Synthesis and characterization of a cationic ruthenium complex featuring an unusual bis(η2-BH) monoborane ligand

Article abstract of DOI:10.1021/ic801300m

The reaction of Cp*Ru(PⁱPr₃)Cl with MesBH ₂ (Mes = 2,4,6-trimethylphenyl), followed by chloride abstraction with LiB(C₆F₅)₄·2.5OEt₂ (LiBF₂₀), afforded the crystallographically characterized complex [Cp*Ru(PⁱPr₃)(BH₂Mes)] ⁺B(C₆F₅)^4_; notably, this represents the first reported cationic complex to feature an η²-BH monoborane ligand, as well as a rare example of bis(η²-BH) ligation.

Full text of DOI:10.1021/ic801300m

Inorg. Chem. 2008, 47, 7471-7473
Synthesis and Characterization of a Cationic Ruthenium Complex
Featuring an Unusual Bis(η2-BH) Monoborane Ligand
Kevin D. Hesp,† Matthew A. Rankin,† Robert McDonald,‡ and Mark Stradiotto*,†
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3, and the
X-Ray Crystallography Laboratory, Department of Chemistry, UniVersity of Alberta, Edmonton,
Alberta, Canada T6G 2G2
Received July 10, 2008
The reaction of Cp*Ru(PiPr3)Cl with MesBH2 (Mes ) 2,4,6-
trimethylphenyl), followed by chloride abstraction with LiB(C6F5)4 ·
2.5OEt2 (LiBF20), afforded the crystallographically characterized
complex [Cp*Ru(PiPr3)(BH2Mes)]+B(C6F5)4-; notably, this repre-
remain quite rare,3-6 especially in comparison to related
complexes featuring η2-HH7 and η2-SiH8 ligands.1a Further-
more, despite the potential utility of cationic, coordinatively
unsaturated platinum group metal complexes in mediating
B-H bond activation chemistry,1 only neutral complexes
supported by η2-BH monoborane ligands have appeared thus
far in the literature.3,4
2
sents the first reported cationic complex to feature an η -BH
2
monoborane ligand, as well as a rare example of bis(η -BH)
ligation.
Coordinatively unsaturated species of the type
[Cp*RuPn]+X- (Cp* ) η5-C5Me5) exhibit rich and diverse
reactivity,9 including reactions with organosilanes leading
to isolable η2-SiH adducts,1a,8 as well as to catalytically active
[Cp*(PR3)(H)2RudSiHR]+X- species generated via double
geminal Si-H bond activation.10 Given the diagonal rela-
tionship between silicon and boron, we became interested
in exploring the B-H activation chemistry of simple borane
substrates within the coordination sphere of [Cp*RuPn]+X-
complexes. Herein we report on the reaction of Cp*Ru-
There is widespread interest in documenting the stoichio-
metric reactivity of transition-metal complexes with B-H-
containing substrates, both in the quest to uncover unusual
metal-ligand bonding motifs and as a means of advancing
our understanding of prominent metal-catalyzed transforma-
tions such as the hydroboration of unsaturated molecules,
the dehydrogenative borylation of hydrocarbons, and the
dehydrogenation of Lewis adducts including ammonia bo-
rane.1 In this regard, η2-BH complexes of simple monobo-
ranes represent appealing synthetic targets because such
species are commonly implicated in B-H oxidative addition/
reductive elimination cycles1 and can serve as models of
reactive σ-CH intermediates in hydrocarbon activation chem-
istry.2 However, isolable examples of such η2-BH complexes
(3) Ruthenium examples: (a) Alcaraz, G.; Clot, E.; Helmstedt, U.; Vendier,
L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2007, 129, 8704. (b) Lachaize,
S.; Essalah, K.; Montiel-Palma, V.; Vendier, L.; Chaudret, B.;
Barthelat, J.-C.; Sabo-Etienne, S. Organometallics 2005, 24, 2935.
(c) Montiel-Palma, V.; Lumbierres, M.; Donnadieu, B.; Sabo-Etienne,
S.; Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624.
(4) Non-ruthenium examples: (a) Crestani, M. G.; Mun˜oz-Herna´ndez, M.;
Are´valo, A.; Acosta-Ram´ırez, A.; Garc´ıa, J. J. J. Am. Chem. Soc. 2005,
127, 18066. (b) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000,
122, 9435. (c) Muhoro, C. N.; He, X.; Hartwig, J. F. J. Am. Chem.
Soc. 1999, 121, 5033.
* To whom correspondence should be addressed. E-mail: mark.stradiotto@
dal.ca.
†
Dalhousie University.
University of Alberta.
(5) Conversely, a number of borane adducts of the type (η1-BH3 ·L)MLn
have been prepared. For MLn ) Cp′RuP2+ see: Kawano, Y.; Hashiva,
M.; Shimoi, M. Organometallics 2006, 25, 4420, and references cited
therein.
‡
(1) For selected reviews, see: (a) Perutz, R. N.; Sabo-Etienne, S. Angew.
Chem., Int. Ed. 2007, 46, 2578. (b) Stephens, F. H.; Pons, V.; Baker,
R. T. Dalton Trans. 2007, 2613. (c) Braunschweig, H.; Kollann, C.;
Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (d) Aldridge, S.;
Coombs, D. L. Coord. Chem. ReV. 2004, 248, 535. (e) Braunschweig,
H.; Colling, M. Coord. Chem. ReV. 2001, 223, 1. (f) Piers, W. E.
Angew. Chem., Int. Ed. 2000, 39, 1923. (g) Braunschweig, H. Angew.
Chem., Int. Ed. 1998, 37, 1786. (h) Irvine, G. J.; Lesley, M. J. G.;
Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E. G.; Roper,
W. R.; Whittell, G. R.; Wright, L. J. Chem. ReV. 1998, 98, 2685. (i)
Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957.
(6) Related borate complexes of the type (BH4)MLn are well established.
For a review, see: (a) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77,
263. (b) For MLn ) Cp*RuP+ see: Suzuki, H.; Lee, D. H.; Oshima,
N.; Moro-oka, Y. Organometallics 1987, 6, 1569.
(7) (a) Heinekey, D. M.; Oldham, W. J., Jr Chem. ReV 1993, 93, 913. (b)
Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155.
(8) (a) Lachaize, S.; Sabo-Etienne, S. Eur. J. Inorg. Chem. 2006, 2115.
(b) Nikonov, G. I. AdV. Inorg. Chem. 2005, 53, 217. (c) Lin, Z. Chem.
Soc. ReV. 2002, 31, 239. (d) Corey, J. Y.; Braddock-Wilking, J. Chem.
ReV. 1999, 99, 175.
(2) For selected reviews pertaining to the metal-mediated C-H activation
of hydrocarbons, see: (a) Crabtree, R. H. J. Organomet. Chem. 2004,
689, 4083. (b) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507.
(c) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.
Acc. Chem. Res. 1995, 28, 154.
(9) (a) Jime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P. Eur. J. Inorg.
Chem. 2004, 17. (b) Davies, S. G.; McNally, J. P.; Smallridge, A. J.
AdV. Inorg. Chem. 1990, 30, 1.
(10) Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 13640.
10.1021/ic801300m CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 17, 2008 7471
Published on Web 07/31/2008

COMMUNICATION
Scheme 1. Synthesis of 2 and 3a
a
LiBF20 ) LiB(C6F5)4 ·2.5OEt2; Mes ) 2,4,6-trimethylphenyl.
Figure 2. ORTEP diagram of 3 shown with 50% ellipsoids. Selected H
atoms and the counteranion have been omitted. Selected interatomic
distances (Å) and angles (deg): Ru-P 2.3911(6); Ru · · ·B 1.921(2); Ru-H1
1.61(3); Ru-H2 1.60(3); B-H1 1.31(3); B-H2 1.28(3); B-C 1.522(3);
Ru-B-C 172.24(17); P-Ru-B 97.88(7); H1-Ru-H2 79.1(14); H1-B-H2
104.5(17); H1-B-C 126.9(11); H2-B-C 125.3(13).
of the progress of the reaction by use of NMR methods
revealed the consumption of 2, along with the formation of
a single new product 3 [δ(31P) ) 72.0].15 Integration of the
1H NMR spectral data for isolated analytically pure 3 (54%)
confirmed the presence of Cp*, PiPr3, Mes, and two hydrides
in this complex. As well, the broad doublet observed in the
Figure 1. ORTEP diagram of 2 shown with 50% ellipsoids. Selected H
atoms have been omitted. Selected interatomic distances (Å) and angles
(deg): Ru-P 2.3589(6); Ru · · ·B 2.162(3); Ru-H1 1.63(3); Ru-H2 1.71(3);
B-H1 1.29(2); B-H2 1.30(3); B-Cl 1.887(3); B-C 1.586(3); Ru-B-C
123.13(16); P-Ru-B 98.63(7); H1-Ru-H2 71.5(12); H1-B-H2
98.2(16).
1
2
hydride region of the H NMR spectrum (-10.3 ppm, JPH
) 15.0 Hz) was observed as a considerably more sharp
doublet in the corresponding 1H{11B} NMR spectrum,
thereby suggesting the presence of B-H linkages in 3. The
solid-state connectivity in 3 was determined on the basis of
a single-crystal X-ray diffraction experiment (Figure 2),13b,d
thereby establishing this complex as the first cationic member
of a very limited class of isolable complexes featuring an
η2-BH monoborane ligand.3,4 Complex 3 can be viewed as
adopting a piano-stool structure, in which the Cp*Ru+
fragment is supported by three neutral two-electron donors:
the PiPr3 coligand as well as two dative interactions involving
the bis(η2-BH)-ligating MesBH2 group, giving an 18-electron
configuration. Notably, the bis(η2-BH) motif in 3 has only
been observed in one other complex [Ru(H)2(PCy3)2-
(BH2Mes), 4], recently reported by Sabo-Etienne and co-
workers.3aWhile the presence of Ru-H coligands in the
neutral complex 4 is of considerable interest in relation
to σ-complex-assisted metathesis processes,1a the absence
of such coligands in the cationic species 3 provides an
opportunity to examine bis(η2-BH) ligation free of po-
tential secondary Ru-H · · · BMesH2 or other interligand
interactions.1a,3b The Ru-H [1.61(3) and 1.60(3) Å] and
B-H [1.31(3) and 1.28(3) Å] distances in 3 are compa-
rable to those found in 2, suggesting significant B-H
interactions that would appear to be inconsistent with an
alternative description of 3 as being the dihydridoborylene
species [Cp*(PiPr3)(H)2RudBMes]+B(C6F5)4-. Notably,
the Ru · · · B contact [1.921(2) Å] in 3 is significantly
shorter than that in the precursor 2; indeed, the Ru · · · B
distance in 3 is statistically equal to that observed in 4
[1.938(4) Å], which represents the shortest reported Ru-B
(PiPr3)Cl (1)11 with MesBH2 (Mes ) 2,4,6-trimethylphenyl)12
to give Cp*Ru(PiPr3)(BH2MesCl) (2), which serves as a
precursor to [Cp*Ru(PiPr3)(BH2Mes)]+B(C6F5)4- (3). Nota-
bly, 3 represents the first reported cationic complex to feature
an η2-BH monoborane ligand, as well as a rare example of
bis(η2-BH) ligation.
Treatment of a dark-blue hexanes solution of 1 with 1 equiv
of MesBH2 resulted in an immediate color change to orange;
31P NMR analysis of the reaction mixture after 0.5 h revealed
the quantitative formation of 2 (Scheme 1),13 which was isolated
as a crystalline solid in 68% yield. The assignment of 2 as a
CS-symmetric chloroborate complex arising from chlorine
transfer from ruthenium to boron is supported by NMR
spectroscopic data [e.g., δ(1H) ) -11.8, Ru(H)2B; δ(31P) )
66.6; δ(11B) ) 46.5], as well as X-ray diffraction data (Figure
1).13b,c The structural features of 2 are in agreement with those
of the only other closely related borate complex
Cp*Ru(PMe3)(BH3Cl),14 and the Ru · · ·B distance in 2 [2.162(3)
Å] is in keeping with other crystallographically characterized
bidentate borohydride complexes.6,14
In viewing 2 as a potential precursor to an unusual cationic
ruthenium complex of MesBH2, a solution of 2 in hexanes
was treated with a solution of LiB(C6F5)4 · 2.5OEt2 in
fluorobenzene (Scheme 1). The addition caused an immediate
color change from orange to orange-yellow, and monitoring
(11) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Chem. Commun. 1988,
278.
(12) Smith, K.; Pelter, A.; Jin, Z. Angew. Chem. Int. Ed. 1994, 33, 851.
(13) (a) Full experimental details are provided in the Supporting Informa-
tion. (b) ORTEP-3 for Windows version 1.074: Farrugia, L. J. J. Appl.
Crystallogr. 1997, 30, 565. All Ru-H-B positions were located in
the Fourier difference map and refined freely. (c) Crystal data for 2
(193 K): monoclinic (P21/c); a ) 16.007(2) Å; b ) 10.4284(14) Å;
c ) 18.504(3) Å; ꢀ ) 106.1982(16) Å; V ) 2966.1(7) Å3; Z ) 4;
GOF ) 1.063; R1 ) 0.0302; wR2 ) 0.0761. (d) Crystal data for 3
(193 K): orthorhombic (Pbca); a ) 18.8734(15) Å; b ) 18.0950(15)
Å; c ) 30.283(3) Å; V ) 10341.9(15) Å3; Z ) 8; GOF ) 1.047; R1
) 0.0342; wR2 ) 0.0900.
(15) We have not been able to observe 11B NMR resonances for 3, despite
prolonged acquisition times using either variable-temperature 11B{1H}
NMR methods or 1H-11B HSQC NMR techniques and by employing
baseline correction routines. Furthermore, we have thus far not been
able to obtain satisfactory IR data for 3 owing to the air sensitivity of
the complex.
(14) Kawano, Y.; Shimoi, M. Chem. Lett. 1998, 935.
7472 Inorganic Chemistry, Vol. 47, No. 17, 2008

COMMUNICATION
linkage.3a The sum of the H-B-H and H-B-C angles
(ca. 357°), as well as the observed Ru-B-C angle
[172.24(17)°], is in keeping with bis(η2-BH) monoborane
ligation within the cationic complex 3. The rare and
unusual bis(η2-BH) coordination featured in 3 can be
viewed as providing empirical support for the viability
of putative platinum group [(alkane)MLn]+X- species that
that feature multiple σ-CH interactions.2,16
SiH8 ligands,17 it is evident that similar comparative reactiv-
ity studies involving η2-BH complexes [including cationic
bis(η2-BH) species such as 3] could figure importantly in
advancing our understanding of this rather poorly explored
class of complexes. We are currently developing variants of
3, in an effort to evaluate how altering the substituents at
Ru and B can be exploited as a means of initiating new and
interesting reactivity involving the Ru(H)2B core. The results
of these and related investigations featuring other cationic
platinum group metal η2-BH complexes will be the focus of
future reports.
In a preliminary reactivity survey, heating of 3 in fluo-
robenzene at 50 °C resulted in clean conversion to the Lewis
i
adduct Pr3P · BH2Mes (5; 11B and 31P NMR). In examining
the ability of 3 to insert unsaturated substrates, this complex
was treated with styrene or diphenylacetylene (1 or 10 equiv);
no reaction was observed at ambient temperature, and heating
of these reaction mixtures at 50 °C afforded 5 as a major
product (31P NMR). By comparison, [Cp*(PiPr3)(H)2-
RudSiHR]+X- complexes react with R-olefins to afford
Si-H insertion products.10
In summary, the synthesis and characterization of 3
establishes for the first time the feasibility of η2-BH
monoborane ligation within the coordination sphere of a
cationic metal complex. Given the significant insights that
have been gained over the past 2 decades through the study
of neutral and cationic complexes featuring η2-HH7 or η2-
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada and Dalhousie
University for their generous support of this work. We also
thank Drs. Michael Lumsden and Katherine Robertson
(Atlantic Region Magnetic Resonance Center, Dalhousie
University) for assistance in the acquisition of NMR data.
Supporting Information Available: Experimental details and
characterization data, including X-ray crystallographic information
files (CIF) for 2 and 3. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC801300M
(16) For the relevance of such species in metal-mediated hydrocarbon C-H
activation chemistry, see: Jones, W. D. Acc. Chem. Res. 2003, 36,
140.
(17) Kubas, G. J. AdV. Inorg. Chem. 2004, 56, 127.
Inorganic Chemistry, Vol. 47, No. 17, 2008 7473