Synthesis and solid-state characterization of platinum complexes with hexadentateamino- and iminophosphine ligands

Abstract

Hexadentate ligands cis,cis-C₆H₉(NCHC₆H₄(PPh₂))₃ (1) and cis,cis-C₆H₉(NHCH₂C₆H₄(PPh₂))₃ (2) were synthesized starting from cis,cis-1,3,5-triaminocyclohexane, and characterized using NMR spectroscopy and single-crystal X-ray diffraction. These ligands can bind both Pt(0) and Pt(II) metal centers using either or both of the soft phosphine moieties and the hard amine/imine moieties. In many cases the resulting complexes are negligibly soluble; hence, ³¹P and ¹⁹⁵Pt solid-state NMR (SSNMR) spectroscopy was applied to analyse the bonding modes of the hexadentate ligands. The ¹⁹⁵Pt SSNMR spectroscopy of these complexes is particularly challenging, since ¹H–¹⁹⁵Pt cross polarization is extremely inefficient, the ¹⁹⁵Pt longitudinal relaxation times are extremely long and the ¹⁹⁵Pt powder patterns are expected to be quite broad due to platinum chemical shift anisotropy. It is demonstrated that the ultra-wideline ¹⁹⁵Pt SSNMR spectra can be efficiently acquired with a combination of frequency-stepped piecewise acquisitions and cross-polarization/Carr–Purcell Meiboom–Gill (CP/CPMG) NMR experiments. The ¹⁹⁵Pt and ³¹P SSNMR data are correlated to important structural features in both Pt(0) and Pt(II) species.