Synthesis and structures of titanium complexes bearing tetradentate tripodal [O2XC] ligands (X = C, P)

Article abstract of DOI:10.1039/c9dt03969d

We report the synthesis and structures of titanium complexes supported by tripodal mixed-donor [O₂CC] and [O₂PC] ligands. Stepwise cyclometallation of a chelating bis(phenoxide) complex led to tetradentate binding of the [O₂CC] ligand to the titanium center. Phosphination of a Ti-C bond of the [O₂CC] complex afforded the tripodal [O₂PC] ligand.

Full text of DOI:10.1039/c9dt03969d

Dalton
Transactions
View Article Online
View Journal | View Issue
COMMUNICATION
Synthesis and structures of titanium complexes
bearing tetradentate tripodal [O2XC] ligands
(X = C, P)†
Cite this: Dalton Trans., 2020, 49, 27
Received 9th October 2019,
Accepted 26th November 2019
Yusuke Nakanishi, Yutaka Ishida
and Hiroyuki Kawaguchi
*
DOI: 10.1039/c9dt03969d
rsc.li/dalton
We report the synthesis and structures of titanium complexes and induce remarkable reactivity patterns.7,11–14 As an exten-
supported by tripodal mixed-donor [O2CC] and [O2PC] ligands. sion of our work on the coordination chemistry of triphenyl-
Stepwise cyclometallation of a chelating bis(phenoxide) complex methane-based ligands, we were motivated to incorporate
led to tetradentate binding of the [O2CC] ligand to the titanium different donor groups into the arm units of the tripodal
center. Phosphination of a Ti–C bond of the [O2CC] complex ligand backbone. Here we describe the synthesis and struc-
afforded the tripodal [O2PC] ligand.
tures of titanium complexes bearing tripodal [O2CC]4− and
[O2PC]3− ligands.
Tridentate and tetradentate ligands with tripodal topology
have received a great deal of attention.1–6 Two main features
make this class of ligands useful in coordination chemistry.
First, tripodal ligands can enforce a single site of reactivity
upon coordination to a metal center, thereby providing a desir-
able platform for small molecule activation and atom transfer
chemistry. Second, the choice of substituents and donor
groups incorporated into the arms of the tripodal ligands
allows for the tailored synthesis of ligands for specific appli-
cations. Our group has been examining the use of tris(2-
hydroxylphenyl)methane as a supporting ligand.8,9 A triphenyl-
methane ligand framework has a propeller conformation with
all ortho-substituted groups aligned with the central methine
hydrogen, which appears to be well suited to accommodate a
metal ion.10 Therefore, deprotonation of three hydroxyl groups
of triphenoxymethane effectively provides a facially tridentate
[O3]3− ligand, and subsequent activation of the bridgehead
methine C–H bond results in a tetradentate [O3C]4− ligand.8,9
These highly charged tripodal [O3] and [O3C] ligands are suit-
able for exploring the chemistry of metal ions in high oxi-
dation states.
A
triphenylmethane
derivative
(2-Br-C6H5)(2,4-tBu2-
C6H2OH)2CH (H2[O2ArBr]) is readily available and used as a
ligand precursor in this study.15 Treatment of TiCl4 with 1
equivalent of H2[O2ArBr] in toluene at room temperature for
12 h gave a red solution along with elimination of HCl. After
removal of all volatiles under vacuum, [O2ArBr]TiCl2(thf) (1)
was obtained as brown crystals in 94% yield upon extraction of
the solid residue with THF, evaporation to dryness, and
washing with pentane (Scheme 1). The molecular structure of
Within the family of tripodal ligands, those containing
different types of donor functions are particularly interesting
to us, because mixed-donor multidentate ligands can bind
various transition metals, stabilize a range of oxidation states,
Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku,
Tokyo 152-8551, Japan. E-mail: hkawa@chem.titech.ac.jp
†Electronic supplementary information (ESI) available: Experimental and
characterization data, and crystallographic data for 1′, 4-thf and 5. CCDC
1957864–1957866. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/C9DT03969D
Scheme 1 Synthesis of titanium complexes.
This journal is © The Royal Society of Chemistry 2020
Dalton Trans., 2020, 49, 27–30 | 27

View Article Online
Communication
Dalton Transactions
Addition of 3 equivalents of PhCH2K to 1 in THF produced
the tribenzyl complex K[(O2ArBr)Ti(CH2Ph)3] (2) according to
1H NMR spectroscopy. The three sets of benzyl protons appear
to be in rapid exchange on the NMR time scale at room temp-
erature, as evidenced by the presence of one broad peak inte-
grating to six protons at 3.03 ppm. The phenoxide groups
remain equivalent. The bridgehead methine proton is
observed at 5.23 ppm, shifted upfield relative to that for 1 at
6.69 ppm. All attempts to isolate 2 were unsuccessful due to its
thermal instability and light-sensitivity.
To determine the identity of decomposition products, the
1
thermal reaction of 2 was monitored by H NMR spectroscopy.
An NMR tube sample of 2 in THF-d8 was heated to 60 °C for
12 h in the absence of light, leading to full conversion to a
mixture of products, as seen by the disappearance of the
signal for the bridgehead methine proton at 5.23 ppm. The
reaction mixture included K[(O2CArBr)Ti(CH2Ph)2] (3) and
toluene concomitant with small amounts of [O2CC]Ti(thf)2
(4-thf) and dibenzyl. The Ti-containing products 3 and 4-thf
were inseparable. However, 3 was light sensitive and cleanly
converted to 4-thf and dibenzyl when exposed to light from a
xenon lamp at room temperature. Although single crystals of
4-thf suitable for X-ray structure determination were grown
from pentane, purification of 4-thf on a preparative scale has
Fig. 1 The molecular structure of 1’ with thermal ellipsoids set at the
50% probability level. All hydrogen atoms are omitted for clarity except
for the methine proton.
the 2-tert-butyl-5-methylphenoxide analogue 1′ has been deter- met with difficulties due to its high solubility in common
mined by X-ray diffraction (Fig. 1), confirming that the organic solvents. However, the [O2CC] complex was isolated as
complex is monomeric in the solid state.16 The structure of 1′ a pyridine adduct. A preparative scale reaction carried out in
closely resembles the chelating bis(phenoxide) ligand complex THF gave a brown solution accompanied by precipitation of
[mbmp]TiCl2(thf) (mbmp
=
2,2′-methylenebis(6-tert-butyl-4- KBr. Addition of pyridine to the supernatant followed by
methylphenoxide)).17 The geometry of the titanium center is tri- workup afforded [O2CC]Ti(py)2 (4-py) as purple powder in 29%
gonal bipyramidal (τ = 0.96)18 with two equatorial chloride yield. The H NMR spectrum of 4-py in THF-d8 shows that the
1
ligands (Ti–Cl = 2.244(1) and 2.2835(9) Å), an axial THF molecule molecule has average Cs symmetry with equivalent pyridine
(Ti–O = 2.181(2) Å), and a bidentate [O2ArBr] ligand spanning ligands, implying reversible pyridine dissociation in solution.
axial and equatorial sites. The equatorial phenoxide ligand is
The molecular structure of 4-thf is shown in Fig. 2. The
more bent (138.7(2)°) than the axial phenoxide ligand (159.4(2)°). bridgehead methine C–H bond of the ligand backbone is acti-
This is likely correlated to the extent of π donation from the vated followed by ortho-metalation of the pendant phenyl ring
oxygen lone pairs to the titanium metal, as the equatorial Ti–O to furnish a four-membered titanabenzocyclobutene, leading to
bond distance (1.819(2) Å) is longer than the axial Ti–O bond dis- tetradentate binding of the [O2CC] ligand to the metal center.
1
tance (1.773(2) Å). The H and 13C{1H} NMR spectra of 1 show Two THF molecules are bound to the metal to complete its
the presence of equivalent phenoxide groups, indicating average coordination sphere. The coordination environment of the tita-
Cs symmetry due to reversible THF dissociation in solution.
nium metal in 4-thf is significantly distorted because of the four-
We previously reported that zirconium and niobium benzyl membered ring. This metallacycle is puckered with the Ti–C(31)–
complexes supported by the triphenoxymethane ligand under- C(30)–C(15) torsion angle of 21.9(1)°, rather than planar as
went C–H activation of the bridgehead methine to provide the observed frequently in the titanacyclobutene fragments of less
tetradentate tripodal [O3C] ligand.8,9 Cyclometalation via σ- preorganized systems,21–24 suggesting that severe strains are
bond metathesis is common in early transition metal com- present in this complex. The Ti–C(sp3) and Ti–C(sp2) bond dis-
plexes with ortho-substituted phenoxide ligands.19,20 For tances of 2.143(2) and 2.071(2) Å are comparable to those found
example, M(OAr)2(CH2Ph)2 (M = Ti, Zr; OAr = 2,6-di-tert-butyl- in titanacyclobutene complexes,21–24 and the C(15)–C(31) bond
phenoxide) has been reported to give
a
cyclometalated distance of 1.523(2) Å is in accordance with a C(sp3)–C(sp2) single
t
complex M(OC6H3 BuCMe2CH2)(OAr)(CH2Ph) with elimination bond (1.51 Å).25 The ortho-metalated benzene ring is completely
of toluene.19 Complex 1′ adopts a flattened boat conformation planar, and its C–C bonds are practically non-alternating and
for the eight-membered TiO2C5 chelate ring, which causes the have normal distances. The bridgehead carbon C(15) atom
bridgehead methine proton to be in close proximity to the tita- adopts a distorted tetrahedral geometry. The Ti–O bond distances
nium center. When the metal center is alkylated, this con- of 1.929(1) and 1.909(1) Å are longer than those in 1.
strained geometry could promote the coordination of the
bridgehead carbon atom via C–H bond activation.
Titanacyclobutene complexes have been reported to react
with PhPCl2 and R2PCl (R
Ph, iPr), yielding
=
28 | Dalton Trans., 2020, 49, 27–30
This journal is © The Royal Society of Chemistry 2020

View Article Online
Dalton Transactions
Communication
Fig. 3 The molecular structure of 5 with thermal ellipsoids set at the
50% probability level. All hydrogen atoms are omitted for clarity.
Fig. 2 The molecular structure of 4-thf with thermal ellipsoids set at
the 50% probability level. All hydrogen atoms are omitted for clarity.
explained by the reversible η1 to η3 slippage of the phenoxide
groups on the NMR time scale. However, we cannot rule out
the possibility that both phenoxide groups are η1-bound to the
metal center in solution.
phosphacyclobutenes26,27 or allylphosphines,28 respectively.
Based on this precedent, we were interested in the synthesis of
a phosphine–phenoxide mixed-donor ligand by phosphination
of a Ti–C bond of 4-py. Treatment of 4-py with a slight excess
of Ph2PCl in THF at room temperature gave a brown solution
from which [O2PC]TiCl(py) (5) was obtained as brown powder
in 85% yield after workup. Product 5 results from selective P–C
bond formation at the C(sp2) carbon of the titanabenzocyclo-
butene ring. The 1H NMR spectrum of 5 in C6D6 shows the
presence of a single complex with Cs symmetry. A 13C{1H}
NMR signal at 114.2 ppm is identified as the metal-bound
bridgehead carbon resonance, which appears as a 31P-coupled
In summary, we have prepared the [O2CC] complex by step-
wise metalation of the bis(phenoxide) ligand. Subsequent
phosphination of the titanabenzocyclobutene unit resulted in
the [O2PC] tripodal ligand. It should be noted that the reaction
of Ti(CH2Ph)4 with the ligand precursor H2[O2P] gave a
mixture of uncharacterized products. Efforts towards the syn-
thesis of [O2CC] complexes of other transition metals using
this methodology are ongoing. We are also attempting the
insertion of different donor groups into a metal–carbon bond
of the [O2CC] complexes to prepare new mixed-donor tripodal
ligands.
3
doublet with JCP = 13.8 Hz. Compared to the free ligand
H2[O2P] (−16.8 ppm), a 31P{1H} NMR signal is shifted down-
field and found at 44.3 ppm.
The molecular structure of 5 has been determined by the
X-ray diffraction study (Fig. 3), displaying the tetradentate
coordination of the [O2PC] ligand with one phosphorus, one
carbon, and two phenoxide oxygen donor atoms bound to the
metal center. Complex 5 possesses a distorted octahedral geo-
metry at titanium, with a chloro and a pyridine ligand trans to
the carbon and the phosphine donor atom, respectively. The
most salient structural feature is the presence of Ti–C(ipso)
(2.423(2) Å) and Ti–C(ortho) (2.448(2) Å) interactions with one
of the phenoxide groups. Similar η3 bonding interactions have
been structurally observed with binaphtholate ligands co-
ordinated to Ru, Rh, and Ir29–31 and are often observed with
benzyl and anilide ligands.32–37 The Ti–O(η3) bond distance
(1.929(1) Å) is longer than the Ti–O(η1) bond distance
(1.897(1) Å). The Ti–P bond distance of 2.5605(5) Å is within
the range reported for Ti(IV) complexes.38–41 The geometry of
the bridgehead carbon C(15) atom is distorted tetrahedral. The
solid-state structure is in contrast to the presence of equivalent
phenoxide groups in solution. This discrepancy could be
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research (No. 18H01992) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Notes and references
1 J. G. Verkade, Acc. Chem. Res., 1993, 26, 483–489.
2 L. H. Gade, Acc. Chem. Res., 2002, 35, 575–582.
3 A. G. Blackman, Eur. J. Inorg. Chem., 2008, 2633–2647.
This journal is © The Royal Society of Chemistry 2020
Dalton Trans., 2020, 49, 27–30 | 29

View Article Online
Communication
Dalton Transactions
4 A. Phanopoulos, P. W. Miller and N. J. Long, Coord. Chem. 23 F. N. Tebbe and R. L. Harlow, J. Am. Chem. Soc., 1980, 102,
Rev., 2015, 299, 39–60.
6149–6151.
5 S. Trofimenko, Chem. Rev., 1993, 93, 943–980.
6 X. Hu and K. Meyer, J. Organomet. Chem., 2005, 690, 5474–
5484.
7 W. A. Chomitz and J. Arnold, Chem. – Eur. J., 2009, 15,
2020–2030.
8 F. Akagi, T. Matsuo and H. Kawaguchi, J. Am. Chem. Soc.,
2005, 127, 11936–11937.
9 F. Akagi, Y. Ishida, T. Matsuo and H. Kawaguchi, Dalton
Trans., 2011, 40, 2375–2382.
10 M. B. Dinger and M. J. Scott, Eur. J. Org. Chem., 2000,
2467–2478.
11 R. J. Burford, A. Yeo and M. D. Fryzuk, Coord. Chem. Rev.,
2017, 334, 84–89.
12 M. D. Fryzuk, J. B. Love, S. J. Rettig and V. G. Young,
Science, 1997, 275, 1445–1447.
13 M. D. Fryzuk, S. A. Johnson and S. J. Rettig, J. Am. Chem.
Soc., 1998, 120, 11024–11025.
24 S. Ogoshi and J. M. Stryker, J. Am. Chem. Soc., 1998, 120,
3514–3515.
25 M. B. Smith and J. March, in Advanced Organic Chemistry,
Wiley, New York, 6th edn, 2007, p. 24.
26 K. M. Doxsee and G. S. Shen, J. Am. Chem. Soc., 1989, 111,
9129–9130.
27 W. Tumas, J. A. Suriano and R. L. Harlow, Angew. Chem.,
Int. Ed. Engl., 1990, 29, 75–76.
28 Y. Zhou, C. Chen, X. Yan and C. Xi, Organometallics, 2014,
33, 844–846.
29 J. M. Blacquiere, R. McDonald and D. E. Fogg, Angew.
Chem., Int. Ed., 2010, 49, 3807–3810.
30 J. M. Blacquiere, C. S. Higman, R. McDonald and
D. E. Fogg, J. Am. Chem. Soc., 2011, 133, 14054–14062.
31 D. Panichakul, Y. Su, Y. Li, W. Deng, J. Zhao and X. Li,
Organometallics, 2008, 27, 6390–6392.
32 M. Wiecko, D. Girnt, M. Rastätter, T. K. Panda and
P. W. Roesky, Dalton Trans., 2005, 2147–2150.
33 M. E. G. Skinner, T. Toupance, D. A. Cowhig, B. R. Tyrrell
and P. Mountford, Organometallics, 2005, 24, 5586–5603.
14 L. N. Grant, M. E. Miehlich, K. Meyer and D. J. Mindiola,
Chem. Commun., 2018, 54, 2052–2055.
15 H.-Y. Chen, Y.-H. Lee, M. Y. Chiang, W.-Y. Lu, H.-C. Tseng,
H.-Y. Tsai, Y.-H. Chen, Y.-C. Lai and H.-Y. Chen, RSC Adv., 34 M. J. Lopez, A. Kondo, H. Nagae, K. Yamamoto, H. Tsurugi
2015, 5, 82018–82026. and K. Mashima, Organometallics, 2016, 35, 3816–3827.
16 The asymmetric unit of the unit cell contains two independent 35 A. L. Odom, P. L. Arnold and C. C. Cummins, J. Am. Chem.
molecules with essentially identical metric parameters, and
the metrical details of one of them are discussed for clarity.
17 J. Okuda, S. Fokken, H.-C. Kang and W. Massa, Chem. Ber.,
1995, 128, 221–227.
18 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and
G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349–
1356.
Soc., 1998, 120, 5836–5837.
36 K. B. P. Ruppa, N. Desmangles, S. Gambarotta, G. Yap and
A. L. Rheingold, Inorg. Chem., 1997, 36, 1194–1997.
37 B. L. Tran, M. Singhal, H. Park, O. P. Lam, M. Pink,
J. Krzystek, A. Ozarowski, J. Telser, K. Meyer and
D. J. Mindiola, Angew. Chem., Int. Ed., 2010, 49, 9871–9875.
38 L. Morello, P. Yu, C. D. Carmichael, B. O. Patrick and
M. D. Fryzuk, J. Am. Chem. Soc., 2005, 127, 12796–12797.
19 I. P. Rothwell, Acc. Chem. Res., 1988, 21, 153–159.
20 S. L. Latesky, A. K. McMullen, I. P. Rothwell and 39 S. Batke, T. Kothe, M. Haas, H. Wadepohl and J. Ballmann,
J. C. Huffman, J. Am. Chem. Soc., 1985, 107, 5981–5987. Dalton Trans., 2016, 45, 3528–3540.
21 T. Shono, T. Nagasawa, A. Tsubouchi, K. Noguchi and 40 R. J. Long, V. C. Gibson, A. J. P. White and D. J. Williams,
T. Takeda, Chem. Commun., 2008, 3537–3539. Inorg. Chem., 2006, 45, 511–513.
22 R. J. McKinney, T. H. Tulip, D. L. Thorn, T. S. Coolbaugh 41 L.-C. Liang, Y.-N. Chang and H. M. Lee, Inorg. Chem., 2007,
and F. N. Tebbe, J. Am. Chem. Soc., 1981, 103, 5584–5586. 46, 2666–2673.
30 | Dalton Trans., 2020, 49, 27–30
This journal is © The Royal Society of Chemistry 2020