57Fe NMR study of ligand effects in cyclopentadienyliron complexes

Article abstract of DOI:10.1021/om950989b

The first systematic ⁵⁷Fe NMR study of ligand effects in cyclopentadienyliron complexes is presented. Four series encompassing a total of 35 compounds have been studied. Among the compounds are five new ring-substituted complexes with the formula (C₅H₄Y)Fe(CO)-(PPh₃)(Me), Y = Me, SiMe₃, NEt₂, I, and Ph. The crystal structure of (C₅H₄I)Fe(CO)(PPh₃)-(Me) (32) was determined. For the series CpFe(CO)₂R (type I), ⁵⁷Fe shielding was found to decrease with the bulkiness of the alkyl ligand R and to correlate with the CO-insertion rate of the complex. In type III complexes CpFe(CO)(L)(COMe), L = PR₃, and CO, it is again the steric requirement of the ligand L that dominates the ⁵⁷Fe chemical shift which increases with larger cone angles (θ). In contrast, a strong electronic effect was found for type II complexes CpFe(CO)(PPh₃)X, X = H, Me, and I. Ligands with higher electronegativity induce a shift of the ⁵⁷Fe resonance to higher frequencies. In the complexes (C₅H₄Y)Fe-(CO)(PPh₃)(Me), Y = Me, H, SiMe₃, NEt₂, I, Ph, COOMe, and COⁱPr (type IV), again electronic effects are dominant, whereby electron acceptor substituents (e.g. Y = COⁱPr, COOMe) cause a deshielding of the iron nucleus, relative to complexes with electron donor substituents (e.g. Y = Me, H, SiMe₃). Dominant electronic ligand effects in type IV complexes are also apparent from the correlation of δ(⁵⁷Fe) with the electronic substituent parameter σ_I. The ⁵⁷Fe chemical shift is shown to be very sensitive to changes in the ligand sphere, and the effects are discussed in terms of the paramagnetic shielding constant (σ_para). The ¹J(⁵⁷Fe,¹³C) coupling constants and longitudinal relaxation times T₁ of ⁵⁷Fe in selected complexes have been determined and ligand effects are discussed.

Full text of DOI:10.1021/om950989b

Organometallics 1996, 15, 2469-2477
2469
57F e NMR Stu d y of Liga n d Effects in
Cyclop en ta d ien ylir on Com p lexes†
Eric J . M. Meier, Wiktor Koz´min´ski, Anthony Linden, Philipp Lustenberger, and
Wolfgang von Philipsborn*
Organisch-chemisches Institut, Universita¨t Zu¨rich, Winterthurerstrasse 190,
CH-8057 Zu¨rich, Switzerland
Received December 28, 1995X
The first systematic 57Fe NMR study of ligand effects in cyclopentadienyliron complexes
is presented. Four series encompassing a total of 35 compounds have been studied. Among
the compounds are five new ring-substituted complexes with the formula (C5H4Y)Fe(CO)-
(PPh3)(Me), Y ) Me, SiMe3, NEt2, I, and Ph. The crystal structure of (C5H4I)Fe(CO)(PPh3)-
(Me) (32) was determined. For the series CpFe(CO)2R (type I), 57Fe shielding was found to
decrease with the bulkiness of the alkyl ligand R and to correlate with the CO-insertion
rate of the complex. In type III complexes CpFe(CO)(L)(COMe), L ) PR3, and CO, it is
again the steric requirement of the ligand L that dominates the 57Fe chemical shift which
increases with larger cone angles (θ). In contrast, a strong electronic effect was found for
type II complexes CpFe(CO)(PPh3)X, X ) H, Me, and I. Ligands with higher electronegativity
induce a shift of the 57Fe resonance to higher frequencies. In the complexes (C5H4Y)Fe-
(CO)(PPh3)(Me), Y ) Me, H, SiMe3, NEt2, I, Ph, COOMe, and COiPr (type IV), again electronic
effects are dominant, whereby electron acceptor substituents (e.g. Y ) COiPr, COOMe) cause
a deshielding of the iron nucleus, relative to complexes with electron donor substituents
(e.g. Y ) Me, H, SiMe3). Dominant electronic ligand effects in type IV complexes are also
apparent from the correlation of δ(57Fe) with the electronic substituent parameter σI. The
57Fe chemical shift is shown to be very sensitive to changes in the ligand sphere, and the
effects are discussed in terms of the paramagnetic shielding constant (σpara). The 1J (57Fe,13C)
coupling constants and longitudinal relaxation times T1 of 57Fe in selected complexes have
been determined and ligand effects are discussed.
In tr od u ction
Cyclopentadienyl complexes of group VIII transition
metals are vital in several important stoichiometric and
catalytic reactions,15 and annually many synthetic ap-
plications are published.16,17 Cyclopentadienyliron com-
plexes in particular have attracted considerable interest
as stoichiometric reagents in stereoselective reactions.18
To date, no systematic 57Fe NMR study of ligand effects
in such complexes has been reported, but preliminary
results have shown that 57Fe shielding is a sensitive
parameter reflecting both electronic and steric ligand
effects.19
While metal NMR investigations are becoming more
and more common,20,21 the number of 57Fe NMR studies
is still limited22,23 because the 57Fe nucleus is one of the
most insensitive nuclei in NMR spectroscopy owing to
its small magnetic moment and low natural abundance
(2.2%).24 Therefore, direct detection of 57Fe NMR
The performance of catalysts, e.g. of a homogeneous
Ziegler-Natta type, depends critically on the nature of
the metal-coordinated ligands. The catalytic activity
has been shown to vary with the metal-alkyl bond
stability, the metal-olefin coordination, and steric
effects.2 However, a more than qualitative character-
ization of ligand effects is often difficult. Ligand effects
on structure and reactivity can be probed by transition
metal NMR spectroscopy, as we have reported for a
variety of cases: 59Co,3,4 103Rh,5-9 55Mn,10,11 and 57Fe.12-14
† Transition Metal NMR Spectroscopy. 31. Part 30: See ref 1.
X Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) Bu¨hl, M.; Hopp, G.; von Philipsborn, W.; Prosenc, M.-H.; Rief,
H.; Brintzinger, H.-H. Organometallics 1996, 15, 778.
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Mynott, R.; von Philipsborn, W.; Egolf, T. J . Organomet. Chem. 1984,
272, 231.
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Philipsborn, W. Helv. Chim. Acta 1990, 73, 1469.
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Organometallics 1991, 10, 3049.
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Chem. Soc. 1993, 115, 5889.
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J . Organomet. Chem. 1996, 513, 193.
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Philipsborn, W. J . Org. Chem. 1989, 54, 5433.
(11) Dowler M. E.; Le, T. X.; DeShong, P.; von Philipsborn, W.;
Vo¨hler, M.; Rentsch, D. Tetrahedron 1993, 49, 5673.
(12) J enny, T.; von Philipsborn, W.; Kronenbitter, J .; Schwenk, A.
J . Organomet. Chem. 1981, 205, 211.
(13) Adams, C. M.; Cerioni, G.; Hafner, A.; Kalchhauser, H.; von
Philipsborn, W.; Prewo, R.; Schwenk, A. Helv. Chim. Acta 1988, 71,
1116.
(14) Hopp, G.; Nanz, D.; Stefanova, R.; von Philipsborn, W. Orga-
nometallics, submitted for publication.
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1980.
(16) Kerber, R. C. J . Organomet. Chem 1994, 477, 119.
(17) Richmond, M. G. J . Organomet. Chem. 1994, 477, 219.
(18) Davies, S. G. Pure Appl. Chem. 1988, 60, 13.
(19) Meier, E. J . M.; von Philipsborn, W. Chimia 1994, 48, 262.
(20) Benn, R.; Rufin´ska, A. Angew. Chem. 1986, 98, 851.
(21) Rehder, D. Coord. Chem. Rev. 1991, 110, 161.
(22) von Philipsborn, W. Pure Appl. Chem. 1986, 58, 513.
(23) Benn, R. In Transition Metal NMR; Pregosin, P. S., Ed.;
Elsevier: Amsterdam, 1991; p 103.
(24) Mason, J . In Multinuclear NMR; Plenum Press: New York,
1987.
S0276-7333(95)00989-7 CCC: $12.00 © 1996 American Chemical Society

2470 Organometallics, Vol. 15, No. 10, 1996
Meier et al.
Sch em e 1
Ch a r t 1
to the literature, in group IV five new complexes (29-
33) were synthesized and completely characterized for
the first time. The selected synthetic path for these
complexes is shown in Scheme 1. As starting materials,
[(C5H4Y)Fe(CO)2]2, Y ) H, Me, Ph, and SiMe3,34,35 and
diazocyclopentadiene36 were used to form [(C5H4Y)Fe-
(CO)2X], X ) hal.37,38 Further reaction with PPh3 in
39
benzene, catalyzed by [CpFe(CO)2]2 and followed by
addition of MeMgI in THF, gave the desired type IV
complexes 29, 30, 32, and 33. [(C5H4NEt2)Fe(CO)-
(PPh3)Me] (31) was synthesized from CpFe(CO)(PPh3)-
Br35,39 by ring substitution with LiNEt2, again followed
by methylation with MeMgI. Two other complexes that
have been synthesized (34, 35) were previously reported
by Abbott et al.,40 but the authors gave no details about
the syntheses and spectroscopic properties. We isolated
them after deprotonation of the cyclopentadienyl ligand
signals requires high sample concentrations,12 high
magnetic fields,13 and long measurement times.20,22,25
Koridze and co-workers overcame the sensitivity prob-
lem by synthesizing 57Fe-enriched (80-90%) com-
plexes.26 More recently, the powerful method of indirect
2D HMQC (31P,57Fe){1H} NMR spectroscopy was intro-
duced by Benn and Brevard, 20,23,27-29 which allows the
i
of CpFe(CO)(PPh3)(COR), R ) Pr, and OMe, with
1
determination of 57Fe chemical shifts and J (57Fe,31P)
n-BuLi, followed by migration of the acyl ligand. Sub-
sequent methylation of the anion with methyl iodide
yielded the two corresponding substituted cyclopenta-
dienyl complexes (34, 35).
coupling constants with dilute 5-mm samples in record-
ing times that permit systematic studies.
In the present work, we report results obtained on
four closely related series of cyclopentadienyliron com-
plexes of the type (C5H4Y)Fe(CO)LR.
57F e NMR Sp ectr oscop y of Com p lexes of Typ es
I-IV. The spectra of type I complexes were recorded
Resu lts a n d Discu ssion
(30) Gmelin, Handbook of Inorganic Chemistry, 8th ed.; Springer
Verlag: Berlin, 1984; Vol. B12.
Syn th eses. The cyclopentadienyliron complexes of
four series with the general formula (C5H4Y)Fe(CO)LR
were prepared for a systematic study of ligand effects
(Chart 1). While the well-known complexes of type
I,30,31 type II,32 and type III33 were obtained according
(31) J ohnson, M. D. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, U.K., 1982; Vol. 4.
(32) Reger, D. L.; Culbertson, E. C. Syn. React. Inorg. Metal-Org.
Chem. 1976, 6, 1.
(33) Bibler, J . P.; Wojcicki, A. Inorg. Chem. 1966, 5, 889.
(34) Coville, N. J .; Loonat, M. S.; White, D.; Carlton, L. Organome-
tallics 1992, 11, 1082.
(25) Dechter, J . J . Prog. Inorg. Chem. 1985, 33, 393.
(26) (a) Koridze, A. A.; Petrovskii, P. V.; Gubin, S. P.; Fedin, E. I. J .
Organomet. Chem. 1975, 93, C26. (b) Koridze, A. A.; Astakhova, N.
M.; Petrovskii, P. V.; Lutsenko, A. I. Dokl. Chem. 1978, 242, 416. (c)
Koridze, A. A.; Astakhova, N. M.; Petrovskii, P. V. J . Organomet. Chem.
1983, 254, 345.
(27) Benn, R.; Brevard, C. J . Am. Chem. Soc. 1986, 108, 5622.
(28) Benn, R.; Brenneke, H.; Frings, A.; Lehmkuhl, H.; Mehler, G.;
Rufin´ska, A.; Wildt, T. J . Am. Chem. Soc. 1988, 110, 5661.
(29) Nanz, D. Ph.D. Thesis, Universita¨t Zu¨rich, 1993.
(35) White, D.; J ohnston, P.; Levendis, I. A.; Michael, J . P.; Coville,
N. J . Inorg. Chim. Acta 1994, 215, 139.
(36) Weil, T.; Cais, M. J . Org. Chem. 1963, 28, 2472.
(37) Piper, T. S.; Wilkinson, G. J . Inorg. Nucl. Chem. 1956, 2, 38.
(38) Herrmann, W. A.; Huber, M. Chem. Ber. 1978, 111, 3124.
(39) Fabian, B. D.; Labinger, J . A. J . Am. Chem. Soc. 1979, 101,
2239.
(40) Abbott, S.; Baird, G. J .; Davies, S. G.; Dordor-Hedgecock, I. M.;
Maberly, T. R.; Walker, J . C.; Warner, P. J . Organomet. Chem. 1985,
289, C13.

Ligand Effects in Cyclopentadienyliron Complexes
Organometallics, Vol. 15, No. 10, 1996 2471
Ta ble 1. 57F e NMR Da ta for Cyclop en ta d ien ylir on
Com p lexes
assumption of Pople44 and the average excitation energy
(AEE) approximation45 changes of σ can be roughly
estimated with the following equation:
T1/s (% CSA)
δ(57Fe)/ 1J (57Fe,31P)/
no. R, L, or Y
ppm
Hz
9.4 T
14.1 T
r-3
∆E
CpFe(CO)2R, Type I
σpara ≈ -
Q
(1)
∑
AB
1
Me
684
696
716
727
784
796
805
959
2
3
4
5
6
7
8
Et
nBu
iBu
In eq 1 r denotes the distance of the valence d
electrons from the 57Fe nucleus, ∆E is the average
electronic excitation energy, and QAB is the bond order
and charge density term. An increase in the 57Fe
chemical shift can then be rationalized as being due to
either a decrease in the d-orbital expansion (nephelaux-
etic effect) or in ∆E that is often approximated by the
HOMO-LUMO gap. Chemical shift trends of the
measured complexes will be discussed on the basis of
these considerations.
Va r ia tion of Alk yl Liga n d s R in Typ e I Com -
p lexes Cp F e(CO)2R. Electronic ligand effects in CpFe-
(CO)2X (X ) alkyl, aryl, hal, CN) complexes have been
reported as early as 1967.46 Linear relationships be-
tween the IR absorption frequency ν(CO) and the half-
wave reduction potentials E1/2 as well as the Hammett
substituent constant σI were found. An increase in the
electron-acceptor capability of X leads to increased IR
neoPe
iPr
sBu
COMe
CpFe(CO)(PPh3)R, Type II
9
H
536
1385
1386
1392
1425
1480
1500
2607
1525
1531
1531
1536
1543
1545
1550
54.4
58.7
58.8
57.0
58.1
60.6
57.9
55.1
59.3
59.5
59.2
58.4
58.9
58.5
60.1
10
11
12
13
14
15
16
17
18
19
20
sBu
nBu
Me
iBu
iPr
neoPe
I
COnBu
COiBu
COMe
COiPr
4.5 (77)
4.0 (96)
4.3 (91)
2.3 (88)
1.8 (98)
2.0 (96)
21A COsBu (A)
21B COsBu (B)
22
4.5 (95)
4.0 (88)
5.0 (100)
2.1 (98)
1.9 (94)
2.1 (100)
COneoPe
frequencies and decreased values for E1/2
. In 1968
CpFe(CO)(L)COMe, Type III
King47 reported a correlation between H chemical shifts
(cyclopentadienyl signal) and IR stretching frequencies
ν(CO) for the same complexes. Gansow et al.48 confirmed
these results and found further linear relationships
between the 13C NMR shifts of the carbonyl and the IR
stretching frequencies ν(CO) as well as the Taft param-
eter σI. All of these previous correlation studies were
attributed to changes of the electronic nature of X.
From earlier 57Fe NMR studies12,23 it was known that
the iron shift is extremely sensitive to electronic ligand
effects, but also steric effects were clearly recognizable
in substituted (diene)Fe(CO)3 complexes.13 In addition,
it was shown by 59Co NMR of alkylcobaloximes4 that
steric ligand effects can be recorded directly at the Co
center. We, therefore, expected that small changes in
the ligand sphere, such as steric ligand effects, might
also be discernible in the spectra of type I complexes.
A comparison of the 57Fe chemical shifts within the
complex series I reveals a dependence on the steric effect
of the alkyl ligands R in CpFe(CO)2R: complexes (1-7)
with sterically more demanding ligands resonate at
higher frequencies as shown in Figure 1. Lengthening
and branching of the alkyl chain results in increased
δ(57Fe) values. The covered range of about 120 ppm
confirms the high sensitivity of δ(57Fe) to this steric
alkyl group effect. The correlation of δ(57Fe) with
different steric ligand parameters (V,49 úf,50 Ωs,51 v,52
1
23
24
25
26
27
19
28
P(OMe)3
P(OiPr)3
PMe3
1159
1268
1374
1414
1424
1531
1683
93.9
93.0
58.3
57.7
57.5
59.2
58.1
12.2 (32)
8.8 (51)
13.7 (18)
11.2 (33)
PMe2Ph
PMePh2
PPh3
PiPr3
(C5H4Y)Fe(CO)(PPh3)Me, Type IV
29
30
31
32
33
34
35
Me
1367
1421
1437
1439
1500
1576
1580
56.4
54.7
56.6
56.5
55.8
56.6
55.9
3.8 (96)
1.8 (98)
SiMe3
NEt2
I
4.0 (88)
3.4 (79)
1.9 (94)
1.7 (90)
Ph
COOMe
COiPr
3.7 (96)
1.7 (98)
by direct 57Fe observation at 19.4 MHz (B0 ) 14.1 T).
The 57Fe chemical shifts and the coupling constants
1J (57Fe,31P) of type II-IV complexes were measured at
12.9 MHz (B0 ) 9.4 T) by inverse 2D (31P,57Fe){1H}
experiments.28 This recording technique has the ad-
vantage that sample concentrations are more than ten
times lower and recording times are at least three times
shorter than for direct observation; however, coupling
to an abundant spin-1/2 nucleus, such as 31P or 1H, is
required. The 57Fe chemical shifts and 1J (57Fe,31P)
coupling constants of all complexes studied in this work
are summarized in Table 1.
The shielding constant σ which is responsible for the
chemical shift δ can be separated into diamagnetic (σdia
and paramagnetic (σpara) components according to the
formalism presented by Ramsey.41 It has been shown
that variations of σdia for light and heavy nuclei in a
range of molecular environments are less than 5%.42 The
)
(44) (a) Pople, J . A. J . Chem. Phys. 1962, 37, 53. (b) Pople, J . A. J .
Chem. Phys. 1962, 37, 60. (c) Pople, J . A.; Santry, D. P. Mol. Phys.
1964, 9, 301.
(45) Webb, G. A. In NMR and the Periodic Table; Harris, R. K.,
Mann, B. E., Eds.; Academic Press: New York, 1978; p 49.
(46) Nesmeyanov, A. N.; Chapovskii, Y. A.; Denisovich, L. I.;
Lokshin, B. V.; Polovyanyuk, I. V. Proc. Acad. Sci. USSR: Chem. 1967,
174, 576.
observed chemical shift range for heavy atoms is
43
dominated by variations of σpara
.
For this reason,
(47) King, R. B. Inorg. Chim. Acta 1968, 2, 454.
(48) Gansow, O. A.; Schexnayder, D. A.; Kimura, B. Y. J . Am. Chem.
Soc. 1972, 94, 3406.
changes in σdia usually are neglected, and with the
(41) Ramsey, N. F. Phys. Rev. 1950, 78, 699.
(42) Webb, G. A. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983; Vol. 1, p 79.
(43) Kidd, R. G. In Annual Reports on NMR Spectroscopy; Webb,
G. A., Ed.; Academic Press: New York, 1980; Vol. 10A, p 1.
(49) Meyer, A. Y. J . Chem. Soc., Perkin Trans. 2 1986, 1567.
(50) Beckhaus, H.-D. Angew. Chem. 1978, 90, 633.
(51) Komatsuzaki, T.; Akai, I.; Sakakibara, K.; Hirota, M. Tetrahe-
dron 1992, 48, 1539.
(52) Charton, M. J . Am. Chem. Soc. 1975, 97, 1552.

2472 Organometallics, Vol. 15, No. 10, 1996
Meier et al.
F igu r e 1. Graphical representation of 57Fe chemical shifts
in complexes of type I.
Ta ble 2. Rea ction Ra tes kobs for th e CO In ser tion
Rea ction of Typ e I Com p lexes w ith P P h 3 a n d 57F e
Ch em ica l Sh ifts
R in
CpFe(CO)2R
kobs/s-1
δ(57Fe)/ppm
Me
2.1 × 10-2
1.6 × 10-1
5.3 × 10-1
1.5
1.6
2.2
684
716
727
784
796
805
nBu
iBu
neoPe
iPr
F igu r e 2. Correlation between the CO insertion rate, kobs
,
for the reaction CpFe(CO)2R + PPh3 f CpFe(CO)(PPh3)COR
sBu
and δ(57Fe) of type I complexes (r ) 0.976).
54
Es,53 and Es′ ) gave plots similar to those found in
alkylcobaloximes4 and alkylrhodoximes.8 Thus, δ(57Fe)
responds very sensitively to steric changes in CpFe-
(CO)2R complexes (1-7) in contrast to δ(1H), δ(13C), or
ν(CO) which vary only within a small range and give
no evidence for such an effect.
available, ab-initio calculations of the geometry57 were
n
carried out for CpFe(CO)2R, R ) Me (1), Bu (3), and
iPr (6). They confirmed our expectations and showed
that the metal-alkyl σ-bond distance increases with the
steric demand of R: Fe-Me ) 2.062 Å; Fe-nBu ) 2.081
Å; Fe-iPr ) 2.116 Å.
To check whether the above steric effect reflects the
strength of the Fe-C σ-bond, the complexes of type I
were reacted with triphenylphosphine in refluxing THF
to obtain the corresponding CpFe(CO)(PPh3)COR com-
plex (type II, 17-22). The CO insertion reaction into
the Fe-C bond was followed by the decrease of the
highest IR CO band at about 2000 cm-1. The reaction
conditions were rendered pseudo-first-order by adding
triphenylphosphine in at least 10-fold excess. Our
results, which are in agreement with earlier CO inser-
tion studies,55 are summarized in Table 2.
Indeed, δ(57Fe) was found to correlate linearly with
kobs of the CO insertion reaction, as shown in Figure 2.
High 57Fe shielding (i.e. a low-frequency resonance)
corresponds to a slow CO insertion reaction and there-
fore a strong Fe-C bond (large ∆Hq).56 Thus, the 57Fe
chemical shift seems to offer a direct indication of the
reactivity of the Fe-C σ-bond in these complexes. The
changes of δ(57Fe) can be explained by the r-3 term in
eq 1: a complex with bulky R ligands which undergoes
a fast CO insertion reaction will have a weaker and,
therefore, longer Fe-C bond resulting in a larger r-3
term (nephelauxetic effect). However, if the weaker
Fe-C bond is associated with a smaller HOMO/LUMO
gap, the ∆E term may also be responsible for the
deshielding effect. Therefore, δ(57Fe) increases, as
observed in the experiment (Figure 2). To support the
explanation of weaker and longer Fe-C bonds for
sterically more demanding R ligands and because no
X-ray crystallographic data for type I complexes are
Va r ia tion of Liga n d s L in Typ e III Com p lexes
Cp F e(CO)(L)(COMe). For the displacement of the
solvent from [CpFe(CO)(COMe)(solvent)]+ radicals by
organic nitriles it has previously been reported58 that
the rate of reaction decreases as the size of the nitrile
increases. A linear correlation was found between log
k and Tolman’s cone angle θ of the nitrile. In the
electrochemically promoted carbonyl insertion reaction
of CpFe(CO)(PR3)Me,59,60 different stereoelectronic ef-
fects of the PR3 ligand on the rates were reported for
each reaction step. In the current study, a steric effect,
which depends on the size of the ligand L, was found
for the neutral type III complexes. Tolman’s cone angle
of the phosphorus ligand61 correlates linearly with
δ(57Fe), as shown in Figure 3. The shifts cover a range
of 500 ppm. The fact that the point L ) CO (8) fits the
correlation very well, even though the electronic behav-
ior of CO is different from that of PR3, strongly supports
the predominant steric influence on δ(57Fe) in this
series.
Va r ia t ion of Liga n d s R in Typ e II Com p lexes
Cp F e(CO)(P P h 3)R. A strong electronic effect was
found in CpFe(CO)(PPh3)R complexes for the series R
) H (9), Me (1), and I (16). The chemical shift δ(57Fe)
varies over more than 2000 ppmsthe more electrone-
gative ligands R causing decreased shielding. A similar
effect has been reported by Benn28 for CpFe(PMe3)2R
(57) Geometries have been optimized at a gradient-corrected level
of density functional theory employing a polarized split-valence bases
set: Bu¨hl, M.; Malkina, O. L.; Malkin, V. G. Helv. Chim. Acta 1996,
in press.
(58) Fernandez, A. L.; Prock, A.; Giering, W. P. Organometallics
1994, 13, 2767.
(59) Woska, D. C.; Prock, A.; Giering, W. P. Organometallics 1992,
11, 3343.
(60) Woska, D. C.; Bartholomew, J .; Greene, J . E.; Eriks, K.; Prock,
A.; Giering, W. P. Organometallics 1993, 12, 304.
(61) Tolman, C. A. Chem. Rev. 1977, 77, 313.
(53) Taft, R. W. in Steric Effects in Organic Chemistry; Newman,
M. S., Ed.; J ohn Wiley & Sons: New York, 1956.
(54) MacPhee, J . A.; Panaye, A.; Dubois, J .-E. Tetrahedron 1978,
34, 3553.
(55) Wojcicki, A. Adv. Organomet. Chem. 1973, 11, 87.
(56) Green, M.; Westlake, D. J . J . Chem. Soc. A 1971, 367.

Ligand Effects in Cyclopentadienyliron Complexes
Organometallics, Vol. 15, No. 10, 1996 2473
F igu r e 3. Correlation of δ(57Fe) with Tolman’s cone angle
θ 61 of ligand L in type III CpFe(CO)(L)COMe complexes
(r ) 0.941).
F igu r e 4. Correlation of δ(57Fe) with the Taft parameter
σI 67 of the substituent Y in type IV (C5H4Y)Fe(CO)(PPh3)-
Me complexes. r ) 0.987 without the data point for Y ) I.
complexes and δ(57Fe) was rationalized by the r-3
dependence (eq 1).
changed sequence of δ(57Fe) values for type II versus
type I complexes.
The substitution of one CO by PPh3 to yield CpFe-
(CO)(PPh3)R complexes (R ) alkyl), results in δ(57Fe)
values that are about 700 ppm higher than in series I.
These findings were already made in type III complexes
and explained by the steric requirement of the PPh3
ligand. Within series II (R ) alkyl; 10-15), δ(57Fe)
again covers a range of about 120 ppm, but the sequence
of the ligands R has changed when compared with type
I complexes (1-7). The δ(57Fe) values of the complexes
with R ) acyl (17-22) all lie within 25 ppm, probably
because the ligands differ only at the â-position relative
to the iron center. In all complexes of type II the
Va r ia tion of Su bstitu en ts Y in Typ e IV Com -
p lexes (C5H4Y)F e(CO)(P P h 3)Me. The rationalization
of the effects induced by the Cp ring in substituted
cyclopentadienyl complexes is of considerable interest.
The catalytical activity of homogeneous Ziegler-Natta
catalysts is strongly influenced by cyclopentadienyl
substituents.64 For complexes of the type (C5H4Y)Fe-
1
(CO)(L)I,65 infrared spectroscopy66 and H NMR spec-
troscopy34,35 document electronic and steric effects of Y.
In our series of type IV complexes, the influence of Y on
δ(57Fe) was found to depend mainly on the electronic
properties of Y, which are displayed via the Cp-Fe
π-bond (Figure 4). Electron acceptor substituents, Y )
COiPr and COOMe, at the Cp ring induce larger 57Fe
chemical shifts compared with the electron donor sub-
stituents, Y ) Me, H, and SiMe3, as expected. The iron
shift covers a range of 200 ppm. Figure 4 shows the
correlation of δ(57Fe) with the electronic substituent
parameter σI (Taft)67 for type IV complexes. Correla-
tions of δ(57Fe) with other electronic substituent pa-
rameters were also linear but of lower significance. The
data point for Y ) I lies outside the linear correlation
curve (r ) 0.987), probably owing to steric interactions
between I and the bulky PPh3 ligand. To provide more
information on that question, the crystal structure of
(C5H4I)Fe(CO)(PPh3)Me (32) was determined. Indeed,
it shows (Figure 5) that the I atom adopts the anti
conformation with respect to the P atom (torsion angle
I-C(1)-Fe-P: -163.3(2)°). Similar anti conformations
were also found in (C5H4I)Fe(CO)(PPh3)I and in (C5H4-
CHPh2)Fe(CO)(PPh3)I (torsion angle Y-C(1)-Fe-P:
-166.7 and 165.5°, respectively). The Y-C(1)-Fe-P
torsion angle is found to be smaller for the complex
1
coupling constants J (57Fe,31P) were found to be similar
(54-61 Hz; Table 1) which indicates little variation in
the Fe-P bond. Indeed, a search on the crystallographic
data in the Cambridge Structural Database62 showed
that the Fe-PPh3 bond in 21 different complexes with
the general structure CpFe(CO)(PPh3)R (type II, R )
alkyl, acyl) varies by only 0.01 Å.
Davies and Seeman63 described the stereochemical
situation in complexes of type II as pseudo-octahedral.
Therefore, the bond angles R-Fe-PPh3 and CO-Fe-
PPh3 lie around 90°. One phenyl ring of the PPh3 ligand
forms a plane approximately parallel to the R-Fe-CO
plane. The distance between these two planes is ca. 3-4
Å.63 Large R ligands would lead to steric interactions
between the R and PPh3 groups. Therefore, the ob-
served ligand effects in type II (R ) alkyl, acyl)
complexes seem to be mainly of steric origin. Further-
more, it could be shown by 1H nuclear Overhauser
experiments for type II complexes with large acyl groups
(e.g. R ) COneoPe) that beside the R/PPh3 interaction
(NOE: 5.6%) an interaction between the R and Cp
groups also exists (NOE: 8.5%). This observation is an
additional indication for the presence of steric interli-
gand effects for type II complexes and could explain the
t
(C5H4 Bu)Fe(CO)(PPh3)I (120.9°).68
(64) Mo¨hring, P. C.; Coville, N. J . J . Organomet. Chem. 1994, 479,
1.
(65) Coville, N. J .; du Plooy, K. E.; Pickl, W. Coord. Chem. Rev. 1992,
116, 1.
(66) du Plooy, K. E.; Ford, T. A.; Coville, N. J . J . Organomet. Chem.
1992, 441, 285.
(62) Allen, F. H.; Davies, J . E.; Galloy, J . J .; J ohnson, O.; Kennard,
O.; Macrae, C. F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J . M.; Watson,
D. G. J . Chem. Info. Comp. Sci. 1991, 31, 187.
(63) Davies, S. G.; Seeman, J . I. Tetrahedron Lett. 1984, 25, 1845.
(67) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.

2474 Organometallics, Vol. 15, No. 10, 1996
Meier et al.
of the field-dependent chemical shift anisotropy mech-
anism (CSA). In the case of cyclopentadienyliron com-
plexes with the PPh3 ligand (types II and IV) CSA is
the dominant relaxation mechanism. The relaxation
times of these complexes lie in the ranges 3.4-5 s and
1.7-2.3 s in the magnetic fields of 9.4 and 14.1 T,
respectively. Complexes with an acyl ligand (17-19,
21, 22) relax slightly more slowly than those with a
methyl ligand and a substituted cyclopentadienyl ring
(29, 31, 32, 35), suggesting a larger CSA in the latter.
In contrast to the 57Fe chemical shifts, the differences
between relaxation times for type IV complexes are
relatively small. Also, in type II complexes, the relax-
ation times are only slightly different for variable
ligands. Drastic changes of longitudinal relaxation and
its mechanism are introduced with substitution of the
PPh3 ligand by PMe3 or P(OMe)3. In both cases (25 and
23) T1 increases by a factor of 4-5 at 14.1 T. The slower
relaxation is expected to be due to shorter correlation
times τc as compared with the PPh3 complexes. In
addition, the observed decrease of the CSA contribution
(Table 1) indicates a reduced asymmetry of the ligand
field.
F igu r e 5. Thermal ellipsoid plot of the structure of 32.
Ta ble 3. On e-Bon d 57F e,13C Cou p lin g Con sta n ts of
Typ e IV Com p lexes
no.
compd
1J (57Fe,13C)/Hz
12
29
30
31
32
34
35
(C5H5)Fe(CO)(PPh3)Me
10.9
11.0
11.2
11.3
11.5
10.3
12.0
(C5H4Me)Fe(CO)(PPh3)Me
(C5H4SiMe3)Fe(CO)(PPh3)Me
(C5H4NEt2)Fe(CO)(PPh3)Me
(C5H4I)Fe(CO)(PPh3)Me
Con clu sion s
The systematic 57Fe NMR study of four series of
cyclopentadienyliron complexes has shown that shield-
ing of the iron nucleus is controlled in a subtle manner
by ligand properties and that electronic and steric effects
can be clearly distinguished.
(C5H4COOMe)Fe(CO)(PPh3)Me
(C5H4COiPr)Fe(CO)(PPh3)Me
The electronic substituent effects of Y on the other
ligands at the iron center were also observed in type
IV complexes: ν(CO) as well as δ(31P) correlate well
with the Hammett parameter σp.67
The electronic ligand effects for ligands in the R-posi-
tion to iron, observed in type II complexes CpFe(CO)-
(PPh3)R, R ) H, Me, and I (9, 12, 16), were found to be
strongest (∆δ ) 2000 ppm). In type IV complexes,
(C5H4Y)Fe(CO)(PPh3)Me, the electronic substituent ef-
fect of Y is 10 times smaller, because Y acts via two
bonds, one being a π-bond. Still smaller, but consistent,
steric effects were discovered when the ligand atom in
the R-position and its hybridization were held constant.
In that case, if an alkyl ligand has sufficient space, as
in CpFe(CO)2R (type I), the volume of R influences the
shielding of the iron nucleus, presumably via the Fe-C
σ-bond strength. This is supported by correlation of
57Fe shielding with the CO insertion rate. Even if steric
changes in the R-position to iron are very large, as in
type III complexes, CpFe(CO)(L)COMe, where the cone
angle (θ) of L correlates with δ(57Fe), the steric effect is
still four times smaller than the electronic R-effect.
When steric changes were made in the â-position, as in
type II complexes, CpFe(CO)(PPh3)R, R ) acyl, only a
moderate effect on the iron shielding was observed.
One-bond 57Fe,13C coupling constants of the Fe-C
σ-bond are not correlated with the chemical shift varia-
tions induced by ligand effects in the investigated
complexes. The dominant T1 relaxation mechanism of
the complexes with PPh3 ligands was found to be due
to chemical shift anisotropy, and no other significant
ligand or substituent effects are observable. However,
T1 increases considerably in P(OMe)3 and PMe3 com-
plexes with a concomitant decrease of the field-depend-
ent CSA contribution.
To observe the possible effects of the Y substituent
1
on the Fe-C σ-bond, the coupling constants J (57Fe,13C)
of type IV complexes were measured by the recently
published method employing (1H,57Fe) inverse correla-
tion with a binomial suppression of signals of the
(57Fe,12C) isotopomer.69 These results are shown in
Table 3. In contrast to the large chemical shift varia-
tion, the 1J (57Fe,13C) coupling constants are rather
insensitive to changes of Y, and their values lie in the
range of 11 ( 1 Hz. Therefore, it can be assumed that
the Fe-C σ-bond in type IV complexes is not strongly
affected by the Y substituent. For type I complexes
1J (57Fe,13C) coupling constants are significantly smaller
(9 ( 1 Hz), which could be due to the missing phospho-
rus donor.70 The reported coupling constants are about
1
three times smaller than J (57Fe,13CO).12
Lon gitu d in a l Rela xa tion Tim es T1. Measure-
ments of T1 relaxation times of 57Fe are difficult by
direct detection, owing to the low receptivity of the
nucleus. Only a limited amount of such data is avail-
able.71 Using the double polarization transfer method
with 31P detection72 we have determined T1 values in
the present series. These values are given in Table 1.
Measurements in two different magnetic fields (9.4 and
14.1 T) enabled the determination of the contribution
(68) du Toit, J .; Levendis, D. C.; Boeyens, J . C. A.; Loonat, M. S.;
Carlton, L.; Pickl, W.; Coville, N. J . J . Organomet. Chem. 1989, 368,
339.
(69) Meier, E. J . M.; Koz´min´ski, W.; von Philipsborn, W. Magn.
Reson. Chem. 1996, 34, 89.
(70) Meier, E. J . M.; Koz´min´ski, W.; von Philipsborn, W. unpublished
results.
(71) Hafner, A.; von Philipsborn, W.; Schwenk A. J . Magn. Reson.
1987, 74, 433.
(72) Koz´min´ski, W.; von Philipsborn, W. J . Magn. Reson. A 1995,
116, 262.
The great sensitivity of 57Fe NMR shielding to elec-
tronic and steric effects renders it a useful probe into
the structure and reactivity of cyclopentadienyliron
complexes.

Ligand Effects in Cyclopentadienyliron Complexes
Organometallics, Vol. 15, No. 10, 1996 2475
Solutions of approximately 100 mg (0.2-0.5 mmol) of iron
complex in C6D6 solution in 5-mm sample tubes at 300 ( 0.1
K were measured on a Bruker AMX-600 spectrometer. In all
cases, 2J (57Fe,1H) of about 2 Hz was used for polarization
transfer. Repetition times were at least 1.5T1(1H). Spectral
widths of 100 Hz in F1 and 250 Hz in F2 were used, 1024
data points were collected in F2, and 64 t1 increments with
about 100 scans for each were acquired and zero-filled to 512
frequency points in F1. The experimental time was ca. 12 h.
Exp er im en ta l Section
Gen er a l Meth od s. Standard 1H and 13C NMR spectra
were recorded on a Bruker ARX-300 spectrometer at 300.1 and
75.4 MHz referenced to residual undeuterated solvent with
chemical shifts being reported as δ/ppm from TMS. 31P NMR
spectra were recorded on a Bruker AM-400-WB spectrometer
at 161.9 MHz externally referenced to 85% H3PO4 (¥ )
40.480747 MHz). IR spectra were recorded on a Perkin-Elmer
298 IR spectrophotometer. All reactions and purifications
were carried out under nitrogen using Schlenk-tube tech-
niques.73 All solvents were deoxygenated, dried, and distilled
prior to use.
Syn th eses. The complexes of types I-III and the precur-
sors of type IV complexes were synthesized by literature
procedures: CpFe(CO)2R (type I, 1-8);74,75 CpFe(CO)(PPh3)R
(type II), R ) alkyl (10-15),32 R ) acyl (17-22);33 CpFe(CO)-
(L)COMe (type III, 19, 23-28);33 (C5H4Y)Fe(CO)(PPh3)X, X )
halogen, Y ) H, Me, SiMe3, Ph,34,35,39,76 X ) Br and Y ) NEt2;77
(C5H4I)Fe(CO)(PPh3)I;35,38,39 CpFe(CO)(PPh3)(COOMe);78-80
CpFe(CO)(PPh3)H (9).30
57Fe NMR Measu r em en ts. Indirect two-dimensional NMR
spectra were recorded at 300 K on a modified Bruker AM-400-
WB spectrometer (B0 ) 9.4 T) with an Aspect 3000 computer.
For the (31P,57Fe){1H} measurements, an additional, third
channel consisting of a PTS-160 synthesizer, a Bruker pulse
modulator, and a B-SV 3 BX heterodecoupler unit with a low-
frequency amplifier was used. A 5-mm inverse triple-
resonance probehead, low-frequency range tuneable between
9 and 41 MHz, was employed. In our experiments, 90° pulses
were 14 µs (31P) and 32 µs (57Fe). For inverse correlation
experiments, a standard HMQC sequence was used with
magnitude calculation in F1. Sample concentrations of about
0.3 M and recording times of 4-8 h were required. Typical
acquisition parameters were relaxation delays of 2 s, spectral
widths in F2 of 250 Hz and in F1 of 10000 Hz, 1k data points
in F2, 512 data points in F1, and addition of 24 scans for each
of 512 increments. To check that the observed 57Fe signals
were not folded a second experiment with a reduced F1
spectral width of 1000 Hz was recorded. Complexes of type I
were recorded by direct 57Fe observation at 19.4 MHz on a
Bruker AMX-600 spectrometer (B0 ) 14.1 T) using a broad-
band probe head. Sample concentrations of about 4 M and
recording times of 12-15 h were required. Typically 14 000
points were sampled for a spectral width of 8000 Hz. A pulse
width of 33 µs (90°) and a relaxation delay of 2.5 s were chosen.
All spectra were recorded at 300 K in C6D6 and 5 mm tubes
at natural 57Fe abundance. The 57Fe chemical shifts are
reported in ppm relative to neat Fe(CO)5 as an external
standard (¥ ) 3.237 798 MHz). The reproducibility was found
to be (0.5 ppm; coupling constants J (57Fe,31P) are ( 0.5 Hz.
For inverse detection experiments on the modified Bruker
AMX-600, the TBI-LR triple resonance inverse probehead (1H,
31P, BB (7-20.5 MHz)) for (1H,57Fe) and (31P,57Fe) experiments
was employed. The 90° pulse angle was ca. 11 µs for 1H, 25
µs for 31P, and 30 µs for 57Fe. For decoupling of 31P, a 90° low-
power pulse of 225 µs was used in the (1H,57Fe){31P} correla-
tions.
P r epar ation of (C5H4Y)Fe(CO)(P P h 3)Me, Y ) Me, SiMe3,
NEt2, I, a n d P h (29-33). Typically 0.50 mmol of the complex
(C5H4Y)Fe(CO)(PPh3)I, Y ) Me, SiMe3, Ph, and I, or (C5H4-
NEt2)Fe(CO)(PPh3)(Br) was dissolved in 15 mL of THF, and
at room temperature 0.55 mmol of Grignard-reagent MeMgI
(Fluka pract ∼22% in THF) was added dropwise.The color of
the reaction mixture changed from green to red. Then the
solvent was removed and the residue was chromatographed
on silica gel with hexane/Et2O (1:1).
(C5H4Me)F e(CO)(P P h 3)Me (29): red crystals (170 mg,
93%). IR (benzene): 1902 (s, CO). 1H NMR (C6D6): 7.85-
7.20 (m, 15H, PPh3); 4.27 (bs, 2H, Cp); 4.05, 3.93 (2 bs, 2H,
Cp); 1.91 (s, 3H, CH3); 0.48 (d, 3J (P,H) ) 6.5, 3H, Fe-CH3).
2
13C{1H} NMR (C6D6): 223.5 (d, J (C,P) ) 30.5, CO); 137.8 (d,
1J (C,P) ) 38.9, PPh3); 134-127 (m, PPh3); 99.4, 88.3, 85.4,
81.8, 79.1 (5 s, Cp); 12.6 (s, CH3); -17.5 (d, 2J (C,P) ) 21.3,
Fe-CH3). 31P{1H} NMR (C6D6): 85.6 (s). 57Fe NMR (C6D6):
1367 (1J (Fe,P) ) 56.4).
(C5H4SiMe3)F e(CO)(P P h 3)Me (30): deep red powder (150
mg, 90%). IR (benzene): 1902 (s, CO). 1H NMR (C6D6): 7.50-
6.80 (m, 15H, PPh3); 4.49, 4.35, 4.11, 3.68 (4 bs, 4H, Cp); 0.29
(s, 9H, Si(CH3)3); 0.19 (d, 3J (P,H) ) 6.2, 3H, Fe-CH3). 13C-
{1H} NMR (C6D6): 223.8 (d, 2J (C,P) ) 33.2, CO); 137.6 (d,
1J (C,P) ) 39.3, PPh3); 134.5-127.5 (m, PPh3); 102.8, 89.1, 86.4,
2
84.8, 81.4 (5 s, Cp); 0.0 (s, Si(CH3)3); -21.1 (d, J (C,P) ) 22.0,
Fe-CH3). 31P{1H} NMR (C6D6): 85.7 (s). 57Fe NMR (C6D6):
1421 (1J (Fe,P) ) 54.7).
(C5H4NEt2)F e(CO)(P P h 3)Me (31): red oil (41 mg, 52%).
IR (benzene): 1892 (s, CO). 1H NMR (C6D6): 7.64-6.92 (m,
15H, PPh3); 4.36, 3.95, 3.46, 2.93 (4 m, 4H, Cp); 2.73, 2.72 (q,
3J (H,H) ) 7.3, 4H, CH2); 0.79 (t, 3J (H,H) ) 7.1, 6H, CH3); 0.20
3
(d, J (P,H) ) 6.0, 3H, Fe-CH3). 13C{1H} NMR (C6D6): 223.2
2
1
(d, J (C,P) ) 29.8, CO); 138.0 (d, J (C,P) ) 36.6, PPh3); 134-
127 (m, PPh3); 78.7, 76.7, 67.4, 57.1, 57.0 (5 s, Cp); 44.6 (s,
Relaxation time measurements in all cases were performed
on the Bruker AM-400 (9.4 T) and Bruker AMX-600 (14.1 T)
spectrometers. Samples (ca. 0.5 M in C6D6) sealed in the 5
mm sample tubes were carefully cleaned and degassed by
several freeze-pump-thaw cycles. The double polarization
transfer method 31P-57Fe-31P was employed, with series of
the 31P 180° pulses spaced by 5 ms during relaxation delay,
without refocusing delay before acquisition and with continu-
ous 1H decoupling.72 A spectral width of 250 Hz, acquisition
time of 2 s, repetition time of 1.5-2T1 (31P), and a tempera-
ture of 300 K were used for all 57Fe measurements. The
experimental time was ca. 12 h. Relaxation rates in two
magnetic fields were calculated on the basis of the logarithm
of the intensity in 16 time points using linear regression. The
intensities of both doublet components were averaged.
CH2); 12.8 (s, CH3); -16.5 (d, J (C,P) ) 20.2, Fe-CH3). 31P-
2
{1H} NMR (C6D6): 87.1 (s). 57Fe NMR (C6D6): 1437 (1J (Fe,P)
) 56.6).
(C5H4I)F e(CO)(P P h 3)Me (32): red oil (254 mg, 53%). IR
(benzene): 1918 (s, CO). 1H NMR (C6D6): 7.60-7.05 (m, 15H,
3
PPh3); 4.56, 4.30, 3.89, 3.79 (4 m, 4H, Cp); 0.53 (d, J (P,H) )
2
6.5, 3H, Fe-CH3). 13C{1H} NMR (C6D6): 221.2 (d, J (C,P) )
31.6, CO); 135.3 (d, 1J (C,P) ) 39.8, PPh3); 132-125 (m, PPh3);
2
92.6, 86.7, 85.2, 82.8, 81.4 (5 s, Cp); -15.7 (d, J (C,P) ) 20.6,
Fe-CH3). 31P{1H} NMR (C6D6): 83.7 (s). 57Fe NMR (C6D6):
1439 (1J (Fe,P) ) 56.5).
(74) Piper, T. S.; Wilkinson, G. J . Inorg. Nucl. Chem. 1956, 3, 104.
(75) Green, M. L. H.; Nagy, P. L. I. J . Organomet. Chem. 1963, 1,
58.
(76) Piper, T. S.; Wilkinson, G. J . Inorg. Nucl. Chem. 1956, 2, 38.
(77) Brun, P.; Vierling, P.; Riess, J . G.; Le Borgne, G. Organome-
tallics 1987, 6, 1032.
1
For the measurement of J (57Fe,13C) coupling constants at
natural isotope abundance, (1H,57Fe){31P} HMQC inverse
correlation with a (1 3h 3 1h) binomial excitation pulse and
continuous WALTZ-16 decoupling of 31P was employed.69
(78) Grice, N.; Kao, S. C.; Pettit, R. J . Am. Chem. Soc. 1979, 101,
1627.
(73) Yamamoto, A. In Organotransition Metal Chemistry. Funda-
mental Concepts and Applications; Wiley: New York, 1986.
(79) Reger, D. L.; Colman, C. J . Organomet. Chem. 1977, 131, 153.
(80) Brunner, H.; Schmidt, E. J . Organomet. Chem. 1973, 50, 219.

2476 Organometallics, Vol. 15, No. 10, 1996
Meier et al.
Ta ble 4. Cr ysta llogr a p h ic Da ta for 32
(C5H4P h )F e(CO)(P P h 3)Me (33): deep red oil (113 mg,
60%). IR (benzene): 1911 (s, CO). 1H NMR (C6D6): 7.85-
7.15 (m, 15H, PPh3, 5H, Ph); 5.04, 4.81, 4.39, 4.32 (4 bs, 4H,
Cp); 0.54 (d, 3J (P,H) ) 6.3, 3H, Fe-CH3). 13C{1H} NMR (THF-
d8): 222.3 (d, 2J (C,P) ) 31.3, CO); 137.0 (d, 1J (C,P) ) 39.4,
PPh3); 135-125 (m, PPh3, Ph); 87.5, 85.9, 84.1, 82.1, 77.9 (5 s,
Cp); -18.8 (d, 2J (C,P) ) 20.8, Fe-CH3). 31P{1H} NMR
(CDCl3): 84.1 (s). 57Fe NMR (CDCl3): 1500 (1J (Fe,P) ) 55.8).
empirical formula
C25H22FeIOP
552.17
fw
cryst color, habit
cryst dimens (mm)
temp (K)
red, prism
0.20 × 0.38 × 0.39
173(1)
cryst system
space group
monoclinic
P21/n (No. 14)
4
Z
P r ep a r a tion of (C5H4COOMe)F e(CO)(P P h 3)Me (34). A
248 mg (0.526 mmol) amount of CpFe(CO)(PPh3)(COOMe) was
dissolved in 20 mL of THF. At -78 °C 0.33 mL (0.526 mmol)
of n-BuLi (Fluka pract, 1.6 M in hexane) was added dropwise
and the solution stirred for 10 min. To the deep violet solution
0.03 mL (0.526 mmol) of CH3I (Fluka purum) was added at
once, and the color changed to orange-red. Chromatography
over silica gel (1:1 hexane/Et2O) afforded 157 mg (0.324 mmol,
61%) of (C5H4COOMe)Fe(CO)(PPh3)Me. IR (benzene): 1917
(s, CO), 1717 (m, COOMe). 1H NMR (C6D6): 7.70-7.15 (m,
15H, PPh3); 5.42, 5.25, 4.17, 4.04 (4 bs, 4H, Cp); 3.71 (s, 3H,
OCH3), 0.65 (d, 3J (P,H) ) 6.0, 3H, Fe-CH3). 13C{1H} NMR
reflcns for cell determn
2θ range for cell determn (deg)
unit cell params
a (Å)
b (Å)
25
39-40
7.787(3)
15.768(3)
17.960(4)
101.58(3)
2160.4(9)
1096
1.697
2.215
ω/2θ
60
0.727, 1.000
6941 (+h,+k,(l)
6291
0.032
4817
c (Å)
â (deg)
V (Å3)
F(000)
Dx (g cm-3
)
µ(Mo KR) (mm-1
scan type
)
2θ(max) (deg)
2
transm factors (min, max)
tot. reflcns measd
sym indepdt reflcns
Rint
(C6D6): 221.7 (d, J (C,P) ) 32.4, CO); 167.0 (s, COOR); 136.5
1
(d, J (C,P) ) 40.7, PPh3); 134-127 (m, PPh3, Cp); 92.0 89.0,
84.2 (3 s, Cp); 50.9 (s, OCH3); -19.8 (d, 2J (C,P) ) 21.1, Fe-
CH3). 31P{1H} NMR (C6D6): 82.7 (s). 57Fe NMR (C6D6): 1576
(1J (Fe,P) ) 56.6).
reflcns used [I > 3σ(I)]
params refined
final R
final wR
weights
284
0.0524
0.0748
P r ep a r a tion of (C5H4COiP r )F e(CO)(P P h 3)Me (35). The
same procedure as for 34 was used to synthesize (C5H4COi-
Pr)Fe(CO)(PPh3)Me, but to 134 mg (0.278 mmol) of CpFe-
(CO)(PPh3)(COiPr) in 20 mL of THF was added 0.18 mL (0.290
mmol) of n-BuLi (Fluka pract, 1.6 M in hexane) dropwise at
room temperature and the solution stirred for 10 min. Addi-
tion of 0.02 mL (0.321 mmol) of CH3I, filtration over silica gel,
and crystallization with hexane yielded 58 mg (0.117 mmol,
42%) of (C5H4COiPr)Fe(CO)(PPh3)Me as an orange powder. IR
(benzene): 1920 (s, CO), 1661 (m, CO-i-Pr). 1H NMR (C6D6):
7.75-7.10 (m, 15H, PPh3); 5.30, 5.10, 4.08, 4.02 (4 bs, 4H, Cp);
3.09 (sept, 3J (H,H) ) 6.8, 1H, CH), 1.38 (d, 3J (H,H) ) 6.3, 3H,
w ) [σ2(Fo) + (0.015Fo)2]-1
2.618
goodness of fit
final ∆max/σ
0.004
1.39, -1.55
∆F(max, min) (e Å-3
)
factors were refined for all H atoms. Refinement of the
structure was carried out on F using full-matrix least-squares
procedures, which minimized the function ∑w(|Fo| - |Fc|)2. The
weighting scheme was based on counting statistics and
included a factor to downweight the intense reflections. Plots
of ∑w(|Fo| - |Fc|)2 versus |Fo|, reflection order in data collection,
(sin θ)/λ, and various classes of indices showed no unusual
trends. A correction for secondary extinction was not applied.
There was poor agreement between the Fo and Fc values of 26
reflections as well as several large peaks of residual electron
density near the Fe atom. Small irregularities in the shape
of the crystal made accurate indexing of the crystal faces
difficult, and therefore, it was necessary to use the ψ-scan
method for absorption correction. It is unlikely that these
discrepancies result from disorder in the structure, and the
cause is most probably the inadequacy of the absorption
correction.
3
3
CH3); 1.36 (d, J (H,H) ) 6.4, 3H, CH3); 0.53 (d, J (P,H) ) 5.9,
3H, Fe-CH3). 13C{1H} NMR (C6D6): 221.7 (d, 2J (C,P) ) 33.0,
CO); 202.0 (s, CO-i-Pr); 136.3 (d, J (C,P) ) 40.7, PPh3); 134-
1
127 (m, PPh3); 93.1, 90.3, 85.1, 84.0, 82.7 (5 s, Cp); 36.8 (s,
CH); 19.0 (s, CH3); 18.8 (s, CH3); -19.9 (d, 2J (C,P) ) 21.2, Fe-
CH3). 31P{1H} NMR (C6D6): 83.0 (s). 57Fe NMR (C6D6): 1580
(1J (Fe,P) ) 55.9).
Cr yst a llogr a p h ic An a lysis of (Cp I)F e(CO)(P P h 3)Me
(32). Crystals of C25H22FeIOP, obtained from hexane/diethyl
ether, were used for a low-temperature X-ray structure de-
termination. All measurements were made on a Rigaku
AFC5R diffractometer using graphite-monochromated Mo KR
radiation (λ ) 0.710 69 Å) and a 12 kW rotating anode
generator. The intensities of three standard reflections were
measured after every 150 reflections and remained stable
throughout the data collection. The intensities were corrected
for Lorentz and polarization effects. An empirical absorption
correction, based on azimuthal scans of several reflections,81
was applied. An attempt with an analytical absorption
correction did not yield better results. The space group was
determined from the systematic absences. Equivalent reflec-
tions were merged. Data collection and refinement parameters
are given in Table 4.
Neutral atom scattering factors for non-hydrogen atoms
were taken from Maslen, Fox, and O’Keefe,83 and the scatter-
ing factors for H atoms were taken from Stewart, Davidson,
and Simpson.84 Anomalous dispersion effects were included
in Fc;85 the values for f ′ and f ′′ were those of Creagh and
McAuley.86 All calculations were performed using the TEX-
SAN crystallographic software package.87
Rea ctivity Stu d ies. To a THF solution of 7 × 10-3
M
CpFe(CO)2R typically at least 0.07 M PPh3 was added. This
mixture was heated under reflux, and in periodic time
intervals the infrared spectra of small samples were recorded
on a Perkin-Elmer 298 IR spectrophotometer. Solution cells
The structure was solved by Patterson methods using
SHELXS86,82 which revealed the positions of the I and Fe
atoms. All remaining non-hydrogen atoms were located in a
Fourier expansion of the Patterson solution. The non-
hydrogen atoms were refined anisotropically. All of the H
atoms were fixed in geometrically calculated positions with a
C-H distance of 0.95 Å. Individual isotropic temperature
(83) Maslen, E. N.; Fox, A. G.; Keefe, M. A. In International Tables
for Crystallography; Wilson, A. J . C., Ed.; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1992; Vol. C, Table 6.1.1.1, pp 477-
486.
(84) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J . Chem. Phys.
1965, 42, 3175.
(85) Ibers, J . A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.
(86) Creagh, D. C.; McAuley, W. J . In International Tables for
Crystallography; Wilson, A. J . C., Ed.; Kluwer Academic Publishers:
Dordrecht, The Netherlands, 1992; Vol. C, Table 4.2.6.8, pp 219-222.
(87) TEXSAN. Single Crystal Structure Analysis Software, Version
5.0; Molecular Structure Corp.: The Woodlands, TX 1989.
(81) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
(82) Sheldrick, G. M. SHELXS-86. Acta Crystallogr. 1990, A46, 467.

Ligand Effects in Cyclopentadienyliron Complexes
Organometallics, Vol. 15, No. 10, 1996 2477
of 0.1 mm path length with sodium chloride windows were
used. The progress of the reaction was followed by observing
Su p p or tin g In for m a tion Ava ila ble: Listings of frac-
tional atomic coordinates and equivalent isotropic temperature
factors, interatomic distances, bond angles, anisotropic tem-
perature factors, and hydrogen atom coordinates and U values
(5 pages). This material is contained in many libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, can be ordered from the ACS, and can
be downloaded from the Internet; see any current masthead
page for ordering information and Internet access instructions.
the disappearance of the highest CO band at about 2000 cm-1
.
The recorded spectra were evaluated with pseudo-first-order
kinetics by ploting ln(At - A∞) against t (At, absorption at time
t; A∞, absorption after completion of the reaction).
Ack n ow led gm en t. The authors thank the Swiss
National Science Foundation and the Dr. Helmut Leger-
lotz-Stiftung for financial support.
OM950989B