Copper(I) complexes containing 1,1′-bis(diphenylphosphino) ferrocene (dppf) as a chelate ligand. X-ray crystal structures of [Cu2(μ-Cl)2(κ2-P,P-dppf)2] and [Cu(CNtBu)2(κ2-P,P-dppf)][BF4]

Article abstract of DOI:10.1016/S0022-328X(01)00963-9

Copper(I) complexes [Cu(CNR)₂(κ²-P,P-dppf)][BF₄] (R=^tBu (1), 2,6-Me₂C₆H₃ (2), Cy (3), Bz (4)) containing the chelating dppf ligand (dppf=1,1′-bis(diphenylphosphino)ferrocene) have been prepared by reaction of the complex [Cu₂(μ-Cl)₂(κ²-P,P-dppf)₂] with the corresponding isocyanide C≡NR (molar ratio 1:1). Alternatively, complexes 1-4 can also be obtained by the reaction of the acetonitrile complexes [Cu(MeCN)₂(κ²-P,P-dppf)][BF₄] with 2.5 equivalents of C≡NR in refluxing dichloromethane. The crystal structures of [Cu₂(μ-Cl)₂(κ²-P,P-dppf)₂] and [Cu(CN^tBu)₂(κ²-P,P-dppf)][BF₄], determined by X-ray diffraction, are also reported.

Full text of DOI:10.1016/S0022-328X(01)00963-9

Journal of Organometallic Chemistry 637–639 (2001) 677–682
www.elsevier.com/locate/jorganchem
Copper(I) complexes containing 1,1%-bis(diphenylphosphino)
ferrocene (dppf) as a chelate ligand. X-ray crystal structures of
[Cu2(m-Cl)2(k2-P,P-dppf)2] and [Cu(CNtBu)2(k2-P,P-dppf)][BF4]
Josefina D´ıez a, M. Pilar Gamasa a, Jose´ Gimeno a,*, Maurizio Lanfranchi b,
Antonio Tiripicchio b,*
a Departamento de Qu´ımica Orga´nica e Inorga´nica, Facultad de Qu´ımica, Instituto de Qu´ımica Organometa´lica ‘Enrique Moles’
(Unidad Asociada al CSIC), Uni6ersidad de O6iedo, 33071 O6iedo, Spain
b Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Uni6ersita` di Parma,
Centro di Studio per la Strutturistica Diffrattometrica del CNR, Parco Area delle Scienze 17A, 43100 Parma, Italy
Received 20 February 2001; received in revised form 9 April 2001; accepted 23 April 2001
Abstract
Copper(I) complexes [Cu(CNR)2(k2-P,P-dppf)][BF4] (R=tBu (1), 2,6-Me2C6H3 (2), Cy (3), Bz (4)) containing the chelating
dppf ligand (dppf=1,1%-bis(diphenylphosphino)ferrocene) have been prepared by reaction of the complex [Cu2(m-Cl)2(k2-P,P-
dppf)2] with the corresponding isocyanide CꢀNR (molar ratio 1:1). Alternatively, complexes 1–4 can also be obtained by the
reaction of the acetonitrile complexes [Cu(MeCN)2(k2-P,P-dppf)][BF4] with 2.5 equivalents of CꢀNR in refluxing
dichloromethane. The crystal structures of [Cu2(m-Cl)2(k2-P,P-dppf)2] and [Cu(CNtBu)2(k2-P,P-dppf)][BF4], determined by X-ray
diffraction, are also reported. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Copper(I) complexes; dppf; Isocyanide complexes
1. Introduction
dppf)2] (C) (R=C6H4CH3-4, Ph, CH2OCH3, CH2CH2-
CH3, (h5-C5H4)Fe(h5-C5H5) [7]. Given that these com-
plexes have been obtained from the reaction of the
chelate dppf complex [Cu2(m-Cl)2(k2-P,P-dppf)2] (D)
with LiCꢀCR, the capability of dppf to act as a bridg-
ing ligand forming the [Cu2(m-dppf)2] framework can
also be observed (Chart 1).
Following our interest in preparing novel polynuclear
copper(I) complexes of type C we have tried to synthe-
size novel m-CNR derivatives containing the [Cu2(m-
dppf)2] framework starting from the precursor complex
[Cu2(m-Cl)2(k2-P,P-dppf)2] (D).
The reactions of D with CꢀNR, in the presence of a
chloride abstractor, however, led to mononuclear
cationic copper(I) complexes [Cu(CNR)2(k2-P,P-
dppf)][BF4] (R=tBu (1), 2,6-Me2C6H3 (2), Cy (3), Bz
(4)) in which dppf behaves as a chelating ligand. Here
we report the synthesis and characterization of these
complexes along with the X-ray crystal structures of
complex 1 and of its precursor derivative [Cu2(m-
Cl)2(k2-P,P-dppf)2] (D).
It is well known that the bidentate ligand 1,1%-bis(-
diphenylphosphino)ferrocene (dppf) shows a versatile
coordination mode adapting its steric bite angle to the
geometric requirement of the metal through appropri-
ate ring twisting or tilting [1,2]. Although dppf usually
shows a chelating ability, its bridging behavior in
homo- and heteronuclear complexes is also well docu-
mented [3].
We have recently reported the synthesis of a series of
trinuclear copper(I) complexes of the type [Cu3(m3-h1-
CꢀCR)n(m-dppm)3][BF4]3−n (A) (n=1, 2) [4,5] and
[Cu3(m3-h1-CꢀCR)(m-CNR)(m-dppm)3][BF4]2 (B) [6].
Moreover, we have shown that dppf can also act as a
bridging ligand giving rise to the formation of binuclear
copper(I) complexes of the type [Cu2(m2-h1-CꢀCR)2(m-
* Corresponding authors. Tel.: +34-985-103-461; fax: +34-985-
103-446 (J.G.).
E-mail addresses: jgh@sauron.quimica.uniovi.es (J. Gimeno),
tiri@unipr.it (A. Tiripicchio).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00963-9

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J. D´ıez et al. / Journal of Organometallic Chemistry 6378–639 (2001) 677–682
2. Results and discussion
where
R=tBu (1), 2,6-Me2C6H3 (2), Cy (3), or Bz (4).
Alternatively, complexes 1–4 can also be obtained by
the reaction of the acetonitrile complexes [Cu(Me-
CN)2(k2-P,P-dppf)][BF4] with 2.5 equivalents of CꢀNR
in refluxing dichloromethane. (Eq. (2))
2.1. Synthesis and characterization of
[Cu(CNR)2(s2-P,P-dppf )][BF4] (R=tBu (1),
2,6-Me2C6H3 (2), Cy (3), Bz (4))
The reactions of [Cu2(m-Cl)2(k2-P,P-dppf)2] (D) with
the corresponding isocyanide CꢀNR (molar ratio 1:1)
in CH2Cl2 and in the presence of AgBF4 at room
temperature, lead to the formation of complexes 1–4
which are isolated from the reaction mixture as yellow
insoluble tetrafluoroborate salts in good yields (Eq. (1))
[Cu(MeCN)2(k2-P,P-dppf)][BF4]+2CꢀNR
“[Cu(CNR)2(k2-P,P-dppf)][BF4]+2MeCN
(2)
All the complexes are air stable in the solid state and
soluble in chlorinated solvents. They have been charac-
terized by elemental analyses, conductance measure-
ments and IR and NMR (1H and 31P{1H}) spectro-
scopies. Selected spectroscopic data are given in
[Cu2(m-Cl)2(k2-P,P-dppf)2]+2CꢀNR+2AgBF4
“2[Cu(CNR)2(k2-P,P-dppf)][BF4]+2AgCl
(1)
Chart 1.
Table 1
IR, 31P{1H}, and1H-NMR data for complexes 1–4
Complexes
IR (w(NC) or w(BF))
NMR
31P{1H}
−5.17
−3.6
1H
[Cu(CNtBu)2(k2-P,P-dppf][BF4] (1)
[Cu(CN2,6-Me2C6H3)2(k2-P,P-dppf][BF4] (2)
[Cu(CNCy)2(k2-P,P-dppf][BF4] (3)
[Cu(CNBz)2(k2-P,P-dppf][BF4] (4)
2175 w, 1086 br
2153 w, 1065 br
2178 w, 1052 br
2198 w, 1053 br
1.3 (s, 18H, (Me3)3C), 4.12 (s, 4H, C5H4), 4.43 (s,
4H,C5H4), 7.3–7.6 (m, 20H, Ph)
2.1 (s, 12H, Me), 4.15 (s, 4H, C5H4), 4.4 (s, 4H,
C5H4), 7–7.7 (m, 23H, Ph)
1.25–2.1 (m, 22H, C6H11) 4.1 (s, 4H, C5H4), 4.45 (s,
4H, C5H4), 7.1–7.7 (m, 20H, Ph)
4.1 (s, 4H, PhCH2), 4.4 (s, 4H, C5H4), 4.8 (s, 4H,
C5H4), 7–7.5 (m, 24H, Ph)
−5.02
−5.25
Spectra recorded in CDCl3; l in ppm.

J. D´ıez et al. / Journal of Organometallic Chemistry 637–639 (2001) 677–682
679
Fig. 1. View of the dimeric centrosymmetrical complex D with the atomic numbering system. The ellipsoids are drawn at 30% probability level.
Table 1. Elemental analyses are consistent with the
proposed formulations and the conductivity data (in
Me2CO) show that complexes 1–4 are 1:1 electrolytes.
The IR spectra (KBr) exhibit the expected absorptions
for the BF−4 anion as well as those characteristic for the
dppf and the rest of the ligands. In particular, the
presence of the coordinated terminal CNR ligands is
evidenced by the appearance of one weak w(CN) ab-
sorption band in the range of 2153–2198 cm−1. IR
spectra of other copper(I) complexes with terminal
isocyanide ligands show comparable w(CN) absorptions
[6].
coordinate preference of dppf to act as a chelating
ligand in the presence of isocyanides. The reactions
performed to obtain bridging isocyanide–dppf cop-
per(I) complexes, analogous to C, afforded only the
chelate dppf complexes 1–4.
2.2. Description of the structures of
[Cu(CNtBu)2(s2-P,P-dppf )][BF4]·CH2Cl2, (1·CH2Cl2),
and of its precursor deri6ati6e
[Cu2(v-Cl)2(s2-P,P-dppf )2]·CH2Cl2 (D·CH2Cl2)
A view of the molecular structure of D is shown in
Fig. 1 together with the atomic numbering system.
Selected bond distances and angles are given in Table 2.
In the crystals of D·CH2Cl2 molecules of solvation
are also present. The complex consists of centrosym-
metrical dimeric units with the two chorine atoms
bridging the two copper atoms. The tetrahedral coordi-
nation of the copper atoms is achieved by two P atoms
of the chelating dppf ligand [CuꢁP bond lengths=
1H NMR spectra of complexes 1–4 at room temper-
ature show the expected signals due to aromatic and
cyclopentadienyl protons, along with those assigned to
the isocyanide ligands. The resonance of the h5-C5H4
protons appears as two unresolved multiplets of the
A2B2 spin system at l ca. 4.1–4.8 in accord with the
proposed formulations. 31P{1H}-NMR spectra of com-
plexes 1–4 (Table 1) at room temperature show a single
resonance at l ca. −5 indicating that all phosphorus
atoms in each complex are chemically equivalent. These
chemical shifts are in the range of those found in
copper(I) complexes containing chelating dppf ligands
[7,8]. In order to confirm these features the structure of
complex 1 as well as that of the precursor complex
[Cu2(m-Cl)2(k2-P,P-dppf)2] (D) have been determined by
an X-ray diffraction study (see below).
,
2.275(1) and 2.270(1) A]. The P1ꢁCuꢁP2 bite angle,
111.26(4)°, of the chelating dppf is very close to the
expected value for this coordination. The other coordi-
nation bond angles are rather distorted [(P2ꢁCuꢁCl=
115.95(4),
113.52(4),
P2ꢁCuꢁCl%=118.74(4),
P1ꢁCuꢁCl%=99.65(4),
P1ꢁCuꢁCl=
ClꢁCuꢁCl%=
96.00(3)°], because of the four-membered Cu2Cl2 ring
and of the steric hindrance of the dppf ligand. The two
CuꢁCl bond lengths are asymmetrical, 2.358(1) and
In summary, although the bridging coordination
mode of dppf is well documented, only few examples
have been previously described for copper(I) complexes
and its coordination behavior is strongly dependent on
the accompanying ligands. In this work it is shown the
,
2.434(1) A. The two substituted Cp rings in the dppf
ligand adopt the usual staggered conformation, which
determines the conformational chirality of the Cp2Fe
fragment. The structure of D is strictly comparable to

680
J. D´ıez et al. / Journal of Organometallic Chemistry 6378–639 (2001) 677–682
that of the iodine derivative [Cu2(m-I)2(k2-P,P-dppf)2]
[9].
Table 3
,
Selected bond lengths (A) and bond angles (°) for 1·CH2Cl2
A view of the molecular structure of 1 is shown
Bond lengths
CuꢁP(1)
in Fig.
2 together with the atomic numbering
2.309(2)
1.957(5)
2.023(5)
2.009(5)
2.054(5)
2.047(6)
2.045(5)
1.817(5)
1.822(5)
1.837(5)
1.134(6)
1.482(6)
CuꢁP(2)
2.328(2)
1.956(6)
2.031(5)
2.028(5)
2.053(6)
2.050(5)
2.027(5)
1.786(5)
1.832(5)
1.826(5)
1.135(7)
1.462(8)
system. Selected bond distances and angles are given
in Table 3.
CuꢁC(35)
FeꢁC(1)
FeꢁC(2)
FeꢁC(3)
FeꢁC(4)
CuꢁC(40)
FeꢁC(18)
FeꢁC(19)
FeꢁC(20)
FeꢁC(21)
FeꢁC(22)
P(2)ꢁC(18)
P(2)ꢁC(23)
P(2)ꢁC(29)
N(2)ꢁC(40)
N(2)ꢁC(41)
In the crystals of 1 cationic [Cu(CNtBu)2(k2-P,P-
dppf)] complexes, BF−4 anions and CH2Cl2 molecules
FeꢁC(5)
Table 2
P(1)ꢁC(1)
P(1)ꢁC(6)
P(1)ꢁC(12)
N(1)ꢁC(35)
N(1)ꢁC(36)
,
Selected bond lengths (A) and bond angles (°) for D·CH2Cl2
Bond lengths
CuꢁP(1)
CuꢁCl
2.275(1)
2.358(1)
2.036(3)
2.048(4)
2.058(4)
2.055(4)
2.028(4)
1.799(4)
1.829(4)
1.825(4)
CuꢁP(2)
CuꢁCl%
2.270(1)
2.434(1)
2.045(3)
2.032(4)
2.043(4)
2.048(4)
2.047(4)
1.819(4)
1.829(4)
1.834(4)
Bond angles
FeꢁC(1)
FeꢁC(2)
FeꢁC(3)
FeꢁC(4)
FeꢁC(5)
P(1)ꢁC(1)
P(1)ꢁC(11)
P(1)ꢁC(17)
FeꢁC(6)
FeꢁC(7)
FeꢁC(8)
FeꢁC(9)
FeꢁC(10)
P(2)ꢁC(6)
P(2)ꢁC(23)
P(2)ꢁC(29)
P(1)ꢁCuꢁP(2)
C(35)ꢁCuꢁP(1)
C(40)ꢁCuꢁP(1)
C(1)ꢁFeꢁC(21)
C(2)ꢁFeꢁC(20)
C(3)ꢁFeꢁC(19)
108.96(5)
114.0(2)
103.8(2)
173.1(2)
172.9(2)
171.9(3)
C(40)ꢁCuꢁC(35)
C(35)ꢁCuꢁP(2)
C(40)ꢁCuꢁP(2)
C(4)ꢁFeꢁC(18)
C(5)ꢁFeꢁC(22)
114.3(2)
102.2(2)
113.8(2)
170.6(3)
171.7(2)
Bond angles
P(2)ꢁCuꢁP(1)
P(2)ꢁCuꢁCl
111.26(4)
115.95(4)
113.52(4)
176.0(2)
175.4(2)
176.5(2)
P(2)ꢁCuꢁCl%
P(1)ꢁCuꢁCl%
ClꢁCuꢁCl%
C(4)ꢁFeꢁC(7)
C(5)ꢁFeꢁC(8)
118.74(4)
99.65(4)
96.00(3)
177.8(2)
177.5(2)
of solvation are present. The cation adopts a pseudo
C2 symmetry, with the two-fold axis passing through
the metal atoms. The tetrahedral coordination around
Cu involves two C atoms from CNtBu ligands [CuꢁC
P(1)ꢁCuꢁCl
C(1)ꢁFeꢁC(9)
C(2)ꢁFeꢁC(10)
C(3)ꢁFeꢁC(6)
,
bonds=1.956(6) and 1.957(5) A] and two P atoms
from the chelating dppf. The CuꢁP bond lengths,
Symmetry transformations used to generate equivalent atoms: −x+
1/2, −y+1/2, −z+1.
,
2.309(2) and 2.328(2) A, are significantly longer that
those found in D. The P1ꢁCuꢁP2 bite angle,
108.96(5)°, of the chelating dppf is very similar to that
found in D. The values of other angles involved in the
tetrahedral coordination are in the range 102.2(2)–
114.3(2)°. As in D the Cp2Fe fragment shows a con-
formational chirality in the crystals, however, both
enantiomers are present.
3. Experimental
The reactions were carried out under dry N2 using
Schlenk techniques. All solvents were dried by stan-
dard methods and distilled under N2 before use. IR
spectra were recorded (4000–400 cm−1) on a Perkin–
Elmer 1720-X FT spectrometer using KBr pellets. The
C, N and H analyses were carried out with a Pelkin–
Elmer 240-B microanalyzer. Conductivities of Me2CO
solutions in ca. 5×10−4 mol dm−3 were measured
with a Jenway PCM3 conductimeter. NMR spectra
were recorded on Bruker AC300 at 300MHz (1H) and
121.5 MHz (31P) using Me4Si or 85% H3PO4 as stan-
dards. The complexes [Cu2(m-Cl)2(k2-P,P-dppf)2] and
[Cu(MeCN)2(k2-P,P-dppf)][BF4] were prepared as de-
scribed in the literature [7]. IR and NMR spectro-
scopic data for all the new complexes are collected in
Table 1.
Fig. 2. View of the cationic complex of 1 with the atomic numbering
system. The ellipsoids are drawn at 30% probability level.

J. D´ıez et al. / Journal of Organometallic Chemistry 637–639 (2001) 677–682
681
3.1. [Cu(CNR)2(s2-P,P-dppf )][BF4] (R=tBu (1),
2,6-Me2C6H3 (2), Cy (3), Bz (4))
N, 3.21%. Anal. Found: C, 63.99; H, 4.73; N, 2.79.
Calc. for C52H46N2BCuF4P2Fe (2): C, 64.58; H, 4.79;
N, 2.89%. Anal. Found: C, 62.69; H, 5.38; N, 2.98.
Calc. for C48H50N2BCuF4P2Fe (3): C, 62.46; H, 5.46;
N, 3.03%. Anal. Found: C, 63.93; H, 4.51; N, 2.91.
Calc. for C50H40N2BCuF4P2Fe (4): C, 63.95; H, 4.51;
N, 2.98%.
3.1.1. Procedure A
A mixture of the corresponding isocyanide (0.30
mmol), [Cu2(m-Cl)2(k2-P,P-dppf)2] (0.1 g, 0.15 mmol)
and AgBF4 (0.059 g, 0.30 mmol) in CH2Cl2 (20 ml) was
stirred at room temperature (r.t.) for 4 h. The resulting
solution after filtration was concentrated under vac-
uum. The addition of Et2O (20 ml) to the concentrated
solution led to the precipitation of yellow solids, which
were recrystallized from CH2Cl2–Et2O. Yields: 50–
65%. \M (V−1 cm2 mol−1) 1=114, 2=111, 3=120,
and 4=116. Anal. Found: C, 59.06; H, 5.28; N, 3.08.
Calc. for C44H46N2BCuF4P2Fe (1): C, 60.67; H, 5.32;
3.1.2. Procedure B
The corresponding isocyanide (0.625 mmol.) was
added to a solution of [Cu(MeCN)2(k2-P,P-dppf)][BF4]
(0.23 g, 0.25 mmol) in CH2Cl2 (20 ml) and the mixture
was stirred and heated to reflux for 4 h. The addition of
Et2O (20 ml) to the concentrated solution led to the
precipitation of yellow solids, which were washed with
Et2O (3×10 ml) and vacuum-dried. Analytically pure
samples were obtained by recrystallization from
CH2Cl2–Et2O. Yields: 60–75%.
Table 4
Crystal data and structure refinement parameters for [Cu2(m-Cl)2(k2-
P,P-dppf)2]·CH2Cl2 and [Cu(CNtBu)2(k2-P,P-dppf)][BF4]·CH2Cl2
3.2. X-ray crystal structure determination of D·CH2Cl2
and 1·CH2Cl2
[Cu2(m-Cl)2-
(k2-P,P-dppf)2]·
CH2Cl2
[Cu(CNtBu)2-
(k2-P,P-dppf)][BF4]·
CH2Cl2
Diffraction data were recorded at 293(2) K with the
q–2q technique on a Siemens AED diffractometer for
both compounds. The structures were solved by direct
methods (SIR92) [10] and refined by full-matrix least-
squares methods based on F2 using the SHELXL-97 [11]
program. All the non-hydrogen atoms were refined
anisotropically. The chlorine atoms of CH2Cl2 in
D·CH2Cl2 were found disordered as well as the methyl
Empirical formula
Formula weight
C69H58Cl4Cu2Fe2P4 C45H48BCl2CuF4FeN2P2
1391.61
0.71073
Monoclinic
C2/c
955.89
,
Wavelength (A)
0.71073
Monoclinic
P21/n
Crystal system
Space group
Unit cell dimensions
,
a (A)
24.801(5)
13.393(3)
18.751(4)
93.130(10)
6219(2)
4
18.280(6)
15.221(5)
18.498(6)
115.65(2)
4640(3)
4
,
b (A)
t
,
c (A)
i (°)
V (A )
carbon atoms of the Bu groups in 1·CH2Cl2. Crystal
data and some details of the structure determination
are listed in Table 4. All calculations were carried out
on the DIGITAL AlphaStation 255 of the ‘Centro di
Studio per la Strutturistica Diffrattometrica’ del CNR,
Parma.
3
,
Z
Dcalc (Mg m−3
)
1.486
1.368
Absorption coefficient 1.449
(mm−1
F(000)
1.004
)
2840
1968
Crystal size (mm)
q range for data
collection (°)
0.12×0.15×0.25
3.04–28.03
0.16×0.21×0.34
3.32–25.00
4. Supplementary material
Index ranges
−325h532,
−65k517,
−245l523
7721
−215h521,
−185k517,
−135l521
8384
Crystalllographic data for the structural analysis
have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC nos. 158660 and 158661
for compounds D·CH2Cl2 and 1·CH2Cl2, respectively.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://www.
ccdc.cam.ac.uk).
Reflections
collected/unique
Unique reflections
Data/restraints/
parameters
Final R indices
[I\2|(I)]
7507 [Rint=0.0477] 8127 [Rint=0.0322]
7507/0/417
8127/24/579
R1=0.0403,
wR2=0.0869
R1=0.0928,
wR2=0.1224
R1=0.0696,
wR2=0.1983
R1=0.1030,
wR2=0.2251
1.096
R indices (all data)
Goodness-of-fit on F2 1.087
Largest difference
0.405 and −0.443 1.285 and −0.647
peak and hole
Acknowledgements
−3
,
(e A
)
This work was supported by the Direccio´n General
de Investigacio´n Cient´ıfica y Te´cnica of Spain (DGI-
CYT). Project PB96-0558.
GOF=[ꢀ[w(F2o−F2c)2]/(n−p)]1/2
,
R1=ꢀꢁ ꢁFoꢁ−ꢁFcꢁ ꢁ/ꢀꢁFoꢁ, wR2=
[ꢀ[w(Fo2−Fc2)2]/ꢀ[w(Fo2)2]]1/2, w=1/[|2(F2o)+(aP)2+bP], where P=
[max(F2o,0)+2F2c]/3.

682
J. D´ıez et al. / Journal of Organometallic Chemistry 6378–639 (2001) 677–682
Os: (k) Y.-Y. Choi, W.-T. Wong, J. Organomet. Chem. 542
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