Synthesis, crystal structure and electrochemistry of tetrahedral mono-β-diketonato titanocenyl complexes

Article abstract of DOI:10.1016/j.ica.2006.11.010

The synthesis of a variety of tetrahedral β-diketonato titanium(IV) complexes of the type [(C₅ H₅)₂ Ti (CH₃ COCHCOR)]⁺ ClO₄^- with R = CF₃, OCH₃, C₆H₅, CH₃ and Fc is described. The Ti^III/Ti^IV couples and the Fc/Fc⁺ couple exhibited chemically and electrochemically reversible cyclic voltammetric behaviour. The formal reduction potential of the Ti^III/IV couple increased as the group electronegativity of the R group of the β-diketonato ligand increased. Bulk electrolysis showed that one electron was transferred in the Ti^III/IV couple and one electron in the ferrocenyl/ferrocenium redox couple in the ligand. The crystal structure for the R = OCH₃ complex showed that this β-keto-ester binds through the carbonyl oxygen of the ester group and not the ether oxygen.

Full text of DOI:10.1016/j.ica.2006.11.010

Inorganica Chimica Acta 360 (2007) 2277–2283
www.elsevier.com/locate/ica
Synthesis, crystal structure and electrochemistry of tetrahedral
mono-b-diketonato titanocenyl complexes
*
*
Elizabeth Erasmus, Jeanet Conradie , Alfred Muller, Jannie C. Swarts
Department of Chemistry, University of the Orange Free State, Bloemfontein 9300, South Africa
Received 10 August 2006; received in revised form 5 November 2006; accepted 11 November 2006
Available online 18 November 2006
Abstract
The synthesis of a variety of tetrahedral b-diketonato titanium(IV) complexes of the type ½ðC5H5Þ TiðCH3COCHCORÞꢀþClO with
ꢁ
2
4
R = CF3, OCH3, C6H5, CH3 and Fc is described. The TiIII/TiIV couples and the Fc/Fc+ couple exhibited chemically and electrochem-
ically reversible cyclic voltammetric behaviour. The formal reduction potential of the TiIII/IV couple increased as the group electroneg-
ativity of the R group of the b-diketonato ligand increased. Bulk electrolysis showed that one electron was transferred in the TiIII/IV
couple and one electron in the ferrocenyl/ferrocenium redox couple in the ligand. The crystal structure for the R = OCH3 complex
showed that this b-keto-ester binds through the carbonyl oxygen of the ester group and not the ether oxygen.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Titanocene; b-diketonates; Cyclic voltammetry; Crystal structure
1. Introduction
C6H5 [5,6] have been reported. The coordination sphere
of these complexes does not change when switching the
redox state of the central coordinating metal fromþTiIII to
Titanocene dichloride is a convenient starting material
for the preparation of many other organometallic com-
pounds of titanium [1] and has found application as a
catalyst or catalyst component for a wide variety of hydro-
metalation and carbometalation reactions as well as other
transformations involving Grignard reagents. A key fea-
ture of this titanocene dichloride (1) is its complex aqueous
chemistry that includes aquated species such as 2 and 3 in
Scheme 1 [2].
TIIV. For the ½ðC5H5Þ TiðCH3COCHCO–OEtÞꢀ ClO
ꢁ
2
4
complex, there are conflicting structural literature reports.
Based on the IR results, some authors concluded that
bonding at the ester takes place via the keto oxygen [3],
while others concluded that bonding takes place via the
‘‘ether’’ type oxygen of the OEt group [7].
Electrochemical studies by Bond et al. on [(C5H5)2-
TiIII(CH3COCHCOR)] with R = CH3 or C6H5 show a
Cationic b-diketonato dicyclopentadienyl titanium(IV)
complexes was first prepared by Doyle and Tobias [3,4].
Although no crystal structure of titanium(IV) complexes
of the type [(C5H5)2TiIV(R1COCHCOR2)]+ is known, the
structures of the analogous titanocene(III) compounds
[(C5H5)2TiIII(R1COCHCOR2)] with R1, R2 = CH3 or
chemically and electrochemically reversible one electron
0
TiIII/IV couple at formal reduction potentials E0 = ꢁ0.86
and ꢁ0.85 V versus Fc/Fc+ in 0.2 M (nBu4N)(PF6)/butyro-
nitrile, respectively [5]. It was concluded that the formal
reduction potentials of these complexes are essentially inde-
pendent of the b-diketonato ligand when R = CH3 or
C6H5. This must be so because the group electronegativities
of the CH3 and C6H5 groups are almost the same
(vCH ¼ 2:34 and vC H ¼ 2:21) [8]. It remains unexplored
*
3
6
5
Corresponding authors. Tel.: +27 51 4012194; fax: +27 51 4446384 (J.
as to what the influence of the b-diketonato side group sub-
Conradie).
stituents would be if their group electronegativities vary
E-mail address: conradj.sci@mail.uovs.ac.za (J. Conradie).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.11.010

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E. Erasmus et al. / Inorganica Chimica Acta 360 (2007) 2277–2283
2.2.1. 1-Methoxy-1,3-butanedionato-j2O,O0-bis(g5-
cyclopentadienyl)titanium(IV)perchlorate (5)
-
2+
-
. 2ClO4
2+
. 2Cl
OH2
OH2
OH2
OH2
Cl
Cl
Ti
H2O
Ti
AgClO4
+
2AgCl
Ti
3
A solution of the aquated bis(cyclopentadienyl)tita-
nium(IV) species, 2, was prepared by stirring a solution
of titanocene dichloride (250 mg, 1.0 mmol) in water
(6 ml) for 30 min before silver perchlorate (376 mg,
1.98 mmol) dissolved in 3 ml water was added. The suspen-
sion was then stirred under nitrogen for an hour. Silver
chloride was filtered off and washed with water (2 ml).
The filtrate was cooled on an ice-bath and an excess methyl
acetoacetate (1160 mg, 10.0 mmol) was added dropwise
while stirring. The crystalline product was filtered after
standing for a day and washed with water and ether to
yield 182 mg (48%) of spectroscopically pure crystals of 5
suitable for single crystal X-ray analysis. M.p. = 230 ꢁC.
3
2
(and other aquated species)
(and other aquated species)
-
+
ClO4
O
O
O
OH
CH3
CH3
R
O
O
R
CH3
R
Ti
R= CF3,4; OCH3,5; C6H5,6; CH3,7; Fc, 8.
Scheme 1. Synthesis of ½ðC5H5Þ TiðCH3COCHCORÞꢀþClO complexes.
ꢁ
2
4
1
IR (cmꢁ1) = 1568. H NMR (d/ppm, CDCl3) 2.11 (s, 3H,
substantially. This study reports the synthesis oþf some
CH3), 3.98 (s, 3H, OCH3), 5.52 (s, 1H, CH), 6.79 (s,
10H, 2 · C5H5). Elemental Anal. Calc. for TiC15H17O7Cl:
C, 45.9; H, 4.4. Found: C, 45.9; H, 4.3%.
known and new ½ðC5H5Þ TiIVðCH3COCHCORÞꢀ ClO
ꢁ
2
4
complexes (Scheme 1) where the R group was chosen to
have widely different group electronegativities to enable
us to determine the extent of intramolecular electronic
communication through an electrochemical study with
2.2.2. 1,1,1-Trifluoro-2,4-pentanedionato-j2O,O0-bis(g5-
cyclopentadienyl)titanium(IV)perchlorate (4)
R = CF3 (4, v
¼ 3:01), OCH3 (5, v
¼ 2:64), C6H5
This compound was prepared as above but by replacing
the methyl acetoacetate with a solution of 154 mg
(1.0 mmol) trifluoroacetylacetone in 1.5 ml ice cold THF.
A yield of 69 mg (16%) was obtained after three days.
M.p. = 230 ꢁC. IR (cmꢁ1) = 1593. 1H NMR (d/ppm,
CDCl3) 2.25 (s, 3H, CH3), 5.96 (s, 1H, CH), 6.88 (s, 10H,
2 · C5H5). Elemental Anal. Calc. for TiC15H14O6ClF3: C,
41.8; H, 3.3. Found: C, 42.1; H, 3.5%.
CF3
OCH3
(6, vC H ¼ 2:21), CH3 (7, vCH ¼ 2:34) and Fc (8,
6
5
3
v
Fc = 1.87). The bonding mode of b-ketoesters to the TiIV
core is also settled from the crystal structure determination
of ½ðC5H5Þ TiIVðCH3COCHCO–OMeÞꢀþClO
.
ꢁ
2
4
2. Experimental
2.1. Materials and apparatus
2.2.3. 1-Ferrocenoyl-1,3-butanedionato-j2O,O0-bis(g5-
cyclopentadienyl)titanium(IV)perchlorate (8)
Solid reagents (Merck, Aldrich and Sigma) were used
without further purification. Liquid reactants and solvents
were distilled prior to use, water was double distilled.
CH3CN was dried over CaH2 and freshly distilled prior
to use. 1-Ferrocenylbutane-1,3-dione [9], the titanocenꢁe
complexes [3] ½ðC5H5Þ TiðCH3COCHCOC6H5ÞꢀþClO
This compound was prepared as for 5 but by replacing
the methyl acetoacetate with a solution of 210 mg
(1.0 mmol) 1-ferrocenoyl-1,3-butanedione in 4 ml ice cold
THF. A yield of 207 mg (48%) was obtained after three
1
days. M.p. = 86–89 ꢁC. IR (cmꢁ1) = 1513. H NMR (d/
2
4
(6) and ½ðC5H5Þ TiðCH3COCHCOCH3ÞꢀþClO (7) were
ppm, CDCl3) 2.25 (s, 3H, CH3), 4.41 (s, 5H, C5H5 from
Fc), 4.79 (s, 2H, C5H4 from Fc), 4.96 (s, 2H, C5H4 from
Fc), 6.22 (s, 1H, CH), 6.75 (s, 10H, 2 · C5H5). Elemental
Anal. Calc. for FeTiC24H23O6Cl: C, 52.7; H, 4.2. Found:
C, 53.5; H, 4.4%.
ꢁ
2
4
synthesized according to published procedures. Caution:
ꢁ
All ClO4 salts should be handled with precautions
against possible explosion. Melting points were determined
with a Reichert Thermopan microscope with a Kofler hot-
stage and are uncorrected. NMR measurements were
recorded on a Bruker Advance DPX 300 NMR spectrom-
eter. Chemical shifts are reported as d values relative to
SiMe4 (0 ppm). IR spectra were recorded from neat sam-
ples on a Digilab FTS 2000 Fourier transform spectrome-
ter utilizing a He–Ne laser at 632.6 nm.
2.2.4. Characterisation data for 1-phenyl-1,3-butanedionato-
j2O,O0-bis(g5-cyclopentadienyl)titanium(IV)perchlorate
(6)
Yield: 65%. M.p. = 189–194 ꢁC. IR (cmꢁ1) = 1547. H
1
NMR (d/ppm, CDCl3) 2.44 (s, 3H, CH3), 6.79 (s, 10H,
2 · C5H5), 6.80 (s, 1H, CH), 7.55–7.70 (m, 3H, C6H5),
7.92–7.98 (m, 2H, C6H5).
2.2. Synthesis
Safety note: Perchlorate complexes are potentially
explosive. We have experienced even small quantities to
explode upon heating during melting point determinations.
They should be treated with caution and handled in small
quantities.
2.2.5. Characterisation data for 2,4-Pentanedionato-j2O,O0-
bis(g5-cyclopentadienyl)titanium(IV)perchlorate (7)
Yield: 86%. M.p. > 230 ꢁC. IR (cmꢁ1) = 1543. 1H NMR
(d/ppm, CDCl3) 2.30 (s, 6H, 2 · CH3), 6.14 (s, 1H, CH),
6.73 (s, 10H, 2 · C5H5).

E. Erasmus et al. / Inorganica Chimica Acta 360 (2007) 2277–2283
2279
2.3. Electrochemistry
trode (isolated from the sample by means of a
0.1 mol dmꢁ3 (nBu4N)(PF6)/CH3CN salt bridge), a sheet
of glassy carbon working electrode submersed into a dept
that gave an electro-active area of approximately 2 cm2
and a Ag/Ag+ (0.010 mol dmꢁ3 AgNO3 in 0.10 mol dmꢁ3
(nBu4N)(PF6)/CH3CN) reference electrode mounted on a
Luggin capillary were employed. Current readings (I) and
the charge transfer (C) were recorded automatically by
using BAS 100 B/W Windows software.
Cyclic voltammetry measurements on 1.0 mmol dmꢁ3
solutions of the complexes in dry acetonitrile containing
0.100 mol dmꢁ3 tetra-n-butylammonium hexafluorophos-
phate ((nBu4N)(PF6), Fluka, electrochemical grade) as sup-
porting electrolyte were made under a blanket of purified
argon at 25.0 ꢁC utilizing a BAS 100 B/W electrochemical
workstation interfaced with a personal computer. A three
electrode cell, which utilized a Pt auxiliary electrode, a
glassy carbon (surface area 0.0707 cm2) working electrode
and a Ag/Ag+ (0.010 mol dmꢁ3 AgNO3 in CH3CN) refer-
ence electrode [10] mounted on a Luggin capillary was used
[11,12]. In this work all cited potentials are referenced
2.4. X-ray crystal structure determination for 5
A dark red regular fragment was used for the X-ray
crystallographic analysis of compound 5 (Table 1). The ini-
tial unit cell and data collection were achieved by the APEX2
software [14] utilizing COSMO [15] for optimum collection of
more than a hemisphere of reciprocal space. The frames
were integrated using a narrow-frame integration algo-
rithm and reduced with the Bruker SAINT-PLUS [16] and
XPREP [16] software packages, respectively. Data were cor-
rected for absorption effects using the multi-scan technique
SADABS [17]. The structure was solved by the direct methods
package SIR97 [18] and refined using the WINGX software
package [19] incorporating SHELXL [20]. The molecular plot
was drawn using the DIAMOND program [21] with a 50%
thermal envelope probability for non-hydrogen atoms.
against the Fc/Fc+ couple as suggested by IUPAC [13].
0
Ferrocene exhibited E0 = 77 mV versus Ag/Ag+, ipc/
ipa = 0.98, DEp = 74 mV under our experimental condi-
tions [8]. Successive experiments under the same experi-
mental conditions showed that all formal reduction
potentials were reproducible within 5 mV. All temperatures
were kept constant to within 0.5 ꢁC.
Bulk electrolysis was carried out utilizing a BAS 100 B/
W voltammograph at 25.0 ꢁC in 2.5 cm3 acetonitrile. A
three electrode cell, which utilized a Pt wire auxiliary elec-
Table 1
Crystal data and structure refinement for ½ðC5H5Þ2TiðCH3COCHCO-
ðOCH3ÞÞꢀþClO (5)
ꢁ
3. Results and discussion
4
Empirical formula
Formula weight
Temperature (K)
C15H17ClO7Ti
392.64
173(2)
3.1. Synthesis of complexes
˚
The complexes ½ðC5H5Þ TiðCH3COCHCOCF3ÞꢀþClO
Wavelength (A)
0.71073
monoclinic
P21/c
ꢁ
2
4
Crystal system
Space group
Unit cell dimensions
½ðC5H5Þ TiðCH3COCHCO–OMeÞꢀþClO
(5),
ꢁ
(4),
2
4
½ðC5H5Þ TiðCH3COCHCOC6H5ÞꢀþClO (6), ½ðC5H5Þ Ti-
ꢁ
2
2
4
ðCH3COCHCOCH3ÞꢀþClO
(7) and ½ðC5H5Þ2TiðCH3-
ꢁ
˚
a (A)
7.5326(2)
20.8283(5)
10.7927(3)
90
103.839(2)
90
1644.13(7)
4
4
COCHCOFcÞꢀþClO4 (8) were obtained by adding either
an excess of the b-diketone (in the case of 5 or 7) or a
THF solution of a stoichiometric amount of the b-dike-
tone dissolved in THF to an aqueous solution of the
aquated bis(cyclopentadienyl)titanium(IV) species (3),
Scheme 1. Coordination of the b-diketonate ligand to
the titanium centre is promoted by the fact that H2O
is a much poorer coordination group than Clꢁ. Removal
of the Clꢁ ion by AgCl precipitation drives chloride dis-
placement from the titanium(IV) centre with water mol-
ecules to the maximum yield of 3. Compounds 5 and 7
precipitated immediately upon b-diketone addition, while
precipitants of 4, 6 and 8 slowly formed from the
reaction mixtures at the bottom of the flask at room
temperature after more than 24 h. If the AgClO4 is
added in excess, the yield of the reaction decreases dra-
matically probably due to side reactions catalyzed by
the excess of AgClO4. The much lower yield of complex
4 may be described to the fact that the electron with-
drawing capability of the CF3 groups, makes the b-dike-
tone a poorer electrophile making complex formation
more difficult. Another contributing factor may be the
˚
ꢁ
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
Crystal size (mm)
h Range for data collection (ꢁ)
1.586
0.717
808
)
0.33 · 0.22 · 0.19
2.18–28.35
ꢁ10 6 h 6 10, ꢁ27 6 k 6 25,
ꢁ14 6 l 6 14
30,323
4091 [0.0295]
99.8
0.8757 and 0.7977
Index ranges
Reflections collected
Independent reflections [Rint
Completeness to h = 28.35ꢁ (%)
Maximum and minimum
transmission
]
Refinement method
Full-matrix least-squares on F2
4091/0/219
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
1.050
R1 = 0.0379, wR2 = 0.0966
R1 = 0.0441, wR2 = 0.1017
Largest difference in peak and hole 0.763 and ꢁ0.404
ꢁ3
˚
(e A
)

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E. Erasmus et al. / Inorganica Chimica Acta 360 (2007) 2277–2283
titanium–fluorine single bond energy of 581 kJ molꢁ1
that is higher than that of the titanium–oxygen single
bond (478 kJ molꢁ1) [22] favoring Ti–F bond formation,
leading to side products and a lower yield of 4.
The synthesized titanocenyl b-diketonato salts are all
insoluble in pure water, hexane and ether, slightly soluble
in organic solvents such as chloroform, dichloromethane,
and acetone and decomposes in ethanol. No decomposition
was detected in samples that were stored for up to two
years. Solutions in pure and dry acetone or acetonitrile
were stable for at least 48 h.
The cation of 5 features a distorted tetrahedral coordina-
tion around the Ti atom. The two planar Cp (cyclopenta-
dienyl) rings form an angle of 48.9ꢁ between them. The
IV
˚
average Ti –C bond length of 2.362 A is not significantly
shorter than the equivalent bond length in the TiIII com-
plexes. [(C5H5)2TiIII(CH3COCHCOCH3)] showed this
average bond length to be 2.374 A, [(C5H5)2TiIII-
˚
˚
(C6H5COCHCOC6H5)] showed it to be 2.383 A and for
[(C5H5)2TiIII(CH3COCHCOC6H5)] it was 2.363 A. The
˚
O–Ti–O bond angle of 86.29(7)ꢁ for 5 is 2–4ꢁ larger than
that in the [(C5H5)2TiIII(b-diketonato)] complexes.
The b-diketonato ligand binds unsymmetrically to the
titanium(IV) core with titanium–oxygen bond lengths of
3.2. X-ray structure of 5
˚
1.944(2) and 2.009(2) A, respectively (see Table 2). These
˚
For the keto ethyl ester complex analogous to 5,
[(C5H5)2Ti(CH3COCHCO–OEt)]+, coordination to the
metal via the ‘‘ether’’-type oxygen of the OC2H5 group
was suggested [4]. In the crystal structure determination
of 5, it was found that ½ðC5H5Þ2TiðCH3CO-
bond lengths are 0.12–0.14 A shorter than the Ti–O dis-
tances in the similar [(C5H5)2TiIII(b-diketonato)] complexes
(2.068(5) and 2.068(5) A in [(C5H5)2TiIII(CH3COCHC-
˚
OCH3)] [5], 2.067 and 2.087 A in [(C5H5)2TiIII-
˚
˚
(C6H5COCHCOC6H5)] [6] and 2.078 and 2.085 A in
CHCO–OMeÞꢀþClO4 coordinates through the carbonyl
[(C5H5)2TiIII(CH3COCHCOC6H5)] [6]). This indicates that
the more electron deficient Ti(IV) centre binds oxygen
stronger than Ti(III). The longer Ti–O bond length trans
to the OCH3 group is consistent with the fact that the O
of C@O nearest to the more electronegative OCH3 group
of the chelate ring (vOCH ¼ 2:64, vCH ¼ 2:21) has the
ꢁ
oxygen of the ester group and not through the ether oxygen
of the (CO)OMe group. This striking result conclusively
proves the interpretations of White [21] who from the IR
and NMR spectral data concluded that [(C5H5)2Ti-
(CH3COCHCO–OEt)]+ must be coordinated through the
carbonyl ester oxygen.
3
3
smallest trans influence [23]. This is in agreement with the
polarization theory, since the oxygen nearest to the more
electronegative group (OCH3) will be less polarizable as a
result of the electron attracting power of the OCH3 group
[24].
A perspective view of 5 showing atom labeling is pre-
sented in Fig. 1. Crystal data and details of data collection
and refinement are given in Table 1. Selected bond lengths
and angles for the molecule are listed in Table 2. The asym-
metric unit contains one cationic complex [(C5H5)2TiIV-
Normal single C–C bond lengths are typically 1.54 A,
˚
(CH3COCHCO(OCH3))]+ and one counter anion ClO4
.
while typical C@C double bonds have a length of
ꢁ
˚
1.337 A [25]. The large double bond character of the C–C
bonds in the chelate ring of the b-diketonato ligand of 5,
˚
˚
1.401(3) A, for C13–C14 and 1.354(3) A for C12–C13, as well
˚
as the similar C–O bond lengths (1.277(2) and 1.282(2) A)
emphasise effective electron delocalisation in the pseudo
aromatic core of the b-diketonato ligand. The fact that
delocalisation enhances electronic communication between
R-groups of the b-diketonato ligand and the Ti centre will
be discussed in the electrochemical section below.
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) for ½ðC5H5Þ2TiðCH3COCHCO-
ðOCH3ÞÞꢀþClO (5)
ꢁ
4
Ti–O1
Ti–O2
1.9435(15)
2.0094(15)
1.282(2)
1.277(2)
1.315(2)
1.462(3)
1.495(3)
1.354(3)
1.401(3)
O1–Ti–O2
C12–O1–Ti
C14–O2–Ti
86.29(7)
133.16(14)
127.80(13)
117.13(17)
122.26(18)
115.69(18)
122.02(18)
124.48(19)
117.62(18)
125.19(18)
117.18(18)
O1–C12
O2–C14
O3–C14
O3–C15
C11–C12
C12–C13
C13–C14
Ct1–Ti–Ct2a
C14–O3–C15
O1–C12–C13
O1–C12–C11
C13–C12–C11
C12–C13–C14
O2–C14–O3
O2–C14–C13
O3–C14–C13
133.74
Fig. 1. A perspective view of ½ðC5H5Þ TiðCH3COCHCOðOCH3ÞÞꢀþClO
ꢁ
2
4
a
(5) showing atom labelling. The ellipsoids are drawn at a 50% probability
Ct1 = centroid of plane defined by C1–C5, Ct2 = centroid of plane
defined by C6–C10.
displacement level.

E. Erasmus et al. / Inorganica Chimica Acta 360 (2007) 2277–2283
2281
3.3. Group electronegativity of OMe
of R = OCH3 as 2.64 on the Gordy scale. This value corre-
sponds reasonably well with vOCH ¼ 2:68 as calculated by
3
It has previously been shown that accurate apparent
group electronegativities, vR, of R for esters of the type
R(C@O)(OCH3) can be obtained from a linear fit between
vR and carbonyl IR stretching frequency [8,9]. The straight
line generated by the fit of vR and m(C@O) (Fig. 2) fits the
equation m(C@O)R = 74.5vR + 1560.5 and was used to
determine the effective or apparent group electronegativity
Huheey [26] using compounds of the type R1R2C@O. The
effective vOCH value is not the same for all complexes. Util-
3
ising PO bond vibrations in compounds of the type
R1R2R3P=O, v
was reported to be 2.89 [27].
OCH3
3.4. Electrochemistry
All the complexes (4–8) exhibit a characteristic one elec-
tron electrochemically reversible TiIII/TiIV couple (see Figs.
3
and 4) with DE < 84 mV. The ½ðC5H5Þ2TiðCH3-
1800
1750
1700
1650
0
COCHCOFcÞꢀþClO4 complex exhibits an additional
reversible ferrocenyl ligand redox couple with
DE = 77 mV. Peak current ratios (ipc/ipa for the titanium
and ipa/ipc for the ferrocenyl couple) for both redox pro-
cesses is nearly 1. Plots of peak current versus m0.5 (m = scan
rate) for all redox processes was linear for all complexes.
This result is demonstrated for compound 8 in the inset
of Fig. 4 and indicated that the electrochemistry was essen-
tially diffusion controlled. The electrochemical scheme
associated with the ferrocenyl complex is
ꢁ
CF3
Cl
-200
-400
-600
-800
-1000
OCH3
CCl3
CHCl2
CH2Cl
CH2Br
CH3
C6H5
H
vs
CF3
Fc
E
C6H5
OCH3
CH3
Fc
[(C5H5)2TiIII(CH3COCHCOFc)]
1.5
2
2.5
3
3.5
R / Gordy scale
χ
+e- -e-
Eo' = -903 mV
Fig. 2. Left axis: The linear relationship between Gordy scale group
electronegativity of an R group, vR, and carbonyl stretching frequencies of
the indicated esters, RC(O)OMe. Calibration data from Refs. [8,9] allowed
[(C5H5)2TiIV(CH3COCHCOFc)]+
the determination of vOMe from m(C@O) = 1757 cmꢁ1 for H3COC(O)OMe.
+e-
0
-e-
Eo' = 296 mV
Right axis: The relationship between the formal reduction potentialþ(E0 ) of
the TiIII/IV couple of the complexes ½ðC5H5Þ TiðCH3COCHCORÞꢀ ClO
ꢁ
2
4
and vR of the R groups on the b-diketonato ligands.
[(C5H5)2TiIV(CH3COCHCOFc+)]2+
200
R = CF3 (4)
40
70
50
30
Free Fc
150
20
10
0
R = OCH3 (5)
ferrocenyl
fragment
oxidation
Free Fc
R = C6H5 (6)
R = CH3 (7)
R = Fc (8)
0
5
10 15 20
100
50
(scan rate)1/2 / (mV/s-1)1/2
30
Free Hfca
10
-10
-30
-50
ferrocenium
fragment
reduction
0
TiIII/TiIV
-1500 -1000 -500
0
500
1000
-50
-1300
Potential / mV
-800
-300
200
700
Fig. 4. Cyclic voltammograms of a 1 mmol dmꢁ3 solution of ½ðC5H5Þ Ti-
Potential / mV vs Fc/Fc+
2
ðCH3COCH2COFcÞꢀþClO (8), in 0.1 mol dmꢁ3 (nBu4N)(PF6)/CH3CN
ꢁ
4
Fig. 3. Cyclic voltammograms of 1.0 mmol dmꢁ3 solutions of
containing free ferrocene as internal standard at scan rates of 50, 100, 150,
200 and 250 mV sꢁ1 on a glassy carbon electrode at 25.0 ꢁC vs. Fc/Fc+.
Inset: The relationship between Ti anodic peak currents and (scan rate)1/2
obeys the Randles–Sevcik equation [28] for 8. Graph slope = 2.14 lA
½ðC5H5Þ TiðCH3COCHCORÞꢀþClO
, free ferrocene (Fc) and free
ꢁ
2
4
CH3COCH2COFc in CH3CN containing 0.1 mol dmꢁ3 (nBu4N)(PF6) as
supporting electrolyte on a glassy carbon working electrode at 25.0 ꢁC and
a scan rate of 100 mV sꢁ1
.
(mVs
)
ꢁ1 ꢁ1/2. Scans were initiated at ꢁ200 mV in the positive direction.

2282
E. Erasmus et al. / Inorganica Chimica Acta 360 (2007) 2277–2283
Table 3
Cyclic voltammetry data (potentials vs. Fc/Fc+) of 1.0 mmol dmꢁ3 CH3CN solutions of the indicated compounds in 0.1 mol dmꢁ3 (nBu4N)(PF6)/CH3CN
measured on a glassy carbon working electrode at a scan rate of 100 mV sꢁ1
0
a
Complex
vR (Gordy scale)
vCO (cmꢁ1
)
Epa (mV)
DEp (mV)
E0 (mV)
ipc (lA)
ipc/ipa
4, R = CF3
5, R = OCH3
6, R = C6H5
7, R = CH3
3.01
2.64b
2.21
2.34
1.87
1593
1561
1547
1543
1513
ꢁ596
ꢁ766
ꢁ820
ꢁ822
ꢁ864
334c
78
78
78
84
78
77
79
74
ꢁ635
ꢁ805
ꢁ859
ꢁ864
ꢁ903
296c
239c
0c
20.02
20.08
23.66
19.11
21.49
21.53e
24.09e
26.28e
0.94
0.84
0.81
0.96
0.88
0.96d
0.97d
0.98d
8, R = Fc
Ferrocenyl fragment from 8
Free CH3COCH2COFc
Free ferrocene
278c
37c
Data tabulated are of the TiIII/IV couple unless stated otherwise.
a
Values from [8,9].
Result from this study.
Potentials for the ferrocenyl/ferrocenium couple.
ipa/ipc value.
b
c
d
e
ipa value.
Electrochemical data of the mono-b-diketonato titanium
A peculiarity of the present titanium system was
observed when complex (8) was studied. Initial studies
showed that peak currents for the reversible TiIII/IV could
be 4–5 times less than for the ferrocenyl/ferrocenium cou-
ple. To come up with comparable peak currents for these
two couples, extreme electrode care had to be resorted to.
A very well polished electrode would give the same peak
currents for the titanium and ferrocenyl couples only for
the first scan at, e.g. 100 mV sꢁ1 scan rate. To get the same
current for the ferrocenyl and titanium couples for the
ensuing 200 mV sꢁ1 and other scans, the electrode had to
be polished between successive scans.
complexes 4–8 are summarized in Table 3. From Table 3
0
and Fig. 2, it can be seen that the E0 for the TiIII/IV couple
decreases with decreasing vR value of the b-diketonato side
groups. This is expected, because the more electron
donating the R group becomes, the more negative the
titanium(IV) centre will become and accordingly the easier
it will be reduced. Even though the relationship between
vR and the formal reduction potential of the titanium
centre is not linear, Fig. 2, it still demonstrates good
electronic communication between the titanium centre
and each of the pendent R-groups to the b-diketonato
ligand.
The ferrocenyl group of the (CH3COCHCOFc)ꢁ ligand
was found to be electroactive at potentials 57 mV higher
in the titanium complex than for the free b-diketone. This
result clearly showed that the electropositive Ti centre with-
draws electron density from the b-diketonato ligand. In
another study focussing on [RhI(CH3COCHCOFc)(CO)2],
the ferrocenyl couple was found to be shifted to 190 mV ver-
sus Fc/Fc+. This is 49 mV lower compared to the free
ligand, emphasising the difference between the electron
withdrawing properties of the electropositive TiIV centre
over the electron donating properties of the electron rich
Rh(I) centre.
1.6
1.0
Fc/Fc+
TiIV/TiIII
1.2
0.8
0.4
0.0
0.5
0.0
I
I
0
2000
4000
6000
A one electron transfer process for both the TiIII/IV cou-
ple and the ferrocene/ferrocenyl couple of 8 was confirmed
by bulk electrolysis for both the reduction of Ti(IV) at an
applied potential of ꢁ1280 mV versus Fc/Fc+ and for the
oxidation of the ferrocenyl fragment at an applied potential
of 520 mV versus Fc/Fc+ (see Fig. 5 and Table 4).
Relative time / s
Fig. 5. Coulometry experiments on ½ðC5H5Þ TiðCH3COCH2COFcÞꢀþ-
2
ꢁ
ClO4 (8) indicate that the overall amount of electrons transferred per
molecule during the oxidation of the ferrocenyl group and the reduction of
TiIV in 8 is n = Q/NF = 1 (left axis). The current–time response is shown
on the right axis.
Table 4
Bulk electrolysis results for the complex ½ðC5H5Þ TiðCH3COCH2COFcÞꢀþClO (8)
ꢁ
2
4
Couple
Mass (mg)
105 N (mol)
Q (C)
Q/NF
n
Eexp (mV) vs. Fc/Fc+
TiIII/TiIV
Fc/Fc+
8.5
8.5
1.56
1.56
1.497
1.511
0.99
1.00
1
1
ꢁ1280
520
Electrolysis was preformed on N mol of the complex in 0.1 mol dmꢁ3 (nBu4N)(PF6)/CH3CN at 25.0 ꢁC, using a carbon sheet working electrode. Q = total
charge collected and n = number of electrons transferred per molecule. Eexp indicates the potential at which the oxidation of the ferrocenyl ligand or the
reduction of the Ti in 8 was performed.

E. Erasmus et al. / Inorganica Chimica Acta 360 (2007) 2277–2283
2283
The formal reduction potentials obtained for (6,
R = C6H5) and (7, R = CH3) during this study, ꢁ0.859
and ꢁ0.864 V, respectively, is mutually consistent with
data published by Bond and co-workers, namely ꢁ0.85
and ꢁ0.86 V versus Fc/Fc+, respectively [5]. We can now,
however, ammend the conclusion drawn by these research-
ers. The TiIII/IV couple will only be independent of the side
groups of the b-diketonato ligand in complexes of the type
[(C5H5)2Ti(RCOCHCOR0)] if the group electronegativities
of the R and R0 side groups are very close to each other.
However, if they differ substantially as was the case in this
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2006.
11.010.
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CF3
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4. Conclusions
New titanocene complexes of the type ½ðC5H5Þ2Ti-
ðCH3COCHCORÞꢀþClO with R = CF3, OMe and Fc
ꢁ
4
were synthesised and characterised. A single crystal X-ray
determination of the structure of [(C5H5)2Ti(CH3COC-
HCO–OMe)]+ClO4 indicates that the b-ketoester ligand is
bonded to the Ti centre via the carbonyl oxygen and not
the ‘‘ether’’-type oxygen of the ester group. This implies
that the bonding mode of the b-ketoester is the same as
that for b-diketonato ligands. Electrochemical experiments
on the [(C5H5)2Ti(CH3COCHCOR)]+ complexes revealed
a chemically and electrochemically reversible one electron
TiIII/IV couple at a potential dependent on the apparent
group electronegativity of the R group of the b-diketonato
ligand.
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Financial assistance by the South African National Re-
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gratefully acknowledged. We acknowledge A. Kuhn for
help in providing crystals suitable for X-ray diffraction of
complex 5.
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Appendix A. Supplementary material
CCDC 626381 contains the supplementary crystallo-
graphic data for 5. These data can be obtained free of
charge
via
http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data