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Improved Syntiietic Routesto Molybdenum-NitridoCompoundsMichael Holrnes a & Shawn C. Sendlinger aa Department of Chemistry , North Carolina CentralUniversity , Durham, NC, 27707Published online: 14 Apr 2008.

To cite this article: Michael Holrnes & Shawn C. Sendlinger (1999) Improved SyntiieticRoutes to Molybdenum-Nitrido Compounds, Synthesis and Reactivity in Inorganic andMetal-Organic Chemistry, 29:1, 143-153, DOI: 10.1080/00945719909349440

To link to this article: http://dx.doi.org/10.1080/00945719909349440

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SYNTH REACT. INORG. MET.-OKG. CHEM., 29(1). 141-153 (1999)

IMPROVED SYNTIIETIC ROUTES TO MOLYBDENUM-NITRIDO

COMPOUNDS

Michael Holrnes and Shawn C. Sendlinger"

Department of Chemistry North Carolina Central University

Durham. NC 27707

ABSTRACT

Convenient synthetic routes to the compounds MoBrJ, MoNBr3, and

MoNRr?(bpy) are described using Mo(CO)6. Br2. (CI 13)3SiN3, and 2,2'-bipyridine

as inexpensive and readily available starting materials. These one-pot syntheses

produce analytically pure compounds in multigram quantities with high yields

under mild conditions. IJse of (CHj)jSiN3 as a nitride source is preferable to that

ol' unstable and explosivc IN? which has been utilized in a previous synthesis of

.M oN R r

INTRODUCTION

Our interest in the synthesis and reactivity of transition metal-nitrido

compounds. and their possible use as monomers in the formation of metal-nitrido

polymers'. led us to seek new synthetic routes to the compounds MoNBr? and

MoNBrz( bpy), where bpy = 2,2'-hipyridine.

Copyrighi 1999 hy Marcel D e k k e ~ . Inc

143

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144 IIOLMES AND SENDLINGER

The first synthesis of MoIWr3 reported' involves addition of INI to a CCII

suspension of MoRr4 at room temperature. This reaction proceeds i n two steps, as

shown in eqs (,1) and (2):

The azide compound, a kinetic product which forms rapidly, decomposes to form

the nitride after prolonged stirring ( I ? h) at room temperature, or more rapidly

upon heating. Purification of MoNBr, involves filtration. vacuum drying. and

cxtraction with liquid bromine to remove cxccss ,MoBr4 and other impurities. A

69% ovcrall yield was reported.' 'Ihe other reported synthesis' of MoNBr3 is shown in eq (3):

[MoCI.1(NSC1)]2 + 10 (C11,)1SiRr -+ 2 MoNBrI + S2Rr2 + 10 (CH3)TSiBr + Br'(3)

nr~)mvtriniethylsilaii~ was acldcd dropwise to a suspension of [h.loC14(NSCI)]? i n

CH2Br:. After stirring for 12 h. MoNBr? was isolated via filtration, washed with

CI12Hr2, and dried under vacuum to give a 93% yield.

The preparation of MoNErlhpy) proceeds4 in near quantitative yield as

shown in eq (4):

This reduction was performed in CH2Br2 nt room temperature in 98% yield.

Both of the reported syntheses of MoNBr3 have obvious drawbacks. The

first route involves use of the unstable and potentially explosive reagent IN;.

Another inconvenience of this method is the Soxhlet extraction of impurities

using liquid bromine, which is reported to take four days.' The second synthetic

method requires preparation of (MoCI ,(NSC1)J2. While eq (3) does provide

MoNBrI in high yield. the preparation5 of [ MoC14(NSC1)]2 requires the synthesis

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SYNTHETIC ROUTES TO MOLY BDENCM-NITRIDO COMPOUNDS 145

of trithiazyl chloride, (NSCI)3, which is produced from the reaction of excess C12

with SiNzC12.6 This necessitates synthesis of SjNzCIz from S ~ C I L and NI11CI.6 In

order to avoid reactions requiring excess Clz or the unstable reagent IN3, we

sought to shorten and simplify the synthesis of MoNBr3 by an alternative route

which utilized inexpensive starting materials that are commercially available.

RESIJLI'S 4 N D DISClJSSION

An early report' on the synthesis of MoBr4 involves the stoichiometric

oxidation of Mo(CO)<, by Br2 as shown in eq (5):

Mo(CO)~ + 7- Br? + MOB^.^ + 6 CO ( 5 )

Subsequent work showed that this method produces pure IMoBr4.* The reaction

was originally performed without solvent. We have found that use of CH2C12 or

CCI, as the solvent is a more convenient rncthod which simplifies isolation of the

solid product, as i t IS now easily filtered. At -18"C, the reaction proceeds

smoothly as shown in eq (6):

cF12c12 ccl& MOBr, + 6 CO Mo(CO), + 2 Br2 -18°C to R.T.

We havc found no evidence for significant chlorine incorporation to form mixed

Mo/CI/Br compounds under these condition^.^^"' After warming to room

temperature and stirring for several hours, the black solid was filtered, washed

with solvent (CHzC12 or CCIa), and dried under vacuum. Extensive drying was

required to completely remove all solvent. Excellent yields (cn. 95%) were

obtained.

The next step involved the use of commercially available (CH&SiN3 as

the nitride source, as shown in eq (7):

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146 IIOLMES AND SENDLINGER

Unlike the previous report using INj, we were not able to isolate the azide.

MoK.3Brj. even at short reaction times. The black MoNBr.3 product was isolated

via filtration, washed, and was thoroughly dried under vacuum to remove all

traces of solvent. Typical yields were above 88%.

We have found that the reactions illustrated in eqs ( 6 ) and (7) can be

combined i n a one-pot synthesis to yield MoNBr? directly. This is easily

accomplished by subsequent additions of Br: and (CH3):JiN? at low temperature,

as shown in eq (8):

+ 2 Br2, CIl,CI, + (CH,),SiN3 - 18°C to R.T., -N2* MoNBr? + (CHj)3SiBr (8) Mo(CO)O - 18"C, -6 CO

After the product was filtered, washed with CH2CI~, and thoroughly dried in

vacuo, this method gave an essentially quantitative yield of MoNBr3 (98%).

Preparation of MoNRr2(bpy) proceeded at room rcniperature using CH2CI2

as solvent according to cq (4). Again. we have not observed any incorporation of

chloridc i n our compound$ as a rcsult of using chlorinated solvents.") The

synthesis of MoNBr2(bpy) can also be performed by subsequcnt additions of the

required reagents in a onr-pot approach, as shown in eq (9):

The brown MoKBrl(bpy) was isolated in high yield (92%).

under vacuum was required to remove all traces of solvent.

Prolonged drying

Care must be taken when performing the reactions shown in eqs ( 6 ) , (8).

and (9) due to the production of CO. An efficient fume hood must be used and

the reaction mixture must be well-cooled (-18°C) before the Brz is added.

Observing these precautions, we have performed the synthesis of MoBrJ shown in

equation (6) using up to 8 grams of Mo(C0)6 without any problem. Further scale-

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SYNTHETIC ROUTES TO MOLYBDENUM-NITRIDO COMPOUNDS 147

up of any of these reactions should be approached with caution. A lower initial

temperature rind/or a slower rate of Rr? addition may be required to ensure a

controlled rate of CO evolution.

Characterization of Compounds

MoBr.1. Molybdenum( IV) bromide proved to be quite sensitive to

hydrolysis, and could only be handled under an inert atmosphere. The

decomposition point (observed in a sealed capillary tube) was difficult to

determine, since the black MoBr.% slowly began to sublime and reform brown

crystals i n cooler parts of the tube outside the heating block of the Mel-Temp

apparatus. The formation of these brown crystals was observed beginning at ca.

200°C. Based on previous studies"." of the thermal decomposition of MoBrd,

these brown crystals were most likely MoBrl which was formed according to eq

(10):

MoBrJ + MoBrj + $5 Br? (10)

Quantitative studies of the thermal decomposition have shown that MoBr4

decomposes completely i n one hour at CCI. 200°C to give pure MoB1-3.l~

Elemental analysis of MoBr.4 gave the expected 1.4 Mo:Br ratio. Residual

solvent proved difficult to remove. Heating the material to ca. 60°C under

vacuum for 6-8 h proved sufficient. As expected, the infrared spectrum of MoBr4

was featureless in the 4400-450 cm-l range accessible to our instrument. Previous

studies' report a terminal Mo-Rr vibration at 28X cm-l with shoulders at 305 and

278 cm I, and an absorption assigned to a bridging Mo-Br vibration at 218 cm I .

MoNBrT. The solubility of MoNBr? proved to be quite low in all but the most

polar solvents (POCII, DMSO). The decomposition temperature was found to be

cu. 240°C. at which point the black color of the substance changed to brown.

Previous have shown that MoNBrj quantitatively decomposes when

heated above 200°C under vacuum to form MoNBr2, as shown in eq ( 1 1 ):

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148 HOLMES AND SENDLINGER

The infrared spectrum of MoNBr3 exhibited a band assignable to the M o m stretch

at 1023 cm I, which agrees with previous results.' Elemental analysis gave the

expected Mo:Rr ratio of 1.3.

MoNBrz(buv). l h e decomposition temperature of MoNBr2(bpy) was 298-

300°C. An intense absorption at 945 cin-' in the infrared spectrum is assignable

to the Mom stretch.' The expected peaks due to a bound 2,2'-bipyridine ligand

were also observed in the infrared spectrum (see Experimental). The 'HNMR

spectrum of MoNBr?(bpy) i n DMSO-dh exhibited paramagnetically broadened

and ill-resolved peaks between 7 and 8 ppm. The elemental analysis of a

MoNBrz(bpy) sample which had undergone prolonged drying under high vacuum

indicated the unsolvated form. This compound exhibits appreciable solubility

only in highly polar solvents. l h i s low solubility suggests that MoNBr'(bpy) may

associate into tetramcrs i n the solid state, as do several other transition metal-

nitrido compound^.'^ A ImssihlP structure based on this conjecturc is shown i n

h g . 1 .

Synthctic and analytical data for MoBr4, MoNBri, and MoNBr2(bpy) have

been collected in Table I.

We have reported the facile synthesis of iMoBr4, MoNBr?. and

MoNBr'(bpy) using relatively inexpensive, commercially available starting

materials. Trimethylsilylazide served as an efficient nitride source, and its use is

preferable to that of unstable IN3. Our synthetic route also avoids the preparation

of' [MoC14(NSC1)]2 and its required precursors. Decomposition data, infrared

spectra. and elemental analyses are all in agreement with literature reports and

expected values. A possible structure of MoNBrz(bpy) has been presented. We

are currently exploring the reactivity of these and other transition metal-nitrido

compounds in order to delineate the basicity of the terminal nitrido group, and to

explore the possibility ol' using these compounds as building blocks i n the

production of polymers which contain a (ransition metalhitrogen linkage along

the backbone. Progress in these areas will be reported in due course.

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SYNTHETIC ROUTES TO MOLYBDENUM-NITRIDO COMPOUNDS 149

FIG. 1. Postulated Structure of MoNBr,(bpy) Based on Literature Precedent

Table I. Svnthetic and Analvtical Data

. _ - I BrdMo Mo 23.09 (22.64)

I Compound FW (gho le )

VOBIA Br: 76.91 (77.27) I

Mo: 27.44 (27.80) 98 I 240 . 349.66 ~. - ~ ~ Br: 68.56 (68.14) .MoNBr?(bpy) C10H8Br2MoN3 C: 28.20 (27.80) 95 1 298

I €1: 1.89 (1.91) i L N: 9.87 (10.06)

EXPERIMENTAL

Gcneral Considerations.

All manipulations were performed under a purified nitrogen atmosphere

employing standard Schlenk techniques and an Innovative Technology glovebox.

Dichloromethane and carbon tetrachloride were distilled under nitrogen from

CaH?: CCls was then stored in an airtight container over activated 4-A molecular

sieves. Trimethylsilylazide and bromine wcre purchased from Aldrich Chemical

Co. and were purged with nitrogen gas before each use. Molybdenum

hexacarbonyl was purchased from Aldrich and was purified by sublimation under

high vacuum prior to use. Decomposition points were measured in sealed glass

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150 HOLMES AND SENDLINGER

tubes with a Mel-Temp I1 apparatus and are uncorrected. Elemcntal analyscs

werc pcrformcd by Dcscrt Analytics. Infrared spcctra werc obtained on a Pcrkin-

Elmer 1600 FTIR spectrophotometer. 'HNMR spectra were obtained on a Varian

300 TJnity INOVA spcctromctcr (299.958 MHL).

__ MoRr4.

To a stirred suspension of Mo(C0)6 (2.00 g, 7.58 rnmol) in 45 rnL CIIZCI: of

CCI.I at - 18°C (orthodichlorobcn7eiie/liqiiid nitrogen bath) was added Brz (0.78 1

mL, 15.2 mmol) dropwise via a syririgc over a period of ca. 1 min. Evolution of

CO gas was evident from the decp red-orange mixture. This

reaction shoitld only he performed in m i ejficientfuiiie hood). After stirring for 5

min, thc cold bath was rcmovcd. Slow gas evolution continued as the color

darkencd to black over a 10 min period. After stirring at room tcmperature for ccz.

3 h, the mixture was filtcrcd giving a black solid which was washed with 3 x 10

niI, portions of CIl$&, and was extensively dried under vacuum. yield 2.98 g

(95% b a d on Mo). IK (KBr pellct, c m ' ) : Featureless from 3400 to 450 mi- ' .

lkcomposition point: cci. 200°C (bec tcxt).

(WARNING:

__ MoN13rTL.Mcthod A

To a htirred suspension of .MoBrd (2.00 g, 3.81 mmol) in 50 ml, CHzCl. at -18°C

was addcd (CN3)lSiN:I (0.670 mI,. 5.05 niniol) via a syringe. After 5 min thc cold

bath was rcniovcd and evolution of N? gas was evident as the mixture warmed to

room temperature. After stirring for scvcrd hours, the black solid was isolated by

filtration, washed with 3 x 10 mL portions of CHzCL, and was extcnsivcly dried

undcr vacuum, yield 1 .SO g (89%. bascd on Mo).

MoNBr?JlCthod B

To a stirred suspcnsion of M O ( C O ) ~ (2.00 g, 7.58 rnmol) in 45 mL CH2CI2 of

CCI4 at -18°C was added Br2 (0.781 rnI,, 15.2 mmol) dropwise via a syring,e over

a pcriod of ca. 1 min. Evolution of CO gas was evident from the dccp red-orange

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SYNTHETIC ROUTES T O MOLYBDENUM-NITRIDO COMPOUNDS 151

mixture. ?his reaction should oiilj be yetfortried in an ejjficient

firnie hood). After 5 min, (CH?)jSiN3 (1.01 mL, 7.61 mmol) was added via a

syringe. After continued stirring at -18°C for fivc min, no apparent color change

occurrcd. The cold bath was rcmovcd and as the mixture warmed to room

tcmpcrature. evolution of Nz gns was evident and the color rapidly (ca. 2 min)

darkened to black, Thc black solid was isolated by filtration, washed with 3 x 10

mL portions of CH2CL, and was extensivcly dried under vacuum, yield 2.61 g

(98% based on Mo). I I i (KBr pellet. cm-’): 1399 (m), 1023 (s), 986 (m), 961 (w).

913 (w), 843 (m). Decomposition point: ca. 240°C (see text).

(WARNING:

MoNRrz(bpv), Method A

In thc glovebox, a 100 mL Schlcnk flask was charged with MoNBr3 (2.39 g, 6.81

mmol) and 7-,7-’-bipyridine (1.08g, 6.91 mmol). To the flask were added ca. 50

mL C I I ~ C I ~ , and a dark solution began to form immediately. After stirring for 16

h , the mixture was a dark brown suspension. The brown solid was isolatcd via

filtration, washed with 3 x 15 mL portions of CHzCIz, and was extensively dried

undcr vacuum. yicld 2.85 g (98% bascd on Mo).

$loNBr4bpy]. Method B

To a stirred suspension of Mo(C0)b (2.00 g, 7.58 mmol) in 50 mL CH?CI? at

- 18°C was added Brz (0.78 1 mL, 15.2 rnmol) dropwise via a syringe ovcr a period

of ca. 1 min. Evolution of CO gas was evident from the deep red-orange mixture.

(WARNING: This reaction should only be perfornied in an efficient fume hood).

After 5 min, (CH3)3SiN1 (1.01 mL, 7.61 mmol) was added via a syringe, then

7,2’-bipyridine (1.08 g, 6.91 mmol) was added via a solids addition tube. The

cold bath was removed and a very dark solution formed as thc flask warmed to

room temperature. Evolution of Nl gas was evident during this period. After

stirring for 5 h, the mixture was filtered, leaving a brown solid which was washed

with 3 x 15 mL portions of CH2C12. This solid was extensively dried undcr

vacuum. Typical yields were 2.97 to 3.12 g (92 to 97% based on Mo). IR (KBr

pellet, cm-I): 3071 (w), 3024 (w), 1600 (s), 1468 (m), 1441 (s). 1317 (rn), 1281

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152 HOLMES AND SENDLINGER

(vw), 1243 (w) . 1176 (sh), 1157 (m), I103 (w), 1062 (w). 1044 (w). 1026 (s), 943

(s), 762 (s), 730 (m), 653 (w), 635 (w). Decomposition point: 298-300°C.

-~ ACKNOWLEDGEMENTS

Acknowledgement is made to the Donors of The Petroleum Research

Fund, administered by the American Chemical Society, for partial support of this

research. M. H. wishes to thank the Lniversity IJndergraduate Research Program

and North Carolina Central 1Jniversity for a Sumnier Research Fellowship,

S. C. S. would likc to thank the Research Corporation for a Cottrell College

Science Award. The Varian Unity INOVA NMR spectrometer was purchased

with funds provided by the Office of Naval Research.

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1 . M . I I . Chisholrn, Angcw. (:hem. Int. Ed. Engl., 3. 673 (1991)

2. K. Dchnicke and N. Kriigcr, Z. Iiarurforsch.. m. 1242 (1978j.

3. K. Hoslcr and K. Dchnickc. Z. Anorg. Allg. Chcni., 554. 108 (1987).

4. K. Dehnicke. H. Prinz, W. Kalitz., and R. Kujanek, Ikb igs . Ann. Cheni., 20, (1981).

5 . K . Dehnicke and I!. Muller. Comments Inorg. Chcni., 4. 213, (1985).

6. W. L. Jolly and K. D. Maguire. Inorg. Synth.. 0, 102. (1967).

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8. S. A. Shchukarev, I.V. Vasil'kova, and N. D. Zaitscva, Vestnik Lenningrad. God. Lniv.. Ser. Fiz.-Khim., 16, 127 (1961); Chcni. Abstr., 56, 9510 (1962).

I). K. Dchnickc, N. Kriigcr, K Kujanek. and F. Wellcr. Z. Krist., 153, 181 ( 1980).

10. Initial elemental analyses of MoBrl, MoNBr3, and MoNRrz(bpy) indicated that C1 was present in amounts of 3-6'3.. 'HNMK spectroscopy revealed the presence of CH2C12 in thcsc samples. Extensive drying of the compounds removed all traces of solvent, and an analysis of thoroughly dried MoNBr?(bpy) had a %CI of <O. 1%.

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SYNTHETIC ROUTES TO MOLYBDENUM-NITRIDO COMPOUNDS 153

1 1 . P. J . 11. Cannell, R. E. McCarley, and R. D. Hogue, Inorg. Synth., l0, 49 ( 1 967).

12. I. V. Vasil’kova, A. I. Efimov, and B. Z. Pitrimov, Khim. Redk. Elem., 47, 44 (1964); Chem. A b s t r . , a , 9165 (1964).

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IS. A number of transition metal-nitrido compounds aggregate in tetrameric form as illustrated in Fig. I . See K. Dehnicke and J . Strahle, Angew. Chem. Int. Ed. Engl.. 3 , 4 1 3 (1981); [l id . 3, 9.55 (1992).

Received: 29 April 1998 Referee 1: J. L. Eglin Accepted: 11 August 1998 Referee 11: E. A. Maata

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