Dinitrogen Scission by a Molybdenum(III) Xylidene Complex

Transcription

1 Dinitrogen Scission by a Molybdenum(III) Xylidene Complex Dr. Mircea D. Gheorghiu mircea@mit.edu, phone: x Introduction. 2. About Dinitrogen. 3. itrogen Fixation. Haber-Bosch ammonia synthesis. itrogenase. 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. MR of paramagnetic compounds. 9. Concluding remarks and future developments. 9/9/03 1

2 1. Introduction. Prof. C. C. Cummins Catalina aplaza (MIT Undergraduate Research Opportunities Program) 9/9/03 2

3 The main reference: Dinitrogen Cleavage by Three-Coordinate Molybdenum(llI) Complexes: Mechanistic and Structural Data Catalina E. aplaza, Marc J. A. Johnson, Jonas C. Peters, Aaron. Odom, Esther Kim, Christopher C. Cummins, Graham. George, and Ingrid J. Pickering; J. Am. Chem. Soc. 1996,118, /9/03 3

4 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) aplaza, C. E.; Odom, A..; Davis, W. M.; Cummins, C. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, aplaza, C. E.; Davis, W. M.; Cummins, C. C. Angew. Chem., Int. Ed. Engl. 1995,34, (3) aplaza, C. E.; Cummins, C. C. Science 1995, 268, 861. See also: aplaza, C. E.; Johnson, A. R.; Cummins, C. C. J. Am. Chem. Soc. 1996, 118, /9/03 4

5 Kit s suggestion (April 1998) 9/9/03 5

6 2. About dinitrogen: On Earth 4/5 of the volume of atmosphere; Titan (moon of Saturn) 82-94%. Bp ºC (O 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. 9/9/03 6

7 3. itrogen fixation (to nif, niffing): 1. Haber-Bosch (1909, 1913) (Fe catalyst; C; atm): 2 + 3H 2 2H 3 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. 9/9/03 7

8 9/9/03 8

9 itrogen is not accessible to plants itrogen-fixing bacteria (symbiotic association with legume plants) enzyme (nitrogenase) converts 2 into metabolically ammonia. itrogenase has two component metalloproteins: the Fe-protein (source of electrons) and the MoFeprotein (interprotein electron transfer and catalyst for binding and reduction). 9/9/03 9

10 is: 4. itrogen binding and scission: t-bu + Mo orange-red CH 3 CH 3 Mo -35 o C 25 o C ether Mo 2 Mo Amber 1042 cm -1 Purple student yield: 75 % (76%) Small substituents allow for metal-metal bonding (Me 2 ; OBu t ): Mo Mo Mo 9/9/03 10

11 Structure: 9/9/03 11

12 Mechanism: 4 A" 2 A'' 1 A 3 B g 4 A' Mo end-on binding of 2 Mo Mo 25 o 3 C Mo Mo -35 o C t 1/2 = 35 min Mo no barrier of activation (theoretical methods) barrier of activation: - computed ca. 18 kcal/mol - experimental 23.3 kcal/mol 1 A' Theoretical works (=H 2 ): Cui, Q; Musaev, D. G.; Svensson, M.; Sieber, S.; Morokuma, K. J. Am. Chem. Soc. 1995, 117, eyman, K. M.; asluzov, V.A.; Hahn, J.; andis, C. R.; Rosch,. Organometallics, 1997, 16, 995. Experimental work: aplaza, C. E.; Cummins, C. C. Science, 1995, 268, 861. apalza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A..; Kim, E.; Cummins, C. C.; George, G..; Pickering, I. J. J. Am. Chem. Soc. 1996, 118, /9/03 12

13 5. Why 2 is activated by Mo(III) complexes? 3 Mo is isolobal to nitrogen atom () and carbyde (CH), with all three d electrons unpaired (high spin). The frontier orbitals 3a 1 2e 2 are derived largely from dz2 (3a) and dxz and dyz, respectively (2e). (see the pictures on right dxz, dyz, dz2). The R 1 R 2 substituent is a strong donor into d orbitals (e MOs) of Mo, increasing the electron density at the metal. 9/9/03 13

14 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(RAr) 3 is isolobal with CH Mo H C Mo Mo Mo CH CH CH 9/9/03 14

15 There are and interactions involved in 2 - Mo 3. 2 provides the electrons to empty orbitals of Mo (s or d). The component is back-donation from occupied d orbitals to the empty * of 2. As result Mo- becomes stronger, the - bond weaker and the reaction 3 Mo 2 Mo 3 2Mo 3 more exothermic. Dissociation energy: Mo triple bond = 155 kcal/mol, 2 is kcal/mol: 2R 3 Mo + 2 2R 3 Mo is exothermic 9/9/03 15

16 6. Starting materials for Mo 3 1. MoCl 5 2. igand from (CH 3 CO) 2 O and diacetone alcohol: OH O 9/9/03 16

17 7. A. Current version for the synthesis of MoCl 3 (THF) 3 Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001, Modified by Stephens, F., August, MoCl 5 cold Et 2 O MoCl4 (OEt 2 ) 2 THF MoCl 3 (THF) 3 Sn, 30 min Sn, 30 min black-brown solid brown solution orange suspension in diethyl ether bright orange powder 9/9/03 17

18 Oxidation states of Mo Mo: Z=42; Ground state: Kr5s 1 4d 5 ( 7 S) MoCl 5 (Mo 2 Cl 10 ): Mo(V) one unpaired electron (readily abstracts oxygen from oxygenated solvents) MoCl 4 (Et 2 O) 2 : Mo(IV) two unpaired electrons mer-mocl 3 (THF) 3 : Mo(III) three unpaired electrons no fac-mocl 3 (THF) 3 9/9/03 18

19 7. B. Synthesis of the m-xylidine ligand: t-bu ca. 4-6 equiv. t-buh 2 H O CH 3 C/RT BF 4 (CH 3 CO) 2 O HBF 4 Bui in thawing pentane ca 90 o C OH t-bu i O student's yield of pyrilium salt: 66% [lit. 72%] Vernaudon, P; Rajoharison H. G.; Roussel, C. Bull. Soc. Chim. Fr. 1987, student's yield of -t-butyl-m-xylidene: 62%. student's yield of i-t-butyl-m-xylidene*et 2 O: 63% [67%] Johnson, A. R.; Cummins, C. C. Inorg. Synth. 32, 1998, /9/03 19

20 iterature data: O t-buh 2/ Et3 (3 eq; 2 : 1) -40 o C/ 6h; yield 60% BF 4 Br t-buh 2 (1. 2 eq) Pd 2 (dba) 3 (0.25 mol%) (R)-BIAP (0.375 mol%) t-buoa/toluene yield 47%; unreacted 45% t-bu H Br t-buh 2 as reagent and solvent ah 2, yield ca. 42%. Separated from its isomer by recrystallization of the hydrochloride 1. Vernaudon, P; Rajoharison H. G.; Roussel, C. Bull. Soc. Chim. Fr. 1987, Furster, A.; Mathes, C.; ehman, C. W. J. Am. Chem. Soc. 1999, 121, see ref 61 in: Cummins, C. C. Chem. Commun. 1998, For the synthesis of the H(t-Bu)Ar] 3 d 6 isotopomer see: Johnson, A. R.; Cummins, C. C. Inorg. Synth. 1998, 32, /9/03 20

21 Synthesis of Mo[(t-Bu)Ar] 3 t-bu i t-bu MoCl 3 (THF) ether -100 o C Mo is exp.: 1:2 orange-red extremly sensitive to moisture and oxygen student's yield: 30% (lit. 70%) aplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A..; Kim, E.; Cummins, C. C.; George, G..; Pickering, I. J. J. Am. Chem. Soc. 1996, 118, /9/03 21

22 8. MR of paramagnetic compounds Diamagnetic compounds (all electrons are paired) display a normal MR, slim signals spread over 0-10 ppm. Paramagnetic compounds (unpaired electrons). 1. MR signals are broadened proportional to /r 6 ( is the gyromagnetic ratio, 1 H: x10-7 radt -1 s -1 ; 2 H: x10-7 radt -1 s -1 ). 2. MR chemical shifts as result of interaction between electron and nuclear spin is the contact shift (in esr the hyperfine coupling): B/B 0 = (a 2 h)/(4 p kt) 9/9/03 22

23 (A) trace: CH 3 H 2 ; (B) trace i(ch 3 H 2 ) 6 2+ ; i 2+ (d 8 ) is octahedral with 2 unpaired electrons:, t 2g6 e g 2 1 H MR 9/9/03 23

24 The 1 H MR of Mo[(t-Bu)(Ar)] 3 t-bu ca 64 ppm (br); Ar-CH3; -10 ppm 9/9/03 24

25 9. Concluding remarks and future developments: The 1995 discovery by MIT scientists that the three-coordinate molybdenum(iii) complex Mo[(t-Bu)Ar] 3 reacts readily with nitrogen, became in 1998 an advanced undergraduate experiment. The (best) synthesis of the ligand (-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, 1 H-MR 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 MoCl 3 (THF) 3, the electronic structures calculated at the DFT level to rationalize the unusual reactivity of Mo 3 and to assess the Mo vibration in 3 Mo. 9/9/03 25