An unusual example of allyl-to-alkynyl migration in a phenylacetylide-bridged heterobinuclear complex of rhodium and iridium

5330

Organometallics 1999, 18, 5330-5343

An Un u su a l Exa m p le of Allyl-to-Alk yn yl Migr a tion in a

P h en yla cetylid e-Br id ged Heter obin u clea r Com p lex of

Rh od iu m a n d Ir id iu m

Darren S. A. George, Robert W. Hilts,^†Robert McDonald,^‡and Martin Cowie*^,§

Department of Chemistry University of Alberta Edmonton, Alberta, Canada T6G 2G2

Received August 16, 1999

The reactivity of the alkynyl-bridged complex [RhIr(CO)₂(µ₂-η¹:η²-C₂Ph)(dppm)₂][X] (X )

BF₄(1a ), SO₃CF₃(1b); dppm ) Ph₂PCH₂PPh₂) with electrophiles has been demonstrated.

Protic acids HX (X ) BF₄, SO₃CF₃) first yield the oxidative-addition products [RhIr(X)(CO)₂-

(µ-H)(µ-C₂Ph)(dppm)₂][X], which under carbon monoxide result in displacement of the weakly

coordinating BF₄^-or SO₃CF₃^-anions and subsequent conversion to the vinylidene-bridged

[RhIr(CO)₄(µ-CC(H)Ph)(dppm)₂][X]₂. Reaction of 1 with allyl halides yields the allyl vinyl-

idene-bridged compounds [RhIr(Y)(CO)(µ-CC(Ph)CH₂CHdCH₂)(µ-CO)(dppm)₂][X] (Y ) Br

(5), Cl (6)), by coupling of the alkynyl and allyl groups at the â-position of the alkynyl moiety.

NMR studies at low temperatures show coordination of allyl halide at Ir at -80 °C, followed

by allyl halide loss and subsequent oxidative addition at -50 °C. The oxidative-addition

intermediates, [RhIr(η¹-CH₂CHdCH₂)(CO)₂(µ-Y)(µ-C₂Ph)(dppm)₂][X] (Y ) Br (9), Cl (10)),

rearrange to the allylvinylidene products (5 and 6) at ambient temperature. Although halide

removal from compounds 5 and 6, using AgBF₄, does not result in destabilization of the

allylvinylidene fragment, resulting instead in replacement of halide by fluoborate ion, the

reaction of 1 with allyl halide in the presence of a silver salt does not lead to coupling of the

allyl and alkynyl moieties, but gives [RhIr(η³-C₃H₅)(CO)(µ-C₂Ph)(µ-CO)(dppm)₂][X]₂(13).

Addition of halide ion to this η³-allyl complex at ambient temperature again leads to formation

of 5 or 6. On the basis of these results a mechanism is proposed for the allyl/alkynyl coupling

reaction.

In tr od u ction

alkynyl complexes have also been implicated in a

number of carbon-carbon bond-forming processes such

as alkyne oligomerization and polymerization⁴and

cycloaddition reactions.⁵As part of an ongoing effort to

determine how adjacent metals can influence the reac-

tivity of hydrocarbyl ligands,⁶we have undertaken a

study of bridging alkynyl groups in binuclear com-

plexes.⁷

The alkynyl (or acetylide) group is a versatile ligand

in organometallic chemistry.¹Not only does it serve as

a common precursor to vinylidenes,²themselves versa-

tile ligands in carbon-carbon bond formation,³but

* To whom correspondence should be addressed.

^†Present address: Grant MacEwan Community College, Edmonton,

AB.

(2) (a) Bruce, M. I.; Swincer, A. G. Adv. Organomet. Chem. 1983,

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^‡Faculty Service Officer, Structure Determination Laboratory.

^§McCalla Research Professor, 1999-2000. Fax: (780)492-8231.

E-mail: martin.cowie@ualberta.ca.

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Publication on Web 11/12/1999

Allyl-to-Alkynyl Migration

Organometallics, Vol. 18, No. 25, 1999 5331

The most common alkynyl bonding mode is the

terminal one (A) in which this group is η¹-bound to a

single metal.^1a,8In this geometry the alkynyl group has

by migration to the alkynyl â-carbon,⁷raising the

possibility that other cases of alkynyl reactivity may also

proceed via initial attack at the metal. One goal of this

study was to extend the reactivity of bridging alkynyl

groups to include electrophilic attack, through initial

involvement of the electron-rich group 9 metals.

Alkynyl groups have also been implicated as inter-

mediates in the transformation of 1-alkynes to vinyli-

denes.¹³Certainly, in binuclear late-metal complexes we

have shown that this transformation is extremely facile

and have proposed that it is facilitated by the µ-η¹:η²-

alkynyl binding mode, in which migration of the hydride

ligand from an intermediate such as C to the alkynyl

â-carbon occurs readily, as diagrammed in eq 1.^6h,14

been shown to be susceptible to electrophilic attack at

the â-carbon to yield vinylidene ligands.^5d,8a,9When it

bridges two metals, it usually adopts the µ-η¹:η²-binding

mode (B), in which it is σ-bound to one metal while

π-bound to the adjacent metal.^8b,10As might be expected,

this bonding mode results in a modified reactivity

compared to that of the η¹-alkynyl, with bridging groups

being susceptible to nucleophilic attack.^11,12Further-

more, Carty¹¹and others¹²have shown that nucleophilic

attack can occur at either the R- or â-positions. We have

also shown, in an alkynyl-bridged, heterobinuclear Rh/

Ir complex, that nucleophilic attack occurs first at Ir

and that in the case of the hydride group this is followed

Intermediate C can be considered as containing a metal-

substituted alkyne moiety (RCtCM), which is π-bound

to the second metal, with the process shown in eq 1

being analogous to migratory insertion involving mutu-

ally cis hydride and alkyne ligands. We sought to extend

this analogy to carbanionic ligands in efforts to induce

migration of an alkyl or related group from one metal

to the adjacent â-carbon of an µ-η¹:η²-alkynyl group.

Certainly the analogous migrations of alkyls to coordi-

nated alkynes are well-known.^14d,15Although most

previous examples of C-C bond formation involving

alkynyl groups occur at the alkynyl R-carbon,^{1f,o,q,4a,b,8b,3l}

we are aware of two examples of apparent migration to

the â-carbon of a bridging alkynyl group to generate

vinylidene-bridged complexes. In the first case, migra-

tion of an aryl group in a Ti/Cu complex occurs,¹⁶

whereas in the second, migration of a methyl ligand in

a diiridium complex was observed.^6hIn this report we

describe our studies aimed at probing this uncommon

mode of carbon-carbon bond formation.

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Can. J . Chem. 1996, 74, 2289. (b) Sterenberg, B. T.; McDonald, R.;

Cowie, M. Organometallics 1997, 16, 2297. (c) Antwi-Nsiah, F. H.;

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Submitted to Inorg. Chim. Acta.

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Exp er im en ta l Section

All solvents were distilled over appropriate drying agents

(sodium/benzophenone for THF, ether, benzene, and pentane;

phosphorus pentoxide for halogenated solvents) before use.

Reactions were done at ambient temperature using standard

Schlenk techniques (under either dinitrogen or argon) unless

otherwise stated. Prepurified dinitrogen, argon, and carbon

monoxide were purchased from Praxair Products, Inc., and

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Valerga, P. J . Am. Chem. Soc. 1997, 119, 6529. (d) J ime´nez, M. V.;

Sola, E.; Mart´ınez, A. P.; Lahoz, F. J .; Oro, L. A. Organometallics 1999,

18, 1125.

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L.-S.; Cowie, M. Organometallics 1995, 14, 2374. (c) Wang, L.-S.; Cowie,

M. Organometallics 1995, 14, 3040. (d) Antwi-Nsiah, F. H.; Oke, O.;

Cowie, M. Organometallics 1996, 15, 506.

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Bergman, R. G. Pure Appl. Chem. 1981, 53, 161. (d) J ordan, R. F.;

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Lang, H.; van Koten, G. J . Organomet. Chem. 1995, 505, 123.

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11, 1701.

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1988, 341, 421.

5332 Organometallics, Vol. 18, No. 25, 1999

George et al.

were used as received. Ammonium hexachloroiridate(IV) and

potassium hexachlororhodate(III) were obtained from Van-

couver Island Precious Metals; hydrated rhodium trichloride

was purchased from Colonial Metals. The compounds [IrAgCl-

(CCPh)(CO)(dppm)₂],¹⁷[Rh₂(CO)₄Cl₂],¹⁸and [Rh₂(COD)₂Cl₂]¹⁹

were prepared by literature methods. Deuterated solvents

(obtained from Cambridge Isotope Laboratories) were distilled

and stored over molecular sieves under argon. The compound

[RhIr(CO)₂(CCPh)(dppm)₂][SO₃CF₃] (1b ) was prepared as

previously reported.^7bThe silver salts (AgBF₄, AgO₃SCF₃) were

obtained from Aldrich and were stored and used in an argon-

filled glovebox. All other compounds were obtained from

Aldrich and used as received. Spectroscopic data for all new

compounds are given in Table 1.

NMR spectra were obtained on either a Bruker 400 or 200

MHz spectrometer. Infrared spectra were run on either a

Perkin-Elmer Model 1600 FTIR or a Nicolet Magna-IR 750

spectrometer as either solids (Nujol mulls on KBr disks) or

solutions (KCl cell with 0.5 mm window path length). Elemen-

tal analyses were conducted by the microanalytical service

within the department.

P r ep a r a tion of Com p ou n d s. (a ) [Rh Ir (CO)₂(µ-CCP h )-

(d p p m )₂][BF ₄] (1a ). A solid sample of [IrAgCl(CO)(CCPh)-

(dppm)₂] (1.000 g, 810.7 µmol) was placed in a flask with

[Rh₂(COD)₂Cl₂] (180.0 mg, 409.9 µmol), NaBF₄(90.0 mg, 820

µmol), and 30 mL of CH₂Cl₂, yielding a brown-red mixture,

which was stirred for 1 h, followed by the introduction of 1

atm of carbon monoxide. The solution was filtered after a

further 3 h of stirring, then the solvent was removed in vacuo.

The resulting solid was recrystallized from 1:3 CH₂Cl₂/ether

and washed three times with 10 mL aliquots of pentane. The

product was dissolved in 10 mL of CH₂Cl₂and the solution

refluxed for 1 h. THF (20 mL) was added, and the reflux

continued for 5 h, after which time the solvent was reduced

to 10 mL and the product was precipitated by the addition of

diethyl ether (60 mL). The red product was then washed three

times with 10 mL aliquots of ether and then recrystallized from

CH₂Cl₂/Et₂O (1:1) at room temperature, affording [RhIr(CO)₂-

(µ-CCPh)(dppm)₂][BF₄]‚1.5CH₂Cl₂(0.8850 g, 76%) as burgundy

crystals. Anal. Calcd for C_61.5H₅₂O₂P₄Cl₃RhIrBF₄: C, 51.47; H,

3.65; Cl, 8.88. Found: C, 51.13; H, 3.56, Cl, 8.42.

could be isolated free from [RhIr(CO)₃(µ-CCPh)(dppm)₂][BF₄],

which is formed via loss of HBF₄, and were characterized in

solution only.

(e) [Rh Ir Br (CO)(µ-CCP h CH₂CHdCH₂)(µ-CO)(d p p m )₂]-

[BF ₄] (5a ). A sample of 1a (140.0 mg, 107.0 µmol) was placed

in a flask with 15 mL of CH₂Cl₂, forming a red-purple solution.

Allyl bromide (9.3 µL, 107.5 µmol) was added via gastight

syringe, causing a color change to brown. After stirring for 3

h, the solution had turned a deep cherry red. The solvent

volume was then reduced to 2 mL under nitrogen flow, and

the product precipitated by the addition of 40 mL of diethyl

ether. The product was recrystallized from 1:1:4 CH₂Cl₂/THF/

ether, then washed three times with ether. Yield: 110.0 mg

(72%). Anal. Calcd for C₆₃H₅₄BrO₂P₄RhIrBF₄: C, 52.95; H,

3.81. Found: C, 52.85; H, 3.94.

(f) [Rh Ir Br (CO)(µ-CCP h CH₂CHdCH₂)(µ-CO)(d p p m )₂]-

[SO₃CF ₃] (5b). A sample of 1b (50.5 mg, 36.9 µmol) was placed

in a flask with 5 mL of benzene and 7 mL of THF. Allyl

bromide (3.2 µL, 44 µmol) was added via gastight syringe,

causing a slow color change to brown. After stirring for 1 h,

the solution was allowed to stand undisturbed overnight,

yielding a crop of dark purple crystals. The red-brown super-

natant was removed, and the product was dried under vacuum.

Yield: 28.1 mg (51% yield). Anal. Calcd for C₆₄H₅₄BrO₅P₄-

RhIrF₃S: C, 51.55; H, 3.65; Br, 5.36. Found: C, 51.56; H, 3.54;

Br, 5.64.

(g) [Rh Ir Cl(CO)(µ-CCP h CH₂CHdCH₂)(µ-CO)(d p p m )₂]-

[BF ₄] (6a ). A sample of 1a (29.4 mg, 22.5 µmol) was placed in

a flask with 5 mL of CH₂Cl₂, forming a red-purple solution.

Allyl chloride (1.8 µL, 22 µmol) was added via gastight syringe,

causing a color change to brown. After stirring overnight, the

solution had turned a deep cherry red. The solvent volume

was then reduced to 1 mL under nitrogen flow, and the product

precipitated by the addition of 10 mL of ether. The product

was recrystallized from 1:5 CH₂Cl₂/ether, then washed twice

with 5 mL of ether, affording [RhIrCl(CO)(µ-CCPhCH₂CHd

CH₂)(µ-CO)(dppm)₂][BF₄](6a )‚1.75CH₂Cl₂as cherry-red crys-

tals. Yield: 15.3 mg (44%). Anal. Calcd for C_64.75H_57.5Cl_4.5O₂P₄-

RhIrBF₄: C, 50.73; H, 3.78; Cl, 10.41. Found: C, 51.09; H,

3.69; Cl, 10.49.

(h ) [Rh Ir Cl(CO)(µ-CCP h CH₂CHdCH₂)(µ-CO)(d p p m )₂]-

[SO₃CF ₃] (6b). Compound 1b (0.200 g, 145 µmol) was placed

in a 100 mL flask along with 30 mL of CH₂Cl₂, affording a

red-purple solution. Allyl chloride (12 µL, 146 µmol) was added

by microliter syringe, and the mixture was stirred overnight

at ambient temperature, during which time the solution color

changed from burgundy to cherry red. The solvent volume was

reduced to ca. 3 mL in vacuo, and 30 mL of diethyl ether was

added, causing the product to precipitate as a red crystalline

solid. Recrystallization from CH₂Cl₂/diethyl ether (1:1) at

ambient temperature afforded red crystals of [RhIrCl(CO)(µ-

CCPhCH₂CHdCH₂)(µ-CO)(dppm)₂][SO₃CF₃] (6b)‚0.5CH₂Cl₂.

Yield: 0.085 g (39%). Anal. Calcd for C_64.5H₅₅O₅Cl₂P₄RhIrF₃S:

C, 52.02; H, 3.72; Cl, 4.76; S, 2.15. Found: C, 52.44; H, 3.65;

Cl, 3.60; S, 2.51.

(b) [Rh Ir (F BF ₃)(CO)₂(µ-H)(µ-CCP h )(d p p m )₂][BF ₄] (2a ).

An NMR tube was charged with 1a (20.2 mg, 15.4 µmol) and

0.4 mL of CD₂Cl₂. Excess HBF₄‚OEt₂(5.1 µL, 37 µmol) was

added, resulting in a color change from red-brown to yellow-

brown. The product was spectroscopically characterized as

[RhIr(FBF₃)(CO)₂(µ-H)(µ-CCPh)(dppm)₂][BF₄] (2a ) but could

not be isolated pure due to facile loss of HBF₄to give the

starting material.

(c) [Rh Ir (OSO₂CF ₃)(CO)₂(µ-H)(µ-CCP h )(d p p m )₂][BF ₄]

(2b). An NMR tube was charged with 1b (20.2 mg, 15.4 µmol)

and 0.4 mL of CD₂Cl₂. Excess HBF₄‚OEt₂(5.1 µL, 37 µmol)

was added, resulting in a color change from red-brown to

yellow-brown. This was shown to contain [RhIr(OSO₂CF₃)-

(CO)₂(µ-H)(µ-CCPh)(dppm)₂][BF₄] (2b) as the only organome-

tallic compound; however, this compound could not be isolated

pure due to facile loss of free acid to give 1 and so was

characterized in solution only.

(d ) Rea ction of 2a w ith CO. An NMR tube was charged

with 1a (20.2 mg, 15.4 mmol) and 0.4 mL of CD₂Cl₂. Excess

HBF₄‚OEt₂was added, forming a brown-yellow solution. This

was placed under an atmosphere of CO, causing a color change

to yellow. The NMR spectra of this sample showed complete

conversion to [RhIr(CO)₃(µ-H)(µ-CCPh)(dppm)₂][BF₄]₂(3). Upon

standing under CO, this converted to the vinylidene complex

[RhIr(CO)₄(µ-CCHPh)(dppm)₂][BF₄]₂(4). Neither compound

(i) Low -Tem p er a t u r e R ea ct ion of 1b w it h CH ₂d

CHCH₂Br or CH₂dCHCH₂Cl. An NMR tube was charged

with 1b (17.5 mg, 12.8 µmol) and 0.4 mL of CD₂Cl₂. The sample

was cooled to -90 °C and treated with allyl bromide (1.1 µL,

13 µmol). The NMR spectra of this sample showed the

formation of the olefin adduct [RhIr(CO)₂(η²-CH₂CHCH₂Br)-

(µ-CCPh)(dppm)₂][SO₃CF₃] (7). Warming to -30 °C resulted

in complete conversion to the oxidative addition product

[RhIrBr(η¹-CH₂CHCH₂)(CO)₂(µ-CCPh)(dppm)₂][SO₃CF₃] (9).

Upon warming to room temperature, this compound reacted

further, slowly giving the allylvinylidene bromo complex 5b

as the major product. The same procedure was used to monitor

the reaction of 1b with allyl chloride. Analogous intermediates

8 and 10 were observed before formation of the final product

6b.

(17) Hutton, A. T.; Pringle, P. G.; Shaw, B. L. Organometallics 1983,

2, 1889.

(18) McCleverty, J . A.; Wilkinson, G. Inorg. Synth. 1990, 28, 85.

(19) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88.

Allyl-to-Alkynyl Migration

Organometallics, Vol. 18, No. 25, 1999 5333

Ta ble 1. Sp ectr a l Da ta ^a

NMR

compound

δ(³¹P{¹H})^b

21.8 (dm,

δ(¹³C{¹H})^c

δ(¹H)^c

IR, cm^{-1 d}

[RhIr(BF₄)(CO)₂(µ-CCPh)(µ-H)-

(dppm)₂][BF₄] (2a )

186.7 (dt, Rh-CO,

¹J _RhC) 81.8 Hz,

²J _PC) 15.5 Hz),

170.6 (t, Ir-CO,

²J _PC) 9.6 Hz)

4.65 (m, PCH₂P),

3.40 (m, PCH₂P),

-19.00 (bd, µ-H,

¹J _RhH) 6 Hz)

2068

1985

¹J _RhP) 91.7 Hz),

-4.5 (m)

[RhIr(O₃SCF₃)(CO)₂(µ-CCPh)(µ-H)-

(dppm)₂][BF₄] (2b)

22.7 (dm,

4.75 (m, PCH₂P),

3.55 (m, PCH₂P),

-19.42 (m, µ-H,

¹J _RhP) 109.2 Hz),

-6.0 (m)

2

¹J _RhH≈ J _PH≈ 6 Hz)

[RhIr(CO)₃(µ-H)(µ-CCPh)(dppm)₂]-

[BF₄]₂(3)

22.0 (dm,

188.1 (dt, Rh-CO,

¹J _RhC) 83.0 Hz,

²J _PC) 16.7 Hz),

161.4 (t, Ir-CO,

²J _PC) 7.7 Hz),

4.45 (m, PCH₂P),

3.70 (m, PCH₂P),

-7.78 (m, µ-H,

2018

1996

1984

¹J _RhP) 105.1 Hz),

-19.1 (m)

¹J _RhH) 11.7 Hz,

²J _PH) 8.0, 2.4 Hz)

154.9 (t, Ir-CO,

²J _PC) 4.4 Hz)

[RhIr(CO)₄(µ-CCHPh)(dppm)₂]-

[BF₄]₂(4)

9.5 (dm,

210.6 (dm, µ-CdCHPh,

¹J _RhC) 28.2 Hz),

188.7 (dm, Rh-CO,

¹J _RhC) 51.0 Hz),

176.1 (dm, Rh-CO,

¹J _RhC) 60.6 Hz),

168.1 (t, Ir-CO,

²J _PC) 9.9 Hz),

4.45 (m, PCH₂P),

4.55 (m, PCH₂P),

6.94 (b, 1H)

2105

2092

2059

1987

¹J _RhP) 79.1 Hz),

-20.9 (m)

161.7 (s, Ir-CO)

185.8 (dm, µ-CO,

¹J _RhC) 39.6 Hz),

178.9 (t, Ir-CO,

²J _PC) 10.4 Hz)

[RhIrBr(CO)(µ-CCPhCH₂CHd

CH₂)(µ-CO)(dppm)₂][BF₄] (5a )

6.2 (dm,

6.20 (b, H_c), 4.15 (d, H_e,

³J _HH) 16 Hz),

1998

1803

¹J _RhP) 105.4 Hz),

-0.96 (m)

4.05 (d, H_d,

³J _HH) 9 Hz),

3.80 (m, PCH₂P),

3.40 (m, H_a+ H_b),

3.20 (m, PCH₂P)

5.90 (b, H_c),^e

6.3 (dm,

186.4 (dm, µ-CO,

¹J _RhC) 38.6 Hz),^e

178.8 (t, Ir-CO,

²J _PC) 9.9 Hz)

¹J _RhP) 105.3 Hz),^e

1.7 (dm,

3.90 (b, H_a+ H_d+ H_e),

3.60, 2.80 (m, PCH₂P),

3.40 (b, H_b)

²J _PP) 260.7 Hz),

-2.4 (dm,

²J _PP) 260.7 Hz)

6.5 (dm,

[RhIrBr(CO)(µ-CCPhCH₂CHd

CH₂)(µ-CO)(dppm)₂][O₃SCF₃] (5b)

186.0 (dm, µ-CO,

¹J _RhC) 41.1 Hz),

179.1 (t, Ir-CO,

²J _PC) 10.3 Hz),

200.4 (m, µ-CdCRPh,

¹J _RhC) 18.7 Hz),

157.4 (t, µ-CdCRPh,

³J _PC) 3.8 Hz),

6.20 (b, H_c), 4.20 (d, H_e,

³J _HH) 16 Hz),

1986

1802

¹J _RhP) 105.4 Hz),

-0.5 (m)

4.05 (d, H_d,

³J _HH) 8 Hz),

3.85 (m, PCH₂P),

3.40 (m, H_a+ H_b),

3.30 (m, PCH₂P)

38.5 (s, CdCPhCH₂-

CHdCH₂),

123.5 (s, CdCPhCH₂-

CHdCH₂),

99.1 (s, CdCPhCH₂-

CHdCH₂)

6.4 (dm,

186.4 (dm, µ-CO,

¹J _RhC) 39.4 Hz),^e

178.9 (t, Ir-CO,

²J _PC) 10.0 Hz)

¹J _RhP) 105.4 Hz),^e

2.1 (dm,

²J _PP) 261.3 Hz),

-1.96 (dm,

²J _PP) 261.3 Hz)

6.2 (dm,

[RhIrCl(CO)(µ-CCPhCH₂CHd

CH₂)(µ-CO)(dppm)₂][BF₄] (6a )

186.1 (dm, µ-CO,

¹J _RhC) 39.6 Hz),

178.9 (t, Ir-CO,

²J _PC) 10.5 Hz)

6.22 (b, H_c), 4.37 (d, H_e,

³J _HH) 16 Hz),

2001

1810

¹J _RhP) 105.1 Hz),

-0.7 (m)

4.02 (d, H_d,

³J _HH) 9 Hz),

3.77 (m, PCH₂P),

3.40 (m, H_a+ H_b),

3.25 (m, PCH₂P)

6.21 (b, H_c), 4.27 (d, H_e,

³J _HH) 17 Hz),

[RhIrCl(CO)(µ-CCPhCH₂CHd

CH₂)(µ-CO)(dppm)₂][CF₃SO₃] (6b)

6.3 (dm,

2015

1794

¹J _RhP) 105 Hz),

-0.5 (m)

4.01 (d, H_d,

³J _HH) 9 Hz),

3.78 (m, PCH₂P),

3.41 (m, H_a+ H_b),

3.29 (m, PCH₂P)

5334 Organometallics, Vol. 18, No. 25, 1999

George et al.

Ta ble 1 (Con tin u ed )

NMR

compound

δ(³¹P{¹H})^b

δ(¹³C{¹H})^c

δ(¹H)^c

IR, cm^{-1 d}

[RhIr(CO)₂(CH₂CHCH₂Br)(µ-CCPh)-

(dppm)₂][O₃SCF₃] (7)

16.7 (ddm,

190.0 (dt, Rh-CO,

²J _PC) 13.9 Hz,

¹J _RhC) 83.6 Hz),^e

5.2 (b, H_a+ H_b),^e

2.0 (b, 1H),

0.0 (b, 1H),

¹J _RhP) 92.7 Hz,

²J _PP) 316.4 Hz),^e

14.8 (ddm,

180.2 (d, Ir-CO,

-0.8 (b, 1H),

4.20, 4.00 (b, PCH₂P)

¹J _RhP) 103.8 Hz,

¹J _RhC) 17.6 Hz)

²J _PP) 316.4 Hz),

-11.7 (m)

[RhIr(CO)₂(CH₂CHCH₂Cl)-

(µ-CCPh)(dppm)₂][O₃SCF₃] (8)

17.8 (ddm,

5.1 (b, H_a+ H_b),

2.0 (b, 1H),

0.7 (b, 1H),

¹J _RhP) 94 Hz,

²J _PP) 314 Hz),

14.8 (ddm,

4.2, 4.0 (b, PCH₂P)

¹J _RhP) 94 Hz,

²J _PP) 314 Hz),

-11.6 (m)

[RhIr(η¹-CH₂CHdCH₂)(µ-Br)(CO)₂-

(µ-CCPh)(dppm)₂][O₃SCF₃] (9)

22.1 (d,

186.3 (dt, Rh-CO,

²J _PC) 15.2 Hz,

¹J _RhC) 83.3 Hz),^f

4.90 (H_c, ddt,

¹J _RhP) 104.8 Hz),

-24.3 (s)

³J _HH) 16, 11, 8 Hz),^f

4.15 (H_d, dd,

167.7 (t, Ir-CO,

³J _HH) 11, 2 Hz),

²J _PC) 7.6 Hz)

3.65 (H_e, dd,

³J _HH) 16, 2 Hz),

2.40 (H_a, H_b, dt,

³J _PH) 9 Hz,

³J _HH) 8 Hz)

4.65, 4.50 (m, PCH₂P)

[RhIr(η¹-CH₂CHdCH₂)(µ-Cl)(CO)₂-

(µ-CCPh)(dppm)₂][O₃SCF₃] (10)

22.5 (d,

4.58 (H_c, ddt,

¹J _RhP) 106.2 Hz),^g

-19.5 (s)

³J _HH) 16,11,8 Hz),^g

4.18 (H_d, dd,

³J _HH) 10, 2 Hz),

3.28 (H_c, dd,

³J _HH) 16, 2 Hz),

2.01 (H_a, H_b, dt,

³J _PH) 9 Hz,

³J _HH) 8 Hz),

4.59, 4.14 (m, PCH₂P,

²J _HH) 7 Hz)

[RhIr(BF₄)(CO)(µ-CCPhCH₂CHd

CH₂)(µ-CO)(dppm)₂][BF₄] (11)

11.9 (dm,

188.1 (dm, µ-CO,

¹J _RhC) 40.1 Hz),

6.15 (m, H_c),

4.56 (m, H_e,

³J _HH) 16 Hz),

4.39 (m, H_d,

³J _HH) 9 Hz),

3.57 (m, H_a,

²J _HH) 14 Hz),

2.95 (m, H_b,

2023 (s)

1814 (s)

¹J _RhP) 115.3 Hz),

1.14 (m)

176.8 (t, Ir-CO,

²J _PC)10.3 Hz),

190.0 (m, µ-CdCRPh,

¹J _RhC) 22.6 Hz),

151.5 (t, µ-C)CRPh,

³J _PC) 3.8 Hz),

38.7 (s, CdCPhCH₂-

CHdCH₂),

³J _HH) 11, 14 Hz),

3.90, 3.85 (m, PCH₂P),

96.4 (s, CdCPhCH₂-

CHdCH₂)

3.45, 3.25 (m, PCH₂P)

[RhIr(CO)₃(µ-CCPhCH₂CHd

CH₂)(dppm)₂][BF₄]₂(12)

23.70 (m,

194.6 (dt, Rh-CO,

¹J _RhC) 53.0 Hz,

²J _PC) 18.5 Hz),

173.6 (t, Ir-CO,

²J _PC) 6.9 Hz),

6.40 (m, H_c),

4.90 (m, PCH₂P),

4.55 (m, H_e),

4.40 (m, H_d),

4.05 (m, PCH₂P),

2019

2037

2086

¹J _RhP) 84.47 Hz,

²J _AB) 386.9 Hz,

^XJ _AC) 14.95 Hz,

²J _AD) 46.96 Hz),

13.58 (m,

160.0 (m, Ir-CO),

3.80 (m, PCH₂P + H_b),

3.60 (m, PCH₂P),

3.25 (d, H_a)

¹J _RhP) 87.8 Hz,

²J _BC) 43.50 Hz,

^XJ _BD) 11.0 Hz),

-23.96(m,

225.3 (m, µ-CdCRPh,

¹J _RhC) 21.0 Hz),

161.7 (m, µ-CdCRPh),

42.2 (s, CdCPhCH₂-

²J _CD) 302.6 Hz),

-29.21 (m)

CHdCH₂),

113.1 (s, CdCPhCH₂-

CHdCH₂),

82.9 (s, CdCPhCH₂-

CHdCH₂)

[RhIr(CO)₂(η³-CH₂CHCH₂)-

(µ-CCPh)(dppm)₂][BF₄]₂(13a )

17.2 (m,

4.98 (d, H_d),

4.60 (m, H_c+ PCH₂P),

4.18 (m, PCH₂P),

2025 (s)

1880 (s)

¹J _RhP) 95.6 Hz,

²J _PP) 318.1 Hz),

15.5 (ddm,

3.60 (m, 2 PCH₂P),

3.45 (m, H_e),

2.10 (m, H_a),

¹J _RhP) 95.5 Hz,

²J _PP) 318.1 Hz),

-12.1 (m)

1.00 (m, H_b)

Allyl-to-Alkynyl Migration

Organometallics, Vol. 18, No. 25, 1999 5335

Ta ble 1 (Con tin u ed )

NMR

compound

δ(³¹P{¹H})^b

δ(¹³C{¹H})^c

δ(¹H)^c

4.98 (d, H_d),

4.55 (m, H_c+ PCH₂P),

4.12 (m, PCH₂P),

3.78 (m, 2 PCH₂P),

3.45 (m, H_e),

IR, cm^{-1 d}

[RhIr(CO)₂(η³-CH₂CHCH₂)-

(µ-CCPh)(dppm)₂][O₃SCF₃]₂(13b)

17.5 (m,

190.6 (dt, Rh-CO,

¹J _RhC) 83.5 Hz,

²J _PC) 15.0 Hz),

180.3 (dt, Ir-CO,

¹J _RhC) 17.0 Hz,

²J _PC≈ 6 Hz),

2013

1882

¹J _RhP) 95.4 Hz,

²J _PP) 316.7 Hz),

15.5 (m,

¹J _RhP) 5.0 Hz,

²J _PP) 316.7 Hz),

-11.6 (dm,

2.06 (m, H_a),

1.00 (m, H_b)

106.1 (s, Ir-C≡CPh,

²J _PP) 335.3 Hz),

-12.2 (dm,

¹J _RhC) 3.7 Hz),

100.0 (s, CH₂CHCH₂),

64.6 (m, Ir-CtCPh,

¹J _RhC) 5.1 Hz,

²J _PP) 335.3 Hz)

²J _PC) 15.0 Hz),

58.0 (s, CH₂CHCH₂),

48.4 (s, CH₂CHCH₂)

a

Abbreviations used: NMR, m ) multiplet, dm ) doublet of multiplets, s ) singlet; d ) doublet, t ) triplet, q ) quartet, dt ) doublet

of triplets, ddt ) doublet of doublets of triplets, dtt ) doublet of triplets of triplets, ddm ) doublet of doublets of multiplets, b ) broad,

b

bd ) broad doublet; IR, w ) weak, m ) medium, s ) strong, b ) broad. Vs 85% H₃PO₄in CD₂Cl₂at 25 °C unless otherwise stated.^cVs

d

TMS in CD₂Cl₂at 25 °C unless otherwise stated. Labeling scheme for carbons and protons is given in ref 31. Nujol mull on KBr disks

g

unless otherwise noted, ν_COunless otherwise noted. ^e-80 °C. ^f-60 °C. 0 °C.

(j) [Rh Ir (BF₄)(CO)(µ-CCP h CH₂CHdCH₂)(µ-CO)(dppm )₂]-

[BF ₄] (11). Compound 1a (140.0 mg, 107.0 µmol) was placed

in a flask and dissolved in 20 mL of CH₂Cl₂. Allyl bromide

(9.2 µL, 106 µmol) was added via gastight syringe, causing a

color change from red-purple to brown. This solution was

stirred for 3 h, over which time the solution changed to a deep

cherry red. A solution of AgBF₄(21.0 mg, 107.9 µmol) in 10

mL of THF was added via cannula, resulting in the formation

of a red-orange suspension. This was stirred 1 h, filtered

through Celite, and taken to dryness in vacuo. The residue

was recrystallized from 1:15 CH₂Cl₂/ether and then again from

1:3:25 CH₂Cl₂/THF/ether. Yield: 122.0 mg (79%). Anal. Calcd

for C₆₃H₅₄O₂P₄RhIrB₂F₈: C, 52.70; H, 3.79. Found: C, 52.53;

H, 3.62.

(k) [Rh Ir (CO)₃(µ-CCP h CH₂CHdCH₂)(dppm )₂][BF₄]₂(12).

An NMR tube was charged with 11 (10.2 mg, 7.1 µmol) and

0.4 mL of CD₂Cl₂. An atmosphere of CO was placed over the

sample, resulting in a color change from red-orange to yellow.

The ³¹P{¹H} NMR spectrum of this sample at -20 °C showed

complete conversion to 12. This compound was not isolated in

the solid state, due to facile loss of CO, and was characterized

in solution only.

(l) [Rh Ir (η³-CH₂CHCH₂)(CO)₂(µ-CCP h )(d p p m )₂][BF ₄]₂

(13a ). Silver fluoborate (28.0 mg, 144 µmol) and 1a (181.0 mg,

138.3 µmol) were dissolved in 20 mL of CH₂Cl₂, forming a

purple solution. Allyl bromide (12.0 µL, 139 µmol) was added,

causing a rapid color change to brown. After stirring for 2 h,

the suspension was filtered to give a clear yellow solution and

the solvent removed under vacuum. The brown residue was

dissolved in 2 mL of CH₂Cl₂and 4 mL of THF, and the product

precipitated by the slow addition of 25 mL of ether. The yellow

solid was washed three times with 15 mL of ether and dried.

Yield: 160.4 mg (81%). Anal. Calcd for C₆₃H₅₄O₂P₄RhIrB₂F₈:

C, 52.70; H, 3.79. Found: C, 52.62; H, 3.79.

mL of CD₂Cl₂and stirred for 5 min. The purple solution was

then transferred to an NMR tube and cooled to -80 °C. The

³¹P{¹H} NMR spectrum of this sample showed the presence

1

of one new compound (³¹P{¹H}, 23.6 (dm, J _RhP) 101.3 Hz),

7.5 (b); ¹H, 4.55 (b), 4.00 (b), each 2H) in addition to small

amounts of 1a . Upon addition of allyl bromide (1.5 µL, 17 µmol)

to this solution at -80 °C, the color changed form purple to

brown. This solution was shown to contain a mixture of 13

and two new species (A, ³¹P{¹H}: 28.3 (dm, ¹J _RhP) 107.9 Hz),

1

-24.2 (b); B 27.8 (dm, J _RhP) 109.3 Hz), -24.2 (b)). Even at

-80 °C, these compounds rapidly transformed into 13.

(o) Rea ction of 13 w ith Ch lor id e An ion . An NMR tube

was charged with 13a (10.8 mg, 7.5 µmol) and PPNCl (11.0

mg, 19.2 µmol). This was cooled to -80 °C, and 0.4 mL of CD₂-

Cl₂was added, forming a red solution. This was shown by ³¹P-

{¹H} NMR spectroscopy to contain a mixture of 13 and 1 in a

5:1 ratio. Warming to -60 °C caused nearly complete conver-

sion to 1. Upon warming to ambient temperature, the allyl-

vinylidene chloride 6a was formed, which reacted further to

give the dichloro species [RhIrCl₂(CO)(µ-CCPhCH₂CHdCH₂)-

(dppm)₂].^7a,20

(p ) Attem p ted Rea ction of 1 w ith CH₃O₃SCF ₃. An NMR

tube was charged with 1b (24.0 mg, 17.5 µmol) and 0.4 mL of

CD₂Cl₂. Methyl triflate (2.0 µL, 16 µmol) was added, resulting

in no change in the ³¹P{¹H} spectrum.

(q) Attem p ted Rea ction of 1 w ith P h CH ₂Br . An NMR

tube was charged with 1b (20.0 mg, 14.6 µmol) and 0.4 mL of

CD₂Cl₂. Benzyl bromide (1.3 µL, 11 µmol) was added. NMR

spectroscopy showed no reaction. The same result was obtained

after the addition of AgO₃SCF₃(2.7 mg, 11 µmol).

(r ) Attem p ted Rea ction of 1 w ith CH₃I. An NMR tube

was charged with 1b (19.9 mg, 14.5 µmol) and 0.4 mL of CD₂-

Cl₂. Methyl iodide (0.9 µL, 15 µmol) was added, resulting in

no change in the ³¹P{¹H} NMR spectrum.

(m ) [Rh Ir (η³-CH₂CHCH₂)(CO)₂(µ-CCP h )(d p p m )₂][SO₃-

CF ₃]₂(13b). Silver triflate (27.1 mg, 106 µmol) and 1b (140.6

mg, 102.6 µmol) were dissolved in 8 mL of CH₂Cl₂, forming a

purple solution. Allyl bromide (8.9 µL, 103 µmol) was added,

causing a rapid color change to brown. After stirring for 1 h,

the suspension was filtered to give a clear yellow solution. The

solution was then reduced to 1 mL, diluted with 3 mL of THF,

and precipitated by the slow addition of 25 mL of ether. The

yellow solid was washed three times with 10 mL of ether and

dried. Yield: 131.0 mg (80%). Anal. Calcd for C₆₅H₅₄O₈S₂P₄-

RhIrF₆: C, 50.04; H, 3.49. Found: C, 50.47; H, 3.50.

(n ) Low -Tem p er a tu r e Rea ction of 1a w ith AgO₃SCF ₃

a n d CH₂dCHCH₂Br . Compound 1a (23.0 mg, 17.6 µmol) was

placed in a flask with AgO₃SCF₃(14.5 mg, 56.4 µmol) and 0.6

X-r a y Da ta Collection a n d Str u ctu r e Solu tion . Deep

red crystals of [RhIrCl(CO)₂(µ-CdC(Ph)CH₂CHdCH₂)(dppm)₂]-

[SO₃CF₃]‚1.5CH₂Cl₂were obtained from layering Et₂O onto a

CH₂Cl₂solution of the compound. Data were collected on a

Bruker P4/RA/SMART 1000 CCD diffractometer using Mo KR

radiation at -80 °C. Unit cell parameters were obtained from

a least-squares refinement of the setting angles of 8192

reflections from the data collection. The systematic absences

indicated the space group to be P2₁/c (No. 14). The data were

corrected for absorption through use of the SADABS proce-

(20) [RhIrCl₂(CO)(µ-CCPhCH₂CHdCH₂)(dppm)₂]‚CH₂Cl₂: Anal. Calcd

for C₆₃H₅₆OCl₄P₄RhIr: C, 54.44; H, 4.06; Cl, 10.20. Found: C, 54.40;

H, 3.91; Cl, 10.52. IR: ν(CO) ) 1991 cm^-1

.

5336 Organometallics, Vol. 18, No. 25, 1999

George et al.

Ta ble 2. Cr ysta l Da ta a n d Su m m a r y of Da ta

Collection a n d Refin em en t for Com p ou n d

6b‚1.5CH₂Cl₂

Sch em e 1

formula

cryst size (mm)

fw

C_65.5H₅₇Cl₄F₃IrO₅P₄RhS

0.28 × 0.24 × 0.22

1573.96

cryst syst

space group

a, Å

monoclinic

P2₁/c (No. 14)

23.707(2)

b, Å

22.365(2)

c, Å

24.030(2)

â, deg

91.715(1)

12735(1)

V, Å³

Z

D

8

_calcd, Mg/M³

1.642

diffractometer

radiation (λ), Å

max 2θ, deg

µ, mm^-1

Bruker P4/RA/SMART 1000 CCD

Mo KR (0.71073)

51.60

2.706

6264

F(000)

T, K

193

no. of indep reflns

24280

no. of obsd reflns (I > 2σ(I)) 15567

no. of params refnd

goodness of fit

1505

1.043

0.0574

0.1538

2.27, -1.80

2

R₁(F_og 2σ(F_o²))

wR₂(all data)

late transition metal is not common but is well docu-

∆F_max, ∆F_min, e/Å³

1

mented.²⁵The hydride signal of 2a appears in the H

NMR spectrum at δ -19.00, and the small observed

coupling to rhodium (6 Hz) is consistent with an iridium-

bound hydride having a weak bridging interaction with

rhodium, as was found for other products of HX addition

to 1.^7a,bThe IR spectrum shows only terminal carbonyls

(ν_CO) 2068, 1985 cm^-1), in agreement with the ¹³C-

{¹H} NMR spectrum, which shows a terminal carbonyl

dure.²¹See Table 2 for a summary of crystal data and X-ray

data collection information.

The structure was solved using the DIRDIF-96 program

system,²²and refinement was completed using the program

SHELXL-93.²³Hydrogen atoms were assigned positions based

on the geometries of their attached carbon atoms and were

given thermal parameters 20% greater than those of the

attached carbons. There are two crystallographically indepen-

dent formula units in the asymmetric unit. One of the two

crystallographically independent triflate anions was found to

be disordered over two orientations occupying the same region

of space, necessitating the use of the following distance

restraints within orientation: d(S-O) ) 1.45 Å; d(S-C) ) 1.80

Å; d(F-C) ) 1.35 Å; d(F‚‚‚F) ) 2.20 Å; d(O‚‚‚O) ) 2.37 Å; d(F‚‚‚

O)(gauche) ) 3.04 Å. Distance restraints were also applied to

achieve idealized geometries within two of the solvent CH₂-

Cl₂molecules (d(Cl-C) ) 1.80 Å; d(Cl‚‚‚Cl) ) 2.95 Å). The final

model refined to values of R₁(F) ) 0.0574 (for 15567 data with

F_o²g 2σ(F_o²)) and wR₂(F²) ) 0.1538 (for all 24280 independent

data).²⁴

1

on rhodium (δ 186.7, J _RhC) 81.8 Hz) and another on

iridium (δ 170.6). A trans arrangement of the hydride

and fluoborate groups is proposed based on analogies

with the trans addition of other polar reagents to square

planar Ir(I) complexes.²⁶Furthermore, it has been

1

shown that in many [Ir(H)L₅]-type complexes, the H

NMR chemical shift of the hydride ligand depends

strongly upon the identity of the ligand in the trans

position.²⁷Compounds in which the trans ligand is a

weakly coordinating σ-donor display the hydride at high

field (cf. [Ir(H)(Cl)(X)(CO)(PPh₃)₂] (X ) BF₄, δ -26.5;

X ) O₃SCF₃, δ -21.8; X ) AlCl₄, δ -20.8),^28a[Ir(H)-

(Cl)(L)(CO)(PPh₃)₂][BF₄] (L ) CH₃C(O)CH₃, δ -21.4;

L ) H₂O, δ -21.1),^28aand [Ir(H)(Cl)(BF₄)(N₂)(PPh₃)₂]

(δ -24.8)).^28bIn complexes in which the hydride is trans

to a carbonyl, the hydride signal appears at much lower

field (cf. [Ir(H)(L)(CO)₂(PPh₃)₂][X] (L ) Cl, X ) BF₄, δ

-8.6; L ) CCPh, X ) PF₆, δ -8.8)^28b,29and [RhIr(X)-

(CO)₂(µ-H)(µ-CCR)(dppm)₂][O₃SCF₃] (X ) CCPh, R )

CH₃, δ -8.5; X ) H, R ) Ph, δ -9.9).^7a,b

Resu lts a n d Com p ou n d Ch a r a cter iza tion

Reaction of the phenylacetylide-bridged A-frame [RhIr-

(CO)₂(µ-CCPh)(dppm)₂][BF₄] (1a ) with an excess of

HBF₄leads to the formation of the hydride-bridged

product [RhIr(BF₄)(CO)₂(µ-H)(µ-CCPh)(dppm)₂][BF₄] (2a),

for which the structure shown in Scheme 1 is proposed,

resulting from formal oxidative addition of HBF₄to the

iridium center. Coordination of the fluoborate ion to a

Although the fluxionality of 2a made it difficult to

confirm the coordination of fluoborate to the metal

without a crystal structure, the proposed structure is

based on analogies with the mononuclear complexes

(21) Programs for diffractometer operation, data collection, data

reduction, and absorption were those supplied by Bruker.

(22) Beurskens, P. T.; Beurskens, G.; Bosman, W. P.; de Gelder, R.;

Garcia Granda, S.; Gould, R. O.; Israel, R.; Smits, J . M. M. The

DIRDIF-96 program system; Crystallography Laboratory, University

of Nijmegen: The Netherlands, 1996.

(25) Beck, W.; Su¨nkel, K. Chem. Rev. 1988, 88, 1405, and references

therein.

(23) Sheldrick, G. M. SHELXL-93. Program for crystal structure

determination; University of Go¨ttingen: Germany, 1993. Refinement

(26) Collman, J . P.; Sears, C. T. Inorg. Chem. 1968, 7, 27.

(27) (a) Chatt, J .; Coffey, R. S.; Shaw, B. L. J . Chem. Soc. 1965,

7391. (b) Vaska, L. J . Am. Chem. Soc. 1966, 88, 5325. (c) Taylor, R.

C.; Young, I. F.; Wilkinson, G. Inorg. Chem. 1966, 5, 20. (d) Singer,

H.; Wilkinson, G. J . Chem. Soc. A 1968, 2516.

(28) (a) Olgemo¨ller, B.; Beck, W. Inorg. Chem. 1983, 22, 997. (b)

Olgemo¨ller, B.; Bauer, H.; Beck, W. J . Organomet. Chem. 1981, 213,

C57.

2

on F_ofor all reflections (all of these having F_og -3σ(F_o²)). Weighted

R-factors wR₂and all goodnesses of fit S are based on F_o²; conventional

R-factors R₁are based on F_o, with F_oset to zero for negative F_o². The

2

observed criterion of F_o> 2σ(F_o²) is used only for calculating R₁and

is not relevant to the choice of reflections for refinement. R-factors

2

based on F_oare statistically about twice as large as those based on

F_o, and R-factors based on ALL data will be even larger.

(29) Walter, R. H.; J ohnson, B. F. G. J . Chem. Soc., Dalton Trans.

1978, 381.

(24) R₁) ∑||F_o| - |F_c||/∑|F_o|; wR₂) [∑w(F_o- F_c²)²/∑w(F_o⁴)]^1/2

.

2

Allyl-to-Alkynyl Migration

Organometallics, Vol. 18, No. 25, 1999 5337

[Ir(H)(Cl)(BF₄)(L)(PPh₃)₂] (L ) CO, N₂)^28band on the

protonation of the triflate salt, compound 1b, by HBF₄,

which yields [RhIr(O₃SCF₃)(CO)₂(µ-H)(µ-CCPh)(dppm)₂]-

[BF₄] (2b), in which the coordinated fluoborate anion is

replaced by triflate. This triflate-containing compound

is less susceptible to deprotonation than 2a , requiring

a smaller excess of acid to effect 100% conversion to the

hydride complex, and the two compounds show signifi-

cant differences in their ³¹P{¹H} NMR spectra for the

iridium-bound phosphines. Not only are the signals for

the two compounds separated by 1.5 ppm, but the

signals for 2b are much sharper than those of 2a ,

suggesting that the coordinating anion in 2a is more

labile than that in the triflate analogue. This is consis-

tent with previous reports that have shown triflate to

be a more strongly coordinating ligand than fluobo-

rate.^25,30Replacing both anions with triflate (by adding

triflic acid to 1b) gives [RhIr(O₃SCF₃)(CO)₂(µ-H)(µ-

CCPh)(dppm)₂][SO₃CF₃], which is indistinguishable

from 2b by ³¹P{¹H} and ¹H NMR spectroscopy. Although

¹⁹F NMR spectroscopy would be expected to confirm the

coordination of the fluoborate ion, the broad signals from

the excess HBF₄in solution mask any potentially

informative signals in this case.

Sch em e 2

spectrum and is replaced by a new broad signal at δ

6.94, which is in the range expected for a vinylidene

hydrogen.^2a,e

Attempts to effect the oxidative addition of methyl

iodide, methyl triflate, and benzyl bromide to compound

1 failed, with no reaction being observed in either case.

However, allyl bromide reacts with compound 1 at room

temperature to give [RhIrBr(CO)(µ-CO)(µ-CdC(Ph)CH₂-

CHdCH₂)(dppm)₂][X] (X ) BF₄, 5a ; X ) O₃SCF₃, 5b),

in which the allyl fragment has condensed with the

alkynyl moiety to give a bridging allylvinylidene ligand,

as outlined in Scheme 2. The ¹³C{¹H} NMR spectrum

of 5b shows the signals for C_Rand C_âat δ 200.4 and δ

157.4, respectively, within the expected range for vi-

nylidenes in analogous systems.²The observed coupling

of C_Rto the rhodium nucleus (¹J _RhC) 18.7 Hz) and to

both sets of phosphines confirms that the vinylidene is

in the bridging position. This spectrum also shows the

presence of two carbonyls, one of which is terminally

bound to iridium (δ 179.1) and one that bridges the two

Addition of carbon monoxide to a solution of 2a results

in the immediate formation of [RhIr(CO)₃(µ-H)(µ-CCPh)-

(dppm)₂][BF₄]₂(3). The IR spectrum of this compound

shows only terminal carbonyl stretches (ν_CO) 2018,

1996, 1984 cm^-1), while the ¹³C{¹H} NMR spectrum

shows three carbonyl signals, one due to a terminal

carbonyl on rhodium (δ 188.1, ¹J _RhC) 83.0 Hz) and two

on iridium (δ 161.4, 154.9). The ¹H NMR spectrum

shows a hydride signal at δ -7.78, with coupling to

rhodium (¹J _RhH) 11.7 Hz) and to both sets of ³¹P nuclei

2

(²J _P(Rh)H) 2.4 Hz, J _P(Ir)H) 8.0 Hz). The lower-field

chemical shift is consistent with the hydride ligand

being trans to a carbonyl. Labeling experiments in

which unenriched 2a is reacted with ¹³CO have shown

that the newly introduced carbonyl ligand is bound to

iridium, giving rise to the signal at 154.9 ppm. The large

coupling seen between this carbonyl and the hydride in

1

metals (δ 186.0, J _RhC) 41.1 Hz). The ¹³C{¹H} NMR

1

the H{³¹P} NMR spectra (²J _CH) 44 Hz) indicates that

signals for the olefinic carbons of the allyl fragment (δ

123.5 (C_δ) and 99.1 (C_ꢀ); see ref 31 for the numbering

scheme for these carbons) are closer to the expected

these ligands are mutually trans, with the carbonyl

occupying the coordination site previously occupied by

the labile fluoborate ion. In contrast, labeling the other

carbonyl positions (by adding ¹²CO to an ¹³CO-enriched

sample of 2a ) gives rise to only small (≈2 Hz) coupling

between the hydride and the labeled carbonyls, indicat-

ing that the carbonyls originally in the complex remain

trans to the alkynyl ligand on each metal.

range for an uncoordinated olefin (RC_aHdC_bH₂, δ_C

=

a

120-140; δ_C= 105-120)³²than for that expected for

b

typical olefins coordinated to rhodium or iridium, which

give rise to ¹³C{¹H} NMR signals in the range δ 40-

85.³³Similarly, the signals for the olefin moiety in the

¹H NMR spectrum (δ 6.20, 4.20, 4.05) fall closer to the

range expected for a noncoordinated alkene (δ 4.5-7.0)³²

than for a typical coordinated olefin (which, for electron-

rich metals, normally are shifted to higher field by 1-5

ppm³⁴). In addition, no coupling of either the olefinic

Under excess carbon monoxide, 3 slowly converts to

complex 4, in which the hydride ligand has migrated

from the metals to the C_âof the alkynyl ligand, forming

a bridging vinylidene group. The C_Rof the vinylidene

appears in the ¹³C{¹H} NMR spectrum at δ 210.6

(¹J _RhC) 28.2 Hz), which is diagnostic for a bridging

vinylidene,²and is similar to those seen in the other

vinylidenes described herein. Also apparent are four

carbonyl signals, two resulting from carbonyls termi-

(31) The labeling scheme for the carbons and hydrogens of the

allylvinylidene complexes is as follows: C_Rand C_ârefer to the two

carbons of the alkynyl ligand (C_Rbeing directly bound to iridium), C_γ,

C_δ, and C_ꢀare those derived from the allyl group with C_γbound to the

original â-carbon of the acetylide (C_â). H_aand H_bare the hydrogens

bound to C_γ; Hc is bound to C_δ; H_dand H_eare bound to C_ꢀ.

(32) Friebolin, H. Basic One- and Two-Dimensional NMR Spectros-

copy; VCH Publishers: New York, 1993.

1

nally bound to rhodium (δ 188.7, J _RhC) 51.0 Hz; δ

1

176.1, J _RhC) 60.6 Hz) and two due to carbonyls

terminally bound to iridium (δ 168.1, 161.7). The

(33) Mann, B. E.; Taylor, B. F. ¹³C{¹H} NMR Data For Organome-

tallic Compounds; Academic Press: London, 1981.

1

hydride signal of 3 has disappeared from the H NMR

(34) Herberhold, M. Metal π-Complexes; Elsevier Scientific Publish-

ers: Amsterdam, 1974; Vol. II, Part 2.

(30) Lawrance, G. A. Chem. Rev. 1986, 86, 17.

5338 Organometallics, Vol. 18, No. 25, 1999

George et al.

Ta ble 3. Selected In ter a tom ic Dista n ces (Å) in

Com p ou n d 6b

molecule A

distance

molecule B

distance

atom 1

atom 2

Ir

Rh

O(1)

O(2)

C(3)

C(4)

C(5)

C(6)

Rh

2.9840(7)

2.347(2)

1.906(9)

2.041(9)

2.043(9)

2.527(2)

2.361(2)

2.399(2)

2.040(9)

2.389(8)

2.403(8)

2.108(8)

1.129(11)

1.163(10)

1.351(13)

1.507(13)

1.513(12)

1.340(12)

1.486(13)

2.9700(7)

2.350(2)

2.345(2)

1.902(10)

2.069(9)

2.036(8)

2.507(2)

2.362(2)

2.397(2)

2.046(8)

2.385(9)

2.413(8)

2.116(8)

1.138(12)

1.158(10)

1.369(13)

1.462(12)

1.492(12)

1.336(12)

1.487(13)

P(1)

P(3)

C(1)

C(2)

C(7)

Cl

P(2)

P(4)

C(2)

C(3)

C(4)

C(7)

C(1)

C(2)

C(4)

C(5)

C(6)

C(7)

C(91)

F igu r e 1. Perspective view of one of the two crystallo-

graphically independent [RhIrCl(CO)(µ-CC(Ph)CH₂CHd

CH₂)(µ-CO)(dppm)₂]⁺cations of compound 6b (molecule A)

showing the atom-labeling scheme. Non-hydrogen atoms

are represented by Gaussian ellipsoids at the 20% prob-

ability level. Only the ipso carbons of the dppm phenyl

rings are shown. Hydrogen atoms are shown with arbi-

trarily small thermal parameters.

Ta ble 4. Selected In ter a tom ic An gles (d eg) in

Com p ou n d 6b

molecule A

angle

molecule B

angle

atom 1

atom 2

atom 3

Rh

P(1)

C(1)

C(2)

Ir

Cl

P(2)

C(2)

C(3)

C(4)

Ir

Rh

Ir

Rh

C(1)

C(2)

C(3)

C(4)

C(5)

C(6)

C(7)

C(1)

C(2)

C(7)

P(3)

C(2)

C(7)

Cl

C(2)

C(3)

C(4)

C(7)

C(2)

C(3)

C(4)

C(7)

P(4)

C(3)

C(4)

C(7)

C(4)

C(7)

O(1)

Rh

O(2)

C(4)

C(3)

C(5)

C(6)

C(7)

C(91)

Rh

143.2(3)

43.0(2)

44.9(2)

163.13(8)

100.2(4)

171.9(4)

87.9(3)

137.10(6)

43.0(3)

145.3(2)

113.6(2)

43.2(2)

94.1(3)

77.6(2)

108.7(2)

177.7(2)

174.64(8)

171.6(4)

155.4(3)

86.2(4)

32.7(3)

102.1(3)

71.4(3)

170.9(9)

94.0(4)

127.3(7)

138.6(7)

74.2(5)

142.4(3)

43.5(2)

45.4(2)

162.07(8)

98.9(4)

172.2(4)

88.9(3)

136.77(6)

44.1(3)

145.9(2)

113.9(2)

43.3(2)

carbons or protons to rhodium or to phosphorus is

observed. This suggests that any bonding between the

olefin and either metal center is weak.

The ³¹P{¹H} NMR spectra of 5a and 5b at room

temperature show typical AA′BB′X patterns, although

the signals for the iridium-bound phosphines are broad.

For example, cooling a sample of 5a to -80 °C results

in the two iridium-bound phosphines becoming in-

equivalent, causing the peak at δ -0.96 to split into a

second-order set of doublets of multiplets, due to two

strongly coupled (260.7 Hz) nuclei at δ 1.7 and -2.4.

The signal for the rhodium-bound phosphines, in con-

trast, does not change significantly with temperature.

Reaction of 1a or 1b with allyl chloride gives the

analogous chloro complexes 6a and 6b, having very

similar spectral parameters to those of 5.

To confirm the proposed geometry, the X-ray structure

of 6b was determined and is shown in Figure 1, with

selected bond lengths and angles shown in Tables 3 and

4, respectively. This determination confirms that the

allyl fragment from the allyl chloride has added to the

â-carbon of the alkynyl ligand, forming an allylvinyli-

dene group. In addition, it shows that the olefinic moiety

of this ligand is coordinated to rhodium, resulting in an

octahedral geometry around this metal (if the Rh-Ir

bond is ignored), with the olefinic and vinylidene func-

tionalities of the allyl vinylidene ligand occupying two

mutually cis coordination sites, with the remaining four

occupied by the mutually trans dppm phosphines, the

chloride, and the bridging carbonyl. The significance of

the allyl-group coordination to rhodium will be made

obvious later. At iridium, the coordination is best

described (again ignoring the metal-metal bond) as a

tetragonal pyramid, with the dppm phosphines, the

bridging vinylidene, and the terminal carbonyl occupy-

ing the basal sites and the bridging carbonyl occupying

92.7(3)

77.3(2)

108.8(2)

178.5(2)

174.61(8)

170.0(3)

156.4(3)

87.3(3)

33.2(3)

102.6(3)

71.4(3)

171.5(10)

92.4(4)

126.9(7)

140.7(7)

74.5(5)

Rh

73.0(5)

72.3(5)

104.5(5)

125.7(9)

107.7(8)

115.2(8)

119.0(8)

125.7(8)

91.9(3)

104.7(5)

129.3(9)

111.2(7)

114.6(8)

118.9(7)

126.4(8)

91.3(3)

C(3)

C(4)

C(5)

C(7)

Ir

Rh

C(6)

144.5(7)

122.3(6)

145.1(7)

122.3(6)

the apical site. Consistent with this geometry descrip-

tion, the basal ligands are clearly bent away from the

bridging carbonyl (P(1)-Ir-P(3) ) 163.13(8)°, 162.07(8)°;

C(1)-Ir-C(7) ) 171.9(4)°, 172.2(4)°).³⁵This combination

of geometries (octahedral around one metal; tetragonal

(35) The two values given for each parameter are for the two

independent molecules.

Allyl-to-Alkynyl Migration

Organometallics, Vol. 18, No. 25, 1999 5339

Ch a r t 1

pyramidal around the other) is fairly common in these

heterobinuclear systems and has been observed in

[RhMn(CO)₄(µ-S)(dppm)₂],³⁶[RhMo(CO)₄(µ-Cl)(dppm)₂],³⁷

and [RhIr(CCPh)(CO)₂(µ-CCPh)(µ-H)(dppm)₂][O₃SCF₃].^14d

One carbonyl of 6b bridges the two metals in an

essentially symmetric fashion (Rh-C(2) ) 2.040(9),

2.046(8) Å, Ir-C(2) ) 2.041(9), 2.069(9) Å; Rh-C(2)-

O(2) ) 138.6(7)°, 140.7(7)°, Ir-C(2)-O(2) ) 127.3(7)°,

126.9(7)°). Despite the long metal-metal separation of

2.9840(7), 2.9700(7) Å, the compound is best formulated

with a metal-metal bond, to satisfy the electron count

at each metal. This abnormally long separation may

result from the presence of the chelating allylvinylidene

ligand, in which the pendent allyl moiety appears to pull

the rhodium center away from Ir. Consistent with this,

the Rh-C(7) distance (2.108(8), 2.116(8) Å) is somewhat

longer than the Ir-C(7) distance (2.043(9), 2.036(8) Å)

and is notably longer than the rhodium-carbon bonds

in comparable vinylidene complexes such as [Rh₂(CO)₂-

(PCy₃)₂(µ-O₂CCH₃)(µ-CdCHPh)][BF₄] (Rh-C ) 2.015-

(5), 1.984(5) Å).³⁸Similarly, the vinylidene moiety is

bent toward rhodium, with the Rh-C(7)-C(6) angle

being significantly smaller (122.3(6)°, 122.3(6)°) than the

Ir-C(7)-C(6) angle (144.5(7)°, 145.1(7)°), showing the

strain within the chelate.

Despite the movement of Rh toward the olefinic

moiety, the Rh-olefin bonds (Rh-C(3), Rh-C(4)) are

still long, ranging from 2.385(9) to 2.413(8) Å (compared

to 2.16 and 2.02 Å found in [CpRh(CH₂dCH₂)(CF₂d

CF₂)]³⁹and between 2.09 and 2.19 Å for a number of

“normal” Rh(I) olefin complexes⁴⁰), a further indication

that the coordination is strained. The olefinic carbon-

carbon bond length (1.35(1), 1.37(1) Å) is not substan-

tially lengthened from that in the free olefin, further

suggesting a weak coordination to Rh.

process in which the olefin dissociates, twists around

the C_γ-C_δsingle bond, and recoordinates via the other

face of the olefin. This causes the vinylidene to bend in

the other direction, resulting in effective exchange of

the phosphine environments. At lower temperatures,

the up-and-down motion of the allylvinylidene group is

sufficiently slow to allow the different environments to

be detected. The top-bottom asymmetry apparently has

a much reduced effect upon the rhodium-bound phos-

phines, which appear degenerate at all temperatures

between ambient temperature and -80 °C.

In an attempt to determine how the allyl fragment

migrated to the alkynyl group, we sought to observe

intermediates in this transformation by monitoring the

low-temperature addition of allyl bromide to compound

1b. The first intermediate, observed at -80 °C, is

proposed to be [RhIr(CO)₂(η²-CH₂dCHCH₂Br)(µ-CCPh)-

(dppm)₂][O₃SCF₃] (7), as shown in Scheme 2. This

species contains a semibridging carbonyl and the olefin

coordinated to iridium in an arrangement similar to that

suggested previously for a number of olefin adducts of

1 and confirmed crystallographically for the dimethyl-

allene adduct.^7bThe olefin signals in the ¹H NMR

spectrum have shifted substantially, from between δ 6.2

and 5.0 in free allyl bromide to δ 2.0, 0.0, and -0.8,

consistent with the upfield shift expected upon coordi-

nation to a metal center.³⁴Unfortunately, these signals

are broad, even at very low temperature (-90 °C), and

thus reveal very little structural information. As a

result, exact assignments of these signals could not be

made.

Although the X-ray study on 6b supports the conclu-

sions from the NMR studies of 5 and 6 that the olefin

moiety is only weakly coordinated to Rh, it is significant

that these compounds do not react with CO, indicating

that the olefin is not displaced by this ligand. Further

reactivity studies of 5 and 6 are underway and will be

the subject of a future paper.

When compound 7 is warmed to -50 °C, it transforms

into a mixture of 1 and the oxidative-addition product

[RhIr(η¹-CH₂CHCH₂)(CO)₂(µ-CCPh)(µ-Br)(dppm)₂][O₃-

SCF₃] (9). The observation of 1 in the reaction mixture

indicates that the allyl bromide ligand does not remain

coordinated at higher temperatures. The structural

formulation of 9, in which an η¹-allyl group is bound to

iridium, is based on the ¹H NMR spectrum, which shows

the characteristic signals;⁴¹the signal for the hydrogens

on the R-carbon appears as a multiplet at δ 2.40,

showing coupling to the iridium-bound phosphines, and

the three olefinic protons appear at δ 4.90, 4.15, and

3.65, showing no coupling to rhodium or to phosphorus.

The equivalence of the hydrogens on the R-carbon also

indicates that the allyl is σ-bound. In an η³-allyl species

all five protons would be expected to couple to the

adjacent phosphines. Although the exact arrangement

of the ligands at iridium is equivocal, the arrangement

shown, containing the bridging bromide trans to the η¹-

allyl, is assumed on the basis that the addition of

organic halides to the analogous mononuclear complex

The observed fluxionality of 5 and 6 can be rational-

ized on the basis of a consideration of the structure

determined in the solid state for 6b. The coordination

of the olefinic fragment to rhodium causes the vi-

nylidene to twist out of the equatorial plane. This results

in the phenyl ring of the allylvinylidene ligand being

thrust into the vicinity of the dppm phenyl rings on one

of the iridium-bound phosphines (toward C(51) in Figure

1), as diagrammed in Chart 1. As a result, the magnetic

environments of the phosphorus atoms bound to iridium

are inequivalent in the static structure. At room tem-

perature, however, the two iridium-bound phosphines

can become equivalent on the NMR time scale by a

(36) Wang, L.-S.; McDonald, R.; Cowie, M. Inorg. Chem. 1994, 33,

3735.

(37) (a) Graham, T. W. Ph.D. Thesis, University of Alberta, 1999,

Chapter 2. (b) Graham, T. W., Van Gastel, F.; McDonald, R.; Cowie,

M. Organometallics 1999, 18, 2177.

(38) Esteruelas, M. A.; Lahoz, F. J .; On˜ate, E.; Oro, L. A.; Rodr´ıguez,

L. Organometallics 1993, 12, 4219.

(39) Guggenberger, L. J .; Cramer, R. J . Am. Chem. Soc. 1972, 94,

3779.

(40) Carr, S. W.; Shaw, B. L.; Thornton-Pett, M. J . Chem. Soc.,

Dalton Trans. 1987, 1763.

(41) Deeming, A. J .; Shaw, B. L. J . Chem. Soc. A 1969, 1562.

5340 Organometallics, Vol. 18, No. 25, 1999

George et al.

-

Sch em e 3

for the weakly coordinating BF₄group in [Ru(BF₄)-

(NO)(CO)(PPh₃)₂].⁴⁴This can be contrasted to the ¹⁹F

-

NMR spectrum expected for a strongly coordinated BF₄

anion,⁴⁵for which two signals for the coordinated (δ

-250 to -420) and noncoordinated fluorines (δ -153

to -159) are observed. Addition of 1 equiv of [Bu₄N]-

[BF₄] causes this signal to sharpen (W_1/2) 113 Hz) and

move slightly downfield to δ -150.8, consistent with the

proposed exchange.

Compound 11 reacts reversibly with carbon monoxide,

forming 12, in which the coordinating fluoborate ion is

replaced by a carbonyl ligand. This exchange is ac-

companied by the movement of the bridging carbonyl

of 11 to a terminal position on the iridium center of 12,

as demonstrated by the IR and ¹³C{¹H} NMR spectra.

Upon broad-band ³¹P decoupling, the iridium-bound

carbonyl multiplet at δ 160.0 appears as a doublet with

6.2 Hz coupling to Rh, probably as a result of trans

coupling through the metal-metal bond. Upon conver-

sion of 11 to 12, binding of the olefinic fragment to Rh

appears to have been strengthened by the addition of

the carbonyl, as seen in the slight high-field shift of the

¹³C{¹H} NMR signals of the olefinic carbons. The ³¹P-

{¹H} NMR spectrum of this compound at low temper-

ature shows that all four phosphorus nuclei are inequiv-

alent, displaying a pattern typical of an ABCDX spin

system (X ) Rh). The top-bottom asymmetry is more

pronounced in this compound than in the dicarbonyl

complex 11; this is probably due to the stronger coor-

dination of the olefinic group to rhodium, which should

cause a more pronounced bending of the vinylidene out

of the equatorial plane. The strong P-P coupling

between the two rhodium-bound phosphines (387 Hz)

and between the two iridium-bound phosphines (303 Hz)

indicates that the trans arrangement of the dppm

ligands has been retained. The ¹⁹F NMR spectrum of

this compound clearly shows that the fluoborate has

been displaced by the carbonyl (W_1/2) 6.5 Hz). Under

vacuum or dinitrogen purge, compound 12 readily loses

CO, reforming 11.

If the addition of allyl bromide to 1a or 1b is carried

out in the presence of either AgBF₄or AgO₃SCF₃, the

condensation of the alkynyl and allyl fragments does

not occur. Rather, the η³-allyl complexes [RhIr(η³-CH₂-

CHCH₂)(CO)₂(µ-CCPh)(dppm)₂][X]₂(X ) BF₄, 13a ; X )

SO₃CF₃, 13b) are formed, as shown in Scheme 4. The

¹H{³¹P} NMR spectrum of 13b shows a pattern consis-

tent with an asymmetrically bound η³-allyl group with

two high-field signals at δ 1.00 and 2.06 for protons H_b

and H_a(see Chart 2 for labeling) and the other three

protons at significantly lower field (δ 3.45-4.98). These

assignments have been confirmed by ¹³C-¹H NMR

correlation experiments. The observed pattern suggests

that the bonding of the allyl ligand approaches the σ,

π-coordination mode, as is found for [RhM(η³-C₃H₅)-

(CO)₃(dppm)₂] (M ) Os, Ru).^6b,43bAlthough the chemical

shifts are also consistent with an η¹-allyl group, which

normally gives rise to signals at δ 2.0-3.5 for the

methylene protons and δ 3.5-5.5 for the olefinic hydro-

[IrCl(CO)(PPh₃)₂] has been found to result in trans

addition to the metal center.⁴²Furthermore, the unusu-

ally small intraligand coupling within the dppm ligands

(²J _PP< 2 Hz) in the ³¹P{¹H} NMR spectrum has been

previously associated with the presence of a bridging

halide in these systems.^40,43aThe chemical shift of the

iridium-bound carbonyl (δ 167.7) in the ¹³C{¹H} NMR

spectrum is also more consistent with a carbonyl angled

away from rhodium. As described previously,^7a,biridium-

bound carbonyls angled toward rhodium appear at lower

field than those angled away, presumably indicating a

weak interaction with Rh in the former case. Warming

a solution of 9 results in transformation to the allylvi-

nylidene product 5, along with small quantities (≈10-

15%) of unidentified decomposition products. No other

intermediate was observed. The reaction of 1 with allyl

chloride at low temperatures parallels that of the allyl

bromide, with analogous intermediates 8 and 10 being

observed (see Scheme 2).

Reaction of 5a with AgBF₄gives [RhIr(BF₄)(CO)(µ-

CO)(µ-CdC(Ph)CH₂CHdCH₂)(dppm)₂][BF₄] (11), in

which the bromide ligand has been replaced by a

coordinating fluoborate, with all other ligands main-

taining the same geometries as in the precursor (see

Scheme 3). The ¹³C{¹H} NMR signals for the carbonyl

and allylvinylidene ligands are very similar to those of

5, indicating a similar structure, and the IR spectrum

shows carbonyl stretches at 2023 and 1814 cm^-1

,

indicating the presence of both terminal and bridging

CO ligands; the shift to higher frequency is consistent

with the replacement of the good donor bromide by the

weakly coordinating fluoborate anion. At both ambient

temperature and -80 °C, only one, very broad peak

(W_1/2) 217 Hz) is seen at δ -150.3 in the ¹⁹F NMR

spectrum, indicating that the coordinating fluoborate

is exchanging with free fluoborate, equilibrating all

eight fluorine nuclei. A similar broad ¹⁹F NMR signal

at δ -160.7, with half-width of 321 Hz, was observed

(42) (a) Leigh, G. J .; Richards, R. L. In Comprehensive Organome-

tallic Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;

Pergamon Press: Oxford, UK, 1982; Vol. 5, pp 563-565. (b) Atwood,

J . D. In Comprehensive Organometallic Chemistry II; Abel, E. W.,

Stone, F. G. A.; Wilkinson, G., Eds.; Elsevier: Oxford, UK, 1995; Vol.

8, p 309.

(44) Ogasawara, M.; Huang, D.; Streib, W. E.; Huffman, J . C.;

Gallego-Planas, N.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J . Am.

Chem. Soc. 1997, 119, 8642.

(45) (a) Su¨nkel, K.; Urban, G.; Beck, W. J . Organomet. Chem. 1983,

252, 187. (b) Appel, M.; Beck, W. J . Organomet. Chem. 1987, 319, C1.

(c) Kuhn, N.; Schumann, H.; Winter, M.; Zauder, E. Chem. Ber. 1988,

121, 111.

(43) (a) Sterenberg, B. T. Ph.D. Thesis, University of Alberta, 1997,

Chapter 2. (b) Chapters 3 and 4.

Allyl-to-Alkynyl Migration

Sch em e 4

Organometallics, Vol. 18, No. 25, 1999 5341

(¹J _RhP) 101 Hz) and δ 7.5, which are only slightly

shifted from the signals of compound 1a (δ 21.0 and δ

6.2). Although it seems clear that this compound is

formed through the interaction of the silver ion with

1a , the actual structure is uncertain, since this inter-

mediate could not be isolated or further characterized.

No coupling was observed in the ³¹P{¹H} NMR spectrum

to either ¹⁰⁷Ag or ¹⁰⁹Ag (52% and 48% natural abun-

dance, respectively).⁴⁷Upon addition of allyl bromide

to this solution at -80 °C, a mixture is formed that

contains 13 and two new compounds, which give rise

to overlapping peaks at δ 27.9 (¹J _RhP) 111 Hz) and δ

27.6 (¹J _RhP) 111 Hz) for the Rh-bound phosphines and

exactly overlapping peaks at δ -24.1 for the Ir-bound

nuclei. Although the nature of these intermediates is

also uncertain, their almost identical ³¹P spectral pa-

rameters suggest that they are isomers (although the

nature of the isomerism is unclear). Furthermore, these

³¹P NMR signals are very similar to those seen for the

oxidative-addition product [RhIr(η¹-CH₂CHdCH₂)Br-

(CO)₂(CCPh)(dppm)₂][O₃SCF₃] (9), so it is suggested

that these intermediates also have an η¹-allyl group

bound to Ir. Unfortunately the spectral data yield no

information pertaining to the bromo, silver, or triflate

moieties, and the instability of these species, even at

low temperature, precluded their characterization. Upon

warming or with time these intermediates transform

into 13.

Ch a r t 2

Compound 13 is susceptible to nucleophilic attack,

and addition of chloride ion causes migration of the allyl

fragment to the alkynyl group, yielding the allylvi-

nylidene chloride 6. At low temperature, the initial

product of chloride addition to 13 is compound 1 and

free allyl chloride, formed either through halide attack

on the terminal carbon of the allyl ligand or through

halide attack at iridium, followed by reductive elimina-

tion. At higher temperatures the reactions leading to

compound 6 occur, as previously described.

The reaction of 13 with other nucleophiles (PMe₃, H^-,

CN^-) does not result in migration of the allyl group to

the alkynyl ligand and results instead in attack directly

at the allyl moiety.^7a

gens,⁴¹coupling observed between the iridium-bound

phosphines and all five hydrogens on the allyl ligand is

consistent with η³-coordination. In addition, the high-

field protons (H_aH_b) would be expected to be equivalent

in an η¹-allyl. In 13, not only are the two protons

inequivalent, but H_bhas a greater coupling to H_c

(³J _bc≈ 9 Hz) than does H_a(³J _ac≈ 5 Hz), indicating that

H_bis anti to H_c. The ¹³C{¹H} NMR spectrum shows that

the Ir-bound carbonyl is semibridging to rhodium

(¹J _RhC) 17.0 Hz), an observation supported by the IR

spectrum (ν_CO) 1882 cm^-1), and the alkynyl ligand

remains in the bridging position, as shown by the

coupling of the R- and â-carbons to rhodium (C_R, δ 64.6,

1

¹J _RhC) 5.1 Hz; C_â, δ 106.1, J _RhC) 3.7 Hz).

Discu ssion

The ³¹P{¹H} NMR spectrum of 13b indicates that this

compound displays top-bottom asymmetry, and the

P-P coupling constants (316.7 and 335.3 Hz for the

rhodium- and iridium-bound phosphines) indicate that

the diphosphine ligands are still mutually trans at the

metals. This arrangement of the phosphines is unex-

pected, since the comparable mononuclear [IrCl(η³-

C₃H₅)(CO)(PPh₃)₂]⁴⁶and the binuclear [RhM(η³-C₃H₅)-

(CO)₃(dppm)₂] (M ) Os, Ru)^6b,43bcompounds have cis

phosphines at the allyl-bound metal, presumably to

minimize steric interactions with the allyl group.

In attempts to probe the steps involved in the forma-

tion of 13, low-temperature addition of allyl bromide to

a solution of 1a and silver triflate was monitored by ³¹P-

{¹H} NMR spectroscopy. Prior to the addition of allyl

bromide, the purple solution of 1a and AgO₃SCF₃shows

the presence of one main compound, which gives rise

to broad signals in the ³¹P{¹H} NMR spectrum at δ 23.6

As noted earlier, π coordination of an alkynyl group

to a second metal, as observed in the doubly bridged,

µ-η¹:η²-binding mode, has a substantial effect on its

reactivity. Whereas η¹-alkynyl groups are susceptible

to electrophilic attack at the â-carbon,^5d,8a,9µ₂-η¹:η²-

alkynyls are susceptible to attack by nucleophiles, at

either the R- or the â-carbon.^11,12This reactivity differ-

ence can be partially rationalized as resulting from π

donation to the second metal, thereby rendering the

alkynyl group more electron deficient. Surprisingly

perhaps, little else is known about the functions of the

two metals in influencing alkynyl reactivity in the

doubly bridging mode. We have previously demon-

strated, in this alkynyl-bridged Rh/Ir system, that

nucleophilic attack occurs preferentially at Ir and that

when the nucleophile is the hydride ligand subsequent

migration to the â-carbon of the alkynyl group occurs,

(46) Kaduk, J . A.; Poulos, A. T.; Ibers, J . A. J . Organomet. Chem.

1977, 127, 245.

(47) Kidd, R. G.; Goodfellow, R. J . In NMR and The Periodic Table;

Harris, R. K., Mann, B. E., Ed.; Academic Press: London, 1978.

5342 Organometallics, Vol. 18, No. 25, 1999

George et al.

Sch em e 5

yielding a bridging vinylidene group.⁷We reasoned that

these low-valent, late metals should also be susceptible

to coordination of electrophiles and that similar migra-

tion of the electrophile to the alkynyl group may

subsequently occur. This has been verified in the

reaction of [RhIr(CO)₂(µ-η¹:η²-CCPh)(dppm)₂][X] (X )

BF₄(1a ), SO₃CF₃(1b)) with fluoboric acid, which

initially yields the oxidative-addition products [RhIr-

(X)(CO)₂(µ-H)(µ-η¹:η²-C₂Ph)(dppm)₂][BF₄] (X ) BF₄(2a ),

SO₃CF₃(2b)). Although hydrogen transfer from the

metals to the alkynyl ligand is not observed in 2,

Further warming generates the final allyl-vinylidene-

bridged products by apparent migration of the η¹-allyl

group to the alkynyl â-carbon, with no other species

being observed. We suggest, in fact, that direct migra-

tion of the allyl group from Ir to the alkynyl group does

not occur. Not only is the alkynyl group oriented

unfavorably for such a direct transfer, but this direct

transfer should yield an isomer of the observed product,

in which the allyl moiety was adjacent to Ir (and

π-bound to this metal) with the phenyl substituent

adjacent to Rh. On the basis of the stronger Ir-olefin

bond,⁴⁸this should be the favored isomer. Although

isomerism via rotation about the vinylidene CdC bond

is known,^14bwe observe no isomerism in this system,

presumably because of the bulk of the vinylidene

substituents in 5 and 6.

The observation that allyl addition in the absence of

halide ion (complexed by Ag⁺) does not lead to alkynyl/

allyl coupling, but instead yields an η³-allyl complex

(Scheme 4), clearly demonstrates the importance of

halide ion in this transformation. Significantly, the

addition of halide ion to the η³-allyl complex at ambient

temperature then leads to alkynyl/allyl coupling, which

is irreversible, since halide removal after formation of

the allylvinylidene product does not lead to destabiliza-

tion of this moiety (Scheme 3). A significant observation

is that halide ion addition to the η³-allyl product at low

temperature results first in loss of allyl halide and

regeneration of starting materials.

On the basis of the above observations we propose a

mechanism for generation of the alkynyl-allyl-coupled

product as shown in Scheme 5 (ignoring the η²-allyl

halide adduct). At low temperature the favored species

is the oxidative-addition product (A) in which the η¹-

allyl group is coordinated to Ir. We propose that this is

in equilibrium with small (undetected) amounts of the

η³-allyl product (B) resulting from halide loss. The

conversion of an η¹- to an η³-allyl upon loss of a two-

electron donor (X^-) is a well-precedented transforma-

tion.⁴⁹Species B differs from compound 13 only by the

presence of halide in the former. In the presence of

halide ion, nucleophilic attack of the halide at the allyl

-

replacement of the weakly coordinating BF₄^-or SO₃CF₃

anions by CO does lead subsequently to migration,

yielding the vinylidene-bridged [RhIr(CO)₄(µ-CC(H)Ph)-

(dppm)₂]²⁺. This demonstration of metal-mediated elec-

trophile transfer to the alkynyl group suggested that

carbon-carbon bond formation at the alkynyl â-carbon

could result if organic electrophiles were used.

Initial attempts to induce carbon-carbon bond forma-

tion by the addition of methyl iodide, methyl triflate,

or benzyl bromide to 1 failed with no reaction observed

in either case. However, reaction of 1 with either allyl

bromide or chloride occurs readily, yielding [RhIrX(CO)-

(µ-CO)(µ-CC(Ph)CH₂CHdCH₂)(dppm)₂]⁺(X ) Br (5), Cl

(6)), in which carbon-carbon bond formation has oc-

curred by coupling of the allyl group with the alkynyl

â-carbon to give an allylvinylidene ligand (Scheme 2).

A series of reactions at low temperatures has estab-

lished that, analogous to the reactions with acids, this

coupling is metal mediated and does not result from

direct electrophilic attack at the alkynyl group. Moni-

toring the above reaction of 1 with the allyl halide

at -80 °C shows initial conversion to the olefin ad-

ducts [RhIr(CO)(η²-CH₂dCHCH₂X)(µ-CCPh)(µ-CO)(d-

ppm)₂]⁺, in which the allyl halide molecules are bound

to Ir. These species are not believed to be directly

involved in the transformations to the final allylvi-

nylidene products, since upon warming, allyl halide

dissociation, regenerating starting materials, is first

observed. Presumably, at -80 °C olefin coordination is

the only accessible pathway. Upon further warming

(-50 °C), oxidative addition of the allyl halide occurs,

yielding [RhIr(η¹-CH₂CHdCH₂)(CO)₂(µ-X)(µ-CCPh)(d-

ppm)₂]⁺, which has the allyl fragment η¹-bound to Ir

and the alkynyl moiety in its original bridging position.

(48) Ziegler, T. Inorg. Chem. 1985, 24, 1547.

(49) Elschenbroich, Ch.; Salzer, A. Organometallics, A Concise

Introduction; VCH Publishers: New York, 1989; Chapter 15.

Allyl-to-Alkynyl Migration

Organometallics, Vol. 18, No. 25, 1999 5343

group can occur regenerating the starting materials, C

and allyl halide. This is the reverse of nucleophilic

displacement of the halide ion by the Ir center in the

oxidative-addition step that initially yields A. Although

the thermodynamically favored oxidative addition is at

Ir, regenerating A, oxidative addition can also occur at

Rh yielding D, in which the η¹-allyl fragment is bound

to Rh. This species is not observed and either loses allyl

halide regenerating C or reacts irreversibly at higher

temperatures by transfer of the allyl group to the

alkynyl â-carbon yielding the allylvinylidene product

(E). Although we have no direct evidence of this last

transformation (D f E), it is consistent with geometry

of the product observed in which the allyl moiety is

adjacent to and π-bound to Rh. Furthermore, the

geometry in D is favorable for allyl migration to the

â-position in the alkynyl group, and our failure to

observe such an intermediate is consistent with the

greater lability of Rh compared to Ir.⁵⁰Although this

transformation (D to E) involving allyl and alkynyl

groups has not been previously reported, the closely

related migration of an aryl group to a µ₂-η¹:η²-alkynyl

ligand in a Ti/Cu complex¹⁶and the migration of a

methyl ligand to a bridging acetylide in a diiridium

complex^6hhave been proposed to rationalize the forma-

tion of vinylidene-bridged products in each case. Despite

their apparent rarity, we suggest that migrations of

alkyl or related hydrocarbyl groups to the â-carbon of

bridging alkynyls may be more common than imagined,

on the basis of the similarities between an alkyne and

a metalloalkynyl moiety, in which the metal can be

viewed as one of the alkyne substituents. In this analogy

the Ir-CtCPh moiety in structure D can be considered

as an alkyne that is π-bound to Rh. Migration of an alkyl

group to the alkynyl â-carbon is analogous to the well-

documented alkyl-to-alkyne migrations in mononuclear

complexes.^14d,15Even more closely related to the trans-

formation D to E is the allyl/alkyne coupling on single

metals⁵¹that has found recent applications in cycload-

dition reactions to generate five-, six-, and seven-

membered carbocycles.⁵²

Certainly, another possibility that must be considered

for the generation of the allylvinylidene compounds 5

and 6 from 1 is the direct electrophilic attack at the

alkynyl â-carbon. However, the failure of 1 to react with

either methyl triflate or benzyl bromide under the same

conditions indicates that the alkynyl group is not very

nucleophilic. In addition, we would expect direct attack

on the alkynyl group to occur on the face away from

Rh, thereby generating the other isomer of 5 or 6 in

which the allyl fragment is adjacent to Ir.

There are two recent reports of related metal-medi-

ated coupling reactions involving alkynyl and allyl

ligands, although both involve coupling at the alkynyl

R-carbon.^53,54In the first case, coupling was initiated

in the presence of silica gel, possibly by initial proton-

ation of the alkynyl group yielding a vinylidene, which

underwent subsequent coupling with the allyl ligand.⁵³

This proposal receives support from the study by Chin

and co-workers⁵⁴in which the coupling of alkynyl and

allyl ligands was shown to be initiated by electrophiles,

again through initial formation of vinylidene ligands,

followed by subsequent coupling. Also in the first

report,⁵³allyl condensation at the â-carbon of an alkynyl

ligand was proposed, although this apparently occurs

by direct electrophilic attack by allyl halides, and is not

metal mediated.

Con clu sion s

This and our previous study^7a,bhave shown that

either nucleophiles or electrophiles can react with

bridging alkynyl groups via a metal-mediated process

in which attack at a metal first occurs, followed by

transfer to the â-carbon of the alkynyl group. It appears

that transfer occurs from the metal that is π-bound to

the alkynyl group and is therefore analogous to the

migratory insertion involving transfer of a hydride or

alkyl group to a coordinated alkyne. Although metal-

mediated coupling of a hydrocarbyl fragment to the

â-carbon of an alkynyl group is rare, the above analogy

with alkyne chemistry suggests that this should be a

general phenomenon.

Ack n ow led gm en t. We thank the Natural Sciences

and Engineering Research Council of Canada (NSERC)

and the University of Alberta for financial support of

this research and NSERC for support of the Bruker P4/

RA/SMART 1000 CCD diffractometer.

(50) Shen, J .-K.; Tucker, D. S.; Basolo, F.; Hughes, R. P. J . Am.

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Article Doi

An unusual example of allyl-to-alkynyl migration in a phenylacetylide-bridged heterobinuclear complex of rhodium and iridium

DOI: 10.1021/om9906565

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