The d3/d2 alkyne complexes [MX2(eta-RC[triple bond, length as m-dash]CR)Tp']z (X = halide, z = 0 and 1+): final links in a d6-d2 redox family tree.

The d4 halide complexes [MX(CO)(eta-RC[triple bond, length as m-dash]CR)Tp'] [R = Me, M = W, X = F; R = Ph, M = Mo or W, X = F or Cl; Tp' = hydrotris(3,5-dimethylpyrazolyl)borate] undergo two-electron oxidation in the presence of a halide source to give the d2 monocations [MX1X2(eta-PhC[triple bond, length as m-dash]CPh)Tp']+ (R = Me, M = W, X1 = X2 = F; R = Ph, M = Mo, X1 = X2 = F or Cl; M = W, X1 = X2 = F or Cl; X1 = F, X2 = Cl). Each monocation (R = Ph) shows two reversible one-electron reductions (the second process was not detected for R = Me) corresponding to the stepwise formation of the neutral d3 and monoanionic d4 analogues, [MX1X2(eta-PhC[triple bond, length as m-dash]CPh)Tp'] and [MX1X2(eta-PhC[triple bond, length as m-dash]CPh)Tp']- respectively; the potentials for the two processes can be 'tuned' over a range of ca. 1.0 V by varying M and X. Chemical one-electron reduction of [MX2(eta-PhC[triple bond, length as m-dash]CPh)Tp']+ gave [MX2(eta-PhC[triple bond, length as m-dash]CPh)Tp'] (M = Mo or W, X = F or Cl). X-Ray structural studies on the redox pairs [WX2(eta-PhC[triple bond, length as m-dash]CPh)Tp']z (X = F and Cl, z = 0 and 1+) show the alkyne to bisect the X-W-X angle in the d2 cations but align more closely with one M-X bond in the neutral d3 molecules, consistent with the anisotropic ESR spectra of the latter; the solution ESR spectrum of [MoF2(eta-PhC[triple bond, length as m-dash]CPh)Tp'] showed equivalent fluorine atoms, i.e the alkyne oscillates at room temperature. The successful isolation of [MX2(eta-PhC[triple bond, length as m-dash]CPh)Tp']+ and [MX2(eta-PhC[triple bond, length as m-dash]CPh)Tp'] completes a series in which d6 to d2 alkyne complexes are linked in a redox family tree by sequential one-electron transfer and substitution reactions. The implications for such trees in the production of new species and the selective synthesis of paramagnetic complexes acting as synthetically useful 'alkyne radicals' are discussed.

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.

The d4/d3 redox pairs [MX(CO)(eta-RC[triple bond, length as m-dash]CR)Tp']z (z=0 and 1): structural consequences of electron transfer and implications for the inverse halide order.

The d4 halide complexes [MX(CO)(eta-RC[triple bond, length as m-dash]CR)Tp'] {X=F, Cl, Br or I; R=Me or Ph; M=Mo or W; Tp'=hydrotris(3,5-dimethylpyrazolyl)borate} undergo one-electron oxidation to the d3 monocations [MX(CO)(eta-RC[triple bond, length as m-dash]CR)Tp']+, isolable for M=W, R=Me. X-Ray structural studies on the redox pairs [WX(CO)(eta-MeC[triple bond, length as m-dash]CMe)Tp']z (X=Cl and Br, z=0 and 1), the ESR spectra of the cations [WX(CO)(eta-RC[triple bond, length as m-dash]CR)Tp']+ (X=F, Cl, Br or I; R=Me or Ph), and DFT calculations on [WX(CO)(eta-MeC[triple bond, length as m-dash]CMe)Tp']z (X=F, Cl, Br and I; z=0 and 1) are consistent with electron removal from a HOMO (of the d4 complexes) which is pi-antibonding with respect to the W-X bond, pi-bonding with respect to the W-C(O) bond, and delta-bonding with respect to the W-Calkyne bonds. The dependence of both oxidation potential and nu(CO) for [MX(CO)(eta-RC[triple bond, length as m-dash]CR)Tp'] shows an inverse halide order which is consistent with an ionic component to the M-X bond; the small size of fluorine and its closeness to the metal centre leads to the highest energy HOMO and the lowest oxidation potential. In the cations [MX(CO)(eta-RC[triple bond, length as m-dash]CR)Tp']+ electronegativity effects become more important, leading to a conventional order for Cl, Br and I. However, high M-F pi-donation is still facilitated by the short M-F distance.

Structural consequences of the one-electron reduction of d4 [Mo(CO)2(eta-PhC[triple bond]CPh)Tp']+ and the electronic structure of the d5 radicals [M(CO)L(eta-MeC[triple bond]CMe)Tp'] {L = CO and P(OCH2)3CEt}.

Reduction of [M(CO)2(eta-RC[triple bond]CR')Tp']X {Tp' = hydrotris(3,5-dimethylpyrazolyl)borate, M = Mo, X = [PF6]-, R = R' = Ph, C6H4OMe-4 or Me; R = Ph, R' = H; M = W, X = [BF4]-, R = R' = Ph or Me; R = Ph, R' = H} with [Co(eta-C5H5)2] gave paramagnetic [M(CO)2(eta-RC[triple bond]CR')Tp'], characterised by IR and ESR spectroscopy. X-Ray structural studies on the redox pair [Mo(CO)2(eta-PhC[triple bond]CPh)Tp'] and [Mo(CO)2(eta-PhC[triple bond]CPh)Tp'][PF6] showed that oxidation is accompanied by a lengthening of the C[triple bond]C bond and shortening of the Mo-C(alkyne) bonds, consistent with removal of an electron from an orbital antibonding with respect to the Mo-alkyne bond, and with conversion of the alkyne from a three- to a four-electron donor. Reduction of [Mo(CO)(NCMe)(eta-MeC[triple bond]CMe)Tp'][PF6] with [Co(eta-C5H5)2] in CH2Cl2 gives [MoCl(CO)(eta-MeC[triple bond]CMe)Tp'], via nitrile substitution in [Mo(CO)(NCMe)(eta-MeC[triple bond]CMe)Tp'], whereas a similar reaction with [M(CO){P(OCH2)3CEt}(eta-MeC[triple bond]CMe)Tp']+ (M = Mo or W) gives the phosphite-containing radicals [M(CO){P(OCH2)3CEt}(eta-MeC[triple bond]CMe)Tp']. ESR spectroscopic studies and DFT calculations on [M(CO)L(eta-MeC[triple bond]CMe)Tp'] {M = Mo or W, L = CO or P(OCH2)3CEt} show the SOMO of the neutral d5 species (the LUMO of the d4 cations) to be largely d(yz) in character although much more delocalised in the W complexes. Non-coincidence effects between the g and metal hyperfine matrices in the Mo spectra indicate hybridisation of the metal d-orbitals in the SOMO, consistent with a rotation of the coordinated alkyne about the M-C2 axis.

Bi- and poly-metallic cyanide-bridged complexes of the redox-active cyanomanganese nitrosyl unit [Mn(CN)(PR3)(NO)(eta-C5H4Me)].

Cationic nitrile complexes and neutral halide and cyanide complexes, with the general formula [MnL1L2(NO)(eta-C5H4Me)]z, undergo one-electron oxidation at a Pt electrode in CH2Cl2. Linear plots of oxidation potential, Eo', vs. nu(NO) or the Lever parameters, EL, for L1 and L2, allow Eo' to be estimated for unknown analogues. In the presence of TlPF6, [MnIL'(NO)(eta-C5H4Me)] reacts with [Mn(CN)L(NO)(eta-C5H4Me)] to give [(eta5-C5H4Me)(ON)LMn(mu-CN)MnL'(NO)(eta5-C5H4Me)][PF6] which undergoes two reversible one-electron oxidations; DeltaE, the difference between the potentials for the two processes, differs significantly for stable cyanide-bridged linkage isomers. Novel pentametallic complexes such as [Mn[(mu-NC)Mn(CNBut)(NO)(eta5-C5H4Me)]4(OEt2)][PF6]2 and [Mn[(mu-NC)Mn(CNXyl)(NO)(eta5-C5H4Me)]4(NO3-O,O')][PF6], containing a trigonal bipyramidal and a distorted octahedral Mn(II) centre, respectively, result either from slow decomposition of the binuclear cyanide-bridged species or from the reaction of anhydrous MnI2 with four equivalents of [Mn(CN)L(NO)(eta5-C5H4Me)] in the presence of TlPF6.

Evidence for an unstable Bi(II) radical from Bi-O bond homolysis. Implications in the rate-determining step of the SOHIO process.

The reaction of BiCl(3) with the lithium salt of o-di-tert-butylphenol under nitrogen forms organic oxidation products rather than the expected Bi(OAr)(3) complex, and bismuth disproportionation products. Likewise, the decomposition of Bi(III) aryloxides Bi(O-2,6-(i)Pr(2)C(6)H(3))(3) and ClBi(O-2,4,6-(t)Bu(3)C(6)H(2))(3) leads to corresponding organic oxidation products. These reactions can be explained by Bi-O bond homolysis to form unstable Bi(II) radicals, analogous to a fundamental step suggested to intervene in the SOHIO process.

The d2/d3 alkyne redox pair [WF2(PhC identical to CPh)Tp']z (z = +1 or 0): missing links in a 'redox family tree'.

The d2/d3 redox pair [WF2(PhC identical to CPh)Tp']z [z = +1 or 0, Tp' = hydrotris(3,5-dimethylpyrazolyl)borate] is the missing link in a 'redox family tree' relating the d6 tricarbonyls [M(CO)3L]- to the d2 trihalides [MX3L] (M = Mo or W, L = Cp or Tp') by a series of stepwise reactions involving sequential one-electron oxidation followed by ligand substitution.

An EPR Study of 2,3-Bis(diphenylphosphino)maleic Anhydride (BMA) Complexes and the BMA Radical Anion.

EPR spectra are reported for four metal complexes of 2,3-bis(diphenylphosphino)maleic anhydride (BMA), [Co(2)(PhCCR)(CO)(4)(eta-BMA)](-), R = Ph, H, [Co(2)(PhCCPh)(CO)(4)(&mgr;-BMA)](-), and [PhCW(CO)(2)(BMA)Cl](-), as well as the radical anions, [BMA](-) and [BPCD](-), BPCD = 4,5-bis(diphenylphosphino)cyclopentene-1,3-dione. At room temperature, all spectra are 1:2:1 triplets due to hyperfine coupling to two equivalent (31)P nuclei with coupling to two equivalent (1)H nuclei for [BPCD](-) and unresolved coupling to one or two (59)Co nuclei for the Co complexes with chelating or bridging BMA, respectively. The (31)P couplings are temperature dependent, ca. -3 and -13 mG K(-)(1) for the metal complexes and ligand radical anions, respectively. At low temperature, the spectrum of [BMA](-) shows the presence of symmetric and asymmetric PPh(2) rotational conformers, related by the thermodynamic parameters DeltaH degrees = -0.8 +/- 0.2 kJ mol(-)(1) and DeltaS degrees = 4 +/- 1 J mol(-)(1) K(-)(1) and interconverted with activation parameters DeltaH() = 18.2 +/- 0.4 kJ mol(-)(1), DeltaS() = -30 +/- 2 J mol(-)(1) K(-)(1). The temperature dependence of the (31)P couplings is explained by a negative spin-polarization contribution to and a positive contribution due to P 3s character; the latter increases with the asymmetry of the PPh(2) conformations. The range of conformations accessible to the metal complexes is less than for the ligand radical anions, and accordingly the temperature dependence is significantly smaller.

A Triangular Copper(I) Complex Displaying Allosteric Cooperativity in Its Electrochemical Behavior and a Mixed-Valence Cu(I)-Cu(I)-Cu(II) State with Unusual Temperature-Dependent Behavior.

Reaction of the tris-chelating hexadentate podand ligand tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate (Tp(Py)) with [Cu(MeCN)(4)][PF(6)] affords [Cu(I)(3)(Tp(Py))(2)][PF(6)] (1), which was crystallographically characterized. 1.(MeCN)(2): C(52)H(44)B(2)Cu(3)F(6)N(20)P, orthorhombic, Pna2(1); a = 24.592(7), b = 16.392(5), c = 13.365(5) Å; Z = 4. Each Cu(I) ion is four coordinated by one N,N '-bidentate arm from each ligand; each ligand therefore donates each bidentate arm to a different Cu(I) ion. The isosceles triangular arrangement of Cu(I) ions with N-donor ligands is reminiscent of the tricopper(I) site of ascorbate oxidase. One-electron oxidation of 1 affords the Cu(I)(2)Cu(II) complex [Cu(3)(Tp(Py))(2)][PF(6)](2) (2). The potentials of the Cu(I)/Cu(II) redox couples are affected by the ease with which the accompanying geometric rearrangement can occur. Thus, the first oxidation of 1 is facile (-0.52 V vs the ferrocene/ferrocenium couple, Fc/Fc(+)), but as a result of the concomitant structural rearrangement the second oxidation is rendered much more difficult (+0.12 V vsFc/Fc(+)) and results in slow decomposition of the product. A third oxidation does not occur at accessible potentials. This complex therefore exhibits negative cooperative behavior, in which the geometric change accompanying one metal-based redox change hinders further redox changes at other sites via an allosteric effect. EPR studies on the mixed-valence complex 2 show that in frozen glasses below 120 K the unpaired electron is delocalized over two metal centers (7-line spectrum), but above 160 K the electron becomes localized and gives a simple axial spectrum. The electronic spectrum of 2 in solution shows an intense band at 910 nm (epsilon 2100 dm(3) mol(-)(1) cm(-)(1)) which we believe to be an IVCT band. The combination of EPR and electronic spectral studies show that 2 is class III (fully delocalized over 2 centers) below 120 K but class II (localized but strongly interacting) at higher temperatures.