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Dinitrogen Scission by a Molybdenum(III) Xylidene Complex

Dr. Mircea D. Gheorghiu

1. Introduction. 2. About Dinitrogen. 3. Nitrogen Fixation. Haber-Bosch ammonia synthesis.

Nitrogenase.4. Dinitrogen binding and scission. Mechanism.5. Why dinitrogen is activated by Mo(III) Complexes?6. Starting materials.7. Synthesis of the molydenum precursors and the ligand.8. NMR of paramagnetic compounds.9. Concluding remarks and future developments.

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The main reference:

� Dinitrogen Cleavage by Three-Coordinate Molybdenum(llI) Complexes: Mechanistic and Structural Data

� Catalina E. Laplaza, Marc J. A. Johnson, Jonas C. Peters, Aaron L. Odom, Esther Kim, Christopher C. Cummins, Graham N. George, and Ingrid J. Pickering;

J. Am. Chem. Soc. 1996,118, 8623-863

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An excerpt from the Kit’s paper references section:

� (1) Dedicated to Professor Alan Davison of the MIT Department of Chemistry on the occasion of his 60th birthday.

� (2) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999. Laplaza, C. E.; Davis, W. M.; Cummins, C. C. Angew. Chem., Int. Ed. Engl. 1995,34, 2042.

� (3) Laplaza, C. E.; Cummins, C. C. Science1995, 268, 861. See also: Laplaza, C. E.; Johnson, A. R.; Cummins, C. C. J. Am. Chem. Soc. 1996, 118, 709.

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(April 1998)

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2. About dinitrogen:On Earth 4/5 of the volume of atmosphere; Titan

(moon of Saturn) 82-94%. Bp -195.8 ºC (O2 -183 ºC).

� The triple bond is very strong:� At 3000 °C do not decompose measurable.� Bond energy is 225 kcal/mole.� Combines with other elements only with

difficulty. At high temperature combines directly with magnesium, titanium, and aluminum, forming nitrides.

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3. Nitrogen fixation (to nif, niffing):

1. Haber-Bosch (1909, 1913)(Fe catalyst; 450-650 °C; 200-600 atm):

N2 + 3H2�2NH3

� Explosives and nitrogen fertilizers.� Half of the total nitrogen input to

agriculture (the rest comes by biological nitrogen fixation).

� About 1% of the world’s total annual energy supply.

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� Nitrogen is not accessible to plants� Nitrogen-fixing bacteria (symbiotic

association with legume plants) enzyme (nitrogenase) converts N2 into metabolically ammonia.

� Nitrogenase has two component metalloproteins: the Fe-protein (source of electrons) and the MoFe-protein (interprotein electron transfer and catalyst for binding and reduction).

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4. Nitrogen binding and scission:

MoL

L

L

MoL

LL

N

N

MoL

LL

Nt-Bu

CH3 CH3

MoL

L

L

N-35 oC 25 oC

L is:

1042 cm-1

N

N

+

Purple

Amber

etherorange-red

Small substituents allow for metal-metal bonding (NMe2;

OBut):

MoL

L

L

Mo MoL

L

L

L

L

L2

student yield: 75 % (76%)

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Structure:

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Mechanism:

MoL

L

L

end-on binding of N2

MoL

LL

N

N

MoL

LL

N

N

MoL

LL

MoL

LL

N

-35 oC

25 oCMoL3

t1/2 = 35 min

no barrier of activation(theoretical methods) barrier of activation:

- computed ca. 18 kcal/mol- experimental 23.3 kcal/mol

Theoretical works (L=NH2):Cui, Q; Musaev, D. G.; Svensson, M.; Sieber, S.; Morokuma, K. J. Am. Chem. Soc. 1995, 117, 12366.Neyman, K. M.; Nasluzov, V.A.; Hahn, J.; Landis, C. R.; Rosch, N. Organometallics, 1997, 16, 995.

Experimental work:Laplaza, C. E.; Cummins, C. C. Science, 1995, 268, 861.Lapalza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996, 118, 8623.

4A'

1A3Bg

4A"2A''

1A'

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5. Why N2 is activated by Mo(III) complexes?

� L3Mo is isolobal to nitrogen atom (N) and carbyde (CH), with all three d electrons unpaired (high spin).

� The frontier orbitals 3a12e2 are derived largely from dz2 (3a) and dxz and dyz, respectively (2e). (see the pictures on right dxz, dyz, dz2).

� The NR1R2 substituent is a strong � donor into d� orbitals (e MOs) of Mo, increasing the electron density at the metal.

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Isolobal analogy (R. Hoffmann, Angew Chem. Int. Ed. Engl. 1982, 21, 711) is relating structures of organometallic compounds to organic ones. Two fragments are isolobal “if the number, symmetry properties, approximate energy, an shape of the frontier orbitals and the number of electrons in them are similar.”

� Mo(NRAr)3 is isolobal with CH

Mo CH

Mo Mo Mo CH CH

CH

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� There are � and � interactions involved in N2-MoL3. N2 provides the � electrons to empty �orbitals of Mo (s or d). The � component is back-donation from occupied d� orbitals to the empty �* of N2. As result Mo-N becomes stronger, the N-N bond weaker and the reaction L3MoN2MoL3 � 2NMoL3 more exothermic.

Dissociation energy: Mo�N triple bond = 155 kcal/mol, N2 is 225.7 kcal/mol:

� 2R3Mo + N2 � 2R3Mo�N is exothermic

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6. Starting materials for MoL3

� 1. MoCl5� 2. Ligand from (CH3CO)2 O and

diacetone alcohol:

O

OH

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7. A. Current version for the synthesis of MoCl3(THF)3

� Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001, 2699.� Modified by Stephens, F. , August, 2002.

MoCl5cold Et2O

MoCl4(OEt2)2THF

Sn, 30 minMoCl3(THF)3

Sn, 30 min

black-brown solidbrown solution

orange suspensionin diethyl ether

bright orange powder

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Oxidation states of Mo� Mo: Z=42; Ground state: Kr5s14d5 (7S)

� MoCl5 (Mo2Cl10): Mo(V) one unpaired electron� (readily abstracts oxygen from oxygenated solvents)

� MoCl4(Et2O)2: Mo(IV) two unpaired electrons

� mer-MoCl3(THF)3: Mo(III) three unpaired electrons� no fac- MoCl3(THF)3

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7. B. Synthesis of the m-xylidineligand:

O

BF4

O

OHN

t-Bu Li

Nt-Bu H

ca. 4-6 equiv. t-BuNH2

CH3CN/RT

(CH3CO)2O

HBF4

ca 90oC

BuLi inthawing pentane

student's yield of pyrilium salt: 66% [lit. 72%]Vernaudon, P; Rajoharison H. G.; Roussel, C. Bull. Soc. Chim. Fr. 1987, 205-211.student's yield of N-t-Butyl-m-xylidene: 62%.student's yield of LiN-t-Butyl-m-xylidene*Et2O: 63% [67%]Johnson, A. R.; Cummins, C. C. Inorg. Synth. 32, 1998, 129.

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O

BF4 Nt-Bu H

t-BuNH2/NEt3 (3 eq; 2 : 1)

-40 oC/ 6h; yield 60%

1. Vernaudon, P; Rajoharison H. G.; Roussel, C. Bull. Soc. Chim. Fr. 1987, 205-211.2. Furster, A.; Mathes, C.; Lehman, C. W. J. Am. Chem. Soc. 1999, 121, 9453.3. see ref 61 in: Cummins, C. C. Chem. Commun. 1998, 1777. For the synthesis of the HN(t-Bu)Ar]3 d6 isotopomer see: Johnson, A. R.; Cummins, C. C. Inorg. Synth. 1998, 32, 123.

Brt-BuNH2 (1. 2 eq)Pd2(dba)3 (0.25 mol%)(R)-BINAP (0.375 mol%)t-BuONa/tolueneyield 47%; unreacted 45%

t-BuNH2 as reagent and solventNaNH2, yield ca. 42%.Separated from its isomer byrecrystallization of the hydrochloride

Br

Literature data:

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Synthesis of Mo[N(t-Bu)Ar]3

MoCl3(THF)3

Nt-Bu Li

+ether

-100 oCMoL

L

LN

t-Bu

L is

3

orange-redextremly sensitive tomoisture and oxygen

student's yield: 30% (lit. 70%)

Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996, 118, 8623.

exp.: 1:2

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8. NMR of paramagnetic compounds

� Diamagnetic compounds (all electrons are paired) display a “normal” NMR, slim signals spread over 0-10 ppm.

� Paramagnetic compounds (unpaired electrons). 1. NMR signals are broadened proportional to �/r6 (� is the

gyromagnetic ratio, 1H: 26.7519x10-7radT-1s-1; 2H: 4.1066x10-7radT-1s-1).

2. NMR chemical shifts as result of interaction between electron and nuclear spin is the contact shift (in esr the hyperfine coupling): �B/B0 = (a�2h)/(4 � pkT)

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The 1H NMR of Mo[N(t-Bu)(Ar)]3t-Bu ca 64 ppm (br); Ar-CH3; -10 ppm

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9. Concluding remarks and future developments:

� The 1995 discovery by MIT scientists that the three-coordinate molybdenum(III) complex Mo[N(t-Bu)Ar]3 reacts readily with nitrogen, became in 1998 an advanced undergraduate experiment.

� The (best) synthesis of the ligand (N-t-butyl-m-xylidine) is performed based on a procedure modified from the literature.

� During the experiment students perform syntheses of diverse organic and inorganic compounds using glove box techniques. Topics like the isolobal concept, 1H-NMR of paramagnetic species, high spin electronic states are also covered.

� The next version of the experiment will also feature a short cut in the synthesis of MoCl3(THF)3, the electronic structurescalculated at the DFT level to rationalize the unusual reactivity of MoL3 and to assess the Mo�N vibration in L3Mo�N.