The titanium tris-anilide cation [Ti(N[tBu]Ar)3]+ stabilized as its perfluoro-tetra-phenylborate salt: Structural characterization and synthesis in connection with redox activity of 4,4′-bipyridine dititanium complexes

Article abstract of DOI:10.1039/c5dt00105f

This work explores the reduction of 4,4′-bipyridine using two equivalents of the titanium(iii) complex Ti(N[^tBu]Ar)₃ resulting in formation of a black, crystalline complex, (4,4′-bipy){Ti(N[^tBu]Ar)₃}₂, for which an X-ray structure determination is reported. The neutral, black, 4,4′-bipyridine-bridged bimetallic was found to be redox active, with mono- and di-anions being accessible electrochemically, and with the mono- and di-cations also being accessible chemically, and isolable, at least when using the weakly coordinating anion [B(C₆F₅)₄]^- as the counter-ion. It proved possible to crystallize the salt [(4,4′-bipy){Ti(N[^tBu]Ar)₃}₂][B(C₆F₅)₄]₂ for a single-crystal X-ray structure investigation; in this instance it was revealed that the aromaticity of the 4,4′-bipyridine ligand, that had been disrupted upon reduction, had been regained. A rare cationic d⁰ metal tris-amide complex, shown by X-ray crystallography to contain an intriguing pyramidal TiN₃ core geometry, namely {Ti(N[^tBu]Ar)₃}⁺, could also be isolated when using [B(C₆F₅)₄] as the essentially non-interacting counter-ion. This highly reactive cation should be considered as a potential intermediate in the plethora of reactions wherein Ti(N[^tBu]Ar)₃ has been shown to effect the reduction of substrates including halogenated organic molecules, carbonyl compounds, organic nitriles, and metal complexes. This journal is

Full text of DOI:10.1039/c5dt00105f

Dalton
Transactions
PAPER
t
+
The titanium tris-anilide cation [Ti(N[ Bu]Ar) ]
3
stabilized as its perfluoro-tetra-phenylborate salt:
structural characterization and synthesis in
connection with redox activity of 4,4’-bipyridine
dititanium complexes†
Cite this: Dalton Trans., 2015, 44,
6784
Heather A. Spinney,‡ Christopher R. Clough§ and Christopher C. Cummins*
This work explores the reduction of 4,4’-bipyridine using two equivalents of the titanium(III) complex
t
t
3 3 2
Ti(N[ Bu]Ar) resulting in formation of a black, crystalline complex, (4,4’-bipy){Ti(N[ Bu]Ar) } , for which an
X-ray structure determination is reported. The neutral, black, 4,4’-bipyridine-bridged bimetallic was found
to be redox active, with mono- and di-anions being accessible electrochemically, and with the mono-
and di-cations also being accessible chemically, and isolable, at least when using the weakly coordinating
]− as the counter-ion. It proved possible to crystallize the salt [(4,4’-bipy){Ti(N[ Bu]Ar)
t
anion [B(C
6
F
5
)
4
3
}
2
]-
[B(C F ) ]
6 5 4 2
for a single-crystal X-ray structure investigation; in this instance it was revealed that the aromati-
city of the 4,4’-bipyridine ligand, that had been disrupted upon reduction, had been regained. A rare cationic
0
d
3
metal tris-amide complex, shown by X-ray crystallography to contain an intriguing pyramidal TiN core
t
+
Received 9th January 2015,
Accepted 12th February 2015
3 6 5 4
geometry, namely {Ti(N[ Bu]Ar) } , could also be isolated when using [B(C F ) ] as the essentially non-inter-
acting counter-ion. This highly reactive cation should be considered as a potential intermediate in the
DOI: 10.1039/c5dt00105f
t
plethora of reactions wherein Ti(N[ Bu]Ar)
3
has been shown to effect the reduction of substrates including
www.rsc.org/dalton
halogenated organic molecules, carbonyl compounds, organic nitriles, and metal complexes.
nitrile, with the molybdenum(III) tris-anilide complex Mo-
1
Introduction
t
(
N[ Bu]Ar) in this case responsible for reductive complexation
3
This investigation took shape as an inquiry into the potential of the organic nitrile prior to coupling with titanium-com-
t
2
ability of the tris-anilide titanium(III) complex Ti(N[ Bu]Ar)
3
plexed pyridine. Related to Rothwell’s para-coupled pyridine
(
Ar = 3,5-Me C H ) to reductively bind or couple pyridine. Pyri- dimer by dehydrogenation is a system in which 4,4′-bipyridine
2
6
3
dine coupling as a route to bipyridines is a reaction of signifi- bridges two titanium centers, and accordingly, this article
t
cant interest, and it has been shown previously by Rothwell treats the ability of Ti(N[ Bu]Ar)
3
(Fig. 1) to reductively complex
et al. that pyridine reductive coupling could be effected by a that organic molecule in a 2 : 1 fashion. A study of the redox be-
t
titanium(II) synthon resulting in a dititanium(III) system con- havior of the (4,4′-bipy){Ti(N[ Bu]Ar)
}
3 2
bimetallic system led to
1
taining a bridging {–NC H C(H)–C(H)C H N–} moiety. In the significant discovery that the highly electrophilic cation {Ti-
addition, we showed previously that Ti(N[ Bu]Ar) was capable (N[ Bu]Ar) } could be isolated when generated in particular as
4 4 4 4
t
t
+
3
3
−
6 5 4
of promoting pyridine reductive cross-coupling with benzo- its {[B(C F ) ]} salt. The synthesis and structural characteriz-
t
+
3
ation of cation {Ti(N[ Bu]Ar) } is a key finding delivered in the
present work, as this species is a plausible reactive intermediate
in a variety of reductive processes effected by the special one-
6
7
-435 Department of Chemistry, Massachusetts Institute of Technology,
7 Massachusetts Avenue, Cambridge, MA 02139-4307, USA.
t
electron reducing agent that is embodied by Ti(N[ Bu]Ar)
3
.
E-mail: ccummins@mit.edu; Tel: +(617) 253-5332
Electronic supplementary information (ESI) available: Spectra, tables of crystal-
†
1
.1 Reductions effected by titanium tris-anilide
lographic data, DFT-optimized coordinates. CCDC 893531–893535. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
C5DT00105F
t
As a one-electron reducing agent, Ti(N[ Bu]Ar)
pared to SmI2, but in general Ti(N[ Bu]Ar) is “hotter” in
being able to effect halogen atom abstraction from less-acti-
vated halogen-containing organic molecules than SmI
3
may be com-
4
t
3
‡
Present address: The Dow Chemical Company, 1776 Building, Midland,
MI 48674, USA.
Present address: SAFC Hitech, 5485 County Road V, Sheboygan Falls,
WI 53085, USA.
2
is
§
capable of; an example of this is X abstraction from stoichio-
6784 | Dalton Trans., 2015, 44, 6784–6796
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
two electron reduction of the 4,4′-bipyridine bridge by the two
1
equivalents of titanium(III) tris-anilide; the H NMR chemical
shifts observed for the bipyridine protons (δ = 5.43 and
6
.97 ppm) are typical of those observed in other doubly
1
3,14
reduced 4,4′-bipyridine species.
t
2
.2 Molecular structure of (4,4′-bipy){Ti(N[ Bu]Ar) }
3 2
t
Crystals of (4,4′-bipy){Ti(N[ Bu]Ar) } that were grown from a
3
2
saturated toluene solution were found to be suitable for analy-
sis by X-ray crystallography; the space group was P ˉ1 with a crys-
tallographic inversion center located at the midpoint of the
t
Fig. 1 Line drawing of neutral Ti(N[ Bu]Ar)
3
based upon coordinates central C–C bond. The observed molecular structure of (4,4′-
3
from an X-ray crystal structure and emphasizing (dashed lines) the two
aryl groups that make close contact with the electrophilic d metal
t
3 2
bipy){Ti(N[ Bu]Ar) } confirms the presence of a reduced 4,4′-
1
bipyridine bridge in the molecule (Fig. 2). The central C–C
bond is markedly contracted (1.394(2) Å) relative to the single
C–C bond in free 4,4′-bipyridine (1.484(2) Å), and is essentially
center.
metric bromobenzene or iodobenzene to produce the corres- equal in length to the double C–C bond observed in bis(tri-
t
ponding titanium(IV) complex XTi(N[ Bu]Ar) rapidly and quan- methylsilyl)dihydro-4,4′-bipyridine (1.382(3) Å), another
3
5
6
13,14
titatively at 25 °C in a nonpolar solvent such as benzene.
While 7-chloronorbornadiene similarly gave a rapid reaction,
doubly-reduced derivative of 4,4′-bipyridine.
The shortest
C–C distance observed in the pyridyl rings of the complex
neat chlorobenzene proved to be less reactive, requiring ca. 8 h (C41–C42 = 1.358(2) Å) also points to the existence of localized
t
to produce ClTi(N[ Bu]Ar)
3
(also in essentially quantitative C–C double bonds in the molecule. The Ti1–N4 distance
yield), while fluorobenzene was observed to be effectively inert (1.9913(13) Å) is similar to the average Ti1–N distance for the
t
5
t
to Ti(N[ Bu]Ar) . Since Ti(N[ Bu]Ar) has little propensity to anilide ligands (1.97 Å), indicating that N4 may be regarded
3
3
capture organic radicals, its halogen abstraction propensity similarly as an amide nitrogen atom, as would be expected in
2
−
could be harnessed in a radical synthesis of PR molecules uti- a system containing a [4,4′-bipyridine] moiety. A search of
3
7
lizing white phosphorus. In general, carbonyl compounds are the Cambridge Structural Database returned one other system
leading to ketyl radical chem- containing two titanium atoms bridged by a 4,4′-bipyridine
istry; this behavior is similar to that reported by Covert and ligand. In 2005, Kraft et al. reported a molecular square
t
attacked rapidly by Ti(N[ Bu]Ar)
3
5
Wolczanski for the related three-coordinate titanium(III)
t
8
complex Ti(OSi Bu
3
)
3
.
tert-Butyl esters react giving substi-
t
tution of tert-butyl radical by Ti(N[ Bu]Ar) , a facile pathway for
the synthesis of formate derivatives. Titanium tris-anilide has
3
9
also been shown to reduce inorganic substrates, for example in
1
0
the synthesis of WCl (THF) from WCl (DME), in the reduc-
3
3
4
3
,11
tive complexation of metal oxo and nitrido systems,
the synergistic (together with Mo(N[ Bu]Ar) ) binding of N .
3 2
Additionally, there is evidence that Ti(N[ Bu]Ar) engages in
reversible) reductive complexation of CO and pyridine.
and in
t
12
t
3
2
(
2
2
Results and discussion
t
2
.1 Reaction of Ti(N[ Bu]Ar)
3
with 4,4′-bipyridine
t
The dinuclear titanium complex (4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
was
synthesized in excellent yield (89%) via treatment of a bright
t
green solution of Ti(N[ Bu]Ar) in thawing Et O (−110 °C) with
3
2
0
.5 equiv. of 4,4′-bipyridine. Addition of the latter reactant eli-
cited a color change from green to black and then the product
t
(
4,4′-bipy){Ti(N[ Bu]Ar) } precipitated as a fine black powder.
3 2
The bimetallic complex is diamagnetic, and its structure (a
single anilide ligand environment and D2h symmetry of the
1
bridging 4,4′-bipyridine ligand) was readily assigned using H
13
1
and C NMR spectroscopies. Integration of H NMR signals
indicated that there were six anilide ligands for every one mole-
cule of 4,4′-bipyridine. The complex is diamagnetic by virtue of the 50% probability level.
t
3 2
Fig. 2 Diagram of (4,4’-bipy){Ti(N[ Bu]Ar) } with thermal ellipsoids at
15
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 6784–6796 | 6785

Paper
Dalton Transactions
formed from four molecules of 4,4′-bipyridine connected by
1
4
four titanocene fragments at the corners. In this case the
,4′-bipyridine ligands were only partially reduced, with intera-
4
tomic distances intermediate between those of neutral 4,4′-
bipyridine and bis(trimethylsilyl)dihydro-4,4′-bipyridine.
t
2
.3 Electronic structure of (4,4′-bipy){Ti(N[ Bu]Ar) }
3
2
To gain insight into the electronic structure of the neutral diti-
tanium complex of 4,4′-bipyridine, density functional theory
(DFT) calculations were performed on a truncated version of
the molecule: (4,4′-bipy){Ti(N[Me]Ph) } ; the model was built
3
2
based upon the crystallographic atomic coordinates for (4,4′-
t
bipy){Ti(N[ Bu]Ar)
3 2
} and then geometry-optimized. The calcu-
lated HOMO (Fig. 3) contains both π-bonding and π-antibond-
ing character, with the most significant contribution coming Fig. 4 Electronic absorption spectrum of (4,4’-bipy){Ti(N[ Bu]Ar) } as a
t
3
2
from the central C–C π-bond; the pattern of π bonding in the solution in THF.
HOMO corresponds nicely to what is likely the dominant
Lewis resonance structure for this system. Notably, the HOMO
the presence of a strong ligand-to-metal charge transfer band
also contains a contribution from Ti–N π-bonding. The LUMO
is composed almost exclusively of metal-centered empty d
orbital character. A significant HOMO–LUMO energy gap is
predicted by the calculation (0.83 eV) and this is supported by
−
1
−1
at 910 nm (ε = 32 000 M cm ) in the UV/Vis spectrum of
t
black (4,4′-bipy){Ti(N[ Bu]Ar) } (Fig. 4).
3
2
2
.4 Electrochemistry and chemical redox of (4,4′-bipy)-
t
{
Ti(N[ Bu]Ar) }
3 2
t
The electronic structure of (4,4′-bipy){Ti(N[ Bu]Ar) } leads to
3
2
interesting electrochemical behavior in solution. Cyclic voltam-
t
metry performed on THF solutions of (4,4′-bipy){Ti(N[ Bu]-
Ar) } (Fig. 5) indicates two reversible oxidation events and two
3
2
quasi reversible reduction events and the electrochemical data
are summarized in Table 1. The observed redox features can
be explained by considering the different charged states of the
molecule (illustrated in Fig. 6 for the isolable neutral, cationic,
and di-cationic species). Starting from the neutral species,
2
−
removal of an electron from the [4,4′-bipy] ligand results in
formation of a radical cation without any change to titanium
oxidation state. Nevertheless, it is possible to draw a resonance
structure for this species wherein the unpaired electron
resides on one of the titanium centers, such that the radical
cation could also be formulated as a mixed-valent titanium(III)/
titanium(IV) species. Removal of an additional electron from
t
the radical cation leads to dicationic [(4,4′-bipy){Ti(N[ Bu]-
2
+
Ar)
bipy ligand.
Sweeping negative, addition of an electron to (4,4′-bipy)-
3 2
} ] , in which aromaticity is restored to the bridging 4,4′-
t
{
3 2
Ti(N[ Bu]Ar) } converts one of the titanium(IV) centers in the
molecule to titanium(III) and generates an anionic mixed-
valent complex. Addition of a second electron reduces the
remaining titanium(IV) center to titanium(III). It is indicated
t
that all five charge states of (4,4′-bipy){Ti(N[ Bu]Ar) } should
3
2
be accessible through chemical oxidation and reduction reac-
tions, with the exception of the second reduction event. At
+
−
2.7 V vs. the Fc/Fc couple, the latter is too near the solvent
window. Here we note that the cyclic voltammogram of bis(tri-
methylsilyl)dihydro-4,4′-bipyridine exhibits two reversible oxi-
dation waves at potentials (vs. Fc/Fc ) nearly identical to those
Fig. 3 Frontier MO energy levels calculated for model system (4,4’-
1
6
+
bipy){Ti(N[Me]Ph)
3
}
2
.
6786 | Dalton Trans., 2015, 44, 6784–6796
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
1
8
Differential pulse voltammetry (DPV) was used to obtain
accurate E1/2 values for each of the one-electron couples
observed in the cyclic voltammogram. The comproportiona-
tion constants K
valence species [(4,4′-bipy){Ti(N[ Bu]Ar) } ] in order to evalu-
c
were calculated for the proposed mixed-
t
+
3
2
ate the extent of electronic communication between the tita-
nium centers. The K values were derived from the potential
c
1
7
separation between ΔE1 between couples (Table 1). For the
cation [(4,4′-bipy){Ti(N[ Bu]Ar)
corresponds to K = 2.6 × 10 , which is in the range usually
c
associated with Class II mixed-valent compounds. Therefore,
some degree of electronic communication is occurring
between the two metal centers. This communication is more
pronounced in the anion, [(4,4′-bipy){Ti(N[ Bu]Ar) } ] , where
the larger ΔE1/2 value of 0.43 V corresponds to K = 1.9 × 10 . A
c
/2
t
+
3
5
}
2
] , the ΔE1/2 value of 0.32 V
1
9
t
−
3
2
7
t
3 2
Fig. 5 Cyclic voltammogram for (4,4’-bipy){Ti(N[ Bu]Ar) } in THF with a
greater degree of electronic delocalization is therefore observed
sweep rate of 800 mV s− referenced vs. Fc/Fc with [N Bu
0.2 M) as the supporting electrolyte. The arrow signifies the rest poten-
1
+
n
F
)
4
]
4
][B(C
6
5
in the mixed-valent anion than in the mixed-valent cation.
(
t
Chemical oxidation reactions of (4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
tial and the direction of the sweep.
have shown that it is possible to release an equivalent of 4,4′-
bipyridine from the molecule. For example, treatment of (4,4′-
t
t
bipy){Ti(N[ Bu]Ar)
3
}
2
with an equivalent of iodine resulted in
Table 1 Electrochemical data for the complex (4,4’-bipy){Ti(N[ Bu]-
t
its quantitative conversion to two equivalents of ITi(N[ Bu]-
Ar)
3
}
2
5
,11
Ar)3
with concomitant release of an equivalent of 4,4′-bipyr-
idine. A similar transformation was also achieved with two
equivalents of the outer-sphere oxidant ferrocenium triflate
(Fc[OTf]), and demonstrates that the reduction of 4,4′-bipyri-
dine by titanium(III) trisanilide Ti(N[ Bu]Ar)
a
b
c
Couple
E
1/2/V
ΔE1/2/V
0.43
K
c
7
5
2
1
0
1
−/1−
−2.65
−2.22
−1.36
−1.04
1.9 × 10
2.6 × 10
−/0
/1+
+/2+
0.32
t
3
is chemically
with a single
t
reversible. Treatment of (4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
a
b
E
1/2 values are referenced versus ferrocene/ferrocenium. Separation equivalent of Fc[OTf] did not result in conversion to a one-
between two adjacent half-wave potentials representing the range
wherein the respective ion exists. Calculated according to ln K =
c
electron oxidation product, but instead, half an equivalent of
the dinuclear complex was consumed in being oxidized by two
electrons, leaving half an equivalent unreacted. This is not
entirely surprising considering how closely spaced the two oxi-
dation events are in the cyclic voltammogram and that only
mild oxidizing agents are required for both transformations.
c
1
7
−nFΔE1/2/RT.
t
The one-electron oxidation product [(4,4′-bipy){Ti(N[ Bu]-
] is available via oxidation with a ferrocenium salt con-
+
Ar)
3
}
2
taining a weakly coordinating anion. For example, treatment of
t
20,21
(
3 2 6 5 4
4,4′-bipy){Ti(N[ Bu]Ar) } with one equivalent of Fc[B(C F ) ]
at room temperature in THF resulted upon mixing in a color
change from black to brown. After separation from ferrocene,
−
t
+
6 5 4 3 2
the [B(C F ) ] salt of [(4,4′-bipy){Ti(N[ Bu]Ar) } ] was isolated
as a brown powder (84% yield). The latter cation proved to be
completely NMR silent. It also was found to be sensitive ther-
mally, appearing gradually to decompose in solution at room
temperature over a period of a few days. Efforts to grow crystals
of this salt at low temperature did not succeed.
t
+
3 2
The radical nature of [(4,4′-bipy){Ti(N[ Bu]Ar) } ] was con-
firmed by EPR spectroscopy. A room-temperature EPR spec-
trum of a toluene solution containing the cation displayed
only one signal (g = 2.0004), with no additional hyperfine
coupling. The g-value of the signal is typical of an organic
π-radical; however, this does not rule out a resonance contribu-
Fig. 6 Isolable in three states of charge, the figure depicts schemati-
t
cally (4,4’-bipy){Ti(N[ Bu]Ar)
3 2
} , its mono-cation, and di-cation, the latter
cations both having been isolated as perfluoro-tetraphenyl borate salts.
1
3
observed here; however, for that molecule the two reversible tor to the radical cation in which the unpaired electron is
reduction waves are absent as there are present at silicon vis-à- located on one of the titanium ions. As the HOMO of the
vis titanium no low-lying empty d orbitals.
neutral precursor is largely π-bonding in nature, removal of an
Dalton Trans., 2015, 44, 6784–6796 | 6787
This journal is © The Royal Society of Chemistry 2015

Paper
Dalton Transactions
electron from this orbital should result in π bonds of reduced
order. Consistent with this, the IR spectrum of [(4,4′-bipy)-
t
+
−
{
Ti(N[ Bu]Ar)
3
}
2
] (as its [B(C
6
F
5
)
4
] salt) displays a CvC stretch
−1
−1
at 1614 cm , compared to the CvC stretch at 1622 cm that
is observed for the neutral precursor. Additionally, the UV/Vis
spectrum of the radical cation in THF shows a band at 965 nm
ε = 10 000 M− cm ), this being less intense than the corres-
1
−1
(
−1
−1
ponding band observed at 910 nm (ε = 32 000 M cm ) in the
unoxidized species. This can be interpreted as a diminution of
the LMCT interaction in the system upon oxidation.
In line with the inference that one-electron oxidation of
t
(
4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
is not accompanied by any major
structural change, we found that the cation [(4,4′-bipy){Ti-
t
+
(
N[ Bu]Ar) } ] can be reduced back to its neutral and diamag-
3 2
netic precursor by treatment with decamethyl cobaltocene.
The cation can also be further oxidized by treatment with an
additional equivalent of Fc[B(C F ) ] to yield the new dicatio-
6
5 4
t
2+
−
nic complex [(4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
]
6 5 4 2
as its ([B(C F ) ] )
salt. The latter red salt, as expected, could also be accessed (in
t
8
4% yield) upon treatment of (4,4′-bipy){Ti(N[ Bu]Ar) } with
3 2
6 5 4
two equivalents of Fc[B(C F ) ]. Additionally, two equiv. of the
t
+
radical cation [(4,4′-bipy){Ti(N[ Bu]Ar)
3 2
} ] are generated quan-
titatively upon mixing of one equivalent each of neutral (4,4′-
t
t
2+
3 2 3 2
bipy){Ti(N[ Bu]Ar) } and [(4,4′-bipy){Ti(N[ Bu]Ar) } ] , demon-
strating the relationship among this series of complexes whose
compositions differ only in terms of their state of charge.
−
Crystals of the ([B(C
6
F
5
)
4
] )
2
salt of dication [(4,4′-bipy){Ti-
t
2+
(
N[ Bu]Ar)
3
}
2
]
suitable for an X-ray diffraction study were Fig. 7 Diagram (50% probability ellipsoids) of dication [(4,4’-bipy){Ti-
grown by liquid–liquid diffusion of hexane into an o-difluoro- (N[ Bu]Ar)
t 2+
−
3
}
2
]
6 5 4
with hydrogen atoms, [B(C F ) ] ions, and an o-difluor-
15
obenzene solvent molecule omitted.
benzene solution. o-Difluorobenzene is polar and has a higher
dielectric constant (D = 13.8) than THF (D = 7.4), but is a
weaker donor. The ([B(C F ) ] ) salt of dication [(4,4′-bipy)-
s
s
2
2
−
6
5 4
2
in rearomatization of the 4,4′-bipyridine unit. In further
support of the latter interpretation, the Ti–N(bipyridine) dis-
tances in [(4,4′-bipy){Ti(N[ Bu]Ar) } ] (2.209(2) Å and 2.213(2)
t
2+
{
Ti(N[ Bu]Ar)
3
}
2
]
is insoluble in alkane and arene solvents,
but dissolves in coordinating solvents including THF and pyri-
dine. However, dissolution in pyridine-d5 results in a ligand
displacement reaction, liberating 4,4′-bipyridine and resulting
t
2+
3
2
Å) are substantially longer than they are in neutral (4,4′-bipy)-
t
{
Ti(N[ Bu]Ar) } (1.9913(13) Å), and are characteristic of the
3 2
t
+
in coordination of pyridine-d
moieties (vide infra). When the dication salt is dissolved in
THF-d , the THF-d solvent molecules compete with 4,4′-bi-
pyridine for binding to the {Ti(N[ Bu]Ar)
in an equilibrium between free and bound 4,4′-bipyridine
5
to the liberated {Ti(N[ Bu]Ar)
3
}
23
interaction between a neutral N-donor and titanium(IV).
Compensating for the weaker Ti–N(bipyridine) bonds, the
Ti–N(anilide) distances in the dication (avg. = 1.92 Å) are sub-
stantially shorter than in the neutral precursor (avg. = 1.97 Å),
consistent with increased π-donation to the cationic titanium
centers from the anilide nitrogens. As well, there appears to be
an additional bonding interaction between C11 and Ti1
C11⋯Ti1 = 2.500(3) Å), and C41 also closely approaches Ti2
C41⋯Ti2 = 2.554(3) Å). It is not unprecedented for N-tert-buty-
lanilide ligands to bind in an η or even in an η fashion in the
8
8
t
+
3
} cations, resulting
1
being observed in the solution H NMR (benzene-d
00 MHz) spectrum. Thus, the use of o-difluorobenzene as a
crystallization solvent was key to determining the solid-state
structure of the dication’s ([B(C F ) ] ) salt. The molecular
6 5 4 2
structure of dication [(4,4′-bipy){Ti(N[ Bu]Ar)
6
, 20 °C,
5
(
(
−
t
2+
}
2
]
is shown in
2
3
3
Fig. 7.
3
process of stabilizing an electrophilic metal center.
The most striking structural feature of dication [(4,4′-bipy)-
is the twist about the central C–C bond of
the bipyridine (torsion angle = 37.16°). A twist of similar mag-
nitude is observed in free 4,4′-bipyridine (torsion angle =
4.20°). Additionally, the central C–C bond is 1.484(4) Å in The dication of the previous section can reasonably be viewed
length, coincident with the corresponding distance in free 4,4′- as two {Ti(N[ Bu]Ar) } cations linked by a neutral 4,4′-bipyri-
bipyridine (1.484(2) Å) and 0.09 Å longer than in neutral (4,4′- dine ligand. As the {Ti(N[ Bu]Ar)
t
2+
{
Ti(N[ Bu]Ar)
3
}
2
]
t
2
.5 Synthesis and molecular structure of cation {Ti(N[ Bu]-
+
−
Ar) } ([B(C F ) ] salt)
3
6 5 4
1
4
3
t
+
3
t
+
3
}
cation had never been
t
bipy){Ti(N[ Bu]Ar)
3
}
2
(1.394(2) A). This demonstrates that the independently synthesized in our laboratory, we set out to
removal of two electrons from (4,4′-bipy){Ti(N[ Bu]Ar) } results accomplish this via oxidation of Ti(N[ Bu]Ar) using [FeCp2]-
t
t
3
2
3
6788 | Dalton Trans., 2015, 44, 6784–6796
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Dalton Transactions
Paper
t
[
B(C
6 5 4
F ) ]. Treatment of a green, ethereal solution of Ti(N[ Bu]-
Ar) with an Et O slurry of [FeCp ][B(C F ) ] at room tempera-
3
2
2
6 5 4
ture resulted upon mixing in a color change to red. After
removal of solvent under reduced pressure, what remained
was a red oil that could be converted to an orange powder
upon repeated trituration using n-hexane. The orange powder
t
was determined to be the salt {Ti(N[ Bu]Ar)
3
}[B(C
6
F
5
)
4
],
obtained (90% yield) in a form that is entirely free of any co-
t
ordinating solvent molecules. In this form, salt {Ti(N[ Bu]Ar)
B(C ] is insoluble in Et O, benzene and toluene, but does
dissolve in these solvents upon addition of a few drops of THF.
3
}-
[
6
F
5
)
4
2
−
6 5 4
The relative inertness of the [B(C F ) ] counter-ion is key to
t
+
the isolation of the {Ti(N[ Bu]Ar)
3
} cation. Prior experiments
t
+
aimed at similarly generating the cation {Ti(N[ Bu]Ar) }
via ferrocenium oxidation using [FeCp
instead resulted in isolation of the fluoride, FTi(N[ Bu]Ar)
is likely that the reactive {Ti(N[ Bu]Ar) } cation is formed
3
2
4
2 6 3 3 2 4
][B[3,5-C H (CF ) ] ]
t
5
3
. It
t
+
3
t
+
Fig. 9 Diagram of cation {(THF)Ti(N[ Bu]Ar)
ellipsoids. The [B(C
3
}
with 50% probability
under the latter conditions but that it abstracts fluoride from
15
−
6 5 4
F ) ] counter-anion, ether molecule of crystalliza-
−
the insufficiently inert [B[3,5-C
6
H
3
(CF
3
)
2
]
4
] anion.
tion and hydrogen atoms are omitted for clarity (no close anion/cation
t
Crystals of {Ti(N[ Bu]Ar) }[B(C F ) ] were grown by liquid– contacts were observed).
3
6 5 4
liquid diffusion of hexane into an o-difluorobenzene solution
of the salt, conducive to an X-ray diffraction study (Fig. 8). The
feature common to all of the neutral M(N[R]Ar)3 complexes
t
+
titanium(IV) ion of cation [Ti(N[ Bu]Ar) ] pulls the N-tert-buty-
3
25
26
3
that have been characterized to date (M = Ti, V, Cr,
lanilide ligands in very close (Ti1–N = 1.8788(15), 1.8830(16),
and 1.8937(16) Å). Additionally, all three anilide ligands in this
otherwise unsolvated cation appear to engage titanium in
12,27–29
Mo
3
). Trigonal pyramidal MN units are, however, a
characteristic structural feature for homoleptic rare earth
30
metal phenyl(trimethylsilyl) amide complexes. In addition,
2
an η -N,C fashion, with the Ti1–C distances (2.3870(18),
Andersen has remarked on the trigonal pyramidal coordi-
2
.3984(18), and 2.4048(19) Å) being shorter than is typically
31
nation environment at uranium in U[N(SiMe3)2]3.
Finally,
2
3
observed for η or η anilide aryl ring ligation to titanium; for
example, see the structure of {(THF)Ti(N[ Bu]Ar)
Hursthouse et al. showed that the coordination environment
at scandium in Sc[N(SiMe is similarly trigonal pyramidal
with N–Sc–N angles of ca. 117°.
The N-tert-butylanilide ligands in the otherwise unsolvated
t
+
3
} , described
3 2 3
) ]
below (Fig. 9).
32
A most unusual feature of the structure (Fig. 8) is the trigo-
nal pyramidal nature of the TiN core unit (the average N–Ti–N
angle is ca. 111.3°); despite the presence of two η anilide
3
t
+
[
3
Ti(N[ Bu]Ar) ] cation can certainly be viewed as providing a
3
protective steric sheath about the electrophilic titanium(IV)
center that is essential, together with the inert counter anion,
for its isolation. Inspection of the packing arrangement in
this crystal structure reveals no significant cation–anion
interactions.
t
3
interactions in the structure of neutral Ti(N[ Bu]Ar) , the TiN
3
3
moiety in that case is rigorously trigonal planar, a structural
t
+
2
.6 Electronic structure of cation {Ti(N[ Bu]Ar)
3
}
t
+
The trigonal pyramidal nature of {Ti(N[ Bu]Ar)
3
}
is not the
only unusual feature of the observed structure. The Ti–N–
CMe3 angles are quite obtuse (average value, 147.08°) while
the Ti–N–C(aryl) angles are correspondingly acute (avg.:
91.23°). The latter acute Ti–N–C(aryl) angles bring the three
aryl ipso carbon atoms in suggestively close to the titanium
center, such that structure drawing programs (based on their
default element radii) draw lines between Ti1 and each of C11,
C21, and C31. The average Ti–C(aryl ipso) distance is 2.3967 Å.
Because of these relatively close Ti–C(aryl ipso) contacts, it is
tempting to conclude that there is an associated chemical
bonding interaction. In order to probe for this, an Atoms in
3
3
34
t
+
Molecules (AIM) analysis was carried out on a wavefunction
Fig. 8 Diagram of cation {Ti(N[ Bu]Ar)
3
}
with 50% probability ellip-
1
5
−
that was calculated at the observed structure (only the hydro-
gen positions were optimized) with the result that no bond
soids. The [B(C F ) ] counter-anion and hydrogen atoms are omitted
for clarity (no close anion/cation contacts were observed).
6 5 4
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 6784–6796 | 6789

Paper
Dalton Transactions
critical points were located between titanium and the aryl ipso
carbon atoms; by this electron density criterion, no chemical
bonds exist between titanium and the aryl ipso carbon atoms
and we must conclude that the relatively close contacts occur
incidentally as a consequence of the titanium–nitrogen mul-
tiple bonding together with unusual hybridization at nitrogen.
t
Regarding the Ti–N bonding being multiple in {Ti(N[ Bu]-
+
35,36
Ar) } , natural bond orbital (NBO)
and natural resonance
3
3
7–39
34
theory (NRT)
AIM leads to the conclusion that each nitrogen is tetravalent
engages in four electron-pair bonds), that the titanium–nitro-
analysis of the same wavefunction used for
(
gen natural bond order is ca. 1.96, and that the natural valence
at titanium is six (titanium engages in six electron-pair
4
0
bonds). This analysis also gives Ti–C(aryl ipso) bond orders
of near zero, consistent with the lack of associated bond criti-
cal points from the AIM analysis. The indicated TivN double
bonding is associated with short titanium–nitrogen inter- Fig. 10 Diagram of n-butoxy complex BuOTi(N[ Bu]Ar)3 with 50%
atomic distances (avg.: 1.885 Å; for reference, an average value probability ellipsoids. The molecule has a crystallographic C
n
t
1
5
3
axis
coincident with the Ti–O vector and the n-butyl group occupies two
of 1.928 Å was reported for the corresponding chloride deriva-
t
41
positions (one shown).
tive, ClTi(N[ Bu]Ar) ).
3
2
.7 Known metal tris-amide cations
yellow powder, characterization of which revealed it to be the
+
n
t
The metal tris-amide cations [M{N(SiMe ) } ] , where M is zir- n-butoxy complex BuOTi(N[ Bu]Ar) that ostensibly arose by
3
2 3
3
conium or hafnium, have been generated by methide abstrac- way of hydride attack on a coordinated THF ligand. Complexa-
4
2
tion using B(C
6
F
5
)
3
.
The latter cations were studied by X-ray tion by strongly Lewis-acidic metal cations is known to activate
4
7
crystallography and found to exhibit short M ← (Si–C) contacts THF to nucleophilic attack; In one example, addition of
interpreted as weak bonding interactions that serve to stabilize KCp* to the cation [Cp* Sm(THF) ] (as its tetraphenylborate
the positive charge in these systems. In view of the AIM and salt) led to THF ring-opening and C–C bond formation, rather
NBO results discussed above for the bonding in similarly trigo- than to the formation of the desired product, SmCp*3.
nal pyramidal {Ti(N[ Bu]Ar)
contacts as the reason for the pyramidal nature of the MN
M = Zr, Hf) unit in these cationic amides may warrant reconsi-
deration. Donor-free, cationic transition-metal tris-amide com-
plexes are otherwise unknown. An example of a donor-stabilized
titanium tris-amide cation that has been characterized crystallo-
+
2
2
4
2
4
8
t
3
}, the invocation of M ← (Si–C)
t
+
2.9 Structure of cation {(THF)Ti(N[ Bu]Ar)
3
}
3
(
Both the n-butoxy complex and the THF adduct {(THF)Ti-
t
+
+
(
N[ Bu]Ar) } (the latter as its [B(C F ) ] salt) have been iso-
3 6 5 4
lated and characterized by single-crystal X-ray diffraction
studies (Fig. 9 and 10). In the case of the cationic THF adduct
graphically is the simple homoleptic dimethylamide-derived
t
+
{
(THF)Ti(N[ Bu]Ar) } , we find that binding of the THF mole-
3
+
system [(Me
2
NH)Ti(NMe
2 3
) ]
of importance in alkyne carbo-
cule displaces two of the three Ti⋯Carylipso close contacts invol-
4
3
amination catalysis. A cationic titanium ferrocene-diamide
system has been generated by reaction of a dimethyl precursor
ving the anilide ligands with respect to the structure observed
t
+
in the solid state for the solvent-free cation, {Ti(N[ Bu]Ar) }
3
4
4
3 6 5 4
with the trityl borate [CPh ][B(C F ) ] reagent.
(
Fig. 8). The relevant distance that characterizes the remaining
Ti1⋯C31 close contact, 2.5250(16) Å, is longer by greater than
.1 Å than any of the three such distances observed for {Ti-
2
.8 Binding and ring-opening of tetrahydrofuran
0
(
We hypothesized that the highly electrophilic titanium(IV)
t
+
N[ Bu]Ar) } (Fig. 8). The Ti1–O1 T distance of 2.0962(12) Å
3
t
+
center in the {Ti(N[ Bu]Ar) } cation might allow us to access
3
characterizing the titanium–THF interaction is not remark-
able, but it should be noted that the crystallographic data
reported here (Fig. 9) represent rare structural characterization
of a group 4 metal cation having the general composition
some previously inaccessible species, one target of interest
being the neutral titanium(IV) hydride, HTi(N[ Bu]Ar)
obtained, the latter would be a member of the small set of
documented titanium(IV) hydride complexes that includes HTi-
(PMe ), where Dipp represents 2,6-C H - Pr , as well
3 3 6 3 2
as {HTiCp*(NP Bu )(THF)} . Addition of stoichiometric [Li]-
t
3
. If
1
2
+
{
(THF)M(NR R ) } .
3
4
5
i
(ODipp)
t
+ 46
3
[
HBEt
of {Ti(N[ Bu]Ar)
in any immediate sign of reaction, but the color of the reaction
mixture gradually turned from red to orange to yellow as the All manipulations were performed under an atmosphere of
solution warmed to room temperature. After workup, a major purified N in a vacuum atmospheres model MO-40M glovebox
product could be obtained in 58% yield as a free-flowing or in standard Schlenk glassware in the hood. Solvents were
3
] (as a 1.0 M solution in THF) to a thawing red solution
3
Experimental details
t
3
6 5 4
}[B(C F ) ] in o-difluorobenzene did not result
3.1 General procedures
2
6790 | Dalton Trans., 2015, 44, 6784–6796
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
obtained anhydrous and oxygen-free from a Contour Glass diffraction were grown from a saturated solution of the
Solvent Purification System, built by SG Water recently material in toluene, which was left in the freezer at −35 °C for
rebranded as Pure Process Technology (http://www.pureproces- one week. Anal. Calcd: C 75.21; H 8.93; N 8.56. Found: C 74.98;
1
stechnology.com/) with the exception of pyridine which was H 8.78; N 8.67. H NMR (benzene-d
6
, 20 °C, 400 MHz): δ = 7.12
distilled from sodium. All anhydrous solvents were stored at (broad s, 4H, CvC(H)–N), 6.97 (s, 12H, HAr), 6.77 (s, 6H, HAr),
room temperature over 4 Å molecular sieves. Deuterated sol- 5.43 (d, JHH = 8 Hz, 4H, (H)CvC–N), 2.34 (s, 36H, Ar–CH ),
, 20 °C,
d6 was degassed and stored over molecular sieves for at least 100.6 MHz): δ = 150.4 (s, aryl ipso), 137.3 (s, m-Ar), 137.0 (s,
two days prior to use. THF-d was distilled from sodium/ben- central CvC), 129.6 (s, p-Ar), 127.3 (s, o-Ar), 118.4 (s, ring
zophenone and stored over molecular sieves at −35 °C. Pyri- CvC), 105.7 (s, ring CvC), 63.8 (s, NC(CH ), 31.0 (s, ArCH ),
dine-d5 was distilled from calcium hydride and stored over 22.1 (s, NC(CH ) ) ppm. UV/Vis Data (THF, 3.05 × 10 M,
3
1
3
3 3 6
vents were purchased from Cambridge Isotope Labs. Benzene- 1.23 (s, 54H, C(CH ) ) ppm. C NMR (benzene-d
8
3
)
3
3
−
5
3 3
−
1
−1
molecular sieves. Celite 435 (EM Science), 4 Å molecular sieves 20 °C): λmax = 287 nm (ε = 32 000 M cm ), λmax = 379 nm (ε
1
−1
−1
−1
(
2
Aldrich), and alumina (EM Science) were dried by heating at = 19 000 M− cm ), λmax = 910 nm (ε = 32 000 M cm ). IR
00 °C under a dynamic vacuum for at least 24 hours prior to Data (nujol mull, KBr plates): 1623 (s), 1586 (vs), 1354 (s), 1287
use. All glassware was oven-dried at temperatures greater than (vs), 1182 (vs), 1148 (vs), 1043 (vs), 982 (vs), 937 (vs), 885 (vs),
t
−1
1
3
70 °C prior to use. Ti(N[ Bu]Ar) was synthesized following a 717 (vs), 686 (s), 638 (vs), 581 (s), 418 (s) cm .
1
1
reported literature procedure that has also been the subject
of a video (see http://web.mit.edu/ccclab/videos/). Ferrocenium
t
3
.3 Iodine oxidation of (4,4′-bipy){Ti(N[ Bu]Ar) }
3
2
t
triflate was prepared by oxidation of ferrocene with silver tri- Complex (4,4′-bipy){Ti(N[ Bu]Ar)
flate in dichloromethane. All other chemicals were pur- (5 mL) to create a black solution (0.100g, 0.0764 mmol). Iodine
chased from Aldrich and used without further purification (0.020 g, 0.079 mmol) was dissolved in THF (2 mL) to create a
3
}
2
was dissolved in THF
4
9
2
unless otherwise stated. NMR spectra were obtained on Bruker red-brown solution. The I solution was added to the titanium
Avance 400, Varian Mercury 300 or Varian Inova 500 spectro- solution dropwise with stirring. After 30 seconds of stirring,
1
meters. H NMR spectra were referenced internally to residual the reaction mixture turned from black to orange. The reaction
solvent resonances (benzene-d
6
, 7.16 ppm; THF-d
8
, 3.58 ppm; mixture was allowed to stir for half an hour at which point the
1
3
pyridine-d , 8.74 ppm). C NMR spectra were referenced solvent was removed under vacuum. The resulting crude
5
internally to solvent signals (benzene-d
6
, 128.39 ppm; THF-d
8
,
6
material (orange powder) was dissolved in benzene-d for ana-
1
9
1
6
7.57 ppm). F NMR spectra were referenced externally to lysis by H NMR spectroscopy. This demonstrated that tita-
t
5
CFCl . EPR spectra were collected on a Bruker EMX spectro- nium(IV) iodide complex ITi(N[ Bu]Ar) was cleanly formed in
3
3
1
meter outfitted with 13-inch magnets, an ER 4102ST cavity and quantitative yield, together with free 4,4′-bipyridine. H NMR
a Gunn diode microwave source producing X-band (8–10 GHz) (benzene-d , 20 °C, 400 MHz): δ = 8.57 (dd, JHH = 4 Hz and 2
6
radiation. Samples for IR spectroscopy were prepared as Nujol Hz, 4H, CvC(H)–N, 4,4′-bipyridine), 6.92 (broad s, 12H, HAr),
mulls on KBr windows and IR spectra were collected on a 6.83 (dd, JHH = 8 Hz, 4H, (H)CvC–N, 4,4′-bipyridine), 6.81
Perkin-Elmer 2000 FT-IR spectrophotometer. UV/Vis spectra (broad s, 6H, HAr), 2.25 (s, 36H, Ar–CH
were collected on a HP8453 spectrophotometer using 1 cm (CH ) ) ppm.
3
), 1.38 (s, 54H, C
3
3
quartz cells manufactured by Starna. Elemental analyses were
performed by Midwest Microlab, LLC, Indianapolis, Indiana.
t
3
.4 Fc[OTf] reaction with (4,4′-bipy){Ti(N[ Bu]Ar) }
3
2
t
Complex (4,4′-bipy){Ti(N[ Bu]Ar) } (0.100 g, 0.0764 mmol) was
dissolved in THF (5 mL) to create a black solution. Ferroce-
3
2
t
3
.2 Preparation of (4,4′-bipy){Ti(N[ Bu]Ar) }
3
2
t
49
Ti(N[ Bu]Ar)
3
(3.01 g, 5.22 mmol) was dissolved in 60 mL of nium triflate (Fc[OTf], 0.026 g, 0.078 mmol) was slurried in
Et O to create an emerald green solution. This solution was THF (3 mL) to create a dark blue suspension. The Fc[OTf] sus-
2
frozen in the dry box cold well. The 4,4′-bipyridine (0.405 g, pension was added to the titanium solution dropwise, with
2
2
.59 mmol) was dissolved in 15 mL of Et O to create a color- stirring. After 30 seconds of stirring, the reaction mixture
less solution. This solution was also frozen in the dry box cold turned from black to dark red-black. The reaction mixture was
well. The thawing bipyridine slurry (4,4′-bipyridine is insoluble allowed to stir for half an hour at which time the solvent was
2
in Et O at low temperatures) was added dropwise with stirring removed under reduced pressure. The resulting crude material
t
to the thawing Ti(N[ Bu]Ar) solution. There was an immediate (dark red powder) was dissolved in benzene-d for analysis by
3
6
1
color change from green to black upon addition. The reaction
H NMR spectroscopy. It is clear from this analysis that no
mixture was allowed to stir for one hour while it slowly one-electron oxidation product was formed. Half of the start-
t
warmed to room temperature. The Et O was removed under ing material (4,4′-bipy){Ti(N[ Bu]Ar) } was oxidized to produce
2
3 2
t
11
vacuum, and the resulting black residue was slurried in 50 mL one equivalent of the triflate complex TfOTi(N[ Bu]Ar)
3
,
one
of n-hexane. The black solid was isolated by filtration and equivalent of ferrocene, and half an equivalent of 4,4′-bipyri-
1
washed with n-hexane until the washings became colorless. dine. H NMR (benzene-d , 20 °C, 400 MHz): δ = 8.57 (dd, J
6
HH
The powder was further dried under dynamic vacuum for two = 4 Hz and 2 Hz, 2H, CvC(H)–N, 4,4′-bipyridine), 7.12 (broad
t
hours. Total yield: 3.12 g, 2.38 mmol, 89% (based on s, 4H, CvC(H)–N, (4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
), 6.97 (s, 6H, HAr,
C H N Ti , 1309.58 g mol ). Crystals suitable for X-ray (4,4′-bipy){Ti(N[ Bu]Ar) } ), 6.83 (dd, J = 8 Hz, 2H, (H)CvC–
HH
−
1
t
8
2
116
8
2
3 2
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 6784–6796 | 6791

Paper
Dalton Transactions
t
N, 4,4′-bipyridine), 6.77 (s, 3H, HAr, (4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
), ferrocene.) After removal of all volatile materials under
t
t
6
.74 (s, 3H, HAr, TfOTi(N[ Bu]Ar) ), 6.56 (broad s, 6H, HAr, reduced pressure, salt [(4,4′-bipy){Ti(N[ Bu]Ar) } ][B(C F ) ] was
), 5.43 (d, JHH = 8 Hz, 2H, (H)CvC–N, (4,4′- isolated as a brown powder. Yield: 0.6382 g, mmol, 84%
3 2 8 2
bipy){Ti(N[ Bu]Ar) } ), 4.00 (s, 10H, ferrocene), 2.34 (s, 18H, (based on C106H116BN F20Ti , 1988.62 g mol ). Anal. Calcd: C
3
3 2
6 5 4
t
TfOTi(N[ Bu]Ar)
3
t
−1
t
Ar–CH , (4,4′-bipy){Ti(N[ Bu]Ar) } ), 2.23 s, 18H, Ar–CH , TfOTi 64.02; H 5.88; N 5.63. Found (Try 1): C 59.76; H 5.67; N 4.85.
3
3 2
3
t
t
(
2
N[ Bu]Ar)
7H, C(CH
3
), 1.24 (s, 27H, C(CH
, (4,4′-bipy){Ti(N[ Bu]Ar) ) ppm.
3
)
3
, TfOTi(N[ Bu]Ar)
3
), 1.23 (s, Found (Try 2): C 59.18; H 5.24; N 5.45. Poor analytical results
t
t
3
)
3
3
}
2
for [(4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
][B(C
6
F
5
)
4
] are likely a result of
partial decomposition of the radical cation at room tempera-
ture. Attempts to further purify the compound failed. The
3
.5 Preparation of [FeCp ][B(C F ) ]
2 6 5 4
This salt was prepared using Geiger’s reported procedure for complex gradually decomposes in solution, even at −35 °C.
2
1
50
Fc[PF6], using [Li(OEt ) ][B(C F ) ] in place of Na[PF ]. In a This is noted by a change in color from brown to green or
2 3
6
5 4
6
1
00 mL round bottom flask, ferrocene (1.00 g, 5.38 mmol) was violet. The salt dissolves in benzene-d
6
to form a dark brown
1
suspended in a mixture of water (20 mL) and acetone (8 mL). solution, but was found to be NMR silent by both H and
To this suspension, FeCl (1.13 g, 6.95 mmol) was added as a
1
9
F NMR spectroscopy. EPR (toluene, 20 °C): singlet at g =
3
−
5
solid, immediately creating a dark green solution and evolving 2.0004. UV/Vis Data (THF, 2.85 × 10 M, 20 °C): λmax = 372 nm
heat. The solution gradually changed from dark green to dark (ε = 20 000 M− cm ), λmax = 965 nm (ε = 10 000 M cm ). IR
blue as it was stirred for 20 minutes. At this point, the dark Data (nujol mull, KBr plates): 1614 (vs), 1587 (s), 1356 (vs),
blue solution was filtered through Celite and a small amount 1279 (vs), 1209 (m), 1167 (vs), 1128 (vs), 1045 (s), 1022 (s),
1
−1
−1
−1
of additional acetone was used to wash the filter cake. The [Li- 974 (vs), 935 (s), 884 (s), 838 (m), 789 (m), 713 (s), 682 (s),
OEt ) ][B(C F ) ] (5.02 g, 5.52 mmol) was added with stirring 667 (s), 638 (s), 566 (w), 423 (w) cm−
1
.
(
2
3
6 5 4
to the dark blue solution. There was no color change, but a
dark blue oil began to form on the sides of the flask. After stir-
ring for 15 minutes, ethanol (10 mL) was added to the reaction (4,4′-bipy){Ti(N[ Bu]Ar) } (0.200 g, 0.153 mmol) was dissolved
t
3
.7 Preparation of [(4,4′-bipy){Ti(N[ Bu]Ar) } ][B(C F ) ]
3 2 6 5 4 2
t
3
2
mixture, causing precipitation of a dark blue solid. The solid in THF (10 mL) to create a black solution. The ferrocenium
was collected by filtration and washed with water (20 mL) and salt, Fc[B(C ] (0.264 g, 0.305 mmol), was dissolved in THF
6 5 4
F )
hexane (20 mL). A second crop of solid was obtained by con- (5 mL) to create a dark blue solution. The ferrocenium solu-
centrating the volume of the filtrate to about 15–20 mL on a tion was added dropwise with stirring to the (4,4′-bipy){Ti-
t
rotary evaporator and then adding additional ethanol (10 mL). (N[ Bu]Ar)
3
}
2
solution, the color of which gradually changed
The blue solid thus obtained was collected and washed in a from black to light red during the course of the addition. The
similar manner. The two crops of solid were combined and reaction mixture was allowed to stir for half an hour at which
dried under vacuum for 48 hours. Yield: 3.28 g, 3.79 mmol, time all volatile materials were removed under vacuum. The
1
9
1
7
2
0%. F{ H} NMR (THF, 20 °C, 282 MHz): 133 (s), 162 (t, J
=
red oil so obtained was dissolved in Et O (5 mL) and to the
2
FF
1
9
1 Hz), 166 (t, JFF = 18 Hz) ppm. Note: the F NMR signals solution was added n-hexane (20 mL). Solvent was removed
show some broadening due to the presence of the paramag- from this mixture under vacuum, yielding a red, oily solid. The
+
netic Fc cation in solution.
oily solid was slurried in n-hexane (20 mL) and then the
mixture was subjected again to solvent removal under reduced
pressure, leaving behind a light red-orange powder. The
t
3
.6 Preparation of [(4,4′-bipy){Ti(N[ Bu]Ar) } ][B(C F ) ]
3
2
6 5 4
t
(
4,4′-bipy){Ti(N[ Bu]Ar) } (0.500 g, 0.382 mmol) was dissolved powder was in turn slurried in n-hexane (20 mL), and then iso-
3 2
in THF (20 mL) to create a black solution. The ferrocenium lated by filtration. The solids were washed with n-hexane until
salt, Fc[B(C ] (0.330 g, 0.381 mmol), was dissolved in THF the washings became colorless. (Note: upon solvent removal
5 mL) to create a dark blue solution. The ferrocenium solu- under reduced pressure, the orange filtrate was found to
tion was added dropwise with stirring to the solution of (4,4′- contain ferrocene.) After complete solvent removal under
6 5 4
F )
(
t
t
bipy){Ti(N[ Bu]Ar)
3 2 3 2 6 5 4 2
} , the color of which gradually changed reduced pressure, salt [(4,4′-bipy){Ti(N[ Bu]Ar) } ]-[B(C F ) ]
from black to brown during the course of the addition. The was isolated as a red powder. Yield: 0.341 g, 0.128 mmol, 84%
116 2 8 2
reaction mixture was allowed to stir for half an hour, at which (based on C130H B N F40Ti
, 2667.66 g mol− ). Crystals suit-
1
time all volatile materials were removed under vacuum. The able for X-ray diffraction were grown by dissolving 0.15 g in
brown oil so obtained was dissolved in Et O (5 mL) and to the 1.5 mL of o-difluorobenzene. The red solution was transferred
2
solution was added n-hexane (20 mL). All volatile materials to an NMR tube, and n-hexane was layered on top. The tube
were again removed under vacuum, yielding a brown, oily was left undisturbed at room temperature for two weeks,
solid. The oily solid was slurried in n-hexane (20 mL) and during which time large, red, shard-like crystals grew on the
then, again, all volatile materials were removed under reduced sides of the tube (crystals began to appear after one day). Anal.
pressure, leaving a brown powder. The brown powder was in Calcd: C 58.50; H 4.38; N 4.20. Found: C 58.38; H 4.34; N 4.19.
1
turn slurried in n-hexane (20 mL), and then isolated by fil-
H NMR (pyridine-d , 20 °C, 400 MHz): δ = 8.86 (d, J = 6 Hz,
5 HH
tration. The solids were washed with n-hexane until the wash- 2H, free 4,4′-bipy), 7.81 (d, JHH = 6 Hz, 2H, free 4,4′-bipy), 6.99
t
+
ings became colorless. (Note: the orange filtrate was (s, 3H, HAr, [(pyridine-d
5
)Ti(N[ Bu]Ar)
3
] ), 6.91 (s, 6H, HAr,
t
+
concentrated to dryness under vacuum and found to contain [(pyridine-d )Ti(N[ Bu]Ar) ] ), 2.31 (s, 18H, Ar-CH , [(pyridine-
5
3
3
6792 | Dalton Trans., 2015, 44, 6784–6796
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
t
+
d
(
5
)Ti(N[ Bu]Ar)
3
] ), 1.29 (s, 27H, C(CH
3
)
3
, [(pyridine-d
5
)Ti- time large, red, block-like crystals grew on the sides of the
t
+
N[ Bu]Ar) ] ) ppm.
tube. Anal. Calcd: C 57.39; H 4.33; N 3.35. Found: C 57.13;
3
1
H 4.61; N 3.07. H NMR (pyridine-d
5
, 20 °C, 400 MHz): δ =
), 1.29
s, 27H, C(CH ) ) ppm. H NMR (THF-d , 20 °C, 400 MHz): δ =
t
3
.8 [(4,4′-bipy){Ti(N[ Bu]Ar) } ][B(C F ) ]: alternative
3 2 6 5 4
preparation
6
.99 (s, 3H, HAr), 6.91 (s, 6H, HAr), 2.31 (s, 18H, Ar–CH
3
1
(
3
3
8
t
[
(4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
][B(C
6
F
5
)
4
]
2
(0.050 g, 0.038 mmol), 7.13 (s, 3H, HAr), 6.84 (s, 6H, HAr), 2.40 (s, 18H, Ar–CH
3
), 1.28
20 °C,
arate vial, (4,4′-bipy){Ti(N[ Bu]Ar) } (0.102 g, 0.0382 mmol) 125.7 MHz): δ = 149 (d, half of doublet hidden under pyridine-
1
3
was dissolved in THF (5 mL) to create a red solution. In a sep- (s, 27H, C(CH
3
)
3
)
ppm.
5
C NMR (pyridine-d ,
t
3
2
was dissolved in THF (5 mL) to create a black solution. The
5
d resonance, C–F), 142.5 (s, aryl ipso), 139.6 (s, m-Ar), 139.3
black solution was added to the red solution dropwise, with (d, JCF = 246 Hz, C–F), 137.4 (d, JCF = 232 Hz, C–F), 132.0 (s,
stirring. Upon completion of the addition, the reaction p-Ar), 130.2 (s, o-Ar), 125.8 (m, C–F), 66.9 (s, NC(CH ) ), 30.7 (s,
3
3
1
3
mixture was a dark brown color. The reaction mixture was ArCH
3 3 3 8
), 21.6 (s, NC(CH ) ) ppm. C NMR (THF-d , 20 °C,
allowed to stir for one hour at which time all volatile materials 125.7 MHz): δ = 149.3 (d, JCF = 238 Hz, C–F), 144.4 (s, aryl ipso),
were removed under reduced pressure to yield a brown oil. The 140.3 (d, JCF = 246 Hz, C–F) 139.8 (s, m-Ar), 137.3 d, JCF
oil was dissolved in Et
=
2
O (3 mL) and n-hexane (15 mL) was 250 Hz, C–F), 131.6 (s, p-Ar), 129.4 (s, o-Ar), 125.5 (broad, C–F),
added to the solution. After removing all volatile materials 67.3 (s, NC(CH
3
)
3
), 30.9 (s, ArCH
3
3 3
), 21.4 (s, NC(CH ) ) ppm.
from the mixture under vacuum, an oily, brown solid was
obtained. The solid was slurried in n-hexane (15 mL) and once
again subjected to the removal of all volatile materials, yield- [Ti(N[ Bu]Ar)
t
3
.10 Crystallization of [(THF)Ti(N[ Bu]Ar) ][B(C F ) ]
3
6 5 4
t
3
][B(C
6
F
5
)
4
] (ca. 100 mg) was slurried in Et
2
O
ing a light brown powder. The brown powder was slurried in (10 mL). THF was added (three to four drops) to completely
n-hexane (10 mL) and the solids were isolated by filtration. The dissolve the material. The orange solution was placed in the
t
material was characterized as [(4,4′-bipy){Ti(N[ Bu]Ar)
3
}
2
]- freezer at −35 °C. After two days, large, orange crystals were
[
B(C F ) ] by its color and by the fact that it was observed to be observed in the vial. The material continued to crystallize over
6 5 4
NMR silent (paramagnetic). Yield: 0.124 g, 0.0616 mmol, 81% the next two days, with large, orange crystals covering the base
−
1
(
based on C106
H
116BN
8
F
20Ti
2
, 1988.62 g mol ).
of the vial. A crystal was selected for the X-ray structure deter-
mination after about a week had passed. Attempts to grow crys-
tals in a similar manner from toluene resulted in the material
t
3
.9 Preparation of [Ti(N[ Bu]Ar) ][B(C F ) ]
3
6 5 4
t
Titanium trisanilide, Ti(N[ Bu]Ar)
3
(0.500 g, 0.867 mmol), was oiling out of solution. The material also did not crystallize out
dissolved in Et O (30 mL) to create a bright green solution. In of concentrated THF solutions stored in the freezer.
2
a separate flask, the ferrocenium salt, Fc[B(C
6 5 4
F ) ] (0.750 g,
n
t
3
.11 Preparation of ( BuO)Ti(N[ Bu]Ar)
3
0
.867 mmol), was slurried in Et O (10 mL). The slurry was
2
t
t
added dropwise, with stirring, to the Ti(N[ Bu]Ar) solution. [Ti(N[ Bu]Ar) ][B(C F ) ] (0.192 g, 0.152 mmol) was dissolved
3
3
6 5 4
Additional Et
2
O (10 mL) was used to transfer the Fc[B(C
6
F
5
)
4
]
in o-difluorobenzene (5 mL) to form a red solution, which was
that remained behind in the original flask. During the course subsequently frozen solid in the dry box cold well. A solution
of the addition, the color of the reaction mixture turned from of [Li][HBEt ] (1.0 M in THF, 152 μL, 0.152 mmol) was added
3
bright green to bright red-orange. The reaction mixture was to the thawing red solution using a microsyringe. There was
allowed to stir for an additional 30 minutes and then were no immediate color change; however, the color of the reaction
removed all volatile materials. The resulting sticky, red oil was mixture gradually changed from red to orange to yellow upon
2
dissolved in Et O (5 mL), n-hexane (30 mL) was added, and warming to room temperature. The reaction mixture was
then from the new mixture were removed all volatile materials. allowed to stir for a total of one hour and then pumped down
This step was repeated, and the resulting orange powder was to dryness. The resulting yellow oil was triturated twice with
slurried in n-hexane (30 mL) before removing once again all n-hexane (10 mL) to give a yellow powder. This material was
6 5 4
volatile materials. It is essential to repeatedly triturate the slurried in n-hexane and filtered to remove [Li][B(C F ) ]. The
material with n-hexane in order to remove all the coordinated white solids present in the fritted funnel were washed with
solvent molecules. Finally, the orange powder was slurried in n-hexane until the washings became colorless. The yellow fil-
n-hexane (30 mL), collected by filtration using a frit, and trate was pumped down to dryness to yield a yellow powder.
washed with n-hexane until the washings became colorless. Yield: 0.058 g, 0.089 mmol, 59%. Crystals suitable for X-ray
The orange filtrate was concentrated under vacuum and ferro- diffraction were grown by first creating a slurry of the yellow
cene (0.1284 g) was recovered (80% of expected). The orange powder in n-pentane and then adding Et
powder in the frit was further kept under vacuum for 1 hour. the material dissolved. The yellow solution so obtained was
Yield: 1.057 g, 0.842 mmol, 97% (based on C60 20Ti, placed in the dry box freezer at −35 °C for five days, during
255.74 g mol− ). Crystals suitable for X-ray diffraction were which time yellow cubic crystals deposited. Anal. Calcd: C
2
O dropwise until all
H
3
54BN F
1
1
t
1
grown by dissolving [Ti(N[ Bu]Ar) ][B(C F ) ] (0.20 g) in 73.92; H 9.78; N 6.47. Found: C 71.05; H 8.91; N 6.09. H NMR
3
6 5 4
o-difluorobenzene. The red solution was transferred to an (benzene-d
NMR tube and n-hexane was layered on top. The tube was left 6H, o-ArH), 4.53 (t, JHH = 7.6 Hz, 2H, OCH
undisturbed at room temperature for five days, during which (s, 18H, Ar–CH ), 1.83 (apparent pentet, J = 7.6 Hz, 2H,
6
, 20 °C, 400 MHz): δ 6.68 (s, 3H, p-ArH), 5.83 (br s,
3
2
2 2 3
CH CH CH ), 2.16
3
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 6784–6796 | 6793

Paper
Dalton Transactions
OCH
2
CH
2
CH
2
CH
3
), 1.41 (apparent sextet, J = 7.6 Hz, 2H, form that is unstabilized by coordination of donor solvent
3
OCH CH CH CH ), 1.10 (s, 27H, C(CH ) ), 0.99 (t, J
= 7.6 molecules. This accomplishment was made possible by the
2
2
2
3
3 3
HH
1
3
Hz, OCH
2
CH
2
CH
2
CH
3
);
C
NMR (benzene-d
6
,
20 °C, judicious choice of a robust counter-anion, namely the per-
−
1
6 5 4
00.6 MHz): δ 152.7 (s, NAr ipso), 136.0 (s, m-Ar), 128.5 (s, fluorinated tetraphenylborate anion, [B(C F ) ] . The latter has
o-Ar), 125.5 (s, p-Ar), 77.0 (s, OCH CH CH CH ), 60.3 been reported previously to permit isolation of extremely
2
2
2
3
(
s, C(CH
3
)
3
), 35.7 (s, OCH
CH
2
CH
CH
2
CH
CH
2
CH
3
), 31.2 (s, C(CH
3
)
3
), 21.7 (s, electrophilic cations, such as in the salt originally formulated as
5
7,58
Ar–CH ), 19.3 (s, OCH
3
2
2
2
3
), 14.3 (OCH
2
CH CH
2
2 3
CH ).
triethylsilyl perfluoro-tetraphenylborate, [Et
3 6 5 4
Si][B(C F ) ],
but recently shown actually to have the composition [Et Si–H–
SiEt ][B(C F ) ], and thus still able to serve as a source of
3 6 5 4
3
3
.12 X-Ray crystallography
59
Crystals were mounted in hydrocarbon oil on a nylon loop or a triethylsilylium ion via release of an equivalent of triethyl-
glass fiber. Low-temperature (100 K) data were collected on a silane. The latter silylium ion source has exhibited impressive
Siemens Platform three-circle diffractometer coupled to a stoichiometric reactivity that has spawned applications in cata-
6
0
t
+
Bruker-AXS Smart Apex CCD detector with graphite-monochro- lysis. The titanium tris-amide cation {Ti(N[ Bu]Ar)
mated Mo-K
radiation (λ = 0.71073) performing ψ- and now be explored in analogous fashion as a unique and power-
ω-scans. A semi-empirical absorption correction was applied ful Lewis acid: based upon the observation that neutral Ti-
3
}
can
α
5
1
t
to the diffraction data using SADABS. All structures were (N[ Bu]Ar)
and including ISiMe , bromobenzene, and triphenyltin chloride to
refined against F on all data by full-matrix least squares with form XTi(N[ Bu]Ar)
3
readily abstracts a halogen atom from substrates
5
2,53
solved by direct or Patterson methods using SHELXS
3
2
t
5–7
3
(X = I, Br, or Cl),
the titanium(IV)–
5
3
SHELXL-97. All non-hydrogen atoms were refined anisotropi- halogen bond energy in this system exceeds those found in the
cally. All hydrogen atoms were included in the model at geo- corresponding group 14 element–halogen compounds.
metrically calculated positions and refined using a riding
model. The isotropic displacement parameters of all hydrogen
atoms were fixed to 1.2 times the U value of the atoms to
Acknowledgements
which they are linked (1.5 times for methyl groups). In struc-
tures where disorders were present, the disorders were refined
The authors are grateful to Syngenta for support and for
within SHELXL with the help of rigid bond restraints as well
helpful discussions with Dr David A. Jackson and Prof. Clark
as similarity restraints on the anisotropic displacement para-
R. Landis. Nicholas Piro is thanked for his help in solving the
meters for neighboring atoms and on 1,2- and 1,3-distances
throughout the disordered components. The relative occu-
pancies of disordered components were refined freely
t
structure of (4,4′-bipy){Ti(N[ Bu]Ar } and Matthew Rankin is
3
2
thanked for aiding with NMR data collection.
5
4
t
within SHELXL. The unit cell of [(4,4′-bipy){Ti(N[ Bu]Ar) } ]-
3
2
6 5 4 2
[B(C F ) ] contains one o-difluorobenzene molecule residing
atop the inversion center which has been treated as a diffuse
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6794 | Dalton Trans., 2015, 44, 6784–6796
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
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0
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