Magnesium complexes supported by pyrrolyl ligands: syntheses, characterizations, and catalytic activities towards the polymerization of ε-caprolactone

Abstract

The syntheses, structures and catalytic activities of five magnesium complexes supported by HL1 (HL1 = N-(1H-pyrrol-2-ylmethylene)(2-pyridinyl)methanimine), HL2 (HL2 = N-(1H-pyrrol-2-ylmethylene)-2-pyridineethanimine), HL3 (HL3 = 2-pyrrolecarbaldmethylimine), HL4 (HL4 = N-((1H-pyrrol-2-yl)methylene)(phenyl)methanimine) and HL5 (HL5 = N-((1H-pyrrol-2-yl)methylene)-2-phenylethanimine) are described. Treatment of MgⁿBu₂ with 2 equiv. of HL1, HL2, HL3, HL4 and HL5, respectively, results in the formation of Mg(L1)₂ (1), Mg(L2)₂ (2), Mg(L3)₂(THF)₂ (3), Mg(L4)₂(THF)₂ (4) and Mg(L5)₂(THF)₂ (5). All complexes have been characterized by elemental analyses and NMR studies. The solid-state structures of complexes 2, 3, and 4 have been further established by single X-ray crystallography. All complexes were found to be active catalysts for the ring-opening polymerization of ε-caprolactone.

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Introduction

Magnesium compounds are among the most common organometallic reagents used as indispensable tools in organic synthesis and catalysis,¹ especially as alkylating reagents,² catalysts for the hydroamination of aminoalkenes,³ and initiators for the polymerization of esters.⁴ As new applications of magnesium compounds continue to be discovered, studies tailoring ligand systems to specific utilizations attract increasing attention. Over the past few years, the exploitation of magnesium compounds supported by bulky β-diketiminato ligand systems has provided a number of spectacular results,⁵ and a plethora of magnesium alkoxides supported by such ligands have been found as initiators for ring-opening polymerization of organic esters.⁶ However, the potential of alkylmagnesium complexes supported by pyrrolyl ligands, especially the pyrrolyl Schiff base ligands that form five-membered ring systems with magnesium ions, to be the catalysts for hydroamination or polymerization reactions has been less explored.⁷ The deprotonated pyrrole, named the pyrrolide anion, is among the most favorable ligands in organometallic chemistry. Isoelectronic with the Cp anion, the pyrrolide anion is also a competent η⁵ ligand and can offer the possibility of forming o-bonds through the ring nitrogen atom. Thus, pyrrolyl ligands provide tunable steric and electronic features required for compensating coordinative unsaturation of metal centers and catalytic activity toward polymerization. It is reasoned that the reaction of MgⁿBu₂ with bi- or tridentate pyrrolyl Schiff base ligands could provide alkylmagnesium complexes, which may show good catalytic activities towards hydroamination and polymerization reactions. Consequently, we have explored the reactions of MgⁿBu₂ with two tridentate ligands HL1 and HL2, and three bidentate ligands HL3–HL5 (Scheme 1), and five complexes of compositions Mg(L1)₂ (1), Mg(L2)₂ (2), Mg(L3)₂(THF)₂ (3), Mg(L4)₂(THF)₂ (4) and Mg(L5)₂(THF)₂ (5) were generated. Herein, we report the syntheses and characterizations of these complexes. The catalytic behavior of these five complexes towards ring-opening polymerization of ε-caprolactone is also presented.

Scheme 1 Structures of the ligands.

Results and discussion

Syntheses of ligands and magnesium complexes

The ligands HL1 (HL1 = N-(1H-pyrrol-2-ylmethylene)(2-pyridinyl)methanimine),⁸ HL2 (HL2 = N-(1H-pyrrol-2-ylmethylene)-2-pyridineethanimine),⁹ HL3 (HL3 = 2-pyrrolecarbaldmethylimine),¹⁰ HL4 (HL4 = N-((1H-pyrrol-2-yl)methylene)(phenyl) methanimine),¹¹ and HL5 (HL5 = N-((1H-pyrrol-2-yl)methylene)-2-phenyl ethanimine)¹¹ were prepared using condensation reactions between pyrrole-2-carboxaldehyde with the corresponding amine. Treatment of MgⁿBu₂ with one equiv. of HL1, HL2, HL3, HL4 and HL5, respectively, led to the formations of 1–5. The one ligand set chelated alkylmagnesium complex was not afforded. Then the reaction of MgⁿBu₂ with 2 equiv. of HL1, HL2, HL3, HL4 and HL5, respectively, was conducted, and complexes 1–5 were readily afforded in good yields after recrystallization in THF/hexane (Scheme 2). They were also characterized by ¹H and ¹³C NMR spectra and elemental analyses.

Scheme 2 Syntheses of complexes 1–5.

Structure descriptions of complexes 2, 3 and 4

The molecular structures of 2, 3 and 4 in the solid states have been confirmed by X-ray analysis and are shown in Fig. 1, 2, and 3, respectively. The crystallographic data and experimental details for structural analyses are summarized in Table 1. Selected bond distances and angles are listed in Table 2.

Fig. 1 ORTEP structural drawing of 2. Ellipsoids are drawn at the 30% probability level, and hydrogen atoms are omitted for clarity.

Fig. 2 ORTEP structural drawing of 3. Ellipsoids are drawn at the 30% probability level, and hydrogen atoms are omitted for clarity.

Fig. 3 ORTEP structural drawing of 4. Ellipsoids are drawn at the 30% probability level, and hydrogen atoms are omitted for clarity.

Table 1 Crystal data and structure refinements for 2–4

	2	3	4
a Including solvate molecules. b Mo-Kα radiation. c R 1 = Σ(|Fo | − |Fc|)/Σ(|Fo|) for observed reflections. d w = 1/[σ^2(Fo^2) + (αP)^2 + bP] and P = [max(Fo^2,0) + 2Fc^2]/3. e wR 2 = {Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]}^1/2 for all data.
Formula^a	C24H24MgN6	C30H34 MgN4O2	C32H38 MgN4O2
M g mol^−1^a	420.80	506.92	534.97
T/K	293(2)	293(2)	293(2)
Wavelength^b/Å	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Monoclinic	Triclinic
Space group	C 2/c	P 21/c	Pī
a/Å	28.215(6)	9.986(2)	10.453(2)
b/Å	16.504(3)	14.269(3)	11.456(2)
c/Å	14.405(3)	19.278(4)	12.516(3)
α/°	90	90	84.02(3)
β/°	98.59(3)	92.42(3)	76.88(3)
γ/°	90	90	89.43(3)
V/Å^3	6633(2)	2744.5(10)	1451.7(5)
Z	12	4	2
ρ/g cm^−3	1.264	1.227	1.224
F(000)	2664	1080	572
Crystal size/mm	0.40 × 0.30 × 0.15	0.23 × 0.21 × 0.19	0.25 × 0.20 × 0.17
θ range/°	3.05 to 25.02°	3.04 to 27.47°	1.68 to 28.36°
Limiting indices	−29 ≤ h ≤ 33	−12 ≤ h ≤ 12	−13 ≤ h ≤ 13
	−19 ≤ k ≤ 18	−18 ≤ k ≤ 16	−15 ≤ k ≤ 15
	−17 ≤ l ≤ 16	−25 ≤ l ≤ 20	−16 ≤ l ≤ 15
Reflections collected/unique	16 422/5828	14 835/6228	26 473/7247
Data/restraints/parameters	5828/0/421	6228/7/334	7247/15/352
GOF	1.181	1.157	1.066
R 1, wR2 [I > 2σ(I)]	R 1 = 0.0927	R 1 = 0.0980	R 1 = 0.0556
	wR 2 = 0.1372	wR 2 = 0.2011	wR 2 = 0.1497
R 1 ^c,wR2^d^,^e (all data)	R 1 = 0.1359	R 1 = 0.1504	R 1 = 0.0685
	wR 2 = 0.1537	wR 2 = 0.2289	wR 2 = 0.1597
Largest diff. peak and hole/e Å^3	0.284 and −0.254	0.488 and −0.736	0.519 and −0.501

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Table 2 Selected bond lengths (Å) and angles (°) for 2–4

2
Mg(1)-N(1)	2.307(4)	Mg(1)-N(4)	2.320(4)
Mg(1)-N(2)	2.176(3)	Mg(1)-N(5)	2.162(3)
Mg(1)-N(3)	2.169(4)	Mg(1)-N(6)	2.156(4)
N(1)-Mg(1)-N(2)	82.67(13)	N(2)-Mg(1)-N(6)	105.47(13)
N(1)-Mg(1)-N(3)	156.89(14)	N(3)-Mg(1)-N(4)	84.44(13)
N(1)-Mg(1)-N(4)	84.47(12)	N(3)-Mg(1)-N(5)	101.98(13)
N(1)-Mg(1)-N(5)	96.59(13)	N(3)-Mg(1)-N(6)	103.93(14)
N(1)-Mg(1)-N(6)	93.27(13)	N(4)-Mg(1)-N(5)	82.49(13)
N(2)-Mg(1)-N(3)	77.97(13)	N(4)-Mg(1)-N(6)	159.88(13)
N(2)-Mg(1)-N(4)	94.09(13)	N(5)-Mg(1)-N(6)	77.90(13)
N(2)-Mg(1)-N(5)	176.56(15)
3
Mg(1)-N(1)	2.144(3)	Mg(1)-N(4)	2.257(3)
Mg(1)-N(2)	2.261(3)	Mg(1)-O(1)	2.121(3)
Mg(1)-N(3)	2.125(3)	Mg(1)-O(2)	2.141(3)
N(1)-Mg(1)-O(1)	92.82(11)	N(2)-Mg(1)-N(4)	178.19(11)
N(1)-Mg(1)-O(2)	87.46(11)	N(3)-Mg(1)-O(1)	90.74(11)
N(1)-Mg(1)-N(2)	78.49(11)	N(3)-Mg(1)-O(2)	89.06(11)
N(1)-Mg(1)-N(3)	175.95(12)	N(3)-Mg(1)-N(4)	78.67(11)
N(1)-Mg(1)-N(4)	103.31(11)	N(4)-Mg(1)-O(1)	89.02(11)
N(2)-Mg(1)-O(1)	90.76(11)	N(4)-Mg(1)-O(2)	88.81(11)
N(2)-Mg(1)-O(2)	91.41(11)	O(1)-Mg(1)-O(2)	177.82(12)
N(2)-Mg(1)-N(3)	99.53(12)
4
Mg(1)-N(1)	2.1393(15)	Mg(1)-N(4)	2.2124(16)
Mg(1)-N(2)	2.1938(17)	Mg(1)-O(1)	2.1406(16)
Mg(1)-N(3)	2.1480(15)	Mg(1)-O(2)	2.1455(15)
N(1)-Mg(1)-O(1)	94.63(6)	N(2)-Mg(1)-N(4)	86.95(6)
N(1)-Mg(1)-O(2)	93.07(6)	N(3)-Mg(1)-O(1)	92.66(7)
N(1)-Mg(1)-N(2)	78.65(6)	N(3)-Mg(1)-O(2)	95.30(6)
N(1)-Mg(1)-N(3)	169.32(6)	N(3)-Mg(1)-N(4)	78.74(6)
N(1)-Mg(1)-N(4)	92.95(6)	N(4)-Mg(1)-O(1)	94.17(6)
N(2)-Mg(1)-O(1)	173.25(6)	N(4)-Mg(1)-O(2)	173.98(6)
N(2)-Mg(1)-O(2)	94.34(6)	O(1)-Mg(1)-O(2)	85.23(7)
N(2)-Mg(1)-N(3)	94.09(6)

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Single crystal analysis revealed that complex 2 crystallizes in monoclinic crystal system of the C2/c space group. The central Mg^II ion possesses a distorted octahedral coordination environment with six nitrogen atoms from two tridentate L1⁻ ligands. Two donor imine nitrogen atoms of the ligands are in trans arrangement, with bond angles of 77.97(13)° (N2–Mg1–N3) and 101.98(13)° (N3–Mg1–N5), summing to 179.97(6)°. The atoms N1, N3, N4 and N6 are almost coplanar, with the bond angles of 103.93(14)° (N3–Mg1–N6), 84.44(13)° (N3–Mg1–N4), 84.47 (12)° (N1–Mg1–N4) and 93.27(13)° (N1–Mg1–N6), respectively, summing to 366.16(6)°, with the deviation being 6.16(6)° compared with 360°. The bond lengths between the magnesium atom and donor pyridyl nitrogen atom (Mg1–N1 = 2.307(4) Å, and Mg1–N4 = 2.320(4) Å) are apparently longer than those of the Mg–N(imine) distances (Mg1–N2 = 2.176(3) Å, and Mg1–N5 = 2.162(3) Å), and also longer than the Mg–N(pyrrolyl) distances (Mg1–N3 = 2.169(4) Å, and Mg1–N6 = 2.156(4) Å). Compared with the four-coordinated magnesium complexes [Mg–N(imine) = 2.0525(12) and Mg–N(pyrrolyl) = 2.1154(13)]^7a,7b, the Mg–N(imine) and Mg–N(pyrrolyl) distances in 2 are all longer.

The crystal structure of 3 revealed that the octahedral Mg^II ion is surrounded by four nitrogen atoms (N1, N2, N3, and N4) from two chelating pyrrolyl ligands and two oxygen atoms (O1 and O2) from two THF molecules, with two oxygen atoms occupying the axial positions. The atoms N1, N2, N3 and N4 form the equatorial plane, with bond angles of 78.49(11)° (N1–Mg1–N2), 99.53(12)° (N3–Mg1–N2), 78.67(11)° (N3–Mg1–N4) and 103.31(11)° (N1–Mg1–N4), respectively, summing to 360.04(5)°, with the deviation being 0.04(5)° compared with 360°. The bond lengths between the magnesium atom and donor nitrogen atoms (Mg1–N2 = 2.261(3) Å, Mg1–N4 = 2.257(3) Å) are apparently longer than the Mg–N(pyrrolyl) distance (Mg1–N1 = 2.144(3) Å , Mg1–N3 = 2.125(3) Å). The two THF ligands are trans to each other, as are the two imines and the two pyrrolyl ligands.

The crystal structure of 4 is similar to that of 3, and the metal ion is also coordinated by two ligands and two THF molecules. The two THF ligands and the two imines are cis to each other, while two pyrrolyl ligands are trans to each other in complex 4.

Ring-opening polymerization of ε-caprolactone by 1–5

The application of structurally well-defined magnesium complexes as initiators for the synthesis of poly(ε-caprolactone) (PCL) via the ring-opening polymerization of lactones has gained particular interest.^4,6 Among the systems evaluated are phenolate-,¹²β-diiminato-,^5a,5c,6a,6b and trisindazalylborate¹³ ligand supported magnesium complexes. To the best of our knowledge, the catalytic activity of magnesium complexes chelated by pyrrolyl Schiff base ligands has not been studied and this represents a deficiency in this area. Therefore, we carried out an assessment of the ring-opening polymerization capability of the five new complexes.

The initial studies were performed using Mg(L1)₂ (1) as the initiator and employing dimethyl ether (DME), tetrahydrofuran (THF), and toluene as the solvent, respectively. It was found that no observable polymerization occurred at 20 °C in each solvent after 24 h. As the temperature was increased to 40 °C, trace amounts of polymers were generated. A good yield (90%) was afforded in toluene, a moderate yield (48%) was provided in DME and a poor yield (23%) was given in THF as the temperature was further increased to 60 °C. When the polymerization was conducted at 80 °C, the comparable yields could be achieved in a shorter time.

We further probed the polymerization activity of complexes 2–5 at 60 °C and 80 °C in DME, THF and toluene, respectively. The polymerization results are summarized in Table 3. It was found that all five complexes can effectively initiate ε-caprolactone polymerization, and all of the obtained polymers have high molecular weights and relatively narrow molecular weight distributions (PDIs). The solvents have the obvious effect on the yields of the polymers. For complexes 1 and 2, when the polymerization was conducted in ethereal solvents, such as DME and THF, poor or moderate yields were afforded; whereas very good yields were obtained when the polymerizations were performed in toluene. We reasoned that DME and THF molecules tend to coordinate with the metal center during the polymerization process, which may hinder the attack of ε-caprolactone to the same metal center. No remarkable differences in different solvent were observed for the polymerization reactions catalyzed by complexes 3–5.

Table 3 Polymerization of ε-caprolactone initiated by 1–5

Entry	Initiator	Solvent	[M/I]	T/h	Yield^a /%	T/°C	Mn^b (calc) (10^4)	Mn^c (10^4)	Mn^d (10^4)	PDI	Efficiency/%
a Yield: weight of polymer obtained/weight of monomer used. b Mn (calc) = Mmono × [M]/[I] × Conv. c Measured by GPC relative to polystyrene standards. d Measured by GPC relative to polystyrene standards with Mark-Houwink corrections^14 for Mn (obsd) = 0.56 Mn (GPC) for ε-caprolactone.
1	1	DME	200	21	48	60	1.09	1.59	0.89	1.47	69.9
2	1	DME	200	5	15	80	0.34	0.55	0.31	1.25	61.8
3	1	THF	200	8	23	60	0.52	0.69	0.39	1.11	75.4
4	1	THF	200	3	18	80	0.41	0.54	0.30	1.16	75.9
5	1	Tol	200	20	90	60	2.05	3.18	1.78	1.78	64.5
6	1	Tol	200	10	92	80	2.10	3.01	1.68	1.77	69.7
7	2	DME	200	10	23	60	0.52	0.72	0.40	1.21	72.2
8	2	DME	200	2	20	80	0.45	0.58	0.32	1.18	77.6
9	2	THF	200	8	32	60	0.73	1.03	0.58	1.30	70.9
10	2	THF	200	3	90	80	2.05	5.38	3.01	1.29	38.1
11	2	Tol	200	20	89	60	2.03	9.87	5.53	1.28	20.6
12	2	Tol	200	12	95	80	2.17	5.41	3.03	1.60	40.1
13	3	DME	200	3	90	60	2.05	5.55	3.11	1.27	36.9
14	3	DME	200	1	93	80	2.12	9.25	5.18	1.44	22.9
15	3	THF	200	4	49	60	1.12	1.25	0.70	1.29	89.6
16	3	THF	200	2	23	80	0.52	0.69	0.39	1.21	75.4
17	3	Tol	200	5	50	60	1.14	6.20	3.47	1.63	18.4
18	3	Tol	200	1	52	80	1.19	1.60	0.90	1.41	74.4
19	4	DME	200	2	72	60	1.64	2.92	1.63	1.46	56.2
20	4	DME	200	0.5	85	80	1.94	3.28	1.84	1.30	59.1
21	4	THF	200	1.5	90	60	2.05	2.88	1.61	1.36	71.2
22	4	THF	200	0.5	76	80	1.73	2.56	1.43	1.34	67.6
23	4	Tol	200	1	86	60	1.96	2.31	1.29	1.43	84.8
24	4	Tol	200	0.4	68	80	1.55	2.33	1.30	1.43	66.5
25	5	DME	200	6	28	60	0.64	0.72	0.40	1.22	88.9
26	5	DME	200	3	23	80	0.52	0.65	0.36	1.20	80.7
27	5	THF	200	6	85	60	1.94	3.82	2.14	1.51	50.8
28	5	THF	200	3	31	80	0.71	0.91	0.51	1.20	78.1
29	5	Tol	200	2	75	60	1.71	2.61	1.46	1.50	65.5
30	5	Tol	200	1	32	80	0.73	1.10	0.62	1.42	66.4

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Conclusions

In conclusion, five new complexes Mg(L1)₂ (1), Mg(L2)₂ (2), Mg(L3)₂(THF)₂ (3), Mg(L4)₂(THF)₂ (4) and Mg(L5)₂(THF)₂ (5) were synthesized and characterized. The catalytic behavior of these complexes towards ring-opening polymerization of ε-caprolactone in different solvents and at different temperatures was investigated. Complexes 1–5 exhibited good catalytic activity for the polymerization of ε-caprolactone.

Experimental section

General considerations

All manipulations of air-sensitive complexes were carried out in a MBraun drybox under a purified nitrogen atmosphere. Anhydrous THF, DME and toluene were freshly distilled from purple sodium benzophenone ketyl for at least 4 days. ¹H and ¹³C spectra were recorded on Innova-400 and Unity Innova-300 spectrometers at ambient temperature using TMS as an internal standard, and chemical shifts were reported in ppm. Elemental analyses for carbon, hydrogen and nitrogen atoms were performed on a Carlo-Erba EA1110 CHNO-S microanalyzer.

X-ray crystallography

Crystals grown from concentrated solutions at room temperature were quickly selected and mounted on a glass fiber in wax. The data collections were carried out on a Mercury CCD detector equipped with graphite-monochromated Mo-Kα radiation using the ϕ/ω scan technique at room temperature. The structures were solved by direct methods with SHELXS-97.^15,16 The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by use of geometrical restraints. A full-matrix least-squares refinement on F² was carried out using SHELXL-97.

General procedure for the polymerization reaction

A THF solution of complex 2 (0.0211 g, 0.05 mmol) and ε-caprolactone (1.14 g, 10 mmol) was added to a 100 mL Schlenk flask equipped with a magnetic stirring bar in a glovebox. Afterwards, the Schlenk flask was taken out of the glovebox. The reaction mixture was heated to 60 °C for 8 h and then terminated by addition of a mixture of conc. HCl/EtOH (1 : 5 v/v) (2 mL). Petroleum ether (20 mL) was added to give a yellow solid. The solid was dissolved in THF (20 mL), and column chromatography on Al₂O₃ gave a white solid.

Syntheses of complexes

Mg(L1)₂ (1). To a solution of MgⁿBu₂ (2.0 mL, 0.5 M in heptane, 1.0 mmol) in tetrahydrofuran (2 mL) was added HL1 (0.3702 g, 2.0 mmol) in tetrahydrofuran at −35 °C. After stirring at room temperature for 24 h, the volatiles were removed under reduced pressure to give a purple solid. Yield: 0.3429 g (87.32%). ¹H NMR (400 MHz, CDCl₃) δ 8.43 (s, 2H, N=CH), 7.93 (d, 2H, J = 4.7 Hz, pyridyl-H), 7.56 (t, 2H, J = 7.5 Hz, pyridyl-H), 7.21 (d, 2H, J = 7.8 Hz, pyridyl-H), 6.99–6.91 (m, 2H, pyridyl-H), 6.81 (s, 2H, pyrrole-H), 6.69 (d, J = 3.0 Hz, 2H, pyrrole-H), 6.11 (d, J = 1.8 Hz, 2H, pyrrole-H), 5.01 (s, 4H, NCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 158.81, 158.48, 147.57, 138.34, 137.46, 136.14, 122.67, 122.00, 115.41, 110.27, 55.25. Anal. calc. for C₂₂H₂₀N₆Mg: C, 67.28; H, 5.13; N 21.40%. Found: C, 67.98; H, 5.51; N, 20.78%.

Mg(L2)₂ (2). To a solution of MgⁿBu₂ (2.0 mL, 0.5 M in heptane, 1.0 mmol) in tetrahydrofuran (2 mL) was added HL2 (0.3985 g, 2.0 mmol) in tetrahydrofuran at −35 °C. The reaction mixture was stirred at room temperature overnight after which time volatiles were removed under reduced pressure to yield a crude product. The final product was obtained as light yellow block crystals after crystallization from tetrahydrofuran/hexane. Yield: 0.3778 g (89.78%). ¹H NMR (400 MHz, CDCl₃) δ 8.15 (s, 2H, N=CH), 7.99 (d, 2H, J = 4.4 Hz, pyridyl-H), 7.53 (td, J = 7.6, 1.6 Hz, 2H, pyridyl-H), 7.16 (d, J = 7.7 Hz, 2H, pyridyl-H), 6.95–6.84 (m, 4H, pyridyl-H+pyrrole-H), 6.55 (d, J = 2.8 Hz, 2H, pyrrole-H), 6.15–6.04 (m, 2H, pyrrole-H), 4.14–4.02 (m, 4H, CH₂-pyridyl), 3.76 (s, 4H, NCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 159.68, 152.77, 149.36, 136.22, 130.00, 123.62, 122.46, 121.29, 114.78, 109.62, 60.25, 39.95. Anal. calc. for C₂₄H₂₄N₆Mg: C, 68.50; H, 5.75; N, 19.97%. Found: C, 68.73; H, 5.69; N, 19.59%.

Mg(L3)₂(THF)₂ (3). Following a procedure similar to that described for the preparation of 1, treatment of tetrahydrofuran (2 mL) and MgⁿBu₂ (2.0 mL, 0.5 M in heptane, 1.0 mmol) with 2 equiv. of HL3 (0.3404 g, 2.0 mmol) in tetrahydrofuran (5 mL) at −35 °C was carried out. The reaction mixture was stirred at room temperature overnight after which time volatiles were removed under reduced pressure. The product was obtained as light yellow block crystal after crystallization from tetrahydrofuran/hexane. Yield: 0.4326 g (85.35%). ¹H NMR (400 MHz, CDCl₃) δ 8.43 (s, 2H, N=CH), 7.36 (dd, J = 15.5, 8H, Ar-H), 7.19 (t, J = 7.0 Hz, 2H, Ar-H), 6.87 (s, 2H, pyrrole-H), 6.78 (s, 2H, pyrrole-H), 6.27 (s, 2H, pyrrole-H), 3.77 (s, 8H, CH₂O of THF), 1.81 (s, 8H, CH₂ of THF).¹³C NMR (75 MHz, CDCl₃) δ 155.70, 149.40, 138.70, 138.40, 129.56, 125.08, 120.94, 120.13, 113.01, 69.22, 25.28. Anal. calc. for C₃₀H₃₄N₄O₂Mg: C, 71.08; H, 6.76; N, 11.05%. Found: C, 71.17; H, 6.63; N, 11.44%.

Mg(L4)₂(THF)₂ (4). Following a procedure similar to that described for the preparation of 1, treatment of tetrahydrofuran (2 mL) and MgⁿBu₂ (2.0 mL, 0.5 M in heptane, 1.0 mmol) with 2 equiv. of HL4 (0.3684 g, 2.0 mmol) in tetrahydrofuran (5 mL) at −35 °C was carried out. The reaction mixture was stirred at room temperature overnight after which time volatiles were removed under reduced pressure. The product crystallized from tetrahydrofuran/hexane as a colorless block crystal. Yield: 0.4765 g (89.07%). ¹H NMR (400 MHz, CDCl₃) δ 8.24 (s, 2H, N=CH), 7.53–7.41 (m, 6H, Ar-H), 7.36 (d, J = 7.3 Hz, 4H, Ar-H), 7.19 (s, 2H, pyrrole-H), 6.89 (s, 2H, pyrrole-H), 6.52 (d, J = 1.5 Hz, 2H, pyrrole-H), 4.72 (s, 4H, NCH₂), 3.70 (s, 8H, CH₂O of THF), 1.85 (s, 8H, CH₂ of THF). ¹³C NMR (100 MHz, CDCl₃) δ 161.16, 139.60, 137.11, 135.08, 128.39, 128.28, 127.03, 116.19, 110.80, 68.39, 60.61, 25.24. Anal. calc. for C₃₂H₃₈N₄O₂Mg: C, 71.84; H, 7.16; N, 10.47%. Found: C, 71.35; H, 6.75; N, 10.94%.

Mg(L5)₂(THF)₂ (5). This complex was prepared as a red solid from reaction of HL5 (0.3965 g, 2.0 mmol) in tetrahydrofuran (5 mL) with MgⁿBu₂ (2.0 mL, 0.5 M in heptane, 1.0 mmol) in tetrahydrofuran (2 mL) at −35 °C and recrystallization from mixed solvents of tetrahydrofuran and hexane by a similar procedure as in the synthesis of 1. Yield: 0.4913 g (87.26%). ¹H NMR (300 MHz, CDCl₃) δ 8.03 (s, 2H, N=CH), 7.37–7.15 (s, 8H, Ar-H), 7.15–7.05 (s, 4H, Ar-H+pyrrole-H), 6.73 (s, 2H, pyrrole-H), 6.44 (s, 2H, pyrrole-H), 3.70 (s, 8H, CH₂O of THF), 3.60 (s, 4H, CH₂-Ar, 2.67 (s, 4H, NCH₂), 1.85 (s, 8H, CH₂ of THF). ¹³C NMR (75 MHz, CDCl₃) δ 160.41, 139.81, 137.27, 134.43, 129.09, 128.50, 126.21, 115.84, 110.84, 68.71, 59.71, 38.40, 25.50. Anal. calc. for C₃₄H₄₂N₄O₂Mg: C, 72.53; H, 7.52; N, 9.95%. Found: C, 72.87; H, 6.81; N, 9.90%.

Acknowledgements

The authors appreciate the financial support of the Hundreds of Talents Program (2005012) of CAS, the Natural Science Foundation of China (20872105), the “Qinglan Project” of Jiangsu Province (Bu109805) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. CCDC reference numbers 794139, 805638 and 843509 for complexes 2, 3 and 4, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra01235a/

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