2
B. Andrews et al. / Polyhedron xxx (2016) xxx–xxx
terminated phosphonate monolayers can be formed on copper
oxide surfaces, such as with perfluoro [29] and pyrrolyl [30]
grade) were obtained from Fisher Scientific. Hydrochloric acid
(ACS grade) was obtained from Fisher Scientific and diluted to
6 M prior to use. Ethanol (200 proof, ACS grade) was obtained from
Pharmco-Aaper. Chloroform-D with 0.3 v/v% tetramethylsilane
(TMS) and methanol-D, both with 99.8%-atom D incorporation,
were obtained from Acros Organics. Dichloromethane (HPLC
grade), tetrahydrofuran (HPLC grade without preservatives), and
acetonitrile (HPLC grade) were obtained from Fisher Scientific
and dried and purified using an MBraun SPS Compact solvent
purification system prior to use. All reagents were used as received
unless otherwise noted.
x
-functional groups for applications in non-wetting surfaces and
in formation of electroactive polymer films, respectively. Analysis
of XPS spectra of copper surfaces before and after deposition of
octadecylphosphonic acid indicates that the formation of the phos-
phonate monolayer is caused by a condensation reaction between
the phosphonic acid head groups and the surface hydroxyl species
(CuAOH) present in the native oxide layer [31]. Employing the
Tethering by Aggregation and Growth (TBAG) deposition method
developed by Schwartz et al. [55–62] may be a way to form com-
prehensive monolayers on the copper oxide surface without the
need to further oxidize the surfaces via hydrogen peroxide, as
employed by Refs. [29,30].
2.2. Characterization techniques
Infrared spectroscopy was conducted using a PerkinElmer
Spectrum RX1 Fourier Transform Infrared Spectrometer. Spectra
of solid compounds were taken as the bulk solid using a Pike MIRa-
cle Attenuated Total Reflectance accessory. IR of modified surfaces
was conducted using a Fixed-Angle Specular Reflectance accessory
versus a blank copper coupon background. All surface IR spectra
were averaged over 256 scans at 4 cmꢁ1 resolution.
NMR spectra were recorded on a 300 MHz/52 MM Bruker spec-
trometer or a 60 MHz Varian spectrometer in deuterated solvents
using TMS as an internal standard.
X-ray photoelectron spectroscopy was conducted using a Kratos
Axis Ultra X-ray photoelectron spectrometer. Survey scan analyses
were conducted with an analysis area of 300 ꢀ 700 microns and a
pass energy of 160 eV, while high-resolution analyses were of the
same sample size were conducted with a pass energy of 20 eV.
In this work, we explore the possibility of using alkylphospho-
nate monolayers on the native oxide layer of bulk copper surfaces
as a platform for tethering a transition metal-based complex. Tran-
sition metal complexes have been immobilized on solid supports
such as polymers and inorganic solids [76–78]. These systems have
a diverse range of applications from sensing and molecular recog-
nition to luminescence, photocatalysis, and electrocatalysis [76].
Functionalizing a surface with a transition metal catalyst should
have the advantage of decreasing the time and cost needed for cat-
alyst recovery and increasing the efficiency of the catalyst through
the direct application of an electrochemical potential, particularly
if the surface can function as an electrode and an electrocatalyst
is bound [33,79,80].
Ruthenium polypyridyl complexes have attracted much atten-
tion since the late 1970s due to their potentialuse in photochemistry
and luminescence, dye-sensitized solar cells, sensors, electrochemi-
cal activation, and catalysis. For comprehensive reviews of the
literature regarding ruthenium polypyridyl complexes see [81,82].
In particular, [Ru(tpy)(bpy)(OH2)]2+ (tpy = 2,20;60,200-terpyridine;
bpy = 2,20-bipyridine) complexes and related compounds have been
targeted as efficient water oxidation catalysts due to the photo-
chemical properties of the bipyridine ligand with recent research
showing that only one metal center is necessary for oxygen
production [83–96]. In an alternate application, [Ru(bpy)(CO)2Cl2]
complexes and related compounds have been reported in the elec-
troreduction of carbon dioxide [97–105]. In this work, an analog of
the reported electrocatalytically active complex [Ru(bpy)(CO)2Cl2]
has been bound to the oxide layer of bulk copper through three syn-
thetic pathways given in Schemes 1 and 2 that result in phosphonate
monolayers of varying alkyl chain lengths and monodentate or
bidentate ligation of the electrocatalyst. We report herein on the
synthesis and characterization of these derivatized surfaces.
2.3. Synthesis of 11-hydroxyundecylphosphonic acid (1)
11-Hydroxyundecylphosphonic acid was synthesized via
a
Michaelis–Arbuzov reaction on the bromo functional group of
11-bromo-1-undecanol following protection of the hydroxyl group
with acetyl chloride. The procedure outlined below is modified from
those previously reported in the literature [106,107]. NMR peak
assignments of products follow those in the literature [106,107].
Acetyl chloride (1.4 mL, 0.020 mol) was added dropwise to a
solution of 11-bromo-1-undecanol (5.06 g, 0.0201 mol) and tri-
ethylamine (2.8 mL, 0.020 mol) in dry dichloromethane (10 ml)
and stirred at 0 °C under nitrogen for 3 h. The solution was washed
with saturated sodium bicarbonate solution (30 mL) and deionized
water (2 ꢀ 25 mL). The organic layer was isolated, dried over
sodium sulfate, and the solvent was removed by rotary evapora-
tion. Yield: 3.79 g, 66%. 1H NMR (CDCl3/TMS, 300 MHz):
d/ppm = 4.05 (t, 2H, CH2O), 3.41 (t, 2H, CH2Br), 2.05 (s, 3H, CH3),
1.85 (m, 2H, ⁄CH2CH2O), 1.62 (m, 2H, ⁄CH2CH2Br), 1.42–1.28 (m,
14H, CH2).
2. Experimental
2.1. Materials
11-Bromoundecylacetate (3.79 g, 0.0129 mol) was refluxed in a
three molar excess of triethyl phosphite (7.5 mL, 0.044 mol) at
150 °C under nitrogen for 5 h. Excess triethyl phosphite was
removed via short path vacuum distillation to give a golden yellow,
oily product. Yield: 4.22 g, 93%. 1H NMR (CDCl3/TMS, 300 MHz):
d/ppm = 4.05 (m, 6H, CH2O), 2.05 (s, 3H, CH3), 1.5–1.75 (m, 4H,
CH2, CH2P), 1.1–1.5 (m, 22H, CH2, CH3).
Acetyl chloride (99+%), 11-bromo-1-undecanol (97%), isonico-
tinic acid (99%), 4-dimethylaminopyridine (99%), N,N0-dicyclo-
hexylcarbodiimide (99%), triethylamine (99%), trimethylsilyl
bromide (99%), carbon tetrachloride (reagent grade), selenium
(IV) oxide (99.8%), and N-bromosuccinimide (99%) were obtained
from Acros Organics. Sodium bicarbonate (certified ACS) and anhy-
drous sodium sulfate (certified ACS) were obtained from Fisher Sci-
entific. Triethyl phosphite (97%) was obtained from MP
Biomedicals, Inc. Tricarbonyldichlororuthenium(II) dimer and azo-
bis(isobutyronitrile) (98%) were obtained from Sigma–Aldrich.
4,40-dimethyl-2,20-bipyridine (97%) was obtained from Astatech.
The copper surface was obtained as a 300 mm ꢀ 300 mm ꢀ 1 mm
sheet from Fisher Scientific. Methanol (certified ACS), 1,2-dichlor-
oethane (certified ACS), 1,4-dioxane (certified ACS), acetone (certi-
fied ACS), concentrated nitric acid (ACS grade), and hexane (HPLC
A
solution
of
diethyl(11-acetoxyundecyl)phosphonate
(4.22 g, 0.0120 mol) and trimethylsilyl bromide (TMSBr, 6.6 mL,
0.050 mol) in dry acetonitrile was stirred under nitrogen at room
temperature for 24 h. Methanol was then added in excess, and
the solution was stirred under nitrogen at room temperature for
24 h [108,109]. The resulting solution was treated with 6 M HCl
to yield a white, crystalline solid (1), recovered by vacuum
filtration. Yield: 2.62 g, 87%. 1H NMR (CD3OD/TMS, 300 MHz):
d/ppm = 4.93 (s, 3H, OH), 3.50 (m, 2H, ⁄CH2OH), 1.2–1.8 (m, 20H,
CH2, CH2P).