Synthesis of monomeric Fe(II) and Ru(II) complexes of tetradentate phosphines

Article abstract of DOI:10.1021/ic102510c

rac-Bis[{(diphenylphosphino)ethyl}-phenylphosphino]methane (DPPEPM) reacts with iron(II) and ruthenium(II) halides to generate complexes with folded DPPEPM coordination. The paramagnetic, five-coordinate Fe(DPPEPM)Cl₂ (1) in CD₂Cl

Full text of DOI:10.1021/ic102510c

ARTICLE
pubs.acs.org/IC
Synthesis of Monomeric Fe(II) and Ru(II) Complexes of Tetradentate
Phosphines
Barun Jana, Arkady Ellern, Oleg Pestovsky,* Aaron Sadow,* and Andreja Bakac*
Ames Laboratory and Chemistry Department, Iowa State University, Ames, Iowa 50011, United States
S Supporting Information
b
ABSTRACT:
rac-Bis[{(diphenylphosphino)ethyl}-phenyl-
phosphino]methane (DPPEPM) reacts with iron(II) and
ruthenium(II) halides to generate complexes with folded
DPPEPM coordination. The paramagnetic, five-coordinate Fe-
(DPPEPM)Cl2 (1) in CD2Cl2 features a tridentate binding
mode as established by 31P{1H} NMR spectroscopy. Crystal
structure analysis of the analogous bromo complex, Fe-
(DPPEPM)Br2 (2) revealed a pseudo-octahedral, cis-R geo-
metry at iron with DPPEPM coordinated in a tetradentate
fashion. However, in CD2Cl2 solution, the coordination of
DPPEPM in 2 is similar to that of 1 in that one of the external
phosphorus atoms is dissociated resulting in a mixture of three
tridentate complexes. The chloro ruthenium complex cis-Ru-
(κ4-DPPEPM)Cl2 (3) is obtained from rac-DPPEPM and
either [RuCl2(COD)]2 [COD = 1,5-cyclooctadiene] or RuCl2-
(PPh3)4. The structure of 3 in both the solid state and in
CD2Cl2 solution features a folded κ4-DPPEPM. This binding
mode was also observed in cis-[Fe(κ4-DPPEPM)(CH3CN)2](CF3SO3)2 (4). Addition of an excess of CO to a methanolic solution
of 1 results in the replacement of one of the chloride ions by CO to yield cis-[Fe(κ4-DPPEPM)Cl(CO)](Cl) (5). The same reaction
in CH2Cl2 produces a mixture of 5 and [Fe(κ3-DPPEPM)Cl2(CO)] (6) in which one of the internal phosphines has been
substituted by CO. Complexes 2, 3, 4, and 5 appear to be the first structurally characterized monometallic complexes of κ4-
DPPEPM.
’ INTRODUCTION
trans compounds with Group 8 metal centers21,22 with structures
similar to those of complexes with two bidentate ligands such as
trans-FeCl2(dmpe)2 (dmpe = bis(dimethylphosphino)ethane,23
[FeH(H2)(dmpe)2][BPh4],24 and FeCl2(DHPePE)2 (DHPePE =
1,2-bis(bis(hydroxypropyl)phosphino)ethane).25 The trans
geometry in these compounds may limit their reactivity by
suppressing processes that require cis-disposed valencies, al-
though cis-iron(II) complexes of bidentate and meso isomers of
linear tetradentate phosphines are also formed on oc-
casion.23,25-30
Ligand modifications can introduce strain, as well as control
the geometry and configuration of late metal polyphosphine
complexes. One approach involves replacing the central ethylene
unit in DPPEPE with a methylene to shorten the ligand back-
bone and provide the ligand Ph2PCH2CH2PPhCH2PPhCH2-
CH2PPh2 (DPPEPM, Chart 1).31 Monometallic rhodium(III)
compounds of the related tetraphosphine-containing diethyl-
phosphino groups, Et2PCH2CH2PPhCH2PPhCH2CH2PEt2
(DEPEPM), have been reported. The configuration of
Multidentate ligands are often used to provide stabilized
coordination complexes through the chelate effect. At the same
time, the structure of a polydentate ligand provides means to
control the bite angle, chelate ring size, and stereochemistry. The
resulting electronic and steric effects can be used to enhance the
reactivity of a metal center. In well-studied organometallic
examples, ansa-metallocenes1-3 and constrained-geometry com-
pounds4-9 provide enhanced reactivity in olefin polymerization
and bond activation, while inhibiting disproportionation reac-
tions. Similarly, bidentate phosphines enforce cis coordination
geometries, even though trans configurations might be sterically
or electronically preferred, and tridentate phosphines such as
MeSi(CH2PMe2)310-12 and [PhB(CH2PiPr2)3]13,14 enforce fac-
coordination modes. Branched tetradentate ligands, such as
P(CH2CH2PR2)3
and P(CH2CH2CH2PMe2)3,17-19 are
15,16
constrained to keep the nonphosphine “reactive” coordination
sites disposed cis.
Linear tetradentate ligands can coordinate in several config-
urations, including trans, cis-R, and cis-β.20 Notably, the linear,
ethylene-bridged, tetraphosphine ligand Ph2CH2CH2PPhCH2-
CH2PPhCH2CH2PPh2 (DPPEPE, Chart 1) typically provides
Received: December 15, 2010
Published: March 07, 2011
r
2011 American Chemical Society
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[(DEPEPM)RhCl2]þ and [(DEPEPM)RhCl(CH2Cl)]þ is cis-
R.32 Palladium and platinum compounds of the perphenylated
DPPEPM, on the other hand, are typically bimetallic,33 although
monometallic complexes containing κ2- or κ3-DPPEPM can
also be obtained under appropriate conditions.34 In group 8
chemistry, a related hexaphosphine ligand (Et2PCH2CH2)2-
PCH2P(CH2CH2PEt2)2 (eHTP) coordinates in a tetradentate,
cis-R configuration to Fe(II).35 Bis(diphospholanylethane) iron(II)
chloride [(BPE5)2FeCl2] adopts a cis-R geometry presumably
for steric reasons,26 while κ4-{P(CH2CH2PMe2)3}FeX2 gives
cis-complexes because of the connectivity of the ancillary
ligand.15,16
by its short backbone into folded geometry, should be more
efficient in enabling monometallic reactivity by positioning the
substrate and reactants adjacent (cis) to each other.
The goal of this study was to synthesize iron(II) and
ruthenium(II) complexes of DPPEPM, establish expected co-
ordination modes, and identify substitution pathways to examine
the potential of such complexes in various catalytic reactions,
including reduction of nitrogen. Even though DPPEPM has been
known for several decades, group 8 metal complexes of this
ligand have not previously been reported. Herein, we describe
the synthesis, spectroscopic and structural characterization, and
reactivity of several DPPEPM coordination complexes.
Our interest in phosphine complexes of group 8 metals arises
from their potential in nitrogen activation and reduction.22,24,36
In a recent example, an iron complex containing two bidentate
phosphine ligands has been shown to react with molecular
hydrogen and nitrogen through a series of intermediates to
generate measurable amounts of ammonia and hydrazine.36 On
the basis of the few examples described above, we expected that a
complex with a tetradentate P4 ligand, such as DPPEPM, forced
’ EXPERIMENTAL SECTION
All the reactions and manipulations were performed under a dry
nitrogen atmosphere in a glovebox or on a Schlenk line. Methylene
chloride, toluene, pentane, diethyl ether, and tetrahydrofuran were dried
and deoxygenated in an IT PureSolv system. Acetone was used
immediately after distillation. Tetrahydrofuran-d8 was heated to reflux
over Na/K alloy and vacuum-transferred. CD2Cl2 was heated to reflux
over CaH2 and vacuum-transferred. CD3OD was distilled and was
stored over molecular sieves.
Chart 1
The ligand DPPEPM was obtained as a mixture of isomers in 90%
yield by a literature procedure.31 The meso and rac diastereomers were
separated by fractional crystallization from ether. The meso form
precipitated first as a white powder and was separated by filtration.
The mother liquor was evaporated to dryness, and the sticky, colorless
1
Table 1. H NMR and 31P{1H} NMR Data and CO/CF Stretching Frequencies of the Reported Compoundsa
compounds
rac-DPPEPM31
solvent: 1H NMR, ppm
solvent: 31P{1H}, ppm
IR, cm-1
CD3OD: 1.82-2.00, 2.01-2.13, 2.14-2.22
(P-CH2-CH2-P); 2.25 (t, P-CH2-P)
C6D6: -25.8 (1Pint, dd); -25.2 (1Pint, dd);
-11.9 (2Pext, dd)
Fe(DPPEPM)Cl2 (1)
CD2Cl2: -3.20 (br), -0.7 to 1.34 (br), 3.21-4.02
(br), 6.25-9.41 (br), 11.09 (br), 13.60 (br)
CD2Cl2: -9.2 (br), 8.9 (br), 16.0 (br), 53.4 (br)
THF-d8: -2.7 (br), 7.4 (br), 15.8 (br), 54.8 (br),
58.8 (br)
CD3CN: 45.2 (2P, t, 2Jpp 35 Hz), 72.3 (2P, t, 2Jpp
35 Hz)
Fe(DPPEPM)Br2 (2)
Ru(DPPEPM)Cl2 (3)
CD2Cl2: 2.3 (br), 3.1 (br), 3.3 (br), 3.5 (br), 3.75
(br), 6.76 (m), 6.95 (m), 7.16 (m), 7.33 (m), 7.47
(m), 8.27 (m)
CD2Cl2: -1.9 (br), 8.8 (br), 14.8 (br), 51.8 (br),
56.9 (br), 65.2 (br)
CD2Cl2: 2.17 (m, PCH2CH2P), 2.36 (m,
PCH2CH2P), 2.95 (m, PCH2CH2P), 3.90 (m,
PCH2P), 6.80-8.00 (PPh)
CD2Cl2: 27.8 (2P, t, 2Jpp 19 Hz), 47.6 (2P, t, 2Jpp
19 Hz)
[Fe(DPPEPM)(CH3CN)2](OTf)2 (4) CD2Cl2: 2.02 (s, CH3CN), 2.20 (m, PCH2CH2P),
2.60 (m, PCH2CH2P), 2.96 (m, PCH2CH2P),
3.44 (m, PCH2CH2P), 4.13 (t, PCH2P), 6.98-
7.85 (m, PPh)
CD2Cl2: 42.8 (2P, t, 2Jpp 35 Hz), 69.7 (2P, t, 2Jpp νC-F = 1209
35 Hz)
[Fe(DPPEPM)Cl(CO)](Cl) (5)
CD3OD: 2.17-2.30 (br m, PCH2CH2P), 2.56-
2.82 (br m, PCH2CH2P), 2.88-3.25 (br m,
PCH2CH2P), 3.30 (br m, (PCH2CH2P and
PCH2P), 6.91 (m, Ph), 7.12 (m, Ph), 7.14 (m,
Ph), 7.20-7.40 (m, Ph), 7.43 (m, Ph), 7.56 (m,
Ph), 7.91 (m, Ph), 8.15 (m, Ph)
CD3OD: 26.7, 52.3, 61.8, 85.3 (all four P ddd, νCdO =1942
2JPP = 117, 115, 48, 31, 26, 22 Hz)
Fe(DPPEPM)Cl2(CO) (6)
CD2Cl2: 1.27-2.98 (br, PCH2CH2P
CD2Cl2: -11.5 (br, uncoordinated P), 20.9 (m), νCdO =1955
28.4 (m), 53.2 (m), 55.2 (m), 63.7 (m), 77.2
(m), 87.2 (m)
and PCH2P), 7.00-8.17 (m, PPh)
a Abbreviations: m = multiplet, s = singlet, d = doublet, t = triplet, br = broad.
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ARTICLE
residue was heated under vacuum at 200 °C for two hours to yield rac-
DPPEPM in 70% yield. 1H and 31P{1H} NMR data agreed well with the
literature values31 and are presented in Table 1.
room temperature. The reaction mixture was heated at 60 °C until
the color turned dark yellow. The solvent was removed under reduced
pressure to give a yellow solid. That solid was washed with 10 mL of
dry pentane, diethyl ether (2 ꢀ 10 mL), and dried under vacuum. Yield:
0.60 g (0.72 mmol, 89%). 1H NMR (CD2Cl2, 400 MHz): δ 2.17
(m, PCH2CH2P), 2.36 (m, PCH2CH2P), 2.95 (m, PCH2CH2P), 3.90
(m, PCH2P), 6.80-8.00 (PPh). 31P{1H} NMR (CD2Cl2, 162 MHz): δ
27.8 (2P, t, 2Jpp 19 Hz), 47.6 (2P, t, 2Jpp 19 Hz). IR (KBr, cm-1): 819 (w),
880 (w), 999 (w), 1028 (w), 1098 (m), 1158 (w), 1189 (w), 1262 (w),
1336 (w), 1409 (w), 1433 (s), 1484 (m), 1570 (w), 1585 (w), 3000 (w),
3074 (m). Anal. Calcd for C41H40P4Cl2Ru: C 59.43, H 4.87; Found C
59.62, H 4.75. mp: 194-196 °C.
Anhydrous FeCl2 (Strem) and FeBr2 (Sigma-Aldrich) were used as
39
received. Fe(CH3CN)2(OTf)2,37 Ru(COD)Cl2,38 and Ru(PPh3)3Cl2
were synthesized by literature procedures. Elemental analyses were
performed with a Perkin-Elmer2400 Series II CHN/S by the Iowa State
Chemical Instrumentation Facility. X-ray diffraction data were collected
with a Bruker-AXS SMART 1000 CCD diffractometer using Bruker-
AXSSHELXTL software. 1H NMR, 31P{1H} NMR, and 19F NMR
spectra were recorded with a Bruker 400 spectrometer. 1H NMR spectra
of the paramagnetic 1, and 31P{1H} NMR spectra of 2 were collected
with a relaxation delay of 130 ms to improve signal-to-noise ratio.
Spectra obtained with the more standard relaxation delay of one second
appeared to be identical but required much longer acquisition times for
Synthesis of [Fe(DPPEPM)(CH3CN)2](OTf)2 (4). DPPEPM
(0.54 g, 0.822 mmol) in 15 mL of CH2Cl2 was added dropwise to a
solution of Fe(CH3CN)2(OTf)2 (0.25 g, 0.822 mmol) in 5 mL of
CH2Cl2 at ambient temperature. The resulting reddish-orange solution
was allowed to stir for 14 h. After the removal of solvent under vacuum,
the reddish-orange residue was washed with 10 mL of pentane,
10 mL of diethyl ether, and dried under vacuum. Yield: 0.76 g (0.68
mmol, 82%). X-ray quality crystals were obtained from a concentrated
1
the same S/N ratio. The signals in the H NMR spectrum of 5 were
broad, but a reasonably sharp 31P{1H} NMR spectrum was obtained.
Paramagnetism of 1 and 2 was confirmed by the Evans method,40 3.6
and 2.1 unpaired electrons for 1 and 2, respectively.
Synthesis of Fe(DPPEPM)Cl2 (1). A solution of 1.17 g (1.78
mmol) of rac-DPPEPM in 40 mL of CH2Cl2 was stirred in a 250 mL
Schenk round-bottom flask for 10 min at room temperature. Solid FeCl2
(0.226 g, 1.78 mmol) was slowly added at ambient temperature. The
resulting mixture was stirred for 8 h, after which time the solvent was
removed under reduced pressure. The violet solid product was washed
with 30 mL of pentane, diethyl ether (2 ꢀ 20 mL), and dried under
vacuum. Yield: 1.12 g (1.32 mmol, 80%). 1H NMR (400 MHz, CD2Cl2):
δ -3.20 (br), -0.7 to 1.34 (br), 3.21-4.02 (br), 6.25-9.41 (br), 11.09
(br), 13.60 (br). 31P{1H} NMR (CD2Cl2, 162 MHz): δ -9.2 (br), 8.9
(br), 16.0 (br), 53.4 (br). 31P{1H} NMR (THF-d8, 162 MHz): δ -2.7
(br), 7.4 (br), 15.8 (br), 54.8 (br), 58.8 (br). 31P{1H} NMR (CD3CN,
162 MHz): δ 45.2 (2P, t, 2Jpp 35 Hz), 72.3 (2P, t, 2Jpp 35 Hz). ESI-MS:
782 (Mþ), 747 (Mþ-Cl). IR (KBr, cm-1): 808 (w), 873 (w), 913 (w),
999 (w), 1026 (w), 1068 (w), 1097 (m), 1159 (w),1180 (w), 1272 (w),
1310 (w), 1331 (w), 1433 (s), 1482 (m), 1571 (w), 1585 (w), 1659 (w),
1815 (w), 1890 (w), 1963 (w), 2908 (w), 2960 (w), 3002 (w), 3057
(m). Anal. Calcd for C41H40P4Cl2Fe: C 62.86, H 5.15; Found C 62.49,
H 5.01. mp: 117-121 °C.
Synthesis of Fe(DPPEPM)Br2 (2). rac-DPPEPM (0.27 g,
0.40 mmol) was dissolved in 15 mL of THF. Solid FeBr2 (0.087 g,
0.40 mmol) was added at room temperature. The reaction mixture was
stirred for 8 h during which time the color changed from light green to
purple and then to gray. After evaporation of the solvent, the dark gray
residue was washed with diethyl ether (2 ꢀ 10 mL) and dried under
vacuum for 2 h. Yield: 0.290 g (3.3 mmol; 82%). Single crystals suitable
for X-ray diffraction were obtained from a concentrated THF solution at
room temperature over two days. 1H NMR (CD2Cl2, 400 MHz): δ 2.3
(br), 3.1 (br), 3.3 (br), 3.5 (br), 3.75 (br), 6.76 (m), 6.95 (m), 7.16 (m),
7.33 (m), 7.47 (m), 8.27 (m). 31P{1H} NMR (CD2Cl2): δ -1.9 (br),
8.8 (br), 14.8 (br), 51.8 (br), 56.9 (br), 65.2 (br). IR (KBr, cm-1): 818
(w), 880 (w), 999 (w), 1028 (w), 1071 (w), 1094 (m), 1159 (w), 1188
(w), 1262 (w), 1314 (w), 1410 (w), 1433 (s), 1482 (m), 1584 (w), 2920
(w), 2949 (w), 3057 (m). Anal. Calcd for C41H40P4Br2Fe: C 56.45,
H 4.62; Found C, 56.18; H, 4.72. mp: 202-206 °C.
1
acetone solution at room temperature after 2 d. H NMR (CD2Cl2,
400 MHz): δ 2.02 (5.6 H s, CH3CN), 2.20 (2.1 H m, PCH2CH2P), 2.60
(2.0 H m, PCH2CH2P), 2.96 (2.0 H m, PCH2CH2P), 3.44 (1.9 H m,
PCH2CH2P), 4.13 (1.8 H t, PCH2P, 2JPH 12 Hz), 6.98-7.85 (30.8 H m,
PPh). 31P{1H} NMR (CD2Cl2, 162 MHz): δ 42.8 (2P, t, 2Jpp 35 Hz),
2
69.7 (2P, t, Jpp 35 Hz). 19F NMR (CD2Cl2, 376 MHz): δ -77.4
(OSO2CF3). IR (KBr, cm-1): 813 (w), 873 (w), 999 (w), 1209
(triflate), 1099 (m), 1152 (s), 1223 (m), 1260 (vs, triflate), 1368 (w),
1483 (m), 1585 (w), 1658 (w, CN), 2922 (w), 2966 (m), 3053 (s). Anal.
Calc. for C47H46F6N2O8P4S2Fe: C 51.66, H 4.24, N 2.56; Found C
51.49, H 3.98, N 2.46. mp: 105-107 °C.
Synthesis of [Fe(DPPEPM)(Cl)(CO)]Cl (5). A violet solution of
Fe(DPPEPM)Cl2 (0.237 g, 0.30 mmol) in 25 mL of CH3OH was
degassed with three freeze-pump-thaw cycles. The solution was
frozen in a liquid nitrogen bath, and the headspace was charged with
carbon monoxide (1 atm). The flask was sealed, and the reaction
mixture was allowed to warm to room temperature and was stirred for
24 h, over which time the color changed from violet to orange. The
solvent was removed under reduced pressure, and the orange residue
was washed with diethyl ether (2 ꢀ 10 mL) and dried. Crystals for
structural analysis were grown from methanol at 0 °C. Yield: 0.16 g
(67%). 1H NMR (CD3OD, 400 MHz): δ 2.17-2.30 (br m,
PCH2CH2P), 2.56-2.82 (br m, PCH2CH2P), 2.88-3.25 (br m,
PCH2CH2P), 3.30 (br m, (PCH2CH2P and PCH2P), 6.91 (m, Ph),
7.12 (m, Ph), 7.14 (m, Ph), 7.20-7.40 (m, Ph), 7.43 (m, Ph), 7.56 (m,
Ph), 7.91 (m, Ph), 8.15 (m, Ph). 31P{1H} NMR (CD3OD, 162 MHz):
δ 26.7, 52.3, 61.8, 85.3 (all four P ddd, 2JPP = 117, 115, 48, 31, 26, 22
Hz). IR (KBr, cm-1): 798 (w), 877 (w), 998 (w), 1026 (vs), 1103 (m),
1159 (w), 1188 (w), 1269 (w), 1311 (w), 1433 (s), 1482 (m), 1584
(w), 1942 (vs, CO), 2827 (w), 2903 (w), 2941 (w), 3077 (s). Anal.
Calcd for C42H40OP4Cl2Fe: C 62.17, H 4.97; Found C 62.49, H 5.01.
mp: 219-222 °C.
Synthesis of Fe[(DPPEPM)Cl2(CO)] (6). Fe(DPPEPM)Cl2
(0.167 g; 0.21 mmol) was dissolved in 15 mL of CH2Cl2 in a 100 mL
Schlenk flask. The violet solution was degassed with three freeze-
pump-thaw cycles and charged with one atmosphere of carbon
monoxide at liquid nitrogen temperature. The sealed solution was
allowed to warm to room temperature and was stirred for 24 h. During
this time, the color of the solution changed from violet to yellow. The
solvent was removed under reduced pressure, and the yellow residue
washed with 2 ꢀ 10 mL of diethyl ether. Single crystals for X-ray crystal
structure determination were obtained by dissolving the residue in
20 mL of CH2Cl2 and placing the solution in a freezer at -30 °C.
The crystals formed within several days. Yield: 0.15 g (0.18 mmol, 87%).
Synthesis of Ru(DPPEPM)Cl2 (3). Method 1. DPPEPM (0.216 g,
0.33 mmol), dissolved in 10 mL of CH2Cl2, was added dropwise to a
suspension of RuCl2(PPh3)4 (0.402 g, 0.33 mmol) in CH2Cl2. Thesolution
turned yellow. After the mixture was stirred for 8 h, the solvent was removed
under reduced pressure to give a light yellow solid that was washed with
10 mL of pentane, diethyl ether (2 ꢀ 20 mL), and dried under vacuum.
Yield: 0.22 g (0.27 mmol; 81%).
Method 2. A solution of DPPEPM (0.54 g, 0.82 mmol), dissolved
in 15 mL of CH2Cl2, was added dropwise to a suspension of [Ru-
(COD)Cl2]2 (0.230 g, 0.82 mmol) in 10 mL of CH2Cl2 over 5 min at
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peak at 747 (Mþ-Cl). Attempts to obtain an X-ray quality single
crystal of 1 were not successful.
Violet solutions of 1 in CD2Cl2 exhibit four broad resonances
in the 31P{1H} NMR spectrum (CD2Cl2) at -9.2, 8.9, 16.0,
and 53.4 ppm, Table 1. (Additional spectroscopic and structural
information for the complexes presented in this work are
given in the Supporting Information). This pattern is indicative
of tridentate DPPEPM coordination.31,35,41 The signal at
-9.2 ppm is close to that for the PPh2 groups in the free
ligand31 (-11.9 ppm, see Table 1) and is assigned to an
uncoordinated external PPh2 group in the complex. The reso-
nances at 8.9 and 16.0 ppm correspond to the internal PPh
groups and that at 53.4 ppm to a coordinated external PPh2.
THF solutions of 1are red and exhibit a 31P{1H} NMR (THF-d8)
spectrum similar to that in CD2Cl2, except that there are now two
downfield resonances corresponding to coordinated PPh2. This
pattern is consistent with a mixture of two complexes, where the
second complex likely contains a coordinated THF molecule.
The complex Fe(DPPEPM)Br2 (2) was synthesized from
FeBr2 and one equivalent of rac-DPPEPM in THF at room
temperature. The compound was isolated as a dark-gray solid
that was poorly soluble in THF and moderately soluble in
CH2Cl2. A single crystal X-ray diffraction analysis revealed four
phosphorus atoms and two bromide ions coordinated in a
pseudo-octahedral, cis-R conformation, Figure 1. Both bromide
ions are trans to the internal phosphorus atoms. In this com-
pound, and in all of the κ4-DPPEPM complexes described in this
work, the Fe-P (internal) bonds are somewhat shorter than
Fe-P (external) bonds. In particular, the Fe1-P1 and Fe1-P3
bonds lengths in the PhPCH2PPh moiety are 2.209(1) and
2.194(1) Å, whereas Fe1-P2 and Fe1-P4 distances are
2.290(1) and 2.320(1) Å.. This is believed to be a result of the
elongation and bending of Fe-P(external) bonds to reduce the
overall strain, but this motion pulls the internal phosphines closer
to the metal. The weaker trans influence of bromide in comparison
to a diarylalkylphosphine may also favor shorter Fe-P(internal)
distances, as was reported for the cis-dichloro complexes (BPE5)2-
FeCl2 and P(CH2CH2PMe2)3FeCl2 complexes.17,26
Figure 1. ORTEP drawing of Fe(DPPEPM)Br2 (2). Thermal ellipsoids
are drawn at 35% probability level. Two molecules of FeBr2(DPPEPM)
are found in a unit cell, and both possess identical connectivity and
similar bond lengths and angles. One molecule and the hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and angles [deg]:
Br1-Fe1 2.5149(7), Br2-Fe1 2.5054(7), Fe1-P3 2.1939(13), Fe1-
P1 2.2092(12), Fe1-P2 2.2897(14), Fe1-P4 2.3204(13), P3-Fe1-
P1 73.12(5), P3-Fe1-P2 98.36(5), P1-Fe1-P2 83.80(5), P3-Fe1-
P4 85.29(5), P1-Fe1-P4 105.80(5), P2-Fe1-P4 170.38(5), P3-
Fe1-Br2 100.08(4), P1-Fe1-Br2 167.72(4), P2-Fe1-Br2 87.19(4),
P4-Fe1-Br2 83.39(4), P3-Fe1-Br1 162.52(4), P1-Fe1-Br1
91.52(4), P2-Fe1-Br1 87.95(4), P4-Fe1-Br1 91.13(4), Br2-
Fe1-Br1 96.48(3).
1H NMR (CD2Cl2, 400 MHz): δ 1.27-2.98 (br, PCH2CH2P and
PCH2P), 7.00-8.17 (m, PPh). 31P{1H} NMR (CD2Cl2, 162 MHz): δ
-11.5 (br, noncoordinated P), 20.9 (m), 28.4 (m), 53.2 (m), 55.2 (m),
63.7 (m), 77.2 (m), 87.2 (m). IR (KBr, cm-1): 806 (w), 869 (w), 999
(w), 1026 (vs), 1099 (m), 1159 (w), 1189 (w), 1255 (w), 1309 (w),
1434 (s), 1484 (m), 1585 (w), 1955 (vs, CO), 2913 (w), 2964 (w), 3053
(s). Anal. Calcd for C42H40OP4Cl2Fe: C 62.09, H 5.09; Found C 61.17,
H 5.16. mp: >300 °C. The 2% discrepancy between the experimental
and calculated values of carbon content is likely caused by the instability
of the complex.
In contrast to the solid state structure, 2 appears to have the P4
ligand bound in a κ3 mode in CD2Cl2 solution, as judged by
31P{1H} NMR spectrum, which exhibits features similar to those
of the dichloro compound. Specifically, the resonance in the
negative range, -1.9 ppm, is indicative of an uncoordinated
external PPh2 group. Also, two resonances are present in the low
field range, and a third one grows in at 56.9 ppm over time
(approximately 10 min at room temperature), indicating that yet
another form is generated. Despite the complexity of the 31P-
{1H} NMR spectra of 2, analytical data confirm that its con-
stitution is identical to that of the crystals characterized by X-ray
diffraction.
’ RESULTS AND DISCUSSION
Synthesis and Characterization. To facilitate the formation
of cis-R metal complexes, we focused on the racemic diastereomer
of the ligand. Work with related rhodium complexes32 has shown
that coordination of the meso form generates a less stable
complex because of unfavorable steric factors.
The reaction of rac-DPPEPM with one equivalent of FeCl2 in
methylene chloride at room temperature, eq 1, yielded Fe(κ3-
DPPEPM)Cl2 (1) in 80% yield. Compound 1 is air sensitive and
soluble in CH2Cl2, alcohols and THF, but insoluble in benzene,
pentane, and toluene. The ESI-MS spectrum of a solution in
CH2Cl2 shows the molecular ion peak at m/z 782 (Mþ) and base
The interpretation of the NMR spectra of compounds 1 and 2
is further complicated by their paramagnetic natures (in contrast,
a related tetradentate phosphine complex FeL4Cl2 bearing a self-
assembled phosphine is diamagnetic)42 and the difference be-
tween solution and solid state structures. To better assess the
solution structures of these compounds, we sought a ruthenium
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Figure 2. ORTEP drawing of Ru(κ4-DPPEPM)Cl2 (3). Thermal
ellipsoids are drawn at 35% probability level. Two molecules of CH2Cl2
and hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: Ru1-P2 2.2489(5), Ru1-P1 2.2657(5), Ru1-P3
2.3564(5), Ru1-P4 2.3650(5), Ru1-Cl2 2.4658(5), Ru1-Cl1
2.4885(5), P2-Ru1-P1 72.12(2), P2-Ru1-P3 83.90(2), P1-
Ru1-P3 96.81(2), P2-Ru1-P4 94.29(2), P1-Ru1-P4 83.607(2),
P3-Ru1-P4 177.91(2), P2-Ru1-Cl2 95.17(2), P1-Ru1-Cl2
166.39(2), P3-Ru1-Cl2 86.375(2), P4-Ru1-Cl2 92.745(2), P2-
Ru1-Cl1 170.05(2), P1-Ru1-Cl1 104.510(2), P3-Ru1-Cl1
87.270(2), P4-Ru1-Cl1 94.614(2), Cl2-Ru1-Cl1 88.824(2).
Figure 3. ORTEP drawing of the cationic portion [Fe(κ4-DPPEPM)-
(Cl)(CO)]Cl (5). Thermal ellipsoids are drawn at 35% probability level.
Hydrogen atoms, two chloride counterions, and two molecules of
methanol (crystallization solvent) are not shown. Selected bond lengths
[Å] and angles [deg]:Fe1-C42 1.800(3), Fe1-P3 2.2389(1), Fe1-P4
2.2458(1), Fe1 P1 2.262(1), Fe1-P2 2.2626(2), Fe1-Cl42 2.312(5),
C42-Fe1-P3 163.1(7), C42-Fe1-P4 85.5(6), P3-Fe1-P4
84.34(5), C42-Fe1-P1 87.8(6), P3-Fe1-P1 102.99(5), P4-Fe1-
P1 172.48(6), C42-Fe1-P2 95.6(7), P3-Fe1-P2 72.60(5), P4-
Fe1-P2 98.21(5), P1-Fe1-P2 85.78(5).
analog as a less labile, diamagnetic derivative. Ru(DPPEPM)Cl2
(3) was prepared from rac-DPPEPM and either RuCl2(PPh3)4
or [RuCl2(COD)]2. The latter reaction is shown in eq 2. The
light-yellow, air-sensitive solid is soluble in THF and CH2Cl2.
There are five well-separated, broad signals of equal intensity in
the 1H NMR spectrum corresponding to five types of methylenic
protons. The diamagnetic 31P{1H} NMR spectrum in CD2Cl2
shows two triplets at 27.8 and 47.6 ppm corresponding to two
internal and two external phosphorus atoms, respectively.
hexacoordinated iron with the two cis positions occupied by
molecules of acetonitrile as shown by crystal structure analysis
of [Fe(DPPEPM)(CH3CN)2](OTf)2 (4). In this structure,
four phosphorus atoms and two acetonitrile molecules are
coordinated to iron(II) in a cis-R geometry. The terminal
Fe-P bonds are again somewhat longer than internal Fe-P
bonds, similar to the trend observed with compounds 2 and 3,
but in general, the average Fe-P bond lengths are not signifi-
cantly different from those in other iron complexes of neutral
phosphine ligands.43-45
The 31P{1H} NMR spectrum of 4 in CD2Cl2 solution shows
two pseudo-triplets at 42.8 and 69.7 ppm associated with two
different types of phosphorus atoms in the κ4-bound DPPEPM.
1
The H NMR (CD2Cl2) spectrum exhibits five well-separated
broad peaks of equal intensities assigned to coordinated DPPEPM,
and a singlet at 1.20 ppm associated with coordinated acetoni-
trile, Table 1. Free triflate anions exhibit a sharp singlet at
-77 ppm in 19F NMR, and the C-F stretch at ν = 1209 cm-1
in the IR spectrum (KBr). All of the NMR data identify the
solution species as κ4-coordinated, in contrast to the κ3 mode
observed in solution for the dichloro and dibromo complexes in
CD2Cl2. However, the 31P NMR spectrum of 1 in acetonitrile-d3
(δ 42 and 72) was similar to the 31P{1H} NMR spectrum of
4 suggesting that the chloride ligands are substituted by acetonitrile.
Substitution Reactions with CO. The reaction of 1 with
excess CO in methanol generated [Fe{DPPEPM}(CO)Cl]Cl
(5) in which one of the original chloride ions was replaced by
CO, eq 3. The 31P{1H} NMR spectrum of 5 exhibits four sets of
signals, each consisting of eight lines (ddd). This pattern is
The solid-state structure of 3 contains the DPPEPM ligand
coordinated to ruthenium in a tetradentate cis-R geometry as in 2
(Figure 2). As in the iron DPPEPM complex, the internal Ru-P
distances that are trans to chloride are shorter than the external
ones. In contrast to 2, however, the solid-state geometry of 3 is
consistent with the solution-state structure suggested by 31P{1H}
NMR spectroscopy.
The reaction between Fe(CH3CN)2(OTf)2 and DPPEPM
might have been expected to generate a product with reduced
coordination number because steric constraints should make it
difficult for two triflate anions to bind cis to each other and because
the labile triflate and acetonitrile ligands could readily dissociate. We
anticipated that a five-coordinate iron center containing an outer-
sphere triflate would have different substitution chemistry than 1.
Contrary to this reasoning, the reddish-orange product contains
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31P{1H} NMR spectrum in CD2Cl2 exhibits three new reso-
nances at 20.9, 55.3, and 77.2 ppm assigned to coordinated
phosphorus atoms. The signal corresponding to the uncoordi-
nated phosphorus at -11.5 pm is very broad suggesting fast
relaxation, caused perhaps by traces of free iron(II) in the sample.
Signals corresponding to dangling phosphorus have been shown
to disappear upon addition of even trace amounts of FeCl2 in
related complexes.35
Interestingly, the coordination of CO to 1 in CH2Cl2 was
accompanied by isomerization from 1,4,6-P3 coordination of
DPPEPM to the iron center into a 1,4,9-P3 bonding mode. Both
external phosphorus atoms in the product (6) are coordinated,
but one of the internal phosphorus atoms is dissociated from the
metal. Although the internal dialkylphenylphosphino group is a
stronger donor than the external alkylphenylphosphines, the
cleavage of an Fe-P(internal) bond in a hexacoordinated
complex appears favorable as it should relieve the strain in the
bound ligand better than the cleavage of an Fe-P(external)
bond would. In the cationic product, 5, all four phosphorus
atoms are coordinated to the metal. The substitution of chloride
by CO appears to be more facile in methanol where 5 was the sole
observed product. The greater polarity of this solvent should
facilitate reactions that proceed through a polar transition state,
such as that expected for the dissociation of chloride from 1.
The apparent partial dissociation and isomerization of coordi-
nated DPPEPM induced by the strain in ligand backbone is one
of the features that may make these complexes catalytically active
in appropriate reactions. The ability of the catalyst to bind the
substrate and release products while preserving its own overall
structural integrity is one of the basic and most essential
requirements in catalytic chemistry.
Figure 4. ORTEP drawing of [Fe(κ3-DPPEPM)(CO)Cl2] (6). Ther-
mal ellipsoids are drawn at 35% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and angles [deg]: Fe1-
C42 1.751(3), Fe1-P2 2.2436(8), Fe-P1 2.2804(8), Fe1-P4
2.2928(8), Fe1-Cl2 2.3329(7), Fe1-Cl1 2.3716(8), P1-C1
1.822(3), C42-Fe1-P2 93.62(9), C42-Fe1-P1 89.86(9), P2-
Fe1-P1 86.74(3), C42-Fe1-P4 93.79(9), P2-Fe1-P4 92.83(3),
P1-Fe1-P4 176.35(3), C42-Fe1-Cl2 172.32(9), P2-Fe1-Cl2
92.35(3), P1-Fe1-Cl2 85.66(3), P4-Fe1-Cl2 90.73(3), C42-
Fe1-Cl1 81.81(9), P2-Fe1-Cl1 174.09(3), P1-Fe1-Cl1 89.50(3),
P4-Fe1-Cl1 91.20(3), Cl2-Fe1-Cl1 91.91(3).
consistent with a κ4 coordination of four nonequivalent phos-
phorus atoms, as in the structure shown in Figure 3. A large trans
coupling of 117 Hz in the 31P spectrum between a cis pair of
internal phosphorus atoms is likely due to a stronger coupling
through both the iron atom and the carbon atom of the
methylene group or due to unusually small P-Fe-P angle of
72.6°. A similar cis iron complex of κ4-eHTP ligand exhibits a
similar NMR spectrum with a large cis coupling constant.35 The
internal phosphine ligands are trans to Cl and CO and might be
used to assess the relative contributions of ligand structure and
trans influence on the geometry of (DPPEPM)Fe(II) com-
pounds. Indeed, the Fe-P(internal) distances are similar to
the Fe-P(external) distances in 5. However, the chloride and
carbonyl ligands are disordered over the two coordination sites
limiting the interpretations of the structural data for 5.
’ CONCLUSIONS
All of the iron complexes of DPPEPM in this work, as well as
ruthenium example, exhibit R (folded) geometry. In solid
complexes 2, 3, 4, and 5, the ligand DPPEPM is coordinated in
a κ4 mode and bound in a cis-R configuration. To the best of our
knowledge, these are first monometallic, κ4 complexes of
DPPEPM ever reported. Typically, these types of ligands either
bind with lower denticity, κ2 or κ3, or form bimetallic
complexes,32,34 although a κ4 Rh(III) complex of the related
Et2PCH2CH2P(Ph)CH2P(Ph)CH2CH2PEt2
has
been
prepared.32
In CD2Cl2 solutions, one of the Fe-PPh2(external) bonds in
the halo complexes 1 and 2 and in chloro carbonyl complex 6 is
cleaved to reduce the denticity of DPPEPM to three. Also,
complexes 1 and 2 appear to be present in solution in more
than one form as shown by H and 31P NMR in THF and
1
CH2Cl2. Moreover, the two chlorides in 1 are labile as shown by
the appearance of the bis-acetonitrile complex 4 upon dissolution
of 1 in CH3CN, Table 1. The apparent kinetic lability of
individual sites, combined with the retention of structural
integrity of the molecule is an essential feature to be exploited
in future design of DPPEPM-based catalysts.
The dicationic bis-acetonitrile complex 4 retains the η4 ligand
binding mode even in solution, as does the singly charged chloro
carbonyl complex 5 in methanol and in CD2Cl2. The fine
interplay between the overall charge, solvent polarity, and ligand
hapticity is another feature that should allow fine-tuning of the
potential catalytic activity of these complexes. Even the net
outcome of the reaction with CO can be altered by the change
The same reaction in CD2Cl2 generated a mixture of 5 and a
comparable amount of another complex that was identified as
(κ3-DPPEPM)FeCl2(CO) (6) by crystal structure analysis,
Figure 4. In addition to the four resonances characteristic of 5
(and shifted in CD2Cl2 to 28.4, 53.2, 63.7, and 87.2 ppm), the
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of solvent,46 as shown in eq 3. One should expect similar sen-
sitivity to reaction conditions with other substrates.
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’ ASSOCIATED CONTENT
S
Supporting Information. Figures S1-S9. This informa-
b
tion is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: pvp@iastate.edu (O.P.); bakac@ameslab.gov (A.B.);
sadow@iastate.edu (A.S.).
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The support for this project from the Iowa Energy Center
(B.J., A.S., A.B., A.E.) and the U.S. Department of Energy,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences through the Ames Laboratory
(O.P.) and is gratefully acknowledged. The Ames Laboratory is
operated for the U.S. Department of Energy by Iowa State
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