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BarunBarun Jana, Steven R. Neal, Aaron Jana, Steven R. Neal, Aaron SadowSadow, , Oleg Oleg PestovskyPestovsky, and , and Andreja Andreja BakacBakac

ACTIVATION OF MOLECULAR NITROGEN ACTIVATION OF MOLECULAR NITROGEN WITH TRANSITION METAL COMPLEXESWITH TRANSITION METAL COMPLEXES

Ammonia ConferenceKansas City, October 12-13, 2009

Iowa State University, Ames, IA

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•Catalytic steam reforming CH4 + H2O → CO + 3H2

•Water gas shift reactionCO + H2O → CO2 + H2

•Catalytic NH3 formationN2 + 3H2 → 2 NH3 ΔHf

0 = - 46.1 kJ/mol300-5500C, up to 250 atmFe or Ru oxide cat

HABER-BOSCH PROCESS

Thermodynamics are favorable, kinetic barriers

•High temperatures lower equilibrium concentrations and yields of ammonia.•125 bar of H2/N2 (3:1 molar ratio),125 °C - yield of ammonia is 91%. •500 °C, yield is 12%. Must increase pressure. •NEED CATALYTIC REACTION TO LOWER KINETIC BARRIER

N N ΔH0 = 944 kJ/mol

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BIOLOGICAL NITROGEN FIXATION

NITROGENASE ENZYMES (Fe, Mo, V)

N2 from air NH3 + H2

Plants:

Enzyme (= catalyst)+ protons + electrons

M. D. Fryzuk, Chem. Rev. 2004, 104, 385;

4M. D. Fryzuk, Chem. Rev. 2004, 104, 385; Acc. Chem. Res. 2009, 42, 127

BINDING IN METAL-N2 COMPLEXES

Function of:MetalOxidation stateLigands

Free N2: PhN NPh NH2 NH2

N-N 1.0975 Å 1.255 1.460νNN 2331 cm-1 1442 1111

(Ta)

M N N M N N

M N N M M N N M

N

NMM

N

NMM

N

NMM

N

NMM

M

NN

M M

NN

M

Weak Activation Strong Activation

End-onMonometallic

End-onBimetallic

Side-onBimetallic

Side-on End-onBimetallic

(unknown)

(unknown)

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Hanna, Lobkovsky, Chirik Organometallis 2009, 28, 4079

EFFECT OF SUBSTITUENTS ON BINDING MODE OF N2

N

R

Ti N2

R

R

TiN2

R

R

N2

Ti N

R

R

N Ti

R

R = SiMe2Ph R = i-Pr R = H , Me

R

Ti N

R

R

N Ti

R

R = Me

N

N

N

Ti

N

N

Ti

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Mo(NRAr)3

CLEAVAGE OF N2 AT Mo(III)

Laplaza, Cummins Science 1995, 268, 861

MoArRN

NRAr

NRAr

Mo

ArRNNRAr

NRAr

N

N

N2

1 atm, -35 ºC

Mo(NRAr)3

Mo

ArRNNRAr

NRAr

N

N

Mo

ArRNNRAr

NRAr

30 ºC

t1/2 = 35 minMo

ArRNNRAr

NRAr

N

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Yield: 55%(24 hours)

Nishibayashi et al, Science 1998, 279, 540

BIMETALLIC SYSTEM FOR N2 ACTIVATION

W

N

PN

PP

P

N

N

P = PMe2Ph

+ 6 [RuCl(!2-H2)(dppp)2]X

6 [RuCl(dppp)2]X

H2

(1 atm)

X = PF6 , BF4 , OTf , BPh4

55 ºC2 NH3 + 6 [RuHCl(dppp)2]

+ W(VI) (?)

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CATALYTIC REDUCTION OF N2 AT SINGLE Mo CENTER

Yandulov, Schrock Science 2003, 301, 76

4 Catalytic cycles

1, 2, 3, 4, 7, 8,12,13 prepared & structurally characterized

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HYDROGENATION AND CLEAVAGE OF N2

Pool, Lobkovsky, Chirik Nature 2004, 427, 527

Zr

N

N

Zr Zr

N

NZr

H2

25 °C

H

H

H

H

ZrH

H

H2

85 °C

(60 %)

+ NH3

(10-15 %)

Zr

N

NH2

Zr

85 °C- H2

10Curley, Cook, Reece, Muller, Cummins, J. Am. Chem. Soc. 2008, 130, 9394

PHOTOCHEMICAL CLEAVAGE OF NN BOND

Mo N N MoAr[t-Bu]N

Ar[t-Bu]NAr[t-Bu]N

N[t-Bu]Ar

N[t-Bu]ArN[t-Bu]Ar

Mo

Ar[t-Bu]NN[t-Bu]Ar

N[t-Bu]Ar

N

MoAr[t-Bu]N

N[t-Bu]Ar

N[t-Bu]Ar

+ 22

h!

-78 °C

N-N bond cleavage N2 extrusion- N2

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REDUCTION OF N2 TO NH3 AT IRON

Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem. Soc. 2005, 127, 10184.

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Tyler , Leigh, et al

P

P

1,2-bis(bis-(methoxypropyl) -phosphino)ethane)

=

Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem. Soc. 2005, 127, 10184.Leigh, G. J.; Jimenez-Tenorio, M. J. Am. Chem. Soc. 1991, 113, 5862.

Orange

Pale yellow

PFeII

P

P

P

Cl

Cl

PFeII

P

P

PH2

H

PFeII

P

P

P

N2

H

PFe0

P

P

PN2

H2/B-

N2/H+

B-

H+

NH4+/N2H5

+H2O

THF/

Et2O

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•Starting (P2)2FeIICl2 not stable in water

(hydrolyzes in 24 hours, H+-catalyzed)

•Protonation of (P2)2Fe0N2 causes hydrolytic destruction of complex

(free protonated ligand observed)

•Every step carried out independently (protonation/deprotonation)

•Need a "single pot" catalytic process

PROBLEMS

14Patrick L. Holland, . Am. Chem. Soc. 2009, 131, 9471

ACTIVATION OF N2 WITH COBALT COMPLEXES

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Fe(I)-NN-Fe(I) BY NITRIDE COUPLING

Betley & Peters, J. Am. Chem. Soc 2004, 126, 6252

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Funding: Iowa Energy Center

Oleg PestovskyBarun Jana

Aaron Sadow

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