Syntheses and structures of magnesium and zinc boraamidinates: EPR and DFT investigations of Li, Mg, Zn, B, and in complexes of the [PhB(N?Bu) 2]?- anion radical

Article abstract of DOI:10.1021/ic0520014

The first magnesium and zinc boraamidinate (bam) complexes have been synthesized via metathetical reactions between dilithio bams and Grignard reagents or MCI₂ (M = Mg, Zn). The following new classes of bam complexes have been structurally characterized: heterobimetallic spirocycles {(L)μ-Li[PhB(μ-N^tBu)₂]}₂M (6a,b, M = Mg, L = Et₂O, THF; 6c, M = Zn, L = Et₂O); bis(organomagnesium) complexes {[PhB(μ₃-N^tBu)₂](Mg ^tBu)₂(μ₃-CI)Li(OEt₂) ₃} (8) and {[PhB(μ₃-N^tBu) ₂](MgR)₂(THF)₂} (9a, R = ⁱPr; 9b, R = Ph); mononuclear complex {[PhB(μ-NDipp)₂]Mg-(OEt ₂)₂} (10). Oxidation of 6a or 6c with iodine produces persistent pink (16a, M = Mg) or purple (16b, M = Zn) neutral radicals {L _x-μ-Li[PhB(μ-N^tBu)₂]₂M} ^? (L = solvent molecule), which are shown by EPR spectra supported by DFT calculations to be C_s-symmetric species with spin density localized on one of the bam ligands. In contrast, characterization of the intensely colored neutral radicals {[PhB(μ-N^tBu) ₂]₂M)^? (5c, M = In, dark green; 5d, M = B, dark purple) reveals that the spin density is equally delocalized over all four nitrogen atoms in these D_2d-symmetric spirocyclic systems. Oxidation of the dimeric dilithio complex {Li₂[PhB(μ₄-N ^tBu)₂]}₂ with iodine produces the monomeric neutral radical {[PhB(μ-N^tBu)₂]Li(OEt₂) _x}^? (17), characterized by EPR spectra and DFT calculations. These findings establish that the bam anionic radical [PhB(N ^tBu)₂]^?- can be stabilized by coordination to a variety of early main-group metal centers to give neutral radicals whose relative stabilities are compared and discussed.

Full text of DOI:10.1021/ic0520014

Inorg. Chem. 2006, 45, 2119−2131
Syntheses and Structures of Magnesium and Zinc Boraamidinates: EPR
and DFT Investigations of Li, Mg, Zn, B, and In Complexes of the
•-
[PhB(NtBu)2] Anion Radical
Tristram Chivers,* Dana J. Eisler, Chantall Fedorchuk, Gabriele Schatte, and Heikki M. Tuononen
Department of Chemistry, UniVersity of Calgary, Calgary, Alberta, Canada T2N 1N4
Rene´ T. Boere´
Department of Chemistry and Biochemistry, UniVersity of Lethbridge,
Lethbridge, Alberta, Canada T1K 3M4
Received November 18, 2005
The first magnesium and zinc boraamidinate (bam) complexes have been synthesized via metathetical reactions
between dilithio bams and Grignard reagents or MCl2 (M Mg, Zn). The following new classes of bam complexes
have been structurally characterized: heterobimetallic spirocycles (L) -Li[PhB( 2M (6a,b, M Mg, L
-NtBu)2]
Et2O, THF; 6c, M Zn, L Et2O); bis(organomagnesium) complexes [PhB( 3-Cl)Li(OEt2)3
3-NtBu)2](MgtBu)2(
(8) and [PhB( (9a, R Ph); mononuclear complex [PhB( -NDipp)2]Mg-
3-NtBu)2](MgR)2(THF)2 iPr; 9b, R
(OEt2)2 Mg) or purple (16b, M Zn)
)
{
µ
µ
}
)
)
)
)
{
µ
µ
}
{
µ
}
)
)
{
µ
}
(10). Oxidation of 6a or 6c with iodine produces persistent pink (16a, M
)
)
•
neutral radicals
{
Lx-
µ
-Li[PhB(
µ
-NtBu)2]2M
}
(L
)
solvent molecule), which are shown by EPR spectra supported
by DFT calculations to be Cs-symmetric species with spin density localized on one of the bam ligands. In contrast,
•
characterization of the intensely colored neutral radicals
{
[PhB(µ
-NtBu)2]2M
}
(5c, M
)
In, dark green; 5d, M
)
B, dark purple) reveals that the spin density is equally delocalized over all four nitrogen atoms in these D2d
-
symmetric spirocyclic systems. Oxidation of the dimeric dilithio complex
{
Li2[PhB(
µ
4-NtBu)2]
}
with iodine produces
2
•
the monomeric neutral radical
{
[PhB(
µ
-NtBu)2]Li(OEt2)x
}
(17), characterized by EPR spectra and DFT calculations.
These findings establish that the bam anionic radical [PhB(NtBu)2]•- can be stabilized by coordination to a variety
of early main-group metal centers to give neutral radicals whose relative stabilities are compared and discussed.
Introduction
(µ-NR′)2][µ-RC(NR′)2κ2N,N′]Mg}2,2d,3 complexes involving
pendant ams,4 and mixed Mg/Al mono-am complexes
{[MeC(µ-NR′)2][Me2Al(µ-NiPr)2]Mg} have been reported.5
Potential applications of Mg am complexes include their use
The coordination chemistry of the monoanionic amidinate
(am) ligand [RC(NR′)2]- (Chart 1) has been extensively
studied.1 For example, am complexes of all the s-block
elements have been reported.1 In the context of the current
work, a variety of magnesium am complexes have been
identified. These include mono- and bis-am complexes of
the general formulas [RC(µ-NR′)2]MgX and [RC(µ-NR′)2]2Mg
(R, R′ ) alkyl, aryl, SiMe3; X ) alkyl, aryl, halide)2 in which
the Mg centers are usually stabilized by a Lewis base. In
addition, dinuclear Mg complexes with bridging ams {[RC-
(2) For examples, see: (a) Boere´, R. T.; Cole, M. L.; Junk, P. C. New. J.
Chem. 2005, 29, 128. (b) Schmidt, J. A. R.; Arnold, J. J. Chem. Soc.,
Dalton Trans. 2002, 2890. (c) Xia, A.; El-Kaderi, H. M.; Heeg, M.
J.; Winter, C. H. J. Organomet. Chem. 2003, 682, 224. (d) Sadique,
A. R.; Heeg, M. J.; Winter, C. H. Inorg. Chem. 2001, 40, 6349. (e)
Srinivas, B.; Chang, C. C.; Chen, C. H.; Chiang, M. Y.; Chen. I.-T.;
Wang, Y.; Lee, G.-H. J. Chem. Soc., Dalton Trans. 1997, 957. (f)
Walther, D.; Gebhardt, P.; Fischer, R.; Kreher, U.; Go¨rls, H. Inorg.
Chim. Acta 1998, 281, 181.
(3) For an example, see: Cotton, F. A.; Haefner, S. C.; Matonic, J. H.;
Wang, X.; Murillo, C. A. Polyhedron 1997, 16, 541.
* To whom correspondence should be addressed. E-mail:
chivers@ucalgary.ca. Telephone: (403) 220-5741. Fax: (403) 289-9488.
(1) For reviews, see: (a) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994,
133, 219. (b) Edelmann, F. T. Coord. Chem. ReV. 1994, 137, 403.
(4) For examples, see: (a) Chen, C.-T.; Huang, C.-A.; Tzeng, Y.-R.;
Huang, B.-H. J. Chem. Soc., Dalton Trans. 2003, 2585. (b) Kincaid,
K.; Gerlach, C. P.; Giesbrecht, G. R.; Hagdorn, J. R.; Whitener, G,
D,; Shafir, A.; Arnold, J. Organometallics 1999, 18, 5360.
10.1021/ic0520014 CCC: $33.50
Published on Web 02/08/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 5, 2006 2119

Chivers et al.
Chart 1
These reagents have provided the predominant method of
transferring the bam ligand to other metal centers.11e The
first X-ray structural determinations of dilithio bam com-
plexes have shown that the extent of aggregation is influ-
enced by the substituent (R) on boron, where the fundamental
building block is the Li2N2B unit A (Chart 1).11b,c In the case
n
of the dimers {Li2[RB(µ4-NtBu)2]}2 (1a, R ) Ph; 1b, Bu;
t
1c, Bu), two of these units participate in a face-to-face
interaction through lithium-nitrogen contacts to give the
bicapped cube B. In the unique example of a trimer
{Li2[RB(µ4-NtBu)2]}3 (2, R ) Me), three Li2N2B units
associate edge-on through lithium-nitrogen contacts to give
the tricapped hexagonal prism C. The solvated complex
{(THF)2(µ-THF)Li2[PhB(µ3-NDipp)2]} (3) (Chart 1)11d is a
unique example of a monomeric dilithio boraamidinate. An
intriguing observation during investigations of the dilithio
bam complexes 1a-c was the formation of red solutions
upon exposure to air or during the course of metathetical
reactions with main-group element halides.11c In related
polyimido anions of p-block elements, such behavior is
associated with the formation of free radicals.12
Until 1993, reports on p-block bam derivatives were
restricted to groups 14-16,11a,c,e,13 with the exception of {[µ-
Mes*B(NMe)2κ2N,N′]AlMe}2 for which a crystal structure
has not been reported.13i Early investigations of the coordina-
tion chemistry of the bam dianion with transition metals were
limited to group 4,11a,14 but recently, Nocera and co-workers
have described complexes of groups 5 and 6 including
octahedral tris-bam complexes and a paramagnetic vanadium-
(IV) complex.15 We have described the characterization of
the spirocyclic anions {[PhB(µ-NtBu)2]2M}- (4a, M ) Al,
4b, M ) Ga; 4c, M ) In) as their lithium derivatives16,17
and showed that the oxidation of 4a,b with iodine produces
as reagents for the synthesis of transition-metal complexes
and in chemical vapor deposition (CVD). Group 13 am
complexes are also of interest as potential single-source
precursors to nitride materials and as selective reagents and
catalysts. The most important classes include mono-am
complexes [RC(µ-NR′)2]MX2 (M ) group 13 element; R,
R′ ) alkyl, aryl, SiMe3; X ) halide, hydride, alkyl, aryl),6
bis-am complexes [RC(µ-NR′)2]2MX,6b,f,7 tris-am complexes
[RC(µ-NR′)2]3M,6f,8 dinuclear group 13 complexes with
bridging am ligands, and complexes of the type {[µ-RC-
(NR′)2κ2N,N′]MX2}2 and {[µ-RC(NR′)2κ2N,N′]2MX}2,6f,9 in-
corporating pendant ams.4a,10
(11) (a) Fest, D.; Habben, C. D.; Meller, A.; Sheldrick, G. M.; Stalke, D.;
Pauer, F. Chem. Ber. 1990, 123, 703. (b) Brask, J. K.; Chivers, T.;
Schatte, G. Chem. Commun. 2000, 1805. (c) Chivers, T.; Fedorchuk,
C.; Schatte, G.; Brask, J. K. Can. J. Chem. 2002, 80, 821. (d) Chivers,
T.; Fedorchuk, C.; Parvez, M. Inorg. Chem. 2004, 43, 2643. (e) Blais,
P.; Brask, J. K.; Chivers, T.; Fedorchuk, C.; Schatte, G. ACS Symp.
Ser. 2002, 822, 195.
(12) Armstrong, A. F.; Chivers, T.; Tuononen, H. M.; Parvez, M.; Boere´,
R. T. Inorg. Chem 2005, 44, 7981 and references therein.
By comparison, the coordination chemistry of the dianionic
boraamidinate (bam) ligand [RB(NR′)2]2- (isoelectronic with
am, Chart 1) is still in its infancy. For s-block metal
complexes, only lithium derivatives have been reported.11
(13) (a) Fusstetter, H.; No¨th, H. Chem. Ber. 1979, 112, 3672. (b) Gudat,
D.; Niecke, E.; Nieger, M.; Paetzold, P. Chem. Ber. 1988, 121, 565.
(c) Heine, A.; Fest, D.; Stalke, D.; Habben, C. D.; Meller, A.;
Sheldrick, G. M. J. Chem. Soc., Chem. Commun. 1990, 742. (d)
Habben, C. D.; Heine, A.; Sheldrick, G. M.; Stalke, D.; Bu¨hl, M.;
Schleyer, P. V. R. Chem. Ber. 1991, 124, 47. (e) Habben, C. D.;
Herbst-Irmer, R.; Noltemeyer, M. Z. Naturforsch. 1991, 46b, 625. (f)
Habben, C. D.; Heine, A.; Sheldrick, G. M.; Stalke, D. Z. Naturforsch.
1992, 47b, 1367. (g) Paetzold, P.; Hahnfeld, D.; Englert, U.;
Wojnowski, W.; Dreczewski, B.; Pawelec, Z.; Walz, L. Chem. Ber.
1992, 125, 1073. (h) Koch, H.-J.; Roesky, H. W.; Besser, S.; Herbst-
Irmer, R. Chem. Ber. 1993, 126, 571. (i) Geschwentner, M.; Nolte-
meyer, M.; Elter, G.; Meller, A. Z. Anorg. Allg. Chem. 1994, 620,
1403. (j) Chivers, T.; Gao, X.; Parvez, M. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2549. (k) Luthin, W.; Stratmann, J.-G.; Elter, G.;
Meller, A.; Heine, A.; Gornitzka, H. Z. Anorg. Allg. Chem. 1995, 621,
1995. (l) Albrecht, T.; Elter, G.; Noltemeyer, M.; Meller, A. Z. Anorg.
Allg. Chem. 1998, 624, 1514.
(5) For an example, see: Li, M.-D.; Chang, C. C.; Wang, Y.; Lee, G.-H.;
Organometallics 1996, 15, 2571.
(6) For examples, see: (a) Ergezinger, C.; Weller, F.; Dehnicke, K. Z. Z.
Naturforsch. 1988, 43b, 1621. (b) Coles, M. P.; Swenson, D. C.;
Jordan, R. F. Organometallics 1997, 16, 5183. (c) Coles, M. P.;
Swenson, D. C.; Jordan, R. F. Organometallics 1998, 17, 4042. (d)
Kottmair-Maieron, D.; Lechler, R.; Weidlein, J. Z. Anorg. Allg. Chem.
1991, 593, 111. (e) Lechler, R.; Hausen, H.-D.; Weidlein, J. J.
Organomet. Chem. 1989, 359, 1. (f) Barker, J.; Blacker, N. C.; Phillips,
P. R.; Alcock, N. W.; Errington, W.; Wallbridge, M. G. H. J. Chem.
Soc., Dalton Trans. 1996, 431.
(7) For an example, see: Duchateau, R.; Meetsma, A.; Teuben, J. H. Chem.
Commun. 1996, 223.
(8) For an example, see: Zhou, Y.; Richeson, D. S. Inorg. Chem. 1996,
35, 2448.
(9) For examples, see: (a) Hausen, H.-D.; Gerstner, F.; Shwarz, W. J.
Organomet. Chem. 1978, 145, 277. (b) Zhou, Y.; Richeson, D. S.
Inorg. Chem. 1996, 35, 1423.
(10) For an example, see: Doyle, D.; Gun’ko, Y. K.; Hitchcock, P. B.;
Lappert, M. F. J. Chem. Soc., Dalton Trans. 2000, 4093.
(14) Koch, H.-J.; Roesky, H. W.; Bohra, R.; Noltemeyer, M.; Schmidt,
H.-G. Angew. Chem., Int. Ed. Engl. 1992, 31, 598.
(15) (a) Manke, D. R.; Nocera, D. G. Inorg. Chim. Acta 2003, 345, 235.
(b) Manke, D. R.; Nocera, D. G. Inorg. Chem. 2003, 42, 4431. (c)
Manke, D. R.; Loh, Z.-H.; Nocera, D. G. Inorg. Chem. 2004, 43, 3618.
(16) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez, M. Inorg. Chem. 2003,
42, 2084.
2120 Inorganic Chemistry, Vol. 45, No. 5, 2006

Magnesium and Zinc Boraamidinates
Scheme 1
Scheme 3
Scheme 2
The proposed pathway involves the initial production of
7, which then undergoes redistribution to produce 6a (89%)
with the elimination of MgtBu2 that was detected by 1H NMR
spectroscopy (∼δ 0.2). The generality of method 1 (Scheme
2) was demonstrated in the reactions of 1a with a variety of
Grignard reagents in diethyl ether, including MesMgBr,
nBuMgCl, and MeMgBr, all of which produce 6a. Similarly,
the THF derivative {(THF)µ-Li[PhB(µ-NtBu)2]}2Mg (6b) is
obtained from the reactions of the Grignard reagents PhMgCl
stable18 neutral radicals {[PhB(µ-NtBu)2]2M}• (5a, M ) Al;
5b, M ) Ga), which were characterized in the solid state by
X-ray crystallography. A combination of EPR spectroscopic
analyses and DFT calculations showed that spin delocaliza-
tion is uniform over both bam ligands in 5a,b and that the
spirocyclic structure is retained in solution (Scheme 1).17
Herein, we describe studies that extend the coordination
chemistry of the bam ligand to magnesium and zinc. The
synthesis and characterization of a series of magnesium bams,
including the heterobimetallic spirocyclic complexes {(Et2O)-
µ-Li[PhB(µ-NtBu)2]}2M (6a, M ) Mg; 6c, M ) Zn), are
discussed. The oxidation of 6a,c with iodine produces
persistent18 radicals, which have been characterized by a
combination of EPR spectra and DFT calculations. In
addition, we have extended our earlier studies of group 13
spirocyclic bam radicals {[PhB(µ-NtBu)2]2M}• to the char-
acterization of the indium- and boron-containing analogues
(Scheme 1; 5c, M ) In; 5d, M ) B). Finally, we describe
the identification of the paramagnetic red species formed
upon oxidation of dilithio bams.11c
i
or PrMgCl in THF and 1a in a 1:1 stoichiometry.
Complexes of type 6 may also be prepared by a direct
approach (method 2, Scheme 2). Treatment of 2 equiv of 1a
with MCl2 (M ) Mg or Zn) in diethyl ether at -78 °C
afforded 6a and the zinc analogue {(Et2O)µ-Li[PhB(µ-Nt-
Bu)2]}2Zn (6c) in 75 and 51% yields, respectively. Colorless
crystals of 6a,c, grown from concentrated solutions in Et2O,
1
7
were characterized by CHN analyses, H, 11B, Li, and 13C
NMR spectra, and X-ray structural analyses (vide infra). For
both 6a,c, the 1H NMR spectra show resonances for the Ph
and tBu substituents and the coordinated Et2O molecules in
1
the appropriate relative intensities. However, the H NMR
spectrum for 6a displays two singlets for the four tBu groups
attached to the amide nitrogens of the bam; consistently, two
t
signals assigned to the methyl groups of these Bu substit-
uents are observed in the 13C NMR spectrum. In contrast,
only one tBu resonance is observed in the 1H NMR and 13
C
NMR spectra for 6c. The 11B NMR spectra of 6a-c exhibit
broad resonances at δ 37-38, consistent with three-
7
coordinate boron centers.19 Singlets are observed in the Li
Results and Discussion
NMR spectra of 6a-c at δ 1.44, 1.43, and 1.16, respectively.
When the reaction of 1a and tBuMgCl was carried out in
a 1:2 stoichiometry, colorless crystals of the bis(organomag-
nesium) complex {[PhB(µ3-NtBu)2](MgtBu)2(µ3-Cl)Li(O-
Syntheses and Spectroscopic Characterization of Mg
and Zn bams. The reaction of Li2[PhB(µ3-NtBu)2] (1a) and
a Grignard reagent in a 1:1 stoichiometry was attempted in
an effort to generate the mixed-metal Mg/Li mono-bam 7
(Scheme 2, method 1). However, the reaction with tBuMgCl
affords the spirocyclic dilithio magnesium bis-bam complex
{(Et2O)µ-Li[PhB(µ-NtBu)2]}2Mg (6a, Scheme 2).
1
Et2)3} (8, Scheme 3) were isolated. The H NMR spectrum
of 8 shows resonances for the Ph and tBu substituents, along
with the coordinated Et2O molecules, in the appropriate
relative intensities. The signals for the methyl groups of the
t
two Bu substituents attached to the amide nitrogen atoms
(17) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuononen, H.
M.; Boere´, R. T. Chem. Commun. 2005, 3930.
appear at δ 1.44, while those on the two magnesium atoms
are found at δ 1.12. The 11B NMR resonance is observed at
(18) The term “stable”, as applied to radicals, is taken to indicate “a species
that can be isolated and shows no sign of decomposition under an
inert atmosphere at room temperature”, whereas a “persistent” radical
has “a relatively long lifetime under the conditions it is generated”:
Power, P. P. Chem. ReV. 2003, 103, 789.
(19) No¨th, H.; Wrackmeyer, B. In Nuclear Magnetic Resonance Spectro-
scopy of Boron Compounds; Springer-Verlag: Berlin, 1978; pp 74-
101.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2121

Chivers et al.
Scheme 4
7
δ 39. A resonance is also observed at δ -0.37 in the Li
NMR spectrum of 8 consistent with the presence of LiCl in
this complex.
Figure 1. Thermal ellipsoid plot of {(Et2O)µ-Li[PhB(µ-NtBu)2]}2M (M
) Mg, 6a; M ) Zn, 6c). For clarity, H atoms are omitted and only the
R-carbons of the tBu substituents and oxygen atoms of the Et2O molecules
are shown.
i
Treatment of 1a with 2 equiv of solutions of PrMgCl or
PhMgCl in THF afforded {[PhB(µ3-NtBu)2](MgiPr)2(THF)2}
(9a) and {[PhB(µ3-NtBu)2](MgPh)2(THF)2} (9b), respectively
(Scheme 3). The 1H NMR spectrum of 9a shows resonances
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for
{(Et2O)µ-Li[PhB(µ-NtBu)2]}2M (M ) Mg, 6a; M ) Zn, 6c)
t
i
param
6a
6c
param
6a
6c
for the Ph, Bu, and Pr substituents and coordinated THF
molecules, with relative intensities indicative of symmetrical-
M(1)-N(1)
M(1)-N(2)
B(1)-N(1)
B(1)-N(2)
N(1)-Li(1)
N(2)-Li(1)
2.093(1) 2.076(4) M(1)-N(3)
2.086(1) 2.013(4) M(1)-N(4)
1.446(2) 1.436(8) B(2)-N(3)
1.449(2) 1.443(7) B(2)-N(4)
2.023(3) 2.01(1) N(3)-Li(2)
2.033(3) 2.03(1) N(4)-Li(2)
2.088(1) 2.024(4)
2.086(1) 2.067(4)
1.446(2) 1.443(7)
1.447(2) 1.425(7)
2.035(3) 2.07(1)
2.045(3) 2.03(1)
t
i
equivalent Bu and Pr substituents and the presence of two
coordinated THF molecules in solution. Similarly, the
intensities of the resonances observed in the 1H NMR
spectrum of 9b indicate three Ph groups, two tBu substituents,
and two coordinated THF molecules. The 11B NMR chemical
shifts for 9a,b are δ 39 and 40, respectively. No signals are
N(1)-M(1)-N(3) 125.45(5) 131.0(2) N(2)-M(1)-N(1) 67.54(5) 68.4(2)
N(4)-M(1)-N(1) 138.54(5) 128.8(2) N(2)-M(1)-N(3) 138.09(5) 136.6(2)
N(1)-B(1)-N(2) 106.7(1) 106.0(5) N(3)-B(2)-N(4) 107.1(1) 106.9(4)
N(1)-B(1)-C(5) 126.7(1) 126.8(5) N(3)-B(2)-C(23) 124.8(1) 126.5(5)
N(2)-B(1)-C(5) 126.6(1) 127.1(5) N(4)-B(2)-C(23) 128.1(1) 126.5(5)
B(1)-N(1)-Li(1) 77.8(1) 78.3(4) B(1)-N(2)-Li(1) 77.4(1) 77.5(4)
C(1)-N(1)-M(1) 132.21(9) 132.0(3) C(11)-N(2)-M(1) 134.57(9) 132.3(3)
B(2)-N(3)-Li(2) 77.2(1) 77.5(4) B(2)-N(4)-Li(2) 76.9(1) 79.2(4)
Li(2)-N(3)-C(19) 134.0(1) 135.0(4) M(1)-N(4)-C(29) 134.45(9) 134.7(3)
7
observed in the Li NMR spectra of 9a,b. Crystals of 9a,b
suitable for X-ray structural determinations could not be
obtained.
11d
The reaction of PhB[N(H)Dipp]2 with n/sBu2Mg (1:1)
in boiling hexane produced the mono-bam complex {[PhB-
(µ-NDipp)2]Mg(OEt2)2} (10, Scheme 4), which was identi-
fied by multinuclear NMR spectroscopy and an X-ray
in 6a,c are equal within experimental error, while the B-N
distances are intermediate between those of single- and
double-bond values.20 The geometry about the central metal
atom is distorted tetrahedral [bond angle range: M ) Mg,
67.54(5)-138.54(5)°; M ) Zn, 68.4(2)-136.6(2)°] as a
result of geometric constraints imposed by the bam ligands.
The metal-nitrogen bond distances observed in complexes
6a,c are typical for Mg-N and Zn-N single bonds.21
The X-ray structural analysis of {[PhB(µ3-NtBu)2](Mgt-
Bu)2(µ3-Cl)Li(OEt2)3} (8) reveals that each nitrogen atom
of the bam ligand coordinates to two magnesium tert-butyl
groups. The molecular geometry and atomic numbering
scheme for 8 are shown in Figure 2, while pertinent structural
parameters are summarized in Table 2. The Mg-N bond
distances are equal within experimental error (ca. 2.13 Å);
cf. ca. 2.09 Å in the spirocyclic complex 6a. The magnesium
atoms in the dimagnesiated derivative {[PhB(µ-NtBu)2](Mgt-
Bu)2} are also bridged by the chloride ion of a ClLi(OEt2)3
molecule. The structure of 8 is reminiscent of the framework
observed for the dilithiated complex 3,11d with the bridging
THF ligand in 3 replaced by ClLi(OEt2)3 in 8.
1
structural analysis (vide infra). The H NMR spectrum of
10 exhibits only one resonance for both the CH and CH3
groups of the Dipp substituents; the corresponding signals
are observed in the 13C NMR spectrum. The 11B NMR
resonance of δ 31 for 10 is shifted upfield compared to those
observed for 6, 8, and 9 (δ(11B) 37-40). Complete depro-
tonation was also indicated by the lack of an N-H stretch
in the IR spectrum. For comparison, the reaction of n/sBu2Mg
with p-tolylC[N(H)Dipp](NDipp) produces the bis-substi-
tuted square-planar complex [p-tolylC(µ-NDipp)2]2Mg.2a
X-ray Structures of Mg and Zn bam Complexes. The
X-ray structures of 6a,c show that the spirocyclic dianion
{[PhB(µ-NtBu)2]2M}2- (M ) Mg, Zn) is N,N′-chelated to
two monosolvated lithium cations. The molecular geometry
and atomic numbering scheme for 6a,c are shown in Figure
1, and pertinent structural parameters are summarized in
Table 1. The two four-membered NBNM (M ) Mg, Zn)
rings are puckered with the M and B atoms on one side of
the plane, while the two four-coordinate nitrogen atoms
reside on the other side of the plane.
The boron atoms in 6a,c exhibit distorted trigonal-planar
geometries with substantial deviations from the ideal bond
angles, although the sum of the angles is 360° in each case.
The geometry about the nitrogen atoms is distorted tetrahe-
dral (bond angle ranges: ca. 77-135°) with the smallest
values associated with the B-N-Li angle. The NBN angles
(20) Paetzold, P. Phosphorus, Sulfur, Silicon 1994, 93-94, 39.
(21) For an example, see: Buijink, J. K.; Noltemeyer, M.; Edelmann, F.
T. Z. Naturforsch., B 1991, 46, 1328. A search for Mg-N distances
in the Cambridge Structural Database provided 1543 values with a
mean of 2.13 Å and 948 distances in the range 1.97-2.15 Å. Similarly,
for Zn-N distances, 10 000 values were found with a mean of 2.11
Å and 5174 distances in the range 2.00-2.12 Å.
2122 Inorganic Chemistry, Vol. 45, No. 5, 2006

Magnesium and Zinc Boraamidinates
Figure 2. Thermal ellipsoid plot of {[PhB(µ3-NtBu)2](MgtBu)2(µ3-Cl)-
Li(OEt2)3} (8). For clarity, H atoms are omitted and only the R-carbons of
t
the Bu substituents and oxygen atoms of the Et2O molecules are shown.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for
{[PhB(µ3-NtBu)2](MgtBu)2(µ3-Cl)Li(OEt2)3} (8)
Mg(1)-N(1)
2.139(2) Mg(2)-N(1)
2.132(2) Mg(2)-N(2)
2.536(1) Mg(2)-Cl(1)
2.131(3) Mg(2)-C(19)
1.446(3) B(1)-N(2)
2.131(2)
2.129(2)
2.530(1)
2.138(3)
1.456(3)
65.73(8)
95.76(7)
Mg(1)-N(2)
Mg(1)-Cl(1)
Mg(1)-C(15)
B(1)-N(1)
N(2)-Mg(1)-N(1)
N(1)-Mg(1)-Cl(1)
N(1)-Mg(1)-C(15) 138.9(1)
C(15)-Mg(1)-Cl(1) 115.37(8)
N(1)-B(1)-N(2)
N(2)-B(1)-C(1)
Figure 3. Thermal ellipsoid plot of {[PhB(µ-NDipp)2]Mg(OEt2)2} (10).
For clarity, H atoms are omitted and only the oxygen atoms of the Et2O
molecules are shown.
65.53(8)
95.36(6)
N(2)-Mg(2)-N(1)
N(1)-Mg(2)-Cl(1)
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for
{[PhB(µ-NDipp)2]Mg(OEt2)2} (10)
N(1)-Mg(2)-C(19) 137.8(1)
C(19)-Mg(2)-Cl(1) 115.11(9)
105.6(2)
127.7(2)
N(1)-B(1)-C(1)
126.7(2)
Mg(1)-N(1)
Mg(1)-O(1)
B(1)-N(1)
1.985(3) Mg(1)-N(2)
2.017(3) Mg(1)-O(2)
1.439(5) B(1)-N(2)
1.973(3)
2.015(3)
1.449(5)
N(1)-Mg(1)-O(1) 129.7(1)
N(1)-Mg(1)-O(2)
O(1)-Mg(1)-O(2)
B(1)-N(2)-C(19)
B(1)-N(2)-Mg(1)
117.4(1)
The mean B-N bond distances [1.451(3) Å] in 8 are equal
within experimental error; cf. |d(B-N)| ) 1.446(4) Å in 3.
While the Li-O distances involving the bridging THF
molecule in 3 are significantly different [2.270(7) and
2.013(6) Å], the chloride ion in 8 bridges the magnesium
atoms symmetrically [2.536(1) Å, Mg(1)-Cl(1); 2.530(1)
Å, Mg(2)-Cl(1)]. The lithio am complex Li[nBuC(NtBu)2]
has been shown to entrap LiX salts to form complexes such
as {(Li[nBuC(NtBu)2])2‚LiCl‚THF}2.22
The boron atom in 8 exhibits a distorted trigonal-planar
geometry [ 360.0(2)°], while the magnesium atoms adopt
distorted tetrahedral geometries [bond angle range ca. 66-
138.5°] with the smallest angles associated with the N-Mg-N
angle. The NBN angle [105.6(2)°] in 8 is similar to those in
6a (ca. 107°), but significantly smaller than that in 3
[111.4(3)°].11d The larger angle in the latter presumably
reflects the accommodation of the bulkier NDipp groups in
3. As a result of the two four-coordinate nitrogen centers,
the two four-membered N-B-N-Mg rings in 8 are
puckered in a manner similar to that observed for 6a.
The X-ray structure of {[PhB(µ-NDipp)2]Mg(OEt2)2} (10)
shows that the [PhB(NDipp)2]2- ligand is coordinated to a
single Mg2+ cation in an N,N′-chelated fashion. The coor-
dination sphere is completed by oxygen atoms from two THF
molecules. The molecular geometry and atomic numbering
scheme are shown in Figure 3, while pertinent structural
parameters are summarized in Table 3.
N(1)-Mg(1)-N(2)
B(1)-N(1)-C(1)
B(1)-N(1)-Mg(1)
C(1)-N(1)-Mg(1) 147.4(2)
N(1)-B(1)-N(2)
N(1)-B(1)-C(13)
74.7(1)
125.4(3)
86.3(2)
96.4(1)
125.7(3)
86.4(2)
C(19)-N(2)-Mg(1) 145.2(2)
N(2)-B(1)-C(13) 123.2(3)
112.5(3)
124.3(3)
geometry [bond angle range: 74.7(1)-129.7(1)°] with the
smallest angle associated with the N-Mg-N bond angle.
The B-N distances [1.439(5) Å, B(1)-N(1); 1.449(5) Å,
B(1)-N(2)] reflect substantial nitrogen-boron multiple-bond
character.
The N-B-N bond angle of 112.5(3)° in 10 is larger than
those observed in 6a and 8 (ca. 107°) but similar to that in
the dilithiated bam complex 3 [111.4(3)°].11d The larger NBN
angles observed in 3 and 10 accommodate the bulkier NDipp
groups in the bam ligand in these complexes. The geometries
at the boron and nitrogen atoms of the [PhB(NDipp)2]2-
ligand in 10 are distorted trigonal planar [ (deg): 360.0(3),
B(1); 359.1(3), N(1); 357.3(3), N(2)]. The four-membered
Mg(1)-N(2)-B(1)-N(1) ring is essentially planar (0.0040
Å mean deviation).
Syntheses and Characterization of Group 13 bam
Complexes. We have previously reported the syntheses and
structural characterization of the monolithiated spirocyclic
complexes {(Et2O)µ-Li[PhB(µ-NtBu)2]2M} (11b, M ) Ga;
11c, M ) In) by the reactions of 1a with MCl3 in a 2:1
molar ratio.16 Attempts to prepare the analogous aluminum
complex {(Et2O)µ-Li[PhB(µ-NtBu)2]2Al} (11a) by this method
were unsuccessful. Complex 11a can be obtained, however,
in a two-step process involving the initial preparation of the
mono-bam complex {[PhB(µ-NtBu)2]AlCl(OEt2)} (12), which
can be isolated in 60% yield from the reaction of 1a and
AlCl3 in a 1:1 molar ratio, followed by treatment of 12 with
1a (Scheme 5).
The Mg-N distances of ca. 1.98 Å in 10 are significantly
shorter than those observed in the dimagnesiated complex 8
(ca. 2.13 Å). The formal 2+ charge on magnesium in 10 is
a likely major contributor to the observed difference. The
magnesium center in 10 adopts a distorted tetrahedral
(22) Chivers, T.; Downard, A.; Parvez, M. Inorg. Chem. 1999, 38, 4347.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2123

Chivers et al.
Scheme 6
Figure 4. Thermal ellipsoid plot of {[PhB(µ-NtBu)2]AlCl(OEt2)} (12).
For clarity, H atoms are omitted.
Scheme 5
range at B(1), 106.36(9)-128.5(1)°; bond angle range at N(1)
and N(2), ca. 87-140°].
The Al-Cl distance of 2.1402(5) Å in 12 falls between
those reported for aluminum mono- and bis-am complexes,
ca. 2.10 and 2.19 Å, respectively.6b The B-N distances
[1.442(2) and 1.455(2) Å] indicate significant multiple-bond
character. The Al-N bond lengths [1.806(1) and 1.802(1)
Å] are shorter by ca. 0.07 Å than the Al-N distances in
related mono-am complexes,6b presumably as a result of the
dianionic charge of the bam ligand in 12. There are no
intermolecular Al‚‚‚Cl interactions present in the unit cell
of 12.
Table 4. Selected Bond Lengths (Å) and Bond Angles (deg) for
{[PhB(µ-NtBu)2]AlCl(OEt2)} (12)
Al(1)-Cl(1)
2.1402(5) Al(1)-O(1)
1.8641(9)
1.802(1)
1.455(2)
Al(1)-N(1)
1.806(1)
1.442(2)
79.99(4)
115.05(5)
132.26(9)
86.83(7)
Al(1)-N(2)
B(1)-N(2)
The reaction of 12 with 1a in a 1:1 molar ratio in boiling
benzene in the presence of a small amount of diethyl ether
produces {(Et2O)µ-Li[PhB(µ-NtBu)2]2Al} (11a) in 33% yield
(Scheme 6). The multinuclear NMR spectra of 11a are
consistent with a spirocyclic structure similar to that previ-
ously established for 11b,c.16 The 1H NMR spectrum of 11a
in C6D6 shows resonances for the Ph and NtBu groups, in
addition to the coordinated Et2O molecule, with the ap-
B(1)-N(1)
N(1)-Al(1)-N(2)
N(1)-Al(1)-O(1)
B(1)-N(1)-C(10)
B(1)-N(1)-Al(1)
N(1)-Al(1)-Cl(1) 124.18(4)
N(2)-Al(1)-Cl(1) 124.62(4)
B(1)-N(2)-C(20)
B(1)-N(2)-Al(1)
131.93(9)
86.62(7)
C(10)-N(1)-Al(1) 140.19(8)
N(1)-B(1)-C(1)
N(1)-B(1)-N(2)
C(20)-N(2)-Al(1) 140.40(8)
N(2)-B(1)-C(1)
128.5(1)
125.2(1)
106.36(9)
The structure of 12 was established by a single-crystal
X-ray analysis (Figure 4), and pertinent structural parameters
are summarized in Table 4.23 Solvation of the aluminum
center by diethyl ether apparently prevents dimerization; cf.
the dimeric structure of the unsolvated complex {[PhB(µ3-
NtBu)(µ-NtBu)]GaCl}2.16 The geometry around the aluminum
center in 12 is distorted tetrahedral [bond angle range:
79.99(4)-124.62(4)°]. The bond angle N-Al-N is acute
[79.99(4)°] but larger than the mean value of 70.91(5)°
observed for aluminum am complexes of the type RC(µ-
1
propriate relative intensities. The H and 13C NMR spectra
of 11a in C6D6 at room temperature reveal the equivalence
of all four NtBu environments indicative of a fluxional
process involving the lithium center, as observed for 11b,c.16
7
The 27Al, 11B, and Li NMR spectra of 11a exhibit singlets
at δ 488.2, 36, and 0.89, respectively.
In an attempt to synthesize the boron analogue of 11a,
the reaction of 1a with BF3‚OEt2 in a 2:1 molar ratio was
1
investigated. The H and 11B NMR spectra recorded as a
i
t
i
NR′)2]AlCl2 [R ) Me, R′ ) Pr; R ) Bu, R′ ) Pr, Cy].6b
The [PhB(µ-NtBu)2]Al core in 12 forms a nearly planar
metallacycle with an N(2)-Al(1)-N(1)-B(1) torsion angle
of 3.05(7)°. The boron and three-coordinate nitrogen atoms
exhibit distorted trigonal-planar coordination ( ca. 360°),
the latter atoms displaying a larger disparity [bond angle
function of time revealed the existence of more than one
reaction pathway. The initial observation of two equally
intense resonances at δ 43 and 30 in the 11B NMR spectrum
(indicative of inequivalent three-coordinate boron environ-
ments), associated with tBu and Ph resonances with relative
intensities of 18:5 in the 1H NMR spectrum, is attributed to
the formation of the complex [PhB(µ-NtBu)2]BF (13)
(Scheme 6). Subsequently, the generation of the spirocyclic
(23) Although complex 12 was mentioned in the preliminary communica-
tion,17 no structural details were given.
2124 Inorganic Chemistry, Vol. 45, No. 5, 2006

Magnesium and Zinc Boraamidinates
(M ) In, B) to evaluate their frontier orbitals. The HOMOs
(highest-occupied molecular orbitals) of these diamagnetic
anions are nondegenerate and exhibit equal delocalization
of spin density over all four nitrogen centers. Thus, like 5a,b,
the corresponding spirocyclic radicals {[PhB(µ-NtBu)2]2In}•
(5c) and {[PhB(µ-NtBu)2]2B}• (5d) are expected to retain
the D2d symmetry of their parent diamagnetic anions in
solution. The calculated structures for the model radicals
{[PhB(µ-NMe)2]2M}• (M ) In, B) can be compared to the
structures determined for the analogous aluminum and
gallium systems by X-ray crystallography.17 As expected,
the M-N bond lengths in M ) In and B derivatives are
considerably longer (2.073 Å) and shorter (1.532 Å),
respectively, than the corresponding bond lengths in M )
Al or Ga derivatives.17 The geometries of the two MNBN
rings are close to square planar in the all-boron system
{[PhB(µ-NMe)2]2B}• (bond angle range: ca. 85-97°),
whereas the indium analogue displays distinctly asymmetric
InNBN rings with very acute N-In-N bond angles (68°);
the geometries of the MNBN rings in the M ) Al and Ga
derivatives fall between the values calculated for the indium
and boron systems.17 These differences in binding of the two
bam ligands to the central atom reflect the different sizes of
the group 13 elements M ) B, Al, Ga, and In.
Figure 5. Photograph of diethyl ether solutions of the neutral spirocyclic
radicals {[PhB(µ-NtBu)2]2M}• (left, M ) In, 5c; right, M ) B, 5d).
complex 11d is indicated by the appearance of a broad
resonance at δ 22 and a sharp singlet at δ 0 in the 11B NMR
spectrum (approximate intensity ratio 2:1), indicative of
three- and four-coordinate boron environments, respec-
t
tively.19 The borazine [PhF2B3N3 Bu3] (14) has been isolated
from this reaction and identified by X-ray crystallography
and multinuclear NMR spectra.24 The NMR resonances for
14 grow at the expense of those attributed to 13 during the
course of the reaction, and the formation of 14 is favored
relative to 11d when the reaction is carried out on a 1:1 molar
ratio. A possible route to 14 involving the cyclodimerization
of 13 to an eight-membered ring 15 is shown in Scheme
6.25 However, in view of the known stability of related B4N4
rings, e.g., (ClBNtBu)4,26 the direct formation of 14 from 13
cannot be ruled out.
EPR and DFT Investigations of Early Main-Group
Element and Zn bam Radicals. In a preliminary com-
munication, we described the one-electron oxidation of the
spirocyclic anions 4a,b to give the stable neutral radicals 5a
(dark red) and 5b (dark green) (Scheme 1).17,18 In this work,
we discuss the characterization of the indium and boron
analogues, 5c,d, respectively.
The oxidation of 11c with 0.5 equiv of iodine generates a
dark green solution of complex 5c (Figure 5) that, in contrast
to 5a,b, is unstable at ambient temperature. Thus, the
oxidation was carried out at -78 °C, and the paramagnetic
species formed were investigated by variable-temperature
(VT) EPR spectroscopy. The bright purple solutions of
complex 5d (Figure 5) formed by air oxidation of the
products of the reaction of 1a with BF3‚OEt2 in a 2:1 molar
ratio (vide supra) were also investigated by EPR spectro-
scopy. Whereas the intense dark green color of 5c lasts only
for minutes at room temperature,27 diethyl ether solutions
of radical 5d remain dark purple and exhibit similar EPR
spectra even after two years.
Considering the composition of the SOMO (singly oc-
cupied molecular orbital) of the model radicals {[PhB(µ-
NMe)2]2M}•, the experimental EPR spectra of both 5c,d will
exhibit hyperfine structure due to interaction of the unpaired
electron with four equivalent nitrogen (14N, I ) 1, 99.6%)
centers. In addition, hyperfine structure due to the central
9
9
metal (5c: 113In, I ) /2, 4.3%; 115In, I ) /2, 95.7%. 5d:
10B, I ) 3, 19.9%; 11B, I ) /2, 80.1%) and two equivalent
3
boron atoms of the bam ligands may be observed due to
spin polarization. The experimental EPR spectra of 5c
obtained at -40 °C and 5d obtained at 25 °C are shown in
Figures 6a and 7a, respectively. The experimental EPR
spectra are best simulated by assuming interactions of the
unpaired electron with the central main group element, four
equivalent nitrogen centers, and two equivalent boron atoms.
The simulations shown in Figures 6b and 7b for 5c,d,
respectively, were obtained by using the hyperfine coupling
(hfc) constants given in Table 5, in which there is a fairly
good agreement between the experimental and the calculated
hfc values (Table 5). Hence, the experimental and simulated
EPR spectra, together with DFT calculations, indicate that
the neutral radicals {[PhB(µ-NtBu)2]2In}• (5c) and {[PhB-
(µ-NtBu)2]2B}• (5d) adopt spirocyclic structures in solution
as depicted in Scheme 1.
(26) Bowser, J. R.; Neilson, R. H.; Wells, R. L. Inorg. Chem. 1978, 17,
1882.
(27) After a few minutes, the EPR spectrum of 5c is replaced by an
asymmetric spectrum indicating the presence of at least two radical
species. The outer ends of this spectrum exhibit a pentet with 34 G
width indicating hfc to two equivalent 14N nuclei (A ) 8.5 G). This
pattern is repeated 10 times suggesting coupling to a single 115In center.
No coupling to boron was resolved. Although a possible candidate
for this radical is the monocyclic bam complex [PhB(µ-NtBu)2In]• ,
Theoretical calculations were first carried out on the D2d-
symmetric diamagnetic model systems {[PhB(µ-NMe)2]2M}-
(24) Chivers, T.; Fedorchuk, C.; Parvez, M. Acta Crystallogr. 2005, C61,
o47.
(25) The factors that determine the outcome of the cyclodimerization of
B2N2 rings are not well understood: Gilbert, T. M.; Gailbreath, B. D.
Organometallics 2001, 20, 4727.
the calculated value of a115 for this radical is much higher than the
In
observed value of ∼21 G.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2125

Chivers et al.
Table 5. Experimental and Calculated Hyperfine Coupling Constants
t
(G) for {[PhB(µ-NR′)2]2M}• (M ) In, B; R′ ) Bu, Me)a
{[PhB(µ-NR)2]2M}
M ) In
isotope
nb exptl (R ) tBu) calcd (R ) Me)
113In/115In
14N
1
4
2
1
4
2
22.50/41.00
4.70
1.70/5.18
2.31/6.92
5.46
c
4.10
-2.03/-6.10
-2.81/-8.43
4.59
10B/11B
10B/11B
14N
M ) B
10B/11B
1.85/5.56
-2.21/-6.64
a Parameters were determined from simulations optimized to match the
experimental spectra. 100% Lorentzian line shapes: g ) 2.00407, M ) In;
g ) 2.00405, M ) B. b Number of nuclei. c Unavailable (see Experimental
Section).
Figure 6. Experimental (a) and simulated (b) X-band EPR spectra of a
diethyl ether solution of {[PhB(µ-NtBu)2]2In}• (5c) at -40 °C.
Figure 8. Photograph of diethyl ether solutions of the radicals
{(Et2O)Li[PhB(µ-NtBu)2]2M}• (left, M ) Mg, 16a; right, M ) Zn, 16b).
Scheme 7
inequivalent nitrogen atoms. In addition, hyperfine structure
arising from interactions with the central metal (16a, 25Mg,
I ) 5/2, 10%; 16b, 67Zn, I ) 5/2, 4.1%), one boron atom, and
minor contributions from the adjacent bam ligand, i.e., two
equivalent nitrogen atoms and/or one boron atom, is expected
due to spin polarization effects.
The experimental EPR spectrum obtained at room tem-
perature for 16b is shown in Figure 10a; the spectrum for
16a exhibits a similar pattern owing to the low natural
abundance of the spin-active isotopes for magnesium and
zinc. Excellent simulations (Figure 10b) were obtained by
using the hfc constants given in Table 6. The experimental
and calculated hfc values are in close agreement. Hence, for
both radicals, the EPR spectra support the prediction from
DFT calculations that the unpaired electron is localized on
one bam ligand. The EPR spectra for the zinc radical 16b
reveal minor interactions with the adjacent bam ligand, as
well as some low-intensity satellites due to coupling of the
unpaired electron to the 67Zn nucleus.
Figure 7. Experimental (a) and simulated (b) X-band EPR spectra of a
diethyl ether solution of {[PhB(µ-NtBu)2]2B}• (5d) at 25 °C.
The oxidation of the dilithiated Mg or Zn spirocyclic bis-
bam complexes 6a,c (Scheme 7) was also investigated by
EPR spectroscopy. Addition of 0.5 equiv of iodine to
colorless solutions of 6a or 6c in diethyl ether at room
temperature generates persistent pink or bright purple solu-
tions, respectively (Figure 8). DFT calculations for the neutral
radicals {(Et2O)Li[PhB(µ-NtBu)2]2M}• (16a, M ) Mg; 16b,
M ) Zn) predict Cs-symmetric structures and localized spin
density on the bam ligand that is not coordinated to Li+ (see
Scheme 7 and Figure 9). Consequently, the EPR spectra of
both radicals are expected to show hyperfine structure
resulting from interaction of the unpaired electron with two
The first indications of the formation of paramagnetic early
main-group metal bam complexes came from investigations
of lithium derivatives of the type 1.11c It was observed that
the initially colorless solutions of 1a-c became red upon
2126 Inorganic Chemistry, Vol. 45, No. 5, 2006

Magnesium and Zinc Boraamidinates
Figure 9. Optimized structure (left; bond lengths in Å, bond angles in deg) and SOMO (right; isosurface values ( 0.06) of {(Me2O)Li[PhB(µ-NMe)2]2Mg}•.
Figure 10. Experimental (a) and simulated (b) X-band EPR spectra of a
diethyl ether solution of {(Et2O)Li[PhB(µ-NtBu)2]2Zn}• (16b) at 25 °C.
Figure 11. Experimental (a) and simulated (b) X-band EPR spectra of a
diethyl ether solution of {[PhB(µ-NtBu)2]Li(OEt2)x}• (17) at -40 °C.
Table 6. Experimental and Calculated Hyperfine Coupling Constants
t
(G) for {(Et2O)Li[PhB(µ-NR′)2]2M}• (M ) Mg, Zn; R′ ) Bu, Me)a
Scheme 8
{(Et2O)Li[PhB(µ-NR)2]2M} isotope nb exptl (R ) tBu) calcd (R ) Me)
M ) Mg
10B/11
B
1
1
1
1
1
2
1
1
1
1
1
1
2
1
3.55/10.64
8.30
8.55
-4.01/-12.30
7.28
7.89
14N
14N
25Mg
10B/11
14N
7Li
1.70
1.85
B
B
n.o.c
-0.02
-0.11
-0.01
-4.06/-12.19
7.97
n.o.
n.o.
3.61/10.70
9.15
9.34
8.80
n.o.
0.45
n.o.
M ) Zn
10B/11
14N
14N
8.36
We have now carried out the oxidation of 1a with 0.5
equiv of iodine in diethyl ether at room temperature (Scheme
8). The bright pink species so formed is unstable at room
temperature. Consequently, the solutions used for VT EPR
studies were prepared at -78 °C. The EPR spectrum obtained
at -40 °C is shown in Figure 11a. This spectrum is best
simulated (see Table 7) with hyperfine interactions of the
unpaired electron with one lithium atom (7Li, I ) 3/2, 92.41%;
6Li, I ) 1, 7.59%), two equivalent nitrogen centers, and one
boron atom, suggesting the formation of the monocyclic
radical {[PhB(µ-NtBu)2]Li(OEt2)x}• (17), in which the radical
monoanion [PhB(NtBu)2]•- is chelated to a diethyl ether-
solvated lithium center.
67Zn
10B/11
14N
-7.82
-0.01
-0.23
-0.03
B
7Li
a Parameters were determined from simulations optimized to match the
experimental spectra. 100% Lorentzian line shapes: g ) 2.00446, M )
Mg; g ) 2.00526, M ) Zn. b Number of nuclei. c n.o. indicates no
observable hfc constant.
exposure to air. In preliminary EPR studies, only coupling
to two equivalent 14N nuclei (a five-line pattern) was
resolved. In the absence of resolved coupling to 7Li centers,
it was not possible to speculate on the state of aggregation
of the radical species.11c
Inorganic Chemistry, Vol. 45, No. 5, 2006 2127

Chivers et al.
Figure 12. Optimized structure (left; bond lengths in Å, bond angles in deg) and SOMO (right; isosurface values ( 0.06) of {[PhB(µ-NMe)2]Li(OMe2)2}•.
Chart 2
Table 7. Experimental and Calculated Hyperfine Coupling Constants
lithium or group 13 metal centers indicate the qualitative
trend in stabilities depicted in Chart 2.
t
(G) for {[PhB(µ-NR′)2]Li}• (R′ ) Bu, Me)a
isotope
nb
exptl (R ) tBu)
calcd (R ) Me)
For the spirocyclic systems, the higher stability of the B-,
Al-, and Ga-containing radicals compared to the Mg and Zn
systems can be attributed to the delocalization of the unpaired
electron over both bam ligands in the group 13 complexes.
The high stability of the boron-centered radical 5d in solution
is noteworthy. In contrast to 5a,b, however, it has not yet
been possible to devise a synthetic route that will allow the
isolation of 5d in the solid state. The lower stability of 5c
may result from a combination of stronger intermolecular,
i.e., radical-radical, interactions as a result of the larger In
center and weaker In-N bonds compared to 5a,b,d. Inter-
molecular association to form larger aggregates is likely the
reason for the low stability of the mononuclear Li complex.
These results suggest that redox behavior is an important
feature of the chemistry of the bam ligand, as implied by
the frequent observation of intensely colored solutions in the
metathetical reactions of Li2bam reagents with main-group
element halides. This fundamental difference between the
behavior of main-group metal complexes of bam ligands
compared to that of am ligands undoubtedly stems from the
dianionic charge in the former.
10B/11
14N
B
1
2
1
3.67/11.18
8.51
2.14
-4.24/-12.72
7.59
-2.39
7Li
a Parameters were determined from simulations optimized to match the
experimental spectra. 100% Lorentzian line shapes: g ) 2.00443. b Number
of nuclei.
The plausibility of the assignment of the experimental EPR
spectrum to the monocyclic radical was supported by DFT
calculations. Figure 12 shows the optimized structure and
SOMO calculated for the model system {[PhB(µ-NMe)2]Li-
(OEt2)x}•; the calculated hfc constants are listed in Table 7.
The excellent match between the calculated and experimental
values provides strong support for the identification of the
transient radical as the monolithium species {[PhB(µ-NtBu)2]-
Li(OEt2)x}•.
Conclusions
A variety of magnesium complexes of the dianionic bam
ligand have been synthesized and structurally characterized.
These magnesium reagents offer a potential alternative to
the widely used lithium derivatives for the transfer of bam
ligands to other metal centers. In common with other early
main-group metal bam complexes, these novel magnesium
and zinc complexes are readily oxidized to paramagnetic
species. The one-electron oxidation of the metal-coordinated
bam dianion [PhB(µ-NtBu)2]2- to the corresponding radical
anion [PhB(NtBu)2]•- can be controlled by using the ap-
propriate amount of iodine as the oxidizing agent. The
combination of EPR spectroscopic analyses and DFT cal-
culations provides information about both the molecular and
electronic structures, e.g., spin distribution, of these para-
magnetic complexes in solution. Similar studies of complexes
in which the radical anion [PhB(NtBu)2]•- is coordinated to
Experimental Section
Reagents and General Procedures. The compounds tBuMgCl
(2.0 M solution in Et2O), MesMgBr (1.0 M solution in Et2O),
nBuMgCl (2.0 M solution in Et2O), MeMgBr (3.0 M solution in
Et2O), MgCl2 (anhydrous beads, 10 mesh, 99.99%), PhMgCl (2.0
i
M solution in THF), PrMgCl (2.0 M solution in THF), n/sBu2Mg
(1.0 M solution in heptane), and BF3‚OEt2 were commercial samples
(Aldrich) and used as received. Anhydrous ZnCl2 was prepared by
heating ZnCl2‚H2O under vacuum (150 °C, 10-2 Torr, 18 h). The
reagents AlCl3 and I2 were sublimed prior to use.
The compounds Li2[PhB(µ3-NtBu)2] (1a),11c PhB[N(H)Dipp]2,11d
and {(Et2O)µ-Li[PhB(µ-NtBu)2]2In} (11c)16 were prepared by the
literature methods.
2128 Inorganic Chemistry, Vol. 45, No. 5, 2006

Magnesium and Zinc Boraamidinates
Solvents were dried with appropriate drying agents and distilled
onto molecular sieves before use. All reactions and the manipulation
of moisture- and/or air-sensitive reagents or products were carried
out under an atmosphere of argon or under vacuum using standard
Schlenk techniques or a glovebox. All glassware was carefully dried
prior to use.
Calcd for C36H62B2Li2MgN4O2: C, 67.28; H, 9.72; N, 8.71.
Found: C, 66.98; H, 10.18; N, 8.66. H NMR: δ 8.18-7.21 (m,
1
10 H, -C6H5), 3.33 [br s, 8 H, O(CH2)2(CH2)2], 1.49 [s, 18 H,
-C(CH3)3], 1.35 [s, 18 H, -C(CH3)3], 1.14 [br s, 8 H, O(CH2)2-
7
(CH2)2]. 11B NMR: δ 38 (br s). Li NMR: δ 1.43 (s). 13C NMR:
δ four resonances in the range 150.0-127.5 (-C6H5), 69.3
[O(CH2)2(CH2)2], 51.0 [-C(CH3)3], 50.9 [-C(CH3)3], 38.1 [-C(C-
H3)3], 38.0 [-C(CH3)3], 25.6 [O(CH2)2(CH2)2]. Complex 6b was
also prepared in the manner described above by using the Grignard
Instrumentation. 1H NMR spectra were recorded on Bruker AC
200 and DRX 400 spectrometers, and chemical shifts are reported
7
relative to Me4Si in CDCl3. Li, 11B, 13C, and 19F NMR spectra
i
reagent PrMgCl (2.0 M solution in THF).
were measured on a Bruker DRX 400 or a Bruker AMX2-300
spectrometer. All NMR spectra were obtained on C6D6 solutions
at 25 °C. Chemical shifts are reported relative to those of BF3‚
OEt2 in C6D6, Me4Si in CDCl3, 1.0 M LiCl in D2O, and CFCl3 in
C6D6, respectively. Infrared spectra were obtained as Nujol mulls
between KBr plates on a Nicolet Nexus 470 FT-IR spectrometer
in the range 4000-350 cm-1. Elemental analyses were provided
by the Analytical Services Laboratory of the Department of
Chemistry, University of Calgary. X-band EPR spectra were
recorded on a Bruker EMX 113 spectrometer equipped with a
variable-temperature accessory. EPR spectral simulations were
carried out by using the XEMR v. 0.728a and WINEPR SimFonia
v. 1.2528b programs.
Preparation of {(THF)µ-Li[PhB(µ-NtBu)2]}2Zn (6c). A solu-
tion of 1a (0.27 g, 1.12 mmol) in Et2O (100 mL) was added to
solid ZnCl2 (0.08 g, 0.56 mmol) at -78 °C producing a dark
burgundy reaction mixture, which was allowed to warm to room
temperature over a period of 20 min and then set to reflux for 18
h. The mixture was filtered and the solvent removed in vacuo. The
residue was washed several times with cold n-hexane to give white
amorphous 6c (0.20 g, 0.29 mmol, 51%). Anal. Calcd for C36H66B2-
Li2N4O2Zn: C, 62.86; H, 9.67; N, 8.15. Found: C, 62.28; H, 9.98;
1
N, 8.12. H NMR: δ 7.74-7.20 (m, 10 H, -C6H5), 3.36 [q, 8 H,
(CH3CH2)2O], 1.39 [s, 36 H, -C(CH3)3], 0.98 [t, 12 H, (CH3-
CH2)2O]. 11B NMR: δ 37 (br s). 7Li NMR: δ 1.16 (s). 13C NMR:
δ four resonances in the range 150.1-125.8 (-C6H5), 65.8
[(CH3CH2)2O], 51.2 [-C(CH3)3], 37.6 [-C(CH3)3], 15.1 [(CH3-
CH2)2O]. A concentrated solution of 6c in Et2O stored at -15 °C
for 6 h provided colorless crystals that were suitable for X-ray
diffraction studies.
Preparation of {(Et2O)µ-Li[PhB(µ-NtBu)2]}2Mg (6a). Method
t
1. A solution of BuMgCl in Et2O (2.0 M, 0.72 mL, 1.43 mmol)
was added to a solution of 1a (0.35 g, 1.43 mmol) in toluene (50
mL) at room temperature producing a white slurry that was stirred
for 18 h. The reaction mixture was filtered and the solvent removed
in vacuo. The resulting residue was dissolved in Et2O, and
concentration of the solution followed by storage at room temper-
ature for 30 min afforded X-ray-quality colorless crystals of 6a
(0.41 g, 0.63 mmol, 89%). Complex 6a was also prepared in the
manner described above by using the Grignard reagents MesMgBr
Preparation of {[PhB(µ3-NtBu)2](MgtBu)2(µ3-Cl)Li(OEt2)3}
(8). A solution of tBuMgCl in Et2O (2.0 M, 1.93 mL, 3.85 mmol)
in toluene (10 mL) was added to a solution of 1a (0.47 g, 1.93
mmol) in toluene (50 mL) at room temperature producing a white
slurry that was stirred for 18 h. The reaction mixture was filtered,
and the solvent was removed in vacuo. The residue was washed
twice with cold n-hexane affording a pale yellow powder (0.58 g,
0.88 mmol, 46%). A concentrated solution of 8 in diethyl ether (1
week, -15 °C) provided colorless X-ray-quality crystals. Anal.
Calcd for C26H51BClLiMg2N2O (8 - 2Et2O): C, 61.29; H, 10.09;
n
(1.0 M solution in Et2O), BuMgCl (2.0 M solution in Et2O), or
MeMgBr (3.0 M solution in Et2O).
Method 2. A solution of 1a (0.75 g, 3.06 mmol) in Et2O (100
mL) was added to solid MgCl2 (0.15 g, 1.53 mmol) at -78 °C
producing a dark burgundy reaction mixture, which was allowed
to warm to room temperature over a period of 20 min and then set
to reflux for 18 h. The resulting colorless solution was filtered and
the solvent removed in vacuo. The residue was washed several times
with cold n-hexane gave white amorphous 6a (0.74 g, 1.14 mmol,
75%). A concentrated solution of 6a in diethyl ether stored at -15
°C for 1 d provided colorless crystals suitable for X-ray diffraction
studies. Anal. Calcd for C36H66B2Li2MgN4O2: C, 66.86; H, 10.29;
1
N, 5.50. Found: C, 61.13; H, 10.81; N, 6.09. H NMR: δ 7.57-
6.99 (m, 5 H, -C6H5), 3.21 [q, 12 H, (CH3CH2)2O], 1.44 [s, 18 H,
-C(CH3)3], 1.12 [s, 18 H, -C(CH3)3], 0.86 [t, 18 H, (CH3CH2)2O].
11B NMR: δ 39 (br s). 7Li NMR: δ -0.37 (s). 13C NMR: δ four
resonances in the range 138.2-126.0 (-C6H5), 65.7 [(CH3CH2)2O],
50.6 [-C(CH3)3], 36.9 [-C(CH3)3], 35.5 [-C(CH3)3], 21.8
[-C(CH3)3], 14.2 [(CH3CH2)2O].
Preparation of {[PhB(µ3-NtBu)2](MgiPr)2(THF)2} (9a). A
1
N, 8.66. Found: C, 66.57; H, 10.86; N, 8.80. H NMR: δ 7.96-
i
solution of PrMgCl in THF (2.0 M, 1.11 mL, 2.22 mmol) was
7.20 (m, 10 H, -C6H5), 3.32 [q, 8 H, (CH3CH2)2O], 1.47 [s, 18 H,
-C(CH3)3], 1.32 [s, 18 H, -C(CH3)3], 0.98 [t, 12 H, (CH3CH2)2O].
added to a solution of 1a (0.27 g, 1.11 mmol) in toluene (50 mL)
at room temperature producing a white slurry that was stirred for
5 h. The reaction mixture was filtered to give a pale yellow filtrate
from which the solvent was removed in vacuo. The residue was
redissolved in Et2O and concentrated to afford 9a as colorless thin
needle crystals (0.21 g, 0.41 mmol, 37%). Anal. Calcd for C24H45-
BMg2N2O (9a - THF): C, 65.96; H, 10.38; N, 6.41. Found: C,
65.41; H, 10.76; N, 6.22. 1H NMR: δ 7.87-7.25 (m, 5 H, -C6H5),
3.59 [br m, 8 H, O(CH2)2(CH2)2], 1.75 [d, 12 H, -CH(CH3)2,
3J(1H-1H) ) 8 Hz], 1.27 [br m, 8 H, O(CH2)2(CH2)2], 1.17 [s, 18
H, -C(CH3)3], 0.32 [septet, 2 H, -CH(CH3)2, 3J(1H-1H) ) 8 Hz].
11B NMR: δ 39 (br s).13C NMR: δ four resonances in the range
133.9-126.6 (-C6H5), 70.5 [O(CH2)2(CH2)2], 50.4 [-C(CH3)3],
37.2 [-C(CH3)3], 26.4 [O(CH2)2(CH2)2], 25.7 [-CH(CH3)2], 9.1
[-CH(CH3)2].
7
11B NMR: δ 38 (br s). Li NMR: δ 1.44 (s). 13C NMR: δ four
resonances in the range 150.9-125.6 (-C6H5), 66.0 [(CH3CH2)2O],
51.0 [-C(CH3)3], 38.3 [-C(CH3)3], 37.9 [-C(CH3)3], 15.1 [(CH3-
CH2)2O].
Preparation of {(THF)µ-Li[PhB(µ-NtBu)2]}2Mg (6b). A solu-
tion of PhMgCl in THF (2.0 M, 0.33 mL, 0.66 mmol) was added
to a solution of 1a (0.16 g, 0.66 mmol) in toluene (25 mL) at room-
temperature producing a cloudy pale yellow mixture that was stirred
for 18 h. The reaction mixture was filtered, and the solvent was
removed in vacuo. The residue was redissolved in THF and
concentration of the solution followed by storage at -15 °C for 3
d afforded colorless crystals of 6b (0.15 g, 0.23 mmol, 71%). Anal.
(28) (a) Eloranta, J. XEMR, version 0.7; University of Jyva¨skyla¨: Jyva¨skyla¨,
Finland, 2004. (b) WINEPR SimFonia, version 1.25; Bruker Ana-
lytische: Messtechnik, Germany, 1996.
Preparation of {[PhB(µ3-NtBu)2](MgPh)2(THF)2} (9b). A
solution of PhMgCl in THF (2.0 M, 0.60 mL, 1.21 mmol) was
Inorganic Chemistry, Vol. 45, No. 5, 2006 2129

Chivers et al.
Table 8. Selected Crystal Data and Data Collection Parameters for {(Et2O)µ-Li[PhB(µ-NtBu)2]}2M (M ) Mg, 6a; M ) Zn, 6c),
{[PhB(µ3-NtBu)2](MgtBu)2(µ3-Cl)Li(OEt2)3} (8), {[PhB(µ-NDipp)2]Mg(OEt2)2} (10), and {[PhB(µ-NtBu)2]AlCl(OEt2)} (12)a
param
formula
6a
6c
8
10b
12
C36H66B2Li2MgN4O2
646.74
P21/n
9.977(2)
17.347(4)
24.129(5)
90
C36H66B2Li2N4O2Zn
687.80
P21/n
13.057(3)
17.268(4)
19.428(4)
90
109.42(3)
90
4131.4(1)
4
C34H71BClLiMg2N2O3
657.75
C76H118B2Mg2N4O4
1221.98
P1h
C18H33AlBClN2O
670.65
P21/n
11.8251(2)
13.4014(2)
14.6888(2)
90
106.965(1)
90
2226.48(6)
4
fw
space group
a, Å
b, Å
P1h
10.142(2)
12.481(3)
18.091(4)
89.81(3)
81.72(3)
73.45
11.008(2)
17.524(4)
21.343(4)
111.85(3)
102.36(3)
90.73(3)
3713.4(2)
2
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
92.00(3)
90
4173.3(1)
4
2170.4(7)
2
1.006
0.146
724
d
calcd, g cm-3
1.029
0.075
1416
0.0484
0.1166
1.106
0.627
1488
0.0749
0.1726
1.093
0.081
1336
0.0754
1.094
0.218
792
0.0400
0.1137
µ, mm-1
F(000)
Rc
0.0609
0.1548
d
Rw
0.1855
a Temperature ) 173(2) K; wavelength ) 0.710 73 Å. b The complex crystallizes with two independent but chemically equivalent molecules in the
2
2
2
asymmetric unit. c R ) Σ||Fo| - |Fc||/Σ|Fo| (I > 2.00σ(I)). d Rw ) {[Σw(Fo - Fc )2]/[Σw(Fo )2]}1/2 (all data).
added to a solution of 1a (0.15 g, 0.61 mmol) in toluene (50 mL)
at room temperature producing a cloudy pale yellow reaction
mixture that was stirred for 12 h. The reaction mixture was filtered,
isolating a pale yellow filtrate from which the solvent was removed
in vacuo. The residue was washed twice with n-hexane affording
9b as a yellow powder (0.23 g, 0.40 mmol, 66%). Thin needle
crystals of 9b were isolated from a concentrated solution in Et2O
(9 d, -15 °C). Anal. Calcd for C30H41BMg2N2O (9b - THF): C,
11B NMR: δ 36 (br s). 13C NMR: δ 155.0 (-C6H5), 132.7
(-C6H5), 127.4 (-C6H5), 126.3 (-C6H5), 66.4 [(CH3CH2)2O], 49.7
[-C(CH3)3], 35.6 [-C(CH3)3], 15.8 [(CH3CH2)2O]. X-ray-quality
crystals of 12 were isolated from a concentrated solution in Et2O
(1 d, -15 °C).
Preparation of {[PhB(µ-NtBu)2]2In}• (5c). A solution of I2 in
Et2O (0.60 mL, 0.0643 M, 0.038 mmol) was added to a colorless
solution of {(Et2O)µ-Li[PhB(µ-NtBu)2]2In} (11c) (0.050 g, 0.076
mmol) in Et2O (15 mL) at -80 °C instantly producing a dark green
solution. The reaction mixture was kept at -80 °C prior to recording
EPR spectra. UV-vis (Et2O): λmax ) 481 (br), 646 nm (br).
Preparation of {(Et2O)Li[PhB(µ-NtBu)2]2Mg}• (16a). A solu-
tion of I2 in Et2O (0.70 mL, 0.024 M, 0.017 mmol) was added to
a colorless solution of {(Et2O)µ-Li[PhB(µ-NtBu)2]}2Mg (6a) (0.022
g, 0.034 mmol) in Et2O (10 mL) at room temperature instantly
producing a bright pink solution. The resulting reaction mixture
was used for EPR studies. UV-vis (Et2O): λmax ) 521 (br).
Preparation of {(Et2O)Li[PhB(µ-NtBu)2]2Zn}• (16b). A solu-
tion of I2 in Et2O (0.84 mL, 0.044 M, 0.037 mmol) was added to
a colorless solution of {(Et2O)µ-Li[PhB(µ-NtBu)2]}2Zn (6c) (0.051
g, 0.074 mmol) in Et2O (10 mL) at room temperature instantly
producing a bright purple solution. The resulting reaction mixture
was studied by EPR spectroscopy. UV-vis (Et2O): λmax ) 562
(br).
1
71.34; H, 8.18; N, 5.55. Found: C, 70.88; H, 8.24; N, 5.82. H
NMR: δ 8.02-7.17 (m, 15 H, -C6H5), 3.60 [br m, 8 H, O(CH2)2-
(CH2)2], 1.33 [s, 18 H, -C(CH3)3], 1.09 [br m, 8 H, O(CH2)2-
(CH2)2]. 11B NMR: δ 40 (br s). 13C NMR: δ eight resonances in
the range 166.3-125.8 (-C6H5), 70.0 [O(CH2)2(CH2)2], 50.7
[-C(CH3)3], 37.5 [-C(CH3)3], 25.4 [O(CH2)2(CH2)2].
Preparation of {[PhB(µ-NDipp)2]Mg(OEt2)2} (10). A solution
of n/sBu2Mg in heptane (1.0 M, 3.16 mL, 3.16 mmol) was added to
a solution of PhB[N(H)Dipp]2 (1.39 g, 3.16 mmol) in n-hexane
(150 mL) at -30 °C producing a red reaction mixture. The mixture
was allowed to warm to room temperature over 20 min and then
set to reflux for 18 h. Concentration of the reaction mixture followed
by layering with diethyl ether and cooling (-15 °C, 2 d) gave X-ray-
quality colorless crystals of 10 (0.62 g, 1.01 mmol, 32%). Anal.
Calcd for C38H59BMgN2O2: C, 74.70; H, 9.73; N, 4.58. Found:
1
C, 74.11; H, 9.50; N, 4.29. H NMR: δ 7.55-7.02 (m, 11 H, Ph
3
and aryl of Dipp), 4.00 [septet, 4 H, -CH(CH3)2, J(1H-1H) ) 8
Preparation of {[PhB(µ-NtBu)2]Li(OEt2)x}• (17). A solution
of I2 in Et2O (0.84 mL, 0.044 M, 0.037 mmol) was added to a
colorless solution of Li2[PhB(µ3-NtBu)2] (1a) (0.051 g, 0.074 mmol)
in Et2O (10 mL) at -80 °C instantly producing a pink solution.
This solution was kept at -80 °C prior to recording EPR spectra.
Hz], 3.16 [q, 8 H, (CH3CH2)2O], 0.97 [d, 24 H, -CH(CH3)2, 3J(1H-
1H) ) 8 Hz], 0.73 [t, 12 H, (CH3CH2)2O]. 11B NMR: δ 31 (br s).
13C NMR: δ eight resonances in the range 152.5-119.4 (Ph and
aryl of Dipp), 67.1 [(CH2CH2)2O], 35.1 [-CH(CH3)2], 22.9 [-CH-
(CH3)2], 14.6 [(CH2CH2)2O]. IR (cm-1): no N-H stretch.
Preparation of {[PhB(µ-NtBu)2]AlCl(OEt2)} (12). A colorless
solution of 1a (0.46 g, 1.86 mmol) in Et2O (50 mL) was added to
solid AlCl3 (0.25 g, 1.86 mmol) cooled to -78 °C producing a
bright purple reaction mixture. After 15 min, the stirred reaction
mixture was allowed to reach room temperature affording a clear
colorless solution. After an additional 15 min at room temperature,
a cloudy pale yellow reaction mixture was produced that was stirred
for 8 h. The resulting mixture was filtered to remove LiCl. Removal
of solvent in vacuo and addition of cold n-hexane gave a pale yellow
precipitate of 12 (0.41 g, 1.11 mmol, 60%) that was washed twice
with cold n-hexane. Anal. Calcd for C14H23AlClBN2: C, 58.95;
H, 9.07; N, 7.64. Found: C, 58.42; H, 9.25; N, 7.72. 1H NMR: δ
7.63 (d, 2 H, -C6H5), 7.24 (m, 3 H, -C6H5), 3.76 [q, 4 H,
(CH3CH2)2O], 1.22 [s, 18 H, -C(CH3)3], 0.81 [t, 6 H, (CH3CH2)2O].
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; 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.: Pittsburgh, PA, 2003.
2130 Inorganic Chemistry, Vol. 45, No. 5, 2006

Magnesium and Zinc Boraamidinates
X-ray Analyses. Relevant parameters for the data collections
and crystallographic data are summarized in Table 8. Thermal
ellipsoid plots are shown at the 30% probability level, and only
selected carbon atoms are labeled.
Computational Details. All calculations were done with the
Gaussian 03 program.29 The structures of all compounds were
optimized in their ground states using density functional theory.
Hybrid PBE0 exchange-correlation functional30 and Ahlrichs’ TZVP
basis sets31 were used in all optimizations (the corresponding ECP
basis set was used for indium). Hyperfine coupling constants of
paramagnetic species were calculated by single point calculations
employing the optimized geometries, PBE0 functional, TZVP basis
sets, and unrestricted Kohn-Sham formalism. For {[PhB(µ-
NMe)2]2In}•, the use of a quasi-relativistic ECP for the heavier
nucleus indium prevents the direct determination of its hfc constants
using the applied method.
Acknowledgment. The authors gratefully acknowledge
financial support from the Natural Sciences and Engineering
Research Council (Canada), the Alberta Ingenuity Fund,
Helsingin Sanomain 100-vuotissa¨a¨tio¨ (H.M.T), and the
University of Calgary.
(30) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865. (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett.
1997, 78, 1396. (c) Perdew, J. P.; Burke, K.; Ernzerhof, M. J. Chem.
Phys. 1996, 105, 9982. (d) Ernzerhof, M.; Scuseria, G. E. J. Chem.
Phys. 1999, 110, 5029.
(31) The basis sets were used as they are referenced in the Turbomole
5.7.1 internal basis set library. These basis sets can be downloaded
from the following: ftp://ftp.chemie.uni-karlsruhe.de/pub/basen/for free
of charge. Site accessed Feb 2005.
Supporting Information Available: Full experimental details
of the X-ray analyses and crystallographic data in CIF format for
complexes 6a,c, 8, 10, and 12. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC0520014
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