NMR Spectroscopic Characterization of Methylcobalt(III) Compounds with Classical Ligands. Crystal Structures of [Co(NH(3))(5)(CH(3))]S(2)O(6), trans-[Co(en)(2)(NH(3))(CH(3))]S(2)O(6) (en = 1,2-Ethanediamine), and [Co(NH(3))(6)]-mer,trans-[Co(NO(2))(3)(NH(3))(2)(CH(3))](2)-trans- [Co(NO(2))(4)(NH(3))(2)]

Co(III) complexes of the pentadentate NHC ligand PY4Im: carbene-induced trans influences and the non-disappearing 13C NMR peak

Co(III) complexes of the N-heterocyclic carbene ligand PY4Im (PY4Im = (1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene)) having the general formula [(PY4Im)Co(X)](ClO4)n (X = NCMe; n = 3: OH-, N3-, NCS-, ONO-, F-; n = 2: O2CO2-, n = 1; (N3-)3, n = 0) were prepared and structurally characterised. X-ray structural data are consistent with the presence of a trans influence due to the coordinated carbene carbon, and this is also supported by computational results. 13C NMR spectra of the complexes did not display peaks corresponding to the carbene carbon, except in the case of the [(PY4Im)Co(O2CO)]+ cation, where a peak at δ = 170.21 ppm was observed. However, HMBC spectra allowed indirect determination of the chemical shifts of the carbene carbon in the remaining complexes, owing to the geometry of the PY4Im ligand. Calculated 13C chemical shifts for the complexes showed very good agreement with the experimental values for all but the carbene carbon atoms in all cases.

Understanding Chemical Selectivity through Well Selected Excited States

In this publication, we propose a new set of reactivity/selectivity descriptors, derived within a Rayleigh-Schrödinger perturbation theory framework, for chemical systems undergoing an electrostatic (point-charge) perturbation. From the electron density polarization at first order, qualitative insight on reactivity is retrieved, while more quantitative information (noteworthy selectivity) can be obtained from either the second-order energy response or the number of shifted electrons under perturbation. Noteworthily, only a small number of excitations contribute significantly to the overall responses to perturbation, suggesting chemical reactivity could be foreseen by a careful scrutiny of the electron density reorganization upon excitation.

Towards the first theoretical scale of the trans effect in octahedral complexes

In this paper, we show that trans effects in octahedral complexes can primarily be related to differences in the ability, for a given ligand, to cede electron density to the metal cation under the influence of the ligand at the trans position.

UV-visible spectroscopy of macrocyclic alkyl, nitrosyl and halide complexes of cobalt and rhodium. Experiment and calculation

Visible light photolysis of Co(NH₃)₅CH₃²⁺ generates alkyl radicals, unlike the halide complexes Co(NH₃)₅X²⁺ which produce halogen anions, X⁻.

The effect of remote substitution on the formation of preferential isomers of cobalt(iii)-tetrazolate complexes by microwave assisted cycloaddition

Cycloaddition of cis-[Co(N₃)₂(en)₂]NO₃1 or trans-[Co(N₃)₂(en)₂]ClO₄1a with organonitriles affords cis/trans cobalt bis-tetrazolate complexes, depending upon ligand substitution, solvent molecules and counteranions.

Solid-state 13C and 59Co NMR spectroscopy of 13C-methylcobalt(iii) complexes with amine ligands

Five octahedral Co(iii) cations, [trans-Co(en)(2)(X)((13)CH(3))](n+) where en = ethylenediamine, X = CN(-), N(3)(-), NH(3), NO(2)(-) or H(2)O and n = 1 or 2, as well as [Co(NH(3))(5)(13)CH(3)](2+), have been investigated by solid-state (13)C and (59)Co NMR spectroscopy. We show that the determination of the (59)Co nuclear quadrupolar parameters both directly via(59)Co NMR and indirectly via(13)C NMR provide complementary information that is unavailable if one investigates only one nucleus. Specifically, (1)J((59)Co,(13)C) and the orientation of the largest component of the EFG were determined via(13)C NMR spectroscopy, which also established the negative sign of C(Q)((59)Co). Cobalt-59 NMR spectroscopy was used to characterize the cobalt magnetic shielding tensor, to verify the magnitudes of C(Q)((59)Co) and to establish the value of eta(Q), which is difficult to determine indirectly. The measurements show that the EFG tensors are either axially symmetric or close to being so, but there is a wide range of C(Q) values, from -40 MHz for the complex with X = H(2)O to -105 MHz with X = CN(-). The Co chemical shift tensors are approximately axially symmetric with the spans, delta(11)-delta(33), ranging from 3700 to 5600 ppm for X = H(2)O and CN(-), respectively. The latter measurements also established the relative orientations of the Co EFG and chemical shift tensors. Density functional theory calculations of the (59)Co EFG and magnetic shielding tensors as well as of (1)J((59)Co,(13)C) for the NO(2)(-) and N(3)(-) complexes were undertaken. These calculations confirm the experimental observation that the sign of C(Q) is negative and that the largest component of the EFG is along the Co-methyl-carbon bond.

Mechanistic Elucidation of the Substitution Behavior of Alkyl Cobaloximes in Water and Methanol as Solvents

The ligand substitution behavior of trans-[Co(Hdmg)2(R)S] (R = CH3, PhCH2; Hdmg = dimethylglyoximate; S = H2O and/or MeOH) was studied for imidazole, pyrazole, 1,2,4-triazole, N-acetylimidazole, 5-chloro-1-methylimidazole, NO2-, Ph3P, Ph3As, and Ph3Sb as entering ligands. In all cases, except for Ph3As and Ph3Sb, the observed kinetics shows a linear dependence on the entering nucleophile concentration with no evidence for a back reaction. In the case of Ph3As and Ph3Sb as entering nucleophiles, kinetic evidence for a reverse solvolysis reaction is at hand. Activation parameters (DeltaH++, DeltaS++, and DeltaV++) were determined from the temperature and pressure dependence of all studied reactions and support the operation of a dissociatively activated substitution mechanism. The rate and activation parameters show that there is an increase in the dissociative character from a dissociative interchange to a limiting dissociative mechanism that depends on the nature of R and the entering nucleophile. The crystal structure of trans-[Co(en)2(Me)H2O]2+ was determined by X-ray analysis. The Co-O and Co-C bond lengths were found to be 2.153(6) and 1.995(10) angstroms, respectively. The kinetic and structural data are discussed in reference to a series of earlier studied systems for which data are reported in the literature.

Spin−Orbit-Induced Anomalous pH-Dependence in 1H NMR Spectra of CoIII Amine Complexes: A Diagnostic Tool for Structure Elucidation

The pH-dependent (1)H NMR characteristics of a series of Co(III)-(polyamin)-aqua and Co(III)-(polyamin)-(polyalcohol) complexes, [Co(tach)(ino-kappa(3)-O(1,3,5))](3+) (1(3+)), [Co(tach)(ino-kappa(3)-Omicron(1,2,6))](3+) (2(3+)), [Co(tach)(taci-kappa-Nu(1)-kappa(2)-O(2,6))](3+) (3(3+)), [Co(ditame)(H(2)O)](3+) (4(3+)), and [Co(tren)(H(2)O)(2)](3+) (5(3+)), were studied in D(2)O by means of titration experiments (tach = all-cis-cyclohexane-1,3,5-triamine, ino = cis-inositol, taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol, tren = tris(2-aminoethyl)amine, ditame = 2,2,6,6-tetrakis-(aminomethyl)-4-aza-heptane). A characteristic shift was observed for H(-C) hydrogen atoms in the alpha-position of a coordinated amino group upon deprotonation of a coordinated oxygen donor. For a cis-H-C-N-Co-O-H arrangement, deprotonation of the oxygen donor resulted in an additional shielding (shift to lower frequency) of the H(-C) proton, whereas for a trans-H-C-N-Co-O-H arrangement, deprotonation resulted in a deshielding (shift to higher frequency). The effect appears to be of rather general nature: it is observed for primary (1(3+)-5(3+)), secondary (4(3+)), and tertiary (5(3+)) amino groups, and for the deprotonation of an alcohol (1(3+)-3(3+)) or a water (4(3+), 5(3+)) ligand. Spin-orbit-corrected density functional calculations show that the high-frequency deprotonation shift for the trans-position is largely caused by a differential cobalt-centered spin-orbit effect on the hydrogen nuclear shielding. This effect is conformation dependent due to a Karplus-type behavior of the spin-orbit-induced Fermi-contact shift and thus only significant for an approximately antiperiplanar H-C-N-Co arrangement. The differential spin-orbit contribution to the deprotonation shift in the trans-position arises from the much larger spin-orbit shift for the protonated than for the deprotonated state. This is in turn due to a trans-effect of the deprotonated (hydroxo or alkoxo) ligand, which weakens the trans Co-N bond and thereby interrupts the Fermi-contact mechanism for transfer of the spin-orbit-induced spin polarization to the hydrogen nucleus in question. The unexpectedly large long-range spin-orbit effects found here for 3d metal complexes are traced back to small energy denominators in the perturbation theoretical expressions of the spin-orbit shifts.

NMR Spectroscopic and X-ray Crystallographic Study of Methylcobalt(III) Compounds with Saturated Amine Ligands

The (13)C chemical shifts of methylcobalt(III) compounds with saturated amine ligands in cis positions to the methyl group and a monodentate ligand, L = CN(-), NH(3), NO(2)(-), N(3)(-), H(2)O, or OH(-), in the trans position are reported. The amine ligands used, 1,2-ethanediamine (en), 1,3-propanediamine (tn), N,N'-bis(2-aminoethyl)-1,3-propanediamine (2,3,2-tet), N,N'-bis(3-aminopropyl)-1,2-ethanediamine (3,2,3-tet), and 1,4,8,11-tetraazacyclotetradecane (cyclam), all exert an apparent cis influence on the (13)C resonance signal of the coordinated methyl group. In the trans-[Co(en)(2)(CH(3))(L)](n+) series the (15)N resonance frequency of the coordinated en has also been measured. The influence of L on the en (15)N chemical shifts is reverse the influence on the methyl (13)C chemical shifts except in the case of L = NO(2)(-), which affects a further deshielding of the amine nitrogen nucleus. The methyl (1)J(CH) coupling constants in the trans-[Co(en)(2)(CH(3))(L)](n+) series range from 128.09 Hz (L = CN(-)) to 134.11 Hz (L = H(2)O). The crystal structures of trans-[Co(en)(2)(CH(3))(ClZnCl(3))], trans-[Co(3,2,3-tet)(CH(3))(N(3))]ClO(4), trans,trans-[(CH(3))(en)(2)Co(CN)Co(en)(2)(CH(3))](PF(6))(3)(CH(3)CN), and cis-[Co(en)(2)(CH(3))(NH(3))]ZnCl(4) were determined from low-temperature X-ray diffraction data.

The profound influence of a single metal–carbon bond on the reactivity of bio-relevant cobalt(iii) complexes

This Perspective reports the development of mechanistic insight over the past 6 years, on the substitution behaviour of cobalamins that contain a single Co-C bond. The effect of the alkyl group, located in the trans position, on the thermodynamic, kinetic and ground state trans effect, was studied in detail. The substitution reactions of different alkylcobalamins with CN- were investigated, the apparent mechanistic discrepancy reported for the co-enzyme B12 was resolved and a logical explanation could be offered. In addition, a complete picture of the effect of pressure on the UV-Vis spectra of different base-on and base-off cobalamins is presented, which clearly shows the role of the alkyl group in controlling the equilibrium between five- and six-coordinate species, and the possible participation of such species in the studied ligand substitution reactions. The kinetics of the base-on/base-off equilibration was studied for the first time using a pH-jump technique. All in all the novel mechanistic information adds to the understanding of the profound effect that a single metal-carbon bond can have on the reactivity of such Co(III) complexes.

Migratory insertion of hydrogen isocyanide in the pentacyano(methyl)cobaltate(III) anion

The preparation of the pentacyano(iminiumacetyl)cobaltate(III) anion and its N-methyl and N,N-dimethyl derivatives is reported. The iminiumacetyl group is formed by migratory insertion of cis hydrogen isocyanide in the pentacyano(methyl)cobaltate(III) anion. The new compounds have been spectroscopically characterized by (1)H, (13)C, (15)N, and (59)Co NMR spectroscopy and by absorption spectroscopy. The iminium carbon atoms yield (13)C NMR signals at 256.7, 247.7, and 240.4 ppm for the parent iminiumacetyl compound and its N-methyl and N,N-dimethyl derivatives, respectively. The (15)N resonance frequencies of the iminium groups and the lack of rotation of the carbon-nitrogen bond both show that this bond is best described as a double bond. The structure of (Et(4)N)(Ph(4)As)(2)[Co(CN)(5)(CH(3))] was determined by X-ray crystallography at 122.0(5) K. The structure displays disorder.

X-ray diffraction, Raman spectroscopic, and solid-state NMR studies of the group 14 metal-(tetracarbonyl)cobalt complexes Ph3MCo(CO)4 (M = Si, Sn, Pb)

Single-crystal X-ray diffraction studies illustrate that the three title compounds are isomorphous, belonging to the triclinic space group P[Formula: see text], with slightly distorted trigonal bipyramidal geometry about cobalt. The solid-state 29Si, 119Sn, and 207Pb cross-polarization magic angle spinning (CP MAS) NMR spectra are presented. The indirect spin–spin coupling constant (J), quadrupolar–dipolar shift (d), direct dipolar coupling constant (D' ), anisotropy in spin–spin coupling (ΔJ), and the chemical shift tensor were extracted. A plot of the reduced coupling constant vs. s-electron densities at the nucleus indicates that the Fermi contact term may be dominant for the tin and lead complexes; however, the large ΔJ for all complexes indicate that there are also significant anisotropic terms. Trends in the Raman scattering spectra are also discussed.Key words: 29Si, 119Sn, and 207Pb CP MAS NMR, tetracarbonyl cobalt, spin–spin coupling, chemical shift tensor, quadrupole coupling, Fermi contact, cobalt–group 14.

Statistical and molecular mechanics analysis of the effects of changing donor type on bond length in the two series [Co(III)N(n)O(6-n)] and [Ni(II)N(n)O(6-n)] (n = 0-6): a new route to bond-stretch Parameters

Changes in bond lengths across the series of complexes [Co(III)N(n)O(6-n)] and [Ni(II)N(n)O(6-n)], (n = 0-6) have been examined by a statistical analysis of the bond lengths in 256 Co(III) and 205 Ni(II) complexes. In both cases a systematic reduction in both metal-N and metal-O bond lengths is observed as oxygen donors replace nitrogen in the coordination sphere. In the case of Co(III), the reduction in bond lengths is linear across the series, whereas, in the case of Ni(II), it is more asymptotic in nature. It was found that this systematic change to the inner coordination sphere produced a much larger range of bond lengths than had previously been observed by changes to the outer sphere. The trends across the two series were reproduced using molecular mechanics; however, the magnitude of the change was not initially predicted correctly in either case. Alterations to molecular mechanics parameters that reproduced the trends in the [Co(III)N(n)O(6-n)] series also resulted in a significant overall improvement in the predictions of Co(III)-N bond lengths in a series of Co(III) hexaamines, with all being reproduced within 0.006 A. This improvement was taken as an indication that the bond-length reduction across the series is largely steric in origin.

Synthesis and DNA binding of novel water-soluble cationic methylcobalt porphyrins

Organocobalt derivatives of tetracationic water-soluble porphyrins are difficult to prepare via the typical reductive alkylation of the Co(II)(por) (porH(2) = porphyrin ligand). None have been reported. The problem may arise because the porphyrin core is made relatively electron poor by the positively charged peripheral groups. We have circumvented this problem by using the [Co(III)(NH(3))(5)CH(3)](2+) reagent, which inserts the Co(III)-CH(3) moiety directly into porH(2) in water under basic conditions. The method afforded two new [CH(3)Co(por)](4+) derivatives, [CH(3)CoTMpyP(4)](4+) and [CH(3)CoTMAP](4+), where [TMpyP(4)](4+) and [TMAP](4+) are the coordinated, NH-deprotonated forms of meso-tetrakis(N-methyl-4-pyridiniumyl)porphyrin and meso-tetrakis(N,N,N-trimethylaniliniumyl)porphyrin, respectively. The binding of the two new [CH(3)Co(por)](4+) cations to DNA and to the synthetic DNA polymers [poly(dA-dT)](2) and [poly(dG-dC)](2) was studied. Using published criteria by which changes in DNA viscosity and in the visible and CD spectra in the Soret region can be used to assess DNA binding, we conclude that both are outside binders. A large hypochromicity of the Soret bands of the [CH(3)Co(por)](4+) cations observed upon outside binding to DNA may indicate a high degree of self-stacking. The visible absorption and CD spectra of the [CH(3)Co(por)](4+) cations in the presence of 1:1 mixtures of [poly(dA-dT)](2) and [poly(dG-dC)](2) are nearly identical to those with [poly(dA-dT)](2) alone and are very different from those of [poly(dG-dC)](2) alone. Thus, both cations show a high preference for outside binding at AT-rich over GC-rich DNA sites. Upon binding of each of the [CH(3)Co(por)](4+) cations to all of the DNA polymers, the Soret bands exhibit blue shifts, whereas the Soret bands of the corresponding [(H(2)O)(2)Co(por)](5+) cations exhibit red shifts. The blue shifts strongly suggest that the [CH(3)Co(por)](4+) cations, particularly [CH(3)CoTMAP](4+), become five-coordinate forms to some extent on DNA binding; this result is the first good evidence for the presence at equilibrium of five-coordinate CH(3)Co(III)(N(4)) forms in water.

Ligand substitution behavior of a simple model for coenzyme B12

The ligand substitution reactions of trans-[CoIII(en)2(Me)H2O]2+, a simple model for coenzyme B12, were studied for cyanide and imidazole as entering nucleophiles. It was found that these nucleophiles displace the coordinated water molecule trans to the methyl group and form the six-coordinate complex trans-[Co(en)2(Me)L]. The complex-formation constants for cyanide and imidazole were found to be (8.3 +/- 0.7) x 10(4) and 24.5 +/- 2.2 M-1 at 10 and 12 degrees C, respectively. The second-order rate constants for the substitution of water were found to be (3.3 +/- 0.1) x 10(3) and 198 +/- 13 M-1 s-1 at 25 degrees C for cyanide and imidazole, respectively. From temperature and pressure dependence studies, the activation parameters delta H++, delta S++, and delta V++ for the reaction of trans-[CoIII(en)2(Me)H2O]2+ with cyanide were found to be 50 +/- 4 kJ mol-1, 0 +/- 16 J K-1 mol-1, and +7.0 +/- 0.6 cm3 mol-1, respectively, compared to 53 +/- 2 kJ mol-1, -22 +/- 7 J K-1 mol-1, and +4.7 +/- 0.1 cm3 mol-1 for the reaction with imidazole. On the basis of reported activation volumes, these reactions follow a dissociative mechanism in which the entering nucleophile could be weakly bound in the transition state.