Unusual stability of reaction intermediates in ortho-metalation reactions of dicyclohexylphenylphosphane with dirhodium(II) tetraacetate

Article abstract of DOI:10.1016/j.jorganchem.2013.02.030

Reaction of dirhodium(II) tetraacetate with 1 molar equivalent of dicyclohexylphenylphosphane afforded the complex [Rh₂(μ-O ₂CCH₃)₃{μ-(C₆H ₄)PCy₂}(CH₃COOH)₂] (1) in which the phosphane ligand is coordinated to the rhodium atoms in a bridging ortho-metalated mode. As a second product, [Rh₂(μ-O ₂CCH₃)₃{μ-(C₆H ₄)PCy₂}(CH₃COOH)(PhPCy₂)] (2) was isolated from the same reaction. 2 proved to be unusually stable toward further reaction to a doubly ortho-metalated complex which has been accessible in the reaction of dirhodium(II) tetraacetate with other phosphane ligands. However, doubly ortho-metalated [Rh₂(μ-O₂CCH₃) ₂{μ-(C₆H₄)PCy₂} ₂(CH₃COOH)₂] (4) was isolated in low yield after a prolonged thermal reaction between dirhodium(II) tetraacetate and 2 equivalents of PhPCy₂. Exchange of the bridging acetate ligands in 1 and 4 by trifluoroacetate ligands afforded [Rh₂(μ-O ₂CCF₃)₃{μ-(C₆H ₄)PCy₂}(CF₃COOH)₂] (5) and [Rh ₂(μ-O₂CCF₃)₂{μ-(C ₆H₄)PCy₂}₂(CF₃COOH) ₂] (7a). Complex 7a without axial ligands, [Rh₂(μ- O₂CCF₃)₂{μ-(C₆H ₄)PCy₂}₂] (7b) was isolated, as well. The crystal structure of complex 5 was determined showing consequences of the bulky cyclohexyl groups on several structural parameters.

Full text of DOI:10.1016/j.jorganchem.2013.02.030

Journal of Organometallic Chemistry 733 (2013) 53e59
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Unusual stability of reaction intermediates in ortho-metalation reactions of
dicyclohexylphenylphosphane with dirhodium(II) tetraacetate
*
Konrad Herbst , Mercedes Sanaú, Francisco Estevan, M. Angeles Úbeda
Departamento de Química Inorgánica, Facultad de Química, Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of dirhodium(II) tetraacetate with 1 molar equivalent of dicyclohexylphenylphosphane afforded
Received 15 January 2013
Received in revised form
14 February 2013
the complex [Rh2(
nated to the rhodium atoms in a bridging ortho-metalated mode. As a second product, [Rh2(
O2CCH3)3{ -(C6H4)PCy2}(CH3COOH)(PhPCy2)] (2) was isolated from the same reaction. 2 proved to be
m-O2CCH3)3{m-(C6H4)PCy2}(CH3COOH)2] (1) in which the phosphane ligand is coordi-
m
-
m
Accepted 19 February 2013
unusually stable toward further reaction to a doubly ortho-metalated complex which has been accessible
in the reaction of dirhodium(II) tetraacetate with other phosphane ligands. However, doubly ortho-
Keywords:
Rhodium
ortho-Metalation
Acetate
Trifluoroacetate
metalated [Rh2(
thermal reaction between dirhodium(II) tetraacetate and 2 equivalents of PhPCy2. Exchange of the
bridging acetate ligands in 1 and 4 by trifluoroacetate ligands afforded [Rh2( -O2CCF3)3{ -(C6H4)
PCy2}(CF3COOH)2] (5) and [Rh2( -O2CCF3)2{ -(C6H4)PCy2}2(CF3COOH)2] (7a). Complex 7a without axial
ligands, [Rh2( -O2CCF3)2{ -(C6H4)PCy2}2] (7b) was isolated, as well. The crystal structure of complex 5
m-O2CCH3)2{m-(C6H4)PCy2}2(CH3COOH)2] (4) was isolated in low yield after a prolonged
m
m
m
m
m
m
was determined showing consequences of the bulky cyclohexyl groups on several structural parameters.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
ligands have been structurally characterized over the last years by
the group of Lahuerta et al. [5e10].
The intramolecular CeH activation of aryl phosphane ligands
by dirhodium(II) tetraacetate has been subject of detailed in-
vestigations since the discovery of this reaction type in 1984 [1,2].
Formally being an acidebase reaction, a phenyl proton in ortho
position to the PeC bond protonates a bridging acetate ligand
which is subsequently removed from the Rh dimer as a molecule of
free acetic acid. The phosphane ligand then adopts a bridging co-
ordination mode to the rhodium atoms with one aryl ring being
metalated at one of the ortho positions. At maximum two of the
Doubly metalated rhodium(II) complexes are inherently chiral
when the phosphane ligands adopt a head-to-tail configuration
[11]. Substitution of the acetate ligands for chiral carboxylate li-
gands leads to diasteromers whose separation by column chro-
matography succeeded in a number of cases [11e13]. The separated
diastereomers may be back-transformed to a pair of (now resolved)
enantiomers by reaction with a non-chiral carboxylic acid. Doubly
ortho-metalated rhodium(II) carboxylate complexes are active
catalysts in CeH insertion and cyclopropanation reactions of
bridging acetate ligands in [Rh(
m-O2CCH3)4] can be substituted by
a-diazo ketones [12e17]. The possibility of applying chiral catalysts
such ortho-metalated phosphane ligands [3]. In terms of a general
reaction scheme, ortho-metalation at a rhodium(II) dimer is initi-
ated by the coordination of the phosphane ligands in axial position
to the rhodium atoms which is possible in solution already at
ambient conditions. Rearrangement of the phosphane ligands into
equatorial position and CeH activation usually requires thermal
reaction conditions (e.g. in mixtures of boiling toluene/acetic acid)
[4]. Several reaction intermediates with various triaryl phosphane
in these types of reactions has in recent years increased the interest
in this type of enantiopure rhodium(II) complexes, since the
product enantioselectivity can be effectively controlled by the
electronic and steric properties of the organic substituents at the
carboxylate and phosphane ligands [18,19]. The experimental re-
sults were also supported by quantum chemical density functional
theory calculations on the electronic properties of the ligands [20].
The reaction pathways of many phosphane ligands toward
dirhodium(II) tetraacetate have been systematically investigated. In
some cases these studies delivered unexpected results, the latest
example being the ring rearrangement of a thiophene ring in a
tris(2-thienyl)phosphane ligand coordinated to the Rh dimer [21].
A previous example is the rearrangement of an ortho-metalated
* Corresponding author. Present address: Haldor Topsøe A/S, Nymøllevej 55, DK-
2800 Lyngby, Denmark. Tel.: þ45 22496679.
E-mail address: knh@topsoe.dk (K. Herbst).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.02.030

54
K. Herbst et al. / Journal of Organometallic Chemistry 733 (2013) 53e59
methyldiphenylphosphane ligand by reaction of the complex with
two equivalents of PPh3 resulting in the metalation of the methyl
group [9].
stirring for 30 min, the solvent was evaporated in a vacuum. The
oily residue can be crystallized from a mixture of CH2Cl2/hexane.
Yield: 110 mg (0.11 mmol, 96%) 1H NMR (CDCl3,
d/ppm): 0.80e1.97
In the present study, we haveinvestigated the reaction of another
mixed alkylearyl phosphane, dicyclohexylphenylphosphane, to-
ward dirhodium(II) tetraacetate. Two factors, the bulkiness of the
cyclohexyl groups and the increase of the basicity at the phosphorus
atom induced by the cyclohexyl groups, make PhPCy2 an interesting
ligand for studies of the coordinative behavior toward dirhodium(II)
acetate.
(m, 44H, C6H11); 1.65 (s, 6H, CH3); 2.09 (s, 3H, CH3); 2.25 (s, 3H,
CH3); 6.35 (t, J ¼ 7.5 Hz, 1H, C6H4); 6.63 (t, J ¼ 7.2 Hz, 1H, C6H4); 6.85
(m, 1H, C6H4); 7.32e7.48 (m, 4H, C6H4 þ C6H5); 7.69 (t, J ¼ 7.4 Hz,
2H, C6H5). 13C{1H} NMR (CDCl3,
d/ppm): 14.2 (s, CH3); 20.8 (s, CH3);
22.7 (s, CH3); 23.8e37.0 (cyclohexyl carbons); 119.0e147.5 (aro-
matics); 157.1 (dd, 1J(RhC) ¼ 32 Hz, 2J(PC) ¼ 20 Hz, metalated C);
176.0 (s, COO); 180.5 (d, J ¼ 2.1 Hz, COO); 188.0 (s, COO). 31P{1H}
NMR (CDCl3,
3J(PP’)
d
/ppm): ꢁ16.8 (ddd, 1J(RhP) ¼ 111 Hz, 2J(RhP) ¼ 33 Hz,
2. Experimental
¼
12 Hz, axial P); 28.3 (ddd, 1J(RhP)
¼
155 Hz,
2J(RhP) ¼ 2 Hz). UV/vis (CHCl3, lmax/nm (ε/dm3 molꢁ1 cmꢁ1)): 258
(9700); 303sh (9000); 338 (16,000); 523 (1200). Anal. Calcd. for
C44H66O8P2Rh2$½C6H14 (1033.85): C, 54.60; H, 7.12. Found: C,
54.36; H, 7.32.
2.1. General
All preparations were carried out under an atmosphere of dry
nitrogen using Schlenk techniques. Solvents were dried and
distilled from standard drying agents prior to use. Solvent mixtures
are on volume/volume basis. 1H, 13C{1H}, 19F{1H} and 31P{1H} NMR
spectra were recorded on a Bruker AC-300 FT spectrometer as
CDCl3 solutions. Chemical shifts are reported in ppm, using TMS
2.4. Synthesis of [Rh2(m-O2CCH3)2(h-O2CCH3){m-(C6H4)PCy2}
(CH3COOH)(PhPCy2)] (3)
A solution of 2 (40 mg, 0.04 mmol) in CDCl3 (0.7 ml) was irra-
diated by a Hgequartz lamp for 1 h. The color of the solution slowly
changed to dark-brown. The reaction product was identified by
(1H, 13C), CFCl3 19F) and 85% H3PO4 31P) as references. The
( (
coupling constants (J) are in hertz (Hz). UV/vis spectra were
recorded on a Varian Cary 300 Bio UV/visible spectrophotometer.
Solutions for UV/vis spectroscopy were prepared by dissolution of a
weighted amount of the complex (10e20 mg) in 50 ml CHCl3
containing 1% of ethanol with addition of one drop of CH3COOH or
CF3COOH. X-ray structure determination was carried out on a
NMR spectroscopy in solution. 1H NMR (CDCl3,
d/ppm): 0.7e2.3 (m,
44H, C6H11); 1.24 (s, 3H, CH3); 2.02 (s, 3H, CH3); 2.11 (s, 6H, CH3);
6.68 (t, J ¼ 7.3 Hz, 1H, C6H4); 6.78 (t, J ¼ 7.5 Hz, 1H, C6H4); 6.94 (t,
J ¼ 7.4 Hz, 1H, C6H4); 7.15e7.57 (m, 3H, C6H5); 7.60e7.80 (m, 2H,
C6H5); 8.02 (m, 1H, C6H4). 31P{1H} NMR (CDCl3,
d/ppm): 18.7 (dd,
BrukereNonius Kappa CCD diffractometer (Mo
K
a
radiation,
1J(RhP) ¼ 141 Hz, 2J(RhP) ¼ 9 Hz), 36.1 (dd, 1J(RhP) ¼ 178 Hz,
l
¼ 0.71073 A). Chemical analyzes were performed at the Centro
2J(RhP) ¼ 4 Hz).
ꢀ
Microanálisis Elemental, Universidad Complutense de Madrid,
Spain.
2.5. Synthesis of [Rh2(
m-O2CCH3)2{m-(C6H4)PCy2}2(CH3COOH)2] (4)
[Rh2(m-O2CCH3)4(CH3OH)2] was purchased from Pressure
Chemical Co. PhPCy2 was purchased from Aldrich.
[Rh2( -O2CCH3)4(CH3OH)2] (170 mg, 0.34 mmol) and PhPCy2
m
(234 mg, 0.85 mmol) in a mixture of toluene:acetic acid (3:1,
30 ml) were heated to reflux for 24 h. After evaporating the sol-
vents to dryness, the crude product was dissolved in CH2Cl2 (4 ml)
for chromatographic purification on silica-gel (hexane, 40 ꢀ 3 cm).
Elution with hexane:CH2Cl2:acetic acid (100:100:1) afforded a vi-
olet band (without acetic acid: green band) which was collected.
Evaporation of the solvents in a vacuum resulted in a violet pow-
2.2. Synthesis of [Rh2(
m-O2CCH3)3{m-(C6H4)PCy2}(CH3COOH)2] (1)
[Rh2( -O2CCH3)4(CH3OH)2] (506 mg, 1.00 mmol) and PhPCy2
m
(275 mg, 1.00 mmol) in a mixture of toluene:acetic acid (3:1, 30 ml)
were heated to reflux for 2 h. After evaporating the solvents to
dryness, the crude product was dissolved in CH2Cl2 (4 ml) for
chromatographic purification on silica-gel (hexane, 40 ꢀ 3 cm).
Elution with hexane:CH2Cl2:CH3COOH (40:20:1 by volume) affor-
der. Yield: 135 mg (0.14 mmol, 41%). 1H NMR (CDCl3,
d/ppm): 0.44e
1.97 (m, 44H, C6H11); 2.03 (s, 6H, CH3); 2.15 (s, 6H, CH3); 6.69 (t,
ded as first fraction a small magenta colored band of [Rh2(
m
-
J ¼ 7.1 Hz, 2H, C6H4); 6.79 (t, J ¼ 7.2 Hz, 2H, C6H4); 6.95 (t,
O2CCH3)3{ -(C6H4)PCy2}(CH3COOH)(PhPCy2)] (2), then a major
m
J ¼ 7.1 Hz, 2H, C6H4); 7.97 (m, 2H, C6H4). 13C{1H} NMR (CDCl3,
d/
blue band of 1. After evaporation of the solvents, a dark blue
ppm): 22.1 (s, CH3); 24.2 (s, CH3); 35.9e37.1 (cyclohexyl car-
bons); 120.2e146.0 (aromatics); 163.6 (m, metalated C); 178.6 (s,
powder was obtained. Yield: 545 mg (0.70 mmol, 70%). 1H NMR
(CDCl3,
(s, 6H, CH3); 2.36 (s, 3H, CH3); 6.95 (m, 2H, C6H4); 7.17 (m,1H, C6H4);
8.61 (m, 1H, C6H4). 13C{1H} NMR (CDCl3,
/ppm): 21.6 (s, CH3); 22.8
d/ppm): 0.83e2.21 (m, 22H, C6H11); 1.68 (s, 6H, CH3); 2.25
COO); 181.4 (s, COO). 31P{1H} NMR (CDCl3,
d/ppm): 18.7 (AA‘XX’
system). UV/vis (CHCl3, lmax/nm (ε/dm3 molꢁ1 cmꢁ1)): 243
(20,400); 295 (13,800); 385 (490); 564 (330). Anal. Calcd. for
C44H66O8P2Rh2$C6H14 (1076.94): C, 55.76; H, 7.49. Found: C, 55.77;
H, 6.91.
d
(s, CH3); 24.0 (s, CH3); 26.3e37.6 (cyclohexyl carbons); 121.3e143.1
(aromatics); 161.1 (dd, 1J(RhC) ¼ 33 Hz, 2J(PC) ¼ 20 Hz, metalated
C); 178.8 (s, COO); 182.2 (d, J ¼ 2.1 Hz, COO trans P); 189.6 (s, COO).
31P{1H} NMR (CDCl3,
d
/ppm): 22.2 (dd, 1J(RhP)
¼
151 Hz,
2.6. Synthesis of [Rh2(m-O2CCF3)3{m-(C6H4)PCy2}(CF3COOH)2] (5)
2J(RhP) ¼ 6 Hz). UV/vis (CHCl3, lmax/nm (ε/dm3 molꢁ1 cmꢁ1)): 243
(10,000); 276 (9900); 381sh (300); 565 (190); 626 (190). Anal.
Calcd. for C28H43O10PRh2 (776.43): C, 43.31; H, 5.58. Found: C,
43.81; H, 5.46.
CF3COOH (2 ml) was added to a solution of 1 (100 mg,
0.13 mmol) in CHCl3 (20 ml). After 24 h of stirring, the solvents were
evaporated in a vacuum. The green solid was re-dissolved in
CHCl3:CF3COOH (10:1) and the process repeated. Yield: 132 mg
2.3. Synthesis of [Rh2(m-O2CCH3)3{m-(C6H4)PCy2}(CH3COOH)(PhPCy2)]
(0.13 mmol, 98%). 1H NMR (CDCl3,
d
/ppm): 0.80e2.48 (m, 22H,
(2)
C6H11); 7.02e7.16 (m, 2H, C6H4); 7.29 (t, J ¼ 5.9 Hz, 1H, C6H4); 8.42
(m, 1H, C6H4); 8.83 (s, br, 2H, COOH). 13C{1H} NMR (CDCl3,
d/ppm):
A solution of PhPCy2 (30 mg, 0.11 mmol) in CHCl3 (2 ml) was
added to a solution of 1 (85 mg, 0.11 mmol) in CHCl3 (20 ml). The
color of the solution changed immediately to magenta. After
25.8e37.8 (cyclohexyl carbons); 111.3 (q, 1J(CF) ¼ 290 Hz, CF3);
114.3 (q, 1J(CF) ¼ 285 Hz, CF3); 115.2 (q, 1J(CF) ¼ 282 Hz, CF3);
123.2e141.8 (aromatics); 153.1 (dd, 1J(RhC) ¼ 31 Hz, 2J(PC) ¼ 17 Hz,

K. Herbst et al. / Journal of Organometallic Chemistry 733 (2013) 53e59
55
Δ
Scheme 1. Synthesis of 1 and 2.
metalated C); 163.5 (q, 2J(CF)
¼
43 Hz, COO); 168.0 (q,
2.9. Synthesis of [Rh2(m-O2CCF3)2{m-(C6H4)PCy2}2] (7b)
2J(CF) ¼ 40 Hz, COO trans P); 174.6 (q, 2J(CF) ¼ 40 Hz, COO); 19F{1H}
NMR (CDCl3,
d
/ppm): ꢁ75.7 (s, 6F); ꢁ75.3 (s, 3F); ꢁ74.9 (6F).
The compound was obtained by heating 7a at 80 ꢂC for 1 h in a
31P{1H} NMR (CDCl3,
d
/ppm): 22.7 (dd, 1J(RhP)
¼
142 Hz,
vacuum. 1H NMR (CDCl3,
/ppm): 0.52e2.10 (m, 44H, C6H11); 6.75
(m, 2H, C6H4); 6.90 (t, J ¼ 7.2 Hz, 2H, C6H4); 7.00 (t, J ¼ 7.2 Hz, 2H,
C6H4); 7.62 (m, 2H, C6H4). 19F{1H} NMR (CDCl3,
d
2J(RhP) ¼ 5 Hz). UV/vis (CHCl3, lmax/nm (ε/dm3 molꢁ1 cmꢁ1)): 245
(12,800); 276sh (8300); 388sh (370); 576 (190); 647 (160). Anal.
Calcd. for C28H28F15O10PRh2 (1046.28): C, 32.14; H, 2.70. Found C,
32.39; H, 2.78.
d/ppm): ꢁ75.0 (s).
Anal. Calcd. for C40H52F6O4P2Rh2 (978.60): C, 49.09; H, 5.36. Found:
C, 49.25; H, 5.68.
2.10. Crystallographic study
2.7. Synthesis of [Rh2(m-O2CCF3)3{m-(C6H4)PCy2}(CF3COOH)(PhPCy2)]
(6)
The structure was solved by direct methods using the SHELXTL
software package [22]. The correct positions for the heavy atoms
were deduced from an E-map. Subsequent least-squares refine-
ment and difference Fourier calculations revealed the positions of
the remaining non-hydrogen atoms. Hydrogen atoms were placed
in geometrically generated positions and refined riding on the atom
to which they are attached.
The same procedure as for the synthesis of 2 was employed
using 5 (115 mg, 0.11 mmol) and PhPCy2 (30 mg, 0.11 mmol).
Yield: 130 mg (0.11 mmol, 98%). 1H NMR (CDCl3,
d/ppm): 0.80e
2.40 (m, 44H, C6H11); 6.44 (t, J ¼ 8.2 Hz, 1H, C6H4); 6.74 (t,
J ¼ 7.1 Hz, 1H, C6H4); 6.90 (m, 1H, C6H4); 7.35e7.52 (m, 4H,
C6H5 þ C6H4); 7.63 (t, J ¼ 8.2 Hz, 2H, C6H5). 13C{1H} NMR (CDCl3,
d/
ppm): 25.6e37.5 (cyclohexyl carbons); 111.1 (q, 1J(CF) ¼ 288 Hz,
CF3); 114.7 (q, 1J(CF) ¼ 288 Hz, CF3); 115.4 (q, 1J(CF) ¼ 282 Hz, CF3);
3. Results and discussion
121.3e148.0 (aromatics); 150.0 (dd, 1J(RhC)
¼
31 Hz,
2J(PC) ¼ 18 Hz, metalated C); 160.2 (q, 2J(CF) ¼ 40 Hz, COO); 166.9
3.1. Syntheses and spectroscopic characterizations
(q, 2J(CF) ¼ 39 Hz, COO); 173.3 (q, 2J(CF) ¼ 39 Hz, COO). 19F{1H}
NMR (CDCl3,
31P{1H} NMR (CDCl3,
2J(RhP)
d
/ppm): ꢁ76.0 (s, br, 3F); ꢁ75.0 (s, 6F); ꢁ74.9 (s, 3F).
An established procedure for the synthesis of singly and doubly
ortho-metalated dirhodium(II) phosphane complexes consists of a
thermal reaction of dirhodium(II) tetraacetate with one or two
equivalents of the phosphane ligand in a boiling mixture of toluene
and acetic acid (3:1). These conditions generally lead within 2 h
reaction time to a formation of the singly or doubly-metalated
complex in high yield, a side product in the synthesis of the
singly-metalated complex being small amounts of the doubly-
metalated compound. However, employing these conditions in
d
/ppm): ꢁ14.4 (ddd, 1J(RhP) ¼ 113 Hz,
¼
25 Hz, 3J(PP’)
¼
10 Hz, axial P); 26.1 (dd,
1J(RhP) ¼ 149 Hz). UV/vis (CHCl3, lmax/nm (ε/dm3 molꢁ1 cmꢁ1)):
243 (13,500); 289 (10,500); 354 (11,800); 526 (1400). Anal. Calcd.
for C44H54F12O8P2Rh2 (1206.64): C, 43.80; H, 4.51. Found: C, 43.58;
H, 4.51.
2.8. Synthesis of [Rh2(m-O2CCF3)2{m-(C6H4)PCy2}2(CF3COOH)2] (7a)
CF3COOH (1 ml) was added to a solution of 4 (50 mg,
0.05 mmol) in CHCl3 (10 ml). After stirring for 1 h, the product was
purified by column chromatography on silica-gel (hexane,
20 ꢀ 2 cm). Elution with CH2Cl2:hexane:CF3COOH (200:100:1)
yielded a green band which was collected and evaporated to
dryness. Repeated evaporation of the residue from hexane resul-
ted in crystallization of the product as a green powder. Yield:
2000
Rh2ac4
(1)
(2)
1500
1000
500
0
53 mg (0.044 mmol, 87%). 1H NMR (CDCl3,
d/ppm): 0.59e2.05 (m,
44H, C6H11); 5.45 (s, br, 2H, COOH); 6.77 (m, 2H, C6H4); 6.92 (t,
J ¼ 7.6 Hz, 2H, C6H4); 7.02 (t, J ¼ 7.8 Hz, 2H, C6H4); 7.65 (m, 2H,
C6H4). 13C{1H} NMR (CDCl3,
d/ppm): 25.5e38.5 (cyclohexyl car-
bons); 114.4 (q, 1J(CF) ¼ 286 Hz, CF3); 114.9 (q, 1J(CF) ¼ 287 Hz,
CF3); 121.9e146.0 (aromatics); 157.5 (m, metalated C); 161.7 (q,
2J(CF) ¼ 43 Hz, COO); 167.3 (q, 2J(CF) ¼ 39 Hz, COO). 19F{1H} NMR
300
400
500
600
700
800
/ppm): ꢁ76.2 (s, 6F); ꢁ75.5 (s, 6F). 31P{1H} NMR (CDCl3,
d/
λ / nm
(CDCl3,
d
ppm): 18.5 (AA‘XX’ system). UV/vis (CHCl3, lmax/nm
(ε/dm3 molꢁ1 cmꢁ1)): 255 (15,300); 293 (12,700); 410 (390); 651
(280). Anal. Calcd. for C44H54F12O8P2Rh2 (1206.64): C, 43.80; H,
4.51. Found: C, 44.31; H, 4.51.
Fig. 1. UV/vis spectra of [Rh2(
PCy2}(CH3COOH)2] (1) and [Rh2(
dissolved in CHCl3 containing 1% of ethanol. One drop of acetic acid was added to each
solution.
m
m
-O2CCH3)4(CH3COOH)2], [Rh2(
m
-O2CCH3)3{
m
-(C6H4)
-O2CCH3)3{m-(C6H4)PCy2}(CH3COOH)(PhPCy2)] (2)

56
K. Herbst et al. / Journal of Organometallic Chemistry 733 (2013) 53e59
ν,
Scheme 2. Reactivity of compound 3.
the reaction between dirhodium(II) tetraacetate and PhPCy2 in a
1:1 ratio lead to a deep-blue solution, from which several com-
pounds could be isolated after column chromatographic separation
on silica (Scheme 1). An initial minor magenta-colored band eluted
with hexane:CH2Cl2:acetic acid (40:20:1) was followed by a major
blue fraction, whereas a green band stayed on the top of the col-
umn. The blue band was identified as the singly ortho-metalated
phosphane ligand is strongly shielded by the rhodium atoms to a
signal centered at
d
¼ ꢁ16.2 ppm. Both phosphane ligands resonate
as ddd signals with a joint coupling constant 3J(PP’) of 12 Hz. In the
electronic spectra of 1 and 2 (Fig. 1), distinct differences are
observed in the visible region. A pale blue solution of 1 in CHCl3
shows two merged bands of low intensity at 626 and 565 nm.
Bands of singly ortho-metalated Rh2-phosphane complexes in this
complex [Rh2(m-O2CCH3)3{m-(C6H4)PCy2}(CH3COOH)2] (1) with
region have previously been assigned to
p*(Rh2) / s*(Rh2) and
two acetic acid ligands in axial position (Scheme 1). 1 was isolated
in a moderate yield of 70%. The magenta-colored band consisted of
a second singly ortho-metalated complex containing one acetic acid
p
*(Rh2) / *(RheLeq) transitions, respectively [25e28]. In 1,
d
however, these bands are red-shifted by 42 and 128 nm in com-
parison with the bands of dirhodium(II) tetraacetate. Red-shifts of
similar size have been observed previously [27]. Coordination of a
second phosphane ligand to 1 in axial RheRh direction creates the
deeply magenta-colored complex 2. In the UV/vis spectrum of 2,
this gives rise to an intensification of the band for the
and one PhPCy2 ligand in axial position, [Rh2(m-O2CCH3)3{m-(C6H4)
PCy2}(CH3COOH)(PhPCy2)] (2). Though the coordination of the
second phosphane is thinkable in any of the two axial positions of 1
resulting in the formation of two isomers, only the one with the
phosphane axially coordinated to the rhodium atom bonded to the
metalated carbon was observed. The formation of both isomers was
observed for other phosphanes [9,23]. In the present case, the size
of the cyclohexyl groups probably prevented the formation of the
second isomer.
The isolation of this complex by column chromatography is
remarkable, since the contact with eluate liquor containing acetic
acid usually substitutes any axially coordinated phosphane ligand
on rhodium(II) complexes [3]. 2 is an intermediate in the formation
of a doubly ortho-metalated diphosphane rhodium(II) complex.
Attempts to suppress the formation of 2 by altering the reaction
time were unsuccessful. Consequently, unreacted dirhodium(II)
tetraacetate was present on the column as a green band in all at-
tempts to optimize the yield of 1. On the other hand, 2 was formed
quantitatively by an instantaneous reaction of 1 and one equivalent
of PhPCy2 in CHCl3 (Scheme 1). It can be crystallized from hexane
containing traces of CH2Cl2 as a magenta-colored powder.
In the 31P{1H} NMR spectrum, the phosphane ligand of 1 reso-
nates in the typical range of mono-metalated rhodium(II) com-
p*(Rh2) /
s
*(Rh2) transition at
l
¼ 523 nm. The second band is
presumably hidden under an intense CT band which absorbs below
420 nm.
The irradiation of a CHCl3 solution of 2 by an Hgequartz lamp
led to the rearrangement of the axial phosphane ligand into an
equatorial position, while the free axial position was occupied
by a molecule of acetic acid (Scheme 2). The complex formed
in this reaction, [Rh2(m-O2CCH3)2(h-O2CCH3){m-(C6H4)PCy2}
(CH3COOH)(PhPCy2)] (3) was, however, only detected in solution
by 1H and 31P{1H} NMR spectroscopy. The rearrangement of the
PhPCy2 ligand into equatorial position caused a 50 ppm low-field
shift of its signal in the 31P{1H} NMR spectrum. Other
rhodium(II)/phosphane systems with this type of geometry have
shown thermodynamic stability for many months. In contrast to
this, 3 rearranged back to 2 over a period of 10 days to an extent
of 60%. The other 40% of 2 reacted to a new species, identified as
the doubly ortho-metalated compound [Rh2(
m-O2CCH3)2
{
m
-(C6H4)PCy2}2(CH3COOH)2] (4) (Scheme 2). Since both bridging
phosphane ligands now are chemically equivalent but magneti-
pounds at
d
¼ 22.2 ppm as a doublet of doublet [24]. 1H and 13C
cally not equivalent, the 31P{1H} NMR spectrum is an AA‘XX’ spin
NMR spectra show only three signals for the acetate/acetic acid
ligands in the ratio 1:2:2, indicating that the two molecules of
acetic acid in axial position are equivalent on the NMR time scale. In
the 31P{1H} NMR spectrum of 2, the chemical shift of the ortho-
metalated phosphane ligand changes only slightly upon coordina-
tion of the second molecule of PhPCy2, whereas the axial
system with the signal centered at
d
¼ 18.7 ppm. The bridging
acetate and the axially coordinated acetic acid ligands resonate as
two singlets, respectively, in the 1H and 13C{1H} NMR spectra.
Attempts to separate a mixture of 2 and 4 by column chroma-
tography were not successful. To obtain 4 in pure form, a thermal
synthesis was employed by reacting dirhodium(II) tetraacetate and
Δ
Scheme 3. Synthesis of 4.

K. Herbst et al. / Journal of Organometallic Chemistry 733 (2013) 53e59
57
Fig. 2. UV/vis spectra of [Rh2(
m-O2CCH3)2{
m-(C6H4)PCy2}2(CH3COOH)2] (4), [Rh2(
m
-O2CCF3)2{
m-(C6H4)PCy2}2(CF3COOH)2] (7a) and [Rh2(
m
-O2CCH3)2{
m
-(C6H4)PPh2}2(CH3COOH)2] (8)
dissolved in CHCl3 containing 1% of ethanol. One drop of acetic/trifluoroacetic acid was added to the solution where indicated.
Cy
Cy
Cy
Cy
Cy
Cy
P
P
P
Rh
O
R
CHCl3/CF3CO2H
(10:1)
R´
R´
O
HO
R
HO
R´
HO
R´
PhPCy2 (1:1)
O
O
O
C
O
C
C
O
O
O
O
O
Rh
O
Rh
O
Ph
Rh
O
OH
R
Rh
Rh
O
OH
R´
O
O
P
O
C
O
C
Cy
O
O
O
Cy
O
R´
R
R´
R´
R
R´
R´= CF3
R = CH3
1
6
5
Scheme 4. Synthesis of 5 and 6.
2 equivalents of PhPCy2 for 2 h. However, the major product of this
reaction in boiling toluene/acetic acid was complex 2, which was
isolated in 85% yield (Scheme 3). This again confirmed the unusual
stability of 2 in thermal reactions. 4 was only present by approxi-
mately 5%. After prolonging the reaction time to 24 h, no signals of
2 were detected by 31P{1H} NMR spectroscopy. Due to the drastic
conditions a considerable amount of the reaction system had begun
to decompose in compounds which were not identified. Complex 4
was enriched in the reaction mixture only to 40%. However, it was
now possible to isolate 4 by column chromatography on silica using
CH2Cl2/hexane/acetic acid (100:100:1) as eluent. From this solvent
mixture, 4 crystallizes in large, violet blocks.
During dissolution of 4 in CHCl3 containing 1% of ethanol for UV/
vis spectroscopic measurements, a color change from violet to
green is observed. This color change is caused by an exchange of the
axial acetic acid ligands for ethanol, however, upon addition of one
drop of acetic acid, the violet color of 4 is restored. A similar color
effect has been observed for the dissolution of [Rh2(m-O2CCH3)2{m-
(C6H4)PPh2}2(CH3COOH)2] (8) [2] in the CHCl3/ethanol mixture,
which changes from blue to violet by addition of acetic acid. In the
visible part of the electronic spectra (Fig. 2), the color change of 4
from green to violet is displayed by
*(Rh2) / *(Rh2) transition from 652 nm to 564 nm. The corre-
sponding blue-shift of 8 is much smaller (571 nme546 nm). A
a blue-shift of the
p
s
Cy
Cy
Cy
Cy
Cy
Cy
P
Rh
O
P
P
R´
R´
O
O
R´
O
HO
R´
CHCl3/
HO
O
O
O
C
C
80ºC
Δ
vacuum
O
CF3CO2H (10:1)
O
O
O
Rh
O
R´
Rh
O
Ph
Cy
OH
R´
Rh
Rh
Rh
O
, 10 h
Δ
P
C
P
P
O
Cy
Cy
Cy
O
O
Cy
R´
Cy
R´
R´
R´
6
R' = CF3
7b
7a
CHCl3/CF3CO2H (10:1)
30´
Cy
Cy
P
R
O
O
HO
R
O
C
O
Rh
OH
R
Rh
C
P
Cy
O
O
Cy
R
R = CH3
4
Scheme 5. Synthesis of 7a and 7b.

58
K. Herbst et al. / Journal of Organometallic Chemistry 733 (2013) 53e59
Table 1
Crystal data for [Rh2(
singly ortho-metalated complex with a second phosphane ligand in
axial position, [Rh2( -O2CCF3)3{ -(C6H4)PCy2}(CF3COOH)(PhPCy2)]
m
-O2CCF3)3{
m
-(C6H4)PCy2}(CF3COOH)2] (5).
m
m
(6), which analogous to 2 was stable during elution from a silica
column.
Empirical formula
C28H28F15O10PRh2
Formula weight
Temperature (K)
Crystal system
Space group
1046.29
153(2)
Triclinic
After heating a solution of 6 in CHCl3/CF3COOH (10:1) to reflux
for 10 h, the doubly ortho-metalated compound [Rh2(m-O2CCF3)2{m-
Pe1 (number 2)
9.5439(8)
12.1501(10)
16.3073(13)
90.1750(10)
104.3930(10)
101.0410(10)
1795.2(3)
2
(C6H4)PCy2}2(CF3COOH)2] (7a) was detected by 31P{1H} NMR
spectroscopy in 25% yield. Attempts to prolong the reaction time
did not increase the yield of 7a, but led to decomposition of the
complexes (Scheme 5).
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
A much easier preparative access to 7a was possible by the
exchange of the acetate ligands of 4 by CF3COOꢁ (Scheme 5). After
30 min of stirring 4 in CHCl3/CF3COOH (10:1), the reaction was
completed and 7a was purified by column chromatography. Iso-
lated in this way, 7a contained two molecules of trifluoroacetic
acid in axial position, which was shown by the presence of two
signals in a 1:1 ratio in the 19F{1H} NMR and a broad signal at
5.5 ppm for the carboxylic protons in the 1H NMR spectrum. In
CHCl3 solution containing 1% of ethanol, 7a is stable against ex-
change of the axial trifluoroacetic acid ligands for ethanol, as
judged by the unchanged position of the bands in the UV/vis
spectrum (Fig. 2) before and after addition of trifluoroacetic acid.
However, by heating 7a at 80 ꢂC for 1 h in vacuum, the axial co-
ordinated CF3COOH ligands were lost. A similar de-coordination of
the axial ligands has been observed in the thermal treatment of
dirhodium tetra(trifluoroacetate) [29]. In spite of its low solubility
in chlorinated hydrocarbon solvents, the resulting complex
3
ꢀ
Cell volume (A )
Z
Dcalc (g cmꢁ3
)
1.936
1.095
1032
m
(Mo K
F(000)
range (ꢂ)
a
) (mmꢁ1
)
q
2.86e23.28
None
Absorption correction
Refinement method
Reflections collected/unique
Full-matrix least-squares on F2
6974/4844 [Rint ¼ 0.0435]
Reflections observed [I > 2
Parameters
s(I)]
4844
507
1.035
Goodness-of-fit on F2
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
s
(I)]
R1 ¼ 0.0336, wR2 ¼ 0.0742
R1 ¼ 0.0456, wR2 ¼ 0.0799
ꢀꢁ3
0.669 and ꢁ0.732 e A
similar propensity of the singly ortho-metalated complexes 1 and 2
to undergo color changes induced by exchange of the axial acetic
acid ligand has not been observed.
An exchange of all acetate and acetic acid ligands in 1 for tri-
fluoroacetate ligands was possible by stirring a solution of 1 in
CHCl3/CF3COOH (10:1) for 24 h at room temperature (Scheme 4).
During the reaction the color changes from blue to green. The re-
[Rh2(m-O2CCF3)2{m-(C6H4)PCy2}2] (7b) could be characterized by
19F{1H} NMR (one singlet at ꢁ75.0 ppm) and 1H NMR spectroscopy,
which did not show any signals of a ligand in axial position.
Furthermore, the absence of axial ligands in 7b was confirmed by a
correct microanalysis.
action product, [Rh2(m-O2CCF3)3{m-(C6H4)PCy2}(CF3COOH)2] (5),
was purified from the liberated CH3COOH by repeated evaporation
from CHCl3/CF3COOH. The 19F{1H} NMR shows three singlets
at ꢁ75.7, ꢁ75.3 and ꢁ74.9 ppm in a 2:1:2 ratio, confirming that two
molecules of CF3COOH are coordinated in axial position. In the 1H
NMR spectrum the carboxylic protons resonate as broad signals at
8.83 ppm. By slow evaporation of a CDCl3 solution, dark-green
single crystals of 5 suitable for an X-ray structure analysis were
obtained (vide infra).
3.2. Crystal structure of 5
Details of the structure determination for complex 5 are shown
in Table 1. Selected bond lengths and angles are shown in Table 2
and the molecular structure of 5 is displayed in Fig. 3. The struc-
ture of 5 can be viewed as a derivative of the paddlewheel struc-
ture of [Rh2(m-O2CCF3)4] in which one of the trifluoroacetate
ligands is replaced by a bridging dicyclohexylphenylphosphane
ligand [29]. The phosphane ligand is coordinated via the phos-
phorus atom to one rhodium atom, while the phenyl ring is
In full analogy to the transformation of 1 to 2, the reaction of 5
with one equivalent of PhPCy2 at room temperature afforded the
Fig. 3. Molecular structure of [Rh2(m-O2CCF3)3{m-(C6H4)PCy2}(CF3COOH)2] (5). The ellipsoids are drawn at the 30% probability level. H atoms have been removed for clarity.

K. Herbst et al. / Journal of Organometallic Chemistry 733 (2013) 53e59
59
Table 2
Appendix A. Supplementary material
Selected bond lengths (A) and angles (ꢂ) for [Rh2(
m
-O2CCF3)3{m-(C6H4)PCy2}
ꢀ
(CF3COOH)2] (5).
The crystallographic data for compound 5 have been deposited at
The Cambridge Crystallographic Data Centre (reference number
916484). These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Rh(1)eRh(2)
Rh(1)eP(1)
Rh(1)eO(2)
Rh(1)eO(4)
2.4476(5)
2.2311(12)
2.045(3)
2.044(3)
2.212(3)
Rh(2)eO(1)
Rh(2)eO(3)
Rh(2)eO(5)
Rh(2)eO(9)
2.055(3)
2.042(3)
2.263(3)
2.339(3)
1.980(4)
Rh(1)eO(6)
Rh(2)eC(12)
Rh(1)eO(7)
2.400(3)
Rh(2)eRh(1)eP(1)
Rh(1)eRh(2)eC(12)
P(1)eRh(1)eO(7)
P(1)eRh(1)eO(6)
O(2)eRh(1)eO(4)
91.53(3)
Rh(2)eRh(1)eO(7)
Rh(1)eRh(2)eO(9)
C(12)eRh(2)eO(9)
C(12)eRh(2)eO(5)
O(1)eRh(2)eO(3)
166.31(8)
168.32(8)
95.54(15)
176.20(15)
174.27(12)
References
96.14(13)
101.74(8)
177.44(9)
173.29(12)
[1] A.R. Chakravarty, F.A. Cotton, D.A. Tocher, J. Chem. Soc. Chem. Commun.
(1984) 501e502.
[2] A.R. Chakravarty, F.A. Cotton, D.A. Tocher, J.H. Tocher, Organometallics 4
(1985) 8e13.
[3] F. Mohr, S.H. Privér, S.K. Bhargava, M.A. Bennett, Coord. Chem. Rev. 250 (2006)
1851e1888.
[4] P. Lahuerta, J. Payá, E. Peris, A. Aguirre, S. García-Granda, F. Gómez-Beltrán,
Inorg. Chim. Acta 192 (1992) 43e49.
[5] P. Lahuerta, F. Estevan, in: Metal Clusters in Chemistry, vol. II, Wiley-VCH,
Weinheim, 1999, p. 678 (and references therein).
[6] P. Lahuerta, J. Payá, M.A. Pellinghelli, A. Tiripicchio, Inorg. Chem. 31 (1992)
1224e1232.
[7] P. Lahuerta, E. Peris, M.A. Ubeda, S. García-Granda, F. Gómez-Beltrán,
M.R. Díaz, J. Organomet. Chem. 455 (1993) C10eC12.
[8] A. Garcia-Bernabé, P. Lahuerta, M.A. Ubeda, S. García-Granda, P. Pertierra,
Inorg. Chim. Acta 229 (1995) 203e209.
coordinated in ortho position to the other rhodium atom. The
framework of four bridging ligands is completed by two tri-
fluoroacetic acid ligand coordinated in axial position to the
ꢀ
rhodium atoms. Compared to the RheRh distance of 2.3813(8) A in
the parent compound [Rh2(m-O2CCF3)4] [29], the RhRh distance in
ꢀ
5 is slightly elongated (2.4476(5) A). A similar RheRh distance of
ꢀ
2.438(1) A has been observed in a closely related complex to 5,
[Rh2(m-O2CCF3)3{m-(C6H4)PPh2}(CF3COOH)2], differing only in the
nature of the organic substituents at the phosphorus atom [30].
However, the sterically more demanding cyclohexyl groups in 5
give rise to a stronger bending of the axial trifluoroacetic acid
ligand away from the phosphane ligand. This is evidenced by a
P(1)eRh(1)eO(7) angle of 101.74(8)ꢂ compared to only 96.5(1)ꢂ in
[9] F. Estevan, S. García-Granda, P. Lahuerta, J. Latorre, E. Peris, M. Sanaú, Inorg.
Chim. Acta 229 (1995) 365e371.
[10] F. Estevan, A. Garcia-Bernabé, S. Gracía-Granda, P. Lahuerta, E. Moreno,
J. Pérez-Prieto, M. Sanaú, M.A. Ubeda, J. Chem. Soc. Dalton Trans. (1999)
3493e3498.
[11] D.F. Taber, S.C. Malcolm, K. Bieger, P. Lahuerta, M. Sanaú, S.-E. Stiriba, J. Pérez-
Prieto, M.A. Monge, J. Am. Chem. Soc. 121 (1999) 860e861.
[12] F. Estevan, K. Herbst, P. Lahuerta, M. Barberis, J. Pérez-Prieto, Organometallics
21 (2001) 950e957.
[13] M. Barberis, J. Pérez-Prieto, K. Herbst, P. Lahuerta, Organometallics 22 (2002)
1667e1673.
[14] F. Estevan, P. Lahuerta, J. Pérez-Prieto, M. Sanaú, S.-E. Stiriba, M.A. Ubeda,
Organometallics 16 (1997) 880e886.
[15] F. Estevan, P. Lahuerta, J. Pérez-Prieto, I. Pereira, S.-E. Stiriba, Organometallics
17 (1998) 3442e3447.
[16] P. Lahuerta, E. Moreno, A. Monge, G. Muller, J. Pérez-Prieto, M. Sanaú, S.-
E. Stiriba, Eur. J. Inorg. Chem. (2000) 2481e2485.
[17] M. Barberis, P. Lahuerta, J. Pérez-Prieto, M. Sanaú, Chem. Commun. (2001)
439e440.
[Rh2(
difference in both mono-metalated complexes is the conformation
of the axial trifluoroacetic acid ligands. In [Rh2( -O2CCF3)3{m-
m-O2CCF3)3{m-(C6H4)PPh2}(CF3COOH)2]. Another structural
m
(C6H4)PPh2}(CF3COOH)2] it was found that only one trifluoroacetic
acid ligand (coordinated to Rh(2)) formed an intramolecular
interaction via hydrogen bonds to the trifluoroacetate ligand
opposite to the bridging phosphane. The other trifluoroacetic acid
ligand did not show any intramolecular interaction [30]. In
contrast to this, both axial ligands in 5 show hydrogen bonding of
their OH functions to the trans(P) trifluoroacetate ligand, resulting
[18] J. Pérez-Prieto, S.-E. Stiriba, E. Moreno, P. Lahuerta, Tetrahedron: Asymmetry
14 (2003) 787e790.
ꢀ
in O(5)eO(10) and O(6)eO(8) distances of 2.608 and 2.633 A,
respectively.
[19] F. Estevan, P. Lahuerta, J. Lloret, M. Sanaú, M.A. Ubeda, J. Vila, Chem. Commun.
(2004) 2408e2409.
[20] P. Hirva, P. Lahuerta, J. Pérez-Prieto, Theor. Chem. Acc. 113 (2005) 63e68.
[21] J. Lloret, F. Estevan, P. Lahuerta, P. Hirva, J. Pérez-Prieto, M. Sanaú, Organo-
metallics 25 (2006) 3156e3165.
4. Conclusion
[22] SHELXTL (Version 6.10), BRUKER, 2000.
[23] M.V. Borrachero, F. Estevan, P. Lahuerta, J. Payá, E. Peris, Polyhedron 12 (1993)
1715e1717.
It has been demonstrated that the presence of cyclohexyl
groups in the PhPCy2 ligand has a significant impact on the chemistry
of the system dirhodium tetraacetate/dicyclohexylphenylphosphane.
The singly ortho-metalated complex [Rh2(m-O2CCH3)3{m-(C6H4)
PCy2}(CH3COOH)(PhPCy2)] containing one axial phosphane ligand
proved to be unusually thermodynamically stable, impeding the
[24] P. Lahuerta, J. Payá, X. Solans, M.A. Ubeda, Inorg. Chem. 31 (1992) 385e391.
[25] L. Natkaniec, F.P. Pruchnik, J. Chem. Soc. Dalton Trans. (1994) 3261e3266.
ꢁ
[26] F.P. Pruchnik, R. Starosta, P. Smolenski, E. Shestakova, P. Lahuerta, Organo-
metallics 17 (1998) 3684e3689.
[27] F.P. Pruchnik, R. Starosta, T. Lis, P. Lahuerta, J. Organomet. Chem. 568 (1998)
177e183.
[28] R. Starosta, F.P. Pruchnik, Z. Ciunik, Polyhedron 25 (2006) 1994e2006.
[29] F.A. Cotton, E.V. Dikarev, X. Feng, Inorg. Chim. Acta 237 (1995) 19e26.
[30] S. García-Granda, P. Lahuerta, J. Latorre, M. Martinez, E. Peris, M. Sanaú,
M.A. Ubeda, J. Chem. Soc. Dalton Trans. (1994) 539e544.
synthesis ofthe doublyortho-metalatedcomplex [Rh2(m-O2CCH3)2{m-
(C6H4)PCy2}2(CH3COOH)2] in high yield. Further work is required to
determine if the cyclohexyl groups also exert an influence on the
catalytic properties of the doubly ortho-metalated complexes.