Coordination chemistry in water of a free and a lipase-embedded cationic NCN-pincer platinum center with neutral and ionic triarylphosphines

Article abstract of DOI:10.1021/om2010832

The coordination chemistry in aqueous media was studied for the platinum center of low-molecular-weight cationic NCN-pincer platinum complexes [RC ₆H₂(CH₂NMe₂)₂-3,5- Pt(H₂O)-4]⁺ (R = -(CH₂)₃P(=O)(OEt) (OC₆H₄NO₂-4) (1(OH₂)), H (2(OH ₂))) as well as of the platinum center of the NCN-pincer platinum cation embedded in the lipase cutinase (cut-1; molecular weight 20 619.3) with various anionic, neutral, and cationic triarylphosphines. A ³¹P NMR study of the coordination of triarylphosphines to the cationic NCN-pincer platinum center in low-molecular-weight [2(OH₂)][OTf] in both D ₂O and Tris buffer (Tris = tris(hydroxylmethyl)aminomethane) showed that the phosphine-platinum coordination is strongly affected by Tris buffer molecules. Two crystal structures of a NCN-pincer platinum-phosphine and a NCN-pincer platinum-ethanolamine coordination complex with ethanolamine as a functional model of Tris with hydrogen bridges, provoking a dimeric supramolecular structure, confirmed that the coordination observed in solution occurred in the solid state as well. A ³¹P NMR and ESI-MS study of the lipase cut-1 showed that the coordination of various triarylphosphines to the enzyme-embedded platinum center is affected by the surrounding protein backbone, discriminating between phosphines on the basis of their size and charge. By using ³¹P NMR spectroscopy and ESI-MS spectrometry, study of the coordination of triarylphosphines to cut-1 was possible, thereby avoiding the need for the application of laborious biochemical procedures. To the best of our knowledge, this is the first example of a study involving the selective binding of organic ligands to the metal center of a semisynthetic metalloprotein, unequivocally demonstrating that the well-established coordination chemistry for small-molecule complexes can be transferred to biological molecules. This initial study allows future explorations in the field of selective protein targeting and identification, as in protein profiling or screening studies.

Full text of DOI:10.1021/om2010832

Article
pubs.acs.org/Organometallics
Coordination Chemistry in Water of a Free and a Lipase-Embedded
Cationic NCN-Pincer Platinum Center with Neutral and Ionic
Triarylphosphines
Birgit Wieczorek,†,⊥ Dennis J. M. Snelders,† Harm P. Dijkstra,† Kees Versluis,‡ Martin Lutz,§
Anthony L. Spek,§ Maarten R. Egmond,∥ Robertus J. M. Klein Gebbink,† and Gerard van Koten*,†
†Organic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
§
∥
‡Biomolecular Mass Spectrometry Group, Crystal & Structural Chemistry, and Membrane Enzymology, Bijvoet Centre for
Biomolecular Research, Padualaan 8, 3584 CH Utrecht, The Netherlands
S
* Supporting Information
ABSTRACT: The coordination chemistry in aqueous media
was studied for the platinum center of low-molecular-weight
c a t i o n i c N C N - p i n c e r p l a t i n u m c o m p l e x e s
[RC6H2(CH2NMe2)2-3,5-Pt(H2O)-4]+ (R = −(CH2)3P(
O)(OEt)(OC6H4NO2-4) (1(OH2)), H (2(OH2))) as well as
of the platinum center of the NCN-pincer platinum cation
embedded in the lipase cutinase (cut-1; molecular weight 20
619.3) with various anionic, neutral, and cationic triarylphos-
phines. A 31P NMR study of the coordination of
triarylphosphines to the cationic NCN-pincer platinum center
in low-molecular-weight [2(OH2)][OTf] in both D2O and
Tris buffer (Tris = tris(hydroxylmethyl)aminomethane)
showed that the phosphine−platinum coordination is strongly
affected by Tris buffer molecules. Two crystal structures of a NCN-pincer platinum−phosphine and a NCN-pincer platinum−
ethanolamine coordination complex with ethanolamine as a functional model of Tris with hydrogen bridges, provoking a dimeric
supramolecular structure, confirmed that the coordination observed in solution occurred in the solid state as well. A 31P NMR
and ESI-MS study of the lipase cut-1 showed that the coordination of various triarylphosphines to the enzyme-embedded
platinum center is affected by the surrounding protein backbone, discriminating between phosphines on the basis of their size
and charge. By using 31P NMR spectroscopy and ESI-MS spectrometry, study of the coordination of triarylphosphines to cut-1
was possible, thereby avoiding the need for the application of laborious biochemical procedures. To the best of our knowledge,
this is the first example of a study involving the selective binding of organic ligands to the metal center of a semisynthetic
metalloprotein, unequivocally demonstrating that the well-established coordination chemistry for small-molecule complexes can
be transferred to biological molecules. This initial study allows future explorations in the field of selective protein targeting and
identification, as in protein profiling or screening studies.
biotin−(strept)avidin system. Different metal (e.g., rhodium)
coordination complexes were noncovalently embedded into
(strept)avidin, thus creating an enantioselective hybrid catalyst;
i.e., the stereochemical properties of the protein backbone are
transferred to the embedded metal center. In addition to the
biotin−(strept)avidin system, also other proteins,21−24 such as
apo-myoglobin and heme-oxygenase,7−11,25,26 papain,27 other
Cys-containing enzymes,28−30 and carbonic anhydrases, were
used as scaffolds to construct artificial metalloenzymes.21,31
When a transition-metal catalyst is strategically embedded into
a chiral protein, the influence of the protein backbone can be
INTRODUCTION
■
The coordination chemistry of metal ions embedded in
proteins plays a crucial role in living systems; e.g., the typical
3D structures and catalytic activities of metalloproteins are
determined by the coordination of the protein backbone to one
or more transition-metal ions.1−5 By modifying specific amino
acid fragments in existing metalloproteins, the exact positioning
of the metal center in the protein backbone and its reactivity
can be elucidated. These studies showed that the nature of both
the metal ions (e.g., Cu(I), Zn(II), Ni(II)) and the amino acids
involved can dramatically influence the tertiary and quaternary
structures and the functionalities of the metalloprotein.6−15
By a combination of protein engineering techniques with
metal coordination chemistry, different artificial metalloproteins
have been developed. Pioneering work was reported by
Whitesides in 197816 and later by Ward et al.6,17−20 using the
Special Issue: F. Gordon A. Stone Commemorative Issue
Received: November 4, 2011
Published: March 13, 2012
© 2012 American Chemical Society
2810
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
optimally exploited to tune the activity of the metal center, as
e.g. in catalytic applications. In addition to catalysis, protein-
embedded metal centers have mainly been used in anticarcino-
genic and biomedical studies32−36 and have been applied as
redox systems,37,38 as probes,39−42 or in arrays and
assemblies,43−46 illustrating the great potential of these
protein−metal hybrid systems.
exposed enough to allow bridge formation by a small halide
atom and thus the assembly of two NCN-pincer platinum
chloride−lipase hybrids to one cationic dimer. This shows that
the metal center is available for further reaction: e.g.,
coordination chemistry and catalysis.
As a follow-up of this work, we decided to explore the
coordination properties of the protein-embedded NCN-pincer
platinum center with a series of cationic, neutral, and anionic
triarylphosphine ligands (Figure 2; vide infra). It was
anticipated that classical coordination chemistry48−55 could be
expanded toward artificial biological hybrid systems, thereby
adding novel, largely unexploited properties to biological
molecules. The various phosphines that have been used in
this model study had been selected for their solubility
properties in water and for their differences in bulkiness.
Furthermore, we set out to develop protocols allowing us to
monitor the coordination reactions in a straightforward
manner, e.g. by 31P NMR and ESI-MS methods, thereby
avoiding laborious biochemical analysis protocols. To evaluate
the influence of the protein backbone on the binding of these
triarylphosphines to the platinum center, we also performed the
same coordination chemistry on the parent low-molecular-
weight NCN-pincer platinum complexes lacking the protein
para substituent.
Recently, our group reported on the covalent anchoring of an
ECE-pincer metal (Pd or Pt) moiety to a lipase,47 where E is a
neutral, two-electron heteroatom donor, such as NMe2 or SMe.
In these ECE-pincer metal complexes, the bis ortho chelation
of the metal ion by the two donating E groups provides further
stability to the central M−Cpincer bond, making the ECE-pincer
metal complexes compatible with aqueous solvent media and
biological molecules, such as proteins. Recently, we were able
to obtain X-ray crystal structures of a number of these pincer−
cutinase hybrids, which allowed us to obtain a detailed picture
of the orientation of the pincer groups in the protein pocket.47
It appeared that in the case of the NCN-pincer platinum
complex RC6H2(CH2NMe2)2-3,5-PtCl-4 (R= −(CH2)3P(
O)(OEt)(O−) (1Cl) embedded in cutinase, depending on the
type of buffer used (either chloride-rich or chloride-poor),
either the monomeric NCN-pincer platinum chloride-lipase
hybrid [cut-1Cl] (Figure 1a) or the dimeric hybrid [cut-1-Cl-
cut-1]+, in which two cationic NCN-pincer platinum−lipase
hybrids are bridged through a single chloride ion, was formed
(Figure 1b).47 From the structural features of the two hybrids it
was obvious that, in the case of the dimer, the two protein-
embedded NCN-pincer platinum cations apparently are
RESULTS AND DISCUSSION
■
As cationic ECE-pincer metal complexes possessing non-
coordinating anions, such as triflate or tetrafluoroborate, are
ideal building blocks in supramolecular coordination chemistry
studies,57−66 it was decided to synthesize the ionic NCN-pincer
platinum complexes [1(OH2)][OTf] and [2(OH2)][OTf]65,67
f r o m t h e i r c o r r e s p o n d i n g h a l i d e a n a l o g u e s
([RC6H2(CH2NMe2)2-3,5-PtCl-4] (R= −(CH2)3P(O)-
(OEt)(OC6H4NO2-4) (1Cl),56 or H (2Cl),65,67 respectively;
Scheme 1a) by reaction with AgOTf. Filtration of the resulting
reaction mixture over Celite resulted in the removal of AgCl.
1
[1(OH2)][OTf] was characterized by elemental analysis, H,
13C, 19F, and 31P NMR spectroscopy, and MALDI-TOF MS.
The cutinase-pincer hybrid molecule [cut-1(OH2)][OTf]
was obtained by inhibition of cutinase (125 mM Tris, pH 8,
acidified with HBF4, i.e. in the absence of chloride anions) with
a solution of [1(OH2)][OTf] (4 equiv in MeCN); the p-
nitrophenolate leaving group of [1(OH2)][OTf] was replaced
by the nucleophilic oxygen atom of the Ser120 residue in the
active side of cutinase. After inhibition, excess inhibitor
[1(OH2)][OTf] and p-nitrophenolate were removed by
dialysis (Scheme 1b). Subsequent analysis of [cut-1(OH2)]-
[OTf] by ESI-MS (m/z 21134.8, corresponding to [cut-1]+
(calcd 21 137.5); note the m/z value of [cut-1Cl] is 21 173.0)
and 31P NMR (phosphonate-P δ 28.7 ppm, Scheme 1) showed
that complete and irreversible inhibition of cutinase had
occurred.
Coordination Studies of [2(OH2)][OTf] with Triaryl-
phosphine Ligands in Aqueous Media Monitored by 31
P
NMR Spectroscopy. To explore the coordination chemistry
of cationic NCN-pincer platinum complexes with various
phosphines (Figure 2) by 31P NMR in aqueous media, a model
study was carried out first with the pincer complex [2(OH2)]-
[OTf] and anionic triarylphosphine Na3[3]68 (TPPTS =
P(C6H4(SO3Na)-3)3), whereby both [2(OH2)][OTf] and
TPPTS were soluble under the aqueous conditions described.
Two aqueous solvent systems were selected for our studies: (1)
neat D2O and (2) a halide ion free Tris buffer (Tris =
Figure 1. (a) Part of the molecular structure of [cut-1Cl] in the solid
state, showing the covalent attachment of the NCN-pincer platinum
moiety. (b) Dimeric structure with the chloride ion bridging the two
platinum(II) cations to form [cut-1-Cl-cut-1]+, as obtained from
crystallization in chloride-poor buffer.47,56
2811
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
Scheme 1. (a) Synthesis of the Cationic NCN-Pincer Platinum Complexes [1(OH2)][OTf] and [2(OH2)][OTf] and (b)
a
Covalent Anchoring of [1(OH2)][OTf] to Cutinase
a
The respective 31P NMR values are given in parentheses.
Figure 2. The various anionic, neutral, and cationic triarylphosphines used in the coordination studies with cationic NCN-pincer platinum triflate
complexes [1(OH2)][OTf] and [cut-1(OH2)][OTf], respectively. Note that [3] = TPPTS minus three Na+ and [4] = TXPTS minus three Na+.
tris(hydroxylmethyl)aminomethane, pH 8.0), as it is a standard
buffer for many enzyme studies. The solvent system with Tris
allowed us to study the influence of Tris on the coordination
chemistry of the phosphines.
Hz) is indicative of a trans coordination of the phosphine with
respect to Cipso (the carbon atom directly bound to
platinum),69−71 i.e. of the presence of a complex with a
phosphine coordination mode as depicted for the complex
[2(TPPTS)][OTf] (Scheme 2).
When more than 1.0 equiv of phosphine TPPTS was added
to a solution of [2(OH2)][OTf], the coordination of a second
phosphine to platinum was observed. At 22.9 and 10.7 ppm,
doublet resonances (2J(31P−31P) = 13−15 Hz) appeared with
1J(31P−195Pt) = 1600 and 3900 kHz, respectively (Table S1,
Supporting Information). These signals are attributed to a
species in which two phosphines are coordinated to the
platinum center and where the two phosphines hold a mutual
cis position (cis-[2(TPPTS)2(D2O)]+, Scheme 2). The
1J(31P−195Pt) = 1600 and 3900 Hz coupling constants of cis-
[2(TPPTS)2(D2O)]+ are attributed to the phosphines bound
For the study in D2O, various amounts (0.1−5.0 equiv) of a
solution of TPPTS (δ(31P) −5.7 ppm) in D2O were added to a
solution of [2(OH2)][OTf]65,67 in D2O. The resulting
solutions were analyzed by 31P NMR spectroscopy 5 min
after mixing as well as after 2 days (Scheme 2 and Table S1
(Supporting Information)). Upon addition of 0.1−1.0 equiv of
TPPTS the only signal observed in the 31P NMR spectra was a
1
sharp resonance at 30.6 ppm with J(31P−195Pt) = 2100 Hz
(Scheme 2). No resonance pointing to the presence of free
phosphine was observed, indicating that all phosphine
molecules had coordinated to platinum. The spectrum revealed
no change after 2 days, clearly indicating the formation of a
stable 1:1 coordination product. The 1J(31P−195Pt) value (2100
2812
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
Scheme 2. Observed Resonances in 31P NMR upon the Addition of 0.1−5.0 equiv of TPPTS to [2(OH2)][OTf] in D2O and the
a
Proposed Species Formed, [2(TPPTS)]+, cis-[2(TPPTS)2(D2O)]+, and trans-[2(TPPTS)2(D2O)]+
a
1
2
The 31P NMR resonances and the J(31P−195Pt) and J(31P−31P) values (for cis-[2(TPPTS)2(D2O)]+), respectively, are given in parentheses.
Figure 3. The two cations and the dianion of [2(OH2)]2[2(3)] (note that (3) = TPPTS minus three Na+; residues 1−3) in the asymmetric unit
(three cocrystallized water molecules are omitted for clarity). Hydrogen atoms are omitted for clarity, and displacement ellipsoids are drawn at the
50% probability level. See also Figure S1 (Supporting Information), showing the infinite 2D network by intermolecular hydrogen bonding.
trans69−71 and cis69,72−74 to Cipso, respectively, which is in good
agreement with literature values for related complexes (e.g.,
1600−2000 Hz for trans coordination and 3100−4400 Hz for
cis coordination). It is noteworthy that previous studies have
shown67,72,75 that free o-CH2NMe2 groupings, as depicted in
cis-[2(TPPTS)2(D2O)]+ (Scheme 2), cannot coordinate to the
platinum metal center. Decoordination of o-CH2NMe2 group-
ings in [2(OH2)]+ followed by cis coordination of stronger
ligands causes the now monodentate C-bonded NCN-pincer
ligand to turn its pincer arene plane perpendicular to the
platinum coordination plane, thus releasing the steric constraint
imposed by the cis-coordinated stronger ligand, in this case the
TPPTS ligand.
When these solutions were left standing for another 2 days, a
singlet signal grew into the spectrum at 23.8 ppm
(1J(31P−195Pt) = 3200 Hz), pointing to the copresence of a
Pt complex with two chemically and magnetically identical
phosphines. These resonances suggested that on standing an
isomerization of cis-[2(TPPTS)2(D2O)]+ to trans-[2-
(TPPTS)2(D2O)]+ had occurred (Scheme 2). The formation
1
of trans-[2(TPPTS)2(D2O)]+ is supported by J(31P−195Pt)
values from the literature ranging from 2980 to 3060 Hz75−78
and a crystal structure of a PCP-pincer platinum complex.75
2813
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
Scheme 3. Observed Resonances in 31P NMR upon the Addition of 0.1−5.0 equiv of TPPTS to [2(OH2)][OTf] in Tris Buffer
a
(125 mM, pH 8.0) and the Different Species Formed
a
The 31P NMR resonances and the 1J(31P−195Pt) and 2J(31P−31P) values (for cis-[2(TPPTS)2(L)]+ and cis-[2(TPPTS)(L)2]+ with L = Tris, H2O)
values are given in parentheses. Tris is also shown.
When 1 equiv of phosphine TPPTS was added to a solution of
independently prepared [2(TPPTS)]+ (vide infra), the same
isomerization behavior was observed, with the initial formation
of the coordination complex cis-[2(TPPTS)2(D2O)]+, which
ultimately converted into trans-[2(TPPTS)2(D2O)]+. Also
here, no free TPPTS was detected after 1 and 2 days,
respectively, indicating once more that the phosphine present
had coordinated quantitatively to the NCN-pincer platinum
complex.
Isolation and Crystallographic Studies. To confirm the
formation of at least one of the in situ observed coordination
complexes, we decided to isolate and fully analyze the
coordination complex [2(TPPTS)]+. Mixing of [2(OH2)]-
[OTf] and TPPTS (1:1) in H2O and subsequent workup
yielded a white powder. Elemental analysis and mass
spectrometry of this powder clearly showed that a 1:1
phosphine-NCN-pincer platinum complex was formed. The
31P NMR resonance (30.6 ppm) and coupling data
(J(31P−195Pt) = 2100 Hz) were identical with those observed
during the titration experiment (vide supra and Scheme 2). The
1H NMR resonances, especially the aromatic and aliphatic
signals of the pincer moiety, indicated a symmetric molecule
with trans coordination of the phosphine with respect to Cipso
(triplet and doublet for the aromatic pincer proton signals with
3JH−H = 7.2 Hz, two singlets for −CH2 and −NMe2 proton
signals at 4.12 and 2.37 ppm, respectively).
S2 in the Supporting Information). In the asymmetric unit,
three pincer platinum(II) cations are present, one of which is
coordinated to the trianionic triarylphosphine, thus forming the
dianionic NCN-pincer phosphine entity [2(3)]2− (residue 1;
note that (3) = TPPTS minus three Na+). The remaining two
NCN-pincer platinum cations are coordinated to water,
[2(OH2)]+ (residues 2 and 3), and act as counter-cations to
yield a charge-neutral unit cell. The sodium and triflate ions
present in the original aqueous solution of TPPTS apparently
did not turn up in the crystallized material. The phosphine in
residue 1 (Figure 3) binds to the NCN-pincer platinum cation
trans with respect to Cipso (Pt1−P1 = 2.3627(12) Å). The
orientation of the phosphine and the Pt−P bond distance
(Table S2, Supporting Information) are comparable to the
crystal structure data of other ECE-pincer platinum phosphine
complexes, which contain a monodentate phosphine that is
coordinated trans to Cipso.
70,71 Selected bond lengths and angles
of [2(OH2)]2[2(3)] are given in Table S2 (Supporting
Information). An interesting additional structural feature of
[2(OH2)]2[2(3)] is that it forms an infinite 2D network by
intermolecular hydrogen bonding. This network is aligned
parallel to the crystallographic ab plane (see Figure S1). The
coordinated and noncoordinated water molecules act as
hydrogen bond donors. Nine hydrogen bonds are accepted
by the oxygen atoms of sulfonate groups and noncoordinated
water, respectively; the hydrogen bond of H26O is accepted by
the π system of the “pincer” phenyl ring C13−C63.
The reason for this surprising crystallization behavior might
originate from the use of CH2Cl2 as cosolvent during
crystallization. As phosphine TPPTS is insoluble in CH2Cl2
and the crystals were formed at the H2O/CH2Cl2 interface, the
different solubilities of the coordination complex [2(TPPTS)]-
[OTf] in the H2O and CH2Cl2 phases might have provoked the
observed crystallization.
To confirm the proposed structure of [2(TPPTS)]+, we tried
to obtain single crystals suitable for an X-ray crystal structure
determination. As initial attempts to crystallize [2(TPPTS)]+
from pure water failed, we added several droplets of CH2Cl2,
after which some crystals were formed at the CH2Cl2/H2O
interface. The X-ray crystal structure determination of these
crystals revealed the unexpected formation of an ionic
coordination complex with an interesting molecular structure
(Figure 3; relevant bond distances and angles are given in Table
NMR Studies with [2(OH2)][OTf] and Phosphine TPPTS
in Tris Buffer. A slightly different coordination behavior was
2814
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
Figure 4. Displacement ellipsoid plot (50% probability level) of the 1:1 coordination complex [2(NH2(CH2)2OH)][OTf]. C−H hydrogen atoms
are omitted for clarity. Symmetry operation: (i) 2 − x, −y, 1 − z.
observed when, instead of D2O, Tris buffer (chloride free) was
used as a solvent for the study of the coordination behavior of
phosphine TPPTS to the cationic pincer platinum complex
[2(OH2)][OTf]. Again for this study, various amounts of
TPPTS (0.1−5.0 equiv in D2O) were added to [2(OH2)]-
[OTf] (in 125 mM Tris buffer, pH 8.0, acidified with HBF4;
final concentration of [2(OH2)][OTf] in Tris 19 mM).
Subsequently, the different solutions were analyzed by 31P
NMR spectroscopy after 5 min and 1, 2, and 19 days.
In a control experiment, where independently synthesized
[2(TPPTS)][OTF] (vide infra) was dissolved in d11-Tris
1
buffer and subsequently analyzed by H NMR and 31P NMR
spectroscopy, exactly the same trans to cis isomerization of the
coordinated phosphine was observed, as described above. This
observation supports the formation of the coordination
complex cis-[2(TPPTS)(Tris)2]+ from the complex [2-
(TPPTS)][OTF], as depicted in Scheme 3.
When more than 1 equiv of phosphine TPPTS was added,
two additional signals were observed, in addition to the signals
of the monocoordinated cis and trans species in its 31P NMR
spectrum. These two signals at 22.1 ppm (1J(31P−195Pt) = 1640
Hz, 1J(31P−31P) = 14 Hz) and 10.1 ppm (1J(31P−195Pt) = 3850
Upon the addition of up to 1 equiv of phosphine TPPTS,
initially the appearance of a 31P resonance peak at 30.6 ppm
(1J(31P−195Pt) = 2100 Hz) was observed (Table S3, Supporting
Information), which is the same signal as was observed in the
coordination study in D2O: i.e., assigned to the phosphine
bound trans with respect to Cipso (see complex [2(TPPTS)]+,
Schemes 2 and 3). However, when the same mixture was
analyzed after 1 day, a second signal at 8.6 ppm (1J(31P−195Pt)
= 4010 Hz) had appeared (Table S3, Supporting Information),
which gradually became more intense over time and ultimately
after 19 days was the only resonance present in the spectrum.
1
Hz, J(31P−31P) = 13 Hz) were both doublets (Table S3,
Supporting Information), indicating the coordination of two
phosphines to the same platinum center in a cis orientation
(complex cis-[2(TPPTS)2(Tris)]+, Scheme 3). An isomer-
ization of the bis-coordinated product, as observed for the study
in D2 O (cis-[2(TPPTS)2 (D2 O)]+ → trans-[2-
(TPPTS)2(D2O)]+, Scheme 2) did not occur when Tris buffer
was used as a solvent.
1
Its J(31P−195Pt) value of 4010 Hz is indicative of phosphine
Interestingly, even with an excess of 5.0 equiv of phosphine
TPPTS present, we could still observe a small amount of the
monocoordinated product, in addition to the bis-coordinated
product. Furthermore, the ratio between mono- and bis-
coordinated product did not change over time. This
observation, which is different from the observations in D2O,
is most probably due to the competitive coordination of Tris to
the cationic NCN-pincer metal center. Apparently, the large
excess of Tris (125 mM Tris to 19 mM [2(OH2)][OTf]) and
its competitive coordination to platinum could only allow for
the partial formation of the bis-coordinated complex cis-
[2(TPPTS)2(Tris)]+.
Coordination of Ethanolamine to [2(OH2)][OTf]. The
coordination of TPPTS to [2(OH2)][OTf] was obviously
influenced by the coordinating properties of Tris. Accordingly it
was attempted to study the various coordination modes of the
potentially tetradentate Tris molecule to [2(OH2)][OTf] by
1H NMR spectroscopy. As the coordination pattern turned out
coordination cis with respect to Cipso,
70,72−74 which in this case
would imply an isomerization of the coordinated phosphine
from trans to cis. As the occurrence of the resonance at 8.6 ppm
with 1J(31P−195Pt) = 4010 Hz was only observed in the
presence of Tris buffer, the formation of the cis-coordination
complex (i.e. cis-[2(TPPTS)(Tris)(L)]+ with L = Tris, H2O)
from the trans coordination complex is most probably assisted
by the presence of Tris molecules. Ample evidence84,85,79,80 is
available that an important prerequisite for the trans to cis
isomerization to occur is the dissociation of at least one Pt−
Npincer bond. The subsequent cis coordination of an external
ligand causes severe steric interference with the decoordinated
o-CH2NMe2 grouping of the pincer ligand, making decoordi-
nation of the second o-CH2NMe2 substituent with concomitant
rotation of the aryl ring out of the Pt coordination plane a
favorable process, as it is releasing the steric strain of the ligands
in the platinum coordination plane, vide supra.
2815
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
Scheme 4. Coordination of Phosphines TXPTS and [5](BF4)3, Respectively, to [2(OH2)][OTf]
to be very complex (see Figure S2 for the spectra, Supporting
Information), it was very difficult to deduce from these spectra
the actual coordination mode of Tris to the cationic NCN-
pincer platinum moiety. Therefore, we chose to study the
coordination of ethanolamine (2-hydroxyethylamine) (pKa =
9.3) to [2(OH2)][OTf] in D2O at pH 7. As ethanolamine
possesses only one hydroxyl group as compared to three for
Tris, ethanolamine was considered to be a suitable, though less
complex, model for Tris.
which two NCN-pincer platinum cations are N bonded. Each
of the OTf anions are H bonded via two of its O atoms to one
of the two H atoms of the H−N−H···O−H bonding motif of
the central 10-membered diaza ring. It must be noted that the
intramolecular N−Pt coordination of the o-NMe2 groups is
retained. These findings indicate that coordination of Tris
molecules to the NCN-pincer platinum cations in both the low-
molecular-weight complex [2] and [cut-1] (vide infra) will be
rather complex because of the distinct role the formation of H-
bonding patterns will play in the resulting species. Nevertheless,
the experiments with ethanolamine suggest that the coordina-
tion of Tris and the NCN-pincer platinum cations will occur via
its amine N donor atom, while the OH groups will most likely
be involved in hydrogen atom bonding patterns.
Coordination of Phosphines TXPTS and [5](BF4)3 to
[2(OH2)][OTf] in D2O. To investigate the influence of the
steric requirements of the selected neutral, cationic, and anionic
triarylphosphines on their coordination to the NCN-pincer
platinum cation, the reactions of trianionic triarylphosphine
TXPTS (note that trianionic (4) = TXPTS minus three Na
cations) and tricationic triarylphosphine [5] to [2(OH2)]-
[OTf] were studied. To a solution of [2(OH2)][OTf] were
added various amounts of a solution of either TXPTS or
[5](BF4)3, and the resulting solutions were analyzed by 31P
NMR spectroscopy (Scheme 4).
When a 1:1 mixture of ethanolamine and [2(OH2)][OTf] in
1
D2O was analyzed by H NMR, it was observed that the
original multiplet signals of uncoordinated ethanolamine (3.47
and 2.59 ppm) shifted to 3.73 and 2.98 ppm. The aliphatic
pincer signals shifted equally from 4.02 and 2.82 ppm to 4.00
and 2.88 ppm, respectively (all values in D2O). These data
pointed to the formation of a coordination complex between
ethanolamine and [2(OH2)][OTf].
When a 1:1 mixture of [2(OH2)][OTf] and ethanolamine
was crystallized from H2O, single crystals were obtained. An X-
ray crystal structure determination of these crystals revealed the
molecular structure of the 1:1 coordination complex [2-
(NH2(CH2)2OH)][OTf] in the solid state (Figure 4; relevant
bond distances are given in Table S4). The ORTEP plot of
[2(NH2(CH2)2OH)][OTf] shows that the NH2 group of
ethanolamine coordinates to the platinum ion of the pincer
moiety trans to Cipso, while the hydroxyl group remains free.
Two [2(NH2(CH2)2OH)]+ cations are arranged into a unique
dimeric structure via mutual N−H···O hydrogen bonding of
one [2(NH2(CH2)2OH)]+ cation with the OH grouping of the
other [2(NH2(CH2)2OH)]+ cation; i.e., the structure can be
considered as comprising a central 10-membered diaza ring to
For trianionic phosphine TXPTS the coordination of only
one phosphine to the NCN-pincer platinum cation was
observed in D2O, even when up to 5 equiv was added. Both
the titration study and the preparative synthesis of [2-
(TXPTS)][OTF] in D2O gave in the 31P NMR spectra a
1
chemical shift of 29.1 ppm with J(31P−195Pt) = 1940 Hz,
2816
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
which pointed to a structure with the phosphine grouping
coordinating trans to Cipso (Scheme 4).
coordination behavior of [5][BF4]3 (vs TXPTS) and [2-
(OH2)][OTf].
31P NMR Study with [cut-1(OH2)][OTf] and Phosphine
TPPTS. From the crystal structures it became apparent that the
cutinase-embedded pincer metal center was exposed to the
solvent (cf. Figure 1a).47 Therefore, it was anticipated that the
pincer metal head group could be available for the coordination
to various phosphines. The coordination of trianionic
triarylphosphine TPPTS to the cationic pincer cutinase hybrid
[cut-1(OH2)][OTf] was studied by adding different amounts
of TPPTS (0.5, 1.0, and 1.5 equiv in D2O) to a solution of
[cut-1(OH2)][OTf] (2 mM) in Tris buffer (125 mM, pH 8.0,
acidified with HBF4, 10% D2O) (Figure 6). The resulting
solution was analyzed by 31P NMR spectroscopy. Interestingly,
we observed only one resonance at about 8.8 ppm
(1J(31P−195Pt) = 3970 Hz) in the spectra, which points to
the presence of one coordinated TPPTS ligand per embedded
platinum, regardless of the number of equivalents of TPPTS
present in solution. See Figure S3 in the Supporting
Information for the spectrum that could be obtained for
[cut-1(OH2)][OTf] in the presence of TPPTS (1.5 equiv).
When various amounts of tricationic [5](BF4)3 were added
to [2(OH2)][OTf] dissolved in a water/acetonitrile mixture
(5/2 v/v), again the 1:1 coordination of one phosphine to the
metal center was observed. However, in this case the
coordination of a phosphine cis with respect to Cipso was
observed as well, with 20% trans (1J(31P−195Pt) = 2070 Hz)
and 80% cis (1J(31P−195Pt) = 3010 Hz) coordination complex
(trans-[2(5)][BF4]3[OTf] and cis-[2(5)(L)2][BF4]3[OTf];
Scheme 4). Also for this phosphine, coordination of a second
phosphine molecule was not observed upon addition of more
than 1 equiv of [5][BF4]3. Complexes [2(TXPTS)][OTf] and
[2(5)][BF4]3[OTf] were synthesized, and the pure white
products were fully analyzed by NMR spectroscopy and high-
resolution ES mass spectrometry (see the Supporting
Information for details).
In addition to steric constraints, also the σ-bond donating
properties of the phosphorus are a determining factor for the
coordination properties of phosphine ligands (such as PR3). It
has been established that the σ-bond donating properties of the
phosphorus atom in substituted triarylphosphines are influ-
enced by the charge of the different R groups.81,82 It appeared
that the nature of the σ-bond donating properties can be
measured by converting the respective phosphines into the
corresponding SePR3 derivatives with selenium and sub-
sequent measurement and comparison of the 1J(31P−77Se)
coupling constant of these compounds.81,82
1
The position of the peak and the observed J(31P−195Pt)
value (3970 Hz; 195Pt satellite signals are marked in Figure S3)
indicate that in the case of the embedded pincer platinum the
TPPTS might be coordinated cis to Cipso.
69,73−75,78,83 However,
due to the fact that one of the satellite signals coincides with the
internal standard resonance (phosphoric acid at 0 ppm),
caution should be taken assigning the TPPTS coordination
mode solely on the basis of the 1J(31P−195Pt) values. In contrast
to what has been observed for the parent low-molecular-weight
NCN-pincer platinum compound, i.e. the formation of the bis-
coordination product trans-[2(3TPPTS)2(D2O)]+ (Scheme
2), the presence of an excess of TPPTS (1.5 equiv) in solution
did not lead to the formation of a bis-TPPTS cutinase pincer
hybrid species.
1
It appeared that the J(31P−77Se) coupling values of sodium
3-phosphoroselenoylbenzene sulfonate (Na3[6]) and N,N,N-
trimethyl-1-(4-phosphoroselenoylphenyl)methanaminium
([7][BF4]3) (Figure 5) are nearly identical, indicating similar σ-
ESI-MS Studies: Titration of [cut-1(OH2)]+ with Various
Amounts of TPPTS. To investigate and quantify the
coordination of phosphines to [cut-1(OH2)]+ further, we
decided to study the coordination of TPPTS to [cut-1(OH2)]+
by ESI-MS. For this purpose we added different portions of a
solution of TPPTS (1.0−5.0 equiv) to a solution of [cut-
1(OH2)]+. In order to obtain high-resolution mass spectra, we
had to perform these experiments in NH4Ac instead of Tris
buffer. More importantly, a separate coordination study in
NH4Ac buffer showed results similar to those obtained in Tris,
which justified the buffer switch required for this ESI-MS study.
Figure 5. 31P NMR data of phosphine selenides Na3[6] and [7][BF4]3
in D2O.84
bond donating properties of TPPTS and [5][BF4]3, which
could indicate similar coordination properties to the platinum
cation. These observations indicate that a difference in σ-
donation properties cannot be a reason for the different
Figure 6. Proposed molecular structure of pincer-phosphine coordination complex cis-[cut-1(TPPTS)]+ formed upon addition of 0.5−1.5 eq. of
TPPTS to [cut-1(OH2)]+. See also Figure S3 for the recorded 31P NMR spectrum.
2817
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
After addition of 1.0 equiv of TPPTS, mostly monocoordi-
nated [cut-1(TPPTS)]+ was observed (Figure 7), along with
i.e., a buffer-soluble complex was formed. In order to test
whether the coordination of 8 to [cut-1(OH2)]+ might be
aspecific, a dilution experiment with ESI-MS analysis of the
solution was carried out. These experiments indicated that the
coordination was indeed selective. Thus, even though 8 itself is
water insoluble, selective coordination to the embedded
platinum to give water-soluble [cut-1(8)]+ does occur.
Subsequently, we also investigated the coordination of the
water-soluble tri- and hexacationic phosphines [5][BF4]3,
[9][BF4]6, and [10][BF4]6.84 Most interestingly, after addition
of 1.0 equiv of [5][BF4]3, [9][BF4]6, or [10][BF4]6, we did not
see any evidence for the presence of [cut-1(phosphine)]+
coordination complexes in the ESI-mass spectra (Table S5),
showing that no detectable binding of these phosphines to the
protein-embedded platinum center had occurred.
CONCLUSIONS
■
Figure 7. Deconvoluted ESI-MS spectrum of a 1:1 mix of [cut-
1(OH2)]+ and TPPTS. The highest peak corresponds to (H3[cut-
1(3)]+); the shoulder peaks at higher masses correspond to the
association of one or two sodium ions (H2Na[cut-1(3)]+ and
HNa2[cut-1(3)]+, respectively). See also Table S5.
This study has shown that the coordination of various
phosphines to an artificial metal center in a semisynthetic
metalloenzyme is feasible. We demonstrated that the well-
established coordination chemistry for low-molecular-weight
NCN-pincer platinum complexes can be transferred to novel
semisynthetic metalloprotein systems having these pincer
complexes embedded. However, it is apparent that in this
semisynthetic protein hybrid system the pincer platinum head
group exerts its own unique coordination behavior dictated by
the specific influence of the protein backbone on the cationic
pincer metal center. The results suggest that these effects are of
both kinetic (cf. effect on the Pt−N dissociation−association
process of the NCN-pincer platinum moiety and the influence
of the charge of the phosphine ligand) and thermodynamic
nature (cf. influence of the size of the phosphine in its complex
with the embedded Pt center on its stability). An intriguing
question arises when the volume and shape of the starting
NCN-pincer platinum moiety, which is rather two-dimensional,
is compared with those of the cis phosphine coordination
product that is clearly three-dimensional with the decoordi-
nated o-CH2NMe2 groups oriented perpendicular to the Pt's
coordination plane and the bulky, negatively charged
phosphine. It is obvious that sterics play a role, because
coordination of two of these phosphines to the protein-
embedded platinum center seems impossible (in contrast with
the case for the bis-phosphine complexes obtained with the
low-molecular-weight NCN-pincer platinum compound). Fur-
ther studies are required to understand how the shapes,
volumes, and polarity of the phosphine ligands used in the
present study and the protein surface surrounding the pincer
metal group affect each other. A first indication for this
interplay can be found on comparing the different orientations
of a NCN-pincer platinum (see Figure 1a) and a SCS-pincer
palladium halide group (SCS represents the
[C6H3(CH2SMe)2] anion), respectively, in the corresponding
solid-state structures [cut-NCN-pincer PtCl] and [cut-SCS-
pincer PdBr] (see overlay of their structures in the solid state
in Figure 5a of ref 47). Moreover, the coordination studies
described here were all performed in a strongly polar medium,
in water or in aqueous Tris, respectively. Due to the fact that all
phosphines except 8 are charged, polar effects influencing the
interaction of the coordinated phosphines with the hydrophilic
and hydrophobic areas of the protein backbone surrounding
the embedded Pt center and the coordinated, charged
phosphines must play a role as well. It seems likely that the
occurrence of intramolecular, interligand Coulombic repulsion
traces of [cut-1]+ and traces of bis-TPPTS coordinated H6[cut-
1(3)2]+ (vide infra). The latter result was surprising, as this
seemed to contrast with the earlier conclusions from the 31P
NMR spectroscopic study (vide supra). It appeared that after
further dilution of the solution (5× and 10×) these peaks
assigned to bis coordination had disappeared and thus turned
out to be aspecific. A control experiment showed that under
concentrated conditions aspecific phosphine binding to free cut
(that lacks the NCN-pincer platinum moiety) indeed occurred
but disappeared upon diluting the solution.
The mass spectrum in Figure 7 showed a characteristic
pattern of three shoulder peaks, with the peak with the highest
intensity corresponding to the coordinated phosphine having
all three Na+ ions replaced by H+ (H3[cut-1(3)]+, Table S5),
i.e. displaying a +1 charged phosphine metallopincer cutinase
coordination complex. The shoulder peaks correspond to the
substitution of two and one Na+ ions by H+, respectively, i.e. of
H2Na[cut-1(3)]+ and HNa2[cut-1(3)]+.
ESI-MS Studies of Reactions of [cut-1(OH2)]+ with
Trianionic TPPTS and TXPTS, Neutral Triphenylphos-
phine, Tricationic [5]3+, and Hexacationic [9]6+ and
[10]6+. Encouraged by these results, we also studied the
coordination behavior of the bulkier and differently charged
triarylphosphines with [cut-1(OH2)][OTf], as this potentially
could be an approach to probe whether the observed binding of
the TPPTS to the embedded platinum center could be the
result of the presence of the negative charge in the TPPTS: i.e.,
by mutual lipase phosphine interactions. Therefore, to [cut-
1(OH2)][OTf] (1 equiv) were added different phosphines (1
equiv) and these samples were subsequently analyzed by ESI-
MS. In Table S5, the results for different water-soluble cationic
and anionic phosphines (Na3[4], [5][BF4]3, [9][BF4]6,
[10][BF4]6) and water-insoluble, neutral phosphine (8) are
given.
With compound TXPTS we observed partial coordination of
the phosphine to [cut-1(OH2)]+ when 1.0 equiv of TXPTS
was added. In this case around 50% of free [cut-1]+ was still
observed according to mass spectrometry.
Triphenylphosphine 8 lacks charged groups and is insoluble
in the aqueous buffer used. When 8 was added to [cut-
1(OH2)]+, again partial formation of [cut-1(8)]+ was observed:
2818
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
(8) Ueno, T.; Abe, S.; Yokoi, N.; Watanabe, Y. Coord. Chem. Rev.
2007, 251, 2717−2731.
(9) Ueno, T.; Koshiyama, T.; Abe, S.; Yokoi, N.; Ohashi, M.;
Nakajima, H.; Watanabe, Y. J. Organomet. Chem. 2007, 692, 142−147.
(10) Ueno, T.; Norihiko, Y.; Abe, S.; Watanabe, Y. J. Inorg. Biochem.
2007, 101, 1667−1675.
can explain the coordination of just one phosphine molecule to
the platinum center. It must be noted that phosphines TPPTS
and TXPTS and [5][BF4]3 carry opposite charges (anionic
versus cationic), which can be expected to affect their
coordination behavior in different ways.85
Notably, we demonstrated for the first time that the
coordination properties of complex metal protein hybrid
systems can be studied using standard chemical analysis
techniques (31P NMR spectroscopy and ESI-MS spectrometry),
thereby creating straightforward analysis protocols without the
need for laborious biochemical analysis procedures. This proof
of principle study opens the door to various potential new
applications in which the unique properties of synthetic metal
complexes and biomolecules are combined. For example, we
are currently studying the catalytic properties of similar pincer
metal protein hybrids86,88 as well as the use of luminescent
NCN-pincer platinum complexes as site-selective protein
labels.87 The latter study can serve as a starting point in
assaying the accessibility and the chemical properties of
protein-embedded ECE-pincer metal centers.
(11) Satake, Y.; Abe, S.; Okazaki, S.; Ban, N.; Hikage, T.; Ueno, T.;
Nakajima, H.; Suzuki, A.; Yamane, T.; Nishiyama, H.; Watanabe, Y.
Organometallics 2007, 26, 4904−4908.
(12) Oshovsky, G. V.; Reinhoudt, D. N.; Verboom, W. Angew. Chem.,
Int. Ed. 2007, 46, 2366.
(13) Slim, M.; Sleiman, H. F. Bioconjugate Chem. 2004, 15, 949−953.
(14) Kam-Wing Lo, K.; Hui, W.-K.; Chun-Ming Ng, D. J. Am. Chem.
Soc. 2002, 124, 9344−9345.
(15) Chen, P. R.; He, C. Curr. Opin. Chem. Biol. 2008, 12, 214−221.
(16) Wilson, M. E.; Whitesides, G. M. J. Am. Chem. Soc. 1978, 100,
306−307.
(17) Collot, J.; Gradinaru, J.; Humbert, N.; Skander, M.; Zocchi, A.;
Ward, T. R. J. Am. Chem. Soc. 2003, 125, 9030−9031.
(18) Steinreiber, J.; Ward, T. R. Coord. Chem. Rev. 2008, 252, 751.
(19) Pierron, J.; Malan, C.; Creus, M.; Gradinaru, J.; Hafner, I.;
Ivanova, A.; Sardo, A.; Ward, T. R. Angew. Chem., Int. Ed. 2008, 47,
701−705.
ASSOCIATED CONTENT
* Supporting Information
■
(20) Ward, T. R. Acc. Chem. Res. 2011, 44, 47−57.
(21) Heinisch, T.; Ward, T. R. Curr. Opin. Chem. Biol. 2010, 14,
184−199 and references cited therein..
S
CIF files giving crystallographic data for [2(OH2)]2[2(3)] and
[2(NH2(CH2)2OH)][OTf] and text, tables, and figures giving
experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org. The crystallographic data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(22) Podtetenieff, J.; Taglieber, A.; Bill, E.; Reijerse, E. J.; Reetz, M.
T. Angew. Chem., Int. Ed. 2010, 49, 5151−5155.
(23) Abe, S.; Hikage, T.; Watanabe, Y.; Kitagawa, S.; Ueno, T. Inorg.
Chem. 2010, 49, 6967−6973.
(24) Kohler, V.; Ward, T. R. ChemBioChem 2010, 11, 1049−1051.
(25) Ohashi, M.; Koshiyama, T.; Ueno, T.; Yanase, M.; Fujii, H.;
Watanabe, Y. Angew. Chem., Int. Ed. 2003, 42, 1005−1008.
(26) Carey, J. R.; Ma, S. K.; Pfister, T. D.; Garner, D. K.; Kim, H. K.;
Abramite, J. A.; Wang, Z.; Guo, Z.; Lu, Y. J. Am. Chem. Soc. 2004, 126,
10812−10813.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+) 31 2523615. E-mail: g.vankoten@uu.nl.
■
(27) Panella, L.; Broos, J.; Jin, J.; Fraaije, M. W.; Janssen, D. B.;
Jeronimus-Stratingh, M.; Feringa, B. L.; Minnaard, A. J.; De Vries, J. G.
Chem. Commun. 2005, 45, 5656−5658.
Present Address
⊥Novo Nordisk A/S, Diabetes Protein & Peptide Chemistry,
Novo Nordisk Park C9.2.23, DK-2760 Maløv, Denmark.
̊
(28) den Heeten, R.; Munoz, B. K.; Popa, G.; Laan, W.; Kamer, P. C.
J. Dalton Trans. 2010, 39, 8477−8483.
Notes
(29) Laan, W.; Munoz, B. K.; den Heeten, R.; Kamer, P. C. J.
ChemBioChem 2010, 11, 1236−1239.
The authors declare no competing financial interest.
(30) Deuss, P. J.; Popa, G.; Botting, C. H.; Laan, W.; Kamer, P. C. J.
Angew. Chem., Int. Ed. 2010, 49, 5315−5317.
ACKNOWLEDGMENTS
We thank Utrecht University (B.W.) and NRSC-Combination
Catalysis (H.P.D.) for funding.
■
(31) Gallagher, J.; Zelenko, O.; Walts, A. D.; Sigman, D. S.
Biochemistry 1998, 37, 2096−2104.
(32) McNae, I. W.; Fishburne, K.; Habtemariam, A.; Hunter, T. M.;
Melchart, M.; Wang, F.; Walkinshaw, M. D.; Sadler, P. J. Chem.
Commun. 2004, 1786−1787.
DEDICATION
Dedicated to the memory of Prof F. Gordon A. Stone.
■
(33) Chen, H.; Parkinson, J. A.; Morris, R. E.; Sadler, P. J. J. Am.
Chem. Soc. 2003, 125, 173−186.
REFERENCES
■
(34) Alberto, R., Bioorganometallics; Wiley-VCH: Weinheim,
Germany, 2006; p 97−124.
(1) Banci, L.; Bertini, I.; Ciofi-Baffoni, S.; Katsari, E.; Katsaros, N.;
Kubicek, K.; Mangani, S. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 3994.
(2) Banci, L.; Bertini, I.; Calderone, V.; Ciofi-Baffoni, S.; Mangani, S.;
Martinelli, M.; Palumaa, P.; Wang, S. Proc. Natl. Acad. Sci. U.S.A. 2006,
103, 8595.
(35) Jaouen, G., Bioorganometallics: Biomolecules, Labelling, Medicine;
Wiley-VCH: Weinheim, Germany, 2006; p 1−472.
(36) Munch, K. H.; Heide, S. T.; Christensen, N. J.; Hoeg-Jensen, T.;
Thulstrup, P. W.; Jensen, K. J. Chem. Eur. J. 2011, 17, 7198−7204.
(37) Peterson, J. R.; Smith, T. A.; Thordarson, P. Chem. Commun.
2007, 1899−1901.
(3) Bertini, I.; Sigel, A.; Sigel, H., Handbook on Metalloproteins;
Marcel Dekker: New York, 2001; p 1−1800.
(4) Forzi, L.; Sawers, R. G. Biometals 2007, 20, 565−578.
(5) Salgado, E. N.; Radford, R. J.; Tezcan, F. A. Acc. Chem. Res. 2010,
43, 661−672.
(38) Meggers, E. Chem. Commun. 2009, 1001−1010.
(39) Louie, M.-W.; Tsz-Him Fong, T.; Kam-Wing Lo, K. Inorg.
Chem. 2011, 50, 9465−9471.
(40) Zaykov, A. N.; Ball, Z. T. Chem. Commun. 2011, 47, 10927−
10929.
(41) Licini, G.; Scrimin, P. Angew. Chem., Int. Ed. 2003, 42, 4572−
4575.
(6) Creus, M.; Pordea, A.; Rossel, T.; Sardo, A.; Letondor, C.;
Ivanova, A.; LeTrong, I.; Stenkamp, R. E.; Ward, T. R. Angew. Chem.,
Int. Ed. 2008, 47, 1400−1404.
(7) Yokoi, N.; Ueno, T.; Unno, M.; Matsui, T.; Ikeda-Sato, M.;
Watanabe, Y. Chem. Commun. 2008, 229−231.
2819
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820

Organometallics
Article
(42) Urig, S.; Fritz-Wolf, K.; Reau, R.; Herold-Mende, C.; Toth, K.;
Davioud-Charvet, E.; Becker, K. Angew. Chem., Int. Ed. 2006, 45,
1881−1886.
(73) Meijer, M. D.; de Wolf, E.; Lutz, M.; Spek, A. L.; van Klink, G.
P. M.; van Koten, G. Organometallics 2001, 20, 4198−4206.
(74) Poverenov, E.; Gandelman, M.; Shimon, L. J. W.; Rozenberg,
H.; Ben-David, Y.; Milstein, D. Organometallics 2005, 24, 1082−1090.
(75) Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. J.
Am. Chem. Soc. 2000, 122, 11822−11833.
(76) Manna, J.; Whiteford, J. A.; Stang, P. J.; Muddiman, D. C.;
Smith, R. D. J. Am. Chem. Soc. 1996, 118, 8731−8732.
(77) Bennett, M. A.; Jin, H.; Willis, A. C. J. Organomet. Chem. 1993,
451, 249−256.
(78) Gorla, F.; Venanzi, L. M.; Albinati, A. Organometallics 1994, 13,
43−54.
(43) Gallagher, J. A.; Levine, L. A.; Williams, M. E. Eur. J. Inorg.
Chem. 2011, 4168−4174.
(44) Coppock, M. B.; Miller, J. R.; Williams, M. E. Inorg. Chem. 2011,
50, 949−955.
(45) Bogdan, N. D.; Matache, M.; Roiban, G. D.; Dobrota, C.; Meier,
V. M.; Funeriu, D. P. Biomacromolecules 2011, 12, 3400−3405.
(46) Bogdan, N. D.; Matache, M.; Meier, V. M.; Dobrota, C.;
Dumitru, I.; Roiban, G. D.; Funeriu, D. P. Chem. Eur. J. 2010, 16,
2170−2180.
(79) van der Zeijden, A. A. H.; van Koten, G.; Luijk, R.; Nordemann,
R. A.; Spek, A. L. Organometallics 1988, 7, 1549−1556.
(80) Sutter, J.-P.; James, S. L.; Steenwinkel, P.; Karlen, T.; Grove, D.
M.; Veldman, N.; Smith, R. D.; Spek, A. L.; van Koten, G.
Organometallics 1996, 15, 941−948.
(47) Rutten, L.; Wieczorek, B.; Mannie, J.-P. B. A.; Kruithof, C. A.;
Dijkstra, H. P.; Egmond, M. R.; Lutz, M.; Spek, A. L.; Gros, P.; Klein
Gebbink, R. J. M.; van Koten, G. Chem. Eur. J. 2009, 15, 4270−4280.
(48) Du, S.; Ellis, D. D.; Jellis, P. A.; Kautz, J. A.; Malget, J. M.; Stone,
F. G. A. Organometallics 2000, 19, 1983−1992.
(49) Franken, A.; McGrath, T. D.; Stone, F. G. A. Organometallics
2010, 29, 2234−2247.
(81) Allen, D. W.; Nowell, I. W.; Taylor, B. F. J. Chem. Soc., Dalton
Trans. 1985, 2505−2508.
(82) Allen, D. W.; Taylor, B. F. J. Chem. Soc., Dalton Trans. 1982,
51−54.
(50) Skopek, K.; Barbasiewicz, M.; Hampel, F.; Gladysz, J. A. Inorg.
Chem. 2008, 47, 3474−3476.
(83) Meijer, M. D.; Kleij, A. W.; Lutz, M.; Spek, A. L.; van Koten, G.
J. Organomet. Chem. 2001, 640, 166−169.
(51) Neo, K.-E.; Huynh, H. V.; Koh, L. L.; Henderson, W.; Hor, T. S.
A. J. Organomet. Chem. 2008, 693, 1628−1635.
(52) Minakawa, M.; Takenaka, K.; Uozumi, Y. Eur. J. Inorg. Chem.
2007, 1629−1631.
(84) Snelders, D. J. M.; van Koten, G.; Klein Gebbink, R. J. M. Chem.
Eur. J. 2011, 17, 42−57.
(85) Snelders, D. J. M.; van Koten, G.; Klein Gebbink, R. J. M. J. Am.
Chem. Soc. 2009, 131, 11407−11416.
(53) Olsson, D.; Arunachalampillai, A.; Wendt, O. F. Dalton Trans.
2007, 5427−5433.
(86) Wieczorek, B.; Klimeck, T.; Dijkstra, H. P.; Egmond, M. R.; van
Koten, G.; Klein Gebbink, R. J. M. To be submitted for publication.
(87) Wieczorek, B.; Lemcke, B.; Dijkstra, H. P.; Egmond, M. R.;
Klein Gebbink, R. J. M.; van Koten, G. Eur. J. Inorg. Chem. 2010,
1929−1938.
(54) Freixa, Z.; van Leeuwen, P. W. N. M. Coord. Chem. Rev. 2008,
252, 1755−1786.
(55) Adams, J. J.; Lau, A.; Aruisamy, N.; Roddick, D. M. Inorg. Chem.
2007, 46, 11328−11334.
(56) Kruithof, C. A.; Casado, M. A.; Guillena, G.; Egmond, M. R.;
van der Kerk-van Hoof, A.; Heck, A. J. R.; Klein Gebbink, R. J. M.; van
Koten, G. Chem. Eur. J. 2005, 11 (23), 6869−6877.
(57) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40,
3750−3781.
(88) Wieczorek, B.; Traff, A.; Krumlinde, P.; Dijkstra, H. P.; Egmond,
̈
M. R.; van Koten, G.; Backvall, J.-E.; Klein Gebbink, R. J. M.
̈
Tetrahedron Lett. 2011, 52, 1601−1604.
(58) Suijkerbuijk, B. M. J. M.; Aerts, B. N. H.; Dijkstra, H. P.; Lutz,
M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Dalton Trans.
2007, 1273−1276.
(59) Huck, W. T. S.; Prins, L. J.; Fokkens, R. H.; Nibbering, N. M.
M.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998,
120, 6240−6246.
(60) Hall, J. R.; Loeb, S. J.; Shimizu, G. K. H.; Yap, G. P. A Angew.
Chem., Int. Ed. 1998, 37, 121−123.
(61) Gerhardt, W. W.; Weck, M. J. Org. Chem. 2006, 71, 6333−6341.
(62) Yount, W. C.; Loveless, D. M.; Craig, S. L. J. Am. Chem. Soc.
2005, 127, 14488−14500.
(63) Terheijden, J.; van Koten, G.; Grove, D. M.; Vrieze, K. J. Chem.
Soc., Dalton Trans. 1987, 1359−1366.
(64) South, C. R.; Higley, M. N.; Leung, K. C.-F.; Lanari, D.; Nelson,
A.; Grubbs, R. H.; Stoddart, J. F.; Weck, M. Chem. Eur. J. 2006, 12,
3789−3797.
(65) Schmulling, M.; Grove, D. M.; van Koten, G.; van Eldik, R.;
̈
Veldman, N.; Spek, A. L. Organometallics 1996, 15 (5), 1384−91.
(66) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2004,
43, 725−733.
(67) Terheijden, J.; van Koten, G.; Vinke, I. C.; Spek, A. L. J. Am.
Chem. Soc. 1985, 107, 2891−2895.
(68) Shaw, C. F.; Isab, A. A.; Hoeschele, J. D.; Starich, M.; Locke, J.;
Schulteis, P.; Xiao, J. J. Am. Chem. Soc. 1994, 116, 2254−2260.
(69) Otto, S.; Samuleev, P. V.; Polyakov, V., A.; Ryabov, A. D.;
Elding, L. I. Dalton Trans. 2004, 3662−3668.
(70) Maassarani, F.; Davidson, M. F.; Wehman-Ooyevaar, I. C. M.;
Grove, D. M.; van Koten, M. A.; Smeets, W. J. J.; Spek, A. L.; van
Koten, G. Inorg. Chim. Acta 1995, 235, 327−338.
(71) Hanan, G. S.; Kickham, J. E.; Loeb, S. J. Organometallics 1992,
11, 3063−3068.
(72) Kleij, A. W.; Klein Gebbink, R. J. M.; Lutz, M.; Spek, A. L.; van
Koten, G. J. Organomet. Chem. 2001, 621, 190−196.
2820
dx.doi.org/10.1021/om2010832 | Organometallics 2012, 31, 2810−2820