In search of the [PhB(μ-NtBu)2]2As ? radical: Experimental and computational investigations of the redox chemistry of group 15 bis-boraamidinates

Article abstract of DOI:10.1021/ic702435e

DFT calculations for the group 15 radicals [PhB(μ-N^tBu) ₂]₂M^? (M = P, As, Sb, Bi) predict a pnictogen-centered SOMO with smaller contributions to the unpaired spin density arising from the nitrogen and boron ato

Full text of DOI:10.1021/ic702435e

Inorg. Chem. 2008, 47, 3823-3831
In Search of the [PhB(µ-NtBu)2]2As• Radical: Experimental and
Computational Investigations of the Redox Chemistry of Group 15
Bis-boraamidinates
Jari Konu,† Heikki M. Tuononen,‡ Tristram Chivers,*,† Andrea M. Corrente,† René T. Boeré,§
and Tracey L. Roemmele§
Department of Chemistry, UniVersity of Calgary, Calgary, Alberta T2N 1N4, Canada, Department
of Chemistry, UniVersity of JyVäskylä, P.O. Box 35, JyVäskylä, FI-40014, Finland, and
Department of Chemistry and Biochemistry, UniVersity of Lethbridge, Lethbridge,
Alberta T1K 3M4, Canada
Received December 17, 2007
DFT calculations for the group 15 radicals [PhB(µ-NtBu)2]2M• (M ) P, As, Sb, Bi) predict a pnictogen-centered
SOMO with smaller contributions to the unpaired spin density arising from the nitrogen and boron atoms. The
t
reactions of Li2[PhB(µ-NR)2] (R ) Bu, Dipp) with PCl3 afforded the unsolvated complex LiP[PhB(µ-NtBu)2]2 (1a)
in low yield and ClP[PhB(µ-NDipp)2] (2), both of which were structurally characterized. Efforts to produce the
arsenic-centered neutral radical, [PhB(µ-NtBu)2]2As•, via oxidation of LiAs[PhB(µ-NtBu)2]2 with one-half equivalent
of SO2Cl2, yielded the Zwitterionic compound [PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph] (3) containing one four-coordinate
boron center with a B-Cl bond. The reaction of 3 with GaCl3 produced the ion-separated salt, [PhB(µ-
NtBu)2]2As+GaCl4- (4), which was characterized by X-ray crystallography. The reduction of 3 with sodium
naphthalenide occurred by a two-electron process to give the corresponding anion [{PhB(µ-NtBu)2}2As]- as the
sodium salt. Voltammetric investigations of 4 and LiAs[PhB(µ-NtBu)2]2 (1b) revealed irreversible processes. Attempts
to generate the neutral radical [PhB(µ-NtBu)2]2As• from these ionic complexes via in situ electrolysis did not produce
an EPR-active species.
Introduction
Perhaps the most intriguing consequence of the 2- charge
is the facile tendency for redox transformations to occur in
which the [bam]2- dianion is oxidized to the corresponding
monoanion radical [bam]–•. This paramagnetic ligand can be
stabilized through chelation, e.g., to early main-group met-
als.3–5 In the case of certain group 13 metals, it has been
possible to isolate stable neutral radicals, for example, the
highly colored spirocyclic systems {M[PhB(µ-NtBu)2]2}• (M
) Al, Ga) and determine their X-ray structures.4 The SOMO
in these radicals is located primarily and equally in p-orbitals
on the four nitrogen atoms of the two bam ligands; there is
very little unpaired electron density on the group 13 centers.
The corresponding boron- and indium-containing radicals
{M[PhB(µ-NtBu)2]2}• (M ) B, In) were characterized in
The dianionic boraamidinate (bam) ligand, [RB(NR′)2]2-,
is formally isoelectronic with the extensively studied monoan-
ionic amidinate (am) ligand [RC(NR′)2]-. Recent work on
early main-group and transition-metal complexes has estab-
lished that the 2- charge on the bam ligand results in some
interesting fundamental differences compared to the behavior
of the monoanionic am ligand. The 2- charge lowers the
requirement for ancillary ligands around high oxidation state
metal centers. Consequently, homoleptic species of the type
[ML2]x- (x ) 0, 1, 2) and [ML3]2- (L ) bam) are common
features of main-group and transition-metal complexes.1–3
* To whom correspondence should be addressed. Telephone: (403) 220-
5741, Fax: (403) 289-9488, E-mail: chivers@ucalgary.ca.
†
University of Calgary.
University of Jyväskylä.
University of Lethbridge.
(3) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez, M. Inorg. Chem. 2003,
42, 2084.
‡
§
(4) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuononen, H. M.;
Boeré, R. T. Chem. Commun. 2005, 3930.
(1) For a recent review, see Fedorchuk, C.; Copsey, M. C.; Chivers, T.
Coord. Chem. ReV. 2007, 251, 897.
(5) Chivers, T.; Eisler, D. J.; Fedorchuk, C.; Schatte, G.; Tuononen, H. M.;
Boeré, R. T. Inorg. Chem. 2006, 45, 2119.
(2) Manke, D. R.; Nocera, D. G. Inorg. Chem. 2003, 42, 4431.
10.1021/ic702435e CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 9, 2008 3823
Published on Web 04/09/2008

Konu et al.
solution by EPR spectroscopy.5 However, the indium radical
is not sufficiently stable to be isolated in the solid state, while
the boron analogue, although it is thermally very stable, could
not be obtained in a pure form.
the synthesis and X-ray structures of the Zwitterionic
complex [PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph] (3) and the
ion-separated salt [PhB(µ-NtBu)2]2As+GaCl4 (4); (d) the
-
chemical reduction of 3 and the cation [PhB(µ-NtBu)2]2As+;
and (e) voltammetric studies of the bis(boraamidinate)arsenic
system.
In a recent investigation, we reported the first examples
of heavy group 15 boraamidinates in which the versatile
coordinating ability of the bam ligand was highlighted.6 In
addition to the monobams ClM[PhB(µ-NtBu)2] (A; M ) As,
Sb, Bi) with a potentially useful M-Cl reactive site, the
unusual 2:3 bam complexes, M2[PhB(µ-NtBu)2]3 (B; M )
Sb, Bi), displayed a unique bonding arrangement in which
two metal centers are each N,N′-chelated by one bam ligand
and the two [M(bam)]+ units are bridged by the third [bam]2-
ligand. For the 1:2 group 15 bam systems, both solvated
complexes (Et2O)LiM[PhB(µ-NtBu)2]2 (C; M ) As, Bi) and
unsolvated ladder structures LiM[PhB(µ-NtBu)2]2 (D; M )
As, Sb) were observed; these complexes showed interesting
fluxional behavior in solution (Berry pseudorotation) in the
case of M ) Sb, Bi.6 Although monofunctional phosphorus-
containing bam complexes of the type XP[PhB(µ-NtBu)2]
(X ) Cl, Br; type A) are known,7 a bis-bam complex
LiP[PhB(µ-NtBu)2]2 (type C or D) has not been reported.
Experimental Section
General Procedures. All reactions and the manipulations of
products were performed under an argon atmosphere by using
standard Schlenk techniques or an inert atmosphere glovebox. The
compounds PhBCl2 (Aldrich, 97%), GaCl3 (Aldrich, 99.99%), AsCl3
(Alfa, 99%), [nBu4N]Br3 (Aldrich, 98%), and NO[SbF6] (Aldrich,
99.9%) were used as received. PCl3 (Aldrich, 99%) was distilled
n
prior to use. LiN(H)tBu was prepared by the addition of BuLi to
t
a solution of anhydrous BuNH2 in n-hexane at -10 °C and its
1
purity was checked by H NMR spectroscopy. The compounds
PhB[N(H)tBu]2,8a Li2[PhB(µ-NtBu)2],8a Li2[PhB(µ-NDipp)2]8b, and
LiAs[PhB(µ-NtBu)2]26 (1b) were prepared as described earlier. The
solvents n-hexane, toluene, Et2O, and THF were dried by distillation
over Na/benzophenone, and the solvents dichloromethane and
acetonitrile were distilled from CaH2 under an argon atmosphere
prior to use. Electrochemical-grade tetrabutylammonium hexafluo-
rophosphate [nBu4N][PF6] (Fluka) was used as the supporting
electrolyte and was kept in a desiccator prior to use. Ferrocene,
prepared following the procedure described by Jolly,9 was sublimed
prior to use. Elemental analyses were performed by Analytical
Services, Department of Chemistry, University of Calgary, and
Canadian Microanalytical Service Ltd., Vancouver, British Colum-
bia.
1
7
Spectroscopic Methods. The H, Li, 11B, 13C, 31P, and 71Ga
NMR spectra were obtained in CD2Cl2, d8-THF or d8-toluene at
23 °C on a Bruker DRX 400 spectrometer operating at 399.59,
155.30, 128.20, 100.49, 161.77, and 121.87 MHz, respectively. 1H
and 13C spectra are referenced to the solvent signal and the chemical
7
shifts are reported relative to (CH3)4Si. Li and 11B NMR spectra
are referenced externally, and the chemical shifts are reported
relative to a 1.0 M solution of LiCl in D2O and to a solution of
BF3 ·Et2O in C6D6, respectively. Similarly, 31P and 71Ga NMR
spectra are referenced externally and the chemical shifts are reported
relative to an 85% solution of H3PO4 and to a solution of Ga(NO3)3
in D2O, respectively.
Electrochemical Methods and Procedures. Cyclic voltammo-
grams (CVs) were obtained at temperatures of 21 ( 2 °C in
acetonitrile, dichloromethane, and tetrahydrofuran solutions contain-
ing 0.1 M, 0.4 M, and 0.4 M [nBu4N][PF6], respectively, as the
supporting electrolyte. CVs were executed by using a PARSTAT
2273 computer-controlled potentiostat with PowerSuite (v 2.58)
software. The cell design consisted of a conventional three-electrode
setup with a 3.0 mm diameter glassy carbon (GC), a 1.6 mm
diameter platinum (Pt), or a 1.6 mm diameter gold (Au) working
electrode; a Pt wire auxiliary, and a silver wire quasi-reference
electrode. The reference electrode was separated from the bulk
solution by a fine-porosity frit. CVs were obtained over scan rates
0.1–20 V s-1. All potentials are reported vs the operative formal
potential, EFc0, for the Fc+/Fc redox couple (Fc ) ferrocene), which
was used as an internal standard. The electrodes were polished with
0.05 µm alumina on a clean polishing cloth (Buehler, USA), rinsed
with distilled water, and dried with tissue paper prior to use. The
In the current study, we were primarily interested in
radicals of the type {M[PhB(µ-NtBu)2]2}• (M ) group 15
element) in order to determine the effect of the additional
pair of electrons on the molecular and electronic structures
of the group 15 systems in comparison with those of the
well-characterized group 13 analogues (M ) Al, Ga).4,5 In
this context, we report (a) the results of DFT calculations of
the molecular structures and EPR parameters of the pnicto-
gen-centered radicals7 [PhB(µ-NR)2]2M• (M ) P, As, Sb,
Bi; R ) Me, tBu); (b) the synthesis and X-ray structures of
LiP[PhB(µ-NtBu)2]2 (1a) and ClP[PhB(µ-NDipp)2]2 (2); (c)
(6) Konu, J.; Balakrishna, M. S.; Chivers, T.; Swaddle, T. W. Inorg.
Chem., 2007, 46, 2627.
(7) In this paper, the term “pnictogen” refers to the central, heavy group
15 element (P, As, Sb, Bi) in all spirocyclic complexes.
(8) (a) Chivers, T.; Fedorchuk, C.; Schatte, G.; Brask, J. K. Can. J. Chem.
2002, 80, 821. (b) Chivers, T.; Fedorchuk, C.; Parvez, M. Inorg. Chem.
2004, 43, 2643.
(9) Jolly, W. L. The Synthesis and Characterization of Inorganic
Compounds; Prentice Hall: Englewood Cliffs, NJ, 1970; p 484.
3824 Inorganic Chemistry, Vol. 47, No. 9, 2008

[PhB(µ-NtBu)2]2As• Radical
Table 1. Crystallographic Data for LiP[PhB(µ-NtBu)2]2 (1a), ClP[PhB(µ-NDipp)2] (2), [PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph] (3), and
a
-
[PhB(µ-NtBu)2]2As+GaCl4 (4)
1a
2
3
4
empirical formula
fw
C28H46B2LiN4P
498.22
C30H39BClN2P
504.86
C28H46AsB2ClN4
570.68
C28H46AsB2Cl4GaN4
746.75
cryst. system
space group
a, Å
b, Å
c, Å
R, deg.
ꢁ, deg.
γ, deg.
V, Å3
monoclinic
C2/c
25.993(5)
8.674(2)
18.342(4)
90.00
134.55(3)
90.00
2947(2)
4
monoclinic
P21/c
15.876(3)
12.416(3)
15.917(4)
90.00
115.22(3)
90.00
2839(1)
4
monoclinic
P21/n
10.700(2)
16.593(3)
17.521(4)
90.00
100.83(3)
90.00
3055(1)
4
monoclinic
P21/n
10.690(2)
37.733(8)
18.605(4)
90.00
91.31(3)
90.00
7503(3)
8
Z
T, °C
-100
-100
-100
-100
F
calcd, g/cm3
1.123
1.181
1.241
1.322
µ(Mo KR), mm-1
crystal size, mm3
F(000)
0.116
0.212
1.223
1.917
0.12 × 0.06 × 0.02
0.10 × 0.08 × 0.06
0.24 × 0.20 × 0.02
0.20 × 0.12 × 0.08
1080
1080
1208
3072
Θ range, deg
reflns collected
unique reflns
Rint
3.40–25.03
4614
2578
3.27–25.03
15549
4804
3.47–25.03
8669
5343
2.18–25.03
22880
13105
0.0228
0.0539
0.0352
0.0511
reflns [I > 2σ (I)]
R1 [I > 2σ (I)]b
wR2 (all data)c
GOF on F2
2200
0.0641
0.1875
1.084
3679
0.0435
0.1100
1.019
3999
0.0435
0.1014
1.048
8641
0.0598
0.1494
1.015
completeness
0.993
0.960
0.990
0.989
c
a λ (Mo KR) ) 0.71073 Å. b R1 ) Σ||Fo|-|Fc||/Σ|Fo|. wR2 ) [Σw(Fo -Fc )2/ΣwFo ]1/2
.
2
2
4
solution was purged with dry nitrogen for 10 min directly before
use, and a blanket of nitrogen gas covered the solution during all
experiments. The cell design for all simultaneous electrochemical
electron paramagnetic resonance (SEEPR) experiments has been
described previously.10 This cell was placed inside a Bruker EMX
113 spectrometer operating at X-band frequencies (9.8 GHz) at 18
( 2 °C.
disorder in the chlorine atom positions. In the final refinement,
the site occupation factors for these atoms were approximately
80/20%. The scattering factors for the neutral atoms were those
incorporated with the programs. Crystallographic data are
summarized in Table 1.
Computational Details. The geometries of all studied com-
pounds were optimized by using DFT. The calculations utilized
both the hybrid PBE0 exchange-correlation functional for Me-
substituted systems as well as its nonhybrid GGA variant PBEPBE
X-ray Crystallography. Crystals of LiP[PhB(µ-NtBu)2]2 (1a),
ClP[PhB(µ-NDipp)2] (2), [PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph]
(3), and [PhB(µ-NtBu)2]2As(GaCl4) (4) were coated with Paratone
8277 oil and mounted on a glass fiber. Diffraction data were collected
on a Nonius KappaCCD diffractometer using monochromated Mo KR
radiation (λ ) 0.71073 Å) at -100 °C. The data sets were corrected
for Lorentz and polarization effects, and empirical absorption correction
was applied to the net intensities. The structures were solved by direct
methods using SHELXS-9711 and refined using SHELXL-97.12 After
the full-matrix least-squares refinement of the nonhydrogen atoms with
anisotropic thermal parameters, the hydrogen atoms were placed in
calculated positions [C-H ) 0.98 Å for C(CH3)3 and 0.95 Å for phenyl
hydrogens]. The isotropic thermal parameters of the hydrogen atoms
were fixed at 1.2 times that of the corresponding carbon for phenyl
hydrogens, and 1.5 times for C(CH3)3. In the final refinement, the
hydrogen atoms were riding on their respective carbon atoms.
Similarly to the structures of LiM[PhB(µ-NtBu)2]2 (M ) As,
Sb),6 the lithium and phosphorus atom positions in the structure
of 1a were disordered with the atoms distributed over the two
atomic sites. The two atoms were not constrained to locate in
the same position, but the anisotropic thermal parameters were
restricted to be equal. The site occupation factors were fixed at
50% for these atoms. The crystal structure of 4 exhibits two
13
t
for Bu-substituted systems. Ahlrichs’ triple-ꢀ valence basis sets
augmented by one set of polarization functions (TZVP) were
employed in all calculations.14 For heavy atoms antimony and
bismuth, the corresponding ECP basis sets were used.14,15 Ap-
propriate molecular point groups and the resolution-of-the-identity
approximation were used to improve the efficiency of the calcula-
tions. Geometry optimizations were done with Turbomole 5.9.116
17
t
(R ) Bu) and Gaussian 03 (R ) Me) program packages.
Hyperfine coupling constants were calculated for all paramagnetic
systems in their geometry-optimized structures by using both a
nonrelativistic and a scalar relativistic (ZORA) approach18 within
the unrestricted Kohn–Sham formalism. The nonrelativistic calcula-
tions utilized the same basis sets as the geometry optimizations,
but only the hybrid version of the PBEPBE functional was
employed. However, for the heavier nuclei antimony and bismuth,
the use of ECP basis sets prevents the direct determination of
(13) (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.; Ernzerhof, M.; Burke, K. J. Chem.
Phys. 1996, 105, 9982. (d) Ernzerhof, M.; Scuseria, G. E. J. Chem.
Phys. 1999, 110, 5029.
(14) The basis sets were used as they are referenced in the Turbomole
5.9.1 internal basis set library. Seeftp://ftp.chemie.uni-karlsruhe.de/
pub/ for explicit basis set listings.
(15) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys.
1993, 80, 1431.
(16) (a) TURBOMOLE, Program Package for ab initio Electronic Structure
Calculations, Version 5.9.1 Theoretical Chemistry Group, University
of Karlsruhe, Karlsruhe, Germany, 2007; (b) Ahlrichs, R.; Bär, M.;
Häser, M.; Horn, H; Kölmel, C. Chem. Phys. Lett. 1989, 162, 165.
-
discrete ion pairs in which the second GaCl4 anion shows
(10) Boeré, R. T.; Bond, A. M.; Chivers, T.; Feldberg, S. W.; Roemmele,
T. L. Inorg. Chem. 2007, 46, 5596.
(11) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Deter-
mination, University of Göttingen, Germany, 1997.
(12) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refine-
ment, University of Göttingen, Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3825

Konu et al.
1
Table 2. Calculated Hyperfine Coupling Constants for
[PhB(µ-NtBu)2]2M• [G]
5.55. Found: C, 71.35; H, 8.24; N, 5.45. H NMR (toluene-d8,
23 °C): δ 7.26–6.37 [m, 11H, aromatic], 4.18 [sept., 2H,
-CH(CH3)2 of Dipp groups, 3J(1H,1H) ) 6.8 Hz], 3.64 [2H,
na
M ) 31P
M ) 75As
M ) 121Sb
M ) 209Bi
-CH(CH3)2 of Dipp groups, J(1H,1H) ) 6.8 Hz], 1.37 [d, 6H,
3
11B
14N
14N
M
2
2
2
1
-1.9
12.8
-1.8
514.4
-3.7
11.4
-2.0
491.4
-3.9
9.3
-2.0
758.0
-5.6
6.2
-0.1
572.4
-CH(CH3)2 of Dipp groups, J(1H,1H) ) 6.8 Hz], 1.27 [d, 6H,
3
-CH(CH3)2 of Dipp groups, J(1H,1H) ) 6.8 Hz], 1.16 [d, 6H,
3
-CH(CH3)2 of Dipp groups, J(1H,1H) ) 6.8 Hz], 1.06 [d, 6H,
3
a Number of equivalent nuclei.
-CH(CH3)2 of Dipp groups, 3J(1H,1H) ) 6.8 Hz]. 13C{1H} NMR:
δ 147.9 [d, J(13C,31P) ) 7.6 Hz], 145.8 [d, J(13C,31P) ) 6.0
2
2
hyperfine couplings using the same method. In addition, relativistic
calculations are essential in order to obtain more than a qualitative
accuracy for systems containing heavy nuclei. Thus, relativistic
calculations for systems with Sb and Bi atoms were carried out.
The calculations utilized the PBEPBE GGA functional13 together
with the large TZ2P STO-type basis sets.19 The hyperfine coupling
constant calculations were done with the Gaussian 0317 (nonrela-
tivistic) and ADF 2007.0120 program packages (relativistic). The
values reported in Table 2 are nonrelativistic for the lighter nuclei
and scalar-relativistic for the heavier atoms antimony and bismuth.
Synthesis of LiP[PhB(µ-NtBu)2]2 (1a). A solution of Li2[PhB(µ-
NtBu)2] (0.244 g, 1.00 mmol) in 15 mL of Et2O was added to a
solution of PCl3 (0.069 g, 0.50 mmol) in 15 mL of Et2O at -80
°C. The reaction mixture was stirred for ½ h at -80 °C and 16 h
at 23 °C. LiCl was removed by filtration and the solvent was
concentrated to ca. 2 mL. Crystallization from diethyl ether afforded
Hz], 134.7, 134.5 [d, J(13C,31P) ) 14.8 Hz], 132.2, 128, 127.7,
3
124.4, 29.72 [d, 4J(13C,31P) ) 26.4 Hz], 28.76, 25.32 [d,
4J(13C,31P) ) 16.8 Hz], 24.78, 24.5, 23.98. 11B NMR: δ 32.8.
31P{1H} NMR: δ 189.8. Colorless crystals of 2 were grown from
n-hexane after 5 d at 5 °C.
Synthesis of [PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph] (3). A solu-
tion of 1b (0.271 g, 0.50 mmol) in diethyl ether (25 mL) was cooled
to -80 °C and a solution of SO2Cl2 (0.040 mL, 0.067 g, 0.50 mmol)
in diethyl ether (1.0 mL) was added via syringe. The reaction
mixture was stirred for ½ h at -80 °C and ca. 4 h at room
temperature. LiCl was removed by filtration and the solvent was
evaporated under vacuum to give 3 as a white solid (0.243 g, 85%).
1H NMR (toluene-d8, 23 °C): δ 6.98–8.24 [m, 10H, C6H5], 1.43
[s, 18H, C(CH3)3], 1.33 [s, 9H, C(CH3)3], 1.25 [s, 9H, C(CH3)3].
13C{1H} NMR: δ 126.7–131.1 [C6H5], 55.6 [s, C(CH3)3], 55.0 [s,
C(CH3)3], 54.3 [s, C(CH3)3], 32.7 [s, C(CH3)3], 32.6 [s, C(CH3)3],
32.5 [s, C(CH3)3]. 11B NMR: δ 37.3, 7.6 ppm. Colorless crystals
of 3 were obtained from Et2O after 8 d at 5 °C.21
1
1a after 5 days at 5 °C (0.062 g, 25%). H NMR (toluene-d8, 23
4
°C): δ 6.16–7.70 [m, 10H, C6H5], 1.46 [d, J(1H,31P) ) 2.4 Hz,
9H, C(CH3)3], 1.34 [s, 18H, C(CH3)3], 1.16 [s, 9H, C(CH3)3]. 11B
–
Synthesis of [PhB(µ-NtBu)2]2As+GaCl4 (4). A solution of 3
21
7
NMR: δ 34.9. Li NMR: δ 0.94. 31P{1H} NMR: δ 87.1.
(0.285 g, 0.50 mmol) in diethyl ether (20 mL) was cooled to -80
°C and a solution of GaCl3 (0.088 g, 0.50 mmol) in diethyl ether
(10 mL) was added via cannula. The reaction mixture was stirred
for ½ h at -80 °C and ca. 2 h at room temperature. The precipitate
was allowed to settle and the solvent was decanted via cannula.
The product was then washed with Et2O (2 × 20 mL) and dried
under vacuum giving 4 as a white, spectroscopically pure powder
Synthesis of ClP[PhB(µ-NDipp)2]2 (2). A solution of Li2[PhB(µ-
NDipp)2] (0.726 g, 1.60 mmol) in Et2O (30 mL) was added to a
solution of PCl3 (0.14 mL, 1.60 mmol) in Et2O (10 mL) at -80
°C. The reaction mixture was stirred for ½ h at -80 °C and 18 h
at 23 °C. The volatiles were removed in Vacuo and the product
was then extracted with n-hexane. The precipitate of LiCl was
removed by filtration and the solvent was evaporated under
vacuum, yielding an oily product that solidified to give a pale
yellow powder of 2 after 24 h at 23 °C (0.648 g, 1.30 mmol,
80%). Anal. Calcd. for C30H39BN2PCl: C, 71.37; H, 7.78; N,
1
(0.306 g, 82%). H NMR (CD2Cl2, 23 °C): δ 7.51–7.56 [m, 10H,
C6H5], 1.44 [s, 36H, C(CH3)3]. 13C{1H} NMR: δ 129.0–131.0
[C6H5], 57.7 [C(CH3)3], 32.3 [C(CH3)3]. 11B NMR: δ 37.8. 71Ga
NMR: δ 249.6. Colorless crystals of 4 were obtained from n-hexane
layered on top of the THF solution after 24 h at 5 °C.21
(17) 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., Pittsburgh, PA, 2003.
(18) (a) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys.
1993, 99, 4597. (b) van Lenthe, E.; Baerends, E. J.; Snijders, J. G.
J. Chem. Phys. 1994, 101, 9783. (c) van Lenthe, E.; Ehlers, A. E.;
Baerends, E. J. J. Chem. Phys. 1999, 110, 8943.
(19) (a) van Lenthe, E.; Baerends, E. J. J. Comput. Chem. 2003, 24, 1142.
(b) Chong, D. P.; Lenthe, E.; van Gisbergen, S. J. A.; Baerends, E. J.
J. Comput. Chem. 2004, 25, 1030.
(20) ADF2007.01, SCM, Theoretical Chemistry, Vrije Universiteit, Am-
sterdam, Netherlands; http://www.scm.com.
Results and Discussion
DFT Calculations of the Neutral Radicals [PhB(µ-
t
NR)2]2M• (M ) P, As, Sb, Bi; R ) Me, Bu). Our earlier
DFT calculations on the model group 13 radicals [PhB(µ-
NMe)2]2M• (M ) B, Al, Ga, In) revealed spirocyclic
structures with spin density equally distributed over the two
bam ligands and the SOMO distributed equally over p-or-
bitals on each of the four nitrogen atoms.4,5 By contrast, DFT
calculations for the corresponding group 15 systems [PhB(µ-
NMe)2]2M• predict two different types of electronic struc-
tures. For the lighter group 15 bis-bams (M ) P, As, Sb),
the calculations reveal a distorted C2 symmetric trigonal
bipyramidal geometry with the SOMO located primarily on
the group 15 center (Figure 1a). For the bismuth-containing
bis-bam radical, the calculations predict a highly distorted
C1 symmetric structure with one long (2.51 Å) and one short
(2.16 Å) Bi-N bond as well as a SOMO which is mainly
located on the nitrogen atoms of one of the two chelating
bam-ligands (Figure 1b). As a consequence, large (several
hundred Gauss) hyperfine coupling constants for the group
(21) Although the product was spectroscopically pure, on the basis of
multinuclear (1H, 13C, 11B) NMR spectra, numerous attempts, by both
in-house and commercial services, gave unsatisfactory elemental
analyses.
3826 Inorganic Chemistry, Vol. 47, No. 9, 2008

[PhB(µ-NtBu)2]2As• Radical
Figure 1. SOMOs (isosurface value (0.06) of the neutral [PhB(µ-
NMe)2]2M• radicals; (a) M ) P, As, Sb; (b) M ) Bi. Electronic structures
of the tBu-derivatives, [PhB(µ-NtBu)2]2M• (M ) P, As, Sb, Bi), are
analogous to that presented in Figure 1a.
Figure 2. Crystal structures of (a) LiP[PhB(µ-NtBu)2]2 (1a), and (b)
ClP[PhB(µ-NDipp)2]2 (2) with the atomic numbering scheme. Hydrogen
15 center are expected in the EPR spectra of the pnictogen-
centered radicals [PhB(µ-NMe)2]2M• (M ) P, As, and Sb).7
In contrast, the EPR spectra of the analogous bismuth species
should display at least 1 order of magnitude smaller Bi
coupling.
a
atoms have been omitted for clarity. Symmetry operation: -x, y, 0.5 - z.
Scheme 1
Interestingly, calculations done for the more realistic NtBu
substituted derivatives gave a C2-symmetric trigonal bipyr-
amidal geometry also for the bismuth species. Hence, the
NtBu-substituted bismuth species is also predicted to be a
pnictogen-centered radical. This change in geometry most
likely originates from a combination of steric reasons (i.e.,
due to increased steric bulk of the NtBu substituents) and
electronic factors (i.e., a slight change in MO characteristics
due to the difference in N-substituent). The optimized
geometries of all of the lighter group 15 congeners are in
good agreement with the structures obtained for the NMe
derivatives and show only small changes in their calculated
hyperfine coupling constants (see Table 2 for values calcu-
lated for NtBu-substituted derivatives [PhB(µ-NtBu)2]2M•).
This fundamental difference between the electronic structures
of the group 13 radicals [PhB(µ-NR)2]2M• (M ) B, Al, Ga,
In) and their group 15 analogues, predicted by the DFT
calculations, provided the incentive for investigations of the
chemical or electrochemical generation of the pnictogen-
centered radicals [PhB(µ-NtBu)2]2M• (M ) P, As, Sb, Bi).7
Synthesis and Chemical Oxidation of LiP[PhB(µ-
NtBu)2]2 (1a): X-ray Structures of 1a and ClP[PhB(µ-
NDipp)2] (2). In order to complete the series of bis-bam
complexes of group 15 elements of the type LiM[PhB(µ-
NtBu)2]2 (M ) P, As, Sb, Bi),6 and to provide a potential
source of the phosphorus-centered radical [PhB(µ-NtBu)2]2P•,
the reaction between PCl3 and Li2[PhB(µ-NtBu)2] was carried
out in a 1:2 molar ratio. The new phosphorus-containing
complex LiP[PhB(µ-NtBu)2]2 (1a) was isolated in 25% yield
(Scheme 1) and characterized in solution by multinuclear
NMR spectroscopy and in the solid state by a single-crystal
X-ray structure determination.
1
The H NMR spectrum of 1a at room temperature in
toluene shows a doublet and two singlets in a 1:2:1 intensity
ratio for the tBu groups and a multiplet for phenyl hydrogens
in the expected intensity ratios. The Li, 11B, and 31P NMR
7
spectra exhibit singlets at 0.94, 34.9, and 87.1 ppm,
respectively. The NMR spectroscopic data for 1a resemble
those of the corresponding arsenic complex LiAs[PhB(µ-
NtBu)2]2 (1b),6 suggesting a similar ladder structure for 1a.
The single-crystal X-ray crystallographic analysis con-
firmed the isostructural nature of 1a and the heavier group
15 analogues LiM[PhB(µ-NtBu)2]2 (M ) As, Sb).6 As
illustrated in Figure 2a, the molecular structure of 1a is
composed of two four-membered rings, BN2Li and BN2P,
connected by Li-N and P-N bonds to form a tricyclic
compound with both three- and four-coordinate nitrogens.
The B-N bond lengths in 1a show a slight inequality of ca.
0.04 Å (Table 3). Expectedly, the P-N bond to three-
coordinate nitrogen is ca. 0.12 Å shorter than those involving
the four-coordinate nitrogens. The central four-membered
PN2Li ring in 1a is nonplanar with mean torsion angles
involving the four atoms of ca. 6.5°. The three-coordinate
boron and nitrogen atoms in 1a show a cis arrangement with
respect to the central four-membered ring similar to that
observed in LiM[PhB(µ-NtBu)2]2 (M ) As, Sb).6
The oxidation of 1a with SO2Cl2 in both 2:1 and 1:1 molar
ratios in diethyl ether was investigated. The attempted one-
Inorganic Chemistry, Vol. 47, No. 9, 2008 3827

Konu et al.
(0.97)22 is somewhat higher than the value of ca. 0.83
observed for P-Br, As-Cl, and Sb-Cl bonds in XM[PhB(µ-
NtBu)2] (X ) Br, M ) P; X ) Cl, M ) As, Sb).6 Steric
crowding of the Dipp groups does not have an effect on the
N-P-N and N-P-Cl bond angles as indicated by the
similarity of these values to those observed in BrP[PhB(µ-
NtBu)2].8a Consistently, the P-N-B and N-B-N angles
are also comparable with those observed in BrP[PhB(µ-
NtBu)2].24
Table 3. Selected Bond Lengths (Å) and Angles (°) in
LiP[PhB(µ-NtBu)2]2 (1a) and ClP[PhB(µ-NDipp)2] (2)
1a
2
1a
2
P1–N1
P1–N2
P1–X
1.690(4)
1.834(4)
1.713(2) B1–N2
1.716(2) Li1a–N2
1.455(4) 1.455(3)
1.97(2)
1.76(3)
1.789(4)b 2.099(1)c Li1a–N1a
B1–N1
1.416(4)
1.447(3) Li1a–N2a
79.4(1) P1–N1–B1
2.01(3)
N1–P1–N2 111.1(2)
N1–P1–X
83.5(2) 91.0(1)
83.5(2)b 103.4(1)c P1–N2a–B1 78.9(2) 90.6(1)d
N2–P1–X 103.8(1)b 104.2(1)c N1a–B1a–N2 107.7(2)
N1–B1–N2a 107.7(2)
98.0(2)d B1a–N2–P1 125.2(2)
ChemicalOxidationofLiAs[PhB(µ-NtBu)2]2 (1b):Synthe-
sis and Characterization of [PhB(µ-NtBu)2As(µ-NtBu)2-
B(Cl)Ph] (3). Since the group 13 radicals [PhB(µ-NtBu)2]2M•
(M ) Al, Ga) are readily obtained from the corresponding
anions by one-electron oxidation with iodine,4 we initially
attempted to generate [PhB(µ-NtBu)2]2As• by oxidation of
1b with 1/2 equiv of I2, but no reaction was observed. The
oxidizing agents [nBu4N]Br3 and NO[SbF6] were also inef-
fective. Consequently, we turned our attention to the stronger
oxidizing agent sulfuryl chloride. The reaction of 1/2 equiv
of SO2Cl2 with 1b produced LiCl and an almost colorless
reaction solution, suggesting the absence of a radical species.
a Symmetry operation: -x, y, 0.5 - z. X ) N2a. c X ) Cl1. d No
b
symmetry operation on N2 atom.
electron oxidation did not generate any EPR-active species,
and both reactions resulted in multiple products. The 11B
NMR spectra of the reaction mixtures showed the presence
of both three- and four-coordinate boron centers, cf. forma-
tion of 3 described below. The 31P NMR spectra revealed
two main products with singlets at 6.0 and -69.9 ppm,
comprising ca. 70% of the phosphorus-containing com-
pounds; however, no tractable products were isolated from
either reaction.
7
A multinuclear NMR spectroscopic analysis (1H, Li, 11B,
13C) of the reaction mixture showed the presence of some
unreacted 1b, as well as a product that exhibits three NtBu
resonances with relative intensities 2:1:1 (1H, 13C), two
different environments for boron centers (11B), and no lithium
atoms (7Li). Specifically, singlets were observed at 37.3 and
7.6 ppm in the 11B NMR spectrum, suggesting the presence
of both three-coordinate and four-coordinate boron atoms,
respectively. In light of these NMR data, the reaction of
SO2Cl2 with 1b was carried out in a 1:1 molar ratio and the
product [PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph] (3) was isolated
in 85% yield (1), and identified by an X-ray crystallographic
analysis.
With a view to generating the sterically protected phos-
phorus-centered radical [PhB(µ-NDipp)2]2P•, the reaction
between PCl3 and Li2[PhB(µ-NDipp)2] was conducted in a
1:2 molar ratio. However, the 31P NMR spectrum revealed
a complex mixture of products. Consequently, we decided
to attempt installation of the two bam ligands on the
phosphorus center in a stepwise fashion, a process that had
previously been found to be necessary for the successful
synthesisofthebis-bamaluminumcomplex(Et2O•Li)Al[PhB(µ-
NtBu)2]2.4 The reaction between PCl3 and Li2[PhB(µ-
NDipp)2] in a 1:1 molar ratio produced an 80% yield of
ClP[PhB(µ-NDipp)2] (2) (Scheme 1), which was character-
ized in solution by multinuclear NMR spectroscopy and in
the solid state by a single crystal X-ray structure determi-
nation. Subsequent reaction between 2 and a second equiva-
lent of Li2[PhB(µ-NDipp)2], however, also failed to generate
the bis-bam complex, LiP[PhB(µ-NDipp)2]2.
LiAs PhB(µ-NtBu)2 2 + SO2Cl2 f
[
]
( )
1b
t
t
(
2
)
PhB(µ-N Bu) As(µ-N Bu) B Cl Ph + LiCl (1)
[
]
2
( )
3
The crystal structure of 3 with the atomic numbering
scheme is depicted in Figure 3a, and the pertinent bond
parameters are summarized in Table 4. The structure consists
of a spirocyclic arsenic complex in which the arsenic center
is N,N′-chelated by a bam ligand and a chlorinated bam
ligand with a B-Cl bond. The solid-state structure is
consistent with the observation of both three- and four-
The 1H NMR spectrum of 2 shows the expected resonances
for the phenyl and Dipp groups. The 11B NMR spectrum
exhibits a singlet at 32.8 ppm and a single resonance is
observed at 189.8 ppm in the 31P NMR spectrum, cf. δ (31P)
181.2 for ClP[PhB(µ-NtBu)2].8a The crystal structure of 2
(Figure 2b) confirms the isostructural relationship with the
other structurally characterized group 15 monobams, XM-
[PhB(µ-NtBu)2] (X ) Br, M ) P; X ) Cl, M ) As, Sb,
Bi).6,8a In common with the lighter group 15 monobams,
XM[PhB(µ-NtBu)2] (X ) Br, M ) P; X ) Cl, M ) As), 2
does not exhibit the significant intermolecular M · · · Cl close
contacts that were observed for the heavier group 15
congeners (X ) Cl, M ) Sb, Bi).6 The P-N bonds in 2
(Table 3) are somewhat elongated (by ca. 0.03 Å) compared
to those in the tert-butyl derivative BrP[PhB(µ-NtBu)2],8a
whereas the B-N bond lengths in 2 are in the typical range
for the bam ligand.1 The calculated P-Cl bond order in 2
(22) The P-Cl bond order was calculated by the Pauling equation N )
10(D-R)/0.71 23
, where R is the observed bond length (Å). The single bond
length D is estimated from the sums of appropriate covalent radii (Å):
23
P-Cl 2.09.
(23) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960.
(24) In view of the multiple products detected by 31P NMR spectroscopy
after chemical oxidation of 1a and the difficulties involved in the
purification of the antimony derivative LiSb[PhB(µ-NtBu)2]2 due to
6
the co-formation of the 2:3 complex, Sb2[PhB(µ-NtBu)2]3, we have
focused our attempts to generate group 15-centered radicals of the
type [PhB(µ-NtBu)2]2M• on the arsenic system 1b. The oxidation of
(Et2O · Li)Bi[PhB(µ-NtBu)2]2 with iodine or SO2Cl2, however, was
conducted and resulted in the formation of a mixture of the known
2:3 bam complex, Bi2[PhB(µ-NtBu)2]3,6 and a number of unidentified
compounds.
3828 Inorganic Chemistry, Vol. 47, No. 9, 2008

[PhB(µ-NtBu)2]2As• Radical
bond involving the three-coordinate nitrogen in 1b has a bond
length of 1.789(3) Å.6 These bond lengths are indicative of
a Zwitterionic compound with the positive charge located
mainly on arsenic and the negative charge on the four-
coordinate boron center. The B-Cl bond length of 1.953(4)
Å in 3 is significantly longer than those observed in amidinate
complexes of the Ph(Cl)B unit (1.847(4)–1.882(6) Å), while
the elongated B-N bond distances in 3 are comparable to
the corresponding B-N bonds in the amidinate com-
plexes.25,26 The lengthened B-N bonds in 3 are, however,
among the longest observed in the three-atom NBN backbone
of the bam ligand; similar bond lengths have been reported
only when four-coordinate nitrogen atoms are involved.1
Elongation of the B-N bonds and contraction of the
As-N bonds result in a wider N-As-N angle (ca. 5.5°)
within the chlorinated ligand and a narrower N-B-N angle
(ca. 8.1°) at the four-coordinate boron. Although the
N-B2-Cl and C-B2-Cl bond angles show ideal tetrahe-
dral values, the remaining bond angles at the B2 atom are
somewhat distorted due to the involvement of this atom in
the four-membered AsNBN ring with a bite angle at boron
of 93.8(8)°. The three-coordinate boron atom shows a
trigonal planar geometry (sum of bond angles is 359.9°), as
is typical for bam ligands.1 The four-membered AsNBN rings
are slightly distorted from planarity with torsion angles of
ca. 2.3° and 4.5°. These two rings are almost perpendicular
to each other with an angle of 87.9° between the two planes.
The geometry at the spirocyclic arsonium center is distorted
tetrahedral; the NAsN bite angles of 75.7(1)° for the
chlorinated bam ligand and 81.2(1)° for the bam ligand, while
the other NAsN bond angles are in the range 125.1(1)–
128.7(1)°.24
Figure 3. (a) Molecular structure of [PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph]
(3) and (b) crystal structure of the [PhB(µ-NtBu)2]2As+ cation in 4 with
the atomic numbering schemes. Hydrogen atoms have been omitted for
clarity. Only one of the two independent cations is shown in 4.
Table 4. Selected Bond Lengths (Å) and Angles (°) in
-
[PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph] (3) and [PhB(µ-NtBu)2]2As+GaCl4
(4)
3
4a
3
4a
As1–N1
As1–N2
As1–N3
As1–N4
B1–N1
B1–N2
B2–N3
1.825(3) 1.800(4) B2–N4
1.833(3) 1.811(4) B2–Cl1
1.755(3) 1.803(4) Ga–Cl1
1.764(3) 1.797(4) Ga–Cl2
1.441(5) 1.466(7) Ga–Cl3
1.448(5) 1.453(7) Ga–Cl4
1.565(5) 1.453(7)
1.569(5) 1.469(7)
1.953(4)
2.176(2)
2.171(2)
2.194(4)
2.175(2)
N1–As1–N2 75.7(1)
N1–As1–N3 127.1(1)
N1–As1–N4 126.6(1)
N2–As1–N3 125.1(1)
N2–As1–N4 128.7(1)
N3–As1–N4 81.2(1)
N1–B1–N2 101.9(3)
77.7(2) N3–B2–C200 116.7(3)
129.3(2) N4–B2–C200 117.9(3)
128.4(2) Cl1–B2–C200 109.4(3)
125.0(2) Cl1–Ga1–Cl2
126.4(2) Cl1–Ga1–Cl3
77.8(3) Cl1–Ga1–Cl4
101.7(4) Cl2–Ga1–Cl3
101.3(4) Cl2–Ga1–Cl4
Cl3–Ga1–Cl4
107.52(8)
109.60(7)
110.66(8)
111.18(8)
108.55(8)
109.32(7)
Chemical Reduction of [PhB(µ-NtBu)2As(µ-NtBu)2-
B(Cl)Ph] (3). Since oxidation of 1b resulted in the formation
of 3 via a two-electron process, we turned our attention to
the reduction of 3 as a possible route to the radical [PhB(µ-
NtBu)2]2As•. Initial attempts to reduce 3 with Ph3Sb in Et2O
or with metallic sodium in THF were unsuccessful. Conse-
quently, we chose sodium naphthalenide as a stronger
reducing agent.27
N3–B2–N4
93.8(8)
N3–B2–Cl1 108.9(3)
N4–B2–Cl1 109.2(2)
a Bond parameters of the second discrete ion pair are identical.
coordinate boron centers and three tBu environments (bonded
to nitrogens N1, N2, and N3/N4, respectively) in the solution
NMR spectra. The B-Cl bond formation in the three-atom
NBN backbone is a novel feature in the chemistry of
boraamidinates.1 A similar arrangement for four-coordinate
boron can be found, however, in amidinate complexes of
the Ph(Cl)B unit.25,26
The B-N bonds in the chlorinated bam ligand in 3 are
ca. 0.12 Å longer than those in the nonchlorinated ligand
owing to the influence of the four-coordinate boron center
in the former. Concomitantly, the As-N bonds involving
the chlorinated bam ligand are ca. 0.07 Å shorter than those
in the bam ligand. The As-N bond lengths to the bam ligand
are comparable to those observed in the monobam, [PhB(µ-
NtBu)2]AsCl (1.839(5) and 1.846(5) Å),6 while the corre-
sponding bonds to the chlorinated ligand are the shortest
reported for the arsenic bam complexes thus far; the As-N
Upon addition of Na[C10H8] to a solution of 3 in THF at
-80 °C, the dark green color of the naphthalenide radical
anion disappeared to give a colorless solution. The 1H NMR
spectrum of the reaction mixture showed the presence of the
compound 3, naphthalene, and a new product with three
singlets for the NtBu-groups at 1.38, 1.29, and 1.20 ppm in
a 1:2:1 intensity ratio. The 11B NMR spectrum exhibited a
relatively low intensity signal at 7.6 ppm for the four-
coordinate boron in 3 and a broad resonance at ca. 36 ppm
attributed to the three-coordinate boron atoms in both 3 (37.3
1
ppm) and the new product. The similarity of the H NMR
spectrum for the new product and that reported for the known
(27) Metallic sodium has a formal reduction potential of -3.04 V (vs Fc)
in THF or glyme, while the formal reduction potential for the
naphthalenide radical anion is -3.10 V (vs Fc) in THF.28
(28) Connelly, N. G.; Geiger, W. E Chem. ReV. 1996, 96, 877. references
therein.
(25) Blais, P.; Chivers, T.; Downard, A.; Parvez, M. Can. J. Chem. 2000,
78, 10.
(29) See for example Cowley, A. H.; Carrano, C. J.; Geerts, R. L.; Jones,
R. A.; Nunn, C. M. Angew. Chem., Int. Ed. 1988, 27, 277.
(26) Findlater, M.; Hill, N. J.; Cowley, A. H. Polyhedron 2006, 25, 983.
Inorganic Chemistry, Vol. 47, No. 9, 2008 3829

Konu et al.
Scheme 2
Figure 4. Cyclic voltammograms of 4 (1.7 mM) and ferrocene (2.7 mM)
in CH3CN at a GC electrode at 21 °C, [nBu4N][PF6] (0.1 M), ν ) 0.2 V
s-1: (a) immediately after the addition of ferrocene, (b) before the addition
of ferrocene, and (c) several minutes after the addition of ferrocene.
compound LiAs[PhB(µ-NtBu)2]2 (three resonances at 1.39,
1.30, and 1.22 ppm with a 1:2:1 intensity ratio)6 suggests
that a two-electron reduction with formation of the analogous
sodium salt NaAs[PhB(µ-NtBu)2]2 has occurred (Scheme 2).
Electrochemistry of [PhB(µ-NtBu)2]2As+GaCl4 (4)
-
and LiAs[PhB(µ-NtBu)2]2 (1b). Voltammetric studies per-
formed on 4 in acetonitrile showed a single wave which was
both chemically and electrochemically irreversible over scan
rates 0.1–20 V s-1, on both Pt and GC electrodes. Multiple
CVs taken at the same scan rate and concentration, both
before and after the addition of ferrocene (as an internal
reference), resulted in variations in the cathodic peak current
height, as indicated in Figure 4, with essentially invariant
peak potentials near -1.3 V. The variation in the magnitude
of the peak current for the reduction process can be attributed
to adsorption of the radical onto the surface of the electrode.
Such adsorption effects have been well-documented for thiols
on metals such as gold and platinum.31 Thus, while the peak
currents can give no information on the number of electrons
transferred, the adsorption phenomenon itself supports a one-
electron reduction to a reactive radical. The complete absence
of a return wave (at an offset potential) suggests that radical
dimerization to give an As-As bonded species is not a
significant pathway under the conditions used in this study.
Radical dimerization has been observed, however, in the one-
electron electrochemical reduction of Ph2AsBr, which pro-
duces Ph2AsAsPh2.32
Voltammetric studies performed on 1b in both dichlo-
romethane and tetrahydrofuran on Pt, GC, and Au electrodes
showed at least one oxidation process, which is also
chemically and electrochemically irreversible, and takes place
near the edge of the solvent window. This initial oxidation
process gives rise to a return wave in THF which is offset
by 1.5 V on Au (Figure 5), and by over 1.8 V on Pt (these
waves are observed only after scanning through the +1.24
V process). The presence of this offset return wave may
Synthesis and Characterization of [PhB(µ-NtBu)2]2-
As+GaCl4– (4). The chemical or electrochemical reduction of
the cation [PhB(µ-NtBu)2]2As+ represents another potential
source of the radical, [PhB(µ-NtBu)2]2As•. With this target in
mind, we prepared the ion-separated salt [PhB(µ-NtBu)2]2-
As+GaCl4– (4) in excellent yield by the reaction of 3 with GaCl3
1
in diethyl ether (eq 2). The H and 11B NMR spectra of 4 in
CD2Cl2 reveal a single environment for all four tBu groups and
a single resonance for three-coordinate boron, consistent with
chloride ion abstraction from the B-Cl unit in 3. The 71Ga
NMR spectrum exhibits a single resonance at 249.6 ppm, which
is a typical chemical shift for the GaCl4- anion.29
[PhB(µ-NtBu)2As(µ-NtBu)2B(Cl)Ph] + GaCl3 f
(3)
[PhB(µ-NtBu)2]2As+GaCl4- (2)
(4)
The crystal structure of 4 exhibits two independent ion
pairs in the crystal lattice. Several data collections were
attempted, but the second GaCl4- anion consistently showed
disorder in the chlorine atom positions. The bond parameters
-
of the other GaCl4 anion, and both of the two [PhB(µ-
NtBu)2]2As+ cations (Figure 3b), however, display reasonable
estimated standard deviations. The As-N and B-N bond
lengths in 4 are intermediate between the values observed
for the two ligands in 3 (Table 4). As expected, however,
they are somewhat closer to the parameters observed for the
bam ligand in 3. The endocyclic N-B-N and N-As-N
bond angles in 4 are also close to those observed in the
nonchlorinated bam ligand in 3. As observed for group 13
neutral radicals [PhB(µ-NtBu)2]2M• (M ) Al, Ga),3,4 the
(30) The one-electron reduction of 4 with Cp2Co was attempted in diethyl
ether at -80 °C. The deep red color of Cp2Co disappeared immediately
upon dropwise addition of the reducing agent; no EPR-active species
could be detected. The 1H NMR spectrum revealed a complex mixture
of products none of which could be isolated.
t
methyl groups in the Bu units of both ligands in 4 are
staggered, whereas the B-Cl bond formation in 3 results in
an eclipsed conformation. The four-membered AsNBN rings
in 4 are slightly distorted from planarity with torsion angles
of 1.9° and 3.4°, and the angle between these planes is only
slightly closer to 90° (88.8°) than that in 3. Finally, the bond
(31) (a) Finklea, H. O. Electrochemistry of Organized Monolayers of Thiols
and Related Molecules on Electrodes. Electroanalytical Chemistry;
Bard, A. J.; Rubinstein, I., Eds. Marcel Dekker: New York, 1996. (b)
Lipkowski, J.; Ross, P. N., Eds. Adsorption of Molecules at Metal
Electrodes; VCH: New York, 1992. (c) Mazur, M.; Krysinski, P. J.
Phys. Chem. B 2002, 106, 10349.
-
angles in the GaCl4 anion are close to those of an ideal
(32) Dessy, R. E.; Chivers, T.; Kitching, W. J. Am. Chem. Soc. 1965, 88,
467.
tetrahedron.30
3830 Inorganic Chemistry, Vol. 47, No. 9, 2008

[PhB(µ-NtBu)2]2As• Radical
in 1b has a powerful stabilizing effect. Thus, the onset of
oxidation of this species is found at least several volts more
anodic than observed for the y ) 0/+1 couple in 4.
Attempts to generate the radical [PhB(µ-NtBu)2]2As• by
SEEPR, an in situ method optimized for the detection of
short-lived, electrochemically generated radicals,10 by reduc-
tive electrolysis of 4 at -1.3 V in CH3CN or oxidative
electrolysis of 1b at +0.9 V in CH2Cl2 or at 1.3 V in THF,
did not produce a detectable EPR-active species.
Conclusions
Intriguingly, DFT calculations predict that the formal
addition of two electrons to the neutral group 13 radicals
[PhB(µ-NtBu)2]2M• (M ) Al, Ga, In) to give the correspond-
ing group 15 systems [PhB(µ-NtBu)2]2M• (M ) P, As, Sb,
Bi) results in a dramatic change in the electronic structure
from one in which the unpaired electron density is equally
localized on the four nitrogen atoms of the two bam ligands
to systems that can be described as pnictogen-centered
radicals. Oxidation of the anion [PhB(µ-NtBu)2]2As– with
sulfuryl chloride resulted in B-Cl bond formation, the first
example of reactivity at the boron center in bam complexes,1
and the subsequent characterization of the spirocyclic cation
[PhB(µ-NtBu)2]2As+ isoelectronic with the neutral germa-
nium spirocycle [PhB(µ-NtBu)2]2Ge.34 Investigations of the
chemical redox behavior of the [PhB(µ-NtBu)2]2As– anion
and the [PhB(µ-NtBu)2]2As+ cation showed that a two-
electron process is favored over a one-electron process.
Although the voltammetric studies suggest one-electron
processes, no direct evidence for the independent existence
of the [PhB(µ-NtBu)2]2As• radical was obtained by SEEPR
experiments.
Figure 5. Cyclic voltammogram of 1b (1.4 mM) and ferrocene (0.6 mM)
in THF at a Au electrode at 21 °C, [nBu4N][PF6] (0.4 M), ν ) 0.2 V s-1
.
indicate that, upon oxidation of 1b, the lithium ion complexes
with THF to give [PhB(µ-NtBu)2]2As• and Li(THF)4 . The
+
intermediacy of such lithium complexes has been implicated
previously in related systems.33 The return wave would then
correspond to reduction of the electrochemically generated
radical at +1.24 V. Two irreversible oxidation processes
occurred in CH2Cl2 on GC, Pt, and Au, at +0.9 V and +1.3
V, respectively, with no evidence for return waves. The
insolubility of 1b in CH3CN precluded electrochemical
measurements in that solvent.
Comparison of redox potentials for irreversible processes
and among different solvent/electrode combinations is fraught
with difficulties. Follow-up reactions, whether decomposition
or adsorption, can significantly displace apparent E° values.
Even with such caveats, these two incompatible sets of peak
potentials (i.e., -1.3 V for the 0/+1 couple in 4 and 1.24 V
for the -1/0 couple in LiAs[PhB(µ-NtBu)2]2 (1b) indicate
that the existence of the neutral radical [PhB(µ-NtBu)2]2As•
is likely to be thermodynamically unfavorable. Insofar as
they do reflect the redox transformations of [Asbam2]y (y )
-1, 0, +1), it appears that the strong complexation of lithium
Acknowledgment. The authors gratefully acknowledge
financial support from the Natural Sciences and Engineering
Research Council (Canada), the Alberta Ingenuity Fund
(T.L.R. and A.M.C.), and the Academy of Finland (H.M.T.).
(33) (a) Armstrong, A.; Chivers, T.; Parvez, M.; Boere, R. T. Angew. Chem.,
Int. Ed. Engl. 2004, 43, 502. (b) Armstrong, A.; Chivers, T.; Parvez,
M.; Schatte, G.; Boeré, R. T. Inorg. Chem. 2004, 43, 3453. (c)
Armstrong, A.; Chivers, T.; Tuononen, H. M.; Parvez, M.; Boeré, R. T.
Inorg. Chem. 2005, 44, 7981.
Supporting Information Available: X-ray crystallographic files
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(34) Konu, J.; Chivers, T. unpublished results.
IC702435E
Inorganic Chemistry, Vol. 47, No. 9, 2008 3831