Advances in F-Element Cyanide Chemistry Jean-Claude Berthet, Pierre Thuéry, Michel Ephritikhine

Advances in f-element cyanide chemistry Jean-Claude Berthet, Pierre Thuéry, Michel Ephritikhine

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Jean-Claude Berthet, Pierre Thuéry, Michel Ephritikhine. Advances in f-element cyanide chemistry. Dalton Transactions, Royal Society of Chemistry, 2015, 44, pp.7727-7742. 10.1039/C5DT00692A. hal-01157634

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Advances in f-element cyanide chemistry

Cite this: Dalton Trans., 2015, 44, Jean-Claude Berthet, Pierre Thuéry and Michel Ephritikhine 7727 This Dalton perspective gives an overview of the development of cyanide chemistry of 4f- and 5f-elements, a ﬁeld which was poorly explored in contrast to the attention paid to the cyanide complexes of the d transition metals. The use of the cyanide ligand led to the discovery of mono- and polycyanide complexes which exhibit unprecedented and unexpected coordination geometries. A new type of linear 3− − metallocenes including [U(Cp*)2(CN)5] (Cp* = C5Me5) and the ﬁrst bent actinocenes [An(Cot)2(CN)]

(An = Th, U; Cot = C8H8) were isolated. Thorocene was found to be much more reactive than uranocene

since a series of sterically crowded cyanide complexes have been obtained only from [Th(Cot)2]. A series of cyanido-bridged dinuclear compounds and mononuclear mono-, bis- and tris(cyanide) complexes + III were prepared by addition of cyanide salts to [MN*3] (M = Ce, U) and [UN*3] [N* = N(SiMe3)2]. The Ce , UIII and UIV ions were clearly diﬀerentiated in these reactions by cyanide linkage isomerism, as shown for III 2− example by the structures of the cyanide complex [U N*3(CN)2] and of the isocyanide derivatives III 2− IV − III IV [Ce N*3(NC)2] and [U N*3(NC)] . While the U–CN/NC coordination preference towards the U /U Received 16th February 2015, pair is related to the subtle balance between steric, covalent and ionic factors, DFT computations and in Accepted 16th March 2015 particular the calculated total bonding energies between the metal and the cyanide ligand allowed the DOI: 10.1039/c5dt00692a observed coordination mode to be predicted. The ability of the cyanide ligand to stabilize the high oxidation www.rsc.org/dalton states was assessed with the synthesis of UV and UVI complexes in the inorganic and organometallic series.

1. Introduction

For a long time, the cyanide ligand has occupied a prominent position in the chemistry of the d transition metals, as demon-

Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07. strated by the number of reviews, some of them quite old, CEA, IRAMIS/NIMBE/LCMCE, CEA/CNRS UMR 3685 NIMBE, CEA/Saclay, Bât. 125, 91191 Gif-sur-Yvette, France. E-mail: [email protected], devoted to the fundamental aspects and applications of these 1–12 − [email protected], [email protected] complexes. This ubiquity of the CN anion in various

Jean-Claude Berthet graduated Pierre Thuéry graduated from from ENSCR (Rennes) and ENSCP (Paris) and obtained his obtained his Ph.D. in 1992 from Ph.D. in 1987 from Orsay Uni- the University of Orsay for versity. After some years in studies on the organometallic industrial research, he joined the chemistry of uranium in CEA’s CEA in 1990. For more than laboratory. Immediately after, he twenty years, he has been active joined the group of Saclay as a in X-ray crystallography, while permanent staﬀ for chemical pursuing his own interest in the developments on the f-elements synthesis and structure of com- in nonaqueous solution. His plexes, coordination polymers or studies are devoted to organo- frameworks, particularly those Jean-Claude Berthet uranium chemistry and UIII/LnIII Pierre Thuéry built by uranyl and lanthanide diﬀerentiation and, during the cations with macrocycles or car- last ten years, much of his research has focused on the structure boxylic acids. n+ and reactivity of actinocenes (U, Th), of the uranyl ion {UO2} (n = 1, 2) and of cyanide compounds of the f-elements.

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domains, from biology to materials science, results from its in low yields.27 The tris(cyclopentadienyl) and bis-(indenyl) η strong coordinating ability, its capacity to stabilize a wide cerium(IV) complexes [Ce(Cp)3(CN)] (Cp = -C5H5) and 28 range of oxidation states and stereochemistries and to adopt [Ce(C9H7)2(CN)2] were reported in 1972, but these results diﬀerent ligation modes, thus giving a rich variety of homo- were strongly questioned.29 The first organouranium cyanide

and heteropolynuclear compounds with interesting structures complexes [U(Cp)2(CN)] and [U(Cp)3(CN)] were synthesized in and physicochemical properties. The research in this field is 1974 by protonolysis of [U(Cp)3] and [U(Cp)4] with HCN in

currently attracting much attention with the discovery of mole- benzene, as well as [Ln(Cp)2(CN)] from [Ln(Cp)3] (Ln = Nd, cular-based assemblies with cyanide bridges, giving magnetic Yb).30 Other syntheses of the tris(cyclopentadienyl) derivatives

materials like Prussian Blue type complexes, and providing sig- [U(C5H4R)3(CN)] were proposed afterwards (Scheme 1): from 9–13 nificant insights into magnetostructural correlations. In salt metathesis reactions of [U(C5H4R)3Cl] (R = H, Me) with striking contrast, the cyanide complexes of the f-elements alkali metal cyanides in aqueous solution or acetonitrile,31 oxi- t 32 have been largely neglected, even though the chemistry of the dation of [U(C5H4R)3](R=H, Bu) with nitrile or isonitrile 30,31 n lanthanides and actinides has witnessed a spectacular overall molecules, and addition of N Bu4CN to the cationic pre- 14–23 t 33,34 development during the recent period. cursors [U(C5H4R)3][BPh4](R= Bu, SiMe3). However, The first report of a uranium cyanide complex dates from attempts at the preparation of the anionic derivatives − 1901 with the formation of an insoluble uranyl species [U(C5H4R)3(CN)2] (R = H, Me) by reaction of [U(C5H4R)3(CN)] obtained by reaction of uranyl acetate with excess KCN. This with an excess of alkali metal cyanide in aqueous or organic 31 material was formulated as [K2][UO2(CN)4] but it was not solutions were unsuccessful. The poor solubility of these characterized.24 After initial attempts in 1964 at the prepa- complexes in organic solvents and their high ν(CN) infrared −1 ration of a uranium(IV) cyanide complex by treatment of the stretching frequencies (between 2090 and 2110 cm ) chloride or thiocyanate precursors with mercuric cyanide or suggested that they adopt a polymeric structure held by strong iodine monocyanide in acetonitrile solution,25 such a com- CN bridges. This hypothesis was confirmed with the character-

pound, [UCl3(CN)]·4NH3, was isolated for the first time in 1970 ization of the cyclic trimeric and hexameric samarium com- 35 36 from the reaction of UCl4 and NaCN in anhydrous liquid pounds [Sm(Cp*)2(μ-CN)(CNCy)]3 and [Sm-(Cp*)2(μ-CN)]6 26 ammonia. Under such conditions, this insoluble complex (Cp* = η-C5Me5) (Fig. 1), isolated in 1988 and 1997 respectively.

did not react further with additional NEt4CN, but the possible These were the first cyanide complexes of the f-elements to formation of the bis(cyanide) [NEt4]2[UCl4(CN)2] from UCl4 have been structurally characterized and they were followed by

and NEt4CN in liquid hydrogen cyanide was suggested. Follow- a series of lanthanide analogues [Ln(Cp*)2(μ-CN)(L)]3 (Ln = La 37,38 t 38,39 ing these results, the homoleptic lanthanide cyanides [Ln or Pr and L = Me3SiCN; Ln = Ce or Sm and L = BuNC; (CN)x]∞ (x = 3 and Ln = Ce, Pr, Sm, Eu, Ho, Yb, or x = 2 and Ln = Sm, Eu, Yb) were obtained by either of two ways: treatment

of metal turnings with Hg(CN)2 in liquid NH3 gave impure products whereas electrolysis of metal pieces in the presence of HCN in liquid ammonia aﬀorded the pure compounds Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07.

Michel Ephritikhine obtained his Ph.D. in 1974 from the Univer- sity of Paris and after a postdoc- toral stay in the group of M. L. H. Green, where he carried out research on metallacyclo- butanes and their role in the metathesis reaction of alkenes, he joined the group of H. Felkin in Gif-sur-Yvette where he studied the chemistry of rhenium polyhydrides and the C–H bond Michel Ephritikhine activation of alkanes. In 1984, he joined the CEA in Saclay, where he presently holds the position of Directeur de Recherche Emérite at the CNRS. His current research interests comprise the diﬀerentiation of trivalent lanthanide and actinide ions and the synthesis, structure, reactivity and physicochemical properties of Scheme 1 Syntheses of tris(cyclopentadienyl) uranium cyanide uranium complexes. complexes.

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Fig. 1 Crystal structures of the ﬁrst crystallographically characterized f-element cyanide complexes [Sm(Cp*)2(μ-CN)(CNCy)]3 (left) and

[Sm(Cp*)2(μ-CN)]6 (right). As in all subsequent ﬁgures, nitrogen atoms are purple and carbon atoms are dark blue.

Ln = Sm and L = tBuCN)40 synthesized either by oxidation of led to the synthesis of a large number of hetero-polynuclear t 40 [Sm(Cp*)2(THF)2] with BuCN, or the salt metathesis reaction d–4f complexes which have opened new perspectives for the n t 39 of [Ce(Cp*)2I] with N Bu4CN in the presence of BuNC, or study of the magnetic interactions between transition and 37,38 sterically induced reductions of [M(Cp*)3] (M = La, Pr, Sm) lanthanide metal ions through the cyanido bridge, with the t in the presence of Me3SiCN, Me3SiCH2NC, C6H11NC, BuNC or emergence of novel properties of the magnetic materials tBuCN (Scheme 2). It is only in 2010 that the uranium counter- resulting from the large and anisotropic magnetic moment of μ t 12,41–44 part [U(Cp*)2( -CN)(CN Bu)]3 was isolated from the reaction of the paramagnetic metal centres. These complexes, t 38 [U(Cp*)3] with BuNC through N–C bond cleavage. which are also attractive for their potential as multifunctional Significant advances in the design of cyanido-bridged mole- systems combining several properties such as magnetism, cular-based magnetic materials during the last decade have luminescence and catalytic activity, have already been reviewed and will not be further described here. By comparison, such compounds of the actinides are quite uncommon, being Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07. 45,46 limited to a few thorium(IV) tetracyanoplatinates and 45,47 uranyl, actinide(IV) (An = Th, U, Np, Pu) and actinide(III)(An =Am,Cf)hexacyanoferrates.46,48,49 These latter species received special attention for their application in nuclear fuel reproces- sing and subsequent lanthanide or minor actinide separation. When we started our studies on the f-element cyanide complexes in 2007, we were surprised that, in contrast to the numerous cyanometalates, no mononuclear lanthanide complex with a terminal cyanide ligand was reported and only two such compounds of uranium were crystallographically

characterized, i.e. the monocyanides [U(C5Me4H)3- 50 t 51 (CN)0.4(Cl)0.6] and [U(C5 Bu3H2)2(CN)(OSiMe3)] (Fig. 2). The former was serendipitously obtained from the decomposition t of the alkyl isocyanide derivative [U(C5Me4H)3(CN Bu)] while the latter bis(cyclopentadienyl) complex was synthesized by t v treating the oxo precursor [U(C5 Bu3H2)2( O)] with Me3SiCN. As for the d transition metals, the wide range of physicochemical properties oﬀered in developing the cyanide chemistry of uranium and the lanthanides, in particular because of the novel structures and reactions which could be expected from their high coordination numbers and possible contri-

Scheme 2 Syntheses of trinuclear cyanide complexes [Ln(Cp*)2(μ-CN)(L)]3. bution of the f orbitals, were an incitement to explore this

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of unique examples of discrete mono- and polycyanide complexes which revealed unprecedented and unexpected coordination geometries. Until recently, the linear metallocenes of the f-elements were limited to three compounds of the divalent lanthanides (Sm, Eu, and Yb) for which the linear geometry was forced by the steric crowding of the bulky substituents on the cyclopenta- – dienyl rings.52 54 All the other derivatives with small substituents, in particular the bisCp* compounds which are the most popular metallocenes, were found exclusively in a bent- sandwich configuration whatever the nature of the f metal ion, its oxidation state and the charge of the complex. The first

linear Mf(C5Me5)2 compounds (Mf = f-element) were synthesized by filling completely the equatorial girdle of the bisCp* uranium fragment with five neutral or anionic donor t − Fig. 2 Crystal structure of the complex [U(C5 Bu3H2)2(CN)(OSiMe3)]. ligands (MeCN, CN ) and such complexes represent a new type of linear metallocenes only observed at that time with 55,56 uranium. Thus, treatment of [U(Cp*)2X2] [X = I, OSO2CF3

field. However, in order to prevent the formation of insoluble (OTf)] with NR4CN gave successively the bent bis- and tris-

cyanide products, soluble organometallic and amide precur- (cyanide) metallocenes [U(Cp*)2(CN)2] and [NR4][U(Cp*)2- sors with NMR signature and limited number of coordination (CN)3], and the penta(cyanide) derivative [NR4]3[U(Cp*)2(CN)5] sites were at first carefully chosen so as to favor the formation (Fig. 3), which adopts a linear configuration (Scheme 3).34,57,58

of soluble and easily detectable cyanide compounds. By using The bis(cyanide) [U(Cp*)2(CN)2] was afterwards isolated from − the alkali metal or ammonium salts of the CN ion as cyana- the reaction of the fluoride [U(Cp*)2F2(py)] with excess 59 IV tion reagents, we isolated a number of mono- and polycyanide Me3SiCN. The tris and penta(cyanide) U complexes were complexes of cerium, thorium and uranium, in the inorganic found to be in equilibrium in solution.34 In contrast, the bent n and organometallic series. These complexes displayed novel tris(cyanide) metallocenes [N Bu4]2[M(Cp*)2(CN)3](M=Ce,U) and unexpected structures, in particular the uranium com- (Fig. 3) were the sole products isolated from the trivalent pre- μ pounds which proved stable both in low and high oxidation cursors [M(Cp*)2I] or [M(Cp*)2( -CN)]n, even in the presence of states. The three metal ions were found to exhibit major diﬀer- excess cyanide.58 Theoretical calculations showed that the ences in their bonding and reactions with the CN group, uranium(III) complex is not stable in the linear configuration revealing the role of the f electrons and orbitals in the metal– because one electron occupies an antibonding orbital, while cyanide bond. uranium(IV) metallocenes are stable in both the bent and linear configurations.57 More generally, a decreasing number Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07. of 5f electrons in the Mf(Cp*)2 fragment would favor the sub- 2. Linear metallocenes, bent sequent addition of ligands and a transition from bent to 0 actinocenes and half sandwiches linear shape, and the existence of the 5f uranium(VI) complex VI − 60 [U (Cp)2(CN)5] was predicted by DFT calculations. The small size, linear shape and strong coordinating ability of The rapid and reversible electron transfer between the UIII IV 2− − the cyanide ligand have been of major interest in the synthesis and U centres of [U(Cp*)2(CN)3] and [U(Cp*)2(CN)3] was

3− − Fig. 3 Crystal structure of the linear anion [U(Cp*)2(CN)5] (left) and of the bent anion [U(Cp*)2(CN)3] (right).

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Scheme 3 Syntheses of bent and linear cyanide metallocenes.

1 58 revealed by H NMR spectroscopy. Comparison of the crystal Since its discovery in 1968, uranocene [U(Cot)2](Cot= n III 64 structures of [N Bu4]2[M (Cp*)2(CN)3] (M = Ce, U) shows that η-C8H8) and its derivatives in the actinide and lanthanide

the U–C(Cp*) and U–C(CN) distances are shorter, by series all exhibited a D8h symmetry and a disappointingly poor 0.02–0.03 Å, than the Ce–C(Cp*) and Ce–C(CN) distances, reactivity which was explained by their inability to coordinate while the ionic radius of uranium(III)is∼0.02 Å larger than supplementary ligands to the metal centre as a result of the that of cerium(III). Such deviations of U–X distances (X = C, N, steric constraints imposed by the two parallel rings. Therefore, I, P, S) from a purely ionic bonding model, which have been it was generally accepted that a bis(cyclooctatetraenyl) complex observed in a variety of analogous uranium(III) and lanthanide could not adopt a bent configuration. Here again, the cyanide 61,62 (III) complexes, are explained by a stronger, more covalent ion was useful in showing that things could be otherwise since actinide–ligand interaction. In these anionic and monomeric it proved to be an eﬃcient wedge for bending the linear urano- complexes, the shortening of the U–C(CN) bond length with cene and thorocene, with formation of the monocyanide com- 65 66 respect to the corresponding Ce–C(CN) could be an indication plexes [M][An(Cot)2(CN)] (An = Th, U; M = Na(18-crown-6) σ of a stronger -donation of the ligand towards uranium rather or NR4) (Fig. 5). Theoretical studies indicate that despite the than π back-bonding, as supported by the ν(CN) frequencies. broken symmetry, the gain in electrostatic interaction and a Crystals of the mixed valence uranium(III/IV) complex significant uranium–CN orbital interaction are suﬃcient to Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07. μ μ [{U(Cp*)2}2( -CN){( -CN)2Na(thf)}2]∞, which is a 1D coordi- stabilize the bent cyanide complex with respect to uranocene. nation polymer (Fig. 4), were isolated from an attempt to The 6d, and to a less extent 5f, uranium orbitals have a signifi- prepare monomeric polycyanide compounds of trivalent cant participation in the interaction both with the aromatic 58 67 uranium by mixing [U(Cp*)2I] with an excess of NaCN. This rings and the cyanide ligand. However, thorocene was found compound, which certainly resulted from oxidation of an UIII to be much more reactive than uranocene since a series of polycyanide species by traces of air, can be formally viewed as sterically crowded cyanide complexes have been obtained only III IV the association of the U and U complexes from [Th(Cot)2], depending on the Th : CN ratio and the nature III IV + [U (Cp*)2(CN)3{Na(thf)}2] and [U (Cp*)2(CN)2]. Together with of the M cation of the MCN reagent (Scheme 4). The remark- IV the trinuclear compound [{U (Cp*)2Cl2(μ-CN)}2Mg(thf)4] able stability of uranocene compared to cerocene and thoro- (Fig. 4) which was serendipitously formed in a mixture of cene was previously noted and theoretically explained by the 34 [U(Cp*)2Cl2], NaCN and residual traces of MgCl2, the poly- greater covalency due to the larger involvement of 6d and 5f 68,69 meric uranium(III/IV) complex is likely to be the first compound orbitals in the uranium–ligand bonding. The coordinating – u – − exhibiting f-element C N M bridges (M = main group or d ability of [Th(Cot)2(CN)] was demonstrated by the structural transition metal) since in the aforementioned lanthanide and characterization of di-, tri- and polynuclear species with [Th]– 65 actinide cyanometalates, the bridging CN ligands are co- CuN–[Na] bridges in [Th(Cot)2(μ-CN)Na(18-crown-6)] or – u – n μ ordinated to the f-element via the nitrogen atom. The [Th] C N [Th] linkages in [N Bu4][{Th(Cot)2}2( -CN)], n U–NuC–Ag linkage was also found in the first cyanometalate [N Bu4]2[{Th(Cot)2(μ-CN)}2Th(Cot)2] and [NR4][Th(Cot)2(μ-CN)] 70 of uranium(IV), [U(Cp*)2(dmf)3(μ-NC)2(AgI)2·2dmf]∞ (Fig. 4), (Fig. 6). The polymeric arrangement of the latter mono- 34 synthesized by treating [U(Cp*)2I2] with AgCN in dmf, which cyanide complex revealed that not one but two coordination is the first linear metallocene of an f-element with distinct sites are available on a bent Th(Cot)2 fragment, and this was 63 n donor ligands in its equatorial girdle, and which crystallizes confirmed with the synthesis of the biscyanide [N Bu4]2- 70 as a 2D assembly. [Th(Cot)2(CN)2] (Fig. 6).

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Fig. 4 Crystal structures of the cyanido bridged complexes [{U(Cp*)2}2(μ-CN){(μ-CN)2Na(thf)}2]∞ (top, with the Cp* methyl groups and thf carbon

atoms omitted for clarity), [{U(Cp*)2Cl2(μ-CN)}2Mg(thf)4] (middle) and [U(Cp*)2(dmf)3(μ-NC)2(AgI)2·2dmf]∞ (bottom, with the Cp* methyl groups and dmf nitrogen and carbon atoms omitted for clarity). Sodium is blue, magnesium green, silver light blue and iodine brick red.

′ 71,72 ff In contrast to the relative inertness of uranocene, the dis- 4,4 -bipy, R4Phen). From DFT calculations, this di erence tinct reactivity of thorocene was also observed in the presence was accounted for by electrostatic effects, the 5f 0 thorium ion of other anions and neutral mono- and bidentate Lewis bases, being more Lewis acidic than its 5f 2 uranium analogue.71 − ffi − with formation of the bent sandwich complexes [Th(Cot)2N3] , The great a nity of the CN ion for uranium(IV) also − t [{Th(Cot)2}2(μ-H)] and [Th(Cot)2(L)] (L = BuNC, py, 2,2′-bipy, favoured the synthesis of some soluble heteroleptic poly-

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3. Cyanide linkage isomerism in CeIII, UIII and UIV complexes

n III After the bent metallocenes [N Bu4]2[M (Cp*)2(CN)3](M=Ce, U) which were the first trivalent molecular polycyanide compounds of an f-element to have been structurally identified, 58 and the first fully characterized actinide(III) cyanides, new CeIII and UIII cyanide compounds were synthesized by addition n of M′CN [M′ =NR4 with R = Me, Et, Bu or K(18-crown-6)] to 78 the tris(silylamide) precursors [MN*3] (M = Ce, U). Thus the III − cyanido-bridged dinuclear compounds [M′][(M N*3)2(μ-CN)] Fig. 5 Crystal structure of the anion [U(Cot)2(CN)] . and the mononuclear mono-, bis- and tris(cyanide) complexes ′ ′ n [M ][MN*3(CN)], [M ]2[MN*3(CN)2] and [N Bu4]2[MN*2(CN)3] (Scheme 5) were formed successively. The synthesis of the ﬀ cyanides which might o er ways to the homoleptic U(CN)n polycyanide derivatives required the use of the more soluble n compounds. For example, the monoCp* and monoCot com- Bu4CN salt. The bis(cyanide) complexes were found to be in plexes [NEt4]3[U(Cp*)(CN)6], [NEt4]3[U(Cot)(CN)5] and [NEt4]2- equilibrium with the mono(cyanide) complexes in solution

[U(Cot)(CN)4] (Fig. 7) were synthesized by treating the iodide and were slowly transformed into an equimolar mixture of the and amide precursors [U(Cp*)I3(THF)2], [U(Cot)I2(THF)2] and mono- and tris(cyanide) derivatives with elimination of a N*

[U(Cot)(N*)2] [N* = N(SiMe3)2] with NEt4CN, whereas the mixed ligand. The crystal structures of [K(18-crown-6)]2- III amido-cyanide complex [NEt4]3[U(CN)6(NEt2)] was obtained [U N*3(μ-CN)2] and its benzene solvate showed unambigu- 73–76 from the cation [U(NEt2)3][BPh4]. Despite its strong coordi- ously the cyanide ligation mode of the CN ligand to the − nating ability, the CN ion proved incapable of displacing a U atom in the U–CuN–K bridges (Fig. 8). The 1D polymeric

Cot ligand of the actinocenes [An(Cot)2] (An = Th, U), in con- structure of the unsolvated complex arises from the presence − i − trast to the amide and alkoxide groups NEt2 and O Pr which of interactions between adjacent K(18-crown-6) fragments − i 77 reacted with [U(Cot)2] to give [U(Cot)X3] (X = NEt2,OPr). which are disrupted by benzene molecules in the solvate ffi ff III When they are of su cient quality to permit the di eren- [K(18-crown-6)]2[U N*3(μ-CN)2]·2C6H6. The analogous cerium tiation between carbon and nitrogen atoms, the X-ray diffrac- complexes are isomorphous with those of the uranium tion data show that the CN group is attached to the metal via counterparts but not isostructural because of the distinct the carbon atom in all the aforementioned complexes, with cyanide linkage in the Ce–NuC–K bridges (Scheme 6). The the exception of the cyanometalates. Ce–NC bonding mode was also clearly determined in the Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07.

Scheme 4 Syntheses of bent thorocene cyanide complexes.

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Fig. 6 From top to bottom and from left to right: crystal structures of the bent thorocene complexes [Na(18-crown-6)][Th(Cot)2(μ-CN)], − 2− − 2− [{Th(Cot)2}2(μ-CN)] , [{Th(Cot)2(μ-CN)}2Th(Cot)2] , [Th(Cot)2(μ-CN)] and [Th(Cot)2(CN)2] . Thorium atoms are green and sodium atoms blue.

3− 3− Fig. 7 Crystal structures of the mono-Cp* and mono-Cot uranium(IV) cyanide complexes [U(Cp*)(CN)6] (left), [U(Cot)(CN)5] (middle) and 2− [U(Cot)(CN)4] (right).

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Scheme 5 Syntheses of uranium(III) cyanide complexes in the [MN*3] series. Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07.

III III Fig. 8 Crystal structures of the Ce and U cyanide complexes in the MN*3 series [K(18-crown-6)]2[UN*3(μ-CN)2] (top), [K(18-crown- 2− 6)]2[UN*3(μ-CN)2]·2C6H6 (middle) and [CeN*3(CN)2] (bottom). Uranium is yellow, cerium light blue, potassium green and silicium blue.

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Scheme 7 Syntheses of uranium(IV) cyanide complexes in the [MN*3] series.

the U4+ ion, a harder Lewis acid than UIII, displays a greater affinity for the harder nitrogen end of the CN ligand. The same arguments can account for the difference in the crystal 2− Scheme 6 Cyanide linkage isomerism in the anions [MN*3(CN)2] (M = 0 t structures of the thorium(IV)5f complex [Th(C5 Bu3H2)2(NC)- Ce, U). 80 (OSiMe3)] which revealed a Th–NC bonding of the CN ligand t while the uranium(IV) analogue [U(C5 Bu3H2)2(CN)(OSiMe3)] shows a U–CN linkage,51 adifference which has not been com- n mononuclear complex [N Bu4]2[CeN*3(CN)2] (Fig. 8). The strik- mented on. DFT calculations on the actual complexes and q − ing difference between the cyanide and isocyanide ligation their hypothetical counterparts [UN*3X] (q = 1, 0) and q modes of the CN ligand in the uranium and cerium species [UN*3X2] (q = −2, −1) (X = CN or NC) show that the stronger can be explained by the UIII ion being softer than the CeIII ion σ-donating ability of cyanide and isocyanide towards the UIII/UIV in the HSAB classification and having a greater affinity for the pair is governed by the best energy matching between 6d/5f softer carbon end of the CN ligand. The preferential coordi- metal and ligand orbitals and covalency contribution (orbital nation of the cyanide and isocyanide ligands towards uranium mixing). This latter effect seems to play a more significant role or cerium in the trivalent bis(cyanide) complexes was corro- in the observed UIII–CN than in the UIV–NC coordination.79 borated by considering the binding energies of these ligands to Very recently, the U–NC bonding was unveiled by the infra- the metal ions and by the comparison of the DFT optimized red spectra and electronic structure calculations of the MeU- Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07. v geometries and the structural crystal data. The electronic struc- (NC) and CH2 U(H)(NC) complexes and the series of U(NC), ture analysis showed that the σ-donating ability of the cyanide U(NC)2 and U(NC)4 compounds obtained by reactions of laser- ligand is stronger with uranium than with cerium cations, due ablated uranium atoms with acetonitrile81 and cyanogen,82 to the better energy matching between 6d/5f metal and ligand respectively, in argon matrices at 4 K. These results indicate orbitals. This difference plays a significant role in the metal– that the isocyanides bond more strongly to the uranium in the ligand coordination preference, leading to a non-negligible +1 to +4 oxidation states than the cyanides when no other covalent character of the bonding in the former case. ligands are present. It is interesting to note that the metal–iso- IV The uranium(IV) analogues of these trivalent cyanide com- cyanide coordination mode was also predicted for the Ti 4−n – plexes in the UN*3 series, [(UN*3)2(μ-CN)][BPh4], [UN*3(CN)] compounds [Ti(CN)n] (n =16), except for n = 6 where the 83 and [M][UN*3(CN)2] (M = NEt4 or K(THF)4) were synthesized by cyanide isomer is preferred.

successive addition of NEt4CN or KCN to the cationic precur- However, the cyanide ligands were found to be attached via IV 84 sor [U N*3][BPh4] (Scheme 7), while crystals of [K(18-crown-6)]- the carbon atom in [NEt4][UN*(N,N)(CN)2] [N,N = (Me3Si)NSi- IV – – [U N*3(CN)2] were obtained by oxidation of [K(18-crown-6)]- Me2CH2CH2SiMe2N(SiMe3)] (Fig. 10) where the N U N angles III 79 ﬀ [U N*3(CN)] with pyridine N-oxide. The striking structural between the amide groups di er from those measured in feature of these complexes (Fig. 9) is the coordination of the [K(18-crown-6)][UN*3(CN)2]. Similar distortions are observed in CN group to the U4+ ion through the nitrogen atom, which is the structure of the dinuclear UIII complex [Na(15-crown-5)]- 85 opposite to the cyanide ligation mode observed in the [{UN*(N,N)}2(μ-CN)] (Fig. 10), in comparison with that of 77 uranium(III) counterparts. These are the first crystallographi- [K(18-crown-6)][(UN*3)2(μ-CN)]. These compounds belong to cally characterized complexes with a U–NC linkage. Here a series of cyanide complexes of (N,C), (N,N) and (O,N) metalla- again, the distinct UIV–N and UIII–C ligation modes of the CN cycles of tri-, tetra- and pentavalent uranium which are deriva-

ligand in these couples of isocyanide and cyanide complexes tives of the (N,C) metallacycles [UN*2(N,C)] and [NaUN*(N,C)2]

can be explained from the HSAB classification by the fact that [N,C =CH2SiMe2N(SiMe3)], as detailed in Scheme 8. The dis-

Dalton Transactions Perspective

IV + Fig. 9 Crystal structures of the U cyanide complexes in the MN*3 series [(UN*3)2(μ-CN)] (top left), [UN*3(CN)] (top right) and [K(THF)4][UN*3(CN)2] (bottom). Uranium atoms are yellow and potassium atoms green. Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07.

− − Fig. 10 Crystal structures of the anions [{UN*(N,N)}2(μ-CN)] (left) and [UN*(N,N)(CN)2] (right).

− tinct U–CN coordination mode in [UN*(N,N)(CN)2] in compar- hypothetical isocyanide complexes is small. More generally, – − ﬃ – III IV ison with U NC in [UN*3(NC)2] is di cult to explain because the U CN/NC coordination preference towards the U /U pair the energy diﬀerence between the actual cyanide and is related to the subtle balance between steric, covalent

Perspective Dalton Transactions Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07.

Scheme 8 Cyanide complexes of (N,C), (N,N) and (N,O) metallacycles of tri-, tetra- and pentavalent uranium.

− 2+ and ionic factors that govern U–CN/NC bonding. However, nation and bonding of the CN group to {UO2} , via U–Cor in all cases, DFT computations and in particular the U–N bonding, was further questioned88,89 and, on the basis of 3− calculated total bonding energies between the metal and the DFT calculations, the penta(cyanide) derivative [UO2(CN)5] cyanide ligand allowed the prediction of the observed coordi- was predicted to be the most stable species in aqueous solu- nation mode.79 tion.89 To conclude the debate, a few uranyl cyanide complexes have been recently characterized. The U–CN ligation mode of the cyanide ion was unambiguously determined in the crystal n 4. High-valent uranium cyanide structure of [N Bu4]3[UO2(CN)5] (Fig. 11) which was obtained n 90 – complexes by reaction of [UO2(OTf)2] with N Bu4CN. The mean U C- (CN) distance is larger than expected by ca. 0.3–0.4 Å The absence of any identified uranyl cyanide complex was sur- when compared to the mean U–C(CN) bond length of 2.62(3) prising in view of the large number of studies devoted to the Å in the eleven-coordinate and geometrically similar IV 57 halides and pseudo-halides (N3, NCS, NCO) of the ubiquitous trianionic U complex [NEt4]3[U(Cp*)2U(CN)5], suggesting 2+ 86 IV– trans-dioxo uranyl(VI) ion, {UO2} . Prior to any experimental a stronger U CN interaction, in agreement with the IR results, the cyanide ligand was claimed to be anathema to data. uranium(VI) because the carbon end of CN was considered as a The aﬃnity of the cyanide ion for the uranyl fragment was strong σ donor and a good π acceptor.87 However, the coordi- also demonstrated by the formation of the cis-amido cyanide

Dalton Transactions Perspective

3− 2− Fig. 11 Crystal structures of the anions [UO2(CN)5] (left) and [UO2(Cp*)(CN)3] (right).

derivative [NEt4]2[UO2(N*)2(CN)2] resulting from addition of ligands which generally stabilize the high metal oxidation 91 IV NEt4CN to [UO2(N*)2] in pyridine. states, the bis metallacyclic complex [NaU N*(O,N)2][O,N = v Stabilization of high-valent organouranium complexes, OC( CH2)SiMe2N(SiMe3)] was considered as a potential pre- 2+ V VI especially those of the trans-dioxo uranyl ion {UO2} which is cursor of U and U compounds. Actually, its reaction with I2 V the most stable species in the +6 oxidation state, is a long- aﬀorded the diuranium(V) “ate” complex [Na][{U N*-

standing challenge. The first cyclopentadienyl uranyl complex, (N,O)2}2(μ-I)] which was transformed into the cyanide V [NEt4]3[UO2(Cp*)(CN)3], was synthesized by oxidation of the [M][U N*-(N,O)2(CN)] [M = NEt4, Na(15-crown-5)] in the pres- IV 84 linear U metallocene [NEt4]3[U(Cp*)2(CN)5] with pyridine ence of MCN (Scheme 8, Fig. 12). However, the tris(amido) V N-oxide. (Scheme 9) Its structure (Fig. 11) represents a novel bis(cyanide) complex [U N*3(CN)2] could not be isolated either IV 79 coordination geometry for the uranyl ion which almost invari- from one electron oxidation of [NEt4][U N*3(CN)2] or substi- 92 V ably adopts a polygonal bipyramidal configuration. It is note- tution of the halide ligands of [U N*3X2] (X = F, Cl, Br) with n worthy that cyclopentadienyl uranyl compounds could not be Me3SiCN or N Bu4CN, a failure which was explained by un- 2+ 94 synthesized by reaction of uranyl {UO2} salts with cyclopenta- favorable thermodynamic factors. dienyl anions, which is in fact a convenient route to penta- At last, the third UV cyanide compound to have been + 93 valent derivatives of the {UO2} ion. reported is [NEt4][UN*3(vO)(CN)] (Fig. 12), readily obtained by V 94 Uranium(V) complexes are reputed to undergo facile dispro- addition of NEt4CN to the oxo precursor [U N*3(vO)]. The Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07. portionation into UIV and UVI derivatives and the factors which U–CN distance of 2.491(7) Å, shorter than that measured in n V 57 V determine their stability are not well understood. The cyanide [N Bu4]2[U (Cp*)2(CN)5] (2.548(7) Å) and [U N*(N,O)2(μ-CN)- ligand proved to be eﬃcient in the stabilization of the +5 oxi- Na(15-crown-5)] (2.567(7) Å),84 was attributed to the inverse n 94–96 dation state, as shown by the linear metallocene [N Bu4]2- trans influence stabilization imparted by the oxo ligand. IV [U(Cp*)2(CN)5] obtained by oxidation of the parent U precursor by traces of oxygen.57 In the 5f 1 UV complex, only one out- of-plane f orbital is occupied, which minimizes the electronic − 5. Conclusion repulsion with the CN lone pair and thus favours the linear form. The chemistry of cyanide complexes of the f-elements wit- Owing to its anionic character which would favour its ready nessed significant advances during the last decade, confirm- one electron oxidation and the presence of amide and alkoxide ing the remarkable coordinating capacity of the small-sized and linearly shaped CN group. The cyanide ligand was useful in the synthesis of a new type of linear metallocenes and of 3− the first actinocenes with a bent geometry, [U(Cp*)2(CN)5] − and [An(Cot)2(CN)] (An = Th, U), thus disproving the generally accepted ideas on the stability and reactivity of these complexes. Most notable is the capacity of the cyanide group to highlight the greater reactivity of thorocene in comparison with uranocene, through occupancy of a second coordination 2− site in the bis(cyanide) [Th(Cot)2(CN)2] and its derivatives. The ability of the cyanide ligand to stabilize uranium com- 3− Scheme 9 Synthesis of the uranyl cyanide [UO2(Cp*)(CN)3] . pounds from the +3 to +6 oxidation states was assessed, and

Perspective Dalton Transactions

V − Fig. 12 Crystal structures of the U monocyanide complex [UN*(N,O)2(CN)Na(15-crown-5)] (left) and the anion [UON*3(CN)] (right).

novel high-valent UV and UVI complexes in the inorganic and 3 A. Ludi and H. U. Güdel, in Inorganic Chemistry, Springer, organometallic series were synthesized. The cyanide ligand 1973, pp. 1–21. was very eﬃcient in the diﬀerentiation of the CeIII,UIII and UIV 4 P. Rigo and A. Turco, Coord. Chem. Rev., 1974, 13, 133.

metal centres in the addition reactions with [MN*3](M=Ce, 5 W. P. Griffith, Coord. Chem. Rev., 1975, 17, 177. + U) and [UN*3] , leading to a large series of mononuclear 6 A. G. Sharpe, in Comprehensive Coordination Chemistry, ed. mono-, bis- and tris(cyanide) complexes in which the UIII–CN, G. Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon, CeIII–NC and UIV–NC linkages were observed. These distinct Oxford, 1987, vol. 2, ch. 12-1, pp. 7–14. coordination modes could be accounted for by DFT compu- 7 T. P. Hanusa and D. J. Burkey, in Encyclopedia of Inorganic tations, in particular the calculated total bonding energies Compounds, ed. R. B. King, J. Wiley and Sons, Chichester, between the metal and the cyanide ligand, showing that the 1994, vol. 2, pp. 943–949. U–CN/NC coordination preference towards the UIII/UIV pair is 8 H. Vahrenkamp, A. Geiss and G. N. Richardson, J. Chem. related to the subtle balance between steric, covalent and ionic Soc., Dalton Trans., 1997, 3643. − factors. More generally, while the CN ion acts as a strong 9 M. Verdaguer, A. Bleuzen, V. Marvaud, J. Vaissermann, Published on 16 March 2015. Downloaded by CEA Saclay 05/05/2015 06:37:07. σ donor and a weak π acceptor ligand with the d transition M. Seuleiman, C. Desplanches, A. Scuiller, C. Train, metals,97 IR spectra and DFT analysis of the f-element cyanide R. Garde, G. Gelly, C. Lomenech, I. Rosenman, P. Veillet, complexes indicate mainly cyanide to metal σ donation with C. Cartier and F. Villain, Coord. Chem. Rev., 1999, 190–192, no π back-donation effects, in contrast to the CO ligand in the 1023. uranium metallocene derivatives where metal–ligand back- 10 M. Ohba and H. Ōkawa, Coord. Chem. Rev., 2000, 198, 313. bonding was observed.98,99 11 P. Przychodzen, T. Korzeniak, R. Podgajny and Further studies including theoretical analysis are necessary B. Sieklucka, Coord. Chem. Rev., 2006, 250, 2234. to specify the influence of the nature of the metal and ancillary 12 S. Tanase and J. Reedijk, Coord. Chem. Rev., 2006, 250, ligands on the coordination mode of the cyanide ligand in 2501. complexes of the f-elements. Of special interest are the mono- 13 S. Wang, X. H. Ding, Y. H. Li and W. Huang, Coord. Chem. nuclear compounds with terminal cyanide ligands which Rev., 2012, 256, 439. could be used as valuable building blocks for the design of 14 M. Ephritikhine, Dalton Trans., 2006, 2501. novel clusters and coordination polymers with interesting 15 S. T. Liddle and D. P. Mills, Dalton Trans., 2009, 5592. physicochemical properties. 16 O. P. Lam, C. Anthon and K. Meyer, Dalton Trans., 2009, 9677. 17 O. P. Lam and K. Meyer, Polyhedron, 2012, 32,1. References 18 R. J. Baker, Coord. Chem. Rev., 2012, 256, 2843. 19 R. J. Baker, Chem. – Eur. J., 2012, 18, 16258. 1 W. P. Griffith, Q. Rev. Chem. Soc., 1962, 16, 188. 20 T. W. Hayton, Chem. Commun., 2013, 49, 2956. 2 B. M. Chadwick and A. G. Sharpe, in Advances in Inorganic 21 B. M. Gardner and S. T. Liddle, Eur. J. Inorg. Chem., 2013, Chemistry and Radiochemistry, ed. H. J. Emeléus and 3753. A. G. Sharpe, Academic Press, 1966, vol. 8, pp. 83–176. 22 M. B. Jones and A. J. Gaunt, Chem. Rev., 2013, 113, 1137.

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