Hydrotris[3-(carboxypyrrolidido)pyrazol-l-yl]borate, the first proven N3O3-hexadentate homoscorpionate ligand

Article abstract of DOI:10.1039/b001651i

The new homoscorpionate ligand, hydrotris[3-(carboxypyrrolidido)pyrazol-l-yl]borate, [Tp^cpd]^-, was synthesized, and it formed with lanthanide(m) ions La, Nd and Sm, structurally characterized [M(Tp^cpd)JPF6 complexes, where the lanthanide ion was in a ten-coordinate enviroment consisting of one N₃O₃ hexadentate Tp^cpd ligand, and one N₂O₂ tetradentate Tp^cpd ligand. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b001651i

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Hydrotris[3-(carboxypyrrolidido)pyrazol-1-yl]borate, the first
proven N3O3-hexadentate homoscorpionate ligand
Arnold L. Rheingold, Christopher D. Incarvito and Swiatoslaw Trofimenko*
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA
Received 29th February 2000, Accepted 2nd March 2000
Published on the Web 30th March 2000
The new homoscorpionate ligand, hydrotris[3-(carboxy-
pyrrolidido)pyrazol-1-yl]borate, [Tpcpd]؊, was synthesized,
and it formed with lanthanide(III) ions La, Nd and Sm,
structurally characterized [M(Tpcpd)2]PF6 complexes, where
the lanthanide ion was in a ten-coordinate enviroment
consisting of one N3O3 hexadentate Tpcpd ligand, and one
N2O2 tetradentate Tpcpd ligand.
Although over 170 different scorpionate ligands are known,1
their maximum denticity is usually equal to the number of
pyrazolyl rings bonded to boron, except for the instance of
oxidative addition of the ortho-CH of a 3-phenyl substituent
from TpPh to Rh().2 The only example where higher denticity,
including κ6, has been realized in a Tpx ligand, was with TpPy,
Fig. 1 TpcpdM where –N᎐N– and – – – O represents the third, hidden,
᎐
where the 3-(2-pyridyl) substituent on each pyrazolyl ring was
also capable of coordination.3–5 No other examples of Tpx
hexadenticity are known, although ligands such as Tpo-An (o-
pzcpd ring.
An = anisole, ortho-methoxyphenyl),6 and Tp2,4-(OMe) Ph,7 might
2
be capable of exhibiting additional coordination, up to κ6,
through the ortho-methoxy groups. However, the structurally
characterized Tl complexes of Tpo-An 8
,
and of Tp2,4-(OMe) Ph
2
showed the ortho-methoxy groups not only uncoordinated, but
actually turned away from the metal ion.
Our approach to N3O3 hexadentate Tpx ligands consisted
of choosing 3-C(O)NR2 as the pyrazolyl 3-substituent, which
would coordinate through the oxygen, bearing a partial
negative charge via the contributing form R Nϩ᎐C–OϪ. As we
᎐
2
did not want the 3-substituent to be overly bulky, we selected as
NR2 the pyrrolidido group, N(C4H8), in which the N-substitu-
ents were tied back into a relatively inflexible five-membered
ring, leading to the ligand Tpcpd (“cpd” standing for carboxy-
pyrrolidido). The five-membered N,O-chelate ring formed by
this substituent per pzcpd unit (pz = pyrazolyl) would be more
compact than the twisted six-membered one, which might form
in the case of an ortho-methoxyphenyl 3-substituent (Fig. 1).
3-(Carboxypyrrolidido)pyrazole, Hpzcpd, was synthesized
by the reaction of diketopiperazine (the tricyclic anhydride
of pyrazole-3-carboxylic acid),9,10 with excess pyrrolidine, as
shown in Scheme 1.† Upon reaction with KBH4 in refluxing
Fig. 2 Structure of the complex Tl(Tpcpd). Selected bond lengths (Å)
and angles (Њ): Tl–N(11) 2.713(7), Tl–N(21) 2.710(7), Tl–N(31)
2.704(7); N(11)–Tl–N(31) 67.2(2), N(11)–Tl–N(21) 69.4(2), N(21)–Tl–
N(31) 72.4(2).
Tl[TpPy] (2.670 Å), while the Tl–O distances averaged 3.004 Å,
thus being moderately shorter than the pyridyl N–Tl distances
in Tl[TpPy] (3.176 Å).9
The reaction of Tpcpd with trivalent lanthanide ions (La, Nd,
Sm) in 2:1 mol ratio yielded cationic species [M(Tpcpd)2]ϩ,†
which were isolated as their PF6 salts, and structures of all three
were determined by X-ray crystallography.‡ They were essen-
tially identical, and of the type shown in Fig. 3 for the Sm()
complex, except that their average M–N and M–O bond lengths
decreased in the La–Nd–Sm series, in line with the “lanthanide
contraction” of their ionic radii. Thus, the average M–N bond
lengths were 2.720, 2.663, and 2.635 Å, respectively, while the
corresponding M–O bonds were 2.594, 2.538 and 2.510 Å.
In each case the lanthanide ion was in a ten-coordinate
environment, being bonded to the three N and three O atoms
of one κ6 Tpcpd ligand, and to two N and two O atoms of the
second Tpcpd ligand, bonded only in κ4 fashion, and contain-
ing the uncoordinated pzcpd arm positioned roughly at right
angles to the B–M axis. Counting the N,O-donor set from each
pzcpd arm as a unit, one could describe the coordination of La
as “octahedral” with one vacant site. Looking down the B–M
bond, viewed as having a square of oxygens in the equatorial
belt, and three capping nitrogens at one apex, and two nitrogens
Scheme 1
4-methylanisole it was converted to hydrotris[3-(carboxy-
pyrrolidido)-pyrazol-1-yl]borate, Tpcpd, which was purified and
characterized as the Tl salt, TlTpcpd. An X-ray crystallographic
structure determination (Fig. 2) showed Tl to be symmetrically
coordinated to the three N atoms.‡ In addition, all three carb-
onyl oxygens were pointed at the Tl ion, although being out of
the bonding range. The N–Tl bond lengths averaged 2.709 Å,
which was slightly longer than the pyrazolyl N–Tl distances in
DOI: 10.1039/b001651i
J. Chem. Soc., Dalton Trans., 2000, 1233–1234
This journal is © The Royal Society of Chemistry 2000
1233

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mined in chloroform-d. Anal. Calc. for C8H11N2O: C, 63.6; H, 7.28;
N, 18.5; Found: C, 63.3; H, 7.49; N, 18.2%.
Tl[Tpcpd]. A mixture of Hpzcpd and KBH4 (3.5:1 mol ratio) was
refluxed in 4-methylanisole until the theoretical amount of hydrogen
was evolved. After distilling the solvent in vacuo, the residue was dis-
solved in THF–DMF (10:1) and treated with excess aqueous TlNO3.
After dilution with much water, the product was extracted with meth-
ylene chloride, the extracts were filtered through alumina, stripped, and
the residue yielded 69% of the product, mp 263–265 ЊC, upon tritur-
ation with ethyl acetate; IR: BH 2500, CO 1606 cmϪ1. 1H NMR (ppm):
7.62 (d, J = 2.2 Hz, 1H, H-5), 6.36 (d, J = 2.2 Hz, 1H, H-4), 3.56 (m, 4H,
CH2), 1.85 (m, 4H, CH2). 13C NMR (ppm): 23.5 and 26.2 (NCH2CH2),
46.4 and 47.8 (NCH2CH2), 105.2 (C-4), 135.3 (C-5), 147.1 (C-3), 162.9
(CO). Anal. Calc. for C24H31BN9O3Tl: C, 43.2; H, 4.65; N, 12.6; Found:
C, 43.1; H, 4.78; N, 12.4%.
[La(Tpcpd)2][PF6]. A mixture of TlTpcpd and La(NO3)3 (2:1 mol ratio)
was stirred with excess NH4PF6 in DMF until a clear solution resulted,
which was diluted with much water, and extracted with chloroform. The
extracts were passed through a layer of alumina, and the residue from
evaporation of the eluate was crystallized from nitromethane–EtOAc.
Mp 296–298 ЊC, decomp.; IR: BH 2474, CO 1592 cmϪ1. 1H NMR (ppm):
7.83 (1H, H-5), 6.43 (1H, H-4), 3.61 (2H, NCH2), 2.92 (2H, NCH2),
1.87 (2H, CH2), 1.63 (2H, CH2). 13C NMR (ppm): 23.4 and 26.4
(NCH2CH2), 47.0 and 47.7 (NCH2CH2), 105.3 (H-4), 136.1 (H-5),
147.0 (H-3), 163.8 (CO). Anal. Calc. for C48H62B2F6LaN12O6P: C, 47.7;
H, 5.13; N, 13.9; Found: C, 48.0; H, 5.28; N, 13.7%.
Fig. 3 Structure of the complex cation [Sm(Tpcpd)2]ϩ. Selected bond
lengths (Å) and angles (Њ): Sm–N(1) 2.596(7), Sm–N(11) 2.673(8),
Sm–N(21) 2.533(8), Sm–N(31) 2.651(7), Sm–N(41) 2.701(7), Sm–O(1)
2.484(7), Sm–O(2) 2.579(7), Sm–O(3) 2.481(7), Sm–O(4) 2.482(6),
Sm–O(5) 2.523(6); N(1)–Sm–N(11) 60.9(2), N(11)–Sm–N(21) 68.9(2),
N(21)–Sm–N(31) 128.5(2), N(31)–Sm–N(41) 67.1(2), O(1)–Sm–O(2)
77.9(2), O(2)–Sm–O(3) 135.4(2), O(3)–Sm–O(4) 70.9(2), O(4)–Sm–O(5)
136.3(2).
The isostructural [Nd(Tpcpd)2]PF6 and [Sm(Tpcpd)2]PF6 complexes
were prepared similarly.
‡ Crystal data. For C24H31BN9O3Tl: orthorhombic, Pbcn, a =
34.899(4), b = 9.26(1), c = 17.409(3) Å, V = 5605(4) Å3, Z = 8, T =
298(2) K, Dcalc = 1.680 g cmϪ3, colorless rod, GOF = 0.625, µ(Mo-
Kα) = 0.71073 Å, R(F) = 4.03% for 6329 observed independent
reflections (4Њ ≤ 2θ ≤ 55Њ).
For C48H62B2F6LaN18O6Pؒ2EtOAc: monoclinic, C2/c, a =
25.7439(3), b = 19.9574(2), c = 25.7150(3) Å, β = 101.1056(5)Њ, V =
12964(3) Å3, Z = 8, T = 173(2) K, Dcalc = 1.470 g cmϪ3, colorless block,
GOF = 2.205, µ(Mo-Kα) = 0.71073 Å, R(F) = 7.81% for 10194
observed independent reflections (4Њ ≤ 2θ ≤ 50Њ).
and one oxygen at the other. All the La–N bond distances were
almost identical, those of the κ6 ligand averaging 2.694 Å.
The shortest bond (2.644 Å) was trans to the vacant site. The
La–O distances were also almost identical, and averaged
2.694 Å in the κ3 ligand, and 2.760 Å in the κ2 ligand.
In the related icosahedral cation [Sm(TpPy)2]ϩ, the pyrazole
N–M bond lengths averaged 2.658 Å, while the pyridyl N–M
bonds were 2.950 Å.11 These values should be compared with
those of [Sm(Tpcpd)2]ϩ, which were 2.635 Å for Sm–N, and
2.510 Å for Sm–O bonds. The fact that the latter distance is
shorter by 0.44 Å as compared with the corresponding Sm–N
distance to the pyridyl nitrogen in [Sm(TpPy)2]ϩ, points to much
tighter κ6 chelation by the Tpcpd ligand, as compared with
TpPy. It is possible that this compactness precluded the accom-
modation of κ6 chelation by the second Tpcpd ligand in the
[La(Tpcpd)2]ϩ complexes, at least in the crystal. At the same
For C48H62B2F6N18NdO6PؒCH2Cl2ؒCHCl3: monoclinic, C2/c,
a = 26.0247(4), b = 19.8463(3), c = 26.3284(4) Å, β = 103.9329(7)Њ,
V = 13198(5) Å3, Z = 8, T = 173(2) K, Dcalc = 1.512 g cmϪ3, purple
block, GOF = 2.040, µ(Mo-Kα) = 0.71073 Å, R(F) = 8.52% for 12673
observed independent reflections (4Њ ≤ 2θ ≤ 52Њ).
For C48H62B2F6N18O6PSmؒ1/2H2Oؒ1/2Me2CO: monoclinic, C2/c,
a = 25.7018(2), b = 19.6629(2), c = 25.5878(3) Å, β = 100.4362(8)Њ,
V = 12717.4(2) Å3, Z = 8, T = 173(2) K, Dcalc = 1.463 g cmϪ3, colorless
plate, GOF = 1.168, µ(Mo-Kα) = 0.71073 Å, R(F) = 8.16% for 10364
observed independent reflections (4Њ ≤ 2θ ≤ 50Њ). CCDC reference
number 186/1885. See http://www.rsc.org/suppdata/dt/b0/b001651i/
for crystallographic files in .cif format.
1
time, the H and 13C NMR spectra of the diamagnetic [La-
(Tpcpd)]ϩ complex cation showed only one type of pzcpd present,
implying a rapid exchange of the coordinated and uncoordin-
ated pzcpd arms on the NMR time scale, as the presence of a
rigid 12-coordinate structure, with two κ6 Tpcpd ligands, was
thought to be less likely. No attempts were made to freeze out
the static structure.
In summary, we have synthesized the first proven N3O3-
hexadentate homoscorpionate ligand, Tpcpd, modifications of
which can be readily visualized, and have demonstrated that
it is capable of coordinating in both, κ4 N2O2 and κ6 N3O3
fashion.
1 S. Trofimenko, Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands, Imperial College Press, London, UK,
1999.
2 R. Krentz, PhD Dissertation, University of Alberta, Edmonton,
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Notes and references
† Syntheses: 3-(carboxypyrrolidido)pyrazole (Hpzcpd). This pyrazole
was synthesized by refluxing diketopiperazine9,10 in THF with a large
excess of pyrrolidine. After stripping the low-boilers, the product was
distilled in vacuo, bp 215–220 ЊC at 3 Torr (58% yield). Mp 145–147 ЊC
(from toluene–heptane). IR: CO 1589 cmϪ1. H NMR (ppm): 7.61 (d,
1
J = 2.2 Hz, 1H, H-5), 6.69 (d, J = 2.2 Hz, 1H, H-4), 3.84 (m, 2H, CH2),
3.70 (m, 2H, CH2), 1.95 (4H, CH2). 13C NMR (ppm): 23.9 and 26.5
(NCH2CH2), 47.0 and 48.5 (NCH2CH2), 107.0 (C-4), 134.1 (C-5), 142.5
(C-3), 161.0 (CO). These, and all the other NMR spectra were deter-
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J. Chem. Soc., Dalton Trans., 2000, 1233–1234