Formation of donor-acceptor Fe(0)-Hg(II) bond in separation and stabilization of optically active iron(0) phosphine complexes. Absolute configuration of (+)-(R)-(CO)4Fe(μ-EtPhPpy)HgCl2

Article abstract of DOI:10.1016/S0020-1693(99)00216-9

The chiral phosphine ligand 2-(ethylphenylphosphino)pyridine (EtPhPpy, R/S mixture) reacts with Fe(CO)₅ to give racemic (±)-Fe(CO)₄(EtPhPpy-P), which undergoes an addition reaction with HgCl₂ to afford the optically active

Full text of DOI:10.1016/S0020-1693(99)00216-9

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 293 (1999) 106–109
Note
Formation of donor–acceptor Fe(0)–Hg(II) bond in separation
and stabilization of optically active iron(0) phosphine complexes.
Absolute configuration of (+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2
Shan-Ming Kuang a, Zheng-Zhi Zhang b, Kandasamy Chinnakali c, Hoong-Kun Fun c,
Thomas C.W. Mak a,*
a The Chinese Uni6ersity of Hong Kong, Shatin, New Territories, Hong Kong, Hong Kong
b Elemento-Organic Chemistry Laboratory, Nankai Uni6ersity, Tianjin, People’s Republic of China
c School of Physics, Uni6ersity Sains Malaysia, 11800 USM Penang, Malaysia
Received 3 December 1998; accepted 16 April 1999
Abstract
The chiral phosphine ligand 2-(ethylphenylphosphino)pyridine (EtPhPpy, R/S mixture) reacts with Fe(CO)5 to give racemic
(9)-Fe(CO)4(EtPhPpy-P), which undergoes an addition reaction with HgCl2 to afford the optically active binuclear Fe(0)–Hg(II)
complexes (+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2 and (−)-(S)-(CO)4Fe(m-EtPhPpy)HgCl2, which can be separated manually in
crystalline form. The absolute configuration of (+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2 has been determined by single crystal X-ray
analysis. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Iron complexes; Mercury complexes; Chiral phosphine complexes
{(Rp)-PMePrPh)}] [9], as well as [Sp-4-4-1(R),4(S)]-
bromo[1-[(dimethylamino)ethyl]-2-naphthalenyl-C,N]-
[1 - (benzylthio) - 2 - (methylphosphino)ethane - P]palla-
dium(II) [10].
1. Introduction
Many chiral phosphorus compounds have interesting
biological properties connected closely with the abso-
lute configuration at the phosphorus centre [1], and
transition metal complexes of optically active phosphi-
nes have been used successfully as efficient catalysts in
asymmetric synthesis [2]. Several transformation meth-
ods have been developed to synthesize optically pure
P-chiral phosphines [3,4], which were the first to be
used for asymmetric hydrogenation [5,6]. However,
published reports about the absolute configuration of
P-chiral phosphines are few in number, some known
examples being [{(+)-(1R,5R)-h3-pinenyl}Ni{(Sp)-
Recent work from our laboratories has demonstrated
that neutral 18-electron organometallic compounds,
such as trans-Fe(CO)3(Ph2Ppy)2 and Fe(CO)4(Ph2Ppy-
P), can react with Lewis acids to form heterobinuclear
complexes that are consolidated by a donor–acceptor
metal–metal bond [11–16]. A study of the catalytic
behaviour of a number of heterobinuclear complexes of
this class showed that when the complex FeRh(m-
Ph2Ppy)2(CO)4Cl is used as a homogeneous catalyst for
the carbonylation of ethanol and iodoethane to form
ethyl propionate, the reaction activity and selectivity
are superior to Rh(Ph3P)3Cl [11].
PMetBuPh)}]
[7],
(+)-[h4-(1,5-cod)Rh{(Rp)-PMe-
(men)Ph}](BF4) [8] and (−)-[FeCoMo(m-S)Cp(CO)7-
We have prepared previously the asymmetrical
phosphine ligand EtPhPpy as a racemic mixture [16].
We report here the reaction of racemic (9)-Fe-
(CO)4(EtPhPpy-P) with HgCl2 that leads to the sepa-
* Corresponding author. Tel.: +852-609 6000; fax: +852-603
5544.
E-mail address: tcwmak@cuhk.edu.hk (T.C.W. Mak)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00216-9

S.-M. Kuang et al. / Inorganica Chimica Acta 293 (1999) 106–109
107
ration and stabilization of optical isomers of iron(0)
phosphine complexes. The absolute configuration of
(+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2 has been deter-
mined by X-ray analysis.
2. Results and discussion
By the oxygen-atom transfer reaction, racemic (9)-
Fe(CO)4(EtPhPpy-P) was obtained from Fe(CO)5,
(9)-EtPhPpy [16] and Me3NO·2H2O [15].
Fe(CO)5+Me3NO·2H2O+(9)-EtPhPpy
“(9)-Fe(CO)4(EtPhPpy-P)+Me3N+CO2
+2H2O
This complex decomposed slowly in CH2Cl2 even under
a N2 atmosphere at room temperature, which may be
due to the decarbonylation reaction. Treatment of a
solution of racemic (9)-Fe(CO)4(EtPhPpy-P) in
CH2Cl2 with HgCl2 at room temperature readily yielded
binuclear Fe(0)–Hg(II) complexes (9)-(CO)4Fe(m-Et-
PhPpy)HgCl2. By layering MeOH on the reaction solu-
tion, (+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2 was obtained
as orange plate-like crystals and (−)-(S)-(CO)4Fe(m-
EtPhPpy)HgCl2 as orange needle-shaped microcrystals,
which were separated manually under a microscope
(Scheme 1).
The binuclear Fe(0)–Hg(II) complexes are stable in
solution and in the solid state even on exposure to air,
which indicates that the formation of a donor–acceptor
Fe(0)–Hg(II) bond leads to stabilization of unstable
electron-rich iron(0) complexes. The change of electron
density at the iron centre can be gauged from the
observed carbonyl stretching vibrations. The IR spec-
trum of (9)-Fe(CO)4(EtPhPpy-P) shows w(CO) at
2045.7, 1980.5 and 1930.4 cm−1, while that of (9)-
(CO)4Fe(m-EtPhPpy)HgCl2 shifted to higher frequencies
at 2097.4, 2046.5, 2015.4 cm−1, which is consistent with
a decrease in electron density at the iron(0) centre.
The 31P{1H} NMR spectrum of (9)-Fe(CO)4-
(EtPhPpy-P) exhibits a singlet at 83.62 ppm, while that
of (9)-(CO)4Fe(m-EtPhPpy)HgCl2 shows a triplet at
95.18 ppm with 2+3J(199Hg–31P)=417 Hz. Such a
coupling constant has also been found in ClFe(CO)2(m-
Fig. 1. ORTEP drawing (35% thermal ellipsoids) showing the molecu-
lar structure of (+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2. Pertinent bond
,
lengths (A) and angles (°): Fe(1)–Hg(1) 2.608(1), Hg(1)–Cl(1)
2.549(1), Hg(1)–Cl(2) 2.445(1), Hg(1)–N(1) 2.530(4), Fe(1)–P(1)
2.297(1); P(1)–Fe(1)–C(1) 169.1(2), C(2)–Fe(1)–C(3) 163.3(2), C(4)–
Fe(1)–Hg(1) 176.0(2), Hg(1)–Fe(1)–C(2) 80.4(2), C(2)–Fe(1)–C(4)
96.2(2), C(4)–Fe(1)–C(3) 100.5(2), C(3)–Fe(1)–Hg(1) 82.9(2), Fe(1)–
Hg(1)–Cl(1) 114.8(1), Fe(1)–Hg(1)–Cl(2) 138.7(1), Fe(1)–Hg(1)–
N(1) 90.0(1), Cl(1)–Hg(1)–Cl(2) 100.8(1).
Ph2Ppy)2HgCl (2+3J(199Hg–31P)=419 Hz) [17] and
mer-[{(MeO)3Si}(CO)3Fe(m-dppm)HgCl{Ph2CH2C(O)-
Ph}] (2+3J(199Hg–31P)=397 Hz) [18].
The molecular structure of (+)-(R)-(CO)4Fe(m-Et-
PhPpy)HgCl2 is depicted in Fig. 1. The chiral phospho-
rus atom exhibits an R configuration. However, as the
phosphorus atom coordinates to the iron centre
through its lone-pair electrons, the absolute configura-
tion of the original phosphine ligand can be deduced as
S (Scheme 1). The iron and mercury centres are linked
by an EtPhPpy-P,N bridge supported by a donor–ac-
ceptor metalꢀmetal bond. The coordination geometry
of the Fe(1) centre is distorted octahedral with bond
angles
P(1)–Fe(1)–C(1)=169.1(2),
C(2)–Fe(1)–
C(3)=163.3(2) and C(4)–Fe(1)–Hg(1)=176.0(2)°. The
sum of the Hg(1)–Fe(1)–C(2), C(2)–Fe(1)–C(4), C(4)–
Fe(1)–C(3), and C(3)–Fe(1)–Hg(1) bond angles is
360°, indicating that the atoms of this C3FeHg frag-
ment are co-planar. The P(1) atom deviates from the
perpendicular to the above fragment, with a somewhat
acute bond angle of P(1)–Fe(1)–Hg(1)=83.3(1)°. The
Hg centre exhibits a distorted tetrahedral coordination
geometry, with bond angles Fe(1)–Hg(1)–Cl(1)=
114.8(1), Fe(1)–Hg(1)–Cl(2)=138.7(1), Fe(1)–Hg(1)–
N(1)=90.0(1) and Cl(1)–Hg(1)–Cl(2)=100.8(1)°. The
,
Fe–Hg distance of 2.608(1) is comparable to 2.546(1) A
in Cl2HgFe(CO)2(PMe2Ph)2(CS2C2(CO2Me)2) [19] and
,
2.570(2) A in (CO)4Fe(m-Ph2Ppy)2Hg(m-Cl)2HgCl2 [8].
The Hg–N distance of 2.530(4) is somewhat longer
,
Scheme 1.
than2.483(11)A in(CO)4Fe(m-Ph2Ppy)2Hg(m-Cl)2HgCl2.

108
S.-M. Kuang et al. / Inorganica Chimica Acta 293 (1999) 106–109
The crystallization of enantiomeric forms of
Found: C, 30.84; H, 2.16; N, 1.77. Calc. for
C17H14NFeHgO4P: C, 31.16; H, 2.14; N, 2.14%.
(CO)4Fe(m-EtPhPpy)HgCl2 in easily distinguishable
habits is a most unusual phenomenon. Thus far we
have had no success in our attempt to recover the
enantiopure (−)-EtPhPpy ligand from the reaction of
(+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2 with bis(diphenyl-
phosphino)methane(dppm),1,2-bis(diphenylphosphino)-
ethane (dppe), HCl or NaOH in DMF solution, and
also failed to obtain the enantiopure phosphine oxide
EtPhP(O)py by treating the binuclear complex with
concentrated HNO3 and H2O2.
3.4. X-ray crystallography
(+)-(R)-(CO)4Fe(m-EtPhPpy)HgCl2, C17H14Cl2Fe-
HgNO4P, M=654.6, orthorhombic, space group
P212121 (No. 19), a=10.224(1), b=10.455(1), c=
3
,
,
19.780(1) A, V=2114.2(1) A , Z=4, Dc=2.057 Mg
m
−3, F(000)=1240, orange plate with dimensions
0.28×0.10×0.08 mm, v(Mo Ka)=8.289 mm−1. A
total of 19 570 reflections with 2q555° were measured
on a Smart CCD system at 20°C using graphite
3. Experimental
,
monochromated Mo Ka radiation (u=0.71073 A), and
empirical absorption correction was applied using the
SADABS [20] program. Refinement of 245 parameters
for 27 non-hydrogen atoms and 5740 observed (ꢀFoꢀ\
4|(Fo) out of 7776 (Rint=4.69%) unique data con-
verged to R(F)=0.038 and wR(F2)=0.056 with
3.1. General
All reactions were carried out under nitrogen using
Schlenk techniques. The solvents were purified by stan-
dard methods. Infrared spectra were recorded on a
Perkin–Elmer 1600 spectrometer as KBr discs. The
31P{1H} NMR spectra were recorded on a Bruker
ARX-500 spectrometer at 202.5 MHz using 85% H2PO3
as the external standard and CDCl3 as solvent.
w
−1=|2(Fo2) and goodness-of-fit=0.933. The Flack x
parameter [21], which refined to a value of 0.010(5), was
used to establish the absolute structure.
4. Supplementary material
3.2. Preparation of (9)-Fe(CO)4(EtPhPpy-P)
The atomic parameters have been deposited at the
Cambridge Crystallographic Data Centre as CCDC
Ref. No. 116304.
Me3NO·H2O (0.55 g, 0.05 mol) dissolved in 20 ml
ethanol was added slowly, over a period of 30 min, to
a mixture of Fe(CO)5 (0.67 ml, 0.05 mol) and (9)
EtPhPpy (1.08 g, 0.05 mol) in 20 ml ethanol under N2
at room temperature, and stirred for a further 5 h. The
solvent was removed in vacuo to give an orange
residue, which was then purified by column chromato-
graphy using 1:1 hexane/dichloromethane as the eluent
under N2. The product was obtained as a yellow pow-
der (1.60 g, 83% yield). IR (KBr disc): w(CO) 2045.7,
Acknowledgements
This work is supported by Hong Kong Research
Grants Council under Grant No. CUHK 4022/98P.
1980.5, 1930.4 cm−1 31P{1H} NMR: l 83.62 ppm.
.
Anal. Found: C, 53.70; H, 3.79; N, 3.30. Calc. for
C17H14NFeO4P: C, 53.26; H, 3.66; N, 3.66%.
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