質(zhì)譜在錒系配位化學研究中的應用

陳秀婷，李晴暖，龔 昱,*

質(zhì)譜在錒系配位化學研究中的應用

陳秀婷1,2，李晴暖1，龔 昱1,*

錒系金屬離子與中性或離子型配體在溶液中所形成的配位化合物是錒系元素的重要存在形式[1]。由于乏燃料后處理過程中涉及到鈾、镎、钚等錒系元素的分離[2-3]，而無論萃取還是離子交換等方法，錒系元素與特定配體所形成配合物的結(jié)構(gòu)特征與反應規(guī)律直接影響了分離效果，因此對于這類配合物體系的化學性質(zhì)開展研究是十分必要的。另一方面，鈾、钚等錒系元素特殊的電子結(jié)構(gòu)(可參與成鍵的5f軌道以及相對論效應等)，導致了它們尤其是輕錒系元素在化學行為上與鑭系元素存在著明顯的差異。一系列不同的穩(wěn)定氧化態(tài)(+3到+7)以及存在形式使溶液中的錒系元素呈現(xiàn)出介于過渡金屬和鑭系元素的化學性質(zhì)[4-6]，因此探索錒系配合物所特有的結(jié)構(gòu)、性質(zhì)與反應方式對于深入理解錒系元素的化學性質(zhì)同樣有著重要的意義。

然而溶劑分子和伴陰離子等環(huán)境因素的存在，致使溶液中錒系金屬離子與配體間的結(jié)合并不完全取決于錒系元素的內(nèi)在性質(zhì)，溶劑化效應以及離子對形成的影響不可避免，這也在一定程度上影響了錒系配合物的反應規(guī)律。相比之下，將溶液中的配合物以離子的方式引入到氣相，利用質(zhì)譜方法研究錒系配位化合物的結(jié)構(gòu)與反應是一種有效的實驗方法[7-9]。由于質(zhì)譜的研究對象是具有特定質(zhì)荷比的離子，選擇合適的離子形式便能使伴陰離子的影響得到控制。通過優(yōu)化離子的形成條件可以在質(zhì)譜中得到不同程度溶劑化和無溶劑分子包圍的金屬配合物離子，這也為研究金屬配合物的內(nèi)在化學性質(zhì)以及溶劑化效應創(chuàng)造了條件。

為了在氣相中形成特定的錒系配合物離子，合適的離子化條件至關(guān)重要。盡管激光濺射金屬或者簡單化合物靶是制備特殊錒系化合物的有效方法[10-11]，但是基于電噴霧技術(shù)的電離方式更適合將錒系配合物離子從溶液轉(zhuǎn)移至氣相。電噴霧電離作為一種常用的軟電離技術(shù)，可以在不顯著改變?nèi)芤褐信浜衔锝Y(jié)構(gòu)和組成的情況下，實現(xiàn)金屬配位化合物的離子化[12]。處于氣相中的離子被束縛在離子阱或者離子回旋共振池等質(zhì)量分析器中，這類“反應容器”為研究氣相錒系配合物的結(jié)構(gòu)和反應提供了必要的實驗條件[13]。處于“反應容器”中的各類配合物離子經(jīng)過質(zhì)量選擇這一“分離”和“純化”步驟可以得到單一的反應物或產(chǎn)物，隨后利用碰撞誘導解離(CID)[14]、電子轉(zhuǎn)移解離(ETD)[15]等技術(shù)以及室溫條件下的分子離子反應[16-17]便能對特定離子的結(jié)構(gòu)和反應展開研究。

近五年來，基于質(zhì)譜方法的氣相錒系化學研究已經(jīng)取得了一系列研究成果，涵蓋了釷、鏷、鈾、镎、钚、镅、鋦等眾多錒系元素。本文將從多電荷錒系配合物離子的形成和反應、錒酰離子的氧化與活化反應等方面介紹錒系元素的氧化態(tài)、存在形式、電荷密度以及配體性質(zhì)對錒系配合物在結(jié)構(gòu)和反應等方面的影響。

1 多電荷錒系配合物離子的形成和反應

紅：O；藍：N；灰：C；黃：An；氫原子省略圖及的結(jié)構(gòu)示意圖Fig.1 Structures of TMOGA(a)，

1.2 An3+和An4+配合物的形成與反應

A：AnⅣ； B：AnⅢ；C：PuⅣ(L)2(86)3+；D：AnⅣ(L)2(L-86)3+圖的碰撞誘導裂解反應[26]Fig.2 Collison induced dissociation of An(L)

綜上所述，四價錒系金屬離子在氣相中的還原穩(wěn)定性順序依次為Th(Ⅳ)>U(Ⅳ)>Np(Ⅳ)>Pu(Ⅳ)，這也與溶液中四者的氧化還原趨勢一致。鑒于四價鏷、镅和鋦同樣具有相當?shù)姆€(wěn)定性，對它們所形成配合物的研究將能更全面地反映出四價錒系金屬離子在反應規(guī)律上的異同及其影響因素。

2 錒酰離子的氧化與活化反應

圖3 UⅤ的氧化反應速率與配體X質(zhì)子親和能的關(guān)系[34]Fig.3 Plot of the rates of O2 addition to UⅤas a function of the gas basicity of X-[34]

紅：O；灰：H；黃：An圖4 錒酰離子與水的氧交換反應勢能面Fig.4 Potential energy profiles for the oxo-exchange

圖的碰撞誘導裂解反應[45]Fig.5 Collision induced dissociation of UO2(NCO)

3 錒酰離子-冠醚配合物的結(jié)構(gòu)

紅：O；灰：C；黃：An；氫原子省略圖6冠醚配合物的結(jié)構(gòu)及其水合行為示意圖Fig.6 Schematic structures of actinyl-crown ether complexes and their reactivities toward H2O

4 配位誘導的化學反應

錒系配合物離子在氣相中的化學反應除了與金屬離子的性質(zhì)密切相關(guān)外，配體自身的反應規(guī)律也有著重要的影響。通常情況下，處于結(jié)合狀態(tài)的配體只有在得到額外能量的時候才能引發(fā)自身的反應，該能量既可以來自于光激發(fā)也可以來自于CID過程，這一點已為人們所熟知。除此以外，錒系配合物與另一分子因配位而釋放的反應熱同樣能夠誘導與金屬結(jié)合配體的化學反應。

5 鈾酰離子的電荷密度

圖7 DAA的脫水反應(上)與鈾酰離子存在下DAA的配位誘導解離反應(下)Fig.7 Dehydrations of DAA in condensedphase induced by spectator-ligand-addition(bottom)

圖配合物的兩種裂解方式Fig.8 Fragmentation patterns

6 結(jié)語與展望

基于質(zhì)譜的氣相化學研究在錒系配合物的制備、表征和反應機理探索方面展現(xiàn)出獨特的作用。由于氣相條件下溶劑分子和伴陰離子等環(huán)境因素的影響可以得到有效地控制，因此實驗結(jié)果能夠更好地反映出錒系元素內(nèi)在性質(zhì)對反應規(guī)律的影響。同時通過選擇合適的離子體系，借助CID、分子離子反應等方法可以在氣相中實現(xiàn)溶液中不易發(fā)生的化學反應，使錒系元素在溶液中被“隱藏”的化學性質(zhì)得以體現(xiàn)。處于氣相中的錒系配合物通常都是簡單分子離子，這也使基于從頭計算和密度泛函理論的量子化學計算能夠?qū)ζ溥M行比較準確地模擬[11,57-58]，通過與實驗現(xiàn)象的對比進而得出諸如電子結(jié)構(gòu)、化學鍵等對錒系化合物結(jié)構(gòu)、性質(zhì)和反應的影響。近些年來，隨著負離子光電子能譜[59-60]、紅外光解離光譜[61-62]等各種質(zhì)譜相關(guān)的新型實驗技術(shù)在氣相錒系化學研究中得到應用，實驗自身已經(jīng)能夠給出許多與配合物離子結(jié)構(gòu)、電子結(jié)構(gòu)直接相關(guān)的信息。另一方面，大氣壓化學電離、激光濺射電離等多種電離方式的應用也使更多性質(zhì)各異的固態(tài)和液態(tài)化合物易于離子化，這些都極大地拓展了氣相化學研究的范圍。盡管質(zhì)譜方法的研究對象是氣相配合物離子，但是實驗得到的結(jié)果往往與配合物在溶液中的化學行為直接相關(guān)。結(jié)合紅外光譜、核磁共振、X射線吸收和衍射等技術(shù)，便可以在不同層次上深入認識各類錒系配位化合物的結(jié)構(gòu)特征和反應規(guī)律，在闡明錒系元素化學性質(zhì)的同時，也能為乏燃料后處理等過程中涉及的錒系配位化學問題提供必要的科學依據(jù)。

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1.中國科學院 上海應用物理研究所,上海 201800；2.中國科學院大學，北京 100049

錒系配位化合物不僅是乏燃料后處理所涉及的重要物種，也是探究錒系元素性質(zhì)對化合物結(jié)構(gòu)特征與反應規(guī)律影響的重要模型，因此對于這類配合物體系開展研究是十分必要的。處于氣相中的錒系配合物離子由于溶劑分子、伴陰離子等環(huán)境因素得到了有效的控制，它們的化學行為更準確地體現(xiàn)出錒系元素內(nèi)在性質(zhì)的影響。本文將對近五年來質(zhì)譜方法在氣相錒系配合物的制備、表征和反應機理研究方面所取得的主要研究成果進行介紹。

錒系元素；配位化合物；質(zhì)譜；氣相

Mass Spectrometric Studies on the Chemistry of Actinide Coordination Complexes

CHEN Xiu-ting1,2, LI Qing-nuan1, GONG Yu1,*

1.Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China；2.University of Chinese Academy of Sciences, Beijing 100049, China

Actinide coordination complexes are not only important species involved in nuclear fuel cycle, but also serve as model systems from which the influence of 5f electron as well as relativistic effect on the structures and reactivities of actinide complexes can be derived. Investigations on actinide coordination complexes in solution are usually complicated due to the presence of solvent molecules and counterpart anions, while all these effects could be under control when the coordination chemistry of actinide complexes is probed in the gas phase. With the help of various gas phase dissociation techniques, ion-molecule reactions as well as quantum chemical calculations, the intrinsic properties of actinide elements and their influences on the gas phase chemistry of actinide complexes are unveiled. Herein, recent progresses on multiply charged actinide complexes, actinyl oxidation and activation, actinyl-crown ether complexes, spectator-ligand-addition induced reaction and effective charge densities of actinyls are presented. The discussion is mainly focused on the formation, characterization and reaction mechanism of these actinide coordination complexes.

actinide; coordination complexes; mass spectrometry; gas phase

2016-06-14；

2016-09-04

中國科學院戰(zhàn)略性先導科技專項(XDA02030000)；“千人計劃”青年人才項目；留學人員科技活動項目

陳秀婷(1991—)，女，江西宜春人，博士研究生，從事錒系配位化合物的氣相化學研究；E-mail: chenxiuting@sinap.ac.cn

*通信聯(lián)系人：龔 昱(1982—)，男，上海人，研究員，從事錒系元素化學研究，E-mail: gongyu@sinap.ac.cn

O641.4；O657.63

A

0253-9950(2017)01-0001-12

10.7538/hhx.2017.39.01.0001