Pentanuclear Sn（Ⅱ）Guanidinate Complex:Synthesis,Structure,and Catalytic Activity for Addition of Arylamines into N,N′-Diisopropylcarbodiimide

WANG Ying-Ying TONG Hong-Bo ZHOU Mei-Su

(Institute of Applied Chemistry,Shanxi University,Taiyuan 030006,China)

Abstract:A pentanuc1ear Sn（Ⅱ）guanidinate comp1ex[PhNC(NMe)2N(H)SnCl]2[PhNC(NMe)2NSnCl]2Sn(1)was synthesized via the reaction of PhN(Li)SiMe3and anhydrous tin（Ⅱ）ch1oride.It was we11 structura11y characterized by1H NMR,13C NMR,e1ementa1 ana1ysis,and X-ray sing1e crystal crystal1ography techniques.Comp1ex 1 is an active cata1yst in the addition reaction of ary1amines into N,N′-diisopropy1carbodiimide giving guanidinates.CCDC:1954650.

Keywords:tin（Ⅱ）;guanidinate;structure;cata1ytic addition

0 Introduction

Guanidinate ligands as one of the important noncyclopentadienyl ancillary ligands have received increasing attention in organometallic chemistry and synthetic chemistry.They are useful in stabilizing a variety of main group,transition metal,andf-block element complexes[1-3].Some of these derivatives have been found to be efficient catalysts or pre-catalysts in organic and polymerization reaction[4-6].The synthetic access for the preparation of most of these ligand species rely on the insertion reaction of carbodiimides RN=C=NR into a metal alkyl amide bond[7-10].The nonsymmetric guanidinato ligands were prepared via the nucleophilic reactions of N-centered anions to nitriles free fromα-hydrogen,migrations of SiMe3group and isomerization[4-5,11].Both the tin（Ⅱ） and cyclo-Sn（Ⅳ）S4complexes bearing symmetric amidinato ligands,much similar to the guanidinato ligands,were outstanding catalysts for the cyclotrimerization of phenyl isocya-nates to triaryl isocynnurates[12-13].Sn（Ⅱ）guanidinate complexes have been limited to a handful of systems which typically contain sterically bulky groups[14-17].Recently,some Sn（Ⅱ）guanidinate complexes were proved to be potential to act as single source precursors to the deposited thin films[18].Very recently,homo-and hetero-mononuclear 1,3,5-triazapentadienyltin（Ⅱ）complexes and organotin（Ⅳ）complexes were synthesized and some of them were efficient catalysts for the cata-lytic addition of arylamine toN,N′-diisopropylcarbodiimide giving guanidine[19-20].Here we report the synthesis and structural features of pentanuclear Sn（Ⅱ）guanidinate complex 1.Its catalytic behaviors towards addition reaction of arylamines intoN,N′-diisopropylcarbodiimide have been investigated and it is active as a catalyst to give a guanidinate.

1 Experimental

1.1 Reagents general procedures

All manipulations were carried out under an atmosphere of argon using standard Schlenk techniques.Solvents were purchased from commercial sources.Deuterated solvent CDCl3was dried over activated mo-lecular sieves(0.4 nm)and vacuum transferred before use.THF was dried and distilled from sodium/benzophenone and stored over a sodium mirror under argon.PhN(Li)SiMe3was synthesized according to literature procedures[4].Glassware was oven-dried at 150℃overnight.The NMR spectra were recorded on Bruker AVANCEⅢ-600MHz spectrometer and recorded in CDCl3.The chemical shifts were reported inδvalues relative to external SiMe4for1H and13C.Melting points were measured in sealed capillaries and uncorrected.Elemental analyses were carried out using a Vario EL-Ⅲanalyzer(Germany).

1.2 Preparation

1.2.1 Synthesis of [PhNC(NMe)2N(H)SnCl]2[PhNC(NMe)2NSnCl]2Sn(1)

(CH3)2NCN(0.24 mL,2.92 mmol)was added to a solution of PhN(Li)SiMe3(0.50 g,2.92 mmol)in THF(30 mL)at-78℃.The resulting mixture was warmed toca.20℃and stirred overnight.SnCl2(0.68 g,3.65 mmol)was added at-78℃.The resulting mixture was warmed toca.20℃and stirred overnight.The solution was concentratedin vacuoand stored at-20℃for one month,affording colorless crystals of 1(0.256 g,28.4%).m.p.138～141 ℃.Anal.Calcd.for C36H46Cl4N12Sn5·2THF(%):C,34.62;H,4.09;N,11.01.Found(%):C,34.59;H,4.07;N,11.03.1H NMR(CDCl3):δ1.695(s,THF),2.565(s,12 H,N(CH3)2),3.209(s,12 H,N(CH3)2),3.764(s,THF),4.105(s,2 H,NH)7.015～7.379(m,20 H,Ph).13C NMR(CDCl3):δ25.61(THF),40.02,42.52(N(CH3)2),68.00(THF),122.18(p-CPh),122.37(m-CPh),129.71(o-CPh),148.00(Cipso-Ph),169.51(NCN).

1.2.2 General addition reaction procedures

A 30 mL Schlenk tube in a dried argon atmosphere was charged with 1(0.015 g,0.01 mmol).To the flask were addedN,N′-diisopropylcarbodiimide(0.31 mL,2.00 mmol)and arylamines(2.00 mmol).The resulting mixture was stirred at 25,60 or 80℃for the desired time.The reaction mixture was then hydrolyzed with water(0.5 mL),extracted with dichloromethane(3×10 mL),dried over anhydrous Na2SO4,and filtered.After the solvent was removed under reduced pressure,the residue was recrystallized in hexane to give the final product as a white solid.

1.2.3 X-ray crystallography

The single crystals of 1 suitable for X-ray diffraction studies were obtained.Data collection was performed with MoKαradiation(λ=0.071 073 nm)on a D8 Venture diffractometer using the Ω 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)[21].The structure was solved by the direct method(SHELXS-97)[22].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 riding modes(SHELXTL)[23].Crystal data and details of data collection and refinements for 1 are summarized in Table 1.

CCDC:1954650.

Table 1 Crystallographic data and structural refinement for 1

2 Results and discussion

2.1 Synthesis and molecular structure of[PhNC(NMe)2N(H)SnCl]2[PhNC(NMe)2NSnCl]2Sn(1)

Compound 1 was prepared according to Scheme 1.Thus,treatment of PhN(Li)SiMe3with one equivalent of Me2NCN in THF at low temperature,and then 1.25 equiv of SnCl2,afforded the Sn compound.The likely course of reaction was successive insertion of Me2NCN into the Li-N(SiMe3)Ph bond,elimination of Me3SiCl molecules,hydrolysis,and metathesis with SnCl2,thus featuring both monoanionic and dianionic guanidinate ligands in 1.Complex 1 has been characterized by1H and13C NMR and elemental analysis.Suitable crystals for X-ray diffraction analysis can be obtained by slowly cooling the saturated THF solution to-20℃for one month.

Fig.1 Molecular structure of 1

Scheme 1 Synthetic route for compound 1

The molecular structure of compound 1 is shown in Fig.1.It crystallizes in the orthorhombic space groupPbcn.In 1,there are two mono-and two dianionic guanidinato ligands.The tetra-coordinated central Sn（Ⅱ）ion bonded to four terminal nitrogen atoms from each of mono-or dianionic guanidinato ligand,located in a tetrahedral environment with a dihedral angle of 89.6°between planes N3Sn2N3iand N6Sn2N6i(Fig.1).Each of other four Sn（Ⅱ）ions coordinates with two terminal nitrogen atoms from each of mono-and dianionic guanidinato ligand and a chlorine ligand in a pyramidal manner.The Sn（Ⅱ）ions are almost in a plane with the mean deviation of 0.007 50 nm.The central of molecule 1 is a tin atom(Sn2)surrounded by four fused sixmembered Sn2N3C rings.The 16-membered macrocycle(SnNCN)4may be regarded as a star containing Sn5N8C4that four N atoms with respect to the central tin atom Sn2(Fig.2).The structure of 1 is different from those clusters or cages,that polynuclear tin complexes possessing N and O donor groups normally have,such as tin（Ⅱ）imido cubanes,distorted cubanes or boat with[SnN]4motif,mix-oxidation-state Sn（Ⅱ）/Sn（Ⅳ） double cubanes with Sn7N8(two Sn4N4cubanes which share a central Sn（Ⅳ）ion)[19,24-26],and a ladder-like or drum structures for the higher oxidative state of organooxotin clusters[27-28].

Fig.2 Core of 1

For the C7N1N2N3 and Cl6N4N5N6 frameworks,the three C-N bond distances in the guanidinate ligands are in a range of 0.133 5(5)～0.136 6(5)nm and 0.133 5(5)～0.137 2(5)nm,respectively.These are comparable to C(sp2)-N(sp2)bonds(ca.0.136 nm)[29],indicating of lone pair donation from the nitrogen atom(dimethylamido group)to the central carbon and concomitant electron delocalization involving all three nitrogen atoms of the chelating ligands.The bond distance of N3-Sn3(0.216 5(3)nm)is slightly shorter than that of N6-Sn1(0.2189(3)nm).The Sn-Clbonddistances of Sn1-Cll(0.248 98(12)nm)and Sn3-Cl2(0.249 50(12)nm)are comparable to that of tri-coordinated tin（Ⅱ）triazapentadienyl complex(0.250 97(9)nm)[19],shorter than that in[(i-Pr)2ATI]SnCl([(i-Pr)2ATI]=N-isopropyl-2-(isopropylamino)troponimine)(0.254 2(2)nm)[30],and in[C5H5]SnCl(0.268 nm)[31].

The bond angles of N1-Sn1-N6,N3-Sn2-N6 and N3-Sn3-N4iare89.43(12)°,106.49(13)°and89.86(12)°,respectively.The bond angles of N-C-N in the CN3frameworks are in a range of 119.0(3)°～120.8(3)°.The Sn1…Sn2,Sn1…Sn3 and Sn2…Sn3 separations are 0.352 4,0.491 4 and 0.351 0 nm,respectively,suggesting the weak tin-tin bonding(Sn-Sn 0.368 nm)[32]between Sn1 and Sn2,Sn2 and Sn3,respectively.

2.2 Catalytic behaviors of the catalyst

The catalytic activity of 1 for catalytic addition of arylamines toN,N′-diisopropylcarbodiimide to guanidinates was evaluated.The optimum conditions and results are listed in Table 2.For comparison,SnCl2was also used in the addition reaction of arylamines toN,N′-diisopropylcarbodiimide(Table 3).All reactions were performed under solvent free condition.

The reaction of aniline withN,N′-diisopropylcarbodiimide was performed at 25℃in the presence of 1.With the increase of the reaction time from 0.25 to 2 h,the yields increased from 16.8% to 84.6%(Table 2,Entry 1～4).At 60 and 80℃,the yields in 0.25 h were 76.8% and 95.9%,respectively(Table 2,Entry 5 and 7),much higher than that of 16.8% at 25℃(Table 2,Entry 1).In 0.5 h,the higher temperature will proceed the transformation and give a higher yield of guanidinate(Table 2,Entry 6 and 8).By comparison,both the complex 1 and SnCl2can accelerate the addition reaction with the increase of the reaction temperature and reaction time in a similar manner,but the catalytic activities of 1 are much higher than those of SnCl2(Table 3).For instance,the reaction with SnCl2afforded guanidinate in only 28.6%(Table 3,Entry 4),34.8%(Table 3,Entry 6),and 51.3%(Table 3,Entry 8)isolated yields,respectively,whereas the yields were 84.6%(Table 2,Entry 4),91.0%(Table 2,Entry 6)and 97.3%(Table 2,Entry 8),respectively,when complex 1 was used.However,the activity of 1 at 80℃in 0.5 h(Entry 8,97.3%)was lower than those of some Lewis acids such as GaCl3,which can afford the product in the yield of 99% in only 2 min at 10℃(5%(n/n)catalyst loading)[33];but comparable to that of anhydrous AlCl3(97%),regardless of the reaction time and reaction temperature[34].When complex 1 was used in the addition reaction of bulky amine andN,N′-diisopropylcarbodiimide,the lower yields were found and neither the higher reaction temperature nor the prolong reaction time can be helpful to the catalytic reaction(Table 2,Entry 9～12).The much less active situation of SnCl2was shown in Table 3(Entry 10～14).The less efficient with lower yields because of the larger steric hindrance of the substrate was also documented in the lanthanide aryloxide complexes[35]and organotin（Ⅳ） complexes[20]reported previously.

Table 2 Addition of ArNH2toiPrNCNiPr by complex 1a

Table 3 Addition of ArNHtoiPrNCNiPr by SnCla22

The proposed pathway involved the amination of 1 by aniline giving an intermediate,then the nucleophilic addition reaction to a carbodiimide yielding an active species,protonolysis by aniline affording the product and the intermediate.The four tri-coordinated tin（Ⅱ）ions(Sn1,Sn1i,Sn3,and Sn3i)having sterically active lone pair,would have been relevant to the catalytic activity of 1.The experimental data on the reaction pathway and the isolation of the intermediate and the active species are worth expecting in the future.

3 Conclusions

In conclusion,a pentanuclear Sn（Ⅱ）guanidinate complex[PhNC(NMe)2N(H)SnCl]2[PhNC(NMe)2NSnCl]2Sn(1)was readily prepared via the reaction of PhN(Li)SiMe3with Me2NCN,and further SnCl2.The structural features were determined by X-ray diffraction study.This complex exhibits good activity for the catalytic addition of aniline toN,N′-diisopropylcarbodiimide.Further studies on the synthesis of new metal guanidinate complexes and use in the reactions of various primary and secondary aromatic amines to carbodiimide are pursuing in our laboratory.