NMR Investigations of Restricted Rotations and Metallotropic Shifts in Rhenium(I) Tricarbonyl Halide Complexes of Pyridyl Carboxamide and Thioamide Ligands

Abstract

A series of rhenium(I) tricarbonyl halide complexes of pyridyl mono‐ and di‐carboxamides and ‐thioamides has been studied by dynamic NMR techniques. Oxygen coordination to ReI of the carboxamide ligand reduced the energy barrier to C–N rotation by 2–13 kJ mol–1 whereas sulfur–ReI coordination of the thioamide ligands led to a reduction of 28–33 kJ mol–1. In the pyridyl dicarboxamide and ‐dithioamide metal chelate complexes metallotropic shifts occur between the three donor atoms, O, N, O or S, N, S, energies [ΔG‡ (298.15 K)] being in the range 78–89 kJ mol–1; these high values arise from the strong electron donating properties of the ‐NR2 (R = Me, Et) groups.

Understanding the Deactivation Pathways of Iridium(III) Pyridine‐Carboxiamide Catalysts for Formic Acid Dehydrogenation

“…The dihedral angle O1‐C16‐N2‐C17 is close to zero and of only 2.52°, suggesting that the N(2) atom has partial sp 2 hybridization. Consistent with this, the C16−N2 and C16−O bond lengths (1.3247(8) and 1.2458(7) Å, respectively), are nearly the same as those observed in 2 (1.324(9) and 1.257(7) Å), [34] indicating that both these bonds possess double‐bond character [46, 48] …”

“…Consistent with this, the C16ÀN2 and C16ÀOb ond lengths (1.3247(8) and 1.2458(7) ,r espectively), are nearly the same as those observed in 2 (1.324(9) and 1.257(7) ), [34] indicating that both these bonds possess double-bondc haracter. [46,48] The isolated 2a_BF 4 was also completely characterizedinsolution by NMR spectroscopy in CD 2 Cl 2 ( Figures S3-S8), which showedt he expected shift towardl ow frequency of the N-Me carbon resonance with respect to 2 (see Ta ble S1). Upon dissolving 2a_BF 4 in D 2 O, the 1 HNMR spectrum ( Figure S9) showedthe same two sets of resonances observed for the mixture of 2 and HNO 3 ,f urther supporting the interpretationo f NMR resultsdiscussed above.…”

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“…The 13 C NMR spectrum also shows the signal of carbons of 2,6‐positions in piperidine unit ( C f ) as two peaks at δ52.5 and 49.9. These results indicate the existence of different conformers resulting from the barrier to internal rotation about the CN bond in the thioamide group 9–11. A similar split of the signals based on the rotational barrier was also observed for Model 1, PA‐1, 1,1′‐(1,4‐phenylenedicarbonyl)dipiperidine, and 1,4‐bis(2,2‐dicyano‐1‐vinylpiperidyl)‐benzene 9.…”

Trimethylplatinum(IV) complexes of dithiocarbamato ligands: an experimental NMR study on the barrier to C–N bond rotation

“…This is in contrast to recent work on the tricarbonylrhenium() complexes of pyridine-2,6-bis(N,N-dialkylcarbothioyl amide, C 5 H 3 N(C(S)NR 2 ) 2 -2,6) (L), [ReX(CO) 3 (L)] (X = Cl, Br, or I), which show that co-ordination of carbothioyl group to the metal centre reduces the barrier to C-N bond rotation significantly. 47…”