6808
J. Am. Chem. Soc. 1998, 120, 6808-6809
Highly Electrophilic [Re(CO)4(PR3)]+ Center Binds
Et2O and CH2Cl2 and Heterolytically Activates H2
Jean Huhmann-Vincent, Brian L. Scott, and
Gregory J. Kubas*
Chemical Science and Technology DiVision, MS J514
Los Alamos National Laboratory
Los Alamos, New Mexico 87545
ReceiVed January 20, 1998
The activation of H2 by transition-metal complexes is an
important step in many chemical processes, and the study of η2-
H2 complexes has provided valuable insight into the activation
of H-X (X ) H, Si, C) bonds in general.1 Metal-H2 binding is
governed by both σ-donation from H2 to metal and π-back-
bonding from metal to H2. The relative strengths of each bonding
component has been quantified theoretically and is dependent on
the electronic nature of the ancillary ligands.1g-i We are currently
interested in synthesizing new extremely electrophilic cationic
systems containing mainly π-acceptor ligands that enhance
σ-donation from H2 to metal at the expense of back-donation.
Such species increase the acidity of coordinated H2 and potentially
other σ-ligands such as alkanes, thus promoting heterolytic
activation via known deprotonation pathways.1b-d,2 Although
many organometallic Lewis acid fragments are known,3 com-
plexes containing only one donor ligand and four electron-
withdrawing carbonyls that bind H2 at or near ambient temperature
are unknown. In this paper, we report such a cationic Re
fragment, [Re(CO)4(PR3)][BArf], that binds and heterolytically
activates H2, as well as coordinates weak bases.
Protonation of the Re(I) methyl complexes [cis-Re(Me)(CO)4-
(PR3)] (R ) Ph (1a),4 Cy (cyclohexyl, 1b)5) with [H(OEt2)2]-
[BArf]6 (BArf ) B[3,5-C6H3(CF3)2]4-) in Et2O proceed with the
elimination of methane to afford [cis-Re(CO)4(PR3)(OEt2)][BArf]
(R ) Ph, 98% yield (2a); Cy, 89% yield (2b)).7 These complexes
form even in CH2Cl2 (crystallize on hexane addition) where the
acid is the ether source and are soluble in halogenated solvents
such as CH2Cl2 and C6H5F. Although some Et2O complexes have
been characterized spectroscopically such as [trans-Pt(H)(PiPr3)2-
(OEt2)][BArf] and [CpRe(NO)(PPh3)(OEt2)][BF4],8 few have been
characterized crystallographically (none for Re). X-ray analysis
of 2a showed Et2O in an octahedral position cis to PPh3 and no
unusual contacts with the BArf anion (Figure 1).9 The Re-O
distance is 2.254(11) Å, and the ether C-O-C angle is 113.9-
(13)°, typical of other M-OEt2 structures.10
Figure 1. ORTEP of 2a with 50% probability ellipsoids: Re(1)-P(1),
2.504(2) Å; C(49)-O(5)-Re(1), 119.3(9)°; C(47)-O(5)-Re(1), 116.7-
(9)°.
The Et2O in 2 is bound strongly in the solid, and the ratio of
BArf/OEt2 did not decrease after exposing the complexes to
vacuum for 12 h. For 2a in CD2Cl2 at -73 °C, resonances at
3.53 (q) and 0.98 (t) ppm in the 1H NMR spectrum are assigned
to the CH2 and CH3 groups of coordinated Et2O (resonances for
free Et2O at this temperature are 3.34 (q) and 1.08 (t)).8a Upon
warming the CD2Cl2 solution of 2a to room temperature,
dissociation and exchange of the Et2O was observed. However,
the Et2O could not be completely removed by recrystallization
from CH2Cl2/hexanes, and decomposition products formed upon
prolonged dissolution in CH2Cl2. The bound Et2O was easily
displaced in CH2Cl2 solution by more basic ligands such as H2O
or THF.11
To obtain the ether-free derivatives of 2, methyl abstraction
from 1 was carried out by reaction with [Ph3C][BArf]12 in CH2-
Cl2 solution. However, instead of producing coordinatively
unsaturated or agostic complexes, the CH2Cl2 adducts [cis-
Re(CO)4(PR3)(CH2Cl2)][BArf] (R ) Ph, 81% yield, (3a); Cy, 80%
yield, (3b))13 were formed. They are moderately air stable in
solid and solution, although the coordinated CH2Cl2 is quickly
replaced by adventitious water. CH2Cl2 solutions decompose at
room temperature within days to form chloride-bridged dimers
{[cis-Re(CO)4(PR3)]2(µ-Cl)}{BArf}.14 The fact that the Et2O and
CH2Cl2 complexes are isolable is attributed to the strong elec-
trophilicity of the [Re(CO)4(PR3)]+ fragment. This is in direct
contrast to Heinekey’s analogous bis-phosphine complexes, [mer-
Re(CO)3(PR3)2][BArf], which are isolated as the agostic com-
pounds in CH2Cl2.15 Furthermore, the importance of a noninter-
acting counterion is reflected by the existence of anion-coordinated
derivatives cis-Re(CO)4(PPh3)(FBF3) and cis-Re(CO)4(PPh3)-
(OTeF5).16
(1) Reviews: (a) Schneider, J. J. Angew. Chem., Int. Ed. Engl. 1996, 35,
1068. (b) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913. (c)
Morris, R. H.; Jessop, P. G. Coord. Chem. ReV. 1992, 121, 155. (d) Crabtree,
R. H. Angew. Chem., Int. Ed. Engl. 1992, 32, 789. (e) Kubas, G. J. Acc. Chem.
Res. 1988, 21, 120. See also: (f) Butts, M. D.; Kubas, G. J.; Luo, X.-L.;
Bryan J. C. Inorg. Chem. 1997, 36, 3341. (g) Li, J.; Dickson, R. M.; Ziegler,
T. J. Am. Chem. Soc. 1995, 117, 11482. (h) Li, J.; Ziegler, T. Organometallics
1996, 15, 3844. (i) Dapprich, S.; Frenking, G. Angew. Chem., Int. Ed. Engl.
1995, 34, 354.
(2) (a) Rocchini, E.; Mezzetti, A.; Rugger, H.; Burckhardt, U.; Gramlich,
V.; Zotto, A. D.; Martinuzzi, P.; Rigo, P. Inorg. Chem. 1997, 36, 711. (b) For
a review of heterolytic H2 activation, see: Brothers, P. J. Prog. Inorg. Chem.
1981, 28, 1.
(9) Crystal data for 2a: monoclinic, P21/c, a ) 12.567(3) Å, b ) 25.370-
(4) Å, c ) 18.999(3) Å, â ) 105.28(2)°, V ) 5843(2) Å3, Z ) 4, R1 )
0.0700 and wR2 ) 0.1313, GOF ) 0.981, colorless plates from Et2O/hexane.
(10) (a) Yi, C. S.; Wodka, D.; Rheingold, A. L.; Yap, G. P. A.
Organometallics 1996, 15, 2. (b) Rix, F. C.; Brookhart, M.; White, P. S. J.
Am. Chem. Soc. 1996, 118, 2436. (c) Solari, E.; Musso, F.; Gallo, E.; Floriani,
C.; R, N.; Chiese-Villa, A.; Rizzoli, C. Organometallics 1995, 14, 2265.
(11) Addition of hexanes to a CH2Cl2 solution of 2a and 5 equiv of THF
provided [cis-Re(CO)4(PPh3)(THF)][BArf] in 91% yield.
(12) Bahr, S. R.; Boudjouk J. Org. Chem. 1992, 57, 5545. Ph3CCl was
used in place of Ph3C(OTf).
(3) See, for example: Beck, W.; Sunkel, K. Chem. ReV. 1988, 88, 1405.
(4) McKinney, R. J.; Kaesz, H. D. J. Am. Chem. Soc. 1975, 97, 3066.
(5) [cis-Re(CO)4(PCy3)Cl] was isolated from the reaction of Re(CO)5Cl
and 1 equiv of PCy3 in refluxing CHCl3. Recrystallization from cyclohexane
gave white microcrystals (85%) used to prepare 1b as a white solid (48%) by
reaction with MeLi in Et2O and chromatography (SiO2/hexanes).
(6) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11,
3920.
(13) Data for 3a: IR (cm-1, Nujol, νCO) 2122 (m), 2051, 2041, 2033, 2019,
2008, 1987 (s, overlapping); 1H NMR (CD2Cl2) δ 5.33 (s, 2H, CH2Cl2);
31P{1H} NMR (CD2Cl2) δ 11.1; Anal. Calcd for C55H29BCl2F24PO4Re: C,
43.79; H, 1.94. Found: C, 44.15; H, 1.75.
(14) Identified by crystallography as the major components from CH2Cl2/
hexanes solutions of 3a or 3b after 2 weeks at -30 °C. Manuscript in
preparation.
(15) Heinekey, D. M.; Radzewich, C. E.; Voges, M. H.; Schomber, B. M.
J. Am. Chem. Soc. 1997, 119, 4172.
(16) (a) Cheng, T. Y.; Bullock, R. M.Organometallics 1995, 14, 4031. (b)
Beck, W.; Schweiger, M. Z. Anorg. Allg. Chem. 1991, 595, 203. (c) Brewer,
S. T.; Buggey, L. A.; Holloway, J. H.; Hope, E. G. J. Chem. Soc., Dalton
Trans. 1995, 2941.
(7) Data for 2a: IR (cm-1, Nujol, νCO) 2118 (m), 2035 (s), 2010 (s), 1988
(s); 31P{1H} NMR (CD2Cl2, -73 °C)
δ 16.2; Anal. Calcd for
C58H37BF24O5PRe: C, 46.51; H, 2.49. Found: C, 46.40; H, 2.36.
(8) (a) Butts, M. D.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc. 1996,
118, 11831. (b) Agbossou, S. K.; Ferna´ndez, J. M.; Gladysz, J. A. Inorg.
Chem. 1990, 29, 476.
S0002-7863(98)00212-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/25/1998