NMR Nuclear Magnetic ResonanceNMR for Organometallic compounds

IndexNMR-basicsH-NMRNMR-SymmetryHeteronuclear-NMRDynamic-NMRNMR and Organometallic compounds

NMR in Organometallic compoundsspins 1/2 nucleiFor small molecules having nuclei I=1/2 : Sharp lines are expectedW1/2 (line width at half height) = 0-10 HzIf 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 compoundsNMR properties of some spins 1/2 nucleiIndex

Spin 1/2

Multinuclear NMRThere 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 structuresNuclear 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 compoundsspins > 1/2 nucleiThese 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 lines can be obtained for low molecular weight (small tc)and if nuclei are embedded in ligand field of cubic (tetrahedral, octahedral) symmetry (qzz blocked)

NMR properties of some spins quadrupolar nuclei

Quadrupolar nuclei: Oxygen-17NMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella

Notable nuclei19F: spin , abundance 100%, sensitivity (H=1.0) : 0.83 2JH-F = 45 Hz, 3JH-F trans = 17 Hz, 3JH-F Cis = 6 Hz 2JF-F = 300 Hz, 3JF-F = - 27 Hz29Si: 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 easy. 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 with other nuclei, the magnitude of the X-P coupling constants is terrific for the assignment of structures Karplus angle relationship works quite well

Notable nuclei31P: 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 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 equivalent) cis phosphines. See Selnau, H. E.; Merola, J. S. Organometallics, 1993, 5, 1583-1591.

Notable nuclei103Rh: 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 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 doublet at d190.18 with 1JRh-C = 84 Hz: See Bitterwolf, T. E., Gambaro, A., Gottardi, F., Valle G Organometallics, 1991, 6, 1416-1420.

Chemical shift for organometallicIn 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 statep - 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 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

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 shiftFurther 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

Proton NMR and other nucleiThe usual range for proton NMR is quite small if we compare to other nuclei:13C => 400 ppm19F => 900 ppm195Pt => 13,000 ppm !!!

Advantage of proton NMR : Solvent effects are relatively smallDisadvantage: 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 briefly discussFor heavier non-metal element we will discuss 77Se and 125Te.For transition metal, we will discuss 55Mn and 195Pt

Index

Alkali organometallics: OrganolithiumFor Lithium: we have the choice between 2 nuclei:6Li : Q=8.0*10-4a=7.4%I=17Li : Q=4.5*10-2a=92.6%I=3/26Li : Higher resolution7Li : Higher sensitivity7Li 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

Organolithium

Boron NMRFor Boron: we have the choice between 2 nuclei:10B : Q= 8.5 * 10-2 a=19.6%I=311B : Q= 4.1 * 10-2a=80.4%I=3/211B : Higher sensitivity

Boron NMR

Boron NMR

11B coupling with Fluorine: 19F-NMR10B : Q= 8.5 * 10-2 a=19.6%n=10.7 I=3NaBF4 / D2O19F-NMR2nI+1 = 72nI+1 = 411BF410BF4Isotopic shift11B : Q= 4.1 * 10-2a=80.4%n=32.1 I=3/2Boron can couple to other nuclei as shown here on 19F-NMRJBF=0.5 HzJBF=1.4 Hz

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

F-19 shifts electronegativityOxidation state of neighborStereochemistryEffect of more distant group 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. Sensitive to:Index

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

Sn shifts

H-NMR of Sn compound3 isotopes with spin :Sn-115 a=0.35%Sn-117 a=7.61%Sn-119a=8.58%2JSN117-H2JSN119-H = 54.3 Hz2JSN119-H = 1.046 * 2JSN117-H(ratio of g of the 2 isotopes)NMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella

Sn-1193 isotopes with spin :Sn-115 a=0.35%Sn-117 a=7.61%Sn-119a=8.58%NMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella

Sn-119 couplingSn-117 a=7.61%Sn-119a=8.58%1- molecule containing 1 Sn-1192- molecule containing Sn119, Sn117 J between Sn-119 and Sn-1173- molecule containing two Sn119 Form an AB spectra (J=684 Hz)4- molecule containing Sn119 and C13 J between Sn119 and C13

Dynamic NMRp261

C13

Cycloheptatriene

Dynamic NMR

1H-NMR

P-31 Shifts - 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 factorsPIII covers the whole normal range: strongly substituent dependantPV narrower range: - 50 to + 100.Unknown can be predicted by extrapolation or interpolationPX2Y or PY3 can be predicted from those for PX3 and PXY2The best is to compare with literature values.

The range of shifts is 250 ppm from H3PO4 Extremes: Index

P-31 ShiftsIndex

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 and 125Te. (see picture next slide)

Other nuclei: Selenium, TeluriumIndex

Correlation between Tellurium and Selenium ShiftsIndex

Manganese-55Manganese-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 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 1500 Mn-I : d 1500 to -3000

55Mn chemical shifts seems to reflect the total electron density on the metal atomIndex

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, 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. IndexLines are broad (large CSA) large temperature dependence (1 ppm per degree)I = a=33.8%K2PtCl6 ref set to 0. Scale: -6000 to + 7000 ppm !!

Pt-195 : coupling with protonsCSA 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!CH2=CH21H-NMRa=33.8%

Pt-195I = a=33.8%H6 : ddJ5-6 = 6.2 HzJ4-6 = 1.3 HzJH6-Pt195 = 26 HzNMR From Spectra to Structures An Experimental approach Second edition (2007) Springler-VerlagTerence N. Mitchellm Burkhard Costisella

Pople Notation Spin > are generally 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 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 NMR shows septet ===> product is then P(SiH3)2-Index

P-31 Spectrum of PF2H(NH2)2 labeled with 15N 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 => quintets2 x 3 x 3 x 5 = 90 lines !t1JP-F1JP-Ft1JP-HTriplet 1JP-NQuintet 2JP-H

Effect of Coupling with exotic nuclei in NMRFor 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 Low abundance nuclei of spin 1/2 13C, 29Si, 117Sn, 119Sn, 183W : should show scalar coupling => satellite signals around the major isotope. Index

Si-29 coupling29Si 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

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 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 => tripletremaining molecule has 2x (1/3 x 2/3) = 4/9 => one Pt with spin 1/2 => doubletAdding 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

Carbon-13 in organometallic NMR13C is extremely useful to organometallic NMRFor example:Palladium complexe has: 4 non-equivalent Methyls 2 methylenes Allyl : 1 methylene, 2 methynylPhenyl: 4 C: mono-subst.Index

29Si-NMRPolymeric siloxanes are easily studied by NMR: These have terminal R3SiO- Chain R2Si (O-)2 Branch R-Si(O-)3 Quaternary Si(O-)4 All these Silicon have different shifts making it possible to study the degree of polymerization and cross-linkingIndex

Coupling with Quadrupolar Nuclei (I>1/2)2nI + 1 linesThe observation of such coupling depends on the relaxation rate of the quadrupolar nuclei (respect to coupling constant)Index

Coupling with Quadrupolar Nuclei (I>1/2)

Factors contributing to Coupling constantMagnetic Moment of one nuclei interact with the field produced by orbital motion of the electrons which in turn interact with the second nuclei.There is a dipole interaction involving the electron spin magnetic momentThere is also a contribution from spins of electrons which have non-zero probability of being at the nucleus => Fermi contact Index

1-bound couplingDepends on s-orbital character of the boundHybridization of the nuclei involved 1JCH => 125 (sp3), 160 (sp2), 250 (sp) Electronegativity is another factor: increase the couplingCCl3H => 1JCH = 209 Hz Coupling can be used to determine coordination number of PF , PH compounds, and to distinguish axial, equatorial orientation of Fluorines.1JPH = 180 (3 coordinate) , 1JPH = 400 (4 coordinate) Coupling can also be used to distinguish single double bondE.g.

Index

2-bound coupling2J can give structural information: There is a relationship between 2J and Bond angle=> coupling range passes through zero. Therefore the sign of the coupling must be determinedIndex

3-bound couplingDepends on Dihedral angle 3JXY = A cos 2f + B cos f + C A, B, C : empirical constantsIndex

Complicated proton spectra : CH3-CH2-S-PF2Almost quintetIndex

Complicated Fluorine spectra : PF2-S-PF2Second order spectra: 19FChemically equivalentMagnetically non-equivalent1JPF different from 3JPHThis type of spectra is frequent in transition metal complex:MCl2(PR3)2Index

Equivalence and non-equivalenceF are Non-EquivalentThe 2 phosphorus are Pro-chiral: non-equivalentIndex

To identify a compound: PF215NHSiH3Use as many techniques as possibleProton nmr spectra is difficult to analyze with so many JsBut with 19F, 15N and 31P spectra its easier (get heteronuclear J)Index

To identify a compound: PF215NHSiH3Use as many techniques as possibleUsing decoupler : easier analysisIndex

Multinuclear ApproachProton NMR spectra: 3 groups of peaks integrating for 12:4:1Resonances due to Methyl and CH2 have coupling with 31PAnd also shows satellites due to mercury coupling (199Hg 16.8%)While third resonance is broadIn 31P, there is a single signal: Symmetrical compound: that has Mercury satellitesIn 199Hg NMR (with proton decoupling): quintet demonstrate the presence of 4 PhosphorusIndex

Heteronuclear NOE NOE enhancement can give useful gain in signal-to-noise It is most efficient when the heteronuclei is bound to proton NOEMAX = 1 + gH/2gX For nuclei having negative g, NOE is negative (for 29Si, max=-1.5)Index

Exchange : DNMR Dynamic NMRNMR is a convenient way to study rate of reactions provided that the lifetime of participating species are comparable to NMR time scale (10-5 s)At low temperature, hydrogens form an A2B2X spin systemAt higher temperature germanium hop from one C to the nextIndex

Paramagnetic compounds in NMRUsually paramegnetic compounds are too braod => give ESRIn NMR, Chemical shift is greatly expandedParamagnetic shifts are made up of 2 component:Through space Dipolar interaction between the magnetic moment of the electron and of the nucleusContact Shift: coupling between electron and nucleus. This interaction would give a doublet in NMR but J ~ millions of Hertz!! With such large coupling, intensity of the 2 resonances are not equal => weighted mean position is not midway With fast relaxation, collapse of the multiplet may fall thousands Hertz away from expected position => Contact ShiftContact Shift give a measure of unpaired spin density at resonating Nucleus.Useful for studying spin distribution in organic radical or in ligands in organo metallic complexes

Paramagnetic compounds in NMR4 sets of resonances:1 symmetrical Fac: the 3 ligand are identical 3 Asymetrical ligand in Mer occur with 3 time the probability.

IndexNMR-basicsH-NMRNMR-SymmetryHeteronuclear-NMRDynamic-NMRNMR and Organometallic compoundsSpecial 1D-NMR

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