Ruthenium maltolato complexes

Article abstract of DOI:10.1039/a606961d

Two maltol molecules can be deprotonated and coordinated to Ru^II to generate a series of octahedral complexes having the general formula Ru(ma)₂L₂ (ma = C₆H₅O₃; L = PPh₃, Me

Full text of DOI:10.1039/a606961d

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Ruthenium maltolato complexes
Michael D. Fryzuk,* Michael J. Jonker and Steven J. Rettig
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T 1ZI
Two maltol molecules can be deprotonated and coordinated
to RuII to generate a series of octahedral complexes having
the general formula Ru(ma)2L2 (ma = C6H5O3; L = PPh3,
Me2SO; L2 = cycloocta-1,5-diene); when L = PPh3, a
precursor for the dimerization of phenylacetylene is ob-
tained.
ruthenium(ii) having only oxygen donors as ancillary ligands to
be structurally characterized. Both structures display similar cis
octahedral geometries; in fact upon close examination of the
two structures in the solid state, the same C1 molecular
symmetry is found for each derivative. Because the maltolato
ligand has two different oxygen donors, it can be considered as
an anionic AB bidentate ligand; for an octahedral molecule
having the general formula M(AB)2L2 (L = monodentate
ligand), such a complex can exist in five stereoisomeric forms
(trans and cis descriptors refer to disposition of L2): two trans
stereoisomers with C2h and C2v symmetry, respectively and
three cis stereoisomers two of which have C2 symmetry while
the last is completely asymmetric (C1). For the situation where
L2 is a symmetrical bidentate ligand, such as cod, only the three
cis isomers are possible.
Maltol 1 is a naturally occurring, water-soluble, non-toxic food
additive.1 When deprotonated (pKa = 8.38),2 it forms an
anionic system capable of acting as a chelating, bidentate, O,OA
ligand of the type shown as 2.
Because this ligand can confer water solubility on its metal
complexes, even when the complexes are uncharged, there have
been many reports on the use of this ligand in biological studies.
For example, Al(ma)3 has been used in studies related to
AlzheimerAs disease,3 Fe(ma)3 has been found to have potential
as a treatment for iron-deficiency anaemia4 and VO(ma)2 has
been tested as an insulin mimetic.5–7 However, little work has
been done on the use of this ligand in organometallic type
complexes or in homogeneous catalysis.8–10 Here we report the
preparation and structure of a series of octahedral bis(maltolato)
complexes of ruthenium11 and show that one of these
complexes shows activity for the dimerization of phenyl-
acetylene.
The reaction of potassium maltolate, Kma (prepared by the
addition of KOBut to 1 in Et2O), with a variety of ruthenium(ii)
complexes in organic solvents proved to be successful as a
preparative route for the incorporation of the maltolato ligand.
Thus, reaction of Kma with [Ru(cod)Cl2]x in thf led to the
formation of Ru(ma)2(cod) 3 as orange–red crystals in 95%
yield; similarly, addition of Kma to RuCl2(PPh3)3 and RuCl2-
(Me2SO)4 generated Ru(ma)2(PPh3)2 4 and Ru(ma)2(Me2SO)2
5, respectively. These reactions are outlined in Scheme 1.
The solid-state structures of the cod complex 3 and the
bis(dimethyl sulfoxide) derivative 5 were obtained and are
shown in Figs. 1 and 2 respectively, along with selected bond
lengths and angles.† Complex 3 is the first alkene complex of
From the NMR spectroscopic data, the solid-state structure of
3 is maintained in solution such that only one species is
observed and it displays peaks characteristic of a molecule with
low symmetry. On the other hand, the bis(phosphine) complex
4 exists as a mixture of three of the possible five stereoisomers;
one isomer shows two inequivalent phosphines by 31P{1H}-
1
NMR spectroscopy and resonances in the H NMR spectrum
that compare to the asymmetric cis isomer found for 3 above.
The remaining two stereoisomers of 4 have equivalent phos-
phines and equivalent maltolato ligands and thus could either
O(4)
C(7)
C(12)
C(8)
C(11)
C(9)
C(19)
C(10)
O(5)
C(18)
C(17)
O(6)
C(20)
M
Ru
O
O
O
O
C(13)
H5
H6
OH
Me
O
C(16)
O(2)
Me
O(3)
C(14)
C(15)
C(3)
C(2)
C(4)
Ru(ma)2(cod)
3
i
O
O
C(5)
C(1)
O(1)
O– K+
Me
ii
C(6)
Ru(ma)2(PPh3)2
4
Fig. 1 X-Ray crystal structure of Ru(ma)2(cod) (3) (ellipsoids drawn at the
33% probability level). Selected interatomic distances (Å) and bond angles
(°): Ru–O(2) 2.126(4), Ru–O(3) 2.098(5), Ru–O(5) 2.072(4), Ru–O(6)
2.114(4), Ru–C(13) 2.138(5), Ru–C(14) 2.157(7), Ru–C(17) 2.153(7), Ru–
C(18) 2.161(6), O(2)–C(3) 1.306(7), O(3)–C(4) 1.288(7), C(3)–C(4)
1.43(1), O(5)–C(9) 1.335(7), O(6)–C(10) 1.262(9), C(9)–C(10) 1.45(1),
C(13)–C(14) 1.34(2), C(17)–C(18) 1.46(1); O(2)–Ru– O(3) 79.5(2),
O(5)–Ru–O(6) 79.5(2), O(3)–Ru–O(5) 156.0(2), O(2)–Ru–O(6) 92.7(2).
iii
Kma
Ru(ma)2(Me2SO)2
5
Scheme 1 Reagents and conditions: i, [Ru(cod)Cl2]x, thf, 65 °C, 16 h; ii,
RuCl2(PPh3)3, thf, 22 °C, 16 h; iii, RuCl2(Me2SO)4, toluene, 90 °C, 16 h
Chem. Commun., 1997
377

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have trans or cis geometries. The situation for the Me2SO
required for activity, and this is reasonable given that this
18-electron complex contains two rather large, monodentate
ligands. One can speculate further that the mechanism of alkyne
dimerization involves a vinylidene intermediate12,13 as required
for the production of both E and Z isomeric forms of the eneyne
product. Internal alkynes were generally found to be unreactive
with all three of the bis(maltolato) complexes of Ru.
In this study we have demonstrated that organometallic type
complexes and catalytic activity can be observed for Ru
derivatives that are stabilized by the maltolato ancillary ligand.
In addition, the cyclooctadiene complex 3 represents the first
reported alkene-type complex of Ru stabilized by only oxygen
donor ligands.
complex is more complex since isomers are also apparent in the
1
crude reaction mixture but their H NMR spectra overlap too
much to be able to distinguish them.
While homoleptic maltolato complexes, M(ma)n, are gener-
ally water soluble we found that only the bis(dimethyl
sulfoxide) complex 5 exhibited any water solubility;10 both the
cod derivative 3 and the bis(phosphine) 4 were insoluble in
H2O. These complexes also displayed differences in their
reactivity patterns. In this case, addition of phenylacetylene to
either the cod complex 3 or the bis(dimethyl sulfoxide) 5 led to
no reaction and recovery of starting materials. However, the
bis(phosphine) derivative 4 was found to dimerize PhC·CH to
give the two possible linear isomers in a 1:1 ratio with turnover
rates of ca. 20 h21 (Scheme 2).
This work was supported by NSERC of Canada. We also
acknowledge Johnson Matthey for the loan of Ru salts.
Attempts to detect intermediates in this process by monitor-
ing the reaction by 31P{1H} NMR spectroscopy were unsuc-
cessful; however, when the dimerization was performed in the
presence of excess PPh3 the rate of the dimerization reaction
was noticeably retarded. Thus, phosphine dissociation from 4 is
Footnote
† Crystal data: 3, C20H22O6Ru, M = 459.46, orthorhombic, space group
Pna21 (no. 33), a = 16.542(2), b = 9.684(2), c = 11.480(8) Å, U
= 1839.2(6) Å3, Z = 4, Dc = 1.659 g cm23, m(Mo-Ka) = 8.68 cm21
,
F(000) = 936. An orange–red prism of dimensions 0.10 3 0.10 3 0.15 mm
was used. 3755 reflections were measured on a Rigaku AFC6S with Mo-Ka
radiation using w–2q scans. The structure was solved by Patterson methods
using full-matrix least-squares on F for all non-hydrogen atoms using
Lorentz polarization and absorption corrections to give R = 0.034 and
Rw = 0.029 for 1775 independent observed reflections with I > 3s(I) and
O(4)
H(7)
H(10)
H(8)
H(13)
H(12)
C(12)
H(9)
C(7)
H(6)
C(8)
C(13)
H(11)
244 variables for 2qmax = 65°.
C(11)
–
C(9)
O(7)
.
5, C16H22O8RuS2 2.5C6H6, M = 702.82, triclinic, space group P1 (no.
C(10)
2), a = 12.015(2), b = 16.509(2), c = 8.7933(8) Å, a = 99.853(9),
b = 100.225(9), g = 94.28(1)°, U = 1681.2(4) Å3, Z = 2, Dc = 1.388
g cm23, m(Mo-Ka) = 6.19 cm21, F(000) = 726. An orange prism of
dimensions 0.30 3 0.35 3 0.45 mm was used. 8104 relections were
measured of which 7729 were unique using a Rigaku AFC6S with Mo-Ka
radiation using w–2q scans. The structure was solved using direct methods
using full-matrix least squares on F for all non-hydrogen atoms using
Lorentz polarization and absorption corrections to give R = 0.038 and
Rw = 0.043 for 5946 independent observed reflections with I > 3s(I) and
379 variables for 2qmax = 55°. Atomic coordinates, bond lengths and
angles, and thermal parameters have been deposited at the Cambridge
Crystallographic Data Centre (CCDC). See Information for Authors, Issue
No. 1. Any request to the CCDC for this material should quote the full
literature citation and the reference number 182/293.
O(5)
H(15)
C(14)
S(1)
O(6)
H(14)
Ru
H(18)
C(15)
H(16)
O(8)
S(2)
O(2)
H(19)
O(3)
H(17)
C(16)
H(22)
C(3)
H(1)
C(4)
H(21)
C(1)
C(2)
H(20)
H(2)
H(4)
C(5)
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H(3)
C(6)
H(5)
O(1)
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Fig. 2 X-Ray crystal structure of Ru(ma)2(Me2SO)2 5 (ellipsoids drawn at
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C
C
H
Ph
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Z
Scheme 2 Reagents and conditions: i, 1 mol% Ru(ma)2(PPh3)2 4, 50 °C,
toluene: E:Z ratio = 1:1
Received, 10th October 1996; Com. 6/06961D
378
Chem. Commun., 1997