Synthesis and Characterization of Structurally Diverse Alkaline-Earth Salen Compounds for Subterranean Fluid Flow Tracking

Article abstract of DOI:10.1021/acs.inorgchem.7b01350

A family of magnesium and calcium salen-derivatives was synthesized and characterized for use as subterranean fluid flow monitors. For the Mg complexes, di-n-butyl magnesium ([Mg(Buⁿ)₂]) was reacted with N,N′-ethylene bis(salicyliden

Full text of DOI:10.1021/acs.inorgchem.7b01350

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis and Characterization of Structurally Diverse Alkaline-Earth
Salen Compounds for Subterranean Fluid Flow Tracking
Timothy J. Boyle,*,† Jeremiah M. Sears,† Jeffery A. Greathouse,‡ Diana Perales,† Roger Cramer,§
Orion Staples,# Arnold L. Rheingold,∥ Eric N. Coker,† Todd M. Roper,⊥ and Richard A. Kemp*,†,#
†Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Boulevard, SE, Albuquerque, New Mexico 87106,
United States
‡Sandia National Laboratories, Geochemistry Department, P.O. Box 5800, MS 0754, Albuquerque, New Mexico 87185-0754, United
States
§University of Hawaii-Manoa, Department of Chemistry, 2545 McCarthy Mall, Honolulu, Hawaii 96822-2275, United States
∥University of California-San Diego, Department of Chemistry and Biochemistry, 9500 Gilman Drive, La Jolla, California 92093,
United States
⊥Carbo Ceramics, Inc., Energy Center II, 575 N. Dairy Ashford Road, Suite 300, Houston, Texas 77079, United States
#Department of Chemistry and Chemical Biology, University of New Mexico, 300 Terrace Street NE, Albuquerque, New Mexico
87131, United States
S
* Supporting Information
ABSTRACT: A family of magnesium and calcium salen-
derivatives was synthesized and characterized for use as
subterranean fluid flow monitors. For the Mg complexes, di-n-
butyl magnesium ([Mg(Bun)2]) was reacted with N,N′-ethylene
bis(salicylideneimine) (H2-salen), N,N′-bis(salicylidene)-1,2-
phenylenediamine (H2-saloPh), N,N′-bis(3,5-di-t-butylsalicyli-
dene)-ethylenediamine (H2-salo-But), or N,N′-bis(3,5-di-t-butyl-
salicylidene)-1,2-phenylenediamine (H2-saloPh-But), and the
products were identified by single-crystal X-ray diffraction as
[(κ3-(O,N,N′),μ-(O′)saloPh)(μ-(O),(κ2-(N,N′),μ-(O′)saloPh)2(μ-(O),κ3-(N,N′,O′)saloPh′)Mg4]·2tol (1·2tol; saloPh′ = an
alkyl-modified saloPh derivative generated in situ), [(κ4-(O,N,N′,O′)saloPh)Mg(py)2]·py (2·py), [(κ4-(O,N,N′,O′)salo-
But)Mg(py)2] (3), [(κ4-(O,N,N′,O′)saloPh-But)Mg(py)2]·tol (4·tol), and [(κ3-(O,N,N′),μ-(O′)saloPh-But)Mg]2 (5), where tol
= toluene; py = pyridine. For the Ca species, a calcium amide was independently reacted with H2-salo-But and H2-saloPh-But to
generate the crystallographcially characterized compounds: [(κ4-(O,N,N′,O′)salo-But)Ca(py)3] (6), [(κ4-(O,N,N′,O′)saloPh-
But)Ca(py)3]·py (7·py). The bulk powders of these compounds were further characterized by a number of analytical tools, where
2−7 were found to be distinguishable by Fourier transform infrared and resonance Raman spectroscopies. Structural properties
obtained from quantum calculations of gas-phase analogues are in good agreement with the single-crystal results. The potential
utility of these compounds as taggants for monitoring subterranean fluid flows was demonstrated through a series of experiments
to evaluate their stability to high temperature and pressure, interaction with mineral surfaces, and elution behavior from a loaded
proppant pack.
laboratories that overcomes these issues utilizes hydrocarbon-
soluble compounds that have been intercalated into porous
ceramic proppants.8 These types of proppants are routinely
used in hydraulic fracturing of oil and gas wells, where they are
pumped into the fractured reservoir rock, creating high
conductivity pathways connecting reservoir fluids to the well
bore. As the reservoir fluids flow through the proppants, the
molecular taggants will be washed into the “produced” fluids
and transported to the surface. For these taggants to be
industrially valuable, they must be easily identified and
distinguished using standard/mobile analytical tools. This
INTRODUCTION
■
Accurately monitoring the fluid flow of deep underground
reservoirs (water or oil) is imperative to maximize the energy
recovery from these wells. Methods developed to track the
fluid’s movements include radioactive tracers,1−3 DNA frag-
ments,4,5 and nanoparticles (i.e., Fe,4,6 Ln/Y2O3 ). Unfortu-
7
nately, these methods have significant limitations: radioactive
materials are not always considered environmentally friendly
and do not yield the desired long-term fluid flow information;
DNA fragments tend to degrade at the high temperatures and
pressures encountered in these underground systems;4,5 and
nanoparticles have been found to bind to subsurface strata6,7
and/or demonstrate poor stability.4,6,7 An alternative approach
for tracking organic fluids (i.e., oil) being developed in our
Received: May 25, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01350
Inorg. Chem. XXXX, XXX, XXX−XXX

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concept has been previously demonstrated using phosphonate
salts infused into porous ceramic particles in produced reservoir
water from hydraulically fractured oil and gas wells.9,10
But)Mg]2 (5) where tol = toluene and py = pyridine. For the
Ca species, calcium bis(trimethylsilyl)amide ([Ca(NR2)2]; eq
2) was independently reacted with H2-salo-But and H2-saloPh-
But to generate the crystallographically characterized complexes
[(κ4-(O,N,N′,O′)salo-But)Ca(py)3] (6) and [(κ4-(O,N,N′,O′)-
saloPh-But)Ca(py)3]·py (7·py). These compounds were fully
characterized using a variety of analytical techniques. Addition-
ally, molecular modeling of the compounds was undertaken to
analyze the impact that the different ligand modifications had
on the degree of charge transfer between the metal ion and the
organic components (saloR′-R moieties and py ligands).
Electronic structure calculations were performed on individual
compounds. The models were validated by comparing
geometric properties with those obtained by single-crystal
diffraction experiments.
Metal derivatives of salens, porphyrins, phthalocyanines, and
other related multidentate ligands that meet the demanding
deep underground system criteria (i.e., solubility, stability,
dispersible) have been preliminarily investigated. Of the
derivatives noted above, the metal-salen complexes came to
the forefront because of the high thermal stabilities reported for
this family of compounds.11−15 Additionally, the facile
syntheses of salen derivatives is attractive, since the ring
substituents can be easily tailored to increase the solubility in
the various underground fluids of interest (water vs hydro-
carbon). Ligand modifications, especially to the electron-rich
rings, and metal selection should result in distinguishable
features via vibrational spectroscopy. As large quantities of
soluble tags will be necessary to map the fluid flow behavior in
real underground systems, the metals employed should be
plentiful, nontoxic, and inexpensive.
[Mg(Bun)2] + H2‐saloR′‐R″ → [(solv)xMg(saloR′‐R)]
(1)
[Ca(NR2)2] + H2‐saloR′‐But → [(solv)xCa(saloR′‐But)]
We therefore selected the alkaline-earth metals for our initial
efforts, and this report details the syntheses and character-
ization of a family of magnesium (Mg) and calcium (Ca) salen
compounds. The salen ligands (H2-saloR′-R; Figure 1a−d)
(2)
R = Si(CH3)3; R′ = C2H4, C6H4; R″ = H, 2, 4‐But
With this unique set of precursors available, preliminary
investigations into the retention of the structure under
simulated deep well conditions were investigated, including
thermal/pressure, interaction with surfaces (SiO2 columns),
and elution data from the proppant. The potential tracker
properties for select compounds were determined and are
briefly discussed.
EXPERIMENTAL SECTION
■
All compounds described below were handled with rigorous exclusion
of air and water using standard Schlenk line and glovebox techniques
unless otherwise discussed. All chemicals were used as received (from
Aldrich and Alfa Aesar) without further purification, including:
methanol (MeOH; anhydrous, 99.8%), diethyl ether (Et2O,
anhydrous, 99.7%), pyridine (py; anhydrous, 99.8%), tetrahydrofuran
(THF; anhydrous, ≥99.9%), toluene (tol; anhydrous, 99.8%), N,N′-
bis(salicylidene)ethylenediamine (H2-salen; 98%), N,N′-bis-
(salicylidene)-1,2-phenylenediamine (H2-saloPh; 97%), calcium iodide
(CaI2), 1.0 M di-n-butyl magnesium solution ([Mg(Bun)2] in
heptane), potassium bis(trimethylsilyl)amide (KNR2), 3,5-di-tert-
butyl-2-hydroxybenzaldehyde (99%), ethylene diamine (99%), pyr-
idine-d5 (py-d5; ≥99.5 atom % D), and chloroform-d (CDCl3; 99.8
atom % D). Ortho-phenylenediamine (98%) was obtained from Acros
Organics. Calcium bis(trimethylsilyl)amide ([Ca(NR2)2]) was synthe-
sized from the reaction of CaI2 and 2 equiv of KNR2 in THF.16 Dried
crystalline materials were used for all analytical analyses. The porous
proppant used was provided by Carbo Ceramics, Inc., and is available
from them under the product name 20/40 CarboUltraLite.
Figure 1. Schematic structures of H2-saloR′-R derivatives: (a) H2-
salen, (b) H2-saloPh, (c) H2-salo-But, (d) H2-saloPh-But, and (e)
dihedral angles analyzed: (i) N−C C−N, (ii) N···N C−C, and C···M···
C.
Fourier transform infrared (FTIR) data were collected on a Nicolet
6700 FTIR spectrometer using a KBr pellet press under a flowing
atmosphere of nitrogen or with iD7 ATR accessory mounted with a
monolithic diamond crystal. Spectra of select species are shown in
Supporting Information (Figures S1 and S2). Elemental analyses were
collected on a PerkinElmer 2400 Series II CHNS/O Analyzer with
samples prepared in an argon-filled glovebox. Melting point data were
collected using a Stanford Research Systems Optimelt MPA100.
Raman spectra were obtained using a Thermo Scientific DXR Smart
Raman instrument. All NMR samples were prepared under an argon
atmosphere and flame-sealed using crystalline material at as high a
concentration as possible. Spectra were collected on a Bruker Avance
studied include: (a) N,N′-ethylene bis(salicylideneimine) (H2-
salen), (b) N,N′-bis(salicylidene)-1,2-phenylenediamine (H2-
saloPh), (c) N,N′-bis(3,5-di-t-butylsalicylidene)-ethylenedi-
amine (H2-salo-But), and (d) N,N′-bis(3,5-di-t-butylsalicyli-
dene)-1,2-phenylenediamine (H2-saloPh-But). The products
isolated from the reaction of di-n-butyl magnesium (Mg(Bun)2;
eq 1) with the H2-saloR′-R derivatives were crystallographically
identified as [(κ3-(O,N,N′),μ-(O′)saloPh)(μ-(O),(κ2-
(N,N′),μ-(O′)saloPh)2(μ-(O),κ3-(N,N′,O′)saloPh′)Mg4]·2tol
(1·2tol; saloPh′ = an alkyl-modified saloPh derivative generated
in situ), [(κ4-(O,N,N′,O′)saloPh)Mg(py)2]·py (2·py), [(κ4-
(O,N,N′,O′)salo-But)Mg(py)2] (3), [(κ4-(O,N,N′,O′)saloPh-
But)Mg(py)2]·tol (4·tol), and [(κ3-(O,N,N′),μ-(O′)saloPh-
1
500 NMR spectrometer under standard experimental conditions: H
analysis was performed with a 4 s recycle delay at 16 scans; 13C
analysis was performed with a 10 s recycle delay with a minimum of 64
scans. Alternatively, a Bruker Fourier 300 HD spectrometer or a
B
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Magritek Spinsolve Phosphorus benchtop spectrometer was also used.
All spectra were referenced to the appropriate residual protonated
species in the solvent. The Thermogravimetric analysis (TGA)/
differential scanning calorimetry (DSC) analyses were obtained on a
Mettler Toledo model TGA/DSC 1. UV−vis absorbance spectral data
were collected on a Photonics CCD Array UV−vis Spectropho-
tometer. All experiments were conducted under an argon atmosphere
at a heating rate of 10 °C min−1 from room temperature to 750 °C in
an alumina crucible.
986(w), 909(w), 878(w), 852(w,sh), 833(w), 806(w), 793(w),
757(w), 744(w), 702(m), 621(w), 476(w), 432(w). 1H NMR
(500.1 MHz, py-d5) δ 8.70 (1H, d, JH−H = 2.1 Hz, NC5H5), 8.31
(1H, s, -C(H)N-), 7.67 (1H, t, JH−H = 2.6 Hz, OC6H2But2), 7.52
(0.5H, t, JH−H = 1.7 Hz, NC5H5), 7.29 (1H, d, JH−H = 2.6 Hz,
OC6H2But2), 7.17 (1H, t, JH−H = 1.7 Hz, NC5H5), 3.39 (2H, s, −N−
CH2−), 1.59 (9H, s, OC6H2(C(CH3)3)2), 1.23 (9H, s, OC6H2(C-
(CH3)3)2). Anal. Calcd for C42H56MgN4O2 (MW = 673.24): C, 74.93;
H, 8.38; N, 8.32. C37H51MgN3O2 (MW = 594.14; 3 − 1 py): C, 74.80;
H, 8.65; N, 7.07. Found: C, 75.43; H, 8.35; N, 6.45%.
Ligand Synthesis. H2-salo-But. 3,5-Di-tert-butyl salicylaldehyde
(4.68 g, 20.0 mmol) and ethylene diamine (0.600 g, 9.98 mmol) were
added to a 500 mL round-bottom flask containing MeOH (∼225 mL).
After it was stirred at 65 °C for 17 h, the reaction was allowed to cool
to room temperature. The resulting precipitate was isolated by
filtration, rinsed with Et2O (3 × 20 mL), and dried in an oven at 100
°C for 24 h affording an off white/light yellow powder. Yield 96.1%
(4.73 g). FTIR (cm−1; iD7 ATR): 3254 (w(br), −OH), 3051 (w),
2956 (s), 2867 (m), 1626 (s, CN), 1594 (m), 1464 (s), 1438 (s),
1392 (m), 1360 (s), 1252 (m), 1172 (s), 1040 (s), 879 (s), 829 (s),
[(κ4-(O,N,N′,O′)saloPh-But)Mg(py)2]·tol (4·tol). [Mg(Bun)2] (1.85
mL, 1.85 mmol), H2-saloPh-But (1.00 g, 1.85 mmol) in toluene (∼15
mL) with py (∼1.5 mL). Yield 93.7% (1.41 g). Reaction turns yellow
in color; powder dries to a light brown. FTIR (cm−1): 3071 (w); 2995
(w); 2951 (s); 2904 (m); 2864 (m); 1604 (s); 1578 (s); 1546 (m);
1464 (s); 1438 (s); 1387 (s); 1360 (s); 1248 (m); 1191 (m); 1159
(s); 1134 (s); 1109 (m); 1042 (m); 978 (m); 836 (m); 795 (s); 746
(s); 699 (s). 1H NMR (500.1 MHz, py-d5) δ 9.25 (1H, s(br), NC5H5),
8.70 (2H, s(br), -C(H)N-, NC5H5), 7.90−7.88 (1H, mult.,
OC6H2But2), 7.68 (1H, t, JH−H = 7.3 Hz, OC6H2But2), 7.45 (1H, t,
JH−H = 7.6 Hz, NC5H5), 7.33−7.22(1H, mult., OC6 H2But2, N−C6H5),
7.18 (2H, t, JH−H = 7.4 Hz, OC6H2But2), 1.78 (9H, s, OC6H2(C-
(CH3)3)2), 1.31 (9H, s, OC6H2(C(CH3)3)2). Anal. Calcd for
C53H64MgN4O2 (MW = 813.43): C, 78.26; H, 7.93; N, 6.89.
C99H120Mg2N8O4 (MW = 1534.71; 4 + 1/2 tol): C, 77.48; H, 7.88;
N, 7.30. Found: C, 77.03; H, 8.62; N, 6.55%.
1
773 (s), 709 (s), 644 (s). H NMR (500.1 MHz, CDCl3): 13.64 (s,
2H, −OH), 8.40 (s, 2H, −CHN-), 7.36 (d, 2H, C6H2(C(CH3)3)2),
7.08 (d, 2H, C6H2(C(CH3)3)2), 3.94 (s, 4H, N(CH2)2N); 1.45 (s,
18H, C(CH3)3); 1.30 (s, 18H, C(CH3)3). Anal. Calcd for C32H48N2O2
(MW = 492.75): C, 78.00; H, 9.82; N, 5.69. Found: C, 78.43; H, 9.96;
N, 5.57%. mp 184−187 °C.
H2-saloPh-But. 3,5-Di-tert-butyl salicylaldehyde (2.24 g, 9.56 mmol)
and o-phenylenediamine (0.59 g, 5.46 mmol) were added to a 250 mL
round-bottom flask containing 75 mL of MeOH. After it was stirred at
65 °C for 17 h, the reaction was allowed to cool to room temperature.
The resulting precipitate was isolated by filtration, rinsed with Et2O (3
× 20 mL), and dried in an oven at 100 °C for 24 h. This afforded a
light-yellow powder. Yield: 72.1% (1.95 g). FTIR (cm−1; iD7 ATR):
3368 (w(br), −OH), 3056 (w), 2951 (s), 2905 (m), 2866 (m), 1805
(w), 1615 (s, CN), 1570 (s), 1479 (m), 1438 (s), 1360 (s), 1251
(m), 1169 (s), 1104 (m), 973 (m), 877 (m), 820 (m), 755 (s), 643
[(κ3-(O,N,N′),μ-(O′)saloPh-But)Mg]2 (5). [Mg(Bun)2] (0.46 mL,
0.46 mmol), H2-saloPh-But (0.25 g, 0.46 mmol) in toluene (∼15 mL).
Yield 92.2% (0.24 g). Reaction turns dark brown in color; powder
dries to orange. FTIR (cm−1): 2956(s), 2905(s), 2867(s), 1614(s),
1581(s), 1528(s), 1463(s), 1438(m), 1409(w), 1385(m), 1360(m),
1333(w), 1259(s), 1197(m), 1167(s), 1136(w), 1108(m), 1025(w),
974(w), 927(w), 872(w), 834(w), 795(w), 749(m), 695(w), 637(w),
535(w), 522(w). Anal. Calcd for C72H92Mg2N4O4 (5, MW =
1126.16): C, 76.79; H, 8.23; N, 4.98. Found: C, 76.60; H, 8.31; N,
4.64%.
General Ca-Salen Reaction. In an inert atmosphere glovebox, a
preweighed sample of the desired H2-saloR′-R was added to a stirring
solution of [Ca(NR2)2] in toluene. After it was stirred for 12 h, py was
added, and the reaction was stirred for at least 1 h and then set aside
with the cap removed until X-ray quality crystals grew.
1
(m). H NMR (43 MHz, CDCl3): 13.52 (s, 2H, −OH), 8.66 (s, 2H,
−CHN-), 7.18−7.41 (m, 4H, C6H2(C(CH3)3)2), 1.45 (s, 18H,
−C(CH3)3), 1.34 (s, 18H, −C(CH3)3). Anal. Calcd for C36H48N2O2
(MW = 540.79): C, 79.96; H, 8.95; N, 5.18. Found: C, 80.40; H, 8.68;
N, 4.97%. mp 194−197 °C.
General Mg-Salen Reaction. In an inert atmosphere glovebox, a
preweighed sample of the desired H2-saloR′-R was added to a stirring
solution of [Mg(Bun)2] in toluene. After it was stirred for 12 h, py was
added (except for 5), and the reaction was stirred for at least an
additional hour. This reaction was set aside until X-ray quality crystals
grew.
[(κ4-(O,N,N′,O′)salo-But)Ca(py)3] (6). [Ca(NR2)2] (0.732 g, 2.03
mmol), H2-salo-But (1.00 g, 2.03) in toluene (∼15 mL) and py (∼1.5
mL). The reaction turns pale yellow in color; powder dried to off-
white powder. Yield 89.8% (1.40 g). FTIR (cm−1): 3060 (w); 2956
(s); 2906 (m); 2867 (s); 1624 (s, CN); 1525 (m); 1464 (s); 1438
(s); 1392 (m); 1359 (s); 1270 (s); 1252 (s); 1200 (m); 1154 (m);
1041 (s); 973 (s); 931 (s); 879 (s); 774 (s); 728 (s); 694 (s); 645
[(κ4-(O,N,N′,O′)saloPh)Mg(py)2]·py (2·py). [Mg(Bun)2] (0.95 mL,
0.95 mmol), H2-salo-Ph (0.30 g, 0.95 mmol) in toluene (∼5 mL).
Reaction turns red-orange in color; powder dries to a pale orange
color. After it was stirred for 12 h, py addition yielded a red product.
Yield 84.2% (0.46 g). FTIR (KBr, cm−1) 3053(s), 2014(s), 2955(s),
2924(s), 2869(m), 1611(s), 1583(s,sh), 1546(s), 1526(s), 1483(s,sh),
1469(s), 1388(s), 1302(s), 1247(w), 1220(w), 1180(s), 1147(s),
1125(m), 1107(m), 1077(w), 1041(m), 1001(m), 976(w), 918(m),
856(w), 840(w), 801(w), 749(s), 701(m), 618(w,sh), 605(w),
1
(m). H NMR (500.1 MHz, py-d5) δ 8.70 (0.5H, d, JH−H = 2.7 Hz,
NC5H5), 8.31 (1H, s, -C(H)N-), 7.64 (1H, s(br), OC6H2But2), 7.54
(0.5H, t, JH−H = 7.5 Hz NC5H5), 7.29 (1H, s(br), OC6H2But2), 7.18
(0.5H, t, JH−H = 7.5 Hz, NC5H5), 3.30 (2H, s, −N−CH2−), 1.37 (9H,
s, OC6H2(C(CH3)3)2), 1.33 (9H, s, OC6H2(C(CH3)3)2). Anal. Calcd
for C47H61CaN5O2 (6, MW = 768.12): C, 73.49; H, 8.01; N, 9.12.
Found: C,72.79; H, 8.22; N, 9.17%.
1
635(m). H NMR (500.1 MHz, py-d5) δ 9.09 (1H, s(br), NC5H5),
[(κ4-(O,N,N′,O′)saloPh-But)Ca(py)3]·py (7·py). [Ca(NR2)2] (0.667
g, 1.85 mmol), H2-saloPh-But (1.00 g, 1.85 mmol) in toluene (∼15
mL) and py (∼1.5 mL). The reaction turns dark yellow in color;
powder dried to yellow-orange powder. Yield 93.2% (1.27 g). FTIR
(cm−1): 3060 (w); 2992 (w); 2946 (m); 2904 (m); 2866 (m); 1594
(s, CN); 1573 (s); 1518 (m); 1436 (s); 1381 (m); 1358 (m); 1236
(m); 1195 (s); 1149 (s); 1032 (m); 976 (m); 875 (m); 796 (s); 744
8.70 (2H, s(br), -C(H)N-, NC5H5), 7.80−7.79 (1H, mult., OC6H5),
7.54 (1H, t, JH−H = 7.6 Hz, OC6H5), 7.48 (1H, d, JH−H = 3.9 Hz,
OC6H5), 7.33 (1H, t, JH−H = 7.6 Hz, NC5H5), 7.24−7.31(1H, mult.,
OC6H5, N−C6H5), 7.21 (2H, t, JH−H = 7.4 Hz, OC6H5, N−C6H5),
6.55 (1H, t, JH−H = 7.4 Hz, O−C6H5). Anal. Calcd for C30H24MgN4O2
(2, MW = 496.85): C, 72.52; H, 4.87; N, 11.28. Found: C, 71.59; H,
5.82; N, 8.27%.
1
[(κ4-(O,N,N′,O′)salo-But)Mg(py)2] (3). [Mg(Bun)2] (2.03 mL, 2.03
mmol), H2-salo-But (1.00 g, 2.03 mmol) in toluene (∼15 mL) and py
(∼1.5 mL). Reaction turns yellow in color; powder dries to a light
brown. Yield 90.0% (1.23 g). FTIR (KBr, cm−1): 2956(s), 2868(s),
1629(s), 1545(w,sh), 1528(m), 1467(s), 1441(s), 1414(m), 1391(m),
1360(m), 1336(w), 1300(w), 1275(w), 1275(w,sh), 1257(m),
1233(w), 1217(w), 1200(w), 1160(m), 1089(w), 1070(w), 1037(w),
(s); 699 (s). H NMR (500.1 MHz, py-d5) δ 8.70 (mult., NC5H5),
8.55 (2H, s, CHN), 7.63 (2H, s, OC6H2But2), 7.55(2H, mult,
NC5H5), 7.33 (2H, s, OC6H2But2),7.19 (mult. NC5H5), 7.05 (4H, s,
C6H4), 1.51 (9H, s, OC6H2But2), 1.37 (9H, s, OC6H2But2). Anal.
Calcd for C56H66CaN6O2 (7·py, MW = 895.26): C, 75.13; H, 7.43; N,
9.39. C51H61CaN5O2 (7 - py, MW = 737.06): C, 74.96; H, 7.66; N,
7.60. Found: C, 75.52; H, 8.10; N, 7.34%.
C
DOI: 10.1021/acs.inorgchem.7b01350
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a
Table 1. Select (Average) Metrical Data (Experimental and Calculated) for 1−7
distances (Å)
angle (deg)
b
b
b
b
compd
Mg−Osal
Mg−Nsal
Mg−Npy
O−M−O
N−M−N
O−M−N
O−M−N′
Npy−M−Npy
c
2
1.97
2.16
2.31
109.12
76.48
87.22
163.49
179.73
163.47
179.73
177.37
176.59
177.26
176.28
calcd
3
1.91
2.11
2.16
2.17
2.15
2.16
2.13
2.13
2.24
2.27
2.24
2.30
2.24
2.13
103.68
116.07
114.83
107.43
110.10
101.47
77.88
75.48
77.08
77.15
77.31
76.38
76.38
89.14
84.39
84.28
87.81
86.29
87.02
85.79
166.73
158.93
160.41
164.47
163.59
154.86
142.73(t)
1.96
calcd
4
1.97
1.94
calcd
4a
1.96
1.93
5
1.93 (t)
115.30 (μ-t)
160.17 (μ)
2.03 (μ)
106.53 (μ′-t)
84.57 (μ−μ)
85.12 (μ-t)
105.91 (μ′-t)
119.16 (μ−μ)
153.11
calcd
1.93 (t)
2.14
75.63
85.67
138.27 (t)
2.01 (μ)
159.13 (μ)
6
2.29
2.27
2.26
2.29
2.56
2.52
2.51
2.52
2.54
2.57
2.54
67.30
68.50
64.88
66.79
70.77
71.89
71.99
72.98
135.76
135.36
136.23
139.20
176.94, 88.47, 88.47
175.34, 86.23, 89.11
171.21, 107.23, 81.66
168.31, 81.21, 87.11
calcd
7
151.94
148.09
calcd
2.52
147.68
a
b
c
(t) = terminal, (μ) = bridging. As within a salen moiety. Two molecules in the unit cell.
General X-ray Crystal Structure Information. For 1, 2, 4, 6,
C, H, N, O, and Mg atoms and a radius of 2.31 Å for Ca atoms.28
Table 1 shows the excellent agreement in metrical data between the
crystal structures and DFT-optimized structures, validating the quality
of the DFT models for these hybrid compounds. The atomic
coordinates and CHELPG charges of Gaussian-optimized structures
can be found in the Supporting Information.
and 7, single crystals were mounted onto a loop from a pool of
Fluorolube or Parabar 10312 and immediately placed in a cold N2
vapor stream on a Bruker AXS diffractometer employing an incident-
beam graphite monochromator, Mo Kα radiation (λ = 0.710 70 Å) and
a SMART APEX CCD detector. Indexing and frame integration were
performed using the APEX-III software suite. Absorption correction
was performed using face-indexing (numerical method) also within the
APEX-III software. The structures were solved using SHELXL-2016/6
and refined using OLEX2 version 1.2.17 All final CIF files were
checked using the CheckCIF program (http://www.iucr.org/).
Specific issues with the characterization of 1, 3, and 5 were due to
the limitation of the Mo source employed. A rational route to 1 has
not been realized, yet, but it is reported for completeness; however,
structural issues with 1 were due to poor diffraction.
For compounds 3, 4a, and 5 alternative single-crystal X-ray systems
were employed with substantially improved results. These were
collected on either a Bruker Ultra system with a Mo rotating-anode
source with Helios multilayer optics (no graphite monochromator)
and an APEXII CCD detector or a system employing a Rigaku FR-X
Cu rotating-anode source with Helios multilayer optics and a
Platium135 CCD detector on a Bruker D8 platform.
Proppant Loading. The pore volume of the proppant is reported to
be 0.112 mL/g, and no more than 90% of pore volume was to be
occupied by complex in solution during the impregnation process.
Therefore, the precursor loading was between 2 and 6 wt % using an
infusion (or impregnation) process that is similar to that used in the
preparation of heterogeneous catalysts. To a preweighed vial, the
desired amount of metal salen complex was added to 10 mL of toluene
(referred to as saloR-But solution). The proppant was weighed (10.0
g) into a separate vial, and 0.8 mL of saloR-But solution was added
dropwise to the vial containing proppant and adsorbed into the pores.
After each addition, the vial was shaken, and the excess toluene was
removed in vacuo. This process was repeated, until the desired wt %
loading was achieved. Loadings: Compound 3 (0.479 g) loading
achieved 4.79 wt % of 3 in proppant, Compound 4 (0.632 g) loading
achieved 6.32 wt % of 4 in proppant, and compound 7 (0.648 g)
loading achieved 6.48 wt % of 7 in proppant.
Column Preparation/Elution. Sand (∼30 g; ACS grade, Aldrich)
was added to a column (226 mm) and saturated with toluene (20
mL). Each compound (0.100 g) was dissolved in toluene (50 mL),
stirred for 30 min, and added to the column in aliquots (10 mL) until
depleted. The sample flask was then rinsed with toluene (∼50 mL),
which was added to the column. This was followed by an additional
toluene (∼50 mL) wash. Every step was run at a flow rate of 0.065
mL/sec. The product was identified by FTIR spectral data. Recovered
yields for: 2, 100% (0.10 g); 3, (100%; 0.10 g); 4, (100%; 0.10 g); 5,
74% (0.074 g).
Additional information concerning the data collection and final
structural solutions can be found in the Supporting Information or by
accessing CIF files through the Cambridge Crystallographic Data Base.
The unit cell parameters for the structurally characterized compounds
1−7 are listed in the Supporting Information.
Computational Modeling. Gas-phase electronic structure calcu-
lations were performed on individual M−L complexes using Gaussian
0918 at the density functional theory (DFT) level. The M062X hybrid
functional19 was used along with the cc-pVDZ basis set19−22 for all
atoms. The M062X functional was used to model spectroscopic
properties of magnesium complexes.23,24 This combination of DFT
methods has previously been found to yield excellent agreement with
experimental structures of related M−L compounds.25,26 All atoms
were allowed to relax in all structures except 2, since the ligand phenyl
rings tilted significantly out of plane. Instead, this structure was initially
optimized without py ligands, and then these atoms were held fixed
while the py ligands were optimized. The resulting structure is lower in
energy and more consistent with the other Mg complexes (3−5). After
geometry optimization, partial atomic charges for all complexes were
calculated using the CHELPG method,27 with default atomic radii for
Solvothermal Processing. The stability of this family of compounds
under the high pressure and temperatures was evaluated using a
solvothermal (SOLVO) approach. For 2−7, a 0.10 g sample was
placed into a Teflon liner of a Parr Digestion bomb in 20 mL of
solvent (toluene or dodecane), sealed, and heated to a preset
temperature (120 or 180 °C) for 24 h. After this time, the residual
product was isolated by centrifugation, washed with hexanes (three
times), and characterized via FTIR spectroscopy (see Supporting
Information) to evaluate structure retention.
D
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alkyl derivative,50 the reaction of [Mg(Bun)2] with the
commercially available H2-salen or H2-saloPh was undertaken
in toluene (eq 1). For the H2-salen ligand, the products formed
were found to be insoluble in toluene, but upon addition of the
strong Lewis base py, clear, orange solutions could be obtained
at elevated temperatures. All attempts to generate single crystals
yielded only amorphous powders. Switching to the H2-saloPh
ligand led to a clear, dark-orange solution from warm toluene.
The crystals that grew upon slow cooling were identified as
compound 1 (shown in Figure 2).
Elution Studies. Laboratory experiments designed to mimic fluid
flow through hydraulically fractured zones in a subsurface well that
contain a ceramic proppant are available. In general, the experimental
setup uses a cartridge pump to send the desired solvent fluid (either
aqueous or hydrocarbon) at a constant rate through a small “proppant
pack” to a fraction collector. The proppant pack consists of porous
proppants that have been infused with the metal salen compounds,
which are packed into a plastic syringe equipped with a fine mesh
proppant retention screen at the entrance tip. The pore volumea
measure of the fluid volume within the pack of proppant pelletsis
measured. The proppant pack is then sealed at the top with a rubber
septum to generate pressure to force the exiting fluid into tubing for
transport to the collection test tubes in the fraction collector. Samples
were collected over 40+ pore volumes, and the concentrations of metal
complexes in the samples were measured by atomic absorption
spectrophotometry (AAS) after 4, 20, and 40 pore volumes had passed
over the proppant pack. A photograph of the elution testing setup is
shown in Figure S3 of the Supporting Information.
For the initial efforts, compound 4 was analyzed. To quantify the
concentration of Mg in the samples recovered from the elution test, a
PerkinElmer PinAAcle 900T Atomic Absorption Spectrophotometer
was used in flame mode. Standard solutions containing known
concentrations of Mg were prepared from mesityl magnesium bromide
(1 M in THF; Aldrich) diluted with n-propyl alcohol and Isopar L
(final solutions contained 20 vol % Isopar L). The Isopar L was added
to standards in order to match the matrix in the samples (Isopar L was
the solvent used in the elution studies, and all samples were diluted 4:1
with n-propyl alcohol before AAS analysis).
RESULTS AND DISCUSSION
■
A search of the structure literature reveals that surprisingly few
alkaline-earth salen derivatives have been reported.29 Of the 22
compounds structurally characterized with at least one alkaline-
earth metal coupled to a salen backbone, the majority of the
family members are heterometallic species with the alkaline-
earth cations located externally to the salen cavity, including:
Mg−Cu,30−34 Mg−Ni,35 Ca−Ni,35 Ca−U,36 Ca−Cu,37,38 Sr−
Cu,39 Ba−Cu,40,41 Ba−Ni,42,43 and Ba−Fe.44 Only six
homometallic compounds have been reported: [Be(μ-(κ4-
(O,N,N′,O′)salen))]2·H2O,45 bis(μ-3,3′-(ethane-1,2-diyl bis-
(iminiomethyl ylidene)) bis(2-oxybenzoate))diaqua, dimagne-
sium hexahydrate),46 aqua,methanol-(ethylendiamine-N,N′-bis-
(2-aldimino-4-methyl-6-formylphenolato)) magnesium,47
(THF)(HOEt)2Ca(κ4-(O,N,N′O′)salo-But,48 (DME)Ca(κ4-
(O,N,N′O′)saloCy-But),49 where saloCy-But = bis(3,5-di-t-
butylsalicylidene)cyclohexane-1,2-diamine, (HOEt)3Sr(κ4-
(O,N,N′O′)salo-But).48 It is of note that [Mg(salen)]2 and
[Mg(salo-Ph)]2 were reported from the reaction of [Mg-
(CH2C6H5)2] and the respective Schiff-base; however, these
products were not crystallographically characterized.39
We were initially interested in Mg and Ca due to their
ubiquity in nature and low cost; however, the diversity of the
coordination numbers (CNs) of the Mg and Ca compounds
(CN ranges from 4 through 7), the number and types of
solvent molecules bound, the torsional angle of the salen
backbone, and the lack of homoleptic Mg derivatives indicated
that a study of this family of compounds was necessary prior to
their use as taggants. Therefore, the syntheses and character-
ization of Mg and Ca derivatives was undertaken. Once isolated
and modeled, their analytical properties were obtained to
determine if they could be individually identified by simple
spectroscopic means, a key feature for the possible use as
monitors in deep well fluid systems.
Figure 2. Structure plot of 1. The heavy-atom thermal ellipsoids are
drawn at 30% level, with C atoms drawn as ball and stick and H atoms
omitted for clarity.
Initial inspection of the structure of 1 reveals four
“Mg(saloPh)” moieties that are interconnected through the
oxygen atoms from the phenoxide rings: two terminal (one
bridges) and two internal (both bridge). The inclusion of the n-
butyl group results in a chain of [(μ-O)-Mg]4 with two trigonal
bipyramidal (τ = 0.37)51 and two pseudo-octahedral (OC-6)
bound Mg metal centers. Upon closer examination of the
second saloPh ligand, surprisingly, an n-butyl group was solved,
branching off the carbon of the “Ph−C(34)N(4)−Mg(2)”
moiety. This indicates that a C−C bond was formed in this
simple reaction. This mechanism is not fully understood, but
the use of the basic [Mg(Bun)2] may lead to deprotonation of
the Ph−C(H)N fragment. Isolation of crystals from this
reaction have not been easily reproducible, and the structure
suffers from weak diffraction. This compound is not included in
the Experimental Section, as an established route to the
molecule is not available; however, the structure properties are
reported (see Table S1 in Supporting Information) due to its
unique tetranuclear nature and the unexpected C−C bond
formation.52
Alternatively, the product isolated from the Lewis basic
solvent py was found to be the mononuclear compound 2
(Figure 3). Compound 2 is the first report of a structurally
characterized, alkaline-earth salen derivative with a bound py
solvent molecule. The coordination of the Mg is best described
as pseudo octahedral (OC-6) using the full complement of
available binding sites on the saloPh-But(κ4-O,N,N′,O′) ligand
along with two trans bound py solvent molecules.
Mg Complexes. Following the report by Corazza et al. on
the successful substitution of Mg into the salen ring through an
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Figure 4. Structure plot of 3. The heavy-atom thermal ellipsoids are
drawn at 30% level, with C atoms drawn as ball and stick and H atoms
omitted for clarity.
Figure 3. Structure plot of 2·py. The heavy-atom thermal ellipsoids are
drawn at 30% level, with C atoms drawn as ball and stick and H atoms
omitted for clarity. The two molecules per unit cell are shown.
their structural arrangement would be. Following the same
reaction pathway noted above but omitting the addition of py
led to the crystallization of compound 5. This compound was
characterized by single-crystal X-ray diffraction as a dinuclear
complex (Figure 6). For 5, each 5-coordinated Mg was solved
in a square base pyramidal geometry (τ = 0.96) binding in a κ3-
(O,N,N′),μ-(O′) chelation mode, where the second O atom
from each saloPh-But bridges two Mg centers.
Salen derivatives with t-butyl groups on the rings were of
interest due to their expected increased solubility in organic
solvents. Both the ethyl53,54 and phenylene55 diamine
derivatives (Figure 1c,d) were synthesized following literature
routes,53−58 wherein 3,5-di-tert-butyl-2-hydroxybenzaldehyde
was condensed with ethylene-1,2-diamine or o-phenylene-1,2-
diamine forming N,N′-bis(3,5-di-t-butylsalicylidene)-ethylene-
diamine (H2-salo-But, Figure 1c) or N,N′-bis(3,5-di-t-butylsa-
licylidene)-1,2-phenylenediamine (H2-saloPh-But: Figure 1d),
respectively. Once isolated and characterized, these ligands
were reacted with [Mg(Bun)2], following eq 1. The products
isolated were identified by single-crystal X-ray diffraction
studies as the monomeric species 3 and 4 (see Figures 4 and
5a, respectively). The saloR-But derivatives act in the same
tetradentate manner binding through the κ4-O,N,N′,O′ atoms
with trans axial py molecules.
Attempts to reproduce an improved crystal of 4 led to the
isolation of 4a (Figure 5b). While the binding mode of the salo-
But to the Mg for both compounds are identical, only one py
solvent molecule was solved bound to the Mg cation of 4a. This
limited solvation places the Mg in a 5-coordinate square base
pyramidal (τ = 0) geometry. A rational route to the mono- (4a)
versus the disubstituted (4) has not been discerned, as of yet.
Since the ability to bind a variable number of solvent molecules
is unexpected and difficult to control, the structure parameters
(see Table S1 in Supporting Information) are reported to allow
for ease of identification. There are some differences in the
metrical data of 4 and 4a. In particular, the Mg−N distance of
the disubstituted species is over 0.15 Å longer. This weaker
interaction is also reflected in the 6° and 10° larger bond angles
of O−M−O and O−M−N, respectively. On the basis of these
metrical data, the number or type of solvent bound to the metal
may play a bigger role in the final stability of these compounds
in real-world efforts than originally anticipated.
Since these are the first Mg/py/salen derivatives reported,
the only viable models to compare the metrical data to are
among themselves (2−5). Surprisingly, there were significant
variations in the bond distances and angles noted based on the
ligand employed, which can only be attributed to the rigidity of
the ethylene diamine backbone (saloPh vs saloPh-But). In
particular, the phenyl ring enforces a more rigorous OC-6
geometry around the Mg, but disorder was noted and is
thought to be due to the strain of binding to the small Mg
cation (0.86 Å, CN-6).59 Figure 1e shows a diagram of some of
the torsional angles discussed below to clarify the following
discussion. The twist of the salen moiety is most evident in the
dihedral angle between the N···N and the C−C of the phenyl
ring (0.7 and 1.92° for 2; 23.6° for 3; 0° for 4; 1.3° for 5). The
N−C−C−N torsion angle was found to be 0.75 and 2.13 o for
2; 44.7° for 3; 0.18°for 4, and 2.45° for 5 demonstrating the
rigidity induced by the phenyl backbone. The bend of the salen
ligand around the metal was determined by measuring the para
C atoms in the opposite rings and the metal (C···M···C): 2
(14.3 and 15.4°); 3 (3.8°); 4 (11.5°); and 5 (41.7°). While the
ethylene group twists more to accommodate the coordination
of the metal, a phenyl backbone alternatively bends the salen
ligand. The bound py molecules remain relatively linear with an
average 177.3° angle (py-Mg-py); however, they can twist
relative to each other (41 and 8.5° for 2; 0.4° for 3; 29.7° for
4). As can be noted by the two values for 2 there is a significant
difference between the py twists of the two molecules present
in the unit cell, possibly due to packing forces. The phenyl ring
of the saloPh species was found to be bent from the Mg−N
bonds by 9.29 and 9.77° for 2, 0.17° for 4, and 20.3° for 5. This
is most likely a result of relieving the strain induced by the bend
It was of interest to determine if the solventless structures for
the saloPh-But−Mg derivatives could be obtained and what
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Figure 6. Structure plot of 5. The heavy-atom thermal ellipsoids are
drawn at 30% level, with C atoms drawn as ball and stick and H atoms
omitted for clarity.
thermal event noted for all of these compounds was observed
above 300 °C. Weight losses stabilized after 550 °C for all
samples except 3, which stabilized after 600 °C. Analyses of the
residual powder indicated that periclase (MgO; PDF 00-045-
0946) formed for all samples. The overall weight losses were
similar to expected conversion percentages to periclase [cmpd
(%calc; %exp): 2·py (7.0; 7.5); 3 (5.3 (+py); 5.0); 4 (5.0; 5.3);
5 (7.2; 7.5). This indicates that the majority of the bulk
powders are consistent with the single-crystal structures with
minor adjustments with solvent addition for 3.
1H NMR spectra of 2−4 were collected in py-d5, and selected
resonances are listed in Table 2. Interpretation of the spectra is
fairly straightforward with easily distinguishable py and saloR′-
But resonances observed. The 8.70 ppm Ph−CHN proton
resonances noted for both 2 and 4 are therefore not a useful
spectroscopic handle for identifying the various precursors. The
−CHN- resonances were expected to be the most
distinguishing resonance; however, shifts of free ligands and
the corresponding complexes were too similar to distinguish
the individual compounds. In contrast, the −N−CH2−
resonances of the salo-But ligated complexes were found to
shift 0.5 ppm upon complexation. The absence of these protons
in the saloPh-But derivatives limits the impact of the observed
shift.
Figure 5. Structure plot of (a) 4·tol and (b) 4a. The heavy-atom
thermal ellipsoids are drawn at 30% level, with C atoms drawn as ball
and stick and H atoms omitted for clarity.
1
of the rest of the saloPh ligand. Larger R1 values attributed to
poor diffraction of the samples were observed for 3 and 5.
Elemental analyses on 2−5 were undertaken to verify purity
(see Experimental Section). The data collected for 2−4 (the
solvated Mg species) were not consistent with their solid-state
structure. This was not unexpected, as solvated species tend to
decompose inconsistently. The variance of these analyses being
due to the solvent is confirmed by the good agreement noted
for 5. Attempts at using complexometric titrations to establish
the purity of the bulk powder were not successful, as Mg is
difficult to solubilize and complex. Therefore, TGA analyses
were undertaken. From these experiments, weight losses were
noted at ∼140 °C for 2−4 but not for 5; therefore, the lack of a
similar loss in the spectrum of 5 at low temperature indicates
this weight change is associated with the loss of py. The next
The H NMR spectrum of compound 5 was collected in
both tol-d8 and py-d5. The spectrum of the dinuclear 5 in
toluene was extremely complicated, and no meaningful
interpretation was made. The numerous peaks were attributed
to the locked-out alkyls of the t-butyl groups, the potential
monomer−dimer equilibrium, and/or partial bridging of the
saloPh-But. The 13C NMR data were collected but due to low
solubility additional information was not garnered. The
spectrum of 5 in py-d5 was identical to that of 4, which
demonstrates the simplicity with which 5 can coordinate py to
render 4.
Alternative analyses were investigated to aid in distinguishing
the different ligands and metal binding noted for 2−5. UV−vis,
FTIR, and Raman spectroscopy proved to be reliable methods
for identifying the free ligand and Mg species. Select features of
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Table 2. Select Analytical Data for 1−7
cmpd no
NMR (py-d5) -C(H)N-
NMR (py-d5) -C(H2)-N-
UV−vis (nm−1
)
FTIR (CN) cm−1
Raman (CN) cm−1
H2-saloPh
8.78
8.40
8.66
8.70
8.31
8.70
353
1613
1626
1615
1611
1629
1604
1614
1624
1594
1609
1635
1617
1617
1617
1642
1611
1629
1616
H2-salo-But
3.94
333
H2-saloPh-But
306
2
3
4
5
6
7
379
3.39
3.30
344, 375
354
353, 378
337, 420
354, 493
8.31
8.55
these spectra are tabulated in Table 2. The UV−vis spectra of
the ligands and the Mg complexes (2−5) were collected in
toluene. For 2−5, a broad peak between ∼343 and ∼419 nm
was observed. The observed maxima are listed in Table 2, and
the spectra are shown in Figure 7. These values are in
agreement with reported ligand-to-metal charge-transfer
transition bands of related metal-salen complexes.60,61
investigated using [Ca(NR2)2] in toluene (eq 2). For both
reactions, the parent metal-salen complex product rapidly
precipitated upon mixing and remains insoluble in hot toluene.
The salo-But and saloPh-But products were soluble in py, and
upon crystallization by slow evaporation yielded the mono-
meric py-containing derivatives 6 (Figure 8) and 7 (Figure 9).
Figure 7. UV−vis spectra of (a) Mg (2−5) and (b) Ca (6 and 7)
derivatives.
Figure 8. Structure plot of 6. The heavy-atom thermal ellipsoids are
drawn at 30% level, with C atoms drawn as ball and stick and H atoms
omitted for clarity.
For the free ligands, the FTIR stretch for the CN was
found at 1626 cm−1 for H2-salo-But and 1615 cm−1 for H2-
saloPh-But. Upon complexation to the metal, both the FTIR
and Raman CN stretch shifted by ∼10 cm−1. As desired, dual
vibrational spectroscopy (FTIR and Raman) characterization
successfully identifies each synthesized complex. This is a key
feature for possible use of these compounds as fluid flow
trackers in underground systems. Upon complexation to Mg,
the FTIR imine stretches were shifted for 2−5 to 1611 (2),
1629 (3), 1604 (4), and 1614 (5) cm−1. Similar shift trends
were noted for the Raman spectral data for 2 (1617 cm−1), 3
(1617 cm−1), 4 (1642 cm−1), and 5 (1611 cm−1).
A CN-7 geometry around the metal occurs from the
tetradentate salo-ligand (κ4-O,N,N′,O′) and the coordination
of 3 py solvent molecules. The phenyl rings of the salo-But are
planar but at the cost of a twisted ethyl group on the diamine
(27.9°). In contrast, the rigidity induced by the phenyl ring of
the saloPh-But causes a substantial bend in the rings of the salo
moiety.
The metrical data of 6 and 7 (Table 1) were not as
influenced by the ligands as noted for 2−5. The majority of the
bond angles are identical with the major difference noted at the
O−M−O angle. This may be due to the electron donation and
steric bulk of the three bound py ligands. The trans py solvent
molecules were found to be only slightly twisted, with an
Ca Complexes. As the Ca(R)2 precursors are not readily
available, derivatization of H2-salen or H2-saloPh was
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were associated with the loss of the py solvent molecules. The
next thermal event occurred greater than 270 °C and was
finalized by 750 °C. Powder X-ray diffraction (PXRD) analyses
of the residual powder revealed lime (CaO; PDF 00-037-1497)
had formed for both samples.
1
Solution H NMR spectra of 6 and 7 were found to be in
agreement with the solid-state structure, with one set of
resonances observed for the various components. Similar Ph-
HCN shifts of 8.31 ppm for 6 and 8.55 ppm for 7 were
observed. Comparison of these spectra with 2−4 revealed a
similar shift for 6 and 3 that limits using this distinguishing
resonance. Because of the success of the UV−vis, FTIR, and
Raman spectral differentiation noted for the free ligands and
their Mg complexes, these techniques were utilized similarly for
the Ca complexes. Broad bands, similar to 2−5, were also
observed in the UV−vis spectra of 6 and 7 (see Figure 7 and
Table 2). The broad peak between ∼338 and ∼419 nm are
again attributed to ligand-to-metal charge-transfer transition
bands.60,61 As for the FTIR spectra, upon complexation to Ca
the H2-salo-But imine stretch was shifted from 1626 to 1624
cm−1 for 6, and the H2-saloPh-But stretch shifted from 1615 to
1594 cm−1 for 7. Similar shifts were noted for the Raman
spectral data: 1635 cm−1 for H2-salo-But versus 1629 cm−1 for 6
and 1617 cm−1 for H2-saloPh-But versus 1616 cm−1 for 7.
Modeling. As an initial step in the development of
molecular simulation methodologies to predict the behavior
of these compounds in underground environments, DFT
calculations of gas-phase clusters of 2−7 were used to compare
the effect of ligand chemistry on structural properties. Such
simulations can be used to assist in identifying the solution
behavior of these compounds in oil (hydrophobic, organic),
water (hydrophilic), or a mixture of the two. Subsurface
behavior can also be simulated in this manner. The models
were developed and compared to the crystal structures (see
Table 1). Overall, a comparison of the model’s angles and
Figure 9. Structure plot of 7. The heavy-atom thermal ellipsoids are
drawn at 30% level, with C atoms drawn as ball and stick and H atoms
omitted for clarity.
average 2.7° torsion angle. The twist of the salo moiety around
the N···N and C−C bonds (see Figure 1e) are 27.9° for 6 but
0.5° for 7. However, the phenyl ring of the diamine and the
phenoxy rings are bent away from each other. The N−C−C−N
dihedral angle was found to be 56.5 and 2.13° for 6 but only
0.88° for 7, showing the induced rigidity of the phenyl
backbone. Further, as noted above, the twist that the backbone
endures forces the phenyl salen species to bend significantly:
18.7° for 6 versus 46.0° for 7. Again, because of this bend the
phenyl ring of the saloPh derivative 7 was angled away from the
Ca−N bonds by 39.1°.
Elemental analyses on 6 and 7 were also measured and, in
contrast to 2−5, were found to be in good agreement with the
solid-state structures. For 7, this required the loss of a py ligand
to garner acceptable percentages. TGA analyses of these
compounds revealed weight losses initiated at ∼140 °C, which
Figure 10. DFT-optimized structures of the Mg-L complexes of (a) “Mg(salen)(py)2”, (b) 2, (c) 3, (d) 4, and (e) 5.
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distances to those of the single-crystal structure were found to
be consistent.
a. Magnesium. The DFT-optimized structures of the
various Mg-salo-R complexes are shown in Figure 10a−e. In
the structural model of the salen ligands, there is significant
twisting about the C−N−N−C dihedral of the ethylenediamine
group due to the strain imposed by the OC-6 coordinated Mg
cation. In fact, the N−C−C−N dihedral angles are ∼49° for
the salen of 3, compared with dihedral angles ∼0° for the
saloPh compounds (2, 4, and 5). The general trend of the
dihedral angles is consistent with the single-crystal structures
noted previously with 2, 4, and 5 being nearly 0°; however, the
large 49° twist noted for the model is more than twice that seen
in the structure of 3.
Another significant difference between the calculated Mg-
(salen) and 2 is the orientation of two axially bound py ligands,
resulting in what initially appears to be C2 rotational symmetry;
however, the py ligands of 2 are perpendicular to each other,
which disrupts the C2 symmetry. A twist in the py orientation
was noted for 2−4 but was not as much as the model indicated;
the largest twist noted was for 4 at 29.7°. Furthermore, the
three phenyl rings of 2 (Figure 10b) are not planar where the
two rings along the x-axis are tilted upward by ∼8° and the
central phenyl ring along the y-axis is tilted downward by ∼8°.
This maximizes the short-range interactions with the two py
ligands. Introduction of the But groups onto the rings
introduces an off-plane tilt by ∼6° (Figure 10d) for 4. The
saloPh-But ligands in compound 5 show significant non-
planarity, likely due to the steric hindrance of the two ligands in
such close proximity. Angles centered on the Mg atoms in 5 are
also reduced compared to the monomer 4 (Table 1). This
general trend of salo ligand bends were noted in the
experimental data as well.
b. Calcium. Optimized structures of 6 and 7 (Figure 11)
were found to be similar to the Mg species, but due to the
larger Ca cation,47 a third py ligand located in the equatorial
plane binds to Ca forming a CN-7 geometry. The ethylenedi-
amine group in 6 is twisted with a N−C−C−N dihedral angle
of 59°, which is significantly larger than noted for the Mg
system (vide infra). This angle is larger than noted for the
experimentally derived structures, but the general trends are
again consistent. Additionally, the four coordinating donor
atoms of the salo backbone were found to be significantly
deviated from planarity. The dihedral angles involving the metal
atom and two coordinated atoms (e.g., N−M−O−C) range
from 34° to 46° for 6 and 7. In contrast, the same angles
observed for 2−4 range from 3° to 9°.
Fluid Flow Parameters. Critical for practical use, these
salen-based tracers must survive the high temperatures and
pressures typically noted for deep-well conditions, easily loaded
into, and freely elute from, the carrier proppants, and flow to
the surface without binding to underground strata. Therefore,
an initial evaluation of 2−7 for use as potential underground
tracers was undertaken, and the properties determined are
discussed according to (i) thermal/pressure stability, (ii)
interaction with surfaces (SiO2 columns), and (iii) elution
data from the proppant. The conditions associated with a deep
well (or “bottom-hole conditions” at 1800 m) will be
considered as follows: 80 °C, 6000 psi pressure in a silica
environment to simulate the rock formations under-
ground.62−66 Because of variations in the contents, location,
and well composition, these values vary considerably; however,
Figure 11. DFT-optimized structures of the Ca-salo complexes (a) 6
and (b) 7.
these conservative parameters are considered at the high end of
the bottom-hole conditions.
I. Thermal/Pressure Stability. To test the stability of the
precursor, several experiments with “model oils” were
conducted on 3−7. The thermal robustness of these
compounds in toluene at reflux temperatures was undertaken.
Toluene was selected due to its boiling point, similarity to some
oil components, and the fact that subsequent computational
modeling uses toluene as a model oil as well. All samples were
dissolved in toluene, heated for 12 h, and then dried. FTIR data
indicated that each sample was found to survive intact.
Next, the samples were dissolved in toluene (30 mL), placed
into a Teflon-lined Parr bomb, and heated to 120 °C for 12 h
under SOLVO conditions. At these conditions, pressures as
high as 1450 psi were calculated to exist. Extraction of the
compounds after this reaction indicated that 5−7 had
maintained their original FTIR spectra (vide supra). The Mg
derivatives (2−4) were slightly altered, which was assumed to
be due to loss of coordinated solvent, which was confirmed by
the similarity of the FTIR spectra of a series of toluene-
generated Mg-saloR-But powders and the material recovered
from the Parr bomb (see Supporting Information, Figure S1).
In comparison to the Mg species, the Ca derivatives (6 and 7)
must have a stronger interaction with the py that prevents this
disassociation. This is borne out by the bond distances reported
in the structural literature (i.e., Ca−N (range 2.495−2.641 Å vs
Mg−N (range 2.117−2.376 Å))67 when the cation variations
(Mg, CN-6, 0.71 Å; Ca CN-6, ;1.14 Å)59 are taken into
account.
Evaluation at higher temperatures were sought, so the
samples of 3−6 were individually placed into a Parr bomb
containing 25 mL of dodecane. Dodecane was selected due to
its longer hydrocarbon chain length and higher boiling point.
The sealed Parr bomb was placed into a box furnace at 180 °C
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Article
for 24 h, with an expected pressure of 2220 psi. After it cooled
to room temperature, the samples were evaluated by FTIR for
their distinguishing features (vide supra). On the basis of these
data, it was evident that none of the samples survived the
process (Supporting Information Figure S2). The soluble
fraction of 3 and 4 were concentrated, and the crystals isolated
from the reaction mixture proved to be the original H2-saloR′-
But ligand; the insoluble fraction was amorphous, but heat
treating at 400 °C indicated magnesium oxide (PDF 01-076-
9192) or periclase (01-071-6488), respectively, had been
isolated. The products isolated from 6 and 7 possessed calcium
oxide (01-077-7243) along with several other Ca species. While
these data indicated that the compounds would not survive at
these high temperatures and pressures, it is of note that these
conditions exceed what would be expected or required for a
bottom-hole.
ii. Retention. The need for these metal−ligand complexes to
remain soluble in the product fluids is obvious, as premature
deposition onto underground rock formations would alter both
qualitative and quantitative results. Attempts to determine the
mobility and potential deposition of these molecules in
underground caverns were initially investigated using columns
of sand (SiO2). The preweighed samples were dissolved in
toluene, a column of packed sand was saturated with toluene,
and the samples were loaded onto the column. The sample was
run through the column at a continuous flow rate of 0.065 mL
sec−1, then flushed with excess toluene to ensure full elution.
The sample was then collected and dried. For three of the four
samples studied (2, 3, and 4) an ∼100% yield by weight
recovery was obtained. A reduced recovery of 74% yield by
weight was noted for 5 and is attributed to partial deposition on
the column’s surface. Follow-on molecular simulations will
determine the extent of interaction of similar compounds with
clay mineral phases. Refinement of these samples for the
various underground minerals (silica, kaolinite, and montmor-
illonite) have been successfully modeled;68 however, exper-
imental verification is necessary to establish the proper
constituents of the precursors for the different regions the
wells may exist in.
volumes of liquid passed over the Mg-salen loaded proppant
package was analyzed.
Second, the testing solvent can be either an aqueous salt
solution that simulates a formation brine or a long-chain
hydrocarbon (such as Isopar L) to mimic the organic products.
Typically, it is much easier to analyze samples in aqueous
media, as inductively coupled plasma techniques can be used to
accurately measure the metal concentration. However, for this
effort we were interested in evaluating the Mg-salen
concentrations in hydrocarbons. As such, a procedure using
AAS (described in the Experimental Section) was developed to
analyze the low Mg levels found in the organic solvent.
Third, we note that it is possible to put a polymeric coating
on proppant particles to retard the elution rate of the Mg-salen
species from the package and thus extend the operating lifetime
of the proppant package. However, our goal in this study was to
experimentally demonstrate the concept that the M-salen (M =
Mg, Ca) complexes developed could function as taggants for
monitoring underground fluid flow. For expediency, we were
interested in the accelerated elution of these compounds during
the lab testing; thus, polymeric coatings were not put on the
proppant after it was loaded.
For this report, our initial analyses were undertaken on a
proppant pack infused with 4, utilizing the equipment shown in
Figure S3. As expected, the level of eluted Mg-salen complex
decreased markedly from the initial phase of the testing (176
ppb, aliquot taken after four pore volumes had been collected)
to the intermediate sampling taken at the 20 pore volume mark
(29 ppb), to the latter stages of testing at the ∼40 pore volume
mark (9 ppb). This decline in measured Mg levels is shown
graphically in Figure 12, and these results indicate that these
iii. Elution Data. As mentioned previously, for the prepared
metal-salen ligand complexes to be useful in tracking fluid flows
the infused complexes must elute from the underground
proppants upon exposure to the produced fluid and remain
soluble until they are analyzed above-ground. As the fluid
flowing from underground is typically a mixture of hydro-
carbons (i.e., organic product) and water, when eluted they will
partition into either the organic or aqueous phases. Laboratory-
scaled elution testing equipment was used to simulate the “real
world” performances of experimental samples by flowing the
desired fluid over a proppant pack and measuring the amount
of complex found in sample aliquots over time (Figure S3).
There are at least three key points to keep in mind when
evaluating these samples.
First, the amount of complex released into the produced fluid
is expected to decrease over time, as the concentration of
infused complex decreases within the proppant. Elution
experiments were conducted at a constant flow rate, and
aliquots were taken as a function of time. The eluted fluid
volume is converted to pore volumes for elution profile
analysis. This pore volume concept is very similar to the
established liquid hourly space velocity (LHSV) concept used
in liquid flow catalytic studies. For this system (vide infra), the
level of Mg found in the aliquots after 4, 20, and ∼40 pore
Figure 12. Concentration of Mg from 4 in eluted Isopar L as a
function of equivalent pore volumes eluted.
salen-metal species loaded into proppants meet the preliminary
requirement that they gradually elute from the proppant
package over time. Further ongoing studies are focused on
evaluating long-term elution performances using both aqueous
and hydrocarbon fluids. These results will be published, when
appropriate, in more industrially focused journals.
CONCLUSION AND SUMMARY
■
A series of pyridine-solvated alkaline-earth salen derivatives
(Mg: 2−4a; Ca: 6 and 7) have been isolated and structurally
characterized as monomeric κ4-(O,N,N′,O′)saloR-R′ species.
Additionally, a solvate-free Mg complex was isolated as the
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dinculear complex 5 adopting a κ3-(O,N,N′),μ-(O′) binding.
For each of the solvates, the salen moiety was found to adopt a
tetradentate (κ4-(O,N,N′,O′)) binding mode. For the smaller
congener Mg, one or two py molecules were bound to the
metal forming CN-5 or CN-6, respectively, whereas the larger
Ca ion binds three py molecules to fill its CN-7 coordination
sphere. The twists of the salen ligands are less dependent on
the metal and more on the backbone of the salen ligand: salo
(twist 23.6° to 27.9°) versus saloPh (twist 0 to 1.3°). Twisting
of the ethylene backbone allows the rest of the salo ligand to
remain relatively planar, whereas the saloPh derivatives have the
phenoxy rings significantly bent away from the central plane to
relieve strain. These trends were also observed in the
complementary DFT calculations. The high agreement
between the DFT calculations and experimental results enables
the development of meaningful models of the subterranean
fluid flow. Eventually, atomic charges and geometric properties
will be used in large-scale molecular simulations to investigate
the behavior of these compounds in oil and/or aqueous
solutions and to understand their adsorption characteristics
onto mineral surfaces found underground. Elution tests using 4
loaded into a proppant and Isopar L as simulant for
underground oil reservoir revealed the desired gradual release
of 4 into the fluid flow, thus verifying the potential use of such
compounds as taggants. Additional work, exploring this family
of compounds and other derivatives69 as a means to monitor
underground fluid flow, is underway.
43580). Sandia National Laboratories is a multimission
laboratory managed and operated by National Technology
and Engineering Solutions of Sandia, LLC, a wholly owned
subsidiary of Honeywell International, Inc., for the U.S.
Department of Energy’s National Nuclear Security Admin-
istration under Contract No. DE-NA0003525.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01350.
Atomic coordinates, data collection parameters, and
CHELPG charges for 1−7 (PDF)
Accession Codes
CCDC 1547258−1547264 and 1571279 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: tjboyle@sandia.gov. Phone: (505) 272-7625. Fax:
(505) 272-7336. (T.J.B.)
■
*E-mail: rakemp@unm.edu. Phone: (505) 277-6655. (R.A.K.)
ORCID
Timothy J. Boyle: 0000-0002-1251-5592
Jeffery A. Greathouse: 0000-0002-4247-3362
Richard A. Kemp: 0000-0002-2063-3812
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Laboratory Directed Research
and Development program at Sandia National Laboratories,
and the authors gratefully acknowledge the use of the Bruker X-
ray diffractometer purchased via the National Science
Foundation CRIF:MU at the Univ. of New Mexico (CHE04-
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