lecture for b.sc. and m.sc. students prof. r....

„31P NMR spectroscopy – an experimentalists view on a powerful tool“

Lecture for B.Sc. and M.Sc. students

Prof. R. Streubel

Institut für Anorganische Chemie, Bonn, Germany

http://anorganik.chemie.uni-bonn.de/akstreubel/Streubel_Home.html

E-mail: r.streubel@uni-bonn.de

Outline

1. Fundamental aspects

2. Phosphanes (phosphines)

3. Phosphane oxides and phosphate esters

4. Phosphane oxides vs. phosphoranes and phosphates

6. Phosphorus compounds in low coordination

5. MLn and BH3 complexes

7. Coupling of phosphorus to main group elements (H, C, F)

8. Coupling of phosphorus to transition metals (W)

9. Chemical shift range for phosphorus compounds

1. Fundamental aspects – a short introduction

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1. Fundamental aspects – a comparison to hydrogen

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Phosphorus contains just one isotope (31P: natural abundance 100%)

γ 31P : γ 1H => 10.841 : 26.7519 = 40.480737 [MHz] (if 1H = 100 MHz)

Relative receptivity of 31P: 0.0665 => less sensitive than 1H (1.00) !

Standard for 31P NMR measurements: H3PO4 (concentration 85%)

Resonances are given in δ [ppm] relative to the standard, „chemical shift=> sign convention: + (= resonance at lower field; „lowfield-shifted“)

Chemical shift (δ) is determined by • electronegativity (χ) - π-electron overlap (nπ)• bond angle (θ)differences occur upon change in the bonding environment(J. R. van Wazer):

2. Phosphanes (phosphines)

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Structural features (I): RmPHm-n(σ3λ3-P, PIII, pyramidal, between p3 and sp3 hybridisation)

Strong downfield-shift upon increasing degree of C substitution at P

2. Phosphanes (phosphines)

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Steric effects !?

2. Phosphanes (phosphines)

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Examples of phosphanes and their chloro derivatives of RmPClm-n(σ3λ3-P, PIII, pyramidal)

Strong downfield-shift upon increasing degree of Cl substitution at P

Introducing an increment system based on:1) Electronegativity of directly bound atoms2) Electronegativity of atoms bonded to the α, β, etc. atom

Electronegative elements such as chlorine lead to a deshielded P center

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2. Estimating chemical shifts: increments systems

2. Phosphanes and chloro derivatives

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Examples of phosphanes and their chloro derivatives of RmPClm-n(σ3λ3-P, PIII, pyramidal)

∆δ values: 1) PMe2Cl vs PMeCl2:: 98.82) P(t-Bu2)Cl vs P(t-Bu)Cl2: 53.3 (!?)

Attention:In addition, the increment system has to take steric and electronic effects into account !

3. Phosphane oxides vs. phosphate esters

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Examples of phosphane oxides and their alkoxy derivatives RmP(O)ORm-n

(σ4λ5-P, PV, tetrahedral; ~ sp3 hybridisation with some d-orbital contribution)

Upfield-shift upon increasing degree of OR/OAr substitution at P

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Examples of phosphane oxides and their SR and NR derivatives RmP(E) (σ4λ5-P, PV, tetrahedral; ~ sp3 hybridisation with some d-orbital contrib

Note: OR and NR derivatives resonate upfield from respective SR deriv.=> SR show weaker π-backbonding

3. Phosphane oxides vs. sulfides and imines

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1) σ4λ5-P, PV, tetrahedral; 2) σ5λ5-P, PV, trigonal bipyramidal; 3) σ6λ5-P, PV, octahedral; ~ sp3 hybridisation with some d-orbital contribution

4. Phosphane oxides vs. phosphoranes vs. phosphates

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Why is their a shielding or deshielding effect in complexes?

5. MLn complexes of phosphanes and derivatives (I)

Why are Cr complexes more deshielding than Mo or W complexes ?

Coordination chemical shift values (∆δ): Cr > Mo > W (~ 20-30 ppm)

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5. MLn complexes of phosphanes and derivatives (II)

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5. BH3 complexes of phosphanes and derivatives

Borane complexation has a deshielding effect – unless β-atoms can deliever electron density !

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6. Phosphorus compounds in low coordination

Phosphaalkynes,

σ1λ3-P, PIII, linear

sp hybridisation

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6. Phosphorus compounds in low coordination

Diphosphenes, σ2λ3-P, PIII, bent, ~ sp2 hybridisation

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6. Phosphorus compounds in low coordination

Phosphaalkenes, σ2λ3-P, PIII, bent, ~ sp2 hybridisation

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6. Low-coordinated phosphorus compounds

In phosphenium compounds substituents have huge effects on thechemical shift !?

Phosphenium salts, σ2λ2-P, PIII, planar, ~ sp2 hybridisation

Phosphenium compounds are charged species and therefore the chemical shift is strongly dependend of c, T and solvent polarity !

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7. Coupling of P to main group elements (H,C,F): 1H

General aspects of couplings of P to H => nJ(P,H):

1J(P,H) is positiv; => the sign is then alternating (f(n))

The magnitude of nJ(P,H): 1J(P,H) > 3J(P,H) > 2J(P,H) > 4J(P,H)

1J(P,H) coupling constants: Na[PH2] < CH2CH2PH < PH3 < PH4

+< PH2F3

139.0 < 158.3 < 188.2 < 547 < 841 Hz

The magnitude of 2J(P,H) in cyclic systems:depends on the angle ϕ

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7. Coupling of P to main group elements (H,C,F): 13C

General aspects of couplings of P to C => nJ(P,C)

1J(P,C) is negativ => the sign is depending on and increases withincreasing electronegativity, coordination number and bond angle sum

Within a series of alkyl phosphanes the magnitude of nJ(P,C)1J(P,C) > 3J(P,C) > 2J(P,C) >

4J(P,C)

The magnitude of 2J(P,C) in cyclic systems:depends also on the angle ϕ(Karplus curve)

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7. Coupling of P to main group elements (H,C,F): 19F

General aspects of couplings of P to F => nJ(P,F)

1J(P,F) is negativ => the sign is depending on and increases with increasing electronegativity and coordination number

Within a class of phosphorus compounds the magnitude of nJ(P,F):1J(P,E) > 3J(P,E) > 2J(P,E) > 4J(P,E)

Selected examples of 1J(P,F) coupling constants: PF3 < P(O)F3 < PF5 < PF6

- < H2P(NH2)F2

-1441 < -1080 < -938 < -706 < -598 Hz

The magnitude of 1J(P,F) depends very much on the s-character of the bond: sp > sp2 > sp3 > dsp2 > dsp3 > d2sp3

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8. Coupling of phosphorus to transition metals (W)

In the case of W(CO)5PR3 complexes:Strong correlation of the 1J(W,P) coupling constants from the Σ EN values (Pauling) of directly to P bonded atoms !

1J(W,P) coupling constant magnitudes [Hz]

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1J(W,P) coupling constant magnitudes [Hz]

8. Coupling of phosphorus to transition metals (W)

9. Chemical shift range for phosphorus compounds

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6. Low-coordinated phosphorus compounds