2-Pyridylmetallocenes: Part 3 [1]. Cyclomercuration of 2-pyridyl-ferrocene and 1-(2-pyridyl)-2-methyl-ferrocene.

Article abstract of DOI:10.1016/j.poly.2012.06.047

Mercuration of 2-pyridylferrocene [(2-C₅H₄N)C ₅H₄]Fe(C₅H₅) (1a) and its 2-methylated derivative 1b with Hg(OAc)₂ followed by treatment with LiCl gave after chromatographic workup

Full text of DOI:10.1016/j.poly.2012.06.047

Polyhedron 44 (2012) 133–137
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Polyhedron
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2-Pyridylmetallocenes: Part 3 [1]. Cyclomercuration of 2-pyridyl-ferrocene and
1-(2-pyridyl)-2-methyl-ferrocene. Molecular structures of
[1-(2-C5H4N)-2-(CH3)C5H3]Fe(C5H5), [1-(ClHg)-2-(2-C5H4N)C5H3]Fe(C5H5) and
[1-(ClHg)-2-(2-C5H4N)-3-(CH3)C5H2]Fe(C5H5)
⇑
Karlheinz Sünkel , Stefan Weigand
Ludwig-Maximilians-Universität Munich, Dept. Chemistry, Butenandtstr. 9, 81377 Munich, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Mercuration of 2-pyridylferrocene [(2-C5H4N)C5H4]Fe(C5H5) (1a) and its 2-methylated derivative 1b with
Hg(OAc)2 followed by treatment with LiCl gave after chromatographic workup the cyclomercurated and
[1-(ClHg)-2-(2-C5H4N)-3-(R)C5H2]Fe(C5H5) (R = H: 2a; Me: 2b)) in 43% and 21% yield. The molecular
structures of 1b, 2a and 2b have been determined by X-ray crystallography.
Received 25 May 2012
Accepted 19 June 2012
Available online 28 June 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Pyridylmetallocenes
Cyclomercuration
Crystal structure determination
1. Introduction
2. Experimental
The mercuration of ferrocene to give chloromercuriferrocene
was reported by Nesmeyanov et al. already in 1954 [2]. Later on
several substituted ferrocenes were also successfully mercurated.
The so-called ‘‘cyclomercuration’’ or ‘‘ortho-mercuration’’ of cyclo-
pentadienyl complexes bearing nitrogen containing substituents
was studied intensively by the group of Y.J. Wu and recently by At-
tar et al. [3]. The mechanism of the mercuration of ferrocene was
studied by A.F. Cunningham [4]. He could show that primary attack
of mercury occurs at the iron atom, followed by an endo attack on
the coordinated cyclopentadienyl ring. Eventually, oxidation of
iron to give a ferricenium species might happen as well, as we
could show earlier for the mercuration with HgCl2 [5]. With a
nitrogen containing substituent on the cyclopentadienyl ring,
alternatively coordination of the nitrogen lone pair might happen
first [6].
All reactions were performed under an atmosphere of argon
using standard Schlenk-techniques. The solvents used for the lith-
iation reactions (THF and Et2O) were obtained from Aldrich in the
highest available quality and used without further purification. The
other solvents were of standard quality. The reagents n-BuLi (2.5 m
in hexane); 2-bromopyridine; Hg(OAc)2 and MeI were obtained
from Aldrich and used without further purification. 2-Pyridylferro-
cene (1a) was prepared as recently reported by us [7].
NMR: JEOL ECP-270 and ECX-400, Ref. CHCl3 d(1H) = 7.240 ppm,
d(13C) = 77.0 ppm;
Ref.
CHDCl2
d(1H) = 5.300 ppm,
d(13C) = 53.8 ppm. The numbering scheme for the assignment of
NMR spectra is shown in Scheme 1.
MS: Finnigan MAT 90 and JEOL Mstation 700.
The elemental analysis of 1b was performed at the microanalyt-
ical laboratory at the chemistry department of Ludwig-Maximil-
ians University, Munich. Due to local restrictions, combustion
analyses of mercury containing compounds could not be
performed.
We recently reported the cyclomercuration of tricarbonylcyclo-
pentadienylrhenium [1] and observed a rather short Hg–N interac-
tion, while the C–Hg–Cl bond still remained linear. Here we
present our results with the mercuration of 2-pyridyl-ferrocene
and its 2-methylated derivative.
2.1. Synthesis of 2-methyl-1-(2-pyridyl)-ferrocene (1b)
A solution of 1a (0.24 g, 0.91 mmol) in Et2O (10 mL) was treated
with n-BuLi solution (1.0 mL, 2.5 mmol) with stirring for 45 min.
Then the mixture was cooled to ꢀ78 °C and MeI (0.14 mL,
2.2 mmol) was added. Stirring was continued for 30 min at this
temperature and then for 2 h at room temperature. Evaporation
⇑
Corresponding author. Tel.: +49 89 2180 77773.
E-mail address: suenk@cup.uni-muenchen.de (K. Sünkel).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.06.047

134
K. Sünkel, S. Weigand / Polyhedron 44 (2012) 133–137
4
was added and stirring was continued for 16 h/3 days. Then a mix-
3
5
ture of CH2Cl2 (40 mL/15 mL) and water (60 mL/20 mL) was added
with vigorous stirring. The aqueous phase was separated and ex-
tracted with CH2Cl2 (80 mL/40 mL). The combined organic phases
were stirred with MgSO4, filtered and evaporated to dryness. The
remaining orange residue was chromatographed on alumina. Elu-
tion with hexane/CH2Cl2 (2:1/1:2) yielded the desired products
after evaporation to dryness as orange solids. Yields: 0.21 g (43%)
2a, 0.050 g (21%) 2b. Recrystallization from CH2Cl2 by slow evapo-
ration at room temperature gave crystals suitable for X-ray struc-
ture analysis.
2
7
8
R
6
N
N
R
11
1. Hg(OAc)2
2. LiCl
11
10
Hg Cl
9
Fe
Fe
R = H
1a
1b
2a
2b
R = CH3
Scheme 1. Synthesis of 2a and 2b and numbering scheme for NMR spectra.
2a: 1H NMR (270 MHz, CD2Cl2): d = 8.40 (‘‘dd’’, 1H, H6), 7.62
(‘‘dt’’, 1H, H4), 7.40 (‘‘d’’ br, 1H, H3), 7.15 (‘‘ddd’’, 1H, H5), 5.00
(br s, 1H, H11), 4.61 (br, s, JH–Hg = 36 Hz, 1H, H10), 4.39 (br s, JH–
Hg = 54 Hz, 1H, H9), 4.06 (s, 5H, Cp). 13C NMR (68 MHz, CD2Cl2):
d = 158.4 (C2), 148.6 (C6), 137.3 (C4), 121.6 (C5), 120.1 (C3), 86.8
(JC–Hg = 403 Hz, C7), 83.6 (C8), 75.8 (JC–Hg = 261 Hz, C10), 73.4 (JC–
Hg = 228 Hz, C9), 70.4 (Cp), 67.9 (C11). MS (DEI+): m/z = 726.2
(L2Hg+), (L ꢁ Fe(Cp)(C5H3C5H4N)), 499.1 (M+), 261.1 (M+ꢀHgCl).
2b: 1H NMR (270 MHz, CD2Cl2): d = 8.47 (‘‘td’’, 1H, H6), 7.63 (m,
2H, H3 + 4), 7.18 (‘‘ddd’’, 1H, H5), 4.47 (m, JH–Hg = 42 Hz, 1H, H10),
4.20 (m, JH–Hg = 31 Hz, 1H, H9), 3.99 (s, 5H, Cp), 2.39 (s, 3H, CH3).
13C NMR (68 MHz, CD2Cl2): d = 159.4 (C2), 148.80 (C6), 137.2
(C4), 121.4 (C5), 121.2 (C3), 85.7 (C7), 84.0 (C8 + C11), 76.9 (C10),
73.8 (C9), 71.0 (Cp), 17.2 (CH3). MS (DEI+): m/z = 513.3 (M+),
276.2 (M+–HgCl, 100%).
of the solvent in vacuo left a red oily residue, which was chromato-
graphed on alumina using CH2Cl2 as eluent. After removal of the
solvent a red sticky oil remained. Yield: 0.20 g (79%). EA (calcd/
found): C: 69.34/68.26; H: 5.46/5.82, N: 5.05/4.65%. 1H NMR
(270 MHz, CDCl3): d = 8.52 (‘‘ddd’’, 1H, H6), 7.58 (‘‘dt’’, 1H, H4)
7.53 (‘‘td’’, 1H, H3), 7.07 (‘‘ddd’’, 1H, H5), 4.67 (‘‘dd’’, 1H, H8),
4.25 (‘‘dd’’, 1H, H10), 4.19 (‘‘t’’, 1H, H9), 4.10 (s, 5H, C5H5), 2.35
(s, 3H, CH3). 13C NMR (67.9 MHz, CDCl3): d = 160.0 (C2), 148.9
(C6), 135.5 (C4), 122.2 (C5), 120.3 (C3), 83.6 (C7), 83.3 (C11),
72.0 (C10), 68.9 (C9), 67.0 (C8), 70.2 (C5H5), 15.4 (CH3). MS
(DEI+): m/z = 277.1 (M+, 100%), 211.1 (M+ꢀC5H6, 31%), 155.1
(M+ꢀC5H6–Fe, 9%).
2.2. Synthesis of chlorido-[2-(2-pyridyl)-ferrocenyl-jC]-mercury (2a)
and chlorido-[3-methyl-2-(2-pyridyl)-ferrocenyl- C]-mercury (2b)
j
2.3. Crystal structure determinations
A solution of the ferrocenylpyridine (1a: 0.26 g, 0.99 mmol/1b:
0.13 g, 0.47 mmol) in CH2Cl2 (20 mL) was treated with a solution of
Hg(OAc)2 (0.32 g/0.16 g, 1.00 mmol/0.50 mmol, respectively) in
MeOH (40 mL/19 mL). After stirring for 90 min a solution of LiCl
(0.070 g, 1.7 mmol/0.040 g, 0.94 mmol) in MeOH (30 mL/14 mL)
Suitable single crystals of 1b and 2a were mounted on the tip of
a glass fiber, and investigated on a Kappa CCD diffractometer, using
MoKa radiation (k = 0.71073 Å). Crystals of 2b were measured on
an Oxford XCalibur diffractometer. Details of data collections and
Table 1
Experimental details of the crystal structure determinations.
Compound
1b
2a
2b
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C
16H15FeN
C15H12ClFeHgN
498.15
C16H14ClFeHg N
512.17
277.14
monoclinic
P21/c
16.3585(10)
8.7964(5)
17.6471(11)
103.170(6)
11.3288(2)
10.3721(3)
12.8948(3)
112.4589(15)
1400.26(6)
4
2.363
12.160
928
0.151 ꢂ 0.087 ꢂ 0.068
3.22° to 27.47°
ꢀ14 6 h 6 14, ꢀ13 6 k 6 13,
ꢀ16 6 l 6 16
22472
8.9743(9)
17.4340(17)
9.9739(8)
107.898(9)
1485.0(2)
4
2.291
11.470
960
0.16 ꢂ 0.14 ꢂ 0.04
4.22° to 26.35°
ꢀ11 6 h 6 10, ꢀ19 6 k 6 21,
ꢀ9 6 l 6 12
5997
b (Å)
c (Å)
ß (°)
Volume (Å3)
Z
2472.6(3)
4 ꢂ 2
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
1.489
)
1.198
1152
Crystal size (mm3)
0.24 ꢂ 0.17 ꢂ 0.1
4.19° to 26.36°
ꢀ20 6 h 6 15, ꢀ10 6 k 6 10,
ꢀ22 6 l 6 19
10042
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta (max)°
Absorption correction
4993 [Rint = 0.0501]
99.0%
Semi-empirical from equivalents
3207 [Rint = 0.0516
99.8%
3012 Rint = 0.0628
99.6%
Maximum and minimum transmission 0.8871 and 0.8652
0.3595 and 0.2355
3207/0/220
1.039
0.6320 and 0.1357
3012/0/183
0.999
Data/restraints/parameters
4993/0/327
Goodness-of-fit on F2
0.820
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0420, wR2 = 0.0781
R1 = 0.0814, wR2 = 0.0839
R1 = 0.0207, wR2 = 0.0432
R1 = 0.0283, wR2 = 0.0451
R1 = 0.0378, wR2 = 0.0789
R1 = 0.0526, wR2 = 0.0883
0.00100(18)
1.627 and ꢀ1.723
Extinction coefficient
Largest difference peak and hole
(e Åꢀ3
CCDC number
0.626 and ꢀ0.392
0.591 and ꢀ1.003
)
883793
883794
883795

K. Sünkel, S. Weigand / Polyhedron 44 (2012) 133–137
135
atives we could show that
a-halogenation was possible using a
reaction sequence of lithiation and electrophilic halogenation. Sim-
ilarly, lithiation of pyridylferrocene 1a followed by methylation
with methyl iodide yielded 1-methyl-2-(2-pyridyl)-ferrocene (1b)
in an isolated yield of 79%. Crystals of this compound were isolated
and examined by X-ray diffraction. The molecular structure is
shown in Fig. 1. Important bond parameters are collected in
Table 2.
1b crystallizes in the monoclinic space group P21/c with two
independent molecules in the unit cell. The distance Ct(1)–Fe
(the abbreviation ‘‘Ct’’ stands for ‘‘centroid’’) of the substituted
cyclopentadienyl ring from the iron atom is slightly shorter than
the distance Ct(2)–Fe of the unsubstituted ring, with both rings
being nearly ideally eclipsed (2.9° and 3.7° in both molecules).
The main difference between the two independent molecules is
the degree of twisting of the cyclopentadienyl and the pyridine
ring as indicated by the torsion angles N–C2–C7–C11. Quite inter-
esting is the fact that contrary to the halogenated pyridylferroc-
enes the methyl substituent is situated next to the pyridine
nitrogen atom. One might speculate on a weak hydrogen bridge
CHꢃ ꢃ ꢃN between the methyl carbon and the pyridine nitrogen as
a reason for this observation.
C111
C112
C110
C109
N100
C106
C104
C107
C108
Fe1
C102
C103
C105
C114
C113
C115
C116
C117
Fig. 1. Molecular structure of 1b.
Table 2
Comparison of important bond parameters in 1b, 2a, and 2b.
1
1b
2a
2b
C2–N
C2–C7
N–Hg
1.340(4)/1.342(4)
1.473(4)/1.481(4)
NA
1.346(5)
1.485(5)
2.713(3)
2.033(4)
2.313(2)
177.0(1)
172.6(2)
NA
ꢀ5.80(5)
3.55(5)
1.654(2)
1.656(2)
178.9(1)
ꢀ18.9
1.346(9)
1.487(10)
2.748(6)
2.036(8)
2.313(3)
177.8(2)
165.1(7)
ꢀ1.6(13)
ꢀ15.7(10)
8.59(9)
1.648(3)
1.656(3)
177.91(17)
ꢀ2.71
C8–Hg
NA
NA
NA
When solutions of 1a,b in CH2Cl2 were treated at room temper-
ature with a slight excess of a MeOH solution of mercuric acetate,
followed by a MeOH solution of lithium chloride, the mercurated
complexes 2 and 3 could be isolated in 43% and 21% yield, respec-
tively, after chromatographic work-up (Scheme 1).
Both compounds were obtained as orange microcrystalline sol-
ids. They were characterized by 1H, 13C NMR and mass spectra, and,
after recrystallization, by X-ray crystallography. The 1H NMR spec-
Hg–Cl
C8–Hg–Cl
N–C2–C7–C11
C2–C7–C11–CR
N–C2–C7–C8
C2–C7–C8–Hg
Ct(1)–Fe
13.4(5)/ꢀ5.4(4)
0.2(6)/2.3(5)
ꢀ166.5(3)/174.5(3)
NA
1.6436(5)/1.6453(4)
1.6518(5)/1.6557(4)
178.63/8)/178.20(8)
2.9/3.7
Ct(2)–Fe
Ct(1)–Fe–Ct(2)
C7–Ct(1)–Ct(2)–Ccp
tra show for the two cyclopentadienyl protons in a- and ß-position
to the HgCl substituent satellites due to 1H–199Hg coupling.
Scheme 2 shows our data in comparison with some compounds
from the literature [9,10]. No satellites are observed for the pyri-
dine-H6 atom, and also its chemical shift is only slightly different
from the starting pyridylferrocenes. This is in sharp contrast to
the situation in mercurated phenylpyridine (see Scheme 2): There
the corresponding proton is shifted to high field by nearly 1 ppm,
and a substantial 3J(H–Hg) of 211 Hz was reported [11]. This hints
at a very weak interaction between the mercury and nitrogen
atoms in 2a and 2b. However, due to the strong dependence ‘‘on
spatial orientation and hybridization of bonds between coupling
nuclei’’ 1H and 199Hg [12] this observation has to be carefully inter-
preted. Thus, in 5-chloromercurio-thiophene-2-carboxylic acid
1
The atom numbering corresponds to Scheme 1, not to the individual atom
names in the structures. Ct(1) and Ct(2) are the centroids of the substituted and
unsubstituted cyclopentadienyl rings. Ccp is the carbon atom of the unsubstituted
cyclopentadienyl ring next to the projection of the C7–Ct(1) vector onto this ring.
refinement are collected in Table 1. For the solution and refine-
ment of all structures the program package WINGX was used [8].
3. Results and discussion
Recently we reported a new synthetic approach towards the
preparation of 2-pyridylmetallocenes
(MLn = CpFe (1a), CpRu and Mn(CO)3) [8]. For the ferrocene deriv-
g
5-[(2-C5H4N)(C5H4)]MLn
[47]
H
[40]
H
[54]
H
[31]
H
CF3
[29]
[21]
[36]
[42]
Cl
C
Cl
Cl
Hg
Hg
Hg
Hg
H
H
H
H
H
H
R
R
H
H
H
H3C
FeCp
FeCp
FeCp
FeCp
R= 2-pyridyl
R= 2-pyridyl
ref. [9]
ref. [10]
2b
2a
HOOC
S
Hg Cl
N
H
H
[211]
[38]
Hg
H
[-]
Cl
ref. [11]
ref. [12]
Scheme 2. Observed J(H–Hg) in 2a, b and some related compounds from the literature, J values (in Hz) in brackets.

136
K. Sünkel, S. Weigand / Polyhedron 44 (2012) 133–137
[195]
[160]
[228]
Cl
Cl
Cl
Hg
[2810]
Hg
Hg
[249]
[261]
[222]
[975]
[403]
[140]
[203]
R
R
FeCp
ref. [9]
FeCp
FeCp
R= CH2NMe2, ref. [14]
R= 2-pyridyl, 2a
Scheme 3. Observed J(13C–199Hg) in 2a and some related compounds from the literature, J values (in Hz) in brackets.
C15
C12
C13
C14
C11
C12
C16
C15
Fe
C13
Fe
C4
C5
C2
C7
C8
C2
C1
C1
C3
C6
C4
C6
N1
Hg1
Hg
Cl
C5
C9
C7
C10
N1
C8
Cl1
C9
C11
Fig. 2. Molecular structure of 2a.
C10
(see Scheme 2) a substantial 4J(H–Hg) was found, while 3J(H–Hg)
was not observed at all [12].
Fig. 3. Molecular structure of 2b.
The 13C NMR spectrum of 2a showed carbon-mercury couplings
for the cyclopentadienyl carbon atoms C7, C9 and C10, 1J(C–Hg) of
C8 and 3J(C–Hg) of C11 were not observed, as well as no carbon-
mercury couplings could be detected for 2b, most likely due to a
poor signal–noise ratio. Atoms were assigned by the known [13]
sequence 1J > 3J > 2J > 4J. Scheme 3 shows our data in comparison
with some literature data [9,14].
The mass spectra of both compounds showed the molecular
ions; additionally the mass peak of the symmetrical bis-ferroce-
nyl-mercury compound could be detected for compound 2a. How-
ever, it cannot be decided, if this known symmetrization reaction
e.g., [14,6] occurred already during dissolution before the measure-
ment or only in the mass spectrometer [13].
Fig. 4. View of the polymeric structure of 2b in the crystal.

K. Sünkel, S. Weigand / Polyhedron 44 (2012) 133–137
137
From CH2Cl2 solutions of both mercurated compounds crystals
separated that were suitable for X-ray diffraction analysis. Views
of the molecular structures of both compounds are depicted in
Figs. 2 and 3, important bond parameters are collected in Table 2.
The Hg–C and Hg–Cl distances and the C–Hg–Cl angle are nearly
identical in both compounds, however, the Hg–N distance is
slightly larger in 3. In comparison to the structure of (2-pyridyl-
phenyl)mercury chloride [15] the bond lengths Hg–Cl are identical
in 2 and 3, while Hg–C are shorter and Hg–N are longer in our com-
pounds. In the structure of the Cr(CO)3 complex of the mercurated
phenylpyridine the Hg–C and Hg–Cl distances are the same as in
the monometallic compound, while the Hg–N distance is elon-
gated, but still shorter than in 2 and 3 [6]. The torsion angle be-
tween cyclopentadienyl and pyridine ring (N–C2–C7–C11 in
Scheme 1) is in 3 significantly larger than in 4, suggesting sub-opti-
mal interaction of the nitrogen lone pair and the mercury atom.
While in 2 both cyclopentadienyl rings are half-way between
eclipsed and staggered conformation (‘‘ideal’’ torsion angles C7–
Ct(1)–Ct(2)–Ccp would be 0 and 36°), the eclipsed conformation
in 1b remains conserved in 3.
One particularly interesting feature in the crystal structure of
the above-mentioned (2-pyridylphenyl)mercury chloride is the
formation of tetranuclear [Hg4L4Cl4] units via stacking of two
Hg2Cl2 rings with two triply bridging chlorine atoms [17]. Within
one Hg2Cl2 dimer there were two short and two long Hg–Cl dis-
tances of 2.31 and ca. 3.4 Å, while the Hg–Cl distance between
two dimers was 3.18 Å.
In the crystal of 2 there is also a weak contact between two
neighboring Hg–Cl groups, however with a distance of more than
3.8 Å.
preparative use of such reactions the yields of the mercuration
reactions have to be optimized first. Studies to this end are
underway.
Appendix A. Supplementary data
CCDC 883793–883795 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
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In the crystal structure of 3 another structural motif is found.
Basic unit is again a dimer, formed between two inversion related
molecules at (x,y,z) and (1 ꢀ x,ꢀy,ꢀz). A central Hg2Cl2 bridge
shows two short and two long Hg0–Cl distances of 2.313 and
3.435 Å, respectively. Two such dimers associate via hydrogen
bridges between the chloride ligand and the hydrogen atom H3
of the substituted cyclopentadienyl ring, with a length of 2.823 Å.
Overall, a polymeric structure is formed along the z direction of
the crystal (Fig. 4).
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570.
4. Conclusion
Pyridylferrocenes can be mercurated with Hg(OAc)2/LiCl to give
[1-(ClHg)-2-(2-C5H4N)-3-R-C5H2]Fe(C5H5) (R = H, CH3) in modest
yields. NMR spectra and crystal structure determinations show
only a very weak interaction between the pyridine nitrogen and
the mercury atom. Preliminary experiments show that the HgCl
group can be substituted by other functional groups, but to make