The F/Ph rearrangement reaction of [(Ph3P)3RhF], the fluoride congener of Wilkinson's catalyst

Published on Web 10/08/2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF], the

Fluoride Congener of Wilkinson’s Catalyst

Stuart A. Macgregor,*^,‡D. Christopher Roe,^†William J. Marshall,^†Karen M. Bloch,^†

Vladimir I. Bakhmutov,^§and Vladimir V. Grushin*^,†

Contribution from Central Research & DeVelopment, E. I. DuPont de Nemours and Company,

Inc., Experimental Station, Wilmington, Delaware 19880-0328, School of Engineering and

Physical Sciences, William H. Perkin Building, Heriot-Watt UniVersity,

Edinburgh EH14 4AS, U.K., and Department of Chemistry, Texas A&M UniVersity,

P.O. Box 30012, College Station, Texas 77842-3012

Received July 7, 2005; E-mail: vlad.grushin-1@usa.dupont.com; s.a.macgregor@hw.ac.uk

Abstract: The fluoride congener of Wilkinson’s catalyst, [(Ph₃P)₃RhF] (1), has been synthesized and fully

characterized. Unlike Wilkinson’s catalyst, 1 easily activates the inert C-Cl bond of ArCl (Ar ) Ph, p-tolyl)

under mild conditions (3 h at 80 °C) to produce trans-[(Ph₃P)₂Rh(Ph₂PF)(Cl)] (2) and ArPh as a result of

C-Cl, Rh-F, and P-C bond cleavage and C-C, Rh-Cl, and P-F bond formation. In benzene (2-3 h at

80 °C), 1 decomposes to a 1:1 mixture of trans-[(Ph₃P)₂Rh(Ph₂PF)(F)] (3) and the cyclometalated complex

[(Ph₃P)₂Rh(Ph₂PC₆H₄)] (4). Both the chloroarene activation and the thermal decomposition reactions have

been shown to occur via the facile and reversible F/Ph rearrangement reaction of 1 to cis-[(Ph₃P)₂Rh(Ph)-

(Ph₂PF)] (5), which has been isolated and fully characterized. Kinetic studies of the F/Ph rearrangement,

an intramolecular process not influenced by extra phosphine, have led to the determination of E_a) 22.7

q

( 1.2 kcal mol^-1, ∆H ^q) 22.0 ( 1.2 kcal mol^-1, and ∆S ) -10.0 ( 3.7 eu. Theoretical studies of F/Ph

exchange with the [(PH₃)₂(PH₂Ph)RhF] model system pointed to two possible mechanisms: (i) Ph transfer

to Rh followed by F transfer to P (formally oxidative addition followed by reductive elimination, pathway 1)

and (ii) F transfer to produce a metallophosphorane with subsequent Ph transfer to Rh (pathway 2). Although

pathway 1 cannot be ruled out completely, the metallophosphorane mechanism finds more support from

both our own and previously reported observations. Possible involvement of metallophosphorane

intermediates in various P-F, P-O, and P-C bond-forming reactions at a metal center is discussed.

generation of highly reactive “naked” fluoride,¹⁹late transition

metal fluorides have attracted much attention due to their

Introduction

The chemistry of late transition metal fluoride complexes has

constituted a new and expanding area for research in recent

years.^1-7In addition to interesting applications in synthesis and

catalysis,^8-16C-H activation,¹⁷materials science,¹⁸and the

uncommon chemical properties and bonding features.^1,5,7,20-52

(17) Cooper, A. C.; Folting, K.; Huffmann, J. C.; Caulton, K. G. Organometallics

1997, 16, 505.

(18) Lobban, J. International Patent Application WO 2003089443 A1, 2003.

(19) Grushin, V. V. Angew. Chem., Int. Ed. 1998, 37, 994.

(20) Poulton, J. T.; Sigalas, M. P.; Eisenstein, O.; Caulton, K. G. Inorg. Chem.

1993, 32, 5490.

^†DuPont CR&D.

^‡Heriot-Watt University.

^§Texas A&M University.

(21) Poulton, J. T.; Sigalas, M. P.; Folting, K.; Streib, W. E.; Eisenstein, O.;

Caulton, K. G. Inorg. Chem. 1994, 33, 1476.

(1) Doherty, N. M.; Hoffmann, N. W. Chem. ReV. 1991, 91, 553.

(2) Holloway, J. H.; Hope, E. G. J. Fluorine Chem. 1996, 76, 209.

(3) Murphy, E. F.; Murugavel, R.; Roesky, H. W. Chem. ReV. 1997, 97, 3425.

(4) Clark, H. C. S.; Holloway, J. H. AdV. Inorg. Fluorides 2000, 51.

(5) Mezzetti, A.; Becker, C. HelV. Chim. Acta 2002, 85, 2686. Also, see:

Becker, C.; Kieltsch, I.; Broggini, D.; Mezzetti, A. Inorg. Chem. 2003, 42,

8417.

(22) Cooper, A. C.; Bollinger, J. C.; Huffman, J. C.; Caulton, K. G. New J.

Chem. 1998, 22, 473.

(23) Coalter, J. N., III; Huffman, J. C.; Streib, W. E.; Caulton, K. G. Inorg.

Chem. 2000, 39, 3757.

(24) Huang, D.; Koren, P. R.; Folting, K.; Davidson, E. R.; Caulton, K. G. J.

Am. Chem. Soc. 2000, 122, 8916.

(6) Braun, T.; Perutz, R. N. Chem. Commun. 2002, 2749.

(7) Grushin, V. V. Chem. Eur. J. 2002, 8, 1006.

(8) Vaska, L.; Peone, J., Jr. J. Chem. Soc., Chem. Commun. 1971, 418. Vaska,

L.; Peone, J., Jr. Inorg. Synth. 1974, 15, 64.

(9) Jones, C. M.; Doherty, N. M. Polyhedron 1995, 14, 81.

(10) Cooper, A. C.; Caulton, K. G. Inorg. Chim. Acta 1996, 251, 41.

(11) Cooper, A. C.; Huffman, J. C.; Caulton, K. G. Inorg. Chim. Acta 1998,

270, 261.

(12) Dorta, R.; Egli, P.; Zuercher, F.; Togni, A. J. Am. Chem. Soc. 1997, 119,

10857.

(25) Caulton, K. G. New J. Chem. 1994, 18, 25.

(26) Veltheer, J. E.; Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1995, 117,

12478.

(27) Fraser, S. L.; Antipin, M. Yu.; Khroustalyov, V. N.; Grushin, V. V. J. Am.

Chem. Soc. 1997, 119, 4769.

(28) Pilon, M. C.; Grushin, V. V. Organometallics 1998, 17, 1774.

(29) Flemming, J. P.; Pilon, M. C.; Borbulevitch, O. Ya.; Antipin, M. Yu.;

Grushin, V. V. Inorg. Chim. Acta 1998, 280, 87.

(30) Marshall, W. J.; Thorn, D. L.; Grushin, V. V. Organometallics 1998, 17,

5427.

(13) Krueger, J.; Carreira, E. M. J. Am. Chem. Soc. 1998, 120, 837.

(14) Barthazy, P.; Togni, A.; Mezzetti, A. Organometallics 2001, 20, 3472.

(15) Vicente, J.; Gil-Rubio, J.; Guerrero-Leal, J.; Bautista, D. Organometallics

2004, 23, 4871.

(16) Renkema, K. B.; Werner-Zwanziger, U.; Pagel, M. D.; Caulton, K. G. J.

Mol. Catal. A 2004, 224, 125.

(31) Grushin, V. V. Organometallics 2000, 19, 1888.

(32) Grushin, V. V.; Marshall, W. J. Angew. Chem., Int. Ed. 2002, 41, 4476.

(33) Marshall, W. J.; Grushin, V. V. Organometallics 2003, 22, 555.

(34) Marshall, W. J.; Grushin, V. V. Organometallics 2004, 23, 3343.

(35) Clark, H. C. S.; Fawcett, J.; Holloway, J. H.; Hope, E. G.; Peck, L. A.;

Russell, D. R. J. Chem. Soc., Dalton Trans. 1998, 1249.

9

15304

J. AM. CHEM. SOC. 2005, 127, 15304-15321

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

It is now recognized that fluoro transition metal complexes are

distinctive, exhibiting reactivity patterns that are remarkably

different from those of their much better known chloro, bromo,

and iodo counterparts. This statement is strongly supported, for

example, by the fluoro congener of Wilkinson’s catalyst,

[(Ph₃P)₃RhF] (1).

[(Ph₃P)₂Rh(Ph₂PF)(Cl)] (2), as a result of C-Cl, Rh-F, and

P-C bond cleavage and C-C, Rh-Cl, and P-F bond forma-

tion. This unusual result provides an interesting comparison with

the “normal” oxidative addition of iodobenzene^53,56and p-

iodotoluene⁵⁷to the chloro and iodo analogues of 1. First, de-

spite the much higher reactivity of the C-I bond, the reac-

tions of [(Ph₃P)₃RhX] (X ) Cl, I) with PhI or TolI require

higher temperatures and longer reaction times to occur. Second,

the latter reactions lead to the formation of the expected

oxidative addition products, σ-aryl Rh(III) complexes [(Ph₃P)₂-

Rh(Ar)X₂], as a result of Ar-I bond cleavage and Rh-I and

Rh-Ar bond formation.^53,56,57

Recently, an efficient, simple synthesis of 1 was com-

municated along with its full characterization and unexpected

reactivity.⁵³On one hand, 1 appeared akin to its famous

counterpart [(Ph₃P)₃RhCl] (Wilkinson’s catalyst),⁵⁴in that both

displayed (i) similar geometry parameters in the solid state, (ii)

the same degree of phosphine dissociation in solution, and (iii)

the ability to decarbonylate DMF to give [(Ph₃P)₂Rh(CO)X]

(X ) F or Cl).⁵³On the other hand, both the thermal

decomposition of 1 in benzene and the reaction of 1 with

chlorobenzene or 4-chlorotoluene take paths that are not even

remotely similar to the reactivity of Wilkinson’s catalyst under

identical conditions. It is known⁵⁴that heating [(Ph₃P)₃RhCl]

in benzene or toluene leads to precipitation of [(Ph₃P)₄Rh₂(µ-

Cl)₂] due to PPh₃dissociation. The same dinuclear complex was

also produced in >95% selectivity when the reaction was

performed in chlorobenzene (100 °C, 2.5 h, eq 1) with only

trace amounts (<2%) of the oxidative addition product [(Ph₃P)₂-

Rh(Ph)Cl₂] being detected by ³¹P NMR.⁵³This is not surprising

because activation of the notoriously inert C-Cl bond of PhCl

and other unactivated chloroarenes with a Rh(I) or Pd(0) species

normally occurs only when the metal center bears a tertiary

phosphine that is bulkier and more electron rich than PPh₃.⁵⁵

Equally unexpected were both the ease and the outcome of

the thermal decomposition of 1 in benzene (eq 3).⁵³After 2-3

h at 80 °C, 1 was smoothly converted to a 1:1 mixture of trans-

[(Ph₃P)₂Rh(Ph₂PF)(F)] (3) containing Rh-F and P-F bonds,

and the long-known⁵⁸cyclometalated complex 4.

To account for the results described in eqs 2 and 3, a unifying

mechanistic scheme was proposed, involving rearrangement of

1 to [(Ph₃P)₂Rh(Ph)(Ph₂PF)] (5; cis or trans) as the first key

step (Scheme 1).⁵³If the fluorine and a phenyl in 1 interchanged

their positions, a σ-phenyl Rh(I) species would be formed (such

as 5), which one might expect⁵⁹to be electron-rich enough to

In sharp contrast,^55a1 was found to easily cleave the C-Cl

bond of chlorobenzene and p-chlorotoluene (TolCl) under

unexpectedly mild conditions (1-3 h at 80-100 °C), as shown

in eq 2.⁵³This reaction gave rise to a new Rh(I) complex, trans-

Scheme 1

(36) Tilset, M.; Fjeldahl, I.; Hamon, J.-R.; Hamon, P.; Toupet, L.; Saillard, J.-

Y.; Costuas, K.; Haynes, A. J. Am. Chem. Soc. 2001, 123, 9984. Tilset,

M.; Hamon, J.-R.; Hamon, P. Chem. Commun. 1998, 765.

(37) (a) Cronin, L.; Higgitt, C. L.; Karch, R.; Perutz, R. N. Organometallics

1997, 16, 4920. (b) Braun, T.; Parsons, S.; Perutz, R. N.; Voith, M.

Organometallics 1999, 18, 1710.

(38) Jasim, N. A.; Perutz, R. N.; Foxon, S. P.; Walton, P. H. J. Chem. Soc.,

Dalton Trans. 2001, 1676.

(39) Jasim, N. A.; Perutz, R. N.; Archibald, S. J. Dalton Trans. 2003, 2184.

(40) Reinhold, M.; McGrady, J. E.; Perutz, R. N. J. Am. Chem. Soc. 2004, 126,

5268.

(41) Gil-Rubio, J.; Weberndorfer, B.; Werner, H. J. Chem. Soc., Dalton Trans.

1999, 1437.

(42) Moigno, D.; Callejas-Gaspar, B.; Gil-Rubio, J.; Werner, H.; Kiefer, W. J.

Organomet. Chem. 2002, 661, 181.

(43) Gorol, M.; Moesch-Zanetti, N. C.; Roesky, H. W.; Noltemeyer, M.; Schmidt,

H.-G. Eur. J. Inorg. Chem. 2004, 2678.

(44) Barthazy, P.; Hintermann, L.; Stoop, R. M.; Worle, M.; Mezzetti, A.; Togni,

A. HelV. Chim. Acta 1999, 82, 2448. Barthazy, P.; Stoop, R. M.; Woerle,

M.; Togni, A.; Mezzetti, A. Organometallics 2000, 19, 2844.

(45) Barthazy, P.; Broggini, D.; Mezzetti, A. Can. J. Chem. 2001, 79, 904.

(46) Kirkham, M. S.; Mahon, M. F.; Whittlesey, M. K. Chem. Commun. 2001,

813.

(55) (a) For reviews, see: Grushin, V. V.; Alper, H. Chem. ReV. 1994, 94, 1047;

Top. Organomet. Chem. 1999, 3, 193. (b) For the earliest original

contributions to Pd/dippp-catalyzed reactions of nonactivated chloroarenes

from Milstein’s group, see: Ben-David, Y.; Portnoy, M.; Milstein, D. J.

Am. Chem. Soc. 1989, 111, 8742; J. Chem. Soc., Chem. Commun. 1989,

1816. For a full citation of Milstein’s catalytic Ar-Cl activation work, see

ref 55a. (c) For the original contribution to Cy₃P/Pd-catalyzed reactions of

chlorobenzene from Osborn’s group, see: Huser, M.; Youinou, M. T.;

Osborn, J. A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1386. For a full

citation of Osborn’s catalytic Ar-Cl activation work, see ref 55a. (d) For

early work on Ar-Cl activation, catalyzed by Rh complexes of bulky, basic

phosphines, see: Grushin, V. V.; Alper, H. Organometallics 1991, 10, 1620.

(e) For the discovery of the currently most popular t-Bu₃P-Pd catalytic

system for chloroarene activation, see: Nishiyama, M.; Koie, Y. Eur. Pat.

Appl. EP 802173 A1, 1997. Yamamoto, T.; Nishiyama, M.; Koie, Y.

Tetrahedron Lett. 1998, 39, 617 and 2367. More recently, t-Bu₃P has been

widely and successfully used by Littke and Fu for various Pd-catalyzed

cross-coupling reactions of chloroarenes.^55f(f) For a review, see: Littke,

A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176.

(47) Braun, T.; Noveski, D.; Neumann, B.; Stammler, H.-G. Angew. Chem.,

Int. Ed. 2002, 41, 2745.

(48) Noveski, D.; Braun, T.; Schulte, M.; Neumann, B.; Stammler, H.-G. Dalton

Trans. 2003, 4075.

(49) Noveski, D.; Braun, T.; Kruckemeier, S. J. Fluorine Chem. 2004, 125,

959.

(50) Nilsson, P.; Plamper, F.; Wendt, O. F. Organometallics 2003, 22, 5235.

(51) (a) Yahav, A.; Goldberg, I.; Vigalok, A. J. Am. Chem. Soc. 2003, 125,

13634. (b) Yahav, A.; Goldberg, I.; Vigalok, A. Inorg. Chem. 2005, 44,

1547.

(52) Vicente, J.; Gil-Rubio, J.; Bautista, D.; Sironi, A.; Masciocchi, N. Inorg.

Chem. 2004, 43, 5665.

(53) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2004, 126, 3068.

(54) Jardine, F. H. Prog. Inorg. Chem. 1981, 28, 63.

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15305

A R T I C L E S

Macgregor et al.

oxidatively add the C-Cl bond of chlorobenzene. A separate

experiment did indicate⁵³that [(Ph₃P)₃RhPh]⁵⁸readily reacted

with PhCl (neat; 80 °C; <1 h) to give [(Ph₃P)₃RhCl] and Ph₂

apparently via C-Cl oxidative addition and C-C reductive

elimination. Therefore, it seemed reasonable to propose that 5

would react with ArCl to produce A, followed by C-C reductive

elimination from A to give Ar-Ph and 2 (Scheme 1), both of

which were indeed the products of the reaction (eq 2). In the

absence of ArCl or any other reactive electrophile, 5 might

undergo P-ligand exchange with the as yet unreacted 1 (Scheme

1), resulting in the observed difluoride 3 (eq 3) and [(Ph₃P)₃-

RhPh]⁵⁸(B). The latter was previously found⁵³to undergo rapid

and clean cyclometalation to 4 even below 80 °C.

Although the proposed F/Ph rearrangement did account for

the reactivity of 1, no evidence was obtained for the intermediate

formation of 5, as shown in Scheme 1.⁵³To establish the mech-

anism of these uncommon reactions of 1 (eqs 2 and 3), we

undertook a detailed investigation of this chemistry. As a result

of these studies, the previously proposed F/Ph exchange reaction

of 1 was indeed observed. In this paper, we report the remark-

ably facile and reversible rearrangement of [(Ph₃P)₃RhF], 1, to

[(Ph₃P)₂Rh(Ph)(Ph₂PF)], 5 (cis form), and the results of both

experimental and theoretical studies of this intriguing exchange.

Figure 1. ORTEP drawing of 1 with thermal ellipsoids drawn to the 50%

probability level and all H atoms omitted for clarity.

Table 1. Geometry Parameters for 1 and [(Ph₃P)₃RhCl]

[(Ph₃P)₃RhCl] (ref 64)

parameter,

Å or deg

[(Ph₃P)₃RhF], 1

(this work)

orange

red

Rh-X

2.070(2)

2.193(1)

2.325(1)

166.9(1)

159.7(1)

2.404(4)

2.225(4)

2.304(4)

2.338(4)

166.7(2)

159.1(2)

2.376(4)

2.214(4)

2.322(4)

2.334(4)

156.2(2)

152.8(1)

Rh-P trans to X

Rh-P trans to P

X-Rh-P

Results

Synthesis and Reactivity of [(Ph₃P)₃RhF] (1). The fluoro

analogue of Wilkinson’s catalyst [(Ph₃P)₃RhF], 1, has been

previously mentioned in the literature,^9,60although without

proper characterization. We found that 1 can be effortlessly

prepared by reacting easily accessible [(COD)₂Rh₂(µ-OH)₂]⁶¹

with Et₃N(HF)₃(TREAT HF)⁶²and excess PPh₃in ether (eq

4). This method furnished analytically pure 1 as an orange-

yellow powder, which precipitated from the reaction mixture

in almost quantitative yield. Multigram quantities of 1 were

prepared using reaction 4. Less conveniently, 1 was synthesized

by reacting [(PPh₃)₄Rh₂(µ-OH)₂]⁶³with TREAT HF to give

[(PPh₃)₄Rh₂(µ-F)₂],⁵³which transformed to 1 upon treatment

with PPh₃.

P-Rh-P

dissociation from 1 (eq 5) in rigorously anhydrous benzene (25

°C) was determined by ³¹P NMR and calculated at ca. 30%,

similar to the value measured for Wilkinson’s catalyst under

identical conditions. In the solid state, 1 revealed a square-planar

geometry around Rh (Figure 1), with the coordination bond

angles and distances being strikingly similar to those⁶⁴of both

the red and the orange forms of Wilkinson’s catalyst (Table 1).

Heating 1 in chlorobenzene at 80 °C for 2-3 h resulted in

the formation of trans-[(Ph₃P)₂Rh(Ph₂PF)(Cl)] (2) and biphenyl,

as shown in eq 2. Repeating the experiment in 4-chlorotoluene

produced 4-methylbiphenyl instead of Ph₂(eq 2). Complex 2

was isolated and fully characterized by X-ray analysis (Figure

2) and NMR data (Table 2). The formation of biphenyl and

4-methylbiphenyl was confirmed by GC-MS.⁵³

Fluoride 1 also readily reacted with iodobenzene and bro-

mobenzene. In both cases, the reaction produced Rh(III)

complexes [(Ph₃P)₂Rh(Ph)X₂], where X ) I or Br, respec-

tively.⁵³It is likely that these reactions first produced [(Ph₃P)₂-

Rh(Ph₂PF)(X)] (X ) I or Br), much like the reaction of 1 with

PhCl (eq 2). Being much more reactive than PhCl, however,

PhI and PhBr would oxidatively add to the initially generated

[(Ph₃P)₂Rh(Ph₂PF)(X)] to give [(Ph₃P)₂Rh(Ph)X₂], a stable and

isolable Rh(III) species (eq 6). Both [(Ph₃P)₂Rh(Ph)Br₂] and

[(Ph₃P)₂Rh(Ph)I₂] were structurally characterized by X-ray

diffraction.⁵³The fluorinated phosphine, Ph₂PF, that is lost in

the second oxidative addition (eq 6) could not be observed due

to its poor stability.⁶⁵However, doublets with J_P-F) 665, 838,

971, and 1017 Hz observed in the range of -35 to -81 ppm in

Complex 1 is air-sensitive, especially in solution. It is

moderately soluble in benzene (ca. 0.6 g in 100 mL at 25 °C),

poorly soluble in toluene and THF, and insoluble in ether,

alkanes, and cycloalkanes. Chlorinated solvents such as dichlo-

romethane and chloroform were found to cause rapid decom-

position of 1. Like Wilkinson’s catalyst,⁵⁴1 also loses PPh₃in

benzene solution, producing [(PPh₃)₄Rh₂(µ-F)₂] (eq 5). This

process is reversible, with the equilibrium being established

within the time of dissolution. The degree of phosphine

(56) Mori, K.; Mizoroki, T.; Ozaki, T. Bull. Chem. Soc. Jpn. 1976, 49, 758.

(57) Ishiyama, T.; Hartwig, J. J. Am. Chem. Soc. 2000, 122, 12043.

(58) Keim, W. J. Organomet. Chem. 1968, 14, 179.

(59) Semmelhack, M. F.; Ryono, L. Tetrahedron Lett. 1973, 14, 2967.

(60) Van Gaal, H. L. M.; Van den Bekerom, F. L. A.; Verlaan, J. P. J. J.

Organomet. Chem. 1976, 114, C35.

(61) Uson, R.; Oro, L. A.; Cabeza, J. A. Inorg. Synth. 1985, 23, 126.

(62) McClinton, M. A. Aldrichimica Acta 1995, 28, 31. Haufe, G. J. Prakt.

Chem. 1996, 338, 99.

(63) Grushin, V. V.; Kuznetsov, V. F.; Bensimone, C.; Alper, H. Organometallics

1995, 14, 3927.

(64) Bennett, M. J.; Donaldson, P. B. Inorg. Chem. 1977, 16, 655.

9

15306 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

Table 2. ¹⁹F and ³¹P NMR Data for 1, 2, and 3 in Benzene-d₆at 20 °C

^aCoupling commonly observed in the presence of excess PPh₃and/or anhydrous CsF.

Figure 2. ORTEP drawing of 2 with thermal ellipsoids drawn to the 50%

probability level and all H atoms omitted for clarity.

Figure 3. ORTEP drawing of 3 with thermal ellipsoids drawn to the 50%

probability level and all H atoms omitted for clarity.

the ¹⁹F NMR spectra of the reaction mixtures provided firm

evidence for P-F bond formation.

for filtration of the reaction mixtures could lead to the formation

of the bifluoride analogue of 3, trans-[(Ph₃P)₂Rh(Ph₂PF)(FHF)],

6, probably due to partial hydrolysis of 3 on the cotton surface.

Although only small quantities of the bifluoride were produced,

a few single crystals of 6 were obtained and found suitable for

an X-ray diffraction study (Figure 4). Table 2 lists NMR data

for the new complexes 1, 2, and 3.

The thermal decomposition of 1 in benzene at 80 °C (eq 3)

readily occurred to >90% conversion after 3 h. The ³¹P and

¹⁹F NMR spectra of the reaction mixture indicated that two

complexes were cleanly produced, trans-[(Ph₃P)₂Rh(Ph₂PF)-

(F)] (3; see Table 2) and the cyclometalated complex 4. The

latter was identified by its characteristic ³¹P NMR spectrum,⁶⁶

whereas the structure of the former (3) was established based

on both the ³¹P and the ¹⁹F NMR data.⁵³Fractional crystalliza-

tion of the reaction mixture afforded X-ray quality crystals of

3 for structural characterization in the solid state (Figure 3).

During these studies, it was also found that using cotton wool

X-ray Structures of [(Ph₃P)₃RhF] and trans-[(Ph₃P)₂Rh-

(Ph₂PF)(X)] (X ) Cl, F, and FHF). Table 3 provides

coordination geometry parameters of the structurally related 1,

2, 3, and 6. A number of interesting conclusions can be drawn,

as follows.

(1) The P-Rh bonds for the Ph₂PF ligands in 2, 3, and 6 are

ca. 0.04-0.09 Å shorter than the Ph₃P-Rh bonds trans to X in

1 (X ) F) and in Wilkinson’s catalyst (X ) Cl, see Table 1).

The shorter bonds to the more π-acidic Ph₂PF ligand indicate

an important contribution of metal to P π-back-donation⁶⁷to

the P-Rh bonding⁶⁸involving an electron-rich metal center.

(2) The Rh-F bond is ca. 0.03 Å shorter when trans to

Ph₂PF (3) rather than to PPh₃(1). The importance of a

π-acceptor trans to F for stabilization of the M-F bond in

(65) (a) Brown, C.; Murray, M.; Schmutzler, R. J. Chem. Soc. C 1970, 878. (b)

Riesel, L.; Haenel, J.; Ohms, G. J. Fluorine Chem. 1988, 38, 335.

(66) Kuznetsov, V. F.; Yap, G. P. A.; Bensimon, C.; Alper, H. Inorg. Chim.

Acta 1998, 280, 172.

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15307

A R T I C L E S

Macgregor et al.

Table 3. Selected Geometry Parameters for 1, 2, 3, and 6

square-planar d⁸complexes has been discussed in reviews^1,7,25

and numerous experimental papers. For instance, the Pd-F bond

in trans-[(Ph₃P)₂Pd(Ar)F] has been found³⁰shorter, by ca. 0.035

Å, for Ar ) 4-NO₂C₆H₄than for Ar ) Ph. Also, stable

complexes of the type trans-[(i-Pr₃P)₂Rh(F)(L)] have been

reported⁴¹to form upon treatment of [(i-Pr₃P)₄Rh₂(µ-F)₂] with

L ) CO, CNR, PhCtCPh, or CH₂dCH₂but not with weaker

π-acids Py or MeCN. Because in square-planar d⁸transition

metal fluorides π-donation from F to the empty d(x²-y²) orbital

is symmetry forbidden, destabilizing filled/filled d_π-p_πM-F

repulsion²⁵can only be alleviated via back-donation from the

filled d orbitals on M to the vacant antibonding orbital of a

π-acidic ligand trans to F. Both push-pull interactions of the

lone pairs on F with a π-acceptor trans to it through filled

d-orbitals on the metal^25,69and electrostatic effects⁵are believed

to diminish the strength of the filled/filled repulsion and hence

stabilize the M-F bond. In contrast with the different Rh-F

bonds in 1 and 3, the Rh-Cl bonds in Wilkinson’s catalyst

(2.404(4) and 2.376(4) Å,⁶⁴see Table 1) and in 2 (2.396(1) Å)

are similar in length, indicating that the M-X filled/filled

repulsion diminishes considerably when going from X ) F to

X ) Cl.

(3) Complex 6 is a new member of the small family of

transition metal complexes containing bifluoride as a lig-

and.^{27,28,31,38,41,46,52,70-79}All geometry parameters found for 6

and the analogous fluoride 3 are similar, except the Rh-F bond

in 6 is elongated by ca. 0.04 Å, as a result of HF hydrogen

bonding. Likewise, the M-F bonds in [(COD)Rh(PPh₃)(FHF)]⁵²

and [(Ph₃P)₂Pd(Ph)(FHF)]⁷⁸are longer than those in [(COD)-

Rh(PPh₃)(F)]⁵²and [(Ph₃P)₂Pd(Ph)(F)],²⁷respectively. The F‚‚‚F

distance (2.329 Å) and the F-F-Rh angle (135.6°) in 6 are

within the range reported for other bifluoride complexes.

(4) All complexes 1, 2, 3, and 6 exhibit CH‚‚‚F interactions,

contacts that are shorter than the sum of the van der Waals radii

of fluorine (1.47 Å) and hydrogen (1.20 Å), a common

phenomenon in organometallic fluorine chemistry.⁸⁰It is

noteworthy that such short CH‚‚‚F contacts are also observed

for the fluorine atoms bonded to phosphorus in the Ph₂PF ligand

of 2, 3, and 6. Because in a recent publication,³⁴CH‚‚‚F contacts

in [(i-Pr₃P)₄Rh₂(µ-F)₂] were overlooked,⁸¹an ORTEP showing

these contacts is presented in Figure 5.

Basic Characteristics of the Thermal Decomposition of

1. Detection, Isolation, and Full Characterization of the

Intermediate, cis-[(Ph₃P)₂Rh(Ph)(Ph₂PF)], 5. The thermal

(67) For a review, see: Dias, P. B.; Minas de Piedade, M. E.; Martinho Simo˜es,

J. A. Coord. Chem. ReV. 1994, 135/136, 737.

(68) (a) Moloy, K. G.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117, 7696. (b)

Huang, J.; Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G. J. Am.

Chem. Soc. 1998, 120, 7806.

(69) Mayer, J. M. Comments Inorg. Chem. 1988, 8, 125.

(70) Coulson, D. R. J. Am. Chem. Soc. 1976, 98, 3111.

(71) Roesky, H. W.; Sotoodeh, M.; Xu, Y. M.; Schrumpf, F.; Noltemeyer, M.

Z. Anorg. Allg. Chem. 1990, 580, 131.

(72) Hintermann, S.; Pregosin, P. S.; Ru¨egger, H. J. Organomet. Chem. 1992,

435, 225.

(73) Whittlesey, M. K.; Perutz, R. N.; Greener, B.; Moore, M. H. Chem.

Commun. 1997, 187.

(74) Murphy, V. J.; Hascall, T.; Chen, J. Y.; Parkin, G. J. Am. Chem. Soc. 1996,

118, 7428. Murphy, V. J.; Rabinovich, D.; Hascall, T.; Klooster, W. T.;

Koetzle, T. F.; Parkin, G. J. Am. Chem. Soc. 1998, 120, 4372.

(75) Holzbock, J.; Sawodny, W.; Walz, L. Z. Kristallogr. 1997, 212, 115.

Bentrup, U.; Harms, K.; Massa, W.; Pebler, J. Solid State Sci. 2000, 2,

373.

(76) Braun, T.; Foxon, S. P.; Perutz, R. N.; Walton, P. H. Angew. Chem., Int.

Ed. 1999, 38, 3326.

(77) Jasim, N. A.; Perutz, R. N. J. Am. Chem. Soc. 2000, 122, 8685.

(78) Roe, D. C.; Marshall, W. J.; Davidson, F.; Soper, P. D.; Grushin, V. V.

Organometallics 2000, 19, 4575.

(79) Archibald, S. J.; Braun, T.; Gaunt, J. A.; Hobson, J. E.; Perutz, R. N. Dalton

Trans. 2000, 2013.

(80) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1,

277.

Figure 4. ORTEP drawing of 6 with thermal ellipsoids drawn to the 50%

probability level and all H atoms, except for FHF, omitted for clarity.

9

15308 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

Figure 6. ORTEP drawing of 5‚2C₆H₆with thermal ellipsoids drawn to

the 50% probability level. The solvent molecules and all H atoms are omitted

for clarity. Selected bond distances (Å) and bond angles (deg): Rh(1)-

C(13) 2.070(2); Rh(1)-P(1) 2.2192(5); Rh(1)-P(3) 2.3183(5); Rh(1)-P(2)

2.3189(5); P(1)-F(1) 1.604(1); C(13)-Rh(1)-P(1) 82.78(5); C(13)-Rh-

(1)-P(3) 170.55(5); P(1)-Rh(1)-P(3) 91.42(2); C(13)-Rh(1)-P(2) 88.03-

(5); P(1)-Rh(1)-P(2) 168.67(2); P(3)-Rh(1)-P(2) 98.53(2); F(1)-P(1)-

C(1) 98.40(8).

Figure 5. ORTEP drawing of [(i-Pr₃P)₄Rh₂(µ-F)₂] showing CH‚‚‚F contacts

that were previously missed.^34,81All other H atoms are omitted for clarity.

Thermal ellipsoids are drawn to the 50% probability level.

decomposition of 1 to 3 and 4 was chosen for studies aimed at

understanding the mechanism of the P-C bond cleavage and

P-F bond formation (eq 3). In a series of preliminary studies

by ¹⁹F and ³¹P NMR, some important observations were made,

as follows.

the ³¹P NMR spectra, pointing to P-F bond formation upon

thermolysis of 1 at 40-80 °C.

(1) The rate of reactions 2 and 3 was not influenced by free

PPh₃(up to 25 equiv), which was deliberately added to the reac-

tion mixtures. This observation was in accord with the enhanced

thermal stability of [(PPh₃)₄Rh₂(µ-F)₂], which forms upon

spontaneous dissociation of PPh₃from 1 in solution (eq 5). In

sharp contrast to >90% conversion of 1 after 3 h at 80 °C in

benzene, [(PPh₃)₄Rh₂(µ-F)₂] did not decompose under these

conditions. This result also indicated that no phosphine pre-

dissociation from 1 was required for reactions 2 and 3 to occur.

(2) In a series of separate experiments, it was found that the

cyclometalation of [(Ph₃P)₃RhPh], B,⁵⁸to 4 (Scheme 1) was

decelerated by extra PPh₃. The ambient temperature ³¹P NMR

spectrum of [(Ph₃P)₃RhPh], with or without extra PPh₃,

displayed an unsymmetrical doublet-like looking signal at 32.7

ppm (J ) 162.4 Hz). The cyclometalated product 4 exhibited

three resonances with the chemical shifts and coupling constants

being in full accord with the literature data.⁶⁶In a freshly

prepared benzene solution devoid of added PPh₃, B underwent

ca. 10% conversion to 4 after just 20 min at room temperature,

as indicated by ³¹P NMR. However, in the presence of 2 equiv

of extra PPh₃, B was much less prone to cyclometalation, and

only trace amounts of 4 (<5%) were formed after 3 days. At

80 °C, B transformed to 4 rapidly, even in the presence of a

10-fold excess of PPh₃.

Having made these observations, we attempted isolation of

this intermediate. Preliminary NMR kinetic studies were

conducted on the thermal decomposition of 1 in the presence

of extra PPh₃. The latter was used (i) to shift equilibrium 5 to

the reactive 1, thus minimizing the presence of phosphine-

deficient, much more thermally stable [(PPh₃)₄Rh₂(µ-F)₂], and

(ii) to slow the irreversible cyclometalation of B (Scheme 1),

which would drive all of the equilibria to the final products, 3

and 4. It was found that the highest concentration of the

intermediate could be reached after 1 and PPh₃(4 equiv) were

heated in benzene at 70 °C for 1 h. These conditions were then

used to scale-up the reaction for isolation of the intermediate.

It was found that unlike the starting material (1) and the product

(3), the intermediate was soluble in ether, albeit only in the

presence of extra PPh₃. This observation led to the development

of an efficient procedure for isolation of the pure intermediate

in 19-29% yield.

Single-crystal X-ray diffraction revealed the structure of the

intermediate as cis-[(Ph₃P)₂Rh(Ph)(Ph₂PF)] (5; Figure 6).⁸²This

product of direct positional exchange between the fluorine and

a phenyl in 1 was the “missing link” of the proposed mechanistic

model, as shown in Scheme 1. Computer simulation of an

ABMXY spin system (A, B, and M ) ³¹P; X ) ¹⁹F; Y ) ¹⁰³Rh)

produced patterns that are in excellent agreement with the

experimental ¹⁹F and ³¹P NMR spectra of 5 (Figures 7 and 8).

A weak broadened doublet at -132 ppm with J_P-F) ca. 860

Hz was commonly observed in the ¹⁹F NMR spectrum of

purified samples of 5 (Figure 7). Because of the low content

(ca. 5% of 5), no attempt was made to characterize this species.

We carefully propose that the weak doublet might be from the

trans isomer of 5 or perhaps from a dimeric species such as

[(PPh₃)₂(PPh₂F)₂Rh₂(µ-F)₂].

(3) In the very slow decomposition reaction of 1 at a low

temperature (25-40 °C), the appearance and subsequent disap-

pearance of B was observed. Even more pronounced was an

intermediacy of another species, which displayed signals with

a large coupling constant J_P-F) 869 Hz in both the ¹⁹F and

(81) W.J.M. and V.V.G. are thankful to Professor Santiago Alvarez and Dr.

Gabriel Aullo´n for bringing this to our attention. Previously, intramolecular

CH‚‚‚F contacts in [(i-Pr₃P)₄Rh₂(µ-F)₂] were missed³⁴using the Cambridge

Mercury program, version 1.3. This is mentioned as a cautionary note that

clicking on “Short Contact < (sum of vdw radii)” does not include

intramolecular contacts unless they are more explicitly defined using the

sub-menu “Define short contacts”.

(82) Two solvates of 5, cis-[(Ph₃P)₂Rh(Ph)(Ph₂PF)]‚Et₂O and cis-[(Ph₃P)₂Rh-

(Ph)(Ph₂PF)]‚2C₆H₆, were studied by single-crystal X-ray diffraction.

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15309

A R T I C L E S

Macgregor et al.

and B, with the latter undergoing the facile irreversible

cyclometalation to 4. Preliminary observations regarding the

thermolysis kinetics of 1 indicated that varying the concentration

of PPh₃did not have a significant impact on the rate (see above).

Likewise, dilution of the starting solution by approximately a

factor of 3 did not lead to a marked decrease in the observed

reaction rate. Therefore, to suppress the formation of the dimer

[(PPh₃)₄Rh₂(µ-F)₂], the kinetic experiments were run in the

presence of a 12-fold excess of extra PPh₃. In this way, the

equilibrium between 1 and the dimer (eq 5) was efficiently

shifted toward 1 without influencing the rate of exchange under

study (eq 7 and Scheme 2). Also, it was reasoned that because

the formation of 3 and B via ligand exchange between 1 and 5

is second order, the fluorine transfer step could be isolated

kinetically by using as low a concentration of 1 or 5 as was

practicable for kinetic observations. Indeed, it was established

that for [1] or [5] ) 4 × 10^-3mol dm^-3reaction 7 was

amenable to a kinetic study by ¹⁹F NMR. At this concentration,

the undesired bimolecular P ligand exchange processes leading

to 3 and B were slow enough to remain kinetically insignificant

in the 30-60 °C temperature range.

Figure 7. Experimental (top) and simulated (bottom) ¹⁹F NMR spectra of

5 in benzene (20 °C). The simulations lead to the following parameters

characterizing the ABMXY spin system: A, PPh₃trans to PFPh₂, 36.6 ppm,

cis-²J_P-P) -29.1 Hz, trans-²J_P-P) 383.1 Hz, trans-³J_P-F) 39.4 Hz,

¹J_P-Rh) -154.8 Hz; B, PPh₃cis to PFPh₂, 37.9 ppm, cis-²J_P-P) -40.3

Hz, cis-³J_P-F) 41.6 Hz, ¹J_P-Rh) -122.0 Hz; M, PFPh₂, 171.9 ppm, ¹J_P-F

1

2

) 868.6 Hz, J_P-Rh) -214.6 Hz; X, PFPh₂, -131.0 ppm, J_F-Rh

)

10.0 Hz.

The F-Ph Rearrangement of 1 to 5 Is Reversible. Kinetic

Studies of the F/Ph Exchange. It was found that 5 readily

decomposes in benzene at 80 °C to give cleanly 3 and 4, just

as if 1 rather than 5 were being used for the reaction (eq 3).

When the thermal decomposition of 5 at 40-60 °C was

monitored by ¹⁹F and ³¹P NMR, the intermediate formation of

1 was observed. These experiments indicated that the F/Ph

exchange reaction of 1 to produce 5 is reversible (eq 7). To

gain insight into the mechanism of this Ph/F rearrangement, a

kinetic study of reaction 7 was carried out, with the equilibrium

being approached from both sides.

Scheme 2

The results of one kinetic run, at 40 °C, are presented in

Figure 9, showing the approach to equilibrium starting from 1

(A) and from 5 (B). Analogous results of the other runs at 30,

50, 60, and 70 °C can be found in the Supporting Information.

As can be seen from Scheme 2, two side processes could

complicate the kinetic measurements (¹⁹F NMR) of the Ph/F

exchange (eq 7), (i) the dimerization of 1 to give [(PPh₃)₄Rh₂-

(µ-F)₂] and (ii) ligand exchange between 1 and 5 to produce 3

Figure 8. Experimental (top) and simulated (bottom) ³¹P NMR spectra of 5 in benzene (20 °C). The simulations lead to the following parameters characterizing

1

the ABMXY spin system: A, PPh₃trans to PFPh₂, 36.6 ppm, cis-²J_P-P) -29.1 Hz, trans-²J_P-P) 383.1 Hz, trans-³J_P-F) 39.4 Hz, J_P-Rh) -154.8 Hz;

1

B, PPh₃cis to PFPh₂, 37.9 ppm, cis-²J_P-P) -40.3 Hz, cis-³J_P-F) 41.6 Hz, J_P-Rh) -122.0 Hz; M, PFPh₂, 171.9 ppm, J_P-F) 868.6 Hz, J_P-Rh

)

2

-214.6 Hz; X, PFPh₂, -131.0 ppm, J_F-Rh) 10.0 Hz.

9

15310 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

the limit of our ability to obtain meaningful data. The trend in

the values obtained for A_eqmand B_eqmin the range 30-60 °C is

for K_eqto decrease slightly from 0.9 to 0.6, whereas at 70 °C

the estimated parameters lead to K_eqcloser to 1.2. The data at

70 °C are therefore somewhat suspect, although the rate estimate

at this temperature does not appear to have a significant impact

on the determination of the activation parameters.

Table 4. Estimated Rate Constants for the Conversion of 1 to 5

as a Function of Temperature

1

temp (°C)

k₁(s^-

,

×

10⁵)

30

40

50

60

70

0.51

1.90

5.38

14.7

43.6

The rate constants obtained in a benzene-benzene-d₆mixture

by this approach are presented in Table 4. The data in Table 4

are well described by the transition state treatment of the

temperature dependence of reaction rate constants as judged by

the linearity of the corresponding Eyring plot in Figure 10. The

activation enthalpy (∆H ^q) and entropy (∆S ^q) were determined

by direct nonlinear least-squares fitting of these parameters to

the Eyring equation (k₁) (k_BT/h) exp(∆S ^q/R) exp(-∆H ^q/RT))

using the temperature and rate constant data in Table 4. This

fitting procedure gave the parameter estimates ∆H ^q) 22.0 (

1.2 kcal mol^-1and ∆S ^q) -10.0 ( 3.7 eu (the corresponding

Arrhenius parameters are E_a) 22.7 ( 1.2 kcal mol^-1and

log(A/s^-1) ) 11.1 ( 0.8). The stated uncertainties represent 95%

confidence limits and were obtained using an estimated error

in the rate constants of 10% and an estimated error in

temperature of 2 °C. When the rate constant at 70 °C was

omitted from the analysis (see above), the estimated activation

Figure 9. (A) The approach to equilibrium at 40 °C starting from 1 (O),

and (B) starting from 5 (0).

q

parameters were ∆H ) 21.8 ( 2.6 kcal mol^-1and ∆S

)

-10.8 ( 8.0 eu.

The approach to equilibrium was modeled according to standard

kinetic equations. ⁸³

A(t) ) (A_init- A_eqm) exp(-(k₁+ k_-1)t) + A_eqm

B(t) ) (B_init- B_eqm) exp(-(k₁+ k_-1)t) + B_eqm

where A(t) and B(t) are the observed intensities at time t, the

subscripts “init” and “eqm” refer to initial and equilibrium

intensities, k₁is the forward rate constant (1 going to 5), and

k_-1is the reverse rate constant. The combined data sets were

subjected to nonlinear least-squares optimization using initial

intensities as parameters that were necessarily distinct between

the two experiments, along with the single best-fitting rate

constant (k₁+ k_-1) and equilibrium intensities consistent with

the entire data set. The best-fitting parameters were used to

obtain the smooth curves in Figure 9 and other kinetic plots

(see Supporting Information), and the rate constant of interest

(k₁) was obtained from the equilibrium intensities, k₁) (k₁+

k_-1)/(1 + A_eqm/B_eqm). At 30 °C, only the reverse reaction could

be analyzed due to the lack of solubility of 1. This, however,

was not a problem for the reverse process as 5 is much more

easily soluble in benzene than 1. At 70 °C, we were clearly at

Figure 10. Eyring plot for the rate data presented in Table 4.

Solvent Effects on the F/Ph Rearrangement. In view of

the possibility of solvent participation in the reaction, the

reaction of 1 at 60 °C was repeated in THF-d₈and led to a rate

constant of 19.1 × 10^-5s^-1(vs 14.7 × 10^-5s^-1in benzene).

Hence, in this case the reaction was not strongly influenced by

solvent.

A series of semiquantitative studies indicated that in benzene-

acetonitrile (78:22 by volume) and in benzene-DMSO (78:22

by volume), the reaction of 5 in the presence of PPh₃to 1 was

faster. Moreover, both MeCN and DMSO promoted phosphine

ligand exchange (Scheme 2), the secondary processes leading

(83) Cox, B. G. Modern Liquid-Phase Kinetics; Oxford University Press:

Oxford, UK, 1994.

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15311

A R T I C L E S

Macgregor et al.

Scheme 3

to [(Ph₂PF)(Ph₃P)₂RhF] (3) and [(Ph₃P)₃RhPh] (B). The latter

did not undergo cyclometalation (³¹P NMR) because the

reactions were run at a low temperature (25-45 °C) in the

presence of a large excess of PPh₃. The reaction of 1 to 5 was

not observed in these solvent mixtures. A precise quantitative

kinetic study in benzene-MeCN or benzene-DMSO could not

be carried out due to the facilitated secondary processes, that

is, P-ligand exchange (see above). When nitrobenzene was used

instead of MeCN or DMSO, the reaction of 1 to 5 was slower

than in pure benzene (by approximately a factor of 2), and the

reaction of 5 to 1 was strongly accelerated, giving ca. 80%

conversion at >90% selectivity to 1 versus <5% conversion

without PhNO₂. Unlike MeCN and DMSO, nitrobenzene did

not promote any secondary ligand exchange processes at all.

of us has identified computationally a metallophosphorane

formed by methoxide transfer from Pt to phosphine as a low

energy intermediate in the isomerization processes of [(H₂PCH₂-

CH₂PH₂)(CO)Pt(Me)(OMe)] model systems.⁸⁸Moreover, the

transfer of an aryl group from a phosphoranide ligand to an

iridium metal center has also been reported.⁸⁹Thus, there is

precedent for both type of processes involved in pathway 2,

the transfer of a hard ligand, such as fluoride, from metal to

phosphine and the transformation of a metallophosphorane into

a metal-phosphine/aryl complex.

Computed profiles for F/Ph exchange via pathway 1 are

shown in Figures 11 (for cis-1′, pathway 1a) and 12 (for

trans-1′, pathway 1b). Figure 11 also includes a comparison of

the computed geometries of cis-1′ and cis-5′ with those

determined crystallographically for 1 and 5. The computed

metal-ligand distances are well reproduced, in particular the

shortening of the Rh-P bond trans to F and the Rh-PH₂F

distance in cis-5′, where the latter is ca. 0.1 Å shorter than the

Rh-PH₃bonds. The Rh-F distance in cis-1′ is somewhat too

short in our calculations, probably due to the simplification of

the phosphine ligands in our model. Indeed, in a second form

of cis-1′ in which a different orientation of the PH₂Ph ligand

allows a short ortho-C-H‚‚‚F distance of 2.108 Å to develop,

a slightly longer Rh-F bond is computed (2.022 vs 2.005 Å),

suggesting that such C-H‚‚‚F contacts can lengthen metal-

fluoride bonds. Overall the product of F/Ph exchange, cis-5′, is

computed to be 2.3 kcal/mol less stable than the model reactant

cis-1′, a trend that is consistent with the equilibrium lying

slightly toward 1 experimentally (25 °C, benzene).

The solvent effects observed were analyzed against the

dielectric constant ꢀ⁸⁴and Gutmann’s donor and acceptor

numbers⁸⁵of the solvents (benzene, ꢀ ) 2.28, DN ) 0.1, AN

) 8.2; THF, ꢀ ) 7.52, DN ) 20.0, AN ) 8.0; MeCN, ꢀ )

36.64, DN ) 14.1, AN ) 19.3; DMSO, ꢀ ) 47.24, DN ) 29.8,

AN ) 19.3; PhNO₂, ꢀ ) 35.6, DN ) 4.4, AN ) 14.8). The

reaction occurred at approximately the same rate in benzene

and in slightly more polar but much more coordinating THF.

All three highly polar solvents (MeCN, DMSO, and PhNO₂)

accelerated the reaction of 5 to 1 and slowed the reverse process.

Of these three polar solvents, the ones with higher donor

numbers (MeCN and DMSO) also promoted the secondary

P-ligand exchange processes, whereas poorly donating nitroben-

zene did not. In addition to the polarity and donicity factors,

the solvent’s ability to form a hydrogen bond to the fluoride on

Rh may also need to be taken into consideration, as MeCN and

DMSO are better H-bond donors than benzene, nitrobenzene,

and THF.

Theoretical Studies on the F/Ph Rearrangement of 1 to

Give 5. We have employed density functional calculations

to study F/Ph exchange in 1 using simplified models of the

type [(PH₃)₂(PH₂Ph)RhF], 1′. Two general mechanistic pos-

sibilities were considered that differ in the order of transfer of

the Ph and F groups to and from the metal (Scheme 3). Path-

way 1 proceeds via Ph group transfer from phosphine to Rh

in what is formally an oxidative addition step. The Rh-phos-

phide intermediate can then undergo P-F bond-forming re-

ductive elimination to produce [(PH₃)₂(PH₂F)RhPh], 5′. In

principle, Ph group transfer may occur from a phosphine either

cis to F (cis-1′, as shown in Scheme 3) or from a site trans to

F (trans-1′). Pathway 2 is characterized by an initial transfer of

F from Rh to phosphine to form a metallophosphorane, from

which Ph can transfer back to Rh to complete the F/Ph exchange

process. The nature of this metallophosphorane mechanism

necessitates that it will originate from cis-1′. In the following,

it will be useful to consider metallophosphoranes formally as

complexes between an anionic [PH₂FPh]^-phosphoranide ligand

and a Rh(I) metal center.

The first step in pathway 1a, transfer of Ph from phosphorus

to Rh, proceeds with a dramatic shortening of the Rh‚‚‚C(ipso)

distance from ca. 3.6 Å in cis-1′ to only 2.22 Å in TS_cis-1′-I1

.

This has the effect of pushing the {PH₂} moiety out of the metal

coordination plane with the result that the phosphide ligand

eventually moves into the axial position in the Rh(III) inter-

mediate (I1, E ) +10.4 kcal/mol). The activation barrier for

P-C bond activation is computed to be 24.7 kcal/mol, and

during this process the phenyl ring adopts the perpendicular

orientation predicted from extended Hu¨ckel calculations.⁹⁰To

our knowledge, this is the first time that the activation of a P-aryl

bond by a transition metal center has been studied using high-

level calculations.⁹¹Interestingly, this process does not cor-

respond to a simple Ph group transfer from phosphorus to metal,

as this would be expected to place Ph in the axial position in

Metallophosphoranes⁸⁶similar to that in Scheme 3 have been

proposed as intermediates in the fluoride-promoted dispropor-

tionation of Pd(II) and Pt(II) phosphine complexes,⁸⁷and one

(84) Lide, D. R. Handbook of Organic SolVents; CRC Press: Boca Raton, FL,

1995.

(85) Gutmann, V. Electrochim. Acta 1976, 21, 661.

(86) For a review of metallophosphoranes, see: Nakazawa, H.; Kubo, K.;

Miyoshi, K. Bull. Chem. Soc. Jpn. 2001, 74, 2255.

(87) Mason, M. R.; Verkade, J. G. Organometallics 1992, 11, 2212.

(88) Macgregor, S. A.; Neave, G. W. Organometallics 2004, 23, 891.

(89) Kajiyama, K.; Nakamoto, A.; Miyazawa, S.; Miyamoto, T. K. Chem. Lett.

2003, 32, 332.

(90) Ortiz, J. V.; Havias, Z.; Hoffmann, R. HelV. Chim. Acta 1984, 67, 1.

9

15312 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

Figure 11. Computed reaction profile (kcal/mol) for pathway 1a in vacuo and in DMSO (ꢀ ) 46.7; italicized) with selected distances in angstroms. For

cis-1′ and cis-5′, comparisons with the experimental distances from 1 and 5 are given in parentheses. In the drawing of cis-1′, the value of 1.836 Å shown

in parentheses is the average of the experimental P-C distances for the cis PPh₃ligands of 1.

Figure 12. Computed reaction profile (kcal/mol) for pathway 1b in vacuo and in DMSO (ꢀ ) 46.7; italicized) with selected distances in angstroms.

I1. Instead, P-C activation appears to involve extrusion of the

{PH₂} moiety; the reverse process from I1 to cis-1′ could

certainly be viewed as PH₂insertion into the Rh-Ph bond.

The geometry of I1, with PH₂cis to F, is set up for P-F

bond-forming reductive elimination, and a transition state

corresponding to this process was located (TS_I1-I2, E ) +30.3

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15313

A R T I C L E S

Macgregor et al.

Figure 13. Computed reaction profile (kcal/mol) for pathway 2a in vacuo and in DMSO (ꢀ ) 46.7; italicized) with selected distances in angstroms.

kcal/mol). Unexpectedly, however, this transition state led to a

new intermediate (I2, E ) +30.3 kcal/mol),⁹²where the PH₂F

ligand appears to bind in an agostic fashion to the metal cen-

ter via the P-F bond (P‚‚‚F ) 1.79 Å). A transition state

to complete the P-F bond formation process was then located

by lengthening the Rh‚‚‚F distance (TS_I2-cis-5′, E ) +31.1

kcal/mol), and this was found to lead to the model F/Ph

exchange product, cis-5′. Intermediate I2 therefore exists as

a very shallow minimum, but IRC calculations do confirm

that both TS_I1-I2and TS_I2-cis-5′link to this structure. Overall,

TS_I2-cis-5′is the highest point along the pathway for conversion

of cis-1′ to cis-5′ via pathway 1a, corresponding to a total

computed activation barrier of 31.1 kcal/mol.

In pathway 1b, Ph group transfer from trans-1′ (E ) -0.3

kcal/mol) proceeds through transition state TS_{trans-1′-I3}(E )

+30.0) to give a new five-coordinate intermediate I3 (E )

+13.1). In sharp contrast to pathway 1a, P-C bond cleavage

in this case does correspond to a Ph group transfer with the

{PH₂} moiety barely moving out of the metal coordination

plane. I3 exhibits a distorted trigonal bipyramidal Y-shaped

geometry with axial PH₃ligands and large F-Rh-C(ipso) and

F-Rh-PH₂angles of 133° and 139°, respectively. Such a

structure is typical of five-coordinate d⁶metal centers featuring

a π-donor ligand.⁹³P-F bond formation from I3 therefore

requires a significant movement of the PH₂ligand, and the

F-Rh-PH₂angle narrows accordingly to a value of 53° in

TS_I3-I4(E ) +25.4 kcal/mol). In a similar way to TS_I1-I2in

pathway 1a, TS_I3-I4does not lead directly to the final product

but instead to an η²-P,F-agostic intermediate (I4, E ) +24.3

kcal/ mol), and a further transition state has to be located to

complete the P-F bond formation process (TS_I4-cis-5′, E )

+24.2).⁹²The structure of TS_I4-cis-5′is rather unusual, however,

being reminiscent of a trigonal bipyramid with one missing axial

ligand. IRC and geometry optimization calculations beyond this

transition state showed that TS_I4-cis-5′links to the F/Ph exchange

product cis-5′ and not, as might have been initially expected

from the geometry of I3, to the alternative form, trans-5′. All

attempts to locate a transition state linking I4 directly to

trans-5′ failed, and converged instead on structures that

ultimately would lead to cis-5′. Overall, the barrier associated

with the conversion of trans-1′ to cis-5′ is +30.0 kcal/mol,

slightly lower than that along pathway 1a, although in this case

the highest point along the profile corresponds to the P-C

activation rather than the P-F bond-forming step.

For pathway 2, two routes for F/Ph exchange via a metallo-

phosphorane structure have been characterized, referred to in

the following as pathways 2a and 2b. The computed profile for

pathway 2a is shown in Figure 13. We initially searched for a

metallophosphorane pathway by systematically decreasing the

relevant P‚‚‚F distance. When this led only to a steady in-

crease in energy, we coupled this motion to rotation about the

Rh-PH₂Ph bond, and this eventually allowed us to locate a

metallophosphorane structure, however not as an inter-

mediate but as a transition state (TS _cis-1′-I5, E ) +36.9 kcal/

mol). The five-coordinate phosphorus center in this species

exhibits a trigonal bipyramidal geometry, with the exchanging

F and Ph in axial positions (angles at P: C_ax-P-Rh )

94.3°; F_ax-P-Rh ) 84.0°; H_eq-P-Rh ) 119°/130°). The

Rh-PH₂FPh bond (2.30 Å) is similar in length to the

Rh-PH₂Ph bond computed in cis-1′, although the elongation

of the trans Rh-PH₃distance (to 2.33 Å from 2.30 Å in cis-1′)

suggests a slightly higher ligand trans influence for PH₂FPh as

compared to PH₂Ph. IRC and geometry optimization calculations

beyond TS_cis-1′-I5led to the new intermediate, I5 (E ) +13.4).

This structure is formed by rotation around the Rh-PH₂FPh

(91) Hoffmann⁹⁰reported a barrier of only 1.8 kcal/mol with EHMO calculations

for Ph group transfer in a [PdH₃(PH₂Ph)]^-model. However, this figure is

relative to a [PdH₃(PH₂Ph)]^-reactant in which the PH₂Ph ligand has a

highly distorted geometry and so omits the reorganization energy associated

with this distortion.

(92) The energy of I2 is computed to be 0.3 kcal/mol more stable than that of

TS_I1-I2before the inclusion of zero-point energy corrections. Similarly, I4

is 0.2 kcal/mol more stable than TS_I4-cis-5′prior to this correction.

(93) Riehl, J.-F.; Jean, Y.; Eisenstein, O.; Pe´lissier, M. Organometallics 1992,

11, 729.

9

15314 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

Figure 14. Computed reaction profile (kcal/mol) for pathway 2b in vacuo and in DMSO (ꢀ ) 46.7; italicized) with selected distances in angstroms.

bond to bring the phenyl ring into proximity with the metal,

allowing an interaction between Rh and the phenyl ring to be

established (Rh-C(ipso) ) 2.25 Å). The {PH₂FPh} moiety is

relatively unchanged in both I5 and TS_cis-1′-I5with only a slight

elongation of the P-C bond in the former. Intermediate I5 could

therefore be described as containing an η²-(P-C) phosphoranide

ligand, similar to those observed with more strongly donating

heteroatom substituents at phosphorus.⁹⁴I5, however, represents

only a shallow local minimum on the potential energy surface,

and the subsequent full cleavage of the P-C bond occurs with

a minimal activation barrier of 0.3 kcal/mol via TS_I5-cis-5′(E

) +13.7 kcal/mol). The overall activation barrier to F/Ph

exchange via pathway 2a is +36.9 kcal/mol, 6-7 kcal/mol

Figure 15. Alternative views of metallophosphorane stationary points along

pathway 2b.

higher than those computed along pathways 1a and 1b.

The second metallophosphorane pathway, pathway 2b, is

shown in Figure 14 and is based on related work on square-

planar Pd systems where transfer of F onto PH₃was found to

be coupled to an isomerization of the metal coordination

geometry.⁹⁵Thus, in this case, as the P‚‚‚F distance is shortened

the PH₃ligand initially trans to PH₂Ph moves with the fluoride

to create a vacant site trans to the developing phosphoranide.

TS_I6-I5(E ) +31.0 kcal/mol), was located that led to the

formation of the same η²-(P-C) intermediate, I5, found along

pathway 2a. The potential energy surface around I6 is therefore

extremely flat, with TS_cis-1′-I6, I6, and TS_16-I5all being within

0.4 kcal/mol of each other. Despite this, these three stationary

points have distinctly different geometries, as seen in the

alternative views shown in Figure 15, which emphasize how

the increasing Rh‚‚‚F distance is coupled both to a decreasing

Rh‚‚‚C(ipso) distance and to the movement of the {(PH₃)₂Rh}

This led ultimately to the location of a metallophosphorane

as an intermediate (I6, E ) +30.9 kcal/mol), formed via

TS_cis-1′-I6(E ) +31.3 kcal/mol). Metallophosphorane I6 again

shows a fairly regular trigonal bypyramidal geometry at

phosphorus (angles at P: C_ax-P-Rh ) 100.5°; F_ax-P-Rh )

79.9°; H_eq-P-Rh ) 123°/124°), and the main difference as

compared to TS _cis-1′-I5of pathway 2a is that the phosphoranide

moiety is now trans to the vacant site. This results in a shortening

of the M-PH₂FPh bond to 2.24 Å, and the fact that I6 is 6

kcal/mol more stable than TS_cis-1′-I5indicates that PH₂FPh also

has a greater trans influence than PH₃. From I6 a transition state,

moiety relative to the PH₂FPh ligand. From I5, the final delivery

of the Ph group to Rh occurs as in pathway 2a via TS_I5-cis-5′

.

Overall, the lower energies of the metallophosphorane

structures along pathway 2b result in an activation barrier of

31.0 kcal/mol, 6 kcal/mol lower than pathway 2a, but now

very similar in magnitude to those computed along pathways

1a/b. The calculations therefore suggest that oxidative addi-

tion/reductive elimination (pathways 1a/b) and a metallophos-

phorane process (pathway 2b) may be competitive as mecha-

nisms for F/Ph exchange in the simple [(PH₃)₂(PH₂Ph)RhF]

model system.

(94) For examples of η²-(P-N) and η²-(P-O) phosphoranides with group 9

metals, see: (a) Toyota, K.; Yamamoto, Y.; Akiba, K.-Y. J. Organomet.

Chem. 1990, 586, 171. (b) De Meester, P.; Lattman, M.; Chu, S. S. C.

Acta Crystallogr., Sect. C 1987 43, 162.

To probe the effect of solvent polarity on the energetics of

F/Ph exchange, we have recomputed the energies of all

stationary points when placed in a continuum with a dielectric

(95) Goodman, J.; Macgregor, S. A., unpublished results.

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15315

A R T I C L E S

Macgregor et al.

constant equivalent to that of DMSO (ꢀ ) 46.7). For the direct

reaction, 1′ to 5′, the results (in italics in Figures 11-14) indicate

that the overall barriers for all four pathways increase in a more

polar medium (pathway 1a, +37.2 kcal/mol; pathway 1b, +31.6;

pathway 2a, +39.5 kcal/mol; pathway 2b, +34.8 kcal/mol).

However, for the reverse reaction, from cis-5′ to cis-1′ (pathways

2a and 2b) and from cis-5′ to trans-1′ (pathway 1b), the

computed barriers are all reduced in DMSO, to +31.5, to +26.8,

and to +23.3 kcal/mol, respectively. Pathway 1a is the only

one of the four that exhibits an increase in the barrier (to +29.2

kcal/mol) for the reaction of 5′ to 1′ in the more polar medium.

versible. Both the experimental and the theoretical studies

indicate that in benzene the interconvertible isomers 1 and 5

are close in energy (K_eq≈ 1). In other words, D_Rh-F

D_P-Ph≈ D_Rh-Ph+ D_P-Ffor the 1/5 pair, in the assumption that

entropy change for the Ph/F rearrangement (eq 7) is small.⁹⁹

+

A

P-F bond is usually stronger than a P-C bond. For instance,

in PPh₃and PF₃the mean P-Ph and P-F bond dissociation

enthalpies are ca. 70-80^100,101and 115-120^101,102kcal/mol,

respectively. These values allow us to carefully propose that

the Rh-F bond in 1 might be considerably stronger than the

Rh-Ph bond in 5 (by ca. 35-50 kcal/mol) and much more

thermodynamically stable than a late transition metal-fluorine

bond is often thought of. In the absence of experimental values

for these bond dissociation enthalpies, support for this state-

ment is gained from the computed homolytic bond dissocia-

tion energies of the Rh-F bond in cis-1′ (111.9 kcal/mol)

and the Rh-Ph bond of cis-5′ (70.4 kcal/mol).⁹⁹The calculated

value for the difference D_Rh-F- D_Rh-Ph(41.5 kcal/mol) is

within the range of the aforementioned experimental estima-

tion (35-50 kcal/mol).

Discussion

A half-dozen or so reactions leading to P-C bond cleavage

and P-F bond formation at a metal center have been reported

in the literature. In 1991, Blum, Frolow, and Milstein⁹⁶described

the first reaction of this type (eq 8). Since then, more examples

of P-F bond-forming reactions at Ni,^37aPd^31-33(eqs 9-12),

Ru⁹⁷(eqs 13 and 14), and Pt⁹⁸(eq 15) centers have been

published.

Various mechanisms have been proposed for reactions 8-15,

including radical, oxidative addition, reductive elimination, and

metallophosphorane pathways.^31,96-98Our calculations on F/Ph

exchange in 1 suggest that, in general, two oxidative addition-

reductive elimination pathways, 1a (Figure 11) and 1b (Figure

12), and two metallophosphorane pathways, 2a (Figure 13) and

2b (Figure 14), are possible. Two of these four, however, may

be disqualified, pathways 1a and 2a. Pathway 2a is less plausible

due to its relatively high activation energy barrier of ca. 37

kcal/mol (Figure 13), as compared to ca. 30-31 kcal/mol for

the other three (Figures 11, 12, and 14). For pathway 1a, the

computed data are inconsistent with the solvent effect data.

Experimentally, it was demonstrated that an increase in solvent

polarity results in slower rates for the direct reaction (1 to 5)

and faster rates for the reverse process (5 to 1). This is in full

accord with the computational data for all the routes, except

pathway 1a, where the computational results predict a decrease

in the rate of the reaction of 5′ to 1′ upon increasing the polarity

of the medium.

For both pathways 1b and 2b remaining under consideration,

the experimental and theoretical data for the activation barrier

(ca. 23 vs ca. 30 kcal/mol) are in reasonable agreement, even

though the simplified cis-[(PH₃)₂(PH₂Ph)RhF] model system

was used for the calculations, and on this basis both pathways

may be feasible. Moreover, there is precedent for key steps in

both classes of pathway: aryl transfer to the metal center (Ir)

in a metallophosphorane is known,⁸⁹and reversible oxidative

addition of the P-C bond of PPh₃to ([(Et₃P)₄Ni]) has also been

(96) Blum, O.; Frolow, F.; Milstein, D. J. Chem. Soc., Chem. Commun. 1991,

258.

(97) (a) den Reijer, C. J.; Woerle, M.; Pregosin, P. S. Organometallics 2000,

19, 309. (b) den Reijer, C. J.; Dotta, P.; Pregosin, P. S.; Albinati, A. Can.

J. Chem. 2001, 79, 693. (c) Geldbach, T. J.; Pregosin, P. S. Eur. J. Inorg.

Chem. 2002, 1907.

(98) Jasim, N. A.; Perutz, R. N.; Whitwood, A. C.; Braun, T.; Izundu, J.;

Neumann, B.; Rothfeld, S.; Stammler, H.-G. Organometallics 2004, 23,

6140.

(99) For D_Rh-F+ D_P-Ph≈ D_Rh-Ph+ D_P-Fin the 1/5 pair, it must also be assumed

that the Rh-PPh₂Ph bond in 1 is similar in strength to the Rh-PPh₂F bond

in 5. Calculations on our model systems show that the Rh-PH₂Ph bond in

cis-1′ is in fact only about 5.3 kcal/mol weaker than the Rh-PH₂F bond

in cis-5′.

(100) Bedford, A. F.; Mortimer, C. T. J. Chem. Soc. 1960, 1622.

(101) Neale, E.; Williams, L. T. D.; Moores, V. T. J. Chem. Soc. 1956, 422.

(102) Brown, T. L.; LeMay, H. E., Jr.; Bursten, B. E. Chemistry: The Central

Science, 7th ed.; Prentice Hall: Upper Saddle River, NJ, 1997.

In contrast with all of the above examples (eqs 8-15), the

F/Ph exchange reaction of [(Ph₃P)₃RhF], 1 (eq 7), is re-

9

15316 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

demonstrated.¹⁰³However, on balance, we believe that the

metallophosphorane mechanism first reported by one of us⁸⁸is

more plausible, for the following reasons:¹⁰⁴

Scheme 4

(1) Oxidative addition to low-valent transition metals such

as Rh(I) and Pd(0) usually requires a vacant coordination site

and hence is commonly decelerated by extra phosphine.¹⁰⁵This

is also true for P-C oxidative addition reactions of PPh₃to

mononuclear Ni(0),¹⁰³Pd(0),¹⁰³and Rh(I),¹⁰⁶which are all

inhibited by extra phosphine. In contrast, reactions 2, 3, and 7

are not influenced by even a large excess of PPh₃. Moreover,

phosphine deficiency slows down the F/Ph exchange, as it leads

to the dimer [(Ph₃P)₄Rh₂(µ-F)₂] that is highly thermally stable

(see above). In this species, the movement of the bridging F is

restricted and the ability of F to act as an internal nucleophile

may be lost or at least diminished. This is further circumstantial

evidence for pathway 2b over 1b, as [(Ph₃P)₄Rh₂(µ-F)₂] still

should be able to undergo P-C bond cleavage even if the

subsequent P-F bond formation were not possible.

arising from H-bonding to MeCN or DMSO may contribute to

the reduced reactivity of 1 observed in these solvents.

Alternatively, the F/Ph rearrangement (eq 7) via a metallo-

phosphorane might be formally viewed as an intramolecular

version of the concerted fragmentation process PR₅a PR₃+

R₂,¹¹⁰in which the R₂product contains a Lewis acid center

and hence appears coordinated to the PR₃formed in the same

reaction (eq 16).

(2) Oxidative addition of the P-C bond of a tertiary phos-

phine to a metal center most frequently leads to di- and poly-

nuclear complexes with PR₂bridges.^103,106,107Such species have

never been observed in our studies of the reactions of 1 or 5.

(3) Attempted preparation of [(Ph₃P)₃IrF] was carried out

from [(COD)₂Ir₂(µ-OH)₂], 12 equiv of PPh₃, and TREAT HF,

or from [(Ph₃P)₃IrCl] and AgF under sonication in benzene. Both

reactions resulted in complex mixtures containing PPh₃-cyclo-

metalated species (upfield multiplet resonances around -40 to

-70 ppm in the ³¹P NMR)¹⁰⁸but no P-F bond containing

products. Hence, we conclude that while the Ir complexes are

apparently more prone to oxidative addition (in this case

cyclometalation) than 1, P-F bond formation at the Ir center

does not occur.

The thermal decomposition of aryl palladium fluorides (eqs

10-12) leading to P-F bond formation might also proceed via

a metallophosphorane. Two independent, competing reaction

paths have been observed for the decomposition of [(Ph₃P)₂-

Pd(C₆D₅)F].³¹One of the two involves reversible P-C reductive

elimination to give a phosphonium cation, a process that is

inhibited by extra phosphine.^31,111The other decomposition path

leads to P-F bond formation and is not influenced by extra

phosphine,³¹much like the F/Ph rearrangement reaction of 1.

In addition, methyl/phenyl exchange has also been observed in

[(Ph₃P)₂Pd(I)Me]¹¹²and has been shown not to be inhibited by

free phosphine, and so we suggest that it too may be mediated

by a metallophosphorane. We also propose that in certain cases,

P-F bond formation via a metallophosphorane might not

involve a metal fluoride intermediate. For instance, oxidative

addition of the reactive C-F bond of hexafluorobenzene and

pentafluoropyridine to some tertiary phosphine metal complexes

may occur via an S_NAr-type mechanism with extra stabilization

of the Meisenheimer intermediate by P-F coordination. The

latter also makes the basic and highly reactive fluoride a better

leaving group (Scheme 5). Such a mechanism might be operative

in reactions similar to those shown in eqs 8 and 15.

In simplified terms, the first step in the new metallophos-

phorane mechanism^88,95may be regarded as somewhat similar

to migratory insertion involving transfer of a metal alkyl ligand

onto CO¹⁰⁹(Scheme 4). Thus, F acts as an internal nucleophile

attacking phosphine, and the reduction in nucleophilic character

(103) Fahey, D. R.; Mahan, J. E. J. Am. Chem. Soc. 1976, 98, 4499.

(104) A reviewer proposed that the F/Ph rearrangement might be catalyzed by

an adventitious source of proton or hydrogen bond donor, which could

coordinate to the F ligand thus weakening the Rh-F bond. In our

experiments, special precautions were taken to avoid the presence of acid

and water. Our kinetic results were consistently reproducible for both the

direct and the reverse reactions, suggesting that a significant contribution

from catalysis by acid or water was unlikely. Following the reviewer’s

recommendation, we ran the reactions of 1 going to 5 and of 5 going to

1 under our standard anhydrous conditions (see the Experimental Section)

in the presence and in the absence of dry CsF as a base. For these

experiments, CsF was calcined under vacuum, brought in the glovebox,

ground up to a fine powder, vacuum-calcined again, and stored in the

glovebox. The reaction of 1 in a Teflon liner at 70 °C was not influenced

by CsF (30 equiv per Rh), reaching equilibrium after 1 h (¹⁹F NMR),

much like the similar reaction previously conducted in the absence of

CsF in glass (see Figure 5A of the Supporting Information). Neither CsF

(20 equiv) nor p-C₆H₄SO₃H‚H₂O (1 mol % to [Rh]) affected significantly

the reaction of 1 at 60 °C. No substantial effect of CsF (22 equiv) was

observed on conversion of 5 to 1 at the same temperature (60 °C) after

20, 50, and 90 min. We conclude therefore that, under our conditions,

acid catalysis is unlikely to be the main channel for the F/Ph rearrange-

ment.

Perutz, Braun, and co-workers⁹⁸recently reported that while

oxidative addition of pentafluoropyridine to [(R₃P)₂Pd] (R )

i-Pr or Cy) produced [(R₃P)₂Pd(C₅F₄N)F], the reaction of the

same substrate with similar platinum complexes [(R₃P)₂Pt] (R

) i-Pr or Cy) led to P-F and Pt-R bond formation (eq 15).

Scheme 5

(105) (a) For instance, [(Ph₃P)₂RhCl] is at least 2 orders of magnitude more

reactive toward oxidative addition of H₂than [(Ph₃P)₃RhCl]: Halpern,

J.; Wong, C. S. J. Chem. Soc., Chem. Commun. 1973, 629. (b) In the

oxidative addition of aromatic iodides to [(Ph₃P)_nPd] (n ) 3, 4), the

reactive Pd(0) species is [(Ph₃P)₂Pd]: Fauvarque, J.-F.; Pflu¨ger, F.;

Troupel, M. J. Organomet. Chem. 1981, 208, 419.

(106) Lewin, M.; Aizenshtat, Z.; Blum, J. J. Organomet. Chem. 1980, 184, 255.

(107) Garrou, P. E. Chem. ReV. 1985, 85, 171.

(108) Dahlenburg, L. J. Organomet. Chem. 1983, 251, 347. Bennett, M. A.;

Bhargava, S. K.; Ke, M.; Willis, A. C. Dalton Trans. 2000, 3537.

(109) For a review, see: Cavell, K. J. Coord. Chem. ReV. 1996, 155, 209.

(110) Hoffmann, R.; Howell, J. M.; Muetterties, E. L. J. Am. Chem. Soc. 1972,

94, 3047.

(111) (a) Kong, K.-C.; Cheng, C.-H. J. Am. Chem. Soc. 1991, 113, 6313. (b)

Goodson, F. E.; Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1997,

119, 12441.

(112) Morita, D. K.; Stille, J. K.; Norton, J. R. J. Am. Chem. Soc. 1995, 117,

8576.

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15317

A R T I C L E S

Macgregor et al.

Attempts to detect a Pt-F intermediate failed as even at lower

temperatures only P-F bond formation was observed. We found

that [(Cy₃P)₂Pt] reacted with C₆F₆in THF at 65 °C ca. 5 times

faster than [(Cy₃P)₂Pd], the reaction products being [(Cy₃P)Pt-

(Cy₂PF)(Cy)(C₆F₅)] and [(Cy₃P)₂Pd(C₆F₅)(F)], respectively (see

the Experimental Section), in full accord with the Perutz-

Braun⁹⁸data on similar reactions with C₅F₅N. In contrast,

reactions of [(Cy₃P)₂M] (M ) Pd, Pt) with neat chlorobenzene

at 25 °C furnished [(Cy₃P)₂M(Ph)(Cl)] regardless of the

nature of the metal, the reaction of [(Cy₃P)₂Pd] being slightly

faster (k_Pd/k_Pt) ca. 1.2; see the Experimental Section). The

different reaction outcomes and the fact that k_Pd/k_Pt< 1 for the

reaction with C₆F₆but k_Pd/k_Pt> 1 for the reaction with PhCl

indicate that different mechanisms might be involved. While

the reaction of C₅F₅N or C₆F₆with [(Cy₃P)₂Pd] might occur

via a three-center mechanism, the more nucleophilic Pt complex

might react via an S_NAr-type path, as shown in Scheme 5.

Likewise, an elegant study by Fo´a and Cassar clearly demon-

strated that oxidative addition of the C-Cl bond of p-XC₆H₄Cl

to [(Ph₃P)₃Ni] can occur via two distinctive mechanisms,

probably S_NAr for strongly electron-withdrawing X and 3-center

for all other substituents.¹¹³

Like fluoride,⁸⁷some hard O-anions¹¹⁴such as acetate¹¹⁵or

hydroxide^116,117have been reported to promote the clean and

often remarkably facile reduction of Pd(II), Pt(II), Rh(III), and

some other metals at the expense of a tertiary phosphine ligand

in the inner sphere. With OH^-(Scheme 6), the reaction is

initiated via M-OH bond formation and subsequently proceeds

with retention of configuration at P.¹¹⁷These observations are

consistent with an intramolecular attack of OH^-on phosphine

and thus may well involve the intermediacy of a metallophos-

phorane.^88,95

the Rh‚‚‚Cl and Rh‚‚‚C(ipso) distances, and these calculations

indicated that any metallophosphorane species would be at least

40 kcal/mol higher in energy than cis-[(PH₃)₂(PH₂Ph)RhCl].

Thus, the barriers for Cl/Ph exchange are at least 10 kcal/mol

higher than those for F/Ph exchange, and these higher barriers

in the case of the chloro species are consistent with the nonob-

servation of processes based on Cl/Ph exchange in Wilkinson’s

catalyst under comparable conditions (80-100 °C). It has indeed

been reported¹⁰⁶that the thermal decomposition of Wilkinson’s

catalyst to Ph₂(and presumably Rh-PPh₂) species requires

hours at 140-180 °C to occur to observable conversions.

Conclusions

The fluoride congener of Wilkinson’s catalyst, [(Ph₃P)₃RhF]

(1), has been synthesized, fully characterized, and found to

exhibit distinctive reactivity. In sharp contrast with Wilkinson’s

catalyst, 1 easily cleaves the inert C-Cl bond of nonactivated

chloroarenes (chlorobenzene, p-chlorotoluene) under mild con-

ditions to produce trans-[(Ph₃P)₂Rh(Ph₂PF)(Cl)] as a result of

C-Cl, Rh-F, and P-C bond cleavage and C-C, Rh-Cl, and

P-F bond formation. The thermal decomposition of 1 in

benzene (2-3 h at 80 °C) leads to a 1:1 mixture of trans-

[(Ph₃P)₂Rh(Ph₂PF)(F)] and [(Ph₃P)₃RhPh], which easily under-

goes cyclometalation to [(Ph₃P)₂Rh(Ph₂PC₆H₄)].

The chloroarene activation and thermal decomposition reac-

tions of 1 have a common first step involving the facile and

reversible F/Ph rearrangement reaction of 1 to cis-[(Ph₃P)₂Rh-

(Ph)(Ph₂PF)] (5). The latter has been isolated and fully

characterized in solution and in the solid state (X-ray). Kinetic

studies of the interconversion of 1 and 5 indicated a unimo-

lecular process that is not influenced by extra phosphine, with

∆H ^q) 22.0 ( 1.2 kcal mol^-1and ∆S ^q) -10.0 ( 3.7 eu, and

the corresponding Arrhenius parameters E_a) 22.7 ( 1.2 kcal

mol^-1and log(A/s^-1) ) 11.1 ( 0.8. Density functional

calculations on a [(PH₃)₂(PH₂Ph)RhF] model system identified

two possibly competing mechanisms: (i) P-C oxidative addi-

tion with subsequent F-P reductive elimination (pathway 1b)

and (ii) F transfer to produce a metallophosphorane,^88,95followed

by Ph transfer to Rh (pathway 2b). Although both pathways

display similar activation barriers for the simple [(PH₃)₂-

(PH₂Ph)RhF] model system, a number of experimental observa-

tions suggest that the metallophosphorane mechanism is more

plausible. Analysis of some literature data likewise strengthens

the notion that certain reactions resulting in P-F, P-O, and

P-C bond formation at a metal center might also be mediated

by metallophosphorane species.^88,95

Scheme 6

In contrast to its fluoro analogue, Wilkinson’s catalyst does

not undergo transformations such as those shown in eqs 2 and

3. Preliminary calculations¹¹⁶on cis-[(PH₃)₂(PH₂Ph)RhCl], the

chloro congener of cis-1′, have established that Cl/Ph exchange

via pathway 1a has a barrier of ca. 41 kcal/mol. To date, we

have not succeeded in locating metallophosphorane structures

analogous to TS_cis-1′-I5(Figure 13) or I6 (Figure 14) that were

located along pathways 2a and 2b in the fluoro system. All

attempts to optimize such structures resulted in elongation of

the P-Cl distance, often with Cl transfer back to Rh. Mapping

of the potential energy surfaces around these putative metallo-

phosphorane structures was performed by systematically varying

Experimental Section

All chemicals were purchased from Aldrich and Strem chemical

companies and used as received. Complexes [(COD)₂Rh₂(µ-OH)₂],⁶¹

[(Ph₃P)₄Rh₂(µ-OH)₂],⁶³[(Ph₃P)₃RhPh],⁵⁸[(Cy₃P)₂Pd],¹¹⁸[(Cy₃P)₂Pt],¹¹⁹

[(COD)₂Pt],¹²⁰[(COD)₂Ir₂(µ-OH)₂],¹²¹and [(Ph₃P)₃IrCl]¹²²were pre-

pared as described in the literature. The solvents were thoroughly dried

using standard techniques and stored over freshly calcined molecular

sieves (4 Å) in a glovebox. All manipulations with the Rh fluoride

(113) Fo´a, M.; Cassar, L. J. Chem. Soc., Dalton Trans. 1975, 2572.

(114) For a recent overview of such reactions, see section 2.2 in: Grushin, V.

V. Chem. ReV. 2004, 104, 1629.

(118) Grushin, V. V.; Bensimon, C.; Alper, H. Inorg. Chem. 1994, 33, 4804.

(119) Fornies, J.; Green, M.; Spencer, J. L.; Stone, F. G. A. J. Chem. Soc.,

Dalton Trans. 1977, 1006.

(120) Walther, D.; Heubach, K.; Bo¨ttcher, L.; Schreer, H.; Go¨rls, H. Z. Anorg.

Allg. Chem. 2002, 628, 20.

(121) Green, L. M.; Meek, D. W. Organometallics 1989, 8, 659.

(122) Bennett, M. A.; Latten, J. L. Inorg. Synth. 1989, 26, 200.

(115) For accounts of the detailed studies of the catalytically important reaction

of Pd(OAc)₂with R₃P leading to Pd(0) and R₃PO, see: Amatore, C.;

Jutand, A. Acc. Chem. Res. 2000, 33, 314; J. Organomet. Chem. 1999,

576, 254.

(116) Gonzalez-Carrera, V.; Goodman, J.; Macgregor, S. A., unpublished results.

(117) Grushin, V. V.; Alper, H. Organometallics 1993, 12, 1890.

9

15318 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

complexes and products of their transformations were carried out under

nitrogen in a glovebox. NMR spectra were obtained with Bruker Avance

DRX400 and Varian Unity Inova systems, both operating at 400 MHz.

NMR spectral simulations were performed using Nuts Professional

processing software (Acorn NMR, Livermore, CA). The kinetic NMR

data in Figure 9 and in the Supporting Information were analyzed as

described in the text using the lsqnonlin function from the MATLAB

Optimization Toolbox (MATLAB Version 7, The Mathworks, Inc.,

Natick, MA). A Bruker-CCD instrument was used for single-crystal

X-ray diffraction studies. Microanalyses were performed by Micro-

Analysis, Inc., Wilmington, DE, and Galbraith Laboratories, Inc.,

Knoxville, TN.

After 4 h at room temperature, yellow crystals of 2 were separated and

dried under vacuum. The yield of spectroscopically pure 2 was 0.103

g (50%). The product was recrystallized from dichloromethane-hexanes

before combustion analysis. Anal. Calcd for C₄₈H₄₀ClFP₃Rh, %: C,

66.5; H, 4.7. Found, %: C, 65.6; H, 4.8. For the NMR characterization,

see Table 2. (b) 1 (63 mg) was reacted with PhCl (0.3 mL) as described

above. The reaction mixture was diluted with pentane (4 mL), filtered

through silica gel, and analyzed by GC-MS to reveal the formation

of biphenyl. (c) 1 (83 mg) was reacted with p-chlorotoluene, as

described above. The reaction mixture was diluted with pentane (4 mL)

and filtered through silica gel. GC-MS analysis of the colorless filtrate

indicated the formation of 4-methylbiphenyl and biphenyl in a 1000:8

ratio.

Thermal Decomposition of 1. A 5-mm NMR tube was charged

with 1 (40 mg) and benzene (0.5 mL), sealed, and heated (with

occasional swirling until 1 had all dissolved) at 80 °C (oil bath).

Reaction progress was monitored by ³¹P and ¹⁹F NMR. After 3 h at 80

°C (ca. 90% conversion), the mixture contained two complexes, 3 and

4 in a 1:1 molar ratio, as indicated by the NMR data (see Table 2 and

ref 66).

Synthesis of [(Ph₃P)₃RhF] (1). (a) To a stirring mixture of

[(COD)₂Rh₂(µ-OH)₂] (0.15 g; 0.33 mmol), PPh₃(1.15 g; 4.25 mmol),

and ether (8 mL) was added Et₃N‚3HF (TREAT HF; 37 µL; 0.23

mmol). After the mixture was vigorously stirred for 2 h, the orange-

yellow microcrystalline precipitate was separated by filtration, thor-

oughly washed with ether, and dried under vacuum. The yield of 1

was 0.56 g (94%). Anal. Calcd for C₅₄H₄₅FP₃Rh, %: C, 71.4; H, 5.0.

1

Found, %: C, 71.2; H, 5.3. H NMR (C₆D₆, 20 °C), δ: 7.0 (m, 3H,

m,p-Ph); 7.9 (m, 2H, o-Ph). For the ¹⁹F and ³¹P data, see Table 2. (b)

TREAT HF (90 µL; 0.56 mmol) was added to a vigorously stirring

mixture of [(COD)₂Rh₂(µ-OH)₂] (0.34 g; 0.75 mmol), PPh₃(2.5 g; 9.5

mmol), and ether (15 mL). After being stirred for 3 h at room

temperature, the orange-yellow precipitate was separated, washed with

ether, and dried under vacuum. The yield of 1 was 1.22 g (90%). X-ray

quality crystals were obtained by quickly dissolving the complex in

warm benzene at vigorous stirring, adding hexanes to the warm, solid-

free orange solution, and leaving the mixture at room temperature

overnight. (c) This experiment exemplifies the synthesis of 1 using

substoichiometric quantities of TREAT HF to avoid contamination with

bifluoride, for kinetic studies. To a stirring mixture of [(COD)₂Rh₂-

(µ-OH)₂] (0.160 g; 0.35 mmol), PPh₃(1.25 g; 4.77 mmol), and ether

(12 mL) was added Et₃N‚3HF (TREAT HF; 35 µL; 0.22 mmol). After

the mixture was vigorously stirred for 2 h, the orange-yellow micro-

crystalline precipitate was separated by filtration, thoroughly washed

with THF, benzene, and ether, and dried under vacuum overnight. The

yield of 1 was 0.38 g (65%, as calculated on the HF source).

Thermal Decomposition of [(Ph₃P)₃RhPh]. A solution of [(Ph₃P)₃-

RhPh] (ca. 30 mg) in benzene (0.6 mL) was heated at 80 °C (oil bath)

for 1 h. ³¹P NMR analysis of the orange reaction solution revealed the

formation of 4 via cyclometalation in ca. 100% selectivity.

Reaction of [(Ph₃P)₃RhPh] with PhCl. A 5-mm NMR tube

was charged with [(Ph₃P)₃RhPh] (ca. 30 mg) and chlorobenzene

(0.6 mL), sealed, and heated at 80 °C (oil bath) for 1 h. ³¹P NMR

analysis of the orange reaction solution revealed the formation of

[(Ph₃P)₃RhCl].

Preparation of cis-[(Ph₃P)₂Rh(Ph)(Ph₂PF)] (5). A mixture of 1

(0.39 g; 0.43 mmol), PPh₃(0.74 g; 2.82 mmol), and benzene (18 mL)

in a closed vessel was stirred at 75 °C (oil bath) for 1 h. The mixture

was worked up in a glovebox. The solution was evaporated to leave a

dark red-brown residue, which was treated with ether (10 mL). After

the mixture was stirred for 10 min, the orange precipitate was filtered

off. The filtrate was reduced in volume to ca. 5 mL and treated with

hexanes (5 mL). Yellow crystals of 5 began to form. After 30 min,

more hexanes (2 mL) were added. After 2 h, the yellow crystals of 5

were separated, washed with ether (3 × 2 mL), hexanes (2 × 2 mL),

and dried. Keeping the combined mother liquor and the washings at

-20 °C overnight produced an extra quantity of 5 as yellow needles.

Both crops of 5 were combined, dissolved in benzene (3 mL), and the

solution was evaporated with a nitrogen flow to ca. 1 mL. Hexanes (1

mL) were added, followed by more hexanes (9 mL) after 10 min. After

the mixture was kept at -20 °C for 2 weeks, yellow crystals of

spectroscopically pure 5 were separated, washed with hexanes, and

dried under vacuum. The yield of 5 was 0.115 g (29%). X-ray quality

crystals of 5‚2C₆H₆and 5‚Et₂O were obtained by recrystallization

from benzene and benzene-ether-hexanes, respectively. Anal. Calcd

for C₅₈H₅₅FOP₃Rh (5‚Et₂O), %: C, 70.9; H, 5.6. Found, %: C, 70.1;

Synthesis of [(Ph₃P)₄Rh₂(µ-F)₂]. Solid [(Ph₃P)₄Rh₂(µ-OH)₂] (0.22

g; 0.17 mmol) was dissolved in hot benzene (12 mL), the heater was

removed, and the orange solution was treated, at vigorous stirring, with

Et₃N‚3HF (TREAT HF; 20 µL; 0.12 mmol). The color turned reddish-

orange immediately. After 15 min of vigorous stirring, the solution

was filtered through cotton wool, reduced in volume to ca. 4 mL, and

treated with hexanes (8 mL). After 1 h, the red-orange crystals were

separated, washed with hexanes (3 × 6 mL), and dried under vacuum.

The yield of [(Ph₃P)₄Rh₂(µ-F)₂] was 0.155 g. The mother liquor and

the washings were combined to produce additional quantities of

[(Ph₃P)₄Rh₂(µ-F)₂] (0.04 g) upon standing at room temperature for 2

days. Overall yield: 0.195 g (88%). This very air-sensitive compound

is insoluble in ether, alkanes, and cycloalkanes, poorly soluble in

benzene and THF, and is decomposed instantly by dichloromethane.

Anal. Calcd for C₇₂H₆₀F₂P₄Rh₂, %: C, 66.9; H, 4.7. Found, %: C,

66.2; H, 4.7. ¹H NMR (C₆D₆, 20 °C), δ: 6.8 (m, 3H, m,p-Ph), 7.8 (m,

2H, o-Ph). ¹⁹F NMR (C₆D₆, 20 °C), δ: -323.7 (m; J_Rh-F) ca. 40 Hz).

1

H, 5.5. H NMR (C₆D₆, 20 °C), δ: 6.6-6.8 (m, 3H, m,p-PhRh), 7.9

(m, 42H, PhP and o-PhRh). For the ¹⁹F and ³¹P NMR spectra, see

Figures 7 and 8.

Reactions of [(Cy₃P)₂M] (M ) Pd, Pt) with PhCl. Both complexes

used for these experiments were >99% pure and extra phosphine-free

(³¹P NMR). Two samples were prepared: [(Cy₃P)₂Pd] (17 mg; 0.026

mmol) in PhCl (0.55 mL) and [(Cy₃P)₂Pt] (20 mg; 0.026 mmol) in

PhCl (0.55 mL). The reactions were monitored via ³¹P NMR. After

20, 90, and 115 h at room temperature, the conversions of the Pd and

Pt complexes to [(Cy₃P)₂M(Ph)Cl] were 25% and 20%, 69% and 55%,

and 84% and 69%, respectively. From these data, the value of k_Pd/k_Pt

was calculated at ca. 1.2. The chemical shifts and J_P-Ptvalues ob-

served for [(Cy₃P)₂Pd],¹¹⁸[(Cy₃P)₂Pt],¹¹⁹[(Cy₃P)₂Pd(Ph)Cl],^55cand

[(Cy₃P)₂Pt(Ph)Cl]¹¹⁹were similar to those reported in the literature.

Oxidative addition reactions of chlorobenzene to [(Cy₃P)₂Pd(dba)],^55c

and to [(Cy₃P)₂Pt],¹¹⁹have been reported previously.

³¹P NMR (C₆D₆, 20 °C), δ: 60.5 (m; J_Rh-P) 207.7 Hz; J_P-F

)

195.5 Hz). The ¹⁹F and ³¹P NMR spectral patterns are complex

and similar to those previously reported⁴¹for the i-Pr₃P analogue,

[(i-Pr₃P)₄Rh₂(µ-F)₂], an AA′A′′A′′′MM′XX′ spin system. For details,

see ref 53.

Reaction of 1 with PhCl or TolCl. (a) A mixture of 1 (0.215 g;

0.24 mmol) and chlorobenzene (1 mL) was stirred at 100 °C (oil bath)

for 2.5 h. The mixture was cooled to room temperature, treated with

toluene (3 mL) and hexanes (3 mL), and left overnight. The crystalline

solid was separated and quickly extracted with toluene (8 mL) at reflux.

The orange extract was filtered hot and treated with hexanes (4 mL).

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15319

A R T I C L E S

Macgregor et al.

Table 5. A Summary of Crystallographic Data for 3, 5‚2C₆H₆, 5‚Et₂O, and 6

3

5

‚

2C₆H₆

5‚

Et₂O

6

empirical formula

FW

C₄₈H₄₀F₂P₃Rh

850.62

C₆₆H₅₇FP₃Rh

1064.94

C₅₈H₅₅FOP₃Rh

982.84

C₄₈H₄₁F₃P₃Rh

870.63

cryst color, form

cryst system

space group

a (Å)

gold, irreg. block

monoclinic

P2(1)/c

12.865(4)

gold, irreg. block

monoclinic

P2(1)/n

13.3165(7)

clear, needle

orthorhombic

P2(1)2(1)2(1)

9.702(6)

gold, irreg. block

monoclinic

P2(1)/n

12.4782(9)

b (Å)

10.424(3)

19.8008(11)

22.373(13)

20.1446(14)

c (Å)

29.519(8)

20.2300(11)

22.947(13)

16.0477(11)

R (deg)

90

99.343(5)

90

4981(5)

90

â (deg)

100.2060(10)

90

5249.8(5)

95.7850(10)

90

4013.3(5)

γ (deg)

V (Å³)

3906.1(19)

Z

4

density (g/cm³)

1.446

0.604

1744

1.347

0.462

2208

1.311

0.482

2040

1.441

0.593

1784

abs. µ (mm^-1

F (000)

)

cryst size (mm)

temp (°C)

scan mode

detector

0.34 × 0.13 × 0.08

0.30 × 0.30 × 0.28

0.22 × 0.01 × 0.01

0.46 × 0.46 × 0.15

-100

ω

Bruker-CCD

θ

_max(deg)

29.19

73 287

10 507

0.0598

28.71

43 223

13 307

0.0393

22.39

30 183

6393

0.1707

28.3

33 129

9955

0.0286

no. obsd. refs

no. uniq. refs

R_merge

no. params

487

641

577

501

R indices [I > 2σ(I)]^a

R indices (all data)^a

S^b

wR₂) 0.089, R₁) 0.037

wR₂) 0.094, R₁) 0.062

0.973

wR₂) 0.0844, R₁) 0.0331

wR₂) 0.092, R₁) 0.045

1.039

wR₂) 0.096, R₁) 0.052

wR₂) 0.107, R₁) 0.093

0.87

wR₂) 0.075, R₁) 0.030

wR₂) 0.078, R₁) 0.042

1.08

max diff peak, hole (e/Å³)

0.85, -0.67

0.43, -0.48

0.58, -0.57

0.58, -0.28

2

^aR₁) ∑||F_o| - |F_c||/∑|F_o|, wR₂) {∑[w(F_o- F_c)²]/∑[w(F_o)²]}^1/2(sometimes denoted as R_w2). ^bGOF ) S ) {∑[w(F_o- F_c)²]/(n - p)}^1/2, where

n is the number of reflections, and p is the total number of refined parameters.

Reactions of [(Cy₃P)₂M] (M ) Pd, Pt) with C₆F₆. Both complexes

used for these experiments were >99% pure and extra phosphine-free

(³¹P NMR). Two samples were prepared: [(Cy₃P)₂Pd] (48 mg; 0.07

mmol) and C₆F₆(10 µL) in THF (0.65 mL) and [(Cy₃P)₂Pt] (55 mg;

0.07 mmol) and C₆F₆(10 µL) in THF (0.65 mL). The reactions were

run in sealed 5-mm NMR tubes at 70 °C (oil bath) and monitored by

³¹P and ¹⁹F NMR. After 24 h, the conversions of the Pd and Pt

complexes to trans-[(Cy₃P)₂Pd(C₆F₅)F] and [(Cy₃P)Pt(Cy₂PF)(Cy)-

(C₆F₅)] were 3% and 15%, correspondingly. NMR data for trans-

[(Cy₃P)₂Pd(C₆F₅)F], δ. ¹⁹F: -110.1 (m, 2F, C₆F₅), -164.8 (m, 1F, C₆F₅),

approximately centered between the Rh-F region (-260 to -285 ppm)

and the P-F region (-110 to -130 ppm). A spectral width of 100

kHz was used to observe additional decomposition products. An

acquisition time of 0.5 s was used in conjunction with a recycle delay

of 1 s and a pulse width of 55°. Temperature calibration using an

ethylene glycol standard indicated insignificant deviations (<(0.2 °C)

from the thermocouple set point in the range used for the kinetic data.

Full details of the kinetic studies are presented in the Supporting

Information.

Solvent Effect Studies. (a) In a drybox, a stock solution was

prepared of 1 (18 mg), PPh₃(146 mg), and dry, O₂-free benzene (4

mL). Three 5-mm NMR tubes were charged with 0.7 mL of this freshly

prepared solution. To these three tubes, rigorously anhydrous benzene

(0.2 mL; control sample A), acetonitrile (0.2 mL; sample B), and DMSO

(0.2 mL; sample C) were added. The tubes were swirled to ensure

homogeneity, sealed, and brought out. The samples were analyzed by

¹⁹F NMR after 17 h at 25 °C, to determine the conversion to 5 at 19%,

<5%, and <5% for A, B, and C, respectively. All three samples were

then placed in an oil bath at 40 °C for 2 h, and analyzed again. The

conversion to 5 was calculated at 19%, <5%, and <5% for A, B, and

C, respectively. Analysis by ³¹P NMR indicated that partial decomposi-

tion of 1 had taken place in samples B (<20% to [(Ph₃P)₃Rh(MeCN)]⁺)

and C (<30-40% to unidentified products). ³¹P NMR characteristics

for the [(Ph₃P)₃Rh(MeCN)]⁺were in accord with the literature data:¹²⁴

33.5 ppm, 2P, dd, J_P-P) 39.4 Hz, J_P-Rh) 139.8 Hz; 45.8 ppm, 1P,

dt, J_P-Rh) 172.7 Hz. (b) Three samples were prepared as described

above, except 5 was used instead of 1. The samples were analyzed by

¹⁹F NMR after 1 h at 40 °C, to determine the conversion to 3 at <5%,

60%, and 60% for A, B, and C, respectively. Analysis of B by ³¹P

NMR indicated the formation of stoichiometric amounts of [(Ph₃P)₃-

Rh(Ph)], due to the facilitated ligand exchange between 1 produced

and the as yet unreacted 5 (see Schemes 1 and 2), and trace quantities

-165.8 (m, 2F, C₆F₅), -325.0 (br s, 1F, Pd-F). ³¹P: 31.6 (d, J_P-F

)

16 Hz). NMR data for [(Cy₃P)Pt(Cy₂PF)(Cy)(C₆F₅)], δ. ¹⁹F: -111.2

(m, 2F, J_F-Pt) 196 Hz, o-C₆F₅), -164.2 (m, 2F, C₆F₅), -164.4 (m,

1F, C₆F₅), -175.8 (ddt, 1F, J_F-P) 904.6 and 37.5 Hz, J_F-Pt) 375

Hz, P-F). ³¹P: 19.2 (dd, 1P, J_P-F) 37.5 Hz, J_P-P) 443.7 Hz, J_P-Pt

) 2722 Hz, Cy₃P), 181.7 (dd, 1P, J_P-F) 904.6 Hz, J_P-P) 443.7 Hz,

J_P-Pt) 3851 Hz, Cy₂PF). These NMR characteristics are similar to

those reported⁹⁸for trans-[(Cy₃P)₂Pd(4-C₅F₄N)F] and [(Cy₃P)Pt-

(Cy₂PF)(Cy)(4-C₅F₄N)].

Kinetic Measurements. Samples of 1 and 5 for kinetic studies were

prepared by the methods described above. Because 1 cannot be purified

by recrystallization, any excess of TREAT HF was strictly avoided in

its synthesis to prevent contamination with bifluoride. Precautions were

taken to avoid the presence of adventitious water in the samples. Only

brand new, previously unused Wilmad 528-PP NMR tubes were

employed, which were carefully vacuum oven-dried prior to sample

preparation.^104,123

One-dimensional ³¹P, ¹⁹F NMR and kinetic data were acquired on a

Varian Unity Inova system operating at 400 MHz. For the kinetic data

obtained by ¹⁹F NMR, the transmitter was placed at -180 ppm,

(123) Our initial kinetic studies were carried out with samples placed in both

5-mm and 10-mm NMR tubes, with and without inert Teflon liners. These

variations in the nature of the surface (glass vs Teflon) and in the glass

surface-to-volume ratio did not cause an observable change in reaction

rates.

(124) Pimblett, G.; Garner, C. D.; Clegg, W. J. Chem. Soc., Dalton Trans. 1985,

1977.

9

15320 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

The F/Ph Rearrangement Reaction of [(Ph₃P)₃RhF]

A R T I C L E S

of [(Ph₃P)₃Rh(MeCN)]⁺(see above). (c) A solution of 1 (20 mg) and

PPh₃(100 mg) in a mixture of benzene (0.7 mL) and PhNO₂(0.2 mL)

was kept at 45 °C for 1 h, and then analyzed by ¹⁹F NMR. The analysis

indicated ca. 15% conversion to 5. No formation of 3 was noticed.

Under similar conditions but in the absence of PhNO₂, ca. 30%

conversion to 5 was observed. (d) A solution of 5 (22 mg) and PPh₃

(95 mg) in a mixture of benzene (0.7 mL) and PhNO₂(0.2 mL) was

kept 25 °C for 1 h, and then analyzed by ¹⁹F NMR. The analysis

indicated ca. 80% conversion to 1. No formation of 3 was noticed,

although a small amount (ca. 5%) of a P-F species (-74.7 ppm, dd,

J_F-P) 1017 Hz, J_F-Rh(P)?) 9 Hz) was observed. Under similar

conditions but in the absence of PhNO₂, no conversion to 1 (<5%)

was observed.

Computational Studies. Details of computational studies are

presented in the Supporting Information.

Acknowledgment. This is DuPont CR&D contribution no.

8591. S.A.M. thanks Heriot-Watt University and the EPSRC

for funding. Computational resources on a HP/COMPAQ ES40

multiprocessor cluster (Columbus) at the Rutherford Appleton

Laboratory (RAL), provided by the EPSRC National Service

for Computational Chemistry Software, are acknowledged.

Supporting Information Available: Details of kinetic, crys-

tallographic (CIF), and computational studies. This material is

available free of charge via the Internet at http://pubs.acs.org.

X-ray Crystallographic Studies. A summary of crystallographic

data for new complexes 3, 5‚2C₆H₆, 5‚Et₂O, and 6 is presented in Table

5. The CIF files are presented in the Supporting Information.

JA054506Z

9

J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15321

Article Doi

The F/Ph rearrangement reaction of [(Ph3P)3RhF], the fluoride congener of Wilkinson's catalyst

DOI: 10.1021/ja054506z

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ja054506z

Full text of DOI:10.1021/ja054506z

Products guided by the article

R&D Labs maybe for 869117-74-8

Relevant to this article

Hot Product