New unsymmetrical thioether- and thiolate-substituted ferrocene ligands and an unusual bridged-Pd dimer complex

Article abstract of DOI:10.1039/b007511f

A series of new unsymmetrical 1, 1'-disubstituted ferrocenediyl ligands featuring thioether or thiolate substituents have been conveniently synthesised and, as an example of their coordinating ability, a bridged palladium dimer has been formed and structurally characterised.

Full text of DOI:10.1039/b007511f

New unsymmetrical thioether- and thiolate-substituted ferrocene ligands and
an unusual bridged-Pd dimer complex
Vernon C. Gibson, Nicholas J. Long,* Andrew J. P. White, Charlotte K. Williams and David J. Williams
Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, UK
SW7 2AY. E-mail: n.long@ic.ac.uk
Received (in Cambridge, UK) 18th September 2000, Accepted 18th October 2000
First published as an Advance Article on the web
A series of new unsymmetrical 1, 1’-disubstituted ferrocene-
diyl ligands featuring thioether or thiolate substituents have
been conveniently synthesised and, as an example of their
coordinating ability, a bridged palladium dimer has been
formed and structurally characterised.
redox properties of the ligands. Finally, a solution of 1 (1 equiv.)
in THF was treated with lithium triethylborohydride (2.2 equiv.)
and stirred for 1.5 h. On evaporation to dryness, a dark red oily
product 4 was formed in quantitative yield but was not isolated
due to its unstable nature. As confirmation of its formation and
as a preliminary investigation into its coordination chemistry, 4
was treated with trans-[Pd(PhCN)2Cl2] (2 equiv.) in toluene and
heated to 60 °C for 16 h. Following work-up and crystallisation
in CH2Cl2–hexane (1+1), the purple–black product 5 was
isolated in 81% yield as a crystalline solid. The X-ray analysis
of 5‡ shows that the desired bidentate coordination of the
ferrocenediyl species has been achieved. Surprisingly, however,
a bridged dimer has been formed where, for each ligand, one of
the sulfur atoms is binucleating and carries a formal negative
charge whereas the other links to a single palladium ion and is
formally neutral (Fig. 1). The complex has non-crystallographic
C2 symmetry about an axis passing through the centre of, and
normal to the plane of, the Pd2S2 ring, i.e. both ferrocenediyl
units lie on the same side of the two linked coordination planes.
Each ferrocenediyl unit adopts a staggered conformation, their
S–C5H4 bonds ‘subtending’ angles of 45 and 46° for Fe(1) and
Fe(2), respectively. The two palladium coordination planes are
folded by ca. 30° out of plane with respect to each other about
the S(3)…S(4) vector. The transannular Pd…Pd and S…S
distances in the central four-membered ring are 3.394 and 2.998
Å, respectively. The geometry at each palladium centre is
distorted square planar with cis angles in the ranges
81.1(2)–101.9(2) and 81.0(2)–102.5(2)° at Pd(1) and Pd(2),
respectively. The Pd–S distances fall into two distinct groups
with those to the negatively charged bridging sulfurs sig-
Although ferrocene was discovered nearly fifty years ago, the
search for new ferrocene derivatives and chemistry continues
apace. In particular, versatile, stable and synthetically un-
demanding ferrocenyl and ferrocenediyl ligands are sought after
due to their extensive coordination chemistry and applications
within catalysis.1,2 Ferrocene species with donor heteroatoms
(e.g. P, N, S, O) substituted onto the cyclopentadienyl rings are
well known, especially those featuring homo-substitution (i.e.
the same donor atom along with the same alkyl or aryl
substituents). Much less well-known are unsymmetrical 1,1A-
disubstituted ferrocenes featuring hetero-combinations of N, P
or S atoms, formed via bromo,3, 4 lithio5, 6 or stannyl7,8
intermediates. These ligands are neutral in character and their
preparations are often non-trivial. So far as we are aware,
examples of unsymmetrical purely S-substituted neutral and
neutral–anionic ferrocenediyl ligands (i.e. analogous to the
hemilabile P/S2,9 P/O2,10,11 and N/O212 (‘SHOP’) based-
catalysts) are hitherto unknown. Here, we report the new and
convenient syntheses of the first examples of these types of
ligands and a preliminary study of their coordination chemistry
which has led to the isolation and structural characterisation of
a novel palladium dimer complex.
The new ligands 1–4 can be synthesised from the well known
1, 2, 3-trithia[3]ferrocenophane13 (Scheme 1). By utilising an
elegant method first described by Herberhold et al,14 the
trisulfur bridge can be cleaved by organolithium reagents to
form (after air oxidation) bis(1A-organylthiolatoferrocenyl)di-
sulfanes. Herberhold et al. have prepared the species RS–Fc–
SS–Fc–SR (R = n-butyl, phenyl; Fc = 1,1A-ferrocenediyl), but
during our research into the formation of redox-active and
sterically hindered ligands for homogeneous catalysis, we
reacted the ferrocenophane (1 equiv.) with mesityllithium (2
equiv.) and formed RS–Fc–SS–Fc–SR 1 (where R = mesityl).
This species may be an interesting, sterically hindered ligand in
its own right, but at present, we have concentrated on the
formation of the monoferrocenediyl species 2–4.† On reaction
of 1 (1 equiv.) with lithium triethylborohydride (2.2 equiv.) in
THF and after acid/base work-up, the air-sensitive orange solid
2 was isolated in 88% yield. 1H NMR spectroscopy showed the
unsymmetrical substitution of the cyclopentadienyl rings with
the presence of signals due to S-mesityl, S–H and four pseudo
triplets for the C5H4 ring protons.
A solution of 1 was also treated with methyllithium (2.2
equiv.) and the reaction mixture stirred for 16 h. Cleavage of the
disulfide linkage was effected and the crude orange mixture
obtained was washed, extracted and purified via column
chromatography [neutral grade II alumina, CH2Cl2–hexane
(1+4)] and 3 was isolated as an air- and moisture-stable orange
oil (50%) (N.B. 25% of 1 was also isolated). The use at this stage
of various organolithium reagents can of course, lead to
derivatisation and a possible ‘fine-tuning’ of the steric and
Scheme 1 Reagents and conditions: (i) 2 MesLi, THF, 20 h; (ii) 2.2
LiEt3BH, THF, 1.5 h, H+; (iii) 2.2 MeLi, THF, 16 h; (iv) 2.2 LiEt3BH, THF,
1.5 h; (v) trans-[Pd(PhCN)2Cl2], toluene, 60 °C, 16 h.
DOI: 10.1039/b007511f
Chem. Commun., 2000, 2359–2360
This journal is © The Royal Society of Chemistry 2000
2359

(1% KOH) (20 cm3), followed by dropwise addition of conc. HCl. The
layers were separated and the aqueous layer extracted with diethyl ether (3
3 20 cm3). The combined organic layers were dried (MgSO4) and
evaporated to dryness to leave an air-sensitive orange solid 2 (0.42 g, 88%);
dH(CDCl3) 2.21 (3H, s, CH3), 2.47 (6H, s, CH3), 2.98 (1H, s, SH), 4.10 (2H,
t, C5H4), 4.21 (4H, t, C5H4), 4.33 (2H, t, C5H4), 6.86 (2H, s, C6H2); m/z 367
(M 2 H+), 334 (M 2 SH+).
3: a solution of 1 (0.58 g, 0.79 mmol) in THF (40 cm3) was treated with
methyllithium (1.6 M solution in diethyl ether, 1.1 cm3, 1.74 mmol) and the
reaction mixture stirred for 16 h. The solvent was evaporated in vacuo, the
residue was dissolved in diethyl ether (50 cm3) and water (10 cm3) was
added. The layers were separated and the aqueous layer extracted with
diethyl ether (3 3 20 cm3). The combined organic layers were dried
(MgSO4), then evaporated to dryness. The product was purified by column
chromatography [neutral grade II alumina, CH2Cl2–hexane (20+80)] and
after removal of the solvent, isolated as an orange oil (0.15 g, 50%);
dH(CDCl3) 2.24 (3H, s, CH3), 2.34 (3H, s, SCH3), 2.51 (6H, s, CH3), 4.13
(2H, t, C5H4), 4.26 (4H, m, C5H4), 4.34 (2H, t, C5H4), 6.88 (2H, s, C6H2);
m/z 382 (M+), 367 (M 2 CH3+), 271 (M 2 C5H4SCH3+).
4 and 5: a solution of 1 (0.10 g, 0.135 mmol) in THF (30 cm3) was treated
with lithium triethylborohydride (1 M solution in THF, 0.3 cm3, 0.30 mmol)
and the reaction stirred for 1.5 h. The air-sensitive dark red solution of 4 was
formed in quantitative yield and due to its instability was reacted in situ
without work-up. Thus, a toluene (30 cm3) solution of 4 was treated with
bis(benzonitrile)palladium(II) dichloride (0.103 g, 0.27 mmol) also in
toluene (30 cm3). There was an immediate darkening of the solution to deep
purple and stirring at 60 °C was continued for 16 h. A black ppt. was filtered
off and the filtrate washed with water (2 3 10 cm3), dried (MgSO4) and
evaporated to dryness. The crude residue was washed with hot hexane (2 3
40 cm3) and recrystallised from CH2Cl2–hexane (1+1) to leave a purple
crystalline powder 5 (0.11 g, 81%); dH(CDCl3) 2.16 (6H, s, CH3), 2.41 (6H,
s, CH3), 3.29 (6H, s, CH3), 4.07 (2H, m, C5H4), 4.28 (2H, m, C5H4), 4.44
(2H, m, C5H4), 4.57 (2H, m, C5H4), 4.60 (2H, m, C5H4), 4.66 (2H, m,
C5H4), 5.07 (2H, m, C5H4), 6.80 (2H, m, C6H2), 6.93 (2H, s, C6H2); m/z 982
(M 2 Cl+), 897 (M 2 mes+).
Fig. 1 The molecular structure of 5. Selected bond lengths (Å) and angles
(°); Pd(1)–Cl(1) 2.332(5), Pd(1)–S(1) 2.365(4), Pd(1)–S(3) 2.305(4),
Pd(1)–S(4) 2.303(5), Pd(2)–Cl(2) 2.334(5), Pd(2)–S(2) 2.354(4), Pd(2)–
S(3) 2.305(5), Pd(2)–S(4) 2.309(4); S(4)–Pd(1)–S(3) 81.1(2), S(4)–Pd(1)–
S(1) 101.9(2), S(3)–Pd(1)–S(1) 176.1(2), S(3)–Pd(2)–S(4) 81.0(2), S(3)–
Pd(2)–S(2) 102.5(2), S(4)–Pd(2)–S(2) 176.2(2), Pd(2)–S(3)–Pd(1) 94.8(2),
Pd(1)–S(4)–Pd(2) 94.7(2).
nificantly shorter than those to the others. Surprisingly, the Pd–
S bridge distances are symmetric, i.e. there is no trans influence
of the chloride ligands. The complexes are linked by a
combination of C–H…p and p–p interactions to form sheets
that extend in the ab plane (Fig. 2). The space between adjacent
parallel sheets is occupied by disordered dichloromethane
molecules. This type of bridged system featuring a thioether/
thiolate ligand appears to be unique, the only other similar
species being a palladium dimer featuring bridging dithiolate
ligands.15
‡
Crystal data for 5: C38H38S4Cl2Fe2Pd2·3.5CH2Cl2, M
= 1315.6,
monoclinic, space group P21/c (no. 14), a = 16.411(4), b = 14.897(3), c =
22.093(4) Å, b = 95.06(1)°, V = 5380(2) Å3, Z = 4, Dc = 1.624 g cm–3
,
m(Mo-Ka) = 18.2 cm21, F(000) = 2620, T = 293 K; deep red plates, 6985
independent reflections, F2 refinement to give R1 = 0.078, wR2 = 0.161 for
3473 independent observed absorption corrected reflections [|Fo|
>
4s(|Fo|), 2q @ 45°], 560 parameters (the high R1 is a consequence of crystal
decomposition and disorder in the CH2Cl2 groups). The platinum analogue
is isomorphous [a = 16.383(3), b = 14.875(2), c = 22.167(3) Å, b =
94.95(1)°, V = 5382(2) Å3].16
CCDC 182/1826. See http://www.rsc.org/suppdata/cc/b0/b007511f/ for
crystallographic files in .cif format.
1 For a detailed literature review, see: Ferrocenes: Homogeneous
Catalysis–Organic Synthesis–Materials Science, ed. A. Togni and T.
Hayashi, VCH, Weinheim, Germany, 1995; Metallocenes, ed. A. Togni
and R. L. Haltermann, Wiley-VCH, Weinheim, Germany, 1998.
2 For a comprehensive overview of ferrocene and other metallocene
chemistry, see: N. J. Long, in Metallocenes: An Introduction to
Sandwich Complexes, Blackwell Science, Oxford, 1998.
3 T. Y. Dong and C.-K. Chang, J. Chin. Chem. Soc., 1998, 45, 577.
4 I. R. Butler, U. Griesbach, P. Zanello, M. Fontani, D. Hibbs, M. B.
Hursthouse and K. L. M. A. Malik, J. Organomet. Chem., 1998, 565,
273.
5 I. R. Butler, J. Organomet. Chem., 1997, 552, 63.
6 N. J. Long, J. Martin, G. Opromolla, A. J. P. White, D. J. Williams and
P. Zanello, J. Chem. Soc., Dalton Trans., 1999, 1981.
7 G. A. Balavoine, J.-C. Daran, G. Iftiem, E. Manoury and C. Moveau-
Bossuet, J. Organomet. Chem., 1998, 567, 191.
8 J. A. Adeleke, Y.-W. Chen and L.-K. Liu, Organometallics, 1992, 11,
2443.
Fig. 2 Part of one of the C–H···p and p–p linked sheets of molecules present
in the structure of 5. The H···p distances (Å) and C–H···p angles (°) are (a)
2.76, 136; (b) 2.72, 141; (c) 2.79, 161; (d) 2.87, 159. The centroid···centroid
and mean interplanar separations between rings A and B are (e) 3.98, 3.73
Å.
9 J. R. Dilworth, J. R. Miller, N. Wheatley, M. J. Baker and J. G. Sunley,
J. Chem. Soc., Chem. Commun., 1995, 1579.
10 U. Klabunde and S. D. Ittel, J. Mol. Catal., 1987, 41, 123.
11 W. Keim, R. Appel, S. Gruppe and F. Knoch, Angew. Chem., Int. Ed.
Engl., 1987, 26, 1012.
12 S. Y. Desjardins, K. J. Cavell, J. L. Hoare, B. W. Skelton, A. N. Sobolev,
A. H. White and W. Keim, J. Organomet. Chem., 1997, 544, 163.
13 J. J. Bishop, A. Davison, M. L. Katcher, D. W. Lichtenberg, R. E.
Merrill and J. C. Smart, J. Organomet. Chem., 1971, 27, 241.
14 M. Herberhold, O. Nuyken and T. Pohlmann, J. Organomet. Chem.,
1991, 405, 217.
Work is in progress to explore the diverse coordination
chemistry expected for these ligands and their role in homoge-
neous catalysis.
We acknowledge financial support from the EPSRC and
Johnson Matthey plc are thanked for the loan of platinum
salts.
Notes and references
† Syntheses: 2: compound 1 (0.50 g, 0.65 mmol) was dissolved in dry,
deoxygenated THF (30 cm3) and to this lithium triethylborohydride (1 M
solution in THF, 1.40 cm3, 1.40 mmol) was added and the reaction mixture
stirred for 1.5 h. The solvent was removed in vacuo and the residue then re-
dissolved in diethyl ether. The dark red solution was poured onto dilute base
15 R. Cao, M. Hong, F. Jiang and H. Liu, Acta Crystallogr., Sect. C, 1995,
51, 1280.
16 V. C. Gibson, N. J. Long, A. J. P. White, C. K. Williams and D. J.
Williams, unpublished results.
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Chem. Commun., 2000, 2359–2360