Synthesis, characterization and reactivity of (dithiolato)indium complexes

Article abstract of DOI:10.1016/j.poly.2017.06.053

We have synthesized indium complexes incorporating tetradentate dithiolate ligands. The 1:1 reaction of InX₃ (X?=?OAc, NO₃) and the corresponding dithiol or dithiolate yielded the compounds [(SOOS)In(py)(NO₃)] (1), [(SNNS)In(OAc)] (2), [In(μ-SNNS)₂(μ-OMe)In][NO₃] (3), [(SNNSPr)In(OAc)] (4), [(NNS₂)In(OAc)] (5) and [(NNS₂)In(NO₃)] (6) [H₂(SOOS)?=?2,2′-(ethylenedioxy)diethanethiol; H₂(SNNS)?=?N,N′-dimethyl-N,N′-bis(2-mercaptoethyl)ethylenediamine; H₂(SNNSPr)?=?N,N′-diethyl-N,N′-bis(2-mercaptoethyl)propanediamine; H₂(NNS₂)?=?N,N-diethyl-N′,N′-bis(2-mercaptoethyl)ethanediamine]. The solid-state structures of 1, 2 and 4–6 are mononuclear and show a tetradentate SOOS/SNNS/NNS₂ ligand and a distorted octahedral (1) or trigonal bipyramidal (2, 4–6) coordination geometry at indium. Compound 3 is dinuclear, with the indium centres bridged by a -OMe oxygen atom and a thiolate sulfur atom of chelating tetradentate ligands, respectively. InX₃ (X?=?Cl, NO₃) were found to be useful Lewis acid catalysts for the aldol reaction of benzaldehyde and 1-(trimethylsiloxy)cyclohexene under ambient conditions, while compounds 1–6 show moderate activity as catalysts for the esterification of stearic acid and transesterification of methyl stearate and glyceryl trioctanoate.

Full text of DOI:10.1016/j.poly.2017.06.053

Accepted Manuscript
Synthesis, Characterization and Reactivity of (Dithiolato)indium Complexes
Timothy S. Anderson, Glen G. Briand, Ralf Brüning, Andreas Decken, Matthew
J. Margeson, Heidi M. Pickard, Eric E. Trevors
PII:
S0277-5387(17)30481-3
DOI:
http://dx.doi.org/10.1016/j.poly.2017.06.053
POLY 12734
Reference:
Polyhedron
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Accepted Date:
31 March 2017
30 June 2017
Please cite this article as: T.S. Anderson, G.G. Briand, R. Brüning, A. Decken, M.J. Margeson, H.M. Pickard, E.E.
Trevors, Synthesis, Characterization and Reactivity of (Dithiolato)indium Complexes, Polyhedron (2017), doi:
http://dx.doi.org/10.1016/j.poly.2017.06.053
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Synthesis, Characterization and Reactivity of (Dithiolato)indium Complexes
Timothy S. Andersona, Glen G. Brianda*, Ralf Brüning,b Andreas Deckenc, Matthew J.
Margesona, Heidi M. Pickarda and Eric E. Trevors
a Department of Chemistry and Biochemistry, Mount Allison University,
Sackville, New Brunswick, Canada E4L 1G8
b Department of Physics, Mount Allison University,
Sackville, New Brunswick, Canada E4L 1E6
c Department of Chemistry, University of New Brunswick,
Fredericton, New Brunswick, Canada E3B 6E2
* To whom correspondence should be addressed. Tel: (506) 364-2346.
Fax: (506) 364-2313. E-mail: gbriand@mta.ca
1

Abstract
We have synthesized indium complexes incorporating tetradentate dithiolate ligands. The 1:1
reaction of InX3 (X = OAc, NO3) and the corresponding dithiol or dithiolate yielded the
compounds [(SOOS)In(py)(NO3)] (1), [(SNNS)In(OAc)] (2), [In(μ-SNNS)2(μ-OMe)In][NO3]
(3), [(SNNSPr)In(OAc)] (4), (NNS2)In(OAc) (5) and (NNS2)In(NO3) (6) [H2(SOOS) = 2,2’-
(ethylenedioxy)diethanethiol; H2(SNNS) = N,N’-dimethyl-N,N‘-bis(2-
mercaptoethyl)ethylenediamine; H2(SNNSPr) = N,N’-diethyl-N,N‘-bis(2-
mercaptoethyl)propanediamine; H2(NNS2) = N,N-diethyl-N’,N‘-bis(2-
mercaptoethyl)ethanediamine]. The solid-state structures of 1, 2 and 4-6 are mononuclear and
show a tetradentate SOOS/SNNS/NNS2 ligand and a distorted octahedral (1) or trigonal
bipyramidal (2, 4-6) coordination geometry at indium. Compounds 3 is dinuclear, with the indium
centres bridged by a -OMe oxygen atom and a thiolate sulfur atom of chelating tetradentate
ligands, respectively. InX3 (X = Cl, NO3) were found to be useful Lewis acid catalysts for the
aldol reaction of benzaldehyde and 1-(trimethylsiloxy)cyclohexene under ambient conditions,
while compounds 1-6 show moderate activity as catalysts for the esterification of stearic acid and
transesterification of methyl stearate and glyceryl trioctanoate.
2

1. Introduction
A major focus in green chemistry over the past two decades has been the development of
synthetic methods that can be carried out in ambient atmosphere conditions and use benign
solvents, notably water [[1]]. To this end, several inorganic metal salts have been recognized as
useful water-tolerant Lewis acid catalysts for a variety of reactions [[2]]. Notable is the seminal
work of Kobayashi et al., who employed the Mukaiyama aldol C-C bond formation reaction in
thf:water to compare the utility of group 1-15 metal chlorides and triflates as water stable Lewis
acids [[3],[4]] The most effective catalysts were rare earth triflates, with main group metal salts
generally performing poorly due to their tendency to readily undergo hydrolysis reactions and
form metal oxides.
Since this time, compounds of environmentally benign main group metals, such as indium
salts (e.g. chloride, bromide, acetate, triflate), have been successfully employed as water tolerant
Lewis acids for a variety of reactions [[5],[6],[7]]. However, very little work has been carried out
with organometallic indium complexes, which provide possible tunability of catalytic activity
through modification of organic ligands [[8]]. We have previously explored the ability of Lewis
acidic methylindium dithiolates to facilitate the ring opening polymerization of cyclic esters
[[9],[10]]. We now report the synthesis and characterization of a series of (dithiolato)indium
acetate/nitrate compounds (1-6), which possess hydrolytically stable In-S bonds. We have
screened these as potential water-tolerant Lewis acid catalysts using the Mukaiyama aldol
reaction, and examined their ability to facilitate the esterification or transesterification of fatty
acids or triglycerides, respectively, which are important reactions for the production of biodiesel
from waste oils.
3

Structural drawings of (di)thiol proligands and 1-6 near here.
2. Results and discussion
2.1 Synthesis of 1-6
Compounds 1-6 were prepared via the 1:1 metathesis reaction of the in situ generated
sodium or potassium dithiolate ligand and indium nitrate, or the dithiol and indium acetate, in
methanol. Reactions were stirred at 23°C for 18 h (1) or heated to reflux for 3 h (2-6), filtered,
and the product crystallized from the reaction filtrate in 38-93 % yield. Six equivalents of
pyridine were added to the reaction of H2(SOOS) and In(NO3)3 in attempt to increase solubility of
the product and promote crystallization, and resulted in formation of the pyridine adduct
[(SOOS)In(py)(NO3)] (1). The reaction of In(NO3)3 with H2(SNNS) resulted in the isolation of
the dinuclear product [(SNNS)In(μ-OMe)In(SNNS)][NO3] (3), respectively. The compound
incorporates a bridging –OMe ligand (vide infra), which results from deprotonation of a methanol
solvent molecule. This appears to be a favorable process given that the reaction does not include
the addition of base.
Despite several attempts, satisfactory elemental analysis data could not be obtained for 3
and 6. Low mass percentage C, H and S values and high N values suggests contamination by
alkali metal nitrate. However, a higher purity product was achieved using KOH versus NaOH as
a base, which yields KNO3 rather than NaNO3 as byproduct. The former precipitates more readily
from the reaction mixture, while the latter co-crystallizes with the product. Due to insolubility,
compound 3 could not be characterized by solution NMR. However, X-ray powder diffraction
4

data confirm the identity of the bulk sample and the presence of a minor amount of amorphous
impurity (see Supplementary data).
2.2 X-ray crystal structures
Crystals suitable for X-ray crystallographic analysis were isolated by cooling the
corresponding reaction mixture at -15°C (1, 3-6) or 4°C (2). X-ray crystal structures are
presented in Figures 1-7. Selected bond distances and angles are included in the figure captions.
The structure of [(SOOS)In(py)(NO3)] (1) (Figure 1) shows indium bonded to a
tetradentate (SOOS)2- ligand, a pyridine nitrogen atom, and a chelating nitrato-κ2O,O’ group. The
geometry at the metal is distorted octahedral if the nitrate group is considered to occupy one
coordination site [S1-In1-S2 = 172.51(3), O1-In1-N1 = 144.55(6), O2-In1-N2 = 162.63(6)]. The
In1-O1 bond distance [2.514(2) Å] is significantly longer than the In1-O2 bond distance [2.468(2)
Å], suggesting a weaker trans influence from the neutral pyridine ligand [O1-In1-N1 =
144.55(6)°] than the anionic nitrate ligand [O2-In1-O3 = 144.0(1)°; O2-In1-O4 = 155.8(1)°; O2-
In1-N2 = 162.63(6)°]. The nitrate ligand is nearly symmetrically bonded to the indium centre
[In1-O3 = 2.443(2), In1-O4 = 2.426(2)) Å].
The structure of [(SNNS)In(OAc)] (2) (Figure 2) shows indium bonded to a tetradentate
(SNNS)2- ligand and a chelating acetate-κ2O,O’ group. The geometry at the metal is distorted
trigonal bipyramidal if the acetate group is considered to occupy one coordination site. An amine
atom and a thiolate sulfur atom occupy axial positions [S2-In1-N1 = 158.36(7)°] and an amine
nitrogen, a thiolate sulfur and the acetate group occupy equatorial positions [S1-In1-N2 =
109.01(8), S1-In1-C9 = 131.4(1), N2-In1-C9 = 117.0(1)°]. The assumption that the acetate
5

group occupies a single coordination site seems reasonable as the structure of [(SNNS)InMe]
yields a very similar distorted trigonal bipyramidal structure with a monodentate methyl group
substituted for the acetate group [Error! Bookmark not defined.]. The cis orientation of the sulfur
atoms of the dithiolate ligand [S1-In1-S2 = 108.62(3)°] is in contrast to the trans orientation in 1
[S1-In1-S2 = 172.51(3)°] and similar to the related complexes (BAT-TM)InX [H2(BAT-TM) =
tetramethylbis(aminoethanethiolate); X = Cl, NCS-N, O2CPh- κ2O,O’], in which the thiolate
sulfur atoms of the tetradentate-SNN’S’ ligand occupy cis positions [S-In-S = 105.7(2)-107.5(1)°]
[[11]] The axial bond distances [In1-S2 = 2.479(1) and In1-N1 =2.386(3) Å] are larger than the
corresponding equatorial bond distances [In1-S1 = 2.451(1) and In1-N2 [2.296(3) Å],
presumably as a result of trans influence in the former. The acetate ligand is asymmetrically
bonded to the indium centre [In1-O1 = 2.430(3) Å; In1-O2 = 2.205(2) Å].
The structure of [In(μ-SNNS)2(μ-OMe)In][NO3] (3) (Figure 3) shows a dinuclear
monocation with one crystallographically unique indium centre. The metal centres are each
chelated by a tetradentate (SNNS)2- ligand, and are bridged by one sulfur atom from each
dithiolate and an –OMe ligand. This results in a distorted octahedral geometry at indium [S1-In1-
S2 = 176.33(4), N1-In1-S1* = 148.9(1), N2-In1-O1 = 168.1(1)°]. Unlike 2, the sulfur atoms of
the (SNNS)2- ligand are in trans positions. The In-S bond distance to the bridging sulfur atom
(S1) is significantly larger [In1-S1 = 2.696(1) versus In1-S2 = 2.446(1) Å], and is larger than the
In1-S1* bond distance [2.614(1) Å].
The structure of [(SNNSPr)In(OAc)] (4) (Figure 4) is very similar to that of 2, and shows
small changes in metrics due to the propyl- versus ethyl- ligand backbone. The N1-In1-N2 bond
angle increases from 76.8(1)° in 2 to 89.3(2)° in 4, affecting an axial S2-In1-N1 bond angle
increase from 158.36(7)° in 2 to 172.7(1)° in 4. Also, In-S and In-N bond distances in 4 [In1-S1
6

= 2.492(1), In1-S2 = 2.460(1), In1-N1 = 2.414(4), In1-N2 = Å] are slightly longer than the
corresponding values in 2, and the acetate ligand is more symmetrically bonded to the indium
centre [In1-O1 = 2.270(3), In1-O2 = 2.315(4) Å]. The asymmetric unit also contains the
enantiomer as a crystallographically unique molecule. It exhibits similar bond distances and angles
(see Table S1).
The structure of (NNS2)In(OAc) (5) (Figure 5) shows indium bonded to a tetradentate
(NNS2)2- ligand and a chelating acetate-κ2O,O’ group. The geometry at the metal centre is
distorted trigonal bipyramidal if the acetate group is considered to occupy one coordination site.
The central ligand amine nitrogen atom and the acetate ligand occupy axial sites [N1-In1-C11 =
169.1(1)°] and two thiolate sulfur atoms and the ligand terminal amine nitrogen atom occupy
equatorial positions [S1-In1-S2 = 135.34(5), S1-In1-N2 = 109.3(1), S2-In1-N2 = 106.7(1)°].
The In1-N1 bond distance [2.449(4) Å] is larger than the In1-N2 [2.353(4) Å], presumably as a
result of the trans influence of the acetate ligand. The acetate ligand is asymmetrically bonded to
the indium centre [In1-O1 = 2.162(4) Å; In1-O2 = 2.693(4) Å].
The structure of (NNS2)In(NO3) (6) (Figure 6) is very similar to that of 5, and shows
small changes in metrics due to the presence of the nitrato-κ2O,O’ versus acetate ligand. The
geometry at the metal centre is also distorted trigonal bipyramidal if the nitrate group is
considered to occupy one coordination site. The geometry at indium in 6 is closer to an ideal
trigonal bipyramid than in 5, with an axial-axial bond angle closer to 180° [N1-In1-N3 =
174.1(1)°] and equatorial bond angles closer to 120° [S1-In1-S2 = 127.47(3), S1-In1-N2 =
116.96(6), S2-In1-N2 = 110.80(6)°]. The equatorial bond distances [In1-S1 = 2.4156(8), In1-S2
= 2.4077(9), In1-N2 = 2.329(2) Å] are slightly shorter than in 5, while the axial In1-N1 bond
distance [2.364(2)] is significantly shorter. This is a result of the more weakly coordinated nitrate
7

ligand [In1-O1 = 2.234(2), In1-O2 = 2.851(4) Å]. Compounds 1 and 6 are very rare examples of
indium complexes containing a chelating nitrato-κ2O,O’ group [[12]].
The In-S bond distances in compounds 2-5 are in the range of previously reported indium
bis- and tris(thiolates) [In-S = 2.4165(6)-2.543(2) Å] [Error! Bookmark not defined.,Error!
Bookmark not defined.,[13],[14],[15]]. However, those of compounds 1 [In1-S1 = 2.4130(7),
In1-S2 = 2.4109(7) Å] and 6 [In1-S1 = 2.4156(8), In1-S2 = 2.4077(9) Å] are at the low end of
this range or are shorter. This is presumably a result of the weak interaction of the nitrate anion,
yielding a more cationic indium centre and stronger In-S bonding interactions. The structure of 3
resembles those of a series of dinuclear [(NNO)InCl(μ-OEt)(μ-Cl)InCl(NNO)] complexes
reported by Mehrkodavandi et al., which are efficient catalysts for the ring opening
polymerization of cyclic esters [[16]]. Like 3, these complexes incorporate a single bridging -OR
group. However, compound 3 differs in that the indium centres are triply bridged and the
dinuclear complex is cationic.
2.3 Aldol reactions
The Mukaiyama aldol reaction was chosen to screen InCl3, In(OAc)3, In(NO3)3 and
(thiolato)indium compounds 1-6 as water stable catalysts due to the availability of data for a large
number of group 1-15 chlorides and triflates for comparison [Error! Bookmark not
defined.,Error! Bookmark not defined.]. Results are reported in Table 1. Kobayashi et al.
reported a 68% product yield with 20 mol% InCl3 catalyst in 9:1 thf:water [Error! Bookmark not
defined.], while Loh et al. obtained a 79% product yield with addition of water only [[17]]. The
latter procedure required "purification" of the InCl3, which involved dissolution of InCl3 in water,
8

filtration and removal of water in vacuo, and likely resulted in the formation of indium
aqua/hydroxy chloride [[18]]. Our trials of the aldol reaction of benzaldehyde and 1-
(trimethylsiloxy)cyclohexene with 20 mol% InCl3 catalyst using the methods of both Kobayashi
and Loh gave low product yields. We subsequently discovered that when the reaction using
Loh’s “purified” InCl3 as catalyst was carried out neat, i.e. without addition of solvent or water, a
quantitative yield of product was obtained. For comparison, analogous reactions using In(OAc)3
and In(NO3)3 were also carried out. In(OAc)3 gave a trace amount of product, while In(NO3)3
gave near quantitative conversion with a 10 mol% catalyst loading. Further, decreasing In(NO3)3
catalyst loading to 5 mol% gave only a trace amount of product. Decreasing the amount of 1-
(trimethylsiloxy)cyclohexene to 1.5 equivalents did not affect yield significantly with 10 mol%
In(NO3)3, while decreasing to 1 equivalent gave a 47% yield. With 10 mol% catalyst loading and
2 equivalents of 1-(trimethylsiloxy)cyclohexene, compounds 1-6 gave only trace amounts of
product.
2.4 Esterification and transesterification reactions
Waste greases and oils are cheap and renewable sources of triglycerides that can be
converted into fatty acid methyl esters (FAME), otherwise known as biodiesel [[19]].
Conventional transesterification processes usually involve strongly acidic or basic solutions as
homogeneous catalysts [[20]]. However, free fatty acid (FFA) impurities react with basic
solutions to form soaps, which makes separation of glycerol problematic [[21]]. Further, the
presence of water leads to hydrolysis of oils and FAME in the presence of strong acids and bases
[[22]]. Therefore, ideal homogeneous catalysts for biodiesel production from waste oils must
9

catalyze esterification reactions of carboxylic acids (fatty acids) with alcohol, as well as
transesterification reactions of triglycerides. We have screened indium salts and compounds 1-6
as homogeneous catalysts for these reactions.
The esterification of stearic acid to methyl stearate was carried out in refluxing methanol
for 4 hours using indium salts and compounds 1-6 as catalysts (Table 2). InCl3 and In(NO3)3 gave
moderate and high yields of 61% and 89%, respectively. Of the indium (di)thiolate compounds,
compound 2 gave a moderate yield (41%), while low yields of 4-28% were found for 1 and 3-6.
The dithiolatoindium acetate compounds 2, 4 and 5 all give higher yields than In(OAc)3, while the
nitrate compounds 1, 3 and 6 give lower yields than In(NO3)3.
The transesterification of methyl stearate to butyl stearate was carried out in refluxing n-
butanol for 19 hours using indium salts and compounds 1-6 as catalysts (Table 3). Both InCl3 and
In(NO3)3 showed near quantitative conversions. Indium thiolate compounds 1-6 gave very high
yields (84-100%), with the exception of compound 3. The reaction was subsequently carried out
for 4 hours with those compounds affording ≥90% product yield, i.e. InCl3, In(NO3)3, 2 and 4-6.
Compound 2 gave the highest yield (81%), outperforming both indium salts.
Finally, the transesterification of glyceryl trioctanoate to methyl stearate was carried out in
refluxing methanol for 19 hours using indium salts and compounds 1-6 as catalysts (Table 4).
In(NO3)3 was the only salt that afforded any appreciable amount of product (23%). Indium
thiolate compounds 1-6 gave very low yields (<1-13%), with the exception of compound 2, which
gave a moderate yield of 31%.
3. Conclusions
10

The stoichiometric reaction of indium salts with polydentate dithiol(ate)s in methanol is a
facile route to the synthesis of mononuclear indium dithiolates or dinuclear indium dithiolates
incorporating bridging –OMe groups. Our optimization of the Mukaiyama aldol reaction with
(hydrolyzed) InCl3 and In(NO3)3 showed that the reaction was best carried out neat in ambient
conditions and without the addition of water or thf solvent, giving near quantitative yields.
Indium dithiolates 1-6 were not effective catalysts for this reaction, but show moderate reactivity
for esterification/transesterification reactions of fatty acids, fatty acid esters and triglycerides.
InCl3 and In(NO3)3 were found to give higher yields than 1-6 as catalysts for the esterification of
stearic acid. However, compound 2 gave the highest yields for the transesterification of methyl
stearate to butyl stearate and the transesterification of glyceryl trioctanoate to methyl octanoate.
Although low overall, this shows that the reactivity of the indium complex can be altered
significantly with the choice of thiolate ligand, affording complexes that are more effective Lewis
acid catalysts than commonly employed indium salts. We are currently studying other thiolate
ligand frameworks and catalyzed chemical reactions.
4. Experimental
4.1 General Considerations
Solution 1H and 13C{1H} spectra were recorded at 23°C on either a JEOL GMX 270 MHz
+ spectrometer (270 and 67.9 MHz, respectively), or a Varian Mercury 200 MHz + spectrometer
(200 and 50 MHz, respectively), and chemical shifts are calibrated to the residual solvent signal.
ATR FT-IR spectra were recorded on a Thermo Nicolet iS5 FT-IR spectrometer in the range of
11

4000-400 cm-1. FT-Raman spectra were recorded on a Thermo Nicolet NXR 9600 Series FT-
Raman spectrometer in the range 3900-70 cm-1. Melting points were recorded on an
Electrothermal MEL-TEMP melting point apparatus and are uncorrected. Elemental analyses
were performed by Laboratoire d'analyse élémentaire, Université de Montréal, Montreal, Canada.
Indium(III) chloride (98%), indium (III) nitrate hydrate (99.9%), indium(III) acetate
(99.99%), 2,2’-(ethylenedioxy)diethanethiol (95%), N,N’-dimethylethylenediamine 85%, N,N’-
diethyl-1,3-propanediamine 97%, 2- N,N-diethylethylenediamine 99%, ethylene sulfide (98%),
benzaldehyde (>99%), 1- cyclohexeneoxy(trimethylsiloxy) (99 %), stearic acid (98.5%), methyl
stearate (99%), 1-butanol and glyceryl trioctanoate (≥99 %) were used as received from Sigma-
Aldrich. Sodium hydroxide (97.0%) and potassium hydroxide (97.0%) was used as received from
Caledon. H2(SNNS), H2(SNNSPr) [[23]], and H2(NNS2) [[24]], were prepared according to
literature methods.
4.2 Synthesis
4.2.1 [(SOOS)In(py)(NO3)] (1).
Under a stream of dinitrogen, pyridine (0.528g, 6.68 mmol) and In(NO3)3 (0.427g, 1.11
mmol) were added to MeOH (15 mL). After 15 minutes, H2(SOOS) (0.200 g, 1.10 mmol) was
added and the reaction mixture was left to stir for 19 h. The solution was filtered and the filtrate
was allowed to sit at -15°C. After 3 d the reaction mixture was filtered to yield 4 as colourless
crystals of (0.429 g, 1.02 mmol, 93%). Anal. Calc. for C11H17InN2O5S2: C, 30.29; H, 3.93; N,
6.42; S, 14.70. Found: C, 29.97; H, 3.87; N, 6.51; S, 13.82. M.p. = 166-169 °C. FT-IR (cm-1):
632 m, 669 s, 694 m, 755 m, 797 w, 811 w, 878 w, 932 w, 995 w, 994 w, 1026 m, 1065 w, 1159
12

w, 1192 w, 1203 w, 1217 w, 1262 m, 1275 w, 1345 m, 1470 s, 1603 m, 2856 w, 2936 w. FT-
Raman (cm-1): 120s, 140s, 214m, 284m, 340vs, 500vw, 649w, 671m, 1011s, 1037m, 1221w,
1
1278w, 1448w, 1603w, 2880w, 2940m, 3971w, 3071m. H NMR (dmso-d6, ppm): 3.57 (m,
12H, CH2), 7.37 (m, 2H, m-NC5H5), 7.77 (tt, JH-H = 7.8, 2.0 Hz, 1H, p-NC5H5), 8.55 (dd, JH-H
3.8, 1.6 Hz, 2H o-NC5H5). 13C{1H} NMR (dmso-d6, ppm): 25.7 (SCH2), 66.2 (SCH2CH2O),
69.9 (OCH2CH2O), 123.6 (m-NC5H5), 135.8 (p-NC5H5), 149.2 (o-NC5H5).
=
4.2.2 [(SNNS)In(OAc)] (2).
Under a stream of dinitrogen, 0.350 g (1.20 mmol) In(OAc)3 was added to a solution of
0.250 g (1.20 mmol) H2(SNNS) in MeOH (10 mL). The reaction mixture was heated to reflux
for 20 h and hot filtered. The filtrate was allowed to sit at 4°C. After 6 d the solution was
filtered to yield 3 as colorless crystals (0.266 g, 0.699 mmol, 58%) Anal. Calc. for
C10H21InN2O2S2: C, 31.59; H, 5.57; N, 7.37; S, 16.87. Found: C, 31.50; H, 5.62; N, 7.26; S,
16.72. M.p. = 155 °C. FT-IR (cm-1): 505 w, 627 m, 685 s, 709 w, 737 m, 946 w, 1014 m, 1043
m, 1095 w, 1198 w, 1447 s, 1526 m, 2930 w. FT-Raman (cm-1): 124m, 187m, 231w, 261w,
1
311vs, 390w, 549w, 678m, 947m, 1052w, 1306w, 1454m, 2929s. H NMR (CDCl3, ppm): 2.10
(s, 3H, InCO2CH3), 2.46 (s, 6H, NCH3), 2.91 (m, 12H, CH2). 13C{1H} NMR (CDCl3,
ppm): 20.4 (CO2CH3), 22.2 (SCH2), 52.2 (SCH2CH2N), 53.5 (NCH2CH2N), 181.9
(CO2CH3).
4.2.3 [(SNNS)In(μ-OMe)In(SNNS)][NO3] (3).
Under a stream of nitrogen, 0.361 g (1.20 mmol) In(NO3)3 was added to a solution of
0.250 g (1.20 mmol) H2(SNNS) and 0.134 g (2.40 mmol) KOH in MeOH (10 mL). The reaction
mixture was heated to reflux for 20 h, then allowed to sit at -15°C. After 5 d the solution was
13

filtered to yield 3 as colorless crystals (0.162 g, 0.219 mmol, 37%). M.p. = 222 °C. FT-IR (cm-
1): 665 m, 754 s, 830 m, 958 s, 1007 s, 1043 s, 1076 m, 1281 s, 1313 s, 1449 m, 2889 w. FT-
Raman (cm-1): 111s, 185s, 240m, 255m, 318s, 341m, 364w, 457m, 667m, 757w, 940w, 959w,
1039m, 1136vw, 1232w, 1283w, 1444m, 2926s, 2947s. NMR data could not be obtained due to
low solubility. Despite several attempts, satisfactory elemental analysis data could not be
obtained.
4.2.4 [(SNNSPr)In(OAc)] (4).
Under a stream of dinitrogen, 0.291 g (0.998 mmol) In(OAc)3 was added to a solution of
0.250 g (0.998 mmol) H2(SNNSPr) in MeOH (10 mL). The reaction mixture was heated to
reflux for 19 h and hot filtered. The filtrate was allowed to sit at -15°C. After 2 d the solution
was filtered to yield 4 as colorless crystals (0.291 g, 0698 mmol, 70%). Anal. Calc. for
C13H27InN2O2S2: C, 36.97; H, 6.44; N, 6.63; S, 15.18. Found: C, 36.84; H, 6.54; N, 6.52; S,
15.53. M.p. = 191 °C. FT-IR (cm-1): 455 w, 521 w, 627 m, 685 s, 709 w, 737 m, 946 w, 1015
w, 1043 m, 1095 w, 1198 w, 1448 s, 1527 m, 2930 w. FT-Raman (cm-1): 144s, 172m, 204s,
284m, 316vs, 342s, 380m, 471m, 530w, 670s, 744w, 764w, 942m, 1211w, 1246w, 1279w,
1
1437w, 1456m, 2823w, 2921s, 3012w. H NMR (CDCl3, ppm): 0.96 (t, JH-H = 8.2 Hz, 6H
NCH2CH3), 2.10 (s, 3H, InCO2CH3), 2.96 (m, 18H, CH2). 13C{1H} NMR (CDCl3, ppm): 20.5
(CO2CH3), 22.2 (SCH2), 30.9 (NCH2CH2CH2N), 52.2 (SCH2CH2N), 53.5 (NCH2CH2CH2N),
181.9 (CO2CH3).
4.2.5 (NNS2)In(OAc) (5).
14

Under a stream of dinitrogen, 0.292 g (0.998 mmol) In(OAc)3 was added to a solution of
0.250 g (0.998 mmol) H2(NNS2) in MeOH (10 mL). The reaction mixture was heated to reflux
for 23 h, then allowed to sit at -15°C. After 1 d the solution was filtered to yield 2 as colorless
crystals (0.223 g, 0.566 mmol, 57%). Anal. Calc. for C12H25InN2O2S2: C, 35.30; H, 6.17; N,
6.68. Found: C, 35.34; H, 6.34; N, 6.87. M.p. = 172 °C. FT-IR (cm-1): 506 m, 599 w, 620 w,
670 s, 722 w, 743 m, 887 w, 930 w, 977 w, 1000 m, 1047 w, 1095 s, 1300 m, 1330 s, 1383 s,
1446 m, 1473 w, 1581 m, 2982 w. FT-Raman (cm-1): 111s, 185vs, 240m, 256m, 318s, 339m,
1
363m, 458m, 543w, 667m, 757w, 941w, 1029m, 1232w, 1284w, 1443m, 2926s, 2947s. H
NMR (CDCl3, ppm): 1.25 (t, JH-H = 7.0 Hz, 6H, NCH2CH3), 1.97 (s, 3H, InCO2CH3) 2.76 (m,
16H, CH2). 13C{1H} NMR (CDCl3, ppm): 9.15 (NCH2CH3), 22.8 (SCH2), 23.5 (CO2CH3), 44.5
(NCH2CH3), 49.5 (SCH2CH2N), 57.5 (NCH2CH2N) 181.7 (CO2CH3).
4.2.6 (NNS2)In(NO3) (6).
Under a stream of dinitrogen, 0.300 g (0.998 mmol) In(NO3)3 and 0.250 g (0.998 mmol)
H2(NNS2) were added to a solution of 0.112 g (0.200 mmol) KOH in MeOH (10 mL). The
reaction mixture was heated to reflux for 21 h, then allowed to sit at -15°C. After 2 d the solution
was filtered to yield 3 as colorless crystals (0.223 g, 0.566 mmol, 57%). M.p. = 172 °C. FT-IR
(cm-1): 663 w, 723 m, 745 m, 809 m, 824 w, 975 w, 1009 s, 1053 w, 1093 m, 1284 vs, 1354 m,
1444 m, 1477 m, 2853 w. FT-Raman (cm-1): 183s, 208s, 240m, 318s, 341m, 364w, 458m, 544w,
1
676m, 758w, 958w, 1038m, 1136w, 1284w, 1443m, 297s, 2948s. H NMR (dmso-d6, ppm):
1.28 (t, 6H, NCH2CH3), 2.92 (m, 16H, CH2). 13C{1H} NMR (dmso-d6, ppm): 10.19 (NCH2CH3),
20.2 (SCH2), 44.5 (NCH2CH3), 51.1 (SCH2CH2N), 56.4 (NCH2CH2N). Despite several attempts,
satisfactory elemental analysis data could not be obtained.
15

4.3 Catalysis
4.3.1 Aldol reaction
5, 10 or 20 mol % of the indium precatalyst and 0.106 g, 1 mmol benzaldehyde were
stirred together for 30 minutes. 1, 2, or 3 mmol of 1-(trimethylsiloxy)cyclohexene was added
and the reaction mixture was stirred at 23°C for 18 h. The resulting product was analyzed by 1H
NMR spectroscopy in CDCl3. Alternatively, 5 mL of water was added and the mixture stirred for
15 min. 15 mL CH2Cl2 was then added and the mixture stirred for 15 min. The organic layer was
removed, dried over anhydrous magnesium sulfate, and filtered. The solution was washed
through ~1-2 cm of silica gel (dessicant, 28-200 mesh) with 30 mL CH2Cl2. The solvent was
then removed in vacuo and the resulting product analyzed by 1H NMR spectroscopy in CDCl3.
Signals for the anti (5.40 ppm, d, 3J = 2.5 Hz) and syn (4.83 ppm, d, 3J = 9.0 Hz) products were
integrated against that of benzaldehyde (10.00 ppm) to determine percent yield and isomeric ratio.
4.3.2 Esterification of stearic acid
To a solution of stearic acid (0.100 g, 0.352 mmol) in MeOH (1 mL) was added 10 mol %
of the desired catalyst (0.035 mmol). The resulting solution was heated to reflux for 4 h, after
which the excess methanol was removed under reduced pressure. The residue was extracted with
diethyl ether and the solvent was removed under vacuum to yield the crude product. Samples
were analyzed by 1H NMR spectroscopy in CDCl3 to determine conversion of stearic acid to
methyl stearate.
16

4.2.3 Transesterification of methyl stearate
Methyl stearate (0.100g, 0.335 mmol) was dissolved in 1-butanol (1 mL) with subsequent
addition of 10 mol % of the desired catalyst (0.035 mmol). The resulting solution was heated at
reflux for 4-18 h, after which the solvent was removed under vacuum. The residue was extracted
with diethyl ether and the solution washed with distilled water (5 mL). The organic layer was
then dried over anhydrous sodium sulfate and the solvent removed under vacuum to yield the
crude product. Samples were analyzed by 1H NMR spectroscopy in CDCl3 to determine
conversion of methyl stearate to methyl stearate.
4.3.4 Transesterification of glyceryl trioctanoate
Glyceryl trioctanoate (0.150g, 0.319 mmol) was dissolved in MeOH (1 mL) with
subsequent addition of 10 mol % of the desired catalyst (0.032 mmol). The resulting solution was
refluxed for 4-18 h, after which excess alcohol was removed under vacuum. The residue was
extracted with diethyl ether, washed with distilled water (5 mL), dried over anhydrous sodium
sulfate, and the solvent removed under vacuum to yield the crude product. Samples were
analyzed by 1H NMR spectroscopy in CDCl3 to determine conversion of glyceryl trioctanoate to
methyl stearate.
4.4 X-ray crystallography
Crystals of 1- were isolated from the reaction mixtures as indicated above. Single crystals
were coated with Paratone-N oil, mounted using a polyimide MicroMount and frozen in the cold
nitrogen stream of the goniometer. A hemisphere of data was collected on a Bruker AXS
17

P4/SMART 1000 diffractometer using  and  scans with a scan width of 0.3  and 10 s exposure
times. The detector distance was 5 cm. The data were reduced (SAINT) [[25]] and corrected for
absorption (SADABS) [[26]]. The structures were solved by direct methods and refined by full-
matrix least squares on F2(SHELXTL) [[27]]. All non-hydrogen atoms were refined using
anisotropic displacement parameters. Hydrogen atoms were included in calculated positions and
refined using a riding model, except for the methanol hydrogen atoms that were omitted (6).
Crystallographic data are found in Table 5.
For 2, all of the C atoms were disordered over two positions and the site occupancies
determined using an isotropic model as 0.7 [C(1)-C(8)] and 0.3 [C(1A)-C(8A)] and fixed in
subsequent refinement cycles. Thermal parameters for the disordered pairs were constrained and
the bond distances within the equivalent bonds restrained. For 3, the methanol molecule was
disordered and the site occupancies determined using an isotropic model as 0.5 (C(10), C(11),
O(10), O(11)) and fixed in subsequent refinement cycles. For 4, the crystal was a multiple twin
and the orientation matrix of the major component was determined (CELL_NOW) [[28]]. For 5
and 6, the crystal was twinned and the orientation matrixes for two components were determined
(CELL_NOW).
4.5 X-ray powder diffraction
X-ray powder diffraction (XRD) measurements were carried out in reflection with a
custom built theta/theta diffractometer equipped with a pyrolytic graphite monochromator and
analyzer crystals. Cu K-alpha radiation (l 1⁄4 0.154178 nm) was used for the measurements, in the
5-100° 2θ range with a 0.1° step size and 10 s count time. Air-scattering was avoided by
18

evacuating the sample space and signal originating from the substrate was minimized by placing
the powder on a Si (511) crystal. The data are shown as a function of the modulus of the
scattering vector, q = 4/λ sin θ, where 2θ is the scattering angle. The power pattern was
calculated from single crystal data using Diamond 3.1 software [[29]]. The experimental data was
fit using a Rietveld refinement [[30]].
Appendix A. Supplementary data
Selected bond distances for both crystallographically unique molecules of 4. Experimental and
calculated X-ray powder diffraction (XRD) pattern for 3. CCDC 1526658-1526660 and
1526662-1526664 contains the supplementary crystallographic data for 1-6. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
We thank the following: Dan Durant for assistance in collecting solution NMR data; Delilah
Brown for XRD data collection; and the Natural Sciences and Engineering Research Council of
Canada, the New Brunswick Innovation Foundation, the Canadian Foundation for Innovation and
Mount Allison University for financial support.
19

20

Structural drawings of dithiol proligands.
Structural drawings of 1-6.
21

Figure 1. X-ray structure of 1 (50% probability ellipsoids). Hydrogen atoms are not shown for
clarity. Selected bond distances (Å) and angles (°): In1-S1 = 2.4130(7), In1-S2 = 2.4109(7), In1-
O1 = 2.514(2), In1-O2 = 2.468(2), In1-N1 = 2.340(2), In1-O3 = 2.443(2), In1-O4 = 2.426(2),
S1-In1-O1 = 76.40(4), S2-In1-O2 = 77.73(4), O3-In1-O4 = 52.71(6), O1-In1-O2 = 69.54(5), S1-
In1-S2 = 172.51(3), O1-In1-N1 = 144.55(6), O2-In1-N2 = 162.63(6).
22

Figure 2. X-ray structure of 2 (50% probability ellipsoids). Hydrogen atoms are not shown for
clarity. Selected bond distances (Å) and angles (°): In1-S1 = 2.451(1), In1-S2 = 2.479(1), In1-
N1 = 2.386(3), In1-N2 = 2.296(3), In1-O1 = 2.430(3), In1-O2 = 2.205(2), S1-In1-N1 =
82.90(8), S2-In1-N2 = 82.02(8), N1-In1-N2 = 76.8(1), S1-In1-N2 = 109.01(8), S1-In1-C9 =
131.4(1), N2-In1-C9 = 117.0(1), S2-In1-N1 = 158.36(7).
23

Figure 3. X-ray structure of the cation of 3 (50% probability ellipsoids). Hydrogen atoms are
not shown for clarity. Symmetry transformations used to generate equivalent atoms (*): -x, -y+1,
z. Selected bond distances (Å) and angles (°): In1-S1 = 2.696(1), In1-S2 = 2.446(1), In1-S1* =
2.614(1), In1-N1 = 2.345(4), In1-N2 = 2.333(4), In1-O1 = 2.174(4), S1-In1-N1 = 76.4(1), S2-
In1-N2 = 84.5(1), N1-In1-N2 = 77.9(1), S1-In1-S2 = 176.33(4), N1-In1-S1* = 148.9(1), N2-
In1-O1 = 168.1(1).
24

Figure 4. X-ray structure of 4 (50% probability ellipsoids). Hydrogen atoms are not shown for
clarity. Selected bond distances (Å) and angles (°): In1-S1 = 2.492(1), In1-S2 = 2.460(1), In1-
N1 = 2.414(4), In1-N2 = 2.352(4), In1-O1 = 2.270(3), In1-O2 = 2.315(4), S1-In1-N1 =
172.7(1), S2-In1-N2 = 114.2(1), S2-In1-C12 = 129.7(1), N2-In1-C12 = 114.8(2), N1-In1-N2 =
89.3(2), S1-In1-N2 = 83.5(1), S2-In1-N1 = 81.7(1).
25

Figure 5. X-ray structure of 5 (50% probability ellipsoids). Hydrogen atoms are not shown for
clarity. Selected bond distances (Å) and angles (°): In1-S1 = 2.425(1), In1-S2 = 2.439(1), In1-
N1 = 2.449(4), In1-N2 = 2.353(4), In1-O1 = 2.162(4), In1-O2 = 2.693(4), S1-In1-S2 =
135.34(5), S1-In1-N2 = 109.3(1), S2-In1-N2 = 106.7(1), N1-In1-C11 = 169.1(1).
26

Figure 6. X-ray structure of 6 (50% probability ellipsoids). Hydrogen atoms are not shown for
clarity. Selected bond distances (Å) and angles (°): In1-S1 = 2.4156(8), In1-S2 = 2.4077(9), In1-
N1 = 2.364(2), In1-N2 = 2.329(2), In1-O1 = 2.234(2), In1-O2 = 2.851(4), S1-In1-S2 =
127.47(3), S1-In1-N2 = 116.96(6), S2-In1-N2 = 110.80(6), N1-In1-N3 = 174.50(7).
27

28

Table 1. Catalyzed aldol reaction of benzaldehyde and 1-trimethylsiloxycyclohexene.
Catalyst
n
mol% catalyst
Yield (%) (anti:syn)
100 (73:27)
<1
InCl3 (purified)
In(OAc)3
In(NO3)3
In(NO3)3
In(NO3)3
In(NO3)3
1-6
2
20
20
10
5
2
2
95 (56:44)
<1
2
1.5
1
10
10
10
93 (46:54)
47 (45:55)
<1
2
29