Synthesis and characterization of iron(ll) complexes with tetradentate diiminodiphosphine or diaminodiphosphine ligands as precatalysts for the hydrogenation of acetophenone

Article abstract of DOI:10.1021/ic801518h

Six complexes of the type trans-[Fe(NCMe)₂(P-N-N-P)]X ₂(X = BF₄^-, B{Ar^t}₄ ^- (Ar^t = 3, 5-(CF₃)₂C ₆H₃) containing diiminodiph

Full text of DOI:10.1021/ic801518h

Inorg. Chem. 2009, 48, 735-743
Synthesis and Characterization of Iron(II) Complexes with Tetradentate
Diiminodiphosphine or Diaminodiphosphine Ligands as Precatalysts for
the Hydrogenation of Acetophenone
Christine Sui-Seng, F. Nipa Haque, Alen Hadzovic, Anna-Maria Pu¨tz, Valerie Reuss, Nils Meyer,
Alan J. Lough, Marco Zimmer-De Iuliis, and Robert H. Morris*
Department of Chemistry, UniVersity of Toronto, 80 St. George St., Toronto,
Ontario M5S 3H6, Canada
Received August 9, 2008
Six complexes of the type trans-[Fe(NCMe)2(P-N-N-P)]X2 (X ) BF4-, B{Arf}4-) (Arf ) 3,5-(CF3)2C6H3) containing
diiminodiphosphine ligands and the complexes trans-[Fe(NCMe)2(P-NH-NH-P)][BF4]2 with a diaminodiphosphine
ligand were obtained by the reaction of Fe(II) salts with achiral and chiral P-N-N-P or P-NH-NH-P ligands,
respectively, in acetonitrile at ambient temperature. The P-N-N-P ligands are derived from reaction of ortho-
diphenylphosphinobenzaldehyde with the diamines 1,2-ethylenediamine, 1,3-propylenediamine, (S,S)-1,2-disopropyl-
1,2-diaminoethane, and (R,R)-1,2-diphenyl-1,2-diaminoethane. Some complexes could also be obtained for the
first time in a one-pot template synthesis under mild reaction conditions. Single crystal X-ray diffraction studies of
the complexes revealed a trans distorted octahedral structure around the iron. The iPr or Ph substituents on the
diamine were found to be axial in the five-membered Fe-N-CHR-CHR-N- ring of the chiral P-N-N-P
ligands. A steric clash between the imine hydrogen and the substituent probably determines this stereochemistry.
The diaminodiphosphine complex has longer Fe-N and Fe-P bonds than the analogous diiminodiphosphine
complex. The new iron compounds were used as precatalysts for the hydrogenation of acetophenone. The complexes
without axial substituents on the diamine had moderate catalytic activity while that with axial Ph substituents had
low activity but fair (61%) enantioselectivity for the asymmetric hydrogenation of acetophenone. The fact that the
diaminodiphosphine complex has a slightly higher activity than the corresponding diiminodiphosphine complex suggests
that hydrogenation of the imine groups in the P-N-N-P ligand may be important for catalyst activation. Evidence
is provided, including the first density-functional theory calculations on iron-catalyzed outer-sphere ketone
hydrogenation, that the mechanism is similar to that of ruthenium analogues.
Introduction
for olefin and aryl azide hydrogenation4-6 and the achiral
iron catalyst recently reported by Casey et al. for ketone and
imine hydrogenation.7 There are also reports of iron-
catalyzed hydrogenation of alkynes8 and transfer hydrogena-
tion of ketones and olefins7-11 and enantioselective hydros-
Replacing homogeneous catalysts based on platinum group
metals with those based on first row transition metals is an
attractive challenge. Iron-based catalysts would be desirable
owing to their potentially lower cost, toxicity, and environ-
mental impact.1 Until recently the known homogeneous
hydrogenation catalysts of iron had low activity relative to
platinum metal catalysts,2,3 apart from Chirik’s iron catalysts
(4) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794–13807.
(5) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J. Organome-
tallics 2005, 24, 5518–5527.
* To whom correspondence should be addressed. E-mail: rmorris@
chem.utoronto.ca. Phone: 1-416-978-6962. Fax: 1-416-978-6962.
(1) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
3317–3321.
(6) Bart, S. C.; Lobkovsky, E.; Bill, E.; Chirik, P. J. J. Am. Chem. Soc.
2006, 128, 5302–5303.
(7) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816–5817.
(8) Bianchini, C.; Meli, A.; Peruzzini, M.; Frediani, P.; Bohanna, C.;
Esteruelas, M. A.; Oro, L. A. Organometallics 1992, 11, 138–145.
(9) Bianchini, C.; Farnetti, E.; Graziani, M.; Peruzzini, M.; Polo, A.
Organometallics 1993, 12, 3753–3761.
(2) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. ReV. 2004, 104,
6217–6254.
(3) Marko, L.; Palagyi, J. Trans. Met. Chem. 1983, 8, 207–209.
10.1021/ic801518h CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 2, 2009 735
Published on Web 11/26/2008

Sui-Seng et al.
Chart 1
to study the effect of the ligand structure on the activity and
selectivity of iron-based asymmetric hydrogenation catalysts.
Thus, the achiral tetradentate diiminodiphosphine ligands
ethP2N2 ({PPh2(2-C6H4)CH)N(CH2)2N)CH(2-C6H4)PPh2})
and prP2N2 ({PPh2(2-C6H4)CH)N(CH2)3N)CH(2-C6H4)-
PPh2}) are employed, as well as the diaminodiphosphine
ligand ethP2(NH)2 ({PPh2(2-C6H4)CH2NHCH2-}2), to probe
the need for the NH groups in ketone hydrogenation. Gao
et al. reported the synthesis of the dicationic complexes
[Fe(NCMe)2(ethP2N2)][ClO4]2 and [Fe(NCMe)2(ethP2(NH)2)]-
[ClO4]2 but did not report the crystal structures or catalytic
activity of these.19 We also use for the first time the chiral
tetradentate diiminodiphosphine ligands {(S,S)-(iPr-ethP2N2)}
((S,S)-{PPh2(2-C6H4)CH)NCH(iPr)-}2) and {(R,R)-(ph-
ilylation of ketones12 under relatively mild conditions. Gao
and co-workers reported that mixing [Et3NH][Fe3H(CO)11]
with chiral diaminodiphosphine P-NH-NH-P ligands
produced moderately active and selective catalysts for the
asymmetric transfer hydrogenation of ketones.13
Gao et al. showed previously that the ruthenium complex
trans-RuCl2{(R,R)-cyP2(NH)2)} with the diaminodiphosphine
ligand ((R,R)-{PPh2(2-C6H4)CH2NHC6H10NHCH2(2-C6-
H4)PPh2} (Chart 1) was a precatalyst for the active and
selective asymmetric transfer hydrogenation of ketones using
basic isopropanol as the source of the hydrogen.14 The
presence of the NH is important since the analogous complex
trans-RuCl2{(R,R)-cyP2N2} containing the diiminodiphos-
phine ligand was found to be much less active. Rauten-
strauch’s group and our group reported that related ruthenium
complexes with the tetradentate P-NH-NH-P ligands
20
ethP2N2)} ((R,R)-{PPh2(2-C6H4)CH)NCH(Ph)-}2 in the
synthesis of iron complexes to probe the effect of substitu-
tions at the diamine on the catalyst activity and enantiose-
lectivity.
Results and Discussion
Synthesis and Characterization. The reaction of FeCl2
or [Fe(H2O)6][BF4]2 with the neutral ligands ethP2N2, prP2N2,
(S,S)-iPr-ethP2N2, and (R,R)-Ph-ethP2N2 in acetonitrile gave
the corresponding iron complexes [Fe(NCMe)2(ethP2N2)]-
[FeCl4] (1), [Fe(NCMe)2(ethP2N2)][BF4]2 (2), [Fe(NC-
Me)2(prP2N2)][BF4]2 (4), [Fe(NCMe)2{(S,S)-(iPr-ethP2-
N2)}][BF4]2 (5), and [Fe(NCMe)2{(R,R)-(ph-ethP2N2)}][BF4]2
(6) in good yields (Scheme 1, Method A and B). We also
found that a new, in situ one-pot template synthesis of
2-(diphenylphosphino)benzaldehyde and ethylenediamine or
1,3-diaminopropane with [Fe(H2O)6][BF4]2 in the presence
of Na2SO4 produces compounds 2 and 4 in 68% and 84%
yield, respectively (Scheme 1, Method C). A different
template synthesis of related iron complexes was reported
recently.21 Further reaction of 2 with the sodium salt
NaB{Arf}4 (Arf ) 3,5-(CF3)2C6H3) produces the complex
[Fe(ethP2N2)(NCMe)2][B{Arf}4]2 (3).
ethP2(NH)2
({PPh2(2-C6H4)CH2NH(CH2)2NHCH2(2-
C6H4)PPh2}) and (R,R)-cyP2(NH)2 (Chart 1) are very active
precatalysts for the H2-hydrogenation of ketones.15-17
Motivated by these results, we were wondering whether
similar well-defined iron complexes could be used as
precatalysts for the hydrogenation of ketones. We recently
reported that the complex trans-[Fe(NCMe)2{(R,R)-cyP2N2}]-
[BF4]2 (Chart 1) containing a diiminodiphosphine ligand
((R,R)-{PPh2(2-C6H4)CH)NC6H10N)CH(2-C6H4)PPh2} was
active and somewhat enantioselective for H2 hydrogenation
of acetophenone to 1-phenylethanol with 27% e.e. while
related complexes with carbonyl or isonitrile ligands (Chart
1) were very active and moderately selective for the
asymmetric transfer hydrogenation of ketones.18
Complexes 1-6 were isolated in 64-93% yields as red-
orange solids that are air stable for a few hours (both in the
solid-state and in solution). They dissolve in CH2Cl2, MeCN,
and DMSO to give a red-orange solution, but they are poorly
soluble in CHCl3, 2-propanol and insoluble in diethyl ether,
THF, and hydrocarbons. Their spectroscopic properties are
similar to those of the perchlorate salts reported by Gao et
al.,19 displaying the characteristic singlet in the 31P{1H} NMR
spectrum at about 51-54 ppm and a singlet for the imine
The present contribution reports the syntheses and char-
acterization of related well-defined Fe complexes where the
diamine precursor to the diiminodiphosphine ligand is varied
(10) Enthaler, S.; Erre, G.; Tse, M. K.; Junge, K.; Beller, M. Tetrahedron
Lett. 2006, 47, 8095–8099.
(11) Enthaler, S.; Hagemann, B.; Erre, G.; Junge, K.; Beller, M. Chem.
Asian J. 2006, 1, 598–604.
(12) Shaikh, N. S.; Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int.
Ed. 2008, 47, 2497–2501.
1
resonance at about 8.9-9.4 ppm in the H NMR spectrum.
The structures of compounds 1, 2, 4, 5, and 6 in the solid
state were also established by X-ray crystallography (Figure
1). Bond distances and angles are shown in Table 1.
Compound 2 crystallizes with three independent molecules
(13) Chen, J. S.; Chen, L. L.; Xing, Y.; Chen, G.; Shen, W. Y.; Dong,
Z. R.; Li, Y. Y.; Gao, J. X. Acta Chim. Sin. (Huaxue Xuebao) 2004,
62, 1745.
(14) Gao, J. X.; Ikariya, T.; Noyori, R. Organometallics 1996, 15, 1087–
1089.
(15) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-Rashid, K.;
Morris, R. H. Chem.sEur. J. 2003, 9, 4954–4967.
(16) Li, T.; Churlaud, R.; Lough, A. J.; Abdur-Rashid, K.; Morris, R. H.
Organometallics 2004, 23, 6239–6247.
(19) Gao, J. X.; Wan, H. L.; Wong, W. K.; Tse, M. C.; Wong, W. T.
Polyhedron 1996, 15, 1241–1251.
(20) Gao, J.-X.; Zhang, H.; Yi, X.-D.; Xu, P.-P.; Tang, C.-L.; Wan, H.-L.;
Tsai, K.-R.; Ikariya, T. Chirality 2000, 12, 383–388.
(21) Mikhailine, A. A.; Kim, E.; Dingels, C.; Lough, A. J.; Morris, R. H.
Inorg. Chem. 2008, 47, 6587–6589.
(17) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004,
248, 2201–2237.
(18) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem.,
Int. Ed. Engl. 2008, 47, 940–943.
736 Inorganic Chemistry, Vol. 48, No. 2, 2009

Achiral and Chiral Iron Complexes Containing P-N-N-P Ligands
Figure 1. Structures of complexes 2, 4, 5, and 6. Thermal ellipsoids are shown at 30% probability. The counter-ions, hydrogen atoms and solvent molecules
are omitted for clarity. Only one of the three independent molecules of compound 2 is shown.
Scheme 1. Preparation of the Fe(II) Complexes 1-6
in the asymmetric unit, but all bond distances and angles
are in a similar range, so only one molecule is discussed.
The complexes are distorted trans octahedral. The Fe-P
distances are between those observed for the low-spin iron(II)
complex cis-[Fe(CO)(NCMe)(PEt2CH2NMeCH2PEt2)(PMe2-
CH2PMe2)][BF4]2 (Fe-P 2.30-2.38 Å)22 and those observed
in the complexes trans-[FeH(NCMe)(PEt2CH2NMeCH2-
PEt2)(PMe2CH2PMe2)][BF4] (Fe-P 2.20-2.22 Å)22 and
trans-[Fe(H2)H(dppe)2][BPh4] (Fe-P 2.23-2.25 Å)23 while
the Fe-N(3) and Fe-N(4) distances to the MeCN ligands
Inorganic Chemistry, Vol. 48, No. 2, 2009 737

Sui-Seng et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 1, 2, 4, 5, 6, and 7a
1
2a
4
5
6
7a
Fe1-N1
2.013(4)
2.011(3)
1.929(3)
1.928(4)
2.2879(13)
2.2621(11)
174.88(15)
82.73(13)
98.83(4)
2.044(8)
1.986(2)
1.903(9)
1.933(9)
2.256(2)
2.279(3)
175.2(23)
82.6(3)
2.026(3)
2.038(3)
1.911(3)
1.914(3)
2.280(1)
2.290(1)
179.39(13)
85.4(1)
2.030(40
2.018(4)
1.916(4)
2.011(3)
2.021(3)
1.910(3)
1.920(4)
2.274(2)
2.268(2)
178.56(15)
81.62(13)
102.84(5)
86.67(10)
2.068(4)
2.087(3)
1.916(4)
1.890(4)
2.3379(14)
2.3221(15)
177.67(19)
82.56(15)
102.32(5)
88.36(11)
Fe1-N2
Fe1-N3
Fe1-N4
1.920(4)
Fe1-P1
2.2825(14)
2.2715(14)
178.05(17)
80.90(16)
101.29(5)
90.03(12)
Fe1-P2
N4-Fe1-N3
N1-Fe1-N2
P2-Fe1-P1
N1-Fe1-P1
102.41(9)
87.6(2)
105.35(4)
84.97(8)
171.89(9)
a Data for one of three molecules in the unit cell. Those for the other two are very similar.
Figure 2. Space filling model of 6 and Newman Projections along the
N1-C1 axis emphasizing the close contact of an equatorial ethylene
hydrogen and an imine hydrogen (R ) CHPh, R′ ) C6H4) in conformation
Figure 3. Structure of complex 7a. Thermal ellipsoids are shown at 30%
probability. The counter-ions, the hydrogen atoms (except NH), and the
solvent molecules are omitted for clarity.
A and the steric clash of conformation B.
are similar to that of the first (the carbonyl) complex. The
main distortions from octahedral angles are due to the
geometric requirements of the P-N-N-P ligands where
the N-Fe-N angles are small (80.9 to 82.7° for the com-
plexes with 5-membered rings and 85.4(1)° for the 6-mem-
bered ring of 4) and the P-Fe-P angles are large (98.8 to
105.4°). In complexes 5 and 6, the (S,S)-substituted iso-
propyl and the (R,R)-substituted phenyl groups occupy the
axial positions of the 5-membered Fe-N-CHR-CHR-N-
ring. A reason for this might be the steric effect of the CH)N
protons. The space filling model of 6 (Figure 2) shows that
the imine protons are close but avoid the ethylene hydrogens
(the favored conformation A in the Newman Projection of
Figure 2) while they would clash with the phenyl rings on
the ethylene backbone if these were in the equatorial position
(disfavored conformation B). Of 204 crystal structures of
complexes containing the diamine NH2CHPhCHPhNH2 as
a bidentate ligand or its derivatives as bidentate or polyden-
tate ligands reported in the Cambridge Crystallographic
Database, we find that only 25 have axial phenyl substituents
on the five-membered chelate rings. Of these, four are
bidentate with bulky groups on nitrogen, one is tridentate,
and the rest are tetradentate. In almost all of these tetradentate
structures there is a steric basis for the axial orientation of
the phenyls. The complex trans-[Fe(MeCN)2{cyP2N2}]-
[BF4]2 has a less sterically demanding cyclohexyl ring fused
to the equatorial positions of the 5-membered ring.
Scheme 2. Preparation of Complex 7 As Two Isomers
(7) (Scheme 2). The 31P{1H} NMR spectra shows two
1
singlets at 38 and 46 ppm, and the H NMR exhibits two
sets of signals indicating that a mixture of two isomers in a
ratio 7a/7b 2.4:1.0 was obtained. The signals for the N-
CH2-CH2-N protons in complex 7 appear as multiplets
centered at 2.57 and 3.12 for 7a and 2.95 and 3.0 ppm for
7b. The H,H-COSY spectrum indicates that these signals
form an AA′BB′ spin system. Unambiguous assignment of
the amine protons was achieved by adding a few drops of
D2O to the NMR sample, which caused the two multiplets
observed for NH at 4.42 for 7a and 4.33 ppm for 7b to
disappear.
The structure of 7a as determined by single crystal X-ray
diffraction is shown in Figure 3. Bond distances and angles
are given in Table 1. The Fe-P and Fe-N bond distances
to the P-NH-NH-P ligand in 7a are all longer than the
corresponding ones in the P-N-N-P ligand in complexes
The reaction of the tetradentate diaminodiphosphine ligand
ethP2(NH)2 with [Fe(H2O)6][BF4]2 in acetonitrile produces
the purple complex trans-[Fe(NCMe)2(ethP2(NH)2)][BF4]2
(22) Henry, R. M.; Shoemaker, R. K.; Newell, R. H.; Jacobsen, G. M.;
DuBois, D. L.; Rakowski DuBois, M. Organometallics 2005, 24,
2481–2491.
(23) Ricci, J. S.; Koetzle, T. F.; Bautista, M. T.; Hofstede, T. M.; Morris,
R. H.; Sawyer, J. F. J. Am. Chem. Soc. 1989, 111, 8823–8827.
738 Inorganic Chemistry, Vol. 48, No. 2, 2009

Achiral and Chiral Iron Complexes Containing P-N-N-P Ligands
Scheme 3. Catalytic Hydrogenation of Acetophenone
Scheme 4. Catalytic Cycle for the Outer Sphere Hydrogenation of
Acetone Using MH(NHCH2CH2NH2)(PH3)2 (M ) Fe or Ru)
Table 2. Catalytic Hydrogenation of Acetophenonea
entry
C
S:C:B T (°C) PH2 (atm) time (h) conv. (%) e.e. (%)
1
2
3
4
5
6
7
8
9
2
2
2
3
4
5
6
7
7
225:1:15
225:1:15
225:1:15
200:1:8
225:1:15
225:1:15
225:1:15
225:1:15
225:1:15
50
20
50
50
50
50
50
20
50
15
25
25
10
25
25
25
25
25
18
18
18
14
18
24
18
18
18
5-15
5
70-95
14
80
3
4
4
<5
61
99
a S ) PhCOMe, C ) catalyst, B ) KOtBu.
1, 2, and 4-6 by 0.03-0.10 Å. Complex 7a has an
approximate C2 axis in the FeP2N2 plane. The NH groups
point to opposite sides of the plane of the P-NH-NH-P
ligand, and the backbone is skewed through this plane. The
other isomer, 7b, likely has the two NH groups on the same
side of the plane. These isomers readily interconvert in
MeCN, possibly by NH deprotonation/reprotonation reactions
as observed for related ruthenium complexes.15,16
diamine before becoming active hydrogenation catalysts.16
The complex trans-FeH2{ethP2(NH)2} would have the hy-
dride and NH groups thought to be necessary for the efficient
outer sphere transfer of hydride and proton to the ketone
that is suspected for analogous ruthenium systems.17 Both
complexes 2 and 7 display low activity at room temperature
(entries 2 and 8) with turnover frequencies (TOF) of about
1 h-1 but good reactivity at 50 °C (TOF at 99% conversion
) 12 h-1 for 7). The current systems have slightly lower
activity than that of Casey’s system (TOF 2 h-1 at 25 °C).7
Complexes 2-7 were also tested for the hydrogenation
of acetophenone to 1-phenylethanol by transfer from basic
isopropanol, but none of these complexes showed catalytic
activity.
Density-Functional Theory (DFT) Calculations. Thus
a possible mechanism for the asymmetric hydrogenation of
acetophenone using catalysts 1-6 may involve dihydride
species, the simplest of these being trans-FeH2{ethP2(NH)2}.
Owing to the large size of this dihydride, the tetradentate
ligand has been simplified to one ethylene diamine and two
PH3 ligands for the present DFT study. Such a simplification
was shown in a previous computational study on the
analogous ruthenium system to have no significant effect on
the core structures and relative energies of species.24
With the assumption that the mechanism of catalysis with
iron is similar to that of ruthenium (with the catalytic cycle
shown in Scheme 4) and that the iron species are low spin,
we can calculate their structures and energetics and compare
them to those of ruthenium.
Hydrogenation of Acetophenone. Complexes 2-7 were
tested as catalyst precursors for the hydrogenation of
acetophenone to 1-phenylethanol in basic isopropanol (Scheme
3) and the results are summarized in Table 2. Complex 2
provides a more active system at 25 atm H2 than at 15 atm
when the temperature is 50 °C (entries 3 vs 1 of Table 2).
This pressure dependence is consistent with dihydrogen
splitting being the turn-over limiting step as it is for the
RuHCl(cyP2(NH)2) catalyst system.15 The use of the
-
-
B{Arf}4 anion (complex 3) instead of the BF4 anion
allowed some activity at 10 atm H2 (entry 4) but the
advantage was not sufficient to warrant further investigation,
and a screening of different anions has not been done yet.
Complex 2 provided a system that was even somewhat active
at room temperature (entry 2). Thus the standard conditions
for iron catalysis became 25 atm H2 and 50 °C when the
substrate to catalyst ratio is 225:1 and base/catalyst ratio is
15:1. Smaller ratios of base/catalyst provided poor reactivity;
there is no hydrogenation if the base is omitted. Under the
standard conditions complexes 2, 4, and 7 constitute mod-
erately active precatalysts (Entries 3, 5, and 9) with activities
comparable to that of trans-[Fe(MeCN)2{cyP2N2}]2+.18 By
contrast the chiral complexes 5 and 6 display poor reactivity
with complex 5 providing essentially racemic 1-phenyletha-
nol while 6 shows some enantioselectivity (61% e.e.) in favor
of the S isomer. The bulky axial substituents of 6 and 7 as
noted above must block access to the iron. In all cases the
catalyst activity dies at the conversion noted in the Table 2.
The somewhat lower activity of 2 (entry 3) compared to
7 (entry 9) suggests that 2 might be converted to the same
active hydride species as 7, possibly trans-FeH2-
{ethP2(NH)2}. In this regard we have demonstrated that
diimine ligands on ruthenium undergo reduction to the
The cycle begins with the hydrido-amido species A
(Scheme 4, Figure 4) and acetone both of which have a
relative free energy of 4.3 kcal/mol versus A and free
isopropanol (Figure 5). For both iron and ruthenium, a very
similar distorted trigonal bipyramidal geometry is predicted
(24) Li, T.; Bergner, I.; Haque, F. N.; Zimmer-De Iuliis, M.; Song, D.;
Morris, R. H Organometallics, 2007, 26, 5940–5949.
Inorganic Chemistry, Vol. 48, No. 2, 2009 739

Sui-Seng et al.
Conclusion
In summary, we have prepared and characterized some
achiral and chiral iron complexes containing P-N-N-P and
P-NH-NH-P ligands in the coordination sphere. These
complexes are easily accessible by reaction of the neutral
ligands with the Fe2+ salts [Fe(H2O)6][BF4]2 or FeCl2. Thus,
the compounds [Fe(NCMe)2(ethP2N2)][FeCl4] (1), [Fe(NC-
Me)2(ethP2N2)][BF4] (2), [Fe(NCMe)2(prP2N2)][BF4]2 (4),
Fe(NCMe)2{(S,S)-(iPr-ethP2N2)}][BF4]2,(5),[Fe(NCMe)2{(R,R)-
(ph-ethP2N2)}][BF4]2 (6), and [Fe(NCMe)2(ethP2(NH)2)]-
[BF4]2 (7) were obtained. We also have shown that 2 and 4
can be prepared in a one-pot template synthesis of 2-(diphe-
nylphosphino)benzaldehyde and ethylenediamine or 1,3-
diaminopropane with the iron precursor [Fe(H2O)6][BF4]2.
We show for the first time that the diaminodiphosphine
complex 7 has longer Fe-N and Fe-P bonds than the
analogous diiminodiphosphine complex 2.
Complexes 2, 4, and 7 have good activity for catalyzing
the hydrogenation of acetophenone under the conditions
reported for the related complex trans-[Fe(NCMe)2-
{cyP2N2}]]2+. All of these complexes have hydrogen atoms
in axial positions of the diamine-derived backbone. Com-
plexes 5 and 6 with axial iPr and Ph groups, respectively,
on the Fe-N-CHR-CHR-N- rings are much less active,
presumably because access to the iron is blocked. In all cases
the catalyst systems dies after 222 turnovers for 7, 214 for
2, 180 for 4, 7 for 5, and 9 for 6.
The fact that the rate of hydrogenation increases with the
pressure of dihydrogen and the fact that NH groups in the
ligand appear to favor catalysis (with complex 7 slightly more
active than 2) point to similarities with the mechanism of
action of related ruthenium catalysts. That is, the heterolytic
splitting of dihydrogen may produce an H-Fe-N-H motif
that permits the efficient outer sphere transfer of a hydride
and a proton to the polar carbonyl group. The first DFT
calculations indicate that this is a viable mechanism for iron.
Considering the fact that almost nothing is known about
the iron-catalyzed hydrogenation of ketones, we plan to test
a number of different ligand systems to get a better
understanding of the influence of the ligand for this reaction.
Mechanistic and kinetic studies are also in progress.
Figure 4. Calculated Structures of the hydrido-amido complex A and the
trans-dihydride species C, where M ) Fe or Ru. For bond distances and
angles see the Supporting Information.
with the amido nitrogen N1, the hydride H2, and the
phosphorus P2 occupying the equatorial positions. The
M-N1 bond is short (1.84 Å for Fe, 1.94 Å for Ru), which
reflects multiple bond character consistent with a dative
p(N)fd(M) amido-metal π-bond. The negative charge
(-0.6) on the amide nitrogen is similar for both species. The
next step in the cycle is the heterolytic splitting of dihydrogen
across the M-N1 bond via transition state B* (Figure 6).
The activation barrier for this step is 20.8 kcal/mol for
iron and 20.9 kcal/mol for ruthenium. In the transition state
structure B*, the H3-H4 distance is elongated from 0.78 Å
in free dihydrogen to nearly 1 Å for both iron and ruthenium.
The APT (atomic polar tensor) charges indicate that the H-H
bond is highly polarized (H3 -0.2, H4 +0.4) in each case.
An analysis of the vibrational mode of the transition state
with frequency 1056i cm-1 for iron and 1172i cm-1 for
ruthenium reveals that the H3-H4 bond elongates and at
the same time the geometry of N1 changes from trigonal
planar to tetrahedral.
The cycle then proceeds to the trans-dihydride C, which,
along with 1 equiv of acetone, has a relative free energy of
5.3 kcal/mol for iron and 2.3 kcal/mol for ruthenium. The
dihydride C is the H2 transfer agent in this cycle as H3 and
H4 are oriented such that a oxygen of a ketone carbonyl
group will interact with the proton on N1 of the diamine
while, at the same time, the carbon of the ketone can orient
close to the hydride on the metal. Such a transition state,
D*, was in fact located, and it has a relative free energy of
15.5 kcal/mol for iron and 4.3 kcal/mol for ruthenium. The
H4-O1 and H3-C1 bond distances decrease while at the
same time the geometry of C1 changes from trigonal planar
to tetrahedral in the transition state vibrational mode with
an imaginary frequency of 161.6i for Fe (c.f. 186.6i for Ru).
The hydride on ruthenium has a greater negative charge than
on iron, and the C-O bond is more polarized in the transition
state with ruthenium than iron. Thus, electrostatics play a
large part in lowering the barrier for ruthenium compared
to iron. Overall, using either iron or ruthenium, the hydro-
genation of acetone via the proposed mechanism is thermo-
dynamically favorable by 4.3 kcal/mol.
Experimental Section
General Procedures. All preparations and manipulations were
carried out under an argon or nitrogen atmosphere using standard
Schlenk and glovebox techniques. Dry, oxygen-free solvents were
prepared by distillation from appropriate drying agents and
employed throughout. The ethylenediamine and the 1,3-diamino-
propane were distilled before use. All other commercially available
reagents were used without further purification. The synthesis of
Na[B{3,5-(CF3)2C6H3}4] (NaB{Arf}4)25,26 and the ligands ethP2N2,
ethP2(NH)2 prP2N2, and (R,R)-ph-ethP2N2 have been reported
previously.20,27 The (S,S)-1,2-bis-isopropyl-1,2-diaminoethane di-
Therefore, the calculations indicate that the splitting of
dihydrogen is the turn-over limiting step in this mechanism
for iron as it is for ruthenium, and the energetics are very
similar. On the other hand, the H+/H- transfer to the ketone
is a higher activation process for iron than ruthenium.
(25) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith,
M. D. Inorg. Chem. 2001, 40, 3810–3814.
(26) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920–3922.
(27) Jeffery, J. C.; Rauchfuss, T. B.; Tucker, P. A. Inorg. Chem. 1980, 19,
3306–3315.
740 Inorganic Chemistry, Vol. 48, No. 2, 2009

Achiral and Chiral Iron Complexes Containing P-N-N-P Ligands
Figure 5. Reaction coordinate diagram (free energies at 298 K, 1 atm) for the hydrogenation of acetone starting with A, acetone, and dihydrogen. The solid
line is the cycle with iron as the metal center, while the dotted line represents the cycle with ruthenium. The free energies reported are relative to the sum
of the free energies of A and isopropanol.
H), 1.78-1.67 (m, CH-CH3), 0.54-0.49 (m, CH3).13C{1H} NMR
(100 MHz, CDCl3): 159.3 (d, J 21, HC)N), 140.1 (d, J 19, Ar),
136.9-136.7 (m, Ar), 134.1-133.8 (m, Ar), 133.1 (s, Ar), 129.7
(s, Ar), 128.7-128.4 (m, Ar), 78.12 (s, CH-iPr), 28.45 (s, CH),
19.7, 18.4 (s, CH3). 31P{1H} NMR (121 MHz; CDCl3): -13.22
(s). Anal. Calcd for C46H46N2P2: C, 80.2; H, 6.73; N, 4.07%. Found:
C, 79.26; H, 6.84; N, 4.96%. HRMS-EI ( m/z): [M]+ calcd for
C46H46N2P2 688.8184, found 688.3248. Selected IR (KBr) 1634
cm-1 (ν CdN).
Synthesis of trans-[Fe(NCMe)2(ethP2N2)][FeCl4], (1).
Method A. FeCl2 (80 mg, 0.6 mmol) was added to a stirred MeCN
Figure 6. Calculated transition state structures of dihydrogen splitting, B*,
and the concerted transfer of dihydrogen to acetone, D*. For the bond
distances and angles see the Supporting Information.
suspension (10 mL) of ethP2N2 (200 mg, 0.3 mmol) at ambient
temperature. The resulting orange-red mixture was stirred for 30
min. The product precipitated as an orange powder and was isolated
by filtration (220 mg, 78%). Recrystallization from a CD3CN
solution in a NMR tube yielded red crystals suitable for X-ray
hydrochloride was provided by DiaminoPharm Inc. The mass
spectroscopy and elemental analyses were performed at the
University of Toronto, and all samples were handled under argon
for the EA. Varian Gemini 400 and 300 MHz spectrometers were
employed for recording 1H (400 and 300 MHz), 13C{1H} (100 and
75 MHz), and 31P{1H} (121 MHz) NMR spectra at ambient
temperature. The 1H and 13C NMR spectra were referenced to
solvent resonances, as follows: 7.26 and 77.16 ppm for CHCl3 and
CDCl3, 1.94 and 1.24 ppm for CH3CN and CD3CN). The 31P NMR
spectra were referenced to 85% H3PO4 (0 ppm). Gas chromatog-
raphy was carried out on a Perkin-Elmer Autosystem XL. All
infrared spectra were recorded on a Nicolet 550 Magna-IR
spectrometer.
1
diffraction studies and elemental analysis. H NMR (300 MHz;
CD3CN): 8.99 (br, CH)N), 7.76-6.63 (m, ArH), 6.03 (br, ArH),
4.31, 2.87 (br, CH2), 2.30 (s, CH3CN). 31P{1H} NMR (161 MHz;
CD3CN): 54.05 (s). Anal. Calcd for C44H40N4P2Cl4Fe2: C, 56.2; H,
4.29; N, 5.96%. Found: C, 56.23; H, 4.45; N, 5.88%.
Synthesis of trans-[Fe(NCMe)2(ethP2N2)][BF4], (2). Method
B. A suspension of ethP2N2 (149 mg, 0.25 mmol) in 5 mL of MeCN
was added to a solution of [Fe(H2O)6][BF4]2 (84 mg, 0.25 mmol)
in MeCN (10 mL). After stirring for 1 h, the red solution was
concentrated to 1 mL, and 10 mL of Et2O was added. A purple
powder precipitated. The powder was isolated and washed with
hexane. (200 mg, 87%). Crystals suitable for X-ray diffraction
studies were obtained from a MeCN/Et2O solution. 1H NMR (400
MHz, CDCl3): 9.46 (s, CH)N), 8.07-6.71 (m, ArH), 4.35 (s, CH2),
2.00 (s, CH3CN); 31P{1H} NMR (161 MHz, CDCl3) 54.4 (s). Anal.
Calcd for C44H40N4B2F8P2Fe: C, 57.68; H, 4.40; N, 6.12%. Found:
C, 57.16; H, 4.40; N, 5.86%.
Method C. A solution of [Fe(H2O)6][BF4]2 (100 mg, 0.29 mmol),
2-(diphenylphosphino)benzaldehyde (172 mg, 0.59 mmol), and 1,2-
diaminoethane (20 µL, 0.29 mmol) in 5 mL of MeCN was refluxed
for 1 h. The resulting red solution was concentrated to about 1
mL, and 10 mL of Et2O were added. A red powder precipitated
Synthesis of (S,S)-{PPh2(2-C6H4)CH)NCH(iPr)-}2. A solution
of (S,S)-1,2-bis-isopropyl-1,2-diaminoethane dihydrochloride (220
mg, 1.01 mmol), 2-(diphenylphosphino)benzaldehyde (579 mg, 1.99
mmol), triethylamine (406 µL, 3.03 mmol), and anhydrous Na2SO4
(834 mg, 5.87 mmol) in CHCl3 was refluxed for 24 h. An orange
suspension was obtained and evaporated to dryness. THF (10 mL)
was added, and the mixture was filtered. The resulting filtrate was
concentrated to about 1 mL and was layered with 15 mL of EtOH.
A beige powder precipitated and was isolated by filtration (619
1
mg, 90%). H NMR (300 MHz; CDCl3): 8.71 (d, J 4.8, CH)N),
8.02-7.98, 7.35-7.16, 6.91-6.78, (m, ArH), 2.99-2.96 (m, N-C
Inorganic Chemistry, Vol. 48, No. 2, 2009 741

Sui-Seng et al.
Table 3. Summary of Crystal Data, Details of Intensity Collection, and Least-Squares Refinement Parameters for Complexes 1, 2, 4, 5, 6, and 7aa
1
2
4
5
6
7a
formula
C44H40B2F8N4P2Fe2
Cl4. 3MeCN
1063.40
C44H40B2F8N4P2Fe
C45H36B2F8N4P2Fe
C50H52B2F8N4P2
Fe. MeCN
1041.42
C56H48B2F8FeN4N2
C44H44B2F8FeN4P2.
MeCN
961.30
Fw
916.21
924.19
1068.39
size, mm
lattice type
space group
T, K
0.16 × 0.16 × 0.16 0.14 × 0.16 × 0.20 0.17 × 0.26 × 0.32 0.13 × 0.16 × 0.20 0.22 × 0.22 × 0.16 0.04 × 0.12 × 0.15
tetragonal
I41/a
orthorhombic
P212121
150
monoclinic
P21/c
orthorhombic
C2221
monoclinic
C2
monoclinic
P21
373
150
150
210
150
a, Å
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V, Å3
33.049(5)
33.049(5)
19.333(4)
90
90
90
21116(6)
16
1.338
0.853
8768
9.8340(3)
32.8133(10)
38.8071(14)
90
90
90
12522.5(7)
12
1.458
0.512
5640
20.1739(10)
10.5412(2)
20.7331(10)
90
101.0440(15)
90
4327.4(3)
4
1.419
13.8857(4)
17.2819(5)
43.4423(9)
90
90
90
10424.9(5)
8
1.327
0.419
4320
22.3773(5)
11.3226(4)
20.5795(7)
90
92.895(2)
90
5207.6(3)
4
1.363
12.5831(6)
13.6522(10)
13.7334(8)
90
107.936(3)
90
2244.6(2)
2
1.422
Z
F
calc/Mg m-3
µ(Mo KR), mm-1
F(000)
0.495
1888
0.422
2200
0.480
992
range θ collected, deg
reflns collected/unique
R1 (I > 2σ(I))
2.7 to 27.5
67418/12075
0.0647
0.1242
1.049
2.6 to 25.0
50075/20664
0.0807
0.1686
1.005
2.6 to 25.0
23795/7163
0.0491
0.1131
0.996
2.6 to 27.5
23299/11696
0.0656
0.1255
0.944
2.62 to 27.5
22054/10781
0.057
0.1597
1.027
2.6 to 27.5
14391/8383
0.0552
0.0973
0.985
wR2 (all data)
goodness of fit
parameters refined
591
1648
559
636
659
577
maximum peak in final 0.738
1.534
0.555
0.540
0.393
0.368
∆F map/e Å-3
a
Definition of R indices: R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
2
2
and was isolated by filtration and washed with hexane to give the
analytically pure product (168 mg, 68%).
etonitrile yielded red crystals suitable for X-ray diffraction studies
and elemental analysis. 1H NMR (300 MHz, CDCl3): 9.39 (s,
CH)N), 8.22-8.20, 7.79-6.73 (m, ArH), 3.91 (d, J 9, N-C H),
2.00 (br, CH), 1.73 (s, CH3CN), 1.18 (d, J 6, CH3), 0.46 (d, J 6,
CH3). 31P{1H} NMR (121 MHz, CDCl3): 52.3 (s). Anal. Calcd for
C50H52N 4B2F8P2Fe1 ·1.25 CHCl3: C, 53.55; H, 4.67; N, 4.87. Found:
C, 53.56; H, 4.69; N, 5.09.
Synthesis of trans-[Fe(NCMe)2(ethP2N2)][B{Arf}4]2 (3).
NaB{Arf}4 (85 mg, 0.096 mmol) was added to a CH2Cl2 solution
(4 mL) of [Fe(MeCN)2(ethP2N2)][BF4]2 (44 mg, 0.048 mmol). After
stirring for 30 min, the resulting orange mixture was filtered through
a small pad of Celite and evaporated to dryness to give an orange
1
powder (110 mg, 93%). H NMR (400 MHz, CDCl3): 8.90 (s,
Synthesis of trans-[Fe(NCMe)2{(R,R)-(ph-ethP2N2)}][BF4]2
(6). A solution of (R,R)-(ph-ethP2N2) (510 mg, 0.78 mmol) and
[Fe(H2O)6][BF4]2 (260 mg, 0.78 mmol) in MeCN (10 mL) was
stirred for 1 h at ambient temperature. The solution was evaporated,
and the remaining red residue was washed with pentane. The
analytically pure product was obtained after crystallization from
MeCN/Et2O as dark red crystals (510 mg, 64%). Recrystallization
from a MeCN/MeOH/Et2O solution yielded crystals suitable for
CH)N), 7.70-6.57 (m, ArH), 3.75 (s, CH2), 2.00 (s, CH3CN).
31P{1H} NMR (161 MHz, CDCl3): 51.5 (s). Anal. Calcd for C108
H
64N4B2F48P2Fe : C, 52.54; H, 2.61; N, 2.27. Found: C, 52.95; H,
1
2.31; N, 2.0.
Synthesis of trans-[Fe(NCMe)2(prP2N2)][BF4]2, (4). Method
B. A solution of prP2N2 (408 mg, 0.66 mmol) in 15 mL of MeCN
was added dropwise to a solution of [Fe(H2O)6][BF4]2 (220 mg,
0.65 mmol) in MeCN (10 mL). After stirring for 3 h, the resulting
red solution was concentrated to about 1 mL, and 30 mL of hexane
were added. A red-orange powder precipitated and was isolated
1
X-ray diffraction studies. H NMR (400 MHz, CD3CN): 9.32 (s,
CH)N), 7.82-7.21 (m, ArH), 6.94 (m, ArH), 6.85 (m, Ar H), 5.97
1
(s, N-C H), 1.96 (s, CH3CN). 31P{1H} NMR (161 MHz, CDCl3):
by filtration and washed with hexane (530 mg, 87%). H NMR
(300 MHz, CD3CN): 8.99 (s, CH)N), 7.82-6.51 (m, ArH), 3.62
(br, N-CH2), 2.28 (br, CH2-CH2-CH2), 1.94 (s, CH3CN). 31P{1H}
NMR (121 MHz, CD3CN): 51.3 (s). Anal. Calcd for
C45H40N4B2F8P2Fe1: C, 58.23; H, 4.34; N, 6.04. Found: C, 57.73;
H, 3.82; N, 5.72.
Method C. A solution of [Fe(H2O)6][BF4]2 (100 mg, 0.29 mmol),
2-(diphenylphosphino)benzaldehyde (172 mg, 0.59 mmol), and 1,3-
diaminopropane (25 µL, 0.29 mmol) in 5 mL of MeCN was stirred
at ambient temperature for 1 h. The resulting red solution was
concentrated to about 1 mL, and 10 mL of Et2O were added. A
red-orange powder precipitated and was isolated by filtration and
washed with hexane (210 mg, 84%).
Synthesis of trans-[Fe(NCMe)2{(S,S)-(iPr-ethP2N2)}][BF4]2
(5). A solution of (S,S)-(eth-iPrP2N2) (239 mg, 0.35 mmol) and
[Fe(OH2)6][BF4]2 (117 mg, 0.35 mmol) in MeCN (10 mL) was
stirred for 20 min at ambient temperature.. The resulting red solution
was concentrated to 1 mL, and 10 mL of Et2O were added. A red-
orange powder precipitated and was isolated by filtration and
washed with Et2O (259 mg, 75%). Vapor diffusion of diethyl ether
into a solution of [Fe(NCMe)2{(S,S)-(iPr-ethP2N2)}][BF4]2 in ac-
51.8 (s). Anal. Calcd for C56H48N4B2F P2Fe1: C, 62.95; H, 4.53;
N, 5.24. Found: C, 62.69; H, 4.79; N, 5.81.
8
Synthesis of trans-[Fe(MeCN)2(ethP2(NH)2)][BF4]2 (7). A
suspension of ethP2(NH)2 (300 mg, 0.49 mmol) in 5 mL of MeCN
was added to a solution of [Fe(H2O)6][BF4]2 (168 mg, 0.49 mmol)
in MeCN (10 mL). After stirring for 1 h, the resulting purple
solution was concentrated to 1 mL, and 10 mL of Et2O were added.
A purple powder precipitated and was isolated by filtration (405
mg, 91%). Recrystallization of a small portion of this solid by
vapor diffusion of Et2O into
a
MeCN solution of
[Fe(NCMe)2(ethP2(NH)2)][BF4]2 under N2 yielded crystals of 7a
1
suitable for X-ray diffraction studies and elemental analysis. H
NMR (400 MHz, CD3CN): 7a+7b (ratio 2.4:1): 7.68-6.31 (m,
ArH); 7a: 4.42 (br, NH), 3.95 (br, CH2), 3.12 (AA′, NCH2CH2N),
2.57 (BB′, NCH2CH2N), 1.96 (s, CH3CN); 7b 4.37 (d, J 14, CH2),
4.33 (br t, J 10, NH), 4.05 (dd, J 14, J 10, CH2), 3.00, 2.95 (AA′BB′,
NCH2CH2N), 1.96 (s, CH3CN); 31P{1H} NMR (121 MHz, CD3CN):
46.1 (s, 7b), 38.5 ppm (s, 7a). Anal. Calcd For C44H44N4B2F8P2Fe:
C, 57.43; H, 4.82; N, 6.09%. Found: C, 57.0; H, 4.73; N, 6.38%.
742 Inorganic Chemistry, Vol. 48, No. 2, 2009

Achiral and Chiral Iron Complexes Containing P-N-N-P Ligands
General Procedure for the Iron Catalyzed Asymmetric
H2-Hydrogenation of Acetophenone. In an Ar or N2 glovebox,
the iron complex (0.008 mmol) was suspended in 2 mL of
2-propanol and acetophenone (225 equiv) in 1 mL of 2-propanol.
The solution of base was prepared by dissolution of KOtBu (15
equiv) in 2 mL of 2-propanol. The substrate solution, then the base
solution, and finally the suspension of precatalyst were injected
into a 50 cm3 Parr hydrogenator reactor at the desired pressure and
temperature, maintained by use of a Fischer Scientific Isotemp
1016D water bath. The conversion and enantiomeric excess of the
products were determined by GC using a Perkin-Elmer Autosystem
XL apparatus with a chiral column (CP Chirasil-Dex CB 25 m ×
2.5 mm), and utilizing an H2 carrier gas at a column pressure of 6
psi, an injector temperature of 250 °C, and a FID of 275 °C. The
retention times were acetophenone 5.0 min, (R)-1-phenylethanol
8.5 min, (S)-1-phenylethanol 9.1 min.
X-ray Crystal Structure Determinations. X-ray crystallo-
graphic data for 1-2 and 4-7 were collected on a Bruker-Nonius
Kappa-CCD diffractometer using monochromated Mo KR radiation
and were measured using a combination of φ scans and ω scans
with κ offsets, to fill the Ewald sphere. The data were processed
using the Denzo-SMN package. Absorption corrections were carried
out using SORTAV. The structure was solved and refined using
SHELXTL V6.1 for full-matrix least-squares refinement that was
based on F2. All H atoms were included in calculated positions
and allowed to refine in riding-motion approximation with Uiso tied
to the carrier atom. Crystallographic data for the compounds is given
in Table 3.
Linux Enterprise Edition. All calculations used the rMPW1PW9129
density functional method. Ruthenium and iron were treated with
the SDD basis set to include relativistic effects and an effective
core potential, and H, C, N, P, and O were treated with the triple-ꢁ
basis set 6-311++G** which includes diffuse functionals and
additional p-orbitals on H, as well as additional d-orbitals on C, N,
P, and O. The structures were optimized in the gas phase at 1 atm
of pressure and 298 K. Full vibrational analyses were performed
on the optimized structure to obtain values of ∆G, ∆H, and ∆S.
The QST2 or QST3 method was utilized to locate transition states.
All transition states were found to have one imaginary frequency.
The calculated APT charges are used.30 The APT charge is the
derivative of the forces on the atoms with respect to an applied
external electric field and yields atomic charges that more accurately
reflect the charge distribution rather than individual atomic charges.
Acknowledgment. NSERC Canada is thanked for a
Discovery Grant to R.H.M., the Petroleum Research Fund,
as administered by the American Chemical Society for a
Type AC grant to R.H.M., le Fonds de Recherche sur la
Nature et les Technologies Que´bec for a fellowship to C.S.S.,
and the Deutscher Akademischer Austauschdienst for a
scholarships to A.M.P. and V.R. We thank Hyunwoo Kim
and Prof. Jik Chin for a gift of (S,S)-1,2-bis-isopropyl-1,2-
diaminoethane dihydrochloride.
Supporting Information Available: Details of the DFT calcula-
tions, a complete citation for reference 28, and crystallographic data
for 1, 2, 4, 5, 6, 7a (cif). This material is available free of charge
via the Internet at http://pubs.acs.org.
Computational Details. All calculations were performed using
Gaussian03.28 The calculations were performed on a workstation
with two 2.8 GHz Opteron X2 with 4 GB of RAM and Red Hat
IC801518H
(28) Frisch, M. J. et al. Gaussian03, Rev. B.03; Gaussian, Inc.: Pittsburgh,
(29) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664–675.
(30) Cioslowski, J. J. Am. Chem. Soc. 1989, 111, 8333–8336.
PA, 2001.
Inorganic Chemistry, Vol. 48, No. 2, 2009 743