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 [1,1'-{5,6-(µ-H)-nido-2,4-(SiMe3)2-2,4-C2B4H4} -2,2',4,4'-(SiMe3)4-1-1'-commo -Ln(2,4-C2B4H4)2, where Ln = Dy, Er, and related species. DFT calculations have been reported for 13C chemical shifts for uranyl sulphene complexes and anions in the gas-phase.

2.4 Compounds of Group 4. – The 1H NMr spectra of (R2NO)2Ti(CH2Ph)3, where R = CH2Ph or Et, show that the hydroxylaminato ligands exhibit both η1- and η2- binding modes. The 1H and 13C NMR spectra of (Me2PMEN)-Ti(CH2Ph)2, where H2(Me2PMEN) = N,N'-dimethyl-N,N'-bis[(S)2-methyl-pyrrolidine]ethylene diamine, show that C2 symmetry is retained on the NMR time-scale between –80 and +30°C.

1H and 13C NMR data for (L)Ti(NEt2)2 and (L)Zr(NEt2)(thf), where H2L = 2,2 -di(3-methylindolyl)methane, have been reported. The zirconium complex is 5-coordinate, with a coordinated thf molecule, as shown. 1H and 31P{1H} spectra for TiI4[o-C6H4(EMe2)], where E = P or As, are consistent with cis, 6-coordinate octahedral geometries. The anion Ti2Cl9 in solution has 47,49Ti NMR spectra showing that it comprises two face-sharing octahedra.

1H NOESY and 1H, 19F NOESY spectra were used to determine the solution-phase structures of metallocenium homogeneous catalyst ion-pairs, e.g. [Cp2ZrMe]+ [MeB(C6F5)3]- and related systems. Cation-like intermediates formed by activation of zirconocenes, L2ZrCl2 (L = Cp, indenyl, fluorenyl) with methylaluminoxane, have been characterised using 1H NMR.

Detailed 1H and 13C{H} assignments were used to deduce the geometry for the substituted salicylaldimine derivatives of zirconium alkyls, Zr(L)R2, where R = CH2Ph or CH2tBu, H2L = derivatives of 2,2'-diamino-6,6'-dimethylbi-phenyl.

1H, 13C and 31P NMR data for (9) and similar complexes gave evidence for the agostic interaction shown. Solution 1H NMR spectra of (10) show that for R = iPr the supine isomer is formed exclusively, while for R = tBu a 2: 1 mixture of the supine and prone isomers is formed. Mesoporous SiO2-ZrO2 aerogels were studied by 29Si liquid-state NMR. The 13C and 31P solution NMR spectra have been reported for (Zr[µ,µ'-O2P(cycl -C6H11)2](OtBu3)}2 and Zr3[µ,µ'-O2(OtBu)2]5 (OtBu)7. The latter shows the presence of three phosphorus environments (ratio approximately 2:2:1).

NMR data were also reported for: CpTiX3, Cp*TiX3 (X = Cl, Br) and related (47,49Ti);, [Ti(η5:η1] -C5H4SiMe2NtBu)Me(NC5H5)]+ (1H); (11), where M = Ti, Zr, X = NMe2, CH2Ph; (1H, 13C); Ti(η5):η1]-(C5H4)B(NR2NPh(NMe2)2 (1H, 11B, 13C, 29Si); Ti[η5:η1-2-methylindenyl)-SiMe2NCMe3]2 [CH2]n (n = 6, 9, 12) (1H, 13C); [Ti(N3)n](n-4), n = 4, 5, 6 (14N); (12), R = Cl or Me (1H, 13C); (13), E = C or Si, (1H, 13C(H}); [(2,6-Ph2-C6H3-η5 -C5H4)Zr(NEt2)3] (1H, 13C(1H}); silsesquioxane-tethered fluorene ligand and their zirconium q-complexes, e.g. Cp*[(c-C5H9)7Si8O12-X-Flu]ZrCl2, where Flu = fluorene, X = CH2, (CH2)3 or C6H4CH2 (13C, 29Si); (14) and related complexes, (1H, 13C); and Cp2Zr(OOCCH2S-κ2-O,S) (µ-O-OOCCH2S-κ1-O, κ2-O,S)(MoCp'2), where Cp = C5EtMe4, Cp' = C5Me4H.

2.5 Compounds of Group 5. – NMR data (1H, 13C, 31P, 51V) were reported for tBuN=VIIICp(PR3)2, where R = Me, Et, nBu, OMe, OPh; R3 = Me2Ph, MePh2 characteristic 51V chemical shifts, J(51V31P) and J(15V14N) coupling constants were determined. The 51V NMR data for VOL(hq), where Hhq = 8-hydroxyquinoline, H2L = dibasic tridentate ONO Mannich bases, all show a single signal, i.e. only one isomer is present. The 1H, 13C and 31P NMR of VO(acac)L, VOCl2L, and VOClL2, where HL = HN(PPh2NR)2, R = Ph, SiMe3, show that they are all monomers, with bidentate L-. Coordination interactions between K3[VO(O2)C2O4]. H2O and imidazole or substituted imidazoles were probed by 1H, 13C and 51V NMR.

The 51V NMR spectra of aqueous solutions of [VO(O2)cmaa(H2O)]2- and [VO(O2)(Hcmaa)(H2O)]-, where H3cmaa = (R,S)-N-(carboxymethyl)aspartic acid, gave evidence for the presence of both exo- and endo- forms. DFT calculations gave a predicted 51V chemical shift for [VO(O2)2(Im)]-, where Im = imidazole. 51V NMR data for [VO(O2)2(phen)]- show that it is substantially more inert to ligand substitution than the bipy analogue. 1H, 13C and 51V NMR, with NOESY experiments, were used to determine the solution structures of VO(OR)(ONNO), where R = iPr, tBu or CH2CF3, H2[ONNO} = bis(phenox-y)amine ligand. The 51V NMR spectra of aqueous solutions of [VO(O2) (oxalate)(L)]-, where L = bipy or phen, show that they are stereochemically rigid.

Proton NMR spectra of solutions of Cp2NbH3 and fluorinated alcohols gave evidence for hydrogen bond formation, i.e. (15). The proton NMR spectrum of [(L)Ta(µ-H)2(µ-O)Ta(L)]-, where L = 2,6-bis(3-tert-butyl-5-methyl-2-hydroxybenzyl)-4-tert-butyl phenol, includes resonances at 10.3 and 12.1 ppm due to bridging hydrides.

NMR data were also reported for: cis-VOL (L = salicylaldehyde semicarbazone and related) (1H, 13C); VO(O2)(bpa), [VO(O2) (heida)]- (Hbpa = bis(picolyl-β-alanine, H2heida = N-(2-hydroxyethyl)iminodiacetic acid) (17O, 51V); [V2O2(O2)2 (R,S-mand)2]2- (mand = mandelato) (51V); ClV [S2P(OR)2]2 (R = Pr, Ph), cyclic ClV[S2POGO]2 (G = -CH2CMe2CH2-, - CMe2CMe2-, -CH2CEt2CH2-); [MNb12O14]16- (17O); NbCl5-(LiCl/KCl), NbCl5-NaCl and NbCl5-CsCl melts; and Ta(V)-1,2,3-triazolato complexes, e.g. Cp*TaPh3[N3C2(COOMe)2].