Probing the intrinisic structure and dynamics of aminoborane coordination at late transition metal centers: Mono(σ-BH) binding in [CpRu(PR 3)2(H2BNCy2)]+

Article abstract of DOI:10.1021/ja203051d

Aminoboranes, H₂BNRR′, represent the monomeric building blocks from which novel polymeric materials can be constructed via metal-mediated processes. The fundamental capabilities of these compounds to interact with metal centers have been probed through the coordination of H ₂BNCy₂ at 16-electron [CpRu(PR₃) ₂]⁺ fragments. In contrast to the side-on binding of isoelectronic alkene donors, an alternative mono(σ-BH) mode of aminoborane ligation is established for H₂BNCy₂, with binding energies only ～8 kcal mol^-1 greater than those for analogous dinitrogen complexes. Variations in ground-state structure and exchange dynamics as a function of the phosphine ancillary ligand set are consistent with chemically significant back-bonding into an orbital of B-H σ* character.

Full text of DOI:10.1021/ja203051d

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pubs.acs.org/JACS
Probing the Intrinisic Structure and Dynamics of Aminoborane
Coordination at Late Transition Metal Centers: Mono(σ-BH) Binding in
[CpRu(PR3)2(H2BNCy2)]þ
Dragoslav Vidovic,* David A. Addy, Tobias Kr€amer, John McGrady,* and Simon Aldridge*
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K.
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, Singapore 637371
S Supporting Information
b
essential to synthetic routes which employ in situ amineborane
dehydrogenation, presumably distorts the structural landscape in
ABSTRACT: Aminoboranes, H2BNRR0, represent the
monomeric building blocks from which novel polymeric
materials can be constructed via metal-mediated processes.
The fundamental capabilities of these compounds to inter-
act with metal centers have been probed through the
coordination of H2BNCy2 at 16-electron [CpRu(PR3)2]þ
fragments. In contrast to the side-on binding of isoelectronic
alkene donors, an alternative mono(σ-BH) mode of ami-
noborane ligation is established for H2BNCy2, with binding
energies only ∼8 kcal molꢀ1 greater than those for ana-
logous dinitrogen complexes. Variations in ground-state
structure and exchange dynamics as a function of the
phosphine ancillary ligand set are consistent with chemi-
cally significant back-bonding into an orbital of BꢀH
σ* character.
favor of the four-electron-donating bis(σ-BH) coordination
mode over the corresponding (two-electron-donating) side-on
π-bound motif. Thus, Alcaraz and Sabo-Etienne report an
energetic preference of 14.3 kcal molꢀ1 for the “end-on” bis-
(σ-BH) coordination geometry of H2BNH2 at [L2Ru(H)2].5a
With a view to investigating the intrinsic two-electron donor
capabilities of aminoborane ligands, we have therefore set out to
examine the coordination of monomeric H2BNCy2 at 16-elec-
tron fragments of the type [CpRu(PR3)2]þ. While such metal
systems are known to be capable of the side-on binding of
alkenes,8 an alternative mono(σ-BH) mode of aminoborane
ligation is established for H2BNCy2. Crystallographic studies in
the solid state, together with DFT calculations and spectroscopic
studies in solution, allow for the elucidation of ground-state
binding motifs/energetics and the dynamic exchange pathways
as a function of the ancillary phosphine co-ligands.
The syntheses of aminoborane complexes [CpRu(PR3)2-
(H2BNCy2)]þ[BArf4]ꢀ [(PR3)2 = (PPh3)2 (5); Cy2PCH2CH2-
PCy2, dcype (6)] are readily accomplished according to the
chemistry outlined in Scheme 1. In each case, cleaner syntheses
are effected through the intermediacy of the dinitrogen com-
plexes [{CpRu(PPh3)2}2(μ-N2)]2þ[BArf4]ꢀ2 (3)9 and [CpRu-
(dcype)(N2)]þ[BArf4]ꢀ (4) (see Supporting Information);
in situ reactivity of 1 (or 2) with Na[BArf4]/H2BNCy2 under
an argon atmosphere invariably generates a mixture of Ru/P-
containing products. In the case of 4, subsequent reaction with
H2BNCy2 generates 6 in nearly quantitative yield (as judged by
multinuclear NMR), with the isolated yield of ca. 30% reflecting
losses inherent in the isolation of this very reactive species.10
minoboranes, H2BNRR0, are the subject of significant inter-
A
est not only as the first-formed products in the dehydrogena-
tion of a class of BN-containing hydrogen storage materials but
also as the monomeric building blocks from which a number of
novel well-defined inorganic polymers can be constructed.1
Thus, for example, Manners and co-workers have reported the
metal-catalyzed polymerization of methylamineborane, H3B
3
NMeH2, by ruthenium, rhodium, or palladium complexes to
give high-molecular-weight poly(aminoboranes), [H2BNMe(H)]n,
i.e., BN analogues of poly(propylene).2 This transformation is
thought to occur via two metal-mediated steps, namely (i)
dehydrogenation of H3B NMeH2 and (ii) polymerization of
3
the monomeric H2BNMe(H) so formed.2 The fundamental
mode(s) of interaction of monomeric aminoboranes with
catalytically relevant late transition metal systems are there-
fore of significant interest.3ꢀ7
5 and 6 have been characterized by 1H, 11B, 13C, 19F, and 31
P
NMR spectroscopy, elemental microanalysis, and single-crystal
X-ray diffraction. Although the 11B NMR spectrum of each
compound features a broad resonance (at δB 35 ppm) which is
not shifted significantly from the free aminoborane (δB 35.4
ppm),11 the corresponding 1H NMR spectra at 20 ꢀC are more
informative, featuring distinct resonances associated with the
RuHB and BH hydrogens (at δH ꢀ11.97 and 6.39 ppm for 5;
ꢀ14.56 and 5.80 ppm for 6, respectively). The 31P NMR
spectrum of both compounds at 20 ꢀC displays a single resonance,
Despite their isoelectronic relationship with 1,1-disubstituted
alkenes, the coordination chemistry of aminoboranes has only
very recently begun to be examined. To date, the only reported
examples of such complexes feature chelating H2BNR2 ligands
coordinated to [L2M(H)2]nþ fragments via two BꢀHꢀM
bridges (M = Ru, n = 0; M = Rh, Ir, n = 1; L = N-heterocyclic
carbene, tertiary phosphine).5 Such a coordination geometry
contrasts with the classical “side-on” binding mode observed for
alkene donors within the same framework.5b,c However, the
utilization of these 14-electron metal systems, while seemingly
Received: April 4, 2011
Published: May 12, 2011
r
2011 American Chemical Society
8494
dx.doi.org/10.1021/ja203051d J. Am. Chem. Soc. 2011, 133, 8494–8497
|

Journal of the American Chemical Society
COMMUNICATION
Scheme 1. Syntheses of j1-Aminoborane Complexes 5 and 6
from the Corresponding Dinitrogen Systemsa
Figure 2. (a) Simplified diagram showing the alignment of the metal
and borane fragments in 6. (b) Alignment of the [CpRu(PR3)2]þ
HOMO and borane BH σ* MO. (c) Alignment of the BN π* MO.
found for k2-bound H2BNCy2 ligands (e.g., 178.4(3)ꢀ for
[(IMes)2Rh(H)2(H2BNCy2)]þ).5b,c,13
Spectroscopically, the presence of a single RuꢀHꢀB bridging
interaction is revealed by distinct RuHB and BH signals in the 1H
NMR spectra of 5 and 6 at 20 ꢀC and is further signaled by
disparate RuꢀH and BꢀH contacts in the solid state (e.g., for 6,
a Key reagents/conditions: (a) Na[BArf4] (1.0 equiv), fluorobenzene,
N2 atmosphere, 10 min at 20 ꢀC, as per ref 8; (b) H2BNCy2 (1.0 equiv),
fluorobenzene, 5 min at 20 ꢀC, 32% isolated yield; (c) Na[BArf4]
(1.0 equiv), fluorobenzene, N2 atmosphere, 10 min at 20 ꢀC, quantita-
tive by NMR, ca. 10% isolated yield; (d) H2BNCy2 (1.0 equiv), fluoro-
benzene, 5 min at 20 ꢀC, 24% isolated yield.
d[Ru(1)ꢀH(351)] = 1.669(1), d[Ru(1) H(352)] = 2.887 Å;
3 3 3
d[B(35)ꢀH(351)] = 1.244(6), d[B(35)ꢀH(352)] = 1.150(5) Å).
Moreover, the presence of only one bridging hydrogen atom is
consistent with Ru B distances [2.430(4) and 2.332(6) Å for
3 3 3
5 and 6, respectively] which are significantly longer than that
found in (Cy3P)2Ru(H)2(k2-H2BNiPr2) [1.980(3) Å].5d The
finding that the Ru B contacts for 5 and 6 are shorter than
3 3 3
that in [CpRu(PMe3)2(k1-H3B NMe3)]þ [2.648(3) Å] pre-
3
sumably reflects the presence of a three (rather than four)-
coordinate boron center and the possibility for back-bonding
from ruthenium.14,15
Three additional observations are consistent with the possi-
bility for back-bonding from the highest occupied molecular
orbital (HOMO) of the [CpRu(PR3)2]þ fragment into a BꢀH
σ* orbital of the coordinated borane in 5 and 6. First, the
alignment of the coordinated BꢀH bond with respect to the
[CpRu(PR3)2]þ fragment [as manifested, for example, by a (Cp
centroid)ꢀRuꢀ(BH centroid)ꢀB torsion angle of 84.3ꢀ for 6]
maximizes the potential for overlap between the respective
orbitals (Figure 2a,b).16 Second, the differences in both the
Figure 1. Structures of the cationic components of 5 and 6. Anions and
hydrogen atoms (except boron-bound hydrogens) are omitted, and
phosphine substituents are shown in wireframe format for clarity.
Thermal ellipsoids are set at the 30% probability level. Key bond
lengths (Å) and angles (ꢀ): for 5, Ru(1)ꢀP(2) 2.354(1), Ru(1)ꢀP-
Ru B [2.430(4), 2.332(6) Å] and BꢀH(Ru) distances
3 3 3
(21) 2.358(1), Ru(1)
B(45) 2.430(4), Ru(1)ꢀH(451) 1.665(4),
[1.207(4), 1.244(6) Å] between 5 and 6 are consistent with
greater back-bonding from the more electron-rich [CpRu(dcype)]þ
fragment (cf. [CpRu(PPh3)2]þ), albeit with some caution at-
tached to the interpretation of the less well defined BꢀH
distances. Finally, the chemical shift of the RuHB bridging
hydrogen is also markedly more hydridic in the case of 6 (δH
ꢀ14.56, cf. ꢀ11.97 for 5). By contrast, the BN distances for the
two compounds are statistically identical [1.376(4) and 1.382(7)
Å], suggesting little population of the BN π* orbital. Such an
observation conflicts with that made for (Cy3P)2Ru(H)2-
(H2BNiPr2) but is not entirely unexpected for the current
[CpRu(PR3)2]þ systems, given that the BN π* orbital in 6, for
example, lies effectively orthogonal to the metal-based HOMO
(Figure 2c).16
3 3 3
B(45)ꢀN(46) 1.376(4), B(45)ꢀH(451) 1.207(4), B(45)ꢀH(452)
1.114(4), Ru(1) B(45)ꢀN(46) 130.2(3); for 6, Ru(1)ꢀP(2)
3 3 3
2.338(1), Ru(1)ꢀP(5) 2.319(1), Ru(1)
B(35) 2.332(6), Ru-
3 3 3
(1)ꢀH(351) 1.669(1), B(35)ꢀN(36) 1.382(7), B(35)ꢀH(351)
1.244(6), B(35)ꢀH(352) 1.150(5), Ru(1) B(35)ꢀN(36) 133.1(4).
3 3 3
The boron-bound hydrogen atoms were located in the crystallo-
graphic Fourier difference maps.
consistent with averaging of the two phosphorus environ-
ments via rapid rotation of the aminoborane ligand about the
Ruꢀ(BH centroid) axis (vide infra).
While the formulations of both 5 and 6 were suggested by
spectroscopic data and the bulk composition confirmed by
microanalysis, unequivocal characterization was additionally
reliant on crystallographic studies (Figure 1).12 The structures
of the cationic components of these compounds feature in common
(i) a three-legged piano stool geometry at ruthenium, with the non-
Cp coordination environment being defined by the two phosphorus
centers and the BꢀH centroid [—(BH centroid)ꢀRuꢀP = 90.3,
96.3ꢀ and 90.9, 92.0ꢀ for 5 and 6, respectively], and (ii) a mono-
Differences in the binding affinity of H2BNCy2 for the
ruthenium center in 5 and 6 presumably also influence the
fluxional behavior of these systems, as determined by variable-
temperature NMR experiments. Thus, 6 is characterized by two
fluxional processes relating to the dynamic behavior of the
coordinated borane. At very low temperatures (T = ꢀ80 ꢀC)
in CD2Cl2 solution, the 31P{1H} NMR spectrum of 6 shows two
resonances (at δP 79.2 and 83.1 ppm; 2JPP = 23 Hz) consistent
with the structure of the cation determined crystallographically
and with slow rotation about the Ruꢀ(BH centroid) vector on
(σ-BH) aminoborane ligand characterized by a bent Ru BꢀN
3 3 3
framework [—Ru BꢀN = 130.2(3)ꢀ for 5, 133.1(4)ꢀ for 6],
3 3 3
which contrasts with the analogous (essentially linear) fragment
8495
dx.doi.org/10.1021/ja203051d |J. Am. Chem. Soc. 2011, 133, 8494–8497

Journal of the American Chemical Society
COMMUNICATION
Scheme 2. Fluxional Processes Involving the BꢀH Bonds in
thereby defining the intrinsic two-electron donor capabilities of
this topical ligand family. Geometric parameters for the mono-
dentate BH-bound ground-state structures and details of dy-
namic fluxional processes have been probed as functions of the
ancillary phosphine co-ligands. Ru(II) systems have been shown
to be among the most active catalysts for the dehydrocoupling/
polymerization of methylamineborane;2 the current study sheds
light on coordination geometries and migratory pathways poten-
tially accessible for the aminoborane monomer in catalytic systems.
Further details of the reactivity of these novel systems will be
reported in due course.
5 and 6
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic and characterization
b
details for 4 and 5; details of all DFT-calculated structures;
crystallographic data for 3ꢀ6 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
the NMR time scale (Scheme 2a). Coalescence of these reso-
nances occurs at T = ꢀ50 ꢀC, and the barrier to rotation (ΔGq =
9.9 kcal molꢀ1) so determined is marginally greater than the
value of 7.4 kcal molꢀ1 reported by Schlecht and Hartwig for a
comparable process in CpMn(CO)2(HBcat).17 By contrast, the
analogous fluxional process for 5 cannot be frozen out at
temperatures in excess of ꢀ90 ꢀC, suggesting that the corres-
ponding barrier to rotation in this case is less than ca. 7 kcal
molꢀ1. A second fluxional process can be identified for both
complexes (Tc = 7 ꢀC, ΔGq = 12.8 kcal molꢀ1 for 5; Tc = 28 ꢀC,
ΔGq = 14.3 kcal molꢀ1 for 6) which results in coalescence of the
RuHB and BH signals and of the two distinct sets of cyclohexyl
methine protons. As such, these spectral changes are assigned to
a process involving exchange of the bound and unbound BH
hydrogens (Scheme 2b), with the markedly higher barrier
associated with this process for 6 (vs 5) being consistent with
the tighter binding of the borane implied crystallographically.
In order to quantify/contextualize the strength of the metalꢀ
ligand interaction associated with this simple mono(σ-BH)
mode of coordination of H2BNCy2, a series of quantum chemical
calculations has been carried out on 5, 6, and related [CpRu-
(PR3)2]þ-containing systems (see Supporting Information).
Consistent with the structural data determined in the solid state,
significantly stronger binding of the aminoborane is found for 6
(ΔG = ꢀ26.0 for ligand association, cf. ꢀ15.2 kcal molꢀ1 for 5).
Moreover, these free energies can be put into context by the
corresponding values of ꢀ46.5, ꢀ19.2, and ꢀ17.2 kcal molꢀ1
calculated for the binding of CO, η2-C2H4, and N2 to the same
cationic [CpRu(dcype)]þ fragment. Experimentally, such data
are consistent with the displacement of N2 by H2BNCy2 in the
synthesis of 6, and with the (synthetically verified) reaction of
6 with CO to generate [CpRu(dcype)(CO)]þ and free amino-
borane (see Supporting Information). Calculations have also
been carried out on 6 to shed light on the likely mechanism for
the exchange process shown in Scheme 2b; a transition state
can be identified (at a free energy of þ17.5 kcal molꢀ1
with respect to 6) featuring a more symmetrically bound
aminoborane ligand (d[RuꢀH] = 2.261, 2.334 Å; d[BꢀH] =
Corresponding Author
Simon.Aldridge@chem.ox.ac.uk
’ ACKNOWLEDGMENT
The authors thank EPSRC for funding (EP/F019181/1) and
the Oxford Supercomputing Centre for computing time.
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Journal of the American Chemical Society
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(16) N.B.: The HOMO-2 orbital for the [CpML2]þ fragment lies
perpendicular to the HOMO: Schilling, B. E. R.; Hoffmann, R.;
Lichtenberger, D. J. Am. Chem. Soc. 1979, 101, 585–591.
(17) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122,
9435–9443.
(9) Shaw, M. J.; Bryant, S. W.; Rath, N. Eur. J. Inorg. Chem. 2007,
3943–3946.
(10) Synthesis of 6: To solution of 4 (0.24 g, 0.16 mmol) in ca.
15 mL of C6H5F was added H2BNCy2 (0.59 mL of a 0.27 M solution in
C6H5F, 0.16 mmol). After being stirred for 5 min, the solution was
filtered and layered with hexanes (30 mL), and yellow crystals suitable
for X-ray crystallography were obtained. Isolated yield, 0.08 g (32%). 1H
NMR (300 MHz, CD2Cl2, ꢀ20 ꢀC): δH ꢀ14.56 (br, 1H, RuHB),
0.76ꢀ1.98 (overlapping m, 68 H, CH2 and Cy of dcype, CH2 of NCy),
2.90 (s, 1H, NCH), 3.30 (s, 1H, NCH), 4.77 (s, 5H, Cp), 5.80 (br, 1H,
BH), 7.49 (s, 4H, para-CH of [BArf4]ꢀ), 7.65, (s, 8H, ortho-CH of
[BArf4]ꢀ). 13C{1H} NMR (126 MHz, CD2Cl2, 20 ꢀC): δC 22.0
1
2
(apparent t, JPC þ JPC = 20 Hz, PCH2 of dcype), 25.8ꢀ26.1 (br,
CH2 of NCy), 26.0, 26.4 (s, Cy C4 of dcype), 26.2, 28.9 (s, Cy C3 of
4
dcype), 27.0, 27.1 (apparent t, 2JPC þ JPC = 10 Hz, Cy C2 of dcype),
4
27.3, 27.4 (apparent t, 2JPC þ JPC = 5 Hz, Cy C6 of dcype), 29.0, 29.2
(s, Cy C5 of dcype), 32.7, 38.1 (br, CH2 of NCy), 38.2, 38.9 (apparent t,
3
1JPC þ JPC = 14 Hz, PCH of dcype), 58.3, 64.2 (br, CH of NCy), 80.4
(s, Cp), 117.9 (para-CH of [BArf4]ꢀ), 125.0 (q, 1JCF = 272 Hz, CF3 of
[BArf4]ꢀ), 129.2 (q, 2JCF = 31 Hz, meta-C of [BArf4]ꢀ), 135.2 (ortho-
CH of [BArf4]ꢀ), 162.0 (q, 1JBC = 50 Hz, ipso-C of [BArf4]ꢀ). 11B{1H}
NMR (96 MHz, CD2Cl2, 20 ꢀC): δB 35 (br, H2BNCy2), ꢀ7.6 ([BArf4]ꢀ).
19F NMR (283 Hz, CD2Cl2, 20 ꢀC): δF ꢀ62.7 (CF3). 31P{1H} NMR
(121MHz, CD2Cl2, 20ꢀC): δP 81.4; (ꢀ80 ꢀC) 79.2 (d, 2JPP = 23 Hz), 83.1
(d, 2JPP = 23 Hz). Microanalysis, calcd for C75H89B2F24NP2Ru: C, 54.76;
H, 5.45; N, 0.85. Found: C, 54.77; H, 5.47; N, 0.71.
(11) Euzenat, L.; Horhant, D.; Ribourdouille, Y.; Duriez, C.; Alcaraz,
G.; Vaultier, M. Chem. Commun. 2003, 2280–2281.
(12) Crystallographic data for 5: C85H71B2F24NP2Ru, Mr 1747.10,
monoclinic, P21/c, a = 17.7779(1), b = 12.4782(1), and c = 35.9708(3)
Å, β = 90.565(1)ꢀ, V = 7979.2(1) Å3, Z = 4, Fc = 1.454 Mg mꢀ3, T =
150 K, λ = 0.71073 Å; 74 381 reflns collected, 18 022 independent
8497
dx.doi.org/10.1021/ja203051d |J. Am. Chem. Soc. 2011, 133, 8494–8497