HAN Hong-Fei LI Wen-Jun ZHANG Sho-Feng WEI Xue-Hongc

a (Department of Chemistry, Taiyuan Normal University, Jinzhong, Shanxi 030619, China)

b (The School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China)

c (Scientific Instrument Center, Shanxi University, Taiyuan 030006, China)

1 INTRODUCTION

The guanidinato ligands involving metals from across the periodic table have been synthesized and applied in many areas, including synthesis[1-5], coordination chemistry[6-9]and material science[10-14].Since starting in the late 1990s,group 13 guanidinate complexes, especially highly electrophilic aluminium species, are widely studied due to their potential applications as excellent candidates for several well-known catalytic applications[15,16]and gaseous precursors to technologically important materials[17-19].

These guanidinatoaluminum complexes containing Al-C bonds are sensitive under normal environment.Because of this reason, the usage range of this kind of complexes is greatly limited.Although the high oxophilicity ability of aluminum(III) has been demonstrated by several examples of the interaction of aluminum complexes with oxygen in the reported papers[20-22], the synthesis of oxo guanidinatoaluminum complexes has not been paid enough attention by the researchers.Considering their potential applications of such complexes, and then inspired by the first oxo amidinatoaluminum complex, [{HC(NDipp)2}Al-Me(μ-OMe)]2, which was isolated unexpectedly in 2010 by Richards’ group[23], a series of oxo guanidinatoaluminum complexes have been synthesized by our group, such as [{(C5H10N)C(NCy)2}-AlMe(μ-OMe)]2[24](a), [{(Bz(Me)N)C(NCy)2}-AlMe(μ-OMe)]2[24](b), [{(Et2N)C(NCy)2}AlMe-(μ-OMe)]2[25](c), [(Et2N)C(NC6H5)(NAr)AlMe-(μ-OMe)]2[25](d), Ar = 2,6-Me2C6H3), and

Each of complexes a～d was obtained by the insertion of oxygen atom into the Al?C bond of mononuclear guanidinatoaluminum complex;while complex e was synthesized by reacting mononuclear guanidinatoaluminum complex with 1 equiv of AlMe3in the presence of H2O.In addition, two oxo guanidinatoaluminum complexes, a and b, have been used to initiate the Meerwein-Ponndorf-Verley (MPV) reaction.The experimental result shows that complex a exhibits better catalytic activity than b in the reduction.In order to further investigate the influence of initiators’ structures on the catalytic activities in the MPV reaction, we herein report the synthesis and characterization of a new guanidinatoaluminum complex,[{(C4H8N)C(NCy)2}AlMe(μ-OMe)]2(1).Our report also includes their applications in MPV reaction.

2 EXPERIMENTAL

2.1 Instruments and reagents

All solvents were freshly distilled and stored over a potassium mirror or activated molecular sieves prior to use.Deuterated solvent C6D6was dried over activated molecular sieves (4 ?) and vacuum transferred before use.CyN=C=NCy (Cy = cyclohexyl,Alfa Aesar) and AlMe3(2.0 M solution in hexane;Alfa Aesar) were obtained commercially and used as received.Pyrrolidine, aliphatic and aromatic aldehydes were dried over MgSO4and redistilled before use.1H NMR (300 MHz) and13C NMR (75 MHz) spectra of the compounds were recorded on a Bruker DRX 300 instrument and referenced internally to the residual solvent resonances (chemical shift data in δ).Elemental analyses were conducted on a Vario EL-III instrument.

2.2 Synthesis of complex 1,[{(C4H8N)C(NCy)2}AlMe(μ-OMe)]2

To a stirred solution of pyrrolidine (0.25 mL,3.0 mmol) in hexane (15 mL), trimethylaluminum (1.50 mL, 3.0 mmol) was added dropwise.The mixture was heated to 60 °C for 12 h and then cooled to room temperature, N,N'-dicyclohexylcarbodiimide (0.678 g, 3.0 mmol) was added.After being stirred for 6 h, dry oxygen was introduced slowly into the solution via a balloon and the reaction was carried out for 30 min at –78 °C.Then the resulting mixture was warmed to room temperature and stirred overnight.The white precipitate was collected by filtration, then washed with hexane (2 × 5 mL)and dried in vacuo to give the product as a white solid, which was recrystallized from hexanetetrahydrofuran (1:2) solution at –10 °C to give the colorless crystals of complex 1 (0.737 g,70.3%).Anal.Calcd.for C38H72Al2N6O2(%): C,65.30; H, 10.38; N, 12.02.Found (%): C, 65.16;H, 10.29; N, 12.51.δ1H (300 MHz, C6D6, 293 K):3.75 (s, 3H, OCH3), 3.11～3.23 (m, 2H, C6H11),2.97～3.09 (m, 4H, C4H8N), 1.90～2.00 (m, 8H,C6H11), 1.68～1.75 (m, 4H, C4H8N), 1.30～1.49(m, 6H, C6H11), 0.98～1.11 (m, 6H, C6H11),–0.17(s, 3H,AlCH3).δ13C NMR (C6D6, 25 °C): 158.43(NCN), 58.43 (OCH3), 53.49, 38.56 (C4H8N),59.88, 57.49, 55.67, 54.72, 39.72, 30.77, 30.35,30.06, 29.56, 29.20 (C6H11) (Al-CH3resonances were not observed due to the poor solubility of complex 1).

2.3 General procedure employed for the MPV reaction

To a solution of catalyst in toluene was added aldehyde, followed by the addition of 2-propanol.The reaction mixture was then refluxed for the required reaction time under an atmosphere of nitrogen.The reaction was then cooled to room temperature and the conversion yield was determined by1H NMR spectroscopic study based on the integration in the methylene and the CHO region.

2.4 Structure determination

A colorless single crystal with dimensions of 0.35mm × 0.30mm × 0.25mm was mounted on a glass fiber in a random orientation and single-crystal X-ray diffraction data for complex 1 were collected with MoKα radiation (λ = 0.71073 ?) on a Bruker Smart Apex CCD diffractometer equipped with multi-scan technique in the range of 1.69≤θ≤25.05oat 223 K.A total of 3612 reflections were collected with 3114 unique ones (Rint= 0.0512), of which 2896 observed reflections with I ＞ 2σ(I) were used in the succeeding refinements.Corrections were applied for Lorentz and polarization effects as well as absorption using multi-scans (SADABS)[26].The structure was solved via direct methods employed in the SHELXS-97[27]program and refined by full-matrix leastsquares methods against F2with SHELXL-2016[28].All non-hydrogen atoms were refined anisotropically,and all hydrogen atoms were located in the calculated positions and refined with a riding model.The final R = 0.0380, wR = 0.0913 (w = 1/[σ2(Fo2) +(0.0447P)2+ 0.6418P], where P = (Fo2+ 2Fc2)/3), S= 0.999, (Δ/σ)max= 0.000, (Δρ)max= 0.216 and(Δρ)min= –0.204 e/?3.The selected bond lengths and bond angles for complex 1 are shown in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (o) of 1

3 RESULTS AND DISCUSSION

3.1 Description of the structure

The synthetic route of complex 1 is shown in Scheme 1 and the formation of 1 can be described in the following steps, which are in accord with those reported for previous structures of complexes a～d[24,25]: (I) treatment of a secondary amine with trimethylaluminum affords amino aluminum containing Al?N bonds; (II) insertion of carbodiimides into Al?N bonds; (III) insertion of oxygen atoms into the Al?C bonds forms the oxygen bridged structure.

Scheme 1. Synthetic route to complex 1

The molecular structure of complex 1 is depicted in Fig.1.Complex 1 crystallizes in the monoclinic P21/c space group.Crystalline 1 is a centrosymmetric dimer with a central, virtually rhombus Al(1)O(1)Al(1)'O(1)' core, which is close to that of complexes a～d[24,25].The geometry around the Al atom in the structure of complex 1 is penta-coordinated with a distorted trigonal bipyramidal environment in which the nitrogen N(2) and oxygen O(1) atoms occupy the axial positions.In the molecular structure of 1, the dihedral angle between planes Al(1)N(1)C(7)N(2) and Al(1)O(1)Al(1)'O(1)'(58.622(4)°) is slightly smaller than that (62.207(4),60.192(4), and 61.179(6)°, respectively) in complexes a～c.However, it is significantly larger than that (52.152(6)°) of complex d.The Al–O bond lengths (1.8329(1)～ 1.9184(1) ?) lie in the reasonable range for those (1.8308(1)～1.9223(1) ?)in complexes a～d.The Al(1)–N(1) and Al(1)–N(2)bonds (1.9289(1) and 2.0338(1) ?, respectively) of the guanidinate ligand also fall in the range for those(1.9229(2)～2.0710(1) ?) in complexes a～d.For the guanidinato moiety C7N3, the bond distances of N(1)–C(7), N(2)–C(7) and N(3)–C(7) are 1.348(2),1.336(2) and 1.369(2) ?, respectively, suggesting a greater degree of electronic delocalization within the guanidinate backbone of 1.In addition, the bite angle of N(1)–C(7)–N(2) in the guanidinato moiety C(7)N3is 110.14(1)°.

Fig.1. ORTEP diagram of 1.Thermal ellipsoids are drawn at the 30% probability level.Hydrogen atoms are omitted for clarity

3.2 Complex 1 as catalyst for the MPV reaction

According to the successful experiences[24], the reductions of a series of aldehydes using 1 as precatalyst (Scheme 2) were carried out in anhydrous toluene at 110 °C for 8 h under the atmosphere of N2,and the molar ratio of aldehyde, complex 1 andiPrOH in the reduction is 20:1:40([PhCHO]/[Al]/[iPrOH] = 20/2/40).The results are shown in Table 2.

Scheme 2. MPV reaction using complex 1 as the pre-catalyst

Table 2. MPV Reaction of Aldehyde Catalyzed by Complex 1a

Four aromatic aldehydes were reduced to the corresponding alcohols under above conditions in 83～99% yield (Table 2, entries 1～4).The reduction of p-anisaldehyde gave an obviously lower yield than p-chlorobenzaldehyde (Table 2, entries 2～3), which is probably due to the increasing positive charge of the aldehyde group carbon atom caused by the chlorine atom on the phenyl ring.In view of the two aldehyde groups on the ring, the conversion level of tereph-thaldehyde into 1,4-benzenedimethanol is acceptable (Table 2, entry 4).In addition, three particular aliphatic aldehydes were also reduced to the corresponding alcohols under our catalytic system in 89～98% yield (Table 2, entries 5～7).Due to the conjugative effect, the yields of cinnamic aldehyde and citral were somewhat lower than that of citronellal.As a result, compared to the two oxo guanidinatoaluminum complexes (a and b)[24], the catalytic activity of complex 1 is similar to that of a in the reduction, while 1 as pre-catalyst in the reduction showed better activity than b.It may be attributed to the similar substituents of complexes a and 1 bound to the N atoms of guanidinato ligand.A proposed mechanism for the reduction of a series of aldehydes catalyzed by 1 would be similar to the reaction catalyzed by aluminum complexes chelated phenoxide ligands[29].

4 CONCLUSION

In summary, a new guanidinatoaluminum complex was synthesized and characterized by satisfactory C,H and N microanalyses,1H,13C NMR spectra and single-crystal X-ray structural data.Complex 1 used as pre-catalysts for the MPV reaction has been studied, and exhibited good catalytic activities toward the MPV reduction.Using 1 as a pre-catalyst at low (5 mol%) loading, a series of aldehydes are reduced to the corresponding alcohols in good yields.

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