Isotope Eﬀects in 195 Pt NMR Spectroscopy: Unique 35/37 Cl- and 16/18 O‑Resolved “Fingerprints” for All [PtCl 6-n (OH) n ] 2- (n =1-5) Anions in an Alkaline Solution and the Implications of the Trans Inﬂuence Leon Engelbrecht, Pieter Murray, and Klaus R. Koch* Department of Chemistry and Polymer Science, University of Stellenbosch, Post Bag X1, Matieland 7602, South Africa * S Supporting Information ABSTRACT: A detailed analysis of the intrinsic 1 Δ 195 Pt( 37/35 Cl) and 1 Δ 195 Pt( 18/16 O) isotope 128.8 MHz 195 Pt NMR proﬁles of the series of kinetically inert [PtCl 6-n (OH) n ] 2- (n =1-5) anions generated in strongly alkaline aqueous solutions shows that each 195 Pt NMR resonance of the [Pt 35/37 Cl 6-n ( 16/18 OH) n ] 2- (n =1-5) anions is resolved only into [(6 - n) + 1 for n =1-5] 35/37 Cl isotopologues at 293 K. Evidently, the greater trans inﬂuence of the hydroxido ligand in the order OH - > Cl - > H 2 O in [PtCl 6-n (OH) n ] 2- (n =1-5) complexes results in somewhat longer Pt-Cl bond displacements trans to the hydroxido ligands, resulting in the absence of isotopomer e ﬀects in the [PtCl 6-n (OH) n ] 2- (n =1-5) anions in contrast to that observed in the corresponding [PtCl 6-n (H 2 O) n ] (2-n)- (n =1-5) complexes. In suitably 18 O-enriched sodium hydroxide solutions, additional intrinsic 1 Δ 195 Pt( 18/16 O) isotope eﬀects are remarkably well-resolved into unique isotopologue- and isotopomer-based 195 Pt NMR proﬁles, ascribable to the higher trans inﬂuence of the OH - ligand. The consequent signiﬁcantly shorter Pt-OH bonds in these anions emphasize 16/18 O isotopomer eﬀects in the 195 Pt NMR peaks of [Pt 35/37 Cl 6-n ( 16/18 OH) n ] 2- (n =1-5) for magnetically nonequivalent 16/18 OH isotopomers statistically possible in some isotopologues. These 195 Pt NMR proﬁles constitute unique NMR “ﬁngerprints”, useful for the unambiguous assignment of the series of [PtCl 6-n (OH) n ] 2- anions including their possible cis/trans/fac/mer stereoisomers in such solutions, without a need for accurate chemical shift measurements. ■ INTRODUCTION Since the ﬁrst accurate measurement of the positive magnetic moment 1 of the only stable magnetically active isotope of 195 Pt (I = 1 / 2 ) at 33.83% natural abundance, 2 195 Pt NMR spectroscopy has developed into a powerful spectroscopic tool for the study of the chemistry of countless platinum- containing compounds with oxidation states of platinum of 0, II, or IV in solution. This subject has been extensively reviewed in recent years. 3-6 The utility of 195 Pt NMR arises inter alia from its relatively high NMR receptivity, 2 with the very large known chemical shift δ( 195 Pt) range exceeding 13000 ppm, which is extremely sensitive to the detailed structure of the platinum-containing molecules or complexes, as well as the generally large scalar ( n J) spin-spin coupling constants to other magnetically active nuclei. 3-6 These NMR parameters are generally excellent spectroscopic probes of the structure of the platinum-containing compound. In solution, the scalar spin- spin coupling constants between 195 Pt and other magnetically active nuclei can range from a few hertz to >140 kHz. The 1 J( 195 Pt- 205 Tl) coupling constant of 148 kHz observed in a dimeric platinum-thallium complex, [{Pt(ONO 2 )(NH 3 ) 2 - (NHCOtBu)}Tl(ONO 2 ) 2 (MeOH)], is reported to be one of the largest known in solution. 7 Overall, the 195 Pt chemical shift range, reﬂecting the magnetic shielding of this nucleus within a given molecule in solution under deﬁned conditions, is generally interpreted to result from several distinct shielding parameter contributions. These additive contributions may traditionally be formulated as σ overall = σ d + σ p + σ so + σ other , where σ d is the diamagnetic, σ p the paramagnetic, and σ so the relativistic spin-orbit coupling shielding contributions, includ- ing eﬀects due to other “extraneous” factors upon shielding generally embodied in a σ other shielding contribution term, respectively. 8-14 In this context, the experimentally observed sensitivity of δ( 195 Pt)/ppm to “other” factors (embodied in σ other ), such as the inﬂuence of the solvent, 15 temperature, 16 pressure, and isotope eﬀects, 17,18,49 is documented in the literature, although the origin of such eﬀects is still relatively poorly understood on a fundamental level. In particular, isotope eﬀects arising from the substitution of a heavier isotope for a lighter one of the same element usually Received: December 8, 2014 Published: February 20, 2015 Article pubs.acs.org/IC © 2015 American Chemical Society 2752 DOI: 10.1021/ic502901d Inorg. Chem. 2015, 54, 2752-2764