NMR Spectroscopy in the Liquid and Gas Phases

BY G. DAVIDSON Formerly University of Nottingham, Nottingham, UK

1 Introduction

The format of this Chapter will be slightly different from that for earlier years. Papers dealing with essentially static situations will be dealt with first – with each Group of the Periodic Table discussed in turn. Results on dynamic systems will then follow – again on the basis of the Periodic Groups, with papers on paramagnetic compounds being dealt with last.

2 Stereochemistry

2.1 Compounds of Group 1. – (6Li, 15N) and (6Li, 13C) couplings were observed for mixed complexes formed between LiCH2CN and chiral lithium amides (1H, 6Li, 13C, 15N data). 7Li and 31P{1H} HMQC experiments were used to assign the structures of benzyllithium complexes of N-methyl-N-benzylphosphinamide, e.g. (1). 1H and 13C NMR and 13C-1H correlation spectra were used to confirm the presence of a C-Si-Ni-Li 4-membered heterocycle in [benzylbis(dimethylamino)-methylsilyl-κ2-C,N](N, N, N',N'-tetramethylenediamine -κ-N,N)lithium(I).

The 7Li NMR spectra of (CpAr5)Li(thf2) and (CpAr5)Li, where Ar = 3,5-tBu2C6H3, suggest the presence of more than one species in solution, e.g. in thf/C6D6 the monomer and [(CpAr5)2Li][Li(thf)x]. 2H NMR spectroscopy was used to study cation π-interactions between LiCl, NaCl, KCl, RbCl, CsCl and AgNO3 solutions with C6D6. The complex (2) gives a 119Sn resonance as a quartet at –819.8 ppm, due to 119SnLi-7 coupling, confirming the covalent Sn-Li bond in solution, even at room temperature.

The 6Li, 15N and 13C NMR spectra of the α-aminoalkoxide-LiHMDS mixed dimer, where LiHMDS = lithium hexamethyldisilazide, showed the presence of a pair of conformers. 6Li and 15N couplings and 6Li, 1H HOESY data gave structural information for chiral lithium amides with chelating sulfide groups, e.g. (3).

7Li pulsed gradient spin-echo (PGSE) measurements on LiPPh2 in thf or Et2O solutions show that the compound is a monomer in the former, but a dimer in the latter solution. Proton NMR chemical shifts have been used to examine perturbations in water structure in LiOH, KF or KCl solutions.

Other lithium-containing systems studied by NMR included: alkyne lithium compounds with ligands tethered at C2 (13C); n-[CM2e{CHMeN(R)2}.Li], where R = 2,6-iPr2C6H3 (1H, 7Li, 13C); (Et2O)LiSnPh2Ar*, (LiSnPh2Ar*)2, where Ar* = C6H3-2,6-Trip2, Trip = C6H2-2,4,6-iPr3, (1H, 7Li, 13C, 119Sn); [Ph2PTe][Li(TMEDA)1.33 (thf)1.33], [Ph2PTe2] [Li(thf)3.5.(TMEDA)0.25.] and related (1H, 13C, 31P); [1-LiNPhCHPh-2-NMe2C6H4]2, [1-LiNPhCHPhCH2-2-NMe2C6H42] (1H, 7Li, 13C); [(RfN)2NLi(solv)2, where Rf = C6F5, solv = Et2O, thf (1H, 13C, 19F); (R-NP)Li(thf)2, where H(R-NP) = N-(2-dip-henylphosphinophenyl)-2,6-di-R-aniline, R = Me, iPr (7Li{1H}, 31P{1H});MeSi(2-C5H4N)3Li(X), where X = 0.8Cl,0.2Br (1H, 7Li); Li[P(NHtBu)2(NtBu)-(NSiMe3)] and related (1H, 7Li, 13C, 31P); [{Ph2P(O)N(CH2Ph)-CH3}. LiOC6H2-2.6-{C(CH3)3}2-4-CH3). C7H8]2 (1H, 7Li, 13C, 31P).

Proton NMR data have established that Na+ or K+ can be encapsulated into a range of new calix[4]crowns-4 and calix[4]crowns-5. Similar data show that Na+ and K+ can bind to a calix[4]semitube having urea functionality. The solution +H NMR spectrum of Na11 (OtBu)10 (OH) includes a peak at 3.21 ppm due to the hydroxyl group. Samples in the NaF-AlF3-Al system at 1030°C were characterised by 19F, 23Na and 27Al NMR spectroscopy. Proton and 133Cs NMR spectroscopy gave evidence for complexation of Cs+ by a p-tert-butylcalix[6]arene hexaacetamide derivative.

2.2 Compounds of Group 2. – 9Be chemical shift data were used to study hydrogen-bonding between Be(H2O)42+ and water in the second coordination sphere.

Evidence was found (1H and 31P NMR) for the formation of (neopentyl)Mg(HMPA) and (neopentyl)Mg2- in solutions containing Mg(ne-opentyl)2 and hexamethyphosphoramide (HMPA). Proton NMR spectra of C6D6 solutions showed the presence of two isomers of (4). The complex (5) was characterised by 1H, 13C and 31P NMR. Characteristic 1H and 13C{1H} data were reported for Br(thf)Mg[oxam(R)2]Mg(thf)Br, where oxam(R)2 = (6), R = OMe or NMe2. 1H and 13C NMR, with (1H, 1H) COSY and (1H, 13C) HETCOR data on [Mg(L)]3+, where the ligands are bis(pendant arm) macrocyclic Schiff bases, suggest that there is approximately pentagonal bipyramidal coordination at the magnesium.

Calculated inter- and intramolecular indirect NMR spin-spin coupling constants and chemical shifts gave predicted values associated with inner- an outer-sphere binding of Mg2+ or Zn2+ to a guanine base. NMR spectra (2H, 23Na and 31P) were used to study the interaction of M2+ (=Mg, Cd or Ni) with liquid crystalline NaDNA solutions. Ab initio and DFT methods were used to calculate 17O NMR shieldings for OM6 (OH)122-, where M = Mg, Ca or Sr.

The 1H NMR spectrum of (7) at low temperatures shows the presence of two diastereoisomers. Ab initio calculations have been made of 15N chemical shift differences induced by Ca2+ binding to EF-hand proteins. 1H and 13C NMR spectra were used to characterise calcium pyrrolates, [Ca{(2-dimethylamino-methyl)pyrrolyl}2(D)n], where D = thf, py, n = 2, D = dmf, TMEDA, n = 1. The 1H NMR spectra of (η5-Gaz)M(thf)2, where M = Ca or Yb, and (η5-Gaz)Yb(py)2, where Gaz = 1,4-dimethyl-7-isopropylazulene, show exclusive formation of N2-ansa-metallocenes. 1H-15N heteronuclear single quantum coherence spectra were used to study and compare the binding of Ca2+ and La3+ to calmodulin and a calmodulin-binding peptide.

1H and 13C NMR spectra of M2+ (M = Ca, Ba, Pb) complexes with the Schiff base formed from gossypol and 5-hydroxy-3-oxapentylamine show the formation of 1: 1 complexes. Complexes [M(L)]2+, where M = Ca, Ba, Zn, Cd, Pb, L = (8) were characterised by 1H NMR. An NMR study has been made of the binding of Ca2+ to synthetic hexasaccharide models of modified heparin.

1H and 13C NMR spectra were used to study [M(thd)2(L)n]m, where M = Ba, L = Hpz, Hpz*, m = 2, n = 2; M = Sr, L = Hpz, Hpz*, m = 1, n = 3; Hthd = 2,2,6,6-tetramethylheptane-3,5-dione, Hpz = pyrazole, Hpz* = 3,5-dime-thylpyrazole.

2.3 Compounds of Group 3 (Yttrium, Lanthanides, Actinides). – The 13C NMR spectrum of (Y2C)@C82 in CS2 solution is consistent with encapsulation of Y2C2 in a C82-C3v (8) cage. The complex Y[CH(SiMe3)(SiMe2OMe)]3 gives 1H, 13C and 29Si NMR spectra in solution consistent with the presence of two isomers. 1H, 13C{1H} and 89Y spectra were reported and assigned for [{η5-C5Me4SiMe2R)Y}4 (µ-H)4 (µ3-H)4(thf)2].

1H, 11B and 13C NMR data were used to characterise...