Synthesis and structural study of an unsymmetrical aliphatic benzamidinato ligand and its use in the formation of selected Cu(I), Mg(II) and Zr(IV) complexes

Abstract

The reaction of a Et₂O solution of LiN(Cy)SiMe₃ (Cy = cyclohexyl) with PhCN in the presence of the Lewis-base donor tetrahydrofuran (THF) or Me₂N(CH₂)₂NMe₂ (TMEDA) gave the THF- or TMEDA-solvated unsymmetrical aliphatic lithium benzamidinato compound [{PhC(NCy)(NSiMe₃)}Li(THF)]₂ (1) or [{PhC(NCy)(NSiMe₃)}Li(TMEDA)] (2). Treatment of LiN(Cy)SiMe₃ with PhCN in diethyl ether, and further with CuCl, MgBr₂(THF)₂ or ZrCl₄ at a ratio of 1 : 1, 2 : 1 and 3 : 1 allowed the formation of novel metal complexes [{PhC(NCy)(NSiMe₃)}Cu]₂ (3), [{PhC(NCy)(NSiMe₃)}₂Mg(OEt₂)] (4) and [{PhC(NCy)(NSiMe₃)}₃ZrCl] (5), respectively. The X-ray structures of complexes 1–4 are presented. Complex 5/MAO system allows the moderate polymerization of ethylene.

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Introduction

Over the past decades, developing various ancillary ligands as alternatives to cyclopentadienyl ligands has attracted much interest in catalytic processes. Accordingly, the coordination chemistry of the heteroatom-containing non-Cp-spectator ligands, especially the nitrogen-based ligand systems have been recently studied, of which the amidinates and the related guanidinates are largely explored.^1–3 The amidinato ligands have been and remain of special interest because of their greater potential in ligand design over Cp ligands by virtue of their different steric and electronic properties due to the variation of the N and C substituents. In addition, they can display a variety of bonding modes to the metals.

The majority of previous publications have dealt with C₂-symmetric ligands where either N, N′-chelating to a metal occurs or there is bridging between two metal atoms,⁴ and some of their group 4 complexes are active for the highly stereoregular polymerization of propylene under pressure.⁵ The C₁-symmetric metal amidinates are expected to be catalysts or reagents for regio- or even enantio-selective transformations.^4,6 Recently, the synthesis, structures and reactivities of N′-trimethylsilyl 2-pyridylamidinate and fluoro-containing amidinates such as lithium fluoroarylamidinates and lithium N-pentafluorophenyl have been investigated.^7–10 Some of them can be used as building blocks in coordination polymers, ladder and cage structures. Very recently, [benzamidinato]³⁻ complexes have been successfully prepared via the reduction of Et₂O- or TMEDA-solvated lithium benzamidinates with lithium metal,¹¹ and they showed dynamic behaviour. The aggregation and solution dynamics of some other lithiated amidinates have also been involved.^12–14 Unusual bridging μ, η² : η¹-benzamidinate ligands were found in bis[bis(1-tert-butyl-3-ethylacetamidinato)magnesium¹⁵ and Fe(PhNC(Ph)NPh)₄,¹⁶ and a μ, η² : η²-benzamidinate ligand was found in K₂((Me₃Si)NC(Ph)N(PhC=C(^tBu)₂)₂.¹⁷

Amongst the many reported amidinato ligands, our group has been interested in developing the coordination chemistry and catalytic behaviour of group 4 transition metal complexes containing unsymmetrical amidinato ligands such as silyl-linked bis(amidinate), silyl-linked amidinate–amidine, mixed silyl-linked bis(amidinate) and cyclopentadienyl.^18–21 Some chiral amidinato- and bis(amidinato-) ligands and their metal complexes derived from the well known scaffold (1R, 2R)-1,2-diaminocyclohexane were also documented in our previous publications.²² These ligands were prepared via the nucleophilic reactions of N-centered anions to nitriles free from α-hydrogen, migrations of SiMe₃ and isomerization.

Following our results with unsymmetrical aromatic and chiral amidinato ligands, we have decided to extend the ligand system to unsymmetrical aliphatic benzamidinato ligand chemistry and further towards its metal ions. Herein we report the synthesis and structural characterization of the unsymmetrical aliphatic benzamidinato lithium ligand: Lewis-base donor tetrahydrofuran(THF) or Me₂N(CH₂)₂NMe₂(TMEDA) solvated compounds [{PhC(NCy)(NSiMe₃)}Li(THF)]₂ (1) and [{PhC(NCy)(NSiMe₃)}Li(TMEDA)] (2). These lithium salts were used as ligand transfer reagents when reacted with metal halides CuCl, MgBr₂(THF)₂ and ZrCl₄ to produce the corresponding complexes [{PhC(NCy)(NSiMe₃)}Cu]₂ (3), [{PhC(NCy)(NSiMe₃)}₂Mg(OEt₂)] (4) and [{PhC(NCy)(NSiMe₃)}₃ZrCl] (5), respectively. To the best of our knowledge, this is a rare example of the synthesis and structural characterization of the unsymmetrical aliphatic benzamidinato ancillary ligand system and metal complexes.

Experimental

All manipulations were carried out under an atmosphere of argon using standard Schlenk techniques. The solvent was purchased from commercial sources. Deuterated solvent C₆D₆, C₅D₅N, CDCl₃ was dried over activated molecular sieves (4 Å) and vacuum transferred before use. Diethyl ether and THF were dried and distilled from sodium/benzophenone and stored over a sodium mirror under argon. Dichloromethane was distilled from activated molecular sieves (4 Å) or CaH₂. CuCl, ZrCl₄, PhCN, and n-butyllithium in hexane were purchased from Alfa Aesar and used without further purification. Lithiation of cyclohexylamine with n-BuLi in hexane, followed by the reaction of the resulting solution with one equivalent of trimethylsilyl chloride gave the silylated CyNH(SiMe₃). Further lithiation of CyNH(SiMe₃) with n-BuLi gave the starting material LiN(Cy)SiMe₃. Glassware was oven-dried at 150 °C overnight. ¹H spectra and ¹³C spectra were recorded with a Bruker DRX-300 spectrometer. Melting points were measured in sealed capillaries and are uncorrected. Elemental analyses were carried out using a Vario EL-III analyzer (Germany).

Synthesis and characterization

[{PhC(NCy)(NSiMe₃)}Li(THF)]₂ (1). PhCN (0.26 mL, 2.65 mmol) was added to a solution of CyN(Li)SiMe₃ (0.47 g, 2.65 mmol) in Et₂O (30 cm³) at −78 °C. The resulting mixture was warmed to ca. 25 °C and stirred overnight. Volatiles were removed in vacuo to yield a white solid, which was extracted with THF. The concentrated extract at −25 °C yielded colorless crystals of compound 1 (0.54 g, 57%). Mp: 107–109 °C. Found: C, 67.64; H, 9.54; N, 8.06. C₄₀H₆₆Li₂N₄O₂Si₂ requires C, 68.14; H, 9.44; N, 7.95%. ¹H NMR (C₅D₅N): δ −0.22 (s, 9 H, SiMe₃), 0. 83 (s, 4 H, Cy), 1.14 (s, 4 H, Cy), 1.35 (s, 4 H, THF), 1.52 (s, 2 H, Cy), 2.84 (s, 1 H, Cy), 3.4 (s, 4 H, THF), 6.96–7.32 (m, 5 H, Ph). ¹³C NMR (C₅D₅N): δ 1.49 (SiMe₃), 24.4 (β-CH₂/THF), 23.4, 23.7, 35.5 (CH₂/Cy), 54.7 (CH/Cy), 65.8 (α-CH₂/THF), 124.3 (p-CPh), 125.0 (m-CPh), 126.0 (o-CPh), 142.3 (Cipso-Ph), 170.1 (NCN). ⁷Li NMR (C₅D₅N): δ 0.578 ppm.

[{PhC(NCy)(NSiMe₃)}Li(TMEDA)] (2). PhCN (0.39 mL, 3.78 mmol) and TMEDA (0.56 mL, 3.78 mmol) were added to a solution of CyN(Li)SiMe₃ (0.67 g, 3.78 mmol) in Et₂O (30 cm³) at −78 °C. The resulting mixture was warmed to ca. 25 °C and stirred overnight. The mixture was filtered; the concentrated filtrate at −25 °C yielded colorless crystals of 2 (0.85 g, 57%). Mp: 122–125 °C. Found: C, 67.13; H, 10.45; N, 13.45. C₂₂H₄₁LiN₄Si requires C, 66.62; H, 10.42; N, 14.13%. ¹H NMR (C₆D₆+C₅D₅N): δ −0.06 (s, 9 H, SiMe₃), 1.00 (s, 4 H, Cy), 1.22 (s, 4 H, Cy), 1.68 (s, 2 H, Cy), 2.03 (s, 12 H, NMe₂), 2.19 (s, 4 H, NCH₂), 2.97 (s, 1 H, Cy), 7.04–7.29 (m, 5 H, Ph). ¹³C NMR (C₆D₆+C₅D₅N): δ −4.28 (SiMe₃), 17.73, 18.69, 29.88 (CH₂/Cy), 37.99 (NCH₃), 48.86 (CH/Cy), 50.04 (N–CH₂–CH₂–N), 118.1 (p-CPh), 119.0 (m-CPh), 120.0 (o-CPh), 136.5 (Cipso-Ph), 165.4 (NCN). ⁷Li NMR (C₆D₆+C₅D₅N): δ 4.82 ppm.

[{PhC(NCy)(NSiMe₃)}Cu]₂ (3). PhCN (0.20 mL, 2.09 mmol) was added to a solution of CyN(Li)SiMe₃ (0.37 g, 2.09 mmol) in Et₂O (30 cm³) at −78 °C. The resulting mixture was warmed to ca. 25 °C and stirred overnight. CuCl (0.207 g, 2.09 mmol) was added at −78 °C. The resulting mixture was warmed to ca.25 °C and stirred for 24 h. The mixture was filtered; the concentrated filtrate at −25 °C yielded colorless crystals of 3 (0.54 g, 77%). Mp: 196–198 °C. Found: C, 57.54; H, 7.78; N, 8.01. C₃₂H₅₀Cu₂N₄Si₂ requires C, 57.02; H, 7.48; N, 8.31%. ¹H NMR (CDCl₃): δ −0.09–0.04 (m, 9 H, SiMe₃), 0.89–0.92 (m, 2 H, Cy), 1.42–1.47 (m, 4 H, Cy), 1.59–1.69 (m, 4 H, Cy), 2.47 (m, 1 H, Cy), 7.06–7.43 (m, 5 H, Ph). ¹³C NMR (CDCl₃): δ 1.67–4.79 (SiMe₃), 26.2, 37.3, 38.6, 59.6 (Cy), 127.9 (p-CPh), 128.8 (m-CPh), 129.6 (o-CPh), 141.5 (Cipso-Ph), 177.3 (NCN).

[{PhC(NCy)(NSiMe₃)}₂Mg(OEt₂)] (4). PhCN (0.23 ml, 2.43 mmol) was added to a solution of CyN(Li)SiMe₃ (0.43 g, 2.43 mmol) in Et₂O (30 cm³) at −78 °C. The resulting mixture was warmed to ca.25 °C and stirred overnight. MgBr₂(THF)₂ (0.40 g, 1.22 mmol) was added at −78 °C. The resulting mixture was warmed to ca.25 °C and stirred for 24 h, and then filtered. The filtrate was concentrated in vacuo and stored at −25 °C for one night, yielding colorless crystals of 4 (0.49 g, 63%). Mp: 170 °C (decomp.). Found: C, 66.38; H, 9.01; N, 8.83. C₃₆H₆₀MgN₄OSi₂ requires C, 67.00; H, 9.37; N, 8.68%. ¹H NMR (CDCl₃): δ −0.20–0.20 (m, 9 H, SiMe₃), 1.14–1.24 (m, 2 H, Cy), 1.58–1.97 (m, 8 H, Cy), 1.36–1.40 (t, Et₂O), 2.85 (s, 1 H, Cy), 3.68–3.75 (m, Et₂O), 7.29–8.13 (m, 5 H, Ph). ¹³C NMR (CDCl₃): δ 2.62–4.98 (SiMe₃), 17.0, 67.6 (Et₂O), 25.9, 26.6, 27.2, 35.6, 37.0 (CH₂/Cy), 59.9 (CH/Cy), 128.8 (p-CPh), 129.6 (m-CPh), 130.8 (o-CPh), 143.2 (Cipso-Ph), 158.4 (NCN).

[{PhC(NCy)(NSiMe₃)}₃ZrCl] (5). PhCN (0.36 ml, 3.50 mmol) was added to a solution of CyN(Li)SiMe₃ (0.62 g, 3.50 mmol) in Et₂O (30 cm³) at −78 °C. The resulting mixture was warmed to ca.25 °C and stirred overnight. ZrCl₄ (0.27 g, 1.17 mmol) was added at −78 °C. The resulting mixture was warmed to ca.25 °C and stirred for 24 h. The solvent was removed in vacuo, and the residue was extracted with dichloromethane and filtered. The filtrate was concentrated in vacuo and stored at −25 °C for three days, yielding colorless crystals of 5 (0.55 g, 55%). Mp: 192 °C (decomp.). Found: C, 60.43; H, 7.90; N, 9.07. C₄₈H₇₅ClN₆Si₃Zr requires C, 60.87; H, 7.98; N, 8.87%. ¹H NMR (C₆D₆): δ 0.47 (s, 9 H, SiMe₃), 1.18–1.20 (m, 4 H, Cy), 1.58–1.82 (m, 6 H, Cy), 3.64 (s, 1 H, Cy), 6.95–7.84 (m, 5 H, Ph).

Ethylene polymerization

Ethylene polymerization was carried out in a 500 mL autoclave stainless steel reactor equipped with a mechanical stirrer and a temperature controller. Briefly, toluene, the desired amount of cocatalyst, and a toluene solution of the catalytic precursor (the total volume was 100 mL) were added to the reactor in this order under an ethylene atmosphere. When the desired reaction temperature was reached, ethylene at 10 atm pressure was introduced to start the reaction, and the ethylene pressure was maintained by constant feeding of ethylene. After 45 min, the reaction was stopped. The solution was quenched with HCl-acidified ethanol (5%), and the precipitated polyethylene was filtered, washed with ethanol, and dried in vacuum at 60 °C to constant weight.

X-ray crystallography. Data collection of 1–4 was performed with Mo-Kα radiation (λ = 0.71073 Å) on a Bruker Smart Apex CCD diffractometer using the omega scan mode yielding a total of N reflections. Crystals were coated in oil and then directly mounted on the diffractometer under a stream of cold nitrogen gas. Corrections were applied for Lorentz and polarization effects as well as absorption using multi-scans (SADABS).²³ The structure was solved by direct method (SHELXS-97).²⁴ The remaining non-hydrogen atoms were obtained from the successive difference Fourier map. All non-H atoms were refined with anisotropic displacement parameters, while the H atoms were constrained to parent sites, using a riding mode (SHELXTL).²⁵ Crystal data and details of data collection and refinements for 1–4 are summarized in Table 1. Selected bond distances and bond angles are listed in Tables 2–5.

Table 1 Crystal data and refinements for compounds 1–4

Compound	1	2	3	4
Formula	C40H66Li2N4O2Si2·C4H8O	C22H41LiN4Si	C32H50Cu2N4Si2	C36H60MgN4OSi2
M	777.13	396.62	674.02	645.37
Crystal system	Orthorhombic	Orthorhombic	Triclinic	Monoclinic
Space group	Pna 21 (No.33)	P212121(No.19)	Pī (No.2)	C2/c (No.15)
a/Å	32.031(19)	10.240(3)	10.101(12)	19.456(17)
b/Å	9.999(6)	12.316(4)	12.616(15)	9.444(8)
c/Å	15.155(9)	20.617(6)	14.018(17)	21.882(20)
α (°)	90.00	90.00	96.910(17)	90.00
β (°)	90.00	90.00	98.657(19)	93.237(17)
γ (°)	90.00	90.00	90.190(19)	90.00
U/Å^3	4854(5)	2600.2(14)	1753(4)	4015(6)
Z	4	4	2	4
Absorption coeff. (mm^−1)	0.111	0.103	1.307	0.134
Flack parameter	0.5(3)	0.2(2)	—	—
Unique reflections, Rint	7313, 0.1433	4610, 0.1701	6180, 0.0605	3562, 0.0762
Reflections with I > 2σ(I)	3780	3344	4570	2039
Final R indices [I > 2σ(I)] R1, wR2	0.0812, 0.1906	0.0593, 0.1520	0.0394, 0.0867	0.0607, 0.1447
R indices (all data) R1, wR2	0.1664, 0.2462	0.0788, 0.1641	0.0616, 0.0977	0.1189, 0.1825

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Results and discussion

Synthesis and characterization

Symmetric amidinato ligands were commonly prepared in situ from an insertion reaction of corresponding carbodiimide into a metal alkyl bond.^26–29 Symmetric benzamidinate ligand [PhC(NSiMe₃)₂]⁻ could be obtained via the reaction of lithium amide LiN(SiMe₃)₂ with benzonitrile.^2,30–32 Unsymmetrical lithium amidinates can be made by first coupling an amine and an isothiocyanate to form a thiourea, followed by sulfur removal with N-bromosuccinimide and treatment of the resulting carbodiimide with an alkyllithium.³³ The unsymmetrical aliphatic benzamidinate ligand, [PhC(NCy)(NSiMe₃)]Li, was prepared from trimethylsilylcyclohexylamine after lithiation with n-BuLi and the concomitant reaction with benzonitrile. The synthetic routes to complexes 1–5 were illustrated in Scheme 1. This involved the insertion of benzonitrile into the Li–N bond of LiN(Cy)SiMe₃ with either a synchronous or subsequent 1,3-SiMe₃ N→N′ rearrangement and salt metathesis reactions.

Scheme 1 Synthesis of complexes 1–5. Reagents and conditions: (i) Et₂O, −78 °C, recrystallization from THF; (ii) Et₂O/TMEDA, −78 °C; (iii) CuCl, Et₂O, −78 °C; (iv) 1/2 MgBr₂(THF)₂, Et₂O, −78 °C; (v) 1/3 ZrCl₄, Et₂O, −78 °C, recrystallization from CH₂Cl₂.

The THF-solvated complex 1 was obtained in 57% yield by treatment of LiN(Cy)SiMe₃ with PhCN at the ratio of 1 : 1 in Et₂O and crystallization from THF. TMEDA-solvated complex 2 was obtained also in 57% yield by treatment of LiN(Cy)SiMe₃ with PhCN and TMEDA at the ratio of 1 : 1 : 1 in Et₂O and crystallization from Et₂O. Both of them are colorless and highly moisture- and air-sensitive. They quickly changed to a pale-yellow solid when exposed to air. In an inert atmosphere they can be stored without decomposition at room temperature.

Treatment of LiN(Cy)SiMe₃ with PhCN at the ratio of 1 : 1 in diethyl ether, and further with anhydrous CuCl, or MgBr₂(THF)₂ at ratios of 1 : 1 or 2 : 1 allowed the formation of 3 and 4 in yields of 77% and 63%, respectively. Colorless single crystals of 3 and 4 suitable for X-ray analysis were obtained by recrystallization in diethyl ether in situ. Colorless crystalline 3 changes into a white solid in 12 h, and then turns pale-green, dark-green and finally black in 5 d. Crystals of 4 change into white solid in 12 h. Complex 5 was obtained by treatment of LiN(Cy)SiMe₃ with PhCN at the ratio of 1 : 1 and then with one third portion of ZrCl₄ in diethyl ether at −78 °C and by recrystallization in dichloromethane. Colorless crystalline 5 was relatively stable in air for several days and finally changed into white solid. The molecular structures of 1–4 are depicted in Fig. 1, 2, 4 and Fig. 5, respectively.

The structure of crystalline lithium benzamidinate 1 is illustrated in Fig. 1 and selected geometric parameters are listed in Table 2. Complex 1 has a ladder-shaped backbone, with a planar Li1N1Li2N4 (mean deviation of 0.0054 Å) rhomboidal core, flanked by the fused planar rings N1C1N2Li2 [mean deviation 0.0905 Å, fold angle to core 53.2°] and Li1N3C17N4 [mean deviation 0.0749 Å, fold angle to core 59.6°]. The angles at the nitrogen atom of the core [69.3(5) and 70.3(4)°] are narrower than the values found for the unsymmetrical lithium guanidinate [(Et₂O)LiN(SiMe₃)C(NMe₂)N(Ph)]₂ [73.5(3)°],³⁴ but those at the Li atom [109.6(5) and 110.8(5)°] are wider than in the latter [106.5(3)°]. The N–Li bond lengths [2.118(12) and 2.162(12) Å] of intermetallacyclic LiNCN in 1 are slightly longer than those in [(Et₂O)LiN(SiMe₃)C(NMe₂)N(Ph)]₂ [2.057(3) Å]. In complex 1, the amidinato nitrogen atoms act as chelates. It is noteworthy that both the N1 (has a Cy arm) and N4 (possesses a SiMe₃ arm) atoms also function as bridges. This novel feature differs from those of [Li{μ:κ²–N(SiMe₃)C(Ph)NBu^t}THF]₂,³⁵ [Li{μ:κ²–N(SiMe₃)C(C₆H₄Me–4)NPh}(OEt₂)]₂⁴ and [(Et₂O)LiN(SiMe₃)C(NMe₂)N(Ph)]₂³⁴ where the bridging atom is regiospecifically restricted to N(SiNe₃) or N(Ph), suggesting that the choice bridging group is governed by steric rather than electronic considerations.

Fig. 1 Molecular structure of 1 (50% thermal ellipsoids).

Table 2 Selected bond lengths (Å) and angles (°) for 1

Li1–Li2	2.477(16)	N1–Li1	2.118(12)
N1–Li2	2.184(14)	C1–N1	1.306(8)
N4–Li1	2.194(12)	C1–N2	1.349(9)
N4–Li2	2.162(12)	Li1–O1	1.904(12)
N1–C1–N2	119.7(6)	Li1–N1–Li2	70.3(4)
N1–Li2–N2	66.5(4)	N1–Li1–N4	110.8(5)
N3–Li1–N4	66.2(4)	N1–Li2–N4	109.6(5)
N3–C17–N4	119.5(6)	Li1–N4–Li2	69.3(5)

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An X-ray study of crystalline 2 (Fig. 2, Table 3) shows that it has a distorted tetrahedral environment in which the lithium atom is arranged symmetrically between the two benzamidinate and the two TMEDA nitrogen atoms (N1–Li1 = 1.976(5) Å, N2–Li1 = 2.013(5) Å; N4–Li1 = 2.105(6) Å, N5–Li1 = 2.100(6) Å). The dihedral angle between N1Li1N2 and N4Li1N5 is 95.5°. The different C–N distances [N1–C7 = 1.309(3) Å, N2–C7 = 1.343(3) Å] suggest dissimilar electronic delocalized throughout the amidinate moiety. The Li–N(SiMe₃) bond length [2.013(5) Å] is slightly longer than that of Li–N(Cy) [1.976(5) Å], suggesting the greater steric nature of the SiMe₃ group than the Cy group and the unsymmetrical coordination of the ligand to the lithium atom.

Fig. 2 Molecular structure of 2 (50% thermal ellipsoids).

Table 3 Selected bond lengths (Å) and angles (°) for 2

Si1–N2	1.703(2)	N2–Li1	2.013(5)
N1–C7	1.309(3)	N4–Li1	2.105(6)
N2–C7	1.343(3)	N5–Li1	2.100(6)
N1–Li1	1.976(5)	—	—
N1–Li1–N2	69.50(17)	Si1–N2–Li1	144.34(18)
N1–C7–N2	118.1(2)	Si1–N2–C7	131.20(19)
C7–N1–Li1	86.7(2)	C7–N2–Li1	84.30(19)

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The key bond lengths of the NCNLi moiety of 2 and its chiral benzamidinate analogue [N(myrtanyl)C(Ph)N(SiMe₃)]Li·TMEDA] I⁶ and aromatic benzamidinate analogue [LiL(TMEDA)] (L = (2,6–Me₂C₆H₃)NC(Ph)N(SiMe₃)) II³⁶ are shown in Fig. 3. The mean Li–N length of 2 [1.994 Å] is comparable to those in the unsymmetrical TMEDA-solvated analogues I [2.022 Å] and II [2.020 Å], and is also comparable to that in the symmetrical benzamidinate analogue Li[PhNC(Ph)NPh](TMEDA) [2.023 Å].¹⁴ The Li–N lengths are slightly shorter than those of 2.076(6)–2.188(6) Å in the monomeric PMDIEN-solvated symmetrical benzamidinate complex Li[PhNC(Ph)NPh](PMDIEN),¹⁴ but similar to the mean value in Li{(2,6–(p-Bu^tC₆H₄)₂C₆H₃)C(NPrⁱ)₂}(TMEDA) [1.997 Å].³⁷

Fig. 3 Bond lengths (Å) of NCNLi for 2, I and II.

The X-ray structure of 3 is shown in Fig. 4 and selected bond distances and angles are listed in Table 4. In complex 3, there are two chemically similar independent molecules that lie about the independent inversion centres in the crystal lattice, but only one molecule is shown in Fig. 3 for clarity. Benzamidinate ligands bridge copper atoms in a μ, η¹ : η¹ fashion, which is similar to the structure of [Cu(I) (N, N′-diisopropylacetamidinate)]₂²⁷ and [CuC(Me)(N^sBu)₂]₂.³³ The average distance of Cu–N is 1.902(3) Å, being slightly longer than those of symmetric acetamidinates [Cu(I) (N, N′-diisopropylacetamidinate)]₂ [1.869(1) Å]²⁷ and [CuC(Me)(N^sBu)₂]₂ [1.877(2) Å].³³ The distance between the two copper atoms is 2.473(2) Å. This is shorter than the “normal” Cu–Cu bond distance of 2.65–2.70 Å, and much shorter than those in nitrogen-based dimeric Cu(I) 1,3,5-triazapentadienyl complexes [Cu{(N(R)C(R′))₂N}]₂ (R = SiMe₃; R′ = dimethylamino or 1-piperidino) [3.636 and 3.536 Å],³⁸ indicating the presence of metal–metal interaction.^39–41 The N–Cu–N bonds are essentially linear with 176.41(10)°.

Fig. 4 Molecular structure of 3 (50% thermal ellipsoids). Symmetry operations used to generate equivalent atoms: 2 − x, 1 − y, −z.

Table 4 Selected bond lengths (Å) and angles (°) for 3

a Mean value. b Amidinate backbone.
Cu–N^a	1.902(3)
Cu–Cu	2.473(2)
N–C^a^b	1.344(4)
N–C^a	1.388(4)
N–Cu–N^a	176.41(10)
N–C–N^b	122.4(3)

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A perspective view of 4 is presented in Fig. 5, along with selected bond lengths and angles in Table 5. Diethyl ether adduct complex 4 exists as a five-coordinate monomer in which the two amidinato ligands are coordinated in a chelating η²-fashion (Fig. 5) and are arranged around the metal in a butterfly-like fashion. Such coordination affords a much distorted trigonal bipyramidal environment with the metal, the fifth position being occupied by the solvent Et₂O. The Mg1 atom and O1 of the diethyl ether ligand lie on a twofold axis. In 4, the pseudo-equatorial sites are taken up by the N2, N2′ (N2 and N2′ bear the Cy substituents) and O atom, and leaving N1 and N1′ (N1 and N1′ bear the SiMe₃ substituents) in axial position. When compared to the dimeric amidinate complexes of magnesium¹⁵ and some heavy alkaline-earth strontium and barium guanidinates,⁴² we propose that the monomeric nature of 4 is a result of steric requirements of the substituents of the ligand. This coordination sphere of metal was also found in the five-coordination, C₂-symmetric calcium complex [Ca{(Cy)NC{N(SiMe₃)₂}N(Cy)}₂·(Et₂O)]⁴³ and the strontium complex [Sr{(iPr)NC{N(SiMe₃)₂}N(iPr)}₂·(Et₂O)]⁴² with bulky guanidinato ligands.

Fig. 5 Molecular structure of 4 (50% thermal ellipsoids). Symmetry operations used to generate equivalent atoms: −x, y, 0.5 − z.

Table 5 Selected bond lengths (Å) and angles (°) for 4

Mg1–N1	2.158(3)	Mg1–O1	2.078(5)
Mg1–N2	2.104(3)	N1–Si1	1.719(3)
C1–N1	1.350(5)	C1–N2	1.319(4)
N1–Mg–N2	64.09(12)	Mg1–N2–C1	91.1(2)
N1–C1–N2	115.9(3)	Mg1–N1–Si1	141.04(18)
Mg1–N1–C1	88.0(2)	N1–Mg1–O1	96.45(9)
N1–Mg–N1′	167.10(19)	N2–Mg–N2′	114.29(19)

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The bond distances of Mg–N in 4 are in the range of 2.104(3)–2.158(3) Å, shorter than those in the magnesium bis[N,N′-bis(trimethylsilyl)benzamidinate] bis(THF) adduct [PhC(NSiMe₃)₂]₂Mg(THF)₂ [2.188(3)–2.208(3) Å],⁴⁴ but longer than those in the homometallic heteroleptic magnesium amidinate [PhC(NSiMe₃)₂Mg{μ-NC(CH₂SiMe₃)Ph}]₂ [2.067(1)–2.089(1) Å].⁴⁵ The bonding distance of Mg1–O1 is 2.078(5) Å. The bond angles of N1–Mg1–N2′, N1–Mg1–N1′ and N2–Mg1–N2′ are 108.37(12)°, 167.10(19)° and 114.29(19)°, respectively. The N–Mg–N angle within the amidinato ligand is 64.09(12)°. For the MgN2C metallacycle, the carbon atom C1 projects 0.0622 Å above the plane defined by Mg1, N1 and N2. The dihedral angle between N1Mg1N2 and N1′Mg1N2′ is 68.0°.

Crystals of 5, obtained from dichloromethane, were of poor quality but the data was adequate to establish its structure of tris(benzamidinato)zirconium chloride, in which the three chelated benzamidinato ligands are disposed around the zirconium atom in a propeller-like fashion. The zirconium atom is the center of a chloride-capped distorted octahedron.

Polymerization experiments

Complex 5 was tested as a procatalyst for ethylene polymerization with methylaluminoxane (MAO) as the activator under varied conditions. The detailed results are summarized in Table 6. At a ratio of Al/Zr 1000, with the temperature from 30 to 70 °C, the activity of 5 increased from no activity to 1.87 × 10³ g PE mol⁻¹ h⁻¹. At 50 °C, with an increase of the Al/Zr molar ratio from 1000 to 3000, complex 5 showed relatively high activity (4.13 × 10³ g PE mol⁻¹ h⁻¹) at a ratio of Al/Zr 2000. Complex 5 reached its highest activity of 5.44 × 10³ g PE mol⁻¹ h⁻¹ with an optimal Al/Zr ratio of 2000 at 70 °C. Overall, complex 5/MAO system displays moderate activity in our case, and the Al/Zr ratio and reaction temperature didn't exert significant influence on the activity.

Table 6 Data for ethylene polymerization catalyzed by complex 5/cocat system^a

Entry	T/°C	Al/Zr	Yield (mg)	Activity^b
a Polymerization conditions: 5 μmol catalyst, Cocat: MAO, 10 atm, 100 ml toluene, 0.75 h. b g mol^−1 h^−1.
1	30	1000	Trace	—
2	50	1000	4.70	1.25 × 10^3
3	70	1000	7.00	1.87 × 10^3
4	50	1500	7.40	1.97 × 10^3
5	50	2000	15.5	4.13 × 10^3
6	60	2000	16.3	4.35 × 10^3
7	70	2000	20.4	5.44 × 10^3
8	80	2000	15.4	4.11 × 10^3
9	50	2500	14.4	3.84 × 10^3
10	50	3000	5.70	1.52 × 10^3

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Compared with the seven-coordination zirconium complexes such as the zirconium benzamidinate complex [N(SiMe₃)C(C₆H₄Me-4)NPh]₃ZrCl] (8.6 × 10³ g PE mol⁻¹ h⁻¹ bar⁻¹)⁴ and zirconium guanidinate [PhNC(NMe₂)NSiMe₃]₃ZrCl (1.05 × 10⁴ g PE mol⁻¹ h⁻¹),³⁴ there were only minor differences in activity between complex 5 and its analogues.

Conclusions

We have described here a simple and effective synthesis for a novel unsymmetrical aliphatic benzamidinato ligand system [{PhC(NCy)(NSiMe₃)]⁻ and its metal complexes via the addition of lithiated cyclohexylamine to a suitable nitrile and with a concomitant 1,3-SiMe₃ N→N′ rearrangement. The coordination about the lithium cation is influenced by the choice of Lewis-base donors. The two ligands in 1 show an interesting feature in that they bind to the metal in different manners. Complex 5 allows the moderate polymerization of ethylene. Effects of the non-donor solvents on the coordination mode of its lithium salts will be published in separate paper. Further work on L₂ZrCl₂ type complexes that will exhibit high activity for olefin polymerization is underway in our laboratory.

Acknowledgements

The authors acknowledge the financial support of the Natural Science Foundation of China (No.20672070, 20772074), Shanxi Scholarship Council of China (No. 201012), and Shanxi International Education Exchange Association.

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Footnote

† CCDC reference numbers 865606–865609. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra20709e

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