Activation of CarbonâFluorine Bonds by Oxidative Addition to Low

Chapter 25 Activation of Carbon—Fluorine Bonds by Oxidative Addition to Low-Valent Transition Metals 1

2

Downloaded by PENNSYLVANIA STATE UNIV on May 2, 2013 | http://pubs.acs.org Publication Date: April 29, 1994 | doi: 10.1021/bk-1994-0555.ch025

Carolyn E. Osterberg and Thomas G. Richmond

1Departmentof Chemistry, Concordia College, Moorhead, MN 56560 2Departmentof Chemistry, University of Utah, Salt Lake City, UT 84112 Several zero valent transition metals [W(0), Mo(0), and Ni(0)] can insert into an aromatic carbon-fluorine bond of suitably designed Schiff base ligands to afford stable divalent oxidative addition products with new metal -carbon and metal-fluorine linkages. New compounds are characterized by multinuclear NMR and IR spectroscopic methods. These reactions proceed readily under mild conditions and show that transition metals can promote cleavage of the strongest single bond to carbon. Qualitatively, the rate of these reactions increase with increasingfluorinationof the aromatic ring consistent with a nucleophilic mechanism for C-F activation. In addition the structure of the chelating ligand is crucial in promoting reactivity in these systems. In some instances, carbon monoxide scavenging blocks the open coordination site at the metal necessary for C-F activation. The crystal structure of a model metal-ligand complex prior to oxidative addition is reported. A summary of related examples of carbon-fluorine activation by organometallic complexes is provided and helps place our research in a broader context. Early work in organometallic chemistry benefited greatlyfromcontemporaneous developments in organofluorine chemistry which provided a new class of ligands which often yielded stable organometallic species (1). In particular, the low reactivity of the strong C-F bond stabilizes perfluoroalkyl complexes to β-elimination pathways (which are often facile in hydrocarbon analogues) and made the synthesis of a wide variety of thermally stable and chemically intriguing molecules. This trend continues to the present (2). Although relatively uncommon, activation of C-F bonds has been noted in a number of reactions of metal complexes withfluorinatedsubstrates usually under rather vigorous conditions and accompanying low chemical yields. These examples may provide insight into the nature of the high temperature defluorination technologies involving metals such as iron used in the synthesis of fluorinated arenesfromsaturated perfluorocarbons (3). Replacement offluorineby hydrogen is sometimes observed in catalytic hydrogénations although usually under forcing conditions compared to the lower halogens (4). Our research group has sought to uncover the fundamental organometallic chemistry offluorocarbonligands as models for catalytic reactions and to develop new methods of C-F bond formation. 0097-6156/94/0555-0392$08.00/0 © 1994 American Chemical Society In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

25.

Activation of Carbon-Fluorine Bonds

O S T E R B E R G AND RICHMOND

393

C-F Bond Activation by Electron Deficient Reagents

A diverse body of chemistry has been developed based on facile fluoride abstraction (5) or halide exchange (6) from perfluoroalkyl ligands by electrophilic reagents to afford perhaloalkylidene ligands(7). These transformations have been the subject of a recent review (8) and will not be discussed in detail since a main group electrophile is usually employed to remove the fluoride. Thermolysis of Cp2Ti(C F )2 at 150 °C for 24 h affords an 8% yield of Cp2Ti(C6F5)F in which a C-F bond is presumably broken to afford the Ti-F bond perhaps through a benzyne intermediate (9). Not surprisingly, other early transition metal systems which have a high affinity for fluoride as a ligand have also been implicated in C-F bond activation. Thus Cp*2ScH reacts with tetrafluoroethylene to afford Cp*2ScF either through an initial insertion of the tetrafluoroethylene into the scandium-hydride bond followed by β-fluoro elimination or by a σ-bond metathesis pathway (10). Another apparent example of β-fluoro elimination at a zirconium metal center has also been noted (Buchwald, S. L., Massachusetts Institute of Technology, personal communication, 1992).


6

5

PMe3

Cp Zr /

CH =CF 2

2

(1)

Cp Zr

2

2

-PMe

3

Attempts to prepare Cp Ti(CF ) invariably afford titanocene difluoride perhaps by an α-elimination pathway (11). Electron transfer is proposed to trigger C-F activation in tetrakis(trifluoromethyl)cyclopentadienone upon reaction with bis(cyclopentadienyl)titanacyclobutanes (12). Organometallic lanthanide and actinide complexes also extract fluoride from organofluorine compounds. Bis(pentamethylcyclopentadienyl)ytterbium reacts with hexafluorobenzene to afford the mixed valence dimer [Cp*2Yb]2(u-F) and Cp*2YbC6F5 (13). The mixed valence dimer was also isolated from similar, but slower reactions with fluorinated ethylenes, PhCF and PhF. Bis(pentamethylcylopentadienyl) complexes of Yb, Sm and Eu have been shown to abstract two fluoride ions from perfluorinated olefins to afford dienes (14). Tungsten lamp photolysis accelerates this defluorination reaction. Examples of C-F activation in the chemistry of (CôFs^Yb also have been noted (15). Fluorocarbons provide a novel mild fluorine source for the preparation of (MeC H4) UF from (MeC H4) UCMe and PhCF (16). These reactions occur under mild conditions and most appear to be irreversible due to the strong metal-fluoride bond formed. Although detailed mechanistic studies have not been reported, initial interaction of these hard, coordinatively unsaturated metals centers with a fluorine lone pair of the fluorocarbon seems likely followed by atom or electron transfer. 2

3

2

3

5

3

5

3

3

3

C-F Activation by Electron Rich Reagents n

Electron rich d (n = 6,8,10) complexes have also been demonstrated to activate C-F bonds by nucleophilic or oxidative addition pathways. Most common is nucleophilic displacement of fluoridefroma perfluoroaromatic or perfluoroolefinic substrate as typified by the chemistry of [CpFe(CO)2]- and related highly nucleophilic anions as in equation 2 (17).

In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

394

INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY

[CpFe(CO) r

+

2

[CpFe(CO) ]


2

_

+

C F 6

F C=CF 2

•

6

•

2

CpFe(CO) C F 2

6

5

CpFe(CO) CF=CF 2

+ 2

F +

F

Despite the strong C-F bond strength, fluoride is a good leaving group in these substitution reactions. Reactions such as these usually require highly fluorinated substrates for reasonable yields. No new metal fluorine bond is formed. Of particular interest to our research are reactions involving oxidative addition of a C-F bond by a transition metal complex which involve formation of both metal-fluorine and metal-carbon bonds. This transformation is the intellectual cousin of the important C-H oxidative addition process in hydrocarbon chemistry. The pioneering orr/io-metallation work carried out by Bruce and co-workers (18) showed that metal-carbon bond formation could be achievedfromfluorinated substrates such as perfluoroazobenzene at low valent metal centers, albeit under forcing conditions and with low yields (equation 3).

Similar reactions occur for Mn(CO) H and CpRu(PPh ) Me (19). The fate of the cleaved fluorine atom was not determined. In partially fluorinated systems, C-H and CF bond activation were found to be competitive (20). Pyrolysis of perfluoroalkyl iron(H) carbonyl complexes sometimes results in fluorocarbon products that involve shifts of C-F bonds although the role of the metal in these transformations is not clear. For example, heating CF3Fe(CO)4l to 160 °C affords tetrafluoroethylene and perfluoropropene perhaps by difluorocarbene formation by α-elimination (21). Quantitative formation of hexafluororcyclobutene is obtained in the decomposition of the octafluoroferrocyclopentane complex C4FgFe(CO)4 (21). βelimination is proposed to account for the formation of hexafluoropropene by thermolysis of CF3CF2CF2Fe(CO)4 (22). Vicinal defluorination has been observed in the reaction of octafluorocyclooctatetraene with dicobalt octacarbonyl forming the (μhexafluorocyclooctatrieneynedicobalt hexacarbonyl (23). In 1987, we reported the first high yield oxidative addition of a C-F bond to a transition metal center by treating the 1:1 Schiff base ligand derivedfrom1,2diaminobenzene and pentafluorobenzaldehyde with W(CO) (PrCN)3 (24). This transformation will be discussed in detail below. Later we demonstrated that ligands derived from M«5ym-N,N-dimethylethylened1amine and pentafluoro-, 2,6-difluoro- and even 2-fluoro- benzaldehydes undergo similar transformations, although heating to 60100 °C is required for the di- and mono-fluoro systems (25). In the latter case, C-F activation takes place even in the presence of the weaker C-H bond. Puddephatt and co-workers have employed these ligand systems at a dimethylplatinum(n) center (equation 4) (26). Interestingly, facile C-F activation is observed for the pentafluorophenyl system, but the product of C-H activation (trapped by loss of methane) is noted for the monofluoro ligand (equation 5). Crespo and co workers (27) have demonstrated that with proper ligand design (28), C-F activation takes place even in the presence of a weaker C-X (X = H, CI, Br) bonds (equation 6). These ligand based systems provided the first examples of high yield C-F oxidative addition and continue to afford excellent opportunities to study these reactions. 5

3 2

3



25. OSTERBERG AND RICHMOND


395

However, intermolecular C-F activation, as noted for some of the electron déficient reagents above, would be most important for catalytic or synthetic applications. Several electron rich systems hold promise in this area. An early report by Fahey and Mahan noted a 7% yield of thermally unstable irarc,s-Ni(PEt3)2(C6F5)FfromCe^e and Ni(PEt3) (COD) over a period of days at 35 °C (29). Although obtained rapidly in 65 % isolated yield, the product of benzoylfluoride oxidative addition, transNi(PEt3)2(COPh)F was also thermally unstable. An unusual case of C-F activation that may involve nickel was noted by Roundhill and co-workers in the novel synthesis of [PPh (C6F H-4)]Br obtained by hydrolysis following reaction of NiBr and CoFsBr in molten PPh at 200 °C (30). Somewhat better defined chemistry is observed for second and third row transition metals. Milstein and coworkers (31) treated with CH3lr(PEt3)3 at 60 °C to afford Ir(PEt3) (PEt2F)C F5 with the expulsion of CH4 and C2H4. In this unique transformation, a C-F bond is broken and new Ir-C and P-F bonds are formed but the metal does not undergo a net change in oxidation state. A multistep mechanism involving electron transferfroma cyclometallated Ir(III) intermediate to hexafluorobenzene, P-C bond cleavage, transfer offluoridefrom [CÔFO]- to iridium and finally to phosphorous was proposed. Only C-H activation is observed for fluorobenzene or 1,3,5-trifluorobenzene under similar conditions. Photochemical C-F activation is observed (equation 7) for Cp*Rh(PMe3)^2-C6F6) but no C-F activation is detected under thermal conditions (32). 2

3

4

2

3

2

6


396


PMeg

hv


Cp*Rh

(7)

Interestingly, the fluoro complex is rather unstable and scavenges chloridefromsolvent to yield the crystallographically characterized chloro complex. Most recently, Hoffmann and Unfiled (33) have reported the quantitative oxidative addition of to a 14-electron Pt(0) fragment generated by reductive eUmination of neopentane in neat (equation 8). M e

3

C N

/

C M e 3

(8)

Although not detected, an tfi-C^e intermediate is suspected. These examples show that both electron deficient and electronrichmetal centers with appropriate ligand complements can serve to activate C-F bonds in well defined reactions. In the case of the early transition and rare earth metals, bimolecular fluoride abstraction seems to be the dominant and irreversible reaction pathway while oxidative addition with formation of new metal-carbon and metal-fluorine bonds is possible for electron rich metals capable of undergoing two electron oxidation. Clearly much mechanistic work remains to be accomplished to understand these important transformations. Herein we report more details on our preliminary report (24) of C-F activation at tungsten(0) including its extension to molybdenum and nickel systems as well as qualitative observations concerning the mechanism of these transformations. Results Condensation in ethanol afford the fluorinated 1:1 Schiff base ligands 1-3 and 8 in high yield as yellow crystalline solids. The inline proton resonance and F NMR resonances are collected in Table I. Insertion of tungsten(O) into the ortho C-F bond of ligands 1-3 is achieved readily at room temperature in THF solution to afford the seven coordinate tungsten(II) products 4-6 in moderate to good yields as air and water stable red-orange crystalline solids (equation 9). The hydrogen bonding ability of the coordinated primary amine is demonstrated by die isolation of THF solvates which are stable to vacuum. Upon coordination to tungsten, the ligand imine resonance shifts downfield by 0.6 -1 ppm and the typical three band pattern in the carbonyl region of the IR spectrum shifts to higher energy relative to the W(CO)3(EtCN)3 starting material (34) (Table Π). Complexes 4-6 exhibit broad fluorine (vi « 25 hz) resonances near -200 ppm in their 19F NMR spectra assigned to the new tungsten-fluoride bond. This resonance shifts further upfield upon addition of water, alcohols or phenols capable of hydrogen bonding to the fluoride (35). 19

/2




397

(9)

1: F = 2,3A5,6-F 2: F = 2,3,5,6-F 3: F = 2,3-F n


n

4: F„.i = 2,3A5-F 5: F . ! = 2,4,5-F 6: F„.i =3-F

5

4

n

n

2

4

3

Table I. Selected NMR Data for Ligands and Metal Complexes Compound Imine

i 9

Aromatic F Resonances

1 2 3

8.75 8.73 8.87

-142.85(dd, 2); -151.16(t, 1); -162.32(q, 2) -139.50(m, 2); -143.64(m, 2) -139.07(t, 1); -146.98(t, 1)

4 5 6 7

9.72 9.70 9.47 9.42

-113.24 (ddd, 1); -139.99(t, 1); 153.56(t, 1), -164.03(t, 1) -89.27(d, 1); -144.56(m, 2) -87.38(m, 1) -114.10(dd, 1); -138.82(t, 1); -153.64(t, 1); -162.97(t, 1)

8 9 10

8.56 9.76 9.46

-141.01(t, 2); -152.00(m, 1); -163.51(q, 2) -115.84(dd, 1); -141.64(m, 1); -155.00(m, 1); -165.81(t, 1) -138.89(m, 2); -152.65(dd, 1); -161.31(m, 2)

11a lib

9.29

-138.91(dd, 2), -154.65(t, 1); -162.25(dd, 2) -136.40(d, 1), -138.44(d, 1); -152.80(t, 1); -161.51(br, 2)

12

8.48

-132.55(dd, 1); -140.52(t, 1); -148.51(t, 1); -161.88(t, 1)

—

aRecorded at 0 °C. bRecorded at -83 °C.

In addition, the fluorine adjacent to the newly formed tungsten-carbon bond shifts significandy to low field relative to the free ligand. The molecular structure of 4 was verified by X-ray crystallography as previously reported (24). The molybdenum analogue of 4 may be convenientiy prepared using Mo(CO)3(THF)3 generated in situ from the toluene complex (36), as starting material. An unknown deep maroon impurity can be removed by crystallization from THF/hexanes but results in the lower isolated yield than for 4.


398


—Ν

NH

N

Mo(CO) (PhMe)

2

3


PhMe

\ι/

F NH

2

(10)

/ \^CO OC CO

Spectroscopic and physical parameters of 7 are quite similar to 4 but the molybdenum complex is slightly air-sensitive.

Table II: Selected Data for Metal Carbonyl C-F Activation Products Compound 4 5 6 7 9

Yield(%)

δ M-F (ppm)a

69 35 61 40 67

-227.4 -231.5 -233.1 -206.1 -208.7

V G O (cm-!)

b

2020(m), 1937(s), 1906(s) 2017(m), 1933(s), 1900(s) 2012(m), 1927(s), 1892(s) 2026(s), 1952(s), 1917(s) 2020(m), 1937(s), 1907(s)

^Recorded in (CD ) CO. ^Recorded in THF. 3

2

The solubility of 4-7 is somewhat limited in solvents of low coordinating ability as a consequence of their hydrogen bonding properties which leads to the formation of headto-tail dimers in the solid state (37). Accordingly we have also explored the chemistry of the Ν,Ν-dimethyl ligand 8. Under the usual synthetic conditions a 67 % yield of oxidative addition product 9 was crystallized and spectroscopic data are in accord with the proposed structure (equation 11).

NMe2

•=N

W(CO) (EtCN) 3

CONMej

3

01)

\ l /

• 3 EtCN 8

10 Interestingly one of the diastereotopic methyl resonances exists as a doublet in the 13C{ Ή} NMR spectrum of 9 with 3 J = 7 Hz. The observation of three distinct CO resonances with coupling to fluoride (δ 234.44 ( 2 J = 41 Hz); δ 225.23 ( 2 J = 13 Hz); (δ 220.65 ( JCF = 8 Hz) indicating that the seven coordinate structure is static and C F

C F

2


C F



399


fluoride does not dissociate on the NMR time scale. The tetracarbonyl complex 10 was isolated from the supernatant in 16 % yield. The solution IR spectrum in the carbonyl region shows the expected 4-band pattern for this geometry. The imine resonance shifts downfield upon coordination but the i?F NMR spectra shows an intact pentafluorophenyl group. Additional insight into the nature of the tungsten(0)-ligand complex prior to oxidative addition is obtained by the generation of the purple mononitrile complex 11 by carrying out the ligand addition in dichloromethane and immediately isolating the precipitate that forms (equation 12).

The low field shift of the imine proton and strong metal carbonyl bands at 1907 and 1792 cm-i are consistent with the proposed structure. Dissolution of the solid 11 in THF results in quantitative spectroscopic conversionto4. Dynamic behavior is observed in the 19F NMR spectra of 11 with the most noticeable change being the inequivalence of the two ortho-fluonncs in the slow exchange limit spectrum recorded at -83 °C. Coalescence was observed at -30 °C and the expected three resonance pattern was obtained at 0 °C. Since later transition metals are often used in catalytic transformation of C-X bonds (38), we also investigated the reactivity of ligand 8 with Ni(0).

19

Four highly coupled aromatic fluorine resonances were observed in the F NMR spectra characteristic of the metallacycle structure. Although the Ni-F resonance could not be located in the 19F NMR spectra, its presence was inferred from the J C F coupling of 6 Hz observed to the carbons of the methyl groups of the ligand. In addition a parent ion was detected in the mass spectrum of 11. 3


400



Discussion Activation of aromatic carbon-fluorine bonds may be achieved at room temperature under mild condition by the use of appropriately designed chelating Schiff base ligands at low valent tungsten or molybdenum carbonyls. This is the first report of C-F bond activation at molybdenum. It is remarkable that the exceedingly strong C-F bond (154 kcal/mole for CeFe (39) ) can be cleaved under such mild conditions. Qualitatively, the pentafluorophenyl system reacts more readily than less heavily fluorinated ligands. For this class of ligands with a 1,2-diarninobenzene backbone, we were unable to detect CF activation in the ligand derivedfrom2,4-difluorobenzaldehyde although the 2,3isomer 3 does react. Kinetic studies show that C-Cl oxidative addition in similar systems is greatly accelerated by electron withdrawing groups on the aromatic ring (40, 41). This is consistent with a nucleophilic mechanism for C-F bond activation akin to nucleophilic aromatic substitution in organic systems (41). Both kinetic and thermodynamic factors appear to favor C-F, in preference to C-H activation in the chemistry of 3. The metal carbonyl fluoride products are stable while we have demonstrated in related systems that the putative tungsten hydride product of C-H activation is thermodynamically unstable and could not be detected even at -80 °C (41). In addition, the carbon of a C-F bond should be more electrophilic than that of a C-H bond favoring attack at the C-F bond by the metal nucelophile. A single crystal X-ray diffraction study of 10 was carried out since this provides a structural model of the metal-ligand complex before C-F bond activation. An ORTEP diagram with selected bond distances and angles is given in Figure 1. The bond distances and angles are unexceptional, but the favorable geometry for attack of an ortho-C-F bond is apparent and probably crucial for the facile reactivity observed. Dynamic 19F NMR studies of 11 provide further insight into the nature of the metalligand interaction prior to oxidative addition. The observation of inequivalent orthofluorines at low temperature suggest that rotation around the FsCô-irnine carbon bond is restricted by the crowded nature of the complex. At the coalescence temperature of -30 °C, AG* = 10.4(5) kcal/mol. Steric interactions between the pentafluorophenyl group and ligands bound to tungsten are probably responsible for this behavior. Thus, after dissociation of the nitrile ligand, complex 11 is in an excellent geometry to insert into the C-F bond. The chelating ligand is crucial in enabling use to observe this reactivity; we have been unable to promote bimolecular C-F activation of Cfie with W(CO)3(RCN)3 (42). The restricted conformation of the imine ligand once coordinated to the metal center seems most important in promoting C-F activation. We have extended this chemistry to the later transition metals in the preparation of the "homoleptic" Ni(0) complex of ligand 8. Based on its diamagnetic behavior, we propose a square planar geometry for 12. Thus our ligand complement also promotes C-F activation with the metal undergoing a dio to d transformation. The coordinatively unsaturated nature of 12 may allow for further reaction chemistry. 8

Future Prospects A wide variety of transition metals have now been shown to be capable of reacting with C-F bonds with highly fluorinated aromatic compounds most susceptible to attack (43). The stability of early transition metal-fluoride bonds suggests that catalytic reactions may be difficult to attain. Thus more work with low valent, later transition metal systems should be pursued. The microscopic reverse of oxidative addition—reductive elimination—may provide a new means of making C-F bonds but this has not yet been demonstrated. Saturated perfluorocarbons provide the next challenge for C-F bond activation by metal reagents (44). Understanding the organometallic chemistry of fluorocarbons may also lend insight into new strategies for destruction of chlorinated organic wastes and development of new synthetic methods for replacement CFC's.





F5

Figure 1. ORTEP representation of 10. Selected bond distances (Â): W-Nl, 2.351(3); W-N2, 2.241(4); W-Cl, 1.954(5); W-C2, 2.027(5); W-C3, 1.953(5); W-C4,2.012(5); N2-C13, 1.263(6). Selected bond angles (deg): N1-W-N2, 71.3(1); W-N2-C13, 132.6(3); N2-C13-C14, 120.1(4); Nl-W-Cl, 172.1(2); N2-W-C3, 168.7(2), C2-W-C4, 172.4(2). Reproduced with permission from ref. 40. Copyright 1990 Elsevier Science Publishers BV.


401

INORGANIC FLUORINE CHEMISTRY: TOWARD T H E 21ST CENTURY

402

Experimental Section All manipulations were carried out under a nitrogen atmosphere using standard Schlenk or glove box techniques. Solvents were distilledfromappropriate drying agents under nitrogen. FT-IR spectra were recorded in 0.1 mm CaF2 solution cells and a Varian XL300 spectrometer was used to record Ή , i3CpH} ^ (internal CCI3F reference) NMR spectra. Elemental analysis were carried out by Desert Analytics, Tucson, AZ. Literature preparations were used for the following starting materials: W(CO)3(PrCN)3 (34), Mo(CO) (C H ) (45), and Ni(COD) (46). Schiff base ligands were isolated in greater than 80% yields as yellow crystalline solids by filtration of the precipitate formed upon condensation of the diamine with the appropriatefluoro-tenzaldehydein ethanol. Selected Ή and 19F NMR data are given in Tables I and II. 4. A 100 mL flask was charged with 1 (0.572 g, 2.00 mmol) and W(CO) (PrCN) (0.878 g, 2.03 mmol) and 30 mL THF. After stirring for 10 min, the volume of the burgundy solution was reduced under vacuum and 8 mL hexanes were added. Cooling this solution to -10 °C afforded 4 (0.90 g, 1.4 mmol) as a THF solvate (MP, 159 °C, dec). Anal. Calcd for C20H15N2F5O4W: C, 38.36; H, 2.41; N, 4.47; F, 15.17. Found: C, 38.03; H, 2.27; N, 4.63; F, 15.36. 5. This compound was prepared in a similar manner to 4fromligand 2 but the reaction mixture was twice evaporated to dryness and redissolved in THF to remove released PrCN prior to crystallization (MP = 141-142 °C, dec). Anal. Calcd for Ci6H N2F 03WO.2 THF: C, 36.39; H, 1.74; N, 5.05. Found: C, 36.19; H, 1.94; N, 5.04. 6. This compound was prepared as for 4 using ligand 3 but allowed to stir overnight prior to crystallization (MP = 138, dec). Anal. Calcd for Ci H N2F O W.0.5 THF: C, 40.32; H, 2.63; N, 5.22. Found: C, 40.57; H, 2.98; N, 5.22. 7. In 25 mL of THF, Mo(CO) (PhMe) (1.07 g, 3.94 mmol) and 1 (1.10 g, 3.85 mmol) were combined. After stirring for 2 h, THF was removed under vacuum and the residue was extracted into 20 mL THF, filtered and evacuated to dryness. This solid was washed with CH2C1 to remove Mo(CO)6 and then recrystallized from THF/hexanes to afford orange highly crystalline 7 as a THF solvate (0.71 g, 1.5 mmol). Anal. Calcd for C20H15N2F5O4M0: C, 44.30; H, 2.79; N, 5.18. Found: C, 44.36; H, 2.75; N, 5.42. 9. This compound was prepared in a similar manner to 5 in 67% isolated yield from ligand 8 (MP = 168, dec). Anal. Calcd for C18H11N2F5O3W: C, 37.14; H, 1.90; N, 4.80. Found: C, 37.28; H, 1.86; N, 4.77. 10. Hexanes were added to the supernatant from the preparation of 9 and cooling to -10 °C produced orange-red crystals in 16 % isolated yield (MP 155-156 °C, dec). IR (THF): 2010(s), 1887(s), 1879(sh), 1844(s) cm-i. Anal. Calcd for C19H11N2F5C4W: C, 37.40; H, 1.82; N, 4.59. Found: C, 37.52; H, 1.69; N, 4.58. 12. Addition of 35 mL of THF to Ni(COD) (2.6 g, 9.5 mmol) and 8 produced a red solution. After stirring overnight, the THF and COD were removed under vacuum. Stirring with an additional 35 mL of THF gave a purple powder (3.0 g, 8.0 mmol) which was isolated by filtration in 86 % yield. A parent peak was observed at 372 amu in the mass spectrum. Anal. Calcd for C15H11N2F5N1: C, 48.31; H, 2.98; N, 7.47. Found: C, 48.17; H, 2.92; N, 7.51. Single crystals of 10 were obtained from THF/hexanes in the monoclinic space group C2/c with a = 22.785(3), b = 13.280(2), c = 11.634(2) Â, β = 116.41(1)° and Ζ = 8. Refinement based on 2521 data with I > 3σΙ collected on a Syntex PI (λ\ΐο Κα ~ 0.71073 Â, μ = 60.23 cm-i) and 280 variables gave R = 0.0240 and R = 0.0275 with GOF = 2.23. 1 9 r ?


3

3

8

6

8

7

8

2

3

4

2

3

3

2

2

w




403

Acknowledgments. We thank Dr. Atta M. Arif for solving the crystal structure reported in this work and the Presidential Young Investigator Program of the National Science Foundation for financial support. T. G. R. is an Alfred P. Sloan Research Fellow (1991-1993).


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

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