An acidity scale for phosphorus-containing compounds including metal hydrides and dihydrogen complexes in THF: Toward the unification of acidity scales

J. Am. Chem. Soc. 2000, 122, 9155-9171

9155

An Acidity Scale for Phosphorus-Containing Compounds Including

Metal Hydrides and Dihydrogen Complexes in THF: Toward the

Unification of Acidity Scales

Kamaluddin Abdur-Rashid, Tina P. Fong, Bronwyn Greaves, Dmitry G. Gusev,^†

Justin G. Hinman, Shaun E. Landau, Alan J. Lough, and Robert H. Morris*

Contribution from the Department of Chemistry, UniVersity of Toronto, 80 St. George Street,

Toronto, Ontario M5S3H6, Canada

ReceiVed December 20, 1999

Abstract: More than 70 equilibrium constants K between acids and bases, mainly phosphine derivatives, have

1

been measured in tetrahydrofuran (THF) at 20 °C by H and/or ³¹P NMR. The acids were chosen or newly

THF

synthesized in order to cover the wide pK_R

range of 5-41 versus the anchor compound [HPCy₃]BPh₄at

9.7. These pK_R^THFvalues are approximations to absolute, free ion pK_a^THFand are obtained by crudely correcting

the observed K for 1:1 ion-pairing effects by use of the Fuoss equation. The acid/base compounds include 14

phosphonium/phosphine couples, 17 cationic hydride/neutral hydride couples, 9 neutral polyhydride/anionic

hydride couples, 14 dihydrogen/hydride couples, and 4 other nitrogen- and phosphorus-based acids. The effects

on pK_Rof the counterions BAr′₄and BF₄vs BPh₄and [K(2,2,2-crypt)]⁺versus [K(18-crown-6)]⁺are

found to be minor after correcting for differences in inter-ion distances in the ion-pairs involved. Correlations

with ν(M-H) noted here for the first time suggest that destabilization of M-H bonding in the conjugate base

hydride is an important contributor to hydride acidity. It appears that Re-H bonding in the anions [ReH₆(PR₃)₂]^-

-

is greatly weakened by small increases in the basicity of PR₃, resulting in a large increase in the pK_Rof the

conjugate acid ReH₇(PR₃)₂. Correlations with other scales allow an estimate of the pK_R

THF

values of more

than 1000 inorganic and organic acids, 20 carbonyl hydride complexes, 46 cationic hydrides complexes, and

dihydrogen gas. Therefore, many new acid-base reactions can be predicted and known reactions explained.

THF, with its low dielectric constant, disfavors the ionization of neutral acids HA over HB⁺, and therefore

separate lines are found for pK_R^THF(HA) and pK_R^THF(HB⁺) when plotted against pK_a

or pK_a^MeCN. The

DMSO

crystal structure of [Re(H)₂(PMe₃)₅]BPh₄is reported.

Introduction

on the basis of NMR and IR measurements.^3-5Metal hydride

MeCN

acid strengths in the range 10 < pK_a

< 27 have been

Acidity scales of organic compounds in nonaqueous solvents

are used extensively to understand and predict reactivity. For

example, a scale of acid dissociation constants, K_a^DMSO(AH f

determined. However, MeCN is not useful for some weakly

acidic neutral hydride complexes which we show in this work

MeCN

to have pK_a

> 34; therefore, the conjugate base form

H(DMSO)_x+ A^-), for over 1000 compounds in dimethyl

MeCN

+

deprotonates CH₃CN (pK_a

) 44),⁶and thus the base

sulfoxide (DMSO) have been determined by Bordwell’s group

strength is leveled by the solvent. It is also not useful for many

η²-dihydrogen complexes because CH₃CN is a better ligand than

η²-H₂. Nevertheless, the first pK_adetermination of a dihydrogen

complex was done in this solvent.⁷Similarly, DMSO is a

medium strength ligand and can substitute H₂. It can also

undergo reduction and be deprotonated by strong bases. We

and others have found that CH₂Cl₂is an excellent solvent for

acidic dihydrogen and dihydride compounds but that it tends

to react with hydride compounds when their conjugate acid

forms have pK_a^aq> 15 on a pseudoaqueous scale.⁸Several pK_a^aq

values of metal hydride complexes in CH₂Cl₂anchored to the

by use of electronic spectroscopy.¹

We are interested in measuring the widest possible range of

transition metal hydride and dihydrogen acid strengths and

relating them to acid dissociation constants for organic com-

pounds so that new acid-base reactions can be predicted.

Tetrahydrofuran (THF) is the best solvent for our studies.

Potentially, pK_a^THFvalues can range from approximately 0 for

H(THF)_x⁺to greater than 50 for the deprotonation of THF. There

are only limited pK_ascales for transition metal hydride

complexes, despite the fact that they often mediate organome-

tallic reactions; these were reviewed in 1991.²Most of these

complexes contain carbonyl and/or phosphine ligands and are

usually insoluble in water, except for a few acidic compounds

aq

pK_aof phosphonium salts have been reported; these are

actually ion-pair pK values because the effects of ion-pairing

(3) Weberg, R. T.; Norton, J. R. J. Am. Chem. Soc. 1990, 112, 1105-

1108 and references therein.

such as HCo(CO)₄with pK_a≈ 0 (pK_a^MeCN) 8.4).²A precise

aq

(4) Kristja´nsdo´ttir, S. S.; Moody, A. E.; Weberg, R. T.; Norton, J. R.

Organometallics 1988, 7, 1983-1987.

scale for about 20 neutral hydrido-carbonyl compounds in

acetonitrile (MeCN) was reported by Norton and co-workers

(5) Kristja´nsdo´ttir, S. S.; Loendorf, A. J.; Norton, J. R. Inorg. Chem.

1991, 30, 4470-4471.

^†Present address: Chemistry Department, Sir Wilfred Laurier University,

Waterloo, ON N2L 3C5, Canada.

(1) Bordwell, F. B. Acc. Chem. Res. 1988, 21, 456-463.

(2) Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides:

Recent AdVances in Theory and Experiment; Dedieu, A., Ed.; VCH: New

York, 1992; Chapter 9, pp 309-359.

(6) Schwesinger, R.; Schlemper, H. Angew. Chem., Int. Ed. Engl. 1987,

26, 1167-1169.

(7) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1987, 109, 5865-

5867.

(8) Jia, G.; Lough, A. J.; Morris, R. H. Organometallics 1992, 11, 161-

171.

Published on Web 09/09/2000

9156 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

have not been considered.^8-13Angelici and co-workers have

ranked the acidity and the bond dissociation energies of about

50 cationic hydrides by protonating neutral metal complexes

in CH₂ClCH₂Cl with triflic acid (HOSO₂CF₃) and measuring

K_d) 3000e^b/(4πNa³)

(4)

b ) -e²/(aꢀkT), N ) 6.02 × 10²³mol^-1, a is the inter-ion

distance, which is equal to r⁺+ r^-in centimeters, e ) 4.80 ×

10^-10esu, ꢀ is the dielectric constant, k ) 1.38 × 10^-16erg/

deg, and T is the temperature in kelvin.

the enthalpy of the reaction.^14,15These fall in the range of 10-

THF

40 kcal/mol (or approximately 5 < pK_a

< 13; see below).

In THF, monocations with Fuoss ion-pair radius r⁺and

monoanions with radius r^-exist almost completely as 1:1 ion-

pairs,²²with K_din the range 10^-8-10^-4M when the salt

concentrations are less than 0.01 M.^17,18,23Typically at concen-

trations greater than 0.01 M, the concentrations of triple ions

(MA₂^-,M₂A⁺) and quadrupoles (M₂A₂) become important,

particularly when specific hydrogen-bonding or covalent inter-

Again, the solvent CH₂ClCH₂Cl is incompatible with the very

basic conjugates of weakly acidic hydrides.

A problem with THF is the low dielectric constant (7.6) and

therefore the complication of ion-pairing; ionic equilibria in CH₂-

Cl₂and CH₂ClCH₂Cl also have this complication. Solvents such

as DMSO and acetonitrile have high enough dielectric constants

(46.6 and 36.0, respectively) that ion-pairs are usually com-

actions or short inter-ion distances are possible.²⁴We find that

DMSO

MeCN

pletely dissociated and the pK_a

or pK_a

values are

THF

independent of the nature of the cation M⁺of eq 1. A pK value,

where K is the constant for eq 1, is determined in the reaction

eq 4 agrees within an order of magnitude with the K_d

that

have been measured conductometrically^17,18,23,25when crystal-

lographically derived values of r⁺and r^-are used. The Fuoss

equation was derived for spherical ions, so its application to

nonsymmetrical ions is somewhat problematic.

of a reference acid A₁H with the conjugate of the unknown

DMSO

acid A₂H in order to determine the pK_a

value of A₂H

according to eq 2.

Substitution of eq 4 for each salt into eq 3 yields eq 5, where

the correction eq 6 is based on inter-ion-pair distances a in

K

A₁H + M⁺A₂^-y

\

z A₂H + M⁺A₁

(1)

(2)

-

THF

angstroms for the two salts MA₁and MA₂. In eq 5, pK_R

is

used instead of pK_a^THFbecause, although these quantities should

be equal, there may be some disagreement because of errors in

the corrections for ion-pairing which must be addressed in future

work.

pK_a^DMSO(A₂H) ) pK_a^DMSO(A₁H) - pK

Streitwieser and co-workers^16,17and Antipin and co-work-

ers^18,19have determined free ion pK_fi

values of neutral

THF

pK_R^THF(A₂H) ) pK_R^THF(A₁H) - pK + ∆pK_d

(5)

hydrocarbon acids AH in THF by use of dissociation constants,

K_d({M⁺,A^-} f M⁺+ A^-), for the ion-pairs {M⁺,A^-} formed

by the conjugate bases. Therefore, eq 2 can be corrected for

ion-pairing as in eq 3.¹⁷They found that, when the pK_a^DMSOof

∆pK_d) -33.5(1/a_MA- 1/a_MA) + 3 log(a_MA/a_MA) (6)

1

2

1

Equations 5 and 6 show the importance of utilizing salts of

similar inter-ion-pair separation in order to minimize this

correction in the construction of a pK_a^THFscale. Small ions with

localized charge should be avoided to minimize ion-pair

aggregation.

pK_fi^THF(A₂H) ) pK_fi^THF(A₁H) - pK -

log(K_d({M⁺,A₂})/K_d({M⁺,A₁})) (3)

-

We are using for synthetic, practical, and solubility reasons

fluorene at 22.9 (per hydrogen) is used as the anchor for the

-

the large, non-hydrogen-bonding anions BPh₄or BAr′₄^-, Ar′

DMSO

THF scale (i.e., pK_fi^THF(fluorene) ) 22.9), the pK_a

and

) C₆H₃-3,5-(CF₃)₂, or cations [K(Q)]⁺, Q ) 18-crown-6 or

[K(Z)]⁺, Z ) 2,2,2-crypt. The ability of such counterions to

effect changes in the pK_ascale needs to be studied because these

are common, weakly ion-pairing counterions used by organo-

metallic chemists.

pK_fi^THFvalues coincide for a variety of unsaturated hydrocarbon

THF

acids.¹⁷However, the absolute pK_a

values are likely to be

quite different from the pK_fi^THFand pK_a^DMSOvalues. The pK_a^THF

for picric acid of 11.6 has been determined potentiometrically

by Coetzee et al.²⁰This is the only experiment we have found

that attempts to establish an absolute pK_a^THFvalue. This value

is different from the pK_a^DMSO(picric acid) of 0.¹

Since 1990, the Morris group ^8,10-12,26and other groups ^27-29

have been working on determining the relative acidity of cationic

dihydrogen and dihydride complexes by measuring by use of

quantitative ³¹P{gated ¹H} and ¹H NMR spectroscopy the

constants for equilibria with phosphonium salts, [HPR₃]BF₄or

[HPR₃]BPh₄, of known pK_a^aq. Our earlier acidity scales covered

smaller pK_aranges than the current work and were not corrected

The 1:1 ion-pair dissociation constants are determined by use

of conductivity measurements or are estimated theoretically, for

example, by use of the Fuoss model of ion-pairs (eq 4),²¹where

(9) Jia, G. C.; Lau, C. P. Coord. Chem. ReV. 1999, 192, 83-108.

(10) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R.

H.; Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375-3388.

(11) Jia, G.; Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875-883.

(12) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R.

H.; Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc.

1996, 118, 5396-5407.

(13) Rocchini, E.; Mezzetti, A.; Ruegger, H.; Burckhardt, U.; Gramlich,

V.; Del Zotto, A.; Martinuzzi, P.; Rigo, P. Inorg. Chem. 1997, 36, 711-

720.

aq

for ion-pairing effects as in eq 5. The use of a variety of pK_a

references for our nonaqueous scales introduced some errors.

Also, the pK_aof many neutral hydrides could not be determined

by use of phosphonium acids because they are much greater

than that of [HP^tBu₃]⁺, the weakest common phosphonium

(21) Fuoss, R. M. J. Am. Chem. Soc. 1958, 80, 5059-5061.

(22) Greater than 98% of the salt added is in this form.

(23) Salomon, M. Electrochim. Acta 1985, 30, 1021-1026.

(24) Chen, Z. D.; Hojo, M. J. Phys. Chem. B 1997, 101, 10896-10902.

(25) Barbosa, J.; Barro´n, D.; Bosch, E.; Rose´s, M. Anal. Chim. Acta

1992, 264, 229-239.

(26) Morris, R. H. Inorg. Chem. 1992, 31, 1471-1478.

(27) Ng, S. M.; Fang, Y. Q.; Lau, C. P.; Wong, W. T.; Jia, G. C.

Organometallics 1998, 17, 2052-2059.

(28) Ng, W. S.; Jia, G. C.; Huang, M. Y.; Lau, C. P.; Wong, K. Y.;

Wen, L. B. Organometallics 1998, 17, 4556-4561.

(29) Jia, G. C.; Lee, H. M.; Williams, I. D.; Lau, C. P.; Chen, Y. Z.

Organometallics 1997, 16, 3941-3949.

(14) Wang, D.; Angelici, R. J. J. Am. Chem. Soc. 1996, 118, 935-942.

(15) Angelici, R. J. Acc. Chem. Res. 1995, 28, 51-60.

(16) Krom, J. A.; Petty, J. T.; Streitwieser, A. J. Am. Chem. Soc. 1993,

115, 8024-8030.

(17) Kaufman, M. J.; Gronert, S.; Streitwieser, A., Jr. J. Am. Chem. Soc.

1988, 110, 2829-2835.

(18) Antipin, I. S.; Gareyev, R. F.; Vedernikov, A. N.; Konovalov, A. I.

J. Phys. Org. Chem. 1994, 7, 181-191.

(19) Antipin, I. S.; Gareev, R. F.; Ovchinnikov, V. V.; Konovalov, A. I.

Dokl. Phys. Chem. 1989, 304, 4-7 (in English).

(20) Coetzee, J. F.; Deshmukh, B. K.; Liao, C.-C. Chem. ReV. 1990, 90,

827-835.

Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9157

Table 1. ³¹P NMR Chemical Shifts of Phosphines and

Phosphonium Salts in THF

used without further purification. Rhenium powder, KH, 2,2,2-crypt,

[NⁿBu₄]BH₄, and NaBPh₄were supplied by Aldrich Chemical Co. OsO₄

was obtained from Johnson Matthey. Fe(C₅Me₅)(CO)₂H and Mo(C₅H₅)-

(CO)₃H were provided by Dr. Morris Bullock, Brookhaven National

Laboratory. The compounds Ru(H₂)₂H₂(PⁱPr₃)₂and [K(18-crown-6)]-

[RuH₅(PⁱPr₃)₂],⁴⁰[K(18-crown-6)][RuH₃(CO)(PⁱPr₃)₂],³²[K(18-crown-

6)][RuH₃(PPh₃)₃],³²[K(18-crown-6)][OsH₃(CO)(PⁱPr₃)₂],³²[K(2,2,2-

crypt)][OsH₃(CO)(PⁱPr₃)₂],³²[K(18-crown-6)][OsH₅(PⁱPr₃)₂],³¹OsH₄(CO)-

(PⁱPr₃)₂,³²ReOCl₃(PPh₃)₂,⁴¹[K(18-crown-6)][ReH₃(NO)(PⁱPr₃)₂],³²[K(18-

crown-6)][ReH₆(PPh₃)₂],⁴²ReH₇(PPh₂C₆H₄F)₂,⁴³ReOCl₃(PCy₃)₂,⁴⁴RuH-

(C₅H₅)(dppe) and RuH(C₅H₅)(dppm),^11,45K₂[OsO₂(OMe)₄],⁴⁶[OsH₃-

(PMe₃)₄]BPh₄, [OsH₃(PEt₃)₄]BPh₄,⁴⁷ReH(PMe₃)₅,⁴⁸and NaBAr′₄⁴⁹were

prepared according to literature procedures. Aniline, morpholine, and

4-aminobenzotrifluoride were dried over KOH and distilled under Ar

before use. The nitrogen-donor bases were converted to the HBF₄salts

by dropwise addition of HBF₄‚Et₂O to ether solutions of the nitrogen-

donor bases under Ar. The protonated bases were carefully crystallized

bases

δ

³¹P

acids^a

δ

³¹P

P^tBu₃

63.4 [HP^tBu₃]⁺

57.9^b

32.4^b,c

42.2^b

44.3^b

-0.6^b

23.7^b

11.4;^d14.4^b

30.8^b

50.3

-2.6

26.8

1.3

-6.6

-2.3

2.6

PCy₃

10.9 [HPCy₃]⁺

P^tBu₂Me

12.1 [HP^tBu₂Me]⁺

20.7 [HPⁱPr₃]⁺

PⁱPr₃

PMe₃

-61.5 [HPMe₃]⁺

-18.8 [HPEt₃]⁺

PEt₃

PⁿBu₃

-31.7 [HPⁿBu₃]⁺

-9.7 [HPⁱPr₂Me]⁺

39.6 [HP^tBu₂Ph]⁺

-25.3 [HPⁿBu₂Ph]^{+ e}

2.5 [HPCy₂Ph]⁺

-16.1 [HPEt₂Ph]^{+ e}

-26.9 [HPMePh₂]^{+ e}

-11.5 [HPEtPh₂]^{+ e}

4.4 PO(OEt)₂PhNH

PⁱPr₂Me

P^tBu₂Ph

PⁿBu₂Ph^e

PCy₂Ph

PEt₂Ph^e

e

PMePh₂

PEtPh₂

[K(crypt)][PO(OEt)₂PhN]

e

1

and characterized by H NMR. Protonated salts that have not already

^aBPh₄^-salts, if not mentioned otherwise. ^b[B(C₆H₃(CF₃)₂)₄]^-salt.

been reported in the literature have been found to have the correct

elemental analyses by the Guelph Microanalytical Laboratory.

-

^cδ 29.95 reported for the corresponding BPh₄salt.^{47 d}In agreement

with reported δ 11.9 for the BPh₄^-salt.^{47 e}These acids and bases are

in fast exchange on the NMR time scale in solution, and single averaged

chemical shifts are observed for their mixtures.

[HNEt₃]BPh₄. NEt₃(100 mg, 1 mmol) and CF₃COOH (225 mg, 2

mmol) were added to 1.5 mL of ethanol and mixed with another solution

containing NaBPh₄(338 mg; 1 mmol) in 3 mL of ethanol, affording a

precipitate. It was separated by filtration, washed with 4 × 2 mL of

ethanol, and dried in vacuo, giving the product in 82% yield (340 mg).

[HPR₃]BPh₄. All phosphonium salts [HPR₃]BPh₄(PR₃) PⁿBu₃,

P^tBu₂Ph, PCy₂Ph, PⁿBu₂Ph, PEt₂Ph, PMePh₂) were isolated according

to the reported preparation of [HNEt₃]BPh₄, with typical yields of 80-

90%. The ³¹P NMR chemical shifts are reported in Table 1.

[HPR₃]BAr′₄. When the BPh₄^-salts proved insoluble in THF (PR₃

) P^tBu₂Me, PⁱPr₃, PMe₃, PEt₃, PⁱPr₂Me), the isolation of [HPR₃]BAr′₄

(BAr′₄^-) [B(C₆H₃(CF₃)₂)₄]^-) was employed. In a typical preparation,

PR₃(0.32 mmol) was protonated by [HOEt₂]BAr′₄(300 mg, 0.32 mmol)

in 1.5 mL of THF. The product was precipitated by addition of hexanes

(10 mL), filtered, washed with 3 × 2 mL of hexanes, and dried in

vacuo to give a white solid. Typical yield: 80%.

[HOEt₂]BAr′₄. This is a more convenient method than the original.⁴⁹

A 1 M solution of anhydrous HCl (1 mL, 0.1 mmol) in Et₂O (Aldrich)

was added to a solution of NaBAr′₄(715 mg, 0.08 mmol) in 1.5 mL of

Et₂O. NaCl precipitated and was filtered off. The filtrate was mixed

with 15 mL of hexane, and an oily residue formed in the vial. After

triturating with a spatula, the oily residue afforded a precipitate. It was

filtered, washed with 3 × 2 mL of hexanes, and dried in vacuo. Yield:

733 mg (96%).

acid.³⁰However, our earlier acidity measurements were useful

in explaining the relative strengths of proton-hydride bonds,^31,32

understanding H/D exchange reactions, and predicting reactivity,

such as the protonation of coordinated dinitrogen.^33,34They

assisted us in understanding the bond dissociation energies and

bonding of the M(η²-H₂) unit and in finding complexes that

displayed, for the first time, proton transfer between dihydrogen

and a thiolate ligand ([M(H₂)(SR)L_n]⁺h [M(H)(SHR)L_n]⁺)^35,36

and between dihydrogen and a cyanide ligand ([M(H₂)(CN)-

L_n]⁺h [M(H)(CNH)L_n]⁺).^37-39

Now many neutral dihydrogen and polyhydride complexes

are available for study, partly because of a high-yield, general

method for the synthesis of the conjugate base anionic hydrides.

This involves the reaction of KH and crown or crypt with MH_l-

Cl_x(PR₃)₂starting materials in THF.^31,32Therefore, we undertook

the creation of a continuous acidity scale in THF to better

understand the acid/base behavior of cationic and neutral metal

hydride and dihydrogen complexes.

Experimental Section

ReH₃(PMePh₂)₄. H₂O₂(30%, 3 mL) was cautiously (strongly

exothermic reaction) added dropwise to rhenium powder (417 mg, 2.24

mmol), and the mixture was stirred for 15 min. Stirring was continued

at 60 °C for an additional 15 min to give a clear grayish solution. The

solvent was removed in vacuo at 40-60 °C. By use of a pipet, 1.5 mL

of HCl (37%) was added to the orange residue, and the resulting solution

was added against a flow of N₂to a solution of PMePh₂(2.7 g, 13.5

mmol) in 20 mL of ethanol. This was repeated with two more batches

of 1.5 mL of HCl until all of the residue was washed from the flask.

The mixture was stirred for 20 h at 75 °C. The golden-yellow precipitate

General. If not specified otherwise, all manipulations and NMR

sample preparations were carried out under N₂with the use of standard

Schlenk and glovebox techniques in dry, oxygen-free solvents. ¹H and

³¹P NMR measurements were done on a Varian Gemini 300 spectrom-

eter; spectral data are reported in Tables 1 and 2.

The phosphine and amine compounds were received from com-

mercial suppliers (Aldrich Chemical Co., Organometallics, Inc.) and

(30) However, P(NMeCH₂CH₂)₃N is a very strong neutral base with

MeCN

pK_a

) 32.9. Kisanga, P. B.; Verkade, J. G.; Schwesinger, R. J. Org.

Chem. 2000, 65, 5431-5432.

(40) Abdur-Rashid, K.; Gusev, D.; Lough, A. J.; Morris, R. H. Orga-

nometallics 2000, 19, 834-843.

(31) Abdur-Rashid, K.; Gusev, D. G.; Landau, S. E.; Lough, A. J.; Morris,

R. H. J. Am. Chem. Soc. 1998, 120, 11826-11827.

(32) Gusev, D. G.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 1998,

120, 13138-13147.

(41) Johnson, N. P.; Lock, C. J. L.; Wilkinson, G. Inorg. Synth. 1969,

9, 145-146.

(42) Baudry, D.; Ephritikhine, M. J. Organomet. Chem. 1986, 311, 189.

(43) Abdur-Rashid, K.; Hinman, J.; Lough, A. J.; Morris, R. H.; Roesche,

A., manuscript in preparation.

(33) Jia, G.; Morris, R. H.; Schweitzer, C. T. Inorg. Chem. 1991, 30,

593-594.

(34) Nishibayashi, Y.; Iwai, S.; Hidai, M. Science 1998, 279, 540-542.

(35) Schlaf, M.; Lough, A. J.; Morris, R. H. Organometallics 1996, 15,

4423-4436.

(44) Kelle Zeiher, E. H.; DeWit, D. G.; Caulton, K. G. J. Am. Chem.

Soc. 1984, 106, 7006.

(45) Abbreviations: PPh₂CH₂PPh₂(dppm), PR₂CH₂CH₂PR₂(R ) Ph

(dppe), R ) C₆H₄CF₃(dtfpe), R ) C₆H₄OMe (dape), R ) PPh₂CH₂CH₂-

CH₂PPh₂(dppp); Q⁺) [K(18-crown-16)]⁺, and Z⁺) [K(2,2,2-crypt)]⁺.

(46) Criegee, R. Justus Liebigs Ann. Chem. 1942, 550, 99.

(47) Gusev, D. G.; Hubener, R.; Burger, P.; Orama, O.; Berke, H. J.

Am. Chem. Soc. 1997, 119, 3716-3731.

(48) Jones, W. D.; Maguire, J. A. Organometallics 1987, 6, 1728-1737.

(49) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,

11, 3920-3922.

(36) Schlaf, M.; Morris, R. H. J. Chem. Soc., Chem. Commun. 1995,

625-626.

(37) Amrhein, P. I.; Drouin, S. D.; Forde, C. E.; Lough, A. J.; Morris,

R. H. J. Chem. Soc., Chem. Commun. 1996, 1665-1666.

(38) Fong, T. P.; Lough, A. J.; Morris, R. H.; Mezzetti, A.; Rocchini,

E.; Rigo, P. J. Chem. Soc., Dalton Trans. 1998, 2111-2114.

(39) Fong, T. P.; Forde, C. E.; Lough, A. J.; Morris, R. H.; Rigo, P.;

Rocchini, E.; Stephan, T. J. Chem. Soc., Dalton Trans. 1999, 4475-4486.

9158 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

Table 2. NMR Data for the Hydride Complexes in THF or THF-d₈

formula

Mo(C₅H₅)(CO)₃H

Re(H)₇(PCy₃)₂

[Re(H)₆(PCy₃)₂][K(2,2,2-crypt)]

Re(H)₇(PPh₃)₂

[Re(H)₆(PPh₃)₂][K(18-crown-6)]

Re(H)₇(PPh₂C₆H₄F)₂

δ

³¹P

pattern

J_PP

δ ¹H

pattern^a

J_HP

J_HH

-5.6

-6.4

-9.62

-4.9

-7.37

-4.92

-7.37

-2

s

48.5

65.9

31.3

44.5

30.4

43.6

55.2

s

t

18.9

18.5

14.7

18.5

14.7

[Re(H)₆(PPh₂C₆H₄F)₂][K(18-crown-6)]

Re(H)₄(NO)(PⁱPr₃)₂

br s

tt

-7.4

-8.3

-5.6

mer-[Re(H)₃(NO)(PⁱPr₃)₂][K(18-crown-6)]

66.7

s

33.5

16

8.3

dt

[Re(H)₄(PMe₃)₄]BPh₄

Re(H)₃(PMe₃)₄

-42.1

-42

s

d

br

-7.76

-4.17

-6.8

p

m

p

20.6

20

[Re(H)₄(PMe₂Ph)₄]BPh₄

Re(H)₃(PMe₂Ph)₄

[Re(H)₄(PMePh₂)₄]BPh₄

Re(H)₃(PMePh₂)₄

[Re(H)₂(PMe₃)₅]BPh₄

ReH(PMe₃)₅

-27

-19.9

-9.7

-1.7

-6.1

-7.63

p

h

20

29.3

-46.3

-44.6

-52.4

12.1

FeH(C₅Me₅)(CO)₂H

-11.8

-8.11

-9.25

-7.7

s

Ru(H₂)₂(H)₂(PⁱPr₃)₂

88

106.3

83.7

s

t

8.1

18.8

[RuH₅(PⁱPr₃)₂][K(18-crown-6)]

Ru(H₂)(CO)(H)₂(PⁱPr₃)₂

br

tt

dt

tt

dt

br

mer-[Ru(H)₃(CO)(PⁱPr₃)₂][K(18-crown-6)]

101.3

-9.97

-9.1

28

20.9

28

6.5

6.6

mer-[Ru(H)₃(CO)(PⁱPr₃)₂][K(2,2,2-crypt)]

101.5

s

-10

-9.1

21

Ru(H₂)(H)₂(PPh₃)₃

58

s

-7.6

[Ru(H)₃(PPh₃)₃][K(18-crown-6)]

[Ru(H)₂(C₅Me₅)(PMe₃)₂]BPh₄

RuH(C₅Me₅)(PMe₃)₂

[Ru(H₂)(C₅Me₅)(PMe₂Ph)₂]BPh₄

RuH(C₅Me₅)(PMe₂Ph)₂

[Ru(H)₂(C₅Me₅)(PMePh₂)₂]BF₄

RuH(C₅Me₅)(PMePh₂)₂

[Ru(H)₂(C₅Me₅)(PPh₃)₂]BF₄

RuH(C₅Me₅)(PPh₃)₂

[Ru(H₂)(C₅Me₅)(dppm)]BF₄

[Ru(H)₂(C₅Me₅)(dppm)]BF₄

RuH(C₅Me₅)(dppm)

[Ru(H)₂(C₅H₅)(PPh₃)₂]BPh₄

RuH(C₅H₅)(PPh₃)₂

59.9

14.4

6.3

-9.95

-10

AA′A′′br

t

32.4

38

-13.8

-9.3

25

30.1

36.8

28.2

35.3

26.5

33.6

26.4

41.4

46.2

61.8

69.2

23.4

4.9

17.5

58.6

67.6

79.6

68.5

92.5

5

-13.2

-8.1

-12.5

-7.3

-11.9

-6.8

br

t

dt

t

-6.1

28.8

32

-10.6

-7.44

-11.1

-9.02

-8.8

3.5

3.8

t

[Ru(H₂)(C₅H₅)(dppe)]BF₄

[Ru(H)₂(C₅H₅)(dppe)]BPh₄

RuH(dppe)(C₅H₅)

br

t

-13.7

-6.89

-11

t

[Ru(H₂)(C₅H₅)(dppm)]BPh₄

RuH(C₅H₅)(dppm)

br

dt

t

20.7

58.5

63.7

50

31.3

Os(H)₆(PⁱPr₃)₂

-10.4

-12.42

-10.4

-11.56

-11.77

-12.05

-13.08

-9.73

-11.26

-7.7

-10.7

-6.7

-10.5

-11

9.45

14.9

9.45

25.5

17.1

15.4

[OsH₅(PⁱPr₃)₂][K(18-crown-6)]

Os(H₂)(H)₂(CO)(PⁱPr₃)₂

mer-[Os(H)₃(CO)(PⁱPr₃)₂]K(18-crown-6)]

t

61.3

6.3

[Os(H)₃(PEt₃)₄]BPh₄

Os(H)₂(PEt₃)₄

-11.6

-18.8

-11.7

-54.6

-52.8

-47.2

-34.2

-30.3

-32.7

-20

s

t

s

t

s

t

s

m

s

p

13.4

m

p

m

p

m

p

m

t

tt

BB′

t

tt

BB′

[Os(H)₃(PMe₃)₄]BPh₄

Os(H)₂(PMe₃)₄

5

18

[Os(H)₃(PMe₂Ph)₄]BPh₄

Os(H)₂(PMe₂Ph)₄

9.9

9.2

15.5

[Os(H)₃(PMePh₂)₄BPh₄

Os(H)₂(PMePh₂)₄

-9.4

-13.9

32.5

11.4

Ir(H)₅(PCy₃)₂

cis-[IrH₄(PCy₃)₂][K(18-crown-6)]

12

13.5

28.5

-14.04

-15.43

-11.29

-14.95

-15.51

4.5

Ir(H)₅(PⁱPr₃)₂

46.2

42.6

s

12.3

13.4

120

cis-[IrH₄(PⁱPr₃)₂][K(18-crown-6)]

4.9

^ap ) pentet.

Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9159

was separated by filtration and then washed with 2 × 5 mL of ethanol,

3 × 3 mL of diethyl ether, and again with 2 × 5 mL of ethanol. Yield:

1.75 g. In the product, the complexes ReCl₃(PMePh₂)₃(70%) and ReCl₄-

ReH₇(PCy₃)₂. The following is a simplified version of the reported

preparation. ReOCl₃(PCy₃)₂(400 mg, 0.48 mmol) and [NⁿBu₄]BH₄(494

mg, 1.92 mmol) were slurried in 10 mL of ethanol. Stirring was

continued for 20 h. Solids were collected by filtration, washed with 3

× 3 mL of ethanol, and dried in vacuo. Yield: 253 mg, 73%. Product

identity was established by a comparison to the literature NMR data.⁴⁴

1

(PMePh₂)₂could be identified by use of H NMR.^50,51This mixture

cleanly afforded the trihydride ReH₃(PMePh₂)₄in an NMR tube reaction

with BH₄^-and PMePh₂, and the trihydride was used in the preparative-

scale reaction as follows.

Crude ReCl₃(PMePh₂)₃(400 mg, ca. 0.45 mmol), [ⁿBu₄N]BH₄(403

mg, 1.57 mmol), and PMePh₂(448 mg, 2.24 mmol) were dissolved in

5 mL of THF. After 2 h, addition of 5 mL of ethanol caused

precipitation of a yellow solid. The precipitate was filtered, washed

with 3 × 3 mL of ethanol, and dried in vacuo. Yield: 300 mg (68%).

The identity of this spectroscopically pure product was established by

comparison to the literature NMR data.⁵¹

OsH₂(PMePh₂)₄. PMePh₂(566 mg, 2.83 mmol) was added to K₂-

[OsO₂(OMe)₄] (200 mg, 0.47 mmol), and the mixture was dissolved

in 6 mL of ethanol. Stirring continued for 20 h. The amorphous OsH₂-

(PMePh₂)₄precipitated and was isolated by filtration, washed with 4

× 1.5 mL of ethanol, and dried in vacuo. Yield: 261 mg (56%). Product

identity was established by a comparison to the literature NMR data.⁵⁴

OsH₂(PMe₂Ph)₄. This complex was prepared as described above

for OsH₂(PMePh₂)₄using PMe₂Ph (683 mg, 4.94 mmol) and K₂[OsO₂-

(OMe)₄] (300 mg, 0.71 mmol). Although the PMe₂Ph product is poorly

soluble in ethanol, it did not precipitate from the reaction solution with

stirring. Precipitation was achieved at -30 °C, and the solid was treated

as above. Yield: 365 mg (70%). Product identity was established by

a comparison to the literature NMR data.⁵⁵

ReH₃(PMe₂Ph)₄and [ReH₄(PMe₂Ph)₄]BPh₄. From rhenium powder

(527 mg, 2.83 mmol) and H₂O₂(3 mL), the orange product was

prepared as described above for ReCl₃(PMePh₂)₃. It was extracted from

the flask with 3 × 1.5 mL of HCl (37%) in ethanol (1:2), and the

washings were added, in turn, against a flow of N₂to a solution of

PMe₂Ph (2.35 g, 17 mmol) in 7 mL of ethanol. The mixture was stirred

for 18 h at 75 °C, cooled, and left at -30 °C overnight. The yellow

precipitate was filtered and washed with 4 × 3 mL of ethanol, 3 mL

of diethyl ether, and 2 × 3 mL of hexanes. Yield: 1.2 g. In the product,

the complexes ReCl₃(PMe₂Ph)₃and ReCl₄(PMe₂Ph)₂could be detected

[OsH₃(PMePh₂)₄]BPh₄. OsH₂(PMePh₂)₄(200 mg, 0.2 mmol) was

dissolved in 7.5 mL of a mixture of ethanol and THF (4:1) containing

CF₃COOH (70 mg, 0.61 mmol). The product was then precipitated by

addition of NaBPh₄(69 mg, 0.20 mmol) in 2 × 1.5 mL of ethanol. It

was separated by filtration, washed with 3 × 3 mL of ethanol, and

dried in vacuo. Yield: 245 mg (86%). Product identity was established

by a comparison to the literature NMR data.⁵⁶

1

by H NMR.^50,52

The crude ReCl₃(PMe₂Ph)₃was successfully used in the preparation

of ReH₃(PMe₂Ph)₄, as reported,⁵⁰from pure ReCl₃(PMe₂Ph)₃. This

method was employed to isolate [ReH₄(PMe₂Ph)₄]BPh₄as well.

Following the reported synthesis, the bright yellow suspension in ethanol

was cooled to room temperature. The yellow precipitate of ReH₃(PMe₂-

Ph)₄was filtered and washed with ethanol. From the filtrate, the cationic

tetrahydride [ReH₄(PMe₂Ph)₄]BPh₄could be precipitated further by the

addition of a solution of NaBPh₄in ethanol. The identity of this

spectroscopically pure product was established by comparison to the

literature NMR data.⁵³

ReH₃(PMe₃)₄. A mixture of ReOCl₃(PPh₃)₂(1.92 g, 2.3 mmol) and

PMe₃(1.83 g, 24 mmol) in THF (10 mL) was stirred for 16 h at 70

°C. Stirring continued for 2 h at 70 °C after addition of [NⁿBu₄]BH₄

(2.16 g, 8.4 mmol). The reaction mixture was evaporated to dryness

under vacuum at 75 °C (0.5 h), and the residue was extracted with

hexanes. The solvent was removed under vacuum, and the solids were

sublimed to afford 154 mg of ReH₃(PMe₃)₄contaminated with 10% of

ReH(PMe₃)₅.^{48 1}H NMR (C₆D₆): δ 1.56 (m, 36H, CH₃), -7.76 (q,

²J(H-P) ) 20.6 Hz, 3H, ReH). ³¹P NMR (C₆D₆): δ -41.3. ³¹P NMR

(THF): δ -42.0.

[OsH₃(PMePh₂)₄]BAr′₄. The method above was utilized with

NaBAr′₄in place of NaBPh₄. Yield: 80%. Anal. Calcd for C₈₄H₆₇-

BF₂₄OsP₄: C, 54.32; H, 3.64. Found: C, 54.52; H, 3.66.

[OsH₃(PMe₂Ph)₄]BPh₄. OsH₂(PMe₂Ph)₄(200 mg, 0.27 mmol)

completely dissolved in 5 mL of ethanol upon addition of CF₃COOH

(61 mg, 0.53 mmol). The product was then precipitated by addition of

NaBPh₄(92 mg, 0.27 mmol) in 2 × 1.5 mL of ethanol. It was separated

by filtration, washed with 3 × 3 mL of ethanol, and dried in vacuo.

Yield: 245 mg (86%). Product identity was established by a comparison

to the literature NMR data.⁵⁶

OsH₆(PⁱPr₃)₂. [K(18-crown-6)][OsH₅(PⁱPr₃)₂] (360 mg, 0.44 mmol)

was dissolved in 3 mL of ethanol at 20 °C and left at -30 °C for 20

h. The white crystalline precipitate was isolated by filtration while cold,

washed with 2 × 1.5 mL of cold (-30 °C) ethanol, and dried under

vacuum to give 125 mg (55%) of the product. Despite the almost

quantitative formation of OsH₆(PⁱPr₃)₂in this reaction (confirmed by

³¹P NMR; a trace impurity was Os(H₂)H₂(CO)(PⁱPr₃)₂), the isolated

yield is relatively low on account of the high solubility. Product identity

was established by a comparison to the literature NMR data.⁵⁷

[ReH₂(PMe₃)₅]BPh₄. THF (4 mL) was added to a mixture of [HNEt₃]-

BPh₄(223 mg, 0.53 mmol) and crude ReH(PMe₃)₅(300 mg, 0.53

mmol),⁴⁸and the resulting solution was stirred for 1 h. Diethyl ether

(15 mL) was then added, and colorless, air-stable crystals of [ReH₂-

[K(crypt)][NCCHCN]. [K][NCCHCN] (40 mg, 0.38 mmol)],

prepared from KOH and NCCH₂CN in MeOH, was added to a solution

of 2,2,2-crypt (144 mg, 0.38 mmol) in THF (10 mL). The reaction

was stirred for 24 h, and the solvent was removed by vacuum. The

resulting pale pink powder was washed with 20 mL of diethyl ether to

yield a white powder. Yield: 116 mg, 63%. ¹H NMR (CD₂Cl₂): δ 3.6

(s, 12H), 3.5 (m, 12H), 2.55 (m, 12H).

1

(PMe₃)₅]BPh₄precipitated. Yield: 442 mg, 94%. H NMR (THF-d₈):

2

δ -7.57 (sxt, J(H,P) ) 28.5 Hz, 2H, ReH), 1.64 (d, J(H,P) ) 7.32

Hz, 45H, CH₃), 6.77-7.34 (m, 20H, Ph). ³¹P{¹H} (THF-d₈): δ -45.2

(s). IR (Nujol): 1852 cm^-1(νRe-H). Anal. Calcd for C₃₉H₆₇BP₅Re:

C, 52.76; H, 7.61. Found: C, 52.79; H, 7.32.

[K(18-crown-6)][Ph₃C]. A mixture of Ph₃CH (0.161 g, 0.659 mmol),

KH (0.042 g, 1.7 mmol), 18-crown-6 (0.192 g, 0.726 mmol), and THF

(5 mL) was stirred under 1 atm of N₂for 12 h. The mixture changed

from colorless to red within the first 5 min of the reaction. The solids

were removed by filtration (glass frit) and washed with THF (3 × 1.5

mL). The combined filtrate and washings was evaporated to dryness

under reduced pressure, and the resulting residue was extracted with 5

mL of hexanes. The red solids were collected by filtration (glass frit),

washed with hexanes (3 × 1.5 mL), and dried under reduced pressure.

Yield: 36%. ¹H NMR (THF-d₈): δ 7.29 (m, 6H, phenyl H), 6.50 (m,

6H, phenyl H), 5.93 (m, 3H, phenyl H), 3.55 (s, 24H, crown H).

ReH(PMe₃)₅. THF (10 mL) was added to a mixture of [ReH₂(PMe₃)₅]-

BPh₄(250 mg, 0.28 mmol), 18-crown-6 (5 mg, 0.28 mmol), and KH

(11 mg, 0.28 mmol), and the resulting suspension was stirred overnight

and then evaporated to dryness. The solids were extracted with 3 × 3

mL of a hexane/diethyl ether mixture (1:1). The combined extracts were

filtered and evaporated to dryness, to give a spectroscopically pure

1

sample of ReH(PMe₃)₅.⁴⁸Yield: 146 mg, 91%. H NMR (C₆D₆): δ

2

-8.77 (qui of d, J(H,P) ) 23.0 and 12.4 Hz, 1H, ReH), 1.55 (br,

36H, CH₃), 1.44 (d, ²J(H,P) ) 4.98 Hz, 9H, CH₃). ³¹P{¹H} (C₆D₆): δ

-45.0 (d), -52.6 (qi), ²J(P,P) ) 10.7 Hz. IR (Nujol) 1757 cm^-1(νRe-

H).

(54) Bell, B.; Chatt, J.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1973,

997.

(50) Chatt, J.; Leigh, G. J.; Mingos, D. M. P.; Paske, R. J. J. Chem.

Soc. A 1968, 2636-2641.

(51) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1989, 28, 2181-2186.

(52) Douglas, P. G.; Shaw, B. L. Inorg. Synth. 1977, 17, 65.

(53) Lunder, D. M.; Green, M. A.; Streib, W. E.; Caulton, K. G. Inorg.

Chem. 1989, 28, 4527-4531.

(55) Bruno, J. W.; Huffman, J. C.; Green, M. A.; Zubkowski, J. D.;

Hatfield, W. E.; Caulton, K. G. Organometallics 1990, 9, 2556-2567.

(56) Rottink, M. K.; Angelici, R. J. Inorg. Chem. 1993, 32, 3282-3286.

(57) Aracama, M.; Esteruelas, M. A.; Lahoz, F. J.; Lopez, J. A.; Meyer,

U.; Oro, L. A.; Werner, H. Inorg. Chem. 1991, 30, 288-293.

9160 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

Reaction of [K(18-crown-6)][Ph₃C] with IrH₅(PⁱPr₃)₂. A mixture

of [K(18-crown-6)][Ph₃C] (0.009 g, 0.013 mmol) and IrH₅(PⁱPr₃)₂(0.009

g, 0.017 mmol) was dissolved in THF (0.65 mL). The resulting solution

changed from red to colorless within 10 s. This indicates complete

conversion of Ph₃C^-to Ph₃CH. The ³¹P{¹H} NMR spectrum exhibited

resonances for cis- and trans-[IrH₄(PⁱPr₃)₂]^-and IrH₅(PⁱPr₃)₂.

from 13 mg of P(C₆H₄-4-OMe)₃(0.038 mmol) and 6.7 mg of C₆H₅-

NH₃BF₄(0.037 mmol) in 1.0 mL of CD₃CN. The NMR spectra were

recorded after 1.5 h. ³¹P NMR chemical shifts of phosphines used are

as follows: δ 4.3 (HP(C₆H₄-4-OMe)₃⁺), -9.4 (P(C₆H₄-4-OMe)₃), and

1

-5.2 (reaction mixture). The H NMR chemical shifts for the methyl

protons of the phosphines were as follows: δ 3.89 (HP(C₆H₄-4-

OMe)₃⁺), 3.76 (P(C₆H₄-4-OMe)₃), and 3.80 (reaction mixture). The

mole fraction of [HP(C₆H₄-4-OMe)₃⁺] was determined to be 0.3 ( 0.1,

and the ratio [HP(C₆H₄-4-OMe)₃⁺]/[P(C₆H₄-4-OMe)₃] was 0.4 ( 0.2.

The ratio [C₆H₅NH₂]/[C₆H₅NH₃⁺] could not be directly determined since

the resonances in the ¹H NMR spectrum were not well resolved. Since

the reaction was performed with the reactants in a 1:1 mole ratio, the

mole fractions [C₆H₅NH₂] and [C₆H₅NH₃⁺] are assumed to be equal to

those of the phosphines, 0.3 ( 0.1 and 0.7 ( 0.1, respectively. The

equilibrium constant for eq 8 is therefore 0.2 ( 0.2. The pK_a^MeCNof

Attempted Reaction of Ph₃CH with [K(18-crown-6)][IrH₄-

(PⁱPr₃)₂]. A mixture of Ph₃CH (0.004 g, 0.02 mmol), [K(18-crown-

6)][IrH₄(PⁱPr₃)₂], and THF-d₈was flame-sealed in an NMR tube under

1 atm of N₂. After 5 days there was still no sign of reaction on the

1

basis of the H NMR spectrum and the lack of color of the solution.

Determination of Equilibrium Constants in THF or THF-d₈.

Samples were mixed as described in Table 3 and then flame-sealed

under Ar. Solutions containing labile dihydrogen complexes were sealed

under H₂(1 atm). In general, equilibrium constants were determined

1

+

by H and gated-decoupled ³¹P NMR (in nondeuterated THF for the

the amine is 10.6,⁵⁸and the pK_a^MeCNof HP(C₆H₄-4-OMe)₃is thus

majority of the ³¹P measurements). The recycling time (D1 + AT) was

determined to be 9.8 ( 0.4.

Determination of pK_a

1

set to more than 3T₁in the H and ³¹P measurements, employing 90°

MeCN

+

of HPMePh₂by Use of Protonated

pulses. The time D1 + AT ) 50 s was used in the ³¹P measurements

when no experimental T₁information was available since, generally,

the T₁times were in the range of 2-8 s for the complexes of this work.

Usually, signals for all of the species in equilibrium could be located

Aniline, C₆H₅NH₃BF₄(Eq 9). In 1.0 mL of CD₃CN, a solution was

prepared consisting of 20 mg of PMePh₂(0.10 mmol) and 18 mg of

C₆H₅NH₃BF₄(0.10 mmol). After 1.5 h, the NMR spectra were recorded.

³¹P NMR chemical shifts of phosphines used are as follows: δ 4.2

(HPMePh₂⁺), -26.0 (PMePh₂), and -18.2 (reaction mixture). The ¹H

NMR chemical shifts for the methyl protons of the phosphines were

as follows: δ 2.49 (HPMePh₂⁺), 1.60 (PMePh₂), and 1.82 (reaction

mixture). The mole fraction [HPMePh₂⁺] was determined to be 0.25

( 0.01, and the average ratio [HPMePh₂⁺]/[PMePh₂] was 0.34 ( 0.01.

The ratio [C₆H₅NH₂]/[C₆H₅NH₃⁺] was estimated on the basis of mass

balance as above. The equilibrium constant is therefore 0.11 ( 0.01.

1

and integrated in the ³¹P{gated H} NMR and, in the case of hydride

complexes, in the ¹H NMR spectra as well. The phosphonium

compounds with pK_a< 5 were found to transfer protons faster than

the phosphorus chemical shift difference in hertz so that only one

averaged peak for each acid/base pair is observed in the ³¹P{gated ¹H}

1

spectrum and averaged resonances are also observed in the H NMR

spectra. In these cases, the limiting chemical shifts for the pure

phosphines and phosphonium salts (Table 1) were used to determine

the ratio of their concentrations from the weighted average chemical

shifts. Mass-balance arguments can also be used to estimate the

equilibrium concentration of the species from their starting concentra-

tions.

The pK_a^MeCNof the amine is 10.6,⁵⁸and the pK_a^MeCNof HPMePh₂is

+

therefore determined to be 9.6 ( 0.1.

Determination of the pK_a^MeCNof HP^tBu₃⁺by Use of Protonated

Morpholine, O(CH₂CH₂)₂NH₂BF₄(Eq 10). A solution was prepared

of 22 mg of P^tBu₃(0.11 mmol) and 17.3 mg of O(CH₂CH₂)₂NH₂BF₄

(0.10 mmol) in 1.0 mL of CD₃CN. The ¹H NMR spectrum was recorded

after 2.5 h. The averaged chemical shift of the tert-butyl group methyls

of 1.46 ppm indicated that 43% P^tBu₃(methyl at 1.28 ppm) and 57%

HP^tBu₃⁺(methyl at 1.59 ppm) were present. The methylene resonances

of O(CH₂CH₂)₂NH are observed at averaged positions of 3.66 and 2.92

ppm. The interpretation of these chemical shifts is complicated by the

fact that O(CH₂CH₂)₂NH is present not only in protonated form

(methylene multiplets at 3.83 and 3.22 ppm in a pure sample) and in

nonprotonated form (methylene multiplets at 3.52 and 2.71 ppm) but

also in the self-associated O(CH₂CH₂)₂NH‚NH₂(CH₂CH₂)₂O⁺form.

Norton and co-workers⁵⁹have described how to treat this problem by

use of the self-association constant, K_f, of 10 M^-1for O(CH₂CH₂)₂-

NH.⁵⁸The concentration of P^tBu₃, the K_f, and the total starting

concentration of O(CH₂CH₂)₂NH (B) derivatives (0.100 M) are used

to determine [BHB⁺] ) 0.017 M. The concentrations of B and HB⁺

are then 0.049 and 0.034 M, respectively. The weighted averages of

chemical shifts for the O(CH₂CH₂)₂NH methylenes are calculated to

be 3.65 and 2.93 ppm, in good agreement with the observed average

chemical shifts; this assumes that the BHB⁺methylene multiplets would

be observed at 3.68 and 2.92 ppm. Therefore, the equilibrium constant

for this reaction is 2.1. A separate ³¹P NMR experiment with starting

concentrations of 0.10 M gave an averaged ³¹P resonance at equilibrium

Determination of Equilibrium Constants in CH₃CN. The follow-

ing equilibria were studied:

PPh₃+ CF₃-4-C₆H₄NH₃⁺h HPPh₃⁺+ CF₃-4-C₆H₄NH₂(7)

+

P(C₆H₄-4-OMe)₃+ C₆H₅NH₃⁺h HP(C₆H₄-4-OMe)₃

+

C₆H₅NH₂(8)

PMePh₂+ C₆H₅NH₃⁺h HPMePh₂⁺+ C₆H₅NH₂

(9)

P^tBu₃+ O(CH₂CH₂)₂NH₂⁺h HP^tBu₃⁺+ O(CH₂CH₂)₂NH (10)

Each equilibrium was established in a 5-mm NMR tube containing a

sealed glass capillary with P(OMe)₃in C₆D₆as a reference, sealed with

a rubber septum and Parafilm, under an inert atmosphere. The ³¹P and

¹H NMR spectra of the reaction mixtures and the separate components

of each equilibrium were recorded.

MeCN

+

Determination of the pK_a

of HPPh₃by Use of Protonated

p-Trifluoromethylaniline, CF₃-4-C₆H₄NH₃BF₄(Eq 7). A solution

consisting of 20 mg of PPh₃(0.076 mmol) and 19 mg of CF₃C₆H₄-

NH₃BF₄(0.076 mmol) in 0.8 mL of CD₃CN was prepared. After 0.5

h, the ¹H and ³¹P NMR spectra were recorded. The ¹H NMR chemical

shifts of the aromatic protons of CF₃C₆H₄NH₂in the reaction mixture

were 7.70 and 7.35 ppm. Relating these values to those of the aromatic

+

of 58.2 ppm compared with 61.5 ppm for HP^tBu₃and 55.7 ppm for

P^tBu₃. From these data, an equilibrium constant of 2.5 is calculated, in

1

reasonable agreement with the H experiment. From the pK_a^MeCNof

O(CH₂CH₂)₂NH₂⁺(16.6),⁵⁸the pK_a^MeCNof HP^tBu₃⁺is then determined

to be 17.0 ( 0.1.

+

protons of CF₃C₆H₄NH₃(7.85, 7.60 ppm) and to those of CF₃C₆H₄-

NH₂(7.35, 6.69 ppm), we determined the mole fractions of CF₃C₆H₄-

+

NH₃⁺to be 0.71 ( 0.01 and the ratio [CF₃C₆H₄NH₂]/[CF₃C₆H₄NH₃

]

X-ray Diffraction Structure Determination of [Re(H)₂(PMe₃)₅]-

BPh₄. Crystals suitable for X-ray diffraction were obtained by layering

a THF solution of the salt with diethyl ether. Data were collected on

a Nonius Kappa-CCD diffractometer using Mo KR radiation (λ )

0.71073 Å). The CCD data were integrated and scaled using the

DENZO-SMN software package, and the structure was solved and

to be 0.40 ( 0.02. Similarly, the ratio [HPPh₃⁺]/[PPh₃] of 0.6 ( 0.2

was determined by use of the following ³¹P NMR chemical shifts: δ

-4.5 (PPh₃), 6.5 (HPPh₃⁺), and -0.46 (reaction mixture). From the

two ratios, the equilibrium constant for eq 7 (0.24 ( 0.05) can be

calculated, which in turn was used in conjunction with the pK_a^MeCNof

the protonated nitrogen-donor base (8.6) to calculate the pK_a^MeCNof

(58) Coetzee, J. F.; Padmanabhan, G. R. J. Am. Chem. Soc. 1965, 87,

+

HPPh₃(8.0 ( 0.1).

Determination of the pK_a

5005-5010.

MeCN

+

of HP(C₆H₄-4-OMe)₃by Use of

(59) Moore, E. J.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1986,

108, 2257-2263.

Protonated Aniline, C₆H₅NH₃BF₄(Eq 8). A solution was prepared

Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9161

Table 3. Thermodynamic Data for Acid/Base Equilibria at 20 °C^a

b

acid

base

time to reach equilibrium

K

error

10

pK

<-2

0.24

<-2

∆pK_d

+1

∆pK_R

<-3

IrH₅(PⁱPr₃)₂

ReH₇(PCy₃)₂

Q[CPh₃]^c

<1 day

<2.5 h

<4 days

<1 day

<1 h

>100

0.57

>100

0.68

1.50^d

428

0.0030

0.004

230

Q[cis-IrH₄(PCy₃)₂]

Q[cis-IrH₄(PⁱPr₃)₂]

Z[Ph₂N]^c

0.0

0.2

0.0 <-2

0.0

0.5

-0.5

0.0

0.5

0.0

-1.4

0.0

20

5

0.17

0.2

[HPCy₂Ph]BPh₄/Q[RuH₃(CO)(PⁱPr₃)₂]^eQ[RuH₅(PⁱPr₃)₂]

-0.18^d

-2.6

2.5

-0.7

-2.1

2.5

OsH₄(CO)(PⁱPr₃)₂

Q[RuH₅(PⁱPr₃)₂]

mer-Z[RuH₃(CO)(PⁱPr₃)₂]

mer- Z[RuH₃(CO)(PⁱPr₃)₂]

Q[mer-RuH₃(CO)(PⁱPr₃)₂]

Q[mer-ReH₃(NO)(PⁱPr₃)₂]

Z[mer-OsH₃(CO)(PⁱPr₃)₂]

Q[OsH₅(PⁱPr₃)₂]

Q[fac-RuH₃(PPh₃)₃]

Q[OsH₅(PⁱPr₃)₂]

Q[ReH₆(PPh₃)₂]

Q[ReH₆(Ph₂PC₆H₄F)₂]

Q[ReH₆(P(C₆H₄F)₃)₂]

ReH(PMe₃)₅

Q[ReH₆(Ph₂PC₆H₄F)₂]

ReH₃(PMe₃)₄

Z[CH(CN)₂]

ReH₃(PMe₂Ph)₄

Z[Ph₂N]/[OsH₃(PEt₃)₄]BPh₄

ReH₃(PMePh₂)₄

OsH₂(PMe₂Ph)₄

NEt₃

e

ReH₇(PCy₃)₂

20

30

20

40

30

20

e

2.4

OsH₄(CO)(PⁱPr₃)₂

-2.4

-1.9

-1.2

1.1

-0.9

1.9

-2.8

4.6

1

-2.4

-1.9

-1.2

0.6

-0.9

3.3

e

74

16

OsH₆(PⁱPr₃)₂

e

OsH₄(CO)(PⁱPr₃)₂

<1 day

<12 h

12-24 h

<1 h

0.073

8.5

0.0136

684

e

OsH₆(PⁱPr₃)₂

e

HPPh₂

P(O)(OEt)₂PhNH

-2.8

4.6

OsH₆(PⁱPr₃)₂

<3 h

0.000025 100

e

Fe(C₅Me₅)(CO)₂H

ReH₇(PPh₃)₂

<1 h

0.1

30

50

20

30

20

50

40

20

10

30

10

30

20

30

1

<1 h

0.0031

0.028

2.3 M^{-1 g}

0.39 M

.078

769

0.04 M

13 M^-1

24

2.5

1.5

2.5

1.5

ReH₇(Ph₂PC₆H₄F)₂

1-4 h

6-12 h

<6 days

<3 days

<2 days

2-6 days

24 h

-0.37 -4^h

4

-4

f

[ReH₂(PMe₃)₅]BPh₄

[ReH₄(PMe₂Ph)₄]BPh₄

CH₂(CN)₂

[OsH₃(PEt₃)₄]BPh₄

[OsH₃(PMe₃)₄]BPh₄

[OsH₃(PMe₂Ph)₄]BPh₄

[OsH₃(PMe₃)₄]BPh₄

[OsH₃(PMe₂Ph)₄]BPh₄

Mo(C₅H₅)(CO)₃H

[OsH₃(PMePh₂)₄]BPh₄

[OsH₃(PMePh₂)₄]BAr′₄

[OsH₃(PMePh₂)₄]BPh₄

[HP^tBu₃]BAr′₄

0.41

1.1

-2.9

1.4

-1.1

-1.4

-1.7^j

-0.8

2.0

4

-0.2

0.0

5

1.3

-2.9

-4

4

-5

-0.1

0.2

0.0

-0.4

-5

-0.6

-0.5

-0.6

0.0

0.4

0.0

-1.3

-1.9

-1.0

2.0

49ⁱ

5.9

<24 h

<1 h

0.0093

0.01

2.0 M^-1

3.2

2.0

2.4

OsH₂(PMePh₂)₄

RuH(C₅Me₅)(PMePh₂)₂

NEt₃

-0.3

-0.51

-0.40

-0.70

1.25

1.07

0.85

-0.66

1.13

1.16

-0.49

0.38

0.52

1.2

5

12-24 h

<24 h

<1 h

0.1

-0.1

1.8

1.1

0.85

-0.7

1.1

1.2

-0.5

0.4

0.5

1.2

0.8

0.6

0.7

1.5

0.9

-1.5

1.6

0.7

2.5

5.0

P^tBu₃

PCy₃

0.056

0.086

0.14

4.60

0.074

0.069

3.1

0.42

0.30

0.063

0.15

0.27

0.18

0.032

0.14

0.12

35

0.023

0.18

0.19

0.030^l

0.013^m

1.0

1.44

0.71

1.2

[P^tBu₃]BPh₄

<1 h

[HPⁱPr₃]BAr′₄

PCy₃

<1 h

[HP^tBu₃]BAr′₄

P^tBu₂Me

<1 h

[HP^tBu₃]BAr′₄

P^tBu₂Me

<1 h

[HPⁿBu₃]BAr′₄

PⁱPr₃

<1 h

[HPⁱPr₃]BAr′₄

PMe₃

<1 h

[HPⁱPr₃]BAr′₄

PEt₃

<1 h

[HPCy₃]BAr′₄

PⁿBu₃

<1 h

[HPCy₃]BPh₄

PⁿBu₃

<1 h

0.82^j

0.57

0.74

1.5

0.85

0.92

-1.5

1.6

[HPⁱPr₃]BAr′₄

PⁱPr₂Me

<1 h

[HP^tBu₂Ph]BPh₄

[HPⁿBu₃]BPh₄

RuH(C₅H₅)(dppm)^k

RuH(C₅H₅)(dppm)

P^tBu₂Ph

<1 h

[HPⁿBu₃]BPh₄

<1 h

[HPⁿBu₃]BPh₄

P^tBu₂Ph

<1 h

[HPCy₂Ph]BPh₄

[HP^tBu₂Ph]BPh₄

[Ru(H₂)(C₅H₅)(dppm)]BPh₄

[HPⁿBu₃]BPh₄

P^tBu₂Ph

<1 h

PCy₂Ph

<1 h

e

<1 h

0.74

0.72

1.5^l

<1 h

RuH(C₅H₅)(dppe)

PCy₂Ph

<1 h

1.5

1.9

[HPⁿBu₃]BPh₄

<1 h

1.9^m

[HNMe₂Ph]BPh₄

[HPCy₂Ph]BPh₄

[HPⁿBu₂Ph]BPh₄

[HPEt₂Ph]BPh₄

[HPCy₂Ph]BPh₄

[HPⁿBu₂Ph]BPh₄

[HPMePh₂]BPh₄

[HPEt₂Ph]BPh₄

<1 h

0.0

-0.4

-0.2

0.1

-0.1

-0.3

-0.1

0.1

PⁿBu₂Ph

PCy₂Ph

PEt₂Ph

PMePh₂

<1 h

-0.16

0.15

-0.08

-0.26

-0.11

0.12

0.02

1.15

1.0

<1 h

1.2

1.8

1.3

<1 h

0.75

0.95

0.07

0.09

[HPCy₂Ph]BPh₄

[HPEt₂Ph]BPh₄

[HPMePh₂]BPh₄

<1 h

0.0

1.2

1.0

PEtPh₂

<1 h

^aSamples in THF in the concentration range 0.02-0.07 M were sealed under Ar unless otherwise specified. ^bSee the Appendix for the definition

of ∆pK_dand ∆pK_R. ^cQ ) [K(18-crown-6)]⁺. Z ) [K(2,2,2-crypt)]⁺salt. ^dCalculated for protonation of [K(18-crown-6)][RuH₅(PⁱPr₃)₂] by

Ru(H₂)(H₂)(CO)(PⁱPr₃)₂under H₂(1 atm). ^eSealed under H₂. ^fPlus 18 mM QBPh₄. ^gQBPh₄) 0.018 M in eq 13. ^hSee eq 14. ⁱCalculated for

protonation of OsH₂(PEt₃)₄by [OsH₃(PMe₃)₄]BPh₄. ∆pK_aof 1.7 was previously reported.⁴⁷^jReference 47. ^kAbbreviations in ref 45. ^lData for the

equilibrium involving [Ru(H)₂(C₅H₅)(dppe)]⁺. ^mData for the equilibrium involving [(C₅H₅)Ru(H₂)(dppe)]⁺.

9162 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

Table 4. Estimates of Ionic Radii To Be Used in the Fuoss

used in this work. The few steps now needed for their syntheses

and the high yields in most cases makes their use as practical

acids and bases more feasible. For example, ReH₃(PR₃)₄, PR₃

) PMePh₂and PMe₂Ph, were prepared in two steps starting

from the commercially available phosphine and rhenium

powder. The cationic complex [ReH₄(PMe₂Ph)₄]BPh₄was

conveniently obtained as a byproduct in the synthesis ReH₃(PMe₂-

Ph)₄. A useful source of hydride in these preparations and also

those of ReH₇(PCy₃)₂is [NⁿBu₄]BH₄. The complexes OsH₂-

(PR₃)₄were obtained from OsO₄efficiently by way of K₂[OsO₂-

(OMe)₄] and its direct reaction with phosphine in ethanol. The

preparation of the anionic hydrides by use of KH/18-crown-6

or KH/2,2,2-crypt in THF has been described elsewhere.^{31,32,40,60,61}

Structures of the Acids and Bases. The structures of most

of the hydride complexes have been determined elsewhere in

the literature (see Figure 2). The coordination numbers of the

complexes are listed in Tables 7 and 8 in the column labeled

∆; ∆ is the change in coordination number from the acid to the

base form. The dihydrogen complex acids of this study are

identified by a ∆ of 6/6 because they are all six-coordinate and,

when they are deprotonated, the base hydride form is also six-

coordinate (cyclopentadienyl ligands are considered to occupy

three coordination sites). One interesting exception to this rule

is the acid/base pair RuH₂(H₂)₂(PⁱPr₃)₂/[RuH₅(PⁱPr₃)₂]^-(Table

8), where the acid is a six-coordinate bis-dihydrogen complex⁶¹

while the base is a seven-coordinate pentagonal bipyramid.⁴⁰

When the polyhydride dihydrogen complexes are deprotonated,

the base forms are too electron rich to retain H-H bonds.

The classical polyhydride acid complexes have coordination

numbers ranging from nine- to six-coordinate (Tables 7 and

8). The acidity of Ir(H)₂(C₅Me₅)(PMe₃) (Table 10) was recently

reported by Bergman and co-workers.⁶²The complex ReH₇-

(PPh₂C₆H₄F)₂may contain an elongated dihydrogen ligand or,

perhaps more suitably, a compressed dihydride moiety, on the

basis of a study of ReH₇(P(C₆H₄F)₃)₂by Crabtree and co-

workers.^63,64The structure of the cation of [Re(H)₂(PMe₃)₅]BPh₄

was determined by single-crystal X-ray diffraction to be a

pentagonal bipyramid (Figure 1). The bond lengths (Table 6)

are similar to those observed for the related dihydride ReH₂-

(hq)(PMe₃)(PPh₃)₂(hq ) 2 hydroxyquinoline monoanion).⁶⁵

Determination of the Equilibrium Constants. As in previ-

ous studies, we have determined the equilibrium constants for

cationic acid-neutral base reactions (eq 11) or neutral acid-

anionic base reactions (eq 12) in THF or THF-d₈by use of

Equation^a

ion-pair

radius, Å

ion-pair

radius, Å

cation

anion

+

-

PHR₃₊

3.0

2.2

3.0

4.7

4.2

4.0

2.0

5.0

BPh₄

4.4^b

-

NHR₃

ReH₆L₂

3.0

+

M(C₅R′₅)H₂L₂

MH₅L₂

3.0

-

MH_x(PMePh₂)₄₊

RuH₃(PPh₃)₃

3.0

-

MH_x(PMe₂Ph)₄

MH₃(CO)(PⁱPr₃)₂

3.5,^c3.3^d

2.0

+

-

MH_x(PMe₃)₄

PPh₂_-

K(18-crown-6)⁺

NPh₂_-

CPh₃

2.0

K(2,2,2-crypt)⁺

2.1^e

2.5^b

-

CF₃_-SO₃

BF₄

^aSee Table 3 for the specific compounds; values not found in the

literature were estimated from related crystal structures; R ) alkyl or

aryl; L₂) (phosphine)₂or (diphosphine); M ) Ru or Os or Re; x ) 2

or 3. ^bReference 23. ^cIn ion-pair with K(crown)⁺because K⁺interacts

with the carbonyl. ^dIn ion-pair with K(crypt)⁺. ^eReference 17.

Table 5. Crystallographic Data for [Re(H)₂(PMe₃)₅]BPh₄

a, Å

26.6894(4)

formula

C₃₉H₆₇BP₅Re

887.79

Pnma

b, Å

16.0571(3)

formula weight

space group

T, K

c, Å

9.9528(1)

R, deg

â, deg

γ, deg

V, Å³

Z

90

150

90

λ, Å

0.71073

1.383

0.042

0.087

90

F

_calc, mg/m³

4265.3(1)

4

R₁(all data)

wR₂

Table 6. Selected Bond Distances and Angles for

[Re(H)₂(PMe₃)₅]BPh₄

Distances, Å

Re(1)-H(1RE)

Re(1)-P(2)

1.53(4)

Re(1)-H(2RE)

Re(1)-P(1 or 1A)

Re(1)-P(4)

1.64(6)

2.388(1)

2.433(1)

2.368(1)

2.441(1)

Re(1)-P(3)

Angles, deg

138(3)

68(2)

H(1RE)-Re(1)-H(2RE)

H(2RE)-Re(1)-P(2)

H(1RE)-Re(1)-P(2)

H(1RE)-Re(1)-P(1

or 1A)

70(2)

90.49(3)

H(2RE)-Re(1)-P(1 or 1A) 89.88(3) P(2)-Re(1)-P(1 or 1A) 90.49(2)

P(1)-Re(1)-P(1A)

178.80(5) H(1RE)-Re(1)-P(4)

66(2) P(2)-Re(1)-P(4)

89.41(2) H(1RE)-Re(1)-P(3)

165(2) P(2)-Re(1)-P(3)

89.96(2) P(3)-Re(1)-P(4)

157(2)

133.12(5)

58(2)

127.9(1)

98.98(5)

H(2RE)-Re(1)-P(4)

P(1 or 1A)-Re(1)-P(4)

H(2RE)-Re(1)-P(3)

P(1 or 1A)-Re(1)-P(3)

1

quantitative ³¹P{gated H} and H NMR.

+

MHL_n+ {HB⁺,Y^-} h {MH₂L_n,Y^-} + B

(11)

B ) phosphine, amine, metal hydride;

Y^-) BF₄, BPh₄, BAr′₄

-

MHL_n+ {Q⁺,A^-} h {Q⁺,ML_n} + AH

(12)

A^-) organic or organometallic anion;

Q⁺) [K(18-crown-6)]⁺(Z⁺) [K(2,2,2-crypt)]⁺also used)

Figure 1. Molecular structure of the cation of [Re(H)₂(PMe₃)₅]BPh₄.

refined using SHELXTL V5.0. The crystallographic data are listed in

Table 5 and selected bond distances and angles in Table 6. The hydrides

were located and refined with isotropic thermal parameters (Figure 1).

(60) Landau, S. E.; Morris, R. H.; Lough, A. J. 1999, in preparation.

(61) Abdur-Rashid, K.; Gusev, D.; Lough, A. J.; Morris, R. H. Orga-

nometallics 2000, 19, 1652-1660.

(62) Peterson, T. H.; Golden, J. T.; Bergman, R. G. Organometallics

1999, 18, 2005-2020.

Results

(63) Luo, X.-L.; Howard, J. A. K.; Crabtree, R. H. Magn. Reson. Chem.

1991, 29, S89-93.

(64) Michos, D.; Luo, X.-L.; Howard, J. A. K.; Crabtree, R. H. Inorg.

Chem. 1992, 31, 3914-3916.

Preparation of the Acids and Bases. New routes were

developed to several of the transition metal hydride complexes

Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9163

Figure 2. Ladder of pK_R^THFranges: (a) 0-25 and (b) 25-50. Selected classical hydride acids are on the left side, while dihydrogen acids are on

the right of each diagram.

Typically constants in the range 10³-10^-3can be measured

with a 10-30% error. An advantage of this NMR method is

that reactions that proceed with the formation of unexpected

NMR-active side products can be easily detected and rejected.

Methods based on electronic spectroscopy rely on the presence

of isosbestic points to prove that the equilibrium is “clean”. An

advantage of NMR over the UV/vis method is that no extinction

coefficients need be determined since NMR is inherently

quantitative.

Equilibrium data could be reliably determined in THF even

for quite unstable compounds such as the dihydrogen complexes

RuH₂(H₂)(CO)(PⁱPr₃)₂and RuH₂(H₂)₂(PⁱPr₃)₂by preparing their

equilibrium mixtures in situ (in sealed NMR tubes) from the

isolable [RuH₃(CO)(PⁱPr₃)₂]^-and [RuH₅(PⁱPr₃)₂]^-anions under

H₂. The use of expensive THF-d₈is not always required since,

in the case of hydride complexes containing phosphine ligands,

the phosphorus signals are easily integrated and the hydride

signals are sufficiently negative of 0 ppm in the ¹H NMR

spectrum to be detectable in nondeuterated THF.

Equilibrium constants determined in this work are listed in

Table 3 along with ∆pK_Rcalculated according to eqs 5 and 6

by use of ion-pair radii of Table 4 (see the Appendix for more

THF

details). The calculated pK_R

values are listed in Tables 7

(pK_R(HB⁺)) and 8 (pK_R(HA)). When more than one isomer

(tautomer) of a complex is present in solution, the stereochem-

istry of the isomer, of which the concentration was used for the

calculation of pK_R, is indicated.

Tricyclohexylphosphonium tetraphenylborate, [HPCy₃]BPh₄,

THF

aq

with pK_R

) pK_a) 9.7, was chosen as the anchor for our

scale. Although somewhat arbitrarily chosen, this anchor

fortuitously leads to a ladder of pK_R^THFvalues that approximate

the true pK_ain THF, as we will demonstrate below.

Overlapping equilibria were examined to more and to less

acidic compounds to create a continuous ladder of values (Tables

7 and 8, Figure 2). For example, clean reactions of PCy₃with

[HP^tBu₃]BPh₄and separately with [HPⁿBu₃]BPh₄in THF allow

(65) McKinney, T. M.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 1999,

38, 1548-1554.

9164 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

Table 7. Ladder of pK_R^THF(HB⁺) Values for Cationic Acids with Different Anions^a

-

acid

base

ReH(PMe₃)₅

∆

BF₄

BPh₄

BAr′₄

[ReH₂(PMe₃)₅]⁺

7/6

8/7

7/6

6/6

7/6

6/6

7/6

24.2

22.9

20.0

18.7

16.5

17.4

16.9

16.6

16.5

15.9

15.8

14.4

14.9

14.7

13.3

12.5

12.4

12.3

13

[ReH₄(PMe₃)₄]⁺

ReH₃(PMe₃)₄

ReH₃(PMe₂Ph)₄

OsH₂(PEt₃)₄

RuH(C₅Me₅)(PMe₃)₂

cis-RuH₂(dape)₂

OsH₂(PMe₃)₄

RuH₂(PMe₃)₄

RuH₂(PEt₃)₄

FeH₂(PMe₃)₄

ReH₃(PMePh₂)₄

RuH(C₅Me₅)(PMe₂Ph)₂

OsH₂(PMe₂Ph)₄

cis-RuH₂(dppe)₂

cis-OsH₂(dppe)₂

NEt₃

[ReH₄(PMe₂Ph)₄]⁺

[OsH₃(PEt₃)₄]⁺

[Ru(H)₂(C₅Me₅)(PMe₃)₂]⁺

trans-[Ru(H₂)H(dape)₂]⁺

[OsH₃(PMe₃)₄]⁺

cis-[Ru(H₂)H(PMe₃)₄]⁺

[RuH₃(PEt₃)₄]⁺

[FeH₃(PMe₃)₄]⁺

7/6 + 6/6

8/7

7/6

6/6

4/3

7/6

6/6

7/6

4/3

6/6

4/3

7/6

4/3

7/6

4/3

7/6

6/6

7/6

6/6

4/3

[ReH₄(PMePh₂)₄]⁺

[Ru(H)₂(C₅Me₅)(PMe₂Ph)₂]⁺

[OsH₃(PMe₂Ph)₄]⁺

trans-[Ru(H₂)H(dppe)₂]⁺

trans-[Os(H₂)H(dppe)₂]⁺

[HNEt₃]⁺

[OsH₃(PMePh₂)₄]⁺

[Ru(H)₂(C₅Me₅)(PMePh)₂]⁺

trans-[Fe(H₂)H(dppe)₂]⁺

[Ru(H)₂(C₅Me₅)(PPh₃)₂]⁺

[HP^tBu₃]⁺

cis-OsH₂(PMePh₂)₄

RuH(C₅Me₅)(PMePh₂)₂

cis-FeH₂(dppe)₂

RuH(C₅Me₅)(PPh₃)₂

P^tBu₃

12.4

12.3

10.9

10.6

9.7

10.8

9.7

9.4

[HPCy₃]⁺

PCy₃

9.7

9.2

8.7

[HP^tBu₂Me]⁺

P^tBu₂Me

[Ru(H₂)(C₅Me₅)(dppm)]⁺

[HPⁱPr₃]⁺

RuH(C₅Me₅)(dppm)

9.2

8.9

PⁱPr₃

9.0

[Ru(H)₂(C₅Me₅)(dppm)]⁺

[HPMe₃]⁺

RuH(C₅Me₅)(dppm)

PMe₃

8.7

8.5

[HPEt₃]⁺

PEt₃

[HPⁿBu₃]⁺

PⁿBu₃

8.9

8.0

[Ru(H)₂(C₅H₅)(dppp)]⁺

[HPⁱPr₂Me]⁺

RuH(C₅H₅)(dppp)

8.7

PⁱPr₂Me

8.5

[HP^tBu₂Ph]⁺

P^tBu₂Ph

[Ru(H)₂(C₅H₅)(PPh₃)₂]⁺

trans-[Ru(H₂)H(dtfpe)₂]⁺

trans-[Os(H₂)H(dtfpe)₂]⁺

trans-[Fe(H₂)H(dtfpe)₂]⁺

[Ru(H₂)(C₅H₅)(dppm)]⁺

[Ru(H)₂(C₅H₅)(dppe)]⁺

[Ru(H₂)(C₅H₅)(dppe)]⁺

[HPⁿBu₂Ph]⁺

RuH(C₅H₅)(PPh₃)₂

cis-RuH₂(dtfpe)₂

cis-OsH₂(dtfpe)₂

cis-FeH₂(dtfpe)₂

RuH(C₅H₅)(dppm)

RuH(C₅H₅)(dppe)

PⁿBu₂Ph

PEt₂Ph

PCy₂Ph

PMePh₂

NMe₂Ph

8.0

7.3

6.7

7.4

7.2

7.4

7.0

6.6

6.5

6.4

6.0

5.3

[HPEt₂Ph]⁺

[HPCy₂Ph]⁺

[HPMePh₂]⁺

[HNMe₂Ph]⁺

[HPEtPh₂]⁺

PEtPh₂

^aAbbreviations in ref 45; ∆ ) change in coordination number.

equilibrium constants of 0.14 and 1/0.15 ) 6.7, respectively,

to be determined by quantitative ³¹P NMR. There is no 1:1 ion-

pair correction ∆K_dfor these equilibria because the inter-ion-

pair distance should be the same for all HPR₃BPh₄salts. In these

ion-pairs the tetraphenylborate ion (r^-) 4.4 Å) is assumed to

be situated near the open hemisphere of the phosphonium ion

containing the P-H bond (r⁺) 3.0 Å). The phosphonium salts

of these compounds with pK_R) 10.6 and 8.9, respectively

(Table 7), then provide secondary standards to proceed up and

down the ladder of hydride- and phosphorus-containing cationic

and neutral acid/base pairs. In earlier work, the tetrafluoroborate

salt, [HPCy₃]BF₄, was used as a reference at 9.7, and a smaller

continuous scale was developed. These values are included in

Table 7 for comparison. Also included are values determined

in previous work but now corrected for 1:1 ion-pairing for the

complexescis-[Fe(H₂)H(PMe₃)₄]BPh₄,⁴⁷[Ru(H)₂(C₅Me₅)(PR₃)₂]-

The reaction between [ReH₂(PMe₃)₅]⁺and [ReH₆(PPh₂-

C₆H₄F)₂]^-provides a link to the neutral/anionic hydride pairs

of Table 8 (eq 13, K ) 0.39 M). So far, this is the only clean

+

-

{ReH₂(PMe₃)₅,BPh₄} + {K(18-crown-6)⁺,ReH₆

-

(PPh₂C₆H₄F)₂} h ReH(PMe₃)₅+ ReH₇(PPh₂C₆H₄F)₂+

{K(18-crown-6)⁺,BPh₄} (13)

-

equilibrium discovered to make this link. Even still, there is a

small amount of decomposition of ReH₇(PPh₂C₆H₄F)₂to Re₂H₈-

(PPh₂C₆H₄F)₄, a reaction that is much slower than reaction 13.

Fortunately, when 1 equiv of [K(crown)]BPh₄is added with

ReH(PMe₃)₅and ReH₇(PPh₂C₆H₄F)₂under H₂, an equilibrium

is established starting from the right-hand side of eq 13. The

pK_R^THFfor ReH₇(PPh₂C₆H₄F)₂is then determined to be 28 ( 3

by use of eq 5 with K ) 0.39 M, pK_R^THF([ReH₂(PMe₃)₅]⁺) )

24.2 ( 1.0, and ∆pK_d) -4 ( 2 from eq 14 instead of eq 6,

where the 1:1 ion-pair dissociation constants are estimated by

use of eq 4 and the ionic radii from Table 4.

BPh₄,⁸and trans-[M(H₂)(H)(diphosphine)₂]BPh₄in THF.

10

For a few phosphonium salts, the BPh₄^-anion did not provide

a sufficiently soluble compound for use by this method. In these

cases, the CF₃-substituted tetraarylborate anion, BAr′₄^-, was

found to be useful. A limited ladder of pK_Rvalues referenced

to [HPCy₃]BAr′₄at 9.7 was also determined (Table 7).

QBPh4

∆pK_d) -log[K_d^QReH6L2K_d^{ReH2L′5BPh4}/K_d

]

(14)

Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9165

Table 8. Ladder of pK_R^THF(HA)^aof Neutral Acids

THF

acid

base

∆

pK_R

pK_fi

pK_a^DMSO

CHPh₃

[CPh₃]^-

g46

g43

30.8^b

24.2^c

23.8^e

30.6^b

IrH₅(PⁱPr₃)₂

[cis-IrH₄(PⁱPr₃)₂]^-

[cis-IrH₄(PCy₃)₂]^-

[ReH₆(PCy₃)₂]^-

[Ph₂N]^-

7/6

9/8

3/2

6/7

6/6

7/6

3/2

6/6

8/7

2/1

6/5

9/8

4/3

7/6

2/1

IrH₅(PCy₃)₂

ReH₇(PCy₃)₂

Ph₂NH

41 ( 4

39 ( 4

38 ( 4

36 ( 4

35 ( 4

32 ( 4

31 ( 4

30 ( 3

28 ( 3

24 ( 2

17 ( 1

10 ( 2

25^d

RuH₂(H₂)₂(PⁱPr₃)₂

RuH₂(H₂)(CO)(PⁱPr₃)₂

Re(H)₄(NO)(PⁱPr₃)₂

HPPh₂

[RuH₅(PⁱPr₃)₂]^-

[RuH₃(CO)(PⁱPr₃)₂]^-

[ReH₃(NO)(PⁱPr₃)₂]^-

[PPh₂]^-

23.1^e

18.3^d

Os(H₂)(H)₂(CO)(PⁱPr₃)₂

RuH₂(H₂)(PPh₃)₃

OsH₆(PⁱPr₃)₂

[OsH₃(CO)(PⁱPr₃)₂]^-

[RuH₃(PPh₃)₃]^-

[OsH₅(PⁱPr₃)₂]^-

[P(O)(OEt)₂PhN]^-

[Fe(C₅Me₅)(CO)₂]^-

[ReH₆(PPh₃)₂]^-

[ReH₆(PPh₂C₆H₄F)₂]^-

[CH(CN)₂]^-

P(O)(OEt)₂PhNH

Fe(C₅Me₅)(CO)₂H

ReH₇(PPh₃)₂

ReH₇(PPh₂C₆H₄F)₂

CH₂(CN)₂

MoH(C₅H₅)(CO)₃

2,4,6-C₆H₂(NO₂)₃OH

12.0^b

11.1^b

0^d

[Mo(C₅H₅)(CO)₃]^-

[2,4,6-C₆H₂(NO₂)₃O]^-

(11.6)^f

^apK_R^THF(HA) is corrected for ion-pairing and should be approximately independent of the cation. See Table 3 for the nature of the cation used.

^bReference 18. ^cReference 16. ^dReference 1. ^eReference 19. ^fpK_a^THFdetermined potentiometrically by Coetzee et al.²⁰

Table 9. Acidity Constants Determined for THF Solutions of Cations and for Other Solvents

pK_a^{H O}or pK_a^MeOH

pK_a^DMSO

pK_a^MeCN

-∆H in CClH₂CClH₂

THF

a

2

acid

pK_R

(P(NMe₂)₃N)₂P(NMe₂)NH^tBu⁺

[P(NMe₂)₃NP(NMe₂)₂NH^tBu]⁺

[ReH₄(PMe₂Ph)₄]⁺

[OsH₃(PMe₃)₄]⁺

[RuH₃(PMe₃)₄]⁺

[OsH₃(PMe₂Ph)₄]⁺

[HNEt₃]⁺

31^b

26.2^c

21.5^c

38.6^c

33.5^c

25.4^d

26^b

20.0

16.9

16.6

14.9

12.5

12.4

10.7

9.7

11.5^e

11.3^e

43.3

39.3

38.8

36.6

33.2

10.7^f,g

9^h

18.5^g

17.0

[OsH₃(PMePh₂)₄]⁺

[HP^tBu₃]⁺

11.4^i,j

9.7^i,k

[HPCy₃]⁺

[HPⁿBu₃]⁺

8.7

8.43^i,k

8.65^l

[HPMe₃]⁺

8.7

[Ru(H)₂(C₅H₅)(PPh₃)₂]⁺

[Ru(H₂)(C₅H₅)(dppm)]⁺

[Ru(H)₂(C₅H₅)(dppe)]⁺

[Ru(H₂)(C₅H₅)(dppe)]⁺

[HPEt₂Ph]⁺

8.0^m

29.7

28.9

29

7.2

7.4

7.0

6.5

6.4

6.0

5.3

29

6.15^f,k

4.57^i,k

5.18^f

[HPMePh₂]⁺

9.6

24.7

28.4

[HNMe₂Ph]⁺

2.45^h

[HPEtPh₂]⁺

4.90^i,k

^aOTf^-salts.^{14,56 b}These are our estimates based on the reported equilibria in THF referenced to 9-phenylfluorene (pK_R

≈ 30) in THF.⁶

THF

^cReference 66. ^dReference 5. ^eIn MeOH relative to pK_a^aq(MeOH) ) 16.7.^{47 f}Determined in H₂O. ^gReference 68. ^hReference 1. ⁱDetermined in

CH₃NO₂and then correlated to the water scale. ^jReference 82. ^kReference 67. ^lDetermined in H₂O by NMR.^{83 m}BF₄salt.¹¹

-

THF

Table 10. Estimated pK_R

Values of Selected Acids

base

THF

a

acid

estimated pK_R

pK_aother

-∆H in CClH₂CClH₂

Ir(H)₂(C₅Me₅)(PMe₃)

H₂

IrH(C₅Me₅)(PMe₃)Li

QH

>51

49

44

38^d

35

32

30

21

18

16

15

12

6

38-41^a

35.3^b

31.0^c

32^e

-

HCPh₃

CPh₃

cyclo-C₆H₁₁OH

HPPh₂

cyclo-C₆H₁₁O^-

-

PPh₂

23.8^f

-

WH(C₅H₅)(PMe₃)(CO)₂

9-phenylfluorene

[PtH(dmpe)₂]PF₆

[NiH(dmpe)₂]PF₆

[PtH(dppe)₂]PF₆

[TMGH]OTf

W(C₅H₅)(PMe₃)(CO)₂

9-phenylfluorenide^-

Pt(dmpe)₂

26.6^g

17.9^e

28.5^h

24.3^h

22^h

Ni(dmpe)₂

Pt(dppe)₂

tetramethylguanidine

OsH(C₅H₅)(PPh₃)₂

Os(C₅Me₅)₂

13.6,ⁱ23.3^j

13.4^l

43.2^k

37.3^k

26.6^k

[Os(H)₂(C₅H₅)(PPh₃)₂]BF₄

[OsH(C₅Me₅)₂]OTf

[ReH₂(C₅H₅)₂]⁺

[HPPh₃]BPh₄

9.9^m

ReH(C₅H₅)

PPh₃

6

5.5ⁿ

3

2.7ⁱ8.0^o

21.2^j

^aOn the basis of the reaction of the anion with H₂or DMSO but lack of reaction with toluene.^{62 b}In THF, see text.^{84 c}pK_a^{DMSO 18}

.

^dBased on

THF 19

^fpK_fi

j

l

the equilibrium with RuH₄(PPh₃)₄.⁸⁵^epK_a^{DMSO 1}

.

^gpK_a^{MeCN 2}^hpK_a^{MeCN 69}ⁱpK_a^aq. pK_a^{MeCN 15}^kReference 14. In CH₂Cl₂.^{28 m}pK_a^{MeCN 86}

. . . .

ⁿ60% dioxane/water mixture.^{87 o}pK_a^MeCN(this work).

the cation [K(2,2,2-crypt)]⁺(Z⁺) was used as well as Q⁺to

demonstrate that the calculation of pK_R^THFvalues is independent

of the cation.

THF

A continuously linked set of neutral acids with pK_R

between 28 and 41 (ReH₇(PCy₃)₂) was then established by use

of the equilibrium method represented by eq 12. In one case,

9166 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

IrH₅(PⁱPr₃)₂was not sufficiently acidic to establish equilibrium

with ReH₇(PCy₃)₂, and so its pK_R^THFis estimated to be greater

than 43. Similarly, triphenylmethane is not acidic enough to

protonate [IrH₄(PⁱPr₃)₂]^-. This is a thermodynamic effect instead

of a kinetic effect because [K(crown)]CPh₃completely depro-

tonates IrH₅(PⁱPr₃)₂. If the pK_R

of HCPh₃is about 44

THF

(estimated from pK_a^DMSO, Table 10), then the pK_R^THFof IrH₅(Pⁱ-

Pr₃)₂and also IrH₅(PCy₃)₂(Table 8) must be about 43.

A useful equilibrium is established between [ReH₄(PMe₂-

Ph)₄]⁺(pK_R^THF) 20.0) and CH₂(CN)₂(eq 15, Table 3, K ) 13

THF

M^-1). Here, pK_R

of CH₂(CN)₂is determined to be 24 ( 2

CH₂(CN)₂+ ReH₃(PMe₂Ph)₄h

{ReH₄(PMe₂Ph)₄,CH(CN)₂} (15)

Figure 3. Correlation between other acid scales and the pK_R^THFscale.

The points are from Tables 8 and 9.

+

-

on the basis of the estimated K_dof the ion-pair of 5 × 10^-6

M

Error in pK_r^THFValues of Neutral Weak Acids. The errors

in these pK_R^THFvalues are indicated in Table 8. They are larger

than the cationic acids because of the error in the estimate of

the K_dvalues in the linking of the cationic and neutral scales

((2), plus an accumulation of errors in the measurement of K

and in the estimate of individual ∆pK_d.

(a ) 6 Å in eq 4). The reverse equation between {Kcrypt⁺,CH-

(CN)₂^-} and {ReH₄(PMe₂Ph)₄⁺,BPh₄^-} also established the

THF

pK_R

of CH₂(CN)₂to be 24 ( 2.

We find that picric acid completely protonates PBu₂Ph

THF

(pK_R

) 6.6) and then oxidizes it to a variety of products

+

THF

Equations Linking Other Acidity Scales. Table 9 compares

(under Ar). It is not as acidic as HPPh₃(pK_R

≈ 3, Table

THF

the pK_R

values for some of the cationic compounds deter-

10), although the slow oxidation of the PPh₃complicates the

experiment. We estimate the pK_R^THFfor picric acid to be 10 (

mined in this work with those determined in other solvents,

often with other anions. Also included are two phosphazene

acids of which the ion-pair pK value has been determined in

THF

2, which is in agreement with the Coetzee value (pK_a

)

11.6)²⁰and indicates that our pK_R^THFvalues are close to absolute

ones.

DMSO

MeCN

THF and pK_a

and pK_a

have also been measured or

estimated.^6,66We estimate the pK_R^THFof [P(NMe₂)₃NP(NMe₂)₂-

NH^tBu]⁺to be 26 (Table 9) on the basis of its reported

equilibrium in THF with 9-phenylfluorene⁶⁶(pK_R^THF) 31) and

an estimated K_dof 10^-5M; the other phosphazenium acid is

MoH(C₅H₅)(CO)₃forms a clean equilibrium upon reaction

with OsH₂(PMePh₂)₄in THF-d₈to give the 1:1 ion-pair {OsH₃-

THF

(PMePh₂)₄,Mo(C₅H₅)(CO)₃}. Therefore, the pK_R

of MoH-

(C₅H₅)(CO)₃is 17, with K_destimated as 10^-5M. Fe(C₅Me₅)-

(CO)₂H reacts with Q[ReH₆(PPh₃)₂] to give an equilibrium with

[K(crown)][Fe(C₅Me₅)(CO)₂] and ReH₇(PPh₃)₂and another

unidentified species in lower concentration with a ³¹P chemical

shift at 50 ppm. Assuming that ion-pair dissociation constants

are comparable, the pK_R^THFof the iron complex is approximately

31. The pK_a^MeCNvalues for these Mo and Fe hydrides are also

reported to be 5 pK units less acidic in THF.⁶⁶

DMSO

A significant observation is that there are separate pK_a

/

pK_R^THFcorrelations for cationic (HB⁺) and neutral (HA) acids

(Figure 3). The data for the two cationic ammonium ions and

the two protonated phosphazene compounds of Table 9 provide

an approximate conversion equation for the line (Figure 3)

DMSO

relating pK_R^THF(HB⁺) and Bordwell’s pK_a

scales (eq 17,

known to be 13.9 and 26.3, respectively.⁵⁹

R²) 0.99). The data for the neutral acids picric acid, CH₂-

THF

Error in pK_r

Values of Cationic Acids. The error in

the equilibrium constant determination depends on the magni-

tude of the constant, the signal-to-noise of each species in the

spectrum of the equilibrium mixture, and whether signals for

all of the species are resolvable. The estimated errors for the

equilibrium constants are listed in Table 3. The ladder approach

provides pK_R^THFvalues that have errors that accumulate as they

become farther removed from the reference value of 9.7 due to

the equilibrium constant errors plus errors in the estimate of

∆pK_dwhich are on the order of (0.1 for the cationic acids.

The absolute values of ∆pK_drange from 0.0 to (0.6, with the

largest ∆pK_dfor the equilibria between the smaller pairs

{NHEt₃⁺,BPh₄^-} and the larger ion-pairs {OsH₃L₄⁺,BPh₄^-},

L ) PMe₂Ph and PMePh₂. However, in these cases and in other

cases, the cumulative errors tend to cancel since the ∆pK_dvalues

tend to alternatively add and subtract as one goes up the ladder

according to the reactions of Table 3. The estimated cumulative

error for a pK_R^THFvalue of the cationic acids is given by eq 16.

pK_a^DMSO) 0.95pK_R^THF(HB⁺) - 3.0

(17)

(CN)₂, P(O)(OEt)₂PhNH, PPh₂H, and NPh₂H (Table 8) are

plotted in Figure 3. The least-squares regression provides eq

THF

18 (R²) 0.99). Since the Streitwieser pK_fi

scale of neutral

pK_a^DMSO) 0.85pK_R^THF(HA) - 9.6

(18)

acids was anchored to the pK_a^DMSOof fluorene and found to be

DMSO

equivalent to the pK_a

values for a variety of hydrocarbon

acids with delocalized anions,¹⁷eq 18 also applies to the many

THF

pK_fi

values reported by Streitwieser and co-workers and

Antipin and co-workers.¹⁸Therefore, the slope of eq 18 should

be 1.0. Errors in our estimates of pK_R^THFprobably account for

this deviation.

A least-squares fit line through eight points (Figure 3) relating

the aqueous scale (pK_a^aq) of the cationic compounds listed in

Table 9 to the pK_R^THFscale gives eq 19 (R²) 0.89). The small

correlation coefficient shows, as expected, that water and THF

cumulative error in pK_R^THF) (0.08|pK_R^THF- 9.7| (16)

THF

(66) Schwesinger, R.; Schlemper, H.; Hasenfratz, C.; Willaredt, J.;

Dambacher, T.; Breuer, T.; Ottaway, C.; Fletschinger, M.; Boele, J.; Fritz,

H.; Putzas, D.; Rotter, H. W.; Bordwell, F. G.; Satish, A. V.; Ji, G. Z.;

Peters, E. M.; Peters, K.; vonSchnering, H. G.; Walz, L. Liebigs Ann. 1996,

1055-1081.

Therefore, the pK_R

of the weakest cationic acid, [ReH₂-

(PMe₃)₅]⁺, is 24 ( 1 relative to HPCy₃⁺at 9.7. The uncertainty

THF

in the pK_R

values will be reduced as more measurements

are made in the future.

Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9167

BPh₄^-counterions. The differences in ion-pairing and hydrogen-

bonding may account for the deviation of the slope of eq 22

(1.8) from that predicted by thermodynamics, RT ln(10) ) 1.35.

pK_a^aq) 1.0pK_R^THF(HB⁺) - 0.7

(19)

are very different in their solvating properties. This may also

reflect the fact that the pK_avalues of some of the protonated

phosphines were actually determined by potentiometric titration

in CH₃NO₂and then correlated to the water scale.⁶⁷The

compounds [HPMePh₂]⁺, [HP^tBu₃]⁺, and [HNEt₃]⁺show the

largest deviations from the correlation. Neutral acids would be

expected to have greater pK_R^THFvalues than those predicted by

the use of eq 19. For example, 2,6-dinitrophenol has a pK_a^aqof

3.6 but has a pK_R^THF(HA) estimated to be 18 on the basis of its

Correlation with Metal Hydride Vibrational Modes. Metal

hydride vibrational data for selected complexes are listed in

Table 11. There is a general trend that, for related complexes,

the metal hydride stretch in both the acid and base forms drops

to lower wavenumber as the complex becomes more basic. Thus,

for the series of Re^V/Re^IIIacid/base pairs of Table 11, a change

in pK_afrom 7 to 20 is reflected in a change in ν(Re-H) from

2061 to 1933 cm^-1for the acid and from 2019 to 1782 cm^-1

for the base (Figure 4). The base form always has lower ν(M-

H) than the acid form. The metal hydride modes for the base

form move to very low frequency for the neutral/anionic

hydrides (Figure 5).

(16.5) values.⁶⁸

DMSO

MeCN

pK_a

(4.9) and pK_a

Utilizing MeCN as a solvent and similar NMR detection, PPh₃

was found to be in equilibrium with CF₃-4-C₆H₄NH₃⁺BF₄

,

+

PMePh₂with C₆H₅NH₃⁺BF₄^-, and P^tBu₃with protonated

morpholine tetrafluoroborate; these provide the pK_a^MeCNvalues

of 8.0, 9.6, and 17.0, respectively. The last two are useful links

to the THF scale (Table 9, Figure 3). When combined with the

values for [HNEt₃]⁺,⁵⁹[Re(H)₄(PMe₂Ph)₄]BPh₄,⁵and the two

phosphazenium acids⁶⁶of Table 9, a linear fit provides the

relationship of eq 20 (R²) 0.99 for six points). This allows

Electrochemical Measurements. Reversible electrochemical

potentials corresponding to the oxidation of the conjugate base

of a hydride or dihydrogen complex along with the pK_aof the

acid are needed to calculate a metal hydride bond dissociation

energy.^10,14,70Unfortunately, the anionic hydrides [MH₃(CO or

NO)L′₂]^-(M ) Re, Os, Ru; L′ ) PⁱPr₃) in THF/NⁿBu₄PF₆

display irreversible oxidations in the range from -0.06 to -0.42

V vs Fe(C₅H₅)₂⁺/Fe(C₅H₅)₂(Table 12).

pK_a^MeCN) 1.13pK_R^THF(HB⁺) + 3.7

(20)

the approximate placement on our scale of eight cationic nickel

group hydrides [MH(diphosphine)₂]⁺determined by DuBois and

co-workers (see examples in Table 10).⁶⁹Triphenylphosphonium

tetraphenylborate is insoluble in THF but soluble in MeCN and

CH₂Cl₂, and so eq 20 can be used to estimate its pK_R^THFto be

3 (Table 10).

Discussion

The Anchor for the Scale. Protonated tricyclohexylphos-

phine with pK_a) 9.7 was chosen as the anchor for our scale

for a variety of reasons. Equilibrium constants are conveniently

determined by ³¹P NMR; several have already been determined.

The base is bulky and rarely substitutes for other ligands in

metal complexes. The phosphonium tetraphenylborate salt is

easily prepared and has a high enough pK_ato place it toward

Two metal hydrides, Mo(C₅H₅)(CO)₃H and Fe(C₅Me₅)-

MeCN

(CO)₂H, with pK_a

values of 13.9 and 26.3, respectively,

THF

were found to have approximate pK_R

values of 17 and 31,

THF

the middle of the scale. Protonated triethylamine with pK_R

respectively. These two points plus the point (pK_R^THF(HA),

pK_a^MeCN) ) (11.6, 11) for picric acid⁶⁸provide correlation eq

21 (R²) 0.99). Equation 21 then provides a link to relate the

) 12.5 is a useful secondary standard. Its pK_a^MeCNand pK_a^DMSO

values have also been reported (Table 9). The enthalpies of

protonation of PCy₃and NEt₃in CHCl₂CHCl₂have been

determined. Therefore, our scale can be related to the other

important acidity scales. In addition, NEt₃is often used as a

base in organometallic reactions.

pK_a^MeCN) 0.81pK_R^THF(HA) + 1.0

(21)

MeCN

pK_a

values of the 20 neutral carbonyl hydride complexes

studied by Norton and co-workers to the THF scale.^2,5The very

basic anionic hydride [RuH₅(PⁱPr₃)₂]^-(pK_R^THFof the acid form

is 39) reacts immediately with MeCN, presumably via an initial

proton-transfer reaction.

Picric acid is another possible choice for an anchor for the

THF scale. Its pK_a^THF(HA) was determined by potentiometric

measurements to be 11.6, while its pK_a^DMSOvalue is about 0.²⁰

As noted above, these values are consistent with eq 18, and

this suggests that our pK_R^THFvalues are close to absolute pK_a^THF

values. As mentioned, the Streitwieser pK_fi^THF(HA) scale

arbitrarily uses the pK_a

This explains the large discrepancy in pK_fi^THF(HA) and

pK_R^THF(HA) values for HPPh₂and HNPh₂(Table 8).

The heats of protonation by trifluoromethylsulfonic acid

(HOTf) of the compounds in CH₂ClCH₂Cl where available from

the literature^15,56are also included in Table 9. The conversion

equation is eq 22 (R²) 0.94 for 10 points neglecting

HPMePh₂⁺). It is interesting that both amines and phosphines

DMSO

value of fluorene for its anchor.

Comparison with Other Scales. A significant observation

-∆H_HM^{CH ClCH Cl}(HB⁺OTf^--) ) 1.8pK_R^THF(HB⁺BPh₄) +

-

2

THF

is that there are separate pK_a^DMSO/pK_R

correlations for

cationic (HB⁺) acids (eq 17) and the neutral (HA) acids (eq

18). The reason is that the dissociation of HA results in a net

increase in the population of ions (eqs 23 and 24), compared to

that of HB⁺(eq 25), that will be opposed by a lowering of the

dielectric constant from DMSO to THF. There will also be two

separate correlations for HB⁺and HA in MeCN (eqs 20 and

21) and in H₂O (eq 19, equation for HA not determined).

16.3 (22)

fall approximately on the same line. If pseudoaqueous pK_a

values are used instead, then distinct lines are obtained for

amines and phosphines (Figure 1 of ref 15). Therefore, solvation

effects are similar in dichloroethane and THF but different in

water. Only the last solvent can act as a hydrogen bond donor.

-

The -∆H_HMvalues were measured for cations with CF₃SO₃

The pK_R^THFvalues for phosphonium salts (Table 7) correlate

with literature values (Figure 3; eqs 19 and 20) with a few

THF

counterions, while the pK_R

values were determined with

value of [HP^tBu₃]BPh₄is 10.6 (Table

THF

(67) Streuli, C. A. Anal. Chem. 1960, 32, 985-987.

(68) Kolthoff, I. M.; Chantooni, M. K.; Bhowmik, S. J. Am. Chem. Soc.

1968, 90, 23-28.

(69) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc. 1999,

121, 11432-11447.

exceptions. The pK_R

+

7), while that of HNEt₃is 12.5. This is a different ordering

than the pK_a^aqvalues of 11.4 and 10.7, respectively. Note also

that the heat of protonation of NEt₃with HOTf in CH₂ClCH₂-

9168 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

THF

Table 11. pK_R

pK_R

Values of Acids and Infrared Wavenumbers of Metal Hydride Modes of the Acid and Conjugate Base Complexes in Nujol

acid

ν(M-H)

2061

2023, 1938

1933

base

ν(M-H)

2019

1956, 1851

1782

6^a

[Re^VH₂(C₅H₅)₂]^{+ b}

Re^IIIH(C₅H₅)₂

Re^IIIH₃(PMePh₂)₄

Re^IIIH₃(PMe₂Ph)₄

Re^IH(PMe₃)₅

15.8

20.0

24.2

6^a

[Re^VH₄(PMePh₂)₄]BPh₄

[Re^VH₄(PMe₂Ph)₄]BPh₄

[Re^IIIH₂(PMe₃)₅]BPh₄

[Os^IVH(C₅Me₅)₂]⁺

1852

1757

2194

Os^II(C₅Me₅)₂

2164^c

12^a

16.9

30

[Os^IVH₂(C₅H₅)(PPh₃)₂]BPh₄

[Os^IVH₃(PMe₃)₄]PF₆

Re^VIIH₇(PPh₃)₂

2163, 2130^d

2043^e

Os^IIH(C₅H₅)(PPh₃)₂

Os^IIH₂(PMe₃)₄

2060^d

1985^e

1984, 1961

1970 w

1935 sh

1915 s

Q[Re^VH₆(PPh₃)₂]

Q[Re^VH₆(PCy₃)₂]

1884, 1853

1935 sh, 1884 m

1823 m

41

Re^VIIH₇(PCy₃)₂

1717 s

35

Os^VIH₆(PⁱPr₃)₂

1980, 1910

Q[Os^IVH₅(PⁱPr₃)₂]

1836

1858,^f1843^f

1965, 1688^g

1960, 1680^g

∼43

Ir^VH₅(PⁱPr₃)₂

Ir^VH₅(PCy₃)₂

1950

1945

Q[cis-Ir^IIIH₄(PⁱPr₃)₂]

Q[cis-Ir^IIIH₄(PCy₃)₂]

^aSee Table 10. ^bReference 88; anion was not specified. ^cReference 89. ^dReference 90. ^eReference 91. ^fIn THF. ^gReference 60.

Table 12. Peak Potentials from Cyclic Voltammetry for Anionic

Hydrides with L′ ) PⁱPr₃

a

b

complex

E_p(V)

E_calc

Q[ReH₃(CO)L′₂]

Q[OsH₃(CO)L′₂]

Z[OsH₃(CO)L′₂]

Q[RuH₃(CO)L′₂]

Q[RuH₅L′₂]

-0.42

-0.32

-0.29

-0.06

-0.6

-0.2

^aVersus Fe(C₅H₅)₂⁺/Fe(C₅H₅)₂in 0.2 M NⁿBu₄PF₆in THF; scan

rates 0.25 V/s. ^bd⁵/d⁶reduction potentials predicted by use of Lever’s

method.^26,92

M ) Ru (16.6, Table 9), Os (16.9), are quite different from

those reported for MeOH solutions of [MH₃(PMe₃)₄]OMe (11.3,

11.5) created by dissolving MH₂(PMe₃)₄in MeOH.⁴⁷The

differences in anions and solvent character make comparisons

difficult, but the MeOH values are probably closer to true

for the series of Re^V/Re^III

THF

Figure 4. Plot of ν(Re-H) versus pK_R

acid/base pairs of Table 11.

aqueous pK_avalues than the THF ones.

MeCN

The pK_a

values of some 20 neutral carbonyl hydride

complexes, mainly of the types MH(CO)_x(L) and MH(C₅R₅)-

(CO)_xL (M ) Cr, Mo, W, Mn, Re, Fe, Ru, Os; L ) CO,

phosphine) have been determined to fall in the range from 8 to

26.8.²By use of eq 21 their pK_R^THFvalues can be estimated to

fall in the range from 8 to 32.

The pK_R^THFvalues of more than 38 cationic hydrides, mainly

of the types [MH(CO)_xL_y](OTf) and [MH(C₅R₅)XL_x](OTf) (X

) halide or hydride), can be estimated by use of eq 22 from

their -∆H_HMvalues determined in CHCl₂CHCl₂.¹⁵These fall

in the range from 0 to 14. For example, [Os(C₅H₅)(H)₂(PPh₃)₂]-

THF

OTf is predicted to have a pK_R

of 12 on the basis of its

-∆H_HMvalue of 37.3 (Table 10). The pK_a(not corrected for

-

ion-pair effects) of the BF₄salt of this complex in CH₂Cl₂

THF

Figure 5. Plot of ν(M-H) versus pK_R

for the anionic hydrides of

was reported to be 13.4 relative to HPCy₃⁺at 9.7.²⁸Most basic

hydrides with conjugate acids with pK_R

Table 11.

THF

> 15 are expected

K₂₃

K_d23

to react with these chlorinated solvents.

HA + xTHF y z {H(THF)_x⁺,A^-} y z H(THF)_x⁺+ A^-

pK_aof Dihydrogen. The correlation of pK_R^THF(HA) with

(23)

(24)

of H₂/Q⁺H^-(not

DMSO

THF

pK_a

allows an estimate of the pK_R

K_R^THF(HA) ) K₂₃K_d23

corrected for ion-pairing) as about 49 on the basis of the work

by Buncel and Menon in 1977.⁷¹Their determination of the

pK_aof H₂at 35.3 (also not corrected for ion-pairing) was based

on the reaction of H₂with Q[CH(C₆H₂-2,4-Me₂)₂] in THF and

anchored to 9-phenylfluorene, estimated to have a pK_aof 18.5.

The pK_R^THFof CH₂Ph₂is estimated to be 48 on the basis of its

K_d25

{HB⁺,Y^-} + xTHF y z HB⁺+ Y^-+ xTHF

K_R^THF(HB )

+

y

z B + H(THF)_x⁺+ Y^-(25)

of 32.3.¹⁸Therefore, the pK_R

Me₂)₂is about 50, and therefore, the ion-pair pK_R

of CH₂(C₆H₂-2,4-

DMSO

THF

Cl is 2.7 kcal/mol greater than that of P^tBu₃(Table 9), also in

keeping with the greater basicity of NEt₃in these aprotic

pK_a

THF

of H₂/

solvents.

The pK_R

(70) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112, 2843.

(71) Buncel, E.; Menon, B. J. Am. Chem. Soc. 1977, 99, 4457-4461.

THF

values for the complexes [MH₃(PMe₃)₄]BPh₄,

Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9169

Q⁺H^-is about 49 according to data from Buncel et al.⁷¹Our

value of 49 (Table 10) still remains an estimate until direct links

are found to fill in the gap between the top of our scale, 41 for

ReH₇(PCy₃)₂, and the pK_R^THFof H₂(g). The pK_aof Ir(C₅Me₅)-

(H)₂(PMe₃)/Ir(C₅Me₅)(H)(Li)(PMe₃) has been reported to be in

the range 38-41 because the lithium salt deprotonates H₂(g)

The acidity of the complexes ReH₇(PR₃)₂(Table 8) is

surprisingly sensitive to substituent effects. Changing from

PPh₂C₆H₄F to PPh₃results in a 2 pK unit change, while changing

from PPh₃to PCy₃results in an 11 pK unit change! While the

IR spectra of the neutral acids ReH₇L₂are comparable (Table

11, Figure 5), there are much lower ν(Re-H) modes for the

anionic PCy₃complex (a strong mode at 1717 cm^-1) compared

to those of the PPh₃complex (1884, 1853 cm^-1). This indicates

that the destabilization of rhenium-ligand bonding in the anions

is a strong determinant of the acidity of the acid form. The effect

of replacing L ) PPh₃by L ) PCy₃on the acidity of some

iridium complexes [Ir(C₅H₅)(CO)(L)(H)]OTf is much smaller

than that observed for our rhenium system. In the case of

iridium, the -∆H_HMchanged from 30 to 32.7 kcal/mol;¹⁴this

represents a pK change of 1.3 units according to eq 22. The

replacement of two L ) PPh₃by two L ) PMe₃in [Ru(C₅-

Me₅)L₂(H)₂]⁺results in a ∆pK of 5.6 (Table 7), again much

smaller than the observed change for ReH₇L₂.

and DMSO but not toluene in THF;⁶²this translates into a

THF

pK_R

> 51 (Table 10). A low ν(Ir-H) would be expected

for this lithium salt on the basis of the IR trends reported in

Table 11, but no IR spectrum was reported in the work.

Equation 26 is useful in relating the bond dissociation of an

element-hydrogen bond BDE{EH/(E^•+ H^•)} in kilocalories

per mole with its pK_a{EH/(E^-+ H⁺)} on the MeCN scale and

+

the reduction potential E_1/2{(E^•+ e^-)/E^-} versus Fe(C₅H₅)₂

/

Fe(C₅H₅)₂.^69,70,72,73Since BDE{H₂(g)/(2H^•)} ) 103.25 kcal/mol

BDE{EH/(E^•+ H^•)} ) 1.37pK_a{EH/(E^-+ H⁺)} +

23.1E_1/2{(E^•+ e^-)/E^-} + 59.5 (26)

A lower metal hydride stretching frequency might be expected

to be indicative of weaker metal hydride bonding. The cationic

polyhydrides of Table 11 show a decrease in ν(MH) with

increasing pK_R. The neutral polyhydride weak acids of Table

11 (ReH₇L₂, OsH₆L₂, and IrH₅L₂) all have metal hydride modes

at about 1950 cm^-1. An increase in the wavenumbers of M-H

modes with increase in pK_Rof the hydrides might have been

expected on the basis of the report of an increase in M-H bond

dissociation energy with a decrease in acidity of related cationic

hydride complexes¹⁴and on the basis of eq 26. But this is not

observed.⁷⁵It is significant that the conjugate base hydrides have

lower metal hydride modes that drop with increasing pK_adown

to 1680 cm^-1for [IrH₄L₂]^-. It appears from these data that the

weak acidity of these polyhydrides is due in part to the

destabilization of bonding in the conjugate base hydride, with

the assumption that there is some direct relationship between

ν(MH) and M-H bond strength. The very low metal hydride

mode for the iridium anions is due to the high trans influence

of the mutually trans hydride ligands. However, caution is

needed in interpreting the IR data of anions because the cation

can greatly affect the metal hydride vibrational modes. The

spectacular example of this is the series of complexes M₂[OsH₆],

where ν(Os-H) is 1500 cm^-1for M ) Ba and 1850 cm^-1for

M ) Mg.⁷⁶In our case, the crown cation remains the same,

although its interaction with the hydrides on the anion probably

varies. In cases where Nujol mull and THF samples have been

compared, the wavenumbers of modes are quite similar.

for dihydrogen and E_1/2{(H^•+ e^-)/H^-} is assigned to be -1.1

V vs Fe(C₅H₅)₂⁺/Fe(C₅H₅)₂in MeCN,⁷⁴the pK_a^MeCN{H₂(g)/(H^-

+ H⁺)} is calculated by use of eq 26 to be 50 on the CH₃CN

scale. Conversion eq 21 provides an extrapolated pK_R^THFvalue

of 60. This value, when compared to the ion-pair pK_R^THF(H₂/

Q⁺H^-) ) 49, would suggest that K_d(Q⁺H^-f Q⁺+ H^-) is

very small (approximately 10^-11M).

Effect of the Substituents and the Metal on pK_a. The

expected substituent effect of increasing acidity of the phos-

phonium with the substitution of Ph for Et or Cy is observed.

+

Thus, there are the pK_Rorderings HPEt₃> HPEt₂Ph⁺

>

HPEtPh₂⁺and HPCy₃⁺> HPCy₂Ph⁺(Table 7). Replacing Me

for Ph in the complexes [Re(H)₄(PR₃)₄]⁺causes a decrease in

pK_R: PMe₃(22.9) > PMe₂Ph (20.0) > PMePh₂(15.8) (Table

7). A similar result is observed for [Os(H)₃(PR₃)₄]⁺: PMe₃

(16.9) > PMe₂Ph (14.9) > PMePh₂(12.4) (Table 7).

An unusual observation is that methyl substituents make

phosphines more basic than do ethyl or butyl substituents, while

the effect is the opposite for the Os complexes [OsH₃(PR₃)₄]⁺.

Compare, for example, the pK_Rof HPMePh₂⁺BPh₄^-(6.4) versus

-

that of HPEtPh₂⁺BPh₄(5.3), and the pK_Rof HPMe₃⁺BAr′₄

(8.7) versus that of HPEt₃⁺BAr′₄^-(8.5). In the gas phase, bases

with Me substituents are known to be more basic than those

with Et or Bu. However, this effect is not observed for heats of

protonation in CH₂ClCH₂Cl where HPMe₃⁺OTf^-(-∆H ) 31.6)

is more acidic than HPEt₃⁺OTf^-(-∆H ) 33.7). In addition,

[Os(H)₃(PMe₃)₄]⁺is more acidic than [Os(H)₃(PEt₃)₄]⁺. There-

fore, solvation effects make the prediction of substituent effects

somewhat difficult.

Dihydrogen Complexes. Previous determinations of con-

stants for equilibria between metal hydrides and phosphonium

Osmium and rhenium hydrides of the types [OsH₃(PR₃)₄]⁺,

[ReH₄(PR₃)₄]⁺, and [ReH₂(PMe₃)₅]⁺were found to be the least

acidic cationic compounds of the scale, with the rhenium

complexes being 4-6 units less acidic than the osmium ones

with the same ligand set (e.g., [ReH₄(PMe₂Ph)₄]⁺with 20.0

versus [OsH₃(PMe₂Ph)₄]⁺with 14.9, Table 7). On a per-proton

basis the Os complexes are even more acidic, considering that

they have three deprotonation sites while the Re complexes have

four. This probably reflects the greater stabilization on going

from seven-coordinate Os^IV(d⁴) to six-coordinate Os^II(d⁶) com-

pared with going from eight-coordinate Re^V(d²) to seven-

coordinate Re^III(d⁴).

salts can be used to place a variety of hydride and dihydrogen

THF

species on the pK_R

scale (Table 7). The tetrafluoroborate

salts of the dihydrogen complexes [Ru(H₂)(C₅H₅)(L)]BF₄(L )

dppm,⁷⁷dppe) and the dihydride tautomer [Ru(H)₂(C₅H₅)(dppe)]-

8

BF₄were previously determined to be 2.2, 2.5, and 2.2 units

-

aq

more acidic, respectively, than HPCy₃⁺BF₄(pK_a) 9.7) in

THF, although these numbers were not corrected for ion-pair

dissociation.¹¹The present work with the BPh₄salts give

-

THF

similar numbers and produces pK_R

values of 7.2, 7.0, and

(75) A reviewer noted that the metal hydride IR wavenumber and the

bond dissociation energy may not be related since the former emphasizes

an ionic component to the bonding (on the way to heterolytic splitting)

while the latter is a homolytic splitting process.

(76) Linn, D. E.; Skidd, G. M.; Tippmann, E. M. Inorg. Chim. Acta 1999,

291, 142-147.

(77) Conroy-Lewis, F. M.; Simpson, S. J. J. Chem. Soc., Chem. Commun.

1987, 1675-1676.

(72) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287-

294.

(73) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989, 111, 6711-6717.

(74) Zhang, X. M.; Bruno, J. W.; Enyinnaya, E. J. Org. Chem. 1998,

63, 4671-4678.

9170 J. Am. Chem. Soc., Vol. 122, No. 38, 2000

Abdur-Rashid et al.

THF

7.4, respectively (Table 7). Similarly, the pK_aof the complexes

[Ru(H)₂(C₅H₅)(PPh₃)₂]BF₄and trans-[M(H₂)(H)(dtfpe)₂]BF₄

(dtfpe ) (CF₃-4-C₆H₄)₂PCH₂CH₂P(C₆H₄-4-CF₃)₂),¹⁰were previ-

ously estimated in THF and anchored to HPCy₃⁺. As reported

previously, the pK_aof the dtfpe dihydrogen complexes increases

in the aperiodic order Fe < Os < Ru. A similar order exists for

the series trans-[M(H₂)(H)(dppe)₂]BPh₄/cis-MH₂(dppe)₂(Table

7). The pK_aorder Os < Ru also applies to the neutral dihydrogen

complexes M(H₂)(H)₂(CO)(PⁱPr₃)₂(Table 8). The Os complex

has a “slow-spinning” dihydrogen ligand with an H-H distance

of 1.14 Å according to the minimum T₁(17 ms, 300 MHz) and

J_HD(16 Hz) data.⁷⁸The Ru complex has a fast-spinning

dihydrogen with an H-H distance of 0.88 Å according to similar

data (T₁^min) 8 ms at 200 MHz, J_HD) 32.4 Hz). The ruthenium

dihydrogen complexes are thought to be less acidic than

corresponding osmium ones because of the stronger (shorter)

H-H bond.¹⁰This would also explain why the dihydrogen

complex Ru(H₂)₂(H)₂(PⁱPr₃)₂is less acidic than the classical

hexahydride OsH₆(PⁱPr₃)₂(Table 8). The usual rule is that the

5d metal hydride is less acidic than the 4d metal hydride.²For

example, on the basis of -∆H_HM, the osmium complex is more

basic by 7.6 kcal mol^-1(approximately 4 pK_aunits) than the

ruthenium one for both M(C₅H₅)H(PPh₃)₂and M(C₅H₅)₂, where

classical hydrides are formed on protonation with HOTf.¹⁴

Similarly, Os(H)₂(CO)₄is less acidic than Ru(H)₂(CO)₄in CH₃-

CN by 2.1 pK units.⁴

pK_R

) 7.4, while HBr has pK_R

≈ 12 on the basis of eq

18, and so it will not react.

Conclusions

A continuous ladder of acid/base equilibria determined by

NMR has now been constructed for THF solutions of phosphorus-

containing species that spans the pK_R

Convenient syntheses of acids and bases that cover this pK range

are reported. The ladder includes 14 phosphonium salts/

phosphine couples, 14 dihydrogen/hydride couples, 17 cationic

hydride/neutral hydride couples, and 9 neutral hydride/anionic

THF

range from 5 to 41.

hydride couples. This is just the starting point. As more rungs

THF

of the ladder are interlinked, the pK_R

values will become

more accurate and converge on absolute pK_a^THF. The effects of

concentration on ion-pairing, ion aggregation, and pK_R^THFvalues

need to be examined more closely. However, the concentrations

used here are practical for NMR experiments.

THF

Correlations with other scales allow an estimate of the pK_R

values of more than 1000 organic acids, 20 carbonyl hydrides,

and of 46 cationic hydrides on the basis of work by the groups

of Bordwell, Streitwieser, Antipin, Norton, Angelici, and

DuBois. Therefore, the various acidity scales have been ap-

proximately united, and new acid-base reactions can be

predicted. Correlations with ν(M-H) noted here for the first

time suggest that destabilization of M-H bonding in the

conjugate base hydride is an important contributor to the pK_a.

It appears that Re-H bonding in the anions [ReH₆(PR₃)₂]^-is

greatly weakened by small increases in basicity of PR₃, resulting

in a large increase in pK_aof the conjugate acid ReH₇(PR₃)₂.

Work is still needed to link H₂(g) to our scale. It is estimated

on our scale to have an ion-pair pK_R

earlier work by Menon and Buncel.

We and others are developing a similar scale in CH₂Cl₂for

very acidic dihydrogen complexes.^13,38Here, THF cannot be

used as a solvent because some of these complexes protonate

THF.

It is interesting to note that the complexes Re(H)₄(NO)(PⁱPr₃)₂

and Os(H₂)(H)₂(CO)(PⁱPr₃)₂have close pK_R

values (Table

THF

8); therefore, replacement of Re(NO) by Os(CO) gives some-

what comparable properties, although the osmium complex is

a dihydrogen complex, while the rhenium complex is classical.⁷⁹

The peak potentials for oxidation of the conjugate base anions

are also similar (Table 12). The dihydrogen complexes Ru(H₂)₂-

THF

of 49 on the basis of

THF

(H)₂(PⁱPr₃)₂and Ru(H₂)(CO)(H)₂(PⁱPr₃)₂have similar pK_R

values; these structures are related by replacing a π-acid H₂

ligand with a π-acid CO ligand.

Kinetics of Proton Transfer. Dramatic differences in the

rates of proton transfer were noted. The equilibria between

complexes with several large ligands and high pK_Rvalues (e.g.,

[Re(H)₄L₄]⁺, [Os(H)₃L₄]⁺, and [Ru(C₅Me₅)(H)₂(PMePh)₂]⁺)

were very slow (12 h to 3 days, Table 3). The most dramatic

effect was observed for [Re(H)₄(PEt₃)₄]⁺, which continued to

Appendix

The definition of ∆pK_Rof Table 3 for reactions like eq 12 is

∆pK_R) pK_R(HA₁) - pK_R(HA₂) ) pK - ∆pK_d

react with Re(H)₃(PMe₃)₄for months without reaching equi-

THF

librium. The exact pK_R

for the PEt₃complex has not yet

as in eqs 5 and 6.

been determined for this reason. By contrast, the acidic

phosphonium salts apparently reached equilibrium in seconds.

Steric effects are very important in such reactions. This was

clearly demonstrated in an earlier study by Hanckel and

Darensbourg, where proton transfer from sterically congested

[MoH(CO)₂(dppe)₂]⁺to F^-was fast (1 h) but that to NEt₂H

was slow (10 days) and that to Mo(¹³CO)₂(dppe)₂was extremely

For reactions like eq 11, the definition of ∆pK_Ris

∆pK_R) pK_R(HB,Y) - pK_R(MH₂L_n,Y) ) pK - ∆pK_d

where the inter-ion distance a for {HB,Y} is used in place of

a_MAin eq 6 for the calculation of ∆pK_dand the a for

2

{MH₂L_n,Y} is used in place of a_MA

.

1

slow.⁸⁰

THF

(81) Basallote, M. G.; Dura´n, J.; Ferna´ndez-Trujillo, M. J.; Ma´n˜ez, M.

A. Organometallics 2000, 19, 695-698.

Use of pK_r

Correlations. The correlation between acid

scales will prove useful in predicting new reactions and

explaining reported ones. For example, it appeared puzzling why

the reaction of RuH(C₅H₅)(dppm) with HBr (pK_a^DMSO0.9) did

not produce {Ru(H₂)(C₅H₅)(dppm)⁺,Br^-} in THF⁸¹while reac-

tion with {HOEt₂,BF₄} did produce {Ru(H₂)(C₅H₅)(dppm),BF₄}.¹¹

The present work shows that the dihydrogen complex has

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Acidity Scale for Phosphorus-Containing Compounds

J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9171

Acknowledgment. We thank NSERC for an operating grant

to R.H.M. and Johnson Matthey Research Centre for a loan of

ruthenium, osmium, and iridium salts. A donation of carbonyl

hydride complexes from Dr. Morris Bullock is gratefully

acknowledged. We are indebted to Professor A. Streitwieser,

Jr., for very constructive comments in his review of this article.

Supporting Information Available: Crystallographic data

for [Re(H)₂(PMe₃)₅]BPh₄(CIF, PDF) and a table of elemental

analyses of representative salts (PDF). This material is available

free of charge via the Internet at http://pubs.acs.org.

JA994428D

Article Doi

An acidity scale for phosphorus-containing compounds including metal hydrides and dihydrogen complexes in THF: Toward the unification of acidity scales

DOI: 10.1021/ja994428d

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