Tin(ii) amide/alkoxide coordination compounds for production of Sn-based nanowires for lithium ion battery anode materials

Article abstract of DOI:10.1039/c2dt30772c

A series of tin(ii) amide alkoxides ([(OR)Sn(NMe₂)]_n) and tin(ii) alkoxides ([Sn(OR)₂]_n) were investigated as precursors for the production of tin oxide (SnO_x) nanowires. The precursors were synthesized from the metathesis of tin dimethylamide ([Sn(NMe₂)₂]₂) and a series of aryl alcohols {H-OAr = H-OC₆H₄(R)-2: R = CH₃ (H-oMP), CH(CH₃)₂ (H-oPP), C(CH₃)₃ (H-oBP)] or [H-OC₆H₃(R)₂-2,6: R = CH₃ (H-DMP), CH(CH₃)₂ (H-DIP), C(CH₃)₃ (H-DBP)]}. The 1:1 products were all identified as the dinuclear species [(OAr)Sn(μ-NMe₂)]₂ where OAr = oMP (1), oPP (2), oBP (3), DMP (4), DIP (5), DBP (6). The 1:2 products were identified as either a polymer ([Sn(μ-OAr)₂]_∞ (where OAr = oMP (7), oPP (8)), dinuclear [(OAr)Sn(μ-OAr)]₂ (where OAr = oBP (9), DMP (10) or DIP/HNMe₂ (11)), or mononuclear [Sn(DBP)₂] (12) complexes. These novel families of compounds (heteroleptic 1-6, and homoleptic 7-12) were evaluated for the production of SnO_x nanowires using solution precipitation (SPPT; oleylamine/octadecene solvent system) or electrospinning (ES; THF solvent) processing conditions. The SPPT route that employed the heteroleptic precursors yielded mixed phases of Sn ^o:romarchite [1 (100:0); 2 (80:20); 3 (68:32); 4 (86:14); 5 (66:35); 6 (88:12)], with a variety of spherical sized particles [1 (350-900 nm); 2 (150-1200 nm); 3 (250-950 nm); 4 (20-180 nm); 5 (80-400 nm); 6 (40-200 nm)]. For the homoleptic precursors, similar phased [7 (80:20); 8 (23:77); 9 (15:85); 10 (34:66); 11 (77:23); 12 (77:23)] spherical nanodots were isolated [7 (50-300 nm); 8: (irregular); 10 (200-800 nm); 11 (50-150 nm); 12 (50-450 nm)], except for 9 which formed polycrystalline rods [Sn^o:romarchite (15:85)] with aspect ratios >100. From ES routes, the heteroleptic species were found to form 'tadpole-shaped' materials whereas the homoleptic species formed electrosprayed nanodots. The one exception noted was for 7, where, without use of a polymer matrix, nanowires of Sn^o, decorated with micron sized 'balls' were observed. Due to the small amount of material generated, PXRD patterns were inconclusive to the identity of the generated material; however, cyclic voltammetry on select samples was used to tentatively identify the final Sn^o (from 7) with the other sample identified as SnO_x (from 1).

Full text of DOI:10.1039/c2dt30772c

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PAPER
Tin(II) amide/alkoxide coordination compounds for production of Sn-based
nanowires for lithium ion battery anode materials†
Timothy J. Boyle,*a Thu Q. Doan,a Leigh Anna M. Steele,a Christopher Apblett,a Sarah M. Hoppe,a
Krista Hawthorne,a Robin M. Kalinicha and Wolfgang M. Sigmundb,c
Received 16th January 2012, Accepted 13th May 2012
DOI: 10.1039/c2dt30772c
A series of tin(II) amide alkoxides ([(OR)Sn(NMe2)]n) and tin(II) alkoxides ([Sn(OR)2]n) were investigated
as precursors for the production of tin oxide (SnOx) nanowires. The precursors were synthesized from the
metathesis of tin dimethylamide ([Sn(NMe2)2]2) and a series of aryl alcohols {H-OAr = H-OC6H4(R)-2:
R = CH3 (H-oMP), CH(CH3)2 (H-oPP), C(CH3)3 (H-oBP)] or [H-OC6H3(R)2-2,6: R = CH3 (H-DMP),
CH(CH3)2 (H-DIP), C(CH3)3 (H-DBP)]}. The 1 : 1 products were all identified as the dinuclear species
[(OAr)Sn(μ-NMe2)]2 where OAr = oMP (1), oPP (2), oBP (3), DMP (4), DIP (5), DBP (6). The 1 : 2
products were identified as either a polymer ([Sn(μ-OAr)2]∞ (where OAr = oMP (7), oPP (8)), dinuclear
[(OAr)Sn(μ-OAr)]2 (where OAr = oBP (9), DMP (10) or DIP/HNMe2 (11)), or mononuclear [Sn(DBP)2]
(12) complexes. These novel families of compounds (heteroleptic 1–6, and homoleptic 7–12) were
evaluated for the production of SnOx nanowires using solution precipitation (SPPT; oleylamine/
octadecene solvent system) or electrospinning (ES; THF solvent) processing conditions. The SPPT route
that employed the heteroleptic precursors yielded mixed phases of Sno : romarchite [1 (100 : 0); 2
(80 : 20); 3 (68 : 32); 4 (86 : 14); 5 (66 : 35); 6 (88 : 12)], with a variety of spherical sized particles
[1 (350–900 nm); 2 (150–1200 nm); 3 (250–950 nm); 4 (20–180 nm); 5 (80–400 nm); 6 (40–200 nm)].
For the homoleptic precursors, similar phased [7 (80 : 20); 8 (23 : 77); 9 (15 : 85); 10 (34 : 66); 11
(77 : 23); 12 (77 : 23)] spherical nanodots were isolated [7 (50–300 nm); 8: (irregular); 10 (200–800 nm);
11 (50–150 nm); 12 (50–450 nm)], except for 9 which formed polycrystalline rods [Sno : romarchite
(15 : 85)] with aspect ratios >100. From ES routes, the heteroleptic species were found to form ‘tadpole-
shaped’ materials whereas the homoleptic species formed electrosprayed nanodots. The one exception
noted was for 7, where, without use of a polymer matrix, nanowires of Sno, decorated with micron sized
‘balls’ were observed. Due to the small amount of material generated, PXRD patterns were inconclusive
to the identity of the generated material; however, cyclic voltammetry on select samples was used to
tentatively identify the final Sno (from 7) with the other sample identified as SnOx (from 1).
However, several drawbacks limit the widespread growth of LIB
technology, such as the need for higher energy capacity, longer
Introduction
Due to their expected high energy density, reasonable cycle life,
long storage times, and ‘green’ status, lithium ion battery (LIB)
power sources are of interest for meeting the consumer’s demand
for cheaper, smaller, and safer but more powerful devices.
cycle life, and faster recharging rates. In particular, the strain the
anode and cathode materials undergo during the cycling of
Li ions can cause significant mechanical failures.1–7 For
example, upon Li ion cycling of Sio (one of the most attractive
LIB anode materials)1,2 undergoes a more than 400% volume
change, which destroys the crystalline material. Recently, a
report by Cui and coworkers demonstrated that Sio nanowires
can survive this cycling strain while maintaining high capacities
(>3500 mAh g−1 for 20 cycles).3 However, several drawbacks
were noted for this system such as irreversible capacity losses,
laborious preparative routes, and limitations to large-scale
production.
aSandia National Laboratories, Advanced Materials Laboratory, 1001
University Boulevard SE, Albuquerque, New Mexico, 87106, USA.
E-mail: tjboyle@Sandia.gov; Fax: +1 (505)272-7336;
Tel: +1 (505)272-7625
bDepartment of Materials Science and Engineering, University of
Florida, Gainesville, Florida 32611-6400, USA
cWCU Department of Energy Engineering, Hanyang University, Seoul,
South Korea
†Electronic supplementary information (ESI) available: Structure plots
of all compounds (1–12 and S1–S3), and the quantitative analyses of the
PXRD of the generated Sno/SnO (17 pages). CCDC 843197–843202,
843204–843212. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30772c
Alternatively, tin oxide (SnOx) has attracted increasing interest
as use as a LIB anode material due to the theoretical value of
990 mAh g−1 expected from the oft-cited Li4.4Sn.4–7 Unfortu-
nately, the formation of this alloy also undergoes a volume
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9349–9364 | 9349

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expansion (300%), which ‘pulverizes’ the material.7 As noted
for the Sio system, Lee and Wang have reported SnOx nanowires
possess a capacity of ∼1100 mAh g−1, however, significant mor-
phological changes were noted upon cycling.8 Since this is
higher than the theoretical capacity of SnO2, we became inter-
ested in generating SnOx nanowires and exploring their utility as
anodes for LIB applications. Previous routes for the production
of SnOx nanowires for LIBs utilized a vapor–liquid–solid (VLS)
mechanism from the thermal evaporation,9,10 hard templating,11
or salt melts.12 Recently, a report by Tatte et al.13 was dissemi-
nated that employs a true sol–gel processing approach by a
‘direct pull’ process of metal tert-butoxide [M(OBut)y] oils
exposed to water to generate MOx (M = Sn, Ti, Zr, Hf, and Ce)
nanowires with an aspect ratio of up to 10 000. These routes are
complicated and/or expensive setups that not necessarily amen-
able to large scale production.
ð2Þ
Experimental section
All reactions were performed under a dry, inert atmosphere using
standard Schlenk line and glovebox techniques. The following
chemicals were used as received from Aldrich: SnCl2, LiNMe2,
H-oMP, H-oPP, H-oBP, H-DMP, H-DIP, H-DBP. All anhydrous
solvents were used as received (Aldrich) in Sure/SealTM bottles.
[Sn(NMe2)2]2 was synthesized from the reaction of SnCl2 and
two equivalents of LiNMe2 in THF at 0 °C.27,28
Solution routes to SnOy nanowires are underexplored but if
successful, they will be simple, fast, relatively inexpensive, and
amenable to large scale syntheses. For these systems, we favor
metal alkoxides (M(OR)y) precursors since they are often com-
mercially available, their physical and chemical properties (i.e.,
hydrolysis susceptibility, solubility, volatility, etc.) can be easily
tailored through ligand set manipulations, and they have all the
necessary components present (i.e., metal and oxygen) prior to
decomposition. The structural arrangement of M(OR)y (directed
by the ligand set employed) has been found to play a role in
determining the properties of the final ceramic material
produced.14–24 Often the conversion of M(OR)y to MOx is
accomplished through hydrolysis by either a one-step polycon-
densation mechanism, upon exposure to water2,25 or by an ether
elimination26 (after a series of reaction processes) when organic
solvents and water are employed. However, in the processes used
in this study, water was omitted to minimize side reactions and
the expected oxalate intermediates.26
General instrumentation
FT-IR spectral data were obtained on a Nicolet 6700 FTIR spec-
trometer using KBr pellets pressed under an argon atmosphere
and handled under an atmosphere of flowing nitrogen. Elemental
analyses were performed on a Perkin-Elmer 2400 CHN-S/O
elemental analyzer. All NMR samples were prepared using crys-
talline material handled under an argon atmosphere and then
flame sealed under vacuum. Spectra were collected on a Bruker
Avance 500 NMR spectrometer, using a 5 mm inverse probe,
under standard experimental conditions: 1H analysis was per-
formed with a 4-second recycle delay at 16 scans; spectra were
referenced to the toluene-d8 peak at 2.09 ppm; 13C spectra were
obtained with a 10-second recycle delay and 4k scans and refer-
enced to the methyl peak of the toluene-d8 at 20.5 ppm; 119Sn
solution NMR analysis was performed with a 4–second recycle
delay, and spectra were externally referenced to the secondary
standard aqueous 1 M SnMe4 at 0 ppm.
Our previous work27 with tin(II) amides [Sn(NMe2)2] and alk-
oxides [Sn(OR)2] led us to investigate the synthesis of tin(II)
amide/aryloxides [Sn(NMe2)(OAr)] and aryloxide [Sn(OAr)2]
for use in the production of nanowires. Following eqn (1) and
(2), the dimethyl amide derivative [Sn(NMe2)2]2 was indepen-
dently reacted with a series of aryl alcohols (H-OAr), including
General synthesis
Due to the similarity of synthesis a general method is presented
with particular details presented for each complex. To a stirring
solution of [Sn(NMe2)2]2 in toluene, the appropriate equivalent
of the desired H-OAr was added, which immediately turned the
reaction mixture to a dark yellow to orange color depending on
the H-OAr used. The reaction mixtures were stirred for 12 h. For
those reactions that formed a small precipitate, it was removed
by centrifugation. The reaction or mother liquor was set aside
open to the glovebox atmosphere to allow the volatile fraction to
slowly evaporate until crystals formed. All analyses were per-
formed on the crystalline material.
H-OC6H4(R)-2:
R = CH3 (H-oMP), CH(CH3)2 (H-oPP),
C(CH3)3 (H-oBP)] or [H-OC6H3(R)2-2,6: R = CH3 (H-DMP),
CH(CH3)2 (H-DIP), C(CH3)3 (H-DBP)]. The products were all
identified as the dinuclear species for the heteroleptic com-
pounds (eqn (1)) as: [(OAr)Sn(μ-NMe2)]2 where OAr = oMP
(1), oPP (2), oBP (3), DMP (4), DIP (5), DBP (6) and for the
homoleptic compounds (eqn (2)) as a mixture of polymers [Sn-
(μ-OAr)2]∞ [where OAr = oMP (7), oPP (8)], dinuclear [(OAr)-
Sn(μ-OAr)]2 (where OAr = oBP (9), DMP (10) or DIP/HNMe2
(11)), and mononuclear [Sn(DBP)2] (12), species. Solution pre-
cipitation (SPPT) and electrospinning (ES) routes were under-
taken using these novel compounds in an attempt to generate
SnOx nanowires. The various results of these studies will be
presented.
[(oMP)Sn(μ-NMe2)]2 (1). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-oMP (0.261 g, 2.42 mmol), and ∼10 mL
toluene. Yield 68.9% (0.450 g). FTIR (KBr pellet, cm−1
)
2962(s), 2929(sh, s), 2895(sh, s), 2860(sh, s), 2729(w), 2703(w),
2589(w), 2364(w), 2340(w), 2254(w), 2118(w), 1884(w),
1649(m), 1597(m), 1466(s), 1382(s), 1357(s), 1234(s), 1190(s),
1046(s), 1023(s), 933(s), 859(w), 841(m), 792(m), 778(sh, m),
756(s), 728(w), 616(s), 569(s), 560(sh, s), 489(s), 472(sh, s),
455(sh, s), 413(sh, s). 1H NMR (500.1 MHz, tol-d8): δ 7.20
ð1Þ
9350 | Dalton Trans., 2012, 41, 9349–9364
This journal is © The Royal Society of Chemistry 2012

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(1 H, d, OC6H4(CH3), JH–H = 3.0 Hz), δ 7.08 (1 H, t,
OC6H4(CH3), JH–H = 5.6 Hz), δ 6.82 (1 H, t, OC6H4(CH3), JH–H
= 5.6 Hz), 6.71 (1 H, d, OC6H4(CH3), JH–H = 3.2 Hz), 2.34
(3 H, s, OC6H4(CH3)), 2.33 (6 H, s, N(CH3)2). 13C NMR
(100.1 MHz, tol-d8): δ 161.2, 131.4, 127.1, 118.5, 117.1
(OC6H4(CH3)), 40.9 (N(CH3)2), 18.4 (OC6H4(CH3)). Anal.
Calcd for C18H26N2O2Sn2 (MW = 539.96): C, 40.04; H, 4.85; N,
5.21. Found: C, 40.53; H, 4.66; N, 4.23.
(6 H, s, OC6H3((CH3)2). 13C NMR (100.1 MHz, tol-d8):
δ 160.4, 126.2, 118.1 (OC6H3((CH3)2)), 40.7 (N(CH3)2), 20.9
(OC6H3((CH3)2)). Anal. Calcd for C20H30N2O2Sn2 (MW
=
567.89): C, 42.30; H, 5.32; N, 4.93. Found: C, 42.36; H, 5.26;
N, 4.28.
[(DIP)Sn(μ-NMe2)]2 (5). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-DIP (0.431 g, 2.42 mmol), and ∼10 mL toluene.
Yield 62.6% (0.514 g). FTIR (KBr pellet, cm−1) 3055(s),
3031(m), 3019(sh, m), 2959(s), 2925(sh, s), 2865(s), 1596 (s),
1460(s), 1442(s), 1426(s), 1381(m), 135(m), 1322(m), 1256(s),
1205(s), 1196(sh, s), 1158(w), 1149(m), 1109(m), 1097(sh, m),
1069(m), 1045(w), 1034(m), 1003(m), 932(w), 883(m), 842(s),
796(w), 752(s), 702(s), 677(w), 620(m), 570(m), 541(s), 461(sh,
m), 420(m). 1H NMR (500.1 MHz, tol-d8): δ 7.14 (2 H, d,
OC6H3((CH(CH3)2)2), JH–H = 3.0 Hz), 6.92 (1 H, t, OC6H3((CH-
(CH3)2)2), JH–H = 6.0 Hz), 3.52 (1 H, m, OC6H3((CH(CH3)2)2),
JH–H = 5.6 Hz), 2.56 (6 H, s, N(CH3)2), 1.27 (12 H, d,
OC6H3((CH(CH3)2)2), JH–H = 5.6 Hz). 13C NMR (100.1 MHz,
tol-d8): δ 156.5, 123.7, 119.1 (OC6H3((CH(CH3)2)2)), 40.9(N-
(CH3)2), 27.9 (OC6H3((CH(CH3)2)2)), 24.3 (OC6H3((CH-
(CH3)2)2)). Anal. Calcd for C28H46N2O2Sn2 (MW = 672.00): C,
50.05; H, 5.70; N, 4.17. Found: C, 48.29; H, 6.64; N, 3.99.
[(oPP)Sn(μ-NMe2)]2 (2). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-oPP (0.329 g, 2.42 mmol), and ∼10 mL toluene.
Yield 72.1% (0.519 g). FTIR (KBr pellet, cm−1) 3070(m),
3060(sh, m), 3044(sh, m), 2998(s), 2956(s), 2933(sh, s), 2935
(sh, s), 2913(sh, s), 2860(s), 2831(sh, s) 2782(s), 1590(s),
1572(s), 1480(s), 1458(s), 1441(sh, s), 1403(s), 1380(m), 1360
(m), 1340(s), 1285(s), 1271(s), 1258(s), 1242(sh, s), 1217(sh, s),
1188(s), 1155(m), 1153(m), 1123(s), 1079(s), 1036(s), 967(w),
927(m), 893(s), 874(sh, m), 845(sh, s), 838(sh, s), 758(s),
739(s), 722(s), 702(m), 602(s), 577(m), 570(sh, m), 562(sh, m),
530(m), 495(m), 460(s), 441(s), 408(s), 406(sh, s). 1H NMR
(500.1 MHz, tol-d8): δ 7.29 (1 H, d, OC6H4(CH(CH3)2), JH–H
=
2.8 Hz), 6.89 (1 H, t, OC6H4(CH(CH3)2), JH–H = 6.0 Hz),
6.71 (1 H, d, OC6H4(CH(CH3)2), JH–H = 3.0 Hz), 3.62 (1 H,
sept, OC6H4(CH(CH3)2), JH–H = 5.6 Hz), 2.39 (6 H, s,
N(CH3)2), 1.33 (6 H, d, OC6H4(CH(CH3)2), JH–H = 4.4 Hz). 13
C
[(DBP)Sn(μ-NMe2)]2 (6). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-DBP (0.499 g, 2.42 mmol), and ∼10 mL
NMR (100.1 MHz, tol-d8): δ 160.4, 126.7, 126.4, 118.9,
117.4 (OC6H4(CH(CH3)2), 41.0 (N(CH3)2), 27.3
toluene. Yield 78.5% (0.698 g). FTIR (KBr pellet, cm−1
)
3056(m), 3054(sh, m), 3071(m), 3019(m), 2954(s), 2900(sh, s),
2870(sh, s), 2799(sh, m), 1580(m), 1475(m), 1468(sh, m),
1444(m), 1425(m), 1399(s), 1383(s), 1359(m), 1351(m),
1319(sh, w), 1258(s), 1219(s), 1190(s), 1151(m), 1119(s),
1104(s), 1067(sh, w), 1036(m), 1009(sh, w), 947(w), 919(w),
886(s), 847(s), 809(s), 797(sh, m), 755(s), 701(w), 634(m),
601(w), 577(w), 533(m), 459(m), 437(s), 426(sh, s), 403(s).
Anal. Calcd for C32H54N2O2Sn2 (MW = 736.45): C, 52.49; H,
7.39; N, 3.80. Found: C, 52.60; H, 7.41; N, 3.45.
(OC6H4(CH(CH3)2), 23.4 (OC6H4(CH(CH3)2). Anal. Calcd for
C22H34N2O2Sn2 (MW = 595.94): C, 44.34; H, 5.75; N, 4.70.
Found: C, 44.85; H, 5.65; N, 3.57.
[(oBP)Sn(μ-NMe2)2]2 (3). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-oBP (0.365 g, 2.42 mmol), and ∼10 mL toluene.
Yield 83.5% (0.629 g). FTIR (KBr pellet, cm−1) 2959(s), 2921
(sh, s), 2888(sh, s), 2859(sh, s), 2721(sh, m), 1467(s), 1454(sh,
s), 1385(s), 1362(s), 1231(s), 1187(s), 1102(w), 1050(w), 1023
(m), 955(s), 914(s), 900(s), 841(w), 804(w), 754(s), 684(w),
584(s), 559(sh, s), 463(s), 449(sh, m), 432(w). 1H NMR
[Sn(μ-oMP)2]∞
1.21 mmol), H-oMP (0.522 g, 4.83 mmol), and ∼10 mL
toluene. Yield 19.2% (0.154 g). FTIR (KBr pellet, cm−1
(7). Used
[Sn(NMe2)2]2
(0.500
g,
(500.1 MHz, tol-d8): δ 7.39 (1 H, d, OC6H4(C(CH3)3), JH–H
=
)
3.0 Hz), 7.06 (1 H, t, OC6H4(C(CH3)3), JH–H = 6.0 Hz), 6.85
(1 H, t, OC6H4(C(CH3)3), JH–H = 6.0 Hz), 6.59 (1 H, d,
OC6H4(C(CH3)3), JH–H = 3.0 Hz), 2.49 (3 H, s, N(CH3)2), 1.57
(9 H, s, OC6H4(C(CH3)3)). 13C NMR (100.1 MHz, tol-d8):
δ 161.5, 139.1, 127.4, 127.1, 118.9, 118.7 (OC6H4(C(CH3)3)),
2953(s), 2922(sh, s), 2895(sh, s), 2860(sh, s), 2703(w), 2367(w),
1888(w), 1653(m), 1592(m), 1466(s), 1382(s), 1357(s), 1233(s),
1188(s), 1092(s), 1046(m), 1023(m), 944(s), 843(m), 757(s),
729(w), 613(sh, s), 565(s), 473(s), 418(sh, m). 1H NMR
(500.1 MHz, tol-d8): δ 7.02 (1 H, d, OC6H4(CH3), JH–H
=
42.2
(N(CH3)2),
35.2
(OC6H4(C(CH3)3)),
30.2
3.0 Hz), 6.95 (1 H, t, OC6H4(CH3), JH–H = 6.0 Hz), 6.77 (2H,
m, OC6H4(CH3)), 2.16 (3H, s, OC6H4(CH3)). 13C NMR
(100.1 MHz, tol-d8): δ 156.9, 131.5, 127.3, 120.9, 120.4
(OC6H4(CH3)), 18.1 (OC6H4(CH3)). Anal. Calcd for
C14H14O2Sn (MW = 332.97): C, 50.50; H, 4.24; N, 0.00. Found:
C, 51.06; H, 4.29; N, 1.07.
(OC6H4(C(CH3)3)). Anal. Calcd for C24H38N2O2Sn2 (MW
624.00): C, 46.20; H, 6.14; N, 4.49. Found: C, 46.66; H, 6.28;
N, 4.32.
=
[(DMP)Sn(μ-NMe2)]2 (4). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-DMP (0.295 g, 2.42 mmol), and ∼10 mL
toluene. Yield 74.8% (0.513 g). FTIR (KBr pellet, cm−1) 3062
(s), 3039(sh, s), 3009(s), 2958(s), 2941(sh, s), 2902(s), 2864(sh,
s), 2788(s), 2772(m), 1589(s), 1464(s), 1419(s), 1372(m),
1290(m), 1262(s), 1219(s), 1200(sh, s), 1159(sh, m), 1124(m),
1094(s), 1037(s), 981(w), 951(sh, w), 913(w), 900(w), 843(s),
804(m), 763(s), 742(m), 684(m), 575(s), 563(s), 522(s), 507(sh,
s), 464(s), 422(sh, m). 1H NMR (500.1 MHz, tol-d8): δ 7.07
(2 H, d, OC6H3((CH3)2), JH–H = 2.8 Hz), 6.76 (1 H, t,
OC6H3((CH3)2), JH–H = 5.6 Hz), 2.40 (6 H, s, N(CH3)2), 2.29
[Sn (μ-oPP)2]∞ (8). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-oPP (0.658 g, 4.83 mmol), and ∼10 mL toluene.
Yield 55.1% (0.518 g). FTIR (KBr pellet, cm−1) 3064(m),
3029(m), 2961(s), 2868(sh, m), 1591(m), 1488(s), 1444(s),
1382(m), 1361(m), 1344(m), 1285(sh, s), 1252(s), 1191(m),
1149(s), 1111(w), 1085(m), 1035(m), 1008(sh, w), 925(s),
888(m), 838(s), 814(sh, s), 775(sh, s), 758(s), 689(sh, w),
637(w), 592(m), 578(m), 539(w), 488(w), 447(w). 1H NMR
This journal is © The Royal Society of Chemistry 2012
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(500.1 MHz, tol-d8): δ 7.15 (1 H, d, OC6H3((CH(CH3)2)2), JH–H
= 3.2 Hz), 6.95 (1 H, t, OC6H3((CH(CH3)2)2), JH–H = 6.0 Hz),
6.83 (1 H, t, OC6H3((CH(CH3)2)2), JH–H = 6.0), 6.53 (1 H, mult,
OC6H3((CH(CH3)2)2)), 3.66 (1 H, sept, OC6H3((CH(CH3)2)2),
137.7 (OC6H3((CH(CH3)2)2)), 36.1 (N(CH3)2), 27.6 (OC6H3((CH-
(CH3)2)2)), 24.5 (OC6H3((CH(CH3)2)2)). Anal. Calcd for
C50H75NO4Sn2 (MW = 991.49): C, 60.57, H, 7.62, N, 1.41.
Found: C, 61.06; H, 7.59; N, 1.15.
JH–H = 5.4 Hz), 1.20 (6 H, d, OC6H3((CH(CH3)2)2), JH–H
=
[(DBP)2Sn] (12). Used [Sn(NMe2)2]2 (0.500 g, 1.21 mmol),
H-DBP (0.998 g, 4.83 mmol), and ∼10 mL toluene. Yield
85.3% (1.09 g). FTIR (KBr pellet, cm−1) 3642(s), 3061(s),
2956(s), 2908(sh, s), 2041(w), 1910(w), 1851(w), 1795(w),
1695(w), 1651(w), 1579(m), 1480(s), 1462(s), 1425(sh, s),
1403(s), 1362(w), 1315(w), 1259(m), 1213(s), 1144(w),
1115(m), 1100(m), 1040(w), 1024(w), 923(w), 882(m), 846(s),
811(s), 798(sh, m), 762(s), 752(s), 647(m), 638(m), 576(m),
536(m), 474(sh, w), 450(s), 421(m), 408(m). 1H NMR
5.6 Hz). 13C NMR (100.1 MHz, tol-d8): δ 148.9, 139.3, 136.4,
126.7, 126.5, 129.9 (OC6H3((CH(CH3)2)2), 26.94 (OC6H3((CH-
(CH3)2)2), 24.10 (OC6H3((CH(CH3)2)2). Anal. Calcd for
C18H22O2Sn (MW = 389.08): C, 55.57, H, 5.70. Found: C,
55.81; H, 6.03.
[(oBP)Sn(μ-oBP)]2 (9). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-oBP (0.730 g, 4.83 mmol), and ∼10 mL toluene.
Yield 60.8% (0.613 g). FTIR (KBr pellet, cm−1) 3055(m), 3008
(s), 2966(s), 2904(s), 2868(s), 2825(sh, m), 2783(m), 1907(w),
1871(w), 1592(s), 1572(sh, m), 1482(s), 1451(s), 1430(s),
1406(m), 1372(w), 1281(sh, s), 1272(sh, s), 1256(s), 1184(s),
1152(m), 1125(m), 1111(s), 1045(s), 1037(s), 981(m), 901(m),
(500.1 MHz, tol-d8): δ 7.32 (2H d, OC6H3((C(CH3)3)2), JH–H
=
3.2 Hz), 6.85 (1 H, t, OC6H3((C(CH3)3)2), JH–H = 6.0 Hz), 1.57
(18 H, s, OC6H3((C(CH3)3)2). 13C NMR (100.1 MHz, tol-d8):
δ
139.8, 119.4 (OC6H3((C(CH3)3)2)), 35.3 (OC6H3-
1
((C(CH3)3)2)), 33.1 (OC6H3((C(CH3)3)2)). Anal. Calcd for
C28H42O2Sn (MW = 529.34): C, 63.53, H, 8.00, N, 0.00.
Found: C, 63.52; H, 8.05; N, 0.82.
856(s), 754(s), 713(s), 597(s), 566(s), 536(s), 472(s). H NMR
(500.1 MHz, tol-d8): δ 7.36 (1 H, d, OC6H4(C(CH3)3), JH–H
=
3.2 Hz), 7.01 (1 H, t, OC6H4(C(CH3)3), JH–H = 6.0 Hz), 6.88
(1 H, d, OC6H4(C(CH3)3), JH–H = 2.8 Hz), 6.82 (1 H, t,
OC6H4(C(CH3)3), JH–H = 6.0 Hz), 1.77 (2 H, s, HNMe2), 1.57
(9 H, s, OC6H4(C(CH3)3)). 13C NMR (100.1 MHz, tol-d8):
δ 160.0, 139.5, 127.5, 127.2, 119.9, 119.1 (OC6H4(C(CH3)3)),
36.1, 35.2 (OC6H4(C(CH3)3)), 30.4 (OC6H4(C(CH3)3)). Anal.
Calcd for C20H26O2Sn (MW = 417.13): C, 57.59, H, 6.28.
Found: C, 61.09; H, 7.50; N, 1.63.
General X-ray crystal structure information29
Crystals were mounted from a pool of Fluorolube™ and immedi-
ately placed in a cold N2 vapor stream, on a Bruker AXS dif-
fractometer equipped with a SMART 1000 CCD detector using
graphite monochromatized MoKα radiation (λ = 0.7107 Å) or a
Bruker AXS diffractometer equipped with a APEX II detector.
Lattice determination and data collection As well as data
reduction and corrected for absorption was performed using the
appropriate software suite, either SMART or APEX II.
Structures were solved by direct methods that yielded the
heavy atoms, along with a number of the lighter atoms or by
using the PATTERSON method, which yielded the heavy atoms.
Subsequent Fourier syntheses yielded the remaining light-atom
positions. The hydrogen atoms were fixed in positions of ideal
geometry and refined using Shelxl-97 software. The final refine-
ment of each compound included anisotropic thermal parameters
for all non-hydrogen atoms. All final CIF files were checked at
http://www.iucr.org/. Additional information concerning the data
collection and final structural solutions can be found in the ESI†
or by accessing CIF files through the Cambridge Crystallo-
graphic Data Base. Data collection parameters for 1–12 are given
in Table 1. For 7, one disordered toluene was removed using
SQUEEZE in the PLATON software suite. Structure plots for each
of the compounds discussed in this report (hetero-, homoleptic, and
oxide compounds) are shown in the ESI† (Fig. S1–S15†). Plots of
representative structure types are shown in Fig. 1–5.
[(DMP)Sn(μ-DMP)]2·tol (10). Used [Sn(NMe2)2]2 (0.500 g,
1.21 mmol), H-DMP (0.590 g, 4.83 mmol), and ∼10 mL
toluene. Yield 78.6% (0.685 g). FTIR (KBr pellet, cm−1
)
3062(m), 3011(m), 2957(s), 2903(s), 2853(m), 2727(m),
2027(w), 1921(w), 1852(w), 1785(w), 1660(w), 1588(s),
1494(s), 1465(s), 1418(s), 1378(m), 1322(w), 1299(m), 1262(s),
1233(sh, m), 1218(s), 1182(s), 1092(s), 1029(m), 981(w),
914(w), 861(sh, m), 835(s), 781(sh, m), 769(s), 760(s), 742(m),
731(m), 698(w), 692(s), 680(s), 575(s), 556(m), 522(s), 507(s),
1
463(w), 430(m), 412(m). H NMR (500.1 MHz, tol-d8): δ 6.95
(2 H, d, OC6H3((CH3)2), JH–H = 3.0 Hz), 6.74 (1 H, t, OC6H3-
((CH3)2), JH–H = 6.0 Hz), and 2.28 (6 H, s, OC6H3((CH3)2)). 13C
NMR (100.1 MHz, tol-d8): δ 135.4, 129.1, 120.4 (OC6H3((CH3)2)),
19.2 (OC6H3((CH3)2)). Anal. Calcd for C32H36O4Sn (MW
361.02): C, 53.23, H, 5.03. Found: C, 52.92; H, 5.09.
=
[(DIP)Sn(μ-DIP)2Sn(DIP)(HNMe2)] (11). Used [Sn(NMe2)2]2
(0.500 g, 1.21 mmol), H-DIP (0.862 g, 4.83 mmol), and
∼10 mL toluene. Yield 50.8% (0.580 g). FTIR (KBr pellet,
cm−1) 3055(s), 3020(m), 2960(s), 2865(s), 2008(w), 1988(w),
1923(w), 1890(w), 1864(w), 1839(w), 1788(w), 1696(w),
1648(w), 1628(w), 1596(s), 1460(s), 1443(sh, s), 1423(s),
1381(s), 1354(s), 1323(s), 1256(s), 1206(s), 1119(s), 1158(s),
1149(m), 1096(sh, m), 1069(m), 1045(sh, m), 1033(s), 1003(s),
932(m), 883(s), 842(s), 796(m), 752(s), 701(s), 677(m), 620(m),
583(sh, w), 571(m), 541(s), 460(w), 421(w). 1H NMR
(500.1 MHz, tol-d8): δ 7.07 (2 H, d, OC6H3((CH(CH3)2)2), JH–H
= 3.2 Hz), 6.88 (1 H, t, OC6H3((CH(CH3)2)2), JH–H = 3.2 Hz),
3.53 (2 H, s(br), OC6H3((CH(CH3)2)2)), 1.90 (2 H, s, H(N-
Solution precipitation (SPPT) route
A SPPT route was used to generate all nanomaterials using crys-
talline materials. The desired precursor (1 mmol) dissolved in
oleylamine (∼2 mL) was added to a heated, stirring solution of
octadecene (∼10 mL) in a glass, three-neck, round bottom flask.
After heating at reflux for 45 min, the reaction was allowed to
cool to room temperature and the resulting powder was isolated
by centrifugation. The grey precipitate was then washed three
(CH3)2)]2), 1.18 (12 H, d, OC6H3((CH(CH3)2)2), JH–H
=
5.2 Hz). 13C NMR (100.1 MHz, tol-d8): δ 137.4, 129.3, 128.6,
9352 | Dalton Trans., 2012, 41, 9349–9364
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Table 1 Data collection parameters for 1–12 and oxide materials (S1–S3)
Compound
1
2
3
4
Chemical formula
Formula weight
Temp (K)
Space group
a (Å)
b (Å)
c (Å)
C18H26N2O2Sn2
539.79
173(2)
Monoclinic, C2/c
26.984(3)
7.4491(7)
C16H22N2OSn
377.05
C24H38N2O2Sn2
623.94
C20H30N2O2 Sn2
567.84
173(2)
Monoclinic, P2(1)/n
12.3543(19)
14.471(2)
173(2)
Triclinic, P1
173(2)
Triclinic, P1
ˉ
ˉ
8.1411(12)
9.4382(14)
11.1451(17)
85.095(2)
82.534(2)
88.505(2)
845.9(2)
2
8.2967(7)
11.3895(9)
21.6629(18)
89.6130(10)
80.5720(10)
79.6220(10)
1985.8(3)
3
21.522(2)
13.135(2)
α (°)
β (°)
106.458(2)
104.226(3)
γ (°)
V (Å3)
4148.9(7)
8
1.729
2276.2(6)
4
1.657
Z
D
calcd (Mg m−3
)
1.480
1.508
1.565
1.907
μ (Mo, Kα) (mm−1
)
)
)
2.419
2.209
(Rint
)
0.035
3.32 (4.00)
6.89 (7.16)
0.011
1.96 (2.01)
4.91 (4.93)
0.027
3.39 (4.49)
7.00 (7.45)
0.047
4.06 (5.37)
9.08 (9.67)
R1a (%) (all data)
wR2b (%) (all data)
Compound
5
6
7
8
Chemical formula
Formula weight
Temp (K)
Space group
a (Å)
b (Å)
c (Å)
C28H46N2O2 Sn2
680.05
173(2)
Monoclinic, P2(1)/c
9.9563(8)
20.5582(17)
15.2269(13)
C32H54N2O2Sn2
736.15
203(2)
Monoclinic, P2(1)/c
15.0543(15)
12.3130(12)
18.4523(18)
C70H70O10Sn5
1664.71
188(2)
C72H88O8Sn4
1556.18
173(2)
ˉ
ˉ
Triclinic, P1
Triclinic, P1
11.166(2)
12.382(2)
26.605(5)
99.731(2)
92.217(2)
111.074(2)
3363 (11)
2
1.644
1.887
0.018
3.48 (4.16)
8.39 (8.74)
12.2459(8)
12.5489(8)
13.8878(9)
101.888(1)
111.323(1)
110.848(1)
1713.48(19)
1
1.507
1.492
0.017
2.59(3.01)
6.49 (6.74)
α (°)
β (°)
101.518(1)
100.9500(1)
γ (°)
V (Å3)
3053.9(4)
4
1.479
1.660
0.021
2.06 (2.53)
4.87 (5.14)
3358.1(6)
4
1.456
1.516
0.019
2.11 (2.38)
5.37 (5.55)
Z
D
calcd (Mg m−3
)
μ (Mo, Kα) (mm−1
Rint
R1a (%) (all data)
wR2b (%) (all data)
Compound
9
10
11
12
Chemical formula
Formula weight
Temp (K)
Space group
a (Å)
b (Å)
c (Å)
C40H52O4Sn2
834.20
173(2)
Triclinic, P1
8.36(4)
11.187(5)
12.069(6)
63.389(4)
71.234(5)
84.577(5)
953.9(8)
1
1.452
1.346
0.016
1.97 (2.10)
4.72 (4.80)
C39H44O4Sn2
814.12
203(2)
Monoclinic, P2(1)/c
14.8587(11)
10.8624(8)
22.554 (16)
C50H75NO4Sn2
991.49
296(2)
Monoclinic, P2(1)/c
19.628(3)
13.8183(19)
19.431(3)
C28H42O2Sn
529.31
203(2)
Monoclinic, P2(1)/n
13.856(5)
13.874(5)
ˉ
13.921(5)
α (°)
β (°)
95.394(1)
111.156(6)
94.788(5)
γ (°)
V (Å3)
3624.1(5)
4
1.492
1.416
0.025
2.97 (3.65)
7.38(7.76)
4915.1(13)
4
1.340
1.057
0.063
3.60 (7.12)
7.39 (9.31)
2666.8(15)
4
1.318
0.978
0.020
2.30 (2.45)
6.02 (6.12)
Z
D
calcd (Mg m−3
)
μ (Mo, Kα) (mm−1
Rint
R1a (%) (all data)
wR2b (%) (all data)
Compound
S1
S2
S3
Chemical formula
Formula weight
Temp (K)
Space group
a (Å)
b (Å)
c (Å)
C46H54N2O8Sn6
1475.05
173(2)
Monoclinic, C2/c
15.9836(18)
19.1076(18)
15.9975(17)
C45H56O9Sn6
1453.04
C39H60O7Sn4
1115.63
173(2)
Orthorhombic, Pbca
22.7442(13)
16.4148(9)
23.7362(13)
188(2)
Triclinic, P1
ˉ
12.3902(17)
13.1348(18)
16.511(2)
α (°)
β (°)
88.778(2)
72.1440(10)
93.263(2)
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Table 1 (Contd.)
Compound
S1
S2
S3
γ (°)
V (Å3)
Z
75.5470(10)
2472.2(6)
2
1.952
3.034
0.033
3.95 (5.48)
10.30 (12.42)
4877.8(9)
4
2.0009
3.077
0.028
2.32 (2.87)
9.11 (11.32)
8861.7(9)
8
1.672
2.271
0.046
3.01 (4.13)
9.38 (10.74)
D
calcd (Mg m−3
)
μ (Mo, Kα) (mm−1
Rint
)
R1a (%) (all data)
wR2b (%) (all data)
a R1 = Σ ||Fo| − |Fc||/Σ|Fo| × 100. b wR2 = [Σw(Fo2 − Fc ) /Σ(w|Fo|2)2]1/2 × 100.
2 2
Fig. 1 Structure plot of 3. Thermal ellipsoids of heavy atoms drawn at the 30% level with carbon atoms drawn as ball and stick for clarity. The two
molecules located in the unit cell are shown.
times with chloroform and the resulting grey powder was
allowed to dry at room temperature.
0.15° s−1 on nanomaterials using a zero background holder.
Representative patterns and calculated percentages are shown in
the ESI† (Fig. S16–S19†).
Transmission electron microscopy (TEM)
Electrospinning (ES) synthesis
An aliquot of the nanopowder dispersed in chloroform was
placed directly onto a holey carbon type-A, 200 mesh, copper
TEM grid that was purchased from Ted Pella, Inc. The aliquot
was then allowed to dry. The resultant particles were studied
using a Philips CM 30 TEM operating at 300 kV accelerating
voltage and equipped with a Thermo Noran System Six Energy
Dispersive X-ray (EDX) System operating at 300 kV accelerating
voltage.
The appropriate precursor was dissolved in THF to generate a
0.66 M solution. This stock solution was transferred to a 3 mL
syringe, fitted with an 18-gauge stainless steel needle (inner
diameter of 0.965 mm). The syringe was fixed horizontally on a
syringe pump (Model: MD-1000, Bioanalytical Systems Inc.).
An electrode connected to a high voltage power supply (Model:
DEL HVPS INST 230 30 KV, Spellman High Voltage Elec-
tronics Corporation) was attached to the tip of the metallic
needle. A grounded stationary square collector (size 10 cm ×
10 cm) consisting of a piece of graphene or cardboard square
covered by a piece of clean aluminum foil was mounted 25 cm
above the tip of the needle and used for the fiber collection.
Powder X-ray diffraction (PXRD)
Powder X-ray diffraction (XRD) was performed by a PANalyti-
cal X’Pert Pro XRD in the 2θ range of 10–100° at a scan rate of
9354 | Dalton Trans., 2012, 41, 9349–9364
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Fig. 2 Structure plot of growth moiety for polymeric 7. Thermal ellipsoids of heavy atoms drawn at the 30% level with carbon atoms drawn as ball
and stick for clarity.
Fig. 4 Structure plot of 11. Thermal ellipsoids of heavy atoms drawn
at the 30% level with carbon atoms drawn as ball and stick for clarity.
Fig. 3 Structure plot of 10. Thermal ellipsoids of heavy atoms drawn
at the 30% level with carbon atoms drawn as ball and stick for clarity.
Scanning electron microscopy (SEM)
A portion of electrospun material was cut from the Al foil or gra-
phene substrate and fixed onto a SEM stub using double-sided
carbon tape. The samples were then analyzed without further
processing. Samples were imaged using a Hitachi S-5200 Nano
SEM field emitter gun scanning electron microscope
(FEGSEM). A PGT EDS detector and Spirit software was used
for the acquisition of EDS spectra.
Electrospinning to produce the Sn-bearing nanowires was carried
out using the stock solution under the following conditions: flow
rate (FR) was 0.9 mL h−1; 15 kV, electric field (EF) was
0.6 kV cm−1. The charge was applied and electrospinning was
performed for ∼60 min. An experimental setup can be found in
the ESI (Fig. S20†).
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by centrifugation. The remaining soluble fraction was then set
aside to slowly evaporate until crystals grew.
FTIR spectroscopic data of the products from these reactions
indicated that a mixture of amide and alkoxide ligands were
present with a loss of the 3000 cm−1 OH stretch. Two stretching
modes for the ν(Sn–O) 476–466 cm−1 and ν(C–O)Sn in the
range of 1030–1070 cm−1 should be observed for Sn(OR)x com-
pounds;30 however, due to the out-of-plane bends of the aryl-
oxide ligands,31–33 these stretches could not be unequivocally
identified. The limited structural identification that could be
discerned from the FTIR data led us to undertake single crystal
X-ray experiments.
Table 1 shows the structural data parameters for 1–6. Each of
these compounds was found to adopt a dinuclear arrangement
with a bridging NMe2 and a terminal OAr ligand. The three
coordinated Sn(II) metal centers have (N–μ)–Sn–(μ–N) angles of
av 79.0° and O–Sn–(μ–N) av 90.85°. Fig. 1 shows compound 3
(two molecules located in the unit cell) as the representative
member of the heteroleptic family isolated (see ESI† for all
structures and packing diagrams). Prior to the isolation of 1–6,
the only OAr/amide complex reported was Sn(IV) species
(DBP-Me)2Sn(NMe2)2 (Sn-O distance av 1.98 Å, Sn–N distance
Fig. 5 Structure plot of 12. Thermal ellipsoids of heavy atoms drawn
at the 30% level with carbon atoms drawn as ball and stick for clarity.
Electrochemical analysis
Electrochemical studies on electrosprayed material were prepared
by changing the working distance between the spray tip and the
substrate to localize all the deposited material. Deposits were
made on graphite paper (Grafoil™). All testing was conducted
under an Ar atmosphere in 1 M TBAP in acetonitrile against a
Ag/Ag+ reference with Pt counter electrode. Cyclic voltammetry
was performed at 25 mV s−1 at room temperature starting at
0.1 V vs. Ag/Ag+ ( just above the Sn0 to Sn2+ potential) and
sweeping cathodically to look for SnO reduction to Sn0 in the
first sweep. Electrospun (ES) samples were punched with a 1 cm
diameter punch to have a defined area of electrode for each
measurement.
av. 1.99 Å).34 Other alkoxide amides include: [((Me3Si)2N)Sn-
35
(μ-OBut)]2
and [((Me3Si)2N)Sn(μ-OCH2CH2N(Me2)c)]2,36
[(ORN)Sn(μ-NMe2)]2 where ORN = 2-(((2,6-diisopropylphenyl)-
imino)methyl-4,6-diiodophenolato).37 The metrical data for 1–6
(see Table 2) are consistent with each other and the appropriate
literature compounds.34–37
Purity of the bulk powders of the heteroleptic compounds was
further investigated using elemental analysis. As is often noted
for these types of compounds, obtaining useful elemental ana-
lyses is difficult due to the high volatility of the compounds,
incomplete or premature decomposition of the precursor, or
inclusion of solvents. Therefore, while the elemental analyses
are within acceptable range for C and H, the percent of N for
each sample was on the low end of the expected range. The one
exception is compound 5 where significant more N was recorded
than would be expected for the crystal structure; however, the C
and H were considered low. Further, the values of the N percen-
tage were extremely varied from sample to sample. This behav-
ior implies a complex decomposition process is occurring for 5
which limits the utility of the data. Overall, the elemental ana-
lyses of 1–6 variability was attributed to the high volatility of the
NMe2 ligand, which makes determining purity difficult.
Results and discussion
The development of a series of Sn(II) compounds was necessary
prior to investigating their utility for the production of SnOx
nanowires. Inert atmosphere methodologies were investigate to
minimize water exposure thereby avoiding oxo intermediate for-
mation throughout.26,27 The synthesis and characterization of the
various hetero- and homoleptic species isolated in this study are
discussed below. This is followed by a discussion of the results
for the SPPT and ES processing routes employing these
compounds.
Therefore, further investigations were undertaken using 1H,
13C, and 119Sn NMR experiments. The resonances of the 1H
spectra for 1–6 in tol-d8 were found to be consistent with the
respective OAr and the NMe2 ligands in a 1 : 1 ratio and the 13C
NMR spectrum also confirmed the presence of only one set of
OAr and NMe2 ligands; however, a number of the OAr aryl
peaks for these compounds were obscured by the tol-d8 peaks.
In an effort to elucidate the powder’s solution behavior and
purity, 119Sn NMR [119Sn 8.58% natural abundance, I = l/2 and
117Sn, 7.61% natural abundance, I = l/2) spectra were collected
and are tabulated in Table 3. Only one 119Sn resonance was
observed for 1–5, ranging from −39 to −52 ppm. Compound 6
had too low a solubility to allow for meaningful analysis. A
correlation between the chemical shift and the coordination
number of the Sn has been previously proffered,38 with
Heteroleptic
The initial precursors of interest were the ‘Sn(OAr)(NMe2)’
compounds, since it was assumed the heteroleptic compounds
would demonstrate an asymmetric decomposition upon thermal
treatment which could lead to wire-like growth. The synthesis of
these compounds was undertaken under an atmosphere of argon,
through the addition of one equivalent of the desired H-OAr to a
clear pale yellow solution of [Sn(NMe2)2]2 dissolved in toluene
(eqn (1)). Dependent on the HOAr employed, the reaction
mixture changed to a dark yellow to orange color. Each reaction
was stirred for 12 h and any precipitates formed were removed
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Table 2 Metrical data summary for 1–12a
Compound
1
2
3
4
5
6
Distances (Å)
Sn⋯Sn
3.30
2.23
2.07
3.46
2.27
2.0719(14)
3.35
2.24
2.053(3)
3.33
2.23
2.05
3.31
2.23
2.06
3.41
2.27
2.09
Sn–(μ–N)
Sn–O
Angle (°)
Sn–μ–N–Sn
(N–μ)–Sn–(μ–N)
O–Sn–(μ–N)
95.51
79.13
87.39
99.43(6)
80.57(7)
88.45
96.99(12)
79.50
92.13
96.7
78.5
90.1
95.8
79.7
89.0
97.7
76.6
98.0
Compound
7
8
9
10
11
12
Distances (Å)
Sn⋯Sn
3.65
2.25
—
3.57
2.14
—
3.53
2.1695(16)
2.0624(17)
3.52
2.18
2.05
3.57
2.27
2.04
—
––
2.02
Sn–μ–O
Sn–O
Angle (°)
O–Sn–O
(O–μ)–Sn–(Ο)
(μ–O)–Sn–(μ–O)
Sn–(μ–O)–Sn
N–Sn–O
—
—
96.5
110.1
—
—
—
92.7
107.4
—
—
—
—
91.7
70.8
107.3
77.6(2)
89.01(7)
—
—
89.6
75.0
105.01(7)
—
88.6
71.3
107.7
—
—
a All values are averaged except when error bars are presented; — not present
Table 3 119Sn NMR data for 1–12a
orange color, independent of the OAr used. After stirring for
12 h, X-ray quality crystals were isolated through slow evapor-
ation of the volatile components. Based on the absence of the
OH stretch at 3000 cm−1 and lack of NMe2 stretches, FTIR data
indicated the ‘Sn(OAr)2’ species had been produced. Again the
stretching modes of the ν(Sn–O) and ν(C–O)Sn were obfuscated
by the OAr bends.30–33 Additional information pertaining to the
structure of the compound was necessary and crystal structures
were obtained when possible.
Compound
δ
119Sn (tol-d8)
Compound
δ
119Sn (tol-d8)
1
2
−42.426
−43.249
7
8
−412.1
−429.2 (75%),
−490.3 (25%)
−315.7 (3%)
−323.0(97%)
−329.1 (100% @50°C)
−293.5
3
−39.349
9
4
5
−44.772
−52.598
10
11
−121.6 (9%)
−340.5 (91%)
−219.0
The structures of the homoleptic products proved to be signifi-
cantly varied from the dinuclear heteroleptic compounds. For 7
(Fig. 2) and 8, polymeric structures were solved where each
Sn(II) metal center adopted a tetra-coordination using μ-OAr
ligands. Compounds 9, 10 (Fig. 3), and 11 were found to adopt
dinuclear arrangements with one terminal and two μ-OAr per
metal center. Compound 11 (Fig. 4) also possesses one HNMe2
6
—
12
a — insoluble.
3-coordinated Sn(II) alkoxide species appearing between δ −350
and −225 ppm and 2-coordinated Sn(II) alkoxide resonances
reportedly fall between δ −100 to −175 ppm. Scheme 1 shows a
graphical representation of the various 119Sn shifts reported for
Sn(OR)x compounds (note: the 4 coordinated Sn(IV) species Sn
(NMe2)4 is observed at δ 0.0 ppm)38 and those determined in
this work. Obviously, the amide alkoxides do not exactly follow
the Sn(OR)2 trend and more work to verify the nuclearity is
necessary. However, the clustering of the chemical shifts for 1–6
and the presence of one 119Sn peak for each sample, implies
these compounds all relatively pure in terms of the Sn precursor
and they all adopt the same structure in solution.
1
bound to one of the Sn metal centers, which was verified by H
NMR and elemental analyses data (vide infra). Finally, a mono-
meric species was isolated with the most sterically hindered
ligand (i.e., DBP) identified as 12 (Fig. 5). The bent geometry
forms an O–Sn–O angle of 89.0° due to the lone pair of elec-
trons. A comparison of the metrical data for 7–12 were found to
be self-consistent with longer Sn–(μ-OR) than Sn–(OR) dis-
tances. Comparable distances for this set of compounds and the
heteroleptic (1–6) species were also found to be in agreement.
Of the more than 500 previously reported, crystallographically
characterized, homometallic Sn species, only five can be con-
sidered ‘simple’ aryloxide derivatives (all of which were mono-
meric) including (DBP,39,40 DBP(Me)-4,40 OC6H2(C6H2Me3-
2,4,6)2,41 OC6H3(C6H3(CH(CH3)2)2-2,6),42 OC6H2(CH2NMe2)3-
2,4,643). The values obtained for 7–12 are in agreement when
comparable angles and distances are analyzed.
Homoleptic
The aminolysis with the series of HOAr was again used to gen-
erate the homoleptic compounds following eqn (2). Upon
addition of the HOAr to the pale yellow solution of [Sn-
(NMe2)2]2 dissolved in toluene, the reaction mixture turned to an
Efforts were undertaken to determine if the bulk powders were
in agreement with the single crystal structures obtained for the
homoleptic species. The elemental analyses of the bulk powder
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resonances (7–12) with the heteroleptic compounds (1–6) and
literature Sn(OR)x are shown in Scheme 1. For 7, 10, and 12,
only one 119Sn resonances was recorded indicates a relatively
pure sample with simple solution behavior exists for these com-
pounds. For 8, 9, and 11 two 119Sn peaks were noted.
For the polymeric compounds 8, the two 119Sn shifts falls
within the expected range noted for the 4-coordinated Sn(OR)x.
Since the previous analytical data indicates the bulk powder of 8
is consistent with the single crystal structure, the 3 : 1 ratio of
119Sn shifts noted, 8 reflects an intermolecular process. This can
be explained either by disruption of the polymer into unequiva-
lent smaller chains or more likely inter-conversion of cis-trans of
isomers around the lone pair of electrons on the Sn. For the
dinuclear 9, the two 119Sn resonances (δ −315.7 to −323.0 ppm)
were noted in a 3 : 97 ratio. The consistency of the shifts indi-
cates that similar coordination environments around the Sn metal
center must exist. Low temperature investigations could not be
undertaken due to preferential precipitation. High temperature
119Sn NMR spectra obtained on 9 revealed at 50 °C that the two
room temperature peaks coalesced to a singlet at −329.1 ppm.
Two potential explanations could account for this behavior: a
cis-trans isomerization or a monomer–dimer equilibrium. The
latter would yield two disparate chemical shifts representing the
2 and 3 coordinated Sn metal centers and therefore, inversion
around the Sn metal center is assumed to be occurring yielding
these two peaks. For 11, two 119Sn peaks were expected for the
inequivalent 3- and 4- coordinated Sn metal centers found in the
single crystal structure; however, while two peaks were observed
(δ −121.6 and −340.5), the ratio was not the expected 1 : 1 but
9 : 91 instead. A number of complex equilibria or isomerizations
can be used to explain the chemical shifts but the ratio implies
an Sn-NMe2 impurity is present. Additional work to further
clarify the complex solution behavior of these homoleptic
species is necessary.
Oxide compounds
From several attempts to crystallize the various compounds dis-
cussed above, a number of oxo- species were also isolated and
crystallographically characterized. While rational routes to these
compounds are being developed, the unit cell parameters and
structural discussion are presented to assist in identifying the
various products that can be generated in these reactions. Unit
cell parameters are supplied in Table 1 for ease of identification
with all structures and packing diagrams supplied in the ESI
(Fig. S13–S15†). A brief description of each compound is sup-
plied below.
Oxide structures of general formula Sn6(μ3-O)4(μ3-oPP)4·solv
(solv = py (S1) or H-oPP (S2)) were isolated from eqn (2) using
H-oPP. The general arrangement consisted of six Sn atoms
arranged in an octahedral geometry that are interconnected by
alternating μ3-O and μ3-oPP ligands. This generates a series of
interconnected ‘Sn(μ3-O)(μ3-oPP)Sn’ squares with a central void
of 5.09 × 4.62 Å. The central core of this compound is consistent
with that of other ‘Sn(O)x(OR)y’ compounds.44–48 The different
solvents for S1 and S2 are in the lattice. Another oxide (S3) was
also formed from the attempt to generate the DIP derivative. The
central core construct consists of a six membered ring of
Scheme 1 119Sn chemical shift (major) of structurally characterized
homoleptic tin alkoxide species: black – shifts from; ref. 38 – 2
coordinated heteroleptic;
4-coordinated homoleptic.
– 2 coordinated homoleptic, and
were found to be in agreement with the single crystal structures
of 8, 10, and 11. For 7, 9, and 12 significant amounts of excess
nitrogen was observed, which was assumed to be residual
HNMe2. The purity of the bulk powders was further investigated
1
using solution state NMR. The H and 13C NMR spectrum of
7–11 revealed only one set of OAr ligand resonances for the
respective compounds; again, some of the OAr peaks were
obfuscated by the tol-d8 resonances. The presence of only one
set of ligand resonances implies that the solid state structures are
disrupted in solution or the ligands are exchanging rapidly to
generate one set of resonances. 119Sn NMR data was collected to
elucidate the solution behavior. The placement of these
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alternating Sn and O atoms from the DIP ligands. Each of the
Sn atoms is bound to a μ3-O atom, forming a corner missing
cube shape. However, from each of these Sn atoms, a methoxide
(OMe) ligand bridges to another Sn(4) atom. This results in the
general construct of {[(μ-DIP)Sn]3(μ3-O)](μ-OMe)3Sn} (S3). For
all of these compounds (S1–S3), the Sn atoms are in the +2 oxi-
dation state, which makes formation of the oxide difficult to
explain. Further, the production of the OMe in the S3 compound
is not fully understood and the reproduction of these compounds
is not high. Therefore, these compounds are reported in Table 1
only to allow for identification of the desired complex. The dis-
tances in these compounds are similar to other oxide/alkoxides
that we previously generated by hydrolysis of the alkoxide.44–48
polycrystalline rods of varied lengths leading to high aspect
ratios (some >100) were noted in a ratio of Sno : romarchite
(15 : 85). Compound 10 (Fig. 7d) generated spheres which had
an approximate ratio of Sno : SnO (34 : 66) that ranged in size of
200–800 nm). In general, for 7–12, these homoleptic species
yielded similar size ranges for the resulting particles, indicating
that the steric impact is much more limited for the homoleptic than
the heteroleptic compounds. The higher degree of oxide to metal
noted for these systems is most likely due to the presence of the
two alkoxide ligands versus the amine in the heteroleptic species.
(b) ES. Since the SPPT products were mainly spherical,
alternative routes were investigated to generate wires. Most of
the routes that are available, such as the direct draw that pro-
duced high quality nanowires of SnO2, are not a continuous
process, which limit their utility in the production of LIB
materials.13 In contrast, ES49–51 is a method that continuously
produces fibers that can range in diameter from a few
micrometers to 100 nanometers. This process works by applying
a high electrical field to a drop of precursor solution pushed
through a needle. The applied high electrical force overcomes
the surface energy of the droplet and eventually forms a Taylor
cone. The ejection of the solution forms a wire, which undergoes
a whipping motion and collects on a metallic platform forming
mats of fibers that possess a high surface area to mass ratio. Poly-
mers are typically used to ensure a continuous linkage between
drops and wires is made. We were interested in non-polymer
based nanowires (i.e., direct ES ceramic nanowires) to minimize
post-processing and garner more control over the final materials
produced. Unless alternating poles are used, ES generates a map
of nanowires, which would still be of use in LIB applications.
For this study, the ES investigations were undertaken in an argon
filled glovebox. Upon application of the voltage, a purple plasma
was formed and electrical discharges randomly occurred in the
glovebox. It was determined that Ar has about 20% of the break-
down strength of N2; therefore, all further experiments were con-
ducted under an atmosphere of N2.
Nanomaterial synthesis
With this newly developed family of hetero-(1–6) and homolep-
tic (7–12) derivatives available, attempts to synthesize SnOx
nanowires were undertaken. The results (i.e., SEM, PXRD, etc)
from these studies are presented for (a) SPPT followed by (b) ES
methodologies.
(a) SPPT. The solvent system of oleylamine and octadecene
was selected based on its propensity to form nanowires.27 For
the heteroleptic monosubstituted OAr derivatives (1, 2, and 3),
large spheres on the order of one micron were observed with
some nanosized (>20 nm) particles present. PXRD patterns for
the materials generated from 1 indicated that only Sno (PDF 04-
0673, tin) had been formed; however, for both 2 and 3 a mixture
of phases [Sno, SnO (PDF 06-0395, romarchite), and SnO (PDF
04-1342, tin oxide)] in approximate ratios of 80 : 20 and 68 : 32,
respectively. There are no indications from the TEM images
(Fig. 6) of a different set of particles that would represent the
oxides. The TEM images revealed that a variety of spherical
sized particles and sizes were available. For compound 1 the par-
ticles ranged in size from 350–900 nm), for
2 from
150–1200 nm, and for 3 from 250–950 nm. There does not
appear to be an influence from the changing sterics of the ortho
substituents. For the di-substituted OAr species (4–6) mixed
phases of Sno (PDF 04-0673) and SnOx (PDF 06-0395) in
approximate ratio of Sno : SnO of 86 : 14, 66 : 35, and 88 : 12,
respectively, were isolated. The majority of the nanomaterials
were Sno nanodots but both spheres and rods were present in
TEM images (Fig. 6). The reduction of the Sn(II) to Sno in each
of these systems is believed to be driven by the lone pair of elec-
trons on the amine surfactant.27 The size of the particles for the
di-substituted OAr ligated compounds ranged for the various
precursors 4 (20–180 nm); 5 (80–400 nm); 6 (40–200 nm) but,
in comparison to 1–3 were smaller. This is attributed to the
increased sterics around the metal center, which allows for
increased solubility and protracted reactivity, resulting in the gen-
eration of simultaneous nucleation sites.
For the ES studies, 0.66 M solutions in THF of the heterolep-
tic (1–6) precursors were employed and the SEM images of the
various materials generated from these precursors are shown in
Fig. 8. Except for 2, the materials generated appear to be a
mixture of electrospraying (i.e., dots) versus ES (i.e., wires)
forming some unusual ‘tadpole’ shaped materials. This appears
to be an effect where the Taylor cone has been initiated but the
drop is dislodged prior to connecting to the next drop or the drop
forms and stretches out to form a tail. The drops are several
microns in size with nanometer wires forming (easily observed
for both 1 and 4). Wires with ‘balls’ strung along them have
been observed previously for electrospun systems that lack chain
entanglement and is often attributed to a high precursor concen-
tration that may collapse the polymer coils and reduce chain
entanglement.51 Attempts to optimize the nanowires formed led
to an exploration of different concentrations; however, the lower
concentration solutions yielded electrosprayed materials and
higher concentration solutions were too thick to be pumped.
Therefore, the 0.66 M was maintained throughout this study.
For the majority of the homoleptic (7–11) species, attempts to
ES these compounds under similar conditions noted for the het-
eroleptic series failed to form wires, instead electrospraying dots.
This is not unexpected since the ability to chemically connect to
The homoleptic species (7–12) were also investigated and
found to form similar spherical nanoparticles (Fig. 7), except for
9. For the particles generated from 7, 8, 11, and 12, all were
found to be spherical and of mixed phase [# (size, approximate
ratio of Sno (04-0673):SnO(06-0395))]: 7 (50–300 nm, 80 : 20),
8
(irregular, 23 : 77), 11 (50–150 nm, 77 : 23), and 12
(50–450 nm, 77 : 23). For 9 (Fig. 7c), ∼100 nm wide
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Fig. 6 TEM of nanomaterials generated from SPPT processing of: (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6.
the next drop is limited with the di- and mono-nuclear com-
pounds. The SEM images of these materials are shown in Fig. 9.
Interesting, for the polymeric compound 7 (Fig. 9a) long nano-
wires with ∼1 μm ‘balls’ strung along them at distant intervals
were observed. These are the first reports of ceramic nanowires
formed by ES methods that employs a single precursor dissolved
in a solvent and does not require the aid of a polymer support or
the addition of water;52–62 however, it is of note that several
other systems have utilized complex ‘sols’ that have successfully
generated ceramic wires and tubes.52–63 The system reported
here employs only the dissolution of a single precursor in a
solvent and the application of an electric field.
Due to small sample size generated by the ES process in this
experiment, identification of the phase formed using PXRD ana-
lyses were not possible. Therefore, it became necessary to utilize
other means to identify whether SnOx or Sno had been formed.
One effort that proved to be useful was cyclovoltammetry elec-
trochemical (CVE) evaluations. By CVE methods, if SnOx had
been formed, then upon a reductive sweep in cyclic voltammetry,
a Sn2+ to Sn0 reduction peak at approximate −0.05 V vs. ref
should be observed. If Sn0 was present, no peak will be noted
upon reduction but when doing an anodic sweep, an oxidation
peak will be observed. Fig. 10 shows the CVE results for the
four separate electrospun samples (2 h deposition time): (i) 1, (ii)
1 for 8 h, (iii) 5, and 7. An additional blank of the graphite foil
was used as a reference background for the voltammetry. All of
the isothermal (25 °C) measurements made were performed at
the same sweep rate (25 mV s−1); therefore, any changes in the
capacitance of the observed voltammetry have been attributed to
changes in surface area due to deposition of material on the
graphite foil. Neither the sample generate from 1 (2 h) nor from
5 yielded meaningful data, presumably due to the low levels of
materials deposited.
For the samples generated from compound 1 (8 h) and 7, suffi-
cient material was present that allowed for electrochemical analy-
sis. For 1 (8 h) electrospun materials, no peaks were noted for
the first sweep. Subsequent sweeps showed similar peaks as
those noted for the 7 materials. For the electrospun materials of
7, no reduction peak on the first cathodic sweep, but a significant
oxidation peak at approximately −50 mV (vs. ref) was observed,
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Fig. 7 TEM of nanomaterials generated from SPPT processing of: (a) 7, (b) 8, (c) 9, (d) 10, (e) 11, and (f) 12.
which is consistent with the potential of Sn0 to Sn2+ oxidation.
Subsequent cathodic sweeps on the same sample also showed a
weak reductive peak around −130 mV, consistent with the re-
reduction of oxidized Sn cations.
Since tin oxides are weakly soluble in CH3CN, the gradual
evolution of these peaks may be a result of the slow dissolution
of the SnOx followed by the electrochemical activation of the Sn
material now present in the solution. Because of the low total
oxidation current over a significant capacitive current on the
samples, it is difficult to definitively identify the observed peaks
as either the oxidation or reduction of the Sn that is present.
However, the appearance of these peaks at the standard
potentials for these reactions provides some argument that for 7,
a reduced state of Sn (i.e., Sno) was deposited. This may also
indicate that for the material deposited using 1 (8 h) was SnOx
instead of the reduced material, since there was no oxidation or
reduction peak in the first sweep.
Summary and conclusion
A series of Sn(II) heteroleptic [(OR)Sn(μ-NMe2)]2 and homolep-
tic [Sn(OR)2]n derivatives were synthesized by aminolysis with
the degree of substitution controlled by the stoichiometry of
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Fig. 8 SEM images of electrospun wires from (a) 1, (b) 2, (c) 3, (d) 4,
(e) 5, and (f) 6.
Fig. 9 SEM of images of electrospun wires from (a) 7, (b) 8, (c) 9, (d)
10, (e) 11, and (f) 12.
HOR modifier. The products isolated were predominately dinuc-
lear (1–6 and 9–11) but polymeric (7 and 8) and mononuclear
(12) species were isolated as well. The changes in the homolep-
tic series of compounds appear to be controlled by the steric
bulk of the aryl substituent. These novel structurally diverse pre-
cursors were employed for production of nanowires. Under
SPPT conditions, micron-sized spheres of mixed phased (Sno,
romarchite, and SnOx) materials were formed, where the hetero-
leptic species favor Sno nanoparticles and the homoleptic com-
pounds favor the oxide phases. The variation in phases obtained
was attributed to the free electrons of the amide moiety for the
heteroleptic species. For 9, rods were formed but were not
regular enough to be of interest for battery applications. Attempts
to electrospin the heteroleptic compounds yielded ‘tadpole’
shaped materials, whereas the homoleptic species merely electro-
sprayed. The one exception was 7, which formed nanowires with
micron-sized ‘balls’ strung along them. Efforts to understand the
impact of the various ligand sets on the different material’s mor-
phologies are underway.
However, it is of note that the resultant ceramic nanowires
were generated using a precursor dissolved in a solvent, which
contrasts to previous efforts that necessitated the use of a
polymer, water, or complex sol synthesis. More work to improve
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the morphological properties of these nanowires and extending
this process to other polymer free ceramic materials is underway.
Preliminary electrochemical studies indicated that compound 7
may have formed Sno nanowires during deposition; whereas the
material generated from 1 (8 h) was preliminarily identified as
SnOx. Additional work employing CVE methods on the precur-
sors are underway to elucidate the mechanism of formation of
these materials, since (under the conditions used in this work)
ES utilizes an electrical field to decompose the precursor and
does not undergo standard hydrolysis condensations.2,25,26
Further work on molecular design or processing conditions to
control the phases formed along with development of new
methods for testing the generated LIB anode materials is
underway.
Acknowledgements
The authors would like to thank Dr M. A. Rodriguez (SNL) for
assistance in the PXRD analyses, Dr T. M. Alam for assistance
with NMR analyses, and Prof. Kemp (UNM) for the use of the
Bruker X-ray diffractometer purchased via the National Science
Foundation CRIF:MU award to the University of New Mexico
(CHE04-43580). This work was supported by the National Insti-
tute for NanoEngineering (NINE) and the Laboratory Directed
Research and Development (LDRD) programs at Sandia
National Laboratories. Sandia National Laboratories is a multi-
program laboratory managed and operated by Sandia Corpor-
ation, a wholly owned subsidiary of Lockheed Martin Corpor-
ation, for the U.S. Department of Energy’s National Nuclear
Security Administration under contract DE-AC04-94AL85000.
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