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NMR N uclear M agnetic R esonance

NMR N uclear M agnetic R esonance. NMR for Organometallic compounds. Index. NMR-basics. H-NMR. NMR-Symmetry. Heteronuclear-NMR. Dynamic-NMR. NMR and Organometallic compounds. NMR in Organometallic compounds spins 1/2 nuclei.

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NMR N uclear M agnetic R esonance

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  1. NMRNuclear Magnetic Resonance NMR for Organometallic compounds Index NMR-basics H-NMR NMR-Symmetry Heteronuclear-NMR Dynamic-NMR NMR and Organometallic compounds

  2. NMR in Organometallic compoundsspins 1/2 nuclei For small molecules having nuclei I=1/2 : Sharp lines are expected W1/2 (line width at half height) = 0-10 Hz If the nuclei has very weak interactions with the environment, Long relaxation time occur (109Ag => T1 up to 1000 s !!!) This makes the detection quite difficult!

  3. Isotope Nat. Abun-dance % () 107 rad T-1 s-1 Frequency (MHz) Rel. Receptivity 1H 99.985 26.7519 100.0 1.00 3H - 28.535 106.7 -- 3He 0.00013 -20.380 76.2 5.8 * 10-7 13C 1.11 6.7283 25.1 1.8 * 10-4 15N 0.37 -2.712 10.1 3.9 * 10-6 19F 100.0 25.181 94.1 8.3 * 10-1 29Si 4.7 -5.3188 19.9 3.7 * 10-4 31P 100.0 10.841 40.5 6.6 * 10-2 57Fe 2.2 0.8661 3.2 7.4 * 10-7 77Se 7.6 5.12 19.1 5.3 * 10-4 89Y 100.0 -1.3155 4.9 1.2 * 10-4 103Rh 100.0 -0.846 3.2 3.2 * 10-5 107Ag 51.8 -1.087 4.0 3.5 * 10-5 109Ag 48.2 -1.250 4.7 4.9 * 10-5 111Cd 12.8 -5.6926 21.2 1.2 * 10-3 113Cd 12.3 -5.9550 22.2 1.3 * 10-3 NMR in Organometallic compoundsNMR properties of some spins 1/2 nuclei Index

  4. Isotope 117Sn Nat. Abundance % 7.6 Magnetogyric ratio () 107 rad T-1 s-1 -9.578 35.6 Relative NMR frequency (MHz) Rel. Receptivity 3.5 * 10-3 119Sn 8.6 -10.021 37.3 4.5 * 10-3 125Te 7.0 -8.498 31.5 2.2 * 10-3 129Xe 26.4 -7.441 27.8 5.7 * 10-3 169Tm 100.0 -2.21 8.3 5.7 * 10-4 171Yb 14.3 4.712 17.6 7.8 * 10-4 183W 14.4 1.120 4.2 1.1 * 10-5 187Os 1.6 0.616 2.3 2.0 * 10-7 195Pt 33.8 5.768 21.4 3.4 * 10-3 199Hg 16.8 4.8154 17.9 9.8 * 10-4 203Tl 29.5 15.436 57.1 5.7 * 10-2 205Tl 70.5 15.589 57.6 1.4 * 10-1 207Pb 22.6 5.540 20.9 2.0 * 10-3 Spin 1/2

  5. Multinuclear NMR • There are at least four other factors we must consider • Isotopic Abundance. Some nuclei such as 19F and 31P are 100% abundant (1H is 99.985%), but others such as 17O have such a low abundance (0.037%). Consider: 13C is only 1.1% abundant (need more scans than proton). • Sensitivity goes with the cube of the frequency. 103Rh (100% abundant but only 0.000031 sensitivity): obtaining a spectrum for the nucleus is generally impractical. However, the nucleus can still couple to other spin-active nuclei and provide useful information. In the case of rhodium, 103Rh coupling is easily observed in the 1H and 13C spectra and the JRhX can often be used to assign structures • Nuclear quadrupole. For spins greater than 1/2, the nuclear quadrupole moment is usually larger and the line widths may become excessively large. • Relaxation time

  6. Q = quadrupole moment qzz = electric field gradient tc = correlation time I = spin quantum number (2I + 3) Q2 q2zztc W1/2 ~ I2 (2I -1) NMR in Organometallic compoundsspins > 1/2 nuclei These nuclei possess a quadrupole moment (deviation from spherical charge distribution) which cause extremely short relaxation time and extremely large linewidth W1/2 (up to 50 KHz) Narrow linescan be obtained for low molecular weight(small tc) and if nuclei are embedded in ligand field of cubic (tetrahedral, octahedral) symmetry (qzz blocked)

  7. Isotope 2H Spin 1 Abun-dance % 0.015 () 107 rad T-1 s-1 4.1066 Freq. (MHz) 15.4 Rel. Recep-tivity 1.5 * 10-6 Quadrupole moment10-28m2 2.8 * 10-3 6Li 1 7.4 3.9371 14.7 6.3 * 10-4 -8 * 10-4 7Li 3/2 92.6 10.3975 38.9 2.7 * 10-1 -4 * 10-2 9Be 3/2 100.0 -3.7596 14.1 1.4 * 10-2 5 * 10-2 10B 3 19.6 2.8746 10.7 3.9 * 10-3 8.5 * 10-2 11B 3/2 80.4 8.5843 32.1 1.3 * 10-1 4.1 * 10-2 14N 1 99.6 1.9338 7.2 1.0 * 10-3 1 * 10-2 17O 5/2 0.037 -3.6279 13.6 1.1 * 10-5 -2.6 * 10-2 23Na 3/2 100.0 7.0801 26.5 9.3 * 10-2 1 * 10-1 25Mg 5/2 10.1 -1.639 6.1 2.7 * 10-4 2.2 * 10-1 27Al 5/2 100.0 6.9760 26.1 2.1 * 10-1 1.5 * 10-1 33S 3/2 0.76 2.055 7.7 1.7 * 10-5 -5.5 * 10-2 35Cl 3/2 75.5 2.6240 9.8 3.6 * 10-3 -1 * 10-1 37Cl 3/2 24.5 2.1842 8.2 6.7 * 10-4 -7.9 * 10-2 39K 3/2 93.1 1.2498 4.7 4.8 * 10-4 4.9 * 10-2 47Ti 5/2 7.3 -1.5105 5.6 1.5 * 10-4 2.9 * 10-1 49Ti 7/2 5.5 -1.5109 5.6 2.1 * 10-4 2.4 * 10-1 51V 7/2 99.8 7.0453 26.3 3.8 * 10-1 -5 * 10-2 55Mn 5/2 100.0 6.608 24.7 1.8 * 10-1 4 * 10-1 NMR properties of some spins quadrupolar nuclei

  8. Quadrupolar nuclei: Oxygen-17 NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-Verlag Terence N. Mitchellm Burkhard Costisella

  9. Notable nuclei • 19F: spin ½, abundance 100%, sensitivity (H=1.0) : 0.832JH-F = 45 Hz, 3JH-F trans = 17 Hz, 3JH-FCis = 6 Hz2JF-F = 300 Hz, 3JF-F = - 27 Hz • 29Si: spin ½, abundance 4.7%, sensitivity (H=1.0) : 0.0078The inductive effect of Si typically moves 1H NMR aliphatic resonances upfield to approximately 0 to 0.5 ppm, making assignment of Si-containing groups rather easy. In addition, both carbon and proton spectra display Si satellites comprising 4.7% of the signal intensity. • 31P: spin ½, abundance 100%, sensitivity (H=1.0) : 0.071JH-P = 200 Hz, 2JH-P ~2-20 Hz,1JP-P = 110 Hz, 2JF-P ~ 1200-1400 Hz, 3JP-P = 1-27 Hzthe chemical shift range is not as diagnostic as with other nuclei, the magnitude of the X-P coupling constants is terrific for the assignment of structuresKarplus angle relationship works quite well

  10. Notable nuclei • 31P: spin ½, abundance 100%, sensitivity (H=1.0) : 0.071JH-P = 200 Hz, 2JH-P ~2-20 Hz,1JP-P = 110 Hz, 2JF-P ~ 1200-1400 Hz, 3JP-P = 1-27 Hzthe chemical shift range is not as diagnostic as with other nuclei, the magnitude of the X-P coupling constants is terrific for the assignment of structuresKarplus angle relationship works quite well 2JH-P is 153.5 Hz for the phosphine trans to the hydride, but only 19.8 Hz to the (chemically equivalent) cisphosphines. See Selnau, H. E.; Merola, J. S.Organometallics, 1993, 5, 1583-1591.

  11. Notable nuclei • 103Rh: spin ½, abundance 100%, sensitivity (H=1.0) : 0.0000311JRh-C = 40-100 Hz, 1JRh-C(Cp) = 4 Hz, For example, in the 13C NMR spectrum of this linked Cp, tricarbonylRhdimer at 240K (the dimer undergoes fluxional bridge-terminal exchange at higher temperatures), the bridging carbonyl is observed at d232.53 and is a triplet with 1JRh-C = 46 Hz. The equivalent terminal carbonyls occur as a doubletat d190.18 with 1JRh-C = 84 Hz: See Bitterwolf, T. E., Gambaro, A., Gottardi, F., Valle G Organometallics, 1991, 6, 1416-1420.

  12. Chemical shift for organometallic In molecules, the nuclei are screened by the electrons. So the effective field at the nucleus is: Beff = B0(1-) Where  is the shielding constant. The shielding constant has 2 terms: d (diamagnetic)and p (paramagnetic) d- depends on electron distribution in the ground state p - depends on excited state as well. It is zero for electrons in s-orbital. This is why the proton shift is dominated by the diamagnetic term. But heavier nuclei are dominated by the paramagnetic term. Index

  13. Symmetry Non-equivalent nuclei could "by accident" have the same shift and this could cause confusion. Some Non-equivalent group might also become equivalent due to some averaging process that is fast on NMR time scale. (rate of exchange is greater than the chemical shift difference) e.g. PF5: Fluorine are equivalent at room temperature (equatorial and axial positions are exchanging by pseudorotation) Index

  14. Symmetry in Boron compounds

  15. M = C M = Si M = Ge MH4 0.1 3.2 3.1 MH3I 2.0 3.4 3.5 MH3Br 2.5 4.2 4.5 MH3Cl 2.8 4.6 5.1 (MH3)2O 3.2 4.6 5.3 MH3F 4.1 4.8 5.7 Proton - NMR Increasing the 1 s orbital density increases the shielding Shift to low field when the metal is heavier (SnH4 -  = 3.9 ppm) Index

  16. Proton – NMR : Chemical shift • Further contribution to shielding / deshielding is the anisotropic magnetic susceptibility from neighboring groups (e.g. Alkenes, Aromatic rings-> deshielding in the plane of the bound) • In transition metal complexes there are often low-lying excited electronic states. When magnetic field is applied, it has the effect of mixing these to some extent with the ground state. • Therefore the paramagnetic term is important for those nuclei themselves => large high frequency shifts (low field).The protons bound to these will be shielded ( => 0 to -40 ppm) (these resonances are good diagnostic. ) • For transition metal hydride this range should be extended to 70 ppm! • If paramagnetic species are to be included, the range can go to 1000 ppm!! Index

  17. Proton NMR and other nuclei • The usual range for proton NMR is quite small if we compare to other nuclei: • 13C => 400 ppm • 19F => 900 ppm • 195Pt => 13,000 ppm !!! • Advantage of proton NMR : Solvent effects are relatively small • Disadvantage: peak overlap Index

  18. Chemical shifts of other element There is no room to discuss all chemical shifts for all elements in the periodical table. The discussion will be limited to 13C, 19F, 31P *as these are so widely used. Alkali Organometallics (lithium) will be briefly discuss For heavier non-metal element we will discuss 77Se and 125Te. For transition metal, we will discuss 55Mn and 195Pt Index

  19. Alkali organometallics: Organolithium For Lithium: we have the choice between 2 nuclei: 6Li : Q=8.0*10-4 a=7.4% I=1 7Li : Q=4.5*10-2 a=92.6% I=3/2 6Li : Higher resolution7Li : Higher sensitivity 7Li NMR : larger diversity of bonding compare to Na-Cs (ionic) • Solvent effects are important (solvating power affects the polarity of Li-C bond and govern degree of association • d covers a small range: 10 ppm • Covalent compound appear at low field (2 ppm range) • Coupling 1JC-Li between carbon and Lithium indicate covalent bond

  20. Organolithium

  21. Boron NMR For Boron: we have the choice between 2 nuclei: 10B : Q= 8.5 * 10-2 a=19.6% I=3 11B : Q= 4.1 * 10-2 a=80.4% I=3/2 11B : Higher sensitivity

  22. Boron NMR

  23. Boron NMR

  24. JF-10B n10B = JF-11B n11B 11B coupling with Fluorine: 19F-NMR 10B : Q= 8.5 * 10-2 a=19.6% n=10.7 I=3 2nI+1 = 7 11B : Q= 4.1 * 10-2 a=80.4% n=32.1 I=3/2 2nI+1 = 4 Boron can couple to other nuclei as shown here on 19F-NMR Isotopic shift 19F-NMR 11BF4 NaBF4 / D2O 10BF4 JBF=0.5 Hz JBF=1.4 Hz

  25. C13 shifts • Saturated Carbon appear between 0-100 ppm with electronegative substituents increasing the shifts. • CH3-X : directly related to the electronegativity of X. • The effects are non-additive: CH2XY cannot be easily predicted • Shifts for aromatic compounds appear between 110-170 ppm • -bonded metal alkene may be shifted up to 100 ppm: shift depends on the mode of coordination • one extreme shift is CI4 = -293 ppm !!! • Metal carbonyls are found between 170-290 ppm. (very long relaxation time make their detection very difficult) • Metal carbene have resonances between 250-370 ppm Index

  26. F-19 shifts Wide range: 900 ppm! And are not easy to interpret. The accepted reference is now: CCl3F. With literature chemical shift, care must be taken to ensure they referenced their shifts properly. • electronegativity • Oxidation state of neighbor • Stereochemistry • Effect of more distant group Sensitive to: Index

  27. F-19 shifts The wide shift scale allow to observe all the products in the reaction of : WF6 + WCl6 --> WFnCln-6 (n=1-6) Index

  28. Sn shifts

  29. NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-Verlag Terence N. Mitchellm Burkhard Costisella 3 isotopes with spin ½ : Sn-115 a=0.35% Sn-117 a=7.61% Sn-119 a=8.58% H-NMR of Sn compound 2JSN119-H = 1.046 * 2JSN117-H (ratio of g of the 2 isotopes) 2JSN119-H = 54.3 Hz 2JSN117-H

  30. NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-Verlag Terence N. Mitchellm Burkhard Costisella Sn-119 3 isotopes with spin ½ : Sn-115 a=0.35% Sn-117 a=7.61% Sn-119 a=8.58%

  31. Sn-119 coupling 1- molecule containing 1 Sn-119 2- molecule containing Sn119, Sn117 J between Sn-119 and Sn-117 3- molecule containing two Sn119 Form an AB spectra (J=684 Hz) 4- molecule containing Sn119 and C13 J between Sn119 and C13 Sn-117 a=7.61% Sn-119 a=8.58%

  32. Dynamic NMR p261

  33. C13

  34. Cycloheptatriene

  35. Dynamic NMR

  36. 1H-NMR

  37. P-31 Shifts The range of shifts is ± 250 ppm from H3PO4 Extremes: • - 460 ppm for P4 • +1,362 ppm phosphinidene complexe: tBuP[Cr(CO)5]2 • Interpretation of the shifts is not easy : there seems to be many contributing factors • PIII covers the whole normal range: strongly substituent dependant • PV narrower range:  - 50 to  + 100. • Unknown can be predicted by extrapolation or interpolation • PX2Y or PY3 can be predicted from those for PX3 and PXY2 • The best is to compare with literature values. Index

  38. P-31 Shifts Index

  39. Other nuclei: Selenium, Telurium There are many analogies between Phosphorus and Selenium chemistry. There are also analogies between the chemical shifts of 31P and 77Se but the effect are much larger in Selenium! For example: Se(SiH3)2 and P(SiH3)3 are very close to the low frequency limit (high field) The shifts in the series SeR2 and PR3 increase in the order R= Me < Et < Pri < But There is also a remarkable correlation between 77Se and125Te. (see picture next slide) Index

  40. Correlation between Tellurium and Selenium Shifts Index

  41. Manganese-55 • Manganese-55 can be easily observed in NMR but due to it’s large quadrupole moment it produces broad lines • 10 Hz for symmetrical environment e.g. MnO4- • 10,000 Hz for some carbonyl compounds. • It’s shift range is => 3,000 ppm • As with other metals, there is a relationship between the oxidation state and chemical shielding • Reference: MnVII : d = 0 ppm (MnO4-) • MnI : d –1000 to –1500 • Mn-I : d –1500 to -3000 • 55Mn chemical shifts seems to reflect the total electron density on the metal atom Index

  42. Pt-195 Shifts I = ½ a=33.8% K2PtCl6 ref set to 0. Scale: -6000 to + 7000 ppm !! Platinum is a heavy transition element. It has wide chemical shift scale: 13,000 ppm! The shifts depends strongly on the donor atom but vary little with long range. For example: PtCl2(PR3)2 have very similar shifts with different R Many platinum complexes have been studied by 1H, 13C and 31P NMR. But products not involving those nuclei can be missed : PtCl42- Major part of Pt NMR studies deals with phosphineligands as these can be easily studied with P-31 NMR. Lines are broad (large CSA) large temperature dependence (1 ppm per degree) Index

  43. Pt-195 : coupling with protons CSA relaxation on 195Pt can have unexpected influence on proton satellites. CSA relaxation increases with the square of the field. If the relaxation (time necessary for the spins to changes their spin state) is fast compare to the coupling, the coupling can even disapear! 1H-NMR CH2=CH2 a=33.8%

  44. NMR – From Spectra to Structures An Experimental approachSecond edition (2007) Springler-Verlag Terence N. Mitchellm Burkhard Costisella Pt-195 I = ½ a=33.8% H6 : dd J4-6 = 1.3 Hz J5-6 = 6.2 Hz JH6-Pt195 = 26 Hz

  45. Pople Notation Spin > ½ are generally omitted. Index

  46. Effect of Coupling with exotic nuclei in NMR Natural abundance 100% 1H, 19F, 31P, 103Rh : all have 100% natural abundance. When these nuclei are present in a molecule, scalar coupling must be present. Giving rise to multiplets of n+1 lines. One bond coupling can have hundreds or thousands of Hz. They are an order of magnitude smaller per extra bound between the nuclei involved. Usually coupling occur up to 3-4 bounds. Example: P(SiH3)3 + LiMe -> Product : P-31 NMRshows septet ===> product is then P(SiH3)2- Index

  47. P-31 Spectrum of PF2H(NH2)2 labeled with 15N t 1JP-H t Triplet1JP-N Quintet 2JP-H 1JP-F 1JP-F 2 x 3 x 3 x 5 = 90 lines ! coupling with H (largest coupling : Doublet) then we see triplet with large coupling with fluorine With further Coupling to 2 N produce triplets, further coupled to 4protons => quintets

  48. Effect of Coupling with exotic nuclei in NMR Low abundance nuclei of spin 1/2 13C, 29Si, 117Sn, 119Sn, 183W : should show scalar coupling => satellite signals around the major isotope. • For example: WF6 as 183W has 14% abundance, the fluorine spectra should show satellite signals separated by the coupling constant between fluorine and tungsten. The central signal has 86% intensity and the satellites have 14%. This will produce 1:12:1 pattern Index

  49. Si-29 coupling • 29Si has 5% abundance. • For H3Si-SiH3 , the chance of finding • H3-28Si--29Si-H3 is 10%. Interestingly we can see that the two kind of protons are no longer equivalent so homonuclear coupling become observable! The molecule with 2 Si-29 is present with 0.25% intensity and is difficult to observe. • The second group gives smaller coupling Index

  50. Coupling with Platinum 195Pt the abundance is 33%. Platinum specie will give rise to satellite signal with a relative ratio of1 : 4 : 1. This intensity pattern is diagnostic for the presence of platinum. If the atom is coupled to 2 Pt, the situation is more complex: 2/3 x 2/3 => no Pt spin (central resonance) 1/3 x 1/3 => two Pt with spin 1/2 => triplet remaining molecule has 2x (1/3 x 2/3) = 4/9 => one Pt with spin 1/2 => doublet  Adding the various components together we now have 1:8:18:8:1 pattern. The weak outer lines are often missed, leaving what appear to be a triplet 1:2:1 !!! Index

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