Bis(imidazolin-2-ylidene-l-yl)borate complexes of the heavier alkaline earths: Synthesis and studies of catalytic hydroamination

Article abstract of DOI:10.1021/om8010933

Heteroleptic complexes of the heavier alkaline earth elements, calcium and strontium, containing bis(imidazolin-2-ylidene-l-yl)borate ligands may be synthesized by deprotonation of boronium salt ligand precursors with [KN(SiMe₃)₂] in the presence of CaI₂ or SrI₂. The silylamide complexes [{H₂B(ImBu) ₂}M{N(SiMe₃)₂}(THF)_n] (M = Ca, n = 1; M = Sr, n = 2), containing N-tert-butyl- substituted imidazolin-2-ylidene fragments, are stable to intermoleeular solution redistribution despite some evidence of solution fluxionality in the case of M = Ca. Attempts to synthesize heteroleptic iodide-containing species led to the isolation of the homoleptic compounds [{H₂B(ImBu)₂}₂MTHF)_n] (M = Ca, n = 1; M = Sr, n = 2) most likely because of the reduced steric demands of the halide coligand. Although extension of the "one pot" synthetic method to precursors containing N-mesityl-substituted imidazoles resulted in extensive ligand degradation, use of [Ca{N(SiMe₃)₂j ₂(THF)₂] as both base and group 2 element source provided the target heteroleptic silylamide complex. X-ray diffraction analyses demonstrated that the borate ligands adopt a facial n³-binding mode in which coordination is provided through the two N-heterocyclic carbene (NHC) donors and an additional agostic-type interacton from one hydrogen of the BH₂ residue. Analysis of this ligand binding by DFT methods indicates that, although the bonds from the carbon centers are similar in character to those provided by neutral NHC ligands, the bonding is supplemented by an electrostatic component which is manifested as a coordination of a hydridic BH bond. The ionic nature of the overall interaction is sufficient to maintain the metal-ligand binding in the presence of amine donors and has allowed the evaluation of the Ca and Sr complexes [{H₂B(ImBu)₂) M{N(SiMe₃)₂}(THF)_n] as precatalysts for the intramolecular hydroamination of ami-noalkenes. While the calcium catalyst provides activities commensurate with our previously reported β-diketiminato complex, [CH{C(Me)N(2,6-Pr₂C₆H ₃)}Ca{N(SiMe₃)₂}(THF)], the strontium derivative is superior in all cases.

Full text of DOI:10.1021/om8010933

1730
Organometallics 2009, 28, 1730–1738
Bis(imidazolin-2-ylidene-1-yl)borate Complexes of the Heavier
Alkaline Earths: Synthesis and Studies of Catalytic Hydroamination
Merle Arrowsmith, Michael S. Hill,* and Gabriele Kociok-Ko¨hn
Department of Chemistry, UniVersity of Bath, ClaVerton Down, Bath, BA2 7AY, United Kingdom
ReceiVed NoVember 17, 2008
Heteroleptic complexes of the heavier alkaline earth elements, calcium and strontium, containing
bis(imidazolin-2-ylidene-1-yl)borate ligands may be synthesized by deprotonation of boronium salt ligand
precursors with [KN(SiMe3)2] in the presence of CaI2 or SrI2. The silylamide complexes
[{H2B(ImtBu)2}M{N(SiMe3)2}(THF)n] (M ) Ca, n ) 1; M ) Sr, n ) 2), containing N-tert-butyl-
substituted imidazolin-2-ylidene fragments, are stable to intermolecular solution redistribution despite
some evidence of solution fluxionality in the case of M ) Ca. Attempts to synthesize heteroleptic iodide-
containing species led to the isolation of the homoleptic compounds [{H2B(ImtBu)2}2MTHF)n] (M )
Ca, n ) 1; M ) Sr, n ) 2) most likely because of the reduced steric demands of the halide coligand.
Although extension of the “one pot” synthetic method to precursors containing N-mesityl-substituted
imidazoles resulted in extensive ligand degradation, use of [Ca{N(SiMe3)2}2(THF)2] as both base and
group 2 element source provided the target heteroleptic silylamide complex. X-ray diffraction analyses
demonstrated that the borate ligands adopt a facial η3-binding mode in which coordination is provided
through the two N-heterocyclic carbene (NHC) donors and an additional agostic-type interacton from
one hydrogen of the BH2 residue. Analysis of this ligand binding by DFT methods indicates that, although
the bonds from the carbon centers are similar in character to those provided by neutral NHC ligands, the
bonding is supplemented by an electrostatic component which is manifested as a coordination of a hydridic
BH bond. The ionic nature of the overall interaction is sufficient to maintain the metal-ligand binding
in the presence of amine donors and has allowed the evaluation of the Ca and Sr complexes
[{H2B(ImtBu)2}M{N(SiMe3)2}(THF)n] as precatalysts for the intramolecular hydroamination of ami-
noalkenes. While the calcium catalyst provides activities commensurate with our previously reported
ꢀ-diketiminato complex, [CH{C(Me)N(2,6-iPr2C6H3)}Ca{N(SiMe3)2}(THF)], the strontium derivative is
superior in all cases.
compound to undergo solution (Schlenk-type) redistribution
equilibria.4 This feature, coupled with a tendency toward
protonation by substrates of lower pKa than the free ꢀ-imi-
doimine ligand precursor, has been a source of particular
frustration in our efforts to understand and study several catalytic
reactions. Chisholm has made similar observations and has
reported, for example, that the reaction of I with iso-propanol
resultsintheisolationofthehomolepticspecies[CH{C(Me)N(2,6-
iPr2C6H3)}2Ca] rather than the desired heteroleptic alkoxide.3b
The observation that the ꢀ-diketiminate anion provided insuf-
ficient kinetic protection when combined with coligands of
reduced steric demands led this group to use the ubiquitous
tris(pyrazolyl)borate ligands in the synthesis of calcium com-
plexes such as II for the catalytic polymerization of rac-lactide.3b
We have recently described the synthesis of several N-
heterocyclic carbene (NHC) adducts of the homoleptic alkaline
earth silylamides, [(NHC)M{N(SiMe3)2}2] (M ) Ca, Sr, Ba).5
Although these complexes are somewhat more tolerant of
ambient moisture than the uncomplexed silylamides, the NHC
ligand is easily displaced by harder neutral bases such as amines
and phosphine oxides. Similar observations in 4f element
chemistry have led to the successful development of NHC-
Introduction
The successful application of coordination complexes in
homogeneous catalysis has relied to a large extent upon the use
of suitable coligands. Selection of appropriate donor identity,
denticity, charge and ligand topology has, particularly in
transition metal catalysis, resulted in a sophisticated appreciation
of steric and electronic factors in the management of catalytic
turnover. Our own,1 and others’,2 efforts in the development of
a defined catalytic reaction chemistry for the heavier group 2
elements (Ca, Sr and Ba) have relied upon compounds such as
Chisholm’s ꢀ-diketiminato complex (I) as a convenient proto-
type reagent and have yielded a plethora of new compounds.3
Although a wide range of derivatives and new reaction types
have been facilitated through the use of compound I, an
underlying concern has always been the propensity of this
* To whom correspondence should be addressed. E-mail: msh27@
bath.ac.uk.
(1) For example: (a) Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am.
Chem. Soc. 2005, 127, 2042. (b) Crimmin, M. R.; Barrett, A. G. M.; Hill,
M. S.; Hitchcock, P. B.; Procopiou, P. A. Organometallics 2007, 26, 2953.
(c) Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; Hitchcock, P. B.;
Procopiou, P. A. Organometallics 2008, 27, 497.
(2) For uses as catalysts for intramolecular hydroamination, see: (a)
Datta, S.; Roesky, P. W.; Blechert, S. Organometallics 2007, 26, 4392. (b)
Datta, S.; Gamer, M. T.; Roesky, P. W. Organometallics 2008, 27, 1207.
(c) Buch, F.; Harder, S. Z. Naturforsch B, J. Chem. Sci. 2008, 63, 169.
(3) (a) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Chem. Comm.
2003, 48. (b) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem.
2004, 43, 6717.
(4) Avent, A. G.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B. Dalton
Trans. 2004, 3166.
(5) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Kociok-Ko¨hn, G.;
MacDougall, D. J.; Mahon, M. F.; Procopiou, P. A. Organometallics 2008,
27, 3939.
10.1021/om8010933 CCC: $40.75
2009 American Chemical Society
Publication on Web 02/27/2009

Synthesis and Studies of Catalytic Hydroamination
Organometallics, Vol. 28, No. 6, 2009 1731
atmosphere of either dinitrogen or argon. NMR experiments were
conducted in Youngs tap NMR tubes made up and sealed in an
MBraun Labmaster Glovebox. NMR spectra were collected on
either a Bruker AV-400 spectrometer (13C{1H} NMR 100 MHz,
11B{1H} NMR 96.3 MHz), or a Bruker AV-300 spectrometer
1
(13C{1H} NMR 75 MHz). BH proton resonances in the H NMR
spectra were observed through selective decoupling from the 11B
Nuclei. Solvents (toluene, benzene, THF, hexane) were dried by
passage through the columns of a commercial solvent purification
system. C6D6 and d8-toluene were purchased from Goss Scientific
Instruments Ltd. and dried over molten potassium before distillation
under nitrogen and storage over molecular sieves. The heavier group
2 amides [M{N(SiMe3)2}2(THF)2] (M ) Ca, Sr),10 tert-butylimi-
dazole,11 mesitylimidazole,12 Me3N.BH2I,13 Me3N.BH2Br14 and all
aminoalkenes were prepared by literature procedures.15 Attempts
to acquire satisfactory elemental analyses on compounds 7, 8 and
containing ligands in which the ligand is tethered to the hard
electropositive center by an anionic alkoxo or amido functional-
ity.6 With these observations in mind, our attention was drawn
to the bis- and tris(imidazolin-2-ylidenyl)borate ligands, III and
IV, as possibly useful scaffolds for the stabilization of catalyti-
cally active heavier group 2 centers. Ligands of the general
structure IV bear a clear topological relationship to the ligand
utilized in the synthesis of II. We also hoped that the anionic
nature of the NHC-based borate ligands would provide a stable
and electrostatically bound coordination pocket for the group 2
center while the carbon-centered donors would display a reduced
tendency toward the µ-metal bridging interactions which are
probable intermediates in the redistributive processes outlined
above.
Although bi- and tridentate ligands exemplified by the general
structures III and IV were first reported by Fehlhammer in the
late 1990s,7 the use of these ligands, in either transition element
or main group chemistry, is relatively limited. Smith has more
recently provided several reports in which the anionic ligand is
generated by reaction of a lithium or magnesium base with a
boronium salt ligand precursor.8,9 While initial studies were
centered upon deprotonation of the potentially tridentate species
[HB(tBuIm)3]Br2 (tBuIm ) 1-tert-butylimidazole), more recent
explorations have advanced to include the bidentate anion
[H2B(tBuIm)2], which has been isolated as the lithium and nickel
derivatives [H2B(tBuIm)2Li]2 and [{H2B(tBuIm)2}2Ni].8d In this
contribution we describe our initial syntheses of a series of
bis(imidazol-2-ylidene)borate complexes of calcium and stron-
tium along with some preliminary observations of silylamide
derivatives in intramolecular hydroamination catalysis.
1
9 were unsuccessful; H spectra of the isolated compounds are
provided in the Supporting Information as corroborative evidence
of purity.
Synthesis of Dihydro-bis(imidazole)boronium Bromides and
Iodides (Compounds 1-4). General Procedure. Two molar
equivalents of the relevant imidazole and one molar equivalent of
Me3NBH2X (X ) Br, I) were stirred at reflux (133 °C) under argon
in dry chlorobenzene for 3-5 h (X ) I) or 10-15 h (X ) Br).
Upon cooling to room temperature, a white precipitate formed. The
solvent was removed in Vacuo, the crude white solid was dissolved
in CH2Cl2, precipitated with Et2O and dried consecutively in air
and on a vacuum line for a day to remove traces of residual
chlorobenzene. Crystallization solvents are noted for the individual
reactions.
[H2B(ImtBu)2]I, 1. 1-tert-Butyl-imidazole (6.00 g, 48.3 mmol)
and Me3NBH2I (4.80 g, 24.2 mmol). Crystallized from water:
1
colorless solid (7.72 g, 82%), mp 135 °C. H NMR ppm (CDCl3,
298 K): 9.21 (s, 1H, ring-NCHN), 7.16 (d, 1H, ring-CHdCHNtBu,
3
3J ) 1.8 Hz), 7.12 (d, 1H, ring-CHdCHNtBu, J ) 1.8 Hz), 3.43
(br. s, 2H, BH2), 1.70 (s, 9H, CH3). 13C{1H} NMR ppm (CDCl3,
298 K): 138.3 (ring-NCHN), 130.0 (ring-CHdCHNtBu), 126.0
(ring-CHdCHNtBu), 59.2 (CCH3), 30.9 (CH3). >11B{1H} NMR ppm
(CDCl3, 298 K): -7.60. IR (Nujol): υB-H 2434 cm-1, br. Anal.
Calcd for C14H26BIN4: C, 43.33; H, 6.75; N, 14.44%. Found: C,
43.0; H, 6.70; N, 14.0%.
[H2B(ImtBu)2]Br, 2. 1-tert-Butyl-imidazole (2.00 g, 16.4 mmol)
and Me3N.BH2Br (1.25 g, 8.2 mmol). Crystallized from CH2Cl2:
colorless solid (2.92 g, 100%), mp 110 °C. 1H NMR ppm (CDCl3,
4
298 K): 9.49 (t, 1H, ring-NCHN, J ) 1.5 Hz), 7.15 (t, 1H, ring-
CHdCHNtBu, 3/4J ) 1.5 Hz), 7.08 (t, 1H, ring-CHdCHNtBu, 3/4J
) 1.5 Hz), 3.43 (br. s, 2H, BH2), 1.70 (s, 9H, C(CH3)3). 13C{1H}
NMR ppm (CDCl3, 298 K): 130.1 (ring-NCHN), 129.0 (ring-
CHdCHNtBu), 126.0 (ring-CHdCHNtBu), 59.0 (CCH3), 30.8
(CH3). 11B{1H} NMR ppm (CDCl3, 298K): -9.09. IR (Nujol): υB-H
2434 cm-1, br. Anal. Calcd for C14H26BBrN4: C, 49.2; H, 7.53; N
16.67%. Found: C, 49.30; H, 7.68; N, 16.43%.
Experimental Section
General Procedures. All manipulations were carried out using
standard Schlenk line and glovebox techniques under an inert
(6) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. ReV. 2007,
36, 1732.
[H2B(ImMes)2]I, 3. 1-Mesityl-imidazole (2.00 g, 10.7 mmol)
and Me3N.BH2I (1.07 g, 5.4 mmol). Crystallized from CHCl3:
(7) (a) Kernbach, U.; Ramm, M.; Luger, P.; Fehlhammer, W. P. Angew.
Chem., Int. Ed. Engl. 1996, 35, 310. (b) Fra¨nkel, R.; Birg, C.; Kernbach,
U.; Habereder, T.; No¨th, H.; Fehlhammer, W. P. Angew. Chem., Int. Ed.
2001, 40, 1907. (c) Fra¨nkel, R.; Kernbach, U.; Bakola-Christianopoulou,
M.; Plaia, U.; Suter, M.; Ponikwar, W.; No¨th, H.; Moinet, C.; Fehlhammer,
W. P. J. Organomet. Chem. 2001, 617-618, 530. (d) Fra¨nkel, R.; Kniczek,
J.; Ponikwar, W.; No¨th, H.; Polborn, K.; Fehlhammer, W. P. Inorg. Chim.
Acta 2001, 312, 23.
1
colorless solid (2.35 g, 85%), mp 172 °C. H NMR ppm (CDCl3
298 K): 9.20 (s, 1H, ring-NCHN), 7.60 (s, 1H, ring-CHdCHNMes),
(10) (a) Hitchcock, P. B.; Lappert, M. F.; Lawless, G. A.; Royo, B.
J. Chem. Soc., Chem. Commun. 1990, 1141. (b) Westerhausen, M. Inorg.
Chem. 1991, 30, 96.
(8) (a) Nieto, I.; Cervantes-Lee, F.; Smith, J. M. Chem. Commun. 2005,
3811. (b) Cowley, R. E.; Bontchev, R. P.; Duesler, E. E.; Smith, J. M.
Inorg. Chem. 2006, 45, 9771. (c) Forshaw, A. P.; Bontchev, R. P.; Smith,
J. M. Inorg. Chem. 2007, 46, 3792. (d) Nieto, I.; Bontchev, R. P.; Smith,
J. M. Eur. J. Inorg. Chem. 2008, 2476. (e) Nieto, I.; Ding, F.; Bontchev,
R. P.; Wang, H.; Smith, J. M. J. Am. Chem. Soc. 2008, 130, 2716. (f)
Scepaniak, J. J.; Fulton, M. D.; Bontchev, R. P.; Duesler, E. E.; Kirk, M. L.;
Smith, J. M. J. Am. Chem. Soc. 2008, 130, 10515.
(11) Hofmann, K. The Chemistry of Heterocyclic Compounds, Imidazole
and Its DeriVatiVes, Part 1; Wiley Interscience: New York, 1956.
(12) Collman, J. P.; Wang, Z.; Zhong, M.; Zeng, L. J. Chem. Soc., Perkin
Trans. 2000, 1217.
(13) Mathur, M. A.; Ryschkewitsch, G. E.; Swicker, B. F.; Carter, J. C.
Inorg. Synth. 1975, 12, 120.
(14) Mathur, M. A.; Moore, D. A.; Popham, R. E.; Sisler, H. H.; Dolan,
S.; Shore, S. G. Inorg. Synth. 1992, 29, 51.
(15) Gagne´, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992,
114, 275.
(9) For an application of a Cu(I) complex, see: Tubaro, C.; Biffis, A.;
Scattolin, E.; Basato, M. Tetrahedron 2008, 64, 4187.

1732 Organometallics, Vol. 28, No. 6, 2009
Arrowsmith et al.
7.05 (s, 1H, ring-CHdCHNMes), 6.98 (s, 2H, Ar-m-H), 3.67 (br.
s, 2H, BH2), 2.33 (s, 3H, p-CH3), 2.04 (s, 6H, o-CH3). 13C{1H}
NMR ppm (CDCl3, 298 K): 141.1 (ring-NCHN), 140.2 (Ar-i-C),
134.8 (ring-CHdCHNMes), 130.1 (Ar-m-C), 126.8 (Ar-p-C), 123.2
(Ar-o-C), 115.1 (ring-CHdCHNMes), 21.5 (p-CH3), 18.2 (o-CH3).
11B{1H} NMR ppm (CDCl3, 298 K): -6.10. IR (Nujol): υB-H 2444
cm-1, br. Anal. Calcd for C24H30BIN4: C, 56.27; H, 5.90; N, 10.94%.
Found: C, 56.7; H, 5.89; N, 10.7%.
[H2B(ImMes)2]Br, 4. 1-mesityl-imidazole (2.00 g, 10.74 mmol)
and Me3N.BH2Br (0.82 g, 5.37 mmol). Crystallized from CHCl3:
white solid (1.30 g, 52%), mp 190-192 °C. 1H NMR ppm (CDCl3,
298 K): 9.55 (s, 1H, ring-NCHN), 7.60 (s, 1H, ring-CHdCHNMes),
7.00 (s, 1H, ring-CHdCHNMes), 6.97 (s, 2H, Ar-m-H), 2.32 (s,
3H, p-CH3), 2.04 (s, 6H, o-CH3). 13C{1H} NMR ppm (CDCl3, 298
K): 140.4 (ring-NCHN), 140.3 (Ar-i-C), 134.3 (Ar-m-C), 129.4
(ring-CHdCHNMes), 126.1 (Ar-p-C), 122.1 (Ar-o-C), 114.4 (ring-
CHdCHNMes), 20.9 (p-CH3), 17.4 (o-CH3). 11B{1H} NMR ppm
(CDCl3, 298 K): -7.90. IR (Nujol): υB-H 2441 cm-1, br. Anal.
Calcd for C24H30BBrN4: C, 61.91; H, 6.50; N, 12.04%. Found: C,
61.7; H, 6.48; N, 11.9%.
(nonagostic B-H), 2252 (agostic B-H) cm-1. Anal. Calcd for
C29.5H61.5BN5O2Si2Sr (937.82): C, 52.63; H, 9.21; N, 10.41%.
Found: C, 52.75; H, 9.43; 10.33.
[{H2B(ImtBu)2}2Ca(THF)], 7. Redistribution product from the
attempted synthesis of [{H2B(ImtBu)2}CaI(THF)]: Compound 1
(0.50 g, 1.29 mmol), calcium iodide (0.38 g, 1.29 mmol) and
KN(SiMe3)2 (0.51 g, 2.58 mmol): colorless crystals after 3 days at
-20 °C (0.20 g, 49% based on CaI2). 1H NMR/ ppm (C6D6): 7.26
3
3
(d, 4H, J ) 1.5 Hz, ring-CHdCHNtBu), 6.60 (d, 4H, J ) 1.5
Hz, ring-CHdCHNtBu), 4.44 (br. s, 2H, BH2), 3.59 (m, 4H, THF),
1.38 (m, 4H, THF), 1.36 (s, 36H, CH3). 13C{1H} NMR ppm (C6D6,
298 K): 195.0 (carbene-C), 124.0 (ring-CHdCHNtBu), 113.3 (ring-
CHdCHNtBu), 66.7 (THF), 53.8 (CCH3), 30.0 (THF), 24.3 (CH3).
11B{1H} NMR ppm (C6D6, 298 K): -5.42. IR (Nujol): υB-H 2363
(nonagostic B-H), 2255 (agostic B-H) cm-1
.
[{H2B(ImtBu)2}2Sr(THF)2], 8. Redistribution product from the
attempted synthesis of [{H2B(ImtBu)2}SrI(THF)]2: Compound 1
(0.50 g, 1.29 mmol), strontium iodide (0.44 g, 1.29 mmol) and
KN(SiMe3)2 (0.51 g, 2.58 mmol): colorless crystals after 3 days at
-20 °C (0.21 g, 43%). Loses one molecule of THF on isolation.
1H NMR ppm (C6D6, 298 K): 7.29 (d, 4H, 3J ) 1.5 Hz,
ring-CHdCHNtBu), 6.63 (d, 4H, 3J ) 1.5 Hz, ring-CHdCHNtBu),
4.44 (br. s, 2H, BH2), 3.67 (m, 4H, THF), 1.30 (s, 36H, CH3), 1.30
(m, 4H, THF). 13C{1H} NMR ppm (C6D6 298 K): 198.9 (carbene-
C), 124.3 (ring-CHdCHNtBu), 113.1 (ring-CH dbd)CHNtBu), 67.2
(THF), 53.7 (CCH3), 30.0 (THF), 24.2 (CH3). 11B{1H} NMR ppm
(C6D6 298 K): -5.64.
Synthesis of Bis(imidazolyl-2-ylidenyl)borate Alkaline Earth
Compounds 5-8. General Procedure. In a glovebox, the ap-
propriate ligand precursor (1-4), calcium iodide and potassium
bis(trimethylsilyl)amide were weighed at a 1:1:3 or 4 ratio into a
dry Schlenk flask. Dry THF was added at -78 °C and the mixture
was left to stir at room temperature overnight until the calcium
iodide beads had been consumed. The solvent was removed in
Vacuo and hexane/toluene added. The resultant milky solution was
stirred for another hour then allowed to settle prior to filtration and
concentration of the solution to incipient crystallization. After 3-7
days at -20 °C, the mother liquor was filtered into a second Schlenk
and the transparent crystals completely dried on vacuum line.
Samples suitable for X-ray diffraction analyses were selected under
a cryogenic flow of dinitrogen to prevent melting at ambient
temperatures.
[{H2B(ImMes)2}Ca{N(SiMe3)2}(THF)], 9. Compound 3 (0.74
g, 1.46 mmol), [Ca{N(SiMe3)2}2(THF)2] (0.50 g, 0.98 mmol): light-
pink powder (0.14 g, 50%). 1H NMR ppm (C6D6 298 K): 7.21 (d,
3
2H, J ) 1.5 Hz, ring-CHdCHNMes), 6.77 (s, 4H, Ar-H), 6.27
3
(d, 2H, J ) 1.5 Hz, ring-CHdCHNMes), 4.41 (br. s, 2H, BH2),
3.52 (m, 4H, THF), 2.12 (s, 6H, Ar-p-CH3), 1.93 (s, 2H, Ar-o-
CH3), 1.14 (m, 4H, THF), 0.12 (s, 18H, Si(CH3)3). 13C{1H} NMR
ppm (C6D6 298 K): 196.0 (carbene-C), 137.1 (Ar-i-C), 136.2 (Ar-
m-C), 135.0 (Ar-p-C), 128.2 (Ar-o-C), 123.6 (ring-CHdCHNMes),
117.7 (ring-CHdCHNMes), 67.6 (THF), 23.7 (THF), 19.6 (Ar-p-
CH3), 16.4 (Ar-o-CH3), 3.9 (Si(CH3)3). 11B{1H} NMR ppm (C6D6
298 K): -5.87.
[{H2B(ImtBu)2}Ca{N(SiMe3)2}(THF)], 5. (a)Compound 2 (2.08
g, 6.1 mmol), calcium iodide (1.79 g, 6.1 mmol) and KN(SiMe3)2
(3.66 g, 18.3 mmol): pale yellow transparent solid, very air- and
temperature- sensitive, oily solid at 21 °C; crystalline at -16 °C
(0.55 g, 18%). (b) Compound 1 (2.00 g, 5.2 mmol), calcium iodide
(1.51 g, 5.2 mmol) and KN(SiMe3)2 (3.08 g, 15.5 mmol): off-white
powder (1.55 g, 56%). 1H NMR ppm (C6D6, 298 K): 7.23 (d, 2H,
ring-CHdCHNtBu, 3J ) 1.5 Hz), 7.13 (s, 2H, ring-CHdCHNtBu),
Crystallographic Data. Data for 1, 3, 6, 7 and 10 were collected
at 150 K on a Nonius KappaCCD diffractometer equipped with an
Oxford Cryosystem, using graphite monochromated Mo KR radia-
tion (λ ) 0.71073 Å). Data were processed using the Nonius
Software.16
3
6.59 (d, 2H, ring-CHdCHNtBu, J ) 1.5 Hz), 6.52 (s, 2H, ring-
For 1, 3, 6 and 10 a symmetry-related (multiscan) absorption
correction had been applied. Crystal parameters and details on data
collection, solution and refinement for the complexes are provided
in Table 1. Structure solution, followed by full-matrix least-squares
refinement was performed using the WINGX-1.70 suite of programs
throughout.18
CHdCHNtBu), 4.31 (br. s, 2H, BH2), 3.63 (m, 4H, THF), 1.36
(m, 4H, THF), 1.31 (s, 18H, CH3), 0.45 (broad s, 18H, Si(CH3)3).
13C{1H} NMR ppm (C6D6, 298 K): 195.0, (carbene-C), 125.0, (ring-
CHdCHNtBu), 114.4 (ring-CHdCHNtBu), 68.6 (THF), 54.8
(THF), 31.1 (CCH3), 25.0 (CCH3), 2.3 (Si(CH3)3). 11B{1H} NMR
ppm (C6D6, 298 K): -5.8. In d8-toluene, the imidazol-2-ylidene
1H NMR signals split at low temperatures. IR (Nujol): υB-H 2385
(non agostic B-H), 2252 (agostic B-H) cm-1. Anal. Calcd for
C24H52BCaN5OSi2 (533.8): C, 54.00; H, 9.82; N, 13.12%. Found:
C, 54.09; H, 9.41; 13.23.
For compounds 1 and 3 the -BH2 hydrogen atoms were identified
in the difference Fourier map and refined freely. In the structure of
6, the main compound contained one disordered (80:20)THF ligand
and the voids contained some solvent which could be modeled as
hexane since the NMR data indicated that hexane was present. Each
main molecule cocrystalized with 1/4 of hexane which was
disordered about a center of inversion. For 7, the sample consisted
of low melting crystals which were selected using a modified device,
similar to that of Veith and Ba¨rnighausen.17 The crystals turned
out to be very weakly diffracting which resulted in a high R(int)
[{H2B(ImtBu)2}Sr{N(SiMe3)2}(THF)2], 6. Compound 1 (1.00
g, 2.6 mmol), strontium iodide (0.88 g, 2.6 mmol) and KN(SiMe3)2
(1.54 g, 7.7 mmol): colorless crystals after 1 day at -20 °C from
hexane (1.85 g, 76%), mp 137-140 °C. 1H NMR ppm (C6D6, 298
K): 7.23 (broad s, 2H, ring-CHdCHNtBu), 6.62 (s, 2H, ring-
CHdCHNtBu), 4.32 (br. s, 2H, BH2), 3.59 (m, 8H, THF), 1.35 (s,
18H, CH3), 1.30 (m, 8H, THF), 0.37 (s, 18H, Si(CH3)3). 13C{1H}
NMR ppm (C6D6 298 K): 201.5 (carbene-C), 126.0 (ring-
CHdCHNtBu), 115.2 (ring-CHdCHNtBu), 69.0 (THF), 55.5
(THF), 32.3 (CCH3), 25.8 (CH3), 5.9 (Si(CH3)3). 11B{1H} NMR
ppm (C6D6 298 K): -5.7. NMR also showed presence of hexane
in the lattice; confirmed by X-ray study. IR (Nujol): υB-H 2363
(16) DENZO-SCALEPACK: Otwinowski Z. and Minor, W. Processing
of X-ray Diffraction Data Collected in Oscillation Mode; Methods in
Enzymology Volume 276: Macromolecular Crystallography, part A; Carter,
C. W., Jr., Sweet R. M., Eds., Academic Press: New York, 1997; pp 307-
326.
(17) Veith, M.; Ba¨rnighausen, H. Acta Crystallogr. B 1974, 30, 1806.
(18) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.

Synthesis and Studies of Catalytic Hydroamination
Organometallics, Vol. 28, No. 6, 2009 1733
Table 1. Crystallographic Data for Compounds 1, 3, 6, 7 and 10
1
3
6
7
10
Molecular formula
C14H26BIN4
388.10
C24H30BIN4
512.23
C29.5H61.5BN5O2Si2Sr C32H56B2CaN8O
C63H80B2Ca2I2N8O2
1336.93
Formula weight (g mol-1
)
672.95
630.55
Crystal system
Space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
monoclinic
P21/c
11.9656(2)
10.0655(1)
15.1879(2)
90
96.405(1)
90
1817.81(4)
4
monoclinic
P21/m
8.2566(1)
19.2495(3)
8.8410(1)
90
115.216(1)
90
1271.25(3)
2
tetragonal
P42/n
20.5450(1)
20.5450(1)
18.9151(2)
90
orthorhombic
Pbca
16.0309(3)
19.9759(4)
23.0604(5)
90
90
90
7384.7(3)
8
0.205
triclinic
P1
j
12.7284(4)
15.4087(6)
18.6707(8)
88.910(1)
84.771(1)
65.664(2)
3321.9(2)
2
90
90
7984.01(10)
8
1.440
Z
µ (mm-1
)
1.759
1.418
1.276
1.338
1.147
1.337
F (g cm-3
)
1.120
1.134
121 > range (deg)
R1,a wR2 [I > 2σ(I)]b
R1,a wR2 (all data)b
3.03-30.00
0.0682, 0.1256
0.0921, 0.1397
3.53-30.01
0.0269, 0.0597
0.0351, 0.0630
3.67-27.49
0.0566, 0.1267
0.0928, 0.1493
3.81-24.21
0.0609, 0.1191
0.1067, 0.1410
3.67-24.94
0.1107, 0.2617
0.1665, 0.2925
Measured/independent reflections/Rint 34042/5294/0.0409 25876/3804/0.0413 132210/9125/0.0736
9209/5875/0.2578 30202/11375/0.1265
b
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. wR2 ) {∑[w(F2o - Fc2)2]/∑[w(Fo2)2]}1/2
.
Scheme 1
resonances at ca. 9.5 ppm attributed to the imidazole H-2 proton
in the H NMR spectra. The structures of compounds 1 and 3
1
are illustrated in Figure 1a and b, while details of the X-ray
analyses and selected bond length and angle data are provided
in Tables 1 and 2, respectively. Although showing some internal
variation, the B-N distances within the cationic units of 1
[1.545(8), 1.568(8) Å] and 3 [1.567(2) Å] are within the range
of distances observed in the previously reported boronium
cations, [(tert-butylamine)(1-methylimidazole)BH2]+ [1.557(4)
Å] and [(Me3N)(1-methylimidazole)BH2]+ [1.555(3), 1.604(3)
Å].19,20 In common with these previous studies the imidazolium
rings within 1 and 3 do not display any significant distortions
due to incorporation into the boronium cation.
and a lower resolution data set (Θmax ) 24.2°). The compound
contains one THF ligand which shows a 1:1 disorder in C32. In 10
the asymmetrical unit consists of two molecules of CaI-dimer and
one solvent molecule of toluene. Both THF groups binding to Ca
show potential disorder. However, due to a very weak data set this
could not be resolved. The -BH2 hydrogens could be located in the
difference Fourier map but had to be constrained.
The majority of our previous work has employed a “one pot”
procedure to provide large quantities of the ꢀ-diketiminate
complex I.3 With a similar motivation in mind, addition of THF
at low temperature to a solid mixture of three equivalents of
[KN(SiMe3)2] and single molar equivalents of the ligand
precursor (1 or 2) and either CaI2 or SrI2 provided after workup
the desired heteroleptic complexes 5 and 6 (Scheme 2). To date,
analogous reactions employing BaI2 as the alkaline earth starting
material have not yielded any tractable products. It is also
notable that NMR analysis of the crude reaction mixtures
indicated the formation of a complex mixture of products
formed, most likely, by the fragmentation of the ligand
precursors during the reactions (Vide infra). Despite these
observations, the materials separated from the KI byproduct were
extremely soluble in hydrocarbon solvents and storage of
concentrated solutions at -20 °C for extended periods allowed
the isolation of both compounds as colorless crystalline solids
Computational Details
The model heteroleptic complexes [{HB{(NMeCH)2C}M-
{N(SiH3)2}(OMe2)2] 11 (M ) Ca) and 12 (M )Sr) were examined
using the B3LYP density functional theory and LAN2DZ pseudo-
potentials (and basis set) implemented in Gaussian03.29 The
geometry optimizations were performed by selecting an initial
geometry based upon that deduced for the structure of the
heteroleptic strontium complex 6 and, in both cases, were confirmed
as true minima by independent frequency calculations. Details of
atomic coordinates for both optimized structures and energies are
available as Supporting Information.
Results and Discussion
Synthetic and Structural Studies of Group 2 Bis(imida-
zolin-2-ylidene-1-yl)borates. The ligand precursor salt
[H2B(tBuIm)2]I, 1, was synthesized in a manner similar to that
recently described by Smith but employing the reaction of two
equivalents tert-butylimidazole with Me3N.BH2I in refluxing
chlorobenzene rather than the lower boiling solvent toluene
(Scheme 1).8d The analogous bromide salt, compound 2, and
the respective 1-mesityl iodide and bromide analogues, com-
pounds 3 and 4, were synthesized by the same method and
isolated as colorless crystalline solids after recrystallization from
water (1), dichloromethane (2), or chloroform (3 and 4).
1
in moderate to good (50-70%) yields. The H and 13C{1H}
NMR data were consistent with the expected structures.
Particularly diagnostic were the signals ascribed to the coordi-
nated C-2 position of the carbenoid carbon centers of the borate
ligands which appeared at 195.0 ppm (5) and 201.5 ppm (6) in
the respective 13C{1H} NMR spectra. We have recently de-
scribed a series of calcium and strontium adducts with neutral
NHC ligands, [(NHC)M{N(SiMe3)2}2], and noted that the
magnitude of the upfield shifts (∆δ) induced in the resonance
of the C-2 center from that of the free carbene were dependent
1
Compound 1 provided H and 13C{1H} NMR chemical shift
data in concordance with those quoted recently by Smith and
co-workers and all four salts displayed characteristic low field
(20) Soutullo, M. D.; Odom, C. I.; Smith, A. B.; McCreary, D. R.;
Sykora, R. E.; Salter, E. A.; Wierzbicki, A.; Davis, J. H., Jr. Inorg. Chim.
Acta 2007, 360, 3099.
(21) Arduengo, A. J.; Davidson, F.; Krafczyk, R.; Marshall, W. J.;
Tamm, M. Organometallics 1998, 17, 3375.
(19) Fox, P. A.; Griffin, S. T.; Reichert, W. M.; Salter, E. A.; Smith,
A. B.; Tickell, M. D.; Wicker, B. F.; Cioffi, E. A.; Davis, J. H., Jr.; Rogers,
R. D.; Wierzbicki, A. Chem. Commun. 2005, 3679.

1734 Organometallics, Vol. 28, No. 6, 2009
Arrowsmith et al.
Figure 1. (a) ORTEP plot of compound 1; (b) ORTEP plot of compound 3. Thermal ellipsoids at 30% probability.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 1 and 3
1
3a
1.545(8) 1.567(2)
1.568(8)
1
3a
109.0(5) 107.38(18)d
C(1)-N(1)-C(3) 106.3(5) 107.58(13)
B-N(1)
B-N(3)
N(1)-B-N(3)
-
C(1)-N(1) 1.343(7) 1.3267(19) C(1)-N(1)-B
C(3)-N(1) 1.383(7) 1.379(2) C(3)-N(1)-B
C(1)-N(2) 1.336(7) 1.3376(19) C(1)-N(2)-C(2) 107.7(5) 107.67(13)
C(2)-N(2) 1.382(7) 1.378(2)
C(8)-N(3) 1.332(7)
C(9)-N(3) 1.382(7)
C(8)-N(4) 1.344(7)
C(10)-N(4) 1.382(7)
B-H(1A)
B-H(1B)
126.5(5) 125.51(16)
127.2(5) 126.91(16)
C(1)-N(2)-C(4) 127.5(5) 126.42(13)
C(2)-N(2)-C(4) 124.8(5) 125.78(13)
-
-
-
-
C(8)-N(3)-C(9) 107.3(5)
-
-
-
-
-
C(8)-N(3)-B
125.5(5)
127.1(5)
-
C(9)-N(3)-B
1.10(6) 1.14(3)b
1.07(7) 1.11(3)c
-
-
-
a Symmetry transformations used to generate equivalent atoms: x, -y
+ 1/2, z. b B(1)-H(1). c B(1)-H(2). d N(1)-B(1)-N(1)′.
Scheme 2
Figure 2. ORTEP Plot of compound 6. Thermal ellipsoids at 30%
probability. Hydrogen and tert-butyl carbon methyl atoms removed
for clarity.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 6 and 7
Table 3. Comparative M-Ccarbene and δ13C NMR data for Ca and Sr
NHC Complexes
6a
7b
6a
7b
compounda
M-CNHC (Å)
δ13C CNHC (ppm)
ref
M-C(1)
M-C(8)
M-N(5)
M-O(1)
M-O(2)
2.757(4) 2.583(3) C(1)-M-C(8) 83.29(11) 83.74(11)i
2.739(3) 2.646(4)d C(1)-M-N(5) 112.20(11) 85.77(11)j
2.510(3) 2.640(4)e C(1)-M-O(1) 86.55(10) 81.75(10)k
2.592(3) 2.607(3)f C(1)-M-O(2) 121.65(14) 79.14(10)l
2.606(4)c 2.467(3)g C(8)-M-N(5) 108.27(11) 105.37(11)m
5
7
9
-
195.0
195.0
196.0
193.3
195.4
196.2
201.5
198.9
199.0
198.2
203.7
This work
This work
This work
5
5
21
This work
This work
5
21
21
2.583(3)-2.646(4)
-
[Ca(L1){N(SiMe3)2}2] 2.598(2)
[Ca(L2){N(SiMe3)2}2] 2.6259(2)
C(8)-M-O(1) 145.81(10) 106.67(11)n
[(Cp*)2Ca{L3}]
6
2.562(2)
M-H(1A) 2.87(3)
B-N(3)
1.544(5) 1.552(6)h C(8)-M-O(2) 85.49(12) 160.72(11)o
C(1)-N(1) 1.367(4) 1.365(4) N(5)-M-O(1) 105.80(10) 108.21(11)p
C(1)-N(2) 1.367(5) 1.378(4) N(5)-M-O(2) 125.67(14)
B-H(1A) 1.12(3)
B-H(1B) 1.14(4)
-
2.739(3)-2.757(4)
8
-
[Sr(L1){N(SiMe3)2}2]
[(Cp*)2Sr{L3}]
[(Cp*)2Sr{L3}2]
2.731(3)
-
-
-
-
O(1)-M-O(2) 72.36(12)
-
2.868(5), 2.854(5)
a M ) Sr. b M ) Ca. c Sr-O(2A), 2.621(19). d Ca-C(4). e Ca-C(7).
f Ca-C(10). g Ca-O. h B(1)-N(3). i C(1)-Ca-C(4). j C(7)-Ca-C(10).
k C(1)-Ca-O. l C(10)-Ca-O. m C(10)-Ca-C(4). n C(7)-Ca-C(4).
o C(1)-Ca-C(10). p C(1)-Ca-C(7).
a L1 ) 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; L2 ) 1,3-bis-
(2,6-di-iso-propylphenyl)imidazol-2-ylidene; L3 ) 1,3-bis(methyl)imid-
azol-2-ylidene.
Scheme 3
temperature 1H and 13C{1H} NMR studies performed on
compound 5 revealed some evidence for fluxional behavior in
solution. Cooling of a sample in d8-toluene revealed a decoa-
lescence of the bis(imidazolin-2-ylidene-1-yl)borate ligand
resonances into a series of singlets of equal intensity as the
temperature was lowered. Although a splitting of the broad B-H
resonance was also observed down to the low temperature limit
1
(208 K) of the H{11B} NMR spectrum, consistent with the
upon the Lewis acidity of the metal (Ca > Sr > Ba).5 A similar
effect is apparent from consideration of the chemical shift data
of 5 and 6, which are closely comparable to those encountered
in the neutral adducts. Examination of these and other relevant
literature data listed in Table 3 reveal that the nature of the
Lewis basic carbene donors are apparently not perturbed by
incorporation into the bidentate anionic borate ligand. Variable
formation of a labile agostic interaction, this process was ill
defined due to the likely diastereotopic nature of the BH2 protons
and the presence of the 20% quadrupolar 10B nucleus. The
fluxional process was best characterized, therefore, by examina-
tion of the aromatic protons of the imidazolin-2-ylidene residues
which provided a value of ∆Gq 48.7 kJ mol-1 for a coalescence
temperature of 238 K. It is noticeable that the C-2 imidazolin-

Synthesis and Studies of Catalytic Hydroamination
Organometallics, Vol. 28, No. 6, 2009 1735
Scheme 4
Scheme 5
group 2 complexes and indicative of equilibration of the
geometric isomers illustrated in Scheme 3. Variable temperature
studies performed upon the strontium analogue of 5, compound
6, revealed similar, but less defined, fluxional behavior most
likely as a consequence of the reduced charge density of the
larger Sr2+ cation.
Crystals of compound 6 suitable for X-ray diffraction analysis
were isolated from hexane solution at -20 °C. The results of
this experiment are illustrated in Figure 2 and details of the
X-ray analysis and selected bond length and angle data are
provided in Tables 1 and 4 respectively. Compound 6 adopts a
distorted pseudo-octahedral coordination geometry in the solid
state with a facial O2N donor set provided by two molecules of
THF and the silylamide coligand. The other face is occupied
by the bis(imidazolin-2-ylidene-1-yl)borate ligand which adopts
a boat conformation and an effective η3-coordination mode. In
this latter case, donation to the strontium cation is provided by
the two imidazolin-2-ylidene carbene centers and an apparent
agostic-type interaction from a boron B-H bond. The Sr-C
distances [Sr-C(1), 2.757(4) Å; Sr-C(8), 2.739(3) Å] are
comparable to that observed within our recently reported three-
coordinate complex [Sr(NHC){N(SiMe3)2}2] (NHC ) 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) [Sr-C ) 2.731(3)
Å] despite the increased coordination number at strontium (Table
3).5 The interaction between Sr and a hydridic hydrogen of the
BH2 unit [2.87(3) Å], although long, is within the range
established [2.59(3)-2.89(3) Å] within several borohydride and
phosphinoborane-substituted carbanion strontium derivatives.24,25
In an attempt to synthesize heteroleptic alkaline earth iodide
derivatives, similar reactions employing the ligand precursor,
compound 1, were attempted using two molar equivalents of
[KN(SiMe3)2] rather than three (Scheme 4). Reactions employ-
ing calcium and strontium iodides provided similar results and
homoleptic species, compounds 7 [Ca{(tBuIm)2BH2}2(THF)]
and 8 [Sr{(tBuIm)2BH2}2(THF)2], which are evidently formed
due to the reduced steric demands of the intended iodide in
2-ylidene resonance in the 13C{1H} NMR spectrum also splits
into two (198.7 and 198.8 ppm) at the low temperature limit
(208K) of the experiment. Smith has recently reported that the
structure of the homoleptic nickel complex [Ni{(tBuIm)2BH2}2]
is best described as square planar despite the boat conformation
that is adopted by the borate ligands, a geometry that is retained
in solution at room temperature.8 This bonding situation
contrasted with that of an octahedral bis(pyrazolyl)borate
complex, [Ni{(tBuPz)2BH2}2], in which two agostic Ni-H-B
bonds completed the metal’s coordination sphere and was
reasoned to be a consequence of the more strongly donating
carbene-based ligand.22 Similar M-H-B interactions are ob-
served in the solid state analyses of the group 2 complexes
reported herein (Vide infra) and are reminiscent of the coordi-
nating behavior of “scorpionate” bis(pyrazolato)borate coordina-
tion complexes.23 Further evidence for the maintenance of an
agostic-type M-H-B interaction in the solid state was provided
by inspection of the IR spectra of compounds 5 and 6, where
two distinct υB-H stretches were observed for both compounds
(5: 2385, 2252 cm-1; 6: 2363, 2252 cm-1). The lower frequency
vibrations are assigned to be a result of the M-H-B agostic
interactions, a deduction supported by the frequency calculations
performed upon the model complexes, 11 and 12 (Vide infra).
Although alternative processes cannot be discounted unequivo-
cally, we propose, therefore, that the fluxional solution behavior
observed for compound 5 is a consequence of the lability of
the metal to ligand interactions typically observed in heavier
Figure 3. ORTEP Plot of compound 7. Thermal ellipsoids at 30%
probability. Hydrogen and tert-butyl carbon methyl atoms removed
for clarity.
Figure 4. ORTEP Plot of compound 10 with thermal ellipsoids at
20% probability.

1736 Organometallics, Vol. 28, No. 6, 2009
Arrowsmith et al.
Figure 5. Representations of the optimized (DFT, B3LYP/LAN2DZ) structures of compounds (a) 11 and (b) 12. Calculated NBO charges
for selected atoms shown in parenthesis. Selected bond lengths (Å); 11: Ca-C(avg.) 2.63; Ca-N 2.47; Ca-HB 2.93; Ca-O(avg.) 2.48.
12: Sr-C(avg.) 2.79; Sr-N 2.55; Sr-HB 2.87; Sr-O(avg.) 2.62.
Table 5. Intramolecular Hydroamination of Terminal Aminoalkenes Catalyzed by Compounds 5 and 6
comparison to the hexamethyldisilazide coligands employed in
the syntheses of compounds 5 and 6. Although both compounds
proved to be extremely air-, moisture- and temperature-sensitive,
preventing the acquisition of accurate microanalytical data,

Synthesis and Studies of Catalytic Hydroamination
Organometallics, Vol. 28, No. 6, 2009 1737
promptly prepared samples analyzed by 1H and 13C{1H} NMR
spectroscopy displayed similar spectral features suggestive of
a single borate ligand environment. The chemical shifts at-
tributed to the donor-carbon carbene resonances [7, 195.0; 8,
198.9 ppm] also bore close comparison to those observed for
the heteroleptic species 5 and 6, indicative of comparable
binding modes and charge donation (Table 3). This deduction
was confirmed by a further X-ray structural analysis obtained
for the calcium compound 7. Although crystals of 7 required
handling under a cryogenic N2 stream to prevent decomposition
and were weakly diffracting, the structure was unambiguous
and revealed a similar η3-binding of the borate ligands to that
observed for 6. The structure of compound 7 is illustrated in
Figure 3 and details of the X-ray analysis and selected bond
length and angle data are provided in Tables 1 and 4 respec-
tively. The range of calcium-carbon distances [2.583(3)-2.646(4)
Å] are similar to those observed within the neutral NHC adducts
[(NHC)Ca{N(SiMe3)2}2] [NHC ) 1,3-bis(2,4,6-trimethylphe-
nyl)imidazol-2-ylidene, 2.598(2) Å; NHC ) 1,3-bis(2,6-di-
isopropylphenyl)imidazol-2-ylidene) 2.6285(16) Å]5 and
[(C5Me5)2Ca{C(NMeCMe)2}] [2.5629(2) Å] (Table 3),21 as well
as the range of distances reported by Harder for several calcium
bis(iminophosphoranyl)methandiyl derivatives [2.528(2)-2.805(1)
Å].26 Although substantially longer than those observed within
our previously reported dialkyldihydridoborate [HC{C(Me)CN-
(2,6-iPr2C6H3)2}CaH2BR2] (R ) the anion formed from
9-borabicyclo[3.3.1]nonane) [2.17(4) - 2.31(4) Å]27 and the
calcium tetrahydoborate [(DME)2Ca(BH4)2] (range: 2.35(3) -
2.58(3) Å),24 the Ca-H(1b/2a) distances within 7 [2.97, 2.83
Å] again describe a structurally significant interaction such that
the calcium center may be depicted as effectively seven-
coordinate. It is also notable that the chelating framework of
the borate ligand evidently possesses little flexibility as the
N-B-N bond angles display little variation across the precursor
1 [109.0(5)°] and the calcium and strontium complexes 7
[111.0(3), 111.2(3)o] and 6 [112.0(3)°] respectively.
substituted precursors, 3 and 4 Via the “one pot” procedure
illustrated in Scheme 2. In these cases the reactions turned deep
crimson in color, which were shown by NMR analysis to contain
a complex and intractable mixture of products. This problem
was partially circumvented through the adoption of an alternative
synthetic route, which utilized the homoleptic calcium amide
[Ca{N(SiMe3)2}2(THF)2] as both base and source of the group
2 metal. Reaction of 1.5 equivalents of [Ca{N(SiMe3)2}2(THF)2]
with the boronium iodide salt precursor compound 3 (Scheme
5) provided the colorless complex 9, which provided NMR
spectra indicative of the intended heteroleptic formulation.
Although the δc carbene resonance of 9 (196.0 ppm) was again
similar to that observed in compounds 5 and 7, variable
temperature NMR studies revealed none of the fluxional
behavior attributed to the heteroleptic species described above.
While compound 9 provided well-formed single crystals at low
temperature, multiple attempts to acquire X-ray diffraction data
were frustrated by the extremely rapid decomposition of the
material once removed from the chilled mother liquor. A data
set collected on a more durable crystal from one of these
reactions yielded the structure of an unanticipated heteroleptic
iodide species, compound 10 (Figure 4). Although the data set
for 10 was very weak and yielded residuals for the structure
which were unacceptably high (R1 ) 0.1107, wR2 ) 0.2617)
precluding any detailed discussion, the connectivity was un-
ambiguous. The borate ligands again adopt a tridentate coor-
dination mode, while the presence of the iodide ligands
highlights the propensity toward redistributative processes of
heavier group 2 centers.
The nature of the metal to ligand coordination in complexes
5-10 was assessed by DFT calculations on the model hetero-
leptic complexes [{HB{(NMeCH)2C}M{N(SiH3)2}(OMe2)2] 11
(M ) Ca) and 12 (M )Sr) using the B3LYP density functional
theory and LAN2DZ pseudopotentials (and basis set) imple-
mented in Gaussian03.29 The geometry optimizations were
performed by selecting an initial geometry based upon that
deduced for the structure of the heteroleptic strontium complex
6 and, in both cases, were confirmed as true minima by
independent frequency calculations. To reduce computational
expense the tert-butyl groups of the borate ligands were replaced
by methyl groups, the methyl groups of the silyl amide ligands
were replaced by hydrogen and the coordinated THF molecules
were simplified to two molecules of dimethyl ether. The second
molecule of coordinated solvent was retained within the
coordination sphere of the calcium center of 11 despite the lower
coordination number determined for the “real case” of com-
pound 5. It was felt that this was necessary to take into account
the reduced steric demands of the truncated ligands and to allow
for internal consistency when comparing the two calculated
structures. Figure 5 illustrates the results of this study. In both
Attempts to elaborate a rational synthesis of these homoleptic
species have so far been unsuccessful. Although an analogous
“one pot” procedure employing four equivalents of the silyla-
mide base produced materials with 1H and 13C{1H} NMR
spectra reminiscent of those of 7, these materials contained no
coordinated THF. Rather, additional singlet resonances at δH
0.84 (9H), 6.36 (1H) and 7.13 (1H) ppm were attributed to
coordinated 1-tert-butylimidazole which was evidently produced
by rupture of the B-N bonds of the ligand precursor during
borate anion formation. Although we have observed no evidence
of ligand degradation subsequent to anion formation, it appears
that such B-N cleavage is a common feature of this ligand
class at some stage in the deprotonation of, and the necessary
charge distribution within, the ligand precursors.28
Similar difficulties have also blighted our attempts to
synthesize analogues of compounds 5-8 utilizing the N-mesityl
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K. Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B. Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S. Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(22) Belderrain, T. R.; Paneque, M.; Carmona, E.; Gutierrez-Puebla, E.;
Monge, M. A.; Ruiz-Valero, C. Inorg. Chem. 2002, 41, 425.
(23) Trofimenko, S. Scorpionates. The Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College Press: London, 1999.
(24) Bremer, M.; No¨th, H.; Thomann, M.; Schmidt, M. Chem. Ber. 1995,
128, 455.
(25) Izod, K.; Wills, C.; Clegg, W.; Harrington, R. W. Inorg. Chem.
2007, 46, 4320.
(26) Orzechowsky, L.; Jansen, G.; Harder, S. J. Am. Chem. Soc. 2006,
128, 14676.
(27) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.;
Procopiou, P. A. Organometallics 2007, 26, 2953.
(28) We have also often observed B-N cleavage in our attempts to
synthesize heavier group 2 derivatives of tris(imidazolin-2-ylidene-1-
yl)borate derivatives; Arrowsmith, M.; Hill, M. S. unpublished results.

1738 Organometallics, Vol. 28, No. 6, 2009
Arrowsmith et al.
cases the calcium and strontium contacts to the carbon donors
of the η3-coordinated ligands replicate the distances observed
within the structures of compounds 7 and 6 respectively to
within ca. 0.02 Å (see Figure 5 caption). Especially notable
was the agreement between the calculated M · · · HB distances
[11, 2.93 Å; 12, 2.87 Å] and the values determined in the X-ray
studies. The calculated υB-H stretching vibrations for both model
reported for the ꢀ-diketiminato calcium precatalyst, I.1a In
common with these previous studies, bulky geminal substituents
in the ꢀ-position of the amine favored cyclization due to a
Thorpe-Ingold effect (entries 3-7), while terminal monosub-
stitution considerably hindered the reaction and terminal dis-
ubstitution prevented it entirely (entries 8-11). Catalytic activity
also rapidly diminished with increasing ring-size (5 > 6 > 7;
entries 12-16). Generally, cyclization occurred faster, with
better conversion, and at lower temperature and catalyst loading
with the strontium complex 6 than with its calcium counterpart.
This is consistent with our observations of similar reactions
catalyzed by heteroleptic calcium and strontium triazenide
complexes but conflicts with the recent reports of Roesky and
Blechert.2,30 In this latter case, aminotropiniminate complexes
[{(iPr)ATI}M{N(SiMe3)2}(THF)2] (M ) Ca or Sr) were applied
to the cyclization of a series of substrates similar to that listed
in Table 5. It was found that the strontium complex was
significantly less active than its calcium counterpart for all
reactions studied and led to the conclusion that the origin of
this effect was the change in radius (for CN 6 (Å); Ca2+ 1.00;
Sr2+ 1.18)31 on descending the group.2b We would suggest at
this time that this is something of an oversimplification and that
any relative reduction in rate is more likely a function of the
kinetic protection and propensity to solution redistribution of
the supporting ligand rather than an intrinsic feature of the group
2 metal selected. We have carried out extensive experimental
and theoretical experiments in support of this statement and will
describe these data in subsequent publications.
compounds (11: 2503 (nonagostic), 2412 (agostic) cm-1
;
12: 2499 (nonagostic), 2401 (agostic) cm-1) are somewhat
higher than those observed for the heteroleptic species 5 and 6.
However this may be ascribed to the decreased reduced mass
of the oscillators within the simplified models and it is notable
that ∆υ, the difference between the stretching vibrations in both
the real and model compounds, is of a similar magnitude, ca.
100 cm-1, across all four sets of data. For both 11 and 12 natural
bond orbital (NBO) calculations were undertaken and the
calculated charges on the Ca2+ (11, +1.83 Å) and Sr2+ (12,
+1.86 Å) centers were suggestive of essentially ionic bonding.
In both cases the donor carbon centers bear only a modest
negative charge (<-0.03) indicating that they retain the es-
sentially pure σ-donor character implicated in the formation of
neutral adducts such as [(NHC)Ca{N(SiMe3)2}2] (NHC ) 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene).5 Although the
majority of the anionic charge of the ligands is borne by the
boron-bound imidazolinylidenyl nitrogen centers (ca. -0.63),
the electrostatic nature of the M · · · HB interactions is indicated
by the hydridic nature (ca. -0.11 for the coordinated hydrogen)
of the borohydride subunits.
Catalytic Reactivity. We have reported previously that
neutral NHC ligands readily dissociate in solution in the
presence of harder (e.g., N- and O-centered) donors. This
observation is especially relevant to the potential application
of group 2 complexes in hydroamination catalysis where the
reaction conditions are innately disposed toward carbene dis-
placement.5 Encouraged by the results described above, there-
fore, compounds 5 and 6 were assessed for the intramolecular
hydroamination of a range of substituted aminoalkenes on an
NMR scale in C6D6. In all cases, careful inspection of the NMR
spectra revealed no observable protonation or dissociation of
the borate ligand throughout reaction times up to a period of
Acknowledgment. We thank the Engineering and Physical
Sciences Research Council (EPSRC) for provision of a
project studentship (MA).
Supporting Information Available: NMR spectra for com-
pounds 7, 8 and 9, atomic coordinates for the calculated structures
of compounds 11 and 12 and further details of the X-ray analysis
of compound 10. Crystallographic information files (CIF) for 1, 3,
6, 7 and 10. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
one week. The imidazole H signals and the 13C{1H} carbene
OM8010933
resonances remained effectively unaltered and no conclusive
evidence for Schlenk-type ligand redistribution was observed.
Examination of the results listed in Table 5 indicated that
the amidocalcium complex 5 displayed similar activity to that
(30) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Kociok-Ko¨hn, G.;
Procopiou, P. A. Inorg. Chem. 2008, 47, 7366.
(31) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.