Solvent Impact on the Diversity of Products in the Reaction of Lithium Diphenylphosphide and a Ti(III) Complex Supported by a tBu2P–P(SiMe3) Ligand

We present two important trends in the reactivity of the titanium complex [MeNacNacTi(Cl){η2-P(SiMe3)-PtBu2}] (MeNacNac– = [Ar]NC(Me)CHC(Me)N[Ar]; Ar = 2,6-iPr2Ph) with nucleophilic reagents RLi (R = Ph2P, tBuO, (Me3Si)2N, and tBu2N) depending on the reaction medium. Reaction in nonpolar solvent (toluene) leads to three main products: via an autoredox process and nucleophilic substitution at the Ti-atom to afford the Ti(IV) complex [MeNacNacTi(R){η2-P-PtBu2}] (1 for R = PPh2), via the elimination of Me3SiR to afford Ti(III) complex [MeNacNacTi(Cl){η2-P-PtBu2}]−[Li(12-crown-4)2]+ (2), and via 2e– reduction process to afford new ionic complex [{ArNC(Me)CHC(Me)}Ti=NAr{η1-P(SiMe3)-PtBu2}]−[Li(12-crown-4)2]+ (3). Quite differently, the complex [MeNacNacTi(Cl){η2-P(SiMe3)-PtBu2}] reacts with Ph2PLi in THF, unexpectedly yielding two new, four-coordinate Ti(IV) imido complexes 4a [{ArNC(Me)=CHC(H)(Me)-P(PtBu2)}Ti=NAr(Cl)]−[Li(12-crown-4)2]+·(toluene)2 and 4b [{ArNC(CH2)CH=C(Me)-P(PtBu2)}Ti=NAr(Cl)]−[Li(12-crown-4)2]+·(Et2O). Complex 2 dissolved in THF converts to 4a and 4b. 1, 2, 3, 4a, and 4b were characterized by X-ray diffraction. 1, 4a, and 4b were also fully characterized by multinuclear NMR spectroscopy.

The reactions of [ Me NacNacTi(Cl){η 2 -P(SiMe 3 )-PtBu 2 }] with other nucleophiles (tBu 2 OLi, tBu 2 NLi, and (Me 3 Si) 2 NLi) were carried out analogously to this reaction with Ph 2 PLi. In the reactions with these nucleophiles, the substitution products were not isolated in crystalline form; therefore, the reaction solutions were investigated by 31 2) ) (for 31 P{ 1 H} NMR spectrum of reaction mixture, see Figure  S2). The 31 P{ 1 H} NMR spectrum confirms the nucleophilic attack of Ph 2 P − on the titanium center and the elimination of the chloride ion as LiCl. Additionally, during this reaction, the titanium atom is oxidized (1e − oxidation). All attempts to isolate the compounds observed in the NMR spectra failed. To obtain the complexes in a crystalline form, the reaction of [ Me NacNacTi(Cl){η 2 -P(SiMe 3 )-PtBu 2 }] with Ph 2 PLi in toluene was conducted in the presence of 12-crown-4 (molar ratio 1:1:2). Importantly, 12-crown-4 was added only 10 min before the end of the reaction. This modification allowed us to isolate the ionic Ti(III) complex [ Me NacNacTi(Cl){η 2 -P-PtBu 2 }] − [Li(12-crown-4) 2 ] + (2) in a crystalline form. After the isolation of 2, the reaction solution was investigated by NMR spectroscopy. 31 P{ 1 H} NMR revealed that the Ti(IV) complex [ Me NacNacTi(PPh 2 ){η 2 -P-PtBu 2 }] (1) was still present in the reaction mixture; therefore, the solution was concentrated and stored at −25°C. After 24 h, dark red crystals had appeared and were characterized as complex 1 by 31 P{ 1 H} NMR spectroscopy. 1 H/ 31 P-HMBC examinations of 1 revealed that the phosphanyl phosphorus atom (104.0 ppm) is only correlated with the tert-butyl groups (0.88 ppm, d, J PH = 14.8 Hz, 18H), while the phosphido phosphorus atom (at 207.7 ppm) correlates with the protons at 8.29 ppm (broad signal), 6.59 ppm (broad signal), 5.89 ppm (s), 2.89 ppm (weak correlation, sept, J HH = 6.7 Hz), and 1.87 ppm (s). The integrations of the above-mentioned proton signals indicate 6 H in the signal at 1.87 ppm, 1 H in the signal at 5.89 ppm and 2 H in the signal at 2.89 ppm (methyl group, γ-proton, and isopropyl group of the Me NacNac skeleton, respectively) (for the spectra of complex 1, see Figures S3−S10). In the next step, the crystals of 1 were isolated, and the solvent was was also created (for the molecular structure of complex 3, see Figure S1). NMR and X-ray results indicated that the reaction of [ Me NacNacTi(Cl){η 2 -P(SiMe 3 )-PtBu 2 }] with Ph 2 PLi in the presence of 12-crown-4 in toluene probably leads to two independent and competitive reactions. The first reaction can be recognized as a lithiation of the phosphido phosphorus atom of the starting titanium(III) complex and proceeds without changing of the oxidation state of the titanium atom. The second reaction can be recognized as the redox reaction in which the new complexes 1, 3, and Ph 2 P-PPh 2 are created. The molecular structure of complex 3 indicates that it is a product Mindiola and co-workers observed similar 2e − reduction process of β-diketiminate titanium(III) and zirconium(IV) complexes in the reactions with KC 8 . 35,58 Tokitoh and coworkers received similar results with the same reagent for βdiketiminate tetravalent complexes of metals of Ti-group. 59 In our previous research work, the reduction process of β- In these redox processes, Ph 2 P(SiMe 3 ) and LiCl are also formed. The above-described results are summarized in Scheme 1. It should be emphasized that the employed reaction conditions aimed to optimize the preparation of compound 1. The reaction carried out in room temperature showed that the dominant processes were the elimination reaction of the SiMe 3 group and lithiation of phosphorus atom. In this case, compound 1 was formed in trace amounts, and its isolation in crystalline form was impossible. The key element in this case was also the time of addition of a crown ether. The addition of a 12-crown-4 at the beginning of the reaction favors the formation of complex 2; therefore, a crown ether was added almost at the end of the reaction.
Moreover, this reaction was also conducted with other nucleophilic reagents: tBuOLi, (Me 3 Si) 2 NLi, and tBu 2 NLi. After each of these reactions, we only isolated paramagnetic complex 2. Furthermore, the formation of 2 was accompanied by the creation of products containing SiMe 3 groups (visible in the 1 H NMR spectrum as tBuO(SiMe 3 ) at 1.25 ppm (s) and X-ray and NMR studies of 4a and 4b revealed the "phospha-Staudinger" displacement of the imide group N−Ar from Me NacNac by the phosphanylphosphinidene P-PtBu 2 moiety and an intermolecular hydrogen shift leading to two fourcoordinate Ti(IV) imido complexes with different NP ligands. It should be noted that a similar displacement involving the imine-aryl functionality of the NacNac ligand has been described by Mindiola and co-workers. 16 Additionally, in complexes 4a and 4b, the oxidation of both Ti atoms is accompanied by the reduction of the two putative hybrid NP ligands [P(PtBu 2 )-CH(Me)CH-C(Me)N(Ar)] derived from the Me NacNac anion (Scheme 1).
4a [{(Ar)NC(Me)CHC(H)(Me)(P-PtBu 2 )}TiNAr-(Cl)] − [Li(12-crown-4) 2 ] + ·(toluene) 2 can be considered as a product of hydrogen addition to the δ-carbon atom in the NP hybrid ligand. The proton connected to the δ-carbon atom in 4a is visible in the 1 H NMR spectrum as a broad multiplet at 3.74 ppm, and the signal of the δ-carbon atom appears in the 13 C NMR spectrum at 55.0 ppm (based on HMQC and 13 C-DEPT-90). An analogous reduction of a NacNac ligand via the addition of a hydrogen atom to the β-carbon of the backbone of the β-diketimante ligand was observed by Uhl and coworkers. The reduction process was revealed in the reaction of NacNacH with AlH 3 and EtNMe 2 , in which the reduced ligand was formed by hydrogen atom transfer from aluminum to a βcarbon atom. 60 4b [{(Ar)NC(CH 2 )CHC(Me)(P-PtBu 2 )}-TiNAr(Cl)] − [Li(12-crown-4) 2 ] + ·(Et 2 O) can be considered as a product of hydrogen elimination from the Me group at the β-carbon atom in the NP hybrid ligand. In the 1 H NMR spectrum, the two protons of the methylene group are observed at 2.96 and 2.63 ppm. These resonances correlate to the sp 2 -hybridized carbon observed at 77.06 ppm (based on HMQC and 13 C-DEPT-135) (see Figure 1).
The signal of the γ-proton in 4b is centered at 4.64 ppm. A similar deprotonation process of the β-diketiminate ligand in the titanium complex was reported by Mindiola and coworkers, although the reduction reaction was conducted with KCH 2 Ph (4.92 ppm for the γ-proton and 3.64 and 3.25 ppm for CH 2 ). 28 It is worth noting that Mindiola and co-workers also reported approximate determination for a β-diketiminate ligand, which can interact with an adjacent phosphinidene ligand in a "phospha-Staudinger" reaction, generating a titanium-imide complex supported by the new NP ligand. 61 Furthermore, both reduction reactions of the β-diketiminate ligands were previously described separately in the literature, but such a conjugated autoredox reaction has never been reported.
We also studied the progress of the reaction according to Scheme 1, i.e., the formation of 4a and 4b over time directly in an NMR tube. Therefore, the [ Me NacNacTi(Cl){η 2 -P(SiMe 3 )-  In order to investigate the influence of temperature on the reaction route, we have carried out the above-described reaction again. The substrates were also mixed at −15°C and immediately after mixing, the 31 P{ 1 H} NMR spectrum was measured and singlet from Ph 2 P(SiMe 3 ) was observed. The NMR tube was heated in the oily bath at +50°C. Next, spectra were acquired after 1, 2, 24, 48, and 72 h and 7 days. The conducted experiment revealed that the temperature has an impact on the reaction rate. Higher temperature causes the reaction proceeded rapidly but no other products are formed. Moreover, after 7 days, complex 4a is no longer visible in the solution.
To better identify the differences between the reactions in polar and nonpolar solvents, we also carried out reactions with the same amounts of substrates and under the same conditions (RT). In both reactions, 12-crown-4 was added at the beginning of the reactions. One reaction was carried out in toluene-d 8 , while the other was carried out in THF-d 8 . Reaction mixtures were sealed in NMR tubes and measured cyclically. Measurements were performed after 1, 24, and 48 h and 5 days from the initiation of the reactions. The conducted experiments revealed that in nonpolar solvent only complex 1 was visible in the 31 P{ 1 H} NMR spectrum, while the high signal of Ph 2 P(SiMe 3 ) indicated on the presence of complex 2 in the reaction mixture. Importantly, the 31 P{ 1 H} NMR measurement conducted after 5 days revealed no signals from 4a and 4b (see Figure S42). The obtained results clearly show, that both complexes 1 and 2 are stable in the nonpolar solvent.
In the reaction carried out in THF-d 8 formation of complexes 4a and 4b was observed, while the formation of complex 1 was not observed (see Figure S43). These experiments clearly prove that the polarity of the solvents is very important in the reaction of [ Me NacNacTi(Cl){η 2 -P(SiMe 3 )-PtBu 2 }] with Ph 2 PLi and 12-crown-4 and has a direct impact on the resulting products.
Finally, we also examined the stability of isolated crystals of 2 in THF at room temperature over 24 h. Therefore, the crystals of 2 were dissolved in THF-d 8 and investigated via NMR. The 31 P{ 1 H} NMR spectrum revealed the formation of complexes 4a and 4b (Scheme 1). Additionally, in the 1 H NMR spectrum the proton connected to the β-carbon was also visible (see Figures S44 and S45). Obtained results may indicate that the protonation of the β-carbon in 4a is directly connected with deprotonation process observed in complex 4b. In this intermolecular process of two molecules of complex 2, in the "phosha-Staudinger, protonation and deprotonation reactions, the one molecule of complex 4a and one molecule of complex 4b are created.
The crystallographic structure shows that the phosphanylphosphinidene ligand coordinates side-on to the metal center. The Ti1−P1 distance (2.3353(17) Å) lies in the range between single and double Ti−P bonds. The reported distances of single Ti−P bonds are 2.585(1) Å for [Cp 2 Ti-(C O ) ( P Et ) 3 ] 6 7 a n d 2 .  (7) Å)). 37 The Ti1−P2 bond distance is typical for a single bond between phosphorus and titanium atoms. The P− P distance (2.1034(19) Å) lies in the range of double bonds, especially for the side-on coordination of a phosphanylphosphinidene ligand to a metal center. A similar distance was reported for the previously described β-diketiminate titanium-(IV) complexes with phosphanylphosphinidene ligands. 37 Compound 4a crystallized as red crystals from a toluene/ pentane solution. The molecule crystallized in the P2 1 /c space group with four molecules in the unit cell. 4a displays a fourcoordinate titanium complex with the metal center in a tetrahedral environment (Figure 4).  61 In 4a, the NCCCP backbone is not planar, and the reduced carbon atom C16 shows a tetrahedral coordination environment (ΣC16 = 331.40(2)°). The H16 atom was localized according to the electron density map (C−H distance is 1.000(2) Å). The N1− C14 distance of 1.416(4) Å is shorter than typical N−C single bonds (1.48 Å) but is comparable to that observed in the reduced ligand reported by Uhl (N−C 1.428(3) Å and 1.425(3) Å) 60 and is significantly longer than the N−C distances in delocalized π-bonding systems of nonreduced βdiketiminate ligands [(NacNac)Ti(OAr)] 2 (μ 2 :η 2 ,η 2 -P 2 ), 1.330, 1.348, 1.367, and 1.341 Å). 17     Crystals of 4b suitable for X-ray analysis were grown from an diethyl ether/pentane mixture. 4b crystallizes in the triclinic system in the P1̅ space group with two molecules in the unit cell. The molecular structure of 4b also displays a fourcoordinate titanium complex bound to two nitrogen atoms, one chloride, and one phosphorus atom ( Figure 5).
The X-ray structure of 4a clearly indicates two stereocenters in the molecule, the δ-carbon and titanium atom, while in complex 4b, only the titanium atom is stereocenter. The 31 P{ 1 H} NMR spectra of 4a reveal the presence of only one set of resonances. This means that this solution contains a racemic mixture of two diastereomers or that the configuration at the Ti-center is not rigid in solution.

CONCLUSIONS
We demonstrated two important reaction pathways of [ Me NacNacTi(Cl){η 2 -P(SiMe 3 )-PtBu 2 }] with nucleophiles (Ph 2 PLi, tBuOLi, (Me 3 Si) 2 NLi, and tBu 2 NLi) in the presence of 12-crown-4, and the reaction outcome depends on the solvent. In toluene, substitution and simultaneous oxidation of titanium occurs with all nucleophiles. Ph 2 PLi afforded the most substitution product, and [ Me NacNacTi(PPh 2 ){η 2 -P-PtBu 2 }] (1) was isolated. [ Me NacNacTi(Cl){η 2 -P-PtBu 2 }] − -[Li(12crown-4) 2 ] + (2) was isolated for all nucleophiles. Additionally, using fairly harsh crystallization, we also received a 2e − reduction product, [{ArNC(Me)CHC(Me)}TiNAr{η 1 -P-(SiMe 3 )-PtBu 2 }] − -[Li(12-crown-4) 2 ] + (3), which completes the stoichiometry of the reaction and allows us to understand occurring processes in a nonpolar solvent. However, the same protocol with the same molar ratio but in THF solution led to two different Ti(IV) products, 4a and 4b. In both complexes (4a and 4b), the "phospha-Staudinger" displacement in the βdiketiminate ligand and formation of a four-coordinate titanium complexes with the NP ligands was observed. Furthermore, X-ray and NMR spectroscopic investigations revealed that the titanium atoms in both complexes are oxidized and that the organic Me NacNac ligands are reduced. The two redox processes are connected with a transfer of one hydrogen atom: In 4b, the Me group attached to the β-carbon is dehydrogenated, while in 4a, a hydrogen adds to the δcarbon of the NP hybrid ligand. We also demonstrated that ionic complex 2 is unstable in THF solution and is transformed to complexes 4a and 4b.
NMR spectroscopic data, crystallographic data, and DFT calculations (PDF) Accession Codes CCDC 1859187, 1859190, 1859191, 1991067, and 2001101 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic