 (d, 6.5 Hz, 2H, m-phenyl (apical)), 7.75 (d, 7.5 Hz,
2H, m-Pyr), 8.15 (d, 7 Hz, 2H, o-phenyl (apical)). 29Si (toluene-d8, -80
°C): δ ) 50.23, -44.58 ppm.
at -78 °C, the solution was warmed to ambient temperature and 1
atm of H2 admitted. The resulting reaction mixture was stirred for
several hours and the solvent removed in vacuo, leaving a nitrogen-
sensitive brown solid identified as 1-H2. Anal. Calcd for C33H45N3Fe:
C, 73.30; H, 8.45; N, 7.83. Found: C, 73.46; H, 8.98; N, 7.60. Magnetic
1
susceptibility (benzene-d6): µeff ) 2.70 µB. H NMR (benzene-d6): δ
) -0.50 (d, 8 Hz, 12H, CHMe2), 0.54 (q, 8 Hz, 4H, CHMe2), 1.14 (d,
8 Hz, 12H, CHMe2), 1.97 (t, 7.5 Hz, 1H, p-Pyr), 7.74 (m, 6H, m,p-
aryl), 10.40 (d, 9 Hz, 2H, m-Pyr), 15.09 (20.78, 6H, C(Me)).
Conclusions
Synthesis and characterization of an unusual high spin, square
pyramidal iron(0) bis(dinitrogen) complex supported by a
pyridinediimine ligand have been accomplished. In solution, this
molecule loses 1 equiv of N2 to form a four-coordinate iron(0)
dinitrogen complex. Both N2 complexes allow access to the rich
chemistry of the L3Fe(0) fragment under mild, thermal condi-
tions. These molecules also serve as precursors for iron(0)
dihydrogen, bis(silane), and alkyne complexes. The silane and
dihydrogen σ-complexes are geometrically and electronically
similar to the dinitrogen complexes, while the alkyne adduct is
distorted toward a pseudotetrahedral geometry. All of these
molecules serve as effective precatalysts for the hydrogenation
and hydrosilation of olefins and alkynes under mild conditions.
The catalytic reactions proceed with levels of activity and
selectivity that rival those of traditional precious metal catalysts.
Isotopic labeling studies established that terminal olefins such
as 1-butene and 1-hexene do not undergo alkyl isomerization
competitive with the catalytic reactions, although olefin isomer-
ization is observed in the absence of H2 or silane. Overall, this
study has demonstrated that iron, when in the appropriate
coordination geometry and spin state, is a reasonable alternative
to toxic, precious metals in designing catalytic processes.
Acknowledgment. We thank the National Science Foundation
(CAREER award to P.J.C.) and the NIH Chemistry and Biology
Interface Training Grant at Cornell University for financial
support, Jeffrey Elich for the initial preparation and crystalliza-
tion of 2-Cl3, Wesley Bernskoetter for computational assistance,
and Ivan Keresztes for guidance with NMR spectroscopy. We
also acknowledge the Wolczanski laboratory for access to a
Toepler pump, the Collum group for assistance with React-IR
experiments, Professor Gordon Yee (Virginia Tech) for SQUID
measurements, and Professor Jonas C. Peters (Caltech) for
disclosing results prior to publication.
Preparation of (iPrPDI)Fe(η2-H2) (1-H2). A 25 mL round-bottomed
flask was charged with 0.073 g (0.123 mmol) of 1-(N2)2, and the green
solid was dissolved in approximately 10 mL of pentane. The flask was
fitted with a 180° needle valve and attached to a high-vacuum line.
After removal of the nitrogen from the drybox atmosphere by evacuation
Supporting Information Available: Additional NMR, com-
putational, UV-vis, and magnetic data and experimental details
for 2-Cl3 (PDF) and crystallographic data for 1-(N2)2, 1-(Ph-
CtCPh), 1-(Si)2, and 2-Cl3 including full atom-labeling
schemes and bond distances and angles (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(91) (a) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 117,
7157. (b) Molander, G. A.; Julius, M. J. Org. Chem. 1992, 57, 6347. (c)
Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 13640.
JA046753T
9
J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13807