A kinetically inert proton on a metal - Metal bond in [{(η5- C5H3)2(SiMe2)2}Ru2(CO)4(μ-H)]+ that promotes reactions with amines [13]

Full text of DOI:10.1021/ja000883u

6130
J. Am. Chem. Soc. 2000, 122, 6130-6131
Scheme 1
A Kinetically Inert Proton on a Metal-Metal Bond
in [{(η5-C5H3)2(SiMe2)2}Ru2(CO)4(µ-H)]+ that
Promotes Reactions with Amines
Maxim V. Ovchinnikov and Robert J. Angelici*
Department of Chemistry, Iowa State UniVersity
Ames, Iowa 50011
ReceiVed March 13, 2000
Unsaturated ligands in transition metal complexes can be
activated to nucleophilic attack by creating a positive charge on
the complex.1 Carbon monoxide ligands are activated to attack
by amine nucleophiles when the positive charge on a complex is
sufficiently high to give CtO stretching force constants, kCO, that
are higher than 16.5 mdyn/Å (or ν(CO) values higher than
approximately 2000 cm-1).2 These reactions lead to carbamoyl
complexes (eq 1), and some reactions give formamides and ureas
the slow rate of deprotonation of 1H+ thereby allowing nucleo-
philic attack on a CO ligand.
10
The reaction of (C5H4)2(SiMe2)2 with Ru3(CO)12 in the
presence of the hydrogen acceptor 1-dodecene furnished {(η5-
C5H3)2(SiMe2)2}Ru2(CO)4 (1) in 72% yield, as an air- and
moisture-stable yellow solid (Scheme 1).11 The hydride-bridged
dinuclear Ru complex12 1H+ was formed in quantitative yield
upon addition of 1 equiv of HBF4‚OEt2 or CF3SO3D to a solution
of complex 1 in CH2Cl2 at room temperature. The Ru-H
catalytically.3 One approach to making a complex more positive
is to add a proton (H+) to the metal (eq 2). While numerous metal
carbonyl complexes have been protonated,4 the CO ligands in
1
resonance in the H NMR spectrum occurs as a singlet at δ
-19.92 ppm. The CO stretching frequencies for 1H+ are
approximately 67 cm-1 higher than those for 1 and fall within
the range where amine attack on the CO groups is expected to
occur.2 An X-ray diffraction study of 1H+BF4- reveals an eclipsed
orientation of the terminal CO ligands on the two Ru atoms. The
M(L)x(CO)y + H+ f H-M(L)x(CO)y+
(2)
these complexes either do not react with amines because their
kCO and ν(CO) values are insufficiently high or the amine bases
simply deprotonate the metal to give the unreactive neutral
complex M(L)x(CO)y. This rapid deprotonation occurs for a wide
range of cationic metal hydride complexes H-M(L)x(CO)y+.5,4b
Neutral H-M(L)x(CO)y complexes often undergo deprotonation
much more slowly,6 but their kCO and ν(CO) values are not
sufficiently high to promote attack by amines. Di- and polynuclear
metal complexes with M-H-M bridging hydrides also undergo
rapid deprotonation with bases.7,8 In this communication we
describe a cationic dinuclear complex [{(η5-C5H3)2(SiMe2)2}Ru2-
(CO)4(µ-H)]+ (1H+) whose high ν(CO) values promote amine
attack but is only slowly deprotonated by amines. The bridging
dicyclopentadienyl (η5-C5H3)2(SiMe2)2 ligand9 with two SiMe2
groups linking the cyclopentadienyl rings is a key contributor to
-
Ru-Ru distance is substantially longer in 1H+BF4 (3.1210(5)
Å) than in 1 (2.8180(3) Å).13
Compound 1H+ is exceptionally stable with respect to depro-
tonation by strong organic bases such as Et3N, quinuclidine, or
pyridine. Less than 2% of the complex was deprotonated after 1
h in CD3NO2 or CD3CN solution in the presence of 10-fold
excesses of these amines. Moreover, the deuterated complex
1D+TfO- in wet acetone solution (∼10% H2O) did not undergo
measurable H-D exchange after 5 days at 25 °C. In contrast to
1H+, the unbridged and monobridged complexes (η5-C5H5)2Ru2-
(CO)4(µ-H)+ 14a and {(η5-C5H4)2(SiMe2)}Ru2(CO)4(µ-H)+ 14b were
deprotonated instantly and quantitatively by bases such as pyridine
or diethylamine. The acidity of 1H+BF4 , estimated as pKaAN from
-
studies of the equilibrium constant for the proton-transfer reaction
(1) Bush, R. C.; Angelici, R. J. J. Am. Chem. Soc. 1986, 108, 2735.
(2) Angelici, R. J. Acc. Chem. Res. 1972, 5, 335.
(3) (a) McCusker, J. E.; Logan, J.; McElwee-White, L. Organometallics
1998, 17, 4037. (b) McCusker, J. E.; Abboud, K. A.; McElwee-White, L.
Organometallics 1997, 16, 3863. (c) McCusker, J. E.; Grasso, C. A.; Main,
A. D.; McElwee-White, L. Org. Lett. 1999, 1, 961. (d) Dombek, B. D.;
Angelici, R. J. J. Catal. 1977, 48, 433.
(10) (a) Siemeling, U.; Jutzi, P.; Neumann, B.; Stammler, H. G.; Hursthouse,
M. B. Organometallics 1992, 11, 1328. (b) Hiermeier, J.; Koehler, F. H.;
Mueller, G. Organometallics 1991, 10, 1787.
(11) In a typical procedure, a solution of Ru3(CO)12 (50.0 mg, 78.2 µmol),
(C5H4)2(SiMe2)2 (28.6 mg, 117.0 µmol), and 1-dodecene (260.0 mg, 2.4 mmol)
in heptane (30 mL) was heated to reflux for 18 h. The mixture was
chromatographed on an alumina column (1 × 20 cm) first with hexanes and
then with a 1:10 (v/v) mixture of CH2Cl2 and hexanes which eluted a yellow
band containing 1 (47 mg, 72%). 1H (400 MHz, CDCl3): δ 0.26 (s, 6 H,
Si(CH3)), 0.46 (s, 6 H, Si(CH3)), 5.37 (d, J ) 1.6 Hz, 4 H, Cp-H), 5.78 (t, J
) 1.6 Hz, 2 H, Cp-H). 13C (100 MHz, CDCl3): δ -2.25 (CH3), 4.53 (CH3),
87.72, 93.95, 95.57 (Cp), 204.57 (CO). IR (CH2Cl2): ν(CO) (cm-1) 2015
(vs), 1952 (vs). Anal. Calcd for C18H18O4Ru2Si2: C, 38.84; H, 3.26. Found:
C, 39.05; H, 3.32.
(4) (a) Angelici, R. J. Acc. Chem. Res. 1995, 28, 52. (b) Kristja´nsdo´ttir, S.
S.; Norton, J. R. In Transition Metal Hydrides: Recent AdVances in Theory
and Experiments; Dedieu, A., Ed.; VCH: New York, 1991; Chapter 10. (c)
Pearson, R. G. Chem. ReV. 1985, 85, 41. (d) Martinho Simo˜es, J. A.;
Beauchamp, J. L. Chem. ReV. 1990, 90, 629. (e) Bullock, R. M. Comments
Inorg. Chem. 1991, 12, 1.
(5) Jia, G.; Morris, R. H. Inorg. Chem. 1990, 29, 582.
(6) (a) Edidin, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1987,
109, 3945. (b) Moore, E. J.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc.
1986, 108, 2257. (c) Jordan, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982,
104, 1255.
(12) 1H+BF4-): 1H (400 MHz, CD2Cl2): δ -19.92 (s, 1 H, Ru-H-Ru),
0.47 (s, 6 H, Si(CH3)), 0.62 (s, 6 H, Si(CH3)), 5.99 (d, J ) 2.0 Hz, 4 H,
Cp-H), 6.02 (t, J ) 2.0 Hz, 2 H, Cp-H). 13C (100 MHz, CD2Cl2): δ -2.57
(CH3), 2.86 (CH3), 88.97, 98.56, 98.81 (Cp), 195.19 (CO). IR (CH2Cl2): ν-
(CO) (cm-1) 2077 (vs), 2050 (w), 2027 (s). Anal. Calcd for C18H19BF4O4-
Ru2Si2: C, 33.55; H, 2.97. Found: C, 33.19; H, 2.90.
(7) (a) Nataro, C.; Thomas, L. M.; Angelici, R. J. Inorg. Chem. 1997, 36,
6000. (b) Nataro, C.; Angelici, R. J. Inorg. Chem. 1998, 37, 2975.
(8) (a) Weberg, R. T.; Norton, J. R. J. Am. Chem. Soc. 1990, 112, 1105.
(b) Kristja´nsdo´ttir, S. S.; Moody, A. E.; Weberg, R. T.; Norton, J. R.
Organometallics 1988, 7, 1983.
(13) Details of the X-ray diffraction studies of 1, 1H+BF4-, and 2a will be
published separately: Ovchinnikov, M. V.; Guzei, I. A.; Angelici, R. J.
Manuscript in preparation.
(9) For recent examples of (η5-C5H3)2(SiMe2)2 complexes, see: (a) Sun,
H.; Teng, X.; Huang, X.; Hu, Z.; Pan, Y. J. Organomet. Chem. 2000, 595,
268. (b) Cano, A. M.; Cano, J.; Cuenca, T.; Go´mez-Sal, P.; Manzanero, A.;
Royo, P. Inorg. Chim. Acta 1998, 280, 1.
(14) (a) Ovchinnikov, M. V.; Angelici, R. J. Unpublished results. (b)
Froehlich, R.; Gimeno, J.; Gonzalez-C. M.; Lastra, E.; Borge, J.; Garcia-G.
S. Organometallics 1999, 18, 3008.
10.1021/ja000883u CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/08/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 25, 2000 6131
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between 1 and HPPh3+BF4 in CD3CN at 25 °C,15 is 6.5((0.2)
Scheme 2
in CD3CN. This pKaAN value clearly indicates that the above-
-
noted amine bases will thermodynamically deprotonate 1H+BF4
easily. Although it is not obvious why the (η5-C5H3)2(SiMe2)2
bridging ligand causes the bridging proton to be so slowly
removed, it may be due to a combination of the bulkiness of the
dimethylsilyl linkers and the rigidity of the molecule. The donor
ability of the (η5-C5H3)2(SiMe2)2 ligand to the Ru atom is probably
similar to that of the Cp ligand based on average ν(CO) values16
for 1H+BF4- (2044 cm-1) and Cp2Ru2(CO)4(µ-H)+ 7a (2046 cm-1).
Reactions of 1H+ with secondary amines (Me2NH, Et2NH,
morpholine, pyrrolidine), primary amines (MeNH2, EtNH2,
BnNH2), or ammonia furnished {(η5-C5H3)2(SiMe2)2}Ru2(CO)3-
(NHRR′) (2) complexes and the corresponding formamides in a
1:1 ratio (Scheme 1). Complexes of type 217 were isolated in 78-
94% yield as air- and moisture-sensitive dark-red solids. The other
Ru-containing product in these reactions was the deprotonated
complex 1 observed in 5-20% yields as a result of direct
deprotonation of 1H+ by amine. The yields of 1 appear to depend
on the steric properties of the amine as less bulky amines (e.g.
NH3) give higher yields of 1. Also, the yields of 1 were lower
when the deuterated 1D+TfO- complex was used. Using a variety
of amines, we determined that 1H+ reacts when the amine has a
pKa value equal to or greater than 8.33 (morpholine)18 and a cone
angle Θ that is equal to or less than 125° (diethylamine).19 Bulky
amines (Bn2NH, i-Pr2NH, Cy2NH) and weakly nucleophilic
amines (aniline) failed to react with 1H+.
the reaction. Experiments using the deuterium-labeled 1D+TfO-
gave formamide products (D(CdO)NRR′) that are completely
deuterated at the formyl position and no other. When 1D+TfO-
rather then 1H+BF4- was used in the reaction, substantially less
deprotonation to 1 was observed, as expected for a deuterium
isotope effect.
Rates of the reaction of 1D+TfO- ([1D+TfO-] ) 8.34 × 10-3-
11.12 × 10-3 M-1) with morpholine ([morpholine] ) 8.56 ×
10-1 to 10.82 × 10-1 M-1) in nitromethane solvent to give {(η5-
C5H3)2(SiMe2)2}Ru2(CO)3{NH(CH2CH2)2O} were followed by
monitoring the disappearance of the ν(CO) bands in the IR spectra
On the basis of studies described below, the amine reactions
are proposed to occur by the mechanism shown in Scheme 2.
This involves initial nucleophilic attack by the amine on a
coordinated CO to produce the cationic intermediate A, which is
rapidly deprotonated to B.2,20 Reductive elimination of the
formamide from B gives an unsaturated diruthenium intermediate
that coordinates an amine to give 2. No intermediates were
observed by FT-IR or NMR spectroscopy during the course of
1
or H NMR signals of 1D+TfO-. The reaction was shown to
follow the second-order rate law, -d[1D+TfO-]/dt ) k2[1D+TfO-]-
[morpholine], where k2 ) (2.3 ( 0.5) × 10-3 M-1 s-1 at 20 °C.
This rate law is consistent with the first step in the mechanism
(Scheme 2) being rate-determining. The subsequent deprotonation
of the nitrogen in A is likely to be fast and the reductive
elimination of the formamide from B must be rapid because there
is no spectroscopic evidence for intermediates in the reaction.
Such a facile reductive elimination is surprising because removal
of the bridging H+ by bases is so slow. Reductive eliminations
-
(15) A solution of 1 (18.6 mg, 31.9 µmol) and HPPh3+BF4 (11.2 mg,
32.0 µmol) in CD3CN (1 mL) was sealed in an NMR tube under an argon
1
atmosphere. After following the reaction for 8 days, the H NMR spectrum
indicated a [1]/[1H+BF4-] ratio of 2.35, and the 31P NMR spectrum established
a [HPPh3+BF4-]/[PPh3] ratio of 2.47. From these two ratios and the pKaAN of
involving a µ-H have only recently been characterized, e.g. in
+ 21
HPPh3 (8.0), the pKaAN of 1H+BF4 was calculated to be 6.5((0.2) in
+
-
the formation of alkanes and arenes from Pd2R2(µ-H)(dppm)2
.
acetonitrile.
In conclusion, we have discovered that protonation of the Ru-
Ru bond in 1 gives a cationic complex (1H+) in which the
bridging proton is removed only very slowly by bases even though
the proton is thermodynamically acidic (pKaAN ) 6.5((0.2)). The
low kinetic acidity of 1H+ allows it to react with alkylamines,
which attack a CO ligand that is activated to such an attack by
the cationic nature of the complex. These amine reactions lead
to the elimination of the µ-H which becomes incorporated into
the formamide product. Mechanistic studies support the pathway
shown in Scheme 2. Further studies of reactions of 1H+ with
nucleophiles are in progress.
(16) Jolly, W. L.; Avanzino, S. C.; Rietz, R. R. Inorg. Chem. 1977, 16,
964.
(17) In a typical procedure, gaseous CH3NH2 was slowly bubbled through
-
a suspension of yellow 1H+BF4 (50.0 mg, 77.6 µmol) in hexanes (25 mL)
for 5 min at ambient temperature. The color of the reaction mixture
immediately changed to wine-red. Solvent was removed in a vacuum, and
the red residue was recrystallized from hexanes (10 mL) at -25 °C to give
1
39 mg (83%) of 2a as dark-red, air- and moisture-sensitive crystals. H (400
MHz, C6D6): δ 0.24 (s, 6 H, Si(CH3)), 0.38 (s, 6 H, Si(CH3)), 1.58 (bs, 2 H,
CH3NH2), 1.67 (t, J ) 6.4 Hz, 3 H, CH3NH2), 4.52 (d, J ) 2.0 Hz, 2 H), 4.77
(t, J ) 2.0 Hz, 1 H), 5.11 (d, J ) 2.4 Hz, 2 H), 5.76 (t, J ) 2.4 Hz, 1 H). 13
C
(100 MHz, C6D6): δ -1.88 (CH3), 5.38 (CH3), 41.39 (CH3NH2), 79.20, 83.59,
90.10, 92.98, 94.37, 95.52 (Cp), 209.07 (CO), 211.98 (CO). IR (hexanes):
ν(CO) (cm-1) 1973 (vs), 1907 (vs), 1884 (m). Anal. Calcd for C18H23NO3-
Ru2Si2‚1/4C6H14: C, 40.30; H, 4.60; N, 2.41. Found: C, 40.77; H, 4.57; N,
2.95.
Acknowledgment. We appreciate the support of the National Science
Foundation through Grant No. CHE-9816342.
(18) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous
Solution; Butterworth: London, 1972.
JA000883U
(19) For amine cone angles, see: Seligson, A. L.; Trogler, W. C. J. Am.
Chem. Soc. 1991, 113, 2520.
(21) Stockland, R. A., Jr.; Anderson, G. K.; Rath, N. P. J. Am. Chem. Soc.
1999, 121, 7945.
(20) Semmelhack, M. F. J. Organomet. Chem., Libs. 1976, 1, 361.