Synthesis and reactivity of a ferrocene-derived PCP-pincer ligand

Article abstract of DOI:10.1039/b108585a

The 1,3-bis(diphosphinomethyl)ferrocene 3 readily reacts with [(C₂H₄)₂RhCl]₂ to form an equilibrating pair of diastereomers 8a and 8b by C-H insertion into the ferrocene.

Full text of DOI:10.1039/b108585a

Synthesis and reactivity of a ferrocene-derived PCP-pincer ligand
Edward J. Farrington,a Eloisa Martinez Viviente,a B. Scott Williams,b Gerard van Koten*b and John
M. Brown*a
a Dyson Perrins Laboratory, South Parks Road, Oxford, UK OX1 3QY. E-mail: bjm@herald.ox.ac.uk
b Dept. of Metal-Mediated Synthesis, Utrecht University, Padualaan 8 3584 CH. Utrecht, The Netherlands.
E-mail: g.vankoten@chem.uu.nl
Received (in Cambridge, UK) 25th September 2001, Accepted 12th December 2001
First published as an Advance Article on the web 21st January 2002
The 1,3-bis(diphosphinomethyl)ferrocene 3 readily reacts
with [(C2H4)2RhCl]2 to form an equilibrating pair of
diastereomers 8a and 8b by C–H insertion into the ferro-
cene.
complexation, however. Reaction of the ferrocene-derived
ligand 3 with (PPh3)3RuCl2 or (MeCN)2PdCl2 likewise failed to
give characterisable products.
Pincer-type ligands have played a significant role in homoge-
neous catalysis.1 With the recent development of effective
catalysts for dehydrogenation that involve Rh, or more
especially Ir, complexes related to compound 1, interest in their
application has been heightened.2 Among the factors that
contribute to their reactivity in the critical C–H activation step
will be the chelate bite angle and the electronic character of both
the s- and n-bonding entities. Synthesis and complexation
chemistry of a higher homologue 2 based on cycloheptatriene
has already been reported.3 For these reasons we are interested
in the synthesis of the ferrocene analogue 3, and report its
successful completion together with some preliminary reactions
that demonstrate the access to pincer-type ligation.
When ligand 3 was reacted with [(C2H4)2RhCl]2 in CD2Cl2 at
ambient temperature a clean transformation occurred within
minutes, the product being characterised by NMR and MS.†
Initially, two sets of resonances were observed in the 1H NMR
spectrum, in particular two distinct Rh–H signals at ca. 227
ppm. Equilibration occurred over 4 h so that the lower field
signal, initially the minor component, became strongly pre-
dominant. The equilibration could also be followed by 31P
NMR, it being observed that the signal at 87.5 ppm, decreases
monotonically over 3 h as the signal at 93.5 ppm increases. The
structures of several (PCP)RhHCl complexes or Ir analogues
have been determined, and exhibit common geometric fea-
tures.11 The ligand and M–Cl form a distorted square plane with
the M–H in orthogonal relationship (Fig. 1).
This structural arrangement gives rise to a single magnetic
environment for the hydride as defined by the high-field signal
in the 1H NMR spectrum, with one reported exception.§ In the
present case there are two distinct structural isomers possible
for the insertion products 8a and 8b in which the (C5H5)Fe
General methods for the preparation of 1,3-disubstituted
ferrocenes are severely lacking.4 In order to synthesise a
suitable precursor, we prepared compound 4 according to
published procedures, involving the photochemical displace-
ment of p-xylene from CpFe[p-xylene]+BF42 in the presence of
3-methoxycarbonyl-6-dimethylaminofulvene.5,6 This was
cleanly reduced by LiAlH4 in refluxing THF to the primary diol
5a in 89% yield.7 Attempts to form the diacetate 5b by
conventional means (Ac2O, py, DMAP) were unsuccessful, but
it was demonstrated that direct SN1 reaction in AcOH at 80 °C
proceeded cleanly, according to precedent.8 Rather than isolate
the diacetate according to conventional phosphination proce-
dures,9 diol 5a was converted directly into the desired
diphosphine 3 by heating with HPBut2 (2 equiv.) in AcOH (80
°C, 1 h), isolating the product† in 62% yield by precipitation
from MeOH.
moiety is respectively cis- (8a) or trans- (8b) to the hydride. The
hydride resonance of the more stable isomer formed on standing
the solution in CD2Cl2 for several hours shows a lack of NOE
communication with the C5H5 ring, but an intense NOE to one
The new ligand was surveyed for its reactivity towards
simple unsaturated metal complexes under mild conditions. A
model reaction was first established, using ligand 1b. By NMR
(C6D6, 65 °C, 30 min), displacement of the pincer diamine
ligand10 in 6 by the diphosphine was observed. The formation of
complex 7 was inferred,‡ although it was not isolated.
Repeating this reaction with ligand 3 gave no evidence of
Fig. 1 Geometry of MHCl pincer complexes.
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CHEM. COMMUN., 2002, 308–309
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however. Further work has confirmed that the pure HRhCl complex,
unambiguously synthesised, gives only a single high-field resonance dH
227.8 ppm, JRhH 52Hz, 2JPH 12 Hz.
pair of tert-butyl groups. On this basis the assignment of cis- or
trans- to the preferred initial and final products of insertion is as
defined in the spectroscopic data.† The results are consistent
with initial intramolecular C–H insertion from the less hindered
exo-face of the ferrocene to give complex 8a. Equilibration of
complexes 8a and 8b may occur through reversal of that step, or
phosphine dissociation and recombination.
The reactivity of ligand 3 towards C–H insertion in a simple
Rh complex demonstrates that the relevant bis-chelate can be
readily formed, despite the additional strain energy engendered
by the smaller template ring of ferrocene vs. arene. The catalytic
chemistry of 3 and its relatives will be explored in future
work.
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We thank EPSRC and Johnson-Matthey for a CASE
Studentship (to E. J. F.). The Seneca Foundation is acknow-
ledged for support of a visit to Oxford by E. M. V.; B. S. W.
thanks NATO for a Fellowship. Johnson-Matthey contributed a
loan of precious metal salts.
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Notes and references
† For 3: Mp 157–158 °C (decomp.). dH (CDCl3): 4.35 (brs, 1H), 4.15 (d, JHP
1 Hz, 2H), 4.06 (s, 5H), 2.59 (d, JHP 2 Hz, 4H, CH2), 1.11 (2 3 d, JHP 11
Hz, 18H, 18H, diastereotopic But). dC (CDCl3): 87.6 (d, JCP 20 Hz, 2C),
70.3 (s, 1 CH), 70.1 (s, 5 CH), 68.9 (d, JCP 6 Hz, 2 CH), 31.8 (2d, JCP 22 Hz,
JCP 7 Hz, 4 C), 30.1 (2d, JCP 13 Hz, JCP 3 Hz, 12 Me), 22.7 (d, JCP 23 Hz,
2 CH2). dP (CDCl3): 34.1 (s). MS AP+ 502.2601 (Calc. for C28H48P2Fe
2
502.2581. For 8a: dH (CD2Cl2): 227.55 (dt, 1H Rh–H, JRhH 47 Hz, JPH
12.5 Hz), 1.23 (9H, virtual q, J 6.5 Hz), 1.52 (9H, virtual q, J = 6.5 Hz), 2.67
(1H, brq), 3.00 (1H, brq), 3.96 (s, 5H), 4.34 (s, 2H). dP (CD2Cl2): 88.45 (d,
JRhP 114Hz). For 8b: dH (CD2Cl2): 227.45 (dt, 1H Rh–H, JRhH 47 Hz, 2JPH
13 Hz), 1.20 (9H, virtual q, J 6.5 Hz), 1.57 (9H, virtual q, J 6.5 Hz), 2.54
(1H, brq), 2.90 (1H, brq), 3.91 (5H, s), 4.42 (2H, s). dP (CD2Cl2): 93.5 (d,
JRhP 115 Hz). ES MS (8a + 8b): 605.1634 (M+ 2 Cl) (Calc. for
10 P. Dani, T. Karlen, R. A. Gossage, S. Gladiali and G. van Koten, Angew.
Chem., Int. Ed., 2000, 39, 743.
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C
28H48FeP2Rh 605.1636); 604.1575 (M+ 2 HCl) (Calc. for C28H47FeP2Rh
604.1557).
‡ For 7: dP (C6D6): 46, 50 (ABq, J 5 Hz), 61 (s).
§ In ref. 11b the HRhCl pincer complex derived from ligand 1a has been
reported as giving rise to two distinct high-field 1H NMR signals considered
to be conformational isomers. This observation was confirmed by us when
the synthesis was repeated starting with the ligand·(HBr)2 salt. We
suspected that a mixture of HRhCl and HRhBr complexes was responsible,
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