Novel bimetallic group 9 metal catalysts containing P,S-chelating o-carboranyl ligand system for the carbonylation of methanol

Article abstract of DOI:10.1246/cl.2000.602

A series of novel group 9 transition metal complexes containing a P,S-chelating ligand have been prepared. The bimetallic rhodium carbonyl complex is much more effective in catalyzing the carbonylation of methanol to acetic acid than the previously known catalyst, [RhI₂(CO)₂]^-.

Full text of DOI:10.1246/cl.2000.602

602
Chemistry Letters 2000
Novel Bimetallic Group 9 Metal Catalysts Containing P,S-Chelating o-Carboranyl Ligand System
for the Carbonylation of Methanol
Heung-Sae Lee, Jin-Young Bae,† Jaejung Ko,* Yong Soo Kang,† Hoon Sik Kim,*† and Sang Ook Kang*
Department of Chemistry, Korea University, Chochiwon, Chung-nam 339-700, Korea
†Korea Institute of Science and Technology, P.O. Box 131, Cheongryangri, Seoul 130-650, Korea
(Received January 28, 2000; CL-000094)
A series of novel group 9 transition metal complexes con-
taining a P,S-chelating ligand have been prepared. The
bimetallic rhodium carbonyl complex is much more effective in
catalyzing the carbonylation of methanol to acetic acid than the
previously known catalyst, [RhI2(CO)2]-.
Rh 2a, Ir 2b). Bubbling carbon monoxide through a
dichloromethane solution of 2 leads to the formation of the
bimetallic P,S-chelate complexes [(CabP,S)M(CO)]2 3 (M = Rh
3a, Ir 3b) as main products, which were isolated in 89-94%
yields. Alternatively, such complexes 3 can be synthesized using
a second route. When compounds [M(µ-Cl)(CO)2]2 (M = Rh, Ir)
were reacted with two equiv of 1, the bimetallic complexes 3a-b
were obtained in good yields (73-81%). Such complexes 3 were
isolated as air-stable orange-yellow colored microcrystalline
solids and were then spectroscopically characterized.8 X-Ray
structure determination of 3a was carried out, and the resulting
ORTEP plot and selected bond distances and angles of 3a are
shown in Figure 1.9 It consists of discrete dinuclear units pos-
sessing a center of symmetry at the midpoint of the Rh(1)-Rh(1)'
vector, which forms an Rh2S2 butterfly core with a thiolate
bridge. The Rh2S2 ring is puckered, the dihedral angle between
the planes Rh1S1Rh1' and Rh1S1'Rh1' being 71.5 (1)o. Each Rh
atom is four-coordinate and displays a square-planar geometry.
The average Rh-S distance of 2.370 Å is typical.10,2a
Bimetallic complexes using sulfur-containing bridging lig-
ands such as thiolate,1 aminothiolate,2 phosphinothiolate,3 to
hold two metal atoms in close proximity have received consid-
erable attention in recent years. One of the main reasons is the
interest in the cooperative influence4 of neighboring metal cen-
ters on catalytic reactions. The design of new bimetallic com-
plexes involves the use of new assembling P,S-chelating ligand
containing bulky o-carborane backbone. Recent reports of
unusually stable C,N-,5 S,N-,6 and S,S'-chelating7 o-carboranyl
metal complexes appear to imply that the chelation, rigid con-
formation, and the o-carboranyl ligand backbone might be ideal
for the stabilization of possible metal intermediates for
organometallic reactions. In addition, the long-term thermal
stability of such metal complexes can be realized by incorporat-
ing thermally robust o-carborane ligand backbone. Here, we
describe new classes of improved methanol carbonylation cata-
lysts containing the o-carboranylphosphinothiolato ligand,
LiCabP,S 1, which show significant improvements in catalytic
activities over those obtained with [RhI2(CO)2]-.
The reaction of [M(µ-Cl)(cod)]2 (M = Rh, Ir; cod =
cycloocta-1,5-diene) with two equiv of 1 produced the four-
coordinated metallacyclic compounds (CabP,S)M(cod) 2 (M =
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
603
Compounds 3a and 3b exhibit one ν(CO) band at 1994 and
1985 cm-1, respectively. The lower frequency shift of CO
stretches in 3 than those of the other carbonyl metal phos-
phinothiolates3 is indicative of a higher basicity in the CabP,S
case. The reactivity of 3 has been explored in order to increase
the nucleophilicity of the metal atoms. Such complexes 3 react
with an excess PEt3 to yield the corresponding mononuclear
metal chelates 4.11
We gratefully acknowlege the financial support from the
Korea Science Foundation and the Korea Science and
Engineering Foundation.
References and Notes
1
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The catalytic activities of the bimetallic complexes 3 were
tested for the carbonylation of methanol and were compared
with those of the mononuclear complexes 4 under the same
conditions. Figure 2 shows the reaction profiles for the various
rhodium and iridium complexes by plotting the quantity of car-
bon monoxide vs time, and Table 1 lists the turnover numbers.
As a control experiment, the catalytic reaction was carried out
with the Monsanto catalyst [RhI2(CO)2]-, which was formed in
situ from the added [RhCl(CO)2]2 under the stated reaction con-
ditions.12 As can be seen in Figure 2 and Table 1, at equivalent
rhodium catalyst concentrations, the bimetallic complex 3a
exhibits significant catalytic activity over the Monsanto cata-
lyst. The increased catalytic activity of 3a is most likely a con-
sequence of the formation of the thermally stable metal chelate
3
4
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that is imposed by bulky o-carborane ligand backbone. The 31
P
NMR spectrum of the inorganic residue remaining after the car-
bonylation reaction by 3a indicates that a high proportion of the
phosphorus remains bound to the rhodium center and retains the
original bimetallic structure as deduced from the chemical shift
and coupling constant.
Data for 3a. Anal. Calcd for C30H40B20O2P2S2Rh2: C, 36.74;
H, 4.11%. Found: C, 36.77; H, 4.18%. IR (KBr, cm-1): ν(BH),
2577; ν(C=O), 1994.; ν(C=C), 1437. 1H NMR (200.13 MHz,
ppm, CDCl3): 8.43 (br, Ph), 7.66 (br, Ph). 13C{1H} NMR
1
(50.3 MHz, ppm, CDCl3): 185.90 (d, JRh-C = 57 Hz, CO),
136.04 − 126.71 (s, 12, Ph), 94.00 (s, C2B10). 31P{1H} NMR
1
(80.0 MHz, ppm, CDCl3): 81.44 (d, JRh-P = 169 Hz, PPh2).
Data for 3b. Anal. Calcd for C30H40B20O2P2S2Ir2: C, 31.08;
H, 3.48%. Found: C, 31.11; H, 3.52%. IR (KBr, cm-1):
ν(BH), 2580; ν(C=O), 1984.; ν(C=C), 1433. 1H NMR
(200.13 MHz, ppm, CDCl3): 8.39 (br, Ph), 7.65 (br, Ph).
13C{1H} NMR (50.3 MHz, ppm, CDCl3): 206.10 (s, CO).
31P{1H} NMR (80.0 MHz, ppm, CDCl3): 53.18 (s, PPh2).
Crystallographic data for 3a: a = 10.8636(9) Å, b =
30.160(2) Å, c = 14.633(2) Å, and β = 92.414(9)o with Z = 4
in space group P21/a (No. 14). R1(wR2) = 0.0756 (0.1968)
for 9374 data with I > 2.0σ(I) and anisotropic refinements of
the model with idealized hydrogen atoms.
9
10 A. Elduque, L. A. Oro, M. T. Pinillos, A. Tiripicchio, and F.
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11 Data for 4a. Anal. Calcd for C21H35B10OP2SRh: C, 41.45; H,
5.80%. Found: C, 41.50; H, 5.88%. IR (KBr, cm-1): ν(BH),
2596; ν(C=O), 1975. 1H NMR (200.13 MHz, ppm, CDCl3):
8.26 (m, 5, Ph), 7.54 (m, 5, Ph), 1.85 (dq, 6, CH2), 1.20 (dt, 9,
Me). 13C{1H} NMR (50.3 MHz, ppm, CDCl3): 187.11 (d,
1
1JRh-C = 58 Hz, CO), 16.45 (d, JP-C = 13 Hz, P(CH2Me)3),
6.93 (s, P(CH2Me)3). 31P{1H} NMR (80.0 MHz, ppm,
1
2
CDCl3): 82.21 (dd, JRh-P = 296 Hz, JP-P = 128 Hz, PPh2),
26.22 (dd, 1JRh-P = 296 Hz, 2JP-P = 128 Hz, PEt3). Data for 4b.
Anal. Calcd for C21H35B10OP2SIr: C, 36.14; H, 5.06%.
Found: C, 36.19; H, 5.10%. IR (KBr, cm-1): ν(BH), 2577;
ν(C=O), 1965. 1H NMR (200.13 MHz, ppm, CDCl3): 8.20
(m, 5, Ph), 7.55 (m, 5, Ph), 2.00 (dq, 6, CH2), 1.18 (dt, 9, Me).
13C{1H} NMR (50.3 MHz, ppm, CDCl3): 206.10 (s, CO),
1
18.31 (d, JP-C = 34 Hz, P(CH2Me)3), 8.52 (s, P(CH2Me)3).
31P{1H} NMR (80.0 MHz, ppm, CDCl3): 79.60 (d, 2JP-P = 130
Hz, PPh2), 23.12 (d, 2JP-P = 130 Hz, PEt3).
12 a) D. Forster, J. Am. Chem. Soc., 98, 846 (1976). b) D.
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