Ruthenium(II) complexes possessing the η6-p-cymene ligand

Article abstract of DOI:10.1016/j.ica.2007.10.012

Complexes possessing a soft donor η⁶-arene and hard donor acetylacetonate ligand, [(η⁶-p-cymene)Ru(κ²-O,O-acac-μ-CH)]₂[OTf]₂ (1) (OTf = trifluoromethanesulfonate; acac = acetylacetonate) and [(η⁶ - p -cymene) Ru (κ² - O, O -acac) (THF)] [BAr₄^′] (2) {Ar′ = 3,5-(CF₃)-C₆H₃}, were prepared and fully characterized. The lability of the μ-CH linkage for complex 1 and the THF ligand of 2 allow access to the unsaturated cation [(η⁶-p-cymene)Ru(κ²-O,O-acac)]⁺. The reaction of [(η⁶ - p -cymene) Ru (κ² - O, O -acac) (THF)] [BAr₄^′] (2) with KTp {Tp = hydridotris(pyrazolyl)borate} produces [TpRu (η⁶ - p -cymene)] [BAr₄^′] (5). The azide complex [(η⁶ - p -cymene) Ru (κ² - O, O -acac) (N₃ Ar)] [BAr₄^′] (6) forms upon reaction of [(η⁶ - p -cymene) Ru (κ² - O, O -acac) (THF)] [BAr₄^′] (2)with N₃Ar (Ar = p-tolyl), and reaction of [(η⁶ - p -cymene) Ru (κ² - O, O -acac) (THF)] [BAr₄^′] (2) with CHCl₃ at 100 °C yields the chloride-bridged binuclear complex [{(η⁶ - p -cymene) Ru}₂ (μ -Cl)₃] [BAr₄^′] (7). The details of solid-state structures of [(η⁶-p-cymene)Ru(κ²-O,O-acac-μ-CH)]₂[OTf]₂ (1), [TpRu (η⁶ - p -cymene)] [BAr₄^′] (5) and [{(η⁶ - p -cymene) Ru}₂ (μ -Cl)₃] [BAr₄^′] (7) are disclosed.

Full text of DOI:10.1016/j.ica.2007.10.012

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 3254–3262
www.elsevier.com/locate/ica
q
Ruthenium(II) complexes possessing the g6-p-cymene ligand
a
a,
b
a
Taisuke Sumiyoshi , T. Brent Gunnoe *, Jeffrey L. Petersen , Paul D. Boyle
a
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, United States
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, WV 26506-6045, United States
b
Received 1 August 2007; accepted 9 October 2007
Available online 16 October 2007
Dedicated to Robert J. Angelici
Abstract
Complexes possessing a soft donor g6-arene and hard donor acetylacetonate ligand, [(g6-p-cymene)Ru(j2-O,O-acac-l-CH)]2[OTf]2 (1)
(OTf = trifluoromethanesulfonate; acac = acetylacetonate) and ½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ½BAr40 ꢀ ð2Þ {Ar0 = 3,5-(CF3)–
C6H3}, were prepared and fully characterized. The lability of the l-CH linkage for complex 1 and the THF ligand of 2 allow access to
the unsaturated cation [(g6-p-cymene)Ru(j2-O,O-acac)]+. The reaction of ½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ½BAr04ꢀ ð2Þ with KTp
{Tp = hydridotris(pyrazolyl)borate} produces ½TpRuðg6-p-cymeneÞꢀ½BAr04ꢀ ð5Þ. The azide complex ½ðg6-p-cymeneÞRuðj2-O; O-acacÞ
ðN3ArÞꢀ½BAr04ꢀ ð6Þ forms upon reaction of ½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ½BAr40 ꢀ ð2Þwith N3Ar (Ar = p-tolyl), and reaction of
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ½BAr40 ꢀ ð2Þ with CHCl3 at 100 ꢀC yields the chloride-bridged binuclear complex
½fðg6-p-cymeneÞRug2ðl-ClÞ3ꢀ½BAr40 ꢀ ð7Þ. The details of solid-state structures of [(g6-p-cymene)Ru(j2-O,O-acac-l-CH)]2[OTf]2 (1),
½TpRuðg6-p-cymeneÞꢀ½BAr04ꢀ ð5Þ and ½fðg6-p-cymeneÞRug2ðl-ClÞ3ꢀ½BAr40 ꢀ ð7Þ are disclosed.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: p-Cymene; Ruthenium; Acetylacetonate; Half-sandwich complex
1. Introduction
In addition, the use of Ru(II)–g6-arene complexes in
medicinal applications has been explored [29–36].
The combination of an g6-arene ligand and a formally
anionic oxygen-based ligand provides a coordination
sphere with both ‘‘soft’’ (i.e., the arene) and ‘‘hard’’ (i.e.,
the oxygen-based ligand) donors. Along these lines, we
report the preparation and preliminary reactivity of Ru(II)
complexes that possess an g6-p-cymene and j2-O,O-acac
(acac = acetylacetonate) ligand.
The coordination chemistry of mono-metallic half-sand-
wich complexes has been of significant interest in inorganic
and organometallic chemistry [1–5]. Formally charge-neu-
tral g6-arene ligands have played a prominent role in this
field and can serve as ancillary ligands as well as intermedi-
ates for arene functionalization [6–13]. In terms of ancillary
ligands, hexa-hapto aromatic Ru(II) complexes provide a
flexible template upon which to build diverse coordination
environments and to develop metal-mediated catalysis
using the g6-aromatic ligand to provide a site to potentially
vary steric and electronic properties of the catalyst [14–28].
2. Experimental
2.1. General procedures
Unless otherwise noted, all synthetic procedures were
performed under anaerobic conditions in a nitrogen-filled
glovebox or using standard Schlenk techniques. Glovebox
purity was maintained by periodic nitrogen purges and
was monitored by an oxygen analyzer (O2 < 15 ppm for
q
Congratulations to Bob Angelici on a career that has advanced the
fields of inorganic and organometallic chemistry as well as inspired
students and colleagues.
*
Corresponding author. Tel.: +1 919 513 3704; fax: +1 919 515 8909.
E-mail address: brent_gunnoe@ncsu.edu (T.B. Gunnoe).
0020-1693/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.10.012

T. Sumiyoshi et al. / Inorganica Chimica Acta 361 (2008) 3254–3262
3255
all reactions in glovebox). Tetrahydrofuran was dried by
distillation from sodium/benzophenone. Acetonitrile was
dried by distillation from CaH2. Hexanes (stored over 4 A
CV (THF, 100 mV/s, TBAH): Ep,a = ꢁ1.13 V, Ru(II/I).
Anal. Calc. for C32H42Ru2F6O10S2: C, 39.67; H, 4.37; O,
16.52. Found: C, 39.10; H, 4.50; O, 16.58%.
˚
molecular sieves) and methylene chloride were purified by
passage through a column of activated alumina. Benzene-
d6 and chloroform-d1 were degassed using three freeze–
pump–thaw cycles and stored under a nitrogen atmosphere
2.2.2. ½ðg6-p-CymeneÞRuðj2-O; O-acacÞðTHF Þꢀ½BAr04ꢀ ð2Þ
[(g6-p-Cymene)Ru(j2-O,O-acac-l-CH)]2[OTf]2
(1)
(1.30 g, 2.7 mmol) was dissolved in 40 mL of THF.
NaBAr04 (4.79 g, 5.4 mmol) was added to the solution,
and the resulting mixture was stirred for 12 h. The volatiles
were removed in vacuo, the residual material was dissolved
in approximately 20 mL of CH2Cl2, and the solution was
filtered through a plug of Celite. The filtrate volume was
reduced to approximately 10 mL in vacuo, and hexanes
(approximately 40 mL) were added to form a precipitate.
The orange solid was collected via vacuum filtration
1
˚
over 4 A molecular sieves. H NMR spectra were recorded
on a Varian Mercury 300 or 400 MHz spectrometer, and
13C NMR spectra (operating frequency 75 MHz) were
recorded on a Varian Mercury 300 MHz spectrometer. All
1H and 13C NMR spectra are referenced against residual
proton signals (1H NMR) or the 13C resonances of the deu-
terated solvent (13C NMR). 19F NMR spectra were
obtained on a Varian 300 MHz spectrometer (operating fre-
quency 282 MHz) and referenced against an external stan-
dard of hexafluorobenzene (d = ꢁ164.9). Electrochemical
experiments were performed under a nitrogen atmosphere
using a BAS Epsilon Potentiostat. Cyclic voltammograms
were recorded in a standard three-electrode cell from
ꢁ2.00 V to +2.00 V with a glassy carbon working electrode
and tetrabutylammonium hexafluorophosphate (TBAH) as
electrolyte. Tetrabutylammonium hexafluorophosphate
was dried under dynamic vacuum at 140 ꢀC for 48 h prior
to use. All potentials are reported versus NHE (normal
hydrogen electrode) using cobaltocenium hexafluorophos-
phate as an internal standard. The preparation, isolation
and characterization of KTp, (g6-p-cymene)Ru(j2-O,O-
acac)Cl and NaBAr40 {Ar0 = 3,5-(CF3)-C6H3} have been
previously reported [2,4,37,38]. All other reagents were used
as received from commercial sources. Elemental analyses
were performed by Atlantic Microlabs, Inc.
1
through a fine porosity frit (2.26 g, 66% yield). H NMR
(CDCl3, d): 7.71 (8H, br s, ortho BAr0 ), 7.54 (4H, br s,
4
paraBAr0 ), 5.50 (2H, d, JHH = 6 Hz, p-cymene aromatic
3
4
3
CH), 5.27 (2H, d, JHH = 6 Hz, p-cymene aromatic CH),
5.27 (1H, s, acac-CH), 3.49 (4H, m, a-THF), 2.79 (1H,
3
sept, JHH = 7 Hz, CH(CH3)2), 2.15 (3H, s, p-cymene C–
CH3), 2.06 (6H, s, acac CH3), 1.74 (4H, m, b-THF), 1.32
3
(6H, d, JHH = 7 Hz, p-cymene CH(CH3)2).13C{1H}
NMR (CDCl3, d): 188.3 (s, acac C–O), 161.8 (1:1:1:1 quar-
tet, JCB = 51 Hz, ipso-C of BAr0 ), 134.9 (s, para-C of
1
4
BAr0 ), 129.1 (q, JCF = 29 Hz, meta-C of BAr0 ), 127.5 (q,
2
4
4
1JCF = 272 Hz, CF3 of BAr0 ), 117.6 (s, ortho-C of BAr0 ),
4
4
101.0, 98.9, 98.2 80.4, 78.7 (all s, p-cymene aromatic and
acac C-H), 70.9 (s, a-C of THF), 31.3 (s, CH(CH3)2),
27.2 (s, acac methyl), 25.5 (s, b-C of THF), 22.2 (s, p-cym-
ene C–CH3), 17.7 (s, p-cymene CH(CH3)2). 19F{1H} NMR
(CDCl3, d): ꢁ62.8 (s, BAr0 ). CV (THF, TBAH, 100 mV/s):
4
E1/2 = ꢁ0.78 V, Ru(II/I). Anal. Calc. for C51H41Ru-
2.2. Preparation of complexes
F24BO3: C, 48.24; H, 3.25. Found: C, 47.87; H, 3.29%.
2.2.1. (g6-p-Cymene)Ru(j2-O,O-acac)OTf (1-OTf)
2.2.3. ½ðg6-p-CymeneÞRuðj2-O; O-acacÞðNCMeÞꢀ½BAr04ꢀ ð3Þ
½ðg6-p-CymeneÞRuðj2-O;O-acacÞðTHFÞꢀ½BAr04ꢀð2Þ (0.53
g, 0.42 mmol) was dissolved in 40 mL of NCMe, and the
solution was stirred for approximately 10 min. The vola-
tiles were removed in vacuo, and the residual material
was dissolved in approximately 5 mL of CH2Cl2. The addi-
tion of hexanes (approximately 40 mL) afforded a precipi-
tate. The orange solid was collected via vacuum filtration
through a fine porosity frit (0.46 g, 88% yield).1H NMR
(g6-p-Cymene)Ru(j2-O,O-acac)Cl (0.765 g, 2.07 mmol)
was dissolved in 40 mL of THF. Upon addition of AgOTf
(0.584 g, 2.27 mmol) a white precipitate (presumably AgCl)
formed. The mixture was stirred for 30 min at room tem-
perature, after which time it was filtered through a plug
of Celite. The filtrate was concentrated to approximately
10 mL in vacuo, and hexanes (approximately 40 mL) were
added to yield a precipitate. The product was collected via
vacuum filtration through a fine porosity frit to give an
(CDCl3, 0 d): 7.71 (8H, br s, orthoBAr0 ), 7.54 (4H, br s,
1
orange solid (0.919 g, 92% yield). H NMR (CDCl3, d):
4
3
3
paraBAr4), 5.52 (2H, d, JHH = 6 Hz, p-cymene aromatic
5.64 (2H, d, JHH = 6 Hz, p-cymene aromatic CH), 5.42
3
3
CH), 5.21 (1H, s, acac-CH), 5.21 (2H, d, JHH = 5 Hz, p-
(2H, d, JHH = 6 Hz, p-cymene aromatic CH), 5.17 (1H,
cymene Ar–CH), 2.75 (1H, sept, 3JHH = 7 Hz, CH(CH3)2),
2.18, 2.11 (each 3H, each a s, p-cymene C–CH3 and
NCCH3), 1.98 (6H, s, acac CH3), 1.28 (6H, d, 3JHH = 7 Hz,
p-cymene CH(CH3)2).13C{1H} NMR (CDCl3, d): 188.1 (s,
3
s, acac-l-CH), 2.97 (1H, sept, JHH = 7 Hz, CH(CH3)2),
2.31 (3H, s, p-cymene C–CH3), 2.05 (6H, s, acac-CH3),
3
1.39 (6H, d, JHH = 7 Hz, CH(CH3)2). 13C{1H} NMR
1
(CDCl3, d): 187.5 (s, acac C–O), 119.4 (q, JCF = 319 Hz,
1
acac0 C–O), 161.9 (1:1:1:1 quartet, JCB = 50 Hz, ipso-C of
SO3CF3), 100.3 (s, acac C-H), 97.5 and 97.1 (each a s, p-
cymene 1,4-positions), 80.5 and 78.3 (each a s, p-cymene
2,3-positions), 31.2 (s, p-cymene CH(Me)2), 27.2 (s, p-cym-
ene C-CH3), 22.5 (s, acac CH3), 17.9 (s, p-cymene
CH(CH3)2). 19F{1H} NMR (CDCl3, d): ꢁ78.0 (s, SO3CF3).
0
2
BAr4), 135.0 (s, para-C of BAr4), 129.1 (q, JCF = 34 Hz,
meta-C of BAr ), 124.8 (q, JCF = 273 Hz, CF3 of BAr0 ),
0
1
4
4
122.7 (s, NCCH3), 117.7 (s, ortho-C of BAr0 ), 103.0,
4
100.7, 99.2, 84.4, 80.4 (all s, p-cymene aromatic and acac

3256
T. Sumiyoshi et al. / Inorganica Chimica Acta 361 (2008) 3254–3262
C-H), 31.1 (s, CH(CH3)2), 27.1 (s, acac methyl), 22.1 (s, p-
cymene C-CH3), 17.6 (s, p-cymene CH(CH3)2), 3.0 (s,
s, Tp 3,5-positions), 135.0 (s, para-C of BAr04), 129.0 (q,
2JCF = 42 Hz, meta-C of BAr0 ), 124.7 (q, JCF = 272 Hz,
1
4
NCCH3). 19F{1H} NMR (CDCl3, d): ꢁ58.8 (s, BAr0 ).
CF3 of BAr04), 117.7 (ortho-C of BAr04), 108.7, 107.5,
4
CV (NCMe, TBAH, 100 mV/s): Ep,a = ꢁ1.24 V, Ru(II/I).
Anal. Calc. for C49H36BF24NO3Ru: C, 47.45; H, 2.93; N,
1.13. Found: C, 47.44; H, 2.92; N, 1.12%.
102.0, 85.7, 85.5 (all s, p-cymene aromatic and Tp 4 posi-
tions), 31.5 (s, CH(CH3)2), 22.7 (s, p-cymene C-CH3),
19.0 (s, p-cymene CH(CH3)2). 19F{1H} NMR (CDCl3,
d): ꢁ 58.8 (s, BAr04). CV (THF, TBAH, 100 mV/s):
Ep,a = ꢁ1.45 V, Ru(II/I). Anal. Calc. for C51H36RuB2F24-
N6: C, 46.70; H, 2.77; N, 6.41. Found: C, 46.85; H, 2.94;
N, 6.18%.
2.2.4. ½ðg6-p-CymeneÞRuðj2-O; O-acacÞðPMe3Þꢀ½BAr04ꢀ ð4Þ
½ðg6-p-CymeneÞRuðj2-O;O-acacÞðTHFÞꢀ½BAr04ꢀð2Þ (0.520
g, 0.41 mmol) was dissolved in 40 mL of dichloromethane.
Trimethylphosphine (0.030 g, 0.45 mmol) was added to the
solution, and the resulting mixture was stirred for 5 min.
The solution volume was reduced to approximately 5 mL
in vacuo, and hexanes (approximately 40 mL) were added
to form a precipitate. The product was collected via vac-
uum filtration through a fine porosity frit to give a yellow
solid (0.46 g, 88% yield). 1H NMR (CDCl3, d): 7.71 (8H, br
s, orthoBAr04), 7.54 (4H, br s, paraBAr40 ), 5.50 (2H, d,
3JHH = 6 Hz, p-cymene aromatic C-H), 5.38 (2H, d,
3JHH = 6 Hz, p-cymene aromatic C-H), 5.38 (1H, s, acac-
2.2.6. ½ðg6-p-CymeneÞRuðj2-O; O-acacÞðN3ðp-tolylÞÞ
½BAr04ꢀ ð6Þ
½ðg6-p-CymeneÞRuðj2-O;O-acacÞðTHFÞꢀ½BAr04ꢀð2Þ (0.334
g, 0.263 mmol) was dissolved in 40 mL of dichloromethane,
and p-tolylazide (0.052 g, 0.394 mmol) was added to the
solution. The mixture was stirred for approximately 1 h.
The solution was filtered through a plug of Celite, and the
filtrate was concentrated to approximately 10 mL under
reduced pressure. Upon addition of hexanes (approximately
40 mL), a black precipitate formed. The solid was collected
via vacuum filtration through a fine porosity frit to give a
3
CH), 2.47 (1H, sept, JHH = 7 Hz, CH(CH3)2), 1.93 (6H,
s, acac CH3), 1.84 (3H, s, p-cymene C–CH3), 1.28 (9H, d,
3
1
2JPH = 12 Hz, P(CH3)3), 1.18 (6H, d, JHH = 7 Hz, p-cym-
red solid (0.170 g, 49% yield). H NMR (CDCl3, d): 7.71
ene CH(CH3)2). 13C{1H} NMR (CDCl3, d): 189.8 (s, acac
(8H, br s, orthoBAr04), 7.53 (4H, br s, paraBAr04), 7.24 (2H,
1
3
C–O), 161.9 (1:1:1:1 quartet, JCB = 50 Hz, ipso-C of
d, JHH = 9 Hz, azide aromatic CH), 6.79 (2H, d,
BAr0 ), 135.0 (s, para-C of BAr0 ), 129.1 (q, JCF = 31 Hz,
3JHH = 9 Hz, azide aromatic CH), 6.04 (1H, s, acac-CH),
5.82, 5.51, 5.13 (4H total, 1:2:1 integration, each a m, p-cym-
ene aromatic CH), 2.79 (1H, sept, 3JHH = 7 Hz, CH(CH3)2),
2.20 (3H, s, p-cymene CH3), 2.18 (3H, s, azide CH3), 1.85
2
4
4
meta-C of BAr0 ), 124.8 (q, JCF = 273 Hz, CF3 of BAr0 ),
1
4
4
117.7 (s, ortho-C of BAr04), 105.0, 101.8, 96.6, 89.2, 87.8
(all s, p-cymene aromatic and acac C-H), 30.7 (s,
CH(CH3)2), 27.2 (s, acac methyl), 21.8 (s, p-cymene C-
CH3), 16.7 (s, p-cymene CH(CH3)2), 14.3 (d, 1JCP = 31 Hz,
3
(6H, s, acac CH3), 1.35 (6H, d, JHH = 7 Hz, p-cymene
CH(CH3)2). 13C{1H} NMR (CDCl3, d): 188.1 (s, acac C–
P(CH3)3). 19F{1H} NMR (CDCl3, d): ꢁ58.8 (s, BAr0 ). CV
O), 161.9 (1:1:1:1 quartet, JCB = 50 Hz, ipso-C of BAr0 ),
1
4
4
(THF, TBAH, 100 mV/s): Ep,a = ꢁ1.63 V, Ru(II/I). Anal.
Calc. for C50H42BF24O2PRu: C, 47.09; H, 3.32; O, 2.51.
Found: C, 46.79; H, 3.29; O, 2.70%.
137.1 (s, ipso-C of azide), 135.0 (s, para-C of BAr04), 131.5
2
(s, para-C of azide), 129.2 (q, JCF = 34 Hz, meta-C of
BAr04), 128.6 (s, ortho-C of azide), 125.0 (s, meta-C of azide),
124.8 (q, JCF = 273 Hz, CF3 of BAr0 ), 117.7 (s, ortho-C of
1
4
BAr04), 86.5, 86.4, 86.2, 83.8, 83.6 (all s, p-cymene aromatic
2.2.5. ½TpRuðg6-p-CymeneÞꢀ½BAr04ꢀ ð5Þ
and acac C-H), 32.0 (s, CH(CH3)2), 24.2 (s, azide CH3),
23.2 (s, acac CH3), 22.3 (s, p-cymene C–CH3), 19.2 (s, p-
cymene CH(CH3)2), 3.0 (s, NCCH3). 19F{1H} NMR
½ðg6-p-CymeneÞRuðj2-O;O-acacÞðTHFÞꢀ½BAr04ꢀð2Þ (0.130
g, 0.102 mmol) was dissolved in 40 mL of dichloromethane,
and KTp (0.037 g, 0.15 mmol) was added to the solution.
The mixture was stirred for approximately 12 h. The solu-
tion was filtered through a plug of Celite, and the filtrate
was concentrated to approximately 10 mL under reduced
pressure. Upon addition of hexanes (approximately
40 mL), a red precipitate formed. The red solid was col-
lected via vacuum filtration through a fine porosity frit
(CDCl3, d): ꢁ62.8 (s, BAr0 ). CV (THF, TBAH, 100 mV/
4
s): Ep,a = ꢁ0.50 V, Ru(II/I). We were unable to obtain sat-
isfactory elemental analysis of this complex.
2.2.7. f½ðg6-p-CymeneÞ Ruꢀ ðl-ClÞ g½BAr04ꢀ ð7Þ
2
2
3
A thick-walled glass tube was charged with ½ðg6-p-
cymeneÞRuðj2-O;O-acacÞðTHFÞꢀ½BAr04ꢀð2Þ (0.200 g, 0.157
mmol) and 30 mL of CHCl3. The solution was heated at
100 ꢀC for three weeks. After filtration through a fine
porosity frit, the solution was concentrated to approxi-
mately 10 mL. Hexanes (approximately 40 mL) were added
to form a dark red precipitate. The dark red solid was col-
lected via vacuum filtration through a fine porosity frit
1
(0.530 g, 45% yield). H NMR (CDCl3, d): 7.99 (3H, d,
3JHH = 2 Hz, Tp 3 or 5 positions), 7.70 (8H, br s, BAr0
4
3
ortho), 7.56 (3H, d, JHH = 2 Hz, Tp 3 or 5 positions),
7.51 (4H, br s, BAr0 para), 6.29 (3H, t, JHH = 2 Hz, Tp
3
4
3
4 positions), 5.68 (2H, d, JHH = 6 Hz, p-cymene aromatic
C-H), 5.55 (2H, d, JHH = 6 Hz, p-cymene aromatic C-H),
2.92 (1H, sept, JHH = 7 Hz, CH(CH3)2), 2.33 (3H, s, p-
3
3
3
1
cymene C–CH3), 1.17 (6H, d, JHH = 7 Hz, p-cymene
(0.090 g, 45% yield). H NMR (CDCl3, d): 7.71 (8H, br s,
CH(CH3)2).13C{1H} NMR (CDCl3, d): 161.8 (1:1:1:1 quar-
orthoBAr04), 7.53 (4H, br s, paraBAr40 ), 5.56 (2H, d,
3JHH = 6 Hz, p-cymene aromatic CH), 5.36 (2H, d,
tet, 1JCB = 50 Hz, ipso-C of BAr0 ), 143.7 and 136.3 (each a
4

T. Sumiyoshi et al. / Inorganica Chimica Acta 361 (2008) 3254–3262
3257
3JHH = 6 Hz, p-cymene aromatic CH), 2.76 (1H, sept,
3JHH = 7 Hz, CH(CH3)2), 2.18 (3H, s, p-cymene C–CH3),
3
1.29 (6H, d, JHH = 7 Hz, p-cymene CH(CH3)2). 13C{1H}
1
NMR (CDCl3, d): 161.8 (1:1:1:1 quartet, JCB = 50 Hz,
ipso-C of BAr04), 135.0 (s, para-C of BAr04), 129.1 (q,
0
1
2JCF = 34 Hz, meta-C of BAr4), 124.7 (q, JCF = 272 Hz,
CF3 of BAr0 ), 117.7 (ortho-C of BAr0 ), 102.5, 97.3, 78.9
4
4
and 78.2 (all s, p-cymene aromatic), 31.7 (s, CH(CH3)2),
22.3 (s, p-cymene C–CH3), 19.0 (s, p-cymene CH(CH3)2).
19F{1H} NMR (CDCl3, d): ꢁ59.1 (s, BAr0 ). CV (THF,
4
TBAH, 100 mV/s): Ep,a = ꢁ0.92 V, Ru(II/I). Anal. Calc.
for C52H40Ru2BF24Cl3: C, 43.37; H, 2.80. Found: C,
42.86; H, 2.89%.
2.2.8. Attempted catalytic aziridination
Fig. 1. ORTEP (scaled to enclose 30% probability) of [(g6-p-cyme-
ne)Ru(j2-O,O-acac-l-CH)]2[OTf]2 (1) (OTf anions are not depicted).
½ðg6-p-CymeneÞRuðj2-O;O-acacÞðTHFÞꢀ½BAr04ꢀð2Þ (0.010
g, 0.008 mmol), styrene (0.002 g, 0.156 mmol) and PhINTs
(0.054 g, 0.156 mmol) were combined in a screw-cap NMR
tube in CDCl3 (0.5 mL). The reaction was monitored by
1H NMR spectroscopy. After 48 h, complete decomposition
of complex 2 was observed without production of aziridine
[39].
Table 1
6
2
˚
Selected bond distances (A) and angles (ꢀ) for [(g -p-cymene)Ru(j -O,O-
acac-l-CH)]2[OTf]2 (1)
˚
Bond lengths (A)
Ru(1)–Centa
Ru(1)–O(1)
Ru(1)–O(2)
Ru(1)–C(1)
Ru(1)–C(2)
Ru(1)–C(3)
1.671
Ru(1)–C(4)
Ru(1)–C(5)
Ru(1)–C(6)
Ru(1)–C(13)b
O(1)–C(12)
O(2)–C(14)
2.189(3)
2.179(3)
2.188(3)
2.315(3)
1.242(3)
1.246(3)
2.085(2)
2.081(2)
2.208(3)
2.190(3)
2.179(3)
2.2.9. Hydrogenation of styrene
½ðg6-p-CymeneÞRuðj2-O;O-acacÞðTHFÞꢀ½BAr04ꢀð2Þ (0.013
g, 0.010 mmol) and styrene (0.002 g, 0.2 mmol) were com-
bined in a J-Young NMR tube in CDCl3 (0.5 mL). The reac-
tion was followed by 1H NMR spectroscopy under 30 psi of
dihydrogen pressure at 60 ꢀC for 36 h. At this time, 31%
yield of ethylbenzene was observed. No additional produc-
tion of ethylbenzene occurred at longer reaction times, and
1H NMR spectroscopy revealed decomposition of 2 to mul-
tiple products.
Bond angles (ꢀ)
O(2)–Ru(1)–O(1)
C(12)–O(1)–Ru(1)
C(14)–O(2)–Ru(1)
C(3)–C(4)–C(10)
O(1)–C(12)–C(13)
a
86.4(1)
128.3(2)
128.3(2)
120.3(3)
126.0(2)
O(1)–C(12)–C(11)
C(13)–C(12)–C(11)
C(12)–C(13)–C(14)
O(2)–C(14)–C(13)
O(2)–C(14)–C(15)
115.9(3)
118.1(3)
120.0(2)
125.6(2)
116.1(3)
Cent corresponds to the centroid of the central six-membered ring of
the p-cymene ligand.
b
Symmetry transformations used to generate equivalent atoms: ꢁx + 2,
ꢁy + 2, ꢁz + 2.
3. Results and discussion
The reaction of previously reported (g6-p-cyme-
ne)Ru(j2-O,O-acac)Cl[2,4] with AgOTf results in chlo-
ride/triflate metathesis to produce (g6-p-cymene)Ru(j2-
O,O-acac)OTf (1-OTf) (Eq. (1)). Consistent with the
exchange of chloride and triflate ligands, the 19F NMR
spectrum of 1-OTf reveals a singlet at ꢁ78.0 ppm. We
anticipated that this reaction would produce the mono-
meric product of simple chloride/triflate exchange, (g6-p-
cymene)Ru(j2-O,O-acac)OTf, however, a single crystal
X-ray diffraction study has revealed that 1-OTf does not
have a Ru-OTf linkage, rather, the C-H moiety of the
acac ligand serves to bridge two Ru centers to form the
bimetallic complex [(g6-p-cymene)Ru(j2-O,O-acac-l-
CH)]2[OTf]2 (1) (Fig. 1, Tables 1 and 2). Thus, in the
solid-state, the acac p-system apparently forms a stronger
bond with Ru than does the weakly coordinating triflate
ligand. However, experimental evidence suggests that the
dimeric l-CH structure does not persist in solution (see
below).
ð1Þ
Examples of complexes with l-CH bridging b-diketonate
ligands are known [40]. The bond distance between Ru and
˚
the bridging acac carbon is 2.315(3) A. This bond distance is
longer than typical RuII–C bond distances of Ru–alkyl link-
ages [41–45], which are typically in the range of 2.16(2)–
2.21(2) A, as well as Ru–C bond distances of g -olefins
2
˚
and vinyl ligands. For example, the Ru–Colefin bond dis-
2
i
˚
tances of cis-Ru(acac)2(g - C2H4)(P Pr3) are 2.172(5) A
˚
and 2.181(5) A,[46] and the Ru–Cvinyl bond distance of
Ru{C(C=CPh)=CHPh}(CO)(PPh3)2(j2-N,S-C4H5N3S) is
˚
2.111(4) A [47]. The average bond distance for Ru–Carene

3258
T. Sumiyoshi et al. / Inorganica Chimica Acta 361 (2008) 3254–3262
Table 2
Selected crystallographic data for complexes 1, 5 and 7
Complex
1
5
7
Empirical formula
Formula weight
T (K)
C16H21F3O5RuS
483.46
223(2)
0.71073
monoclinic
P21/n
10.1067(8)
16.674(1)
11.752(1)
90
C56H46B2F24N6Ru
1381.68
293(2)
0.71073
monoclinic
P21/c
12.9695(8)
18.945(1)
24.865(2)
90
98.393(1)
90
C52H40BC13F24Ru2
1440.14
110
0.71070
triclinic
˚
k (A)
Crystal system
Space group
ꢀ
P1
˚
a (A)
12.6470(3)
14.174(4)
16.992(5)
72.119(2)
73.622(1)
86.217(1)
2780.5(1)
2
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
108.291(2)
90
1880.4(3)
4
3
˚
V (A )
6044.2(6)
4
Z
qcalc (g/cm3)
Crystal size (mm)
Goodness-of-fit
R1, wR2 {I > 2r(I)}
1.708
0.26 · 0.32 · 0.34
1.049
1.518
0.12 · 0.28 · 0.54
0.963
1.720
0.20 · 0.18 · 0.16
1.034
0.0332, 0.0912
0.0534, 0.1407
0.0331, 0.0775
˚
interactions is 2.189(3) A. There are several examples of
structurally characterized Ru(II) systems with g6-p-cymene
ligands. For instance, the [(g6-p-cymene)Ru(1,2,3,4-Me4-
1,3-butadiene)Cl][ClO4] [48] system has an average Ru–
In THF solution, 1 does not coordinate THF, which is
consistent with the proposed (g6-p-cymene)Ru(j2-O,O-
acac)OTf (1-OTf) formulation (Chart 1). In contrast, the
reaction of 1 and NaBAr04 {Ar0=3,5-(CF3)2C6H3} in
THF produces ½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ
½BAr04ꢀ ð2Þ (Eq. (3)). Hence, THF does not compete with
OTf for coordination to Ru but does displace the l-CH
˚
Carene bond distance of 2.280(4) A, the average Ru–Carene
6
+
˚
distance of [(g -p-cymene)Ru(PPh3)2Cl] [5] is 2.278(2) A
while those of [(g6-p-cymene)Ru(1,2-S2C2B10C10-S,S0)(aryl-
amine)] [49] and [(g6-p-cymene)Ru[S2C2(B10H10)](PPh3)]
1
linkage (Scheme 1). The H NMR spectrum of 2 reveals
˚
˚
[50] are 2.227(8) A and 2.263(4) A, respectively.
resonances at 3.49 and 1.74 ppm with resonances at 70.9
and 25.5 ppm in the 13C NMR spectrum due to the a-
and b-positions, respectively, of the coordinated THF. In
comparison, free THF peaks resonate at 3.76 and
1.85 ppm in 1H NMR spectrum and at 68.0 and
25.6 ppm in the 13C NMR spectrum. Thus, coordination
of THF to the cationic Ru(II) results in a slight upfield shift
of resonances (1HMR spectrum) of the THF ligand.
It is likely that the l-CH bridging acac structure of
[(g6-p-cymene)Ru(j2-O,O-acac-l-CH)]2[OTf]2 (1) does
not persist in solution. For example, the chemical shift of
the acac-CH moiety of 1 in CDCl3 is 5.17 ppm. This value
is quite similar to the analogous chemical shift of other
acac complexes reported herein (the range is 6.04 ppm to
5.21 ppm), which is suggestive of a simple j2-O,O coordi-
nation mode. Other evidence suggests that (g6-p-cyme-
ne)Ru(j2-O,O-acac)OTf (1-OTf) is present in solution
(see below). In addition, if the l-CH bridging acac struc-
ture persists in solution, it is easily displaced. For example,
placing 1 in CDCl3 with acetonitrile produces [(g6-p-cyme-
ne)Ru(j2-O,O-acac)(NCMe)][OTf] (3-OTf) within 10 min
at room temperature (Eq. (2)). The production of 3-OTf
has been observed by 1H NMR spectroscopy while
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðNCMeÞꢀ½BAr04ꢀ ð3Þ has
been isolated and characterized (see below).
ð3Þ
ð2Þ
Chart 1. Competition between coordination of acac-CH and OTf.

T. Sumiyoshi et al. / Inorganica Chimica Acta 361 (2008) 3254–3262
3259
Fig. 2. ORTEP (30% probability) of ½TpRuðg6-p-cymeneÞꢀ½BAr40 ꢀ ð5Þ (the
BAr04 counterion is not depicted).
Table 3
Selected
˚
(A)
Scheme 1. Equilibria for coordination of THF as
counterion.
a
function of
bond
distances
and
angles
(ꢀ)
for
½TpRuðg6-p-cymeneÞꢀ½BAr04ꢀ ð5Þ
˚
Bond lengths (A)
Ru(1)–N(3)
Ru(1)–N(1)
Ru(1)–N(5)
Ru(1)–Centa
Ru(1)–C(15)
Ru(1)–C(14)
Ru(1)–C(12)
Ru(1)–C(11)
Ru(1)–C(10)
2.096(3)
Ru(1)–C(13)
C(10)–C(15)
C(10)–C(11)
C(10)–C(16)
C(11)–C(12)
C(12)–C(13)
C(13)–C(14)
C(13)–C(19)
C(14)–C(15)
2.236(4)
1.397(5)
1.429(5)
1.494(6)
1.371(5)
1.416(5)
1.408(5)
1.491(5)
1.398(5)
2.103(2)
2.106(3)
1.694
2.174(3)
2.182(3)
2.183(3)
2.185(3)
2.236(3)
The THF ligand of 2 is quite labile. For example, the com-
bination of and PMe3 in CH2Cl2 produces
2
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðPMe3Þꢀ½BAr04ꢀ ð4Þ (Eq. 4).
Performing this reaction in an NMR tube in CDCl3 reveals
the formation of free THF. Complex 2 also reacts with
NCMe to produce ½ðg6-p-cymeneÞRuðj2-O; O-acacÞ
ðNCMeÞꢀ½BAr04ꢀ ð3Þ (Eq. (5)). Performing the reaction in
NCCD3 reveals complete conversion to 3 within 10 min with
concomitant formation of free THF.
Bond angles (ꢀ)
N(1)–Ru(1)–Centa
N(3)–Ru(1)–Centa
N(5)–Ru(1)–Centa
N(3)–Ru(1)–N(1)
N(3)–Ru(1)–N(5)
a
129.0
127.1
131.5
86.7(1)
84.3(1)
N(1)–Ru(1)–N(5)
N(6)–B(1)–N(4)
N(2)–B(1)–N(4)
N(2)–B(1)–N(6)
81.9(1)
106.8(3)
108.6(3)
107.1(3)
Cent corresponds to the centroid of the six-membered ring of the
ð4Þ
ð5Þ
p-cymene molecule.
given in Table 2. The p-cymene ligand is asymmetrically
coordinated to Ru with Ru–Carene bond lengths of the C-
substituted aromatic positions {2.236(3) A and 2.236(4) A}
longer than the Ru–Carene bond distances of unsubstituted
positions {average Ru–Carene bond distance of unsubstitut-
˚
˚
˚
ed positions = 2.181(3) A}, which likely reflects a steric
influence.
Thinking that the formally anionic and six-electron
donating ligand Tp {Tp = hydridotris(pyrazolyl)borate}
might displace the p-cymene ligand to form a charge neutral
TpRu(j2-O,O-acac)L system, we reacted 2 with KTp.
Instead of Tp/p-cymene exchange, the 0 reaction of
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ½BAr4ꢀ ð2Þ
and
ð6Þ
KTp results in the displacement of the anionic acac ligand
and THF to produce ½TpRuðg6-p-cymeneÞꢀ½BAr40 ꢀ ð5Þ (Eq.
1
(6)). In the downfield region of the H NMR spectrum of
5, there are three resonances due to the Tp ligand, which is
consistent with the presence of a molecular C3 axis of rota-
tion. A single crystal X-ray diffraction study of 5 has con-
firmed its identity (Fig. 2). Bond distances and angles are
presented in Table 3 with selected crystallographic data
The reaction of complex 2 with N3(p-tolyl) results in
ligand exchange to produce ½ðg6-p-cymeneÞRuðj2-O; O-
formation of 6 includes resonances in the 1H NMR spectrum
acacÞfN3ðp-tolylÞgꢀ½BAr0 ꢀ ð6Þ (Eq. (7)). Evidence for the
4

3260
T. Sumiyoshi et al. / Inorganica Chimica Acta 361 (2008) 3254–3262
Table 4
in the aromatic region due to the p-tolyl group (7.24 and
6.29 ppm) as well as a singlet at 2.18 ppm assigned to the
methyl of the p-tolyl fragment. Heating complex 6 does not
result in clean transformation to a Ru imido complex but
rather decomposition to multiple intractable systems.
6
˚
Selected bond distances (A) and angles (ꢀ) for ½fðg -p-cymeneÞ2Rug2
ðl-ClÞ3ꢀ½BAr04ꢀ ð7Þ
˚
Bond lengths (A)
Ru(1)–C(1)
Ru(1)–C(2)
Ru(1)–C(3)
Ru(1)–C(4)
Ru(1)–C(5)
Ru(1)–C(6)
Ru(1)–C(l1)
Ru(1)–C(l2)
Ru(1)–C(l3)
2.178(2)
2.153(2)
2.160(2)
2.181(2)
2.170(2)
2.159(2)
2.4320(4)
2.4185(4)
2.4502(4)
Ru(2)–C(11)
Ru(2)–C(12)
Ru(2)–C(13)
Ru(2)–C(14)
Ru(2)–C(15)
Ru(2)–C(16)
Ru(1)–C(l1)
Ru(1)–C(l2)
Ru(1)–C(l3)
2.172(2)
2.152(2)
2.162(2)
2.186(2)
2.169(2)
2.151(2)
2.4380(4)
2.4217(4)
2.4497(4)
ð7Þ
Bond angles (ꢀ)
Heating (100 ꢀC) complex 2 in CHCl3 or CH2Cl2 forms
the binuclear complex f½ðg6-p-cymeneÞ2Ruꢀ2ðl-ClÞ3g
½BAr04ꢀ ð7Þ (Eq. (8)). Qualitatively, the rate of the formation
of 7 is faster in CHCl3 than CH2Cl2. An X-ray diffraction
study of a single crystal of complex 7 has confirmed its
identity (Fig. 3). Bond distances and angles are presented
in Table 4 with selected crystallographic data given in
Table 2. The average bond distance from Ru to arene car-
C(1)–Ru(1)–Cl(1)
C(1)–Ru(1)–Cl(2)
C(1)–Ru(1)–Cl(3)
C(4)–Ru(1)–Cl(3)
C(4)–Ru(1)–Cl(2)
C(4)–Ru(1)–Cl(1)
C(7)–C(1)–Ru(1)
C(10)–C(4)–Ru(1)
Ru(1)–Cl(1)–Ru(2)
Ru(1)–Cl(2)–Ru(2)
Ru(2)–Cl(3)–Ru(1)
96.94(4)
112.64(4)
166.70(4)
93.71(5)
126.60(5)
150.20(5)
128.5(1)
128.1(1)
84.12(1)
84.76(1)
83.49(1)
C(9)–C(7)–C(8)
C(1)–C(7)–C(9)
C(1)–C(7)–C(8)
111.0(2)
113.6(2)
108.9(1)
120.9(2)
96.98(4)
127.15(5)
165.56(5)
149.30(5)
165.98(5)
91.70(5)
79.60(1)
C(3)–C(4)–C(10)
C(11)–Ru(2)–Cl(1)
C(12)–Ru(2)–Cl(1)
C(13)–Ru(2)–Cl(1)
C(14)–Ru(2)–Cl(1)
C(15)–Ru(2)–Cl(2)
C(16)–Ru(2)–Cl(1)
Cl(2)–Ru(2)–Cl(3)
˚
bon is 2.167(2) A, which is the closest Ru–Carene contact
among the complexes 1, 5 and 7. The short Ru–Carene bond
distances of 7 are likely due to the less sterically crowded
coordination sphere versus complexes 1 and 5. In addition,
the Ru–Carene bond distance of the C-substituted positions
3.1. Catalysis
˚
˚
{2.178(2) A and 2.181(2) A} are longer than the average
distance of Ru–Carene of the unsubstituted positions
Since the THF ligand of ½ðg6-p-cymeneÞRu
ðj2-O; O-acacÞðTHFÞꢀ½BAr04ꢀ ð2Þ is labile, we suspected that
complex 2 might serve as a catalyst precursor. In order to
test the ability of 2 to coordinate and activate substrates,
we chose two reactions with substantial precedent: olefin
aziridination and olefin hydrogenation. For the former,
complex 2 shows no activity. For example, a CDCl3 solu-
tion of PhINTs, styrene and 5 mol% of 2 results in no pro-
duction of aziridine reaction after 48 h at room
temperature. At this time, complex 2 is observed to decom-
pose to multiple uncharacterized complexes. In contrast, a
solution of styrene and 5 mol% 2 in CDCl3 under 30 psi of
dihydrogen results in the formation of ethylbenzene (Eq.
(9)). Monitoring the reaction at 60 ꢀC reveals 31% yield
˚
{2.160(2) A}.
BAr'4
BAr'4
Ru
Cl
Cl
CHCl3
100 ºC
Cl
Ru
ð8Þ
(8)
O
O
Ru
O
(2)
(7)
1
of ethylbenzene after 36 h. H NMR spectroscopy reveals
decomposition of 2 into multiple products, and prolonged
reaction times do not result in additional production of
ethylbenzene.
ð9Þ
3.2. Cyclic voltammetry
The combination of the ‘‘soft’’ donor g6-arene and
‘‘hard’’ oxygen-donor acac ligand sets up an interesting elec-
tronic mix. In order to probe the electron density of the new
Ru systems, we performed cyclic voltammetry. Table 5 dis-
plays a list of complexes and observed potentials.
Fig. 3. ORTEP (50% probability) of ½fðg6-p-cymeneÞ2Rug2ðl-ClÞ3ꢀ
½BAr04ꢀ ð7Þ (the BAr04 counterion is not depicted).

T. Sumiyoshi et al. / Inorganica Chimica Acta 361 (2008) 3254–3262
3261
Table 5
Appendix A. Supplementary material
The redox potentials for the various Ru(II) complexes
Complex
Ru(II/I)a,b (V)
Details of single crystal X-ray diffraction studies are
provided. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2007.10.012.
½ðg6-p-cymeneÞRuðj2-O; O-acacÞfN3ðp-tolylÞgꢀ½BAr04ꢀ ð6Þ ꢁ0.50
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ½BAr04ꢀ ð2Þ
f½ðg6-p-cymeneÞ2Ruꢀ2ðl-ClÞ3g½BAr04ꢀ ð7Þ
ꢁ0.78
ꢁ0.92
ꢁ1.13
ꢁ1.24
ꢁ1.45
ꢁ1.63
[(g6-p-cymene)Ru(j2-O,O-acac)OTf (1-OTf)
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðNCMeÞꢀ½BAr04ꢀ ð3Þ
½TpRuðg6-p-cymeneÞꢀ½BAr04ꢀ ð5Þ
References
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðPMe3Þꢀ½BAr04ꢀ ð4Þ
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Organomet. Chem. 690 (5) (2005) 1218.
a
Reported vs. NHE.
b
All potentials are irreversible (Ep,a) except for complex 2, which is
reversible (E1/2).
[3] T. Ohnishi, Y. Miyaki, H. Asano, H. Kurosawa, Chem. Lett. (8)
(1999) 809.
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For all complexes, scans to positive potentials (2.0 V
versus NHE) revealed no redox activity. Thus, the combi-
nation of the g6-arene and acac or Tp ligands and overall
cationic charge results in electron-deficient Ru(II) systems,
which suggests that such complexes might be strongly
Lewis acidic. In contrast, scans at negative potentials (ver-
sus NHE) result in reduction of the Ru complexes, which is
consistent with Ru(II) to Ru(I) transformations. With the
exception of complex 2, the reductions are irreversible.
The order of the absolute value of the reduction potentials
for complexes of the general formula ½ðg6-p-cymeneÞRu
ðj2-O; O-acacÞðLÞꢀ½BAr04ꢀ, 6 < 2 < 1 < 3 < 4, suggests that
the relative donating ability of the ligands (from most
donating to least donating) is: PMe3 > NCMe > l-CH/
OTf > THF > N3(p-tolyl). The more negative reduction
potential of complex 1 relative to 2 is consistent with the
lack of coordination of THF upon dissolution of 1 in
THF. The Ru(II/I) reduction potentials suggest that the
l-Cl3 moiety is more strongly donating than the combina-
tion of the j2-acac ligand and THF, but more weakly
donating than the j2-acac ligand in combination with
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We have synthesized a series of complexes possessing
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characterization of three of these systems. The complexes
[(g6-p-cymene)Ru(j2-O,O-acac-l-CH)]2[OTf]2 (1) and
½ðg6-p-cymeneÞRuðj2-O; O-acacÞðTHFÞꢀ½BAr04ꢀ ð2Þ allow
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We thank the National Science Foundation (CAREER
Award: CHE 0238167) and the Alfred P. Sloan Founda-
tion (Research Fellowship) for support of this research.

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