Highly bulky and electron-rich terminal ruthenium phosphido complexes: New donor ligands for palladium-catalyzed Suzuki cross-couplings

Article abstract of DOI:10.1021/ic020589y

Secondary phosphine complexes of the formula [(η⁵-C₅H₅)Ru(L)₂- (PHR₂)]⁺ BAr_F^- are prepared from cationic ruthenium N₂ complexes and PHR₂ (R = Ph (a), t-Bu (b), Cy (c)). Additions of t-BuOK or NaN(SiMe₃)₂ give the phosphido complexes (η⁵-C₅H₅)Ru(L)₂ (PR₂) ((L)₂ = (PEt₃)₂ (5a-c), depe (6a, b)) in high NMR yields. These rapidly oxidize in air to give isolable RuP(=O)R₂ species. Complex 5a is more basic than the rhenium analogue (η⁵-C₅H₅)Re(NO)-(PPh₃) `(PPh₂), and 6b is more basic than P-t-Bu₃. Complexes 5a-c and 6b are effective ligands for palladium-catalyzed Suzuki reactions. The catalyst from 6b is nearly as reactive as that from the benchmark ligand P-t-Bu₃.

Full text of DOI:10.1021/ic020589y

Inorg. Chem. 2002, 41, 6947−6949
Highly Bulky and Electron-Rich Terminal Ruthenium Phosphido
Complexes: New Donor Ligands for Palladium-Catalyzed Suzuki
Cross-Couplings
Jose´ Giner Planas and J. A. Gladysz*
Institut fu¨r Organische Chemie, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg,
Henkestrasse 42, 91054 Erlangen, Germany
Received October 1, 2002
Secondary phosphine complexes of the formula [(η5-C H )Ru(L)2-
and nucleophilic than in organic analogues PR3.2,4 However,
5
5
(PHR )]+ BArF- are prepared from cationic ruthenium N complexes
we sought to develop new series of phosphido complexes
that would be still more electron-rich. Our attention was
drawn to ruthenium(II) species of the type (η5-C5R5)Ru-
(PR3)2(X), in which the strongly π accepting NO ligand of
the rhenium has been eliminated. Many such adducts are
known where X is a halide, alkoxide, or thiolate ligand, or
(PR3)2 constitutes a chiral diphosphine ligand.5,6 However,
to our knowledge no phosphido complexes are yet de-
scribed.7,8 In this communication, we report that such species
are (1) easily generated, (2) among the most electron rich
trivalent phosphorus compounds known, and (3) highly
effective ligands for palladium-catalyzed Suzuki coupling
reactions.
2
2
and PHR (R ) Ph (a), t-Bu (b), Cy (c)). Additions of t-BuOK or
2
NaN(SiMe3)2 give the phosphido complexes (η5-C H )Ru(L)2(PR )
5
5
2
((L)2 ) (PEt3)2 (5a−c), depe (6a,b)) in high NMR yields. These
rapidly oxidize in air to give isolable RuP(dO)R species. Complex
2
5a is more basic than the rhenium analogue (η5-C H )Re(NO)-
5
5
(PPh3)(PPh2), and 6b is more basic than P-t-Bu3. Complexes 5a−c
and 6b are effective ligands for palladium-catalyzed Suzuki
reactions. The catalyst from 6b is nearly as reactive as that from
the benchmark ligand P-t-Bu3.
Over the past few years, numerous new types of phos-
phorus donors have been evaluated as ligands in metal-
catalyzed organic transformations. Improved performance
characteristics are often found with species that are bulkier
and/or more electron-rich.1 In this context, we have been
interested in coordinatively saturated phosphido complexes
of the type LnMPR2: as ligands for metal-catalyzed reac-
tions.2,3 Our efforts to date have focused on rhenium(I)
systems of the formula (η5-C5R5)Re(NO)(PPh3)(PR2), in
which the pyramidal PR2 moiety is much more congested
In view of the congested nature of the target compounds,
building blocks with good leaving groups were sought. Our
attention was drawn to the recently reported and easily
prepared bis(tertiary phosphine) dinitrogen complexes 1 and
2 shown in Scheme 1.9 These were treated with the secondary
phosphines PHPh2 (a), PH-t-Bu2 (b), and PHCy2 (c) under
homogeneous conditions in C6H5F at room temperature. As
shown in Scheme 1, workups gave the new secondary
-
phosphine complexes [(η5-C5H5)Ru(PEt3)2(PHR2)]+ BArF
(3a-c) and [(η5-C5H5)Ru(depe)(PHR2)]+ BArF- (4a,b)10 in
* Author to whom correspondence should be addressed. E-mail: gladysz@
organik.chemie.uni-erlangen.de.
(4) Buhro, W. E.; Zwick, B. D.; Georgiou, S.; Hutchinson, J. P.; Gladysz,
J. A. J. Am. Chem. Soc. 1988, 110, 2427.
(1) The following references treat the Suzuki reaction, but many additional
examples are cited therein: (a) Wolfe, J. P.; Singer, R. A.; Yang, B.
H.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 9550. (b) Littke,
A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020. (c)
Zapf, A.; Ehrentraut, A.; Beller, M. Angew. Chem., Int. Ed. 2000, 39,
4153; Angew. Chem. 2000, 112, 4315. (d) Clarke, M. L.; Cole-
Hamilton, D. J.; Woolins, J. D. J. Chem. Soc., Dalton Trans. 2001,
2721. (e) Li, G. Y. J. Org. Chem. 2002, 67, 3643. (f) Kataoka, N.;
Shelby, Q.; Stambuli, J. P.; Hartwig, J. F. J. Org. Chem. 2002, 67,
5553.
(2) (a) Kromm, K.; Zwick, B. D.; Meyer, O.; Hampel, F.; Gladysz, J. A.
Chem. Eur. J. 2001, 7, 2015. (b) Kromm, K.; Hampel, F.; Gladysz, J.
A. HelV. Chim. Acta 2002, 85, 1778. (c) Eichenseher, S.; Kromm, K.;
Delacroix, O.; Gladysz, J. A. Chem. Commun. 2002, 1046. (d) Kromm,
K.; Osburn, P. L.; Gladysz, J. A. Organometallics 2002, 21, 4275.
(3) This is part of a general effort involving donor ligands that incorporate
some type of “spectator metal”, outside of metallocenes such as
ferrocene. See: Delacroix, O.; Gladysz, J. A. Chem. Commun., in press
(feature article).
(5) (a) Bennett, M. A.; Bruce, M. I.; Matheson, T. W. In ComprehensiVe
Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford,
1982; Vol. 4, pp 775-796. (b) Bennett, M. A.; Khan, K.; Wenger, E.
In ComprehensiVe Organometallic Chemistry II, Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 7, pp
476-527. (c) Consiglio, G.; Morandini, F. Chem. ReV. 1987, 87, 761.
(6) Bell, P. T.; Cagle, P. C.; Vichard, D.; Gladysz, J. A. Organometallics
1996, 15, 4695.
(7) For ruthenium phosphido complexes of the formula (η5-C5Me5)Ru-
(CO)2(PRX), see: (a) Stasunik, A.; Wilson, D. R.; Malisch, W. J.
Organomet. Chem. 1984, 270, C18. (b) Weber, L.; Reizig, K.; Boese,
R. Organometallics 1985, 4, 2097.
(8) For ruthenium phosphido complexes of the formula Ru(CO)2(PPh3)2-
(Cl)(PRH), see: (a) Bohle, D. S.; Jones, T. C.; Rickard, C. E. F.;
Roper, W. R. Organometallics 1986, 5, 1612. (b) Bohle, D. S.; Clark,
G. R.; Rickard, C. E. F.; Roper, W. R.; Taylor, M. J. J. Organomet.
Chem. 1988, 348, 385.
(9) Aneetha, H.; Jimenez-Tenorio, M.; Puerta, M. C.; Valerga, P.
Organometallics 2002, 21, 628.
10.1021/ic020589y CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/21/2002
Inorganic Chemistry, Vol. 41, No. 26, 2002 6947

COMMUNICATION
Scheme 1. Syntheses and Reactions of New Ruthenium Complexesa
a Conditions: (a) PHR2, C6H5F; (b) t-BuOK (3a, 4a) or NaN(SiMe3)2 (3b,c, 4b); (c) air.
Table 1. Suzuki Coupling Reactions
65-78% yields as air-stable powders. Each was characterized
by microanalysis and NMR, as summarized in the Supporting
Information. The 31P NMR spectra exhibited typical AX2
patterns arising from the secondary and two equivalent
tertiary phosphine ligands (2J(PP) ) 31.3-42.0 Hz). The
1H NMR spectra showed diagnostic doublets of triplets for
the PH signals (1J(HP)/3J(HP) ) 352.9/4.0 (3a), 350.5/5.2
(4a), 327.7/4.0 (3c), 318.9/2.0 (4b), 316.5/2.4 (3b) Hz).
Deprotonations of the secondary phosphine complexes
were attempted in NMR tubes.11 Yellow acetone-d6 solutions
of 3a and 4a were treated with 1.0-1.1 equiv of t-BuOK.
conversionb
(PhBr) (%)
yieldb
(Ph2) (%)
entrya
ligand/Pd
time (h)
1
2
3
4
5
6
6b/Pd(OAc)2
5c/Pd(OAc)2
5b/Pd(OAc)2
5a/Pd(OAc)2
5a/Pd2(dba)3
none/Pd(OAc)2
100
100
100
96
95
97
96
94
81
59
1
2
2
32
32
48
88
63
a General reaction conditions: bromobenzene (0.432-0.526 mmol, 1.0
equiv), phenylboronic acid (1.5 equiv), K3PO4 (2.0 equiv), Pd(OAc)2 (1.0
mol %) or Pd2(dba)3 (0.5 mol % ) 1.0 mol % Pd), 2 mol % ligand
(deprotonated in situ), toluene, 80 °C. b Conversion and yield determined
by GC (average of at least two runs), based on tridecane standard.
1
The solutions turned bright orange, and H and 31P NMR
spectra (Supporting Information)12 were consistent with the
formation of target phosphido complexes (η5-C5H5)Ru(PEt3)2-
(PPh2) (5a; 93% conversion) and (η5-C5H5)Ru(depe)(PPh2)
(6a; 90% conversion). Under analogous conditions, 3b,c and
4b did not react. However, when the much stronger base
NaN(SiMe3)2 was added to THF-d8 solutions (1.1-1.5
equiv), high conversions to phosphido complexes 5b (80%),
5c (95%), and 6b (90%) were observed.11 Attempts to isolate
5a-c and 6a,b always gave some of the corresponding
phosphine oxide, and traces were always detected in the
NMR tubes. When air was deliberately added, conversions
were complete within seconds, much faster than for the
analogous rhenium complexes. In the case of 5a, the oxide
(η5-C5H5)Ru(PEt3)2(P(dO)Ph2) (7a) was isolated in >99%
yield.
for which we reported data with the rhenium complexes
(η5-C5R5)Re(NO)(PPh3)(PR2) earlier. The conditions sum-
marized in entries 1-4 of Table 1 (Pd(OAc)2/K3PO4/toluene,
80 °C) share certain features with those used by Buchwald
and Fu,1a,b and 5a-c and 6b were generated with 2.0 equiv
of the bases described above (complete conversions).14
Phenyl bromide was consumed as depicted in Figure 1. The
catalysts derived from P-t-Bu2 and PCy2 complexes 5b,c were
distinctly more reactive than that derived from PPh2 complex
5a (less bulky and electron-rich ligand). The depe-substituted
P-t-Bu2 complex 6b gave a still more active catalyst. The
palladium source Pd2(dba)310c did not give improved results,
and in the absence of a phosphido complex rates were much
slower and only partial conversion could be effected.
The activity of the catalyst system derived from 6b is very
slightly lower than that of the related rhenium complex
(η5-C5R5)Re(NO)(PPh3)(P-t-Bu2) reported earlier,2c but higher
than those of RePPh2 and RePMe2 analogues. In order to
verify that the ruthenium complexes are more electron rich,
an NMR tube was charged with a 1:1 mixture of 3a and the
corresponding rhenium secondary phosphine complex
[(η5-C5R5)Re(NO)(PPh3)(PHPh2)]+ TfO-,2a,4 and 1.0 equiv
of t-BuOK in acetone-d6 was added. NMR spectra showed
Hence, by analogy to protocols developed for other
oxidation-sensitive electron-rich phosphines used in metal-
catalyzed reactions,2c,13 5a-c and 6a,b were “stored” as the
precursors 3a-c and 4a,b, and generated by in situ depro-
tonation. They were first evaluated in the palladium-catalyzed
Suzuki reaction of bromobenzene and phenylboronic acid,
(10) Abbreviations: (a) BArF- ) B[3,5-(C6H3(CF3)2)]4-; (b) depe ) Et2-
PCH2CH2PEt2; (c) dba ) dibenzylidene acetone.
(11) For the simultaneous deprotonation and complexation of the ruthenium
secondary phosphine complex [(η5-C5H5)Ru(CO)2(PPh2H)]+ BF4- by
a platinum compound, see: Powell, J.; Fuchs, E.; Gregg, M. R.;
Phillips, J.; Stainer, M. V. R. Organometallics 1990, 9, 387.
1
(12) The cyclopentadienyl H NMR signals of 5a-c and 6a,b were 0.5-
0.8 ppm upfield from those of 3a-c and 4a,b, and the J(P,P) values
decreased markedly. When the samples were stored for 24 h, significant
reprotonation to 5a-c and 6a,b occurred, perhaps due to slow-reacting
hydroxylic moieties associated with the glass.
(14) A detailed procedure is provided in the Supporting Information. A
31P NMR experiment shows that the phosphido complex 5a reacts
upon addition of Pd(OAc)2, and that subsequent addition of phenyl-
boronic acid does not give detectable amounts of the protonation
product 3a.
(13) Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295.
6948 Inorganic Chemistry, Vol. 41, No. 26, 2002

COMMUNICATION
lack of reactivity of 3b,c and 4b toward t-BuOK and P-t-
Bu3 indicates that the title compounds are among the most
basic trivalent phosphorus compounds.16 Accordingly, they
should be excellent synthetic building blocks. The title com-
pounds are also very effective ligands for metal-catalyzed
reactions, and they approach the reactivities of the best
organophosphines in palladium-catalyzed Suzuki reactions.
These promising lead results are easily amenable to further
optimization and extendable to chiral, nonracemic ruthenium
systems. In the former context, we are currently attempting
to more efficiently enter the catalytic cycle with preformed
MPR2Pd systems, such that the steric and electronic effects
of these novel metal-containing phosphorus donor ligands
upon reactivity can be more exactly evaluated.
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft (DFG, GL 300/4-1), the von Humboldt Foun-
dation (fellowship to J.G.P.), and Johnson Matthey PMC
(palladium and ruthenium loans) for support, and Prof. M.
Jime´nez-Tenorio (University of Ca´diz) for helpful discus-
sions.
Figure 1. Consumption of bromobenzene under the conditions of Table
1.
the complete conversion of the latter to (η5-C5R5)Re(NO)-
(PPh3)(PPh2), and no reaction of 3a. Hence, 5a has a much
higher Brønsted basicity than its rhenium counterpart, and
catalytic activity is not directly correlated to this property.
Similarly, the best RuP-t-Bu2 and ReP-t-Bu2 catalyst systems
remain slightly less active than that derived from P-t-Bu3,2c
which is one of the best ligands for the Suzuki reaction.1b
An NMR tube was therefore charged with a 1:1 mixture of
Supporting Information Available: Experimental procedures
and NMR data for all compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC020589Y
(15) There is furthermore, within homologous series of compounds, a
correlation of 1J(HP) values of protonated phosphines and basicity.
The couplings of 3b and 4b are much lower than that of [HP-t-Bu3]+
-
BF4 (318.9-316.5 vs 436 Hz), consistent with greater p character
in the phosphorus bonding orbital and enhanced basicity. See: Hudson,
H. R. In The Chemistry of Organophosphorus Compounds; Hartley,
F. R., Ed.; Wiley: New York, 1990; Vol. 1, Chapter 12.
- 13
4b and [HP-t-Bu3]+ BF4 , and 1.0 equiv of NaN(SiMe3)2
in THF-d8 was added. NMR spectra showed complete
deprotonation of the latter, and no reaction of 4b. Hence,
6b has a much higher Brønsted basicity than P-t-Bu3.15
In summary, this study has significantly expanded the scant
literature on ruthenium phosphido complexes.7,8,11 Although
no quantitative acid/base measurements were conducted, the
(16) Recent lead references: (a) Alder, R. W.; Butts, C. P.; Orpen, A. G.;
Read, D.; Oliva, J. M. J. Chem. Soc., Perkin Trans. 2 2001, 282. (b)
Abdur-Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman,
J. G.; Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc.
2000, 122, 9155. (c) Kisanga, P. B.; Verkade, J. G.; Schwesinger, R.
J. Org. Chem. 2000, 65, 5431. Note that in this case the conjugate
acid is a hypervalent phosphorus compound.
Inorganic Chemistry, Vol. 41, No. 26, 2002 6949