Reactions of β-diketiminate-stabilized calcium amides with 9-borabicyclo[3.3.1]nonane (9-BBN)

Article abstract of DOI:10.1021/om070083t

The reaction of a β-diketiminate-stabilized calcium diphenylamide with the dialkylborane 9-BBN allows the synthesis of a β-diketiminate- stabilized calcium borohydride along with an amidoborane reaction coproduct via a likely σ-bond metathesis mechanism.

Full text of DOI:10.1021/om070083t

4076
Organometallics 2007, 26, 4076-4079
Reactions of â-Diketiminate-Stabilized Calcium Amides with
9-Borabicyclo[3.3.1]nonane (9-BBN)
Anthony G. M. Barrett,*,† Mark R. Crimmin,† Michael S. Hill,*,† Peter B. Hitchcock,‡ and
Panayiotis A. Procopiou§
Department of Chemistry, Imperial College London, Exhibition Road, South Kensington,
London SW7 2AZ, U.K., The Chemistry Laboratory, UniVersity of Sussex, Falmer, Brighton,
East Sussex BN1 9QJ, U.K., and GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road,
SteVenage, Hertfordshire SG1 2NY, U.K.
ReceiVed January 29, 2007
Scheme 1. σ-Bond Metathesis
Summary: The reaction of a â-diketiminate-stabilized calcium
diphenylamide with the dialkylborane 9-BBN allows the syn-
thesis of a â-diketiminate-stabilized calcium borohydride along
with an amidoborane reaction coproduct Via a likely σ-bond
metathesis mechanism.
Scheme 2. Acid-Base Reactions of Heavier Alkaline Earth
Amides
σ-bond metathesis is a reaction of fundamental importance
to synthetic chemistry. As shown in Scheme 1, this reaction
permits the simultaneous construction and destruction of two
σ-bonds in a single step. Usually catalyzed by d0 transition
metal, lanthanide (4fn5d0), or actinide (5fn6d0) based catalysts,
the σ-bond metathesis mechanism has found application in a
host of dehydrocoupling reactions for the catalytic construction
of new heteroatom-heteroatom and carbon-heteroatom bonds,
important examples of which include the dehydropolymerization
of silanes1 and C-H activation of simple hydrocarbons.2-4 More
recently, σ-bond metathesis processes have received attention
for potential application in hydrogen generation and storage.
As work toward extending the scope of catalytic group 2
reactivity, we now report that the heteroleptic calcium amide 1
and the related â-diketiminate-stabilized calcium diphenylamide
2 participate in selective σ-bond metathesis (or acid-base)
reactions with the dialkylborane 9-borabicyclo[3.3.1]nonane (9-
BBN). These metathesis reactions allow for the synthesis of
the novel â-diketiminate-stabilized calcium borohydride 3 and,
perhaps more importantly, in the case of 2 an amidoborane
byproduct through the construction of a new nitrogen-boron
σ-bond.
Our interest in such reactivity stems from the study of well-
defined complexes of the heavier alkaline earths (Ca, Sr, and
Ba) and the supposition that direct parallels may be drawn
between reactions described as occurring via σ-bond metathesis
and classical acid-base chemistry. While stoichiometric acid-
base reactions have long been applied to the construction of
new alkaline earth-heteroatom bonds in both homoleptic and
heteroleptic organometallic complexes (Scheme 2),5 it has only
recently been shown that such reactivity may be included into
catalytic cycles reminiscent of trivalent organolanthanide chem-
istry, allowing the heterofunctionalization of unsaturated carbon-
carbon bonds.5g,6
The reaction of 1 with 1 equivalent of dimeric 9-BBN was
conducted on an NMR scale using C6D6 as a solvent and
monitored by H and 11B NMR spectroscopy. A slow reaction
1
occurred at room temperature over a period of 12 h, apparent
from the appearance of a series of new resonances in the H
NMR that were consistent with a coordination complex contain-
ing both the â-diketiminate ligand and a cyclic borane moiety.
1
11
11B
1
B NMR revealed a new product peak at -12.1 ppm (t, J H-
1
11B
) 68 Hz) similar to that of -13.3 ppm (d, J H- ) 50 Hz)
reported for the calcium borohydride [{1,2,4-C5(SiMe3)3H2}-
* To whom correspondence should be addressed. M.S.H.: fax, +44 (0)-
20 7594 5804; tel, +44 (0)207 594 5709; e-mail, mike.hill@
imperial.ac.uk.
(5) For general reviews, see: (a) Westerhausen, M. Coord. Chem. ReV.
1998, 176, 157. (b) Alexander, J. S.; Ruhlandt-Senge, K. Eur. J. Inorg.
Chem. 2002, 11, 2761. Specifically for the synthesis of Ca-X bonds in
heteroleptic â-diketiminato stabilized calcium complexes, see: (c) Chisholm,
M. H.; Gallucci, J. C.; Phomphari, K. Inorg. Chem. 2004, 43, 6717. (d)
Chisholm, M. H.; Gallucci, J. C.; Phomphari, K. Chem. Commun. 2003,
48. (e) Avent, A. G.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B. Dalton
Trans. 2004, 3166. (f) Avent, A. G.; Crimmin, M. R.; Hill, M. S.; Hitchcock,
P. B. Dalton Trans. 2005, 278. (g) Crimmin, M. R.; Barrett, A. G. M.;
Hill, M. S.; Hitchcock, P. B.; Procopiou, P. Organometallics 2007, 26, 2953.
(h) Avent, A. G.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.
Organometallics 2005, 24, 1184. (i) Ruspic, C.; Nembenna, S.; Hofmeister,
A.; Magull, J.; Harder, S.; Roesky, H. W. J. Am. Chem. Soc. 2006, 128,
15000. (j) Harder, S.; Brettar, J. Angew. Chem., Int. Ed. 2006, 45, 3474.
(k) Nembenna, S.; Roesky, H. W.; Nagendran, S.; Hofmeister, A.; Magull,
J.; Wilbrandt, P. J.; Hahn, M. Angew. Chem., Int. Ed. 2007, 46, 2512.
(6) (a) Crimmin, M. R.; Casely, I. J.; Hill. M. S. J. Am. Chem. Soc.
2005, 127, 2042. (b) Buch, F.; Brettar, J.; Harder, S. Angew. Chem., Int.
Ed. 2006, 45, 2741.
† Imperial College London.
‡ University of Sussex.
§ GlaxoSmithKline Medicines Research Centre.
(1) (a) Woo, H.-G.; Tilley, T. D. J. Am. Chem. Soc. 1989, 111, 8043.
(b) Woo, H.-G.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114,
5698. (c) Woo, H.-G.; Walzer, J. F.; Tilley, T. D. J. Am. Chem. Soc. 1992,
114, 7047. (d) Sadow, A. D.; Tilley, T. D. Organometallics 2001, 20, 4457.
(e) Sadow, A. D.; Tilley, T. D. Organometallics 2003, 22, 3577. (f) Tilley,
T. D. Acc. Chem. Res. 1993, 26, 22 and references therein.
(2) (a) Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276. (b)
Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6941. (c) Watson, P. L.;
Parshall, G. W. Acc. Chem. Res. 1985, 18, 51.
(3) (a) Bruno, J. W.; Marks, T. J.; Day, V. W. J. Am. Chem. Soc. 1982,
104, 7357. (b) Fendrick, C. M.; Marks, T. J. J. Am. Chem. Soc. 1984, 106,
2214.
(4) (a) Sadow, A. D.; Tilley, T. D. Angew. Chem., Int. Ed. 2003, 42,
803. (b) Sadow, A. D.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 7971.
(c) Sadow, A. D.; Tilley, T. D. J. Am. Chem. Soc. 2005, 127, 643.
10.1021/om070083t CCC: $37.00 © 2007 American Chemical Society
Publication on Web 06/29/2007

Notes
Scheme 3. Reaction of Calcium Amides 1 and 2 with the
Organometallics, Vol. 26, No. 16, 2007 4077
Dialkylborane 9-BBN
Figure 1. Isomers 3a and 3b of the isolated calcium borohydride.
Table 1. Selected Bond Distances (Å) and Bond Angles (deg)
for 3a and 3b
bond length
bond angle
3a
3b
3a
3b
Ca-H1a 2.27(4) 2.15(4) N2-Ca-N1
Ca-H1b 2.17(4) 2.31(4) O-Ca-H34
81.36(14) 82.18(13)
152.8(12)
87.4(10)
50.0(15)
B-H1a
B-H1b
1.17(4) 1.25(4) H1a-Ca-H1b 50.8(16)
Ca(HBEt3)(THF)2].7 The reaction time could be shortened to 2
h by performing the experiment at 60 °C. Accordingly, repetition
of the reaction on a preparative scale using toluene as a solvent,
extraction into hexane, and concentration by evaporation in
vacuo gave the molecular borohydride 3 as a colorless crystalline
solid (Scheme 3).8 Examination of the reaction byproducts by
1H NMR spectroscopy revealed a number of new species
incorporating the trimethylsilyl group. Similarly 11B NMR
revealed multiple products, including a resonance at 56.2 ppm
tentatively assigned as the amidoborane (Me3Si)2N-BR2 (vide
infra).
Due to the apparent fragility of the nitrogen-silicon bond
under these reaction conditions,9 we sought to synthesize a more
robust heteroleptic calcium amide species and study its reaction
with 9-BBN. We have previously reported that 1 reacts with
relatively acidic amines to yield the corresponding heteroleptic
calcium amides with liberation of HN(SiMe3)2.5e,f Indeed, the
stoichiometric reaction of 1 with diphenylamine allowed the
isolation of the â-diketiminate-stabilized calcium diphenylamide
2 (Scheme 3).10
1.28(5) 1.19(5) O-Ca-H1a
96.2(11) 140.1(11)
Ca‚‚‚H34 2.33(5) 2.30(4) O-Ca-H1b
81.1(12)
2.549(6) 2.543(6) H1a-Ca-H34 71.9(17)
C34-H34 1.05(5) 1.06(4) H1b-Ca-H34 69.4(16)
H1a-B-H1b 102(3)
90.6(12)
73.1(15)
68.2(15)
102(3)
Ca‚‚‚B
h at room temperature, most likely due to the decreased steric
demands of the diphenylamide coligand as well as the increased
Lewis basicity of the coordinated diphenylamide during the
course of reaction (vide infra). Analysis of the reaction by 11
B
NMR revealed, in addition to 3, a coproduct observed as a broad
singlet resonance at 53.0 ppm in C6D6 solution, a value
comparable to that of 49.6 ppm (in CCl4) reported for Me2B-
NPh2.11 The amidoborane 4, which was isolated by sublimation
directly from the crude reaction mixture, was characterized in
both solution and the solid state. Mass spectrometry showed a
1
diagnostic set of peaks at m/z 289/290 for [M]+, and both H
and 13C NMR are consistent with the proposed structure, which
has been confirmed unambiguously by an independent synthesis
(see the Supporting Information).12 Given that the diphenylamide
2 is synthesized from diphenylamine and 1, this two-step
sequence represents a calcium-mediated dehydrocoupling of an
amine with a dialkylborane.
The heteroleptic calcium diphenylamide 2 also reacted with
9-BBN to yield the calcium complex 3. In this case the reaction
was considerably faster than with 1 and was complete within 1
The calcium borohydride 3 was characterized by multinuclear
NMR and infrared spectroscopy and by single-crystal X-ray
diffraction. X-ray analysis of a single crystal of 3 revealed the
presence of two unique conformers, 3a and 3b, within the
asymmetric unit (Figure 1). The H atoms involved in interactions
with the calcium centers were refined, while other H atoms were
included in riding mode. Selected bond angles and distances
are presented in Table 1, and complete data are provided in the
Supporting Information. Each conformer demonstrates pseudo-
octahedral geometry at calcium, with the coordination sphere
being provided by the â-diketiminate ligand, two hydride units
(7) Harvey, M. J.; Hanusa, T. P.; Pink, M. Chem. Commun. 2000, 489.
(8) Reaction of 1 with 9-BBN: a stirred solution of 1 (500 mg, 0.74
mmol) and 9-BBN dimer (244 mg, 0.74 mmol) in toluene (10 mL) was
heated to 60 °C for 2 h. The reaction mixture was cooled to room
temperature and the solvent removed in vacuo. The crude product was then
extracted into hexane and the extract filtered. Slow removal of the solvent
induced crystallization, and the product 3 was isolated as a white crystalline
solid (116 mg, 0.178 mmol, 24%). Mp (hexane): 135 °C dec. 1H NMR
(d8-tol, 400 MHz, 298 K, δ): 0.67 (broad singlet, 2H), 1.14 (d, 12H, J )
6.8 Hz), 1.25 (d, 12H, J ) 6.8 Hz), 1.28 (m, 4H), 1.48-1.60 (m, 4 H),
1.57 (s, 6H), 1.70-1.78 (m, 4 H), 1.86-1.96 (m, 4 H), 3.06 (hept, 4H, J
) 6.8 Hz), 3.64 (m, 4H), 4.67 (s, 1H), 6.97-7.07 (m, 6H). 13C NMR (d8-
tol, 100 MHz, 298 K, δ): 24.3, 24.5 (broad signal), 25.0, 25.3, 26.0, 28.6,
34.6, 69.8, 93.4, 123.2, 137.4, 141.5, 145.4, 166.2. 11B NMR (d8-tol, 128
Hz, 298 K): -12.5 (t, J ) 68 Hz). Infrared (KBr disk, cm-1): 3057, 2960,
2923, 2867, 2827, 2754, 2054, 2023, 1931, 1618, 1543, 1459, 1398, 1314,
1262, 1169. Anal. Found for C41H65BCaN2O: C, 75.52; H, 9.88; N, 4.40.
Calcd: C, 75.43; H, 10.00; N, 4.30. Yield based upon NMR analysis using
tetrakis(trimethylsilyl)silane as an internal standard: 81%.
(11) Casanova, J.; Geisel, M. Inorg. Chem. 1974, 13, 2783.
(12) Reaction of 2 with 9-BBN: a solution of 2 (700 mg, 1.0 mmol)
and 9-BBN dimer (1.0 mmol) in toluene was stirred for 1 h at room
temperature. The crude product was then extracted into hexane and the
extract filtered. Slow removal of the solvent induced crystallization, and
the product 3 was isolated as a colorless crystalline solid (132 mg, 0.20
mmol, 20%). The solvent was removed from the filtrate, and the remaining
crude product sublimed at 130 °C and 4 × 10-1 mbar to yield the
amidoborane 4 as a colorless crystalline solid (35 mg, 0.12 mmol, 12%).
This latter product is moisture sensitive and could not be purified by
fractional crystallization, due to contamination by 3. Although 4 has been
reported in the literature,21 no characterization data have been described.
1H NMR (C6D6, 400 MHz, 298 K, δ): 1.44 (m, 2H), 1.48-1.55 (m, 2H),
1.82-1.87 (m, 8H), 1.92-2.02 (m, 2H), 6.91-6.95 (m, 2H), 7.05-7.07
(m, 8H). 11B NMR (C6D6, 128 Hz, 298 K, δ): 53.0. 13C NMR (C6D6, 100
MHz, 298 K, δ): 23.8, 24.7 (broad signal), 33.8, 125.2, 127.5, 129.0, 148.8.
Infrared (KBr disk, cm-1): 2917, 2885, 2837, 1591, 1560, 1491, 1451,
1409, 1338, 1296, 1259. MS (CI, ammonia, positive ion, m/z): 289/290
([M]+, 30%), 170 ([M - C8H13B]+, 100%). The structure was confirmed
by an independent synthesis from the reaction of B-MeO-9-BBN with
LiNPh2 (see Figure S1, Supporting Information).
(9) Wrackmeyer, B.; Schwarze, B.; Milius, W. J. Organomet. Chem.
1995, 489, 201.
(10) Synthesis of [[ArNC(Me)CHC(Me)NAr]Ca(NPh2)(THF)] (2): to an
unstirred solution of 1 (500 mg, 0.74 mmol) in hexane (20 mL) was added
diphenylamine (125 mg, 0.74 mmol) as a solution in the same solvent (5
mL). The addition was undertaken as to layer the two solutions, and the
reaction mixture was left to stand indisturbed. After 14 h the reaction yielded
large colorless crystals of the â-diketiminate-stabilized calcium amide (206
mg, 0.30 mmol, 41%). 1H NMR (400 MHz, C6D6, δ): 0.97 (m, 4H), 1.12
(d, 12H, J ) 6.8), 1.19 (d, 12H, J ) 6.8), 1.69 (s, 6H), 3.11 (m, 4H), 3.20
(hept, 4H, J ) 6.8), 4.85 (s, 1H), 6.53-6.56 (m, 6H), 6.91 (dd, 4H, J )
7.6, 8.0), 7.12-7.15 (m, 6H). 13C NMR (100.7 MHz, C6D6, δ): 24.5, 24.8,
25.1, 28.6, 69.0, 94.3, 115.8, 118.5, 124.4, 125.1, 130.4, 141.9, 146.0, 155.7,
166.4. Anal. Found for C45H59CaN3O: C, 77.55; H, 8.60; N, 5.97. Calcd:
C, 77.42; H, 8.51; N, 6.01.

4078 Organometallics, Vol. 26, No. 16, 2007
Notes
14 h resulted in only a small amount of irreversible (Schlenk-
like) redistribution to the known homoleptic compound [{(Ar)-
1
NC(Me)CHC(Me)N(Ar)}2Ca].15 In the H NMR spectrum of
compound 3 the protons of the methine unit of the borohydride
ligand exhibited a considerable upfield shift (ca. 0.7 ppm)
compared to those in both 9-BBN and the amidoborane 4 and
were apparent as an unresolved broad singlet at 0.67 ppm. This
assignment was confirmed by additional COSY experiments.
The observation of this increased shielding of the methine unit
implies that the agostic interaction of the H2BR2 moiety with
the calcium center is retained in solution on the NMR time scale.
Cooling of a d8-toluene solution of 3 resulted in a noticeable
broadening of the resonances attributed to the diisopropyl moiety
of the â-diketiminate ligand in the 1H NMR spectrum. At
temperatures below 223 K the isopropyl methine resonance,
Figure 2. ORTEP representations of 3a (left) and 3b (right)
(thermal ellipsoids at 20% probability). H atoms (with the exception
of the hydridic and agostic H atoms) are omitted for clarity.
of the H2BR2 moiety, a molecule of THF, and an apparent
agostic interaction with the methine unit adjacent to boron upon
the borohydride ligand (Figure 2). The conformers differ by
their coordination geometry at calcium. Conformer 3a displays
a pseudo-trans relationship between the agostic hydrogen and
the coordinated THF, while 3b demonstrates a pseudo-cis
relation between the same two substituents. In both isomers the
borate anion binds to calcium via two three-center/two-electron
Ca-H-B bridging interactions. The Ca-H bond distances take
values between 2.17 and 2.31(4) Å and compare well to the
calcium-hydride bond lengths of 2.21(4) and 2.10(3) Å quoted
for the calcium borohydride [{1,2,4-C5(SiMe3)3H2}Ca(HBEt3)-
(THF)2] and the recently described calcium hydride [{(Ar)NC-
(Me)CHC(Me)N(Ar)}CaH(THF)]2, respectively.5j,7 The geom-
etries at the calcium metal centers deviate significantly from
ideal octahedra, due to the tight bite angle provided by not only
the â-diketiminate (N1-Ca-N2 bond angles 81.36(14) and
82.18(13)°) but also the H2BR2 ligands (H-Ca-H bond angles
of 50.8(16) and 50.0(15)°). The geometry at boron is unambigu-
ously tetrahedral, and the boron-hydride bond lengths range
between 1.17(4) and 1.28(5) Å. In both conformers there is a
close contact between the calcium and boron metal centers with
Ca‚‚‚B distances of 2.549(6) Å in 3a and 2.543(6) Å in 3b.
The agostic interactions of the calcium centers with the methines
of the borohydride ligands are significant, with Ca‚‚‚H bond
lengths of 2.33(5) and 2.30(4) Å in 3a and 3b, respectively.
These interactions are only marginally longer than the calcium-
hydride distances quoted above and are considerably shorter
than the Ca‚‚‚H agostic interaction of 2.582 Å observed in the
related heteroleptic calcium anilide [{(Ar)NC(Me)CHC(Me)N-
(Ar)}Ca(NHAr)(THF)],5f suggestive of a significant degree of
bonding such that the borohydride moiety may be realistically
described as a tridentate ligand. Similar agostic interactions have
been observed in homoleptic lanthanide and alkali-metal boro-
hydride complexes13,14 and in the related compound [{1,2,4-
C5(SiMe3)3H2}Ca(HBEt3)(THF)2].7
3
apparent as a heptet at 3.06 ppm (4H, JH-H ) 6.8 Hz) in the
room-temperature spectrum, appeared as two broad but distinct
(2H by integration) resonances at 2.85 and 3.34 ppm. Over the
same temperature range the isopropyl methyl peaks changed in
a similar manner, and both sets of data yielded a Gibbs free
energy of activation, ∆Gq, of 43.8 kJ mol-1 for this process.
This fluxionality may be attributed to a hindered rotation about
the N-aryl bonds due to steric congestion provided by the
auxiliary ligands around calcium. We have previously observed
a similar behavior in the related heteroleptic compounds
[{(Ar)NC(Me)CHC(Me)N(Ar)}Ca(CtC-t-Bu)]2 and [{(Ar)NC-
(Me)CHC(Me)N(Ar)}Ca(Cp4Me)(THF)].5h Despite the observa-
tion of temperature-dependent fluxional behavior in the â-diketim-
inate spectator ligand, no decoalescence of the 11B NMR
resonance of 3 was observed down to temperatures of 193 K,
suggesting facile interconversion of the extremes represented
by the conformers 3a and 3b in solution.
The ease of this interconversion was assessed by DFT
calculations on the model complexes 5 and 6 [{(PhNC(H))2CH}-
Ca(OMe2)(µ-H2BMe2)], using the B3LYP density functional
theory and LANL2DZ pseudopotentials (and basis set) imple-
mented in Gaussian 03.16 The geometry optimizations were
performed by selecting initial geometries in which the sixth
coordination site about the calcium center was provided by a
short “agostic-type” interaction to the boron-bonded methyl
group. This was oriented with either a trans or cis configuration
to the coordinated dimethyl ether based upon simplified
coordinates provided by the solid-state configurations of 3a (for
5) and 3b (for 6), respectively. As expected, there was negligible
difference (ca. 3 kJ mol-1) between the overall energies of the
two calculated conformations, while the bond lengths and angles
of the fully optimized structures were, in general, within 2% of
the experimentally determined parameters. It is notable, how-
ever, that the Ca‚‚‚HC agostic distances were overestimated by
approximately 0.3 Å in both model structures. The calculated
ν(CH) vibrations associated with the “coordinated” methyls (5,
2897 cm-1; 6, 2918 cm-1), although significantly lowered by
interaction with the calcium centers, are also somewhat higher
than those observed in the solid state. It seems likely that both
of these discrepancies may be ascribed to the simplifications
(i.e., modified steric demands and decreased reduced mass),
which were introduced with the structures of 5 and 6 to reduce
computational cost.
Infrared analysis (KBr disk) confirmed the presence of the
borohydride moiety and showed both symmetric and asymmetric
ν(BH) stretching modes at 2054 and 2023 cm-1, respectively.
These values lie well within the range established for lanthanide-
(II) dihydridoborates of 2118-2012 cm-1 13
Notably, the
.
infrared spectrum also displayed a low-intensity and low-
frequency ν(CH) vibration at 2754 cm-1, which may be
attributed to the CH stretch of the methine unit weakened by
the agostic interaction with the calcium center.
The â-diketiminate-stabilized calcium borohydride proved
stable in solution. Heating samples of 3 in C6D6 to 60 °C for
We propose that the reaction leading to the formation of
compounds 3 and 4 proceeds via (i) initial formation of a borate
complex by coordination of the borane to the nitrogen of the
(13) (a) Chen, X.; Lim, S.; Plecnik, C. E.; Liu, S.; Du, B.; Meyers, E.
A.; Shore, S. G. Inorg. Chem. 2004, 43, 692. (b) Chen, X.; Lim, S.; Plecnik,
C. E.; Liu, S.; Du, B.; Meyers, E. A.; Shore, S. G. Inorg. Chem. 2005, 44,
6052.
(15) Harder, S. Organometallics 2002, 21, 3782.
(16) Frisch, M. J., et al. Gaussian 03, Revision C.02; Gaussian, Inc.,
Wallingford, CT, 2004.
(14) Knizek, J.; No¨th, H. J. Organomet. Chem. 2000, 614-615, 168.

Notes
Organometallics, Vol. 26, No. 16, 2007 4079
diphenylamide ligand of 2, (ii) decomposition of the latter
species to a coordinatively unsaturated heteroleptic calcium
hydride and 4, and (iii) subsequent trapping of this calcium
hydride with a further 1 equiv of the dialkylborane to form the
calcium borohydride 3. This hypothesis is supported by several
literature precedents. While the reaction of group 1 amides and
alkoxides with hydridodialkylboranes to yield the corresponding
metal borohydride complexes is well established,17 the inter-
mediacy of a heteroleptic calcium hydride is also supported by
the documented synthesis of Ca(BH4)2 by the reaction of CaH2
with B2H6.18,19 Further evidence for this hydride intermediate
was provided by an NMR-scale reaction of [{(Ar)NC(Me)CHC-
(Me)N(Ar)}CaH(THF)]2, generated in situ, with 9-BBN, which
also gave the borohydride 3.
dialkylborane yielded the novel calcium borohydride 3 along
with an amidoborane side product. The latter compound
represents the construction of a new nitrogen-boron bond by
a dehydrocoupling reaction, and the potential remains to render
this reaction catalytic with respect to the group 2 organometallic
reagent by interception of the calcium hydride intermediate. We
are continuing to study the scope of this reaction and to
investigate the possible application of group 2 mediated σ-bond
metathesis reactions for the catalytic construction of new
heteroatom-heteroatom and carbon-heteroatom bonds.
Acknowledgment. We thank GlaxoSmithKline for a gener-
ous endowment (to A.G.M.B.), the Royal Society for a
University Research Fellowship (M.S.H.) and Royal Society
Wolfson Research Merit Award (A.G.M.B.), and the Engineer-
ing and Physical Sciences Research Council and GlaxoSmith-
Kline for generous support of our studies. We also thank Peter
Haycock for invaluable assistance with variable-temperature and
multinuclear NMR spectroscopy.
In conclusion, the stoichiometric σ-bond metathesis reaction
of a readily accessible heteroleptic calcium amide with a
(17) Singaram, B.; Jackson, T. C.; Harrison, J. Synlett 1996, 383.
(18) Koester, R.; Huber, H. Inorg. Synth. 1977, 17.
(19) The reaction of 1 with some alternative common group 13 hydrides
proved unsuccessful. Thus, the reaction of 1 with BH3‚OEt2 resulted in the
isolation of a product in which the â-diketiminate ligand had undergone
reduction by the borane complex (see Figure S2, Supporting Information).
These results demonstrate the fragility of the complex 1 as a model for
developing a reaction chemistry of calcium and suggest the properties of
the substrate are important for a selective σ-bond metathesis reaction to
occur. In this respect, it is likely that the bulky bicycloborane moiety acts
to temper further reactivity by sterically encapsulating the reactive calcium
center.
Supporting Information Available: Text and figures giving
full experimental details and characterization data and CIF files
giving crystal data for 3 and 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM070083T