N-丁基吡啶六氟磷酸盐离子液体异构体的晶体结构及其异构化对热性质的影响

Crystal structures and thermal properties of isomers of N-butyl pyridine ionic liquids

以具有直链或支链结构的溴代正丁烷、溴代异丁烷为烷基化试剂,设计合成了互为异构体的N-正丁基吡啶六氟磷酸盐([n-C4pyr][PF6])和N-异丁基吡啶六氟磷酸盐([i-C4pyr][PF6])离子液体,在配比为V甲醇∶V甲苯=1∶3和V乙醇∶V甲苯=1∶3的混合溶剂体系中,培养出了晶型完整的异构体单晶体,并通过单晶X衍射技术,确定了异构体单晶体的晶体结构。[n-C4pyr][PF6]属于单斜晶系,空间群为P2(1)/n,晶胞参数为a=2.62(3)nm,b=0.68(7)nm,c=0.72(7)nm,α=90.00(4)°,β=90.00(5)°,γ=90.00(4)°,Z=4.00,V=1.27(2)nm3,Dc=1.47 mg/m3,μ=0.27mm-1,F(000)=576.00;[i-C4pyr][PF6]属于单斜晶系,空间群为P2(1)/c,晶胞参数为a=1.06(19)nm,b=0.97(18)nm,c=1.33(3)nm,α=90.00(4)°,β=113.00(3)°,γ=90.00°(4)°,Z=4.00,V=126.33(4)nm3,Dc=1.48 mg/m3,μ=0.27mm-1,F(000)=576.00。晶体结构分析表明:N-丁基吡啶六氟磷酸盐离子液体异构体的空间群存在一定的联系,P2(1)/n属于P2(1)/c的变型,异构化影响了阴阳离子之间的相互作用以及晶体堆积的空间网络形成,异构化对N-丁基吡啶六氟磷酸盐离子液体异构体的晶格能的影响显著,对热分解温度的影响较小;异构体的熔点相差29.10℃而凝固点相差为14.57℃,在冷却结晶过程中均出现过冷现象,进一步有效验证了其晶体学的研究结果。

N-n-butylpyridinium hexafluorophosphate( [n-C4pyr][PF6]) and N-isobutylpyridium hexafluorophosphate [i-C4pyr][PF6]have been synthesized using n-butyl bromide and isobutylbromide as alkylating reagents.Large single crystals were formed in the methanol-toluene system [Vmethanol∶ Vtoluene= 1∶ 3]and the ethanol-toluene system [Vethanol∶ Vtoluene= 1∶ 3]. The crystal structures were determined by single crystal X-ray diffraction. The crystal structure of [n-C4pyr][PF6]was determined to be monoclinic( P21/n,Z = 4. 00) with a = 2. 62( 3) nm,b = 0. 68( 7) nm,c = 0. 72( 7) nm,α = 90. 00( 4) °,β = 90. 00( 5) °,γ = 90. 00( 4) °,Dc= 1. 47 mg /m3,μ =0. 27 mm- 1,and V =1. 27( 2) nm3. The crystal structure of [i-C4pyr][PF6]was determined to be monoclinic( P21/c,Z =4. 00) with unit-cell parameters a =1. 06( 19) nm,b =0. 97( 18) nm,c =1. 33( 3) nm,α =90. 00( 4) °,β =113. 00( 3) °,γ = 90. 00( 4) °,V = 126. 33( 4) nm3,Dc= 1. 49 mg /m3,μ =0.27 mm- 1. Analysis shows that the space group of [n-C4pyr][PF6]is the variant of that of [i-C4pyr][PF6],and the different isomers of the ionic liquids have different interactions between the ions and different crystal packing space networks. The melting point of [i-C4pyr][PF6]is higher than that of[n-C4pyr][PF6],by 29.10℃. There is also an obvious difference between the freezing points of the two isomers. In addition,supercooling was found during the crystallization.