Synthesis of a heterobimetallic rhodium-iron complex containing an η3-interaction between rhodium and the B-Cipso-C ortho unit of a triarylborane

Article abstract of DOI:10.1021/om060903m

Reaction of [{RhCl(CO)₂}₂] with a phosphine/ thioether/borane ligand (TXPB) gave [Rh([μ-Cl)(CO)(TXPB)] (1), a rare example of a complex containing a M-Cl-BR₃ bridging interaction. Reaction of 1 with K[CpFe(CO)₂] gave [(TXPB)Rh([μ-CO)₂Fe(CO)Cp] (2), which contains an unprecedented η³-interaction between rhodium and the B-C_ipso-C_ortho unit of a triarylborane. DFT calculations suggest a bonding description intermediate between that expected for an isolated borane/alkene complex and a fully delocalized allyl-like complex.

Full text of DOI:10.1021/om060903m

© Copyright 2006
Volume 25, Number 25, December 4, 2006
American Chemical Society
Communications
Synthesis of a Heterobimetallic Rhodium-Iron Complex Containing
an η3-Interaction between Rhodium and the B-Cipso-Cortho
Unit of a Triarylborane
Simon R. Oakley, Kyle D. Parker, David J. H. Emslie,* Ignacio Vargas-Baca,
Craig M. Robertson, Laura E. Harrington, and James F. Britten
Department of Chemistry, McMaster UniVersity, 1280 Main Street West,
Hamilton, Ontario, L8S 4M1, Canada
ReceiVed October 3, 2006
Summary: Reaction of [{RhCl(CO)2}2] with a phosphine/
thioether/borane ligand (TXPB) gaVe [Rh(µ-Cl)(CO)(TXPB)]
(1), a rare example of a complex containing a M-Cl-BR3
bridging interaction. Reaction of 1 with K[CpFe(CO)2] gaVe
[(TXPB)Rh(µ-CO)2Fe(CO)Cp] (2), which contains an unprec-
edented η3-interaction between rhodium and the B-Cipso-Cortho
unit of a triarylborane. DFT calculations suggest a bonding
description intermediate between that expected for an isolated
borane/alkene complex and a fully delocalized allyl-like
complex.
Beyond alkyl or hydride abstraction, reported modes of
reactivity between an arylborane and an organometallic complex
include phosphine abstraction,4 adduct formation,5 or reaction
with an ancillary ligand (e.g., to form a boratacyclopentadienyl6
dianion). In late transition metal chemistry, arylboranes have
also been shown to react directly with the metal7-9 to form
unusual metal borane complexes. Herein, we describe the
preparation of a rhodium complex containing an η3-interaction
between rhodium and the B-Cipso-Cortho unit of a triarylborane.
We recently reported the preparation of a rigid phosphine-
thioether-borane ligand (TXPB, Scheme 1), which has been
shown to be effective in positioning a borane in close proximity
to palladium.10 Reaction of TXPB with [{Rh(µ-Cl)(CO)2}2] gave
yellow [Rh(µ-Cl)(CO)(TXPB)] (1) in 83% isolated yield
(Scheme 1). Selected spectroscopic features include a sharp
doublet (1JRh,P 161 Hz) at 63.8 ppm in the 31P NMR spectrum
and ν(CO)(Nujol) ) 2010 cm-1. The solid-state structure of 1
(Figure 1) reveals that the TXPB ligand is P,S-coordinated to
square planar rhodium, and the borane unit of TXPB engages
in a rare M-Cl-B bridging interaction.9,11 Boron is consider-
ably pyramidalized [C-B-C ) 111.9(7)°, 113.5(6)°, and 114.4-
(7)°], and the B-Cl distance [1.995(9) Å] is only 0.05-0.15 Å
longer than that observed for aryl-substituted chloroborane
Lewis base adducts or chloroborates (cf. 1.893(2) Å in Ph2-
BCl(THF)12 and 1.937(5) Å in [PPN][{PhClB(η5-C5H4)2}-
A thorough understanding of the interactions possible between
arylboranes and transition metal complexes is of particular
importance given their prominent position as cocatalysts and
reagents in transition metal chemistry. For example, arylboronic
esters play a key role as reagents in Suzuki-Miyaura coupling1
and triarylboranes occupy a position of prime importance as
activators for insertion polymerization.2 Until recently, the vast
majority of homogeneous polymerization catalysis involved the
use of d0 metal complexes. However, the discovery of effective
mid and late transition metal catalysts3 greatly expands the range
of metal-borane reactivity that may be encountered.
* Corresponding author. Fax: (905)-522-2509. Tel: (905)-525-9140.
E-mail: emslied@mcmaster.ca.
(1) Beller, M.; Bolm, C. Transition Metals for Organic Synthesis, 2nd
ed.; Wiley-VCH: Weinheim, 2004.
(2) (a) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (b)
Arndt, S.; Spaniol, T. P.; Okuda, J. Chem. Commun. 2002, 896. (c) Piers,
W. E. AdV. Organomet. Chem. 2005, 52, 1.
(4) Dioumaev, V. K.; Plo¨ssl, K.; Carroll, P. J.; Berry, D. H. Organo-
metallics 2000, 19, 3374.
(3) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
10.1021/om060903m CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/01/2006

5836 Organometallics, Vol. 25, No. 25, 2006
Communications
In order to probe whether a metal-boron interaction might
be accessible by replacement of chloride with a nucleophilic
metal anion, complex 1 was reacted with K[CpFe(CO)2],
resulting in the formation of olive green [(TXPB)Rh(µ-CO)2Fe-
(CO)Cp] (2) in 69% yield (Scheme 1). The presence of a doublet
(1JRh,P 175 Hz) at 46.0 ppm in the 31P NMR spectrum and
carbonyl stretching frequencies of 1960 and 1771 cm-1 (Nujol)
confirm that 2 contains both bridging and terminal carbonyl
ligands and that the TXPB ligand remains bound to rhodium.
A broad singlet in the 11B NMR spectrum at 2 ppm demonstrates
that 2 does not contain a neutral three-coordinate triarylborane.
1
The H NMR of 2 shows broad P-Ph and B-Ph signals and a
single CMe2 peak at 20 °C,16 but at -75 °C is well resolved
and corresponds to a single C1 symmetric product. Of particular
interest are five signals originating from one phenyl ring, several
of which are observed at unusually high field [7.16 (o), 6.93
(m), 6.52 (p), 6.25 (m), 3.60 (o) ppm].
Figure 1. ORTEP of [Rh(µ-Cl)(CO)(TXPB)]·hexane (1) with
solvent and hydrogen atoms omitted for clarity (50% thermal
ellipsoids). Selected bond lengths (Å): Rh-C(48) 1.817(11), Rh-P
2.205(2), Rh-S 2.379(2), Rh-Cl 2.381(2), B-Cl 1.995(9), B-C(42)
1.611(12), B-C(36) 1.619(12), B-C(5) 1.612(12).
After many attempts, crystals of 2·solvent were obtained by
slow diffusion of hexanes into a 1,2-dme/toluene solution of 2
at -30 °C. The solid-state structure of 2 shows that the TXPB
ligand is bound to rhodium not only via phosphorus and sulfur
but also through boron and the ortho- and ipso-carbons of one
B-phenyl ring (Figure 2). This coordinated phenyl ring provides
an explanation for the unusually shielded aromatic protons in
the 1H NMR spectrum. The complex has a total of 36 electrons
and a Rh-Fe single bond with two approximately symmetrical
bridging carbonyl ligands (located in a plane with Rh and Fe)
and one terminal carbonyl on iron. Rhodium is trigonal
bipyramidal, excluding the Rh-Fe bond and with C(42), C(49),
and P in the trigonal plane. The geometry at boron is more planar
than pyramidal; ∑(C-B-C) ) 357(3)°, although C42 lies
0.421(66) Å above the C5-B-C36 plane.17
Scheme 1. Preparation of Complexes 1 and 2
Complex 2 could be considered to contain either (A) an η2-
coordinated phenyl ring and a σ-interaction between rhodium
and an approximately planar borane or (B) a conjugated
borataallyl (B-C-C) ligand (Scheme 1). Neither class of
metal-ligand interaction has precedent in the literature. How-
ever, calculations18 have shown that in certain cases an
unsymmetrical µ-boryl complex18,19 can be considered a metal-
ZrCl2]13), indicative of a strong B-Cl interaction. In keeping
with this observation, the 11B NMR chemical shift for 1 occurs
at 11.5 ppm, compared with 69 ppm in free TXPB. However,
the Rh-Cl bond in 1 seems largely unperturbed, with Rh-Cl
) 2.381(2) Å, compared to 2.381(1) Å in [{o-C6H4(PPh2)-
(SMe)}RhCl(CO)]14 and 2.370(2) Å in [(POT)RhCl(CO)]15
(POT ) a neutral, bidentate, chiral triarylphosphine/dialkylth-
ioether ligand). For comparison, the B-Cl interaction in [Pd-
(η3-allyl)(µ-Cl){(Cy2B)(iPr2P)C6H4-o}], a close relative of 1,
appears to be substantially weaker, with B-Cl ) 2.16 Å, ∑-
(C-B-C) ) 349°, and 11B NMR δ ) 47 ppm.9
(6) Choukroun, R.; Lorber, C.; Donnadieu, B. Organometallics 2004,
23, 1434.
(7) (a) Burlitch, J. M.; Burk, J. H.; Leonowicz, M. E.; Hughes, R. E.
Inorg. Chem. 1979, 18, 1702. (b) Burlitch, J. M.; Leonowicz, M. E.;
Petersen, R. B.; Hughes, R. E. Inorg. Chem. 1979, 18, 1097.
(8) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou,
D. Angew. Chem., Int. Ed. 2006, 45, 1611.
(9) Bontemps, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. J. Am. Chem.
Soc. 2006, 128, 12056.
(10) Emslie, D. J. H.; Blackwell, J. M.; Britten, J. F.; Harrington, L. E.
Organometallics 2006, 25, 2412.
(5) See for example adducts with organometallic complexes bearing the
following ligands: (a) oxo: Sa´nchez-Nieves, J.; Frutos, L. M.; Royo, P.;
Castan˜o, O.; Herdtweck, E., Organometallics 2005, 24, 2004. (b) nitrido:
Crevier, T. J.; Bennett, B. K.; Soper, J. D.; Bowman, J. A.; Dehestani, A.;
Hrovat, D. A.; Lovell, S.; Kaminsky, W.; Mayer, J. M. J. Am. Chem. Soc.
2001, 123, 1059. (c) carbyne: van der Eide, E. F.; Piers, W. E.; Romero,
P. E.; Parvez, M.; McDonald, R. Organometallics 2004, 23, 314. (d)
alkylidene: Cook, K. S.; Piers, W. E.; Woo, T. K.; McDonald, R.
Organometallics 2001, 20, 3927. (e) halide: References 9 and 11a. (f) cyano
or vinyl cyanide: Brunkan, N. M.; Brestensky, D. M.; Jones, W. D. J. Am.
Chem. Soc. 2004, 126, 3627. Rocchini, E.; Rigo, P.; Mezzetti, A.; Stephan,
T.; Morris, R. H.; Lough, A. J.; Forde, C. E.; Fong, T. P.; Drouin, S. D.
Dalton Trans. 2000, 3591. (g) carbonyl: Choukroun, R.; Lorber, C.; Lepetit,
C.; Donnadieu, B. Organometallics 2003, 22, 1995. (h) nitrile: Choukroun,
R.; Lorber, C.; Vendier, L.; Donnadieu, B. Organometallics 2004, 23, 5488.
Garc´ıa, J. J.; Are´valo, A.; Brunkan, N. M.; Jones, W. D. Organometallics
2004, 23, 3997. (i) enolate: Spaether, W.; Klass, K.; Erker, G.; Zippel, F.;
Fro¨hlich, R. Chem. Eur. J. 1998, 4, 1411. (j) acyl: Anderson, G. D. W.;
Boys, O. J.; Cowley, A. R.; Green, J. C.; Green, M. L. H.; Llewellyn, S.
A.; von Beckh, C. M.; Pascu, S. I.; Vei, I. C. J. Organomet. Chem. 2004,
689, 4407. (k) enone: Ogoshi, S.; Yoshida, T.; Nishida, T.; Morita, M.;
Kurosawa, H. J. Am. Chem. Soc. 2001, 123, 1944. Reference 10.
(11) (a) Lancaster, S. J.; Al-Benna, S.; Thornton-Pett, M.; Bochmann,
M. Organometallics 2000, 19, 1599. (b) Crevier, T. J.; Bennett, B. K.; Soper,
J. D.; Bowman, J. A.; Dehestani, A.; Hrovat, D. A.; Lovell, S.; Kaminsky,
W.; Mayer, J. M. J. Am. Chem. Soc. 2001, 123, 1059.
(12) Cross, W. I.; Lightfoot, M. P.; Mair, F. S.; Pritchard, R. G. Inorg.
Chem. 2000, 39, 2690.
(13) Burns, C. T.; Stelck, D. S.; Shapiro, P. J.; Vij, A.; Kunz, K.; Kehr,
G.; Concolino, T.; Rheingold, A. L. Organometallics 1999, 18, 5432.
(14) Steeg, N.; Kramolowsky, R. Z. Kristallogr.-New Cryst. Struct. 1997,
212, 273.
(15) R´ıos-Moreno, G.; Toscano, R. A.; Redo´n, R.; Nakano, H.; Okuyama,
Y.; Morales-Morales, D. Inorg. Chim. Acta 2005, 358, 303.
1
(16) The H NMR spectrum of 2 at 20 °C is consistent with exchange
between coordinated and noncoordinated B-Ph rings, presumably via a
σ-bonded M-BR3 intermediate or by complete B-Cipso-Cortho dissociation.
(17) Accurate B-C bond lengths were not accessible from the X-ray
crystal structure of 2, and hydrogen atoms were not located.
(18) Westcott, S. A.; Marder, T. B.; Baker, R. T.; Harlow, R. L.;
Calabrese, J. C.; Lam, K. C.; Lin, Z. Polyhedron 2004, 23, 2665.
(19) Curtis, D.; Lesley, M. J. G.; Norman, N. C.; Orpen, A. G.; Starbuck,
J. Dalton Trans. 1999, 1687.

Communications
Organometallics, Vol. 25, No. 25, 2006 5837
Figure 2. ORTEP of [(TXPB)Rh(µ-CO)2Fe(CO)Cp]·solvent (2)
with disordered solvent (∼6.5 carbon atoms) and hydrogen atoms
omitted for clarity (50% thermal ellipsoids). Selected bond lengths
(Å): Rh-Fe 2.665(3), Fe-C(48) 1.73(2), Fe-C(49) 1.92(2), Fe-
C(50) 1.88(2), Rh-C(49) 1.97(2), Rh-C(50) 2.03(2), Rh-P 2.346-
(5), Rh-S 2.354(5), Rh-B 2.63(2), Rh-C(42) 2.33(2), Rh-C(43)
2.46(2), B-C(42) 1.50(3), B-C(36) 1.57(3), B-C(5) 1.63(3),
C(42)-C(43) 1.41(2).
borane adduct, even in the absence of significant pyramidal-
ization at boron. In addition, allyl-like (C-B-C) chelation of
a borane-bridged diylide to Zr and Pd was recently reported.20
The Rh-B distance of 2.63(2) Å is longer than that observed
in Hill, Bourissou, Parkin, and Connelly’s rhodium borane
complexes [2.09-2.31 Å]8,21 or Rh-B in Westcott et al.’s
unsymmetrical µ-boryl complex [2.444(9) Å].18 However, M-B
is intermediate between that observed in Shapiro’s palladium
[2.200(5) Å] and zirconium [2.754(4) Å] boron-bridged diylide
complexes20 and is considerably shorter than that observed for
group 8 metallocenylboranes [2.8-3.3 Å].22
In complex 2, as with metallocenylboranes, the M-B distance
will be strongly influenced by geometric constraints imposed
by the rigid ligand backbone. However, the 11B NMR chemical
shift of 2 ppm rules out the possibility that the borane is three-
coordinate and neutral and is held in close proximity to rhodium
solely as a consequence of a Rh-(η2-arene) interaction. In
addition, the B···P distance of 4.770(21) Å in 2, which is
considerably shorter than that of 5.442(9) Å in 1 or 5.774(4) Å
in [Pd(dba)(TXPB)],10 demonstrates that the borane unit is not
positioned in close proximity to rhodium as a simple conse-
quence of ligand rigidity and that a significant Rh-(BCC)
interaction must exist. In fact, while important to control and
direct borane reactivity, it is conceivable that the rigidity of
TXPB may in fact enforce longer bond lengths between Rh and
the BCC unit (especially Rh-B) than would otherwise be
observed.
Figure 3. Localized molecular orbitals of 2. Peripheral Me and
CMe3 groups and the o-, m-, and p-carbon atoms of three phenyl
rings hidden for clarity.
The bridging carbonyl ligands in 2 give rise to ν(CO) ) 1771
cm-1, compared with 1673 and 1712 cm-1 for [(Me3P)2Rh(µ-
CO)2Fe(PMe3)Cp]23 and 1708 cm-1 for [Cp*Rh(µ-CO)2Fe(C6-
Me6)],24 despite a greater electron count on 2 (36 versus 34 if
Rh-Fe bonds can be considered to be single in all complexes).
Complex 2 also undergoes an irreversible reduction at Ep(red)
) -1.81 V in THF (cf. E1/2 ) -1.97 V for TXPB reduction in
THF), consistent with reduction of the Rh-Fe core of 2. While
it is tempting to interpret these data with respect to formal
charges on the Rh-Fe core as a result of either borane/alkene
or borataallyl ligand bonding, there are problems inherent with
assigning formal charge to ligands such as these in significantly
covalent complexes. This point has been highlighted in a recent
article focused on the nature of metal-boron σ-bonding in
transition metal borane complexes, [LxM-BR3].25 Therefore,
for 2, the major distinction between a borane/alkene complex
(A) and a borataallyl complex (B) must be the nature of the
B-Cipso-Cortho molecular orbitals involved in M-(BCC) bond-
ing (Figures 3 and 4), rather than the charge on the BCC unit.
The interaction between Rh and the BCC unit was further
examined using DFT (ADF 2005.01, TZP, VWN, PW91,
ZORA). The geometry of a model analogue of 2 with H in place
of CMe3 groups was fully optimized using frozen cores (up to
3d for Rh and below the valence shell for all other atoms) and
closely matches the experimental structure. It also reveals
nonplanarity at both B and C43 (∑(C-B-C) ) 354.4°; C42
lies 0.605 Å above the C5-B-C36 plane; C46-C43-H43 )
168.58°) and a slight shortening of B-C42 relative to B-C36
and B-C5 (1.567 vs 1.587 and 1.593 Å).17
(20) Jiang, F.; Shapiro, P. J.; Fahs, F.; Twamley, B. Angew. Chem., Int.
Ed. 2003, 42, 2651.
(21) (a) Crossley, I. R.; Foreman, M. R. S.-J.; Hill, A. F.; White, A. J.
P.; Williams, D. J. Chem. Commun. 2005, 221. (b) Crossley, I. R.; Hill, A.
F.; Humphrey, E. R.; Willis, A. C. Organometallics 2005, 24, 4083. (c)
Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2006, 25, 289.
(d) Blagg, R. J.; Charmant, J. P. H.; Connelly, N. G.; Haddow, M. F.; Orpen,
A. G. Chem. Commun. 2006, 2350. (e) Landry, V. K.; Melnick, J. G.;
Buccella, D.; Pang, K.; Ulichny, J. C.; Parkin, G. Inorg. Chem. 2006, 45,
2588.
(22) Scheibitz, M.; Bolte, M.; Bats, J. W.; Lerner, H.-W.; Nowik, I.;
Herber, R. H.; Krapp, A.; Lein, M.; Holthausen, M. C.; Wagner, M. Chem.
Eur. J. 2005, 11, 584, and references therein. Note: calculations suggest
that in borylferrocenes, a direct Fe-B interaction does not exist. However,
an interaction between an iron d orbital and the Cipso-B π system may
play an important role in the observed bending of BR2 out of the plane of
the cyclopentadienyl ring to which it is attached.
The compositions of relevant orbitals were examined by
applying the Boys-Foster localization method to the result of
an all-electron calculation. Three localized molecular orbitals
involve significant Rh-(BCC) bonding interactions (Figure 3):
(23) Dahlenburg, L.; Hache, R. Inorg. Chim. Acta 2003, 350, 77.
(24) Ho¨rlein, R.; Herrmann, W. A. J. Organomet. Chem. 1986, 303, C38.
(25) Parkin, G. Organometallics 2006, 25, 4744.

5838 Organometallics, Vol. 25, No. 25, 2006
Communications
plex. This is expected since overlap between B and Cipso must
exist, but the extent of mixing will depend on the electronega-
tivity difference between boron and carbon (Figure 4).
In summary, reaction of [Rh(µ-Cl)(CO)(TXPB)] (1) with
K[CpFe(CO)2] gave [(TXPB)Rh(µ-CO)2Fe(CO)Cp] (2), in
which a triarylborane group is found to engage in an η3-
interaction with rhodium via B-Cipso-Cortho. This type of
interaction is without precedent, but could have unrealized
significance in transition metal chemistry involving aryl or
vinylboranes, especially where metal oxidation is possible. The
nature of complex 2 and the Rh-(B-Cipso-Cortho) interaction
was investigated by NMR and IR spectroscopy, cyclic volta-
mmetry, X-ray crystallography, and DFT calculations, which
suggest a bonding description intermediate between that ex-
pected for an isolated borane/alkene complex and a fully
delocalized allyl-like complex. Future work will include studies
into the reactivity of 2 with nucleophiles and unsaturated
molecules and attempts to probe the generality of the η3(BCC)-
arylborane coordination mode.
Figure 4. Schematic representations of the π-manifold for an ECC
ligand (E ) B or C) in the two extreme bonding descriptions (an
isolated borane/alkene ligand and a fully delocalized allyl ligand)
and an intermediate situation for a conjugated BCC ligand.
The HOMO-18 shows π-delocalization across the BCC frag-
ment, consistent with π-bonding borataallyl character. The
HOMO-4 is π*(Cipso-Cortho) in character with no significant
contribution from boron, as would be expected for a borane/
alkene complex. The HOMO involves a bonding interaction
between π*(Cipso-Cortho) and pπ-B. In a symmetrical π-allyl
ligand, this orbital would correspond to the familiar πn orbital,
containing a central node as a result of equal magnitude bonding
and antibonding contributions between the central carbon atom
and the two equivalent terminal carbons. Overall, these calcula-
tions confirm the existence of a significant Rh-(BCC) bonding
interaction and are indicative of Rh-BCC bonding character
intermediate between that expected for an allyl-like ligand with
fully delocalized molecular orbitals and a borane/alkene com-
Acknowledgment. D.J.H.E. and I.V.B. thank NSERC of
Canada for Discovery Grants and Canada Foundation for
Innovation (CFI) and Ontario Innovation Trust (OIT) for New
Opportunities Grants. D.J.H.E. thanks McMaster University for
support in the form of a start-up grant. K.D.P thanks NSERC
of Canada for a USRA.
Supporting Information Available: Experimental, NMR, CV,
computational, and crystallographic details. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
OM060903M