Facile syntheses of homoleptic diarylmercurials via arylboronic acids

Article abstract of DOI:10.1016/j.jorganchem.2008.10.024

A general procedure for the syntheses of diarylmercurials is presented. Reactions proceeded in isopropanol in the presence of a base and arylboronic acid. With one exception, all reactions proceeded in good to excellent yields, and this procedure was appl

Full text of DOI:10.1016/j.jorganchem.2008.10.024

Journal of Organometallic Chemistry 694 (2009) 213–218
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Facile syntheses of homoleptic diarylmercurials via arylboronic acids
b
David V. Partyka a, , Thomas G. Gray
*
a Creative Chemistry, LLC, Cleveland, OH 44106, United States
b Department of Chemistry, Case Western Reserve University, Millis 418, Cleveland, OH 44106, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A general procedure for the syntheses of diarylmercurials is presented. Reactions proceeded in isopropa-
nol in the presence of a base and arylboronic acid. With one exception, all reactions proceeded in good to
excellent yields, and this procedure was applicable to a variety of aromatic and heteroaromatic boronic
acids. Products were characterized by multinuclear NMR spectroscopy and microanalysis, and investi-
gated by DFT calculations. The structure of di(4-pyridyl)mercury (6) was further authenticated by
X-ray crystallography. Combined with previous work on the formation of arylgold(I) complexes via aryl-
boronic acids, this procedure may be generally useful for the arylation of late transition metals.
Ó 2008 Elsevier B.V. All rights reserved.
Received 16 September 2008
Received in revised form 12 October 2008
Accepted 13 October 2008
Available online 22 October 2008
Keywords:
Diarylmercurials
Arylboronic acid
Protodemercuration
Kohn–Sham orbitals
Quaternization
1. Introduction
which produce HgX2 as a byproduct can never yield 100% based
on mercury atoms, as elemental mercury or an inorganic mercury
Although it is one of the most toxic non-radioactive elements
known, there is much current interest in mercury in various appli-
cations. Mercury pollution continues to be a cause of concern for
countries around the world, and the European Union has even
called for a ban on mercury exports by 2011 [1]. Therefore, it is
not surprising that much of this interest is in the selective sensing
of mercury ions in solution and the extraction of mercury from the
environment [2–6]. However, less attention has currently been gi-
ven to the syntheses of various mercury-containing species.
Diarylmercurials have traditionally been synthesized by a vari-
ety of methods [7]. Seyferth and co-workers treated monoorgano-
mercurials RHgX (X = acetate, halide, hydroxide, etc.) with a
polyethyleneimine in the presence of water to produce diorgano-
mercurials in a reaction which also produced HgX2 as a byproduct
[7a]. Other investigators have used different methodologies,
including the treatment of mercuric chloride with phenylmagne-
sium bromide (to form diphenylmercury) [7a] and the treatment
of mercuric bromide with aryllithium reagents in THF to form a
variety of diarylmercurials [7g] in addition to other methods.
While all of these procedures are suitable to the syntheses of
diarylmercurials in a laboratory, the chief disadvantages include
air and water sensitivity of starting materials (aryllithium and
Grignard methods in particular), functional group incompatibility
(e.g., a ketone or nitro group would be incompatible with Grignard
or aryllithium reagents), or lack of atom economy (the reactions
salt must be extruded by stoichiometric necessity).
An impressive variety of functionally-substituted arylboronic
acids and esters have become commercially available in recent
years. Already well-known as successful transmetalating reagents
in Suzuki–Miyaura coupling [8], boronic acids and esters have also
been successfully applied to the monoarylation of various gold(I)
bromides [9], lead acetates [10], and mercuric chloride [11]. In
one investigation [12], Challenger and Richards reported that mer-
curic oxide reacts with arylboronic acids in water to yield diaryl-
mercurials, but the products were only characterized by melting
point and in one case, elemental analysis. In this investigation
we describe a general procedure for the high-yield syntheses of
fully-characterized diarylmercurials in analytical purity starting
from readily-available, air and water stable boronic acids and mer-
cury(II) acetate in the presence of base.
2. Results and discussion
2.1. Synthesis
In contrast to previous synthetic approaches, a one-step proce-
dure (done in isopropyl alcohol) starting from mercury(II) acetate
and the appropriate boronic acid (>2 equiv.) in the presence of ce-
sium carbonate was found to be a convenient, general procedure
for the syntheses of homoleptic diarylmercurials. This is not the
first time that mercury(II) acetate has been used as a precursor
in the synthesis of organomercurials. Hanke has demonstrated that
the reaction of mercury(II) acetate with neat phenyl halides at
* Corresponding author. Tel.: +1 2163921772.
E-mail address: c2chemicals@gmail.com (D.V. Partyka).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.024

214
D.V. Partyka, T.G. Gray / Journal of Organometallic Chemistry 694 (2009) 213–218
140 °C yields (para-halophenyl)mercuric acetate complexes [13],
whereas Rausch and co-workers mercurated ferrocene in glacial
acetic acid using mercury(II) acetate [14]. Even more relevant is
a Hg–C bond formation observed when mercury(II) acetate is trea-
ted with a vinylboronate; the observed product is a bis(vinylated)-
mercury(II) complex [15]. Also, as mentioned in the Introduction,
Challenger and Richards reported that the reaction of mercuric
oxide and arylboronic acids yields diarylmercurials [12], but the
Table 1
Bis(arylmercury(II)) products, yields, and designation of compoundsa
Arylboronic acid
Equivalents of boronic acid
2.6
Product
Isolated yield (%)
77
1
2
O
B(OH)2
B(OH)2
O
Hg
O
2.6
2.8
62
83
O
O
Hg
O
3
4
B(OH)2
Hg
2.6
78
B(OH)2
Fe
Fe
Hg
Fe
2.6
5b
90
CO2Et
B(OH)2
Hg
EtO2C
EtO2C
2.6
2.6
6
7
12
83
N
F
B(OH)2
N
F
Hg
N
F
B(OH)2
B(OH)2
Hg
Hg
F
F
F
F
F
F
2.1
8
58
2.1
9c
80
S
B(OH)2
Hg
S
S
a
Hg(OAc)2 is the mercury precursor for each reaction.
Solvent is ethanol (190 proof).
Reaction time is 12 h.
b
c

D.V. Partyka, T.G. Gray / Journal of Organometallic Chemistry 694 (2009) 213–218
215
products were only characterized by melting point and in one case,
elemental analysis.
mer to latter (as observed by 1H NMR). It appears that even milder
conditions than those used to synthesize the homoleptic mercury
complexes are not conducive to the preparation of heteroleptic
diarylmercury complexes.
In this investigation, the base is necessary for the clean forma-
tion of products, as a control reaction in the absence of base (using
4-tolylboronic acid, data not included) yielded a mixture of prod-
ucts including the desired product. This methodology is similar
to the approach of Gray and co-workers in their protocol for the
syntheses of phosphine-ligated arylgold(I) complexes starting from
boronic acids [9]. In that system, a control reaction conducted in
the absence of base yielded no product. It is possible that the base
serves multiple functions including quaternization of boron (as
was shown to be necessary in Suzuki–Miyaura coupling [16] and
more recently in a system where transmetalation was effected
from boron to bismuth [17]) and prevention of protodemercura-
tion [18], although transmetalation by mono- and diarylmercurials
does not require a base [19]. Table 1 summarizes the synthetic
details.
All products, with the exception of 6, were synthesized in good
to excellent isolated yields. As can be seen, all varieties of function-
alities and size were tolerated under these reaction conditions. The
acetyl and ethoxycarbonyl functionalities of 2 and 5, respectively,
would not necessarily survive the conditions of Grignard or butyl-
lithium methodology, thus our methodology provides a convenient
entrée into diarylmercurials with Grignard and butyllithium sensi-
tive functional groups. Workups (all of which were performed in
air) are simple, and all products are nearly insoluble in the reaction
milieu, such that precipitation appears to help drive the reaction.
However, the successful formation of 5 indicates that precipitation
is not necessarily imperative for product formation.
2.2. NMR spectroscopy
All products were analyzed by 1H NMR as a confirmation of pur-
ity, and new products 6–8 were further analyzed by 13C NMR (and
in the case of 7 and 8, 19F NMR). Product 9 has no appreciable sol-
ubility in any common solvents with the exception of DMSO, and
even in this solvent, the solubility is high enough only to record
a proton NMR spectrum.
The spectra of all products are consistent with Cs symmetry,
which indicates that there is fast (relative to the NMR timescale)
rotation about the Hg–C bonds in solution. In the case of fluori-
nated products 7 and 8, 1H–19F and 13C–19F couplings were ob-
served in the 9H and 13C NMR spectra, respectively, which
accounts for the observed multiplets. Broad 199Hg (I = ½) satellite
peaks were observed in the 1H NMR of all compounds for only
one of the signals (presumably the ortho protons). In a recent
investigation by Parkin and co-workers, 199Hg satellite peaks of
the ethyl resonances of thimerosal were significantly broadened
on a 400 MHz NMR spectrometer (relative to a 300 MHz NMR
spectrometer), similar to what was observed here [20].
2.3. Computational results and crystal structure of 6
The electronic structure of the representative complex 6 was
examined with nonlocal density-functional theory calculations. A
harmonic frequency calculation confirms the calculated structure
of 6 to be a potential-energy minimum. The molecular orbital dia-
gram is shown as Fig. 1.
Not unexpectedly, the frontier orbitals are ligand-predominated
as gauged by percentage contributions of metal and ligands to the
orbitals’ electron density. Electron densities are apportioned
according to Mulliken’s scheme [21]. The geometry was optimized
with imposed D2d symmetry, despite that the crystal structure of 6
(Fig. 2) shows only a D2 structure.
We attempted to extend this specific methodology to the prep-
aration of mixed diarylmercury (II) complexes (i.e., R–Hg–R0) using
phenylmercury(II) acetate. Even at room temperature over the per-
iod of 12 h in the same solvent for several boronic acids (1.3 equiv.
boronic acid and base), the majority products observed by NMR
were the ligand rearrangement products diphenylmercury and
diarylmercury (depending on which boronic acid was used). In
the case of mesitylboronic acid, dimesitylmercury was observed
along with a small amount of a different mesityl containing species
(probably the mixed diarylmercury complex) in an 8:1 ratio of for-
Fig. 1. Kohn–Sham orbital correlation diagram of 6, optimized in full D2d symmetry. Plots of selected orbitals (0.03 au) appear at right. Methylene chloride solvation is
included approximately through a polarized continuum model (PCM).

216
D.V. Partyka, T.G. Gray / Journal of Organometallic Chemistry 694 (2009) 213–218
tion, and the residue extracted with toluene or toluene/THF (ex-
cept 9, which was collected by filtration), and filtered. The
filtrate was reduced to dryness via rotary evaporation, leaving a
pure solid/residue which was triturated with pentane, dried, and
collected.
3.1.1. [Hg(4-anisyl)2] (1)
Fig. 2. ORTEP representation of one crystallographically-independent molecule of
6, showing0 50% probability ellipsoids and atom-labeling scheme. Selected bonds
distances (ÅA) and bond angles (°): Hg1–C1, 2.069(4); Hg1–C6, 2.083(4). C1–Hg1–C6,
177.70(17). The corresponding values for the second crystallographically-indepen-
dent molecule of 6 are not significantly different.
The isolated solid was analytically pure. Yield: 45 mg (77%). 1H
NMR (CDCl3): d 7.34–7.39 (m, C6H4OCH3, 4H), 7.02 (d, C6H4OCH3,
J = 8.8 Hz, 4H), 3.83 (s, C6H4OCH3, 6H) ppm. Anal. Calc. for
C14H14HgO2: C, 40.53; H, 3.40. Found: C, 40.89; H, 3.25%.
3.1.2. [Hg(4-acetylphenyl)2] (2)
0
The isolated solid was analytically pure. Yield: 41 mg (62%). 1H
NMR (CDCl3): d 8.03 (d, C6H4COCH3, J = 8.4 Hz, 4H), 7.56 (d,
C6H4COCH3, J = 8.4 Hz, 4H), 2.62 (s, C6H4COCH3, 6H) ppm. Anal.
Calc. for C16H14HgO2: C, 43.92; H, 3.68. Found: C, 43.79; H, 3.22%.
The calculated Hg–C bond length is 2.1240ÅA compared to the
experimental values of 2.069(4) and 2.083(4) ÅA (for two crystallo-
graphically independent molecules). The highest-occupied
Kohn–Sham orbital (HOMO) and the HOMO – 1 are each singly
degenerate functions delocalized over the ligands’
r-bonding
framework; each nears significant lone-pair character on nitrogen
(38.1%, HOMO; 59.3%, HOMO – 1). In a crude simplification, the a1
HOMO – 1 can be viewed as the in-phase, and the b2 HOMO as the
antiphase combinations of the pyridyl nitrogen lone pairs. These
results are not surprising considering the large number of sub-
van der Waals contacts (involving the pyridyl nitrogen atom) iden-
tified in the crystallographic packing of 6 (these data suggest that
molecules such as 6, which feature outwardly-directed nitrogen
atoms, would be suitable for applications in supramolecular chem-
istry). Moreover, this is interesting because crystal engineering
3.1.3. [Hg(mesityl)2] (3)
The product was spectroscopically pure. Yield: 60 mg (83%). A
recrystallized sample (THF/pentane) gave analytically pure mate-
rial. The 1H NMR spectral data matched the literature spectral data
[23]. Anal. Calc. for C18H22Hg: C, 49.25; H, 5.05. Found: C, 49.13; H,
4.96%.
3.1.4. [Hg(ferrocenyl)2] (4)
The isolated solid was analytically pure. Yield: 65 mg (78%). 1H
NMR (CDCl3): d 4.49 (s br, C5H4, 4H), 4.24 (s, C5H5, 10H), 4.11 (s br,
C5H4, 4H) ppm. Anal. Calc. for C20H18Fe2Hg: C, 42.10; H, 3.18.
Found: C, 41.87; H, 3.11%.
more commonly utilizes hydrogen bonding or
p–p stacking
xinteractions as rational means to create supramolecular architec-
tures [22].
A toroidal sd-hybrid orbital is evident at mercury in the a1
HOMO, but this orbital is essentially nonbonding toward the
3.1.5. [Hg(3-carboxyethylphenyl)2] (5)
The isolated solid was analytically pure. Yield: 71 mg (90%). 1H
NMR (CDCl3): d 8.12–8.16 (m, C6H4(CO2CH2CH3), 2H), 7.94 (d,
C6H4(CO2CH2CH3), J = 7.6 Hz, 2H), 7.62 (d, C6H4(CO2CH2CH3),
J = 7.2 Hz, 2H), 7.53 (t, C6H4(CO2CH2CH3), J = 7.6 Hz, 2H), 4.39 (q,
C6H4(CO2CH2CH3), J = 7.2 Hz, 4H), 1.41 (t, C6H4(CO2CH2CH3),
J = 7.2 Hz, 6H) ppm. Anal. Calc. for C16H18HgO4: C, 43.32; H, 3.64.
Found: C, 43.05; H, 3.51%.
r
-bond 4-pyridyl carbon atoms. The LUMOs of 6 form a doubly
degenerate set, each being localized on a different 4-pyridyl ligand;
they overlap with the vacant mercury 6s orbital. The a1 HOMO – 7,
which lies some 2.09 eV below the HOMO, accounts for Hg–C
bonding. This stabilization concurs with substantial covalency of
the Hg–C bond, as expected from their similar Pauling electroneg-
ativities: 2.00 for Hg, 2.55 for C.
3.1.6. [Hg(4-pyridyl)2] (6)
3. Experimental section
The full procedure is included for this product, as it is slightly
different than the general procedure. A flask was loaded with
mercury(II) acetate (59 mg, 0.19 mmol), 4-pyridylboronic acid
(2.6 equiv., 59 mg, 0.45 mmol) and cesium carbonate (2.6 equiv.,
150 mg, 0.46 mmol). Isopropanol (3 mL) was added, and the flask
sealed with a septum. The contents were evacuated and backfilled
with argon twice, and heated in a 50 °C oil bath for 20 h. The mix-
ture was cooled, the solvent removed by rotary evaporation, and
the residue extracted with toluene/THF and filtered. The filtrate
solvent was removed by rotary evaporation, the residue triturated
with pentane, and the solid collected and dried. As the product
mixture could not be washed with methanol (6 is soluble in meth-
anol), the product was recrystallized by vapor diffusion of pentane
into a saturated THF/toluene solution and collected in analytical
purity. Yield: 19 mg (12%). 1H NMR (CDCl3): d 8.64 (d, C5H4N, 4H,
J = 5.6 Hz), 7.33 (d, C5H4N, 4H, J = 5.6 Hz) ppm. 13C NMR (CDCl3):
d 177.46 (s), 149.46 (s), 133.20 (s) ppm. Anal. Calc. for C10H8HgN2:
C, 33.67; H, 2.26; N, 7.85. Found: C, 33.81; H, 2.46; N, 7.74%.
All solvents and reagents were used as received. Arylboronic
acids were purchased from Acros Organics and Frontier Scientific.
Caution! Organomercury compounds are extremely toxic, and
appropriate safety measures should be taken when working with
these compounds. All reactions were set up in air but stirred under
argon for reasons of safety. Microanalyses (C, H, and N) were per-
formed by Robertson Microlit Laboratories, Inc (Madison, NJ). NMR
spectra (1H, 13C{1H}, and 19F) were recorded on a Varian AS-400
spectrometer operating at 399.7, 100.5, and 376.1 MHz, respec-
tively. For 1H and 13C{1H} NMR spectra, chemical shifts were deter-
mined relative to the solvent residual peaks. For 19F NMR spectra,
chemical shifts were determined relative to an internal standard
(CFCl3 in CDCl3). All 1H NMR spectra possessed broad 199Hg satel-
lite peaks (I = ½).
3.1. General procedure
A flask was loaded with mercury(II) acetate (0.09–0.21 mmol),
arylboronic acid (2.1–2.8 equiv., see Table 1) and cesium carbonate
(equimolar with arylboronic acid). Isopropyl alcohol (3 mL) was
added, and the flask sealed with a septum. The contents were
heated under argon in a 50–55 °C oil bath for 12–20 h (see Table 1).
The mixture was cooled, the solvent removed by rotary evapora-
3.1.7. [Hg(3,5-difluorophenyl)2] (7)
The solid (which was pure by NMR) could be recrystallized in
xylene/pentane (vapor diffusion) to give analytically pure material.
Yield: 64 mg (83%). 1H NMR (CDCl3): d 6.96–7.01 (m, ortho-C6F2H3,
4H), 6.72 (tt, para-C6F2H3, J = 2.4, 9.2 Hz, 2H) ppm. 13C NMR
(CDCl3): d 164.62 (d, J(13C–19F) = 10.2 Hz), 162.11 (d, J(13C–19F) =

D.V. Partyka, T.G. Gray / Journal of Organometallic Chemistry 694 (2009) 213–218
217
10.6 Hz), 119.28 (dd, J(13C–19F) = 5.3, 17.9 Hz), 103.95 (t,
J(13C–19F) = 24.9 Hz) ppm. 19F NMR (CDCl3): d ꢀ110.55 (d, J =
5.3 Hz) ppm. Anal. Calc. for C12H6F4Hg: C, 33.77; H, 1.42. Found:
C, 34.05; H, 1.54%.
and the associated basis set [28], which was contracted as fol-
lows: (8s,7p,6d) ? [6s,5p,3d]. Full D2d symmetry was imposed
during optimization; a harmonic frequency calculation of the
optimized structure confirmed it to be a potential-energy mini-
mum. A single-point calculation was performed on the resulting
optimized geometry. In this calculation, nonmetal atoms were de-
scribed with the TZVP basis set of Godbout and co-workers [29].
The Stuttgart effective core potential and basis set were again
used for mercury. Relativistic effects with the Stuttgart ECP and
its associated basis set are introduced with a potential term
(i.e., a one-electron operator) that replaces the two-electron ex-
change and Coulomb operators resulting from interaction be-
tween core electrons and between core and valence electrons.
In this way relativistic effects, especially scalar effects, are in-
cluded implicitly rather than as four-component, one-electron
functions in the Dirac equation. Methylene chloride solvation ef-
fects were included implicitly with the polarized continuum mod-
el (PCM) of Tomasi and co-workers [30,31]. Percentage
compositions of molecular orbitals, overlap populations, and bond
orders between fragments were calculated using the AOMIX pro-
gram [32,33].
3.1.8. [Hg(2-methyl-5-fluorophenyl)2] (8)
The isolated solid was analytically pure. Yield: 50 mg (58%). 1H
NMR (CDCl3): d 7.28 (dd, C6FH3(CH3), J = 5.2, 8.4 Hz, 2H), 7.09 (dd,
C6FH3(CH3), J = 2.8, 8.4 Hz, 2H), 6.85 (td, C6FH3(CH3), J = 2.8, 8.4 Hz,
2H), 2.53 (s, C6FH3(CH3), 6H) ppm. 13C NMR (CDCl3): d 162.83 (s),
160.37 (s), 139.87 (d, J(13C–19F) = 3.4 Hz), 131.10 (d, J(13C–19F) =
6.7 Hz), 123.09 (d, J(13C–19F) = 18.1 Hz), 114.60 (d, J(13C–19F) =
21.1 Hz), 24.55 (s, C6H3F(CH3)) ppm. 19F NMR (CDCl3):
d
(ꢀ119.08)–(ꢀ119.29) (m) ppm. Anal. Calc. for C14H12F2Hg: C,
40.15; H, 2.89. Found: C, 39.99; H, 2.66%.
3.1.9. [Hg(2-benzothienyl)2] (9)
The isolated solid was analytically pure. Yield: 34 mg (80%). The
product is sparingly soluble only in DMSO, and only 1H NMR solu-
tion data could be obtained. 1H NMR (DMSO-d6): d 7.95 (d,
J = 8.0 Hz, 2H), 7.86 (d, J = 6.8 Hz, 2H), 7.51 (s, 2H), 7.25–7.36 (m,
4H) ppm. Anal. Calc. for C16H10HgS2: C, 41.15; H, 2.16. Found: C,
40.27; H, 1.97%.
Acknowledgements
3.2. Crystal structure determination of compound 6
D.V.P. gratefully acknowledges investors in Creative Chemistry,
LLC and Case Western Reserve University for support. T.G.G. grate-
fully acknowledges the National Science Foundation (Grant CHE-
0749086), the donors of the Petroleum Research Fund adminis-
tered by the American Chemical Society (Grant 42312-G3 to
T.G.G.), and Case Western Reserve University for support. Mr.
James B. Updegraff III is thanked for assistance with crystallo-
graphic work for 6.
3.2.1. Crystal data
Compound 6: C10H8HgN2, M = 356.77, orthorhombic, space
group Ibca, a = 11.0574(17) Å, b = 21.975(4) Å, c = 30.710(5) Å, U =
7462(2) Å3, Z = 32, Dcalc = 2.543 Mg mꢀ3, k (Mo k
a) = 0.71069 Å,
l
= 16.450 mmꢀ1, F(000) = 5184, T = 100(2) K.
3.2.2. Data collection and reduction
A colorless block ca. 0.53 ꢁ 0.31 ꢁ 0.05 mm3 (grown by slow
diffusion of pentane into a saturated 1,2-dichloroethane solution)
was mounted in paratone oil on a mitogen tip. A total of 40290
reflections were collected on a Bruker AXS SMART APEXII CCD dif-
fractometer, h ranging from 1.85–27.23° for data collection. A Lor-
entz-polarization type absorption correction was applied using
AXScale. Merging equivalents gave 4139 independent reflections
(Rint = 0.0678).
Appendix A. Supplementary material
CCDC 702269 contains the supplementary crystallographic data
for compound 6. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.jorganchem.2008.10.024.
3.2.3. Structure solution and refinement
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The unit cell was determined using APEX2 Crystallographic
Suite. The structure of 6 was solved by direct methods and refined
by full matrix least squares against F2 with all reflections using
SHELXTL. Refinement of extinction coefficients was found to be insig-
nificant. All non-hydrogen atoms were refined anisotropically. All
other hydrogen atoms were placed in standard calculated positions
and all hydrogen atoms were refined with an isotropic displace-
ment parameter 1.5 (CH3) or 1.2 (all others) times that of the adja-
cent carbon or nitrogen atom. The refinement proceeds to R1 =
0.0231, wR2 = 0.0823 with a goodness of fit on F2 0.820 for 299 re-
fined parameters and 0 restraints, and R1 = 0.0367, wR2 = 0.1095
for all data with electron density ranging from 1.417 to
ꢀ1.781 e Å3 in the final Fourier synthesis.
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