Reactivity of quinoline-8-carbaldehyde toward platina-β-diketone and Acetyl(amine)platinum(II) complexes. Formation of Acyl(hydroxyalkyl)platinum(IV)

Article abstract of DOI:10.1021/om4011774

The reaction of the platina-β-diketone [Pt₂{(COMe) ₂H}₂(μ-Cl)₂] (1) with quinoline-8- carbaldehyde (C₉H₆NCHO) in MeOH/H₂O (Pt/C ₉H₆NCHO = 1/2) leads to the cleavage of the chloride bridges in 1 with acetaldehyde displacement and to the formation of the acetylacyl(hydroxyalkyl)platinum(IV) complex [Pt(COMe)Cl(C₉H ₆NCO-κN,κC)(C₉H₆NCHOH-κN, κC)] (3) as a single diastereomer (OC-6-46). Water-assisted activation of the OC-H bond of one aldehyde and H transfer to the oxygen atom of the second aldehyde occurs. In the presence of sodium methoxide, dehydrochlorination of the platinum starting material affords the dinuclear [Pt₂(C ₉H₆NCO-κN,κC)₂(μ-COMe) ₂] (4) with acetyl bridges in a head-to-tail fashion. Pyridine or tetrahydrofuran fail to cleave the acetyl bridges, which are readily cleaved by triphenylphosphane to afford the mononuclear [Pt(C₉H ₆NCO-κN,κC)(COMe)(PPh₃)] (5) with the two acyl groups in cis positions. The reaction of the diacetylbis(amine)platinum(II) complexes [Pt(COMe)₂(NH₂R)₂] (R = CH ₂Ph (2a), Et (2b)) with quinoline-8-carbaldehyde produced the Schiff base products [Pt(COMe)₂(C₉H₆NCHi - NR-κN,κN′)] (R = CH₂Ph (6a), Et (6b)). The complexes have been fully characterized, and X-ray diffraction structures of 3 and 4 are reported.

Full text of DOI:10.1021/om4011774

Article
pubs.acs.org/Organometallics
Reactivity of Quinoline-8-carbaldehyde toward Platina-β-diketone
and Acetyl(amine)platinum(II) Complexes. Formation of
Acyl(hydroxyalkyl)platinum(IV)
Itziar Zumeta,† Tim Kluge,‡ Claudio Mendicute-Fierro,† Cristoph Wagner,‡ Lourdes Ibarlucea,†
Tobias Ruffer,§ Virginia San Nacianceno,† Dirk Steinborn,*,‡ and María A. Garralda*,†
̈
†Faculty of Chemistry at San Sebastian, University of the Basque Country, UPV/EHU, P° Manuel Lardizab
́
al, 3, 20018 San Sebastian
́
,
Spain
‡Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Straße 2, D-06120 Halle, Germany
§Institute of Chemistry, Chemnitz University of Technology, Straße der Nationen 62, D-09111 Chemnitz, Germany
S
* Supporting Information
ABSTRACT: The reaction of the platina-β-diketone
[Pt2{(COMe)2H}2(μ-Cl)2] (1) with quinoline-8-carbaldehyde
(C9H6NCHO) in MeOH/H2O (Pt/C9H6NCHO = 1/2) leads
to the cleavage of the chloride bridges in 1 with acetaldehyde
displacement and to the formation of the acetylacyl-
(hydroxyalkyl)platinum(IV) complex [Pt(COMe)Cl-
(C9H6NCO-κN,κC)(C9H6NCHOH-κN,κC)] (3) as a single
diastereomer (OC-6-46). Water-assisted activation of the OC−
H bond of one aldehyde and H transfer to the oxygen atom of
the second aldehyde occurs. In the presence of sodium
methoxide, dehydrochlorination of the platinum starting
material affords the dinuclear [Pt2(C9H6NCO-κN,κC)2(μ-
COMe)2] (4) with acetyl bridges in a head-to-tail fashion.
Pyridine or tetrahydrofuran fail to cleave the acetyl bridges, which are readily cleaved by triphenylphosphane to afford the
mononuclear [Pt(C9H6NCO-κN,κC)(COMe)(PPh3)] (5) with the two acyl groups in cis positions. The reaction of the
diacetylbis(amine)platinum(II) complexes [Pt(COMe)2(NH2R)2] (R = CH2Ph (2a), Et (2b)) with quinoline-8-carbaldehyde
produced the Schiff base products [Pt(COMe)2(C9H6NCHNR-κN,κN′)] (R = CH2Ph (6a), Et (6b)). The complexes have
been fully characterized, and X-ray diffraction structures of 3 and 4 are reported.
(PR3)x(C2H4)3−x] (x = 1, 2) react with PPh2(o-C6H4CHO) or
C9H6NCHO to afford the square-planar acylhydridoplatinum-
(II) derivative [PtH{PPh2(o-C6H4CO)-κP,κC}(PPh3)] or
[PtH(C9H6NCO-κN,κC)(PR3)].7,8 The iridium(III) complex
[IrHCl{PPh2(o-C6H4CO)-κP,κC}(Cod)] (Cod = 1,5-cyclo-
octadiene) is also believed to undergo the oxidative addition
of C9H6NCHO, followed by an Ir to O hydrogen transfer, to
afford the coordinatively saturated hydrido irida-β-diketone
[IrHCl{(PPh2(o-C6H4CO)-κP,κC)(C9H6NCO-κN,κC)H}].9 In
contrast, the reaction of halo- or alkylplatinum(II) compounds
with tethered aldehydes affords complexes with P- or N-
coordinated monodentate ligands such as [NBu4][Pt-
(C6F5)3{PPh2(o-C6H4CHO)-κP}],10 cis-[PtX2{PPh2(o-
C6H4CHO)-κP}2] (X = Me, Cl),4a,11 or trans-
[PtCl2(C9H6NCHO-κN)(PR3)].12 The neutral complexes
may afford the acyl derivatives [PtX{PPh2(o-C6H4CO)-κP,κC}-
{PPh2(o-C6H4CHO)-κP}] (X = Me, Cl),4a [Pt{PPh2(o-
C6H4CO)-κP,κC}2],11 and [PtCl(C9H6NCO-κN,κC)(PR3)],12
INTRODUCTION
■
Aldehydes are useful reagents that can undergo different types
of transformations. Among others, the activation of the OC−H
bonds mediated by late-transition-metal complexes is involved
in many stoichiometric and catalytic reactions and has long
been an attractive subject in organometallic and organic
chemistry.1,2 Aldehydes are also widely used for the
condensation reaction with primary amines, often coordinated
to transition metals, to afford imines. A large number of Schiff
bases, including chiral and macrocyclic ligands, have thus been
prepared.3 Tethered aldehydes, such as phosphino-aldehydes4
or quinoline-8-carbaldehyde (C9H6NCHO),5 have proved
useful to stabilize the intermediates in the reactions of
aldehydes involving metal atoms, and quinoline-8-carbaldehyde
has been recently used to prepare polydentate ligands used in
the synthesis of platinum complexes for chemotherapeutic drug
design.6
When metals in low oxidation states are used, the activation
of the OC−H bond is generally assumed to proceed via
classical oxidative addition pathways, leading to acyl hydrido
metal complexes. The zerovalent platinum complexes [Pt-
Received: December 5, 2013
© XXXX American Chemical Society
A
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Scheme 1
1
and proposed mechanisms for their formation have been the
oxidative addition of OC−H followed by reductive elimination
of HX or direct electrophilic attack on the formyl group by
platinum(II) with displacement of a proton.4a,11,13 The
hydridoplatinum(II) derivative [PtH{(PPh2O)2H}(PPh2OH)],
containing hydrogen-bonded secondary phosphane oxide,
reacts with PPh2(o-C6H4CHO) to undergo the insertion of
the aldehyde into the Pt−H bond to give the cyclic platinum
alkoxide [Pt{(PPh2O)2H}{PPh2(o-C6H4CH2O)-κP,κO}].14
Platina-β-diketones [Pt2{(COR)2H}2(μ-Cl)2] are unsatu-
rated acyl(hydroxycarbene) complexes stabilized by an intra-
molecular O−H···O hydrogen bond. They show unique
reactivity in comparison to 18-valence-electron metalla-β-
diketones due to their electronic unsaturation and their
kinetically labile ligand sphere.15 The platina-β-diketone
[Pt2{(COMe)2H}2(μ-Cl)2] (1) reacts with mono- or bidentate
L/(LL) donors to yield the acetylhydridoplatinum(IV)
complexes [PtHCl(COMe)2(LL)], which are highly stable
when they contain hard bidentate N donors.16 Furthermore,
oxidative addition of halogens and halides to diacetylplatinum-
(II) results in the requisite platinum(IV) complexes.17 In the
presence of a base, a reductive HCl elimination on [PtHCl-
(COMe)2(LL)] can be enforced to selectively yield the
diacetylplatinum(II) complexes [Pt(COMe)2(LL)].18 Recently
we have reported that PPh2(o-C6H4CHO) reacts with 1 to
produce [PtCl{PPh2(o-C6H4CO)-κP,κC}{PPh2(o-C6H4CHO)-
κP}], while in the reactions with the diacetylbis(amine)-
platinum(II) complexes [Pt(COMe)2(NH2R)2] (2), the Schiff
base reaction products [Pt(COMe)2{PPh2(o-C6H4CHNR)-
κP,κN}] and the intermediate hemiaminal-containing species
[Pt(COMe)2(PPh2{o-C6H4CH(OH)(NHR)}-κP,κN)] are ob-
tained, depending on the reaction conditions.19 We report now
on the reaction of 1 with quinoline-8-carbaldehyde leading to
unprecedented acyl(hydroxyalkyl)platinum(IV) derivatives or
to dinuclear acetyl-bridged platinum(II) complexes, depending
on the reaction conditions. The reaction of C9H6NCHO with 2
is also reported.
For complex 3 the H NMR spectrum shows a resonance at
2.68 ppm, in the range expected for an acetyl ligand,16,20,21 and
the JPt,H = 11.4 Hz coupling constant is also characteristic of
3
this type of acetylplatinum(IV) complex. The hydroxyl proton
of the formed hydroxyalkyl group appears at 6.27 ppm as a
doublet with satellites. The pattern observed is due to coupling
to platinum (3JPt,H = 39.2 Hz) and to the alkyl CH (3JH,H = 11.4
Hz) proton, which appears at 7.22 ppm also as doublet with
platinum satellites (2JPt,H = 107.2 Hz). By addition of 1 drop of
CD3OD to a CD2Cl2 solution of 3, the signal at 6.27 ppm
disappears due to H/D exchange and the resonance at 7.22
ppm becomes a singlet with platinum satellites. In the 13C{1H}
NMR spectrum the carbon atom of the hydroxyalkyl group
appears at 62.6 ppm coupled with platinum (1JPt,C = 841 Hz).
The 13C{1H} NMR spectrum also contains resonances at 196.4
and 197.6 ppm, indicating the presence of ligands, with
1
characteristic JPt,C coupling constants of 1142 and 901 Hz,
respectively.16 The platinum chemical shift, −1421 ppm, is as
expected for a platinum(IV) complex.21 Complex 3 contains a
newly formed asymmetric sp3 carbon atom. This complex could
exist in several diastereomeric forms because the platinum atom
is also stereogenic, but the reaction is highly diastereoselective.
Single crystals of 3·2CHCl3 for X-ray diffraction analysis were
grown overnight, at room temperature, from a chloroform
solution. Complex 3·2CHCl3 was found to crystallize in the
triclinic system with two crystallographically identical molecules
in the unit cell and four molecules of chloroform. Selected
bond lengths and angles are given in Figure 1.
The geometry around the metal atom is a slightly distorted
octahedron (configuration index OC-6-46), with the three
organyl ligands trans to N and Cl ligands, thus avoiding an
energetically unfavorable trans arrangement of two strong σ-
donor ligands. The highest deviation from linearity corresponds
to the C3−Pt−Cl angle of 173.4(2)°. The Pt atom is displaced
from the best least-squares C1C13N2N1 plane by 0.065 Å
toward the chlorine atom. The Pt−N1 bond length trans to the
hydroxyalkyl fragment (2.179(6) Å) is equal to the Pt−N2
bond length trans to the acetyl ligand (2.187(4) Å), as
expected. The Pt−C13 distance trans to nitrogen is longer than
the Pt−C3 distance trans to chlorine (2.068(7) and 2.005(5) Å,
respectively). This difference may be viewed as a consequence
of the differences in the trans influence (N > Cl) and of the
different hybridization for both carbon atoms: sp3 for C13 and
sp2 for C3.22 Both distances are longer than the Pt−C bonds in
1 (1.95(1) Å), which have substantial double-bond character.23
The C3−O2 distance (1.213(8) Å) in complex 3 is similar to
RESULTS AND DISCUSSION
■
The reaction of complex 1 with quinoline-8-carbaldehyde in
MeOH/H2O at room temperature (Pt/C9H6NCHO = 1/2)
leads to the cleavage of the chloride bridges in 1 and the
formation of the acetylacyl(hydroxyalkyl)platinum(IV) com-
plex [Pt(COMe)Cl(C9H6NCO-κN,κC)(C9H6NCHOH-
κN,κC)] (3), shown in Scheme 1 ,with displacement of
acetaldehyde. Complex 3 was isolated as a colorless powder in
moderate yield (53%). It is stable at room temperature in the
solid state and decomposes in chloroform or methylene
chloride solution within 1 day. Complex 3 was characterized
by NMR (1H, 13C{1H}, 195Pt{1H}) as well as by microanalysis
and single-crystal X-ray diffraction analysis.
12b
that observed for [PdCl(C9H6NCO-κN,κC)(PPh3)]·PPh3
and shorter than the C13−O3 distance (1.411(8) Å), as
expected. An intramolecular O3−H···O2 hydrogen bond
between the hydroxyalkylquinoline and the acylquinoline
units is observed, with an O3···O2 distance of 2.708(7) Å
(O2···H, 1.90 Å; O3−H···O2, 155°). π−π interactions
B
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(3DH) was obtained, due to the easy H/D exchange between
1
the hydroxyl group and the solvents. The corresponding H
NMR spectrum in CDCl3 contains a singlet, instead of a
doublet, with satellites at 6.41 ppm and no resonance at 7.22
ppm. From these deuteration experiments we conclude that
activation of the aldehyde OC−H bond occurs in only half of
the molecules of C9H6NCHO, while in the other half this bond
remains unaltered and protonation of the oxygen occurs. As
expected, when using a Pt/C9H6NCDO/C9H6NCHO = 1/1/1
ratio, in a CH3OH/H2O mixture, an equimolar mixture of 3
and 3DH was obtained.
According to this evidence we believe that the reaction of 1
with C9H6NCHO starts with cleavage of the chloride bridges,
N coordination of two molecules of the ligand, and O to Pt
hydrogen transfer to form A, followed by acetaldehyde
elimination, leading to B, as shown in Scheme 2.
Scheme 2
Figure 1. ORTEP view of 3 in crystals of 3·2CHCl3 (30% probability
ellipsoids; H atoms omitted for clarity, except those of the hydrogen
bond and of the chiral C atom). Selected bond lengths (Å) and angles
(deg): Pt−N1, 2.179(6); Pt−N2, 2.187(4); Pt−Cl 2.481(1); Pt−C1,
2.052(6); Pt−C13, 2.068(7); Pt−C3, 2.005(5); C3−O2, 1.213(8);
C13−O3, 1.411(8); C1−O1, 1.196(8); C1−Pt−C3, 89.2(2); C1−Pt−
C13, 91.9(2); C1−Pt−N1, 91.4(2); C1−Pt−N2, 174.0(2); C1−Pt−
Cl, 93.6(2); C3−Pt−C13, 94.2(2); C3−Pt−N1, 82.0(2); C3−Pt−N2,
88.1(2); C3−Pt−Cl, 173.4(2); C13−Pt−N1, 174.9(2); C13−Pt−N2,
82.9(2); C13−Pt−Cl, 91.7(2); N1−Pt−N2, 93.6(2); N1−Pt−Cl,
92.1(1); N2−Pt−Cl, 89.6(1); C14−C13−O3, 110.6(5); C4−C3−O2,
122.5(5); C2−C1−O1, 122.5(6).
involving adjacent molecules between the hydroxyalkylquino-
line moieties (3.356 Å) and between the carboxyquinoline
moieties (3.612 Å) are also observed.24
DFT calculations show that, out of the 20 possible
diastereomers, the most stable one is that which was found in
crystals of 3·2CHCl3 (configuration index OC-6-46) with a C-
configured platinum atom25 and the asymmetric C atom in an
R configuration (abbreviated CR) or the other way a round, the
AS enantiomer (for more details see the Supporting
Information)
Possible paths for the transformation of the monodentate
C9H6NCHO ligands in B into the acylquinoline and the
hydroxyalkylquinoline fragments observed in 3 are an electro-
philic activation of one aldehyde OC−H by the metal to
generate a positively charged hydrogen that would be
transferred to the other aldehyde to form the hydroxyalkyl
fragment13,27 and the chelate-assisted oxidative addition of one
of the two aldehydes followed by insertion of the second
aldehyde into the Pt−H bond, as in the reaction of o-
(diphenylphosphino)benzaldehyde with rhodium(I) complex-
es.4c
Both types of mechanisms could be assisted by the presence
of water, which may form a hydrogen bond with the oxygen
atom of aldehydes. Transfer of a water proton to one of the
aldehydes and electrophilic OC−H activation of the other
aldehyde, with transfer of proton to the oxygen of water (see a
in Chart 1), appears feasible. Transfer of a proton from water to
an oxygen atom doubly bonded to carbon plays a role in the
selective hydrogenation of the CO bond of α,β-unsaturated
aldehydes28 and in the hydrogenation of CO2 promoted by
Ir(III) complexes through heterolysis of H2.29 The oxidative
addition of one aldehyde to afford an acyl−hydrido
intermediate followed by transfer of the water proton to the
other aldehyde and hydrogen transfer from platinum to the
oxygen atom of water via a six-centered transition state (see b
in Chart 1) can also afford 3. The ability of late-transition-metal
hydrides to transfer a hydride or a proton is well known.30
When the reaction of complex 1 with quinoline-8-
carbaldehyde is performed in dichloromethane in the presence
The reaction of the platina-β-diketone 1 with C9H6NCHO to
afford the Pt(IV) derivative 3 requires a Pt/(C9H6NCHO) =
1/2 ratio and the presence of water. Complex 1 is known to
react with quinoline (quin) with cleavage of the chloride
bridges to give the mononuclear platina-β-diketone [PtCl-
{(COMe)2H}(quin)],26 and C9H6NCHO has been reported to
behave as a monodentate ligand in [PtCl2(C9H6NCHO-
κN)(PEt3)].12a 1 also reacts with bidentate N-donor ligands
(N∧N) such as 2,2′-bipyridine and 1,4-diazabuta-1,3-dienes to
afford, among others, [PtCl(COMe)(N∧N)] compounds with
acetaldehyde displacement.16,20 To gain insight into the
mechanism of the reaction shown in Scheme 1, we performed
several experiments using the deuterated aldehyde
C9H6NCDO. When a Pt/C9H6NCDO = 1/2 ratio was used,
in a CD3OD/D2O mixture, the deuterated derivative [Pt-
(COMe)Cl(C9H6NCO-κN,κC)(C9H6NCDOD-κN,κC)]
1
(3DD) was obtained, as shown by the H NMR spectrum
lacking the resonances of the CHOH fragment. When the
reaction with a Pt/C9H6NCDO = 1/2 ratio was performed in a
CH3OH/H2O mixture, the only partially deuterated [Pt-
(COMe)Cl(C9H6NCO-κN,κC)(C9H6NCDOH-κN,κC)]
C
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a
terminal in the dinuclear Ir complex [{Ir(C9H6NCO-κN,κC)-
(py){μ-PPh2(o-C6H4CO)-κP,κC:κO}}2](ClO4)2, which also
contains a bridging acylphosphane ligand.35 To ascertain
which acyl group behaves as a bridge, an X-ray diffraction
study was undertaken. Single crystals of 4·0.5H2O suitable for
X-ray diffraction analysis were grown by slow diffusion of
diethyl ether onto a dichloromethane solution. The complex 4·
0.5H2O crystallizes with two crystallographically independent
dimers, namely A and B, and a molecule of water. No
significant differences are observed in the molecular geometries
of A and B. An ORTEP view of 4A and selected bond lengths
and angles are shown in Figure 2.
Chart 1.
a
C(O)Me and Cl ligands omitted for clarity.
Complex 4 consist of dimers with two bridging acetyl ligands
in a head-to-tail fashion, forming a six-membered ring in a
slightly twisted boat conformation with the two Pt atoms at the
apexes. The coordination around the platinum can be described
as square planar and deviates only slightly from planarity in all
cases. The deviation of the platinum atom from the
corresponding least-squares best-fit mean plane formed by its
coordinated atoms is between 0.016 and 0.056 Å. The bond
distances involving the metal are as expected, and the C−O
bond lengths are equal within the 3σ criterion, regardless of the
acyl groups belonging to an acetyl bridge (mean 1.25(2) Å) or
to a terminal chelating ligand (mean 1.22(2) Å). The
coordination planes are bent at an angle of 102.2(6)°, larger
than in the related platina-β-diketonates of platina-β-diketones
[{X2Pt(μ-COMe)2Pt{(COMe)2H}}2]z (X/X = Cl/NH2Ph′, z
= 0; X/X = Cl/Cl, z = 2−; X/X = bpy, z = 2+) with a head-to-
head arrangement of the bridging acetyl ligands, where the
angles are 76.3(4)−89.6(4)°.32,36 The platinum−platinum
distances, Pt1···Pt2 = 3.462(1) Å for 4A and Pt3···Pt4 =
3.438(1) Å for 4B, are longer than those in platina-β-
diketonates of platina-β-diketones (3.160(1)−3.358(1) Å).
Two kinds of intermolecular interactions can be detected.
Hydrogen bonds are observed between the O1w atom of the
water molecule, the O1 atom of one of the acylquinoline
ligands of an A dimer, and the O5 atom of one of the
acylquinoline ligands of a B dimer, so that the water molecule
bridges both types of dimers (O1···O1w = 2.861(3) Å, O5···
O1w = 2.951(2) Å; O1w−H2···O1 = 154°, O1w−H1···O5 =
151°) to form [A···H−O−H···B] units. Additionally, the
quinoline rings form a net of π−π interactions with distances
in the range 3.33−3.40 Å.
of sodium methoxide, dehydrochlorination and activation of the
OC−H bonds occurs with formation of the dinuclear complex
[Pt2(C9H6NCO-κN,κC)2(μ-COMe)2] (4) and displacement of
acetaldehyde, irrespective of the Pt/C9H6NCHO ratio
employed (1/1 or 1/2) (Scheme 3). Complex 4 could be
isolated as a yellow powder in moderate yield (63%). Complex
4 is also formed in the reaction of 3 with NaOMe, which occurs
with displacement of C9H6NCHO, as shown in Scheme 3. It is
stable at room temperature in the solid state but decomposes in
chloroform or dichloromethane solution within 1 day. Complex
4 was characterized by microanalysis, high-resolution ESI mass
spectrometry, and NMR and IR spectroscopy as well as by
single-crystal X-ray diffraction analysis.
In the ESI mass spectra signals due to dinuclear protonated
([Pt(C9H6NCO-κN,κC)(μ-COMe)]2 + H+) or aggregated with
Na+ ([Pt(C9H6NCO-κN,κC)(μ-COMe)]2 + Na+) were identi-
fied. In the IR spectrum of complex 4, absorptions due to acyl
groups were observed. The signals at lower wavenumbers
(1499, 1512 cm−1) are in accord with the presence of bridging
acyl groups.31 Bridging acetyl ligands in platina-β-diketonates of
platina-β-diketones show absorptions in the 1514−1534 cm−1
range.32 The signal at 1629 cm−1 can be assigned to terminal
acyl groups. In line with these observations, the 13C{1H} NMR
spectrum shows a resonance at very low field, 261.6 ppm,
characteristic of bridging acyl groups, and a singlet with
platinum satellites (1JPt,C = 1523 Hz) at 194.7 ppm due to
terminal acyl groups.33 The platinum chemical shift, −3143
ppm, is as expected for a platinum(II) complex.34
Acetyl groups are well-known as bridging ligands in platinum
dinuclear complexes, while the acylquinoline ligand behaves as
a bridge in the heterodinuclear Rh−Ir derivative [IrCl{PPh2(o-
C6H4CO)-κN,κC}(μ-H)(μ-C9H6NCO-κN,κC:κO)Rh(Cod)],
which also contains a terminal acylphosphane ligand, or as a
DFT calculations give proof that complex 4 (configuration
index SP-4-3) found in crystals of 4·0.5H2O is the most stable
isomer. The diastereomer having two acyl ligands in mutually
Scheme 3
D
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Figure 2. ORTEP view of 4A in crystals of 4·0.5H2O (30% probability ellipsoids; H atoms omitted for clarity). Selected bond lengths (Å) and angles
(deg) (the values of the two crystallographically independent molecules are given separated by a slant): Pt1−C1, 1.96(2)/1.98(1); Pt2−C15,
1.97(2)/1.95(1); Pt1−N1, 2.092(9)/2.09(1); Pt2−N2, 2.11(1)/2.12(2); Pt1−C13, 1.97(1)/1.96(2); Pt2−C11, 1.98(2)/1.97(2); Pt1−O2, 2.15(2)/
2.15(1); Pt2−O3, 2.13(1)/2.135(9); C1−O1, 1.20(2)/1.21(2); C15−O4, 1.21(2)/1.24(2); C11−O2, 1.26(2)/1.27(2); C13−O3, 1.25(2)/1.23(2);
C1−Pt1−N1, 83.9(6)/83.6(5); C15−Pt2−N2, 84.1(6)/83.4(6); N1−Pt1−O2, 88.5(5)/88.6(5); N2−Pt2−O3, 88.1(5)/87.3(5); C13−Pt1−O2,
92.6(6)/91.5(6); C11−Pt2−O3, 92.3(6)/92.8(7); C1−Pt1−C13, 95.0(7)/96.3(6); C15−Pt2−C11, 95.5(7)/96.4(8); C1−Pt1−O2, 172.3(6)/
172.2(5); C15−Pt2−O3, 172.1(6)/170.6(6); C13−Pt1−N1, 178.4(6)/178.2(6); C11−Pt2−N2, 177.4(6)/174.9(7); O1−C1−C2, 117(2)/119(1);
O4−C15−C16, 118(1)/199(2); O3−C13−C14, 115(1)/115(2); O2−C11−C12, 115(2)/113(1).
a
trans positions (configuration index SP-4-2) is 43.9 kcal/mol
higher in energy. The configurational isomer with bridging
acylquinoline ligands (μ-C9H6NCO-κN,κC:κO) proved to be
also 43.6 kcal/mol higher in energy (for details see the
Supporting Information).
Scheme 4.
As shown in Scheme 3, complex 4 reacts with triphenyl-
phosphane to undergo acyl bridge cleavage to give the
mononuclear complex [Pt(C9H6NCO-κN,κC)(COMe)(PPh3)]
(5) with cis acyl groups. Complex 5 was isolated as a red
powder in 55% yield. At room temperature, the solid is stable,
but after 2 days in chloroform solution decomposition products
appear. The IR spectrum shows absorptions due to terminal
acyl groups and the 13C{1H} NMR spectrum shows resonances
at 231.7 (d) ppm, due to the acylquinoline fragment trans to
phosphorus (2JP,C = 120 Hz),and at 222.8 (d) ppm, due to the
acetyl group cis to phosphorus (2JP,C = 8 Hz). The 31P NMR
spectrum of complex 5 contains a singlet with satellites at 28.1
a
Legend: (i) C9H6NCHO; (ii) C9H6NCHNEt. R = CH2Ph (a), Et
(b).
obtained nor detected by NMR, thus showing that the
condensation reaction is much faster when using quinoline-8-
carbaldehyde than when using the more flexible o-
(diphenylphosphino)benzaldehyde. Complexes 6a,b could
both be isolated as a brown or red powder in a yield of 35−
85%. Complexes 6 show resonances due to two nonequivalent
acetyl groups. The chemical shifts of the acetyl ligands in the
13C{1H} NMR spectra, in the 227−238 ppm range, agree with
acetyl groups trans to nitrogen ligands in acetylplatinum(II)
complexes.34 The appearance of new resonances at 163.4 (6a)
and 161.5 (6b) ppm confirms the formation of a new CN
bond. The platinum chemical shifts are in the range of typical
platinum(II) complexes. Furthermore, high-resolution ESI
mass spectra (cationic mode) of complexes 6 showed the
molecular peaks of 6 ([M + H]+). Complex 6b can also be
easily prepared by the reaction of 2b with the iminoacylquino-
line C9H6NCHNEt (7) (Scheme 4ii), which leads to
displacement of the ethylamine.
1
ppm, and the JPt,P = 1755 Hz coupling constant agrees with a
phosphorus atom trans to an acyl group.13,26,37 The platinum
chemical shift, −3630 (d) ppm, is as expected for a
platinum(II) complex.34 Other donors such as pyridine and
tetrahydrofuran are unable to cleave the acetyl bridges in 4
under the same reaction conditions. In line with this behavior,
complex 4 fails to react with C9H6NCHO to afford 3.
Finally the reaction of the diacetylbis(amine)platinum(II)
complexes [Pt(COMe)2(NH2R)2] (R = CH2Ph (2a), Et
(2b))38 with quinoline-8-carbaldehyde, either in dichloro-
methane at −78 °C or in methanol at room temperature,
affords the Schiff base products [Pt(COMe)2(C9H6NCH
NR-κN,κN′)] (R = CH2Ph (6a), Et (6b)), shown in Scheme 4i.
CONCLUSIONS
■
At variance with the reaction of
2 with o-
The reaction of quinoline-8-carbaldehyde with platina-β-
diketones leads to metal-mediated and water-assisted carbon
to oxygen proton transfer between two aldehydes and to the
formation of new (acylquinoline)(hydroxyalkylquinoline)-
platinum(IV) species. In the presence of bases dehydrochlori-
(diphenylphosphino)benzaldehyde, where the imine−phos-
phane complexes and the intermediate hemiaminal−phosphane
complexes could be isolated and characterized,19 in the present
case intermediate hemiaminal derivatives could be neither
E
dx.doi.org/10.1021/om4011774 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Reaction of Complex 3 with NaOMe. To a methylene chloride
(3 mL) solution of complex 3 (10 mg, 0.02 mmol) was added NaOMe
(0.9 mg, 0.02 mmol) to afford a yellow solution. After the mixture was
stirred for 1 h at room temperature, the addition of diethyl ether (3
mL) gave a yellow precipitate, which was filtered off, washed with
diethyl ether (5 mL), and dried under vacuum. The 1H NMR
spectrum shows the solid to be a mixture of 4 and C9H6NCHO.
Preparation of [Pt(C9H6NCO-κN,κC)(COMe)(PPh3)] (5). To a
methylene chloride (3 mL) solution of [Pt2(C9H6NCO-κN,κC)2(μ-
COMe)2] (4; 40 mg, 0.05 mmol) was added a stoichiometric amount
of PPh3 (27 mg, 0.10 mmol) to afford a red solution. After the mixture
was stirred for 1.5 h at room temperature, the addition of diethyl ether
(3 mL) gave a red precipitate, which was filtered off, washed with
diethyl ether (5 mL), and dried under vacuum. Yield: 36 mg (55%).
Anal. Calcd for C30H24NO2PPt (656.60): C, 54.88; H, 3.68; N, 2.13.
Found: C, 55.07; H, 3.96; N, 2.35. HRMS (ESI): m/z calcd
nation occurs to afford dinuclear platinum(II) derivatives with
preferred acetyl bridges and chelating acylquinoline ligands. In
the solid state these dimers are connected via hydrogen bonds
involving the acylquinoline oxygen atoms. Pyridine or THF fails
to cleave the acetyl bridges, while triphenylphosphane easily
affords mononuclear Pt(II) species. The reaction of quinoline-
8-carbaldehyde with diacetylbis(amine)platinum(II) leads read-
ily to new diacetyl(iminoacylquinoline)platinum(II) complexes.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were performed under an
argon atmosphere using the standard Schlenk techniques. Solvents
were dried (Et2O and n-pentane over Na/benzophenone; MeOH over
Mg; CHCl3/CH2Cl2 over CaH2) and distilled prior to use. NMR
spectra were, unless otherwise specified, recorded at 27 °C on Varian
Gemini 200, VXR 400, and Unity 500 NMR spectrometers. TMS (1H,
13C{1H}) was used as internal reference and H2PtCl6 (δ(195Pt) 0 ppm)
or H3PO4 (δ(31P) 0 ppm) as external reference. IR spectra were
recorded on a Bruker Tensor 27 spectrometer with a Platinum ATR
unit. The positive ion high-resolution ESI mass spectra were measured
with a Bruker Apex III Fourier transform ion cyclotron resonance (FT-
ICR) mass spectrometer (Bruker Daltonics, Billerica, MA, USA).
Microanalyses were performed by the University of Halle micro-
analytical laboratory using a CHNS-932 (LECO) elemental analyzer.
The platinum complexes [Pt2{(COMe)2H}2(μ-Cl)2] (1) and [Pt-
(COMe)2(NH2R)2] (2) as well as C9H6NCHO and C9H6NCDO
were prepared according to literature methods.13,23,38,39
1
[C30H24NO2PPt + Na]+ 679.1085, found 679.1090. H NMR (200
MHz, CDCl3): δ 1.97 (s+d, 3JPt,H = 19.0 Hz, 3H, COCH3), 6.94, 7.39,
7.70, 7.95, 8.11, and 8.34 (m, 21H, aromatic protons). 31P{1H} NMR
(81 MHz, CDCl3): δ 28.1 (s+d, 1JPt,P = 1755 Hz). 13C{1H} NMR (125
MHz, CD2Cl2): δ 42.1 (s+d, 2JPt,C = 310 Hz, 3H, COCH3), 121.9 (s),
123.8 (s), 128.7 (d, 3JP,C = 10 Hz), 129.1 (s), 130.4 (s), 130.7 (d, 4JP,C
= 2 Hz), 131.4 (s), 131.8 (s), 134.5 (d, 2JP,C = 13 Hz), 138.0 (s), 146.9
1
3
(d, JP,C = 25 Hz), 151.7 (s), 152.9 (d, JP,C = 3 Hz), (13
2
nonequivalent aromatic C atoms), 222.8 (d, JP,C = 8 Hz, COMe),
231.7 (d, 2JP,C = 120 Hz, COquin). 195Pt{1H} NMR (86 MHz, CDCl3):
δ −3630 (d, JPt,P = 1764 Hz). IR: 1636 (m), 1619 (m) cm−1.
1
Preparation of [Pt(COMe)2(C9H6NCHNR-κN,κN′)] (R =
CH2Ph (6a), Et (6b)). Method a. To a methanol (3 mL) solution
of [Pt(COMe)2(NH2R)2] (R = CH2Ph (2a), Et (2b); 0.11 mmol) or
to a methylene chloride (3 mL) solution of 2a or 2b at −78 °C was
added C9H6NCHO (16.9 mg, 0.11 mmol) to afford a red solution at
room temperature. After the mixture was stirred for 4.5 h at room
temperature, the addition of n-hexane for 6a and diethyl ether for 6b
gave a brown or a red precipitate, which was filtered off, washed with
n-hexane or diethyl ether, and dried under vacuum. Data for 6a (R =
CH2Ph) are as follows. Yield: 21 mg (36%). Anal. Calcd for
C21H20N2O2Pt·CH2Cl2 (612.43); C, 43.15; H, 3.62; N, 4.57. Found:
C, 43.28; H, 3.39; N, 4.85. HRMS (ESI): m/z calcd [C21H20N2O2Pt +
H]+ 528.1256, found 528.1254 (in addition to other signals). 1H NMR
(200 MHz, CDCl3): δ 2.06 (s+d, 3JPt,H = 22.7 Hz, 6H, COCH3), 5.19
(s+d, 3JPt,H = 21.5 Hz, 2H, CH2), 7.29, 7.52, 7.70, 7.87, 8.12, and 8.37
Preparation of [Pt(COMe)Cl(C9H6NCO-κN,κC)(C9H6NCHOH-
κN,κC)] (3). To
a freshly prepared suspension of
[Pt2{(COMe)2H}2(μ-Cl)2] (1; 80 mg, 0.13 mmol) in methanol/
water (3/0.75 mL) was added C9H6NCHO (79 mg, 0.50 mmol),
whereupon dissolution of 1 and precipitation of a colorless product
occurred. The solid was filtered off, washed with diethyl ether, and
dried under vacuum. Yield: 88 mg (53%). Finally, the complex was
recrystallized from chloroform/diethyl ether. Anal. Calcd for
C22H17ClN2O3Pt·CHCl3 (707.31): C, 39.06; H, 2.57; N, 3.96.
1
Found: C, 38.71; H, 2.67; N, 3.77. H NMR (500 MHz, CD2Cl2):
3
3
δ 2.68 (s+d, JPt,H = 11.4 Hz, 3H, COCH3), 6.27 (d+d, JH,H = 11.4
3
3
2
Hz, JPt,H = 39.2 Hz, 1H, OH), 7.22 (d+d, JH,H = 11.5 Hz, JPt,H
=
107.2 Hz, 1H, CH(OH)), 7.03, 7.44, 7.66, 7.84, 8.16, 8.24, 8.66, and
10.01 (m, 12H, aromatic protons). 13C{1H} NMR (125 MHz,
CD2Cl2): δ 37.7 (s+d, 2JPt,C = 168 Hz, COCH3), 62.6 (s+d, 1JPt,C = 841
Hz, CH(OH)), 121.8, 123.9, 126.5, 128.7, 128.8, 128.9, 129.0, 129.5,
131.7, 133.1, 138.8, 139.4, 140.9, 146.4, 147.6, 148.2, and 150.6 (s,
3
(m, 10H, aromatic protons), 8.33 (s+d, JPt,H = 30 Hz, 1H, NCH),
3
9.45 (m+d, JPt,H = 23.3 Hz, 1H, CH H2 quinoline). 13C{1H} NMR
(100 MHz, CDCl3): δ 43.5 (s, COCH3), 43.9 (s, COCH3), 68.2 (s,
CH2), 122.7, 127.0, 128.0, 128.6, 128.9, 129.4, 130.1, 135.1, 135.9,
139.9, 140.5, 141.1, and 156.8 (s, 13 nonequivalent aromatic C atoms),
163.4 (s, NCH), 227.9 (s, COMe), 236.9 (s, COMe). 195Pt{1H}
NMR (86 MHz, CDCl3): δ −3199 (s). IR: 1574 (m), 1551 (m) cm−1.
Data for 6b (R = Et) are as follows. Yield: 44 mg (85%). Anal. Calcd
for C16H18N2O2Pt·CH2Cl2 (549.06); C, 37.10; H, 3.66; N, 5.09.
Found: C, 36.27; H, 4.05; N, 5.21. HRMS (ESI): m/z calcd
[C16H18N2O2Pt + H]+ 466.1100, found 466.1104. 1H NMR (200
1
18C, aromatic carbon atoms), 196.4 (s+d, JPt,C = 1142 Hz, COMe),
1
197.6 (s+d, JPt,C = 901 Hz, COquin). 195Pt{1H} NMR (107 MHz,
CD2Cl2): δ −1421 (s). IR: 1697 (m), 1673 (s), 1638 (s) cm−1.
Preparation of [Pt2(C9H6NCO-κN,κC)2(μ-COMe)2] (4). To a
methylene chloride (3 mL) solution of [Pt2{(COMe)2H}2(μ-Cl)2] (1;
90 mg, 0.14 mmol) at −78 °C were added C9H6NCHO (44.5 mg,
0.28 mmol) and NaOMe (15.3 mg, 0.28 mmol). The resulting orange
solution became a black suspension after it was stirred overnight at
room temperature. The precipitate was filtered off, and the solution
was concentrated under vacuum (0.5 mL). Addition of diethyl ether (3
mL) gave a yellow precipitate that was filtered off, washed with diethyl
ether (5 mL), and dried under vacuum. Yield: 71 mg (63%). Anal.
Calcd for C24H18N2O4Pt2 (788.60): C, 36.55; H, 2.30; N, 3.55. Found:
C, 36.43; H, 1.86; N, 3.77. HRMS (ESI): m/z calcd [C24H18N2O4Pt2 +
H]+ 789.0634, found 789.0646; calcd [C24H18N2O4Pt2 + Na]+
3
MHz, CDCl3): δ 1.37 (t, JH,H = 7.2 Hz, 3H, CH2CH3), 2.07 (s+d,
3
3JPt,H = 23.5 Hz, 3H, COCH3), 2.34 (s+d, JPt,H = 24.0 Hz, 3H,
COCH3), 3.94 (m, 2H, CH2CH3), 7.50, 7.70, 7.93, 8.09, and 8.38 (m,
5H, aromatic protons), 8.38 (s+d, 3JPt,H = 30.6 Hz, 1H, NCH), 9.39
3
(s+d, JPt,H = 26.9 Hz, 1H, aromatic proton). 13C{1H} NMR (100
MHz, CDCl3): δ 17.1 (s, CH2CH3), 43.7 (s, COCH3), 43.8 (s,
COCH3), 60.9 (s, CH2CH3), 122.7, 127.0, 129.3, 130.3, 134.8, 139.8,
140.0, 141.1, and 156.7 (s, 9C, aromatic carbon atoms), 161.5 (s, N
CH), 227.1 (s, COMe), 237.4 (s, COMe). 195Pt{1H} NMR (86 MHz,
CDCl3): δ −3194 (s). IR: 1617 (m), 1579 (m) cm−1.
1
811.0448, found 811.0465. H NMR (400 MHz, CDCl3): δ 2.87 (s
+d, 3JPt,H = 23.5 Hz, 3H, COCH3), 7.59, 7.97, 8.11, 8.45, and 9.03 (m,
6H, aromatic protons). 13C{1H} NMR (100 MHz, CDCl3): δ 41.6 (s
+d, 2JPt,C = 260 Hz, COCH3), 122.6, 124.0, 128.2, 128.9, 129.4, 137.4,
145.8, 149.0, and 150.9 (s, 9C, aromatic carbon atoms), 194.7 (s+d,
1JPt,C = 1523 Hz, COquin), 261.6 (s, COMe). 195Pt{1H} NMR (86
MHz, CDCl3): δ −3143 (s). IR: 1629 (m), 1576 (m), 1512 (m), 1499
(m) cm−1.
Method b. To
a methanol (3 mL) solution of [Pt-
(COMe)2(NH2Et)2] (2b; 100 mg, 0.27 mmol) was added
C9H6NCHNEt (7; 50 mg, 0.27 mmol) to afford an orange solution.
After the mixture was stirred for 3 h at room temperature, the addition
of diethyl ether gave an orange precipitate of 6b, which was filtered off,
washed with diethyl ether, and dried under vacuum. Yield: 106 mg
(85%). Analytical data are as given above.
F
dx.doi.org/10.1021/om4011774 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Preparation of C9H6NCHNEt (7). To a solution of
C9H6NCHO in methanol (100 mg, 0.64 mmol) cooled to −78 °C
was added ethylamine. The reaction mixture was warmed to room
temperature and stirred for 4 h. The reaction mixture was
concentrated under vacuum to afford an oil that was identified
5206−5215. (g) Beck, R.; Florke, U.; Klein, H. F. Inorg. Chem. 2009,
̈
48, 1416−1422. (h) Adams, J. J.; Arulsamy, N.; Roddick, D. M.
Organometallics 2012, 31, 1439−1447. (i) Roa, A. E.; Salazar, V.;
́
Lopez-Serrano, J.; Onate, E.; Paneque, M.; Poveda, M. L. Organo-
̃
metallics 2012, 31, 716−721.
1
3
spectroscopically. H NMR (400 MHz, CDCl3): δ 1.36 (t, JH,H = 7.3
(2) For hydroacylation reactions see for example: (a) Roy, A. H.;
Lenges, C. P.; Brookhart, M. J. Am. Chem. Soc. 2007, 129, 2082−2093.
(b) Jun, C. H.; Jo, E. A.; Park, J. W. Eur. J. Org. Chem. 2007, 1869−
1881. (c) Shen, Z.; Khan, H. A.; Dong, V. M. J. Am. Chem. Soc. 2008,
130, 2916−2917. (d) Omura, S.; Fukuyama, T.; Horiguchi, J.;
Murakami, Y.; Ryu, I. J. Am. Chem. Soc. 2008, 130, 14094−14095.
(e) Moxham, G. L.; Randell-Sly, H.; Brayshaw, S. K.; Weller, A. S.;
Willis, M. C. Chem. Eur. J. 2008, 14, 8383−8397. (f) Pawley, R. J.;
Huertos, M. A.; Lloyd-Jones, G. C.; Weller, A. S.; Willis, M. C.
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3
Hz, 3H, CH2CH3), 3.79 (m, 2H, CH2CH3), 7.39 (dd, J2,3 = 4.2 Hz,
3
3
3J4,3 = 8.3 Hz, H3quin), 7.56 (dd, J5,6 = 8.1 Hz, J7,6 = 7.3 Hz, H6quin),
7.84 (dd, J7,5 = 1.2 Hz, H5quin), 8.12 (dd, J2,4 = 1.7 Hz, H4quin), 8.39
(dd, H7quin), 8.93 (dd, H2quin), 9.64 (s, NCH). 13C{1H} NMR (100
MHz, CDCl3): δ 16.4 (s, CH2CH3), 56.2 (s, CH2CH3), 121.2 (s,
C3quin), 126.5 (s, C6quin), 127.3 (s, C7quin), 128.2 (s, C10quin), 130.1 (s,
C5quin), 133.3 (s, C8quin), 136.3 (s, C4quin), 146.6 (s, C9quin), 149.9 (s,
C2quin), 158.1 (s, NCH).
4
4
X-ray Structure Determination of [Pt(COMe)Cl(C9H6NCO-
κN,κC)(C9 H6 NCHOH-κN,κC)]·2CHCl3 (3·2CHCl3 ) and
[Pt2(C9H6NCO-κN,κC)2(μ-COMe)2]·0.5H2O (4·0.5H2O). Data for
X-ray diffraction analyses were collected on Stoe-IPDS 2T (3·
2CHCl3) and Oxford Gemini S (4·0.5H2O) diffractometers using
Mo Kα and Cu Kα radiation, respectively (λ = 0.71073/1.54184 Å, 3/
4, graphite monochromator). A summary of the crystallographic data,
the data collection parameters, the refinement parameters, and details
of the absorption corrections is given in Table S1 in the Supporting
Information. The structures were solved with direct methods using
SHELXS-9740 and refined using full-matrix least-squares routines
against F2 with SHELXL-97.41 All non-hydrogen atoms were refined
with anisotropic displacement parameters and hydrogen atoms with
isotropic ones. H atoms were placed in calculated positions according
to the riding model, except that of the O−H···O hydrogen bond in 3·
2CHCl3 and those of the solvate molecule in 4·0.5H2O, which were
located in the electron density map.
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ASSOCIATED CONTENT
* Supporting Information
■
S
A table, figures, and a CIF file giving crystallographic data for
complexes 3 and 4 and details of DFT calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Chem. 1985, 284, 409−419.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mariaangeles.garralda@ehu.es (M.A.G.); dirk.
steinborn@chemie.uni-halle.de (D.S.).
(8) Ghilardi, C. A.; Midollini, S.; Moneti, S.; Orlandini, A. J. Chem.
Soc., Dalton Trans. 1988, 1833−1836.
■
(9) (a) Ciganda, R.; Garralda, M. A.; Ibarlucea, L.; Pinilla, E.; Torres,
M. R. Dalton Trans. 2009, 4227−4235. (b) Garralda, M. A. Dalton
Trans 2009, 3635−3645 and references therein.
Notes
(10) Casas, J. M.; Fornies
1997, 1559−1563.
́
, J.; Martín, A. J. Chem. Soc., Dalton Trans.
The authors declare no competing financial interest.
(11) Vaughan, T. P.; Koedyk, D. J.; Spencer, J. L. Organometallics
2011, 30, 5170−5180.
ACKNOWLEDGMENTS
Partial financial support by the Ministerio de Economıa y
Competitividad (CTQ2011-24859), Gobierno Vasco, and
■
(12) (a) Albinati, A.; Anklin, C. G.; Pregosin, P. S. Inorg. Chim. Acta
1984, 90, L37−L38. (b) Albinati, A.; Anklin, C. G.; Ganazzoli, F.;
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Ruegg, H.; Pregosin, P. S. Inorg. Chem. 1987, 26, 503−508.
̈
́
Universidad del Paıs Vasco (UPV/EHU) are gratefully
(13) Anklin, C. G.; Pregosin, P. S.; Wombacher, F. J.; Ruegg, H.
̈
acknowledged. I.Z. is grateful to Gobierno Vasco for a
Organometallics 1990, 9, 1953−1958.
(14) van Leeuwen, P. W. N. M.; Roobeek, C. F.; Orpen, A. G.
Organometallics 1990, 9, 2179−2181.
scholarship. We also thank Dr. Jurgen Schmidt (Leibniz
̈
Institute of Plant Biochemistry, Halle, Germany) and Dr.
Martin Bette (University of Halle) for ESI-MS determinations
and discussions.
(15) Steinborn, D. Dalton Trans. 2005, 2664−2671.
(16) Gerisch, M.; Bruhn, C.; Vyater, A.; Davies, J. A.; Steinborn, D.
Organometallics 1998, 17, 3101−3104.
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Met. Chem. 2009, 34, 61−74. (c) Bette, M.; Schmidt, J.; Steinborn, D.
Eur. J. Inorg. Chem. 2013, 2395−2410.
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