Enantiomerically Pure Tetravalent Neptunium Amidinates: Synthesis and Characterization

Abstract The synthesis of a tetravalent neptunium amidinate [NpCl((S)‐PEBA)3] (1) ((S)‐PEBA=(S,S)‐N,N′‐bis‐(1‐phenylethyl)‐benzamidinate) is reported. This complex represents the first structurally characterized enantiopure transuranic compound. Reactivity studies with halide/pseudohalides yielding [NpX((S)‐PEBA)3] (X=F (2), Br (3), N3 (4)) have shown that the chirality‐at‐metal is preserved for all compounds in the solid state. Furthermore, they represent an unprecedented example of a structurally characterized metal–organic Np complex featuring a Np−Br (3) bond. In addition, 4 is the only reported tetravalent transuranic azide. All compounds were additionally characterized in solution using para‐magnetic NMR spectroscopy showing an expected C 3‐symmetry at low temperatures.

, N 3 (4)) have shown that the chirality-at-metali sp reserved for all compounds in the solid state. Furthermore, they represent an unprecedented example of as tructurally characterized metal-organicN pc omplex featuring aN p ÀBr (3)b ond. In addition, 4 is the only reported tetravalentt ransuranic azide. All compounds werea dditionally characterized in solutionu sing para-magnetic NMR spectroscopy showing an expected C 3 -symmetry at low temperatures.
The chemistry of neptunium has alwaysl agged behind that of its lightern eighbor uranium (U) [1] due to its radiotoxicity and hence small amountso ft his man-madee lement whichm ust be handled in dedicated research facilities. [2] Additionally,t he lack of availables tarting compounds confined its use until only recently.I n2 014 Reilly et al. published the synthesis of NpCl 4 (dme) 2 [3] (dme = dimethoxyethane), which enables the synthesis of an umber of Np coordination compounds in recent years. [4] However,m any recent studies are limited to one-step reactions chemes targeting the desired tetravalent Np complex. Additionalr eactivitys tudieso ft he compounds are scarced ue to the small amountso fN ps tarting material availablea nd the precautions neededw hen handling this material.
Enantiopurec omplexes of the transuranium (TRU)e lements are basically unknown. To the best of our knowledge,nos ingle crystal structureo fa nenantiopure TRU compound has been reported yet. There is only one publication on Cm complexes ligatedb yc hiral cages;t hese compounds were characterized in solution and by elemental analysis. [5] In prior work some of us established as eries of rare earth metalc omplexes ligated by ac hiral amidinate. [6] By using N,N'bis-(1-phenylethyl)benzamidinate (PEBA) [7] as al igand,as eries of mono-, bis-, and tris(amidinate) lanthanide complexesw ere synthesized. [6b, c] All chiral bis-and tris(amidinate) complexes have an additional axial chirality.S ome of these compounds were investigated as suitablee nantioselective catalysts in the hydroamination/ cyclization reactions and the ring opening polymerization of rac-lactide. [8] Herein, we report the first synthesis of aN p IV complex bearing ac hiral ligand,w hich is also the first example of as tructurally characterized enantiopure TRU compound. Additionally, the new complexes also represent the first TRU amidinate compounds, an important class of N-donorl igands, which has already been introduced in main group, transition metal, lanthanidea nd actinide( Th, U) chemistry. [9] Furthermore, we were able to functionalize the synthesizedc ompounds by halogen resp. pseudohalogen exchanger eactions to yield the first structurally characterized metal-organic Np complexf eaturing aN p-Br bond,a sw ell as an unprecedentedN p IV azide.
The Np IV amidinate 1 was obtaineda sc rystalline solid after workup (see Experimental Procedurei nS upportingI nformation). This chloro complex wasused as precursor for several exchange reactions in order to investigatet he conformational stabilityo ft he enantiopurecompound. Thus, 1 was reacted with AgPF 6 ,T MSBr and NaN 3 to give the respective fluoro (2), bromo (3)a nd azido (4)N p IV trisamidinates(see Scheme 2).
All reactions were carried out under inert conditions yielding the complexes in pure form in high yields and no indication of remaining chloro complex 1 (see NMR spectra in SI). Similar halide substitution reactions have already been reportedf or thorium and uranium compounds yielding the respectivef luoride, [10] bromide, [11] and azide [10b, 12] compounds.W ec ould show that theser eactionc onditions are also applicable to Np. The use of AgPF 6 as af luorinating agenth as been reported by Liddle et al. starting from aU V compound in ar edox fluorination.
[10b] However, we appliedt his reagent in ar edox-neutral approachb ys tarting from the tetravalent Np amidinate 1 to give the tetravalent Np fluoride 2.T he mechanism of the substitutionr eaction is not fully understood yet (see Supporting Information for more details).
All Np compounds 1-4 crystallize in the chiral space group P2 1 2 1 2 1 with similar cell parameters (see Table S7 in Supporting Information) reflecting their isostructurality and even isochirality (see Flack parameters) at the metal center. They exclusively crystallize in the D-configuration. [13] Note, compound 2 shows deviation of the Flack parameter from 0, indicating some degree of racemization. The molecular structure of 1-4 is shown in Figure 1. The Np atom is 7-fold coordinated by three amidinatel igands and one (pseudo)halide. The amidinate ligands are coordinating asymmetrically to the Np centers rep-resented by al ong NpÀN a (N a = N1, N3, N5;2 .49-2.52 )a nd a short NpÀN b (N b = N2, N4, N6;2 .36-2.39 )b ond. The variance betweent he different compounds is small. Only the Np fluoride 2 possesses slightly longerN p ÀNb ond lengths (max. 3pm) than compounds 1, 3, 4.This may be causedb ythe relatively strong NpÀFb ond thus weakening the NpÀNb onds. All NpÀNd istances are in good agreement with literature data [4b, g, 14] (see Table S4 in the Supporting Information). Within the amidinate units, the CÀNd istances are similari na ll compounds (ranging from 1.30 to 1.40 ,s ee Ta ble S3 in Supporting Information) suggesting ad elocalization of electron density.F urthermore, the amidinatel igands are tilted against the NpÀXb ond in such way that ap ropeller-like structure is formed witht he halide lying on the rotation axis (see Figure 1). Surprisingly,a ll three ligandsp ossess ad ifferent tilting angle (see Table S2 and Figure S9 in the Supporting Information) pointing to only a C 1 -symmetry of the molecules in the solids tate. The NpÀXd istances are growing, as expected, with increasing ion radius of X( NpÀF: 2.166 (13) (2) (1) (3)). The NpÀCl distance is in accordance with availablel iterature data, [4c, 15] and distances to the halidesi n2 and 3 are similart o inorganic Np IV halides [16] (see Table S5 in the SupportingI nformation).
Compounds 1-4 werea lso characterized by infrared spectroscopy in the solid state (see Figure S15). The characteristic CÀNs tretching vibration of the amidinate unit (1418 cm À1 )i s similar for 1-4 indicating also an egligible influence of the halide.T he NpÀXs tretching vibration (X = F, Cl, Br) is anticipated to appear below 650 cm À1 [ 17] and thus could not be observed with the experimental setup. The only difference between the compounds is the expected intense asymmetric stretching of the azide ion coordinated to the neptunium in 4 (n as (N 3 ) = 2092 cm À1 ). Thisi si ng ood agreement with the only other reported Np azido compound ([NpO 2 (N 3 )(Phen)(H 2 O)] 2 ·3H 2 O) where an asymmetrics tretching frequency of 2090 cm À1 is reported. [18] NMR spectra of the paramagnetic compounds 1-4 were recorded at low temperature (243 K) to improvet he line width. However,o nly one set of signals, which is consistent with a C 3 -symmetry of the molecules in solution,i so bserved. This is another indication of the conformational stabilityo ft he chiral Np amidinate ligandsi n  solution.U pon coordination to the Np ion, the symmetry within the ligand is broken. Thus, the number of NMR signals doubles compared to Li(S)-PEBA (see Figure 2). The paramagnetic influence of the Np IV center increases the differences between the chemical shifts of similar nucleii ne ach set of signals. This is most likely due to as trong distance and angle dependency of ap seudo-contact contribution of the paramagnetic Np IV center. The paramagnetic influence is best observed in the 1 HNMR spectra showingasignal range from À11 to 51 ppm. Ac utout of the 1 HNMR spectra of all Np complexes is shown in Figure 2. The complete spectraa re depicted in the Supporting Information.
As seen from the spectra,i ti sc lear that the halide ligand has an influence onto the chemical shift of the protonso ft he attached amidinate ligands. However,t his influencei sq uite small when comparing the spectrao ft he chloro (1), bromo (3) and azido (4)c omplexes. By considering that the solid-state structureso fa ll compounds do not differ substantially from each other and considering that their structures are preserved in solution, the similarities of the 1 HNMR spectra are not surprising. Interestingly,t he fluorides eems to have the strongest influence onto the chemical shifts, as the overall range of signals is significant smaller compared to all other compounds (À6t o4 0ppm). We suggest an electronic influence of the fluoro ligand on the electronic structureo ft he Np IV ion and hence its paramagnetism as ap ossible explanation for this observation. Similar effectsh ave already been described for lanthanidea nd uranium complexes. [19] Complexes 1-4 were additionally characterized in solution using UV/visible/NIR spectroscopy. The spectra are shown in Figure 3. In the region between 300 and 600 nm p-p*t ransitions of the amidinate ligandsa re visible. The region between 600 and 1050 nm is dominated by f-f transitions as already observedb yB rown et al. foraNp IV triamidoamine complex. [4b] A comparison between the spectra of different complexesr evealed only minor difference induced by the auxiliary halide ligand.H owever,s ubtle differences are again visible for the fluoro compound 2 with an additional transition at 725 nm. This is in good agreementw ith the observations from NMR spectroscopy,s uggestinga ne lectronic influence of the fluoro ligand.
In summary,aseries of isostructural enantiomeric pure tetravalent neptunium amidinates have been presented. They possess the first example of transuranic amidinate molecules and the first structurally characterizedc hiral TRU complexes. Starting from the newly reported Np chloro complex 1,h alide and pseudohalide exchange reactions are possible. For the first time, this gave access to as tructural characterized metal-organic Np complexh aving aN p ÀBr bond.A ll complexes (1-4) possess the same chirality-at-metal, showing the conformational stability of the complexes upon halide exchange. Ac omparison of the Np IV complexes in solution revealed the influence of the fluoro ligand onto the electronic structure, potentially altering the crystal field splitting. [20] Thish as been comprehensively shown by using paramagnetic NMR and UV/visible spectroscopy. The rich chemistry of the Np IV amidinate system pre-  (3), N 3 (4)). Similar protonsare colored with similar colors (blue/pale blue:N C-H; green/pale-green: ortho-Ph;r ed/orange: meta-Ph, brown: para-Ph). sented, leads us to assume that the (S)-PEBA ligand could serve as suitable spectator ligand for other actinidec ompounds and would enablefurther reactivity.