A ¹³C NMR investigation of some tetrakisisocyanocopper(I) tetrafluoroborate complexes: Chemical shift of, and coupling to, the isocyano carbon

Abstract

Abstract The 13 C n.m.r. spectra twelve isocyanides, RNC, and their Cu(I) complexes, Cu(RNC) 4 BF 4 , were recorded in the solvents CDCl 3 and DMSO. The resonances of the isocyano carbon, C 0 , could be observed for both free and coordinated isocyanides. In the spectra of the complexes this C 0 resonance was present as a broad line. For some complexes the 14 N 13 C 0 coupling could only be detected after heating the sample or by adding small amounts (<7 mo1%) of Cu(CH 3 CN) 4 BF 4 . For the remaining complexes neither J ( 14 N 13 C 0 ) nor J ( 14 N 13 C 1 ), could be detected. No carbon‐copper coupling could be detected. The line shape of C 0 is discussed in terms of exchange of an isocyanide ligand and in terms of quadrupolar nitrogen and copper relaxation. In most complexes exchange seems to be dominant, whereas in other complexes quadrupolar nitrogen relaxation is also of importance. On coordination of the isocyanide, the C 0 resonance shifts upfield (15 to 20 ppm) and J ( 14 N 13 C 0 ) shows a threefold increase. The upfield shift is associated with a larger excitation energy while the increase of J ( 14 N 13 C 0 ) is ascribed to a larger effective nuclear charge on C 0 . The chemical shifts of the α‐carbon, the β‐carbon and the solvent effect on all n.m.r. parameters in both free and coordinated isocyanides are also discussed.