Slow migration of a phosphorus ligand between two metal centers

Article abstract of DOI:10.1002/anie.200702234

(Chemical Equation Presented) Slow dance: Long believed to be impossible, migration of a phosphorus ligand via a bridging intermediate is observed between two metal centers in a diiron complex containing an asymmetric bridging ligand. This process occurs in a similar fashion to the migration of carbonyl ligands but at a rate slow enough to be studied by NMR spectroscopy.

Full text of DOI:10.1002/anie.200702234

Communications
DOI: 10.1002/anie.200702234
Phosphorus Ligand Migration
Slow Migration of a Phosphorus Ligand between Two Metal Centers**
Huailin Sun,* Jianli Gu, Zhensheng Zhang, Hai Lin, Fei Ding, and Qiuchang Wang
Fluxional behavior is an important phenomenon often
observed in many transition-metal complexes by means of
dynamic NMR spectroscopy.[1–4] One of the most frequently
mentioned textbook examples is the fluxional behavior of the
carbonyl ligands in the bis(cyclopentadienyl)diiron complex
[(h5-C5H5)2Fe2(m-CO)2(CO)2] (1), which proceeds via inter-
change of the bridging and terminal COligands according to
the Adams–Cotton mechanism.[2–6] While many ligands, such
as CNR and NO, exhibit similar fluxional behavior, it has long
been asserted that phosphorus ligands PR3 are unable to
participate in such fluxional processes.[5] This belief is most
clearly illustrated by the dynamic NMR-spectroscopic study
of the ring-bridged system [(h5,h5-C5H4Me2CCMe2C5H4)2Fe2-
(m-CO)2(CO)P(OPh)3] (2), in which even the migration of CO
the inability of PR3 to participate in the classical Adams–
Cotton fluxional process is still generally accepted, and thus it
has been difficult to classify PR3 as a normal “fluxional
ligand”. Herein we report the first observation of PR3
participation in the Adams–Cotton fluxional process, which
occurs at such a slow rate that is not detectable by dynamic
NMR spectroscopy.
We detected PR3 migration by the simple strategy of using
an asymmetric bridge connecting two cyclopentadienyl rings.
This approach was found to be an ideal choice because other
methods, such as introducing substituents onto the cyclo-
pentadienyl rings of the symmetrically bridged derivatives
such as 2, could cause steric problems that hinder free
migration of PR3.[12] Although systems with symmetrical
silicon or carbon bridges have been extensively studied in the
past, complexes with asymmetric bridges have been seldom
reported.[13,14] In the course of our studies into silicon–silicon
bond activation by transition-metal complexes, Me2SiSiPh2
and CH2SiMe2 have been used as the bridging groups in the
diiron complex, and the PR3-substituted derivatives [(h5,h5-
C5H4Me2SiSiPh2C5H4)Fe2(m-CO)2(CO)PR3] (R = OMe (3a),
OEt (3b), Me (3c)) and [(h5,h5-C5H4CH2SiMe2C5H4)Fe2(m-
CO)2(CO)PR3] (R = OMe (4a), OEt (4b), OPh (4c)) have
been synthesized. These complexes are mixtures of cis (3a’–c’
and 4a’–c’) and trans (3a’’–c’’ and 4a’’–c’’) isomers (where cis
and trans refer to the relationship between PR3 and bridging
group B, see Equation (1)). Fortunately, the pure cis or trans
isomers could be successfully isolated by fractional crystal-
lization of the mixtures at À308C, and their structures have
been fully characterized by X-ray diffraction (see Supporting
Information).
is blocked by P(OPh)3.[7] This phenomenon has usually been
attributed to PR3 not being able to occupy the bridging
positions between two metal centers and hence only being
permanently bonded to one metal atom as a terminal ligand.
In the last few years however breakthroughs have been
achieved in the isolation of stable complexes with bridging
PR3,[8,9] and more recently PR3 migration between metal
centers via bridging PR3 intermediates or transition states has
also been detected by dynamic NMR-spectroscopic studies of
several systems, including a metal dimer [(h5-C5Me5)2Ru2(m-
H)2PR3] (R = Me, Et, iPr, Cy, Bz, OMe, OPh)[10] and metal
clusters, such as [PtRu5(m6-C)(CO)15(PMe2R)] (R = Ph and
Me),[11a] and [RuOs3(m-H)2(CO)12(m6-C)PPh3].[11b] Even so,
When the pure isomers were redissolved in solvent, it was
surprising to find that they gradually transformed into the
other isomer and ultimately resulted in an equilibrium
mixture of the two isomers in a ratio of about 1:1 [Eq. (1)].
[*] Prof. H. Sun, J. Gu, Z. Zhang, F. Ding, Prof. Q. Wang
Department of Chemistry and the
State Key Laboratory of Elemento-Organic Chemistry
Nankai University
94 Weijin Road, Tianjin 300071 (P. R. China)
Fax: (+86)22-2349-8132
E-mail: sunhl@nankai.edu.cn
H. Lin
Institute of Macromolecular Chemistry
Nankai University
94 Weijin Road, Tianjin 300071 (P. R. China)
Interestingly, this transformation is such a slow process that
may be monitored by means of conventional 1H NMR
spectroscopy (Figure 1). This finding clearly demonstrates
that PR3 can freely migrate between two iron centers, and the
previous inability to detect such PR3 migration in the
[**] We thank NSFC (nos. 20372036 and 20421202), the Natural Science
Foundation of Tianjin (no. 05YFJMJC06800) and the Ministry of
Education of China (no. 03406) for financial support of this work.
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
7498
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Chemie
Scheme 2. Proposed mechanism for the migration of PR3 between two
metal centers via formation of a bridging PR3 intermediate.
migration of PR3. Such a migration between two iron centers
must pass through a bridging PR3 intermediate or transition
state, similar to the formation of bridging CO(Scheme 2).
The existence of stable complexes and unstable intermediates
or transition states having bridging PR3 has already been
established and theoretical calculations have also indicated
that PR3 should be able to form the bridging structures with
pentacoordinate phosphorus.[8b,9,10] In fact, pentacoordination
is not new in phosphorus chemistry.[15] Thus there should be
little doubt about the formation of the bridging PR3 during
the PR3 migration in the present systems, although it could
not be detected by either IR or NMR spectroscopy.
Figure 1. 1H-NMR-spectroscopic monitoringof phosphorus-ligand
migration. A 3a’’; B 3a’. a) P(OMe)3 signal change with time. From
left: 7, 20, 40, 67, 105, and 201 min after re-dissolution of 3a’’ in C6D6.
b) Relative concentrations versus time for both isomers.
symmetrical systems such as 2 arises from the chemical
equivalence of the two iron centers rather than the stability of
the coordination bond of P(OPh)3.
Running the migration in the presence of another free
ligand, for example the reaction of 3a’ or 3b’, that contain
ligands P(OMe)3 and P(OEt)3, in the presence of the free
ligands P(OEt)3 and P(OMe)3, respectively, gave no ligand-
exchange products. The same result was observed for the
complex 3c’ with ligand PMe3 in
To gain further insight into details of the migration
process, the kinetics of the reaction were studied by 1H NMR
spectroscopy. The results showed standard first-order kinetics
apply for both the forward and the reverse processes. The
enthalpy of activation (DH°) for all the compounds studied
were below 22.3 kcalmolÀ1 (Table 1), which is less than the
energy needed for dissociation of CO[16] or PR3,[17] and even
À1 [18]
À
for breaking the weakest Fe Fe bond (ca. 27 kcalmol ).
the presence of the deuterated
Table 1: Activation parameters of phosphorus ligand migration determined by kinetic studies.
ligand [D9]PMe3, and vice versa.
An experiment using complex 4c’
containing the ligand P(OPh)3 in
the presence of P(OMe)3 also gave
no exchange (Scheme 1). These
results unambiguously rule out
the possibility of ligand migration
through dissociative release of PR3
followed by its recoordination to
the metal center.
[a]
[b]
[c]
Entry Reactions Solvent
DH°1/DH°
DS°/DS°
DG°1/DG°
À1
À1
À1
1
2
3
4
5
6
7
3a’Q3a’’
3a’Q3a’’
3b’Q3b’’
3c’Q3c’’
4a’Q4a’’
4b’Q4b’’
4c’Q4c’’
[D6]benzene
21.0Æ0.7/21.1Æ0.6
À3.6Æ2.2/À3.4Æ2.0 22.0/22.1
À8.1Æ2.1/À8.3Æ2.1 22.5/22.5
À5.6Æ2.0/À6.2Æ1.8 22.3/22.4
[D]chloroform 20.1Æ0.6/20.0Æ0.6
[D6]benzene
[D6]benzene
[D6]benzene
[D6]benzene
20.7Æ0.6/20.6Æ0.6
18.4Æ0.5/18.9Æ0.5 À10.4Æ1.7/À9.0Æ1.7 21.5/21.6
21.8Æ0.3/21.8Æ0.3
22.2Æ0.4/22.3Æ0.4
[d]
À2.8Æ1.0/À2.8Æ0.9 22.7/22.6
À2.1Æ1.3/À1.8Æ1.2 22.8/22.8
[d]
[d]
[a] Units: kcalmolÀ1. [b] Units: calKÀ1 molÀ1. [c] at 298.2 K; units: kcalmolÀ1. [d] Not determined as the
reaction was too rapid to monitor by 1H NMR spectroscopy.
A nondissociative mechanism
is therefore responsible for the
This rules out the possibility of the involvement of such steps
in the PR3 migration process and therefore confirms the
above mechanism that involves only concurrent migration of
CO, which has a rather low energy barrier of approximately
11 kcalmolÀ1; this is only half the height of the energy barrier
for the present PR3 migration.[6] Interestingly, the DH° values
are not very sensitive to the electronic effects of the alkyl and
alkoxyl substituents of the small phosphorus ligands. This
result is consistent with there being no obvious difference in
related bond lengths and angles in the molecular structures of
Scheme 1. PR3 migration in the presence of another phosphorus ligand
(shown in the square brackets), demonstratingthat the latter did not
exchange with the PR3 ligand in the reaction substrates.
these complexes as determined by X-ray diffraction (Table 2).
The only exception was that the larger ligand P(OPh)3 in 4c
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Lengths/
Angles[b]
À
Fe Fe
2.5220(7)
2.1287(10)
2.3969(12)
2.5381(14)
2.1409(18)
2.362(2)
2.5205(14)
2.189(2)
2.460(3)
2.5290(7)
2.1420(9)
2.5311(8)
2.1465(12)
2.516(2)
2.113(3)
98, 5817 – 5826.
À
Fe P
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À
Si Si
[d]
[c]
À
C Si
1.870(3)
105.76(3)
1.863(5)
104.42(4)
–
À
À
P Fe Fe
106.47(3)
103.85(5)
107.88(7)
104.50(8)
[a] For details of the X-ray diffraction studies, see SupportingInformation. [b] Bond lentghs [ꢀ],
angles [8]. [c] Not accurately determined owingto substantial thermal motion of the carbon atom. [d] on
the bridge.
Use of a polar solvent did not significantly change the
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the nonpolar intermediates/transition states. Of particular
note is the small negative entropies of activation (DS°) that
have been observed, which is in accord with the nondissocia-
tive mechanism (in which the formation of bridging PR3
À
restricts rotation of the M P bonds, leading to an increase
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[12] Introduction of different substituents on the two cyclopenta-
dienyl rings of unbridged complexes 1 is not suitable for the
present study because intermolecular Fe-Fe bond metathesis
extensively occur for such complexes, especially at elevated
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The results obtained show that PR3 migration in the diiron
complexes can in fact take place in a similar fashion to CO
migration, except that a higher energy is required and as a
result a slow migration rate is exhibited. This assertion is
especially reasonable considering the relatively low—albeit
well-established—ability of PR3 to form bridging structures.
The observation of slow migration of PR3 indicates that such
fluxional behavior in transition-metal complexes must occur
much more extensively than has been detected using dynamic
NMR-spectroscopic methods alone. The influence of this
feature of PR3 on the chemistry and properties of related
systems, ranging from simple dimers through polynuclear
clusters to nanocrystals of transition metals, will have to be
considered in the future.[19] It must be noted that slow
migration of chain-linked diphosphorus ligands, such as
Ph2P(CH2)nPPh2, via bridging intermediates/transition states
in metal clusters was reported very recently. Unfortunately,
such migration only occurred for the chain length n = 2 and
not for n = 1, 3, and 4. Such a strong dependence on the chain
length indicates that the migration cannot be regarded as free
migration of simple monophosphorus ligands PR3.[20]
[17] G. Distefano, G. Innorta, J. Organomet. Chem. 1968, 14, 465 –
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[18] A. R. Culter, M. Rosenblum, J. Organomet. Chem. 1976, 120,
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[20] S. Kandala, M. G. Richmond, Inorg. Chem. 2006, 45, 5976 – 5979,
and references therein.
Received: May 21, 2007
Published online: August 14, 2007
Keywords: fluxionality · kinetics · ligand migration ·
.
NMR spectroscopy · phosphane ligands
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