The crystal structure of [Ni(dmf)6][NiCl4] and comments on the hydrolysis of coordinated amides and peptides in metal complexes

• Published in: Transition metal chemistry (Weinheim) Vol. 23; no. 3; p. 257
• Main Authors: Hay, Robert W, Albedyhl, Sabine, Lightfoot, Philip
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01.06.1998
• Subjects: Crystal structure; Enzymes; Nickel
• ISSN: 0340-4285; 1572-901X
• DOI: 10.1023/A:1015744413792

Solutions of nickel(II) chloride in N,N-dimethylformamide (DMF) in the presence of Et^sub 2^O slowly give bluegreen crystals of [Ni(dmf)^sub 6^][NiCl^sub 4^] whose structure has been confirmed by X-ray crystallography. The complex crystallises with three crystallographically distinct [Ni(dmf)^sub 6^]^sup 2+^ cations in the asymmetric unit, Ni(1) on a general position and Ni(2) and Ni(3) on centres of symmetry. There are also two unique [NiCl^sub 4^]^sup 2-^ anions, both on general positions. N,N-Dimethylformamide is O-bonded to nickel as coordination via nitrogen would cause the loss of the resonance energy of the amide group. All Ni--O bond lengths are within the expected range for such complexes [2.029(4)-2.084(4) \rA]. The C=O bond lengths, which range from 1.219(7) to 1.259(7) \rA, are not significantly different from those for uncomplexed dmf (1.232 ± 0.004 \rA), suggesting that there is very little polarisation of the C=O bond by nickel(II). Oxygen-bonded dmf undergoes rapid hydrolysis in metal complexes, for example the base hydrolysis of [Co(NH^sub 3^)^sub 5^dmf]^sup 3+^ to give the formato complex is some 10^sup 4^-fold faster than that of the free ligand. Copper(II)-catalysed amide and peptide bond hydrolysis is subject to ca. 10^sup 6^-fold rate enhancements. The origin of these rate enhancements, which appear to be due to transition state effects rather than ground state effects, is discussed in detail.[PUBLICATION ABSTRACT]