Syntheses, Characterization, and Redox Activity of Ferrocene-Containing Titanium Complexes

The chemistry of bis(π–η5:σ–η1-pentafulvene)titanium complexes is characterized by a broad range of E–H activation and Ti–C functionalization reactions, whereas ferrocene derivatives are easily accessible and redox-active compounds. The reaction of ferrocenealdehyde and -ketones with bis(π–η5:σ–η1-pentafulvene)titanium complexes result in the formation of bimetallic complexes via insertion of the C=O double bond of the aldehyde/ketone into the Ti–Cexo bond of the pentafulvene moiety. The reaction of bis(π–η5:σ–η1-pentafulvene)titanium complexes with ferrocenyl alcohols leads to alcoholate complexes via deprotonation of the OH group by the pentafulvene ligand. Because of the one remaining pentafulvene unit, further functionalization of the complexes is possible. In this work, we proceeded with 1,1′-bifunctionalized ferrocene derivatives for intramolecular follow-up reactions. 1,1′-Ferrocenedimethanol reacts with bis(π–η5:σ–η1-pentafulvene)titanium complexes in a double O–H deprotonation reaction to yield the dialcoholate complex. 1,1′-bis(phenylphosphine)ferrocene reacts differently as the double P–H deprotonation reaction results in the formation of a P–P linked phosphine. Therefore, we studied the reactivity of 1,1′-bis(phenylphosphine)ferrocene toward Rosenthal’s reagent. As Rosenthal’s reagent is regarded as a masked titanocene(II) species, it undergoes redox reactions toward H-acidic substrates, forming a paramagnetic Ti(III) complex.


■ INTRODUCTION
−13 Because of the manifold derivatization options of the ferrocene framework, there is a huge number of polymetallic complexes containing ferrocenes which are linked with different bridges (C; 14−17 N; 18−22 O; 23−25 P; 26 S; 27,28 Se; 29,30 N,O 28,31,32 ).−40 Because the chemistry of bis(π−η 5 :σ−η 1pentafulvene)titanium complexes is characterized by a broad range of E−H activation and Ti−C functionalization reactions, 41 they are excellent precursors for the generation of bimetallic Ti/Fe complexes. 42,43As we reported earlier, 44 both monofunctional and bifunctional transformations are possible depending on the substrate.Herein, we report novel bimetallic Ti/Fe complexes based on ferrocene aldehydes, ketones, alcohols and 1,1′-bis(phenylphosphine)ferrocene.Although there are several examples of ferrocene-containing titanium alcoholate complexes, we present the first ferrocenebased titanium phosphido complex.The complexes were characterized using NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), computational studies, cyclic voltamme-try (CV) and electron paramagnetic resonance (EPR) spectroscopy.

■ RESULTS AND DISCUSSION
The reaction of ferrocenealdehyde Fe1 and ferroceneketones Fe2 and Fe3 with bis(π−η 5 :σ−η 1 -pentafulvene)titanium complex I results in the formation of insertion products Ti1a,b,c (Scheme 1).The complexes were formed via the insertion of the C�O double bond of the aldehyde or ketones into the Ti−C exo bond, thus forming alcoholate ligands.
The products were characterized via NMR spectroscopy (SI, Figures S1−S6) and additionally, the structure of Ti1a was determined by single-crystal X-ray diffraction (Figure 1).The 1 H NMR spectra reveal two sets of signals per complex due to the asymmetric Ti center and the former prochiral aldehyde/ ketone, which become chiral after the insertion reaction, forming two diastereomers.One diastereomer is favored in each complex, revealing product ratios of 1:0.4 (Ti1a), 1:0.2 (Ti1b) and 1:0.25 (Ti1c).The eight different signals of the Cp-protons of the main diastereomers indicate an asymmetry between both Cp-rings and therefore shows that the reaction occurred only at one of the pentafulvene units.The 1 H NMR chemical shift of the former aldehyde signal of Ti1a at 5.76 ppm indicates that the insertion reaction occurred because the signal is shielded more strongly in comparison with the respective ferrocenealdehyde (9.93 ppm). 45In addition, the We abstained from a detailed discussion of bond length, etc. for Ti1a due to poor data quality of the structure.
The reactions of ferrocenyl methanol Fe4 and 1-ferrocenyl ethanol Fe5 with the bis(π−η 5 :σ−η 1 -pentafulvene)titanium complex I result in the formation of the alcoholate complexes Ti2a,b via deprotonation of the OH group by the pentafulvene ligand (Scheme 2).
While two diastereomers are formed in Ti2b (ratio of 1:0.8) due to the asymmetric Ti center and the racemic Fe5, there is only one set of signals for Ti2a as Fe4 is achiral.The eight different signals of the Cp-protons in the 1 H NMR spectra (SI, Figures S7 and S9) indicate that deprotonation of one equivalent of the alcohols occurred due to the asymmetry of the Cp rings.Both complexes were additionally characterized by single-crystal X-ray diffraction (Figure 2).
The 1 H NMR spectra (SI, Figures S11 and S13) indicate that double deprotonation reactions occurred because of the reduced amount of signals.Two signals correspond to the titanium Cp protons, two signals to the ferrocene Cp protons and one signal to the methylene groups, displaying high symmetry.In addition, the molecular structure of the dialcoholate complex Ti3b was revealed by single-crystal Xray diffraction (Figure 3).
To determine the redox properties of Ti3b, we performed cyclic voltammetric measurements (Figure 4).Oxidation of the ferrocene moiety (Fe(II)/Fe(III) E ox,Fc = −0.06V) and reduction of the titanocene moiety (Ti(IV)/Ti(III) E red,Ti = −2.18V) were observed.Both redox processes are irreversible, and while the Ti(IV)/Ti(III) reduction of titanocene alcoholates is often irreversible, 50 the Fe(II)/Fe(III) oxidation of Fe6 is reversible (SI, Figure S22).Therefore, the irreversibility of the oxidation of Ti3b was unexpected.
The frontier orbitals of Ti3b (Figure 5) reveal a ferrocenecentered highest occupied molecular orbital (HOMO) and a titanocene-centered lowest unoccupied molecular orbital (LUMO) with no interactions between the metals.The density functional theory (DFT) calculations support the cyclic voltammetry (CV) studies because the ferrocene oxidizes while the titanocene is reduced.Furthermore, we used 1,1′-bis(phenylphosphine)ferrocene Fe7 to demonstrate the reactivity of bis(π−η 5 :σ−η 1pentafulvene)titanium complexes toward bidentate phosphine ligands.A double deprotonation reaction of the two PH groups and the formation of a bis(phosphido)titanium complex was expected, however, the reactions resulted in the formation of the P−P-linked ferrocenephosphine Fe8 (Scheme 4).This is evident from the NMR measurements as the 31 P{ 1 H} NMR spectra (SI, Figures S17 and S20) show only one singlet at 5.3 ppm, corresponding to other P−P-linked ferrocenephosphines (8.7, 20.6 ppm). 53The attempted synthesis of Fe8 was also reported and a chemical shift of 4.6 ppm was found. 53In contrast, the precursor Fe7 shows a doublet at −60.9 ppm, which corresponds to secondary phosphines (SI, Figure S21).The 1 H NMR spectra (SI, Figures S15 and S18) show the respective signals of Fe8 along with broad signals of the respective titanium complex species, which were formed as byproducts.The nature of these titanium complex species could not be determined and the product mixture could not be separated.
We also studied the reactivity of Fe7 toward the titanocenebis(trimethylsilyl)acetylene complex III, which resulted in the phosphidophosphine titanium(III) complex Ti4 (Scheme 5).This redox reaction occurred via the masked titanocene(II) species by release of BTMSA and the reduction of one proton to hydrogen.
The structure of the resulting paramagnetic phosphidophosphine complex Ti4 was revealed via single-crystal X-ray diffraction (Figure 7), showing that only one P−H group was deprotonated.
The diffuse SOMO (Figure 9, middle) of Ti4 is mainly located at the titanocene moiety and the phosphido ligand and partly in the ferrocene moiety.The SUMO (Figure 9, right) is located at the phosphine ligand and partly at both metallocenes.
This bonding situation is supported by the EPR spectrum of Ti4, showing a double doublet coupling pattern, which correlates with the two different phosphine ligands (Figure 10).A similar coupling pattern was reported for another phosphidophosphine titanium(III) complex. 58 attempted to chemically oxidize Ti4 with AgOTf.However, this resulted in the formation of the di(1,1′bis(phenylphosphine)ferrocene) silver(I) triflate complex AgP and other byproducts in a nonstoichiometric reaction (Scheme 6).

■ SUMMARY AND CONCLUSIONS
In this work, we showed the syntheses of further tunable ferrocene-containing titanium complexes via insertion of ferrocene aldehydes and ketones or deprotonation of ferrocene alcohols by bis(π−η 5 :σ−η 1 -pentafulvene)titanium complexes.The second pentafulvene moiety was used to introduce further functionalities and generate bimetallic complexes with O-and P-based bridges.Using the bifunctional ferrocene derivatives, double deprotonation occurred to either generate dialcoholate titanium complexes or a P−P-linked ferrocenephosphine.This unprecedented phosphidophosphine titanium complex is the first example of a ferrocene-containing phosphido titanium complex and was analyzed using NMR measurements, singlecrystal X-ray diffraction, EPR spectroscopy, cyclic voltammetry and computational data.

■ EXPERIMENTAL SECTION
All reactions were carried out under a dry nitrogen or argon atmosphere using standard Schlenk and glovebox techniques.
Caution! Extreme care should be taken both in the handling of the cryogen liquid nitrogen and its use in the Schlenk line trap to avoid the condensation of oxygen from air.Solvents were dried according to standard procedures over Na/K alloy with benzophenone as indicator and subsequently distilled and stored under a nitrogen atmosphere.Bis(π−η 5 :σ−η 1pentafulvene)titanium complexes I and II 52 and titanocenebis-(trimethylsilyl)acetylene complex III 63 were prepared according to published procedures.Ferrocene derivatives Fe1, 45 Fe2, 46 Fe3, 46 Fe4, 64 Fe5, 65 Fe6, 66 and Fe7 26 were prepared according to published procedures.NMR spectra were recorded on a Bruker AVANCE III 500 spectrometer ( 1 H 500 MHz).IR spectra were recorded on a Bruker Tensor 27 spectrometer using an attenuated total reflection (ATR) method.Elemental analyses were carried out on a Euro EA 3000 Elemental Analyzer.Melting points were determined using a Mettler Toledo MP30.EPR spectra were recorded on a Magnettech ESR spectrometer MiniScope MS300.The cyclovoltammetric measurements were performed using Metrohm Autolab PGSTAT204.Platinum was used as the counter electrode, Ag/Ag + as reference electrode and glassy carbon as working electrode.All electrode potentials are referenced versus the internal standard ferrocene/ferrocenium (Fc/Fc + ) couple.Cyclic voltammetry measurements were performed in a dry and degassed 0.1 M N(n-Bu) 4 PF 6 /THF solution at room temperature under an N 2 atmosphere.All redox potentials are referenced to Fc/Fc + .All DFT (density functional theory) calculations were performed with the B3LYP/Def2-TZVP level of theory.
Further exact details of syntheses, crystallographic data, NMR spectra and cyclic voltammograms are given in the Supporting Information (SI).

Figure 5 .
Figure 5. Calculated structure of Ti3b without (left) and with the calculated surface diagram of the HOMO (middle) and the LUMO (right)(isodensity value 0.01 at B3LYP/Def2-TZVP).