Borane adducts of the water soluble phosphine PTA (PTA = 1,3,5-triaza-7-phosphaadamantane)

Article abstract of DOI:10.1016/j.inoche.2006.01.003

Boronation of the water soluble phosphine PTA (PTA = l,3,5-triaza-7- phosphaadamantane) in dry THF easily affords the N-boronated adduct N-B-PTABH₃ which represents the first N-coordinated adduct of this popular water soluble phosphine (N-B-PTABH₃ = 1-boranyl-1,3,5-triaza- 7-phosphaadamantane). In situ ³¹P NMR spectroscopy suggests the occurrence of a stepwise boronation resulting in the addition of as many as four equivalents of BH₃ to PTA to give the polyboranyl species N _x, P_y-B-PTA(BH₃)_(x+y) (x = 1-3, y = 0, 1). Hydrolysis of the monoboranyl adduct occurs slowly in the presence of water yielding back PTA and boric acid, H₃BO₃.

Full text of DOI:10.1016/j.inoche.2006.01.003

Inorganic Chemistry Communications 9 (2006) 360–363
www.elsevier.com/locate/inoche
Borane adducts of the water soluble phosphine PTA
PTA = 1,3,5-triaza-7-phosphaadamantane)
(
a,b
b
a
c
c,
*
Sandra Bola n˜ o , Alberto Albinati , Jorge Bravo , Luca Gonsalvi , Maurizio Peruzzini
a
Departamento de Quimica Inorganica, Facultade de Quimica, Universidade de Vigo, Campus Universitario, E-36310 Vigo, Spain
b
Department of Structural Chemistry (DCSSI), University of Milan, 21, via G. Venezian I-20133 Milan, Italy
Istituto di Chimica del Composti Organometallici, Consiglio Nazionale delle Ricerche (ICCOM-CNR), Via Madonna del Piano,
c
5
0019 Sesto Fiorentino, Firenze, Italy
Received 2 January 2006; accepted 10 January 2006
Available online 2 March 2006
Abstract
Boronation of the water soluble phosphine PTA (PTA = l,3,5-triaza-7-phosphaadamantane) in dry THF easily affords the N-boro-
nated adduct N-B-PTABH which represents the first N-coordinated adduct of this popular water soluble phosphine (N-B-PTABH = 1-
boranyl-1,3,5-triaza-7-phosphaadamantane). In situ P NMR spectroscopy suggests the occurrence of a stepwise boronation resulting in
the addition of as many as four equivalents of BH to PTA to give the polyboranyl species N , P -B-PTA(BH (x+y) (x = 1–3, y = 0, 1).
BO
3
3
3
1
3
x
y
3
)
Hydrolysis of the monoboranyl adduct occurs slowly in the presence of water yielding back PTA and boric acid, H
3
3
.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Water soluble phosphines; Borane; Amino-borane; Phosphino-borane NMR spectroscopy
Among water soluble phosphines, particular attention
has been recently paid to the cage-like tertiary phosphine
l,3,5-triaza-7-phosphaadamantane (PTA, I) [1] whose high
solubility in water has been ascribed to the extensive partic-
ipation of its three nitrogen atoms in hydrogen bonding
interactions with water molecules [2]. The attractiveness
of this ligand stems from its unique properties which make
it an hydrosoluble ligand amenable for aqueous biphasic
catalysis and metal carrier in biological systems. Indeed,
with respect to most of the commonly used water soluble
phosphines [3], PTA is a neutral and air-stable molecule
which is both sterically (the cone angle is 103°) and elec-
tronically comparable with the extremely air and moisture
P
N
N
N
I
Following the recent interest in PTA chemistry, a variety
of PTA metal complexes, invariably coordinated via the
bridgehead P-atom, have been synthesized [1,4–6] and suc-
cessfully used in aqueous phase catalysis [1,4e,4f,5] as well
as in biologic tests aimed at evaluating their activity as
selective anticancer agents [1,4a,4h,6]. In contrast, little
efforts have so far been addressed to explore the coordina-
tion properties of the potentially multidonor triaza-phos-
sensitive PMe [1].
3
+
+
phaadamantane moiety. Electrophiles such as H and R
R = Me, Et, CH Ph, CH CH CH CH I [1], CH C H (p-
[
2
2
2
2
2
2
6
4
MeO)[7]] are known to selectively bind to one nitrogen
atom, but rather surprisingly N-coordination of PTA was
never observed until our recent report describing the
*
Corresponding author. Tel.: +39 055 5225289; fax: +39 055 5225203.
E-mail address: mperuzzini@iccom.cnr.it (M. Peruzzini).
1
387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.01.003

S. Bola n˜ o et al. / Inorganic Chemistry Communications 9 (2006) 360–363
361
1
straightforward formation of the heterobimetallic coordi-
quartet at ꢀ10.55 ppm ( J = 98.3 Hz), which matches the
BH
nation polymer [{CpRu(PTA) (dmso-jS)} {AgCl }] by
literature values reported for other aminoboranes [13].
2
2
the reaction of silver triflate with [CpRu(PTA) Cl] in
Monitoring the reaction between PTA and BH . THF
2
3
3
1
1
DMSO [8]. In such water soluble polymer, the AgCl anion
by P{ H} NMR provides useful mechanistic information
and shows that all of the four donor atoms of PTA may
coordinate borane, fully exploiting the coordination capa-
bilities of PTA (Scheme 1). Thus, adding more than one
equivalent of borane to a THF solution of PTA shows that
the formation of 1 is followed by that of the bis-boranyl
species 2 existing as a pair of coordination isomers depend-
ing on whether the boronation occurs either at a second
nitrogen atom (2N, d ꢀ92.37) or at the phosphorus (2P,
d ꢀ43.41). Further addition of borane produces a similar
pair of tris-boronated PTA isomers (3N, d ꢀ93.45; 3P; d
2
links two nitrogen PTA atoms of two distinct
{
CpRu(PTA) } units, whereas the PTA P-atoms are coor-
2
dinated to the ruthenium centre.
Herein, we report on the simple preparation of new N-
adducts of PTA which pave the way to the synthesis of
new PTA derivatives which, in turn, might be successfully
used as modified versions of PTA in organometallic chem-
istry and catalysis as well as in medicinal chemistry [9].
The plain reaction of 1.1 excess of BH ÆTHF (1 M in
3
THF) with PTA in THF gives, after work-up, a white solid
of formula [P(CH NCH ) {CH N(BH )CH }] (N-B-
ꢀ34.01). Eventually, when a large excess of BH is added
2
2 2
2
3
2
3
PABH3; (1)) (N-B-PTABH = 1-boranyl-1,3,5-triaza-7-
to the solution, the signal due to the tetrakis-boranyl
adduct 4 (d ꢀ28.83) becomes the only resonance visible
in the NMR spectrum. Remarkably, the signals of the
P-boranyl complexes 2P, 3P and 4 are highly deshielded
3
phosphaadamantane) which was characterized by
conventional spectroscopic techniques and elemental
analysis (Scheme 1) [10]. The coordination of borane to
nitrogen rather than phosphorus is indirectly confirmed by
with respect to the N-boranyl adducts ½Dd ¼ dPTA
ꢀ
3
1
the sharp P NMR resonance [d = ꢀ94.85, Dd ¼ dPTA
ꢀ
d
ðPꢀBꢀPTABH Þave ¼ ꢀ101:28 to ꢀ 35:42 ¼ 65:86 and feature
3
d
NꢀBꢀPTABH3 ¼ ꢀ101:28 to ꢀ 94:85 ¼ 6:43], and directly
the typical broadness of boronated phosphines [11]. Only
the first equivalent of borane is transferred to the phospha-
adamantane cage with complete regioselectivity preferring
one of the nitrogen atoms rather than the bridgehead phos-
phorus [14]. In contrast, the bis- and tris-boranyl adducts 2
and 3 are formed without regioselective control. Thus, for 2
N-coordination is favoured whereas for the tris-boranyl
species 3, P-coordination represents the preferred pathway.
Fig. 1 shows the progress of the PTA boronation as a func-
1
3
1
inferred from inspection of both the IR and of the C{ H}
NMR spectra [11]. A diagnostic B-N stretching vibration is
ꢀ
1
visible in the solid state IR spectrum (KBr) at 1171 cm
12], while the carbon-13 NMR spectrum shows the loss of
[
the ternary symmetry of PTA showing four different CH2
resonances in keeping with the regioselective delivery of bor-
1
1
ane to one nitrogen atom. The B NMR spectrum displays a
tion of the equivalents of BH ÆTHF added to a THF solu-
tion of PTA (36 mg, 0.23 mmol, in 0.8 ml of THF locked
3
P
P
a
b
b
BH3.THF
H2O
N
g
N
with a CDCl capillary) with the different species quantified
3
N
N
BH3
31
N
N
by careful NMR integration of their P NMR resonances.
g
-
H3BO3
d
Attempts to isolate 2, 3 or 4 in the solid state failed
because when THF solutions enriched in these polybora-
nated species were evaporated to dryness, only the monob-
oranyl adduct 1 could be obtained as a solid.
Complex 1, dissolved in acetone, slowly reacts with
water to give back the free PTA and orthoboric acid
1
BH3.THF
BH3
P
P
H B
3
N
N
N
N
N
N
BH3
BH3
N
N
(Scheme 1). The reaction is accelerated at higher tempera-
2
P
2N
ture and completes in ca. 48 h when a sample of 1 in ace-
BH3.THF
tone/H O (1:1 v/v) is kept at 80 °C. The presence of
2
H B
3
H BO in the hydrolised solution was confirmed by the
3
3
P
P
appearance in the IR spectrum of absorptions ascribable
ꢀ
1
H B
3
to B–O bonds (1241 < mBO < 1435 cm ) replacing those
H B
3
N
N
BH3
ꢀ1
BH3
N
assigned in 1 to B–H stretchings (2386 < m < 2275 cm ).
BH
N
3
1
1
In the B NMR spectrum, a signal at 19.52 ppm was
attributed to boric acid [13]. The transformation of 1 into
PTA and H BO is quantitative under the experimental
H B
3
BH3.THF
P
3N
3
3
BH3
conditions used and resembles the well-known acid-cataly-
sed hydrolysis of aminoboranes for which a mechanism
involving the reversible protonation of 1 with release of
H and formation of B(OH) has been proposed [15].
P
H B
3
N
N
BH3
N
2
3
H B
The general reactivity and, in particular, the coordina-
tion properties of 1 and the higher boronated adducts
(2,4) are under investigation in our laboratories [9]. Preli-
3
4
Scheme 1.

362
S. Bola n˜ o et al. / Inorganic Chemistry Communications 9 (2006) 360–363
4
1
00 PTA
1
3P
80
60
40
20
0
2N
3
N
2P
0
1
2
3
4
5
6
7
8
9
10
11
BH3.THF equivalents
Fig. 1. Distribution of the different PTA borane adducts (1–4) as a function of the equivalents of BH
3
.THF added to a THF solution of PTA.
minary studies indicate that the boronated PTA-adducts
behave as monofunctional P-ligands towards a variety of
transition metal ligands including rhodium, iridium and
ruthenium. In some cases the new metal complexes exhibit
a substantial enhancement of their water solubility in com-
parison to the related PTA species, which could make them
suitable as potential water soluble catalysts and in medici-
nal chemistry. Catalytic studies aimed at exploring the per-
formance of these ligands in both water and under biphasic
aqueous conditions are in progress.
(d) A.M.M. Meij, S. Otto, A. Roodt, Inorg. Chim. Acta 358 (2005)
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e) D.A. Krogstad, A.J. DeBoer, W.J. Ortmeier, J.W. Rudolf, J.A.
Halfen, Inorg. Chem. Commun. 8 (2005) 1141;
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(g) B.J. Frost, S.B. Miller, K.O. Rove, D.M. Pearson, J.D. Korinek,
J.L. Harkreader, C.A. Mebi, J. Shearer, Inorg. Chim. Acta 359 (2006)
1
(
(
2
83;
(
h) A. Romerosa, T. Campos-Malpartida, C. Lidrissi, M. Saoud, M.
Serrano-Ruiz, M. Peruzzini, J.A. Garrido-C a´ rdenas, F. Garc ´ı a-
Maroto, Inorg. Chem. 45 (2006) 1289.
[
5] (a) For recent catalytic applications of PTA complexes see: S. Bola n˜ o,
L. Gonsalvi, F. Zanobini, F. Vizza, V. Bertolasi, A. Romerosa, M.
Peruzzini, J. Mol. Catal. A 224 (2004) 61;
Acknowledgements
(
b) A.M. Trzeciak, B. Borak, Z. Ciunik, J.J. Ziołkowski, M.F.C.
Guedes da Silva, A.J.L. Pombeiro, Eur. J. Inorg. Chem. (2004) 1411;
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We thank the EC for promoting this scientific activity
with financial support including grants (to S.B.) through
the Marie Curie Research Training Network ‘‘HYDRO-
CHEM’’ (Contract HPRN-CT-2002-00176). Thanks are also
expressed to ‘‘FIRENZE HYDROLAB’’, a project sponsored by
Ente Cassa di Risparmio di Firenze, for the use of the Bru-
ker 400 DRX NMR spectrometer. S.B. thanks Dr. P. Bar-
(
(d) D.A. Krogstad, J. Cho, A.J. DeBoer, J. Klitzke, W.R. Sanow,
H.A. Williams, J.A. Halfen, Inorg. Chim. Acta 359 (2006) 136.
6] (a) A. Romerosa, P. Bergamini, V. Bertolasi, A. Canella, M.
Cattabriga, R. Gavioli, S. Ma n˜ as, N. Mantovani, L. Pellicani, Inorg.
Chem. 43 (2004) 905;
[
[
(
b) A. Dorcier, P.J. Dyson, C. Gossens, U. Rothlisberger, R.
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c) B. Serli, E. Zangrando, T. Gianferrara, C. Scolaro, P.J. Dyson, A.
1
1
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(
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towards different transition metal moieties. A. Albinati, S. Bola n˜ o, L.
Gonsalvi, M. Peruzzini, in preparation.
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Anal. for C H15BN P (170.99 g/mol) Calcd. C 42.15, H 8.84, N 24.57%;

S. Bola n˜ o et al. / Inorganic Chemistry Communications 9 (2006) 360–363
363
ꢀ
1
found C 42.17, H 8.80, N 24.15%. IR (KBr pellets, cm ): mBH = 2275
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1
(
m), 2317 (m), 2386 (s); mBN = 1171 (s). H NMR (acetone-d
6
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4
8
1
C
00.13 MH
H, CH ); 4.30–4.53 (m, 4H, CH
00.68 MHz, TMS ref): d (ppm) 43.42 (d, C
z 3
, TMS ref): d (ppm) 0.77–1.99 (br, 3H, BH
13
1
2
2
6
1
b
1
3
3
a
, JCP = 25.8 Hz); 75.10 (d, C
c
, JCP = 1.1 Hz); 77.69 (d, C
, 294 K, 128.3 MHz, BF .OEt
). P{ H} NMR (acetone-d
ref): ꢀ94.85 ppm (s).
11] Coordination of borane to phosphorus should have broadened the
d
, JCP
=
[14] Computational investigation of reaction energies for Lewis acid
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New Theoretical and Spectroscopical Approaches to Inorganic
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txos, Spain, 4–9, September, 2004, Book of Abstract, Abstract
58.
11
0.4 Hz). B NMR (acetone-d
6
3
2
ref): d
1
31
1
(
2
ppm) ꢀ10.55 (q, JBH = 98.3 Hz, BH
3
6
,
94 K, 162.02 MHz, 85% H PO
3
4
[
31
P NMR resonance of 1. Such kind of broadening is a common
NMR feature exhibited by boronated phosphines and is observed also
for the bis-, tris- and tetrakis-boranyl PTA adducts 2P, 3P and 4.
12] See for example: C.A. Taylor II, S.W. Brown, V. Subramaniam, S.
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