Synthesis and structural characterization of a family of modified hafnium tert -butoxide for use as precursors to hafnia nanoparticles

Article abstract of DOI:10.1021/ic300622h

A series of modified, hafnium tert-butoxide ([Hf(OBu^t) ₄]) compounds (1-26) were crystallographically characterized, and representative species were then used to produce HfO₂nanoparticles. This systematically varied family of [Hf(OR)₄] compounds was developed from the reaction of [Hf(OBu^t)₄] with a series of (i) Lewis basic solvents, tetrahydrofuran, pyridine, or 1-methylimidazole; (ii) simple phenols, HOC₆H₄(R)-2 or HOC₆H ₃(R)₂-2,6 where R = CH₃, CH(CH ₃)₂, or C(CH₃)₃; and (iii) complex polydentate alcohols, tetrahydrofuran methanol (H-OTHF), pyridinecarbinol (H-OPy), and tris(hydroxymethylethane) (THME-H₃). The solvent-modified products were crystallographically characterized as [Hf(OBu^t)₄(solv)_n] (1-3). The phenoxide (OAr)-exchanged [Hf(OBu^t)₄] products isolated from toluene were characterized as dimeric [Hf(OAr)_n(OBu^t) _4-n]₂ (4 and 5) or [Hf(μ-OH)(OAr)₃(HOBu ^t)]₂ (6 and 7) for the less sterically demanding OAr ligands and [Hf(OAr)_n(OBu^t)_4-n(HOBu ^t)] (8 and 9) monomers for the larger OAr ligands. When Lewis basic solvents were employed, solvated monomers of varied OAr substitutions were observed as [Hf(OAr)_n(OBu^t)_4-n(solv) _x], where solv = THF (10, 11, and 13-15) and py (16 and 19-21). The nuclearities of the remaining complex polydentate alcohol derivatives ranged from monomers (24, OPy) to dimers (22, OTHF; 23, OPy) to tetramers (25 and 26, THME). On the basis of their nuclearities, select members of this family of [Hf(OR)₄] compounds (monomer, [Hf(OBu^t)₄], 8; dimer, 19a, 22; tetramer, 25) were used to determine the validity of using [Hf(OR)₄] precursors for the production of hafnia (HfO₂) nanoparticles under solvothermal (oleylamine/oleic acid) conditions. After a 650 °C thermal treatment, the resulting powder X-ray diffraction pattern for each powder was found to be consistent with HfO₂ (PDF 00-040-1173), and after a 1000 °C treatment, larger particles of HfO₂ (PDF 00-043-1017) were reported. Transmission electron microscopy images confirmed that nanomaterials had formed. Because identical processing conditions had been employed for each HfO₂ nanomaterial, the morphological variations observed in this study may be attributed to the individual precursors ( precursor structure affect ).

Full text of DOI:10.1021/ic300622h

Article
pubs.acs.org/IC
Synthesis and Structural Characterization of a Family of Modified
Hafnium tert-Butoxide for Use as Precursors to Hafnia Nanoparticles
Timothy J. Boyle,*,† Leigh Anna M. Steele,† Patrick D. Burton,‡ Sarah M. Hoppe,† Chelsea Lockhart,†
and Mark A. Rodriguez†
†Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Boulevard SE, Albuquerque, New Mexico 87106,
United States
‡Department of Chemical and Nuclear Engineering, Center for Micro-Engineered Materials, University of New Mexico, Albuquerque,
New Mexico 87131, United States
ABSTRACT: A series of modified, hafnium tert-butoxide
([Hf(OBut)4]) compounds (1−26) were crystallographically char-
acterized, and representative species were then used to produce
HfO2 nanoparticles. This systematically varied family of [Hf(OR)4]
compounds was developed from the reaction of [Hf(OBut)4]
with a series of (i) Lewis basic solvents, tetrahydrofuran, pyr-
idine, or 1-methylimidazole; (ii) simple phenols, HOC6H4(R)-2
or HOC6H3(R)2-2,6 where R = CH3, CH(CH3)2, or C(CH3)3;
and (iii) complex polydentate alcohols, tetrahydrofuran methanol
(H-OTHF), pyridinecarbinol (H-OPy), and tris(hydroxymethyl-
ethane) (THME-H3). The solvent-modified products were crys-
tallographically characterized as [Hf(OBut)4(solv)n] (1−3). The phenoxide (OAr)-exchanged [Hf(OBut)4] products isolated from
toluene were characterized as dimeric [Hf(OAr)n(OBut)4−n]2 (4 and 5) or [Hf(μ-OH)(OAr)3(HOBut)]2 (6 and 7) for the less
sterically demanding OAr ligands and [Hf(OAr)n(OBut)4−n(HOBut)] (8 and 9) monomers for the larger OAr ligands. When Lewis
basic solvents were employed, solvated monomers of varied OAr substitutions were observed as [Hf(OAr)n(OBut)4−n(solv)x], where
solv = THF (10, 11, and 13−15) and py (16 and 19−21). The nuclearities of the remaining complex polydentate alcohol derivatives
ranged from monomers (24, OPy) to dimers (22, OTHF; 23, OPy) to tetramers (25 and 26, THME). On the basis of their
nuclearities, select members of this family of [Hf(OR)4] compounds (monomer, [Hf(OBut)4], 8; dimer, 19a, 22; tetramer, 25) were
used to determine the validity of using [Hf(OR)4] precursors for the production of hafnia (HfO2) nanoparticles under solvothermal
(oleylamine/oleic acid) conditions. After a 650 °C thermal treatment, the resulting powder X-ray diffraction pattern for each powder
was found to be consistent with HfO2 (PDF 00-040-1173), and after a 1000 °C treatment, larger particles of HfO2 (PDF 00-043-1017)
were reported. Transmission electron microscopy images confirmed that nanomaterials had formed. Because identical processing
conditions had been employed for each HfO2 nanomaterial, the morphological variations observed in this study may be attributed to
the individual precursors (“precursor structure affect”).
(“getters”) for nuclear control rods, electroceramic devices such
as nonvolatile computer memories, high dielectric constant (k)
barrier materials for metal oxide semiconductor field effect tran-
sistors, heat mirrors, and sensors to mention a few.
INTRODUCTION
■
Oxide materials of the group 4 metals have found widespread
use in a variety of daily applications, such as food coloring
agents, transistors, photocatalysts, dental implants, sunscreens,
paints, thermal barrier coatings, and many more. While a
number of precursors have been used to generate these ceramic
oxides, metal alkoxides ([M(OR)x]) have found favor because
of the ease with which their physical properties (i.e., high solu-
bility, low thermal decomposition, low C retention, etc.) can be
tuned simply by altering the ligand set. Because [M(OR)x]
precursor structures have been found to play a role in deter-
mining the final materials’ properties,1−11 understanding their
coordination behavior is of interest. Thus, a great deal of effort
has focused on structurally identifying [Ti(OR)4] and [Zr(OR)4]
precursors;12 however, significantly less effort has been proffered
concerning the identification of [Hf(OR)4].12−16 This is surpris-
ing because hafnia (HfO2)-based materials are employed in a
number of important applications, including neutron absorbers
We are interested in exploiting the high-k properties of HfO2
nanowires for semiconductor applications; however, only a
handful of synthesis efforts concerning HfO2 nanomaterials
have been reported.17−22 Of the different methods available, solu-
tion routes are preferred because of the simple experimental
setup and their amenability to large-scale production. The
reported solution routes to HfO220−22 were found to employ in
situ generated [Hf(OR)4]; therefore, it was reasoned that by
starting with well-characterized [Hf(OR)4] compounds,
higher purity HfO2 with more control over the final mor-
phologies would be available. Because of the impact that the
Received: December 5, 2011
Published: November 6, 2012
© 2012 American Chemical Society
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a
arrangement of [M(OR)4] has on the final properties of nano-
materials,1−11 coupled with the void in structurally characterized
[Hf(OR)4],13−16,23 it was of interest to generate a variety of
[Hf(OR)4] prior to initiating a study on their utility for the
production of tailored HfO2 nanomaterials.
Table 1. List of Products from Equation 1
The commercially available monomeric11 precursor hafnium
tert-butoxide ([Hf(OBut)4]) was selected as the starting synthon
because it is a soluble oil, which was thought to facilitate the sub-
stitution reactions shown in eq 1. The ligands employed included
(a) simple phenols, (HOAr) = HOC6H4(R)-2 or HOC6H3-
(R)2-2,6 where R = CH3, CH(CH3)2, C(CH3)3; (b) complex
polydentate alcohols, tetrahydrofuran methanol (H-OTHF or
(OC4H7)cy(CH2OH)-2), pyridinecarbinol (H-OPy or (NC5H4)cy-
(CH2OH)-2) where “cy” denotes cyclic, and tris(hydroxymethyl-
ethane) (THME-H3 or (HOCH2)3CCH3) in a series of
Lewis basic solvents of (i) tetrahydrofuran (THF), (ii)
pyridine (py), or (iii) 1-methylimidazole (MeIm). Sche-
matics of the ligands investigated in this effort are shown in
Figure 1a−j. Table 1 lists the products isolated, and the full
list of compounds follows.
a
− = crystal not isolated. Blue = solvates of [Hf(OBut)4]. Orange =
phenoxide (i) tol, (ii) THF, and (iii) py. Green = polydentate.
[Hf(μ-OC6H4(CH3)-2)(OC6H4(CH3)-2)3(HOBut)]2 (4),
[Hf2(μ3-O)(μ-OH)(μ-OC6H4(CH3)-2)(OC6H4(CH3)-2)4-
(HOBut)]2·tol (4a), [Hf(μ-OC6H4(CH(CH3)2)-2)-
(OBut )3 ]2 (5), [H][Hf3(μ3 -O2 (μ-OBut )3 (OC6 H4 -
(CH(CH3)2)-2)6] (5a), [Hf(μ-OH)(OC6H4(C(CH3)3)-2)3-
(HOBut)]2·2tol (6), [Hf(μ-OH)(OC6H3(CH3)2-2,6)3(HOBut)]2
(7), [Hf(OC6H3(CH(CH3)2)2-2,6)4(HOBut)] (8), [Hf(OC6H3-
(C(CH3)3)2-2,6)(OBut)3(HOBut)] (9), [Hf(OC6H4(CH3)-2)4-
(THF)2] (10), [Hf(OC6H4(CH(CH3)2)-2)2(OBut)2(THF)2]
(11), [Hf(OC6H3(CH3)2-2,6)4(THF)] (13), [Hf(OC6H3-
(CH(CH3)2)2-2,6)3(OBut)(THF)] (14), [Hf(OC6H3(C-
(CH3)3)2-2,6)(OBut)3(THF)] (15), [Hf(OC6H4(CH3)-2)4-
(py)2]·py (16), [Hf(OC6H3(CH3)2-2,6)4(py)2] (19), [Hf(μ-
OH)(OC6H3(CH3)2-2,6)3(py)]2·3py (19a), [Hf(OC6H3(CH-
(CH3)2)2-2,6)3(OBut)(py)] (20), and [Hf(OC6H3(C(CH3)3)2-
2,6)(OBut)3(py)] (21). The remaining complex polydentate alcohol
derivatives were solved as [Hf(μc-OTHF)(OBut)3]2 (22),
solv
[Hf(OBut)4] + nHOR ⎯⎯→⎯ [Hf(OBut)4−n(OR)n] + nHOBut
(1)
[Hf2(OBut)4(μc-OPy)3(OPy)] (23), [(OPy)c Hf(OBut)2]
2
(24), and [(μ-THME)(μ-OR)Hf2(OR)4]2 [OR = OBut (25)
and ONep (OCH2C(CH3)3 (26) where “c” denotes chelation].
Note: Crystals could not be isolated in our hands for com-
pounds 12 [OC6H4(C(CH3)3)-2/THF], 17 [OC6H4(CH-
(CH3)2)-2/py], and 18 [OC6H4(C(CH3)3)-2/py]. The syn-
thesis and characterization of the precursor compounds 1−26
are discussed in detail.
With this novel set of well-characterized precursors (1−26)
now available, select members having varied nuclearity (monomer,
[Hf(OBut)4], 8; dimer, 19a, 22; tetramer, 25) were processed
under solvothermal (SOLVO) conditions to generate the first
alkoxide precursor solution route to HfO2 nanoparticles. The com-
pounds were chosen based on their nuclearities to
demonstrate their utility as HfO2 precursors and initiate a
study on the precursor structure affect (PSA).4,9,10 Details of
the synthesis, crystal structures, and preliminary investiga-
tion into the nanomaterials generated from this novel family
of [Hf(OR)4] compounds will be discussed.
Figure 1. Schematic representation of the various ligands employed
in this study: (a) HOC(CH3)3 (HOBut), HOC6H3(R)-2,(R′)-6 where
R′ = H and R = (b) CH3, (c) CH(CH3)2, and (d) C(CH3)3,
HOC6H3(R)-2,(R′)-6 where R′ = R = (e) CH3, (f) CH(CH3)2,
(g) C(CH3)3, (h) (OC4H7)cy(CH2OH)-2 (H-OTHF), (i)
(NC5H4)cy(CH2OH)-2 (H-OPy), and (j) (HOCH2)3CCH3
(THME-H3).
EXPERIMENTAL SECTION
■
All reactions were performed under a dry, inert atmosphere using
standard Schlenk-line and glovebox techniques. The following chem-
icals were used as received from Aldrich: [Hf(OBut)4], HOAr =
HOC6H4(R)-2 and HOC6H3(R)2-2,6 (where R = CH3, CH(CH3)2,
C(CH3)3), (OC4H7)cy(CH2OH)-2 (H-OTHF), (NC5H4)cy(CH2OH)-2
(H-OPy), (HOCH2)3CCH3 (THME-H3). All anhydrous solvents
The solvent derivatives (Table 1) of [Hf(OBut)4] were
characterized as [Hf(OBut)4(solv)n] [n = 1: THF (1);
n = 2: py (2), MeIm (3)]. The phenoxide-modified products
were also identified by single-crystal X-ray diffraction as
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1
(tol, THF, py, and MeIm) were used as received (Aldrich) in Sure/Seal
bottles and handled under an inert atmosphere.
472(s), 465(s,sh). H NMR (500.1 MHz, CDCl3): δ 7.23 (0.8H, d,
OC6H4(CH(CH3)2)-2), JH−H = 7.8 Hz), 7.01 (1.2H, d, OC6H4(CH-
Fourier transform infrared (FTIR) spectral data were obtained on a
Nicolet 6700 FTIR spectrometer 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 by
dissolving crystalline material in the appropriate deuterated solvent
followed by flame sealing under vacuum. Spectra were collected on a
Bruker Avance 500 NMR spectrometer, using a 5 mm inverse probe,
(CH3)2-2), JH−H = 7.3 Hz), 6.91 (1.1H, t, OC6H4(CH(CH3)2-2), JH−H =
6.9 Hz), 6.73 (0.9H, t, OC6H4(CH(CH3)2)-2), JH−H = 7.3 Hz), 3.32 (0.9,
sept, OC6H4(CH(CH3)2-2), JH−H = 6.7 Hz), 1.17 (5.0H, d, OC6H4(CH-
(CH3)2-2), JH−H = 6.7 Hz), 0.90 (27H, s, OC(CH3)3). Elem anal. Calcd
for C42H76Hf2O8 (MW = 1066.01): C, 47.32; H, 7.19. Found: C, 45.57;
H, 7.18.
[Hf(μ-OH)(OC6H4(C(CH3)3)-2)3(HOBut)]2·2tol (6). [Hf(OBut)4]
(0.47 g, 1.0 mmol) and H-OC6H4(C(CH3)3)-2 (0.60 g, 4.0 mmol)
in ∼3 mL of tol. Yield: 67% (0.54 g). FTIR (KBr, cm−1): 3853(w),
3095(w), 3077(w), 3047(w), 3009(m), 2969(s), 2361(w), 1917(w),
1879(w), 1594(m), 1569(m), 1485(s), 1438(s), 1386(m), 1362(s),
1294(s), 1270(s), 1233(s), 1195(s), 1130(m), 1090(m), 1055(m),
1030(s), 998(s), 875(s), 861(s), 829(m), 788(m), 756(s), 742(s),
693(m), 616(m), 569(m), 542(m), 525(m), 487(m), 441(m). 1H
1
under standard experimental conditions: H analyses were performed
with a 4-s recycle delay at 16 scans; spectra were referenced to the
residual proton peak of chloroform-d (CDCl3) at 7.24 ppm, toluene-d8
(tol-d8) peak at 2.09 ppm, tetrahydrofuran-d8 (THF-d8) at 1.79 ppm,
or pyridine-d5 (py-d5) at 8.75 ppm.
General Synthesis of [Hf(OBut)4(solv)x]. A sample of [Hf-
(OBut)4] (1.0 g, 2.1 mmol) was added to a vial containing the
desired solvent (∼3 mL). The resultant precipitate was heated
until the reaction turned clear. After cooling to room temperature,
the reaction mixture was allowed to set uncovered until crystals
formed.
NMR (500.1 MHz, tol-d8): δ 7.29 (6.0H, d, OC6H4(C(CH3)3-2), JH−H
=
7.6 Hz), 6.79 (5.8H, t, OC6H4(C(CH3)3-2), JH−H = 7.9 Hz), 1.53
(24.1H, s, OC6H4(C(CH3)3-2)), 1.23 (14.1H, s, OC(CH3)3). Elem
anal. Calcd for C82H114Hf2O10 (MW = 1616.76): C, 60.92; H, 7.11.
Found: C, 58.71; H, 6.89.
[Hf(OBut)4(THF)] (1). Solvent used: THF. Yield: 0.68 g (59%). FTIR
(KBr, cm−1): 2970(s), 2916(m,sh), 2888(m,sh), 2851(m,sh), 1459(m),
1384(m), 1358(s), 1243(m,sh), 1228(m,sh), 1191(s), 1018(s), 991(s),
903(m), 781(br,m), 701(w), 669(w), 568(m), 527(m), 493(m), 475(m).
1H NMR (500.1 MHz, THF-d8): δ 1.63 (OC(CH3)3). Elem anal.
Calcd for C20H44HfO5 (MW = 543.05): C, 44.23; H, 8.17. Found: C,
37.40; H, 7.37.
[Hf(μ-OH)(OC6H3(CH3)2-2,6)3(HOBut)]2 (7). [Hf(OBut)4] (0.50 g,
1.1 mmol) and H-OC6H3(CH3)2-2,6 (0.52 g, 4.3 mmol) in ∼10 mL of
tol. Yield: 72% (0.48 g). FTIR (KBr, cm−1): 3422(s,br), 3068(w),
3039(w), 3019(w), 3010(w), 2962(s), 2924(s), 2855(s), 2733(w),
1592(m), 1561(w), 1508(w), 1473(s), 1437(s,sh), 1427(s), 1401(w),
1372(w), 1276(s), 1231(s), 1170(w), 1161(w), 1092(s), 1044(m),
1033(m), 983(w), 888(s), 863(m), 801(m), 760(s), 735(m), 719(m),
1
[Hf(OBut)4(py)2] (2). Solvent used: py. Yield: 1.3 g (97%). FTIR
(KBr, cm−1): 2970(s), 2916(m,sh), 2888(m,sh), 2851(m,sh), 1460(m),
1425(m), 1385(m), 1458(m), 1359(s), 1243(m,sh), 1214(m,sh), 1201(s),
1025(s,sh), 1021(s), 995(s), 908(m), 901(m), 850(w), 682(w), 667(w),
550(m), 489(w), 467(m), 155(m), 446(m). H NMR (500.1 MHz,
tol-d8): δ 6.88 (2.4H, d, OC6H3(CH3)2-2,6, JH−H = 7.3 Hz), 6.69
(1.4H, t OC6H3(CH3)3-2), 2.20 (6.0H, s(br), OC6H4(CH3)2-2,6), 0.98
(2.6H, s, OC(CH3)3). Elem anal. Calcd for C56H76Hf2O10 (MW =
1266.18): C, 53.12; H, 6.05. Found: C, 58.71; H, 6.89.
1
568(m), 530(m), 492(m), 476(m). H NMR (500.1 MHz, py-d5):
[Hf(OC6H3(CH(CH3)2)2-2,6)4(HOBut)] (8). [Hf(OBut)4] (0.47 g, 1.0
mmol) and H-OC6H3(CH(CH3)2)2-2,6 (0.71 g, 4.0 mmol) in ∼3 mL
of tol. Yield: 60% (0.57 g). FTIR (KBr, cm−1): 3905(w), 3886(w),
3855(w), 3822(w), 3808(w), 3753(w), 3745(w), 3713(w), 3676(w),
3657(w), 3630(w), 3566(s), 3175(w), 3062(m), 3024(m), 2963(s),
2868(s), 2586(m), 2302(w), 1902(w), 1845(w), 1794(w), 1697(w),
1641(w), 1588(m), 1435(s), 1382(s), 1361(s), 1325(s), 1259(s),
1200(s), 1159(s), 1112(s), 1095(s), 1041(s), 1016(s), 951(w),
936(m), 892(s), 870(s), 822(m), 805(m), 793(s), 749(s), 704(s),
623(w), 587(w), 551(w), 536(w), 473(m), 445(w), 422(m), 405(m).
1H NMR (500.1 MHz, tol-d8): δ 7.03 (2.0H, d, OC6H3(CH(CH3)2-
δ 1.40 (OC(CH3)3). Elem anal. Calcd for C26H46HfN2O4 (MW =
629.14): C, 49.64; H, 7.37; N, 4.45. Found: C, 48.15; H, 7.57; N, 4.29.
[Hf(OBut)4(MeIm)2] (3). Solvent used: tol with MeIm (∼5 drops).
Washed repeatedly with hexanes and toluene. Yield: 1.2 g (89%).
FTIR (KBr, cm−1): 2963(s), 2910(m,sh), 2897(m,sh), 2866(m,sh),
2842(m,sh), 1560(m), 1530(m), 1473(m), 1457(m), 1422(m),
1374(m), 1348(m), 1199(s), 1108(m), 1085(m), 1019(s), 936(m),
830(w), 825(w), 739(m), 661(m), 617(w), 500(m), 478(m). Elem
anal. Calcd for C24H48HfN4O4 (MW = 635.15): C, 45.38; H, 7.62; N,
8.82. Found: C, 41.27; H, 6.84; N, 7.29.
General Synthesis of HOAr-Modified [Hf(OBut)4]. The desired
ligand was added to a vial containing [Hf(OBut)4] fully dissolved in
the solvent of choice. In some instances (noted below), the reaction
mixture was placed directly into a freezer at −40 °C. Otherwise, the
reaction mixture was stirred for 24 h and then allowed to sit with
the cap loose until crystals formed. Yields were based on the first
crystalline batch. Analytical data were collected on dried crystalline
material.
2,6), JH−H = 7.6 Hz), 6.88 (1.0H, t, OC6H3(CH(CH3)2-2,6), JH−H
=
7.6 Hz), 3.38 (1.6H, s(br), OC6H3(CH(CH3)2-2,6), 1.33 (2.2H, s,
OC(CH3)3), 1.12 (11.4H, s(br), OC6H3(CH(CH3)2-2,6)). Elem anal.
Calcd for C52H78HfO5 (MW = 961.66): C, 64.95; H, 8.18. Found: C,
62.25; H, 7.62
[Hf(OC6H3(C(CH3)3)2-2,6)(OBut)3(HOBut)] (9). [Hf(OBut)4] (0.41 g,
0.87 mmol) and H-OC6H3(C(CH3)3)-2,6 (0.18 g, 0.87 mmol) in ∼3 mL
of tol. Yield: 44% (0.26 g, crystals isolated at −40 °C). FTIR (KBr, cm−1):
3062(w), 2966(s), 2926(s), 2870(m), 1420(m), 1389(m), 1360(m),
1262(m), 1231(w), 1189(m), 1094(s), 1050(s), 1016(s), 889(m),
[Hf(μ-OC6H4(CH3)-2)(OC6H4(CH3)-2)3(HOBut)]2 (4). [Hf(OBut)4]
(0.50 g, 1.1 mmol) and H-OC6H4(CH3)-2 (0.46 g, 4.3 mmol) in
∼5 mL of tol. Yield: 39% (0.28 g). FTIR (KBr, cm−1): 2969(s),
2960(w), 1813(w), 1672(m, sh), 1487(s), 1360(m), 1292(m,sh),
1263(s), 1191(s), 1113(m), 1043(s), 899(w), 848(m), 793(w,sh),
757(s), 700(s), 633(s), 598(s), 538(m), 479(s). 1H NMR (500.1
MHz, CDCl3): δ 7.10 (4.0H, d, OC6H4(CH3)-2), JH−H = 6.8 Hz), 6.93
(3.9H, s, br), OC6H4(CH3)-2)), 6.82 (4.1H, t, OC6H4(CH(CH3)2-2),
JH−H = 7.1 Hz), 6.73 (3.7H, s(br), OC6H4(CH3)-2)), 2.04 (12.4H,
s(br), OC6H4(CH3)-2), 1.29 (10.4H, s, OC(CH3)3)). Elem anal.
Calcd for C64H76Hf2O10 (MW = 1362.26): C, 56.43; H, 5.62. Found:
C, 54.24; H, 5.49.
1
844(w), 797(s), 746(s), 747(m), 676(w), 469(s,br). H NMR (500.1
MHz, CDCl3): δ 7.23 (2.0H, d, (OC6H3(C(CH3)3)2-2,6), JH−H = 7.7
Hz), 6.78 (0.8H, s(br), (OC6H3(C(CH3)3)2-2,6)), 1.50 (17H, s,
O(C(CH3)3)2-2,6), 1.33 (27H, s, OC(CH3)3). Elem anal. Calcd for
C30H58HfO5 (MW = 677.27): C, 53.20; H, 8.63. Found: C, 50.47; H,
8.11.
[Hf(OC6H4(CH3)-2)4(THF)2] (10). [Hf(OBut)4] (0.50 g, 1.1 mmol)
and H-OC6H4(CH3)-2 (0.46 g, 4.3 mmol) in ∼3 mL of THF. Yield:
79% (0.63 g). FTIR (KBr, cm−1): 3066(m), 3016(m), 2982(m),
2966(m), 2926(m), 2882(m), 2871(m), 1596(s), 1578(m), 1546(w),
1487(s), 1460(m), 1387(w), 1280(sh,s), 1277(s), 1265(s), 1230(m),
1208(w), 1188(w), 1167(w), 1155(w), 1114(s), 1042(s), 1042(m),
984(w), 932(w), 895(s), 884(s), 858(m), 805(w), 773(sh,m), 752(s),
734(sh,m), 714(m), 630(m), 582(m), 559(w), 526(m), 463(m),
[Hf(μ-OC6H4(CH(CH3)2)-2)(OBut)3]2 (5). [Hf(OBut)4] (0.50 g, 1.1
mmol) and H-OC6H4(CH(CH3)2)-2 (0.14 g, 1.1 mmol) in ∼1.5 mL
of tol. Yield: 37% (0.21 g, crystals isolated at −40 °C). FTIR (KBr, cm−1):
3065(w), 3021(w), 2967(s), 2928(s) 2908(s,sh), 2868(s,sh), 1655(w),
1593(m), 1577(m), 1572(m), 1560(m), 1508(m), 1485(s), 1445(s),
1382(m,sh), 1361(s,br), 1291(s), 1262(s), 1232(m), 1197(s), 1146(m),
1086(s), 1033(s), 902(m), 872(m), 789(m), 755(s), 483(s,sh),
1
412(w). H NMR (500.1 MHz, THF-d8): δ 6.69 (1H, d, OC6H4-
(CH3)-2, JH−H = 3.6 Hz), 6.83 (1H, t, OC6H4(CH3)-2, JH−H = 7.4 Hz),
12077
dx.doi.org/10.1021/ic300622h | Inorg. Chem. 2012, 51, 12075−12092

Inorganic Chemistry
Article
6.74 (1H, d, OC6H4(CH3)-2, JH−H = 3.6 Hz), 6.53 (1H, t, OC6H4-
(CH3)-2, JH−H = 7.3 Hz), 2.14 (3H, s, OC6H4(CH3)-2). Elem anal.
Calcd for C36H44HfO6 (MW = 751.22): C, 57.56; H, 5.90. Found: C,
57.04; H, 6.10.
[Hf(OC6H3(CH(CH3)2)2-2,6)3(OBut)(py)] (20). [Hf(OBut)4] (0.50 g,
1.1 mmol) and H-OC6H3(CH(CH3)2)2-2,6 (0.57 g, 3.2 mmol) in
∼3 mL of py. Yield: 83% (0.76 g). FTIR (KBr, cm−1): 3060(br,w),
3029(w), 2963(s), 2925(m), 2869(m), 149(m), 1438(s), 1382(m),
1329(m), 1262(s), 1207(s,br), 1151(m,br), 1115(s), 1096(s), 1060(m),
1044(m), 1018(m), 955(w), 935(w), 895(s), 873(s), 801(s), 795(s),
[Hf(OC6H4(CH(CH3)2)-2)2(OBut)2(THF)2] (11). [Hf(OBut)4] (0.50 g,
1.1 mmol) and H-OC6H4(CH(CH3)2)-2 (0.30 g, 2.1 mmol) in ∼3 mL
of THF. Yield: 68% (0.53 g). FTIR (KBr, cm−1): 3023(m), 3066(m),
2961(s), 2926(m), 2909(m), 2867(m), 1593(m), 1574(m), 1483(s),
1444(s), 1382(m), 1361(m), 1345(m), 1285(s), 1255(br,s), 1271-
(m,sh), 1193(m), 1174(m), 1147(m), 1111(w), 1085(m), 1033(m),
1012(w), 901(s), 874(s), 860(s), 841(sh,m), 753(s), 627(m), 572(m),
1
749(s), 704(m), 669(m), 591(m), 475(br,m). H NMR (500.1 MHz,
py-d5): δ 8.74 (C5H5), 7.58 (C5H5), 7.21 (C5H5), 6.90 (6.1H, d,
OC6H3(CH(CH3)2)2-2,6, JH−H = 3.7 Hz), 6.63 (3.1H, t, OC6H3(CH-
(CH3)2)2-2,6, JH−H = 7.4 Hz), 3.76 (6.0H, sept, OC6H3(CH(CH3)2)2-
2,6, JH−H = 6.8 Hz), 1.39 (9.5H, s, OC(CH3)3), 1.16 (33.1H, d,
OC6H3(CH(CH3)2)2-2,6, JH−H = 3.3 Hz). Elem anal Calcd for
C45H65HfNO4 (MW = 862.50): C, 62.67; H, 7.60; N, 1.62. Found:
C, 61.92; H, 7.42; N, 2.49.
1
533(w), 497(w), 468(w), 426(w). H NMR (500.1 MHz, CDCl3): δ
7.08 (1H, d, OC6H4(CH(CH3)2-2), JH−H = 7.0 Hz), 7.04 (1H, d,
OC6H4(CH(CH3)2-2, JH−H = 6.9 Hz), 6.60 (1H, multiplet,
OC6H4(CH(CH3)2-2)), 6.54 (1H, multiplet, OC6H4(CH3)-2), 3.72
(1H, multiplet, OC6H4(CH(CH3)2-2), 1.27 (8.5H, s, OC(CH3)3),
1.16 (6.7H, d, OC6H4(CH(CH3)2-2), JH−H = 6.9 Hz). Elem anal.
Calcd for C34H56HfO6 (MW = 739.29): C, 55.24; H, 7.63. Found: C,
58.88; H, 6.81.
[Hf(OC6H3(C(CH3)3)2-2,6)(OBut)3(py)] (21). [Hf(OBut)4] (0.25 g,
0.53 mmol) and H-OC6H3(C(CH3)3)2-2,6 (0.11 g, 0.53 mmol) in ∼3 mL
of py. Yield: 67% (0.24 g). FTIR (KBr, cm−1): 3016(w), 2964(s),
2923(m), 2958(m), 1477(s), 1468(s),1456(m), 1426(s), 1374(w,br).
1278(s,br), 1266(s), 1230(s), 1092(s), 1031(m,br), 982(w), 957(w),
890(s), 864(m), 802(m,br), 763(s), 734(m), 721(m), 706(w), 563(w),
[Hf(OC6H3(CH3)2-2,6)4(THF)] (13). [Hf(OBut)4] (0.50 g, 1.1 mmol)
and H-OC6H3(CH3)2-2,6 (0.52 g, 4.3 mmol) in ∼3 mL of THF. Yield:
50% (0.39 g). FTIR (KBr, cm−1): 3067(m), 3040(m), 3015(m),
2944(s), 2919(s), 2854(m), 2731(w), 1608(w), 1592(m), 1473(s),
1444(m,sh), 1428(s), 1276(s), 1230(s), 1159(w), 1092(s), 1072(w),
1043(w), 1015(w), 983(w), 889(s), 879(m,sh), 761(s), 735(s),
1
551(w,br), 475(w), 463(w), 420(w). H NMR (500.1 MHz, py-d5): δ
8.72 (C5H5), 7.57 (C5H5), 7.43 (2.0H, d, OC6H3(C(CH3)3)2-2,6), 7.22
(C5H5), 6.90 (0.8H, t, OC6H3(C(CH3)3)2-2,6), 1.61 (18.0H, σ, OX-
(XH3)3), 1.36 (32.0H, s, OC6H3(C(CH3)3)2-2,6). Elem anal. Calcd for
C31H53HfNO4 (MW = 682.25): C, 54.57; H, 7.83; N, 2.05. Found: C,
53.01; H, 7.78; N, 1.93.
1
719(s), 699(w), 633(w), 550(m), 488(m), 460(m). H NMR (500.1
[Hf(μc-OTHF)(OBut)3]2 (22). [Hf(OBut)4] (1.0 g, 2.1 mmol) and H-
OTHF (0.22 g, 2.1 mmol) in ∼1.5 mL of THF. Yield: 52% (0.55 g).
FTIR (KBr, cm−1): 2966(s), 2918(m,sh), 2888(m,sh), 2836(m,sh),
2760(w), 2685(w), 1465(m), 1458(m), 1380(m), 1355(s), 1205-
(m,sh), 1205(m,sh), 1197(s), 1020(m,sh), 1010(s), 927(m), 815(m),
780(m), 756(m), 522(m), 480(m). NMR (500.1 MHz, CDCl3): δ
4.58−4.27 (1.4H, multiplet, OC4H7(CH2O)), 4.16−3.71 (5.5H,
multiplet, OC4H7(CH2O)), 1.94 (4.2H, s(br), OC4H7(CH2O)), 1.25
(13.5H, s, OC(CH3)3), 1.16 (13.5H, s, OC(CH3)3). Elem anal. Calcd
for C34H72Hf2O10 (MW = 997.91): C, 40.92; H, 7.27. Found: C,
41.08; H, 7.32.
MHz, THF-d8): δ 6.76 (2H, d, OC6H3(CH3)2-2,6, JH−H = 2.5 Hz),
6.45 (1H, t, OC6H3(CH3)2-2,6, JH−H = 7.3 Hz), 2.08 (6.2H, s, OC6H3-
(CH3)2-2,6). Elem anal. Calcd for C36H44HfO5 (MW = 735.22): C,
58.81; H, 6.03. Found: C, 58.24; H, 6.03.
[Hf(OC6H3(C(CH3)3)2-2,6)(OBut)3(THF)] (15). [Hf(OBut)4] (0.25 g,
0.53 mmol) and H-OC6H3(C(CH3)3)2-2,6 (0.11 g, 0.53 mmol) in
∼3 mL of THF. Yield: 28% (0.10 g). FTIR (KBr, cm−1): 2970(s),
2940(s,sh), 2928(s,sh), 2916(s,sh), 2868(s), 2812(w), 1469(m),
1458(m), 1426(m), 1387(s), 1359(s), 1314(w), 1249(s), 1230(s),
1202(s), 1193(s), 1146(w) 1123(w), 1093(w), 1036(s,br), 1022(s,br),
1001(s), 910(s), 894(s), 844(w), 822(w), 806(w), 781(m), 767(m),
746(m), 727(m), 679(w), 670(w), 857(s), 567(s), 531(s), 521(s,sh),
493(s), 474(s), 448(s). 1H NMR (500.1 MHz, THF-d8): δ 7.03 (2.8H,
d, OC6H3(C(CH3)3)2-2,6), 6.46 (0.70H, d, OC6H3(C(CH3)3)2-2,6),
1.42 (18.0H, s, OC6H3(C(CH3)3)2-2,6), 1.27 (26.6H, σ, OX(XH3)3).
Elem anal. Calcd for C30H56HfO5 (MW = 675.25): C, 53.36; H, 8.36.
Found: C, 42.13; H, 7.27.
[(OPy)c2Hf(OBut)2] (24). [Hf(OBut)4] (3.0 g, 6.3 mmol) and H-OPy
(1.4 g, 13 mmol) heated in ∼3 mL of tol. Yield: 56% (1.9 g). 1H NMR
(500.1 MHz, tol-d8): δ 8.71 (1H, d, NC5H5(CH2O), JH−H = 5.3 Hz),
6.81 (1.1H, t, NC5H5(CH2O), JH−H = 7.9 Hz), 6.51 (1.2H, t,
NC5H5(CH2O), JH−H = 6.2 Hz), 6.43 (1.1H, d, NC5H5(CH2O),
JH−H = 7.9 Hz), 5.51 (2.1H, s(br), NC5H5(CH2O), JH−H = 5.3 Hz), 1.38
(9.3H, s, OC(CH3)3). Elem anal. Calcd for C20H30HfN2O4 (MW =
540.95): C, 44.41; H, 5.59; N, 5.18. Found: C, 44.08; H, 5.46; N, 5.23.
[(μ-THME)(μ-OBut)Hf2(OBut)4]2 (25). [Hf(OBut)4] (0.40 g, 0.85
mmol) and H3-THME (0.050 g, 0.42 mmol) in ∼1.5 mL of tol. Yield:
82% (0.29 g). FTIR (KBr, cm−1): 2968(s), 2922(m), 2865(m),
1458(m,br), 1406(m), 1383(m), 1358(m), 1206(s,br), 1128(m),
1066(m,sh), 1010(s,br), 917(w), 880(w), 781(w), 621(w), 485(s,br).
Elem anal. Calcd for C50H108Hf4O16 (MW = 1679.36): C, 35.76; H,
6.48. Found: C, 36.16; H, 6.62.
[Hf(OC6H4(CH3)-2)4(py)2]·py (16). [Hf(OBut)4] (0.50 g, 1.1 mmol)
and H-OC6H4(CH3)-2 (0.46 g, 4.3 mmol) in ∼3 mL of py. Yield: 75%
(0.68 g). FTIR (KBr, cm−1): 3064(m), 3013(m), 2963(m), 2944(m),
2925(m), 2855(m), 1624(m), 1594(m), 1485(s), 1446(m), 1324(m),
1280(s), 1266(s,sh), 1214(m), 1187(m), 1150(m), 1112(m),
1069(m), 1043(m), 1013(m), 982(w), 928(w), 907(m,sh), 896(m,sh),
881(m), 846(w), 805(w), 770(m), 753(m), 716(w), 700(m), 629(m),
617(m), 562(m), 466(m). 1H NMR (500.1 MHz, py-d5): δ 8.71
(C5H5), 7.56 (C5H5), 7.18 (1.3H, multiplet, OC6H4(CH3)-2 and
C5H5), 7.04 (1.1H, t, OC6H4(CH3)-2, JH−H = 7.4 Hz), 6.91 (1.0H,
br s, OC6H4(CH3)-2), 6.80 (1.1H, d, OC6H4(CH3)-2, JH−H = 3.6 Hz),
2.29 (3.0H, s, OC6H4(CH3)-2). Elem anal. Calcd for C43H43HfN3O4
(MW = 845.27): C, 61.17; H, 5.13; N, 4.98. Found: C, 60.52; H, 5.25; N,
3.66.
General X-ray Crystal Structure Information.24 Crystals were
mounted onto a glass fiber from a pool of Fluorolube and immediately
placed in a cold N2 vapor stream, on a Bruker AXS diffractometer equipped
with a SMART APEX CCD detector using graphite-monochromatized
Mo Kα radiation (λ = 0.7107 Å). Lattice determination and data collec-
tion were carried out using SMART, version 5.054, software or the
APEXII software suite. Data reduction and absorption correction were
performed using either SAINTPLUS, version 6.01, software (absorp-
tion correction via the SADABS program within the SAINT software
package) or the APEXII software suite (absorption correction
performed using face indexing).
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 H atoms
were fixed in positions of ideal geometry (riding model) and refined
using SHELXS or XSHELL. The final refinement of each compound
included anisotropic thermal parameters for all non-H atoms. All final
[Hf(OC6H3(CH3)2-2,6)4(py)2] (19). [Hf(OBut)4] (0.50 g, 1.1 mmol)
and H-OC6H3(CH3)2-2,6 (0.52 g, 4.3 mmol) in ∼3 mL of py. Yield:
60% (0.52 g). FTIR (KBr, cm−1): 3060(w), 2963(s), 2927(m),
2869(s), 1474(m), 1459(m), 1438(s), 1382(m), 1363(m), 1329(m),
1262(s), 1207(s), 1151(m), 1113(s), 1096(s), 1060(m), 1044(m),
1018(s), 955(m), 936(m), 895(s), 873(s), 803(s), 795(s), 749(s),
1
704(m), 659(m), 591(m), 475(m,br). H NMR (500.1 MHz, py-d5):
δ 8.57 (C5H5), 7.41 (C5H5), 7.05 (C5H5), 6.90 (2.0H, d, OC6H3-
(CH3)2-2,6, JH−H = 3.7 Hz), 6.63 (1.0H, t, OC6H3(CH3)2-2,6, JH−H
=
7.4 Hz), 1.90 (6.1H, s, OC6H3(CH3)2-2,6). Elem anal. Calcd for
C42H46HfN2O4 (MW = 821.32): C, 61.42; H, 5.65; N, 3.41. Found: C,
63.42; H, 5.74; N, 4.67.
12078
dx.doi.org/10.1021/ic300622h | Inorg. Chem. 2012, 51, 12075−12092

Inorganic Chemistry
Article
Table 2. Data Collection Parameters for 1−26
c
1
2
3
4
4a
chemical formula
fw
C20H44HfO5
543.05
C26H46HfN2O4
629.14
C24H48HfN4O4
635.15
C64H74Hf2O10
1360.21
C92H104Hf4O16
2179.71
temp (K)
space group
a (Å)
188(2)
173(2)
173(2)
193(2)
173(2)
orthorhombic, Cmc2(1)
11.2622(12)
19.504(2)
monoclinic, Cc
12.4267(16)
14.5716(18)
16.503(2)
triclinic, P1
monoclinic, C2/c
17.841(2)
triclinic, P1
̅
̅
9.4760(13)
9.6926(14)
16.645(2)
94.277(2)
93.582(2)
100.798(2)
1493.0(4)
2
12.868(5)
14.209(5)
14.610(5)
111.724(5)
103.867(5)
107.372(5)
2175.8(13)
1
b (Å)
14.6484(16)
24.992(3)
c (Å)
12.2942(13)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
96.077(2)
96.5190(10)
2700.5(5)
4
2971.6(7)
4
6489.3(12)
4
D
calcd(Mg/m3)
1.256
1.406
1.413
1.392
1.664
μ,(Mo Kα) (mm−1
)
3.881
3.540
3.525
3.294
4.819
a
R1 (all data) (%)
3.85 (5.10)
9.99 (10.85)
5
2.00 (2.06)
5.05 (5.02)
5a
2.02 (2.06)
4.96 (5.00)
6
4.82 (8.17)
12.17 (14.68)
7
4.19 (5.97)
10.11 (11.29)
8
b
wR2 (all data) (%)
chemical formula
fw
C42H76Hf2O8
1066.01
C66H94Hf3O11
C82H112Hf2O10
1614.70
C56H72Hf2O10
1262.12
C52H77HfO5
1598.91
960.63
temp (K)
space group
a (Å)
173(2)
188(2)
173(2)
193(2)
173(2)
orthorhombic, Pbca
20.414(3)
monoclinic, P21/c
19.5770(13)
13.9040(9)
25.4289(17)
triclinic, P1
triclinic, P1
monoclinic, P2(1)/c
13.0352(5)
13.5007(5)
27.8053(9)
̅
̅
11.8206(6)
11.8510(6)
14.7547(8)
81.465(2)
80.888(3)
74.560(2)
1954.89(18)
1
11.598(3)
12.002(3)
12.264(3)
95.273(2)
111.814(3)
102.161(3)
1521.9(6)
1
b (Å)
19.223(3)
c (Å)
24.999(4)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
103.8040(10)
94.921(2)
9810(2)
6721.8(8)
2
4875.3(3)
4
8
D
calcd(Mg/m3)
1.444
1.574
1.372
1.377
1.309
μ,(Mo Kα) (mm−1
)
4.272
4.676
2.708
3.457
2.183
a
R1 (all data) (%)
3.56 (5.75)
11.31 (12.43)
9
4.16 (7.67)
7.77 (9.23)
10
4.15 (5.65)
10.41 (12.68)
11
5.99 (7.04)
15.10 (16.13)
13
3.332 (6.19)
9.21 (11.96)
b
wR2 (all data) (%)
c
14
chemical formula
fw
C30H57HfO5
676.25
C36H42HfO6
C34H56HfO6
C36H44HfO5
C44H68HfO5
855.50
749.19
188(2)
739.28
735.20
temp (K)
space group
a (Å)
188(2)
188(2)
188(2)
188(2)
monoclinic, P2(1)/m
9.5174(5)
18.1890(10)
10.3225(6)
102.9030(10)
1741.83(17)
2
tetragonal, P4b2
monoclinic, P2(1)/n
14.8446(10)
15.2135(10)
16.5388(11)
104.2950(10)
3619.4(4)
4
monoclinic, P2(1)/c
12.0537(9)
35.859(3)
16.6506(13)
103.8060(10)
6989.0(9)
8
monoclinic, P2(1)/n
10.926(7)
20.224(13)
19.759(13)
91.281(7)
4365(5)
̅
18.094(6)
18.094(6)
10.231(3)
b (Å)
c (Å)
β (deg)
V (Å3)
Z
3349.3(19)
4
4
D
calcd(Mg/m3)
1.289
1.486
1.357
1.397
1.290
μ,(Mo Kα) (mm−1
)
3.025
3.157
2.920
3.022
2.429
a
R1 (all data) (%)
2.22 (2.34)
7.12 (7.51)
15
5.21 (5.94)
14.37 (15.52)
16
2.83 (4.22)
9.80 (11.82)
19
3.41 (5.18)
8.21 (9.08)
19a
13.17 (14.44)
33.69 (34.17)
20
b
wR2 (all data) (%)
chemical formula
fw
C30H56HfO5
675.24
C43H42HfN3O4
843.29
C42H46HfN2O4
821.30
C68H74Hf2N4O8
1432.29
C45H65HfNO4
862.47
temp (K)
space group
a (Å)
188(2)
188(2)
173(2)
188(2)
188(2)
monoclinic, P2(1)/n
12.0962(7)
17.8254(11)
15.7668(10)
93.7640(10)
3392.3(4)
4
orthorhombic, Pbca
18.942(3)
orthorhombic, Pbca
15.417(3)
monoclinic, P2(1)/n
11.8132(10)
18.9819(16)
16.0251(14)
103.3240(10)
3496.7(5)
2
monoclinic, P2(1)/n
12.6897(9)
18.8428(13)
18.9880(13)
105.1500(10)
4382.4(5)
4
b (Å)
14.616(2)
19.797(4)
c (Å)
27.444(4)
28.940(6)
β (deg)
V (Å3)
Z
7598(2)
8
8833(3)
8
D
calcd(Mg/m3)
1.322
1.474
1.235
1.360
1.307
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Table 2. continued
15
16
19
19a
20
μ,(Mo Kα) (mm−1
)
3.106
2.791
2.398
3.018
2.419
a
R1 (all data) (%)
3.20 (4.00)
9.85 (10.94)
21
7.78 (11.85)
17.67 (20.30)
4.85 (5.65)
12.08 (12.46)
23
2.49 (2.99)
8.66 (9.39)
3.41 (6.44)
9.40 (12.37)
25
b
wR2 (all data) (%)
c
22
24
chemical formula
fw
C31H53HfNO4
682.23
C34H72Hf2O10
997.91
C40H60Hf2N4O8
1081.90
C80H120Hf4N8O16
2161.81
C50H108Hf4O16
1679.32
temp (K)
space group
a (Å)
188(2)
173(2)
173(2)
173(2)
173(2)
monoclinic, P2(1)/n
19.892(5)
17.489(5)
19.892(5)
98.82
monoclinic, C2/c
24.768(3)
9.9099
monoclinic, P2(1)/c
20.6679(15)
9.6220(7)
24.0736(17)
111.5430(10)
4453.0(6)
4
monoclinic, P2(1)/c
15.339(2)
18.253(2)
16.805(2)
102.467(2)
4594.2(10)
4
monoclinic, C2/c
39.060(5)
15.805(2)
21.544(3)
100.258(2)
13087(3)
8
b (Å)
c (Å)
19.2242
113.452
4328.7(10)
4
β (deg)
V (Å3)
Z
6839(3)
8
D
calcd(Mg/m3)
1.325
1.346
1.614
1.508
1.705
μ,(Mo Kα) (mm−1
)
3.081
4.830
4.710
4.562
6.380
a
R1 (all data) (%)
5.06 (10.28)
12.81 (15.65)
12.88
3.89 (5.96)
8.94 (10.08)
26
3.16 (3.46)
10.75 (11.12)
2.30 (3.01)
5.62 (6.29)
b
wR2 (all data) (%)
35.60
chemical formula
fw
C60H128Hf4O16
1819.58
temp (K)
space group
a (Å)
173(2)
monoclinic, C2/c
18.564(2)
23.319(3)
18.727(2)
112.775(2)
7474.7(16)
4
b (Å)
c (Å)
β (deg)
V (Å3)
Z
Dcalcd (Mg/m3)
μ,(Mo Kα) (mm−1
1.617
)
5.592
a
R1 (all data) (%)
2.37 (3.02)
b
wR2 (all data) (%)
6.06 (6.72)
a
b
c
R1 = ∑||Fo| − |Fc||/∑|Fo| × 100. wR2 = [∑w(Fo2 − Fc2)2/∑(w|Fo|2)2]1/2 × 100. Full structure not reported because of poor quality of the final
structure model. Crystal unit cell data reported for convenience. If additional information is required concerning these structures, please contact the
authors.
Table 3. Metrical Data for the OAr Derivatives
Hf−OAr
Hf−OBut
Hf−HOBut
Hf−solv
OAr−Hf−OAr
solv−Hf−OAr
OBut−Hf−OAr OBut−Hf−OBut
o-alkyl group compd nucl
(av Å)
(av Å)
(av Å)
(av Å)
(av deg)
(av deg)
(av deg)
115.54
(av deg)
(CH3)-2
4
2
1
1
2
1
1
1
1
1
1
1
1
1
2.02
1.98
1.98
2.20
2.03
1.94
1.98
1.95
1.96
1.955
2.00
1.99
1.99
2.290
98.24
94.76
10
16
5
2.26
2.39
87.05
109.35
113.96
CH(CH3)2-2
(CH3)-2,6
1.91
1.92
111.38
96.63
102.57
103.38
11
13
19
8
2.34
2.29
2.40
157.5
97.19
117.71
112.3
109.53
106.5
128.3
105.75
106.5
CH(CH3)2-2,6
2.32
2.37
128.4
110.1
112.2
108.1
112.2
112.2
14
20
9
1.91
1.90
1.92
1.92
1.91
2.26
2.42
106.5
106.2
C(CH3)3-2,6
101.04
100.2
100.9
15
21
2.38
2.48
104.8
105.0
CIF files were checked at http://www.iucr.org/. Additional infor-
mation concerning the data collection and final structural solutions can
be found by accessing CIF files through the Cambridge Crystallo-
graphic Data Base. The data for the crystal structures of 1−26 have
been deposited at the Cambridge Crystallographic Data Centre and
allocated the deposition numbers CCDC 855821−855848. These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. [fax (44)
01223-336033; e-mail deposit@ccdc.cam.ac.uk] or via http://www.
ccdc.cam.ac.uk/conts/retrieving.html. Table 2 lists the data collection
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Table 4. Metrical Data for the OAr/Oxide Derivatives
Hf−OAr
Hf−HOBut
Hf−μ-O or
Hf---Hf
(av Å)
OAr−Hf−OAr
OR−Hf-OAr μ-O−Hf−μ-O solv−M− OAr
o-alkyl group compd nucl
(av Å)
(av Å)
Hf−μ-OH (av Å)
(av deg)
(av deg)
(av deg)
(av deg)
(CH3)-2
4a
4
3
2
2
2
1.97
1.94
1.97
1.97
1.98
2.21
(μ3) 2.09, (μ) 2.15
3.358
3.17
3.54
3.52
3.56
99.42
101.42
96.82
99.41
98.41
92.67
99.66
115.6
115.5
72.33
66.72
69.46
69.45
68.67
CH(CH3)2-2 5a
2.19
2.13
2.15
2.15
C(CH3)3-2
(CH3)-2,6
6
2.27
2.26
7
19a
111.8
monomeric or dinuclear species have been reported. Because of
this void, a great deal of [Hf(OR)4] precursor development was
warranted prior to our investigation into HfO2 nanomaterials.
The following discussion is organized by characterization of
the following: (A) solvates, unraveling the structural impact on
[Hf(OBut)4] from a series of Lewis basic solvents (i.e., THF,
py, and MeIm); (B) phenoxides, attempts to determine the
extent of aryloxide substitution and structural impact, further
subdivided by solvent (i) toluene, (ii) THF, and (iii) py; (C)
complex polydentate, alkoxide ligands to induce additional
structural variants; (D) nanoparticle synthesis, first route to
employ [Hf(OR)4] precursors and a preliminary investigation
into the applicability of the PSA4,9,10 on generating
morphologically controlled HfO2 nanoparticles.
A. Solvates (1−3). The solution molecular complexity of
[Hf(OBut)4] has not been reported but was considered to be
monomeric in toluene based on the fact that [Ti(OBut)4] and
[Zr(OBut)4] reportedly have a molecular complexity of 1.0,11
the similarity in size of Zr4+ and Hf4+,36 and that hafnium
tertiary alkoxides possess greater volatility than zirconium or
titanium.11 Therefore, it was assumed that [Hf(OBut)4] in
Lewis basic solvents would yield monomeric complexes. The
addition of a Lewis base to [Hf(OBut)4] immediately formed a
white precipitate, which facilely dissolved with heating. Upon
cooling, crystals of the monomeric solvates of THF (1), py (2),
and MeIm (3), shown in Figure 2a−c, respectively, were for-
med. Compounds 1 and 3 represent the first THF and MeIm
solvate derivatives of [Hf(OR)4]. Previously, the only mono-
meric [Hf(OR)4] complex reported employed the polydentate
MMPO ligand.15,16 While the quality of the crystal structure
solution for 1 (ball and stick shown in Figure 2a) was poor, the
connectivity unequivocally established it as a five-coordinated
monomer. Upon drying under vacuum, crystals of 1 re-formed
a clear liquid, which was attributed to the loss of the weakly
bound THF solvent molecule. This volatility explains the
difficulty in obtaining high-quality single crystals, the lower than
expected yield after drying the crystals, and poor elemental
analysis results. The only other structurally characterized five-
coordinated hafnium compounds reported that are bound
solely by O atoms are the siloxides: [Hf((μ-OBut)OSi(OBut)2)]-
[Hf(OSi(OBut)3)4]37 and [Hf(OSi(OBut)3)4(H2O)].38
parameters for 1−26. Tables 2−4 tabulate the metrical data (presented
by ligand) for the OAr, OAr/oxide, and polydentate ligated com-
pounds, respectively, isolated in this work.
Specific issues associated with individual structures are discussed
below. The significant disorder observed for the pendant chains for 1,
14, and 22 resulted in resolution of the basic connectivity only. For
compound 5a, the disorder in the OC6H4(CH(CH3)2)-2 ligand led to
it being refined isotropically, with no H atoms; however, the H atoms
were added back into the molecular formula in the crystal (Table 2) to
reflect charge neutrality. The molecular weights of compounds 4 and
6−10 reported in Table 2 reflect the final CIF files and do not include
the protons detailed in the Experimental Section because they could
not be unequivocally located in the final models. For 24, three H
atoms could not be located on one of the methyl groups of a
disordered tert-butyl group but were added into Table 2 to maintain
charge neutrality. The electron densities of disordered solvent mole-
cules in the lattice were successfully modeled using the PLATON/
SQUEEZE program for compounds 4, 5, 7, 13, 19, and 19a. The
“squeezed” molecule atoms were not added to the molecular formula(s)
in Table 2. Slightly disordered ligands were modeled using standard
crystallographic restraints for 5, 5a, 15, 24, and 26.
General Nanoparticle Synthesis. Under an argon atmosphere, a
0.5 M solution of the desired Hf precursor ([Hf(OBut)4], 8, 19a, 22,
and 25) in a mixture of oleic acid and oleylamine (2 mL/20 mL) was
added to a Parr Acid Digestion bomb, sealed, transferred to an oven,
and heated at 185 °C for 24 h. The nanomaterials were isolated by
centrifugation followed by washing with acetone and toluene. The final
nanoparticles could be dispersed with toluene.
Transmission Electron Microscopy (TEM). An aliquot of the
HfO2 nanopowder dispersed in toluene 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 resul-
tant particles were analyzed using a Philips CM 30 transmission
electron microscope operating at 300 kV accelerating voltage and
equipped with a Thermo Noran System 6 energy-dispersive X-ray sys-
tem operating at 300 kV accelerating voltage.
Powder X-ray Diffraction (PXRD). Powder samples were
mounted directly onto a zero background holder (The Gem Dugout).
The average Bravais lattice crystal symmetry and cell parameters were
found using PXRD patterns collected on a PANalytical powder dif-
fractometer with Cu Kα radiation (1.5406 Å) and an RTMS
X′Celerator detector. Patterns were analyzed with the JADE software
program.25
RESULTS AND DISCUSSION
■
A search of the crystallographically characterized compounds
that possess one Hf and four OR ligands yields a surprisingly
small set of compounds.12 Removing compounds from this list
that possess problematic anions for oxide materials produc-
tion26,2728−32 or oxo-containing species33,34 leaves only a hand-
ful of “simple” [Hf(OR)4] that have been fully characterized,
including [Hf(OPri)4·HOPri],13 [Hf(OPri)4(solv)]2 [solv =
HOPri or HOPri/py],14 and [(MMPOc)2Hf(OR)2] (OR =
OBut,15 MMPO16 or methoxy-2-methyl-2-propanolato).
Recently, we added to this family of compounds using the
ONep ligand: {[H][(μ-ONep)3Hf2(ONep)5(OBut)]}.35 From
this limited number of simple [Hf(OR)4] precursors, only
In contrast to 1, compounds 2 and 3 were solved as six-
coordinated, pseudooctahedrally (OC-6) coordinated mono-
mers. This was achieved by the binding of two solvent
molecules. While the previously reported [Hf(OPri)4(py)]214 deriv-
ative was isolated as a dimer, the nuclearity variation is attrib-
uted to the decreased steric bulk of OPri versus OBut. The yield
of 3 was initially higher than calculated because of the difficulty
in removing excess MeIm from in vacuo drying. After numerous
washings with hexanes and toluene, the product weight was
constant but led to a reduction in the overall yield, presumably
because of lost product. Elemental analyses of 2 and 3 were
found to be incongruous with the calculated values, which was
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with the ν(C−O)Hf.41 The ν(Hf−O) stretch is reported to be
at 567 and 526 cm−1 and was observed in both 1 and 2 but not
3. This is attributed to the influence of the strong Lewis base
MeIm solvate, where the introduction of coordinating solvents
has been reported to shift M−O bands to lower frequencies.41
NMR data for 1 and 2 revealed the single OBut resonance in
the parent deuterated solvent; however, compound 3 was
surprisingly not soluble enough in py-d5 or CDCl3 to garner an
acceptable NMR spectrum.
From this simple set of compounds, the monomeric nature
of [Hf(OBut)4] in the presence of a Lewis basic solvent has
been verified. Therefore, modification of this precursor with the
more sterically demanding OAr ligands was expected to yield
monomeric species as well in Lewis basic solvents; however,
nonpolar solvents were not as predictable.
B. Phenoxide Derivatives. The synthesis of this series of
OAr-modified [Hf(OBut)4] compounds was undertaken using
commercially available, sterically varied, 2,6-substituted phenols
(HOAr, eq 1). The Hf-OAr derivatives were of interest because
the steric bulk around the metal center can be easily mani-
pulated based on the ortho substitution. The series of solvents
investigated included (i) toluene, (ii) THF, and (iii) pyridine,
and the products are discussed in order below. Figures 3−8
show the thermal structure plots of the various ligand/solvent
systems for HOC6H4(R)-2 where R = CH3 (Figure 1b), CH-
(CH3)2 (Figure 1c), and C(CH3)3 (Figure 1d) and for HOC6H3-
(R)2-2,6 where R = CH3 (Figure 1e), CH(CH3)2 (Figure 1f),
and C(CH3)3 (Figure 1g), respectively.
i. Toluene Products (4−9). Initial efforts focused on gen-
erating the fully substituted compounds using toluene as the
solvent (eq 1). After stirring for 12 h, the resulting modified
[Hf(OR)4] products were isolated by crystallization. For the
majority of samples, the loss of the broad −OH stretch
around 3000 cm−1 and/or the inclusion of OAr stretches and
bends in the FTIR spectra of these crystals indicated that
some degree of substitution had occurred. A strong peak
around 1000 cm−1 was noted in each spectrum, which is con-
sistent with ν(C−O)Hf,41 but the ν(Hf−O) stretches noted
for Nujol samples of [Hf(OBut)4]41 were not readily observed
for all samples. For 7 and 8, in addition to the OAr stretches and
bends, a broad stretch around 3500 cm−1 was observed, indicat-
ing the presence of a hydroxide or phenol moiety; however,
there was no stretch noted in this range for 4, 6, or 9, which
were solved (vide infra) as a HOBut derivative.
To assist in understanding the various structural changes
wrought from these substitutions, single-crystal X-ray studies
were undertaken and are discussed below based on increasing
steric bulk of the OAr ligand. For a number of the compounds
synthesized in this study, more ligands are present than can be
accounted for by the tetravalent nature of the Hf metal center.
This requires the assignment of H atoms in the final structure,
but unambiguous assignment of their proper location was not
always possible. While the delocalized protons can be placed
outside of the formula as we did for [H][(μ-ONep)3Hf2-
(ONep)5(OBut)],35 we have attempted to identify their location
based on metrical data, literature structures, and logical deduc-
tion. The source of the oxide/hydroxide moiety identified in the
following structures is currently unknown; however, oxo species
are prevalent in the synthesis of [M(OR)x] and reportedly
produced by several mechanisms including hydrolysis due to
adventitious water, ligand degradation (i.e., ether, ester, or
alkene elimination), and aerobic oxidation.11,41 Each of these
routes has been considered for the toluene system, but a great
Figure 2. Structure plots of solvates: (a) 1 (ball and stick); (b) 2; (c)
3. Thermal ellipsoids of heavy atoms drawn at the 30% level with C
atoms drawn as ball and stick for clarity.
attributed to the volatility of the bound solvent at the high
temperatures used for analysis. The metrical data indicate that 2
and 3 possess similar Hf−O bond distances (1.96 and 1.98 Å,
respectively) and are in agreement with those reported for the
[(MMPOc)2Hf(OBut)2]15 (av 1.93 Å) complex. The Hf−Nsolv
distances of 2.45 and 2.40 Å for 2 and 3, respectively, are
also consistent with those noted for other Hf−N distances
(range 2.0639−2.4640 Å).12 The FTIR spectrum of 1−3
displayed a strong peak around 990 cm−1, which is consistent
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Figure 3. Structure plots of OC6H4(CH3)-2 phenoxide derivatives: (a) 4; (b) 4a·tol; (c) 10 (two molecules per unit cell); (d) 16. Thermal ellipsoids
of heavy atoms drawn at the 30% level with C atoms drawn as ball and stick for clarity.
deal more work is necessary to verify the proper mechanism(s) for
formation of the oxo moiety. The structures isolated from toluene
are discussed below according to (a) the 2-monosubstituted and
the (b) 2,6-disubstituted OAr derivatives.
Using the more sterically demanding OC6H4(CH(CH3)2)-2
ligand led to the dinuclear complex 5 (see Figure 4a),
which had successfully metathesized only one OBut ligand.
The resulting Hf metal centers adopted a distorted square-
base-pyramidal (SBP; τ = 0.21) geometry, using two μ-OC6H4-
(CH(CH3)2)-2 ligands and three terminal OBut ligands per
metal. The metrical data (Table 3) are consistent with the other
compounds in this investigation. One attempt to generate
higher-quality crystals from a slower growth process led to the
formation of the oxo species 5a. The structure of 5a adopts a
standard M3O12 arrangement with two μ3-O, three μ-OBut, and
six terminal OC6H4(CH(CH3)2)-2 ligands (shown in Figure 4b).
The necessary phenol(s) or hydroxide(s) ligands could not be
discerned from the metrical data (Table 4) or from Hf3(μ3-O)
literature compounds12,33,44,45 and were therefore considered dis-
tributed around the molecule.
All attempts to generate a Hf-OC6H4(C(CH3)3)-2 derivative
led to isolation of the oxo species 6 (Figure 5), with two
toluene molecules located in the unit cell. This compound
adopts a standard M2O10 dinuclear arrangement with two μ-O,
six terminal OC6H4(C(CH3)3)-2, and two terminal OBut
ligands. In agreement with the similar literature structures46−49
and confirmed by the metrical data (Table 4), the complex was
assigned as μ-OH with an axial HOBut ligand. Interestingly, the
FTIR data do not reveal the expected strong −OH stretch
around 3500 cm−1, but this may be a reflection of the bridging
nature of μ-OH.
a. Monosubstituted OAr Derivatives (4−6). The mono-
ortho-substituted OAr derivatives of [Hf(OBut)4] (Figure 1b−d)
were all found to form dinculear complexes. Compound 4 had
two pseudo-OC-6-bound Hf metal centers that were bridged by
two μ-OC6H4(CH3)-2 ligands (see Figure 3a). The remaining
coordination sites were filled by six terminal OC6H4(CH3)-2
and two OBut ligands. Charge balance requires that two of
these ligands are protonated. Because the axial Hf−OBut dis-
tances of 2.29 Å for 4 (see Table 3) were significantly longer
than those observed for 2 and 3 (av 1.97 Å), they were assigned
as HOBut ligands. The structure of 4 is similar to constructs
of [M(OPri)4(HOPri)]2 (M = Zr,42,43 Hf14), which also pos-
sess distinguishing, longer, axial M−HOPri distances (i.e., Hf−
HOPri = av 2.21 Å14). During one attempt to recrystallize 4
from a protracted crystal growth, the oxo species 4a·tol (Figure 3b)
was generated. This compound adopts a M4O16 structure, with two
μ3-O, two μ-O, and two μ-OC6H4(CH3)-2 holding the central
core together. In addition, there are four OC6H4(CH3)-2 and
two OBut terminal ligands. To maintain charge neutrality, based
on the metrical data (Table 4) and what is typically reported
for solvated M4O16 structures,12 the OBut ligands were formally
protonated and the bridging oxide ligands were assigned as a
μ-OH.
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Figure 5. Structure plot of OC6H4(C(CH3)3)-2 phenoxide derivative 6.
Thermal ellipsoids of heavy atoms drawn at the 30% level with C
atoms drawn as ball and stick for clarity.
the obtained percentages are in agreement with the loss of
ligand moieties similar to what was observed for the oxide
1
crystal structures of 4a and 5a, respectively. The simple H
NMR spectra obtained for 4 and 5 in CDCl3 were consistent
with the observed crystal structure; however, the low solubility
of these compounds prevented the collection of useful 13C
NMR spectra. For 6, the removal of a toluene solvent molecule
from the formula leads to acceptable percentages.
b. Disubstituted Derivatives (7−9). Modifying [Hf(OBut)4]
with the 2,6-disubstituted OAr ligands (Figure 1e−g) was
initiated using HOC6H3(CH3)2-2,6; however, only oils were
isolated. From a long-time reaction mixture, 7 (Figure 6a) was
solved as a dinuclear species with a metal center that uses three
terminal OC6H3(CH3)2-2,6, two bridging O atoms, and one
terminal OBut ligand. The metrical data (Table 4) for 7 are
consistent with a [Hf(μ-OH)]2 central core and a HOBut
ligand.44,46−54 For the OC6H3(CH(CH3)2)2-2,6 (8; Figure 7a)
and OC6H3(C(CH3)3)2-2,6 (9; Figure 8a) derivatives, oxo-free
monomers were identified. Compound 8 was solved in an
irregular SBP (τ = 0.12)55 arrangement using four OC6H3(CH-
(CH3)2)2-2,6 and one OBut ligand. Compound 9 possesses
only a single OC6H3(C(CH3)3)2-2,6 with retention of four
OBut ligands, forming a very distorted SBP (τ = 0.41) geo-
metry around the Hf metal center. For both 8 and 9, the
substantially longer Hf−OBut distance observed in 8 [Hf(1)−
O(8) = 2.32 Å] and 9 [Hf(1)−O(3) = 2.37 Å] allows for these
ligands to be assigned as a charge-balancing alcohol (HOBut).
Elemental analyses of the crystalline material of 7 (when two
toluene molecules are included) and 9 were found to be in
agreement with the calculated values. For 8, the conversion of
several ligands [HOBut and one OC6H3(CH(CH3)2)2-2,6 ligand]
to −OH results in acceptable values.
Figure 4. Structure plots of OC6H4(CH(CH3)2)-2 phenoxide
derivatives: (a) 5; (b) 5a; (c) 11. Thermal ellipsoids of heavy atoms
drawn at the 30% level with C atoms drawn as ball and stick for clarity.
Further characterization of the bulk powders of 7−9 using
1H and 13C NMR spectroscopy was undertaken in tol-d8. Again,
the compounds' low solubility prevented useful information
from being obtained for the 13C NMR spectra; however, the 1H
1
Elemental analyses were undertaken to further determine the
purity of the bulk powders of 4−6. It is of note that for [M(OR)x]
it is often difficult to obtain acceptable analyses because of the
properties that make them of interest for materials applications:
low decomposition temperatures, high volatility, rapid hydrolysis,
and inclusion of solvents (i.e., increased solubility). Therefore,
it is not surprising that most of the experimental data are not
consistent with the calculated percentages. For both 4 and 5,
NMR data did lend some insight into their purity. For the H
NMR spectrum of 7, three sets of resonances were expected for
the −OH, OC6H3(CH3)2-2,6, and HOBut ligands. The latter
two resonances were observed but were overlapped with the
tol-d8 peaks, which makes accurate integration difficult. The OH
resonance was not unequivocally observed. For 8, the methyl
resonances of OC6H3(CH(CH3)2)2-2,6 to the OBut resonances
were found to be in rough agreement with a 15.5:3 ratio
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Figure 6. Structure plots of OC6H3(CH3)2-2,6 phenoxide derivatives: (a) 7; (b) 13 (two molecules per unit cell); (c) 19; (d) 19a. Thermal
ellipsoids of heavy atoms drawn at the 30% level with C atoms drawn as ball and stick for clarity.
compared to the expected 16:3 ratio. For 9, the spectrum in
CDCl3 showed the expected 1:4 OC6H3(C(CH3)3)-2,6 to OBut
ratio. The NMR data coupled with the acceptable elemental
analyses argue for an agreement between the observed structure
of 7−9 and the bulk powder.
ii. Tetrahydrofuran Derivatives (10−15). The introduction
of THF was expected to reduce the nuclearity of the compounds
based on the expected coordination of the Lewis basic sol-
vent. In contrast to the toluene derivatives, the FTIR data for
the THF adducts (10−15) had no −OH stretches and the
remaining stretches and bends were noticeably less complicated
but fully consistent with the various OAr ligands investigated.
The ν(C−O)Hf and ν(Hϕ−O) stretching frequencies of
the THF versus toluene derivatives were slightly shifted. The
degree of substitution could not be unequivocally established,
and crystal structures were obtained when possible to under-
stand the modifications.
The OC6H4(CH3)-2 ligated compound 10 (Figure 3c) was
solved as a fully substituted mononuclear species, generating an
OC-6 Hf metal center by also binding two THF solvent mole-
cules. Two molecules were solved in the unit cell. As the steric
bulk was slightly increased, only the disubstituted species 11
was formed (see Figure 4c). Again, the hafnium adopts a
pseudo-OC-6 arrangement using two OC6H4(CH(CH3)2)-2,
two OBut, and two THF molecules. All attempts to generate the
OC6H4(C(CH3)3)-2-ligated species yielded only oils. The
OC6H3(CH3)2-2,6 derivative 13 (Figure 6b) was found to be
a fully substituted monomer that also binds a single THF solvent
molecule, thereby generating a distorted, trigonal-bipyramidal
(TBP; τ = 0.59) coordination around the hafnium. Employing
the larger OC6H3(CH(CH3)2)2-2,6 ligand led to 14 (Table 2),
which because of poor crystal quality could only have its con-
nectivity identified as [Hf(OC6H3(CH(CH3)2)2-2,6)3(OBut)-
(THF)]. Finally, the steric bulk of the OC6H3(C(CH3)3)2-2,6
modifier allowed for only one successful ligand substitution,
forming 15 (Figure 8b). In addition, one THF solvent molecule
bound to the Hf metal center, yielding a pseudo-SBP (τ = 0.37)
geometry.
The solution state of the THF adducts was investigated
1
through H NMR studies using crystals of 10−15 individually
dissolved in THF-d8. For the majority of compounds, the
appropriate signals for the ligands (OAr/OBut) in the ratio
1
noted in the solid state were observed in the H NMR spec-
trum. For 11, the OBut peaks were not readily discernible, but
on the basis of integration of the doublet for the propyl group
of the OC6H4(CH(CH3)2)-2 ligand, it is apparent that these
peaks coincidentally overlap. Peaks consistent with the free
HOC6H4(CH(CH3)2)-2 ligand were present in the spectrum at
low levels, which is consistent with the elemental analysis data
(vide infra). We were not able to successfully grow crystals of
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Figure 7. Structure plots of OC6H3(CH(CH3)2)2-2,6 phenoxide
derivatives: (a) 8; (b) 20. Thermal ellipsoids of heavy atoms drawn at
the 30% level with C atoms drawn as ball and stick for clarity.
the OC6H4(C(CH3)3)-2 derivative 12. Experimental elemental
analyses obtained for 10 and 13 were found to be in agreement
with the calculated values. For 11, it appears that the variation
may be due to the inclusion of residual HOC6H4(CH3)-2
solvent molecules. For 15, the variation is more in line with the
premature loss of the OC6H3(C(CH3)3)2-2,6 ligand.
iii. Pyridine Derivatives (16−21). When py was used as
the solvent (eq 1), the FTIR spectra of the various products
(16−21) appeared to be very similar to the previously dis-
cussed THF adducts. Again, the −OH peaks were not observed
in these spectra and the ν(C−O)Hf and ν(Hϕ−O) stretching
frequencies were slightly shifted in comparison to the tol or
THF system. Crystal structures were obtained for each reaction
mixture when possible, and all nonoxide species were isolated
as solvated monomers.
For the OC6H4(CH3)-2 derivative, monomeric 16 (see Figure 3d)
had an OC-6 Hf metal center [four OC6H4(CH3)-2 ligands and
two py solvent molecules] similar to what was noted for the
THF adduct 10. An additional molecule of py was located
within the unit cell for 16. Both the OC6H4(CH(CH3)2)-2
(17) and OC6H4(C(CH3)3)-2 (18) derivatives formed oils,
and thus crystal structures were not available. For the dis-
ubstituted species, the OC6H3(CH3)2-2,6 derivative yielded com-
pound 19 (Figure 6c), which displayed full ligand substitution.
Figure 8. Structure plots of OC6H3(C(CH3)3)-2,6 phenoxide
derivatives: (a) 9; (b) 15; (c) 21 (two molecules per unit cell).
Thermal ellipsoids of heavy atoms drawn at the 30% level with C
atoms drawn as ball and stick for clarity.
The pseudo-OC-6 geometry was generated through the binding
of two py solvent molecules, as shown in Figure 6c. A long-
term crystallization mixture led to the dinuclear species 19a
(Figure 6d), where each hafnium possesses three OC6H3-
(CH3)2-2,6 ligands, a py (Hf−Npy = 2.42 Å), and a bridging
OH ligand. Again, the assignment of an −OH in the structure is
consistent with the metrical data (Table 4) and literature
reports.23,26,44,46,48−54,56−58 Increasing the steric bulk to the OC6H3-
(CH(CH3)2)2-2,6 ligand allows for three ligand substitutions,
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a
Table 5. Substitution Number of the OAr Ligands
OC6H4(R)-2
OC6H3(R)-2,6
CH(CH3)2
solvent/R
CH3
CH(CH3)2
C(CH3)3
CH3
C(CH3)3
tol
4 (4)
1 (5)
2 (11)
−
3 (6, oxo)
3 (7, oxo)
4 (13)
4 (8)
1 (9)
THF
py
4 (10)
4 (16)
−
−
3 (14)
3 (20)
1 (15)
1 (21)
3 (19)
3 (19a, oxo)
a
Compound number listed in parentheses. − = compound not isolated.
yielding a distorted SBP geometry (τ = 0.31) for 20 (Figure 7b)
through coordination of a py solvent molecule. The OC6H3-
(C(CH3)3)2-2,6 reaction (eq 1) again formed the monosub-
stituted complex, forming an irregular SBP (τ = 0.33) geometry
for the hafnium of 21 (Figure 8c; two molecules per unit cell).
Elemental analyses for the py adducts 16 and 19−21 were
not in agreement with the proposed values. For all of these
compounds, the addition or subtraction of py solvent molecules
will place the obtained values in the acceptable range. Again,
crystalline material was dissolved in the parent deuterated sol-
vent (i.e., py-d5 for 16−21) to elucidate structural behavior and
purity. For the monomeric py adduct (16 and 19−21), not
surprisingly only one set of ligands was noted for these com-
pounds, which argues for retention of their structure in solution.
iv. Metrical Data. Metrical data of the OAr (tol, THF, and py)
derivatives are listed in Table 3 by ligand. As can be dis-
cerned, the bond distances and angles of this family of com-
pounds are self-consistent. The two dinuclear OAr derivatives
(4 and 5) possessed Hf---Hf distances of 3.38 and 3.57 Å and
μ-OAr−M−μ-OAρ angles of 70.72° and 66.52°, respectively.
The average Hf−OAr distance is 2.00 Å, which is longer
than the average Hf−OBut distance of 1.92 Å. In contrast, the
ligands that were assigned as HOBut have Hf−O distances
that average 2.33 Å, which is in line with other solvates (THF
and py) with a Hf−solv distance of 2.37 Å. Coordination around
the hafnium metal was individually discussed above, and it was
found that the majority of five-coordinated species were distorted
SBP, with the six-coordinated complexes adopting pseudo-OC-6
geometries.
v. Structural Ligand Effect Summary. It was originally
thought that the substitution pattern of OAr for OBut would
be directed by the steric bulk of the ortho substituent(s) and
the pKa59 of the phenol [i.e., (CH3)-2 < (CH(CH3)2)-2 <
(C(CH3)3)-2 < (CH3)2-2,6 < (CH(CH3)2)2-2,6 < (C(CH3)3)2-2,6].
Table 5 shows the general metathesis that occurred in the
solvents employed (eq 1). At the extremes, the steric bulk trend
holds, where, independent of the solvent used, the OC6H4-
(CH3)-2 derivatives (Figure 3) demonstrate full exchange while
the OC6H3(C(CH3)3)2-2,6 system (Figure 8) generates only
one substitution. The rest of the ligands do not appear to follow
any specific trend. The OC6H4(CH(CH3)2)-2 derivatives
demonstrated limited metathesis, while the more sterically
demanding OC6H4(C(CH3)3)-2 (Figure 5), OC6H3(CH3)2-2,6
(Figure 6), and OC6H3(CH(CH3)2)2-2,6 (Figure 7) products
had three or four exchanged products. This alters the exchange
pattern to (CH3)-2 > (C(CH3)3)-2 ∼ (CH3)2-2,6 ∼ (CH-
(CH3)2)2-2,6 > (CH(CH3)2)-2 > (C(CH3)3)2-2,6. It is also of
note that each molecule had sufficient room to bind at least one
solvent ligand for the THF or py systems. Therefore, it is clear
that the substitution behavior is more complex than being
dependent on either the steric bulk of the ortho substituent or
the acidity of the phenol protons. However, these compounds
set the framework for a more in-depth study that is necessary
Figure 9. Structure plots of OR* derivatives: (a) 22 (ball and stick);
(b) 23; (c) 24 (two molecules per unit cell). Thermal ellipsoids of
heavy atoms drawn at the 30% level with C atoms drawn as ball and
stick for clarity.
to understand and control substitution of these HOAr with
[Hf(OBut)4].
C. Complex Polydentate Alcohols (22−26). Because
compounds 1−21 were monomeric or dimeric, it was of interest
to explore whether the polydentate alcohols could generate
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Table 6. Metrical Data for the Polydentate Derivatives
M−OL
M−OBut
M---M
(av Å)
OL−M−OL
OBut−M−OBut
OBut−M−OL
μ-OL−M−OBut
μ-OL−M−μ-OL
ligand
OPy
compd nucl
(av Å)
(av Å)
(av deg)
(av deg)
(av deg)
(av deg)
(av deg)
23
2
(O) 2.00
(μ-O) 2.17
(N) 2.40
(O) 2.04
(N) 2.38
2.15
1.93
1.94
3.27
(N) 155.2
102.23
100.4
116.6
71.45
24
26
27
1
4
4
−
(N) 78.73
(O) 148.7
77.89
103.1
96.58
94.25
(N) 126.0
(O) 99.5
−
−
THME
(O) 1.96
3.52
3.49
(O) 124.8
(μ-OBut) 69.96
85.12
90.58
(μ-O) 2.20
(O) 1.94
2.14
90.58
−
(O) 100.4
(μ-O) 2.19
(μ-ONep) 69.05
Figure 11. PXRD of nanomaterials generated from 25 (a) as prepared,
(b) annealed at125 °C, and (c) annealed at 650 °C in air.
The bidentate OTHF ligand (Figure 1h) was introduced to
[Hf(OBut)4] in a variety of stoichiometries, but as noted for the
titanium system,60 only the monosubstituted complex was
isolated as 22 (Figure 9a). This compound is dinuclear, where
each hafnium binds an OTHF ligand that acts as a chelating
bridge (μc-OTHF) while retaining the original OBut ligands.
The metrical data for this structure solution are not reliable
because of severe disorder in the pendant ligand chains. Ele-
mental analyses were consistent with the observed solid-state
structure. The NMR data for 22 present resonances that are
broad and varied, with two sharp resonances for OBut present
in a 1:1 ratio. The breadth of the resonances and two equal
OBut peaks indicate that either an equilibrium between the
monomer and dinuclear structures exists or the THF moiety of
the OTHF ligand may bind and unbind, allowing for more
variations in the solution structure. The later process is more
consistent with what was noted for the titanium system.60
Variable-temperature NMR data were not pursued because of
the preferential crystallization of this compound at even slightly
lower temperatures.
Figure 10. Structure plots of polydentate THME derivatives: (a) 25;
(b) 26. Thermal ellipsoids of heavy atoms drawn at the 30% level with
C atoms drawn as ball and stick for clarity.
alternative [Hf(OR)4] arrangements. These are discussed
below, focusing on a series of solvent-like-substituted methanol
derivatives termed HOR* [i.e., H-OTHF (Figure 1h) and
H-OPy (Figure 1i)] and the tridentate THME ligands.
a. HOR*. Previously, the HOR* bidentate ligands were
found to form dinuclear and mononuclear titanium com-
pounds,60 but the impact that the larger Hf metal center would
have on the final structure was not known. After the desired
HOR* was mixed with [Hf(OBut)4], the reaction was stirred
for 12 h and then the volatile portion was allowed to slowly
evaporate until crystals formed. The FTIR data clearly showed
that the OR* ligands had reacted with a number of the OBut
ligands.
The reaction of [Hf(OBut)4] with 2 equiv of H-OPy (Figure 1i)
led to the dinuclear 23 (Figure 9b); however, the substitution is
not symmetrical. The Hf(1) atom possesses three terminal
OBut ligands and binds to three O atoms of the OPy bridging
ligands. Two of these OPy ligands [O(1) and O(2)] chelate
bridge [μc-OPy] to Hf(2). The remaining OPy O atom[O(3)]
is bridging only. The other Hf atom in 23 possesses a terminal
OBut, a terminal OPy, two μc-OPy, and a μ-OPy ligand. This
means the first Hf atom adopts a pseudo-OC-6 and the second
Hf atom is seven-coordinated. Heating a solution of 23 dis-
solved in pyridine led to the isolation of 24, as shown in Figure 9c.
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For the two mononuclear molecules per unit cell of 24, there
are two chelating OPyc ligands and two OBut ligands bound to
the pseudo-OC-6 Hf metal center. Not surprisingly, the metrical
data for 23 and 24 (see Table 6) are very similar to each other
and the (OPy)2Ti(OR)2 derivatives (upon compensation for the
change in the cation size).60 NMR data of 24 dissolved in py-d5
were consistent with the monomeric species observed in the solid
state. The bulk powder elemental analysis was found to be in
agreement with the observed crystal structure.
b. THME. Using the tridentate ligand THME (Figure 1j)
resulted in the formation of the tetranuclear species 25 and
is shown in Figure 10a. The central core of this compound
resembles the same structure noted previously for the Zr-
THME derivative,61 where two μ-THME ligands bridge four Hf
metal centers. In addition, two μ-OR groups are present and
located off of the same Hf metal center. The pseudo-OC-6
metal center is completed through the binding of two terminal
OBut groups. The metrical data for 25 are in agreement throughout
the distances and angles reported and with those of the zirconium
derivative.61 The low solubility of 25 prevented one from obtaining
meaningful NMR data; however, elemental analysis of the bulk
powder was found to be in agreement with the crystal structure.
The THME/ONep derivative 26 (Figure 10b) was also synthesized
under conditions similar to those noted for 25 but using {[H][(μ-
ONep)3Hf2(ONep)5(OBut)]}35 instead of [Hf(OBut)4]. The final
product was found to adopt an arrangement and properties
identical with those noted for 25, but for this report, only the
structural properties are reported. Metrical data for 25 and 26 are
shown in Table 6, and the distances and angles were found to be in
agreement with each other and the literature61 data when the ionic
radius is taken into account.
D. Nanoparticle Synthesis. The two methods that have
been reported for the production of HfO2 nanomaterials are
sputtering17−19 and solution20−22 routes. For solution routes,
only the solvothermal (SOLVO) processing of hafnium chloride
in the presence of benzyl alcohol22 led to HfO2 nanomaterials.
The other two efforts focused on making (a) nickel-doped HfO220
from the powder processing of mixed-metal oxide powders in
triethanolamine or (b) hafnium oxide@gold (core@shell) nano-
particles21 using tetrakis(dimethylamino)hafnium processed in the
presence of 1,2 hexadecandiol and gold acetate. All three solution
routes go through uncharacterized, in situ generated, alkoxide-like
intermediates. Therefore, it was of interest to determine the utility
of the newly characterized [Hf(OR)4] precursors (1−26) in
the production of HfO2 nanomaterials and the impact that their
structural arrangements would have on the nanomaterials’ final
morphology.
The compounds were selected to represent the mononuclear
([Hf(OBut)4] and 8), dinuclear (19a and 22), and tetranuclear
(25) species. A SOLVO route using oleylamine/oleic acid as
the solvent system was selected because of the propensity of
this system to generate nanowires.62−65 After the reaction had
been properly processed, the resulting dark-brown powders were
found by PXRD to be amorphous (see Figure 11a). Thermal
processing at 125 °C (Figure 11b) did not improve the crys-
tallinity but did remove the organic species noted in the low 2θ
range. Final processing at 650 °C in air led to a light-tan material
that possessed a broad PXRD pattern that was consistent with
crystalline HfO2 [PDF 00-040-1173 (HfO2); Figure 11c]. Scherrer
equation analyses did not lead to valid particle size determination
because of the breadth of the peaks and their variability in the half-
height peak width. TEM images (see Figure 12) confirmed that
nanomaterials had been successfully synthesized, and EDS analyses
Figure 12. TEM images of nanomaterials generated from (a) Hf(OBut)4
(scale bar = 50, 50 nm), (b) 8 [OC6H3(CH3)2-2,6; 20, 50 nm], (c) 19a
[OC6H3(CH(CH3)2)2-2,6: 20, 50 nm], (d) 22 (OTHF; 20, 20 nm), and
(e) 25 (THME; 50, 50 nm) after annealing at 650 °C in air.
revealed the presence of Hf atoms and a trace amount of O atoms
with no C atoms (note: the weak oxygen peak was attributed to
the low signal available because of the sample proximity to the
copper grids used). In order to improve the crystallinity of HfO2,
powders from 22 were processed at 1000 °C. The sharper PXRD
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Figure 13. (a) PXRD and (b) TEM images (the scale bar for both is 50 nm) from SOLVO-generated powders from 22 processed at 1000 °C in air.
patterns (see Figure 13a) of the resulting off-white powder
were found to be in agreement with HfO2 (PDF 00-043-
1017).25 TEM images (Figure 13b) revealed that the higher
processing temperature had generated some sintering of the
particles with sizes now ranging between 20 and 25 nm. EDS
analysis was consistent with Hf and O atoms being present as
well as a trace amount of C atoms. These data, for the first time,
demonstrate the utility of [Hf(OR)4] precursors for the
production of HfO2 nanomaterials through the SOLVO route
discussed.
Previously, we have clearly demonstrated the role that the
precursor can play in directing the final morphology of the
nanomaterials.4,9,10 However, the PSA focused on zinc-,4
germanium-,9 and cadmium10-based ceramic materials. These
data allowed us to determine whether the PSA could be
employed for tailored HfO2 nanomaterials. Hafnia reportedly
adopt several crystal structures (i.e., monoclinic, cubic, and
orthorhombic), which lend themselves to more rodlike struc-
tures.66,67 The initial parent [Hf(OBut)4] precursor was inves-
tigated to establish the baseline morphologies that could be
obtained under these conditions. On the basis of the mono-
meric nature of [Hf(OBut)4],11,36 upon decomposition, the
resulting nucleation shower should produce a uniform set of
individual growth nuclei and thus produce nanodots. While a
variety of different-sized dots ranging in size from 2 to 25 nm
were present, a number of unexpected nanorods were observed.
These nanorods had an aspect ratio of over 5 (the smallest rod
was ∼80 nm × 400 nm), but the fine structure of the rods
appears to be agglomerated nanoparticles (Figure 12a). In
contrast, the other monomeric precursor 8 generated only nano-
dots on the order of 2−4 nm (Figure 12b). For dinuclear species,
larger dots or thin rods were expected based on the M2O2 central
core growth nuclei. As can be observed in Figure 12c, plates
ranging from 20 to 30 nm were isolated from 19a. The other
dinuclear precursor 22 yielded only nanodots (10−30 nm;
Figure 12d). No rods were noted. Finally, for the tetranuclear
precursor 25, the plane of the Hf4O8 central core is capped by
the two THME ligands. These growth nuclei should lead to
large rod growth or plates. The TEM images revealed large
plates on the order of 30−50 nm, as well as a number of rods
(Figure 12e). The rods varied in aspect ratios ranging from 4 to
>9 (∼100 × 900 nm) and appear to have a microstructure that
is less granular than that noted for the rods of [Hf(OBut)4].
The precursors’ growth nuclei and decomposition tempera-
ture are speculated to be directed by the various ligand sets.
While a great deal more work is necessary to understand, opti-
mize, and verify the PSA for this system, the initial efforts indi-
cate that precursor-induced morphological variations may exist.
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This lends promise to the design of tailored HfO2 nanomate-
rials based on designer [Hf(OR)4] precursors.
the National Science Foundation CRIF:MU award to Professor
Kemp of the University of New Mexico (Grant CHE04-
43580). Sandia National Laboratories is a multiprogram
laboratory managed and operated by Sandia Corp., a wholly
owned subsidiary of Lockheed Martin Corp., for the U.S.
Department of Energy’s National Nuclear Security Admin-
istration under Contract DE-AC04-94AL85000.
SUMMARY AND CONCLUSION
■
For the first time, the coordination chemistry of [Hf(OBut)4]
was systematically investigated using (i) Lewis basic solvents,
(ii) simple phenoxides, and (iii) complex polydentate alcohols.
For the Lewis basic solvates, monomeric complexes [Hf-
(OBut)4(solv)n] (1−3) were isolated. The structurally identi-
fied OAr products proved to be either [Hf(OAr)n(OBut)4‑n]2
(4 and 5), [Hf(μ-OH)(OAr)3(HOBut)]2 (6 and 7), or
[Hf(OAr)n(OBut)4−n(HOBut)] (8 and 9) when toluene is
used as the solvent or solvated monomeric compounds [Hf-
(OAr)n(OBut)4−n(solv)x] (10−21) when Lewis basic solvents
are used. The majority of these compounds (4−21) were found
to have Hf metal centers that adopt either a distorted five-
coordinate (SBP or TBP, with the former being the preferred
mode) or a six-coordinate (pseudo-OC-6) geometry. In several
instances, a retained OBut was identified as a coordinated
HOBut (4 and 6−9) versus a solvated HOAr complex; how-
ever, a bound HOR was not found for any of the crystals that
employed a Lewis basic solvent. The steric bulk of OC6H3-
(C(CH3)3)2-2,6 and OBut only allowed for one ligand exchange,
as noted for 9, 15, and 21. Interestingly, for the OC6H4-
(C(CH3)3)-2 systems investigated, only oils were isolated unless
an oxo species (6) formed. Oxo formation was noted for long-
term crystal growth of the toluene species, yielding dinuclear
complexes with OH bridging ligands (4a, 5a, 6, and 7). The
oxo formation occurred only for the less sterically hindered
OAr derivatives; however, this might be due to their longer
crystallization time and thus greater chance of potential expo-
sure to adventitious oxygen or water versus the shorter, quicker
crystal growth of the larger OAr species (8 and 9). Additionally,
19a showed that, even for Lewis basic solvents, oxo formation
was possible. Polydentate ligands were found to generate more
complex precursors (22−26), but the final structures are similar
to the smaller titanium congener species previously isolated.
The low nuclearity observed in this study is believed to be due
to the steric bulk of OBut, so additional studies that use the less
sterically demanding OEt (i.e., [Hf(OEt)4]) and polydentate
ligands are underway to elicit more complex precursor types.
SOLVO processing of select, structurally varied alkoxide
precursors (monomer, [Hf(OBut)4] and 8; dimer, 19a and
22; tetramer, 25) were evaluated for the production of HfO2
nanomaterials under identical processing conditions. The result-
ing products were found to generate HfO2 dots, plates, and rods,
indicating that the precursors may have an impact on the final
morphology. Additional work to exploit the PSA to obtain
designed HfO2 nanomaterial morphologies is underway.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tjboyle@Sandia.gov. Phone: (505)272-7625. Fax: (505)
272-7336.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors thank the National Institute for Nanoengineering
and Laboratory Directed Research and Development programs
at Sandia National Laboratories for support of this work and are
grateful for use of the Bruker X-ray diffractometer purchased via
12091
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