Table salt and other alkali metal chloride oligomers: structure, stability, and bonding

We have investigated table salt and other alkali metal chloride monomers, ClM, and (distorted) cubic tetramers, (ClM)(4), with M = Li, Na, K, and Rb, using density functional theory (DFT) at the BP86/TZ2P level. Our objectives are to determine how the structure and thermochemistry (e.g., Cl-M bond lengths and strengths, oligomerization energies, etc.) of alkali metal chlorides depend on the metal atom and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The analyses confirm the high polarity of the Cl-M bond (dipole moment, VDD, and Hirshfeld atomic charges). They also reveal that bond overlap derived stabilization (approximately -26, -20, and -8 kcal/mol), although clearly larger than in the corresponding F-M bonds, contributes relatively little to the (trend in) bond strengths (-105, -90, and -94 kcal/mol) along M = Li, Na, and K. Thus, the Cl-M bonding mechanism resembles more closely that of the even more ionic F-M bond than that of the more covalent C-M or H-M bonds. Tetramerization causes the Cl-M bond to expand, and it reduces its polarity.     

Insights into the metathesis reaction involving M-M,C-C,and M-C triple bonds from computations employing density functional theory on model compounds M2(OH)6 and M2(SH)6, where M = Mo and W

Calculations employing density functional theory (Gaussian 98, B3LYP, LANL2DZ, 6-31G) have been undertaken to interrogate the factors influencing the metathesis reaction involving M-M, C-C, and M-C triple bonds for the model compounds M(2)(EH)(6), M(2)(EH)(6)(mu-C(2)H(2)), and [(HE)(3)M(tbd1;CH)](2), where M = Mo, W and E = O, S. Whereas in all cases the ethyne adducts are predicted to be enthalpically favored in the reactions between M(2)(EH)(6) compounds and ethyne, only when M = W and E = O is the alkylidyne product [(HO)(3)W(tbd1;CH)](2) predicted to be more stable than the alkyne adduct. For the reaction M(2)(EH)(6)(mu-C(2)H(2)) --> [(HE)(3)M(tbd1;CH)](2), the deltaG degrees values (kcal mol(-)(1)) are -6 (M = W, E = O), +5 (M = Mo, E = O), +18 (M = W, E = S), and +21 (M = Mo, E = S) and the free energies of activation are calculated to be deltaG() = +19 kcal mol(-)(1) (M = W, E = O) and +34 kcal mol(-)(1) (M = Mo, E = O), where the transition state involves an asymmetric bridged structure M(2)(OH)(4)(mu-OH)(2)(CH)(mu-CH) in which the C-C bond has broken; C.C = 1.89 and 1.98 A for W and Mo, respectively. These results are discussed in terms of the experimental observations of the reactions involving ethyne and the symmetrically substituted alkynes (RCCR, where R = Me, Et) with M(2)(O(t)()Bu)(6) and M(2)(O(t)()Bu)(2)(S(t)()Bu)(4) compounds, where M = Mo, W.     

Dissection of the difference between the group I metal ions in inhibiting GSK3β: a computational study

Glycogen synthase kinase 3β (GSK3β) is a serine/threonine kinase that requires two cofactor Mg(2+) ions for catalysis in regulating many important cellular signals. Experimentally, Li(+) is a competitive inhibitor of GSK3β relative to Mg(2+), while this mechanism is not experienced with other group I metal ions. Herein, we use native Mg(2)(2+)-Mg(1)(2+) GSK3β and its Mg(2)(2+)-M(1)(+) (M = Li, Na, K, and Rb) derivatives to investigate the effect of metal ion substitution on the mechanism of inhibition through two-layer ONIOM-based quantum mechanics/molecular mechanics (QM/MM) calculations and molecular dynamics (MD) simulations. The results of ONIOM calculations elucidate that the interaction of Na(+), K(+), and Rb(+) with ATP is weaker compared to that of Mg(2+) and Li(+) with ATP, and the critical triphosphate moiety of ATP undergoes a large conformational change in the Na(+), K(+), and Rb(+) substituted systems. As a result, the three metal ions (Na(+), K(+), and Rb(+)) are not stable and depart from the active site, while Mg(2+) and Li(+) can stabilize in the active site, evident in MD simulations. Comparisons of Mg(2)(2+)-Mg(1)(2+) and Mg(2)(2+)-Li(1)(+) systems reveal that the inline phosphor-transfer of ATP and the two conserved hydrogen bonds between Lys85 and ATP, together with the electrostatic potential at the Li(1)(+) site, are disrupted in the Mg(2)(2+)-Li(1)(+) system. These computational results highlight the possible mechanism why Li(+) inhibits GSK3β.     

Synthesis and characterization of (smif)2M(n) (n = 0, M = V, Cr, Mn, Fe, Co, Ni, Ru; n = +1, M = Cr, Mn, Co, Rh, Ir; smif =1,3-di-(2-pyridyl)-2-azaallyl)

A series of Werner complexes featuring the tridentate ligand smif, that is, 1,3-di-(2-pyridyl)-2-azaallyl, have been prepared. Syntheses of (smif)(2)M (1-M; M = Cr, Fe) were accomplished via treatment of M(NSiMe(3))(2)(THF)(n) (M = Cr, n = 2; Fe, n = 1) with 2 equiv of (smif)H (1,3-di-(2-pyridyl)-2-azapropene); ortho-methylated ((o)Mesmif)(2)Fe (2-Fe) and ((o)Me(2)smif)(2)Fe (3-Fe) were similarly prepared. Metatheses of MX(2) variants with 2 equiv of Li(smif) or Na(smif) generated 1-M (M = Cr, Mn, Fe, Co, Ni, Zn, Ru). Metathesis of VCl(3)(THF)(3) with 2 Li(smif) with a reducing equiv of Na/Hg present afforded 1-V, while 2 Na(smif) and IrCl(3)(THF)(3) in the presence of NaBPh(4) gave [(smif)(2)Ir]BPh(4) (1(+)-Ir). Electrochemical experiments led to the oxidation of 1-M (M = Cr, Mn, Co) by AgOTf to produce [(smif)(2)M]OTf (1(+)-M), and treatment of Rh(2)(O(2)CCF(3))(4) with 4 equiv Na(smif) and 2 AgOTf gave 1(+)-Rh. Characterizations by NMR, EPR, and UV-vis spectroscopies, SQUID magnetometry, X-ray crystallography, and DFT calculations are presented. Intraligand (IL) transitions derived from promotion of electrons from the unique CNC(nb) (nonbonding) orbitals of the smif backbone to ligand π*-type orbitals are intense (ε ≈ 10,000-60,000 M(-1)cm(-1)), dominate the UV-visible spectra, and give crystals a metallic-looking appearance. High energy K-edge spectroscopy was used to show that the smif in 1-Cr is redox noninnocent, and its electron configuration is best described as (smif(-))(smif(2-))Cr(III); an unusual S = 1 EPR spectrum (X-band) was obtained for 1-Cr.     

Role of alkali metal cation size in the energy and rate of electron transfer to solvent-separated 1:1 [(M+)(acceptor)] (M+ = Li+, Na+, K+) ion pairs.

The effect of cation size on the rate and energy of electron transfer to [(M(+))(acceptor)] ion pairs is addressed by assigning key physicochemical properties (reactivity, relative energy, structure, and size) to an isoelectronic series of well-defined M(+)-acceptor pairs, M(+) = Li(+), Na(+), K(+). A 1e(-) acceptor anion, alpha-SiV(V)W(11)O(40)(5-) (1, a polyoxometalate of the Keggin structural class), was used in the 2e(-) oxidation of an organic electron donor, 3,3',5,5'-tetra-tert-butylbiphenyl-4,4'-diol (BPH(2)), to 3,3',5,5'-tetra-tert-butyldiphenoquinone (DPQ) in acetate-buffered 2:3 (v/v) H(2)O/t-BuOH at 60 degrees C (2 equiv of 1 are reduced by 1e(-) each to 1(red), alpha-SiV(IV)W(11)O(40)(6-)). Before an attempt was made to address the role of cation size, the mechanism and conditions necessary for kinetically well behaved electron transfer from BPH(2) to 1 were rigorously established by using GC-MS, (1)H, (7)Li, and (51)V NMR, and UV-vis spectroscopy. At constant [Li(+)] and [H(+)], the reaction rate is first order in [BPH(2)] and in [1] and zeroth order in [1(red)] and in [acetate] (base) and is independent of ionic strength, mu. The dependence of the reaction rate on [H(+)] is a function of the constant, K(a)1, for acid dissociation of BPH(2) to BPH(-) and H(+). Temperature dependence data provided activation parameters of DeltaH = 8.5 +/- 1.4 kcal mol(-1) and DeltaS = -39 +/- 5 cal mol(-1) K(-1). No evidence of preassociation between BPH(2) and 1 was observed by combined (1)H and (51)V NMR studies, while pH (pD)-dependent deuterium kinetic isotope data indicated that the O-H bond in BPH(2) remains intact during rate-limiting electron transfer from BPH(2) and 1. The formation of 1:1 ion pairs [(M(+))(SiVW(11)O(40)(5-))](4-) (M(+)1, M(+) = Li(+), Na(+), K(+)) was demonstrated, and the thermodynamic constants, K(M)(1), and rate constants, k(M)(1), associated with the formation and reactivity of each M(+)1 ion pair with BPH(2) were calculated by simultaneous nonlinear fitting of kinetic data (obtained by using all three cations) to an equation describing the rectangular hyperbolic functional dependence of k(obs) values on [M(+)]. Constants, K(M)(1)red, associated with the formation of 1:1 ion pairs between M(+) and 1(red) were obtained by using K(M)(1) values (from k(obs) data) to simultaneously fit reduction potential (E(1/2)) values (from cyclic voltammetry) of solutions of 1 containing varying concentrations of all three cations to a Nernstian equation describing the dependence of E(1/2) values on the ratio of thermodynamic constants K(M)(1) and K(M)(1)red. Formation constants, K(M)(1), and K(M)(1)red, and rate constants, k(M)(1), all increase with the size of M(+) in the order K(Li)(1) = 21 < K(Na)(1) = 54 < K(K)(1) = 65 M(-1), K(Li)(1)red = 130 < K(Na)(1)red = 570 < K(K)(1)red = 2000 M(-1), and k(Li)(1) = 0.065 < k(Na)(1) = 0.137 < k(K)(1) = 0.225 M(-1) s(-1). Changes in the chemical shifts of (7)Li NMR signals as functions of [Li(5)1] and [Li(6)1(red)] were used to establish that the complexes M(+)1 and M(+)1(red) exist as solvent-separated ion pairs. Finally, correlation between cation size and the rate and energy of electron transfer was established by consideration of K(M)(1), k(M)(1), and K(M)(1)red values along with the relative sizes of the three M(+)1 pairs (effective hydrodynamic radii, r(eff), obtained by single-potential step chronoamperometry). As M(+) increases in size, association constants, K(M)(1), become larger as smaller, more intimate solvent-separated ion pairs, M(+)1, possessing larger electron affinities (q/r), and associated with larger k(M)(1)() values, are formed. Moreover, as M(+)1 pairs are reduced to M(+)1(red) during electron transfer in the activated complexes, [BPH(2), M(+)1], contributions of ion pairing energy (proportional to -RT ln(K(M)(1)red/K(M)(1)) to the standard free energy change associated with electron transfer, DeltaG degrees (et), increase with cation size: -RT ln(K(M)(1)red/K(M)(1)) (in kcal mol(-1)) = -1.2 for Li(+), -1.5 for Na(+), and -2.3 for K(+).     

Ab Initio Studies on MCH2(OH) and CH3OM(M=H, -, Li, Na)

IntroductionThe greatsynthetic utility of organolithium reagents has been extended by the introduc-tion ofα-lithium-etherreagents[1— 4] .Those reagentsareeasily prepared,and they can be usedas anionic resources to synthesize a large variety of compounds stereo-selectively[5— 8] .Fur-thermore,such reagents can react with nucleophiles like RLi,only a typical reaction of car-benoid[9,1 0 ] .Though the ambidentnature isof greatinterest,only a little work has been doneon model molecule Li CH2 …     

Remarkable dynamical opening and closing of platinum and palladium pentaruthenium carbido carbonyl cluster complexes

The reaction of Ru(5)(CO)(15)(mu(5)-C), 1, with Pt(PBu(t)(3))(2) at room temperature yielded the mixed-metal cluster complex PtRu(5)(CO)(15)(PBu(t)(3))(C), 2, in 52% yield. Compound 2 consists of a mixture of two interconverting isomers in solution. One isomer, 2A, can be isolated by crystallization from benzene/octane solvent. The second isomer, 2B, can be isolated by crystallization from diethyl ether. Both were characterized crystallographically. Isomer 2A consists of a square pyramidal cluster of five ruthenium atoms with a phosphine-substituted platinum atom spanning the square base. Isomer 2B consists of a square pyramidal cluster of five ruthenium atoms with a phosphine-substituted platinum atom on an edge on the square base. The two isomers interconvert rapidly on the NMR time scale at 40 degrees C, deltaG(313)++ = 11.4(8) kcal mol(-1), deltaH++ = 8.8(5) kcal mol(-1), deltaS++ = -8.4(9) cal mol(-1) K(-1). The reaction of Pd(PBu(t)(3))(2) with compound 1 yielded two new cluster complexes: PdRu(5)(CO)(15)(PBu(t)(3))(mu(6)-C), 3, in 50% yield and Pd(2)Ru(5)(CO)(15)(PBu(t)(3))(2)(mu(6)-C), 4, in 6% yield. The yield of 4 was increased to 47% when an excess of Pd(PBu(t)(3))(2) was used. In the solid state compound 3 is structurally analogous to 2A, but in solution it also exists as a mixture of interconverting isomers; deltaG(298)++ = 10.6(6) kcal mol(-1), deltaH++ = 9.7(3) kcal mol(-1), and deltaS++ = -3(1) cal mol(-1) K(-1) for 3. Compound 4 contains an octahedral cluster consisting of one palladium atom and five ruthenium atoms with an interstitial carbido ligand in the center of the octahedron, but it also has one additional Pd(PBu(t)(3)) grouping that is capping a triangular face of the ruthenium cluster. The Pd(PBu(t)(3)) groups in 4 also undergo dynamical interchange that is rapid on the NMR time scale at 25 degrees C; deltaG(298)++ = 11(1) kcal mol(-1), deltaH++ = 10.2(4) kcal mol(-1), and deltaS++ = -3(2) cal mol(-1) K(-1) for 4.     

Mechanism of a novel exchange process in alkali metal salts of 1,5-dicyclooctatetraenylnaphthalene dianion

A novel low-temperature intramolecular exchange was detected by (13)C NMR spectrometry in the Na(+) and K(+) salts of the title compound. The process causes the pairwise exchange in the dianion ring of C(2"), C(3"), and C(4") with C(8"), C(7"), and C(6"), respectively. The free energy of activation (DeltaG()(exch)) for the dipotassium salt (2(2-)/2K(+)) in THF-d(8) at 230 K is 12.6 kcal mol(-1). Two key questions are addressed: (1) Why are these carbons anisochronous and (2) what is the mechanism of exchange? NMR data for 1-cyclooctatetraenylnaphthalenedipotassium (3(2-)/2K(+)) as well as ab initio HF/3-21G(++) calculations for 3, 3(2-), and 3(2-)/2K(+) indicate that the nonequivalence is due to both slow rotation across a barrier at which the naphthalene and COT(2)(-) rings are approximately coplanar and slow inversion of the neutral COT ring. This results in the noteworthy circumstance of diastereotopic carbons being observed in a molecule that does not possess either a stereogenic or a prostereogenic center. Comparison of DeltaG()(exch) and DeltaG(++)(BS) for 2(2-)/2K(+) with the corresponding values for 2(2-)/2Na(+) and 2(2-)/2Li(+) and of DeltaG(++)(exch) with DeltaG(++) for ring inversion in 1,4-dicyclooctatetraenylnaphthalene leads to the conclusion that COT(2-) ring rotation and COT ring inversion both contribute to exchange in 2(2-)/2K(+) in a 3:1 ratio, but that exchange occurs exclusively by ring rotation in 2(2-)/2Li(+). The latter result is attributed to looser ion pairing in the dilithium (and disodium) salts.     

Arachno, nido, and closo aromatic isomers of the Li6B6H6 molecule

We analyzed chemical bonding in low-lying isomers of the recently computationally predicted B(6)H(6)Li(6) molecule. According to our calculations the benzene-like B(6)H(6)Li(6) (D(2h), (1)A(1g)) arachno structure with the planar aromatic B(6)H(6)(6-) anion is the most stable one. A nido isomer with two aromatic B(6)H(6)(4-) (pentagonal pyramid) and Li(3)(+) (triangular) moieties, which can be considered as derived from the global minimum structure through a two-electron intramolecular transfer from B(6)H(6)(6-) to three Li(+) cations, was found to be 10.7 kcal/mol higher in energy. A closo isomer with three aromatic moieties (octahedral B(6)H(6)(2-) and two Li(3)(+)) was found to be 31.3 kcal/mol higher in energy than the global minimum. Another isomer with three aromatic moieties (two B(3)H(3)(2-) and Li(3)(+)) was found to be substantially higher in energy (74.4 kcal/mol). Thus, the intramolecular electron transfers from the highly charged B(6)H(6)(6-) anion to cations are not favorable for the B(6)H(6)Li(6) molecule, even when a formation of three-dimensional aromatic B(6)H(6)(2-) anion and two sigma-aromatic Li(3)(+) cations occurs in the closo isomer.     

On the viability of small endohedral hydrocarbon cage complexes: X@C4H4, X@C8H8, X@C8H14, X@C10H16, X@C12H12, AND X@C16H16

Small hydrocarbon complexes (X@cage) incorporating cage-centered endohedral atoms and ions (X = H(+), H, He, Ne, Ar, Li(0,+), Be(0,+,2+), Na(0,+), Mg(0,+,2+)) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C(4)H(4), T(d)()) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C(8)H(8), O(h)()) and bicyclo[2.2.2]octane (C(8)H(14), D(3)(h)()) minima are limited to encapsulating species smaller than Ne and Na(+). Despite its intermediate size, adamantane (C(10)H(16), T(d)()) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li(0,+), Be(0,+,2+), Na(0,+), and Mg(2+). In contrast, the truncated tetrahedrane (C(12)H(12), T(d)()) encapsulates fewer species, while the D(4)(d)() symmetric C(16)H(16) hydrocarbon cage (see Table of Contents graphic) encapsulates all but the larger Be, Mg, and Mg(+) species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H antibonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (E(endo)) to the sum of their isolated components (E(inc) = E(endo) - E(cage) - E(x)) and to their exohedral isomer energies (E(isom) = E(endo) - E(exo)). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e., E(isom) is always exothermic), Be(2+)@C(10)H(16) (T(d)(); -235.5 kcal/mol), Li(+)@C(12)H(12) (T(d)(); 50.2 kcal/mol), Be(2+)@C(12)H(12) (T(d)(); -181.2 kcal/mol), Mg(2+)@C(12)H(12) (T(d)(); -45.0 kcal/mol), Li(+)@C(16)H(16) (D(4)(d)(); 13.3 kcal/mol), Be(+)@C(16)H(16) (C(4)(v)(); 31.8 kcal/mol), Be(2+)@C(16)H(16) (D(4)(d)(); -239.2 kcal/mol), and Mg(2+)@C(16)H(16) (D(4)(d)(); -37.7 kcal/mol) are relatively stable as compared to experimentally known He@C(20)H(20) (I(h)()), which has an E(inc) = 37.9 kcal/mol and E(isom) = -35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets.     

Reversal of stability on metalation of pentagonal-bipyramidal (1-MB6H7(2-) 1-M-2-CB5H7(1-) and 1-M-2,4-C2B4H7) and Icosahedral (1-MB11H12(2-) 1-M-2-CB10H12(1-) and 1-M-2,4-C2B9H12) boranes (M = Al, Ga, In, and Tl): energetics of condensation and relationship to binuclear metallocenes

The usual assumption of the extra stability of icosahedral boranes (2) over pentagonal-bipyramidal boranes (1) is reversed by substitution of a vertex by a group 13 metal. This preference is a result of the geometrical requirements for optimum overlap between the five-membered face of the ligand and the metal fragment. Isodesmic equations calculated at the B3LYP/LANL2DZ level indicate that the extra stability of 1-M-2,4-C(2)B(4)H(7) varies from 14.44 kcal/mol (M = Al) to 15.30 kcal/mol (M = Tl). Similarly, M(2,4-C(2)B(4)H(6))(2)(1-) is more stable than M(2,4-C(2)B(9)H(11))(2)(1-) by 9.26 kcal/mol (M = Al) and by 6.75 kcal/mol (M = Tl). The preference for (MC(2)B(4)H(6))(2) over (MC(2)B(9)H(11))(2) at the same level is 30.54 kcal/mol (M = Al), 33.16 kcal/ mol (M = Ga) and 37.77 kcal/mol (M = In). The metal-metal bonding here is comparable to those in CpZn-ZnCp and H(2)M-MH(2) (M= Al, Ga, and In).     

Complexes of a dianionic bis(diphosphinomethanide) with lithium, sodium, potassium and zirconium. Effect of substitution on the Schlenk dimerisation of vinylidene phosphines

The vinylidene phosphine (Pr(n)(2)P)(2)C=CH(2) (1) undergoes Schlenk dimerisation on treatment with an excess of any of the alkali metals Li, Na or K to give the butane-1,4-diide complexes [(L)M{(Pr(n)(2)P)(2)CCH(2)}](2)[(L)M =(THF)(2)Li (6), (THF)(3)Na (7b), (DME)(2)K (8b)], after recrystallisation. Whereas the reaction between the analogous phenyl derivative (Ph(2)P)(2)C=CH(2) and K results in cleavage of a P-C bond, 1 reacts smoothly with K to give 8, with no evidence for P-C cleavage. Compound 6 is an excellent ligand transfer reagent: metathesis reactions between either 6 or its phenyl analogue [(THF)(2)Li{(Ph(2)P)(2)CCH(2)}](2) (2) and two equivalents of Cp(2)ZrCl(2) in THF give the corresponding dinuclear zirconocene derivatives [Cp(2)Zr(Cl){(R(2)P)(2)CCH(2)}](2) in good yields [R = Ph (11), Pr(n)(12)]. Compounds 6, 7b, 8b, 11 and 12 have been characterised by multi-element NMR spectroscopy and, where possible, by elemental analysis; compounds 6, 7b, 11 and 12 have additionally been characterised by X-ray crystallography.     

Oxidative reactivity difference among the metal oxo and metal hydroxo moieties: pH dependent hydrogen abstraction by a manganese(IV) complex having two hydroxide ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of nature's choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).     

Activation of the sulfhydryl group by Mo centers: kinetics of reaction of benzyl radical with a binuclear Mo(micro-SH)Mo complex and with arene and alkane thiols

This paper provides evidence from kinetic experiments and electronic structure calculations of a significantly reduced S-H bond strength in the Mo(micro-SH)Mo function in the homogeneous catalyst model, CpMo(micro-S)(2)(micro-SH)(2)MoCp (1, Cp = eta(5)-cyclopentadienyl). The reactivity of 1 was explored by determination of a rate expression for hydrogen atom abstraction by benzyl radical from 1 (log(k(abs)/M(-)(1) s(-)(1)) = (9.07 +/- 0.38) - (3.62 +/- 0.58)/theta) for comparison with expressions for CH(3)(CH(2))(7)SH, log(k(abs)/M(-)(1) s(-)(1)) = (7.88 +/- 0.35) - (4.64 +/- 0.54)/theta, and for 2-mercaptonaphthalene, log(k(abs)/M(-)(1) s(-)(1)) = (8.21 +/- 0.17) - (4.24 +/- 0.26)/theta (theta = 2.303RT kcal/mol, 2sigma error). The rate constant for hydrogen atom abstraction at 298 K by benzyl radical from 1 is 2 orders of magnitude greater than that from 1-octanethiol, resulting from the predicted (DFT) S-H bond strength of 1 of 73 kcal/mol. The radical CpMo(micro-S)(3)(micro-SH)MoCp, 2, is revealed, from the properties of slow self-reaction, and exclusive cross-combination with reactive benzyl radical, to be a persistent free radical.     

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.     

Hydride donor abilities and bond dissociation free energies of transition metal formyl complexes

The hydride complex [Pt(dmpe)2H]+ (dmpe = 1,2-bis(dimethylphosphino)ethane) reversibly transfers H- to the rhenium carbonyl complex [CpRe(PMe3)(NO)(CO)]+, giving the formyl CpRe(PMe3)(NO)(CHO). From the equilibrium constant for the hydride transfer (16.2), the DeltaGdegrees for the reaction was determined (-1.6 kcal/mol), as was the hydride-donating ability of the formyl (44.1 kcal/mol). The hydride-donating ability, DeltaGdegrees(H-), is defined as the energy required to release the hydride ion into solution by the formyl complex [i.e. M(CHO) right arrow M(CO)+ + H-]. Subsequently, the hydride-donating ability of a series of formyl complexes was determined, ranging from 44 to 55 kcal/mol. With use of this information, two rhenium carbonyl complexes, [CpRe(NO)(CO)2]+ and [Cp*Re(NO)(CO)2]+, were hydrogenated to formyls, employing [Pt(dmpp)2]2+ and Proton-Sponge. Finally, the E(1/2)(I/0) values for five rhenium carbonyl complexes were measured by cyclic voltammetry. Combined with the known DeltaGdegrees(H-) values for the complexes, the hydrogen atom donating abilities could be determined. These values were all found to be approximately 50 kcal/mol.     

Hydrocarbon species mu3-CCH2-, mu3-CCH3 and mu-CHCH3 supported on Ti3O3

Treatment of the mu3-ethylidyne complex [{TiCp*(mu-O)}3(mu3-CMe)](1), (Cp*=eta5-C5Me5) with alkali metal amides leads to the oxoheterometallocubane derivatives [M(mu3-O)3{(TiCp*)3(mu3-CCH2)}] [M = Li (2), Na (3), K (4), Rb (5), Cs (6)] containing the naked carbanion mu3-CCH2-; the addition of triphenylmethanol and tert-butanol to the compounds 2-6 gives rise to the oxoderivatives [{TiCp*(mu-O)}3(mu-CHMe)(OCR3)][R = Me (7), Ph (8)] which show a mu-ethylidene bridge on the surface model Ti3O3.     

Synthesis, structure, and reactivity of O-donor Ir(III) complexes: C-H activation studies with benzene

Various new thermally air- and water-stable alkyl and aryl analogues of (acac-O,O)2Ir(R)(L), R-Ir-L (acac-O,O = kappa2-O,O-acetylacetonate, -Ir- is the trans-(acac-O,O)2Ir(III) motif, R = CH3, C2H5, Ph, PhCH2CH2, L = Py) have been synthesized using the dinuclear complex [Ir(mu-acac-O,O,C3)-(acac-O,O)(acac-C3)]2, [acac-C-Ir]2, or acac-C-Ir-H2O. The dinuclear Ir (III) complexes, [Ir(mu-acac-O,O,C3)-(acac-O,O)(R)]2 (R = alkyl), show fluxional behavior with a five-coordinate, 16 electron complex by a dissociative pathway. The pyridine adducts, R-Ir-Py, undergo degenerate Py exchange via a dissociative mechanism with activation parameters for Ph-Ir-Py (deltaH++ = 22.8 +/- 0.5 kcal/mol; deltaS++ = 8.4 +/- 1.6 eu; deltaG++298 K) = 20.3 +/- 1.0 kcal/mol) and CH3-Ir-Py (deltaH++ = 19.9 +/- 1.4 kcal/mol; deltaS++ = 4.4 +/- 5.5 eu; deltaG++298 K) = 18.6 +/- 0.5 kcal/mol). The trans complex, Ph-Ir-Py, undergoes quantitatively trans-cis isomerization to generate cis-Ph-Ir-Py on heating. All the R-Ir-Py complexes undergo quantitative, intermolecular CH activation reactions with benzene to generate Ph-Ir-Py and RH. The activation parameters (deltaS++ =11.5 +/- 3.0 eu; deltaH++ = 41.1 +/- 1.1 kcal/mol; deltaG++298 K = 37.7 +/- 1.0 kcal/mol) for CH activation were obtained using CH3-Ir-Py as starting material at a constant ratio of [Py]/[C6D6] = 0.045. Overall the CH activation reaction with R-Ir-Py has been shown to proceed via four key steps: (A) pre-equilibrium loss of pyridine that generates a trans-five-coordinate, square pyramidal intermediate; (B) unimolecular, isomerization of the trans-five-coordinate to generate a cis-five-coordinate intermediate, cis-R-Ir- square; (C) rate-determining coordination of this species to benzene to generate a discrete benzene complex, cis-R-Ir-PhH; and (D) rapid C-H cleavage. Kinetic isotope effects on the CH activation with mixtures of C6H6/C6D6 (KIE = 1) and with 1,3,5-C6H3D3 (KIE approximately 3.2 at 110 degrees C) are consistent with this reaction mechanism.     

Interaction of triatomic germanium with lithium atoms: electronic structure and stability of Ge3Lin clusters

Quantum chemical calculations were applied to investigate the electronic structure of mono-, di-, and tri- lithiated triatomic germanium (Ge3Lin) and their cations (n = 0-3). Computations using a multiconfigurational quasi-degenerate perturbation approach (MCQDPT2) based on complete active space CASSCF wavefunctions, MRMP2 and density functional theory reveal that Ge3Li has a 2A' ground state with a doublet-quartet gap of 24 kcal/mol. Ge3Li2 has a singlet ground state with a singlet-triplet (3A' '-1A1) gap of 30 kcal/mol, and Ge3Li3 a doublet ground state with a doublet-quartet (4A' '-2A') separation of 16 kcal/mol. The cation Ge3Li+ has a 1A' ground state, being 18 kcal/mol below the 3A' state. The computed electron affinities for triatomic germanium are EA(1) = 2.2 eV (experimental value is 2.23 eV), EA(2) = -2.5 eV, and EA(3) = -5.9 eV, for Ge3-, Ge32-, and Ge33-, respectively, indicating that only the monoanion is stable with respect to electron detachment, in such a way that Ge3Li is composed of Ge3-Li+ ions. An atoms in molecules (AIM) analysis shows the absence of a Ge-Ge-Li ring critical point in Ge3Li. An electron localization function (ELF) map of Ge3Li supports the view that the Ge-Li bond is predominantly ionic; however, a small covalent character could be anticipated from the Laplacian at the Ge-Li bond critical point. The ionic picture of the Ge-Li bond is further supported by the natural bond orbital (NBO) results. The calculated Li affinity value for Ge3 is 2.17 eV, and the Li+ cation affinity value for Ge3- amounts to 5.43 eV. The larger Li+ cation affinity of Ge3- favors an electron transfer, resulting in a Ge3-Li+ interaction.     

Direct Electrochemical Investigations of 17-Electron Complexes of CpM(CO)(3)(*) (M = Mo, W, and Cr)

Photolysis of complexes of the type M(2)(CO)(6)(RC(5)H(4))(2) (where M = W, Mo, Cr and R = H (Cp) or CH(3) (Cp')) leads to the production of short lived 17-electron radicals. Direct electrochemical characterization of these intermediates has been achieved using a technique known as photomodulated voltammetry (PMV). The results from PMV analysis are in excellent agreement with literature estimates for CpMo(CO)(3)(*) and CpCr(CO)(3)(*). However, CpW(CO)(3)(*) is found to be shifted oxidatively 115 mV relative to previous literature estimates. The change in the value for the tungsten complex changes previous estimates to the bond dissociation energy for tungsten metal hydrides by 3.0 +/- 0.9 kcal/mol. Lifetime information on the radicals is also reported based on the phase shift of the electrochemical signal observed by PMV under limiting current conditions.