Reductive coupling of [(RO)2Ti(L2)2] complexes containing a chelating bis(aryloxide) ligand with ketones (L 2 = bpy, dmbpy, or phen)

Article abstract of DOI:10.1021/om050231k

The reactivity of [(RO)₂Ti(L₂)₂] complexes (L₂ = bpy, dmbpy, or phen; (RO)₂ = DMSC or MBMP) with ketones was investigated (DMSC = 1,2-alternate dimethylsilyl-bridged p-tertbutylcalix[4]arene dianion; MBMP = 2,2′-methylenebis(6-tert-butyl-4- methylphenol) dianion). The molecular structures of [(DMSC)Ti(bpy)2] (6a) and [(DMSC)Ti(bpy)₂] (7a) were characterized by X-ray crystallography. Their structural data taken together with UV-visible and magnetic susceptibility data suggest some electron transfer into the LUMO (π* orbital) of the diimine ligands. [(RO)₂Ti(L₂)₂] complexes undergo light-assisted reaction with aromatic ketones in a reversible manner to afford Ti-η²-ketone complexes [(RO)₂Ti(η ²-OCArR)(L₂)] (Ar = aryl while R = aryl or alkyl), which undergo further reaction with ketone to give the corresponding 2-aza-5-oxa-titanacyclopentene. Qualitatively, the efficacy of the formation of 2-aza-5-oxa-titanacyclopentene [(RO)₂Ti(L₂)₂] complexes increased with increasing ketone concentration and in the order L ₂ = bpy < dmbpy < phen. This order is best explained in terms of the dependence of the equilibrium between [(RO)₂Ti(L ₂)₂] and [(RO)₂Ti(η²-OCArR) (L₂)] on relative abilities of the ketone and the diimine to accept π-electron density, as well as in terms of the order of the rate of reaction of [(RO)₂Ti(η²-OCArR)(L₂)] with ketone.

Full text of DOI:10.1021/om050231k

Organometallics 2005, 24, 3995-4002
3995
Reductive Coupling of [(RO)2Ti(L2)2] Complexes
Containing a Chelating Bis(aryloxide) Ligand with
Ketones (L2 ) bpy, dmbpy, or phen)
David Owiny,‡ Jesudoss V. Kingston,‡ Marc Maynor,‡ Sean Parkin,‡
Jeff W. Kampf,§ and Folami T. Ladipo*,‡
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, and
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055
Received March 24, 2005
The reactivity of [(RO)2Ti(L2)2] complexes (L2 ) bpy, dmbpy, or phen; (RO)2 ) DMSC or
MBMP) with ketones was investigated (DMSC ) 1,2-alternate dimethylsilyl-bridged p-tert-
butylcalix[4]arene dianion; MBMP ) 2,2′-methylenebis(6-tert-butyl-4-methylphenol) dianion).
The molecular structures of [(DMSC)Ti(bpy)2] (6a) and [(DMSC)Ti(bpy)2] (7a) were character-
ized by X-ray crystallography. Their structural data taken together with UV-visible and
magnetic susceptibility data suggest some electron transfer into the LUMO (π* orbital) of
the diimine ligands. [(RO)2Ti(L2)2] complexes undergo light-assisted reaction with aromatic
ketones in a reversible manner to afford Ti-η2-ketone complexes [(RO)2Ti(η2-OCArR)(L2)]
(Ar ) aryl while R ) aryl or alkyl), which undergo further reaction with ketone to give the
corresponding 2-aza-5-oxa-titanacyclopentene. Qualitatively, the efficacy of the formation
of 2-aza-5-oxa-titanacyclopentene [(RO)2Ti(L2)2] complexes increased with increasing ketone
concentration and in the order L2 ) bpy < dmbpy < phen. This order is best explained in
terms of the dependence of the equilibrium between [(RO)2Ti(L2)2] and [(RO)2Ti(η2-OCArR)-
(L2)] on relative abilities of the ketone and the diimine to accept π-electron density, as well
as in terms of the order of the rate of reaction of [(RO)2Ti(η2-OCArR)(L2)] with ketone.
Introduction
pinacolate complexes [(DMSC)Ti(OCAr2CAr2O)] (1a, Ar
) p-MeC6H4; 1b, Ar ) Ph; DMSC ) 1,2-alternate
Low-valent titanium-mediated reductive coupling re-
actions of unsaturated organic substrates continues to
grow in importance as a synthetic methodology.1,2
However, better understanding of the synthesis, struc-
ture, and reactivity of reduced titanium complexes is
needed in order to achieve greater mechanistic under-
standing and better control over selectivity of the
reactions. Hence, increased effort has been focused on
developing the synthesis and reactivity of well-charac-
terized low-valent group 4 metal complexes.3,4 Over the
last several years, we have been developing the syn-
thesis and reactivity of well-characterized Ti(II) syn-
thons containing calix[4]arene-derived chelating bis-
(aryloxide) ligation.4 Recently, we found that titana-
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* To whom correspondence should be addressed. E-mail: fladip0@
uky.edu.
‡ University of Kentucky.
§ University of Michigan.
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10.1021/om050231k CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/06/2005

3996 Organometallics, Vol. 24, No. 16, 2005
Owiny et al.
Scheme 1
dimethylsilyl-bridged p-tert-butylcalix[4]arene dianion)
react with aromatic diimines, such as 2,2′-bipyridine
(bpy), 4,4′-dimethyl-2,2′-dipyridyl (dmbpy), or 1,10-
phenanthroline (phen), to produce Ti-η2-ketone com-
plexes [(DMSC)Ti(η2-OCAr2)(L2)] (2a-d, L2 ) bpy,
dmbpy, or phen).4b The latter compounds undergo light-
assisted reaction with 1 equiv of ketone to yield 2-aza-
5-oxa-titanacyclopentenes 3a-d (Scheme 1), in which
C-H activation of the diimine ligand and hydride
migration to a Ti-bound ketone to form an alkoxide
group has occurred. Titanium-promoted reductive coup-
ling of ketones with 2,2′-bipyridine or 1,10-phenanthro-
line is attractive because 6-(1-hydroxyalkyl)-2,2′-bipy-
ridines and 2-(1-hydroxyalkyl)-1,10-phenanthrolines,
especially chiral derivatives, are of interest as ligands
for catalyst exploration.5,6 Also, there is currently ample
need of additional general methods for synthesis of this
class of compounds.5,7 Helquist and co-workers have
recently reported SmI2-promoted coupling of aliphatic
ketones and aldehydes with 1,10-phenanthroline to
produce 2-(1-hydroxyalkyl)-1,10-phenanthrolines, but
aromatic ketones and aldehydes were not useful sub-
strates.7a,b
Herein, we describe results from our studies of the
reactions of ketones with titanium bis(diimine) com-
plexes [(DMSC)Ti(L2)2] (6a-c) and [(MBMP)Ti(L2)2]
(7a and 7b, MBMP ) 2,2′-methylenebis(6-tert-butyl-4-
methylphenol) dianion). We chose to study this class of
compounds because aromatic diimines are redox-active
π-acid ligands that have been shown to support electron-
rich reduced titanium centers.8 Furthermore, thermal
or photochemical substitution of an aromatic diimine
ligand can be used to open up coordination sites at metal
centers.9
Experimental Section
General Details. All experiments were performed under
a dry nitrogen atmosphere using standard Schlenk techniques
or in a Vacuum Atmospheres, Inc. glovebox. Solvents were
dried and distilled by standard methods before use.10 All
solvents were stored in the glovebox over 4A molecular sieves
that were dried in a vacuum oven at 150 °C for at least 48 h
prior to use. Unless otherwise stated all reagents were
purchased from Aldrich. Benzophenone, 1,10-phenanthroline,
4,4′-dimethylbenzophenone, 2,2′-bipyridine, and 4,4′-dimethyl-
2,2′-dipyridyl were sublimed, while 2,2′-methylenebis(6-tert-
butyl-4-methylphenol) was dried under vacuum for 24 h before
use. [(DMSC)TiPh2] (4), [(DMSC)Ti(bpy)2] (6a), and [(DMSC)-
Ti(dmbpy)2] (6b) were prepared according to published
procedures.4d [(MBMP)TiPh2] (5)11 was prepared via modifica-
tion of the reported procedure (see Supporting Informa-
tion). 1H and 13C NMR spectra were recorded on a Varian
Gemini-200 spectrometer or a Varian VXR-400 spectrometer
at room temperature unless otherwise stated. 1H and 13C
chemical shifts were referenced to residual solvent peaks.
Infrared spectra were recorded on a Nicolet Magna 560
spectrometer. Electronic spectra were recorded on a Hewlett-
Packard 8453 series UV-visible spectroscopy system. GC-MS
analyses were performed on a Hewlett-Packard 5890 series II
gas chromatograph with a Hewlett-Packard 5972 series mass
selective detector at an ionizing potential of 70 eV or at
University of Kentucky Mass Spectrometry Center on a
Thermo Finnigan (San Jose, CA) Polaris Q (quadruple ion
trap) or JEOL JMS-700T MStation spectrometer. Elemental
analyses were performed by Complete Analysis Laboratories,
Inc., Parsippany, NJ.
[(DMSC)Ti(phen)2] (6c). To a toluene (15 mL) solution of
[(DMSC)TiPh2] (4) (1.55 g, 1.71 mmol) was added 1,10-
phenanthroline (0.617 g, 3.42 mmol). The initially bright
yellow solution immediately turned dark olive-green. After
stirring for 2 min, pentane (30 mL) was added into the olive-
green solution and the resulting mixture was cooled at -15
°C for 10 min. The olive-green solid isolated by filtration was
washed with pentane (3 × 5 mL) and dried under vacuum.
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Reductive Coupling of [(RO)2Ti(L2)2] Complexes
Organometallics, Vol. 24, No. 16, 2005 3997
Yield: 1.44 g, 75.8%. 1H NMR (C6D6): δ 10.11 (d, J ) 6.0 Hz,
2H, phen), 9.25 (d, J ) 6.0 Hz, 2H, phen), 7.48 (d, J ) 2.8 Hz,
2H, arom CH), 7.47 (d, J ) 2.8 Hz, 2H, arom CH), 6.95 (d, J
) 2.8 Hz, 2H, arom CH), 6.84 (s, 2H, phen), 6.78 (d, J ) 7.2
Hz, 2H, phen), 6.75 (d, J ) 2.8 Hz, 2H, arom CH), 6.49 (s, 2H,
phen), 6.13 (d, J ) 7.2 Hz, 2H, phen), 6.05 (dd, J ) 7.2, 5.6
Hz, 2H, phen), 5.75 (dd, J ) 7.2, 5.6 Hz, 2H, phen), 4.82 (d, J
) 16.0 Hz, 2H, calix-CH2), 4.57 (d, J ) 15.2 Hz, 1H, calix-
CH2) 4.35 (d, J ) 16.0) Hz, 2H, calix-CH2) 3.92 (d, J ) 13.6
Hz, 1H, calix-CH2), 3.35 (d, J ) 15.2 Hz, 1H, calix-CH2), 3.22
(d, J ) 13.6 Hz, 1H, calix-CH2), 1.51 (s, 18 H, t-Bu), 0.88 (s,
18H, t-Bu), 0.32 (s, 3H, exo-SiMe), -0.74 (s, 3H, endo-SiMe).
13C NMR (C6D6): δ 163.6 (Ti-OC), 152.2, 150.7, 145.4,
142.8, 142.1, 140.2, 135.6, 132.1, 131.5, 131.3, 131.2, 130.9,
130.1, 129.7, 129.4, 127.0, 126.9, 126.5, 126.2, 125.9,
123.1, 115.8, 115.3, 41.2 (calix-CH2), 38.7 (calix-CH2), 38.5
(calix-CH2), 34.5 (C(CH3)3, 33.8 (C(CH3)3), 32.6 (C(CH3)3), 31.6
(C(CH3)3), 2.9 (exo-SiMe), -1.1 (endo-SiMe). Anal. Calcd for
C70H74N4O4SiTi: C, 75.65; H, 6.71; N, 5.04. Found: C, 75.29;
H, 6.57; N, 4.95.
[(MBMP)Ti(bpy)2] (7a). To a solution of [(MBMP)TiPh2]
(5) (3.00 g, 5.60 mmol) in 15 mL of ether was added 2,2′-
bipyridine (1.74 g, 11.2 mmol). The reaction mixture im-
mediately turned dark blue and was stirred for 30 min. The
solvent was removed under reduced pressure, and the dark
blue residue was washed with pentane (3 × 10 mL) and dried
under vacuum. Yield: 3.40 g, 86.9%. 1H NMR at -20 °C
(toluene-d8): δ 10.69 (d, J ) 5.2 Hz, 1H, bpy), 9.20 (d, J ) 5.6
Hz, 1H, bpy), 8.41 (d, J ) 5.6 Hz, 1H, bpy), 8.37 (d, J ) 6.0
Hz, 1H, bpy), 7.67 (s, 1H, MBMP arom CH), 7.60 (s, 1H,
MBMP arom CH), 7.32 (s, 1H, MBMP arom CH), 6.68 (d, J )
7.2 Hz, 2H, bpy), 5.89-6.00 (m, 4H, bpy), 5.85 (d, J ) 12.4
Hz, 1H, MBMP-CH2), 5.60-5.70 (m, 2H, bpy), 5.40 (t, 1H, bpy),
5.03 (t, 1H, bpy), 4.06 (d, J ) 12.4 Hz, 1H, MBMP-CH2), 2.47
(s, 3H, Me), 2.40 (s, 3H, Me), 1.37 (s, 9H, t-Bu), 0.70 (s, 9H,
t-Bu). Anal. Calcd for C43H46N4O2Ti: C, 73.91; H, 6.63; N, 8.01.
Found: C, 73.79; H, 6.37; N, 7.92. Single crystals of 7a suitable
for X-ray diffraction study were obtained by slow evaporation
of a benzene solution.
[(MBMP)Ti(dmbpy)2] (7b). To a solution of [(MBMP)-
TiPh2] (5) (0.250 g, 0.463 mmol) in 5 mL of toluene was added
4,4′-dimethyl-2,2′-dipyridyl (0.171 g, 0.930 mmol). The reaction
mixture immediately turned dark blue-green and was stirred
for 30 min. The solvent was removed under reduced pressure,
and the dark blue residue was washed with pentane (3 × 5
mL) and dried under vacuum. Yield: 0.289 g, 82.7%. 1H NMR
at -75 °C (toluene-d8): δ 10.92 (br s, 1H, dmbpy), 9.39 (br s,
1H, dmbpy), 8.85 (br m, 2H, dmbpy), 8.71 (br d, 1H, dmbpy),
8.56 (d, J ) 4.8 Hz, 1H, dmbpy), 7.74 (br s, 1H, MBMP arom
CH), 7.71 (br s, 1H, MBMP arom CH), 7.48 (br s, 1H, MBMP
arom CH), 6.81 (br s, 1H MBMP arom CH), 6.59 (s, 1H, dmbpy)
6.46 (br d, J ) 4.8 Hz, 1H, dmbpy), 6.14 (br d, 1H, MBMP-
CH2), 5.75 (br d, 1H, dmbpy), 5.68 (s, 1H, dmbpy), 5.24 (br d,
1H, dmbpy), 4.91 (br d, 1H, dmbpy), 4.21 (br d, 1H, MBMP-
CH2), 2.55 (br s, 3H, MBMP-Me) 2.43 (br s, 3H, MBMP-Me),
1.99 (br s, 3H, dmbpy-Me), 1.82 (s, 3H, dmbpy-Me), 1.68 (br s,
6H, 2 dmbpy-Me), 1.58 (br s, 9H, t-Bu), 0.86 (br s, 9H, t-Bu).
Anal. Calcd for C47H54N4O2Ti: C, 74.78; H, 7.21, N, 7.42.
Found: C, 74.62; H, 7.25; N, 7.34.
[(MBMP)Ti{K3-OCPh2C10H7N2}(OCHPh2)] (8a). To a 10
mL ether solution of [(MBMP)TiPh2] (5) (0.101 g, 0.187 mmol)
in a glass pressure tube was added 2,2′-bipyridine (0.0613 g,
0.392 mmol). The resulting dark blue reaction mixture was
stirred for 10 min and then charged with benzophenone
(0.0910 g, 0.500 mmol). The reaction mixture was heated with
stirring at 65 °C for 12 h. After cooling to ambient temperature,
the mixture was filtered and the orange precipitate was
washed with THF (2 × 3 mL) followed by toluene (2 × 3
mL) and dried under vacuum. Yield: 0.141 g, 83.2%. 1H NMR
(CD2Cl2): δ 8.69 (d, J ) 5.2 Hz, 1H, arom CH), 8.18 (d, J )
8.4 Hz, 1H, arom CH), 7.89 (t, J ) 7.6 Hz, 1H, arom CH), 7.80
(t, J ) 7.6 Hz, 1H, arom CH), 7.69 (m, 2H arom CH), 7.62 (d,
J ) 8.0 Hz, 1H arom CH), 7.59 (d, J ) 8.0 Hz, 1H arom CH),
7.41 (d, J ) 8.0 Hz, 1H arom CH), 7.31 (br t, 1H, arom CH),
7.26-6.70 (m, 19 H, arom CH), 6.65 (s, 1H, MBMP arom CH),
6.48 (s, 1H, MBMP arom CH), 5.92 (d, 1H, J ) 13.6 Hz,
MBMP-CH2), 5.56 (s, 1H, OCHPh2), 2.73 (d, 1H, d ) 13.6 Hz,
MBMP-CH2), 2.19 (s, 3H, Me), 2.11 (s, 3H, Me), 1.50 (s, 9H,
t-Bu), 0.38 (s, 9H, t-Bu). 13C NMR (CD2Cl2): δ 170.4, 152.9,
151.3, 148.6, 147.05, 140.1, 139.6, 129.5, 128.7, 128.5, 128.4,
128.2, 127.9, 126.4, 126.3, 125.84, 125.7, 124.7, 121.9, 119.2,
99.5 (OCPh2C10H5Me2N2), 86.8 (Ti-OCHPh2), 35.5 (MBMP-
CH2), 33.9(C(CH3)3), 31.3 (C(CH3)3), 29.7 (C(CH3)3), 21.2
(dmbpy CH3), 20.9 (dmbpy CH3). MS (EI, 25 eV) m/z: 906 [M]+,
724 [M - Ph2CO]+. Single crystals of 8a suitable for X-ray
diffraction study were obtained by slow evaporation of a dilute
benzene solution.
Typical Procedure for NMR Study of the Reaction
between [(DMSC)Ti(L2)2] (6a-c) and 4,4′-Dimethylben-
zophenone. A 0.094 mL portion of a 0.100 M stock solution
of [(DMSC)Ti(bpy)2] (6a) was added into a screw-capped NMR
tube, followed by 0.180 mL of a 0.100 M stock solution of (p-
MeC6H4)2CO and finally 0. 518 mL of C6D6. This resulted in
0.800 mL of a 0.0117 M solution of 6a and a 0.0225 M solution
of (p-MeC6H4)2CO. The tube was vigorously shaken and placed
into the NMR spectrometer at 22 °C. The reaction was
monitored by recording 1H NMR spectra immediately after
inserting the sample in the spectrometer and every 10 min
thereafter. The dependence of the reaction on (p-MeC6H4)2CO
was obtained by varying the concentration of (p-MeC6H4)2CO
while conducting each experiment in C6D6 at the same
temperature, using an identical amount of 6a and the same
total volume (0.800 mL). The dependence on the nature of the
diimine was obtained by conducting analogous reactions for
[(DMSC)Ti(dmbpy)2] (6b) and [(DMSC)Ti(phen)2] (6c).
Typical Procedure for Reaction of [(MBMP)Ti(bpy)2]
(7a) with Ketones. To an ether (10-15 mL) solution of
[(MBMP)TiPh2] (5) in a glass pressure tube was added 2,2′-
bipyridine (2 equiv). The dark blue mixture was stirred for 2
min, and then the ketone (g2 equiv) was added. The reaction
mixture was heated at 65 °C for a period of time (18-72 h)
during which the reaction progress was monitored via 1H and
GC-MS analysis of aliquots taken under N2. After cooling the
reaction mixture to ambient temperature, the orange suspen-
sion was filtered. The precipitate was washed twice with ether
and then dissolved in a ∼2:1 mixture of CH2Cl2 and aqueous
NH4Cl. After stirring until the CH2Cl2 layer was yellow, the
organic layer was separated and then subjected to chromato-
graphic separation.
2,2′-Bipyridinyl-6-yl-diphenylmethanol (9a). From reac-
tion of [(MBMP)TiPh2] (5) (0.352 g, 0.653 mmol) with 2,2′-
bipyridine (0.203 g, 1.30 mmol) and Ph2CO (0.238 g, 1.30
mmol) for 18 h, 0.164 g (88% yield) of 9a was isolated as an
off-white solid after chromatographic purification on a silica
column using 5:1 CH2Cl2/heptane as eluent. 1H NMR
(CDCl3): δ 8.71-8.66 (m, 2H, bipyridyl), 8.45-8.37 (m, 3H,
bipyridyl), 7.84-7.72 (m, 3H, arom CH), 7.42-7.26 (m, 8H,
arom CH), 7.17 (dd, J ) 7.6, 1.2 Hz, 1H, bipyridyl), 6.55 (s,
1H, OH). 13C NMR (CDCl3): δ 162.4, 155.3 154.2, 149.3, 146.3,
137.6, 137.0, 128.4, 128.1, 127.5, 124.1, 123.8, 123.3, 121.3,
120.0, 81.1 (C-OH). HRMS (EI, 25 eV): m/z calcd for C23H18N2O
(M+) 338.4066, found 338.1415.
2,2′-Bipyridinyl-6-yl-1-phenylethanol (9b). From reac-
tion of [(MBMP)TiPh2] (5) (0.316 g, 0.585 mmol) with 2,2′-
bipyridine (0.185 g, 1.18 mmol) and PhCOMe (0.141 g, 1.18
mmol) for 48 h, 0.0360 g (28% yield) of 9b was isolated as a
faint pink oil after two chromatographic purification steps on
silica columns using 5:1 CH2Cl2/heptane and then followed by
1
7:1 acetone/heptane as eluent. H NMR (CDCl3): δ 8.71 (d, J
) 4.4 Hz, 1H, bipyridyl), 8.46 (d, J ) 8.0 Hz, 1H, bipyridyl),
8.34 (d, J ) 8.0 Hz, 1H, bipyridyl), 7.87 (t, J ) 8.0 Hz, 1H,
bipyridyl), 7.81 (t, J ) 7.6 Hz, 1H, bipyridyl), 7.56 (dd, J )

3998 Organometallics, Vol. 24, No. 16, 2005
Owiny et al.
7.4, 1.2 Hz, 2H, arom CH), 7.40-7.22 (m, 5H, arom CH), 6.07
(s, 1H, OH), 2.00 (s, 3H, Me). 13C NMR (CDCl3): δ 165.4, 155.9,
154.4, 148.1, 147.2, 139.3, 138.6, 128.6, 127.4, 126.4, 124.7,
122.1, 121.6, 120.2, 75.8 (C-OH), 29.6 (CH3). HRMS (EI, 25
eV): m/z calcd for C18H16N2O (M+) 276.3356, found 276.1257
2,2′-Bipyridinyl-6-yl-1,2-diphenylethanol (9c). From re-
action of [(MBMP)TiPh2] (5) (0.201 g, 0.373 mmol) with 2,2′-
bipyridine (0.116 g, 0.744 mmol) and PhCOCH2Ph (0.225 g,
1.15 mmol) for 36 h, 19.0 mg (15% yield) of 9c was isolated as
an off-white solid after chromatographic purification on a silica
column using 4:1 ethyl acetate/heptane as eluent. 1H NMR
(CDCl3): δ 8.76 (d, J ) 4.4 Hz, 1H, bipyridyl), 8.36 (d, J ) 8.0
Hz, 1H, bipyridyl), 8.21 (d, J ) 8.0 Hz, 1H, bipyridyl), 7.96 (t,
J ) 8.0 Hz, 1H, bipyridyl), 7.83 (t, J ) 8.0 Hz, 1H, bipyridyl),
7.66 (dd, J ) 8.0, 1.2 Hz, 2H, arom), 7.50 (d, J ) 8.0 Hz, 1H,
arom CH), 7.45 (t, J ) 8.0 Hz, 1H, arom CH), 7.35 (t, J ) 8.0
Hz, 2H, arom CH), 7.30-7.24 (m, 2H, arom CH), 7.16-7.08
(m, 3H, arom CH), 6.98-7.05 (m, 2H, arom CH), 5.38 (br s,
1H, OH), 3.80 (d, J ) 13.6 Hz, 1H, CH2Ph), 3.67 (d, J ) 13.6
Hz, 1H, CH2Ph). 13C NMR (CDCl3): δ 163.5, 154.0, 147.5,
145.9, 139.4, 138.4, 136.6, 131.1, 128.5, 128.0, 127.5, 126.6,
126.5, 124.6, 122.3, 120.2, 78.0 (C-OH), 47.5 (CH2Ph). HRMS
(EI, 25 eV): m/z calcd for C24H20N2O (M+) 352.4320, found
352.1566.
or yellow-brown upon exposure to air in the solid state
or in solution. [(DMSC)Ti(L2)2] compounds (6a-c) show
higher solubility in organic solvents than [(MBMP)-
Ti(L2)2] compounds (7a,b). For instance, 6a-c are
highly soluble in THF and aromatic hydrocarbon sol-
vents and moderately soluble in ether. In contrast, 7a,b
are highly soluble in THF but only moderately soluble
in aromatic hydrocarbon solvents and ether. All of the
compounds are insoluble in pentane. They react with
polyhalogenated hydrocarbons, such as CH2Cl2 and
CHCl3, to yield a mixture of unidentified Ti(IV) prod-
ucts.
The formulation and structure of [(DMSC)Ti(phen)2]
(6c), [(MBMP)Ti(bpy)2] (7a), and [(MBMP)Ti(dmbpy)2]
(7b) were characterized by microanalysis and spectro-
scopic data. 1H and 13C NMR data for 6c displayed
sharp resonances that are consistent with Cs-symmetry
1
in solution at ca. 22 °C. The H NMR spectrum of 6c
shows two singlets (integrating in 1:1 ratio) for the tert-
butyl groups and six doublets for bridging methylene
groups of the calixarene ligand. Two of the six doublets
integrate as two protons each and represent the meth-
ylene protons that are reflected by the mirror plane,
which contains the remaining four methylene protons.
That the DMSC ligand exists in 1,2-alternate conforma-
tion is evidenced by the shift to high field (δ ) -0.74
ppm) of the endo-Me of the bridging SiMe2 group
(located inside the calixarene cavity) compared to the
exo-Me of the bridging SiMe2 group (located outside the
calixarene cavity), which shows at δ 0.32 ppm.4
Results and Discussion
Synthesis and Characterization of [(RO)2Ti(L2)2]
Compounds. We have previously reported that
[(DMSC)Ti(bpy)2] (6a) and [(DMSC)Ti(dmbpy)2] (6b) are
conveniently synthesized via bipyridine-induced reduc-
tive elimination of biphenyl from [(DMSC)TiPh2] (4).4d
[(DMSC)Ti(phen)2] (6c), [(MBMP)Ti(bpy)2] (7a), and
[(MBMP)Ti(dmbpy)2] (7b) are similarly obtained in high
yield from reaction of the appropriate [(RO)2TiPh2]
compound with 2 equiv of aromatic diimine (eq 1).12 The
At 25 °C, solution 1H NMR spectra of [(MBMP)-
Ti(bpy)2] (7a) and [(MBMP)Ti(dmbpy)2] (7b) displayed
mostly broad resonances, suggesting that the com-
pounds are fluxional on the NMR time scale. Hence we
conducted variable-temperature 1H NMR studies of 7a
and 7b in toluene-d8 from 295 to 193 K. The peaks in
the NMR spectrum for 7a sharpened as the temperature
was lowered, and the 1H NMR data at 253 K are
consistent with 7a adopting C1-symmetry in solution:
each of the two tert-butyl groups, the two methyl groups,
and the four aromatic protons of the MBMP ligand was
observed as a singlet resonance. In addition, each proton
of the bridging CH2 unit of the MBMP ligand showed
as a doublet resonance, and all of the bipyridine protons
were inequivalent. While the peaks in the 1H NMR
spectrum of 7b sharpened considerably as the temper-
ature was lowered, they remained somewhat broad even
at 193 K. Nonetheless, the 1H data similarly established
that 7b is C1-symmetric in solution (see Experimental
Section). It is likely that 7a and 7b owe their stereo-
chemical nonrigidity to isomerization via a twist mech-
anism; a number of such mechanisms have been re-
ported for isomerization of M(chelate)3 complexes.14
Molecular structures of [(DMSC)Ti(bpy)2] (6a) and
[(MBMP)Ti(bpy)2] (7a) were also established by single-
crystal X-ray diffraction studies. Crystallographic data
and selected metrical parameters are given in Tables 1
and 2, respectively. The molecular structures of 6a and
7a (depicted in Figures 1 and 2, respectively) support
their C1-symmetry in solution as assigned by NMR
spectroscopy. In 6a, the geometry about Ti is twisted
away from trigonal prismatic toward octahedral by
∼25°, whereas 7a is distinctly octahedral, albeit dis-
torted. These structures presumably reflect acute bite
related deep blue-green complex [(2,6-Pri2C6H3O)2Ti-
(bpy)2] was similarly prepared by Rothwell.8a Com-
pounds 6a,b and 7a,b were isolated as deep dark blue
solids, while 6c was obtained as a dark olive-green
powder. All of the compounds are stable in the solid
state under N2 atmosphere but are best stored for
extended periods of time at low temperature (below -15
°C) and protected from light.13 They are highly sensitive
to hydrolysis and oxidation and immediately turn yellow
(12) A dark purple precipitate is formed immediately from similar
reaction of [(MBMP)TiPh2] (5) with phen (2 equiv) in ether. This
product is tentatively assigned as [(MBMP)Ti(phen)2] on the basis of
microanalysis data (Anal. Calcd for C47H46N4O2Ti: C, 75.60; H, 6.16;
N, 7.50. Found: C, 75.92; H, 6.39; N, 7.29). The compound is
moderately soluble in THF but only slightly soluble in aromatic
solvents. Its poor solubility prevented structural characterization by
spectroscopic methods and thwarted exploration of its reactivity with
ketones.
(13) Compounds 6a-c can be stored in the solid state at room
temperature and exposed to light for 48 h without significant decom-
position.

Reductive Coupling of [(RO)2Ti(L2)2] Complexes
Organometallics, Vol. 24, No. 16, 2005 3999
Table 1. Crystallographic Data for 6a‚(C10H8N2)1.5
,
7a, and 8a
6a‚(C10H8N2)1.5
7a
8a
formula
fw
C81H88N7O4SiTi C55H58N4O2Ti C65H64N2O4Ti
1299.57
150.0(2)
triclinic
854.95
985.08
T, K
90.0(2)
90.0(2)
monoclinic
P21/c
cryst syst
orthorhombic
P21212
space group P1h
Z
2
4
4
a, Å
15.1753(13)
15.2345(13)
17.0936(15)
99.096(2)
113.247(2)
101.233(2)
3438.2(5)
1.255
16.9420(6)
17.5720(7)
15.4750(9)
90
90.0000(17)
90
4607.0(4)
1.233
0.0621, 0.1156 0.0508, 0.1068
0.0991, 0.1256 0.0958, 0.1224
9.54100(10)
26.3230(3)
20.9570(3)
90
95.9410(5)
90
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
5235.03(11)
1.250
dcalc, g/cm3
R indices
[I>2σ(I)]:
R1, wR2
0.0593, 0.1545
R1, wR2 0.1040, 0.1742
(all data)
Table 2. Selected Bond Distances (Å) and Angles
(deg) for 6a and 7a
Figure 1. Molecular structure of [(DMSC)Ti(bpy)2] (6a)
(50% probability ellipsoids).
6a
7a
Ti(1)-O(1)
Ti(1)-O(2)
Ti(1)-N(2)
Ti(1)-N(3)
Ti(1)-N(1)
Ti(1)-N(4)
N(1)-C(1)
N(1)-C(5)
N(2)-C(10)
N(2)-C(6)
N(3)-C(11)
N(3)-C(15)
N(4)-C(20)
N(4)-C(16)
C(1)-C(2)
1.880(2)
1.926(2)
2.129(3)
2.156(3)
2.156(3)
2.178(3)
1.366(5)
1.385(5)
1.365(5)
1.388(5)
1.345(5)
1.369(4)
1.351(5)
1.370(5)
1.356(5)
1.394(6)
1.354(6)
1.406(5)
1.411(6)
1.397(6)
1.359(6)
1.399(6)
1.371(5)
1.370(5)
1.390(6)
1.366(5)
1.393(5)
1.445(5)
1.389(5)
1.378(6)
1.396(6)
1.361(5)
72.30(12)
71.36(11)
Ti(1)-O(1)
1.882(3)
1.908(3)
2.070(4)
2.165(4)
2.174(4)
2.216(4)
1.359(6)
1.361(6)
1.351(6)
1.374(6)
1.348(6)
1.369(6)
1.363(6)
1.407(6)
1.373(7)
1.389(7)
1.377(7)
1.407(7)
1.454(7)
1.386(7)
1.380(7)
1.376(7)
1.363(7)
1.353(7)
1.399(7)
1.360(7)
1.415(7)
1.426(7)
1.408(7)
1.369(7)
1.404(7)
1.355(7)
72.52(16)
75.36(17)
Ti(1)-O(2)
Ti(1)-N(4)
Ti(1)-N(1)
Ti(1)-N(3)
Ti(1)-N(2)
N(1)-C(24)
N(1)-C(28)
N(2)-C(33)
N(2)-C(29)
N(3)-C(34)
N(3)-C(38)
N(4)-C(43)
N(4)-C(39)
C(24)-C(25)
C(25)-C(26)
C(26)-C(27)
C(27)-C(28)
C(28)-C(29)
C(29)-C(30)
C(30)-C(31)
C(31)-C(32)
C(32)-C(33)
C(34)-C(35)
C(35)-C(36)
C(36)-C(37)
C(37)-C(38)
C(38)-C(39)
C(39)-C(40)
C(40)-C(41)
C(41)-C(42)
C(42)-C(43)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
Figure 2. Molecular structure of [(MBMP)Ti(bpy)2] (7a)
(50% probability ellipsoids).
C(8)-C(9)
C(9)-C(10)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)
C(18)-C(19)
C(19)-C(20)
dine ligands of 6a and 7a (Table 2).16 In addition, the
bridge C-C bonds of the bipyridine ligands are inter-
mediate between single and double bonds (average
bridge C-C ) 1.428(6) and 1.440(7) Å for 6a and 7a,
respectively). These data are indicative of some disrup-
tion of the aromaticity of the bipyridine ligands and
suggest electron transfer into the LUMO (π* orbital) of
each bipyridine ligand. Comparable structural data and
conclusions have been outlined for related bipyridine
and biphosphinine complexes of Zr(II), [Zr(bpy)3]2-, and
N(2)-Ti(1)-N(1)
N(3)-Ti(1)-N(4)
N(1)-Ti(1)-N(2)
N(4)-Ti(1)-N(3)
angles of the bidentate bipyridine ligands (which range
from 71° to 75°, Table 2), as well as steric constraints
imposed by the chelating bis(aryloxide) ligand. Ti-O
and Ti-N bond distances of 6a and 7a essentially
parallel those reported for [(2,6-Pri2C6H3O)2Ti(bpy)2].8a
However unlike in free bipyridine,15 a small but sys-
tematic alternation of short and long C-C bond dis-
tances is apparent within pyridine units of the bipyri-
[Zr(P2C10H4Me4)3]2- 8d as well as for [Mo(OPri)2(bpy)2]15
,
and a bipyridine dianion.17
Formally, [(RO)2Ti(L2)2] complexes 6a-c and 7a,b
are Ti(II) complexes. In fact, the oxidation state of
Ti in [(2,6-Pri2C6H3O)2Ti(bpy)2] was assigned as +2
on the basis of electrochemical studies.8a However,
(16) The systematic nature of the alternation of short and long C-C
bond distances is obvious, although the magnitude of the alternations
is similar to accuracy limitations of the spherical atom scattering factor
approximation (see, Coppens, P.; Sabine, T. M.; Delaplane, R. G.; Ibers,
J. A. Acta Crystallogr. 1969, B25, 2451).
(17) Bock, H.; Lehn, J.-M.; Pauls, J.; Holl, S.; Krenzel, V. Angew.
Chem., Int. Ed. 1999, 38, 952.
(14) (a) Serpone, N.; Bickley, D. G. In Prog. Inorg. Chem. 1972, 17,
p 391. (b) Montgomery, C. D.; Shorrock, C. J. Inorg. Chim. Acta 2002,
328, 259. (c) Rodger, A.; Johnson, B. F. G. Inorg. Chem. 1988, 27, 3061.
(15) Chisholm, M. H.; Huffman, J. C.; Rothwell, I. P.; Bradley, P.
G.; Kress, N.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 4945.

4000 Organometallics, Vol. 24, No. 16, 2005
Owiny et al.
Scheme 2
[(RO)2Ti(L2)2] complexes possess a delocalized electronic
structure; hence redox properties of Ti will be strongly
influenced by redox properties of the diimine ligands.
In addition, excited electronic states may be thermally
and/or photochemically accessible. For example, EPR
and magnetic susceptibility studies of [(η5-C5H5)2Ti(L2)]
(L2 ) bpy or phen) compounds have revealed that they
possess a triplet excited state, which is thermally
accessible from the ground-state singlet.8b,c The intense
color of 6a-c and 7a,b is due to metal to (diimine)
ligand charge-transfer (MLCT) transition.4b,8 Hence
electronic spectra of [(DMSC)Ti(dmbpy)2] (6b) and
[(DMSC)Ti(phen)2] (6c) in toluene display strong bands
below 400 nm. In the visible region, the absorbance
maxima for 6b and 6c are observed at 640 and 712 nm,
respectively. Bathochromic shift of the absorbance
maximum of 6c reflects the lower energy of the π* state
of 1,10-phenanthroline.18 Magnetic susceptibility studies
(by the Evans method)19 of 6a and 6c over the 296
to 353 K temperature range revealed that the com-
pounds are weakly paramagnetic.20 For example, 6c
has a magnetic moment (µeff) of 0.6 µB at 296 K, simi-
lar to the magnetic moment (0.6-0.8 µB) reported for
[(η5-C5H5)2Ti(bpy)].8f,g
deep dark green before eventually becoming orange at
1
completion. H NMR monitoring of the reaction mix-
tures revealed [(DMSC)Ti{η2-OC(p-MeC6H4)2}(L2)]
(2a-c)4b as intermediates (Scheme 2). However, reac-
tions of 6a-c with (p-MeC6H4)2CO proceed reversibly
and the equilibrium between 6a-c and 2a-c depends
on relative abilities of (p-MeC6H4)2CO and the diimine
(L2) to accept π-electron density.
Hence the equilibrium shifts increasingly toward
2a-c with decreasing diimine π-acidity.22 For example,
reaction of [(DMSC)Ti{η2-OC(p-MeC6H4)2}(dmbpy)] (2b)
with 1 equiv of dmbpy in C6D6 furnished a ∼3:22:1 ratio
1
of 6b, 2b, and 3b after 2 h (by H NMR). Conversely,
similar reaction of [(DMSC)Ti{η2-OC(p-MeC6H4)2}(bpy)]
(2a) with 1 equiv of bpy in C6D6 for 2 h produced a
∼2:2:1 ratio of 6a, 2a, and 3a. Reaction of [(DMSC)
Ti{η2-OC(p-MeC6H4)2}(phen)] (2c) with phen (1.4 equiv)
in C6D6 was complete in 1 h, affording 6c and 3c in ∼1:1
ratio.23 These results also imply that 2a-c react faster
with aromatic diimines than (p-MeC6H4)2CO. Thus, the
rate of formation of 2-aza-5-oxa-titanacyclopentenes
3a-c from 6a-c will show dependence on ketone con-
centration. Accordingly, only 95% of [(DMSC)Ti(bpy)2]
(6a) had reacted after 21 h in the presence of 2 equiv of
1
Reactivity of [(RO)2Ti(L2)2] with Ketones.
[(DMSC)Ti(L2)2] (6a-c) undergo light-assisted reac-
tion21 with g2 equiv of (p-MeC6H4)2CO to cleanly yield
corresponding 2-aza-5-oxa-titanacyclopentenes (3a-c).4b
Dark blue or olive-green C6D6 solutions of 6a-c turn
(p-MeC6H4)2CO in C6D6 (by H NMR), while complete
consumption of 6a occurred in ∼10.5 h with 3 equiv of
(p-MeC6H4)2CO. Moreover, quantitative formation of
3a was observed in the latter reaction in ∼16 h, while
the former reaction was only ∼90% complete even after
36 h.
(18) Reduction potentials for the diimine ligands are listed in the
table below. The data suggest that the thermodynamic feasibility of
one-electron transfer to the π* orbital of the diimine ligand follows
the order dmbpy < bpy < phen and reflects their ability to stabilize
low-valent titanium.
Qualitatively, the rate of formation of 3a-c from
[(DMSC)Ti(L2)2] (6a-c) followed the order L2 ) bpy <
dmbpy < phen. Thus, formation of 3b was complete
in 12 h when [(DMSC)Ti(dmbpy)2] (6b) was reacted
in C6D6 with 3 equiv of (p-MeC6H4)2CO, while ana-
logous reaction of [(DMSC)Ti(phen)2] (6c) with (p-
MeC6H4)2CO (3 equiv) yielded 3c in 8 h. Interestingly,
the rate of the reaction of [(DMSC)Ti{η2-OC(p-MeC6H4)2}-
(L2)] (2a-c) with (p-MeC6H4)2CO (1 equiv) in C6D6 was
previously shown to increase in the order L2 ) dmbpy
< bpy , phen.24 Consequently, the efficacy of the
(22) Reference 18 lists reduction potentials for the diimine ligands
and (p-MeC6H4)2CO. It is apparent from the data that one-electron
transfer to (p-MeC6H4)2CO is thermodynamically favored over one-
electron transfer to diimine.
(19) Evans, B. D. F. J. Chem. Soc. 1959, 2003.
(20) Apparently, NMR spectra of 6a-c are not significantly affected
by any unpaired electron density. Sharp resonances were similarly
observed in 1H and 13C NMR spectra of 2a-c even though preliminary
data from room- and low-temperature (110 K) ESR studies of [(DMSC)-
Ti{η2-OC(p-MeC6H4)2}dmbpy] (2b) in the solid state and in toluene
indicate the presence of two radical species.4b
(23) [(DMSC)Ti{η2-OC(p-MeC6H4)2}(phen)] (2c) could only be iso-
lated along with [(DMSC)Ti{κ3-OC(p-MeC6H4)2C12H7N2}{OCH(p-
MeC6H4)2}] (3c).4b Thus, reaction between 40 mg of a 1:4 molar mixture
of 2c (6.95 mg, 0.00622 mmol) and 3c (33 mg, 0.0249 mmol) in C6D6
and 1.6 mg (0.0088 mmol) of 1,10-phenanthroline (∼1.4 equiv relative
to 2c) was followed by 1H NMR spectroscopy. Formation of 3c is
irreversible and has no effect on the reaction, and the amount of 3c
initially present in solution was accounted for in our analysis.
(24) This reactivity order parallels the decrease in the energy gap
between π and π* states of the diimines. We believe that metal to
diimine ligand charge transfer (MLCT) transitions and the resulting
radical species play important roles in the transformation.4b
(21) Reactions of 6a-c with (p-MeC6H4)2CO proceed at a much
slower rate when their solutions are protected from light. For instance,
the reaction of [(DMSC)Ti(dmbpy)2] (6b) with 3 equiv of (p-MeC6H4)2-
CO in C6D6 to give [(DMSC)Ti{κ3-OC(p-MeC6H4)2C10H5Me2N2}{OCH-
(p-MeC6H4)2}] (3b) was complete in <7 h when a NMR tube containing
the solution under N2 was left exposed to ambient light. The reaction
took much longer when an identical solution in a NMR tube was left
spinning in the NMR probe (see Results and Discussion).

Reductive Coupling of [(RO)2Ti(L2)2] Complexes
Organometallics, Vol. 24, No. 16, 2005 4001
Table 3. Reductive Coupling of [(MBMP)Ti(bpy)2]
(7a) with Ketones
Table 4. Selected Bond Distances (Å) and Angles
(deg) for 8a
Ti(1)-O(2)
1.8502(17)
1.8651(17)
1.8850(17)
1.9075(17)
2.185(2)
2.277(2)
99.09(7)
91.09(7)
97.71(8)
94.00(7)
98.66(7)
161.86(8)
172.25(8)
74.68(7)
85.26(8)
91.55(7)
115.73(8)
145.16(8)
81.55(7)
80.56(7)
70.54(8)
Ti(1)-O(1)
Ti(1)-O(4)
Ti(1)-O(3)
Ti(1)-N(2)
Ti(1)-N(1)
O(2)-Ti(1)-O(1)
O(2)-Ti(1)-O(4)
O(1)-Ti(1)-O(4)
O(2)-Ti(1)-O(3)
O(1)-Ti(1)-O(3)
O(4)-Ti(1)-O(3)
O(2)-Ti(1)-N(2)
O(1)-Ti(1)-N(2)
O(4)-Ti(1)-N(2)
O(3)-Ti(1)-N(2)
O(2)-Ti(1)-N(1)
O(1)-Ti(1)-N(1)
O(4)-Ti(1)-N(1)
O(3)-Ti(1)-N(1)
N(2)-Ti(1)-N(1)
RCOR′
entry
R
R′
rxn time
% yield of 9
1
2
3
4
5
6
Ph
Ph
Ph
Ph
Me2NC6H4
Et
Ph
18 ha
36 hb
48 hb
72 hb
72 hb
72 hb
88c (9a)
28c (9b)
15c (9c)
traced
Me
CH2Ph
i-Bu
Me2NC6H4
Et
traced
traced
a 2 equiv of Ph2CO used. b 4 equiv of ketone used. c Unoptimized
isolated yield. dDetermined by GC-MS.
formation of 2-aza-5-oxa-titanacyclopentenes 3a-c from
[(DMSC)Ti(L2)2] (6a-c) must depend on the relative
concentrations of 6a-c and 2a-c in solution (hence
diimine π-acidity), as well as on the rate of reaction of
2a-c with (p-MeC6H4)2CO. For example, while reaction
of [(DMSC)Ti{η2-OC(p-MeC6H4)2}(bpy)] (2a) with (p-
MeC6H4)2CO is faster than reaction of [(DMSC)Ti-
{η2-OC(p-MeC6H4)2}(dmbpy)] (2b) with (p-MeC6H4)2CO,
the equilibrium between [(DMSC)Ti(L2)2] and
[(DMSC)Ti{η2-OC(p-MeC6H4)2}(L2)] lies further toward
[(DMSC)Ti{η2-OC(p-MeC6H4)2}(L2)] when L2 ) dmbpy
than when L2 ) bpy (vide infra), and the net result is
that complete conversion of 6b into 3b occurs faster
than complete conversion of 6a into 3a.
bon (Ti-OCHPh2) at δ 86.8 ppm in the 13C NMR
spectrum.4b,26 The resonance for the Ti-O-C carbon of
the 2-aza-5-oxa-titanacyclopentene ring shows at δ 99.5
ppm.27 X-ray analysis of single crystals of 8a confirmed
the structure assigned by spectroscopy. Crystallographic
data and selected metrical parameters are given in
Tables 1 and 4, respectively. The molecular structure
of 8a shows that it has a distorted octahedral structure
and a meridional tridentate alkoxy-bipyridyl ligand
(Figure 3). The distortion from idealized octahedral
geometry arises from acute bite angles of the tridentate
alkoxy-bipyridyl ligand [ca. 74° for O(1)-Ti(1)-N(1) and
70° for N(1)-Ti(1)-N(2)], as well as steric constraints
imposed by the MBMP chelate ligand. The Ti-N and
Ti-O bond lengths are essentially identical to those of
the related 2-aza-5-oxa-titanacylcopentene derivative
[(DMSC)Ti{κ3-OC(p-MeC6H4)2C10H7N2}{OCH(p-
MeC6H4)2}] (3a).4b
MBMP is obtained from inexpensive 2,2′-methyl-
enebis(6-tert-butyl-4-methylphenol); hence the reactivity
of [(MBMP)Ti(bpy)2] (7a) with ketones was explored
with the aim of developing a viable synthetic route to
6-(1-hydroxyalkyl)-2,2′-bipyridines.25 We typically gen-
erated 7a in a glass pressure tube by reaction of
[(MBMP)TiPh2] (5) with 2 equiv of bpy in ether. Next,
2-4 equiv of ketone is introduced into the resulting dark
blue solution. The tube is then sealed with a Teflon
screw cap and heated at 65 °C for a period of time during
which the reaction progress was monitored via GC-MS
analysis of aliquots. [(MBMP)Ti(κ3-OCPh2C10H7N2)-
(OCHPh2)] (8a) was isolated as an orange powder in
good yield from reaction of 7a with Ph2CO (∼2.7 equiv)
in ether at 65 °C for 12 h, while 6-(1-hydroxyalkyl)-2,2′-
bipyridines (9a-c, Table 3) were isolated after hydroly-
sis and workup of corresponding reaction mixtures. The
structure of 8a was characterized by solution NMR data.
Both the 1H and 13C NMR data are consistent with C1-
1
symmetry in solution. In its H NMR spectrum, two
singlets at δ 0.38 and 1.50 ppm, two singlets at δ 2.11
and 2.16 ppm, and two doublets at δ 2.73 and 5.92 ppm
are observed for inequivalent tert-butyl groups, p-tolyl
methyl groups, and bridging methylene protons of
the MBMP ligand, respectively. The alkoxide proton
(Ti-OCHPh2) of 8a is observed as a singlet at δ 5.56
Figure 3. Molecular structure of [(MBMP)Ti{κ3-
OCPh2C10H7N2}(OCHPh2)] (8a) (50% probability ellip-
soids).
(26) For typical 1H and 13C NMR chemical shift values for the Ti-
OCHAr2 group, see also: (a) Agapie, T.; Diaconescu, P. L.; Mindiola,
D. J.; Cummins, C. C. Organometallics 2002, 21, 1329. (b) Covert, K.
J.; Wolczanski, P. T.; Hill, S. A.; Krusic, P. J. Inorg. Chem. 1992, 31,
66.
(27) Similar chemical shift values have been reported for related
compounds. 13C NMR (CD2Cl2): δ 93-97 ppm for [{κ2-OCAr2C5H4N}2-
Ti(NMe2)2] (Ar ) various aryl groups)28 and 13C NMR (CDCl3): δ 107
ppm for [CpTi{κ2-OCR2C5H4N}Cl2] (R ) Pri or Ph).29
1
ppm in the H NMR spectrum and the alkoxide car-
(25) Our preliminary studies suggest that coordination of a chelating
bis(aryloxide) ligand in [(RO)2Ti(bpy)2] compounds facilitates reductive
coupling with ketones. Presumably, the chelating bis(aryloxide) ligand
enforces cis-arrangement of diimine and ketone molecules. For ex-
ample, Ti(OPri)4/2PriMgCl did not promote reductive coupling of bpy
with Ph2CO under a variety of conditions.

4002 Organometallics, Vol. 24, No. 16, 2005
Owiny et al.
Reactions that yield 2-aza-5-oxa-titanacyclopentenes
invariably turn dark green before eventually becoming
orange. The dark green color is identical to that for
solutions of [(DMSC)Ti{η2-OC(p-MeC6H4)2}(L2)] com-
plexes (2a-c),4b implying [(MBMP)Ti(η2-ketone)(bpy)]
intermediates in the reaction. As in the formation of
2-aza-5-oxa-titanacyclopentenes 3a-c from [(DMSC)-
Ti(L2)2] (6a-c), the efficacy of reductive coupling reac-
tion between [(MBMP)Ti(bpy)2] (7a) and ketones is
increased by increasing ketone concentration. Appar-
ently, the formation of the 2-aza-5-oxa-titanacyclopen-
tene product is increasingly inhibited by bipyridine
released from 7a as the reaction proceeds. The efficacy
of the reaction also depends on the relative abilities of
the ketone and the diimine to accept π-electron den-
sity, consistent with an equilibrium between 7a and
[(MBMP)Ti(η2-ketone)(bpy)]. The yield of the 2-aza-5-
oxa-titanacyclopentene product decreases with decreas-
ing ketone π-acidity in the order Ph2CO > PhCOR .
R2CO (R ) alkyl) ∼ (p-Me2NC6H4)2CO (Table 3). Ali-
phatic ketones and (p-Me2NC6H4)2CO probably react
sluggishly with 7a because formation of [(MBMP)Ti(η2-
ketone)(bpy)] species is thermodynamically unfavorable
since aliphatic ketones and (p-Me2N-C6H4)2CO are
weaker π-acids than bipyridine.30 These results strongly
support a mechanism that involves reversible dissocia-
tion of a diimine ligand (L2) from [(RO)2Ti(L2)2] and
reversible coordination of a ketone to titanium prior to
the rate-limiting step. Attempts to improve the ef-
ficiency of the reductive coupling of 7a with ketones by
conducting the reaction at higher temperatures in
toluene or THF led to complicating side reactions,
including formation of [(MBMP)2Ti].31,32
from reductive coupling of [(DMSC)Ti(L2)2] (6a-c) with
(p-MeC6H4)2CO increased with increasing ketone con-
centration and in the order L2 ) bpy < dmbpy < phen.
This order may be explained by reversible formation of
[(DMSC)Ti{η2-OC(p-MeC6H4)2}(L2)] (2a-c) intermedi-
ates and a rate-limiting step that is dependent on ketone
concentration. In reactions of [(MBMP)Ti(bpy)2] (7a)
with ketones and aldehydes, the yield of 2-aza-5-oxa-
titanacyclopentene (and hence 6-(1-hydroxyalkyl)-2,2′-
bipyridine) decreased with decreasing ketone π-acidity
in the order Ph2CO > PhCOR . R2CO (R ) H, Me, Et,
Pri, But, Bui, or C5H9) ∼ (p-Me2NC6H4)2CO. We are
continuing our investigations of the synthesis, electronic
structure, and reactivity of reduced titanium complexes.
Acknowledgment. Thanks are expressed to the
U.S. National Science Foundation (grant numbers
CHE-9984776 and CHE-0416098) for financial sup-
port of this work. NMR instruments utilized in this
research were funded in part by the CRIF program of
the U.S. National Science Foundation (grant number
CHE-9974810). The authors also thank Zuzanna T.
Cygan and Dr. Mark M. Banaszak Holl (University of
Michigan) for single crystals of 6a.
Supporting Information Available: A summary of crys-
tallographic parameters, atomic coordinates and equivalent
isotropic displacement parameters, bond lengths and angles,
anisotropic displacement parameters, and hydrogen coordi-
nates and isotropic displacement parameters for 6a‚(C10H8N2)1.5
,
7a, and 8a. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM050231K
Summary
(30) Mixtures of aliphatic ketones or (p-Me2NC6H4)2CO with [(MB-
MP)Ti(bpy)2] (7a) in ether at 65 °C in a sealed tube slowly turn green.
However, the reaction mixtures either remain green even after 7 days
at 65 °C or exist as green solutions containing orange-green precipitate.
This observation suggests that the equilibrium between 7a and
[(MBMP)Ti(η2-ketone)bpy] lies decidedly toward 7a and that conversion
of [(MBMP)Ti(η2-ketone)bpy] into 2-aza-5-oxa-titanacyclopentene is
very slow in these cases.
Reactions of [(RO)2Ti(L2)2] (L2 ) bpy, dmbpy, or phen;
(RO)2 ) DMSC or MBMP) with ketones yield corre-
sponding 2-aza-5-oxa-titanacyclopentenes. Efficacy of
the formation of 2-aza-5-oxa-titanacyclopentenes 3a-c
(28) Kim, I.; Nishihara, Y.; Jordan, R. F.; Rogers, R. D.; Rheingold,
A. L.; Yap, G. P. A. Organometallics 1997, 16, 3314.
(29) Doherty, S.; Errington, R. J.; Jarvis, A. P.; Collins, S.; Clegg,
W.; Elsegood, M. R. J. Organometallics 1998, 17, 3408.
(31) Okuda, J.; Fokken, S.; Kang, H.-C.; Massa, W. Chem. Ber. 1995,
128, 221.
(32) 1H NMR showed that [(MBMP)2Ti] is formed from prolonged
heating (at 70 °C) of 7a in C6D6 under N2 atmosphere.