Synthesis and solution behaviour of metal-carbonyl amphiphiles with an Fp (CpFe(CO)2) junction

Article abstract of DOI:10.1016/j.jorganchem.2017.09.003

Metallo-amphiphilic macromolecules with an iron-carbonyl junction (PEGPPh₂-FpR, PEGPPh₂: polyethylene glycol diphenyl phosphine, M_{n, PEG} = 550 or 2000 g/mol; R = octadecyl or hexyl; Fp = CpFe(CO)₂) are synthesiz

Full text of DOI:10.1016/j.jorganchem.2017.09.003

Accepted Manuscript
Synthesis and solution behaviour of metal-carbonyl amphiphiles with an Fp
(CpFe(CO) ) junction
2
Nimer Murshid, Abdelrahman El-Temtamy, Xiaosong Wang
PII:
S0022-328X(17)30514-4
10.1016/j.jorganchem.2017.09.003
JOM 20078
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 26 June 2017
Revised Date: 27 August 2017
Accepted Date: 2 September 2017
Please cite this article as: N. Murshid, A. El-Temtamy, X. Wang, Synthesis and solution behaviour of
metal-carbonyl amphiphiles with an Fp (CpFe(CO) ) junction, Journal of Organometallic Chemistry
2
(2017), doi: 10.1016/j.jorganchem.2017.09.003.
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ACCEPTED MANUSCRIPT
Synthesis and Solution Behaviour of Metal-Carbonyl Amphiphiles
with an Fp (CpFe(CO)2) Junction
Nimer Murshid, Abdelrahman El-Temtamy and Xiaosong Wang*
Department of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, 200
University Avenue West, Waterloo, Ontario, N2L 3G1, Canada.
E-mail: xiaosong.wang@uwaterloo.ca
Abstract
Metallo-amphiphilic macromolecules with an iron-carbonyl junction (PEGPPh2-FpR,
PEGPPh2: polyethylene glycol diphenyl phosphine, Mn, PEG = 550 or 2000 g/mol; R = octadecyl
or hexyl; Fp = CpFe(CO)2) are synthesized via migration insertion reaction of FpR in the
1
presence of PEGPPh2 and are characterized using 31P, H NMR and IR. The prepared
amphiphiles are air stable, soluble in most organic solvents, e.g. THF, DCM, DMF, and exhibit a
reversible redox activity as indicated by cyclic voltammetry (CV) experiments. Surface tension
measurements indicate that the prepared molecules exhibit typical amphiphilic properties with
critical micelle concentrations (CMC) in water ranging between 30 and 113 µM. The aqueous
micelles aggregated from PEGPPh2-FpR agglomerate into suspensions, which is similar with the
solution behaviour of Brij® surfactants, the organic analogue without the Fp junction. The redox
activity of Fp in 1-4, which is sensitive to assembling structures,will be a possible advantage for
further investigation on this agglomeration behaviour.
KEYWORDS: metal-carbonyl complex; organometallic amphiphiles; Fp-surfactants; migration
insertion reaction; iron-phosphine amphiphiles; iron-carbonyl PEG amphiphile
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1. Introduction
Metal carbonyl (MC) complexes have been explored for biomedicine applications.[1-7]
The strong IR absorption of the CO groups at the wavelengths between 1800 and 2200 cm-1, a
biologically transparent window, renders the complexes useful as vibrational probes for
biosensors[1, 5] cell imaging[7] and bioassay[2], etc.[1],[5] The CO molecules released from
MC complexes have functions of anti-bacteria,[8] and anti-inflammation[6]. However, MC
complexes are normally unstable and insoluble in water.[3, 6] Supramolecular[9] and colloidal
chemistry[3, 4] have been explored to address these challenges.
Taking advantage of well-studied migration insertion reaction (MIR),[10-13] we have
recently synthesized a number of air-stable CpFe(CO)2 (Fp) derivatives including small
molecules [14-17] and macromolecules.[18-21] These molecules are able to assemble in water
resulting in colloids stabilized by water-carbonyl interactions.[15-17, 22] The association of the
highly polarized CO groups provides an aggregation-induced local electric field, which results in
a significant enhancement in the IR signal of the CO groups.[14-17] To extend this research for
potential biomedical applications, synthesis and self-assembly of Fp amphiphilic molecules,
PEGPPh2-FpR (PEGPPh2 = polyethylene glycol, Mn, PEG = 550 or 2000 g/mol; R = octadecyl or
hexyl; Fp = CpFe(CO)2) are designed and performed. These molecules are expected to assemble
in water into MC colloids with a biocompatible PEG shell[3, 4] and functions derived from the
aggregation of the Fp groups. The synthesis, characterization and preliminary investigation on
the solution behavior of PEG-PPh2-FpR are reported in this article.
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2. Results and discussion
2.1. Synthesis and characterization of amphiphiles 1-4
Scheme 1. Schematic illustration for the synthesis of PEGPPh2-FpCn amphiphiles.
The synthetic protocol for the preparation of PEGPPh2-FpCn (1-4) is depicted in Scheme
1. PEG-Cl is converted from PEG-OH (Figure S1),[23, 24] and subsequently reacts with
diphenyl phosphine sodium (PPh2Na), which results in the generation of PEGPPh2 (5).[18] FpCn
(6) is prepared via the reaction of cyclopentadienyldicarbonyliron potassium (FpK) with
corresponding alkyl halide (Scheme 1). Subsequently, migration insertion reaction (MIR) of 6 in
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the presence of 5 was performed in THF at 70 C for three days. After purification, the product
was characterized using electrospray ionisation mass spectrometry (ESI-MS), NMR and IR
spectroscopes. The ESI-MS spectra indicate that PEG segments with mixed degree of
polymerization (DP) are incorporated in the amphiphiles (Figure S7-S10). 31P NMR spectra
(Figure 1) depict that the phosphine signal due to 5 at -20.2 ppm is totally disappeared and a
peak at 69.5 ppm due to the coordinated phosphine appears suggesting that the MIR occurs.[18]
Figure 1. 31P NMR (CDCl3, 25 oC, 300 MHz) spectra for (a) 5 and (b) 1
The success of the MIR was also proved by FT-IR spectroscopy. As shown in Figure 2,
FT-IR spectrum for 6 shows two peaks at 1934 and 1990 cm-1 corresponding to the symmetric
and asymmetric stretching of the two terminal carbonyl groups, respectively. After MIR, the two
peaks are shifted towards lower frequencies and are observed at 1912 and 1607 cm-1
corresponding to the terminal and acyl CO groups, respectively (Figure 2). Particularly, the
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appearance of the frequency at a low wavenumber (1607 cm-1) indicates the formation of metal-
acyl bond resulting from MIR.[18]
Figure 2. Partial FT-IR (KBr pellet) spectra of (a) 6 and (b) 1 before and after the migration
insertion reaction.
1H NMR spectra for 1 and 6 are compared in Figure 3. As shown in the figure, the upfield
chemical shift for Cp protons from 4.71 in 6 to 4.36 ppm in 1 confirms that MIR occurred.[18]
The appearance of the signals due to PEG chains in the spectrum for 1 indicates that 5 has
coordinated with 6 as a result of the MIR. In addition, the chemical shift at 1.4 ppm for the
methylene protons,
α to iron, in 6 downfield shifts and splits into two peaks at 2.61 and 2.85 ppm
in the spectrum of 1 (inset in Figure 3). The splitting of the signal is caused by the chiral center
of iron in 1, which further supports the occurrence of MIR.[18]
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Figure 3. 1H NMR (CDCl3, 25 oC, 300 MHz) spectra for
6 (down) and 1 (up).
The synthesized amphiphiles are soluble in most organic solvents, e.g. THF, DCM, DMF,
and stable in air over several months. The molecules also exhibit electrochemical stability. As
shown in Figure 4 and S2, the cyclic voltammetry (CV) experiments of 1-4 in DMF reveal that
the anodic and cathodic currents are reversible with oxidation peaks appearing at ca. 0.60 V (for
1 and 2 with shorter PEG chain) and ca. 0.55 V (for 3 and 4 with longer PEG chain), and with
reduction peaks appearing at ca. 0.44 V (relative to Ag electrode).
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1 µA
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Potential (V)
Figure 4. The CV curve (relative to silver electrode) for
1 in DMF solution with TBAP as
electrolytes (scan rate 50 mV/s).
2.2. Amphiphilic behaviour of 1-4
As indicated by surface tension measurements, the surface tensions of the aqueous
solutions of 1-4 gradually decrease with increasing concentrations and eventually reach constant
values (less than 45 mN/m) at concentrations known as critical micelle concentrations (CMC)
(Figure S3). The CMC values for 1-4 are summarized in Table 1. As shown in Table 1, the CMC
value for 1 and 2 with a shorter hydrophilic PEG (Mn = 550 g/mol) is 29.9 ± 2.3 and 40.0 ± 6.2
µM, respectively. These values are lower than that for 3 (82.1 ± 4.5 µM) and 4 (113.3 ± 23.0
µM) with a longer PEG (Mn = 2000 g/mol). For the molecules with the same PEG chain length
(1 vs 2 or 3 vs 4), the larger the hydrophobic alkyl group; the smaller the CMC values (Table 1).
These results suggest that 1-4 are typical amphiphilies, because the molecules with shorter
hydrophilic segments and longer hydrophobic chains result in stronger hydrophobic interactions
of the alkyl chains and consequently a lower CMC.[25-27]
The maximum surface excess concentration (ꢀ) is calculated from the slope (ꢁꢂ/ꢁꢃꢄꢅꢆ)
of surface tension ( ) as a function of logC (C < CMC) using Gibbs adsorption equation (Figure
ꢂ
S3 in SI),[27, 28] from which the area per molecule (A) occupied at the water/air interface is
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calculated using the formula 1/(ꢀ × ꢇꢈ), where NA = Avogadro’s number. The results are
included in Table 1 for comparison. As shown in Table 1, the
A for 1 and 3 with a C18 group is
41.3 and 52.9 Å2, respectively. These values are close to the dimensions of the Fp junction,
which is ca. 49 Å2 as estimated from a crystal structure of Fp derivative.[14] It suggests that 1
and 3 are compactly packed at the water/air interface due to a stronger hydrophobic interaction
between C18 groups.[27] The molecule weight of PEG in 1 is 550 g/mol, which is shorter than
that in 3 (2000 g/mol), so the relatively larger
A
for 3 is caused by the steric repulsion of flexible
PEG2000 in water. This effect of PEG length on
Å2) and 4 with PEG2000 (A = 132 Å2) are compared. The
A is also observed, when 2 with PEG550 (A = 91.8
A
values for 2 and 4 with a C6 group are
obviously larger than those for 1 and 3 with a C18 group, because the hydrophobic interaction
between C6 groups is smaller. The solution behaviour indicates that the shorter alkyl group has
weaker hydrophobic interactions and thus larger CMC (Table 1).
Table 1. Critical micelle concentration (CMC) and area per molecule (
water/air interface of in water.*
A) occupied at the
1-4
Molecules
CMC
M)
ꢀ
x 106 Area/molecule
(Å2)
41.3
91.8
52.9
132
(Mole/m2)
4.023
1.809
3.137
1.25
(
µ
(1) PEG550-Fp-C18
(2) PEG550-Fp-C6
(3) PEG2000-Fp-C18
(4) PEG2000-Fp-C6
29.9 ± 2.3
40.0 ± 6.2
82.1 ± 4.5
113.3 ± 23
* The date are calculated from surface tension measurements
2.3. Aqueous behaviour of 1-4
To investigate the assembling behavior of 1-4 in water, the aqueous solutions with 2.0
mg/mL of 1-4 were prepared and characterized using dynamic light scattering (DLS). The
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hydrodynamic diameters (Dh) for the micelles assembled from 1 and 2 increase within a few
days after preparation. As shown in Figure 5a and b, the Dh for the solutions increases on the
second day, but there are a significant amount of micelles with smaller Dh. On the 7th day, the
smaller Dh disappears and only larger agglomerates are measurable. The colloids assembled from
3 and 4 with longer PEG2000 are relatively stable within 7 days, but also grow into larger one in a
month (Figure 5 c, d). All micelles assembled from 1-4 eventually agglomerated into cotton-like
suspensions that could be re-dispersed upon shaking.
Figure 5. DLS curves for the aqueous solutions of 1- 4 over time.
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Figure 6 compares the time when the cotton-like suspension appears for the aqueous
solution of 1 – 4. As shown in Figure 6, it takes much longer time for the solution of 3 and 4 due
to the shielding effect of the PEG2000 chains. The agglomeration of 1 and 3 with a C18 group is
relatively slower as compared with 2 and 4 with a C6 group, suggesting that the stronger
hydrophobic interaction of C18 groups slows the agglomeration rate. This agglomeration
behaviour is also known for their organic analogue without Fp-junction, e.g. Brij® S10 (C18H37-
b-PEG10), which has been used for a variety of biomedical applications such as drug
delivery.[29-31] Hydrophobic interaction is commonly used to explain this agglomerations
behaviour.[32, 33] Further investigation to understand this behaviour will be performed using 1-
4 by taking advantage of the Fp junction.
Figure 6. The time required for 1 – 4 in water (2 mg/mL) to agglomerate into suspension.
3. Conclusion
The first example of organometallic amphiphile with an Fp junction, PEGPPh2-FpR, was
prepared via migration insertion reaction of Fp alkyl derivatives in the presence of PEGPPh2.
The prepared macromolecules are stable in air and have a reversible redox activity in DMF.
Surface tension measurements indicate that the prepared molecules exhibit typical amphiphilic
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behaviour with critical micelle concentrations (CMC) in water ranging from 0.03 to 0.113 mM
depending on the chemical structure. The prepared aqueous micelles agglomerate over time into
re-dispersible cotton-like aggregates. Their organic analogues without Fp junction (Brij®
amphiphilic molecules) have similar agglomeration behavior, suggesting that the Fp junction has
little effect on the solution behaviour of PEGPPh2-FpR. Further research to investigate the
agglomeration behaviour of the molecules by taking advantage of the redox activity will be
performed.
4. Experimental Section
4.1. Materials and instrumentations
1-Chlorooctadecan (> 98.0%), and thionyl chloride (
America and Fulka Analytical, respectively. 1-bromohexane (> 98.0%), Cyclopentadienyliron
(II) dicarbonyl dimer (99%), triethylamine ( 99%), Chlorodiphenylphosphine (98%),
≥ 99.0%) were purchased from TCI
≥
benzophenone (99%) and poly(ethylene glycol) methyl ether (PEG, Mn = 550 and 2000), were
purchased from Sigma-Aldrich and used as received. Potassium metal (99.5%, rods in mineral
oil) and sodium metal (99.9%, cubs in mineral oil) were purchased from Sigma-Aldrich, cut into
small pieces and washed in hexane before using. Tetrahydrofuran (THF) was distilled over
sodium/benzophenone before use. All other solvents were obtained from local commercial
providers and used as received.
1H and 31P nuclear magnetic resonance (NMR) spectra were recorded on Bruker-300 (300
MHz) spectrometer at room temperature using appropriate solvents as reported in each case.
Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27
spectrophotometer with a resolution of 1 cm-1 at room temperature. Samples were prepared using
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KBr pellets by grinding and compressing of the amphiphile in anhydrous KBr using Nujol mulls.
Electrospray ionisation mass spectrometry (ESI-MS) were performed using a Thermo Scientific
Q Exactive mass spectrometer. Molecules 1 – 4 were dissolved in acetonitrile with 0.4 w%
formic acid and the solutions were infused at 10 µL/min with spray voltage of 3.2 kV and
capillary temperature of 280 oC. Dynamic light scattering (DLS) experiments were performed at
25 °C using Zetasizer Nano Series (Nano-S90, Malvern Instruments) with laser wavelength of
633 nm at fixed angle of 90o. Surface tension of pure water and the aqueous solutions of the
o
prepared metallo-amphiphiles with different concentrations were measured at 22 C using
tensiometer Data Physics DCAT 21 system. Cyclic voltammetry (CV) measurements were
obtained using DY2000 Multi-Channel Potentiostat (Digi-Ivy Inc.) workstation with a scan rate
of 50 mV/sec and silver wire as a reference electrode. All samples were measured in DMF using
tetrabutylammonium perchlorate (TBAP, 2mg/mL).
4.2. Synthesis of 1-4.
Synthetic procedures of 1 (PEG550-Fp-C18) that contains PEG (Mn = 550 g/mol) and
octadecyl chain, as representative amphiphile, is discussed in details in this section. All synthesis
steps were conducted under dry N2 atmosphere using Schlenk line techniques.
4.2.1. Synthesis of (5)
Synthesis of PEG550-Cl.[24, 25] A toluene solution (17 mL) of poly(ethylene glycol)
methyl ether, Mn = 550 (2.0 g, 3.6 mmol) and triethylamine (C2H5)3N (0.40 g, 4.0 mmol) was
added dropwise into a 100-mL Schlenk flask containing thionyl chloride SOCl2 (2.60 g, 22
o
mmol) at 0 C (ice bath). After the addition, the ice bath was replaced with oil bath and the
solution is gradually heated up to 65 °C. After stirring at 65 °C overnight, the solution was
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passed through a celite column to remove the resulted triethylamine hydrochloride salt and the
filtrate was subject to rotary evaporation to remove toluene. The crude product was then
dissolved in a minimal THF and precipitated out in hexane as cloudy oil. The final product was
1
collected and dried under vacuum, yield PEG550-Cl (1.74 g, 71%). H NMR (300 MHz, CDCl3,
δ
): 3.75 (t, 2H, -CH2Cl), 3.55-5.65 (s, 44H, CH2CH2O), 3.50 (t, 2H, - O - CH2- CH2Cl), 3.37 (s,
3H, CH3O). 1H NMR results for PEG2000-Cl as follow. 1H NMR (300 MHz, CDCl3,
δ): 3.74 (tbr,
2H, -CH2Cl), 3.58-5.68 (sbr, 176H, CH2CH2O), 3.54 (brt, 2H, - O - CH2- CH2Cl), 3.37 (s, 3H,
CH3O).
Synthesis of NaPPh2.[18, 21] Sodium diphenylphosphide (NaPPh2) was prepared as
previously reported by our group. 31P NMR (300 MHz, THF,
δ): -20 (s,1P, Ar2PNa)
Synthesis of PEG550-PPh2 (5a). PEG550-Cl (1.01 g, 1.78 mmol) was added to a 100-mL
Schlenk flask and dissolved in dry THF (5 mL). The flask was placed in an ice bath. To the flask,
5.5 mL sodium diphenylphosphide solution (0.55 g, 2.65 mmol; excess) was added dropwise.
The resulting mixture was left under stirring for 2 hours. Then, degassed methanol (2 mL) was
added dropwise to react with the excess sodium diphenylphosphide. As a result, the color of the
solution was changed from deep red to pale yellow. After the solution was passed through a
celite column, the mixed solvents of THF/methanol were removed under vacuum, leaving pale
yellow oil as crude product. The crude product was dissolved in a minimal THF and
subsequently precipitated out in degassed hexane. After this step of purification, the final product
was collected and dried under vacuum, yield 0.89 g (69.6 %). 31P NMR (300 MHz, CDCl3,
21 ppm; 31P NMR (300 MHz, THF, ): -19 ppm; 1H NMR (300 MHz, CDCl3,
δ): 7.15, 7.55 (sbr,
δ): -
δ
10H, ArH), 3.48 (sbr, 48H, CH2CH2O), 3.20 (s, 3H, CH3O).
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4.2.2. Synthesis of (6)
Fp-Cn (6) was synthesized by coupling of cyclopentadienyldicarbonyliron potassium
(FpK),[34] with 1-chlorooctadecane for 6a or 1-chlorohexane for 6b. For 6a, under stirring at 0
°C, a solution of C18H37Cl (2.5 cmL, 7.3 mmol) in THF (5 mL) was added to an orange
suspension of FpK (2 g, 9.3 mol) in THF (20 mL). The orange suspension gradually turned to
brown and was kept stirred at room temperature for 2 hours. THF was then removed by rotary
evaporation and the residue was extracted using degassed hexane. The supernatant was collected
and rotary evaporated. The resulting solid was chromatographed using a plug of silica column
and degassed hexane/DCM (4:1 v/v) are used as eluent. The first yellow band was collected and
solvents were removed using rotary evaporation. The bright yellow crystals were collected and
dried under vacuum. Yield 2.8 g (89.2 %). 1H NMR (300 MHz, CDCl3,
δ): 4.71 (s, 5H, Cp ring),
1.44 (br, 2H, Fe-CH2-alkyl chain), 1.25 (br, 32H, CH2-alkyl chain), 0.87 (t, 3H, terminal CH3);
=1990 and 1934 cm-1 (two CO groups stretching).
FT-IR (KBr):
ν
4.2.3. Synthesis of PEG550PPh2-FpC18 (1)
A solution of 5a (1.0 g, 1.39 mmol) and 6a (0.72 g, 1.7 mmol) in THF (60 mL) was
o
prepared in a 100-mLSchlenk flask. The solution was then heated up and refluxed at 70 C for
three days. THF was subsequently removed via rotary evaporation, resulting in a dark-brown
crude product. The crude product was then suspended in a minimal amount of degassed hexane
and transferred to a silica column. The column was first flushed using degassed hexane to
remove the excess Fp-C18 (6a), and subsequently washed using dry THF. A dark brown band
was collected. After removing solvent, viscous dark brown oil was collected and dried under
31
1
vacuum, 1.27 g (79.5 %). P NMR (300 MHz, CDCl3,
δ): 69.5 (Ph2-P-C); H NMR (300 MHz,
CDCl3, ): 4.36 (s, 5H, Cp ring), 2.61 and 2.85 (m, 2H, acyl-CH2-alkyl chain), 1.25 (br, 32H,
δ
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CH2-alkyl chain), 0.87 (t, 3H, terminal CH3), 3.6 (br, 42H, CH2CH2O), 3.37 (s, 3H, CH3O), 2.78
(t, 2H, CH2-P, overlapped), 7.2-7.4 (br, 10H, aromatic protons); FT-IR (KBr):
ν
=1912 cm-1
(terminal CO stretching), 1607 cm-1 (migrated acyl carbonyl), Figure 1-3. Molecule 2-4 are
prepared and characterized in the same fashion as for 1; detailed synthesis for 2-4 is described in
the Supporting Information (Figure S4-S6).
4.3. Preparation of the micelles
Typical, 2.0 mg/mL aqueous solution is prepared by adding 10.0 mL of water into a THF
solution of 1-4 (20 mg in 0.5 mL THF) under stirring. Subsequently, THF was removed via
either bubbling with N2 for an hour or dialysis over night against water.
5. Acknowledgements
Nature Science and Engineering Research Council of Canada (RGPIN-2016-04497), the
University of Waterloo are acknowledged for financial support.
Supporting Information Available:
Supplementary data related to this article, including additional synthesis procedure, NMR
spectra, cyclic voltammetry and surface tension results, can be found at http://
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Synthesis and Solution Behaviour of Metal-Carbonyl Amphiphiles
with an Fp (CpFe(CO)2) Junction
Nimer Murshid, Abdelrahman El-Temtamy and Xiaosong Wang*
Highlights:
•
•
Metallo-amphiphilic macromolecules with an iron-carbonyl junction are synthesised
The prepared amphiphiles are redox active and able to self-assemble in water