Keith G. Orrell

Two-dimensional NMR exchange spectroscopy. Quantitative treatment of multisite exchanging systems

Abstract A general method for evaluating rate constants in complex exchange networks with N-sites from two-dimensional EXSY (NOESY) NMR spectra is proposed. A computer program D2DNMR capable of performing signal intensity to exchange rate calculations (and vice versa), based on a matrix formalism, is outlined. The method is illustrated by 195 Pt 2D NMR studies of the A ⇌ B ⇌ C spin system arising from pyramidal sulfur inversion in platinum(IV) complexes of type [Pt X Me 3 (MeSCH 2 CH 2 SMe)] ( X = Cl , I ). Comparison with 1 H NMR bandshape analyses of the same compounds shows high agreement between the rate constants and activation parameters determined by both techniques. Mechanisms of 195 Pt spin-lattice relaxation are briefly discussed.

2,2′:6′,2″-Terpyridine (terpy) acting as a fluxional bidentate ligand. Part 2. Rhenium carbonyl halide complexes, fac-[ReX(CO)3(terpy)](X = Cl, Br or I): NMR studies of their solution dynamics, synthesis of cis-[ReBr(CO)2(terpy)] and the crystal structure of [ReBr(CO)3(terpy)]

Under mild conditions pentacarbonylhalogenorhenium(I) complexes react with 2,2′:6′,2″-terpyridine (terpy) to form stable octahedral tricarbonyl complexes fac-[ReX(CO)3(terpy)](X = Cl, Br or I) in which the terpyridine acts as a bidentate chelate ligand. Under more severe reaction conditions fac-[ReBr(CO)3(terpy)] can be converted to cis-[ReBr(CO)2(terpy)]. In solution the tricarbonyl complexes are fluxional with the terpyridine oscillating between equivalent bidentate bonding modes. At low temperatures rotation of the unco-ordinated pyridine ring is restricted and in CD2Cl2 solution two preferred rotamers exist in approximately equal abundances. Rotational energy barriers have been estimated for the X = Cl and I complexes. The X-ray crystal structure of fac-[ReBr(CO)3(terpy)] confirms the bidentate chelate bonding of terpy with a N–Re–N angle of 74.3°. The pendant pyridine ring is inclined at an angle of 52.9° to the adjacent co-ordinated ring and the unco-ordinated nitrogen is directed towards the axial carbonyl and trans to Br.

Quantitative investigations of molecular stereodynamics by 1D and 2D NMR methods

In the first part of the review mention will be made of some recent developments in NMR techniques which are potentially important for future 2D-EXSY and 2D-NOESY studies of molecular motions. The second part of the review illustrates recent applications of NMR methods to studies of stereochemical non-rigidity in liquid phase inorganic and organic systems

[3]Ferrocenophane bridge reversal barriers : I. Sulphur, selenium and tellurium bridging atoms

Abstract Dynamic NMR studies have yielded accurate energy data for the bridge reversal fluxion of [3]ferrocenophanes with Group VI bridging atoms. This process, whilst appearing very analogous to the chair-to-chair reversal of corresponding 6-membered heterocyclic rings, appears to be a much higher energy process, its associated Δ G ≠ values being in the range 59 to 81 kJ mol −1 depending on the types of Group VI bridging atoms. These data allow estimates to be made for the first time of the relative magnitudes of torsional barriers about single bonds involving like and unlike Group VI atoms. For example, the SS torsion is shown to be 3.9 kJ mol −1 higher in energy than the SSe torsion and 5.8 kJ mol −1 higher than the SeSe torsion. The probable mechanism of the bridge reversal process is discussed.

2,2′:6′,2″-Terpyridine (terpy) acting as a fluxional bidentate ligand. Part 4. cis-[M(C6F5)2(terpy)](M = Pd or Pt): nuclear magnetic resonance studies of their solution dynamics and crystal structure of cis-[Pd(C6F5)2(terpy)]

2,2′:6′,2″-Terpyridine reacted with trans-[M(C6F5)2(diox)2](M = Pd or Pt, diox = 1,4-dioxane) to form the square-planar complexes cis-[M(C6F5)2(terpy)] in which the terpyridine acts as a bidentate chelate ligand. In solution these complexes are fluxional with the terpyridine oscillating between equivalent bidentate modes by a mechanism consisting of a ‘tick-tock’ twist of the metal moiety through an angle equal to the N–M–N angle of the metal centre. Rates of this fluxion were measured by NMR spectroscopy from the exchange effects on the 1H signals of the aromatic hydrogens and in the 19F signals of two C6F5 groups. The ΔG‡ values for the fluxion were ca. 71 and 94 kJ mol–1 for the complexes of PdII and PtII respectively. At below-ambient temperatures further changes in the 19F NMR spectra of both complexes were interpreted in terms of varying rates of rotation of the unco-ordinated pyridine ring, with the rates of rotation of the C6F5 rings being substantially slower at all temperatures and not separately measurable. The lowest-temperature spectra suggested the presence of a pair of degenerate rotamers each having the planes of both C6F5 rings and the unco-ordinated pyridine ring closely parallel and orthogonal to the remainder of the ligand ring system. The crystal structure of [Pd(C6F5)2(terpy)] confirms the bidentate chelate bonding of terpy with a N–Pd–N angle of 77.9°, and the pendant ring oriented at an angle of 46° to the adjacent co-ordinated ring.

2,2′:6′,2″-Terpyridine (terpy) acting as a fluxional bidentate ligand. Part 1. Trimethylplatinum(IV) halide complexes [PtXMe3(terpy)](X = Cl, Br or I): nuclear magnetic resonance studies of their solution dynamics and crystal structure of [PtIMe3(terpy)]

2,2′:6′,2″-Terpyridine (terpy) reacts with trimethylplatinum halides [(PtXMe3)4](X = Cl, Br or I) to form stable octahedral complexes fac-[PtXMe3(terpy)](X = Cl, Br or I) in which the terpy molecule is acting as a bidentate chelate ligand. In solution the complexes are fluxional with the ligand oscillating between equivalent bidentate bonding modes by a mechanism consisting of ‘tick-tock’ twists of the metal moiety through an angle equal to the N–Pt–N angle of the octahedral centre. At below-ambient temperatures rotation of the unco-ordinated pyridine ring is severely restricted with the most favoured rotamers having the plane of the pendant pyridine ring at an angle of ca. 52° with respect to the adjacent co-ordinated pyridine ring plane. The X-ray crystal structure of [PtIMe3(terpy)] depicts the pendant pyridine N atom cis to iodine and this is the predominant species in solution at low temperatures. At above-ambient temperatures the complexes exhibit intramolecular Pt–Me exchange of axial and equatorial environments. Energy data based on accurate dynamic NMR fittings are reported for the three dynamic processes, namely pendant pyridine rotation, 1,4-Pt–N metallotopic shifts and Pt–Me scramblings.

Dynamic NMR studies of ring rotation in substituted ferrocenes and ruthenocenes

Variable temperature 1H and 13C{1H} NMR studies on 1,1′,3,3′-tetra(alkyl)-ferrocenes and -ruthenocenes (alkyl = t-pentyl,t-butyl) have provided accurate barrier energies for restricted rotations of the substituted cyclopentadienyl rings. Energies (ΔG† (298.15 K)) are in the range 40–57 kJ mol−1 and are dependent on the ‘sandwich’ metal (Fe ⋙ Ru) and alkyl substituent (t-pentyl < t-butyl). Ring rotation in 1,1′,3.3′-tetra(phenyl)ferrocene was too rapid for measurement even at 173 K. Spectral changes were analysed primarily on the basis of exchange between a mirror pair of structures having both 5-membered rings eclipsed and the alkyl substituents staggered. In the case of 1,1′,3,3′-tetra(t-butyl)ferrocene a pair of staggered ring rotamers also contributed to the observed NMR bandshape changes.

The first examples of 2,2′:6′,2″-terpyridine as a fluxional bidentate ligand

2,2′:6′,2″-Terpyridine (terpy) forms octahedral complexes fac-[ReBr(CO)3(terpy)], cis-[W(CO)4(terpy)] and fac-[PtClMe3(terpy)] in which the ligand oscillates between equivalent bidentate forms by a mechanism involving a ‘tick-tock’ twist of the metal moiety through an angle equal to the N–M–N angle of the octahedral centre and involving a seven-coordinate metal intermediate.

Synthesis and dynamic NMR studies of fluxionality in palladium(II) and platinum(II) complexes of 2,4,6,-tris(2-pyridyl)-1,3,5-triazine (TPT) and 2,4,6-tris(2-pyridyl)-pyrimidine (TPP)

Complexes of general formulae cis-[M(C6F4CF3)2L] (M = PdII, PtII; L = 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPT) and 2,4,6-tris(2-pyridyl)-pyrimidine (TPP) were isolated as air-stable solids. In all cases cis square-planar complexes were formed with the nitrogen ligands acting as bidentate chelates towards each metal moiety. The complexes exhibited various modes of fluxionality in solution, namely 1,4-metallotropic shifts, a new ‘metal hurdling’ fluxion and, at below-ambient temperatures, restricted rotation of the pendant pyridyl ring adjacent to the metal chelate ring. Dynamic NMR experiments (one-dimensional bandshape analysis and two-dimensional EXSY experiments) provided activation energy data for these processes. Gibbs free energy values (ΔG≠ (298.15 K)) were in the ranges 74–113 (metal hurdling). 69–118 (metal 1,4-shifts) and 37–43 (pendant pyridyl rotations) kJ mol−1. Energies of any of these fluxions were considerably higher in the PtII complexes than in the PdII complexes. To aid understanding of the low temperature fluxionality of the TPT complexes, the complex [Pd(C6F4CF3)2(mstd)] (mstd = meso-stilbenediamine) was synthesised. At low temperatures, C6F4CF3 ring rotations and five-membered ring puckering in this complex were arrested, ΔG≠ (208K) for the latter process being 42.8 kJ mol−1.

The syntheses, structures, and stereodynamics of [3]ferrocenophane complexes: I. Group 6 metal tetracarbonyl complexes, cis-[M(CO)4{(C5H4ECH3)2Fe}]. (M = Cr, Mo, W; E = S or Se). Crystal structure of 1,1′-bis(methylseleno)ferrocene tetracarbonyltungsten

Abstract The complexes cis-[M(CO)4{(C5H4ECH3)2Fe}] (M = Cr, Mo or W; E = S or Se) have been synthesised. Pyramidal inversion of the coordinated S or Se atoms was arrested in most cases at low temperatures (∼ −80°C), when the DL forms of the complexes predominated (⩾ 78%). At higher temperatures combined 1D and 2D-EXSY NMR studies led to energies (ΔG‡(298 K) values) for chalcogen inversion in the range 31–50 kJ mol−1. Fairly large ΔS‡ values for these processes were attributed to the rapid reversal of the EM(CO)4E portion of the ferrocenophane ring. The crystal structure of 1,1′-bis(methylseleno)ferrocene tetracarbonyltungsten has been determined. The crystals have space group P21/a with a 17.580(3), b 9.665(1), c 11.059(1) A, β 107.33(1)° and Z = 4. Least-squares refinement gave R = 0.031 for 2369 independent significant reflections. The WSe bond lengths are 2.674(4) and 2.692(4) A, and the SeMe groups adopt a DL relationship. The SeWSe bond angle is 86.3° and the non-bonded SeSe separation 3.67 A. The cyclopentadienyl rings adopt an eclipsed conformation with the ring planes parallel.