Synthesis and Structures of TiIII and TiIV Complexes Supported by a Bulky Guanidinate Ligand

In this work, titanium complexes of the bidentate bulky guanidine ligand [{(Dip)N}2CNR2]H (where Dip = C6H3iPr2-2,6 and R = CH(CH3)2) (LH) were prepared. Reaction of LH with one equivalent of [(CH3)2NTiCl3] underwent amine elimination to afford the monomeric complex [LTiCl3] (1) in high yield. Attempts to reduce 1 with potassium graphite (KC8) in tetrahydrofuran (THF) were unsuccessful. However, reacting 1 with 3.3 equivalents of KC8 in hexane led to the first example of structurally characterized mono-guanidinate ligand stabilized dimeric TiIII complex [LTiCl(μ–Cl)]2 (2). The synthesized complexes were characterized by NMR spectroscopy and the structures were further confirmed by X-ray crystallography.


Introduction
Stabilization of highly reactive low coordinate and low valent early transition metal complexes has long been an area of interest for chemists, not only from a structural point of view but also due to their reactivity pattern. The strategy that has been widely sought is the application of steric bulk and the mono-anionic nature of the stabilizing ligands. In this regard, N-containing chelating bidentate ligands such as amidinate [1][2][3], guanidinate [1,2,4,5], β-diketiminate [6][7][8], and aminopyridinate [9][10][11] have recently attracted enormous attention ( Figure 1).

Introduction
Stabilization of highly reactive low coordinate and low valent early transition metal complexes has long been an area of interest for chemists, not only from a structural point of view but also due to their reactivity pattern. The strategy that has been widely sought is the application of steric bulk and the mono-anionic nature of the stabilizing ligands. In this regard, N-containing chelating bidentate ligands such as amidinate [1][2][3], guanidinate [1,2,4,5], β-diketiminate [6][7][8], and aminopyridinate [9][10][11] have recently attracted enormous attention ( Figure 1). The unusual oxidation state of +1 is known for all members of first row early transition metals except titanium, and these complexes are mainly stabilized by N-containing ligands [12][13][14][15][16]. Compared to other N-containing bidentate ligands, bulky guanidine ligands seem to be more suitable due to the possibility of varying steric bulk on the NCN moiety that may push the phenyl rings down towards each other to stabilize (to form metal-metal bond) and protect metals in unusually low oxidation states [4,17,18]. Thus, we became interested to explore the possible isolation of titanium (I) species by applying guanidine ligands. Divalent titanium has already been widely used for a variety of metalpromoted organic transformations, which shows that Ti I species might be very interesting in terms of reactivity studies [19]. The chemistry of Ti II complexes is mainly dominated by cyclopentadiene (Cp) ligands, however, N-containing ligands (aminopyridine) have also been successfully applied for Cp-free Ti II species [20]. In comparison to Ti II , isolation of Ti I Figure 1. Guanidinate ligands (left) and other related bidentate monoanionic N-Ligands (R, R' and R", for instance, alkyl or aryl substituents).
The unusual oxidation state of +1 is known for all members of first row early transition metals except titanium, and these complexes are mainly stabilized by N-containing ligands [12][13][14][15][16]. Compared to other N-containing bidentate ligands, bulky guanidine ligands seem to be more suitable due to the possibility of varying steric bulk on the NCN moiety that may push the phenyl rings down towards each other to stabilize (to form metal-metal bond) and protect metals in unusually low oxidation states [4,17,18]. Thus, we became interested to explore the possible isolation of titanium (I) species by applying guanidine ligands. Divalent titanium has already been widely used for a variety of metal-promoted organic transformations, which shows that Ti I species might be very interesting in terms of reactivity studies [19]. The chemistry of Ti II complexes is mainly dominated by cyclopentadiene (Cp) ligands, however, N-containing ligands (aminopyridine) have also been successfully applied for Cp-free Ti II species [20]. In comparison to Ti II , isolation of Ti I species is a challenge to chemists [21]. Here, we describe our attempt to isolate Ti I species Crystals 2021, 11, 886 2 of 7 using the steric bulk and the mono-anionic nature of the guanidinate ligands, and report the synthesis and structures of Ti IV guanidinate and its reduction to Ti III instead of Ti I complex.

General Information
All manipulations were performed with rigorous exclusion of oxygen and moisture in Schlenk-type glassware on a dual manifold Schlenk line or in N 2 filled glove box (mBraun 120-G) with a high-capacity recirculator (<0.1 ppm O 2 ). Solvents were dried by distillation from sodium wire/benzophenone. Deuterated solvents were obtained from Cambridge Isotope Laboratories and were degassed, dried, and distilled prior to use. [(CH 3 ) 2 NTiCl 3 ] and guanidine ligand [LH] were prepared according to the published procedures [22,23]. Commercial TiCl 4 (Acros) was used as received. NMR spectra were recorded on Varian 300 and Varian 400 MHz at ambient temperature. The chemical shifts are reported in ppm relative to the internal TMS. Elemental analyses (CHN) were determined using a Vario EL III instrument. The effective magnetic moments were determined using Sherwood Scientific Magnetic Susceptibility Balance. X-ray crystal structure analyses were performed using a STOE IPDSII equipped with an Oxford Cryostream low-temperature unit. Structure solution and refinement was accomplished using SIR97 [24], SHELXL97 [25] and WinGX [26]. Data collection and cell refinement by X-AREA-STOE. The single crystal was irradiated with Mo-Kα at 133 K. The non-hydrogen atoms were refined with anisotropic thermal parameters. All hydrogen atoms were added at calculated positions and refined using a riding model. No absorption correction was applied to the data. Some of the reflections at certain angles were omitted in the refinement of 2 and that might be the reason for the B-alert in the checkcif. Selected crystallographic data are gathered in Table 1.

Results
Reacting one equivalent of the bulky guanidine ligand [{(Dip)N} 2 CNR 2 ]H (where Dip = C 6 H 3 iPr 2 -2,6 and R = CH(CH 3 (1) in 82% yield (Scheme 1). Compound 1 was characterized using 1 H and 13 C NMR spectroscopy along with elemental analysis. The 1 H-NMR data was in accordance with the nature of the compound formed, showing three doublets for the isopropyl CH 3 protons and two septets for the isopropyl CH protons of the guanidinate ligand.

Syntheses
Synthesis of 1: LH (0.928 g, 2 mmol) was added to [(CH3)2NTiCl3] (0.397 g, 2 mmol) in toluene (50 mL) at room temperature. The resulting brown-red solution mixture was then heated overnight at 80 °C. After cooling to room temperature, the solution was filtered. Volume of the filtrate was reduced to ca. 20 mL under vacuum. After standing at room temperature the solution afforded red crystals of 1  3.60 (sep, 4 H, J = 6.9 Hz, CH(CH3)2), 3.86 (sep, 2 H, J = 6.9 Hz, CH(CH3)2), 7.01-7.11 (m, 6 H, C6H3) ppm. 13   The molecular structure of 1 was confirmed by single crystal structure analysis. The structure analysis revealed the expected mono(guanidinate)titanium(IV) trichloride complex. A distorted trigonal bi-pyramidal coordination around titanium was observed (Figure 2). Titanium is coordinated by two nitrogen and three chlorine atoms. The Ti-N [Ti- The molecular structure of 1 was confirmed by single crystal structure analysis. The structure analysis revealed the expected mono(guanidinate)titanium(IV) trichloride complex. A distorted trigonal bi-pyramidal coordination around titanium was observed ( Figure 2). Titanium is coordinated by two nitrogen and three chlorine atoms. The Ti-N [Ti-N1 2.008 (2) and Ti-N2 2.049 (2) Å] and Ti-Cl [Cl1-Ti1 2.2461(9), Cl2-Ti1 2.2565 (9) and Cl3-Ti1 2.2185(8) Å] bond lengths were comparable to values in the literature [27,28]. bridized nitrogen and carbon atoms. This shows the role of the lone pair of the non-coordinating N-atom in the π system of the ligand that can lead to an increased electron density at the metal center and may result in stronger bonding of the guanidinate ligand. To explore the possible reduction of 1 to Ti I species, we analyzed its reaction with KC8 in THF and found that it didn't lead to the isolation of any characterizable product. However, in hexane its reaction with 3.3 equivalents of KC8 (Scheme 2) led to a green solution. Filtration and reducing the volume of solvent led to the isolation of green crystalline material in a 28% yield. The low yield may be attributed to the low solubility of the product in hexane. X-ray analysis showed 2 to be dimeric Ti III complex (Figure 3) where guanidinate ligand is η 2 -coordinated. Compare to Ti IV , Ti III guanidinates are rare [29][30][31] and dimeric structures of Ti III guanidinates are not known, to the best of our knowledge. The geometry around titanium can be best described as distorted triangular bi-pyramidal with two Natoms of the chelating guanidinate and three halide ligands (Figure 2). The distortion is mainly caused by the NCN moiety of the ligand. The N-Ti-N bond angle [64.08 (19)°] in 2 is comparable to that in 1 [64.76(7)°]. The Ti-N bonds are slightly longer than those in 1. To explore the possible reduction of 1 to Ti I species, we analyzed its reaction with KC 8 in THF and found that it didn't lead to the isolation of any characterizable product. However, in hexane its reaction with 3.3 equivalents of KC 8 (Scheme 2) led to a green solution. Filtration and reducing the volume of solvent led to the isolation of green crystalline material in a 28% yield. The low yield may be attributed to the low solubility of the product in hexane. bridized nitrogen and carbon atoms. This shows the role of the lone pair of the non-coordinating N-atom in the π system of the ligand that can lead to an increased electron density at the metal center and may result in stronger bonding of the guanidinate ligand. To explore the possible reduction of 1 to Ti I species, we analyzed its reaction with KC8 in THF and found that it didn't lead to the isolation of any characterizable product. However, in hexane its reaction with 3.3 equivalents of KC8 (Scheme 2) led to a green solution. Filtration and reducing the volume of solvent led to the isolation of green crystalline material in a 28% yield. The low yield may be attributed to the low solubility of the product in hexane. X-ray analysis showed 2 to be dimeric Ti III complex ( Figure 3) where guanidinate ligand is η 2 -coordinated. Compare to Ti IV , Ti III guanidinates are rare [29][30][31] and dimeric structures of Ti III guanidinates are not known, to the best of our knowledge. The geometry around titanium can be best described as distorted triangular bi-pyramidal with two Natoms of the chelating guanidinate and three halide ligands (Figure 2). The distortion is mainly caused by the NCN moiety of the ligand. The N-Ti-N bond angle [64.08 (19)°] in 2 is comparable to that in 1 [64.76(7)°]. The Ti-N bonds are slightly longer than those in 1.

Scheme 2. Synthesis of mono(guanidinate) titanium(III) complex (2).
X-ray analysis showed 2 to be dimeric Ti III complex ( Figure 3) where guanidinate ligand is η 2 -coordinated. Compare to Ti IV , Ti III guanidinates are rare [29][30][31] and dimeric structures of Ti III guanidinates are not known, to the best of our knowledge. The geometry around titanium can be best described as distorted triangular bi-pyramidal with two Natoms of the chelating guanidinate and three halide ligands (Figure 2). The distortion is mainly caused by the NCN moiety of the ligand. The N-Ti-N bond angle [64.08 (19) • ] in 2 is comparable to that in 1 [64.76 (7)  As expected, the Cl-Ti bond for the bridging chloride ligand [Cl1-Ti1 2.403(2) Å] is longer than the terminal chloride ligand [Cl2-Ti1 2.276(2) Å]. The long Ti-Ti distance of 3.127(2) Å rules out any possible metal-metal bonding interaction. The magnetic susceptibility experiments show the magnetic moment of μeff(298 K) of 0.95 μB which is comparable to values found in the literature [32,33]. The purity of the compounds was further confirmed by elemental analysis. To satisfy our curiosity as to whether 2 can be reduced further it was reacted with two equivalents of KC8 in THF. Despite a color change from green to red, all attempts to produce isolatable material for characterization were unsuccessful.

Conclusions
To isolate titanium in the unusual oxidation state of +1, the reduction of monomeric titanium IV and subsequently isolated dimeric Ti III complexes, supported by a bulky guanidine ligand, were studied using THF and hexane as reaction solvents. Despite the fact that in the present study the reduction of T III/IV complexes didn't lead to the desired Ti I species, it nevertheless highlights the challenges faced in search of the isolation of these complexes. One possibility might be the use of aromatic solvents, as the highly reactive Ti I complexes (if formed) might lead to arene sandwiched Ti I complexes. During these studies the first example of structurally characterized dimeric mono(guanidinate) Ti III complex has been isolated and structurally characterized. To satisfy our curiosity as to whether 2 can be reduced further it was reacted with two equivalents of KC 8 in THF. Despite a color change from green to red, all attempts to produce isolatable material for characterization were unsuccessful.

Conclusions
To isolate titanium in the unusual oxidation state of +1, the reduction of monomeric titanium IV and subsequently isolated dimeric Ti III complexes, supported by a bulky guanidine ligand, were studied using THF and hexane as reaction solvents. Despite the fact that in the present study the reduction of T III/IV complexes didn't lead to the desired Ti I species, it nevertheless highlights the challenges faced in search of the isolation of these complexes. One possibility might be the use of aromatic solvents, as the highly reactive Ti I complexes (if formed) might lead to arene sandwiched Ti I complexes. During these studies the first example of structurally characterized dimeric mono(guanidinate) Ti III complex has been isolated and structurally characterized.