Zwitterionic relatives to the classic [(P-P)Rh(solv)2]+ ions: Neutral catalysts active for H-E bond additions to olefins (E = C, Si, B)

Article abstract of DOI:10.1002/anie.200250378

Formally zwitterionic bis(phosphanyl)-and bis(amino)borate rhodium(1) complexes (see picture) can catalytically mediate the hydrogenation, hydroacylation, hydroboration, and hydrosilation of double bonds. These neutral systems are shown to be highly active, even under conditions incompatible with their isostructural, but formally cationic, relatives.

Full text of DOI:10.1002/anie.200250378

Angewandte
Chemie
Zwitterionic Catalysts
Zwitterionic Relatives to the Classic [(P–P)-
Rh(solv)2]+ Ions: Neutral Catalysts Active for
À
H E Bond Additions to Olefins (E = C, Si, B)**
Theodore A. Betley and Jonas C. Peters*
Since their introduction by Osborne and Schrock three
decades ago, cationic rhodium [(P–P)Rh(solv)2]+ complexes
À
have been used as precatalysts for a wide range of E H (E =
C, Si, B) bond-forming processes.[1–4] Herein, we introduce a
new and complementary class of neutral, formally zwitter-
ionic rhodium complexes supported by bis(phosphanyl)- and
bis(amino)borate ligands that provide an alternative
[*] Prof. J. C. Peters, T. A. Betley
Division of Chemistry and Chemical Engineering
Arnold and Mabel Beckman Laboratories of Chemical Synthesis
California Institute of Technology, Pasadena, CA 91125 (USA)
Fax: (+1)626-577-4088
E-mail: jpeters@caltech.edu
[**] We thank the NSF (CHE-0132216), the ACS Petroleum Research
Fund, and the Dreyfus Foundation for financial support. T.A.B.
thanks the Department of Defense for a graduate research fellow-
ship. Brian Stoltz and Jeremy May provided helpful discussions and
a sample of 4-methyl-4-pentenal.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 2385 – 2389
DOI: 10.1002/anie.200250378
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2385

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approach to the chemistry of the [(P–P)Rh(solv)2]+ com-
Rh(CH3CN)2] (2) (Scheme 1). The related amine-chelat-
ed complexes [{Ph2B(CH2NMe2)2}Rh(nbd)] (3) and
[{Ph2B(CH2NMe2)2}Rh(CH3CN)2] (4) were similarly pre-
pared.[7] The isostructural but cationic systems [{Ph2Si-
plexes.[5] The rhodium(i) zwitterions introduced herein
(Figure 1) feature a borate counteranion that is fastened
within the ligand backbone and partially insulated from the
coordinated metal center by tertiary phosphanes and
amines.[6,7] These complexes are distinct from previously
prepared bis(pyrazolyl)borate rhodium complexes,[8] where
resonance delocalization of the borate charge is likely to be
more prevalent.
(CH2PPh2)2}Rh(nbd)][PF6]
(5),
[{Ph2Si(CH2PPh2)2}-
Rh(CH3CN)2][PF6] (6), [{Ph2Si(CH2NMe2)2}Rh(nbd)][PF6]
(7), and [{Ph2Si(CH2NMe2)2}Rh([D6]acetone)2][PF6] (8)
were independently prepared to allow comparisons of the
reactivity.[9,10] The acetonitrile adducts for the cationic
systems supported by Ph2PCH2CH2PPh2 (dppe) and
Ph2PCH2CH2CH2PPh2 (dppp), [(dppe)Rh(CH3CN)2][PF6]
(9) and [(dppp)Rh(CH3CN)2][PF6] (10), were derived in a
similar fashion from norbornadiene precursors that were
previously reported.[11,4a]
Owing to their novelty, it was of interest to briefly canvass
the structural characteristics and substitution chemistry of the
Figure 1. Catalyst systems featured herein. E=P, R=phenyl; E=N,
R=methyl.
phosphane-based
zwitterions
[{Ph2B(CH2PPh2)2}RhL2]
(Scheme 1). Treatment of the cyclooctadiene analogue of
1a, [{Ph2B(CH2PPh2)2}Rh(cod)] (1b), with CO gas produced
the dicarbonyl complex[{Ph 2B(CH2PPh2)2}Rh(CO)2] (11)
quantitatively. Dicarbonyl 11 underwent single substitution
by PMe3, PPh3, and an N-heterocyclic carbene to produce the
monocarbonyl adduct complexes [{Ph2B(CH2PPh2)2}-
Rh(CO)(PMe3)] (12), [{Ph2B(CH2PPh2)2}Rh(CO)(PPh3)]
(13), and [{Ph2B(CH2PPh2)2}Rh(CO)(carbene)] (14), respec-
The key findings described in this report pertain to the
catalytic activity and electrophilicity of the structurally similar
zwitterionic and cationic complexes that are depicted in
Figure 1. This comparative study shows that the phosphane-
supported zwitterions are not only catalytically competent,
but that they maintain their catalytic activity in both polar and
nonpolar solvents, in contrast to their isostructural but
cationic phosphane relatives. It is also shown that amine-
chelated zwitterions are promising candidates for further
catalytic development. The activity exhibited by these zwit-
terionic P- and N-chelated systems appears to be distinct from
that of their corresponding N-donor bis(pyrazolyl)borate
rhodium relatives, which seem to be relatively unsuitable for
the reactions described herein.[8]
The synthetic and characterization data for each of the
phosphane-chelated zwitterions is provided in the Supporting
Information. The synthetic details for the amine-chelated
complexes were recently communicated elsewhere.[7] Briefly,
zwitterionic phosphane precatalysts were prepared in high
yield by reaction of the diene–rhodium precursors
[{(nbd)RhCl}2] and [{(cod)RhCl}2] with [Ph2B(CH2PPh2)2]À
(nbd = norbornadiene; cod = cyclooctadiene; Figure 1).
Hydrogenative removal of the norbornadiene ligand of
[{Ph2B(CH2PPh2)2}Rh(nbd)] (1a) occurred in the presence
of acetonitrile to afford the complex[{Ph 2B(CH2PPh2)2}-
tively. The tetrakis(phosphane) complex[{Ph B(CH2PPh2)2}-
2
Rh(PMe3)2] (15) was available directly from 1a by treatment
with excess PMe3.
The solid-state structures of complexes 1a, 2, 11, and 14
were determined by X-ray diffraction studies.[12] The struc-
tures of 2 and 11 are displayed in Figure 2, and those of 1a and
14 are available in the Supporting Information. In each case,
the borate unit is locked rigidly in the backbone of the
[Ph2B(CH2PPh2)2]À ligand at a distance of approximately 4 Š
from the coordinated rhodium center. No inter- or intra-
molecular ion-pairing interactions are observed, consistent
with the amine-chelated zwitterion that was characterized
previously.[7] The [Ph2B(CH2PPh2)2]À ligand conformation for
dicarbonyl 11 is distinct from its conformation in the other
three structures and places the borate unit slightly closer
À
(ꢀ 0.1 Š) to the bound rhodium center (Rh B 3.908(3) Š in
11). One possible explanation for this ligand distortion is that
the borate unit is drawn closer to the metal center to
Scheme 1. Synthesis of zwitterionic [{Ph2B(CH2PPh2)2}RhL2] complexes.
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rapidly, whereas the conversion of styrene to ethylbenzene
proceeded more slowly (Table 1). The cationic systems [(P–
P)Rh(nbd)]+ ((P–P) = dppe, dppp) gave similar results. A
possible mode of catalyst deactivation for the zwitterion 1a is
the formation of an arene-bridged Rh–Rh dimer. Unlike the
cationic dimers that are broken up by coordinating solvents to
provide catalytically active species,[4a–c] the zwitterionic dimer
is very robust and requires forcing conditions to reform a
monomeric species.[14]
Table 1: Styrene hydrogenation results.[a]
Figure 2. Molecular structures of 2 and 11 (ellipsoid representations,
50%; hydrogen atoms and residual solvent molecules are omitted for
clarity).
Entry Catalyst
T [h][b] TOF [hÀ1 [c]
]
1
2
3
4
5
6
[{Ph2B(CH2PPh2)2}Rh(nbd)]
[{Ph2Si(CH2PPh2)2}Rh(nbd)][PF6]
[(dppe)Rh(nbd)][PF6]
[(dppp)Rh(nbd)][PF6]
[{Ph2B(CH2NMe2)2}Rh(nbd)]
[{Ph2Si(CH2NMe2)2}Rh(nbd)][PF6]
1a 26
19
5
–
–
3
7
34
44
28
15
11.4
18
compensate for the increased electrophilicity at the rhodium
center because of the strongly p-acidic carbonyl ligands.
Complexes 1–4 are most simply described as zwitterions in
which some of the borate unit's charge is dissipated through
space and/or through the s-bond framework of the ligand
arms to the coordinated rhodium center. This situation might
be anticipated to result in an appreciably more electron-rich
metal center by comparison to an isostructural but cationic
complex. This view is supported by the comparative IR
data obtained for the carbonyl complexes 11 and
[{Ph2Si(CH2PPh2)2}Rh(CO)2][PF6] (16). The symmetric and
antisymmetric vibrations for the zwitterion 11 in dichloro-
methane (2080 and 2029 cmÀ1, respectively) are much lower
in frequency than those for the cation 16 (2099 and 2056 cmÀ1,
respectively). Cationic 16 is electronically similar to the dppe-
and dppp-supported dicarbonyl complexes, where both of the
latter species exhibit vibrations at 2100 and 2056 cmÀ1,
respectively.[13] A similar trend has been observed for the
amine-based systems.[7] The increased electron-richness of the
zwitterions in comparison to their corresponding cations
might be expected to attenuate or shut down their ability to
mediate catalytic transformations, especially those reactions
typical of electrophilic [(P–P)Rh(solv)2]+ complexes. Our
data, however, suggests otherwise.
Comparative Catalytic Studies: Four model reactions were
probed as exemplary of the [(P–P)RhL2]+ ions. These include
the hydrogenation, hydroboration, and hydrosilation of
styrene,[1–3] in addition to the hydroacylation of 4-methyl-4-
pentenal, a transformation popularized by Bosnich and co-
workers.[4] For each model study, we compared the catalytic
activity of the isolated zwitterions to their cationic but
structurally analogous cousins that incorporate a diphenylsi-
lane unit in the phosphane backbone. To ensure that these
Ph2Si-based cationic systems give rise to activities similar to
more conventional phosphane systems with all-carbon back-
bones, we also canvassed the reactivity of cationic congeners
supported by dppe and dppp.
2.5
5
200
100
[a] Conditions: Rh catalyst (0.1 mol%) in [D6]acetone (0.65 mL); H2
(1 mL) at 1 atm added by syringe at 258C. [b] Time T to consume 50% of
styrene as determined by 1H NMR spectroscopy. [c] Turnover frequency
(TOF) values expressed as mol of product per mol of catalyst per h.
Perhaps more intriguing was that the zwitterionic amine
complex 3 exhibited activity far superior to both the neutral
and cationic phosphane systems, and was twice as active as its
cationic amine counterpart 7 (Table 1). Zwitterion 3 also
proved tolerant to repeated addition of fresh styrene sub-
strate. For further comparison, the related bis(pyrazolyl)bo-
rate complex[{Ph 2B(Pz)2}Rh(nbd)] was found to be com-
pletely inactive for the hydrogenation of styrene under similar
conditions.[8] In fact, attempts to hydrogenatively remove the
norbornadiene ligand from [{Ph2B(Pz)2}Rh(nbd)] under the
mild conditions used were unsuccessful.
The catalytic activity of the zwitterionic systems was more
pronounced in the hydroacylation of the model substrate 4-
methyl-4-pentenal ([Eq. (1)]; Table 2). The precatalysts used
to canvass the hydroacylation of this substrate to racemic 3-
methylcyclopentanone were the acetonitrile adduct com-
plexes 2, 4, 6, 9, 10, and the acetone adduct derivative 8, which
required in situ generation.[10] The turnover frequency (TOF)
for hydroacylation of 4-methyl-4-pentenal by 2 was 40 times
greater than its cationic phosphane counterparts 6, 9, and 10.
The data collected for the conventional phosphane systems 9
and 10 compares well with that for the diphenylsilane system
6. This comparison suggests that the [Ph2Si(CH2PPh2)2] ligand
provides
a
good structural model for the anionic
[Ph2B(CH2PPh2)2]À ligand, and also a good benchmark for
comparing the activity of the zwitterionic systems to more
familiar bis(phosphane)-supported catalysts. The activity of 2
also compares well with the BINAP catalyst systems of
Bosnich and co-workers.[4] Their studies were typically
performed in acetone, and more strongly coordinating
solvents such as CH3CN were strictly excluded as they act
as catalyst poisons. The presence of CH3CN likely attenuated
the activity observed for the cationic species in entries 3–5 of
Table 2. The disparity in hydroacylation activity between
zwitterionic 2 and the cationic, phosphane catalysts could be
As a first catalytic screen, the norbornadiene adduct
complexes 1a, 3, 5, and 7 were assessed for their ability to
mediate the hydrogenation of styrene to ethylbenzene in
1
[D6]acetone (monitored by H NMR spectroscopy). For the
neutral and cationic phosphane systems 1a and 5, hydro-
genative removal of the norbornadiene ligands occurred
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Table 2: Hydroacylation results.[a]
even when excess styrene was used.
The cationic phosphane complexes
6, 9, and 10 exhibited very similar
activities that compared favorably
with data that has been previously
reported.[16] These cationic phos-
phane complexes were in general
three to four times more active
than the zwitterionic phosphane
and amine complexes. However,
the catalytic activity for all the
cationic phosphane systems was
completely shut down when an
appreciable quantity of acetonitrile
was present. Remarkably, the
(phosphanyl)- and (amino)borate
zwitterions were extremely toler-
O
0.5 mol%
catalyst
H
(1)
acetone
RT
O
Entry
Catalyst
Yield [%][b]
T[c]
TOF [hÀ1
]
1
2
3
4
5
6
[{Ph2B(CH2PPh2)2}Rh(CH3CN)2]
[Ph2B(CH2NMe2)2]Rh(CH3CN)2
[{Ph2Si(CH2PPh2)2}Rh(CH3CN)2][PF6]
[(dppe)Rh(CH3CN)2][PF6]
[(dppp)Rh(CH3CN)2][PF6]
[{Ph2Si(CH2NMe2)2}Rh(solv)2][PF6]
2
4
6
99
95
78
84
70
0
5 min
30 min
3 h
3 h
3 h
2400
380
52
56
47
9
10
8[d]
3 h
0
[a] Conditions: Rh catalyst (0.5 mol%) in [D6]acetone (1.0 mL) at 258C. [b] Determined by 1H NMR
spectroscopy. [c] Time T to consume starting material. [d] Hydrogenated in [D6]acetone prior to
substrate introduction; two equivalents of CD3CN added to reaction mixture.
further amplified as a function of the solvent medium. For
example, the activity of cationic 6 fell precipitously when
solvents of varying polarities such as benzene, THF, and
acetonitrile were employed (TOF for 6 = 15, 8, and 0 in these
solvents, respectively). The decreasing activity is likely due to
strong coordination of the donor solvent (CH3CN), inability
of THF or the substrate to break up intermediate Rh–Rh
dimers, and poor solubility of the cation in benzene. In sharp
contrast, zwitterionic 2 maintained TOF values near 2400 in
all three solvent systems. Furthermore, the amine zwitterion 4
was also a relatively active catalyst for the hydroacylation
reaction, superior to all cationic systems examined. Like
zwitterionic 2, 4 maintained its catalytic activity in benzene
and in CH3CN. While the in-situ-
ant to both acetonitrile and THF. For example, virtually no
loss of activity was observed in 50/50 solvent mixtures of
CH3CN/THF and CH3CN/acetone.
In summary, this study shows that zwitterionic phosphane
and amine rhodium systems are promising catalysts for H–E
addition reactions to olefins. The reactivity that the zwitter-
ionic systems display parallels that of the cationic [(P–P)-
RhL2]+ systems with two important distinctions: The zwitter-
ions are active in the presence of large quantities of a strongly
coordinating donor solvent such as acetonitrile, which may
reflect their attenuated electrophilicity in comparison to the
cationic systems. Moreover, because the neutral systems are
appreciably soluble in nonpolar hydrocarbons such as ben-
Table 3: Hydroboration and hydrosilation results.[a]
generated catalyst system [{Ph2Si-
(CH2NMe2)2}Rh(acetone)2]+ did
afford racemic 3-methylcyclopen-
tanone, the reaction was only 90%
complete after 2 h and was com-
pletely inhibited when two equiv-
alents of CH3CN were added with
substrate to the solution.
HBCat
CatB
or
SiPh2
H2SiPh2
or
(2)
Ph
Ph
Reagent
Ph
0.01 mol % cat
Catalyst
Solvent
Yield [%][b]
T [h][c]
TOF [hÀ1
]
2
2
4
4
6
6
9
9
10
10
HBCat
HBCat
HBCat
HBCat
HBCat
HBCat
HBCat
HBCat
HBCat
HBCat
THF
THF/CH3CN
THF
THF/CH3CN
THF
THF/CH3CN
THF
THF/CH3CN
THF
THF/CH3CN
95
96
93
94
>99
0
92
0
98
0
1.25
1.5
1.5
1.65
0.5
24
1
24
0.75
24
7600
6400
6200
5700
20000
0
9200
0
13000
0
The precatalysts 2, 4, 6, 9, and
10 were next examined for the
hydroboration and hydrosilation
of styrene ([Eq. (2)]; Table 3).
The cationic precatalyst 8 was also
examined, though the data
obtained for this system were
highly irreproducible due to a pro-
pensity for the complexto precip-
itate Rh0 upon reaction with the
borane and silane reagents. For the
hydroboration system, monitored
in situ by 11B NMR spectroscopy,
only the branched product was
obtained for all cases as deter-
mined by 1H NMR spectroscopy.[15]
The hydrosilation reactions, moni-
tored by GC/MS and 1H NMR
spectroscopy, in all cases afforded
a single anti-Markovnikov product,
2
2
4
4
6
6
9
9
10
10
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
acetone
acetone/CH3CN
acetone
acetone/CH3CN
acetone
acetone/CH3CN
acetone
acetone/CH3CN
acetone
acetone/CH3CN
98
96
91
88
>99
0
96
0
>99
0
1.5
1.5
2
2
0.5
24
0.5
24
0.5
24
6500
6400
4600
4400
20000
0
19200
0
20000
0
[a] Conditions: Rh catalyst (0.01 mol%). [b] For hydroborations: determined by 11B NMR spectroscopy
based on borane conversion; for hydrosilations: determined by 1H NMR spectroscopy based on styrene
conversion. [c] Time elapsed to convert styrene.
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bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
zene, they maintain their activity over a rather broad solvent
polarity range. This solvent range is not spanned by more
typical cationic catalysts, as illustrated by the cationic model
complexes described in this study. We are encouraged to
suggest that a zwitterionic approach to catalyst design will
provide new systems with the potential for a high degree of
functional group and solvent compatibility.
[13] [(dppp)Rh(CO)2][PF6] was not previously reported and thus
prepared in an analogous fashion to 14 and is reported in the
Supporting Information. [(dppe)Rh(CO)2][PF6] was previously
reported in ref [4b].
[14] The neutral “[{Ph2B(CH2PPh2)2}RhI]” unit can dimerize to a
catalytically inactive [{[Ph2B(CH2PPh2)2]Rh}2] complexpresum-
ably stabilized by phosphine aryl-rhodium(i) interactions, a
process also common of cationic “[(P–P)Rh]+” systems.[4] The
dimer has limited solubility but has been characterized by 31P
Received: October 16, 2002
Revised: February 10, 2003 [Z50378]
1
and H NMR spectroscopy, as well as ES·MS and combustion
Keywords: borates · boron · hydrogenation · rhodium ·
.
analysis (see Supporting Information). The presence of the
[{[Ph2B(CH2PPh2)2]Rh}2] dimer has been detected by 31P NMR
spectroscopy in situ during hydrogenation reactions with a 2%
catalyst loading.
zwitterions
[1] a) R. R. Schrock, J. A. Osborne, J. Am. Chem. Soc. 1971, 93,
3091; b) R. H. Crabtree, Acc. Chem. Res. 1979, 12, 331;
c) “Homogeneous Catalysis”: R. H. Crabtree, The Organome-
tallic Chemistry of the Transition Metals, 3rd ed., Wiley, New
York, 2001, chap. 9, pp. 206–236.
[15] C. Elschenbroich, A. Salzer, Organometallics: A Concise Intro-
duction, 2nd ed., VCH, New York, 1992.
[16] M. D. Fryzuk, L. Rosenberg, S. J. Rettig, Organometallics 1996,
15, 2871.
[2] I. Ojima, The Chemistry of Organic Silicon Compounds (Eds.: S.
Patai, Z. Rappoport), Wiley, New York, 1989, chap. 25.
[3] a) K. Burgess, M. J. Ohlmeyer, Chem. Rev. 1991, 91, 1179, and
references therein; b) D. A. Evans, G. C. Fu, A. H. Hoveyda, J.
Am. Chem. Soc. 1988, 110, 6917.
[4] a) D. P. Fairlie, B. Bosnich, Organometallics, 1988, 7, 936; b) D. P.
Fairlie, B. Bosnich, Organometallics, 1988, 7, 946; c) B Bosnich,
Acc. Chem. Res. 1998, 31, 667.
[5] For another class of rhodium(i) zwitterions see: a) R. R. Schrock,
J. A. Osborn, Inorg. Chem. 1970, 9, 2339; b) Z. Zhou, G. Facey,
B. R. James, H. Alper, Organometallics 1996, 15, 2496; c) B. G.
Van den Hoven, H. Alper, J. Am. Chem. Soc. 2001, 123, 10214.
[6] a) J. C. Thomas, J. C. Peters, J. Am. Chem. Soc. 2001, 123, 5100;
b) C. C. Lu, J. C. Peters, J. Am. Chem. Soc. 2002, 124, 5272.
[7] T. A. Betley, J. C. Peters, Inorg. Chem. 2002, 41, 6541.
[8] For examples of bis(pyrazolyl)borate–rhodium(i) complexes see:
a) M. Bortolin, U. E. Bucher, H. Ruegger, L. M. Venanzi, A.
Albinati, F. Lianza, S. Trofimenko, Organometallics 1992, 11,
2514; b) M. J. Baena, M. L. Reyes, L. Rey, E. Carmona, M. C.
Nicasio, P. J. Perez, E. Guitierrez, A. Monge, Inorg. Chim. Acta
1998, 273, 244. Hydrogenation-like catalysis by bis(pyrazolyl)-
borate – rhodium(i) complexes has not, to our knowledge, been
reported, in accord with our own observations using [{Ph2B(pyr-
azolyl)2}Rh(nbd)].
[9] The Supporting Information contains complete synthetic and
characterization data for all new compounds.
[10] The complex[{Ph Si(CH2NMe2)2}Rh(CH3CN)2][PF6] could not
2
be isolated due to problematic precipitation of Rh0.
[{Ph2Si(CH2NMe2)2}Rh(acetone)2][PF6] could be generated,
but only in situ as it also proved unstable to a typical workup
procedure.
[11] R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 4450.
[12] Summarized crystallographic data: [{Ph2B(CH2PPh2)2}Rh(CO)2]
(11): (C40H34BO2P2Rh), Mr = 722.33, yellow plate, collection
temperature = 96 K, monoclinic, space group P21/n
,
a =
9.8984(9), b = 18.7925(16), c = 18.1113(16) Š, b = 98.108(2),
V= 3335.3(5) Š3, Z = 4, R1 = 0.0433 [I > 2s(I)], GOF = 1.468.
[{Ph2B(CH2PPh2)2}Rh(CH3CN)2]·THF (2): (C42H40BN2P2Rh
·C4H8O), Mr = 820.52, yellow cube, collection temperature =
98 K, monoclinic, space group P21/n
16.4484(13), c = 21.5822(17) Š, b = 102.1030(10),
,
a = 11.8228(9), b =
V=
4103.7(6) Š3, Z = 4, R1 = 0.0425 [I > 2s(I)], GOF = 1.192.
CCDC-195381 (1a), CCDC-195383 (2), CCDC-195382 (11),
CCDC-195384 (14) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
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