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NEW SI6MA—ETHY L. COMPOUNDS OF DIMOLYBDENUM ANO EVIDENCE FOR DINU—— ETC(w~aUN 78 N H CHISNOLN. 0 A HAITKQ. C A NURILLO N000fl—76—C—ofl6

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OFFICE OF NAVAL RESEARCH

Cont~~~~~~~Ø4-76-C-fi826]Task No. NR 056-625

(Th ~~~~~CHNICAL *~~~~T~~ O. 78-13

\~4 New i-Ethyl Compounds of Dimolybdenum (MEM) and —

ir~ ~ Evider e for Dinuclear Reductive Elimination with a Concomitant

Metal-Metal Triple to Q.iadruple Bond Transformation:

Et-MEM-Et -~ ?Y~M + C2H4 + C 2H5.

D. A./Hait~~JjC. A./Mur~~~~7

Prepared for Publication

in

Journal of the American Chemical Society

Department of Chemistry 0C..) Princeton University

L.LJ Princeton, New Jersey 08540

June 6, 1978

-

!

Reproduction in whole or in part is permitted for

any purpose of the United States Government

Approved for Public Release: Distribution Unlimited

7i~ ~~~~~~~~~~~~~, 26

~9

~~iJL.._ - ~~~~~~~ .,~~~; ~~~~

SECURITY CLASS IFICAT ION OF THIS PAGE (WI,,te O.Ia Ent•rsd)

D~~DnDT l I L k A ~r lAU DA I’~~ READ INSTRUCTIONSI’IU ‘.#I~ I ~#‘J’.. U lVlLI~ I ~~ I I~.1I~ l

~~~~~~~~ BEFORE COMPLETIN G FORMI. REPORT NUMBER 2. GoVT ACCESSION NO. 2. RECiPIENT’S CATALOG NUMSER

4. TITLE (ond SubtW.) New a—Ethyl Compounds of Dimo ‘1. TYPE OF RE P oR T 6 PERIOD cov r~~~o

ybdenum(MEM) and Evidence for Dinuclear Technical Report 1978~eduçtive ~liu4nation witF~ a çoncocx4tant~‘1eta1-MetaL Triple to Quaarupie Bone Trans• a. PERFORMING ORG. REPORT NUMBER

formation: Et-MRM-Et-’M~M+C2H4-fC2H6. TR-78-13”~7. AUTNOA( .) • S. CONTRACT OR GRANT NUM SER(S)

~~. H. Chishoim , D. A. Haitko and~~. A. Murillo NOOO14_76~C_0826

1

S. PERFORMING ORGANIZATION NAME AND AD DRESS 10. PROGRAM ELEMENT. PROJECT . 1A3~Department of Chemistry~

AREA & WO RK U N I T NUMBERS

Princeton UniversityPrincton , N. J. _______________________

11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

Office of Naval Research June 6, 1978

Department of the Navy ~ NUMB~~RO F PAG ES

• . 14. MONITORING AGENCY NAME & AOORESSI’I( dlii iront from Cont:oilin~ Offic.) IS. SECURI TY CLASS. (et thIs e.poel)

So. DECLASS I Fl CAT IOM/ DOWNGRADINGSCHEDULE

II. DISTRIBUTION STATEMENT (of this R.port)

Approved for public release; distribution unlimited

17. DISTRIBUTION STATEMENT (of th. ab.Izac t .nt.r.d in Block 20, U dIU.r.n( from R.port)

II. SUPPLEMENTARY NOTES

IS. KEY WOROS (Continu. on r.v.r .. .id. ii n.c..amy ~~d id.ni ity by block numb.r)

o-Ethyl, Dimo].ybderium, Metal-Metal Triple and Quadruple Bonds,Dinuclear Reductive Elimination.

20. ASS1 RAC T (ConUnu. on ,.v .ra. old. U n.c.•.~~~ id Sd.ntll~’ by block m th.r)

The preparation and properties of Mo2Et2(NMe2)4 and Mo 2Et(OBu ) 5

are described. Reaction of the former with CO2(~ 4 equiv) leads

to Mo2 (O2CNMe 2)4, C2H6 and C2H4.

DD JAN 73 1473 EDITION OF I NOV 61 IS OBSOLETE Unclassified

....!?8_. __ :0 ~~~~~~~~~~~~~~~~~~~~~~~~

~

$ICUAItY CLAU IPIGATION OF IHIS PAGE (ISbn D.b. tnt.rsd )

- — —-—----—

~~~~~

-•.---—.. -‘- . • .

New a-Ethyl Compounds of Dimolybdenum (MRM) andEvidence for Dinuclear Reductive Elimination with a Concomitant

Metal-Metal Triple to Quadruple Bond Transformation:

Et-MEM-Et -, ~4~N + C2H4 + C2H6.

Sir :Transition metal complexes containing a-ethyl ligands are

prone to thermal decomposition by an initial step involvings-hydrogen elimination:’ N-C2H5 ~ N-H + C2H4. This reaction is

surpressed when the metal is coordinativety saturated and attains

an 18-valence shell electronic configuration.2 We report here Ci)the preparation of the thermally stable a-ethyl dimolybdenum corn-

pounds Mo2Et2(NMe 2)4 and Mo2Et(OBut)5, in which the ethyl ligands

are coordinated to uns,~iturated metal centers3 and (ii) the reaction

between Mo2Et2(NMe 2)4 and CO2 which proceeds according to eq 1

and provides a model reaction for studies of dinuclear reductiveelim ination.4

1. ).lo2Et2(NMe2)4 + 4C02 -, Mo2(O2CNMe2)4 + C2H4 + C2H6

Mo2C12(NMe2)45 reacts smoothly at -78°C with EtLi (2 equiv)in hydrocarbon solvents to give the yellow, crystallin e compoundNo2Et2(NNe 2)4 which may be obtained analytically pure6 by sublim-ation (60-70°C, iO~~ rnmHg) in greater than 7O~ yield based upon eq 2.

2 Mo2C12(NMe 2)4 + 2LiEt -‘ Mo2Et2(I~Me2)4 + 2LiC1

The ‘H NMR spectrum of Mo2Et2(NMe 2)4 obtained in toluene-d8at -61°C at 270 MHz is shown in Figure 1. This corresponds to the

low temperature limiting spectrum of a mixture of anti and gauche-

rotamers of an ethane-like molecule (Me2N)2EtMoiMoEt(NMe 2)2.7 Note

the methylene protons of the gauche rotamer (but not the anti-rotamer)

— - ~~r ’~

2

are diastereotopic and form part of an ABX 3 spectrum. At 90°Crotation about the N-N bonds is rapid on the NMR tirnescale leadingto the coalescence of proximal and distal N-Me signals but rotation

about the MBM bond (anti ~ gauche isomerization) is still slow.

In the mass spectrometer there is a strong molecular ion

Mo2(NMe2)4Et2+ (m/e = 430) and an ion Mo2(NMe 2)4

+ (m/ = 372)

corresponding to the loss of 2Et.

Mo2Et2(NMe 2)4 reacts rapidly at room temperature with tert-

butanol in benzene according to eq ~~~.

3 Mo2Et2(NMe 2)4 ÷ ButOH(ex) Mo2Et(OBut)5 + 4 HNMe2 + C2H6

• •

Mo2Et (OBut)

~ is a burgundy-red solid which sublimes with some

decomposition at 60-70°C i0 4 mt~Hg. The ‘H NMR spectrum at lowtemperature (-76°C) at 270 MHz in toluene-d8 consists of a simpletriplet and quartet for the Et ligand and two resonances in the

8ratio of 3:2 for the OBu groups. This is consistent with an

ethane-like molecule (ButO)2(Et)}4o~Mo(OBut)s in which rotation

about the M0BM0 bond is rapid on the NMR timescale.9

Mo2Et2(NMe 2)4 in toluene reacts rapidly with CO2 (� 4equiv) to give a pale-yellow finely divided precipitate. This

compound has not been structurally characterized but is considered

to be No2(O2CNMe2)4 and to have the dimolybdenum tetraacetatestructure (?~ M)’° on the following grounds: Ci) analytical data’1,

(ii) infrared data’2, and (iii) the appearance in the mass spectrum

of a very strong ion corresponding to Mo2(O2CNNe2)4+ (this is the

ion of highest mass) and the doubly charged ion Mo2(O3CNMe2)42+.The compound is not appreciably soluble in hydrocarbon solvents ,nor CD2C12, but is sparingly soluble in pyridine.13

In a sealed NMR tube reaction Mo2Et2(NMe 2)4 in toluene-d8was reacted with CO2 (> 4 equiv). The finely divided pre-

cipitate was centrifuged to the top of the tube and the ‘H NMRspectrum of the clear, virtually colorless solution was recorded.

3

The only proton signals observed corresponded to ethylene and

ethane which were in the integral ratio of 4:6, respectively.’4

We conclude that the reaction between Mo2Et2(NMe 2)4 and CO2 proceedsstoichiometricall y according to eq 1 and as such provides a modelreaction for detailed studies of dinuclear reductive elimination.’5

A simple intramolecular mechanism involving an initial s-hydrideelimination, Et-MoEM-H + C2H4, followed by C-H reductive elimination

across the Mo~Mo bond, Et-M0EM0-H -, Et-H + Mo~Mo, satisfiei all our

observations.

In contrast to the above we find that Mo2Me2(NNe 2)4• reacts

with CO2 according to eq ~~~.

4 Mo2Me2(NMe2)4 + 4C02 -, M02Me2CO2(NMe2)4

The compound Mo2Me 2(O2CNMe2)416 is of sufficient thermalstability to allow the detection of the molecular ion Mo2Me2(O 2CNMe2)4

+

in the mass spectrometer. In the solid state and in solution

Mo2Me2(O2CNMe2)4 is believed to share the W2Me2(O2CNEt2)4 structure17

which has a planar C-W~W-C unit with a C-W-W angle equal to 106°.

Acknowledgements.

We thank the Office of Naval Research for support of this

work and Professor D. C. Bradley, Queen Mary College, London whothrough the auspices of a NATO Grant kindly provided mass spectral

results.

AuthorsMalcolm H. Chisholm*’S

_____ / Deborah A. Haitko

IiB ’ ~~ S~~Tho n Carlos A. Murillo

0Department of ChemistryPrinceton UniversityPrinceton, New Jersey 08540

UISJR~~C~ ~~~~~ !.~ UY ~~• •LIA L

References

1. a) G. Wilkinson, Science, 185, 109 (1974).

b) p. j . Davidson, M. F. Lappert and R. Pearce, Ace. Chem. Res.

7, 209 (1974).

c) R. R. Schrock and G. W. Parshall, Chem. Rev. 76, 243 (1976).

2. See,for example, the detailed studies of the thermal reaction(1~5-C5H5)Fe(CO)(PPh3)(alkyl) -, (fl 5-C5H5)Fe(CO)(H)(PPH3) + olefin:

D. L. Reger and E. C. Culbertson, J. Am. Chern. Soc. ~~~~~~~, 2789 (1976).

3. In M2X6 and N2XS...nYn compounds (X , Y are uninegative mojiodentate

ligands) the metals attain only 12 valence shell electrons as a

result of M-X a bonds and the M-M triple bond. The metals are

capable of increasing their coordination number and number of

valence electrons by Lewis base association reactions e.g.,

Mo2(OSiNe3)6 ÷ 2HNMe2 ~ Mo2(OSiMe3)6(HNNe 2)2 M. H. Chisholm ,

F. A. Cotton, N. W. Extine and W. W. Reichert, J. Am. Chem. Soc.

100, 153 (1978).

4. For reductive elimination in mononuclear chemistry see C. A. Tolman,

Chem. Soc. Rev. 1, 357 (1972).

5. M. Akiyama, N. H. Chisholm, F. A. Cotton, M. W. Extine and

C. A. Murillo, Inorg. Chem. 16, 320 (1977). Note all operations

must be carried out in dry and oxygen free solvents and atmosphere.6. Found (Calcd): C, 33.55 (33.79); H, 7.89 (8.04); N, 12.99 (13.15).

7. See the structural and dynamic behavior of the related compound

W2Me2(NEt2)4: N. H. Chisholm, F. A. Cotton, M. W. Extine, M. Millarand B. R. Stults, Inorg. Chemn. 15, 2244 (1976).

8. Et group: 8(CH2) = 2.98, 6(CH3) 1.78, J 7.9Hz.

OBu groups at -76°C: 6 — 1.60 and 1.56 in the integral ratio

3:2, respectively. Chemical shifts (6) given in ppm downfield from TMS.

9. Accidental magnetic degeneracy could account for the observed

3:2 spectrum (c.f. predicted low temperature limiting spectrum

2:2:1).

• ~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ — — .---

~-. .

10. For a recent review of compounds containing M-M quadruple

bonds see F. A. Cotton, Chem. Soc. Rev. 4, 27 (1975) (b) The

diethylcarbamate Cr2(O2CNEt2)4 •(HNEt2)2 has recently beenstructurally characterized and shown to have a Cr~Cr bond.

M. H. Chisholm, F. A. Cotton, M. W. Extine and D. C. Rideout,

Inorg. Chem ., submitted for publication.

11. Found (Calcd): C, 26.23 (26.48); H, 4.25 (4.44);

N, 10.09 (10.29).

12. In particular the presence of a strong absorption at 1560 ~~~~

assignable to v(NCO2) of a bridging bidentate carbamate ligand.See N. H. Chisholm and M. W. Extine, J. Am. Chem. Soc. 99,

782 (1977).

13. ‘H nmr data recorded at 100MHz, 25°C in pyridine-d5:

6(O2CNNe2) = 2.93 ppm relative to TMS.14. Found by weighing the traces 38:62. Any departure from the

predicted ratio, 4:6, may be due to their differing solubilities.

15. Labelling studies are planned in order to investigate (i) the

reversibility of $-hydrogen elimination and (ii) the intra vs.

intermolecular nature of the reaction.

16. Analysis Found (Calcd): C, 29.50 (29.29); H, 5.14 (5.23);

H ,). 1, 9. 65 (9. 75) .

17. N. H. Chisholm, F. A. Cotton, N. W. Extine and B. R. Stults,

Inorg. Chem. 16, 603 (1977).

18. Alfred P. Sloan Fellow, 1976-78.

Caption to Figure 1

‘H nmr spectrum of a mixture of anti- and gauche-

Et(Me2N)2Mo~Mo(NMe 2)2Et recorded in toluene-d8 at -61°C

and 270MHz .

II

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