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

Abstract

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.

Structural Insights from 59Co Solid‐State NMR Experiments on Organocobalt(I) Catalysts

“…These values are of the same order of magnitude as the values obtained from the 59 Co solid‐state NMR study of Co(C 2 B 9 H 11 ) 2 − and Co(C 5 H 5 ) 2 + salts as well as the Ω value reported for the 1,2‐polybutadiene catalyst, [Co(C 8 H 13 )(C 4 H 6 )], of 5260(500) ppm . Additionally, Ω values of similar order of magnitude as those of 1 – 5 have been observed for Co III complexes, such as the [Co(bis(2‐pyrimidylcarbonyl)amidate) 2 ] + complexes and the trans ‐[Co(en) 2 (CH 3 )(X)] compounds …”

“…[24] The rather small range in 1 J( 13 C, 59 Co) values is not surprising given the very minimal differences in their respective dipolar coupling constants, R DD ,a nd their nearly identicale lectronic environments based on the C Q ( 59 Co) values (see Ta ble3). The negligible variation of the 1 J( 13 C, 59 Co) values is consistent with observations by Ooms et al [25] on as eries of trans-[Co(en) 2 (CH 3 )(X)] cobalt(III) compounds. The authorso ft hat study showedt hat, even when varying the Xl igand, the 1 J( 13 C, 59 Co) values of the 59 Co-13 CH 3 spin pair did not vary by more than 20 Hz.…”

“…[20] Additionally, W values of similar order of magnitudea st hose of 1-5 have been observed for Co III complexes, such as the [Co(bis(2-pyrimidylcarbonyl)amidate) 2 ] + complexes [36] and the trans-[Co(en) 2 (CH 3 )(X)] compounds. [25]…”

A Quadrupole‐Central‐Transition 59Co NMR Study of Cobalamins in Solution

“…[2] MeCbl and AdoCbl are the two bioactive forms of Cbl and serve as co-factors for methionine synthase and methylmalonyl-CoA mutase, respectively. [3,4] In the context of NMR studies of Cbl compounds, since they are relatively small molecules with molecular masses between 1300 and 1600 Da, their 1 H, 13 C, 31 P NMR studies are straightforward. [5][6][7] It is also possible to use 31 P NMR to study Cbl-containing proteins.…”

“…successfully detected 59 Co NMR signals from a single‐crystal sample of CNCbl. However, while solid‐state 59 Co NMR has proven useful for studying small molecules, biological macromolecules larger than Cbl have appeared to be out of reach to date.…”