Control of electronic and magnetic coupling via bridging ligand geometry in a bimetallic ytterbocene complex

The ligand 1-methyl-3,5-bis(2,2':6',2' '-terpyridin-4'-yl)benzene has been employed in the synthesis of a new bimetallic ytterbocene complex [(Cp*)2Yb](1-methyl-3,5-bis(2,2':6',2' '-terpyridin-4'-yl)benzene)[Yb(Cp*)2] (1) and the doubly oxidized congener [1]2+ in an attempt to determine the impact of the bridging ligand geometry on the magnetic/electronic properties as compared to the previously reported 1,4-analog [(Cp*)2Yb](1,4-di(terpyridyl)benzene)[Yb(Cp*)2] (2). Electrochemical, electronic, and magnetic data provide compelling evidence that the 1,3-geometry associated with the bridging ligand of 1 has done an effective job of inhibiting electronic communication between metal centers and magnetic coupling of spin carriers at room temperature as compared to 2. In fact, the physical data associated with 1 are quite similar to those reported for the monometallic analog (Cp*)2Yb(tpy) (3). In particular, the f-f profile of [1]2+ is nearly identical to that of [3]+ in its spectral features but with an almost exact doubling of the intensities. Further, the electronic coupling between metal centers as manifested in the potential separation between metal-based reduction waves has for the first time in these bimetallic ytterbocene complexes been found to go to zero for 1. Thus, the linkage isomerism at the phenyl coupling unit has induced a change in the ground-state electronic configuration from the singlet dianion-bridged (4f)13(pi*)2(4f)13 state found in 2 to the diradical-bridged (4f)13(piA*)1(piB*)1(4f)13 state in 1. This diradical formulation on the bridging ligand in 1 is supported by DFT calculations for the uncomplexed doubly reduced ligand that indicate the ground-state configuration is a singlet diradical state with the triplet-diradical state lying to slightly higher energy. Magnetic characterization of 1 is most consistent with the behavior previously observed for monometallic analogs such as 3, and there is no evidence of long-range magnetic ordering such as that observed for 2. In addition, X-ray crystallographic characterization of 1 represents the first case of a structurally characterized 2:1 metal-to-ligand adduct of the 1,3-bis(tpy) framework.     

Unprecedented polymerization of epsilon-Caprolactone initiated by a single-site lanthanide borohydride complex, [Sm(eta-C5Me5)2(BH4)(thf)]: mechanistic insights

The monoborohydride lanthanide complex [Sm(Cp*)2(BH4)(thf)] (1a) (Cp* = eta-C5Me5), has been successfully used for the controlled ring-opening polymerization of epsilon-caprolactone (epsilon-CL). The organometallic samarium(III) initiator 1 a produces, in quantitative yields, alpha,omega-dihydroxytelechelic poly(epsilon-caprolactone) displaying relatively narrow polydispersity indices (<1.3) within a short period of time (30 min). The polymers have been characterized by 1H and 13C NMR, SEC, and MALDI-TOF MS analyses. Use of the single-site initiator 1 a allows a better understanding of the polymerization mechanism, in particular with the identification of the intermediate compound [Sm(Cp*)2(BH4)(epsilon-CL)] (1b). Indeed, one molecule of epsilon-CL initially displaces the coordinated THF in 1 a to give 1 b. Then, epsilon-CL opening (through cleavage of the cyclic ester oxygen-acyl bond) and insertion into the Sm--HBH3 bond followed by reduction of the carbonyl function by the BH3 end-group ligand, leads to the samarium alkoxyborane derivative [Sm(Cp*)2[O(CH2)6O(BH2)]] (2). This compound subsequently initiates the polymerization of epsilon-CL through a coordination-insertion mechanism. Finally, upon hydrolysis, alpha,omega-dihydroxypoly(epsilon-caprolactone), HO(CH2)5C(O)[O(CH2)5C(O)]nO(CH2)6OH (4) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.     

Characterization and reactivity of peralkylated LnIIAlIII heterobimetallic complexes

The heterobimetallic peralkylated complexes [Ln(AlR4)2]n (Ln = Sm, Yb; R = Me, Et) were synthesized by a silylamide elimination route from Ln[N(SiMe3)2]2(THF)2 and an excess of AlR3. The solid-state structure of [Sm(AlEt4)2]n is isomorphous to that of the ytterbium derivative. Polymeric [Yb(AlMe4)2]n was examined by 1H and 13C MAS NMR spectroscopy revealing the presence of distinct bridging methyl groups. The reaction of [Yb(AlMe4)2]n and 1,10-phenanthroline (Phen) afforded the monomeric donor adduct Yb(AlMe4)2(Phen), while the protonolysis reaction with 2 equiv. C5Me5H (HCp*) yielded a separated ion pair of composition [Cp*Yb(THF)(4)][AlMe(4)]. Single-crystal X-ray diffraction data are provided for both ytterbium(II) complexes. Solid-state magnetic measurements (SQUID) were performed on [Sm(AlMe4)2]n, [Sm(AlEt4)2]n, SmI2(THF)2 and Sm[N(SiMe3)2]2(THF)2 showing high effective magnetic moments 3.67micro(B) < micro(eff) < 4.43micro(B).     

Mechanistic aspects of samarium-mediated sigma-bond activations of arene C-H and arylsilane Si-C bonds.

To investigate the potential role of Sm-Ph species as intermediates in the samarium-catalyzed redistribution of PhSiH3 to Ph2SiH2 and SiH4, the samarium phenyl complex [Cp*2SmPh]2 (1) was prepared by oxidation of Cp2*Sm (2) with HgPh2. Compound 1 thermally decomposes to yield benzene and the phenylene-bridged disamarium complex Cp*2Sm(mu-1,4-C6H4)SmCp*2 (3). This decomposition reaction appears to proceed through dissociation of 1 into monomeric Cp*2SmPh species which then react via unimolecular and bimolecular pathways, involving rate-limiting Cp* metalation and direct C-H activation, respectively. The observed rate law for this process is of the form: rate = k1[1] + k2[1]2. Complex 1 efficiently transfers its phenyl group to PhSiH3, with formation of Ph2SiH2 and [Cp*2Sm(mu-H)]2 (4). Quantitative Si-C bond cleavage of C6F5SiH3 is effected by the samarium hydride complex 4, yielding silane and [Cp*2Sm(mu-C6F5)]2 (5). In contrast, Si-H activation takes place upon reaction of 4 with o-MeOC6H4SiH3, affording the samarium silyl species [structure: see text] Cp*2SmSiH2(o-MeOC6H4) (7). Complex 7 rapidly decomposes to [Cp*2Sm(mu-o-MeOC6H4)]2 (6) and other samarium-containing products. Compounds 5 and 6 were prepared independently by oxidation of 2 with Hg(C6F5)2 and Hg(o-MeOC6H4)2, respectively. The mechanism of samarium-mediated redistribution at silicon, and chemoselectivity in sigma-bond metathesis reactions, are discussed.     

Observation of internal electron transfer in bulky allyl ytterbium complexes with substituted terpyridine ligands

A series of new bulky allyl terpyridyl-ytterbium complexes have been synthesized to determine the effect of allyl ligands on the internal charge-transfer process that exists in these materials. Compared to the pentamethylcyclopentadienyl-ytterbocene compound Cp*2Yb(tpyCN) (nu(C(triple bond)N) = 2172 cm(-1)), the symmetrically substituted allyl complex [1,3-(SiMe3)2C3H3]2Yb(tpyCN) possesses a markedly lowered C(triple bond)N frequency of 2130 cm(-1). Furthermore, the electronic nature of these bulky allyl complexes can be tuned, as demonstrated by the C(triple bond)N frequency of the asymmetric derivatives [1-(SiMe3)C3H4]2Yb(tpyCN) and [1-(SiPh3)-3-(SiMe3)C3H3]2Yb(tpyCN) (2171 and 2164 cm(-1), respectively). The differences in these frequencies can be attributed to differences in the ligands' steric and electronic character. Single-crystal X-ray characterization of [1,3-(SiMe3)2C3H3]2Yb(tpy) reveals that the allyl moiety possesses shorter Yb-C and Yb-N bond distances than the Cp* analogue. The magnetic susceptibility data for [1,3-(SiMe3)2C3H3]2Yb(tpy) departs dramatically from the Curie law, with a room-temperature magnetic moment of 2.95 mu(B).     

Ytterbocene charge-transfer molecular wire complexes

A systematic study of the novel charge-transfer [(f)14-(pi)0-(f)14 --> (f)13-(pi)2-(f)13] electronic state found in 2:1 metal-to-ligand adducts of the type [(Cp)2Yb](BL)[Yb(Cp)2] [BL = tetra(2-pyridyl)pyrazine (tppz) (1), 6',6' '-bis(2-pyridyl)-2,2':4',4':2',2'-quaterpyridine (qtp) (2), 1,4-di(terpyridyl)-benzene (dtb) (3), Cp = (C5Me5)] has been conducted with the aim of determining the effects of increased Yb-Yb separation on the magnetic and electronic properties of these materials. The neutral [(f)13-(pi)2-(f)13], cationic [(f)13-(pi)1-(f)13] and dicationic [(f)13-(pi)0-(f)13] states of these complexes were studied by cyclic voltammetry, UV-vis-NIR electronic absorption spectroscopy, NMR, X-ray crystallography, and magnetic susceptibility measurements. The spectroscopic and magnetic data for the neutral bimetallic complexes is consistent with an [(f)13(pi)2(f)13] ground-state electronic configuration in which each ytterbocene fragment donates one electron to give a singlet dianionic bridging ligand with two paramagnetic Yb(III) centers. The voltammetric data demonstrate that the electronic interaction in the neutral molecular wires 1-3, as manifested in the separation between successive metal reduction waves, is large compared to analogous transition metal systems. Electronic spectra for the neutral and monocationic bimetallic species are dominated by pi-pi and pi-pi transitions, masking the f-f bands that are expected to best reflect the electronic metal-metal interactions. However, these metal-localized transitions are observed when the electrons are removed from the bridging ligand via chemical oxidation to yield the dicationic species, and they suggest very little electronic interaction between metal centers in the absence of pi electrons on the bridging ligands. Analysis of the magnetic data reveals that the qtp complex displays antiferromagnetic coupling of the type Yb(alpha)(alphabeta)Yb(beta) at approximately 13 K.     

4f-5f heterotrimetallic complexes exhibiting electrochemical and magnetic communication

Novel 4f-5f complexes of U(IV) and Th(IV) with Yb have been prepared using a terpyridyl-functionalized ketimide linking group. These 4f-5f heterotrimetallic complexes (C5Me5)2An[-N=C(CH2C6H5)(tpyYb(C5Me5)2)]2 (where An = Th, U) exist in Yb(II/III) valence equilibria and exhibit rich electrochemical behavior consistent with electronic coupling between the actinide and Yb(II/III)tpy*- moieties. The magnetic response of the uranium complex, studied qualitatively using a subtraction method, is indicative of a coupled magnetic state between the U(IV) and Yb(III)tpy*- groups at low temperatures. Both the electrochemical and magnetic data are in agreement and are consistent with differences in participation of the actinide valence orbitals in the ketimide bonding.     

Ytterbocenes as one- and two-electron reductants in their reactions with diazadienes: YbIII mixed-ligand bent-sandwich complexes containing a dianion of diazabutadiene

Ytterbocene [Yb(C(5)MeH(4))(2)(thf)(2)] reacts with diazabutadiene 2,6-iPr(2)C(6)H(3)-N=CH-CH=N-C(6)H(3)iPr(2)-2,6 (DAD) as a one-electron reductant to afford a bis(cyclopentadienyl) Yb(III) derivative containing a DAD radical anion [Yb(C(5)MeH(4))(2)(dad(-.))]. However, ytterbocenes [YbCp*(2)(thf)(2)] (Cp*=C(5)Me(5), C(5)Me(4)H) coordinated by sterically demanding cyclopentadienyl ligands act as two-electron reductants in their reactions with DAD. These reactions occur by abstraction of one Cp* ring and result in the formation of novel Yb(III) mixed-ligand bent-sandwich complexes, [YbCp*(dad)(thf)], in which the dianion of DAD has an uncommon terminal eta(4)-coordination to the ytterbium atom. The variable-temperature magnetic measurements of complex [Yb(C(5)Me(5))(dad)(thf)] suggest the existence of redox tautomerism for this compound.     

Synthesis and solid-state structures of pyrazolylmethane complexes of the rare earths

In this paper, we report the first examples of trispyrazolylmethane complexes of rare earths. Reaction of LnCl3 with Tpm* (tris(3,5-dimethylpyrazolyl)methane) in THF or acetonitrile gives good yields of the [Ln(Tpm*)Cl3] (Ln = Y, Ce, Nd, Sm, Gd, Yb). Tpm* adducts of the lanthanide triflates [Ln(Tpm*)(OTf)3(THF)] (Ln = Y, Ho, Dy) may also be prepared. The X-ray crystal structures of [Y(Tpm*)Cl3], [Sm(Tpm*)Cl3(THF)], and [Ln(Tpm*)(OTf)3(THF)] (Ln = Y, Ho) are reported. The halide/triflate complexes may be used to prepare the aryloxide complexes [Ln(Tpm*)(OArMe2)3] (Ln = Y, Nd, Sm, Yb; ArMe2 = C6H3-2,6-(CH3)2), which are fluxional in solution as a result of interactions between the Tpm* and the aryloxide groups. The structures of the Nd and Sm complexes have been determined. Finally, the reaction of [Nd(BH4)3(THF)3] with Tpm* in THF results in the displacement of two THF molecules to give [Nd(Tpm*)(BH4)3(THF)]. Infrared spectra are consistent with tridentate borohydride coordination. The X-ray structures of these compounds indicate that the Tpm* ligand is less strongly bound than its anionic trispyrazolylborate analogues.     

Electrochemical and spectroscopic characterization of the novel charge-transfer ground state in diimine complexes of ytterbocene

The novel charge-transfer ground state found in alpha,alpha'-diimine adducts of ytterbocene (C(5)Me(5))(2)Yb(L) [L = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen)] in which an electron is spontaneously transferred from the f(14) metal center into the lowest unoccupied (pi*) molecular orbital (LUMO) of the diimine ligand to give an f(13)-L(*)(-) ground-state electronic configuration has been characterized by cyclic voltammetry, UV-vis-near-IR electronic absorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligand LUMO is stabilized and the metal f orbital is destabilized by approximately 1.0 V each upon complexation for both bpy and phen adducts. The separation between the ligand-based oxidation wave (L(0/-)) and the metal-based reduction wave (Yb(3+/2+)) in the ytterbocene adducts is 0.79 V for both bpy and phen complexes. The UV-vis-near-IR absorption spectroscopic data for both the neutral adducts and the one-electron-oxidized complexes are consistent with those reported recently, but previously unreported bands in the near-IR have been recorded and assigned to ligand (pi*)-to-metal (f orbital) charge-transfer (LMCT) transitions. These optical electronic excited states are the converse of the ground-state charge-transfer process (e.g., f(13)-L(*-) <--> f(14)-L(0)). These new bands occur at approximately 5000 cm(-1) in both adducts, consistent with predictions from electrochemical data, and the spacings of the resolved vibronic bands in these transitions are consistent with the removal of an electron from the ligand pi* orbital. The unusually large intensity observed in the f --> f intraconfiguration transitions for the neutral phenanthroline adduct is discussed in terms of an intensity-borrowing mechanism involving the low-energy LMCT states. Raman vibrational data clearly reveal resonance enhancement for excitation into the low-lying pi* --> pi* ligand-localized excited states, and comparison of the vibrational energies with those reported for alkali-metal-reduced diimine ligands confirms that the ligands in the adducts are reduced radical anions. Differences in the resonance enhancement pattern for the modes in the bipyridine adduct with excitation into different pi* --> pi* levels illustrate the different nodal structures that exist in the various low-lying pi* orbitals.     

Novel RhCp*/GaCp* and RhCp*/InCp* cluster complexes

The reaction of 6 equivalents of GaCp*(Cp*= pentamethylcyclopentadienyl) with [{Cp*RhCl2}2] yields the complex [Cp*Rh(GaCp*)3(Cl)2] (1) exhibting a cage-like intermetallic RhGa3 center with Ga-Cl-Ga bridges. Treatment of this complex with GaCl3 gives the Lewis acid-base adduct [Cp*Rh(GaCp*)2(GaCl3)]. (2) Reaction of [{Cp*RhCl2}2] with understoichiometric amounts of E(I)Cp*(E = Al, Ga, In) leads to a variety of products strongly dependent on the molecular ratio of the reactants. Thus, the reduction of [{Cp*RhCl2}2] with one equivalent of E(I)Cp*(E = Al, Ga, In) gives the RhII dimer [{Cp*RhCl}2]. The insertion of 3 equivalents of InCp* into the Rh-Cl bonds of [{Cp*RhCl2}2] yields the salt [Cp*2Rh]+[Cp*Rh(InCp*){In2Cl4(mu2-Cp*)}]- (3), the anion exhibiting an intermetallic RhIn(3) center with an intramolecularly bridging Cp* ring. The reaction of [{Cp*RhCl}2] with Cp*Ga yields various insertion products. In trace amount the "all hydrocarbon" cluster complex [(RhCp*)2(GaCp*)3] (6) is obtained. The corresponding ethylene containing cluster complex [{RhCp(GaCp*)(C2H4)}2] (7) can be prepared by treatment of [RhCp*(CH3CN)(C2H4)] with GaCp*.     

Electronic structure of mononuclear bis(1,2-diaryl-1,2-ethylenedithiolato)iron complexes containing a fifth cyanide or phosphite ligand: a combined experimental and computational study

A series of mononuclear square-based pyramidal complexes of iron containing two 1,2-diaryl-ethylene-1,2-dithiolate ligands in various oxidation levels has been synthesized. The reaction of the dinuclear species [Fe(III)2(1L*)2(1L)2]0, where (1L)2- is the closed shell di-(4-tert-butylphenyl)-1,2-ethylenedithiolate dianion and (1L*)1- is its one-electron-oxidized pi-radical monoanion, with [N(n-Bu)4]CN in toluene yields dark green crystals of mononuclear [N(n-Bu)4][Fe(II)(1L*)2(CN)] (1). The oxidation of 1 with ferrocenium hexafluorophosphate yields blue [Fe(III)(1L*)2(CN)] (1ox), and analogously, a reduction with [Cp2Co] yields [Cp2Co][N(n-Bu)4][Fe(II)(1L*)(1L)(CN)] (1red); oxidation of the neutral dimer with iodine gives [Fe(III)(1L*)2I] (2). The dimer reacts with the phosphite P(OCH3)3 to yield [Fe(II)(1L*)2{P(OCH3)3}] (3), and [Fe(III)2(3L*)2(3L)2] reacts with P(OC6H5)3 to give [Fe(II)(3L*)2{P(OC6H5)3}] (4), where (3L)2- represents 1,2-diphenyl-1,2-ethylenedithiolate(2-). Both 3 and 4 were electrochemically one-electron oxidized to the monocations 3ox and 4ox and reduced to the monoanions 3red and 4red. The structures of 1 and 4 have been determined by X-ray crystallography. All compounds have been studied by magnetic susceptibility measurements, X-band EPR, UV-vis, IR, and M?ssbauer spectroscopies. The following five-coordinate chromophores have been identified: (a) [Fe(III)(L*)2X]n, X = CN-, I- (n = 0) (1ox, 2); X = P(OR)3 (n = 1+) )3ox, 4ox) with St = 1/2, SFe = 3/2; (b) [Fe(II)(L*)2X]n, X = CN-, (n = 1-) (1); X = P(OR)3 (n = 0) (3, 4) with St = SFe = 0; (c) [Fe(II)(L*)(L)X]n <--> [Fe(II)(L)(L*)X]n, X = CN- (n = 2-) (1red); X = P(OR)3 (n = 1-) (3red, 4red) with St = 1/2, SFe = 0 (or 1). Complex 1ox displays spin crossover behavior: St = 1/2 <--> St = 3/2 with intrinsic spin-state change SFe = 3/2 <--> SFe = 5/2. The electronic structures of 1 and 1(ox) have been established by density functional theoretical calculations: [Fe(II)(1L*)2(CN)]1- (SFe = 0, St = 0) and [Fe(III)(1L*)2(CN)]0 (SFe = 3/2, St = 1/2).     

Lanthanide Structures, Coordination, and Extraction Investigations of a 1,3-Bis(diethyl amide)-Substituted Calix[4]arene Ligand

The synthesis and structure determinations of lanthanum, samarium, ytterbium, and lutetium complexes of 5,11,17,23-tetra-tert-butyl-25,27-bis((diethylcarbamoyl)methoxy)-26,28-dihydroxycalix[4]arene (L) are described. The four structures display similar characteristics with the trivalent lanthanide cation being encapsulated in an eight-coordinate oxygen environment, consisting of six oxygens from the calixarene, a water molecule, and unidentate picrate for lanthanum [La(L-2H)(picrate)(H(2)O)]; and bidentate chelating picrate for the other lanthanides [Ln(L-2H)(picrate)]Ln = Sm, Yb, Lu. Under optimised experimental conditions solvent extraction investigations showed the calix[4]arene ligand L exhibited generally very high percentage extractabilities of lanthanide cations into dichloromethane, presumably on account of the ligand's unique lower rim oxygen containing coordination sphere and its lipophilic exterior.     

Hybrid materials containing organometallic cations and 3-D anionic metal dicyanamide networks of type [Cp*2M][M'(dca)3

A new series of hybrid materials of type [Cp*2M][M'(dca)3] has been prepared by cation templation and structurally characterised (M = Fe(III), Co(III); M'= Mn(II), Fe(II), Co(II), Ni(II), Cd(II); dca-= N(CN)2-). The crystallographic analysis of [Cp*2Fe][Cd(dca)(3)] showed that the [Cd(dca)3]- anionic framework is of a symmetrical 3-D alpha-polonium type, containing octahedral Cd nodes and micro (1,5)-dca bridging ligands. The [Cp*2Fe]+ cations occupy the cube-like cavities within the framework. The cationic and anionic-framework sublattices remain magnetically independent and display susceptibilities, over the range 300 to 2 K, of a Curie-Weiss nature obtained by adding a S= 1/2 (Cp*2Fe+) or a S= 0 (Cp*2Co+) contribution to those of the weakly antiferromagnetically coupled frameworks of M'. These hybrid species do not show any intrinsic long-range magnetic order. The present [Cp*2Fe]M'(dca)3] series display the characteristic, unusually shaped [Cp*2Fe]+ Mossbauer line, in the range 295-5 K, assigned (below 101 K) as the sum of a narrow and a broad line. Relaxation effects were evident. The [Fe(dca)3]- compound showed superimposed low-spin Fe(III) and high-spin Fe(II) lines, the latter giving relaxation broadening effects.     

Synthesis and study of the spectroscopic and redox properties of Ru(II),Pt(II) mixed-metal complexes bridged by 2,3,5,6-tetrakis(2-pyridyl)pyrazine

The mixed-metal supramolecular complexes [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 (tpy = 2,2':6',2'-terpyridine and tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were synthesized and characterized. These complexes contain ruthenium bridged by tppz to platinum centers to form stereochemically defined linear assemblies. X-ray crystallographic determinations of the two complexes confirm the identity of the metal complexes and reveal intermolecular interactions of the Pt sites in the solid state for [(tpy)Ru(tppz)PtCl](PF6)3 with a Pt...Pt distance of 3.3218(5) A. The (1)H NMR spectra show the expected splitting patterns characteristic of stereochemically defined mixed-metal systems and are assigned with the use of (1)H-(1)H COSY and NOESY. Electronic absorption spectroscopy displays intense ligand-based pi --> pi* transitions in the UV and MLCT transitions in the visible. Electrochemically [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 display reversible Ru (II/III) couples at 1.63 and 1.83 V versus Ag/AgCl, respectively. The complexes display very low potential tppz (0/-) and tppz(-/2-) couples, relative to their monometallic synthons, [(tpy)Ru(tppz)](PF6)2 and [Ru(tppz)2](PF6)2, consistent with the bridging coordination of the tppz ligand. The Ru(dpi) --> tppz(pi*) MLCT transitions are also red-shifted relative to the monometallic synthons occurring in the visible centered at 530 and 538 nm in CH3CN for [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4, respectively. The complex [(tpy)Ru(tppz)PtCl](PF6)3 displays a barely detectable emission from the Ru(dpi) --> tppz(pi*) (3)MLCT in CH 3CN solution at RT. In contrast, [ClPt(tppz)Ru(tppz)PtCl](PF6)4 displays an intense emission from the Ru(dpi) --> tppz(pi*) (3)MLCT state at RT with lambda max(em) = 754 nm and tau = 80 ns.     

Syntheses and skeletal transformations of NCNH- and NCN-bridged tetrairidium(III) cages

The diiridium complex [Cp*IrCl2]2 (Cp* = eta5-C5Me5) reacts with 2 equiv of Na(NCNH) at room temperature to afford the 16-membered macrocyclic tetrairidium complex [Cp*IrCl(mu2-NCNH-N,N')]4 (1a). Treatment of 1a with 4 equiv of triethylamine at room temperature leads to the formation of the "C3-elongated cubane-like" tetrairidium complex [Cp*Ir(mu3-NCN-N,N,N')3(IrCp*)3(mu3-NCN-N,N,N)] (2) as the major product, which is further converted into the cubane-type complex [Cp*Ir(mu3-NCN-N,N,N)]4 (3) on refluxing in p-xylene. The molecular structures of [Cp*IrI(mu3-NCNH-N,N')]4.C7H8 (1b.C7H8), 2.0.5C7H8, and 3 have been determined by X-ray analyses.     

Molecular and electronic structure of square-planar gold complexes containing two 1,2-Di(4-tert-butylphenyl)ethylene-1,2-dithiolato ligands: [Au(2L)2]1+/0/1-/2-. A combined experimental and computational study

From the reaction of in situ generated 1,2-di(4-tert-butylphenyl)ethylene-1,2-dithiol, 2LH2, and Na[AuCl4].2H2O in 1,4-dioxane, green brown crystals of diamagnetic [N(n-Bu)4][AuIII(2L)2] (1) were obtained. As shown by cyclic voltammetry, 1 is a member of an electron-transfer series comprising the dianion [AuII(2L)2]2-, the monoanion [AuIII(2L)2]-, the neutral species [AuIII(2L*)(2L)]0 <--> [AuIII(2L)(2L*)]0, and the monocation [AuIII(2L*)2]+. (2L*)1- represents the pi radical anion (Srad = 1/2) of the one-electron oxidized closed-shell dianion (2L)2-. Oxidation of 1 in CH2Cl2 with ferrocenium hexafluorophosphate affords green, paramagnetic microcrystals of [AuIII(2L*)(2L)] <--> [AuIII(2L)(2L*)] (2) (S = 1/2). Complexes 1 and 2 have been characterized by X-ray crystallography. Both species possess square-planar monoanions and neutral molecules, respectively. From the oxidation reaction of 1 or [N(n-Bu)4][AuIII(3L)2] with 2-3 equiv of [NO]BF4 in CH2Cl2, a green solution of [AuIII(2L*)2]+ and green microcrystals of [AuIII(3L*)2]BF4 (3) were obtained, respectively; (3L)2- represents the dianion 1,2-di(4-diphenyl)ethylene-1,2-dithiolate, and (3L*)1- is its pi radical monoanion. The electronic structures of this series of gold species have been elucidated by UV-vis, EPR spectroscopies, and DFT calculations. It is shown computationally by density functional theoretical (DFT) methods that the electronic structure of [AuIII(1L*)2]+ is best described as a singlet diradical (St = 0); the ligand mixed valency in the neutral species 2 is of class (III) (delocalized); the monoanion in 1 contains a AuIII ion and two closed-shell dianionic ligands; and the corresponding dianions [Au(L)2]2- are best described as an intermediate AuII/AuIII species with a metal-ligand delocalized SOMO (25% Au 5d, 75% 3p of four S atoms). (1L)2- is the dianion 1,2-di(phenyl)ethylene-1,2-dithiolate, and (1L*)1- is the pi radical monoanion. The neutral species [PdII(2L*)2] (4) has also been synthesized and characterized by X-ray crystallography. Its electronic structure is the same as described for [AuIII(1L*)2]+ (singlet diradical), whereas that of the monoanion [PdII(2L*)(2L)]- <--> [Pd(2L)(2L*)]- corresponds to that of the neutral gold complex 2. Anodic oxidation of the analogous monoanion [AuIII(mnt)2]-, where mnt = maleonitriledithiolate, gave the neutral complex [Au(mnt)(mnt*)] (E1/2 = 0.91 V vs Fc+/Fc). The optical and EPR spectroscopies of [Au(mnt)(mnt*)] were consistent with those observed for the corresponding di(tert-butylphenyl)ethylenedithiolate complex 2.     

Ligand substituent effect observed for ytterbocene 4'-cyano-2,2':6',2' '-terpyridine

A new N-heterocyclic complex of ytterbocene (Cp(2)Yb(II), Cp = C(5)Me(5)) has been prepared by the addition of 4'-cyano-2,2':6',2' '-terpyridine (tpyCN) to Cp(2)Yb(II)(OEt(2)) in toluene to give a dark blue species designated as Cp(2)Yb(tpyCN). The effect of the electron-withdrawing group (-CN) on the redox potentials of the charge-transfer form of this complex [in which an electron is transferred from the f(14) metal center to the lowest unoccupied (pi) molecular orbital of the tpyCN ligand to give a 4f(13)-pi(1) electronic configuration] has been quantified by cyclic voltammetry. The tpyCN ligand stabilizes this configuration by 60 mV more than that in the unsubstituted tpy ligand complex and by 110 mV more than that in the unsubstituted bpy ligand complex. Magnetic susceptibility measurements corroborate the enhanced stabilization of the 4f(13)-pi(1) configuration by the substituted terpyridyl ligand complex. Furthermore, the temperature dependence of the magnetic data is most consistent with a thermally induced valence tautomeric equilibrium between this paramagnetic 4f(13)-pi(1) form that dominates near room temperature and the diamagnetic 4f(14)-pi(0) form that dominates at low temperature. Differing coordination modes for the tpyCN ligand to the ytterbocene center have also been confirmed by isolation and X-ray crystallographic characterization of complexes binding through either the cyano nitrogen of tpyCN or the three terpyridyl nitrogen atoms of tpyCN.     

Diverse heteroleptic ytterbium(III) thiocyanate complexes by oxidation from bis(thiocyanato)ytterbium(II)

The new ytterbium(II) thiocyanate complex [Yb(NCS)2(thf)2] (1), synthesised by redox transmetallation between [Hg(SCN)2] and ytterbium metal in THF at room temperature, gave monomeric, eight coordinate [Yb-(NCS)2(dme)3] (2, dme = 1,2-dimethoxyethane) on crystallisation from DME, and is a powerful, synthetically useful reductant. Thus, oxidation of 1 with Hg(SCN)2, Hg(C6F5)2/HOdpp (HOdpp = 2,6-diphenylphenol), TlCp (Cp = C5H5 or CH3C5H4), Tl(Ph2pz) (Ph2pz = 3,5-diphenylpyrazolate) and CCl3CCl3 in THF yielded the ytterbium(II) complexes [Yb(NCS)3(thf)4] (3), [Yb-(NCS)2(Odpp)(thf)3](4), [Yb(NCS)2Cp-(thf)3] (Cp = C5H5 (5), CH3C5H4 (6)), [Yb(NCS)2(Ph2pz)(thf)4] (7) and [Yb(NCS)2Cl(thf)4] (8). In the solid state, complexes 4, 6 and 7 were shown by X-ray crystallography to be six, eight and eight coordinate monomers, respectively. Exclusively terminal, N-bound transoid thiocyanate bonding is observed with eta1-Odpp (4), eta5/-C5H4Me (6) and eta2-Ph2Pz (7) ligands attached approximately perpendicular to the N...N vector. The chloride complex 8 is not a molecular species, but consists of discrete, seven coordinate [YbCl2(thf)5] cations and [Yb(NCS)4(thf)3] anions. By contrast, oxidation of 1 with TlO2CPh gave a mixture of [[Yb(NCS)-(O2CPh)2(thf)2]2] (9) and 3 through rearrangement of an initially formed [Yb(NCS)2(O2CPh)] species. The X-ray structure of 9 indicates a dimeric complex with a (Yb(mu-O2CPh)4Yb] core that contains both bridging bidentate and bridging tridentate benzoate groups, and with a terminal N-bound thiocyanate and two THF ligands on each ytterbium. Reduction of Ph2CO with 1 in THF yielded the dinuclear complex [[Yb(NCS)2(thf)3]2(mu-OC(Ph)2C(Ph)2O)] (10), in which two octahedral Yb centres are bridged by a 1,1,2,2-tetraphenylethane-1,2-diolate ligand, derived from reductive coupling of the benzophenone reagent.     

Synthesis and structural characterization of a series of lanthanide(II) amides: steric effect of amido ligand

The synthesis and structures of a series of lanthanide(II) and lanthanide(III) complexes supported by the amido ligand N(SiMe3)Ar were described. Several lanthanide(III) amide chlorides were synthesized by a metathesis reaction of LnCl3 with lithium amide, including {[(C6H5)(Me3Si)N]2YbCl(THF)}2.PhCH3 (1), [(C6H3-iPr2-2,6)(SiMe3)N]2YbCl(mu-Cl)Li(THF)3.PhCH3 (4), [(C6H3-iPr2-2,6)(SiMe3)N]YbCl2(THF)3 (6), and [(C6H3-iPr2-2,6)(SiMe3)N]2SmCl3Li2(THF)4 (7). The reduction reaction of 1 with Na-K alloy afforded bisamide ytterbium(II) complex [(C6H5)(Me3Si)N]2Yb(DME)2 (2). The same reaction for Sm gave an insoluble black powder. An analogous samarium(II) complex [(C6H5)(Me3Si)N]2Sm(DME)2 (3) was prepared by the metathesis reaction of SmI2 with NaN(C6H5)(SiMe3). The reduction reaction of ytterbium chloride 4 with Na-K alloy afforded monoamide chloride {[(C6H3-iPr2-2,6)(SiMe3)N]Yb(mu-Cl)(THF)2}2 (5), which is the first example of ytterbium(II) amide chloride, formed via the cleavage of the Yb-N bond. The same reduction reaction of 7 gave a normal bisamide complex [(C6H3-iPr2-2,6)(SiMe3)N]2Sm(THF)2 (8) via Sm-Cl bond cleavage. This is the first example for the steric effect on the outcome of the reduction reaction in lanthanide(II) chemistry. 5 can also be synthesized by the Na/K alloy reduction reaction of 6. All of the complexes were fully characterized including X-ray diffraction for 1-7.