Trapping of unsaturated small molecules through cyclization with iron-iminophosphorane complexes

Article abstract of DOI:10.1021/om0510728

The iron-iminophosphorane-carbonyl complex Cp*(CO) ₂Fe{P(NPh)(OMe)₂} (1; Cp* stands for η⁵-C₅Me₅) reacted with the activated alkyne dimethyl acetylenedicarboxylate (DMAD) to give Cp*(CO)-Fe{P(OMe) ₂N(Ph)C(CO₂Me)C(CO₂Me)C(O)}, in which the alkyne was trapped between the imino nitrogen and one carbonyl carbon to form a six-membered metallacycle. In contrast, 1 reacted with CO₂ to afford aza-Wittig-type metathesis products, PhN=C=NPh and oxophosphorane (phosphonate) complex Cp*(CO)₂Fe{P(O)(OMe)₂}, probably via a four-membered aza-phosphacycle as an intermediate. Free acetonitrile did not react with 1, while the coordinated acetonitrile in [Cp*(CO)(NCMe) Fe{P(NHPh)-(OMe)₂}]PF₆ was incorporated into a five-membered metallacycle through a base-catalyzed rearrangement to give [Cp*(CO)Fe{P(OMe)₂N(Ph)C(Me)N(H)}]PF₆, in which the nitrile carbon was bonded to the imino nitrogen. It was proposed that this intramolecular cyclization reaction is initiated by the formation of the neutral iminophosphorane complex Cp*(CO)(NCMe)Fe{P(NPh)(OMe)₂} as an intermediate, followed by the prompt nucleophilic attack of the resulting imino nitrogen to the carbon atom of the coordinated acetonitrile.

Full text of DOI:10.1021/om0510728

3238
Organometallics 2006, 25, 3238-3244
Trapping of Unsaturated Small Molecules through Cyclization with
Iron-Iminophosphorane Complexes
Kazuyuki Kubo, Takayuki Baba, Tsutomu Mizuta, and Katsuhiko Miyoshi*
Department of Chemistry, Graduate School of Science, Hiroshima UniVersity,
Higashi-Hiroshima, 739-8526, Japan
ReceiVed December 16, 2005
The iron-iminophosphorane-carbonyl complex Cp*(CO)2Fe{P(NPh)(OMe)2} (1; Cp* stands for η5-
C5Me5) reacted with the activated alkyne dimethyl acetylenedicarboxylate (DMAD) to give Cp*(CO)-
Fe{P(OMe)2N(Ph)C(CO2Me)C(CO2Me)C(O)}, in which the alkyne was trapped between the imino
nitrogen and one carbonyl carbon to form a six-membered metallacycle. In contrast, 1 reacted with CO2
to afford aza-Wittig-type metathesis products, PhNdCdNPh and oxophosphorane (phosphonate) complex
Cp*(CO)2Fe{P(O)(OMe)2}, probably via a four-membered aza-phosphacycle as an intermediate. Free
acetonitrile did not react with 1, while the coordinated acetonitrile in [Cp*(CO)(NCMe)Fe{P(NHPh)-
(OMe)2}]PF6 was incorporated into a five-membered metallacycle through a base-catalyzed rearrangement
to give [Cp*(CO)Fe{P(OMe)2N(Ph)C(Me)N(H)}]PF6, in which the nitrile carbon was bonded to the imino
nitrogen. It was proposed that this intramolecular cyclization reaction is initiated by the formation of the
neutral iminophosphorane complex Cp*(CO)(NCMe)Fe{P(NPh)(OMe)2} as an intermediate, followed
by the prompt nucleophilic attack of the resulting imino nitrogen to the carbon atom of the coordinated
acetonitrile.
as P+-N- isoelectronic to the PdC bond in phosphorus ylides
Introduction
(R3PdCR2 ≈ R3P+-C-R2) well-known as Wittig reagents. In
Iminophosphoranes (R3PdNR), which are variously referred
to as phosphinimines, phosphoranimines, or monophosphazenes,
have been of special interest not only because of their
characteristic structures and versatile reactivities1,2 but also
because of their utilities as practical ligands in coordination
chemistry3 and as precursors of promising materials.4-6 The
PdN bond is described alternatively in an ionic representation
fact, iminophosphoranes readily react with unsaturated mol-
ecules, e.g., CO2, CS2, RNCO, SO2, etc., as well as carbonyl
compounds to provide aza-Wittig products, and so they have
been playing an important role in the syntheses of nitrogen-
containing compounds.1,2,7 The reactions with activated alkynes
have also been reported in relation to similar aza-Wittig
reactions.2,7a,8 These reactions are generally accepted to proceed
through a four-membered cycloadduct involving a pentacoor-
dinated phosphorus as an intermediate,1,2,7 which was isolated
and fully characterized in some cases (eq 1).8i,9
* To whom correspondence should be addressed. E-mail:
kmiyoshi@sci.hiroshima-u.ac.jp.
(1) Emsley, J.; Hall, D. In The Chemistry of Phosphorus: EnVironmental,
Organic, Inorganic, Biochemical and Spectroscopic Aspects; Harper &
Row: New York, 1976; Chaper 10.
(2) Johnson, A. W. In Ylides and Imines of Phosphorus; John Wiley &
Sons: New York, 1993.
(3) For recent review articles, see: (a) Dehnicke, K.; Stra¨hle, J.
Polyhedron 1989, 8, 707. (b) Dehnicke, K.; Weller, F. Coord. Chem. ReV.
1997, 158, 103. (c) Majoral, J.-P.; Igau, A. Coord. Chem. ReV. 1998, 176,
1. (d) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. ReV. 1999, 182,
19. (e) Cavell, R. G.; Kamalesh Babu, R. P.; Aparna, K. J. Organomet.
Chem. 2001, 617-618, 158. (f) Baier, F.; Fei, Z.; Gornitzka, H.; Murso,
A.; Neufeld, S.; Pfeiffer, M.; Ru¨denauer, I.; Steiner, A.; Stey, T.; Stalke,
D. J. Organomet. Chem. 2002, 661, 111. (g) Steiner, A.; Zacchini, S.;
Richards, P. I. Coord. Chem. ReV. 2002, 227, 193. (h) Weber, K.; Korn,
K.; Schorm, A.; Kipke, J.; Lemke, M.; Khvorost, A.; Harms, K.; Sunder-
meyer, J. Z. Anorg. Allg. Chem. 2003, 629, 744. (i) Raab, M.; Tirree´, J.;
Nieger, M.; Niecke, E. Z. Anorg. Allg. Chem. 2003, 629, 769. (j) Stephan,
D. W. Organometallics 2005, 24, 2548.
(4) Wisian-Neilson, P., Allcock, H. R., Wynne, K. J., Eds. In Inorganic
and Organic Polymers II. AdVanced Materials and Intermediates; ACS
Symposium Series 572; American Chemical Society: Washington, DC,
1994.
(5) For example, see: (a) Allcock, H. R. Chem. Eng. News 1985, 63,
22. (b) Manners, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1602. (c) Gates,
D. P.; Manners, I. J. Chem. Soc., Dalton Trans. 1997, 2525. (d) Allcock,
H. Phosphorus, Sulfur Silicon Relat. Elem. 2004, 179, 661.
(6) For reviews on iminophosphoranes as precursors of polyphosp-
hazenes, see: (a) Neilson, R. H.; Wisian-Neilson, P. Chem. ReV. 1988, 88,
541. (b) Wisian-Neilson, P.; Neilson, R. H. Inorg. Synth. 1989, 25, 69. (c)
Wang, B.; Klaehn, J.; Neilson, R. Phosphorus, Sulfur Silicon Relat. Elem.
2004, 179, 821.
We recently reported the syntheses of transition-metalated
iminophosphoranes, Cp*(CO)2M{P(NPh)(OMe)2} (M ) Fe (1),
(7) For recent review articles on the aza-Wittig reactions, see: (a)
Gololobov, Y. G.; Kasukhin, L. F. Tetrahedron 1992, 48, 1353. (b) Molina,
P.; Vilaplana, M. J. Synthesis 1994, 1197.
(8) For recent examples, see: (a) Bellan, J.; Sanchez, M.; Marre, M.-
R.; Beltran, A. M. Bull. Soc. Chim. Fr. 1985, 491. (b) Barluenga, J.; Lo´pez,
F.; Palacios, F. J. Chem. Soc., Chem. Commun. 1985, 1681. (c) Barluenga,
J.; Lo´pez, F.; Palacios, F. J. Chem. Soc., Chem. Commun. 1986, 1574. (d)
Barluenga, J.; Lo´pez, F.; Palacios, F. J. Chem. Soc., Perkin Trans. 2 1989,
2273. (e) Anders, E.; Markus, F. Chem. Ber. 1989, 122, 119. (f) Palacios,
F.; Ochoa de Rettana, A. M.; Pagalday, J. Tetrahedron 1999, 14451. (g)
Uchiyama, T.; Fujimoto, T.; Kakehi, A.; Yamamoto, I. J. Chem. Soc., Perkin
Trans. 1 1999, 1577. (h) Kano, N.; Xing, J.-H.; Kikuchi, A.; Kawashima,
T. Heteroat. Chem. 2001, 12, 282. (i) Kano, N.; Kikuchi, A.; Kawashima,
T. Chem. Commun. 2001, 2096.
(9) (a) Kano, N.; Xing, J.-H.; Kawa, S.; Kawashima, T. Tetrahedron
Lett. 2000, 41, 5237. (b) Kano, N.; Xing, J.-H.; Kawa, S.; Kawashima, T.
Polyhedron 2002, 21, 657.
10.1021/om0510728 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 05/19/2006

Cyclization with Iron-Iminophosphorane Complexes
Organometallics, Vol. 25, No. 13, 2006 3239
Ru), and demonstrated that the imino nitrogen is fairly basic.10
In general, transition-metal-functionalized phosphorus com-
pounds display unique reactivities, in part because of the
enhanced electron density on the phosphorus by the metal
fragment, if covalently bonded to it. One of the simplest
examples is a phosphide complex, that is, metallaphosphine,
which is highly nucleophilic and susceptible to oxidation. For
instance, Paine et al. reported that an iron-phosphide-carbonyl
complex, Cp(CO)2Fe[PPh{N(SiMe3)2}], traps an activated
alkyne, hexafluoro-2-butyne (F3CCtCCF3), to form a conden-
sation product with a five-membered metallacycle (eq 2).11 The
related cyclization reactions with various unsaturated organic
small molecules have also been reported for several phosphide
complexes.12
Figure 1. ORTEP drawing of 2 showing the non-hydrogen atoms
as 50% probability thermal ellipsoids with a numbering scheme.
Selected bond distances (Å) and angles (deg): Fe(1)-C(26) )
1.9286(18), Fe(1)-P(1) ) 2.1120(5), P(1)-N(1) ) 1.7275(16),
N(1)-C(20) ) 1.398(2), C(20)-C(23) ) 1.351(3), C(23)-C(26)
) 1.513(2), C(26)-Fe(1)-P(1) ) 85.97(6), N(1)-P(1)-Fe(1) )
113.29(6), C(20)-N(1)-P(1) ) 122.67(13), C(23)-C(20)-N(1)
) 124.68(16), C(20)-C(23)-C(26) ) 121.32(16), C(23)-C(26)-
Fe(1) ) 116.73(12).
The above-mentioned parallel features between organic
iminophosphoranes and transition-metalated phosphorus com-
pounds prompted us to investigate the reactions of the transition-
metalated iminophosphoranes with various organic and inorganic
molecules having multiple bonds. To date, quite limited reports
deal with the reactivities of metalated iminophosphoranes.
Majoral et al. reported that a zirconium-iminophosphorane,
which easily rearranges into an N-coordinated zirconium-
phosphinoamide at room temperature, affords a variety of
products by breaking the zirconium-phosphorus bond in the
reactions with several reagents including some unsaturated
molecules (vide infra).13 Because the iron-iminophosphorane
1 is stable even at elevated temperatures,10 its reactions might
proceed with the iron-phosphorus bond retained, to provide
unprecedented heterocycles containing phosphorus, nitrogen, and
transition-metal atoms, which might exhibit fascinating
properties.14-16
starting complex 1 (122.3 ppm),10 indicating the lowered valency
1
of the phosphorus in 2. In the H NMR spectra, 1 shows one
doublet at 3.65 ppm (JHP ) 11.4 Hz) assignable to the
spectroscopically identical POMe groups, while 2 shows two
doublets at 3.45 (JHP ) 9.9 Hz) and 3.50 ppm (JHP ) 10.5 Hz)
due to the diastereotopic POMe groups, suggesting the asym-
metric geometry around the iron in 2. In addition to the POMe
groups, two inequivalent CO2Me groups are also observed as
two singlets. The 13C{1H} NMR spectrum of 2 shows a doublet
at 260.8 ppm (JCP ) 49.8 Hz), which is in a typical region for
the Fe-acyl carbonyls, together with a doublet at 218.2 ppm
(JCP ) 37.7 Hz) due to the terminal carbonyl.
Results and Discussion
Reaction with Activated Alkyne. The iron-iminophospho-
rane 1 was treated with DMAD (dimethyl acetylenedicarboxy-
late) in benzene overnight to give a yellow powder of 2 in
moderate yield (28%, eq 3). The IR spectrum shows only one
strong absorption band at 1943 cm-1, assignable to a terminal
carbonyl, together with absorptions at 1735, 1700 (sh), and 1577
cm-1 assignable to acyl carbonyls. The 31P{1H} NMR signal
of 2 (180.2 ppm) appears fairly downfield from that of the
(10) Kubo, K.; Nakazawa, H.; Inagaki, H.; Miyoshi, K. Organometallics
2002, 21, 1942.
(11) McNamara, W. F.; Duesler, E. N.; Paine, R. T. Organometallics
1986, 5, 1747.
The structure of 2 was unambiguously determined by a single-
crystal X-ray diffraction, and the ORTEP drawing is illustrated
in Figure 1 with selected bond distances and angles. Crystal
data and a summary of data collection and structure refinement
are reported in Table 1. The iron has a distorted piano-stool
configuration with one terminal carbonyl ligand. DMAD has
been incorporated between the imino nitrogen and one carbonyl
carbon to form a six-membered metallacycle, in which the bond
length of C(20)-C(23) (1.351(3) Å) is in a normal range for
CdC double bonds. The N(1) atom in 2 has a perfectly planar
geometry, which is nearly perpendicular to the adjacent phenyl
ring plane, suggesting that its lone pair electrons are π-donated
rather toward the olefinic carbon and/or the phosphorus atom-
(12) For example, see: (a) Ashby, M. T.; Enemark, J. H. Organometallics
1987, 6, 1323. (b) Adams, H.; Bailey, N. A.; Day, A. N.; Morris, M. J.;
Harrison, M. M. J. Organomet. Chem. 1991, 407, 248. (c) Mercier, F.;
Ricard, L.; Mathey, F. Organometallics 1993, 12, 98. (d) Malisch, W.; Spo¨rl,
A.; Pfister, H. J. Organomet. Chem. 1998, 568, 241. (e) Malisch, W.;
Thirase, K.; Reising, J. J. Organomet. Chem. 1998, 568, 247. (f) Malisch,
W.; Rehmann, F.-J.; Jehle, H.; Reising, J. J. Organomet. Chem. 1998, 570,
107. (g) Malisch, W.; Klu¨pfel, B.; Schumacher, D.; Nieger, M. J.
Organomet. Chem. 2002, 661, 95.
(13) (a) Igau, A.; Dufour, N.; Mahieu, A.; Majoral, J.-P. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 95. (b) Mahieu, A.; Igau, A.; Jaud, J.; Majoral,
J.-P. Organometallics 1995, 14, 944.
(14) Witt, M.; Roesky, H. W. Chem. ReV. 1994, 94, 1163.
(15) Weber, L. Coord. Chem. ReV. 1997, 158, 1.
(16) Caminade, A.-M.; Majoral, J.-P. Chem. ReV. 1994, 94, 1183.

3240 Organometallics, Vol. 25, No. 13, 2006
Table 1. Summary of Crystal Data for 2, 4, and 7
Kubo et al.
2
4
7
empirical formula
fw
C26H32FeNO8P
573.35
C21H30F6FeN2O3P2
590.26
C21H30F6FeN2O3P2
590.26
cryst size (mm)
temperature (K)
radiation
cryst syst
space group
unit cell dimens
a (Å)
0.33 × 0.28 × 0.18
0.31 × 0.26 × 0.13
0.55 × 0.48 × 0.38
200(1)
200(1)
200(1)
Mo ΚR (0.71073 Å)
orthorhombic
Pbca (#61)
Mo ΚR (0.71073 Å)
orthorhombic
P212121 (#19)
Mo ΚR (0.71073 Å)
monoclinic
P2/c (#13)
15.4540(3)
14.6940(3)
23.7370(3)
8.5180(2)
15.7840(4)
19.4580(4)
11.1990(2)
17.4430(3)
14.0260(2)
103.4380(10)
2664.88(8)
4
b (Å)
c (Å)
â (deg)
V (Å3)
5390.22(17)
8
1.413
2616.09(11)
4
1.499
Z
D
calcd (Mg/m3)
1.471
F(000)
2400
0.668
2.98 to 27.89
0 e h e 20
0 e k e 19
-29 e l e 0
6184
6184
1216
0.767
1.66 to 27.93
0 e h e 11
0 e k e 20
0 e l e 25
3507
3507
1216
0.753
µ (mm-1
)
θ range for data collection (deg)
index range
1.17 to 27.90
0 e h e 14
0 e k e 22
-18 e l e 17
6370
no. of reflns collected
no. of indep reflns
6370
no. of refined params
struct soln
462
325
473
direct methods (SHELX-97)
1.073
R1 ) 0.0441, wR2 ) 0.1105
R1 ) 0.0478, wR2 ) 0.1124
0.360 and -0.718
direct methods (SHELX-97)
1.080
R1 ) 0.0580, wR2 ) 0.1238
R1 ) 0.0738, wR2 ) 0.1411
0.440 and -0.751
0.06(4)
direct methods (SHELX-97)
1.029
R1 ) 0.0414, wR2 ) 0.1036
R1 ) 0.0471, wR2 ) 0.1056
0.707 and -0.855
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole (e/Å3)
absolute struct param
(s). The P(1)-N(1) bond in 2 (1.7275(16) Å) is significantly
longer than the corresponding PdN bond in 1 (1.574(1) Å),10
indicative of its reduced bond order in 2. This observation
reveals that the phosphorus in 2 is thus that of coordinated
phosphite in nature rather than that of metalated iminophos-
phorane, consistent with the 31P{1H} NMR spectrum. In other
words, the Fe-P bond has correspondingly changed in character
formally from covalent to dative through the reaction.
For related cyclization reactions studied for transition-metal-
carbonyl-phosphide and -thiolate complexes with alkenes or
alkynes, two possible mechanisms have been proposed.12a,17 One
is a concerted mechanism involving a cyclic transition state,
and the other is a stepwise mechanism involving an initial
nucleophilic attack of the P or S atom to an unsaturated carbon
to form a charge-separated intermediate. In either mechanism,
the more electron-rich the P or S atom, the more readily the
cyclization proceeds. In the corresponding cyclization of 1, the
strong nucleophilicity of the imino nitrogen must be similarly
crucial. Furthermore, the electron-withdrawing groups on the
alkyne play an essential role; when nonactivated alkynes, such
as phenyl- and diphenyl-acetylenes, were treated with 1 in place
of DMAD, no reaction occurred.
authentic sample18 reveals that 3 is a phosphonate complex with
a PdO group. The formation of the other aza-Wittig product
PhNdCdNPh was also confirmed by the GC-MS analysis of
the reaction mixture, indicating that PhNdCdO afforded by
the single aza-Wittig reaction reacted further with 1 to eventually
give PhNdCdNPh. This reaction is rationally expected to
proceed through a four-membered cycloadduct containing a
P-N-C-O linkage (eq 4) like the usual Wittig reaction. Unlike
in the reaction with DMAD, the formation of a six-membered
metallacycle was not observed. An extensive investigation on
aza-Wittig reactions of 1 is now under way with a variety of
carbonyl and thiocarbonyl compounds.
In the reaction with DMAD, 2 was the only identified product,
and there was no evidence for the formation of A and/or B
(eq 3), as expected in eq 1, although they could have im-
mediately decomposed in some way.
Reaction with CO2. In the Wittig-type reaction, the transient
formation of the highly strained four-membered phosphacycle
would be fully compensated for in energy by the subsequent
rearrangement leading to an extremely stable PdO bond. Such
energetic benefit might provide aza-Wittig-type products rather
than the metallacycle products even in the reaction of 1 with
carbonyl molecules. Thus 1 was treated with dry CO2 in benzene
for 30 min to give a pale yellow powder of 3 (85% yield, eq
4). Comparison of the spectroscopic data of 3 with those of the
The reactions with DMAD and with CO2 reveal that the
nucleophilic iron-iminophosphorane complex serves as an
efficient trapping reagent for various organic and inorganic
unsaturated molecules. Its reactions involve either of the two
different pathways, in both of which the iron-phosphorus bond
(18) Nakazawa, H.; Ichimura, S.; Nishihara, Y.; Miyoshi, K.; Nakashima,
S.; Sakai, H. Organometallics 1998, 17, 5061.
(17) Ashby M. T.; Enemark, J. H. Organometallics 1987, 6, 1318.

Cyclization with Iron-Iminophosphorane Complexes
Organometallics, Vol. 25, No. 13, 2006 3241
is retained, i.e., the metallacycle formation with the molecule
trapped between the imino nitrogen and carbonyl carbon for
DMAD, or the formation of a four-membered cycloadduct
bearing pentacoordinated phosphorus, which rearranges subse-
quently to aza-Wittig-type products for CO2.
Reaction with Acetonitrile. Organic iminophosphoranes
react similarly with nitriles via a four-membered cyclic inter-
mediate (eq 5). However, such nucleophilic additions of
iminophosphoranes occur only toward the activated nitriles
bearing strongly electron-withdrawing groups, such as CN, CF3,
or CCl3.2 Because a transition-metal fragment would enhance
the nucleophilicity of the phosphorus fragment bonded co-
valently to it, the metalated, i.e., activated, iminophosphoranes
might react even with nonactivated nitriles. Thus 1 was dissolved
in neat acetonitrile, but no reaction took place, with no expected
cyclization product being obtained (eq 6). This fact indicates
that the nitrile carbon as it is would not be electrophilic enough
to be attacked by the activated imino nitrogen in 1, and some
activation of the nitrile should be needed.
Figure 2. ORTEP drawing of 4 showing the non-hydrogen atoms
as 50% probability thermal ellipsoids with a numbering scheme.
The PF6 anion have been omitted for clarity. Selected bond distances
(Å) and angles (deg): Fe(1)-N(2) ) 1.936(5), Fe(1)-P(1) )
2.1753(16), P(1)-N(1) ) 1.657(5), N(2)-C(20) ) 1.130(8), N(2)-
Fe(1)-P(1) ) 92.24(15), N(1)-P(1)-Fe(1) ) 113.63(19), C(20)-
N(2)-Fe(1) ) 177.5(5), N(2)-C(20)-C(21) ) 178.4(8).
Because nitriles act as two-electron donor ligands and the
metal coordination will reduce the electron density on the nitrile
carbon, metal-coordinated nitriles are more susceptible to
nucleophilic attack than free nitriles.19 Thus the cationic
acetonitrile complex 4 was allowed to reacted with a base
(NaBH4), which might abstract the PN-H proton to provide a
neutral iminophosphorane fragment in situ (eq 7). The reaction
proceeded quantitatively, and the starting suspension im-
mediately turned into a clear solution. The product 7 was isolated
by column chromatography as an air- and moisture-stable yellow
Figure 3. ORTEP drawing of 7 showing the non-hydrogen atoms
as 50% probability thermal ellipsoids with a numbering scheme.
The PF6 anion have been omitted for clarity. Selected bond distances
(Å) and angles (deg): Fe(1)-N(2) ) 1.9642(16), Fe(1)-P(1) )
2.1331(5), P(1)-N(1) ) 1.7077(15), N(1)-C(20) ) 1.377(2),
N(2)-C(20) ) 1.285(2), N(2)-Fe(1)-P(1) ) 81.90(5), N(1)-
P(1)-Fe(1) ) 103.26(5), C(20)-N(1)-P(1) ) 114.18(12), C(20)-
N(2)-Fe(1) ) 123.65(13), N(2)-C(20)-N(1) ) 116.85(16).
1
powder (76% yield). The H NMR spectrum of 7 shows two
doublets assignable to the diastereotopic OMe groups on the
phosphorus atom, and the difference in chemical shifts of the
two groups is 0.38 ppm for 7 and only 0.07 ppm for 4. The
N(1)-H proton in 4 appears at 5.45 ppm as a broad doublet
(JHP ) 17.15 Hz), whereas the N(2)-H proton in 7 appears at
6.17 ppm as a broad singlet without JHP coupling. In contrast
to the 1H NMR, the 13C{1H} NMR spectrum shows a singlet at
133.6 ppm due to the NCMe carbon for 4, while a doublet (JCP
) 26.1 Hz) at further deshielded 173.5 ppm for 7. Similarly,
the NCMe carbon in 7 appears at 21.1 ppm, which is ca. 17
ppm downfield of that in 4. These observations indicate a
significant structural change in the nitrile ligand in 7. In the
31P{1H} NMR spectrum, 7 appears at 196.2 ppm, which is
downfield from that of 4 (169.7 ppm). The observation of a
septet 31P signal assignable to the PF6 anion indicates that the
complex formed remains positively charged.
The X-ray structural analyses were performed on both 4 and
7, and their ORTEP drawings are illustrated in Figures 2 and
3, respectively. 4 and 7 both have distorted piano-stool
geometries with one terminal carbonyl ligand on the iron atom.
The structural parameters of 4 are basically similar to those of
its protonated dicarbonyl analogue, [1H]+PF6-, previously
reported.10 The somewhat shorter Fe(1)-C(11) bond in 4
-
(1.747(7) Å) than the two Fe-CO bonds in [1H]+PF6
(1.784(2), 1.780(3) Å) indicates that the back-donation from
(19) For review articles on the reactions of transition-metal-coordinated
nitriles, see: (a) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord. Chem.
ReV. 1996, 147, 299. (b) Kukushkin, V. Y.; Pombeiro, A. J. L. Chem. ReV.
2002, 102, 1771.

3242 Organometallics, Vol. 25, No. 13, 2006
Kubo et al.
Scheme 1. Plausible Mechanism of the Base-Catalyzed
Rearrangement of 4 into 7
was obtained for the presence of an equilibrium between 7 and
4. It is noteworthy that the iron atom in the intermediate 5 plays
the following important roles in this rearrangement process: (1)
activation of the nucleophilic iminophosphorane fragment by
electron donation through the Fe-P covalent bond, (2) activation
of the electrophilic nitrile carbon by accepting the lone-pair
electrons of the nitrile through the Fe-N dative bond, and (3)
promotion of the reaction by keeping the iminophosphorane and
nitrile ligands close to each other in its coordination sphere.
Several related intramolecular cyclization reactions of metal-
coordinated nitriles have been reported.19 For instance, the
iridium-hydroxymethyl-acetonitrile complex [Ir(CH2OH)-
(NCMe)(PMe3)4]2+ undergoes a base-catalyzed intramolecular
cyclization via the nucleophilic attack of the hydroxymethyl
oxygen toward the nitrile to form a five-membered metalla-
cycle.20 This cyclization also takes place without a base, but
much more slowly. Similar metallacycle formation was also
reported for nitrile-containing complexes in the reactions with
hydroxylamine, hydrazines, sulfimides, pyrazoles, etc.19
Recently, the metallacycle formation involving aminophos-
phine ligands was reported for ruthenium-vinylidene or -al-
lenylidene complexes (eq 8),21 where intramolecular bond
formation took place between a weakly nucleophilic amino
nitrogen and a highly electrophilic R-carbon of the vinylidene
or the allenylidene ligand. In the corresponding cyclization of
4, the formation of the highly nucleophilic iminophosphorane
intermediate 5 is an essential step, since pure 4 itself is stable
and is not susceptible to the cyclization. Thus it should be
emphasized that the cyclization of 4 contrasts sharply with those
of the above ruthenium-aminophosphine complexes, in that the
former reaction is due largely to the nucleophilicity of the imino
nitrogen, whereas the latter to the electrophilicity of the R-carbon
of the vinilidene or the allenylidene.
the iron to the carbonyl carbon is more extensive in 4, suggesting
the ample electron donation from the accompanying acetonitrile
ligand to the iron. The amino nitrogen N(1) probably has a
trigonal-planar geometry (P(1)-N(1)-C(14) angle of 130.0-
(4)°), like in the usual P-NR2 compounds, and so no interaction
is anticipated between the lone-pair electrons on N(1) and the
nitrile carbon C(20) at this stage. The lone-pair electrons are
donated rather to the σ*-orbital on P(1), suggesting that the
nucleophilicity of the N(1) atom in 4 must be fairly low.
7 has a five-membered metallacycle structure with a new
bridging bond formed between the imino nitrogen N(1) and the
nitrile carbon C(20), in which the five ring atoms are located
practically on the same plane. The location of the N(2)-H
proton in 7 was determined experimentally; the N(2) atom,
originating from acetonitrile, is no doubt protonated. That is to
say, the N(1)-H proton in 4 is once abstracted with NaBH4,
but the N(2) is in turn protonated in 7, meaning that this
cyclization proceeds catalytically with a base (vide infra). The
N(2)-C(20) bond (1.285(2) Å) is fairly shorter than the N(1)-
C(20) bond (1.377(2) Å) in 7, but it is much longer than the
corresponding N(2)-C(20) bond (1.130(8) Å) in 4. This
indicates that the N(2)-C(20) bond has been reduced in bond
order in 7 compared with that in 4, but it still assumes some
multiple-bond character. Thus N(2) in 7 is depicted as an imino
nitrogen bonded datively to the iron in the formal electronic
structure. The P(1)-N(1) bond in 7 (1.7077(15) Å) is compa-
rable to that in 2 (1.7275(16) Å), suggesting that the phosphorus
in 7 is regarded also as that of phosphite coordinated to the
iron atom.
The plausible mechanism for the present cyclization is as
follows (Scheme 1): the starting cationic complex 4 reacts with
a base (NaBH4) to give a small amount of neutral metallaimi-
nophosphorane 5 as an intermediate, in which a highly nucleo-
philic lone pair on sp2-hybridization has been now generated
on the P-N nitrogen upon deprotonation. The subsequent
nucleophilic attack of the P-N nitrogen to the coordinated nitrile
carbon provides 6 with a five-membered metallacycle structure.
The resulting Fe-N nitrogen in 6 then promptly abstracts the
PN-H proton in the remaining 4 to give the final product 7
and to reproduce the neutral intermediate 5 as well. The
intermediates 5 and 6 could not be detected, and no evidence
Majoral et al. reported that a zirconium-iminophosphorane
reacts with nitriles to afford zirconacycles, in which the nitrile
is inserted into the zirconium-phosphorus bond and the PdN
nitrogen is additionally coordinated to the zirconium center (eq
9).13 In this reaction, the phosphorus fragment serves formally
as if it is iminophosphoranide Me2P-(dNAr) with the zirconium-
phosphorus bond broken, and its phosphorus atom, not its imino
nitrogen, attacks the nitrile carbon. Such a high reactivity of
the zirconium-phosphorus bond would reflect the unfavorable
combination of hard zirconium and soft phosphorus under the
(20) Thorn, D. L.; Calabrese, J. C. J. Organomet. Chem. 1984, 272, 283.
(21) Pavlik, S.; Mereiter, K.; Puchberger, M.; Kirchner, K. Organo-
metallics 2005, 24, 3561.

Cyclization with Iron-Iminophosphorane Complexes
Organometallics, Vol. 25, No. 13, 2006 3243
referenced to SiMe4 as an internal standard, and 31P NMR data to
85% H3PO4 as an external standard. IR spectra were recorded on
a Perkin-Elmer Spectrum One spectrometer. Elemental analysis data
were obtained on a Perkin-Elmer 2400 CHN elemental analyzer.
A Shimadzu QP-2000 was used to obtain GC-MS spectra. A Riko-
Kagaku Sangyo UVL-400HA high-pressure mercury lamp was used
for UV light irradiation.
HSAB concept, in contrast to the favorable soft-soft combina-
tion of the iron and phosphorus in 1. In addition, the resulting
zirconium complex is favorably stabilized by the coordination
of the hard CdN and PdN nitrogen atoms to form a metalla-
cycle.
Reaction of 1 with DMAD. To a benzene solution (10 mL) of
1 (777 mg, 1.80 mmol) was added DMAD (0.25 mL, 2.03 mmol),
and the mixture was stirred overnight at room temperature. After
removal of volatile components under reduced pressure, the residual
yellow oil was washed with pentane (10 mL) twice and then
dissolved in a small amount of benzene. The benzene solution was
charged on a silica gel column and eluted with benzene and then
with a mixture of benzene/acetone (1:1). A yellow band eluted was
collected and dried under vacuum. The resulting yellow residue
was washed with pentane until it became a fine yellow powder
and then dried under reduced pressure to give 2 (287 mg, 0.50
mmol, 28% yield). Anal. Calcd for C26H32NFeO8P: C, 54.47; H,
5.63; N, 2.44. Found: C, 54.07; H, 5.48; N, 2.39. IR (νCO, in
In the metallacycle 7, the P(OMe)2N(Ph)C(Me)N(H) moiety
is serving as a P-N chelate ligand toward the iron center. This
type of ligands has been of special interest because of their
potentially hemilabile behavior as well as their diversity in the
coordination modes, i.e., monodentate, bidentate, and bridging
toward homo or hetero metals.22,23 In this respect, the metal-
assisted cyclization of iminophosphoranes with nitriles can be
a new direct synthetic route toward the novel transition-metal
complexes bearing such fascinating P-N chelate ligands.
CDCl3): 1943, 1735, 1700(sh), 1577 cm-1 1H NMR (δ, in
.
CDCl3): 1.78 (s, 15H C5Me5), 3.35 (s, 3H,CO2Me), 3.45 (d, JHP
) 9.9 Hz, 3H, 3.50 (d, JHP ) 10.5 Hz, 3H, POMe), 3.67 (s, 3H,
CO2Me), 7.14-7.29 (m, 5H, Ph). 13C{1H} NMR (δ, in CDCl3):
9.4 (s, C5Me5), 51.5 (s, CO2Me), 51.6 (s, CO2Me), 52.9 (d, JCP
)
Concluding Remarks
9.1 Hz, POMe), 53.0 (d, JCP ) 9.1 Hz, POMe), 96.3 (s, C5Me5),
127.4 (s, Ph), 128.5 (s, Ph), 129.0 (s, Ph), 138.4 (d, JCP ) 0.9 Hz,
NCdC), 138.4 (s, NCdC), 142.9 (d, JCP ) 13.6 Hz, Ph), 164.3 (d,
JCP ) 8.2 Hz, NCC(O)OMe), 164.7 (s, CCC(O)OMe), 218.2 (d,
JCP ) 37.7 Hz, FeCO), 260.8 (d, JCP ) 49.8 Hz, FeC(O)C). 31P-
{1H} NMR (δ, in CDCl3): 180.2(s).
In the present paper, it was demonstrated that the highly
nucleophilic iron-iminophosphoranes trap various organic and
inorganic molecules having multiple bonds in two different
ways: one provides condensation products bearing a six- or
five-membered metallacycle structure, and the other provides
aza-Wittig-type metathesis products probably via a four-
membered aza-phosphacycle as an intermediate. The phosphorus-
nitrogen systems have been of special interest in the search for
wide-range applications to industrial and biomedical uses.4-6
In this respect, the reactive metallaiminophosphoranes and their
cyclic derivatives as well reported here are potential candidates
for building blocks for transition-metal-containing materials,
which might provide versatile physical and chemical proper-
ties.24
Reaction of 1 with CO2. After passing dry CO2 through a
benzene solution (2 mL) of 1 (113 mg, 0.26 mmol) for 30 min,
volatile components were removed under vacuum. The residue was
then dissolved in a small amount of acetone and charged on a silica
gel column. A yellow band eluted with EtOH was collected and
dried under reduced pressure to give a pale yellow powder of 3
(79 mg, 0.22 mmol, 85% yield), which was unambiguously
characterized by comparison of its spectroscopic data with those
of the authentic sample.18 The formation of PhNdCdNPh was
confirmed by the GC-MS analysis of the reaction mixture.
Preparation of 4. An acetonitrile solution (53 mL) of [Cp*-
(CO)2Fe(NCMe)]PF6 (734 mg, 1.70 mmol) was irradiated with UV
light for 1 h at 0 °C, and then volatile components were removed
under vacuum. After washing with ether several times, the residue
was dissolved in CH2Cl2 (25 mL), to which P{N(SiMe3)Ph}(OMe)2
(0.44 mL, 1.67 mmol) was added. The mixture was refluxed
overnight and then evaporated to dryness. The residue was dissolved
in THF (53 mL) containing H2O (2 mL) and stirred for 1 h at 45
°C. After removal of volatile components under vacuum, the
resulting crude product was dissolved in a small amount of CH2-
Cl2 and charged on a silica gel column. A reddish-yellow band
eluted with a mixture of CH2Cl2/acetone (9:1) was collected and
dried under vacuum. The residue was washed with ether several
times and dried under reduced pressure to give a yellow powder
of 4 (694 mg, 1.18 mmol, 69% yield). Anal. Calcd for C21H30N2F6-
FeO3P2: C, 42.73; H, 5.12; N, 4.75. Found: C, 42.77; H, 5.10; N,
Experimental Section
All reactions were carried out under an atmosphere of dry
nitrogen by using Schlenk-tube techniques and purified solvents.
Column chromatography was done quickly in the air. CH2Cl2 and
acetonitrile were purified by distillation from P2O5, while benzene,
pentane, ether, hexane, and THF were distilled from sodium metal
(with benzophenone ketyl for benzene, ether, and THF), and they
were stored under a nitrogen atmosphere. Other solvents, NaBH4
and DMAD, were obtained from common commercial sources and
used without further purification. CO2 gas was supplied from a
steam of dry ice flakes, which had been passed through a desiccant
(Sicapent, Merck) column. Cp*(CO)2Fe{P(NPh)(OMe)2} (1),10
10
[Cp*(CO)2Fe(NCMe)]PF6,25 and P{N(SiMe3)Ph}(OMe)2 were
prepared according to the published procedures.
A JEOL LA-300 spectrometer was used to obtain 1H NMR, 13
C
1
4.66. IR (νCO, in THF): 1970. H NMR (δ, in CDCl3): 1.68 (s,
15H, C5Me5), 2.38 (s, 3H, NCMe), 3.68 (d, JHP ) 12.0 Hz, 3H,
1
NMR, and 31P NMR spectra. H NMR and 13C NMR data were
POMe), 3.75 (d, JHP ) 11.8 Hz, 3H, POMe), 5.45 (br d, JHP
)
(22) For example, see: Wong, W.-Y.; Wong, W.-K.; Sun, C.; Wong,
W.-T. J. Organomet. Chem. 2000, 612, 160, and references therein.
(23) For example, see: Milton, H. L.; Wheatley, M. V.; Slawin, A. M.
Z.; Woollins, J. D. Inorg. Chim. Acta 2005, 358, 1393, and references
therein.
(24) (a) Allcock, H. A.; Desorcie, J. L.; Riding, G. H. Polyhedron 1987,
6, 119. (b) Manners, I. Science 2001, 294, 1664. (c) Manners, I. J. Polym.
Sci. Part A: Polym. Chem. 2002, 40, 179.
17.15 Hz, 1H, NH), 6.97-7.29 (m, 5H, Ph). 13C{1H} NMR (δ, in
CDCl3): 4.4 (s, NCMe), 9.2 (s, C5Me5), 53.0 (d, JCP ) 6.2 Hz,
POMe), 53.0 (d, JCP ) 5.6 Hz, POMe), 94.9 (d, JCP ) 1.2 Hz,
C5Me5), 118.7 (d, JCP ) 5.0 Hz, Ph), 122.6 (s, Ph), 129.5 (s, Ph),
133.6 (s, NCMe), 139.9 (d, JCP ) 3.7 Hz, Ph), 216.3 (d, JCP
)
37.9 Hz, CO). 31P{1H} NMR (δ, in CDCl3): 169.7 (s), -143.7
(sep, JPF ) 712.4 Hz. PF6).
(25) Catheline, D.; Astruc, D. Organometallics 1984, 3, 1094.

3244 Organometallics, Vol. 25, No. 13, 2006
Kubo et al.
Reaction of 4 with NaBH4. To a THF suspension (14 mL) of
4 (485 mg, 0.82 mmol) was added NaBH4 (31 mg, 0.82 mmol),
and the mixture was stirred for 30 min at ambient temperature.
The resulting solution was evaporated to dryness, and then the
residue was dissolved in a small amount of CH2Cl2. The solution
was charged on a silica gel column and eluted with a mixture of
CH2Cl2/acetone (9:1). A reddish-yellow band was collected and
dried under vacuum. The residue was washed with ether and then
with pentane several times, and dried under reduced pressure to
give 7 as a yellow powder (371 mg, 0.62 mmol, 77% yield). Anal.
Calcd for C21H30N2F6FeO3P2: C, 42.73; H, 5.12; N, 4.75. Found:
(λ ) 0.710 69 Å) at 200 K and then processed using the HKL
program package.26 The structures were determined by direct
methods and then were refined by full-matrix least-squares methods
on F2 using the SHELX-97 program package.27 ORTEP drawings
were made using the ORTEP III program.28 Positions of hydrogen
atoms of 4 were calculated by assuming idealized geometries,
whereas those of 2 and 7 were determined from subsequent
difference Fourier maps. The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were not refined for 4, but they
were refined isotropically for 2 and 7. Crystal data, data collection
parameters, and results of the analyses are summarized in Table 1.
Further details are included in the Supporting Information.
1
C, 42.63; H, 5.20; N, 4.69. IR (νCO, in THF): 1961. H NMR (δ,
in CDCl3): 1.72 (s, 15H, C5Me5), 1.89 (s, 3H, N(H)CMe), 3.33 (d,
JHP ) 10.6 Hz, 3H, POMe), 3.71 (d, JHP ) 13.1 Hz, 3H, POMe),
6.17 (br s, 1H, N(H)C), 7.10-7.39 (m, 5H, Ph). 13C{1H} NMR (δ,
in CDCl3): 9.4 (s, C5Me5), 21.1 (d, N(H)CMe), 53.8 (s, POMe),
55.5 (d, JCP ) 13.7 Hz, POMe), 94.6 (d, JCP ) 1.9 Hz, C5Me5),
129.1 (br s, Ph), 129.5 (s, Ph), 130.0 (s, Ph), 135.8 (d, JCP ) 6.2
Hz, Ph), 173.5 (d, JCP ) 26.1 Hz, NCMe), 215.3 (d, JCP ) 31.1
Hz, CO). 31P{1H} NMR (δ, in CDCl3): 196.2 (s), -143.7 (sep,
JPF ) 712.5 Hz. PF6).
Acknowledgment. This work was supported by Grants-in-
Aid for Scientific Research (No. 16033245) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
K.K. thanks Hiroshima University for a research grant.
Supporting Information Available: Crystallographic data for
2, 4, and 7 are available in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
X-ray Structure Determination. A single crystal of 2 was
obtained through recrystallization from hexane at room temperature,
while that of 4 was obtained from CH2Cl2 at 0 °C. A single crystal
of 7 was obtained by slow diffusion of hexane in a CH2Cl2 solution
of 7 at room temperature. Each crystal coated with inert oil was
mounted on a glass fiber and fixed in the cold nitrogen gas stream.
Intensity data were collected on a Mac Science DIP2030 imaging
plate diffractometer with graphite-monochromated Mo ΚR radiation
OM0510728
(26) Otwinowski, Z.; Minor, W. In Methods in Enzymology. Macro-
molecular Crystallography Part A; Sweet, R. M., Carter, C. W., Jr., Eds.;
Academic Press: New York, 1997; Vol. 276, p 307.
(27) Sheldrick, G. M. SHELX-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(28) Burnett, M. N.; Johnson, C. K. ORTEP-3; Report ORNL-6895; Oak
Ridge National Laboratory: Oak Ridge, TN, 1996.