Matrix isolation infrared spectroscopic study of the photochemistry of bis(cyclopentadienyl)dicarbonyl titanium in solid nitrogen

Journal Pre-proof Matrix isolation infrared spectroscopic study of the photochemistry of Bis(cyclopentadienyl)dicarbonyl titanium in solid nitrogen Hiroshi Manda, Jun Miyazaki, Yasuhiro Yamada PII:

S0022-2860(19)31466-8

DOI:

https://doi.org/10.1016/j.molstruc.2019.127357

Reference:

MOLSTR 127357

To appear in:

Journal of Molecular Structure

Received Date: 24 April 2019 Revised Date:

31 October 2019

Accepted Date: 2 November 2019

Please cite this article as: H. Manda, J. Miyazaki, Y. Yamada, Matrix isolation infrared spectroscopic study of the photochemistry of Bis(cyclopentadienyl)dicarbonyl titanium in solid nitrogen, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127357. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

UV

UV

UV

Δ TiCp2(CO) S=0

Δ TiCp2(CO)2 S=0

TiCp2(CO)(N2) S=0

Ar matrix N2 matrix

TiCp2(N2) S=1

TiCp2(N2)2 S=0

1

Matrix Isolation Infrared Spectroscopic Study of the Photochemistry of Bis(cyclopentadienyl)dicarbonyl Titanium in Solid Nitrogen Hiroshi Manda1†, Jun Miyazaki2, and Yasuhiro Yamada1* 1 2

†

Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan

Hokuriku University, Ho-3, Kanagawa-machi, Kanazawa, Ishikawa 920-1181, Japan

Current affiliation is Kanto Bureau of Economy, Ministry of Economy, Trade and

Industry, Japan *

Corresponding author: [email protected]

ABSTRACT:

Infrared

spectra

of

the

photochemical

products

of

bis(cyclopentadienyl)dicarbonyl titanium, TiCp2(CO)2, isolated in an Ar matrix and a N2 matrix were measured. UV-irradiation of TiCp2(CO)2 produced TiCp2(CO) in the Ar matrix, and three types of nitrogen-containing compounds, TiCp2(CO)(N2), TiCp2(N2), and TiCp2(N2)2 were produced in the N2 matrix. The yields of the species changed with the duration of UV-irradiation. Annealing of the sample resulted in the disappearance of unstable TiCp2(N2) and an increase of TiCp2(N2)2. Isotope shifts of the IR spectra were measured using

15

N2 to confirm the assignments. The structures of the species were

estimated using a double hybrid density functional theory calculation (mPW2PLYP/ccpVTZ), and the calculated infrared frequencies were in very good agreement with the experimentally measured spectra.

INTRODUCTION Spectroscopic studies of species isolated in an inert matrix provide useful information on the chemical properties and structures of novel molecules without intermolecular effects or intervening effects of solvents. We have previously reported the infrared spectra of tris(cyclopentadienyl) lanthanides (LnCp3; Ln = lanthanide, Cp = C5H5–) complexes using a matrix isolation technique and found that Cp had different bonding natures in a monomeric molecule and in a crystal lattice. The stable structure of

2 tris(cyclopentadienyl)scandium (ScCp3) isolated in an Ar matrix was found to be Sc(η5Cp)3, while the solid consists of polymeric structures with η1 bridging bonds of the Cp ring [1]. The monomeric structure of tris(cyclopentadienyl)ytterbium (YbCp3) isolated in an Ar matrix was the Yb(η5-Cp)3 structure [2], while the crystal structure of YbCp3 had two Cp rings located between Yb atoms in η5 and η1 fashions. The

structures

of

tetra(cyclopentadienyl)titanium

(TiCp4)

and

bis(cyclopentadienyl)titanium (TiCp2) have attracted the attention [3] because titanium has both the Ti4+ and Ti2+ valence states. The solid structure of TiCp4 was reported to have two η5- and two η1-Cp rings in a molecule [4]. On the other hand, TiCp2 with the typical sandwich structure as with ferrocene is not available, although the structure has been theoretically predicted [5]. The production of a divalent titanium sandwich complex, bis(cyclopentadienyl)titanium (η5-C5Me4R)2Ti) was reported [6-8]. One of the stable

complexes

that

consists

of

Ti2+

and

two

Cp

rings

is

bis(cyclopentadienyl)dicarbonyl titanium, TiCp2(CO)2, which has a unique distorted structure and satisfies the 18-electron rule, assuming each Cp ring and CO donates 6 eand 2 e-, respectively, to the Ti2+ ion. It has been reported that the photolysis of TiCp2(CO)2 produces TiCp2(CO) in an Ar matrix [9]. Although it was expected that further irradiation would produce TiCp2, this species is not readily obtained with 14 electrons at a Ti atom. However, TiCp2 could be a precursor to produce novel species with a bond to Ti metal [10] because of its high reactivity. The chemistry of titanium compounds with nitrogen has received considerable attention because it provides important information on the activation and functionalization of dinitrogen. The synthesis of bis(cyclopentadienyl)metal compounds coordinated by dinitrogen has been studied, for example, (η5-C5Me4CHMe2)2Ti(N2)2 [11]. Bond cleavage of the triple bond in N2 using nitrogen complexes is a very attractive

topic

[12].

The

coordination

of

N2

to

a

family

of

bis(cyclopentadienyl)titanium sandwich complexes, (η5-C5Me4R)2Ti, has been studied using infrared spectroscopy [10]. It was reported that the N2 coordination strength is affected by the R substituent and that Ti-N2 bonding is largely dictated by σ rather than π

effects.

Most

of

the

Ti-N2

bonds

have

end-on

structure;

however,

bis(cyclopentadienyl)titanium dinitrogen compounds have been reported to have a side-

3

on N2 [11]. Studies using simple TiCp2 compounds may provide clarification into the bonding nature of such complexes. The photolysis of TiCp2(N3)2 in a CO-doped Ar matrix produced TiCp2(N2)2 [9]. However, the production of simple TiCp2(N2) or TiCp2(CO)(N2) by the photolysis of TiCp2(CO)2 has yet to be reported. CO and N2 ligands are known to be isoelectronic, and have similar chemical bonding natures. The reaction of a compound with nitrogen could be induced by photolysis of the compound in a nitrogen matrix. For example, the photolysis of [Ir(η5-C5H5)(CO)H2] in a nitrogen matrix yielded [Ir(η5-C5H5)(CO)(N2)] [13]. In this study, the photochemical reaction of TiCp2(CO)2 with N2 molecules in a lowtemperature N2 matrix was studied using infrared spectroscopy associated with density functional theory (DFT) calculations.

EXPERIMENTAL The powder sample of TiCp2(CO)2 (Strem Chemicals, Inc., >98%) was handled in a glove box filled with Ar and was stored in a glass tube equipped with a valve. The sample tube was attached to a cryostat and residual gases were evacuated before introduction to the cryostat. A CsI plate was cooled down to 18 K using a closed cycle refrigerator (Iwatani Cryo Techno, CryoMini D510). TiCp2(CO)2 vapor was introduced at room temperature, and the matrix gas (Ar or N2) in a vessel was introduced simultaneously through a needle valve. The mixture ratio was estimated by measuring the change of the pressures of the matrix gases, and by weighing the sample tube before and after introduction to the cryostat. To see the isotope effects of N2,

14

N2/15N2 was

prepared by mixing N2 (natural abundance; >99.99995%; Taiyo Nissan Sanso) and 15N2 (98%; Spectra Gases Inc.). Infrared spectra of the samples were measured using a Fourier transform-infrared (FT-IR) spectrometer (Perkin Elmer, Spectrum One) with a resolution of 0.5 cm-1. TiCp2(CO)2 was isolated in Ar or N2 matrices at 18 K, and UV irradiation (270 < λ < 390 nm) was performed using a super high-pressure mercury lamp (Ushio, USH-250D) with water and glass filters. The sample was kept at the desired temperature using a heater and a controller. The intensity of the light on the surface of the samples was kept at 65 mW. Theoretical calculations were performed using the Gaussian 09 program set [14].

4

RESULTS AND DISCUSSION TiCp2(CO)2 isolated in an Ar matrix The IR spectrum of the TiCp2(CO)2/Ar = 1/2000 (molar ratio) sample measured at 18 K is shown in Fig. 1a; the strong absorptions at 1904 cm-1 and 1983 cm-1 correspond to the symmetric and asymmetric stretching of the two CO ligands, respectively. These spectroscopic results were identical to results previously reported in the literature [9]. Besides the absorptions of CO, the CH in-plane bending vibration modes of the Cp ring were also measured at 1003, 1006, and 1017 cm-1, and the CH out-of-plane bending vibration modes of the Cp ring at 789, 803, 815, and 818 cm-1; TiCp2(CO)2 was labeled A in Fig. 1. The assignments were confirmed by theoretical calculations, as described later. After UV-photolysis of the sample for 360 min (Fig. 1b), new bands appeared at 772, 796, 822, 1010, and 1890 cm-1, which were assigned to TiCp2(CO) (labeled B in Fig. 1) produced by the photodissociation of TiCp2(CO)2 accompanied by the release of CO. The absorption at 1890 cm-1 has been reported in the literature [9], while the other bands were assigned according to our theoretical calculation results. The intensities of peaks of TiCp2(CO)2 (1983 and 1904 cm-1) decreased by 43% by the UV-irradiation. The peak intensity of TiCp2(CO) (1890 cm-1) was smaller than that expected assuming that TiCp2(CO) was the sole photoproducts in this experiment; the yield of TiCp2(CO) was estimated to be 50% using the calculated infrared intensities (S7). The possibility that the UV-photolysis produced other byproducts that were not found in the spectrum cannot be excluded. Weak absorption band at 2138 cm-1 was assigned to an isolated CO molecule [15, 16], which was produced by the photodecomposition of TiCp2(CO)2. The band at 2153 cm-1 was assigned to CO attached to an impurity water molecule (H2O...CO) [17]. Other bands were observed at 2184 and 2108 cm-1; the peak at 2184 cm-1 corresponded to the in-band lattice mode of CO in the Ar matrix [16]. The peak at 2108 cm-1 was assigned to CO having a weak bond to a photoproduct. One of the possible species was TiCp2(CO)...OC complex in which CO attached by O atom to TiCp2(CO); the calculated structure and frequencies are shown in S6, S7, and S8, and the frequencies other than CO stretching were very similar to those of TiCp2(CO). The yield of TiCp2(CO) was very small, and the further photoirradiation did not form other species. The sample was then annealed at 30 K for 360 min, and the peaks of TiCp2(CO) decreased by 36% (Fig. 1c); TiCp2(CO) recombined with dissociated CO

5 upon annealing at 30 K. The peak of H2O...CO at 2153 cm-1 increased by the annealing (the difference spectrum on annealing is shown in S1). It was demonstrated that the cage effect of the Ar matrix is too strong to produce a large amount of TiCp2(CO), and the recombination of CO was dominant. TiCp2 was not observed in these measurements; the density functional calculation indicated that the TiCp2 is expected to have a peak at ~766 cm-1 (S8), which was not observed in the spectrum (Fig. 1c). The observed peaks and assignments are listed in Table 1.

TiCp2(CO)2 isolated in a N2 matrix Similar experiments were performed using a N2 matrix to determine the reactions of TiCp2(CO)2 with N2 molecules. The IR spectrum of TiCp2(CO)2/N2 = 1/2000 at 18 K had absorptions at 1900 and 1979 cm-1, which correspond to the CO stretching bands of the symmetric and asymmetric modes, respectively, while the peak positions were slightly different from those observed in the Ar matrix because of a matrix shift (Fig. 2a). After UV irradiation (270 < λ < 390 nm) of the sample for 360 min, the peak intensities of TiCp2(CO)2 (labeled A in Fig. 2) decreased, and the products obtained after irradiation were completely different from the products obtained in the Ar matrix (Fig. 2b). The intensity ratio of the peaks at 1900 and 1979 cm-1 was different from that of TiCp2(CO)2 after irradiation; the band at 1900 cm-1 became stronger due to the production of a new species coincidently having an absorption at the same position of 1900 cm-1. The absorption of TiCp2(CO) at 1890 cm-1 was not found in the spectrum. Carbon monoxide CO was observed at 2140 cm-1, which was in agreement with the value for CO in a N2 matrix reported in the literature [18, 19]. The absorptions at 2040 and 2131 cm-1 increased after UV-irradiation, which were in the region of N2 stretching, and were assigned to N2 symmetric and asymmetric modes of TiCp2(N2)2 (labeled E in Fig. 2) by analogy with the CO stretching modes of TiCp2(CO)2. The IR spectrum of TiCp2(N2)2 in a N2 matrix was reported in the literature [9], in which the peaks were reported to have different values (2080 and 2140 cm-1) from the present experimental values. However, considering the broad absorption peaks reported in the literature, these values were in agreements with the present results. New bands also appeared at 781 and 2120 cm-1. After the annealing of the sample at 30 K for 120 min (Fig. 2c), the intermediate species (TiCp2(N2); labeled D in Fig. 2) disappeared

6 and the bands of TiCp2(N2)2 (labeled E in Fig. 2) at 2040 and 2131 cm-1 increased. The difference spectrum is shown in S2. Bands around the 800 cm-1 and 1000 cm-1 regions that corresponded to C-H stretching and bending vibrations in Cp were also measured. The observed absorptions are summarized in Table 1. In contrast to the photolysis of TiCp2(CO)2 in an Ar matrix, the reactions followed by the dissociation of CO were enhanced in the N2 matrix.

Change of the yields during UV-irradiation IR spectra were measured with variation of the irradiation time to clarify the assignments of the absorption peaks with the overlapping of multiple components. The intensities of the selected absorption peaks (1900, 1979, 2042, 2120, 2131, 2140, and 2159 cm-1) were plotted with variation of the irradiation time in Fig. 3. The absorption at 1900 cm-1 was attributed to the combination of two species, whereas the peak at 1979 cm-1 did not overlap with that for other species. The ratio of the peak intensities at 1900 and 1979 cm-1 of TiCp2(CO)2 was 1.35 in the spectrum observed prior to irradiation (Fig. 2a), and thus the portion of the TiCp2(CO)2 in the absorption at 1900 cm-1 was estimated using the intensity at 1979 cm-1, which is marked as X in Fig. 3. The intensities of the residual components were calculated by subtracting the intensity of X from the observed intensity at 1900 cm-1, which is marked as Y in Fig. 3. It became obvious that the absorption at Y and that at 2159 cm-1 behaved similarly, and the intensity ratio was kept at almost 2.3; therefore, the two peaks (Y and 2159 cm-1) were attributed to the same species. The portion of X of 1900 cm-1 was due to C-O stretching and the band at 2159 cm-1 was due to N-N stretching. Thus, the species with 1900 cm-1 (Y) and 2159 cm-1 absorptions was assigned to TiCp2(CO)(N2) (labeled C in Fig. 2), which was confirmed by theoretical calculations, as described later. While TiCp2(CO)2 decreased by UV-irradiation, TiCp2(CO)(N2) increased first and then decreased, which is typical behavior of an intermediate species in a chain reaction. The peak intensities of TiCp2(CO)(N2) remained almost constant while annealing at 30 K, and the species did not increase by the recombination reaction of dissociated CO. Two absorptions at 2042 and 2131 cm-1, which corresponded to TiCp2(N2)2 (labeled E in Fig. 2), increased before the 180 min irradiation time, and became constant after 180 min, keeping the same intensity ratio. The absorptions increased rapidly by

7

annealing, which indicated that an unstable intermediate species was present as a precursor of TiCp2(N2)2, which reacted with surrounding N2 molecules. The absorption at 2140 cm-1 that was assigned to CO increased by irradiation and the intensity became almost saturated after 100 min. The absorption did not decrease by annealing, which indicates that CO did not recombine with the intermediate species. The reactions with surrounding N2 molecules overcame the reactions with CO. The absorption at 2120 cm-1 increased by irradiation, and decreased rapidly by annealing at 30 K. Therefore, this absorption corresponds to an unstable intermediate that easily reacts with N2 to produce TiCp2(N2)2. The intermediate species was assigned to TiCp2(N2) (labeled E in Fig. 2), which was confirmed by theoretical calculations, as shown later. The peak of TiCp2(N2) at 2120 cm-1 had two sidebands at 2115 and 2111 cm-1, which could be attributed to the site effects of the solid N2 matrix. The absorptions at 2163 appeared by UV-irradiation and the intensity of the peak increased with prolonged irradiation. This peak disappeared rapidly by annealing at 30 K. The absorptions of isolated librating CO (∆nlib = 1) at 2156.7 and 2160.5 cm-1 in a Ne matrix have been reported [16]. The infrared spectra of CO diluted in a N2 matrix have been reported to show a broad absorption associated with N2 phonons in the region of 2170-2200 cm-1 [20]; however, this absorption was not in agreement with the sharp peak observed. Therefore, the peak at 2163 cm-1 was assigned to librating CO, of which the intensity was strongly temperature dependent. TiCp2(CO)2 isolated in a 14N2/15N2matrix The assignment of TiCp2(N2)2 and TiCp2(N2) was confirmed by measurements using a matrix consisting of a mixture of

14

N2/15N2 (molar ratio of approximately

14

N2/15N2 =

2/1; Fig. 4). A very small absorption of 15N218O impurity at 2159 cm-1 was observed in the spectrum (Fig. 4a) [22]. Other bands of

15

N218O, ν1 (1232 cm-1) and ν2 (566 cm-1),

were also observed (S3). Impurities of commercial 15N2 sample have been discussed in the literature [23]. TiCp2(N2)2 and TiCp2(N2) species containing

14

N15N ligands were

not observed in the IR spectra. After UV-irradiation, several new absorptions appeared (Fig. 4b), among which was a large absorption at 2120 cm-1 that was assigned to TiCp2(14N2). After annealing of the sample at 30 K for 160 min, the absorptions of TiCp2(N2)2 increased (Fig. 4c); TiCp2(14N2)2 had absorptions at 2042 and 2131 cm-1,

8 TiCp2(14N2)(15N2) had absorptions at 1994 and 2106 cm-1, and TiCp2(15N2)2 had absorptions at 1975 and 2060 cm-1. Although the absorptions of TiCp2(CO)(N2) were relatively weak in the spectra, isotope shifts of TiCp2(CO)(N2) were observed; the absorption at 2089 cm-1 was assigned to TiCp2(CO)(15N2) and the shift of another absorption of CO stretching appeared at an almost identical position of 1900 cm-1. The observations of the isotope shifts confirmed the assignments of TiCp2(N2)2, TiCp2(N2), and TiCp2(CO)(N2). The assignments of the peaks are listed in Table 2. The spectra of CH bending vibration modes are shown in S3, which did not show significant isotope effects. Theoretical frequency calculations of the isotope-substituted species were also performed, as described in the next section.

Theoretical calculations Theoretical calculations were performed using the Gaussian 09 program set [14]. Before calculating unknown species, we searched for adequate functionals and basis sets to estimate the IR frequencies of the species determined experimentally. Benchmark calculations were performed for frequencies of well-assigned species, TiCp2(CO)2 and TiCp2(N2)2. At first, the popular hybrid density functional B3LYP was employed using 6-31+G* and cc-pVTZ basis sets. There was no significant difference between the results obtained using the two basis sets, and the calculated frequencies below 1100 cm-1 were in very good agreement with the observed frequencies. However, the calculated frequencies were too large to fit into the observed frequencies at 2042 and 2131 cm-1 of TiCp2(N2)2. The large discrepancy between the calculated and observed values may be due to the weak bonding nature of Ti-N2. Next, calculations were performed using two double-hybrid density functionals, B2PLYP [21] and mPW2PLYP [24], which include perturbative second-order correlation. When using both B2PLYP and mPW2PLYP, no significant differences were observed between the selection of the two basis sets, 6-31+G* and cc-pVTZ. The frequencies calculated using B2PLYP were too small for N-N in TiCp2(N2)2. However, the results obtained using mPW2PLYP were in good agreement with the observed values, and it was concluded that mPW2PLYP/ccpVTZ was an adequate calculation method for the present system (Supplemental Material S5). Calculations for the other species, TiCp2(CO), TiCp2(CO)(N2), and TiCp2(N2), were conducted using mPW2PLYP/cc-pVTZ to confirm the assignments.

9

The results of frequency calculations for TiCp2(CO)2, TiCp2(N2)2, TiCp2(CO), TiCp2(CO)(N2), and TiCp2(N2) using mPW2PLYP/cc-pVTZ are summarized in Table 1 (A list of the calculated frequencies are available as Supplementary Materials S7 and S8). The harmonic frequencies were calculated and a scaling factor 0.976 was applied in a region between 750 and 1050 cm-1, whereas a scaling factor 0.998 was applied in a region between 1850 and 2250 cm-1. The optimized structures are shown in Fig. 5. The species were calculated assuming both singlet (S=0) and triplet (S=1) states, and all of the species were determined to have singlet ground states, except for TiCp2(N2) had a triplet ground state; the triplet TiCp2(N2) was 0.11 eV more stable than the singlet TiCp2(N2) (Fig. 6). The N-N stretching mode of TiCp2(N2) (S=1) was calculated to be 2125 cm-1, while that of TiCp2(N2) (S=0) was calculated to be 2068 cm-1, which indicates that the Ti-N bond of TiCp2(N2) became weaker in the S=1 state than in the S=0 state. The experimentally observed peak at 2120 cm-1 was thus confirmed to be the absorption of TiCp2(N2) (S=1). The bonding natures of TiCp2 with CO and N2 in relation to the structures of TiCp2(CO) and TiCp2(N2) have been discussed in the literature [25]; it was predicted that TiCp2(N2) is stabilized with the singlet state (S=0), analogous to the structure of TiCp2(CO). It was also predicted that the edge-on geometry could be stable because N2 should not be as good an σ donor nor a π acceptor as CO. Therefore, the TiCp2(N2) with the edge-on structure was also calculated using mPW2PLYP/cc-pVTZ, and the S=1 state was identified as a stable structure. The calculated frequency of N-N stretching for the edge-on structure was 1829 cm-1, which was too small for the experimental value, and the calculated energy was 0.42 eV higher than that of TiCp2(N2) (end-on geometry with S=1), which indicated that this species was not produced in the present experiment. To elucidate the reason why the triplet state of TiCp2(N2) was more stable than the singlet state, consideration based on molecular orbitals was made. An interaction diagram for the bending TiCp2 fragment, TiCp2(N2) and TiCp2(CO) is shown in Fig. 7, based on the energies obtained using the Harris functional [26] to simplify the diagram. Three molecular orbitals of the bending TiCp2 fragment (1a1, b2, and 2a1), which interact with CO or N2 ligands, are shown in the middle of Fig. 7. In the case of TiCp2(CO)2, the two carbonyl ligands form two σ bonds using metal fragment orbitals

10 2a1 and b2, and the 1a1 orbital is stabilized by a combination of the π* orbitals of carbonyls [25]. In the case of TiCp2(CO), only 2a1 forms a σ bond with the carbonyl ligand. The highest occupied molecular orbital (HOMO) of TiCp2(CO) (S=0) is π-back bonding from the d-orbital of Ti to the C=O antibonding orbital, and the lowest unoccupied molecular orbital (LUMO) is the 1a1 orbital. The Ti-C bonding is strong; therefore, the energy difference between the HOMO and LUMO is too large to stabilize in TiCp2(CO) (S=1). On the other hand, the bonding of Ti-N is not strong in TiCp2(N2), and the Ti-N π-back bonding (b2) and the Ti-Cp2 bonding (1a1) had almost the same energy to result in the triplet state (S=1). The frequencies of the isotope-substituted species were also calculated and the results are summarized in Table 2. The isotope effect on the stretching mode of 14N2 and 15N2 was sufficiently large, and thus the calculated results of TiCp2(14N2) and TiCp2(15N2) were in good agreement with the observed frequencies. The stretching mode of CO in TiCp2(CO)(N2) was influenced by the isotope effect of nitrogen (14N2 and

15

N2),

although the difference was very small. The calculated shift of the isotope effects was 0.2% while the observed shift was 0.1%. The interaction energies between the ligands (CO and N2) and Ti metal were estimated using B3LYP/cc-pVTZ. The second-order perturbation energies calculated by natural bond order (NBO) analysis [27] are summarized in Table 3. It became obvious that Ti-CO bonding was stronger than Ti-N2 bonding. Comparison of TiCp2(CO)2 and TiCp2(CO) revealed that both the σ and π bondings of those species had almost the same energies. In contrast, the π bonding of TiCp2(N2) had smaller energy than that of TiCp2(N2)2 because TiCp2(N2) was a triplet state and the spin was located at the π bonding; only the up-spin electron occupies the π bonding.

Reaction mechanism Energies of the species based on the geometry optimized structures using mPW2PLYP/cc-pVTZ calculations are summarized in Fig. 6. To compare the stabilities of the species with different compositions, the energies of the same constituents with reactants were summarized in the procession from TiCp2(CO)2 + 2 N2 to TiCp2(N2)2 + 2 CO. While most of the species in this study were stable in the singlet state (S=0), only

11

TiCp2(N2) was stable in the triplet state (S=1). A small amount of TiCp2(CO) was produced only in the Ar matrix. Photolysis of TiCp2(CO)2 excited the charge transfer band (308 nm) followed by the dissociation of CO. UV-vis absorptions of TiCp2(CO) was calculated using TD-B3LYP/6-31+G*, and TiCp2(CO) was estimated to have charge transfer bands at 302 and 275 nm. Therefore, it might be predicted that UV-light (270 – 390 nm) could photolyze TiCp2(CO). But unstable TiCp2 was not trapped in an Ar matrix. In an Ar matrix, CO and TiCp2(CO) were trapped in a short distance and the decomposed CO and TiCp2(CO) may easily recombine to form TiCp2(CO)2. On the contrary, in the N2 matrix, photolysis of TiCp2(CO)2 produced TiCp2(CO)(N2). Even when TiCp2(CO) was temporarily produced in a N2 matrix, the reaction with surrounding N2 molecules occurred readily to produce the more stable compound, TiCp2(CO)(N2). The yields of TiCp2(CO)(N2) decreased by further photolysis; the photolysis of TiCp2(CO)(N2) induced the release of CO to produce TiCp2(N2). A small amount of TiCp2(N2)2 appeared by UV photolysis and the intensity was increased by annealing of the sample. The calculations indicate that TiCp2(N2) in the triplet electronic state (S = 1) is more stable than that in the singlet state (S = 0). When TiCp2(N2) (S = 1) is trapped in the N2 matrix, it does not react with surrounding N2 molecules to produce TiCp2(N2)2 (S = 0) at 18 K, because the difference of the spin states hinders the reaction. The reason why TiCp2(N2) was stabilized in the N2 matrix while TiCp2(CO) was not trapped could be attributed to the difference in the electronic states of the compounds. The activation energy for the reaction was too small to hinder the reaction at 30 K: TiCp2(N2) (S = 1) + N2 (S = 0) → TiCp2(N2)2 (S = 0).

CONCLUSIONS The UV-photolysis of TiCp2(CO)2 in an Ar matrix produced TiCp2(CO); however, the yields were very small due to the cage effects. In contrast, the UV-photolysis of TiCp2(CO)2 in a N2 matrix produced various species, TiCp2(CO)(N2) and TiCp2(N2)2. TiCp2(CO) was not trapped in the N2 matrix, because of the rapid reaction with surrounding N2 molecules. Further UV-photolysis of TiCp2(CO)(N2) produced TiCp2(N2), and the subsequent thermal reaction of TiCp2(N2) with a N2 molecule to produce TiCp2(N2)2 was observed at 30 K. Isotope-substituted experiments using

15

N2

12

were performed to confirm the assignments of the species. Double hybrid density functional calculations (mPW2PLYP/cc-pVTZ) of the species were performed and the frequency calculation results were in very good agreement with the observed IR spectra. The calculation also indicated that the intermediate species TiCp2(N2) had the triplet ground state. It was thus successfully demonstrated that the UV-photolysis of TiCp2(CO)2 in a N2 matrix could produce a compound with N2 molecules.

13

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Table 1. Observed and calculated wavenumbers. Scaling factors 0.976 and 0.997 were applied below 1055 cm-1 and above 1850 cm-1, respectively. The numbers in parentheses denote the calculated IR intensities (KM/Mole).

Ar matrix

N2 matrix

Assignments

Calculated.

As

After

As

After

After

mPW2PLYP/c

deposited

irradiation

deposited

irradiation

annealing

c-pVTZ

772 vw 780 w

TiCp2(CO)

CH oop

779 (94)

TiCp2(N2)2

CH oop

789 (98)

TiCp2(N2)

CH oop

787 (218)

787 w

TiCp2(CO)(N2)

CH oop

793 (84)

795 vw

TiCp2(CO)2

CH oop

799 (96)

TiCp2(CO)

CH oop

807 (79)

TiCp2(N2)

CH oop

794 (7)

TiCp2(N2)2

CH oop

809 (82)

TiCp2(CO)(N2)

CH oop

808 (91)

TiCp2(CO)2

CH oop

810 (85)

TiCp2(N2)

CH oop

810 (16)

TiCp2(CO)2

CH oop

818 (4)

TiCp2(CO)

CH oop

816 (16)

TiCp2(CO)(N2)

CH oop

819 (21)

TiCp2(CO)2

CH oop

819 (19)

TiCp2(CO)

CH oop

841 (2)

TiCp2(CO)2

CH ip

1004 (9)

TiCp2(CO)

CH ip

1001 (14)

TiCp2(CO)(N2)

CH ip

1005 (11)

TiCp2(N2)

CH ip

1006 (15)

TiCp2(CO)(N2)

CH ip

1007 (9)

TiCp2(CO)2

CH ip

1008 (11)

TiCp2(CO)

CH ip

1011 (26)

TiCp2(CO)2

CH ip

1018 (14)

TiCp2(CO)2

CH ip

1019 (14)

TiCp2(N2)2

CH ip

1020 (14)

TiCp2(CO)(N2)

CH ip

1019 (15)

TiCp2(CO)

CO str.

1927 (901)

TiCp2(CO)2

CO str. asym.

1938 (1053)

TiCp2(CO)(N2)

CO str.

1943 (1035)

780 w

781 w 787 vw 789 m

789 m

795 m

796 vw 803 vw 805 vw

803 m

815 w

803 m

815 w

808 m

808 vw

819 w

822 vw 818 w

818 w

805 w

822 vw

824 w

822 w 1003 w

1003 w

1004 w

1002 vw

1003 vw

1005 vw

1006 w

1006 w

1007 w

1010 w 1017 w

1015 vw 1019 w

1019 vw

1019 vw

1890 m 1904 vs

1904 vs

1900 vs

1900 s

1900 s

1983 vs

1983 vs

1979 vs

1979 s

1979 m

TiCp2(CO)2

CO str. sym.

2014 (703)

2042 s

TiCp2(N2)2

NN str. asym.

2052 (1091)

TiCp2(N2)

NN str.

2119 (1256) 2134 (634)

(2038 w) 2042 s

(2047 w) TiCp2(CO)…OC

2108 vw (2111 w) (2115 w) 2120 m 2131 m

2131 s

TiCp2(N2)2

NN str. sym.

2138 w

2140 s

2140 s

CO

∆nlib = 0

2153 w

(2155 vw)

H2O...CO

2159 w

2159 w

(2163 m)

(2162 w)

TiCp2(CO)(N2)

NN str.

CO

∆nlib = 1

2147 (611)

Table 2. Observed and calculated frequencies of isotope substituted compounds TiCp2(CO)(N2), TiCp2(N2)2, and TiCp2(N2). A scaling factor 0.997 was applied. The numbers in parentheses denote the calculated IR intensities (KM/Mole).

Observed in a N2 matrix Calculated frequencies using Before

After

After

irradiation

irradiation

annealing

1898 m

1898 s

1900 s

1990 s

1975 vw

1975 w

1979 w

1979 w

1995 w

1994 s

1998 w

1998 w

(2041 w)

(2041 m)

2042 w

2042 s

1900 s

1979 s

mPW2PLYP/cc-pVTZ

TiCp2(CO)2

1939(1054)

TiCp2(CO)(15N2)

1943(1034)

TiCp2(CO)(14N2)

1982(1018)

TiCp2(15N2)2

2004(981)

TiCp2(14N2)(15N2)

2052(1091)

TiCp2(14N2)2

2047(1173)

TiCp2(15N2)

TiCp2(CO)2

2050 m 2060 vw

2060 w

2062(592)

TiCp2(15N2)2

2089 vw

2089 vw

2078(547)

TiCp2(CO)(15N2)

2094 vw

2094 vw 2112(687)

TiCp2(14N2)(15N2)

2119(1256)

TiCp2(14N2)

2134 (634)

TiCp2(14N2)2

2147(612)

TiCp2(CO)(14N2)

2106 m 2110 w

2109 w

(2115 w) 2120 s

2159 s

2131 w

2131 m

2140 s

2140 s

CO

2159 s

2159 s

15

(2163 m)

2162 vw

N218O

CO

Table 3. Interaction energies between the ligand and Ti of the species calculated using B3LYP/cc-pVTZ. The second order perturbation E(2) / kcal mol-1 are listed. Species

CO (σ) → Ti

Ti → CO(π∗)

TiCp2(CO)2

257.72

53.39

TiCp2(CO)

251.93

57.08

TiCp2(CO)(N2)

202.33

46.90

NN (σ) → Ti

Ti → NN(π∗)

128.03

34.14

TiCp2(N2)2

131.61

34.49

TiCp2(N2)

115.55

25.64

2250 2200 2150 2100 2050 B

2000 1950 1900

Wavenumber / cm -1

1850

A = 0.01

A = 1.0

1000 950

A = 0.01

A = 1.0

B

A = 0.1

A A

B

800

A = 0.1

A = 0.01

AA A

A = 0.1

A = 0.01

A

A = 0.01

A = 1.0

A = 0.01

Absorbance

A A A

(a)

CO B

(b) B

CO

(c)

750

A

A

A = 0.02

(a)

A+C

E D

C

A

CO

(b)

D E

A = 0.01

A+C

A ED C C

A = 0.05

E

E A+C+E

CO

A = 0.2

E

A+C

A

C

E

A = 0.01

A+C

E C

C A

A = 0.05

A = 0.2

A A

C+D A+C+E

CO

Absorbance

A

AA A

A = 1.0

A

A = 0.1

A

(c)

2200

2100

2000

1900 1050 1000 -1 Wavenumber / cm

850

800

750

-1

1900 cm (X+Y)

20

Y 10

X -1

Area intensity / arb. unit

2159 cm

0 -1

1979 cm

10 -1

2042 cm

5 -1

2131 cm

0

-1

2120 cm

5 -1

0

2140 cm

0

100

200

Irradiation time / min

300

0

150 Annealing time / min

A

15

A = 1.0

A 18

N2 O

(a) 15

18

D

CO

D

(b) 15

E

E

E

A A

A = 0.2

CO

Absorbance

N2 O + C

18

CO

N2 O + C

C

2150

C

E E

2100

E

A = 0.2

CO

E

(c)

A+C

E

A

E

2050

2000 -1

Wavenumber / cm

1950

1900

1850

2.42 eV 1.99 eV 1.49 eV

2.16 eV

S=1

1.97 eV

1.46 eV

S=0 1.44 eV

1.35 eV 0.73 eV

0 eV TiCp2(CO)2

TiCp2 (CO)

TiCp2(CO)N2

TiCp2 (N2 )

TiCp2 (N2 )2

+ 2 N2

+ CO + 2 N2

+ CO + N2

+ 2 CO + N2

+ 2 CO

π-

*+

π,"

*+

σ σ y

z x

!"

%&#'"(!")

%&#'"

%&#'"(#$)

#$

UV-photolysis of TiCp2(CO)2 in an Ar matrix produced TiCp2(CO). UV-photolysis of TiCp2(CO)2 in a N2 matrix produced TiCp2(CO)(N2) and TiCp2(N2). The thermal reaction of TiCp2(N2) with a N2 molecule to produce TiCp2(N2)2 at 30 K. TiCp2(N2) had a triplet ground state.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: