The transition metal coordination chemistry of anion radicals

The stable diiron(2.5) complex ion [(NC)(5)Fe(mu-tz)Fe(CN)5](5-), tz = 1,2,4,5-tetrazine, and its neighboring oxidation states

The conceptually simple mixed-valent diiron compound (NEt(4))(5)[(NC)(5)Fe(mu-tz)Fe(CN)(5)] with the 1,2,4,5-tetrazine (tz) bridging ligand was obtained as a thermally and air-stable material that displays large and highly variable electrochemical comproportionation constants between about 10(8) (in water) and 10(19.0) (in acetonitrile). Strong metal-metal interaction is also evident from spectroscopic results obtained for the solid and for the dissolved species. The rather intense intervalence charge-transfer band occurs around 2400 nm; infrared and Mössbauer spectra reveal the high spectroscopic symmetry of the system according to an (Fe(2.5))(2) formulation. DFT calculations on the [(NC)(5)Fe(mu-tz)Fe(CN)(5)](6-) ion confirm the presence of very low-lying pi(tz) and high-lying d(Fe) orbitals.

Spectroscopic studies of metal complexes with redox-active hydrogenated Schiff bases

Synthesis and spectroscopic (IR, UV-visible, ESR) characterization of metal(II) complexes M(Lx')2, (where M = Co(II), NI(II), VO(II), Pd(II), Lx' = L1', L2', L3' are monoanion of unsubstituted, 5-Cl and 5-Br substituted-2-hydroxybenzylamine) with redox-active N-(3,5-ditert-butyl-4-hydroxyphenyl)-2-hydroxybenzylamine ligands as well as radical species generated from these compounds by the oxidation with PbO2 are reported. ESR studies indicate that the VO(Lx')2 and Ni(Lx')2 complexes, in opposite to their salicylaldimine precursors, are more readily oxidized with lead dioxide and results in the formation of the indophenoxyl type stable radical. The formed radical species are very similar to each other and quite different from those of the salicylaldimine analogous according to their g-factors and hyperfine coupling constants. The nine line radical spectra observed in the oxidation of Co(Lx')2, on standing under vacuum, gradually converted to the signals characteristic of the low-spin Co(II) (g(x,y) = 2.276, g(z) = 1.998, A(xy)Co = 122.7 G, A(z)Co = 150 G) and radical containing Co(III) intermediate with g(x,y) = 2.015, A(xy)Co = 4.66 G, g(z) = 1.989, A(z)Co = 10 G were also observed.

Coupling of Electron Transfer and Bond Dissociation Processes in Dinuclear Complexes with Rhodium and Iridium Reaction Centres Bridged by 2,2'-Bipyrimidine

The complexes [{MCl(η5-C5Me5)}2(μ-bpym)](PF6)2, bpym = 2,2'-bipyrimidine and M = Rh or Ir, were obtained as isomerically pure species (M = Ir) or as mixtures of cis/trans isomers (M = Rh). Even though these compounds undergo partial dissociation into the mononuclear bpym- and solvento-complexes in DMF or acetonitrile, cyclic voltammetry and, in part, spectroscopy (NMR, EPR, UV-VIS) could be used to analyze their reduction with up to five electrons. In acetonitrile at room temperature, the dirhodium compound displays two sequential chloride-dissociative two-electron cathodic steps, leading to the very reactive [{Rh(η5-C5Me5)}2(μ-bpym)]. At -15 °C in DMF, the diiridium compound was found to be sufficiently inert towards dissociation; it is then reversibly reduced to an EPR-detectable radical cation [{IrCl(η5-C5Me5)}2(μ-bpym)]•+ before two separated chloride-dissociative steps occur.

Molecular and electronic structures of bis(pyridine-2,6-diimine)metal complexes [ML2](PF6)n (n = 0, 1, 2, 3; M = Mn, Fe, Co, Ni, Cu, Zn)

A series of mononuclear, octahedral first-row transition metal ion complexes mer-[M(II)L0(2)](PF6)2 containing the tridentate neutral ligand 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (L0) and a Mn(II), Fe(II), Co(II), Ni(II), Cu(II), or Zn(II) ion have been synthesized and characterized by X-ray crystallography. Cyclic voltammetry and controlled potential coulometry show that each dication (except those of Cu(II) and Zn(II)) can be reversibly one-electron-oxidized, yielding the respective trications [M(III)L0(2)]3+, and in addition, they can be reversibly reduced to the corresponding monocations [ML2]+ and the neutral species [ML2]0 by two successive one-electron processes. [MnL2]PF6 and [CoL2]PF6 have been isolated and characterized by X-ray crystallography; their electronic structures are described as [Mn(III)L1(2)]PF6 and [Co(I)L0(2)]PF6 where (L1)1- represents the one-electron-reduced radical form of L0. The electronic structures of the tri-, di-, and monocations and of the neutral species have been elucidated in detail by a combination of spectroscopies: UV-vis, NMR, X-band EPR, Mossbauer, temperature-dependent magnetochemistry. It is shown that pyridine-2,6-diimine ligands are noninnocent ligands that can be coordinated to transition metal ions as neutral L0 or, alternatively, as monoanionic radical (L1)1-. All trications are of the type [M(III)L0(2)]3+, and the dications are [M(II)L0(2)]2+. The monocations are described as [Mn(III)L1(2)]+ (S = 0), [Fe(II)L0L1]+ (S = 1/2), [Co(I)L0(2)]+ (S = 1), [Ni(I)L0(2)]+ (S = 1/2), [Cu(I)L0(2)]+ (S = 0), [Zn(II)L1L0]+ (S = 1/2) where the Mn(II) and Fe(II) ions are low-spin-configurated. The neutral species are described as [Mn(II)L1(2)]0, [Fe(II)L1(2)]0, [Co(I)L0L1]0, [Ni(I)L0L1]0, and [Zn(II)L1(2)]0; their electronic ground states have not been determined.

Electron paramagnetic resonance and spectroscopic characteristics of electrogenerated mixed-valent systems [(eta 5-C5Me5)M(mu-L)M(eta 5-C5Me5)]+ (M = Rh, Ir; L = 2,5-diiminopyrazines) in relation to the radicals [(eta 5-C5Me5)ClM(mu-L)MCl(eta 5-C5Me5)]+ and [(eta 5-C5Me5)M(mu-L)MCl(eta 5-C5Me5)]2+

Electrochemical reduction of the dinuclear [(eta 5-C5Me5)ClM(mu-L)MCl(eta 5-C5Me5)]2+ ions (M = Rh, Ir; L = 2,5-bis(1-phenyliminoethyl)pyrazine (bpip) and 2,5-bis[1-(2,6-dimethylphenyl)iminoethyl]pyrazine (bxip)) proceeds via the paramagnetic intermediates [(eta 5-C5Me5)ClM(mu-L)MCl(eta 5-C5Me5)]+ (L = bpip) or [(eta 5-C5Me5)M(mu-L)MCl(eta 5-C5Me5)]2+ (L = bxip) and [(eta 5-C5Me5)M(mu-L)M(eta 5-C5Me5)]+. Whereas the first is clearly a radical species with a small g anisotropy, the chloride-free cations are distinguished by structured intervalence charge transfer (IVCT) bands in the near-infrared region and by rhombic electron paramagnetic resonance features between g = 1.9 and g = 2.3, which suggests considerable metal participation at the singly occupied MO. Alternatives for the d configuration assignment and for the role of the bisbidentate-conjugated bridging ligands will be discussed. The main difference between bpip and bxip systems is the destabilization of the chloride-containing forms through the bxip ligand for reasons of steric interference.

Synthesis and Characterization of Verdazyl Radicals Bearing Pyridine or Pyrimidine Substituents: A New Family of Chelating Spin-Bearing Ligands

The syntheses and characterization of two new 1,5-dimethyl-6-oxoverdazyl radicals bearing 2-pyridine and 4,6-dimethyl-2-pyrimidine rings as substituents are described. The radical precursors, the corresponding 1,2,4,5-tetrazanes, were prepared by condensation of the bis(1-methylhydrazide) of carbonic acid with the appropriate aromatic aldehyde. Oxidation of 3-(4,6-dimethyl-2-pyrimidyl)-1,5-dimethyl-1,2,4,5-tetrazane 6-oxide (7) with sodium periodate afforded 1,5-dimethyl-3-(4,6-dimethyl-2-pyrimidyl)-6-oxoverdazyl (4), which could be isolated and stored without decomposition. In contrast, attempts to oxidize the analogous 3-(2-pyridyl)-1,5-dimethyl-1,2,4,5-tetrazane 6-oxide (6) with periodate produced the 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl (3) which could not be isolated. However, oxidation of this tetrazane with benzoquinone produced the pyridylverdazyl 3 as a 1:1 complex with hydroquinone. This complex is indefinitely stable in the solid state and provides a means of long-term storage of the pyridylverdazyl. The electronic properties of both radicals have been characterized by EPR spectroscopy, cyclic voltammetry, and MNDO calculations. The radicals have a wide electrochemical window of stability (>1.8 V), and the EPR and computational studies indicate a large spin density residing on N2 and N4.

Variable Reduction Sequences for Axial (L) and Chelate Ligands (NwedgeN) in Rhenium(I) Complexes [(NwedgeN)Re(CO)(3)(L)](n)()

The rhenium(I) compounds [(NwedgeN)Re(CO)(3)(MQ)](PF(6))(2) (NwedgeN = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym), 3,3'-bipyridazine (bpdz), or 1,4,7,10-tetraazaphenanthrene (tap) and MQ(+) = N-methyl-4,4'-bipyridinium) undergo four one-electron reduction steps which could be analyzed using cyclic voltammetry, EPR, IR, and UV/vis spectroelectrochemistry. Due to the rather low-lying pi orbital of tap, the corresponding compound shows electron uptake by NwedgeN each time before MQ(+) is reduced. The opposite is observed for the complexes of the other chelate ligands NwedgeN, however, and the pi(NwedgeN) orbital aproaches the pi(MQ(+)) level in the order bpy < bpym < bpdz. Remarkably, the reduction processes of MQ(+) and bpdz in [(bpdz)Re(CO)(3)(MQ)](PF(6))(2) are separated by only 74 mV as deduced from IR spectroelectrochemical analysis. On reduction of the related compound [(bpy)Re(CO)(3)(mpz)](PF(6))(2) (mpz(+) = N-methylpyrazinium), the first two electrons are added to the axial ligand which has a lower-lying pi orbital than MQ(+) and cannot undergo intramolecular twisting.

Paramagnetic Cluster Ions [B(6)Hal(n)()Hal'(6)(-)(n)()](*)(-) (Hal, Hal' = Cl, Br, I). EPR Evidence for Radical Stabilization through Electronic Effects of the Halogen Substituents

Monoanionic hexaborate cluster radicals [B(6)Hal(n)()Hal'(6)(-)(n)()](*)(-) with mixed halogen substitution were prepared from oxidizable dianionic precursors and were characterized by vibrational and UV-vis spectroscopy. EPR studies of these and the structurally established homoleptic species [B(6)Hal(6)](*)(-) (Hal = Cl, Br, I) reveal strongly increasing g anisotropy and relaxation rate on replacing Cl by Br and especially I substituents; the very stable B(6)I(6)(*)(-) ion (g(1) = 2.04, g(2) = 1.66, g(3) = 1.15) thus exhibits an EPR spectrum only at 4 K. The extent of these effects is attributed to the Jahn-Teller situation in [B(6)X(6)](*)(-) with only partial occupancy of a degenerate MO. Both the absence of B-H bonds and the evidently strong participation of the halogen substituents in the singly occupied MO contribute to the extraordinary stability of these cluster radicals.

The mpz(+)/mpz(*) Pair as Organic Analogue of the NO(+)/NO(*) Ligand Redox System (mpz = N-Methylpyrazinium). A Combined Electrochemical and Spectroscopic Study (UV/Vis/NIR, IR, EPR) of Complexes [(mpz)M(CN)(5)](2)(-)(/3)(-) (M = Fe, Ru, Os)

The oxidation and reduction behavior of complexes [(mpz)M(CN)(5)](2)(-) between the group 8 pentacyanometalates(II) and the N-methylpyrazinium ion mpz(+) was studied using spectroelectrochemical techniques (UV/vis/NIR, IR, EPR) in nonaqueous and, in part, aqueous media. As a further source of information on the electronic structure, we studied the variable solvatochromism of MLCT absorption bands in the precursor ions [(mpz)M(CN)(5)](2)(-). The mpz-centered one-electron reduction showed a tendency for concomitant loss of cyanide (EC process) to yield ions [(mpz)M(CN)(4)](2)(-), the lability increasing in the order M = Os << Ru < Fe. In contrast to the EPR spectra obtained from reduction with S(2)O(4)(2)(-) in aqueous media, which showed no evidence for close mpz(*)/metal association, the intra muros electrolysis experiments in DMSO or acetonitrile yielded the following results: The EPR spectrum that was previously assigned to [(mpz)Fe(CN)(5)](3)(-) is now ascribed to [(mpz)Fe(CN)(4)](2)(-). Whereas the dissociatively inert osmium radical complex [(mpz)Os(CN)(5)](3)(-) is distinguished by a broad EPR signal with a rather large value g(iso) = 2.0157, the EPR spectrum of [(mpz)Ru(CN)(4)](3)(-) reveals well-resolved separate coupling with the (99)Ru and (101)Ru isotopes in natural abundance. Spectroelectrochemistry and reactivity patterns of the complex ions [(mpz)M(CN)(5)](2)(-) are reminiscent of results for corresponding nitrosyl complexes [(NO)M(CN)(5)](2)(-), which suggests that the mpz(+)/mpz(*) ligand redox pair can function as an organic analogue of the NO(+)/NO(*) redox system.

Bonding Properties of a Novel Inorganometallic Complex, Ru(SnPh(3))(2)(CO)(2)(iPr-DAB) (iPr-DAB = N,N'-Diisopropyl-1,4-diaza-1,3-butadiene), and its Stable Radical-Anion, Studied by UV-Vis, IR, and EPR Spectroscopy, (Spectro-) Electrochemistry, and Density Functional Calculations

Ru(SnPh(3))(2)(CO)(2)(iPr-DAB) was synthesized and characterized by UV-vis, IR, (1)H NMR, (13)C NMR, (119)Sn NMR, and mass (FAB(+)) spectroscopies and by single-crystal X-ray diffraction, which proved the presence of a nearly linear Sn-Ru-Sn unit. Crystals of Ru(SnPh(3))(2)(CO)(2)(iPr-DAB).3.5C(6)H(6) form in the triclinic space group P&onemacr; in a unit cell of dimensions a = 11.662(6) Å, b = 13.902(3) Å, c = 19.643(2) Å, alpha = 71.24(2) degrees, beta = 86.91(4) degrees, gamma = 77.89(3) degrees, and V = 2946(3) Å(3). One-electron reduction of Ru(SnPh(3))(2)(CO)(2)(iPr-DAB) produces the stable radical-anion [Ru(SnPh(3))(2)(CO)(2)(iPr-DAB)](*-) that was characterized by IR, and UV-vis spectroelectrochemistry. Its EPR spectrum shows a signal at g = 1.9960 with well resolved Sn, Ru, and iPr-DAB (H, N) hyperfine couplings. DFT-MO calculations on the model compound Ru(SnH(3))(2)(CO)(2)(H-DAB) reveal that the HOMO is mainly of sigma(Sn-Ru-Sn) character mixed strongly with the lowest pi orbital of the H-DAB ligand. The LUMO (SOMO in the reduced complex) should be viewed as predominantly pi(H-DAB) with an admixture of the sigma(Sn-Ru-Sn) orbital. Accordingly, the lowest-energy absorption band of the neutral species will mainly belong to the sigma(Sn-Ru-Sn)-->pi(iPr-DAB) charge transfer transition. The intrinsic strength of the Ru-Sn bond and the delocalized character of the three-center four-electron Sn-Ru-Sn sigma-bond account for the inherent stability of the radical anion.

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.

Bonding Properties of the 1,2-Semiquinone Radical-Anionic Ligand in the [M(CO)(4-n)(L)(n)(DBSQ)] Complexes (M = Re, Mn; DBSQ = 3,5-di-tert-butyl-1,2-benzosemiquinone; n = 0, 1, 2). A Comprehensive Spectroscopic (UV-Vis and IR Absorption, Resonance Raman, EPR) and Electrochemical Study

Rhenium and manganese complexes of the 3,5-di-tert-butyl-1,2-benzosemiquinone (DBSQ) ligand, [M(CO)(4)(DBSQ)], fac-[M(CO)(3)(L)(DBSQ)], and cis,trans-[M(CO)(2)(L)(2)(DBSQ)], with a widely varied nature of co-ligand(s) (L = THF, Me(2)CO, MeC(O)Ph, py, NEt(3), Ph(3)PO, SbPh(3), AsPh(3), PCy(3), P(OPh)(3), PPh(3), dppe-p, PPh(2)Et, P(OEt)(3), PEt(3)) were generated in solution and characterized as valence-localized molecules containing the radical-anionic DBSQ ligand bound to Re(I) or Mn(I) metal atoms. This is evidenced by the following. (i) Carbonyl stretching frequencies nu(C&tbd1;O) and average force constants k(av) are typical for Mn(I) or Re(I) carbonyls. (ii) Frequencies of the intra-dioxolene C=O bond stretching vibration, nu(C=O), lie within the 1400-1450 cm(-1) range which is diagnostic for coordinated semiquinones. (iii) EPR spectra indicate a very small spin density on the metal atom (0.2% < a(M)/A(iso) > 2.6%). (iv) Absorption spectra show Re(I) --> DBSQ MLCT electronic transitions characterized by a resonant enhancement of the Raman peaks due to the nu(C&tbd1;O) and intra-DBSQ nu(C=O) vibrations. (iv) Finally, the electrochemical pattern consists of DBSQ/DBQ and DBSQ/DBCat ligand-localized redox couples. All these properties are, in a limited range, dependent on the nature and, especially, the number of co-ligands L, indicating a small delocalization of the singly occupied MO of the DBSQ ligand over the metal atom. The extent of this delocalization may be finely tuned by changing the co-ligands, although in absolute terms, it remains rather limited, and the DBSQ ligand behaves toward Re(I) and Mn(I) as a very weak pi-acceptor only. The changes of the electronic properties of the metal center induced by the co-ligands are mostly compensated by more flexible M --> CO pi back-bonding as is manifested by large variations of the average C&tbd1;O stretching force constant.

pi Molecular Orbital Crossing a(2)(chi)/b(1)(psi) in 1,10-Phenanthroline Derivatives. Ab Initio Calculations and EPR/ENDOR Studies of the 4,7-Diaza-1,10-phenanthroline Radical Anion and Its M(CO)(4) Complexes (M = Cr, Mo, W)

Ab initio, semiempirical, and HMO perturbation calculations were employed to assess the relative positioning of the closely situated low-lying unoccupied pi MOs a(2)(chi) and b(1)(psi) in 1,10-phenanthroline (phen) and its 3,4,7,8-tetramethyl (tmphen) and four symmetrical diaza derivatives (n,m-dap). Compared to a(2)(chi), the b(1)(psi) pi MO is distinguished by markedly higher MO coefficients at the chelating nitrogen pi centers in 1,10-positions; eventually, a higher Coulomb integral value at those positions will thus always favor the lowering of b(1) beyond a(2). Using the Coulomb integral parameter at the chelating 1,10-nitrogen pi centers as the HMO perturbation variable, the crossing of both energy levels in terms of decreasing preference for the a(2)(chi) over the b(1)(psi) orbital as the lowest unoccupied MO follows the sequence 5,6-dap > 2,9-dap > 4,7-dap > phen > 3,8-dap. The calculations reveal a(2)(chi) as the LUMO in 5,6-dap for all reasonable perturbation parameters, in agreement with previous observations for ruthenium(II) complexes which reveal a discrepancy between the lowest-lying "redox pi orbital" (a(2)) and the "optical pi MO" (b(1)) to which the most intense low-energy MLCT transition occurs. According to the HMO calculations, the situation is more ambiguous for the 4,7-dap analogue, yet EPR/ENDOR studies clearly show that the one-electron-reduced ligand and its tetracarbonylmetal(0) complexes (Cr, Mo, W) have the b(1)(psi) orbital singly occupied. Only ab initio calculations with double-zeta basis and inclusion of d polarization functions reproduced correctly the experimentally observed orbital ordering for tmphen (a(2) < b(1)) and for phen and 4,7-dap (b(1) < a(2)).