Substitution reactions of dichlorobis(betadiketonato-O,O′) titanium(IV) complexes with aryl diolato ligands: An experimental and computational study

Article abstract of DOI:10.1016/j.poly.2013.09.004

Substitution reactions of complexes Ti(β-diketonato) ₂Cl₂, in which the two monodentate Cl^- ligands are replaced by different bidentate aryl-diolato ligands (catechol, naphthol, biphenol, binaphthol, or methylenebinaphthol), produce remarkably hydrolytically stable titanium(IV) complexes with 5-, 7- or 8-membered chelating rings. The lability of the chlorine ligand in the parent compound is dependent on the strength of the electron donating properties of the spectator β-diketonato ligand around the titanium centre. The reactivity of Ti(RCOCHCOR′) ₂Cl₂ complexes according to the β-diketonato ligand (RCOCHCOR′)^- follows the order (PhCOCHCOPh)^- > (PhCOCHCOCH₃)^- > (CH₃COCHCOCH ₃)^-, in line with results for other known transition metal complexes. Detailed substitution kinetics along with the X-ray crystal structure of a mono-chloride Ti(β-diketonato)₂(Cl)(naphthol) reaction intermediate are reported. DFT calculations on the reaction of Ti(acac)₂Cl₂ with different aryl-diolato ligands reveal that chlorine substitution proceeds via a two-step interchange mechanism with the formation of two seven-coordinated transition states and one six-coordinated intermediate. The computed mechanism agrees very well with experimental kinetic and X-ray data. . 2013 Elsevier Ltd. All rights reserved.

Full text of DOI:10.1016/j.poly.2013.09.004

Polyhedron 67 (2014) 231–241
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Substitution reactions of dichlorobis(betadiketonato-O,O0)
titanium(IV) complexes with aryl diolato ligands: An
experimental and computational study
Kathrin H. Hopmann a,b, Annemarie Kuhn c, Jeanet Conradie a,c,
⇑
a Center for Theoretical and Computational Chemistry (CTCC), University of Tromsø, N-9037 Tromsø, Norway
b Department of Chemistry, University of Tromsø, N-9037 Tromsø, Norway
c Department of Chemistry, University of the Free State, 9300 Bloemfontein, South Africa
a r t i c l e i n f o
a b s t r a c t
Substitution reactions of complexes Ti(b-diketonato)2Cl2, in which the two monodentate Clꢀ ligands are
replaced by different bidentate aryl-diolato ligands (catechol, naphthol, biphenol, binaphthol, or methy-
lenebinaphthol), produce remarkably hydrolytically stable titanium(IV) complexes with 5-, 7- or 8-mem-
bered chelating rings. The lability of the chlorine ligand in the parent compound is dependent on the
strength of the electron donating properties of the spectator b-diketonato ligand around the titanium
centre. The reactivity of Ti(RCOCHCOR0)2Cl2 complexes according to the b-diketonato ligand (RCOCH-
COR0)ꢀ follows the order (PhCOCHCOPh)ꢀ > (PhCOCHCOCH3)ꢀ > (CH3COCHCOCH3)ꢀ, in line with results
for other known transition metal complexes. Detailed substitution kinetics along with the X-ray crystal
structure of a mono-chloride Ti(b-diketonato)2(Cl)(naphthol) reaction intermediate are reported. DFT cal-
culations on the reaction of Ti(acac)2Cl2 with different aryl-diolato ligands reveal that chlorine substitu-
tion proceeds via a two-step interchange mechanism with the formation of two seven-coordinated
transition states and one six-coordinated intermediate. The computed mechanism agrees very well with
experimental kinetic and X-ray data.
Article history:
Received 19 June 2013
Accepted 11 September 2013
Available online 23 September 2013
Keywords:
Titanium
Substitution
Kinetics
DFT
Mechanism
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The Ti(b-diketonato)2Cl2 complex and related compounds are
good models for studying ligand substitution because only the
The mechanistic behaviour of titanium(IV) complexes is of fun-
damental interest because of their wide spread use as catalysts in
different organic reactions [1] and in medical applications in anti-
cancer therapy [2]. The origin of the tremendous catalytic activity
of titanium(IV) catalysts is the high Lewis acidity or electron defi-
ciency, which can be easily tuned by variations on the electronic
properties of the ligands, but the down fall is its high sensitivity
to hydrolysis. Titanium alkoxy systems, for example, are effective
catalysts in a variety of processes, but the sensitivity to hydrolysis
results in some Ti–OR bond cleavage when exposed to water that is
produced as a by-product in the reaction (esterification reactions
[3]). Studies have shown that the resistance towards hydrolysis
can be increased by introducing bulky3 or chelating electron-rich
oxygen-based ligands [4]. A very successful series of enantioselec-
tive catalysts, based on chelating binaphtholate ligand (BINOL), is
widely employed [5].
chloride coordinated ligand is labile. The bidentate b-diketonato
ligand enhances the stability of the 12-electron mononuclear
cis-Ti(b-diketonato)2Cl2 complex through ligand p ? metal d
p
p
donation. Each b-diketonato ligand has one
interacts with an empty Ti d-orbital in this
p
-donor orbital which
p-electron bonding [6].
In this paper we have carried out kinetic and computational
studies in order to extend our understanding of the mechanistic
behaviour of titanium in substitution reactions. We describe the
synthesis of a remarkably hydrolytically stable series of complexes
containing three-electron-rich oxygen-based bidentate ligands:
bis(acetylacetonato)-aryl-diolato-titanium(IV) complexes. The
aryl-diolato-ligands form 5-, 7- and 8-membered chelating rings.
Two complexes, the complex with 2,3-dihydroxynaphthalene,
and an intermediate containing 2,3-dihydroxynaphthalene, are re-
ported here for the first time. For the later, we report a crystal
structure that corresponds to the mono-chloride intermediate of
the substitution reaction. Computational studies on the formation
of the a 5-, 7- or 8-membered Ti(acac)2(aryl-diolato) complex pro-
vide insight into the mechanistic details of the substitution
reactions.
⇑
Corresponding author at: Department of Chemistry, University of the Free State,
9300 Bloemfontein, South Africa. Tel.: +27 51 4012194; fax: +27 51 4446384.
E-mail address: conradj@ufs.ac.za (J. Conradie).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.09.004

232
K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
methane (3 ml), dichlorobis(acetylacetonato-O,O0)titanium(IV),
2. Experimental section
2.1. Materials and apparatus
Ti(acac)2Cl2, (0.431 g/1 mmol) in dichloromethane (2 ml) was syr-
inged dropwise at room temperature with an immediate colour
change. The reaction mixture instantly turned dark red and the
dihydroxy-aryl ligand dissolved on shaking. After standing 5 h at
room temperature, the reaction mixture was layered with hexane
(7 ml) and allowed to stand for 2 days. The precipitate was isolated
by filtration and washed with hexane and recrystallized from
dichloromethane/n-hexane.
Solid reagents used in preparations (Merck, Aldrich and Fluka)
were used without further purification. Liquid reactants and sol-
vents were distilled prior to use; water was doubly distilled. Or-
ganic solvents were dried according to published methods [7].
2.2. Synthesis
2.2.2.1. Ti(acac)2cat 2. Yield 12% (0.043). M.p. > 200 °C. Colour: red/
brown. 1H NMR (300 MHz, d/ppm, CDCl3): 2.20 (s, 12H, 4ꢁ CH3),
5.90 (s, 2H, 2ꢁ CH), 6.28 (m, 2H, catH), 6.65 (m, 2H, catH). Elemen-
tal Anal. Calc. for TiC16H18O6: C, 61.2; H, 5.8. Found: C, 61.0; H, 5.9%.
All operations were carried out under anhydrous conditions in a
dry N2 atmosphere. All glassware was flame-dried and was allowed
to cool in a stream of dry N2. The N2 atmosphere was maintained
throughout the reaction. Filtrations and recrystallisations were
performed under a blanket of N2. All solvents were dried and dis-
tilled under N2 immediately before use. Complex numbering is
according to Scheme 1.
2.2.2.2. Ti(acac)2binaph 5. Yield 60% (0.311). M.p. > 200 °C. Colour:
red/brown. 1H NMR (600 MHz, d/ppm, CDCl3): 2.00 (sbr, 12H, 4ꢁ
CH3), 5.77 (s, 2H, 2ꢁ CH), 7.12 (t, 2H, binaphH), 7.25 (t, 2H, bi-
naphH), 7.27 (d, 2H, binaphH), 7.34 (d, 2H, binaphH), 7.83 (d, 4H,
binaphH). Elemental Anal. Calc. for TiC30H26O6: C, 73.5; H, 5.3.
Found: C, 73.7; H, 5.4%.
2.2.1. Dichlorobis(b-diketonato-O,O0)titanium(IV), 1a–1c
(Ti(b-diketonato)2Cl2)
The Ti(b-diketonato)2Cl2 complexes were synthesized according
to published methods [8,9] as described for Ti(acac)2Cl2 1a, Ti(ba)2-
Cl2 1b and Ti(dbm)2Cl2 1c, with acac = acetylacetonato = (CH3-
2.2.3. Bis(acetylacetonato-O,O0)(2,3-naphthalenediolato-
O,O0)titanium(IV), 3 (Ti(acac)2naph)
COCHCOCH3)ꢀ,
ba = benzoylacetonato = (PhCOCHCOCH3)ꢀ,
A 10 times excess of 2,3-dihydroxynaphthalene (H2naph) was
added to Ti(acac)2(Cl)2 in dichloromethane, stirred for 2 h. After
standing 5 h at room temperature, the reaction mixture was lay-
ered with hexane (7 ml) and allowed to stand for 2 days. The pre-
cipitate was isolated by filtration and washed with hexane and
recrystallized from dichloromethane/n-hexane.
dbm = dibenzoylmethanato = (PhCOCHCOPh)ꢀ and Ph = C6H5.
2.2.2. Bis(acetylacetonato-O,O0)(1,2-benzenediolato-O,O0)titanium(IV),
2 (Ti(acac)2cat) and bis(acetylacetonato-O,O0)(2,20-binaphthyldiolato-
O,O0)titanium(IV), 5 (Ti(acac)2binaph)
To a stirred solution of 1 mmol dihydroxy-aryl ligand (either
M.p. > 200 °C. Colour: red/brown. 1H NMR (300 MHz, d/ppm,
CDCl3): 2.16 (s, 12H, 4ꢁ CH3), 5.90 (s, 2H, 2ꢁ CH), 6.64 (s, 1H,
1,2-dihydroxybenzene or 2,20 dihydroxybinaphthyl) in dichloro-
CH3
CH3
O
Ti
O
O
Ti
O
H3C
H3C
H3C
H3C
O
O
O
O
O
O
O
O
H2naph
HO
H2cat
HO
HO
[2]
CH3
CH3
[3]
HO
R'
CH3
R'
R
R
O
O
Ti
O
O
R
R
H3C
H3C
O
O
O
O
Cl
Cl
O
O
O
O
O
O
DCM / N2
-2 HCl
HO
HO
HO
HO
Ti
O
Ti
O
H2biphen
H2binaph
R'
CH3
R'
[4a-c]
[5]
[1a-c]
R'
R
R'
R
R'
HO
HO
1a CH3 CH3
1b Ph CH3
1c Ph Ph
4a CH3 CH3
4b Ph CH3
4c Ph Ph
O
R
R
O
O
Ti
O
H2mbinaph
O
O
R'
[6]
Scheme 1. Synthesis of Ti(b-diketonato)2(L) complexes with L = cat 2, naph 3, biphen 4a, binaph 5 and mbinaph 6 with H2cat = 1,2-dihydroxybenzene or catechol;
H2naph = 2,3-dihydroxynaphthalene or naphthol; H2biphen = 2,20-dihydroxybiphenyl or biphenol; H2binaph = 2,20-dihydroxybinaphthyl or binaphthol; H2mbinaph = 1,10-
dihydroxy-methylene-binaphthyl or methylenebinaphthol. b-diketones; Hacac = 2,4-pentanedione = CH3COCH2COCH3, for 1a, 2, 3, 4a,
butanedione = PhCOCH2COCH3 for 1b and 4b and Hdbm = 1,3-diphenyl-1,3-propanedione PhCOCH2COPh for 1c and 4c.
5 and 6; Hba = 1-phenyl-1,3-

K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
233
naphH), 7.18 (dd, 2H, naphH), 7.54 (dd, 2H, naphH). Elemental
(e
/dm3molꢀ1cmꢀ1 is 1830(20) for 1a and 4890(30) for 4a). The ob-
Anal. Calc. for TiC20H20O6: C, 66.0; H, 5.5. Found: C, 66.1; H, 5.5%.
served first-order rate constants were obtained from least-square
fits of absorbance vs. time data [12].
2.2.4. Bis(b-diketonato-O,O0)(2,20-biphenyldiolato-O,O0)titanium(IV),
4a–4c (Ti(b-diketonato)2(biphen))
2.5. Calculations
The Ti(b-diketonato)2(biphen) complexes were synthesized
according to published methods [10,11] as described for Ti(acac)2
(biphen) 4a, Ti(ba)2(biphen) 4b and Ti(dbm)2(biphen) 4c.
Pseudo-first-order rate constants, kobs, were calculated by
fitting kinetic data [12] to the first-order equation [13] [A]t = [A]0
e(ꢀkobs t) with [A]t and [A]0 the absorbance of the indicated species
at time t and at t = 0 (UV–Vis). The experimentally determined
pseudo first order rate constants were converted to second order
rate constants, k1 (for the first reaction step), by determining the
slope of the linear plots of kobs against the concentration of the
incoming biphenolato ligand. Non-zero intercepts implied that
kobs = k1[biphen] + ks and that the first order rate constant for a sol-
vent pathway, ks, in the proposed reaction mechanism exists. The
first order rate constant for the second reaction step will be de-
noted by k2. All kinetic mathematical fits were done utilizing the
fitting program MINSQ [12]. The error of all the data are presented
according to crystallographic conventions, for example
kobs = 0.0236(1) sꢀ1 implies kobs = (0.0236 0.0001) sꢀ1. The acti-
vation parameters were determined from the Eyring relationship
2.2.5. Bis(acetylacetonato-O,O0)(1,10-methylenebinaphthyldiolato-
O,O0)titanium(IV), 6 (Ti(acac)2 mbinaph)
To a stirred solution of 2,20-dihydroxy-methylene-binaphthyl
(0.300 g, 1 mmol) in CH3CN (15 ml), dichlorobis(acetylacetonato-
O,O0)titanium(IV), Ti(acac)2Cl2 (1.0 mmol) in CH3CN (5 ml) was syr-
inged in dropwise at room temperature with an immediate colour
change (clear/grey to red). The reaction mixture was stirred and
purged with a slow stream of N2 (to evolve the hydrogen chloride
gass) for 20 min and then refluxed for 4–6 h. The reaction mixture
was cooled to room temperature and the solvent evaporated to
dryness. The product was obtained in two ways (i) the residue
was redissolved in CH3CN and allowed to crystallise out or (ii)
the residue was recrystallized from dichloromethane/n-hexane.
Yield 62% (0.3368 g). M.p. > 200 °C. Colour: red. 1H NMR
(300 MHz, d/ppm, CDCl3): 2.07 (s, 12H, 4ꢁ CH3), 4.79, 5.16 (br,
br, 1H each, CH2), 5.80 (s, 2H, 2ꢁ CH), 7.01 (br, 2H, mbinaphH),
7.27 (br, 2H, mbinaphH), 7.40 (br, 2H, mbinaphH), 7.54 (d, 2H,
mbinaphH), 7.73 (d, H, mbinaphH), 8.36 (br, 2H, mbinaphH). Ele-
mental Anal. Calc. for TiC31H28O6: C, 73.8; H, 5.6. Found: C, 73.7;
H, 5.5%.
[13] and the activation free energy
D D D
Gà = Hà – T Sà.
2.6. Computational methods
Calculations of the reaction pathways were performed with the
B3LYP functional as implemented in the GAUSSIAN 03 package [14].
Geometries were optimized in gas phase with a triple-f basis set,
6-311G(d,p). Solvation effects were computed by performing sin-
gle-point calculations on the optimized geometries with the
IEFPCM model, using CH3CN as solvent and a dielectric constant
of 36.64. Thermochemical quantities were calculated from fre-
quency calculations at the same level of theory as optimizations.
The frequency calculations were also employed to confirm the nat-
ure of the obtained stationary points, which exhibited only positive
eigenvalues for minima and one imaginary frequency for transition
states.
2.2.6. ChloroBis(acetylacetonato-O,O0)(3-hydroxy-2-naphtholato-
O,O0)titanium(IV), 7 (Ti(acac)2(Cl)(Hnaph))
The synthetic route of 2 and 5 was followed, using 2,3-dihy-
droxynaphthalene (H2naph) as ligand. The product obtained was
a mixture of 3 and 7. A crystal of 7 suitable for X-ray studies was
isolated from the solution. The NMR of the crystal of 7 also yielded
a mixture of 3 and 7.
M.p. > 200 °C. Colour: red/brown. 1H NMR (300 MHz, d/ppm,
CDCl3): 2.01–2.31 (br, 12H, 4ꢁ CH3), 5.89 (s, 2H, 2ꢁ CH), 7.25 (s,
1H, naphH), 7.27 (t, 1H, naphH), 7.31 (t, 1H, naphH), 7.34 (s, 1H,
naphH), 7.63 (d, 1H, naphH), 7.64 (d, 1H, naphH), 7.75 (br, 1H, OH).
2.7. X-ray crystal structure determination
Crystals of Ti(acac)2(Cl)(Hnaph) 7 were obtained from recrystal-
lization in chloroform. The crystal of 7 was mounted on a glass fi-
ber and used for the X-ray crystallographic analysis. The X-ray
intensity data were measured on a Bruker X8 Apex II 4K CCD dif-
fractometer area detector system equipped with a graphite mono-
2.3. Spectroscopy and spectrophotometry
NMR measurements at 25 °C were recorded on a Bruker Avance
II 600 NMR spectrometer [1H(600.130 MHz)]. The chemical shifts
were reported relative to SiMe4 (0.00 ppm). Positive values indi-
cate downfield shift. UV–Vis spectra were recorded on a Cary 50
Probe UV–Vis spectrophotometer.
chromator and Mo K
a fine-focus sealed tube (k = 0.71073 Å)
operated at 1.5 KW power (50 kV, 30 mA). The detector was placed
at a distance of 3.75 cm from the crystal. Crystal temperature dur-
ing the data collection was kept constant at 100(2) K using an Ox-
ford 700 series cryostream cooler.
2.4. Kinetic measurements
The initial unit cell and data collection were achieved by the
APEX2 software [15] utilizing COSMO [16] for optimum collection of
more than a hemisphere of reciprocal space. A total of 1709 frames
The substitution reaction was monitored on the UV–Vis (by
monitoring the change in absorbance at the 380 nm for complex
1b and at 450 nm for 1c) spectrophotometers. All kinetic measure-
ments were monitored under pseudo-first-order conditions with
[H2biphen] 10 to 200 times the concentration of the Ti(b-diketo-
nato)2Cl2 complex in CH3CN solution. The concentration Ti(b-dike-
were collected with a 0.5° scan width in
time of 90 s frameꢀ1 was used. The frames were integrated using
a narrow-frame integration algorithm and reduced with the SAINT
u and x. An exposure
-
Plus [17] and XPREP [17] software packages respectively. The inte-
gration of the data using a triclinic cell yielded a total of 18989
reflections to a maximum h angle of 28.33°, of which 4996 were
independent with a Rint = 0.0365. Analysis of the data showed no
significant decay during the data collection. Data were corrected
for absorption effects using the multi-scan technique SADABS [18]
with minimum and maximum transmission coefficients of 0.9543
and 0.8663 respectively. The structure was solved by the direct
methods package SIR97 [19] and refined using the WINGX software
tonato)2Cl2 ffi 0.0002 mol dmꢀ3
.
Kinetic measurements, under
pseudo-first-order conditions for different concentrations of Ti(b-
diketonato)2Cl2 at a constant [H2biphen], confirmed that the con-
centration of Ti(b-diketonato)2Cl2 did not influence the value of
the observed kinetic rate constant. A linear relationship between
UV absorbance, A, and concentration, C, confirmed the validity of
the Beer Lambert law (A =
eCl with l = path length = 1 cm) for the
complexes 1a and 4a at experimental wave lengths kmax = 340 nm

234
K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
package [20] incorporating SHELXL [21]. The final anisotropic full-
matrix least-squares refinement on F2 with 258 parameters con-
verged at R1 = 0.0348 for the observed data and wR2 = 0.0871
for all data. The GOF was 1.064. The largest peak on the final differ-
ence electron density synthesis was 0.49 e Åꢀ3 at 0.67 Å from O4
and the deepest hole ꢀ0.31 e Åꢀ3 at 0.67 Å from Ti1.
since the aryl-diolato ligands are more electron-donating than Cl.
The size of the chelated aryl-diolato ring (5-, 7- and 8-membered
chelated rings) or the addition of an extra phenyl ring on the
aryl-diolato ligand has only a very slight effect on the shift of the
acac methine proton.
The aryl-diolato ring protons of the chelated complex are
shifted slightly upfield relative to the uncoordinated ligand. In con-
trast, the resonance for the chelated b-diketonato ligand is always
shifted downfield relative to the uncoordinated b-diketone (in the
studied complexes); electric field effects and pi bonding could con-
tribute to the downfield shifts [28].
The aromatic, methylene, methyl and hydroxyl H atoms were
placed in geometrically idealized positions (C/O–H = 0.84–0.98 Å)
and constrained to ride on their parent atoms with Uiso
(H) = 1.2Ueq(C) for aromatic, methylene and Uiso(H) = 1.5Ueq(C/O)
for methyl and hydroxyl. The methyl and hydroxyl H’s were
located from a Fourier difference map and refined with ideal
geometries. Non-hydrogen atoms were refined with anisotropic
displacement parameters. Atomic scattering factors were taken
from the International Tables for Crystallography Volume C [22].
The molecular plot was drawn using the DIAMOND program [23] with
a 50% thermal envelope probability for non-hydrogen atoms.
Hydrogen atoms were drawn as arbitrary sized spheres with radius
of 0.135 Å.
3.2. The Ti(b-diketonato)2Cl2 + H2biphen reaction
In the following section we discuss in detail the parent com-
plexes Ti(b-diketonato)2Cl2 (b-diketonato = acac [10], ba and
dbm, 1a–c) and the kinetics and the product complexes 4a–c of
the reaction of 1a–c with H2biphen, involving replacement of the
two monodentate Clꢀ ligands by one bidentate biphen. Under sec-
ond order conditions, the related reaction, Ti(b-diketonato)2Cl2 + -
H2naph reaction yielded a mixture of 3 and the mono-chloride
complex 7 (Ti(b-diketonato)2(Cl)(Hnaph)) as products. 7 was suc-
cessfully crystallized, see Section 3.3.
3. Results and discussion
3.1. Synthesis of bis(acetylacetonato-O,O0)(aryl diolato-O,O0) Ti(IV)
complexes
3.2.1. The reactants
The series of Ti(acac)2L complexes with L = cat 2, naph 3, biphen
4a, binaph 5, mbinaph 6, was synthesised according to Scheme 1.
The structures and names of the investigated complexes are sum-
marised in Scheme 1. The complexes 2 [24] 4a [10] 5 [25] and 6
[26] have been reported previously, while complex 3 is reported
here for the first time. The reaction, initiated by adding Ti(acac)2-
Cl2, 1a (see Section 3.2.1. for the synthesis of 1a), to the poorly dis-
solved ligand in dichloromethane, changes colour immediately to
dark red in all cases. These aryl-diolato ligands, which are slightly
soluble in dichloromethane at room temperature, dissolve rapidly
(as the reaction proceeds) on shaking. After standing 5 h at room
temperature, the mixture is layered with hexane and allowed to
stand for 2 days, before the red crystalline product is isolated by fil-
tration and washed with hexane.
The reactant complexes Ti(acac)2Cl2 1a, Ti(ba)2Cl2 1b and
Ti(dbm)2Cl2 1c were prepared by treating TiCl4 with two equiva-
lents of the appropriate b-diketone, isolated and purified by recrys-
tallisation before use [8,9]. Complexes 1a–1c are extremely
moisture and oxygen sensitive but are stable if sealed and stored
under Argon, at room temperature. If exposed to atmospheric oxy-
gen and water vapour, the complex converts to a fine, white pow-
der, titanium dioxide (TiO2), consistent with the complete
hydrolysis and decomposition, proposed by Keppler and Heim
[27]. The Ti(b-diketonato)2Cl2 complexes adopt cis configurations
with Cl in the cis position (as opposed to the trans configurations
where Cl is in the trans position). This allows for the bidentate
b-diketonato ligand to enhance the stability of the 12-electron
Ti(b-diketonato)2Cl2 complex through ligand ? metal
p-electron
Complexes 1a–1c are extremely moisture sensitive and hydro-
lyze easily forming the fine white powder TiO2 [27], whereas com-
plexes 2–6 exhibit a high hydrolytic stability (no precipitate was
observed when dissolved in 0.01% water/CH3CN within 6 weeks)
and are ‘‘air stable’’ for more than 3 years. The strong electron
donation to the titanium centre from the aryl diolato ligand’s oxy-
gens relative to the electron withdrawing Clꢀ-ions, may contribute
to the enhanced hydrolytic stability.
Complexes in this series form 5-, 7- and 8-membered chelated
rings while keeping the acac ligands constant. The chemical shifts
of the methine proton of the acac-ligand in the coordinate complex
and for the uncoordinated Hacac (included for comparison) are
listed in Table 1. The acac methine proton of Ti(acac)2L complexes
2–6 (d = 5.77–5.90 ppm) is slightly upfield shifted relative to the
parent complex Ti(acac)2Cl2 (d = 6.00 ppm) 1. This is expected
donation [6]. Complex 1b, containing unsymmetrical b-diketonato
ligands, exists as three distinct cis geometrical isomers see Fig. 1,
while 1a and 1c, with symmetrical b-diketonato ligands, exist as
a single cis conformer. This is in agreement with NMR data, force
field [29] and DFT calculations [11]. Variable-temperature NMR
data further indicated that in solution the three cis isomers of 1b
exist in a fast equilibrium with cis–trans–cis as the major isomer
[11] which is the same isomer that was characterized by single
crystal X-ray data [30]. The geometry of all of the reactant com-
plexes in the solid state are known from crystal X-ray studies,
viz. 1a [31], 1b [30] and 1c [32].
R
R
R'
H
O
H
O
H
O
R'
R
R'
O
Ti
O
O
Ti
O
O
O
O
Cl
Cl
Cl
Ti
Cl
Table 1
Cl
Cl
1H NMR chemical shifts for the methine protons of complexes 1–6.
R
R'
R
O
O
No
Compound
Ring size
1H/ppm methine H
H
H
H
R'
R
R'
–
1a
2
3
4a
5
Hacac
Ti(acac)2Cl2
Ti(acac)2cat
Ti(acac)2naph
Ti(acac)2biphen
Ti(acac)2binaph
Ti(acac)2mbinaph
–
–
5
5
7
7
8
5.50
6.00
5.90
5.90
5.79
5.77
5.82
cis-cis-trans (C2)
cis-trans-cis (C2)
cis-cis-cis (C1)
Fig. 1. The stereochemistry of cis-Ti(b-diketonato)2Cl2 complexes (position accord-
ing to Cl, R and R0): cis–cis–cis isomer (point group C1), cis–cis–trans (point group C2)
and cis–trans–cis (point group C2). The rotation axis, where applicable, is indicated
with a dotted blue line. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
6

K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
235
R
R
R'
H
O
H
O
H
O
R'
R
R'
H
O
Ti
O
O
Ti
O
O
Ti
O
O
O
O
O
O
O
O
O
O
R
R'
R
H
H
R'
R
R'
cis-cis-trans (C2)
cis-trans-cis (C2)
cis-cis-cis (C1)
Fig. 2. The stereochemistry of Ti(b-diketonato)2biphen complexes (position according to Obiphen, R and R0): cis–cis–cis isomer (point group C1), cis–cis–trans (point group C2)
and cis–trans–cis (point group C2). The rotation axis, where applicable, is indicated with a dotted blue line. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
3.2.2. The products
complexes 1a–1c are given in Table 2 and the results plotted as a
function of the incoming H2biphen concentration in Fig. 3. The
non-zero intercept of these graphs, implies that the solvent, CH3CN,
contributes to the reaction mechanism, although only in a relatively
minor way. The results obtained are thus consistent with a reaction
scheme (Scheme 2) with two kinetically distinct steps (including
also a solvent pathway), yet the exact coordination nature of the
reaction intermediate (i.e. associative, dissociative or interchange
mechanism) requires elucidation.
The bis(b-diketonato) (biphenyldiolato)titanium(IV) product
complexes Ti(acac)2(biphen) 4a, Ti(ba)2(biphen) 4b and Ti(dbm)2
(biphen) 4c were obtained by the substitution of two monodentate
Cl- ligands in Ti(b-diketonato)2Cl2 for the bidentate biphen. The
Ti(b-diketonato)2biphen complexes 4a–4c adopt one or more of
three possible cis configurations, with the bidentate biphen O
atoms in cis positions, see Fig. 2. Similar to the reactants, com-
plexes (4b) containing unsymmetrical b-diketonato ligands, exist
as three distinct cis geometrical isomers whereas 4a and 4c, with
symmetrical b-diketonato ligands, exist as a single cis conformer.
Variable temperature NMR studies and DFT calculations [11]
showed the cis–trans–cis isomer of 4b to be the main isomer in
solution, the same isomer that was isolated and characterized by
X-ray crystallography [11]. The geometries of Ti(acac)2(biphen)
4a [10] and Ti(ba)2(biphen) 4b [11] are known from single crystal
X-ray data.
The general rate law applicable to the first [H2biphen] depen-
dent step is given by Rate = kobs[Ti(b-diketonato)2Cl2] [34,35] with
the pseudo-first-order rate constant kobs = kS + k1[H2biphen] and k1
the second-order rate constant for the first step of the substitution
process, k2 the first-order rate constant for the second step of the
substitution process and kS the first order rate constant for a sol-
vent pathway.
The activation parameters, the entropy of activation,
D
Sà, and
activation enthalpy,
D
Hà, determined from a temperature depen-
dence study, is also given in Table 2. The large negative activation
entropy obtained suggests that the substitution process proceeds
via an associative or interchange mechanism. The structural
changeover from six- to seven-coordinate is possible since the 12-
electron (d0) Ti(b-diketonato)2Cl2 species is coordinatively unsatu-
rated. A 5-coordinated cationic species, [Ti(acac)2Cl]+, formed by
breaking the first Ti–Cl bond (in a dissociative mechanism), is con-
siderably less likely. Stable Ti(IV) complexes with coordination
numbers greater than six, for example, 7- and 8-coordination, are
known [36,37]. From the experimental observations, it is not possi-
ble to determine the structure of the reaction intermediate.
From the experimental rate constants, it is observed that the
reactivity of Ti(RCOCHCOR0)2Cl2 complexes towards ligand substi-
tution follows the order Ti(PhCOCHCOPh)2Cl2 > Ti(PhCOCHC-
OCH3)2Cl2 > Ti(CH3COCHCOCH3)2Cl2. This is the same trend of
reactivity reported for various transition metal b-diketonato com-
plexes containing the same b-diketonato ligands [38].
3.2.3. Experimental substitution kinetics
The substitution of the two monodentate Clꢀ ligands for biden-
tate H2biphen in Ti(b-diketonato)2Cl2 complexes 1a–1c, was fol-
lowed on the UV–Vis spectrophotometer. Two reaction steps were
observed, a relatively fast first step (with a half-life of 1–20 min
depending on the reactant and [H2biphen]) and a second step c.a.
10 times slower than the first reaction step. The rate constants of
the two steps were determined independently. The results obtained
are in agreement with those obtained by Burgess et al. [33]; each
step involves the replacement of a chloride ion, with rate-limiting
ring closure occurring in the second step. Since the reaction was fol-
lowed under pseudo first order conditions with H2biphen in excess,
first-order kinetics in [H2biphen] was observed for the first step, the
first reaction being second order overall with a rate constant k1 -
= kobs/[H2biphen]. The second step was [H2biphen] independent,
i.e. first order with rate constant k2. Rate constants for the first
(k1) and second (k2) stages of substitution of Ti(b-diketonato)2Cl2
Table 2
Kinetic data and activation parameters for the substitution of biphen for the two monodentate Clꢀ ligands Ti(b-diketonato)2Cl2 in CH3CN as solvent, and the ADF/PW91 calculated
HOMO energy (EHOMO) of 1a–1c.
T (°C)
k1 (dm3molꢀ1sꢀ1
)
kS (sꢀ1
)
k2 (sꢀ1
)
D
Hà (kJ molꢀ1
)
D
Sà (J molꢀ1 Kꢀ1
)
D
Gà (kJ molꢀ1
)
Reference
[10]
Ti(acac)2Cl2 1a
15.0
25.0
36.2
0.062(2)
0.153(4)
0.365(5)
0.00012(3)
0.00461(7)
0.0133(1)
0.0015(2)
0.0009(2)
0.0003(1)
59.6
ꢀ60.8
77.7
Ti(ba)2Cl2 1b
10.0
25.0
This study
This study
0.141(4)
0.331(9)
0.691(2)
0.0004(1)
0.0028(3)
0.0082(3)
36.0(6)
52.6(8)
ꢀ133(2)
ꢀ79(1)
76(1)
76(1)
40.0
Ti(dbm)2Cl2 1c
9.5
25.5
40.0
0.1439(6)
0.485(9)
1.35(4)
0.00055(9)
0.0029(2)
0.0097(5)

236
K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
Fig. 3. Temperature and H2biphen concentration dependence of the first reaction step of the substitution of biphen for the two monodentate Clꢀ ligands in Ti(b-
diketonato)2Cl2 in CH3CN, b-diketonato = ba = (PhCOCHCOCH3)ꢀ (left) and dbm = (PhCOCHCOPh)ꢀ (right). [Ti complex] = 0.1 mmol dmꢀ3. Inset: Linear dependence between
ln(k1/T) and 1/T, as predicted by the Eyring equation (k1 is the second order rate constant for the first reaction step).
Table 3
k2, -HCl
k-2
Ti(β-diketonato)2 Cl2
reaction
k1, -HCl
k-1
Ti(β-diketonato)2 biphen
intermediate
+ H2 biphen
Hydrogen bonds for 7 [Å and °].
ks
D–HÁ Á ÁA
O(6)–H(6)Á Á ÁO(4)
d(D–H)
d(HÁ Á ÁA)
d(DÁ Á ÁA)
<(DHA)
-HCl
+ CH3CN
0.84
0.98
0.95
0.95
0.95
0.95
0.98
0.98
0.98
1.95
2.85
2.86
2.78
2.39
2.97
2.82
2.65
2.51
2.7828(16)
3.8142(18)
3.7425(17)
3.6688(16)
3.310(2)
3.524(2)
3.595(2)
3.555(2)
3.458(2)
169.4
169.4
154.9
155.4
162.9
118
136
153
163
C(5)–H(5A)Á Á ÁCl(1)#1
C(8)–H(8)Á Á ÁCl(1)#2
C(20)–H(20)Á Á ÁCl(1)#1
C(3)–H(3)Á Á ÁO(6)#3
C(16)–H(16)Á Á ÁCt(1)#4
C(5)–H(5C)Á Á ÁCt(2)#1
C(6)–H(6A)Á Á ÁCt(3)#5
C(10)–H(10C)Á Á ÁCt(3)#6
solvent-coordinated species
+ H2biphen
Scheme 2. Proposed scheme showing both the direct and solvent pathways.
3.3. Crystal structure of Ti(acac)2(Cl)(Hnaph) 7
Symmetry transformations used to generate equivalent atoms: #1 ꢀ x, ꢀy + 1,
ꢀz + 2 #2 ꢀx + 1, ꢀy + 1, ꢀz + 1 #3 x ꢀ 1, y, z #4 x, y, z + 1 #5 x, y, z ꢀ 1 #6 x ꢀ 1, ꢀy,
ꢀz + 2.
Under second order conditions, the Ti(acac)2Cl2 + H2naph reac-
tion yielded a mixture of Ti(acac)2(Cl)(Hnaph) (7) and the aryl-dio-
lato complex Ti(acac)2(naph) (3) as products. Complex 7 was
successfully crystallized, revealing that it is formed from the
Ti(acac)2Cl2 parent compound (1a) through substitution of a single
chloride ligand. The mono-chloride complex could correspond to
an intermediate in the substitution reaction resulting in formation
of the aryl-diolato complex (3) from 1a. This is confirmed by theo-
retical studies (vide infra). A molecular diagram showing the num-
bering scheme of 7 is presented in Fig. 4. Crystal data and details
for data collections and refinements are summarized in Table S1
of the Supplementary data.
(See Table 3) of the monodentate 2,3-naphthyldiol to a neighbouring
O4 of one of the acac’s is one of the reasons of the resulting orienta-
tion1 and also attributes to the distortion coordination axis. The acac
ligands are also not planar to their coordination planes (9.087(3)2 and
8.93(7)°.3 In addition the five coordinating oxygens have varying bond
lengths, primarily ascribed to the induced trans effect. For the trans O2
and O4 atoms (both from acac ligands) the distances differ marginally
0
(1.9493(11) vs. 1.9876(11) ÅA). The remaining two oxygens on the two
Molecule 7 crystallized on general positions with the central Ti
ion showing severe distortions in its octahedral coordination envi-
ronment. These distortions are due to both steric and electronic ef-
fects. To exemplify this observation, the expected three
perpendicular axis of the ideal coordination environment are bent
(Cl1–Ti1–O1, O4–Ti1–O2 and O3–Ti1–O5 are 174.35(4), 170.39
and 172.37(5)° respectively), possibly to accommodate the chela-
tion of the acac molecules. The strong hydrogen bonding interaction
acac’s are even further weakened by the differing trans ligands. For O1
the chloride trans to it results in a Ti1–O1 distance of 2.0124(12) ÅAand
for O3 the str0ong bonding from the 2,3-naphthyldiol (Ti1–
0
O5 = 1.7914(11) ÅA) trans to it weakens the Ti1–O3 bond to
0
2.0009(12) AÅ.
Despite the numerous aromatic rings in the complex, the only
possible
slippage values of >3 Å. Packing in the crystal is governed by
C–HÁ Á ÁCl/O and C–HÁ Á Á interactions (Fig. 5a). Of notice are the
interactions that generate dimeric units of the compound
p–p stackings in the crystal packing arrangement have
p
C–HÁ Á Á
p
situated over inversion centres, whereas C–HÁ Á ÁO and tri-furcated
1
The 2,3-naphthyldiol is not parallel to the O2–O3–O4–O5 plane. Due to the
twisted nature of the coordination environment the angle of 14.15(3)° between these
two planes should be used with caution (the fitted atoms of the O2–O3–O4–O5 plane
has a rms deviation of 0.0775 whereas the fitted atoms of the 2,3-naphthyldiol has a
rms = 0.0326).
2
Angle take between planes formed by O1–Ti1–O2 and O1–C1–C2–C3–O2.
Deviations of atoms in acac backbone from the plane: O1 = 0.0339(8) Å,
C1 = ꢀ0.0500(11) Å, C2 = 0.0187(15) Å, C3 = 0.0404(9) Å and O2 = -ꢀ0.0430(8) Å.
RMS deviation of fitted atoms = 0.0387 Å.
3
Angle take between planes formed by O3–Ti1–O4 and O3–C7–C8–C9–O4.
Deviations of atoms in acac backbone from the plane: O3 = 0.0057(7) Å, C7 = -
ꢀ0.0093(11) Å, C8 = 0.0066(11) Å, C9 = -ꢀ0.0007(11) Å and O4 = -ꢀ0.0023(7) Å. RMS
deviation of fitted atoms = 0.0058 Å.
Fig. 4. The molecular structure (30% probability displacement ellipsoids) of
Ti(acac)2(Cl)(Hnaph), 7, showing the numbering scheme.

K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
237
C–HÁ Á ÁCl link these together to be propagated along the a and b
computed, amounting to a value of ꢀ46.7 J molꢀ1 Kꢀ1(Table 4). Gi-
ven that entropy calculations are very sensitive to low frequency
modes, this value should be considered an estimation only. None-
theless, it compares well with the experimentally obtained activa-
tion entropy of ꢀ60.8 J molꢀ1 Kꢀ1. The computed activation Gibbs
axis respectively, see Fig. 5b.
3.4. Computational study of the Ti(acac)2Cl2 + aryl-diolato reaction
free energy
tally determined value of 77.7 kJ molꢀ1
D
Gà is 74.7 kJ molꢀ1, which is close to the experimen-
.
3.4.1. Computational study of the Ti(acac)2Cl2 + H2biphen reaction
DFT calculations [11] in agreement with crystal X-ray studies,
show that the Ti(b-diketonato)2Cl2 reactant 1a–1c and the Ti(b-
diketonato)2(biphen) product complexes 4a–4c all adopt an octa-
hedral geometry around the Ti centre. In addition to experiment,
the quantum chemical approach makes it possible to calculate
the energy and structure of transition states and reaction interme-
diates, not always observed experimentally. DFT calculations are
here presented for the Ti(acac)2Cl2 + H2biphen reaction to gain
more insight into the structure of the transition states and possible
reaction intermediates for formation of 4a from 1a.
The reactant structure (Reacbiphen) comprising Ti(acac)2Cl2 and
biphen is shown in Fig. 6a. From here the computed mechanism
proceeds through two reaction steps as found experimentally. The
optimized transition state of the first reaction step (TS1biphen) indi-
cates that attack of the hydroxyl oxygen on the Ti metal, dissociation
of chloride and proton transfer from the attacking hydroxyl to the
dissociating chloride ion can occur in a single step. The optimized
TS1biphen structure displays a 7-coordinated pentagonal bipyramidal
geometry. The attacking oxygen atom is positioned 2.14 Å from the
Ti metal, whereas the scissile Ti–Cl bond is elongated to 3.17 Å
(Fig. 6b), which is 0.85 Å longer than in the reactant complex
(Fig. 6a). The proton of the attacking hydroxyl interacts strongly
with the dissociating chloride ion with a ClÁ Á ÁH–O- distance of
2.02 Å. Dissociation of the chloride ion is further facilitated by a
hydrogen bonding interaction with the free hydroxyl group of the
incoming ligand. Analysis of the transition state eigenvector shows
dominating contributions from four bonds, the scissile Ti–Cl and
O–H bonds and the forming Ti–O and Cl–H bonds, indicating that
dissociation and formation of these bonds occurs concertedly. The
computed imaginary frequency for TS1 is low, ꢀ73.96 cmꢀ1, which
can be explained with the contribution of several heavy atoms to
the reaction coordinate (given that the frequency is inversely
proportional to the square root of the reduced mass of the involved
nuclei) [39]. The computed activation enthalpy (including solvent
effects and thermal corrections at 298.15 K) for TS1 is 60.8 kJ molꢀ1
(Table 4), in good agreement with the experimentally determined
reaction enthalpy of 59.6 kJ molꢀ1. Entropy contributions were also
The intermediate geometry is six-coordinated and exhibits
coordination of the biphenol molecule to the Ti through one oxy-
gen atom (Interbiphen, Fig. 6c). A stable 7-coordinated intermediate
could not be obtained. The released HCl molecule is hydrogen-
bonded to the free hydroxyl of biphenol. The computed
DG value
for the intermediate is ꢀ12.2 kJ molꢀ1. The estimated entropy is
slightly positive and amounts to only +4.0 J molꢀ1 Kꢀ1. The com-
puted results for the first reaction step, involving a seven-coordi-
nated transition state and
a six-coordinated intermediate,
indicate that the substitution reaction occurs through an inter-
change mechanism (as an associative mechanism would require
formation of a 7-coordinated intermediate).
The second transition state (TS2biphen) for attack of the second
hydroxyl oxygen on the Ti centre is similar to the first transition
state, exhibiting a 7-coordinated pentagonal bipyramidal geometry
and involving concerted oxygen attack, chloride release and proton
transfer (Fig. 6d). At the TS2 geometry, the attacking oxygen atom
is positioned 2.24 Å from the Ti atom. The scissile Ti–Cl bond is
elongated to 3.13 Å. The O–H bond of the attacking hydroxyl is
slightly elongated from 0.98 Å at the intermediate to 1.03 Å at
TS2. The ClÁ Á ÁH–O- interaction is strong, with a distance of
1.83 Å. The computed activation enthalpy for the second step is
76.4 kJ molꢀ1 relative to the intermediate (65.4 kJ molꢀ1 relative
to the reactant, Table 4). The computed activation Gibbs free en-
ergy
D
Gà is 91.9 kJ molꢀ1 relative to the intermediate
(79.7 kJ molꢀ1 relative to the reactant, Table 4). These values agree
well with experimental results, 77.7 and 89.1 kJ molꢀ1 for the two
steps respectively, see Table 4, and also indicate that the second
step of the reaction is slower than the first step.
The optimized product structure shows a six-coordinated Ti-
complex (Fig. 6e). The two released HCl molecules are found
hydrogen bonded to the complex. The computed reaction energy
for the overall reaction is 10.0 kJ molꢀ1, which indicates that the
product is slightly less stable than the reactant. Removal of HCl,
either by evaporization (during synthesis) of by diffusing away into
solution (during reaction kinetics) drives the reaction to
completion.
Fig. 5. (a) Packing diagram of 7 showing only C–HÁ Á Áp interactions. (b) Packing diagram of 7 showing the C–HÁ Á Áp interaction that generates dimeric units of the compound
situated over inversion centres, as well as the C–HÁ Á ÁO and tri-furcated C–HÁ Á ÁCl interactions that link these units together along the a and b axis respectively. Hydrogen
atoms not part of the interactions as well as parts of the neighbouring molecules have been omitted for clarity in this figure.

238
K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
Fig. 6. DFT calculated optimized structures of the Ti(acac)2Cl2 + H2biphen reaction (leading to formation of 4a) with (a) Reacbiphen, (b) TS1biphen, (c) Interbiphen, (d) TS2biphen and
(e) Prodbiphen (bonds breaking or forming are shown with dashed lines in the TS structures).
Table 4
Computed energies for biphenol substitution at Ti(acac)2Cl2 (298.15 K).
Ti(b)2Cl2
Computed
Experimental
Hà (kJ molꢀ1
D
H (kJ molꢀ1
)
D
S (J molꢀ1 Kꢀ1
)
D
G (kJ molꢀ1
)
D
Gà (kJ molꢀ1
)
D
)
D
Sà (J molꢀ1 Kꢀ1
)
D )
Gà (kJ molꢀ1
Reacbiphen
TS1biphen
Interbiphen
TS2biphen
Prodbiphen
0.0
60.8
0.0
0.0
74.7
ꢀ46.7
74.7
91.9
59.6
–
ꢀ60.8
77.7
89.1
ꢀ11.0
4.0
ꢀ12.2
65.4
ꢀ47.8
13.2
79.7
–
13.9
10.0
Based on the DFT calculations, the proposed reaction pathway
for the substitution reaction between Ti(b-diketonato)2Cl2 and H2-
biphen may thus be presented as in Scheme 3 for b-diketo-
nato = acac, where the first [H2biphen] dependent step is the
by chloride. The critical distances at TS1naph are slightly different
than with biphenol as substrate, showing that proton transfer to
chloride has progressed somewhat further at TS1naph (the ClÁ Á ÁH
distance is 1.80 Å, Fig. 7b) than at TS1biphen (ClÁ Á ÁH is 2.02 Å,
Fig. 6b). The computed energies show that the barrier for the first
reaction step with H2naph is 74.2 kJ molꢀ1 (Table 5), which is
essentially the same as for biphenol (74.7 kJ molꢀ1, Table 4).
The six-coordinated intermediate geometry Internaph (Fig. 7c)
has a computed relative energy of 2.8 kJ molꢀ1 (Table 5). Note
that the crystallographically obtained structure of the mono-
chloride complex 7 reported above (Fig. 4) corresponds to the
complex Internaph (Fig. 7c).4 The X-ray structure thus provides
additional support for the proposed interchange substitution
mechanism (Scheme 3). From the intermediate, the reaction pro-
gresses through the seven-coordinated TS2naph (Fig. 7d), involving
attack of the second hydroxyl oxygen on the Ti centre. At the opti-
mized geometry of TS2naph, the critical distances are similar to
TS1naph except for the TiÁ Á ÁO distance, which is somewhat shorter
(2.02 Å at TS2naph). The computed barrier for TS2naph is
66.0 kJ molꢀ1 (Table 5), which is lower than for TS1naph. Thus
the first step is rate-limiting for the reaction of Ti(acac)2Cl2 and
formation of
a 6-coordinated [Ti(b-diketonato)2(Cl)(Hbiphen)]
intermediate, and the second [H2biphen] independent step being
rate-limiting ring closure. Both reaction steps involve simulta-
neous chloride dissociation and Ti–Onaph bond formation, implying
an overall interchange mechanism for this substitution reaction.
3.4.2. Computational study of the Ti(acac)2Cl2 + H2naph reaction
The X-ray crystal structure of 4a containing a 7-membered aryl
diolato ring [10] and 6 containing an 8 membered aryl diolato
ring26 were reported previously, while no structure of 2 or 3 con-
taining a 5- membered aryl diolato ring is published to date. To
provide more insight in the structure of a dichlorobis(betadiketo-
nato-O,O0)titanium(IV) complex containing a 5-membered aryl dio-
lato ring, and in order to compare to the results obtained above for
4a, we also performed a theoretical study on the Ti(acac)2Cl2
(1a) + H2naph reaction leading to complex 3.
The optimized geometry for the transition state of the first reac-
tion step (TS1naph, Fig. 7) shows that chloride substitution occurs in
the same fashion as with biphenol above (Fig. 6). At TS1naph, attack
of one hydroxyl oxygen onto the Ti centre occurs simultaneously
with chloride dissociation and abstraction of the hydroxyl proton
4
The X-ray structure of 7 (Fig. 3) corresponds to the enantiomer of Internaph
(Fig. 6c). However, the mechanism for the reaction between the achiral H2naph and
1a will be the same for the two possible enantiomers of 1a.

K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
239
7-coordinated TS1
First reaction step
O
Cl
intermediate
HO
O
Ti
O
Substrates
O
HO
Cl
O
O
Ti
O
O
O
Cl
Cl
O
O
O
O
HO
HO
Ti
HCl
+
+
k1
Cl
O
7-coordinated TS2
Second reaction step
O
intermediate
O
O
O
O
O
Ti
O
O
HO
O
Ti
HCl
+
H
O
O
O
O
Cl
O
O
O
Ti
O
Cl
k2
Product
Scheme 3. Schematic presentation of the proposed interchange mechanism of the substitution reaction between Ti(acac)Cl2 (1a) and H2biphen to afford Ti(acac)2(biphen)
(4a).
Fig. 7. DFT calculated optimized structures of the Ti(acac)2Cl2 + H2naph reaction (leading to formation of 3) with (a) Reacnaph, (b) TS1naph, (c) Internaph, (d) TS2naph, and (e)
Prodnaph (bonds breaking or forming are shown with dashed lines in the TS structures).
H2naph, in contrast to the reaction with biphenol (where the sec-
ond step was rate-limiting). The product state (Fig. 7e) has a rel-
ative Gibbs free energy of 4.1 kcal molꢀ1 (Table 5). The calculated
near zero relative Gibbs free energies of the intermediate Internaph
(Ti(CH3COCHCOCH3)2(Cl)(Hnaph) 7) and the product Prodnaph
(Ti(acac)2(naph) 3) are in agreement with the experimentally

240
K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
Table 5
ven-coordinated transition states and one six-coordinated inter-
mediate. The first reaction step involves the attack of hydroxyl
oxygen of the incoming ligand on Ti, dissociation of the chloride
ion and proton transfer from the attacking hydroxyl to the chlo-
ride. The second reaction step involves attack of the second hydro-
xyl oxygen on Ti, resulting in displacement of the second chloride
ion and ring closure. This step corresponds to an intramolecular
reaction, which is in agreement with the experimental kinetics.
The computed energies for the formation of 4a agree remarkably
well the experimental thermodynamic values. The proposed inter-
change mechanism is further supported by the X-ray structure of 7,
which corresponds to the six-coordinated reaction intermediate.
Computed energies for the reaction of Ti(acac)2Cl2 and H2naph (298.15 K).
Ti(b)2Cl2
Computed
H (kJ molꢀ1
D
)
D
S (J molꢀ1 Kꢀ1
)
D )
G (kJ molꢀ1
Reacnaph
TS1naph
Internaph
TS2naph
Prodnaph
0.0
55.8
1.0
52.9
1.1
0.0
0.0
74.2
2.8
66.0
4.1
ꢀ62.0
3.8
ꢀ43.8
ꢀ10.0
found equilibrium between
conditions.
3
and
7
under second order
Acknowledgements
3.4.3. Computational study of the Ti(acac)2Cl2 + H2mbinaph reaction
We have also performed calculations on the reaction of
Ti(acac)2Cl2 with methylenebinaphthol (H2mbinaph), which re-
sults in formation of product complex 6 containing an 8-mem-
bered aryl diolato ring. The chlorine substitution with
H2mbinaph occurs as with the substrates H2biphen and H2naph,
proceeding through two transition states and a 6-coordinated
intermediate. (Scheme 3). The optimized geometries for the H2-
mbinaph reaction are shown in Figure S1 (Supporting Information).
The critical distances are similar to the H2biphen and H2naph reac-
tions (Figs. 6 and 7). The computed energies (Supporting Informa-
tion, Table S2) show that the barriers with H2mbinaph are
101.4 kJ molꢀ1 for the first step and 99.7 kJ molꢀ1 for the second
step (relative to the reactant species). These values are too close
to determine which step is rate-limiting for formation of 6. The
computed barriers are also higher than for formation of 4a and 3
(Tables 4 and 5). The computational results indicate that this is
mainly due to a higher enthalpy of activation (compare Table S2
with Tables 4 and 5). The computed activation enthalpies increase
from 55.8 kJ molꢀ1 for H2naph (formation of 3) to 65.4 kJ molꢀ1 for
H2biphen (formation of 4a) to 89.3 kJ molꢀ1 for H2mbinaph
(formation of 6). Coordination of the dihydroxyl-substrate to the
Ti-complex will result in a strain in the substrate compared to
the free (unbound) state; this strain is expected to increase in
going from H2naph to H2mbinaph, which can explain the increase
in the computed enthalpies.
Financial assistance by the South African National Research
Foundation and the Central Research Fund of the University of
the Free State is gratefully acknowledged. This work was supported
by the Research Council of Norway through a Centre of Excellence
Grant (Grant No. 179568/V30). A grant of computer time from the
Norwegian supercomputing program NOTUR (Grant No.
NN4654 K) is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.09.004.
References
[1] (a) Y. Qian, J. Huang, M.D. Bala, B. Lian, H. Zhang, H. Zhang, Chem. Rev. 103
(2003) 2633;
(b) R. Beckhause, C. Santamaria, J. Organomet. Chem. 617 (2001) 81;
(c) E. Manek, D. Hinz, G. Meyer, Coord. Chem. Rev. 164 (1997) 5;
(d) J.C. Vites, M.M. Lynam, Coord. Chem. Rev. 146 (1995) 1;
(e) J.C. Vites, M.M. Lynam, Coord. Chem. Rev. 138 (1995) 71;
(f) R.O. Duthaler, A. Hafner, Chem. Rev. 92 (1992) 807;
(g) B.K. Keppler, C. Friese, H.G. Moritz, H. Vongerichten, E. Vogel, Struct. Bond.
78 (1991) 98.
[2] E. Meléndez, Crit. Rev. Oncol. Hematol. 42 (2002) 309.
[3] J.P. Corden, W. Errington, P. Moore, M.G. Partridge, M.G.H. Wallbridge, J. Chem.
Soc., Dalton Trans. (2004) 1846.
[4] M. Aizwa, Y. Nosaka, N. Fujii, J. Non-Cryst. Solids 128 (1991) 77.
[5] (a) J. Balsells, T.J. Davis, P. Carroll, P.J. Walsh, J. Am. Chem. Soc. 124 (2002)
10336;
4. Conclusion
(b) A. van der Linden, C.J. Schaverien, N. Meijboom, C. Ganter, A.G. Orpen, J.
Am. Chem. Soc. 117 (1995) 3008;
(c) T.J. Boyle, N.W. Eilerts, J.A. Heppert, Organometal 13 (1994) 2218;
(d) K. Narasaka, Synthesis 1 (1991) 1;
(e) K. Mikami, M. Shimizu, Chem. Rev. 92 (1992) 1021.
In this paper we have carried out synthetic, kinetic, and compu-
tational studies on a series of a remarkably hydrolytically stable
bis(acetylacetonato-O,O0)(aryl-diolato-O,O0)titanium(IV)
com-
[6] (a) S.N. Brown, E.T. Chu, M.W. Hull, B.C. Noll, J. Am. Chem. Soc. 127 (2005)
16010;
plexes (2–6) containing bidentate ligands (catechol, naphthol,
biphenol, binaphthol, or methylenebinaphthol). The aryl-diolato-
ligands form 5-, 7- and 8-membered chelating rings. We have also
reported the X-ray crystal structure of 7, corresponding to the
mono-chloride intermediate of the substitution reaction resulting
in formation of 3.
N. Kongprakaiwoot, Chem. Sci. 2 (2011) 331.
[7] B.S. Furniss, A.J. Hannaford, P.W.G. Smith, A.R. Tatchell, Vogel’s Textbook of
Practical Organic Chemistry, fifth ed., John Wiley & Sons, New York, 1994. 409.
[8] R.C. Fay, R.N. Lowry, Inorg. Chem. 6 (1967) 1512.
[9] N. Serpone, R.C. Fay, Inorg. Chem. 6 (1967) 1835.
[10] T.A. Tsotetsi, A. Kuhn, A. Muller, J. Conradie, Polyhedron 28 (2009) 209.
[11] A. Kuhn, T.A. Tsotetsi, A. Muller, J. Conradie, Inorg. Chim. Acta 362 (2009) 3088.
[12] L. Helm, MINSQ, non-linear parameter estimation and model development,
least squares parameter optimization, V3.12, MicroMath Scientific Software,
Salt Lake City, UT, 1990.
The reactivity of Ti(RCOCHCOR0)2Cl2 complexes (1a-c) towards
chloriꢀde substitution according to the b-diketonato ligand (RCOCH-
COR0) follows the order (PhCOCHCOPh)ꢀ > (PhCOCHCOCH3)ꢀ
>
[13] J.H. Espenson, Chemical Kinetics and Reaction Mechanisms, second ed.,
McGraw-Hill, New York, 1995. 15, 49, 70–75, 156.
(CH3COCHCOCH3)ꢀ. This is the same order found experimentally
for various transition metal b-diketonato complexes containing
the same b-diketonato ligands. The kinetics for the reaction of
1a–c with biphenol show two kinetically distinct steps, with the sec-
ond step (which is independent on the concentration of the incom-
ing ligand) being slightly slower than the first. The thermodynamic
parameters support an associative or interchange mechanism.
DFT calculations on the reaction of Ti(acac)2Cl2 with different
aryl-diolato ligands reveal that chlorine substitution proceeds via
a two-step interchange mechanism with the formation of two se-
[14] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.

K.H. Hopmann et al. / Polyhedron 67 (2014) 231–241
241
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Germany, 1993, p. 297;
Pople, GAUSSIAN 03, Revision E.01, GAUSSIAN Inc., Wallingford CT, 2004.
[15] APEX2. (Version 1.0-27), Bruker AXS Inc., Madison, Wisconsin, USA, 2005.
[16] COSMO. Version 1.48, Bruker AXS Inc., Madison, Wisconsin, USA, 2003.
[17] SAINT-Plus. Version 7.12 (including XPREP), Bruker AXS Inc., Madison, Wisconsin,
USA, 2004.
[18] SADABSVersion 2004/1, Bruker AXS Inc., Madison, Wisconsin, USA, 1998.
[19] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A.
Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32
(1999) 115.
(b) D.C. Bradley, C.E. Holloway, J. Chem. Soc. A. (1969) 282.
[30] E. Dubler, R. Buschmann, H.W. Schmalle, J. Inorg. Biochem. 95 (2003) 97.
[31] G. Ferguson, C. Glidewell, Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 57
(2001) 264.
[32] L. Matilainen, I. Mutikainen, M. Leskelä, Acta Chem. Scand. 50 (1996) 755.
[33] J. Burgess, S.A. Parsons, Appl. Organomet. Chem. 7 (1993) 343.
[34] F. Basolo, R. Pearson, Mechanism of Inorganic Reactions, second ed., John
Wiley & Sons, Inc, New York, 1965 (K.F. Purcell, J.C. Kotz, Inorganic Chemistry,
International Edition; Saunders: London, 1977, pp. 697).
[35] R.G. Wilkins, Kinetics and Mechanism of Reactions of Transition Metal
Complexes 2nd thoroughly revised edition, VCH, Weinheim, 1991. pp. 103,
232–242.
[20] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[21] G.M. Sheldrick, SHELXL97 program for crystal structure refinement, University of
Göttingen, Göttingen, Germany, 1997.
[22] International Tables Vol
C
A.J.C. Wilson (Ed.), International Tables for
[36] C.A. McAuliffe, D.S. Barratt, Comprehensive Coordination Chemistry, in: G.
Wilkinson, R.D. Gillard, J.A. McCleverty (Eds.), vol. 3, Pergamon Press, Oxford,
1987, pp. 323–360.
Crystallography, Volume C, Kluwer Academic Publishers, Dordrecht, The
Netherlands, 1995.
[23] K. Brandenburg, H. Putz, H. Diamond, Release 3.1a, Crystal Impact GbR, Bonn,
Germany, 2005.
[37] M. Colapietro, A. Vaciago, D.C. Bradley, M.B. Hursthouse, I.F. Rendall, J. Chem.
Soc., Dalton Trans. (1972) 1052.
[24] G.P. Dahl, B.P. Block, Inorg. Chem. 5 (1966) 1394.
[38] (a) J.G. Leipoldt, G.J. Lamprecht, E.C. Steinberg, J. Organometal. Chem. 397
(1990) 239;
[25] S.N. Brown, E.T. Chu, M.W. Hull, B.C. Noll, J. Am. Chem. Soc. 127 (2005) 16010.
[26] P.V. Rao, C.P. Rao, E.K. Wegelius, E. Kolehmainen, K. Rissanen, J. Chem. Soc.,
Dalton Trans. (1999) 4469.
(b) J.G. Leipoldt, S.S. Basson, J.J.J. Schlebush, E.C. Grobler, Inorg. Chim. Acta 62
(1982) 113;
[27] B.K. Keppler, M.E. Heim, Drugs Future 13 (1988) 637.
[28] (a) R.C. Fay, N. Serpone, J. Am. Chem. Soc. 90 (1968) 5701;
(b) F.A. Cotton, R.H. Holm, J. Am. Chem. Soc. 80 (1958) 5658.
[29] (a) B.K. Keppler, C. Friesen, H.G. Moritz, H. Vongerichten, H. Vogel, Metal
Complexes in Cancer Chemotherapy, in: B.K. Keppler (Ed.), VCH, Weinheim,
(c) J.G. Leipoldt, E.C. Grobler, Trans. Met. Chem. 11 (1986) 110;
J.G. Leipoldt, S.S. Basson, J. Organomet. Chem. 418 (1991) 113.
[39] C.J. Cramer, Essentials of Computational Chemistry: Theories and Models,
second ed., John Wiley & Sons, Inc., New York, 2004.