Synthesis and Structures of Ti^III and Ti^IV Complexes Supported by a Bulky Guanidinate Ligand

Abstract

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 Ti^III complex [LTiCl(μ–Cl)]₂ (2). The synthesized complexes were characterized by NMR spectroscopy and the structures were further confirmed by X-ray crystallography.

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 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 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.

2. Materials and Methods

2.1. 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 N₂ filled glove box (mBraun 120-G) with a high-capacity recirculator (<0.1 ppm O₂). 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. [(CH₃)₂NTiCl₃] and guanidine ligand [LH] were prepared according to the published procedures [22,23]. Commercial TiCl₄ (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. Crystallographic data of the compounds 1 and 2.

Compound	1	2
Empirical formula	C31H48Cl3N3Ti	C68H110Cl4N6Ti2
Formula weight	616.97	1249.22
crystal system	orthorhombic	monoclinic
space group	Pna2(1)	C2/c
a [Å]	19.4050(9)	36.6350(15)
b [Å]	10.5550(4)	11.2480(8)
c [Å]	16.1720(7)	19.9120(15)
α [deg]	90.00	90.00
β [deg]	90.00	120.746(6)
γ [deg]	90.00	90.00
V, [Å3]	3312.3(2)	7051.8(8)
crystal size, [mm3]	0.43 × 0.32 × 0.25	0.38 × 0.36 × 0.33
ρcalcd, [g cm−3]	1.237	1.177
µ, [mm−1] (Mo Kα)	0.524	0.420
T, [K]	133(2)	133(2)
2θ range, [deg]	2.52–52.06	2.59–45.42
no. of reflections unique	6232	6661
no. of reflections obs. [I > 2σ (I)]	5081	2099
no. of parameters	343	374
wR2 (all data)	0.0687	0.1999
R value [I > 2σ (I)]	0.0351	0.0747

.

2.2. 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. Yield: 1.01 g (82%). C₃₁H₄₈Cl₃N₃Ti (616.96): Calcd. C 60.35 H 7.84 N 6.81; found C 59.95 H 7.79 N 6.77. ¹H NMR: (C₆D₆, 400 MHz): δ = 0.66 (d, J = 6.9 Hz, 12 H, CH(CH₃)₂), 1.20 (d, J = 6.9 Hz, 12 H, CH(CH₃)₂), 1.58 (d, J = 6.9 Hz, 12 H, CH(CH₃)₂), 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. ¹³C NMR (100 MHz, C₆D₆, 298 K): δ = 23.0 (NCHCH₃), 23.1 (NCHCH₃), 24.0 (CH(CH₃)₂), 24.1 (CH(CH₃)₂), 26.5 (CH(CH₃)₂), 26.5 (CH(CH₃)₂), 29.2 (CH(CH₃)₂), 51.1 (NCH), 124.8 (C^meta), 143.2 (C^ipso), 145.3 (C^ortho), 170.2 (NCN) ppm.

Synthesis of 2: Hexane (50 mL) was added to 1 (3.040 g, 4.93 mmol) and potassium graphite (16.27 mmol) at −30 °C. The suspension was then allowed to come to room temperature and stirred overnight. The resulting green solution was then filtered. The volume of the filtrate was reduced to ca. 10 mL as green crystalline material of 2 started to precipitate. Filtrate was kept at room temperature to afford further material of 2. Yield: 0.850 g (28 %). C₆₄H₉₆Cl₄N₆Ti₂.C₆H₁₄ (1249.22): Calcd. C 64.03 H 8.32 N 7.23; found C 63.86 H 8.6 N 7.78. ¹H NMR: (C₆D₆, 300 MHz): δ = −1.98 (br s), 0.47 (d), 0.56 (d), 1.16–1.25 (m), 1.48 (d), 1.57 (d), 1.87 (s), 2.09 (s), 3.11–3.30 (m), 3.48 (sep), 4.09 (s), 6.35 (br tr), 7.33 (d) ppm. µeff(298 K) = 0.95 µB.

3. Results

Reacting one equivalent of the bulky guanidine ligand [{(Dip)N}₂CNR₂]H (where Dip = C₆H₃iPr₂-2,6 and R = CH(CH₃)₂) (LH) with [(CH₃)₂NTiCl₃] in toluene at 80 °C afforded selectively red crystalline material of mono(guanidinate) Ti^IV complex [{(Dip)N}₂CNR₂]TiCl₃] (1) in 82% yield (Scheme 1). Compound 1 was characterized using ¹H and ¹³C NMR spectroscopy along with elemental analysis. The ¹H-NMR data was in accordance with the nature of the compound formed, showing three doublets for the isopropyl CH₃ protons and two septets for the isopropyl CH protons of the guanidinate ligand.

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]. The nearly identical C-N bond lengths [C1-N2-1.350(3), C1-N3-1.353(3), C1-N1 1.373(3) Å] and the sum of the bond angles around N3 and C1 was approximately 360°, confirm sp²-hybridized 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 η²-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 N-atoms 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. 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.

4. 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.