A THEORETICAL ASSESSMENT OF NEW LIGANDS IN THE … · Model systems: (Peters et al.)...

Model systems: (Peters et al.)

Tris(phosphino)silyl ligands, (SiP3)a,b

• Complexed to iron, binds dinitrogen in three formal oxidation states

• Reduction-driven exchange of NH3 for N2 ligands observed for iron

Scorpionate-type phosphine ligand, (PhBP3)c

• Dinitrogen bound, bridged between two Fe[PhBP3] fragments

• Dinitrogen cleavage observed

The SiP3 ligand The PhBP3 ligand

aWhited, M. T.; Mankad, N. P.; Lee, Y. H.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48,2507.

bLee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558.cBetley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252.

Introduction

Catalytic dinitrogen reduction

• In nature, nitrogenase enzymes produce ammonia from dinitrogen

• The current industrial method, Haber-Bosch, requires elevated tem-peratures and pressures

• A catalyst which can produce ammonia at standard temperature andpressure is highly sought after

• Current lead system: Schrock catalyst, Mo[HIPTN3N],a producing65% yield of ammonia in 6 turnovers

⇒ Catalyst easily protonated underreaction conditions, which leadsto loss of catalytic activity

Mo(N2)

Mo N N

Mo N N H

Mo N NH3

Mo N NH2

Mo N NH3

Mo NMo NH

Mo NH

Mo NH2

Mo NH2

Mo NH3

Mo NH3

e-, -NH3e-

e-

e-

e-

e-

H+

H+

H+

H+

H+

H+

N2

-NH3

H+

[LH+]Mo(N2)

e-, H+1

2a

3

4

5

6

78

9

10

11

12

13

2b

Mo

N

N

NNHIPT

HIPT

iPr

iPr

iPr

iPr

iPr

iPr

(HIPT)

Mo =Searching for the ideal catalyst

• Theoretical investigations providedgreat insight into the Schrock-cycleb, c

• Key steps:(a) The first protonation/reduction step and(b) The ammonia-dinitrogen exchange step

⇒ New systems must be active, but stable in acidic and reducing con-ditions

aYandulov, D. V.; Schrock, R. R. Science 2003, 301, 76.bSchenk, S.; Kirchner, B.; Reiher, M. Chem. Eur. J. 2009, 15, 5073.cSchenk, S.; Le Guennic, B.; Kirchner, B.; Reiher, M. Inorg. Chem. 2008, 47, 3634.

The Schrock SystemFirst protonation/reduction step

[Mo] = Mo(N(C2H4N-HIPT)3)

[Mo]-N2 [Mo]-NNH+

[Mo]-N2- [Mo]-NNH

-185.2(+304.3)

+e- +e--559.0(-69.5)

-989.3 (+11.8)

-1363.2 (-362.1)

+H+

+H+

[Mo]-N

N Ph

H

N

[Mo]-N

N Ph

H

N

[Mo]-N

N Ph

H

NH

+e- -457.2 (+32.3)

-1094.6(-93.5)

+H+

-1037.8 (-36.7)+H+

+1041.2(+40.1)

-H+

(-57.7)

• Initial protonation is exothermic, but ligand protonation preferred

• Ligand protonation does not hinder subsequent dinitrogen proto-nation

• Initial reduction of the N2 species ruled out, but reduction of theNNH+ species is facile

• Overall step is exothermic

Exchange of NH3 by N2

[Mo] = Mo(N(C2H4N-HIPT)3)

[Mo]-NH3-

[Mo]-NH3

[Mo]-NH3+

[Mo]-N2-

[Mo]-N2

[Mo]-N2+

[Mo]

[Mo]+

[Mo]-

CATIONIC

NEUTRAL

ANIONIC

-NH3

-NH3

-NH3

+N2

+N2

+N2

-e-

+e-+e-

+e-

+82.7

+116.1

+120.6

-79.9 +185.2

-543.0-466.0

-84.2

-156.7

-228.5

+N2

+N2 -NH3

-NH3

-15.7

-26.9 -16.4

-27.6

[Mo](NH3)(N2)

[Mo](N2)(NH3)

• Reduction of the NH3+ species is facile

• Neutral exchange clearly preferred to cationic exchange

Aims and Computational DetailsObjectives

• Determine the suitability of the model ligands within a Schrock-cycle

• Compute energetics of the two key reaction steps for each ligand

• Describe the kinetic barrier to the exchange process

Structure optimization with TURBOMOLE

• BP86 exchange–correlation functional, with RI approximation

• Metal, N, Si, P atoms with def2-TZVP basis set, and def2-SVP basisset on C, H

Scans and transition state optimizations with GAUSSIAN 09

• BP86 exchange–correlation functional with the VWN5 local-exchange part (BVP86 keyword)

• def2-TZVP and def2-SVP basis sets taken from EMSL database,a andapplied as for TURBOMOLE calculations

• Auxiiary basis sets applied from the TZVPFit and SVP libraries withinGAUSSIAN 09

Energies are intrinsic and reported in kJ mol−1, numbers in brackets de-note reaction energies obtained relative to Cp∗

2Cr or lutidinuim

ahttps://bse.pnl.gov/bse/portal

The Fe[SiP3] SystemFirst protonation/reduction step

[Fe] = Fe(Si(C6H4PiPr2)3)

[Fe]-N2 [Fe]-NNH+

[Fe]-N2- [Fe]-NNH

-138.5(+351.0)

+e- +e--476.7(+12.8)

-980.3 (+20.8)

-1318.5 (-317.4)

+H+

+H+

[Fe]

N

N

[Fe]

N

N

[Fe]

N

NH

+e- -511.1 (-21.6)

-966.5(+34.6)

+H+

-1071.1 (-70.0)+H+

+1091.7(+90.6)

-H+

H

H H

(+33.6)

• Metal protonation is strongly exothermic, but dinitrogen protona-tion is endothermic

• Subsequent steps to the product are endothermic

• Overall reaction endothermic, suggests poor N2 basicity whenbound to iron

Exchange of NH3 by N2

CATIONIC

NEUTRAL

ANIONIC

[Fe] = Fe(Si(C6H4PiPr2)3)

[Fe]-NH3-

[Fe]-NH3

[Fe]-NH3+

[Fe]-N2-

[Fe]-N2

[Fe]-N2+

[Fe]

[Fe]+

[Fe]-

-NH3

-NH3

-NH3

+N2

+N2

+N2

-e-

+e-+e-

+e-

+38.8

+60.1

+88.7

-82.0 +138.5

-476.2-394.9

-80.6

-133.3

-168.5

+N2

+N2 -NH3

-NH3

-5.2

-27.8 -45.4

-68.0

[Fe]*(NH3)(N2)

[Fe]*(N2)(NH3)

+N2

+N2 -NH3

-NH3

+77.4

+29.2 -21.1

-69.3

[Fe]+*(NH3)(N2)

[Fe]+*(N2)(NH3)

• The NH3+ intermediate is not easily reduced, exchange should oc-

cur via the cationic pathway

• A Phosphine arm of the ligand must dissociate before allowing dini-trogen to bind

⇒ The Fe[SiP3] system does not perform well in either key step

The Mo[SiP3] SystemFirst protonation/reduction step

[Mo] = Mo(Si(C6H4PiPr2)3)

[Mo]-N2 [Mo]-NNH+

[Mo]-N2- [Mo]-NNH

-135.5(+354.0)

+e- +e--476.7(+12.8)

-1036.9 (-35.8)

-1378.1 (-377.0)

+H+

+H+

[Mo]

N

N

[Mo]

N

N

[Mo]

N

NH

+e- -482.7 (+6.1)

-1005.3(-4.2)

+H+

-1114.4 (-113.3)+H+

+1088.1(+87.0)

-H+

H

H H

(-23.0)

• Initial protonation exothermic, but metal site greatly preferred tonitrogen protonation

• Even with metal protonated, subsequent dinitrogen protonation isfeasible

• NNH+ reduction is less favourable than in the Schrock system

• Overall reaction exothermic, but not as good as the Schrock system

Exchange of NH3 by N2

[Mo] = Mo(Si(C6H4PiPr2)3)

CATIONIC

NEUTRAL

ANIONIC

[Mo]-NH3-

[Mo]-NH3

[Mo]-NH3+

[Mo]-N2-

[Mo]-N2

[Mo]-N2+

[Mo]

[Mo]+

[Mo]-

-NH3

-NH3

-NH3

+N2

+N2

+N2

-e-

+e-+e-

+e-

+44.4

+60.3

+78.4

-79.8 +135.5

-472.1-406.1

-66.3

-114.2

-154.0

+N2

+N2 -NH3

-NH3

-82.2

-54.0 +0.1

+29.0

[Mo](NH3)(N2)

[Mo](N2)(NH3)

+N2-NH3

-67.8 +79.8

[Mo]+(NH3)(N2)

• The NH3+ species is not as easily reduced as the Schrock system

• Reductant strength will determine if exchange occurs via a neutralor cationic pathway

The Mo[PhBP3] SystemExchange of NH3 by N2

[Mo] = Mo(PhB(CH2PiPr2)3)

[Mo]-NH3-

[Mo]-NH3

[Mo]-NH3+

[Mo]-N2-

[Mo]-N2

[Mo]-N2+

[Mo]

[Mo]+

[Mo]-

CATIONIC

NEUTRAL

ANIONIC

-NH3

-NH3

-NH3

+N2

+N2

+N2

-e-

+e-+e-

+e-

+66.0

+73.6

+181.1

-100.6 +173.4

-487.7-439.1

-174.6

-115.7

-180.7

+N2 -NH3

-135.6 +93.6

[Mo](NH3)(N2)

• Reduction of the NH3+ intermediate is facile

• Exchange should occur through the neutral pathway

⇒ The scorpionate ligand performs well in both key steps

The Mo[PhBP3] SystemFirst protonation/reduction step

[Mo] = Mo(PhB(CH2PiPr2)3)

[Mo]-N2 [Mo]-NNH+

[Mo]-N2- [Mo]-NNH

-173.4(+316.1)

+e- +e--513.4(-23.9)

-1032.1 (-31.0)

-1372.1 (-371.0)

+H+

+H+

[Mo]

N

N

[Mo]

N

N

[Mo]

N

NH

+e- -547.2 (-57.7)

-990.1(+11.0)

+H+

-1041.9 (-40.9)+H+

+1033.8(+32.7)

-H+

H

H H

(-54.9)

• Initial protonation is exothermic

• Terminal-nitrogen protonation is competitive with the metal proto-nation pathway

• The NNH+ species is easily reduced

• Overall reaction as exothermic as the Schrock system

AcknowledgmentsThis work has been supported by the Swiss National Science FoundationSNF (project 200020-132542).

Barriers to NH3/N2 exchange in Mo[SiP3]

Energies relative to [Mo]-NH3 + free N2

Observations

• Barrier of 30.3 kJ mol−1 to add N2 is lower than the cost of dissociat-ing NH3 (at least 60 kJ mol−1)

• Subsequent loss of NH3 is facile

⇒ The process proceeds via an associative mechanism

0.0

+30.3

-82.2

-35.2

-60.5

+N2

-NH3

Mo-NH3 = 2.34 ÅMo-N2 = 3.12 Å

Mo-NH3 = 2.38 ÅMo-N2 = 1.97 Å

Mo-NH3 = 2.83 ÅMo-N2 = 1.97 Å

Mo-N2 = 2.03 Å

Mo-NH3 = 2.37 Å

A THEORETICAL ASSESSMENT OF NEW LIGANDS IN

THE CATALYTIC REDUCTION OF DINITROGEN

STEVEN M. A. DONALD, MARKUS REIHER

Laboratorium fur Physikalische Chemie, ETH Zurich, Wolfgang-Pauli-Strasse 10,8093 Zurich, Switzerland

{steven.donald, markus.reiher}@phys.chem.ethz.ch