The Jahn–Teller distorted Cu(II) complex [Cu(en)2](OTf)21 (en = 1,2-diaminoethane) has been reported and characterised using X-ray crystallography, EPR and ENDOR spectroscopy, and DFT calculations. The solid state structure shows an intra- and inter-molecular hydrogen-bonded network via the N–H groups and the coordinated triflate anions. CW and pulsed EPR/ENDOR were used to determine the spin Hamiltonian parameters of the Cu(II) complex, which were in excellent agreement with the DFT. The structure of the complex, as determined by angular selective ENDOR, is also in good agreement with the crystal structure, confirming the axial coordination of the counter-ion(s) in the frozen solution. The small 14N superhyperfine couplings are also consistent with the sp3 hybridised nature of the coordinating nitrogens. These results show that the correlation between the 14N hyperfine coupling and hybridisation of donor nitrogens can be useful to determine not only the coordination around the Cu(II) metal centre but also the nature of the donor in unknown Cu(II) systems.
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