Synthesis and reactivity of half-sandwich ruthenium 2- aminoborane complexes

Article abstract of DOI:10.1071/CH13106

Cationic half-sandwich ruthenium complexes featuring 2-bound aminoborane ligands can readily be accessed from 16-electron precursors via chloride abstraction in the presence of H2BNR2 (R≤iPr, Cy). Complexes [Cp*Ru(L)(2-H2BNR2)][BArf4] (2a:R≤iPr, L≤PCy3; 2b:R≤iPr, L≤PPh3; 2c: R≤iPr, L≤1,3-bis-(2,4,6-trimethylphenyl)-imidazol-2- ylidene; 3a:R≤Cy, L≤PCy3; Arf≤C6H3(CF3)2-3,5) were isolated in yields of ～60%, and characterised in the solid state by X-ray crystallography (for 2a, 2c, and 3a). Low-field 11B NMR shifts for the coordinated aminoborane fragment, together with short RuB contacts (of the order of 1.97A) imply a relatively tightly bound borane ligand, a finding which is given further credence by the results of density functional theory studies (e.g. bond dissociation energies in the range 24kcalmol-1; 1kcalmol-1≤4.186kJmol-1). In terms of reactivity, 2 systems of this type, while potentially offering a versatile route to asymmetric 1 systems, in fact undergo borane extrusion even in the presence of a single equivalent of added ligand.

Full text of DOI:10.1071/CH13106

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 1211–1218
Full Paper
http://dx.doi.org/10.1071/CH13106
Synthesis and Reactivity of Half-Sandwich Ruthenium
k2-Aminoborane Complexes
A
A
A
A
David A. Addy, Joshua I. Bates, Michael J. Kelly, Joseph Abdalla,
A
A
A B
,
Nicholas Phillips, Ian M. Riddlestone, and Simon Aldridge
A
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Road, Oxford, OX1 3QR, UK.
B
Corresponding author. Email: Simon.Aldridge@chem.ox.ac.uk
Cationic half-sandwich ruthenium complexes featuring k2-bound aminoborane ligands can readily be accessed from
16-electron precursors via chloride abstraction in the presence of H2BNR2 (R ¼ iPr, Cy). Complexes [Cp*Ru(L)
(k2-H2BNR2)][BArf4] (2a: R ¼ iPr, L ¼ PCy3; 2b: R ¼ iPr, L ¼ PPh3; 2c: R ¼ iPr, L ¼ 1,3-bis-(2,4,6-trimethylphenyl)-
imidazol-2-ylidene; 3a: R ¼ Cy, L ¼ PCy3; Arf ¼ C6H3(CF3)2-3,5) were isolated in yields of ,60 %, and characterised in
the solid state by X-ray crystallography (for 2a, 2c, and 3a). Low-field 11B NMR shifts for the coordinated aminoborane
˚
fragment, together with short Ru?B contacts (of the order of 1.97 A) imply a relatively tightly bound borane ligand,
a finding which is given further credence by the results of density functional theory studies (e.g. bond dissociation energies
in the range 24 kcal molꢀ1; 1 kcal molꢀ1 ¼ 4.186 kJ molꢀ1). In terms of reactivity, k2 systems of this type, while potentially
offering a versatile route to asymmetric k1 systems, in fact undergo borane extrusion even in the presence of a single
equivalent of added ligand.
Manuscript received: 5 March 2013.
Manuscript accepted: 27 May 2013.
Published online: 27 June 2013.
mesitylborane, H2BMes.[7–9] Thus, the reaction of Cp*Ru(PiPr3)
Cl with (H2BMes)n followed by halide abstraction yields a k2
borane complex featuring the 14-electron [Cp*Ru(PiPr3)]þ
fragment.[8] With this in mind, we postulated that similar
chemistry applied to the monomeric secondary aminoboranes
H2BNR2 (R ¼ iPr or Cy) might provide access to half sandwich
aminoborane complexes of the type [Cp*Ru(L)(k2-H2BNR2)]þ.
Such systems might provide an instructive comparison – both in
terms of structural and reaction chemistry - with related
k1-complexes of the type [Cp*Ru(L)2(k1-H2BNR2)]þ recently
developed in our program.[6]
Introduction
Aminoboranes, H2BNRR0 have been the focus of considerable
recent research effort. This reflects not only their potential
relevance to energy storage applications as the products of
dehydrogenation of high hydrogen content BN-containing
materials, but also as the monomeric building blocks from
which a class of novel inorganic polymer can be assembled.[1,2]
Despite their isoelectronic relationship with alkenes, however,
and the wealth of metal complexes known for C¼C double
bonds, the coordination chemistry of aminoboranes is more
limited, being restricted to a dozen or so structurally char-
acterised examples.[3–6] Typically such complexes feature
chelating H2BNR2 ligands coordinated to [L2M(H)2]nþ frag-
ments via two B-H-M bridges (M ¼ Ru, n ¼ 0; M ¼ Rh, Ir,
n ¼ 1; L ¼ N-heterocyclic carbene, tertiary phosphine) and are
formed either by the direct coordination of an aminoborane or
by the in situ dehydrogenation of the corresponding amine-
borane, H3BꢁNR2H.[3–6] Thus, for example, in earlier reports
we have characterised k2 s-complexes of aminoboranes at
Group 9 metal centres featuring N-heterocyclic carbene
co-ligands generated by the in situ dehydrogenation of
H3BꢁNR2H (R ¼ Me, iPr, Cy) (Scheme 1).[5b,d,h,i] Related
charge neutral ruthenium aminoborane complexes such as Ru
(PCy3)2(H)2(k2-H2BNiPr2) have been prepared by Weller,
Alcaraz, and Sabo-Etienne either via an analogous dehydroge-
nation approach or by the direct reaction of an aminoborane with
a ruthenium bis(dihydrogen) complex.[5a,e,f,g] Moreover, reports
from both Sabo-Etienne and Stradiotto confirm that such
a coordination mode is not confined to aminoboranes, with
a similar motif having been established for the more electrophilic
Experimental
General
Manipulation of air-sensitive reagents was carried out in a
glove-box, or by means of Schlenk-type techniques involving
the use of a dry argon or nitrogen atmosphere. With the
exception of fluorobenzene which was distilled from CaH2,
HPLC grade solvents were purified, dried, and degassed before
use by a commercial Braun Solvent Purification System
(SPS 500). [D2]dichloromethane was pre-dried over molecular
sieves before use. The known compounds H2BNR2 (R ¼ iPr,
Cy), Cp*Ru(L)Cl (L ¼ PCy3, PPh3, IMes ¼ 1,3-bis-(2,4,6-
trimethylphenyl)-imidazol-2-ylidene) and Na[BArf4] [where
Arf ¼ C6H3(CF3)2-3,5] were prepared according to literature
t
procedures[10]; BuNC was used as supplied (Sigma Aldrich).
Spectroscopic data for compound [2a][BArf4] were included in
a preliminary communication.[6b] 1H and 13C NMR spectra were
measured on a Bruker AVII 500 FT-NMR or Varian Mercury
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

1212
D. A. Addy et al.
[BArf4]ꢀ
[BArf4]ꢀ
N
N
N
N
N
N
Mes
Mes
Mes
Mes
H3B·NR2H
H
H
H
R
R
H
H
ꢂ
M
ꢂ
Na
M
B
N
Cl
H
M ꢁ Rh, Ir
R ꢁ iPr, Cy
Mes
Mes
N
N
PCy3
PCy3
H
iPr
iPr
H2BNiPr2
H
H
H
H
H
H
H
Ru
Ru
B
N
ꢀ 2H2
H
H
PCy3
PCy3
[B(C6F5)4]ꢀ
(H2BMes)n
Li[B(C6F5)4]
ꢂ
Ru
Mes
H
H
Ru
Ru
Cl
B
B
iPr3P
Mes
iPr3P
H
H
iPr3P
Cl
Scheme 1. Previous reports of k2 borane coordination of primary relevance to the current study.
VX-300 spectrometer, at 258C, and calibrated using the residual
proton or natural abundance 13C resonances of the solvent; 11
ipso-quaternary C of [BArf4]ꢀ); 11B NMR (96 MHz, [D2]
dichloromethane): dB 56.5 (br s, fwhm ¼ 300 Hz, H2BNCy2),
ꢀ6.1 ([BArf4]–); 19F NMR (282 MHz, [D2]dichloromethane): dF
ꢀ62.0 (CF3); 31P{1H} NMR (121 MHz, [D2]dichloromethane):
dP 54.4; m/z (ESIþ) 710.5 (100 %, [M]þꢂ); Found: C 54.93,
H 5.40, N 0.89 %; Mþꢂ, 710.4505. C72H85B2F24NPRu requires
C 55.18, H 5.32, N 0.73 %; Mþꢂ, 710.4550.
B
and 19F spectra were referenced with respect to Et2OꢁBF3 and
CFCl3, respectively. Mass spectra were measured by the EPSRC
National Mass Spectrometry Service, Swansea University or
in-house using a Bruker MicroToF instrument linked to an inert
atmosphere dry box for sample injection. Elemental micro-
analyses were carried out at London Metropolitan University.
Data for [2a][BArf4]: 1H NMR (300 MHz, [D2]dichloro-
methane): dH ꢀ11.21 (br s, 2H, RuBH), 1.11–1.87 (m, 33H,
Syntheses of New Compounds
3
PCy3), 1.22 (d, JHH ¼ 6.6 Hz, 12H, CH3 of iPr), 1.81 (s, 15H,
[Cp*Ru(L)(k2-H2BNR2)][BArf4], [2a–c][BArf4]
3
i
CH3 of Cp*), 3.39 (sept, JHH ¼ 6.6 Hz, 2H, CH of Pr), 7.47
(s, 4H, para-CH of [BArf4]–), 7.64 (s, 8H, ortho-CH of [BArf4]–);
13C NMR (126 MHz, [D2]dichloromethane): dC 11.9 (CH3 of
and [3a][BArf4]
The four compounds were prepared by a common method,
exemplified here for [3a][BArf4]. H2BNCy2 (0.37 cm3 of
a 0.29 M solution in C6H5F, 0.11 mmol) was added to a solution
of Cp*Ru(PCy3)Cl (0.03 g, 0.05 mmol) in fluorobenzene
(,0.7 mL). The blue reaction mixture was then added to
Na[BArf4] (0.05 g, 0.06 mmol) and an instant colour change to
yellow was observed. The solution was filtered, layered with
hexanes (30 mL) and red crystals suitable for X-ray crystallo-
graphy were obtained in 65 % yield. [2a–c][BArf4] were
prepared in analogous fashion from H2BNiPr2 and Cp*Ru(L)Cl
(1a: L ¼ PCy3; 1b: L ¼ PPh3; 1c: L ¼ IMes) in yields of ,60,
50, and 60 %, respectively.
2
Cp*), 24.7, 26.7 (Cy CH2-3,4), 27.9 (d, JPC ¼ 10.1 Hz, Cy
CH2-2), 30.3 (CH3 of iPr), 37.7 (d, 1JPC ¼ 21.1 Hz, Cy CH), 49.9
i
(CH of Pr), 95.3 (Cp*), 117.9 (para-CH of [BArf4]–), 125.0
(q, 1JCF. ¼ 274.0 Hz, CF3 of [BArf4]–), 129.3 (q, 2JCF. ¼ 31.0 Hz,
meta-quaternary C of [BArf4]–), 135.1 (ortho-CH of [BArf4]–),
1
162.1 (q, JCB ¼ 50.1 Hz, ipso-quaternary C of [BArf4]–); 11B
NMR (96 MHz, [D2]dichloromethane): dB 56.2 (br s, fwhm¼
300Hz, H2BNiPr2), ꢀ6.1 ([BArf4]–); 19F NMR (282 MHz, [D2]
dichloromethane): dF ꢀ62.0 (CF3); 31P{1H} NMR (121 MHz,
[D2]dichloromethane): dP 53.3; m/z (ESIþ) 630.4 (100 %,
[M]þꢂ); Found: C 52.87, H 4.92, N 0.86 %; Mþꢂ, 630.3980.
Data for [3a][BArf4]: 1H NMR (300 MHz, [D2]dichloro-
methane): dH ꢀ11.22 (br s, 2H, RuBH), 0.97–2.16 (overlapping
m, 53H, CH and CH2 of PCy3, CH2 of NCy2), 1.78 (s, 15H, CH3
of Cp*), 2.89 (m, 2H, CH of NCy2), 7.46 (s, 4H, para-CH of
[BArf4]ꢀ), 7.63 (s, 8H, ortho-CH of [BArf4]–); 13C NMR
(126 MHz, [D2]dichloromethane): dC 11.9 (CH3 of Cp*), 25.6
(NCy2 CH2-4), 26.5 (NCy2 CH2-3), 26.7 (PCy3 CH2-4), 27.9
C66H76B2F24NPRu requires C 53.08, H 5.09, N 0.94 %; Mþꢂ
630.3922.
,
Data for [2b][BArf4]: 1H NMR (300 MHz, [D2]dichloro-
methane): dH ꢀ10.45 (br s, 2H, RuBH), 1.47 (d, 3JHH ¼ 6.6 Hz,
i
12H, CH3 of Pr), 1.56 (s, 15H, CH3 of Cp*), 3.04 (sept,
i
3JHH ¼ 6.6 Hz, 2H, CH of Pr), 7.48 (s, 4H, para-CH of
[BArf4]–), 7.34–7.66 (overlapping m, 15H, PPh3), 7.64 (s, 8H,
2
(d, JPC ¼ 10.3 Hz, PCy3 CH2-2), 30.3 (NCy2 CH2-2), 36.0
ortho-CH of [BArf4]–); 13C NMR (126 MHz, [D2]dichloro-
(d, 3JCP ¼ 2.7 Hz, PCy3 CH2-3), 37.6 (d, 1JCP ¼ 20.7 Hz, PCy3
CH), 58.9 (NCy2 CH-1), 95.3 (Cp*), 117.9 (para-CH
methane): dC 10.8 (CH3 of Cp*), 32.0 (CH3 of Pr), 50.6 (CH
i
of iPr), 97.7 (Cp*), 117.9 (para-CH of [BArf4]–), 125.0
(q, 1JCF. ¼ 274.0 Hz, CF3 of [BArf4]–), 129.3 (q, 2JCF. ¼ 31.0 Hz,
meta-quaternary C of [BArf4]–), 130.3 (d, 4JCP ¼ 10.5 Hz, para-
1
of [BArf4]–), 125.0 (q, JCF. ¼ 274.0 Hz, CF3 of [BArf4]ꢀ),
2
129.3 (q, JCF. ¼ 31.0 Hz, meta-quaternary C of [BArf4]ꢀ),
1
3
135.1 (ortho-CH of [BArf4]ꢀ), 162.1 (q, JCB ¼ 50.1 Hz,
CH of Ph), 133.7 (d, JCP ¼ 11.7 Hz, meta-CH of Ph), 134.4

Half-Sandwich Ru k2-Aminoborane Complexes
1213
(d, 2JCP ¼ 12.3 Hz, ortho-CH of Ph), 134.8 (d, 1JCP ¼ 50.0 Hz,
150 K. Data were processed using either the DENZO-SMN
package or CrysAlis, and structures solved using SIR92.
Refinement was carried out using full-matrix least-squares
within the CRYSTALS suite.[11] Full crystallographic data for all
structures have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC references 909183 and 927354–
927356. Copies of these data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
1
ipso-C of Ph), 135.1 (ortho-CH of [BArf4]–), 162.1 (q, JCB
¼
50.1 Hz, ipso-quaternary C of [BArf4]–); 11B NMR (96 MHz,
[D2]dichloromethane): dB 57.2 (br s, fwhm ¼ 380 Hz,
H2BNiPr2), ꢀ6.1 ([BArf4]–); 19F NMR (282 MHz, [D2]dichloro-
methane): dF ꢀ62.8 (CF3); 31P{1H} NMR (121 MHz, [D2]
dichloromethane): dP 50.5; m/z (ESIþ) 612.2 ([M]þꢂ); m/z
(HR-MS) 612.2454; [M]þ requires 612.2513; The fact that this
compound is an oil prevented its isolation at a level of purity
suitable for elemental microanalysis.
[2a][BArf4]:C66H76B2F24NPRu, Mw ¼ 1492.95, monoclinic,
Data for [2c][BArf4]: 1H NMR (300 MHz, [D2]dichloro-
P21/c, a ¼ 19.2398(1), b ¼ 14.1958(1), c ¼ 25.3427(2)ꢀA3,
˚
3
3
˚
methane): dH ꢀ9.71 (br s, 2H, RuBH), 1.10 (d, JHH ¼ 6.6 Hz,
b ¼ 99.7258(3)8, V ¼ 3157.5(4) A , Z ¼ 4, rc ¼ 1.453 Mg m
,
i
12H, CH3 of Pr), 1.35 (s, 15H, CH3 of Cp*), 1.98 (br s, 12H,
˚
t ¼ 150 K, l ¼ 0.71073 A. 185601 reflections collected, 15523
independent [R(int) ¼ 0.025] and used in all calculations.
R1 ¼ 0.0428, wR2 ¼ 0.1011 for observed unique reflections
[F2 .2s (F2)] and R1 ¼ 0.0660, wR2 ¼ 0.1119 for all unique
reflections. Max. and min. residual electron densities 0.93 and
ortho-CH3 of Mes), 2.27 (s, 6H, para-CH3 of Mes), 3.14 (sept,
i
3JHH ¼ 6.6 Hz, 2H, CH of Pr), 6.97 (s, 4H, meta-CH of Mes),
7.04 (s, 2H, NCH of IMes), 7.48 (s, 4H, para-CH of [BArf4]–),
7.64 (s, 8H, ortho-CH of [BArf4]–); 13C NMR (126 MHz, [D2]
dichloromethane): dC 11.0 (CH3 of Cp*), 19.6 (para-CH3 of
Mes), 20.2 (ortho-CH3 of Mes), 21.1 (CH3 of iPr), 49.4 (CH of
ꢀ0.82 e Aꢀ3. CCDC reference: 909183.
˚
[2c][BArf4]: C69H67B2F24N3Ru, Mw ¼ 1517.42, triclinic,
iPr), 91.7 (Cp*), 117.9 (para-CH of [BArf4]–), 124.9 (q, 1JCF.
¼
P-1, a ¼ 12.8454(3), b ¼ 15.9928(4), c ¼ 18.3511(5) A,
˚
273.0 Hz, CF3 of [BArf4]–), 125.6 (NCH of IMes), 129.2
a ¼ 78.7012(10)8,
b ¼ 76.2493(11)8,
g ¼ 77.1539(10)8,
2
3
(q, JCF. ¼ 32.0 Hz, meta-quaternary C of [BArf4]–), 130.0
V ¼ 3529.3(2) A , Z ¼ 2, rc ¼ 1.425 Mg mꢀ3
,
t ¼ 150 K,
˚
˚
(ortho-C of Mes), 135.2 (meta-C of Mes), 137.3 (para-C of
Mes), 140.1 (ipso-C of Mes), 135.1 (ortho-CH of [BArf4]–),
162.2 (q, 1JCB ¼ 50.1 Hz, ipso-quaternary C of [BArf4]–), 180.2
(carbene quaternary-C of IMes); 11B NMR (96 MHz, [D2]
dichloromethane): dB 49.9 (br s, fwhm ¼ 340 Hz, H2BNiPr2),
ꢀ6.1 ([BArf4]–); 19F NMR (282 MHz, [D2]dichloromethane): dF
ꢀ62.8 (CF3); m/z (ESIþ) 654.4 (100 %, [M]þꢂ); Found: C 54.76,
H 4.27, N 2.65 %; Mþꢂ, 654.3542. C69H67B2F24N3Ru requires
C 54.61, H 4.45, N 2.77 %; Mþꢂ, 654.3542.
l ¼ 0.71073 A. 20604 reflections collected, 12713 independent
[R(int) ¼ 0.038] and used in all calculations. R1 ¼ 0.0729, wR2 ¼
0.1832 for observed unique reflections [F2 .2s (F2)] and
R1 ¼ 0.1003, wR2 ¼ 0.02081 for all unique reflections. Maꢀx3.
˚
and min. residual electron densities 1.45 and ꢀ1.18 e A
.
CCDC reference: 927354.
[3a][BArf4]: C72H85B2F24NPRu, Mr ¼ 1573.08, monoclinic,
˚
P21, a ¼ 12.7773(1), b ¼ 17.3289(1), c ¼ 17.9242(1)ꢀA3,
3
˚
b ¼ 98.4578(3)8, V ¼ 3925.55(4) A , Z ¼ 2, rc ¼ 1.331 Mg m
,
˚
t ¼ 150 K, l ¼ 0.71073 A. 69624 reflections collected, 16317
independent [R(int) ¼ 0.045] and used in all calculations.
R1 ¼ 0.0503, wR2 ¼ 0.1134 for observed unique reflections
[F2 .2s (F2)] and R1 ¼ 0.0453, wR2 ¼ 0.1083 for all unique
reflections. Max. and min. residual electron densities 0.95 and
[Cp*Ru(PCy3)(CNtBu)2)][BArf4], [4a][BArf4]
A solution of [2a][BArf4] (0.020 g, 0.013 mmol) in fluoro-
benzene (,0.7 mL) was added to a solution of tBuNC
(0.034 mL, 0.030 mmol) also in fluorobenzene (,0.7 mL) and
the reaction mixture was sonicated for 5 min. In situ monitoring
by 11B NMR spectroscopy revealed the generation of free
H2BNiPr2 at this point (dB ¼ 35.4, t, 1JBH ¼ 126 Hz). Filtration
and layering with hexanes (30 mL) led to the formation of
yellow crystals suitable for X-ray crystallography in 60 % yield.
Data for [4a][BArf4]: 1H NMR (300 MHz, [D2]dichloro-
methane): dH 1.10–1.90 (m, 33H, PCy3), 1.36 (s, 18H, tBuNC),
1.73 (s, Cp*), 7.48 (s, 4H, para-CH of [BArf4]ꢀ), 7.64 (s, 8H,
ortho-CH of [BArf4]–); 13C NMR (126 MHz, [D2]dichloro-
methane): dC 10.8 (CH3 of Cp*), 26.6 (Cy CH2-4), 27.8
(d, 2JPC ¼ 10.4 Hz, Cy CH2-2), 30.6 (Cy CH2-3), 31.0 (CH3 of
tBuNC), 37.4 (d, 1JPC ¼ 20.0 Hz, Cy CH-1), 57.9 (tBu quaternary-
ꢀ0.49 e Aꢀ3. CCDC reference: 927355.
˚
[4a][BArf ]: C70H78BF24N2PRu, Mw ¼ 1546.23, monoclinic,
4
˚
P21/c, a ¼ 13.7490(1), b ¼ 19.5577(2), c ¼ 27.5241(2)ꢀA3,
3
˚
b ¼ 99.970(1)8, V ¼ 7400.1(1) A , Z ¼ 4, rc ¼ 1.388 Mgm
,
˚
t ¼ 150 K, l ¼ 1.54180 A. 46253 reflections collected, 15366
independent [R(int) ¼ 0.027] and used in all calculations.
R1 ¼ 0.0681, wR2 ¼ 0.1718 for observed unique reflections
[F2 .2s (F2)] and R1 ¼ 0.0743, wR2 ¼ 0.1810 for all unique
reflections. Max. and min. residual electron densities 1.71 and
ꢀ1.04e Aꢀ3. CCDC reference: 927356.
˚
Computational Method
Density functional theory (DFT) calculations were performed
using the Amsterdam Density Functional (ADF) Package
Software 2012.[12a–c] Calculations were performed using
the Vosko-Wilk-Nusair local density approximation with
exchange from Becke,[12d] and correlation corrections from
Perdew (BP).[12e] Slater-type orbitals (STOs) were used for the
triple zeta basis set with an additional set of polarisation func-
tions (TZP).[12f] The large frozen core basis set approximation
was applied with no molecular symmetry, and the general
numerical integration was six. Frequency calculations were
performed for all freely optimised species and no significant
imaginary frequencies were observed. Estimates of binding
energies were obtained following the strategy outlined by
Baerends,[12g] using the counterpoise method.[12h] Given that
basis set superposition errors (BSSE) tend to be overestimated
t
C of BuNC), 96.3 (Cp*), 117.9 (para-CH of [BArf4]–), 125.0
(q, 1JCF. ¼ 274.0 Hz, CF3 of [BArf4]–), 129.3 (q, 2JCF. ¼ 31.0 Hz,
meta-quaternary C of [BArf4]–), 135.1 (ortho-CH of [BArf4]–),
153.7 (CN of tBuNC), 162.1 (q, 1JCB ¼ 50.1 Hz, ipso-quaternary
C of [BArf4]–); 19F NMR (282 MHz, [D2]dichloromethane): dF
ꢀ62.0 (CF3); 31P{1H} NMR (121 MHz, [D2]dichloromethane):
dP 57.4; m/z (ESIþ) 683.4 (100 %, [M]þꢂ); m/z (HR-MS)
683.4011; [M]þ requires 683.4012.
Crystallographic Method
With the exception of 4a, data were collected on a NoniusKappa
˚
CCD diffractometer (Mo-Ka radiation, l ¼ 0.71073 A) at
150 K. Data for 4a were collected on an Oxford Diffraction
˚
Supernova diffractometer (Cu-Ka radiation, l ¼ 1.54180 A) at

1214
D. A. Addy et al.
by the counterpoise method,[12i] and that the BSSE for [2a]þ and
model versions were found to be less than 2 kcal molꢀ1, reaction
energies are discussed without BSSE correction. Bonding
analyses employed a combined charge and energy decomposi-
tion scheme based on the extended transition state method and
natural orbitals for chemical valence theory (ETS-NOCV).[12j]
Calculations of 11B NMR chemical shifts were performed using
the NMR program contained in the ADF Package.[12k-o]
Chemical shifts are referenced to H2BNMe2 (d ¼ 37.9 ppm) as
the experimental standard. For computational efficiency,
a frozen-core approximation was used for all calculations,
including the NMR chemical shifts. Run files can be found in the
Supplementary Material.
(for [2a]þ) and dB ¼ 56.5, dP ¼ 54.4 ppm (for [3a]þ). In both
cases, large red crystals could subsequently be obtained in
,60 % yield by layering the reaction mixture with hexanes.
NMR data obtained for crystalline samples support
the formation of [Cp*Ru(PCy3)(k2-H2BNR2)][BArf4] (R ¼
iPr: [2a][BArf4]; R ¼ Cy: [3a][BArf4]; Scheme 2). Thus, the
respective 1H NMR spectra feature signals associated with the
Cp*, PCy3, and [BArf4]– moieties. In addition, for [2a][BArf4]
a single set of signals corresponding to the isopropyl CH (sept,
3JHH ¼ 6.6 Hz) and CH3 protons (d, 3JHH ¼ 6.6 Hz) is observed,
consistent with the equivalent NiPr2 isopropyl substituents
expected for a symmetrically bound k2 complex.[5] Similar
findings have been reported previously by Tang et al. in the
synthesis of [M(IMes)2(H)2(k2-H2BNiPr2)]þ (M ¼ Rh, Ir).[5b,d]
Furthermore, in each case, a broad signal in the hydride region
([2a]þ: dH ¼ ꢀ11.21 ppm; [3a]þ: dH ¼ ꢀ11.22 ppm), which can
Results and Discussion
Initial studies targeted the interaction of the aminoborane
H2BNiPr2 with a series of cationic half-sandwich ruthenium
centres. Utilising halide abstraction chemistry and the
16-electron starting material Cp*Ru(PCy3)Cl (1a), a putative
14-electron system can be accessed in situ, thereby promoting
on the basis of simple electron counting considerations, a che-
lating k2 coordination mode of the aminoborane (Scheme 2). In
practice, and in contrast to chemistry reported by Stradiotto and
co-workers for the more electrophilic borane [H2BMes]n,[8] pre-
mixing of either H2BNiPr2 or H2BNCy2 with the 16-electron
ruthenium starting material 1a in fluorobenzene leads to no
change in either the 11B or 31P NMR spectrum. Subsequent
treatment of these intensely blue coloured solutions with a single
equivalent of Na[BArf4], however, results in an immediate
colour change to bright yellow; 11B and 31P NMR spectroscopic
analyses reveal quantitative conversion to a single complex in
each case, characterised by signals at dB ¼ 56.2, dP ¼ 53.3 ppm
1
be resolved into a doublet in the H{11B} spectrum ([2a]þ:
2JHP ¼ 15.0 Hz; [3a]þ: 2JHP ¼ 14.8 Hz), provides evidence for
coordination of the aminoborane to the [Cp*Ru(PCy3)]þ frag-
ment in solution. By means of comparison, the corresponding
data reported for the k2 mesitylborane complex [Cp*Ru(PiPr3)
2
(k2-H2BMes)]þ are dH ¼ ꢀ10.3 ppm, JPH ¼ 15.0 Hz.[8] For
both [2a]þ and [3a]þ, the ESI mass spectrum shows the expected
[M]þ envelope, with the identity of the molecular ion being
confirmed by accurate mass measurements. Bulk sample purity
was confirmed by elemental microanalysis and the structures
of both [2a][BArf4] and [3a][BArf4] in the solid state were
determined by X-ray crystallography (Fig. 1).
The molecular structures of both cations contain an amino-
borane ligand featuring a near planar C2NBH2 skeleton coordi-
nated to the metal via two B-H-M interactions. The essentially
linear Ru?B–N framework [+Ru?B–N ¼ 171.6(2),
169.8(3)o for [2a]þ and [3a]þ, respectively] is similar in nature
to that observed for other k2-bound aminoborane ligands
{e.g. 179.6(3)8 for [(IMes)2Rh(H)2(H2BNiPr2)]þ},[5b,d] and the
H2BNR2
Na[BArf4]
[BArf4]ꢀ
˚
R
B–N distances [1.362(3), 1.353(6) A] are consistent with con-
˚
H
ꢂ
Ru
Ru
Cl
B
N
siderable p bond character [compare with 1.58 A for the sum of
˚
ꢀ NaCl
H
R
the respective covalent radii and 1.380(6) A for the B–N
separation in Ph2NBCl2].[13,14] The Ru?B separations
L
L
[2a][BArf4]: L ꢁ PCy3, R ꢁ iPr
[2b][BArf4]: L ꢁ PPh3, R ꢁ iPr
[2c][BArf4]: L ꢁ IMes, R ꢁ iPr
[3a][BArf4]: L ꢁ PCy3, R ꢁ Cy
1a: L ꢁ PCy3
1b: L ꢁ PPh3
1c: L ꢁ IMes
˚
[1.965(3), 1.986(4) A] are statistically identical to that reported
by Sabo-Etienne and co-workers for the non-Cp system
2
i
(Cy3P)2Ru(H)2(k -H2BN Pr2) [1.980(3) A],[5e] but significantly
˚
shorter than that in [CpRu(PMe3)2(k1-H3BꢁNMe3)]þ [2.648
(3) A],[15] presumably reflecting the presence of a three (rather
˚
Scheme 2. Syntheses of half-sandwich ruthenium k2-aminoborane com-
plexes from 16-electron Cp*Ru(L)Cl precursors.
than four)-coordinate boron centre and the possibility for Ru to
B back-bonding. By means of further comparison, the Ru?B
[8]
separation in [Cp*Ru(PiPr3)(k2-H2BMes)]þ is 1.921(2) A.
˚
Fig. 1. Molecular structures of [2a][BArf4] (left), [3a][BArf4] (centre) and [2c][BArf4] (right) as determined by X-ray crystallography. Thermal
ellipsoids set at the 35 % probability level; H atoms (except metal-bound Hs) and counter-ion omitted, and Mes/Cy groups shown in wireframe format
1
˚
for clarity. Key bond lengths (A) and angles (8):(for [2a] ) Ru(1)?B(31) 1.965(3), B(31)–N(32) 1.362(3), Ru(1)–P(1) 2.388(1), Ru(1)?B(31)–N(32)
171.6(2); (for [3a]1): Ru(1)?B(20) 1.986(4), B(2)–N(5) 1.353(6), Ru(1)–P(2) 2.384(1), Ru(1)?B(20)–N(5) 1.698(3); (for [2c]1) Ru(1)?B(35)
1.972(6), B(35)–N(36) 1.372(7), Ru(1)–C(2) 2.113(5), Ru(1)?B(31)–N(32) 170.6(5).

Half-Sandwich Ru k2-Aminoborane Complexes
1215
Variation in the ancillary two-electron donor ligand, L is also
possible. Thus, the corresponding reactions in fluorobenzene of
Cp*Ru(L)Cl (1b: L ¼ PPh3; 1c: L ¼ IMes) with H2BNiPr2/
Na[BArf4] yield, in each case, a k2-aminoborane adduct closely
related to [2a]þ (Scheme 2). Both [Cp*Ru(PPh3)
(k2-H2BNiPr2)][BArf4] ([2b][BArf4]) and [Cp*Ru(IMes)
(k2-H2BNiPr2)][BArf4] ([2c][BArf4]) have been characterised
by multinuclear NMR spectroscopy and mass spectrometry,
with the 11B NMR spectrum of each compound featuring a broad
downfield shifted resonance ([2b]þ: dB ¼ 57.2 ppm; [2c]þ:
dB ¼ 49.9 ppm). Additionally, the corresponding 1H NMR spec-
tra feature Ru-H-B resonances in the expected chemical shift
range ([2b]þ: dH ¼ ꢀ10.45 ppm; [2c]þ: dH ¼ ꢀ9.71 ppm).
While [2b][BArf4] appears to be an oil, the IMes derivative
[2c][BArf4] can be obtained as a crystalline solid allowing for
(i) establishment of its bulk purity by elemental microanalysis;
and (ii) structure determination by single crystal X-ray diffrac-
tion (Fig. 1). The structure of the [2c]þ cation is very similar
to those of [2a]þ/[3a]þ, being based around a familiar linear
Ru?B–N framework [+Ru?B-N ¼ 170.6(5)8] and featuring
(41.8 kcal molꢀ1), the latter being the difference between steric
(40.1 kcal molꢀ1) and orbital interaction (81.9 kcal molꢀ1
)
energies. In the system featuring Ru–H bond constraints, the
corresponding binding energy is marginally less (18.5 kcal molꢀ1),
reflecting increases in both the distortion energy (37.3 kcal molꢀ1
)
and steric energy (81.5 kcal molꢀ1) which are not quite offset by
that in the orbital interaction energy (137.3 kcal molꢀ1). In terms
of the importance of s-donation and p-back-bonding,
ETS-NOCV analyses imply that the relative energetic contribu-
tions are in the ratio of ,1 : 2 in favour of back-bonding
(e.g. 1 : 2.3 for the freely optimised system; 1 : 1.8 for the
Ru–H constrained system). The predominant ligand-to-metal
s-bonding interaction originates in the higher lying (out of
phase) B–H bonding orbital, while the major component of
metal-to-ligand p back-bonding involves the aminoborane B–N
p* orbital.[5e,6a]
Despite their isoelectronic relationship with 1,1-disubstituted
alkenes, the coordination chemistry of aminoboranes was first
reported in the literature as late as 2010.[4,5a,b] Cations [2a–c]þ
and [3a]þ thus represent significant additions to a limited class
of complexes featuring k2-aminoborane ligation – and the first
such systems featuring a half-sandwich metal fragment. Such
a coordination geometry contrasts with the classical ‘side-on’
binding mode typically observed for alkene donors, and can be
rationalised by the more hydridic nature of the B–H bonds
(compared with C–H), and the significantly lower energy of the
B¼N p system (compared with C¼C).[17,18] Consistently,
Alcaraz and Sabo-Etienne report an energetic preference of
14.3 kcal molꢀ1 for the ‘end-on’ bis(s-BH) coordination geom-
etry of H2BNH2 at charge neutral bis(phosphine)ruthenium
fragments of the type [L2Ru(H)2].[5a] Attempts to encourage
coordination through the B¼N p system by employing non-
hydridic aminoboranes such as Me2BNMe2,[19] however, do not
result in side-on coordination. Rather, the very weakly interact-
ing nature of the p system is emphasised by the fact that the
reaction of 1a with Na[BArf4]/Me2BNMe2 in fluorobenzene
leads to the generation of the known phosphino-allyl complex
[Cp*Ru(H){k2-PCy2(C6H8)}]þ (Scheme 3),[6b,20] presumably
via multiple C–H activation within one of the cyclohexyl
substituents of the putative 14-electron intermediate [Cp*Ru
(PCy3)]þ.[6b] Moreover, the fact that the Me2BNMe2 molecule
is not found to coordinate to the [Cp*Ru(PCy3)]þ fragment is
consistent with the predictions of DFT calculations which imply
˚
a Ru?B separation [1.972(6) A] which is statistically identical
to the two PCy3-ligated compounds.
In the cases of half-sandwich complexes outlined above, the
˚
Ru?B contacts [1.965(3)-1.986(4) A] fall comfortably within
the sum of the respective covalent radii (2.12 A),[14] and the 11
B
˚
chemical shifts (49.9 to 57.2 ppm) are also shifted significantly
downfield, both with respect to the free aminoborane
(35.4 ppm),[10a,b] and to cationic rhodium and iridium com-
plexes featuring a similar k2-mode of ligand coordination
(e.g. 35.9, 37.9 ppm for [M(IMes)2(H)2(k2-H2BNiPr2)]þ, where
M ¼ Rh or Ir, respectively).[5b,d] These observations prompted
us to investigate the nature of the interaction between the
ruthenium centre and the aminoborane ligand in systems such
as [2a]þ by DFT methods. Of particular interest was the extent to
which the metal-borane interaction might be augmented by
back-bonding from the metal into boron-centred molecular
orbitals (e.g. of BH s* or BN p* character).[5e]
While there is general agreement between the structures
[2a]þ obtained by X-ray crystallography and DFT, several
small yet significant differences are noted. Notably, the
˚
˚
Ru?B (2.091 A) and Ru–H (1.855 and 1.860 A) distances are
˚
longer (by 0.126 and 0.214/0.197 A, respectively), the B–H
˚
˚
(1.297 and 1.299 A) distances are shorter (by 0.037/0.036 A),
and the Ru?B-N (168.28) angle is slightly more distorted from
linear (by 3.48) in the calculated structure. These values suggest
less activated B–H bonds in the calculated structure, a hypothe-
sis also consistent with the relatively upfield calculated 11B
NMR shift of 41.2 ppm (compared to 56.2 ppm measured
experimentally). Previous reports imply that DFT methods tend
to underestimate the strength of bonding interactions involving
bridging hydrogens.[16] Consistently, re-optimising the geome-
that dissociation is favoured by 5.4 kcal molꢀ1
.
While the dominance of the s-borane mode of coordination
of aminoboranes of the type H2BNR2 has thus been established,
the utilisation of a 14-electron metal fragment such as [L2Ru
(H)2], [L2M(H)2]þ (M ¼ Rh, Ir), or [(Z5-C5R5)Ru(L)]þ, might
be viewed as distorting the structural landscape in favour of the
four-electron-donating bis(s-BH) coordination mode over
a possible (two-electron-donating) side-on p-bound motif. With
this in mind, we have also sought to determine the intrinsic
two-electron donor capabilities of aminoborane ligands, by
employing 16-electron metal centres.[6a]
Two complementary synthetic strategies have been
employed to target aminoborane complexes of 16-electron
metal fragments (Scheme 4), namely (i) displacement of a
labile ligand from an 18-electron precursor by an aminoborane
(e.g. route 1, L0 ¼ N2); and (ii) modification of a k2-aminobor-
ane complex by the assimilation of an additional two electron
donor (route 2). While the former strategy has been shown to be
successful in the synthesis of k1 complexes of H2BNCy2
featuring [CpRu(PPh3)2]þ and [CpRu(dcype)]þ fragments,[6a]
˚
try but constraining the Ru–H distances to be 1.65 A (as in the
X-ray structure) results in a structure ,5 kcal molꢀ1 higher in
energy, but possessing metrics closer to those observed in the
solid state [d(B–N) ¼ 1.386, d(Ru?B) ¼ 1.967, d(B–H) ¼
˚
1.402, 1.405 A; +(Ru?B-N) ¼ 172.48] and in solution
(dB ¼ 54.4 ppm).
The binding energy for the aminoborane ligand in [2a]þ,
reflecting the difference in energy between the freely opti-
mised complex and [Cp*Ru(PCy3)]þ/H2BNiPr2 fragments, is
calculated to be 23.6 kcal molꢀ1. This reflects the energy
required to distort the free fragments to their complex geome-
tries (18.3 kcal molꢀ1) and the instantaneous interaction energy

1216
D. A. Addy et al.
[BArf4]ꢀ
Me
[BArf4]ꢀ
Na[BArf4]
Me2BNMe2
Me
ꢂ
Ru
N
ꢂ
Ru
Ru
Cl
C6H5F
ꢀ H2
B
H
Cy2P
Me
Cy3P
Cy3P
Me
1a
Scheme 3. Reaction of 1a with Na[BArf4]/Me2BNMe2: CH activation vs borane ligation.
Rn
Rn
Rn
R
ꢂ L
ꢂ H2BNR2
ꢀ Lꢃ
R
R
H
H
H
ꢂ
Ru
ꢂ
Ru
ꢂ
Lꢃ
Ru
N
B
N
B
L
L
R
L
L
L
Route 1
H
Route 2
Scheme 4. Potential routes to half-sandwich ruthenium k1-aminoborane complexes.[6a]
[BArf4]ꢀ
R
[BArf4]ꢀ
ꢂ
tBuNC
(2 equiv.)
H
H
CNtBu
Ru(1)
C(31)
ꢂ
Ru
ꢂ
B
N
Ru
ꢀ
H2BNR2
CNtBu
R
P(2)
Cy3P
Cy3P
[4a][BArf4]
N(32)
C(33)
[2a][BArf4]
Scheme 5. Reaction of [2a][BArf4] with tBuNC; crystal structure of [4a][BArf4] with H atoms and counter-ion omitted
and Cy groups shown in wireframe format for clarity; thermal ellipsoids are set at the 40 % probability level.
the second route was perceived as offering a potential route to
unsymmetrically ligated systems of the type [Cp*Ru(L[1])(L2)
(k1-H2BNR2)]þ via reactions of [2a–c]þ or [3a]þ with
two-electron donors.
instantaneous interaction energy to compensate for the greater
distortion energy (,3.5 kcal molꢀ1). In considering a side-on
p-bound motif for H2BNMe2 (or indeed the isosteric/isoelec-
tronic alkene H2CCMe2), the orientation in which the Me groups
point away from the larger phosphorus substituent (rather than
the smaller Cp ligand) is favoured on steric grounds. However,
not surprisingly, the ‘side-on’ B¼N bond binds significantly
less strongly (15.9 kcal molꢀ1) than the corresponding C¼C
bond (22.1 kcal molꢀ1), or than either of the mono s-(BH)
possibilities (23.4/22.9 kcal molꢀ1).
With this in mind, DFT calculations were carried out on the
exemplar model system [CpRu(PMe3)(CNMe)(H2BNMe2)]þ
with the aim of probing (i) the relative energies of different
modes of aminoborane attachment (e.g. via B–H or B¼N
donation); and (ii) whether the presence of electronic asymme-
try within the ancillary ligand set (based on the differing s donor
and p acceptor properties of the tertiary phosphine and isonitrile
donors) leads to any orientational preference in the aminoborane
binding. For the model k2 system [CpRu(PMe3)(H2BNMe2)]þ,
the bis(s-BH) binding mode (42.1 kcal molꢀ1) is significantly
favoured over p-BN coordination (19.8 kcal molꢀ1). With
a mean contribution of 21 kcal molꢀ1 per bridging Ru-B-H
interaction, this suggests that a single s-BH interaction might
still be favoured over the p-BN motif. Upon introducing a
CNMe ligand, the ruthenium centre adopts a pseudo-tetrahedral
geometry and becomes chiral, resulting in subtle differences
depending on the relative orientation of the H2BNMe2 ligand. In
the two local minima, the B–H bond is orientated either towards
the phosphine or the isonitrile co-ligand (and thus is pseudo-
trans to the other ligand); these rotamers have very similar
binding energies (23.4 and 22.9 kcal molꢀ1, respectively).
While such a small difference (,0.5 kcal molꢀ1) precludes
significant interpretation, it is interesting that the lower energy
species, with the B–H bond aligned towards the phosphine, is
The reactions of [2a][BArf4] with a series of neutral
two-electron donor ligands were therefore probed by NMR
t
and (in the case of BuNC) crystallographic techniques. These
studies revealed that, while [2a]þ is unreactive towards the
weaker donor THF, its reactions towards tBuNC, Ph2CO, PMe3,
PPh3, and PCy3 are characterised by extrusion of the borane
fragment. The related reaction between [2a]þ and [PPh4]Cl
results in regeneration of the intensely blue coloured starting
material Cp*Ru(PCy3)Cl (1a) and H2BNiPr2. In the case of the
t
reaction with BuNC, extrusion of the borane was not only
confirmed by the identification (via multinuclear NMR) of the
free borane H2BNiPr2, but also by the isolation and structural
characterisation of the metal-containing product [Cp*Ru(PCy3)
(CNtBu)2][BArf4] ([4a][BArf4]) (Scheme 5). Although the
quality of the structure solution is not sufficient to justify
discussion of metrics, atomic connectivity and the formation
of the 18-electron bis(isonitrile) cation are thus confirmed.
While the formation of [4a][BArf4] from [2a][BArf4] implies
a 2 : 1 stoichiometry for isonitrile uptake, studies carried out
with a molar ratio of 1 : 1 give no indication of the formation of
˚
slightly further along the oxidative pathway (the B–H is 0.026 A
˚
longer and the Ru–B is 0.024 A shorter) with a larger

Half-Sandwich Ru k2-Aminoborane Complexes
1217
Me
H
CNtBu
CNtBu
Me
Me
H
H
ꢂ
Ru
ꢂ
Ru
ꢂ
Ru
2
N
B
N
ꢂ
ꢂ
B
Me3P
Me
Me3P
H2BNMe2
Me3P
MeNC
H
Scheme 6. Ligand redistribution in the model k1-aminoborane complex [CpRu(PMe3)(CNMe)(k1-H2BNMe2)]þ probed by DFT methods.
the k1 complex [Cp*Ru(PCy3)(CNtBu)(k1-H2BNiPr2)]þ. More-
over, in-depth analysis of the thermodynamics of the related
model system [CpRu(PMe3)(CNMe)(k1-H2BNMe2)]þ by DFT
methods implies that the ligand redistribution reaction repre-
sented by Scheme 6 is exergonic by 7.6 kcal molꢀ1. As such, the
implication for isonitrile systems at least, is that k1 systems are
unlikely to be accessible from related k2 complexes via a ligand
addition protocol, being clearly disfavoured with respect to
more symmetrically substituted systems.
[3] (a) For recent reviews of s-borane complexes see, for example: R. N.
Perutz, S. Sabo-Etienne, Angew. Chem. Int. Ed. 2007, 46, 2578.
doi:10.1002/ANIE.200603224
(b) Z. Lin, Struct. Bond. 2008, 130, 123. doi:10.1007/430_2007_074
(c) G. Alcaraz, S. Sabo-Etienne, Coord. Chem. Rev. 2008, 252, 2395.
doi:10.1016/J.CCR.2008.02.006
(d) K. K. Pandey, Coord. Chem. Rev. 2009, 253, 37. doi:10.1016/
J.CCR.2007.11.026
(e) G. Alcaraz, M. Grellier, S. Sabo-Etienne, Acc. Chem. Res. 2009, 42,
1640. doi:10.1021/AR900091A
(f) G. Alcaraz, S. Sabo-Etienne, Angew. Chem. Int. Ed. 2010, 49, 7170.
doi:10.1002/ANIE.201000898
Conclusions
[4] For an early spectroscopic report examining aminoborane interactions
with transition metals see: G. Schmid, Chem. Ber. 1970, 103, 528.
doi:10.1002/CBER.19701030222
Cationic half-sandwich ruthenium complexes featuring
k2-bound aminoborane ligands can readily be accessed from
16-electron precursors via chloride abstraction in the presence
of H2BNR2 (R ¼ iPr, Cy). Low-field 11B NMR shifts for the
coordinated aminoborane fragment, together with short Ru?B
[5] (a) For previous reports of k2-coordinated aminoboranes, see:
G. Alcaraz, L. Vendier, E. Clot, S. Sabo-Etienne, Angew. Chem. Int.
Ed. 2010, 49, 918. doi:10.1002/ANIE.200905970
˚
contacts (of the order of 1.97 A) imply a relatively tightly bound
(b) C. Y. Tang, A. L. Thompson, S. Aldridge, Angew. Chem. Int. Ed.
2010, 49, 921. doi:10.1002/ANIE.200906171
(c) M. A. Esteruelas, F. J. Fernandez-Alvarez, A. M. Lopez, M. Mora,
E. Onate, J. Am. Chem. Soc. 2010, 132, 5600. doi:10.1021/JA101806X
(d) C. Y. Tang, A. L. Thompson, S. Aldridge, J. Am. Chem. Soc. 2010,
132, 10578. doi:10.1021/JA1043787
(e) G. Alcaraz, A. B. Chaplin, C. J. Stevens, E. Clot, L. Vendier,
A. S. Weller, S. Sabo-Etienne, Organometallics 2010, 29, 5591.
doi:10.1021/OM1004995
borane ligand, a finding which is given further credence by the
results of DFT studies (e.g. bond dissociation energies in the
range 24 kcal molꢀ1). Reactivity-wise k2 systems of this type,
while potentially offering a versatile route to asymmetric k1
systems, in fact undergo borane extrusion even in the presence
of a single equivalent of added ligand.
Supplementary Material
(f) C. J. Stevens, R. Dallanegra, A. B. Chaplin, A. S. Weller,
S. A. Macgregor, B. Ward, D. McKay, G. Alcaraz, S. Sabo-Etienne,
Chem. – Eur. J. 2011, 17, 3011. doi:10.1002/CHEM.201002517
(g) G. Be´nac-Lestrille, U. Helmstedt, L. Vendier, G. Alcaraz, E. Clot,
S. Sabo-Etienne, Inorg. Chem. 2011, 50, 11039. doi:10.1021/
IC201573Q
Full details of DFT calculations (optimised geometries and run
files) and CIFs for all X-ray structures are available on the
Journal’s website.
Acknowledgements
We thank the EPSRC for a doctoral training grant studentship and post-
doctoral support (DAA), and NSERC for a post-doctoral fellowship (JIB).
(h) C. Y. Tang, N. Phillips, J. I. Bates, A. L. Thompson, M. J. Gutmann,
S. Aldridge, Chem. Commun. 2012, 48, 8096. doi:10.1039/C2CC33361A
(i) C. Y. Tang, N. Phillips, M. J. Kelly, S. Aldridge, Chem. Commun.
2012, 48, 11999. doi:10.1039/C2CC35947B
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