Iron complexes for the catalytic transfer hydrogenation of acetophenone: Steric and electronic effects imposed by alkyl substituents at phosphorus

Article abstract of DOI:10.1021/ic101366z

A series of iron(II) complexes, trans-[Fe(NCMe)₂(PR ₂CH₂CH=NCH₂CH₂N=CHCH ₂PR₂)][BPh₄]₂ (5, R = Cy; 7, R = iPr; 9, R = Et) were prepared via the template synthesis in one-pot involving air-stable phosphonium dimers, [cyclo-(-PR₂CH₂CH(OH)-) ₂](Br)₂ (4, R = Cy; 6, R = iPr; 8, R = Et), KOtBu, [Fe(H₂O)₆][BF₄]₂ and ethylenediamine in acetonitrile. In the synthesis of 9, a methanol/acetonitrile solvent mixture was required; otherwise an intermediate iron bis(tridentate) complex, [Fe(PEt₂CH₂CH=NCH₂CH₂NH ₂)₂]²⁺, formed as determined by electrospray ionization mass spectrometry (ESI-MS). The crude iron(II) complexes from a template synthesis with ethylenediamine or (S,S)-1,2-diphenylethylenediamine are stirred in acetone under a CO atmosphere (～2 atm) overnight to displace a NCMe ligand; however, in addition to this, bromide displaces an NCMe ligand as well to form a new class of the iron complexes trans-[Fe(CO)(Br)(PR ₂CH₂CH=NCHR′CHR′N=CHCH₂PR ₂)]⁺ (10 R = Cy, R′ = H; (S,S)-11, R = Cy, R′ = Ph; 12, R = iPr, R′ = H; (S,S)-13, R = iPr, R′ = Ph; 14, R = Et, R′ = H; (S,S)-15, R = Et, R′ = Ph). These complexes were isolated in moderate yields (55-84%) as tetraphenylborate salts. Complexes 10-15 were tested for the catalytic transfer hydrogenation of acetophenone in basic iso-propanol at 25 and 50 °C. The complexes 10-13 (where R = Cy or iPr) were inactive while the complexes 14 and (S,S)-15 (where R = Et) were active at 25 °C but had better activity at 50 °C. Complex (S,S)-15 was higher in activity than complex 14, achieving turnover frequencies as high as 4100 h ^-1, conversions of acetophenone to (R)-1-phenylethanol as high as 80% and an enantiomeric excess (e.e.) of 50% in the product. As catalysis progressed, the e.e. diminished to as low as 26%.

Full text of DOI:10.1021/ic101366z

Inorg. Chem. 2010, 49, 10057–10066 10057
DOI: 10.1021/ic101366z
Iron Complexes for the Catalytic Transfer Hydrogenation of Acetophenone:
Steric and Electronic Effects Imposed by Alkyl Substituents at Phosphorus
Paraskevi O. Lagaditis, Alan J. Lough, and Robert H. Morris*
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
Received July 8, 2010
A series of iron(II) complexes, trans-[Fe(NCMe)2(PR2CH2CHdNCH2CH2NdCHCH2PR2)][BPh4]2 (5, R = Cy; 7, R =
iPr; 9, R = Et) were prepared via the template synthesis in one-pot involving air-stable phosphonium dimers, [cyclo-
(-PR2CH2CH(OH)-)2](Br)2 (4, R = Cy; 6, R = iPr; 8, R = Et), KOtBu, [Fe(H2O)6][BF4]2 and ethylenediamine in
acetonitrile. In the synthesis of 9, a methanol/acetonitrile solvent mixture was required; otherwise an intermediate iron
bis(tridentate) complex, [Fe(PEt2CH2CHdNCH2CH2NH2)2]2þ, formed as determined by electrospray ionization
mass spectrometry (ESI-MS). The crude iron(II) complexes from a template synthesis with ethylenediamine or (S,S)-
1,2-diphenylethylenediamine are stirred in acetone under a CO atmosphere (∼2 atm) overnight to displace a NCMe
ligand; however, in addition to this, bromide displaces an NCMe ligand aþs well to form a new class of the iron
complexes trans-[Fe(CO)(Br)(PR2CH2CHdNCHR0CHR0NdCHCH2PR2)] (10 R = Cy, R0 = H; (S,S)-11, R = Cy,
R0 = Ph; 12, R = iPr, R0 = H; (S,S)-13, R = iPr, R0 = Ph; 14, R = Et, R0 = H; (S,S)-15, R = Et, R0 = Ph). These complexes
were isolated in moderate yields (55-84%) as tetraphenylborate salts. Complexes 10-15 were tested for the
catalytic transfer hydrogenation of acetophenone in basic iso-propanol at 25 and 50 °C. The complexes 10-13 (where
R = Cy or iPr) were inactive while the complexes 14 and (S,S)-15 (where R = Et) were active at 25 °C but had better
activity at 50 °C. Complex (S,S)-15 was higher in activity than complex 14, achieving turnover frequencies as high as
4100 h-1, conversions of acetophenone to (R)-1-phenylethanol as high as 80% and an enantiomeric excess (e.e.) of
50% in the product. As catalysis progressed, the e.e. diminished to as low as 26%.
Introduction
ization,2-6 oxidation,7-13 or cycloaddition.14,15 The advantages
of iron are its low toxicity, low cost, and high abundance. In
the field of cross-coupling, iron catalysts have demonstrated
their value as they have high reactivity toward certain
substrates (i.e., secondary alkyl halides) which are trouble-
some to activate for well established palladium and nickel
catalysts.16-22 Since the majority of active iron catalysts for
this transformation are generated in situ, they are not practical
for large-scale (industrial) applications such that well-defined
iron catalysts are sought after.23,24
We have entered an “iron-age of chemistry” the period in
;
which iron catalysts are being found to replace platinum metal
catalysts for organic transformations,1 such as polymer-
*To whom correspondence should be addressed. E-mail: rmorris@
chem.utoronto.ca.
(1) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
3317–3321.
(2) Gibson, V. C.; Solan, G. A. Top. Organomet. Chem. 2009, 26, 107–158.
€
€
(3) Muller, G.; Klinga, M.; Leskela, M.; Rieger, B. Z. Anorg. Allg. Chem.
2002, 628, 2839–2846.
(4) Kermagoret, A.; Tomicki, F.; Braunstein, P. Dalton Trans. 2008,
2945–2955.
(14) Lai, T. S.; Chan, F. Y.; So, P. K.; Ma, D. L.; Wong, K. Y.; Che, C. M.
(5) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120–2130.
(6) Britovsek, G. J. P.; Gibson, V. C.; Mastroianni, S.; Oakes, D. C. H.;
Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Eur. J. Inorg.
Chem. 2001, 431–437.
Dalton Trans. 2006, 4845–4851.
€
´
(15) Furstner, A.; Majima, K.; Martın, R.; Krause, H.; Kattnig, E.;
Goddard, R.; Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 1992–2004.
(16) Czaplik, W. M.; Mayer, M.; Cvengros, J.; von Wangelin, A. J.
ChemSusChem. 2009, 2, 396–417.
(17) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104,
6217–6254.
ꢁ
(7) Rose, E.; Ren, Q.-Z.; Andrioletti, B. Chem. Eur. J. 2004, 10, 224–230.
;
(8) Yeung, H. L.; Sham, K. C.; Tsang, C. S.; Lau, T. C.; Kwong, H. L.
Chem. Commun. 2008, 3801–3803.
(9) Oldenburg, P. D.; Que, J. L. Catal. Today 2006, 117, 15–21.
(10) Bruijnincx, P. C. A.; Buurmans, I. L. C.; Gosiewska, S.; Moelands,
M. A. H.; Lutz, M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M.
(18) Liu, L. X. Curr. Org. Chem. 2010, 14, 1099–1126.
(19) Bauer, E. B. Curr. Org. Chem. 2008, 12, 1341–1369.
€
(20) Sherry, B. D.; Furstner, A. Acc. Chem. Res. 2008, 41, 1500–1511.
~
Chem. Eur. J. 2008, 14, 1228–1237.
(21) Correa, A.; Mancheno, O. G.; Bolm, C. Chem. Soc. Rev. 2008, 37,
;
(11) Suzuki, K.; Oldenburg, P. D.; Que, L. Angew. Chem., Int. Ed. 2008,
47, 1887–1889.
1108–1117.
€
(22) Furstner, A. Angew. Chem., Int. Ed. 2009, 48, 1364–1367.
ꢀ
(12) Gomez, L.; Garcia-Bosch, I.; Company, A.; Benet-Buchholz, J.; Polo,
(23) Chowdhury, R. R.; Crane, A. K.; Fowler, C.; Kwong, P.; Kozak,
A.; Sala, X.; Ribas, X.; Costas, M. Angew. Chem., Int. Ed. 2009, 48, 5720–5723.
(13) Muthupandi, P.; Alamsetti, S. K.; Sekar, G. Chem. Commun. 2009,
3288–3290.
C. M. Chem. Commun. 2008, 94–96.
(24) Cahiez, G.; Habiak, V.; Duplais, C.; Moyeux, A. Angew. Chem., Int.
Ed. 2007, 46, 4364–4366.
r
2010 American Chemical Society
Published on Web 10/06/2010
pubs.acs.org/IC

10058 Inorganic Chemistry, Vol. 49, No. 21, 2010
Lagaditis et al.
Scheme 1. Synthesis of the trans-[Fe(NCMe)2(P-N-Q-N-P)][BPh4]2
Complexesa
Figure 1. Precatalyst for the asymmetric transfer hydrogenation
of ketones.
Another breakthrough organic transformation for this metal
that has garnered much attention is the asymmetric reduction
of prochiral ketones to alcohols.25-27 This is an important
transformation for it allows the synthesis of valuable enantio-
pure alcohols used for pharmaceuticals, fragrances, and flavors.
Current efficient and enantioselective catalysts used in industry
for this transformation are based on ruthenium and rhodium
complexes.28-30 Many well-defined iron catalysts have emerged
in the literature and have garnered recognition as sustainable
alternative catalysts for various modes of reduction. They in-
clude Casey et al.’s system for reduction via molecular H2,31-33
Chirik and co-workers’ catalysts for hydrosilylation,34-36 and
Beller and co-workers’37-39 for transfer hydrogenation (where
the source of hydrogen is iso-propanol or HCOOH/NEt3
solution). More recent examples of new iron catalysts for
transfer hydrogenation include studies from the groups of
Le Floch40 and Peris and Royo.41 Many more are yet to come
as we continue to discover new ligands that enhance the activ-
ity of iron to make it a catalytically competitive metal.
Our work in recent years has focused on the development
of iron catalysts specifically for asymmetric transfer hydro-
genation.26 This led to the discovery of the precatalyst (R,R)-1,
which when activated with base is the most active and enantio-
selective iron complex for asymmetric reduction of prochiral
ketones to date (Figure 1).42 Complex (R,R)-1 displayed a
high activity for the transfer of hydrogen from the iso-propanol
solvent to acetophenone with a turnover frequency (TOF) of
greater than 3600 h-1 to give (S)-1-phenylethanol in 82% e.e.
(where the catalyst/substrate ratio is 1:2000).
a (i) 2KOtBu, 1.5[Fe(H2O)6][BF4]2,diamine((R,R)-dpen for 3;enfor5, 7
and 9); solvent (MeCN/MeOH for 3, 9 or MeCN for 5 and 7); T = 25°C for
3, 5, 7 or 60°C for 9; (ii) 2.2 NaBPh4, MeOH.
Precatalyst (R,R)-1 was made in a two step procedure that
first involved the synthesis of the iron precursor trans-[Fe-
(NCMe)2(PPh2CH2CHdNCHPhCHPhNdCHCH2PPh2)]-
(BPh4)2 ((R,R)-3) via the template synthesis from a useful
phosphino-aldehyde precursor, 2, in the presence of [Fe(H2O)6]-
[BF4]2 and (R,R)-1,2-diphenylethylenediamine (dpen)
(Scheme 1).42,43 A template approach is required to synthesize
this type of ligand (abbreviated as P-N-Q-N-P) since
neither the diamine nor the phosphino-aldehyde contain any
conjugation to favor the condensation reaction.44,45 The
optimum reaction solvent for this reaction was found to be
a methanol/acetonitrile mixture; if acetonitrile alone is used,
a bis(tridentate) iron complex formed which very slowly con-
verted to the desired tetradentate iron product.43 (R,R)-3 was
isolated as a tetraphenylborate salt (BPh4-) to remove multiple
-
2-
counter-anions, BF4 and FeBr4 (the latter anion formed
in situ), then subjected to a CO atmosphere to replace a NCMe
ligand with a CO ligand to make the precatalyst (R,R)-1.
In this report, we examine the effect on the activity of
analogues to complex (R,R)-1 in the catalytic reduction of
ketonesby changing thePhsubstituents at phosphorusatoms
of (R,R)-1 to alkyl substituents. A plethora of examples exist
in the literature where the change in electronic or sterics of
substituents on the ligands (whether it be NHC’s, phosphines,
etc.) of catalysts completely changed their activity and/or
reactivity.46-56 One famous example is the Grubbs’ first
(25) Gaillard, S.; Renaud, J.-L. ChemSusChem. 2008, 1, 505–509.
(26) Morris, R. H. Chem. Soc. Rev. 2009, 2282–2291.
(27) Chen, J. S.; Chen, L. L.; Xing, Y.; Chen, G.; Shen, W. Y.; Dong,
Z. R.; Li, Y. Y.; Gao, J. X. Acta Chim. Sinica 2004, 62, 1745–1750.
(28) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97–102.
(29) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300–1308.
(30) Wang, C.; Wu, X. F.; Xiao, J. L. Chem. Asian J. 2008, 3, 1750–1770.
(31) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816–5817.
(32) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2009, 131, 2499–2507.
(33) Bullock, R. M. Angew. Chem., Int. Ed. 2007, 46, 7360–7363.
(34) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126,
13794–13807.
(43) Mikhailine, A. A.; Kim, E.; Dingels, C.; Lough, A. J.; Morris, R. H.
Inorg. Chem. 2008, 47, 6587–6589.
€
(44) Hoss, R.; Vogtle, F. Angew. Chem., Int. Ed. Engl. 1994, 33, 375–384.
(45) Busch, D. H.; Stephenson, N. A. Coord. Chem. Rev. 1990, 100, 119–154.
(46) Kermagoret, A.; Braunstein, P. Dalton Trans. 2008, 822–831.
(47) Page, M. J.; Wagler, J.; Messerle, B. A. Dalton Trans. 2009, 7029–
7038.
(48) Namorado, S.; Antunes, M. A.; Veiros, L. F.; Ascenso, J. R.; Duarte,
M. T.; Martins, A. M. Organometallics 2008, 27, 4589–4599.
(49) van der Veen, L. A.; Boele, M. D. K.; Bregman, F. R.; Kamer,
P. C. J.; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J.; Schenk, H.; Bo,
C. J. Am. Chem. Soc. 1998, 120, 11616–11626.
(35) Tondreau, A. M.; Darmon, J. M.; Wile, B. M.; Floyd, S. K.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2009, 28, 3928–3940.
(36) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Org. Lett. 2008, 10,
2789–2792.
ꢀ
(50) Zuidema, E.; Daura-Oller, E.; Carbo, J. J.; Bo, C.; van Leeuwen,
(37) Enthaler, S.; Erre, G.; Tse, M. K.; Junge, K.; Beller, M. Tetrahedron
Lett. 2006, 47, 8095–8099.
P. W. N. M. Organometallics 2007, 26, 2234–2242.
€
(51) Heinicke, J.; Kohler, M.; Peulecke, N.; Keim, W. J. Catal. 2004, 225,
(38) Enthaler, S.; Hagemann, B.; Erre, G.; Junge, K.; Beller, M. Chem.
Asian J. 2006, 1, 598–604.
(39) Enthaler, S.; Spilker, B.; Erre, G.; Junge, K.; Tse, M. K.; Beller, M.
Tetrahedron 2008, 64, 3867–3876.
16–23.
(52) Blaser, H.-U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.; Studer,
M. Adv. Synth. Catal. 2003, 345, 103–151.
(53) Klemps, C.; Payet, E.; Magna, L.; Saussine, L.; Le Goff, X. F.;
(40) Buchard, A.; Heuclin, H.; Auffrant, A.; Le Goff, X. F.; Le Floch, P.
Dalton Trans. 2009, 1659–1667.
(41) Kandepi, V. V. K. M.; Cardoso, J. M. S.; Peris, E.; Royo, B.
Organometallics 2010, 29, 2777–2782.
(42) Mikhailine, A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2009,
131, 1394–1395.
Le Floch, P. Chem. Eur. J. 2009, 15, 8259–8268.
;
€
(54) Jahr, H. C.; Nieger, M.; Dotz, K. H. Chem. Eur. J. 2005, 11, 5333–
;
5342.
(55) Xie, X.; Zhang, T. Y.; Zhang, Z. J. Org. Chem. 2006, 71, 6522–6529.
(56) Lane, D. R.; Beavers, C. M.; Olmstead, M. M.; Schore, N. E.
Organometallics 2009, 28, 6789–6797.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 10059
generation catalyst in which a balance of the electronics and
sterics of the substituents on phosphorus affected the cata-
lytic activity of the catalyst such that PCy3 proved to be the
best phosphine ligand. It was basic enough to stabilize key
intermediates in the ring-opening metathesis reactions and
yet the Cy substituents were bulky enough to prevent the
phosphine ligandfromcoordinating toostrongly.57-59 In this
study, we substitute on phosphorus atoms of the P-N-Q-
N-P ligand the alkyl moieties cyclohexyl, iso-propyl, and
ethyl, explore the catalytic activity of these iron complexes
with these new ligands for the transfer hydrogenation of
acetophenone, and discuss the effects these changes have on
catalytic activity.
solvent such as methanol is needed to convert the reaction
mixtures to the desired iron tetradentate complexes when
smaller substituents on phosphorus (Et, Ph) are present
while acetonitrile suffices with the bulkier Cy and iPr
substituents.
The new complexes 7 and 9 were characterized by
NMR spectroscopy, ESI-MS, and X-ray diffraction stud-
ies (Figure 2). Pink solutions of the complexes in CD3CN
display singlets in their 31P{1H} NMR spectra at 75.9 and
70.8 ppm, respectively. The ESI-MS spectra only show
evidence of free tetradentate ligands. The crystal struc-
tures reveal a distorted octahedral dicationic complex
with trans CH3CN ligands. The P-Fe-P angles are
wide and increase as 9 (R =Et) 108.97(3)° <7 (R= iPr)
112.05(6)° < 5 (R = Cy) 112.92(2)°. This increase in the
bond angle as the bulkiness of the substituent increases is
not surprising as it follows the trend expected on the basis
of Tolman’s cone angles.62 Even though the bond angle
widens, there is no lengthening of the Fe-P bond, as is
expected since there is no significant electronic difference
between the three alkyl substituents.63
Results and Discussion
Synthesis of trans-[Fe(NCMe)2(PR2CH2CHdN-Q-Nd
CHCH2PR2)][BPh4]2 Complexes Where R=Cy, iPr, or Et;
Q = -CH2CH2-. We first studied the generality of the
template synthesis shown in Scheme 1 when the size of the
alkyl group on phosphorus is changed. We recently repor-
ted the smooth synthesis of the complex 5 (R=Cy, R0 =H)
by the reaction of the phosphonium salt 4 (R=Cy) with
KOtBu, ethylenediamine and Fe2þ in CH3CN at room
temperature.60 The reaction of the phosphonium salt 6
(R = iPr)61 proceeded in a similar fashion to produce
complex 7 (R=iPr, R0 =H) with the tetradentate ligand.
In these cases no methanol was needed to drive the reac-
tion to completion. However the reactions likely proceed
via bis(tridentate) complex intermediates since these have
been observed for 2 (R = Ph)43 and 4 (R = Cy).60
Thus, template syntheses can be used for all of these
substitutions at phosphorus. During the study of the use
of such template reactions in the synthesis of the mono-
carbonyl precatalysts, we discovered that the intermediate
step of the precipitation of the trans-[Fe(NCMe)2(P-N-
Q-N-P)]2þ complexes by use of NaBPh4 is unnecessary.
Instead they are converted directly to monocarbonyl
precatalysts as described below. However multiple counter-
2-
anions (FeBr4 and BF4-) are carried over to the next
The synthesis using the ethyl-substituted phosphonium
salt 8 (R = Et)61 was more challenging since the bis(tri-
dentate) complex [Fe(PEt2CH2CHdNCH2CH2NH2)2]2þ
formed whether the solvent was acetonitrile or a methanol/
acetonitrile mixture. The 31P{1H} NMR spectroscopy of
the reaction mixtures revealed major and minor species
with singlets at 65.5 ppm and 70.7 ppm, respectively. We
assign the former signal to the bis(tridentate) complex
and the latter to complex 9 containing the tetradentate
ligand; the latter is similar to the chemical shift of pure 9 in
CD2Cl2 (70.5 ppm, see below). This is consistent with the
chemical shifts of 63 and 74 ppm observed for the anal-
ogous complexes [Fe(PPh2CH2CHdNCH2CH2NH2)2]2þ
and 2, respectively.43 Isolation of the reaction mixtures
via salt metathesis with sodium tetraphenylborate (NaBPh4)
revealed two masses of interest by electrospray ionization
mass spectrometry (ESI-MS): one being for mer-[Fe-
(PEt2CH2CHdNCH2CH2NH2)2]2þ (m/z = 403.2) and
the other for the free protonated P-N-Q-N-P ligand
(m/z=289.2) from decomposition in the gas phase of the
[Fe(NCMe)2(P-N-Q-N-P)]2þ complex. Refluxing the
dark red reaction mixture in acetonitrile/methanol fol-
lowed by addition of NaBPh4 to the resulting pale pink
solution gave the pure complex 9 (R=Et, R0 =H) as the
tetraphenylborate (BPh4-) salt. It is unclear why a protic
step of the synthesis and this results in the surprising for-
mation of carbonyl bromide complexes [Fe(CO)(Br)-
(P-N-Q-N-P)]þ (Scheme 2).
Acetonitrile/Carbon Monoxide Exchange Reactions.
After following the template procedure as described in
step (i) of Scheme 1, the solvent was evaporated, acetone
was added to the residue, and the mixture stirred over-
night under CO (∼2 atm). The red-pink mixtures turned
to brown-yellow, a good indication of a successful CO
1
ligand substitution reaction. The H and 31P{1H} NMR
spectra of the yellow solids obtained after the salt meta-
thesis with NaBPh4 revealed full conversion to complexes
10-15 in 55-84% overall yield. The 31P {1H} NMR spectra
in CD2Cl2 of achiral complexes 10, 12, and 14 (where Q=
-CH2CH2-) revealed a singlet while the spectra of com-
plexes (S,S)-11, (S,S)-13 and (S,S)-15 (where Q = (S,S)-
CH(Ph)CH(Ph)-) revealed two doublets. In the 1H NMR
spectra, the most distinguishable feature which further
supported evidence of the bromide trans to carbonyl geom-
etry was by inspection of the -CH2P protons (Figure 3).
These protons become diastereotopic since they are in
different environments, where one H is on the same side of
the tetradentate ligand plane as the CO ligand while the
other is on the side with the bromide ligand. The 13C{1H}
NMR spectra are consistent with the structures shown in
Scheme 2; however, the resonance for the CO ligand was
too weak to be detected.
(57) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1997,
119, 3887–3897.
(58) Grubbs, R. H. Tetrahedron 2004, 60, 7117–7140.
(59) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993,
115, 9858–9859.
IR spectra of complexes 10-15 revealed one CO stretch
and no CN stretch from coordinated acetonitrile (Table 1).
(60) Lagaditis, P. O.; Mikhailine, A. A.; Lough, A. J.; Morris, R. H.
Inorg. Chem. 2010, 49, 1094–1102.
(61) Mikhailine, A. A.; Lagaditis, P. O.; Sues, P. E.; Lough, A. J.; Morris,
R. H. J. Organomet. Chem. 2010, 695, 1824–1830.
(62) Tolman, C. A. Chem. Rev. 1977, 77, 313–348.
(63) Liu, H. Y.; Eriks, K.; Prock, A.; Giering, W. P. Organometallics 1990,
9, 1758–1766.

10060 Inorganic Chemistry, Vol. 49, No. 21, 2010
Lagaditis et al.
Figure 3. 1H NMR (CD2Cl2) spectra of complexes 5 (top) and 10
(bottom) showing the CH2N and CH2P resonances which are homotopic
in 5 but diastereotopic in 10.
Scheme 2. Synthesis of trans-[Fe(CO)(Br)(P-N-Q-N-P)][BPh4]
Complexesa
a (i) 2 KOtBu, 1.5 [Fe(H2O)6][BF4]2, ethylenediamine, or (S,S)-1,2-
diphenylethylenediamine, conditions as in Scheme 1, then evaporate;
(ii) CO (∼2 atm), acetone; (iii) 1.1 NaBPh4, MeOH.
Table 1. Infrared Data for Complexes 10-15
compound
ν(CtO), cm-1 (KBr pellet)
(R-R)-1
10
(S,S)-11
12
(S,S)-13
14
(S,S)-15
2001
1948
1945
1948
1956
1951
1951
Figure 2. ORTEP plot (thermal ellipsoids at 50% probability) of the X-ray
crystal structures of 7 (top) and 9 (bottom). The BPh4- counteranions, most
hydrogen atoms, and disordered molecules were removed for clarity (see
Supporting Information). Selected bond lengths (A) and angles (deg):
for 7, Fe(1)-N(1), 1.962(4); Fe(1)-N(3), 1.910(5); Fe(1)-P(1), 2.281(2);
Fe(1)-P(2), 2.275(2); N(1)-Fe(1)-N(2), 82.8(2); N(3)-Fe(1)-N(4),
173.4(5); P(1)-Fe(1)-P(2), 112.05(6); for 9, Fe(1)-N(1), 1.958(2);
Fe(1)-N(3), 1.921(2); Fe(1)-P(1), 2.225(8); Fe(1)-P(2), 2.215(8); N(1)-
Fe(1)-N(2), 83.37(1); N(3)-Fe(1)-N(4), 177.76(1); P(1)-Fe(1)-P(2),
108.97(3).
studies, bond lengths and angles of interest are summarized in
Table 2, and selected crystal data in Table 3. All of the
aforementioned complexes have a pseudo trans-octahedral
geometry. The most noticeable feature is the large P-Fe-P
bond angles as in the [Fe(NCMe)2(P-N-Q-N-P)]2þ
complexes (above). This is due primarily to the con-
strained geometry of the tetradentate ligands that form
three 5,5,5- membered rings with the metal ion. There is
also a steric effect from the alkyl substituents at the phos-
phorus atoms in which 10 (R=Cy) with the bulkiest substi-
tuents has the largest bond angle (112.73(3)°) followed
sequentially by (S,S)-13, 14, and (S,S)-15. There is not a
significant difference between the P-Fe-P bond angles of
the [Fe(CO)(Br)(P-N-Q-N-P)]þ and the [Fe(NCMe)2-
(P-N-Q-N-P)]2þ complexes (5 vs 10, 7 vs 13, and 9 vs
14). Hence, neither of the axial ligands has an effect on the
P-Fe-P bond angle and thus this angle is directly related
As expected, the CO stretch decreases in wavenumber as
the substituent on phosphorus increases in donicity. Thus,
the more donating Cy group causes the lowest CO wave-
numbers. The substituent on the diamine has little effect
on the electronics at the metal since the CO wavenumber
changes little, at least on going from 10 to (S,S)-11 and 14
to (S,S)-15.
Once crystals suitable for X-ray diffraction were grown,
we found there was no MeCN ligand coordinated to iron
but rather a bromide ligand (Figure 4). Complexes 10, (S,S)-
13, 14, and (S,S)-15 were characterized by X-ray diffraction

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 10061
Figure 4. ORTEP plot (thermal ellipsoids at 50% probability) of the X-ray crystal structures of 10 (top, left), (S,S)-13 (top, right), 14 (bottom, left) and
(S,S)-15 (bottom, right). The counteranions, most hydrogen atoms, and any other disordered molecules were removed for clarity (see Supporting Information).
Table 2. Selected Bond Lengths (A) and Angles (deg) for 10, (S,S)-13, 14, and
(S,S)-15
to the substituents at phosphorus as well as the geometry
of the 5,5,5-membered rings.
The preferred formation of the complexes with CO
trans to Br rather than to acetonitrile is explained by the
fact that Br is an anionic σ- and π-donor ligand while
acetonitrile is a neutral σ-donor and π-acceptor ligand.
This makes Br a more suitable ligand when trans to CO, a
π-acceptor. The π-donating nature of the Br ligand in com-
plexes 10-15 is evident from the IR spectra. The IR νCtO
absorption peaks are lowered to the range of 1940-1960
cm-1, in comparison with complex (R,R)-1 whose νCtO
absorption peak is at 2001 cm-1 (Table 1). The νCtO peaks
of complexes 10-15 fall in the range for other known iron
complexes that also have a π-donating halide coordinated
trans to a CO ligand.64-67 Furthermore, the preferred
formation of CO trans to Br is an advantage for synthetic
considerations. The CO exchange reaction to form complex
10
(S,S)-13
14
(S,S)-15
Bond Lengths (A)
Fe-Br
2.487 (8)
2.284 (8)
1.980 (2)
1.731 (5)
1.165 (8)
2.508 (7)
2.259 (1)
1.970 (3)
1.757 (5)
1.115 (5)
2.487 (6)
2.236 (1)
1.974 (3)
1.739 (4)
1.153 (4)
2.422 (1)
2.219 (2)
1.958 (4)
1.814 (9)
1.162 (1)
Fe-P(1)
Fe-N(1)
Fe-Ca
Ca-O(1)
Bond Angles (deg)
P(1)-Fe-P(2)
N(1)-Fe-N(2)
112.72 (3)
81.83 (1)
111.73 (5)
82.55 (2)
110.66 (4)
82.43 (1)
108.23 (5)
83.01 (2)
a 10: C(7); (S,S)-13: C(19); 14, (S,S)-15: C(15).
(R,R)-1 is more tedious as the acetone solvent must be
replenished at least once to completely remove all the dis-
sociated acetonitrile and hence to drive the substitution
reaction to completion; otherwise trace amounts of (R,R)-
3 remained as determined by 31P{1H} NMR spectroscopy.
Nevertheless, these newly synthesized complexes with car-
bonyl and bromide as ligands are not inert toward acet-
onitrile substitution when the isolated solids are taken up
in acetonitrile. For example, a 31P{1H} NMR spectrum of
(64) Venturi, C.; Bellachioma, G.; Cardaci, G.; Macchioni, A. Inorg.
Chim. Acta 2004, 357, 3712–3720.
(65) Gao, Y.; Holah, D. G.; Hughes, A. N.; Spivak, G. J.; Havighurst,
M. D.; Magnuson, V. R.; Polyakov, V. Polyhedron 1997, 16, 2797–2807.
(66) Bellerby, J. M.; Mays, M. J.; Sears, P. L. J. Chem. Soc., Dalton Trans.
1976, 1232–1236.
(67) Albertin, G.; Baldan, D.; Bordignon, E. J. Chem. Soc., Dalton Trans.
1986, 329–332.

10062 Inorganic Chemistry, Vol. 49, No. 21, 2010
Lagaditis et al.
Table 3. Selected Crystal Data, Data Collection, and Refinement Parameters for 7, 9, 10, (S,S)-13, 14, and (S,S)-15
7
9 CH2Cl2
3
10 THF
3
(S,S)-13
14
(S,S)-15
empirical formula
C70H84B2-
FeN4P2
1120.82
monoclinic
P21/n
C67H78B2Cl2-
FeN4P2
1149.64
monoclinic
P21/n
C59H82BBr-
FeN2O2P2
1059.78
monoclinic
P21/c
C55H66BBr-
FeN2OP2
979.61
orthorhombic
P212121
150(1)
C39H50Bbr-
FeN2OP2
771.32
monoclinic
P21/c
150(1)
C51H58BBr-
FeN2OP2
923.50
orthorhombic
P212121
150(1)
FW
lattice type
space group
T, K
150(1)
150(1)
150(1)
a, A
b, A
c, A
R, deg
β, deg
γ, deg
V, A3
Z
13.0271(2)
26.9138(7)
17.7660(4)
90
91.0020(1)
90
6228.0(2)
4
1.195
0.338
2392
23.6919(6)
11.9363(2)
24.7617(6)
90
118.3990(8)
90
6159.8(2)
4
1.240
0.427
2432
11.1247(2)
21.3615(3)
25.1343(5)
90
92.2340(9)
90
5968.38(2)
4
1.179
1.015
2248
0.20ꢀ0.15ꢀ0.14
2.58 to 27.48
38865/13607
semiempirical from equivalents
0.913 and 0.867
12.8978(3)
16.6086(3)
23.5406(5)
90
90
90
5042.74(2)
4
1.290
1.195
2056
25.8207(5)
29.1920(6)
10.1299(2)
90
98.109(1)
90
7559.1(3)
8
1.356
1.574
3216
12.4275(3)
15.4890(5)
24.0452(7)
90
90
90
4628.4(2)
4
1.325
1.297
1928
F
calc/Mg m-3
μ(Mo, KR), mm-1
F(000)
cryst size, mm3
range θ collected, deg
reflns collected/unique
abs cor
0.15ꢀ0.15ꢀ0.10
2.75 to 25.00
40634/10946
0.30ꢀ0.20ꢀ0.16
2.59 to 27.51
48792/14056
0.20ꢀ0.20ꢀ0.15
2.60 to 27.54
42429/11494
0.36ꢀ0.12ꢀ0.04
2.63 to 27.47
60298/17278
0.25ꢀ0.18ꢀ0.09
2.63 to 26.72
34619/9506
max and min
0.976 and 0.804
0.966 and 0.832
0.838 and 0.776
0.982 and 0.684
0.890 and 0.771
transmn coeff.
goodness of fit
R1(I>2σ(I))a
1.044
0.0828
0.2573
1.564 and -0.637
1.028
0.0560
0.1589
0.959 and -0.641
1.031
0.0550
0.1410
0.523 and -0.400
1.022
0.0553
0.1183
0.605 and -0.535
0.972
0.0545
0.1286
0.540 and -0.674
1.036
0.0556
0.1203
0.313 and -0.326
wR2 (all data)a
peak and hole, e A-3
P
P
P
P
a Definition of R indices: R1 = (FO - FC)/ (FO); wR2 = [ [w(FO2 - FC2)2]/[ [w(FO2)2]]1/2
.
the reaction in which complex 12 was taken up in acet-
onitrile-d3 revealed two species, a singlet at 76.0 ppm for 7
and a singlet at 77.9 ppm for 12. In addition, when (S,S)-
15 was dissolved in acetonitrile-d3, the spectrum showed
after 1 day the presence of two species, a singlet at 69.4
ppm and a doublet of doublets centered at 66.9 ppm for the
complexes trans-[Fe(NCCH3)2(PEt2CH2CHdNCHPh-)2]2þ
and (S,S)-15, respectively.
At 25 °C, complex 14 catalyzed the reduction of aceto-
phenone to 1-phenylethanol to 15% conversion in 1 h
(Table 4, entry 1) and only to a maximum of 20% conversion
after overnight; whereas, a maximum conversion of 70% was
achieved after 12 h at 50 °C. At a higher substrate loading,
complex 14 led to a maximum 50% conversion of aceto-
phenone to 1-phenylethanol after 1 h at 50 °C (Table 4,
entry 3). In general, complex (S,S)-15 is a better catalyst
than 14 in the reduction of acetophenone (i.e., Table 4, entry
6 vs entry 3; Figure 5) in which there is a substantial increase
in the TOF (h-1). The activity of the catalyst does not
appear to be affected by the choice of base since the use of
sodium iso-propoxide or potassium hydroxide in the same
catalyst/base ratio of 1/8 gave similar conversions and TOF
as with potassium tert-butoxide (Table 4, entries 6-8).
The enantioselectivity in the reduction of acetophenone
catalyzed by (S,S)-15 (e.e. = 55%) was much lower than
that of complex (R,R)-1 (e.e.=82%). Hence, it appears as
though some degree of rigid steric bulk is required at the
phosphorus to maintain high enantioselectivity. At 25 °C,
the enantioselectivity of (S,S)-15 for (R)-1-phenylethanol
was fairly constant such that the e.e. degraded from 57 to
44% when the reaction mixture was left stirring for 12 h.
At 50 °C, however, the alcohol product racemized early in
the catalysis such that only 26-29% e.e. was observed at
1 h (Table 4, entry 10 and 11). Racemization of the alcohol
product has been observed previously in the transfer
hydrogenation using complex (R,R)-1.42 An increase in
the precatalyst loading caused an increase in e.e. as long
as the product is analyzed as soon as the equilibrium is
reached (entry 4 vs 6).
Transfer Hydrogenation of Ketones. We have previously
shown that the iron bis(acetonitrile) complex (R,R)-3 was
inactive for the transfer hydrogenation of ketones in basic
iso-propanol. Complexes 5, 7, and 9 were also tested, and
they too were inactive for the transfer hydrogenation of
acetophenone in the same conditions.42 As demonstrated by
(R,R)-1, for reasons unknown yet, a carbonyl ligand is
necessary to make these types of iron complexes active
transfer hydrogenation catalysts. Hence, complexes 10-15
were tested for the transfer hydrogenation of acetophenone
to 1-phenylethanol in basic iso-propanol. To our surprise,
complexes 10-13 were inactive for transfer hydrogenation
at room temperature and even at 50 °C. Complexes 14 and
(S,S)-15, on the other hand, showed activity for the transfer
hydrogenation of acetophenone at room temperature but
better activity at 50 °C (Table 4). Complexes 14 and (S,S)-15
were inactive in the absence of base, a common observation
with analogous iron complexes42,68,69 and well established
ruthenium-based complexes for transfer hydrogenation.70-72
(68) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem.,
Int. Ed. 2008, 47, 940–943.
(69) Meyer, N.; Lough, A. J.; Morris, R. H. Chem. Eur. J. 2009, 15,
;
5605–5610.
Figure 5 depicts the catalytic behavior of 14 and (S,S)-
15 where the catalyst/base/substrate ratio is 1/8/500. In all
cases, no induction period was observed and rapid con-
version was observed in the first 10 min; afterward, the
catalysis began to plateau. This is a strong indication the
(70) Gao, J.-X.; Ikariya, T.; Noyori, R. Organometallics 1996, 15, 1087–
1089.
(71) Gao, J.-x.; Xu, P.-p.; Yi, X.-d.; Yang, C.-b.; Zhang, H.; Cheng, S.-h.;
Wan, H.-l.; Tsai, K.-r.; Ikariya, T. J. Mol. Catal. A: Chem. 1999, 147, 105–111.
(72) 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.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 10063
Table 4. Catalytic Transfer Hydrogenation of Acetophenone to 1-Phenylethanol in iso-Propanol
entry/
complex
conversionb/
2 min (%)
conversionb/
10 min (%)
conversionb/
60 min (%)
e.e. (%)c 2 min/
60 min
C/B/Sa ratio
temperature (°C)
TOFd (h-1
)
1/14
2/14
3/14
4/15
5/15
6/15
7/15 f
8/15h
9/15
10/15
11/15
1/8/200
1/8/200
1/8/500
1/8/50
1/8/100
1/8/200
1/8/200
1/8/200
1/8/500
1/8/500
1/8/1000
25
50
50
50
50
50
50
50
25
50
50
10
46
34
15
66
47
119
735
1458
21
13
92
86
66
62
54
6
60/-
91
89
90
90
19
72
25
54/24
55/47e
57/47e
58/47e
57/51
56/26
53/29
2421
2341
2245
563
4171
1551
35
81
50
29
5
a C/B/S: catalyst/base/substrate. b Conversions were determined by GC and were reported as an average of two runs. c enantiomeric excess in (R)-1-
phenylethanol. d TOF calculated from slope of initial linear portion of catalysis. e %e.e. at 2 min/10 min. f NaOiPr used as base. h KOH used as base.
Figure 5. Catalytic transfer hydrogenation of acetophenone to 1-phenyl-
ethanol in the presence of 14 or (S,S)-15, potassium tert-butoxide, and
Figure 6. Second addition of acetophenone (260 mg, C/S = 1/200) after
iso-propanol (6 mL). C/B/S = 1/8/500. (a) precatalyst: (S,S)-15; T = 50 °C;
20 min of transfer hydrogenation of acetophenone in the presence of
(b) precatalyst: 14; T = 50 °C; (c) precatalyst: (S,S)-15; T = 25 °C.
(S,S)-15, potassium tert-butoxide and iso-propanol (6 mL) at 50 °C(C/B/S=
1/8/200).
active catalyst slowly deactivates during the course of the
catalysis.
was added after 60 min. A third addition of (S,S)-15 had
little effect on the conversion, indicating that equilibrium
was reached. These experiments provide clear evidence
for the decomposition of the catalyst during catalysis.
In spite of the activity of complex 14 and the increased
activity of complex (S,S)-15 at 50 °C, they are less active
than complex (R,R)-1. At a catalyst/substrate ratio of
1/500, complex (S,S)-15 displayed a TOF of 4100 h-1 at
50 °C, while both complex (R,R)-1 and the bromo variant
trans-[Fe(Br)(CO)(PPh2CH2CHdNCH2CH2NdCH-
CH2PPh2)]þ,75 at similar concentrations have a TOF of
3600 h-1 at the lower temperature of 22 °C. The higher
activity of the PPh2-substituted complexes is clearly an
electronic effect in which phenyl substituents at phos-
phorus are necessary to increase the activity of these
tetradentate iron complexes.
To test this, additional acetophenone (200 equiv) was
added after 20 min to a catalytic reaction mixture that had
a catalyst/substrate ratio of 1/200 (Figure 6). Thus, upon
the second addition of acetophenone, the catalyst dis-
played different catalytic behavior. The initial sharp,
rapid increase in catalysis was not observed; rather it
was a slow and steady increase such that it took more than
twice the amount of time to achieve almost the same
concentration of 1-phenylethanol. We attribute this to
decomposition of the active species during catalysis.
To further demonstrate the catalyst decomposition
process, additional precatalyst was added during the
course of catalysis (Figure 7). In the case of the catalyst
to ketone ratio of 1:500 (Figure 7, a), additional precata-
lyst was added after 60 min; this had a small effect which
indicated that the initial (S,S)-15 to ketone loading of
1/500 allowed for an almost complete approach to equi-
librium before catalyst decomposition. However, in the
case of the catalyst to ketone ratio of 1:1000 (Figure 7, b)
catalysis resumed after an additional 10 mg of catalyst
An electronic effect is also evident by the comparison of
the catalytic activities between complexes 14 and (S,S)-15
(Figure 5, a and b). The phenyl groups on the NN-linker
of the ligand of (S,S)-15, not conjugated to the tetradentate

10064 Inorganic Chemistry, Vol. 49, No. 21, 2010
Lagaditis et al.
Conclusion
We have made a series of new iron complexes, [Fe(CO)(Br)-
(P-N-Q-N-P)][BPh4] for the transfer hydrogenation of
ketones that feature alkyl substituents (Cy, iPr, Et) at phos-
phorus. They are first made as bis-acetonitrile iron complexes
in a facile one-pot template reaction and then subjected to a
CO atmosphere; thereafter, the complexes are isolated as
BPh4 salts in moderate to good yields. The complexes that
contained Cy or iPr substituents (10-13) at phosphorus were
inactive while those with Et substituents (14 and (S,S)-15)
were active for the catalytic reduction of acetophenone. In
addition, complex (S,S)-15, with Ph substituents on the NN-
linker backbone, displayed higher activity than complex 14.
Despite the activity of complexes 14 and (S,S)-15, they are
less active than complex (R,R)-1. This observation clearly
indicates that electron-withdrawing groups are necessary for
the high enzymatic-like activity of complex (R,R)-1. Further-
more, the inactivity of complexes 10-13, indicate that there
cannot be too much steric bulk at the phosphorus donor
atoms of the ligand. Yet the poor enantioselectivity of
complex (S,S)-15 indicated there some degree of steric bulk
is required; thus, this report showed by comparison that the
Ph groups on the phosphorus atoms of the ligand of complex
(R,R)-1 provide a fine balance between sterics and electronics.
Figure 7. Addition of 10 mg of (S,S)-15 to catalytic transfer hydrogena-
tion reactions at 50 °C in the presence of potassium tert-butoxide. (a) Second
addition added after 60 min (initial C/S = 1/500). (b) Second addition
added after 60 min, third addition added after 120 min (initial C/S =
1/1000).
ligand, increased the catalytic activity for the reduction
of acetophenone. However, this electronic withdrawing
effect had a smaller effect such that catalytic activity is still
inferior to (R,R)-1. The difference in the NN-linker had no
effect in increasing the activity of (S,S)-11 and (S,S)-13
over 10 and 12, respectively, indicating the substituents at
the phosphorus have a dominant role in the activity of
these iron complexes for transfer hydrogenation of
ketones. We also believe steric factors are involved which
make complexes 10-13 inactive for transfer hydrogena-
tion but make complexes 14 and (S,S)-15 active. The Cy
or iPr groups may block a key site on the P-N-Q-N-P
ligand backbone; mechanistic investigations of the
Noyori catalyst system RuCl2(P-N-N-P) and its ana-
logues have shown that the P-N-N-P ligand system is
not an ancillary ligand and is involved in the catalytic
Experimental Section
General Comments. All procedures and manipulations involv-
ing air-sensitive materials were performed under an argon or
nitrogen atmosphere using Schlenk techniques or a glovebox
with N2(g). Solvents were degassed and dried using standard
procedures prior to all manipulations and reactions. Deuterated
solvents were purchased from Cambridge Isotope Laboratories,
degassed and dried over activated molecular sieves prior to use.
Phosphonium dimers 4, 6, and 8, were synthesized following a
literature procedure.60,61 All other reagents used were purchased
from commercial sources and utilized without further purifications.
NMR spectra were recorded at ambient temperature and pres-
sure using Varian Gemini 400 MHz [1H (400 MHz), 13C{1H}
(100 MHz) and 31P{1H} (161 MHz)]. The 31P NMR spectra were
referenced to 85% H3PO4 (0 ppm). The elemental analyses were
performed at the Department of Chemistry, University of Toronto,
on a Perkin-Elmer 2400 CHN elemental analyzer. Some com-
plexes gave inconsistent carbon analyses but acceptable hydrogen
and nitrogen contents; we attribute it as a combustion problem
due to tetraphenylborate.76
cycle this mode of action has been coined as “ligand-
;
metal bifunctional catalysis”.73,74 Hence, this blockage
created by those substituents may prevent either iso-
propanol and/or the substrate (acetophenone) to interact
with the P-N-Q-N-P ligand and iron metal center of
the active complex.
This rationale also leads to an additional possibility in
which dissociation of a phosphine donor atom of the
ligand from iron is required in the catalytic cycle. Since
electron donating alkyl substituents render phosphines
more basic, the ligands would be less likely to dissociate.
Even though this latter scenario appears to explain the
difference in catalytic activity between complex (R,R)-1
and complexes 10-15, we are not in favor of it because it
is insufficient to explain the difference between the in-
activity of complexes 10-13 and the activity of complexes
14-15.
Precursor Solution A. A vial was charged with 4 (100 mg,
0.156 mmol), KOtBu (35 mg, 0.311 mmol), and CH3CN (5 mL).
After stirring for 5 min, [Fe(H2O)6][BF4]2 (79 mg, 0.234 mmol)
in CH3CN (2 mL) was added to the white slurry. The solution
turned gray-yellow after 5 min.
trans-[Fe(CO)(Br)(PCy2CH2CHdNCH2CH2NdCHCH2-
PCy2)][BPh4] (10). To precursor solution A, ethylenediamine
(0.17 mL from a stock solution of 200 mg in 4 mL of CH3CN)
was added. The mixture turned pink immediately. After the reac-
tion had gone to completion overnight, the mixture was filtered
through a pad of Celite to remove a gray-white precipitate.
Solvent was removed under reduced pressure to give a red-pink
residue, which was then dissolved in acetone (6 mL) and stirred
overnight under CO (∼ 2 atm). Acetone solvent was removed
under pressure, taken up in MeOH (1.5 mL), and added to a solu-
tion of NaBPh4 (80 mg, 0.234 mmol) in MeOH (1 mL) to cause
precipitation of a yellow solid. The solid was filtered and washed
with MeOH (2 ꢀ 1 mL) and dried under vacuum. Yield: 80%
These theories are still speculation as the mechanism is
not known. Density functional theory (DFT) analysis of
the mechanism is currently under investigation by our group.
(73) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev.
2004, 248, 2201–2237.
(74) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-Rashid,
K.; Morris, R. H. Chem. Eur. J. 2003, 9, 4954–4967.
;
(75) Mikhailine, A. A., Morris, R. H. Inorg. Chem. submitted for publication,
2010.
(76) Marco, A.; Compano, R.; Rubio, R.; Casals, I. Microchim. Acta
2003, 142, 13–19.

Article
Inorganic Chemistry, Vol. 49, No. 21, 2010 10065
(123 mg). Single crystals suitable for an X-ray diffaction study
were obtained by slow diffusion of hexanes into tetrahydrofuran
(THF). 1H NMR (400 MHz, CD2Cl2) δ: 1.20-1.39 (m, HCy),
2.50 (br. m, 2H, HCyP) 2.86 (dd, JHP = 19.0 Hz, 2H, H2CP),
3.27 (dd, JHP =20.0 Hz, 2H, H2CP), 3.44 (m, 2H, H2CN), 3.81
residue, which was then dissolved in acetone (6 mL) and stirred
overnight under CO (∼2 atm). Acetone solvent was removed
under pressure, taken up in MeOH (1.5 mL) and added to a
solution of NaBPh4 (106 mg, 0.311 mmol) in MeOH (1 mL) to
cause precipitation of a yellow solid. The solid was filtered and
washed with MeOH (2 ꢀ 1 mL) and dried under vacuum. Yield:
58% (100 mg). 1H NMR (400 MHz, CD2Cl2) δ: 1.26 (m, 24H,
CH3), 2.28 (m, 2H, MeCH), 2.86 (m, 4H, MeCH, CH2P), 3.24
(dd, JHH = 7.9 Hz, JHP = 25.2 Hz, 2H, CH2P), 3.43 (m, 2H,
CH2N), 3.75 (m, 2H, CH2N), 6.86-7.39 (m, 22H, HAr,
HCdN). 13C {1H} NMR (100 MHz, CD2Cl2) δ: 19.8 (CH3),
20.0 (CH3), 26.6 (t, JCP=8.9 Hz, CHMe2), 27.5 (t, JCP=8.3 Hz,
CHMe2), 40.5 (dd, JCP = 12.1, 16.6 Hz, CH2P), 60.6 (CH2N),
122.5 (CArB), 126.3 (CArB), 136.4 (CArB), 164.5 (m, JCB = 49
Hz, CArB), 174.6 (HCdN). 31P {1H} NMR (161 MHz, CD2Cl2)
δ: 77.7 (s) ppm. IR (KBr) 1948 cm-1 (νCtO). Anal. Calcd for
C43H58 N2OP2FeBrB: C, 62.42; H, 7.07; N, 3.39; Found: C,
64.45; H, 7.33; N, 3.90. MS (ESI, methanol/water; m/zþ): 507.1
[C19H38N2OP2FeBr]þ.
(m, 2H, H2CN), 6.88-7.39 (m, HAr), 7.49 (m, 2H, HCdN). 13
C
{1H} NMR (100 MHz, CD2Cl2) δ: 26.4 (d, JCP = 11.0 Hz,
CyP), 27.8 (m, CCyP), 29.5 (t, JCP=3.4 Hz, CCyP), 30.2 (CCyP),
C
30.8 (d, JCP =8.7 Hz, CCyP), 38.3 (q, JCP =7.8 Hz, CCyP), 39.7
(dd, JCP = 11.7, 16.9 Hz, CCyP), 60.9 (CH2N), 122.7 (CPhB),
126.5 (CPhB), 136.7 (CPhB), 164.7 (m, JCB=49 Hz, CPhB), 174.4
(HCdN). 31P {1H} NMR (161 MHz, CD2Cl2) δ: 69.8 ppm. IR
(KBr) 1948 cm-1 (νCtO). Anal. Calcd for C55H74N2OP2FeBrB:
C, 66.88; H, 7.55; N, 2.84; Found: C, 65.30; H, 7.89; N, 3.15. MS
(ESI, methanol/water; m/zþ): 667.2 [C31H53N2OP2FeBr]þ.
trans-(S,S)-[Fe(CO)(Br)(PCy2CH2CHdNCH(Ph)CH(Ph)Nd
CHCH2PCy2)][BPh4] ((S,S)-11). A solution of (1S,2S)-(-)-1,2-
diphenylethylenediamine (33 mg, 0.156 mmol) in CH3CN (1 mL)
was added to precursor solution A. Thereafter the mixture was
handled in the exact same manner as 10. Yield: 66% (118 mg).
1H NMR (400 MHz, CD3CN) δ: 1.35-1.93 (br. m, HCy), 2.48
(m, 1H, HCCyP), 2.81 (br. m, 2H, H2CP, HCCyP), 3.04 (br. m,
2H, H2CP), 3.54 (m, 1H, H2CP), 4.99 (d, 1H, JHH = 10.9 Hz,
HCPh), 5.55 (d, 1H, JHH =11.7 Hz, HCPh), 6.86-7.50 (br. m,
HAr, HCdN), 7.88 (m, 2H, HCdN). 13C {1H} NMR (100
MHz, CD3CN) δ: 26.7-40.5 (CCyP, CH2P), 77.2 (CHPh), 80.3
(CHPh), 121.1 (CPhB), 124.9 (CPhB), 128.3 (CPh), 128.5 (CPh),
128.8 (CPh), 128.9 (CPh), 129.0 (CPh), 129.1 (CPh), 131.6 (CPh),
132.2 (CPh), 135.0 (CPhB), 162.6 (m, JCB = 49 Hz), 172.0
(HCdN), 172.4 (HCdN). 31P {1H} NMR (161 MHz, CD3CN)
δ: 67.1 (d, JPP=29.8 Hz), 70.4 (d, JPP=29.8 Hz). IR (KBr) 1945
cm-1 (νCtO). Anal. Calcd for C67H82N2OP2FeBrB: C, 70.60;
H, 7.25; N, 2.46. Found: C, 68.09; H, 7.03; N, 2.62. MS (ESI,
methanol/water; m/zþ): 819.3 [C43H62N2OP2FeBr]þ.
trans-(S,S)-[Fe(CO)(Br)(PiPr2CH2CHdNCH(Ph)CH(Ph)Nd
CHCH2PiPr2)][BPh4] ((S,S)-13). A solution of (1S,2S)-(-)-1,2-
diphenylethylenediamine (44 mg, 0.207 mmol) in CH3CN (1 mL)
was added to precursor solution B. Thereafter the mixture was
handled in the exact same manner as 12. Yield: 60% (121 mg).
Single crystals suitable for an X-ray diffaction study were ob-
tained by slow diffusion of pentane into CH2Cl2. 1H NMR (400
MHz, CD3CN) δ: 1.32 (m, 24H, CCH3), 2.35 (m, 2H, Me2CH),
2.72 (m, 2H, Me2CH), 3.03 (m, 3H, CH2P), 3.59 (m, 2H, CH2P),
5.06 (d, 1H, JHH = 12 Hz, PhCH), 5.47 (d, 1H, JHH = 12 Hz,
PhCH), 6.87-7.55 (m, 32H, HAr, HCdN). 13C {1H} NMR (100
MHz, CD3CN) δ: 19.41 (Me), 19.45 (Me), 19.5 (Me), 19.8 (Me),
19.9 (Me), 20.3 (Me), 20.4 (Me), 20.6 (Me), 26.2 (d, JCP =18.9
Hz, MeCH), 26.9 (d, JCP=19.7 Hz, MeCH), 27.6 (d, JCP=20.1,
MeCH), 29.1 (d, JCP = 16.0 Hz, MeCH), 39.6 (dd, JCP = 3.8,
23.2 Hz, CH2P), 40.6 (dd, JCP = 3.2, 25.3 Hz, CH2P), 77.6
(PhCH), 81.2 (PhCH), 122.4 (CArB), 126.2 (CArB), 129.6 (CAr),
129.9 (CAr), 130.4 (CAr), 130.6 (CAr), 132.8 (CAr), 134.1 (CAr),
136.5 (CArB), 164.5 (m, JCB = 49.0 Hz, CArB), 174.9 (HCdN),
175.0 (HCdN). 31P {1H} NMR (161 MHz, CD3CN) δ: 67.1 (d,
trans-[Fe(NCMe)2(PiPr2PCH2CHdNCH2CH2NdCHCH2-
PiPr2)][BPh4]2 (7). A vial was charged with 6 (50 mg, 0.104 mmol),
KOtBu (24 mg, 0.207 mmol), and CH3CN (2 mL). After stirring for
5 min, [Fe(H2O)6][BF4]2 (52 mg, 0.156 mmol) in CH3CN (1 mL)
was added to the white slurry. The solution turned gray-yellow after
5 min. To this mixture, ethylenediamine (0.12 mL from a stock
solution of 200 mg in 4 mL of CH3CN) was added. After the reac-
tion stirred overnight, the pink mixture was filtered through a pad
of Celite. The solvent was removed under reduced pressure, taken
up in MeOH (1.5 mL), and added to NaBPh4 (78 mg, 0.228 mmol)
in MeOH (1 mL) to cause the precipitation of a pale pink solid. The
solid was filtered and washed with MeOH (2ꢀ1 mL) and dried
under vacuum. Yield: 65% (75 mg). Single crystals suitable for an
X-ray diffaction study were obtained by slow diffusion of hexanes
J
PP = 29.8 Hz), 70.4 (d, JPP = 29.8 Hz). IR (KBr) 1956 cm-1
(νCtO). Anal. Calcd for C55H66N2OP2FeBrB: C, 67.43; H, 6.79;
N, 2.86. Found: C, 70.96; H, 6.38; N, 2.19. MS (ESI, methanol/
water; m/zþ): 661.2 [C31H47N2OP2FeBr]þ.
trans-[Fe(NCMe)2(PEt2CH2CHdNCH2CH2NdCHCH2-
PEt2)][BPh4]2 (9). A vial was charged with 8 (60 mg, 0.141 mmol),
KOtBu (32 mg, 0.282 mmol), MeOH (5 mL). After stirring for
5 min, [Fe(H2O)6][BF4]2 (71 mg, 0.211 mmol) in CH3CN (1 mL)
was added to the white slurry the solution turned gray-yellow after
5 min. To this mixture, ethylenediamine (0.16 mL from a stock
solution of 200 mg in 4 mL of CH3CN) was added, and the reaction
was refluxed overnight. The solvent was then removed, taken up in
acetonitrile, and filtered through a pad of Celite to remove a gray-
white precipitate. The solvent was then removed again taken up in
MeOH (1 mL) and added to NaBPh4 (105 mg, 0.310 mmol) in
MeOH (1 mL) to cause precipitation of a pale pink solid. The solid
was filtered and washed with MeOH (2 ꢀ 1 mL) and dried under
vacuum. Yield: 70% (105 mg). Single crystals suitable for an X-ray
diffaction study were obtained by slow diffusion of Et2O into
1
into a 1:1 solution of THF and MeCN. H NMR (400 MHz,
CD3CN) δ: 1.23 (m, 24H, CH(CH3)2), 1.98 (s, NCCH3), 2.35 (m,
4H, CH(CH3)2), 3.35 (d, JHP = 7.00 Hz, CH2P), 4.00 (s, 4H,
CH2N), 6.87-7.30 (m, 40H, HAr), 8.50 (m, 2H, HCdN). 13C {1H}
NMR (100 MHz, CD3CN) δ: 18.8 (d, JCP = 6.6 Hz, CH(CH3)2),
24.8 (dd, JCP=7.2, 8.7 Hz, CH(CH3)2), 37.0 (dd, JCP=9.9,14.6Hz,
CH2P), 60.6 (CH2N), 122.2 (CArB), 126.0 (CArB), 136.2 (CArB),
164.2 (m, JCB = 49.0 Hz, CArB), 177.6 (HCdN). 31P {1H} NMR
(161 MHz, CD2Cl2) δ: 75.9 (s) ppm.
Precursor Solution B. A vial was charged with 6 (100 mg,
0.207 mmol), KOtBu (47 mg, 0.415 mmol), and CH3CN (4 mL).
After stirring for 5 min, [Fe(H2O)6][BF4]2 (105 mg, 0.311 mmol)
in CH3CN (2 mL) was added to the white slurry. The solution
turned gray-yellow after 5 min.
1
CH2Cl2. H NMR (400 MHz, CD3CN) δ: 1.13 (m, 12H, CH3),
1.80 (m, CH2CH3), 3.41 (m, 4H, CH(CH3)2), 3.35 (d, JHP =7.74
Hz, CH2P), 4.01 (s, 4H, CH2N), 6.85-7.28 (m, 40H, HAr), 8.41
(m, 2H, HCdN). 13C {1H} NMR (100 MHz, CD3CN) δ: 9.9
(t, JCP =2.0 Hz, CH3), 18.1 (dd, JCP =9.1, 11.4 Hz, CH(CH3)2),
41.8 (dd, JCP =11.4, 15.1 Hz, CH2P), 61.4 (CH2N), 122.6 (CArB),
126.3 (CArB), 136.2 (CArB), 163.9 (m, JCB=49.0 Hz, CArB), 175.4
(HCdN). 31P {1H} NMR (161 MHz, CD2Cl2) δ: 70.5 (s) ppm.
Precursor Solution C. A vial was charged with 8 (150 mg,
0.352 mmol), KOtBu (79 mg, 0.704 mmol), and MeOH (5 mL).
After stirring for 5 min, [Fe(H2O)6][BF4]2 (178 mg, 0.528 mmol)
trans-[Fe(CO)(Br)(PiPr2CH2CHdNCH2CH2NdCHCH2-
PiPr2)][BPh4] (12). To precursor solution B, ethylenediamine
(0.22 mL from a stock solution of 200 mg in 4 mL of CH3CN)
was added. The mixture turned pink immediately. After the
reaction has gone to completion overnight, the mixture was fil-
tered through a pad of Celite to remove a gray-white precipitate.
Solvent was removed under reduced pressure to give a red-pink

10066 Inorganic Chemistry, Vol. 49, No. 21, 2010
Lagaditis et al.
in CH3CN (2 mL) was added to the white slurry. The solution
turned gray-yellow after 5 min.
study were obtained by slow diffusion of hexanes into CH2Cl2.
1H NMR (400 MHz, CD3CN) δ: 1.18 (m, 12H, CH3), 1.94 (m,
4H, CH2CH3), 2.42 (m, 2H, CH2CH3) 2.64 (m, 2H, CH2CH3),
trans-[Fe(CO)(Br)(PEt2CH2CHdNCH2CH2NdCHCH2P-
Et2)][BPh4] (14). To precursor solution C, ethylenediamine (0.38
mL from a stock solution of 200 mg in 4 mL of CH3CN) was
added. The mixture turned red immediately and was allowed to
reflux overnight (the reaction mixture turned to a pale orange-
yellow color). The solvent was removed, the residue was taken
up with acetonitrile, and filtered through a pad of Celite to
remove a gray-white precipitate. The acetonitrile solvent was
removed under reduced pressure to give a pale-pink residue,
which was then dissolved in acetone (6 mL) and stirred over-
night under CO (∼ 2 atm). Acetone solvent was removed under
pressure, taken up in MeOH (2 mL), and added to a solution of
NaBPh4 (180 mg, 0.528 mmol) in MeOH (1.5 mL) to cause
precipitation of a yellow solid. The solid was filtered and washed
with MeOH (2ꢀ1 mL) and dried under vacuum. Yield: 55%
(150 mg). Single crystals suitable for an X-ray diffaction study
2.98 (m, 3H, CH2P), 3.42 (m, 1H, CH2P), 5.13 (d, 1H, JHH
=
11.4 Hz, CH(Ph)), 5.37 (d, 1H, JHH =11.4 Hz, CH(Ph)), 6.84-
7.32 (m, 32H, HAr, HCdN). 13C {1H} NMR (100 MHz, CD3CN)
δ: 13C {1H} NMR (100 MHz, CD3CN) δ: 8.11 (CH3), 8.17
(CH3), 8.50 (CH3), 8.58 (CH3), 8.78 (CH3), 17.7 (d, JCP = 26.2
Hz, CH2CH3), 19.2 (d, JCP=24.0 Hz, CH2CH3), 20.4 (d, JCP
=
27.6 Hz, CH2CH3), 20.6 (d, JCP = 25.6 Hz, CH2CH3), 77.8
(CH(Ph)), 81.7 (CH(Ph)), 122.3 (CArB), 126.2 (CArB), 127.5
(CAr), 128.7 (CAr), 129.4 (CAr), 129.7 (CAr), 130.0 (CAr), 130.1
(CAr), 130.2 (CAr), 130.4 (CAr), 132.9 (CAr), 134.5 (CAr), 136.3
(CArB), 164.4 (m, JCB = 49 Hz, CArB), 173.4 (HCdN), 174.2
(HCdN). 31P {1H} NMR (161 MHz, CD3CN) δ: 66.8 (d, JPP
=
34.3 Hz), 67.9 (d, JPP = 34.3 Hz). IR (KBr) 1951 cm-1 (νCtO).
Anal. Calcd for C51H59N2OP2FeBrB: C, 66.25; H, 6.43; N, 3.03.
Found: C, 66.81; H, 6.29; N, 2.81. MS (ESI, methanol/water;
m/zþ): 603.1 [C27H38N2OP2FeBr]þ.
1
were obtained by slow diffusion of hexanes into CH2Cl2. H
NMR (400 MHz, CD2Cl2) δ: 1.10 (m, 12H, CH3), 1.89 (m, 6H,
CH2CH3), 2.90 (dd, 2H, JHH = 7.01 Hz, JHP = 19.20, CH2P),
3.19 (dd, 2H, JHH = 7.63 Hz, JHP = 19.2 Hz, CH2P), 3.58 (m,
2H, CH2N), 3.76 (m, 2H, CH2N), 6.89-7.35 (m, 22H, HAr,
HCdN). 13C {1H} NMR (100 MHz, CD2Cl2) δ: 8.08 (Me), 8.26
(Me), 17.9 (t, JCP = 11.8 Hz, CH2Me), 19.9 (t, JCP = 12.7 Hz,
CH2Me), 41.3 (dd, JCP = 13.5, 16.8 Hz, CH2P), 59.8 (CH2N),
122.3 (CArB), 126.1 (CArB), 136.1 (CArB), 164.2 (m, JCB = 49
Hz, CArB), 173.2 (HCdN). 31P {1H} NMR (161 MHz, CD2Cl2)
δ: 66.8 ppm. IR (KBr) 1951 cm-1 (νCtO). Anal. Calcd for
C39H50N2OP2FeBrB: C, 60.73; H, 6.53; N, 3.63; Found: C,
58.16; H, 6.26; N, 3.40. MS (ESI, methanol/water; m/zþ): 451.0
[C15H30N2OP2FeBr]þ.
General Procedure for Transfer Hydrogenation Studies. A solu-
tion of potassium tert-butoxide (10 mg, 0.089 mmol) in iso-
propanol (4 mL) was added via syringe to a Schlenk flask charged
with a mixture of (S,S)-15 (10 mg, 0.011 mmol), acetophenone
(651 mg, 5.418 mmol) in 2 mL of iso-propanol at 50 °C under an
atmosphere of argon. Samples were taken from the reaction mix-
ture periodically via a syringe and needle and were quenched upon
exposure to air. The samples were analyzed by gas chromatography
(GC) using a Perkin-Elmer Clarus 400 chromatograph equipped
with a chiral column (CP chirasil-Dex CB 25 m ꢀ 2.5 mm).
Hydrogen was used as a mobile phase at a column pressure of
5 psi with a split flow rate of 50 mL/min. The injector temperature
was 250 °C, FID temperature was 275 °C, and the oven tempera-
ture was 130 °C. Retention times (tR/min) for acetophenone, 4.33;
(R)-1-phenylethanol, 7.17; (S)-1-phenylethanol, 7.67. All conver-
sions were reported as an average of two GC runs.
trans-(S,S)-[Fe(CO)(Br)(PEt2CH2CHdNCH(Ph)CH(Ph)Nd
CHCH2PEt2)][BPh4] ((S,S)-15). A solution of (1S,2S)-(-)-1,2-
diphenylethylenediamine (75 mg, 0.352 mmol) in CH3CN (1 mL)
was added to precursor solution C. Thereafter the mixture was
handled in the exact same manner as 14. An extra purification
step is required, as the product does not precipitate effectively
from methanol (an appreciable amount remains dissolved in
methanol). The methanol solution was concentrated to a resi-
due, taken up with CH2Cl2 (1 mL), and filtered to remove excess
NaBPh4. Et2O (15 mL) was added to cause precipitation. Yield:
84% (275 mg). Single crystals suitable for an X-ray diffaction
Acknowledgment. We thank NSERC Canada for a Discovery
grant to R.H.M.
Supporting Information Available: Complete crystallographic
data in CIF format for complexes 7, 9 CH2Cl2, 10 THF, (S,S)-
13, 14, and (S,S)-15. This material is available free of charge via
3
3
the Internet at http://pubs.acs.org.