Stoichiometric and catalytic oxidation of BINAP by dioxygen in a rhodium(I) complex

Article abstract of DOI:10.1021/om020241a

The square planar complex (BINAP)Rh(CO)Cl (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) was synthesized from [Rh(COD)Cl]₂ (COD = 1,5 cyclooctadiene) and BINAP under a CO atmosphere. It reacts with oxygen to form the square planar (BINAP(O))Rh(CO)Cl in approximately 50% yield, along with molar equivalents of free BINAP dioxide (BINAP(O)₂) and CO₂. The oxygen atom of the BINAP(O) was shown crystallographically to be trans to the CO ligand. In the presence of excess BINAP under CO/O₂ gas mixtures, the reaction is catalytic with a TOF of 0.16 h^-1 at ambient temperature in chloroform. The kinetics of this transformation were investigated and conditions for optimum selectivity for BINAP(O) formation suggested. A stoichiometric mechanism is proposed that involves initial formation of an O₂ adduct, followed by oxygen atom transfer to both phosphorus atoms of BINAP or to one phosphorus atom and the CO ligand trans to it. These processes might be concerted, or stepwise with intermediate Rh=O species. To the best of our knowledge, this chemistry represents the first example of a reaction exhibiting oxygen atom addition from O₂ either to both phosphorus atoms in a bisphosphine ligand or to one phosphorus atom and a CO ligand in the same complex.

Full text of DOI:10.1021/om020241a

3344
Organometallics 2002, 21, 3344-3350
Stoich iom etr ic a n d Ca ta lytic Oxid a tion of BINAP by
Dioxygen in a Rh od iu m (I) Com p lex
Kevin A. Bunten, David H. Farrar, Anthony J . Poe¨,* and Alan Lough
Department of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario, Canada M5S 3H6
Received March 28, 2002
The square planar complex (BINAP)Rh(CO)Cl (BINAP ) 2,2′-bis(diphenylphosphino)-1,1′-
binaphthyl) was synthesized from [Rh(COD)Cl]2 (COD ) 1,5 cyclooctadiene) and BINAP
under a CO atmosphere. It reacts with oxygen to form the square planar (BINAP(O))Rh-
(CO)Cl in approximately 50% yield, along with molar equivalents of free BINAP dioxide
(BINAP(O)2) and CO2. The oxygen atom of the BINAP(O) was shown crystallographically to
be trans to the CO ligand. In the presence of excess BINAP under CO/O2 gas mixtures, the
reaction is catalytic with a TOF of 0.16 h-1 at ambient temperature in chloroform. The
kinetics of this transformation were investigated and conditions for optimum selectivity for
BINAP(O) formation suggested. A stoichiometric mechanism is proposed that involves initial
formation of an O2 adduct, followed by oxygen atom transfer to both phosphorus atoms of
BINAP or to one phosphorus atom and the CO ligand trans to it. These processes might be
concerted, or stepwise with intermediate RhdO species. To the best of our knowledge, this
chemistry represents the first example of a reaction exhibiting oxygen atom addition from
O2 either to both phosphorus atoms in a bisphosphine ligand or to one phosphorus atom
and a CO ligand in the same complex.
In tr od u ction
1,1′-binaphthyl) rhodium and ruthenium complexes are
well-studied catalysts for enantioselective hydrogena-
tion18-22 of olefins and ketones. The presence of a mono-
oxidized BINAP in an organometallic catalyst may have
interesting consequences.9
Selective oxidation of bisphosphines to BPMOs is
difficult,23-26 and general routes are very rare. Ad-
ditionally, there is very little mechanistic research into
this area of chemistry,24 although Grushin has recently
reported a full account25 of his catalytic and selective
oxidation of several bisphosphines by Pd(II) catalysts
in the presence of alkali and 1,2-dibromoethane.
The syntheses of square planar complexes of the type
RhL2(CO)Cl are well documented in the literature.27-29
However, square planar complexes of chelating phos-
phine ligands are quite rare because many bisphosphine
Synthesis and properties of bisphosphine monoxide
(BPMO) complexes are currently of interest. The com-
bination of a hard oxygen moiety with a soft phosphorus
center in the same ligand may lead to hemilabile1-7
behavior, resulting in the generation of potential coor-
dination sites required for substrate activation. These
unsymmetrical ligands can also generate chirality and
electronic asymmetry at the metal center, and this may
lead to efficient chiral catalysis.8-11
Phosphorus ligands play an important role in deter-
mining the outcome of many catalytic processes, and
complexes containing BPMO ligands have been success-
fully employed as catalysts for a variety of synthetic
applications.12-17 BINAP (2,2′-bis(diphenylphosphino)-
* To whom correspondence should be addressed. E-mail: apoe@
chem.utoronto.ca.
(1) Bader, A.; Lindner, E. Coord. Chem. Rev. 1991, 108, 27.
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10.1021/om020241a CCC: $22.00 © 2002 American Chemical Society
Publication on Web 07/03/2002

Oxidation of BINAP by Dioxygen
Organometallics, Vol. 21, No. 16, 2002 3345
ligands prefer a bridging orientation rather than an η2-
binding mode as in Rh(η2-dppe)(CO)Cl.30-32
Sch em e 1. Gen er a l Rea ction Sch em e for th e
Syn th esis of 1, 2, a n d 3
Reactions of metal carbonyl complexes of the type
RhL2(CO)Cl with oxygen have received relatively little
attention. Foote et al.33,34 demonstrated that reaction
of Rh(PPh3)2(CO)Cl with singlet oxygen at -40 °C forms
an analogue of the well-known Vaska’s adduct (η2-O2)-
Ir(PPh3)2(CO)Cl,35-38 but the Rh complex rapidly loses
its oxygen on warming to room temperature. When
triplet oxygen is used, in the presence or absence of
excess phosphine, reaction leads to the formation of
phosphine oxide, CO2, and mixtures of rhodium com-
plexes of the formula Rh4Cl4(CO)4(O2)2L2 (L ) PPh3,
PPhEt2, PEt3, and PPh2(O-i-Bu)).39,40 Others have man-
aged to prepare a variety of Rh dioxygen complexes
using phosphines,41-44 iminophosphines,45 and ben-
zenethiolate ligands.46
We report herein the synthesis, via [Rh(BINAP)Cl]2
(1) and Rh(BINAP)(CO)Cl (2), of the square planar
rhodium BPMO complex (BINAP(O))Rh(CO)Cl (3), which
is formed by O2 oxidation of 2. BINAP dioxide (BINAP-
(O)2) and CO2 are formed in equivalent amounts, and
some unidentified products are also formed. In the
presence of excess BINAP under CO/O2 gas mixtures,
2 behaves as an oxygenation catalyst transforming
BINAP quantitatively into a mixture of BINAP(O) and
BINAP(O)2. Kinetics for the formation of 3 from 2, as
well as for the catalytic oxidation of BINAP, are
described.
integrated and scaled using the Denzo-SMN package.48 Data
were corrected for absorption effects by the program Denzo-
SMN, which uses the high redundancy of the data to apply a
scale to each frame of data. The structures were solved and
refined on F2 using the SHELXTL\ PC V5.03 package.49
Concentrations of oxygen in chloroform were estimated by
the method of Wilhelm et al.50 and corrected for solvent vapor
pressure.51 They were based on tabulated solubilities52 in
chloroform, and solubilities in 1,2-dichloroethane were as-
sumed to be the same. Concentrations of CO2 were based on
molar absorbances estimated from solutions of known concen-
tration of Ru3(CO)12 to which an excess of [n-Bu4][OH] had
been added, so generating an exact equivalent of CO2.53
Reactions 2 f 3, in chloroform at various temperatures and
under atmospheres of pure O2 or O2/N2 mixtures, were
monitored by IR spectrophotometry in thermostated cells, and
rate constants were obtained from single-exponential analysis
of absobance changes of the C-O stretching bands of the
complexes involved. Corresponding reactions in 1,2-dichloro-
ethane at 25 °C were carried out in Schlenk tubes with
continuous bubbling of the O2 or O2/N2 gases and with periodic
sampling and IR spectral measurements.
Ca ta lytic Rea ction s in Ch lor ofor m a n d Tolu en e. Cata-
lytic reactions using excess BINAP were all performed in a
similar manner. In a typical experiment, mixtures of 2 and a
10-fold or greater excess of BINAP in chloroform or toluene
were reacted under a CO/O2 gas mixture controlled by gas-
flow meters. After a predetermined time, the IR spectrum was
recorded and the solvent was subsequently evaporated. The
residue was extracted into CDCl3 and the solution 31P{1H}
NMR spectrum was recorded and integrated to give yields of
BINAP, BINAP(O), and BINAP(O)2.
Exp er im en ta l Section
Su m m a r y. Details of all experimental procedures are given
in the Supporting Information (SI). All chemical manipulations
were carried out under an atmosphere of nitrogen using
standard Schlenk techniques47 unless otherwise stated. Syn-
theses of compounds 1, 2, and 3 according to Scheme 1 are
described in full in the SI pages. The products were character-
ized by solution FTIR and NMR spectroscopy, mass spectros-
copy, and single-crystal X-ray crystallography.
X-ray data were collected on a Nonius Kappa CCD diffrac-
tometer using graphite-monochromated Mo KR radiation (λ
) 0.71073 Å). A combination of 1° φ and ω (with κ offsets) scans
were used to collect sufficient data. The data frames were
(30) Hughes, R. P. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, G., Abel, E., Eds.; Pergamon Press: Oxford, 1982;
Vol. 5, pp 277-540.
(31) Dyer, G.; Wharf, R. M.; Hill, W. E. Inorg. Chim. Acta 1987,
133, 137.
(32) Sanger, A. J . Chem. Soc., Dalton. Trans. 1977, 120.
(33) Selke, M.; Foote, C. S. J . Am. Chem. Soc. 1993, 115, 1166.
(34) Selke, M.; Foote, C. S.; Karney, W. L. Inorg. Chem. 1993, 24,
5425.
(35) Vaska, L. Acc. Chem. Res. 1976, 9, 175.
(36) Vaska, L. Inorg. Chim. Acta 1971, 5, 295.
(37) Vaska, L.; Bath, S. S. J . Am. Chem. Soc. 1966, 88, 1333.
(38) Vaska, L. Science 1963, 140, 840.
(39) Cullen, W. R.; J ames, B. R.; Strukul, G. Inorg. Chem. 1978,
17, 484.
(40) Geoffroy, G.; Denton, D. A.; Keeney, M. E.; Bucks, R. R. Inorg.
Chem. 1976, 15, 2382.
(41) Bennett, M. J .; Donaldson, P. B. Inorg. Chem. 1977, 16, 1581.
(42) Bennet, M. J .; Donaldson, P. B. J . Am. Chem. Soc. 1971, 93,
3307.
(43) Aresta, M.; Quartanta, E.; Ciccarese, A. C1. Mol. Chem. 1985,
1, 267.
Resu lts
Syn th esis a n d Str u ctu r e. Syn th esis. Complex 2
was prepared from 1 by the well-known bridge splitting
method29-32 (Scheme 1) and undergoes slow oxidation
in solution or in the solid state forming (BINAP(O))Rh-
(CO)Cl (3) in up to ca. 50% yield. However, in the
presence of a 2-fold excess of BINAP, under a CO/O2
(1:1) gas mixture, 3 is formed quantitatively, as ob-
served by monitoring the intensity of the infrared
(48) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(49) Sheldrick, G. M. SHELXTL/ PC V5.1; Bruker Analytical X-ray
Systems: Madison, WI, 1997.
(44) Bennet, M. J .; Donaldson, P. B. Inorg. Chem. 1977, 16, 1585.
(45) Ghilardi, C. A.; Midollini, S.; Moneti, S.; Orlandini, A.; Scapacci,
G. J . Chem. Soc., Dalton Trans. 1992, 3371.
(46) Oskada, K.; Hataya, K.; Yamamoto, T. Inorg. Chem. 1993, 32,
2360.
(47) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-
Sensitive Compounds, 2nd ed.; Wiley: New York, 1986.
(50) Wilhelm, E.; Battino, R. Chem. Rev. 1973, 73, 1.
(51) Timmermans, J . Physicochemical Constants of Pure Organic
Compounds; Elsevier: Amsterdam, 1950; pp 218-221.
(52) Solubilities of Inorganic and Organic Compounds; Stephen, H.,
Stephen, T., Eds.; The MacMillan Company: New York, 1963; Vol. 1,
pp 574-575.
(53) Gross, D. C.; Ford, P. J . Am. Chem. Soc. 1985, 107, 585.

3346 Organometallics, Vol. 21, No. 16, 2002
Bunten et al.
Ta ble 1. In fr a r ed νCO (cm -1) Absor ba n ce Ba n d s for
Com p ou n d s 2 a n d 3
solvent
CHCl3
CH2Cl2
ClCH2CH2Cl
2
3
2018
2010
2007
1986
1979
1977
Ta ble 2. 31P {1H} NMR Da ta for Selected
Com p ou n d s
compound
31P{1H} NMR (δ ppm)
1a
2b
50 (d, J RhP ) 195 Hz)
46.1 (d of d, J RhP ) 163 Hz, J PP ) 44 Hz),
24.9 (d of d, J RhP ) 128 Hz, J PP ) 44 Hz)
48.6 (d, J RhP ) 175 Hz), 41.5 (s)
28.1 (s), -14.3
3b
BINAP(O)b
BINAP(O)2
b
29.3 (s)
a
C6D6. bCDCl3
carbonyl stretching bands. The addition of excess BI-
NAP to solutions of 3 leads to rapid and quantitative
re-formation of 2. Compounds 1, 2, and 3 were charac-
terized by X-ray crystallography (Table S1), solution
FTIR analysis of the νCO stretching frequencies (Table
1), solution 31P{1H} NMR (Table 2), and mass spectrom-
etry. The purity of the compounds was demonstrated
by the sharpness of the bands in the NMR spectra and
the absence of any that were attributable to possible
significant impurities (Figures S1-S3). The IR spectra
of 2 and 3 also showed sharp single bands with no
shoulders, and the presence of the 789-28 and 805-28
mass unit bands in the mass spectra provided additional
support. These represent the expected molecular masses
minus the mass of the CO ligands that are evidently
easily displaced in the mass spectrometric measure-
ments.
X-r a y Cr ysta llogr a p h y. The dimeric nature of 1 was
confirmed by its X-ray structure, and two molecules of
THF were cocrystallized in the lattice. An ORTEP plot
of its structure is given in Figure S4. The two BINAP
ligands exhibit unexceptional bite angles and Rh-Cl-
Rh angles (Table S2). Lengths of the Rh(1 or 2)-Cl and
Rh(1 or 2)-P bonds are also unexceptional (Table S3).
This dimer is not perfectly flat, as the molecule is bent
to an angle of 124.8°, the bending angle being defined
as the angle between the two squares of the rhodium
subunits.54
Complexes 2 and 3 (Figures S5 and 1) are square
planar Rh(I) monomers. Complex 2 shows disorder in
the placement of the CO and Cl ligands with 87% of
the unit cells having the CO group trans to P(1) and
the Cl trans to P(2). The bite angle of the BINAP ligand
is quite normal, and other bond angles are typical for a
square planar complex (Table S2). Bond lengths are also
unexceptional (Table S3).
Compound 3 is best described as a mono-oxidized
phosphine bound to the rhodium center with the car-
bonyl group trans to the oxygen. This complex has a
P-Rh-O bite angle of 90.69(8)° and internal angles
near 90° (Table S2). The complex has Rh-P and Rh-O
bond lengths of 2.242(1) and 2.123(3) Å, respectively.
The rhodium-carbonyl bond is considerably shorter
than in 2 (1.774(5) vs 1.905(5) Å) (Table S3).
F igu r e 1. ORTEP plot of the X-ray structure of 3 showing
30% thermal ellipsoids.
and 1978 cm-1, respectively, in chloroform. There is a
shift of ca. 10 cm-1 to lower carbonyl stretching fre-
quencies as the solvents are changed to dichloromethane
and 1,2-dichloroethane.
P h osp h or u s NMR Sp ectr oscop y. The solution 31P-
{1H} NMR spectrum in C6D6 of 1 exhibits a single
doublet at 50 ppm (J RhP ) 195 Hz). The 31P{1H} NMR
spectrum of 2 in CD6D6 consists of a doublet of doublets
at 46.5 and 26.0 ppm (J Rh-P ) 162 and 128 Hz, J P-P
44 Hz), and 3 exhibits a doublet at 49.2 ppm (J Rh-P
)
)
175 Hz) and a singlet at 41.9 ppm. No phosphorus-
phosphorus coupling was observed for complex 3 (Table
2).
Rea ction s a n d Kin etics. Reactions of 2 with oxygen
in chloroform were monitored by infrared spectroscopy
(Figure 2A), using a temperature-controlled infrared
cell. A clear isosbestic point is visible for 90% of the
reaction, and very weak peaks are observed growing and
decreasing at 2080 and 2070 cm-1. After the oxidation
is complete, the 2080 cm-1 peak is gone, but some of
the 2070 cm-1 absorbance remains. After separation
from an unidentified solid residue, this minor product
was isolated by precipitation with hexanes, but no
crystals suitable for crystallographic analysis could be
isolated. The compound exhibits infrared bands at 2070
and 1991 cm-1. No phosphorus peaks were observed in
the 31P{1H} NMR spectrum.
In the absence of added CO, compound 3 was formed
from 2 in up to ca. 50% yield as estimated from the
intensity of the carbonyl infrared stretching bands and
the measured molar absorbance coefficients (see SI
pages). The solution 31P{1H} NMR spectrum of the
reaction mixture obtained after stirring 2 with oxygen
clearly indicates that BINAP(O)2 and 3 are formed in
nearly 1:1 ratios. The rate of reaction of 2 with oxygen
(monitored at 2018 cm-1) followed first-order behavior
and was accompanied by growth of infrared bands at
2338 and 1986 cm-1 corresponding to CO2 and 3,
respectively (Figure 2A). Rate constants for the loss of
2 are shown in Table 3. Rate constants for growth of
product bands were generally in agreement. Plots of kobs
vs [O2] (Figure 3) for reactions at 25 °C were linear with
In fr a r ed Sp ect r oscop y. The carbonyl infrared
stretching regions for 2 and 3 were recorded in chloro-
form, dichloromethane, and 1,2-dichloroethane (Table
1). Both compounds exhibit a single νCO band, at 2018
(54) Aullo´n, G.; Ujaque, G.; Lledo´s, A.; Alvarez, S.; Alemany, P.
Inorg. Chem. 1998, 37, 804.

Oxidation of BINAP by Dioxygen
Organometallics, Vol. 21, No. 16, 2002 3347
F igu r e 3. Plot of kobs vs [O2] for the reaction of 2 at 25 °C
in the presence of pure O2 and N2/O2 gas mixtures.
Schlenk flask containing the reaction mixture, which
was subject to continuous bubbling with pure O2 or O2/
N2 mixtures. The second-order rate constant was de-
termined as before from kobs vs [O2] plots: 102k2 ) 9.37
( 1.26 M-1 s-1
.
Effect of Ca r bon Mon oxid e. Oxidation of 2 in
chloroform under CO/O2 (1:1) gas mixtures results in
the formation of new carbonyl bands in the infrared
spectrum at 2099, 2087, 2070, 2055, and 2020 cm-1
(Figure 2B), and no isosbestic points are observed in this
region. Yields of 3 were generally higher (65-70%)
under CO/O2 (1:1) than in reactions under N2/O2 (1:1),
but the rate of reaction of 2 is unaffected by the presence
of CO (Table 3).
Effect of Ad d ed BINAP . When reaction of 2 with
an O2/CO mixture is carried out in the presence of 1
molar equiv of BINAP, 2 is completely converted into
BINAP(O)Rh(CO)Cl and none of the high-energy bands
mentioned above for reaction under CO are formed.
Rea ction s of 3 w ith Ca r bon Mon oxid e. A sample
of pure 3 reacts reversibly but incompletely with CO in
chloroform, giving infrared bands at 2090 and 2020 cm-1
with 10% the intensity of the initial absorbance of 3.
Solutions of 3 and the other oxidation products of
reactions of 2 give spectra similar to Figure 2B upon
treatment with CO. Mixtures of 3 and these unidentified
products react quantitatively and rapidly with BINAP,
forming 2 exclusively.
F igu r e 2. Spectroscopic changes for kinetic runs in
chloroform: (A) pure O2; (B) CO/O2 (1:1).
Ta ble 3. Kin etic Da ta for Rea ction s of 2 in P u r e
O2, N2/O2, a n d CO/O2 Sa tu r a ted Ch lor ofor m
Solu tion s
temp (°C) 103 [O2] (M) 104 kobs (s-1
)
102 kobs/[O2] (M-1 s-1
)
40a
25a
25a
25a
25a
14.9a
8.3a
25b
25b
25b
25c
25c
25c
25d
4.81
6.74
6.74
6.74
6.74
7.39
8.06
3.39
3.39
3.39
3.39
3.39
3.39
1.35
7.64 ( 0.06
3.41 ( 0.02
4.05 ( 0.03
4.15 ( 0.12
4.48 ( 0.04
1.34 ( 0.003
0.93 ( 0.004
2.07 ( 0.04
1.93 ( 0.04
1.95 ( 0.03
2.60 ( 0.04
1.87 ( 0.01
1.93 ( 0.02
1.03 ( 0.01
15.89
5.84
5.06
6.64
6.16
1.82
1.15
a
∆Hq ) 14.6 ( 1.1 kcal mol-1; ∆Sq ) -15.3 ( 3.8 cal mol-1
K-1; σ(k2) ) 17%, pure O2. CO/O2 (1:1). N2/O2 (1:1). dN2/O2 (80:
20).
b
c
an intercept close to zero. The second-order rate con-
stant was determined from a linear least-squares analy-
sis of the dependence of kobs on [O2], using proportional
weighting, and 102k2 ) 5.35 ( 0.53 M-1 s-1, the
intercept being given by 105int ) 2.88 ( 1.76 s-1, with
σ(kobs) ) 14%. Activation parameters, also shown in
Table 3, were determined from rate constants for
reactions under pure O2 with second-order rate con-
stants estimated from k2 ) kobs/[O2]; that is, it was
assumed the contribution of the intercept to kobs was
negligible.
Ca ta lytic Oxid a tion of BINAP . Reactions of 2 with
as much as a 50-fold excess of BINAP in chloroform or
toluene were followed under CO/O2 gas mixtures, with
varied temperatures and reaction times. Using large
excesses of BINAP was impractical, as it results in
extremely long reaction times, spanning several weeks,
before all the BINAP was consumed. Infrared monitor-
ing of the reaction indicated no degradation of 2 until
all of the BINAP had been consumed. The yields of
BINAP, BINAP(O), and BINAP(O)2 were estimated
from integration of their 31P{1H} NMR spectra (Figure
S6). The product yields were dependent on solvent,
temperature, gas ratios, and reaction time. Thus, suit-
Oxygenation kinetics were also measured in 1,2-
dichloroethane by taking aliquots of solution from a

3348 Organometallics, Vol. 21, No. 16, 2002
Bunten et al.
able reaction conditions can be inferred to optimize
product yields and minimize reaction times. Reaction
of BINAP with oxygen in the absence of 2 in chloroform
shows no appreciable oxidation after 24 h, but reactions
using a 10-fold excess of BINAP relative to catalyst show
no traces of unoxidized BINAP after periods as short
as 72 h. The approximate yield ratios [BINAP(O)2]/
[BINAP(O)] were ∼2:1 and 1:1 in chloroform and
toluene, respectively (Figure S6). Turnover frequencies
for the catalytic reaction in chloroform and toluene are
0.16 and 0.12 h-1, with turnover numbers of at least 70
and 10, respectively. Attempts to get larger turnover
numbers were impractical because of the long reaction
times involved at ambient pressures. Reactions under
CO/O2 (3:1) mixtures in both solvents result in higher
monoxide formation, but the turnover frequency is
drastically reduced to 0.016 h-1, making the process
very slow. Reactions under pure O2 lead to incomplete
BINAP oxidation, even after long reaction times, and
consequently very low yields of BINAP(O) and BINAP-
(O)2 were observed. Elevated temperatures increase the
overall rate of reaction, but the ratios of BINAP(O)2 to
BINAP(O) are significantly increased.
Use of Oth er Liga n d s. Attempts to use ligands other
than CO for the catalytic reactions were unsuccessful.
Reactions with 10-fold excess of BINAP and 100-fold
excesses of norbornylene or cyclohexylisocyanide lead
to incomplete oxidation of BINAP. The former produced
only trace amounts of norcamphor and norborneneol,
whereas the later reacts rapidly with 2 to form mixtures
of isocyanide complexes,55,56 as evidenced by changes in
the infrared frequencies of the νCN bands.
Use of dppm in place of the excess BINAP led to
similar catalytic formation of the monoxide and dioxide
products in CDCl3, but reactions with dppe gave less
clear results. No further studies were undertaken.
The BINAP in complex 1 is oxidized by O2 to BINAP-
(O)2, but not catalytically.
Complex 2 (Figure S5) is a rare example of a crys-
tallographically characterized square planar chelate
bisphosphine complex of rhodium. A search of the
Cambridge Structure Database (CSD)65 yielded only two
chelated bisphosphine structures where L ) carbonyl-
chloro((2,2′-biphenyl-1,1′-diyl)bis(6,6′-di-tert-butyl-4,4′-
dimethoxy-2,2′-biphenyl-1,1′-diyl)diphosphite), which
crystallized in two different space groups.66 However,
other cis-chelate complexes, where L ) bis(diphen-
ylphosphino)ethane (dppe) and three o-carborane
phosphino derivatives,67,68 Ph2P(C2B10H10)PPh2, Ph2P-
(C2B10H10)P(NMe2)2, and (Me2N)P(C2B10H10)P(NMe2)2,
have been isolated and characterized by infrared and
31P{1H} NMR spectroscopy,32 but not by X-ray crystal-
lography. Phosphorus-rhodium bond lengths in 2 (2.341-
(1), 2.250(1) Å) are close to the average value (2.320 Å)
for a sample of 38 structures with the fragment RhP2-
(CO)Cl found on the CSD. Of these complexes, 24
contained trans-monophosphine ligands, 7 contained
bridging bisphophine ligands, 9 incorporated trans-
chelating bisphosphine ligands, and 2 were the cis-
chelate complexes mentioned above.
The action of oxygen on 2 results in the formation of
3 in approximately 50% yield, along with approximately
equivalent yields of BINAP(O)2 and CO2 and trace
amounts of an unidentified CO-containing product. The
crystal structure proved the existence of the mono-
oxidized BINAP coordinated to the rhodium center. A
search of the CSD revealed only two similar structures
published with BPMO ligands in Rh(I) complexes, the
ligands being [bis(isopropoxy)phosphoryl(diphenylphos-
phino)methane-O,P] ((O-i-Pr)2P(O)CH2PPh2)69 and [(di-
phenylphosphinomethylene)diphenylphosphineoxide-
O,P] (dppm(O)).16 Both complexes have Rh-P and
Rh-O bond lengths and phosphorus-oxygen bite angles
that are similar to those found for 3. Another interesting
feature of these complexes is the strong trans effect of
the oxygen donor on the carbonyl ligand. Rh-C bond
lengths are significantly shortened from 1.905(5) to
1.774(5) Å when changing from 2 to 3. Similar Rh-C
bond lengths of 1.79(1)69 and 1.78716 Å are observed for
the above analogous complexes. Infrared spectroscopy
of 3 in the C-O stretching region show a significant
decrease in νCO of 31 cm-1 in all solvents (Table 1).
Analogous complexes containing dppe(O),16 dppm(O),16
((O-i-Pr)2P(O)CH2PPh2), and ((O-Et)2P(O)CH2PPh2)69
ligands exhibit similar νCO stretching frequencies of
1995, 1985, 1990, and 1990 cm-1 in CH2Cl2, respec-
tively. The first compound shows a decrease in νCO of
15 cm-1 (Nujol) when going from dppe to dppe(O). The
decrease in Rh-C bond length and νCO stretching
frequencies of the BPMO complexes are conventionally
attributed to increased π-back-bonding from the metal
to the carbonyl carbon as a result of π-electron donation
of electrons from the oxygen to the metal. Gladiali et
al.9 have isolated some BINAP(O) complexes of Rh(I)
such as [{BINAP(O)}Rh(diolefine)][BF4], and labiliza-
Discu ssion
Syn th esis a n d Ch a r a cter iza tion . Rea ction s a n d
P r od u cts. The reaction of [Rh(COD)Cl]2 with 2 equiv
of (R)-BINAP leads to the formation of 1 in high yield.57
This complex is characterized as a bis bridged dimer
and is bent by an angle θ of 124.8°. Such bending is
common for this class of compounds ([L2RhCl]2), and a
large variety of bending angles ranging from practically
planar (160° < θ < 180°) or strongly bent (θ < 150°)
are observed.54,58-64 Bond lengths and angles for 1 are
characteristic for this class of complex.
(55) J ones, W. D. J .; Hessell, E. T. Organometallics 1990, 9, 718.
(56) J ones, W. D.; Hessell, E. T. New J . Chem. 1990, 14, 481.
(57) Although (R)-BINAP was used, the primary interest was in the
oxidation reactions and not the stereochemistry of the BINAP complex.
(58) Feiken, N.; Pregosin, P.; Trabesinger, G. Organometallics 1998,
17, 4510.
(59) Roucoux, A.; Thieffry, L.; Carpentier, J .-F.; Devocelle, M.;
Meliet, C.; Agbossou, F.; Mortreux, A.; Welch, A. J . Organometallics
1996, 15, 2440.
(60) Wang, K.; Goldman, M. E.; Emge, T. J .; Goldman, A. S. J .
Organomet. Chem. 1996, 518, 55.
(61) Hofmann, P.; Meir, C.; Hiller, W.; Heckel, M.; Riede, J .;
Schmidt, M. U. J . Organomet. Chem. 1995, 490, 51.
(62) Binger, P.; Haas, J .; Glaser, G.; Goddard, R.; Kru¨ger, C. Chem.
Ber. 1994, 127, 1927.
(63) Schnabel, C. R.; Roddick, D. M. Inorg. Chem. 1993, 32, 1513.
(64) Curtis, D. M.; Butler, W. M.; Greene, J . Inorg. Chem. 1978, 17,
2928.
(65) 3D Search and Research Using the Cambridge Structural
Database. Allen, F. H.; Kennard, O. Chem. Des. Automat. News 1993,
8 (1), 1 and 31-37.
(66) Paciello, R.; Siggel, L.; Kneuper, H.-J .; Walker, N.; Roper, M.
J . Mol. Catal. A: Chem. 1999, 143, 85.
(67) Hill, W. E.; Silva-Trivino, L. M. Inorg. Chem. 1979, 18, 361.
(68) Hill, W. E.; Silva-Trivino, L. M. Inorg. Chem. 1978, 17, 2495.
(69) Le Gall, I.; Laurent, P.; Soulier, E.; Salaun, J .-Y. J . Organomet.
Chem. 1998, 567, 13.

Oxidation of BINAP by Dioxygen
Organometallics, Vol. 21, No. 16, 2002 3349
Sch em e 2. Over a ll Cycle for Ca ta lytic Oxid a tion of BINAP w ith 2
tion of groups trans to the oxygen atom of some Pt(II)
BINAP(O) complexes was observed.70
ligand (left-hand path), yielding “(BINAP(O))RhCl” (c
in Scheme 2) with a vacant coordination site and 1 mol
of CO2. This intermediate must in turn react very
rapidly with any CO in solution to form 3. In the
absence of added CO, the only source must be the short-
lived species “Rh(CO)Cl” (b in Scheme 2). This must
supply the needed CO quite effectively because the yield
of 3 can be as high as ∼50%, which is the maximum
possible if it is generated by CO released in forming
BINAP(O)2. The resulting “RhCl” must be the insoluble
uncharacterized residue found after these reactions.
The presence of excess CO has no effect on the rate
of disappearance of 2, but the increased yield of 3 (to
∼60-70%) reflects the fact that its yield is no longer
dependent on the generation of CO from “Rh(CO)Cl”.
Of the high-energy peaks observed in the infrared
spectrum of reaction mixtures containing CO/O2 (1:1),
two can be attributed to the reaction of 3 with CO and
the others may be attributed to side products formed
by complexation of the “Rh(CO)Cl” species with CO. In
the presence of excess BINAP, none of these bands are
detected, and “Rh(CO)Cl” and these other products must
react with BINAP to re-form 2 (Scheme 2). The yield of
3 is then 100% with respect to the amount of added 2.
Ca ta lytic Oxid a tion of BINAP . In the presence of
excess BINAP and under a CO/O2 (1:1) gas mixture, 2
will behave as an oxygenation catalyst to form BINAP-
(O) and BINAP(O)2. Solvent plays a role in determining
the extent of monoxide and dioxide formation. The use
of toluene favors ∼50% monoxide formation, and chlo-
roform results in ∼30% monoxide yields. Thus, the
solvent has an ability to control the relative rates of the
two competing reaction pathways of the O2 adduct.
The reactions are also dependent on CO/O2 gas ratios.
Reactions in 3:1 CO/O2 mixtures yield more BINAP(O)
vs BINAP(O)2, but the decreased [O2] results in very
low yields even after long reaction times. Increasing
temperature increases the relative yields of BINAP(O)2,
but decreasing O2 levels must lower the overall turnover
frequency of the process. Reaction under pure O2 leads
to low yields of BINAP oxides, presumably due to
catalyst degradation resulting from CO2 loss and forma-
tion of uncharacterized rhodium species. Reactions
Kin et ics. R ea ct ion of (BINAP )R h (CO)Cl w it h
Oxygen . The sharp isosbestic point between the 2018
and 1986 cm-1 bands shows the cleanness of this
transformation in chloroform and 1,2-dichloroethane.
The rate constants in Table 3 show good reproducibility
with standard deviations of the averages being about
5%. The reactions of 2 follow the rate equation kobs
)
int + k2[O2], where the intercept is statistically almost
indistinguishable from zero. This, combined with a
negative entropy of activation (∆Sq ) -15.3 cal mol-1
K-1), indicates that the first step of the mechanism most
likely involves dioxygen coordination to the rhodium
complex, forming a short-lived η2-rhodium dioxygen
intermediate. The kinetics of similar reactions of analo-
gous iridium complexes were studied in detail by
Vaska36,71-73 and Halpern.74 These exhibit highly nega-
72
tive ∆Sq values between -24 and -50 cal mol-1 K-1
,
but the Ir complexes do not activate the oxygen toward
phosphine oxidation, simple oxidative addition being
preferred. Oxygen adducts of Rh(I) complexes are only
observed when the electron density on the Rh atom is
made high enough, usually by the presence of three or
more P-donor ligands.36,41,73,75 The presence of the strong
π-acid CO ligand precludes adduct formation unless it
is by addition of singlet oxygen.34
Effect of Ca r bon Mon oxid e. In the absence of
excess BINAP and CO, the reaction yields roughly
equimolar amounts of 3 and BINAP(O)2, together with
an equivalent amount of CO2. This result suggests there
are two reaction pathways after initial O2 coordination
(Scheme 2). The dioxygen in the adduct can proceed to
either form two phosphorus-oxygen bonds (right-hand
path) or bond to one phosphorus atom and to the CO
(70) Kolla`r, L.; Gladiali, S.; Tenorio, M. J .; Weissensteiner, W. J .
Clust. Sci. 1998, 9, 321.
(71) Vaska, L.; Chem, L. S.; Miller, W. V. J . Am. Chem. Soc. 1971,
93, 6671.
(72) Vaska, L.; Chen, L. S.; Senoff, C. V. Science 1971, 174, 587.
(73) Vaska, L.; Chen, L. S. J . Chem. Soc., Chem. Commun. 1971,
1081.
(74) Chock, P. B.; Halpern, J . J . Am. Chem. Soc. 1966, 88, 3512.
(75) McGinnety, J . A.; Payne, N. C.; Ibers, J . A. J . Am. Chem. Soc.
1969, 91, 6301.

3350 Organometallics, Vol. 21, No. 16, 2002
Bunten et al.
under high pressures of O2/CO mixtures and low tem-
peratures would therefore be needed to produce im-
proved selectivity of the monoxide product with reason-
able turnover frequencies.
CO can raise the overall yield of 3 by ∼20%. The
implication is that the added CO rapidly fills the vacant
coordination site on “(BINAP(O))RhCl” made available
after loss of CO2. The observation of additional peaks
in the IR spectrum for reactions under CO suggests the
formation of (η1-BINAP(O))Rh(CO)2Cl (d in Scheme 2)
and other dicarbonyl rhodium species in solution (e in
Scheme 2).
(iii) In the presence of excess BINAP, 3 was shown to
react quantitatively to form 2 and thus complete the
catalytic cycle. The reaction must be conducted under
CO/O2 gas mixtures, thus ensuring the quantitative
regeneration of 2 because, in the absence of CO, the
catalyst would degrade by 50% per cycle. Under cata-
lytic conditions, no higher energy IR bands, normally
found under noncatalytic conditions, are observed, thus
lending support to the idea that BINAP will rapidly
scavenge the short-lived “Rh(CO)Cl” and other Rh
carbonyl species that may exist in solution so as to re-
form 2 (Scheme 2).
Th e In tim a te Mech a n ism of th e Oxid a tion . Al-
though the stoichiometric mechanism outlined in Scheme
2 is well established by the nature and yields of the
various products and by the kinetics, the details of how
the putative dioxygen adduct reacts with two phospho-
rus atoms, or one phosphorus atom and the CO ligand
trans to it, remain obscure. Coordination of the oxygen
will have the effect of homolytically weakening the O-O
bond, and the P-donor atoms and the CO ligand share
a thermodynamic tendency to accept oxygen atoms
which are available from the weakened oxygen mol-
ecule. One possibility is that they do this in an unusual
concerted manner that effectively involves five-member
transition states, and another is that stepwise processes
are involved in which one oxygen atom remains at-
tached to the Rh atom while the other attaches to a
phosphorus or carbon atom. The oxygen atom from the
rhodium then completes the process. The transition
state for the first of these two steps would resemble the
four-member transition states of alkene metathesis.76
Su m m a r y. (i) A mechanism for the stoichiometric or
catalytic conversion of 2 to 3 (Scheme 2) is proposed
partly on the basis of the following observations. The
negative entropy of activation and a linear dependence
of kobs on oxygen concentration are indicative of an
initial addition of oxygen to 2, forming a short-lived
analogue of Vaska’s complex (η2-O2)Ir(PPh3)2(CO)Cl.
The cleanness of the overall reaction from 2 to 3 is
evidenced by a clear isosbestic point; so there are no
side reactions not specified in Scheme 2. Reactions of 2
with oxygen proceed cleanly to form 3 but in up to ca.
50% yield, along with equivalent amounts of BINAP-
(O)2 and CO2, and two minor unidentifiable species
shown in the IR spectrum. Thus, after initial reaction
of 2 with O2, the dioxygen complex can react via two
independent pathways. One route involves oxidation of
both P atoms on BINAP, forming BINAP(O)2 and the
reactive intermediate “Rh(CO)Cl” (right-hand path).
Alternatively, the oxygen ligand can add to one P atom
on BINAP and to CO, forming CO2 and the coordina-
tively unsaturated intermediate “(BINAP(O))RhCl” (left-
hand path), which can efficiently scavenge any free CO
to form 3. It is not possible to decide whether the
oxidations proceed by single concerted steps or via two-
step processes that involve RhdO intermediates.
(ii) Other reactions of the species involved in Scheme
2 support the proposed scheme. Addition of CO does not
affect the rate of decay of 2, but the presence of excess
Con clu sion s
The complex (BINAP)Rh(CO)Cl is an active, albeit
nonselective, oxygenation catalyst for the formation of
BINAP(O) and BINAP(O)2 from solutions of excess
BINAP under CO/O2 gas mixtures at ambient pressures,
and a probable mechanism is proposed for this trans-
formation. The TOF and TON for this process are 0.16
h-1 and 70, respectively, in chloroform at 25 °C. The
rather low reactivity of this process provides an op-
portunity to identify the key species in the catalytic
cycle, but conditions can be envisaged that would
improve the rates and selectivity. To our knowledge, this
is the first example of a catalytic cycle that occurs with
both oxidation of the two phosphorus atoms on a
chelating bisphosphine ligand and oxidation of one
phosphorus atom and a CO ligand. However, because
of the need to form the relatively unusual mononuclear
chelate complexes, this catalytic reaction cannot easily
be generalized.
Ack n ow led gm en t. The support of the Natural
Science and Engineering Research Council, Ottawa, is
gratefully acknowledged. We also thank Professors
Warren Giering and Alfred Prock (Boston University)
for the loan of two gas flow meters and Digital Specialty
Chemicals for the gift of (R)-BINAP.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails and crystallographic data tables of all bond lengths,
angles, atomic coordinates, and thermal parameters, along
with 31P{1H} NMR spectra for complexes 1, 2, and 3 (Figures
S1-S3), ORTEP drawings for 1 and 2 (Figures S4 and S5),
and figures showing yields of BINAP(O) and BINAP(O)2, are
available free of charge via the Internet at http://pubs.acs.org.
(76) Cotton, F. A.; Wilkinson, G. In Advanced Inorganic Chemistry,
5th ed.; Wiley: New York, 1988; Chapter 28, pp 1265-1269.
OM020241A