吡啶-2%2C6-二甲酸氢锂的合成、结构及热化学性质

Vol.34高等学校化学学报No.52013年5月 CHEMICAL JOURNAL OF CHINESE UNIVERSITIES 　1208~1213doi:10.7503/cjcu20120879吡啶⁃2,6⁃二甲酸氢锂的合成㊁结构及热化学性质高振飞,邸明哲†,邸友莹(山东省化学储能与新型电池技术重点实验室,聊城大学化学化工学院,聊城252059)摘要　以甲醇和水的混合溶液为溶剂,合成了吡啶⁃2,6⁃二甲酸氢锂Li(HDPC)(H 2O)(s),利用X 射线单晶衍射法表征了其晶体结构.用精密自动绝热热量计测量了其在78~378K 温区的低温热容.通过最小二乘法拟合得到摩尔热容随折合温度变化的多项式方程,利用此方程计算出了化合物的舒平热容和各种热力学函数.设计合理的热化学循环,利用等温环境溶解⁃反应热量计分别测定所设计热化学反应的反应物和产物在选定溶剂中的溶解焓,通过计算得到反应焓为-(46.83±0.16)kJ /mol.利用Hess 定律计算出吡啶⁃2,6⁃二甲酸氢锂的标准摩尔生成焓为-(747.90±1.46)kJ /mol.利用紫外-可见光谱仪对反应物和产物溶液的测量证实所设计热化学循环的可靠性.关键词　吡啶⁃2,6⁃二甲酸氢锂;晶体结构;低温热容;标准摩尔生成焓;热化学循环中图分类号　O642.3 文献标志码　A收稿日期:2012⁃09⁃24.基金项目:国家自然科学基金(批准号:20673050,20973089,21273100)资助.联系人简介:邸友莹,男,博士,教授,主要从事热化学和物理无机化学研究.E⁃mail:diyouying@126com;diyouying@ †现在陕西省三原县妇幼保健院工作.吡啶⁃2,6⁃二甲酸,简称H 2DPC,是一种配位形式灵活多变的刚性配体[1,2],具有显著的生物活性,在医学上已得到广泛的应用.在食物中加入吡啶⁃2,6⁃二甲酸盐,则可减少食物中维生素C 的氧化[3,4].锂是对动物和人体有医用价值和生理作用的一种重要元素.人体内钠元素含量过高时会引起高血压等心脑血管疾病,锂在人体内能够置换并取代钠,从而降低心脑血管疾病的发病率.锂对中枢神经活动具有调节作用,可以安定情绪.此外,锂还与许多疾病的防治有关.到目前为止,吡啶⁃2,6⁃二甲酸类金属配合物的合成㊁结构表征及性质研究等已取得很大的进展,但其低温热容和溶解焓等热化学性质方面的研究报道仍很少.本文在合成吡啶⁃2,6⁃二甲酸氢锂的基础上,利用X 射线单晶衍射方法表征了其晶体结构,并考察了其热化学性质.1　实验部分1.1　试剂与仪器吡啶⁃2,6⁃二甲酸和氢氧化锂等均为A.R.级试剂(百灵威科技有限公司),溶剂水为二次去离子水.Smart 1000X 射线单晶衍射仪(德国布鲁克公司),SRC⁃100型溶解⁃反应热量计(武汉大学),精密自动绝热热量计(大连化学物理研究所).1.2　样品的合成按照H 2DPC 和LiOH㊃H 2O 的反应摩尔比(1∶1)分别精确称量各反应物,并先后溶于甲醇和水(体积比1∶1)的混合溶剂中,在电热炉上加热至形成无色澄清溶液.将所得到的混合溶液过滤,室温下静置,得到白色针状晶体,抽滤,用甲醇重结晶2~3次,最后将产物在真空干燥箱中干燥后,置于称量瓶中备用.利用元素分析法确定了化合物中C,H 和N 的含量,利用化学分析法确定了Li 的含量,样品的纯度(质量分数)在99.6%以上.1.3　样品低温热容的测定样品低温热容的测定是在精密自动绝热热量计中进行的[4],样品质量为1.52033g,即0.0080mol,Li(HDPC)(H 2O)的摩尔质量为191.07.测量温度范围为78~378K,液氮为冷冻剂.在热容测量过程中,内屏与样品池之间的温差可以自动控制在±0.001K,样品池在平衡期的温度变化率可以控制在±10-4K /min.为了证实绝热热量计测量结果的准确性,预先测量了量热参考标准物质α⁃Al 2O 3在78~400K 温度范围内的摩尔热容,实验数据与美国标准与技术研究院(NIST)的推荐值[5]之间的相对偏差为±0.30%.1.4　样品标准摩尔生成焓的测定利用SRC⁃100型溶解⁃反应热量计测定样品的溶解焓[4].溶解实验和电标定实验的温差皆通过等面积法[6]校正得到,恒温槽控温精度为±0.001K,量热腔内温度计测温精度为±0.0001K,实验温度控制在(298.15±0.001)K.用原美国标准局提供的量热标准物质THAM(NBS⁃742a)在0.1mol /L 盐酸中的反应焓和KCl 在二次蒸馏水中溶解焓的测量来验证仪器的准确度,结果表明,实验数据与文献[7,8]标准参考数据之间的相对偏差在±0.30%以内.2　结果与讨论2.1　单晶结构分析通过X 射线单晶衍射所得的Li(HDPC)(H 2O)(s)的晶胞参数㊁键长和键角分别列于表1和表2.从表1中可以得出,Li(HDPC)(H 2O)(s)属于单斜晶系,空间群为P 2(1)/m .晶胞参数为a =0.3519(5)nm,b =1.1817(18)nm,c =0.9196(14)nm,α=90°,β=99.962(17)°,γ=90°,Z =2.在剑桥晶体结构数据库中CCDC 编号为900576.Li(HDPC)(H 2O)(s)的分子结构图和晶胞堆积图分别如图1和图2所示.Table 1　Crystal data and structure refinement for the complex C 7H 6LiNO 5Empirical formula C 7H 6LiNO 5Formula weight 191.07Wavelength /nm0.071073Crystal system,space group Monoclinic,P 2(1)/mUnit cell dimensions a =0.3519(5)nm,b =1.1817(18)nm,c =0.9196(14)nmα=90°,β=99.962(17)°,γ=90°Volume /nm 30.3766(10)Z ,Calculated density /(g㊃cm -3)2,1.685Absorption coefficient /mm -10.143F (000)196Data /restraints /parameters683/0/71Goodness⁃of⁃fit on F20.988Final R indices[I >2σ(I )]R 1=0.1695,wR 2=0.4865R indices(all data)R 1=0.1765,wR 2=0.4491Largest diff.peak and hole /(e㊃nm -3)964and -779Table 2　Selected bond lengths (nm )and bond angles (°)for the complex C 7H 6LiNO 5Li1 N10.201(3)O3 Li1 O198.1(10)C2#2 N1 Li1120.7(7)Li1 O10.2155(12)N1 Li1 O174.7(8)C2 N1 Li1120.7(7)Li1 O1#20.2155(12)O1#2 Li1 O1147.8(15)C1 O1 Li1118.7(11)O3 Li1 N1138.1(17)O3 Li1 Li1#1148.9(9)C1 O1 H1109.8O3 Li1 O1#298.1(10)N1 Li1 Li1#173.0(10)Li1 O1 H1130.1N1 Li1 O1#274.7(8)C2#2 N1 C2118.6(14)Li1 O3 H3109.6 从图1可见,Li +分别和吡啶⁃2,6⁃二甲酸配体分子的2个羧基的羟基氧原子和1个氮原子配位,另与1个水分子的氧原子配位.在Li +周围形成2个共边五元环,分别是Li1 N1 C2 C1 O1 Li1和Li1 N1 C2A C1A O1A Li1.在此配合物中Li1 N1键长为0.201(3)nm,Li1 O1键长为0.2155(12)nm.另外,从晶体学数据可知,配体中2个羟基上的氢原子的占有率分别为0.5.与文献[2]的结果对比,本文报道的晶体结构是在不同温度下测量的,符合温度越高键长和晶胞尺寸越大的规律.然而,与文献[3]中报道的吡啶⁃2,6⁃二甲酸氢钾晶体结构相比,二者有很大的不同,这主要是K O 和Li O 键长和极性不同所致.9021　No.5　高振飞等:吡啶⁃2,6⁃二甲酸氢锂的合成㊁结构及热化学性质Fig.1　Molecular structure of the title complex Fig.2　Packing structure of the title complex2.2　热容与温度的关系样品的实验摩尔热容见图3.可以看出,在78~378K 范围,Li(HDPC)(H 2O)(s)的摩尔热容随着Fig.3　C p ,m ⁃T plot of Li (HDPC )(H 2O )(s )温度的升高而升高,整个热容曲线是平滑递增的,这说明该样品在此温度区间结构稳定,没有相变㊁分解和缔合等热异常现象发生.利用最小二乘法将此温度范围内的摩尔热容实验值对折合温度进行多项式拟合,得到热容随温度变化的多项式方程:C p ,m =293.713+205.009X +24.108X 2+11.519X 3+14.890X 4 式中,X 为折合温度[4],X =(T -228)/150,拟合相关系数R 2=0.998.通过热容随温度变化的多项式方程可算出78~378K 温度范围内每个温度点的舒平热容值.利用以下热力学函数关系式:H T -H 298.15=∫T298.15C p ,m d T (1)S T -S 298.15=∫T298.15C p ,m ㊃T -1d T(2)G T -G 298.15=∫T298.15C p ,m d T -T∫T298.15C p ,m ㊃T -1d T(3) 用摩尔热容多项式方程进行数值积分可得78~378K 温度范围内的舒平热容和热力学函数值(表3).Table 3　Smoothed heat capacties and thermodynamic functions of the complex Li (HDPC )(H 2O )(s )T /K C p ,m /(J ㊃K -1㊃mol -1)(H T -H 298.15)/(kJ ㊃mol -1)(S T -S 298.15)/(J ㊃K -1㊃mol -1)(G T -G 298.15)/(kJ ㊃mol -1)T /K C p ,m /(J ㊃K -1㊃mol -1)(H T -H 298.15)/(kJ ㊃mol -1)(S T -S 298.15)/(J ㊃K -1㊃mol -1)(G T -G 298.15)/(kJ ㊃mol -1)80117.95-53.78-285.8-30.91235303.33-22.03-82.57-2.62585122.50-53.18-278.6-29.50240310.27-20.50-76.11-2.23090127.21-52.55-271.4-28.12245317.28-18.93-69.63-1.86795132.06-51.90-264.4-26.79250324.34-17.32-63.15-1.536100137.06-51.23-257.5-25.48255331.48-15.68-56.65-1.237105142.20-50.54-250.7-24.22260338.69-14.01-50.15-0.9699110147.45-49.81-243.9-22.98265345.98-12.30-43.63-0.7352115152.83-49.06-237.2-21.78270353.35-10.55-37.10-0.5328120158.31-48.29-230.6-20.61275360.81-8.763-30.55-0.3628125163.89-47.48-224.0-19.48280368.37-6.940-23.98-0.2252130169.56-46.65-217.5-18.37285376.04-5.080-17.40-0.1201135175.33-45.79-211.0-17.30290383.82-3.180-10.80-0.0478*******.18-44.90-204.5-16.26295391.71-1.242-4.180-0.008499145187.10-43.97-198.1-15.25298.15396.75000150193.10-43.02-191.6-14.28300399.740.73662.463-0.002310121高等学校化学学报 Vol.34　Continued T /K C p ,m /(J ㊃K -1㊃mol -1)(H T -H 298.15)/(kJ ㊃mol -1)(S T -S 298.15)/(J ㊃K -1㊃mol -1)(G T -G 298.15)/(kJ ㊃mol -1)T /K C p ,m /(J ㊃K -1㊃mol -1)(H T -H 298.15)/(kJ ㊃mol -1)(S T -S 298.15)/(J ㊃K -1㊃mol -1)(G T -G 298.15)/(kJ ㊃mol -1)155199.16-42.04-185.2-13.34305407.902.7559.131-0.029********.29-41.03-178.8-12.42310416.204.81515.82-0.0903*******.47-39.99-172.4-11.54315424.666.91722.55-0.1850170217.71-38.92-166.0-10.70320433.299.06129.30-0.3138175224.01-37.82-159.6-9.883325442.0911.2536.08-0.4770180230.36-36.68-153.2-9.100330451.0713.4842.90-0.6749185236.75-35.51-146.8-8.350335460.2615.7649.75-0.9075190243.20-34.31-140.4-7.632340469.6518.0856.64-1.175195249.69-33.08-134.0-6.946345479.2720.4563.57-1.478200256.23-31.82-127.6-6.293350489.1222.8770.55-1.816205262.81-30.52-121.2-5.672355499.2125.3477.56-2.189210269.44-29.19-114.8-5.084360509.5727.8784.62-2.598215276.12-27.82-108.4-4.528365520.2030.4491.73-3.041220282.85-26.43-101.9-4.004370531.1333.0798.88-3.519225289.62-24.99-95.48-3.512375542.3535.75106.1-4.031230296.45-23.53-89.03-3.0522.3　各生成物与产物的溶解焓和设计反应的反应焓选择LiOH㊃H 2O 作为设计反应的反应物.目标产物的标准摩尔生成焓的测定是在常压和298.15K 下进行的,按下列化学反应方程式设计Hess 热化学循环:H 2DPC(s)+LiOH㊃H 2O(s)=Li(HDPC)(H 2O)(s)+H 2O(l)(4) 以体积分数5%稀氨水作为量热溶剂,测量反应物和产物的溶解焓.将反应物和产物分别在玛瑙研钵中研细,以化学反应方程式的计量系数为配比在T =298.15K 时,溶解在100mL 溶剂中,5次平行测量所得到的溶解焓(Δsol H 0m )结果列在表4中.以 sol”代表所选溶剂,则H 2DPC(s)+sol =溶液A′.将LiOH㊃H 2O(s)溶解在溶液A′中,5次平行测量所得到的溶解焓(Δsol H 0 m )结果列于表4中.LiOH㊃H 2O(s)+溶液A′=溶液A.按照反应计量数准确称量已烘干并恒重的Li(HDPC)(H 2O)(s),将其溶解Table 4　Dissolution enthalpies of reactants and products in the selected solvents *SystemSolvent No.m /gΔE s /ΔE e t e /sQ s /JΔsol H 0m /(kJ㊃mol -1)H 2DPC s10.083640.996864.468-31.182-62.36420.083501.003164.359-31.325-62.69730.083880.787881.437-31.132-62.02740.083490.996264.906-31.373-62.80050.083420.993564.577-31.131-62.367-(62.45±0.14)LiOH㊃H 2OSolution A′10.020220.317818.687-2.891-6.02220.020440.394916.250-3.123-6.41330.020110.372316.375-2.967-6.19540.020610.371716.344-2.957-6.03450.020700.371916.328-2.955-6.031-(6.14±0.08)Li(HDPC)(H 2O)s10.091530.782927.469-10.468-21.85420.091650.782327.438-10.448-21.78030.091420.777427.344-10.347-21.64740.091440.790927.235-10.487-21.89150.091200.775427.359-10.326-21.648-(21.76±0.05) *Δsol H 0m =Q s /n =-(ΔE s /ΔE e )I 2Rt e (M /m )[4],m :mass of sample;t e :heating period of electrical calibration;Q s :heat effect during thesample dissolution;ΔE s :the voltage change during the sample dissolution;ΔE e :the voltage change during the electrical calibration,Δsol H 0 m :molar enthalpy of dissolution,n is mole number of the sample,R is the electro⁃resistance(R =1216.9Ω,T =298.15K),I is the current(I =20.015mA),and M is the molar mass of the sample.1121　No.5高振飞等:吡啶⁃2,6⁃二甲酸氢锂的合成㊁结构及热化学性质在所选的溶剂中,5次平行测量的溶解焓(Δsol H 0m )的结果列在表2中.此溶解过程可以表示为Li(HDPC)(H 2O)(s)+sol =溶液B′.作为反应物之一的H 2O(l),在溶液B′中稀释焓的测定可以表示为:溶液B′+H 2O(l)=溶液B.按照产物水在该设计反应中的化学计量系数可推得:由于产物水的摩尔数非常小,该过程的稀释焓可忽略不计,即焓变为零.根据Hess 定律,设计反应的反应焓(Δr H m )可以通过C 7H 5NO 4(s)(ΔH 1),LiOH㊃H 2O(s)(ΔH 2),Li(HDPC)(H 2O)(s)(ΔH 3)和H 2O(l)(ΔH 4)在所选溶剂中的溶解焓计算得到:Δr H m =ΔH 1+ΔH 2-ΔH 3-ΔH 4=-(46.83±0.16)kJ /mol(5)2.4　吡啶⁃2,6⁃二甲酸氢锂盐的标准摩尔生成焓由反应焓与一些辅助的热力学数据,如Δf H 0 m [C 7H 5NO 4,s]=-(787.78±1.45)kJ /mol [9],Δf H 0m[LiOH ×H 2O,s]=-484.93kJ /mol [10]和Δf H 0 m [H 2O,l]=-(285.83±0.04)kJ /mol [11]结合,可计算出化合物Li(HDPC)(H 2O)(s)的标准摩尔生成焓,其反应式及数据列于表5.Δf H 0m [Li(HDPC)(H 2O),s]=ΔH 8=ΔH 1+ΔH 2-ΔH 3-ΔH 4+(ΔH 5+ΔH 6-ΔH 7)=Δr H m +(Δf H 0 m [C 7H 5NO 4,s]+Δf H 0 m [LiOH㊃H 2O,s]-Δf H 0m [H 2O,l])=-(747.90±1.46)kJ /mol(6)Table 5　Reaction scheme used to determine the standard molar enthalpy of formation of Li (HDPC )(H 2O )(s )*No.Reaction Δf H 0 m or (Δsol H 0m +σa )/(kJ㊃mol -1)1{C 7H 5NO 4(s)}+sol =solution A′-(62.45±0.14)(ΔH 1)2{LiOH㊃H 2O(s)}+solution A′=solution A -(6.14±0.08)(ΔH 2)3{Li(HDPC)(H 2O)(s)}+sol =solution B′-(21.76±0.05)(ΔH 3)4H 2O(l)+solution B′=solution B 0(ΔH 4)57C(s)+2.5H 2(g)+0.5N 2(g)+2O 2(g)=C 7H 5NO 4(s)-(787.78±1.45)(ΔH 5)61.5H 2(g)+O 2(g)+Li(s)=LiOH㊃H 2O(s)-484.93(ΔH 6)7H 2(g)+0.5O 2(g)=H 2O(l)-(285.83±0.04)(ΔH 7)87C(s)+3H 2(g)+2.5O 2(g)+N 2(g)+Li(s)=Li(HDPC)(H 2O)(s)-(747.90±1.46)(ΔH 8)*σa =[∑5i =1(x i -x )2]/n (n -1),in which n is the experimental number;x i ,a single value in a set of dissolution measurements;x ,themean value of set of measurement results.Fig.4　UV⁃Vis spectra of solutions A and B根据Hess 定律,只有当溶液A 和溶液B 在浓度㊁组成㊁结构和物理化学性质等方面都相同时,即处于相同的热力学状态时,设计的热化学循环才合理.溶液A 和溶液B 具有几乎完全相同的UV⁃Vis 光谱,如图4所示.溶液A 的折光指数为1.3321±0.0007,溶液B 为1.3320±0.0005.由此可知,溶液A 和溶液B 处于等同的热力学状态.因此,所设计的Hess 热化学循环是合理的,能够用于计算目标化合物Li(HDPC)(H 2O)(s)的标准摩尔生成焓.3　结 论合成吡啶⁃2,6⁃二甲酸氢锂的反应焓Δr H m 为-(46.83±0.16)kJ /mol,说明该反应放热,是焓驱动过程.另外,从反应前后混乱度变化考虑,反应熵变应大于零,因而,此反应也是熵驱动过程.根据热力学原理,吡啶⁃2,6⁃二甲酸氢锂的合成是一个焓与熵共同驱动的反应.吡啶⁃2,6⁃二甲酸氢锂的标准摩尔生成焓Δf H 0m [Li(HDPC)(H 2O),s]=-(747.90±1.46)kJ /mol,是个绝对值较大的负值.因此,与构成它的稳定单质相比,其能量更低,结构更稳定.本文研究的化合物其热容比文献[3]报道的化合物的热容大,这主要是由于Li O 比K O 的键长短㊁键能大造成的.2121高等学校化学学报 Vol.34　参　考　文　献[1]　Tang R.L.,Yan Z.E.,Guo C.C.,Luo Y.M.,Chem.J.Chinese Universities ,2006,27(3),472 477(唐瑞仁,严子耳,郭灿城,罗一鸣.高等学校化学学报,2006,27(3),472 477)[2]　Odoko M.,Kusano 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of Pyridine⁃2,6⁃dicarboxylic Acid Lithium HydrogenGAO Zhen⁃Fei,DI Ming⁃Zhe,DI You⁃Ying *(Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology ,College of Chemistry and Chemical Engineering ,Liaocheng University ,Liaocheng 252059,China )Abstract Pyridine⁃2,6⁃dicarboxylic acid lithium hydrogen[Li(HDPC)(H 2O)]was synthesized in methanol and water mixed solvent.X⁃ray crystallography was applied to characterize its crystal structure.A polynomial equation of experimental molar heat capacities as a function of the temperature was obtained.The smoothed molar heat capacities and thermodynamic functions of the compound were calculated based on the fitted polyno⁃mial equation.A reasonable thermochemical cycle was designed on the basis of the preparation reaction of the substance,and the standard molar enthalpies of dissolution for the reactants and products of the hydrothermal reaction in the selected solvent were measured by an isoperibol solution⁃reaction calorimeter,respectively.The reaction enthalpy was determined to be -(46.83±0.16)kJ /mol.Eventually,the standard molar enthalpy offormation of the title complex was derived to be -(747.90±1.46)kJ /mol by the Hess thermochemical cycle.In addition,the reliability of the designed thermochemical cycle was verified by UV⁃Vis spectroscopy and re⁃fractive indexes.Keywords Pyridine⁃2,6⁃dicarboxylic acid lithium hydrogen;Crystal structure;Low⁃temperature heatcapacity;Standard molar enthalpy of formation;Thermochemical cycle(Ed.:S ,Z )3121　No.5　高振飞等:吡啶⁃2,6⁃二甲酸氢锂的合成㊁结构及热化学性质。