Redox-active tetrahydrosalen (salan) complexes of titanium

Article abstract of DOI:10.1039/c1dt11228g

A diarylamino-substituted N-methyl tetrahydrosalen (salan) ligand, ^An2NLH₂, is prepared in four steps and overall 53% yield from 5-bromosalicylaldehyde, with the key step a palladium-catalysed Hartwig-Buchwald amination of the tert-b

Full text of DOI:10.1039/c1dt11228g

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PAPER
Redox-active tetrahydrosalen (salan) complexes of titanium†
Mauricio Quiroz-Guzman, Allen G. Oliver, Andrew J. Loza and Seth N. Brown*
Received 28th June 2011, Accepted 8th August 2011
DOI: 10.1039/c1dt11228g
A diarylamino-substituted N-methyl tetrahydrosalen (salan) ligand, An2NLH2, is prepared in four steps
and overall 53% yield from 5-bromosalicylaldehyde, with the key step a palladium-catalysed
Hartwig–Buchwald amination of the tert-butyldimethylsilyl-protected 5-bromo-N-methylsalan ligand.
Reaction of An2NLH2 or its bromo analogue with Ti(OiPr)4 or TiF4 results in metalation of the ligand.
The isopropoxide groups are readily exchanged with a- or b-hydroxyacids to form chelated complexes.
X-ray crystallography and NMR spectroscopy indicate that the salan ligands are quite flexible, with
An2NLTiF2, for example, showing four stereoisomers in its 19F NMR spectrum. The major stereoisomer
of (salan)Ti(X)(Y) depends principally on the trans influence of the X and Y groups. Complexes of
An2NL show two reversible, closely spaced redox couples at approximately + 0.1 V vs. ferrocene/
ferrocenium, and a second set of two closely spaced redox couples at ~ + 0.8 V vs. Fc/Fc+.
We recently reported the preparation of tripodal ligands
Introduction
t
N(CH2C6H2-3-R-5-N[C6H4OMe]2-2-OH)3 (R = H, Bu) contain-
ing para-diarylaminophenol groups.10 Titanium binds readily to
this ligand and forms complexes that show six reversible oxidation
reactions. Here we describe the preparation of an analogous
salan ligand, its metalation by titanium, and the electrochemical
properties of the metal complexes.
Bis(salicylidene)ethylenediamine (salen) ligands and their satu-
rated analogues (salan) are an important class of tetradentate
dianionic ligands that have been used extensively in transition
metal chemistry.1 While they are most typically used as chemically
inert supporting ligands, a number of late-metal salen and salan
complexes have been observed to undergo outer-sphere oxidation
to form stable complexes of aroxyl radicals,2 with particularly
electron-rich examples showing very facile oxidation.3 Oxidized
copper salen and salan complexes in particular have been studied
for their ability to mediate the oxidation of alcohols, by analogy
to the enzyme galactose oxidase.4
Aryloxide complexes of the early transition metals are markedly
more difficult to oxidize than those of the later transition metals
due to the strong metal–oxygen p bonding exhibited by these
metals.5 Thus, while salen and salan complexes of titanium have
been used as catalysts for oxidation reactions, these reactions have
relied on the Lewis acidity of titanium to activate oxidants such as
hydrogen peroxide rather than the ability of the ligands (or metal)
to undergo redox reactions.6,7 A ferrocenylethynyl–salen titanium
complex has been observed to show activity for polymerization
of lactones that is modulated by the redox state of the ferrocenes,
but that effect was attributed to a change in inductive effects,8
consistent with observations in other systems that oxidation of
remote ferrocenes produces only small orbital perturbations at
titanium.9
Experimental
General procedures
Unless otherwise noted, all procedures were carried out inside
a drybox or using vacuum line techniques. Chlorinated solvents
˚
were dried over 4 A molecular sieves, followed by CaH2. Benzene
was dried over sodium. Deuterated solvents were obtained from
Cambridge Isotope Laboratories, dried using the same procedures
as their protio analogues, and were stored in the drybox prior
to use. All other reagents were commercially available and used
without further purification. NMR spectra were measured on a
Varian VXR-300 or VXR-500 spectrometer. Chemical shifts for
1
1H and 13C{ H} NMR spectra are reported in ppm downfield of
TMS, using the known chemical shifts of the solvent residuals.
Chemical shifts for 19F NMR spectra are reported in ppm
downfield of CFCl3. Infrared spectra were recorded as Nujol mulls
on NaCl plates on a Nicolet 670 FT-IR spectrometer and are
reported in cm-1. ESI mass spectra were obtained using a Bruker
micrOTOF-II mass spectrometer. Samples were analyzed either
by first flushing the system with MeOH–CH2Cl2–0.5% formic acid
and then introducing the samples into the ion source by direct flow
of a solution of the analyte in anhydrous CH2Cl2 with a flow rate
of 10 mL min-1 in fast-forward mode, or by first flushing the source
with isopropanol and introducing the sample into the source by
direct flow of a solution of the analyte in anhydrous isopropanol
Department of Chemistry and Biochemistry, 251 Nieuwland Science
Hall, University of Notre Dame, Notre Dame, IN, 46556-5670, USA.
E-mail: Seth.N.Brown.114@nd.edu; Fax: 01 574 631 6652; Tel: 01 574
631 4659
† CCDC reference numbers 832123–832125. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt11228g
11458 | Dalton Trans., 2011, 40, 11458–11468
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at 20 mL min-1. FAB mass spectra were obtained on a JEOL
LMS-AX505HA mass spectrometer using 3-nitrobenzyl alcohol
or nitrophenyl octyl ether as a matrix. Peaks reported are the mass
number of the most intense peak of isotope envelopes. UV-visible
spectra were measured in dichloromethane solutions (except as
noted) in 1-cm screw cap quartz cuvettes using a Beckman DU-
7500 diode array spectrophotometer, and are reported as lmax/nm
(e/M-1cm-1). Elemental analyses were performed either by M-
H-W Laboratories (Phoenix, AZ) or Midwest Microlab, LLC
(Indianapolis, IN).
821 (s), 793 (w), 761 (s), 719 (w), 649 (s), 625 (s), 550 (m). Anal.
Calcd for C18H22Br2N2O2: C, 47.19; H, 4.84; N, 6.11. Found: C,
47.16; H, 4.63; N, 5.84.
N,N¢-Dimethyl-N,N¢-bis(2-(tert-butyldimethylsiloxy)-5-bromo-
benzyl)ethylenediamine dihydrochloride. In the air, BrLH2 (3.09 g,
6.74 mmol), tert-butyldimethylsilyl chloride (3.06 g, Acros,
20.3 mmol, 3.02 equiv), 60 mL CH2Cl2, and a stirbar were added to
a round-bottom flask. The flask was sealed with a rubber septum,
and through the septum 3.05 mL DBU (20.4 mmol, 3.02 equiv)
was added via syringe over 3 min. After stirring for 30 min, the
mixture was poured into a separatory funnel and shaken with
75 mL 1 M aqueous HCl. The cloudy organic layer was drained
into a round-bottom flask, reduced in volume to about 10 mL
on the rotary evaporator, and poured into 80 mL ether, causing a
voluminous white precipitate to form. The precipitate was suction
filtered on a glass frit, washed thoroughly with 2 ¥ 20 mL ether, and
air-dried for 20 min to give 4.75 g (93%) of the protected amine
Ligand synthesis
N,N¢-Bis(2-hydroxo-5-bromobenzyl)ethylenediamine. To
a
stirred solution of 13.08 g 5-bromosalicylaldehyde (Aldrich,
65.1 mmol) in 300 mL methanol in the air was slowly added
a solution of 1.95 g (32.5 mmol) ethylenediamine in methanol
(15 mL). The solution turned bright yellow as the amine addition
proceeded and deposited a yellow precipitate. After stirring
the slurry for 5 min, 4.95 g solid NaBH4 (131 mmol) was
added in small portions over 40 min. The color of the reaction
mixture decreased in intensity as the borohydride addition
proceeded, with the bright yellow precipitate dissolving and a
pale yellow precipitate forming. After stirring overnight, the pale
yellow solid was isolated by suction filtration, washed with 2 ¥
15 mL methanol, and air-dried for 1 h to yield 12.74 g of t◦he
bromo-substituted diamine (91%). Mp 160–162◦ (lit.11 175–176 ).
1H NMR (CDCl3): d 2.83 (s, 4H, NCH2CH2N), 3.96 (s, 4H,
ArCH2N), 6.72 (d, 9 Hz, 2H, Ar 3-H), 7.10 (d, 3 Hz, 2H, Ar 6-H),
7.26 (dd, 9, 3 Hz, 2H, Ar 4-H). The compound is too insoluble
for 13C NMR analysis. IR (cm-1): 3421 (br, nOH), 3270 (m, nNH),
2410 (br), 1585 (m), 1413 (m), 1296 (m), 1274 (s), 1255 (m), 1202
(s), 1178 (m), 1115 (s), 1076 (m), 1017 (w), 945 (m), 937 (m), 888
(s), 851 (w), 829 (vs), 777 (m), 723 (s), 628 (s).
1
dihydrochloride. H NMR (CD2Cl2): d 0.30 (s, 12H, Si(CH3)2),
1.01 (s, 18H, SiC(CH3)3), 2.75 (s, 6H, NCH3), 3.83 (br s, 4H,
NCH2CH2N), 4.29 (br s, NCH2Ar), 6.81 (d, 9 Hz, 2H, Ar 3-H),
7.43 (dd, 9, 3 Hz, 2H, Ar 4-H), 8.03 (d, 3 Hz, 2H, Ar 6-H), 12.77
1
(v br s, 2H, NH). 13C{ H} NMR (CD2Cl2): d -3.74 (Si(CH3)2),
18.81 (SiC(CH3)3), 26.28 (SiC(CH3)3), 39.55 (v br), 50.80 (br),
54.45 (v br), 114.20, 121.19, 121.79, 135.05, 136.69, 154.78. IR:
3420 (m, br, nNH), 2322 (m, br), 1487 (s), 1405 (w), 1282 (s), 1263
(s), 1184 (w), 1132 (m), 945 (w), 917 (m), 906 (m), 861 (s), 846 (s),
832 (m), 805 (m), 782 (s), 671 (w). ESI-MS: 687.1843 ((M+H)+,
calcd 687.1833). Anal. Calcd for C30H52Br2Cl2N2O2Si2: C, 47.43;
H, 6.90; N, 3.69. Found: C, 46.38; H, 7.10; N, 3.40.
N,N¢-Dimethyl-N,N¢-Bis(2-hydroxo-5-(di-(4-methoxyphenyl)-
amino)benzyl)ethylenediamine, An2NLH2. Into a 50-mL Erle-
nmeyer flask in the air was weighed 1.1005 g N,N¢-dimethyl-N,N¢-
bis(2-(tert-butyldimethylsiloxy)-5-bromobenzyl)ethylenediamine
dihydrochloride (1.45 mmol). The solid was dissolved in 30 mL
CH2Cl2 and shaken with 25 mL 5% aqueous NaOH in a
separatory funnel. The dichloromethane layer was drained off
and the aqueous layer washed with 10 mL additional CH2Cl2.
The combined organic layers were dried over MgSO4, filtered, and
stripped down on the rotary evaporator to yield a pale yellow oil.
This oil was taken directly into the drybox. In the box, 663.1
mg 4,4¢-dimethoxydiphenylamine (Alfa-Aesar, 2.89 mmol, 2.00
equiv), 416.9 mg sodium tert-butoxide (Aldrich, 4.34 mmol, 2.99
equiv) and 47.5 mg Pd2(dba)3 (Strem, 0.104 mmol Pd, 7 mol%)
were weighed into a 20-mL scintillation vial. The free-base aryl
bromide was dissolved in ~12 mL benzene and added to the solid
reagents. A solution of 16.6 mg tri-tert-butyl phosphine (Strem,
0.082 mmol, 5.6 mol%) in ~3 mL benzene was then added to this
mixture. A stirbar was added, the vial sealed with a Teflon-lined
screw-cap, and the reaction mixture taken out of the drybox and
stirred at room temperature overnight.
N,N¢-Dimethyl-N,N¢-Bis(2-hydroxo-5-bromobenzyl)ethylenedi-
amine, BrLH2. To a stirred mixture of 5.82 g N,N¢-bis(2-hydroxo-
5-bromobenzyl)ethylenediamine (13.5 mmol), 175 mL CH3CN
and 30 mL glacial acetic acid in a 500 mL Erlenmeyer flask in the
air was added 11 mL 37% aqueous formaldehyde (400 mmol, 15
equiv). After stirring for 40 min, 2.23 g solid NaBH4 (58.9 mmol,
2.2 equiv) was added in portions over 10 min. The mixture
warms and eventually becomes turbid. After stirring overnight,
the cloudy mixture was poured into a round-bottom flask, the
Erlenmeyer rinsed with 15 mL H2O, and the rinse combined with
the reaction mixture. The volume of the mixture was reduced to
75 mL on the rotary evaporator. To the stirred mixture was slowly
added 10% aqueous NaOH until the pH was neutral (~80 mL).
After stirring for 10 min to allow the effervescence to cease, the
mixture was suction filtered and the white solid washed with 2 ¥
20 mL H2O, then 2 ¥ 30 mL CH3OH, then air-dried for 15 min
1
to furnish 5.93 g BrLH2 (96%). H NMR (CDCl3): d 2.28 (s, 6H,
NCH3), 2.64 (s, 4H, NCH2CH2N), 3.66 (s, 4 H, ArCH2N), 6.72
(d, 9 Hz, 2H, Ar 3-H), 7.07 (d, 2.5 Hz, 2H, Ar 6-H), 7.26 (dd, 9,
After 17 h, the mixture was opened in the air and poured into
10 mL saturated aqueous ammonium chloride solution, and the
reaction flask washed with a few mL ether, which was added
to the organic/aqueous mixture. The organic/aqueous mixture
was stirred for 5 min; it rapidly turned dark red-brown and
palladium black was deposited. The layers were then separated in
a separatory funnel, the organic layer dried over MgSO4, filtered,
1
2.5 Hz, 2H, Ar 4-H). 13C{ H} NMR (CDCl3): d 41.85 (NCH3),
54.08 (NCH2CH2N), 61.47 (ArCH2N), 111.08, 118.31, 123.71,
131.28, 131.93, 157.17. IR (Nujol mull, cm-1): 2600 (v br, nOH),
1877 (w), 1643 (w), 1602 (s), 1578 (s), 1524 (w), 1412 (m), 1295 (s),
1255 (s, br), 1196 (s), 1178 (m), 1164 (w), 1132 (m), 1115 (m), 1099
(s), 1074 (s), 1019 (s), 973 (s), 939 (w), 915 (m), 878 (s), 847 (m),
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and stripped down on the rotary evaporator to give the silyl-
protected compound as a brown oil.
NCHH¢CHH¢N), 3.79 (s, 12H, OCH3), 4.59 (d, 13 Hz, 2H,
NCHH¢Ar), 5.10 (sept, 6 Hz, 2H, CH(CH3)2), 6.55 (d, 8.5 Hz,
2H, Ar 3-H), 6.68 (d, 2 Hz, 2H, Ar 6-H), 6.78 (d, 9 Hz, 8H,
NC6H4OCH3 2,6-H), 6.87 (dd, 8.5, 2.5 Hz, 2H, Ar 4-H), 6.97 (d,
The oil was extracted into 15-mL portions of methanol,
requiring ~90 mL methanol total. To the methanol solution
was added a stirbar and 0.5 g ammonium fluoride (Alfa-Aesar,
13.5 mmol, 4.5 equiv per silyl group) and the mixture stirred
vigorously overnight at room temperature. Within an hour, the
product had begun to precipitate as a tan solid. After stirring
overnight, the solid was collected by suction filtration, washed
thoroughly first with 10 mL H2O, then 2 ¥ 15 mL methanol, then
air-dried for 1 h to yield 0.714 g An2NLH2 (65%). 1H NMR (C6D6):
d 1.73 (s, 6H, NCH3), 2.02 (s, 4H, NCH2CH2N), 2.99 (s, 4H,
ArCH2N), 3.28 (s, 12H, OCH3), 6.73 (d, 9 Hz, 8H, NC6H4OMe
2,6-H), 6.80 (d, 2 Hz, 2H, Ar 6-H), 6.97 (d, 9 Hz, 2H, Ar 3-H),
7.01 (dd, 9, 2 Hz, 2H, Ar 4-H), 7.10 (d, 9 Hz, 8H, NC6H4OMe 3,5-
1
9 Hz, 8 H, NC6H4OCH3 3,5-H). 13C{ H} NMR (CDCl3): d 25.87
(OCH(CH3)(CH¢3), 26.20 (OCH(CH3)(CH¢3)), 47.41 (NCH3),
51.94 (NCH2CH2N), 55.72 (OCH3), 64.49 (NCH2Ar), 78.01
(OCH(CH3)2), 114.56, 118.19, 124.46, 125.50, 125.83, 139.06,
142.59, 154.67, 158.05. IR: 1604 (w), 1503 (w), 1419 (m), 1302
(m), 1273 (s), 1239 (s), 1180 (w), 1161 (w), 1109 (w), 1039 (s),
997 (w), 967 (w), 886 (s), 811 (m), 800 (m), 774 (w), 728 (s), 667
(m), 559 (w). UV-vis: 365 (3.1 ¥ 104), 299 (5.5 ¥ 104). ESI-MS:
918.4064 (M+, calcd 918.4047). Anal. Calcd for C52H62N4O8Ti: C,
67.97; H, 6.80; N, 6.10. Found: C, 67.85; H, 6.70; N, 5.89.
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-(di-(4-methoxyphenyl)-
amino)benzyl)-ethylenediamine]titanium(IV) difluoride, An2NLTiF2.
An2NLH2 (0.1000 g, 0.115 mmol), TiF4 (Strem, 0.0284 g,
0.229 mmol), CH2Cl2 (10 mL), and a stir bar were placed in a
20 mL scintillation vial. The vial was tightly sealed with a Teflon-
lined screw cap and the mixture heated with vigorous stirring
in a 70 ◦C oil bath for 16 h. The reaction mixture was cooled,
filtered on a glass frit under nitrogen to remove unreacted titanium
tetrafluoride, and the residue washed with CH2Cl2 (5 mL). The
combined filtrates were dried under vacuum to give a sticky dark
red solid. The solid was slurried with hexanes (20 mL) and filtered
on a glass frit. After drying under vacuum for 1 h, the yield of red
difluoride complex was 0.0298 g (31%). 1H NMR (CD2Cl2, major
isomer, added Cp*2Fe): d 2.15 (s, 3H, NCH3), 2.21 (d, 13 Hz,
1H, NCH2CHH¢N), 2.40 (d, 11 Hz, 1H, NCHH¢CH2N), 2.82 (d,
4JFH = 4 Hz, 3H, NCH3), 3.06 (t, 14 Hz, 1H, NCH2CHH¢N), 3.12
(d, 12.5 Hz, 1H, NCHH¢Ar), 3.27 (d, 14.5 Hz, 1H, NCHH¢Ar),
3.51 (t, 14 Hz, 1H, NCHH¢CH2N), 3.76 (s, 12H, OCH3), 4.43 (d,
14 Hz, 1H, NCHH’Ar), 4.70 (d, 13 Hz, 1H, NCHH’Ar), 6.45 (d,
9 Hz, 1H, Ar 3-H), 6.57 (d, 9 Hz, 1H, Ar 3-H), 6.74–6.80 (m, 12H,
ArH and NC6H4OCH3 2,6-H), 6.96 (d, 9 Hz, 8H, NC6H4OCH3
1
H). 13C{ H} NMR (C6D6): d 41.70 (NCH3), 54.29 (NCH2CH2N),
55.42 (OCH3), 61.77 (NCH2Ar), 115.40, 117.87, 123.22, 125.27,
125.54, 125.74, 141.44, 143.20, 154.63, 155.95. IR: 1606 (w), 1508
(s), 1493 (s), 1327 (w), 1302 (w), 1283 (w), 1245 (s), 1181 (w), 1104
(m), 1039 (s), 983 (m), 866 (w), 836 (s), 728 (s). ESI-MS: 755.3817
(M+H+, calcd 755.3803). Anal. Calcd for C46H50N4O6: C, 73.19;
H, 6.68; N, 7.42. Found: C, 73.03; H, 6.65; N, 7.29.
Metalation of salan ligands
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-bromobenzyl)ethylenedi-
amine]titanium(IV) diisopropoxide, BrLTi(OiPr)2. To a suspension
of the ligand BrLH2 (0.8000 g, 1.75 mmol) in benzene (10 mL)
was added a solution of Ti(OiPr)4 (Aldrich, 0.4962 g, 1.75 mmol)
in 10 mL benzene dropwise with vigorous stirring. The initial
pinkish-white suspension became translucent yellow after the
addition was completed. Stirring was stopped after 35 min, and
the volatiles were evaporated under vacuum to give a pale yellow
foam. The residue was triturated with 10 mL hexane and allowed
to stand for 10 min. The precipitate was filtered on a glass frit,
washed with hexanes (2 ¥ 10 mL), and dried. Yield 0.8843 g
(81%). 1H NMR (CDCl3): d 1.24 (d, 6 Hz, 6H, CH(CH3)(CH¢3)),
1.23 (d, 6 Hz, 6H, CH(CH3)(CH¢3)), 1.84 (pseudo-d, 10 Hz, 2H,
NCHH¢CHH¢N), 2.43 (s, 6H, NCH3), 2.97 (pseudo-d, 9 Hz, 2H,
NCHH¢CHH¢N), 3.09 (d, 14 Hz, 2H, NCHH¢Ar), 4.58 (d, 14 Hz,
2H, NCHH¢Ar), 4.97 (sept, 6 Hz, 2H, CH(CH3)2), 6.55 (d, 9 Hz,
2H, Ar 3-H), 7.07 (d, 2.5 Hz, 2H, Ar 6-H), 7.24 (dd, 9, 3 Hz, 2H,
1
3,5-H). 13C{ H} NMR (CD2Cl2): d 43.79 (NCH3), 48.93 (d,
14 Hz), 52.63 (NCH3), 55.95 (OCH3), 57.97 (NCH2CH2N), 63.99
(NCH2CH2N), 64.77 (d, 9 Hz), 115.00 (2C, NC6H4OCH3 2,6-
C), 115.93, 116.96, 123.64, 124.12, 124.16, 124.42, 125.69, 125.80
(2C, NC6H4OCH3 3,5-C), 126.30, 142.10 (2C, NC6H4OCH3 1-C),
142.42, 143.02, 155.43, 155.83 (2C, NC6H4OCH3 4-C), 156.16. 19
F
1
Ar 4-H). 13C{ H} NMR (CDCl3): d 25.82 (OCH(CH3)(CH¢3)),
NMR (CD2Cl2): d 100.15 (d, 40 Hz, cis-b-trans-NCH3, F trans to
N), 103.07 (s, trans-a), 103.62 (d, 36 Hz, cis-b-cis-NCH3, F trans
to N), 146.39 (d, 40 Hz, cis-b-trans-NCH3, F trans to O), 150.36 (d,
36 Hz, cis-b-cis-NCH3, F trans to O), 168.48 (s, cis-a). IR: 1499
(w), 1461 (vs), 1377 (s), 1232 (s), 1176 (w), 1102 (w), 1029 (m),
888 (m), 813 (m), 776 (w), 721 (m). UV-vis: 421 (2.1 ¥ 104), 297
(5.7 ¥ 104). ESI-MS: 838.3024 (M+, calcd 838.3021). Anal. Calcd
for C46H48F2N4O6Ti: C: 65.87; H, 5.77; N, 6.68. Found: C, 65.78;
H, 5.86; N, 6.49.
26.12 (OCH(CH3)(CH¢3)), 47.39 (NCH3), 51.97 (NCH2CH2N),
63.97 (NCH2Ar), 78.62 (OCH(CH3)2), 108.98, 119.59, 126.48,
132.03, 132.07, 161.22. IR: 1586 (m), 1560 (m), 1406 (s), 1363
(s), 1302 (m), 1290 (s), 1185 (w), 1163 (m), 1118 (s), 1074 (w), 1053
(w), 980 (m), 961 (w), 944 (w), 900 (m). UV-vis: 303 (2.2 ¥ 104).
ESI-MS: 562.9872 ((M–OiPr)+, calcd 562.9847). Anal. Calcd for
C24H34Br2N2O4Ti: C, 46.33; H, 5.51; N, 4.50. Found: C, 46.40; H,
5.40; N, 4.36.
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-(di-(4-methoxyphenyl)-
Exchange of ancillary ligands
amino)benzyl)-ethylenediamine]titanium(IV)
diisopropoxide,
An2NLTi(OiPr)2. An2NLH2 (0.5000 g, 0.662 mmol) was reacted
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-bromobenzyl)](2-methyl-
with Ti(OiPr)4 as described above for the bromo analogue to yield
2-oxidopropanoato)titanium(IV),
BrLTi(OC(CH3)2C(O)O).
1
0.5169 g (85%) of An2NLTi(OiPr)2. H NMR (CDCl3): d 1.24 (d,
BrLTi(OiPr)2 (0.1500 g, 0.241 mmol) and a-hydroxyisobutyric acid
(Aldrich, 0.0251 g, 0.241 mmol) were dissolved in chloroform
(5 mL each). The a-hydroxyisobutyric acid solution was added
dropwise to the titanium precursor with vigorous stirring,
6 Hz, 6H, CH(CH3)(CH¢3)), 1.30 (d, 6 Hz, 6H, CH(CH3)(CH¢3))
1.85 (pseudo-d, 9 Hz, 2H, NCHH¢CHH¢N), 2.45 (s, 6H, NCH3),
3.00 (d, 14 Hz, NCHH¢Ar), 3.04 (pseudo-d, 10.5 Hz, 2H,
11460 | Dalton Trans., 2011, 40, 11458–11468
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yielding a yellow solution. Stirring was stopped after 15 min,
and the volatiles were evaporated under vacuum. The pale yellow
solid was suspended in hexanes (20 mL), filtered through a frit,
and vacuum dried overnight to give 0.1380 g product (94%).
1H NMR (CDCl3): d 1.40 (s, 3H, OC(O)C(CH3)(CH¢3)O),
1.40 (s, 3H, OC(O)C(CH3)(CH¢3)O), 2.08 (s, 3H, NCH3), 2.24
(dd, 13, 2.5 Hz, 1H, NCH2CHHN), 2.45 (dd, 13, 2.5 Hz, 1H,
NCHHCH2N), 2.83 (s, 3H, NCH3), 3.02 (td, 13, 2.5 Hz, 1H,
NCH2CHHN), 3.18 (d, 13.5 Hz, 1H, NCHH¢Ar), 3.48 (d, 14 Hz,
1H, NCHH¢Ar), 3.59 (td, 13, 2.5 Hz, 1H, NCHHCH2N), 4.66
(d, 14 Hz, 1H, NCHH¢Ar), 5.00 (d, 13.5 Hz, 1H, NCHH¢Ar),
6.61 (d, 8.5 Hz, 1H, Ar 3-H), 6.65 (d, 8.5 Hz, 1H, Ar 3-H),
7.13 (d, 2 Hz, 1H, Ar 6-H), 7.28 (d, 2 Hz, 1H, Ar 6-H), 7.30
(dd, 9, 3 Hz, 1H, Ar 4-H), 7.34 (dd, 9, 2.5 Hz, 1H, Ar 4-H).
12 Hz, 1H, NCHH¢CH2N), 2.75 (s, 3H, NCH3), 3.11 (d, 14 Hz,
1H, NCHH¢Ar), 3.10 (t, 11 Hz, 1H, NCH2CHH¢N), 3.34 (d,
14 Hz, 1H, NCHH¢Ar), 3.52 (t, 11 Hz, 1H, NCHH¢CH2N), 3.79
(s, 12H, OCH3), 4.63 (d, 14 Hz, 1H, NCHH¢Ar), 4.92 (d, 14 Hz,
1H, NCHH¢Ar), 6.58 (d, 9 Hz, 1H, Ar 3-H), 6.61 (d, 9 Hz, 1H, Ar
3-H), 6.67 (d, 2 Hz, 1H, Ar 6-H), 6.77-6.84 (m, 3H, ArH), 6.79 (d,
9 Hz, 4H, NC6H4OCH3 2,6-H), 6.80 (d, 9 Hz, 4H, NC6H4OCH3
2,6-H), 6.98 (d, 9 Hz, 4H, NC6H4OCH3 3,5-H), 6.99 (d, 9 Hz,
1
4H, NC6H4OCH3 3,5-H). 13C{ H} NMR (CDCl3): d 26.74
(OC(O)C(CH3)(CH¢3)O), 27.92 (OC(O)C(CH3)(CH¢3)O), 43.21
(NCH3), 50.48 (NCH3), 51.99 (NCH2CH2N), 55.71 (OCH3),
57.40 (NCH2CH2N), 64.15 (NCH2Ar), 64.58 (NCH2Ar), 92.78
(OC(O)C(CH3)2O), 114.73, 114.84, 116.55, 117.01, 123.52,
123.59, 123.85, 124.58, 125.31, 125.42, 125.74, 125.98, 141.64,
141.88, 142.05, 142.70, 155.22, 155.59, 156.29, 187.07 (C O).
IR: 1676 (s, nC O), 1604 (w), 1504 (sh), 1489 (sh), 1300 (w), 1262
(w), 1239 (s), 1197 (w), 1120 (w), 1034 (m), 979 (w), 887 (s), 823
(s), 781 (m), 729 (s), 664 (w). UV-vis: 405 (2.6 ¥ 104), 297 (7.1 ¥
104). FABMS: 903 (M+). Anal. Calcd for C50H54N4O9Ti: C, 66.52;
H, 6.03; N, 6.21. Found: C, 66.46; H, 6.01; N, 5.98.
1
13C{ H} NMR (CDCl3): d 26.48 (OC(O)C(CH3)(CH¢3)O), 27.63
(OC(O)C(CH3)(CH¢3)O), 42.85 (NCH3), 50.84 (NCH3), 52.12
(NCH2CH2N), 57.23 (NCH2CH2N), 63.46 (NCH2Ar), 63.85
(NCH2Ar), 94.31 (OC(O)C(CH3)2O), 112.41, 113.20, 118.00,
118.40, 125.84, 127.10, 131.75, 132.24, 132.61, 133.31, 159.35,
159.83, 186.66 (C(O)O). IR: 1685 (m, nC O), 1410 (w), 1268 (s),
1192 (m), 1121 (w), 973 (w), 820 (m), 723 (m), 675 (m). UV-vis:
334 (2.4 ¥ 104), 301 (1.9 ¥ 104). FAB-MS: 605 (M+H+). Anal.
Calcd for C22H26Br2N2O5Ti: C, 43.59; H, 4.32; N, 4.62. Found: C,
43.33; H, 4.29; N, 4.43.
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-(di-(4-methoxyphenyl)-
amino)benzyl)ethylenediamine](2,2-dimethyl-3-oxidopropanoato)-
titanium(IV), An2NLTi(OCH2C(CH3)2C(O)O). The compound
was prepared from An2NLTi(OiPr)2 (0.1500 g, 0.163 mmol) and
hydroxypivalic acid (0.0193 g, 0.163 mmol) using the same
procedure employed for the a-hydroxybutyrate compound to
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-bromobenzyl)ethylenedi-
amine](2,2-dimethyl-3-oxidopropanoato)titanium(IV),
BrLTi(O-
CH2C(CH3)2C(O)O). BrLTi(OiPr)2 (0.1500 g, 0.241 mmol) and
hydroxypivalic acid (Aldrich, 0.0285 g, 0.241 mmol) were reacted
as described for the a-hydroxyisobutyrate derivative to yield
0.1330 g (89%) of the hydroxypivalate complex. 1H NMR (CDCl3):
d 1.14 (s, 3H, C(CH3)(CH¢3)), 1.40 (s, 3H, C(CH3)(CH¢3)), 2.13 (s,
3H, NCH3), 2.21 (dd, 14, 2 Hz, 1H, NCHH¢CH2N), 2.38 (dd, 13,
2 Hz, 1H, NCH2CHH¢N), 2.77 (s, 3H, NCH3), 3.03 (td, 13, 2 Hz,
1H, NCHH¢CH2N), 3.11 (d, 14 Hz, 1H, NCHH¢Ar), 3.40 (d,
14 Hz, 1H, NCHH¢Ar), 3.69 (td, 13, 3 Hz, 1H, NCH2CHH¢N),
3.89 (d, 12 Hz, 1H, NCHH¢Ar), 4.51 (d, 12 Hz, 1H, NCHH¢Ar),
4.58 (d, 13 Hz, 1H, OC(O)C(CH3)2CHH¢O), 4.99 (d, 14 Hz, 1H,
OC(O)C(CH3)2CHH¢O), 6.57 (d, 8.5 Hz, 1H, Ar 3-H), 6.65 (d,
8.5 Hz, 1H, Ar 3-H), 7.13 (d, 2 Hz, 1H, Ar 6-H), 7.24 (d, 2 Hz, 1H,
Ar 6-H), 7.29 (dd, 7, 2 Hz, 1H, Ar 4-H), 7.32 (dd, 9, 2.5 Hz, 1H,
yield 0.1264 g (84%) of the hydroxypivalate complex. H NMR
1
(CDCl3): d 1.13 (s, 3H, OCH2C(CH3)(CH¢3)C(O)O), 1.44 (s, 3H,
OCH2C(CH3)(CH¢3)C(O)O), 2.11 (d, 12 Hz, 1H, NCH2CHH¢N),
2.21 (s, 3H, NCH3), 2.70 (d, 12 Hz, 1 H, NCHHCH2N), 2.69
(s, 3H, NCH3), 3.01 (d, 13.5 Hz, 1H, NCHH¢Ar), 3.11 (t,
13 Hz, 1H, NCH2CHH¢N), 3.28 (d, 14 Hz, 1H, NCHH¢Ar),
3.63 (t, 13 Hz, 1H, NCHH¢CH2N), 3.79 (s, 12H, OCH3), 3.89
(d, 12 Hz, 1H, NCHH¢Ar), 4.45 (d, 12 Hz, 1H, NCHH¢Ar),
4.53 (d, 13 Hz, 1H, OCHH¢C(CH3)2C(O)O), 4.92 (d, 13 Hz, 1H,
OCHH¢C(CH3)2C(O)O), 6.55 (d, 8.5 Hz, 1H, Ar 3-H), 6.60 (d,
8.5 Hz, 1H, Ar 3-H), 6.66 (d, 2.5 Hz, 1H, Ar 6-H), 6.74 (d, 3 Hz,
Ar 6-H), 6.79-6.85 (m, 10H, NC6H4OCH3 2,6-H and Ar 4-H), 6.97
(d, 9 Hz, 4H, NC6H4OCH3 3,5-H), 6.99 (d, 9 Hz, 4H, NC6H4OCH3
1
3,5-H). 13C{ H} NMR (CDCl3): d 22.86 (OC(O)C(CH3)(CH¢3)O),
1
Ar 4-H). 13C{ H} NMR (CDCl3): d 22.63 (C(CH3)(CH¢3)), 24.08
24.26 (OC(O)C(CH3)(CH¢3)O), 43.47 (OCH2C(CH3)2C(O)O),
43.56 (NCH3), 50.19 (NCH3), 52.65 (NCH2CH2N), 55.72 (OCH3),
56.92 (NCH2CH2N), 64.11 (NCH2Ar), 64.71 (NCH2Ar), 83.23
(OCH2C(CH3)2C(O)O), 114.73, 114.81, 116.63, 117.22, 123.47,
123.71, 123.92, 124.03, 125.12, 125.39, 125.54, 125.65, 141.77,
141.86, 142.02, 142.05, 155.23, 155.44, 155.60, 156.18, 181.14
(C O). IR: 1652 (m, nC O), 1502 (w), 1295 (w), 1261 (sh), 1236
(s), 1171 (m), 1034 (m), 888 (w), 813 (w). UV-vis (toluene): 419
(2.0 ¥ 104), 299 (5.9 ¥ 104). FAB-MS: 917 (M+H+). Anal. Calcd
for C51H56N4O9Ti: C, 66.81; H, 6.16; N, 6.11. Found: C, 65.93; H,
5.97; N, 5.80.
(C(CH3)(CH¢3)), 43.31 (OC(O)C(CH3)2CH2O), 43.36 (NCH3),
50.50 (NCH3), 52.74 (NCH2CH2N), 56.79 (NCH2CH2N), 63.52
(NCH2Ar), 64.07 (NCH2Ar), 83.98 (OC(O)C(CH3)2CH2O),
112.34, 112.38, 118.21, 118.63, 126.38, 126.80, 131.80, 132.05,
132.48, 133.18, 159.54, 159.67, 180.65 (TiOC(O)). IR: 1653 (s,
nC O), 1408 (m), 1261 (m), 1170 (m), 1119 (w), 1064 (m), 894 (w),
814 (m), 721 (w), 676 (s). UV-vis: 332 (2.3 ¥ 104), 303 (1.7 ¥ 104).
FAB-MS: 619 (M+H+). Anal. Calcd for C23H28Br2N2O5Ti: C,
44.54; H, 4.55; N, 4.52. Found: C, 44.65; H, 4.33; N, 3.48.
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-(di-(4-methoxyphenyl)-
amino)benzyl)ethylenediamine](2-methyl-2-oxidopropanoato)titan-
ium(IV), An2NLTi(OC(CH3)2C(O)O). The compound was
prepared as described for the bromosalan analogue using
An2NLTi(OiPr)2 (0.1500 g, 0.163 mmol) to give 0.1180 g (79%)
of the chelated product. 1H NMR (CDCl3): d 1.40 (s, 3H,
OC(O)C(CH3)(CH¢3)O), 1.63 (s, 3H, OC(O)C(CH3)(CH¢3)O),
2.14 (d, 12 Hz, 1H, NCH2CHH¢N), 2.20 (s, 3H, NCH3), 2.40 (d,
[N,N¢-Dimethyl-N,N¢-bis-(2-oxido-5-(di-(4-methoxyphenyl)-
amino)benzyl)ethylenediamine]-titanium(IV) dichloride, An2NLTiCl2.
An2NLTi(OiPr)2 (0.1600 g, 0.174 mmol) was weighed into a
50 mL round-bottom flask with a stir bar and dissolved in
CH2Cl2 (15 mL). To this solution, trimethylsilyl chloride (Aldrich,
0.0464 g, 0.427 mmol) in CH2Cl2 (5 mL) was added dropwise
with vigorous stirring. The round-bottom flask was capped with
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a needle valve and the reaction mixture stirred under nitrogen for
48 h. The volatiles were evaporated and the purple residue was
allowed to stand under vacuum for 4 h. The flask was returned to
the drybox and the solid dissolved in CHCl3 (8 mL), layered with
hexanes (5 mL) and Et2O (5 mL), and stored at -35 ◦C. After 4 d,
the supernatant was decanted and the dark purple solid vacuum
dried for 30 min to give 0.1523 g (73%) of the dichloride complex.
The analytical sample contained 0.5 mol CH2Cl2 and 0.5 mol
hexane per mol complex. 1H NMR (CDCl3, added Cp*2Fe): d 2.28
(s, 3H, NCH3), 2.31 (d, 12 Hz, 1H, NCH2CHH¢N), 2.41 (d, 12 Hz,
1H, NCHH¢CH2N), 3.12 (s, 3H, NCH3), 3.19 (d, 13.5 Hz, 1H,
NCHH¢Ar), 3.36 (d, 15 Hz, 1H, NCHH¢Ar), 3.68 (t, 11 Hz, 1H,
NCH2CHH¢N), 3.78 (partially obscured t, 1H, NCHH¢CH2N)
3.79 (s, 6H, OCH3), 3.80 (s, 6H, OCH3), 5.00 (d, 14 Hz, 1H,
NCHH¢Ar), 5.29 (d, 13 Hz, 1H, NCHH¢Ar), 6.51 (d, 8 Hz, 1H,
Ar 3-H), 6.72 (m, 5H, ArH), 6.81 (d, 9 Hz, 4H, NC6H4OCH3
2,6-H), 6.83 (d, 9 Hz, 4H, NC6H4OCH3 2,6-H), 7.01 (d, 9 Hz,
4H, NC6H4OCH3 3,5-H), 7.03 (d, 9 Hz, 4H, NC6H4OCH3 3,5-
or glassy carbon counter electrode, and a silver/silver chloride
pseudo-reference electrode. The electrodes were connected to
the potentiostat through electrical conduits in the drybox wall.
Samples were 1 mM in CH2Cl2, using 0.1 M Bu4NPF6 as the
electrolyte. Potentials were referenced to ferrocene/ferrocenium
at 0 V, with the reference potential established by spiking the
test solution with a small amount of decamethylferrocene (E◦
=
-0.565 V). Cyclic voltammograms were recorded with a scan rate
of 60 mV s-1. Square wave voltammograms were acquired with a
step size of 3 mV, a pulse height of 30 mV, and a frequency of
15 Hz.
X-ray crystallography
Crystals of An2NLTi(OiPr)2 were grown overnight from a so-
lution in hexanes at room temperature. The crystals of
BrLTi(OCH2CMe2COO)·0.5 HOCH2CMe2CO2H·0.5 CHCl3 were
deposited on storage of the filtrate from the preparative procedure
at -35 ◦C for 2 weeks. In each case, the crystal to be analyzed
was placed in inert oil and transferred to a MiTeGen loop in the
cold N2 stream of a Bruker Apex CCD diffractometer. Data were
corrected for absorption and polarization, and the space group
was assigned using standard methods.12 The structures were solved
using direct methods, and all nonhydrogen atoms were treated
anisotropically. The model was refined by full-matrix least-squares
analysis of F2 against all reflections. Hydrogen atoms were placed
in calculated positions, with thermal parameters for the hydrogens
tied to the isotropic thermal parameter of the atom to which they
are bonded (1.5¥ for methyl, 1.2¥ for all others). Calculations
used SHELXTL (Bruker AXS),13 with scattering factors and
anomalous dispersion terms taken from the literature.14 Further
details about the structures are in Table 1.
1
H). 13C{ H} NMR (CDCl3): d 45.88 (NCH3), 53.83 (NCH3),
54.00 (NCH2CH2N), 55.71 (OCH3), 58.12 (NCH2CH2N), 65.96
(NCH2Ar), 66.22 (NCH2Ar), 114.80, 114.91, 115.72, 116.37,
120.79, 121.54, 121.86, 122.04, 126.09, 126.15, 126.58, 128.23,
140.94, 141.36, 143.95, 144.43, 155.00, 155.71, 156.01, 156.03. IR:
1502 (s), 1484 (s), 1299 (w), 1267 (m), 1239 (s), 1183 (m), 1113 (w),
1088 (w), 1059 (w), 1043 (m), 888 (m), 825 (m), 780 (m), 706 (m),
678 (w). UV-vis: 543 (1.9 ¥ 104), 298 (6.0 ¥ 104). Anal. Calcd for
C49.5H56Cl3N4O6Ti: C, 62.11; H, 5.90; N, 5.85. Found: C, 61.57; H,
6.10; N, 5.95.
Electrochemistry
Electrochemical measurements were performed in the drybox
using a BAS Epsilon potentiostat. A standard three-electrode
setup was used, with a glassy carbon working electrode, Pt
Crystals of An2NLTi(OCMe2CO2) were deposited on standing
of the mother liquor from its preparation. An arbitrary sphere
Table 1 X-ray crystallographic details for An2NLTi(OiPr)2, An2NLTi(OCMe2CO2) and BrLTi(OCH2CMe2CO2)·0.5 HOCH2CMe2CO2H·0.5 CHCl3
BrLTi(OCH2CMe2CO2)·0.5
An2NLTi(OiPr)2
An2NLTi(OCMe2CO2)
HOCH2CMe2CO2H·0.5 CHCl3
Molecular formula
Formula weight
T/K
C52H62N4O8Ti
918.96
100(2)
C50H54N4O9Ti
902.87
150(2)
C26H33.5Br2Cl1.5N2O6.5Ti
738.94
120(2)
Monoclinic
P21/c
0.71073 (Mo-Ka)
87013
12258
0.0438
9986
19.9705(15)
21.2780(16)
14.1667(11)
90
98.6784(12)
90
5951.0(8)
8
3.152
Crystal system
Triclinic
Triclinic
¯
¯
Space group
P1
P1
˚
l/A
0.71073 (Mo-Ka)
0.7749 (synchrotron)
23821
Total data collected
No. of indep reflns.
Rint
34937
9682
8573
0.0845
4662
0.0338
6830
Obsd refls [I > 2s(I)]
˚
a/A
11.6904(6)
14.3149(8)
15.9465(8)
66.280(2)
87.702(2)
78.741(2)
2393.9(2)
2
12.703(2)
13.811(2)
14.969(3)
69.740(2)
79.478(2)
73.082(2)
2347.3(7)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
m/mm-1
0.237
0.26 ¥ 0.16 ¥ 0.10
608
0.266
0.08 ¥ 0.04 ¥ 0.02
745
Crystal size/mm
No. refined params
R1, wR2 [I > 2s(I)]
0.20 ¥ 0.19 ¥ 0.09
718
0.0338, 0.0794
0.0608, 0.1490
0.0552, 0.1142
11462 | Dalton Trans., 2011, 40, 11458–11468
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of data was collected at beamline 11.3.1 at the Advanced Light
Source, Lawrence Berkeley National Laboratory on a red 0.08 ¥
0.04 ¥ 0.02 mm plate on a Bruker APEX-II diffractometer using
a combination of w- and f-scans of 0.3◦ at 150(2) K. Data were
reduced and analyzed as described above. The titanium atom and
the first coordination-sphere atoms were found using a Patterson
map, with remaining heavy atoms found on subsequent difference
Fourier syntheses. Hydrogen atoms were found on difference maps
and refined isotropically, except for the hydrogens on the methoxy
groups, which were placed in calculated positions.
of the salicylaldehyde, but found that silylation was incomplete
and the resulting silyl compounds hydrolyzed readily. In contrast,
in the preparation of the salan derivative, reductive amination
proceeds even with the unprotected phenol, and silylation of
the aminomethyl phenols is readily accomplished under standard
conditions (tert-butyldimethylsilyl chloride, DBU). The free amine
is an oil, but can be precipitated from dichloromethane as the
dihydrochloride in pure form and in high yield. Neutralization of
the hydrochloride salt, followed by palladium-catalyzed substitu-
tion of the aryl bromides with bis(4-methoxyphenyl)amine using
Pd0/PtBu3 gives the silyl-protected ligand. Desilylation of the
19
crude material with ammonium fluoride in methanol produces the
free ligand An2NLH2. The overall yield for the four-step synthesis
is 53%, and each synthetic intermediate is isolated in pure form
directly by precipitation, making this an efficient route to this
electron-rich salan ligand.
Results and discussion
Synthesis of a diarylamino-substituted salan ligand
Preparation of the diarylamino-substituted salan ligand begins
with commercially available 5-bromosalicylaldehyde. The tetrahy-
drosalen derivative of this aldehyde had previously been prepared
in two steps, with isolation of the intermediate salen ligand;11,15
we find that generation and in situ reduction of the salen ligand
proceeds in excellent yield (Scheme 1). Methylation at nitrogen
to prepare the bromosalan derivative BrLH2 is accomplished by
reductive amination with formaldehyde as described by Walsh and
coworkers for the 3,5-di-tert-butyl-substituted analogue.16 This
compound was previously prepared by Mannich condensation
of N,N¢-dimethylethylenediamine with 4-bromophenol in a single
step, albeit in lower yield.17
Synthesis and structures of titanium(IV) salan complexes
Both An2NLH2 and its bromo analogue BrLH2 are rapidly metalated
by titanium(IV) isopropoxide to give the corresponding LTi(OiPr)2
complexes (eqn (1)). In situ measurements indicate that the
reactions are quantitative, and pure material can be isolated in high
yield. NMR spectra indicate that these isopropoxide complexes
exist as single geometric isomers with C2 symmetry (e.g., the
benzylic methylene protons and the isopropoxide methyl groups
are diastereotopic). The geometry is confirmed to be cis-a (rather
than trans) by X-ray crystallography of An2NLTi(OiPr)2 (Tables 1–2,
Table 2 Selected bond distances (A) and angles (◦) in
˚
An2NLTi(OiPr)2, An2NLTi(OCMe2CO2) and BrLTi(OCH2CMe2CO2)·0.5
HOCH2CMe2CO2H·0.5 CHCl3
BrLTi(OCH2CMe2CO2)a
An2NLTi(OiPr)2 An2NLTi(OCMe2CO2) Molecule 1 Molecule 2
Ti–O1
Ti–O2
Ti–O3
Ti–O4
Ti–N1
Ti–N2
1.916(2)
1.879(2)
1.813(2)
1.823(2)
2.332(2)
2.366(2)
1.844(2)
1.842(2)
1.849(2)
1.999(2)
2.270(3)
2.295(3)
100.15(10)
105.34(11)
93.03(10)
82.25(10)
159.38(10)
96.66(10)
166.79(10)
96.12(10)
82.66(10)
80.57(10)
163.70(10)
94.52(11)
84.69(10)
84.67(9)
1.851(2)
1.869(2)
1.809(2)
1.986(2)
2.274(2)
2.281(2)
96.92(9)
105.06(8)
93.21(8)
83.07(8)
160.30(9)
94.27(9)
169.03(8)
92.00(9)
83.69(8)
87.14(8)
169.02(9)
94.50(9)
85.02(8)
85.35(8)
77.23(8)
140.2(2)
141.0(2)
134.7(2)d
133.1(2)d
1.857(2)
1.851(2)
1.811(2)
1.982(2)
2.263(2)
2.299(2)
95.55(8)
102.81(8)
94.79(8)
83.85(8)
161.24(8)
98.54(9)
167.22(8)
92.36(8)
82.87(8)
86.44(8)
166.54(9)
95.90(8)
81.30(8)
84.92(8)
77.56(8)
139.9(2)
144.1(2)
135.0(2)d
133.6(2)d
O1–Ti–O2 165.75(9)
O1–Ti–O3 95.26(9)
O1–Ti–O4 90.68(9)
O1–Ti–N1 81.80(8)
O1–Ti–N2 87.42(8)
O2–Ti–O3 94.37(9)
O2–Ti–O4 96.51(9)
O2–Ti–N1 88.03(8)
O2–Ti–N2 80.32(8)
O3–Ti–O4 107.03(10)
O3–Ti–N1 88.25(9)
O3–Ti–N2 162.68(9)
O4–Ti–N1 163.59(9)
O4–Ti–N2 90.01(9)
N1–Ti–N2 75.16(8)
Ti–O1–C11 139.82(18)
Ti–O2–C21 142.45(17)
Ti–O3–Cx 141.7(3)b
Ti–O4–Cy 140.9(2)b
Scheme 1 Synthesis of An2NLH2.
77.14(10)
138.4(2)
In the preparation of the redox-active tripodal ligands
t
142.9(2)
N(CH2C6H2-3-R-5-N(C6H4OMe)2-2-OH)3 (R = H, Bu), protec-
120.6(2)c
116.6(2)c
tion of the phenol group was required prior to reductive amination
of the salicylaldehyde. A benzyl group was used to protect the
starting salicylaldehyde, as has been done in other preparations of
aminetrisphenolates,18 even though deprotection was difficult in
the ortho-substituted derivative. We had explored silyl protection
a Numbers in atom labels for each independent molecule are preceded by
the molecule number (e.g., C11 in molecule 2 is C211). b x = 6A, y = 9. c x =
5, y = 6. d x = 5, y = 7.
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(1)
Crystallography
of
An2NLTi(OCMe2CO2)
and
BrLTi(OCH2CMe2CO2) (Fig. 2–3) indicates that the geometry
of these compounds is cis-b, with the alkoxide group of the
alkoxy-carboxylate trans to one of the amine nitrogens while
the other amine is trans to one of the aryloxide groups. (The
latter compound cocrystallizes with a disordered molecule of
hydroxypivalic acid linking two complexes in the asymmetric
unit by hydrogen bonding to the carbonyl oxygens of the bound
hydroxypivalates.) The cis-b geometry is less commonly observed
in titanium salan complexes than the cis-a geometry, but has
been previously seen in catecholate21a and m-oxo complexes.7,20
Replacement of an alkoxide with a carboxylate renders the
titanium centres more electron-deficient, as judged by the
contraction of the titanium–aryloxide and titanium–amine
Fig. 1 Thermal ellipsoid plot of An2NLTi(OiPr)2, with hydrogen atoms
omitted for clarity. Only the major orientation of the isopropyl group
bonded to O3 is shown.
bonds by ~0.05 A and ~0.07 A, respectively, compared to the
diisopropoxide complex. The five-membered chelate ring size
has a noticeable effect on the bonding of the alkoxide group
to titanium. While the alkoxides normally form the shortest
˚
˚
bonds to Ti (1.818(7) A avg. in An2NLTi(OiPr)2 and 1.810(2)
˚
A avg. in BrLTi(OCH2CMe2CO2)), the Ti–alkoxide bond in
˚
Fig. 1). This geometry is hardly surprising, since a large number
of (salan)titanium(IV) dialkoxides with varying substituents on the
nitrogens and aryl rings have been prepared, and structurally char-
An2N
˚
LTi(OCMe2CO2) is appreciably longer (1.849(2) A) and is
actually slightly longer than the Ti–aryloxide bonds in that
complex. The small ring imposes a relatively acute Ti–O–C
angle (120.6(2)◦, compared to 135–142◦ observed in the other
alkoxides), which likely diminishes its Ti–O p interactions.25,26 The
half-chair conformation of the six-membered hydroxypivalate
ring has been observed previously in titanium complexes.
6c,16,20,21
acterized examples are cis-a
with only a single exception.
(The parent salan complex (salan)Ti(OiPr)2 is cis-b in the solid
state, though it appears to be cis-a in solution.22) Bond distances
and angles in the diarylamino-substituted complex described here
are fairly typical. For example, while one of the aryloxide-titanium
˚
distances (Ti–O2, 1.879(2) A) is (barely) outside of the range
˚
observed to date (1.884–1.967 A in 44 examples), the chemically
(but not crystallographically) equivalent distance (Ti–O1, 1.916(2)
˚
˚
A) is quite typical (literature average 1.92(2) A). Thus, on average,
the aryloxide–Ti distances in An2NLTi(OiPr)2 are at the low end of,
but well within, the observed range. The triarylamine fragment is
essentially planar at nitrogen (sum of angles = 358.8◦ and 358.9◦
at N3 and N4, respectively), with the three aryl rings forming a
propeller shape (angles with the NC3 planes of 46(16)◦). These
features are typical of neutral triarylamines.23
(2)
We have previously used the formation of titanium chelates of w-
hydroxycarboxylic acids as a way of probing the effects of ancillary
ligation with two groups of dissimilar trans influence.24 The
titanium isopropoxide complexes LTi(OiPr)2 react rapidly with a-
hydroxyisobutyric acid or 2,2-dimethyl-3-hydroxypropanoic acid
(hydroxypivalic acid) to form chelated alkoxy-carboxylates with
5- or 6-membered chelate rings, respectively (eqn (2)). NMR
spectra of the bromo-substituted and diarylamino-substituted
salan complexes are very similar to each other, and in each of
the four compounds only one C1-symmetric isomer is apparent by
NMR.
The dark purple dichloride complex An2NLTiCl2 can be prepared
by reaction of the corresponding diisopropoxide complex with
Me3SiCl (eqn (3)), a procedure which has also been used to pre-
pare bis(diketonato)titanium(IV) dichlorides.9,27 The monochloro
complex An2NLTi(OiPr)Cl can be observed as an intermediate in
this reaction by in situ NMR monitoring. NMR resonances for
the isolated dichloride complex are broadened due to partial
oxidation of the triarylamine groups. Addition of traces of
decamethylferrocene (to fully reduce any radical cations) causes
the spectra to sharpen, showing that a single, C1-symmetric isomer
is present in solution. Based on its low symmetry and the similarity
11464 | Dalton Trans., 2011, 40, 11458–11468
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(3)
Attempts to prepare the corresponding fluoro complex by ligand
replacement reactions of An2NLTi(OiPr)2 with, e.g., ammonium
fluoride, were not successful. The fluoride complex could be
prepared in moderate yield by metalation of the free ligand
1
with titanium(IV) fluoride (eqn (4)). As judged by H and 19F
NMR spectroscopy (Fig. 4), An2NLTiF2 exists as a mixture of
isomers. The major isomer, accounting for 80% of the material,
is assigned as the cis-b isomer on the basis of the similarity of
1
its H NMR spectrum to those of the other cis-b compounds.
It displays two doublets with a coupling constant JFF = 40 Hz
typical of cis-TiF2 units28 in its 19F NMR spectrum, consistent
with inequivalent fluorine environments. Based on the firmly
established observation that fluorines trans to fluorine resonate
substantially upfield of fluorines trans to neutral donors in L2TiF4
and [LTiF5]- complexes,29,30 the upfield resonance (d 100.1) is
assigned to the fluorine trans to aryloxide and the downfield
resonance (d 146.4) to the fluorine trans to nitrogen. A smaller pair
of doublets (5% of the total intensity) with similar chemical shifts
and coupling constant (JFF = 36 Hz) to those of the major isomer
are also assigned as originating from a cis-b isomer, differing from
the major isomer in the stereochemistry of the ethylenediamine
ring by having the nitrogen substituents cis rather than trans. This
arrangement of nitrogen substituents is much rarer than the trans
arrangement but has been observed in two di-m-oxodititanium
salan complexes.7b
Fig. 2 Thermal ellipsoid plot of An2NLTi(OCMe2CO2), with hydrogen
atoms omitted for clarity.
(4)
Two other singlets of unequal intensity, therefore attributed to
two different symmetrical difluoride complexes, are also observed
in the 19F NMR. The only possible isomers with equivalent fluorine
environments are the cis-a and the trans isomer. Based on the
chemical shifts, the upfield signal (13%) is assigned to the trans
isomer (F atoms mutually trans) and the downfield signal (2%)
to the cis-a isomer (F trans to N). While the trans isomer has
not been previously observed in a simple salan ligand bonded to
Fig.
3
Thermal ellipsoid plot of one of the crystallographi-
cally inequivalent titanium complexes in BrLTi(OCH2CMe2CO2)·0.5
HOCH2CMe2CO2H·0.5 CHCl3, with hydrogen atoms omitted for clarity.
1
of the H NMR spectra to those of the alkoxy-carboxylates, the
compound is assigned as the cis-b isomer.
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the aryloxide or alkoxide ligands trans to nitrogen, not chloride.
In the salan complex, the trans isomer (which would allow both
aryloxides to be trans to nitrogen) may be disfavored by the
relatively obtuse O–Ti–O angle (~110◦) imposed by geometrical
constraints of the chelate. A wide angle diminishes the ability of
the aryloxide ligands to donate to the B-symmetry dp orbital and
would thus decrease the stability of the trans isomer. This effect
would be much less important in the difluoride complex (which is
13% trans) because of the greater ability of fluoride compared to
chloride to donate to this dp orbital.
The geometric flexibility of the salan ligand may have implica-
tions in the reactivity of its complexes. For example, abstraction
of a methyl group from a cis-a (salan)ZrMe2 complex leads to
rearrangement to form a compound assigned on the basis of
NMR and DFT evidence to a square pyramidal geometry with
the methyl group apical (i.e., trans to the vacant coordination
site) and the salan ligand adopting a trans geometry.37 Thus,
while the neutral compound is geometrically similar to a C2-
symmetric dialkylzirconocene, the cationic species involved in
olefin polymerization differs dramatically in its stereochemistry,
with implications for both reactivity and stereoselectivity in
enchainment.
Fig. 4 19F NMR spectrum of An2NLTiF2 (CD2Cl2, chemical shifts in ppm
downfield of CFCl3). The spectrum was taken in the presence of traces of
decamethylferrocene to reduce adventitiously oxidized triarylamines.
titanium, it is certainly geometrically accessible for the ligand,
as witnessed by numerous late metal salan complexes with this
configuration and by structures of salan-like hexadentate ligands
bonded to titanium.31
A superficial examination of the literature of (salan)Ti(IV)
complexes might lead to the conclusion that the C2-symmetric
cis-a configuration is strongly preferred by this ligand, and indeed
this observation has been adduced as the basis for examining these
ligands for preparing isospecific Ziegler–Natta polymerization
catalysts.32 The complexes prepared here suggest that the prepon-
derance of the cis-a configuration observed in the literature is not
a reflection of the intrinsic geometric predilection of the salan
ligand but rather an artifact of the predominance of dialkoxide
complexes in the literature. Rather than depending primarily on
the salan ligand, the observed geometric preferences in solution
can be best rationalized as reflecting the competing trans influences
of the amine, aryloxide, and other ligands at titanium. Because
alkoxide ligands are more strongly donating than aryloxides, they
prefer to be trans to the weakly donating amine ligands, giving cis-
a complexes. When one alkoxide is replaced by a carboxylate, the
aryloxide has the second-strongest trans influence in the complex,
and so the cis-b geometry, which allows the alkoxide and one
aryloxide to be trans to the amines, is adopted. The preference
of the dichloride complex to be cis-b can be rationalized if the
trans influence of chloride is even lower than that of amine, as
this geometry allows one aryloxide to be trans to chloride (and no
geometry would allow both to be trans to chloride). Finally, the
structural diversity exhibited by An2NLTiF2 emphasizes the intrinsic
flexibility of the salan ligand itself and is consistent with fluoride
having a rather similar trans influence to aryloxide, rendering all
geometries electronically quite similar. While fluoride is a weaker
Brønsted base than aryloxide, titanium is more electropositive
than hydrogen and is thus expected to form a relatively stronger
Optical and electrochemical properties of diarylamino-substituted
(salan)titanium complexes
The UV-visible spectra of An2NLTi(X)(Y) complexes show, in addi-
tion to a very intense ligand-centered p–p* transition at 300 nm,
ligand-to-metal charge transfer transitions at longer wavelengths
(Fig. 5). As expected for a LMCT transition, the absorption feature
moves to lower energy as the titanium becomes more electron-
poor, with lmax shifting from 365 nm for the diisopropoxide to
543 nm for the dichloride. The alkoxy-carboxylate complexes
show peaks at intermediate wavelengths. The difluoride complex
shows an optical spectrum that is very similar to those of the
alkoxy-carboxylates, suggesting that fluoride is intermediate in its
bond to fluorine; note that typical Ti–F distances in neutral
33
˚
octahedral titanium(IV) fluorides (1.75–1.82 A) are similar to
the bond lengths in titanium aryloxides.
From the structural trends observed in these complexes, one
may thus infer a ladder of trans influences of OR > OAr ª F >
O2CR > NR3 > Cl. The trans influence is known to be important
in determining the geometry of titanium(IV) complexes,34 and the
ordering seen here is mostly in agreement with previous studies.
The exception is that chloride usually appears to exert a stronger
trans influence than neutral nitrogen donors. For example, (2,6-
Me2C6H3O)2TiCl2(diamine)35 and (TADDOL)TiCl2(bpy)36 have
Fig.
5
UV-visible spectra (CH2Cl2) of An2NLTi(OiPr)2 (solid line),
An2NLTi(OCH2CMe2CO2) (dashed line), An2NLTiF2 (dotted line) and
An2NLTiCl2 (dot-dash line).
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Table 3 Redox potentials for triarylamine oxidation in An2NLTi complexes
(CH2Cl2, 0.1 M Bu4NPF6, V vs. Cp2Fe+/Cp2Fe)
Conclusions
A para-diarylamino-substituted salan ligand is readily prepared
in four steps from 5-bromosalicylaldehyde in 53% overall yield.
It, like the bromosalan analogue, readily forms complexes of
titanium(IV) with a variety of chelating and monodentate ancil-
lary ligands. The intrinsic geometrical preferences of the salan
backbone appear to be small, with the observed geometry
governed mainly by the trans influence of the ancillary groups.
Each diarylaminoaryloxide group usually undergoes two pairs of
reversible oxidations, at ~0.1 V and ~0.8 V, with modest (50–
150 mV) separations between the redox potentials of the two arms
of the salan ligand. The ability of these ligands to reversibly store
oxidizing equivalents will be explored as a way to mediate redox
reactions at the titanium(IV) centre.
Complex
E◦
E◦
E◦
E◦
1
2
3
4
An2NLTi(OCMe2CO2)
An2NLTi(OCH2CMe2CO2)
An2NLTiF2
0.059
0.059
0.063
0.147
0.119
0.143
0.739
0.715
0.719
0.875
0.843
0.887
donor ability between alkoxide and carboxylate, consistent with
its position in the trans influence series described above.
Para-trisubstituted triarylamines typically show reversible elec-
trochemistry, and electron-rich triarylamines often show two
reversible oxidations, one from the neutral species to the radical
cation, and a subsequent oxidation of the radical cation to form
a quinonoid dication.38 This pattern is observed in most of
the An2NLTi complexes (e.g., Fig. 6), where two closely spaced
oxidations are observed at ~ +0.1 V (vs. Cp2Fe+/Cp2Fe) and a
second set of closely spaced oxidations are observed at ~ +0.8 V
(Table 3). The Ti(IV)/Ti(III) reduction can also be observed,
at very negative potentials (~ -2 V). These potentials, as well
as the 50–100 mV separation between the oxidation waves, are
similar to those previously observed in tripodal (N[CH2C6H3-5-
NAn2-2-O]3)TiX complexes. The bromosalan complexes show no
oxidations at potentials below 1 V.
Acknowledgements
We gratefully acknowledge the donors of the Petroleum Research
Fund, administered by the American Chemical Society, for
financial support of this work. A. J. L. thanks the U. S. National
Science Foundation (CHE-0518243) for summer support. The
sample of An2NLTi(OCMe2CO2) was submitted for synchrotron
crystallographic analysis through the SCrALS (Service Crystal-
lography at Advanced Light Source) program. Crystallographic
data were collected at Beamline 11.3.1 at the Advanced Light
Source (ALS), Lawrence Berkeley National Laboratory. The ALS
is supported by the U.S. Dept. of Energy, Office of Energy Sciences,
under contract DE-AC02-05CH11231.
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