1、NMR Nuclear Magnetic Resonance,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,For small molecules having nuclei I=1/2 : Sharp lines are expected W1/2 (line width at h
2、alf 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!,NMR in Organometallic compounds NMR properties of some spins 1/2 nuclei,Index,Spin 1/2,Multinuclear NMR,There are at l
3、east four other factors we must considerIsotopic 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
4、. 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
5、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,NMR in Organometallic compounds spins 1/2 nuclei,These nuclei possess a quadrupole mom
6、ent (deviation from spherical charge distribution) which cause extremely short relaxation time and extremely large linewidth W1/2 (up to 50 KHz),Narrow lines can be obtained for low molecular weight (small tc) and if nuclei are embedded in ligand field of cubic (tetrahedral, octahedral) symmetry (qz
7、z blocked),NMR properties of some spins quadrupolar nuclei,Quadrupolar nuclei: Oxygen-17,NMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-Verlag Terence N. Mitchellm Burkhard Costisella,Notable nuclei,19F: spin , abundance 100%, sensitivity (H=1.0) : 0.83 2JH-F
8、 = 45 Hz, 3JH-F trans = 17 Hz, 3JH-F Cis = 6 Hz 2JF-F = 300 Hz, 3JF-F = - 27 Hz 29Si: spin , abundance 4.7%, sensitivity (H=1.0) : 0.0078 The 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 eas
9、y. In addition, both carbon and proton spectra display Si satellites comprising 4.7% of the signal intensity. 31P: spin , abundance 100%, sensitivity (H=1.0) : 0.07 1JH-P = 200 Hz, 2JH-P 2-20 Hz, 1JP-P = 110 Hz, 2JF-P 1200-1400 Hz, 3JP-P = 1-27 Hz the chemical shift range is not as diagnostic as wit
10、h other nuclei, the magnitude of the X-P coupling constants is terrific for the assignment of structures Karplus angle relationship works quite well,Notable nuclei,31P: spin , abundance 100%, sensitivity (H=1.0) : 0.07 1JH-P = 200 Hz, 2JH-P 2-20 Hz, 1JP-P = 110 Hz, 2JF-P 1200-1400 Hz, 3JP-P = 1-27 H
11、z the 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 structures Karplus 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 equ
12、ivalent) cis phosphines.,See Selnau, H. E.; Merola, J. S. Organometallics, 1993, 5, 1583-1591.,Notable nuclei,103Rh: spin , abundance 100%, sensitivity (H=1.0) : 0.000031 1JRh-C = 40-100 Hz, 1JRh-C(Cp) = 4 Hz,For example, in the 13C NMR spectrum of this linked Cp, tricarbonyl Rh dimer at 240K (the d
13、imer 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 doublet at d190.18 with 1JRh-C = 84 Hz:,See Bitterwolf, T. E., Gambaro, A., Gottardi, F., Valle G
14、 Organometallics, 1991, 6, 1416-1420.,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 - depend
15、s 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,Symmetry,Non-equivalent nuclei could “by acc
16、ident“ 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 (equ
17、atorial and axial positions are exchanging by pseudorotation),Index,Symmetry in Boron compounds,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,Proton NMR : Chemical shift,Further contribution to shielding
18、/ 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
19、 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 sho
20、uld be extended to 70 ppm! If paramagnetic species are to be included, the range can go to 1000 ppm!,Index,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 effect
21、s are relatively small Disadvantage: peak overlap,Index,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 bri
22、efly discuss For heavier non-metal element we will discuss 77Se and 125Te.For transition metal, we will discuss 55Mn and 195Pt,Index,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 resolut
23、ion 7Li : 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 associationd covers a small range: 10 ppmCovalent compound appear at low field (2 ppm range)Coupling 1JC-L
24、i between carbon and Lithium indicate covalent bond,Organolithium,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,Boron NMR,Boron NMR,11B coupling with Fluorine: 19F-NMR,10B : Q= 8.5 * 10-2 a=19.6% n
25、=10.7 I=3,NaBF4 / D2O,19F-NMR,2nI+1 = 7,2nI+1 = 4,11BF4,10BF4,Isotopic shift,11B : Q= 4.1 * 10-2 a=80.4% n=32.1 I=3/2,Boron can couple to other nuclei as shown here on 19F-NMR,JBF=0.5 Hz,JBF=1.4 Hz,C13 shifts,Saturated Carbon appear between 0-100 ppm with electronegative substituents increasing the
26、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 CI
27、4 = -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,F-19 shifts,electronegativity Oxidation state of neighbor Stereochemistry Effect of more distant group,Wide range: 900
28、 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.,Sensitive to:,Index,F-19 shifts,The wide shift scale allow to observe all the products in the reaction of : WF6 + WCl6 WFnCln
29、-6 (n=1-6),Index,Sn shifts,H-NMR of Sn compound,3 isotopes with spin : Sn-115 a=0.35% Sn-117 a=7.61% Sn-119 a=8.58%,2JSN117-H,2JSN119-H = 54.3 Hz,2JSN119-H = 1.046 * 2JSN117-H,(ratio of g of the 2 isotopes),NMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-Verla
30、g Terence N. Mitchellm Burkhard Costisella,Sn-119,3 isotopes with spin : Sn-115 a=0.35% Sn-117 a=7.61% Sn-119 a=8.58%,NMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-Verlag Terence N. Mitchellm Burkhard Costisella,Sn-119 coupling,Sn-117 a=7.61% Sn-119 a=8.58%,
31、1- molecule containing 1 Sn-119,2- molecule containing Sn119, Sn117J between Sn-119 and Sn-117,3- molecule containing two Sn119Form an AB spectra (J=684 Hz),4- molecule containing Sn119 and C13J between Sn119 and C13,Dynamic NMR,p261,C13,Cycloheptatriene,Dynamic NMR,1H-NMR,P-31 Shifts,- 460 ppm for
32、P4 +1,362 ppm phosphinidene complexe: tBuPCr(CO)52 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 PX
33、2Y or PY3 can be predicted from those for PX3 and PXY2 The best is to compare with literature values.,The range of shifts is 250 ppm from H3PO4,Extremes:,Index,P-31 Shifts,Index,There are many analogies between Phosphorus and Selenium chemistry.There are also analogies between the chemical shifts of
34、 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 and 125Te. (see picture
35、 next slide),Other nuclei: Selenium, Telurium,Index,Correlation between Tellurium and Selenium Shifts,Index,Manganese-55,Manganese-55 can be easily observed in NMR but due to its large quadrupole moment it produces broad lines 10 Hz for symmetrical environment e.g. MnO4- 10,000 Hz for some carbonyl
36、compounds. Its 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 1500Mn-I : d 1500 to -300055Mn chemical shifts seems to reflect the total electron density on the metal atom,
37、Index,Pt-195 Shifts,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,
38、 13C and 31P NMR. But products not involving those nuclei can be missed : PtCl42- Major part of Pt NMR studies deals with phosphine ligands as these can be easily studied with P-31 NMR.,Index,Lines are broad (large CSA) large temperature dependence (1 ppm per degree),I = a=33.8% K2PtCl6 ref set to 0
39、. Scale: -6000 to + 7000 ppm !,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 couplin
40、g, the coupling can even disapear!,CH2=CH2,1H-NMR,a=33.8%,Pt-195,I = a=33.8%,H6 : dd,J5-6 = 6.2 Hz,J4-6 = 1.3 Hz,JH6-Pt195 = 26 Hz,NMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-Verlag Terence N. Mitchellm Burkhard Costisella,Pople Notation,Spin are generally
41、 omitted.,Index,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 thous
42、ands 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 NMR shows septet = product is then P(SiH3)2-,Index,P-31 Spectrum of PF2H(NH2)2 labeled with 15N,coupling with H (largest
43、coupling : Doublet) then we see triplet with large coupling with fluorine With further Coupling to 2 N produce triplets, further coupled to 4protons = quintets,2 x 3 x 3 x 5 = 90 lines !,t,1JP-F,1JP-F,t,1JP-H,Triplet 1JP-N Quintet 2JP-H,Effect of Coupling with exotic nuclei in NMR,For example: WF6 a
44、s 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,Low abundance nuclei of spin 1/2,13C, 29Si, 117Sn, 119Sn,
45、183W : should show scalar coupling= satellite signals around the major isotope.,Index,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 obse
46、rvable! The molecule with 2 Si-29 is present with 0.25% intensity and is difficult to observe. The second group gives smaller coupling,Index,Coupling with Platinum,195Pt the abundance is 33%. Platinum specie will give rise to satellite signal with a relative ratio of 1 : 4 : 1. This intensity patter
47、n 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 = doubletAdding the various components
48、 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,Carbon-13 in organometallic NMR,13C is extremely useful to organometallic NMR,For example: Palladium complexe has:4 non-equivalent Methyls2 methylenesAllyl : 1 methylene
49、, 2 methynyl Phenyl: 4 C: mono-subst.,Index,29Si-NMR,Polymeric siloxanes are easily studied by NMR: These have terminal R3SiO-Chain R2Si (O-)2Branch R-Si(O-)3Quaternary Si(O-)4,All these Silicon have different shifts making it possible to study the degree of polymerization and cross-linking,Index,Coupling with Quadrupolar Nuclei (I1/2),2nI + 1 lines The observation of such coupling depends on the relaxation rate of the quadrupolar nuclei (respect to coupling constant),
