1,3-dipolar cycloaddition to the Fe-O=C fragment (see abstract)

Article abstract of DOI:10.1021/om020605z

1,3-dipolar cycloaddition to the Fe-O=C fragment was investigated. First X-ray crystal structure of the initial bicyclo[2.2.1] adduct was also reported. The cycloaddition reaction of complexes with DMAD in the presence of PR₃ (k: R = OMe; l: R = Ph) resulted without detectable intermediates in the formation of the Fe(CO)₂PR₃(butenolide) complexes.

Full text of DOI:10.1021/om020605z

5628
Organometallics 2002, 21, 5628-5641
1,3-Dip ola r Cycloa d d ition to th e F e-OdC F r a gm en t. 19.
Syn th esis a n d P r op er ties of
F e(CO)2(P R3)(R1NdC(R2)-C(R3)dO) a n d
F e(CO)(P h 2P CH2CH2P P h 2)(R1NdC(R2)-C(R3)dO) a n d
Th eir Rea ctivity tow a r d Dip ola r op h iles. F ir st X-r a y
Cr ysta l Str u ctu r e of th e In itia l Bicyclo[2.2.1] Ad d u ct
Ron Siebenlist, Hans-Werner Fru¨hauf,*,† and Kees Vrieze
University of Amsterdam, Institute of Molecular Chemistry, Nieuwe Achtergracht 166,
1018 WV Amsterdam, The Netherlands
Wilberth J . J . Smeets and Anthony L. Spek
Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry,
Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Received J uly 24, 2002
t
Complexes 7a k -a n , 7bk ,bm [(R1NdC(R2)-C(R3)dO)Fe(CO)2PR3 a : R1 ) Bu, R2 ) H,
n
R3 ) Ph; b: R1 ) Me, R2 ) Ph, R3 ) Ph; k : R ) OMe; l: R ) Ph; m : R ) Et; n : R ) Pr],
and 7a o,b o [(R1NdC(R2)-C(R3)dO)Fe(CO)Ph2PCH2CH2PPh2] have been prepared by
substitution of CO in the respective tricarbonyl complexes 1a ,b at room temperature. Various
dynamic processes have been investigated by 13C and 31P NMR. On the IR time scale, complex
7bk exists in two distinct isomeric forms. Complexes 7 have been characterized by IR, UV-
1
vis, and H, 13C, and 31P NMR spectroscopy. Surprisingly, in the 1,3-dipolar cycloaddition
reaction of complexes 7 with the dipolarophiles dimethyl acetylenedicarboxylate (DMAD,
v), methyl propynoate (w ), phenyl acetylene (x), and p-methoxyphenyl-isothiocyanate (y)
the bicyclic ferra[2.2.1] complexes 8, i.e., the initial cycloadducts, are stable and isolable.
With dimethyl maleate (z) it is formed reversibly. Complexes 8 containing one phosphorus
ligand are formed as two non-interconverting isomers. The molecular structure of 8a ov,
C45H45FeNO6P2, has been determined by single-crystal X-ray diffraction. In pentane, the
initial cycloadduct (8bv) of the tricarbonyl 1b with DMAD is insoluble and precipitates. In
solution above -30 °C 8bv reacts further by insertion of CO into the Fe-O bond to the
1
Fe(CO)3(butenolide) complex 10bv. No intermediates can be detected by H, 13C, and IR
monitoring. The other complexes 8 do not insert CO and slowly decompose on stirring at
room temperature in THF. From the decomposition of complexes 8 with R-iminoketone a ,
the organic compound 11a can be isolated. The cycloaddition reaction of 1a with DMAD (v)
in the presence of PR3 (k : R ) OMe; l: R ) Ph) results without detectable intermediates in
the formation of the Fe(CO)2PR3(butenolide) complexes 10a k v,a lv. They are formed as three
and two non-interconverting isomers, respectively. Reaction of 8bv with PEt3 also results
in the formation of the butenolide complex 10bm v; however, here the PEt3 ligand occupies
exclusively one of the equatorial positions. An explanation for the isomeric ratios of complexes
8 and 10 is discussed.
In tr od u ction
resulting in synthetically interesting coordinated het-
erocycles such as in complexes 4 and 5 (cf. Scheme 1).
In the previous parts of this series we have reported
on the reactions of M(CO)3-n(CNR)n(R1-DAB)1 (M ) Fe,
Ru; n ) 0, 1, and 3) and Fe(CO)3(R1NdC(R2)-C(R3)dO)
complexes (1) with unsaturated substrates, e.g., acti-
vated alkynes,2-7 alkenes,8,9 and isothiocyanates,10-12
The initial step in these reactions consists of an
oxidative 1,3-dipolar cycloaddition of the activated
alkyne (the dipolarophile) across the M-XdC fragment
(the 1,3-dipole), which results in the formation of the
† E-mail: hwf@science.uva.nl. Fax: +31-20-525 6456.
(3) de Lange, P. P. M.; Fru¨hauf, H.-W.; Kraakman, M. J . A.; van
Wijnkoop, M.; Kranenburg, M.; Groot, A. H. J . P.; Vrieze, K.; Fraanje,
J .; Wang, Y.; Numan, M. Organometallics 1993, 12, 417-427.
(4) de Lange, P. P. M.; van Wijnkoop, M.; Fru¨hauf, H.-W.; Vrieze,
K.; Goubitz, K. Organometallics 1993, 12, 428-439.
(5) van Wijnkoop, M.; Siebenlist, R.; de Lange, P. P. M.; Fru¨hauf,
H.-W.; Vrieze, K.; Smeets, W. J . J .; Spek, A. L. Organometallics 1993,
12, 4172-4181.
(1) The abbreviations used throughout this text are as follows: R1-
DAB: 1,4-diaza-1,3-butadienes of formula R1-NdC(H)-C(H)dN-R1;
DMAD: dimethyl acetylenedicarboxylate; MP: methyl propynoate;
DMM: dimethyl maleate; DPPE: 1,2-bis(diphenylphosphanyl)ethane.
(2) Fru¨hauf, H.-W.; Seils, F.; Roma˜o, M. J .; Goddard, R. J . Angew.
Chem. 1983, 95, 1014-16; Angew. Chem., Int. Ed. Engl. 1983, 22, 992;
Angew. Chem. Suppl. 1983, 1435.
10.1021/om020605z CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/07/2002

1,3-Dipolar Cycloaddition to the Fe-OdC Fragment
Organometallics, Vol. 21, No. 25, 2002 5629
Sch em e 1. Rea ction s of F e(CO)2L(R1-DAB) a n d F e(CO)3(r-im in ok eton e) w ith Electr on -Deficien t Alk yn es
bicyclo[2.2.1] intermediate 2. The subsequent reaction
of this intermediate and accordingly the product forma-
tion depend on the co-ligand L, the additional ligand
L′, the metal M, and the nature of the 1,4-dihetero-1,3-
butadiene system. In case of R1-DAB (X ) NR1), this
mechanism is firmly supported by the isolation and
characterization of the N-protonated ferrabicyclo[2.2.1]
complex13 (6) and the isostructural N-protonated bicyclo-
[2.2.1]Ru(CO)3+[BF4-] complex,7 which both closely
resemble intermediate 2, as well as by the formation of
the M(CO)2(L)(THPP) complexes (5). In the case of iron,
the bicyclo[2.2.2] complex 3 formed upon insertion of CO
into the Fe-N bond and uptake of an additional ligand
L′ is thermally labile when L ) L′ ) CO and rearranges,
via reductive elimination, to the Fe(CO)3(pyrrolinone)
complex 4. When a more σ-donating/less π-accepting
ligand than CO is used as co-ligand L or as additional
ligand L′, e.g., CNR or P(OMe)3, the bicyclo[2.2.2]
complex 3 is stable. The stabilizing effect has been
attributed to the strengthening of the Fe-C bond trans
to the σ-donor ligand, i.e., reinforcement of one of the
bonds that is broken in the rearrangement giving
complex 4.14
Reaction of the closely related Fe(CO)3(R1NdC(R2)-
C(R3)dO) (R3 ) Me, Ph) complexes showed that these
complexes exhibit similar reactivity (cf. Scheme 1, X )
O),5,6 whereby the 1,3-dipolar cycloaddition proceeds
regioselective exclusively over the Fe-OdC fragment
with formation of the Fe(CO)3(butenolide) complexes 4.
As opposed to the analogous reactions of M(CO)3(R1-
DAB), intermediates 2 and 3 have not been observed
in this reaction even at very low temperature. Therefore
the reaction mechanism is based on the analogy with
the reactions of M(CO)3(R1-DAB) (M ) Fe, Ru). The
stabilizing effect of σ-donor ligands on the bicyclo[2.2.2]
complex 3 prompted us to synthesize the Fe(CO)2(PR3)-
(R1NdC(R2)-C(R3)dO) (R ) OMe, Ph, Et, nPr) and Fe-
(CO)(DPPE)(R1NdC(R2)-C(R3)dO) (DPPE ) 1,2-bis-
(diphenylphosphino)ethane) complexes and study their
1,3-dipolar reactivity. Second, PR3 can be used as
additional ligand. The objective was to see if we could
stabilize intermediates 2 and 3 in this way and (spec-
troscopically) characterize them. Furthermore, the sub-
stitution of CO for the better σ-donating/less π-accepting
phosphorus ligands was expected to lead to an enhanced
1,3-dipolar reactivity. Analogously, substitution of CO
for the better σ-donating/less π-accepting CNR ligands
in Fe(CO)3(R1-DAB) resulted in the highly activated Fe-
(CNR)3(R1-DAB) complexes, which also react with less
activated dipolarophiles such as alkenes9 and hetero-
allenes,10-12 resulting in complexes analogous to 3.
(6) van Wijnkoop, M.; Siebenlist, R.; Ernsting, J . M.; de Lange, P.
P. M.; Fru¨hauf, H.-W.; Horn, E.; Spek, A. L. J . Organomet. Chem. 1994,
482, 99-109.
(7) van Wijnkoop, M.; de Lange, P. P. M.; Fru¨hauf, H.-W.; Vrieze,
K.; Wang, Y.; Goubitz, K.; Stam, C. H. Organometallics 1992, 11, 3607-
3617.
(8) van Wijnkoop, M.; de Lange, P. P. M.; Fru¨hauf, H.-W.; Vrieze,
K.; Smeets, W. J . J .; Spek, A. L. Organometallics 1995, 14, 4781-4791.
(9) de Lange, P. P. M.; de Boer, R. P.; van Wijnkoop, M.; Ernsting,
J . M.; Fru¨hauf, H.-W.; Vrieze, K.; Smeets, W. J . J .; Spek, A. L.; Goubitz,
K. Organometallics 1993, 12, 440-453.
In the present paper we report on the synthesis and
properties of the Fe(CO)2(PR3)(R1NdC(R2)-C(R3)dO)
and Fe(CO)(DPPE)(R1NdC(R2)-C(R3)dO) complexes
(7). The 1,3-dipolar cycloaddition reaction of complexes
7 and Fe(CO)3(MeNdC(Ph)-C(Ph)dO) (1b) with DMAD1
and some other dipolarophiles are studied. Also the
reactions of Fe(CO)3(tBuNdC(H)-C(Ph)dO) (1a ) with
(10) de Lange, P. P. M.; Alberts, E.; van Wijnkoop, M.; Fru¨hauf, H.-
W.; Vrieze, K.; Kooijman, H.; Spek, A. L. J . Organomet. Chem. 1994,
465, 241-249.
(11) Feiken, N.; Fru¨hauf, H.-W.; Vrieze, K.; Fraanje, J .; Goubitz,
K. Organometallics 1994, 13, 2825-2832.
(12) Feiken, N.; Fru¨hauf, H.-W.; Vrieze, K.; Veldman, N.; Spek, A.
L. J . Organomet. Chem. 1996, 511, 281-291.
(13) Fru¨hauf, H.-W.; Seils, F.; Stam, C. H. Organometallics 1989,
8, 2338-2343.
(14) Fru¨hauf, H.-W.; Seils, F. J . J . Organomet. Chem. 1987, 323,
67-76.

5630 Organometallics, Vol. 21, No. 25, 2002
Siebenlist et al.
Ta ble 1. Syn th esis of Com p lexes 8a k v,a lv,a m v,a n v,
DMAD in the presence of PR3 and of the bicyclo[2.2.1]-
Fe(CO)3 complex 8bv in the presence of PEt3 are
studied.
8a ov-x, 8bk v,bm v,bov, a n d 8a k k v,bk k v
reaction elution
color of the
yield
complex temp (°C) ratioa fraction/complex (%) crystallizationb
8a k v
8a k k v
8a lve
8a m v
8a n v
8a ov
8a ow
8a oxg
8a oyg
8bk v
8bk k v
8bm v
8bov
-80
-80
-80
-80
-80
-80
-30
RT
9:1
9:1
yellow
yellow
42c
61d pentane/Et2O
pentane/Et2O
Exp er im en ta l Section
Ma ter ia ls a n d Ap p a r a tu s. 1H and 13C NMR spectra were
recorded on a Bruker AMX-300 spectrometer. The IR spectra
were recorded on a Biorad FTIR-7 spectrophotometer. Elemen-
tal analyses were carried out by Dornis & Kolbe, Microana-
lytisches Laboratorium, Mu¨lheim a. d. Ruhr, Germany. The
solvents were carefully dried and distilled under nitrogen prior
to use. All preparations were carried out under an atmosphere
of dry nitrogen by conventional Schlenk techniques. Silica gel
for column chromatography (Kieselgel 60, 70-230 mesh, E.
Merck, Darmstadt, Germany) was dried and activated prior
to use by heating to 180 °C under vacuum for 16 h. Fe(CO)5
(Strem Chemicals), P(OMe)3 (k ), PPh3 (l), PEt3 (m ), P(nPr)3
(n ), and DPPE (o) were used as commercially obtained. Fe2-
CH2Cl2 yellow
48
61
63
76
73
43
77
52c
Et2O/CH2Cl2
pentane/Et2O
pentane/Et2O
f
Et2O/CH2Cl2
pentane
9:1
9:1
Et2O
Et2O
h
9:1
9:1
9:1
9:1
2:1
yellow
yellow
yellow/orange
orange/yellow
orange
red/brown
yellow
yellow
yellow
yellow/green
RT
Et2O/CH2Cl2
Et2O/CH2Cl2
-40
-40
-40
-40
63d Et2O/CH2Cl2
62
69
Et2O/CH2Cl2
Et2O/CH2Cl2
a
b
Ratio of Et2O/CH2Cl2 for elution of 8. Solvent(s) from which
the respective complex is crystallized. c Yield when reacted with
d
1 equiv of P(OMe)3. Yield when reacted with 2 equiv of P(OMe)3.
e Formed as two isomers. f Crystals of 8a ov are obtained by
diffusion of pentane in a concentrated CH2Cl2 solution of 8a ov at
(CO)9 and the starting complexes Fe(CO)3(R1NdC(R2)-
15
C(R3)dO) (R1 ) tBu, R2 ) H, R3 ) Ph (1a ); R1 ) Me, R2 ) Ph,
R3 ) Ph (1b)) were prepared according to published proce-
dures.5,16
g
room temperature. The dipolarophile (1.3 equiv) is added in one
lot and the reaction mixture stirred at room temperature until
complete conversion of the starting complex is reached. Complex
h
Syn th esis of Fe(CO)2(P R3)(R1NdC(R2)-C(R3)dO) (7ak-
n , 7bk ,bm ) (w ith P R3 ) P (OMe)3 (k ), P Et3 (m ), P (n P r )3
(n )). To a solution of the appropriate Fe(CO)3(R1NdC(R2)-
C(R3)dO) (1) (1.00 mmol) in 30 mL of pentane was added 1.1
equiv of the respective PR3. The reaction was followed by IR
spectroscopy. For PEt3 and P(nPr)3 complete conversion of the
starting complex took approximately 10 min. During the
reaction the color changed from dark purple to dark black-
purple. For P(OMe)3 complete conversion took approximately
16 h, during which time the color of the solution hardly
changed. When the reaction was carried out with 2 equiv of
P(OMe)3, the reaction time was reduced to 4 h and almost no
(<5%) disubstitution was observed. The obtained solutions
were used for further reactions, assuming a quantitative
formation of 7a k -n and 7bk ,bm .
8a ox is eluted with pentane/Et2O (2:1).
were synthesized as follows. To a solution of Fe(CO)3(R1Nd
C(R2)-C(R3)dO) (1a ,b) (0.25 mmol) in 20 mL of pentane was
added 0.9 equiv of DPPE, and the mixture was stirred for 24
h. The resulting voluminous blue precipitate was washed three
times with 15 mL of pentane, yielding 7a o,bo in approximately
60% yield as blue powders. The powder was transferred in a
NMR tube and cooled to -30 °C, whereupon 0.5 mL of cold
(-30 °C) acetone-d6 was added.
31P NMR a n d IR Mea su r em en t of Com p lexes 7bl,bn .
To a weighted amount of 1b in a NMR tube at -78 °C was
added an equimolar amount of P(nPr)3 (n ), or 2 equiv of PPh3
(l) in 0.5 mL of acetone-d6. The reaction was followed by 31P
NMR. After the reaction was completed the solution was
evaporated to dryness and the IR was measured.
Syn t h esis of F e(CO)2(P P h 3)(t Bu NdC(H )-C(P h )dO)
(7a l). To a solution of Fe(CO)3(tBuNdC(H)-C(Ph)dO) (1a )
(1.00 mmol) in 20 mL of pentane and 10 mL of THF was added
1.1 equiv of PPh3. The reaction was followed by IR spectroscopy
and revealed that complete conversion took 36 h. During the
reaction the color of the solution became black-purple. Addition
of 2 equiv of PPh3 reduced the reaction time to 12 h, and
formation of the disubstituted product was not observed. The
obtained solution was used for further reactions, assuming a
quantitative formation of 7a l.
Syn t h esis of F e(CO)(DP P E )(R1NdC(R 2)-C(R 3)dO)
(7a o,bo). To a solution of the appropriate Fe(CO)3(R1Nd
C(R2)-C(R3)dO) (1) (1.00 mmol) in 20 mL of pentane and 10
mL of THF was added 1.1 equiv of DPPE. The reaction was
followed by IR spectroscopy and revealed a complete conversion
of the starting complex to the intensely blue 7a o and purple-
blue 7bo, respectively, within 5 h. The obtained solutions were
directly used for further reactions, assuming a quantitative
formation of 7a o,bo.
Elem en ta l An a lyses a n d Ma ss Sp ectr om etr y of Com -
p lexes 7. Elemental analyses of complexes 7 were not possible
because the obtained solutions were not analytically pure and
they did not crystallize. Unfortunately, also the mass spectra
could not be obtained due to the extreme sensitivity to air.
Experimental details for the following general procedures
for the formation of complexes 8 are given in Table 1.
Syn th esis of Bicyclo[2.2.1] Com p lexes 8a k v, 8bk v,
8a k k v, a n d 8bk k v. The reaction of Fe(CO)2(P(OMe)3)(tBuNd
C(H)-C(Ph)dO) (7a k ) with DMAD (v) is described as an
example. A solution of 1.00 mmol 7a k in 40 mL of pentane/
THF (3:1) was cooled to the appropriate temperature (cf. Table
1), whereupon a solution of 136 µL (1.1 mmol) of DMAD in 20
mL of pentane/THF (1:1) was added dropwise during 30 min.
The resulting yellow-brown reaction mixture was evaporated
to dryness, and the crude product was purified by column
chromatography on silica gel. The reaction with 7bk was
performed analogously. Elution with pentane/Et2O (1:1) gave
a dark brown fraction containing impurities. Subsequent
elution with Et2O/CH2Cl2 (9:1) afforded a yellow fraction,
which after evaporation of the solvent gave a mixture of 8a k v
and 8a k k v as a yellow powder in a total yield of about 60%.
The ratio of 8a k v:8a k k v was approximately 4:1, and in the
case of the reaction of 7bk approximately 7:1 (8bk v:8bk k v).
Pure complexes 8a k k v,bk k v could be obtained by stirring a
mixture of 8a k v,bk v at room temperature in THF with 1 equiv
of P(OMe)3. The reaction was followed by IR spectroscopy.
Complete conversion to 8a k k v,bk k v took ca. 2 and 4 h,
respectively. The crude product was purified by column
chromatography on silica gel (see Table 1). When the substitu-
tion reaction 7a ,b f 7a k ,bk was performed with 2 equiv of
P(OMe)3, the crude reaction mixture was, after cycloaddition,
NMR Sa m p les of F e(CO)(DP P E)(R1NdC(R2)-C(R3)dO)
(7a o,bo). The NMR samples of 7a o,bo obtained from the
reaction mixture (see above) were paramagnetic, which made
characterization by NMR difficult. Therefore the NMR samples
(15) Fe2CO9 was prepared by a slightly modified literature proce-
dure, using a quartz Schlenk tube and starting with 25 mL of Fe(CO)5,
150 mL of glacial acetic acid, and 10 mL of acetic anhydride. The latter
was added to prevent the mixture from containing too much water. A
Rayonet RS photochemical reactor (λmax ) 2500 Å) was used for
irradiation, and a continuous stream of air was used to cool the reaction
mixture. Filtration, washing with water, ethanol, and ether, and
subsequently drying in vacuo gave Fe2CO9 in usually more than 90%
yield: Braye, E. H., Hu¨bel, W. Inorg. Synth. 1966, 8, 178.
(16) Siebenlist, R.; Fru¨hauf, H.-W.; Vrieze, K.; Smeets, W. J . J .;
Spek, A. L. Eur. J . Inorg. Chem. 2000, 907-919.

1,3-Dipolar Cycloaddition to the Fe-OdC Fragment
Organometallics, Vol. 21, No. 25, 2002 5631
stirred at room temperature for 2 and 4 h, respectively.
Crystalline samples of 8a k k v,bk k v were obtained by slow
addition of Et2O to a suspension of 8a k k v,bk k v in pentane,
followed by cooling to -30 °C for several days.
tion of the solvent gave 10bv as a yellow-brown powder in
about 75% yield.
(b) To a solution of 1.00 mmol of Fe(CO)3(MeNdC(Ph)-
C(Ph)dO) (7b) in 40 mL of pentane/THF (3:1) was added
dropwise a solution of 136 µL (1.1 mmol) of DMAD in 20 mL
of pentane/THF (3:1) during ca. 30 min under an atmosphere
of CO. After all the DMAD was added the reaction mixture
was stirred for another 2 h and then chromatographed as
described under method a, giving 10bv in 75% yield.
NMR Stu d y of th e Rea r r a n gem en t of (13CO-En r ich ed )
8bv to 10bv. A solution of 10 mg of 8bv in 0.5 mL of CDCl3
at -50 °C was transferred to an NMR tube under an
atmosphere of (13CO) carbon monoxide. Immediately a 1H
spectrum was recorded which revealed that 8bv was the only
complex present. Upon warming to room temperature, 8bv
starts to isomerize to the corresponding Fe(CO)3(butenolide)
complex (10bv). During the isomerization the bicyclo[2.2.2]
Syn t h esis of t h e Bicyclo[2.2.1] Com p lexes 8a lv,-
a k v,a m v,a n v, 8a ov-y, a n d 8bm v,bov. The reaction of Fe-
(CO)2(PEt3)(tBuNdC(H)-C(Ph)dO) (7a m ) with DMAD is de-
scribed as a typical example. A solution of 1.00 mmol of 7a m
in 40 mL of pentane/THF (3:1) was cooled to the appropriate
temperature (cf. Table 1), whereupon a solution of 136 µL (1.1
mmol) of DMAD in 20 mL of pentane/THF (1:1) was added
dropwise during 30 min. The resulting yellow-brown reaction
mixture was evaporated to dryness, and the crude product was
purified by column chromatography on silica gel. The other
reactions were performed analogously, except for the reactions
with the less activated dipolarophiles w -y (cf. Table 1 for
details). Elution with pentane/Et2O (1:1) gave a dark brown
fraction containing impurities. Subsequent elution with Et2O/
CH2Cl2 (10:1) afforded a yellow fraction, which after evapora-
tion of the solvent gave 8a m v as a yellow powder. The other
complexes were chromatographed analogously, except for 8a ox
(cf. Table 1). Crystalline samples of 8a lv-a n v, 8a ov-y, a n d
8bm v,bov were obtained by slow addition of Et2O to a
suspension of the appropriate complex in pentane, or by slow
addition of CH2Cl2 to a suspension of the appropriate complex
in Et2O, followed by cooling to -30 or -60 °C for several days
(cf. Table 1). Crystals of 8a ov suitable for X-ray diffraction
were obtained by diffusion of pentane into a concentrated CH2-
Cl2 solution of 8a ov at room temperature.
1
intermediate 9bv was not detectable by H and 13C NMR.
Syn th esis of F e(CO)2(P R3)(bu ten olid e) (R ) OMe, P h )
(10a k v,a lv). A solution of 1.00 mmol of 7a k in 40 mL of
pentane/THF (3:1) was cooled to -40 °C, and 1 equiv of
P(OMe)3 was added, directly followed by the dropwise addition
of a solution of 136 µL (1.1 mmol) of DMAD in 20 mL of
pentane/THF (3:1) during ca. 30 min. The resulting yellow-
brown reaction mixture was evaporated to dryness and the
crude product purified by column chromatography on silica
gel. Elution with Et2O afforded an orange fraction, which after
evaporation of the solvent gave 10a k v as a yellow-orange
powder in about 35% yield. Complex 10a k v was formed as
three non-interconverting isomers in a ratio of 20:14:1. The
reaction with PPh3 was performed analogously. The IR re-
vealed that the butenolide complex 10a lv was formed in about
12% yield. The reaction mixture was evaporated to dryness
and was washed three times with pentane/Et2O (2:1). The
product was not further purified, and the spectroscopic data
were obtained from the raw product. Complex 10a lv was
formed as two isomers in a ratio of 9:1.
Rea ction of th e Bicyclo[2.2.1] Com p lex 8bv w ith P Et 3.
To a solution of 8bv (200 mg, 0.039 mmol) in 20 mL of CH2Cl2
at -50 °C was added 1 equiv of PEt3. The reaction mixture
was allowed to warm to room temperature, and the reaction
was followed by IR spectroscopy. Complete conversion of the
starting complex took approximately 3 h. The IR revealed that
two products were formed. The main product was Fe(CO)2-
(PEt3)(butenolide) (10bm v), which was formed in about 30%
yield. As second product 8bm v was formed (IR and 31P NMR)
in about 5% yield by substitution of CO for PEt3 in 8bv. The
reaction mixture was evaporated to dryness and was washed
three times with pentane/Et2O (2:1). The product was not
further purified, and the spectroscopic data were obtained from
the raw products.
Syn th esis of 11a . A weighted amount of complexes 8a k v-
a m v was dissolved in THF and stirred at room temperature
under an atmosphere of CO or N2. The reaction was followed
by IR spectroscopy. Complete decomposition of the starting
complexes took in both cases between 24 and 48 h, depending
on the PR3 ligand. The reaction mixture was evaporated to
dryness and the crude organic product was purified by column
chromatography on silica gel. Elution with pentane/Et2O (1:
2) afforded a pale yellow fraction, which after evaporation of
the solvent gave 11a as a yellow oil in about 25% yield. In the
case of 8a ov, complete decomposition took approximately 10
days at room temperature and after workup on silica gel gave
11a in about 15% yield.
Cr ysta l Str u ctu r e Deter m in a tion of 8a ov. A transpar-
ent orange block-shaped crystal was mounted on top of a glass-
fiber and transferred to the cold nitrogen stream of an Enraf-
Nonius CAD4T diffractometer for data collection at 150 K
[rotating anode, 50 kV, 100 mA, graphite-monochromated Mo
KR radiation]. Unit cell parameters were determined from a
least squares treatment of the SET4 setting angles of 25
Syn th esis of th e Bicyclo[2.2.1] Com p lexes 8a oz. To a
solution of 1.00 mmol of 7a o in 40 mL of pentane/THF (3:1)
at room temperature was added 2 equiv of dimethyl maleate
(DMM)1 in one lot. IR monitoring revealed that the reaction
started instantly but did not reach completion. Addition of ca.
20 equiv of DMM resulted in complete conversion to 8a oz.
When the reaction was carried out at -20 °C, complete
conversion was reached with 3 equiv of DMM. Upon warming
the reaction mixture to room temperature it became deep blue,
indicating the formation of the starting complex 7a o, and an
equilibrium between 7a o and 8a oz was reached. During
workup on silica gel 8a oz totally decomposed, and thus 8a oz
could not be isolated in this way. Addition of pentane to the
cold reaction mixture (-30 °C) resulted in precipitation of 8a oz
(IR). However, even after several washings with pentane/Et2O
(1:1) the precipitate still contained paramagnetic species,
which made characterization by NMR spectroscopy impossible.
P r ep a r a tion of 13CO-En r ich ed F e(CO)3(MeNdC(P h )-
C(P h )dO) (7b). A pentane solution of 7b (1.00 mmol, 30 mL)
was stirred at room temperature for 24 h in a sealed tube (200
mL) under an atmosphere of 13CO. The enrichment was
estimated to be about 25% on the basis of IR spectroscopy.
The 13CO-enriched 7b was used to follow the rearrangement
reaction 8bv f 10bv by 13C NMR.
Syn th esis of (13CO-En r ich ed ) Bicyclo[2.2.1] Com p lex
8b v. A solution of 1.00 mmol of Fe(CO)3(MeNdC(Ph)-
C(Ph)dO) (7b) in 30 mL of pentane was cooled to 0 °C,
whereupon a solution of 136 µL (1.1 mmol) of DMAD in 20
mL of pentane/Et2O (3:1) was added dropwise during ca. 30
min. The resulting beige precipitate was washed three times
with pentane and dried in vacuo, yielding 8bv as a beige
powder in 90% yield.
Syn th esis of F e(CO)3(bu ten olid e) (10bv). (a) A solution
of 8bv (200 mg, 0.039 mmol) in 20 mL of THF was stirred
under an atmosphere of CO at room temperature. The reaction
was followed by IR spectroscopy. Complete conversion of the
starting complex 8bv to the butenolide 10bv took approxi-
mately 3 h. The resulting yellow-brown reaction mixture was
evaporated to dryness, and the crude product was purified by
column chromatography on silica gel. Elution with Et2O/CH2-
Cl2 (3:1) afforded a yellow-brown fraction, which after evapora-

5632 Organometallics, Vol. 21, No. 25, 2002
Ta ble 2. Cr ysta l Da ta a n d Deta ils of th e Str u ctu r e Deter m in a tion of 8a ov
Siebenlist et al.
8a ov
8a ov
Crystal Data
formula
molecular weight
cryst syst
space group
a (Å)
Data Collection
temperature (K)
C
45H45FeNO6P2
150
1.6, 25.00
0.71073 (monochr.)
813.61
θ
Min, θMax (deg)
monoclinic
P21/c (No. 14)
11.5238(7)
23.0239(15)
18.7570(9)
125.76(1)
4038.3(5)
4
1.338
1704
5.02
0.15 × 0.30 × 0.50
wavelength (Mo KR) (Å)
scan type
∆ω (deg)
ω
0.50 + 0.35 tan θ
b (Å)
c (Å)
X-ray exposure time (h)
linear decay (%)
dataset
total data
total unique data
obsd data (I>2σ(I))
12
0.7
â (deg)
-13:13, 0:27, -13:22
V (Å3)
7395
7114
3857
Z
D
calc (g/cm3)
F(000)
µ (cm-1) [Mo KR]
crystal size (mm)
Refinement
no. of refined params
final R1
501
0.069
final wR2
0.136
goodness of fit
0.94
w-1
σ2(Fo2) + (0.0438P)2
0.01, 0.00
-0.40, 0.51
(∆/σ)av, (∆/σ)max
min. and max. resd dens (e/Å3)
Sch em e 2. F or m a tion of F e(CO)2(P R3)(im k et)
(7a ,bk -n ) a n d F e(CO)(DP P E)(im k et) (7a ,bo)
reflections with 9.96° < θ < 13.82°. The unit cell parameters
were checked for the presence of higher lattice symmetry.17
A
total of 7395 reflections were collected and merged into a
unique data set of 7114 reflections. Data were corrected for
Lp but not for absorption. The structure was solved with direct
methods (SHELXS86)18 and subsequent difference Fourier
analyses. Refinement on F2 was carried out by full matrix least
squares techniques. H atoms were introduced on calculated
positions and included in the refinement riding on their carrier
atoms. All non-hydrogen atoms were refined with anisotropic
thermal parameters; H atoms with isotropic thermal param-
eters were related to Ueq of their carrier atoms. Weights were
introduced in the final refinement cycles. The monoclinic unit
cell contains two solvent accessible voids of 55 Å3 each around
inversion centers. No significant residual density was found
in that region.
Crystal data and numerical details of the structure deter-
mination are given in Table 2. Scattering factors and anoma-
lous dispersion corrections were taken from the International
Tables for Crystallography.19 All calculations were performed
with SHELXL9720 and the PLATON21 package (geometrical
calculations and illustrations). A CIF-file has been deposited
with the CCDC: #189587.
Syn th esis of F e(CO)2(P R3)(im k et) a n d F e(CO)-
(DP P E )(im k et ) (7). The complexes Fe(CO)2(PR3)-
(imket) (7a k -n ,b k ,b m ) and Fe(CO)(DPPE)(imket)
(7a o,bo) are prepared via substitution of one or two CO
ligands by the respective phosphorus ligand in Fe(CO)3-
(imket) (1) at room temperature (cf. Scheme 2). The
reactions are followed by IR spectroscopy. In the case
of the relatively small basic phosphines, PEt3 and
P(nPr)3, the substitution is fast and completed within
10 min with exclusive formation of the monosubstituted
complexes. Reactions with the less basic P(OMe)3 and
also the more bulky PPh3 take longer reaction times of
16 and 36 h, respectively. With 2 equiv of phosphorus
ligand present these reaction times are reduced to 4 and
12 h. However, in the case of P(OMe)3, the appearance
of a CO stretching band at 1917 cm-1 indicates that
(10% of the disubstituted product Fe(CO)[P(OMe)3]2-
(imket) is also formed.
Resu lts a n d Discu ssion
The complexes discussed in this paper are shown in
Schemes 2-6. The type of complexes is identified by
Arabic numbers. The two R-iminoketone ligands are
differentiated by the letters a and b. The phosphorus
ligands are denoted by the letters k -o [P(OMe)3 (k ),
PPh3 (l), PEt3 (m ), P(nPr)3 (n ), and DPPE (o)], and the
dipolarophiles by the letters v-z [dimethyl acetylene
dicarboxylate (DMAD) (v), methyl propynoate (MP) (w ),
phenyl acetylene (x), p-methoxyphenyl-isothiocyanate
(y), and dimethyl maleate (DMM) (z)]. The R-iminoke-
tone ligands are abbreviated in the text as imket.
With the very bulky PcHex3 no CO substitution is
observed. When 1a is reacted with 2 equiv of P(OMe)3,
PEt3, or P(nPr)3, approximately 30% disubstitution has
occurred after 2 days. Complete conversion to the
disubstituted Fe(CO)(PR3)(imket) complexes requires 10
equiv of PR3. However, on basis of the intensity of the
CO stretching bands in the IR, extensive decomposition
has taken place. The Fe(CO)(DPPE)(imket) (7a o,bo)
complexes are prepared by reaction of 1a ,b with DPPE
in pentane/THF (2:1). IR monitoring indicated that the
substitution of the two CO ligands is completed within
(17) Spek, A. L. J . Appl. Crystallogr. 1988, 578.
(18) Sheldrick, G. M. SHELXS86, Program for crystal structure
determination; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(19) Wilson, A. J . C. International Tables for Crystallography;
Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; Vol.
C.
(20) Sheldrick, G. M. SHELXL97; University of Go¨ttingen: Go¨ttin-
gen, Germany, 1997.
(21) Spek, A. L. Acta Crystallogr. 1990, A46, C-34.

1,3-Dipolar Cycloaddition to the Fe-OdC Fragment
Organometallics, Vol. 21, No. 25, 2002 5633
Sch em e 3. Obser ved Rea ction s of Com p lexes 1b, 7a k -o, a n d 7bk -o w ith th e Dip ola r op h iles v-y
scopically (IR, 1H, 13C, 31P NMR, FD-mass) and by
elemental analyses.
Sch em e 4. Rever sible Cycloa d d ition of
F e(CO)(DP P E(tBu NdC(H)-C(P h )dO) (7a o) w ith
DMM (z)
The molecular structure of complexes 8 is established
by a single-crystal X-ray structure determination of the
bicyclo[2.2.1]Fe(CO)(DPPE) complex 8a ov. In this com-
plex the CO ligand is positioned trans to the Fe-O bond,
and accordingly insertion of CO into the Fe-O bond
cannot take place. However, in the bicyclo[2.2.1]Fe(CO)3
and bicyclo[2.2.1]Fe(CO)2(PR3) complexes (8) one of the
CO ligands is cis to the Fe-C bond, which means that
insertion of CO is in principle possible. This is indeed
found for the Fe(CO)3[2.2.1] bicyclic complex 8bv. When
kept below -30 °C, complex 8bv is stable in solution
under an atmosphere of CO. Above this temperature,
however, it reacts further by insertion of CO into the
Fe-O bond. Subsequent uptake of an additional CO
ligand results in the bicyclo[2.2.2] intermediate 9, which
in turn very rapidly isomerizes (reductive elimination
of the two Fe-C bonds followed by recoordination of the
double bond) to form the Fe(CO)3(butenolide) complex
10bv. The reaction 8bv f 10bv is followed by NMR.
During the conversion the formation of the bicyclo[2.2.2]
intermediate 9bv is not detected, which means that the
isomerization 9bv f 10bv is fast. Reaction of the
bicyclo[2.2.1]Fe(CO)2(PR3) complexes 8 in THF or tolu-
ene at room temperature under an atmosphere of CO
or N2 results in decomposition, and the formation of the
corresponding complexes 10 is not detected. From the
decomposition of complexes 8a k v-a n v in THF the
organic compound 11a is isolated in 25% yield. The
DPPE-containing complexes 8 are stable for several
days to weeks in THF, but they finally decompose to
unidentified compounds. In the case of 8a ov compound
11a is isolated in 15% yield.
4 h, and the formation of the Fe(CO)2(DPPE)(imket)
intermediate with a η1-coordinated DPPE ligand is not
detected. Reaction in pentane takes 16 h due to the
limited solubility of DPPE in pentane. In all cases, the
substitution is irreversible. Reaction of complexes 7
under CO (1 bar) for 2 days does not lead to any re-
formation of the starting complexes 1. Complexes 7 are
extremely sensitive to oxygen and moisture. Their
structures have been assigned on basis of their IR, UV-
vis, and NMR (1H, 13C, and 31P) spectroscopic properties.
Syn th esis of th e Bicyclo[2.2.1] Com p lexes (8)
a n d th e Bu ten olid e Com p lexes (10). The 1,3-dipolar
cycloaddition reactions of all complexes 7 with DMAD
and in the case of 7a o also with some other dipolaro-
philes result in the formation of the stable bicyclo[2.2.1]
complexes 8, i.e., the initially formed cycloadduct (cf.
Scheme 3). This is surprising because in the reactions
of Fe(CO)3(imket) (1a ) and the analogous M(CO)n-
(CNR)3-n(R-DAB) (M ) Fe; n ) 0, 1, 3) and Fe(CO)2-
(P(OMe)3)(R-DAB) with DMAD, the initial cycloadduct
could never be observed. Even more surprising is the
reaction of the tricarbonyl 1b with DMAD in pentane,
which also gives the initial cycloadduct 8bv. Interest-
ingly, the bicyclo[2.2.1]Fe(CO)2(PR3) complexes 8 are
obtained as two non-interconverting isomers (cf. NMR
section). Complexes 8 have been characterized spectro-
In the case of the reaction with 7a k ,bk , the initially
formed complexes 8a k v,bk v react further with P(OMe)3
that is present in solution to form the corresponding
disubstituted complexes 8a k k v,bk k v. The additional
P(OMe)3 is supplied by partial decomposition. Addition
of one extra equivalent of P(OMe)3 results in the

5634 Organometallics, Vol. 21, No. 25, 2002
Siebenlist et al.
Sch em e 5. Rea ction of F e(CO)3(tBu NdC(H)-C(P h )dO) (1a ) w ith DMAD a n d P R3
Sch em e 6. Rea ction of Bicyclo[2.2.1]F e(CO)3 Com p lex (8bv) w ith P Et3
exclusive formation of 8a k k v,bk k v. Reaction of the Fe-
(CO)2(PR3)(imket) complexes 7a k -m ,bk with MP at
-40 °C probably also results in the formation of the
initial adducts 8; however, no identifiable product could
be isolated. Complexes 7a k -m ,bk do not react with the
less activated dipolarophiles phenyl acetylene and p-
methoxyphenyl-isothiocyanate.
With the dipolarophile dimethyl maleate (DMM, z),
the 1,3-dipolar cycloaddition is reversible (cf. Scheme
4). Complete conversion to 8a oz at room temperature
requires 20 equiv of DMM. At -20 °C, complete conver-
sion to 8a oz is reached with 3 equiv of DMM. However,
upon warming the solution to room temperature the
color of the solution became deep blue due to the
formation of the starting complex 1a o, and accordingly
8a oz could not be purified on silica gel. Unfortunately,
complex 8a oz could also not be characterized after
precipitation because the isolated powder still contains,
even after repeated washings, small amounts of para-
magnetic species, which makes the characterization by
NMR spectroscopy impossible.
F igu r e 1. 50% probability displacement ellipsoid plot of
8a ov. Hydrogen atoms are omitted for clarity.
In the presence of PR3 (R ) OMe, Ph) as an addi-
tional ligand, reaction of Fe(CO)3(tBuNdC(H)-C(Ph)dO)
(1a ) with DMAD at -40 °C results in the formation of
the corresponding Fe(CO)2(PR3)(butenolide) complexes
(10a k v,a lv) (cf. Scheme 5), and the bicyclo[2.2.2] inter-
mediate (9) is not observed. Analogous to the reactions
with Fe(CO)3(R1-DAB), the additional PR3 ligand in
intermediate 9 is most probably incorporated exclusively
trans to the Fe-C(O) bond. However, despite the
strengthening effect of the PR3 σ-donor ligand on the
Fe-C(O) bond, it is obviously still very weak and
directly reacts further via reductive elimination to form
10a k v,a lv in low yield. Interestingly, 10a k v is formed
as three isomers in a ratio of 20:14:1 and 10blv as two
isomers in a ratio of 9:1 (cf. NMR section). The better
stabilizing σ-donor ligand PEt3 cannot be used in this
case because CO substitution is still very fast at -40
°C, resulting in the formation of Fe(CO)2(P(Et3)(imket).
Reaction of 8bv in the presence of PEt3 results also
in the formation of the Fe(CO)2PEt3(butenolide) (10bm v)
in low yield as one isomer, together with traces of 8bm v
(cf. Scheme 6). Complexes 10 have been characterized
spectroscopically (IR, 1H NMR, 13C NMR, and 31P NMR)
and by elemental analysis of 10bv.
Molecu la r Str u ctu r e of Bicyclo[2.2.1] Com p lex
8a ov. An ORTEP drawing of the molecular structure
of 8a ov together with the atomic numbering is shown
in Figure 1. Selected bond distances and angles are
given in Table 3.
The molecular structure of 8a ov consists of an iron-
(II) center coordinated by a newly formed [2.2.1] bicyclic
structure, in which the iron and the former carbonyl
carbon C7 occupy the bridgehead positions, a chelating
DPPE ligand, and a CO ligand. The coordination
geometry around the central iron atom is strongly

1,3-Dipolar Cycloaddition to the Fe-OdC Fragment
Organometallics, Vol. 21, No. 25, 2002 5635
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles (d eg) for 8a ov (SDs in p a r en th eses)
Fe-P1
Fe-P2
Fe-O5
Fe-N1
Fe-C2
Fe-C19
P1-C32
2.2289(18)
2.2997(16)
1.944(4)
2.025(4)
1.975(5)
1.742(6)
1.835(5)
P2-C33
O5-C7
O6-C19
N1-C8
N1-C9
C1-C2
1.814(7)
1.410(5)
1.154(7)
1.274(7)
1.488(8)
1.354(8)
C1-C5
C1-C7
C2-C3
C7-C8
C7-C13
C32-C33
1.465(7)
1.537(7)
1.481(7)
1.521(9)
1.492(8)
1.522(7)
P1-Fe-P2
P1-Fe-O5
P1-Fe-N1
P1-Fe-C2
P1-Fe-C19
P2-Fe-O5
P2-Fe-N1
P2-Fe-C2
P2-Fe-C19
O5-Fe-N1
O5-Fe-C2
O5-Fe-C19
85.61(6)
83.18(11)
163.88(15)
93.84(17)
96.04(18)
79.74(11)
90.58(11)
162.20(18)
105.69(17)
80.73(17)
82.5(2)
N1-Fe-C2
N1-Fe-C19
C2-Fe-C19
Fe-C19-O6
Fe-O5-C7
Fe-N1-C8
Fe-N1-C9
C8-N1-C9
C2-C1-C5
C2-C1-C7
C5-C1-C7
Fe-C2-C1
85.03(18)
100.1(2)
92.1(2)
Fe-C2-C3
C1-C2-C3
O5-C7-C1
O5-C7-C8
O5-C7-C13
C1-C7-C8
C1-C7-C13
C8-C7-C13
N1-C8-C7
P1-C32-C33
P2-C33-C32
127.0(4)
121.4(4)
106.6(4)
106.0(4)
109.2(4)
100.0(4)
117.5(5)
116.4(4)
114.6(4)
111.0(3)
107.1(4)
174.3(5)
102.4(3)
109.5(4)
129.3(3)
120.6(4)
124.5(4)
112.0(4)
121.3(4)
109.8(3)
174.5(2)
distorted octahedral. This distortion is a consequence
of its [2.2.1] bicyclic structure and the chelating DPPE
ligand. The [2.2.1] bicyclic terdentate ligand is not able
to span three regular octahedral coordination sites (O5-
Fe-N1 ) 80.73(17)°, O5-Fe-C2 ) 82.5(2)°, and N1-
Fe-C2 ) 85.03(18)°). These angles are comparable with
those of the isostructural [2.2.1] iron13 (6) and ruthe-
nium complex.7 In addition, the P1-Fe-P2 angle of
85.61(6)° is also smaller than 90° but compares well
with the P-Fe-P angles reported in the literature for
chelating DPPE ligands.22-24 The Fe-P1 and Fe-P2
bond distances differ significantly. The rather long Fe-
P2 (2.2997(16) Å) distance as compared to the normal
Fe-P1 (2.2289(18) Å) distance22-25 can be explained by
the large trans-influence of C2, which is known to
elongate this bond.7 The Fe-C2 distance of 1.975(5) Å
is somewhat shorter than the comparable Fe-C bond
(2.001(4) Å) in the iron [2.2.1] bicyclic structure 6, which
is probably due to the trans positioned σ-donor P2,
which strengthens the Fe-C2 bond. The intact imine
(N1-C8 ) 1.274(7) Å) is coordinated to iron by a
σ-donative Fe-N1 bond of 2.025(5) Å. As a conse-
quence of the C-C coupling, the former ketone bond
C7-O5 is reduced to a single bond, which is reflected
in the bond distance of 1.410(5) Å. Furthermore, the
central bond of the former alkyne (C1-C2 ) 1.354(8)
Å) is reduced to a double bond. The olato oxygen O5 is
coordinated to iron by a bond of 1.944(4) Å, which
compares well with the Fe-O bond length in Fe2(CO)5-
[iPrNdC(H)C(OMe)(O)C(CO2Me)C(CO2Me)]26 and Fe2-
(CO)6[CEtdCEt(CO2)].27 The rather short Fe-C19 bond
of 1.742(6) Å reflects the electron-donating capacity of
both the DPPE phosphorus atoms, which enhances
π-back-donation to this carbonyl. This is corroborated
by the low-frequency position of the CO stretching band
in the IR (1910 cm-1) and the high-frequency position
of this CO in the 13C NMR (219.7 ppm).
The most important result in the context of this paper,
however, is the confirmation of the [2.2.1] bicyclic
structure, which proves that the Fe(CO)3(R-iminoke-
tone) complexes react via a mechanism similar to that
of the analogous M(CO)3(R1-DAB) (M ) Fe, Ru) com-
plexes.
IR a n d UV-Vis Sp ectr oscop y. The IR data of the
complexes 1, 7, 8, 10, and compound 11a are collected
in Table 4 (see Supporting Information), together with
the elemental analyses, the FD mass, and the UV-vis
data.
Com p lexes 7. The IR spectra of the Fe(CO)2(PR3)-
(imket) (7a k -n ,bk -n ) complexes show two intense CO
stretching bands in the ν(CO) region, and the Fe(CO)-
(DPPE)(imket) (7a ,bo) complexes give rise to one broad
CO stretching band. Compared to the starting Fe(CO)3-
(imket) complexes (1), the CO stretching bands of
complexes 7 are shifted to lower frequency (30-100
cm-1). This is due to the introduction of the strong
σ-donating phosphorus ligands as compared to CO,
which enhances π-back-donation to the remaining car-
bonyl(s).
The most pronounced shifts are, as expected, observed
for the Fe(CO)(DPPE)(imket) complexes as a result of
the substitution of two CO ligands for the two good
σ-donor phosphorus atoms of DPPE. The CO stretching
frequencies of the monosubstituted Fe(CO)2(PR3)(imket)
complexes nicely parallel the σ-donor capacity28 (basic-
ity) of the PR3; that is, the shifts decrease in the order
P(nPr)3 ≈ PEt3 > PPh3 > P(OMe)3. Interestingly, in case
of the P(OMe)3 complex 7bk , clearly four bands are seen
in the IR, which points to two isomeric forms of 7bk .
The ratio is approximately 1:1. For all other Fe(CO)2-
(PR3)(imket) complexes only two CO bands are observed.
Analogously, for Fe(CO)2(2,6-di-iPr-C6H3-DAB)PR3 (R )
OMe)29 three isomers have been detected, of which two
have clearly separated CO bands, while for a series of
n
other PR3 (R ) Ph, c-hex, Bu, OPh) ligands only one
isomer was observed. The isomeric forms of 7bk are
most probably geometrical isomers of a square pyrami-
dal (sqp) structure because the solid state structure of
the starting complex 1b has approximately this geom-
etry16 (see NMR section). In the NMR only one set of
(22) Le Narvor, N.; Toupet, L.; Lapinte, C. J . Am. Chem. Soc. 1995,
117, 7129.
(23) Knorr, M.; Mu¨ller, J .; Schubert, U. Chem. Ber. 1987, 120, 879.
(24) Battaglia, L. P.; Delledonne, D.; Nardelli, M.; Pellizzi, C.;
Prediere, G. J . Organomet. Chem. 1987, 330, 101.
(25) Luh, L.-S.; Liu, L.-K. Organometallics 1995, 14, 1514.
(26) Siebenlist, R.; de Beurs, M.; Feiken, N.; Fru¨hauf, H.-W.; Vrieze,
K.; Kooijman, H.; Veldman, N.; Lakin, M. T.; Spek, A. L. Organome-
tallics 2000, 19, 3032-3053.
(28) Tolman, A. C. Chem. Rev. 1977, 77, 313.
(29) van Dijk, H. K.; Kok, J . J .; Stufkens, D. J .; Oskam, A. J .
Organomet. Chem. 1989, 362, 163.
(27) Aime, S.; Milone, L.; Sappa, E.; Tiripicchio, A.; Tiripicchio
Camellini, M. J . Chem. Soc., Dalton Trans. 1979, 1155.

5636 Organometallics, Vol. 21, No. 25, 2002
Siebenlist et al.
resonances is observed, which shows that the geo-
metrical isomers of 7bk interconvert rapidly on the
NMR time scale (104 s-1). On the IR time scale (1012
s-1), however, the two geometrical isomers of 7bk can
be distinguished. This has also been observed for Fe-
(CNR)3(iPr-DAB) (R ) tBu, c-hex)4 and Fe(CO)3(η4-
diene).30 The somewhat lower CO stretching frequencies
for complexes 7a k -o compared to those of 7bk -o are
due to the better π-accepting properties of R-iminoke-
tone b.
F igu r e 2. Three different geometries of complexes 7.
The UV-vis spectra of complexes 7 all show an
intense absorption band in the visible region between
538 and 599 nm, which unambiguously shows that the
R-iminoketone is coordinated in a σ-N, σ-O chelate
fashion. The bands are shifted to lower energy compared
to those of the parent complexes, and the shifts increase
with increasing σ-donor capacity of the phosphorus
ligand.
Com p lexes 8. The IR spectra of complexes 8 show
one, two, or three CO stretching bands in the terminal
ν(CO) region depending on the number of terminal
carbonyls present. As a consequence of the oxidative
cycloaddition, the iron is oxidized from Fe(0) to Fe(II)
in the [2.2.1] bicyclic complexes 8, and accordingly the
CO stretching bands are shifted ca. 60 cm-1 to higher
frequency as compared to their precursors 1b and 7.
Obviously, the lowest CO stretching frequencies are
observed for the DPPE-containing complexes 8a ov-
z,bov. Within the DPPE series 8a ov-x the CO stretch-
ing band shifts to lower frequency as a result of the
cycloadded alkyne bearing two, one, and no electron-
withdrawing ester groups. The CO stretching bands of
8a oy and 8a oz are found at somewhat higher frequency
than that of 8a ox, indicating that in both cases the
π-back-donation to the remaining carbonyl is reduced.
Apparently, the C(sp3) and S atoms induce less electron
density at the iron center than the C(sp2) carbon in
8a ov-x. The CO stretching bands of the mono-PR3-
substituted complexes 8 nicely reflect the basicity of the
PR3 ligand, as those of complexes 7. The CO stretch-
ing bands of the bis-phosphite-substituted complexes
8a k k v,bk k v are found at ca. 1945 and 1950 cm-1. This
is at approximately 35 cm-1 higher wavenumber than
in the corresponding DPPE complexes. When the cy-
cloaddition is performed with either DMAD or MP, one
broad band of medium intensity is observed in the
F igu r e 3. Different ligand arrangements for complexes
8. Unprimed compound numbers indicate the major (or
only) isomer and primed numbers the minor isomer.
F igu r e 4. Two isomeric forms of Fe(CO)2(PR3)(butenolide)
(10).
frequency compared to those of the earlier reported
10a v.5 The CO stretching bands of 10a k ,lv are observed
at 40 cm-1 (P(OMe)3) and 50 cm-1 (PPh3) to lower
frequency, relative to those of 10a v, due to the introduc-
tion of the PR3 σ-donor ligand. The CO stretching bands
of 10bm v are shifted even further to lower frequency
(60 cm-1) compared to those in the corresponding
tricarbonyl complex 10bv, due to the more basic PEt3
ligand.
1
NMR Sp ectr oscop y. The H, 13C{1H}, and 31P{1H}
NMR data of complexes 1, 7, 8, 10, and 11a are collected
in Tables 5, 6, and 7, respectively (see Supporting
Information). The atomic numbering of selected impor-
tant proton and carbon atoms is given in Scheme 3 and
Figures 2-4.
organic carbonyl region between 1707 and 1694 cm-1
.
This band is assigned to the carbonyls of the ester
substituents. In the case of complex 8a oy, a band at
1562 cm-1 is observed for the CdN double bond of
p-methoxyphenyl-isothiocyanate, which shows that the
CdS bond is cycloadded.
Com p lexes 10. The three terminal CO ligands of
complex 10bv give rise to an absorption pattern that is
characteristic for Fe(CO)3(butenolide) complexes,5,6,30
i.e., one sharp absorption at higher frequency and two
partially resolved absorptions at lower frequency. The
CO bands of 10bv are shifted ca. 35 cm-1 to lower
frequency compared to those of 8bv because the iron is
reduced from Fe(II) (8) to Fe(0) (10). As a result of the
better π-accepting properties of the R-iminoketone b, the
CO bands of 10bv are shifted ca. 10 cm-1 to higher
Com p lexes 7. The imine proton resonances H(8) in
complexes 7a l-n are observed as singlets between 8.14
and 8.43 ppm, and in complex 7a k (P(OMe)3) it is
observed as a doublet at 8.84 ppm. These resonances
are comparable with that at 8.71 ppm for the Fe(CO)3
precursor 1a . Only in the case of 7a k is a small coupling
(4J (P,H) ) 2.6 Hz) observed on the imine proton as a
result of the larger s-character in the lone pair of
P(OMe)3 as compared to the other phosphines, which
is known to lead to larger coupling constants.28 Analo-
gously, all the P(OMe)3-containing complexes 7, 8, and
10 also show approximately twice as large P,C coupling
constants in the 13C NMR spectra compared to those
found for the corresponding PR3 (R ) Ph, Et, nPr)-
containing complexes. The resonances of the imine C(8)
(30) Angermund, H.; Grevels, F.-W.; Moser, R.; Benn, R.; Kru¨ger,
C.; Roma˜o, M. J . Organometallics 1988, 7, 1994.

1,3-Dipolar Cycloaddition to the Fe-OdC Fragment
Organometallics, Vol. 21, No. 25, 2002 5637
and ketone C(7) atoms show coordination shifts of 14-
16 and 21-37 ppm, respectively, to lower frequency as
a result of extensive π-back-donation from the iron into
the low lying π*-orbital of the R-iminoketone. Compared
to the resonances of the imine and ketone carbon atoms
in the precursor complexes 1a ,b, an additional shift of
2-7 ppm (CdN) and 4-12 ppm (CdO) to lower fre-
quency is observed in complexes 7. The shifts clearly
reflect the σ-donor capacity of the phosphorus ligand
used; that is, the shifts increase in the order P(OMe)3
< PPh3, PEt3 ≈ P(nPr)3 < DPPE. Comparable shifts to
lower frequency have been observed for the imine carbon
atoms in Fe(CO)3(R1-DAB) upon substitution of one31
or three4 CO ligands for the more σ-donating/less
π-accepting isocyanide ligands. Like in the precursor
complexes 1a ,b, the larger shifts of the ketone carbon
in comparison with the imine carbon atom indicate that
the chelate coordination exerts a larger effect on the
ketone carbon atom. This may be related to the observed
regioselective addition over the Fe-OdC fragment.
As a result of enhanced π-back-donation the reso-
nances of the remaining terminal CO ligand(s) in 7 are
shifted to higher frequency. As expected, the shift
increases in the order P(OMe)3 < PPh3, PEt3 ≈ P(nPr)3
< DPPE, reflecting the σ-donor capacity of the phos-
phorus ligand. The two terminal CO carbons C(19) and
C(19′) in Fe(CO)2(PR3)(imket) (7a k -n ,bk ,bm ) are iden-
tical on the NMR time scale down to 193 K; that is, they
are observed as one doublet due to coupling with
phosphorus. This may be explained by Berry pseudoro-
tations32,33 which scramble the CO ligands and inter-
change the two faces of the R-iminoketone ligand.
Similar observations have been reported for the CO
ligands in M(CO)3(R1-DAB) (M ) Fe,34 Ru35), Fe(CO)2-
(CNR)(R1-DAB),31 and Fe(CO)3(R-iminoketone)5 and for
the CNR ligands in Fe(CNR)3(R1-DAB).4
with PEt3 coordinated in an apical position and P-Fe-
C(O) angles of 80°. Since very small P,C coupling
constants are observed on the terminal carbonyl car-
bons, this geometry is probably preserved in solution.
Furthermore, these complexes do not show P,C coupling
on the imine carbon atoms. In all complexes 7 a
3
relatively large J (P,C) coupling constant (6-10.6 Hz)
on the ketone carbon atom C(7) is found. Only in the
case of complexes 7 containing ligand b is a 3J (P,C)
coupling (3.8-6.0 Hz) on the imine carbon C(8) ob-
served. A similar effect is found for complexes 8.
To explain the P,C coupling constants found for
complexes 7bk ,bm , a distorted sqp geometry is pro-
posed, as shown in Figure 2 (isomer A). The PR3 is
probably somewhat bent away from the N-Fe-O plane,
3
which might explain the rather large J (P,C) coupling
on the ketone carbon C(7). Obviously, due to this
deformation, also the P-Fe-CO angles change com-
pared to those in Fe(CO)2(PEt3)(p-anisyl-DAB), which
results in somewhat larger P,C coupling constants on
the terminal carbonyl carbons C(19) and C(19′). It has
been shown that 1b (X-ray)16 has approximately a
square pyramidal geometry (89% along the Berry pseu-
dorotation axes)37 in which the nitrogen of the R-imi-
noketone occupies a basal and the oxygen an apical
position. The two isomeric forms of 7bk , seen in the IR,
may thus be explained by interconversion between a
basal (N)-basal (O) and a basal (N)-apical (O) coordi-
nation of the R-iminoketone. However, because on
average relatively small coupling constants are found
on C(19) and C(19′), the P(OMe)3 is apparently coordi-
nated in a cis fashion to both CO ligands (isomer B),
i.e., trans to the nitrogen σ-donor. This agrees well with
the observation that only with the weakest P-donor,
P(OMe)3, isomer B is detected.
In complexes 7a k -n significantly larger coupling
constants are found on C(19) and C(19′). This may be
rationalized by a molecular geometry closely approach-
ing a trigonal bipyramidal (tbp) structure and placing
the phosphorus in an equatorial position (cf. Figure 2,
isomer C). Because the two CO ligands rapidly inter-
change positions, the large equatorial coupling constant
contributes strongly to the observed averaged value. The
change to a tbp structure is in agreement with the
decreased π-acceptor capacity of ligand a in comparison
with that of b. From X-ray studies of five-coordinate Fe-
(R-diimine) complexes it is known31,36,38,39 that their
geometries change from near tbp to sqp with increasing
π-acceptor capacity of the R-diimine.
Interestingly, the 2J (P,C) coupling constants on C(19)
and C(19′) differ significantly between the Fe(CO)2(PR3)-
(imket) complexes 7a k -n and 7bk ,bm , respectively.
This might indicate that these complexes have different
coordination geometries and/or have a different ligand
arrangement. This is supported by the difference in the
31P chemical shifts of 7a k -n and 7bk -n . Complexes
7bl and 7bn are characterized only by 31P NMR and
IR. For complexes 7bk , bm relatively small P,C coupling
constants of 9.1 (7bm ) and 17.5 Hz (7bk ) are observed
on the terminal carbonyl carbons C(19) and C(19′). In
the corresponding complexes 7a k -n these coupling
constants are approximately twice as large and lie
between 21.8 (7a l-n ) and 36.2 Hz (7a k ). The isostruc-
tural Fe(CO)2(PR3)(R1-DAB)36 (R1 ) iPr, p-anisyl) com-
plexes show even smaller P,C coupling constants on the
terminal carbonyl carbon atoms which lie between 4.2
Hz (R ) Me, Et) and 8.4 Hz (R ) OMe). The solid state
structure of Fe(CO)2(PEt3)(p-anisyl-DAB) showed that
this complex has a square pyramidal (sqp) geometry
The coupling constants of 7a o,bo are almost equal,
which indicates that they probably have the same ligand
arrangement. Interestingly, analogous to the ketone
carbon C(7), the carbonyl carbon C(19) in Fe(CO)-
(DPPE)(imket) (7a o,bo) is observed as a triplet due to
the coupling with phosphorus. This indicates that the
DPPE phosphorus atoms are equivalent, which is
confirmed by the singlet observed in the 31P NMR for
the two DPPE phosphorus atoms. Since the R-iminoke-
(31) de Lange, P. P. M.; Kraakman, M. J . A.; van Wijnkoop, M.;
Fru¨hauf, H.-W.; Vrieze, K.; Smeets, W. J . J .; Spek, A. L. Inorg. Chim.
Acta 1992, 196, 151-160.
(32) Albright, T. A.; Hoffmann, R.; Thibeault, J . C.; Thorn, D. L. J .
Am. Chem. Soc. 1979, 101, 3801.
(33) Berry, R. S. Chem. Phys. 1960, 32, 933.
(34) Leibfritz, D.; tom Dieck, H. J . Organomet. Chem. 1976, 105,
255.
(37) Holmes, R. R. In Progress in Inorganic Chemistry; Lippard, S.
J ., Ed.; Wiley: Cambridge, MA, 1984; Vol. 32, pp 119-236.
(38) De Paoli, M. A.; Fru¨hauf, H.-W.; Grevels, F.-W.; Koerner von
Gustorf, E. A.; Riemer, W.; Kru¨ger, C. J . Organomet. Chem. 1977, 136,
219-233.
(35) Mul, W. P.; Elsevier: C. J .; Fru¨hauf, H.-W.; Vrieze, K.; Pein,
I.; Zoutberg, M. C.; Stam, C. H. Inorg. Chem. 1990, 29, 2336-2345.
(36) Fru¨hauf, H.-W.; Pein, I. Unpublished results, 1988.
(39) Kokkes, M.; Stufkens, D. J .; Oskam, A. J . Chem. Soc., Dalton
Trans. 1983, 439.

5638 Organometallics, Vol. 21, No. 25, 2002
Siebenlist et al.
tone ligand is asymmetric, this means that the two
phosphorus atoms of DPPE rapidly exchange on the
NMR time scale. Unfortunately, variable-temperature
31P NMR experiments (293-193 K) did not change the
31P spectrum; that is, no broadening of the signal was
observed. Because two chelating ligands are present in
7a o,bo, the observed exchange cannot be explained by
Berry pseudorotations. Therefore a decoordination-
rotation-coordination mechanism of one of the chelates
must occur to explain the dynamic behavior. As a result
of the extra electron density donated by the DPPE
ligand, the R-iminoester will be strongly bonded due to
extensive π-back-donation into the low lying π*-orbital.
Consequently, we assume that one of the phosphorus
atoms decoordinates and after rotation coordinates
again, thus exchanging the two phosphorus sites. The
phosphorus resonances of 7a o,bo, observed at ap-
proximately 93 ppm, are shifted exceptionally far (105
ppm) to higher frequency compared to the free ligand
(-11.9 ppm). Similar shifts to higher frequency are
observed for the isostructural complexes Fe(CO)3-
(DPPE)24 and Fe(CO)(DPPE)(C4H6),40 characteristic for
five-membered chelate rings.41,42 Also the resonances of
7a l-n ,bl-n are strongly shifted to higher frequency
(75-92 ppm) upon coordination, while for 7a k ,bk the
coordination shift is reduced to approximately 40 ppm.
The position of the resonances and the coordination
shifts compare very well with those obtained for Fe-
(CO)2(PR3)(R1-DAB).36
shifted ca. 29 ppm to higher frequency in comparison
with that in 1b due to a decrease in π-back-donation to
the isolated imine fragment. Similar shifts are found
for complexes 3, 4, and 6. The three terminal carbonyl
carbon atoms (C(19), C(19′), and C(19′′)) give rise to
three separate resonances between 206.1 and 201.8
ppm.
The positions of the resonances in the 13C NMR
spectra of complexes 8bk v,bm v closely resemble those
of 8bv except for the terminal carbonyl carbons C(19)
and C(19′), which are shifted significantly to higher
frequency due to the phosphorus σ-donor ligand. The
13C data of 8a k v-a n v compare very well with those of
8bk v,bm v except for the iron-bonded C(2) resonances
which are slightly shifted (9 ppm) to higher frequency
and the resonance for the imine carbon C(8) which is
shifted ca. 12 ppm to lower frequency due to the
substitution of a phenyl for a proton in ligand a . The
resonances in the 1H NMR are observed at their
expected positions.
Interestingly, the bicyclo[2.2.1]Fe(CO)2(PR3) complex
8 can form two non-interconverting isomers (cf. Figure
3). This is best to be seen in the reaction of 7a k with
DMAD, which leads to two isomers, 8/8′a lv, in a ratio
of 2:1. The major isomer is denoted by its Arabic
number, and the minor isomer is primed. These isomers
show large differences in their P,C coupling constants,
particularly on the iron-bonded carbon C(2). Further-
more, in the 31P NMR two resonances are observed,
which differ significantly in their chemical shift (cf.
Table 7). Both findings are indicative for coordination
of PPh3 at different coordination sites. The position of
PR3 seems to depend on both the R-iminoketone and the
PR3 ligand. Complexes 8bk v,bm v and 8a lv show large
2J (P,C) coupling constants (44-68 Hz) on the iron-
bonded carbon C(2), which clearly indicates that the PR3
ligand is positioned trans to C(2), a ligand arrangement
as shown in Figure 3 (isomer D). On the former ketone
carbon C(7) relatively large P,C coupling constants (8.3-
10.6 Hz) are found. This coupling information may be
conveyed via the C(2)dC(1) bond in trans-position to the
phosphorus.
Com p lexes 8. In the 13C NMR spectrum of the
tricarbonyl complex 8b v only four resonances are
observed that exceed 190 ppm. This indicates that only
four carbon atoms are directly bonded to iron. In the
related [2.2.2] bicyclic complexes 3 (cf. Scheme 1; X )
NR1, L ) L′ ) CO) except for these resonances, a fifth
signal at 204 ppm is present for the inserted carbonyl.
Furthermore, in the mass spectrum of 8bv a molecular
ion peak of 505 mass units is found that corresponds to
a 1:1 stoichiometry of 1b and DMAD. Taken together,
this clearly shows that 8bv has a [2.2.1] bicyclic
structure without an inserted CO.
1
The resonances in the H spectrum are observed at
their expected positions. The most pronounced changes
of chemical shift in the 13C NMR spectrum of 8bv are
observed for the carbon atoms directly involved in the
C-C coupling reaction. The resonance of the former
ketone carbon atom C(7) is observed at 102.7 ppm, a
shift of 78 ppm to lower frequency compared to that in
1d as a consequence of the change in hybridization from
sp2 to sp3. The former alkynic carbon C(2) resonates at
154.5 ppm. This is at ca. 25 ppm to higher frequency
compared to those in complexes 3 and 6, which is
probably a result of the C-C coupling with a ketone
carbon in 8 instead of an imine carbon atom in 3 and 6.
The metal-bonded, sp2-hybridized, former alkynic car-
bon C(1) resonates at 192.0 ppm, which compares well
with complexes 3 and 6. The additional shift to high
frequency of this carbon atom is caused by the σ-bond
to iron, which enhances the carbanionic character of this
carbon, resulting in unusually high paramagnetic shield-
ing. The resonance of the intact imine carbon C(8) is
In the corresponding complexes 8a k v,a m v,a n v and
8′a lv (isomer E), the P,C coupling constants on the iron-
bonded carbon C(2) are strongly reduced and lie between
25.7 and 38.5 Hz. Also the coupling constant on the
former ketone carbon C(7) is reduced to ca. 3 Hz, and
no observable coupling on the imine carbon atom C(8)
1
is observed. In the H NMR a very small P,H coupling
(<0.7 Hz) on the imine proton H(8) is observed. Super-
ficially, the small couplings suggest that the PR3 ligand
is not in a trans-position to nitrogen. This would also
avoid two σ-donor atoms, P and N, in mutual trans-
positions. However, the coordination site trans to oxygen
is probably even less favorable because the olato oxygen
has no π-accepting properties and is also a fairly good
donor. The ligand arrangement seen in the solid state
structure of 8a ov has one of the P-donors approximately
trans to C(2) (P(2)-Fe-C(2) ) 162.28(18)°) and the
other P-donor approximately trans to N(1) (P(1)-Fe-
N(1) ) 163.87(17)°). The latter P-donor in 8a ov, like
the P-donor in 8a k v,a m v,a n v and 8′a lv, does not
couple with imine carbon C(8) and the imine proton
H(8). Furthermore the coupling constants on C(2) and
(40) Ungermann, C. B.; Caulton, K. G. J . Organomet. Chem. 1975,
94, C9.
(41) Garrou, P. E. Inorg. Chem. 1975, 14, 1435.
(42) Garrou, P. E. Chem. Rev. 1981, 81, 229.

1,3-Dipolar Cycloaddition to the Fe-OdC Fragment
Organometallics, Vol. 21, No. 25, 2002 5639
C(7) in 8a ov are almost similar to those in 8a k v,-
a m v,a n v, and 8′a lv. Consequently, we assume that the
PR3 ligand in the latter complexes is coordinated in a
similar position as in 8a ov, i.e., more or less trans to
nitrogen, as shown in Figure 3 (isomer E). The very
small coupling on C(8) and H(8) may indicate that in
solution the P(1)-Fe-N(1) angle is smaller than 164°
(see above), whereas the P(2)-Fe-C(2) angle is probably
more close to 180°, reflected in the large coupling
constant on C(2).
In the 31P spectrum of the crude complex 8bm v, a
second resonance was observed at 50.9 ppm (5%) apart
from that at 32.4 ppm. Since the ppm value compares
well with that found for the corresponding complex
8a m v (52.2 ppm), it probably originates from the isomer
(8′bm v) with the PEt3 ligand approximately trans to
nitrogen.
reported in the literature.43,44 The bis-phosphite-sub-
stituted complexes 8a k k v and 8bk k v show similar P,C
coupling patterns, which means that they probably have
the same ligand arrangement as 8a ov-y,bov. As
2
expected, significantly larger J (P,P) coupling constants
are observed (117 and 103 Hz) in these complexes
because phosphites give rise to larger coupling con-
stants28 and the P-Fe-P angle will probably be sig-
nificantly larger than in the DPPE complexes 8a ov-
y,bov.
For complexes 8a ow -y two regioisomeric complexes
are possible because the dipolarophiles are asymmetric.
1
It is evident from the H, 13C, and 31P NMR data that
in all three cases only one of the isomers is formed. In
the case of the p-methoxyphenyl-isothiocyanate (8a oy)
it was shown by the ν(CdN) in the IR spectrum that
the CdS bond had reacted. The high-frequency positions
of the former alkene protons H(2) (11.43 and 9.67 ppm)
in 8a ow ,a ox clearly show that the alkyne carbon atom
bearing the proton is bonded to iron, resulting in
substantial deshielding. This is in agreement with the
high-frequency positions reported by van Wijnkoop et
al.45 and Muller et al.46,47 for M-C(H)dC proton reso-
nances.
The preference for one of the two coordination sites,
trans to C(2) or more or less trans to N, probably
depends (i) on the steric interactions between the R1
substituent on the imine nitrogen and the PR3 ligand
and (ii) on electronic factors. From the solid state
structure of 8a ov it can be seen that the tert-butyl group
at N(1) sterically interacts with the coordination site
trans to C(2). The coordination site trans to N(1) is
sterically more favorable, although with very bulky PR3
ligands a steric interaction with the ester at C(2) may
be expected. However, this coordination site is thermo-
dynamically less favorable than the coordination site
trans to the C(2) because it brings the two σ-donors, N
an P, approximately in mutual trans-positions. With
this in mind it is conceivable that for steric reasons
complexes 8a k v,a m v,a n v, i.e., with the bulky tert-butyl
group on nitrogen, form the isomer with the PR3 ligand
(R ) OMe, Et, nPr) in a trans-position to nitrogen. With
the small methyl group on nitrogen as in complexes
8bk v,blv and bm v the coordination site trans to C(2)
is sterically not hindered and the phosphorus ligand is
found exclusively (8bk v,blv) or preferentially (8bm v)
in this position. Complex 8blv was synthesized in a
small-scale reaction and is identified by IR and 31P
NMR. The position of the phosphorus resonance clearly
shows that the isomer with PPh3 trans to C(2) is formed
(cf. Table 7). Surprisingly however, in the reaction of
1a with DMAD and the bulky PPh3 (l) ligand the major
of the two formed isomers has the PPh3 in the sterically
less favorable position trans to the C(2). With respect
to the choice of coordination site for the phosphorus
ligand, it can be concluded that a delicate balance exists
between (i) the steric interactions of the P-ligand with
the substituent on nitrogen and the ester group on C(2)
and (ii) electronic factors that disfavor two donor atoms
in mutual trans-positions.
Com p lexes 10. The 13C NMR data of complex 10bv
compare well with those of the earlier reported closely
related complex 10a v.5 The three carbonyl ligands
(C(19), C(19′), and C(19′′)) give rise to three separate
resonances at somewhat higher frequency in comparison
with those of the corresponding 8bv due to increased
π-back-donation as a result of the reduction from Fe-
(II) (8) to Fe(0) (10). The most pronounced changes in
chemical shift, compared with those in 8bv, are ob-
served for the two former alkyne carbon atoms C(1) and
C(2) which are involved in the isomerization step, i.e.,
formation of the coordinated butenolide five-membered
ring. The alkene carbons C(1) and C(2) in 10bv resonate
at 83.1 and 54.6 ppm, which is within the range
normally observed for coordinated alkene carbons. In
10a v they are found at 79.3 and 56.1 ppm, respectively.
All other signals are found at their expected positions.
Interestingly, the Fe(CO)2(PR3)(butenolide) complexes
10 can form three non-interconverting isomers. In the
case of Fe(CO)2(P(OMe)3)(butenolide) three isomers are
formed, of which two (10a k v:10′a k v ) 10:7) are identi-
fied by 1H and 13C NMR. Like for complexes 8, the major
isomer is denoted by its Arabic number and the minor
isomer is primed. The third isomer (10′′a k v) could be
identified only by 31P NMR because it is formed in low
(3%) yield. For PPh3, two isomers are formed in a ratio
of 9:1, of which only the major isomer (10a lv) is
identified by 1H and 13C NMR. The 1H and 13C chemical
shift positions of 10/10′a k v and 10a lv closely resemble
those of 10a v except for the terminal carbonyl C(19) and
C(19′) carbon resonances, which experience a consider-
able shift to higher frequency due to the P-donor. The
The DPPE-containing complexes 8a ov-y,bov give
rise to comparable coupling patterns on both the ter-
minal carbonyl carbons C(19) and the iron-bonded
carbons C(2). This indicates that these complexes all
have the same ligand arrangement, similar to that in
the solid state structure of 8a ov. This is supported by
the resonances in the 31P spectra, which are found at
similar positions. The 2J (P,P) coupling constants of
12.5-34.5 Hz in 8a ov-y,bov compare well with those
(43) Schubert, U.; Gilbert, S.; Knorr, M. J . Organomet. Chem. 1993,
454, 79.
(44) Coco, S.; Mayor, F. J . Organomet. Chem. 1994, 464, 215.
(45) van Wijnkoop, M. PhD Thesis, Universiteit van Amsterdam,
Amsterdam, The Netherlands, 1992; Chapter 3, pp 71-88.
(46) Muller, F.; van Koten, G.; Vrieze, K.; Heijdenrijk, D. Inorg.
Chim. Acta 1989, 158, 69.
(47) Muller, F.; van Koten, G.; Kraakman, M. J . A.; Vrieze, K.; Zoet,
R.; Duineveld, K. A. A.; Heijdenrijk, D.; Stam, C. H.; Zoutberg, M. C.
Organometallics 1989, 8, 982.

5640 Organometallics, Vol. 21, No. 25, 2002
Siebenlist et al.
P,C coupling constants on C(19) and C(19′) differ
significantly in the isomers 10a k v and 10′a k v, indicat-
ing a great difference in the P-Fe-CO angles. Also very
different P,C coupling constants are observed on the two
coordinated alkene carbon atoms C(1) and C(2), respec-
tively. In 10a k v a large coupling is observed on C(1)
(32.1 Hz) and a small coupling (6.8 Hz) for C(2), whereas
in 10′a k v these coupling constants are reverse, 31.8 Hz
on C(2) and 6.8 Hz on C(1). From the solid state
structures of Fe(CO)3(butenolide) 10a v5 it is known that
complexes 10 have a trigonal bipyramidal geometry. The
observed P,C coupling constants can be explained by the
ligand arrangements shown in Figure 4.
data. The FD-mass spectrum shows the correct molec-
ular ion peak at 317 mass units. The presence of the
new N-H bond in 11a is confirmed in the IR spectrum
by a broad absorption at 3390 cm-1. The two ester
carbonyl groups give rise to one broad absorption at
1
1734 cm-1 and a shoulder at 1700 cm-1. In the H NMR
the N-H proton is observed as a broad doublet at 4.25
ppm due to the coupling with the former imine proton
H(8), which gives rise to a doublet at 6.66 ppm. The
large coupling of 13.8 Hz between these two protons
indicates that the N-H and C-H bonds are nearly
coplanar. Comparable large couplings are observed for
the N-protonated butenolide derivatives26 which are
obtained upon decomplexation of the heterocyclic buteno-
An equatorial position of the PR3 ligands is plausible
for electronic reasons because in this way it avoids a
trans arrangement with the σ-donating imine nitrogen.
However, in the case of P(OMe)3 a third resonance is
observed in the 31P NMR, which probably originates
from the isomer with P(OMe)3 in the trans-position to
nitrogen. As expected, it is formed in very low yield (3%)
and only with P(OMe)3, the weakest σ-donor. In the
reaction of 1a with DMAD and PPh3, preferentially
(90%) the isomer is formed with the PPh3 ligand trans
to C(2), while in 10bm v the PEt3 ligand is exclusively
found in this position. In the cycloaddition of DMAD to
Fe(CO)3(R-diimine) with P(OMe)3 as additional ligand,
only one example is known where the [2.2.2] bicyclic
complex 3 undergoes reductive elimination to form the
pyrrolinone complex 4.14 (cf. Scheme 1). There, the
phosphite ligand is coordinated exclusively trans to C(2),
which is the thermodynamically most favorable position
because C(2) is conjugated with the inserted CdO and
thus is a better π-acceptor. Consequently, one would
expect that in complexes 10 the PR3 ligand is coordi-
nated trans to C(2). This is found for the PEt3 ligand in
10bm v and for the PPh3 ligand in 10a lv, which is
preferentially formed (90%). In the latter case also a
small amount of the other equatorial isomer 10′a lv is
formed, which might be due to steric interactions of the
bulky PPh3 with the bulky tert-butyl group at nitrogen
in ligand a . In the case of the reaction of 1a with DMAD
and P(OMe)3, the two equatorial isomers are formed in
almost equal amounts. Obviously, with the less basic
phosphite there is no clear preference for either coor-
dination site.
t
lide ligand in 4 (cf. Scheme 1; X ) O, R1 ) Bu, R3 )
t
OMe). These compounds possess a similar BuN(H)-
C(H)dC fragment. The formation of the new double
bond is supported by the low-frequency shift of the
former imine proton H(8) to 6.66 ppm, a normal position
for an alkene proton. Furthermore, both the shift of ca.
27 ppm to lower frequency of the former imine carbon
C(8) and the high-frequency shift (42 ppm) of C(7) are
t
indicative of the BuN(H)-C(H)dC fragment. The new
alkene proton at C(2) is observed as a singlet at 4.93
ppm.
The two hydrogen atoms needed for the formation of
11a are probably abstracted from THF. Similar proton
abstractions48 from THF have been encountered before
in the coordination chemistry of iron carbonyl with
R-iminoesters. However, the mechanism is unclear, and
because 11a is a only a side product, no further
mechanistic studies were performed.
F or m a tion of Com p lexes 8 a n d 10. The product
formation in the reactions of 1b and complexes 7 with
the employed dipolarophiles can be explained by the
reaction steps depicted in Scheme 3. In the earlier
parts3,7,9,13 of this series we have demonstrated the
isolobal analogy49,50 between the M-NdC fragment (M
) Fe, Ru) in M(CO)n(CNR)3-n(R1-DAB) and an azome-
thine ylide as a typical representative of a Huisgen 1,3-
dipole.51 Likewise, the Fe-OdC fragment5,6 in Fe(CO)3-
(R-iminoketone) is isolobally related to a carbonyl ylide,
which is also a well-known organic 1,3-dipole. On this
basis, the initial reaction is described as an oxidative
1,3-dipolar cycloaddition of the activated alkynes DMAD
and MP across the Fe-OdC fragment, resulting in the
bicyclo[2.2.1] intermediate 2 (cf. Scheme 1; X ) O).
However, it must be noted that the first products that
could be characterized and isolated from these earlier
reactions were the Fe(CO)3(butenolide) complexes (4).
The intermediates were proposed on basis of the analogy
with the reaction of M(CO)n(CNR)3-n(R1-DAB) (M ) Fe,
Ru) with activated alkynes.
The observation that the PR3 ligand in 10bm v, and
less so for 10a lv, ends up in the most favorable position
agrees well with the idea of a coordinatively unsaturated
intermediate during the rearrangement of 9 f 10, when
the bond between C(2) and the inserted carbonyl carbon
has already been formed, but the resulting five-ring has
not yet become coordinated via its C(1)dC(2) bond. In
this intermediate, the rigidity of 9 is broken and the
ligands attached to iron can (pseudo)rotate to their most
favorable position before the C(1)dC(2) bond coordi-
nates. This could also explain the low preparative yields
for 10, especially in the case of PPh3, because bulky
substituents hinder recoordination of the double bond
and accordingly the coordinatively unsaturated inter-
mediate can decompose via other reaction pathways.
The isolation and characterization of the initially
formed bicyclo[2.2.1] complexes 8 in the present reac-
tions (cf. Scheme 3) unequivocally proves that the first
step is a 1,3-dipolar cycloaddition reaction over the Fe-
OdC fragment. Complex 8bv could be isolated only
when the reaction is performed in pentane because it
then directly precipitates out of the solution and inser-
Sp ectr oscop ic Ch a r a cter iza tion a n d Str u ctu r a l
Assign m en t of 11a . The structure of compound 11a
shown in Scheme 3 is tentatively assigned on the basis
of its IR, FD-mass, 1H NMR, and 13C NMR spectroscopic
(48) Siebenlist, R.; Fru¨hauf, H.-W.; Vrieze, K.; Kooijman, H.; Smeets,
W. J . J .; Spek, A. L. Organometallics 2000, 19, 3016-3031.
(49) Hoffmann, R. Science 1981, 211, 995.
(50) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711-800.
(51) Huisgen, R. Angew. Chem. 1963, 75, 604.

1,3-Dipolar Cycloaddition to the Fe-OdC Fragment
Organometallics, Vol. 21, No. 25, 2002 5641
tion of CO does not occur. However, in solution above
-30 °C 8bv immediately reacts further by insertion of
CO into the Fe-O bond. No matter which additional
ligand is offered, CO or PEt3, the resulting bicyclo[2.2.2]
intermediate 9 rapidly isomerizes, via reductive elimi-
nation of the two Fe-C bonds followed by recoordination
of the double bond, to form the Fe(CO)n(PEt3-n)(buteno-
lide) (n ) 0, 1) complexes 10 (cf. Scheme 6). Analogously,
the reaction of 1a with DMAD and PR3 (R ) OMe, Ph)
results in the immediate formation of the corresponding
Fe(CO)2(PR3)(butenolide) complexes 10 (cf. Scheme 5).
This means that intermediate 9, despite the stabilizing
P-donor ligand, still rapidly isomerizes. This instability
may be attributed to the presence of substituents on the
bridgehead carbons of the R-iminoketone, which tend
to promote reductive elimination. This was also ob-
served for the reaction of the biacetyl-derived Fe(CO)3-
(Np-DAB)14 with DMAD, where the first observable
product is the isomerized pyrrolinone complex (4), even
when P(OMe)3 is used as additional ligand. Further-
more, the substitution of the more electronegative
oxygen in the Fe-C(O)-X fragment instead of NR1
might weaken the Fe-C(O) bond.
Rea ction s w ith Less Activa ted Dip ola r op h iles.
Reaction of the Fe(CO)2(PR3)(imket) complexes 7 with
the less reactive alkyne MP proceeds readily at -40 °C,
which is indicative for the increased reactivity of these
complexes. However, no identifiable products can be
isolated, which means that the initially formed com-
plexes 8 decompose. Analogously, reaction of Ru(CO)3-
(R1-DAB) results initially in the formation of the
bicyclo[2.2.2] Ru(CO)3 adduct at -78 °C. Upon warming,
however, it decomposes.52 A similar type of decomposi-
tion could take place in the above-described reactions.
The Fe(CO)2(PR3)(imket) complexes 7 do not react with
phenyl acetylene and p-methoxyphenyl-isothiocyanate.
Besides with DMAD and MP, the Fe(CO)(DPPE)-
(imket) complexes 7a o,bo react also with the less
activated dipolarophiles phenyl acetylene and p-meth-
oxyphenyl-isothiocyanate to give the corresponding
initial adducts 8. The enhanced 1,3-dipolar reactivity
of complexes 7 can be rationalized by electronic argu-
ments. In earlier parts we have shown that in analogy
with the organic carbonyl ylide the Fe-OdC fragment
can be described as a nucleophilic, HOMO-controlled
1,3-dipole, according to the Sustmann classification.53-58
This is supported by CAS-SCF calculations.59-61 It is
known that the reactivity of HOMO-controlled cycload-
ditions is enhanced by the introduction of electron-
donating substituents on the dipole. Consequently, the
introduction of a PR3 and especially a DPPE ligand
leads to an enhanced 1,3-dipolar reactivity.
Ack n ow led gm en t. The investigation was in part
supported (W.J .J .S. and A.L.S.) by The Netherlands
Foundation for Chemical Research (SON) with financial
aid from The Netherlands Organization for Scientific
Research (NWO).
Su p p or tin g In for m a tion Ava ila ble: Table 4 with IR
1
data, elemental analyses, and mass data, Tables 5-7 with H,
13C, and 31P NMR data. Tables S1-S6 of crystal data and
details of structure determination, final coordinates, and
equivalent isotropic displacement parameters of the non-
hydrogen atoms, Hydrogen atom positions and isotropic dis-
placement parameters, (an)isotropic displacement parameters,
and bond distances and angles for compound 8a ov. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM020605Z
(53) Sustmann, R. Tetrahedron Lett. 1971, 2721.
(54) Sustmann, R.; Trill, H. Angew. Chem., Int. Ed. Engl. 1972, 11,
838.
(55) Sustmann, R. Pure. Appl. Chem. 1974, 40, 569.
(56) Houk, K. N.; Yamaguchi, K. 1,3-Dipolar Cycloaddition Chem-
istry; J ohn Wiley and Sons Inc.: New York, 1984.
(57) Fukui, K. Acc. Chem. Res. 1971, 4, 57.
(58) Herdon, W. C. Chem. Rev. 1972, 72, 157.
(59) Houk, K. N.; Sims, J .; Duke, R. E.; Strozier, R. W.; George, J .
K. J . Am. Chem. Soc. 1973, 95, 7287.
(60) Houk, K. N. Acc. Chem. Res. 1975, 8, 361.
(61) Dedieu, A.; Liddel, M. J . Unpublished results.
(52) van Wijnkoop, M. Personal communication.