Comparative structural studies of iodide complexes of uranium(III) and lanthanide(III) with hexadentate tetrapodal neutral N-donor ligands

The syntheses, the solution structures, and the crystal structures of the two new tetrapodal N-donor ligands N,N,N',N'-tetrakis(2-pyrazylmethyl)-1,3-trimethylenediamine (tpztn), 1, and N,N,N',N'-tetrakis(2-pyrazylmethyl)-trans-1,2-cyclohexanediamine (tpzcn), 2, are described. Two different geometric isomers of the cation [La(tpztn)I(2)](+) were isolated in which the ligand adopts two different conformations leading to strong differences in the metal-ligand bond distances. The crystal structure of isostructural complexes of La, U, Ce, and Nd were determined by X-ray diffraction studies for the ligands tpztn and tpzcn. In both series of complexes the two methylpyrazyl arms and the diamine spacer (trimethylene or cyclohexane) around each aliphatic nitrogen adopt the same helical configuration. The complexes crystallize as a racemic mixture of Lambda,Lambda and Delta,Delta enantiomers with distorted square antiprism geometries. In these complexes the M-N(pyrazine) distances show a decrease from La to Ce and from La to Nd which corresponds well to the decrease in ionic radius as expected in a purely ionic bonding model. Conversely the mean value of the U-N(pyrazine) distances is shorter (0.043(3) A for tpztn and 0.054(11) A for tpzcn) than the mean value of the La-N(pyrazine) distances. These differences are significantly larger than the decrease expected from the variation of the ionic radii and can be interpreted in terms of a stronger M-N interaction for U(III). Previously reported extraction studies have shown that while the tripod tris[(2-pyrazyl)methyl]amine (tpza) containing three pyrazyl nitrogens extracts An(III) preferentially to Ln(III), tpztn and tpzcn display no selectivity despite the presence of four pyrazyl groups connected to a different spacer. The structural studies described here show that despite the lack of selectivity observed in the extraction conditions, the arrangement of pyrazyl nitrogens in the tetrapodal architectures of tpztn and tpzcn allows for metal-ligand interaction similar to that observed for tpza.     

U(SMes*)n, (n = 3, 4) and Ln(SMes*)3 (Ln = La, Ce, Pr, Nd): lanthanide(III)/actinide(III) differentiation in agostic interactions and an unprecedented eta3 ligation mode of the arylthiolate ligand, from X-ray diffraction and DFT analysis

Reaction of U(NEt(2))(4) with HS-2,4,6-(t)Bu(3)C(6)H(2) (HSMes) gave U(SMes)(3)(NEt(2))(py) (1), whereas similar treatment of U[N(SiMe(3))SiMe(2)CH(2)][N(SiMe(3))(2)](2) afforded U(SMes)[N(SiMe(3))(2)](3) (2) and U(SMes)(3)[N(SiMe(3))(2)]. The first neutral homoleptic uranium(IV) thiolate to have been crystallographically characterized, U(SMes)(4) (4), was isolated from the reaction of U(BH(4))(4) and KSMes. The first homoleptic thiolate complex of uranium(III), U(SMes)(3) (5), was synthesized by protonolysis of U[N(SiMe(3))(2)](3) with HSMes in cyclohexane. The crystal structure of 5 exhibits the novel eta(3) ligation mode for the arylthiolate ligand. Comparison of the crystal structure of 5 with those of the isomorphous lanthanide congeners Ln(SMes)(3) (Ln = La, Ce, Pr, and Nd) indicates that the U-S, U-C(ipso)(), and U-C(ortho)() bond lengths are shorter than the corresponding ones in the 4f-element analogues, when taking into account the variation in the ionic radii of the metals. The distance between the uranium and the carbon atoms involved in the U...H-C epsilon agostic interaction of each thiolate ligand is shorter, by approximately 0.05 A, than that expected from a purely ionic bonding model. The lanthanide(III)/actinide(III) differentiation was analyzed by density functional theory (DFT). The nature of the M-S bond is shown to be ionic strongly polarized at the sulfur for M = U and iono-covalent (i.e. strongly ionic with low orbital interaction), for M = Ln. The strength of the U...H-C epsilon agostic interaction is proposed to be controlled by the maximization of the interaction between U(+) and S(-) under steric constraints. The eta(3) ligation mode of the arylthiolate ligand is also obtained from DFT.     

Selective complexation of uranium(III) over lanthanide(III) triflates by 2,2':6',2"-terpyridine. X-Ray crystal structures of [M(OTf)3(terpy)2] and [M(OTf)2(terpy)2(py)][OTf](M = Nd, Ce, U) and of polynuclear mu-oxo uranium(IV) complexes resulting from hydrolysis

Reactions of Ln(OTf)3(Ln = Ce, Nd) or [U(OTf)3(dme)2](OTf = OSO2CF3, dme = dimethoxyethane) with 2 mol equivalents of 2,2':6',2"-terpyridine (terpy) in pyridine or acetonitrile led to the quantitative formation of the bis(terpy) complexes which crystallized as the discrete cation-anion pairs [M(OTf)2(terpy)2(py)][OTf] x 0.5py from pyridine or neutral derivatives [M(OTf)3(terpy)2] x nMeCN from acetonitrile (M = Ce, Nd, U). The crystal structures of these complexes show the differences in the M-O bond lengths to follow the variation of the ionic radii of the metals, while the U-N(terpy) and U-N(py) bonds are shorter than those expected from a purely ionic bonding model. The better affinity of terpy for U(III) over Ce(III) and Nd(III) was evidenced by the thermodynamic parameters (K, DeltaH, DeltaS) corresponding to the equilibrium between the bis- and tris(terpy) complexes in acetonitrile. Hydrolysis of the bis(terpy) compounds followed different courses; whereas the aquo compound [Ce(OTf)2(terpy)2(H2O)][OTf] crystallized readily from pyridine, the uranium complexes [UX2(terpy)2(py)]X (X = I, OTf) were oxidized into the tri- and tetranuclear mu-oxo U(IV) compounds [{UI(terpy)2(mu-O)}2{UI2(terpy)}]I4 x 2MeCN x H2O and [{U(OTf)(terpy)2(mu-O)(mu-OTf)U(terpy)}2(mu-OTf)2(mu-O)][OTf]4 x py x MeCN. The crystal structures of these first examples of uranium(IV) compounds with terpy ligands show the almost linear arrangement of the metal atoms.     

The distinct affinity of cyclopentadienyl ligands towards trivalent uranium over lanthanide ions. Evidence for cooperative ligation and back-bonding in the actinide complexes

The mono and bis(cyclopentadienyl) compounds [M(C5H4Bu t)I2] and [M(C5H4Bu t)2I](M = U, La, Ce, Nd) were formed in thf by comproportionation reactions of [M(C5H4Bu t)3] and LnI3 or [UI3(L)4](L = thf or py) in the molar ratio of 1 : 2 and 2 : 1, respectively, while treatment of [UI(3)(py)(4)] or LnI(3)(Ln = La, Ce, Nd) with 1 or 2 mol equivalents of LiC5H4Bu t in thf afforded the [M(C5H4Bu t)I2] and [M(C5H4Bu t)2I2]- compounds, respectively. The X-ray crystal structures of [M(C5H4Bu t)I2(py)3](M = U, La, Ce, Nd), [{Ce(C5H4Bu t)2(mu-I)}2] and [M(C5H4Bu t)2I(py)2](M = U, Nd) have been determined; the differences between the average M-C distances in the mono(cyclopentadienyl) complexes correspond to the variation in the ionic radii of the trivalent uranium and lanthanide ions while the U-N and U-I bond lengths seem to be smaller than those predicted from a purely ionic bonding model. The distinct affinity of the cyclopentadienyl ligands towards Ln(III) and U(III) was revealed by two series of competing reactions: the ligand exchange reactions between [Ln(C5H4Bu t)(n')I(3-n')](Ln = La, Ce, Nd) and [U(C5H4Bu t)(n')I(3-n')] species (1 < or = n'+n' =n < or = 5), and the addition of n mol equivalents of LiC(5)H(4)Bu(t)(1 [less-than-or-equal]n[less-than-or-equal] 5) to a 1 : 1 mixture of LnI3 and [UI3(thf)4] or [UI3(py)4]. The stability of the [M(C5H4Bu t)I2] species was found to vary in the order Nd > Ce > U > La, a trend which is in accord with an electrostatic bonding model. However, the bis and tris(cyclopentadienyl) complexes of uranium are more stable than their lanthanide analogues. This difference can be accounted for by a higher degree of covalency in the U-C5H4Bu t bond, resulting from the late appearance of back-bonding which would emerge only after the first cyclopentadienyl ligand is bound.     

Uranium tris-aryloxide derivatives supported by triazacyclononane: engendering a reactive uranium(III) center with a single pocket for reactivity

The synthesis and spectroscopic characterization of the mononuclear uranium complex [((ArO)(3)tacn)U(III)(NCCH(3))] is reported. The uranium(III) complex reacts with organic azides to yield uranium(IV) azido as well as uranium(V) imido complexes, [((ArO)(3)tacn)U(IV)(N(3))] and [((ArO)(3)tacn)U(V)(NSi(CH(3))(3))]. Single-crystal X-ray diffraction, spectroscopic, and computational studies of this analogous series of uranium tris-aryloxide complexes supported by triazacyclononane are described. The hexadentate, tris-anionic ligand coordinates to the large uranium ion in unprecedented fashion, engendering coordinatively unsaturated and highly reactive uranium centers. The macrocyclic triazacyclononane tris-aryloxide derivative occupies six coordination sites, with the three aryloxide pendant arms forming a trigonal plane at the metal center. DFT quantum mechanic methods were applied to rationalize the reactivity and to elucidate the electronic structure of the newly synthesized compounds. It is shown that the deeply colored uranium(III) and uranium(V) species are stabilized via pi-bonding interaction, involving uranium f-orbitals and the axial acetonitrile and imido ligand, respectively. In contrast, the bonding in the colorless uranium(IV) azido complex is purely ionic in nature. The magnetism of the series of complexes with an [N3O3-N(ax)] core structure and oxidation states +III, +IV, and +V is discussed in context of the electronic structures.     

Lanthanide(III)/actinide(III) differentiation in coordination of azine molecules to tris(cyclopentadienyl) complexes of cerium and uranium

Reaction of azine molecules L with the trivalent metallocenes [M(C5H4R)3](M = Ce, U; R = But, SiMe3) in toluene gave the Lewis base adducts [M(C5H4R)3(L)](L = pyridine, 3-picoline, 3,5-lutidine, 3-chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine), except in the cases of M = U and L = 3-chloropyridine, pyridazine, pyrazine and s-triazine where oxidation of U(III) was found to occur. In the pairs of analogous compounds of Ce(III) and U(III), i.e.[M(C5H4But)3(L)](L = pyridine, picoline) and [M(C5H4SiMe3)3(L)](L = pyridine, lutidine, pyrimidine and dimethylpyrazine), the M-N and average M-C distances are longer for M = Ce than for M = U; however, within a series of azine adducts of the same metallocene, no significant variation is noted in the M-N and average M-C distances. The equilibria between [M(C5H4R)3], L and [M(C5H4R)3(L)] were studied by 1H NMR spectroscopy. The stability constants of the uranium complexes, KUL, are greater than those of the cerium counterparts, KCeL. The values of KML are much greater for R = SiMe3 than for R = But and a linear correlation is found between the logarithms of KML and the hydrogen-bond basicity pKHB scale of the azines. Thermodynamic parameters indicate that the enthalpy-entropy compensation effect holds for these complexation reactions. Competition reactions of [Ce(C5H4R)3] and [U(C5H4R)3] with L show that the selectivity of L in favour of U(III) increases with the [small pi] donor character of the metallocene and is proportional to the pi accepting ability of the azine molecule, measured by its reduction potential.     

Computational study of analogues of the uranyl ion containing the -N=U=N- unit: density functional theory calculations on UO2(2+), UON+, UN2, UO(NPH3)3+, U(NPH3)2(4+), [UCl4[NPR3]2] (R = H, Me), and [UOCl4[NP(C6H5)3]

The electronic and geometric structures of the title species have been studied computationally using quasi-relativistic gradient-corrected density functional theory. The valence molecular orbital ordering of UO2(2+) is found to be pi g < pi u < sigma g < sigma u (highest occupied orbital), in agreement with previous experimental conclusions. The significant energy gap between the sigma g and sigma u orbitals is traced to the "pushing from below" mechanism: a filled-filled interaction between the semi-core uranium 6p atomic orbitals and the sigma u valence level. The U-N bonding in UON+ and UN2 is significantly more covalent than the U-O bonding in UON+ and UO2(2+). UO(NPH3)3+ and U(NPH3)2(4+) are similar to UO2(2+), UON+, and UN2 in having two valence molecular orbitals of metal-ligand sigma character and two of pi character, although they have additional orbitals not present in the triatomic systems, and the U-N sigma levels are more stable than the U-N pi orbitals. The inversion of U-N sigma/pi orbital ordering is traced to significant N-P (and P-H) sigma character in the U-N sigma levels. The pushing from below mechanism is found to destabilize the U-N f sigma molecular orbital with respect to the U-N d sigma level in U(NPH3)2(4+). The uranium f atomic orbitals play a greater role in metal-ligand bonding in UO2(2+), UN2, and U(NPH3)2(4+) than do the d atomic orbitals, although, while the relative roles of the uranium d and f atomic orbitals are similar in UO2(2+) and U(NPH3)2(4+), the metal d atomic orbitals have a more important role in the bonding in UN2. The preferred UNP angle in [UCl4(NPR3)2] (R = H, Me) and [UOCl4(NP(C6H5)3)]- is found to be close to 180 degrees in all cases. This preference for linearity decreases in the order R = Ph > R = Me > R = H and is traced to steric effects which in all cases overcome an electronic preference for bending at the nitrogen atom. Comparison of the present iminato (UNPR3) calculations with previous extended Hückel work on d block imido (MNR) systems reveals that in all cases there is little or no preference for linearity over bending at the nitrogen when R is (a) only sigma-bound to the nitrogen and (b) sterically unhindered. The U/N bond order in iminato complexes is best described as 3.     

The uranium-nitrogen bond in U IV complexes supported by the hydrotris(3,5-dimethylpyrazolyl)borate ligand

Insertion of benzonitrile and acetonitrile into the U-C bond of [U(Tp(Me2))Cl(2)(CH(2)SiMe(3))](Tp(Me2)= HB(3,5-Me(2)pz)(3)) gives the ketimide complexes [U(Tp(Me2))Cl(2){NC(R)(CH(2)SiMe(3))}](R = Ph (1); Me (2)). The identity of complex was ascertained by a single-crystal X-ray diffraction study. In the solid state exhibits octahedral geometry with a short U-N bond length to the ketimide ligand. We also report herein the synthesis and the X-ray crystal structures of the uranium amide complexes [U(Tp(Me2))Cl(2)(NR(2))](R = Et (3); Ph (4)). A detailed comparison of the U-N bond lengths in these compounds with other known U-N (and Th-N) distances in amide and ketimide actinide(IV) complexes is performed, confirming the short character of the U-N bond length in 1.     

Synthesis and luminescence studies of mono- and C3-symmetric, tris(ligand) complexes of Sm(III), Y(III) and Eu(III) with sulfur-bridged binaphtholate ligands

A series of trivalent mono- and tris(ligand) lanthanide complexes of a sulfur-bridged binaphthol ligand [1,1'-S(2-HOC(10)H(4)Bu(t)(2)-3,6)(2)] H(2)L(SN), have been prepared and characterised both structurally and photophysically. The H(2)L(SN) ligand provides an increased steric bulk and offers an additional donor atom (sulfur) as compared with 1,1'-binaphthol (BINOL), a ligand commonly used to complex Lewis acidic lanthanide catalysts. Reaction of the diol H(2)L(SN) with [Sm[N(SiMe(3))(2)](3)] affords silylamido- and amino- derivatives [Sm(L(SN))[N(SiMe(3))(2)][HN(SiMe(3))(2)]] and the crystallographically characterised [Sm(L(SN))[N(SiMe(3))(2)](thf)(2)] with different degrees of structural rigidity, depending on the presence of coordinating solvents. The binaphthyl groups of the L(SN) ligand act as sensitisers of the metal centred emission, which is observed for the Eu(III) and Sm(III) complexes studied. We have therefore sought to use emission spectroscopy as a non-invasive technique to monitor a monomer-dimer equilibrium in these complexes. A dramatic difference between the emission properties of the unreactive dimeric Sm(III) aryloxide complex, the solvated monomeric analogues and the amido adduct demonstrated the potential use of such a technique. For a few representative lanthanides (Ln = Sm, Eu and Y) the reaction of the dilithium salt Li(2)L(SN) with either [Ln[N(SiMe(3))(2]3)] or [LnCl(3)(thf)(3)] affords only the homoleptic complex [Li(S)(3)][LnL(SN)(3)](S = thf or diethyl ether); we report the structural characterisation of the Sm complex. However, the reactions of this dipotassium salt K(2)L(SN) with [Sm[N(SiMe(3))(2)](3)] or [SmCl(3)(thf)(3)] give only [SmL(SN)N(SiMe(3))(2)], or intractable mixtures respectively, in which no (tris)binaphtholate is observed. The only isolable lanthanide-L(SN) halide adduct so far is [YbL(SN)I(thf)].     

Lanthanide complexes of tritopic bis(hydrazone) ligands: single-molecule magnet behavior in a linear Dy(III)3 complex

Tritopic pyridinebis(hydrazone)-based ligands typically produce square M(9) [3 × 3] grid complexes with first-row transition-metal ions (e.g., M = Mn, Fe, Co, Cu, Zn), but with larger lanthanide ions, such coordination motifs are not produced, and instead linear trinuclear complexes appear to be a preferred option. The reaction of 2pomp [derived from pyridine-2,6-bis(hydrazone) and 2-acetylpyridine] with La(III), Gd(III), and Dy(III) salts produces helical linear trinuclear [Ln(3)(2pomp)(2)]-based complexes, where each metal ion occupies one of the three tridentate ligand pockets. Two ligands encompass the three metal ions, and internal connections between metal ions occur through μ-O(hydrazone) bridges. Coligands include benzoate, nitrate, and N,N-dimethylformamide. The linear Dy(III)(3) complex exhibits single-molecule magnet behavior, demonstrated through alternating-current susceptibility measurements. Slow thermal magnetic relaxation was detected in an external field of 1800 Oe, where quantum-tunneling effects were suppressed (U(eff) = 14 K).     

Lanthanide(III)/actinide(III) differentiation in the cerium and uranium complexes [M(C5Me5)2(L)]0,+ (L=2,2'-bipyridine, 2,2':6',2''-terpyridine): structural, magnetic, and reactivity studies

Treatment of [Ce(Cp*)(2)I] or [U(Cp*)(2)I(py)] with 1 mol equivalent of bipy (Cp*=C(5)Me(5); bipy=2,2'-bipyridine) in THF gave the adducts [M(Cp*)(2)I(bipy)] (M=Ce (1 a), M=U (1 b)), which were transformed into [M(Cp*)(2)(bipy)] (M=Ce (2 a), M=U (2 b)) by Na(Hg) reduction. The crystal structures of 1 a and 1 b show, by comparing the U-N and Ce-N distances and the variations in the C-C and C-N bond lengths within the bidentate ligand, that the extent of donation of electron density into the LUMO of bipy is more important in the actinide than in the lanthanide compound. Reaction of [Ce(Cp*)(2)I] or [U(Cp*)(2)I(py)] with 1 mol equivalent of terpy (terpy=2,2':6',2'-terpyridine) in THF afforded the adducts [M(Cp*)(2)(terpy)]I (M=Ce (3 a), M=U (3 b)), which were reduced to the neutral complexes [M(Cp*)(2)(terpy)] (M=Ce (4 a), M=U (4 b)) by sodium amalgam. The complexes [M(Cp*)(2)(terpy)][M(Cp*)(2)I(2)] (M=Ce (5 a), M=U (5 b)) were prepared from a 2:1 mixture of [M(Cp*)(2)I] and terpy. The rapid and reversible electron-transfer reactions between 3 and 4 in solution were revealed by (1)H NMR spectroscopy. The spectrum of 5 b is identical to that of the 1:1 mixture of [U(Cp*)(2)I(py)] and 3 b, or [U(Cp*)(2)I(2)] and 4 b. The magnetic data for 3 and 4 are consistent with trivalent cerium and uranium species, with the formulation [M(III)(Cp*)(2)(terpy(*-))] for 4 a and 4 b, in which spins on the individual units are uncoupled at 300 K and antiferromagnetically coupled at low temperature. Comparison of the crystal structures of 3 b, 4 b, and 5 b with those of 3 a and the previously reported ytterbium complex [Yb(Cp*)(2)(terpy)] shows that the U-N distances are much shorter, by 0.2 A, than those expected from a purely ionic bonding model. This difference should reflect the presence of stronger electron transfer between the metal and the terpy ligand in the actinide compounds. This feature is also supported by the small but systematic structural variations within the terdentate ligands, which strongly suggest that the LUMO of terpy is more filled in the actinide than in the lanthanide complexes and that the canonical forms [U(IV)(Cp*)(2)(terpy(*-))]I and [U(IV)(Cp*)(2)(terpy(2-))] contribute significantly to the true structures of 3 b and 4 b, respectively. This assumption was confirmed by the reactions of complexes 3 and 4 with the H(.) and H(+) donor reagents Ph(3)SnH and NEt(3)HBPh(4), which led to clear differentiation of the cerium and uranium complexes. No reaction was observed between 3 a and Ph(3)SnH, while the uranium counterpart 3 b was transformed in pyridine into the uranium(IV) compound [U(Cp*)(2){NC(5)H(4)(py)(2)}]I (6), where NC(5)H(4)(py)(2) is the 2,6-dipyridyl(hydro-4-pyridyl) ligand. Complex 6 was further hydrogenated to [U(Cp*)(2){NC(5)H(8)(py)(2)}]I (7) by an excess of Ph(3)SnH in refluxing pyridine. Treatment of 4 a with NEt(3)HBPh(4) led to oxidation of the terpy(*-) ligand and formation of [Ce(Cp*)(2)(terpy)]BPh(4), whereas similar reaction with 4 b afforded [U(Cp*)(2){NC(5)H(4)(py)(2)}]BPh(4) (6'). The crystal structures of 6, 6' and 7 were determined.     

Uranium(III)/(IV) nitrile adducts including UI4(N[triple bond]CPh)4, a synthetically useful uranium(IV) complex

The synthesis of complexes used to elucidate an understanding of fundamental An(III) and An(IV) coordination chemistry requires the development of suitable organic-soluble precursors. The reaction of oxide-free uranium metal turnings with 1.3 equivalents of elemental iodine in acetonitrile provided the U(III)/U(IV) complex salt, [U(N[triple bond]CMe)9][UI6][I] (1), in which the U(III) cation is surrounded by nine acetonitrile molecules in a tricapped trigonal prismatic arrangement, a [UI6]2- counterion, and a noncoordinating iodide. The U-N distances for the prismatic and capping nitrogens are 2.55(3) and 2.71(5) A, respectively. The same reaction performed in benzonitrile afforded crystalline UI4(N[triple bond]CPh)4 (3) in 78% isolated yield. In the solid state, 3 shows an eight-coordinate U(IV) atom in a "puckered" square antiprismatic geometry with U-N and U-I distances of 2.56(1) and 3.027(1) A, respectively. This benzonitrile UI4 adduct is a versatile U(IV) synthon that is soluble in methylene chloride, benzonitrile, and tetrahydrofuran, and moderately soluble in toluene and benzene, but decomposes in benzonitrile at 198 degrees C to [UI(N[triple bond]CPh)8][UI]6 (4), a U(III)/U(IV) salt analogous to 1. A toluene slurry of 3 treated with 2.2 equiv of Cp*MgCl.THF (Cp* = pentamethylcyclopentadienide) provided Cp*2UI2(N[triple bond]CPh) (5) in low yields. Single-crystal X-ray structure determination shows that the iodide ligands in 5 are in a rare cis configuration with an acute I-U-I angle of 83.16(7) degrees . Treatment of a methylene chloride solution of 3 with KTp* (Tp* = hydridotris(3,5-dimethylpyrazolylborate)) formed green TpUI3 (6) which was converted to yellow Tp*UI3(N[triple bond]CMe) (7) by rinsing with acetonitrile. Addition of 2.2 equiv of KTp* to a toluene solution of 3 followed by heating at 95 degrees C, filtration, and crystallization led to the isolation of the dinuclear species [Tp*UI(dmpz)]2[mu-O] (9) (dmpz = 3,5-dimethylpyrazolide), presumably formed by hydrolytic cleavage of excess KTp* by adventitious water. The Tp* complexes 6, 7, and 9 were characterized by single-crystal X-ray diffraction, NMR, FT-IR, and optical absorbance spectroscopies.     

Synthesis, structure, and magnetism of an f element nitrosyl complex, (C5Me4H)3UNO

(C(5)Me(4)H)(3)U, 1, reacts with 1 equiv of NO to form the first f element nitrosyl complex (C(5)Me(4)H)(3)UNO, 2. X-ray crystallography revealed a 180° U-N-O bond angle, typical for (NO)(1+) complexes. However, 2 has a 1.231(5) ? N═O distance in the range for (NO)(1-) complexes and a short 2.013(4) ? U-N bond like the U═N bond of uranium imido complexes. Structural, spectroscopic, and magnetic data as well as DFT calculations suggest that reduction of NO by U(3+) has occurred to form a U(4+) complex of (NO)(1-) that has π interactions between uranium 5f orbitals and NO π* orbitals. These bonding interactions account for the linear geometry and short U-N bond. The complex displays temperature-independent paramagnetism with a magnetic moment of 1.36 μ(B) at room temperature. Complex 2 reacts with Al(2)Me(6) to form the adduct (C(5)Me(4)H)(3)UNO(AlMe(3)), 3.     

Experimental and theoretical comparison of actinide and lanthanide bonding in M[N(EPR(2))(2)](3) complexes (M = U, Pu, La, Ce; E = S, Se, Te; R = Ph, iPr, H)

Treatment of M[N(SiMe3)2]3 (M = U, Pu (An); La, Ce (Ln)) with NH(EPPh2)2 and NH(EPiPr2)2 (E = S, Se), afforded the neutral complexes M[N(EPR2)2]3 (R = Ph, iPr). Tellurium donor complexes were synthesized by treatment of MI3(sol)4 (M = U, Pu; sol = py and M = La, Ce; sol = thf) with Na(tmeda)[N(TePiPr2)2]. The complexes have been structurally and spectroscopically characterized with concomitant computational modeling through density functional theory (DFT) calculations. The An-E bond lengths are shorter than the Ln-E bond lengths for metal ions of similar ionic radii, consistent with an increase in covalent interactions in the actinide bonding relative to the lanthanide bonding. In addition, the magnitude of the differences in the bonding is slightly greater with increasing softness of the chalcogen donor atom. The DFT calculations for the model systems correlate well with experimentally determined metrical parameters. They indicate that the enhanced covalency in the M-E bond as group 16 is descended arises mostly from increased metal d-orbital participation. Conversely, an increase in f-orbital participation is responsible for the enhancement of covalency in An-E bonds compared to Ln-E bonds. The fundamental and practical importance of such studies of the role of the valence d and f orbitals in the bonding of the f elements is emphasized.     

Complexation of lanthanides, actinides and transition metal cations with a 6-(1,2,4-triazin-3-yl)-2,2':6',2'-terpyridine ligand: implications for actinide(III)/lanthanide(III) partitioning

The quadridentate N-heterocyclic ligand 6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2'?:?6',2'-terpyridine (CyMe(4)-hemi-BTBP) has been synthesized and its interactions with Am(III), U(VI), Ln(III) and some transition metal cations have been evaluated by X-ray crystallographic analysis, Am(III)/Eu(III) solvent extraction experiments, UV absorption spectrophotometry, NMR studies and ESI-MS. Structures of 1:1 complexes with Eu(III), Ce(III) and the linear uranyl (UO(2)(2+)) ion were obtained by X-ray crystallographic analysis, and they showed similar coordination behavior to related BTBP complexes. In methanol, the stability constants of the Ln(III) complexes are slightly lower than those of the analogous quadridentate bis-triazine BTBP ligands, while the stability constant for the Yb(III) complex is higher. (1)H NMR titrations and ESI-MS with lanthanide nitrates showed that the ligand forms only 1:1 complexes with Eu(III), Ce(III) and Yb(III), while both 1:1 and 1:2 complexes were formed with La(III) and Y(III) in acetonitrile. A mixture of isomeric chiral 2:2 helical complexes was formed with Cu(I), with a slight preference (1.4:1) for a single directional isomer. In contrast, a 1:1 complex was observed with the larger Ag(I) ion. The ligand was unable to extract Am(III) or Eu(III) from nitric acid solutions into 1-octanol, except in the presence of a synergist at low acidity. The results show that the presence of two outer 1,2,4-triazine rings is required for the efficient extraction and separation of An(III) from Ln(III) by quadridentate N-donor ligands.     

Isolation of a Uranium(III)-Carbon Multiple Bond Complex

Low-valent uranium-element multiple bond complexes remain scarce, though there is burgeoning interest regarding to their bonding and reactivity. Herein, isolation of a uranium(III)-carbon double bond complex [(Cp*)₂U(CDP)](BPh₄) ( 1 ) comprising a tridentate carbodiphosphorane (CDP) was reported for the first time. Oxidation of 1 afforded the corresponding U(IV) complex [(Cp*)₂U(CDP)](BPh₄)₂ ( 2 ). The distance between U and C in 2 is 2.481 Å, indicating the existence of a typical U=C double bond, which is further confirmed by quantum chemical calculations. Bonding analysis suggested that the CDP also serves as both σ- and π-donor in complex 1 , though a longer U−C bond (2.666(3) Å) is observed. It implies that 1 is the first isolable mononuclear uranium(III) carbene complex. Moreover, these results suggest that CDPs are promising ligands to establish other low-valent f-block metal-carbon multiple bond complexes.     

Theoretical study of metal-ligand interaction in Sm(III), Eu(III), and Tb(III) complexes of coumarin-3-carboxylic acid in the gas phase and solution

The interaction of lanthanide(III) cations (Ln(III) = Sm(III), Eu(III), and Tb(III)) with the deprotonated form of the coumarin-3-carboxylic acid (cca-) has been investigated by density functional theory (DFT/B3LYP) and confirmed by reference MP2 and CCSD(T) computations. Solvent effects on the geometries and stabilities of the Ln(III) complexes were computed using a combination of water clusters and a continuum solvation model. The following two series of systems were considered: (i) Ln(cca)2+, Ln(cca)2+, Ln(cca)3 and (ii) Ln(cca)(H2O)2Cl2, Ln(cca)2(H2O)2Cl, Ln(cca)3. The strength and character of the Ln(III)-cca- bidentate bonding were characterized by calculated Ln-O bond lengths, binding energies, ligand deformation energies, energy partitioning analysis, sigma-donation contributions, and natural population analyses. The energy decomposition calculations predicted predominant electrostatic interaction terms to the Ln-cca bonding (ionic character) and showed variations of the orbital interaction term (covalent contributions) for the Ln-cca complexes studied. Electron distribution analysis suggested that the covalent contribution comes mainly from the interaction with the carboxylate moiety of cca-.     

Investigations into the synthesis and fluorescence properties of Eu(III), Tb(III), Sm(III) and Gd(III) complexes of a novel bis-beta-diketone-type ligand

A novel bis-beta-diketon ligand, 1,1'-(2,6-bispyridyl)bis-3-phenyl-1,3-propane-dione (L), was designed and synthesized and its complexes with Eu(III), Tb(III), Sm(III) and Gd(III) ions were successfully prepared. The ligand and the corresponding metal complexes were characterized by elemental analysis, and infrared, mass and proton nuclear magnetic resonance spectroscopy. Analysis of the IR spectra suggested that each of the lanthanide metal ions coordinated to the ligand via the carbonyl oxygen atoms and the nitrogen atom of the pyridine ring. The fluorescence properties of these complexes in solid state were investigated and it was discovered that all of the lanthanide ions could be sensitized by the ligand (L) to some extent. In particular, the Tb(III) complex was an excellent green-emitter and would be a potential candidate material for applications in organic light-emitting devices (OLEDs) and medical diagnosis.     

Synthesis,characterization and cytotoxic/cytostatic activity of Sm(III) and Gd(III) complexes

New Sm(III) and Gd(III) complexes of deprotonated 4-hydroxy-3[1-(4-nitrophenyl)-3-oxobutyl]-2H-1-benzopyran-2-one (Acenocoumarol) were synthesized and characterized using FT-IR, FT-Raman, NMR spectra, and elemental analyses. The vibrational study gave evidence for the coordination of ligand to lanthanide ions. The ligand and its lanthanide(III) complexes were tested for their cytotoxic/cytostatic activity against two tumor cell lines and peritoneal mouse macrophages. The Sm(III) and Gd(III) complexes exhibit good activity against melanoma B16 and fibrosarcoma L929 and are stronger inhibitors of tumor cell proliferation than the ligand. Besides their cytotoxicity to tumor cells, Acenocoumarol and its gadolinium(III) and samarium(III) complexes modulate NO generation in activated macrophages.     

Stepwise formation of quasi-octahedral macrocyclic complexes of rhodium(III) and iridium(III) bearing a pentamethylcyclopentadienyl group

Reactions of [[MCl2(Cp*)]2] (1: M=Ir, 2: M=Rh) with bidentate ligands (L) such as 1,4-diisocyano-2,5-dimethylbenzene (a), 1,4-diisocyano-2,3,5,6-tetramethylbenzene (b), pyrazine (c) or 4,4'-dipyridyl (d) gave the corresponding dinuclear complexes [[MCl2(Cp*)]2(L)] (M=Ir: 3a, 3b, 5c, 5d; M=Rh: 4b, 6c, 6d), which were converted into tetranuclear complexes [[M2(mu-Cl)2(Cp*)2]2(L)2](OTf)4 (M=Ir: 7c, 7d, 9a, 9b; M=Rh: 8e, 8d, 10b) on treatment with Ag(OTf). X-ray analyses of 8c and 8d revealed that each of four pentamethylcyclopentadienyl metal moieties was connected by two mu-Cl-bridged atoms and a bidentate ligand to construct a rectangular cavity with the dimensions of 3.7 x 7.0 A for 8c and 3.7 x 11.5 A for 8d. Both the Rh2Cl2 and pyrazine (or 4,4'dipyridyl) ring planes are perpendicular to the Rh4 plane. Treatment of Cl-bridged complexes (7c, 7d, 8e, 8d, 9b, and 10b) with a different ligand (L') resulted in cleavage of the Cl bridges to produce two-dimensional complexes [[MCl(Cp*)]4[(L)-(L')]2](OTf)4 (11ac, 11bc, 11bd, 12bc, and 12bd) with two different ligand "edges". Complex 10b reacted readily with 1,4-diisocyano-2,3,5,6-tetramethylbenzene (b) to give a tetranuclear rhodium(III) complex 12bb. The structure of tetranuclear complexes was confirmed by X-ray analysis of 11bc. Each [MCp*] moiety is surrounded by a Cl atom, isocyanide, and pyrazine (or 4,4'-dipyridyl) and the dimensions of its cavity are 7.0 x 11.6 A.