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Computational and Experimental Structural Studies of Selected Chromium(0) Monocarbene Complexes

Marilé LandmanUniversity of Pretoria

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Contents

1. Conformational analysis of heteroarene carbene complexes

2. Comparison of experimental and theoretical data

3. Electronic and steric factors4. Electrochemistry5. Redox behaviour: Theoretical investigation

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Syn vs Anti conformation

X

(OC)5Cr

Y

X

(OC)5Cr

Y

Syn Anti

X = OEt, NH2Y = O, S, NMe

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List of complexes

X

(OC)5Cr

Z Complex X Substituent Z Substituent

1 OEt 2-Thienyl

2 OEt 2-Furyl

3 OEt 2-(N-Methyl)pyrrolyl

1* NH2 2-Thienyl

2* NH2 2-Furyl

3* NH2 2-(N-Methyl)pyrrolyl

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Scan • Density Functional Theory calculations, using the GAUSSIAN09 • Dihedral scan of X-C-C-Y• Singlet spin state using the hybrid functional B3LYP;

Stuttgart/Dresden (SDD) pseudo potential used to describe Cr electronic core while the valence electrons were described with the Karlsruhe split-valence basis set with polarization functions (def-SV(P))

• Scan performed in steps of 36°

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Scan profiles of 1-3*

1 2 3

1* 2* 3*

0

5

10

15

20

25

30

35

40

45

50

0 90 180 270 360

Ener

gy k

J/m

ol

Dihedral angle (degrees)

0

5

10

15

20

25

30

35

40

45

50

0 90 180 270 360

Ener

gy k

J/m

ol

Dihedral angle (degrees)0

5

10

15

20

25

30

35

40

45

50

0 90 180 270 360

Ener

gy k

J/m

ol

Dihedral angle (degrees)

0

5

10

15

20

25

30

35

40

45

50

0 90 180 270 360

Ener

gy k

J/m

ol

Dihedral angle (degrees)

0

5

10

15

20

25

30

35

40

45

50

0 90 180 270 360

Ener

gy k

J/m

ol

Dihedral angle (degrees)

0

5

10

15

20

25

30

35

40

45

50

0 90 180 270 360

Ener

gy k

J/m

olDihedral angle (degrees)

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Optimization

• No symmetry constraints applied for 0° and 180°; only default convergence criteria were used during the geometric optimizations

• Vibrational frequencies were calculated at the optimized geometries and no imaginary frequencies were observed for the Emin conformers

• TS 90° calculation froze dihedral angle at 90°• Donor−acceptor interactions have been computed

using the natural bond orbital (NBO) method

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Results after optimizationComplex Dihedral Angle

X-C-C-Y (°)

Conformation Energy (kJ/mol) Boltzmann

Distribution

1 0.0

177.3

90.0

Syn

Anti

TS

0.0

10.6

43.2

98.7%

1.3%

2 0.0

180.0

90.0

Syn

Anti

TSa

7.6

0.0

50.3

4.2%

95.8%

3 4.2

153.1

90.0

Syn

Anti

TS

0.0

18.5

34.1

99.9%

0.1%

1* 23.5

148.9

90.0

Syn

Anti

TS

0.0

8.5

16.6

97.0%

3.0%

2* 0.0

170.7

90.0

Syn

Anti

TS

0.0

15.6

45.9

99.8%

0.2%

3* 26.9

144.0

90.0

Syn

Anti

TS

0.0

1.2

18.7

62.1%

37.9%

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Comparing experimental and theoretical data

Complex Dihedral Angle X-C-C-Y (°) Designation

Calculated Crystal

1 0.0 0.0 Syn

2 180.0 180.0 Anti

3 4.2 0.0 Syn

1* 23.5 21.3 Syn

2* 0.0 6.0

-2.0*

Syn

3* 26.9 29.3 Syn

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Crystal structures of 1-3*

1 2 3

1* 2* 3*

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Structural comparisonBond length (Å) 1c 2c 3c 1* 2* 3* Cr-C6carbene 2.063(4) 2.054(5) 2.107(3) 2.096(1) 2.092(1)

2.089(1) 2.103(2)

Cr1-C(carbonyla 1.903(3) 1.897(10) 1.900(3) 1.902(20 1.896(2)

1.899(2) 1.897(3)

Cr1-C1b 1.873(4) 1.891(6) 1.874(3) 1.874(1) 1.876(2) 1.878(1)

1.871(2)

C6-O6/N6 1.337(5) 1.445(17) 1.331(3) 1.318(2) 1.315(2) 1.316(2)

1.312(3)

C6-C7 1.451(6) 1.344 (20) 1.432(4) 1.466(2) 1.452(2) 1.450(2)

1.449(3)

Bond angle (°) C1-Cr1-C6 177.1(2) 177.1(9) 177.1(1) 175.5(1) 177.9(1)

176.1(1) 176.6(1)

Cr1-C6-O6/N6 129.6(3) 126.8 (11) 127.6(2) 121.1(1) 123.4(1) 122.0(1)

120.4(2)

Cr1-C6-C7 124.9(3) 129.4 (11) 124.7(2) 125.6(1) 124.2(1) 125.4(1)

124.9(1)

O6/N6-C6-C7 105.4(3) 103.8(4) 107.7(2) 113.1(2) 112.4(1) 112.6(1)

114.6(2)

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NBO: Donor-acceptor interaction in 2

E = -4.9 kJ/molHighest rotation barrier around the C(carbene)-C(aryl) bond for 2,with a value of 50.3 kJ/mol

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Steric interaction in 2• Delocalization of the lone pairs of electrons on the heteroatom of

thiophene and furan, forms part of the aromatic system• Oxygen more electronegative

heteroatom, furan shows less delocalization of electrons compared to thiophene

• O6…O7 distance is 2.488 Å in 2 (syn) Mulliken charges on these atoms are -0.410 and -0.395, respectively.

• O6…S1 distance in thienyl complex 1 (syn) is 2.724 ÅMulliken charges -0.425 and +0.293.

O

[M]

O

2 (syn)

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Electrochemistry study

CrOC

OCCO

CO

CO

H3CH2CO

R

R = S 1 = O 2 = NCH3 3

CrOC

OCCO

CO

CO

H3CH2CO

S

R'

R' = NCH3 4 = O 5

• Redox behaviour of monomeric heteroarene carbene complexes

• Extend heteroarene substituent to dimeric heteroarene

• DFT study to understand redox behaviour

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Crystal structures of 4 and 5

4 5

Bond length (Å) 4 5

Cr-C6carbene 2.084(2) 2.066(3)

Cr1-C(carbonyl)a 1.902(2) 1.902(3)

Cr1-C1b 1.884(2) 1.885(3)

C6-O6 1.333(2) 1.332(3)

C6-C7 1.442(2) 1.442 (4)

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Electrochemistry• The Cr ethoxycarbene complexes of this study represent

molecules with two redox active centres: the Cr metal and the carbene as “non-innocent” ligand

• Three main redox processes observed:• one reduction process: reduction of the carbene carbon

atom • two oxidation processes: the oxidation of the Cr(0) metal

centre to Cr(I) and the oxidation of electrochemically generated Cr(I) species to either Cr(II) or (CO)5Cr(I)=C(OEt)R(+).

• Comparing the LSV of the processes observed with that of ferrocene, it is concluded that each redox process represents a one electron process only

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Electrochemistry

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Reduction process• Reduction of a complex involves the addition of an electron to

the LUMO of the complex. • The character of the LUMO of a complex should indicate where

the reduction process will occur; the SOMO of the reduced complex will show where the first reduction took place.

• Visualization of the (a) LUMOs of the neutral 1-5, (b) the SOMOs of the reduced (charge q = -1) 1-5 and (c) the spin density of the reduced radical anions of 1-5 provide the same information: the reduction involves the electrophilic carbene carbon and the added electron density is delocalized over the heteroarene five-membered rings.

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Molecular orbitals of reduction process

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Oxidation processes• Oxidation of a complex involves the removal of an electron from the

HOMO of the complex. The character of the HOMO of the neutral complex will thus show where the oxidation will take place

• First oxidation process: Cr(0)-Cr(I) oxidation

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Oxidation processes• Second oxidation process: Cr(I)-Cr(II) or Cr(I)-

(CO)5Cr(I)=C(OEt)R(+) oxidation?

• Removal of an electron from the HOMO of the oxidized radical cation of 1-5• 1-3: Second oxidation involves the removal

of a dyz electron from the Cr(I)-metal centre • 4-5: Involves the removal of an

electron from dimeric heteroarene; leads to Cr(I)-(CO)5Cr(I)=C(OEt)R(+) radical species

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Conclusion• The R group in [(CO)5Cr=C(OEt)R] plays a significant role in the

energy, shape and distribution of the LUMO orbital, in other words, to the extent of electron delocalization, while the HOMO is Cr-based. Consequently the reduction of [(CO)5Cr=C(OEt)R] is sensitive to the electrophilic nature of the R substituent

• The anodic peak potential of the first oxidation process of 1-5 is Cr-based and is only sensitive to the electrophilic character of the heteroarene ring directly attached to the carbene carbon.

• Second oxidation process different for monomeric and dimeric heteroarene complexes

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Acknowledgements

• Students– Roan Fraser Tamzyn Levell– Stephen Thompson Wynand Louw– René Pretorius

• Prof J Conradie, R Lui, UFS• Prof PH van Rooyen, UP• NRF• University of Pretoria