Synthesis and characterization of a tert-butyl ester-substituted titanocene dichloride: t-BuOOCCp2TiCl2

The synthesis and characterization of a new titanocene dichloride complex with tert-butyl esters appended to the cyclopentadiene rings is reported.


Chemical context
Molecules exhibiting charge-separated excited states have been shown to be useful in photocatalysis (Prier et al., 2013), dye-sensitized photoelectrochemical cells (Hammarströ m, 2015;Kalyanasundaram & Grä tzel, 1998) and dye-sensitized solar cells (DSSCs) (Ji et al., 2018;Kalyanasundaram & Grä tzel, 1998). One architecture used in compounds of this type is the Donor-bridge-Acceptor (D--A) architecture, where absorption of a photon results in the transfer of charge from an electron-rich donor portion of the molecule to an electron-poor acceptor portion through a conjugatedlinkage (Ji et al., 2018). Alkynyl titanocenes utilizing titanocene acceptors and ferrocenyl or arylamine donors are promising candidates for sensitizers in DSSCs (Turlington et al., 2016;Pienkos et al., 2016Pienkos et al., , 2018Livshits et al., 2019). In photovoltaic technologies, the sensitizer must be attached to a semiconductor substrate, commonly TiO 2 , using an anchoring group exhibiting a high binding affinity for the substrate (Zhang & Cole, 2015;Kalyanasundaram & Grä tzel, 1998). The most common anchoring group used with TiO 2 semiconductors is the carboxylate, chosen for its strong binding and conjugated -electron system (Galoppini, 2004). Anchoring groups with conjugated systems allow for improved device efficiency in DSSCs compared to anchoring groups with aliphatic or unconjugated linkages (Zhang & Cole, 2015). In alkynyl titanocene sensitizers, the alkynyltitanium bond is sensitive to acid hydrolysis. As a result, the carboxylate anchor must be masked with a protecting group to avoid carboxylic acid intermediates. Our research group has focused primarily on the tert-butyl protecting group, because tbutyl esters are relatively stable and have well documented deprotection strategies under mild conditions (Jung & Lyster, 1977;Theodorou et al., 2018;Shaw et al., 2008). Herein, we report the synthesis, crystallization, and structural analysis of a t-butyl ester substituted titanocene dichloride that will serve as a convenient synthon for D--A titanocenes with carboxylate anchoring groups.

Structural commentary
While many titanocene and metallocene compounds have been characterized by X-ray diffraction, structures of estersubstituted metallocenes are comparatively rare. Here we present the structure of the t-butyl ester-substituted complex t-BuOOC Cp 2 TiCl 2 (Fig. 1). Though the data pool is small, our findings follow trends seen in previously reported structures. In the metallocenes of both vanadium and titanium, the addition of the ester shortens the metal-chlorine bond length by 0.02-0.04 Å [2.3222 (10) and 2.3423 (10) Å in the present titanocene] compared to the parent Cp 2 VCl 2 (Tzavellas et al., 1996) and Cp 2 TiCl 2 (Clearfield et al., 1975) complexes. A similar M-Cl bond contraction was not observed in the recent report of a titanocene with a bulky alkyl substituent appended to the Cp ring, (CpC(CH 3 ) 2 CH 2 CH(CH 3 ) 2 ) 2 TiCl 2 (Ceballos-Torres et al., 2019), suggesting that the change is likely due to the interplay of the electron-withdrawing nature of the ester and the -donor character of the chlorido ligand. Furthermore, substitution at the Cp ring results in a slight elongation of the titanium-cyclopentadiene centroid distance [2.070 (3) and 2.074 (3) Å ] by 0.011 to 0.015 Å in the estersubstituted titanocene here and as much as 0.016 Å in the alkyl substituted titanocene (Ceballos-Torres et al., 2019). However, this trend is not noticeable between ester-substituted and unsubstituted vanadocene dichloride (Klepalová et al., 2013;Tzavellas et al., 1996). Substitution of esters at the Cp ring has little effect on the bond angles formed about the central metal in both titanium and vanadium compounds, with a centroid-Ti-centroid angle of 129.90 (12) and a Cl-Ti-Cl angle of 95.23 (4) observed here. In titanocenes, substi-tution at the Cp ring results in a decrease of the dihedral angle formed between the planes of the two Cp rings. This angle is 58.5 in titanocene dichloride (Clearfield et al., 1975), but is 52.56 (13) in this titanocene and 52.2 in the alkyl substituted titanocene (Ceballos-Torres et al., 2019). This trend is not observed between substituted and unsubstituted vanadocene dichloride, where the dihedral angle is approximately 48 for both (Tzavellas et al., 1996;Klepalová et al., 2013). The dihedral angle formed between the esters and their associated Cp rings differs more in the titanocene than in other estersubstituted metallocenes. In t-BuOOC Cp 2 TiCl 2 , these two angles are 8.2 (6) and 15.7 (3) . In the other ester substituted metallocenes, the angles differ by less than a degree (18.37 and 18.37 in PhOOC Cp 2 VCl 2 and 10.78 and 11.36 in PhOOC Cp 2 NbCl 2 ) (Klepalová et al., 2013). The appended esters in t-BuOOC Cp 2 TiCl 2 extend from the same sides of both Cp rings, and occur on the same side of the complex as the chlorido ligands (Fig. 2). This is a similar arrangement to what occurs in EtOOC Cp 2 NbBr 2 and MeOOC Cp 2 NbBr 2 ÁCH 2 Cl 2 , but differs from PhOOC Cp 2 VCl 2 , PhOOC Cp 2 NbCl 2 , and MeOOC Cp 2 NbBr 2 , where the substituting esters are on opposing sides of their respective Cp rings, and also do not overlap with the halides ( Structure of t-BuOOC Cp 2 TiCl 2 shown as 50% probability ellipsoids, with H atoms as small arbitrary spheres.

Supramolecular features
Intermolecular contact geometries are shown in Table 1. Neighboring molecules are connected along the c-axis direction via C9-H9Á Á ÁO1, C8-H8Á Á ÁCl1, and C4-H4Á Á ÁCl1 interactions to form chains (Fig. 3). Both Cp groups are angled toward the neighboring chlorine atom to enable these interactions. Neighboring molecules along the a-axis are connected in a dimerized fashion via C7-H7Á Á ÁCl1 interactions. The resulting packing diagram is shown in Fig. 4.

Database survey
A CSD search revealed nearly 200 hits for metallocene dichloride complexes, where the two cyclopentadiene ligands were monosubstituted (CSD Version 5.41, Update 2, May 2020; Groom et al., 2016). Of these, only two, CSD entries CICPIP (vanadium) and CICPOV (niobium) are substituted by a protected carboxylate (Klepalová et al., 2013). Both of these utilize a phenyl-protecting group, and the carboxylate carbon is bound to the Cp ring, similar to the tert-butylprotected titanocene of the present study. Methyl-and ethylprotected carboxylate-substituted Cp ligands are reported in the niobium dibromide complexes CICPUB, CICQAI, and CICQEM (Klepalová et al., 2013).

Synthesis and crystallization
Lithium tert-butyl ester cyclopentadienide (Shaw et al., 2008) (2.0278 g, 11.78 mmol, 1 eq) was dissolved by the addition of THF (15 mL) under an argon atmosphere. The reaction solution was chilled to 195 K and 1 M TiCl 4 solution in toluene (6 ml, 6 mmol, 0.5 eq) was added via syringe. The solution changed from pale yellow to red-brown. After 5 minutes, the reaction was allowed to gradually warm to room temperature and stirred overnight. Solid impurities were filtered from the reaction mixture and the solvent was removed from the filtrate. Pentane (5 mL) was added, the mixture was filtered, and the solid impurities were washed with pentane and toluene. The solvent was removed from the filtrate and the resulting red porous solid was dissolved in CH 2 Cl 2 (3 mL), and pentane (50 mL) was added to the solution. The solution was filtered, and the filtrate immediately began to form a precipitate in the filter flask. The resulting suspension was filtered yielding a red-orange powder (0.2823 g, 5.3% yield). 1 H NMR (400 MHz, C 6 D 6 ) 6.95 (2H), 6.04 (2H), 1.42 (9H).
Single crystals suitable for X-ray analysis were grown by slow evaporation of a hexanes solution of the crude product, following the removal of solid impurities. The mixture was chilled to 243 K to encourage further crystallization.

Figure 3
Chains of t-BuOOC Cp 2 TiCl 2 propagating along the c axis. Close contacts are depicted as dotted lines.

Figure 4
Packing of molecules viewed along the b axis.

Figure 2
Ligand orientation in the structure of t-BuOOC Cp 2 TiCl 2 .

Bis[η 5 -(tert-butoxycarbonyl)cyclopentadienyl]dichloridotitanium(IV)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.