Intrinsic 37/35Cl and 18/16O isotope shifts in 195Pt and 103Rh NMR of purely inorganic Pt and Rh complexes as unique spectroscopic fingerprints for unambiguous assignment of structure

Abstract

Well-resolved intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope shifts (where M = ¹⁹⁵Pt or ¹⁰³Rh) are visible in the ¹⁹⁵Pt NMR peak profiles of relatively kinetically inert [PtCl_n(H₂O)_6−n]⁴⁻ⁿ (n = 1–6) complexes, their corresponding hydroxido [PtCl_6−n(OH)_n]²⁻ (n = 1–5/6) anions, and [RhCl_n(H₂O)_6−n]³⁻ⁿ (n = 3–6) complexes in aqueous solutions at ca. 293 K. Although some such isotope effects have been previously reported, there are very limited published data in the open literature, and the first systematic studies of such intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope effects are reviewed in this perspective. In high magnetic-field NMR spectrometers, the ¹⁹⁵Pt and ¹⁰³Rh NMR peak profiles acquired within a relatively narrow temperature range (288–300 K) constitute unique ‘spectroscopic fingerprints’, which allow unambiguous structural assignment in solution. Available data for Pt(IV) and Rh(III) complexes give rise to intrinsic isotope ¹Δδ¹⁹⁵Pt/¹⁰³Rh(^37/35Cl) profiles, which are extraordinarily sensitive to the structure of a particular complex or its geometric isomer. The profiles of aquated Pt(IV) and Rh(III) complexes in acidic solutions may be resolved at either an isotopologue level only or at both an isotopologue and an isotopomer level depending on the structure. By contrast, in the series of [PtCl_6−n(OH)_n]²⁻ (n = 1–6) anions, ¹Δδ¹⁹⁵Pt(^37/35Cl) isotope shifts are resolved only at an isotopologue level. Relatively larger ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts obtained by the partial ¹⁸O enrichment of both the [PtCl_n(H₂O)_6−n]⁴⁻ⁿ (n = 1–6) and [PtCl_6−n(OH)_n]²⁻ (n = 1–6) series give rise to remarkable ¹⁹⁵Pt NMR peak profiles showing both ^37/35Cl and ^18/16O shifts. In the [PtCl_6−n(OH)_n]²⁻ (n = 1–5/6) anions a typical NMR peak profile spanning ∼2 ppm only may be resolved at both the isotopologue and isotopomer levels, depending on whether ^18/16OH⁻ ions are coordinated trans to chloride ions or not. The potential utility of such ¹Δ¹⁹⁵Pt(^37/35Cl) and ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts in selected practical applications involving such complexes is briefly illustrated.