Sunlight photolysis of cyclopentadienyl–tethered titanium(iv) permethyltitanocene chlorides

Article abstract of DOI:10.1016/j.jorganchem.2020.121536

Solutions of permethylcyclopentadienyl-tethered titanium(IV) chlorides incorporating one double bond in their tethers were exposed in glass vessels to sunlight, which triggered their photolytical reactions providing mixtures of products. Although the formation of several different products was apparent from ¹H NMR spectra, only a single product could be identified in the crude reaction mixtures. This product was dicyclopentadiene [C₁₀Me₁₀], which arose from the recombination of the generally photodissociated C₅Me₅ radical. Amongst products that contained titanium, three complexes (3a, 3b and 4a) could be isolated after employing fractional crystallization and these complexes were characterized. The mechanism of 3a formation involves photodissociation of one η⁵-C₅Me₅ ligand from the parent complex, complemented by chlorine abstraction from another reactant molecule. Complexes 3b and 4a become formed via tether rearrangement, which starts with dissociating the tether Ti–C bond and is followed by rotating the tether, reattaching it subsequently through its available radical terminus.

Full text of DOI:10.1016/j.jorganchem.2020.121536

Journal of Organometallic Chemistry 927 (2020) 121536
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Sunlight photolysis of cyclopentadienyl–tethered titanium(iv)
permethyltitanocene chlorides
Jirˇí Pinkasa, Jirˇí Kubištaa, Karel Macha, Róbert Gyepesa,b, Michal Horácˇeka,∗
a J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 3, 182 23 Prague 8, Czech Republic
b Department of Inorganic Chemistry, Faculty of Science, Charles University, Hlavova 2030, 128 43 Prague 2, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Solutions of permethylcyclopentadienyl-tethered titanium(IV) chlorides incorporating one double bond in
their tethers were exposed in glass vessels to sunlight, which triggered their photolytical reactions pro-
viding mixtures of products. Although the formation of several different products was apparent from
1H NMR spectra, only a single product could be identified in the crude reaction mixtures. This product
was dicyclopentadiene [C10 Me10 ], which arose from the recombination of the generally photodissociated
C5Me5 radical. Amongst products that contained titanium, three complexes (3a, 3b and 4a) could be iso-
lated after employing fractional crystallization and these complexes were characterized. The mechanism
of 3a formation involves photodissociation of one η5-C5Me5 ligand from the parent complex, comple-
mented by chlorine abstraction from another reactant molecule. Complexes 3b and 4a become formed
via tether rearrangement, which starts with dissociating the tether Ti–C bond and is followed by rotating
the tether, reattaching it subsequently through its available radical terminus.
Received 23 July 2020
Revised 14 September 2020
Accepted 16 September 2020
Available online 17 September 2020
Keywords:
Titanium
Photolysis
Isomerization
Solid-state structures
Density functional calculation
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
chlorinated solvents affording [CpTiCl3] as the sole product con-
taining titanium [10]c. While this photodecomposition was later
Sunlight photolysis belongs to the family of methods used to
run chemistry in sustainable development mode [1]. Especially in
the field of organic chemistry, a considerable number of large-scale
syntheses has been driven by photolysis [2,3]. The contemporary
goal for utilizing artificial photosynthesis lies in catalytic water ox-
idation [4,5] and in the electrolysis of carbon dioxide using photo-
voltaic electricity [6]. The cost-effective utilization of hydrogen as a
“green” fuel has turned attention to transition metal hydrides and
their photochemical behaviour with the aim to obtain recoverable
hydrogen storing systems [7]. Unfortunately, thermally stable ti-
tanocene(TiIII) hydrides [8] are low on their bonded hydrogen con-
tent and are additionally inert towards visible light. Amongst other
titanocene compounds, great attention has been devoted to the
photolytic behaviour of titanocene dichloride [Cp2TiCl2] (Cp = η5-
C5H5), which is the raw material for the synthesis of other ti-
tanocene derivatives and parent compound in titanium-based ethy-
lene polymerization catalysts [9]. Early photolytic studies indicated
the decomposition of [Cp2TiCl2] to Cp radical and (CpTiCl2), where
the products were characterized by EPR spectra of their adducts
with spin traps [10]a,b and by the photolysis of [Cp2TiCl2] in
found to proceed analogously for the titanocene difluoride and di-
bromide analogues, the photolysis of Cp2TiI2 resulted only in the
elimination of I2 [11]a. This behaviour was due to its increased
halogenide→Ti LM–CT transition energy with respect to the lighter
halides, which results in swapping the frontier orbitals for Cp2TiI2
[11]b-d. Nevertheless, the substitution of both Cp ligands in Cp2TiI2
for their more electron donating analogues η5-C5Me5 (Cp∗) de-
creased the Cp∗→ Ti transition energy, which was demonstrated
experimentally by the elimination of the C5Me5 radical detected
by spin trapping it with nitrosodurene and also by the forma-
tion of Cp∗TiI3 upon performing the photolysis in iodoform [11]c.
Similarly, the photolysis of [Cp2TiMe2] resulted mostly in methane
elimination and yielded only some poorly soluble “polytitanocene”
of decreased hydrogen content [12].
Further research of photoassisted water dissociation was until
recently hampered by the observation, that the photolysis of per-
methyltitanocene dihydroxide Cp∗2Ti(OH)2 by visible light provides
the C5Me5 radical instead of the desired OH radical [13]a. However,
since properties of titanocene complexes are often altered substan-
tially upon introducing suitable functional groups on their cyclic
ligand(s) [14], the linking of cyclopentadienyl ligands with a di-
atomic ansa-bridge enhanced the photolytic stability of the per-
methyltitanocene moiety significantly [13]b,c. For the sake of ex-
ample, ansa-bis(dimethylsilylene)(C5Me4)2Ti(OH)2 exhibited selec-
∗
Corresponding author.
E-mail address: michal.horacek@jh-inst.cas.cz (M. Horácˇek).
https://doi.org/10.1016/j.jorganchem.2020.121536
0022-328X/© 2020 Elsevier B.V. All rights reserved.

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
Scheme 1. Preparation of permethylcyclopentadienyl-tethered titanocene (Ti(IV)) derivatives 1a–1g according to ref. [16]).
Scheme 2. Preparation of permethylcyclopentadienyl-tethered titanocene (Ti(IV)) derivatives 2a and 2b (according to ref. [16]).
tive OH radical elimination upon its photolysis using wavelengths
1.76 ppm in intensity ratio 1:2:2 were indicating the presence
of bis-(1,2,3,4,5-pentamethylcyclopenta-2,4-diene-1-yl) [C10Me10],
which arose from recombination of the C5Me5 radicals (see Figure
A.1 in Supplementary material). Since the measurements were con-
ducted in C6D6, the above 1H chemical shifts are shifted downfield
with respect to the values reported by Jutzi and Hielscher [17] for
[C10Me10] in CDCl3. The chemical shifts and linewidths exhibited
slight variation also due to the presence of varying amounts of
paramagnetic Ti(III) intermediates and products, which were indi-
cated by their EPR spectra showing mostly singlet signals in the
range giso~ 1.96 – 1.99 and exhibiting varying intensities during the
photolytical process.
over 420 nm [13]d, while ansa-titanocene(III/IV) triflate complexes
exhibited properties that made them capable of acting in a closed
catalytic cycle for overall water splitting [13]e.
Herein we report the results of an exploratory research on sun-
light photolysis of permethyltitanocene-tethered (TiIV) monochlo-
rides, which were obtained by inserting various internal acetylenes
into the titanium-methylene bond of the singly tucked-in ti-
tanocene [Cp∗Ti(η5: η1-C5Me4CH2)] [15] and the Ti(III) products
formed were subsequently chlorinated with PbCl2 in tetrahydro-
furan (Scheme1 and Scheme2) [16]. All photolytic reactions were
carried out in C6H6 and C6D6.
The progress of sunlight photolysis for 1a-1g, 2a and 2b in their
C6D6 solutions was followed by 1H NMR and exposure to sunlight
was terminated after reaching the consumption of reactant around
90%. Amongst the generated products, only those that could be
isolated by fractional crystallization were investigated further and
were identified. The reaction mechanisms of the photolytical pro-
cesses were studied by means of Density Functional Theory (DFT)
computations.
The sunlight photolysis of 1a–g (Scheme 1) and 2a and 2b
(Scheme 2) resulted always in complex product mixtures. Pure
products could successfully be obtained from these mixtures only
in three cases upon using repeated fractional crystallization from
hexane. Amongst all the photolyzed compounds, only products
from 1a, 1b, and 2a could be isolated in crystalline form from
scaled-up photolytic experiments carried out in benzene. These
isolated products (3a, 3b, and 4a, respectively) were characterized
by EI-MS, 1H and 13C NMR, IR spectra and by X-ray single-crystal
diffraction and the mechanism of their formation was elucidated
by DFT computations.
2. Results and discussion
Compounds 1a-1g, 2a and 2b dissolved in C6H6 or C6D6 turned
their colour after exposing their solutions in NMR sample tubes
to sunlight for up to four days commonly from red to brownish-
yellow. 1H NMR measurements were used to follow the pho-
tolytic decay of reagents, needed for indicating the time neces-
sary to terminate the photolytic experiments on a larger scale.
No assignment for the newly formed products could be drawn
from these spectral data, only the signals at δH 1.14, 1.66 and
2.1. Photolysis of 1a to give 3a
The sunlight photolysis of 1a dissolved in C6H6 lasting 12 h in a
rotated ampule afforded 3a in 28% isolated yield. This means that
after photodissociation of the Cp∗ ligand, the resulting Ti(III) inter-
mediate had to acquire a further chloride ligand from an external
source, presumably from another molecule of 1a (Scheme 3).
2

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
2.2. Photolysis of 1b to give 3b
Exposing benzene solution of 1b to sunlight for 24 h resulted
in the initial red colour turning to dark yellow. Crystallization from
hexane afforded a pale red crystalline solid product 3b, obtained in
36% yield after recrystallization (Scheme 4).
Although EI-MS spectra of 3b displayed the molecular ion m/z
548 that could be ascribed also to 1b, the fragmentation of both
compounds followed completely different pathways. No molecu-
lar ion was observed for 1b [16] since it underwent an abundant
≡
loss of its whole tether L2 (L2 = h-t-t dimer of HC CSiMe3) [18],
Scheme 3. Photolysis of 1a yielding 3a.
whereas no [M – L2]+ ion was observed for 3b. The common fea-
ture for both compounds was the presence of base peak m/z 218
[M – C5Me4CH2 – L2]+. The tether isomerization in 1b, indicated
by EI-MS spectra of 3b has become evident after complete assign-
ment of 1H and 13C NMR spectra of 3b based on APT, 29Si INEPT,
1DNOESY, gCOSY, gHMBC, and NOESY experiments. The arrange-
ment around the two double bonds of 3b has been derived from
1DNOESY experiments. These showed a strong interaction of the
=CHSiMe3 (5.26 ppm) proton with one proton of the CH2 group
(2.94 ppm) and with protons of SiMe3 group (0.241 ppm) in one
case and the interaction of the TiCH= proton (8.16 ppm) with the
protons of the other SiMe3 group (0.247 ppm) and the Cp∗ protons
(1.76 ppm) in the other case. The IR (KBr) spectrum of 3b exhibited
a broadened band of medium intensity at 1593 cm–1 that could be
attributed to both double bonds. The stretching C-H vibrations of
double bonds should be overlapped with the strong C–H valence
vibrations of the methyl groups. The X-ray single-crystal diffrac-
tion experiment yielded a structure model of 3b in agreement with
spectral data (Fig. 2, Table 2). The asymmetric part of the unit cell
incorporates two molecules that are differing mostly in the orien-
tation of their tether arms. (Fig. 3).
The sunlight-induced tether isomerization is to be initiated by
dissociating the tether Ti–C bond, which is followed by rotation of
the π-isomerized dienyl tether around the CH2–Cquaternary bond, al-
lowing the most distal tether sp2 carbon atom to restore a regular
Ti–C overlap (Scheme 5).
Fig. 1. Solid-state model of 3a with thermal ellipsoids drawn at 30% probability
level. Atom C(5) is the cyclopentadienyl carbon atom residing between C(1) and
C(4). Hydrogen atoms are drawn with rods without reflecting the values of their
thermal parameters.
A similar isomerization mechanism, albeit induced by adding
dihydrogen has been recently observed for the permethylti-
tanocene tethered complexes bearing a methyl substituent on the
tether carbon attached to the cyclopentadienyl methylene [19].
DFT studies suggested the energy required to generate a singlet-
state excited molecule of 1b to be 474 nm via vertical excitation.
Taking into consideration the possibility of intersystem crossing
(which cannot be ruled out entirely due to the presence of the
metal and the prolonged exposition times), the first vertical exci-
tation energy to the electronic triplet-state molecule decreases to
564 nm. In both cases, the excitation of 1b occurs within the visi-
ble region of light, and irrespective of the actual excited-state mul-
tiplicity, the photolytical process follows the same pathway. Both
excited-state geometry optimizations arrived to molecular geome-
tries having their tether dissociated from the metal (hereafter de-
noted as 1b∗; Fig. 4). The tether arm becomes subsequently free to
rotate around the σ bond between the tether carbon and its neigh-
bouring atom bonded to the cyclopentadienyl skeleton. After rota-
tion to a sterically less congested position and bringing the distal
methylene group to the vicinity of the metal, the tether becomes
reattached to the metal. The isomerization process is driven by en-
thalpy, as the total energy of 1b and 3b differs by 49.49 kJ/mol in
favour of the latter.
Table 1
˚
Selected bond lengths (A), bond angles (˚) and torsion angles (˚) for 3a.
Bond lengths
Ti(1)–Cg(1)
2.0040(11)
1.351(4)
Ti(1)–C(12)
C(12)–C(13)
C(13)–C(14)
Ti(1)–Cl(2)
2.082(3)
1.495(4)
1.521(4)
2.2631(8)
C(11)–C(12)
C(13)–C(17)
1.320(4)
Ti(1)–Cl(1)
2.2595(8)
Bond angles
Cl(1)–Ti(1)–Cl(2)
Cg(1)–Ti(1)–Cl(2)
Torsion angles
C(1)–C(6)–C(11)–C(12)
Ti(1)–C(12)–C(11)–C(6)
105.98(3)
117.05(4)
Cg(1)–Ti(1)–Cl(1)
Cg(1)–Ti(1)–C(12)
118.28(4)
104.40(7)
-5.3(3)
8.3(3)
aCg(1) denotes the centre of gravity of the cyclopentadienyl ring C(1)–C(5).
The EI-MS spectra of 3a showed the molecular ion (m/z 388)
of moderate abundance, which fragmentated by HCl elimination to
the base peak m/z 352 or by the tether L1 (L1 = head-to-tail dimer
of 1-pentyne [18]) elimination affording m/z 252 ([M – L1]+; 48).
The 1H NMR and 13C NMR spectra were fully assigned to the teth-
ered permethylcyclopentadienyl ligand containing one exomethy-
lene double bond and one double bond vicinal to titanium, which
were detected also in the solid-state structure of 3a (Fig. 1 and
Table 1). Both double bonds were detected also in the IR (KBr)
spectra of 3a — the double bond vicinal to titanium at 1553 cm–1
and the exo-methylene double bond at 1603 cm–1. This assignment
agrees with the corresponding double bond distances established
from the solid-state model.
2.3. Photolytic conversion of 2a to 4a
Another example of photolysis that is capable to yield an
isolable isomerization product with a surprisingly high selectivity
occurred with 2a (Scheme 6). The isolated yield of 63% after pho-
3

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
Scheme 4. The photolytically induced rearrangement of 1b to 3b.
Table 2
˚
Selected bond lengths (A), angles (°) and torsion angles (°) for the solid-state structure
of 3b. The left part of the table includes data for the first molecule depicted in Fig. 2,
the right part the corresponding quantity for the second molecule.
Bond lengths
Ti(1)–Cl(1)
2.362(3)
2.140(10)
1.518(14)
2.103(5)
2.114(5)
Ti(2)–Cl(2)
Ti(2)–C(57)
C(31)–C(36)
Ti(2)–Cg(3)
Ti(2)–Cg(4)
2.361(3)
2.138(10)
1.493(14)
2.104(5)
2.113(5)
Ti(1)–C(27)
C(1)–C(6)
Ti(1)–Cg(1)∗
Ti(1)–Cg(2)∗∗
Bond angles
Cg(1)–Ti(1)–Cg(2)
Cg(1)–Ti(1)–Cl(1)
Cg(2)–Ti(1)–Cl(1)
Cg(1)–Ti(1)–C(27)
Cg(2)–Ti(1)–C(27)
Cl(1)–Ti(1)–C(27)
Ti(1)C(27)C(26)
Torsion angles
C(1)–C(6)–C(21)–C(26)
Ti(1)–C(27)–C(26)–C(21)
139.1(2)
104.66(18)
106.57(19)
102.3(3)
102.9(3)
90.9(3)
Cg(3)–Ti(2)–Cg(4)
Cg(2)–Ti(2)–Cl(2)
Cg(3)–Ti(2)–Cl(2)
Cg(3)–Ti(2)–C(57)
Cg(4)–Ti(2)–C(57)
Cl(2)–Ti(2)–C(57)
Ti(2)–C(57)–C(56)
139.3(2)
104.01(19)
105.7(2)
103.4(3)
103.8(3)
90.1(3)
141.8(8)
141.2(8)
-57.3(11)
3.5(16)
C(31)–C(36)–C(51)–C(56)
Ti(2)–C(57)–C(56)–C(51)
-56.3(13)
1.7(18)
∗
∗∗
Cg(1) denotes the centre of gravity of the cyclopentadienyl ring C(1)–C(5).
Cg(2) denotes the centre of gravity of the cyclopentadienyl ring C(11)–C(15).
Fig. 3. Superposition of the two molecules residing in the asymmetric part of unit
cell of 3b. The tether arms are drawn towards the right side. The two molecules are
differentiated by using different colours for all atoms of each molecule. Hydrogen
atoms are omitted.
Fig. 2. Solid-state structure of 3b with thermal ellipsoids drawn at 30% probabil-
ity level and with the atom labelling scheme. Only one of the two symmetrically
independent molecules is drawn. Hydrogen atoms are drawn with rods without re-
flecting the values of their thermal parameters.The skeleton of the upper cyclopen-
tadienyl ligand is constructed from atoms C(1)–C(5). Atom C(1) is the ring atom
bonded to the tether C(6).
Scheme 5. Proposal for the isomerization of 1b to 3b.
4

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
Fig. 4. Perspective view of optimized 1b (1b∗) after singlet excitation. Colour leg-
end: Ti – yellow, Cl – green, C – dark grey, Si – brown, H – light grey.
Fig. 5. The solid-state structure of 4a with thermal ellipsoids drawn at 30% proba-
bility level. Hydrogen atoms are omitted for clarity. Only one of the two symmetri-
cally independent molecules is drawn.
Scheme 6. Photolytically induced rearrangement of 2a to 4a.
tolyzing 2a for 12 h showed the formation of 4a to be the over-
whelming process in the system.
The EI-MS spectra of 4a displayed the molecular ion (m/z 572)
and the [M – C5Me5]+ ion of low intensity and the base peak aris-
ing from the loss of the permethylcyclopentadienyl tether [M –
C5Me4CH2 – L3]+ (L3 = Me2Si(C Ct-Bu)2) differing for 2a [16] only
≡
slightly in intensities. The 1H NMR spectrum of 4a preserved most
features of the tether from 2a and contained a doublet which
had to be assigned to a methylene group residing on the tita-
nium. The chemical shifts of its protons differed strongly accord-
ing to the extent of their interaction with the titanium ligand field
electron density. The proton residing close to the titanocene pseu-
doequatorial plane with high electron density was shifted upfield
(−0.84 ppm),while the other was found at 3.40 ppm. Compared
with the 13C NMR resonance at 240.69 ppm of 2a, due to the pres-
ence of a quaternary carbon of the silacycle residing on the tita-
nium atom [16] the quaternal carbon directly connected to C5Me4
ring in 4a was shifted strongly highfield to 153.2 ppm. Similarly,
the highfield shift could be observed for remaining carbon atom
signals of silacycle in 4a compared to 2a. The signal of C=C-Si was
found at 159.7 ppm in 4a (compared to 170.4 ppm in 2a) and
the signal of C-C-Si was localized at 135.8 ppm in 4a (compared
to 149.1 ppm in 2a). Indeed, a formal displacement of methylene
group from cyclopentadienyl ring to titanium atom led to its 13C
NMR signal downfield shift (from 28.5 ppm in 2a to 79.9 ppm in
4a). The 29Si NMR signal at 3.72 ppm for 4a that appeared shifted
downfield from that of 2a at −6.89 ppm [16] was also compatible
with the separation of the silacycle from titanium by the methy-
lene group. In IR spectra, the two very weak absorption bands in
ν(C=C) region at 1592 cm–1 and 1558 cm–1 for 2a were replaced
by a medium intensity band at 1615 cm–1 for 4a, which could be
assigned to the both double bonds with only marginally differing
surroundings. The above spectral assignments are in good agree-
Fig. 6. Superposition of the two molecules residing in the asymmetric part of unit
cell of 4a. The tether arms are orientated towards the reader. The two molecules are
differentiated by using different colours for all atoms of each molecule. Hydrogen
atoms are omitted.
ment with the solid-state model of 4a (Fig. 5). Selected bond an-
gles and distances are provided in Table 3.
The asymmetric part of the unit cell includes two molecules
that are two rotamers of 4a. The most important difference be-
tween the two molecules lies in the orientation of their tether
arms (Fig. 6). Although this difference is modest, it is manifested
well by the torsion angles on the tether arm (Table 3).
The molecular geometry of both 2a and 4a was optimized by
DFT, yielding the total energy of 4a lower by 3.69 kJ/mol and the
Gibbs energy at 298.15 K by 19.97 kJ/mol. This difference is to be
accounted for a steric relief at the central atom, suggested by a no-
table decrease of internuclear repulsion energy of 152.42 hartrees
in favour of 4a.
5

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
Table 3
˚
Selected bond lengths (A), angles (°) and torsion angles (°) for the solid-state structure
of 4a. The left part of the table includes data for the first molecule depicted in Fig. 5,
the right part the corresponding quantity for the second molecule.
Bond lengths
Ti(1)–Cl(1)
2.338(3)
2.260(11)
1.473(13)
2.111(5)
2.153(5)
Ti(2)–Cl(2)
Ti(2)–C(54)
C(35)–C(44)
Ti(2)–Cg(3)
Ti(2)–Cg(4)
2.347(4)
2.268(12)
1.472(14)
2.102(5)
2.156(5)
Ti(1)–C(20)
C(1)–C(10)
Ti(1)–Cg(1)∗
Ti(1)–Cg(2)∗∗
Bond angles
Cg(1)–Ti(1)–Cg(2)
Cg(1)–Ti(1)–Cl(1)
Cg(2)–Ti(1)–Cl(1)
Cg(1)–Ti(1)–C(20)
Cg(2)–Ti(1)–C(20)
Cl(1)–Ti(1)–C(20)
Ti(1)–C(20)–C(19)
Torsion angles
136.8(2)
106.79(16)
104.05(17)
104.2(3)
105.0(3)
90.8(3)
Cg(3)–Ti(2)–Cg(4)
Cg(3)–Ti(2)–Cl(2)
Cg(4)–Ti(2)–Cl(2)
Cg(3)–Ti(2)–C(54)
Cg(4)–Ti(2)–C(54)
Cl(2)–Ti(2)–C(54)
Ti(2)–C(54)–C(53)
136.7(2)
106.75(17)
105.69(18)
104.4(3)
104.1(3)
89.1(3)
125.5(7)
129.0(7)
C(1)–C(10)–C(12)–C(13)
C(1)–C(10)–C(11)–C(19)
Ti(1)–C(20)–C(19)–C(11)
-10(2)
22(2)
C(35)–C(44)–C(46)–C(47)
C(35)–C(44)–C(45)–C(53)
Ti(2)–C(54)–C(53)–C(45)
-8(2)
13(2)
-50(1)
-56(1)
∗
∗∗ Cg(1) denotes the centre of gravity of the cyclopentadienyl ring C(1)–C(5).
Cg(2) denotes the centre of gravity of the cyclopentadienyl ring C(25)–C(29).
step to be the scission of the tether methylene group from the cy-
clopentadienyl ring, in conjunction with methyl migration to this
liberated position on the cyclopentadienyl. Such a mechanism ar-
ranges all the necessary changes on the cyclopentadienyl ligand
needed for the formation of 4a (Scheme 7). Unfortunately, this sug-
gested mechanism could not be verified by DFT, as the computa-
tional demands necessary to locate a proper transition state of an
excited-state molecule incorporating a transition metal were pro-
hibitive.
The importance of steric and electronic effects exerted by the
seemingly uninvolved and innocent substituents (t-Bu and SiMe3)
was demonstrated by the unsuccessful attempts at obtaining any
crystalline product from the analogous photolysis of 2b containing
bis(trimethylsilylethinyl)dimethylsilane.
2.4. Conclusions
Fig. 7. Perspective view of optimized 2a∗ in singlet excitation. Colour legend: Ti –
Exploratory results obtained for the sunlight photolysis of
permethylcyclopentadienyl-tethered titanium(IV) chlorides 1a–1g
and 2a and 2b indicate that both their Ti–Cp∗ and Ti-C(tether)
moieties become activated upon photoexcitation. Photolysis of
the former moiety resulted always in the C5Me5 radical elimi-
nation in various abundance, as indicated by 1H NMR evidence
yellow, Cl – green, C – dark grey, Si – brown, H – light grey.
DFT computations allowed for a deeper insight into the mech-
anism by identifying the first excited-state molecular geome-
try. The singlet excitation energy for 2a was computed to be
446.99 nm with the excitation occurring from the orbital incor-
porating mostly the Ti–Csilacycle σ-overlap. As a result, the first
excited-state molecule of 2a (denoted hereafter 2a∗; Fig. 7) showed
after geometry optimization its Ti–Csilacycle bond becoming pro-
for [C10Me10
] in all solutions photolyzed. The complementary
titanium-containing product 3a could be obtained only from the
photolysis of 1a. The structure of 3a indicates the transiently
formed Ti(III) intermediate abstracting a chlorine from another
molecule of 1a as its only source in the system, while the tether
undergoes no change. Products requiring the Ti–C(tether) bond to
photodissociate were obtained from 1b and 2a. The tether radi-
cal rotating at the CH2 group of 1b allowed to restore the Ti–C
overlap by involving the most distal sp2 carbon and the reaction
yielded 3b. The analogous photodissociation of the Ti–C bond in
2a is a necessary prerequisite for tether rearrangement to yield
4a; however, the overall mechanism is less straightforward than
for 3b. DFT studies suggested 2a to undergo a significant weak-
ening of two of its Ti–C bonds upon excitation — the carbon of
the silacycle and its closest neighbour on the cyclopentadienyl ring.
These two carbons become bonded mutually, while the cyclopen-
tadienyl carbon achieves transient pentacoordination. To reach 4a,
the cyclopentadienyl methyl migrates to the neighbouring atom
what destabilizes that atom’s bond to the tether. The tether CH2
˚
˚
longed from 2.170 A to 2.541 A with the concurrent decrease of
the respective Mayer Bond Order (MBO) value to 0.212, indicating a
substantial release of the silacycle from the metal. Simultaneously,
the silacycle carbon established a bonding overlap with the closest
˚
cyclopentadienyl carbon atom (interatomic distance 1.631 A; MBO
˚
0.798) after its release from the titanium to 2.515 A and decreasing
the respective MBO to 0.081. This carbon atom becomes formally
pentacoordinated [20], having its methyl substituent deviated con-
siderably from the cyclopentadienyl least square plane orientated
outwards from the titanium.
After converting the multiplicity of this excited-state molecule
to a triplet leads to a decrease in its total energy and the sila-
cyclopentadienyl ligand becomes completely detached from the
˚
metal (acquiring the Ti–C distance 2.990 A). We assume the next
6

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
Scheme 7. Proposal for the rearrangement mechanism of 2a to 4a.
of green dimeric titanocene [(μ-η5:η5-C5H4C5H4)(μ-H)2{Ti(η5-
C5H5)}2] [21]. Benzene-d6 (99.6% D) (C6D6) or toluene-d8 (C7D8)
(both Sigma Aldrich) was degassed, distilled under vacuum onto
singly tucked-in permethyltitanocene [Ti(C5Me5)(C5Me4CH2)]
[15] and stored as its solution on a vacuum line. Solutions of
compounds 1a-1g and 2a and 2b in C6D6 were identical with
those described in ref. [16]. Compounds 1a and 1b were ob-
tained newly from 2.0 mmol of [Ti(C5Me5)(C5Me4CH2)] and
excess of head-to-tail dimer of 1-pentyne and (trimethylsi-
group then links to the titanium and the methyl group becomes
attached to the liberated position on the cyclopentadienyl ring.
3. Experimental
3.1. Methods
Synthesis, treatment, and subsequent reactions of low-valent ti-
tanium compounds were carried out on a vacuum line in sealed
all-glass devices equipped with magnetically breakable seals. 1H
(300 MHz), 13C (75 MHz) and 29Si (59.6 MHz) NMR spectra were
recorded on a Varian Mercury 300 spectrometer in C6D6 at 25 °C,
unless stated otherwise. Chemical shifts (δ/ppm) are given relative
to the residual solvent signal δH 7.15 ppm and to the solvent reso-
nance δC 128.0 ppm. 29Si NMR spectra were referenced to an exter-
nal signal of SiMe4 (δSi = 0.0 ppm). EI-MS spectra were obtained
on a VG-7070E mass spectrometer at 70 eV. Crystalline samples
in sealed capillaries were opened and inserted into the direct in-
let under argon. The spectra are represented by peaks of relative
abundance higher than 7% and by important peaks of lower in-
tensity. EPR spectra were recorded on a MiniScope MS400 (Mag-
nettech GmbH, Berlin, Germany) equipped with a microwave fre-
quency counter FC 400 and a temperature controller H03. Crys-
talline samples for EI-MS measurements and melting point deter-
minations were placed in glass capillaries in a glovebox Labmaster
130 (mBraun) under purified nitrogen (concentrations of oxygen
and water were lower than 2.0 ppm) and sealed with flame. KBr
pellets of solid samples were prepared in the glovebox and mea-
sured in an air-protected pellet holder on a Nicolet Avatar FT IR
spectrometer in the range 400−4000 cm−1. Melting points were
measured on a Koffler block in sealed glass capillaries under nitro-
gen, and are uncorrected.
≡
lyl)ethyne (RC CC(R)=CH2;
R
=
Pr, SiMe3), respectively,
compound 2a by reacting [Ti(C5Me5)(C5Me4CH2)] with di-
tert.butylethinyldimethylsilane and compound 2b by reacting
it with bis(trimethylsilylethinyl)dimethylsilane. The obtained Ti(III)
tethered products were chlorinated with PbCl2 (see Scheme 1 and
Scheme 2) [16]. The above acetylene dimers were obtained from
terminal alkynes using Ti catalysts [18].
3.3. Photolysis of 1a to 3a
A solution of 1a in benzene (0.1 M, 20 mL) was exposed to sun-
light for 12 h, turning its red colour to dark yellow. Crystallization
from hexane afforded orange needles of 3a.
3a: Yield 0.22 g, 28%. Mp. 108–110 °C. EI-MS (160 °C): m/z
(relative abundance) 392 (10), 391 (12), 390 (29), 389 (17), 388
(M.+; 38), 387 (9), 386 (8), 356 (9), 355 (15), 354 (45), 353 (36),
352 ([M − HCl]+; 100), 351 (22), 350 (31), 349 (8), 348 (11), 347
(11), 346 (8), 345 (13), 338 (10), 337 ([M – HCl − Me]+; 17), 336
(10), 335 (9), 325 (10), 324 (12), 323 (13), 321 (9), 317 (11), 316
([M − 2 HCl]+; 23), 256 (12), 255 (11), 254 (36), 253 (17), 252 ([M
– L1]+; 48), 236 (13), 218 (15), 219 (8), 218 (15), 217 (11), 216 ([M
– L1 − HCl]+; 18), 215 (9), 214 (8), 213 (11), 134 (15), 119 (17), 105
3
(9), 91 (15), 79 (12), 69 (13). 1H NMR (C6D6): 0.85 (t, JHH = 7.5 Hz,
Photolysis of benzene solutions of 1a−1 g and 3a and 3b in
NMR sample tubes (C6D6) and ampules (0.1 M in 20 mL of C6H6)
were performed in summer (Prague, 50.08° N, 14.42° E) with di-
rect sunlight that passed through 2 × 4.0 mm glass windows and
2.0 mm Pyrex glass of the ampule. The photolysis was terminated
when a contemporaneously photolyzed sample in an NMR tube
showed no more than 10% of the initial compound signal inten-
sity. The products in ampules were processed under diffuse day-
light. Benzene was removed by evaporating it under vacuum, the
residues were then dissolved in 4.0 mL of dry, vacuum-distilled
hexane and this solution was transferred into an ampoule with 3
arms equipped with breakable seals. The ampule was placed into
a refrigerator by one arm close to a cooling side for slow solvent
condensation. After the formation of crystalline solid, the mother
liquor and washings were collected in the cooled arm. Hexane was
distilled back to dissolve the crystalline product and the arm was
sealed off. The crystallization was repeated using the other arm,
and finally the pure crystalline product was collected in the third
arm, dried in vacuum, sealed off, and transferred to the glovebox
where it was weighed and handled further for analytical methods.
3
3H, =CCH2CH2Me); 0.96 (t, JHH = 7.2 Hz, 3H, C(=CH2)CH2CH2Me);
1.31–1.46 (m, 2H, =CCH2CH2); 1.46–1.62 (m, 2H, C(=CH2)CH2CH2);
1.77 (pseudo t, 2H, =CCH2); 1.93, 2.02 (2 × s, 2 × 6H, C5Me4);
2.17–2.29 (m, 2H, C(=CH2)CH2); 3.09 (s, 2H C5Me4CH2); 5.27 (dd,
4
2
2JHH = 3.6 Hz, JHH = 1.8 Hz, 1H, =CH2); 5.40 (dd, JHH = 3.6 Hz,
4JHH = 1.8 Hz, 1H, =CH2). 13C {1H}(C6D6): 12.9, 13.2 (C5Me4); 14.3
(2 × CH2CH2Me); 20.8, 22.4 (2 × CH2CH2Me); 34.1 (C5Me4CH2);
35.4, 37.3 (2 × CH2CH2Me); 115.8 (=CH2); 136.0, 138.8 (C5Me4);
150.2 (C=CH2); 167.3 (Ti-C=C); 234.7 (Ti-C=C); (Cipso, C5Me4) not
detected. IR (KBr, cm–1): 3071 (w), 2957 (vs), 2929 (m), 2905 (m),
2869 (m), 2835 (w), 2726 (vw), 1603 (m), 1553 (s), 1493 (w,b),
1463 (m), 1452 (w), 1432 (m), 1416 (w), 1379 (m), 1310 (w), 1257
(w), 1218 (vw), 1082 (vw), 1025 (w), 987 (w), 914 (w), 889 (m),
872 (w), 755 (s), 513 (m), 464 (s), 409 (vs).
3.4. Photolysis of 1b to give 3b
Compound 1b (0.62 g, 1.13 mmol) in 12 mL of benzene was ex-
posed to sunlight for 24 h turning its red colour to dark yellow.
Crystallization from hexane afforded dark yellow needles of 3b.
3b: Yield 0.23 g, 36%. Mp. 215 °C. EI-MS (160 °C): m/z (rela-
tive abundance) 550 (3), 549 (3), 548 (M.+; 7), 533 ([M – Me]+; 4),
512 ([M – HCl]+; 4), 475 ([M – SiMe3]+; 2), 413 ([M – C5Me5]+;
2), 398 ([M – C5Me5 – Me]+; 3), 222 (12), 221 (20), 220 (89),
3.2. Chemicals
The solvents benzene, toluene, hexane, and tetrahydrofuran
(THF) were dried by refluxing over LiAlH4 and stored as solutions
7

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
219 (74), 218 ([M – C5Me4CH2 – L2]+; 100), 217 (57), 216 (55),
215 (16), 214 (12), 213 (21), 203 ([M – C5Me4CH2 – L2 – Me]+;
17), 182 (10), 181 (12), 180 (11), 178 (14), 135 (11), 119 (18),
105 (10), 91 (12), 74 (19), 73 ([SiMe3]+; 96). 1H NMR (C6D6):
0.25, 0.26 (2 × s, 2 × 9H, SiMe3); 1.52, 1.69 (2 × s, 2 × 3H,
C5Me4); 1.77 (s, 15H, C5Me5); 1.86, 2.14 (2 × s, 2 × 3H, C5Me4);
due to hardware limits. Their partial separation was done during
postprocessing, but since the twinning occurred under a low angle
(about 3.5°), this caused many diffraction spots to overlap generat-
ing numerous symmetry equivalent violations. These inconsistent
reflections were discarded from the final data set, which decreased
the data completeness.
2
2
2.95 (d, JHH = 16.2 Hz, 1H, CH2); 3.13 (dd, JHH = 16.2 Hz,
4JHH = 1.8 Hz, 1H, CH2); 5.27 (br s, 1H, =CHSiMe3); 8.17 (s,
1H, Ti=CH). 13C {1H}(C6D6): 0.7, 1.6 (SiMe3); 10.7, 11.2 (C5Me4);
12.8 (C5Me5); 13.5, 14.0 (C5Me4); 43.0 (CH2); 117.0, 118.8 (C5Me4);
122.9 (=CHSiMe3); 123.7 (C5Me5); 127.5, 129.3 (C5Me4); 142.1
(C=CHSiMe3); 161.4 (Ti-CH=C); 220.2 (Ti-CH=C). The 5-th signal of
(C5Me4) is overlapped by C6D6 signal. 29Si {1H}(C6D6): −11.1, −10.3
The asymmetric part of the unit cell contains two symmetri-
cally independent molecules, which both have the same chemical
composition. One of these molecules has its SiMe3 group disor-
dered, where its methyl groups become rotated by about 45° These
two positions have been refined by setting the occupancy factors
atoms of all methyl group atoms to 0.5. The carbon atoms could
be refined only isotropically.
1
(SiMe3). 1DNOESY (C6D6, selected): Hirr/1Hres: 2.94/1.51, 3.12, 5.26
Similarly, the crystal lattice of 4a was struck by twinning, which
could be accounted for after data reduction. The asymmetric part
of the unit cell again incorporated two independent molecules.
Both molecules have been refined using anisotropic thermal pa-
rameters for their non-hydrogen atoms.
(CH2/C5Me4, CH2, =CHSiMe3); 3.12/2.13 (CH2/C5Me4); 5.26/0.24,
1.51, 2.94 (=CHSiMe3/SiMe3, C5Me4, CH2); 8.16/0.25, 1.76 (Ti=CH/
SiMe3, C5Me5). IR (KBr, cm–1): 2985 (m,sh), 2952 (m), 2900 (s),
2720 (vw), 1593 (m), 1485 (w), 1433 (w,b), 1377 (m), 1331 (vw),
1245 (s), 1209 (w), 1079 (vw), 1064 (vw), 1022 (w), 940 (w), 929
(w), 870 (m), 851 (s), 837 (vs), 749 (w), 687 (w), 648 (vw), 625
(vw), 520 (vw), 486 (vw), 415 (m).
3.7. Computational methods
Computational studies were carried out using Gaussian 16,
Revision C.01 [25]. All computations used the M11 functional
[26] and the 6–31G(d,p) basis set on all atoms. The wavefunctions
of ground-state molecules were always checked for stability. The
optimized molecules of 2a and 4a in their ground states were ad-
ditionally checked for the presence of no imaginary vibrations. The
optimization of excited-state 2a was done using time-dependant
DFT computing simultaneously the five lowest-lying excitations
and optimizing the geometry on the first root. Convergence diffi-
culties during SCF were managed by employing quadratic conver-
gence when needed.
3.5. Photolysis of 2a to give 4a
Compound 2a (0.52 g, 0.91 mmol) in 10 mL of benzene was
exposed to sunlight for 12 h turning its red colour to dark yellow.
Crystallization from hexane afforded yellow-brown plates of 4a.
4a: Yield 0.33 g, 63%. M.p. 204 °C. EI-MS (200 °C): m/z (rel-
ative abundance) 574 (11), 573 (12), 572 (M.+; 19), 539 (6), 538
(5), 537 ([M – Cl]+; 9), 522 ([M – Cl – Me]+; 2), 439 (7), 438
(6), 437 ([M – C5Me5]+; 13), 422 ([M – C5Me5 – Me]+; 3), 380
([M – C5Me5 – Bu]+; 2), 365 ([M – C5Me5 – Bu – Me]+; 2), 323
([M – C5Me5 – 2 Bu]+; 2), 221 (8), 220 (39), 219 (22), 218 ([M –
C5Me4CH2 – L3]+; 100), 217 (13), 216 (15), 73 ([SiMe3]+; 21), 59
Declaration of Competing Interest
(11), 57 (14); (L3 = Me2Si(C CtBu)2). 1H NMR (C6D6): −0.83 (d,
≡
2JHH = 11.1 Hz, 1H, TiCH2); 0.53, 0.63 (2 × s, 2 × 3H, SiMe2); 1.15,
1.25 (2 × s, 2 × 9H, CMe3); 1.40 (2 × s, 2 × 3H, C5Me4); 1.77
(s, 3H, C5Me4); 1.83 (s, 15H, C5Me5); 2.34 (s, 3H, C5Me4); 3.41 (d,
2JHH = 11.1 Hz, 1H, TiCH2). 13C {1H}(C6D6): 1.6, 1.8 (SiMe2); 10.5
(C5Me4); 12.5 (C5Me5); 12.8, 15.9 (C5Me4); 30.7, 31.0 (CMe3); 35.6,
39.5 (CMe3); 79.9 (TiCH2); 115.4, 116.2 (C5Me4); 123.7 (C5Me5);
124.1, 133.0, 134.2 (C5Me4); 135.8 (TiCH2C=C); 153.2 (C5Me4C=);
158.6 (TiCH2C=C); 159.7 (C5Me4C=C). 29Si {1H}(C6D6): 3.7 (SiMe2).
IR (KBr, cm–1): 2955 (vs), 2899 (s), 2862 (m), 2719 (vw), 1615 (m),
1548 (vw), 1477 (m), 1460 (m), 1432 (m), 1379 (s), 1356 (m), 1275
(vw), 1243 (s), 1202 (w), 1196 (w), 1158 (vw), 1120 (vw), 1089
(vw), 1047 (w), 1018 (w), 1007 (vw), 845 (vs), 828 (m), 775 (s),
711 (w), 691 (m), 636 (vw), 560 (vw).
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could influence the
work reported in this paper.
Acknowledgements
Support for this work was provided by the internal grant pro-
vided by the J. Heyrovský Institute of Physical Chemistry.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2020.
121536.
3.6. Solid-state structure determination
Appendix A. Supplementary Data
Single crystals of 3a, 3b, and 4a were mounted into Lindemann
glass capillaries in a Labmaster 130 glovebox (mBraun) under puri-
fied nitrogen atmosphere. Diffraction data were collected on a No-
nius KappaCCD diffractometer equipped with a Bruker APEX II de-
Deposition Number 1,970,667–1,970,669 contain supplementary
crystallographic data for 3a, 3b and 4a. These data are provided
free of charge by the joint Cambridge Crystallographic Data Centre
and Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
˚
tector (MoK radiation, λ = 0.71073 A) at 150 K by using an Oxford
α
Cryostream cooler. Collected data were processed by the diffrac-
tometer software. The phase problem was solved by intrinsic phas-
ing and structure models were refined by full matrix least squares
on F2 using the SHELX program suite [22]. All non-hydrogen atoms
were refined anisotropically. Any hydrogen atoms were placed in
idealized positions and were refined isotropically. Molecular graph-
ics was generated using PLATON [23] and Raster3D [24]. Relevant
crystallographic data are gathered in the Supplementary material.
The crystal lattice of 3b was suffering from twinning and the
two lattices could not be separated properly during data collection
Table of crystallographic data, data collection and structure re-
finement data for 3a, 3b, and 4a. 1H NMR records of photolysis of
1a; 1H NMR, 13C NMR and 29Si NMR spectra of 3a, 3b, and 4a.
Cartesian coordinates of optimized 1b, 1b∗, 2a, 2a∗ and 4a.
References
[1] M. Oelgemöller, Chem. Rev. 116 (2016) 9664–9682.
[2] J.P. Knowles, L.D. Elliott, k.I. Booker-Milburn, Beilstein J. Org. Chem 8 (2012)
2025–2052.
8

J. Pinkas, J. Kubišta, K. Mach et al.
Journal of Organometallic Chemistry 927 (2020) 121536
[3] D. Kim, K.K. Sukimoto, D.C. Hong, P.D. Yang, Angew. Chem. Int. Ed. 54 (2015)
3259–3266.
[15] J.E. Bercaw, J. Am. Chem. Soc 96 (1974) 5087–5095.
[16] J. Pinkas, I. Císarˇová, R. Gyepes, M. Horácˇek, J. Kubišta, J. Cejka, S. Gómez-Ruiz,
ˇ
[4] M.D. Karkas, O. Verho, E.V. Johnston, B. Akermark, Chem. Rev. 114 (2014)
11863–12001.
E. Hey-Hawkins, K. Mach, Organometallics 27 (2008) 5532–5547.
[17] P. Jutzi, B. Hielscher, Organometallics 5 (1986) 1201–1204.
ˇ
[5] A. Vogler, H. Kunkely, Coord. Chem. Rev. 250 (2006) 1622–1626.
[6] T. Haas, R. Krause, R. Weber, M. Demler, G. Schmid, Nature Catalysis 1 (2018)
32–39.
[7] R.N. Perutz, B. Procacci, Chem. Rev. 116 (2016) 8506–8544.
[8] A.J. Hoskin, D.W. Stephan, Coord. Chem. Rev. 233–234 (2002) 107–129.
[9] M. Bochmann, Organometallics 29 (2010) 4711–4740 and references therein.
[10] (a) Z.-T. Tsai, C.H. Brubaker, J. Organomet. Chem 166 (1979) 199–210. (b) P.
B. Brindley, A. G. Davies, J. A.-A. Hawari, J. Organomet. Chem. 250 (1983)
247–256. (c) R.W. Harrigan, G. S. Hammond, H. B. Gray, J. Organomet. Chem.
81 (1974) 79–85.
[18] (a) V. Varga, L. Petrusová, J. Cejka, V. Hanuš, K. Mach, J. Organomet. Chem 509
ˇ
(1996) 235–240. (b) V. Varga, L. Petrusová, J. Cejka, K. Mach, J. Organomet.
Chem. 532 (1997) 251–259. (c) P. Šteˇpnicˇka, R. Gyepes, I. Císarˇová, M. Horácˇek,
J. Kubišta, K. Mach: Organometallics 18 (1999) 4869–4880.
[19] J. Pinkas, J. Kubišta, K. Mach, R. Gyepes, M. Horácˇek, Eur. J. Inorg. Chem. (2020)
128–136.
[20] G. Erker, Chem. Soc. Rev. 28 (1999) 307–314.
[21] H. Antropiusová, A. Dosedlová, V. Hanuš, K. Mach, Transition Met. Chem. (Lon-
don) 6 (1981) 90–93.
[22] G.M. Sheldrick, Acta Cryst C71 (2015) 3–8.
[11] (a) M.R.M. Bruce, D.R. Tyler, Organometallics 4 (1985) 528–533. (b) M. R. M.
Bruce, A. Kenter, D. R. Tyler, J. Am. Chem. Soc. 106 (1984) 639–644. (c) . M. R.
M. Bruce, A. Sclafani, D. R. Tyler, Inorg. Chem. 25 (1986) 2546–2549. (d) J. W.
Kenney, III, D. R. Boone, D. R. Striplin, Y.-H. Chen, K. B. Hamar, Organometallics
12 (1993) 3671–3676.
[12] (a) E. Samuel, P. Maillard, C. Giannotti, J. Organomet. Chem 142 (1977)
289–298. (b) H. Alt, M. D. Rausch, J. Am. Chem. Soc. 96 (1974) 5936–5937.
[13] (a) M. Kessler, S. Schüller, D. Hollmann, M. Klahn, T. Beweries, A. Spannenberg,
A. Brückner, U. Rosenthal, Angew. Chem. Int. Ed. 51 (2012) 6272–6275. (b) M.
Kessler, S. Hansen, C. Godemann, A. Spannenberg, T. Beweries, Chem. – Eur. J.
19 (2013) 6350–6357. (c) C. Godemann, E. Barsch, A. Spannenberg, R. Ludwig,
T. Beweries, Eur. J. Inorg. Chem. (2014) 4068–4072. (d) C. Godemann, L. Dura,
D. Hollmann, K. Grabow, U. Bentrup, H. Jiao, A. Schulz, A. Brückner, T. Bew-
eries, Chem. Commun. 51 (2015) 3065–3068. (e) C. Godemann, D. Hollmann,
M. Kessler, H. Jiao, A. Spannenberg, A. Brückner, T. Beweries, J. Am. Chem. Soc.
137 (2015) 16187–16195.
[23] A.L. Spek, Acta Cryst D65 (2009) 148–155.
[24] E.A. Merritt, D.J. Bacon, Methods Enzymol 227 (1997) 505–524.
[25] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man, G. Scalmani, V. Barone, G.A. Petersson, H. Nakatsuji, X. Li, M. Caricato,
A.V. Marenich, J. Bloino, B.G. Janesko, R. Gomperts, B. Mennucci, H.P. Hratchian,
J.V. Ortiz, A.F. Izmaylov, J.L. Sonnenberg, D. Williams-Young, F. Ding, F. Lip-
parini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe,
V.G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toy-
ota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, K. Throssell, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M.J. Bearpark,
J.J. Heyd, E.N. Brothers, K.N. Kudin, V.N. Staroverov, T.A. Keith, R. Kobayashi,
J. Normand, K. Raghavachari, A.P. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi,
M. Cossi, J.M. Millam, M. Klene, C. Adamo, R. Cammi, J.W. Ochterski, R.L. Mar-
tin, K. Morokuma, O. Farkas, J.B. Foresman, D.J. Fox, Gaussian, Inc., Wallingford
CT, 2019.
[26] R. Peverati, D.G. Truhlar, J. Phys. Chem. Lett 2 (2011) 2810–2817.
[14] J. Pinkas, M. Lamacˇ, Coord. Chem. Rev 296 (2015) 45–90.
9