Martin Minelli

Applications of 95Mo NMR to inorganic and bioinorganic chemistry

Abstract A variety of dinuclear Mo(V), Mo(IV), Mo(III) and Mo(I) complexes, and tri- and tetranuclear homo- and heterometallic complexes of molybdenum have been studied by 95 Mo NMR. The Mo(V) complexes, Cp′ 2 Mo 2 Y 2 (μ-X) 2 (Y = O or S, X = S or Se, Cp′ = η 5 -C 5 Me 5 ), exhibit resonances in the −93 to 478 ppm region, the chemical shifts being sensitive to changes in the oxygen, sulfur and selenium content of the coordination spheres. The Mo(IV) complexes, Cp′ 2 Mo 2 (μ-X) 2 (μ-X 2 ) (X = S or Se), and their derivatives exhibit resonances in the 382–790 ppm region: isomers of Cp′ 2 Mo 2 (μ-S) 2 (μ-SH) 2 which differ in the arrangement of the bridging ligands were also detected. Dinuclear Mo(III) complexes of the form Mo 2 L 6 (L = amido, alkoxy or alkyl ligand) exhibit very deshielded resonances (2430–3624 ppm). The complexes Cp 2 Mo 2 {μ-S 2 C 2 (CF 3 ) 2 } 2 and Cp 2 Mo 2 (CO) 2 {μ-S 2 C 2 (CF 3 ) 2 } 2 (Cp = η 5 -C 5 H 5 ) exhibit resonances consistent with their formulation as distinct Mo(III) and mixed-valence Mo(IV-II) complexes, respectively. The Mo(I) complexes exhibit resonances which are very sensitive to the bond order of the MoMo bond: the resonances of the L 2 Mo 2 (CO) 6 complexes (L = Cp, −1856 ppm; L = Cp′, −1701 ppm) are more than 1800 ppm more shielded than the triply metalmetal bonded complexes, L 2 Mo 2 (CO) 4 (L = Cp, 182 ppm; L = Cp′, 133 ppm). The molybdenum containing homo- and heterometallic complexes exhibit resonances in the −133 to −1619 ppm region. The Fe 2 Mo 2 cubane complex, Cp′ 2 Mo 2 Fe 2 (μ 3 -S) 4 (CO) 4 , exhibits a resonance at −506 ppm. The ease of observation of the sulfurized complexes suggests that 95 Mo NMR may be a valuable technique for the study of hydro-desulfurization processes.

Kinetik und mechanismus der reversiblen öffnung von metall—metall-bindungen am beispiel von Cp(CO)2MnFe2(CO)6PR☆

Abstract The closed tetrahedral clusters Cp(CO)2MnFe2(CO)6(μ3-PR) (1) reversibly add two-electron donors L to give the compounds Cp(CO)2MnFe2(CO)6(μ3-PR)L (2) with an opened MnFe bond. The kinetics of the reaction 1 + L ʗ 2 have been carefully analyzed under a variety of conditions. The experimental results are consistent with the following mechanistic model: The closed clusters, 1, are in equilibrium with their valence tautomeric, unsaturated form with an opened MnFe bond, 1★. The enthalpy of activation ΔH‡ for this process is around 90 kJ/mol, closely comparable with the bond enthalpy of an MnFe bond. The activated complex, 1★, formed in the preequilibrium, consecutively adds L to yield 2. The reaction 2 → 1 is initiated by the loss of L. The corresponding enthalpies of activation (ΔH‡ ⩾ 100 kJ/mol) are within the range expected for the dissociation of L; the entropies of activation are positive in all cases. This mechanistic model is experimentally substantiated here for the first time. Its basic significance towards the understanding of cluster reactivity is discussed.

Synthese und struktur von [(Cp(CO)2Mn)2TePh]x (x = ± 1) ein beitrag zum verständnis der valenztautomerie bei zweikernkomplexen (LnM)2XR

Abstract The salts [(Cp(CO)2Mn)2TePh]+PF6− (III) and [(Cp(CO)Mn)2TePh]−NMe4+ (II) are obtained from (Cp(CO)2MnTePh)2 by AgPF6 oxidation or NaBH4 reduction, respectively. The Te coordination in II is pseudotetrahedral; the cation of III, albeit isoelectronic to stibinidene complexes (Cp(CO)2MN)2SbR, shows pyramidal coordination around Te with a long MnMn bond joining the two metal centers.

95Mo NMR spectral studies on seven-coordinate molybdenum(II) isocyanide complexes

Abstract The 95 Mo NMR spectra of a series of seven-coordinate molybdenum(II) isocyanide complexes of the types [Mo(CNR) 7- n L n ](PF 6 ) 2 (R = CH 3 , CHMe 2 , CMe 3 , C 6 H 11 , CH 2 Ph; L = py, bpy, Me 2 bpy, phen, dppe, P-n-Bu 3 ; n = 0,1,2) [Mo(CNC-Me 3 ) 6 X]PF 6 (X = Cl, Br, I) and [{Mo(CNCMe 3 ) 4 (NN)} 2 (μ-CN)](PF 6 ) 3 (NN = bpy, Me 2 bpy, phen) have been studied. The 95 Mo chemical shift range for this group of complexes is about 1100 ppm. An increase in the size of the R group attached to the isocyanide ligand generally tends to shield the 95 Mo nucleus. Replacement of the isocyanide ligand with a phosphorus ligand also increases the shielding, whereas the replacement of isocyanide with a heterocyclic nitrogen donor leads to deshielding by 800–900 ppm. This group of complexes shows a normal halogen dependence, i.e. replacement of Cl − by Br − and I − increases the shielding of the 95 Mo nucleus. The cyano-bridged cations [{Mo(CNCMe 3 ) 4 (NN)} 2 (μ-CN)] 3+ (NN = bpy, Me 2 bpy, or phen) show two 95 Mo NMR signals, one for the molybdenum coordinated to the carbon of the bridging CN and one for the N -coordinated molybdenum. Comparison of the chemical shifts and linewidths of the cyano-bridged species with those of the corresponding mononuclear molybdenum(II) complexes [Mo(CNCMe 3 ) 5 (NN)](PF 6 ) 2 leads to the assignment of the more deshielded signal to the N -coordinated molybdenum. The 14 N and 31 P NMR spectra for these complexes have also been measured, as have the 13 C NMR spectra of the pairs of complexes [Mo(CNCMe 3 ) 5 (NN)](PF 6 ) 2 and [{Mo(CNCMe 3 ) 4 (NN)} 2 (μ-CN)](PF 6 ) 3 (NN = bpy or phen). The 183 W NMR spectra for [W(CNR) 5 (bpy)](PF 6 ) 2 (R = CMe 3 and CH 2 Ph), show that the δ( 183 W)/δ( 95 Mo) chemical shift ratios for isocyanide complexes are different from the ratio found for M 0 and M VI .

Direct observation of diastereomers with opposite Mo configurations by 95Mo NMR

Abstract Molybdenum-95 NMR has been used to directly detect the two diastereomers differing only in the Mo configuration in the square pyramidal molybdenum(II) complex [C 5 H 5 Mo(CO) 2 NN★]PF 6 , where NN★ is the chiral pyridine-2-carbaldimine ligand derived from pyridine-2-carbaldehyde and (S)(-)-1-phenylethylamine. The simplicity of the spectra clearly reveals the optical purity of the complex with respect to the metal center.

STRUCTURAL, SPECTROSCOPIC AND ELECTROCHEMICAL STUDIES ON A NEW MO(VI) CATECHOLATE COMPLEX

Abstract The reaction of cis-[MoO2(C5H10NO)2] (C5H10NOH = 1-piperidinol) with benzene 1,2-diol(catechol) in CHCl3 leads to the formation of [MoO(C5H10NO)2(C6H4O2)], which has been characterized by X-ray crystallography, NMR spectroscopy and cyclic voltammetry.

A molybdenum-95 and nitrogen-14 nuclear magnetic resonance study of six-co-ordinate hydrotris(3,5-dimethyl-1-pyrazolyl)borate complexes containing a sixteen-electron {Mo(NO)}4 core

The mononitrosyl complexes [Mo{HB(Me2pz)3}(NO)XY][HB(Me2pz)3–= hydrotris (3,5-dimethyl-1-pyrazolyl)borate; X = Y = F, Cl, I, OEt, OPh, SPh, or NHPh; X = Cl, Y = SPh, OPh, OEt, NHC6H4Br-p, NHC6H4Me-p, or NHEt; X = I, Y = OPh, NHC6H4Me-p, NHEt, or NHNMe2] have been studied in solution by 95Mo and 14N n.m.r. spectroscopy. The iodo complex, [Mo{HB(Me2pz)3}(NO)I2], exhibits a 95Mo resonance at δ 2 272, the most deshielded monomeric MoII resonance reported to date. Replacement of the iodo ligands of [Mo{HB(Me2pz)3}(NO)I2] by other ligands increases the shielding of the 95Mo nucleus according to the general trend, I– < Cl– < F– < thiolato < alkoxy < amido. A chemical shift range of δ 161–2 272 was observed for the complexes studied. The complexes exhibit an inverse halogen dependence of the chemical shift. The nitrosyl ligand 14N chemical shifts follow the same shielding trend as the 95Mo chemical shifts.