Embed Size (px)
Citation preview
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
