Diboryl and diboranyl porphyrin complexes: Synthesis, structural motifs, and redox chemistry: Diborenyl porphyrin or diboranyl isophlorin?

Article abstract of DOI:10.1002/chem.200700046

The syntheses of diboryl porphyrin complexes [(BX₂) ₂(ttp)] (ttp: dianion of tetra-p-tolylporphyrin) and the B - B single-bond diboranyl complexes [(BX)₂(ttp)] (X = F, Cl, Br, I) are given. The former are prepared from the

Full text of DOI:10.1002/chem.200700046

FULL PAPER
DOI: 10.1002/chem.200700046
5982
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 5982 – 5993

FULL PAPER
Diboryl and Diboranyl Porphyrin Complexes: Synthesis, Structural Motifs,
and Redox Chemistry: Diborenyl Porphyrin or Diboranyl Isophlorin?
Andre Weiss,[a, b] Michael C. Hodgson,[b] Peter D. W. Boyd,[b] Walter Siebert,*[a] and
Penelope J. Brothers*[b]
Dedicated to Professor Hubert Schmidbaur
Abstract: The syntheses of diboryl por-
phyrin complexes [(BX2)2(ttp)] (ttp:
dianion of tetra-p-tolylporphyrin) and
calculated for related dipyrromethene
complexes) indicate that a combination
of the reducing ability of bromide and
iodide ions combined with the con-
strained environment of the porphyrin
ligand contribute to the driving force.
The reductive coupling is also observed
rin complex, [B
ride abstraction from [(BCl)
gives the planar dication [B2
(ttp)]2+
whereas chemical reduction of [(BCl)2-
(ttp)] by using magnesium anthrace-
nide gives a neutral complex, [B2(ttp)],
in which the TTP ligand has been re-
duced by two electrons to give an un-
usual example of an isophlorin com-
plex. The cationic and neutral com-
2(O2C6H4)
ACHTREUNG
U
ACHTREUNG
U
,
ꢀ
the B B single-bond diboranyl com-
plexes [(BX)2(ttp)] (X=F, Cl, Br, I)
ACHTREUNG
are given. The former are prepared
from the reactions of BX3 (X=F, Cl)
ACHTREUNG
with [Li
B2Cl4 (X=Cl), the reaction of SbF3
with [(BCl)2(ttp)] (for X=F), and, in
2(ttp)] and the latter from
in the reaction of [(BCl2)2
nBuLi to give [(BnBu)2(ttp)], which
was characterised crystallographically.
The reaction of [(BCl)2(ttp)] with cate-
chol gives a boron catecholato porphy-
A
AHCTREUNG
ACHTREUNG
the cases of X=Br or I, in a remark-
able reductive coupling reaction result-
ing directly from the reaction of BBr3
G
plexes [B
N
N
characterised through a combination of
spectroscopic data that is supported by
DFT calculations on the porphine ana-
logues.
or BI3 with [Li2(ttp)]. Density function-
T
Keywords: boron
· density func-
al theory (DFT) calculations on the
thermochemical parameters for the re-
ductive coupling reactions (and those
tional calculations · isophlorin · por-
phyrinoids · redox chemistry
Introduction
with examples of several different geometries and structural
types.[4–7] Interest in boron–porphyrin chemistry derives
from intrinsic factors, in particular, the novelty of a porphy-
rin that contains two central coordinated elements rather
than the more usual single atom; the size mismatch between
the small boron atom and the coordination site in the mac-
rocycle; and the incompatibility of the usual square-planar,
square-pyramidal or octahedral geometry of conventional
porphyrin complexes with the tetrahedral or trigonal geome-
try adopted by boron compounds.
The chemistry of main group porphyrin complexes has
evolved later than that of the biologically relevant transi-
tion-metal porphyrins, but in recent years there has been re-
newed interest and new results in this area.[1–3] One of our
own contributions to this area has been the establishment of
the first fully characterised boron–porphyrin complexes,
[a] Dr. A. Weiss, Prof. Dr. W. Siebert
Particular features of boron–porphyrin complexes include,
firstly, the coordination of two boron atoms per porphyrin,
in which each boron atom is bonded to two adjacent nitro-
gen atoms. Secondly, a marked tetragonal elongation of the
porphyrin ligand is observed, as measured by the non-
bonded N···N distance that is parallel to the B···B axis,
which is 0.8–1.2 Š longer than the perpendicular N···N dis-
tance.[3] In a series of communications we have reported
three key structural types for which X-ray crystal structures
Anorganisch-Chemisches Institut, Universität Heidelberg
Im Neuenheimer Feld 276, 69121 Heidelberg (Germany)
Fax: (49)6621-545609
E-mail: walter.siebert@urz.uni-heidelberg.de
[b] Dr. A. Weiss, Dr. M. C. Hodgson, Prof. P. D. W. Boyd,
Prof. P. J. Brothers
Department of Chemistry, The University of Auckland
Private Bag 92019, Auckland (New Zealand)
Fax : (+64)9373-7422
E-mail: p.brothers@auckland.ac.nz
Chem. Eur. J. 2007, 13, 5982 – 5993ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5983

P. J. Brothers, W. Siebert et al.
have been obtained. The first is [B2OF2
A
the latter, the product was 2,[5] which precipitated from the
reaction mixture and was only stable in solution in the pres-
ence of excess boron halide. Chromatography of 2 on basic
cified porphyrin dianion), which contains an F B O B F
chain threaded through the centre of the porphyrin,[4,7] and
the second is [B2O2
A
(por)] (2) in which a four-mem-
alumina produced [B2O(OH)2(por)], which was the hy-
A
bered B2O2 ring is coordinated in the porphyrin cavity per-
droxy-substituted analogue of 1. The formation of 1 or 2 is
likely to proceed through hydrolysis of initially formed
products of the reactions of boron halides with porphyrin
molecules that do not contain oxygen.
pendicular to the porphyrin plane.[5] A nBu B B nBu
ꢀ ꢀ ꢀ
singly bonded diborane(6) appears in the third type,
[(BnBu)
spectroscopically, but not yet crystallographically, is repre-
sented by the diboryl complex [(BCl2)2
(por)] (4).[6] Butyl
complex 3 can be prepared either by substitution of the
chloro ligands in [(BCl)2(por)] (5) by using nBuLi (precur-
sor 5 is synthesised from [Li2(por)] with B2Cl4) or directly
E
To demonstrate this hypothesis, the reaction of BCl3 with
U
dilithium tetra-p-tolylporphyrin [Li
strictly anhydrous conditions was undertaken. The green,
moisture-sensitive product, [(BCl2)2(ttp)] (4a), precipitated
2(ttp)][8] in hexane under
H
ACHTREUNG
C
from the reaction mixture and could be isolated by filtra-
from the reaction of 4 with nBuLi in an unusual and unex-
tion. Compound 4a can also be produced from the reaction
pected reductive coupling reaction.[6]
of the free-base porphyrin H2(ttp) with BCl3 in pentane, but
A
the reaction with [Li2(ttp)] gives a superior yield. The
A
11B NMR spectrum of the product shows a single resonance
1
at d=5.6 ppm, and the H NMR spectrum shows two sin-
glets (1:1 ratio) for the four tolyl methyl groups and a dis-
tinctive AB quartet pattern for the Hb protons of the pyr-
role rings, which is consistent with C2h or C2v symmetry in
the product. The most probable geometry is a transoid ar-
rangement of two BCl2 groups displaced above and below
the porphyrin ring (C2h symmetry), in which each boron
atom is coordinated to two porphyrin nitrogen atoms. This
geometry is supported by density functional calculations
In this paper we present experimental details for the for-
ꢀ
mation of the B B single-bond porphyrin complexes 3 and
(B3LYP/6-311G(d,p)) on unsubstituted porphyrin analogue
A
5, diboryl complex 4 and significant new results. These com-
prise reactions of free-base and lithiated porphyrins with all
four boron halides BF3, BCl3, BBr3 and BI3; further exam-
ples of the unexpected reductive coupling reactions to pro-
4, which showed that this geometry is an energy minimum.
The calculated bond lengths and angles are consistent with
this formulation (Figure 1a). The reaction of 4a with H2O
duce [(BBr)
new complexes [B
reduction, respectively, of 5. The two new species [B2-
(por)]2+ and [B2
(por)] are the first examples of porphyrin
complexes to contain three-coordinate, trigonal-planar
boron. The neutral complex [B2(por)], formally boron(I),
offers the intriguing possibility that it may contain a dibor-
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
ꢀ
ꢀ
ene ( B=B ) moiety. However, spectroscopic and density
functional theory (DFT) computational evidence indicates
that a more appropriate description is a diborane(4) iso-
phlorin.
Figure 1. Calculated structures of a) compound 4 and b) compound 5
(B3LYP/6-311G(d,p)).
A
(2 equiv) resulted in hydrolysis to form equimolar amounts
of [B2O2(BCl3)2(ttp)] (2a) and the porphyrin salt [H4-
(ttp)]Cl2, which were both identified by 1H NMR spectrosco-
A
ACHTREUNG
Results and Discussion
ACHTREUNG
py, and indicated that the oxygen-containing boron–porphy-
rin products produced in the presence of trace amounts of
water are most likely to occur from hydrolysis of 4a formed
initially.
Reactions of H2
and chlorides: Our early work focussed on the reaction of
the simple boron halides BF3·OEt2 and BCl3·MeCN with
N
G
free-base porphyrins H
porphyrin), H2(tpClpp) (tpClpp: dianion of 5,10,15,20-tetra-
p-chlorophenylporphyrin) and H2(oep) (oep: dianion of
A
The cross-ring, non-bonded B···B distance in [B2O2-
G
U
A
tetragonal in-plane elongation of the porphyrin macrocycle
ꢀ
2,3,7,8,12,13,17,18-octaethylporphyrin) in the presence of
trace amounts of water, which results in complexes that con-
should readily accommodate a B B single bond (rcov(B)=
0.85 Š[9]). To investigate this possibility, the reaction of
[10,11]
ꢀ
ꢀ
tain B O bonds in which the boron halogen bonds had
been partially (BF3·OEt2) or completely (BCl3·MeCN) hy-
drolysed. In the former case, the product was 1[4,7] and in
B2Cl4
with either [Li2
G
G
ꢀ1008C gave the product [(BCl)
ACHTREUNG
[6]
ꢀ
B B single bond present in the B2Cl4 precursor is retained.
5984
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Chem. Eur. J. 2007, 13, 5982 – 5993

Boron–Porphyrin Complexes
The reaction containing [Li2
FULL PAPER
1
G
pyrrole protons in their H NMR spectra. The diboryl com-
plexes 4a and 6a show a distinct AB quartet for Hb, which
appear as two clearly separated doublets or multiplets that
each correspond to four protons upon integration. On the
other hand, the Hb resonances in the diboranyl compounds
5a and 7a appear as one multiplet that corresponds to eight
LiCl, whereas in the reaction with H
ACHTREUNG
ACHTREUNG
AHCTREUNG
ACHTREUNG
ꢀ
tion medium, whereas the diboranyl porphyrin remains in
solution and allows separation of the products by filtration.
The 11B NMR spectrum of 5a shows a single resonance at
protons upon integration. In each case, the B B bonded
compound (5a and 7a) shows a more upfield chemical shift
11
ꢀ
in the B NMR spectrum than the compound with no B B
1
d=ꢀ12 ppm, and the H NMR spectrum is indicative of C2h
bond (4a and 6a) does.
or C2v symmetry with two singlets for the tolyl methyl
groups and a multiplet for the Hb protons of the pyrrole
Unexpected reductive coupling—Reaction of H2Por and
rings.
A
transoid arrangement (C2h symmetry) of the
Li2Por with boron tribromide and boron triiodide: The reac-
ꢀ ꢀ ꢀ
Cl B B Cl moiety in which the two boron atoms are dis-
placed slightly above and below the porphyrin ring (Fig-
ure 1b) is consistent with the 1H NMR spectroscopy data ob-
tained and is also an energy minimum calculated for 5
ꢀ
tion of either H
with the intention of preparing the bromo analogues of 4a,
6a, and [(BBr2)2(ttp)] (8a). However, the reaction produced
G
U
a mixture of two compounds that were identified by two
1
(B3LYP/6-311G
1.74 Š, which is typical for a B B single bond.
Views of the computed structures of 4 and 5 are given in
Figure 1, and bond lengths and angles in Table 1. It is inter-
esting to compare the extent of the tetragonal porphyrin
(d,p)). The B B distance calculated for 5 is
sets of resonances in the H NMR spectrum. The most nota-
[9]
ꢀ
ble feature was that the Hb resonances of the two com-
pounds took two forms, one with an AB quartet similar to
that observed for 4a and 6a, whereas the second exhibited a
multiplet of the form seen for 5a and 7a. This result indicat-
elongation D
A
ed that both compounds 8a and [(BBr)
formed in the reaction of BBr3 with either H2
(ttp)]. An apparently spontaneous reductive coupling of two
ACHTREUNG
the N···N distances parallel and perpendicular to the B B
axis) computed for 4 and 5, which have values of 1.27 and
0.84 Š, respectively. The porphyrin elongates to a greater
extent to accommodate the two bulkier BCl2 groups in 4,
ACHTREUNG
AHCTREUNG
BBr2 groups, which formally contain boron(III), occurred to
G
ꢀ ꢀ ꢀ
ꢀ
produce the Br B B Br moiety and formed a B B single
bond between two boron(II) centres. The reductant in this
system is bromide, although detection of the Br2 oxidation
product proved to be difficult. The proportions of 8a and 9a
that are formed vary with reaction conditions, but 8a is the
whereas the much more compact Cl B B Cl moiety in 5
requires less distortion of the macrocycle. The porphyrin in
4 also shows a considerable out-of-plane distortion, which is
apparent from the view of the molecule in Figure 1a.
The reaction of BF3·OEt2 with [Li2T
the diboryl product [(BF2)2(ttp)] (6a). The diboranyl com-
plex [(BF)2(ttp)] (7a) is prepared by substitution of the
chlorine atoms in 5a by using SbF3 as the fluorinating agent.
The two sets of compounds [(BX2)2(ttp)] (X=Cl (4a),
F (6a)) and [(BX)2(ttp)] (X=Cl (5a), F (7a)) have the
G
major product. For example, in a typical reaction of [Li2-
1
U
A
G
spectroscopy is approximately 3:1.
In contrast, the corresponding reaction of [Li
ACHTREUNG
BI3 gives only the dehalogenated product [(BI)2
ACHTREUNG
G
with no [(BI2)2
ACHTREUNG
same symmetry (C2h), but can be distinguished from each
other, in addition to chemical shift differences, by the cou-
pling patterns for the resonance observed for the eight Hb
could be isolated and characterised fully, in contrast to 9a
which was always formed in the presence of 8a. The exclu-
sive formation of the reductively coupled product in the re-
Table 1. Calculated structural parameters for [(BX2)2
A
G
N
ꢀ
ꢀ
ꢀ
24-atom
Mean N4
N···N
[Š][b]
DN···N
B X
[Š]
B N
[Š]
B B
[Š]
X-B-X
[8]
X-B-B
[8]
plane[a] [Š]
plane[a] [Š]
[Š][c]
A
N
6 (X=F)
4 (X=Cl)
8 (X=Br)
10 (X=I)
1.189
1.338
1.351
1.331
0.912
0.950
0.946
0.922
3.615, 2.468
3.725, 2.443
3.732, 2.440
3.734, 2.431
1.15
1.28
1.29
1.30
1.367, 1.396
1.824, 1.926
1.997, 2.138
2.224, 2.505
1.580
1.563
1.556
1.538
110.3
104.8
102.2
99.02
[(BX)2(porphine)]
N
7 (X=F)
0.443
0.393
0.365
0.291
0.495
0.499
0.401
0.353
0.327
0.261
0.432
0.437
3.317, 2.496
3.316, 2.480
3.319, 2.476
3.326, 2.466
3.314, 2.494
3.284, 2.495
0.82
0.84
0.84
0.86
0.82
0.79
1.427
1.957
2.169
2.252
1.640
1.611
1.586
1.554
1.543
1.520
1.581
1.589
1.727
1.730
1.724
1.707
1.794
1.764
113.8
108.6
106.5
102.0
110.3
112.9
5 (X=Cl)
9 (X=Br)
11 (X=I)
3 (X=nBu)
3a [(BnBu)2
A
[a] Displacement from B. [b] Non-bonded N···N distances parallel and perpendicular to the B–B axis. [c] Difference between N···N
N···N(perpendicular). [d] Data from X-ray crystal structure.[6]
A
Chem. Eur. J. 2007, 13, 5982 – 5993ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5985

P. J. Brothers, W. Siebert et al.
action with BI3 is consistent with the stronger reducing
power of iodide relative to the lighter halides.
Synthetic routes for the preparation of [(BX2)2
ACHTREUNG
6a and 8a) and [(BX)2
ACHTREUNG
marised in Scheme 1.
Figure 2. Superposition of structures calculated for [(BF2)
and [(BI2)(porphine)] (10) (B3LYP/6-311G(d,p)). Hydrogen atoms omit-
ted for clarity
(porphine)] (6)
A
ACHTREUNG
The driving force for the spontaneous reductive dehaloge-
ꢀ
nation and B B bond formation to give 9 and 10 is likely to
derive from a combination of the forced close proximity of
the two boron atoms within the sterically constrained mac-
rocycle, and the presence of bromide and iodide ions that
are able to act as reductants. This hypothesis was examined
by thermochemical calculations performed for the coupling
reaction shown in Scheme 2a for X=F, Cl, Br or I. The re-
Scheme 1. Synthetic routes for the preparation of [(BX2)2
ACHTREUNG
(6a), Cl (4a), Br (8a)) and [(BX)2
(11a)).
ACHTREUNG
Scheme 2. Reductive coupling reactions for a) porphyrin and b) dipyrro-
methene (dpm) complexes.
DFT calculations of the optimised molecular structure of
chloro complex 4 showed that the complex has the largest
D(N···N) in-plane tetragonal distortion of any of the dibor-
G
on–porphyrins prepared to date (Table 1). In addition, the
calculated structure of 4 showed a marked out-of-plane dis-
tortion of the macrocycle (Figure 1a), which indicates that
there is considerable crowding in the molecule in order for
the porphyrin to accommodate the two bulky BCl2 groups
with tetrahedral geometry at boron. This steric effect should
increase in the bromo and iodo analogues. Corresponding
structural data have been calculated for the series of com-
sults, given in Table 2, clearly show that for X=Br and I,
the reaction is both exothermic and spontaneous in the gas
phase, whereas the opposite is true for X=F and Cl, which
is in agreement with experimental observations. Whereas
this calculation is consistent with the spontaneous reduction
reactions, the question of whether or not steric congestion
provides the driving force for the reaction was tested by per-
forming similar calculations for the same reaction with the
corresponding boryl dipyrromethene (dpm) complexes
(Figure 3). These complexes, exemplified by the fluores-
plexes [(BX2)2(por)] (X=F (6), Cl (4), Br (8) and I (10))
A
and selected structural parameters are given in Table 1.
These data show that on going from F to Cl the porphyrin
becomes more distorted, which is evident from D(N···N) and
A
Table 2. Thermochemical calculations for [(BX2)2
G
(por)]
the displacement of boron from both the mean N4 and 24-
atom planes, but for the Br and I complexes the distortion is
no more marked. However, on going down the series of all
four halides there is a significant change in the X-B-X angle,
and 2
A
(dpm)]![(BX)2
N
X
C
(por)]![(BX)2
(por)]
2
N
(dpm)]![(BX)2
]
]
F
715.8
95.9
667.4
46.7
815.7
375.8
249.4
184.8
ꢀ
which becomes progressively smaller and the outer B X
Cl
Br
I
bond becomes markedly longer than the other. This distor-
tion is evident in Figure 2, which shows a superposition of
the calculated molecular structures of (6) and [(BI2)2-
ꢀ70.9
ꢀ119.4
ꢀ212.9
ꢀ261.3168.6
[a] Optimised structures B3LYP/6-311G
unscaled ZPE corrections.
A
A
5986
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Chem. Eur. J. 2007, 13, 5982 – 5993

Boron–Porphyrin Complexes
FULL PAPER
demonstrates a new geometry for boron–porphyrins. The re-
action of 5a with catechol in the presence of free-base por-
phyrin (1 equiv; to act as a base for the catechol protons) in
toluene at ꢀ788C produces a compound identified as [B2-
A
N
1
tensities of the peaks in the H NMR spectrum indicate that
only one catecholate ligand is present and the resonances
are observed at d=4.1 (H3, H6) and 5.3ppm ( H4, H5). The
coordination of catechol to the boron centres was confirmed
by the observation of significantly upfield-shifted resonances
for the aromatic protons in the catecholate ligand, which is
induced by the porphyrin diamagnetic ring current. On the
basis of the presence of one catecholate ligand per porphy-
rin and the symmetry of the overall porphyrin complex ob-
Figure 3. Structures calculated for boryl and diboranyl dipyrromethene
(dpm) complexes (B3LYP/6-311G(d,p)).
A
cence dye BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-inda-
cene) in which X=F,[12,13] are well known. A [(BX2)
(dpm)]
complex can be considered to be a model for half of a
[(BX2)2(por)] complex. The corresponding reaction for
AHCTREUNG
ACHTREUNG
1
which the second set of thermochemical calculations were
carried out is shown in Scheme 2b. For the dipyrromethene
series, the reductive elimination reaction was not favoured
for any halogen, which supports the hypothesis that steric
crowding in the porphyrin species contributes to the driving
force.
served by H NMR spectroscopy, 12a is proposed to retain
ꢀ
the B B single bond and to contain the B
A
one face of the porphyrin, with both boron atoms displaced
to the same side of the N4 plane (Scheme 3, Figure 4). Fur-
ther evidence for the formulation of 12a comes from the ob-
servation of the molecular ion peak by fast atom bombard-
ment mass spectrometry (FABMS).
ꢀ
The spontaneous reductive coupling and B B bond for-
mation to form [(BX)
2(ttp)] (X=Br (9), I (10)) is observed
only in the reactions of BBr3 and BI3 with [Li2
A
as BCl3 gives the expected product, 4a. However, chemical
reduction of 4a by using sodium/potassium alloy as the re-
ductant at ꢀ788C in toluene effected reductive dehalogena-
tion to produce 5a, and provides an alternative synthesis of
5a that does not rely on the use of B2Cl4 as the precursor
(Scheme 1). Similarly, because the reaction of BI3 with [Li2-
ꢀ
A
Scheme 3. Reaction of [(BCl)2
N
N
vides a clean and convenient route to the diboranyl-contain-
ing porphyrin that does not require the use of B2Cl4.
ACHTREUNG
11B NMR data were collected for the series [(BX)2
(ttp)]
G
for X=Cl, Br and I. It is difficult to compare the chemical
shifts in the 11B NMR spectrum for the diboranyl porphyrin
species because the 11B NMR chemical shifts for the simple
boron halides do not vary monotonically down the group
owing to the heavy-atom effect.[14] 11B NMR chemical shifts
have been calculated (DFT) for the model [(BX)2-
A
with the experimental values observed for [(BX)2(ttp)] (5a,
9a and 11a) in Table 3.
A
Figure 4. Structure calculated for [B2
G
A
C6H4O2) (B3LYP/6-311G(d,p)).
AHCTREUNG
Reaction of 5a with catechol: The chlorine atoms in the di-
boryl and diboranyl porphyrin 5a can be substituted by cat-
echol to give a catecholate complex as the product that
A DFT calculation of the molecular structure of model
complex 12, Figure 4, gave a minimum energy structure in
which the plane of the catecholate moiety is tilted (158) by
folding with respect to the O···O vector from the normal to
the porphyrin 24-atom mean plane. The B atoms are 0.82 Š
out of the porphyrin plane. However, the higher symmetry,
C2v, arrangement in which the catecholate (C6H4O2) plane
orthogonal to the mean porphyrin plane is <0.1 kcalmolꢀ1
higher in energy. There is a small imaginary frequency that
corresponds to the movement of the catecholate between
the two possible tilted arrangements indicating that at room
temperature an average structure would be observed in the
NMR spectrum.
Table 3. Experimental and calculated 11B NMR chemical shifts for dibor-
yl porphyrin complexes.[a]
Calculated[b]
Experimental[c]
[(BF)2
[(BCl)2
[(BBr)2
[(BI)2(por)]
A
ꢀ17.5
ꢀ19.0
ꢀ18.6
ꢀ8.4
–
G
U
ꢀ12 to ꢀ14 (br)
G
ACHTREUNG
ꢀ17.8
G
ꢀ13.3
[a] Structures optimised B3LYP/6-311G(d,p). Chemical shifts referenced
N
to those calculated for BF3·Et2O. [b] Calculated for Por: porphine by
using the relativistic ZORA method, which includes spin-orbit contribu-
tions (ADF 2003) BPW91/TZP/ZORA. [c] Por: TTP.
Chem. Eur. J. 2007, 13, 5982 – 5993ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5987

P. J. Brothers, W. Siebert et al.
Reactions of 4a and 5a with nBuLi—Formation of a butyl-
substituted complex and a further example of unexpected
reductive coupling: The dichlorodiboranyl complex 5a is ex-
tremely hydrolytically sensitive and formation of a deriva-
tive by replacing the chloro substituents was undertaken to
ꢀ
provide further evidence for the presence of a B B single
bond. The reaction of 5a with nBuLi (2 equiv) at ꢀ788C in
hexane was successful in producing the di-n-butyldiboranyl
derivative [(BnBu)2(ttp)] (3a). Complex 3a, which was char-
A
Figure 5. Upfield region of the 1H NMR spectrum of [(BnBu)2
showing experimental and calculated (in brackets) (GIAO B3LYP/6-
(ttp)] 3a
acterised by spectroscopy and an X-ray crystal structure
(see below), could also be produced directly by the reaction
of 4a with nBuLi (4 equiv) in hexane at ꢀ788C (Scheme 4).
31G(d)//B3LYP/6-311+G(2d,p)) chemical shifts for the n-butyl hydrogen
N
atoms.
ꢀ
The presence of both the butyl substituents and the B B
single bond were confirmed by an X-ray crystal structure
determined for a crystal of 3a grown from dichlorome-
thane.[6] The molecule has an inversion centre, and the two
boron atoms are found displaced 0.44 Š above and below
ꢀ
the mean N4 plane. The B B distance is 1.769(7) Š, which
[9]
ꢀ
falls within the range for B B single bonds. The average
ꢀ
B N bond distance, 1.58 Š, is consistent with the sum of the
covalent radii of boron (0.89 Š, assumed to be half the B B
ꢀ
bond length in this structure) and sp2-hybridised nitrogen
atom, 0.70 Š.[18] The geometry around the boron atoms is
close to tetrahedral, with the six angles around B1 ranging
from 104 to 1128. The porphyrin ligand is not completely
planar, with the two pyrrole rings bound to B1 tilted up by
6.6 and 8.48, whereas the other two pyrrole rings are tilted
downwards by about the same amount. The porphyrin also
Scheme 4. Reactions of [(BCl2)2
N
N
nBuLi.
This reaction results from both substitution and reduction at
the boron centre in precursor 4a. The expected intermediate
shows
0.79 Š. As expected, this is less than the D
1.09, 1.14, and 1.14 Š observed for the crystal structures of
[B2OF2 (BCl3)2-
(TpClpp)],[4] [B2OF2(oep)],[7] and [B2O2
ꢀ ꢀ
a
marked tetragonal distortion with
D
A
ACHTREUNG
is the bis(di-n-butyl)boryl complex [(BnBu2)2(ttp)], which
A
may then undergo spontaneous reductive coupling as a
result of steric crowding. The fate of the butyl moieties is
not clear, and attempts to find suitable trapping reagents for
butyl radicals were unsuccessful.
G
G
(TpClpp)],[5] in which B O B bridges occur. The more
ꢀ
compact B B single-bond moiety requires less elongation of
the porphyrin macrocycle. Good agreement is obtained be-
tween the structural parameters calculated by using DFT
calculations for 3 (Figure 6) and those measured by X-ray
crystallography for 3a.
1
Molecular structure and spectroscopy of 3a: The H NMR
spectrum of the porphyrin resonances in 3a indicate that the
complex has the same symmetry as precursor 5a. In addi-
tion, the four multiplets that correspond to the butyl reso-
nances are observed, they are well-separated and shifted up-
field (ranging from d=ꢀ0.56 for the CH3 group to
ꢀ5.70 ppm for the a-CH2 group) by the diamagnetic por-
phyrin ring current effect. Similar shifts are observed for the
butyl proton resonances in the diamagnetic Group 13por-
phyrin complexes [M
U
G
n-butyl groups above and below the porphyrin plane. The
1
structure and H and 11B NMR spectra of unsubstituted por-
phyrin complex 3 have also been calculated (GIAO B3LYP/
6-31G(d)//B3LYP/6-311+G(2d,p)), and the upfield chemical
E
shifts calculated for the butyl groups qualitatively match
1
those observed for 3a in the H NMR spectrum (Figure 5).
The 11B NMR chemical shift observed for 3a is d=ꢀ6 ppm
and that calculated for 3 is d=ꢀ12.5 ppm.
Figure 6. Calculated structure for [(BnBu)2
G(d)). Hydrogen atoms have been omitted for clarity.
A
5988
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 5982 – 5993

Boron–Porphyrin Complexes
FULL PAPER
Halogen abstraction from 5a to form the dication [B2-
brown to red-brown was observed, and MgCl2 was formed
along with the extremely sensitive black reaction product
A
N
tion of the chloride ions from 5a by treatment with a
weakly coordinating anion. Whereas the reactions of 5a
with either TlPF6 in toluene or NaBPh4 in CH2Cl2 at ambi-
ent temperatures were unsuccessful, the addition of two
[B2(ttp)] (14a), which decomposed immediately on contact
with air or moisture (Scheme 5b).
The key question concerning the neutral complex 14a is
whether or not it can be described as a diborene, containing
A
equivalents of NaB
(ArF)4 (ArF: 3,5-C6H3
(CF3)2)[19] in CH2Cl2
boron(I) and a B=B moiety coordinated to the porphyrin
dianion, or as a diborane(4) in which the diboranyl unit con-
tains boron(II) and the porphyrin ligand has been reduced
by two electrons to form the 20-p-electron isophlorin tet-
raanion. A useful complex for comparison is the cationic di-
boranyl(4) complex 13a which formally contains the same
ꢀ
ꢀ
resulted in the precipitation of NaCl from the reaction
medium and the formation of the extremely air- and mois-
ture-sensitive salt [B
N
N
II
II
ꢀ
boron(II) (B B ) moiety, but retains the 18-p-electron por-
phyrin ligand. Two lines of evidence point towards the for-
mulation of 14a as a diboranyl(4) isophlorin.
Scheme 5. Preparation of a) [B2
N
N
1
The complex was characterised by H NMR spectroscopy
and by observation of the molecular ion peak by means of
FABMS. The computed structure (B3LYP/6-311G
A
the dication [B
ACHTREUNG
1
planar, in which the two three-coordinate, trigonal-planar
boron atoms are in the plane of the porphyrin, and the com-
plex has overall D2h symmetry (Figure 7). The B B and
The first line of evidence comes from the H NMR spec-
trum of 14a in which dramatic chemical shift differences are
observed relative to the cationic complex 13a. These are
summarised in Table 4, in which the chemical shifts for 5a
ꢀ
ꢀ
B N distances are calculated to be 1.70 and 1.45 Š, which
Table 4. Comparison of 1H NMR data for selected boron[a] and zinc[b]
porphyrin complexes.
5a
13a
[Zn
(ttp)][22]
14a
[Zn
ꢀ0.9
4.95
6.05
1.50
U
Hb
Ho
9.16
9.52
8.20
7.737.65
2.75 2.70
8.95
8.10
7.55
2.70
1.05 0.51
5.84 5.69
6.38 6.27
1.64 1.62
8.22 8.14
7.71 7.62
2.76 2.71
Hm
CH3
Figure 7. Calculated structure (B3LYP/6-311-G
(porphine)]2+ (13).
A
ACHTREUNG
[a] CDCl3, 400 MHz. [b] [D8]THF, 400 MHz.
are both slightly shorter than the corresponding distances
(1.73and 1.557 Š) calculated for precursor 5. The N-B-N
angles in 15 open out to 1138 compared with N-B-N angles
of 105.68 in 5. These observations are consistent with the
presence of three-coordinate, trigonal-planar boron in 13
versus four-coordinate, tetrahedral boron in 5.
are also given. Both 5a and 13a show chemical shifts that
are typical for a diamagnetic tetraarylporphyrin complex, in
which, for example, the pyrrole Hb protons appear near d=
9 ppm. In contrast, the resonances for the Hb protons in 14a
are shifted significantly upfield to d=0.51 and 1.05 ppm.
The data in Table 4 also show upfield shifts for the other
resonances in 14a relative to those in 5a and 13a (Ho, Hm
and CH3). Very similar upfield shifts are observed for the
Chemical reduction of 5a to 14a: The formal two-electron
chemical reduction of 4a by using sodium potassium alloy
as the reductant was described above. Compound 4a con-
ꢀ
anionic zinc porphyrin species [Zn
ACHTREUNG
tains boron
A
compared with the neutral complex [Zn
ACHTREUNG
single bond and boron(II). The prospect of a further two-
electron reduction of 5a to form a diborene species, which
would formally contain boron(I) and a B=B double bond,
was intriguing. The chemical reduction of 5a was achieved
by using magnesium anthracenide[20,21] as the reductant in
THF at ꢀ308C. An immediate colour change from green-
9.95 ppm).[22] 1H NMR data for both zinc complexes are also
given in Table 4. Two-electron reduction of the neutral com-
plex [Zn(ttp)] results in reduction of the porphyrin macrocy-
A
cle rather than reduction of the d10 zinc(II) metal centre.
1
The H NMR chemical shifts observed for [Zn
A
been interpreted as arising from the 20-p-electron, antiaro-
Chem. Eur. J. 2007, 13, 5982 – 5993ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5989

P. J. Brothers, W. Siebert et al.
matic isophlorin macrocycle that results from two-electron
reduction of the porphyrin ring. More recently, an antiaro-
+23.6 and +26.9 ppm) and in values reported for the [Si-
A
A
matic silicon porphyrin derivative [Si
G
G
served pyrrole 1H NMR spectra indicate a rapid interconver-
sion of the single and double bonds with a single signal ob-
served for the zinc and silicon systems and two signals ob-
served for the boron system. A symmetric D2h transition
state has been calculated for the interconversion of the two
been characterised by X-ray structural analysis and NMR
spectroscopy.[23] In the solid state the highly ruffled porphy-
rin shows bond length alternation and in solution there are
similar upfield shifts of the pyrrole protons to d=1.29 ppm.
The similarity of the NMR data for 14a, [Zn
A
[B2(por)] structures. In the gas phase the enthalpy and free
G
A
G
energy of activation have been calculated to be 5.48 and
6.19 kcalmolꢀ1, respectively, which is indicative of a rapid in-
terconversion rate.
drawn for the formulation of 14a as the diboranyl(4) iso-
phlorin complex.
Further support for the diboranyl(4) isophlorin formula-
The reduction of 13a to 14a was also investigated by
cyclic voltammetry. Two reversible reduction waves at ꢀ1.05
and ꢀ1.66 V were observed for 13a at ꢀ308C in CH2Cl2.
tion comes from DFT calculations (B3LYP/6-311G
A
the molecular structure of [B2(por)] (14). The calculated
AHCTREUNG
structure shows a planar molecule with the two boron atoms
lying in the porphyrin plane, and in that sense is similar to
The free-base porphyrin (H2(ttp) also undergoes two one-
U
electron reductions under these conditions, at ꢀ1.39 and
ꢀ1.79 V, which indicates that 13a is more easily reduced
than the free-base porphyrin. The opposite situation is ob-
ꢀ
cationic complex 14. The calculated B B distance is 1.73Š,
ꢀ
which is similar to the B B distances calculated for 13
(1.70 Š), 5 (1.737 Š) and 3 (1.790 Š), and observed by X-
ray crystallography for 3a (1.769(7) Š). In particular, the
served for the electrochemical reduction of [Zn(ttp)], for
G
which two reversible one-electron reductions were reported
ꢀ
calculated B B distance is slightly longer for 14 than for 13,
at ꢀ1.44 and ꢀ1.98 V, compared with ꢀ1.19 and ꢀ1.54 V re-
and there is no evidence for the bond shortening that might
be observed if a diborene B=B bond was present in 14. A
further feature of the calculated structure of 14 is that the
pyrrole rings are differentiated. For example, two diagonally
ported for H
2(ttp) under comparable conditions.[22] The
G
higher potential for reduction of the macrocycle in boron
complex 13a relative to [Zn(ttp)] is consistent with the
higher net positive charge on dicationic 13a relative to neu-
tral [Zn(ttp)].
AHCTREUNG
ꢀ
opposite pyrrole rings have shorter Cb Cb bond lengths
(1.35 Š), whereas the other two pyrrole rings have longer
ACHTREUNG
ꢀ
Cb Cb bond lengths (1.40 Š). Overall, 14 has lower symme-
try (C2h) than 13 (D2h) for which all the pyrrole rings are
Conclusion
ꢀ
equivalent and have, for example, Cb Cb, bond lengths of
1.369 Š. The calculated alternating bond lengths in the mac-
rocycle in 14 are consistent with the formulation as an iso-
phlorin complex, in which the 20-electron antiaromatic mac-
rocycle has undergone a Jahn–Teller distortion and some al-
ternation of bond lengths is evident. The HOMO calculated
by DFT also shows a 20-electron p system (Figure 8).
The chemistry of boron–porphyrins has evolved beyond the
simple isolation and characterisation of some unusual com-
plexes obtained as a result of the combination of the small
element boron with the relatively rigid porphyrin macrocy-
cle. This evolution is marked by the observations, reported
in this paper, of unusual chemistry for both the boron and
the porphyrin ligand, each occurring as a result of the pres-
ence of the other. The remarkable chemistry observed for
boron is the spontaneous reductive coupling, first communi-
cated[6] for the reaction of 4a with nBuLi to give the dibo-
ranyl species [(BnBu)
ACHTREUNG
tions of BBr3 and BI3 with [Li2
AHCTREUNG
yl species 9a and 11a. This chemistry does not normally
occur with the simple boron halides, and DFT calculations
on the thermochemical parameters support the hypothesis
that the sterically constrained environment of the porphyrin
Figure 8. HOMO for [B2
A
G
in the [(BX2)2(ttp)] precursors contributes to the driving
A
force for the reaction.
Unusual chemistry for the porphyrin ligand was observed
for the chemical reduction of 5a, which gave the neutral
species 14a, and was characterised as a complex that con-
tained the 20-electron isophlorin anion. The marked tetrago-
nal elongation of the porphyrin ligand observed in all the
boron–porphyrin complexes is another unusual feature of
these species, and may give rise to novel electronic and
spectroscopic features, which are currently the subject of
further investigations.
Nucleus-independent chemical shifts (NICS)[24] calculated
for 14 and 13 at the centre of the pyrrole rings (NICS(0))
and at a distance of 1 Š above this centre (NICS(1)) clearly
indicate a paratropic shift[25] from the aromatic porphyrin di-
cation (NICS(1): d=ꢀ16.1 ppm) to the antiaromatic neutral
isophlorin (NICS(1): d=+20.2 and +23.4 ppm). Similar
shifts have been calculated in the [Zn
A
(NICS(1): d=ꢀ9.5 ppm) and [Zn
AHCTREUNG
5990
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 5982 – 5993

Boron–Porphyrin Complexes
FULL PAPER
product was separated from LiCl by extraction into toluene. After re-
moval of the toluene the product was isolated as a dark-violet powder
(0.94 g, 80%). The solid product contained one equivalent of toluene
Finally, the increasing sophistication of DFT calculations
has proved invaluable to the work reported here by comple-
menting and in some cases extending the experimental re-
sults.
that could not be removed even under high vacuum. M.p. >3008C;
3
1H NMR (200 MHz, CD2Cl2): d=9.16 (AB-q, 8H; Hb), 8.22 (d, JH,H
=
3
7.9 Hz, 4H; Hortho), 8.14 (d, 3JH,H =7.9 Hz, 4H; Hortho), 7.71 (d, JH,H
=
7.9 Hz, 4H; Hmeta), 7.62 (d, 3JH,H =7.9 Hz, 4H; Hmeta), 2.76 (s, 6H; CH3),
2.71 ppm (s, 6H; CH3); 11B NMR (64.2 MHz, CD2Cl2): d=ꢀ12 ppm (br);
UV/Vis (CH2Cl2): lmax (loge)=370 (3.78), 425 (4.86), 559 (3.60), 605 nm
(3.70); LTFABMS (toluene): m/z (%): 760 (1) [M]+, 725 (7) [MꢀCl]+,
690 (3) [Mꢀ2Cl]+, 671 (4) [Mꢀ2Bꢀ2Cl+H]+. Compound 5a was also
Experimental Section
All syntheses were carried out under an atmosphere of dry argon or ni-
trogen. Glassware was heated under vacuum and flushed with inert gas
prior to use. Solvents were dried according to the usual methods and de-
gassed prior to use. NMR spectra were obtained by using a Bruker
DRX 200 spectrometer operating at 200.13( 1H), 64.21 (11B) or
50.32 MHz (13C), an Avance 300 spectrometer operating at 300.13 MHz
(1H) or a Bruker DRX 400 spectrometer operating at 400.13MHz ( 1H).
prepared from the reaction of H2(ttp) (0.57 g, 0.85 mmol) with B2Cl4
G
(0.16 g, 0.95 mmol) in hexane (20 mL), in a procedure similar to that de-
scribed for [Li2(ttp)]. The product was separated from the [H4(ttp)]Cl2
residue by extraction with toluene (0.33 g, 92%).
[(BCl)2(TpClpp)] 5b: The reaction of H2(TpClpp) (2.16 g, 2.88 mmol)
A
ACHTREUNG
A
N
ACHTREUNG
with B2Cl4 (0.47 g, 2.88 mmol) in hexane (120 mL) was carried out as de-
scribed for 5a to give 5b (0.95 g, 68%). 1H NMR (200 MHz, CD2Cl2):
d=9.10 (AB-q, 8H; Hb), 8.22 (d, 3JH,H =8.4 Hz, 4H; Hortho), 8.14 (d,
3JH,H =8.1 Hz, 4H; Hortho), 7.83(d, 3JH,H =8.4 Hz, 4H; Hmeta), 7.74 ppm (d,
3JH,H =8.2 Hz, 4H; Hmeta); 11B NMR (64.2 MHz, CD2Cl2): d=ꢀ14 ppm
(br); FABMS (2-nitrophenyl octyl ether (NPOE)): m/z (%): 807 (5)
[MꢀCl]+, 788 (4) [Mꢀ2Cl+O]+, 753(5) [ Mꢀ2Clꢀ2B+2H]+.
For H and 13C NMR spectroscopy, the residual solvent signals were used
1
as an internal reference, whereas 11B NMR spectra were referenced to
the external reference BF3·OEt2. UV/Vis spectra were recorded by using
a Perkin–Elmer Lambda 12 spectrometer. Melting points were measured
by using a Büchi instrument and are uncorrected. Elemental analyses
were determined by using a Heraeus C,H,N,O-Rapid instrument. Mass
spectroscopic measurements were made by using a variety of instruments
and ionisation methods, which include Varian MAT CH-7 (EI, chemical
ionisation (CI)), Jeol JMS 700 (FABMS, low-temperature FABMS
(LTFAB)), VG ZAB-2F (EI, high-resolution EI (HREI)), Finnigan
TSG 700 (EI, ESI), VG 70-SE (FAB) and HP-5890 II (HP-5971 MSD)
(GC–MS(EI)) instruments. Electrochemical measurements were per-
formed by using solutions in dichloromethane that contained 0.1m
A
G
N
(80 mL) and BF3·OEt2 (0.25 g, 1.76 mmol) was added (by syringe) to the
reaction mixture at ꢀ788C. After stirring for 2 h at ꢀ788C, the mixture
was allowed to warm to RT and was then stirred for a further 16 h,
during which time the product precipitated from the solution. The insolu-
ble LiF was removed by filtration, and the product was isolated as a mix-
ture of the desired compound together with H2(ttp) by removal of the
T
[nBu4N]
Calomel reference electrode. Chemicals were either obtained commer-
cially or were prepared according to literature procedures (H2
(ttp),[26] H2-
(TpClpp),[26] [Li2(ttp)],[27, 28] B2Cl4,[10,11] magnesium anthracenide,[20,21] Na-
{[3,5-
(CF3)2C6H3]4B} [19]).
[(BnBu)
2(ttp)] 3a:[6] nBuLi (2.5 molLꢀ1; 0.3mL, 0.75 mmol) was added
[PF6] as the electrolyte, a silver working electrode and a standard
toluene from the filtrate under reduced pressure (0.38 g, mixture). M.p.
>3008C; 1H NMR (400 MHz, CDCl3): d=8.64 (d, 3JH,H =4.8 Hz, 4H;
AHCTREUNG
3
Hb), 8.42 (d, JH,H 4.8 Hz, 4H; Hb), 8.13(d, 3JH,H =7.7 Hz, 4H; Hortho),
A
ACHTREUNG
3
3
8.10 (d, JH,H =7.5 Hz, 4H; Hortho), 7.62 (d, JH,H =7.4 Hz, 8H; Hmeta), 2.76
(s, 6H; CH3), 2.71 ppm (s, 6H; CH3); 11B NMR (64.2 MHz, CD2Cl2): d=
ꢀ4.0 ppm; FABMS (NBA): m/z (%): 766 (2) [M]+, 744 (7) [Mꢀ2F+O]+,
725 (9) [Mꢀ3F+O]+; high-resolution FABMS (HRFABMS): calcd for
C48H3611B2F4N4: 766.3062; found: 766.3062.
G
ACHTREUNG
U
ACHTREUNG
to a suspension of 5a (300 mg, 0.35 mmol) in hexane (20 mL) at ꢀ788C.
The reaction mixture was allowed to warm up to RT and was stirred for
16 h. The resulting precipitate was filtered, washed with hexane and ex-
[(BF)2(ttp)] 7a: Compound 5a (0.08 g, 0.09 mmol) was dissolved in tolu-
G
tracted with CH2Cl2 to yield 3a as an olive-green solid (200 mg, 71%).
ene (15 mL) and SbF3 (24 equiv) was added. The reaction mixture was
stirred at RT for 16 h, filtered, and the deep-violet product 7a was isolat-
ed from the filtrate by removal of the solvent under reduced pressure
1H NMR (200 MHz, CD2Cl2): d=8.34 (AB-q, 8H; Hb), 7.99 (d, JH,H
=
3
8.0 Hz, 4H; Hortho), 7.93(d, 3JH,H =7.4 Hz, 4H; Hortho), 7.58 (d, JH,H
=
3
7.9 Hz, 4H; Hmeta), 7.47 (d, 3JH,H =7.4 Hz, 4H; Hmeta), 2.67 (s, 6H; CH3),
2.63(s, 6H; CH 3), ꢀ0.56 (t, 3JH,H =7.3Hz, 6H; CH 3), ꢀ1.13(m, 4H; g-
CH2), ꢀ3.47 (m, 4H; b-CH2), ꢀ5.70 ppm (m, 4H; a-CH2); 11B NMR
(64.2 MHz, CD2Cl2): d=ꢀ6 ppm (br); UV/Vis (CH2Cl2): lmax (loge)=419
(5.28), 443(4.92), 517 (4.12), 553nm (4.00); LTFABMS (toluene): m/z
1
(0.05 g, 69%). M.p. >3008C; H NMR (200 MHz, CD2Cl2): d=9.03(AB-
3
3
q, 8H; Hb), 8.12 (d, JH,H =8.0 Hz, 4H; Hortho), 8.07 (d, JH,H =7.9 Hz, 4H;
Hortho), 7.62 (d, 3JH,H =7.8 Hz, 4H; Hmeta), 7.54 (d, 3JH,H =7.8 Hz, 4H;
Hmeta), 2.68 (s, 6H; CH3), 2.63ppm (s, 6H; CH 3); 11B NMR (64.2 MHz,
CD2Cl2): d=ꢀ5.3ppm; FABMS (NPOE): m/z (%): 744 (10) [M+O]+,
725 (9) [MꢀF+O]+.
(%): 804 (3) [M]+, 747 (7) [MꢀC4H9]+, 690 (15) [Mꢀ2
(C4H9)]+.
[(BCl2)2
R
A
A
G
G
G
(80 mL) and BCl3 (1.00 g, 8.53mmol) was condensed into the reaction
mixture at ꢀ788C. After stirring for 2 h at ꢀ788C the mixture was al-
lowed to warm to RT and was then stirred for a further 16 h, during
which time the product precipitated from the solution. The product was
isolated by filtration and washed several times with pentane, extracted
into toluene solution and then isolated as a dark blue-violet solid by re-
moval of the toluene under reduced pressure (0.32 g, 95%). The ex-
tremely hydrolytically sensitive product formed green solutions in tolu-
ene or halogenated hydrocarbons. Compound 4a could also be prepared
AHCTREUNG
ACHTREUNG
AHCTREUNG
0.97 mmol) and BBr3 (0.50 g, 2.00 mmol) in hexane (20 mL) to give a
mixture of 8a and 9a (0.18 g).
Compound 8a: 1H NMR (200 MHz, CD2Cl2): d=9.47 (d, 3JH,H =4.9 Hz,
4H; Hb), 9.11 (d, 3JH,H =4.9 Hz, 4H; Hb), 8.17 (d, 3JH,H =7.8 Hz, 8H;
Hortho), 7.65 (d, 3JH,H =7.7 Hz, 8H; Hmeta), 2.76 (s, 6H; CH3), 2.65 ppm (s,
6H; CH3); 11B NMR (64.2 MHz, CD2Cl2): d=15.7 ppm.
from the reaction of H2(ttp) (1.58 g, 2.35 mmol) and BCl3 (1.55 g,
A
13.2 mmol) in pentane (50 mL) by using a similar procedure (0.36 g,
18%). M.p. >3008C; 1H NMR (200 MHz, CD2Cl2): d=9.45 (brm, 4H;
Hb), 9.11 (brm, 4H; Hb), 8.45 (d, 3JH,H =7.9 Hz, 8H; Hortho), 7.63(d,
3JH,H =7.8 Hz, 8H; Hmeta), 2.74 (s, 6H; CH3), 2.65 ppm (s, 6H; CH3);
11B NMR (64.2 MHz, CD2Cl2): d=5.6 ppm.
Compound 9a: 1H NMR (200 MHz, CD2Cl2): d=9.33 (AB-q, 8H; Hb),
3
3
8.34 (d, JH,H =7.8 Hz, 4H; Hortho), 8.28 (d, JH,H =7.9 Hz, 4H; Hortho), 7.81
(d, 3JH,H =7.6 Hz, 4H; Hmeta), 7.73(d, 3JH,H =7.6 Hz, 4H; Hmeta), 2.79 (s,
6H; CH3), 2.71 ppm (s, 6H; CH3); 11B NMR (64.2 MHz, CD2Cl2): d=
ꢀ17.8 ppm.
[(BCl)
G
G
(80 mL) was cooled to ꢀ1008C in an ethanol/N2 (l) bath, and B2Cl4
(0.29 g, 1.77 mmol) was condensed into the reaction mixture, which was
then stirred for 3h, then allowed to warm to RT and stirred for a further
12 h. The resulting black powder was collected by filtration, then the
[(BI)2
N
N
(0.48 g, 1.23mmol) in pentane (60 mL) was carried out as described for
4a (except for the addition of solid BI3 to the reaction mixture) to give
1
10a (0.35 g, 64%). H NMR (200 MHz, CDCl3): d=9.61 (AB-q, 8H; Hb),
Chem. Eur. J. 2007, 13, 5982 – 5993ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5991

P. J. Brothers, W. Siebert et al.
3
3
8.26 (d, JH,H =7.9 Hz, 4H; Hortho), 8.19 (d, JH,H =7.9 Hz, 4H; Hortho), 7.72
(d, 3JH,H =7.8 Hz, 4H; Hmeta), 7.64 (d, 3JH,H =7.8 Hz, 4H; Hmeta), 2.72 (s,
6H; CH3), 2.66 ppm (s, 6H; CH3); 11B NMR (64.2 MHz, CD2Cl2): d=
ꢀ13.3 ppm.
Acknowledgements
The authors are grateful to the Marsden Fund (administered by the
Royal Society of New Zealand), the University of Auckland Research
Committee and the Deutsche Forschungsgemeinschaft (DFG) for finan-
cial support.
[B
2(cat)
U
A
0.31 mmol) were dissolved in toluene (30 mL) and cooled to ꢀ788C
before the addition of catechol (0.34 g, 0.31 mmol). The mixture was al-
lowed to warm to RT and the insoluble [H4(ttp)]Cl2 salt was removed by
G
filtration. The solvent was removed from the filtrate to give 12a as a
[1] J. K. M. Sanders, N. Bampos, Z. Clyde-Watson, S. L. Darling, J. C.
Hawley, H. J. Kim, C. C. Mak, S. J. Webb in The Porphyrin Hand-
book, Vol. 3 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Aca-
demic Press, San Diego, 1999, pp. 1.
[2] P. J. Brothers, Adv. Organomet. Chem. 2001, 48, 289.
[3] P. J. Brothers, J. Porphyrins Phthalocyanines 2003, 6, 259.
[4] W. J. Belcher, P. D. W. Boyd, P. J. Brothers, M. J. Liddell, C. E. F.
Rickard, J. Am. Chem. Soc. 1994, 116, 8416.
[5] W. J. Belcher, M. Breede, P. J. Brothers, C. E. F. Rickard, Angew.
Chem. 1998, 110, 1133; Angew. Chem. Int. Ed. 1998, 37, 1112.
[6] a) A. Weiss, H. Pritzkow, P. J. Brothers, W. Siebert, Angew. Chem.
2001, 113, 4311; Angew. Chem. Int. Ed. 2001, 40, 4182; b) A. Weiss,
PhD thesis, University of Heidelberg (Germany), 2002.
[7] T. Kçhler, M. C. Hodgson, D. Seidel, J. M. Veauthier, S. Meyer, V.
Lynch, P. D. W. Boyd, P. J. Brothers, J. L. Sessler, Chem. Commun.
2004, 1060.
black solid that was washed with hexane (0.98 mg, 40%). M.p. >3008C;
3
1H NMR (200 MHz, CD2Cl2): d=9.10 (AB-q, 8H; Hb), 8.10 (brd, JH,H
=
3
7.9 Hz, 8H; Hortho), 7.58 (d, 3JH,H =7.9 Hz, 4H; Hmeta), 7.48 (d, JH,H
=
7.8 Hz, 4H; Hmeta), 5.43(m, 2H; H
N
N
(s, 6H; CH3), 2.68 ppm (s, 6H; CH3); 11B NMR (64.2 MHz, CD2Cl2): d=
ꢀ15 ppm (br); UV/Vis (CH2Cl2): lmax (loge)=370 (3.71), 419 (5.12),
516 nm (4.23); FABMS (NPOE): m/z (%): 798 (10) [M]+, 690 (18)
[MꢀC6H4O2]+, 671 (40) [MꢀC6H4O2B2+H]+.
[B
A
ACHTREUNG
in CH2Cl2 (5 mL) before Na[B
AHCTREUNG
the solution was stirred for 10 min at RT. NaCl was removed by filtration
and the solvent was removed from the filtrate under vacuum to yield 13a
as a green solid (80 mg, 94%); M.p. >3008C; 1H NMR (200 MHz,
CD2Cl2): d=9.52 (AB-q, 8H; Hb), 8.20 (brm, 8H; Hortho), 7.73(d, 4H;
Hmeta), 7.65 (d, 4H; Hmeta), 2.75 (s, 6H; CH3), 2.70 (s, 6H; CH3), 7.57 (s,
16H; Hortho[B
A
N
[8] J. Arnold, D. Y. Dawson, C. G. Hoffman, J. Am. Chem. Soc. 1993,
115, 2707.
[9] A. Moezzi, M. M. Olmstead, P. P. Power, J. Am. Chem. Soc. 1992,
114, 2715.
(64.2 MHz, CD2Cl2): d=ꢀ6.8 ppm ([B
U
m/z (%): 707 (100) [M+O+H]+, 691 (60) [M+H]+, 671 (5) [Mꢀ2B+H]+.
[B2(ttp)] 14a: Compound 5a (0.24 g, 0.28 mmol) was dissolved in THF
G
[10] P. L. Timms, J. Chem. Soc. Dalton Trans. 1972, 830.
[11] H. Schulz, Diplomarbeit, Universität Heidelberg (Germany), 1988.
[12] E. Vos De Wael, E. J. A. Pardoen, J. A. van Koeveringe, J. Lugten-
burg, Recl. Trav. Chim. Pays-Bas 1977, 96, 306.
[13] J. V. Bonfiglio, R. Bonnett, D. G. Buckley, D. Hamzetash, M. B.
Hursthouse, K. M. A. Malik, A. F. McDonagh, J. Trotter, Tetrahe-
dron 1983, 39, 1865–1874.
(20 mL) and cooled to ꢀ308C, then C10H14Mg
N
10ꢀ4 mbar to give 14a as
a
black solid. M.p. >3008C; 1H NMR
3
(200 MHz, CD2Cl2): d=6.38 (d, JH,H =7.6 Hz, 4H; Hmeta), 6.27 (d, JH,H
7.6 Hz, 4H; Hmeta), 5.84 (d, 3JH,H =7.6 Hz, 4H; Hortho), 5.69 (d, JH,H
[14] Personal communication, P. D. W. Boyd, 2004.
[15] R. Guilard, A. Zrineh, A. Tabard, A. Endo, B. C. Han, C. Lecomte,
M. Souhassou, A. Habbou, M. Ferhat, K. M. Kadish, Inorg. Chem.
1990, 29, 4476.
[16] K. M. Kadish, B. Boisselier-Cocolios, A. Coutsolelos, P. Mitaine, R.
Guilard, Inorg. Chem. 1985, 24, 4521.
[17] P. Cocolios, R. Guilard, P. Fournari, J. Organomet. Chem. 1979, 179,
311.
[18] J. E. Huheey, E. A. Keiter, R. L. Keiter, Inorganic Chemistry, 4th ed,
Harper Collins, New York, 1993.
[19] D. L. Reger, T. D. Wright, C. A. Little, J. J. S. Lamba, M. D. Smith,
Inorg. Chem. 2001, 40, 3810.
[20] B. Bogdanovic, S. Liao, R. Mynott, K. Schlichte, U. Westeppe,
Chem. Ber. 1984, 117, 1387.
[21] B. Bogdanovic, N. Janke, C. Krüger, R. Mynott, K. Schlichte, U.
Westeppe, Angew. Chem. 1985, 97, 972; Angew. Chem. Int. Ed.
Engl. 1985, 24, 960.
[22] R. Kosmo, C. Kautz, K. Meerholz, J. Heinze, K. Müllen, Angew.
Chem. 1989, 101, 63 8;Angew. Chem. Int. Ed. Engl. 1989, 28, 604.
[23] J. A. Cissell, T. P. Vaid, A. L. Rheingold, J. Am. Chem. Soc. 2005,
127, 12212.
7.6 Hz, 4H; Hortho), 2.12 (s, 6H; CH3), 1.63(s, 6H; CH 3), 1.05 (d, JH,H
4.4 Hz, 4H; Hb), 0.51 ppm (d, 3JH,H =4.4 Hz, 4H; Hb); 11B NMR
(64.2 MHz, CD2Cl2): d=ꢀ18 ppm.
Computational details: DFT calculations were carried out by using the
[29]
Gaussian 03Program.
[(BX2)2
12, 13, 14 and [Zn
311G(d,p) basis sets for all elements except for iodine in which a SDD
Full geometry optimisations were carried out for
(porphine)], [(BX2)(dpm)] (X=F, Cl, Br, I),
(ttp)]2ꢀ by using the B3LYP density functional with 6-
G
A
ACHTREUNG
ACHTREUNG
pseudopotential and basis set were used.[30] Harmonic vibrational fre-
quencies were calculated for each structure to verify that each stationary
point was a minimum on the molecular hypersurface. These vibrational
calculations were used as a basis for the calculation of enthalpies and
Gibbs energies in the thermochemical analysis of the reductive elimina-
tion reactions of [(BX2)2
A
[(BnBu)2(porphine)] molecule was optimised by using the B3LYP/
ACHTREUNG
6-31G(d) model.
NMR chemical shieldings were calculated by using the GIAO method
with the B3LYP/6-311+G
A
chemical shifts were then calculated with reference to chemical shieldings
calculated by using the same models for BF3·(CH3CH2)2O) and tetrame-
A
[24] Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta, P. von R.
Schleyer, Chem. Rev. 2005, 105, 3842.
thylsilane, respectively.
For the boron complexes [(BX)2(porphine)] with heavier halogens,
G
[25] J. A. Pople, K. G. Untchl, J. Am. Chem. Soc. 1966, 88, 4811.
[26] A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour,
L. Korsakoff, J. Org. Chem. 1967, 32, 476.
11B NMR spectroscopy chemical shifts were calculated by using the ADF
program.[31,32] Calculations were carried out with the BPW91 density
functional and TZP, triple zeta all electron basis sets by using the two-
component zero-order regular approximation (ZORA) method,[33] which
includes spin-orbit coupling. The ADF NMR property module was used
to calculate the nuclear shieldings by using the spin-orbit ZORA method
[27] J. Arnold, J. Chem. Soc. Chem. Commun. 1990, 976.
[28] H. Brand, J. A. Capriotti, J. Arnold, Inorg. Chem. 1994, 33, 4334.
[29] Gaussian 03, Revision B.03, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomer-
y, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyen-
gar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
with the geometries obtained from above (B3LYP/6-311G
(d,p)).[33,34] Cal-
A
culated chemical shifts were determined by the difference between the
shielding in these molecules and those calculated for the reference
A
N
5992
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 5982 – 5993

Boron–Porphyrin Complexes
FULL PAPER
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
[31] Amsterdam Density Functional (ADF), Release 2003, Theoretical
Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amster-
dam, The Netherlands.
[32] G. te Velde, F. M. Bickelhaupt, E. J. Baerends, C. Fonseca Guerra,
S. J. A. van Gisbergen, J. G. Snijders, T. Ziegler, J. Comput. Chem.
2001, 22, 931.
[33] S. K. Wolff, T. Ziegler, E. van Lenthe, E. J. Baerends, J. Chem. Phys.
1999, 110, 7689.
[34] E. van Lenthe, E. J. Baerends, J. G. Snijders, J. Chem. Phys. 1993, 99,
4597.
Received: January 11, 2007
[30] G. Igel-Mann, H. Stoll, H. Preuss, Mol. Phys. 1988, 65, 1321.
Published online: June 15, 2007
Chem. Eur. J. 2007, 13, 5982 – 5993ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5993